THERMAL AND PHOTOCHEMICAL C-H BOND ACTIVATION REACTIONS AT IRIDIUM-PI-COORDINATION VS C-H CLEAVAGE OF ETHENE, STYRENE, AND PHENYLACETYLENE

Thermolysis of [(triphos)Ir(H)2(C2H5)] (1) in various solvents (benzen e, THF, MeCN, DMF) results in formation of the 16-electron fragment [( triphos)IrH] and ethane. The reactive intermediate is capable of inser tion into the C-H bonds of either solvents or appropriate substrates ( 1-alkynes, ethene), generally affording sigma-organyl dihydrides of th e formula [(triphos)Ir(H)2-(sigma-organyl)] (organyl = C6H5, CH2CN, C= CPh, C=CCO2Et, CH=CH2). The hydride carbonyl [(triphos)IrH(CO)] and di methylamine are produced by reaction with N,N-dimethylformamide. Therm olysis of 1 in THF at 66-degrees-C in the presence of ethene (1-4 atm) gives the vinyl hydride [(triphos)Ir(H)2(CH=CH2)] (8) and the pi-comp lex [(triphos)IrH(C2H4)] (7) in a kinetic product ratio showing that p i-complexation is not a precursor for C-H insertion. Under the same re action conditions, styrene gives selectively the pi-complex [(triphos) IrH(CH2=CHPh)] (9). UV irradiation of 7 and 9 in either benzene or THF at 20-degrees-C promotes insertion of iridium into the C-H bonds to g ive 8 and a 1:1 mixture of the E and Z styryl complexes [(triphos)Ir(H )2-(CH=CHPh)] (11, 12), respectively. Secondary photolysis on either 8 or 11 and 12 results in photoejection of ethene and dihydrogen, respe ctively. In the case of the vinyl complex, the reactive intermediate i s trapped by the solvent, whereas the styryl dihydrides are converted to the stable pi-alkyne hydride [(triphos)IrH(HC=CPh)]. The latter com plex is also obtained by photolysis of [(triphos)Ir(H)2(C=CPh)] in THF at 20-degrees-C. Both the vinyl dihydride and the styryl dihydrides a re thermodynamically unstable and convert to the corresponding pi-olef in complex in refluxing THF. Irradiation of 1 in either THF or benzene at 20-degrees-C produces several metal products and gases (C2H6, C2H4 , and H-2) due to the occurrence of both primary and secondary photoly sis reactions. Irradiation of the trihydride [(triphos)Ir(H)3] in benz ene results in formation of the phenyl dihydride [(triphos)Ir(H)2(Ph)] and evolution of H-2. Possible mechanisms for the thermal and photoch emical reactions are discussed.