Article pubs.acs.org/Organometallics
Ruthenium Carbene−Diether Ligand Complexes: Catalysts for Hydrogenation of Olefins Tongen Wang, Conor Pranckevicius, Clinton L. Lund, Michael J. Sgro, and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 S Supporting Information *
ABSTRACT: A series of carbene−diether ligands were prepared and the corresponding Ag salts used to prepare the complexes RuHCl(PPh 3 ) 2 (Im(OR) 2 ) (Im(OR) 2 = C3H2(NCH2CH2OR)2; R = Me (4a), t-Bu (4b), tert-hexyl (4c), Ph (4d), 2,6-i-Pr2C6H3 (4e)). In an analogous fashion the species RuHCl(PPh3)2(Y2Im(OMe)2) (Y2Im(OMe)2 = Y2C3(NCH2CH2OMe)2; Y2 = C6H4 (4f), Y = Cl (4g), Me (4h)) were also synthesized. Similarly RuHCl(CO)(PPh3)2(Im(OMe)2) (5) was prepared and readily converted to RuHCl(CO)(SIMes)(Im(OMe)2) (6) via treatment with SIMes. The reaction of 4a with SIMes afforded RuHCl(SIMes)(Im(OMe)2)(PPh3) (7), which reacts subsequently with Na[BPh4] to give [RuH(Im(OMe)2)(SIMes)][(η6-Ph)BPh3] (8). In a series of tests, the species 4a−h, 5, 6, and 8 were shown to catalyze the hydrogenations of 1-hexene, cyclohexene, and dimethyl itaconate. From the activity of 4a−h it is clear that the capability of the carbene−ether substituents to coordinate to the metal as well as electron-donating substituents on the carbene fragment enhances catalytic activity. Other variations such as in 5, 6, and 8 resulted in terminal-olefin-selective hydrogenation catalysts, although the zwitterionic species 8 showed significantly enhanced activity.
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INTRODUCTION Heterogeneous hydrogenation catalysis has evolved since the seminal discovery of Sabatier.1 With the emergence of organometallic chemistry in the 1960s, homogeneous catalysts have become an active area of study. Most notably, the discovery of RhCl(PPh3)3 by Wilkinson and co-workers2 was followed by the classic work of Osborn and Schrock3−5 and subsequently Crabtree and Morris6 reporting the cationic precatalysts [(COD)Rh(Ph2PCH2CH2PPh2)]+ and [(COD)Ir(py)(PCy3)]+, respectively. These findings have led to numerous applications, and indeed hydrogenation catalysis is the most common transformation used in the chemical industry.7 While asymmetric variations of these catalysts have garnered much attention over the last 30 years,8−10 more recent work has focused on catalysts derived from cheaper and more readily accessible metals such as Fe.11 In the case of Ru, Wilkinson reported the use of RuHCl(PPh3)3 as a catalyst for olefin hydrogenation and showed that it has activity comparable to that of his Rh catalyst.12 While in this particular example the high air sensitivity has limited applications, the last 20 years has seen the emergence of Ru-based hydrogenation catalysts as powerful tools for the asymmetric hydrogenation of polar functional groups.13−20 In targeting Ru-based olefin hydrogenation catalysts, we noted prior reports from Nolan and Fogg and others, who developed effective catalysts using RuHCl(CO)(NHC)(PPh3),21−24 as well as the bis-carbene chelate Ru hydrogenation catalysts developed by Albrecht and co-workers.25 In a recent communication we adopted an approach employing a carbene ligand with pendant methyl ether arms, © XXXX American Chemical Society
anticipating that the ethers could be labile donors that might stabilize transient and highly reactive intermediates.26 This strategy proved successful, as the species RuHCl(PPh3)(Im(OMe)2)(SIMes)27 (Im(OMe)2 = C3H2(NCH2CH2OMe)2) affords facile access to the cis-bis-carbene derivative RuHCl(Im(OMe)2)(SIMes)(PPh3), which is a highly selective catalyst for olefin reduction in the presence of a variety of functional groups. In this full study we explore these systems, describing the preparation of a family of ligand variants and exploring their utility in olefin hydrogenation.
