Complexes of the general formula W[SNS]2M(dppe) (M = Pd, Pt; [SNS]H3 = bis(2-mercapto-p-tolyl)amine; dppe = 1,2-bis(diphenylphosphino)ethane) were prepared by combining the corresponding (dppe)MCl2 synthon with W[SNS]2 under reducing conditions. X-ray diffraction studies revealed the formation of a heterobimetallic complex supported by a single thiolate bridging ligand and a short metal–metal bond between the tungsten and palladium or platinum. Electrochemical and computational results show that the frontier orbitals lie predominantly on the W[SNS]2 fragment suggesting that it behaves as a redox-active metalloligand in these complexes.

A new multimetallic construct has been developed utilizing a redox-active metalloligand. The molybdenum complex, Mo[SNS]2 {1; [SNS]H3 = bis(2-mercapto-p-tolyl)amine}, has been shown to coordinate to Ni(dppe) {dppe = 1,2-bis(diphenylphosphanyl)ethane} through two cis thiolate donors to generate heterobimetallic Mo[SNS]2Ni(dppe) (2) and heterotrimetallic Mo[SNS]2{Ni(dppe)}2 (3). X-ray diffraction studies confirm the presence of formal metal–metal bonds between the molybdenum and nickel centers in the solid state; however, NMR spectroscopic studies show that intracluster interactions are dynamic in solution. The Mo[SNS]2 metalloligand engenders rich redox chemistry in 2 and 3, and in the latter case, electrochemical and spectroscopic data suggest that 3+ is a localized mixed-valence complex, despite the metal–metal bonding network.

A new bimetallic platform comprising a six-coordinate Fe(ONO)2 unit bound to an (ONO)M (M = Fe, Zn) has been discovered ((ONOcat)H3 = bis(3,5-di-tert-butyl-2-phenol)amine). Reaction of Fe(ONO)2 with either (ONOcat)Fe(py)3 or with (ONOq)FeCl2 under reducing conditions led to the formation of the bimetallic complex Fe2(ONO)3, which includes unique five- and six-coordinate iron centers. Similarly, the reaction of Fe(ONO)2 with the new synthon (ONOsq˙)Zn(py)2 led to the formation of the heterobimetallic complex FeZn(ONO)3, with a six-coordinate iron center and a five-coordinate zinc center. Both bimetallic complexes were characterized by single-crystal X-ray diffraction studies, solid-state magnetic measurements, and multiple spectroscopic techniques. The magnetic data for FeZn(ONO)3 are consistent with a ground state S = 3/2 spin system, generated from a high-spin iron(II) center that is antiferromagnetically coupled to a single (ONOsq˙)2− radical ligand. In the case of Fe2(ONO)3, the magnetic data revealed a ground state S = 7/2 spin system arising from the interactions of one high-spin iron(II) center, one high-spin iron(III) center, and two (ONOsq˙)2− radical ligands.

A new series of square-planar nickel(II) donor–acceptor complexes exhibiting ligand-to-ligand charge-transfer (LL'CT) transitions have been prepared. Whereas the use of a catecholate donor ligand in conjunction with a bipyridyl acceptor ligand affords a complex that absorbs throughout the visible region, the use of a azanidophenolate donor ligands in conjunction with a bipyridyl acceptor ligand affords complexes that absorbs well into the near-IR region of the solar spectrum. Three new complexes, (cat)Ni(bpytBu2) (1; (cat)2− = 3,5-di-tert-butyl-1,2-catecholate; bpytBu2 = 4,4′-di-tert-butyl-2,2′-bipyridine), (ap)Ni(bpytBu2) (2; (ap)2− = 4,6-di-tert-butyl-2-(2,6-diisopropylphenylazanido)phenolate), and (apPh)Ni(bpytBu2) (3; (apPh)2− = 10-(2,6-diisopropylphenylazanido)-9-phenanthrolate), have been prepared and characterized by structural, electrochemical, and spectroscopic methods. Whereas all three square-planar complexes show multiple reversible one-electron redox-processes and strong LL'CT absorption bands, in azanidophenolate complexes 2 and 3, the LL'CT absorption covers the near-IR region from 700–1200 nm. The electronic absorption spectra and ground state electrochemical data for 2 and 3 provide an estimate of their excited-state reduction potentials, E+/*, of −1.3 V vs. SCE, making them as potent as the singlet excited state of [Ru(bpy)3]2+.

