Three diiron carbonyl complexes with different carbonyl ligands have been synthesized. Pentacarbonyl and tetracarbonyl complexes 2 and 3 were prepared by the substitution of the bound CO of complex 1 by PPh3. The photo-induced CO-releasing behaviours of these complexes correlate linearly to their reduction potentials and further to the energy level of their LUMOs. The more negative the reduction potentials, the more reactive the complexes. The photo-induced CO-release can be further promoted when a nucleophile is presented such as thiopronin.The photochemical reactivity of the diiron carbonyl complexes increases linearly with their LUMO energies which correlate linearly with their reduction potentials.Download high-res image (84KB)Download full-size image

•Diiron hexacarbonyl complexes with naphthalene moiety bearing functional groups.•The functional groups exert hardly electronic influence on the metal center but can ease the kinetics of the catalysis of proton reduction.•Carboxylic acid and amine groups act as a proton relay and thus improve catalytic efficiency.Three diiron hexacarbonyl complexes (2, 3 and 4) with naphthalene-1,8-bis(thiolate) skeleton as their bridging linkages are reported. For comparison, the protonated form of complex 4 was also prepared (4H+). They bear respectively a functional group on the naphthalene ring at the position 2, i.e. −CH2OH (2), −COOH (3), −CH2N(Et)2 (4) and −CH2NH+(Et)2 (4H+). Complex 2 was derived from the direct reduction of its precursor bearing aldehyde group (−CHO, 1) by NaBH4 while complexes 3 and 4 were routinely synthesized by reacting Fe3(CO)12 with ligands L2 and L3, respectively, which were derived from ligand L1, naphtho[1,8-cd][1,2]dithiole-3-carbaldehyde. These complexes were fully characterized and complexes 3 and 4 were analyzed using X-ray single crystal diffraction. Electrochemistry of these complexes was also investigated by cyclic voltammetry. The carboxylic acid of complex 3 shows significant influence on the second reduction due to the acid group involving reaction with the reduced species. Both infrared spectral data and the first reduction potentials of the complexes suggest that these functional groups exert hardly electronic influence on the metal center. However, the functional groups which can carry proton (−COOH and −CH2N+H(Et)2) can ease the kinetics of the catalysis of proton reduction via probably PCET (proton-coupled electron transfer) mechanism. These proton carriers can also improve catalytic efficiency by acting a proton relay during the catalysis as suggested by the linear plots of peak current against acid concentration for the three complexes.Download high-res image (132KB)Download full-size imageAn internal proton acceptor (base group) can ease the kinetics of the catalysis of proton reduction via probably PCET (proton-coupled electron transfer) mechanism by relaying proton during catalysis.

The reaction of ligand L, N,N-bis(pyridin-2-ylmethyl)acetamide, with five transition metal salts, FeCl3·6H2O, CuCl2·2H2O, Cu(ClO4)2·6H2O, ZnCl2 and K2PtCl4/KI, produced five metal complexes, [(μ-O)(FeClL′)(FeCl3)] (1), [CuLCl2] (2), [CuBPA(ClO4)(CHCN)] ClO4 (3), [ZnLCl2] (4) and [PtLI2] (5), where L′ = 1-(2,4,5-tri(pyridin-2-yl)-3-(pyridin-2-ylmethyl)imidazolidin-1-yl)ethanone which formed in situ, and BPA = bis(pyridin-2-ylmethyl)amine. The ligand and complexes were characterized by a variety of spectroscopic techniques including X-ray single crystal diffraction where applicable. Depending on the metal ion and auxiliary ligand of the complex, the acetyl group of the ligand L could be either intact or cleaved. When ferric chloride hexahydrate was used, the deacetylation proceeded even further and a novel heterocyclic compound (L′) was formed in situ. A possible mechanism was proposed for the formation of the heterocyclic compound found in complex 1. Our results indicate that to cleave effectively an amide bond, it is essential for a metal centre to bind to the amide bond and the metal centre is of sufficient Lewis acidity.

