Interconversion of Ruthenium-O(CH2CH2PCy2)2 Alkylidene and

Sep 13, 2010 - Synopsis. The species alkylidene and alkylidyne hydride salt species (POP-Cy)RuX2(CHPh) and [(POP-Cy)RuHX(CPh)][GaX4] are interconverte...
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Organometallics 2010, 29, 4369–4374 DOI: 10.1021/om100707a

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Interconversion of Ruthenium-O(CH2CH2PCy2)2 Alkylidene and Alkylidyne Hydride Complexes Michael P. Boone, Christopher C. Brown, Travis A. Ancelet, and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 Received July 19, 2010

The species (Cy3P)2RuX2(CHPh) (X = Cl, Br) react with the ancillary ligand O(CH2CH2PCy2)2 to yield the alkylidene species (POP-Cy)RuX2(CHPh) (X = Cl 1, Br 2). Subsequent reaction of 1 and 2 with GaX3 generates the alkylidyne hydride salts [(POP-Cy)RuHX(CPh)][GaX4] (X = Cl 3, Br 4). Treatment of 3 and 4 with the donor ligand pyridine converts these alkylidyne hydrides to the Rualkylidene complexes [(POP-Cy)Ru(py)X(CHPh)][GaX4] (X = Cl 5, Br 6). The complexes 5 and 6 are also formed directly by addition of the Lewis acid-base adduct (py)GaX3 to 1 and 2, respectively. The alkylidyne hydride species 3 and 4 are also quantitatively converted back to alkylidene species 1 and 2 by addition of excess Bu4NX (X = Cl, Br), respectively. Similarly treatment of 5 and 6 with [Et3NH]X or [Bu4N]X (X = Cl, Br) resulted in the re-formation of 1 and 2. These data demonstrate that the interconversion of alkylidene and alkylidyne hydride is energetically facile. This view is supported by crystallographic and preliminary DFT data.

Introduction The chemistry of alkylidene complexes has been widely exploited for alkene metathesis catalysis. This work has had a broad impact on the chemical community and has changed the chemical landscape for synthetic organic and polymer chemistry.1-7 Among the routes for the installation of alkylidene fragments on transitions metals, a long-established methodology involves the protonation of alkylidyne complexes. In the 1980s, classic work by Schrock et al.8-11 on W-alkylidynes showed that such protonations proceed via a 1,2-migration from a metal hydride to C. The interconversion of alkylidynes to alkylidenes or vinylidenes via protonation

or hydride migration has been reported for a variety of W,12-17 Mo,18 Re,19 Os,20-25 and Ru26 species. The reverse reaction in which a 1,2-shift of a H atom from alkylidene-C to a metal giving rise to an alkylidyne hydride species has also been observed in Os24 and Re19 systems. Of particular interest to us are the studies by Esteruelas and co-workers. These authors reported the first Os hydride-alkenylcarbyne species [OsCl2H(CCH2R)(PiPr3)2] and also demonstrated that upon coordination of ligands such as CH3CN or CO they are converted to the corresponding hydride alkenylcarbene complexes [OsHCl(CHdCHdCPh2)(CH3CN)(L)(PiPr3)2]þ.24 Recent studies by the groups of Fogg27 and Johnson28 have

*To whom correspondence should be addressed. E-mail: dstephan@ chem.utoronto.ca. (1) Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem. Res. 1995, 28, 446. (2) Grubbs, R. H.; Lynn, D. M. Aqueous-Phase Organometallic Catalysis 1998, 466. (3) Demonceau, A.; Simal, F.; Delfosse, S.; Noels, A. F. NATO Sci. Ser., II 2002, 56, 91. (4) Semeril, D.; Dixneuf, P. H. NATO Sci. Ser., II 2003, 122, 1. (5) Krause, J. O.; Wang, D.; Anders, U.; Weberskirch, R.; Zarka, M. T.; Nuyken, O.; Jaeger, C.; Haarer, D.; Buchmeiser, M. R. Macromol. Symp. 2004, 217, 179. (6) Brenneman, J. B.; Martin, S. F. Curr. Org. Chem. 2005, 9, 1535. (7) Diesendruck, C. E.; Tzur, E.; Lemcoff, N. G. Eur. J. Inorg. Chem. 2009, 4185. (8) Wengrovius, J. H.; Schrock, R. R.; Churchill, M. R.; Wasserman, H. J. J. Am. Chem. Soc. 1982, 104, 1739. (9) Holmes, S. J.; Clark, D. N.; Turner, H. W.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 6322. (10) Holmes, S. J.; Schrock, R. R. J. Am. Chem. Soc. 1981, 103, 4599. (11) Freudenberger, J. H.; Schrock, R. R. Organometallics 1985, 4, 1937. (12) Blosch, L. L.; Abboud, K.; Boncella, J. M. J. Am. Chem. Soc. 1991, 113, 7066. (13) Garrett, K. E.; Sheridan, J. B.; Pourreau, D. B.; Feng, W. C.; Geoffroy, G. L.; Staley, D. L.; Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 8383.

