Half-Sandwich Silane σ-Complexes of Ruthenium ... - ACS Publications

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Half-Sandwich Silane σ‑Complexes of Ruthenium Supported by NHC Carbene Van Hung Mai,† Ilia Korobkov,‡ and Georgii I. Nikonov*,† †

Chemistry Department, Brock University, Niagara Region, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada X-ray Core Facility, Faculty of Science, University of Ottawa, 150 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada

‡

S Supporting Information *

ABSTRACT: Reactions of carbene complex [Cp(IPr)Ru(pyr)2][BF4] (6, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with excess acetonitrile and LiCl in THF afford complexes [Cp(IPr)Ru(NCCH3)2][PF6] (7) and Cp(IPr)RuCl (8), respectively. Complex 8 was characterized by NMR and X-ray diffraction analysis. Addition of hydrosilanes to 8 results in silane σ-complexes Cp(IPr)Ru(η2-HSiR3) Cl (4), which were characterized by NMR and X-ray studies of Cp(IPr)Ru(η2-HSiMeCl2)Cl (4b) and Cp(IPr)Ru(η2-H3SiPh)Cl (4d). The hydrogen−silicon coupling constants of complexes 4 show an unusual trend in that the J(H−Si) values increase from the less-chlorinated complex Cp(IPr)Ru(η2-HSiMe2Cl)Cl (4c) to the trichloro derivative Cp(IPr)Ru(η2-HSiCl3)Cl (4a). Reaction of Cp(IPr)RuCl (8) with two equivalents of HSiCl3 gave the ruthenate complex [IPrH]+[CpRuCl(H) (SiCl3)2]−, characterized by NMR and X-ray study. Addition of hydrosilanes to the cationic complex [Cp(IPr)Ru(NCCH3)2][BArF4] (9) furnished very unstable cationic silane σ-complexes [Cp(IPr)Ru(η2HSiR3)(NCCH3)]+ (5), characterized by low-temperature NMR. Reaction of complex 9 with two equivalents of HSiCl3 gives the neutral bis(silyl) complex CpRu(NCCH3)(H)(SiCl3)2 and [IPrH][BArF4]. Catalytic studies showed that 9 is a poorer catalyst for hydrosilylation of benzaldehyde, benzonitrile, and pyridine than its phosphine analogue [Cp(iPr3P)Ru(NCCH3)2][BArF4]. The reason for this reduced activity was assigned to the easy dissociation of carbene from the former catalyst.

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INTRODUCTION

RESULTS AND DISCUSSION Starting Materials. We have recently prepared the cationic carbene complex [Cp(IPr)Ru(pyr)2 ][PF 6 ] (6) (pyr = pyridine),10 which is a convenient precursor to complexes 4 and 5. Dissolution of 6 in acetonitrile followed by removal of volatiles afforded the related acetonitrile complex [Cp(IPr)Ru(NCCH3)2][PF6] (7). Addition of compound 6 to a solution of LiCl in THF resulted in quantitative formation of a novel 16e complex, Cp(IPr)RuCl (8, Scheme 1), which was isolated in

Silane σ-complexes have received increased attention as likely intermediates in metal-catalyzed hydrosilylation, alcoholysis, and coupling reactions.1 However, only in a few cases has their direct involvement in catalytic reactions been documented.2−5 The cationic silane σ-complexes are a particularly challenging class of compounds to synthesize due to the high electrophilicity of the silylium center. Not surprisingly, only two representatives have been structurally characterized.3a,6 We have recently prepared a series of surprisingly stable cationic complexes [Cp(iPr3P)Ru(η2-HSiR3)(NCCH3)]+ (1) and reported their involvement in hydrosilylation reactions.2 In a separate study, we prepared related neutral complexes Cp′(iPr3P)Ru(η2-HSiR3)Cl (Cp′ = Cp (2) or Cp* (3)) and showed that these compounds have additional interligand interactions between the ruthenium-bound chloride and the silicon center.7 Other workers showed that similar secondary interligand interactions can play an important directing role in metal-catalyzed hydrometalation reactions.8 Given the very well established analogy between phosphines and NHC carbenes,9 we became curious about the availability of NHC-supported analogues of complexes 1 and 2 and their potential activity in catalysis. Here we report the syntheses and X-ray studies of some new neutral and cationic silane σ-complexes, Cp(IPr)Ru(η2-HSiR3)Cl (4) and [Cp(IPr)Ru(η2-HSiR3)(NCCH3)]+ (5), supported by the IPr carbene (IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene). © XXXX American Chemical Society

Scheme 1. Preparation of Complexes 7 and 8

80% yield after recrystallization from hexane as a deep blue crystalline material. Compound 8 was characterized by NMR spectroscopy and X-ray diffraction. Other related 16e species, Cp′LRuCl (L = phosphine or carbene), are known from the literature.7b,11,12 Counteranion exchange between complex 7 and LiBArF4 (BArF4 = tetrakis(pentafluorophenyl)borate) in Received: November 17, 2015

A

DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Table 1. H−Si Coupling Constants J(H−Si) (Hz) in Complexes 4a−e and Related Phosphine Complexes 2a−e and 3a−e

acetonitrile cleanly generates the related complex [Cp(IPr)Ru(NCCH3)2][BArF4] (9), characterized by NMR and IR. The molecular structure of compound 8 is shown in Figure 1. The geometry can be described formally as a three-coordinate

4 2 3

SiCl3 (a)

SiMeCl2 (b)

SiMe2Cl (c)

SiH2Ph (d)

SiHMePh (e)

