Hydrogenation Catalyst Containing an N-Heterocyclic - American

Article pubs.acs.org/Organometallics

Factors Favoring Efficient Bifunctional Catalysis. Study of a Ruthenium(II) Hydrogenation Catalyst Containing an N-Heterocyclic Carbene with a Primary Amine Donor Wylie W. N. O, Alan J. Lough, and Robert H. Morris* Davenport Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: An alcohol-assisted outer-sphere bifunctional mechanism is proposed for the H2 hydrogenation of ketones catalyzed by the ruthenium(II) precatalyst [RuCp*(C−NH2)py]PF6 (2; Cp* = pentamethylcyclopentadienyl ligand, C−NH2 = N-heterocyclic carbene (NHC) with a tethered primary amine donor, py = pyridine) when activated by an alkoxide base. It has a high activity (turnover frequency (TOF) of up to 17 600 h−1) and selectivity for the H2 hydrogenation of ketones at 25 °C and 8 bar of H2 pressure in the presence of alcohols. Computational studies of a model neutral ruthenium(II) hydride amine complex, RuCp(C−NH2)H (Cp = cyclopentadienyl ligand), using density functional theory (DFT) methods reveal a low free energy barrier for the transfer of a proton/hydride couple to the ketone in the outer coordination sphere. In contrast, the related iridium(III) hydride amine complex [IrCp(C−NH2)H]+ is predicted to have a high barrier for hydride transfer to the ketone due to the poor nucleophilicity of this cationic hydride. A 2-propanol molecule acts as a proton shuttle in the heterolytic splitting of the η2-H2 ligand on ruthenium according to deuterium labeling and computational studies. The carbonyl stretching wavenumbers of the complexes [RuCp*(D−NH2)CO]X, including the new complexes [RuCp*(C−NH2)CO]PF6 (4) and [RuCp*(P−NH2)CO]PF6 (5; P−NH2 = 2-(diphenylphosphino)benzylamine), decrease as D is changed in the order phosphine, NHC, 2′-pyridine, amine. Thus, the NHC complex 2 is more electron rich and more active as a hydrogenation catalyst than the phosphine complex [RuCp*(P−NH2)py]PF6 (3). We conclude that complex 2 outperforms other ruthenium(II) NHC complexes and some other phosphine−amine complexes for the H2 hydrogenation of ketones due to the following factors: (1) the ease of formation of a highly reactive neutral hydride, (2) the presence of an NH2 donor ligand required for bifunctional catalysis, and (3) the presence of an NHC donor to achieve the right balance of hydricity of the metal hydride and acidity of the protic amine group for the efficient H+/H− transfer to the polar double bond.

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coordinated η2-H2 ligand is rate determining.11d−f,i−l,12,13 The key intermediates of the catalytic cycle, the trans-dihydride− amine and the monohydrido−amido catalysts of RuXY(diamine)(phosphine)2 or RuXY(diamine)(diphosphine) systems (X = H, Cl; Y = H, BH4, OR), have been identified and isolated.13,14 Ruthenium(II) alkoxide complexes have been observed at low temperature prior to the formation of an amido species,15 and one of these has been isolated recently.16 Under optimum conditions, such catalysts allow hydrogenation to occur under mild conditions: room temperature and low hydrogen pressures with no added base.13,14,17 The importance of the N−H group was demonstrated by the fact that metal hydride amine systems that lacked the N−H were found to be less active and selective and required forcing conditions in the hydrogenation of ketones.18 The effect of alcohol on bifunctional catalysis with the “NH effect” has also been well studied. Combined computational and experimental studies on many catalyst systems have shown that

INTRODUCTION Catalytic hydrogenation of unsaturated bonds using dihydrogen is an attractive, atom-economical, chemical process.1 The activation of dihydrogen by homogeneous catalysts is a process of continuing interest.2 The use of an ancillary ligand such as an amido3 or an amine donor4 to heterolytically split dihydrogen has emerged as an effective strategy in the design of catalysts for the hydrogenation of polar double bonds.5 This idea has been extended to the development of catalysts for C−C,5e,6 C−O,7 and C−N8 bond formation to provide access to new synthetic building blocks which are otherwise difficult to synthesize using conventional methods.9 The “NH effect”, which involves the bifunctional action of M−H/N−H groups to attack a polar bond in the outer coordination sphere, has been studied extensively both experimentally10 and theoretically.11 The ketone hydrogenation is proposed to proceed via a six-membered pericyclic transition state involving hydrogen bonding of the N−H group with the oxygen of the ketone and attack of the carbonyl group by the metal hydride.3a,5c Many research groups,11e,12 including ours,11k,13 have shown that the heterolytic splitting of a © 2012 American Chemical Society

Received: February 9, 2012 Published: March 6, 2012 2137

dx.doi.org/10.1021/om300108p | Organometallics 2012, 31, 2137−2151

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Figure 1. Late-transition-metal complexes containing a chelating N-heterocyclic carbene (NHC)−primary amine (C−NH2) or a chelating phosphine−primary amine (P−NH2) ligand.

an alcohol molecule serves as a proton shuttle that assists in the heterolytic splitting of the η2-H2 ligand.5f,11e,12a,13b,17b,18b,19 This decreases the energy barrier for the activation of H2 but does not significantly change the energy barrier for the transfer of a proton/hydride couple to the ketone in a hydrogen-bonded network.17b Certain ruthenium(II) complexes bearing a phosphine−amine (P−NH2) ligand, including the complexes trans-RuHCl((S)BINAP)(P−NH2)20 and RuCp*(κ2(P,N)-PPh2CH2CH2NH2)Cl21 (Figure 1), when activated with a base, catalyze efficiently the hydrogenation of a variety of polar bonds, including those of ketones,18h,20,22 imines,14b esters and lactones,23 epoxides,21 and cyclic imides.24 The notion of replacing a phosphine with a more electron-donating N-heterocyclic carbene (NHC) donor is attractive with the promise of a reduction in the toxicity of catalyst precursors and contaminants in the hydrogenated products leading to greener catalysis.25 Thus, the replacement of phosphine by an NHC donor would give a donor-functionalized NHC26 containing an NHC ligand and a primary amine donor (C−NH2). Douthwaite and co-workers reported the synthesis of the first palladium complex containing such a C−NH2 ligand (Figure 1).27 We began to explore the use of a more conveniently prepared C−NH2 ligand in metal complexes for the catalytic hydrogenation of ketones using H2 or 2-propanol as the hydrogen source. The complexes [M(p-cymene)(C−NH2)Cl]PF6 (M = Ru,28 Os;29 Figure 1) were prepared by a transmetalation reaction28 from a nickel(II) complex, [Ni(C−NH2)2](PF6)2 (1; Figure 1), to the appropriate precursor complex. The hydride complex [Ru(p-cymene)(C−NH2)H]PF6 was prepared, but this failed to transfer its proton/hydride couple to acetophenone in either stoichiometric or catalytic reactions. Experimental and computational studies suggested that an outer-sphere bifunctional mechanism was not viable for this cationic hydride− amine complex and that an inner sphere mechanism involving the decoordination of the primary amine group was more likely to occur (Figure 2).29 Although there are many metal complexes containing an amine-functionalized NHC ligand,27,30 none were reported to be useful for ketone hydrogenation30c,g,j apart from those in our work.31 We have prepared the ruthenium(II) complex [RuCp*(C−NH2)py]PF6 (2; Cp* = pentamethylcyclopentadienyl ligand, py = pyridine; Figure 1) and found it to catalyze

