Structure and photochemistry of dicyanocobalt(III) tetraphenylporphyrin. Photochromic reaction caused by photodissociation of axial ligand

Article abstract of DOI:10.1021/ic000408x

Chlorocobalt(III) tetraphenylporphyrin, (Cl)Co(III)TPP, reacts with potassium cyanide in dichlommethane or benzene containing 18-crown-6 to give a green solution of [crown-K⁺][(CN)₂Co(III)TPP^-]. The molecular structure of [crown-K⁺][(CN)₂Co(III)TPP^-] is identified by X-ray crystallography. In methanol, (Cl)Co(III)TPP plus KCN also gives a green solution of [(CN)₂Co(III)TPP^-]. The green methanol solution containing 1.4 x 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(III)TPP^-] dissociates CN^- by continuous photolysis, giving rise to the formation of the orange species, (CH₃OH)(CN)Co(III)TPP. The photoproduct, (CH₃OH)(CN)-Co(III)TPP, regenerates the green species, [(CN)₂Co(III)TPP^-], by reaction with CN^-. The laser photolysis study of [(CN)₂Co(III)TPP^-] 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(III)TPP. These transients eventually return to [(CN)₂Co(III)TPP^-]. The decay of species I follows first-order kinetics with a rate constant k = 2 x 10⁶ s^-1, independent of the concentration of KCN. Species II is identified as (CH₃OH)(CN)Co(III)TPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K⁺][(CN)₂Co(III)TPP^-] in dichloromethane gives the transient species, which goes back to the original complex according to first-order kinetics with a rate constant k = 5 x 10⁶ s^-1. [crown-K⁺][(CN)₂Co(III)TPP^-] is concluded to photodissociate the axial CN^- to form [crown-K⁺CN^-][(CN)Co(III)TPP] in which an oxygen atom of the crown moiety in [crown-K⁺CN^-] is coordinated to the cobalt(III) atom of [(CN)Co(III)TPP] at the axial position. The intracomplex reverse reaction of [crown-K⁺CN^-][(CN)Co(III)TPP] leads to the regeneration of [crown-K⁺][(CN)₂Co(III)TPP^-]. The structure and the reaction of the transient species I observed for [(CN)₂Co(III)TPP^-] in methanol are discussed on the basis of the laser photolysis studies of [crown-K⁺][(CN)₂Co(III)TPP^-] in dichloromethane.