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′-H<sub>2</sub>Bim), a ligand exhibiting multidirectional hydrogen-bonds, were investigated in Ag<sup>I</sup>, Cu<sup>II</sup>, and Ni<sup>II</sup> complexes. The most intriguing feature of this system is that the directionality of hydrogen-bonds varies depending on the <i>cis</i>/<i>trans</i>-conformations of the ligand and the coordination geometries of the metal atoms. The [Ag<sup>I</sup><sub>2</sub>(4,4′-H<sub>2</sub>Bim)<sub>3</sub>] complex included ligands of both <i>cis</i>- and <i>trans</i>-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<sup>II</sup>(4,4′-H<sub>2</sub>Bim)<sub>2</sub>] complexes showed a square-planar coordination sphere by the chelating coordination of two 4,4′-H<sub>2</sub>Bim units having the <i>cis</i>-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<sup>II</sup>(4,4′-HBim<sup>−</sup>)<sub>2</sub>] including the monodeprotonated ligand had a square-planar coordination geometry similar to those of [Cu<sup>II</sup>(4,4′-H<sub>2</sub>Bim)<sub>2</sub>] 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<sup>II</sup>(4,4′-H<sub>2</sub>Bim)<sub>3</sub>] complexes had an octahedral coordination sphere including three chelating 4,4′-H<sub>2</sub>Bim ligands of the <i>cis</i>-conformation. The N−H···X···H−N hydrogen-bonds across counteranions established two- or three-dimensional networks.