Syntheses and structural studies of hexa- and pentacoordinated Zn complexes derived from 2-(aminomethyl)benzimidazole and water

The complexes trans-bis[2-(aminomethyl)-1H-benzimidazole-κ ² N′,N″]diaquazinc(II) dichloride dihydrate 1 and trans-bis[2-(aminomethyl)-1H-benzimidazole-κ ² N′,N″]aquazinc(II) dichloride dihydrate 2 were synthesized selectively by the promotion of O–H···Cl hydrogen bond interactions. The hexacoordinated complex 1 was synthesized at pH 4.5. The dilution of 1 in deionized water produced the pentacoordinated complex 2. NMR and vibrational spectroscopies corroborated the presence of these compounds. Moreover, mass spectrometry and thermogravimetric (TGA) studies demonstrated that chloride ions and crystallization water molecules are essential for the stabilization of 1 but not for complex 2. X-ray diffraction crystallography studies indicated that the presence of two water molecules bonded to the Zn atom elongated all of the coordination bonds. The incidence of a network of hydrogen bond interactions compensates for the unstable hexacoordination. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) studies of the crystal structures of 1 and 2 were used to explain the nature of the coordination bonds and the complexes’ stability .