Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium

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Organometallics 2009, 28, 5466–5477 DOI: 10.1021/om900128s

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Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium Trimethylphosphine Complexes: A Mechanistic Investigation Using High-Pressure NMR Spectroscopy April D. Getty,‡ Chih-Cheng Tai,† John C. Linehan,*,‡ Philip G. Jessop,*,^, Marilyn M. Olmstead,† and Arnold L. Rheingold§

Chemical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999 MS: K2-57, Richland, Washington 99352, †Department of Chemistry, University of California, Davis, California 95616-5295, ^ Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6, and §University of Delaware, Newark, Delaware 19716. Previous address: Department of Chemistry, University of California, Davis, California 95616-5295 )

‡

Received February 17, 2009

Complex cis-(PMe3)4RuCl(OAc) (1) acts as a catalyst for CO2 hydrogenation into formic acid in the presence of a base and an alcohol cocatalyst. NMR spectroscopy has revealed that 1 exists in solution in equilibrium with fac-(PMe3)3RuCl(η2-OAc) (2), [(PMe3)4Ru(η2-OAc)]Cl (3a), and free PMe3. Complex 2 was found to be a poor CO2 hydrogenation catalyst under the conditions of catalysis used for 1. Complex 3a can be prepared by adding certain alcohols, such as MeOH, EtOH, or C6H5OH, to a solution of 1 in CDCl3. The chloride ion of 3a was exchanged for the noncoordinating anion BPh4- or B(ArF)4- (B(ArF)4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to produce [(PMe3)4Ru(η2-OAc)]BPh4 (3b) and [(PMe3)4Ru(η2-OAc)]B(ArF)4 (3c). Complexes 3b and 3c were found to be as efficient as 1 in the catalytic hydrogenation of CO2 to formic acid in the presence of an alcohol cocatalyst. In contrast to 1, 3b and 3c continued to show high catalytic activity in the absence of the alcohol cocatalyst. High-pressure NMR spectroscopy was used to investigate the mechanism of CO2 hydrogenation via 3b,c in the presence of base. The observations were inconsistent with the previously reported phosphine-loss mechanism; a new mechanism is proposed involving an unsaturated, cationic ruthenium complex of the form [(PMe3)4RuH]þ (B) as the active catalyst. The role of the base in this system includes not only trapping of the formic acid product but also initiation of the catalysis by aiding the conversion of 3b,c to B. Crystallographically determined structures are reported for complexes 2, 3b, {[(PMe3)3Ru]2(μ-Cl)2(μOAc)}BPh4, and {[(PMe3)3Ru]2(μ-Cl)3}Cl. Introduction Carbon dioxide recovered from power plant stacks or the atmosphere can be used to produce chemicals and fuels. The reasons for using CO2 as a starting material are numerous.1 Carbon dioxide is cheap, nontoxic, and abundant and is a totally renewable feedstock compared to oil or coal. One chemical that can be catalytically produced from CO2 is formic acid. Formic acid is used for such varied processes as leather tanning, in dye baths for dyeing natural and synthetic fibers, as an additive for cleaning agents, for coagulating latex in the production of natural rubber, and in the synthesis of the artificial sweetener aspartame.2 *To whom correspondence should be addressed. E-mail: john. [email protected] (J.C.L.); [email protected](P.G.J.). (1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. Rev. 2001, 101, 953, and references therein. (2) (a) Kirk, R. E.; Othmer, D. F.; Kroschwitz, J. I.; Howe-Grant, M. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley & Sons: New York, 1991. (b) Reutemann, W.; Kieczka, H. In Ullman’s Encyclopedia of Industrial Chemistry [Online]; Wiley-VCH: Weinheim, 2002. pubs.acs.org/Organometallics

Published on Web 08/26/2009

There have been several reports of the hydrogenation of carbon dioxide to produce formic acid using homogeneous RuII catalysts.3-5 For instance, Jessop et al.3 performed an investigation of CO2 hydrogenation in supercritical CO2 that (3) (a) Munshi, P.; Main, A. D.; Linehan, J. C.; Tai, C.-C.; Jessop, P. G. J. Am. Chem. Soc. 2002, 124, 7963. (b) Tai, C.-C.; Pitts, J.; Linehan, J. C.; Main, A. D.; Munshi, P.; Jessop, P. G. Inorg. Chem. 2002, 41, 1606. (c) Yin, C.; Xu, Z.; Yang, S.-Y.; Ng, S. M.; Wong, K. Y.; Lin, Z.; Lau, C. P. Organometallics 2001, 20, 1216. (d) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 344. (e) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Chem. Soc., Chem. Commun. 1995, 707. (f) Jessop, P. G.; Ikariya, T.; Noyori, R. Nature 1994, 368, 231. (g) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1994, 116, 8851. (4) (a) Urakawa, A.; Jutz, F.; Laurenczy, G.; Baiker, A. Chem.;Eur. J. 2007, 13, 3886–3899. (b) Ogo, S.; Kabe, R.; Hayashi, H.; Harada, R.; Fukuzumi, S. Dalton Trans. 2006, 4657–4663. (c) Ng, S. M.; Yin, C.; Yeung, C. H.; Chan, T. C.; Lau, C. P. Eur. J. Inorg. Chem. 2004, 1788–1793. (d) Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa, H.; Kasuga, K. Organometallics 2004, 23, 1480–1483. (e) Kayaki, Y.; Shimokawatoko, Y.; Ikariya, T. Adv. Synth. Catal. 2003, 345, 175–179. (f) Man, M. L.; Zhou, Z.; Ng, S. M.; Lau, C. P. J. Chem. Soc., Dalton Trans. 2003, 3727–3735. (g) Elek, J.; Nadasdi, L.; Papp, G.; Laurenczy, G.; Joo, F. Appl. Catal. A: Gen. 2003, 255, 59–67. (h) Yi, C. S.; Liu, N. Organometallics 1995, 14, 2616. (i) Koinuma, H.; Kawakami, F.; Kato, H.; Hirai, H. J. Chem. Soc., Chem. Commun. 1981, 213. (j) Inoue, Y.; Izumida, H.; Sasaki, Y.; Hashimoto, H. Chem. Lett. 1976, 863. (k) Inoue, Y.; Sasaki, Y.; Hasimoto, H. J. Chem. Soc., Chem. Commun. 1975, 718. (l) Kokomnikov, I. S.; Lobeeva, T. S.; Vol'pin, M. E. Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.) 1972, 2263. r 2009 American Chemical Society

Article

Organometallics, Vol. 28, No. 18, 2009

included the use of trans-(PMe3)4RuCl2, cis-(PMe3)4RuH2, or cis-(PMe3)4RuCl(OAc) (1, OAc is acetate) as catalyst precursor. We have focused on complex 1 as the catalyst precursor because it is as active as the Ru-dihydride, but is far more easily prepared and handled. We earlier reported the effects of certain alcohols and bases on the yield of formic acid in this system (eq 1).3a

Near-stoichiometric amounts of base are required for the reaction shown in eq 1, as well as a catalytic amount of alcohol in order to obtain reasonable rates of catalysis. Acidic, nonbulky alcohols and triflic acid increase the rate of hydrogenation an order of magnitude above that which can be obtained using methanol or water. Similarly, the use of the more basic DBU (1,8-diazobicyclo[5.4.0]undec-7-ene) rather than NEt3 increases the rate of reaction by an order of magnitude. The combination of DBU and an alcohol cocatalyst leads to a side reaction (eq 2) giving alkyl carbonate salts;3a whether those salts have a role in the catalysis is unknown. Determining the role of the alcohol in the hydrogenation, and being able to obtain reasonable rates of catalysis without it, would be very beneficial. Herein we report the solution chemistry of 1 and the use of high-pressure NMR spectroscopy to further investigate the mechanism of CO2 hydrogenation catalyzed by complex 1 and its derivatives.

Experimental Section General Comments. All manipulations were carried out under N2 or by using high-vacuum techniques, unless otherwise noted. All solvents, liquid alcohols, triethylamine, and DBU (1,8diazobicyclo[5.4.0]undec-7-ene) were degassed by repeated freeze-pump-thaw cycles before use. Chloroform, chloroform-d, and dichloromethane were stirred at least 5 min with basic alumina (activated by heating at 150 C under vacuum for 5 h) before use. Tetrahydrofuran was dried over sodium benzophenone ketyl; however THF-d8 was used as received (packaged in 0.7 mL ampules by Aldrich). Trimethylphosphine was distilled from sodium prior to use. Hydrogen gas (Oxarc, 5.0 grade), deuterium gas (Spectra Gases deuterium-UHP grade 99.5%), CO2 gas (Scott Specialty Gases, SFC grade 99.993%), and 13CO2 gas (Aldrich Chemical Co., 99 atom % 13C and 6 atom % 18O) were used as received. Unless otherwise specified, reagents and solvents were used as purchased from Aldrich Chemical Co., Strem, or Boulder Scientific Company in the case of NaB(ArF)4 (ArF =3,5-bis(trifluoromethyl)phenyl). NMR spectra were recorded using a 7.01 T Varian VXR-300 spectrometer, and chemical shifts (δ) are reported in ppm referenced to residual proton peaks in the deuterated solvent and reported relative to TMS. 13C{1H} NMR spectra were (5) For reviews of transition metal-catalyzed homogeneous CO2 hydrogenation, see: (a) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259. (b) Leitner, W. Angew. Chem., Int. Ed. Engl. 1995, 34, 2207. (c) Jessop, P. G.; Joo, F.; Tai, C.-C. Coord. Chem. Rev. 2004, 248, 2425–2442.

