SYNTHESIS, CHARACTERIZATION, AND REACTIVITY OF NEW CLUSTERS THAT CONTAIN THE [MFE3S4]0 CORE, M = MO, W - A WEAKLY PERTURBED [MFE3S4]0 UNIT STRUCTURALLY AND ELECTRONICALLY ANALOGOUS TO THE REDUCED 3-IRON CENTERS IN FERREDOXINS

The synthesis, characterization, and reactions of the new "cubane"-type clusters (Et4N)3[Fe3S4L3M(CO)3], M = Mo, L = EtS-, BzS-, p-OMePhS-, PhO-, Cl-; M = W, L = p-OMePhS-, are described in detail. These clusters are obtained in a good yield as (Et4N)+ salts, by the reaction of the [Fe3S4L4]3- linear trimer with M(CO)3(MeCN)3, M = Mo, W. The (Et4N)3[Fe3S4(EtS)3Mo(CO)3].CH3CN cluster crystallizes in the Pbcm space group with cell dimension a = 16.557 angstrom, b = 18.915 angstrom, c = 17.110 angstrom; alpha = beta = gamma = 90-degrees, z = 4. Intensity data were collected using a four-circle computer-controlled diffractometer. Refinement of 198 parameters on 1290 data, (F(o)2 > 3-sigma(F(o)1), converged to a final R(w) value of 0.065. The structure of the cubic [Fe3S4Mo]0 core shows the Mo(CO)3 fragment interacting weakly with the Fe3S4 ligand with a Mo-S bond length of 2.63 (1) angstrom and a Fe-Mo distance of 3.274 (5) angstrom. Electrochemical measurements on the [Fe3S4L3M(CO)3]3-complexes in CH2Cl2 show two reversible reduction waves that are assigned for the 2-/3- and 3-/4- redox couples. The H-1 NMR spectra of the complexes exhibit isotropically shifted resonances. The Moessbauer spectra of the complexes show broad doublets. The zero field spectra of (Et4N)3[Fe3S4(EtS)3M(CO)3] and (Et4N)3[Fe3S4(BZS)3M(CO)3] at 4.2 K show structure and can be fit with two or three component doublets in a 2:1 or 1:1:1 ratios. The magnetically perturbed spectra, recorded in an applied field of 4.0 and 6.0 T, can be interpreted in terms of a model where a ferromagnetically coupled (S1,2 = 9/2) Fe2+/Fe3+ pair couples antiferromagnetically with the S = 5/2 Fe3+ Site to give an S = 2 ground state. The fine and hyperfine parameters, obtained by simulations of the Moessbauer spectra, are remarkably similar to parameters reported previously for the Fe3S4 centers in Desulfovibrio gigas Fd II and aconitase. The oxidative decarbonylation reactions of the (Et4N)3-[Fe3S4L3Mo(CO)3] with quinone afford the solvated single cubane (Et4N)2[Fe3S4Cl3Mo(MeCN)(cat)] when the terminal ligand on the iron site is Cl- and the doubly-bridged double-cubane (Et4N)4[Fe3S4(EtS)3Mo(cat)]2 when the terminal ligand is EtS-. The reaction of (Et4N)3[Fe3S4(EtS)3Mo(CO)3] with diethyl disulfide afforded the triply-bridged double-cubane cluster (Et4N)3[Fe6S8(EtS)9Mo2] in moderate yield. Upon heating in solution the (Et4N)3[Fe3S4L3Mo(CO)3] Clusters (L = Cl-, p-COMePhO-) are converted to the corresponding, prismane-adduct, octanuclear clusters (Et4N)4[Fe6S6(L)9{Mo(CO)3}2]. In CH3CN solution the Mo(CO)3 fragment in the [Fe3S4(EtS)3Mo(CO)3]3- cluster can be replaced by FeCl+. In this reaction a statistical mixture of [Fe4S4(Cl)x(EtS)4-x]2- clusters is obtained, as indicated by H-1 NMR spectroscopy.