Bifunctional Mechanism with Unconventional Intermediates for the

Article pubs.acs.org/Organometallics

Bifunctional Mechanism with Unconventional Intermediates for the Hydrogenation of Ketones Catalyzed by an Iridium(III) Complex 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 for the H2 hydrogenation of ketones catalyzed by an iridium system is presented on the basis of experimental and theoretical evidence. An iridium(III) complex containing an N-heterocyclic carbene (NHC) with a tethered primary amine donor (C−NH2), [IrCp*(C−NH2)Cl]PF6 (3; Cp* = pentamethylcyclopentadienyl ligand), when activated by an alkoxide base, catalyzed the H2 hydrogenation of acetophenone and benzophenone under 25 bar of H2 pressure at 50 °C, achieving a maximum turnover frequency (TOF) of 416 h−1. The presence of 2-propanol accelerates catalysis only when the alkoxide base is used in large excess with respect to iridium. This system has activity for ketone hydrogenation that is inferior to that of the structurally similar ruthenium(II) complex [RuCp*(C−NH2)py]PF6 (2; py = pyridine, TOF of up to 17 600 h−1 for the hydrogenation of acetophenone). On the other hand, an iridium(III) complex that contains a cyclopentadienyl ligand (Cp), [IrCp(C−NH2)Cl]PF6 (5), has activity that is superior to that of its Cp* analogue (3) in the catalytic H2 hydrogenation of acetophenone when activated by an alkoxide base, reaching a TOF of up to 687 h−1. Consistent with our previous computational studies, an iridium(III) hydride−amine complex, [IrCp*(C−NH2)H]PF6 (6), was isolated and was found to be inactive as a catalyst for ketone hydrogenation. The cationic charge is thought to contribute to a diminished hydricity and reactivity of the iridium(III) hydride in comparison to the neutral ruthenium(II) analogue. The important role of the N−H group is illustrated as well by the poor catalytic activity of the structurally similar iridium(III) complex [IrCp*(C−NMe2)Cl]PF6 (8), which does not contain an N−H group. Nevertheless, we present evidence that complex 3, in the presence of an excess alkoxide base, does efficiently hydrogenate ketones via an outer-sphere bifunctional mechanism involving the novel, neutral hydride intermediate Ir(η4-Cp*H)(C−NH2)H. The formation of this intermediate relies on the uncommon migration of a hydride ligand to the η5-Cp* ligand which appears to be promoted by the unique NHC ligand of the present system.

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INTRODUCTION The use of bifunctional catalysts that utilize the “NH effect” is an effective strategy for the catalytic hydrogenation of polar double bonds using molecular hydrogen.1 The key intermediate step is proposed to have a six-membered pericyclic transition state involving a hydrogen-bonding interaction between the N−H group and the oxygen of the ketone in the outer coordination sphere and an attack of the carbonyl group by the metal hydride.1a,b We2 and others3 have shown that the heterolytic splitting of a coordinated η2-H2 ligand is rate-determining. Catalysis is found to proceed under very mild reaction conditions (room temperature and low H2 pressure) with no added base, and a high selectivity to the reduction of the polar double bond is normally achieved.2a−c,3a,4 In addition, the importance of the N−H group in the hydrogenation of ketones has been demonstrated for these bifunctional catalysts by the fact that systems containing only a metal hydride were found to be less active and selective and required forcing reaction conditions.5 In most cases, an inner-sphere mechanism which involves the © 2012 American Chemical Society

coordination of the ketone to the metal center is operative for these catalytic systems.5a,c−e We have previously reported nickel(II) and ruthenium(II) complexes that contained an N-heterocyclic carbene (NHC) ligand with a tethered primary amine donor (C−NH2, Figure 1).6 The ruthenium(II) complex [RuCp*(C−NH 2 )py]PF 6 (2; Cp* = pentamethylcyclopentadienyl ligand, py = pyridine; Figure 1) is a very active catalyst for the hydrogenation of a variety of polar double bonds under very mild reaction conditions (8 bar of H2 pressure, 25 °C). Turnover frequencies (TOF) as high as 17 300 h−1 can be achieved with this catalyst in the H2 hydrogenation of acetophenone in 2-propanol.6b In an accompanying paper,7 we showed that catalysis proceeds via an alcohol-assisted outer-sphere bifunctional mechanism, in which the alcohol solvent or the product alcohol participates in catalysis by acting as a proton shuttle to aid the heterolytic splitting of the coordinated η2-H2 ligand.1c,2b,3a,c,4c,5b,8 Received: January 29, 2012 Published: March 6, 2012 2152

dx.doi.org/10.1021/om300071v | Organometallics 2012, 31, 2152−2165

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mechanism: (a) the heterolytic splitting of dihydrogen at the active metal center, aided by a 2-propanol molecule acting as an proton shuttle (step A in Scheme 1) and (b) the transfer of an Ir−H/N−H couple to the ketone in the outer sphere (step B in Scheme 1). Of note, diagonal relationships between late transition metals were studied for the dihydrogen complexes [M(η2-H2)2(H)2(PCy3)]n+ (M = Ru, n = 0; M = Ir, n = 1)12 and, more recently, their use in the activation of B−H bonds.13 Also relevant to the current work is a computational study of the transfer hydrogenation of ketones catalyzed by structurally similar ruthenium(II) and iridium(I) systems.14

Figure 1. Nickel(II) and ruthenium(II) complexes containing a chelating N-heterocyclic carbene (NHC)−primary amine (C−NH2) ligand.

Iridium complexes containing a chelating phosphine−amine (P−NHR) ligand are known to catalyze the hydrogenation of ketones using 2-propanol or hydrogen as the hydrogen source (Figure 2).9 On the other hand, iridium(I) NHC and