(14) Giannini, L.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 823. (15) Doyle, R. A.; Angelici, R. J. Organometallics 1989, 8, 2207. (16) Mayr, A.; Asaro, M. F.; Kjelsberg, M. A.; Lee, K. S.; Engen, D. V. Organometallics 1987, 6, 432. (17) Bastos, C. M.; Daubenspeck, N.; Mayr, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 743. (18) Hills, A.; Hughes, D. L.; Kashef, N.; Lemos, M. A. N. D. A.; Pombeiro, A. J. L.; Richards, R. L. Dalton Trans. 1992, 1775. (19) Ozerov, O. V.; Watson, L. A.; Pink, M.; Caulton, K. G. J. Am. Chem. Soc. 2007, 129, 6003. (20) Espuelas, J.; Esteruelas, M. A.; Lahoz, F. J.; Oro, L. A.; Ruiz, N. J. Am. Chem. Soc. 1993, 115, 4683. (21) Bola no, T.; Castarlenas, R.; Esteruelas, M. A.; O nate, E. Organometallics 2007, 26, 2037. (22) Castarlenas, R.; Esteruelas, M. A.; O nate, E. Organometallics 2007, 26, 2129. (23) Esteruelas, M. A.; L opez, A. M.; Oliv~an, M. Coord. Chem. Rev. 2007, 251, 795. (24) Bola no, T.; Castarlenas, R.; Esteruelas, M. A.; Modrego, F. J.; O nate, E. J. Am. Chem. Soc. 2005, 127, 11184. (25) Jia, G. Coord. Chem. Rev. 2007, 251, 2167. (26) Esteruelas, M. A.; Lahoz, F. J.; Onate, E.; Oro, L. A.; Zeier, B. Organometallics 1994, 13, 4258. (27) Conrad, J. C.; Amoroso, D.; Czechura, P.; Yap, G. P. A.; Fogg, D. E. Organometallics 2003, 22, 3634. (28) Caskey, S. R.; Stewart, M. H.; Ahn, Y. J.; Johnson, M. J. A.; Rowsell, J. L. C.; Kampf, J. W. Organometallics 2007, 26, 1912.

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identified four-coordinate Ru-alkylidyne complexes, obtained by dehydrohalogenation of Ru-alkylidene species. Nonetheless, examples of mononuclear Ru-alkylidyne hydride species are rare, although Werner and co-workers described the six-coordinate alkylidyne hydride cations [(Cy3P)2(L)RuClH(CMe)]X (L = OEt2, H2O, NMe2Ph) some years ago.29 Although alkylidene and alkylidyne hydride species are predicted by DFT calculations to be of similar energies, Esteruelas et al.24 note that the activation barrier between these fragments is determined by the nature of the ancillary ligands, which tunes the electron density at the central metal. This suggests that it should be possible to uncover systems where conversion of Rubased alkylidenes to alkylidyne hydride species might be controllable and reversible. Herein, we describe the synthesis of Ru-alkylidene complexes that incorporate the ancillary ligand O(CH2CH2PCy2)2 (POP-Cy). These species are used to access cationic alkylidyne hydride species that are subsequently converted to new alkylidene cations by the addition of a donor ligand. Alternatively, using a halide source, the original alkylidene species are regenerated, providing examples of the direct interconversion of corresponding alkylidene and alkylidyne hydride species.

