1,2,3-Triazolylidene ruthenium(ii)-cyclometalated complexes and olefin selective hydrogenation catalysis

Article abstract of DOI:10.1039/c4dt03156c

Silver(i) 1,2,3-triazol-5-ylidenes [(RCH₂C₂N₂(NMe)Ph)₂Ag][AgCl₂] (R = Ph 3a, C₆H₂iPr₃3b, C₆H₂Me₃3c) and [(PhCH₂C₂N₂(NMe)R)₂Ag][AgCl₂] (R = C₆H₄Me 3d, C₆H₄CF₃3e) were synthesized and subsequently treated with RuHCl(PPh₃)₃ and RuHCl(H₂)(PCy₃)₂. The reaction of 3a with RuHCl(PPh₃)₃ gave RuHCl(PPh₃)₂(PhCH₂C₂N₂(NMe)Ph) (4a₁) as the minor product and the cyclometalated complex RuCl(PPh₃)₂(PhCH₂C₂N₂(NMe)C₆H₄) (4a₂) as the major product. However, similar reaction with 3b selectively formed the cyclometalated complex RuCl(PPh₃)₂((C₆H₂iPr₃)CH₂C₂N₂(NMe)C₆H₄) (4b₂). Similarly the silver(i) triazolylidenes 3a and 3b were reacted with RuHCl(H₂)(PCy₃)₂; gave RuHCl(PCy₃)₂(PhCH₂C₂N₂(NMe)Ph) (5a₁), RuCl(PCy₃)₂(PhCH₂C₂N₂(NMe)C₆H₄) (5a₂) and RuCl(PCy₃)₂((C₆H₂iPr₃)CH₂C₂N₂(NMe)C₆H₄) (5b₂), respectively. Species 3c, 3d and 3e resulted in the cyclometalated complexes (5c₂, 5d₂ and 5e₂) as the major products as well as the ruthenium-hydride complexes (5c₁, 5d₁ and 5e₁) as the minor products. The cyclometalated species are derived from the ruthenium-hydride complexes via C(sp²)-H activation. These Ru-complexes were shown to act as hydrogenation catalyst precursors for olefinic substrates including those containing a variety of functional groups. This journal is

Full text of DOI:10.1039/c4dt03156c

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1,2,3-Triazolylidene ruthenium(II)-cyclometalated
complexes and olefin selective hydrogenation
catalysis†
Cite this: DOI: 10.1039/c4dt03156c
Bidraha Bagh, Adam M. McKinty, Alan J. Lough and Douglas W. Stephan*
Silver(I) 1,2,3-triazol-5-ylidenes [(RCH2C2N2(NMe)Ph)2Ag][AgCl2] (R = Ph 3a, C6H2iPr3 3b, C6H2Me3 3c)
and [(PhCH2C2N2(NMe)R)2Ag][AgCl2] (R
= C6H4Me 3d, C6H4CF3 3e) were synthesized and sub-
sequently treated with RuHCl(PPh3)3 and RuHCl(H2)(PCy3)2. The reaction of 3a with RuHCl(PPh3)3 gave
RuHCl(PPh3)2(PhCH2C2N2(NMe)Ph) (4a1) as the minor product and the cyclometalated complex
RuCl(PPh3)2(PhCH2C2N2(NMe)C6H4) (4a2) as the major product. However, similar reaction with 3b
selectively formed the cyclometalated complex RuCl(PPh3)2((C6H2iPr3)CH2C2N2(NMe)C6H4) (4b2).
Similarly the silver(I) triazolylidenes 3a and 3b were reacted with RuHCl(H2)(PCy3)2; gave RuHCl-
(PCy3)2(PhCH2C2N2(NMe)Ph) (5a1), RuCl(PCy3)2(PhCH2C2N2(NMe)C6H4) (5a2) and RuCl(PCy3)2((C6H2iPr3)-
CH2C2N2(NMe)C6H4) (5b2), respectively. Species 3c, 3d and 3e resulted in the cyclometalated complexes
(5c2, 5d2 and 5e2) as the major products as well as the ruthenium-hydride complexes (5c1, 5d1 and 5e1) as
the minor products. The cyclometalated species are derived from the ruthenium-hydride complexes via
C(sp2)–H activation. These Ru-complexes were shown to act as hydrogenation catalyst precursors for
olefinic substrates including those containing a variety of functional groups.
Received 15th October 2014,
Accepted 7th November 2014
DOI: 10.1039/c4dt03156c
www.rsc.org/dalton
1,3,4-Trisubstituted-1,2,3-triazol-5-ylidenes (Fig. 1A) are a
recent addition to the family of mesoionic N-heterocyclic car-
Introduction
The most common chemical transformation used in chemical benes (NHCs) that have attracted considerable attention in last
industry is hydrogenation. Indeed, this reaction is essential for five years.8 Precursors to these ligands namely 1,2,3-triazole
the preparation of a vast array of materials, polymers, pharma- (Fig. 1B) are readily synthesized by the Cu(I)-catalyzed 1,3-
ceuticals, agrochemicals, fine chemicals and foodstuffs.1 cycloaddition of organic azides with terminal alkynes. This
Among homogeneous catalysts used for hydrogenations, those so-called ‘click’ reaction9 is highly modular and useful as it
derived from rhodium2 and iridium3 complexes are common. proceeds under mild reaction conditions, with high tolerance
While such precious metal systems are employed because of of functional groups and excellent yields. In 2008 the first tran-
their activity, the expense and toxic nature of these metals has sition metal complex of 1,2,3-triazolydene was reported by
prompted effort to employ alternatives.4 Ruthenium com- Albrecht et al. using the 1,2,3-triazolium salt (Fig. 1C) as pre-
pounds of the type RuHCl(CO)(NHC)(PPh3) are effective cata- cursor.10 While Bertrand et al. synthesized free 1,3,4-trisubsti-
lysts for the hydrogenation of olefins.5 Albrecht et al. showed tuted-1,2,3-triazol-5-ylidenes by the deprotonation of the 1,2,3-
that ruthenium complexes of chelating NHCs acted as a very triazolium salt with KN(SiMe3)2 or KOtBu,11 these species are
robust catalysts.6 In our own efforts we have recently communi- conveniently stabilized by complexation with Ag(I). These latter
cated that a cis-bis-NHC ruthenium hydride complex exhibited species can be exploited for transmetallation reactions and
remarkably selective catalyst for olefin hydrogenation.7 Noting have been further exploited as ligands for novel metal catalysts
that these catalysts do indeed contain electron rich Ru-centers,
we considered the possibility of using of alternative donors.
Department of Chemistry, University of Toronto, 80 St George St, Toronto, Ontario,
Canada, M5S3H6. E-mail: dstephan@chem.utoronto.ca; Tel: +1-416-946-3294
†Electronic supplementary information (ESI) available. CCDC 1028679–1028683.
For crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt03156c
Fig. 1 1,2,3-Triazole species. R1 and R2 = alkyl, benzyl or aryl; R3 = alkyl
or benzyl; X = Br, I, OTf, PF6, BF4.