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RESULTS AND DISCUSSION Synthesis of Iridium(III) Complexes Containing C−NH2 and P−NH2 Ligands. The air-stable iridium(III) complex [IrCp*(C−NH2)Cl]PF6 (3) was prepared by a transmetalation reaction6a,b of 1 and [IrCp*Cl2]2 in refluxing acetonitrile, similar to the preparation of the ruthenium(II) complex 2 (Scheme 2).6b The solid-state structure of 3 has a piano-stool geometry at the iridium center with coordinated Cp*, C−NH2, and chloride ligands (Figure 3). The Ir−Ccarbene distance and the Ir−Ccarbene resonance in its 13C{1H} NMR spectrum are typical of most iridium(III) complexes containing an NHC ligand.10b,d,f,15 The diphenylphosphine-containing complex [IrCp*(P−NH2)Cl]PF6 (4; P−NH2 = 2-(diphenylphosphino)benzylamine) was also prepared by the reaction of the P−NH2 ligand and [IrCp*Cl2]2, followed by the addition of AgPF6 to the reaction mixture (Scheme 3). This was also characterized by NMR (1H, 13C{1H}, and 31P{1H}) and X-ray diffraction studies (Figure 4). The ruthenium(II) analogue [RuCp*(P−NH2)py]PF6 was previously reported.6b We have also prepared the iridium(III) complex [IrCp(C−NH2)Cl]PF6 (5), which contains a cyclopentadienyl ligand (Cp), by a transmetalation reaction6a,b from 1 to [IrCpCl2]216 in refluxing acetonitrile for comparison purposes. The reaction between 1 and [IrCpBr2]216 did not result in the desired complex containing a bromide ligand. The Ir−Ccarbene resonance of 5 in acetonitrile-d3 was observed at 149.0 ppm in its 13C{1H} NMR spectrum. The iridium(III) hydride−amine complex [IrCp*(C−NH2)H]PF6 (6) was prepared from a warm 2-propanol solution of 3 containing 3 equiv of sodium 2-propoxide. The analytically pure compound can be isolated in moderate yields (Scheme 2).6c In determining its solid-state structure (Figure 5), the hydride position in the piano-stool complex was not refined but instead was located approximately by use of an electron density difference map (see the Supporting Information). The Ir−H distance was thus determined to be 1.54 Å, which is typical of iridium hydride complexes containing a Cp* ligand.17 The Ir− Ccarbene distance for 6 (2.015(5) Å) is shorter than that of 3 (2.067(7) Å). The Ir−Ccarbene and Ir−H resonances of 6 in acetonitrile-d3 were observed at 157.0 and −13.59 ppm in the 13C{1H} and 1H NMR spectra, respectively. The characteristic Ir−H stretch was observed at 2068 cm−1 in the infrared spectrum. For comparison, the analogous iridium(III) hydride−amine complex which contains a Cp ligand ([IrCp*(C−NH2)H]PF6, 7) was prepared in a fashion similar to that of 6 starting from the Cp complex 5 and 2-propoxide in 2-propanol, but this complex was not isolated. The Ir−Ccarbene and Ir−H resonances of 7 in tetrahydrofuran-d8 were observed at 149.6 and −14.13 ppm in the 13C{1H} and 1H NMR spectra, respectively. Synthesis of an Iridium(III) Complex Containing a C−NMe2 Ligand. We also prepared an analogous iridium(III)

Figure 2. Iridium catalysts for ketone hydrogenation containing a chelating phosphine−amine (P−NHR) ligand.

half-sandwich iridium(III) NHC complexes are shown to have exceptional activity in the transfer hydrogenation of ketones in 2-propanol solution in the presence of an alkoxide base.10 However, there have been no reports to date for the H2 hydrogenation of polar double bonds using an iridium NHC complex. The successful replacement of phosphine in the P−NHR ligand on iridium by an NHC donor would give a donorfunctionalized NHC11 containing an NHC ligand and an amine donor, which might give promising activity in the catalytic H2 hydrogenation of ketones. In the current study, we show that an iridium(III) complex that is structurally similar to the ruthenium(II) complex 2, when it is activated with an excess of alkoxide base in 2propanol, likely undergoes an alcohol-assisted outer-sphere bifunctional mechanism for the H2 hydrogenation of ketones involving iridium(I) intermediates (Scheme 1). We will present Scheme 1. Alcohol-Assisted Outer-Sphere Bifunctional Mechanism of H2 Hydrogenation of Ketones Catalyzed by an Iridium(I) System Containing a C−NH2 Ligand

both experimental and theoretical evidence to support the two elementary steps in the alcohol-assisted outer-sphere bifunctional 2153


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Scheme 2. Synthesis of Iridium(III) Complexes Containing a C−NH2 Ligand

Figure 4. ORTEP diagram of 4 ([IrCp*(P−NH2)Cl]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): Ir(1)−P(1), 2.307(2); Ir(1)− N(1), 2.146(8); Ir(1)−Cl(1), 2.407(2); Ir(1)−C(24), 2.208(9); P(1)−Ir(1)−N(1), 88.9(2); P(1)−Ir(1)−Cl(1), 89.02(9); Cl(1)− Ir(1)−N(1), 82.3(2).

Figure 3. ORTEP diagram of 3 ([IrCp*(C−NH2)Cl]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): Ir(1)−C(1), 2.067(7); Ir(1)− N(3), 2.127(5); Ir(1)−Cl(1), 2.424(2); Ir(1)−C(13), 2.199(6); C(1)−Ir(1)−N(3), 90.9(2); C(1)−Ir(1)−Cl(1), 92.2(2); Cl(1)− Ir(1)−N(3), 80.7(1).

dazolium chloride hydrochloride (HC−NMe2·HCl)18 and then the transmetalation of the C−NMe2 ligand from silver(I) to [IrCp*Cl2]2 in the same pot (Scheme 4).6c,19 A salt metathesis reaction with AgPF6 afforded a pale yellow powder upon isolation, which was identified as [IrCp*(C−NMe2)Cl]PF6 (8) by NMR spectroscopy and an X-ray diffraction study (Figure 6). In comparison to 3, the piano-stool complex has a shorter Ir− Ccarbene distance (3, 2.067(7) Å; 8, 2.036(4) Å), a longer Ir−N distance (3, 2.127(5) Å; 8, 2.225(4) Å), and a smaller C(1)− Ir(1)−N(3) bite angle of the chelate (3, 90.9(2)°; 8, 87.5(2)°); otherwise it has NMR spectroscopic properties similar to those of other iridium(III) complexes containing an NHC ligand.15b General Features of the H2 Hydrogenation of Ketones Catalyzed by Iridium(III) Complexes. The chloride complex 3 catalyzed the H2 hydrogenation of acetophenone in THF in the presence of potassium tert-butoxide (KOtBu) as the base to 1-phenylethanol in 3 h at 98% conversion under 25 bar of H2 pressure at 50 °C, with a catalyst/base/substrate (C/B/S) ratio

Scheme 3. Synthesis of an Iridium(III) Complex Containing a P−NH2 Ligand

complex that is structurally similar to 3 with an NHC containing a tethered tertiary amine donor (C−NMe2). When chelated, this ligand forms a seven-membered ring including the metal center. The desired complex was synthesized in one pot in two steps, by first the in situ generation of a silver(I) complex containing the C−NMe2 ligand from the reaction of silver(I) oxide and 1-(N,N-dimethylamino)propyl-3-methylimi2154


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Scheme 5. H2 Hydrogenation of Acetophenone Catalyzed by Iridium(III) Complexesa

Figure 5. ORTEP diagram of 6 ([IrCp*(C−NH2)H]PF6) depicted with thermal ellipsoids at 30% probability. The counteranion and most of the hydrogens have been omitted for clarity. The position of the hydride ligand was not refined and thus is not shown. Selected bond distances (Å) and bond angles (deg): Ir(1)−C(1), 2.015(5); Ir(1)− N(3), 2.151(4); Ir(1)−C(14), 2.233(3); C(1)−Ir(1)−N(3), 89.6(2).

a

The catalyst/base/substrate (C/B/S) ratio was 1/8/200. bTOF = turnover frequency, measured from the slope of the linear portion of [alcohol] versus time plot.

