Co-reporter:Rafal Kulmaczewski, Oscar Cespedes, and Malcolm A. Halcrow
Inorganic Chemistry March 20, 2017 Volume 56(Issue 6) pp:3144-3144
Publication Date(Web):February 28, 2017
DOI:10.1021/acs.inorgchem.7b00071
The Fe[BF4]2 complex of the Schiff base podand tris[4-(thiazol-4-yl)-3-aza-3-butenyl]amine exhibits gradual thermal spin-crossover with T1/2 ≈ 208 K in the solid state. A weak discontinuity in the magnetic susceptibility curve at 190 K is associated with a reentrant symmetry-breaking transition involving a trebling of the unit cell volume (from P21/c, Z = 4, to P21, Z = 12). The intermediate phase contains six independent cations in puckered layers of low-spin, and high-spin or mixed-spin, molecules with an overall 30% high-spin population at 175 K.
Co-reporter:Izar Capel Berdiell, Rafal Kulmaczewski, and Malcolm A. Halcrow
Inorganic Chemistry August 7, 2017 Volume 56(Issue 15) pp:8817-8817
Publication Date(Web):July 12, 2017
DOI:10.1021/acs.inorgchem.7b00699
Seven [FeL2][BF4]2 complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD3CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yl)pyridine, 2,6-di(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-1-yl)pyrimidine derivatives often exhibit thermal spin-crossover behavior. Gas-phase density functional theory calculations confirm the high-spin form of [Fe(bpt)2]2+ and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fe/pyrazolyl σ-bonding interaction, which we attribute to a small narrowing of the chelate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence, the high-spin state of [Fe(bpt)2]2+ centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of the bpt derivatives was also synthesized.
Co-reporter:Malcolm A. Halcrow, Guillaume Chastanet
Polyhedron 2017 Volume 136(Volume 136) pp:
Publication Date(Web):4 November 2017
DOI:10.1016/j.poly.2017.01.029
Co-crystallization of [Fe(bpp)2][BF4]2 and [Co(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine) from nitromethane-diethyl ether yields homogeneous polycrystalline materials analyzing as [FexCo1‒x(bpp)2][BF4]2 (1.00 ≤ x ≤ 0.77). Thermal spin-crossover in these materials only involves the iron centers, and increasing the cobalt dopant concentration leads to a reduction in T½ and a loss of cooperativity. The materials exhibit the LIESST effect, with all three samples presenting the same T(LIESST) value. LIESST relaxation kinetics have a clear multistep character, which has not been detected before in samples derived from [Fe(bpp)2][BF4]2. Magnetic susceptibility and low-temperature crystallographic data are also presented for the pure precursor complex [Co(bpp)2][BF4]2.Thermal spin-crossover in [FexCo1‒x(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine; 1.00 ≤ x ≤ 0.77) only involves the iron centers. Increasing the cobalt dopant concentration leads to a reduction in T½ and a loss of cooperativity. The materials exhibit the LIESST effect, with all three samples presenting the same T(LIESST) value. LIESST relaxation kinetics have a clear multistep character, which has not been detected before in samples derived from [Fe(bpp)2][BF4]2.Download high-res image (150KB)Download full-size image
Co-reporter:Rafal Kulmaczewski;Elzbieta Trzop;Laurence J. Kershaw Cook;Eric Collet;Guillaume Chastanet
Chemical Communications 2017 vol. 53(Issue 99) pp:13268-13271
Publication Date(Web):2017/12/12
DOI:10.1039/C7CC07990G
Light-Induced Excited Spin State Trapping (LIESST) data are reported for seven isostructural solvate salts from the iron(II)/2,6-di(pyrazol-1-yl)pyridine family. A complicated relationship between their spin-crossover T1/2 and T(LIESST) values may reflect low-temperature thermal and light-induced symmetry breaking, which is shown by one of the compounds but not by two others.
Co-reporter:Sam Greatorex and Malcolm A. Halcrow
CrystEngComm 2016 vol. 18(Issue 25) pp:4695-4698
Publication Date(Web):16 May 2016
DOI:10.1039/C6CE00966B
Solvate crystals of 9,10-dimethyl-2,3,6,7,14,15-hexa(hydroxy)-triptycene (1) form a variety of 3D hydrogen-bonded topologies, including bcu, acs, bsn and an apparently new 7-connected net. Several of these networks contain 1D or 2D arrays of solvent-filled channels, amounting to up to 60% solvent-accessible void space.
Co-reporter:Rafal Kulmaczewski and Malcolm A. Halcrow
CrystEngComm 2016 vol. 18(Issue 14) pp:2570-2578
Publication Date(Web):14 Mar 2016
DOI:10.1039/C6CE00163G
The phase behaviour and spin states of [Fe(NCS)2(dpq)2] (1; dpq = dipyrido[3,2-f:2′,3′-h]quinoxaline), [Fe(NCS)2(dppz)2] (2; dppz = dipyrido[3,2-a:2′3′-c]phenazine) and [Fe(NCS)2(dppn)2] (3; dppn = dipyrido[3,2-a:2′3′-c]benzophenazine) have been investigated. Solvent-free 1 and 2 are isostructural and low-spin in the crystalline state, in contrast to previously published 2·py (py = pyridine) which exhibits a hysteretic spin-crossover (SCO) transition near 140 K. The inactivity of 1 and 2 towards SCO may relate to their more crowded intermolecular lattice environment, particularly two very short intermolecular anion⋯π contacts involving the NCS− ligands. Two solvate phases of 1 are also described, including 1·2py which undergoes gradual SCO with T½ca. 188 K. Bulk samples of 2 and 3 are predominantly low-spin and isostructural with the crystals of 2 by powder diffraction, but bulk samples of 1 contain an extra phase that exhibits hysteretic SCO, but was not crystallographically characterised. Crystal structures of low-spin [Fe(dppz)3][ClO4]2 (4) and a solvate of [Fe(dppn)3][BF4]2 (5) are also described, which are the first homoleptic complexes of these ligands to be crystallographically characterised.
Co-reporter:Dr. Laurence J. Kershaw Cook;Dr. Rafal Kulmaczewski;Dr. Oscar Cespedes; Malcolm A. Halcrow
Chemistry - A European Journal 2016 Volume 22( Issue 5) pp:1789-1799
Publication Date(Web):
DOI:10.1002/chem.201503989
Abstract
The complex [FeL2][BF4]2 (1; L=4-(isopropylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) forms solvate crystals 1⋅solv (solv=MeNO2, MeCN, EtCN, or Me2CO). Most of these materials lose their solvent sluggishly on heating. However, heating 1⋅MeNO2 at 450 K, or storing 1⋅EtCN under ambient conditions, leads to single-crystal to single-crystal exchange of the organic solvent for atmospheric moisture, forming 1⋅H2O. Solvent-free 1 (1⋅sf) can be generated in situ by annealing 1⋅H2O at 370 K in the diffractometer or magnetometer. The different forms of 1 are isostructural (P21/c, Z=4) and most of them exhibit spin-crossover (SCO) at 141≤T ≤212 K, depending on their solvent content. The exception is the EtCN solvate, whose pristine crystals remain high-spin between 3–300 K. The cooperativity of the spin-transitions depends on the solvent, ranging from gradual and incomplete when solv=acetone to abrupt with 17 K hysteresis when solv=MeCN. Our previously proposed relationship between molecular structure and SCO explains some of these observations, but there is no single structural feature that correlates with SCO in all the 1⋅solv materials. However, changes to the unit cell dimensions during SCO differ significantly between the solvates, and correlate with the SCO cooperativity. In particular, the percentage change in unit cell volume during SCO for the most cooperative material, 1⋅MeCN, is 10 times smaller than for the other 1⋅solv crystals.
Co-reporter:Thomas D. Roberts, Malcolm A. Halcrow
Polyhedron 2016 Volume 103(Part A) pp:79-86
Publication Date(Web):8 January 2016
DOI:10.1016/j.poly.2015.09.054
Two new tridentate ligands 2,6-bis(5-ethyl-1H-pyrazol-3-yl)pyridine and 2,6-bis(5-benzamido-1H-pyrazol-3-yl)pyridine, have been synthesized. These ligands have been used in a new series of six complexes of formula “RuCl2(PPh3)2(LR)·nH2O” (n = 1 or 2) where LR is 2,6-bis(5-R-1H-pyrazol-3-yl)pyridine (R = Me, Et, tBu, NH2, NHC{O}tBu and NHC{O}Ph). Crystal structures of [RuCl(PPh3)2(LMe)]Cl·MeOH and [Ru(OH2)(PPh3)2(LtBu)]Cl2·4CDCl3 contain six-coordinate complex centers with trans-phosphine ligands, and show that the chloride ions can occupy the first or second coordination spheres of the complexes. The latter structure demonstrates that the chloride ions in this type of compound can be labile under ambient conditions, which is an essential pre-requisite for catalytic activity. Anion metathesis yielded [Ru(OH2)(PPh3)2(LtBu)][PF6]2, which was also crystallographically characterized. All the complexes (except air-sensitive [RuCl2(PPh3)2(LNH2)]) were screened for activity towards transfer hydrogenation of acetophenone in refluxing 2-propanol. The chloride salt catalysts are active but show a significant induction period, which may imply decomposition of the complexes during the reaction. However the activity of the PF6− salt is much higher, which shows that competition between chloride and substrate for the metal center is a significant factor in catalysis by these compounds.Six new complexes of type “[RuCl(PPh3)2(LR)]Cl” have been prepared, where LR is a disubstituted 2,6-di(1H-pyrazol-3-yl)pyridine derivative. Crystal structures have shown that the chloride ligand in these compounds can be labile under ambient conditions, which is a requirement for catalysis. One complex salt [Ru(OH2)(PPh3)2(LR)][PF6]2 (R = tBu) was also obtained. All the complexes tested are moderately active towards transfer hydrogenation of acetophenone, with the PF6− salt having a much higher activity than the chloride salts.
Co-reporter:Jonathan J. Loughrey, Nathan J. Patmore, Amgalanbaatar Baldansuren, Alistair J. Fielding, Eric J. L. McInnes, Michaele J. Hardie, Stephen Sproules and Malcolm A. Halcrow
Chemical Science 2015 vol. 6(Issue 12) pp:6935-6948
Publication Date(Web):20 Aug 2015
DOI:10.1039/C5SC02776D
Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl {tBu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6− catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or tBu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.
Co-reporter:Laurynas Pukenas, Florence Benn, Edmund Lovell, Amedeo Santoro, Laurence J. Kershaw Cook, Malcolm A. Halcrow and Stephen D. Evans
Journal of Materials Chemistry A 2015 vol. 3(Issue 30) pp:7890-7896
Publication Date(Web):27 May 2015
DOI:10.1039/C5TC01233C
Drop-casting acetone solutions of [Fe(bpp)2][BF4]2 (bpp = 2,6-di[pyrazol-1-yl]pyridine) onto a HOPG surface affords unusual chain-of-beads nanostructures. The beads in each chain are similar in size, with diameters in the range of 2–6 nm and heights of up to 10 Å, which is consistent with them containing between 10–50 molecules of the compound. The beads can be classified into two types, which exhibit different conduction regimes by current-imaging tunnelling spectroscopy (CITS) which appear to correlate with their positions in the chains, and may correspond to molecules containing high-spin and low-spin iron centres. Similarly drop-cast films of the complex on a gold surface contain the intact [Fe(bpp)2][BF4]2 compound by XPS. 4-Mercapto-2,6-di[pyrazol-1-yl]pyridine undergoes substantial decomposition when deposited on gold, forming elemental sulfur, but 4-(N-thiomorpholinyl)-2,6-di[pyrazol-1-yl]pyridine successfully forms SAMs on a gold surface by XPS and ellipsometry.
Co-reporter:Shinya Hayami, Stephen M. Holmes and Malcolm A. Halcrow
Journal of Materials Chemistry A 2015 vol. 3(Issue 30) pp:7775-7778
Publication Date(Web):14 Jul 2015
DOI:10.1039/C5TC90128F
A graphical abstract is available for this content
Co-reporter:Amedeo Santoro, Laurence J. Kershaw Cook, Rafal Kulmaczewski, Simon A. Barrett, Oscar Cespedes, and Malcolm A. Halcrow
Inorganic Chemistry 2015 Volume 54(Issue 2) pp:682-693
Publication Date(Web):January 7, 2015
DOI:10.1021/ic502726q
Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2]2+, [Fe(1,2-bip)2]2+, and [Fe(1-ipp)2]2+ are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2]2+ and [Fe(2-ipp)2]2+ are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2]2+ exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16–20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers.
