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Complexation Kinetics of Nickel(II) Reacting with py[13]aneN4

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The complexation reaction between Ni(II) and the 13-membered pyridine containing tetraaza macrocycle, py[13]aneN4, (3,7,10,16-tetra-azabicyclo[10.3.1]hexadeca-1(16),11,13-triene), VII, was investigated in aqueous solution in the temperature range (15-35)° C by UV-Vis spectrophotometry. Two consecutive reactions were observed in two distinct time ranges, at λmax 395 nm for the Ni(II) macrocycle absorption. The initial fast step was found to be a second-order reaction (first-order in both Ni(II) and ligand) generating an intermediate species. The subsequent slower reaction was found to be first-order in the intermediate species as it is converted to the final product. It was concluded from the sizes of the second-order rate constant (= 74 (M s)−1 and the first-order rate constant (= 23 × 10−3 s−1) of the second step that initial metal complexation with the ligand occurs, forming an intermediate Ni(II) complex (NiHL3+) followed by deprotonation and possible rearrangements of Ni-N bond to form the thermodynamically stable stereoisomer (NiL)2+. All kinetic measurements were carried out under pseudo first-order conditions in the pH range 5.2-6.1 with the total metal ion concentration in at least ten-fold excess of the total concentration of the ligand. The activation parameters ΔH # and ΔS # for the initial fast reaction were calculated.

The visible spectra of the species indicate formation of a square-planar metal complex with the tetradentate ligand.

Potentiometric titrations did not produce reliable values for the formation constants due to the long equilibration period. However, spectrophotomeric measurements reproduced the literature value for log K NiL (found: 17.3, lit.: 16.81 [Cost and Delgado, Inorg. Chem. 32, 23 (1993)]).
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Keywords: Complexation; Kinetic study; Nickel; Tetraaza macrocycle

Document Type: Research Article

Publication date: May 20, 2003

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