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in reactions such as oxidation of amines,12 and water,13 alco- δ 2.42 (s, 3H, CH3), 4.23 (s, 3H, N–CH3), 5.80 (s, 2H, CH2),
hols,12 oxidative coupling,12 Suzuki coupling,14 and ring- 7.33–7.63 (m, 9H, C6H5 and C6H4), 8.68 (s, 1H, triazolium-H).
opening and ring-closing metathesis.11a In addition, in a 13C NMR (CD2Cl2): δ 21.58 (CH3), 39.03 (N–CH3), 57.91 (CH2),
recent report we have described three half-sandwich Ru(II) 119.20, 128.57, 129.43, 129.74, 129.97, 180.27, 130.68, 131.80,
hydride complexes with 1,2,3-triazolylidene ligands which 143.19, 144.13 (C6H5, C6H4 and trizolium-C). Anal. Calcd for
proved to be good hydrogenation catalysts for olefins.15 In this C18H18F3N3O3S (413.41): C, 52.29; H, 4.39; N, 10.16. Found:
manuscript, we demonstrate that Ru-phosphine complexes of C, 52.32; H, 4.34; N, 10.11.
1,2,3-triazolylidenes are readily accessible and highly effective
2e: 1e (3.035 g, 10.00 mmol) and MeOTf (1.805 g,
and selective catalyst precursors for hydrogenation of functio- 11.00 mmol) yielded 2e (4.229 g, 90%). 1H NMR (CD2Cl2):
nalized olefins.
δ 4.26 (s, 3H, N–CH3), 5.82 (s, 2H, CH2), 7.40–7.87 (m, 9H, C6H5
and C6H4), 8.78 (s, 1H, triazolium-H). 13C NMR (CDCl3):
δ 39.27 (N–CH3), 58.14 (CH2), 126.00, 126.91, 129.45, 129.80,
130.03, 130.39, 130.60, 131.49, 142.57 (C6H5, C6H4 and trizo-
lium-C). Anal. Calcd for C18H15F6N3O3S (467.39): C, 46.26;
H, 3.23; N, 8.99. Found: C, 46.21; H, 3.25; N, 9.02.
Experimental section
General procedure
Syntheses of 1a–e were performed in air with ordinary solvents.
Synthesis of [(PhCH2C2N2(NMe)R)2Ag][AgCl2] (R = C6H4Me
All other manipulations were carried out under an atmosphere 3d, C6H4CF3 3e). Identical synthetic procedures were followed
of dry, oxygen free nitrogen atmosphere employing an Innova- for the preparation of 3d and 3e. A mixture of triazolium salt
tive Technology glove box and a Schlenk vacuum-line. Solvents (5.00 mmol), Ag2O (2.75 mmol) and NMe4Cl (5.50 mmol) in a
(pentane, hexanes, toluene, CH2Cl2) were purified with a 1 : 1 mixture of CH2Cl2 (10 mL) and CH3CN (10 mL) was
Grubbs-type column system manufactured by Innovative Tech- stirred at r.t. for 24 h under dark resulting in yellow solution
nology and dispensed into thick-walled Schlenk glass flasks with grey precipitate. All volatiles were removed under vacuum
equipped with Teflon-valve stopcocks and stored over mole- to give a grey solid which was extracted with CH2Cl2 (20 mL).
cular sieves. CH3CN was stored over CaH2, distilled and The solution was concentrated to approximately one fourth to
degassed before use. Dry benzene was purchased from Aldrich its original volume and filtered through a plug of Celite to get
and degassed before use. Deuterated solvents (C6D6, CDCl3 a clear solution. The solution was added dropwise to well-
and CD2Cl2) were dried over the appropriate agents, vacuum- stirred hexanes (20 mL). This yielded a sticky precipitate with
transferred into storage flasks with Teflon stopcocks, and pale yellow solution. The solid was dried under vacuum
degassed accordingly. 1H, 13C and 31P NMR spectra were resulted in a foamy solid. The solid was dissolved in minimum
recorded at 25 °C on a Bruker 400 MHz spectrometer. Chemi- amount of CH2Cl2 (ca. 4–5 mL) and the solution was added
cal shifts are given relative to SiMe3 and referenced to the dropwise to well-stirred hexanes (20 mL) to give an off-white
residual solvent signal (1H and 13C) or relative to an external solid with colorless solution. The liquid was syringed off and
standard (31P, 85% H3PO4). Chemical shifts are reported in the solid was dried under high vacuum to give pure product.
ppm. Mass spectra were measured on a AB Sciex QStar and
3d: 2d (2.068 g, 5.00 mmol), Ag2O (0.637 g, 2.75 mmol) and
were reported in the form m/z (%) [M+] where “m/z” is the NMe4Cl (0.603 g, 5.50 mmol) yielded 3d (1.811 g, 89%). 1H
mass observed, the intensities of the most intense peaks are NMR (CD2Cl2): δ 2.41 (s, 3H, CH3), 4.10 (s, 3H, N–CH3), 5.54 (s,
reported, and “M+” is the molecular ion peak. Combustion 2H, CH2), 7.21–7.47 (m, 9H, C6H5 and C6H4). 13C NMR
analyses were performed in house, employing a Perkin-Elmer (CD2Cl2): δ 21.43 (CH3), 37.71 (N–CH3), 59.82 (CH2), 124.79,
CHN Analyzer. All reagents were purchased from Aldrich and 128.69, 129.22, 129.28, 129.55, 130.06, 134.81, 140.87, 149.60
were used as received. 1a, 1b, 1c, 1d, 1e, 2a, 2b, 2c, 3a, 3b and (C6H5, C6H4 and trizolium-C). MS (70 eV, ESI): m/z (rel intens)
3c were synthesized according to literature procedure.16 RuHCl 633 (100) [C34H34N6Ag+]. HRMS (ESI; m/z): calcd for
(PPh3)3 and RuHCl(H2)(PCy3)2 was prepared following a modi- C34H34N6Ag, 633.1890; found, 633.1885.
fied literature procedure.17 As repeated elemental analysis of
3d and 3e failed to produce acceptable results, HRMS was per- NMe4Cl (0.603 g, 5.50 mmol) yielded 3e (1.934 g, 84%). 1H
formed as a further characterization. NMR (CD2Cl2): δ 4.15 (s, 3H, N–CH3), 5.61 (s, 2H, CH2),
3e: 2e (2.338 g, 5.00 mmol), Ag2O (0.637 g, 2.75 mmol) and
Synthesis of [PhCH2C2HN2(NMe)R][OTf] (R = C6H4Me 2d, 7.25–7.75 (m, 9H, C6H5 and C6H4). 13C NMR (CD2Cl2): δ 38.03
C6H4CF3 2e). Identical synthetic procedures were followed for (N–CH3), 59.86 (CH2), 125.19, 126.27, 128.72, 129.30, 130.35,
the preparation of 2d and 2e. MeOTf (11.00 mmol) was added 131.58, 131.88, 132.14, 134.70, 148.09 (C6H5, C6H4 and trizo-
dropwise to a solution of 1,2,3-triazole (10.00 mmol) in CH2Cl2 lium-C). MS (70 eV, ESI): m/z (rel intens) 741 (100)
(20 mL) at r.t. The reaction mixture was stirred for 40 h result- [C34H28N6F6Ag+]. HRMS (ESI; m/z): calcd for C34H28N6F6Ag,
ing in a colorless solution. All volatiles were removed under 741.1327; found, 741.1325.
