| Abstract
| - A series of stable complexes, (PMe3)3Ru(SiR3)2(H)2 ((SiR3)2 = (SiH2Ph)2, 3a; (SiHPh2)2, 3b; (SiMe2CH2CH2SiMe2), 3c), has been synthesized by the reaction of hydridosilanes with (PMe3)3Ru(SiMe3)H3 or(PMe3)4Ru(SiMe3)H. Compounds 3a and 3c adopt overall pentagonal bipyramidal geometries in solutionand the solid state, with phosphine and silyl ligands defining trigonal bipyramids and ruthenium hydridesarranged in the equatorial plane. Compound 3a exhibits meridional phosphines, with both silyl ligandsequatorial, whereas the constraints of the chelate in 3c result in both axial and equatorial silyl environmentsand facial phosphines. Although there is no evidence for agostic Si−H interactions in 3a and 3b, theequatorial silyl group in 3c is in close contact with one hydride (1.81(4) Å) and is moderately close to theother hydride (2.15(3) Å) in the solid state and solution (ν(Ru···H···Si) = 1740 cm-1 and ν(RuH) = 1940cm-1). The analogous bis(silyl) dihydride, (PMe3)3Ru(SiMe3)2(H)2 (3d), is not stable at room temperature,but can be generated in situ at low temperature from the 16e- complex (PMe3)3Ru(SiMe3)H (1) and HSiMe3.Complexes 3b and 3d have been characterized by multinuclear, variable temperature NMR and appear tobe isostructural with 3a. All four complexes exhibit dynamic NMR spectra, but the slow exchange limitcould not be observed for 3c. Treatment of 1 with HSiMe3 at room temperature leads to formation of (PMe3)3Ru(SiMe2CH2SiMe3)H3 (4b) via a CH functionalization process critical to catalytic dehydrocoupling of HSiMe3at higher temperatures. Closer inspection of this reaction between −110 and −10 °C by NMR reveals aplethora of silyl hydride phosphine complexes formed by ligand redistribution prior to CH activation. Aboveca. 0 °C this mixture converts cleanly via silane dehydrogenation to the very stable tris(phosphine) trihydridecarbosilyl complex 4b. The structure of 4b was determined crystallographically and exhibits a tetrahedralP3Si environment around the metal with the three hydrides adjacent to silicon and capping the P2Si faces.Although strong Si···HRu interactions are not indicated in the structure or by IR, the HSi distances (2.00(4)− 2.09(4) Å) and average coupling constant (JSiH = 25 Hz) suggest some degree of nonclassical SiHbonding in the RuH3Si moiety. The least hindered complex, 3a, reacts with carbon monoxide principallyvia an H2 elimination pathway to yield mer-(PMe3)3(CO)Ru(SiH2Ph)2, with SiH elimination as a minor process.However, only SiH elimination and formation of (PMe3)3(CO)Ru(SiR3)H is observed for 3b−d. The mosthindered bis(silyl) complex, 3d, is extremely labile and even in the absence of CO undergoes SiH reductiveelimination to generate the 16e- species 1 (ΔHSiH-elim = 11.0 ± 0.6 kcal·mol-1 and ΔSSiH-elim = 40 ± 2cal·mol-1·K-1; Δ= 9.2 ± 0.8 kcal·mol-1 and Δ= 9 ± 3 cal·mol-1·K-1). The minimumbarrier for the H2 reductive elimination can be estimated, and is higher than that for silane elimination attemperatures above ca. −50 °C. The thermodynamic preferences for oxidative additions to 1 are dominatedby entropy contributions and steric effects. Addition of H2 is by far most favorable, whereas the relativeaptitudes for intramolecular silyl CH activation and intermolecular SiH addition are strongly dependent ontemperature (ΔHSiH-add = −11.0 ± 0.6 kcal·mol-1 and ΔSSiH-add = −40 ± 2 cal·mol-1·K-1; ΔHβ-CH-add =−2.7 ± 0.3 kcal·mol-1 and ΔSβ-CH-add = −6 ± 1 cal·mol-1·K-1). Kinetic preferences for oxidative additionsto 1 intermolecular SiH and intramolecular CH have been also quantified: Δ= −1.8 ± 0.8kcal·mol-1 and Δ= −31 ± 3 cal·mol-1·K-1; Δ= 16.4 ± 0.6 kcal·mol-1 and Δ=−13 ± 6 cal·mol-1·K-1. The relative enthalpies of activation are interpreted in terms of strong SiH σ-complexformation and much weaker CH coordination in the transition state for oxidative addition.
|
