Homogeneous catalytic and stoicheiometric hydrogenation of ethylene by trishydridocarbonylbis(triphenylphosphine)iridium(III)

Abstract

The complex trishydridocarbonylbis(triphenylphosphine)iridium(III), IrH₃(CO)(PPh₃)₂, has been found to hydrogenate ethylene catalytically and stoicheiometrically in dimethylformamide at 50°. The rates observed were predicted by equation (i) which could be deduced from a similar equation established previously for the catalyst hydridocarbonyltris(triphenylphosphine)iridium(I), IrH(CO)(PPh₃)₃, where [IrH₃(CO)(PPh₃)₂]₀ represents the initial concentration of IrH₃(CO)(PPh₃)₂. –d[C₂H₄]//_dt=0·01[IrH₃(CO)(PPh₃)₂]₀[C₂H₄]//_{1 + 828[H2]+ 247[PPh3]}(i) The reaction of ethylene with IrH₃(CO)(PPh₃)₂ controls the rate of ethylene activation while the inhibitory effect of hydrogen and triphenylphosphine is thought to originate in subsequent competition between reactions of the ethylene intermediate giving ethane and those with hydrogen and phosphine reforming ethylene. The inhibition cannot be explained solely by the effect of the equilibria (ii) and (iii). IrH₃(CO)(PPh₃)₂⇌ IrH(CO)(PPh₃)₂+ H₂(ii), IrH(CO)(PPh₃)₃⇌ IrH(CO)(PPh₃)₂+ PPh₃(iii)