Figure 2. Inner sphere mechanism involving the decoordination of the primary amine group proposed for the H2 hydrogenation of ketones catalyzed by [Ru(p-cymene)(C-NH2)Cl]PF6 in the presence of an alkoxide base.

with high turnover frequency (TOF) the hydrogenation of a variety of ketones (up to 17 300 h−1 for the hydrogenation of acetophenone), styrene oxide, an aromatic ester, and a ketimine.31 The substrate to catalyst loading is high, and the reaction conditions are very mild (8 bar of H2 pressure, 25 °C). This system has activity that is superior to that of the related catalysts RuCp*(κ2(P,N)-PPh2CH2CH2NH2)Cl and RuCp*(diamine)Cl.11e,18b,21 This complex (2) is, to date, the most active ruthenium(II) catalyst containing an NHC for the H2 hydrogenation of ketones.32 We believe that the unique catalyst architecture for this complex is responsible for the very high activity observed in the ketone hydrogenation and in the activation of H2. In the current study we show that when 2 is activated by an alkoxide base, it follows an alcohol-assisted outer-sphere bifunctional mechanism in the H2 hydrogenation of ketones involving the action of a metal hydride and a protic amine group. A structurally similar iridium(III) complex, [IrCp*(C− NH2)Cl]PF6, when it is activated with an excess of alkoxide base in 2-propanol, is a less active catalyst in ketone hydrogenation, as described in the following paper.33 Herein, we will present both experimental and theoretical evidence to support the two elementary steps in the alcohol-assisted outer-sphere bifunctional mechanism: (a) the heterolytic splitting of dihydrogen at 2138


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the active metal center, aided by a 2-propanol molecule acting as a proton shuttle (step A in Scheme 1), and (b) the transfer

Article

RESULTS AND DISCUSSION

Synthesis of Ruthenium(II) Complexes Containing C−NH2 and P−NH2 Ligands. The ruthenium(II) complex, [RuCp*(C−NH2)py]PF6 (2), was prepared by a transmetalation reaction28,31 of 1 and RuCp*(cod)Cl in refluxing acetonitrile followed by the addition of pyridine in tetrahydrofuran (THF; Scheme 2). The characterization of this complex has been reported previously.31 The diphenylphosphino-containing complex [RuCp*(P−NH2)py]PF6 (3; P−NH2 = 2-(diphenylphosphino)benzylamine herein) was prepared by the reaction of the P−NH2 ligand and RuCp*(cod)Cl. After the crude reaction mixture was treated with AgPF6 in acetonitrile followed by the addition of pyridine in THF, complex 3 was isolated in good yields (Scheme 3).31 We also set out to prepare the carbonyl complexes [RuCp*(C−NH2)(CO)]PF6 (4) and [RuCp*(P−NH2)(CO)]PF6 (5) in order to compare their electronic properties (Schemes 2 and 3). Both complexes 4 and 5 were characterized by NMR and an X-ray diffraction study (Figures 3 and 4). The carbonyl stretching wavenumbers of 4 and 5 are 1940 and 1952 cm−1, respectively. General Features of the H2 Hydrogenation of Ketones Catalyzed by Complexes 2 and 3. The ruthenium(II) complex 2 catalyzed the H2 hydrogenation of a variety of ketones and N-(1-phenylethylidene)aniline with high substrate to catalyst loadings (catalyst to substrate (C/S) ratio up to 1/11 500) under very mild reaction conditions (8 bar of H2 pressure, 25 °C) using either THF or 2-propanol as the reaction medium and potassium tert-butoxide (KOtBu) as the base (catalyst to base (C/B) ratio 1/8). The solvent 2-propanol is an excellent choice for achieving a TOF of 17 300 h−1 in the H2 hydrogenation of acetophenone, under the same catalysis conditions as above (Figure 5).31 The transfer hydrogenation of acetophenone in 2-propanol has activity that is inferior to that of hydrogenation using H2 (TOF = 1270 h−1, 25 °C under argon). Significantly, complex 2 catalyzed the H2 hydrogenation

Scheme 1. Alcohol-Assisted Outer-Sphere Bifunctional Mechanism of H2 Hydrogenation of Ketones Catalyzed by a Ruthenium(II) Complex Containing a C−NH2 Ligand

of an Ru−H/N−H couple to the ketone in the outer-sphere (step B in Scheme 1). We also show here that the structurally similar cationic iridium(III) system is unlikely to catalyze ketone hydrogenation by this mechanism. A novel alternative mechanism involving neutral iridium(I) intermediates will be described in the accompanying paper.33 In addition, a comparison between the C−NH2 and analogous P−NH2 ligand in the ruthenium(II) catalyst allows us to evaluate the relative merits of these ligands in the homogeneous hydrogenation of ketones.

Scheme 2. Synthesis of Ruthenium(II) Complexes Containing a C−NH2 Ligand

Scheme 3. Synthesis of Ruthenium(II) Complexes Containing a P−NH2 Ligand

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conversion) and none of the saturated ketone 4-phenylbutan2-one within 1 h (C/B/S = 1/8/600). This corresponds to a maximum TOF value of 938 h−1 in the conversion to trans4-phenyl-but-3-en-2-ol (Figure 5). The selectivity that 2 exhibits is characteristic of an outer-sphere bifunctional mechanism, although the reduction of the olefin might proceed via a 1,4-addition reaction utilizing the same mechanism.34 The catalytic activity for the hydrogenation of acetophenone using H2 (C/B/S = 1/8/200) of the phosphine−amine complex 3, on the other hand, is inferior to that of 2. It produces a maximum conversion of 8% in 4 h under 25 bar of H2 pressure in THF at 50 °C. Complex 2 is, by far, the most active ruthenium(II) catalyst containing an NHC for the H2 hydrogenation of ketones.32 Effect of Alcohols and Other Additives on the H2 Hydrogenation of Acetophenone Catalyzed by Complex 2. Reactions carried out in THF gave sigmoidal-type reaction profiles with variable induction periods (10−60 min). On the other hand, a reaction that was conducted in 2-propanol did not show any sigmoidal behavior in the reaction profile (Figure 6). The effects of different additives on this induction period were investigated. The addition of the product alcohol, 1-phenylethanol, at the beginning of the reaction (up to 0.4 M in THF) decreased the induction period. In addition, the induction period increased by roughly 6-fold when the pressure of H2 was decreased from 8 to 2 bar. There was little effect on changing the concentrations of acetophenone or the catalyst by 2-fold (see the Supporting Information, Figure S1). The addition of pyridine to the reaction mixture retarded catalysis (Figure 6). In addition, the possibility of ruthenium nanoparticle formation was considered; the hydrogenation of 4′-chloroaccetophenone in THF catalyzed by 2 with KOtBu added was not affected by the addition of excess mercury, an indication that heterogeneous Ru(0) particles are not the active species for catalyzed ketone hydrogenation.31 It appears that there exists a competition between H2 and pyridine to coordinate to the catalytically active species. The presence of alcohol selectively favors the binding and heterolytic splitting of η2-H2 by acting as a proton shuttle.5f,11e,17b,18b,19a This system will have a rate law different from that reported for the amido complex RuH((S)-BINAP)(app) (app = 2-amido-2(2-pyridyl)propane), which displayed autocatalysis in the presence of alcohols, because of competing coordination between H2 and pyridine to the active species.17b Deuterium Labeling Studies Using Complex 2 and 2-Propanol. In order to find evidence for an alcohol-assisted mechanism in the hydrogenation of ketones catalyzed by complex 2, acetophenone was deuterated using D2 gas in 2-propanol (OH) and 2-propanol-d1 (OD, Table 1). Full conversion to the deuterated product was achieved in 30 min. Analysis of the 1 H NMR spectra of the products revealed significant deuteration at the α-carbon and the hydroxyl group of 1-phenylethanol when both D2 and 2-propanol-d1 were used, but less when D2 and 2-propanol were used (Table 1). These results suggest significant H/D scrambling took place at the active ruthenium species with participation of the alcohol solvent. Further analysis of the 2 H NMR spectra showed deuteration of the hydroxyl group of 2-propanol but not at its α carbon. On the other hand, the hydroxyl deuterium of 2-propano1-d was not retained after catalysis (lowered from 100% to 84% deuteration, Table 1), further supporting the idea that H/D scrambling has taken place. This is evidence that 2-propanol is acting as a proton shuttle.18b

