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 kiikiii/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, kii > 10–2 s–1 while at 50 °C kii > 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).