The migratory insertion of carbon monoxide in pentamethylcyclopentadienyliridium (III) complexes. Structural effects on reactivity and mechanism for rhodium and iridium systems

Abstract

The reactions of complex (C5Me5)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh2, PMePh2, PMe2Ph) give the products of alkyl migratory insertion (C5Me5Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C5Me5)Ir(Cl)(CO)(Ph) or the iodide complexes (C5Me5)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C5Me5)Ir(Cl)(CO)(Me) with PMePh2 and PMe2Ph in acetonitrile yields the chloride substitution product [(C5Me5)Ir(CO)(L)(Me)]+Cl−. Kinetic measurements for the reactions of (C5Me5)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C5Me5)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step (K = 1.5 (CyNC), 7.3 (PHPh2), 7.1 (PMePh2) dm3 mol−1 at 323 K) and k2 for the slow carboncarbon bond formation (k2 (105) = 6.9 (CyNC), 1.2 (PHPh2), 1.0 (PMePh2) s−1 at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh2 obtained between 308 and 338 K, are ΔH≠ = 106±16 kJ mol−1 and ΔS≠ = − 14±5 J K−1 mol−1. The reaction of 1a with PMePh2 proceeds at similar rates in tetrahydrofuran (K = 3.7 dm3 mol−1, k2 (105) = 1.2 s−1, 323 K). The crystal structure of (C5Me5)Ir(Cl)(COMe) (PMe2Ph) has been determined by X-ray diffraction. C20H29ClOPIr: Mr = 544.1, monoclinic, P21/n, a = 8.084 (2), b = 9.030(2), c = 28.715 (3) A, β = 91.41 (3)°, Z = 4, Dc = 1.71 g cm−3, V = 2095.5 A3, room temperatyre, Mo Kα, γ = 0.71069, μ = 65.55 cm−1, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η5-C5Me5 molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the IrP and the IrCOMe bonds are eclipsed with respect to C(ring)-methyl bonds.