Engendering structural and chemical diversity in metal-organic framework materials via ligand elaboration and post-synthesis modification

Families of metal-organic frameworks (MOFs), possessing high levels of structural and chemical diversity were synthesized via the covalent elaboration of the constituent organic ligands both pre- and post-MOF synthesis. To engender structural diversity, a series of 4,4'-ethynylenedibenzoic acids were synthesized and used in the construction of Zn-based, mixed-ligand metal-organic frameworks. Through variation of the organic functionalities in the 3- and 3'-positions of these linkers, a collection of MOFs with differing connectivities and varying levels of interpenetration was obtained. This strategy was then employed in attempts to obtain catenation isomers via post-synthesis modification. A series of 3-ethynyl-4-[2-(4-pyridinyl)ethenyl]pyridine ligands with different chemically labile acetylene protecting groups were synthesized. The sizes of the protecting groups were used to affect the levels of catenation in Zn-based, mixed-ligand MOFs constructed using these ligands and 4,4'-(1,2-ethynediyl)bisbenzoic acid. The protecting groups may eventually be deprotected to generate catenation isomers.Post-synthesis elaboration of MOFs was employed to introduce chemical diversity both at the external surfaces of MOF crystals and throughout the entire framework. In the first case, the external surface of a 2-fold catenated, Zn-cornered, mixed-ligand MOF bearing trimethylsilyl-protected acetylenes was selectively deprotected and its surface was subsequently covalently decorated with azide-containing organic molecules via "click" chemistry. In the second case, a non-catenated Zn-based MOF material bearing silyl-protected acetylenes was first constructed. Via a solvent-based selective deprotection strategy, two different organic azides were then "clicked" onto the MOF, resulting in a material whose internal and external surfaces are functionalized differently. Finally, to demonstrate the utility of covalent post-synthesis modification in the construction of otherwise inaccessible MOFs, a glycol-bearing, Zn-based, mixed-ligand MOF was reacted with succininc anhydride. This reaction proceeded quantitatively to yield a new MOF containing free carboxylic acids that can be further modified via complexation with Cu 2+ ions.