Figure 3. ORTEP diagram of 4 ([RuCp*(C−NH2)CO]PF6) depicted with thermal ellipsoids at 30% probability. The counteranion and most of the hydrogens have been omitted for clarity. Selected bond distances (Å) and bond angles (deg): Ru(1)−C(1), 2.065(4); Ru(1)− N(3), 2.178(3); Ru(1)−C(22), 1.859(4); Ru(1)−C(15), 2.223(4); C(22)−O(1), 1.149(5); C(1)−Ru(1)−N(3), 90.8(2); C(1)−Ru(1)− C(22), 95.1(2); C(22)−Ru(1)−N(3), 91.7(2).

Figure 4. ORTEP diagram of 5 ([RuCp*(P−NH2)(CO)]PF6) depicted with thermal ellipsoids at 30% probability. The counteranion and most of the hydrogens have been omitted for clarity. Selected bond distances (Å) and bond angles (deg): Ru(1)−P(1), 2.320(1); Ru(1)−N(1), 2.166(4); Ru(1)−C(30), 1.877(6); Ru(1)−C(24), 2.218(5); C(30)−O(1), 1.140(6); P(1)−Ru(1)−N(1), 87.3(1); P(1)−Ru(1)−C(30), 88.6(2); C(30)−Ru(1)−N(1), 90.8(2).

of methyl benzoate to methanol and benzyl alcohol in THF, reaching a TOF value of 209 h−1 at 25 °C and 838 h−1 at 50 °C and 8 bar of H2 pressure (C/B/S = 1/8/1500; Figure 5). Under similar reaction conditions to the hydrogenation of ketones using KOtBu as the base (8 bar of H2 pressure, 25 °C), complex 2 catalyzed the selective reduction of the polar bond in trans-4-phenyl-but-3-ene-2-one to give trans-4-phenyl-but-3en-2-ol (89% conversion) and 4-phenylbutan-2-ol (4% 2140


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Figure 5. Substrate scope and TOF value reported for the H2 hydrogenation of organic molecules with polar double bonds catalyzed by complex 2 in the presence of KOtBu at 25 °C and 8 bar of H2 pressure. Details of the reaction conditions were reported in ref 31.

Observation and Reactivity of Hydride Complexes of Ruthenium(II). Hydride intermediates have been proposed in the catalytic cycles of the hydrogenation of polar bonds and the racemization of chiral alcohols. The complex RuCp*(κ2(P,N)PPh2CH2CH2NH2)H, which was suggested to be present during the catalytic hydrogenolysis of epoxides, has been observed but not isolated.21 Hydride intermediates have been observed spectroscopically in the racemization catalyst system containing Ru(η5-Cp*)(ICy)Cl, NaOtBu, and (S)-1-phenylethanol (ICy = 1,3-dicyclohexylimidazol-2-ylidene).35 The complex Ru(η5C5Ph5)(CO)2H has been isolated, and this catalyzes the racemization of (S)-1-phenylethanol36 via an inner-sphere mechanism upon the dissociation of the carbonyl ligand.37 We sought to observe or isolate the hydride−amine complex RuCp*(C−NH2)H, which can potentially transfer a proton/ hydride couple from its Ru−H/N−H group to the ketone (step B in Scheme 1).3a,5e,f Attempts to isolate such a complex were not successful. Reactions of excess or stoichiometric amounts of base (KOtBu, NaOiPr, NaOMe, NaBH4, KOH, KH, or K-Selectride) with complex 2 in THF under a hydrogen atmosphere (up to 8 bar) or in 2-propanol solution gave intractable products. Nevertheless, a hydride peak in the 1H NMR spectrum was observed at −9.23 ppm in THF-d8 when the reaction was carried out under a hydrogen atmosphere in the presence of KOtBu. Attempts to utilize such a reaction mixture upon removal of excess KOtBu in the catalytic hydrogenation of acetophenone (0.15 or 1.9 M) using 8 bar of H2 pressure at 25 °C in the absence of base gave no conversion to the product alcohol. However, reactions of complex 2 with sodium 2-propoxide (2.6 equiv) or potassium 1-phenylethoxide (1.3 equiv) in THFd8 at 25 °C under argon gave a species showing a new hydride peak at −9.47 ppm in the 1H NMR spectrum. The relative integration of this peak against the alkoxide suggested that 6% of the starting material was converted to this hydride. A hydride peak at −9.23 ppm was also observed in a smaller amount in these two experiments. Acetone and acetophenone that were produced from β-hydride elimination of the alkoxides were observed as well with 2% and 8% conversion, respectively. After the solutions were left for 1 day in sealed NMR tubes at 25 °C, the hydride peaks disappeared and both samples contained less than 2% of the ketone. These results suggest that the hydridecontaining species at −9.47 ppm in THF-d8 may represent the true catalytically active species, instead of the one that was observed at −9.23 ppm. These hydride complexes, however, are difficult to identify due to the complexity of the reaction mixture and their high reactivity.

Figure 6. Reaction profiles showing the effect of alcohols and pyridine on the hydrogenation of acetophenone (S = substrate) catalyzed by complex 2 (C = catalyst). All of the reactions were conducted with 8 bar of H2 at 25 °C with KOtBu (B) added: (a) in THF, red circles; (b) in 2-propanol, blue diamonds; (c) in THF, [1-phenylethanol] = 0.2 M, green squares; (d) in THF, [1-phenylethanol] = 0.4 M, orange triangles; (e) in THF, [pyridine] = 0.030 M, purple crosses. The C/B/S ratio was 1/8/2515.

Table 1. Deuteration of Acetophenone and 2-Propanol Catalyzed by Complex 2 D content in 1-phenylethanol (%)b

D source

α-CD

OD group

D2/iPrOH D2/iPrOD

42 80

11 76

a

CDnH3−n group (n = 1 − 3)

D content in 2-propanol (%)b OD group

− 71

27 84

a

Reactions were carried out in a 50 mL Parr hydrogenation reactor at 8 bar of D2 pressure at 25 °C using the appropriate solvent (6 mL). KOtBu was used as the base. The catalyst/base/substrate/solvent ratio was 1/8/898/17 060. Complete conversion to the product alcohol was achieved in 30 min. bThe deuterium content of 1-phenylethanol and 2-propanol were determined by 1H and 2H NMR.

