Cobalt complexes thiolates

A chemistry of cobalt-sulfide-thiolate molecular clusters comparable with that of iron systems has also begun to emerge. Treatment of [Co4( -SPh)6(SPh)4]2- with HS- in acetone affords the octanuclear cluster [Co8(ji4-S)6(SPh)8]4 isolated as its Pr4N+ salt.988 In MeCN solution the complex is red-purple with intense sulfur-core charge transfer bands which obscure the Co" d-d transitions. This behaviour contrasts with that of both mono- and poly-nuclear cobalt"-thiolate complexes, which all display LMCT bands below 440 nm and have well-developed v2 and v3 features. The [Co8(/j4-S6)]4+ core sustains reversible one-electron oxidation and reduction (E]l2 = —0.54, — 1.18 V, MeCN) and chemical reduction with sodium acenaphthylenide in THF gives [Co8(/r4-... 

In contrast to iron and manganese, the number of trinuclear cobalt complexes is comparatively small. The reaction of [Co2(CO)s] with 2-propane thiolate or with C3H7SSC3H7 in methanol under solvothermal conditions afforded the electroneutral complex (Co3(SC3H7)s(CO)4], which contains a triangle of cobalt atoms. Its structure is shown in Fig. 16. 

Several synthetic Co(III) and Fe(III) complexes have been generated as models for the active-site metal center in nitrile hydratases [8, 10]. However, few have been examined in terms of water coordination and acidity. Cobalt complexes supported by Ns-type donor ligands, with two carboxamido nitrogen donors, exhibit a pfCa near 7 (Fig. 8.14a and b) [59, 60]. Introduction of two thiolate sulfur donors into the Co(III) coordination sphere increases the pK of the bound water to 8.3 (Fig. 8.14c) [61]. Interestingly, oxidation of one of the sulfur donors to a S-bound sulfmate (Fig. 8.14d) reduces the pK by around 1 unit [62]. As the active-site metal center in nitrile hydratases contain oxidatively modified cysteine residues coordinated to the metal center [8], it has been suggested that the oxidized sulfur donors play a role in modulating the acidity of the metal-bound water molecule. 

Chelation of ixo-maleonitriledithiolate (imdt) has been structurally characterized in the octahedral cobalt(III) complex trmw-[Co(imdt)2(P(ra-Bu)3)2], formed via reaction of cobalt(II) ion with K2(imdt) in the presence of the phosphine.1037 Simple chelating thiolates such as SCH2CH2S not only form mononuclear compounds, but participate in bridging in clusters such as [Co3(SCH2CH2S)3(PEt3)3]3+ (243).1038... 

The nucleophilicity of coordinated sulfur in the complex [Cr(SCH2CH2NH2)(en)2]2+ towards Mel (equation 51) has been measured in DMF/H20 and compared with sulfur nucleophilicities in thiolatocobalt(III) systems.975 As in the case of the H202 oxidations, the nucleophilicity of thiolate coordinated to chromium(III) is only slightly less than when it is coordinated to cobalt(III), implying that nucleophilic attack by coordinated sulfur does not involve any appreciable distortion in the first coordination sphere of the metal. 

The formation of complexes of l,2,3,4-thiatriazole-5-thiol has been well described in CHEC-II(1996) 1,2,3,4-thiatriazole-5-thiol can form complexes with various metals such as palladium, nickel, platinum, cobalt, zinc, etc. <1996CHEC-II(4)691>. These complexes can be prepared either by cycloaddition reactions of carbon disulfide with metal complexes of azide anion (Equation 20) or directly from the sodium salt of l,2,3,4-thiatriazole-5-thiol with metal salts. For instance, the palladium-thiatriazole complex 179 can be obtained as shown in Equation (20) or it may be formed from palladium(ll) nitrate, triphenylphosphine, and sodium thiatriazolate-5-thiolate. It should be noted that complexes of azide ion react with carbon disulfide much faster than sodium azide itself. 

The role of Coball-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co11 species catalysis by the phthalocyanine complex [Con(TSPc)]4 apparently proceeds via a Co1 intermediate and without participation of Co—02 species,680 whereas catalysis by [CoH(TPP)] appears to involve initial formation of an >/ cobalt-dioxygen complex from which Of is displaced by thiolate.681 Several reviews giving extensive coverage to oxidations catalyzed by cobalt(II) complexes are available.649,650,682 683... 

However, as observed elsewhere, the slightly less steric hindered thiolate TEMT ligands display rather different chemistry in analogous reactions. The resultant complex is a dimer, [ Co(TEMT)2(bipy) 2], with two five-coordinate cobalts linked by two asymmetric thiolate bridges, as shown in Fig. 16 (73). 

The principal impetus to the study of zinc thiolates has undoubtedly been the search for structural models for the metal coordination in zinc metalloproteins such as the Zn(S-Cys)2(His)2 center in the transcription factor IIIA and other zinc fingers that feature in protein-DNA interactions (93, 94). The 2 1 complexes [M(SR)2l (M = Zn, Cd SR = TIPT, TEMT) were used as the principle precursors for the synthesis of such models for the Zn(S-Cys)2(His)2. Thus complexes of the types [Cd(TIPT)2(l-CH3-imid)2], [Zn(TIPT)2(bipy)], and [Zn(TEMT)2(l-CH3-imid)2l (imid = imidazole, bipy = bipyridine) were ssmthesized by addition of a nitrogeneous base to the precursor in MeCN. All of them have been characterized by X-ray crystallography and were found to have distorted tetrahedral geometries. It was noticed that the cobalt analogue binds a solvent molecule MeCN to form five-coordinate [Co(TIPT)2(bipy)(CH3CN)], whereas [Zn(TIPT)2(bipy)] does not 96,97). 

Dealkylation of alkyl cobalt species by thiolate was suggested as a reductive cleavage process, involving reduction of alkylcobalamin with thiol to form a trans axial thiol complex . This accounts for the low yield of thioether . Carbon-13 and P studies... 

Finally it should be pointed out that a number of studies have been published purporting to show the nucleophilic displacement of cobalt(I) chelates from alkylcobalt complexes by thiolate anions in base (Eqn. 44) [77,78]. 

Within this context, the present article concentrates on transition metal cluster complexes of cobalt, iron and manganese with mixed chalcogen/carbonyl ligand spheres obtained by reaction of simple binary metal carbonyls with alkali-metal sulfides, alkali-metal thiolates or transition-metal thiolate complexes and their selenium or tellurium counterparts. 

A hexanuclear chalcogen/carbonyl compound of cobalt containing no thiolate ligands, [Co6E2(E2)(CO)i4] (E = S, Se), is shown in Fig. 27 for E = The cluster complex is assembled from two identical trinuclear Co3S(CO)vS2 sub-units which share the S atoms of the disulfide ligand. They possess principle features already known from the [Fe3E(CO)9]- (E = S, Se, Te) series of complexes (see Fig. 9, Section 1.10.5). 

B7.17 Cobalt (iron) thiolato complexes containing the co-li-gand phosphine and reaction products of the structural fragment ML2L (L = 1,2-bidentate thiolate,... 

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