Thermal and Photochemical C-H Bond Activation Reactions at Iridium.- Coordination vs. C-H Cleavage of ethene, Styrene and Phenylacetylene

Thermolysis of [ (triphos)Ir(H)2(C2H5)1 (1) in various solvents (benzene, THF, MeCN, DMF) results in formation of the 16-electron fragment [(triphos)IrH] and ethane. The reactive intermediate is capable of insertion into the C-H bonds of either solvents or appropriate substrates (1-alkynes, ethene), generally affording a-organyl dihydrides of the formula [(triphos)Ir(H)a- (a-organyl)] (organyl = CeHb, CHzCN, CSCPh, C=CC02Et, CH=CH2). The hydride carbonyl [(triphos)IrH(CO)] and dimethylamine are produced by reaction with Nfl-dimethylformamide. Thermolysis of 1 in THF at 66 "C in the presence of ethene (1-4 atm) gives the vinyl hydride [(triphos)Ir(H)2(CH=CH2)](8) and the a-complex [(triphos)IrH(CzH4)] (7) in a kinetic product ratio showing that a-complexation is not a precursor for C-H insertion. Under the same reaction conditions, styrene gives selectively the a-complex [(triphos)IrH(CH2=CHPh)l (9). UV irradiation of 7 and 9 in either benzene or THF a t 20 OC promotes insertion of iridium into the C-H bonds to give 8 and a 1:l mixture of the E and 2 styryl complexes [(triphos)Ir(H)z- (CH=CHPh)] (11, 12), respectively. Secondary photolysis on either 8 or 11 and 12 results in photoejection of ethene and dihydrogen, respectively. In the case of the vinyl complex, the reactive intermediate is trapped by the solvent, whereas the styryl dihydrides are converted to the stable a-alkyne hydride [(triphos)IrH(HC=CPh)l. The latter complex is also obtained by photolysis of [(triphos)Ir(H)2(C=CPh)l in THF at 20 "C. Both the vinyl dihydride and the styryl dihydrides are thermodynamically unstable and convert to the corresponding a-olefin complex in refluxing THF. Irradiation of 1 in either THF or benzene at 20 "C produces several metal products and gases (C2H6, CzH4, and H2) due to the occurrence of both primary and secondary photolysis reactions. Irradiation of the trihydride [ (triphos)Ir(H)3] in benzene results in formation of the phenyl dihydride [(triphos)Ir(H)2(Ph)] and evolution of H2. Possible mechanisms for the thermal and photochemical reactions are discussed.