Synthesis, characterization and crystal structures of Cd(II) and Zn(II) complexes with 2,2′-biimidazole

The reactions of zinc and cadmium salts with 2,2′-biimidazole (H2biim) yielded a series of compounds in which the ligand is coordinated in the chelating bidentate mode. ZnCl2 and [Ag(H2biim)](NO3) in methanol in a 2:1 proportion produced Zn(H2biim)Cl2, in which the metal has a distorted tetrahedral coordination. A 1:2 ratio led to [Zn(H2biim)2(CH3OH)2](NO3)2, containing an octahedrally coordinated Zn(II) center with the O-bonded methanol ligands occupying trans positions. The corresponding [Cd(H2biim)2(CH3OH)2](NO3)2 compound was obtained from CdCl2. By starting with Cd(NO3)2 and Cd(ClO4)2 in aqueous media, the related octahedral bis-chelate compounds [Cd(H2biim)2(NO3)(H2O)](NO3) and Cd(H2biim)2(ClO4)2, respectively, were isolated, the apical positions being filled by perchlorate oxygens in the latter case. With Cd(BF4)2, the glass container participated in the reaction and a tris-chelate complex [Cd(H2biim)3]2(SiF6)(BF4)2 · 6EtOH was isolated. The [Cd(H2biim)3]2+ and SiF62- ions define an extended hydrogen-bonded network, in which BF4-ions surrounded by disordered ethanol molecules occupy large cavities. The two free N–H groups provide H2biim with a unique ability to form hydrogen bonds and their interactions with counter anions or other acceptors play a determining role in controlling molecular packing. The IR spectra of all compounds are discussed.

Graphical abstractZn(NO3)2, Cd(NO3)2 and Cd(ClO4)2 form trans-octahedral [M(H2biim)2L2]n+ species, where H2biim is chelating bidentate and the apical ligands are H2O, CH3OH or counter-anions, whereas tetrahedral Zn(H2biim)Cl2 is obtained from ZnCl2. The tris-chelate [Cd(H2biim)3]2(SiF6)(BF4)2 · 6EtOH is isolated from Cd(BF4)2 reacted in a glass vessel. Hydrogen bonding plays a determining role in crystal packing.Figure optionsDownload full-size imageDownload as PowerPoint slide