Phosphine- and arsine-containing metal-organic frameworks as solid-state ligands




Sikma, Ronald Eric

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Metal-Organic Frameworks (MOFs) are an exciting new class of porous materials that display exceptional structural diversity and broad functional tunability. These materials are of particular interest for gas storage, separations, and catalysis. This work centers on the creation of new MOF materials incorporating P(III) and As(III) Lewis basic sites, known as Phosphine/Arsine Coordination Materials (PCMs/ACMs). These permit the incorporation of secondary 4d/5d metals bound to the soft P(III)/As(III) lone pairs before, during, or after MOF formation. In this sense, PCMs/ACMs can act as solid-state ligands, binding reactive metals in well-defined coordination environments while orienting them periodically throughout a porous crystalline structure. The presence of these secondary metals can drastically enhance the performance of the materials for targeted applications. The first reported triarylarsine-containing MOF, ACM-1, was obtained using a unique arsine linker with phenylpyridyl linkages. ACM-1 was metallated with Au(I) post-synthetically to yield Au-ACM-1, which was characterized by single-crystal X-ray diffraction. The material incorporated Au₂Cl₂ dimers in a sterically constrained cis-As2 pocket, which resulted in an exceptionally short Au···Au aurophilic contact of 2.76 Å. Incorporation of Au(I) allowed the material to function as a heterogeneous catalyst for the hydroalkoxylation of 4-pentyn-1-ol, and also enhanced the solid-state luminescent properties of the material. A broad library of triaryl arsine ligands and new ACMs was achieved, most of which were structurally characterized and evaluated as solid-state ligands. Other materials were obtained with trans-oriented Lewis basic sites. PCM-102 was structurally characterized and found to be a versatile solid-state ligand. Upon addition of low-valent metals, the framework undergoes a distortion to form trans-P₂M complexes (M = Cu(I), Ag(I), Au(I), Rh(I)). Metallation of PCM-102 was found to drastically enhance the gas uptake properties of the resulting materials, and the Ag(I) material was evaluated for the separation of propane and propylene. Additionally, one of the first examples of Lewis acid pillaring was achieved in Ag-PCM-201, in which Ag(I) connects adjacent two-dimensional sheets to form a three-dimensional structure. This was also accomplished using an Os(I) dimer in Os₂-PCM-201, which displayed interesting reactivity and selectivity for adsorption of H₂.



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