Milstein and co-workers have reported that the pincer complexes trans-[Ru(H)2(PNN)(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru-H bonds to produce the six-coordinate trans-[Ru(OCH(OMe)2)(H)(PNN)(CO)] intermediate and (ii) a metal-ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH(OMe)2]- anion along with cleavage of a C-OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru(H)(PNN)(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru(H)2(PNN)(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH(OMe)2]- anion in which the C-H bond is facing the metal center, (ii) reorientation of the [OCH(OMe) 2]- anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C-OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and trans-[Ru(H)(OMe)(PNN)(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal-ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru(H)2(PNN)(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru(H)(OCH(OMe)2)(PNN)(CO)] and [Ru(H)(OCH 2OMe)(PNN)(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry