Abstract/Summary

A new mononuclear Ni(II) complex [NiL] (1) of an unsymmetrically dicondensed N2O3 donor ligand, H2L (N-α-methylsalicylidene-N′-3-methoxysalicylidene-1,3-propanediamine), has been synthesized. Complex 1 on reaction with Mn(ClO4)2·6H2O and NaN3 in different molar ratios yielded three novel heterometallic NiII–MnII complexes, [(NiL)2Mn(N3)](ClO4) (2), [(NiL)2Mn2(N3)2(μ1,1-N3)2(CH3OH)2] (3), and [{(NiL)2Mn}2(μ1,3-N3)(H2O)]·(CH3OH),(ClO4)3 (4). The single crystal structure analyses show a trinuclear NiII2MnII structure for complex 2 and a tetranuclear NiII2MnII2 structure where two dinuclear NiIIMnII units are connected via μ1,1-azido and phenoxido bridges for complex 3. Complex 4 possesses a hexanuclear structure where two trinuclear NiII2MnII units are connected via a μ1,3-azido bridge. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 3 and 4 are antiferromagnetically coupled with the exchange coupling constants (J) of −4.97, −0.14, −0.55 cm–1 for 3 and −3.94 cm–1 for 4. All complexes 2–4 show biomimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (Kcat) are 768, 1985, and 2309 h–1 for complexes 2–4, respectively, whereas for phenoxazinone synthase like activity, the turnover numbers are 3240, 3360, and 13 248 h–1 for complexes 2–4, respectively. This difference in catalytic efficiencies is attributed to the variations in structures of the complexes and formation of active NiII–MnII species in solution during catalysis. The mass spectral analyses suggest the probable intermediate formation and cyclic voltammetry measurement suggest the reduction of Ni(II) to Ni(I) during catalytic reaction. The very high catalytic efficiencies for aerial dioxygen activation of all these heterometallic complexes as well as the highest activity of 4 is attributed to the coordinatively unsaturated penta-coordinated geometry or hexa-coordinated geometry with a solvent water molecule around Mn(II).