Homogeneous hydrogenation of buta-1,3-diene and ethylene catalysed by carbonylhydridotris(triphenylphosphine)iridium(I) and by carbonyltrihydridobis(triphenylphosphine)iridium(III)

Abstract

Buta-1,3-diene is catalytically hydrogenated to a mixture of butenes and butane by carbonylhydridotris(triphenylphosphine)iridium(I), IrH(CO)(PPh₃)₃, and by carbonyltrihydridobis(triphenylphosphine)iridium(III), IrH₃(CO)(PPh₃)₂, at 50° in dimethylformamide. The rates observed could be predicted by the empirical rate equation, –d[C₄H₆]//_dt=(1·86 × 10^–3+ 0·681[PPh₃])[Ir]₀[C₄H₆]//_{( 1 + 1·42 × 10⁴[PPh3])} where [Ir]₀=[IrH(CO)(PPh₃)₃]+[IrH₃(CO)(PPh₃)₂].

This equation and additional spectrophotometric and kinetic studies of the individual steps in the catalytic cycle are quantitatively interpreted in terms of a mechanism based on the reductive elimination of butenes by Ir(σ-C₄H₇)H₂(CO)(PPh₃)₂ formed from Ir(π-C₄H₇)(CO)(PPh₃)₂ produced by the initial reactions of buta-1,3-diene with both IrH(CO)(PPh₃)₃ and IrH₃(CO)(PPh₃)₂. The intermediate Ir(π-C₄H₇)(CO)(PPh₃)₂ has been isolated and characterised. Rate data for the hydrogenation of ethylene catalysed by the same system have also been quantitatively interpreted by a fundamentally similar mechanism based on the formation of Ir(σ-C₂H₅)(CO)(PPh₃)₂ and its subsequent unimolecular decomposition.