SYNTHESIS AND COMPARATIVE REACTIVITY AND ELECTRONIC STRUCTURAL FEATURES OF [MFE3S4]Z+ CUBANE-TYPE CLUSTERS (M = FE, CO, NI)

The heterometal cubane-type clusters [CoFe3S4(Smes)4]2- (7, 80%) and [NiFe3S4(PPh3)(SMes)3]2- (12, 57-71%) have been prepared in good yield as Et4N+ salts by reductive rearrangement reactions of the linear cluster [Fe3S4(Smes)4]3-(4, Smes = mesitylthiolate(1-)) with Co(I) and Ni(0) reactants, respectively. (Et4N)2[7] crystallizes in orthorhombic space group Pbcn with a = 20.673 (3) angstrom, b = 16.600 (3) angstrom, c = 17.259 (2) angstrom, and Z = 4. (Et4N)2[NiFe3S4(PPh3)(SMes)3].2MeCN was obtained in triclinic space group P1BAR with a = 13.138 (3) angstrom, b = 15.461 (4) angstrom, c = 19.622 (4) angstrom, alpha = 107.12 (2)-degrees, beta = 94.54 (2)-degrees, gamma = 108.47 (2)-degrees, and Z = 2. The crystal structures confirm the cubane-type structures and tetrahedral coordination at the M = Fe, Co, and Ni subsites of the [MFe3S4]z+ cores. In 7, the Co and Fe subsites are disordered and in 12 the phosphine ligand is bound to the Ni subsite. The clusters [NiFe3S4(SMes)4]3- (10) and [Fe4S4(Smes)4]2- (6) were obtained as an ca. 1:1 mixture by the reaction of Ni(AsPh3)4 and 4. Potential and actual synthetic routes to [MFe3S4]z+ clusters are outlined. The species 6, 7, 10 form a comparative set with equivalent structures and identical terminal ligands. These species and 12 are best distinguished by their H-1 NMR spectra which manifest contact-shifted resonances that are oppositely signed for substituents at the Fe and M = Co/Ni subsites. In 7, the Fe subsites appear to be more reactive to ligand substitution by thiol than is the Co subsite; both subsites are substituted in the ligand redistribution system 5/7. The three-member electron transfer series [CoFe3S4]3+/2+/1+ and [NiFe3S4]2+/1+/0 have been established. For the reversible couples [MFe3S4(SMes)4]2-/3- the order of potentials is M = Fe < Co (0.18 V) < Ni (0.30 V), with the indicated potential differences vs M = Fe. Mossbauer spectroscopy reveals that 7 and the protein-bound [CoFe3S4]2+ Cluster of D. gigas ferredoxin II have equivalent electronic structures at 4.2 K. As judged by isomer shifts at 1.5 K, the [NiFe3S4]1+ core of polycrystalline 12 contains three equivalent iron sites. However, the 4.2 K Mossbauer spectra obtained in strong applied magnetic fields show clearly that the three sites are magnetically distinct. Interestingly, the room temperature solution H-1 NMR data of 7 and 12 indicate equivalent sites. Isomer shifts imply the fragment formulations Co2+ (S = 3/2) + [Fe3S4]0 (S = 2) and Ni2+ (S = 1) + [Fe3S4]1- (S = 5/2), with antiparallel spin coupling affording the observed S = 1/2 and 3/2 ground states, respectively. Comparison of the isomer shifts of the [NiFe3S4]1+ core with those of other [MFe3S4]1+ cubanes (M = Fe, Zn, Cd) suggests a shifted electron density from the [Fe3S4]1- fragment to the nickel site. The close correspondence of Mossbauer and EPR parameters of synthetic clusters (7, 12) with those of protein [CoFe3S4]2+ and [NiFe3S4]1+clusters indicates that the latter contain the tightly bound cubane-type structures established by X-ray diffraction for the synthetic species.