Synthesis of Ni(II)-Mn(II) complexes using a new mononuclear Ni(II) complex of an N2O3 donor unsymmetrical ligand: structures, magnetic properties and catalytic oxidase activity

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed N2O3 donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (H2L), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL)2Mn(NCS)2(CH3OH)2]·CH3OH (2) [(NiL)2Mn(N(CN)2)2(CH3OH)2]·CH3OH (3) and [(NiL)2Mn2(N3)2(μ1,1-N3)2(CH3OH)2] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O3 donor (NiL) units are coordinated to the central octahedral Mn(ii) centre, whereas complex 4 has a centrosymmetric tetranuclear structure where two binuclear (NiL)Mn units are linked via two phenoxido and two μ1,1-N3 bridges. Among the heterometallic complexes (2-4), only 4 is active towards the catalytic oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone. The turnover number for the aerobic oxidation of 3,5-DTBC is 935 h-1. ESI-mass spectra have been recorded to scrutinize the mechanistic pathway of this catalytic reaction. Variable temperature magnetic susceptibility measurements suggest that complexes 2-4 are antiferromagnetically coupled with coupling constants (J) of -4.84 and -5.23 cm-1 for complexes 2 and 3, respectively and J1 = -2.20 cm-1, J2 = 1.13 cm-1 and J3 = -1.12 cm-1 for complex 4. DFT calculations have been used to rationalize the magnetic super-exchange in complexes 2-4, by computing the theoretical coupling constants and analyzing the spin density plots.