Structure and Photochemistry of Dicyanocobalt(III) Tetraphenylporphyrin. Photochromic Reaction Caused by Photodissociation of Axial Ligand

Chlorocobalt(III) tetraphenylporphyrin, (Cl)CoIIITPP, reacts with potassium cyanide in dichloromethane or benzene containing 18-crown-6 to give a green solution of [crown-K+][(CN)2CoIIITPP-]. The molecular structure of [crown-K+][(CN)2CoIIITPP-] is identified by X-ray crystallography. In methanol, (Cl)CoIIITPP plus KCN also gives a green solution of [(CN)2CoIIITPP-]. The green methanol solution containing 1.4 × 10-4 M KCN turns orange by continuous photolysis with a 250-W mercury lamp for 5 min. The orange solution returns to green when it is kept in the dark for 5 min. The kinetic study suggests that [(CN)2CoIIITPP-] dissociates CN- by continuous photolysis, giving rise to the formation of the orange species, (CH3OH)(CN)CoIIITPP. The photoproduct, (CH3OH)(CN)CoIIITPP, regenerates the green species, [(CN)2CoIIITPP-], by reaction with CN-. The laser photolysis study of [(CN)2CoIIITPP-] in methanol demonstrates that photodissociation of CN- takes place within 20 ns after the 355-nm laser pulse, resulting in the formation of two transients, I (short-lived) and II (long-lived). The absorption spectra of both transients are similar to that of (CH3OH)(CN)CoIIITPP. These transients eventually return to [(CN)2CoIIITPP-]. The decay of species I follows first-order kinetics with a rate constant k = 2 × 106 s-1, independent of the concentration of KCN. Species II is identified as (CH3OH)(CN)CoIIITPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K+][(CN)2CoIIITPP-] in dichloromethane gives the transient species, which goes back to the original complex according to first-order kinetics with a rate constant k = 5 × 106 s-1. [crown-K+][(CN)2CoIIITPP-] is concluded to photodissociate the axial CN- to form [crown-K+CN-][(CN)CoIIITPP] in which an oxygen atom of the crown moiety in [crown-K+CN-] is coordinated to the cobalt(III) atom of [(CN)CoIIITPP] at the axial position. The intracomplex reverse reaction of [crown-K+CN-][(CN)CoIIITPP] leads to the regeneration of [crown-K+][(CN)2CoIIITPP-]. The structure and the reaction of the transient species I observed for [(CN)2CoIIITPP-] in methanol are discussed on the basis of the laser photolysis studies of [crown-K+][(CN)2CoIIITPP-] in dichloromethane.