Microporous Metal-Phosphonates with a Novel Orthogonalized Linker and Complementary Guests: Insights for Trivalent Metal Complexes from Divalent Metal Complexes

Abstract

The reliable self-assembly of microporous metal-phosphonate materials remains a longstanding challenge. This stems from, generally, more coordination modes for the functional group allowing more dense structures, and stronger bonding driving less crystalline products. Here, a novel orthogonalized aryl-phosphonate linker, 1,3,5-tris(4'-phosphono-2',6'-dimethylphenyl) benzene (H₆ L3) has been used to direct formation of open frameworks. The peripheral aryl rings of H₆ L3 are orthogonalized relative to the central aromatic ring giving a tri-cleft conformation of the linker in which small aromatic molecules can readily associate. When coordinated to magnesium ions, a series of porous crystalline metal-organic, and hydrogen-bonded metal-organic frameworks (MOFs, HMOFs) are formed (CALF-41 (Mg), HCALF-42 (Mg), -43 (Mg)). While most metal-organic frameworks are tailored based on choice of metal and linker, here, the network structures are highly dependent on the inclusion and structure of the guest aromatic compounds. Larger guests, and a higher stoichiometry of metal, result in increased solvation of the metal ion, resulting in networks with connectivities increasingly involving hydrogen-bonds rather than direct phosphonate coordination. Upon thermal activation and aromatic template removal, the materials exhibit surface areas ranging from 400-600 m² /g. Self-assembly in the absence of aromatic guests yields mixtures of phases, frequently co-producing a dense 3-fold interpenetrated structure (1). Interestingly, a series of both more porous (530-900 m² /g), and more robust solids is formed by complexing with trivalent metal ions (Al, Ga, In) with aromatic guest; however, these are only attainable as microcrystalline powders. The polyprotic nature of phosphonate linkers enables structural analogy to the divalent analogues and these are identified as CALF-41 analogues. Finally, insights to the structural transformations during metal ion desolvation in this family are gained by considering a pair of structurally related Co materials, whose hydrogen-bonded (HCALF-44 (Co)) and desolvated (CALF-44 (Co)) coordination bonded networks were fully structurally characterized.

Keywords: host-guest chemistry; ligand design; metal-organic frameworks; phosphonates; porous solids.