A Bio-Inspired Switch Based on Cobalt(II) Disulfide/Cobalt(III) Thiolate Interconversion

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 V^IV complex containing an unbound disulfide moiety, [V^IV(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 [V^V(PS3″)(PS2″S^H)] (2) (PS2″S^H = [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 V^IV complex, [V^IV(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 V^V-dithiolate and a V^IV-(thiolate)(thiyl-radical) species. This work demonstrates an unprecedented example of reversible disulfide/dithiolate interconversion mediated by a V^IV center, as well as provides insights into understanding the function of V^V reductases in vanabins.

Cleaner and stronger: how 8-quinolinolate facilitates formation of Co(III)-thiolate from Co(II)-disulfide complexes

The formation of Co(III)-thiolate complexes from Co(II)-disulfide complexes using the anionic ligand 8-quinolinolate (quin^-) has been studied experimentally and quantum chemically. Two Co(II)-disulfide complexes [Co₂(L^xSSL^x)(Cl)₄] (x = 1 or 2; L¹SSL¹ = 2,2'-disulfanediylbis(N,N-bis(pyridin-2-ylmethyl)ethan-1-amine; L²SSL² = 2,2'-disulfanedylbis (N-((6-methylpyridin-2-yl)methyl)-N-(pyridin-2-ylmethyl) ethan-1-amine) have been successfully converted with high yield to their corresponding Co(III)-thiolate complexes upon addition of the ligand 8-quinolinolate. Using density functional theory (DFT) computations the d-orbital splitting energies of the cobalt-thiolate compounds [Co(L¹S)(quin)]⁺ and [Co(L²S)(quin)]⁺ were estimated to be 3.10 eV and 3.07 eV, indicating a slightly smaller ligand-field strength of ligand L²SSL² than of L¹SSL¹. Furthermore, the orientation of the quin^- ligand in the thiolate compounds determines the stability of the thiolate complex. DFT computations show that the thiolate structure benefits from more electrostatic attraction when the oxygen atom of the quin^- ligand is positioned trans to the sulfur atom of the [Co(L¹S)]²⁺ fragment. Quin^- is the first auxiliary ligand with which it appeared possible to induce the redox-conversion reaction in cobalt(II) compounds of the relatively weak-field ligand L²SSL².

Probing the redox-conversion of Co(II)-disulfide to Co(III)-thiolate complexes: the effect of ligand-field strength

The redox-conversion reaction of cobalt(II)-disulfide to cobalt(III)-thiolate complexes triggered by addition of the bidentate ligand 2,2'-bipyridine has been investigated. Reaction of the cobalt(II)-disulfide complex [Co2(L1SSL1)(X)4] (L1SSL1 = di-2-(bis(2-pyridylmethyl)amino)-ethyldisulfide; X = Cl or Br) [1X] with 2,2'-bipyridine (bpy) resulted in the formation of two different products, namely the cobalt(III)-thiolate complex [Co(L1S)(bpy)]X2 and the unexpected side product [Co2(L1SSL1)(bpy)2(X)2]X2. Crystals of [Co2(L1SSL1)(bpy)2(Cl)2](BPh4)2 [2Cl](BPh4)2 obtained after anion exchange showed the cobalt(II) ions to be in octahedral geometries with the nitrogen donors of the disulfide ligand arranged in a facial conformation and the chloride ion trans to the tertiary amine nitrogen. Remarkably, this side product cannot be converted to the cobalt(III)-thiolate compound [Co(L1S)(bpy)](SbF6)2 [3](SbF6)2 by removal of the chloride ion with use of a silver salt, as this causes scrambling of the ligands, resulting in the formation of [Co(bpy)3]n+. [Co(L1S)(bpy)](SbF6)2 was obtained in a pure form by addition of bpy to a solution in acetonitrile of the compound [Co(L1S)(MeCN)2]2+ [4]2+. Addition of NEt4Cl to [3](SbF6)2 regenerates the cobalt(II)-disulfide complex [1Cl] as confirmed spectroscopically. DFT studies revealed that the conversion from [1Cl] to [3]2+ most likely occurs via the hypothetical intermediate species [2Cl]2+mer, in which the nitrogen atoms of the disulfide ligand are arranged in a meridional conformation. Interestingly, the estimated d-orbital splitting energy of [3]2+ is lower than that of [4]2+, indicating that the ligand-field strength of bpy is lower than anticipated, which hampers clean conversion in the redox-conversion reaction. This study shows that the redox-conversion reaction between cobalt(II)-disulfide and cobalt(III)-thiolate complexes is intricate rather than straightforward.

A New Bis(thioether)-Dipyrrin N2S2 Ligand and Its Coordination Behaviors to Nickel, Copper and Zinc

Tetradentate N2S2 coordination platforms are widespread in biological system and have endowed the metalloenzymes and metalloproteins with abundant reactivities and functions. However, there have only three types of N2S2 scaffolds...

