Divanadium Complex Research Articles

A Dinitrogen Divanadium Complex Supported by a Trisamidophosphine Ligand

Comprehensive SummaryThe reaction between tripodal trisamidophosphine ligand H3PN3Ar and V(Mes)3(THF) (Mes = mesityl) yields the vanadium(III) complex (PN3Ar)V (1) with an open site in the axial position, which could coordinate with THF, pyridine, and NH3 to form the corresponding adducts (2—4). The vanadium(III) center is redox‐active, as demonstrated by cyclic voltammetry methods and chemical oxidation or reduction. Notably, a novel dinitrogen divanadium complex with a bridging N2 ligand, {K(THF)}2{[PN3Ar]V}2(μ‐N2) (6), was synthesized via treatment of 1 with 1 equivalent of potassium naphthalenide in THF under a N2 atmosphere. The electronic structures and binding properties of 6 are evaluated and discussed based on its DFT calculations. Additionally, these vanadium complexes (1, 5, 6) can serve as catalysts for the conversion of N2 into N(SiMe3)3 in the presence of reductants and Me3SiCl.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate.

The catalytic transformation of N2 to NH3 by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N2 fixation, well-defined vanadium complexes that can catalyze the conversion of N2 to NH3 are scarce. In particular, a V(NxHy) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(tBu2PO)2-C6H3) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N2 to NH3 and N2H4. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH2), which mediated 15N2 conversion to 15NH3, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH2 possessed a N-H bond dissociation free energy (BDFEN-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH2) with [Ph2NH2][OTf] resulted in the release of NH3 and the formation of a vanadium triflate complex, which upon reduction under N2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH3. These findings provide unprecedented insights into the mechanism of N2 reduction related to FeV nitrogenase.

The Steric Effect in Preparations of Vanadium(II)/(III) Dinitrogen Complexes of Triamidoamine Ligands Bearing Bulky Substituents.

The reactions of newly designed lithiated triamidoamines Li3LR (R = iPr, Pen, and Cy2) with VCl3(THF)3 under N2 yielded dinitrogen–divanadium complexes with a μ-N2 between vanadium atoms [{V(LR)}2(μ-N2)] (R = iPr (1) and Pen (2)) for the former two, while not dinitrogen–divanadium complexes but a mononuclear vanadium complex with a vacant site, [V(LCy2)] (R = Cy2 (3)), were obtained for the third ligand. The V–NN2 and N–N distances were 1.7655(18) and 1.219(4) Å for 1 and 1.7935(14) and 1.226(3) Å for 2, respectively. The ν(14N–14N) stretching vibrations of 1 and 2, as measured using resonance Raman spectroscopy, were detected at 1436 and 1412 cm–1, respectively. Complex 3 reacted with potassium metal in the presence of 18-crown-6-ether under N2 to give a hetero-dinuclear vanadium complex with μ-N2 between vanadium and potassium, [VK(LCy2)(μ-N2)(18-crown-6)] (4). The N–N distance and ν(14N–14N) stretching for 4 were 1.152(3) Å and 1818 cm−1, respectively, suggesting that 4 is more activated than complexes 1 and 2. The complexes 1, 2, 3, and 4 reacted with HOTf and K[C10H8] to give NH3 and N2H4. The yields of NH3 and N2H4 (per V atom) were 47 and 11% for 1, 38 and 16% for 2, 77 and 7% for 3, and 80 and 5% for 4, respectively, and 3 and 4, which have a ligand LCy2, showed higher reactivity than 1 and 2.

Decarboxylative alkylarylation of alkenes with PhI(O2CR)2 to access benzimidazo[2,1-a]isoquinolin-6(5H)-ones catalyzed by a low-valent divanadium complex

Herein, we report a divanadium-catalyzed decarboxylative alkylarylation of alkenes with hypervalent iodine(III) reagents (HIRs) for the rapid construction of tetracyclic benzimidazo[2,1-a]isoquinolin-6(5H)-ones. This transformation proceeds regioselectively with viarously substituted N-methacryloyl-2-arylbenzoimidazoles and diverse aliphatic acid-derived HIRs under mild conditions. Furthermore, mechanistic investigations revealed that the in situ generated alkyl radical triggered this regioselective 6-exo cyclization.

Lantern-Type Divanadium Complexes with Bridging Ligands: Short Metal-Metal Bonds with High Multiple Bond Orders.