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referenced to solvent peaks relative to TMS, 31P{1H} NMR spectra were referenced externally to 85% H3PO4 (downfield being positive), and the 2H NMR spectra were referenced to THF-d8. NMR spectra recorded at atmospheric pressure were obtained using a standard broadband 5 mm NMR probe, and NMR spectra recorded at high pressures were conducted using a 10 mm broadband probe with a 10 mm o.d., 3.5 mm i.d. PEEK (polyether ether ketone) NMR cell run unlocked.6 Elemental analyses were performed by Atlantic Microlab, Inc. The complexes (PMe3)4RuCl(OAc) (1),7 cis-(P(OEt)3)4RuCl2,8 and cis-(P(OEt)3)4RuH28 were prepared according to literature procedures. The catalytic runs in the multivial reactor were performed with equipment described in an earlier paper.9 Pressure vessels were loaded under dry nitrogen atmosphere. Safety Warning. Operators of high-pressure equipment such as that required for these experiments should take proper precautions, including but not limited to the use of blast shields and pressure-relief mechanisms, to minimize the risk of personal injury. Synthesis of fac-(PMe3)3RuCl(η2-OAc) (2). In the drybox, 175 mg (0.35 mmol) of cis-(PMe3)4RuCl(OAc) (1) was dissolved in 6 mL of CH2Cl2, and the solution was filtered through glass wool layered with Celite to remove a small amount of insoluble material. The filtrate was dispensed into a glass bomb, removed from the drybox, and placed on a high-vacuum apparatus. The CH2Cl2 was removed under reduced pressure, and the bomb was placed in a sand bath adjusted to ca. 150 C. The sealed bomb was periodically opened to vacuum for ca. 30 s every 10-15 min during the 2 h heating period. After cooling, the bomb was brought into the drybox and the solid was dissolved in 4 mL of CH2Cl2. This solution was filtered and the product crystallized by vapor diffusion with pentane. The yellow crystalline solid was collected by decanting the mother liquor, and the crystals were washed with pentane (3 2 mL). Yield: 102 mg (0.24 mmol), 69%. Anal. Calcd for C11H30O2P3ClRu: C, 31.17; H, 7.14. Found: C, 31.15; H, 7.17. 1H NMR (CDCl3): δ 1.40-1.50 (overlapping dd’s, 27H, P(CH3)3), 1.97 (s, 3H, O2CCH3). 31P{1H} NMR (CDCl3): δ 22.2 (t, 1P, P(CH3)3 trans to Cl, 2JP-P = 37.0 Hz), 29.4 (d, 2P, P(CH3)3 trans to η2-OAc, 2JP-P=36.5 Hz). 13 C{1H} NMR (CDCl3): δ 18.53 (d, 3C, P(CH3)3 trans to Cl, 1 JP-C = 29.34 Hz), 19.60 (d, 6C, P(CH3)3 trans to η2-OAc, 1 JP-C = 14.71 Hz), 25.14 (s, 1C, O2CCH3), 186.90 (s, 1C, O2CCH3). X-ray Structure Determination of fac-(PMe3)3RuCl(η2-OAc) (2). Crystals of 2 suitable for analysis by X-ray diffraction were obtained by sublimation of 43 mg of finely powdered complex 1 in a Schlenk tube fitted with a coldfinger, under strong vacuum at 160 C. Data collection was carried out using a Bruker SMART 100010 and Mo KR radiation. No absorption correction was applied. The structure was solved by direct methods (SHELXS-9710) in the space group Pmn21(31). It could not be solved in the alternative space group Pmmn(59). It was refined by full-matrix least-squares on F2 (SHELXL-9710). The structure is disordered with respect to a crystallographic mirror plane that passes through Ru1, Cl1, P1, C7, and C8. Thus, the atoms C1, C2, C3, C4, C5, C6, C40 , C50 , and C60 all have disordered counterparts and are refined at 0.50 occupancy. The H atoms were located on a difference map and fixed for C1, C2, C3, and C8. Other H atoms were riding. All H atoms had isotropic thermal parameters set to 1.2 times that of the equivalent isotropic thermal parameter of the bonded carbon. All (6) Yonker, C. R.; Linehan, J. C. J. Organomet. Chem. 2002, 650, 249. (7) Mainz, V. V.; Anderson, R. A. Organometallics 1984, 3, 675. (8) Peet, W. G.; Gerlach, D. H. Inorg. Synth 1974, 15, 38. (9) Thomas, C. A.; Bonilla, R. J.; Huang, Y.; Jessop, P. G. Can. J. Chem. 2001, 79, 719. (10) Data collection: SMART 1000, v. 5.054. Data reduction: SAINT, v. 6.22; Bruker Analytical X-ray Instruments, Inc.: Madison, WI, 2001. Sheldrick, G. M. SHELXS-97, 2001. Sheldrick, G. M. SHELXTL v. 5.10; Bruker Analytical X-ray Instruments, Inc.: Madison, WI, 1997.

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non-hydrogen atoms were refined with anisotropic thermal parameters. Handedness of the crystal was not determined, as the Flack parameter 0.36(6) indicated the likelihood of racemic twinning. The largest peak in the final difference map was 2.13 e A˚-3, 0.97 A˚ from Ru1.10 Selected details of the crystal data collection and refinement for 2 are listed in Table 1. Synthesis of [(PMe3)4Ru(η2-OAc)]X (X = BPh4-, 3b; X = B(ArF)4-, 3c). In the drybox, 206 mg (0.41 mmol) of cis-(PMe3)4RuCl(OAc) (1) was dissolved in 17 mL of CHCl3 and 5.5 mL of MeOH (24% MeOH/CHCl3 v/v solution). After 20 min of stirring, a solution of NaBPh4 (141 mg, 0.41 mmol) in 3 mL of MeOH was added. The slightly cloudy (NaCl) mixture was removed from the drybox and filtered through glass wool layered with Celite. The pale yellow filtrate was concentrated and the product crystallized with pentane. The solid was further purified by recrystallization from CH2Cl2/pentane at -23 C to yield a yellow crystalline solid (3b, 283 mg, 0.36 mmol, 89% yield). Anal. Calcd for C38H59O2P4BRu: C, 58.23; H, 7.59. Found: C, 57.79; H, 7.60. 1H NMR (CDCl3): δ 1.20 (virtual triplet (vt), 18H, P(CH3)3, 2JP-H = 4.5 Hz), 1.28 (vt, 18H, P(CH3)3, 2JP-H = 3.0 Hz), 1.89 (t, 3H, O2CCH3, 5JP-H = 1.0 Hz), 6.89 (t, 4H, B(p-C6H5)4, 3JH-H=6.9 Hz), 7.04 (t, 8H, B(oC6H5)4, 3JH-H = 7.2 Hz), 7.39 (m, 8H, B(m-C6H5)4). 31P{1H} NMR (CDCl3): δ -1.43 (t, 2P, P(CH3)3, 2JP-P = 29.9 Hz), 21.7 (t, 2P, P(CH3)3, 2JP-P = 29.7 Hz). 13C{1H} NMR (CDCl3): δ 17.02 (vt, 6C, P(CH3)3, 1JP-C=13.2 Hz), 21.45 (vt, 6C, P(CH3)3, 1 JP-C = 17.5 Hz), 24.77 (s, 1C, O2CCH3), 121.88 (s, 4C, B(pC6H5)4), 125.72 (s, 8C, B(m-C6H5)4), 136.56 (s, 8C, B(o-C6H5)4), 164.46 (q, 4C, B(i-C6H5)4, 1JB-C = 48.3 Hz), 186.75 (s, 1C, O2CCH3). 3c was prepared similarly, giving 396 mg (0.30 mmol), 75% yield. Anal. Calcd for C46H51O2P4F24BRu: C, 41.61; H, 3.87. Found: C, 41.47; H, 3.85. 1H NMR (CDCl3): δ 1.34 (vt, 18H, P(CH3)3, 2JP-H =4.5 Hz), 1.37 (vt, 18H, P(CH3)3, 2JP-H =3.3 Hz), 1.91 (t, 3H, O2CCH3, 5JP-H = 1.0 Hz), 7.53 (s, 4H, B(pC6H3(CF3)2)4, 7.71 (m, 8H, B(o-C6H3(CF3)2)4). 31P{1H} NMR (CDCl3): δ -1.45 (t, 2P, P(CH3)3, 2JP-P = 30.2 Hz), 21.6 (t, 2P, P(CH3)3, 2JP-P=30.2 Hz). 19F NMR (CDCl3): δ -67.8 (s). 13 C{1H} NMR (THF-d8): δ 16.82 (vt, 6C, P(CH3)3, 1JP-C = 12.2 Hz), 21.43 (vt, 6C, P(CH3)3, 1JP-C = 18.3 Hz), 24.91 (s, 1C, O2CCH3), 122.08 (d, 8C, B(C6H3(CF3)2)4, 1JF-C=275 Hz), 127.47 (s, 4C, B(p-C6H3(CF3)2)4), 130.18 (m, 8C, B(m-C6H3(CF3)2)4), 135.73 (s, 8C, B(o-C6H3(CF3)2)4, 163.00 (q, 4C, B(i-C6H3(CF3)2)4, 1JB-C = 48.8 Hz), 187.69 (s, 1C, O2CCH3). X-ray Structure Determinations of [(PMe3)4Ru(η2-OAc)]BPh4 (3b) and {[(PMe3)3Ru]2(μ-Cl)2(μ-OAc)}BPh4 (4). Both structures were determined using a Bruker platform diffractometer equipped with an APEX detector operated at 100 K using Mo KR radiation. Diffraction symmetry and systematic absences in the data were used to determine uniquely the space groups. The structures were solved by direct methods and completed from subsequent difference Fourier syntheses. All non-hydrogen atoms were refined with anisotropic thermal parameters, and all hydrogen atoms were treated as idealized contributions. All software used in the collection and analysis of data is contained in the current libraries provided by Bruker AXS (Madison, WI). Selected details of the crystal data collection and refinement for 3b and 4 are listed in Table 1. General Procedure for Monitoring CO2 Hydrogenation in THF by High-Pressure NMR Spectroscopy. The ruthenium compound was weighed into a small vial and brought into the drybox (ca. 8 mg, 0.010 mmol of 3b, or ca. 16 mg, 0.012 mmol of 3c). Then 500 μL of THF or THF-d8 and ca. 10 equiv of DBU were added to the vial, and 200 μL of this solution was dispensed into the PEEK NMR cell, which was protected from the atmosphere through use of tubing/valve assembly. This was attached to a high-pressure gas manifold. Upon insertion of the PEEK cell into the NMR probe, 6.7 bar (100 psi) of H2 was introduced, and the system was heated at 50 C for ca. 3 h. Formation of