high vacuum resulting in a colorless oil which solidified on
standing. The solid was washed with hexane (3 × 20 mL) and RuCl(PPh3)2(PhCH2C2N2(NMe)C6H4) (4a2). Toluene (30 mL)
dried under vacuum to give pure product. was added to a mixture of 3a (0.395 g, 0.50 mmol) and RuHCl
Synthesis of RuHCl(PPh3)2(PhCH2C2N2(NMe)Ph) (4a1) and
2d: 1d (2.495 g, 10.00 mmol) and MeOTf (1.805 g, (PPh3)3 (0.926 g, 1.00 mmol). The reaction mixture was stirred
11.00 mmol) yielded 2d (3.747 g, 91%). 1H NMR (CD2Cl2): at 25 °C for 48 h resulting in a dark red solution with brown
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precipitate. The precipitate was filtered off and the solution 140.07, 149.56, 149.91, 154.64 (Ar–C), 174.27, 175.45
was concentrated to ca. one fourth to its original volume. The (Ru–C(C6H4) and Ru–C(triazolylidene)). 31P NMR (CD2Cl2): δ 39.95
concentrated solution was added dropwise to well stirred (PPh3). Anal. Calcd for C61H62ClN3P2Ru (1035.64): C, 70.74;
hexanes (30 mL) resulting in a red precipitate with pale red H, 6.03; N, 4.06. Found: C, 71.01; H, 5.99; N, 4.09.
solution. The liquid was syringed off and the solid was washed
Synthesis of RuHCl(PCy3)2(PhCH2C2N2(NMe)Ph) (5a1) and
with hexanes (3 × 10 mL). The red solid was dried to give crude RuCl(PCy3)2(PhCH2C2N2(NMe)C6H4) (5a2). Benzene (10 mL)
product 4a2 which was dissolved in appropriate solvent and was added to a mixture of 3a (0.197 g, 0.25 mmol) and RuHCl
crystallization gave dark red crystals as pure product 4a2. Dark (H2)(PCy3)2 (0.350 g, 0.50 mmol). The reaction mixture was
red crystals were deposited from the pale red solution on stirred at room temperature for 48 hours resulting in a red
standing, which were found to be a mixture of 4a1 and 4a2 in solution with brown precipitate. The brown solid was filtered
the ratio of 8.5 : 1.5.
off. The red solution was concentrated to ca. 2–3 mL and
4a2: 3a (0.395 g, 0.50 mmol) and RuHCl(PPh3)3 (0.926 g, added dropwise to hexanes (15 mL) while stirring vigorously.
1.00 mmol) yielded a red solid (0.482 g) as crude product. The This resulted in a red solution with orange precipitate. The
crude product was dissolved in benzene (20 mL). Slow solid was filtered off and dried under high vacuum to give 5a2
diffusion of hexanes into the benzene solution resulted in (0.291 g, ca. 60%) [it contains 5a1 as impurity (9%) and could
dark red crystals. Crystals were dried under high vacuum to not be isolated in pure form.]. The red solution was allowed to
1
give pure 4a2 (0.191 g, 21%). H NMR (CDCl3): δ 3.34 (s, 3H, rest 18 hours at room temperature resulting in an orange semi-
N–CH3), 4.86 (s, 2H, CH2), 6.29–6.36 (m, 1H, Ar–H), 6.41–6.49 crystalline precipitate and red solution. The semicrystalline
3
(m, 2H, Ar–H), 6.80 (d, JHH = 8 Hz, 2H, Ar–H), 6.95–7.03 (m, precipitate, which was a mixture of 5a1 and 5a2, was discarded.
3
3H, Ar–H), 7.09–7.45 (m, 30H, Ar–H), 8.08 (d, JHH = 8 Hz, 1H, The red solution was left at −35 °C for 48 hours resulting in
Ar–H). 13C NMR (CDCl3): δ 36.59 (N–CH3), 55.99 (CH2), 118.83, red crystals. The crystals were dried to give pure 5a1 (0.038 g, 8%).
120.08, 122.96, 127.73, 128.70, 129.35, 134.48, 134.79, 135.09,
5a1: 1H NMR (CD2Cl2): δ −26.40 (t, 2JPH = 24 Hz, 1H, Ru–H),
136.01, 139.77, 141.09, 153.53 (Ar–C), 173.39, 180.64 (Ru–C 0.81–2.05 (m, 66H, PCy3), 4.15 (s, 3H, N–CH3), 5.70 (s, 2H,
(C6H4) and Ru–C(triazolylidene)). 31P NMR (CDCl3): δ 37.70 CH2), 6.57 (d, 3JHH = 8 Hz, 2H, Ar–H), 7.33–7.40 (m, 6H, Ar–H),
3
(PPh3). Anal. Calcd for C52H44ClN3P2Ru (909.40): C, 68.68; 7.51 (t, JHH = 8 Hz, 2H, Ar–H). 13C NMR (CD2Cl2): δ 27.55,
H, 4.88; N, 4.62. Found: C, 68.73; H, 4.85; N, 4.60.
28.52, 28.67, 30.01, 30.85, 36.53 (PCy3), 38.75 (N–CH3), 56.22
4a1: 4a1 could not be isolated as a pure compound. The (CH2), 124.33, 127.93, 128.04, 128.56, 128.80, 130.92, 131.59,
crude product (0.122 g) contained a mixture of 4a1 and 4a2 136.15, 144.87 (Ar–C). 31P NMR (CD2Cl2): δ 41.08 (PCy3). Anal.
(8.5 : 1.5). Further crystallization from the mixture increased Calcd for C52H82ClN3P2Ru (947.70): C, 65.90; H, 8.72; N, 4.43.
1
2
the amount of 4a2. H NMR (CDCl3): δ −27.49 (t, JPH = 24 Hz, Found: C, 66.00; H, 8.68; N, 4.44.
1H, RuH), 3.25 (s, 3H, Me), 4.77 (s, 2H, CH2), 6.45–7.35
5a2: 1H NMR (CD2Cl2): δ 0.75–2.11 (m, 66H, PCy3), 4.24
(s, 3H, N–CH3), 6.04 (s, 2H, CH2), 7.28–7.74 (m, 9H, Ar–H).
(m, 40H, Ar–H). 31P NMR (CDCl3): δ 46.33 (PPh3).
Synthesis of RuCl(PPh3)2((C6H2iPr3)CH2C2N2(NMe)C6H4) 31P NMR (CD2Cl2): δ 24.28 (PCy3).
(4b2). Toluene (30 mL) was added to a mixture of 3b (0.520 g,
Synthesis of RuCl(PCy3)2((C6H2iPr3)CH2C2N2(NMe)C6H4)
0.50 mmol) and RuHCl(PPh3)3 (0.926 g, 1.00 mmol). The reac- (5b2). Benzene (10 mL) was added to a mixture of 3b (0.261 g,
tion mixture was stirred at r.t. for 48 h resulting in a dark red 0.25 mmol) and RuHCl(H2)(PCy3)2 (0.350 g, 0.50 mmol). The
solution with brown precipitate. The precipitate was filtered reaction mixture was stirred at room temperature for 48 hours
off and the solution was concentrated to ca. one fourth to its resulting in a red solution with brown precipitate. The brown
original volume. The concentrated solution was added drop- solid was filtered off. All volatiles were removed from the red
wise to well stirred hexanes (30 mL) resulting in a red precipi- solution resulting in a red solid which was washed with
tate with pale red solution. The liquid was syringed off and the hexane (3 × 10 mL). The solid was dried under high vacuum to
solid was washed with hexanes (3 × 10 mL). The red solid give 5b2 as pure product (0.351 g). The hexane phase was
(0.69 g) was dried to give crude product 4b2 which was dis- allowed to rest for 48 hours during which time red crystals
solved in CH2Cl2 (12 mL). Slow diffusion of Et2O into the solu- formed (0.058 g) as pure product 5b2. The solids were com-
tion resulted in dark red crystals. Dark red crystals were also bined and dried thoroughly to give 5b2 (0.409 g, 76%).