Reaction of a heptadentate ligand (H2LI) and a hexadentate ligand (H2LII) with three equivalents of Cu(ClO4)2·6H2O in methanol under basic conditions afforded a hexanuclear cluster [Cu6(LI)2(OH)4](ClO4)4·2DMF·3Et2O (1) and a nonanuclear cluster [(Cu9(LII)3(OH)7)](ClO4)5·0.25CH3OH·1.15H2O (2), respectively, where H2LI is 2,2′-(((pyridine-2,6-diylbis(methylene))bis((pyridin-2-ylmethyl)azanediyl))bis(methylene))diphenol and H2LII 2,2′-((((5-methyl-1,3-phenylene)bis(methylene))bis((pyridin-2-ylmethyl)azane-diyl))bis(methylene))bis(4-methylphenol). The structures of both clusters in their solid states have been determined by X-ray crystallography. Their magnetic susceptibilities reveal that both clusters are antiferromagnetic due to the coupling between the copper centers in the clusters. NMR spectroscopic analysis, conductivity measurements and ESI-MS analysis suggest that the clusters retain their structural integrities in solution. Both clusters show catalytic activity towards the hydroxylation of benzene into phenol with hydrogen peroxide (H2O2) as an oxidant at 80 °C in aqueous acetonitrile. The conversion rate is about 20% and their TON/TOF are 564/188 and 905/302 for clusters 1 and 2, respectively.

•The LUMOs are almost solely located on the metallic centre.•The composition of the HOMOs has a mixed contribution.•Intramolecular hydrogen bonding interactions affect the composition of the HOMOs.Seven diiron hexacarbonyl complexes, [Fe2(L)(CO)6](L = two monothiolate or dithiolate ligands), as the mimics of the diiron subunit of [FeFe]-hydrogenase were theoretically investigated using density functional theory (DFT) to examine the electronic influences of the bridging linkages of the complexes on their electrochemical behaviours. In the calculations, the energies, the compositions of the frontier orbitals and the one-electron redox potentials of these complexes were estimated. Charge distributions of the complexes were calculated using natural population analysis (NPA). The influence of the functional groups of the bridging linkages on the diiron core via intramolecular hydrogen interactions was explored using topological analysis. The results indicated that the bridging linkages of the complexes affect significantly the compositions of the HOMOs whereas hardly alter the compositions of the LUMOs. The correlations of the energies of the frontier orbitals to the redox potentials of the examined complexes were also discussed.The bridging linkages affect significantly the electrochemical behaviours of the diiron carbonyl complexes via alternating both the compositions and energies of the frontier orbitals. DFT calculations reveal that the electronic effect exerted by bridging linkages is not only achieved through intramolecular hydrogen bonding interactions, but also the direct bonding between the iron core and the bridging linkages.

Three diiron complexes (1-Ph, 2-OH, and 3-OCOFc) as mimics of the diiron subunit of [FeFe]-hydrogenase were electrochemically investigated in 0.1 mol L−1 [NBut4]BF4–acetonitrile (MeCN) under CO and Ar atmosphere. Complex 3-OCOFc was prepared from the reaction of complex 2-OH with ferrocenylacyl chloride (FcOCCl). The complex was fully characterised using a variety of spectroscopic techniques. Its structure was established using X-ray single crystal diffraction analysis. In addition to the well-established ECE (E for electrochemical and C for chemical) mechanism, it was revealed that a further reversible reduction at a potential more negative by ca. 600 mV was observed under CO atmosphere. It was further proposed based on the analysis of electrochemical and infrared spectroscopic data that the second redox was due to a two-electron process of supposedly a tetrairon cluster. This product was formed in situ from the reaction between the dianion generated from the ECE process and its parent complex (1-Ph, 2-OH, and 3-OCOFc) and is supposedly of a core “Fe4(II)”. This reaction occurred only when CO was presented. Under Ar atmosphere, bulk electrolysis led to fully-reduced products, that is, with the iron at the oxidation state of zero, but complex 2-OH was an exception. An overall mechanism to describe the electron transfer and coupled chemical reactions under CO atmosphere was proposed.

