Reduced Mono-, Di-, and Tetracubane-Type Clusters Containing the [MoFe₃S₄]²⁺ Core Stabilized by Tertiary Phosphine Ligation

Treatment of oxidized clusters [(Cl4cat)(MeCN)MoFe3S4Cl3]2- (1) and [(Meida)MoFe3S4Cl3]2- (2) with tertiary phosphines in the presence of NaBPh4 in acetonitrile results in chloride substitution at the iron sites and the formation 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 rhomboidal Fe2S2 bridge, and 6 and 7 are tetracubanes. In the latter, four Meida oxygen atoms from different cubanes bind each of two central high-spin Fe(II) atoms in trans-Fe(μ-Cl)LO4 coordination. The topology of these clusters is not precedented. Zero-field Mössbauer parameters for all clusters are reported. Isomer shift considerations suggest the formulation [Mo3+Fe2+2Fe3+S4] for reduced clusters. Voltammetry of 3 and 4 reveals four-member electron transfer series encompassing the oxidation levels [MoFe3S4]4+,3+,2+,+ in the potential interval +1.0 to −1.3 V vs SCE in dichloromethane. Compared to the clusters with monoanionic ligands at the iron sites, phosphine ligation shifts redox potentials to more positive values. This effect arises from reduction of cluster negative charge and the 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, indicating that the phosphine is the reductant. This result implies a similar function of tertiary phosphines in the synthesis of 3 and 5−7. (Cl4cat = tetrachlorocatecholate(2−); Meida = N-methyliminodiacetate(2−).)