Co-reporter:Laurence J. Kershaw Cook, Flora L. Thorp-Greenwood, Tim P. Comyn, Oscar Cespedes, Guillaume Chastanet, and Malcolm A. Halcrow
Inorganic Chemistry 2015 Volume 54(Issue 13) pp:6319-6330
Publication Date(Web):June 8, 2015
DOI:10.1021/acs.inorgchem.5b00614
The synthesis of 4-methyl-2,6-di(pyrazol-1-yl)pyridine (L) and four salts of [FeL2]X2 (X– = BF4–, 1; X– = ClO4–, 2; X– = PF6–, 3; X– = CF3SO3–, 4) are reported. Powder samples of 1 and 2 both exhibit abrupt, hysteretic spin-state transitions on cooling, with T1/2↓ = 204 and T1/2↑ = 209 K (1), and T1/2↓ = 175 and T1/2↑ = 193 K (2). The 18 K thermal hysteresis loop for 2 is unusually wide for a complex of this type. Single crystal structures of 2 show it to exhibit a Jahn–Teller-distorted six-coordinate geometry in its high-spin state, which would normally inhibit spin-crossover. Bulk samples of 1 and 2 are isostructural by X-ray powder diffraction, and undergo a crystallographic phase change during their spin-transitions. At temperatures below T1/2, exposing both compounds to 10–5 Torr pressure inside the powder diffractometer causes a reversible transformation back to the high-temperature crystal phase. Consideration of thermodynamic data implies this cannot be accompanied by a low → high spin-state change, however. Both compounds also exhibit the LIESST effect, with 2 exhibiting an unusually high T(LIESST) of 112 K. The salts 3 and 4 are respectively high-spin and low-spin between 3 and 300 K, with crystalline 3 exhibiting a more pronounced version of the same Jahn–Teller distortion.
Co-reporter:Amedeo Santoro, Carlo Sambiagio, Patrick C. McGowan and Malcolm A. Halcrow
Dalton Transactions 2015 vol. 44(Issue 3) pp:1060-1069
Publication Date(Web):07 Nov 2014
DOI:10.1039/C4DT02824D
A new synthesis of 1,1,1-tris(pyrid-2-yl)ethane (L), and a survey of its coordination chemistry, are reported. The complexes [ML2]n+ (Mn+ = Fe2+, Co2+, Co3+, Cu2+ and Ag+), [PdCl2L] and [CuI(L)] have all been crystallographically characterised. Noteworthy results include an unusual square planar silver(I) complex [Ag(L)2]X (X− = NO3− and SbF6−); the oxidative fixation of aerobic CO2 by [CuI(L)] to yield [Cu2I(L)2(μ-CO3)]2[CuI3] and [Cu(CO3)(L)]; and, water/carbonato tape and water/iodo layer hydrogen bonding networks in hydrate crystals of two of the copper(II) complexes. Cyclic voltammetric data on [Fe(L)2]2+ and [Co(L)2]2+/3+ imply that the peripheral methyl substituent has a weak influence on the ligand field exerted by L onto a coordinated metal ion.
Co-reporter:Laurence J. Kershaw Cook, Julie Fisher, Lindsay P. Harding and Malcolm A. Halcrow
Dalton Transactions 2015 vol. 44(Issue 20) pp:9417-9425
Publication Date(Web):16 Apr 2015
DOI:10.1039/C5DT00732A
The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published “back-to-back” bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(II) and zinc(II). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n]2n+ and [{Zn(μ-bppSSbpp)}n]2n+ form tetranuclear metallacycles in nitromethane solution (n = 4), although 1H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2]2+ and [{Fe(μ-bppSSbpp)}n]2n+ exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures.
Co-reporter:Laurence J. Kershaw Cook, Rufeida Mohammed, Grant Sherborne, Thomas D. Roberts, Santiago Alvarez, Malcolm A. Halcrow
Coordination Chemistry Reviews 2015 s 289–290() pp: 2-12
Publication Date(Web):
DOI:10.1016/j.ccr.2014.08.006
Co-reporter:Dr. Amedeo Santoro;Dr. Rianne M. Lord;Dr. Jonathan J. Loughrey;Dr. Patrick C. McGowan; Malcolm A. Halcrow;Adam F. Henwood;Connor Thomson;Dr. Eli Zysman-Colman
Chemistry - A European Journal 2015 Volume 21( Issue 19) pp:7035-7038
Publication Date(Web):
DOI:10.1002/chem.201406255
Abstract
Condensation of a pyridyl-2-carbaldehyde derivative with 2-(bromoethyl)amine hydrobromide gave tetracyclic pyrido[1,2-a]pyrido[1′,2′:3,4]imidazo-[2,1-c]-6,7-dihydropyrazinium dications in excellent yields. Crystal structures and NOE data demonstrated the helical character of the dications, the dihedral angles between the two pyrido groups ranging from 28–45°. An intermediate in the synthesis was also characterized. A much brighter emission compared to literature helicenes has been found, with quantum yields as high as 60 % in the range of λ=460–600 nm. Preliminary cytotoxicity studies against HT-29 cancer cells demonstrated moderate-to-good activity, with IC50 values 12–30× that of cisplatin.
Co-reporter:Dr. Laurence J. KershawCook;Dr. Helena J. Shepherd;Dr. Tim P. Comyn;Dr. Chérif Baldé;Dr. Oscar Cespedes;Dr. Guillaume Chastanet; Malcolm A. Halcrow
Chemistry - A European Journal 2015 Volume 21( Issue 12) pp:4805-4816
Publication Date(Web):
DOI:10.1002/chem.201406307
Abstract
Crystalline [Fe(bppSMe)2][BF4]2 (1; bppSMe=4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) undergoes an abrupt spin-crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit-cell a axis to one-third of its original value (high-temperature phase 1; Pbcn, Z=12; low-temperature phase 2; Pbcn, Z=4). The SCO-active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140–300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)2][BF4]2 (2; bppBr=4-bromo-2,6-di(pyrazol-1-yl)pyridine) and [Fe(bppI)2][BF4]2 (3; bppI=4-iodo-2,6-di(pyrazol-1-yl)-pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1–3 reveals that the more cooperative spin transition in 1, and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light-induced excited-spin-state trapping (LIESST) effect with T(LIESST=70–80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.
Co-reporter:Laurence J. Kershaw Cook, Malcolm A. Halcrow
Polyhedron 2015 Volume 87() pp:91-97
Publication Date(Web):17 February 2015
DOI:10.1016/j.poly.2014.10.021
Two ruthenium compounds, [Ru(κ3C,N,C-bip)2][BF4]2 (bip = 2,6-di{1-methylimidazol-2-ylidene-3-yl}pyridine) and [Ru(terpy)(κ3N,N,C-terpy∗)][BF4]2 (terpy = 2,2′:6′,2″-terpyridine and terpy∗ = 2,2′:6′,4″-terpyridine), have been investigated as dopants for the spin-crossover lattice [Fe(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine). While [Fe(bpp)2][BF4]2 and [Ru(bip)2][BF4]2 did not co-crystallize, five different compositions of solid solutions [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1–x[BF4]2 were prepared, with 0.96 ⩾ x ⩾ 0.13. The materials with intermediate compositions (0.58 ⩾ x ⩾ 0.28) contained a mixture of crystalline and amorphous material by powder diffraction. The spin-crossover midpoint temperature (T½) in [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1–x[BF4]2 decreases smoothly with x, as the larger ruthenium dopant expands the host lattice and stabilizes its high-spin state. That contrasts with our previously published materials [Fe(bpp)2]z[Ru(terpy)2]1–z[BF4]2, which show a more complicated relationship between T½ and their composition.The fluorescent compounds [Ru(bip)2][BF4]2 (bip = 2,6-di{1-methylimidazol-2-ylidene-3-yl}pyridine) and [Ru(terpy)(terpy∗)][BF4]2 (terpy = 2,2′:6′,2″-terpyridine and terpy∗ = 2′:6′,4″-terpyridine) have been investigated as dopants for the spin-crossover lattice [Fe(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine). While [Fe(bpp)2][BF4]2 and [Ru(bip)2][BF4]2 did not co-crystallize, five different solid solutions [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1−x[BF4]2 were prepared. The spin-crossover midpoint temperature (T½) in [Fe(bpp)2]x[Ru(terpy)(terpy∗)]1−x[BF4]2 decreases smoothly with x.
Co-reporter:Jonathan J. Loughrey, Tim P. Comyn, David C. Apperley, Marc A. Little and Malcolm A. Halcrow
Chemical Communications 2014 vol. 50(Issue 57) pp:7601-7603
Publication Date(Web):29 May 2014
DOI:10.1039/C4CC02198C
[FeL3][BF4]2·xH2O (L = 3-(pyrazinyl)-1H-pyrazole) shows negative thermal expansion between 150–240 K but positive thermal expansion at 240–300 K, linked to rearrangement of anions and water molecules within pores in the lattice.
Co-reporter:Rafal Kulmaczewski, Helena J. Shepherd, Oscar Cespedes, and Malcolm A. Halcrow
Inorganic Chemistry 2014 Volume 53(Issue 18) pp:9809-9817
Publication Date(Web):August 29, 2014
DOI:10.1021/ic501402q
Four new iron(II) complexes [Fe(H2Bpz2)2(L)] were prepared (pz = pyrazolyl), where L is dipyrido[3,2-f:2′,3′-h]quinoxaline (dpq), dipyrido[3,2-a:2′3′-c]phenazine (dppz), dipyrido[3,2-a:2′3′-c]benzo[i]-phenazine (dppn), and dipyrido[3,2-a:2′,3′-c](6,7,8,9-tetrahydro)phenazine (dppc). Crystal structures of [Fe(H2Bpz2)2(dpq)], [Fe(H2Bpz2)2(dppz)], and [Fe(H2Bpz2)2(dppn)] all reveal stacks of complex molecules formed through π–π stacking between interdigitated bipyridyl chelate ligands, often with additional intercalated toluene or uncoordinated bipyridyl ligand (dpq). Molecules of [Fe(H2Bpz2)2(dppc)] form a different stacking motif in the crystal, with weaker contacts between individual molecules. Many of the structures also contain channels of disordered solvent, running between the molecular stacks. Despite their different stacking motifs, all these compounds exhibit very gradual thermal spin-crossover (SCO) on cooling, which occur over different temperature ranges but are otherwise quite similar in form. Weak thermal hysteresis in one of these spin equilibria can be attributed to the effects of a change in bipyridyl ligand conformation in the molecular stacks around 150 K, which was observed crystallographically. These results demonstrate that strong mechanical coupling between molecules in a crystal is not sufficient to engineer cooperative SCO switching, if other regions of the lattice are less densely packed.
Co-reporter:Thomas D. Roberts, Marc A. Little, Laurence J. Kershaw Cook and Malcolm A. Halcrow
Dalton Transactions 2014 vol. 43(Issue 20) pp:7577-7588
Publication Date(Web):31 Mar 2014
DOI:10.1039/C4DT00355A
Syntheses of 2,6-di(5-aminopyrazol-3-yl)pyridine (L1), 2,6-di(5-tertbutylcarboxamidopyrazol-3-yl)pyridine (L2), 2,6-di(5-tertbutylpyrazol-3-yl)pyridine (L3), 2-(5-tertbutylpyrazol-3-yl)-6-(5-methylpyrazol-3-yl)pyridine (L4) and 2-(5-tertbutylpyrazol-3-yl)-6-(5-aminopyrazol-3-yl)pyridine (L5) are reported. Iron complex salts of the first four ligands were crystallographically characterised. The structures exhibit intermolecular hydrogen bonding between the cations and the anions and/or solvent, leading to a fluorite (flu) net, a 1D ladder structure, and a homochiral self-penetrating helical network related to the (10,3)-a (srs) topology. All the complexes are high-spin in the crystal, and bulk samples are also fully or predominantly high-spin at room temperature and below although two of the dried materials exhibit partial spin-state transitions on cooling.
Co-reporter:Jérôme Elhaïk;Colin A. Kilner
European Journal of Inorganic Chemistry 2014 Volume 2014( Issue 26) pp:4250-4253
Publication Date(Web):
DOI:10.1002/ejic.201402623
Abstract
The ClO4– salt of [FeL2]2+ {L = 2,6-bis(3-methylpyrazol-1-yl)pyridine} undergoes very gradual thermal spin-crossover centered just below room temperature. In contrast, the BF4– salt of the same complex exhibits an abrupt and structured spin-transition at lower temperature, with a complicated structural chemistry. The difference can be attributed to a much larger change in molecular structure between the spin states of the complex in the more cooperative BF4– salt, leading to an increased kinetic barrier for their interconversion. Consistent with that suggestion, the high-spin and low-spin structures of weakly cooperative [FeL2][ClO4]2 are almost superimposable.
