Tuning Oxygen Occupancy within Metal-Oxo Clusters for Enhancing Catalytic Activity of Zr-MOFs

Metal organic frameworks containing Lewis acidic metals such as Zr(IV) are highly promising biomimetic catalysts, able to perform a variety of biologically inspired reactions in a range of different environments. By modulation of the building blocks of a parent MOF, structural changes can be induced that may result in increased catalytic activity. Typically, the organic components of the MOF are targeted for such modulation, through either removal of organic linkers or addition of functional groups. Modifying the inorganic component and transforming the nature of the inorganic cluster of the MOF in a controlled manner are more challenging and have been less explored as means of enhancing the catalytic activity of MOFs. Here, we demonstrate that tuning oxygen coordination within the metal-oxo cluster via postsynthetic exchange of Mg(II) into Zr₆O₈-MOF-808 in aqueous solution results in coordinatively unsaturated Zr(IV) metal sites with enhanced catalytic activity. Advanced structural characterization that includes quantitative extended X-ray absorption fine structure (EXAFS) analysis and detailed XRD Rietveld refinement, combined with computational studies based on DFT and DFT-based molecular dynamics, confirmed that single and double-substitution of Zr(IV) with Mg(II) occurred within the cluster core. This resulted in the formation of Zr₅MgO₇ and Zr₄Mg₂O₇ mixed-metal-oxo catalytic sites with decreased overall oxygen occupancy due to the lower coordination number of Mg(II) compared to Zr(IV). The effect of substitution on cluster symmetry and stability has been investigated, and the mechanism of postsynthetic exchange, the stability of resulting structures, and enhancements to catalysis are supported by an in-depth computational study. We show both experimentally and using computational techniques that such modifications promote MOF-substrate interaction and decrease the activation energy (E_act) for catalyzing biologically relevant reactions such as the hydrolysis of peptide bonds and dephosphorylation of amino acids. This work presents an innovative strategy for enhancement of MOFs' catalytic activity through controlled modulation of the inorganic cluster and demonstrates that the reduced oxygen occupancy of a cluster promotes more efficient catalysis.