Experimental Section General Remarks. All manipulations were carried out under an atmosphere of dry, O2-free N2 employing an Innovative Technology glovebox and a Schlenk vacuum line. Solvents were purified with a Grubbs-type column system manufactured by Innovative Technology and dispensed into thick-walled Schlenk glass flasks equipped with Teflon-valve stopcocks (hexanes and CH2Cl2) or were dried over the appropriate agents and distilled into the same kind of storage flasks (C5H5N and C6H12). All solvents were thoroughly degassed after purification (repeated freeze-pump-thaw cycles). Deuterated solvents were dried over the appropriate agents, vacuum-transferred into storage flasks with Teflon stopcocks, and degassed accordingly (CD2Cl2). 1H, 13 C, and 31P NMR spectra were recorded at 25 °C on Bruker 400 MHz spectrometers. Chemical shifts are given relative to SiMe4 and referenced to the residual solvent signal (1H, 13C) or relative to an external standard (31P: 85% H3PO4). In some instances, signal and/or coupling assignment was derived from two-dimensional NMR experiments. Observation of some 13C carbyne and carbene resonances was done by HMBC or HSQC experiments, respectively. In these cases additional coupling constants were not resolved. Chemical shifts are reported in ppm and coupling constants as scalar values in Hz. Combustion analyses were performed in-house employing a Perkin-Elmer CHN analyzer. The ligand O(CH2CH2PCy2)2 (POP-Cy) was prepared by a literature method.30 Synthesis of (POP-Cy)RuX2(CHPh) (X = Cl 1, Br 2). These compounds were prepared in a similar manner, and thus only one procedure is detailed. POP-Cy (0.104 g, 0.22 mmol) in CH2Cl2 (5 mL) was transferred to (Cy3P)2RuCl2(CHPh) (0.166 g, 0.20 mmol) in CH2Cl2 (5 mL) and let stir for 4 h, yielding a dark green solution. The reaction was then pumped dry, and the resulting dark green residue was washed with hexanes (4  10 mL), resulting in a green solid (0.114 g, 78%). 1: 1H NMR (CD2Cl2): 20.03 (s, 1H, RudCH), 8.49 (d, 2H, o-Ph, 1JHH = 8.0 Hz) 7.46 (t, 1H, p-Ph, JHH = 7.2 Hz), 7.28 (t, 2H, m-Ph, JHH = 7.6 Hz), 4.11-4.00 (m, 4H, CH2 backbone), 2.62-2.50 (m, 4H, CH2 backbone), 2.25-2.18 (m, 4H, Cy), 1.71-1.61 (m, 8H, Cy), 1.59-1.01 (m, 32H, Cy). 31P{1H} (29) St€ uer, W.; Wolf, J.; Werner, H.; Schwab, P.; Schulz, M. Angew. Chem., Int. Ed. 1998, 37, 3421. (30) Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L.; Bernatis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois, D. L. Organometallics 1994, 13, 4844.