Scheme 4. Synthesis of an Iridium(III) Complex Containing a C−NMe2 ligand

C/S = 1/600 requires a longer reaction time (37% conversion in 7 h) and a 3-fold decrease in TOF to 26 h−1. The hydrogenation of benzophenone with increased substrate loading led to similar TOF values (TOF = 364 h−1 for C/S = 1/200; TOF = 472 h−1 for C/S = 1/400; see the Supporting Information, Table S3 and Figure S1). Benzophenone cannot form an enolate in the presence of base, unlike acetophenone. Thus, the coordination of an enolate of acetophenone to iridium may inhibit catalysis, as observed elsewhere.2b,6c The H2 hydrogenation of benzophenone in THF is affected by the concentrations of the catalyst and hydrogen pressure but is independent of the ketone concentration (see the Supporting Information, Figure S1) as expected. Complex 3 also catalyzed the transfer hydrogenation of acetophenone in a solution of KOtBu in 2-propanol at 75 °C, giving 69% conversion to 1phenylethanol in 17 h (Table 1). Under a hydrogen atmoTable 1. Transfer Hydrogenation of Acetophenone Catalyzed by Iridium(III) Complexes conversnb (%/h) a

entry

complex

1 2 3

3 4 6

28/3 27/3 23/3

69/17 65/20 44/19

Reactions were carried out in iPrOH (6 mL) at 75 °C under an argon atmosphere; the C/B/S ratio was 1/8/200. KOtBu was used as the base. b Conversions were determined by GC and are reported as an average of two runs. a

Figure 6. ORTEP diagram of 8 ([IrCp*(C−NMe2)Cl]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): Ir(1)−C(1), 2.036(4); Ir(1)− N(3), 2.225(4); Ir(1)−Cl(1), 2.426(1); Ir(1)−C(11), 2.264(4); C(1)−Ir(1)−N(3), 87.5(2); C(1)−Ir(1)−Cl(1), 95.0(1); Cl(1)− Ir(1)−N(3), 84.4(1).

sphere (25 bar), complex 3 catalyzed the H2 hydrogenation of acetophenone in 2-propanol at 50 °C very slowly (C/B/S ratio = 1/8/200), giving 28% conversion to 1-phenylethanol in 3 h with a very small TOF value (17 h−1, Scheme 5). The diphenylphosphine-containing complex 4, on the other hand, has lower activity in H2 hydrogenation than 3 but with similar activity in the transfer hydrogenation of acetophenone

of 1/8/200 (Scheme 5), reaching a TOF of 154 h−1. These are conditions more forcing than those of the analogous ruthenium(II) complex 2.6b An increase in substrate loading to 2155


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equiv of base (see the Supporting Information, Table S2). The addition of excess metal alkoxides is reported to accelerate the H22b,22 and transfer hydrogenation23 of ketones catalyzed by certain ruthenium(II) systems. Hartmann and Chen proposed that the cation of the added base acts as a Lewis acid to stabilize the transition state for the H+/H− transfer from the catalyst to the ketone.22 The addition of [2.2.2]cryptand in an equimolar amount to the potassium ions (C/B/S = 1/16/200) gave full conversion to 1-phenylethanol in 1 h, but with a smaller TOF value (152 h−1). This suggests that the cations do play a minor role.2b,6c In fact, stoichiometric reactions of 3 with different amounts of KOtBu in 2-propanol gave drastically different products as determined by 1H NMR (vide infra). Interestingly, when the ratio of the cyclopentadienyl catalyst 5 to the alkoxide base (KOtBu) is increased from 1/8 to 1/16, it achieves an activity for catalysis (96% conversion to 1-phenylethanol in 0.5 h, TOF = 480 h−1) similar to the reaction that was carried out in the presence of 8 equiv of base. This observation is different from that of the Cp* analogue (the chloride complex 3). Some conclusions can be drawn from the catalytic results obtained by these experiments. (a) Reactions conducted in 2-propanol solution starting from the chloride complex 3 and an alkoxide base lead to the formation of the hydride−amine complex 6 (Scheme 2), which has a poor catalytic activity if the concentration of the alkoxide base present in the beginning of the reaction is small (for example, a C/B ratio of 1/8). This explains the similar catalytic behavior when 3 was used in either the H2 or transfer hydrogenation of acetophenone in 2-propanol. (b) A critical ratio of catalyst to base (C/B = 1/8 for the hydride−amine complex 6 and C/B = 1/16 for the chloride complex 3) is needed for comparable H2 hydrogenation activity. It appears that 6 is the resting state in catalysis and that the alkoxide base must play a crucial role in the activation of this hydride. (c) Catalysis by 6 conducted in 2-propanol was faster than in THF, an aprotic solvent, using the same concentration of the alkoxide base. This is opposite to the behavior of complex 3. The alcohol solvent must participate in catalysis, acting as a reactant and/or as a proton shuttle. Selectivity in the H2 Hydrogenation of an α,βUnsaturated Ketone Catalyzed by Complexes 3 and 5. Complex 3 catalyzed the selective reduction of the ketone in trans-4-phenyl-but-3-en-2-one to give trans-4-phenyl-but-3-en2-ol (88% conversion) and 4-phenylbutan-2-ol (8% conversion) and the saturated ketone 4-phenylbutan-2-one (3% conversion) in 5 h, under 25 bar of H2 pressure at 50 °C in 2-propanol and in the presence of an excess of alkoxide base (16 equiv with respect to catalyst; Scheme 6). Under similar reaction conditions, complex 5 also catalyzed the selective reduction to give mostly trans-4-phenyl-but-3-en-2-ol

