Structure and Reactivity of Bis(silyl) Dihydride Complexes (PMe₃)₃Ru(SiR₃)₂(H)₂:  Model Compounds and Real Intermediates in a Dehydrogenative C−Si Bond Forming Reaction

Abstract

A series of stable complexes, (PMe(3))(3)Ru(SiR(3))(2)(H)(2) ((SiR(3))(2) = (SiH(2)Ph)(2), 3a; (SiHPh(2))(2), 3b; (SiMe(2)CH(2)CH(2)SiMe(2)), 3c), has been synthesized by the reaction of hydridosilanes with (PMe(3))(3)Ru(SiMe(3))H(3) or (PMe(3))(4)Ru(SiMe(3))H. Compounds 3a and 3c adopt overall pentagonal bipyramidal geometries in solution and the solid state, with phosphine and silyl ligands defining trigonal bipyramids and ruthenium hydrides arranged in the equatorial plane. Compound 3a exhibits meridional phosphines, with both silyl ligands equatorial, whereas the constraints of the chelate in 3c result in both axial and equatorial silyl environments and facial phosphines. Although there is no evidence for agostic Si-H interactions in 3a and 3b, the equatorial silyl group in 3c is in close contact with one hydride (1.81(4) A) and is moderately close to the other hydride (2.15(3) A) in the solid state and solution (nu(Ru.H.Si) = 1740 cm(-)(1) and nu(RuH) = 1940 cm(-)(1)). The analogous bis(silyl) dihydride, (PMe(3))(3)Ru(SiMe(3))(2)(H)(2) (3d), is not stable at room temperature, but can be generated in situ at low temperature from the 16e(-) complex (PMe(3))(3)Ru(SiMe(3))H (1) and HSiMe(3). Complexes 3b and 3d have been characterized by multinuclear, variable temperature NMR and appear to be isostructural with 3a. All four complexes exhibit dynamic NMR spectra, but the slow exchange limit could not be observed for 3c. Treatment of 1 with HSiMe(3) at room temperature leads to formation of (PMe(3))(3)Ru(SiMe(2)CH(2)SiMe(3))H(3) (4b) via a CH functionalization process critical to catalytic dehydrocoupling of HSiMe(3) at higher temperatures. Closer inspection of this reaction between -110 and -10 degrees C by NMR reveals a plethora of silyl hydride phosphine complexes formed by ligand redistribution prior to CH activation. Above ca. 0 degrees C this mixture converts cleanly via silane dehydrogenation to the very stable tris(phosphine) trihydride carbosilyl complex 4b. The structure of 4b was determined crystallographically and exhibits a tetrahedral P(3)Si environment around the metal with the three hydrides adjacent to silicon and capping the P(2)Si faces. Although strong Si.HRu interactions are not indicated in the structure or by IR, the HSi distances (2.00(4) - 2.09(4) A) and average coupling constant (J(SiH) = 25 Hz) suggest some degree of nonclassical SiH bonding in the RuH(3)Si moiety. The least hindered complex, 3a, reacts with carbon monoxide principally via an H(2) elimination pathway to yield mer-(PMe(3))(3)(CO)Ru(SiH(2)Ph)(2), with SiH elimination as a minor process. However, only SiH elimination and formation of (PMe(3))(3)(CO)Ru(SiR(3))H is observed for 3b-d. The most hindered bis(silyl) complex, 3d, is extremely labile and even in the absence of CO undergoes SiH reductive elimination to generate the 16e(-) species 1 (DeltaH(SiH)(-)(elim) = 11.0 +/- 0.6 kcal x mol(-)(1) and DeltaS(SiH)(-)(elim) = 40 +/- 2 cal x mol(-)(1) x K(-)(1); Delta = 9.2 +/- 0.8 kcal x mol(-)(1) and Delta = 9 +/- 3 cal x mol(-)(...

σ‐Silane Ruthenium Complexes: The Crucial Role of Secondary Interactions

“…As a matter of fact, there is no decoalescence of the hydride resonance for RuH 3 (SiMe 2 CH 2 SiMe 3 )(PMe 3 ) 3 (12c) in which the phosphane groups are rather small. [22] In this series of complexes, Xray data show that the Ru-Si bond lengths range from 2.2760 (4) Table 2). They overlap the distances reported for silyl and σ-silane species and they are mainly in the lower limit region (taking into account the nature of the silicon substituents).…”

“…[21][22][23][24][25] Depending on the activation degree of the Si-H bond, they can be interpreted as σ-silane or silyl hydride complexes, namely RuH 2 (σ-HSiR 3 )L 2 LЈ or RuH 3 (SiR 3 )L 2 LЈ, respectively (L, LЈ = PR 3 , CO, σ-H 2 ). The NMR spectroscopic data are very similar for most of the complexes, as can be seen from In the case of symmetrical complexes (C 3 axis), the equivalency of the hydrides prevents any further interpretation regarding the bonding mode.…”

“…Orbital representation of Ru-H···Si bonding adapted from ref. [19,21] The fluxionality observed at the NMR time scale for RuH 3 (SiMeCl 2 )(PPh 3 ) 3 , [21] (10) but also for RuH 2 (η 2 -HSiPh 3 )(η 2 -H 2 )(PCy 3 ) 2 (13), [28] (14), [29] and RuH 3 (SiMe 2 CH 2 SiMe 3 )(PMe 3 ) 3 (12c), [22] is consistent with exchange processes characterized by low activation energy barriers. In 10 and 12c, the silane ligand is bearing different substituents (a situation not depicted in Figure 7, a).…”

“…The interpretation of the raw values has to be done cautiously since the typical ranges for which the Si-H bonds were considered to be either fully activated (Ͻ 20 Hz) or just partially elongated (20-70 Hz) have been recently reconsidered. [22,30] (15) to give an idea of the upper limit corresponding to an agostic Si-H interaction. [32] We can then consider the fac and mer isomers of RuCl-(CH 2 SiHMe 2 )(PMe 3 ) 3 described by Berry et al that reflect a further point on the oxidative addition process (still close to the agostic interaction type).…”

Reactions of Hydrosilanes with Transition Metal Complexes and Characterization of the Products

“…The related ruthenium complex [(Me 3 P) 3 HRu-(SiMe 3 )] 2 (N 2 ) provided (Me 3 P) 3 H 2 Ru(SiMe 3 ) 2 , 3-123, on reaction with excess Me 3 SiH. 49 Hydride-and chloride-bridged dimers have been employed as reactants with silanes. The complexes with hydride bridges between two metals tend to give mononuclear silyl complexes.…”