Boone et al. NMR (CD2Cl2): 43.5 (s, -PCy2). 13C{1H} NMR (CD2Cl2): 304.48 (t, RudCH, 2JCP = 8.0 Hz), 155.38 (s, ipso-C, Ph), 131.30 (o-C, Ph), 128.84 (p-C, Ph), 128.55 (m-C, Ph), 69.16 (s, CH2 backbone), 32.35 (t, ipso-C, Cy, 1JCP = 10.5 Hz), 29.22 (s, Cy), 28.71 (s, Cy), 27.76-27.33 (m, Cy), 26.39 (s, Cy), 24.79 (t, CH2 backbone, 1JCP = 9.6 Hz) Anal. Calcd for C35H58Cl2P2ORu 3 CH2Cl2 (813.69): C, 53.14; H, 7.43. Found: C, 53.70; H, 7.08. 2: green solid (0.070 g, 86%). 1H NMR (CD2Cl2, 5.32 ppm): 20.20 (s, 1H, RudCH), 8.53 (d, 2H, o-Ph, JHH = 8.0 Hz) 7.52 (t, 1H, p-Ph, JHH = 7.4 Hz), 7.33 (t, 2H, m-Ph, JHH = 7.7 Hz), 4.11-4.03 (m, 4H, CH2 backbone), 2.94-2.83 (m, 4H, CH2 backbone), 2.28-2.20 (m, 4H, Cy), 1.81-1.52 (m, 8H, Cy), 1.32-1.10 (m, 32H, Cy). 31P{1H} NMR (CD2Cl2): 41.6 (s, -PCy2). 13C{1H} NMR (CD2Cl2): 307.01 (RudCH), 155.61 (s, ipso-C, Ph), 131.44 (o-C, Ph), 129.33 (p-C, Ph), 128.48 (m-C, Ph), 69.35 (s, CH2 backbone), 33.99 (t, ipso-C, Cy, 1JCP = 10.9 Hz), 29.55 (s, Cy), 29.17 (s, Cy), 27.85-27.42 (m, Cy), 26.49 (s, Cy), 24.83 (t, CH2 backbone, 1JCP = 10.4 Hz) Anal. Calcd for C35H58Br2OP2Ru (817.64): C, 51.41; H, 7.15. Found: C, 51.83; H, 7.44. Synthesis of [(POP-Cy)RuHX(CPh)][GaX4] (X = Cl 3, Br 4). These compounds were prepared in a similar manner, and thus only one procedure is detailed. To a stirring solution of 1 (7 mg, 0.01 mmol) in CD2Cl2 (0.5 mL) was added a solution of GaCl3 (2 mg, 0.01 mmol) in CD2Cl2 (0.5 mL), resulting in a bright yellow solution. The resulting complex is not stable enough to isolate. 3: 1 H NMR (CD2Cl2): 8.01-7.91 (m, 3H, o,p-Ph), 7.60 (t, 2H, mPh, JHH = 8.0 Hz), 4.35-4.25 (m, 2H, CH2 backbone), 3.97-3.86 (m, 2H, CH2 backbone), 2.72-2.53 (m, 6H, ipso-CH and CH2 backbone), 2.50-2.37 (m, 2H, CH2 backbone), 2.10-1.18 (m, 40H, PCy), -7.91 (t, 1H, Ru-H, 3JPH = 14 Hz). 31P{1H} NMR (CD2Cl2): 65.5 (s, -PCy2).13C{1H} NMR (CD2Cl2): 311.04 (RutC), 140.50 (s, ipso-C, Ph), 138.74 (s, p-C, Ph), 132.45 (s, o/m-Ph), 130.50 (s, o/m-Ph), 76.46 (s, CH2 backbone), 39.47 (t, ipso-C, Cy, 1JCP = 12.0 Hz), 30.04 (t, ipso-C, Cy, 1 JCP = 13.0 Hz), 31.88 (s, Cy), 31.09 (s, Cy), 29.50 (s, Cy), 29.03 (s, Cy), 27.09 (t, CH2 backbone, 1JCP = 7.2 Hz), 27.9825.03 (m, Cy). EA: Not stable enough to isolate. 4: orange needles (20 mg, 76%). 1H NMR (CD2Cl2): 7.89-7.79 (m, 3H, o, p-Ph), 7.49 (t, 2H, m-Ph, JHH = 7.8 Hz), 4.07-3.96 (m, 2H, CH2 backbone), 3.69-3.58 (m, 2H, CH2 backbone), 3.002.88 (m, 2H, CH2 backbone), 2.44-2.21 (m, 6H, ipso-CH and CH2 backbone), 2.12-1.11 (m, 40H, Cy), -5.17 (t, 1H, Ru-H, 3 JPH = 15 Hz). 31P{1H} NMR (CD2Cl2): 62.9 (s, -PCy2). 13 C{1H} NMR (CD2Cl2): 300.71 (RutC), 140.90 (s, ipso-C, Ph), 136.12 (s, p-C, Ph), 131.55 (s, o/m-Ph), 129.62 (s, o/m-Ph), 72.86 (s, CH2 backbone), 37.36 (t, ipso-C, Cy, 1J CP = 12.0 Hz), 35.98 (t, ipso-C, Cy, 1 J CP = 13.2 Hz), 31.27 (s, Cy), 30.70 (s, Cy), 29.01 (s, Cy), 28.33 (s, Cy), 28.11 (s, Cy), 28.06 (s, Cy), 27.36 (t, CH2 backbone, 1JCP = 7.5 Hz), 26.98-26.79 (m, Cy), 26.44 (t, CH2 backbone, 1JCP = 6.9 Hz), 26.32-25.68 (m, Cy). IR: 1958 cm-1 (ν RuH). Anal. Calcd for C 35H 58Br5P 2OGaRu (1127.09): C, 37.30; H, 5.19. Found: C, 37.49; H, 5.63. Synthesis of [(POP-Cy)Ru(py)X(CHPh)][GaX4] (X = Cl 5, Br 6). These compounds were prepared in a similar manner, and thus only one procedure is detailed. Method i: To a stirring solution of 1 (14 mg, 0.02 mmol) in CH2Cl2 (0.5 mL) was added a solution of GaCl3 (3 mg, 0.02 mmol) in CH2Cl2 (0.5 mL), resulting in a bright yellow solution. To this mixture was added pyridine (0.01 mL), yielding a green-colored solution. The reaction was then pumped dry and washed with hexanes (2  2 mL), yielding a light purple solid (7 mg, 71%) Method ii: To a stirring solution of 1 (56 mg, 0.0768 mmol) in CH2Cl2 (2 mL) was added a solution of the pyridine GaCl3 adduct (24 mg, 0.0941 mmol) in CH2Cl2 (5 mL), and the mixture was stirred for 12 h. The reaction was then pumped dry, washed with hexanes (3  5 mL), and dried in vacuo. The purple solid was then taken up in CH2Cl2 (2 mL) and layered with C6H12 (5 mL) and upon diffusion yielded purple crystals (55 mg, 73%). 5: 1H NMR (CD2Cl2): 18.95 (t, 1H, RudCH, 3JPH = 2.6 Hz), 9.40 (d, 1H, Ph, JHH = 5.9 Hz), 8.63 (d,

