Selective Formation of Unsymmetric Multidentate Azine-Based Ligands in Nickel(II) Complexes

Abstract

A mixture of 2-pyridine carboxaldehyde, 4-formylimidazole (or 2-methyl-4-formylimidazole), and NiCl2·6H2O in a molar ratio of 2:2:1 was reacted with two equivalents of hydrazine monohydrate in methanol, followed by the addition of aqueous NH4PF6 solution, afforded a NiII complex with two unsymmetric azine-based ligands, [Ni(HLH)2](PF6)2 (1) or [Ni(HLMe)2](PF6)2 (2), in a high yield, where HLH denotes 2-pyridylmethylidenehydrazono-(4-imidazolyl)methane and HLMe is its 2-methyl-4-imidazolyl derivative. The spectroscopic measurements and elemental analysis confirmed the phase purity of the bulk products, and the single-crystal X-ray analysis revealed the molecular and crystal structures of the NiII complexes bearing an unsymmetric HLH or HLMe azines in a tridentate κ3N, N’, N” coordination mode. The HLH complex with a methanol solvent, 1·MeOH, crystallizes in the orthorhombic non-centrosymmetric space group P212121 with Z = 4, affording conglomerate crystals, while the HLMe complex, 2·H2O·Et2O, crystallizes in the monoclinic and centrosymmetric space group P21/n with Z = 4. In the crystal of 2·H2O·Et2O, there is intermolecular hydrogen-bonding interaction between the imidazole N–H and the neighboring uncoordinated azine-N atom, forming a one-dimensional polymeric structure, but there is no obvious magnetic interaction among the intra- and interchain paramagnetic NiII ions.