Synthesis, crystal structure, and catecholase activity of three trinuclear heterometallic Ni^II₂-Mn^II complexes derived from a salen-type Schiff base ligand

Abstract

Three new trinuclear heterometallic nickel (II)–manganese (II) complexes, [(NiL)₂Mn(NCS) ₂] (1), [(NiL) ₂Mn(NCO) ₂] (2) and [{NiL(EtOH)} ₂Mn(NO₂)₂]•2EtOH (3), have been synthesized by using [NiL] as the so-called “ligand complex” [where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine] and have been structurally characterized. Crystal structure analyses revealed that complexes 1 and 2 are angular trinuclear species, in which two terminal four-coordinate square planar [NiL] moieties are coordinated to a central Mn^II through double phenoxido bridges. The Mn^II is in a six-coordinate distorted octahedral environment that is bonded additionally to two mutually cis nitrogen atoms of terminal thiocyanate (in 1) and cyanate (in 2). In complex 3, in addition to the double phenoxo bridge, the two terminal Ni^II ions are linked to the central Mn^II by means of a nitrite bridge (1κN:2κO) that, together with a coordinated ethanol molecule, gives rise to an octahedral environment around the Ni^II ions and consequently the structure becomes linear. Catecholase activity of these three complexes was examined by using 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. All three complexes mimic catecholase activity and the rate of catechol oxidation follows saturation kinetics with respect to the substrate and first-order kinetics with respect to the catalyst. The EPR spectra of the complexes exhibit characteristic six line spectra, which indicate the presence of high-spin octahedral Mn^II species in solution state. The ESI-MS positive spectrum of 1 in the presence of 3,5-DTBC has been recorded to investigate possible complex–substrate intermediates.