Getty et al. Table 1. Crystallographic Data Collection Parameters for 2, 3b, and 4

empirical formula fw temp (K) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) μ (mm -1) radiation, λ (A˚) θ range (deg) index ranges no. of reflns collected final R, Rw [I > 2σ(I)]

2

3b

4

C11H30O2P3ClRu 423.78 91(2) orthorhombic Pmn21 9.985(2) 9.103(2) 10.243(2) 90 90 90 931.0(3) 1.237 Mo KR, 0.71073 2.24-30.02 -14 e h e 14 -12 e k e12 -11 e l e 14 8228

C38H59BO2P4Ru 783.61 100(2) monoclinic P21/n 14.058(11) 18.919(14) 14.875(11) 90 99.348(13) 90 3904(5) 0.597 Mo KR, 0.71073 2.14-26.50 -16 e h e 17 -23 e k e 18 -17 e l e 18 17 598

C44H77BCl2O2P6Ru2 1107.73 100(2) monoclinic P21/c 9.1922(6) 31.816(2) 17.6521(12) 90 91.7780(10) 90 5160.0(6) 0.909 Mo KR, 0.71073 2.22-28.29 -12 e h e 11 -42 e k e 40 -18 e l e 23 32 756

0.0368, 0.0785

0.0631, 0.1498

0.0505, 0.1123

cis-(PMe3)4RuH2 (5) was observed using 31P{1H} NMR and 1H NMR .11 Upon complete conversion of 3b,c to 5, the NMR probe was cooled to -80 C before introduction of carbon dioxide into the cell. The NMR probe was raised to -50 C after addition of carbon dioxide, and within an hour conversion of 5 to 6 (cis-(PMe3)4RuH(O2CH)) was observed by NMR spectroscopy. Production of the [HþDBU][O2CH-] salt was observed after ca. 3 h at 24 C, as well as an unidentified ruthenium product postulated to be {[(PMe3)4Ru]2(μ-OAc)2}X2 (9, X = BPh4-, B(ArF)4-, or O2CH-). The NMR cell was then vented to the atmosphere, and the contents were collected using aliquots of CDCl3 or CH2Cl2 (5 0.5 mL) to rinse the cell. A stream of N2 was used to evaporate the volatiles and produce a pale yellow oil. The oil was dissolved in CDCl3, and proton, phosphorus, and carbon (in the cases of using 13CO2) NMR spectra were recorded. NMR Data for 5. 1H NMR at 50 C (THF-d8): δ -10.0 (br, RuH), DBU signals obscure signals for PMe3. 31P{1H} NMR at 50 C (THF-d8): δ -6.75 (t, 2P, P(CH3)3, 2JP-P=26.0 Hz), 0.98 (t, 2P, P(CH3)3, 2JP-P=26.0 Hz). 1H NMR at -80 C (THF-d8): δ -10.25 (dt, 2H, RuH, 2JHtransP =52.2 Hz, 2JHcisP =30.0 Hz), DBU signals obscure signals for PMe3. 31P{1H} NMR at -80 C (THF-d8): δ -4.93 (t, 2P, P(CH3)3, 2JP-P = 26.7 Hz), 2.61 (t, 2P, P(CH3)3, 2JP-P =26.0 Hz). NMR Data for 6. 1H NMR at -50 C (THF-d8): δ -8.15 (dq, 1H, RuH, 2JHtransP = 95.4 Hz, 2JHcisP = 27.8 Hz), 8.11 (s, 1H, RuO2CH), DBU signals obscure signals for PMe3. 31P{1H} NMR at -50 C (THF-d8): δ -10.66 (m, 1P, P(CH3)3 trans to H, 2JP-P = 19.9 Hz), 1.27 (m, 2P, trans P(CH3)3’s, 2JP-P = 26.7 Hz), 21.07 (m, 1P, P(CH3)3 trans to O2CH, 2JP-P = 19.8 Hz). NMR Data for 9. 1H NMR at 24 C (THF-d8): δ 2.02 (s, O2CCH3), DBU signals obscure signals for PMe3. 31P{1H} NMR at 24 C (THF-d8): δ 2.07 (t, 2P, P(CH3)3, 2JP-P = 29.7 Hz), 17.36 (t, 2P, P(CH3)3, 2JP-P = 30.6 Hz). 1H NMR at 24 C (CDCl3): δ 1.99 (s, O2CCH3), DBU signals obscure signals for PMe3. 31P{1H} NMR at 24 C (CDCl3): δ 0.40 (t, 2P, P(CH3)3, 2JP-P = 29.7 Hz), 16.98 (t, 2P, P(CH3)3, 2JP-P = 29.7 Hz). (11) Gusev, D. G.; H€ ubener, R.; Burger, P.; Orama, O.; Berke, H. J. Am. Chem. Soc. 1997, 119, 3716.

Article


Figure 1. Thermal ellipsoid drawing of fac-(PMe3)3RuCl(η2OAc) (2) showing 50% probability contours. Hydrogen atoms have been omitted for clarity.

Results Solution Equilibrium of cis-(PMe3)4RuCl(OAc) (1). In solution, complex 1 is in equilibrium with two other ruthenium-containing species, both of which have been isolated and characterized by X-ray crystallography.12 The 31P{1H} NMR spectrum of a solution of 1 in CDCl3 contains signals for 1 with the expected A2BC splitting pattern3a as well as signals for free PMe3, fac-(PMe3)3RuCl(η2-OAc) (2), and [(PMe3)4Ru(η2-OAc)]Cl (3a, δ 22.2, overlap t; δ -1.0, t). The approximate relative amounts of the ruthenium compounds, determined by 31P NMR spectroscopy, were 73% 1, 12% 2, and 10% 3a.13 In the presence of excess PMe3 (>10 equiv) the 31 P{1H} NMR spectrum in C6D6 contained only signals expected for cis-(PMe3)4RuCl(OAc) and free PMe3. Complex 2 can be prepared by heating or subliming 1 under vacuum, yielding crystals that were analyzed by X-ray crystallography, NMR spectroscopy, and elemental analysis. The molecular structure of 2 is shown in Figure 1, and the summary of crystal data and details of the structure and its refinement are shown in the Supporting Information.14 The structure of 2 can be best described as a six-coordinate pseudo-octahedron, with some distortion of the chloride and phosphines toward the bidentate acetate ligand. The Ru-P bond length trans to chloride is slightly greater than those trans to acetate (ca. 0.03 A˚) and roughly equal to those found in the Ru-trichloride dimer {[(PMe3)3Ru]2(μ-Cl)3}Cl (see Supporting Information). (12) Attempted crystallizations of complex 1 from methanol or other solvents were unsuccessful, generating only small quantities of crystals of {[(PMe3)3Ru]2(μ-Cl)3}Cl, which were crystallographically characterized (see Supporting Information). Crystals of the same dimer were found in small quantities when trans-(PMe3)4RuCl2 was recrystallized. The dimer cation has been reported previously (with BF4 counterion): Statler, J. A.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1984, 1731. (13) The relative amounts of 1, 2, and 3a were obtained by integration of the 31P{1H} NMR spectra (1 s delay, and assuming the T1 values for the different Ru-bound phosphines are identical) of four different samples of 1 dissolved in CDCl3 (the values shown are averages, and the standard deviation is e1%). In addition to these ruthenium compounds, a small amount of cis-(PMe3)4RuCl2 (ca. 5%) was present in some of the samples, which was carried over from the synthesis of 1 (see ref 7). (14) Complete details of the X-ray crystal structure determination are provided in the Supporting Information.