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RESULTS AND DISCUSSION
Complex Synthesis. In a recent communication, we described a synthetic strategy to the species RuHCl(PPh3)2(Im(OMe)2) (4a; Im(OMe)2 = C3H2(NCH2CH2OMe)2).27 In a similar fashion, a series of related species were targeted. Thus, imidazolium salts of the form [C3H3(NCH2CH2OR)2]Cl (R = Me (2a), t-Bu (2b), tert-hexyl (2c), Ph (2d), i-Pr2C6H3 (2e)) were prepared via reactions of the corresponding species ClCH2CH2OR (1a−e) with trimethylsilylimidazole (Scheme 1). Subsequent reactions with Ag2O yielded the silver salts AgCl[(C3H2(NCH2CH2OR)2] (R = Me (3a), t-Bu (3b), terthexyl (3c), Ph (3d), 2,6-i-Pr2C6H3 (3e)), while further reaction with RuHCl(PPh 3 ) 3 gave the Ru complexes RuHCl(PPh3)2(Im(OR)2) (R = Me (4a), t-Bu (4b), tert-hexyl (4c), Ph (4d), 2,6-i-Pr2C6H3 (4e)) (Scheme 1). Received: January 18, 2013
A
dx.doi.org/10.1021/om400044w | Organometallics XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis of 2−4
Figure 2. POV-ray depiction of the molecular structure of 4d. All hydrogen atoms are omitted for clarity. Color scheme: C, black; O, red; Cl, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg): Ru−C = 1.987(3), Ru−P = 2.328(1), 2.337(1), Ru−Cl = 2.456(1), Ru−H = 1.56(4); C−Ru−P = 90.5(1), 90.8(1), P−Ru−P = 178.60(4), C−Ru−Cl = 167.91(11), P−Ru−Cl = 90.22(3), 88.62(3), P−Ru−H = 88(1), 91(1), Cl−Ru−H = 106(1).
The complexes 4b−e give rise to triplet resonances in the 1H NMR spectra at −32.37, −32.34, −32.14, and −31.98 ppm, respectively, which are attributable to the corresponding Ru hydride fragments. These data stand in contrast to the triplet at −23.54 ppm previously reported for 4a. The upfield shift of the hydride resonance suggests the absence of any ligand in the site trans to the hydride. In addition, in the case of 4a, the appearance of a singlet at 44.61 ppm in the 31P NMR spectrum stands differs from the corresponding resonances in 4b−e, which are observed ca. 47.5 ppm. This too is consistent with a structural perturbation. X-ray structural studies of 4b−d (Figures 1 and 2) confirmed that the geometry about Ru is five-coordinate in which two phosphines adopt a trans
disposition. The coordination of an ether arm is not present in any of the compounds 4b−d, which is thought to be due to the steric effects of the bulky ethereal substituents. Substitutions to the backbone carbons of an NHC have been shown to have an impact on the activity of catalysts for olefin metathesis.28−32 Thus, when a related synthetic strategy was employed, compounds RuHCl(PPh3)2(Y2Im(OMe)2) (Y2Im(OMe)2 = Y2C3(NCH2CH2OMe)2; Y2 = C6H4 (4f), Y = Cl (4g), Me (4h)) were prepared from the corresponding benzimidazole, dichloroimidazole, or dimethylimidazole precursors (Scheme 2). In a similar fashion, once the substituted imidazolium salt was obtained, the corresponding Ag carbene complex was generated and used to effect transmetalation to Ru. Scheme 2. Ru Complexes 4f−h
The compounds 4f−h give rise to triplet resonances in the H NMR spectra at −22.59, −22.68, and −22.98 ppm, respectively, which are attributable to the corresponding Ru hydride fragments. These chemical shifts for Ru−H are similar to the Ru−H signal for 4a (−23.54 ppm), implying the coordination of one of the ether arms to the site trans to the hydride. The 13C{1H} resonances for the carbenic carbons of 4f−h are observed at 209.82, 197.15, and 190.61 ppm, respectively. Nolan et al. have recently examined the effect of various substitutions to the NHC backbone on the 13C{1H} chemical shift of carbenic carbons and have found that electrondonating groups on the backbone lead to an upfield shift of the carbenic carbon signal.31 Thus, the present data suggest that the carbenes in 4f−h exhibit electron-donating ability following the trend 4h > 4g > 4f. It is interesting that the NHC in 4g appears to be a better donor than that in 4f, suggesting that the resonance-withdrawing ability of the benzimidazole ring is 1
