COMPETITION BETWEEN INTRAMOLECULAR AND INTERMOLECULAR C-H ACTIVATION IN IRIDIUM ETHENE COMPLEXES

The ethene complexes CpIr(L)(C2H4) (Cp = eta5-C5H5, L = PPh3, CO) unde rgo two competing photochemical reactions in solution: (i) isomerizati on to the vinyl hydride CpIr(L)(C2H3)H and (ii) dissociation of ethene and insertion into solvent C-H bonds to form CpIr(L)(R)H. The vinyl h ydride is favored over the intermolecular product by longer wavelength photolysis, lower temperature, and a more rigid surrounding medium an d by L = PPh3 compared to L = CO. The vinyl hydride isomer is the excl usive product of UV irradiation of CpIr(CO)(C2H4) in solid toluene (77 K) or solid argon (12 K) but competes with formation of CpIr(CO)2 in CO-doped Ar matrices and CpIr(CO)(CH3)H in methane matrices. The latte r is also formed by secondary photolysis of the vinyl hydride. The bar rier for the thermal conversion of CpIr(L)(C2H3)H to the parent ethene complex CpIr(L)(C2H4) increases in the following order: L = C2H4, DEL TAH(double dagger) = 67 +/- 5 kJ mol-1; L = CO, DELTAH(double dagger) = 95 +/- 4 kJ mol-1; L = PPh3, DELTAG(double dagger) = 135 +/- 3 kJ mo l-1 at 318 K. These findings are consistent with our proposal that C-H bond activation occurs via a ''cage-complex'' intermediate. However, the marked influence of temperature on the reaction indicates that int ermolecular C-H bond activation involves a thermally activated step. T he photochemical reaction of CpIr(C2H4)2 with PPh3 in CD3CN generates the vinyl hydride CpIr(PPh3)(C2H3)H via CpIr(PPh3)(C2H4). The correspo nding reaction occurs in benzene with the additional formation of the solvent activation product CpIr(PPh3)(Ph)H.