Sulfur and carbon as heteroatoms in ferrathiacarboranes

Abstract

The complete series of ferrathiacarboranes CpFeCHSBn−3Hn−3 (n=8, 9, 10, 11, 12) related to the experimentally known CpFeC[B(OH)2]SB8H8 and CpFeC(NHCMe3)SB9H9 have been investigated by density functional theory. Deltahedral structures having the iron atoms located at degree 5 or 6 vertices, the carbon and sulfur atoms located at degree 4 vertices, and Fe–S edges are found to be energetically preferred. In general the most spherical closo deltahedra found in the deltahedral borane anions BnHn2− are energetically preferred as central FeCSBn−3 polyhedra in these CpFeCHSBn−3Hn−3 structures. However, for the 9-vertex CpFeCHSB6H6 system, structures having a central FeCSB6isocloso deltahedron with the iron atom located at the unique degree 6 vertex and the sulfur and carbon atoms located at adjacent degree 4 vertices are comparable in energy with closo structures having a central FeCSB6closo tricapped trigonal prism. For the 11-vertex CpFeCHSB8H8 system the unique closo deltahedral structure having the iron atom located at the unique degree 6 vertex and the carbon and sulfur atoms located at the two degree 4 vertices adjacent to the iron atom is particularly favorable since it lies ∼14kcal/mol in energy below the next lowest energy CpFeCHSB8H8 isomer. The central FeCSBn−3 deltahedra found experimentally by X-ray crystallography for the ferrathiacarboranes CpFeC[B(OH)2]SB8H8 and CpFeC(NHCMe3)SB9H9 correspond to the lowest energy CpFeCHSB8H8 and CpFeCHSB9H9 structures, respectively, with similar Fe–S, Fe–C, and Fe–B edge lengths.