63.8 33 40 Hz, which indicate the formation of silane σ-complexes (Table 1).1,13 Comparison with the related phosphine-supported complexes Cp(iPr3P)Ru(η2-HSiR3)Cl (2) revealed an interest-

Figure 2. Molecular structure of compound 4b. Hydrogen atoms, except the hydride on Ru, are omitted for clarity. B

DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

structurally characterized analogue with a phosphine supporting ligand to make a comparison. Attempted preparation of the silane complex 4a in the presence of excess silane resulted in an unexpected reactivity. Thus, addition of two equivalents of HSiCl3 to a toluene solution of 8 at −30 °C led to the formation of an imidazolium salt of the ruthenate complex [CpRuClH(SiCl3)2]− (10). This compound is likely formed by dissociation of the carbene ligand to give electron-deficient species CpRuCl(H)(SiCl3) (11), which then adds another equivalent of HSiCl3 to form silane complex 12 (Scheme 3). Deprotonation of the latter by IPr can

the silicon center, which, neglecting the hydride atom bridging the Ru−Si bond, can be described as a distorted trigonal bipyramid, with one Si-bound chloride being equatorial and the other in the apical position, trans to the Ru-bound chloride (the Cl−Si···Cl angle is 164.9°). This trans chloride Cl2 has an elongated Si−Cl distance of 2.1199(17) Å compared to the Si− Cl(eq) distance of 2.1134(16) Å, indicating that the Cl(trans)− Si···ClRu unit is involved in a hypervalent interaction.1b In the family of complexes Cp′(L)Ru((η2-HSiR3)Cl, the closest comparison can be made with the phosphine complex 2b, having the same silane HSiMeCl2. The Si−Cl(trans) bond is slightly longer in 4b than in 2b (2.1199(17) vs 2.1104(9) Å), which correlates well with the shorter RuCl···Si distance. On the other hand, the Ru−Si distances in both 4b and 2b are the same within three sigmas, 2.3587(11) and 2.3564(7) Å, respectively, and the Ru−Cl bonds are also very close (2.3984(9) and 2.4199(6) Å, respectively), which makes us conclude that the phosphine PiPr3 and the carbene IPr ligands behave very similarly. The shorter Si−H distance of 1.750 Å in 4b than 1.855 Å in 2b correlates well with the larger J(H−Si) value in the former compound (Table 1). But these data should be taken with care because of the well-known inaccuracy in finding hydrogen atoms in a heavy element environment. Previous thermochemical measurements by Nolan et al. for complexes Cp*LRuCl (L = PR3 or NHC) established the following strength of Ru−L bonds: IMes > PCy3 > PiPr3. But in the case of sterically encumbered carbenes, such as IAd, the bond strength drops more dramatically (e.g., 6.8 kcal/mol for IAd and 15.6 kcal/mol for IMes vs 10.5 kcal/mol for PCy3 and 9.4 kcal/mol for PiPr3).11b Unfortunately, no thermochemical data are available for the carbene used in this study, IPr. Thus, on the basis of the structural comparison given above and the fact that IPr is bulkier than IMes, we can assume that the binding capability of IPr is similar to PiPr3, and both stabilize Si−H interligand interactions in a comparable way. The molecular structure of complex 4d is shown in Figure 3. The overall appearance of the structure is similar to 4b except that the apical position is now occupied by a less-electronwithdrawing phenyl group (the Ph−Si···Cl angle is 165.5°). As a result, the hypervalent interaction is weakened, and the Si···Cl contact elongates to 2.889 Å. Unfortunately, there is no

Scheme 3. Possible Mechanism of the Formation of Complex 10

furnish the final product 10. This reaction sequence accounts for the fact that imidazolium ion only forms in the presence of excess silane.14 Compound 10 was characterized by NMR spectroscopy, and its structure was confirmed by X-ray diffraction analysis. In the 1H NMR spectrum recorded in CDCl3, the Ru-bound hydride gives rise to a high-field singlet at −10.59 ppm, and its presence is further supported by the observation of an IR stretch at 2012 cm−1. The silicon atoms give rise to an upfield 29Si NMR signal at −10.7 ppm showing moderate coupling to the hydride (J(H−Si)= 14.5 Hz). The formation of compound 10 highlights a possible decomposition pathway for the NHC-supported silane σ-complexes, which will be discussed further in the following section. The molecular structure of the ruthenate part of salt 10 is shown in Figure 4. This is a typical, four-legged piano-stool geometry. The two silyl ligands are in the trans disposition, as are the hydride and chloride ligands. The Ru−Si bond lengths are 2.3055(9) and 2.3094(11) Å, which is longer than in the neutral compound Cp*(Me3P)2RuSiCl3 (2.265(2) Å) but shorter than in silane σ-complexes [Cp*(Me3P)2Ru(η2HSiCl3)]+ (2.329(1) Å),6a Cp*(Pri2MeP)Ru(η2-HSiCl3)Cl (2.3152(8) Å), and Cp*(Pr i Me 2 P)Ru(η 2 -HSiCl 3 )Cl (2.3111(5) Å).15 The Ru−Cl bond (2.3988(9) Å) is comparable with the Ru−Cl distances in complexes 4b and 4d (2.3984(9) and 2.4279(8) Å, respectively). The distances from the hydride to silicon atoms are 2.129 and 2.156 Å. Previous DFT studies of isoelectronic half-sandwich complexes Cp*Rh(SiR 3 ) 2 H 2 , 16 CpRh(SiR 3 ) 3 H, 17 [Cp*(Me 3 P)Rh(SiR3)2H]+, and [Cp*(Me3P)Rh(SiR3)H2]+18 showed that these compounds have highly delocalized interligand interactions R3Si···H···SiR3 and H···SiR3···H.19 Therefore, we can assume that similar multicenter interactions may be present in compound 10.

