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Cp*(PiPr3)RuOTf: A Reagent for Access to Ruthenium Silylene Complexes Meg E. Fasulo, Paul B. Glaser,† and T. Don Tilley Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460, United States
bS Supporting Information ABSTRACT:
The ruthenium triflate complex Cp*(PiPr3)RuOTf (1) was generated from the reaction of Cp*(PiPr3)RuCl with Me3SiOTf in dibutyl ether. Complex 1 reacted with primary and secondary silanes to produce a family of Ru(IV) silyl dihydride complexes of the type Cp*(PiPr3)Ru(H)2(SiRR0 OTf) (312). Structural analyses of complexes 8 (R = R0 = Ph) and 12 (R = R0 = fluorenyl) revealed the presence of a tetrahedral silicon center and a four-legged piano stool geometry about ruthenium. Anion abstraction from Cp*(PiPr3)Ru(H)2(SiHROTf) by [Et3Si 3 toluene][B(C6F5)4] afforded hydrogen-substituted cationic ruthenium silylene complexes [Cp*(PiPr3)Ru(H)2(dSiHR)][B(C6F5)4] (R = Mes (13), R = Si(SiMe3) (14)) that display a significant RuH 3 3 3 Si interaction, as indicated by relatively large 2JSiH coupling constants (2JSiH = 58.2 Hz (13), 2JSiH = 37.1 Hz (14)). The syntheses of secondary silylene complexes [Cp*(PiPr3)Ru(H)2(dSiRR0 )][B(C6F5)4] (R = R0 = Ph (15); R = Ph, R0 = Me (16), R = R0 = fluorenyl (17)) were also achieved by anion abstraction with [Et3Si 3 toluene][B(C6F5)4]. Complexes 1517 do not display strong RuH 3 3 3 Si secondary interactions, as indicated by very small 2JSiH coupling constant values.
’ INTRODUCTION Transition metal complexes with silylene ligands have been an active area of research for the last twenty years due to their potential applications in transformations involving organosilanes, such as hydrosilylation and silane redistribution.1 A silylenemediated olefin hydrosilylation mechanism that fundamentally differs from the ChalkHarrod mechanism2 has been proposed for two systems, involving [Cp*(PiPr3)Ru(H)2(dSiHPh 3 Et2O)][B(C6F5)4]3 and [PNPIrH(dSiHPh)][B(C6F5)4]4 (Scheme 1). These hydrosilylations are selective for the anti-Markovnikov product and occur only with primary silanes. Importantly, the cationic sp2-hybridized silicon center is required for this reactivity.5 Several reliable synthetic routes to silylene complexes have been established, including the general reaction types of Scheme 2.6 One straightforward strategy involves the coordination of a stable, free silylene to a transition metal fragment (Scheme 2a).7 However, the reactivity of complexes synthesized via this method often results in displacement of the silylene ligand7b or the generation of tetravalent silicon.7c A widely employed procedure is based on abstraction of an anionic substituent, such as a halogen or triflate, from the silicon atom (Scheme 2b).3,4,8 Recently, this chemistry has been extended to include abstraction of hydride from silicon.4 Importantly, anion abstraction inherently yields cationic silylene species. Another effective approach has been termed “silylene extrustion”, which involves two sequential r 2011 American Chemical Society
Scheme 1
Received: August 24, 2011 Published: September 22, 2011 5524
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Scheme 2
SiH activations (e.g., Scheme 2c).9 In an example of this process, a silane reacts with a metal alkyl complex to undergo oxidative addition, followed by a CH reductive elimination. Finally, an α-hydride migration occurs to produce the silylene ligand. This methodology can result in cationic9a or neutral complexes.9f Transition metal silylene complexes are extremely oxygen and moisture sensitive, and a number of examples are thermally unstable. This inherent instability has been assuaged by the use of bulky substituents on the metal fragment and the silicon atom.10 Additionally, the use of electron-donating substituents on the silyl ligand allowed for the first base-free examples of silylenes.11 Because the substituents at silicon play a strong role in determining stability and reactivity for a silylene complex, it is important to establish versatile synthetic methods that allow access to a wide range of new silylene complexes. Herein we describe the synthesis and reactivity of Cp*(PiPr3)RuOTf (1), a new metal complex that activates a number of primary and secondary silanes. Several of the resulting metal silyl species have proven to be useful precursors to new ruthenium silylene complexes.