under identical reaction conditions (Scheme 5 and Table 1, entry 2). The structurally similar ruthenium(II) complex [RuCp*(P−NH2)py]PF6 also showed poor activity in the catalytic H2 hydrogenation of acetophenone in comparison to 2.6b We showed in the accompanying paper that the donor ability of the D−NH2 type ligand (D = NHC, phosphine) is an important factor in determining the catalytic activity of ketone hydrogenation.7 The iridium(III) complex 5, which contains a Cp ligand, has catalytic activity that is superior to that of its Cp* analogue, complex 3. Under reaction conditions similar to those of 3 (25 bar of H2 pressure, 50 °C, C/B/S ratio 1/8/200, KOtBu used as the base), this catalyzed the H2 hydrogenation of acetophenone in 2-propanol to 1-phenylethanol in 0.5 h at 97% conversion, achieving a TOF of 504 h−1 (Scheme 5), and in 1 h at 99% conversion with a TOF of 687 h−1 when the reaction was carried out in THF. Attempts at catalysis using the hydride−-amine complex 6 in the absence of an alkoxide, whether conducted as direct hydrogenation under H2 in THF or 2-propanol (25 bar H2 pressure, 50 °C) or transfer hydrogenation under argon at 75 °C in 2-propanol, resulted in no conversion of the starting ketone. Such observations are similar to those reported for the cationic [Ru(η6-p-cymene)(C−NH2)H]+ system.6c The neutral complexes containing a cyclometalated C−NH2 ligand, IrCp*(κ2(C,N)-2-C6H4CR2NH2)H20 and Ru(η6-C6H6)(κ2(C,N)-2C6H4CR2NH2)H,21 are active in the transfer hydrogenation of ketones in the absence of base at room temperature. Significantly, with an alkoxide base, 6 becomes active in 2propanol (TOF = 213 h−1) but less active in THF for the hydrogenation of acetophenone using H2 gas (Scheme 5). Under argon, 6 is somewhat active for the transfer hydrogenation of acetophenone in 2-propanol solution (Table 1, entry 3). Complex 8, which does not contain an N−H functionality, catalyzed the hydrogenation of acetophenone and benzophenone in THF and KOtBu very slowly to 1-phenylethanol in 2 h to 7% conversion and to diphenylmethanol in 3 h to 25% conversion, respectively, under 25 bar of H2 pressure and 50 °C (C/B/S = 1/8/200, Scheme 5). Catalysis was even slower when 2-propanol was used as the solvent (see the Supporting Information, Tables S2 and S3). This is in contrast to the previously reported systems [Ru(η6-p-cymene)(C−NH2)Cl]+ and [Ru(η6-p-cymene)(C−NMe2)Cl]+, both of which showed similar activities in catalyzing the hydrogenation of acetophenone.6c The N−H group of our iridium(III) system, therefore, is required for catalysis, consistent with an outer-sphere bifunctional mechanism for ketone hydrogenation (step B in Scheme 1). Effect of Alkoxide Base on the H2 Hydrogenation of Acetophenone Catalyzed by Complexes 3 and 5. When the ratio of catalyst to the alkoxide base (KOtBu) is increased from 1/8 to 1/16, the chloride complex 3 achieves an activity (99% conversion to 1-phenylethanol, TOF = 416 h−1) similar to that of the hydride−amine complex 6 in the presence of 8

Scheme 6. H2 Hydrogenation of trans-4-Phenyl-1-but-3-en-2-one Catalyzed by Complexes 2, 3, and 5

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A sigmoidal-type reaction profile was observed with variable induction periods when either acetophenone or benzophenone was hydrogenated using either 3 in THF or 6 in 2-propanol as the catalyst with KOtBu as the base. The addition of 1-phenylethanol at the beginning of the reaction (up to 0.030 M in THF, 40 mol %) decreases the induction period when 3 is present but increases the induction period when 6 is present (Figures 7 and 8). The product alcohol reacts with base to form 1-phenylethoxide. This readily reacts with 3, forming the hydride−amine complex 6 and acetophenone (vide infra). This has the effect of decreasing the concentration of enolate of acetophenone in the reaction mixture and makes it less available to coordinate to the active iridium species. The presence of enolate ions is believed to slow down catalysis by competing with the coordination of H2, as observed elsewhere.2b,6c Deuterium labeling studies support the formation of enolate (see below). On the other hand, the presence of 1-phenylethanol at the beginning of catalysis using the hydride−amine complex 6 and KOtBu changes the basicity of the reaction mixture. This might be important to slow down the formation of the active iridium species during catalysis. The fact that the solvent alcohol and an excess of alkoxide base is required for catalysis involving either catalyst 3 or 6 in order to achieve high and comparable activity (Figures 7 and 8) suggests that they have a role to play in catalysis. The alcohol may play a role similar to that in the ruthenium(II) system (complex 2) to accelerate catalysis by acting as a proton shuttle in the heterolytic splitting of η2-H2 in step A of Scheme 1.7 Deuterium Labeling Studies Using the Iridium(III) Complex 3. To gain further insight into the effect of 2propanol and 1-phenylethanol on the hydrogenation of acetophenone catalyzed by complex 3, acetophenone was first deuterated using D2 gas in THF in the presence of 20 mol % of 1-phenylethanol and KOtBu (Table 2, entry 1). Full conversion

(77% conversion) and more of the saturated ketone 4phenylbutan-2-one (20% conversion) in 5 h (Scheme 6). The ruthenium(II) complex 2 catalyzed the selective reduction of the ketone in the same substrate, giving trans-4-phenyl-but-3en-2-ol (89% conversion) as the major product within 1 h (Scheme 6) under very mild conditions (8 bar of H2 pressure at 25 °C). The high selectivity that was exhibited by these iridium catalysts is characteristic of bifunctional catalysis for ketone hydrogenation.24 Effect of Alcohol on the H2 Hydrogenation of Acetophenone Catalyzed by Iridium(III) Complexes 3 and 6. The effect of alcohols on the hydrogenation of acetophenone catalyzed by the chloride complex 3 and the hydride− amine complex 6 was further investigated (Figures 7 and 8).

Figure 7. Reaction profiles showing the effect of alcohols on the H2 hydrogenation of acetophenone (S) catalyzed by the chloride complex 3 (C) in the presence of KOtBu (B): (a) THF, red circles, C/B = 1/8; (b) 2-propanol, blue diamonds, C/B = 1/8; (c) THF, [1-phenylethanol] = 0.015 M, C/B = 1/8, green squares; (d) 2-propanol, C/B = 1/16, purple triangles. All of the reactions were conducted using 25 bar of H2 at 50 °C. The C/S ratio was 1/200.

Table 2. Deuteration of Acetophenone and 1-Phenylethanol Catalyzed by Complex 3 D content in 1-phenylethanol (%)b reacn entry conditionsa 1 2 3 4

10 bar D2, THF, 18 hc 10 bar D2, THF, 6 hc 8 bar D2, i PrOH, 6 he 10 bar D2, i PrOH, 6 he

CDnH3−n group (n = 1−3)

substrate

α-CD

acetophenone, 1phenylethanol 1-phenylethanold

80

14

36

9

42

15

acetophenone

30

f

1

1-phenylethanol

−

negligiblef

−

OD group

a

Reactions were carried out in a 50 mL Parr hydrogenation reactor at the required D2 pressure at 50 °C using the appropriate solvent (6 mL). KOtBu was used as the base. bThe deuterium contents of 1phenylethanol and 2-propanol were determined by 1H and 2H NMR; no deuteration was observed on the phenyl ring of the product alcohol. c The C/B/S ratio was 1/8/200. d 20 mol % of 1-phenylethanol was used with respect to 3. eThe C/B/S ratio was 1/ 16/200. fDeuteration at the OD group of 2-propanol was observed.

to the product alcohol was achieved in 18 h. Analysis of the 1 H NMR and 2H NMR spectra of the deuterated product suggested 80% deuteration at the α-carbon, 14% at the hydroxyl group, and 36% at the methyl group of 1-phenylethanol. No deuteration was observed at the phenyl ring. Base-catalyzed deuteration of the enolizable acetophenone is responsible for

