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

Chlorocobalt(III) tetraphenylporphyrin, (Cl)Co<sup>III</sup>TPP, reacts with potassium cyanide in dichloromethane or benzene containing 18-crown-6 to give a green solution of [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>]. The molecular structure of [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] is identified by X-ray crystallography. In methanol, (Cl)Co<sup>III</sup>TPP plus KCN also gives a green solution of [(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>]. The green methanol solution containing 1.4 × 10<sup>-4</sup> 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)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] dissociates CN<sup>-</sup> by continuous photolysis, giving rise to the formation of the orange species, (CH<sub>3</sub>OH)(CN)Co<sup>III</sup>TPP. The photoproduct, (CH<sub>3</sub>OH)(CN)Co<sup>III</sup>TPP, regenerates the green species, [(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>], by reaction with CN<sup>-</sup>. The laser photolysis study of [(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] in methanol demonstrates that photodissociation of CN<sup>-</sup> takes place within 20 ns after the 355-nm laser pulse, resulting in the formation of two transients, <b>I</b> (short-lived) and <b>II</b> (long-lived). The absorption spectra of both transients are similar to that of (CH<sub>3</sub>OH)(CN)Co<sup>III</sup>TPP. These transients eventually return to [(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>]. The decay of species <b>I</b> follows first-order kinetics with a rate constant <i>k</i> = 2 × 10<sup>6</sup> s<sup>-1</sup>, independent of the concentration of KCN. Species <b>II</b> is identified as (CH<sub>3</sub>OH)(CN)Co<sup>III</sup>TPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] in dichloromethane gives the transient species, which goes back to the original complex according to first-order kinetics with a rate constant <i>k</i> = 5 × 10<sup>6</sup> s<sup>-1</sup>. [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] is concluded to photodissociate the axial CN<sup>-</sup> to form [crown-K<sup>+</sup>CN<sup>-</sup>][(CN)Co<sup>III</sup>TPP] in which an oxygen atom of the crown moiety in [crown-K<sup>+</sup>CN<sup>-</sup>] is coordinated to the cobalt(III) atom of [(CN)Co<sup>III</sup>TPP] at the axial position. The intracomplex reverse reaction of [crown-K<sup>+</sup>CN<sup>-</sup>][(CN)Co<sup>III</sup>TPP] leads to the regeneration of [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>]. The structure and the reaction of the transient species <b>I</b> observed for [(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] in methanol are discussed on the basis of the laser photolysis studies of [crown-K<sup>+</sup>][(CN)<sub>2</sub>Co<sup>III</sup>TPP<sup>-</sup>] in dichloromethane.