B-phenylated o-carboranes and its chromium derivatives: Synthesis, electrochemical properties, and X-ray structural studies

Abstract

The electron accepting capability of B-phenylated o-carborane is greatly improved through favorable electronic interactions between the two adjoining phenyl-π* and cage carbon-σ* orbitals. Phenyl substitution at the boron atom in the 3-position directs the two phenyl groups of the cage carbon atoms to a face-to-face position and locks them into a rigid conformation, which gives rise to extensive electronic delocalization and maximum energy stabilization from the lowest unoccupied molecular orbital (LUMO). A reversible reduction peak from the cyclovoltammogram (CV) at −1.72 V and a broad and extended UV absorption (λmax = 277 nm) substantiate such LUMO stabilization, thereby augmenting the electron accepting capability of o-carborane. Due to electron depletion from the adjacent o-carborane, the adjoined phenyl groups act as potential π-acid; indeed, when B-phenylated 1,2-diphenyl-o-carborane was reacted with Cr(CO)6, bi- and trimetallic chromium complexes were selectively formed in an η6-bonding fashion between the chromium atoms and phenyl groups.