Synthesis of Ni(ii)-Mn(ii) complexes using a new mononuclear Ni(ii) complex of an unsymmetrical N 2 O 3 donor ligand: structures, magnetic properties and catalytic oxidase activity.

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed N ₂ O ₃ donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (H ₂ L), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL) ₂ Mn(NCS) ₂ (CH ₃ OH) ₂ ]·CH ₃ OH (2) [(NiL) ₂ Mn(N(CN) ₂ ) ₂ (CH ₃ OH) ₂ ]·CH ₃ OH (3) and [(NiL) ₂ Mn ₂ (N ₃ ) ₂ (μ _1,1 -N ₃ ) ₂ (CH ₃ OH) ₂ ] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O ₃ 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 _1,1 -N ₃ 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 J ₁ = -2.20 cm ^-1 , J ₂ = 1.13 cm ^-1 and J ₃ = -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.