Vanadium forms binuclear complexes with a variety of ligands often containing V≡V triple bonds. Many tetragonal divanadium paddlewheel complexes with bridging bidentate ligands have been experimentally characterized. This research exhaustively treats model tetragonal, trigonal, and digonal paddlewheel-type divanadium complexes V2 Lx (L=formamidinate, guanidinate, and carboxylate; x=2, 3, 4), each in the three lowest-energy spin states. The V-V formal bond orders are obtained from metal-metal MO diagrams for representative structures. A number of short V-V multiple bonds of order 3, 3.5, and 4 are found in these model complexes. The short V≡V triple bonds and singlet ground state predicted here for the model tetragonal complexes correspond well with the limited experimental results for the series of known tetragonal paddlewheels. Digonal divanadium lanterns with very short V-V quadruple bonds are predicted as interesting synthetic targets. The V-V bond distances are categorized into distinct ranges according to the formal bond order values from 0.5 to 4. These bond length ranges are compared with the ranges compiled for other divanadium complexes including carbonyl complexes.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation.

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Electronic structure study of divanadium complexes with rigid covalent coordination: potential molecular qubits with slow spin relaxation.

The electronic structures of homovalent [V2(μ-S2)2(R2dtc)4] (R = Et, iBu) and mixed-valent [V2(μ-S2)2(R2dtc)4]+ are reported here. The soft-donor, eight-coordinate ligand shell combined with the fully delocalised ground state provides a highly rigid and covalent environment that will nurture long spin relaxation times in vanadyl-based molecular qubits.

Syntheses, Characterizations, and Crystal Structures of Dinitrogen‐Divanadium Complexes Bearing Triamidoamine Ligands

We recently reported the syntheses, crystal structures, and protonation reactions of dinitrogen divanadium complexes bearing triamidoamine ligands, ([{V(LR)}2(µ‐N2)] (R = iBu (1), EtBu (2), and iPr2Bn (3)). In this study, we have additionally synthesized two novel dinitrogen‐divanadium complexes with similar derivatives (R = Bn (4) and MeBn (5)), which are characterized spectroscopically and structurally. The X‐ray structures have both revealed a divanadium complex with a bridging N2 ligand. The protonation reactions of the µ‐N2 ligands for 4 and 5 in the presence of M+[C10H8]– (M+ = Na+, K+) and proton source (HOTf or [LutH](OTf)) have given ammonia. Particularly, the NH3 yield from the reaction of 5 with K+[C10H8]– and [LutH](OTf) has been 341 %. Furthermore, mixed valence Na+ adduct of 1, [Na{V(LiBu)}2(µ‐N2)] (6), which is prepared by the reaction of 1 with sodium metal, has been prepared and its crystal structure has also been determined. Compound 6 is electrochemically and spectroscopically characterized, which is also evaluated by DFT calculation method.

Spin–orbit coupling and d2–d2 interactions in dinuclear bioctahedral metal clusters

The electronic structure and the metal–metal d2–d2 interactions in dinuclear bioctahedral face-shared cluster complexes [M2X9]3− (M=V, Nb, Ta, and X=Cl, Br, I) have been studied with DFT and topological methods QTAIM and ELF. The ground states of the complexes are shown to be spin-paired and nonmagnetic only in the approximation of spin–orbit interaction. The result correlates well with experimental data on magnetic susceptibility for solid-state complexes containing [Nb2X9]3− and [Ta2X9]3− anions as well as for [V2X9]3− anions in low-temperature spin-paired modifications of Cs3V2Cl9 and Rb3V2Br9 complexes. The transition to the high-temperature spin-paramagnetic state of [V2X9]3− anions is shown to be related to the bifurcation of d2–d2 interaction at some critical distance DCR(M–M) and to weakening of spin–orbit coupling in divanadium complexes.

Selective binding by a divanadium complex of arsenate over phosphate: A computational study

Density functional theory (M06-2X) calculations on divanadium complexes [(VO) 2(bpbp)(A)] 2+ of the oxo anions formate, phosphate, and arsenate (A = HCOO −, H 2 PO 4 - , H 2 AsO 4 - ) as well as the diaquo complex [(VO) 2(bpbp)(OH 2) 2] 3+ indicate increased binding in the series H 2O < HCOO − < H 2 PO 4 - < H 2 AsO 4 - . Aqueous solvation enhances the experimentally observed selective recognition of arsenate over phosphate indicating differences in hydration enthalpies of the respective complexes as the main reason for the surprising differentiation between dihydrogenphosphato and dihydrogenarsenato by the divanadyl complex [(VO) 2(bpbp)(OH 2) 2](ClO 4) 3·H 2O. Deprotonation of the dihydrogenphosphato/arsenato complexes to [(VO) 2(bpbp)(HXO 4)] + further increases preferential solvation of the arsenato over the phosphato complex.