deposited from the pale red solution on standing. Crystals 1H NMR (CD2Cl2): δ 0.92–2.16 (m, 84H, PCy3 and CH3 of iPr),
3
3
were combined and dried under high vacuum to give pure 4b2 2.97 (sept, JHH = 7 Hz, 1H, CH of iPr), 3.07 (sept, JHH = 7 Hz,
1
3
(0.383 g, 37%). H NMR (CD2Cl2): δ 0.93 (d, JHH = 7 Hz, 12H, 2H, CH of iPr), 4.11 (s, 3H, N–CH3), 5.58 (s, 2H, CH2), 6.51
3
3
3
CH3 of iPr), 1.26 (d, JHH = 7 Hz, 6H, CH3 of iPr), 2.28 (sept, (t, JHH = 8 Hz, 1H, Ar–H), 6.60 (t, JHH = 8 Hz, 1H, Ar–H), 7.08
3JHH = 7 Hz, 1H, CH of iPr), 2.89 (sept, JHH = 7 Hz, 2H, CH of (d, JHH = 8 Hz, 1H, Ar–H), 7.18 (s, 2H, Ar–H), 8.25 (d, JHH
=
3
3
3
3
iPr), 3.30 (s, 3H, N–CH3), 5.22 (s, 2H, CH2), 6.17 (d, JHH
=
8 Hz, 1H, Ar–H). 13C NMR (CD2Cl2): δ 24.12, 26.99, 28.05,
8 Hz, 1H, Ar–H), 6.40–6.45 (m, 2H, Ar–H), 7.01 (s, 2H, Ar–H), 28.33, 28.59, 30.69, 30.97, 31.53, 34.70, 37.09, 38.20 (PCy3, CH
3
7.05–7.42 (m, 30H, PPh3), 7.91 (d, JHH = 8 Hz, 1H, Ar–H). and CH3 of iPr), 49.28 (N–CH3), 66.06 (CH2), 117.52, 118.91,
13C NMR (CD2Cl2): δ 24.04 (CH3 of iPr), 24.98 (CH of iPr), 122.03, 122.34, 125.50, 139.91, 143.65, 149.44, 150.04, 154.62
30.30 (CH3 of iPr), 34.60 (CH of iPr), 36.49 (N–CH3), 48.98 (Ar–C), 181.59, 182.66 (Ru–C(C6H4) and Ru–C(triazolylidene)).
(CH2), 118.75, 120.46, 121.85, 122.94, 124.38, 127.66, 128.70, 31P NMR (CD2Cl2): δ 24.49. Anal. Calcd for C61H98ClN3P2Ru
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(1071.92): C, 68.35; H, 9.22; N, 3.92. Found: C, 68.22; H, 9.21;
N, 3.87.
5d2: 1H NMR (CD2Cl2): δ 0.89–2.05 (m, 66H, PCy3), 2.24
(s, 3H, CH3), 4.17 (s, 3H, N–CH3), 5.81 (s, 2H, CH2), 6.48
3
Synthesis of RuHCl(PCy3)2((C6H2Me3)CH2C2N2(NMe)Ph) (d, JHH = 8 Hz, 1H, Ar–H), 7.06 (d, 1H, Ar–H), 7.31–7.42 (m,
(5c1) and RuCl(PCy3)2((C6H2Me3)CH2C2N2(NMe)C6H4) (5c2). 5H, Ar–H), 7.96 (s, 1H, Ar–H). 13C NMR (CD2Cl2): δ 21.78,
Benzene (10 mL) was added to a mixture of 3c (0.230 g, 22.75, 27.08, 28.44, 30.31, 30.44, 30.63, 33.28, 34.54, 36.84,
0.25 mmol) and RuHCl(H2)(PCy3)2 (0.350 g, 0.50 mmol). The 37.12 (PCy3, CH3 and N–CH3), 56.20 (CH2), 118.82, 118.94,
reaction mixture was stirred at room temperature for 48 hours 127.84, 127.98, 128.69, 132.61, 136.51, 137.11, 143.38, 154.31
resulting in a red solution with brown precipitate. The brown (Ar–C) 183.11, 183.96 (Ru–C(C6H4) and Ru–C(triazolylidene)).
solid was filtered off. The red solution was concentrated to ca. 31P NMR (CD2Cl2):
δ
24.17 (PCy3). Anal. Calcd for
2–3 mL and added dropwise to hexanes (15 mL) while stirring C53H82ClN3P2Ru (959.71): C, 66.33; H, 8.61; N, 4.38. Found:
vigorously. This resulted in a red solution with orange precipi- C, 66.25; H, 8.56; N, 4.42.
tate. The solid was filtered off and dried under high vacuum to
5d1: 1H NMR (CD2Cl2): δ −26.30 (t, 2JPH = 24 Hz, 1H, Ru–H),
give 5c2 (0.281 g) as crude product. Crystallization from 0.84–2.05 (m, 66H, PCy3), 2.24 (s, 3H, CH3), 4.17 (s, 3H,
toluene solution at −35 °C gave pure 5c2 (0.202 g, 40%). The N–CH3), 5.81 (s, 2H, CH2), 6.40–6.57 (m, 2H, Ar–H), 7.01–7.11
red solution was allowed to rest 18 hours at room temperature (m, 1H, Ar–H), 7.29–7.43 (m, 4H, Ar–H) 7.97 (s, 1H, Ar–H).
resulting in an orange semicrystalline precipitate and red solu- 31P NMR (CD2Cl2): δ 42.27 (PCy3).
tion. The semicrystalline precipitate, which was a mixture of
Synthesis of RuHCl(PCy3)2(PhCH2C2N2(NMe)(C6H4CF3))
5c1 and 5c2, was discarded. The red solution was left at −35 °C (5e1) and RuCl(PCy3)2(PhCH2C2N2(NMe)(C6H3CF3)) (5e2). Ben-
for 48 hours resulting in red crystals. The crystals were dried to zene (10 mL) was added to a mixture of 3c (0.240 g,
give pure 5c1 (0.044 g, 9%).
0.25 mmol) and RuHCl(H2)(PCy3)2 (0.350 g, 0.50 mmol). The
5c1: 1H NMR (CD2Cl2): δ −26.49 (t, 2JPH = 24 Hz, 1H, Ru–H), reaction mixture was stirred at room temperature for 48 hours
0.83–2.23 (m, 66H, PCy3), 2.30 (s, 3H, CH3), 2.31 (s, 6H, CH3), resulting in a red solution with brown precipitate. The brown
4.04 (s, 3H, N–CH3), 5.28 (s, 2H, CH2), 6.47 (d, 3JHH = 8 Hz, 2H, solid was filtered off. The red solution was concentrated to ca.