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Figure 2. Thermal ellipsoid drawing of the cation in [(PMe3)4Ru(η2-OAc)]BPh4 (3b) showing 50% probability contours. Hydrogen atoms and the BPh4- anion have been omitted for clarity.

Addition of methanol to a solution of 2 in CDCl3 leads to the formation of a new species. The 31P{1H} NMR spectrum acquired 10 min after addition of methanol showed small broadened signals for 2 in addition to a sharp intense A2B pattern at δ 23.8 (d) and 20.4 (t) and two very small broad signals at δ 24.5 and 21.4 ppm. The proton NMR signals shift from a single resonance at 1.97 ppm to two singlets at 2.03 and 1.88 ppm in ca. 1:1 ratio for the acetate ligand when CD3OD is added to 2. The PMe3 protons change from an overlapping singlet and doublet between 1.49 and 1.40 ppm to two sets of signals, a doublet at 1.55 ppm (2JP-H = 8.7 Hz) and a virtual triplet at 1.39 ppm, upon addition of CD3OD. The spectroscopic data are consistent with a ruthenium dimer structure {[(PMe3)3Ru]2(μ-Cl)(μ-OAc)2}þ similar to {[(PMe3)3Ru]2(μ-Cl)2(μ-OAc)}þ (complex 4, vide infra). Addition of MeOH to a CDCl3 solution of 1 (from 7 to 75% MeOH) results in conversion of 1 to 3a. The signals in the 31P{1H} NMR spectrum for 1, 2, and free PMe3 disappear, while the signals for 3a increase. In addition to the signals for 3a, there are very small signals due to the species formed upon the reaction of 2 with MeOH in CDCl3 (vide supra). Complex 3a can also be formed in supercritical CO2 from 1 in the presence of MeOH (ca. 30 equiv). Other methods to prepare 3a include using methanol, ethanol, or o-cresol in CDCl3 or C6D6 (25% solution), using nitromethane in CDCl3 (25% solution), or abstracting the chloride with AgBF4 in CDCl3 (in this case the counterion is BF4- rather than Cl-). The formation of 3a was unaffected by the presence of base. Scheme 1 summarizes the different methods we have discovered to prepare complex 3, including the preparative scale method we used to obtain 3b and 3c. Interestingly, using tert-butanol, 2-propanol, 2,6-di-tertbutylphenol, acetone, or acetonitrile (all ca. 25% solutions in CDCl3) resulted in little or no conversion of 1 to 3a. This trend in effectiveness of the alcohols to promote the conversion of 1 to 3 parallels the observed trend in effectiveness of the alcohols to promote the hydrogenation of CO2. Munshi et al. reported that the yield of formic acid was best with C6F5OH, moderate with MeOH or EtOH, and very poor with t-BuOH or 2,6-tBu2C6H3OH as the added alcohol when

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Scheme 1. Various Methods To Prepare [(PMe3)4Ru(η2-OAc)]X (3a, X = Cl-; 3b, X = BPh4-; 3c, X = B(ArF)4-)

using complex 1 and Et3N for the CO2 hydrogenation reaction.3a The two ionic ruthenium complexes, [(PMe3)4Ru(η2OAc)]BPh4 (3b) and [(PMe3)4Ru(η2-OAc)]B(ArF)4 (3c), were prepared on a large scale by dissolving 1 in a 25% solution of MeOH in CHCl3 followed after ca. 20 min by addition of a solution of either NaBPh4 or NaB(ArF)4,15 in MeOH (Scheme 1). The crystalline sample of 3b obtained from a solution of 3b in CH2Cl2 layered with pentane and stored in the freezer (-23 C) for three days contained large block crystals (3b), as well as a small amount of fine needles (4). These two types of crystals were separated. 3b was recrystallized again by dissolving the separated sample in a minimal quantity of CH2Cl2 and cooling to -10 C. The structure of 4 was determined without further recrystallization. The structure of the cation in 3b is shown in Figure 2. Selected bond lengths and angles are listed in the Supporting Information.14 The structure of 3b is best described as a distorted octahedral, as opposed to a trigonal-bipyramidal structure in which the acetate group is viewed as occupying a single coordination site. The Ru-P bonds for trans-related P(1) and P(2) average about 0.1 A˚ longer than those trans to the acetate group. The small volume occupied by the acetate group has allowed the four phosphine ligands to move toward the acetate group; for example, the P(1) and P(2) phosphines are bent toward the acetate group and the P(3)-Ru(1)-P(4) angle has opened to 96.9. In the unit cell, there are no close contacts made between ions. The thermal ellipsoid drawing of the cation in 4 is shown in Figure 3, and the structure is a ruthenium dimer of the form {[(PMe3)3Ru]2(μ-Cl)2(μ-OAc)}BPh4. The relevant crystallographic parameters are found in the Supporting Information.14 (15) B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate.

The coordination geometry at each ruthenium atom of 4 is more nearly that of a regular octahedron. There are no significant differences in the Ru-P distances as a function of trans relationships. Again, there are no close contacts between ions in the unit cell. Close examination of the 31P{1H} NMR spectrum (acetone-d6) of the mixture of 3b and 4 used for the X-ray structure determinations reveals that 4 constitutes approximately 5% of the sample.16 Catalytic Activity of fac-(PMe3)3RuCl(η2-OAc) (2), [(PMe3)4Ru(η2-OAc)]X (3b, X = BPh4-; 3c, X = B(ArF)4-), cis-(P(OEt)3)4RuCl2, and cis-(P(OEt)3)4RuH2. The catalytic activity of complexes 2, 3b, 3c, cis-(P(OEt)3)4RuCl2, and cis-(P(OEt)3)4RuH2 toward the hydrogenation of CO2 was investigated. The catalytic activities of 1 and 2 were compared directly using a multivial reactor.17 In a typical reaction, NEt3 (0.5 mL), the catalyst (3 μmol), and MeOH (0.5 mL) were kept at 50 C under 40 bar of H2 for 1 h prior to the addition of CO2. Upon addition of CO2 (60 bar), the reaction was allowed to proceed for 1 h at 50 C. The formic acid to amine mole ratio (AAR = mol HCO2H/mol NEt3) generated with 1 as the catalyst was 0.50 ( 0.05 (an average of three runs), whereas the AAR using 2 was only 0.13 ( 0.01. Under identical experimental conditions, complex 1 produced almost 4 times as much formic acid as complex 2. The catalytic activities of 1, 3b, and 3c were compared directly using the same multivial reactor described in the experiment above, with and without alcohol. Each vial in the reactor contained CHCl3 (0.5 mL), NEt3 (0.5 mL), and the catalyst (3 μmol), except for one vial, which contained no catalyst (the control). The experiments involving alcohol contained C6F5OH (18.4 mg, 0.1 mmol), and the conditions for temperature, H2, CO2, and reaction times are the same as (16) The relative amount of 3b to 4 in this sample was obtained by integration of the signals for each compound in the 31P{1H} NMR spectrum taken overnight and with a 30 s relaxation delay. (17) For description of the reactor, see ref 3b.

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Figure 3. Thermal ellipsoid drawing of the cation in {[(PMe3)3Ru]2(μ-Cl)2(μ-OAc)}BPh4 (4) showing 50% probability contours. Hydrogen atoms and the BPh4- anion have been omitted for clarity. Table 2. Comparison of Catalytic Activity of 1, 3b, and 3c catalyst 1 3b 3c 1 3b 3c no catalyst

alcohol present (yes/no)

average AARa

standard deviation for AAR

yes yes yes no no no yes

0.12 0.18 0.19 0.06 0.14 0.16 0.00

0.006 0.001 0.000 0.001 0.018 0.047 0.000

a The average AAR is for two runs (AAR = formic acid to amine mole ratio).

described above. Table 2 lists the results. As expected, 3b and 3c perform about equally well compared to each other with or without alcohol present, and both ionic catalysts perform better than 1 in the absence of alcohol. With alcohol present, 1 has slightly less catalytic activity than 3b and 3c. The known compounds cis-(P(OEt)3)4RuCl2 and cis-(P(OEt)3)4RuH28 have substantial solubility in supercritical CO2. However, these complexes demonstrated no activity toward CO2 hydrogenation with or without alcohol. NMR Investigation of CO2 Hydrogenation by 3b,c in scCO2. Experiments were performed with compounds 3b and 3c in which the solids were loaded into the PEEK NMR tube under nitrogen, along with 50 μL of DBU (0.33 mmol, ca. 37 equiv for 3b and ca. 35 equiv for 3c) and 15 μL of C6H6 (for shimming purposes). Hydrogen (20 bar) and carbon dioxide (94 bar) were introduced into the cell, and the probe was heated at 50 C. The 31P{1H} NMR spectrum of the reaction mixture obtained after 30 min of heating contained small signals at δ 21.0 (t) and 5.8 (t), corresponding to a new ruthenium-trimethylphosphine complex (9); however, the S/N ratio of the spectrum was very poor. Unfortunately, the ruthenium compounds and the [HþDBU][O2CH-] salt formed during the reaction were not soluble enough in the supercritical CO2/H2 mixture to obtain good-quality 31P{1H} or 1H NMR spectra. After heating the mixture for ca. 2 h, the NMR probe was cooled to room temperature and the PEEK tube vented to the atmosphere. The contents of the tube were transferred to a vial, the tube was rinsed several times with CH2Cl2, and the volatiles were removed under a flow of nitrogen. The resulting yellow oil was dissolved in

