Oxidation-reduction Reactions of Complexes with Macrocyclic Ligands. Oxygen Uptake Kinetics, Equilibriums and Intermediates in Aqueous CoII(N₄) Systems

Abstract

The reactions of Co([14]aneN4)(OH2)2 2+ and Co([15]aneN4)(OH2)2 2+ with O2 to form [H2OCo([n]aneN4)]2O2 4+ have been investigated in water at 25°C. The reactions were found to occur in two stages. The initial stage involves the generation of a very reactive 1:1 adduct. At very low ionic strengths the initial step can be isolated from the second reaction stage and rate and equilibrium constants may be determined: k1 = (5.0 ± 1.5) × 105 M-1 s-1 and K1 = (8 ± 2) × 103 M-1 for the low-spin [14]aneN4 complex; k1 = (3.8 ± 1.1) × 103 M-1 s-1 and K1 ≃ 4 × 103 M-1 for the high-spin [15]ancN4 complex For the formation of the μ-peroxo product in the second reaction stage, the respective rate constants are k2 = (4.9 ± 0.4) 105 and (5 ± 1) × 104 M-1 s-1 (μ = 0.4, 25°C). More limited studies of several other reactions of macrocyclic cobalt(II) complexes with O2 have demonstrated that the uptake steps are always rapid, but the extent of the overall equilibrium, CoII(N4)(OH2)2/O2 with a μ-peroxo species, depends somewhat on the cobalt(III)-(II) reduction potential. Most of the variation appears to be due to the second reaction stage (K2). The formation constant for Co(N4)(OH2)O2 2+ (from Co(N4)(OH2)2 3+ and O2 -) is found to be surprisingly large, Kf = 5 × 1021 M-1, suggestive of a significant covalent component to the Co-O2 interaction The intrinsic or reorganizational barrier to adduct formation is small and not very sensitive to variations in the nature of the cobalt complex. In particular, differences in the spin state of the cobalt complex probably affect the rate of substitution into the Co-OH2 bond, but do not seem otherwise to influence the intrinsic reactivity of the complex. © 1980 American Chemical Society.

Department(s)

Chemistry

International Standard Serial Number (ISSN)

0002-7863

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 1980 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Aug 1980

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