(764g) Framework-Topology-Dependent Catalytic Activity of Zirconium-Based, Porphyrinic Metal-Organic Frameworks

One of the goals of rational catalyst design is to increase the rates at which reactions occur in catalytic systems. For suitable reactions, a possible strategy to achieve such rate increase is to design catalysts with active sites that are both abundant and spatially distributed in such fashion that concerted binding of reactants to neighboring sites facilitates the formation of a transition state or intermediate. Metal-organic frameworks (MOFs) are porous crystalline materials constituted by interconnected organic and inorganic building blocks,which can provide well-defined active sites. Because the spatial distribution of building blocks is a function of the underlying framework topology, the topological diversity of MOFs can be exploited to alter the concentration, position and orientation of catalytic sites to enhance catalytic performance. Metalated porphyrins are attractive catalytic sites due to their known ability to catalyze small molecule transformations.

To study to what extent the topological arrangement of immobilized porphyrin metal centers affects catalytic activity of an AB + C -> A + BC reaction, in this work we synthesized three MOFs interconnecting metalated porphyrins and zirconium oxoclusters—which are known to provide high MOF stability—into frameworks of three distinct topologies (ftw, csq, and scu) and tested their catalytic activity for the acyl transfer from N-acetylimidazole (NAI) to three different pyridylcarbinol (PC) regioisomers (2-PC, 3-PC, and 4-PC). The synthesized and tested catalytic MOFs correspond to the well-known MOF-525 and PCN-222 materials and the newly introduced NU-902. Collected kinetic data reveals rate dependence on both framework topology and PC regioisomerism, which is discussed in the light of molecular modeling results.

Initial reaction rates for 3-PC show that the immobilization of metalated porphyrins into a heterogeneous MOF catalyst increases the reaction rate 40- to 140-fold with respect to the uncatalyzed reaction and 20- to 70-fold with respect to the homogeneous catalyst. The enhancement with respect to the homogenous catalysts is due to pre-concentration (i.e. binding of NAI or PC to porphyrin centers) and orientation effects. Initial reaction rates for all PC/MOF combinations show a dependence on the MOF (topology) and the PC regioisomer. The differences for PC/MOF combinations arise from the different spatial arrangement of porphyrins in the three MOFs. Pre-concentration effects are found to be proportional to the density of Zn sites in each MOF and orientation effects to be proportional to the density of “suitable” Zn pairs. Suitable pairs for each MOF were determined based on MM calculations determining the strain of anchored intermediates. DFT calculations show that contrary to 3-PC and 4-PC, Zn-bound 2-PC cannot orient properly relative to Zn-bound NAI on suitable pairs as for its reaction rate to benefit from orientation effects. Thus, we demonstrated how a high density of catalytic sites with rationally selected spatial orientations can be provided with MOFs to facilitate the formation of relevant transition states and enhance reaction rates. We hope this will motivate exploiting the topological diversity of MOFs for multisite-catalyzed chemical reactions.

References

1. Deria, P., Gomez-Gualdron, D.A., Hod, I. Snurr., R.Q., Hupp, J.T., Farha, O.K. J. Am. Chem. Soc.138, 14449 (2016)