Reductive Coupling of Carbon Monoxide by an Anionic Calcium Hydride: A Computational Mechanistic Study

We recently reported that a dimeric, anionic calcium hydride complex can perform the selective reduction and C-C coupling of carbon monoxide. Here, the mechanism of this reaction is investigated computationally. Stepwise coordination and reduction of CO is calculated, with the first molecule of CO being transformed into a Ca-bound formyl ligand. Subsequently, a second CO molecule coordinates to the same calcium center, and C-C bond formation proceeds via insertion of this second CO molecule into the Ca-C^formyl bond. This is in contrast to mechanisms reported for CO reduction with dimeric neutral and cationic Group 2 molecular hydrides, in which both Group 2 centers are involved in this key C-C bond-forming step. In a final step, the remaining hydride ligand located on the second calcium center is transferred to the newly formed CO-derived ligand, yielding a cis-ethenediolate unit, the single experimentally observed product. The cis selectivity can be explained by electrostatic repulsion in the pathway to the trans isomer. NBO/NLMO and energy decomposition analyses show that, in general, electrostatic interactions dominate the interaction between the CO-derived ligands and the calcium center.