VENKATAPURAM RAMANUJAM VIJAYARAGHAVAN
Articles written in Journal of Chemical Sciences
Volume 125 Issue 5 September 2013 pp 1123-1132
The kinetics of oxidation of H2O2 by [NiIII(cyclam)]3+, [NiIIIL1], was studied in aqueous acidic media at 25°C and I = 0.5M (NaClO4). The [NiIIIL1] to [NiIIL1] reduction was found to be fast in the presence of Cu(II) ion than the oxidation of the cyclam ligand by ·OH. The rate constant showed an inverse acid dependence on H+ ion at the pH range 1-1.5. The presence of sulphate retards the reaction. Macrocylic ligand oxidation was followed spectrophotometrically by examining the oxidation of nickel(II) complexes of macrocyclic ligands such as 1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane (L2), 𝑚𝑠-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L3),
Volume 127 Issue 7 July 2015 pp 1287-1298 Regular Articles
Cu(II) ion catalyzed kinetics of oxidation of H2O2 by [NiIIIL2] (L2 = 1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane) was studied in aqueous acidic medium in the presence of sulphate ion. The rate of oxidation of H2O2 by [NiIIIL2] is faster than that by [NiIIIL1] (L1 = 1,4,8,11-tetraazacyclote-tradecane) in sulphate medium. DFT calculations at BP86/def2-TZVP level lead to different modes of bonding between [NiL]II/III and water ligands (L = L1 and L2). In aqueous medium, two water molecules interact with [NiL]II through weak hydrogen bonds with L and are tilted by $\sim$23° from the vertical axis forming the dihydrate [NiL]2+.2H2O. However, there is coordinate bond formation between [NiL1]III and two water molecules in aqueous medium and an aqua and a sulphato ligand in sulphate medium leading to the octahedral complexes [NiL1(H2O)2]3+ and [NiL1(SO4)(H2O)]+. In the analogous [NiL2]III, the water molecules are bound by hydrogen bonds resulting in [NiL2]3+.2H2O and [NiL2(SO4)]+.H2O. As the sulphato complex [NiL2(SO4)]+.H2O is less stable than [NiL1(SO4)(H2O)]+ in view of the weak H-bonding interactions in the former it can react faster. Thus the difference in the mode of bonding between Ni(III) and the water ligand can explain the rate of oxidation of H2O2 by [NiIIIL] complexes.
Volume 129 Issue 2 February 2017 pp 193-202 Regular Article
The Cu(II) ion-catalysed kinetics of oxidation of H ₂O ₂ by [NiIIIL] [where L = L₁ (cyclam) and L ₂ (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 [NiIIIL ₂] was observed to be more stable than [NiIIIL₁]. The rate of the reaction of both complexes with hydrogen peroxide shows contrastingbehaviour at pH > 2.5 when compared to the same reaction in perchloric acid medium. DFT calculations performed on the complexes [NiIIIL₁ (SO₄)(OAc)] and [NiIIIL ₂ (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 ordersin 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 L ₂ forms a strong hydrogen bond with thesulphate ligand which leads to additional stability in [NiIIIL ₂ (SO₄)(OAc)].
Volume 134, 2022
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