Figure 8. Reaction profiles showing the effect of alcohols on the H2 hydrogenation of acetophenone catalyzed by the hydride−amine complex 6: (a) 2-propanol, blue diamonds; (b) THF, red squares; (c) 2-propanol, [1-phenylethanol] = 0.030 M, green triangles. All of the reactions were conducted using 25 bar of H2 at 50 °C, and KOtBu was used as the base. The C/B/S ratio was 1/8/200. 2157


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such a deuterium distribution on 1-phenylethanol.2b,6c In contrast, when 1-phenylethanol was deuterated for 6 h under reaction conditions similar to those above, deuteration was also observed at the α-carbon, hydroxyl, and the methyl group, but to a minor extent (Table 2, entry 2). This supports the formation of acetophenone and the hydride−amine complex 6 via the formation of an alkoxide intermediate, [IrCp*(C−NH2) (O−CHPhCH3)]+, in which the β-hydrogen of 1-phenylethoxide ligand can eliminate.1b,2a,25 Acetophenone that formed was then deuterated in the presence of an active iridium species. When acetophenone was deuterated using D2 gas in 2-propanol in the presence of complex 3 and excess base (Table 2, entry 3), deuteration at the α carbon was significantly smaller (30% deuteration) and deuteration at the hydroxyl and the methyl group was negligible. However, 2H NMR of the reaction mixture showed that the extent of deuteration of the hydroxyl group of 2-propanol was much greater than that at the α carbon of 1-phenylethanol. In fact, the effect of 2-propanol by acting as a proton shuttle in the heterolytic splitting of D2 is more important than the product alcohol, as evidenced by a reaction of 1-phenylethanol with 0.5 mol % of complex 3 in 2-propanol (Table 2, entry 4). In this case, the hydroxyl group of 2propanol was deuterated but not that of 1-phenylethanol. Stoichiometric Reactions Using Complexes 3 and 6. The reaction of the hydride−amine complex 6 and acetophenone in THF-d8 at 50 °C did not result in any product alcohol. The addition of 1.5−2 equiv of KOtBu prior to or after the addition of acetophenone to 6 in THF-d8 did not give any reduced product. It appears that metal hydride is not hydridic enough and therefore not reactive toward the polar double bond, in line with the properties of the cationic ruthenium(II) complex [Ru(p-cymene)(C−NH2)H]+ that we have isolated.6c Crabtree and co-workers have isolated an iridium(III) complex, [Ir(η5-HOC5H4)(PPh3)2H]BF4̧, which failed to react with aldehydes.26 Our attempts to observe the iridium(III) dihydrogen complex [IrCp*(C−NH2)(η2-H2)]2+ similar to the bisNHC complex [IrCp*(C−C)(η2-H2)]2+ 27 and measure its pKa value by equilibrating known amounts of 6, the phosphonium salts [HPCy3]BPh4 (pKαTHF = 9.7),28 or [HPtBu3]BPh4 (pKαTHF = 10.6)28 in THF-d8 were not successful due to the facile displacement of the coordinated dihydrogen by the phosphine that forms. When complex 3 was reacted with KOtBu under 25 bar of H2 pressure at 50 °C in 2-propanol, the products that were formed depended on the amount of base that was added. The addition of 8 equiv of KOtBu at the beginning of the reaction gave, upon evaporation of the solvent and dissolution in acetonitrile-d3, at least three species that were signaled by three resonances at around 3.6 ppm in the 1H NMR spectrum for the CH3 group on the C−NH2 ligand associated with 6 (63%) and two other species that did not contain any hydride peaks (37%, CH3

resonates at 3.69 and 3.72 ppm). When more base was added (16 equiv), the species that has the CH3 peak at 3.69 ppm was observed in 67% abundance. There were two other species that were observed in equal amounts as well, one of which gave a hydride peak at −19.72 ppm. The signals pertaining to the hydride−amine complex 6 were lost. Although the species in this complicated mixture were not identified further, this shows that complex 6 reacts further with excess alkoxide base. Further, a reaction of complex 3 with potassium 1-phenylethoxide in warm THF (50 °C) under argon afforded 6 and acetophenone in 21% conversion as a result of β-hydrogen elimination of 1-phenylethoxide. This also left behind the starting complex as identified by 1H NMR in acetonitrile-d3. Heating such a sample overnight in a sealed NMR tube afforded conversion to 6 in 63% and acetophenone in 70% (see the Supporting Information, Figure S2). We conclude from these experiments that the hydride− amine complex 6 is unlikely to be an active species involved in step B of the alcohol-assisted outer-sphere bifunctional mechanism for ketone hydrogenation (Scheme 1). This is also consistent with our calculations that the cationic model complex [Ir(η5-Cp)(C−NH2)H]+ is the resting state of the outer-sphere bifunctional mechanism in the H2 hydrogenation of ketone.7 These results also establish the formation of complex 6 at the initial stage of catalysis, especially when the concentration of the active catalyst is low and there exists a similar amount of alkoxide anions containing β-hydrogen in the reaction mixture. The activation of such a complex by an alkoxide base, therefore, becomes important at a later stage of catalysis. Outer-Sphere Bifunctional Mechanism Involving Iridium(I) Intermediates. As the cationic hydride−amine complex 6 must be activated during catalysis by an alkoxide base, H2, and 2-propanol, we proposed, on the basis of computational results (density functional theory, DFT),29 an alcoholassisted outer-sphere bifunctional mechanism for ketone hydrogenation involving iridium(I) intermediates. Simplifications were made by replacing the Cp* ligand with Cp; acetone and 2-propanol were used to model the ketone and the product alcohol. When a 2-propoxide anion is reacted with the cationic hydride−amine model complex [IrCp(C-NH2)H]+, the neutral amido−hydride complex R is formed as a result of the deprotonation of the N−H group on the C−NH2 ligand. This has a 2-propanol molecule hydrogen bonded to the amido nitrogen (Scheme 7). An alternative structure Q can also form by the reaction of the cationic hydride-amine complex, [IrCp(C−NH2)H]+, and 2-propoxide. This has a coordinated η5-Cp, hydride and 2-propoxide and a decoordinated amine group of the C−NH2 ligand. This structure is thermodynamically less stable than R (ΔG = 11.4 kcal/mol, Scheme 7). As a coordination site is required for the activation of H2 starting from

Scheme 7. Computed Pathway for Hydride Migration from the Ir−H Bond to the Coordinated Cp Ligand Starting from R

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Scheme 8. Computed Alcohol-Assisted Outer-Sphere Bifunctional Mechanism in the H2 Hydrogenation of Acetone Catalyzed by Complexes of Iridium(I)

