Syntheses and Crystal Structures of Metal Complexes with 2,2′-Biimidazole-like Ligand and Chloride: Investigation of X−H···Cl (X = N, O, and C) Hydrogen Bonding and Cl−π (imidazolyl) Interactions

Six complexes containing the 2,2′-biimidazole-like ligand and chloride, namely, trans-[Ni(H₂biim)₂(H₂O)₂]Cl₂ (1), trans-[Co_1.5(H₂biim)₃(H₂O)₃]Cl₃ (2), [Zn(H₂biim)₂Cl]Cl (3), [Ni₂(H₂bbim)₄(bbim)]Cl₂ (4), [Co(H₂bbim)₃]Cl₂·2H₂O (5), and [Zn₂(Hbbim)₂(bbim)_0.5Cl] (6) (where H₂biim = 2,2′-biimidazole, H₂bbim = 2,2′-bibenzimidazole) have been synthesized by the reactions of the H₂biim or H₂bbim ligand with the metal chloride salts. The structures of the complexes were determined by single-crystal X-ray diffraction analyses, and the results revealed that the distances of N···Cl varied from 3.10 to 3.29 Å and the angles of N−H···Cl varied from 148 to 169°, O···Cl varied from 3.09 to 3.16 Å, O−H···Cl varied from 151 to 178°, C···Cl varied from 3.49 to 3.69 Å, and C−H···Cl varied from 122 to 167°. The chloride may act as a multi hydrogen bonded acceptor varying from 3 to 6, resulting in an important collective contribution to cohesion. The data observed in this study seem to suggest that a variety of X−H···Cl (X = N, O, and C) synthons identified here play a crucial role in the formation and further stabilization of supramolecular architecture, for instance, to link the discrete (0D) or low-dimensional (1D) entities into high-dimensional frameworks. In particular, the coplanar [(H₂O)₂Cl₂]²⁻ guests are encapsulated in the cavities formed by four [Co(H₂bbim)₃]²⁺ cations and stabilized via accepting 6-fold hydrogen bonds from three H₂bbim ligands in 5. The distance between the chloride anion and the imidazolyl ring (Cl···centroid, 3.30 Å and 80°) demonstrates the possible existence of Cl···π (imidazolyl) charge-assisted interactions because the coordination of a positively charged Co(II) ion greatly enhances the electron-deficient character of the imidazolyl ring and provides sufficient polarization to produce an anion−π charge-assisted interaction.