VIV Complex Research Articles

Reversible Conversion of Disulfide/Dithiolate Occurring at a Vanadium(IV) Center: A Biomimetic System for Redox Exchange in Vanabin.

Ascidians use a class of cysteine-rich proteins generally referred to as vanabins to reduce vanadium ions, one of the many biological processes that involve the redox conversion between disulfide and dithiolate mediated by transition-metal ions. To further understand the nature of disulfide/dithiolate exchange facilitated by a vanadium center, we report herein a six-coordinate non-oxido VIV complex containing an unbound disulfide moiety, [VIV(PS3″)(PS1″S-S)] (1) (PS3″ = [P(C6H3-3-Me3Si-2-S)3]3-, where PS1″S-S is a disulfide form of PS3″). Complex 1 is obtained from a reaction of previously reported [VV(PS3″)(PS2″SH)] (2) (PS2″SH = [P(C6H3-3-Me3Si-2-SH)(C6H3-3-Me3Si-2-S)2] with TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) via hydrogen atom transfer. Importantly, complex 1 can be reduced by two electrons to form an eight-coordinate VIV complex, [VIV(PS3″)2]2- (4). The reaction can be reversed through a two-electron oxidation process to regenerate complex 1. The redox pathways both proceed through a common intermediate, [V(PS3″)2]- (3), that has been previously reported as a resonance form of VV-dithiolate and a VIV-(thiolate)(thiyl-radical) species. This work demonstrates an unprecedented example of reversible disulfide/dithiolate interconversion mediated by a VIV center, as well as provides insights into understanding the function of VV reductases in vanabins.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry.

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph2N)3V(μ-N)B(C6F5)2 (1a) and (N(CH2CH2N(C6F5))3)V(μ-N)B(C6F5)2 (1b). Redox control of the VIV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the VV complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the VIV complex, [CoCp2*][(N(CH2CH2N(C6F5))3)V(μ-N)B(C6F5)2] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the VIV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

VIVO and VIV Species Formed in Aqueous Solution by the Tridentate Glutaroimide–Dioxime Ligand – An Instrumental and Computational Characterization

Complexation of VIV in aqueous solution with glutaroimide–dioxime (H3L), a ligand proposed for the possible sequestration of uranium from seawater, was studied by the combined application of spectroscopic (EPR and UV/Vis), spectrometric (ESI‐MS), electrochemical (CV), and computational (DFT) techniques. The results indicate that a rare non‐oxido VIV species, with formula [VIVL2]2–, is formed in the pH range 3–5. It transforms into a usual VIVO complex, [VIVOL(OH)]2–, at pH > 6. The non‐oxido species is characterized by a “type 3” EPR spectrum with Az ≈ 126 × 10–4 cm–1 and a UV/Vis signal with ε > 2000 m–1 cm–1 in the visible region. The detection of VV species by ESI‐MS spectrometry was related to two possible oxidation processes, the first one in solution and the second one in‐source during the recording of the spectra. The cyclic voltammogram of [VIVL2]2– shows two quasi‐reversible processes, at E1/2 = –0.75 V and E1/2 = 0.03 V, assigned to the VIV/VIII reduction and VIV/VV oxidation, respectively. All the experimental results were verified by DFT calculations, which indicated that the geometry of the non‐oxido VIV complex is intermediate between an octahedron and a trigonal prism and allowed us to predict its 51V hyperfine coupling (HFC) tensor A, the electron absorption spectrum, the two redox processes in the cyclic voltammogram, and the electronic structure that, in turn, determines its EPR and UV/Vis behavior.

Formation in aqueous solution of a non-oxido VIV complex with VN6 coordination. Potentiometric, ESI-MS, spectroscopic and computational characterization

The behaviour of the system formed by V(IV)O(2+) ion with all-cis-2,4,6-trimethoxycyclohexane-1,3,5-triamine (tmca) was characterized in aqueous solution through the combined application of electron paramagnetic resonance (EPR) and UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), pH-potentiometry and DFT methods. The formation of an unusual non-oxido [V(tmcaH-2)2] species with VN6 coordination, with the ligand in the bianionic form, was demonstrated. The geometry, EPR and UV-Vis spectra and electronic structure of [V(tmcaH-2)2] were simulated with Gaussian 09 and ORCA software and the results were compared with those of similar oxido and non-oxido vanadium(iv) species formed by other polyamine and polyol related ligands, such as 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci), 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci), cis-inositol (ino) and 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci). The results indicate that V(IV)O(2+) species are formed in acid and weakly basic solution and that [V(tmcaH-2)2] is observed above pH 10. In the non-oxido complex, DFT calculations suggest that the two -NH2 groups are in trans position and that the pre-organization of the ligands favours the metal complexation and the formation of the hexa-coordinated species with VN6 coordination.

