ELECTRON-RICH IRIDIUM COMPLEXES WITH MIXED-DONOR POLYDENTATE LIGANDS - CHEMOSELECTIVE CATALYSTS IN HYDROGEN-TRANSFER REDUCTION OF ALPHA,BETA-UNSATURATED KETONES

The mixed-donor polydentate ligands Prn-N(CH2CH2PPh2)2 (PNP) and Et2NCH2CH2N(CH2CH2PPh2)2(P2N2) react in THF with [Ir(cod)(OMe)]2 (cod = cycloocta-1,5-diene) yielding the σ,η2-cyclooctenyl complexes [(PNP)Ir(σ,η2-C8H13) (1) and [(P2N2)Ir(σ,η2-C8H13)] (2). The crystal structure of 1 has been determined by X-ray methods. The iridium atom is coordinated to the phosphorus and nitrogen donors of PNP and to a cyclooetenyl group via σ and η bonding in a distorted trigonal-bipyramidal geometry. The same coordination geometry is assigned to the P2N2 derivative that exhibits a free diethylamino group. In solution, above ca. -30 °C, compounds 1 and 2 are in equilibrium with the hydride η4-cod isomers [(PNP)IrH(η4-cod)] (3) and [(P2N2)IrH(η4-cod)] (4) via a β-H elimination/hydride migration pathway. The equilibrium constants for the 1 ⥦ 3 and 2 ⥦ 4 interconversions have been obtained at different temperatures by 31P NMR integration in the range 298-348 K. The thermodynamic functions ΔH° and ΔS° for the 1 ⥦ 3 and 2 ⥦ 4 isomerization reactions have been calculated. They are rather similar with each other, but the enthalpy contribution appears slightly more favorable for the latter reaction, in nice accord with the higher concentration of the hydride species observed for the P2N2 system at comparable temperatures. Compounds 1 and 2 arc good catalyst precursors for the chemoselective hydrogen-transfer reduction of α,β-unsaturated ketones such as bcnzylidcncacctonc to allylic alcohols. The catalytic activity of 1 and 2 has been compared and contrasted to that exhibited by the system “[Ir(cod)(OMe)]2 + PNP or P2N2” prepared in situ as well as other related complexes like [(PNP)lr(η4-cod)]BPh4 (5), [(P2N2)Ir(η4-cod)]BPh4 (6), [(triphos)Ir(σ,η2-C8H13)] (7) and [(triphos)Ir(σ,η2-C8H13)]BPh4 (8) [triphos = MeC(CH2PPh2)3], Through this comparison, valuable mechanistic information on the catalysis cycle has been obtained. © 1990, American Chemical Society. All rights reserved.