Figure 3. Molecular structure of compound 4d. Hydrogen atoms, except the hydride on Ru, are omitted for clarity. C

DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX

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the main component of which shows a hydride signal at −9.65 ppm and the Cp signal at 5.30 ppm. This hydride peak displays 29 Si satellites integrated as 9.4% to the main signal (90.6%), indicating the presence of two equivalent silicon atoms (the natural abundance of 29Si isotope is 4.7%) coupled to the hydride atom. The value of the J(H−Si) constant in this species found from the 29Si satellites and from a 1D 1H−29Si HSQC experiment was 24.9 Hz (the 29Si NMR signal was observed at 39.0 ppm). These features are reminiscent of the situation observed for the ate complex 11, but the larger value of the J(H−Si) shows the formation of a different compound. The observation of a coordinated nitrile peak at 2.28 ppm allows us to formulate tentatively this compound as a neutral bis(silyl) hydride, CpRu(NCCH3)(H)(SiCl3)2 (13). The reaction between HSiMe2Ph and 9 was of particular interest to us because this was the silane that performed the best in the 1-catalyzed hydrosilylation.2 We observed that a noticeable addition of HSiMe2Ph (taken in excess) takes place only at −80 °C signified by the appearance of the RuH signal at −7.79 ppm in the 1H NMR. Unfortunately, the NMR spectra were too broad at this temperature to allow for a more in-depth analysis of this compound. This addition of silane is reversible because an increase of temperature or removal of volatiles under vacuum regenerates the starting compound 9. Attempted reaction of 9 with silanes HSi(OEt)2Me, HSi(OEt)3, and HSiEt3 in CD2Cl2 at −40 °C did not result in the formation of silane σ-complexes. Catalytic Studies. Encouraged by the analogy between the cationic complexes 5 and 2 and the participation of the latter species in catalytic hydrosilylation,2 we attempted hydrosilylation by the precursor compound 9. Benzaldehyde, benzonitrile, and pyridine were chosen as the test substrates, and the results are gathered in Table 3. Compared to the

Figure 4. Molecular structure of compound 10.

Cationic Silane σ-Complexes. Addition of hydrosilanes at low temperature (−30 to −40 °C) to the cation 9 results in equilibrium mixtures with the cationic silane σ-complexes [Cp(IPr)Ru(η2-HSiR3)(NCCH3)]+ (5, Scheme 4). Unlike Scheme 4. Preparation of Complexes 5

related compounds 1, complexes 5 are very labile and defied isolation in crystalline form. Therefore, they were characterized only by low-temperature NMR in solutions. Moreover, there was no addition of HSiMe2Cl to ruthenium, even when excess (10 equiv) silane was used. The interesting feature of compounds 5 is that, unlike the neutral analogues 4, their J(H−Si) values correlate well with the H−Si coupling in the PiPr3 congeners 1 (Table 2). At the moment we do not have a clear-cut explanation for the divergent trends in the related series of compounds 4 and 5.

Table 3. Hydrosilylation Catalyzed by Complex 9a H3SiPh PhC(O)H PhCN C5H5N

SiCl3 (a)

SiMeCl2 (b)

60.8 53

47.3 45

SiMe2Cl (c)

SiH2Ph (d)

SiHMePh (e)

45

50.6 48

45.6 48

H2SiMePh 36%, 1 h, 25 °C 21%, 3 h, 50 °C 17%, 2 h, 50 °C

HSiMe2Ph 18%, 1 h, 25 °C 11%, 3 h, 50 °C 9%, 2 h, 50 °C

HSiMeCl2 23%, 1 h, 25 °C 18%, 3 h, 50 °C N/A

a 0.14 mmol of silane, 0.1 mmol of substrates, 0.6 mL of CD2Cl2, 5 mol % catalyst; NMR yield.

Table 2. H−Si Coupling Constants J(H−Si) (Hz) in Complexes 5a,b,d,e and Related Phosphine Complexes 1a−e 5 1

47%, 1 h, 25 °C 26%, 3 h, 50 °C 12%, 2 h, 50 °C

hydrosilylation catalyzed by phosphine complex [Cp(iPr3P)Ru(NCCH3)2]+ (1)2 the activity is significantly reduced, which can be accounted for by the diminished propensity of 9 to react with silanes to give species 5. For example, heating was required for benzonitrile and pyridine, whereas reactions catalyzed by 1 occur at room temperature and are complete in less than 1 h. Of four tested silanes, in no case was complete hydrosilylation achieved, indicating deactivation of the catalyst. A closer inspection of the reaction mixtures by 1H NMR revealed the appearance of signals due to the imidazolium ion [IPrH]+. This observation, coupled with the formation of compound 10 described above, strongly suggests that the main decomposition pathway in hydrosilylation includes the dissociation of IPr from the catalytically active species followed by proton abstraction from the cationic ruthenium hydride.