Co-reporter:Simon A. Barrett and Malcolm A. Halcrow
RSC Advances 2014 vol. 4(Issue 22) pp:11240-11243
Publication Date(Web):12 Feb 2014
DOI:10.1039/C4RA00230J
The spin-crossover equilibrium midpoint temperature (T1/2) in [Fe(3-bpp)2]X2 (3-bpp = 2,6-di{pyrazol-3-yl}pyridine) varies from 259 K when X− = BPh4− to 277 K when X− = Br−, at 10 mM concentrations in an acetone–water solvent mixture.
Co-reporter:Malcolm A. Halcrow
New Journal of Chemistry 2014 vol. 38(Issue 5) pp:1868-1882
Publication Date(Web):30 Aug 2013
DOI:10.1039/C3NJ00835E
Developments in the chemistry of 2,6-di(pyrazol-1-yl)pyridine (1-bpp) and 2,6-di(pyrazol-3-yl)pyridine (3-bpp), their derivatives and their complexes are surveyed, with emphasis on the last eight years. Particular advances include the synthesis of multi-functional spin-crossover switches; the incorporation of emissive f-element podand centres into biomedical sensors; the self-assembly of a variety of functional soft materials and surface structures; and, the use of 3-bpp complexes in catalysis.
Co-reporter:Marc A. Little, Jonathan J. Loughrey, Amedeo Santoro, Malcolm A. Halcrow, Michaele J. Hardie
Tetrahedron Letters 2014 Volume 55(Issue 15) pp:2530-2533
Publication Date(Web):9 April 2014
DOI:10.1016/j.tetlet.2014.03.025
The synthesis of four 3,4-di(alkylsulfanyl)benzyl alcohol derivatives is described, in five steps from methyl 3,4-di(hydroxy)benzoate via a Newman–Kwart rearrangement. Incubation of these derivatives in formic acid affords 2,3,7,8,12,13-hexakis(alkylsulfanyl)-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene products, which are hexa-sulfanyl analogues of the well-known supramolecular cavitand host, cyclotriveratrylene (CTV). The yield of this cyclization depends strongly on the alkylsulfanyl substituents present, in the order SMe > SEt ≈ SiPr ≫ SBn. A crystal structure determination of one of the cyclotrimers shows a mode of self-association that is commonly exhibited by CTV itself.
Co-reporter:Dr. Jonathan J. Loughrey;Dr. Stephen Sproules; Eric J. L. McInnes; Michaele J. Hardie; Malcolm A. Halcrow
Chemistry - A European Journal 2014 Volume 20( Issue 21) pp:6272-6276
Publication Date(Web):
DOI:10.1002/chem.201304848
Abstract
Three diplatinum(II) complexes [{PtL}2(μ-thea)] (H4thea=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene) have been prepared, with diphosphine or bipyridyl “L” co-ligands. One-electron oxidation of these complexes gave radical cations containing a mixed-valent [thea]3− ligand with discrete catecholate and semiquinonate centers separated by quaternary methylene spacers. The electronic character of these radicals is near the Robin–Day class II/III border determined by UV/Vis/NIR and EPR spectroscopies. Crystal-structure determinations and a DFT calculation imply that oxidation of the thea4− ligand may lead to an increased through-space interaction between the dioxolene π systems.
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2013 vol. 42(Issue 4) pp:1784-1795
Publication Date(Web):11 Sep 2012
DOI:10.1039/C2CS35253B
This tutorial review discusses the structural and electronic consequences of the Jahn–Teller effect in transition metal complexes, focussing on copper(II) compounds which tend to be the most studied. The nature of a Jahn–Teller distortion in molecular complexes and extended lattices can be manipulated by application of pressure or temperature, by doping a molecule into a host lattice, or simply by molecular design. Many of these results have been achieved using compounds with a trans-[CuX4Y2] coordination sphere, which seems to afford copper centres that are particularly sensitive to their environment. Jahn–Teller distortions lead to some unusual phenomena in molecular magnetism, and are important to the functionality of important classes of conducting and superconducting ceramics.
Co-reporter:Thomas D. Roberts, Marc A. Little, Floriana Tuna, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2013 vol. 49(Issue 56) pp:6280-6282
Publication Date(Web):30 May 2013
DOI:10.1039/C3CC43613F
Two salts of [FeL2]2+ (L = 2,6-bis[5-methyl-1H-pyrazol-3-yl]pyridine) are isostructural under ambient conditions but show different thermal spin-crossover behaviour, involving a variety of crystallographic phase changes.
Co-reporter:Laurence J. Kershaw Cook, Floriana Tuna and Malcolm A. Halcrow
Dalton Transactions 2013 vol. 42(Issue 6) pp:2254-2265
Publication Date(Web):19 Nov 2012
DOI:10.1039/C2DT31736B
The syntheses of 2,6-di(pyrid-2-yl)pyrazine (L1), 2,6-di(pyrazinyl)pyridine (L2), 2,2′:6′,2′′-terpyrazine (L3), 2,6-di(pyrimidin-4-yl)pyridine (L4), 2,6-di(1,2,4-triazin-3-yl)pyridine (L5), 4-hydroxy-2,6-di(pyrazinyl)pyridine (L6) and 4-hydroxy-2,6-di(pyrimidin-2-yl)pyridine (L7) are described. Homoleptic iron(II) and cobalt(II) complexes of these ligands have been prepared and, in four cases, structurally characterised. The iron complexes are all low-spin. However, while the cobalt complexes of the pyrazine-rich ligands L2, L3 and L6 are all predominantly low-spin in the solid state, the other cobalt complexes are essentially high-spin between 5–300 K. The voltammetric M(III)/(II) (M = Fe or Co) oxidations and metal- or ligand-based reductions all become more anodic as the nitrogen content of the ligands increases, which correlates well with Lever's additive electrochemical parameters for the heterocyclic donor groups in each complex.
Co-reporter:Thomas D. Roberts, Marc A. Little, Laurence J. Kershaw Cook, Simon A. Barrett, Floriana Tuna, Malcolm A. Halcrow
Polyhedron 2013 Volume 64() pp:4-12
Publication Date(Web):12 November 2013
DOI:10.1016/j.poly.2013.01.057
2,6-Di(1-methyl-pyrazol-3-yl)pyridine (LMe), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (LAll), 2,6-di(1-benzyl-pyrazol-3-yl)pyridine (LBz) and di(1-isopropyl-pyrazol-3-yl)pyridine (LiPr) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(LR)2]2+ complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(LMe)2]X2 (X− = BF4−, ClO4− and PF6−), [Fe(LAll)2][BF4]2, [Fe(LBz)2][BF4]2 and [Fe(LiPr)2][PF6]2 have been determined at 150 K. All of these contain high-spin iron centres except [Fe(LMe)2][BF4]2·xH2O, which is predominantly low-spin at that temperature. All the complexes are high-spin between 5 and 300 K as solvent-free bulk powders, and are also high-spin in (CD3)2CO solution between 193 and 293 K. This was unexpected, since the parent complex [Fe(3-bpp)2]2+ undergoes spin-crossover in the same solvent with T½ = 247 K [40]. The high-spin nature of the [Fe(LR)2]2+ complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand ‘R’ substituents.Graphical abstractFour different complexes of the type shown, [Fe(LR)2]2+, have been prepared, with R = methyl, allyl, benzyl and isopropyl. All the compounds are high-spin in acetone solution and the solid state at room temperature and below, except for one salt of [Fe(LMe)2]2+ which is predominantly low-spin at 150 K in the crystal. This contrasts with the parent complex (R = H), which exhibits thermal spin-crossover just below room temperature.Highlights► The syntheses of four 2,6-di(pyrazol-3-yl)pyridine (3-bpp) derivatives bearing pyrazolyl N1 substitutents, and salts of their iron(II) complexes. ► Magnetic measurements and crystal structures show that the complex salts (with one exception) are high-spin between 5 and 300 K. ► The complexes are also high-spin in (CD3)2CO solution between 190 and 300 K. ► Distal substitution of 3-bpp suppresses spin-crossover for all the substituents examined, regardless of their steric and inductive properties.
Co-reporter:Thomas D. Roberts, Floriana Tuna, Tamsin L. Malkin, Colin A. Kilner and Malcolm A. Halcrow
Chemical Science 2012 vol. 3(Issue 2) pp:349-354
Publication Date(Web):04 Nov 2011
DOI:10.1039/C1SC00584G
[FeL2][BF4]2·2H2O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1:1 high:low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1AA) or by thermal dehydration (phase 1BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.
Co-reporter:Rebecca Docherty, Floriana Tuna, Colin A. Kilner, Eric J. L. McInnes and Malcolm A. Halcrow
Chemical Communications 2012 vol. 48(Issue 34) pp:4055-4057
Publication Date(Web):06 Mar 2012
DOI:10.1039/C2CC30873H
The electronic structures of [Cu(terpy)2]2+ and [Cu(bpp)2]2+ (bpp = 2,6-di[pyrazol-1-yl]pyridine) are different, when doped into [M(bpp)2][BF4]2 (M2+ = Fe2+ or Zn2+). The [Cu(terpy)2]2+ dopant is a typical pseudo-Jahn–Teller elongated copper(II) center. However, the [Cu(bpp)2]2+ sites show EPR spectra consistent with a tetragonally compressed {dz2}1 configuration.
Co-reporter:Sara A. Diener, Amedeo Santoro, Colin A. Kilner, Jonathan J. Loughrey and Malcolm A. Halcrow
Dalton Transactions 2012 vol. 41(Issue 13) pp:3731-3739
Publication Date(Web):11 Jan 2012
DOI:10.1039/C2DT11911K
New iron(II) podand complexes have been prepared, by condensation of 2-(aminomethyl)-2-methyl-1,3-diaminopropane with 3 equiv of a heterocyclic aldehyde in the presence of hydrated Fe[BF4]2 or Fe[ClO4]2 as templates. The 2-(aminomethyl)-2-methyl-1,3-diaminopropane is prepared in situ by deprotonation of its trihydrochloride salt. The chloride must be removed from these reactions by precipitation with silver, to avoid the formation of the alternative 2,4,6-trisubstituted-7-methyl-1,3,5-triazaadamantane condensation products, or their FeCl2 adducts. The crystal structures of two 2,4,6-tri(pyridyl)-7-methyl-1,3,5-triazaadamantane-containing species are presented, and contain two different geometric isomers of this tricyclic ring with three equatorial, or two equatorial and one axial, pyridyl substituents. Both structures feature strong C–H⋯X (X = Cl or F) hydrogen bonding from the aminal C–H groups in the triazaadamantane ring. Five iron(II) podand complexes were successfully obtained, all of which contain low-spin iron centres.
Co-reporter:Guillaume Chastanet, Clare A. Tovee, Geoffrey Hyett, Malcolm A. Halcrow and Jean-François Létard
Dalton Transactions 2012 vol. 41(Issue 16) pp:4896-4902
Publication Date(Web):08 Mar 2012
DOI:10.1039/C2DT12122K
The photomagnetic properties of two series of spin-crossover solid solutions, [Fe(1-bpp)2]x[Ru(terpy)2]1−x(BF4)2 and [Fe(1-bpp)2]x[Co(terpy)2]1−x(BF4)2 (1-bpp = 2,6-bis[pyrazol-1-yl]pyridine), have been investigated. For all the materials, the evolution of the T(LIESST) value, the high-spin → low-spin relaxation parameters and the LITH loops were thoroughly studied. Interestingly in the Fe:Co series, along the photo-excitation, cobalt ions are concomitantly converted from low-spin to high-spin states with the iron centres, and also fully relax after light excitation.
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2011 vol. 40(Issue 7) pp:4119-4142
Publication Date(Web):11 Apr 2011
DOI:10.1039/C1CS15046D
Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).
Co-reporter:James J. Henkelis, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2011 vol. 47(Issue 18) pp:5187-5189
Publication Date(Web):30 Mar 2011
DOI:10.1039/C1CC00054C
Recrystallisation of Ag[L1] (HL1 = 3{5}-[pyrid-2-yl]-5{3}-tert-butylpyrazole) in the presence of halide anions leads to two polymorphs of [Ag3(μ-Br)(μ-L1)2], which differ in their mode of supramolecular association, and the cluster [Ag10(μ-L1)8]Cl2. In contrast, Ag[L2] (HL2 = 3{5}-[isoquinol-1-yl]-5{3}-tert-butyl-pyrazole) crystallises as a cyclic tetrameric molecule.