Reaction of Fe3(CO)12 with pyridinyl thioester ligand PyCH2SCOCH3 (L1, Py = pyridin-2-yl) produced complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)5] (1) (PyCH2S = pyridin-2-ylmethanethiolate). When complex 1 reacted with PPh3, a monosubstituted complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)4PPh3] (2), was derived. Reaction of the same precursor with analogous thioester ligand PyCH2SCOPy (L2) generated three novel diiron complexes, [Fe2(κ-Py)(μ-SCH2Py)(CO)5] (3), [Fe2(κ-Py)′(μ-SCH2Py)(CO)5] (4), and [Fe2(κ-Py)(μ-SCH2Py)(CO)6] (5). Complexes 3 and 4 are structural isomers. Complex 5 could be converted into complex 4 but the conversion from complex 5 to the isomer 3 was not observed. All the five complexes were fully characterised using FTIR, NMR, and other techniques. Their structures were determined using X-ray single crystal diffraction analysis. The oxidative formation of complexes 1, 3, 4, and 5 involved C–S and/or C–C bonds cleavages. To probe possible mechanisms for these cleavages, DFT calculations were performed. From the calculations, viable reaction pathways leading to the formation of all the isolated products were delineated. The results of the theoretic calculations also allowed rationalisation of the experimental observations.

A novel multidentate ligand, 5-(bis(pyridin-2-ylmethyl)amino)quinolin-8-ol (HL) was synthesised and characterised. Its coordination modes with a variety of metal ions (Mg2+, Co2+, Cu2+, Zn2+, and Hg2+) were investigated using UV–Vis spectroscopic titration. Among the examined metal ions, coordination ratios (M2+: HL) between the metal ion and the ligand at 1:2, 1:1, and 3:2 were observed due to the ditopic nature of the ligand. In acetonitrile, Mg2+ showed relatively strong fluorescent response upon binding to the ligand among the examined metal ions, Li+, K+, Mg2+, Ca2+, Al3+, Cu2+, Fe3+, Cr3+, Zn2+, Co2+, Ni2+, and Hg2+.Graphical abstractAn 8-hydroxyquinoline-based ditpic ligand. Spectroscopic titration of the ligand against selected metal ions suggests that the metal ions bind progressively to its two binding sites. The binding of Mg2+ enhanced significantly the fluorescent response of the ligand among the examined metal ions.Highlights► A hydroxylquinoline-based novel ditopic ligand was described. ► Metal ions bind progressively to the two binding sites of the ditopic ligand. ► Intramolecular charge-transfer involving d electron would be the major fluorescence-quenching mechanism.

Two diferrous complexes, [Fe2(μ-SCH2CH3)3(CO)5I] and [Fe2(μ-SCH2CH2CH3)3(CO)5I] were synthesised via reaction of a monoiron carbonyl precursor, [Fe(CO)4I2], with ethanethiolate and propanethiolate, respectively. The complexes were fully characterised using spectroscopic techniques, for instance, FTIR and NMR. Their crystal structures were determined using single crystal diffraction analysis. Electrochemical reduction of these complexes are temperature-dependent. At room temperature, the diferrous complexes undergo one-electron reduction. The reduction-initiated cleavage of one bound thiolate and one iodide as a radical, takes the oxidation states of the diiron core from {Fe(I)Fe(II)} to {Fe(I)Fe(I)}. This reduction-initiated transformation can be suppressed by lowering the temperature to 195 K, further reduction of the monoanion was observed at a potential very close to that of the first reduction, which is analogous to the mechanism observed for diiron complexes with a core of {Fe(I)Fe(I)}.