The tungsten complex W[SNS]2 ([SNS]H3 = bis(2-mercapto-4-methylphenyl)amine) was bound to a Ni(dppe) [dppe = 1,2-bis(diphenylphosphino)ethane] fragment to form the new heterobimetallic complex W[SNS]2Ni(dppe). Characterization of the complex by single-crystal X-ray diffraction revealed the presence of a short W–Ni bond, which renders the complex diamagnetic despite formal tungsten(V) and nickel(I) oxidation states. The W[SNS]2 unit acts as a redox-active metalloligand in the bimetallic complex, which displays four one-electron redox processes by cyclic voltammetry. In the presence of the organic acid 4-cyanoanilinium tetrafluoroborate, W[SNS]2Ni(dppe) catalyzes the electrochemical reduction of protons to hydrogen coincident with the first reduction of the complex.

New catecholate ligands containing protected phosphonate anchoring groups in the 4-position of the catecholate ring were synthesized. The catechol 4-diethoxyphosphorylbenzene-1,2-diol, (Etphoscat)H2, was prepared in three steps from pyrocatechol; whereas, the catechol 4-(diethoxyphosphorylmethyl)benzene-1,2-diol, (EtBnphoscat)H2, containing a methylene spacer between the catecholate ring and phosphonate anchor, was prepared from protocatechuic acid in six linear steps. Both catechol derivatives were further elaborated to their trimethylsilyl-protected counterparts to facilitate their binding to nanocrystalline metal oxides. Electronic spectroscopy and cyclic voltammetry were used to probe the electronic properties of the phosphonate-functionalized catecholates in charge-transfer complexes of the general formula (catecholate)Pd(pdi) (pdi = N,N′-bis(mesityl)phenanthrene-9,10-diimine). These studies show that attachment of the phosphonate anchor directly to the 4-position of the (Etphoscat)2– ligand significantly perturbs the donor ability of the catecholate ligand; however, incorporation of a single methylene spacer group in (EtBnphoscat)2– helps to isolate catecholate from the electron-withdrawing phosphonate group.

A family of charge-transfer chromophores comprising square-planar nickel(II) complexes with one catecholate donor ligand and one α-diimine acceptor ligand is reported. The nine new chromophores were prepared using three different catecholate ligands and three different α-diimine ligands. Single-crystal X-ray diffraction studies on all members of the series confirm a catecholate donor–nickel(II)−α-diimine acceptor electronic structure. The coplanar arrangement of donor and acceptor ligands manifests an intense ligand-to-ligand charge-transfer (LL′CT) absorption band that can be tuned incrementally from 650 nm (1.9 eV) to 1370 nm (0.9 eV). Electrochemical studies of all nine complexes reveal rich redox chemistry with two one-electron reduction processes and two one-electron oxidation processes. For one dye, both the singly reduced anion and the singly oxidized cation were prepared, isolated, and characterized by EPR spectroscopy to confirm ligand-localization of the redox processes. The optical and electrochemical properties of these new complexes identify them as attractive candidates for charge-transfer photochemistry and solar-energy conversion applications.

Isostructural vanadium, niobium and tantalum complexes of bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H3), were prepared and characterized to evaluate the impact of the metal ion on redox-activity of the ligand platform. New vanadium and niobium complexes with the general formula, [ONO]MCl2L (M = V, L = THF, 1-V; M = Nb, L = Et2O, 1-Nb) were prepared and structurally analysed by X-ray crystallography. The solid-state structures indicate that the niobium derivative is electronically analogous to the tantalum analog 1-Ta, containing a reduced (ONO) ligand and a niobium(V) metal ion, [ONOcat]NbVCl2(OEt2); whereas, the vanadium derivative is best described as a vanadium(IV) complex, [ONOsq]VIVCl2(THF). One-electron oxidation was carried out on all three metal complexes to afford [ONO]MCl3 derivatives (3-V, 3-Nb, 3-Ta). For all three derivatives, oxidation occurs at the (ONO) ligand. In the cases of niobium and tantalum, electronically similar complexes characterized as [ONOsq]MVCl3 were obtained and for vanadium, ligand-based oxidation led to the formation of a complex best described as [ONOq]VIVCl3. All complexes were characterized by spectroscopic and electrochemical methods. DFT and TD-DFT calculations were used to probe the electronic structure of the complexes and help verify the different electronic structures stemming from changes to the coordinated metal ion.

The redox-active pincer ligand derived from bis(3,5-di-tert-butyl-2-phenol)amine, [ONOcat]H3, enables reductive elimination of di-tert-butyldisulfide from a putative iron(III) dithiolate complex. The quinonate synthon of the ligand, [ONOq]K, was used to prepare [ONOq]FeX2 complexes (1, X = Cl; 2, X = N(SiMe3)2), which were characterized by single-crystal X-ray diffraction, EPR and Mössbauer spectroscopies and identified to be high-spin iron(III) complexes. The protonolysis of 2 with tetrachlorocatechol afforded either monomeric [ONOq]Fe(ortho-C6O2Cl4)(py) (3) or dimeric {[ONOq]Fe(ortho-C6O2Cl4)}2 (4). In contrast, the protonolysis of 2 with tert-butylthiol resulted in the extrusion of di-tert-butyldisulfide and the formation of a [ONOcat]Fe fragment trapped with pyridine as monomeric [ONOcat]Fe(py)3 (5) or dimeric {[ONOcat]Fe(py)}2 (6). These results indicate that the [ONO]Fe platform can promote reductive elimination of disulfide without incurring changes to the metal oxidation state.

