Complete methane-to-methanol catalytic cycle: A DFT study of oxygen atom transfer from N2O to late-row (M=Ni, Cu, Zn) β-diketiminate C-H activation catalysts

DFT calculations indicate methane C-H activation and functionalization is thermodynamically and kinetically feasible using N2O as an oxidant and a beta-diketiminate (L-n) supported catalyst [M] = LnM, when the metal M = Ni, Cu or Zn. Further studies of previously investigated Ni-oxo methane functionalization systems, which undergo hydrogen atom abstraction (HAA), radical rebound (RR), and catalyst regeneration via N2O oxygen atom transfer (OAT) have yielded energetically accessible methane-to-methanol cycles. Methane activation and functionalization is found to be the highest rate limiting step for the cycle (Ni < Zn < Cu), with the HAA step restricting the [Ni] system, and RR has the highest barriers in the [Cu] and [Zn] systems. The entire methane to methanol transformation is endergonic for copper and more notably the zinc complex, leaving the exergonic nickel system as the more favorable metal among the systems modeled. Once methanol is displaced by N2O in the metal inner coordination sphere, [M]-N2O undergoes either monometallic or bimetallic OAT to regenerate the [M]=O active species. For both Ni and Cu, the latter pathway is most favorable, whilst as anticipated Zn systems struggle to reform the [M]=O active species. In terms of overall energetics, [Ni]=O species are promising targets not merely for HAA/RR, but for the entire methane-to-methanol cycle, although over oxidation of the methanol product is unfortunately also feasible. (C) 2012 Elsevier Ltd. All rights reserved.