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

Abstract

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(μ_1,1-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 μ_1,1-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.