Co-reporter:Simon A. Barrett, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2011 vol. 40(Issue 45) pp:12021-12024
Publication Date(Web):16 May 2011
DOI:10.1039/C1DT10620A
The temperature of spin-crossover in [Fe(3-bpp)2][BF4]2 (3-bpp = 2,6-di{pyrazol-3-yl}pyridine) tends to increase in associating solvents. In particular, T½ shifts to 60–70 K higher temperature in water compared to organic solvents.
Co-reporter:Malcolm A. Halcrow
Chemical Communications 2010 vol. 46(Issue 26) pp:4761-4763
Publication Date(Web):03 Jun 2010
DOI:10.1039/C0CC00603C
Phase-pure solid solutions of [Co(terpy)2][BF4]2 and [M(bpp)2][BF4]2 (M = Fe or Ru, bpp = 2,6-dipyrazol-1-ylpyridine) have been prepared. The spin-state of the [Co(terpy)2]2+ dopant is modulated by the spin-state of the [Fe(bpp)2]2+ centres in the iron-containing materials.
Co-reporter:James J. Henkelis, Leigh F. Jones, Marcelo P. de Miranda, Colin A. Kilner, and Malcolm A. Halcrow
Inorganic Chemistry 2010 Volume 49(Issue 23) pp:11127-11132
Publication Date(Web):November 10, 2010
DOI:10.1021/ic1020093
The reaction of CuX2 (X− ≠ F−) salts with 1 equiv of 3-pyridyl-5-tert-butylpyrazole (HL) in basic methanol yields blue solids, from which disk complexes of the type [Cu7(μ3-OH)4(μ-OR)2(μ-L)6]2+ and/or the cubane [Cu4(μ3-OH)4(HL)4]4+ can be isolated by recrystallization under the appropriate conditions. Two of the disk complexes have been prepared in crystalline form: [Cu7(μ3-OH)4(μ-OCH2CF3)2(μ-L)6][BF4]2 (2) and [Cu7(μ3-OH)4(μ-OCH3)2(μ-L)6]Cl2·xCH2Cl2 (3·xCH2Cl2). The molecular structures of both compounds as solvated crystals can be described as [Cu⊂Cu6(μ-OH)4(μ-OR)2(μ-L)6]2+ (R = CH2CF3 or CH3) adducts. The [Cu6(μ-OH)4(μ-OR)2(μ-L)6] ring is constructed of six square-pyramidal Cu ions, linked by 1,2-pyrazolido bridges from the L− ligands and by basal, apical-bridging hydroxy or alkoxy groups, while the central Cu ion is bound to the four metallamacrocyclic hydroxy donors in a near-regular square-planar geometry. The L− ligands project above and below the metal ion core, forming two bowl-shaped cavities that are fully (R = CH2CF3) or partially (R = CH3) occupied by the alkoxy R substituents. Variable-temperature magnetic susceptibility measurements on 2 demonstrated antiferromagnetic interactions between the Cu ions, yielding a spin-frustrated S = 1/2 magnetic ground state that is fully populated below around 15 K. Electrospray ionization mass spectrometry, UV/vis/near-IR, and electron paramagnetic resonance measurements imply that the heptacopper(II) disk motif is robust in organic solvents.
Co-reporter:Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2010 vol. 39(Issue 38) pp:9008-9012
Publication Date(Web):19 Aug 2010
DOI:10.1039/C0DT00295J
Single crystal and powder samples of [Co(terpy)2][BF4]2 are low spin below 100 K but show a typically gradual thermal spin-transition on warming, centred near 270 K. However, the spin-crossover exhibits an unusual and pronounced discontinuity above room temperature, when the material is ca. 87% high-spin. A crystallographic study at nine temperatures between 30–375 K showed that the discontinuity is not caused by a phase transition or by changes in anion disorder. Rather, it may reflect the steric consequence of small changes in the nearest neighbour interactions between the cations as the spin transition progresses.
Co-reporter:Jonathan Day, Katie E. R. Marriott, Colin A. Kilner and Malcolm A. Halcrow
New Journal of Chemistry 2010 vol. 34(Issue 1) pp:52-60
Publication Date(Web):26 Oct 2009
DOI:10.1039/B9NJ00412B
The influence of the pyrazole ligand on the previously reported anion host [ZnCl(HpztBu)3]+ (HpztBu = 5-tert-butylpyrazole) has been investigated. Reaction of ZnCl2 with 3 equiv. of 3{5}-cyclohexylpyrazole (HpzCy) or 3{5}-phenylpyrazole (HpzPh) affords [ZnCl(HpzR)3]Cl (R = Cy or Ph), both of which undergo decomposition to [ZnCl2(HpzR)2] upon recrystallisation. A similar reaction using 3{5}-(thien-2-yl)pyrazole (HpzTn) affords [ZnCl2(HpzTn)2] only. The salts [ZnCl(HpzPh)3]BF4, [ZnCl(HpzCy)3]X (X− = NO3−, ClO4−, CF3SO3− or ½SO42−) and [ZnBr(HpzCy)3]NO3·H2O have been isolated, by performing the above reactions in the presence of 1 equiv. of AgX. The cations in [ZnCl(HpzCy)3]NO3 associate into a dimeric capsule encapsulating two nitrate anions. In contrast, [ZnBr(HpzCy)3]NO3·H2O contains hydrogen-bonded [NO3·H2O]nn− chains enclosed within channels formed by the complex cations, while [ZnCl(HpzCy)3]ClO4 forms a different hydrogen-bonded dimer with the anions occupying two shallow cavities in its surface. These data imply that anion binding by [ZnCl(HpzR)3]+ depends strongly on the steric and inductive properties of the pyrazole ‘R’ substituents.
Co-reporter:Clare A. Tovee;Colin A. Kilner;Simon A. Barrett;James A. Thomas
European Journal of Inorganic Chemistry 2010 Volume 2010( Issue 7) pp:1007-1012
Publication Date(Web):
DOI:10.1002/ejic.200901107
Abstract
Reaction of 4-bromomethyl-2,6-bis(pyrazol-1-yl)pyridine with dipicolylamine yields 4-bis(pyrid-2-ylmethyl)aminomethyl-2,6-bis(pyrazol-1-yl)pyridine (L). Treatment of L with [MCl2(NCPh)2] (M = Pd, Pt) in the presence of AgPF6 affords [MCl(L)]PF6, whose palladium or platinum centre is bound exclusively by the dipicolylamino moiety of L, as established by NMR spectroscopy. The pendant dipyrazolylpyridine residue in these compounds is complexed by iron(II) to form [Fe{MCl(μ-L)}2][PF6]4 (M = Pd, 1; M = Pt, 2·nH2O). The nitromethane solvate crystal of 1 contains low-spin iron centres at 150 K. However, dried 1 and 2·nH2O are predominantly high-spin at room temperature, undergoing very gradual thermal spin transitions upon cooling to ≤ 50 % completeness. The platinum compound also undergoes a thermal spin transition in CD3NO2 solution, with T1/2 = 253 K.
Co-reporter:Clare A. Tovee;Colin A. Kilner;Simon A. Barrett;James A. Thomas
European Journal of Inorganic Chemistry 2010 Volume 2010( Issue 7) pp:
Publication Date(Web):
DOI:10.1002/ejic.201090017
Abstract
The cover picture shows the crystal structure of a heteronuclear trimetallic complex of a “back-to-back” ligand superimposed on a view of the wild heather near Kettlewell in the Yorkshire Dales national park, near Leeds. The ligand contains two different metal-binding domains that can be sequentially metallated with high selectivity – there is no detectable exchange of metal ions between the domains of the ligand in solution. The iron(II) ion at the centre of the complex undergoes a spin transition on cooling, in solution and in the solid state. Details are discussed in the Short Communication by M. A. Halcrow et al. on p. 1007 ff.
Co-reporter:Leigh F. Jones;Colin A. Kilner
Journal of Cluster Science 2010 Volume 21( Issue 3) pp:279-290
Publication Date(Web):2010 September
DOI:10.1007/s10876-010-0283-0
Treatment of FeCl3 with 1 equiv of 3-(pyrid-2-yl)-5-(tertbutyl)-1H-pyrazole (L) in basic methanol affords crystalline [Fe3Cl5(μ3-O)(μ-OMe)2L2(MeOH)] in moderate yield. The compound has an unusual isosceles triangular [Fe3(μ3-O)(μ-OMe)2]5+ core with one unbridged edge. Two of the iron(III) centres in the compound are approximately octahedral, while the third has a five-coordinate geometry. Magnetic susceptibility measurements show antiferromagnetic coupling between the iron centres, leading to a S = 3/2 magnetic ground state and S = 5/2 excited state that are almost accidentally degenerate according to simulation. An analogous reaction using Fe[ClO4]3 as starting material instead affords the low-spin iron(II) complex [FeL3][ClO4]2.
Co-reporter:Malcolm A. Halcrow
Coordination Chemistry Reviews 2009 Volume 253(21–22) pp:2493-2514
Publication Date(Web):November 2009
DOI:10.1016/j.ccr.2009.07.009
Synthetic procedures are available for the derivatisation of almost every position of the 2,6-di(pyrazol-1-yl)pyridine (1-bpp) skeleton. This has led to the preparation of a large number of iron(II) complexes of the [Fe(1-bpp)2]2+ type, many of which undergo spin-crossover transitions at accessible temperatures, sometimes around room temperature. This review surveys these compounds, and presents a wider discussion of their structural chemistry and photomagnetic properties. The availability of a series of chemically similar spin-transition materials has allowed trends in their behaviour to be identified and correlated with their molecular and lattice structures. This represents a rare, positively identified structure:function relationship for spin-crossover materials. Other topics under discussion are the existence of a novel angular Jahn–Teller distortion in several of these compounds and its consequences for spin-crossover; and, the normal and abnormal behaviour of different [Fe(1-bpp)2]2+ materials trapped in metastable high-spin states at low temperature.
Co-reporter:Clare A. Tovee, Colin A. Kilner, James A. Thomas and Malcolm A. Halcrow
CrystEngComm 2009 vol. 11(Issue 10) pp:2069-2077
Publication Date(Web):12 Jun 2009
DOI:10.1039/B904528G
Two series of solid solutions based on the [Fe(bpp)2][BF4]2 (bpp = 2,6-di(pyrazol-1-yl)pyridine) spin-crossover lattice are described. Materials of formula [FeyNi1−y(bpp)2][BF4]2 (0.95 ≥ x ≥ 0.15) are all phase-pure. In contrast, [Fe(bpp)2]x[Ru(terpy)2]1−x[BF4]2 (0.95 ≥ x ≥ 0.28) crystallise as a mixture of the neat [Fe(bpp)2][BF4]2 (space group P21) and [Ru(terpy)2][BF4]2 (Cc) phases as x approaches 0.5, although there is no evidence of segregation of the iron and ruthenium centres between the two. Spin-crossover in [FeyNi1−y(bpp)2][BF4]2 shows a normal variation in T½, ΔH, ΔS and lattice elasticity with decreasing y, for an isomorphously doped material. However, T½ in [Fe(bpp)2]x[Ru(terpy)2]1−x[BF4]2 shows an unusual dependence on x, which decreases slightly for 1.00 ≥ x ≥ 0.57 but increases as x is reduced further. We attribute this anomaly to the change in structure of the solid solutions, from predominantly the P21 lattice when x ≥ 0.47 to the Cc lattice when x = 0.28. The ruthenium centres in those compounds retain their low-temperature fluorescence, albeit with strongly reduced emission intensities. An unusual dependence of ΔH and ΔS for the spin transition in [Fe(bpp)2][BF4]2 on the crystallisation solvent used is also noted.
Co-reporter:Malcolm A. Halcrow
Dalton Transactions 2009 (Issue 12) pp:2059-2073
Publication Date(Web):23 Jan 2009
DOI:10.1039/B815577A
This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal–organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.
