Synthesis, crystal structure and magnetic properties of three unprecedented tri-nuclear and one very rare tetra-nuclear copper(ii) Schiff-base complexes supported by mixed azido/phenoxo/nitrato or acetato bridges

Abstract

Three novel mixed bridged trinuclear and one tetranuclear copper(II) complexes of tridentate NNO donor Schiff base ligands [Cu₃(L¹)₂(μ_1,1-N₃)₂(CH₃OH)₂(BF₄)₂] (1), [Cu₃(L¹)₂(μ_1,1-N₃)₂(μ-NO₃-1κO:2κO′)₂] (2), [Cu₃(L²)₂(μ_1,1-N₃)₂(μ-NO₃-1κO:2κO′)₂] (3) and [Cu₄(L³)₂(μ_1,1-N₃)₄(μ-CH₃COO-1κO:2κO′)₂] (4) have been synthesized by reaction of the respective tridentate ligands (L¹ = 2-[1-(2-dimethylamino-ethylimino)-ethyl]-phenol, L² = 2-[1-(2-diethylamino-ethylimino)-ethyl]-phenol, L³ = 2-[1-(2-dimethylamino-ethylimino)-methyl]-phenol) with the corresponding copper(II) salts in the presence of NaN₃. The complexes are characterized by single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. Complex 1 is composed of two terminal [Cu(L¹)(μ_1,1-N₃)] units connected by a central [Cu(BF₄)₂] unit through nitrogen atoms of end-on azido ligands and a phenoxo oxygen atom of the tridentate ligand. The structures of 2 and 3 are very similar; the only difference is that the central unit is [Cu(NO₃)₂] and the nitrate group forms an additional μ-NO₃-1κO:2κO′ bridge between the terminal and central copper atoms. In complex 4, the central unit is a di-μ_1,1-N₃ bridged dicopper entity, [Cu₂(μ_1,1-N₃)₂(CH₃COO)₂] that connects two terminal [Cu(L³)(μ_1,1-N₃)] units through end-on azido, phenoxo oxygen and μ-CH₃COO-1κO:2κO′ triple bridges to result in a tetranuclear unit. Analyses of variable-temperature magnetic susceptibility data indicates that there is a global weak antiferromagnetic interaction between the copper(II) ions in complexes 1–3, with the exchange parameter J of −9.86, −11.6 and −19.98 cm⁻¹ for 1–3, respectively. In complex 4 theoretical calculations show the presence of an antiferromagnetic coupling in the triple bridging ligands (acetato, phenoxo and azido) while the interaction through the double end-on azido bridging ligand is strongly ferromagnetic.