Three polymers functionalised with diiron carbonyl units, PVC–Fe-A, -B, and -C, were prepared using commercially available polymer PVC (polyvinyl chloride). PVC–Fe-A resulted from the reaction of the reduced form of a diiron complex, [Fe2(μ-S)2(CO)6], with PVC, whereas PVC–Fe-B and PVC–Fe-C were, respectively, prepared by reacting PVC–N3, the polymer functionalised with azide groups by substitution of the chloride of the polymer, with two diiron complexes, [Fe2(μ-SCH2CCH)2(CO)6] (1) and [Fe2(μ-SnBut)(μ-SCH2CCH)(CO)6] (2, nBut = –CH2CH2CH2CH3), via “click chemistry” under the catalysis of CuI. Those polymers were characterised using infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA). Film electrodes were assembled using a spin-coating technique by casting a mixture of the functionalised polymer, MWCNTs (multi-wall carbon nanotubes), and Nafion onto the surface of a vitreous carbon electrode. The assembled electrodes exhibited electrochemical responses and catalysis on proton reduction in a medium of acetonitrile-acetic acid with a positive shift in reduction potential by over 400 mV compared to the precursor diiron complexes (1 and 2).

A diiron hexacarbonyl complex possessing an alkynyl group as a model complex of the diiron sub-unit of [FeFe]-hydrogenase was polymerized under the catalysis of WCl6-SnPh4. The polymer (Poly-{Fe2}) functionalized with {Fe2(CO)6} units which is dominated by cis-form was fully characterised using FTIR, NMR, SEM, TEM, and TGA techniques. Through modifying the monomer, the properties, for example, solubility, of the resultant polymer could be tuned and much larger molecule weight, which was estimated as 7.93 × 105 g mol−1 using static light scattering technique was achieved without compromising its solubility. Spin-coating the functionalized polymer onto the surface of vitreous carbon electrode with or without multi-wall carbon nanotubes (MWCNTs) produced film electrodes which show electrochemical responses. Adding MWCNTs into the film enhances significantly the electrochemical response probably via not only improving the conductivity of the film, but also the increase in its effective surface area after being doped with MWCNTs.Highlights► Polymerizing a diiron mimic achieved a polymer related to [FeFe]-hydrogenase. ► Film electrode was assembled using the polyene-based polymer via spin-coating. ► The film electrode showed electrochemical catalysis on proton reduction.

Re-examining electrocatalytic substitution reactions of two diiron hexacarbonyl complexes, [Fe2(μ-pdt)(CO)6] (1) (pdt = propane-1,3-dithiolate) and [Fe2(μ-padmt)(CO)6] (2) (padmt = (propylazanediyl)dimethanethiolate), reveals that the monoanion of these complexes is the catalytic species rather than their dianion. Detailed mechanisms of electrocatalytic substitution reactions for the two complexes were proposed based on investigations into their electrochemistry without and with the presence of a monodentate ligand PPh3 by using a variety of cyclic voltammetric techniques and rationalised by digital simulations. Our investigations also demonstrate an alternative methodology for synthesis of substituted diiron carbonyl complexes which have widely been employed as models of the sub-unit of the [FeFe]-hydrogenase. Electrochemical mean requires less organic solvents, no harsh reaction conditions, and no auxiliary agents and thus environmentally more benign than chemical synthesis.

By using “click” chemistry between a diazide and a diiron model complex armed with two alkynyl groups, two polymeric diiron complexes (Poly-Py and Poly-Ph) were prepared. The two polymeric complexes were investigated using infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), Mössbauer spectroscopy, and cyclic voltammetry (Poly-Py only, due to the insolubility of Poly-Ph). To probe the coordinating mode of the diiron units in the two polymeric complexes, two control complexes (3 and 4) were also synthesised using a monoazide. Complexes 3 and 4 were well characterised and the latter was further crystallographically analysed. It turns out that in both complexes (3 and 4) and the two polymeric diiron complexes, one of the two iron atoms in the diiron unit coordinates with one of the triazole N atoms. Our results revealed that both morphologies and properties of Poly-Py and Poly-Ph are significantly affected by the organic moiety of the diazide. Compared to the protonating behaviour of the complexes 3 and 4, Poly-Py exhibited proton resistance. In electrochemical reduction, potentials for the reduction of the diiron units in Poly-Py and hence its catalytic reduction of proton in acetic acid–DMF shifted by over 400 mV compared to those for complexes 3 and 4. It is likely that the polymeric nature of Poly-Py offers the diiron units a “protective” environment in an acidic medium and more positive reduction potential.