Co-reporter:Ruth Pritchard, Hanane Lazar, Simon A. Barrett, Colin A. Kilner, Saket Asthana, Chiara Carbonera, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2009 (Issue 33) pp:6656-6666
Publication Date(Web):17 Jul 2009
DOI:10.1039/B907094J
The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L1), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L2) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L3) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L1)2][BF4]2 crystallises in two different solvent-free polymorphs. The tetragonal (α) form crystallises in a known version of the “terpyridine embrace” structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (β) form exhibits a modified form of the same packing motif, containing two unique iron sites in a 2 : 1 ratio. One-third of the complex molecules in that material undergo a very gradual thermal spin-crossover centred at 137 K. Comparison of the two structures implies that spin-crossover cooperativity in the α-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L2)2][BF4]2 is isostructural with α-[Fe(L1)2][BF4]2 and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L3)2][BF4]2 is low-spin as a powder at 360 K and below and can be crystallised as two different solvates from acetone solution. All three compounds exhibit the LIESST effect at 10 K, with photoconversions of 40–100%. Their LIESST relaxation temperatures obey the empirical T(LIESST) = T0− 0.3T1/2 (T0 = 150 K) law that we have previously proposed for this class of compound.
Co-reporter:Ruth Pritchard, Colin A. Kilner, Simon A. Barrett, Malcolm A. Halcrow
Inorganica Chimica Acta 2009 Volume 362(Issue 12) pp:4365-4371
Publication Date(Web):15 September 2009
DOI:10.1016/j.ica.2009.01.022
Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130 °C for 5 days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L1), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L2) as a significant byproduct. Reduction of L1 with excess NaBH4 in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L3) in low yield. The crystalline complex [Fe(L1)2][BF4]2 · 2CF3CH2OH is low-spin at 150 K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L3)2][BF4]2·1.4CH3CN have a mixed spin-state population, with the low-spin state predominating at 150 K. The [Fe(L3)2(BF4)]+ moieties in the lattice associate into 1-D chains through intermolecular O–H⋯O and O–H⋯F hydrogen bonding. Bulk samples of [Fe(L3)2][BF4]2 · H2O are fully low-spin below 200 K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322 K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD3)2CO solution, with transition midpoints near 250 K.The syntheses of two new 2,6-di(pyrazol-1-yl)pyridines substituted at the pyrazole 4-positions are presented. The Fe[BF4]2 complex of 2,6-di(4-[hydroxymethyl]pyrazol-1-yl)pyridine (shown) undergoes a gradual thermal spin transition above room temperature in the solid state, that is triggered by loss of lattice water.
Co-reporter:LeighF. Jones Dr.;ColinA. Kilner;MalcolmA. Halcrow Dr.
Chemistry - A European Journal 2009 Volume 15( Issue 18) pp:4667-4675
Publication Date(Web):
DOI:10.1002/chem.200802368
Co-reporter:Ruth Pritchard, Colin A. Kilner, Malcolm A. Halcrow
Tetrahedron Letters 2009 50(21) pp: 2484-2486
Publication Date(Web):
DOI:10.1016/j.tetlet.2009.03.035
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2008 vol. 37(Issue 2) pp:278-289
Publication Date(Web):02 Oct 2007
DOI:10.1039/B701085K
This tutorial review describes how complexes of iron(II) (and, rarely, other metal ions) can be switched between their high- and low-spin states by different physical stimuli. At low temperatures, it is possible to trap a compound in a metastable excited spin-state which, in favourable cases, may be stable to thermal relaxation below temperatures as high as 130 K. The selective switching and trapping of individual spin centres in polynuclear compounds, and the interplay between spin centres as they relax back to their ground states, are also discussed. Similar phenomena, in which spin transitions are coupled to charge transfer phenomena, can also occur in inorganic and metal–organic cyanometallate compounds and in cobalt–semiquinonate complexes.
Co-reporter:A. Abibat Salaudeen, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2008 (Issue 41) pp:5200-5202
Publication Date(Web):12 Sep 2008
DOI:10.1039/B810393C
A polycatenate network, whose rings are formed from metal-templated hemispheres linked by hydrogen bonds, is reported.
Co-reporter:Ruth Pritchard, Simon A. Barrett, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2008 (Issue 24) pp:3159-3168
Publication Date(Web):24 Apr 2008
DOI:10.1039/B801892H
We report the syntheses of six new salts of the iron(III) complexes of four hexadentate Schiff bases of the saltrien-type derived from 1,8-diamino-3,6-diazaoctane and 2 equiv. of the appropriate salicylaldehyde derivative. Three of these new compounds undergo gradual spin-transitions centred between 155–204 K in the solid state, that proceed to 35–50% completeness. One of the other compounds remains high-spin on cooling, another is low-spin, while the last undergoes spin-crossover centred above room temperature. This spin-state variability is typical of solid compounds of this type. As an aid to understanding this behaviour, the crystal structures and magnetochemistry of the known spin-crossover salts [Fe(saltrien)]PF6, [Fe(saltrien)]ClO4 and [Fe(saltrien)]BPh4 have also been reexamined. The structural chemistry of all three salts was found to differ significantly from what has been previously reported. Correlation of the crystal structures and magnetic behaviour of the compounds in this and previous work suggests that the disposition of the two ligand phenoxy groups plays an important role in determining whether high-spin [Fe(saltrien)]+ complexes can undergo thermal spin-transitions.
Co-reporter:Ruth Pritchard, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2007 (Issue 6) pp:577-579
Publication Date(Web):14 Nov 2006
DOI:10.1039/B613402E
Six structurally related iron(II) complexes show remarkably similar abrupt thermal spin-transitions.
Co-reporter:Chiara Carbonera, Colin. A. Kilner, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2007 (Issue 13) pp:1284-1292
Publication Date(Web):23 Feb 2007
DOI:10.1039/B618480D
The complex dications in the cooperative spin-crossover compound [FeL2][BF4]2 (2,6-di(pyrazol-1-yl)pyridine) pack through π–π interactions into a 2-D layered structure (a “terpyridine embrace” motif). The effects of doping the larger ClO4− ion into this lattice have been investigated. The bulk solids [FeL2][ClO4]x[BF4]2−x are isostructural with [FeL2][BF4]2 when x = 0.30 and 0.98, and isostructural with (stucturally distinct) [FeL2][ClO4]2 when x = 1.89. When x = 1.68, powder samples are a mixture of both phases, but crystalline material adopts purely the ClO4− structure. Increasing the perchlorate content in the lattice for 0 ≤
x
≤ 1.68 causes a small decrease in T1/2 and a narrowing of hysteresis in their spin-crossover, but with no significant reduction in cooperativity. It also leads to more pronounced decreases in ΔH [by up to 3.2(5) kJ mol−1] and ΔS [by up to 10(2) J mol−1 K−1] for the transition by DSC. Single crystals of formula [FeL2][ClO4]y[BF4]2−y (y = 0.44 and 1.13) are isostructural with the pure BF4− salt. While their molecular structures are indistinguishable, the distances between cations in the lattice increase in the doped materials. Weakening of intermolecular π–π interactions between cations is the likely reason for the reduced enthalpy of spin-crossover as x increases. However, the biggest stuctural change is an increase in the spacing between the 2-D layers with increased ClO4−. These results suggest that cooperativity in this material is transmitted within the terpyridine embrace layers.
Co-reporter:Hanane Z. Lazar, Thibaut Forestier, Simon A. Barrett, Colin A. Kilner, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2007 (Issue 38) pp:4276-4285
Publication Date(Web):02 Aug 2007
DOI:10.1039/B708971F
The syntheses of [FeL][BF4]2·H2O, [FeL][ClO4]2·H2O, [FeL][NO3]2·CH3NO2 and [FeL][CF3SO3]2 (L = tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine) are described. The isostructural BF4− and ClO4− salts are high-spin between 5–300 K, while the other two compounds are high-spin at room temperature but undergo gradual high→low spin transitions upon cooling. For [FeL][NO3]2 this transition is centred at 139 K and proceeds to near-completeness, while for [FeL][CF3SO3]2 it is centred at 144 K and only proceeds to 50% conversion. The CF3SO3− salt also undergoes spin-crossover centred at 200 K in (CD3)2CO solution, and exhibits dynamic inversion of its helical ligand conformation. All these compounds except the triflate salt have been crystallographically characterised, and show capped trigonal antiprismatic [6 + 1] coordination geometries. The NO3− and CF3SO3− salts undergo quantitative conversion to trapped, high-spin excited states upon irradiation with a green laser at 10 K (the LIESST effect; LIESST = Light-Induced Excited Spin State Trapping). The thermal stabilities of their LIESST excited states (T(LIESST) = 80–82 K) resemble those found for iron(II) complexes of bidentate N-heterocyclic ligands. Hence, the LIESST properties of [FeL]2+ are those of a complex of three rigid bidentate domains linked by a flexible spacer, rather than of a single encapsulating podand.
Co-reporter:Q. Folshade Mokuolu, Dolos Foguet-Albiol, Leigh F. Jones, Joanna Wolowska, Radoslaw M. Kowalczyk, Colin A. Kilner, George Christou, Patrick C. McGowan and Malcolm A. Halcrow
Dalton Transactions 2007 (Issue 14) pp:1392-1399
Publication Date(Web):09 Mar 2007
DOI:10.1039/B618780C
Treatment of CuF2 with 2 equiv of 3{5}-[pyrid-2-yl]pyrazole (HpzPy), 3{5}-phenylpyrazole (HpzPh) or 3{5}-[4-fluorophenyl]pyrazole (HpzPhF) in MeOH, followed by evaporation to dryness and recrystallisation of the solid residues, allows solvated crystals of [{Cu(µ-pzPy)(pzPy)}2] (1), [{Cu(µ-pzPh)2}4] (2) and [Cu4F2(µ4-F)(µ-pzPhF)5(HpzPhF)4] (3) to be isolated in moderate-to-good yields. Similar reactions of these three pyrazoles with Cu(OH)2 in refluxing MeOH respectively afford 1, 2 and [Cu(pzPhF)2(HpzPhF)2] (4) in ca. 10% yield. Crystalline 1·1/2H2O·2CHCl3 contains two independent dinuclear molecules with a puckered di-(1,2-pyrazolido) bridge motif, linked by a bridging, hydrogen-bonding water molecule. Compound 2·1/2C5H12, containing cyclic, square tetranuclear complex molecules, is the first homoleptic divalent metal pyrazolide to have a discrete molecular rather than polymeric structure, for a metal other than Pd or Pt. The two independent complex molecules in 3·3/4CH2Cl2·HpzPhF contain a unique tetrahedral [Cu4(µ4-F)]7+ core, three of whose edges are spanned by bridging pyrazolido groups. Magnetic data show that the copper centres in 1 and 3 are antiferromagnetically coupled, but that dried bulk samples of 2 do not retain their molecular structure.
Co-reporter:Q. Folshade Mokuolu, Colin A. Kilner, Patrick C. McGowan, Malcolm A. Halcrow
Inorganica Chimica Acta 2007 Volume 360(Issue 14) pp:4025-4030
Publication Date(Web):1 November 2007
DOI:10.1016/j.ica.2007.04.046
Reaction of bis(2-{pyrid-2-yl}ethyl)amine with 2-bromoethanol in the presence of Na2CO3 yields the title ligand, LH. Treatment of LH with the CuBr2 or Zn(O2CMe)2 · 2H2O yields pure crystalline [CuBr(LH)]Br · H2O (1 · H2O) and [Zn2(O2CMe)2(μ-O2CMe)(μ-L)] (2). Reaction of LH with Cu(O2CMe)2 · H2O affords a low yield of [Cu2Cl2(μ-O2CMe)(μ-L)] (3), the Cl− ligands apparently originating from the CH2Cl2 crystallization solvent. Compound 1 · H2O is a near-regular square-pyramidal complex with a neutral, protonated LH ligand. In contrast, 2 and 3 are both unusual unsymmetric dinuclear complexes, with a five-coordinate [ML(O2CMe)] (M = Zn or Cu) unit linked to a second metal ion through the deprotonated ligand alkoxide donor and O,O′-bridging acetate ligand.Reaction of bis(2-{pyrid-2-yl}ethyl)amine with 2-bromoethanol in the presence of Na2CO3 yields the title ligand, LH. Treatment of LH with the appropriate metal salt yields pure crystalline [CuBr(LH)]Br · H2O and [Zn2(O2CMe)2(μ-O2CMe)(μ-L)] (shown). The crystal structure of [Cu2Cl2(μ-O2CMe)(μ-L)] is also presented.
Co-reporter:Leigh F. Jones Dr.;Colin A. Kilner;Marcelo P. de Mira Dr.;Joanna Wolowska Dr.;Malcolm A. Halcrow Dr.