Of note, deuteration of the methyl group of 1-phenylethanol was observed when D2 and 2-propanol-d1 were used in catalysis. This is expected, due to the base-catalyzed deuteration of enolizable ketones as reported elsewhere.13b,29 These experiments support the alcohol-assisted heterolytic splitting of η2-H2 on the active ruthenium species starting from complex 2 when activated (step A in Scheme 1). 2141


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Scheme 4. Computed Outer-Sphere Bifunctional Mechanism for the Hydrogenation of Acetone Catalyzed by Complexes of Ruthenium(II) and Iridium(III)

Conventional Non-Alcohol-Assisted Outer-Sphere Bifunctional Mechanism. The catalytic cycles that involve the outer-sphere bifunctional mechanism in the absence of alcohol for the ruthenium(II) (complex 2) system was investigated by density functional theory (DFT). The MPW1PW91 functional was used, as this gives a better prediction of energy barriers and transition states for 4d and 5d metals.38 A catalytic cycle pertaining to a structurally similar cationic iridium(III) system was also computed for comparison, although the experimental results and the preferred, alternative mechanism for this less active catalyst system will be reported elsewhere.33 Simplifications were made to ease computation by replacing the Cp* ligand with Cp; acetone and 2-propanol were used to model the ketone and the product alcohol. In the conventional outer-sphere mechanism that involves the action of M−H and N−H groups (Scheme 4), the addition of base to the precatalyst afforded the amido complex A, which results from the deprotonation of the N−H group. Such a complex is responsible for activating dihydrogen to yield the metal hydride complex D via an H2 addition step to the metal center (TSB,C) and subsequent heterolytic splitting at the metal center (TSC,D). The activation of H2 at the amido complex is rate-determining, and this should have the largest energy barrier of the overall catalytic cycle. The fast, low barrier step then follows for the transfer of H+/H− from D to the ketone in the outer-sphere fashion to afford the product alcohol and A.5f,11e,j,l,12a,13a,b,17b According to calculations (Figure 7), the free energy barriers (ΔG⧧) for the activation of H2 (the coordination and heterolytic

splitting of H2) by the neutral ruthenium(II) and cationic iridium(III) species are 14.1 (ΔH⧧ = 7.3 kcal/mol) and 22.6 kcal/mol (ΔH⧧ = 14.8 kcal/mol), respectively. The heterolytic splitting of H2 leads to a more reactive ruthenium(II) hydride DRu (−8.8 kcal/mol relative to ARu and H2) but a more thermodynamically stable iridium(III) hydride DIr (−23.5 kcal/mol relative to AIr and H2). The free energy barriers for the transfer of H+/H− from DRu to acetone and to acetophenone are 16.9 (ΔH⧧ = 2.2 kcal/ mol) and 12.3 kcal/mol (ΔH⧧ = −1.7 kcal/mol), respectively (see the Supporting Information, Figure S2). In contrast, it is unfavorable to transfer H+/H− from DIr to acetone (ΔG⧧ = 36.7 kcal/mol; ΔH⧧ = 62.6 kcal/mol). In addition to a stronger Ir−H bond that is formed due to relativistic effects on going from a 4d (Ru) to a 5d (Ir) metal, the cationic charge of the iridium may deactivate the metal hydride, as was observed for the [Ru(pcymene)(C−NH2)H]+ system.29 The computed results suggest well-balanced energy barriers for the ruthenium(II) system. The hydride complex (DRu) which is formed upon the activation of H2 by ARu is readily consumed when it transfers its H+/H− to the ketone in the outer sphere. This provides evidence to support a bifunctional step involving the action of Ru−H/N−H outlined in step B from Scheme 1. The replacement of an arene ligand (η6-C6H6) in [Ru(η6-C6H6)(C−NH2)H]+ with the cyclopentadienyl ligand (η5-Cp) to give Ru(η5-Cp)(C−NH2)H results in a significant energy difference of the ground-state energies of the hydride− amine complexes and the starting ruthenium amido complexes: calculations using the same functional and basis sets suggest that the cationic hydride−amine complex [Ru(η6-C6H6)(C−NH2)H]+ 2142


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Figure 7. Free energy profile for the outer-sphere bifunctional mechanism in the H2 hydrogenation of acetone, starting from A and moving to the right. The pathway in blue represents the ruthenium(II) system, and that in red represents the iridium(III) system. The gas-phase free energies (1 atm, 298 K) are reported relative to A, H2, and acetone in kcal/mol.

Figure 8. Computed transition state structures for the heterolytic splitting of H2 (TSC,D) of the ruthenium(II) (left) and the iridium(III) system (right). The bond lengths (Å) are given in the structures. Color code for the atoms: (orange) ruthenium; (yellow) iridium; (blue) nitrogen; (gray) carbon; (white) hydrogen.

[Ir(η5-Cp*)(C−NH2)H]PF6 does not react with a ketone as well.33 Of note, the neutral, well-defined hydride−amine complexes containing a cyclometalated C−NH2 ligand and an anionic carbon donor IrCp*(κ2(C,N)-2-C6H4CR2NH2)H39 and Ru(η6-C6H6)(κ2(C,N)-2-C6H4CR2NH2)H40 react with acetophenone by transferring their M−H/N−H couples to the ketone in the absence of base. Their mechanisms have not yet been explored computationally, however. It appears that the charge of these hydride−amine complexes is an important factor in determining the reactivity of the M−H/N−H couple to a ketone. Of interest, the geometric parameters for the optimized transition state structures for the heterolytic splitting of H2 (TSC,D, Figure 8) and the transfer of H+/H− to acetone (TSE,F,

is the resting state of the outer-sphere bifunctional mechanism in the H2 hydrogenation of ketone (22.4 kcal/mol more stable than the amido complex and H2). This supports the experimental observation that the cationic hydride−amine complex [Ru(η6-p-cymene)(C−NH2)H]PF6 does not react with a ketone.29 On the other hand, the neutral complex Ru(η5Cp)(C−NH2)H is only 8.8 kcal/mol more stable than the amido complex Ru(η5-Cp)(C−NH) (Figure 7). Our calculated results also predict that the cationic iridium(III) hydride complex (DIr), if formed during catalysis, is the resting state of the catalytic cycle (23.5 kcal/mol more stable than the iridium(III) amido complex and H2), and the transfer of H+/ H− to the ketone in the outer sphere will be very difficult. In fact, we will show that the cationic hydride−amine complex 2143