An anthracene-based novel ligand (L), 9,10-bis((4,6-dimethylpyrimidin-2-ylthio)methyl)anthracene, was synthesised and fully characterised. Interactions of the ligand with selected metal ions, Hg(II), Cu(II), Ag(I), Pb(II), Zn(II), Ni(II), Co(II), and Cr(III), were spectroscopically investigated. Of the examined metal ions, both Hg(II) and Cu(II) showed responses in both UV–Vis and fluorescent spectroscopy towards the ligand in acetonitrile solution. Spectroscopic titration indicated that the ligand forms complexes with the two metal ions in 1:1 and 1:2 ratios, respectively. DFT calculations revealed that Hg(II) binds possibly with two pairs of donor-set {SN} of the ligand to form a mononuclear complex in a distorted planar geometry whereas Cu(II) forms likely a binuclear complex in a tetrahedral geometry in which each Cu(II) is further coordinated with possibly two acetonitrile molecules.An anthracene-based novel ligand derived from, 9,10-bis((4,6-dimethylpyrimidin-2-ylthio)methyl)anthracene, shows selectively fluorescent recognitions in acetonitrile towards both Hg(II) and Cu(II) ions among the investigated metal ions, Hg(II), Cu(II), Ag(I), Pb(II), Zn(II), Ni(II), Co(II), and Cr(III).

Two Hg(II) complexes [HgL′(ClO4)2] (1) and [HgL(ClO4)]ClO4 (2) derived from the macrocyclic ligands, 4-(pyridin-2-ylmethyl)-1,7-dithia-4,10-diazacyclododecane (L′) and 7-(pyridin-2-ylmethyl)-1,4,10-trithia-7,13-diazacyclopentadecane (L), have been crystallographically characterised. Ligand L and its Hg(II) complex were isolated unexpectedly, and a possible formation pathway of the ligand is proposed. By including weakly bound O atoms from the perchlorate ions, the Hg atoms in both complexes are seven-coordinate and possess capped trigonal prismatic geometries. These uncommon structures for Hg(II) complexes were achieved mainly by the relatively large size of the metal ion and the steric effect from the macrocycles. In both complexes, strong hydrogen bonding between the amine hydrogen atom and a perchlorate ion was observed. For complex 1, the interaction is N(3)–H(15)···O(8) at 2.08(12) Å where O(8) is of the same anion as one of the coordinated O atoms; in complex 2, a similar hydrogen bond, N(7)–H(7)···O(32), with a distance of 2.25 Å, is formed to the coordinated anion, but the second anion remains discrete.Two Hg(II) complexes [HgL′(ClO4)2] (1) (left) and [HgL(ClO4)]ClO4 (2) (right), with distorted capped trigonal prismatic geometries, were isolated by reacting Hg(ClO4)2·3H2O with the two macrocyclic ligands, 4-(pyridin-2-ylmethyl)-1,7-dithia-4,10-diazacyclododecane (L′) and 7-(pyridin-2-ylmethyl)-1,4,10-trithia-7,13-diazacyclopentadecane (L). The steric tension imposed by the macrocycle and the size of the Hg(II) centre presumably lead to the geometry observed.

Reacting a bidentate ligand H2L, 2-(2-methoxybenzyl)-2-methylpropane-1,3-dithiol, with Fe3(CO)12 formed a diiron hexacarbonyl complex (1Me) from which a diiron hexacarbonyl complex (1H) pendant with a phenolic group was derived via in-situ demethylation. Further deprotonation of complex 1H gave a diiron pentacarbonyl species (1) in which a rare bond between the soft metal FeI and the relatively hard base phenolate formed, FeI-OR (R = phenolic moiety). This bonding may be a suitable mimic of the bonding feature, {FeIFeI}R-OH/OH2 found in the [FeFe]-hydrogenase.Deprotonation of complex 1H led to the formation of a diiron pentacarbonyl species 1, possibly via an anionic intermediate, which possesses a rare bond between a soft metal FeI and relatively hard base phenolate and ought to have some relevance to the structural feature, {FeIIFeI}–OH2/OH−, found in the diiron sub-unit of [FeFe]-hydrogenase.

