Abstract:
The CO2 insertion reactions of [L4Ru(eta(2)-CH2C6H4)] (1), [L4Ru(eta(2)-OC6H3Me)] (2), and [L4Ru(eta(2)-NHC6H4)] (3), where L = PH3 and PMe3, are modeled using density functional theory methods. In 1 and 2, the metal-assisted CO2 insertion occurs because of the favorable initial axial phosphine dissociation mechanism, whereas in 3, the ligand (NHC6H4)-assisted mechanism operates (DeltaG(⧧) = +19.0 kcal/mol), wherein the nucleophilic affinity of the -NHC6H4 moiety aids the CO2 insertion process. The modeled mechanisms are consistent with the experimental findings by Hartwig et al. (J. Am. Chem. Soc, 1991, 113, 6499), in which the rate of the reactions of 1 and 2 depends on the added phosphine concentration, whereas the rate of the reaction of 3 is independent of the added phosphine concentration. In 1 and 2, CO2 is preferably inserted into the Ru-Caryl bond rather than the competitive Ru-CH2 and Ru-O bonds, respectively. In 1, the pi-type orbital interaction of the aryl ring with the metal center is found to stabilize the transition state for Ru-Caryl bond insertion (DeltaG(⧧) = +25.7 kcal/mol). In 2, the Ru-Caryl insertion (DeltaG(⧧) = +23.0 kcal/mol) is thermodynamically preferred, while the kinetically preferred Ru-O bond insertion (DeltaG(⧧) = +17.4 kcal/mol) is highly reversible. The more electron-donating and sterically bulky PMe3 facilitates the CO2 insertion of 1 and 2 because the initial dissociation of axial PMe3 is easier than that of PH3 by ca. +11.0 kcal/mol, whereas in the case of 3, the effect of PMe3 slightly increases the DeltaG(⧧) value of 3. The increase in the nucleophilic affinity of amido nitrogen in 3 and the increase in the polarity of the solvent decrease the DeltaG(⧧) value of 3 by 48%. The inclusion of the chelating dimethylphosphinoethane ligand in 3 along with the electron-donating substituent at the -NHC6H4 moiety and the polar solvent further reduces the DeltaG(⧧) value of 3 by 62%, which demonstrates the role of the chelating ligand, electron-donating substituent, and polar solvent in the ligand-assisted CO2 insertion reactions.
