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

What is it about?

Cu(II) ion catalyzed kinetics of oxidation of H2O2 by [Ni(III)L2] (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 [Ni(III)L2] is faster than that by [Ni(III)L1] (L1 = 1,4,8,11-tetraazacyclotetradecane = cyclam) 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 ~23 from the vertical axis forming the dihydrate [NiL2]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 [Ni(III)L] complexes.

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Why is it important?

Complementary kinetic study and quantum chemical calculations describe the bonding in low-spin aqua and sulfato Ni(II) and Ni(III) macrocyclic complexes. Quantum chemical calculations demonstrate that the electronic and structural factors involving the macrocyclic ligand cause different modes of bonding between axial water and Ni(III) ion.

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