A family of tantalum compounds was prepared to probe the electronic effects engendered by the addition of electron-donating or electron-withdrawing groups to the 4/4′ positions of the redox-active ligand derived from bis(2-isopropylamino-4-X-phenyl)amine [X,iPr(NNNcat)H3, X = F, H, Me, tBu]). A general synthetic procedure for the X,iPr(NNNcat)H3 ligand family was developed starting from the 4/4′ disubstituted diphenylamine derivative. A second ligand modification, incorporation of aromatic substituents at the flanking nitrogen moieties, was achieved via palladium-catalyzed cross-coupling to afford bis(2-3,5-dimethylphenylamino-4-methoxy-phenyl)amine OMe,DMP(NNNcat)H3 (DMP = 3,5-C6H3Me2), allowing a comparative study to the less sterically hindered isopropyl derivative. Treatment of the triamines with 1 equiv of TaMe3Cl2 generated the corresponding dichloro complexes X,R(NNNcat)TaCl2(L) (L = empty or Et2O) in high yields. These neutral dichloride derivatives reacted with [NBnEt3][Cl] to produce the anionic trichloride derivatives [NBnEt3][X,R(NNNcat)TaCl3], whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives X,R(NNNsq)TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the X,R(NNNcat)TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to give X,R(NNNq)TaCl2(═NR′) (R = iPr; X = OMe, F, H, Me; R′ = p-C6H4tBu, p-C6H4CF3; and R = 3,5-C6H3Me2; X = OMe; R′ = p-C6H4CH3). Electrochemistry, UV–vis–NIR, IR, and EPR spectroscopies along with X-ray diffraction methods were used to characterize and compare complexes with different redox-active ligand derivatives in each oxidation state. This study demonstrates that while the ligand redox potentials can be adjusted over a 270 mV range through substitutions at the 4/4′ ring positions, the coordination chemistry and reactivity patterns at the bound tantalum center remain unchanged, suggesting that such ligand modifications can be used to tune the redox potentials of a complex for a particular substrate of interest.

A new tridentate redox-active ligand platform, derived from bis(2-mercapto-p-tolyl)amine, [SNScat]H3, has been prepared in high yields by a four-step procedure starting from commericially available bis(p-tolyl)amine. The redox-active pincer-type ligand has been coordinated to tungsten to afford the six-coordinate, homoleptic complex W[SNS]2. To benchmark the redox behavior of the [SNS] ligand, the analogous tungsten complex of the well-known redox-active bis(3,5-di-tert-butylphenolato)amide ligand, W[ONO]2, also has been prepared. Both complexes show two reversible reductions and two partially reversible oxidations. Structural, spectroscopic, and electrochemical data all indicate that W[ONO]2 is best described as a tungsten(VI) metal center coordinated to two [ONOcat]3– ligands. In contrast, experimental data suggests a higher degree of S→W π donation, giving the W[SNS]2 complex non-innocent electronic character that can be described as a tungsten(IV) metal center coordinated to two [SNSsq]2– ligands.

Group- and atom-transfer is an attractive reaction class for the preparation of value-added organic substrates. Despite a wide variety of known early-transition metal oxo and imido complexes, these species have received limited attention for atom- and group-transfer reactions, owing to the lack of an accessible metal-based two-electron redox couple. Recently it has been shown that redox-active ligands can support the multi-electron changes required to promote group-transfer reactivity, opening up new avenues for group- and atom-transfer catalyst design. This Perspective article provides an overview of group transfer reactivity in early-transition metal complexes supported by traditional ligand platforms, followed by recent advances in the atom- and group-transfer reactivity of d0 metal complexes containing redox-active ligands.

The treatment of (dpp-nacnacR)Rh(phdi) {(dpp-nacnacR)− = CH[C(R)(N-iPr2C6H3)]2–; R = CH3, CF3; phdi = 9,10-phenanthrenediimine} with X2 oxidants afforded octahedral rhodium(III) products in the case of X = Cl and Br. The octahedral complexes exhibit well-behaved cyclic voltammograms in which a two-electron reduction is observed to regenerate the initial rhodium(I) complex. When treated with I2, (dpp-nacnacCH3)Rh(phdi) produced a square pyramidal η1-I2 complex, which was characterized by NMR and UV–vis spectroscopies, mass spectrometry, and X-ray crystallography. The more electron poor complex (dpp-nacnacCF3)Rh(phdi) reacted with I2 to give a mixture of two products that were identified by 1H NMR spectroscopy as a square pyramidal η1-I2 complex and an octahedral diiodide complex. Reaction of the square pyramidal (dpp-nacnacCH3)Rh(I2)(phdi) with HBF4 resulted in protonation of the (dpp-nacnacCH3)− backbone to provide an octahedral rhodium(III) diiodide species. These reactions highlight the impact that changes in the electron-withdrawing nature of the supporting ligands can have on the reactivity at the metal center.

