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Homogeneous hydrogenation catalysed by square planar iridium (I) complexes Memon, Nazir Ahmed


A study is described of the kinetics of reactions in which the complexes trans-Ir (CO)X(PPh₃)₂, where X = Cl, Br and I, are used for the catalytic homogeneous hydrogenation of olefins, in particular maleic acid. The catalytic activity is poor in benzene solutions but is enhanced in coordinating solvents such as N, N'-dimethylacetamide. The kinetics were studied by measurements of hydrogen uptake, at various experimental conditions in which iridium, olefin and hydrogen concentrations were varied. The rate-law is complex showing between zero and first order in each of iridium, substrate and hydrogen concentrations. The dependence of rate on the halogen followed the order I>Br>Cl, and quite remarkably traces of oxygen in the system enhance the hydrogenation rate considerably. Supplementary spectrophotometry studies have elucidated some of the equilibria involved and the mechanisms proposed show the importance of a solvent assisted dissociation step in making available a coordination site on the transition metal complex. A detailed path for the catalytic hydrogenation is suggested.

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