Cooperative influence of pseudohalides and ligand backbone of Schiff-bases on nuclearity and stereochemistry of cobalt(III) complexes: experimental and theoretical investigation

Four cobalt(III) complexes, [Co(HL1)(NCS)(2)(EtOH)] (1), [Co-2(L-1)(2)(N-3)(2)] (2) and [Co(HL2)(NCS)(2)(EtOH)] (3), [Co(HL2)(N-3)(2)] (4) were synthesized from two Schiff-base ligands namely, (E)-2-((2-(2-hydroxyethylamino) ethylimino) methyl) phenol (H2L1) and (E)-2-((3-(2-hydroxyethylamino) propylimino) methyl) phenol (H2L2), respectively. All the four complexes have been thoroughly characterised by using various physicochemical studies such as UV-Vis, FT-IR, ESI-MS, EPR and single crystal X-ray diffraction. Depending on flexibility of the ligand backbone subtle structural differences are observed in the synthesized complexes. In complex 1 the two thiocyanate ligands are trans to each other whereas in complex 3 they are cis to each other with addition of one additional methylene (-CH2-) group to the ligand system. Complex 2 is dinuclear while complex 4 is mononuclear in the presence of the azide co-anionic ligand. Theoretical studies are carried out in order to rationalize the structural differences observed in the complexes. Catecholase like activity of all the four complexes were performed in N, N-dimethylformamide (DMF) using 3,5-di-tert-butylcatechol (3,5-DTBC) as a model substrate. Complex 2 was found to exhibit the highest activity. Mechanistic investigation of the catecholase like activity revealed the formation of the imine radical during catalytic reactions of complexes 2 and 4 which are further corroborated by the EPR study.