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CDCl3, and NMR spectra were recorded. The 1H NMR spectrum had a large signal at 8.7 ppm (s) corresponding to the formate group of [HþDBU][O2CH-] and a broad signal at 11.2 ppm corresponding to the protonated base, HþDBU. In addition, the spectrum had signals for DBU, PMe3, and OAc- ranging from 1.4 to 3.5 ppm and signals for B(ArF)4ranging from 7.5 to 7.8 ppm. The only signals in the 31P{1H} NMR spectrum were two triplets at δ 16.8 and 0.3, corresponding to the new ruthenium product 9. Since the ruthenium compounds have poor solubility in the supercritical CO2/H2 mixture, the remainder of the NMR investigation was performed with THF or THF-d8 as the solvent. NMR Investigation of the Reaction of 3b,c with H2 in THF. In a typical experiment, 3b or 3c was dissolved in THF or THF-d8 (ca. 0.02 M solutions) in the drybox and ca. 10 equiv DBU was added. A 200 μL aliquot of this solution was dispensed into the PEEK NMR tube. Upon insertion of the PEEK tube into the NMR probe, hydrogen (6.7 bar) was introduced and NMR spectra were recorded. Figure 4 illustrates the 31P{1H} NMR spectra obtained at various times during the production of formic acid catalyzed by 3c in the presence of DBU as the base. The following NMR descriptions are for 3b, and there are no significant differences in the NMR spectra for the experiments in which 3c was used as the catalyst. Without heating above room temperature, no reaction occurs between 3b (or c), H2, and DBU over the course of a day (Figure 4a). After only 20 min at 50 C, a significant amount of the starting material (3b or 3c) converts to two compounds, each having a 31 1 P{ H} NMR A2B2 splitting pattern (Figure 4b). One of the compounds was the known species cis-(PMe3)4RuH2 (5),11 which has two triplets at δ 1.0 and -6.7 in the 31P{1H} NMR spectrum. The other compound, represented by two triplets at δ 17.1 and 1.9 ppm, was an unidentified Ru-PMe3 compound that never exceeds ca. 10% of the total 31P signal. In addition to the signals for these ruthenium compounds and the starting material, there is also a small signal that corresponds to free PMe3. As the reaction proceeds, the intensity of the signals for 3b or 3c decrease, while those for 5 increase and those for the unidentified Ru-PMe3 compound and free PMe3 disappear after ca. 3 h at 50 C. The 1H NMR spectrum at this point has a broad signal at -10.0 ppm, consistent with the reported NMR data for cis-(PMe3)4RuH2 (5). In addition, the H2 dissolved in THF was observed at 4.3 ppm. No conversion of 3b or 3c to 5 occurs in the absence of base, even after 7 h at 50 C. The 1H NMR spectrum of 5 at -80 C is nearly the same as that at 50 C, with slight changes in some of the chemical shifts, and the Ru-hydride signal at -10.25 ppm has become a sharp doublet of quartets. The 31P{1H} NMR spectrum contains only resonances for 5; however the signals are shifted downfield by ca. 1 ppm compared to the spectrum recorded at 50 C. NMR Investigation of the Reaction of 5 with CO2 and H2. Upon complete formation of 5, the NMR probe was cooled to -80 C and CO2 (27 bar) was added. The NMR spectra recorded before and after the addition of CO2 (27 bar) to the PEEK tube at -80 C were identical. However, raising the probe temperature to -50 C results in conversion of 5 to a species with an A2BC splitting pattern in the phosphorus NMR spectrum within 1 h (Figure 4c). The phosphorus NMR data for this new compound (δ 21.0, m; 1.3, m; -10.6, m in a 1:2:1 ratio) is consistent with the NMR data reported

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Figure 4. THF-d8.

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P{1H} NMR spectra of [(PMe3)4Ru(η2-OAc)]B(ArF)4 (3c) combined with DBU and H2 (6 bar), then CO2 (27 bar), in

for cis-(PMe3)4Ru(O2CH)H (6).3d Changes in the proton NMR spectrum occur in the hydride and the formate regions. The signal for the Ru-H was -8.15 ppm (dq), while the signal corresponding to the Ru-O2CH appeared at 8.11 ppm (s). This experiment was repeated using 13CO2. The signal corresponding to the Ru-O213CH functionality appeared as a doublet centered at 8.11 ppm (1JC-H = 188 Hz) in the 1H NMR spectrum. In the 13C{1H} NMR spectrum, the signal corresponding to the Ru-O213CH group appeared as a singlet at 174.0 ppm, and this signal became a doublet (1JC-H = 190 Hz) when the spectrum was recorded with the proton-decoupler turned off. Similar results were obtained for 3c; however only one side of the doublet assigned as the Ru-O213CH group was observed in the 1H NMR spectrum when using 13CO2 due to overlap with the B(ArF)4- signal at 7.7 ppm. The NMR probe temperature was raised to 0 C, following which 31P{1H} NMR spectra were recorded at 5 min intervals for ca. 30 min. The intensities of the signals for complex 6 decreased as those corresponding to several intermediates increased. After 15 min at 0 C, there were four sets of triplets corresponding to four Ru-phosphine compounds with A2B2 splitting patterns in approximately equal concentrations to each other and to 6. Three possible structures consistent with such patterns are cis-(PMe3)4Ru(O2CH)2 (7), [(PMe3)4Ru(η2-O2CH)]O2CH (8), and {[(PMe3)4Ru]2(μ-OAc)2}X2 (9, X = BPh4-, B(ArF)4-, or O2CH-) (see Discussion). In addition, there were two other sets of signals corresponding to Ru-phosphine complexes with A2B2 splitting patterns in lower concentrations. The temperature of the NMR probe was raised to 24 C, and within ca. 3 h all of the Ru-phosphine species converged to a single ruthenium product, 9, with an A2B2 splitting pattern in the 31P{1H} NMR spectrum (δ 17.3, t; 2.1, t,

Figure 4d). The 1H NMR spectrum no longer contained the signal at -8.15 ppm for the Ru-H group, and no other signals were observed upfield of 0 ppm. The singlet at 8.11 ppm corresponding to the Ru-O2CH functionality is no longer present, and the only signal in this region (8.65 ppm, s) is assigned to the formate proton of the [HþDBU][O2CH-] salt.18 The small singlet at 4.3 ppm is no longer present in the spectrum, indicating that the hydrogen has been consumed. When 13CO2 was used, the signal for the carbon-bound formate proton in the 1H NMR spectrum of the [HþDBU][O213CH-] salt appeared as a doublet centered at 8.5 ppm (1JC-H = 181 Hz). The corresponding 13C{1H} NMR spectrum contained a singlet at 167.8 ppm corresponding to the [HþDBU][O213CH-] salt, which became a doublet (1JC-H = 183 Hz) when the proton-decoupler was turned off. Since the formate regions of the proton and carbon (in the cases of using 13CO2) NMR spectra of the final reaction mixture contained only the corresponding signals for the [HþDBU][O2CH-] salt, 9 is most likely not a Ru-formate complex. A possible structure for 9 is a Ru-acetate complex of the form {[(PMe3)4Ru]2(μ-OAc)2}X2 (X = BPh4-, B(ArF)4-, (18) This assignment is based on the 1H NMR spectrum of the [HþDBU][O2CH-] salt prepared independently. Into a 50 mL beaker was dispensed 1.5 mL of DBU (0.010 mol), 10 mL of Et2O, and then 0.48 mL of formic acid (0.012 mol). The cloudy mixture was stirred vigorously for ca. 10 min. Upon letting the mixture stand for 5 min, an oil separated from the Et2O. The ether was decanted and the oil was washed with fresh ether (3 5 mL). The beaker was placed under vacuum overnight to remove residual Et2O and H2O. The resulting white solid was stored in the drybox (1.41 g, 7.11 mmol, 71% yield). 1H NMR (THF-d8): δ 1.65-1.73 (m, 6H, DBU), 1.94 (m, 2H, DBU), 2.89 (m, 2H, DBU), 3.35 (m, 2H, DBU), 3.46 (m, 2H, DBU), 3.52 (m, 2H, DBU), 8.46 (s, 1H, O2CH). 13C{1H} NMR (THF-d8): δ 20.84 (s, 1C, DBU), 25.99 (s, 1C, DBU), 28.03 (s, 1C, DBU), 30.16 (s, 1C, DBU), 32.30 (s, 1C, DBU), 38.91 (s, 1C, DBU), 49.30 (s, 1C, DBU), 54.45 (s, 1C, DBU), 166.87 (s, 1C, DBU), 167.16 (s, 1C, O2CH).

