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.

Structural and Spectroscopic Evidence for a Side-on Fe(III)-Superoxo Complex Featuring Discrete O-O Bond Distances.

The O-O bond length is often used as a structural indicator to determine the valence states of bound O2 ligands in biological metal-dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O-O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Šat 100 K) of a side-on iron-dioxygen species. Despite their different O-O bond distances, all forms possess the same electronic structure of Fe(III)-O2 •-, as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η2-O2 motif binding to the iron center regarding the O-O distance. Therefore, the discrete O-O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O-O distance is not a reliable benchmark to unequivocally identify the valence state of O2 ligands for metal-dioxygen species in O2-utilizing metalloproteins and synthetic complexes.

The dual roles of a V(III) centre for substrate binding and oxygen atom abstraction; nitrite reduction mediated by a V(III) complex.

A V(iii) complex bearing a tris(thiolato)phoshine derivative mediates the reduction of nitrite without the assistance of external protons or oxophilic substrates. The metal site plays dual roles for nitrite binding and deoxygenation. The reaction is monitored by spectroscopy combined with isotopic labeling experiments. The formed product, a {VNO}4 species, is isolated and characterized.

Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt Complexes Binding with (Thiolato)phosphine Derivatives.

In our efforts to understand the nature of metal thiolates, we have explored the chemistry of cobalt ion supported by (thiolato)phosphine ligand derivatives. Herein, we synthesized and characterized a square-planar CoII complex binding with a bidentate (thiolato)phosphine ligand, Co(PS1″)2 (1) ([PS1″]- = [P(Ph)2(C6H3-3-SiMe3-2-S)]-). The complex activates O2 to form a ligand-based oxygenation product, Co(OPS1″)2 (2) ([OPS1″]- = [PO(Ph)2(C6H3-3-SiMe3-2-S)]-). In addition, an octahedral CoIII complex with a tridentate bis(thiolato)phosphine ligand, [NEt4][Co(PS2*)2] (3) ([PS2*]2- = [P(Ph)(C6H3-3-Ph-2-S)2]2-), was obtained. Compound 3 cleaves the C-Cl bond in dichloromethane via an S-based nucleophilic attack to generate a chloromethyl thioether group. Two isomeric products, [Co(PS2*)(PSSCH2Cl*)] (4 and 4') ([PSSCH2Cl*]- = [P(Ph)(C6H3-3-Ph-2-S)(C6H3-3-Ph-2-SCH2Cl)]-), were isolated and fully characterized. Both transformations, oxygenation of a CoII-bound phosphine donor in 1 and alkylation of a CoIII-bound thiolate in 3, were monitored by spectroscopic methods. These reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.