Figure 3. Cumulative moles of CO2 formed normalized by the total iron content (left axis) and per Fe3+ open-metal site (right axis) with increasing activation temperature (423 - 523 K). Reaction conditions: 473 K, 14.5 kPa N2O, 1.5 kPa CH4, t = 2 h, activated at temperature for 12 h in He.
Gagliardi, Snurr, and co-workers have reported density functional theory (DFT)-derived energy profiles for the decomposition of N2O over M2+ sites to form M(IV)=O species;39,40,53 crucially, calculations reported by Barona and Snurr suggest the potential for M3+coordinatively unsaturated sites situated in trimeric MOF nodes to participate in methane oxidation with N2O.54 For example, a V3+ open-metal site in the (Ni2+)(V3+)2mixed-metal node was proposed to be oxidized by N2O to Ni2+(V4+=O)V4+, with both of the trivalent vanadium centers donating electrons to form the oxo intermediate. N2O activation barriers over Fe3+ open-metal sites in [(Cr2+)(Fe3+)2] and [(Mn2+)(Fe3+)2] mixed-metal nodes (144 and 146 kJ mol-1, respectively) were found to be comparable to those predicted for Fe2+ open-metal sites on an (Fe2+)(Fe3+)2 node (146 kJ mol-1). The involvement of Fe3+ sites in methane oxidation may proceed through the involvement of metal-oxo bond formation steps of such a nature, and a fuller understanding of these steps may require knowledge of changes in formal oxidation states over the entire trimer rather than merely one of three metal atoms constituting the node.