3
Ar–H), 6.93 (s, 2H, Ar–H), 7.34 (t, JHH = 8 Hz, 2H, Ar–H), 7.50 2–3 mL and added dropwise to hexanes (15 mL) while stirring
3
(t, JHH = 8 Hz, 1H, Ar–H). 13C NMR (CD2Cl2): δ 21.08, 22.43, vigorously. This resulted in a red solution with orange precipi-
23.07, 25.65, 27.01, 27.49, 28.06, 28.59, 30.77, 31.55, 32.00, tate. The solid was filtered off and dried under high vacuum to
35.03, 38.78 (PCy3 and CH3), 52.38 (N–CH3), 68.16 (CH2), give 5e2 (0.305 g) as crude product. Crystallization from
124.21, 127.60, 129.23, 130.92, 131.84, 138.54, 138.97, 145.26 toluene solution at −35 °C gave pure 5e2 (0.193 g, 38%). The
(Ar–C). 31P NMR (CD2Cl2): δ 41.71 (PCy3). Anal. Calcd for red solution was allowed to rest 48 hours at room temperature
C55H88ClN3P2Ru (989.78): C, 66.74; H, 8.96; N, 4.25. Found: resulting in an orange crystalline precipitate, which was a
C, 66.67; H, 8.93; N, 4.20.
mixture of 5e1 and 5e2. 5e1 could not be isolated in pure form.
5c2: 1H NMR (CD2Cl2): δ 1.08–2.23 (m, 66H, PCy3), 2.30
5e2: 1H NMR (CD2Cl2): δ 0.83–2.04 (m, 66H, PCy3), 4.25
3
(s, 3H, CH3), 2.41 (s, 6H, CH3), 4.23 (s, 3H, N–CH3), 5.96 (s, 3H, N–CH3), 5.85 (s, 2H, CH2), 6.91 (d, JHH = 8 Hz, 1H,
(s, 2H, CH2), 6.98 (m, 1H, Ar–H), 7.52–7.79 (m, 4H, Ar–H), 8.01 Ar–H), 7.23 (d, JHH = 8 Hz, 1H, Ar–H), 7.31–7.44 (m, 5H,
3
(m, 1H, Ar–H). 13C NMR (CD2Cl2): δ 20.15, 21.18, 24.35, 27.14, Ar–H), 8.43 (s, 1H, Ar–H). 13C NMR (CD2Cl2): δ 26.98, 28.28, 28.31,
28.28, 29.42, 30.24, 30.64, 32.32, 33.22, 38.19, 39.26 (PCy3 and 28.36, 30.48, 30.53, 36.70, 36.76, 36.82, 37.54 (PCy3 and
CH3), 49.04 (N–CH3), 62.86 (CH2), 122.37, 125.36, 128.64, N–CH3), 56.37 (CH2), 114.08, 118.42, 127.95, 128.81, 129.67,
129.35, 129.81, 130.04, 132.28, 139.16, 140.65, 143.54 (Ar–C) 130.34, 136.04, 138.53, 143.40, 153.15 (CF3 and Ar–C) 184.81,
[Note: Tertiary carbons were not detected]. 31P NMR (CD2Cl2): 186.88 (Ru–C(C6H4) and Ru–C(triazolylidene)). 31P NMR
δ 23.96 (PCy3). Anal. Calcd for C55H86ClN3P2Ru (987.76): (CD2Cl2): δ 23.58 (PCy3). Anal. Calcd for C53H79ClF3N3P2Ru
C, 66.88; H, 8.78; N, 4.25. Found: C, 66.81; H, 8.91; N, 4.26.
(1013.68): C, 62.80; H, 7.86; N, 4.15. Found: C, 62.87; H, 7.83;
Synthesis of RuHCl(PCy3)2(PhCH2C2N2(NMe)(C6H4Me)) (5d1) N, 4.18.
and RuCl(PCy3)2(PhCH2C2N2(NMe)(C6H3Me)) (5d2). Benzene
5e1: 1H NMR (CD2Cl2): δ −25.86 (t, 2JPH = 8 Hz, = 24 Hz, 1H,
(10 mL) was added to a mixture of 3c (0.210 g, 0.25 mmol) and Ru–H), 0.82–2.11 (m, 66H, PCy3), 4.28 (s, 3H, N–CH3), 5.94
RuHCl(H2)(PCy3)2 (0.350 g, 0.50 mmol). The reaction mixture (s, 2H, CH2), 7.38–7.49 (m, 3H, Ar–H), 7.59–7.71 (m, 2H,
was stirred at room temperature for 48 hours resulting in a red Ar–H), 7.76–7.93 (m, 3H, Ar–H) 9.37 (s, 1H, Ar–H). 31P NMR
solution with brown precipitate. The brown solid was filtered (CD2Cl2): δ 40.96 (PCy3).
off. The red solution was concentrated to ca. 2–3 mL and
added dropwise to hexanes (15 mL) while stirring vigorously.
Hydrogenation of olefins in J. Young NMR tube
This resulted in a red solution with orange precipitate. The In a glove box, a sample of the appropriate metal complex 4a1
solid was filtered off and dried under high vacuum to give 5d2 (4.5 mg, 5 μmol, Note: 4a1 contains 15% of 4a2 as impurity) or
(0.295 g) as crude product. Crystallization from toluene solu- 4a2 (4.5 mg, 5 μmol) or 4b2 (5.0 mg, 5 μmol) or 5b2 (2.2 mg,
tion at −35 °C gave pure 5d2 (0.213 g, 42%). The red solution 2 μmol), deuterated solvent (0.5 mL) (C6D6 for 4a1 and CD2Cl2
was allowed to rest 48 hours at room temperature resulting in for 4a2 and 4b2) and substrate (0.1 mmol) were combined in a
an orange crystalline precipitate, which was a mixture of 5d1 vial. The mixture was transferred to a J. Young tube and the
and 5d2. 5d1 could not be isolated in pure form.
J. Young tube was sealed. On a Schlenk line, the reaction
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mixture was degassed four times using the freeze–pump–thaw residual electron densities in each case were of no chemical
method. The sample was then frozen once more in liquid significance. For more information see ESI.†
nitrogen and 4 atm of H2 was added. The J. Young tube was
sealed again and warmed to room temperature and then
placed in an oil bath pre-heated to 50 °C. 1H NMR spectra were
measured at appropriate intervals and relative integration of
substrate and product peaks were used to determine the com-
position of the mixture.
Results and discussion
Synthesis and characterization
The 1,2,3-triazoles [RCH2C2HN3Ph] (R = Ph 1a, C6H2iPr3 1b,
C6H2Me3 1c) and [PhCH2C2HN3R] (R = C6H4Me 1d, C6H4CF3
1e) were readily synthesized in excellent yield by treating a
mixture of appropriate chloro-derivatives, terminal alkyne and
sodium azide in distilled water in presence of catalytic amount
Hydrogenation of olefins in Parr reactor
In a glove box, a sample of the appropriate metal complex 5a1/
5b2/5c1/5c2/5d2/5e2 (2 μmol), CD2Cl2 (0.5 mL) and substrate
(0.1 mmol) were combined in a vial. The vial was placed in the
Parr reactor and was sealed inside the glove box. The Parr
reactor was pressurized with 50 atm of H2 after purging five
times with 50 atm of H2. The hydrogenation was run for 3 h.
