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

Chlorocobalt(III) tetraphenylporphyrin, (Cl)Co^IIITPP, reacts with potassium cyanide in dichloromethane or benzene containing 18-crown-6 to give a green solution of [crown-K⁺][(CN)₂Co^IIITPP^-]. The molecular structure of [crown-K⁺][(CN)₂Co^IIITPP^-] is identified by X-ray crystallography. In methanol, (Cl)Co^IIITPP plus KCN also gives a green solution of [(CN)₂Co^IIITPP^-]. 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)₂Co^IIITPP^-] dissociates CN^- by continuous photolysis, giving rise to the formation of the orange species, (CH₃OH)(CN)Co^IIITPP. The photoproduct, (CH₃OH)(CN)Co^IIITPP, regenerates the green species, [(CN)₂Co^IIITPP^-], by reaction with CN^-. The laser photolysis study of [(CN)₂Co^IIITPP^-] 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)Co^IIITPP. These transients eventually return to [(CN)₂Co^IIITPP^-]. 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)Co^IIITPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K⁺][(CN)₂Co^IIITPP^-] 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)₂Co^IIITPP^-] is concluded to photodissociate the axial CN^- to form [crown-K⁺CN^-][(CN)Co^IIITPP] in which an oxygen atom of the crown moiety in [crown-K⁺CN^-] is coordinated to the cobalt(III) atom of [(CN)Co^IIITPP] at the axial position. The intracomplex reverse reaction of [crown-K⁺CN^-][(CN)Co^IIITPP] leads to the regeneration of [crown-K⁺][(CN)₂Co^IIITPP^-]. The structure and the reaction of the transient species I observed for [(CN)₂Co^IIITPP^-] in methanol are discussed on the basis of the laser photolysis studies of [crown-K⁺][(CN)₂Co^IIITPP^-] in dichloromethane.