Supramolecular Architectures and Hydrogen-Bond Directionalities of 4,4′-Biimidazole Metal Complexes Depending on Coordination Geometries

Assembled metal complexes of 4,4′-biimidazole (4,4′-H₂Bim), a ligand exhibiting multidirectional hydrogen-bonds, were investigated in Ag^I, Cu^II, and Ni^II complexes. The most intriguing feature of this system is that the directionality of hydrogen-bonds varies depending on the cis/trans-conformations of the ligand and the coordination geometries of the metal atoms. The [Ag^I₂(4,4′-H₂Bim)₃] complex included ligands of both cis- and trans-conformations, the latter of which linked two metal atoms to form a planar dinuclear complex. The triple N−H···X···H−N hydrogen-bonds across the counteranions (X) formed a one-dimensional hydrogen-bond chain. The [Cu^II(4,4′-H₂Bim)₂] complexes showed a square-planar coordination sphere by the chelating coordination of two 4,4′-H₂Bim units having the cis-conformation. The complexes were linked by the double N−H···X···H−N hydrogen-bonds across counteranions or solvent molecules to form one-dimensional chains. The [Cu^II(4,4′-HBim⁻)₂] including the monodeprotonated ligand had a square-planar coordination geometry similar to those of [Cu^II(4,4′-H₂Bim)₂] complexes. The deprotonated nitrogen atom acted as a proton acceptor having a tetrahedral geometry. This complex formed a three-dimensional network by the π-stacks and N−H···O−H···N hydrogen-bonds across crystalline water molecules. The [Ni^II(4,4′-H₂Bim)₃] complexes had an octahedral coordination sphere including three chelating 4,4′-H₂Bim ligands of the cis-conformation. The N−H···X···H−N hydrogen-bonds across counteranions established two- or three-dimensional networks.