[(Me3P)(3)Co(Bcat)(3)]: Equilibrium Oxidative Addition of a B-B Bond and Interconversion between the fac-Tris-Boryl and the mer-Tris-Boryl Complex

摘要

The tris-boryl complex [(Me3P)(3)Co(Bcat)(3)], obtained by oxidative addition of B(2)cat(2) to [(Me3P)(4)Co-Me] via the mono-boryl complex [(Me3P)(4)Co-Bcat], exhibits unique structural and electronic properties. While it is formed in an equilibrium reaction (Delta G approximate to 0 kJ mol(-1)) from the intermediate mono-boryl complex [(Me3P)(4)Co-Bcat], it is isolated in the solid state, depending on the temperature of crystallization, as the fac or mer isomer. Further studies reveal that the fac and the mer isomers of [(Me3P)(3)Co(Bcat)(3)] are comparable in Gibbs energy and their interconversion is thermodynamically feasible. The mer isomer, comprising an unexpected trans B-Co-B motif, was further studied by means of an AIM analysis of the computed (DFT) and experimental electron densities, giving evidence for appreciable B center dot center dot center dot B interactions between two of the boryl ligands. The complex mer-[(Me3P)(3)Co(Bcat)(3)], featuring three distinct boryl ligands, may, as a limiting case, be described as a diborane(4) complex of [(Me3P)(3)Co-Bcat]. DFT computations suggest that those B center dot center dot center dot B interactions are crucial for the stability of the trans B-Co-B complex mer-[(Me3P)(3)Co(Bcat)(3)].