Angewandte Chemie 2007 Volume 119(Issue 22) pp:
Publication Date(Web):25 APR 2007
DOI:10.1002/ange.200700584
Randvoll: Die im Bild gezeigte [12]MC-6-Fluorometallakronenverbindung wurde sowohl als metallfreies Rad als auch in Gegenwart komplexierter Natrium- oder Kaliumionen kristallisiert. Die magnetischen und Struktureigenschaften werden durch die Metallkomplexierung nur wenig verändert. Gegenüberliegende Molekülflächen bilden schalenförmige Hohlräume, in denen H2O oder CH2Cl2 über H-Brücken an die F-Akzeptoren gebunden sind.
Co-reporter:Victoria A. Money Dr.;Chiara Carbonera Dr.;Jérôme Elhaïk Dr.;Malcolm A. Halcrow Dr.;Judith A. K. Howard ;Jean-François Létard Dr.
Chemistry - A European Journal 2007 Volume 13(Issue 19) pp:
Publication Date(Web):8 MAR 2007
DOI:10.1002/chem.200601312
This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL2][BF4]2⋅x H2O (L=2,6-bis{3-methylpyrazol-1-yl}pyridine; x=0–1/3) at 1 K min−1 leads to a cooperative spin transition, taking place in two steps centered at 147 and 105 K, that is only 54 % complete by magnetic susceptibility. Annealing the sample at 100 K for 2 h results in a slow decrease in χMT to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL2][BF4]2⋅x H2O are isostructural (C2/c, Z=8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z=24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110±20 K when x=1/3 and 155±5 K when x=0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10 K, and in approximately 70 % yield by rapidly quenching the sample to 10 K. This metastable high-spin state relaxes back to the low-spin ground state at 87±1 K in one, not two, steps, and without passing through the intermediate phase. This implies that thermal spin-crossover and thermally activated high-spin–low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T(LIESST) being greater than T1/2.
Co-reporter:LeighF. Jones Dr.;SimonA. Barrett;ColinA. Kilner ;MalcolmA. Halcrow Dr.
Chemistry - A European Journal 2007 Volume 14( Issue 1) pp:223-233
Publication Date(Web):
DOI:10.1002/chem.200701318
Abstract
Reaction of CuF2 with one equivalent of 3{5}-(pyrid-2-yl)-5{3}-(tertbutyl)pyrazole (HL) and excess NH4OH in MeOH affords crystalline [NH4{Cu(μ-F)(μ-L)}6(CH2Cl2)2]HF2 in moderate yield. This compound contains the 12-MC-6 metallacrown [{Cu(μ-F)(μ-L)}6] (1) with a NH4+ ion at its center, and CH2Cl2 molecules complexed in bowl-shaped cavities above and below the Cu6F6 ring. Similar reactions using the bases MeNH2, glycine, l-alanine or β-alanine afforded solvated crystals of [1(H3NMe)2]Cl2, [1(gly)2], [1(l-ala)2], and [1(β-ala)2], respectively. The metallacrown 1 in these products contains methylammonium and zwitterionic amino-acid guests in its two bowl-shaped cavities; each of the amino acids hydrogen-bonds to three F atoms. A related reaction using 1,6-diaminohexane resulted in fixation of CO2 from the air to give solvated [1(H3NC6H12NHCO2)2], again with a zwitterionic guest. NMR, ESI-MS and UV/vis measurements suggest that the metallacrown 1 retains its integrity in several organic solvents, although it is unclear to what extent guest binding takes place in solution.
Co-reporter:Leigh F. Jones Dr.;Colin A. Kilner;Marcelo P. de Mira Dr.;Joanna Wolowska Dr.;Malcolm A. Halcrow Dr.
Angewandte Chemie International Edition 2007 Volume 46(Issue 22) pp:
Publication Date(Web):25 APR 2007
DOI:10.1002/anie.200700584
A brimming bowl: The [12]MC-6 fluoro metallacrown (MC) shown in the picture is crystallized both as a metal-free wheel and in complex with Na+ or K+ ions. The magnetic and structural properties are only slightly altered upon metal complexation. Opposite faces of the molecule form bowl-shaped cavities, in which H2O or CH2Cl2 molecules are hydrogen-bonded to the F acceptors.
Co-reporter:Christopher M. Pask, Kenneth D. Camm, Neil J. Bullen, Michael J. Carr, William Clegg, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2006 (Issue 5) pp:662-664
Publication Date(Web):08 Nov 2005
DOI:10.1039/B514634H
The title ligand (‘L’) in crystalline [CuBrL2]Br·CH3OH and [Cu(OH2)L2]SO4·H2O·CH3OH chelates to both the copper(II) cations and the charge-balancing anions.
Co-reporter:Chiara Carbonera, José Sánchez Costa, Victoria A. Money, Jérôme Elhaïk, Judith A. K. Howard, Malcolm A. Halcrow and Jean-François Létard
Dalton Transactions 2006 (Issue 25) pp:3058-3066
Publication Date(Web):17 May 2006
DOI:10.1039/B601366J
The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2 (1), [Fe(L2)2][BF4]2 (2), [Fe(L2)2][ClO4]2 (3), [Fe(L3)2][BF4]2 (4), [Fe(L3)2][ClO4]2 (5) and [Fe(L4)2][ClO4]2 (6) (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds 1–6 display a complete thermal spin transition centred between 200–300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS → LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T1/2: T(LIESST) = T0
− 0.3T1/2. T0 for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms of 5 and 6, the light-induced high-spin state of 5, and the low-spin complex [Fe(L4)2][BF4]2 (7), are described.
Co-reporter:Isabelle Sylvestre, Joanna Wolowska, Colin A. Kilner, Eric J. L. McInnes and Malcolm A. Halcrow
Dalton Transactions 2005 (Issue 19) pp:3241-3249
Publication Date(Web):26 Aug 2005
DOI:10.1039/B505972K
New syntheses are reported of 5-tert-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde, 5-tert-butyl-2-hydroxy-3-phenylsulfanyl-benzaldehyde, and salcyen (H2L1–H2L3) and salcyan (H2L4–H2L6)-type ligands derived from these aldehydes and from 5-tert-butyl-2-hydroxybenzaldehyde. The complexes [CuL]
(L2−
=
[L1]2−–[L6]2−) bearing sulfanyl substituents each show two distinct voltammetric ligand-based oxidations under the same conditions, the first of which is chemically reversible. The first oxidation product is much longer lived by coulometry for the salcyen than for the salcyan ligand complexes, despite the latter having a substantially lower oxidation potential. The lifetimes of all the ligand oxidation products in this system are substantially smaller than for similar compounds derived from 3,5-di(tert-butyl)-2-hydroxybenzaldehyde (Dalton Trans., 2004, 2662). Attempted chemical oxidation of the Schiff base compounds using AgBF4 yielded instead stable silver(I) adducts. A crystal structure of one such compound showed that the Ag atom was coordinated in a slightly bent geometry by the two ligand sulfanyl groups, with two additional long-range Ag⋯O interactions to the phenoxide donors. EPR spectra showed that some of these silver adducts dimerise in CH2Cl2, probably through basal, apical intermolecular Cu–O⋯Cu bridging. In contrast the parent copper(II) complexes are all monomeric in this solvent by EPR.
Co-reporter:Jérôme Elhaïk, David J. Evans, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2005 (Issue 9) pp:1693-1700
Publication Date(Web):05 Apr 2005
DOI:10.1039/B502175H
Single crystal X-ray structures and susceptibility data are described for six homoleptic iron(II) complex salts, of 2,6-di(pyrazol-1-yl)pyridine or a 3,3″-disubstituted derivative of it. Zero field Mössbauer spectroscopic data for four of the complexes, and one previously reported analogue, are also discussed. Four of these compounds exhibit an unusual angular Jahn–Teller distortion towards C2 symmetry to differing degrees, while the other two exhibit structures close to the ‘ideal’
D2d symmetry for this ligand set. This structural distortion has two components: a twisting of the plane of one ligand relative to the other about the N{pyridine}–Fe–N{pyridine} vector, so that the two ligands are no longer perpendicular; and a rotation of one ligand about the Fe ion, so that the N{pyridine}–Fe–N{pyridine} angle <180°. Susceptibility data show that all the complexes are fully high-spin between 5 and 300 K, but yield an unusually wide range of zero-field splitting parameters for the different compounds of between 2.6 and 13.4 cm−1. Magnetostructural correlations suggest that a low value of |D| is diagnostic for a high degree of ‘rotation’ distortion. The Mössbauer spectra imply that an increased quadrupole splitting might also be diagnostic for the presence of the angular distortion.
Co-reporter:Jérôme Elhaïk, Colin A. Kilner and Malcolm A. Halcrow
CrystEngComm 2005 vol. 7(Issue 23) pp:151-157
Publication Date(Web):15 Feb 2005
DOI:10.1039/B417718E
The synthesis and crystal chemistry of [Fe(L3)2][ClO4]2
(1; L3
= 2,6-di[3-hydroxymethylpyrazol-1-yl]pyridine) is described. This compound crystallises as a mixture of three different phases from MeCN/Et2O: two solvent-free polymorphs, termed α-1
(space group P2/c) and β-1
(P21/c); and, a solvate of approximate formula 1·xCH3CN (x
≈ 0.75). The coordination geometries of the complex dications in these three crystals differ significantly, reflecting differing degrees of an unusual angular Jahn–Teller distortion at their high-spin iron centres. The solvated compound 1·xCH3CN undergoes a spin-state transition upon cooling, which is 42% complete at 100 K. This transition cannot probably proceed to more than 50% completeness, owing to a steric clash between neighbouring complex molecules when both are low-spin.
Co-reporter:Antonia T. Rizk, Colin A. Kilner and Malcolm A. Halcrow
CrystEngComm 2005 vol. 7(Issue 59) pp:359-362
Publication Date(Web):31 May 2005
DOI:10.1039/B506015J
Molecules of crystalline 1-(pyridin-2-yl)tetrazole associate through three unique C–H⋯N interactions into a 3
34
2 sheet structure. In contrast, 5-(pyridin-2-yl)-1H-tetrazole forms a six-connected 2-D network, that can be described as a self-penetrating 4
6 net.
Co-reporter:Isabelle Sylvestre, Joanna Wolowska, Eric J.L. McInnes, Colin A. Kilner, Malcolm A. Halcrow
Inorganica Chimica Acta 2005 Volume 358(Issue 4) pp:1337-1341
Publication Date(Web):1 March 2005
DOI:10.1016/j.ica.2004.11.004
The complexes [CuL(TpPh)] (HL = 5-tertbutylsalicylaldehyde, 5-tertbutyl-3-methylsulfanylsalicylaldehyde or 5-tertbutyl-3-phenylsulfanylsalicylaldehyde; [TpPh]− = tris-{3-phenylpyrazolyl}hydridoborate) have been prepared, and adopt square-pyramidal coordination geometries. Each compound exhibits a ligand-based oxidation in CH2Cl2 that is chemically reversible by voltammetry. However, Coulometric determinations showed that the resultant phenoxyl radical products decomposed rapidly at low temperatures in bulk solution. This instability may reflect intramolecular steric repulsions between the phenoxide tertbutyl substituents, and a pyrazolylborate phenyl group. These results contrast with a previously reported analogous compound, bearing a 5-methyl-3-methylsulfanylsalicylaldehydato ligand, which yields a phenoxyl radical oxidation product that is stable for hours under the same conditions.The two complexes shown exhibit ligand-based oxidations yielding phenoxyl radical-containing products, that are chemically reversible on the voltammetric timecale but which decompose rapidly when generated in bulk. This contrasts with an analogous complex lacking the phenoxide tertbutyl group, which yields a phenoxyl radical species with a lifetime of hours under the same conditions.
Co-reporter:Ruth Pritchard, Hanane Lazar, Simon A. Barrett, Colin A. Kilner, Saket Asthana, Chiara Carbonera, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2009(Issue 33) pp:NaN6666-6666
Publication Date(Web):2009/07/17
DOI:10.1039/B907094J
The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L1), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L2) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L3) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L1)2][BF4]2 crystallises in two different solvent-free polymorphs. The tetragonal (α) form crystallises in a known version of the “terpyridine embrace” structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (β) form exhibits a modified form of the same packing motif, containing two unique iron sites in a 2 : 1 ratio. One-third of the complex molecules in that material undergo a very gradual thermal spin-crossover centred at 137 K. Comparison of the two structures implies that spin-crossover cooperativity in the α-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L2)2][BF4]2 is isostructural with α-[Fe(L1)2][BF4]2 and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L3)2][BF4]2 is low-spin as a powder at 360 K and below and can be crystallised as two different solvates from acetone solution. All three compounds exhibit the LIESST effect at 10 K, with photoconversions of 40–100%. Their LIESST relaxation temperatures obey the empirical T(LIESST) = T0− 0.3T1/2 (T0 = 150 K) law that we have previously proposed for this class of compound.
