Hydroxide Complex of the Magnesium Octaethylporphyrin Cation: Spectrum and Formation Constant in Alcohol-Water Solvents

Article abstract of DOI:10.1021/j100230a028

The formation of the complex between photogenerated magnesium octaethylporphyrin cation and hydroxide has been studied in mixed alcohol/water solvents.This complex has been postulated as a participant in the transport of hydroxide and/or hydrogen ion across bilayer lipid membranes.The absorption spectrum of the complex (λ_max = 665 nm) is red shifted from that of the cation (λ_max = 680 nm) in both methanol/water (M/W) and ethanol/water (E/W) solvents.The formation constants for the complex in the two solvent systems are 16 (in M/W) and 190 M^-1 (in E/W).The difference in the two constants is too great to be explained entirely by the Born equation (with correction for ion screening) in which the dielectric constant of the solvent system is the pertinent variable.Differential solvation effects appear to be involved.The rate constants, k₂, for the reaction of the cation with the p-dinitrobenzene anion, p-DNB^- (p-DNB is the electron acceptor for the photogeneration of the magnesium octaethylporphyrin cation), and k₄, for the reaction of the complex with p-DNB^-, are 1.1*10⁹ and 2.1*10⁹ M^-1 s^-1 in M/W and 1.3*10⁹ and 1.2*10⁹ M^-1 s^-1 in E/W.Although the rate constants are within an order of magnitude of being diffusion controlled, the fact that k 4% >= k₂ is indicative of some degree of activation control.