According to the mechanism shown in Scheme 7, the amido−hydride complex S is formed by the loss of a 2propanol molecule from R. The hydride ligand from S can then add to the Cp ligand, which is an uphill process and is slightly entropically disfavored (ΔG⧧ = 35.1 kcal/mol from S, ΔS⧧ = −3.0 cal/(mol K); Scheme 7). Interestingly, replacing the Cp with a Cp* ligand gave a very similar barrier to this process (ΔG⧧ = 35.2 kcal/mol from S*, ΔS⧧ = −1.2 cal/(mol K); see the Supporting Information, Figures S4 and S5). This contributes to the induction period that was observed in the H2 hydrogenation of acetophenone in 2-propanol catalyzed by the hydride−amine complex 6 in the presence of KOtBu. The analogous hydride migration product of ruthenium(0), Ru(η4CpH)(C−NH2), is thermodynamically less stable than Ru(η5Cp)(C−NH2)H (ΔG = 23.8 kcal/mol), although the barrier for hydride migration is similar in energy (ΔG⧧ = 35.3 kcal/mol from Ru(η5-Cp)(C−NH2)H; see the Supporting Information, Figure S4). Significantly, this neutral iridium(I) system, containing an η4cyclopentadiene ligand, allows efficient bifunctional catalysis via the outer-sphere mechanism (Scheme 8). The square-planar amido complex HI1 can heterolytically split a coordinated H2 molecule (ΔG⧧ = 28.1 kcal/mol, ΔH⧧ = 19.7 kcal/mol) in the transition state TS(HI)3,4 (Figure 9). The distorted-squarepyramidal hydride-amine complex HI4, which has an apical hydride ligand (Ir−H distance 1.64 Å), then completes the cycle via the transition state TS(HI)5,6 by transferring its

structure Q or R, we have evaluated three feasible pathways, which included reductive elimination of 2-propanol from Q, ring slippage of the Cp ligand of R, and hydride migration to the Cp ligand of R. Bergman and co-workers proposed similar pathways in the reductive elimination of ethanol from IrCp* (PPh3)(H)(OEt) and subsequent coordination of a neutral ligand L forming the iridium(I) species IrCp*(PPh3)L.17b To our surprise, the alcohol-assisted outer-sphere bifunctional mechanism involving a hydride migration to the Cp* ligand was an energetically favorable pathway for ketone hydrogenation: this will be presented in detail. The migration of a coordinated ligand (hydrides,30 alkyl or aryl groups31) to the η5-Cp* ligand,30b,c in particular in an endo fashion,30a,b,31b,32 is not common, but there are examples of stable iron(II) and ruthenium(II) complexes containing an η4-CpH ligand as a result of an exo33 addition of the hydride ligand to the η5-Cp ring. The two other pathways, reductive elimination of alcohol and the ring slippage of the η5-Cp* ring, are characterized by higher free energy barriers to transfer a hydride to acetone in the outer sphere (71.5 kcal/mol for the reductive elimination mechanism and 34.1 kcal/mol for the ring slippage mechanism). The reductive elimination pathway, however, is not feasible, as this involves the decoordination of the amine throughout the catalytic cycle, while the experimental evidence supports the role of the N−H group. The complete free energy profiles for these cycles are given in the Supporting Information, sections 7 and 8. 2159


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Figure 9. Computed transition state structures for the non-alcohol-assisted (TS(HI)3,4, left) and the alcohol-assisted heterolytic splitting of H2 (TS(HI)3,4alc, right) by the iridium(I) system. The bond lengths (Å) are given in the structures. Color code for the atoms: (yellow) iridium; (blue) nitrogen; (red) oxygen; (gray) carbon; (white) hydrogen.

Figure 10. Free energy profile for (a) the alcohol-assisted outer-sphere bifunctional mechanism (blue pathway) and (b) the non-alcohol-assisted outer-sphere bifunctional mechanism (red pathway) in the H2 hydrogenation of acetone starting from the amido complex HI1 and moving to the right. The gas-phase free energies (1 atm, 298 K) are reported relative to HI1, H2, acetone, and 2-propanol for the blue pathway, and to HI1, H2, and acetone for the red pathway, in kcal/mol.

Although the replacement of Cp* ligand in the chloride complex 3 with the Cp ligand favors facile ring slippage from Ir(η5-Cp)(C−NH)H to Ir(η3-Cp)(C−NH)H during catalysis (ΔG⧧ = 35.4 kcal/mol and ΔH⧧ = 25.4 kcal/mol for ring slippage and the activation of H2; see the Supporting Information, Figure S12), this also favors hydride migration from the hydride ligand of [Ir(η5-Cp)(C−NH)H] to the Cp ring leading to the complex Ir(η4-CpH)(C−NH) (ΔG⧧ = 35.1 kcal/mol for hydride migration to the Cp ring; Scheme 7). We attributed the activity that the Cp* (complex 3) and the Cp (complex 5) catalysts exhibited in catalysis to the lower free energy barrier to

proton/hydride couple to acetone in the outer sphere, forming the amido complex HI1 and 2-propanol, with a free energy barrier of 16.5 kcal/mol (ΔH⧧ = 1.8 kcal/mol; Figure 10). The transfer of a H+/H− couple from Ir(η4-CpH)(C−NH2)H to a ketone in the outer coordination sphere has a similar free energy barrier in comparison to that of Ru(η5-Cp)(C−NH2)H, but smaller compared to that of the cationic hydride−amine complex [Ir(η5-Cp)(C−NH2)H]+.7 It appears that the neutral charge of the iridium(I) system is an important factor in determining the reactivity of the bifunctional M−H/N−H pair during catalysis. 2160


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transfer a H+/H− couple to a ketone in the outer coordination sphere from Ir(η4-CpH)(C−NH2)H (ΔG⧧ = 16.5 kcal/mol, ΔH⧧ = 1.8 kcal/mol) in comparison to the ring-slipped dihydride complex Ir(η3-Cp)(C−NH2)H2 (ΔG⧧ = 34.1 kcal/ mol, ΔH⧧ = 18.9 kcal/mol). Alcohol-Assisted Outer-Sphere Bifunctional Mechanism Involving Iridium(I) Intermediates. The presence of 2-propanol in the system (Scheme 8 and Figure 10) effectively lowers the energy barrier for the heterolytic splitting of H2 by acting as a proton shuttle (ΔG⧧ = ΔG(TS(HI)3,4alc) - ΔG (HI1alc) = 20.7 kcal/mol; ΔH⧧= 9.6 kcal/mol). The transition state structure TS(HI)3,4alc shows that the protons neighboring the hydroxyl oxygen are in close proximity by the formation of two hydrogen-bonds via O···H interactions (H2−O = 1.66 Å, H3−O = 1.24 Å; Figure 9). On the other hand, the energy barriers for the coordination of H2 to HI1alc (ΔG⧧ = ΔG(TS(HI)2,3alc) − ΔG(HI1alc) = 20.5 kcal/mol; ΔH⧧ = 10.1 kcal/mol) and the transfer of the proton/hydride couple to acetone in the outer sphere (ΔG⧧ = 17.0 kcal/mol; ΔH⧧ = 2.0 kcal/mol) are less affected by the presence of a hydrogenbonded 2-propanol molecule in the system.4c,7 All these provide evidence to support an alcohol-assisted outer-sphere bifunctional mechanism (Scheme 1) and serve to explain the important role of 2-propanol as a solvent during catalysis.