Addition of an equivalent of HSiCl3 to a CD2Cl2 solution of 9 at −78 °C followed by slow warming to −30 °C resulted in the formation of [Cp(IPr)Ru(η2-HSiCl3)(NCCH3)]+ (5a) in the course of 11 h. This species gives rise to a hydride signal at −7.61 ppm flanked by 29Si satellites with a large J(H−Si) of 60.8 Hz. The Cp signal was observed at 4.95 ppm (5 H), and the NCCH3 singlet was found at 1.94 ppm. The 2,6-iPr2C6H3 group of the NHC carbene gives rise to eight doublets for the eight nonequivalent methyl groups, consistent with the C1 symmetry. Warming a solution of 5a in CD2Cl2 to room temperature resulted in the formation of an equivalent of imidazolium ion [IPrH]+ and a mixture of hydride compounds, D

DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX

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7.20 (m, 6H, C6H3). 13C NMR (−5 °C, toluene-d8): δ 15.8 (SiCH3), 25.2 (s, CH3 of iPr), 26.4 (s, CH3 of iPr), 22.7 (s, CH3 of iPr), 21.6 (s, CH3 of iPr), 28.4 (s, CH of iPr), 85.0 (Cp), 125.2 (NCH), 124.8 (mC6H3), 130.3 (p-C6H3), 184.9 (Ru-CN2). 29Si NMR (1H−29Si HSQC): δ 39.1 (J(Si−H, = 43.6 Hz). Anal. Calcd for C33H45RuN2SiCl3 (705.240): C, 56.20; H, 6.43; N, 3.97. Found: C, 55.95; H, 6.81; N, 3.84. Cp(IPr)RuCl(η2-HSiClMe2) (4c). Addition of HSiClMe2 to a blue solution of 0.050 g (0.085 mmol) of Cp(IPr)RuCl in 5 mL of toluene at −30 °C results in the immediate color change to yellow. Hexane was added, and the product was recrystallized from a mixture of toluene and hexane (4:1) at −30 °C. Isolated yield: 79% (0.046 g). IR (Nujol): ν(Ru−H) = 1993 cm−1. 1H NMR (−23 °C, toluene-d8): δ −7.80 (1H, RuH), 0.92 (br, 6H, CH3 of iPr), 1.01 (br, 6H, CH3 of i Pr), 1.14 (br, 6H, CH3 of iPr), 1.45 (d, 3J(H−H) = 6.8 Hz, 6H, CH3 of iPr), 0.18 (s, 3H, CH3 of Si(CH3)2, 1.26 (s, 3H, CH3 of Si(CH3)2), 2.55 (br, 2H, CH of iPr), 3.42 (br, 2H, CH of iPr), 4.31 (s, 5H, Cp), 6.46 (s, 2H, NCH), 7.06−7.23 (m, 6H, C6H3). 13C NMR (−23 °C, toluene-d8): δ 10.2 (Si(CH3)2), 23.6 (Si(CH3)2), 24.5 (CH3 of iPr), 26.6 (CH3 of iPr), 23.4 (CH3 of iPr), 22.7 (CH3 of iPr), 28.4 (CH of i Pr), 85.0 (Cp), 125.1 (NCH), 128.4 (s, m-C6H3), 130.4 (s, p-C6H3), 185.9 (Ru-CN2). 29Si NMR (1H−29Si HSQC): δ 29.5 (1J(Si−H) = 56.4 Hz, SiClMe2). Anal. Calcd for C34H48RuN2SiCl2 (684.821): C, 59.63; H, 7.06; N, 4.08. Found: C, 60.06; H, 7.06; N, 3.57. Cp(IPr)RuCl(η2-H3SiPh) (4d). Addition of H3SiPh to a blue solution of 0.050 g (0.085 mmol) of Cp(IPr)RuCl in 5 mL of toluene at −40 °C results in the immediate color change to yellow. 4d was recrystallized from a mixture of toluene and hexane (4:1) at −30 °C. Yield: 76% (0.045 g). IR (Nujol): ν(Ru−H) = 1980 cm−1. 1H NMR (−30 °C, toluene-d8): δ −8.96 (br s, 1H, RuH), 0.90−1.57 (br, 24H, CH3 of iPr), 2.41, 2.77, 3.28, 3.79 (br, 4H, CH of iPr), 3.41 (br, 2H, CH of iPr), 3.94 (s, 5H, Cp), 4.4 (d, 2J(H−H) = 9.5 Hz, 1H, SiH2), 5.39 (d, 2J(H−H) = 9.5 Hz, 1H, SiH2). 6.52 (s, 2H, NCH), 7.18−7.26 (m, C6H3+Ph), 7.65 (d, 3J(H−H) = 6.9 Hz 2H, ortho of SiH2Ph). 13C NMR (−58 °C, toluene-d8): δ 22.1 (CH3 of iPr), 23.7 (CH3 of iPr), 26.7 (CH3 of iPr), 28.6 (CH3 of iPr), 25.4, 26.3, 28.0, 29.6 (CH of iPr), 81.7 (Cp), 125.5 (NCH), 138.6 (m-C6H3), 135.4 (p-C6H3), 147.9 (meta of SiPh), 145.9 (para of SiPh), 148.6 (ortho of SiPh), 186.8 (RuCN2). 1H−29Si HSQC (−58 °C, toluene-d8): δ −22.7 (SiH2Ph). 1D 29 Si INEPT+ (−58 °C, toluene-d8): δ −22.7 (1J(Si−H(Ru)) = 43.2 Hz, 1J(Si−H of SiH2) = 218.2 Hz, SiH2Ph). Anal. Calcd for C38H49RuN2SiCl: (698.42) C, 65.35; H, 7.07; N, 4.01. Found: C, 65.01, H, 6.35; N, 4.07. NMR Reaction of 8 with H2SiMePh. Addition of H2SiMePh to a blue solution of 0.050 g (0.085 mmol) of Cp(IPr)RuCl in 5 mL of toluene at −30 °C results in immediate formation of a yellow silane σcomplex, Cp(IPr)RuCl(η2-HSiHMePh) (4e). 1H NMR spectra showed the presence two isomers of Cp(IPr)RuCl(η2-HSiHMePh). Attempted scale-up of this reaction failed because of the thermal instability of the product. 1H NMR (toluene-d8, −40 °C): δ −8.89 (RuH, major isomer), −8.57 (RuH of minor isomer), 0.93−1.63 (br, CH3 of iPr of both isomers), 2.33−3.41 (br, CH of iPr of both isomers), 4.41 (H, SiH(CH3)Ph major isomer), 4.47 (H, SiH(CH3)Ph minor isomer), 1.14 (H, SiH(CH3)Ph major isomer), 0.49 (H, SiH(CH3)Ph minor isomer), 3.93 (s, H, Cp of major isomer), 4.03 (s, H, Cp of minor isomer), 6.46−6.48 (br, H, NCH of both isomers), 6.92−7.27 (m, 6H, C6H3), 7.31 (H, ortho of SiHMePh of major isomers. 13C NMR (toluene-d8, −40 °C): δ 25.3−26.8 (CH3 of iPr of both isomers), 28.5 (CH of iPr of both isomers), 81.2 (Cp of major isomer), 81.8 (Cp of minor isomer), 124.08 and 124.56 (NCH of both isomers), 122.9−130.5 (C6H3), 187.3 (Ru-CN2 of major isomer), 187.8 (Ru-CN2 of minor isomer). 29Si NMR (1H−29Si HSQC, −40 °C): δ −4.2 (SiHMePh major isomer, 1J(Si−H)Ru)) = 40.5 Hz, 1 J(Si−H of SiHCH3) = 212.4 Hz), −7.38 (SiHMePh minor isomer). Generation of [Cp(IPr)Ru(NCCH3)(η2-HSiCl3)]BArF4 (5a) and CpRu(NCCH3)(H)(SiCl3)2 (13). Addition of HSiCl3 (42 μmol) to 0.050 g (38 μmol) of [Cp(IPr)Ru(NCCH3)2]BArF4 in 0.5 mL of CD2Cl2 at −78 °C followed by warming to −30 °C resulted in clean formation of [Cp(IPr)Ru(NCCH3)(η2-HSiCl3)]BArF4. Warming this sample to room temperature generated a mixture of three other