Figure 1. POV-ray depictions of the molecular structures of (a) 4b and (b) 4c. All hydrogen atoms, except for the hydride, are omitted for clarity. Color scheme: C, black; O, red; Cl, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg) are as follows. For 4b: Ru−C = 1.997(5), Ru−P = 2.303(1), 2.310(1), Ru−Cl = 2.448(1, Ru−H = 1.59(7); C−Ru−P = 92.2(1), 90.6(1), P−Ru−P = 174.38(5), C−Ru−Cl = 164.5(2), P− Ru−Cl = 89.10(5), 89.56(5), C−Ru−H = 93(2), P−Ru−H = 89(2), 86(2), Cl−Ru−H = 103(2). For 4c: Ru−C = 1.969(5), Ru−P = 2.3283(12), 2.3301(11), Ru−Cl = 2.4394(11), Ru−H = 1.41(6); C− Ru−P = 92.14(15), 94.1(2), P−Ru−P = 169.51(4), C−Ru−Cl = 161.2(1), P−Ru−Cl = 87.64(4), 89.19(4), C−Ru−H = 88(2), P−Ru− H = 88(2), 84(2), Cl−Ru−H = 110(2). B
dx.doi.org/10.1021/om400044w | Organometallics XXXX, XXX, XXX−XXX
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Compounds 4a−h are examples of five- and six-coordinate Ru hydride complexes stabilized by a carbene ligand. Morris et al. reported the SIMes complex RuH(PPh3)2((CH2C6H2Me2)SIMes), in which a methyl group from a mesityl substituent undergoes C−H activation.33 Similarly, Fogg et al. reported that the corresponding reaction of IMes with RuHCl(PPh3)3 under N2 gave the unique species (IMes)2RuHCl(N2),34 while Burling et al. described the species Ru(ICy)2(PPh3)H2, which appears to be stabilized by an agostic interaction between Ru and a proton on one of the α-Cy substituents.35 Further Reactivity. It should be noted that compounds 4a−h are acutely sensitive to air. Coordination of CO has been shown to diminish such sensitivity in previously reported ruthenium hydride species.36 The reaction of RuHCl(CO)(PPh3)3 with 3a afforded the white solid 5 in 88% yield (Scheme 3). The 1H NMR spectrum for 5 revealed a triplet at δ
more effective in this case than for the chloro substituents of 4g. X-ray diffraction studies of compounds 4f (Figure 3) and 4g (Figure 4) confirm that in both cases one of the ether arms
Scheme 3. Synthesis of 6−8 Figure 3. POV-ray depiction of the molecular structure of 4f. All hydrogen atoms, except for the hydride, are omitted for clarity. Color scheme: C, black; O, red; Cl, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg): Ru−C = 1.968(3), Ru−P = 2.3323(7), 2.3354(7), Ru−O = 2.379(2), Ru−Cl = 2.4886(7), Ru−H = 1.55(3); C−Ru−P = 93.31(8), 93.20(8), P−Ru− P = 167.15(3), C−Ru−O = 90.09(9), P−Ru−O = 96.21(5), 94.84(5), C−Ru−Cl = 177.56(8), P−Ru−Cl = 88.89(2), 84.44(2), O−Ru−Cl = 90.71(5), C−Ru−H = 87(1), P−Ru−H = 86(1), 83(1), O−Ru−H = 176(1), Cl−Ru−H = 92(1).
−15.13 ppm consistent with Ru−H adjacent to two phosphine ligands. This downfield shift suggests that CO or Cl replaces the ethereal fragment trans to the hydride in a six-coordinate ruthenium species. The 13C{1H} NMR spectrum for 5 includes shifts at 183.78 and 204.76 ppm attributable to the carbene carbons of the NHC and CO ligands, respectively. X-ray diffraction experiments confirmed 5 to be RuHCl(CO)(PPh3)2(Im(OMe)2) (Figure 5). The two molecules in the asymmetric unit exhibited a pseudo-octahedral geometry about Ru. The hydrides were located in the electron density map, and the Ru−H bond length averaged 1.58(3) Å, while the PPh3 ligands were trans to one another. Similarly, the hydride and chloride and CO and NHC adopt trans dispositions. As anticipated, complex 5 is not air-sensitive and can readily be handled in air without any noticeable change in its physical appearance or spectral properties. Related Ru carbene complexes derived from RuHCl(CO)(PPh3)3 such as RuHCl(CO)(NHC)(PPh3) (NHC = IMes, SIMes)22,37 have been reported by Fogg and co-workers. Subsequent treatment of 5 with a stoichiometric amount of SIMes and heating at 60 °C for 24 h produced a bright yellow suspension, from which bright orange crystals of 6 were isolated in 69% yield (Scheme 3). The 1H NMR spectrum for 6 revealed a singlet at δ −25.46 ppm, indicating that the product is phosphine-free. The corresponding 13C{1H} NMR spectrum revealed signals at 190.10, 201.98, and 219.80 ppm that were assigned to the ipso carbon of the unsaturated carbene, the CO ligand, and the ipso carbon of the saturated carbene, respectively. The relative position of the ligands was confirmed
Figure 4. POV-ray depiction of the molecular structure of 4g. All hydrogen atoms, except for the hydride, are omitted for clarity. Color scheme: C, black; O, red; Cl, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg): Ru−C = 1.966(4), Ru−P = 2.333(1), 2.343(1), Ru−O = 2.374(3), Ru−Cl = 2.5213(9), Ru−H = 1.521(5); C−Ru−P = 90.8(1), 93.7(1), P−Ru−P = 171.57(4), C−Ru−O = 90.22(14), P−Ru−O = 95.11(8), 92.0(8), C−Ru−Cl = 177.4(1), P−Ru−Cl = 88.14(4), 87.06(3), O−Ru−Cl = 92.28(8).