The pressure was released and 1H NMR spectra were measured
from the reaction mixture. Relative integration of substrate
and product peaks was used to determine the composition of
the mixture.
of Cu(I) (Scheme 1).16 The reaction is regioselective and 1,4-dis-
ubstituted 1,2,3-triazoles were the only products. Thereafter,
1a, 1b, 1c, 1d and 1e were methylated selectively at N3-possi-
tion by reacting them with methyl triflate and thus generating
[RCH2C2HN2(NMe)Ph)][OTf] (R = Ph 2a, C6H2iPr3 2b, C6H2Me3
2c) and [PhCH2C2HN2(NMe)R][OTf] (R
= C6H4Me 2d,
C6H4CF3 2e), respectively (Scheme 1). By analogy to previous
reports10a,16a subsequent reactions with Ag2O afforded the
stable silver(I) triazolylidenes species 3a–e (Scheme 1). Mass
spectrometry analysis were consistent with the formulation of
these products with the general formula [L2Ag][AgCl2]16a as the
major peaks at m/z = 605.16, 857.44, 689.25, 633.19 and 741.13
were observed in the mass spectra of 3a–e, respectively.
X-ray data collection and reduction
Crystals were coated in Paratone-N oil in the glovebox,
mounted on a MiTegen Micromount and placed under an N2
stream, thus maintaining a dry, O2-free environment for each
crystal. The data were collected on a Kappa Bruker Apex II
diffractometer. Data collection strategies were determined
using Bruker Apex 2 software and optimized to provide >96.6%
complete data. In The data were collected at 150( 2) K for all.
Data for compound 5e2 were collected with Cu radiation while
the others were done with Mo radiation. The data integration
and absorption corrections were performed with the Bruker
Apex 2 software package.18
The silver(I)-triazolylidene complex 3a was reacted with
RuHCl(PPh3)3 to yield ruthenium-hydride complex RuHCl-
(PPh3)2(PhCH2C2N2(NMe)Ph) (4a1) as the minor product and
the cyclometalated complex RuCl(PPh3)2(PhCH2C2N2(NMe)-
C6H4) (4a2) as the major product (Scheme 2). The cyclometa-
lated complex 4a2 was isolated in pure form and fully charac-
terized, whereas the ruthenium-hydride complex 4a1 was
contaminated with 4a2 (12–20%). The presence of triazolyli-
1
dene moiety was observed in H NMR spectra of 4a2 and 4a1.
1
For 4a1, a triplet at −27.49 ppm in the H NMR spectrum and
a doublet at 46.33 ppm in the 31P NMR spectrum were
observed, consistent with the presence of a hydride coupled to
X-ray data solution and refinement
Non-hydrogen atomic scattering factors were taken from the
literature tabulations.19 The heavy atom positions were deter-
mined using direct methods employing the SHELX-2013 direct
methods routine. The remaining non-hydrogen atoms were
located from successive difference Fourier map calculations.
The refinements were carried out by using full-matrix least
squares techniques on F, minimizing the function ω (Fo − Fc)2
2
2
where the weight ω is defined as 4Fo /2σ (Fo ) and Fo and Fc are
the observed and calculated structure factor amplitudes,
respectively. In the final cycles of each refinement, all non-
hydrogen atoms were assigned anisotropic temperature factors
in the absence of disorder or insufficient data. In the latter
cases atoms were treated isotropically. C–H atom positions
were calculated and allowed to ride on the carbon to which
they are bonded assuming a C–H bond length of 0.95 Å.
H-atom temperature factors were fixed at 1.20 times the isotropic
temperature factor of the C-atom to which they are bonded.
The H-atom contributions were calculated, but not refined.
The locations of the largest peaks in the final difference
Fourier map calculation as well as the magnitude of the Scheme 1 Synthesis of 1–3.
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Scheme 2 Synthesis of 4a1, 4a2 and 4b2.
two phosphine moieties on ruthenium. A singlet at 37.70 ppm
was observed in the 31P NMR spectrum of 4a2. In the 13C NMR
spectrum of 4a2, the Ru–C(C6H4) and Ru–C(triazolylidene) reso-
nances were observed at 173.39 and 180.64 ppm. The corres-
ponding reaction of 3b with RuHCl(PPh3)3 gave selectively the
cyclometalated complex (4b2), which was isolated as dark red
1
crystals (Scheme 2). H and 13C NMR spectra of 4b2 confirmed
the presence of the triazolylidene moiety in the complex while
the 13C resonances attributable to Ru–C(C6H4) and Ru–C(tria-
zolylidene) were observed at 174.27 and 175.45 ppm. A singlet
at 39.95 ppm in the 31P NMR spectrum was consistent with
the presence of two equivalent PPh3 moieties.
Complexes 4a1, 4a2 and 4b2 were characterized by X-ray
molecular structure analysis. Single-crystal X-ray analysis of 4a1
confirmed the formulation and revealed a five-coordinate
square-pyramidal Ru-center where the triazolylidene moiety,
chloride, and two phosphine ligands form the base of the
pyramid and the hydride occupies the apex (Fig. 2a). The two
phosphine ligands are trans to each other with Ru–P bond dis-
tances of 2.3171(5) and 2.3253(5) Å. The Ru–H bond distance
in 4a1 is 1.4809(9) Å, which is consistent with the Ru–H bond
distances (1.41–1.59 Å) in previously report ruthenium-imid-
azolylidene complexes5,7 and in contrast to that seen in a
recently published half-sandwich Ru-triazolylidene complex
(1.7310(23) Å).15 Nonetheless, the Ru–C bond distance (1.9886(6)
Å) in 4a1 is consistent with other ruthenium-triazolylidene
complexes [1.98–2.10 Å]. Ru–Cl bond distance [2.4812 (4) Å] is
found to be in the expected range.11a,12,15,16 The ortho-H of the
phenyl moiety is in close proximity of the metal center;
(2.441 Å). The sum of P–Ru–Cl angles [90.03(2)° and 87.37(2)°]
and P–Ru–C angles [95.09(5)° and 89.20(5)°] is ca. 362°. The
benzyl group of the triazolylidene moiety is oriented away
from the ruthenium center.
Single-crystal X-ray analyses of 4a2 and 4b2 revealed the five-
coordinate distorted triagonal bipyramidal Ru centers where
the two trans phosphine ligands occupy the apexes (Fig. 2b
and 2c). In 4a2, the Ru–P distances are 2.3337(6) Å and 2.3412(6)
Å and P–Ru–P angle is 175.67(2)°. The Ru–P distances
(2.3457(8) Å and 2.3481(9) Å) and P–Ru–P angle (170.91(3)°) in
4b2 are consistent with 4a2. The Ru–C(triazolylidene) bond dis-
tances (4a2: 2.0079(3) Å, 4b2: 1.9789(3) Å) are slightly shorter
than the Ru–C(C6H4) bond distances (4a2: 2.0309(3) Å, 4b2:
2.0356(3) Å). The Ru–Cl distances in 4a2 (2.4582(7) Å) and 4b2
(2.4634(8) Å) are comparable. In 4b2 the C–Ru–C angle is 76.75
(3)° which is much narrower than ideal 120°, whereas Cl–Ru–C
Fig. 2 POV-ray depiction of (a) 4a1, (b) 4a2 and (c) 4b2: C, black; Cl,
green; P, orange; N, blue; Ru, purple; H, gray. All hydrogen atoms except
the hydride and the ortho-H of the phenyl moiety are omitted for clarity.
angles (Cl–Ru–C(triazolylidene): 139.63(1)°; Cl–Ru–C(C6H4):
143.62(1)°) are much wider than that expected for an ideal geo-
metry. While the C–Ru–C angle (77.16(1)°) in 4a2 is similar to
that in 4b2 (76.75(3)°), the Cl–Ru–C angles vary widely (4a2: Cl–
Ru–C(triazolylidene): 165.01(8)°; Cl–Ru–C(C6H4): 117.38(8)°.
