LASER-PHOTOLYSIS STUDIES OF ISOCYANIDE COMPLEXES OF CHLOROCHROMIUM(III) TETRAPHENYLPORPHYRIN IN TOLUENE SOLUTIONS

Chloroaquachromium(III) tetraphenylporphyrin, Cl(H2O)(CrTPP)-T-III, re versibly reacts with isocyanide, CN-R, to form Cl(CN-R)(CrTPP)-T-III i n toluene solutions containing 4.4 x 10(-3) M H2O and CN-R. The equili brium constant for the formation of CI(CN-R)(CrTPP)-T-III obtained by spectroscopic measurements ranges from 10 to 10(2) depending on the na ture of the isocyanide used. The laser-photolysis studies of the tolue ne solution demonstrate that CN-R is photodissociated from Cl(CN-R)(Cr PP)-P-III to produce (ClCrTPP)-T-III with a quantum yield ca. 0.1. The five-coordinate (ClCrTPP)-T-III reacts with H2O and CN-R to give Cl(H 2O)(CrTPP)-T-III and Cl(CN-R)(CrTPP)-T-III, respectively. The transien t product Cl(H2O)(CrTPP)-T-III further reacts with CN-R to regenerate Cl(CN-R)(CrTPP)-T-III. The rates for the decay of Cl(H2O)(CrTPP)-T-III are measured as a function of the concentration of CN-R in order to d etermine the rate constants of the forward and backward reactions for the formation of Cl(CN-R)(CrTPP)-T-III. The equilibrium constants obta ined from the kinetic analysis for the decay of Cl(H2O)(CrTPP)-T-III a re in good agreement with those determined by spectroscopic measuremen ts. The mechanism for the equilibrium reaction is discussed in detail on the basis of the rate constants for the formation of Cl(H2O)(CrTPP) -T-III and Cl(CN-R)(CrTPP)-T-III from (ClCrTPP)-T-III, measured in the temperature range 200-300 K. The quantum yield for the photodissociat ion of CN-R is not affected by the presence of oxygen. Since the lowes t excited state T-6(1) Of Cl(CN-R)(CrTPP)-T-III observed by laser phot olysis is effectively quenched by oxygen, this state is not responsibl e for the ligand ejection from CI(CN-R)(CrTPP)-T-III. The S-4(1) State is assumed to be the reactive excited state of Cl(CN-R)(CrTPP)-T-III.