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Figure 9) of both systems are surprisingly similar, except for a longer O−H interaction in TSE,F (O−H···N distance: 1.61 Å for ruthenium and 1.10 Å for iridium) for the ruthenium(II) system.10h,11a,b,e,13a,17b An intrinsic reaction coordinate (IRC) calculation41 that was performed on TS(Ru)E,F located a local minimum, which revealed a 2-propoxide anion that is hydrogen bonded via oxygen to the N−H group on the C−NH2 ligand, along with an agostic C−H interaction of the 2-propoxide anion with the ruthenium(II) center. Subsequent geometry optimization and normal-mode analysis revealed that this structure (FRu′) is 0.9 kcal/mol more stable than TS(Ru)E,F (ΔG = 7.2 kcal/mol relative to ARu, H2, and acetone). This has a O−H distance of 1.39 Å, a C−H distance of 1.26 Å, and a Ru···H distance of 1.85 Å (see Figure S4 in the Supporting Information). These geometric parameters are similar to an analogous structure reported by Gusev and co-workers.10h A QST3 or a QST2 calculation that was performed for the protonation of the 2-propoxide anion by the N−H group on the C−NH2 ligand, giving FRu, failed to locate a transitionstate structure. All these suggest that the transfer of a H+/H− couple from the hydride−amine complex to the ketone in the outer sphere for the ruthenium(II) system might occur in a stepwise fashion.10h,15,42 Alcohol-Assisted Outer-Sphere Bifunctional Mechanism for the Ruthenium(II) System. In the presence of 2-propanol (Scheme 5), or when the concentration of the product alcohol builds up during catalysis, the free energy barrier for the activation of dihydrogen becomes lower. The highest barrier is now ΔG⧧ = 11.1 kcal/mol measured from the energy of A in the presence of alcohol, Aalc (or F in Scheme 4), to the

Figure 9. Computed transition state structures for the transfer of a proton/hydride pair to acetone (TSE,F) from the ruthenium(II) (top) and iridium(III) hydrides (bottom). The bond lengths (Å) are given in the structures. Color code for the atoms: (orange) ruthenium; (yellow) iridium; (blue) nitrogen; (blue) oxygen; (gray) carbon; (white) hydrogen.

Scheme 5. Computed Reaction Pathway of the Activation of H2 by Complex A in the Presence of 2-Propanol

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Figure 10. Free energy profile for the outer-sphere bifunctional mechanism in the activation of H2, starting from A and moving to the right (blue pathway), and in the presence of 2-propanol, starting from Aalc and moving to the right (red pathway). The gas-phase free energies (1 atm, 298 K) are reported relative to A, H2, and 2-propanol for the red pathway and to A and H2 for the blue pathway, in kcal/mol.

Figure 11. Computed transition state structures for the heterolytic splitting of H2 (TSC,Dalc) by the ruthenium(II) system in the presence of 2propanol. The bond lengths (Å) are given in the structure. Color code for the atoms: (orange) ruthenium; (blue) nitrogen; (red) oxygen; (gray) carbon; (white) hydrogen.

alcoholic oxygen are held in close proximity by the formation of two hydrogen bonds via O···H interactions (H(2)−O = 1.74 Å, H(3)−O = 1.36 Å; Figure 11). These results are supportive of an alcohol-assisted mechanism where the alcoholic proton is shuttled between the coordinated dihydrogen and the amido nitrogen via a six-membered-ring transition state (step B in Scheme 1).5f,11e,l,12a,13b,17b,18b,19 Andersson and co-workers have calculated a similar transition state structure for the

transition state for H2 coordination to Aalc (TSB,Calc, Figure 10). This is lower than the 14.1 kcal/mol value for the case of no alcohol present. Similarly the barrier for the heterolytic splitting of H2 becomes lower (ΔG⧧ = ΔG(TSC,Dalc) − ΔG(Aalc) = 8.5 kcal/mol) in comparison to that with no alcohol present (13.1 kcal/mol). The transition state structure of the alcohol-assisted heterolytic splitting of H2 shows that the protons neighboring the 2145


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Figure 12. Carbonyl stretching wavenumbers (cm−1 in KBr) of a series of ruthenium(II) complexes, [RuCp*(D−NH2)CO]+, where D is a donor group. The complexes [RuCp*(κ2(N,N)-N(CH3)2CH2CH2NH2)CO]OTf, [RuCp*(κ2(N,N)-(2′-C5H4N)CH2NH2)CO]OTf, and [RuCp*(κ2(P,N)-PPh2CH2CH2NH2)CO]OTf were prepared by Ikariya and Ito. See refs 5f and 43.

an analogous C−NH2 ligand with an aniline-type N−H group are not very active in the catalytic transfer hydrogenation of ketones.30j The phosphine−amine complex 3 is a poor catalyst for the hydrogenation of acetophenone when activated by base.31 This P−NH2 ligand is the least donating in the series, judging from its carbonyl stretching wavenumber in complex 5. The chelation effect of the phosphine−amine ligand that forms a five-membered or a six-membered ring including the metal may contribute to the difference in electronics at the metal center.45 Overall, the hydricity of the hydride complex must also contribute to the reactivity exhibited by these complexes;46,47a good balance of the hydricity of the hydride ligand and the acidity of the protic amine group, which is dependent on the donor strength of the D−NH2 ligand, is required to promote a successful transfer of H+/H− to the polar bond of interest and, therefore, result in a catalyst of high activity.3a,5e,f The rich substrate scope and the high turnover frequencies of 2 as a hydrogenation catalyst (ketones, an epoxide, a ketimine, and an ester) suggests that a correct electronic balance is achieved in such an NHC−amine system in contrast to its other counterparts.

alcohol-assisted heterolytic splitting of H2 of a structurally similar RuCp*(diamine)X system; the effect of methanol on the free energy barriers of H2 coordination and its activation are pronounced.11e In addition, the free energy barrier for the transfer of a proton/hydride couple to acetone is 16.2 kcal/mol (ΔH⧧ = 2.6 kcal/mol) if a six-membered-ring transition state is assumed (see the Supporting Information, Scheme S1 and Figures S4 and S6).17b The alcohol, therefore, has a minor effect on such a transition state as the energy barriers with and without alcohol were similar (ΔG⧧ = 16.9 kcal/mol), consistent with our observations reported previously.17b On the other hand, the induction period that was observed when an aprotic solvent was used during catalysis can be explained by a competition between pyridine and dihydrogen to coordinate to the amido complex RuCp*(C−NH). The energy barriers for the coordination of pyridine and hydrogen to A are very similar (ΔG⧧ = 15.7 and 14.1 kcal/mol, respectively; ΔH⧧ = 4.6 and 7.3 kcal/mol, respectively; see the Supporting Information, Scheme S2 and Figure S6), and the presence of alcohol provides a lower energy pathway to effectively convert Aalc and H2 to the hydride complex Dalc, which will be readily consumed by transferring its H+/H− couple to acetone in the outer sphere. This has the effect of driving the equilibrium between A plus pyridine and its adduct, RuCp*(C−NH)(py), to the left. Electronic Properties of Ruthenium(II) Complexes That Relate to Their Reactivity in Catalytic Hydrogenation. The electronic properties of a series of ruthenium(II) complexes formulated as RuCp*(D−NH2)Cl, where D is a donor group, have been probed by preparing the corresponding cationic carbonyl complexes.5f,43 In particular, the carbonyl stretching wavenumber of complex 4 containing the C−NH2 ligand lies between those of the complex [RuCp*(κ2(P,N)-PPh2CH2CH2NH2)CO]OTf, in which the D−NH2 ligand is less donating, and complexes containing nitrogen donors, which are more electron donating (Figure 12). Of note, the complexes RuCp* (κ2(N,N)-N(CH3)2CH2CH2NH2)Cl and RuCp*(κ2(N,N)(2′-C5H4N)CH2NH2)Cl are ketone and ester44 hydrogenation catalysts upon activation by KOH, yet are poor catalysts for the hydrogenolysis of epoxides; the complex RuCp*(κ2(P,N)PPh2CH2CH2NH2)Cl, on the other hand, shows the reverse trend.18b,21 Ikariya and co-workers have attributed the facile proton and hydride transfer to the polar bond to a greater acidity of the coordinated NH2 group of the latter.5f A counter example to this is the rhodium(I) amine complex [Rh(η4-diene)(C−NHR)]BF4 (C−NHR = 1-mesityl-3-(2-(mesitylamino)ethyl)imidazolylidene), which contains an acidic aniline-type N−H group and is not active toward the catalytic transfer hydrogenation of benzophenone.30c The iridium(III) and ruthenium(II) complexes prepared by Cross and co-workers that contain