cyclometalated C−NH2 ligand and an anionic carbon donor IrCp*(κ2(C,N)-2-C 6H 4CR2 NH2)H 20 and Ru(η6 -C 6H 6)(κ2 (C,N)-2-C6H4CR2NH2)H21 react with acetophenone in the absence of base, while the cationic complex [Ru(p-cymene)(C−NH2)H]PF6 does not.6c The decreased hydricity of such cationic piano-stool metal hydrides in catalysis appears to be a general phenomenon.28,34 The activation of the iridium(III) hydride−amine complex 6 by alkoxide base seems to be crucial for catalysis. Our computational studies support the proposal that the base deprotonates the amine group of the cationic hydride−amine complex 6 and this then triggers the migration of the hydride to the η5-Cp ring, producing a neutral iridium(I) amido complex. This amido complex with an η4-cyclopentadiene (CpH) group splits η2-H2 with the assistance of alcohol to produce the Ir−H/N−H couple required for the outer-sphere hydrogenation of ketones in the bifunctional mechanism. These steps involve novel iridium(I) intermediates and proceed with reasonable free energy barriers. The two other possibilities, which are the reductive elimination of 2-propanol and the ring slippage pathway, are characterized by a high energy barrier to transfer the metal hydride to the ketone in the outer sphere within the catalytic cycle in comparison to our proposed catalytic cycle. As experimental evidence supports the role of the N−H group, the reductive elimination pathway is not feasible, as this involves the decoordination of the amine throughout the catalytic cycle. Although the migration of the hydride ligand to the η5-Cp* ligand is not common but is precedented,17b,30b,c this might be favored by the presence of the unique NHC ligand in our catalytic system.11b,d It appears that the activation of the cationic hydride− amine complex 6 is responsible for the poor activity of the iridium(III) system for catalytic ketone hydrogenation in comparison to its ruthenium(II) counterpart. This role of the alkoxide base in the catalysis should be considered for other systems involving catalytic ketone hydrogenation. Although it has been suggested that the alkali-metal cations of these bases may play a crucial role in stabilizing the transition state during hydride transfer to the ketone,22,23 we have found that the alkoxide base reacts with the metal hydride complex by changing the geometry and the electronics of the piano-stool complex in order to make it reactive. It is known that certain ruthenium(II)-based catalysts require a high base loading (C/B > 1/1000) in the hydrogenation of ketones using H2.2b,35 The role of the alkoxide base remains unclear, but it may play a similar role in transforming the metal hydride complex to a more reactive species which activates hydrogen and promotes H+/H− transfer to the ketone. Bergens suggests that the presence of excess alkoxide anions promotes the elimination of alkoxide from the complex Ru(H)((R)-tol-BINAP)((R,R)-dpen)(2-propoxide) (dpen = 1,2-diphenylethylenediamine) to form the corresponding hydride−amido complex and 2-propanol.36 We conclude from the present and our previous studies on the analogous ruthenium(II) complex7 that a fine tuning of the Brønsted acidities of the N−H and the M−H groups is crucial to maximize the activity for the hydrogenation of polar bonds. This is accomplished by choosing the right donor ligand and the right metal center, as well as the correct overall charge of the catalyst. Both studies will provide guidelines in catalyst architecture and in the rational use of alkoxide base to maximize the potential of bifunctional catalysts in the direct hydrogenation of polar bonds.

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CONCLUDING REMARKS An alcohol-assisted outer-sphere bifunctional mechanism is proposed for the iridium(III) catalyst system (complexes 3 and 6), which involves iridium(I) intermediates and has the following features: (a) an N−H group is required by the chloride complex 3 for catalysis, since the structurally similar iridium(III) complex 8 with no N−H group is much less active (b) the high selectivity that complex 3 and the Cp complex 5 exhibited toward the reduction of the carbonyl group in an α,β-unsaturated ketone (c) the pronounced effect of alcohols in catalysis with the hydride−amine complex 6 and with the chloride complex 3 in the presence of an excess alkoxide base; labeling studies further support the role of alcohol during catalysis (d) a high free energy barrier is calculated for the transfer of a proton/hydride couple from the cationic hydride−amine complex [Ir(η5-Cp)(C−NH2)H]+,7 but an accessible energy barrier is calculated for the transfer of a proton/ hydride couple from the neutral hydride−amine complex Ir(η4-CpH)(C−NH2)H to acetone in the outer sphere (e) a significant decrease in the free energy barrier to the heterolytic splitting of the η2-H2 ligand on Ir(η4CpH)(C−NH) is calculated when a 2-propanol molecule acts a proton shuttle by participating in a six-memberedring transition state;1c,2b,3a,c,4c,5b,8 the same effect was observed in the ruthenium(II) system which involves an amido intermediate, Ru(η5-Cp)(C−NH)7 A comparison of the activity and selectivity of two structurally similar ruthenium(II) (2) and iridium(III) (3 and 6) catalyst systems in the H2 hydrogenation of ketones reveals that the ruthenium system has higher activity than the iridium system, but similar selectivity in ketone hydrogenation. More importantly, the cationic charge of the intermediates of the iridium(III) system lead to a diminished ability of the hydride− amine complex 6 to react with a ketone in the absence of base. The neutral, well-defined complexes containing a 2161


