Structural divergence in binuclear Cu(ii) pyridoxal Schiff base complexes probed by co-ligands: catecholase mimetic activity and sulphide ion sensing

Abstract

Three hydroxymethyl bridged Cu(II) complexes of a pyridoxal Schiff base ligand 4-((E)-(2-(pyridin-2-yl)ethylimino)methyl)-5-(hydroxymethyl)-2-methylpyridin-3-ol (LH) have been synthesized and characterized on the basis of spectroscopic, elctrochemical and structural properties. The X-ray crystal structures of the complexes reveal dual denticity of the ligand, bidenticity in the absence of a co-ligand as in complex 1, and tridenticity in the presence of a co-ligand such as SCN⁻/N(CN)₂⁻ as in complexes 2 and 3. The complexes, though binuclear in the solid state, exist as a monomeric unit in solution due to the exceptionally long axial Cu–O_{hydroxymethyl} (2.4–2.5 Å) bond. All three complexes show efficient catalytic activities towards the aerial oxidation of 3,5-ditertiarybutylcatechol (DTBCH₂) with k_cat values of 5.38 × 10⁴ h⁻¹, 1.18 × 10⁵ h⁻¹ and 1.06 × 10⁵ h⁻¹ in methanol. Complexes 1 and 2 also act as a selective sulphide ion sensor with K_b values of 6.6 × 10³ M⁻¹ and 8.1 × 10³ M⁻¹, respectively, while their respective L.O.D. values are 3.4 μM and 3.2 μM.