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

The ethene complexes CpIr(L)(C2H4) (Cp = eta5-C5H5, L = PPh3, CO) undergo two competing photochemical reactions in solution: (i) isomerization 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 hydride is favored over the intermolecular product by longer wavelength photolysis, lower temperature, and a more rigid surrounding medium and by L = PPh3 compared to L = CO. The vinyl hydride isomer is the exclusive 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 latter is also formed by secondary photolysis of the vinyl hydride. The barrier for the thermal conversion of CpIr(L)(C2H3)H to the parent ethene complex CpIr(L)(C2H4) increases in the following order: L = C2H4, DELTAH(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 mol-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 intermolecular C-H bond activation involves a thermally activated step. The photochemical reaction of CpIr(C2H4)2 with PPh3 in CD3CN generates the vinyl hydride CpIr(PPh3)(C2H3)H via CpIr(PPh3)(C2H4). The corresponding reaction occurs in benzene with the additional formation of the solvent activation product CpIr(PPh3)(Ph)H.