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or O2CH-). The proton and phosphorus NMR spectra described above are identical to the ones obtained by performing the catalysis reaction with 3b,c in supercritical CO2 (vide supra). In addition, complex 9 is obtained by releasing the pressure on the PEEK NMR cell after conversion of 3b,c to 5, without the addition of CO2, further supporting our assignment of 9 as a Ru-acetate complex rather than a Ruformate. When CO2 was added to the PEEK cell containing the ruthenium-dihydride (5) prepared in situ, and the temperature was kept at 50 C, complex 6 was never observed by phosphorus NMR, as 5 was rapidly converted to 9 and [HþDBU][O2CH-] was produced (the reaction was complete within 1 h). Free PMe3 was observed in the phosphorus NMR spectra recorded during this conversion period (in either THF or CO2 solvent). In addition, a small amount of trimethylphosphine oxide was observed as a singlet at δ 33.1. After the reaction mixture was left overnight at room temperature to confirm that no further changes were occurring, the PEEK NMR cell was vented to the atmosphere. The contents of the cell were removed by pipet, the cell was washed with five 0.5 mL aliquots of CH2Cl2 or CHCl3, and the volatiles were removed from the collected solution under a flow of nitrogen. The pale yellow oily residue was dissolved in CDCl3, and NMR spectra were recorded. The 31P{1H} NMR spectrum contains two large triplets at δ 17.0 and 0.40 (9) and a small singlet at δ 39.2 (OPMe3). This spectrum is slightly different than the one collected before the PEEK cell was vented to the atmosphere. The 31P{1H} NMR spectrum recorded while the solution was still under CO2 pressure contained the two triplets with a separation (Δδ) of 15.3 ppm (in THF). However, the two triplets in the spectrum obtained after release of the CO2 and washes with CDCl3 have a Δδ of 16.6 ppm (in CDCl3). The difference in the Δδ values for these spectra is small, and there are no significant differences in the proton or carbon NMR spectra taken in THF or THFd8 before venting the cell versus the spectra taken in CDCl3 after venting and workup in air; therefore, we conclude that 9 remains unchanged. The signals for DBU and PMe3 range from 1.3 to 3.3 ppm in the 1H NMR spectrum, and the signal corresponding to OAc- is a singlet at 1.99 ppm. The broad signal that was once at ca. 11 ppm (HþDBU) moved upfield to ca. 4 ppm. The carbon-bound formate proton of the [HþDBU][O2CH-] salt appears as a singlet at 8.75 ppm; however, this signal appeared as a doublet centered at 8.78 ppm (1JC-H = 188 Hz) in the experiments where 13CO2 was used. For these experiments, carbon NMR spectra were recorded and the formate signal appears as a singlet at 169.4 ppm, whereas it appears as a doublet centered at 169.4 ppm (1JC-H = 190 Hz) when the proton-decoupler is turned off. The NMR experiment was also performed using D2 instead of H2. The ruthenium-bound deuterium atoms of 5-d2 appear as a multiplet at -9.9 ppm at -80 C in the 2H NMR spectrum (THF). After addition of CO2 at -80 C, raising the probe to -50 C for 1 h, then lowering the temperature back to -80 C in order to acquire the 2H NMR spectrum without further reactivity, the Ru-D signal of 6-d2 appears as a multiplet at -8.2 ppm and the Ru-O2CD signal appears as a singlet at 8.6 ppm. After sitting overnight at 24 C, the sample’s 2H NMR spectrum shows a signal at 8.1 ppm for the O2CD of the [HþDBU][O2CD-] salt. This signal moved to 8.6 ppm when the contents of the PEEK


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NMR tube were removed and analyzed in CH2Cl2 by 2H NMR. The 31P{1H} NMR spectra recorded during this experiment were identical to those described above when using H2 rather than D2. NMR Investigation of the Reactivity of 9. The CDCl3 was evaporated from the product mixture obtained originally from the reaction of 3b with H2, DBU, and CO2 in THF, and the resulting pale yellow oily residue was redissolved in 0.5 mL of THF-d8 under an inert atmosphere. DBU was added (15 μL, 0.1 mmol), 200 μL of this solution was dispensed into the PEEK NMR cell, and the cell was then sealed. Hydrogen (6.7 bar) was added to the cell once it was inserted into the NMR probe, and the probe was heated to 50 C while collecting NMR spectra to observe the progress of the reaction. Complex 9 was converted to cis-(PMe3)4RuH2 (5) within ca. 2 h. The NMR probe was cooled to -80 C, CO2 was added (10 bar), and the probe was raised to -50 C. The reaction was monitored by 31P{1H} NMR spectroscopy for 1 h until all of 5 had been converted to cis-(PMe3)4RuH(CO2H) (6). Finally, the probe was raised to room temperature, and the ruthenium product 9 was observed once again by 1H and 31P{1H} NMR spectroscopy. Reaction of 3b with d9-PMe3. The exchange of phosphine ligands of 3b was confirmed by running a series of experiments with d9-PMe3 and 3b containing protonated PMe3 in THF. In all cases, the 31P{1H} signals of the free phosphines contained both deuterated and protonated PMe3. NMR Investigation of CO2 Hydrogenation in the Presence of Excess PMe3. A solution containing 3b, 10 equiv of DBU, and 20 equiv of PMe3 in THF-d8 was pressurized with 6.7 bar of H2 in a PEEK NMR cell and heated to 50 C. Significant conversion of the starting material occurred within 30 min to give two new Ru-phosphine complexes as observed by 31P{1H} NMR spectroscopy, the expected cis-(PMe3)4RuH2 complex (5) and the other assigned as [(PMe3)5RuH]BPh4 (10) (-9.0 ppm, d, 4P; -22.3 ppm, quintet, 1P; 2JPP = 26 Hz) based on the known compound [(PMe3)5RuH]PF6 (-8.5 ppm, d; -21.9 ppm, quintet; 2JPP = 26 Hz in acetone-d6).19 The Ru-H signal for 10 appeared at -11.5 ppm (dq) in the 1H NMR spectrum (-11.3 ppm according to the literature19). Complexes 5 and 10 appeared to reach equilibrium after ca. 5 h at 50 C and were in a 1:1 ratio. The probe temperature was lowered to -80 C, and CO2 (27 bar) was added to the cell. No reactivity was observed until the NMR probe temperature was raised from -80 to -50 C. Within 15 min, a significant amount of cis-(PMe3)4RuH2 (5) had converted to cis-(PMe3)4RuH(O2CH) (6) as expected; however, very little of complex 10 appeared to react. Over the course of ca. 3 h at -50 C, the concentration of complex 10 diminished appreciably but remained observable in the 31P{1H} NMR spectrum. Upon warming to 24 C, the expected ruthenium product, 9, was the only Ru-phosphine complex observed. The contents of the NMR cell were dissolved in CDCl3 after the gases were vented. The 31P{1H} NMR spectrum showed both complex 9 and complex 10 in a ratio of 1.4:1 (9:10). The 1H NMR spectrum contained signals for [HþDBU][O2CH-] and 9, as well as a Ru-H signal corresponding to 10. (19) (a) Werner, H.; Werner, R. J. Organomet. Chem. 1981, 209, C60– C64. (b) Kayaki, Y.; Ikeda, H.; Tsurumaki, J.-I.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 2008, 81, 1053–1061.

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Scheme 2. Proposed Mechanism for Generation of 5 from 3b,c, H2, and a Base (DBU)

Discussion Mechanism of CO2 Hydrogenation Catalyzed by the RuII Compounds 3b,c, 5, and 10. Generation of cis-(PMe3)4RuH2 (5) from 3b,c. Scheme 2 outlines our proposed mechanism for the formation of the CO2 hydrogenation catalyst precursor, 5, from the [(PMe3)4Ru(η2-OAc)]þ cation. While the PMe3 ligands of 3b,c are relatively labile, as demonstrated by the exchange reaction with d9-PMe3 at room temperature, we believe a pathway involving phosphine loss is not the predominant mechanism for the conversion of 3b,c to 5. The rate of 5 formation in the presence of excess PMe3 was not significantly slower than in the absence of added PMe3, and a significant rate decrease should have been observed if phosphine loss from 3b,c was required in making the Ru-dihydride, 5. This result is consistent with the results published by Jessop et al. for the hydrogenation of CO2 with various RuII-phosphine complexes in supercritical carbon dioxide in which the presence of excess phosphine (6-20 equiv) did not affect the yield of formic acid obtained after 30 min.3b,3d,3g,9 Instead of phosphine loss from 3b,c, we propose that hydrogen binds to the ruthenium center at a site made available by the opening of the acetate chelate to form intermediate A. The η2-dihydrogen ligand is relatively acidic and can be deprotonated by the relatively basic acetate ligand to give the unsaturated RuII-hydride intermediate B and acetic acid, which is trapped by the DBU. Intermediate B has a vacant site that would allow another dihydrogen molecule to bind to the ruthenium center and produce the RuII-η2-dihydrogen-hydride intermediate C. The reversible binding and loss of H2 through a mechanism similar to B and C has been reported.11 The DBU present in the solution could deprotonate the η2-dihydrogen ligand to form cis-(PMe3)4RuH2 (5) and the [HþDBU][X-] salt. There are numerous examples of syntheses of RuII-hydride complexes using H2 and a base.20 For instance, Cappellani et al. reported the first example of two successive heterolytic cleavages of hydrogen where the Ru-η2-dihydrogen intermediates were identified, and the proposed mechanism is similar to the one shown in Scheme 2.20a The cis-(depe)2RuCl2 (depe = Et2PCH2CH2PEt2) complex reacts with 1 equiv of NaBPh4 and 1 atm of H2 in THF to form (20) (a) Cappellani, E. P.; Maltby, P. A.; Morris, R. H.; Schweitzer, C. T.; Steele, M. R. Inorg. Chem. 1989, 28, 4437. (b) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.; Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991, 113, 4876. (c) Mezzetti, A.; Del Zotto, A.; Rigo, P.; Farnetti, E. J. Chem. Soc., Dalton Trans. 1991, 1525. (d) Frediani, P.; Faggi, C.; Salvini, A.; Bianchi, M.; Piacenti, F. Inorg. Chim. Acta 1998, 272, 141.