TD-DFT assessment of UV-vis spectra palladium and platinum complexes with thiols and disulfides

Time-dependent density functional theory (TD-DFT) and spectrophotometric methods were used for speciation analysis in systems disulfides (cystine, cystamine, homocystine, 3,3-dithiodipropionic acid) - [PdCl₄]^2- or [PtCl₄]^2-. We use the M06-2X and CAM-B3LYP density functionals with Def2-SVP basis set to reproduce the experimental UV-vis spectra; the polarized continuum solvation model (PCM) was fitted to take into account solvation effects of the medium (water). Used methods have shown the good agrees with the experiment - theoretical values of transition energies differ from real parameters within ±0.15 eV for functional CAM-B3LYP. Binuclear disulfide complexes of Pd(II) with cystine and cystamine have form S,N-coordination sites, instead of S,S-conformation. It was shown that Pd(II) thiolate complexes formed by cleavage of the disulfide bond exist as [PdCl₃L] and [Pd₂S₂L₂]. Pt(II)-disulfide systems have confirmed the presence of [Pt₂Cl₆(R-SS-R)] and [PtCl₄(S-R)] complex species. The DFT/CAM-B3LYP/Def2-SVP/SMD level can be recommended for theoretical estimations of absorption spectra of complexes of palladium or platinum and sulfur-containing ligands.

Bio-inspired, Multifunctional Metal-Thiolate Motif: From Electron Transfer to Sulfur Reactivity and Small-Molecule Activation

Sulfur-rich metalloproteins and metalloenzymes, containing strongly covalent metal-thiolate (cysteinate) or metal-sulfide bonds in their active site, are ubiquitous in nature. The metal-sulfur motif is a highly versatile tool involved in various biological processes: (i) metal storage, transport, and detoxification; (ii) electron transfer; (iii) activation of the sulfur atom to promote different types of S-based reactions including S-alkylation, S-oxygenation, S-nitrosylation, or disulfide or thiyl radicals formation; (iv) activation of small earth-abundant molecules (such as water, dioxygen, superoxide radical anion, carbon oxides, nitrous oxide, and dinitrogen).This Account describes our investigations carried out during the past 10 years on bio-inspired and biomimetic low-nuclearity complexes containing metal-thiolate bonds. The general objective of these structural, spectroscopic, electrochemical, and catalytic studies was to determine structure-properties-function correlations useful to (i) understanding the peculiar features or the mechanism of the mimicked natural systems and/or (ii) reproducing enzymatic reactivities for specific catalytic applications.By employing a unique highly preorganized N₂S₂-donor ligand with two thiolate functions, in combination with different first-row transition metals (Mn, Fe, Co, Ni, Cu, Zn, or V), we got access to a series of bio-inspired sulfur-rich complexes displaying a widespread spectrum of structures, properties, and functions. We isolated a dicopper(I) complex that, for the first time, mimicked concomitantly the key structural, spectroscopic, and redox features of the biological Cu_A center, a highly efficient electron transfer agent involved in the respiratory enzyme cytochrome c oxidase. In the field of sulfur activation, we explored (i) sulfur methylation promoted by a Zn-dithiolate complex that mimics Zn-dependent thiolate alkylation proteins and shows different selectivity compared to the Ni and Co congeners and (ii) a series of Co, Fe, and Mn complexes as the first copper-free systems able to promote thiolate/disulfide interconversion mediated by (de)coordination of halides. Concerning metal-centered reactivity, we investigated two families of metal-thiolate catalysts for small-molecule activation, especially relevant in the fields of sustainable fuel production and energy conversion: (i) two isostructural Mn and Fe dinuclear complexes that activate and reduce dioxygen selectively, either to hydrogen peroxide or water as a function of the experimental conditions; (ii) a family of dinuclear MFe (M = Ni or Fe) hydrogenase mimics active for catalytic H₂ evolution both in organic solution and on modified electrodes in water.This Account thus illustrates how the versatility of thiolate ligation can support selected functions for transition metal complexes, depending on the nature of the metal, the nuclearity of the complex, the presence and type of co-ligands, the second coordination sphere effects, and the experimental conditions.

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 Co^II 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 Co^III 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*)(PSS^CH₂Cl*)] (4 and 4') ([PSS^CH₂Cl*]^- = [P(Ph)(C₆H₃-3-Ph-2-S)(C₆H₃-3-Ph-2-SCH₂Cl)]^-), were isolated and fully characterized. Both transformations, oxygenation of a Co^II-bound phosphine donor in 1 and alkylation of a Co^III-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.

