Engineering site-isolated reactive metal complexes within a Metal-organic Framework

Abstract

Metal-organic frameworks (MOFs) are a comparatively new, fascinating class of porous materials. They are organic-inorganic hybrid compounds consisting of organic ligands acting as linkers/spacers and metal ions or clusters as nodes/vertices. Combinations of these building blocks allow formation of three-dimensional periodic lattices with different pore shapes and sizes. In the development of MOF chemistry, different methodologies have emerged to imbue MOFs with catalytically active metal centers. One of the most attractive approaches is immobilizing well-defined homogeneous catalysts via postsynthetic modification (PSM). The well-defined architecture of MOFs in combination with the isolation that the framework provides to the guest metal complex allows the elucidation of the coordination environment by X-ray crystallography. This excellent combination of properties provides the opportunity to structurally characterize organometallic species isolated within MOF architectures and explore the potential of these unique systems for application in commercially attractive processes. The Mn-based MOF ([Mn₃L₂L’)] where L = (bis(4-(4-carboxyphenyl)-1H-3,5-dimethylpyrazolyl)methane) (MnMOF-1) has proven to be an exceptional platform for showcasing these properties. This high crystalline MOF has demonstrated a degree of flexibility and possesses free chelating sites capable of binding reactive metal complexes. In Chapter 2, the underlying conformational flexibility of the framework was demonstrated to be solvent dependent and to influence the outcome of postsynthetic metalation. Most of the investigations in heterogenous catalysis using MOFs have focused on reactions for the synthesis of fine chemicals. Generally, this chemistry is carried out in solution and therefore does not take full advantage of the MOF’s intrinsic characteristics such as high surface area, crystallinity, and permanent porosity. Thus, the judicious choice of chemistry and reaction conditions can showcase the unique properties of MOFs. In Chapter 3 and 4, gas-phase reactions with cationic rhodium(I) and iridium(I) bis-ethylene complexes immobilized within MnMOF-1 are reported. The mononuclear Rh(I) species were demonstrated to be highly active towards 1-butene isomerization, while both the Rh(I) and Ir(I) complexes successfully catalyze gas-phase ethylene hydrogenation. The gas phase reactions carried out in these Chapters are an important step towards fully exploiting the capacity of MOFs to act a supports for highly reactive metal centers. Finally in Chapter 5, the concept of isolating reactive metal complexes within MOFs was expanded to include the metalation of MnMOF-1 with a Cu(I) chloride complex. A new methodology was developed to convert the Cu(I) chloride moiety into a series of labile Cu(I) complexes which are otherwise difficult to characterize and which are of interest in copper-centered catalysis. In accordance with the porous nature of the host framework, the weakly bound ‘place-holder’ ligands readily undergo substitution with a broad repertoire of small guest molecules and weakly coordinating anions. Due to the highly crystalline and robust host framework, these sequential processes are mapped via X-ray crystallography, providing exquisite insight into the exchange processes of site-isolated Cu(I) moieties within MnMOF-1, pertinent to Cu-centered catalysis.