Structurally diverse copper(ii) complexes with pyridazine-integrated with pyrazine–Schiff baseligand featuring an easily lost proton in the hydrazone backbone

Abstract

To probe the coordination approach of pyridazine and pyrazine ligands, five new complexes of [Cu^II₂L₂Cl₂] (1), [Cu^II(L-H)(NO₃)(H₂O)₂](NO₃)(H₂O) (2), {[Cu^II(L-H)(H₂O)](ClO₄)₂(H₂O)}_n (3), [Cu^II₂L₂(OH)](NO₃)(H₂O)₂ (4), [Cu^II₂L₂(OH)](ClO₄) (5) based on Schiff base ligand (E)-3-chloro-6-[2-(pyrazin-2-ylmethylene)hydrazinyl]pyridazine (HL) bearing pyridazinyl and pyrazinyl groups have been synthesized and characterized by IR spectra, elemental analyses, the bond valence sum (BVS) analyses, powder X-ray diffraction (PXRD) analyses, X-ray single crystal diffraction and density function theory calculations. The structures of 1, 2 and 3 suggest that the second nitrogen donors of the pyrazinyl unit can link with other metal ions to construct a higher dimensional architecture by the reaction of HL with Cu(ClO₄)₂ [CuCl₂ for 1, Cu(NO₃)₂ for 2 and Cu(ClO₄)₂ for 3]. The structures of 4 and 5 indicate that additional donor units of the pyridazinyl chromophore can bind multiple metal ions to form a multi-metallic cluster, when the protons of the hydrazone backbones are abstracted in the presence of triethylamine to strengthen the coordination behavior of the second nitrogen atom. However, for 1, the protons of the hydrazone backbone are lost with or without triethylamine and the additional nitrogen atoms of the pyrazinyl and pyridazinyl groups are not found to connect to other metal ions, which can probably be assigned to the strong coupling effect of chlorine ions. Magnetic measurements display weak antiferromagnetic coupling between the copper(II) ions in 1 and 5, and ferromagnetic coupling in 3. In addition, the antioxidant activities of HL and 1–5 were also studied.