Controlled Formation of Thiol-Thiolate Hydrogen versus Disulfide Bonds between Two Iridium(III) Centers

Here, we report an iridium(III) coordination system with 2-aminoethanethiolate (aet), which shows the formation of S-H⋅⋅⋅S hydrogen and S-S disulfide bonds in a controlled manner. Treatment of fac-[Ir(aet)₃ ] with aqueous HBF₄ under aerobic conditions gave dinuclear [Ir₂ (aet)₄ (cysta)]²⁺ ([1]²⁺ ; cysta=cystamine) with a single S-S disulfide bond, while dimeric [Ir₂ (aet)₃ (Haet)₃ ](BF₄ )₃ ([2](BF₄ )₃ ) with a triple S-H⋅⋅⋅S hydrogen bond was formed by similar treatment under anaerobic conditions. Upon exposure to air, [2]³⁺ was converted to dinuclear [Ir₂ (aet)₂ (Haet)₂ (cysta)]⁴⁺ ([3]⁴⁺ ), in which two Ir^III centers are spanned by a double S-H⋅⋅⋅S hydrogen bond and a single S-S disulfide bond. Complex [3]⁴⁺ was interconvertible with [1]²⁺ via the removal/addition of protons on S donors, accompanied by the intermolecular exchange of the fac-[Ir(aet)₃ ] units. Complexes [1]²⁺ , [2]³⁺ , and [3]⁴⁺ , isolated as BF₄ ^- salts, were fully characterized by single-crystal X-ray crystallography.

Palladium(II) Ion Mediated Disulfide/Thiolate Interconversion: Predicting the Disulfide Group State from First Principles

Different reactivity of homologous disulfides toward Pd²⁺ was previously reported: stepwise complexation to Pd²⁺ for l-cystine and cystamine ligands, while for dl-homocystine and 3,3'-dithiodipropionic acid, disulfide's disproportionation toward thiolate and sulfinic acid complexes is observed. The disulfide/thiolate interconversion of four different disulfide ligands in the presence of nonredox metal cation Pd²⁺ in aqueous solution has been computationally investigated. We see this different reactivity in different capacities of considered homologous disulfides to stabilize forming S,S'-binuclear complexes, which are believed to be key intermediates toward interconversion products. We thus devise a theoretical model that rationalizes experimentally observed phenomenon of disulfides different reactivity toward nonredox transition metal cation Pd²⁺.

Redox Interconversion between Cobalt(III) Thiolate and Cobalt(II) Disulfide Compounds

The redox interconversion between Co(III) thiolate and Co(II) disulfide compounds has been investigated experimentally and computationally. Reactions of cobalt(II) salts with disulfide ligand L¹SSL¹ (L¹SSL¹ = di-2-(bis(2-pyridylmethyl)amino)-ethyl disulfide) result in the formation of either the high-spin cobalt(II) disulfide compound [Co^II₂(L¹SSL¹)Cl₄] or a low-spin, octahedral cobalt(III) thiolate compound, such as [Co^III(L¹S)(MeCN)₂](BF₄)₂. Addition of thiocyanate anions to a solution containing the latter compound yielded crystals of [Co^III(L¹S)(NCS)₂]. The addition of chloride ions to a solution of [Co^III(L¹S)(MeCN)₂](BF₄)₂ in acetonitrile results in conversion of the cobalt(III) thiolate compound to the cobalt(II) disulfide compound [Co^II₂(L¹SSL¹)Cl₄], as monitored with UV–vis spectroscopy; subsequent addition of AgBF₄ regenerates the Co(III) compound. Computational studies show that exchange by a chloride anion of the coordinated acetonitrile molecule or thiocyanate anion in compounds [Co^III(L¹S)(MeCN)₂]²⁺ and [Co^III(L¹S)(NCS)₂] induces a change in the character of the highest occupied molecular orbitals, showing a decrease of the contribution of the p orbital on sulfur and an increase of the d orbital on cobalt. As a comparison, the synthesis of iron compounds was undertaken. X-ray crystallography revealed that structure of the dinuclear iron(II) disulfide compound [Fe^II₂(L¹SSL¹)Cl₄] is different from that of cobalt(II) compound [Co^II₂(L¹SSL¹)Cl₄]. In contrast to cobalt, reaction of ligand L¹SSL¹ with [Fe(MeCN)₆](BF₄)₂ did not yield the expected Fe(III) thiolate compound. This work is an unprecedented example of redox interconversion between a high-spin Co(II) disulfide compound and a low-spin Co(III) thiolate compound triggered by the nature of the anion.

Low-spin Co^III−thiolate compounds and high-spin Co^II and Fe^II disulfide compound have been synthesized from reactions of a disulfide ligand with Co^II and Fe^II salts. The redox interconversion between the cobalt compounds has been investigated. It is shown that addition of chloride ions to a solution of the Co^III−thiolate compound results in the formation of the Co^II-disulfide compound, whereas removal of chloride anions from the Co^II disulfide compound regenerates the Co^III−thiolate complex.