Synthesis and crystal structure of [Co(CO)4(H7Si8O12)]. A new type of monosubstituted octanuclear silasesquioxane with a silicon–cobalt bond

Abstract

Reaction of octacarbonyldicobalt [Co₂(CO)₈] with octahydrosilasesquioxane H₈Si₈O₁₂ in toluene leads to [Co(CO)₄(H₇Si₈O₁₂)], a new monosubstituted, octanuclear silasesquioxane with a silicon–cobalt bond. The product has been analysed by ¹H, ¹³C, ²⁹Si NMR and IR spectroscopies, mass spectrometry and microanalysis. The crystal structure of [Co(CO)₄(H₇Si₈O₁₂)] has been determined by single-crystal X-ray diffraction and is compared with the structures of H₈Si₈O₁₂, [Co(CO)₄(SiCl₃)] and [Co(CO)₄(SiF₃)]. The H₇Si₈O₁₂ fragment exhibits structural distortions consistently around, and originating from the Si atom to which the Co(CO)₄ fragment binds. The remaining part of the cage essentially retains C₃ symmetry, but is, nevertheless, in principle closer to the ideal O_h symmetry for this type of cage, than is crystalline H₈Si₈O₁₂, for which a non-crystallographic molecular symmetry of T_h is present. The comparison of the structures of the Co(CO)₄ fragments shows that these are consistently distorted and deviate from ideal C_3v symmetry. The distortions are essentially of the same kind in the three compounds and differ only in magnitude. The Si–Co distance in [Co(CO)₄(H₇Si₈O₁₂)] is 2.285 Å. The experimental findings have been compared with extended-Hückel molecular orbital calculations. Interaction between the Co(d_z²) and the Si(P_z) orbitals leads to the bond between the two fragments. Overlap population analysis indicates small but significant bonding interaction between Si and C_eq and antibonding interaction between the Co and the nearest O(Si). Fractional molecular orbital analysis indicates that the electronic structure of [Co(CO)₄(H₇Si₈O₁₂)] consists of three parts: orbitals belonging only to one of the two fragments and orbitals shared by both fragments. The highest-occupied molecular orbital (HOMO) of [Co(CO)₄(H₇Si₈O₁₂)] consists of oxygen lone pairs localised on H₇Si₈O₁₂. The lowest-unoccupied molecular orbital (LUMO) is identical with the LUMO of Co(CO)₄ and the first electronic transitions observed in the near UV are of H₇Si₈O₁₂(oxygen lone pair) to Co(CO)₄ fragment charge-transfer type.