4b2: Cl–Ru–C(triazolylidene): 139.63(1)°; Cl–Ru–C(C6H4):
143.62(1)°).
The silver(I)-triazolylidene complexes 3a–e were also reacted
with RuHCl(H2)(PCy3)2 resulting in the formation of Ru–H
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Scheme 3 Synthesis of 5a1, 5a2, 5b2, 5c1, 5c2, 5d1, 5d2, 5e1 and 5e2.
complexes (5a1, 5c1, 5d1 and 5e1) as the minor products and
the cyclometalated complexes (5a2, 5c2, 5d2 and 5e2) as the
major products (Scheme 3). In the case of 3b, reaction with
RuHCl(H2)(PCy3)2 gave exclusively the cyclometalated complex
5b2 (Scheme 3). The complexes 5a1, 5b2, 5c1, 5c2, 5d2 and 5e2
were isolated as pure compounds and fully characterized (1H,
13C and 31P NMR spectroscopy). However, isolation of pure
5a2, 5d1 and 5e1 proved problematic and thus were characte-
rized by 1H and 31P NMR spectroscopy alone. Similar to the
PPh3-analogues, the present Ru–H complexes 5a1, 5c1, 5d1 and
5e1 displayed a triplet in the range of −25 to −27 ppm (5a1:
−26.40, 5c1: −26.49, 5d1: −26.30, 5e1: −25.86) in the respective
1H NMR spectra and a doublet was observed in the range of 40
to 43 ppm (5a1: 41.08, 5c1: 41.71, 5d1: 42.27, 5e1: 40.96) in the
respective 31P NMR spectra. A singlet in the range of 23 to
25 ppm (5a2: 24.28, 5b2: 24.49, 5c2: 23.96, 5d2: 24.17, 5e2:
23.58) was observed in the 31P NMR spectra of cyclometalated
complexes 5a2–e2. In the 13C NMR spectra of the isolated cyclo-
metalated species, the Ru–C(triazolylidene) and Ru–C(C6H4)/
Ru–C(C6H3) resonances were observed in the range of 180 to
190 ppm (5b2: 181.59 and 182.66, 5d2: 183.11 and 183.96, 5e2:
184.81, 186.88).
Fig. 3 POV-ray depiction of (a) 5a1 and (b) 5e2: C, black; Cl, green; P,
orange; N, blue; F, pink; Ru, purple; H, gray. All hydrogen atoms except
the ruthenium bound hydrogens omitted for clarity.
Formulation of complexes 5a1 was further confirmed by
X-ray crystallography revealing six-coordinate distorted octa-
hedral geometries about Ru if one considers the agostic inter-
action with the ortho-hydrogen of the pendant phenyl ring (5a1
Ru–Hortho: 2.058 Å; 4a1: 2.441 Å) (Fig. 3a). The Ru–H(hydride)
bond distance in 5a1 (1.545(4) Å) is slightly longer than that
found in 4a1 (1.4809(9) Å) while the trans phosphine give rise
to Ru–P bond distances of 2.3700(6) and 2.3743(5) Å. The Ru–C
and Ru–Cl bond distances are found to be 1.987(2) and
2.4896(6) Å, respectively. The Ru–P, Ru–C and Ru–Cl bond dis-
tances in both 4a1 and 5a1 are similar while the C–Ru–P bond
angles are 4a1: 93.92(6)° and 5a1: 94.48(6)°, and the Cl–Ru–P
bond angles are 86.34(2)° and 87.00(2)°. Similar to 4a1, the
benzyl group of the triazolylidene moiety in 5a1 is oriented
away from the ruthenium center.
It is interesting to note that reaction of [RuCl2(p-cymene)]2
with silver(I) triazolylidenes 3a, 3b and 3c, gave cyclometalated
species as minor products (2–5%) with the (p-cymene)RuCl2-
(triazolylidene) as major product (80–90%).16a Herein, use of
RuHCl(PPh3)3 as the synthon is shown to reverse this pattern.
Precedent for such intramolecular C–H activation of iridium
and ruthium-triazolylidene complexes have been reported
recently by the groups of Albrecht13,20 and Fukuzawa.21 In
addition, a cyclometalated Pd-triazolylidene complex has also
been described.22 Conceptually these species are similar to
metallated NHC complexes.23
In the formation of 4 and 5, it is reasonable to suggest that
the Ru–H species is formed initially followed by intramolecular
C(sp2)–H bond activation resulting in cyclomatalation with libe-
ration of H2 being the driving force. To confirm this, 5a1 was
dissolved in CD2Cl2 and the solution was monitored by 31P
NMR spectroscopy. A doublet at 41.1 ppm arising from 5a1
slowly decreased while a singlet at 24.3 ppm corresponding to
5a2 grew in (Fig. 4). After 6 days complete conversion to 5a2 was
observed. Similarly in the 1H NMR spectra, the hydride triplet at
−26.40 ppm corresponding to 5a1 disappeared in 6 days.
Compound 5e2 is
a pseudo six-coordinate octahedral
species (Fig. 3b), in contrast to 4a2 and 4b2 as it includes an
agostic interaction between the Ru and a H on one of the PCy3
ligands. However, this Ru–H(C6H11) distance (2.2442 Å) is
slightly longer than the Ru–H(C6H5) distance (2.058 Å) seen in
5a1. The bond distances and angles about Ru in 5e2 are
similar to those seen in species 4a1, 4a2 and 5a1.
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The ratio of the triazolium-hydrogen peak and the broad
hydride peak was 1 : 4, suggested the formulation of 5b4 as
RuH2(H2)(PCy3)2 (Scheme 4). Although this species could not
1
be isolated, the broad H NMR signal is consistent with facile
interchange of the hydrides and η2-H2 sites. Interestingly the
closely related species Ru2H4(H2)(PCy3)4 displays a similarly
broad peak at −12.5 ppm24 while the 31P NMR resonance from
5b4 (53.53 ppm) is similar to those seen for RuHI(H2)(PCy3)2
(56 ppm) and RuHCl(H2)(PCy3)2 (54 ppm).25
Hydrogenation catalysis
The catalytic activity of 4a1, 4a2, 4b2 and 5b2 for hydrogenation
of alkenes and alkyne was investigated (Table 1). At 50 °C
under 4 atm of H2, with catalyst precursor loadings of 2 or
5 mol%, hydrogenation of olefins was performed and moni-
1
tored by H NMR spectroscopy. In the presence of 4a1 or 4a2,
quantitative reduction of 1-hexene to hexane was observed in
6
h. Similarly complete hydrogenation of 1-hexene was
observed in 5 h for species 4b2. In contrast, 5b2 led to com-
plete hydrogenation of 1-hexene in just 2 h while the reduction
of 2-hexene to hexane was observed in 4 h. Species 4a1, 4a2
and 4b2 displayed much slower activity for the hydrogenation
of 2-hexene (4a1: 100% in 12 h, 4a2: 100% in 12 h, 4b2: 100%
in 10 h). Similarly the conversion of styrene to ethylbenzene by
5b2 (100% in 8 h) was much faster than 4a1 (16 h), 4a2 (14 h)
and 4b2 (12 h). Similarly 5b2 hydrogenated phenylacetylene to
styrene and styrene to ethylbenzene simultaneously. Thus the
reaction mixture yielded phenylacetylene: styrene: ethylben-
zene in a ratio of 30 : 50 : 20 after 6 h, whereas the phenyl-
Fig. 4 Conversion of 5a1 to 5a2 monitored by 31P NMR spectroscopy.