coordinates to the Ru center, similar to the case for complex 4a. Notably, the added substituents on the NHC backbone shorten the Ru−C bonds (4f, 1.968(3) Å; 4g, 1.966(4) Å) and Ru−H bonds (4f, 1.55(3) Å; 4g, 1.521(5) Å) relative to those in 4b− d. Ru−Cl (4f, 2.4886(7) Å; 4g, 2.5213(9) Å) and Ru−O bonds (4f, 2.379(2) Å; 4g, 2.374(3) Å) in 4f,g in the solid state are longer than those in the unsubstituted complex 4a (Ru−Cl = 2.475(1) Å, Ru−O = 2.369(3) Å). Despite the slightly altered bond lengths, the geometry about Ru is not very sensitive to substitution on the NHC backbone. C
dx.doi.org/10.1021/om400044w | Organometallics XXXX, XXX, XXX−XXX
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We have previously communicated the reaction of 4a with SIMes to effect replacement of phosphine, yielding RuHCl(SIMes)(Im(OMe)2)(PPh3) (7).27 This is a rare example of a cis-bis-carbene Ru hydride complex; although several other Ru bis-carbene complexes are known to adopt a cis disposition,25,38−51 the majority of such species adopt a trans-NHC geometry.36,52−61 Dixneuf and co-workers prepared cis-carbene Ru derivatives using a sterically unencumbered carbene,62 while Whittlesey et al. studied a series of Ru cis-bis-carbene complexes employing NHCs with N-bound isopropyl63 or cyclohexyl35 substituents. Treatment of 7 with 1 equiv of Na[BPh4] in methylene chloride produces a gradual dramatic color change from dark red to pale yellow over 16 h. From this reaction, pale yellow crystals of 8 were isolated in 95% yield. The hydride resonance appeared as a singlet at δ −9.28, indicating a phosphine-free species. Intriguingly, five multiplets were located between 3.95 and 5.22 ppm, each of which integrated to one proton. The location of these signals is reminiscent of a monosubstituted arene bound to a ruthenium metal center. If these interpretations are true, the ruthenium metal center should be bound with four ligands: a hydride, two NHCs, and a monosubstituted η6-bound arene. Careful analysis of the corresponding 13C{1H} NMR and 2-D HSQC spectra further supports this interpretation. First, in the HSQC cross-peaks for the protons in the range of 3.95−5.22 ppm were located in the 13 C{1H} NMR spectrum in the range of 86.79−98.81 ppm, well within the normal range for a monosubstituted arene η6-bound to ruthenium.64 The ipso carbons of NHCs were assigned to signals located at 184.13 and 214.22 ppm in the 13C NMR spectrum. In the 11B NMR spectrum of 8, a single sharp singlet indicative of a borate was observed at δ −8.11 ppm. To confirm these interpretations, single-crystal X-ray analysis on 8 confirmed the formulation as [RuH((CH3OCH2CH2)2Im)(SIMes)][(η6-Ph)BPh3] (Figure 7). One arene of tetraphenylborate is η6-bound to the ruthenium metal center with a Ru−Ccentroid distance of 1.799 Å. The 18-electron Ru complex has a piano-stool geometry, where the legs are comprised of a
Figure 5. POV-ray depiction of the molecular structure of 5. Cocrystallized CH2Cl2 and hydrogen atoms, except for hydride, are omitted for clarity. Color scheme: C, black; O, red; Cl, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg): Ru1−CCO = 1.885(5), Ru−Ccarbene = 2.149(4), Ru−P = 2.349(1), 2.355(1), Ru−Cl = 2.597(1), O−CCO = 1.097(5), Ru−H = 1.58(3); CCO−Ru−Ccarbene = 167.0(2), P−Ru−P = 176.04(4), Ccarbene−Ru−Cl = 98.7(1), Ccarbene−Ru−H = 77(1).