Three novel solid phase extraction agents were developed by functionalising sub-micron sized silica gel with organic functional moieties possessing {SN}-ligating atoms. The extractors were characterised by FTIR and TGA. Their capability of adsorbing the ions Fe(III), Cu(II), Zn(II), Cd(II), Cr(VI), Hg(II), Pb(II), Co(II), Ni(II), and Ag(I) is described. The extractors show pH-tunable selectivity for Ag(I) and/or Pb(II). By adjusting the pH to 5 or 6, high affinity is found for both Ag(I) and Pb(II), with little or no interference by the other metal ions. At pH values of <2, the extractors become highly selective for Ag(I), with an adsorption capacity of 35 mg g−1. Little mechanical stirring is required due to the size of the particles. The recovery rates for both Ag(I) and Pb(II) were better 90% even after five repetitive adsorption-desorption cycles.

Electrochemical investigations of a tri-iron cluster, [Fe3(μ3-S)2(CO)9] (designated as Fe3CO thereafter), which possesses a core of [FeIIFeIFeI], is described. In 0.5 M [NBu4]BF4/dichloromethane, the cluster exhibits a fairly reversible redox process at −1.03 V and irreversible reduction wave at ca. −1.75 V. The latter is attributed to the reductions of both the decomposed product of the monoanion, [Fe3CO]−, and the isomer of this anion. This cluster catalyses proton reduction with the presence of acid HBF4·Et2O in dichloromethane. Possible mechanisms were proposed to elucidate its electrochemistry and electrocatalytic behaviours on proton reduction. Digital simulations were performed to verify the proposed mechanisms and kinetic parameters were generated at our best estimation in the simulations. The simulated cyclic voltammograms fit well with the experimentally observed ones, which largely supports the proposed mechanisms.

The synthesis, characterisation of three diiron tetracarbonyl complexes, [Fe2(SCH2)2C(Me)(CH2OR)(PNP)(CO)4] (R = H and PNP = L11: 2; R = H and PNP = L22: 3; R = Ts and PNP = L11: 4) as models for the diiron sub-unit of [FeFe]-hydrogenase are described, where OTs, L11 and L22 are toluenesulfonate, (Ph2P)2NCH2(2-C5H4N) and (Ph2P)2NCH2Ph, respectively. These complexes are fully characterised and the structure of complex 4 is crystallographically determined. Protonation of these complexes with HBF4·Et2O is probed by using infrared and NMR spectroscopies which reveals that no hydride can be formed upon addition of the acid. Instead addition of excess of the acid leads to protonating the N atom of the PNP skeleton, which is a weak base due to participating conjugating interactions with the Fe–Fe centre, as revealed by crystallographic analysis. Electrochemistry of these complexes and their electrocatalytic reduction of protons are also investigated. Our results suggest that the existence of the pendant pyridine group can lower the overpotential for proton reduction but does not seem to enhance electrocatalytic efficiency in our case.

A macrocyclic ligand possessing a donor set of {N3S2} synthesised via Cs+-templation, 4-(pyridin-2-ylmethyl)-1,7-dithia-4,10-diazacyclododecane (L) and its Cu(II) complex, [CuL(NCMe)]2+ (6), are described. This Cu(II) complex interacts with a range of anions, F−, Cl−, Br−, I−, HCOO−, AcO−, CO32−, NO3−, C2O42−, H2PO4−, SCN−, CN−, BF4−. Of the investigated anions, I−, SCN−, and CN−, show strong interaction with the Cu(II) centre as indicated by their spectral variations. The iodide particularly demonstrates distinct change in colour. This change originates from a newly appeared band at 471 nm upon iodide binding, which arises from the ligand (I−) to Cu(II) charge transfer (LMCT) in the iodide-substituted Cu(II) complex, [CuLI]+ (7). All organic compounds are characterised by NMR spectroscopy and/or microanalysis. The identities of the two Cu(II) complexes are confirmed by using microanalysis and the complex 6 is crystallographically analysed.A macrocyclic ligand synthesised via Cs+-templation forms a complex with Cu(II) with a loosely bound solvent molecule as its sixth coordinated ligand. This solvent molecule is readily substituted by other anions, for example, I−, CN− and SCN−. But only the iodide binding produces distinct change in colour.