DFT Calculations of Space Structures of MII Complexes with (N,N,N,N)-Coordinating Macroheterocyclic Ligand – 1,8-Dioxa-3,6,10,13-tetraazacyclotetradecanetetrathione-4,5,11,12

Geometric parameters of MnII, CoII, NiII and CuII complexes with 1,8-dioxa-3,6,10,13-tetraazacyclotetradecanetetrathione-4,5,11,12 macrohetrocyclic ligand with N,N,N,N-coordination mode, which can be formed as a result of template processes in MII – ethanedithioamide-1,2 – methanal triple systems, were calculated using DFT B3LYP method with 6-31G(d) basic set. Additional 6-membered metal chelate cycles formed as a result of template “stitching” are not planar and considerably tilted in respect to two 5-membered cycles. In addition, in VIV complex, both these cycles are on one side from the mean N,N,N,N-plane formed by coordinating nitrogen atoms, in all other complexes – on different sides of this plane.

Air oxidation of divanadium(IV) complexes

The dinuclear VIV=O complex of 2,6-bis{[N,N-bis(2-pyridylmethyl)amino]methyl}-4-tert-butylphenolato (bpbp-), [(V=O)2bpbp(OH2)2](ClO4)3.H2O, is oxidised by air. Two products have been characterised in the solid state, namely a VVVV diperoxido complex, [(V=O)2bpbp(eta2-O2)2](ClO4), and a VVVIV trioxido complex, [V(O)2V(O)bpbp(OH2)](ClO4)(2).2H2O.EtOH, . The rate of formation of is solvent-dependent and is fastest in 2-propanol, compared to methanol or tetrahydrofuran. A transient species postulated to be a VV hydroperoxido species was detected during the course of this relatively slow reaction, which takes place on a timescale of hours to days. Concentration appears to be the factor determining whether complex or complex is obtained from reactions of with air: at higher complex concentrations formation of rather than is presumed to occur via a bimolecular reaction between the starting VIV complex and a reactive oxidized adduct.

A new Schiff base oxovanadium (IV) complex: [N,N′-bis(4-hydroxysalicylidene)-o-phenylenediamine]oxovanadium(IV) bis(dimethyl sulfoxide) dihydrate

The synthesis and characterization of the title VIV complex of a salicyl­ald­imine, [V(C20H14N2O4)O]·2C2H6OS·2H2O, is described. The reaction of 2,4-di­hydroxy­benz­aldehyde and 1,2-phenyl­enedi­amine with vanadyl sulfate produces a monomeric VIV complex. Its structure reveals that the vanadium(IV) ion is pentacoordinated and situated in a distorted square-pyramidal environment.

Piperazinium aquabis(oxalato)oxovanadate(IV) sesquihydrate

A mononuclear VIV complex, (C4H12N2)[VO(C2O4)2(H2O)]·1.5H2O, with bis­(oxalate) ligands, has been synthesized under hydro­thermal conditions. The structure contains aqua­bis­(oxalato)­oxovanadium(IV) complex anions, [VO(C2O4)2(H2O)]2−, and diprotonated piperazine cations, [C4N2H12]2+. The ions are connected through hydrogen bonds to form a three-dimensional network with micropores, where uncoord­inated water mol­ecules are located with the some degree of disorder. The V center has distorted octahedral geometry with one terminal O atom, four O atoms from two oxalate groups and one water O atom. The two oxalate groups are cis arranged. The V—O bond lengths are in the range 1.599 (2)–2.216 (2) Å.

