Tri- and hexa-nuclear NiII–MnII complexes of a N2O2 donor unsymmetrical ligand: synthesis, structures, magnetic properties and catalytic oxidase activities

Abstract

A new mononuclear Ni(II) complex, [NiL] (1) of an unsymmetrically dicondensed N2O2 donor ligand, H2L (where H2L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine), has been synthesized. Complex 1, on reaction with Mn(II) in the presence of different co-anions, yielded three heterometallic NiII–MnII complexes, [(NiL)2Mn(NCS)2]·CH3CN (2), [(NiL)2Mn(N3)(H2O)](ClO4)·H2O (3) and [{(NiL)2Mn(H2O)}2(μ1,3-N3)](ClO4)3 (4). Single crystal structural analyses show that complexes 2 and 3 contain similar bent trinuclear NiII2MnII structures, with the difference that in complex 2 two SCN− ions and in 3, one N3− and one H2O molecule are coordinated to the Mn(II) centre. Complex 4 possesses a hexanuclear structure, in which two trinuclear NiII2MnII units are connected via a single μ1,3-azido bridge. Variable temperature dc molar magnetic susceptibility measurements reveal that the two Mn(II) centres in complex 4 are antiferromagnetically coupled with an exchange coupling constant (J) of −3.73 cm−1. Among the three heterometallic complexes, only 3 and 4 show bio-mimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (Kcat) of complexes 3 and 4 are 984 and 2081 h−1, respectively, whereas for phenoxazinone synthase-like activity, the turnover numbers of complexes 3 and 4 are 6351 and 10 545 h−1, respectively. The high catalytic oxidase activities of complexes 3 and 4 in contrast to the inactivity of complex 2 are attributed to the coordination of a water molecule to the Mn(II) centre in the former structures, which facilitates the binding of the substrate with the catalyst. Mass spectral analyses suggest the probable formation of a complex–substrate intermediate in these catalytic reactions and cyclic voltammetry measurements show that Ni(II) is reducible to Ni(I).