Tailored Microporous Solids to Bridge the Gap Between Homogeneous and Heterogeneous Catalysis

SSCI-VIDE+ING+JEC; National audience; Heterogeneous catalysis allows to circumvent the problem of separation of the catalyst from the products and to simplify its recyclability. The integration of the catalytically active centers into a solid support without loss of performance compared to the homogeneous analog is still a major challenge. In this context, a molecularly defined support as macroligand, i.e. a solid acting like the ligand in the corresponding molecular complex, can be considered as a key to bridge the gap between molecular and heterogeneous catalysis. Metal-Organic Frameworks (MOF) and purely organic microporous polymers are promising candidates. In particular, porous frameworks made by the repetition of a coordinating motif, like the bipyridine motif are of a high interest as far as bipyridines are widely used as chelating ligand for molecular catalysts.[1,2]Here we present series of heterogeneous catalysts based on MOFs and microporous polymers used as macroligands for heterogenized organometallic complexes. As host for molecular Rh-based bypiridine catalysts, we prepared porous solids, which are bpy-containing MOF, namely UiO-67 and MOF-253, and four newly synthesized porous polymers. As shown in the Figure 1, these platforms present different Hammet constant at the meta-position of the bpy ligand. Figure 1. Representatives meta substituted 2,2'-bipyridine ligands embedded in MOF or CMP materials studied herein (σm = Hammett constant).We show that both homogeneous and heterogenized catalysts follow the same linear correlation between the electronic effect of the ligand, described by the Hammett parameter, and the catalytic activity (Figure 2). This correlation highlights the crucial impact of the local electronic environment surrounding the active catalytic center over the long-range framework structure of the porous support. The conception of highly efficient heterogeneous catalyst based on porous polymer support and driven by the Hammett parameter of bipyridine-chelating macroligand is demonstrated here for two Rh-catalyzed reactions: The photoreduction of carbon dioxide with turnover frequencies (TOF) up to 28 h-1,[1] among the highest reported for heterogeneous photocatalytic formate production and the transfer hydrogenation reaction of α-aryl ketones with TOFs of up to 3.3 h-1.[2] Figure 2. Linear correlation between TOF and electronic properties of the macroligand.The results obtained on two different reaction with six different heterogenous catalysts evidence the linear correlation between the activity of heterogenized RhCp*-based catalyst and the local electron density around this isolated active site. This linear relationship make it possible to predict the catalytic activity of such a system based on the local electronic contribution of the ligand, as described by the Hammett constant. The rational design of polymeric platforms envisioned as macroligands and guided by the Hammett parameter will enable the production of ever better-performing microporous solid-based heterogeneous catalysts. The gap between molecular and heterogeneous catalysis has never been so close to being bridged. References[1] F. M. Wisser, P. Berruyer, L. Cardenas, Y. Mohr, E. A. Quadrelli, A. Lesage, D. Farrusseng, J. Canivet, ACS Catal., 8 (2018) 1653.[2] F. M. Wisser, Y. Mohr, E. A. Quadrelli, D. Farrusseng, J. Canivet, ChemCatChem, 10 (2018) 1778.