High-spin ground states via electron delocalization in mixed-valence imidazolate-bridged divanadium complexes

The field of molecular magnetism has grown tremendously since the discovery of single-molecule magnets, but it remains centred around the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. We now demonstrate that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence molecules of the type [(PY5Me(2))V(II)(micro-L(br)) V(III)(PY5Me(2))](4+) were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange for [(PY5Me(2))(2)V(2)(micro-5,6-dimethylbenzimidazolate)](4+). Combined modelling of the magnetic data and spectral analysis leads to an estimate of the double-exchange parameter of B = 220 cm(-1) when vibronic coupling is taken into account.

Selective recognition and binding of arsenate over phosphate

A divanadium complex shows remarkable selectivity for arsenate in substitution reactions with oxoanions.

The Si-doped planar tetracoordinate carbon (ptC) unit CAl3Si− could be used as a building block or inorganic ligand during cluster-assembly

Currently, the molecular assembly and growth from a small building block to the bulk compounds have become a focus in various fields. Ever being chemical curiosities, the “anti-van’t Hoff/Le Bel” realm that is associated with tetracoordinate or hypercoordiate planar centers has made vast progress. Being important in the fundamental research areas, the ptC species have potential applications in materials science. The existence of ptC in a divanadium complex and a large number of organometallic compounds have since been reported to possess ptC and these provide us with great hope that many more compounds with ptC building blocks may be synthesized in future. Herein, we report the assembly and stabilization of CAl3Si− in both the “homo-decked sandwich” and “hetero-decked sandwich” schemes at the B3LYP/6-311+G(d) level. We show that while the Si-doped indeed introduces much complexity during assembly, the electronic and structural integrity feature of CAl3Si− is well conserved during cluster-assembly, characteristic of a “superatom”. This study should be helpful in understanding the hetero-doped assembly mechanism of the ptC chemistry. Moreover, the present results are expected to enrich the flat carbon chemistry, superatom chemistry, metallocenes and combinational chemistry.

Tetracyanide-Bridged Divanadium Complexes: Redox Switching between Strong Antiferromagnetic and Strong Ferromagnetic Coupling

Reaction of [(Me3tacn)V(CF3SO3)3] (Me3tacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) with LiCN in DMF results in oligomerization of cyanide to form [(Me3tacn)2V2(CN)4(mu-C4N4)]. The structure of this binuclear complex features a planar tetracyanide unit bridging two VIV centers via imido type linkages. The conjugated pathway provided by the bridging ligand leads to strong antiferromagnetic coupling (J = -112 cm-1) and an S = 0 ground state. Reduction of the complex with cobaltocene generates the Class III mixed-valence anion [(Me3tacn)2V2(CN)4(mu-C4N4)]1-, wherein resonance exchange induces strong ferromagnetic coupling to give a well-isolated S = 3/2 ground state.

Catalytic Cycle of a Divanadium Complex with Salen Ligands in O2Reduction: Two-Electron Redox Process of the Dinuclear Center (salen =N,N‘-Ethylenebis(salicylideneamine))

In an attempt to provide confirmation for the postulated mechanism of O2 reduction in vanadium-mediated oxidative polymerization of diphenyl disulfide, a series of divanadium complexes containing salen ligand (salen = N,N‘-ethylenebis(salicylideneamine)) were prepared, characterized, and subjected to reactivity studies toward dioxygen. A divanadium(III, IV) complex, [(salen)VOV(salen)][I3] (II), was yielded both by treatments of solutions of [(salen)VOV(salen)][BF4]2 (I) in acetonitrile with excess tetrabutylammonium iodide and by electroreduction of I followed by anion exchange with tetrabutylammonium triiodide. The complex II was characterized by a near-infrared absorption at 7.2 × 103 cm-1 (ε = 60.1 M-1 cm-1 in acetonitrile) assigned to an intervalence transfer band. A crystallographically determined V(III)−V(IV) distance of 3.569(4) Å is consonant with the classification of II as a weakly coupled Type II mixed-valence vanadium (α = 3.0 × 10-2). Oxidation of the cation [(salen)VOV(salen)]+ with O2 in dichloromethane yielded spontaneously the deep blue, mixed valent, divanadium(IV, V) species [(salen)VOVO(salen)]+ which was structurally characterized both as its triiodide (III) and perchlorate (IV) salts. Crystal data for III: triclinic space group P1̄ (no. 2), a = 14.973(2) Å, b = 19.481(2) Å, c = 14.168(2) Å, α = 107.00 (1)°, β = 115.56(1)°, γ = 80.35(1)°, V = 3561.3(9) Å3, Z = 4, Dcalc = 1.953 g/cm3, μ (MoKα) = 31.74 cm-1, final R = 0.057 and Rw = 0.065. Crystal data for IV: triclinic space group P1̄ (no. 2), a = 11.923(3) Å, b = 14.25(1) Å, c = 11.368(7) Å, α = 112.92(5)°, β = 92.76(4)°, γ = 99.13(4)°, V = 1743(1) Å3, Z = 2, Dcalc = 1.537 g/cm3, μ (CuKα) = 57.69 cm-1, final R = 0.042 and Rw = 0.061. The complexes III and IV were deoxygenated in strongly acidic nonaqueous media to produce [(salen)VOV(salen)]3+ as a high-valent complex whose reversible two-electron redox couple (VOV3+/VOV+) at 0.44V vs Ag/AgCl has been confirmed. Its ability to serve as a two-electron oxidant provided a unique model of a multielectron redox cycle in oxidative polymerization.