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CONCLUSIONS There are several pieces of evidence to support an alcoholassisted outer-sphere bifunctional mechanism for the ruthenium(II) complex 2 when activated by an alkoxide base in the hydrogenation of ketones using hydrogen gas: (a) the high selectivity for the reduction of the ketone over the olefin in an α,β-unsaturated ketone, a characteristic feature of a catalyst operating by the outer-sphere bifunctional mechanism5e,f,11e,14b (b) the pronounced effect of alcohols on increasing the rate of catalysis, decreasing the period of activation of the catalyst, and promoting H/D scrambling when deuterated alcohol or deuterium gas is used (c) the low free energy barrier calculated for the transfer of a proton/hydride couple from the neutral ruthenium(II) hydride Ru(η5-Cp)(C−NH2)H to the ketone in the outer sphere; in contrast, a high energy barrier is calculated for this transfer to acetone from the cationic hydride complex [Ir(η5-Cp)(C−NH2)H]+ (d) a significant decrease in the calculated free energy barriers to the heterolytic splitting of the η2-H2 ligand on Ru(η5-Cp)(C−NH) when a 2-propanol molecule acts a proton shuttle by participating in a six-membered-ring transition state5f,11e,12a,13b,17b,18b,19 The donor ability of the D−NH2 type ligand (D = NHC, phosphine, amine, 2′-pyridine) was also investigated by use of the carbonyl stretching wavenumbers of the complexes 2146


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atmosphere using standard Schlenk-line and glovebox techniques. Dry and oxygen-free solvents were always used. The syntheses of [1(2-aminomethylphenyl)-3-methylimidazol-2-ylidene](η5-pentamethylcyclopentadienyl)(pyridine)ruthenium(II) hexafluorophosphate (2) and [2-(diphenylphosphino)benzylamine](η5-pentamethylcyclopentadienyl)(pyridine)ruthenium(II) hexafluorophosphate (3) have been reported previously.28,31 The syntheses of RuCp*(cod)Cl49 and 2-(diphenylphosphino)benzylamine (P−NH2)50 were reported in the literature. All other reagents and solvents were purchased from commercial sources and were used as received. Deuterated solvents were purchased from Cambridge Isotope Laboratories and Sigma Aldrich and degassed and dried over activated molecular sieves prior to use. NMR spectra were recorded on a Varian 400 spectrometer operating at 400 MHz for 1H, 100 MHz for 13C, 161 MHz for 31P, and 376 MHz for 19F. The 1H and 13 C{1H} NMR spectra were measured relative to partially deuterated solvent peaks but are reported relative to tetramethylsilane (TMS). All 19 F chemical shifts were measured relative to trichlorofluoromethane as an external reference. All 31P chemical shifts were measured relative to 85% phosphoric acid as an external reference. All infrared spectra were recorded on a Nicolet 550 Magna-IR spectrometer. The elemental analysis was performed at the Department of Chemistry, University of Toronto, on a Perkin-Elmer 2400 CHN elemental analyzer. Samples were handled under argon where it was appropriate. Single-crystal X-ray diffraction data were collected using a Nonius Kappa-CCD diffractometer with Mo Kα radiation (λ = 0.710 73 Å). The CCD data were integrated and scaled using the Denzo-SMN package. The structures were solved and refined using SHELXTL V6.1. Refinement was by full-matrix least squares on F2 using all data. Details are given in Table S1 (Supporting Information). Synthesis of [1-(2-Aminomethylphenyl)-3-methylimidazol2-ylidene]carbonyl(η 5 -pentamethylcyclopentadienyl)ruthenium(II) Hexafluorophosphate (4; [RuCp*(C−NH2)(CO)]PF6). A Schlenk flask was charged with 2 (32 mg, 0.049 mmol). It was evacuated and back-filled with a CO atmosphere (1 atm) two times. A solution of tetrahydrofuran (10 mL) saturated with Ar was injected into the Schlenk flask against a flow of CO by means of a syringe and a needle. The solution turned immediately from orange-yellow to pale yellow upon dissolution. The solution was stirred at room temperature (25 °C) for 3 h. The volume of the solvent was reduced (2 mL). Addition of diethyl ether or hexanes (12 mL) to this and slow cooling of the solution at −25 °C afforded a pale yellow crystalline solid, which was filtered on a glass frit and dried in vacuo. Yield: 22 mg, 75%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of hexanes into a saturated solution of 4 in tetrahydrofuran under a nitrogen atmosphere. 1H NMR (CD2Cl2, δ): 7.62 (m, 3-CH and 5-CH of Ph, 2H), 7.51 (m, 4-CH and 6-CH of Ph, 2H), 7.35 (d, JHH = 2.07 Hz, 5-CH of imid, 1H), 7.29 (d, JHH = 2.07 Hz, 4-CH of imid, 1H), 4.21 (td, JHH = 2.69, 11.53 Hz, CH2, 1H), 3.89 (s, CH3, 3H), 3.84 (br, NH2, 1H), 2.98 (m, CH2, 1H), 2.17 (m, br, NH2, 1H), 1.40 (s, CH3 of Cp*, 15H). 19F NMR (CD2Cl2, δ): −72.5 (d, JPF = 712 Hz). 13 C{1H} NMR (CD2Cl2, δ): 207.1 (Ru−CCO), 181.8 (Ru−Ccarbene), 139.5 (CPh), 132.6 (CPh), 131.8 (CPh), 130.5 (CPh), 129.7 (CPh), 126.2 (CPh), 125.0 (Cimid), 124.7 (Cimid), 94.6 (CAr‑Cp*), 48.9 (CH2), 39.4 (CH3), 9.5 (CH3 of Cp*). IR (KBr, cm−1): 1940 (ν(CO)). MS (ESI, methanol/water; m/z): 452.1 [M]+. HRMS (ESI, methanol/water; m/z): calcd for C22H28N3ORu+ [M]+ 452.1270, found 452.1269. Attempts at elemental analyses failed to give an acceptable carbon content, while hydrogen and nitrogen contents are in the acceptable range. Typical results are as follows. Anal. Calcd for C22H38F6N3OPRu: C, 44.30; H, 4.73; N, 7.04. Found: C, 42.99; H, 4.64; N, 7.35. Synthesis of [2-(Diphenylphosphino)benzylamine]carbonyl(η5-pentamethylcyclopentadienyl)ruthenium(II) Hexafluorophosphate (5; [RuCp*(P−NH2)(CO)]PF6). A scintillation vial with a threaded screw cap was charged with RuCp*(cod)Cl (50 mg, 0.13 mmol) in dry dichloromethane (5 mL) under a nitrogen atmosphere. A solution of 2-(diphenylphosphino)benzylamine (40 mg, 0.13 mmol) in dry dichloromethane (5 mL) was added to the aforementioned yellow solution and stirred for 1 h at room temperature (25 °C), whereupon the reaction mixture turned orange. Silver hexafluorophosphate (33 mg, 0.13 mmol) in dry acetonitrile (1 mL) was added

