Kinetics and mechanism of the reaction of hydrogen and of the homogeneous catalytic hydrogenation of ethylene with hydridocarbonyltris(triphenylphosphine)iridium(I)

Abstract

Hydrogen and hydridocarbonyltris(triphenylphosphine)iridium(I) have been found to establish equilibrium (i). IrH(CO)(PPh₃)₃+ H₂⇌ IrH₃(CO)(PPh₃)₂+ PPh₃(i) The equilibrium constant K₁ is 5·63 ± 0·33 at 25 °C and 2·19 ± 0·18 at 50 °C. The kinetics of the reaction suggest that it occurs through steps (ii) and (iii). The combined rate constant k_iik_iii/k_–iii is (1·52 ± 0·08)× 10^–3 s^–1 at IrH(CO)(PPh₃)₃⇌ IrH(CO)(PPh₃)₂+ PPh₃(ii), IrH(CO)(PPh₃)₂+ H₂⇌ IrH₃(CO)(PPh₃)₂(iii) 25 °C and (4·88 ± 0·05)× 10^–3 s^–1 at 50 °C. At 25 °C, k_ii > 10^–2 s^–1 while at 50 °C k_ii > 10^–1 s^–1. The rate of the catalysed hydrogenation of ethylene at 50 °C obeys the rate equation (iv) in which k=(10·0 ± 0·3)× 10^–3–d[C₂H₄]//_dt=k[IrH(CO)(PPh₃)₃]₀[C₂H₄]//_{1 +k′[H2]+k″[PPh3]}(iv) l mol^–1 s^–1, k′= 828 ± 33 l mol^–1, and k″= 247 ± 13 l mol^–1. The term [IrH(CO)(PPh₃)₃]₀ represents the initial complex concentration and [PPh₃] is calculated from the equilibrium constant K₁. The results suggest that the active catalyst is hydridocarbonylbis(triphenylphosphine)iridium(I).