Photochemical Generation of Reactive Transition-Metal Intermediates from Air-Stable Precursors. Carbon-Hydrogen Bond Activation via Near-Ultraviolet Photolysis of CpIr(L) (oxalate) and CpIr(L)(N₃)₂ Complexes

We have synthesized several new oxalate and azide complexes of the form CpIr(Ox)L (Cp = pentamethylcyclopentadienyl, Ox = oxalate and L = P(Me)₃, P(Cy)₃) and CpIr(N₃)₂L (L = P(Me)₃, P(Ph)₃, P(Cy)₃, and r-BuNC) respectively. Upon photolysis, these complexes undergo reactions that result in the elimination of CO₂ or N₂ to give reactive intermediates that undergo net oxidative-addition reactions with C-H and C-Cl bonds. We have observed either intermolecular (reaction with solvent) or intramolecular (reaction with a ligand bond) oxidative addition to the Ir center. For photolysis of the azide complexes in benzene solutions, the mode of reaction is determined by L. An exclusively intermolecular reaction is observed when L = P(Me)₃, while products indicative of inter- and intramolecular reactivity are observed with P(Ph)₃. The P(Cy)₃ complexes produce two different orthomctalated complexes we identify as the cis and trans isomers, which arise from activation of either the axial or equatorial hydrogen on the a carbon of a cyclohexyl ring. Photolysis of the oxalate complexes in CC1₄, CHC1₃, or CH₂C1₂ produces the corresponding dichloride. For the photolysis of CpIr(Ox)(P(Me)₃) in CHC1₃ and CH₂Cl₂, intermediates are observed. These intermediates result from the initial oxidative addition of C-Cl bonds of the solvent. The quantum yields for the oxalate photochemical reactions with halocarbons are in the range 0.02-0.28. For the azide complexes, the identity of L has little effect on the quantum yield. Similar quantum yields are observed for reaction in C₆H₆, CHCI₃, and CH₂C1₂; substantially larger quantum yields arc observed for CCI₄ solutions. In the case of the oxalate complexes, the quantum yield correlates inversely with the hydrogen-bonding ability of the solvent, resulting in the curious quantum yield ordering CHC1₃ ≈ CH₂Cl₂ < C₆H₆. IR spectral data indicate significant shifts for the v(CO) frequencies of the oxalate bands in solvents of even moderate H-bonding ability. We suggest that H-bonding enhances nonradiative excited-state decay, which controls the quantum yields of the oxalate complexes. The enhanced reactivity observed for the azide and oxalate complexes in CC1₄ is due to an electron-transfer mechanism from the photogenerated excited state to the solvent.