Bond Activation and Catalysis through Solid Frustrated Lewis Pairs

A fundamental concept in chemistry is that of Lewis bases and acids. Lewis acids are chemical species that accepts electron pairs, while Lewis bases donate electron pairs. Usually, Lewis acids will react with Lewis bases to form an adduct, a new compound with a new bond. Examples of such adducts are the reactive centers in enzymes. However, in so called frustrated Lewis pairs (FLP), the Lewis acid and base cannot come close enough to each other for bond formation. In these cases, the chemical reactivity of the Lewis acid and base can manifest in different and intriguing ways. Frustrated Lewis pairs were originally discovered and investigated in solution, using bulky molecules. FLPs have displayed the ability to break the bonds in small molecules like hydrogen and dinitrogen or make them more reactive. As a consequence, these molecules can possibly be used in chemical transformations under milder conditions than usually, potentially resulting in chemical processes that are more environmentally friendly and less energy demanding. A possible application for which these properties are relevant in the use of hydrogen in hydrogenation reactions and the activation and transformation of other small molecules. Examples are ammonia synthesis and hydrogenation of industrially relevant aromatic compounds. This project will attempt to introduce the catalytic properties of the FLP into porous solids, metal-organic frameworks (MOFs), that have well defined crystal structures and properties. Solids are easily separated from gas or liquid phase reactants, which would be a significant advantage in development of industrial processes if new hydrogenation catalysts with suitable properties for commercial application were discovered. The project focuses on materials discovery, investigation of the properties of the materials with experimental and theoretical methods, and evaluation of whether the materials are suitable for further process development. As part of the project, we have developed synthetic procedures for several organic linker molecules that carry either Lewis acid or base functional groups in addition to the carboxylic acid groups that are required to construct the network of the metal-organic framework. These organic linker molecules were and are used in exploratory materials synthesis with the aim of making MOFs that carry the required functional groups and have large enough pores for reacting molecules to enter and leave. The new materials are investigated using X-ray diffraction, which reveals their crystal structures, i.e. how molecules and atoms are connected, thermal analysis and gas sorption, which reveals how stable they are and how they interact with various guest molecules of interest. Results from calculations performed as part of the project indicate the optimal distance of Lewis acid and base to be active in hydrogen and small molecule activation. We are using the values from these calculations to evaluate from the crystal structure whether new materials made experimentally in the project are promising for the targeted application. Of particular interest among the materials investigated so far is a series of compounds that has the same basic structure, but cancontain either the Lewis base linker, the Lewis acid linker, or a mixture thereof. The mixed linker compound is in principle exactly the type of structure and material the project is looking for. Unfortunately, the distance between Lewis acid and base is too large in this material for meaningful effect on hydrogen activation. However, the pores appear large enough for a small molecules to enter as guests in the framework structure. We currently plan to explore the approach that a small molecule with Lewis acid or base can diffuse into the MOF, arrange itself in closer vicinity to the conjugated acid/base, and in the process create a useful frustrated Lewis pair. Results from the project have been disseminated at international scientific conferences and workshops. Several manuscripts for scientific publications are currently in preparation.

The aim of this project is the incorporation of chemical properties exhibited by frustrated Lewis pairs into metal-organic frameworks with their well-defined crystalline structure and investigation of the properties of these solid systems towards small-molecule activation. Frustrated Lewis pairs (FLPs) are systems, in which a Lewis acid and a Lewis base are prevented - typically due to stereochemical reasons - to approach close enough for formation of a dative bond. Since the discovery that FLPs can dissociate the dihydrogen molecule under formation of a zwitter-ionic compound, their reactivity has been shown to be even more versatile in many types of catalytic and non-catalytic reactions. Prime among these is the use as hydrogenation catalysts. In addition, FLPs have been found to activate many more small molecules beyond dihydrogen. FLP chemistry has been explored almost exclusively in solution to date. Here, we propose to prepare metal-organic frameworks (MOFs) that carry strong Lewis acid and Lewis base functionalities oriented in such a fashion that FLP reactivity is preserved in the solid. To achieve this goal, bifunctional organic linkers carrying the network creating carboxylate groups and Lewis basic phosphorus will be used in the construction of the MOFs. The Lewis acidic functionality will be incorporated into the material either by impregnation with a soluble strong Lewis acid or in the form of coordinatively unsaturated metal sites on the inorganic nodes of the network. After successful preparation of the FLP-MOF material, its reactivity towards hydrogen dissociation and dinitrogen activation will be investigated and its activity in catalytic hydrogenation of aromatic substrates and nitrogen to ammonia will be evaluated. As a consequence, the project may open up a path towards preparation of a heterogeneous system with FLP functionality and (and beyond that a general approach to preparation of single-site catalyst systems using phosphine based MOFs).