[RuCp*(D−NH2)CO]X to help explain the poorer performance in the catalytic H2 hydrogenation of ketones of the phosphine−amine complex [RuCp*(P−NH2)py]PF6 (3) in comparison to the C−NH2 analogue. The C−NH2 ligand was found to be more donating than the P−NH2 ligand. We conclude that a fine tuning of the Brønsted acidities of the N−H and the M−H groups is required by choosing the right donor ligand to achieve maximum activity of polar double bond reduction by H2. The replacement of an arene ligand (η6-p-cymene) in [Ru(η6-p-cymene)(C−NH2)H]+ with the pentamethylcyclopentadienyl ligand (η5-Cp*) to give Ru(η5-Cp)(C−NH2)H results in a significant energy difference of the ground state energies of the hydride−amine complexes with respect to the starting amido complexes. This is important in explaining their reactivity toward polar double bonds. Our computational results show that the cationic model complexes, [Ru(η6-C6H6)(C− NH2)H]+ and [Ir(η5-Cp)(C−NH2)H]+, are the resting states of the outer-sphere bifunctional mechanism in the H2 hydrogenation of ketones. The high free energy barriers that were calculated for these hydride−amine complexes prevent them from transferring their proton/hydride couple to a ketone. This is evident from our experimental studies.29,33 We also showed that it is feasible for the neutral hydride−amine complex Ru(η5Cp)(C−NH2)H to transfer its proton/hydride couple to a ketone in the outer sphere. Ikariya and Pfeffer have showed that the neutral hydride−amine complexes of iridium(III) and ruthenium(II) containing a cyclometalated C−NH2 ligand react with acetophenone in the absence of base at ambient temperature.39,40 It appears that the charge of these hydride− amine complexes is an important factor in determining the reactivity of the M−H/N−H pair. The presence of an N-heterocyclic carbene ligand, an amine group, and a pentamethylcyclopentadienyl ligand in our system, therefore, provides the following unique features favoring efficient bifunctional catalysis: (a) an NH2 group required for bifunctional catalysis utilizing the “NH effect”3a,5e (b) the ability to form a highly reactive neutral hydride (c) the use of an NHC ligand to achieve the right balance of the hydricity of the metal hydride and the acidity of the protic amine group for the efficient H+/H− transfer to the polar double bond These features make the present system outperform other ruthenium(II) NHC complexes29,32 and some other phosphine−amine complexes12a,13a,b,18b,20,48 for the H2 hydrogenation of ketones, judging from the reaction conditions and the TOF values reported for some of these systems in ketone hydrogenation (Table 2). Our results show that an alcoholassisted outer-sphere bifunctional mechanism accounts for such a difference in catalytic activity. We will further discuss the effect of the metal center on ketone hydrogenation by comparing the catalytic activity of the current ruthenium(II) system with that of a structurally similar iridium(III) system in an accompanying article.33 The potential for this ruthenium(II) system in the H2 hydrogenation of other polar double bonds is currently being investigated by our research group.

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EXPERIMENTAL SECTION

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Table 2. Reaction Conditions and Turnover Frequencies Reported for Some Ketone Hydrogenation Catalysts of Ruthenium(II)

cat.

ketone

reacn conditions

TOF/h−1 a

ref

RuH(NH2CMe2CMe2NH)(PPh3)2 trans-RuH2(NH2CMe2CMe2NH2)- ((R)-BINAP) RuHCl(P−NH2)2 RuHCl((R)-BINAP)(P−NH2) RuCl2((S)-tol-BINAP)((S,S)-dpen) RuCl2(ethP2(NH)2) RuH(η1-BH4)((S)-tol-BINAP)((S,S)-dpen) RuCp*(NH2CH2CH2NMe2)Cl RuCp*(P−NH2)Cl RuH(η2-BH4)(IPr)2(CO) [RuH(IMes)2(CO)]BArF4 Ru(p-cymene)(C−O-carboxylate)Cl [Ru(p-cymene)(C−NH2)Cl]PF6 [RuCp*(C−NH2)py]PF6 [IrCp*(C−NH2)Cl]PF6

acetophenone acetophenone acetophenone acetophenone acetophenone acetophenone acetophenone acetophenone acetophenone acetophenone acetone benzophenone tert-butyl phenyl ketone acetophenone benzophenone

6 atm H2, 20 °C, C6D6 14 atm H2, 20 °C, iPrOH 2.0 atm H2, 5 °C, iPrOH 2.0 atm H2, 5 °C, iPrOH 45 bar H2, 30 °C, KOiPr/iPrOH 45 bar H2, 30 °C, KOiPr/iPrOH 4 atm H2, 30 °C, KOiPr/iPrOH 20 atm H2, 30 °C, KOH/iPrOH 1 atm H2, 30 °C, KOH/iPrOH 10 atm H2, 75 °C, iPrOH 2 atm H2, 60 °C, C6D6 60 bar H2, 80 °C, THF/EtOH 25 bar H2, 50 °C, KOtBu/THF 8 bar H2, 25 °C, KOtBu/iPrOH 25 bar H2, 50 °C, KOtBu/THF

6 686 505 2 500b 1 250b 50 000b 3 300 000b 37 440 1 170b 16b 11b 0.42b 64b 461 17 600 472

13a 13a 20 20 13b, 48 13b 12a 18b 18b 32a 32b 32c 29 31 33

a

TOF = turnover frequency, measured from the slope of the linear portion of [alcohol] versus time plot unless otherwise stated. bTOF measured as TON (turnover number) divided by time at a certain percentage conversion. dichloromethane under a nitrogen atmosphere. 1H NMR (CD2Cl2, δ): 7.56−7.49 (m, Ar-CH of Ph-PPh2, 6H), 7.47 (m, Ar-CH of Ph-PPh2, 1H), 7.45−7.36 (m, Ar-CH of Ph-PPh2, 5H), 7.32−7.25 (m, Ar-CH of Ph-PPh2, 2H), 4.00 (m, CH2, 1H), 3.71 (br, NH2, 1H), 3.58 (m, CH2, 1H), 3.05 (br, NH2, 1H), 1.67 (s, CH3 of Cp*, 15H). 19F NMR (CD2Cl2, δ): −72.6 (d, JPF = 712 Hz). 31P{1H} NMR (CD2Cl2, δ): 42.1 (s), −144.5 (sept, JPF = 712 Hz). 13C{1H} NMR (CD2Cl2, δ): 203.9 (d, JCP = 16.83 Hz, Ru−CCO), 140.7 (d, JCP = 15.08 Hz, CPh‑PPh), 134.1 (d, JCP = 12.48 Hz, CPh‑PPh), 133.4 (d, JCP = 10.93 Hz, CPh‑PPh), 132.5 (d, JCP = 22.03 Hz, CPh‑PPh), 132.3 (d, JCP = 31.09 Hz, CPh‑PPh), 131.8 (d, JCP = 2.20 Hz, CPh‑PPh), 131.6 (d, JCP = 2.11 Hz, CPh‑PPh), 131.4 (d, JCP = 2.51 Hz, CPh‑PPh), 131.3 (CPh‑PPh), 130.9 (d, JCP = 1.95 Hz, CPh‑PPh), 129.7 (CPh‑PPh), 129.4 (d, JCP = 10.38 Hz, CPh‑PPh), 129.3 (d, JCP = 10.51 Hz, CPh‑PPh), 129.1 (d, JCP = 7.11 Hz, CPh‑PPh), 96.8 (d, JCP = 1.91 Hz, CAr−Cp*), 50.4, (d, JCP = 9.41 Hz, CH2), 9.7 (CH3 of Cp*). IR