CONCLUSIONS In conclusion, neutral and cationic NHC-supported silane σcomplexes Cp(IPr)Ru(η2-HSiR3)Cl (4) and [Cp(IPr)Ru(η2HSiR3)(NCCH3)]+ (5) can be prepared by silane addition to the precursor compounds 8 and 9. Compared to their iPr3Psupported analogues, complexes 4a−f show an unusual trend in that the J(H−Si) values increase from the HSiMe2Cl derivative to the HSiCl3 derivative. The comparison of molecular structures of Cp(IPr)Ru(η2-HSiMe2Cl)Cl (4b) and Cp(iPr3P)Ru(η2-HSiMe2Cl)Cl as well as the analysis of bonding capabilities of the IPr carbene and iPr3P did not reveal a reason for this divergent behavior. Addition of silanes to the cationic complex [Cp(IPr)Ru(NCCH3)2][BArF4] (9) leads to equilibrium mixtures of 9 with very unstable cationic silane σcomplexes [Cp(IPr)Ru(η2-HSiR3)(NCCH3)]+ (5). The stability of 5 is much reduced in comparison with the isolobal analogues [Cp(iPr3P)Ru(η2-HSiR3)(NCCH3)]+. Complex 9 was found to be a catalyst for the hydrosilylation of benzaldehyde, benzonitrile, and pyridine but is much less active than the related compound [Cp(iPr3P)Ru(NCCH3)2]+. This difference in reactivity can be traced to the ease of IPr dissociation from ruthenium, which is further supported by the formation of the ruthenate complex [IPrH]+[CpRuCl(H)(SiCl3)2]− upon the reaction of Cp(IPr)RuCl with two equivalents of HSiCl3.