New rhodium complexes of bis(3,5-di-tert-butyl-2-phenol)amine ([ONOcat]H3) were synthesized, and their electronic properties were investigated. These compounds were prepared by combining [ONOq]K and [(cod)Rh(μ-Cl)]2 in the presence of an auxiliary donor ligand to yield complexes of the type [ONO]RhLn (n = 3, L = py (1); n = 2, L = PMe3 (2a), L = PMe2Ph (2b), PMePh2 (2c), PPh3 (2d)). Single-crystal X-ray diffraction studies on [ONO]Rh(py)3 (1) revealed a six-coordinate, octahedral rhodium complex. In the case of [ONO]Rh(PMe3)2 (2a), X-ray diffraction showed a five-coordinate, distorted square-pyramidal coordination environment around the rhodium center. While 1 is static on the NMR time scale, complexes 2a–d are fluxional, displaying both rapid isomerization of the square-pyramidal structure and exchange of coordinated and free phosphine ligands. UV–vis spectroscopy shows stark electronic differences between 1 and 2a–d. Whereas 1 displays a strong absorbance at 380 nm with a much weaker band at 585 nm in the absorption spectrum, complexes 2a–d display an intense (ε > 104 M–1 cm–1), low-energy absorption band in the region 580–640 nm; however, in the cases of 2a and 2b, the addition of excess phosphine resulted in changes to the UV–vis spectrum indicating the formation of six-coordinate adducts [ONO]Rh(PMe3)3 (3a) and [ONO]Rh(PMe2Ph)3 (3b), respectively. The experimental and DFT computational data for the six-coordinate complexes 1, 3a, and 3b are consistent with their formulation as classical, d6, pseudo-octahedral, coordination complexes. In the five-coordinate complexes 2a–2d, π-bonding between the rhodium center and the [ONO] ligand leads to a high degree of covalency and metal–ligand electron distributions that are not accurately described by formal oxidation state assignments.

A series of aluminum complexes containing the tridentate, redox-active ligand bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H3) in three different oxidation states were synthesized. The aluminum halide salts AlCl3 and AlBr3 were reacted with the doubly deprotonated form of the ligand to afford five-coordinate [ONHOcat]AlX(solv) complexes (1a, X = Cl, solv = OEt2; 1b, X = Br, solv = THF), each having a trigonal bipyramidal coordination geometry at the aluminum and containing the [ONHOcat]2− ligand with a protonated, sp3-hybridized nitrogen donor. The [ONO] ligand platform may also be added to aluminum through the use of the oxidized ligand salt [ONOq]K, which was reacted with AlCl3 in the presence of either diphenylacetylacetonate (acacPh2−) or 8-oxyquinoline (quinO−) to afford [ONOq]Al(acacPh2)Cl (2) or [ONOq]Al(quinO)Cl (3), respectively, with well-defined [ONOq]− ligands. Quinonate complexes 2 and 3 were reduced by one electron to afford the corresponding complexes K{[ONOsq]Al(acacPh2)(py)} (4) and K{[ONOsq]Al(quinO)(py)} (5), respectively, containing well-defined [ONOsq]2− ligands. The addition of tetrachloro-1,2-quinone to 1a in the presence of pyridine resulted in the expulsion of HCl and the formation of an aluminum complex with two different redox active ligands, [ONO]Al(o-O2C6Cl4)(py) (6). Similar results were obtained when 1a was reacted with 9,10-phenanthrenequinone to afford [ONO]Al(o-O2C14H8)(py) (7) or with pyrene-4,5-dione to afford [ONO]Al(o-O2C16H8)(py) (8). Structural, spectroscopic and preliminary magnetic measurements on 6–8 suggest ligand non-innocent redox behavior in these complexes.

