1.  Introduction
The selective, low-temperature oxidation of methane to methanol, if successfully accomplished, could enable valorization of vast reserves of shale gas resources becoming increasingly abundant in the United States and around the world. 1–3 High-valent metal-oxo complexes serve as promising active centers for low-temperature methane oxidation, and are exploited in a variety of biological and synthetic systems;4–8 for instance, iron(IV)-oxo centers have been long hypothesized as the key oxidizing species in non-heme biological complexes including R2 proteins of ribonucleotide reductase (RNR R2), Fe2+/α-ketoglutarate (αKG)-dependent hydroxylates, and soluble methane monooxygenase (sMMO) enzymes.9–14 Efforts aimed at investigating such active centers have for the most part been focused on iron-zeolites in the heterogeneous catalysis literature15–19 and homogenous complexes in the bioinorganic chemistry literature,20–23 with both classes of materials exhibiting unique limitations with respect to low temperature methane oxidation. Homogeneous complexes, on one hand, often display a propensity towards polynuclear aggregation, thereby limiting somewhat their use in catalysis applications.24–28 Iron-based zeolites, on the other hand, while ideal for investigating iron clusters that do not evolve significantly under reaction conditions, exhibit active site heterogeneity not only greater in degree than that exhibited by homogeneous complexes, but also to an extent that varies significantly with synthesis protocol, thermal treatment, and iron loading.29–31
Metal-organic framework materials (MOFs) potentially offer a solution to the challenge of synthesizing and evaluating materials that carry well-defined, structurally uniform metal-oxo moieties that remain isolated in nature subsequent to their involvement in catalytic redox cycles, with several copper and iron-containing MOFs having been evaluated for the oxidation of light alkanes including methane,32–35 ethane,36–38 and propane.39 Specifically, MIL-100 (MIL =Materials of Institut Lavoisier ) is a MOF that exhibits interesting properties in the partial oxidation of light alkanes.35,39–41 First discovered by Gérard Férey and coworkers, MIL-100 is comprised of trimetallic clusters [(M(III)33-O)] coordinated by trimesate linkers to form a porous structure featuring an MTN (Mobile Thirty Nine) topology (Scheme 1a).42 Removal of terminal ligands (H2O or X-) through thermal activation under inert or vacuum at temperatures below 523 K (Scheme 1b) creates unsaturated open-metal sites over mixed valence nodes [(M(II)M(III)23-O)].43–45The propensity of these nodes to convert methane to methanol at low temperatures (423
Scheme 1. (a) Structure of MIL-100 comprised of trimesic acid linkers and µ3-oxo centered trimer nodes. (b) Formation of M2+ and M3+ open-metal sites in MIL-100 through thermal activation resulting in the elimination of anionic ligands and coordinated water molecules, respectively.