Catalysis by a zinc-porphyrin-based metal-organic framework: From theory to computational design

Catalytic metal-organic frameworks (MOFs) have captured widespread attention for displaying shape, size, chemical, and enantiomeric selectivity characteristic of biological enzymes. Nguyen and co-workers [J. Am. Chem. Soc.2009, 131, 4204-4205] synthesized a stable, crystalline, microporous, zinc-porphyrin-based metal-organic framework (ZnPO-MOF) that incorporates the catalytic activity of metalloporphyrins. They observed a 236-fold initial acceleration of an acyl-transfer reaction between 3-pyridylcarbinol and N-acetylimidazole by ZnPO-MOF, and attributed this catalysis primarily to a high local concentration or "preconcentration" of reactants at the porphyrin-Zn active sites of the framework. We report a detailed theoretical investigation of the framework-promoted reaction using the full atomic structure of the MOF. The three-step first-principles-based method developed for this purpose can be applied to study other catalytically active MOFs, and can help in the computational design of frameworks with optimum preconcentration and chemical sensing properties. Calculations show that reactants bind to the closely packed porphyrin-Zn sites of ZnPO-MOF, resulting in a high local concentration of catalyst-bound reactant. Our model predicts that reactant preconcentration in ZnPO-MOF can increase the initial rate of the acyl-transfer reaction by about 2 orders of magnitude relative to the uncatalyzed reaction, and can be the dominant catalytic mechanism in this system.