• Venkatapuram Ramanujam Vijayaraghavan

      Articles written in Journal of Chemical Sciences

    • Oxidation of hydrogen peroxide by [NiIII(cyclam)]3+ in aqueous acidic media

      Sankaran Anuradha Venkatapuram Ramanujam Vijayaraghavan

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      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), rac-Me6[14]-4,11-dieneN4 (L4) by reaction with hydrogen peroxide. The rate constant for the cross reaction is discussed in terms of Marcus relationship.

    • Kinetic measurements and quantum chemical calculations on low spin Ni(II)/(III) macrocyclic complexes in aqueous and sulphato medium

      Anuradha Sankaran E J Padma Malar Venkatapuram Ramanujam Vijayaraghavan

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      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.

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

      ANURADHA SANKARAN E J PADMA MALAR VENKATAPURAM RAMANUJAM VIJAYARAGHAVAN

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      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)].

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