Selective manganese(III) and vanadium(IV) catalysts for the oxidation of dialkyl sulfides in microemulsion media

The oxidation of n-Bu2S by t-BuOOH in the presence of␣the catalysts [Mn2L2(MeCO2)2 (μ-O)] (ClO4)2 (L= Me3tacn=1,4,7–trimethyl-1,4,7–triazacyclononane) and VO(acac)2 has been studied using microemulsion media (BRIJ 97 and SD2 microemulsions). In the presence of the MnIII complex oxidation to the sulfone occurs, while in the presence of the VIV complex the sulfide is oxidised to the sulfoxide. The oxidation of mustard gas, (ClCH2CH2)2S, has also been studied. The reactions have been monitored by total internal reflectance i.r. spectroscopy. The use of optically transparent microemulsions provides a useful method for carrying out oxidations involving inorganic reagents and water-insoluble substrates.

Bimetallic Reactivity. Oxo Transfer Reactions with a Heterobimetallic Complex of Iron(II) and Vanadium(III).

Several mono- and bimetallic complexes of Fe(II) and V(III) with binucleating macrocyclic systems have been prepared. Electrochemical studies of complexes containing Fe(II) in the 6-coordinate site have shown that the Fe(II/III) potential is affected by the length of the diamine chelate in this site and by the occupancy of the open site. Oxidation of the FeII−VIII complex, A, with 2,4,6-trimethyliodosobenzene produced the FeII−VIV complex, B. Both A and B have been characterized by X-ray crystallography, cyclic voltammetry, and infrared and electronic spectroscopy. The origins of deactivation of two-metal oxidation in bimetallic complexes are discussed primarily in terms of the ligand reorganizational energy, which is called mechanical coupling.

Characterization of bio-related vanadium and zinc complexes containing tetradentate dithiolate-disulfide, -diamine and -amine-amide ligands

The reaction of [VCl3(PMe2Ph)3] with HSS′S′SH (where the HS are thiophenolate and the S′ thioether functions, respectively), H2–1, yields [VCl(μ-SS′S′S)]2 (3) with one of the thiolate groups of each of the two ligands in the bridging mode. Reaction of Na2–1 with [VOCl2(thf)2] leads to a polymeric product of composition [VO(SS′S′S)]x (4). The products obtained from the reaction between [VOCl2(thf)2] and NaSNNSNa, Na2–2, (S is thiophenolate, N the amine function) depend on subtle changes in the diamine backbone of this ligand: If the amine functions are linked by -CH2CH2– (2a), the tetranuclear VIV complex [V(SNNS)μ-O]4 (5) is formed alongside the VIII complex [VCl(SNNS)]. If the backbone is -CH(Me)CH(Me)- (2b), [VO(SNNS)] (7) and the dinuclear, asymmetrically oxo-bridged VIV complex [{(SNN – S)(thf)V}μ-O{V(SNN – S)}] (8) are obtained. In 8, one amine of each of the two ligands is deprotonated to the amide group. In either case, the complexation is accompanied by oxidation of the thiolates to disulfides, leading to the generation of teraazatetrathio-cycloeicosanes (6a/b). Compounds 5 and 8·2THF have been structurally characterized by X-ray analyses. The connectivities have further been established for 3·2CH2Cl2 and for 6b, which exhibits the same conformation as formally characterized 6a. The cluster compound 5 is stabilized by an extended intramolecular N-H...O and N-H...S) hydrogen-bonding network. In 7·2THF, one of the THFs of crystallization is hydrogen-bonded to the NH of the penta-coordinated {VO(SNN – S)} moiety; further, there is an intramolecular hydrogen bond between one of the thiolates of this tetragonal-pyramidal half of the molecule and the NH of the octahedral {VO(SNN – S)thf} half. The generation of the ligand 2b from its precursor compound, the zinc complex [Zn(SNNS)] (9) leads to the structural characterization of 9·CH3OH with a large SZnS bite angle and a strong hydrogen bond between the methanolic OH and one of the thiolate sulfurs. The relevance of these compounds in biological systems is discussed.

Catalytic oxidation of vanadyl salts by oxygen in the presence of sodium molybdate

The catalytic oxidation of vanadyl by oxygen in the presence of sodium molybdate has been studied. Kinetic and spectrophotometric data show that the activation of oxygen is due to its interaction with a MoVI−VIV complex formed in the solution. A salt of this complex has been isolated, its composition is [(C2H5)4N]2H4[Mo3V3O18].