The reaction of a tetradentate ligand, 2,2′-(pyridin-2-ylmethylazanediyl)diethanethiol (H2L), with dodecacarbonyltriiron in dry toluene leads to the formations of a hexacarbonyl complex, [Fe2(EDT)(CO)6] (EDT = ethylenedithiolate), 1a, which is fully characterised. The formation of this complex is via intramolecular C–S/N bonds formation/cleavage promoted by iron-sulfur coordination chemistry. A possible mechanism for the C–S/N bonds formation/cleavage in the formation of the complex is proposed.A diiron hexacarbonyl complex, [Fe2(EDT)(CO)6] (EDT = ehtylenedithiolate), forms in the reaction of a dithiol ligand, 2,2′-(pyridin-2-ylmethylazanediyl)diethanethiol, with Fe3(CO)12 involving intramolecular C–S/N bonds cleavage/formation. Possible mechanism for its formation is discussed.

Infrared data for mono-iron complexes possessing two cis-CO together with Mössbauer data for the enzyme and a model complex support the assignment that the iron centre of the cluster-free hydrogenase Hmd is low-spin Fe(II).

Carbon monoxide binding by displacement of a pendant hemi-labile ligand at a di-iron site can be substantially ‘switched-on’ via a ligand protonation pathway which is competitive with metal-metal bond protonation.

Carbon monoxide binding by displacement of a pendant hemi-labile ligand at a di-iron site can be substantially ‘switched-on’ via a ligand protonation pathway which is competitive with metal-metal bond protonation.

The reaction of ligand L, N,N-bis(pyridin-2-ylmethyl)acetamide, with five transition metal salts, FeCl3·6H2O, CuCl2·2H2O, Cu(ClO4)2·6H2O, ZnCl2 and K2PtCl4/KI, produced five metal complexes, [(μ-O)(FeClL′)(FeCl3)] (1), [CuLCl2] (2), [CuBPA(ClO4)(CHCN)] ClO4 (3), [ZnLCl2] (4) and [PtLI2] (5), where L′ = 1-(2,4,5-tri(pyridin-2-yl)-3-(pyridin-2-ylmethyl)imidazolidin-1-yl)ethanone which formed in situ, and BPA = bis(pyridin-2-ylmethyl)amine. The ligand and complexes were characterized by a variety of spectroscopic techniques including X-ray single crystal diffraction where applicable. Depending on the metal ion and auxiliary ligand of the complex, the acetyl group of the ligand L could be either intact or cleaved. When ferric chloride hexahydrate was used, the deacetylation proceeded even further and a novel heterocyclic compound (L′) was formed in situ. A possible mechanism was proposed for the formation of the heterocyclic compound found in complex 1. Our results indicate that to cleave effectively an amide bond, it is essential for a metal centre to bind to the amide bond and the metal centre is of sufficient Lewis acidity.

Infrared data for mono-iron complexes possessing two cis-CO together with Mössbauer data for the enzyme and a model complex support the assignment that the iron centre of the cluster-free hydrogenase Hmd is low-spin Fe(II).

Two diferrous complexes, [Fe2(μ-SCH2CH3)3(CO)5I] and [Fe2(μ-SCH2CH2CH3)3(CO)5I] were synthesised via reaction of a monoiron carbonyl precursor, [Fe(CO)4I2], with ethanethiolate and propanethiolate, respectively. The complexes were fully characterised using spectroscopic techniques, for instance, FTIR and NMR. Their crystal structures were determined using single crystal diffraction analysis. Electrochemical reduction of these complexes are temperature-dependent. At room temperature, the diferrous complexes undergo one-electron reduction. The reduction-initiated cleavage of one bound thiolate and one iodide as a radical, takes the oxidation states of the diiron core from {Fe(I)Fe(II)} to {Fe(I)Fe(I)}. This reduction-initiated transformation can be suppressed by lowering the temperature to 195 K, further reduction of the monoanion was observed at a potential very close to that of the first reduction, which is analogous to the mechanism observed for diiron complexes with a core of {Fe(I)Fe(I)}.