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EXPERIMENTAL SECTION

General Details. All manipulations were carried out using conventional inert atmosphere glovebox and Schlenk techniques. Grubbs-type columns were used to dry all solvents except THF, which was distilled from sodium benzophenone ketyl solution. NMR spectra were obtained with Bruker DPX-600 instrument (1H: 600 MHz; 13C: 151 MHz; 29Si: 119.2 MHz; 11B NMR: 160 MHz; 19F NMR: 564 MHz), unless otherwise stated. J(Si−H) constants were measured from 1D HSQC. IR spectra were measured on an ATI Mattson FTIR spectrometer. Anhydrous LiCl and silanes were purchased from Sigma-Aldrich. Complex [Cp(IPr)Ru(pyr)2][PF6] was prepared according to the previously published procedure.10 Cp(IPr)RuCl(η2-HSiCl3) (4a). Addition of HSiCl3 to a blue solution of Cp(IPr)RuCl (0.050 g, 0.085 mmol) in 5 mL of toluene at −30 °C results in the immediate color change to yellow. Hexane was added and the product was recrystallized at −30 °C from a mixture of toluene/hexane (4:1), resulting in crystallization of 4a. The product was filtered and dried under vacuum. Yield: 62% (0.038 g). IR (Nujol): ν(Ru−H) = 2030 cm−1. 1H NMR (−23 °C, toluene-d8): δ −7.57 (s, 1H, RuH), 0.85 (br, 6H, CH3 of iPr), 0.96 (br, 6H, CH3 of i Pr), 1.03 (d, 3J(H−H) = 6.4 Hz, 3H, CH3 of iPr), 1.16 (d, 3J(H−H) = 6.4 Hz, 3H, CH3 of iPr), 1.32 (d, 3J(H−H) = 6.4 Hz, 6H, CH3 of iPr), 2.87 (br, 2H, CH of iPr), 3.41 (br, 2H, CH of iPr), 4.38 (s, 5H, Cp), 6.41 (s, 2H, NCH), 7.03−7.23 (m, 6H, C6H3). 13C NMR (−23 °C, toluene-d8): δ 25.3 (CH3 of iPr), 26.3 (CH3 of iPr), 23.4 (CH3 of iPr), 23.5 (CH3 of iPr), 28.4 (CH of iPr), 86.0 (Cp), 125.5 (NCH), 122.9 (s, m-C6H3), 130.9 (s, p-C6H3), 179.0 (Ru-CN2). 29Si NMR (1H−29Si HSQC): δ 13.6 (1J(Si−H) = 64.3 Hz, SiCl3). Anal. Calcd for C32H42RuN2SiCl4 (725.658): C, 52.89; H, 5.96; N, 3.86. Found: C, 52.46; H, 5.67; N, 3.73. Cp(IPr)RuCl(η2-HSiCl2Me) (4b). Addition of HSiCl2Me to a blue solution of 0.050 g (0.085 mmol) of Cp(IPr)RuCl in 5 mL of toluene at −30 °C results in the immediate color change to yellow. The product was recrystallized from a mixture of toluene and hexane (4:1) at −30 °C. Isolated yield: 84% (0.050 g). IR (Nujol): ν(Ru−H) = 2054 cm−1. 1H NMR (−5 °C, toluene-d8): δ −8.10 (1H, RuH), 0.87 (d, 3J(H−H) = 6.7 Hz, 6H, CH3 of iPr), 0.97 (d, 3J(H−H) = 6.7 Hz, 6H, CH3 of iPr), 1.22 (br, 6H, CH3 of iPr), 1.36 (d, 3J(H−H) = 6.7 Hz, 6H, CH3 of iPr), 0.54 (s, 3H, SiCH3), 2.49 (br, 2H, CH of iPr), 3.33 (br, 2H, CH of iPr), 4.48 (s, 5H, Cp), 6.49 (s, 2H, NCH), 6.99− E

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gave a deep blue solution of Cp(IPr)RuCl. Volatiles were removed under vacuum, and the residue was extracted by toluene, filtered, and dried to give 0.138 g (0.23 mmol) of Cp(IPr)RuCl. This product was further recrystallized from a mixture of toluene and hexane (2:3) at −30 °C. Isolated yield: 80%. The compound is highly moisture sensitive. 1H NMR (C6D6): δ 1.16 (d, 3J(H−H) = 6.8 Hz, 12H, CH3 of iPr), 1.49 (d, 3J(H−H) = 6.8 Hz,12H, CH3 of iPr), 3.43 (sept, 3 J(H−H) = 6.8 Hz, 4H, CH of iPr), 3.68 (s, 5H, Cp), 6.68 (s, 2H, NCH), 7.19−7.27 (m, 6H, C6H3). 13C NMR (C6D6): δ 26.4 (s, CH3 of iPr), 22.9 (s, CH3 of iPr), 28.7 (s, CH of iPr), 61.9 (Cp), 124.4 (NCH), 123.7 (s, m-C6H3), 129.8 (s, p-C6H3), 198.7 (Ru-CN2). Anal. Calcd for C32H41RuN2Cl·0.5toluene (636.274): C, 67.01; H, 7.13; N, 4.40. Found: C, 67.34; H, 6.94; N, 4.81. [Cp(IPr)Ru(NCCH3)2]BArF4 (9). Reaction of 0.200 g (0.34 mmol) of Cp(IPr)RuCl with 0.296 g (0.34 mmol) of LiBArF in acetonitrile at room temperature yields a solution of [Cp(IPr)Ru(NCCH3)2]BArF4 and a precipitate. The precipitate was filtered off, and the filtrate was dried under vacuum to yield 0.420 g of [Cp(IPr)Ru(NCCH3)2]BArF4. Yield: 93% (0.420 g). 1H NMR (CD3CN): δ 1.14 (d, 3J(H−H) = 7.0 Hz, 12H, CH3 of iPr), 1.28 (d, 3J(H−H) = 7.0 Hz, 12H, CH3 of iPr), 1,96 (s, 6H, CH3CN), 2.81 (sep, J(H−H) = 7.0 Hz, 4H, CH of iPr), 3.77 (s, 5H, Cp), 7.29 (s, 2H, NCH), 7.36−7.5 (m, 6H, C6H3). 13C NMR (CD3CN): δ 0.8 (NCCH3), 25.0 (CH3 of iPr), 21.9 (CH3 of i Pr), 28.5 (CH of iPr), 73.2 (Cp), 125.9 (NCH), 123.8 (m-C6H3), 130.4 (p-C6H3), 137.0 (NCCH3), 185.4 (Ru-CN2). 11B NMR (CD3CN): δ −16.69. 19F NMR (CD3CN): δ −169.3, −163.8 (t, 3 J(F−F) = 20.4 Hz), −133.7 (t, 3J(F−F) = 20.4 Hz). Anal. Calcd for C60H47RuBF20N4 (1315.891): C, 54.76; H, 3.60; N, 4.26. Found: C, 54.54; H, 3.84; N, 4.55. [IPrH][CpRuCl(H)(SiCl3)2] (10). Two equivalents of HSiCl3 were added by syringe to a precooled (−30 °C) solution of 0.050 g (0.085 mmol) of Cp(IPr)RuCl in 5 mL of toluene. The mixture was charged with hexanes and kept at −30 °C to result in crystallization of compound 10. Yield: 0.057 g (0.067 mmol, 82%). IR (Nujol): ν(Ru− H) = 2012 cm−1. NMR spectra were taken on a Bruker DPX-400 spectrometer: 1H NMR (400 MHz, CDCl3): δ −10.59 (1H, RuH), 5.20 (s, 5H, Cp). 13C NMR (CDCl3): δ 88.7 (Cp). 29Si NMR (H−Si HSQC): δ −10.7 (SiH), 1J(Si−H(Ru), 1D 29Si INEPT+)) = 14.5 Hz. Anal. Calcd for C32H45RuN2Cl7Si (825.35): C, 46.53; H, 5.25; N, 3.39. Found: C, 46.44; H, 5.86; N, 3.23. Crystal Structure Determinations. Data collection results for compounds 4b, 4f, 8, and 10 represent the best data sets obtained in several trials for each sample (Tables S1). The crystals were mounted on thin glass fibers using paraffin oil. Prior to data collection the crystals were cooled to 200.15 K. Data were collected on a Bruker AXS SMART single-crystal diffractometer equipped with a sealed Mo tube source (wavelength 0.710 73 Å) APEX II CCD detector. Raw data collection and processing were performed with the APEX II software package from Bruker AXS.20 Initial unit cell parameters were determined from 60 data frames with 0.3° ω scan each collected at the different sections of the Ewald sphere. Semiempirical absorption corrections based on equivalent reflections were applied.21 The structures were solved by direct methods, completed with difference Fourier syntheses, and refined with full-matrix least-squares procedures based on F2. For 4b, 4d, and 10 the hydride atom positions were found as residual electron density peaks from the Fourier maps. All other hydrogen atoms were placed at calculated position and restrained to riding models. All scattering factors are contained in several versions of the SHELXTL program library, with the latest version used being v.6.12.22

