Metal organic frameworks as Lewis acid catalysts

Abstract

Lewis acids are widely used in the pharmaceutical industry, generally homogeneously, to perform reactions such as C-C or C=N bond formation and acetalisation. Typically, metal salts such as those of Ti, Fe and especially Sc are used, the last typically as the triflate. Metal organic frameworks (MOFs) containing such metals should act as heterogeneous, removable and reusable catalysts for similar reactions if they can be prepared in stable forms and with large, open pores and metal cation sites that can be rendered coordinatively unsaturated. Families of novel MOFs with different structure types and cations have therefore been prepared and their activity has been examined in carbonyl ene C-C bond forming reactions, Friedel-Crafts-Michael additions and in imine formation reactions. Their activities have been compared with those of the well-known HKUST-1(Cu), MIL-100(Fe) and MIL-101(Cr) solids examined as catalysts previously. In particular, divalent transition metal bisphosphonates and dicarboxylates with pore sizes from 10 – 20 Å and scandium carboxylates (MIL-68(Sc), MIL-88D(Sc), MIL-100(Sc), MIL-101(Sc)) have been tested. Synthetic procedures were optimised according to commercial constraints for the known MOFs STA-12(Ni) and MIL-100(Sc). While good activities are observed for Ni-based MOFs and in a number of the scandium-based solids, MIL-100(Sc) is by far the best Lewis acid catalyst for a range of reactions. In particular, MIL-100(Sc) is very active even when used without pre-dehydration, is readily recyclable with minor loss of activity and shows fully heterogeneous activity. It outperforms both MIL-100(Fe) and MIL-101(Cr), each commonly reported as versatile catalysts in the literature. Careful synthesis of bulky substrates shows that the activity is derived from reactions within the internal pore system. Furthermore, MIL-100(Sc) is able to perform tandem reactions - such as dehydration followed by carbonyl ene reaction - in which the Lewis acid sites catalyse two steps. The Lewis acidic sites of the excellent Lewis acid catalyst MIL-100(Sc) has been examined in detail by in situ IR using adsorption of CO and CD₃CN as probe molecules and compared with other MIL-100 materials. The work has been extended to the examination of MOFs containing two different metals, by substitutional approaches within the metal nodes (e.g. Sc-Al, Sc-Fe, Sc-Cr, Sc-Ni, Sc-Co within the trimeric M₃O(O₂C-)₆ nodes of MIL-100). In addition, series of Sc-Fe MIL-100 materials have been prepared that contain α-Fe₂O₃ nanoparticles in the pores of the structure. These composites show higher specific catalytic activity for Lewis acid catalysis than MIL-100(Sc), even though some scandium has been replaced with iron: the origin of this behaviour is discussed. MIL-100(Sc/Fe) has also been explored as a bifunctional catalyst in tandem Friedel-Crafts-oxidation reactions. MIL-100(Sc₆₀/Fe₄₀) was found to give exceptionally high conversions in the Friedel-Crafts-oxidation tandem reaction of 2-methyl indole and ethyl trifluoropyruvate to form a ketone, outperforming the many other materials tested and giving the best balance of the two different types of catalytic sites required to catalyse the reaction. MIL-100(Sc) has also been prepared containing 50% of mono-fluorinated trimesate ligands in the framework for the first time. This fluorinated MIL-100(Sc) has been post-synthetically modified by addition of a di-phenylphosphino group as confirmed by solid state NMR. This can act as a starting point for the future generation of MOF-supported metal phosphine catalysts.