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



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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