Nitrene transfer catalyzed by a d0zirconium(IV) complex with a redox-active ligand is reported. The redox-active ligand, bis(2-isopropylamido-4-methoxyphenyl)amide ([NNNcat]3−), afforded zirconium(IV) complexes, [NNNcat]ZrClL2 (1a, L = THF; 1b, L = CNtBu; 1c, L = py), upon reaction with ZrCl4(THF)2. Complex 1a was oxidized by one and two electrons using PhICl2, affording [NNNsq•]ZrCl2(THF) (2) and [NNNq]ZrCl3 (3), respectively. Aryl azides reacted with 1a to afford zirconium imide dimers, including the crystallographically characterized species {[NNNq]ZrCl(μ2-p-NC6H4tBu)}2 (4). The formation of 4 is the result of the addition of an aryl nitrene to the zirconium(IV) metal center. When 1b was reacted with organoazides, the dimer was not observed, but rather the nitrene group was transferred to the isonitrile to form a carbodiimide. In the presence of excess organoazide and isonitrile, catalytic carbodiimide formation occurred, showing that a redox-active ligand and a d0 metal center can work in concert to effect nitrene group transfer reactivity.

In this Forum Article, we discuss the use of redox-active pincer-type ligands to enable multielectron reactivity, specifically nitrene group transfer, at the electron-poor metals tantalum and zirconium. Two analogous ligand platforms, [ONO] and [NNN], are discussed with a detailed examination of their similarities and differences and the structural and electronic constraints they impose upon coordination to early transition metals. The two-electron redox capabilities of these ligands enable the transfer of organic nitrenes to tantalum(V) and zirconium(IV) metal centers despite formal d0 electron counts. Under the correct conditions, the resulting metal imido complexes can participate in further multielectron reactions such as imide reduction, nitrene coupling, or formal nitrene transfer to an isocyanide.

New square-planar rhodium complexes of the redox-active ligand 9,10-phenenthrenediimine (phdi) have been prepared and studied. The complexes [dpp-nacnacCH3]Rh(phdi) (2a; [dpp-nacnacCH3]− = CH[C(Me)(N-iPr2C6H3)]2−) and [dpp-nacnacCF3]Rh(phdi) (2b; [dpp-nacnacCF3]− = CH[C(CF3)(N-iPr2C6H3)]2−) have been prepared from the corresponding [nacnac]Rh(CO)2 synthons by treatment with Me3NO in the presence of the phdi ligand. Complexes 2a and 2b are diamagnetic, and their absorption spectra are dominated by intense charge-transfer transitions throughout the visible region. Electrochemical studies indicate that both the phdi ligand and the rhodium metal center are redox-active, with the [nacnac]− ligands serving to modulate the one-electron-oxidation and -reduction redox potentials. In the case of 2a, chemical oxidation and reduction reactions provided access to the one-electron-oxidized cation, [2a]+, and one-electron-reduced anion, [2a]−, the latter of which has been characterized in the solid state by single-crystal X-ray diffraction. Solution electron paramagnetic resonance spectra of [2a]+ and [2a]− are consistent with S = 1/2 spin systems, but surprisingly the low-temperature spectrum of [2a]− shows a high degree of rhombicity, suggestive of rhodium(II) character in the reduced anion.

A redox-active, tetradentate ligand, N,N′-bis-(3-dimethylamino-propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N2N2cat]2−), was developed, and the six-coordinate metal complexes [N2N2cat]TiCl2 (3) and [N2N2cat]ZrCl2 (4) were synthesized. The tetradentate ligand was determined to be fluxional in 3 and 4, enabled by reversible dissociation of the neutral amine groups of the [N2N2cat]2− ligand. Both amine arms of 3 could be replaced by N,N-dimethylaminopyridine with an overall free energy change of −4.64(3) kcal mol−1 at 298 K. Cyclic voltammetry experiments were used to probe the redox capabilities of the [N2N2cat]2− ligand: complex 3 exhibited two one-electron oxidations at −0.19 and −0.52 V versus [Cp2Fe]+/0 while 4 exhibited a single two-electron oxidation at −0.55 V. Substitution of the chlorides in 3 for an imide afforded the dimer {[N2N2cat]Ti(μ-p-NC6H4Me)}2, in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N2N2cat]2− ligand. While the bis-azide complex [N2N2cat]Ti(N3)2 was stable toward elimination of N2, the bis-phenylacetylide complex [N2N2cat]Ti(C≡CPh)2 could be oxidized by PhICl2, resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.

The reactivity of tantalum dimethyl and diphenyl complexes (ONOcat)TaMe2 (2) and (ONOcat)TaPh2 (3) with aryl azides and 1,2-diphenylhydrazine is reported. p-Tolyl azide inserts into a Ta–C bond of 2 or 3 to afford (ONOcat)TaMe(η2-MeNNN-p-C6H4Me) (6) or (ONOcat)TaPh(η2-PhNNN-p-C6H4Me) (7), respectively. In the presence of another 1 equiv of azide, another insertion occurs, affording the bis(triazenido) complexes (ONOcat)Ta(η2-MeNNN-p-C6H4Me)2 (4) and (ONOcat)Ta(η2-PhNNN-p-C6H4Me)2 (5). Complexes 2 and 3 also react with 1,2-diphenylhydrazine, resulting in the loss of 2 equiv of methane or benzene, respectively, along with 1/2 equiv of azobenzene. The tantalum-containing product was isolated as the pyridine adduct {(ONOcat)Ta(μ2-NPh)(py)}2 (8), which implicates a tantalum(III) intermediate. The viability of this (ONOcat)TaIII intermediate was proven by KC8 reduction of (ONOcat)TaCl2 (1) in THF, resulting in activation of a C–O bond of THF and formation of {(ONOcat)Ta(μ-O(CH2)3CH2)}2 (10), which was characterized by single-crystal X-ray diffraction studies.

