Synthesis of a Zinc(II) Imidazolium Dicarboxylate Ligand Metal−Organic Framework (MOF): a Potential Precursor to MOF-Tethered N-Heterocyclic Carbene Compounds

Abstract

Two new isomeric dinitrile ligands containing imidazolium salt cores have been synthesized from cyanoanilines in good yield. These have been converted to the corresponding dicarboxylic acids using hydrobromic acid, or the dicarboxylic acids were synthesized directly from the analogous cyanoaniline starting material in a two-step, one-pot reaction. The crystal structures of three of the four compounds are reported. Two reaction pathways with metals are possible for the dicarboxylic acids, initially giving rise to metal carboxylate based metal-organic frameworks (MOFs) as described in this work or N-heterocyclic carbene (NHC) metal complexes en route to the synthesis of MOFs containing tethered NHC complexes. The reaction of 1,3-bis(4-carboxyphenyl)imidazolium bromide with zinc nitrate in dimethylformamide (DMF) gave a one-dimensional (1D) MOF containing the intact imidazolium salt. The potentially porous structure, formed from close packing of the 1D necklace-type polymers, contains channels occupied by disordered DMF solvate molecules. A formate anion also bridges the zinc secondary building units in the 1D polymer, which has an undulating structure resulting from the U-shaped conformation of the dicarboxylate imidazolium ligand.