Addition of hydrogen (4 atm) to a solution of the cyclometa-
lated species 5b2 in a sealed J. Young NMR tube and monitor-
ing by 1H and 31P NMR spectroscopy revealed a fast reaction in
which 5b2 reacted with one equivalent of H2 yielding the
hydride complex 5b1 (Scheme 4) with the characteristic 1H
NMR hydride resonance at −26.13 ppm and the doublet
(41.05 ppm) in 31P NMR spectrum. After 4 h under H2 5b1
reacted further, generating the triazolium cation [(C6H2iPr3)-
CH2C2HN2(NMe)Ph]+ and anionic ruthenium-dihydride
complex 5b3 as evidenced by the 1H NMR resonances at
10.51 ppm and −8.37 ppm respectively and the 31P resonance
at 75.79 ppm. The latter species 5b3 slowly degraded after 30 h
affording a new species 5b4 which exhibited a broad hydride
resonance at −12.25 ppm and 31P resonance at 53.53 ppm.
acetylene was fully consumed in 10
h
affording
a
styrene : ethylbenzene of 40 : 60 and complete conversion to
ethylbenzene in 14 h. The related hydrogenation of phenyl-
acetylene was achieved using 4a1, 4a2 or 4b2 as a catalyst pre-
cursor although these were much slower, yielding
styrene : ethylbenzene ratios of 4 : 96 for 4b2, 16 : 84 for 4a2 and
23 : 77 for 4a1 after 24 h.
The ability of the derived catalysts to tolerate functional
groups was also investigated. Using similar reaction conditions
allylalcohol, acrylaldehyde, 3-buten-2-one, methyl-3-buteneo-
ate, allylamine, acrylonitrile, 1-vinylimidazole, tert-butyl vinyl
ether and phenyl vinyl sulfide were used as substrates for cata-
lytic hydrogenations. In the presence of 5b2, fast and complete
reduction of the olefinic residues in allylalcohol (3 h), acrylal-
dehyde (3 h), 3-buten-2-one (6 h) and methyl-3-buteneoate
(4 h) was observed. In contrast, the hydrogenation of olefins
with donor groups such as allylamine (8 h), acrylonitrile
(14 h), 1-vinylimidazole (10 h), tert-butyl vinyl ether (65% in
24 h) and phenyl vinyl sulfide (78% in 24 h) were much slower.
Catalysts derived from 4a1, 4a2 and 4b2 displayed lower reacti-
vity for most of these functionalized substrates, although quan-
titative reduction was observed for allylalcohol, acrylaldehyde,
3-buten-2-one, methyl-3-buteneoate and allylamine after 24 h.
Nonetheless, it should be noted that in all cases the functional
groups remained unaltered, leading only to exclusive hydro-
genation of the olefinic residues. It is interesting to note that
complexes with general formula RuHCl(p-cymene)(triazolylidene)
Scheme 4 Generation of 5b1, 5b3 and 5b4.
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a
Table 1 Hydrogenation catalysis with 4a1, 4a2, 4b2 and 5b2
t/Conversionb
Cat. (h/%)
t/Conversionb
Cat. (h/%)
t/Conversionb
Cat. (h/%)
t/Conversionb
Cat. (h/%)
Entry Substrate
1
Product
4a1
4a1
6/100
4a2
4a2
6/100
4b2
4b2
5/100
5b2
5b2
2/100
4/100
2
12/100
12/100
10/100
3
4
4a1
4a1
16/100
24/84c
4a2
4a2
14/100
24/84c
4b2
4b2
12/100
24/96c
5b2
5b2
8/100
14/100
5
4a1
12/100
4a2
12/100
4b2
10/100
5b2
3/100
6
7
4a1
4a1
12/100
20/100
4a2
4a2
10/100
18/100
4b2
4b2
8/100
5b2
5b2
3/100
6/100
16/100
8
9
4a1
4a1
14/100
20/100
4a2
4a2
14/100
20/100
4b2
4b2
12/100
16/100
5b2
5b2
4/100
8/100
10
11
4a1
4a1
24/89
24/77
4a2
4a2
24/92
24/86
4b2
4b2
24/100
24/100
5b2
5b2
14/100
10/100
12
13
4a1
4a1
24/19
24/25
4a2
4a2
24/22
24/32
4b2
4b2
24/31
24/38
5b2
5b2
24/65
24/78
a Conditions: 0.20 mmol of substrate and 5 mol % (4a1, 4a2, 4b2) and 2 mol % (5b2) of catalyst precursor in CD2Cl2 at 50 °C under 4 atm of H2.
b Conversions were determined by 1H NMR spectroscopy. c Rest of the product was observed to be styrene.
has been reported to be an olefin selective hydrogenation cata- The conversion of 5b1 to 5b3 and 5b4 observed on prolonged
lyst15 with activity similar to that of 4a1, 4a2 and 4b2.
exposure to H2 were slow and these species are thought not to
Given the previous success with previously reported Ru- impact on the catalytic cycle. Nonetheless, mechanistic studies
NHC-carbene complexes,7 2 mol% 5a1, 5b2, 5c1, 5c2, 5d2 or 5e2 are continuing.
were tested under similar conditions. Thus hydrogenations of
a series of linear and cyclic olefins were examined at 25 °C
under high pressure (50 atm) of H2 in a Parr reactor (Table 2).
Of the species tested, 5b2 was found to be most effective, None-
Conclusions
theless all complexes effected quantitative reduction of A series of 1,2,3-triazolylidene complexes of Ru have been pre-
1-hexene, 2-hexene, cyclopentene, cyclohexene, cyclooctene, pared and characterized. While ruthenium-hydride complexes
allylalcohol, acrylaldehyde, 3-buten-2-one and methyl-3-buteneo- are seen as minor by-products, the major products are ones in
ate. Reduction of allylamine, tert-butyl vinyl ether and phenyl which the ligand is cyclometalated. The cyclometalated
vinyl sulfide was either complete or close, but showed mode- species are generated via C(sp2)–H activation by the ruthe-
rate activity for acrylonitrile and 1-vinylimidazole.
nium-hydride with liberation of H2. Ruthenium-triphenylphos-
The mechanism of the hydrogenation catalysis is thought phine complexes (4a1, 4a2 and 4b2) were effective catalyst
to begin with the rapid conversion of the cyclometallated precursors for hydrogenation of olefins. Interestingly the C–H
species to 5b1. Dissociation of PCy3 from 5b1 is then thought activated catalyst precursor 4b2 showed better activity presum-
to provide the catalytically active species allowing for a cycle of able a result of the additional steric protection of the metal
reactivity involving coordination of olefin, insertion into centre by the tris-isopropylphenyl substituent. Analogous
the Ru–H and reaction with H2 to regenerate the catalyst. ruthenium-PCy3 complexes 5a1, 5b2, 5c1, 5c2, 5d2 and 5e2 dis-
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played enhanced reactivity. This is thought to result from the
greater electron donating ability of PCy3 compared to PPh3. In
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carbene-complexes are the subject of continuing study in our
efforts to develop new olefin-selective hydrogenation catalysts.
The results of these efforts will be reported in due course.
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Acknowledgements
The financial support of LANXESS Inc., the NSERC of Canada,
and the Ontario Centres of Excellence are gratefully acknow-
ledged. DWS is grateful for the award of a Canada Research
Chair.
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