’ RESULTS AND DISCUSSION Synthesis, Characterization, and Reactivity of Cp*(PiPr3)RuOTf (1). The complex Cp*(PiPr3)RuCl has been shown to
activate phenylsilane, and the resulting product, Cp*(PiPr3)RuHCl(SiH2Ph), undergoes chloride abstraction to give a basestabilized silylene complex that exhibits unusually high antiMarkovnikov regioselectivity as a catalyst for alkene hydrosilylations.3 Attempts to extend this methodology to additional catalysts of the type [Cp*(PiPr3)Ru(H)2(dSiHR)]+ have proven difficult, especially given the limited reactivity of Cp*(PiPr3)RuCl toward more sterically demanding primary silanes. Because the anion abstraction method to afford cationic silylene complexes has been successful for both SiCl and SiOTf derivatives, Cp*(PiPr3)RuOTf was targeted as a potential starting material for access to new cationic silylene complexes. Initial attempts to synthesize Cp*(PiPr3)RuOTf from the reaction of Cp*(PiPr3)RuCl with Me3SiOTf at room temperature in diethyl ether for 24 h resulted in an inseparable mixture of starting material and product. Slow removal of volatile materials was somewhat successful in driving the reaction toward completion, but paramagnetic side products and inconsistent yields were problematic. The use of a high boiling ether solvent such as Bu2O (bp 143 °C) allowed for access to pure Cp*(PiPr3)RuOTf (1) (eq 1). After stirring Cp*(PiPr3)RuCl with 1.1 equiv of Me3SiOTf (bp 140 °C) at room temperature for 1 h, the volatile materials were slowly removed under vacuum over 6 h. In this
way, removal of Me3SiCl, the volatile product of the reaction (bp 56 °C), forced the reaction to completion. Subsequent drying under vacuum gave 1 as a purple solid in high isolated yield (89%).
The 1H NMR spectrum of 1 reveals a single peak for the Cp* methyl groups, a septet for the methine protons of the isopropyl groups, and a doublet of doublets for the methyl protons of the isopropyl groups, indicative of a highly symmetric molecule. The 31 1 P{ H} NMR spectrum displays a single peak at 48.0 ppm that is downfield from that of free PiPr3 (19.0 ppm). A single resonance at 76.7 ppm is observed by 19F NMR spectroscopy. The multiple NMR active nuclei of 1 make for convenient NMR spectroscopic handles for exploring reactivity. X-ray quality crystals were obtained as purple blocks from a solution of 1 in (Me3Si)2O at 30 °C (Figure 1). The triflate ligand binds to the Ru center through one oxygen atom, with a RuO bond length of 2.136(2) Å. Although complex 1 formally possesses a 16 electron count, no additional inter- or intramolecular contacts are observed between the Ru center and the triflate ligand. To verify that the bulky phosphine ligand would not preclude the formation of a complex containing more ligands, complex 1 in CH2Cl2 was exposed to 1 atm of CO gas (eq 2). At room temperature, the dark purple solution quickly faded to a dark yellow color. The presence of a CO ligand was observed in the infrared spectrum, as a ν(CO) stretch at 1945 cm1, indicating that there is less back-bonding to the CO of 2 than in Cp*(PiPr3)RuCl(CO) (1910 cm1).12 Additionally, an X-ray structure of 2 confirms the proposed structure.13
Complex 1 readily decomposes in the presence of many arenes to form complexes of the type [Cp*Ru(arene)][OTf].14 These complexes precipitate from solution as white solids and can easily be differentiated from dark purple 1. It is therefore important to avoid the use of arene solvents such as benzene and toluene during manipulations of 1. 5525
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Figure 1. Molecular structure of 1 displaying thermal ellipsoids at the 50% probability level. H atoms have been omitted for clarity. Selected bond lengths (Å): Ru(1)O(1) = 2.136(2), Ru(1)P(1) = 2.4088(9), S(1)O(1) = 1.474(2), S(1)O(2) = 1.431(2), S(1)O(3) = 1.434(2).
Synthesis and Characterization of Cp*(PiPr3)Ru(H)2(SiRR0 OTf) Complexes. Complex 1 was found to react cleanly with 1 equiv of
H3SiPh at room temperature in ether in 30 min to give Cp*(PiPr3)Ru(H)2(SiHPhOTf) (3) as a light tan solid in very good yield (eq 3). Whereas the reaction of Cp*(PiPr3)RuCl with PhSiH3 gives the simple oxidative addition product Cp*(PiPr3)RuHCl(SiH2Ph),15 the Ru(IV) complex 3 is the result of two SiH bond cleavages and migration of the triflate anion to the Si center.
