Synthesis of Ni(ii)-Mn(ii) complexes using a new mononuclear Ni(ii) complex of an unsymmetrical NO donor ligand: structures, magnetic properties and catalytic oxidase activity.

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed NO donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (HL), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL)Mn(NCS)(CHOH)]·CHOH (2) [(NiL)Mn(N(CN))(CHOH)]·CHOH (3) and [(NiL)Mn(N)(μ-N)(CHOH)] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O 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 μ-N 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. 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 for complexes 2 and 3, respectively and J = -2.20 cm, J = 1.13 cm and J = -1.12 cm 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.