Titanium complexes of N,N′-bis(arylimino)acenaphthylene (BIAN) α-diimine ligands with varied steric profiles have been prepared. Coordination of the BIAN ligand derivatives to TiCl4 afforded the adducts (dpp-BIAN)TiCl4 (1a), (tmp-BIAN)TiCl4 (1b), and (dmp-BIAN)TiCl4 (1c) (dpp = 2,6-diisopropylphenyl; tmp = 2,4,6-trimethylphenyl; dmp = 3,5-dimethylphenyl). While the least sterically crowded complex 1c is robust toward loss of the diimine ligand, the dpp-BIAN and tmp-BIAN ligands are readily displaced by pyridine from the more crowded derivatives 1a and 1b, respectively. The crowded profiles engendered by the tmp-BIAN and dpp-BIAN ligands result in the formation of five-coordinate titanium-imide complexes, (dpp-BIAN)TiCl2(═NtBu) (2a) and (tmp-BIAN)TiCl2(═NtBu) (2b), upon addition of tBuNH2 to solutions of 1a or 1b, respectively. Single-crystal X-ray diffraction studies reveal a square pyramidal coordination environment with an apical imide ligand and a short Ti−N distance, consistent with a Ti−N triple bond. Conversely, the less crowded dmp-BIAN ligand affords a six-coordinate titanium imido complex, (dmp-BIAN)TiCl2(═NtBu)(NH2tBu) (4), upon treatment of 1c with tBuNH2. Surprisingly the imido ligand is coordinated trans to one arm of the diimine. This six coordinate species is fluxional in solution, and exchange and variable temperature 1H NMR experiments suggest dissociation of the coordinated tBuNH2 ligand to generate a five-coordinate imido intermediate analogous to 2a and 2b.

While a dimethyltantalum(V) complex of the [ONOcat]3− ligand ([ONOcat]H3 = N,N-bis(3,5-di-tert-butyl-2-phenol)amine) reacted with diazoalkane to afford products of insertion into the Ta−Me bond, the dichlorotantalum(V) complex [ONOcat]TaCl2(OEt2) reacted with 2 equiv of Ph2CN2 to form the ketazine adduct [ONOcat]TaCl2(η2(N,N′)-Ph2C═NN═CPh2). When Ph2CN2 was added to the dichloride in neat styrene, catalytic carbene transfer was observed to form the corresponding cyclopropane

An isostructural series of titanium, zirconium, and hafnium complexes, M[ap]2L2 (M = Ti, Zr, Hf; L = THF, pyridine), of the redox-active 4,6-di-tert-butyl-2-tert-butylamidophenolate ligand ([ap]2−) have been prepared. The zirconium and hafnium derivatives react readily with halogen oxidants such as XeF2, PhICl2, and Br2, leading to products in which one-electron oxidation of each [ap]2− ligand accompanies halide addition to the metal center. Iodine proved to be too weak of an oxidant to yield the corresponding oxidative addition product, and under no conditions could halogen oxidative addition products be obtained for titanium. According to X-ray crystallographic studies, the zirconium and hafnium oxidation products are best formulated as MX2[isq·]2 ([isq·]− = 4,6-di-tert-butyl-2-tert-butylimino-semiquinonate; M = Zr, Hf; X = F, Cl, Br) species, in which the molecule is symmetric with each redox-active ligand in the semiquinone oxidation state. Temperature-dependent magnetization measurements suggest a singlet (S = 0) ground-state for the diradical complexes with a thermally accessible triplet (S = 1) excited state. Solution electron paramagnetic resonance (EPR) spectra are consistent with this assignment, showing both Δms = 1 and Δms = 2 transitions for the antiferromagnetically coupled electrons.

The tridentate [NNO]− Schiff-base ligand supports the carbonate-templated synthesis of a cluster with ten manganese(II) ions arranged in two parallel [MnO]5 rings.

