A mechanistic study of thermal C-H reductive elimination from a six-coordinate d6 iridium complex

The thermolysis of trans-IrL2(CO)Cl(H)(C6H5) (1a; L=P(i-Pr)(3); H trans to CO) produces benzene and the Vaska-type complex IrL2(CO)Cl. A mechanistic study of the reaction has shown that la reversibly loses CO at 120 degrees C (as evidenced by the incorporation of (CO)-C-13) and isomerizes to the previously unreported 1b (H trans to Cl). It was found that 1b is the complex primarily responsible for the formation of benzene upon thermolysis under CO atmosphere; direct loss of benzene from 1a was determined to be, at most, a minor pathway. Benzaldehyde was also formed as a product of thermolysis of la under CO atmosphere. The first-order rate constant for benzene elimination in the absence of CO was found to be 8.5 x 10(-5) s(-1). The presence of only 5 Torr CO results in a decrease to 2.0 x 10(-5) s(-1), but little further inhibition is observed above 5 Torr CO. Added dihydrogen (100 Torr) was found to effect a novel catalysis of benzene elimination from 1a in the absence of CO atmosphere; it is suggested that trace amounts of dihydrogen, present in solutions of 1a, are responsible for the enhanced rate of elimination in the absence of CO. The thermolysis of 1-d(6) in toluene was found to proceed without any toluene incorporation, implying that arene loss is irreversible. (C) 1998 Elsevier Science S.A.