PHOTOCHEMICAL GENERATION OF REACTIVE TRANSITION-METAL INTERMEDIATES FROM AIR-STABLE PRECURSORS - CARBON HYDROGEN-BOND ACTIVATION VIA NEAR-ULTRAVIOLET PHOTOLYSIS OF CP-STAR-IR(L)(OXALATE) AND CP-STAR-IR(L)(N3)2 COMPLEXES

We have synthesized several new oxalate and azide complexes of the form Cp*Ir(Ox)L (Cp* = pentamethylcyclopentadienyl, Ox = oxalate and L = P(Me)3, P(Cy)3) and Cp*Ir(N3)2L (L = P(Me)3, P(Ph)3, P(Cy)3, and t-BuNC) respectively. Upon photolysis, these complexes undergo reactions that results 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 orthometalated complexes we identify as the cis and trans isomers, which arise from activation of either the axial or equatorial hydrogen on the alpha carbon of a cyclohexyl ring. Photolysis of the oxalate complexes in CCl4, CHCl3, or CH2Cl2 produces the corresponding dichloride. For the photolysis of Cp*Ir(Ox)(P(Me)3) in CHCl3 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, CHCl3, and CH2Cl2; substantially larger quantum yields are observed for CCl4 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 CHCl3 almost-equal-to CH2Cl2 < C6H6. IR spectral data indicate significant shifts for the nu-(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 CCl4 is due to an electron-transfer mechanism from the photogenerated excited state to the solvent.