hydride species, the main of which is tentatively assigned the structure CpRu(NCCH3)(H)(SiCl3)2 (13). Significant dissociation of the imidazolium ion characterized by the imidazole proton peak at 8.58 ppm in the 1H NMR spectra was also noticed. 5a: 1H NMR (−40 °C, CD2Cl2): δ −7.81 (s + satt, 1J(Si−H) = 64.4 Hz, RuH), 0.97 (d, 3J(H−H) = 6.7 Hz, 3H, CH3 of iPr), 1.09 (d, 3 J(H−H) = 7.5 Hz, 3H, CH3 of iPr), 1.10 (d, 3J(H−H) = 7.5 Hz, 3H, CH3 of iPr), 1.21 (d, 3J(H−H) = 6.7 Hz, 3H, CH3 of iPr), 1.22 (d, 3 J(H−H) = 6.7 Hz, 3H, CH3 of iPr), 1.38 (d, 3J(H−H) = 6.8 Hz, 3H, CH3 of iPr), 1.47 (d, 3J(H−H) = 6.8 Hz, 3H, CH3 of iPr), 1.49 (d, 3 J(H−H) = 7.5 Hz, 3H, CH3 of iPr), 1.90 (s, NCCH3), 2.75 (m, 2H, CH of iPr), 4.92 (s, 5H, Cp). 13: 1H NMR (−40 °C, CD2Cl2): δ −9.79 (s + satt, 1J(Si−H) = 24.8 Hz, 1H, RuH), 2.30 (s, 3H, NCCH3), 5.26 (s, 5H, Cp). 1H NMR (23 °C, CD2Cl2): δ −9.65 (s, 1H, RuH), 2.37 (s, 3H, NCCH3), 5.27 (s, 5H, Cp). 13C NMR (23 °C, CD2Cl2): δ 4.5 (NCCH3), 89.6 (s, Cp). 29 Si NMR (1H−29Si HSQC, 23 °C): δ 39.3 (1J(Si−H) = 24.9 Hz). Generation of [Cp(IPr)Ru(NCCH3)(η2-HSiMeCl2)]BArF4 (5b). Reaction of 0.050 g (38 μmol) of 9 in 5 mL of CD2Cl2 at −40 °C with 42 μmol of HSiMeCl2 yielded an equilibrium mixture of 5b, 9, and another, yet unknown, hydride species formed in a 1:0.37:0.14 mol ratio. Addition of nine more equivalents of silane changes the ratio of 5b and 9 to 1:0.39. The formation of imidazolium salt [HIPr]BArF4 in 16% yield was also noticed. 5b: 1H NMR (−30 °C, CD2Cl2): δ −7.94 (1H, RuH), 0.85 (s, 3H, SiCH3), 0.99 (br, 3H, CH3 of iPr), 1.07 (br, 3H, CH3 of iPr), 1.18 (br d, 3J(H−H) = 7.2 Hz, 6H, CH3 of iPr), 1.26 (br d, 3J(H−H) = 7.2 Hz, 6H, CH3 of iPr), 1.35 (br, 3H, CH3 of iPr), 1.38 (br, 3H, CH3 of iPr), 2.65 (br s, 1H, CH of iPr), 2.37 (br s, 1H, CH of iPr), 2.30 (br s, 2H, CH of iPr), 4.74 (s, 5H, Cp), 7.41 (br, 4H, m-C6H3), 7.54 (br t, 3J(H− H) = 7.8 Hz, 2H, p-C6H3). 13C NMR (−30 °C, CD2Cl2): δ 2.24 (NCCH3), 4.63 (SiCH3), 21.4 (CH3 of iPr), 22.2 (CH3 of iPr), 28.6 (CH of iPr), 86.5 (Cp), 125.2 (NCH), 127.8 (m-C6H3), 130.0 (pC6H3), 177.9 (NCCH3), 207.7 (Ru-CN2). 29Si NMR (1H−29Si HSQC): δ 56.6 (SiMeCl2), 1J(Si−H(Ru), 1D 29Si INEPT+) = 49.9 Hz, 1J(Si−H of SiH, 1D 29Si INEPT+) = 206.6 Hz. The minor hydride species: 1H NMR (−30 °C, CD2Cl2): δ −8.40 (1H, RuH), 0.03 (s, 3H, SiCH3). 29Si NMR (1H−29Si HSQC): δ 41.1 (SiMeCl2, coupling constant cannot be measured). Generation of [Cp(IPr)Ru(NCCH3)(η2-HSiH2Ph)]BArF4 (5d). Reaction of 0.050 g (38 μmol) of 9 in CD2Cl2 at −40 °C with 42 μmol of PhSiH3 yielded a mixture of [Cp(IPr)Ru(NCCH3)2]BArF and [Cp(IPr)Ru(NCCH3)(η2-HSiH2Ph)]BArF4 in a 1.15:1 mol ratio. Less than 10% of imidazolium salt was also formed. 1H NMR (−30 °C, CD2Cl2): δ −8.28 (1H, RuH), 1.06−1.27 (br m, 24H, CH3 of iPr), 2.09 (s, 3H, CH3CN), 2.45 (br, CH of iPr), 4.18 (s, 5H, Cp), 4.05 (d, 2 J(H−H) = 9.0 Hz, 1H, SiH2), 4.33 (d, 2J(H−H) = 9.0 Hz, 1H, SiH2), 7.35 (s, 2H, NCH in IPr). 13C NMR (−30 °C, CD2Cl2): δ 4.8 (NCCH3), 22.6 (CH3 of iPr), 25.9 (CH3 of iPr), 29.0 (CH of iPr), 83.6 (Cp), 117.8 (NCCH3), 126.0 (NCH), 125.6, 130.6, 135.2 (ortho, meta, and para C of SiH2Ph). 29Si NMR (1H−29Si HSQC): δ −23.4 (SiH2Ph), 1J(Si−H(Ru), 1D 29Si INEPT+) = 50.9 Hz, 1J(Si−H of SiH, 1D 29Si INEPT+) = 208.4 and 211.0 Hz. Generation of [Cp(IPr)Ru(NCCH3)(η2-HSiHMePh)]BArF4 (5e). Reaction of 0.050 g (38 μmol) of 9 with H2SiMePh (42 μmol) in CD2Cl2 at −40 °C resulted in an equilibrium between the starting compound, silane, and two isomers of [Cp(IPr)Ru(NCCH3)(η2HSiHMePh)]BArF4 (formed in a 1:2 ratio). At −40 °C, the ratio of 9 to 5e was 10:1. Major isomer. 1H NMR (−30 °C, CD2Cl2): δ −8.39 (1H, RuH), 0.36 (SiMe), 3.83 (SiH), 4.30 (s, 5H, Cp). 29Si NMR (1H−29Si HSQC): δ 0.4 (1J(Si−H(Ru), 1D 29Si INEPT+) = 46.2 Hz, 1J(Si−H of SiH, 1D 29Si HSQC 1D JC) = 207.4 Hz. Minor isomer. 1H NMR (−30 °C, CD2Cl2): δ −8.15 (1H, RuH), 0.61 (SiMe), 4.14 (SiH), 4.16 (s, 5H, Cp). 29Si NMR (1H−29Si HSQC): δ −2.5 (1J(Si−H (Ru), 1D 29Si INEPT+) = 46.1 Hz, 1J(Si−H of SiH, 1D 29Si HSQC 1D JC) = 195.2 Hz. Cp(IPr)RuCl (8). Addition of 0.012 g (0.29 mmol) of LiCl to a THF solution of 0.250 g (0.29 mmol) of [Cp(IPr)Ru(pyr)2][PF6]

