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Dimerization reaction of copper(II), zinc(II), nickel(II) and manganese(III) porphyrin complexes in aqueous solutions[1]

1975; Pergamon Press; Volume: 37; Issue: 1 Linguagem: Inglês

10.1016/0022-1902(75)80142-4

1878-1225

Radha R. Das, Robert A. Plane,

Surface Chemistry and Catalysis

Copper, zinc, manganese and nickel were incorporated in the water soluble porphyrin obtained by the addition of four molecules of ethylenediamine to protoporphyrin IX dimethyl ester. Spectrophotometric and temperature-jump relaxation measurements on the aqueous solutions of copper, zinc and manganese porphyrins revealed a monomer-dimer equilibria. At 25°C, the rate constant for dimerization determined by temperature-jump relaxation studies of these three metalloporphyrins are, 4·1 × 106 M−1 sec−1, 7·5 × 107 M−1 sec−1 and 4·0 × 108 M−1 sec−1 respectively. The dimer dissociation rate constants are 2·0 × 101 sec−1, 3·0 × 102 sec−1, and 1·1 × 103 sec−1. Beer's law studies showed nickel porphyrin to be existing mostly as a single molecular aggregate in aqueous medium. Stopped flow kinetic measurements of nickel porphyrin were done in 20 volume percent alcohol water mixture, in which monomer-dimer equilibrium exists. The rate constants for the dimerization and dimer-dissociation thus obtained are 4·0 × 105 M−1 sec−1 and 2·0 sec−1 respectively. The dimerization constants determined spectroscopically under similar conditions are 3·1 × 105 M−1, 2·0 × 106 M−1 and 1·0 × 106 M−1 for copper, zinc, manganese and nickel porphyrin respectively and these values agree reasonably well with those from the kinetic results. Ethanol shifts the equilibrium toward monomer.