Porous metal-phosphonate materials with orthogonalized linkers

Abstract

Overall, this thesis describes the reliable synthesis of metal phosphonate materials with tailored ultramicropores for gas separations. Metal-phosphonate materials have an inherent proclivity to self-assemble as efficiently packed structures. This predisposition is countered by a crystal engineering approach that centres on the development of novel aryl-phosphonate linkers that are designed to pack inefficiently. The new linkers are rigidified via the installation of methyl groups, which restrict the torsional angles between adjacent aryl groups, making them approximately orthogonal. Chapter 3 highlights the synthesis and self-assembly of the first orthogonalized linker, 1,3,5-tris(4’-phosphonophenyl)-2,4,6-trimethylbenzene, which has three methyl groups installed on the central aromatic ring. Depending on the size of the metal ion, a unique network structure forms. Coordination to Sr or Ba yields network structures with pores lined with methyl groups, resulting in a measured surface area over 300 m2/g, while coordination to Zn yields a water-stable network with large square channels, resulting in a surface area over 500 m2/g. Chapter 4 further explores the self-assembly of this linker. Self-assembly with trivalent ions, La and Ce, yields isomorphous networks to the Sr network, described in Chapter 3. The use of trivalent metals results in more robust structures, yielding materials that retain order and porosity when immersed in aqueous solutions (pH 1-11). It was recognized that the use of trivalent ions marginally contracted the aperture of the 1-D channels, making them of similar dimensions to industrially relevant compounds. A thorough investigation of both the equilibrium uptakes and kinetic diffusion rates of several key adsorptives was performed, revealing that the contracted pores in the La structure have much lower capacities and diffusion kinetics for larger compounds, while retaining high capacity for carbon dioxide. Chapters 5 and 6, highlight the synthesis and self-assembly of the second orthogonalized linker, 1,3,5-tris(4’-phosphono-2’,6’-dimethylphenyl)benzene, which has six methyl groups installed on the peripheral aromatic rings. This similarly results in orthogonalization, but prevents the installed methyl groups from occupying a portion of the generated pores. The resulting linker exhibits a tri-cleft conformation that readily associates with aromatic groups. Initial self-assembly yielded a three-fold interpenetrated structure, in which adjacent nets interact via an abundance of CH-π interactions. To prevent this interpenetration, small aromatic compounds were added to provide competitive CH-π interactions. A plethora of porous metal-phosphonate structures is presented, whose network architectures are dependant on the identity and stoichiometry of the metal ion, and the presence of included aromatic guests. Coordinating to trivalent ions yields robust porous solids (500-900 m2/g) that retain porosity when immersed in boiling water.