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Table 1. Crystallographic Data 1

2

4

5

C35H58Br2OP2Ru C36H60Br5Cl2GaOP2Ru C41H63Cl7GaNOP2Ru 817.62 1211.97 1066.80 monoclinic orthorhombic monoclinic Pnma P21/c P21/c 17.6657(6) 28.177(10) 10.713(2) 12.1392(4) 18.222(6) 22.239(5) 18.6142(7) 9.131(4) 20.945(4) 116.226(1) 90 100.093(7) 3580.9(2) 4688(3) 4912.9(17) 1.517 1.717 1.442 4 4 4 150 150 150 2.785 5.367 1.333 8209 5528 9098 6653 3602 7344 0.0345 0.0637 0.0539 370 249 541 0.0296 0.0627 0.0399 0.0755 0.1855 0.1118 1.014 1.003 0.957 P P P a 2 2 2 P ˚ These data were collected with Mo KR radiation (λ = 0.71069 A). R = (Fo - Fc)/ Fo; Rw = ( [w(Fo - Fc ) ]/ [w(Fo)2])1/2.

formula Mr [g 3 mol-1] cryst syst space group a [A˚] b [A˚] c [A˚] β [deg] V [A˚3] Fcalc [g 3 cm-3] Z T [K] μ [mm-1] reflns (coll) reflns (unique) Rint params R1 [I > 2σ(I )]/ wR2a R1/ wR2 (all data)a GOF

C36H60Cl4OP2Ru 813.65 orthorhombic Pbca 12.3266(4) 20.0488(7) 31.3531(9) 90 7748.4(4) 1.395 8 150 0.790 8866 6387 0.0535 397 0.0402 0.0894 1.024

3H, py, JHH = 7.1 Hz), 7.94 (t, 1H, Ph, JHH = 7.5 Hz), 7.75 (t, 1H, Ph, JHH = 7.3 Hz), 7.66 (t, 1H, py/Ph, JHH = 6.6 Hz), 7.57-7.49 (m, 3H, py/Ph), 4.44-4.30 (m, 2H, CH2 backbone), 4.21-4.11 (m, 2H, CH2 backbone), 2.98-2.86 (m, 2H, CH2 backbone), 2.48-2.38 (m, 2H, CH2 backbone), 2.11-0.38 (m, 44H, Cy). 31P{1H} NMR (CD2Cl2): 38.5 (s, -PCy2). 13C{1H} NMR (CD2Cl2): 314.72 (RudCH), 154.57 (s, p-C, py), 152.45 (s, o-C, py), 151.95 (s, m-C, py), 137.94 (s, ipso-C, Ph), 132.24 (s, Ph), 128.94 (s, Ph), 126.58 (s, Ph), 69.64 (s, CH2 backbone), 37.16 (t, ipso-C, Cy, 1JCP = 9.6 Hz), 31.69 (t, ipso-C, Cy, 1JCP = 9.6 Hz), 30.14 (s, Cy), 29.59 (s, Cy), 28.48 (s, Cy), 27.64-27.41 (m, Cy), 27.34 (t, CH2 backbone, 1JCP = 5.1 Hz), 26.14 (s, Cy), 25.88 (s, Cy), 24.73 (t, CH2 backbone, 1JCP = 10.3 Hz). Anal. Calcd for C40H63Cl5P2NOGaRu (983.94): C, 48.83; H, 6.45; N, 1.42. Found: C, 48.57; H, 6.49; N, 1.50. 6: green solid (20 mg, 76%). 1H NMR (CD2Cl2): 18.97 (t, 1H, RudCH, 3 JPH = 3.2 Hz), 9.35 (d, 1H, Ph, JHH = 5.4 Hz), 8.58 (d, 2H, py, JHH = 6.9 Hz), 8.50 (d, 1H, py, JHH = 4.6 Hz), 7.95 (t, 1H, Ph, JHH = 7.0 Hz), 7.78-7.67 (m, 2H, py/Ph), 7.56-7.44 (m, 3H, py/Ph), 4.46-4.38 (m, 2H, CH2 backbone), 4.26-4.23 (m, 2H, CH2 backbone), 3.18-3.06 (m, 2H, CH2 backbone), 2.59-2.47 (m, 2H, CH2 backbone), 2.45-0.94 (m, 44H, Cy). 31 P{1H} NMR (CD2Cl2): 35.2 (s, -PCy2). 13C{1H} NMR (CD2Cl2): 316.44 (RudCH), 154.33 (s, py), 152.23 (s, py), 152.04 (s, py), 138.19 (s, Ph), 132.42 (s, Ph), 128.47 (s, Ph), 126.75 (s, Ph), 70.02 (s, CH2 backbone), 37.66 (t, ipso-C, Cy, 1 JCP = 8.9 Hz), 33.31 (t, ipso-C, Cy, 1JCP = 9.2 Hz), 30.47 (s, Cy), 29.87 (s, Cy), 29.55 (s, Cy), 28.26-28.12 (m, Cy), 27.74-27.24 (m, Cy), 26.76-25.80 (m, Cy), 25.07-24.66 (m, Cy). EA: Compound 6 could not be separated from traces of 2. 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 Bruker Apex II diffractometer using a graphite monochromator with Mo KR radiation (λ = 0.71073 A˚). The data were collected at 150((2) K for all crystals. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for absorption effects using the empirical multiscan method (SADABS). Structure Solution and Refinement. The structures were solved by direct methods using XS and refined by full-matrix leastsquares on F2 using XL as implemented in the SHELXTL suite of programs. All non-hydrogen atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed in calculated positions using an appropriate riding model and coupled isotropic temperature factors. In the case of 4, the Ru-hydride was

