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–Cformyl 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.