Theoretical investigation of [2.2]paracyclophane as a donor toward a Cr(CO)3 group

A comparative molecular orbital study of 2.2]paracyclophane and simple arenes as ligands toward a Cr(CO)₃ group was performed with the aim of accounting for the observed coordination patterns. While the inter-ring repulsion is an important factor in 2.2]paracyclophane activation, it is not the only one. The molecular orbitals of two arene rings stacked parallel to each other were analyzed in some detail. The inward hybridization (toward the other ring) of the (arene)₂ HOMO was shown to reduce the strength of consequent bonding with the Cr(CO)₃ is fragment. The overall stabilization of 2.2]paracyclophane complex with Cr(CO)₃ is enhanced by a reduction of the inter-ring repulsion and strengthening of the Ar−Cr bond, and reduced by weakening of the intra-ring carbon-carbon bonds. The inter-ring repulsion increases with approach of the arenes to each other, as appears to happen in the structure of 2.2]paracyclophane complex with Cr(CO)₃. This explains the high donor ability of the free ring of the (arene)₂Cr(CO)₃ complex toward another Cr(CO)₃ fragment. It was proposed that the stabilization of the 2.2]paracyclophane complex with Cr(CO)₃ results ultimately from the relaxation of the strained framework of 2.2]paracyclophane. The kinetic factor in Cr(CO)₃ complexation was also studied by analyzing the charges on competing arene rings in monoaryl-substituted derivatives of 2.2]paracyclophanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 151–157, January, 1998.