Application of a Universal Force Field to Mixed Fe/Mo−S/Se Cubane and Heterocubane Clusters. 1. Substitution of Sulfur by Selenium in the Series [Fe₄X₄(YCH₃)₄]^2-; X = S/Se and Y = S/Se^†

A series of Fe−S and Fe−Se cubane clusters containing all four combinations of the general formula [Fe₄X₄(Y−CH₃)₄]^2- (X = S/Se, Y = S/Se) is investigated with FTIR and Raman spectroscopy. The terminally selenolate coordinated clusters (Y = Se) are prepared by a new synthetic route. All four cluster compounds are structurally characterized by X-ray single-crystal structure determination. Infrared and Raman spectra of all compounds are presented and interpreted with normal coordinate analysis. The corresponding force fields are based on that developed for the Fe₄S₄−benzyl cluster (Czernuszewicz, R. S.; Macor, K. A.; Johnson, M. K.; Gewirth, A.; Spiro, T. G. J. Am. Chem. Soc. 1987, 109, 7178−7187). An empirical procedure is presented to convert Fe−S into Fe−Se force constants. Only minor changes in force constants are found upon S → Se exchange, reflecting the similarity of the Fe−S and Fe−Se bonds. The drastic frequency shifts in the metal−ligand region observed upon substitution of sulfur by selenium are, therefore, primarily due to the corresponding mass changes.