SELENIUM SUBSTITUTION IN [FE4S4(SR)4]2- - SYNTHESIS AND COMPARATIVE PROPERTIES OF [FE4X4(YC6H5)4]2-(X, Y = S, SE) AND STRUCTURE OF [(CH3)4N]2[FE4SE4(SC6H5)4]

By a combination of direct synthesis and ligand-exchange reactions the series of tetranuclear dianions [Fe4X4 (YPh)4]2- (X, Y = S, Se) has been synthesized. The series consists of [Fe4S4 (SPh)4]2-, [Fe4S4 (SePh)4]2-, [Fe4Se4 (SPh)4]2-, and [Fe4Se4 (SePh)4]2-, in which sets of four equivalent sulfur and selenium atoms act as terminal and bridging ligands. Properties of series members were compared in order to examine the consequence of S/Se substitution. The cubane-type geometry previously established for [Fe4S4 (SPh)4]2- has been confirmed for [Fe4Se4 (SPh)4]2- as its Me4N salt. This compound crystallizes in space group P212121 of the orthorhombic system with dimensions a = 11.930 (3) Å, b = 24.126 (5) Å, and c = 14.782 (4) Å (Z = 4). The Fe4Se4 core is distorted toward D2d symmetry with four short (2.385 (2) Å) and eight long (2.417@r@n (5) Å) Fe-Se distances, giving an average of 2.406 Å; the mean Fe— Fe distance is 2.783 Å. Differences in the core structures of [Fe4S4 (SPh)4]2- and [Fe4Se4 (SPh)4]2- are small and fully accountable in terms of the larger covalent radius of and the smaller angles generally formed by selenium. Compared to [Fe4S4 (SPh)4]2- selenium-containing complexes exhibit red-shifted charge-transfer spectra and small positive shifts of potentials for the couples [Fe4X4 (YPh)4]2-/3-. EPR spectra of trianions are axial and indicate equivalent ground-state electronic structures for all members of the series. The collective comparative properties of the series confirm selenium as a fully functional substitute for sulfur and support the possibility that selenium could be found in protein 4-Fe centers. © 1978, American Chemical Society. All rights reserved.