Structure and redox kinetics of nickel(II) tetraazamacrocycles containing pendant carboxylate groups

The nickel(II) complex of the deprotonated pendant-arm macrocycle 5,12-dime thyl-7,14-diphenyl-1,4,8,11-tetraazacyclotetradecane-4,11-diacetate (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, wit h a = 12.981(8), b = 16.474(6), c = 14.262(7) Angstrom and beta = 98.81(4)d egrees. 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 a rms 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-diacetate) and Ni(II)L-2 complexes undergo reversible one-electron oxidation, with re dox potentials of 0.717 and 0.766 V (vs. S.H.E.), respectively. Chemical ox idation 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 val ues are 41 +/- 5 kJ mol(1) and -126 +/- 15 J K-1 mol(1). Outer-sphere oxida tion 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).