located and its position refined (see Table 1 and Supporting Information). DFT calculations were performed using Gaussian 03.31 Geometry optimizations were carried out using the B3LYP functional and LanL2DZ basis sets implemented in Gaussian 03 software. Frequency analysis performed after geometry optimization confirmed that no imaginary frequency was present. Transition states contained only one imaginary frequency, and the transition states were connected to the minimized structures via IRC analysis.

Results and Discussion Initiating these efforts, we strategized that a tridentate ligand comprised of two strong sigma donors and an additional weak donor would be suitable to stabilize a Rualkylidene and yet block reactivity. To this end, the ligand POP-Cy was reacted with (Cy3P)2RuX2(CHPh) (X = Cl or Br) in CH2Cl2 to give dark green solutions. Removal of the solvent and washing of the residue with hexanes afforded the green solids 1 and 2 in 78% and 86% yields, respectively. In the case of 1, a single resonance is observed at 43.5 ppm in the 31 P{1H} NMR spectrum. The 1H NMR spectrum of 1 exhibits a new alkylidene proton signal at 20.03 ppm, while the alkylidene carbon is observed at 304.4 ppm. In addition, resonances consistent with the presence of the bound POPCy ligand are evident. The corresponding alkylidene signals for 2 were observed at 20.20 and 307.01 ppm in the 1H and 13 C spectra, respectively. On the basis of these data the (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; G. E., S.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; ; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyoto, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Lahan, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.

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Boone et al.

Scheme 1. Synthesis and Interconversions of 1-6

Figure 1. POV-ray depiction of 1. P: orange, Cl: green, O: red, C: black, Ru: hunter green.

products 1 and 2 were formulated as (POP-Cy)RuX2(CHPh) (X = Cl 1, Br 2) (Scheme 1). This was unambiguously confirmed via crystallography (Figures 1, 2). The tridentate POP ligand adopts a meridional binding mode with the two mutually trans halides and the alkylidene completing the sixcoordinate geometry about Ru. The Ru-P and Ru-chloride bond distances in 1 were found to be 2.3678(8), 2.3754(8), 2.4102(7), and 2.4177(7) A˚, respectively, whereas in 2 the corresponding distances were 2.3743(7), 2.3857(7), 2.5492(3), and 2.5538(3) A˚. These are similar to those reported by Gusev et al.32 for the five-coordinate species (POP-tBu)RuCl2. The Ru-O bond in 1 (2.3163(19) A˚) is longer than that seen in 2 (2.3004(17) A˚), and both are longer than those in five-coordinate (POP-tBu)RuCl2 (2.123(2) A˚)32 and the Grubbs-Hoveyda complexes, (Ph3P)RuCl2(CH(C6H4)OCHMe2) (2.309(2) A˚) and (Cy3P)RuCl2(CH(C10H6)OMe) (2.257(7) A˚).33 The Ru-C distances in 1 and 2 are very similar at 1.866(3) and 1.870(3) A˚ and are longer than that seen in (Cy3P)2RuCl2(CH(C6H4Cl)) (1.839(3) A˚).34,35 The C-C-Ru angles in 1 and 2 differ slightly at 139.3(2)o and 137.9(2)o. Compound 1 reacts with GaCl3 in CH2Cl2 to generate a bright yellow solution of a new species, 3. This reaction is reminiscent of that described by Piers et al. in which they employ B(C6F5)3 to abstract halide from Ru to generate Ruphosphonium alkylidene complexes.36 This present product (3) exhibits a 31P{1H} NMR signal at 65.5 ppm. However the alkylidene proton resonance is lost and instead a Ru-hydride signal at -7.91 ppm shows coupling of 14 Hz to two P atoms. These data infer that 3 is formulated as the salt [(POP-Cy)RuHCl(CPh)][GaCl4] containing a Ru-H and an alkylidyne fragment. Compound 3 proved to be an extremely sensitive species, thus frustrating all efforts to isolate this compound. (32) Major, Q.; Lough, A. J.; Gusev, D. G. Organometallics 2005, 24, 2492. (33) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791. (34) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2039. (35) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (36) Leitao, E. M.; Eide, E. F. v. d.; Romero, P. E.; Warren, E. P.; McDonald, R. J. Am. Chem. Soc. 2010, 132, 2784.

