(158a) Investigation of Framework Effects On Epoxidation Catalysis in a Metal-Organic Framework

Metal-organic frameworks (MOFs) show great promise for combining the best features of homogeneous and heterogeneous catalysis. MOFs are nanoporous materials formed from self-assembly of metal ion vertices and organic linkers into regular crystalline structures. An appealing strategy for developing MOF catalysts is to use a homogeneous asymmetric catalyst as the organic linker [1], but immobilization of the catalysts in the crystal presents a challenge for preserving high enantioselectivity. Recently, a MOF was synthesized from a (salen)Mn catalyst and biphenyldicarboxylate as linkers with zinc paddlewheel corners [1]. The MOF performed enantioselective epoxidation catalysis with ee values only slightly lower than the homogeneous catalyst (82% ee vs. 88% ee). It is unknown whether electronic effects or steric effects of the MOF environment lead to the decrease in enantioselectivity. In this work, a combination of modeling techniques is used to decouple these effects and determine the dominating factor affecting the asymmetric induction of the catalyst in the MOF.

It is well known that the electronic nature of the 5,5'-substituents on the salen ligand strongly affects the enantioselectivity of the catalyst [2], and density functional theory (DFT) calculations have correlated the modified Hammett parameter σ⁺ of the 5,5'-substituents to reactivity properties [3]. To study the electronic effect of coordination of the 5,5'-substituents to zinc ions in the MOF structure on the enantioselectivity of the (salen)Mn catalyst, hybrid quantum mechanics/molecular mechanics (QM/MM) calculations were performed first to validate these methods for investigating electronic effects of salen ligands and second to determine the electronic effects of the ligand coordination in the MOF. From these calculations, it can be shown whether the coordination geometry in the MOF has a significant effect on the catalysis.

The direction in which the reactant approaches the active site is also believed to play an important role in the asymmetric induction of the (salen)Mn catalyst [4]. To understand the role of the MOF in guiding the reactant to the active site, classical optimizations and molecular dynamics simulations have been performed. These calculations reveal the preferred approach to the homogeneous catalyst and to the catalyst in the MOF. They also provide information about the preferred location of the reactant molecules within in the pores of the MOF. With the insight gained from these calculations as well as from the QM/MM calculations, more effective (salen)Mn MOF catalysts can be designed.

References

[1] S.-H. Cho, B. Ma, S. T. Nguyen, J. T. Hupp and T. E. Albrecht-Schmitt, Chem. Commun., 2563 (2006).

[2] E. N. Jacobsen, W. Zhang and M. L. Guler, J. Am. Chem. Soc. 113, 6703 (1991).

[3] L. Cavallo and H. Jacobsen, J. Org. Chem. 68, 6202 (2003).

[4] T. Katsuki, Adv. Synth. Catal. 344, 131 (2002).