| Abstract
| - The reaction of clusters [(Cl4cat)(CH3CN)MoFe3S4Cl3]2- and [(Meida)MoFe3S4Cl3]2- with tertiary phosphines and NaBPh4 has been investigated. Depending on the nature of Mo ligation and employed phosphine, the products are either neutral Fe−S bridged mono- and dicubane or cationic Mo−carboxylate bridged tetracubane clusters. The MoIII−2FeII−FeIII composition of the [MoFe3S4]2+ cores in the products is suggested by 57Fe Mössbauer spectroscopy.
- Treatment of oxidized clusters [(Cl4cat)(MeCN)MoFe3S4Cl3]2- (1) and [(Meida)MoFe3S4Cl3]2- (2) with tertiaryphosphines in the presence of NaBPh4 in acetonitrile results in chloride substitution at the iron sites and theformation of clusters with the reduced [MoFe3S4]2+ core. Thus, 1 is a precursor to [(Cl4cat)(MeCN)MoFe3S4(PR3)3] (R = But (3), Pri (4)) and [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] (5). Cluster 2 affords [{(Meida)MoFe3S4(PCy3)3}4Fe2(μ-Cl)L2]3+ (L = THF (6), MeCN (7)). The structures of 3−7 were established by X-ray analysis.Clusters 3 and 4 are single cubanes, centrosymmetric 5 (previously reported in a different space group: Demadis,K. D.; Campana, C. F.; Coucouvanis, D. J. Am. Chem. Soc.1995, 117, 7832) is a double cubane with a rhomboidalFe2S2 bridge, and 6 and 7 are tetracubanes. In the latter, four Meida oxygen atoms from different cubanes bindeach of two central high-spin Fe(II) atoms in trans-Fe(μ-Cl)LO4 coordination. The topology of these clusters isnot precedented. Zero-field Mössbauer parameters for all clusters are reported. Isomer shift considerations suggestthe formulation [Mo3+Fe2+2Fe3+S4] for reduced clusters. Voltammetry of 3 and 4 reveals four-member electrontransfer series encompassing the oxidation levels [MoFe3S4]4+,3+,2+,+ in the potential interval +1.0 to −1.3 V vsSCE in dichloromethane. Compared to the clusters with monoanionic ligands at the iron sites, phosphine ligationshifts redox potentials to more positive values. This effect arises from reduction of cluster negative charge andthe tendency of phosphines to stabilize lower oxidation states. The synthesis of reduced clusters 4 from 1 and of[Fe4S4(PPri3)4]+ from [Fe4S4Cl4]2- is accompanied by the formation of Pri3PS, detected by 31P NMR, indicatingthat the phosphine is the reductant. This result implies a similar function of tertiary phosphines in the synthesisof 3 and 5−7. (Cl4cat = tetrachlorocatecholate(2−); Meida = N-methyliminodiacetate(2−).)
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