Figure 2. POV-ray depiction of 2. P: orange, Br: red-brown, O: red, C: black, Ru: hunter green.

However, the analogous reaction of 2 with GaBr3 resulted in the successful isolation of bright orange needles of 4 in 76% yield (Scheme 1). This species also showed no evidence of an alkylidene resonance, but rather a Ru-H signal was seen at -5.17 ppm in the 1H NMR spectrum. In addition, the infrared spectrum of 4 showed an absorption at 1958 cm-1 typical of a Ru-H fragment. The 13C{1H} NMR signal at 300.8 ppm is consistent with the presence of the proposed alkylidyne fragment. The proposed formulations of these alkylidyne hydride salts were further supported with crystallographic data for 4 (Figure 3). In the solid-state structure of 4, the GaBr4 anion was well separated from the six-coordinate, Ru-alkylidyne hydride cation (Ru 3 3 3 Ga 6.208 A˚). These ions reside on a crystallographically imposed 2-fold axis. The Ru-P and Ru-O distances in 4 are 2.3618(18) and 2.241(6) A˚, respectively. These distances are shorter than those in 2, consistent with the cationic character of the Ru center. In contrast, the Ru-Br distance of 2.5920(14) A˚ in 4 is slightly longer than those in 2. This results from the trans influence of the Ru-hydride, trans to the bromide ion. This hydride on Ru was located and refined to a resulting Ru-H distance of 1.53 A˚. The alkylidyne fragment gives rise to a Ru-C distance of 1.713(8) A˚ and the Ru-C-C angle of 177.5(7)o. This Ru-C distance is slightly longer than those reported by Johnson et al. for (Cy3P)2RuF3(C(C6H4Me)) (1.703(9) A˚), (Cy3P)RuI3(C(C6H4Me)) (1.670(5) A˚), and [(Cy3P)2RuCl2(C(C6H4Me))]þ (1.678(3) A˚) and similar to that seen for (Cy3P)2RuFCl2(C(C6H4Me)) (1.714(3) A˚).28 It (37) Clark, G. R.; Marsden, K.; Roper, W. R.; Wright, L. J. J. Am. Chem. Soc. 1980, 102, 6570.

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Scheme 2. DFT Calculation of Barrier between Alkylidene and Alkylidyne Hydride Complexes

Figure 3. POV-ray depiction of the cation of 4. P: orange, Br: red-brown, O: red, C: black, Ru: hunter green, H: gray.

Figure 4. POV-ray depiction of the cation 5. P: orange, Cl: green, O: red, C: black, N: light green, Ru: hunter green.

