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

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 Ni^II complex with two unsymmetric azine-based ligands, [Ni(HL^H)₂](PF₆)₂ (1) or [Ni(HL^Me)₂](PF₆)₂ (2), in a high yield, where HL^H denotes 2-pyridylmethylidenehydrazono-(4-imidazolyl)methane and HL^Me 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 Ni^II complexes bearing an unsymmetric HL^H or HL^Me azines in a tridentate κ³ N, N’, N” coordination mode. The HL^H complex with a methanol solvent, 1·MeOH, crystallizes in the orthorhombic non-centrosymmetric space group P2₁2₁2₁ with Z = 4, affording conglomerate crystals, while the HL^Me 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 Ni^II ions.