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

Abstract

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 oxynickel 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 = −NH 2 , −OCH 3 , −CH 3 , −H, −F, and −NO 2 ) where the substituents were chosen to present similar steric demand, but varying electron-donating or electron-withdrawing properties. Rbenzoate-engendered shifts in the node-based aqua O−H stretching frequency for NU-1000, as measured by DRIFTS (diffusereflectance infrared Fourier-transform spectroscopy), together with systematic shifts in Ni 2p peak energies, as measured by Xray 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.

Pyrene-based metal organic frameworks: from synthesis to applications

“…In the case of NU-1000, SALI approach takes advantage of the ability of the Zr 6 nodes to be altered through the reaction between the free Zr-OH moieties (labile -OH and H 2 O ligands) and introduced nonstructural organic ligands. 90 The functional ligands may be carboxylic acid containing molecules, [265][266][267][268][269] phosphonic acid containing molecules, 270 or perfluoroalkane chains, 271 in which their addition in NU-1000 results in enhanced activity in different catalytic reactions. Moreover, the further modification of the ligands with varying electron-donating or electron-withdrawing groups can be performed to tune the electronic environment provided by the Zr 6 nodes.…”

“…Moreover, the further modification of the ligands with varying electron-donating or electron-withdrawing groups can be performed to tune the electronic environment provided by the Zr 6 nodes. 267 SALI-modification of NU-1000 can be performed with an organic ligand, 268 or a metal complex featuring an organic ligand. 269 NU-1000 functionalized by 5,5di-thio-bis(2-nitrobenzoic acid) ligand (DTNB@NU-1000) was found to be catalytically active for the degradation and subsequent detection of a nerve agent, which was observed by the release of the chromophore degradation product by visual inspection of filtered samples.…”

Hammett Relationship in Oxidase‐Mimicking Metal–Organic Frameworks Revealed through a Protein‐Engineering‐Inspired Strategy

“…Although the variations for some substituents were small, the overall trend illustrated that the presence of an electron‐withdrawing group, especially a nitro group—the strongest typical electron‐withdrawing group, clearly shifted the binding energy to a lager value, agreeing well with a previous report. [ 13 ] When using Hammett's σ m values to describe the electronic effects of these substituted ligands, an H‐SALR was identified (Figure 2c), which is a typical one in the field of quantitative structure–activity relationship. The positive correlation indicated that a negative charge developed during the rate‐limiting step of the reaction.…”

Mixed‐Metal MOF‐74 Templated Catalysts for Efficient Carbon Dioxide Capture and Methanation

“…[ 5 ] Atomic layer deposition has been used to control the formation of nanoparticles within MOF pores. [ 6 ] However, this approach requires specialized equipment and expensive volatile metal precursors. Alternately, MOFs have been grown directly on metallic nanoparticles, leading to core–shell structures.…”