Title

Kinetics and Mechanism of the Reduction of Tetrakis(4-7V-methylpyridyl)porphinecobalt(III) by Chromium(II)

Document Type

Article

Publication Date

8-1-1974

Abstract

The reduction of tetrakis(4-N-methylpyridyl)porphinecobalt(III) (CoinTMpyP) by chromium(II) has been studied as a function of pH and added anions. In perchlorate medium, the observed rate constant may be written as K = (k + k ′[H ])[Cr ], whereas in the presence of chloride ion and thiocyanate ion, catalytic pathways are introduced. For chloride k = (k + k ′/[H ])[Cr ] + (k + k /[H ])[Cl ][Cr ], and for thiocyanate k = (k + k ′/[H ])[Cr ] + (k + k ′[H ])[SCN ][Cr ] + (k + k ′[H ])[SCN ] [Cr ]. The relative rates for the acid-independent paths are k :k :k :k = 1: 30:2 × 10 ; 2 × 10 while for the acid-dependent paths k ′ k ′: k ′: k ′ = 1: 3: 3 × 10 :4 × 10 . A comparison of these relative rate constants with those for the chromium(II) reduction of tris(l.10-phenanthroline)cobalt(III), hexaamminecobalt(III), and tetrakis(4-pyridyl)porphineiron(III) leads to the conclusion that the acid-dependent pathways involve hydroxy-bridged transition states and that reduction of Co TMpyP occurs through the axially bound ligands rather than through the porphyrin ring system. The reduction of the cobalt(III) porphyrin proceeds faster than the replacement of the axial water molecules by chloride or thiocyanate ions, and, therefore, in contrast to the situation for the iion(lll) porphyrin which undergoes rapid ligand replacement, the added anions are not directly bonded to the cobalt(III) center in the transition state for the reduction. These studies show that the mechanism of electron transfer in these porphyrin systems is largely determined by the ease of ligand replacement at the metal center. © 1974, American Chemical Society. All rights reserved. obsd 0 0 obsd o o 1 1 obsd o o 2 2 3 3 0 1 2 3 0 1 2 3 + 2+ + 2+ + - 2+ + 2+ + - 2+ + - 2 2+ 3 5 2 4 III

Publication Name

Inorganic Chemistry

Volume Number

13

First Page

1956

Last Page

1960

Issue Number

8

DOI

10.1021/ic50138a034

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