KINETICS AND MECHANISM OF ACID-CATALYZED DECOMPOSITION REACTIONS OF NICKEL(II) ETHYLENEDIAMINE COMPLEXES AND REACTIONS OF COPPER(II/III) PEPTIDE COMPLEXES WITH MOLECULAR OXYGEN

RITA ANN READ, Purdue University

Abstract

The rate of dissociation of Ni(en)('2+) increases linearly with acid concentration of pH values below 1.5. General-acid catalysis of the dissociation by carboxylic acids and hydrogen phosphates also occurs. Contrary to the usual behavior of general acids, the rate constants for these acids increase as the acid strength decreases. The rate constant for the solvent is 0.13 s('-1) and for the general acids, the rate constants (M('-1), s('-1)) at 25(DEGREES)C are: H(,3)O('+), 0.056; H(,3)PO(,4), 0.71; H(,2)PO(,4)('-), 1.23; HPO(,4)('2-), 11.7; Cl(,3)CCOOH, 0.10; Cl(,2)CHCOOH, 0.08; ClCH(,2)COOH, 0.141; and CH(,3)COOH, 0.30. These results indicate that the dissociation of chelate complexes is accelerated by protonation of the donor while it is still within the first coordination sphere. Acid attack occurs prior to or during solvent separation of the metal and amine donor. The unusual general-acid effect for Ni(en)('2+) reflects the ability of the acids to associate with the metal complex and to transfer a proton to a donor that is in the first coordination sphere. Copper complexes of tetrapeptides, oligopeptides, tri- and tetra-peptide amides having a glycyl residue in the third position react autocatalytically with molecular oxygen. The Cu(III) peptide is the autocatalyst and its decomposition rate determines the rate of O(,2) uptake. The rates of O(,2) uptake range from 10('-8) to 10('-6) M s('-1). The Cu(III) tripeptide complexes take up oxygen as they decompose, but the reaction is not autocatalytic. The rate of the oxygen reaction is influenced by excess peptide, buffer, and pH. Peptide complexes having a substituted amino acid in the third position do not take up oxygen.

Degree

Ph.D.

Subject Area

Chemistry

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