Hydrogenation of Dimethyl Carbonate to Methanol by <i>trans</i>-[Ru(H)<sub>2</sub>(PNN)(CO)] Catalysts: DFT Evidence for Ion-Pair-Mediated
Metathesis Paths for C–OMe Bond Cleavage
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Abstract
Milstein and co-workers have reported
that the pincer complexes <i>trans</i>-[Ru(H)<sub>2</sub>(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 <i>trans</i>-[Ru(OCH(OMe)<sub>2</sub>)(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)<sub>2</sub>]<sup>−</sup> 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)<sub>2</sub>(PNN)(CO)] to
the carbonyl of dimethyl carbonate to give an ion pair of the cationic
metal fragment and the [OCH(OMe)<sub>2</sub>]<sup>−</sup> anion
in which the C–H bond is facing the metal center, (ii) reorientation
of the [OCH(OMe)<sub>2</sub>]<sup>−</sup> 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 <i>trans</i>-[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)<sub>2</sub>(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)<sub>2</sub>)(PNN)(CO)]
and [Ru(H)(OCH<sub>2</sub>OMe)(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