is also comparable to the Os-C distances seen in the Osalkylidyne hydride cation [OsHCl(CPh)(IPr)(PiPr3)]OTf (1.717(2) A˚)22 and slightly shorter than the Os-C distance in (Ph3P)2OsCl(CO)(C(C6H4Me)) (1.77 A˚).37 The above reactions demonstrate the formation of the alkylidyne hydride species 3 and 4 from alkylidene complexes 1 and 2. The reverse reaction was also probed. Addition of the donor ligand pyridine to a solution of 3 resulted in the formation of the new purple product 5 in a 71% yield (Scheme 1). This new species 5 exhibits an alkylidene CH signal at 18.95 ppm in the 1H NMR spectrum, with the 13C{1H} NMR signal at 314.7 ppm shown to be an alkylidene carbon by the HSQC correlation. In addition, resonances attributable to coordinated pyridine were also evident. However, when 4 was treated with the donor ligand pyridine, the corresponding product 6 could not be isolated quantitatively. It appears that in this case base competes for the GaBr3, resulting in varying degrees of re-formation of 2. Thus mixtures of 2 and 6 were formed and separation of 2 and 6 was not possible. Nonetheless, the latter species was spectroscopically characterized. The alkylidene 1H resonance for 6 was seen at 18.97 ppm with the corresponding 13 C signal observed via HSQC correlation at 316.4 ppm. These data support the formulation of 5 and 6 as Rualkylidene complexes [(POP-Cy)Ru(py)X(CHPh)][GaX4]

(X = Cl 5, Br 6). This was also further supported with the crystallographic study of 5 (Figure 4). The Ru-C distance and Ru-C-C angle reflect the presence of the alkylidene unit. The complexes 5 and 6 (Scheme 1) are also formed directly by addition of the Lewis acid-base adduct (py)GaX3 to 1 and 2, respectively. The latter result infers some degree of dissociation of the pyridine-gallium adduct allows sequential halide abstraction and pyridine coordination to the resulting Ru cation. It is noteworthy that the formation of 5 and 6 occurs instantaneously upon addition of pyridine. This stands in contrast to the addition of CH3CN to the Os-hydride alkenecarbyne, where refluxing conditions are required to afford the corresponding alkenecarbene species.24 The above reactivity demonstrates that formation of a vacant coordination site on the Ru alkylidene complexes 1 and 2 induces sufficient electrophilicity at Ru to induce the migration of the alkylidene proton to Ru, affording the alkylidyne hydride species 3 and 4. Herein it is shown that this conversion is readily reversed. The alkylidyne hydride species 3 and 4 are converted to the alkylidene cations 5 and 6 by simple addition of the donor ligand pyridine. In a similar fashion, reactions of 3 and 4 with excess Bu4NX (X = Cl, Br), respectively, at 25 °C resulted in the quantitative reformation of 1 and 2, respectively, as confirmed by 1H and 31 P{1H} NMR data (Scheme 1). Similarly treatment of 5 with [Et3NH]Cl or [Bu4N]Cl resulted in the re-formation of 1 in 50% and 60% yield, respectively, as evidenced by NMR spectroscopy (Scheme 1). These data demonstrate that the interconversion of alkylidene and alkylidyne hydride is energetically facile. To probe this aspect further, the gas phase geometries of model cations [Me2PCH2CH2)2ORuCl(CHPh)]þ (7a) and [Me2PCH2CH2)2ORuHCl(CPh)]þ (7b) were optimized via DFT calculations using the B3LYP functional and LanL2DZ basis set. A transition state for conversion of 7a to 7b was also computed and optimized. These data revealed an energy barrier of 16.6 kcal/ mol for the conversion of 7a to 7b, with an overall energy difference of only þ2.8 kcal/mol (Scheme 2). The transition-state species 7-TS is characterized by an agostic interaction of the alkylidene proton with the Ru center. This geometry is reminiscent of that seen in W-alkylidene complexes originally structurally characterized by Schrock and co-workers.8-11 While expanded models and consideration of solvent and counterion effects would refine the calculated barrier, these preliminary

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computational data further support the notion that the interconversion of alkylidene and alkylidyne hydride species is close to thermodynamically neutral with a relatively small activation barrier. This is consistent with the experimental observations.

Boone et al.

rare example of the direct interconversion of corresponding alkylidene and alkylidyne hydride systems. Research to apply this facile switching of functional fragments in subsequent chemistry of the resulting complexes and catalysis is the subject of current efforts.

Concluding Remarks Herein, we have described Ru-based systems where conversion of alkylidene to alkylidyne hydride system is readily achieved by halide abstraction. The resulting alkylidyne hydride is transformed to a cationic alkylidene via simple addition of a donor ligand. Alternatively the original alkylidenes are regenerated from the alkylidyne hydride species by addition of excess halide ion. To our knowledge, this is a

Acknowledgment. D.W.S. gratefully acknowledges the financial support of NSERC of Canada, the award of a Canada Research Chair, and a Killam Research Fellowship. Supporting Information Available: Calculated 3D coordinates of compounds 7a, 7b, and 7-TS and CIF files giving crystallographic data for compounds 1, 2, 4, and 6 are available free of charge via the Internet at http://pubs.acs.org.