Modeling intermediates in carbon monoxide coupling reactions using cyclooctatetraene thorium derivatives

Abstract

The interaction of carbon monoxide with organoactinides has recently been shown experimentally, particularly by Cloke and co-workers, to result in coupling to give the oligomeric anions C_nO_n²⁻ (n = 2, 3, 4). In order to model possible intermediates in reactions of this type, we have used density functional theory to explore the systems (C₈H₈)Th(CO)_n (n = 1 to 5) and (C₈H₈)₂Th₂(CO)_n (n = 2 to 7) related to the known “thorocene”, (η⁸-C₈H₈)₂Th. Thorium was chosen as the actinide for this work since its chemistry almost entirely involves the single diamagnetic +4 oxidation state. All of the binuclear (C₈H₈)₂Th₂(CO)_n structures found in this work have long Th⋯Th distances ranging from 4.4 to 5.0 Å suggesting the absence of direct Th–Th bonds. Two (C₈H₈)₂Th₂(CO)₂ isomers of similar energies in which the two CO groups have coupled to form trans and cis isomers of a bridging η⁴-μ-C₂O₂ ligand are low energy structures. These bridging η⁴-μ-C₂O₂ ligands exhibit ultralow ν(CO) frequencies around 1000 cm⁻¹ indicating strong back donation of thorium d and f electrons into C–O antibonding orbitals. Most of the carbonyl richer (C₈H₈)₂Th₂(CO)_n (n = 3 to 7) structures are derived from one of these basic (C₈H₈)₂Th₂(CO)₂ structures by addition of terminal CO groups. An exception is the lowest energy (C₈H₈)₂Th₂(CO)₄ structure which has C_4v symmetry with four equivalent separate η²-μ-CO groups bridging the thorium atoms. The thermochemistry of these systems suggest (C₈H₈)Th(CO)₄ and (C₈H₈)₂Th₂(CO)_n (n = 2, 4) to be the most promising synthetic objectives, which are potentially obtainable by reductive carbonylation of the known (C₈H₈)ThCl₂.