[(depe)2RuCl(η2-H2)]þ as the major product. Adding 1 equiv of a base (OR- or Et3N) results in the formation of [(depe)2RuH(η2-H2)]þ, and further addition of base gives (depe)2RuH2. There have also been several studies related to the acidity of metal-bound dihydrogen,11,21 and the pKa of [(PMe3)4RuH(η2-H2)]þ (intermediate C in Scheme 2) has recently been reported to be 16.6 on the THF scale and 11.2 on the aqueous scale.21a Although we propose that intermediate B is formed by intramolecular deprotonation of A by OAc-, it is also possible that intermediate B is formed by intermolecular deprotonation of intermediate A by DBU followed by dissociation of acetate. The proposed mechanism in Scheme 2 is consistent with the fact that no reaction occurs between 3b,c, H2, and CO2 in the absence of base. The base is required not only to trap the formic acid product that is formed during this catalytic process, but also to generate the RuII-dihydride complex, 5. Mechanism of Insertion of CO2 into the Ru-H Bond of 5. The insertion of CO2 into one of the Ru-H bonds of 5 to generate cis-(PMe3)4RuH(O2CH) (6) was observed even at -50 C. It therefore has a very low activation energy. There are several known examples of stoichiometric insertion of CO2 into RuII-H bonds.22 For complex 5, we can envisage three plausible mechanisms for the insertion: (a) direct concerted reaction of the Ru-H bond with CO2 via “σ-bond metathesis”23 without prior coordination of CO2 to the complex, (b) loss of a phosphine ligand followed by CO2 coordination and then insertion, or (c) effective loss of a hydride (by protonation and then H2 loss). Each of these three possibilities will now be assessed. (21) (a) Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000, 122, 9155. For reviews, see: (b) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155. (c) Heinekey, D. M.; Oldham, W. J., Jr. Chem. Rev. 1993, 93, 913. (22) (a) Komiya, S.; Yamamoto, A. J. Organomet. Chem. 1972, 46, C58. (b) Kolomnikov, I. S.; Lobeeva, T. S.; Vol'pin, M. E. Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.) 1972, 2263. (c) Kolomnikov, I. S.; Gusev, A. I.; Aleksandrov, G. G.; Lobeeva, T. S.; Struchkov, Yu. T.; Vol'pin, M. E. J. Organomet. Chem. 1973, 59, 349. (d) Jia, G.; Meek, D. W. Inorg. Chem. 1991, 30, 1953. (e) Whittlesey, M. K.; Perutz, R. N.; Moore, M. H. Organometallics 1996, 15, 5166. (f) Reference 3d. (g) Yin, C.; Xu, Z.; Yang, S.-Y.; Ng, S. M.; Wong, K. Y.; Lin, Z.; Lau, C. P. Organometallics 2001, 20, 1216. (23) For examples of CO2 insertion into the Ru-H bond in which the proposed mechanism is direct attack of the Ru-H moiety on CO2, see refs 22e-22g. For reviews discussing the mechanisms of CO2 insertion into M-H bonds (M = transition metal) in relation to transition metalcatalyzed homogeneous CO2 hydrogenation, see ref 5.

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The first mechanism, CO2 insertion without prior coordination, was evaluated by Musashi and Sakaki using DFT calculations on the (PH3)4RuH2 complex as a model for 5.24 Their calculations indicated that the activation energy, Ea, for CO2 insertion into the Ru-H bond of the six-coordinate (PH3)4RuH2 complex was much higher (29.3 kcal/mol) than the Ea for CO2 insertion into the Ru-H bond of the fivecoordinate (PH3)3RuH2 complex that is generated by phosphine dissociation (10.3 kcal/mol). The conclusion was that generation of an empty site is necessary for the CO2 insertion reaction. Our low-temperature spectroscopic measurements confirm that the reaction must have a very low activation energy. We therefore consider CO2 insertion without an empty site an unlikely mechanism. The second plausible mechanism is phosphine loss followed by CO2 coordination and then insertion. This is the mechanism that was first proposed for this catalytic system.3d However, Jessop et al. reported previously that adding 20 equiv of PMe3 to cis-(PMe3)4RuH2 in supercritical carbon dioxide for the CO2 hydrogenation reaction had essentially no effect; even a very large amount of added PMe3 (13 000 equiv, enough for a solvent effect to appear) only slowed the rate of reaction and did not stop it.3d We also found (vide supra) that the insertion of CO2 into the Ru-H bond of 5 is not significantly inhibited, even at -50 C, by the presence of a 20-fold excess of PMe3. Thus we conclude that CO2 insertion into the Ru-H bond of electron-rich complexes (such as 5) does not involve loss of a phosphine ligand. The third plausible mechanism is generation of an open site by effective loss of a hydride. This could take place by protonation of one of the hydride ligands of 5 by protonated DBU, generating a RuII-η2-dihydrogen-hydride intermediate (C) that would lose H2 to make a site available for CO2 (the sequence 5 f C f B in Scheme 2 and 5 f B f E f F f 6 in Scheme 3). This mechanism is consistent with Musashi and Sakaki’s conclusion that generation of an empty site is required for CO2 insertion, but, unfortunately, those authors did not evaluate this “hydride-loss” route for the generation of the empty site. We know that such a proton transfer from HþDBU to 5 is feasible because [(PMe3)4Ru(η2-H2)H]þ (pKa 16.6 on the THF scale)20a and HþDBU (pKa 16.8 on the same scale)25 are reported to have almost identical acidity. The analogous reaction of 5 with [NH4]PF6 to give [(PMe3)4Ru(NH3)H]þ is known in the literature.26 The hydride-loss pathway, too, could be inhibited by the presence of excess PMe3, by the conversion of 5 via B to [(PMe3)5RuH]þ. To what extent, then, would one expect added PMe3 to inhibit this pathway? Fortunately, the equilibrium between 5 and [(PMe3)5RuH]þ seems to favor 5 except at very high PMe3 concentrations, whereas the equilibrium between [(PMe3)3RuH2] and 5 favors 5 even in the absence of added PMe3. Therefore one would expect that small amounts of added PMe3 would not significantly inhibit the hydride-loss pathway nearly as readily as they would the phosphine-loss pathway. For these reasons, we conclude that the hydride-loss mechanism is consistent with the observations and Musashi and Sakaki’s conclusion that an empty site is required. The other two mechanisms are considered much less likely. (24) Musashi, Y.; Sakaki, S. J. Am. Chem. Soc. 2000, 122, 3867. (25) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (26) Rappert, T.; Yamamoto, A. Organometallics 1994, 12, 4984– 4993.


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Catalytic Production of Formic Acid. The insertion of CO2 into a Ru-H bond of 5 forms a key part of the mechanism for the hydrogenation of CO2 catalyzed by this complex. While the published mechanism3d,8 shows CO2 insertion after phosphine loss, that mechanism is no longer considered likely, as discussed above. The new “hydride-loss” mechanism for CO2 insertion should then be incorporated into the proposed catalytic cycle for CO2 hydrogenation (as shown in Scheme 3). The generation of the active catalyst B from the ruthenium-dihydride, 5, must be possible because the NMR experiments were designed such that 5 was allowed to form from 3b,c in situ before CO2 was added to the system. However, in the catalytic experiments 3b,c likely reacts with H2 and CO2 in the presence of a base to produce formic acid without generating 5. Instead, once intermediate B forms, it can react with CO2 to generate E, then the CO2 can insert into the Ru-H bond to give the intermediate F, which reacts with H2 to produce D, and D undergoes an intramolecular deprotonation of the η2-dihydrogen ligand by the formate ligand to release formic acid and regenerate B (Scheme 3). Thus, under many of the reactor-type conditions used to study the yield of formic acid produced via complexes 1 or 3b,c, compounds 5 and 6 may not be intermediates in the mechanism. Transient species B, E, F, and D in the proposed cycle are not detected spectroscopically, but are in equilibrium with more stable coordinatively saturated complexes. Due to the transient nature of these intermediates, none of them were detected during the NMR experiments. This mechanism involves a cationic ruthenium(II) species as the active catalyst, which is similar to the proposed mechanism for CO2 hydrogenation via the cationic rhodium complex [(PMe2Ph)3RhH2(solv)]BF4 (solv = H2O or THF).27 During the experiments in which excess PMe3 was present, CO2 was added to the 1:1 mixture of cis-(PMe3)4RuH2 (5) and [(PMe3)5RuH]BPh4 (10) at -50 C. Complex 10 appeared to react more slowly than 5 and did not completely convert to 6. This lower reactivity of 10 with CO2 is consistent with our proposal that insertion of CO2 into a Ru-H bond requires an empty site at the Ru. In the catalytic experiments described here, and in those performed in supercritical CO2, the concentration of added phosphine (20 equiv) is not large enough to successfully compete for B with the large concentration of CO2 (the mole fraction of CO2 in THF at 27 bar is greater than 10%). Only in a situation where the added phosphine concentration is closer to that of CO2 would formation of [(PMe3)5RuHþ] be expected with an apparent decrease in the rate of CO2 hydrogenation. Therefore we would expect that the formation of [(PMe3)5RuH]þ would inhibit the insertion of CO2 by the hydride-loss mechanism only if the concentration of PMe3 is very high, especially if it is higher than the concentration of CO2. In addition, we propose that the formic acid is released from the six-coordinate intermediate D (Scheme 3) via a σbond metathesis pathway, rather than reductive elimination of HO2CH from a ruthenium-hydrido-formate complex followed by H2 oxidative addition to regenerate 5. This is in agreement with the calculations presented by Musashi and Sakaki as the lowest energy mechanism for formic acid release. (27) Tsai, J.-C.; Nicholas, K. M. J. Am. Chem. Soc. 1992, 114, 5117.

