A density functional study of the structure and potential energy surface of dibenzenechromium and its alkylated derivatives

Abstract

Density functional calculations were performed with the nonempirically constructed PBE functional and TZ2p basis set to study the potential energy surfaces of several dibenzenechromium derivatives containing methy1 and tert-buty1 groups in aromatic ligands. The method was shown to correctly describe the structure and intramolecular dynamics of bis-arenechromium complexes. It allows barriers to rotation about the metal-ligand bond to be calculated and detailed information to be obtained about the shape of the potential curves that correspond to this rotation. The potential energy surfaces of polymethylated dibenzenechromium derivatives contained several minima close in energy; these minima largely corresponded to eclipsed conformers. The potential curves for rotation about the metal-ligand bond in dibenzenechromium and its methylated derivatives reveal the presence of staggered conformers as transition states. Irrespective of the number of methyl groups present in both ligands, we observed no substantial changes in barrier heights, which were about 1 kcal/mol. Conversely, the introduction of bulky tert-butyl groups destabilized eclipsed conformations. Ensuing steric strain caused substantial out-of-ring-plane displacements of the Cipso-t-Bu bonds away from the chromium atom, distorted aromatic ring planarity, and substantially increased barriers to rotation, to 8–10 kcal/mol.