SYNTHESIS AND CHARACTERIZATION OF THE IRON SULFUR CLUSTER COMPOUNDS [FE4S5(MECP)4](PF6)Y (Y = 0-2) - X-RAY STRUCTURE DETERMINATION OF [FE4S5(MECP)4](PF6)

The series of iron-sulfur cluster compounds [Fe4S5(MeCp)4](PF6)y (y = 0-2) is readily prepared by reaction of [Fe(MeCp)(CO)2]2 with an excess of sulfur in refluxing toluene followed by a number of suitable synthesis and purification steps. The structure of [Fe4S5(MeCp)4](PF6) was determined crystallographically. [Fe4S5(MeCp)4](PF6) crystallizes in the triclinic system, space group P1BAR, with cell dimensions a = 9.876 (1) angstrom, b = 10.683 (1) angstrom, c = 14.642 (1) angstrom, alpha = 87.86 (3)-degrees, beta = 108.67 (2)-degrees, gamma = 90.66 (2)-degrees, V = 1462.4 (5) angstrom 3, and Z = 2. Refinement of 388 parameters on 5127 "observed" reflections gave a final R value of 0.054 (R(w) = 0.078). The structure consists of an asymmetric Fe4S5 core with each iron involved in three Fe-S bonds. The disulfur group in the iron-sulfur core is mu(3)-bonded with one sulfur atom coordinating to two iron atoms and the other sulfur atom to a third iron atom. This coordination mode differs from coordination mode of the S2 group in the related compound [Fe4S5(Cp)4]-[MoOCl4(THF)], which is side-on bonded to one iron atom and end-on bonded to two iron atoms. From H-1 NMR spectra recorded at several temperatures in the range 208-302 K and H-1 COSY spectra recorded at 208 K it is concluded that in acetone-d6 solution the iron-sulfur core of the [Fe4S5(MeCp)4]2+ ion is rigid with the disulfur group coordinating two neighboring iron atoms in an equivalent way and a third iron atom in a unique way. The H-1 NMR spectrum of [Fe4S5(MeCp)4]0 recorded in CDCl3 at 319 K indicates that the iron-sulfur cluster is fluxional with the S2 group rotating around a pseudo-3-fold axis. The electrochemical behavior of the compounds [Fe4S4(MeCp)4](PF6), [Fe4S5(MeCp)4](PF6)2, and Fe4S6(MeCp)4 is very similar to the electrochemical behavior of the related compounds [Fe4S4(Cp)4](PF6), [Fe4S5(Cp)4](PF6)2, and Fe4S6(Cp)4. As expected because of the electron-releasing properties of the methyl groups, the redox potentials have shifted slightly in negative direction.