Introducing Nonstructural Ligands to Zirconia-like Metal–Organic Framework Nodes To Tune the Activity of Node-Supported Nickel Catalysts for Ethylene Hydrogenation

Previous work has
shown that introduction of hexafluoroacetylacetone
(Facac) units as nonstructural ligands for the zirconia-like nodes
of the eight-connected metal–organic framework (MOF), NU-1000,
greatly alters the selectivity of node-supported oxy-nickel clusters
for ethylene dimerization vs oligomerization. Here we explore a related
concept: tuning of support/catalyst interactions, and therefore, catalyst
activity, via parallel installation of organic modifiers on the support
itself. As modifiers we focused on para-substituted
benzoates (R-BA–; R = −NH2, −OCH3, −CH3, −H, −F, and −NO2) where the substituents were chosen to present similar steric
demand, but varying electron-donating or electron-withdrawing properties.
R-benzoate-engendered shifts in the node-based aqua O–H stretching
frequency for NU-1000, as measured by DRIFTS (diffuse-reflectance
infrared Fourier-transform spectroscopy), together with systematic
shifts in Ni 2p peak energies, as measured by X-ray photoelectron
spectroscopy, show that the electronic properties of the support can
be modulated. The vibrational and electronic peak shifts correlate
with the putative electron-withdrawing vs electron-donating strength
of the para-substituted benzoate modifiers. Subsequent
installation of node-supported, oxy-Ni­(II) clusters for ethylene hydrogenation
yield a compelling correlation between log (catalyst turnover frequency)
and the electron donating or withdrawing character of the substituent
of the benzoate units. Single crystal X-ray diffraction measurements
reveal that each organic modifier makes use of only one of two available
carboxylate oxygens to accomplish grafting. The remaining oxygen atom
is, in principle, well positioned to coordinate directly to an installed
Ni­(II) ion. We postulate that the unanticipated direct coordination
of the catalyst by the node-modifier (rather than indirect modifier-based
tuning of support­(node)/catalyst electronic interactions) is the primary
source of the observed systematic tuning of hydrogenation activity.
We suggest, however, that regardless of mechanism for communication
with active-sites of MOF-supported catalysts, intentional elaboration
of nodes via grafted, nonstructural organic species could prove to
be a valuable general strategy for fine-tuning supported-catalyst
activity and/or selectivity.