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Activation of molecular hydrogen, molecular oxygen, and olefins by solutions containing some univalent iridium complexes Chan, Cheuk-Yin

Abstract

Kinetic and spectroscopic studies on solutions of two iridium(I) complexes—trans-chlorocarbonylbis(triphenylphosphine)iridium(I), Ir(CO)Cl(PPh3)2, and μ-dichlorotetrakis (cyclooctene)-di-iridium(I) , [IrCl(C8H,14)2]2—are described, especially for reactions involving activa-tion of molecular H2, molecular O2, and olefins. The studies also illustrate the importance of solvent effects. The catalytic activity of Ir(CO)Cl(PPh3)2 for hydrogenation of maleic acid has been surveyed using a range of solvents—pyridine, dimethylsulfoxide (DMSO), dimethylacetamide (DMA), dimethylformamide (DMF), acetone, sulfolane, acetonitrile, nitromethane and formamide. Where activity is observed, the mechanism appears to involve activation of hydrogen by a square-planar four-coordinate Ir(I) olefin complex. The DMA, DMF and DMSO solvent systems, which are very similar in terms of coordinating ability and dielectric constant, do show catalytic activity and this results from the dissociation of a phosphine molecule from the iridium at some stage to form the required four-coordinate catalyst 1: [series of chemical reactions] The sulfolane system is more active than the DMA, DMF and DMSO systems, but shows much more complicated kinetics. The hydrogenation appears to proceed in part via the phosphine dissociation path outlined in the above scheme, but the major pathway involves a cationic inters mediate Ir (CO) (PPh3)2 (olefin)+, 2, formed via chloride dissociation from the five-coordinate olefin complex. Diethylmaleate is hydrogenated in sulfolane, however, primarily via the phosphine dissociation path. Solvents that are too strongly coordinating (pyridine) or too weakly coordinating (nitromethane) lead to catalytically inactive systems. The catalytic homogeneous hydrogenation of hexene-1, cyclooctene using DMA solutions of [IrCl(C8H14)2]2 involves a monomeric species. The strongly coordinating solvent or the added olefin are thought to cleave the chloride bridge in [IrCl^gH^^J^* The hydrogenation mechanism can be outlined as [series of chemical reactions] where Ir is a complex already containing coordinated olefin. Selective hydrogenation of cyclooctene in a mixture of cyclooctene and hexene-1, the catalytic isomerization of hexene-2 and the catalytic hydrogenolysis of molecular 02 to water, all using [IrCl(C8H 14)2]2 complex in DMA are described and discussed. Molecular 0 is activated by DMA solutions of [IrCl2(C8H14)2]2 containing excess chloride; the major species believed to be present in solution is [IrCl2(C8H14)2]2⁻ . The solution initially absorbs one 02 per iridium. Product characterization proved to be difficult but the solutions catalytically oxidize cumene likely via a hydroperoxide free radical intermediate and the data are discussed in terms of the following equilibria: [series of chemical reactions] During some preliminary studies to investigate possible activation of CO under mild conditions using Ir complexes in aqueous solutions, the iridium(III) dicarbonyl [Ph4As]⁺ [Ir(C0)2C14]⁻•2H20, and a new cluster carbonyl tentatively formulated [Ir(CO)2]n, were synthesized.

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