Study of behaviour of Ni(III) macrocyclic complexes in acidic aqueous medium through kinetic measurement involving hydrogen peroxide oxidation and DFT calculations

Abstract

The Cu(II) ion-catalysed kinetics of oxidation of H2O2 by [Ni IIIL] [where L = L1 (cyclam) and L2 (1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane)] was studied in the pH range of 3.6–5.6 in acetic acid-acetate buffer medium at 25°C in the presence of sulphate ion. The ionic strength (I) was maintained at 0.5 M (NaClO4). The rate constants showed an inverse acid dependence and [Ni IIIL2] was observed to be more stable than [Ni IIIL1]. The rate of the reaction of both complexes with hydrogen peroxide shows contrasting behaviour at pH > 2.5 when compared to the same reaction in perchloric acid medium. DFT calculations performed on the complexes [Ni IIIL1(SO4)(OAc)] and [Ni IIIL2(SO4)(OAc)] reveal that both the acetate and sulphate ligands are axially coordinated to the metal centre. In addition, there is strong hydrogen bonding between the axial ligand and NH hydrogen of the macrocyclic ligand. The computed covalent bond orders in the aqueous medium predict that the acetate forms stronger coordinate bond with Ni ion than the sulphate ligand. The hydroxyl group present in one of the pendant groups of L2 forms a strong hydrogen bond with the sulphate ligand which leads to additional stability in [Ni IIIL2(SO4)(OAc)]. [Figure not available: see fulltext.]