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Scheme 3. Proposed Mechanism for CO2 Hydrogenation into Formic Acid via 5 in the Presence of a Base (DBU)

The lack of reactivity of cis-(P(OEt)3)4RuH2 toward CO2 insertion could be a result of the electron-poor nature of this complex that does not allow for protonation of the Ru-H moiety by DBUHþ. The phosphite ligands do not appear to be labile,28 thus the open site required at the electron-poor ruthenium center cannot be made available for CO2 coordination when using cis-(P(OEt)3)4RuH2. Fate of the Catalyst after Termination of the Catalysis. The termination of the catalysis occurs when the H2 and DBU (the limiting reagents in the NMR experiments) are consumed. In the absence of H2, the catalysis should stall at species F (Scheme 3), which is likely in equilibrium with the coordinatively saturated species [(PMe3)4Ru(η2-O2CH)]X (X = BPh4-, B(ArF)4-, or O2CH-). The 31P{1H} NMR spectrum of the ruthenium-containing product after catalysis has an A2B2 splitting pattern for the phosphines, which is consistent with the [(PMe3)4Ru(η2-O2CH)]X structure but has no additional formate signals in the 1H NMR or 13C NMR spectra other than those for [HþDBU][O2CH-]. To explain this, we suggest the Ru-coordinated formate ligand (28) Gerlach et al. have reported the reaction chemistry of various sixcoordinate RuII-dihydride complexes containing phosphine or phosphite ligands ( Gerlach, D. H.; Peet, W. G.; Muetterties, E. L. J. Am. Chem. Soc. 1972, 94, 4545). They observe that exchange reactions are much more facile at the RuII-phosphine complexes compared to the RuII-phosphite compounds. We obtained similar results during an attempt to prepare 5 from cis-(P(OEt)3)4RuH2 and PMe3. Since PMe3 is a much better donor compared to P(OEt)3, exchange of the phosphites for the phosphines at the RuII center was expected to occur. However, combinations of either a PMe3/hexane solution of cis-(P(OEt)3)4RuH2 or a neat PMe3 solution of cis-(P(OEt)3)4RuH2 and heating for four days at 85 C resulted in no changes in the appearance of the solutions or the NMR spectra.

Scheme 4. Proposed Termination of the Catalytic Cycle and Structure of the Ruthenium Product {[(PMe3)4Ru]2(μ-OAc)2}X2 (9, X = BPh4-, B(ArF)4-, or O2CH-)

of [(PMe3)4Ru(η2-O2CH)]X exchanges with the more basic acetate anion of the [HþDBU][OAc-] salt to generate the ruthenium dimer {[(PMe3)4Ru]2(μ-OAc)2}X2 (9) and [HþDBU][O2CH-] (Scheme 4). We do not observe the Ru-acetate complex 3b,c as the ruthenium product of the CO2 hydrogenation reaction, but the NMR spectra at the termination of the reaction indicate

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the ruthenium product (9) is very similar to 3. Complex 9 is stable to the atmosphere and high-vacuum, as are 3b and 3c, and 9 can be obtained by releasing the pressure on the PEEK cell upon formation of the Ru-dihydride (5) when no CO2 has been added to the system. Thus, we suggest that the ruthenium product (9) is the ruthenium-acetate dimer shown in Scheme 4. The difference in energy between 3 (kinetic product) and 9 (thermodynamic product) is expected to be very small, and 9 may be slightly lower in energy due to less strain in the eight-membered Ru-OAc-Ru-OAc ring versus the four-membered Ru-OAc ring in 3. Acetate ligands are known to act as bridges between two ruthenium centers,29 and we observed such a bridge in the crystal structure of 4 (see Figure 3). Our attempts to isolate and characterize 9 have been unsuccessful. However, we have observed that the product mixture containing 9 will facilitate CO2 hydrogenation when fresh DBU, H2, and CO2 are added. The NMR spectra obtained before catalysis show that the rutheniumdihydride (5) is formed initially after ca. 2 h at 50 C in the absence of CO2, and the ruthenium-hydrido-formate (6) is generated at low temperature upon addition of CO2, just as if complex 3b,c had been used as the starting material. These results are consistent with our proposal that the final ruthenium-containing product, 9, is very similar to 3.

Conclusions One role of the alcohol cocatalyst in the CO2 hydrogenation to formic acid system using cis-(PMe3)4RuCl(OAc) (1) and a base is to facilitate the isomerization of 1 to [(PMe3)4Ru(η2-OAc)]Cl (3a). The chloride ion of 3a was exchanged for noncoordinating ions to produce the airstable catalyst precursors [(PMe3)4Ru(η2-OAc)]BPh4 (3b) and [(PMe3)4Ru(η2-OAc)]B(ArF)4 (3c). These complexes proved to perform as well as 1 in the presence of an alcohol cocatalyst and, more importantly, in the absence of added alcohol. Removing the alcohol cocatalyst from this system eliminates the side reaction between base, CO2, and alcohol to form alkyl carbonate salts. (29) For examples of complexes that have been characterized by X-ray crystallography containing at least two ruthenium centers bridged by OAc-, see: (a) Collin, J.-P.; Joouaiti, A.; Sauvage, J.-P.; Kaska, W. C.; McLoughlin, M. A.; Keder, N. L.; Harrision, W. T. A.; Stucky, G. D. Inorg. Chem. 1990, 29, 2238. (b) Binamira-Soriaga, E.; Keder, N. L.; Kaska, W. C. Inorg. Chem. 1990, 29, 3167. (c) Barral, M. C.; Jimenez-Aparicio, R.; Kramolowsky, R.; Wagner, I. Polyhedron 1993, 12, 903. (d) Mintert, M.; Sheldrick, W. S. Inorg. Chim. Acta 1995, 236, 13. (e) Tanase, T.; Yamada, Y.; Tanaka, K.; Miyazu, T.; Kato, M.; Lee, K.; Sugihara, Y.; Mori, W.; Ichimura, A.; Kinoshita, I.; Yamamoto, Y.; Haga, M.; Sasaki, Y.; Yano, S. Inorg. Chem. 1996, 35, 6230. (f) Ota, K.; Sasaki, H.; Matsui, T.; Hamaguchi, T.; Yamaguchi, T.; Ito, T.; Kido, H.; Kubiak, C. P. Inorg. Chem. 1999, 38, 4070.

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Using high-pressure NMR spectroscopy and 3b,c as precatalysts for CO2 hydrogenation in the presence of a base (DBU), we have determined that the likely active catalyst is an unsaturated, cationic ruthenium complex of the form [(PMe3)4RuH]þ (B). The active catalyst can be generated via cis-(PMe3)4RuH2 (5) by reaction of 3b,c with H2 and DBU at elevated temperature. The formation of B does not require phosphine dissociation from 3b,c or 5. No reaction occurs without the DBU present, indicating it is involved not only in trapping the formic acid product but also in the generation of the catalyst precursor 5 when 3b,c reacts with H2 in the absence of CO2 or in the generation of the catalyst B when 3b,c reacts with a mixture of H2 and CO2. We suggest that CO2 insertion into the Ru-H bond involves precoordination of CO2 to the metal. The cis-(P(OEt)3)4RuCl2 and cis-(P(OEt)3)4RuH2 complexes were completely inactive toward CO2 hydrogenation. Likely reasons are the electron-withdrawing nature of the phosphite ligands and their inability to dissociate, causing the Ru-phosphite complexes to be unable to create an open site by either phosphite dissociation or protonation of a hydride. Termination of the catalytic cycle occurs when the H2 and DBU (the limiting reagents) have been consumed, and a new Ru-phosphine product (9) is generated. Complex 9 has not been isolated and characterized; however, the product mixture containing 9 can facilitate CO2 hydrogenation when fresh DBU, H2, and CO2 are added. We propose that 9 is very similar to 3 and is a ruthenium(II)-acetate dimer of the form {[(PMe3)4Ru]2(η2-OAc)2}2þ that is produced by exchange of acetate for the formate ligand in [(PMe3)4Ru(η2-O2CH)]þ followed by dimerization.

Acknowledgment is made by A.D.G and J.C.L. for support from the Director, Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy, under Contract DE-ACO676RLO 1830. Battelle operates the Pacific Northwest National Laboratory for the Department of Energy. P. G.J. and C.-C.T. gratefully acknowledge support from the Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy, under grant number DE-FG03-99ER14986. Supporting Information Available: Thermal ellipsoid drawings for the X-ray crystal structure of {[(PMe3)3Ru]2(μ-Cl)3}Cl, and the detailed crystallographic data for that complex, plus 2, 3b, and 4 are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium

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