Three diiron complexes (1-Ph, 2-OH, and 3-OCOFc) as mimics of the diiron subunit of [FeFe]-hydrogenase were electrochemically investigated in 0.1 mol L−1 [NBut4]BF4–acetonitrile (MeCN) under CO and Ar atmosphere. Complex 3-OCOFc was prepared from the reaction of complex 2-OH with ferrocenylacyl chloride (FcOCCl). The complex was fully characterised using a variety of spectroscopic techniques. Its structure was established using X-ray single crystal diffraction analysis. In addition to the well-established ECE (E for electrochemical and C for chemical) mechanism, it was revealed that a further reversible reduction at a potential more negative by ca. 600 mV was observed under CO atmosphere. It was further proposed based on the analysis of electrochemical and infrared spectroscopic data that the second redox was due to a two-electron process of supposedly a tetrairon cluster. This product was formed in situ from the reaction between the dianion generated from the ECE process and its parent complex (1-Ph, 2-OH, and 3-OCOFc) and is supposedly of a core “Fe4(II)”. This reaction occurred only when CO was presented. Under Ar atmosphere, bulk electrolysis led to fully-reduced products, that is, with the iron at the oxidation state of zero, but complex 2-OH was an exception. An overall mechanism to describe the electron transfer and coupled chemical reactions under CO atmosphere was proposed.

Reaction of Fe3(CO)12 with pyridinyl thioester ligand PyCH2SCOCH3 (L1, Py = pyridin-2-yl) produced complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)5] (1) (PyCH2S = pyridin-2-ylmethanethiolate). When complex 1 reacted with PPh3, a monosubstituted complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)4PPh3] (2), was derived. Reaction of the same precursor with analogous thioester ligand PyCH2SCOPy (L2) generated three novel diiron complexes, [Fe2(κ-Py)(μ-SCH2Py)(CO)5] (3), [Fe2(κ-Py)′(μ-SCH2Py)(CO)5] (4), and [Fe2(κ-Py)(μ-SCH2Py)(CO)6] (5). Complexes 3 and 4 are structural isomers. Complex 5 could be converted into complex 4 but the conversion from complex 5 to the isomer 3 was not observed. All the five complexes were fully characterised using FTIR, NMR, and other techniques. Their structures were determined using X-ray single crystal diffraction analysis. The oxidative formation of complexes 1, 3, 4, and 5 involved C–S and/or C–C bonds cleavages. To probe possible mechanisms for these cleavages, DFT calculations were performed. From the calculations, viable reaction pathways leading to the formation of all the isolated products were delineated. The results of the theoretic calculations also allowed rationalisation of the experimental observations.

By using “click” chemistry between a diazide and a diiron model complex armed with two alkynyl groups, two polymeric diiron complexes (Poly-Py and Poly-Ph) were prepared. The two polymeric complexes were investigated using infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), Mössbauer spectroscopy, and cyclic voltammetry (Poly-Py only, due to the insolubility of Poly-Ph). To probe the coordinating mode of the diiron units in the two polymeric complexes, two control complexes (3 and 4) were also synthesised using a monoazide. Complexes 3 and 4 were well characterised and the latter was further crystallographically analysed. It turns out that in both complexes (3 and 4) and the two polymeric diiron complexes, one of the two iron atoms in the diiron unit coordinates with one of the triazole N atoms. Our results revealed that both morphologies and properties of Poly-Py and Poly-Ph are significantly affected by the organic moiety of the diazide. Compared to the protonating behaviour of the complexes 3 and 4, Poly-Py exhibited proton resistance. In electrochemical reduction, potentials for the reduction of the diiron units in Poly-Py and hence its catalytic reduction of proton in acetic acid–DMF shifted by over 400 mV compared to those for complexes 3 and 4. It is likely that the polymeric nature of Poly-Py offers the diiron units a “protective” environment in an acidic medium and more positive reduction potential.