Complexes of the general formula W[SNS]2M(dppe) (M = Pd, Pt; [SNS]H3 = bis(2-mercapto-p-tolyl)amine; dppe = 1,2-bis(diphenylphosphino)ethane) were prepared by combining the corresponding (dppe)MCl2 synthon with W[SNS]2 under reducing conditions. X-ray diffraction studies revealed the formation of a heterobimetallic complex supported by a single thiolate bridging ligand and a short metal–metal bond between the tungsten and palladium or platinum. Electrochemical and computational results show that the frontier orbitals lie predominantly on the W[SNS]2 fragment suggesting that it behaves as a redox-active metalloligand in these complexes.

A new bimetallic platform comprising a six-coordinate Fe(ONO)2 unit bound to an (ONO)M (M = Fe, Zn) has been discovered ((ONOcat)H3 = bis(3,5-di-tert-butyl-2-phenol)amine). Reaction of Fe(ONO)2 with either (ONOcat)Fe(py)3 or with (ONOq)FeCl2 under reducing conditions led to the formation of the bimetallic complex Fe2(ONO)3, which includes unique five- and six-coordinate iron centers. Similarly, the reaction of Fe(ONO)2 with the new synthon (ONOsq˙)Zn(py)2 led to the formation of the heterobimetallic complex FeZn(ONO)3, with a six-coordinate iron center and a five-coordinate zinc center. Both bimetallic complexes were characterized by single-crystal X-ray diffraction studies, solid-state magnetic measurements, and multiple spectroscopic techniques. The magnetic data for FeZn(ONO)3 are consistent with a ground state S = 3/2 spin system, generated from a high-spin iron(II) center that is antiferromagnetically coupled to a single (ONOsq˙)2− radical ligand. In the case of Fe2(ONO)3, the magnetic data revealed a ground state S = 7/2 spin system arising from the interactions of one high-spin iron(II) center, one high-spin iron(III) center, and two (ONOsq˙)2− radical ligands.

A new series of square-planar nickel(II) donor–acceptor complexes exhibiting ligand-to-ligand charge-transfer (LL'CT) transitions have been prepared. Whereas the use of a catecholate donor ligand in conjunction with a bipyridyl acceptor ligand affords a complex that absorbs throughout the visible region, the use of a azanidophenolate donor ligands in conjunction with a bipyridyl acceptor ligand affords complexes that absorbs well into the near-IR region of the solar spectrum. Three new complexes, (cat)Ni(bpytBu2) (1; (cat)2− = 3,5-di-tert-butyl-1,2-catecholate; bpytBu2 = 4,4′-di-tert-butyl-2,2′-bipyridine), (ap)Ni(bpytBu2) (2; (ap)2− = 4,6-di-tert-butyl-2-(2,6-diisopropylphenylazanido)phenolate), and (apPh)Ni(bpytBu2) (3; (apPh)2− = 10-(2,6-diisopropylphenylazanido)-9-phenanthrolate), have been prepared and characterized by structural, electrochemical, and spectroscopic methods. Whereas all three square-planar complexes show multiple reversible one-electron redox-processes and strong LL'CT absorption bands, in azanidophenolate complexes 2 and 3, the LL'CT absorption covers the near-IR region from 700–1200 nm. The electronic absorption spectra and ground state electrochemical data for 2 and 3 provide an estimate of their excited-state reduction potentials, E+/*, of −1.3 V vs. SCE, making them as potent as the singlet excited state of [Ru(bpy)3]2+.

The redox-active pincer ligand derived from bis(3,5-di-tert-butyl-2-phenol)amine, [ONOcat]H3, enables reductive elimination of di-tert-butyldisulfide from a putative iron(III) dithiolate complex. The quinonate synthon of the ligand, [ONOq]K, was used to prepare [ONOq]FeX2 complexes (1, X = Cl; 2, X = N(SiMe3)2), which were characterized by single-crystal X-ray diffraction, EPR and Mössbauer spectroscopies and identified to be high-spin iron(III) complexes. The protonolysis of 2 with tetrachlorocatechol afforded either monomeric [ONOq]Fe(ortho-C6O2Cl4)(py) (3) or dimeric {[ONOq]Fe(ortho-C6O2Cl4)}2 (4). In contrast, the protonolysis of 2 with tert-butylthiol resulted in the extrusion of di-tert-butyldisulfide and the formation of a [ONOcat]Fe fragment trapped with pyridine as monomeric [ONOcat]Fe(py)3 (5) or dimeric {[ONOcat]Fe(py)}2 (6). These results indicate that the [ONO]Fe platform can promote reductive elimination of disulfide without incurring changes to the metal oxidation state.