ChemInform Abstract: First Dinuclear Complexes with Fourfold Alkyl‐Bridging.

AbstractThe new divanadium complex (IV) is prepared starting with (II), which contains a loosely bound naphthalene ligand.

Oxidative-reductive transmetallation reactions of germylmercury compounds with bis(ethylbenzene)vanadium and vanadocene

The ionic complex [{(C 6F 5) 3Ge} 3Hg] −[V(C 6H 5C 2H 5) 2] + has been obtained in the reaction of [(C 6F 5Ge] 2Hg with bis(ethylbenzene)vanadium. Dicyclopentadienylvanadium displaces mercury in [(C 6F 5) 3Ge] 2Hg to give the covalent product Cp 2VGe(C 6F 5) 3 (Cp = η 5-C 5H 5). The 4-nuclear product, (C 6F 5) 3GeHg Cp 2VGe(C 6F 5) 3, of the insertion of Cp 2V into the GeHg bond, is formed in the first step of the reaction. It is thought that the next step of the process leads to an unstable divanadium complex with the GeVHgVGe chain. [(C 6F 5) 3Ge] 2Cd reacts similarly. Interaction of Cp 2V with the thallium mercurate [(C 6F 5) 3Ge] 3HgTl · 1.5DME (DME = 1,2-dimethoxyethane) gives Tl, Hg and Cp 2VGe(C 6F 5) 3. The germylvanadium complex Cp 2VGe(C 6F 5) 3 is readily oxidized by silver and mercury chlorides to give Cp 2V(Cl)Ge(C 6F 5) 3 derivatives. ESR spectra of vanadium d 1-complexes are presented.

Electronic Structure of V2(CO)8(μ-PPh2)2 and Mn2(CO)8(μ-AsPh2)2 by Electron Spin Resonance Study of Their Ion Radicals

Abstract The anion radical of V2(CO)8(μ-PPh2)2 and the cation radical of Mn2(CO)8(μ-AsPh2)2 are generated by chemical reduction of the vanadium complex with LiBHEt3 or potassium naphthalenide and by radiochemical oxidation of the manganese complex in frozen Freon mixture, respectively. Their anisotropic ESR spectra show that the V2P2 and the Mn2As2 rhomboids are planar. Their g and hyperfine splitting tensors are analyzed on the basis of a qualitative molecular orbital scheme for metal-metal bonds developed from a hypothesis that metal-ligand interactions are larger than metal-metal interactions. The odd electron orbital of V2(CO)8(μ-PPh2)2−· is assigned to the metal-metal π* MO and the odd electron density on the vanadium d AO is estimated as 0.21. The odd electron of Mn2(CO)8(μ-AsPh2)2+· is concluded to be accomodated in the metal-metal σ* MO and odd electron densities on manganese and arsenic atoms are evaluated as ρ(Mn d)=0.25 and ρ(As p)=0.08 or 0.16, respectively. Order of magnitude estimates of orbital energy differences between metal-metal MO’s are obtained from analyses of g tensors as ε(π*)−ε(σ)\simeq0.4 and ε(π*)−ε(δ)\simeq0.9 eV for the anionic divanadium complex and ε(σ*)−ε(δ*)\simeq1.1 and ε(σ*)−ε(π)\simeq1.4 eV for the cationic dimanganese complex.