A radical pathway in catecholase activity with nickel(ii) complexes of phenol based “end-off” compartmental ligands

Seven dinuclear and one dinuclear based dicyanamide bridged polymeric NiII complexes of phenol based compartmental ligands (HL1–HL4) have been synthesized with the aim to investigate their catecholaselike activity and to evaluate the most probable mechanistic pathway involved in this process. The complexes have been characterized by routine physicochemical studies as well as by X-ray single crystal structure analyses namely [Ni2(L2)(SCN)3(H2O)(CH3OH)] (1), [Ni2(L4)(SCN)3(CH3OH)2] (2), [Ni2(L2)(SCN)2(AcO)- (H2O)] (3), [Ni2(L4)(SCN)(AcO)2] (4), [Ni2(L2)(N3)3(H2O)2] (5), [Ni2(L4)(N3)3(H2O)2] (6), [Ni2(L1)(AcO)2- (N(CN)2)]n (7) and [Ni2(L3)(AcO)2(N(CN)2)] (8), [SCN = isothiocyanate, AcO = acetate, N3 = azide, and N(CN)2 = dicyanamide anion; L1–4 = 2,6-bis(R2-iminomethyl)-4-R1-phenolato, where R1 = methyl and tert-butyl, R2 = N,N-dimethyl ethylene for L1–2 and R1 = methyl and tert-butyl, R2 = 2-(N-ethyl) pyridine for L3–4]. A UV-vis spectrophotometric study using 3,5-di-tert butylcatechol (3,5-DTBC) reveals that all the complexes are highly active in catalyzing the aerobic oxidation of (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) in methanol medium with the formation of hydrogen peroxide.