Structure and redox kinetics of nickel(II) ...

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Title Structure and redox kinetics of nickel(II) tetraazamacrocycles containing pendant carboxylate groups
Author(s) Robert I. Haines, D. R. Hutchings, R. J. Lucas, D. Miller
Journal Canadian Journal of Chemistry-Revue Canadienne De Chimie
Date 2001
Volume 79
Issue 1
Start page 54
End page 62
Abstract The nickel(II) complex of the deprotonated pendant-arm macrocycle 5,12-dimethyl-7,14-diphenyl-1,4,8,11-tetraazacyclotetradecane-4,11-diace tate (L-2)(2-) has been structurally characterized by X-ray crystallography. The Ni(II)L-2 complex crystallizes in the monoclinic system, space group P2(1)/c, with a = 12.981(8), b = 16.474(6), c = 14.262(7) Angstrom and beta = 98.81(4)degrees. The complex shows a distorted cis-geometry about the metal centre, the macrocyclic ligand being folded about a N-Ni-N axis, with the pendant arms coordinated cis to one another. The Ni(II)L-1 (where (L-1)(2-) is meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane-4,11-di acetate) and Ni(II)L-2 complexes undergo reversible one-electron oxidation, with redox potentials of 0.717 and 0.766 V (vs. S.H.E.), respectively. Chemical oxidation of the complexes by peroxodisulfate obey overall second-order rate laws. Enthalpies and entropies of activation for Ni(II)L-1 are 38 +/- 3 kJ mol(1) and -106 +/- 9 J K-1 mol(1); and for Ni(II)L-2 the corresponding values are 41 +/- 5 kJ mol(1) and -126 +/- 15 J K-1 mol(1). Outer-sphere oxidation of Ni(II)L-1 by bis-(triazacyclononane)nickel(III) ([Ni(III)(tacn)(2)](3+)) has been studied, and a self-exchange rate constant for the [Ni(II)L-1](0/+) system has been estimated at 55 mol(1) dm(3) s(1).

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