to the reaction mixture, and a yellow-brown suspension was obtained. After the reaction mixture was stirred for 0.5 h, it was filtered through a pad of Celite, and the solvent was collected into a Schlenk flask under a nitrogen atmosphere. The solvent was then removed under reduced pressure. The Schlenk flask containing the residue was evacuated and back-filled with a CO atmosphere (1 atm) two times. A solution of tetrahydrofuran (10 mL) saturated with Ar was injected into the Schlenk flask against a flow of CO by means of a syringe and needle. The solution turned immediately from orange-yellow to pale yellow upon dissolution. The solution was stirred at room temperature for 3 h. The volume of the solvent was reduced (2 mL). Addition of diethyl ether (15 mL) afforded a yellow precipitate, which was filtered and dried in vacuo to give a pale yellow solid. Yield: 72 mg, 78%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 5 in 2148


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(KBr, cm−1): 1952 (ν(CO)). MS (ESI, methanol/water; m/z): 556.1 [M]+. HRMS (ESI, methanol/water; m/z): calcd for C30H33NOPRu+ [M]+ 556.1337, found 556.1355. Anal. Calcd for C30H33NF6OP2Ru: C, 51.43; H, 4.75; N, 2.00. Found: C, 51.10; H, 4.70; N, 1.91. Representative Example of a Stoichiometric Reaction Using High Pressure of H2. Complex 2 (10 mg, 15 μmol) and potassium tert-butoxide (9 mg, 0.080 mmol) were dissolved separately in THF (4 and 2 mL, respectively) under a nitrogen atmosphere. These solutions were taken up by means of two separate syringes and needles in a glovebox. The needles were stoppered, and the syringes were taken to the reactor. The solutions were then injected into the reactor against a flow of hydrogen gas. The hydrogen gas was adjusted to 8 bar, and the reaction mixture was stirred at 25 °C. After 2 h of reaction, the reactor was detached from the hydrogen source and a H2 pressure of 2−4 bar was maintained. The reactor was attached to a Schlenk line and back-filled with argon gas using standard Schlenk-line techniques. The reaction mixture was then transferred to an empty Schlenk flask filled with argon by syringe and a needle, and the solvent was removed under vacuum. The residue was taken up in diethyl ether (10 mL) and filtered through a pad of Celite. The solvent was then evaporated, and a 1 H NMR spectrum of the crude reaction mixture was measured in THF-d8. Catalysis. Oxygen-free tetrahydrofuran (THF) used for all of the catalytic runs was stirred over sodium for 2−3 days under argon and freshly distilled from sodium benzophenone ketyl prior to use. Oxygen-free 2-propanol used for all catalytic runs was stirred over magnesium turnings and a single crystal of iodine for several hours under argon and freshly distilled prior to use. Acetophenone was vacuum-distilled over phosphorus pentoxide (P2O5) and stored under nitrogen prior to use. 2-Propanol-d (purchased from Cambridge Isotope Laboratories) and 1-phenylethanol were vacuum-distilled, dried over activated molecular sieves, and stored under nitrogen prior to use. D2 gas was purchased from Cambridge Isotope Laboratories. All of the hydrogenation reactions were performed at constant pressures using a stainless steel 50 mL Parr hydrogenation reactor. The temperature was maintained at 25 °C using a constant-temperature water bath. The reactor was flushed several times with hydrogen gas at 2−4 bar prior to the addition of catalyst and substrate and of base solutions. In a typical run (Figure 6), catalyst 2 (3 mg, 4.6 μmol), acetophenone (1.4 g, 11.6 mmol), and 1-phenylethanol (147 mg, 1.2 mmol) and potassium tert-butoxide (4 mg, 0.036 mmol) were dissolved in THF (4 and 2 mL, respectively) under a nitrogen atmosphere. The catalyst/substrate and base solutions were taken up by means of two separate syringes and needles in a glovebox. The needles were stoppered, and the syringes were taken to the reactor. The solutions were then injected into the reactor against a flow of hydrogen gas. The hydrogen gas was adjusted to 25 bar. Small aliquots of the reaction mixture were quickly withdrawn with a syringe and a needle under a flow of hydrogen at timed intervals by venting the Parr reactor at reduced pressure. Alternatively, small aliquots of the reaction mixture were sampled from a stainless steel sampling dip tube attached to a modified Parr reactor. The dip tube was 30 cm in length with an inner diameter of 0.01 in., and a swing valve was attached to the end of the sampling tube. Other technical details were previously reported.11k Two small aliquots of samples were thereby withdrawn quickly at timed intervals by opening the swing valve, and the first two aliquots were discarded. All samples for gas chromatography (GC) analyses were diluted to a total volume of approximately 0.50 mL using oxygenated THF. A Perkin−Elmer Clarus 400 chromatograph equipped with a chiral column (CP chirasil-Dex CB 25 m × 2.5 mm) with an autosampling capability was used for GC analyses. Hydrogen was used as the mobile phase at a column pressure of 5 psi with a split flow rate of 50 mL/min. The injector temperature was 250 °C, the FID temperature was 275 °C, and the oven temperature was 130 °C. Retention times (tR/min): acetophenone, 4.56; (R)-1-phenylethanol, 7.58; (S)-1phenylethanol, 8.03. All of the conversions were reported as an average of two GC runs. The reported conversions were reproducible. Computational Details. All density functional theory (DFT) calculations were performed using the Gaussian 0351 and 0952 packages

with the restricted hybrid mPW1PW91 functional.53 Ruthenium and iridium were treated with the SDD54 relativistic effective core potential and an associated basic set. All other atoms were treated with the double-ζ basis set 6-31++G**, which includes diffuse functionals55 and additional p orbitals on hydrogen as well as additional d orbitals on carbon, nitrogen, and oxygen.56 All geometry optimizations were conducted in the gas phase, and the stationary points were characterized by normal-mode analysis. Reported free energies were obtained at 1 atm and 298 K using unscaled vibrational frequencies. The QST3 method was used to locate transition states. All transition states reported were found to have a single imaginary frequency.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving X-ray structural data for complexes 4 and 5, tables, text, and figures giving details for catalysis, Cartesian coordinates, energies for all of the computed structures, and the complete citations for refs 51 and 52, and AVI files giving animations for loose vibration characterizing the computed transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The NSERC of Canada is thanked for a Discovery Grant to R.H.M. and a graduate scholarship to W.W.N.O. REFERENCES

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