Co-reporter:Ruth Pritchard, Simon A. Barrett, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2008(Issue 24) pp:NaN3168-3168
Publication Date(Web):2008/04/24
DOI:10.1039/B801892H
We report the syntheses of six new salts of the iron(III) complexes of four hexadentate Schiff bases of the saltrien-type derived from 1,8-diamino-3,6-diazaoctane and 2 equiv. of the appropriate salicylaldehyde derivative. Three of these new compounds undergo gradual spin-transitions centred between 155–204 K in the solid state, that proceed to 35–50% completeness. One of the other compounds remains high-spin on cooling, another is low-spin, while the last undergoes spin-crossover centred above room temperature. This spin-state variability is typical of solid compounds of this type. As an aid to understanding this behaviour, the crystal structures and magnetochemistry of the known spin-crossover salts [Fe(saltrien)]PF6, [Fe(saltrien)]ClO4 and [Fe(saltrien)]BPh4 have also been reexamined. The structural chemistry of all three salts was found to differ significantly from what has been previously reported. Correlation of the crystal structures and magnetic behaviour of the compounds in this and previous work suggests that the disposition of the two ligand phenoxy groups plays an important role in determining whether high-spin [Fe(saltrien)]+ complexes can undergo thermal spin-transitions.
Co-reporter:Chiara Carbonera, Colin. A. Kilner, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2007(Issue 13) pp:NaN1292-1292
Publication Date(Web):2007/02/23
DOI:10.1039/B618480D
The complex dications in the cooperative spin-crossover compound [FeL2][BF4]2 (2,6-di(pyrazol-1-yl)pyridine) pack through π–π interactions into a 2-D layered structure (a “terpyridine embrace” motif). The effects of doping the larger ClO4− ion into this lattice have been investigated. The bulk solids [FeL2][ClO4]x[BF4]2−x are isostructural with [FeL2][BF4]2 when x = 0.30 and 0.98, and isostructural with (stucturally distinct) [FeL2][ClO4]2 when x = 1.89. When x = 1.68, powder samples are a mixture of both phases, but crystalline material adopts purely the ClO4− structure. Increasing the perchlorate content in the lattice for 0 ≤
x
≤ 1.68 causes a small decrease in T1/2 and a narrowing of hysteresis in their spin-crossover, but with no significant reduction in cooperativity. It also leads to more pronounced decreases in ΔH [by up to 3.2(5) kJ mol−1] and ΔS [by up to 10(2) J mol−1 K−1] for the transition by DSC. Single crystals of formula [FeL2][ClO4]y[BF4]2−y (y = 0.44 and 1.13) are isostructural with the pure BF4− salt. While their molecular structures are indistinguishable, the distances between cations in the lattice increase in the doped materials. Weakening of intermolecular π–π interactions between cations is the likely reason for the reduced enthalpy of spin-crossover as x increases. However, the biggest stuctural change is an increase in the spacing between the 2-D layers with increased ClO4−. These results suggest that cooperativity in this material is transmitted within the terpyridine embrace layers.
Co-reporter:Laurence J. Kershaw Cook, Rafal Kulmaczewski, Simon A. Barrett and Malcolm A. Halcrow
Inorganic Chemistry Frontiers 2015 - vol. 2(Issue 7) pp:NaN670-670
Publication Date(Web):2015/06/12
DOI:10.1039/C5QI00083A
Oxidation of 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (LSMe) with hydrogen peroxide or mCPBA yields 4-(methylsulfinyl)-2,6-di(pyrazol-1-yl)pyridine (LSOMe) and 4-(methylsulfonyl)-2,6-di(pyrazol-1-yl)pyridine (LSO2Me), respectively. Solid [Fe(LSMe)2][ClO4]2 (1[ClO4]2) is high-spin at room temperature, and exhibits an abrupt spin-transition at T1/2 = 256 K. A shoulder on the cooling side of the χMT vs. T curve is associated with a hysteretic crystallographic phase change, occurring around T↓ = 245 K and T↑ = 258 K. The phase change involves a 180° rotation of around half the methylsulfanyl substituents in the crystal. This contrasts with the previously reported BF4− salt of the same compound, which is isostructural to 1[ClO4]2 at room temperature but transforms to a different crystal phase in its low-spin state. Solid [Fe(LSOMe)2][BF4]2 (2[BF4]2) and [Fe(LSO2Me)2][BF4]2 (3[BF4]2) both exhibit gradual spin-crossover equilibria centred significantly above room temperature. Solution measurements show that the oxidised sulfur centers in 2[BF4]2 and 3[BF4]2 stabilise the low spin states of those complexes.
Co-reporter:Q. Folshade Mokuolu, Dolos Foguet-Albiol, Leigh F. Jones, Joanna Wolowska, Radoslaw M. Kowalczyk, Colin A. Kilner, George Christou, Patrick C. McGowan and Malcolm A. Halcrow
Dalton Transactions 2007(Issue 14) pp:NaN1399-1399
Publication Date(Web):2007/03/09
DOI:10.1039/B618780C
Treatment of CuF2 with 2 equiv of 3{5}-[pyrid-2-yl]pyrazole (HpzPy), 3{5}-phenylpyrazole (HpzPh) or 3{5}-[4-fluorophenyl]pyrazole (HpzPhF) in MeOH, followed by evaporation to dryness and recrystallisation of the solid residues, allows solvated crystals of [{Cu(µ-pzPy)(pzPy)}2] (1), [{Cu(µ-pzPh)2}4] (2) and [Cu4F2(µ4-F)(µ-pzPhF)5(HpzPhF)4] (3) to be isolated in moderate-to-good yields. Similar reactions of these three pyrazoles with Cu(OH)2 in refluxing MeOH respectively afford 1, 2 and [Cu(pzPhF)2(HpzPhF)2] (4) in ca. 10% yield. Crystalline 1·1/2H2O·2CHCl3 contains two independent dinuclear molecules with a puckered di-(1,2-pyrazolido) bridge motif, linked by a bridging, hydrogen-bonding water molecule. Compound 2·1/2C5H12, containing cyclic, square tetranuclear complex molecules, is the first homoleptic divalent metal pyrazolide to have a discrete molecular rather than polymeric structure, for a metal other than Pd or Pt. The two independent complex molecules in 3·3/4CH2Cl2·HpzPhF contain a unique tetrahedral [Cu4(µ4-F)]7+ core, three of whose edges are spanned by bridging pyrazolido groups. Magnetic data show that the copper centres in 1 and 3 are antiferromagnetically coupled, but that dried bulk samples of 2 do not retain their molecular structure.
Co-reporter:Laurynas Pukenas, Florence Benn, Edmund Lovell, Amedeo Santoro, Laurence J. Kershaw Cook, Malcolm A. Halcrow and Stephen D. Evans
Journal of Materials Chemistry A 2015 - vol. 3(Issue 30) pp:NaN7896-7896
Publication Date(Web):2015/05/27
DOI:10.1039/C5TC01233C
Drop-casting acetone solutions of [Fe(bpp)2][BF4]2 (bpp = 2,6-di[pyrazol-1-yl]pyridine) onto a HOPG surface affords unusual chain-of-beads nanostructures. The beads in each chain are similar in size, with diameters in the range of 2–6 nm and heights of up to 10 Å, which is consistent with them containing between 10–50 molecules of the compound. The beads can be classified into two types, which exhibit different conduction regimes by current-imaging tunnelling spectroscopy (CITS) which appear to correlate with their positions in the chains, and may correspond to molecules containing high-spin and low-spin iron centres. Similarly drop-cast films of the complex on a gold surface contain the intact [Fe(bpp)2][BF4]2 compound by XPS. 4-Mercapto-2,6-di[pyrazol-1-yl]pyridine undergoes substantial decomposition when deposited on gold, forming elemental sulfur, but 4-(N-thiomorpholinyl)-2,6-di[pyrazol-1-yl]pyridine successfully forms SAMs on a gold surface by XPS and ellipsometry.
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2008 - vol. 37(Issue 2) pp:NaN289-289
Publication Date(Web):2007/10/02
DOI:10.1039/B701085K
This tutorial review describes how complexes of iron(II) (and, rarely, other metal ions) can be switched between their high- and low-spin states by different physical stimuli. At low temperatures, it is possible to trap a compound in a metastable excited spin-state which, in favourable cases, may be stable to thermal relaxation below temperatures as high as 130 K. The selective switching and trapping of individual spin centres in polynuclear compounds, and the interplay between spin centres as they relax back to their ground states, are also discussed. Similar phenomena, in which spin transitions are coupled to charge transfer phenomena, can also occur in inorganic and metal–organic cyanometallate compounds and in cobalt–semiquinonate complexes.
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2011 - vol. 40(Issue 7) pp:NaN4142-4142
Publication Date(Web):2011/04/11
DOI:10.1039/C1CS15046D
Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).
Co-reporter:Malcolm A. Halcrow
Dalton Transactions 2009(Issue 12) pp:NaN2073-2073
Publication Date(Web):2009/01/23
DOI:10.1039/B815577A
This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal–organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.
Co-reporter:A. Abibat Salaudeen, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2008(Issue 41) pp:NaN5202-5202
Publication Date(Web):2008/09/12
DOI:10.1039/B810393C
A polycatenate network, whose rings are formed from metal-templated hemispheres linked by hydrogen bonds, is reported.
Co-reporter:Hanane Z. Lazar, Thibaut Forestier, Simon A. Barrett, Colin A. Kilner, Jean-François Létard and Malcolm A. Halcrow
Dalton Transactions 2007(Issue 38) pp:NaN4285-4285
Publication Date(Web):2007/08/02
DOI:10.1039/B708971F
The syntheses of [FeL][BF4]2·H2O, [FeL][ClO4]2·H2O, [FeL][NO3]2·CH3NO2 and [FeL][CF3SO3]2 (L = tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine) are described. The isostructural BF4− and ClO4− salts are high-spin between 5–300 K, while the other two compounds are high-spin at room temperature but undergo gradual high→low spin transitions upon cooling. For [FeL][NO3]2 this transition is centred at 139 K and proceeds to near-completeness, while for [FeL][CF3SO3]2 it is centred at 144 K and only proceeds to 50% conversion. The CF3SO3− salt also undergoes spin-crossover centred at 200 K in (CD3)2CO solution, and exhibits dynamic inversion of its helical ligand conformation. All these compounds except the triflate salt have been crystallographically characterised, and show capped trigonal antiprismatic [6 + 1] coordination geometries. The NO3− and CF3SO3− salts undergo quantitative conversion to trapped, high-spin excited states upon irradiation with a green laser at 10 K (the LIESST effect; LIESST = Light-Induced Excited Spin State Trapping). The thermal stabilities of their LIESST excited states (T(LIESST) = 80–82 K) resemble those found for iron(II) complexes of bidentate N-heterocyclic ligands. Hence, the LIESST properties of [FeL]2+ are those of a complex of three rigid bidentate domains linked by a flexible spacer, rather than of a single encapsulating podand.
Co-reporter:Sara A. Diener, Amedeo Santoro, Colin A. Kilner, Jonathan J. Loughrey and Malcolm A. Halcrow
Dalton Transactions 2012 - vol. 41(Issue 13) pp:NaN3739-3739
Publication Date(Web):2012/01/11
DOI:10.1039/C2DT11911K
New iron(II) podand complexes have been prepared, by condensation of 2-(aminomethyl)-2-methyl-1,3-diaminopropane with 3 equiv of a heterocyclic aldehyde in the presence of hydrated Fe[BF4]2 or Fe[ClO4]2 as templates. The 2-(aminomethyl)-2-methyl-1,3-diaminopropane is prepared in situ by deprotonation of its trihydrochloride salt. The chloride must be removed from these reactions by precipitation with silver, to avoid the formation of the alternative 2,4,6-trisubstituted-7-methyl-1,3,5-triazaadamantane condensation products, or their FeCl2 adducts. The crystal structures of two 2,4,6-tri(pyridyl)-7-methyl-1,3,5-triazaadamantane-containing species are presented, and contain two different geometric isomers of this tricyclic ring with three equatorial, or two equatorial and one axial, pyridyl substituents. Both structures feature strong C–H⋯X (X = Cl or F) hydrogen bonding from the aminal C–H groups in the triazaadamantane ring. Five iron(II) podand complexes were successfully obtained, all of which contain low-spin iron centres.