Nitrene transfer catalyzed by a d0zirconium(IV) complex with a redox-active ligand is reported. The redox-active ligand, bis(2-isopropylamido-4-methoxyphenyl)amide ([NNNcat]3−), afforded zirconium(IV) complexes, [NNNcat]ZrClL2 (1a, L = THF; 1b, L = CNtBu; 1c, L = py), upon reaction with ZrCl4(THF)2. Complex 1a was oxidized by one and two electrons using PhICl2, affording [NNNsq•]ZrCl2(THF) (2) and [NNNq]ZrCl3 (3), respectively. Aryl azides reacted with 1a to afford zirconium imide dimers, including the crystallographically characterized species {[NNNq]ZrCl(μ2-p-NC6H4tBu)}2 (4). The formation of 4 is the result of the addition of an aryl nitrene to the zirconium(IV) metal center. When 1b was reacted with organoazides, the dimer was not observed, but rather the nitrene group was transferred to the isonitrile to form a carbodiimide. In the presence of excess organoazide and isonitrile, catalytic carbodiimide formation occurred, showing that a redox-active ligand and a d0 metal center can work in concert to effect nitrene group transfer reactivity.

Isostructural vanadium, niobium and tantalum complexes of bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H3), were prepared and characterized to evaluate the impact of the metal ion on redox-activity of the ligand platform. New vanadium and niobium complexes with the general formula, [ONO]MCl2L (M = V, L = THF, 1-V; M = Nb, L = Et2O, 1-Nb) were prepared and structurally analysed by X-ray crystallography. The solid-state structures indicate that the niobium derivative is electronically analogous to the tantalum analog 1-Ta, containing a reduced (ONO) ligand and a niobium(V) metal ion, [ONOcat]NbVCl2(OEt2); whereas, the vanadium derivative is best described as a vanadium(IV) complex, [ONOsq]VIVCl2(THF). One-electron oxidation was carried out on all three metal complexes to afford [ONO]MCl3 derivatives (3-V, 3-Nb, 3-Ta). For all three derivatives, oxidation occurs at the (ONO) ligand. In the cases of niobium and tantalum, electronically similar complexes characterized as [ONOsq]MVCl3 were obtained and for vanadium, ligand-based oxidation led to the formation of a complex best described as [ONOq]VIVCl3. All complexes were characterized by spectroscopic and electrochemical methods. DFT and TD-DFT calculations were used to probe the electronic structure of the complexes and help verify the different electronic structures stemming from changes to the coordinated metal ion.

Group- and atom-transfer is an attractive reaction class for the preparation of value-added organic substrates. Despite a wide variety of known early-transition metal oxo and imido complexes, these species have received limited attention for atom- and group-transfer reactions, owing to the lack of an accessible metal-based two-electron redox couple. Recently it has been shown that redox-active ligands can support the multi-electron changes required to promote group-transfer reactivity, opening up new avenues for group- and atom-transfer catalyst design. This Perspective article provides an overview of group transfer reactivity in early-transition metal complexes supported by traditional ligand platforms, followed by recent advances in the atom- and group-transfer reactivity of d0 metal complexes containing redox-active ligands.

The tridentate [NNO]− Schiff-base ligand supports the carbonate-templated synthesis of a cluster with ten manganese(II) ions arranged in two parallel [MnO]5 rings.

A series of aluminum complexes containing the tridentate, redox-active ligand bis(3,5-di-tert-butyl-2-phenol)amine ([ONO]H3) in three different oxidation states were synthesized. The aluminum halide salts AlCl3 and AlBr3 were reacted with the doubly deprotonated form of the ligand to afford five-coordinate [ONHOcat]AlX(solv) complexes (1a, X = Cl, solv = OEt2; 1b, X = Br, solv = THF), each having a trigonal bipyramidal coordination geometry at the aluminum and containing the [ONHOcat]2− ligand with a protonated, sp3-hybridized nitrogen donor. The [ONO] ligand platform may also be added to aluminum through the use of the oxidized ligand salt [ONOq]K, which was reacted with AlCl3 in the presence of either diphenylacetylacetonate (acacPh2−) or 8-oxyquinoline (quinO−) to afford [ONOq]Al(acacPh2)Cl (2) or [ONOq]Al(quinO)Cl (3), respectively, with well-defined [ONOq]− ligands. Quinonate complexes 2 and 3 were reduced by one electron to afford the corresponding complexes K{[ONOsq]Al(acacPh2)(py)} (4) and K{[ONOsq]Al(quinO)(py)} (5), respectively, containing well-defined [ONOsq]2− ligands. The addition of tetrachloro-1,2-quinone to 1a in the presence of pyridine resulted in the expulsion of HCl and the formation of an aluminum complex with two different redox active ligands, [ONO]Al(o-O2C6Cl4)(py) (6). Similar results were obtained when 1a was reacted with 9,10-phenanthrenequinone to afford [ONO]Al(o-O2C14H8)(py) (7) or with pyrene-4,5-dione to afford [ONO]Al(o-O2C16H8)(py) (8). Structural, spectroscopic and preliminary magnetic measurements on 6–8 suggest ligand non-innocent redox behavior in these complexes.