Filtration of the reaction solution through a Celite plug and removal of solvent in vacuo resulted in analytically pure 3. In an analogous manner, reactions of 1 with H3SiMes, H3Si(C6F5), H3SiSiPh3, H3SiSi(SiMe3)3, H2SiPh2, H2SiPhMe, H2SiEt2, H2SiiPr2, and 9-silafluorene gave complexes 412, respectively. A number of these organosilanes do not react cleanly with Cp*(PiPr3)RuCl to afford similar complexes, and thus 1 allows for the synthesis of a number of silylene precursor complexes that were not previously accessible. Due to the multiple NMR-active nuclei present in 312, a large amount of structural information can be obtained for these complexes to support the structures proposed in eq 3. For example, the 1H NMR spectrum for 4 reveals an SiH group that gives rise to a resonance at 7.02 ppm, with a characteristically
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large JSiH coupling constant of 215.4 Hz. The two hydride ligands (11.81 and 12.86 ppm) are diastereotopic due to the chiral Si center and are associated with small JSiH coupling constants (JSiH = 11.4 Hz), indicating that there is not a strong interaction between the hydrides and the silicon center. The 29Si NMR spectrum displays a single resonance at 62.4 ppm, in the region typical for a silyl ligand (SiR3) bound to a transition metal.16 The 31 1 P{ H} NMR spectrum exhibits a resonance at 78.2 ppm that is downfield of that observed for 1 (48.0 ppm). Additionally, 19F NMR spectroscopy confirms the presence of the triflate anion with a peak at 76.3 ppm. The NMR data for 312 follow similar trends and are tabulated in Table 1. Most noteworthy are the small JSiH coupling constants observed for all complexes (JSiH e 20 Hz), indicating very weak secondary interactions between the hydrides and the silicon. The range of 29Si NMR resonances from 44.3 ppm for 5 to 118.9 ppm for 11 is typical of silyl ligands with a variety of substituents. Similar complexes containing a (triflate)silyl ligand have previously been synthesized, of the type Cp*(PMe3)2MSiR2OTf (M = Ru, Os).8b,17 For the complex M = Os and R = iPr, the triflate anion reversibly dissocates to provide access to the transient silylene complex, [Cp*(PMe3)2OsSi(iPr)2][OTf]. The degree of dissociation was found to depend on the polarity of the solvent; the 29Si NMR resonance for this compound in C6D6 (100 ppm) is at much higher field than that observed with CD2Cl2 solvent (223 ppm). This behavior is not observed for 312, as NMR spectra for samples in C6D6 and CD2Cl2 are very similar. For example, complex 9 displays a 29Si resonance at 84.1 ppm in C6D6 and at 87.4 ppm in CD2Cl2. Therefore, a transient silylene species does not appear to significantly contribute to the 29 Si NMR shifts of 312. Crystals of 8 suitable for X-ray crystallography were grown from a pentane solution at 30 °C. Two independent molecules per asymmetric unit are present, and all hydride ligands were located (Figure 2). The silicon center is close to tetrahedral (sum of angles around Si = 343.4°) and displays a bonding interaction with the triflate anion. Additionally, the RuSi distance of 2.3138(17) Å is in the range expected for RuSi single bonds.16 The RuH 3 3 3 Si distances are all greater than 2 Å, indicating that full oxidative addition has occurred. However, the hydride ligands are somewhat canted toward the silyl ligands and away from the phosphine, as indicated by the average PRuH angle of 77.7°, which is greater than the average SiRuH angle of 63.7°. The short SiO(1) bond length (1.815(3) Å) further suggests that this complex possesses little silylene character. The solid-state structure of 12 varies little from that of 8 (Figure 3). Additionally, the solid-state structures of 8 and 12 are remarkably similar to that of Cp*(PiPr3)Ru(H)2(SiHMesCl), indicating that the anionic substituent (Cl or OTf) on silicon does not significantly influence the solid-state molecular geometry.18 Synthesis and Characterization of Ruthenium Silylene Complexes [Cp*(PiPr3)Ru(H)2(dSiRR0 )][B(C6F5)4]. A number of cationic silylene complexes have been synthesized via an anion or hydride abstraction, and the triflate complexes 312 seemed potentially suitable for this purpose. Initial attempts to replace the triflate anion with a more weakly coordinating counterion focused on use of the Li(Et2O)2[B(C6F5)4] salt. Addition of 1 equiv of Li(Et2O)2[B(C6F5)4] to 8 in C6D5Br produced a bright orange solution, and 1H NMR spectroscopy revealed quantitative conversion to a new species. However, the 29Si resonance at 127.8 ppm indicated the likely formation of a base-stabilized silylene 5526
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Table 1. NMR Data for Complexes 312 complex
δ 1H (SiH) (2JSiH)
δ 1H (RuH) (2JSiH)
δ 29Si (RuSi)
Cp*(PiPr3)RuH2(SiHPhOTf) (3)
6.76 (211.5)
11.51, 12.49 (18.8)
65.1
Cp*(PiPr3)RuH2(SiHMesOTf) (4)
7.02 (215.4)
11.81, 12.86 (10.4)
62.4
Cp*(PiPr3)RuH2(SiH(C6F5)OTf) (5)
6.75 (227.9)
11.57, 12.26 (
