Sulfated Zirconium Metal–Organic Frameworks as Well-Defined Supports for Enhancing Organometallic Catalysis

Understanding heterogeneous catalysts is a challenging
pursuit
due to surface site nonuniformity and aperiodicity in traditionally
used materials. One example is sulfated metal oxides, which function
as highly active catalysts and as supports for organometallic complexes.
These applications are due to traits such as acidity, ability to act
as a weakly coordinating ligand, and aptitude for promoting transformations
via radical cation intermediates. Research is ongoing about the structural
features of sulfated metal oxides that imbue the aforementioned properties,
such as sulfate geometry and coordination. To better understand these
materials, metal–organic frameworks (MOFs) have been targeted
as structurally defined analogues. Composed of inorganic nodes and
organic linkers, MOFs possess features such as high porosity and crystallinity,
which make them attractive for mechanistic studies of heterogeneous
catalysts. In this work, Zr6-based MOF NU-1000 is sulfated
and characterized using techniques such as single crystal X-ray diffraction
in addition to diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS). The dynamic nature of the sulfate binding motif is found
to transition from monodentate, to bidentate, to tridentate depending
on the degree of hydration, as supported by density functional theory
(DFT) calculations. Heightened Brønsted acidity compared to the
parent MOF was observed upon sulfation and probed through trimethylphosphine
oxide physisorption, ammonia sorption, in situ ammonia DRIFTS, and
DFT studies. With the support structure benchmarked, an organoiridium
complex was chemisorbed onto the sulfated MOF node, and the efficacy
of this supported catalyst was demonstrated for stoichiometric and
catalytic activation of benzene-d6 and
toluene with structure–activity relationships derived.