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)3(µ3-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)2(µ3-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.