DFT study of the reactivity of methane and dioxygen with d10-L2M complexes

A density functional theory analysis of the reactions of methane and O2 with d10-L2M complexes (M = Pd, Pt; L = N-heterocyclic carbene (NHC), PMe3) is presented. Computations suggest that reaction of L2M0 with O2/CH4 to form cis- (L)2M(OOH)(CH3) is only slightly uphill (ΔG ∼ 10–11 kcal/mol). Based on calculated thermodynamics, reaction of (L)2Pt0 with CH4 and O2 is predicted to be more favorable by first addition of CH4 and then reaction of O2 with the resulting methyl–hydrido complex. However, oxidative addition for either the C–H bond of methane or of O2 to d10-L2M complexes is kinetically prohibitive. If barriers to oxidative addition to d10-L2M systems could be reduced, conversion of L2M(H)(CH3) to L2M(OOH)(CH3) via hydrogen atom abstract/radical rebound is calculated to be thermodynamically and kinetically feasible, particularly for NHC and Pd. As (NHC)2Pd also has a lower free energy to methane C–H oxidative addition than does (NHC)2Pt, the former combination would appear to be a promising starting point in the search for catalysts for partial hydrocarbon oxidation.

The present research is a modest first step to unravel what are assuredly complicated pathways by which low-valent organometallics may undergo aerobic oxidation. A DFT analysis of the reaction of methane and O2 with d10-L2M complexes (M = Pd, Pt; L = NHC, PMe3) is presented.Figure optionsDownload as PowerPoint slideHighlights
► Reactions of methane and O2 with d10-L2M complexes are only slightly endergonic.
► Reaction of L2Pt0 is favored by addition of CH4 and then reaction with O2.
► Oxidative addition of the methane C–H bond or of O2 to d10-L2M is prohibitive.
► Conversion of L2M(H)(CH3) to L2M(OOH)(CH3) is feasible, especially for NHC and Pd.
► (NHC)2Pd is a promising starting point for hydrocarbon oxidation catalysis.