KINETIC AND MECHANISTIC STUDIES: I. LIGAND SUBSTITUTION REACTIONS OF TRIPEPTIDOCOPPER(II) COMPLEXES WITH SOME POLYAMINES; II. THE REACTION OF IODIDE WITH IODINE MONOCHLORIDE MEASURED BY THE PULSED-ACCELERATED-FLOW METHOD

PETER NATHANIEL DICKSON, Purdue University

Abstract

Rates and mechanisms of polypyridine substitution reactions of tripeptidocopper(II) complexes are investigated. Reactions of bipy and phen with complexes of tripeptides containing glycyl and alanyl residues proceed via the formation of stable ternary complexes. The peptide is then displaced in a series of steps that includes the proton transfer from water to the ternary complex. The overall rate of the substitution process is subject to steric hindrance from methyl residues on the peptide backbone. The reaction of the tri-L-alanine complex at pH 9 is almost 100 times slower than the corresponding reaction of the tri-glycine complex. The dependence of the reaction rate on polypyridine concentration, pH, and nature of the steric hindrance permit elucidation of the reaction mechanism in detail. The kinetics of the reaction of terpy with tripeptide complexes is determined by the initial coordination of terpy. The Cu-N(peptide) bonds are weak in the mixed-ligand complex formed and the tripeptide is displaced rapidly after the initial coordinations. The tri-(alpha)-aminoisobutyric acid complex has steric hindrance to the formation of chelated mixed-ligand complexes, thus the reaction is limited by the proton transfer to Cu(H(,-2)Aib(,3))('-). The reaction of triethylenetetramine with Cu(II)-tripeptide complexes is subject to steric hindrance from methyl groups on the peptide backbone. Activation parameters for several trien substitution reactions are determined. It is found that steric hindrance is reflected in (DELTA)H('(DBLDAG)) rather than in (DELTA)S('(DBLDAG)). The pulsed-accelerated-flow (PAF) method is used to measure the kinetics of irreversible reactions with pseudo-first-order rate constants as large as 179,000 s('-1). A procedure for correction of light scattering interference in PAF studies is presented and applied to the reaction system Fe(,aq)('2+) + IrCl(,6)('2-). The PAF method is used to study the reaction of ICl with I('-) as a function of I('-), Cl('-), and H('+) concentrations. The second-order rate constant for ICl + I('-) is 1.1 x 10('9) M('-1) s('-1) (independent of H('+)). It is shown that the reaction of ICl(,2)('-) with I('-) is at least 1000 times slower than the reaction of ICl with I('-).

Degree

Ph.D.

Subject Area

Chemistry

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