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00958. NMR spectra for new compounds (PDF) Crystallographic data (CIF) F

DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX

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MeiPr2: Tenorio, M. J.; Puerta, M. C.; Valerga, P. J. Organomet. Chem. 2000, 609, 161. (13) Alcaraz, G.; Sabo-Etienne, S. Coord. Chem. Rev. 2008, 252, 2395−2409. (14) Alternatively but less likely, the mechanism can include C−H elimination of the imidazolium ion [IPrH]+ from the silane complex Cp(IPr)Ru(η2-HSiCl3)Cl (9a) to give the intermediate [CpRuCl(SiCl3)]−, which is then intercepted by excess silane to furnish 10. (15) Osipov, A. L.; Gerdov, S. M.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Organometallics 2005, 24, 587. (16) Fernandez, M.-J.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle, T. F.; Maitlis, P. M. J. Am. Chem. Soc. 1984, 106, 5458. (17) Duckett, S. B.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1991, 28. (18) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004, 23, 886. (19) (a) Vyboishchikov, S. F.; Nikonov, G. I. Organometallics 2007, 26, 4160−4169. (b) For related iron complexes, see: Vyboishchikov, S. F.; Nikonov, G. I. Chem. - Eur. J. 2006, 12, 8518−8533. (c) For related Ru complexes, see: Lee, T. Y.; Dang, L.; Zhou, Z.; Yeung, C. H.; Lin, Z.; Lau, C. P. Eur. J. Inorg. Chem. 2010, 2010, 5675−5684. (20) APEX Software Suite v.2010; Bruker AXS: Madison, WI, 2005. (21) Blessing, R. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, A51, 33. (22) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112.

Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail (G. Nikonov): [email protected]. Tel: +1 (905) 6885550, ext 3350. Fax: +1 (905) 6829020. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a DG NSERC grant (3268992012) to G.I.N. Dedicated to Prof. Judith A. K. Howard (FRS) on the occasion of her 70th birthday.

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00958 Organometallics XXXX, XXX, XXX−XXX