Insertion of CO into metal alkyl bonds is a key step in important industrial processes like hydroformylation and methanol carbonylation as well as many transformations used in the synthesis of fine chemicals. Polar solvents and polar additives (e.g. phosphine oxides) have long been known to promote CO insertion but the mechanism of this process was not understood. We recently conducted a DFT-based study of the CO insertion reaction of Mn(CO)5Bz which offers the first such mechanistic explanation, particularly one that is consistent with results of detailed kinetic studies by Halpern. Specifically, our calculations indicated that CO insertion actually occurs, unassisted, quite rapidly; however, it forms, reversibly, an agostic acyl complex which does not readily react with traps such as phosphines. The polar molecules, however, were calculated to displace the agostic C-H bond, and they are then displaced by the acyl O atom to give an η2-acyl complex. The ability of the polar molecules to displace the agostic C-H bond, although the polar molecules are much poorer ligands than the traps, is the key to this mechanism. The calculations indicate that this key step proceeds through an interaction with the agostic C-H bond which has a significant hydrogen-bonding component. Mn(CO)5Bz and CpMo(CO)3CH3 behave similarly. We are conducting kinetic isotope effect experiments on the Mo system to shed light on the role of the polar additives and the nature of their proposed reaction with the agostic intermediate.

Kinetic studies on the effect of polar additives on CO insertion in CpMo(CO)3CH3

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.