by a single-crystal X-ray analysis of RuHCl(CO)(SIMes)(Im(OMe)2) (6) (Figure 6). The two molecules in the asymmetric
Figure 6. POV-ray depiction of the molecular structure of 6. Cocrystallized CH2Cl2 and hydrogen atoms are omitted for clarity. Color scheme: C, black, O, red, Cl, green, P, orange, N, aquamarine, Ru, salmon. Selected bond lengths (Å) and angles (deg): Ru−CCO = 1.934(10), Ru−COCOMe = 2.087(4), Ru−CSIMes = 2.091(4), Ru−Cl = 2.440; CCO−Ru−COCOMe = 90.0(2), CCO−Ru−CSIMes = 91.0(2), COCOMe−Ru−CSIMes = 178.5(2), CCO−Ru−Cl = 172.8(3), COCOMe− Ru−Cl = 86(1), CSIMes−Ru−Cl = 93.2(1).
unit showed the SIMes carbene ligand trans to the Im(OMe)2 carbene ligand, while the CO is bound trans to chloride. In this case, the hydride could not be located in the electron density map but its location was inferred as one of the two vacant coordination sites apical to the pseudo-square-planar RuC3Cl ligand plane. The C−Ru−Cl angle is 172.8(3)°, and the C− Ru−Cl angle is 86.0(1)°, reflecting a slight distortion from a pseudo-square-planar arrangement of the ligands. The Ru−CO bond length of 1.93(1) Å in 6 is markedly longer than the corresponding bond in 5, presumably a result of the trans chloride ligand in 6. On the other hand, the Ru−Cl bond length of 2.440(1) Å in 6 is much shorter than the corresponding value in 5, again reflecting the differing trans ligation.
Figure 7. POV-ray depiction of 8. Two cocrystallized benzene molecules and hydrogen atoms, except for the hydride, are omitted for clarity. Color scheme: C, black; O, red; B, green; P, orange; N, aquamarine; Ru, salmon; H, blue. Selected bond lengths (Å) and angles (deg): Ru−COCOMe = 2.036(3), Ru−CSIMes = 2.054(3), Ru−H = 1.58(3), Ru−Ccentroid = 1.799; COCOMe−Ru−CSIMes = 90(1), Ccentroid−Ru−H = 126.29, Ccentroid−Ru−COCOMe = 122.73, Ccentroid− Ru−CSIMes = 138.71, COCOMe−Ru−H = 81.4(9), CSIMes−Ru−H = 78.9(9). D
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remained intact. Similarly, substitutions on the NHC backbone were also observed to inhibit decomposition of Ru-based catalysts in metathesis reactions.28,29 We have previously communicated the utility of 7 in the selective hydrogenation of terminal olefins.27 In that case, 7 catalyzed the quantitative hydrogenation of 1-hexene in 8 h. Interestingly, use of the closely related species 6 results in poorer activity in the hydrogenation of 1-hexene. This observation is consistent with the view that the position of the CO cis to the hydride in 6 deters coordination of the olefin in the necessary position for insertion. In addition, it is also noteworthy that in 7 the carbene ligands are oriented in a cis disposition, while in 6 the carbene donors are trans. The further impact of unsaturation is inferred by the quantitative reduction of hexene in 6 h employing 8 as the catalyst. Arene ring slippage or dissociation is thought to generate vacant coordination sites on Ru, which allow interaction with the substrate. Nonetheless, this reactivity is observed to be significantly reduced for the internal olefin cyclohexene (Table 1). Further comparison of these catalysts requires a detailed kinetic study.
hydride and two NHCs. The hydride was located in the electron density map, and a Ru−H bond length of 1.58(3) Å was found. The NHC ipso carbon−ruthenium bond lengths were Ru−COCOMe = 2.036(3) Å and Ru−CSIMes = 2.054(3) Å, which correspond to those typically seen for coordinated unsaturated and saturated NHCs, respectively. Olefin Hydrogenation. The ability of these species 4a−e to act as catalysts for the hydrogenation of 1-hexene, cyclohexene, and dimethyl itaconate was investigated and compared. Reactions were monitored over 24 h (Table 1). In Table 1. Hydrogenation of Olefins Catalyzed by 4a−h,a 5, 6, and 8 4a 4b 4c 4d 4e 4f 4g 4h 5 6 8
1-hexeneb
cyclohexene
dimethyl itaconate
100 96/4 86/14 91/9 94/6 91/9 96/4 97/3 91/9 16/84 100d
68 100 54 86 84 64 87 100 6
