Four new mononuclear nickel(II) complexes, namely [NiL1(H2O)3](NO3)2 (1), [NiL2(H2O)3](NO3)2 (2), [NiL3(H2O)3](NO3)2 (3) and [NiL4(ClBz)(H2O)] 1.25(H2O) (4) have been synthesized via Schiff-base formation by condensation between 2-benzoylpyridine and N-(2-aminoethyl)pyrrolidine for L1, salicylaldehyde and N-(2-aminoethyl)piperazine (L2), 5-chlorosalicylaldehyde and N-(2-aminoethyl)piperazine (L3), and 5-chlorosalicylaldehyde and N-(2-aminoethyl)morpholine (L4). These complexes are comprehensively characterized via routine physicochemical techniques as well as by single crystal X-ray structural analyses. Despite all the nickel complexes are mononuclear, the catecholase activity shows prominent variation depending on the coordination environment around the metal center. Complexes 2 and 3 derived from same amine bear an extra positive charge on the ligand system facilitating the substrate–catalyst interaction to promote the oxidation of 3,5-DTBC to 3,5-DTBQ. On the contrary complexes 1 and 4 remain inert in nature, although 1 shows structural similarities in terms of coordination environment with nickel substituted catechol oxidase.
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