Reductive coupling of carbon monoxide by U(iii) complexes—a computational study

Abstract

The role of U((η-C₈H₆{SiⁱPr₃-1,4}₂)(η-C₅Me₅) and U((η-C₈H₆{SiⁱPr₃-1,4}₂)(η-C₅Me₄H) in the reductive di- tri- and tetramerization of CO has been modelled using density functional methods and U(C₈H₈)(C₅H₅) as the metal fragment. The orbital structure of U(C₈H₈)(C₅H₅) is described. CO binding to form a monocarbonyl U(C₈H₈)(C₅H₅)(CO) is found, by a variety of methods, to place spin density on the CO ligand via back-bonding from the U5f orbitals. A possible pathway for formation of the yne diolate complex [U(C₈H₈)(C₅H₅)]₂C₂O₂ is proposed which involves dimerization of U(C₈H₈)(C₅H₅)CO via coordination of the CO O atoms to the opposing U atoms followed by C–C bond formation to form a zig-zag intermediate, stable at low temperatures. The intermediate then unfolds to form the yne diolate. The structures of [U(C₈H₈)(C₅H₅)]C₂O₂, the deltate complex [U(C₈H₈)(C₅H₅)]C₃O₃ and the squarate complex [U(C₈H₈)(C₅H₅)]C₄O₄ are optimized and provide good models for the experimental compounds. The reaction of further CO with a zig-zag intermediate to form deltate and squarate complexes was explored using Th(C₈H₈)(C₅H₅) as a model and low energy pathways are proposed.