Co-reporter:Malcolm A. Halcrow
Chemical Communications 2010 - vol. 46(Issue 26) pp:NaN4763-4763
Publication Date(Web):2010/06/03
DOI:10.1039/C0CC00603C
Phase-pure solid solutions of [Co(terpy)2][BF4]2 and [M(bpp)2][BF4]2 (M = Fe or Ru, bpp = 2,6-dipyrazol-1-ylpyridine) have been prepared. The spin-state of the [Co(terpy)2]2+ dopant is modulated by the spin-state of the [Fe(bpp)2]2+ centres in the iron-containing materials.
Co-reporter:Thomas D. Roberts, Marc A. Little, Laurence J. Kershaw Cook and Malcolm A. Halcrow
Dalton Transactions 2014 - vol. 43(Issue 20) pp:NaN7588-7588
Publication Date(Web):2014/03/31
DOI:10.1039/C4DT00355A
Syntheses of 2,6-di(5-aminopyrazol-3-yl)pyridine (L1), 2,6-di(5-tertbutylcarboxamidopyrazol-3-yl)pyridine (L2), 2,6-di(5-tertbutylpyrazol-3-yl)pyridine (L3), 2-(5-tertbutylpyrazol-3-yl)-6-(5-methylpyrazol-3-yl)pyridine (L4) and 2-(5-tertbutylpyrazol-3-yl)-6-(5-aminopyrazol-3-yl)pyridine (L5) are reported. Iron complex salts of the first four ligands were crystallographically characterised. The structures exhibit intermolecular hydrogen bonding between the cations and the anions and/or solvent, leading to a fluorite (flu) net, a 1D ladder structure, and a homochiral self-penetrating helical network related to the (10,3)-a (srs) topology. All the complexes are high-spin in the crystal, and bulk samples are also fully or predominantly high-spin at room temperature and below although two of the dried materials exhibit partial spin-state transitions on cooling.
Co-reporter:Thomas D. Roberts, Floriana Tuna, Tamsin L. Malkin, Colin A. Kilner and Malcolm A. Halcrow
Chemical Science (2010-Present) 2012 - vol. 3(Issue 2) pp:NaN354-354
Publication Date(Web):2011/11/04
DOI:10.1039/C1SC00584G
[FeL2][BF4]2·2H2O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1:1 high:low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1AA) or by thermal dehydration (phase 1BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.
Co-reporter:Malcolm A. Halcrow
Chemical Society Reviews 2013 - vol. 42(Issue 4) pp:NaN1795-1795
Publication Date(Web):2012/09/11
DOI:10.1039/C2CS35253B
This tutorial review discusses the structural and electronic consequences of the Jahn–Teller effect in transition metal complexes, focussing on copper(II) compounds which tend to be the most studied. The nature of a Jahn–Teller distortion in molecular complexes and extended lattices can be manipulated by application of pressure or temperature, by doping a molecule into a host lattice, or simply by molecular design. Many of these results have been achieved using compounds with a trans-[CuX4Y2] coordination sphere, which seems to afford copper centres that are particularly sensitive to their environment. Jahn–Teller distortions lead to some unusual phenomena in molecular magnetism, and are important to the functionality of important classes of conducting and superconducting ceramics.
Co-reporter:Jonathan J. Loughrey, Tim P. Comyn, David C. Apperley, Marc A. Little and Malcolm A. Halcrow
Chemical Communications 2014 - vol. 50(Issue 57) pp:NaN7603-7603
Publication Date(Web):2014/05/29
DOI:10.1039/C4CC02198C
[FeL3][BF4]2·xH2O (L = 3-(pyrazinyl)-1H-pyrazole) shows negative thermal expansion between 150–240 K but positive thermal expansion at 240–300 K, linked to rearrangement of anions and water molecules within pores in the lattice.
Co-reporter:Simon A. Barrett, Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2011 - vol. 40(Issue 45) pp:NaN12024-12024
Publication Date(Web):2011/05/16
DOI:10.1039/C1DT10620A
The temperature of spin-crossover in [Fe(3-bpp)2][BF4]2 (3-bpp = 2,6-di{pyrazol-3-yl}pyridine) tends to increase in associating solvents. In particular, T½ shifts to 60–70 K higher temperature in water compared to organic solvents.
Co-reporter:Rebecca Docherty, Floriana Tuna, Colin A. Kilner, Eric J. L. McInnes and Malcolm A. Halcrow
Chemical Communications 2012 - vol. 48(Issue 34) pp:NaN4057-4057
Publication Date(Web):2012/03/06
DOI:10.1039/C2CC30873H
The electronic structures of [Cu(terpy)2]2+ and [Cu(bpp)2]2+ (bpp = 2,6-di[pyrazol-1-yl]pyridine) are different, when doped into [M(bpp)2][BF4]2 (M2+ = Fe2+ or Zn2+). The [Cu(terpy)2]2+ dopant is a typical pseudo-Jahn–Teller elongated copper(II) center. However, the [Cu(bpp)2]2+ sites show EPR spectra consistent with a tetragonally compressed {dz2}1 configuration.
Co-reporter:Guillaume Chastanet, Clare A. Tovee, Geoffrey Hyett, Malcolm A. Halcrow and Jean-François Létard
Dalton Transactions 2012 - vol. 41(Issue 16) pp:NaN4902-4902
Publication Date(Web):2012/03/08
DOI:10.1039/C2DT12122K
The photomagnetic properties of two series of spin-crossover solid solutions, [Fe(1-bpp)2]x[Ru(terpy)2]1−x(BF4)2 and [Fe(1-bpp)2]x[Co(terpy)2]1−x(BF4)2 (1-bpp = 2,6-bis[pyrazol-1-yl]pyridine), have been investigated. For all the materials, the evolution of the T(LIESST) value, the high-spin → low-spin relaxation parameters and the LITH loops were thoroughly studied. Interestingly in the Fe:Co series, along the photo-excitation, cobalt ions are concomitantly converted from low-spin to high-spin states with the iron centres, and also fully relax after light excitation.
Co-reporter:Ruth Pritchard, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2007(Issue 6) pp:NaN579-579
Publication Date(Web):2006/11/14
DOI:10.1039/B613402E
Six structurally related iron(II) complexes show remarkably similar abrupt thermal spin-transitions.
Co-reporter:Laurence J. Kershaw Cook, Floriana Tuna and Malcolm A. Halcrow
Dalton Transactions 2013 - vol. 42(Issue 6) pp:NaN2265-2265
Publication Date(Web):2012/11/19
DOI:10.1039/C2DT31736B
The syntheses of 2,6-di(pyrid-2-yl)pyrazine (L1), 2,6-di(pyrazinyl)pyridine (L2), 2,2′:6′,2′′-terpyrazine (L3), 2,6-di(pyrimidin-4-yl)pyridine (L4), 2,6-di(1,2,4-triazin-3-yl)pyridine (L5), 4-hydroxy-2,6-di(pyrazinyl)pyridine (L6) and 4-hydroxy-2,6-di(pyrimidin-2-yl)pyridine (L7) are described. Homoleptic iron(II) and cobalt(II) complexes of these ligands have been prepared and, in four cases, structurally characterised. The iron complexes are all low-spin. However, while the cobalt complexes of the pyrazine-rich ligands L2, L3 and L6 are all predominantly low-spin in the solid state, the other cobalt complexes are essentially high-spin between 5–300 K. The voltammetric M(III)/(II) (M = Fe or Co) oxidations and metal- or ligand-based reductions all become more anodic as the nitrogen content of the ligands increases, which correlates well with Lever's additive electrochemical parameters for the heterocyclic donor groups in each complex.
Co-reporter:Jonathan J. Loughrey, Nathan J. Patmore, Amgalanbaatar Baldansuren, Alistair J. Fielding, Eric J. L. McInnes, Michaele J. Hardie, Stephen Sproules and Malcolm A. Halcrow
Chemical Science (2010-Present) 2015 - vol. 6(Issue 12) pp:NaN6948-6948
Publication Date(Web):2015/08/20
DOI:10.1039/C5SC02776D
Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl {tBu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6− catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or tBu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.
Co-reporter:Thomas D. Roberts, Marc A. Little, Floriana Tuna, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2013 - vol. 49(Issue 56) pp:NaN6282-6282
Publication Date(Web):2013/05/30
DOI:10.1039/C3CC43613F
Two salts of [FeL2]2+ (L = 2,6-bis[5-methyl-1H-pyrazol-3-yl]pyridine) are isostructural under ambient conditions but show different thermal spin-crossover behaviour, involving a variety of crystallographic phase changes.
Co-reporter:Amedeo Santoro, Carlo Sambiagio, Patrick C. McGowan and Malcolm A. Halcrow
Dalton Transactions 2015 - vol. 44(Issue 3) pp:NaN1069-1069
Publication Date(Web):2014/11/07
DOI:10.1039/C4DT02824D
A new synthesis of 1,1,1-tris(pyrid-2-yl)ethane (L), and a survey of its coordination chemistry, are reported. The complexes [ML2]n+ (Mn+ = Fe2+, Co2+, Co3+, Cu2+ and Ag+), [PdCl2L] and [CuI(L)] have all been crystallographically characterised. Noteworthy results include an unusual square planar silver(I) complex [Ag(L)2]X (X− = NO3− and SbF6−); the oxidative fixation of aerobic CO2 by [CuI(L)] to yield [Cu2I(L)2(μ-CO3)]2[CuI3] and [Cu(CO3)(L)]; and, water/carbonato tape and water/iodo layer hydrogen bonding networks in hydrate crystals of two of the copper(II) complexes. Cyclic voltammetric data on [Fe(L)2]2+ and [Co(L)2]2+/3+ imply that the peripheral methyl substituent has a weak influence on the ligand field exerted by L onto a coordinated metal ion.
Co-reporter:James J. Henkelis, Colin A. Kilner and Malcolm A. Halcrow
Chemical Communications 2011 - vol. 47(Issue 18) pp:NaN5189-5189
Publication Date(Web):2011/03/30
DOI:10.1039/C1CC00054C
Recrystallisation of Ag[L1] (HL1 = 3{5}-[pyrid-2-yl]-5{3}-tert-butylpyrazole) in the presence of halide anions leads to two polymorphs of [Ag3(μ-Br)(μ-L1)2], which differ in their mode of supramolecular association, and the cluster [Ag10(μ-L1)8]Cl2. In contrast, Ag[L2] (HL2 = 3{5}-[isoquinol-1-yl]-5{3}-tert-butyl-pyrazole) crystallises as a cyclic tetrameric molecule.
Co-reporter:Colin A. Kilner and Malcolm A. Halcrow
Dalton Transactions 2010 - vol. 39(Issue 38) pp:NaN9012-9012
Publication Date(Web):2010/08/19
DOI:10.1039/C0DT00295J
Single crystal and powder samples of [Co(terpy)2][BF4]2 are low spin below 100 K but show a typically gradual thermal spin-transition on warming, centred near 270 K. However, the spin-crossover exhibits an unusual and pronounced discontinuity above room temperature, when the material is ca. 87% high-spin. A crystallographic study at nine temperatures between 30–375 K showed that the discontinuity is not caused by a phase transition or by changes in anion disorder. Rather, it may reflect the steric consequence of small changes in the nearest neighbour interactions between the cations as the spin transition progresses.
Co-reporter:Shinya Hayami, Stephen M. Holmes and Malcolm A. Halcrow
Journal of Materials Chemistry A 2015 - vol. 3(Issue 30) pp:NaN7778-7778
Publication Date(Web):2015/07/14
DOI:10.1039/C5TC90128F
A graphical abstract is available for this content
Co-reporter:Laurence J. Kershaw Cook, Julie Fisher, Lindsay P. Harding and Malcolm A. Halcrow
Dalton Transactions 2015 - vol. 44(Issue 20) pp:NaN9425-9425
Publication Date(Web):2015/04/16
DOI:10.1039/C5DT00732A
The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published “back-to-back” bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(II) and zinc(II). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n]2n+ and [{Zn(μ-bppSSbpp)}n]2n+ form tetranuclear metallacycles in nitromethane solution (n = 4), although 1H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2]2+ and [{Fe(μ-bppSSbpp)}n]2n+ exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures.
![Thiourea, N-methyl-N'-[5-(2-pyridinyl)-1H-pyrazol-3-yl]-](http://img.cochemist.com/ccimg/886000/885946-03-2.png)
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