Formation and isomerization of cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 and cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(--Ph)(PPh 3) 2

Abstract

Oxidative addition of H–R (H--Ph and H 2) to trans-Ir(--Ph)(CO)(PPh 3) 2 ( 2) gives the initial products, cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 3a) and cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 3b), respectively. Both cis-bis(PPh 3) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh 3) complexes, trans, trans-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 4a) and cis, trans-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 4b). The isomerization, 3b→ 4b is first order with respect to 3b with k 1=6.37×10 −4 s −1 at 25°C under N 2 in CDCl 3. The reaction rate ( k 1) seems independent of the concentration of H 2. A large negative entropy of activation (Δ S ≠=−24.9±5.7 cal deg −1 mol −1) and a relatively small enthalpy of activation (Δ H ≠=14.5±3.3 kcal mol −1) were obtained in the temperature range 15∼35°C for the isomerization, 3b→ 4b under 1 atm of H 2.