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obtained. After the reaction mixture was stirred further for 0.5 h, it was filtered through a pad of Celite under a nitrogen atmosphere. The solvent was removed at reduced pressure. The yellow crude product was recrystallized with dichloromethane (2 mL) and a diethyl ether and pentane mixture (1/8, 12 mL) to yield a yellow solid, which was filtered and dried in vacuo. Yield: 54 mg, 57%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 4 in dichloromethane under a nitrogen atmosphere. 1H NMR (CD2Cl2, δ): 7.76 (m, Ar-CH of PPh2, 2H), 7.59 (m, Ar-CH of PPh2, 8H), 7.44 (m, 3-CH of Ph, 1H), 7.38 (m, 4CH and 5-CH of Ph, 2H), 7.22 (m, 6-CH of Ph, 1H), 5.52 (br, NH2, 1H), 4.39 (m, CH2, 1H), 3.73 (m, br, CH2 and NH2, 2H), 1.55 (s, CH3 of Cp*, 15H). 19F NMR (CD2Cl2, δ): −72.3 (d, JPF = 712 Hz). 31P{1H} NMR (CD2Cl2, δ): 4.5 (s), −144.4 (sept, JPF = 712 Hz). 13 C{1H} NMR (CD2Cl2, δ): 138.5 (d, JCP = 13.62 Hz, CPPh), 135.4 (d, JCP = 12.07 Hz, CPPh), 134.3 (d, JCP = 9.54 Hz, CPPh), 132.8 (d, JCP = 2.54 Hz, CPPh), 132.3 (d, JCP = 2.69 Hz, CPPh), 131.8 (CPh), 131.7 (CPh), 131.6 (d, JCP = 2.42 Hz, CPh), 131.0 (d, JCP = 2.36 Hz, CPh), 130.8 (d, JCP = 55.83 Hz, CPPh), 129.6 (d, JCP = 10.96 Hz, CPh), 129.0 (d, JCP = 11.08 Hz, CPh), 127.5 (d, JCP = 60.80 Hz, CPPh), 124.8 (d, JCP = 56.53 Hz, CPPh), 94.5 (CAr−Cp*), 48.8, (d, JCP = 11.00 Hz, CH2), 8.8 (CH3 of Cp*). MS (ESI, methanol/water; m/z): 654.2 [M]+, 618.2 [M − Cl]+. HRMS (ESI, methanol/water; m/z): calcd for C29H33NPClIr+ [M]+ 654.1663, found 654.1638. Anal. Calcd for C29H33NClF6P2Ir: C, 43.58; H, 4.16; N, 1.75. Found: C, 43.11; H, 4.11; N, 1.79. Synthesis of [1-(2-Aminomethylphenyl)-3-methylimidazol2-ylidene]chloro(η5-cyclopentadienyl)iridium(III) Hexafluorophosphate (5; [IrCp(C−NH2)Cl]PF6). A Schlenk flask was charged with 1 (66 mg, 0.091 mmol) and [IrCpCl2]2 (60 mg, 0.091 mmol). Dry acetonitrile (12 mL) was added to the reaction mixture, and it was refluxed under an argon atmosphere for 2.5 h until a deep green solution was obtained. The solvent was evaporated under reduced pressure, and the residue was extracted with a tetrahydrofuran and dichloromethane mixture (1/1 by volume, 10 mL) and filtered through a pad of Celite under a nitrogen atmosphere. Addition of pentane (15 mL) to this afforded a yellow precipitate. This was collected on a glass frit, washed with pentane (1 mL) and dried in vacuo. Yield: 57 mg, 50%. 1H NMR (CD3CN, δ): 7.68 (m, 3-CH of Ph, 1H), 7.61 (m, 4-CH and 5-CH of Ph, 2H), 7.54 (m, 6-CH of Ph, 1H), 7.47 (d, JHH = 2.07 Hz, 5-CH of imid, 1H), 7.45 (d, JHH = 2.07 Hz, 4-CH of imid, 1H), 6.24 (br, NH2, 1H), 5.22 (s, CH of Cp, 5H), 4.31 (br, NH2, 1H), 4.12 (m, CH2, 1H), 4.04 (s, CH3, 3H), 3.26 (dt, JHH = 2.92, 12.37 Hz, CH2, 1H). 19F NMR (CD3CN, δ): −73.3 (d, JPF = 707 Hz). 13C{1H} NMR (CD3CN, δ): 149.0 (Ir−Ccarbene), 140.0 (CPh), 132.6 (CPh), 132.2 (CPh), 131.0 (CPh), 129.8 (CPh), 126.3 (Cimid), 126.2 (CPh), 124.3 (Cimid), 79.3 (CAr‑Cp), 47.3 (CH2), 39.4 (CH3). MS (ESI, methanol/water; m/z): 480.1 [M]+. HRMS (ESI, methanol/water; m/z): calcd for C16H18N3ClIr+ [M]+ 480.0813, found 480.0823. Anal. Calcd for C16H18N3ClF6PIr: C, 30.75; H, 2.90; N, 6.72. Found: C, 30.80; H, 2.91; N, 6.04. Synthesis of [1-(2-Aminomethylphenyl)-3-methylimidazol2-ylidene]hydrido(η5-pentamethylcyclopentadienyl)iridium(III) Hexafluorophosphate (6; [IrCp*(C−NH2)H]PF6). A Schlenk flask was charged with 3 (50 mg, 0.072 mmol) in 2-propanol solution (14 mL). The solution was warmed to 50 °C under an argon atmosphere. A solution of sodium 2-propoxide (18 mg, 0.22 mmol) in 2propanol (6 mL) was added to this stirring solution over the course of 0.5 h, whereupon the reaction mixture turned from yellow to red and then to deep brown. The solution was stirred for a further 3 h. After the reaction had gone to completion, the solvent was removed under vacuum. The solid residue was extracted with THF (4 mL) and filtered through a pad of Celite. The addition of pentane (16 mL) to the THF solution yielded a beige precipitate which was collected and dried in vacuo. Yield: 36 mg, 76%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 5 in THF under a nitrogen atmosphere at −30 °C. 1H NMR (CD3CN, δ): 7.60 (m, 3-CH and 4-CH of Ph, 2H), 7.53 (m, 5-CH of Ph, 1H), 7.49 (m, 6-CH of Ph, 1H), 7.37 (d, JHH = 2.13 Hz, 5-CH of imid, 1H), 7.34 (d, JHH = 2.13 Hz, 4-CH of imid, 1H), 4.80

EXPERIMENTAL SECTION

Synthesis. All of the preparations and manipulations, except where otherwise stated, were carried out under a nitrogen or argon atmosphere using standard Schlenk-line and glovebox techniques. Dry and oxygen-free solvents were always used. The synthesis of bis[1-(2aminomethylphenyl)-3-methylimidazol-2-ylidene]nickel(II) hexafluorophosphate (1) has been reported previously.6a The syntheses of [IrCp*Cl2]2,37 [IrCpCl2]2,16 2-(diphenylphosphino)benzylamine (P−NH2),38 and 1-(N,N-dimethylamino)propyl-3-methylimidazolium chloride hydrochloride (HC−NMe2·HCl)18 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 13C{1H} NMR were measured relative to partially deuterated solvent peaks but are reported relative to tetramethylsilane (TMS). All 19F 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 DenzoSMN 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 in the Supporting Information. Synthesis of [1-(2-Aminomethylphenyl)-3-methylimidazol2-ylidene]chloro(η5-pentamethylcyclopentadienyl)iridium(III) Hexafluorophosphate (3; [IrCp*(C−NH2)Cl]PF6). A Schlenk flask was charged with 1 (71 mg, 0.098 mmol) and [IrCp*Cl2]2 (78 mg, 0.098 mmol). Dry acetonitrile (12 mL) was added to the reaction mixture, and it was refluxed under an argon atmosphere for 2.5 h until a deep green solution was obtained. The solvent was evaporated under reduced pressure, and the residue was extracted with tetrahydrofuran (4 mL) and filtered through a pad of Celite under a nitrogen atmosphere. Addition of diethyl ether (15 mL) to this and slow cooling of the solution at −25 °C afforded a yellow precipitate. This was collected on a glass frit, washed with diethyl ether (1 mL), and dried in vacuo. Yield: 127 mg, 93%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated solution of 3 in acetonitrile under a nitrogen atmosphere. 1H NMR (CD2Cl2, δ): 7.75 (dd, JHH = 1.70, 7.32 Hz, 3-CH of Ph, 1H), 7.59 (m, 4-CH and 5-CH of Ph, 2H), 7.48 (dd, JHH = 1.51, 7.55 Hz, 6-CH of Ph, 1H), 7.34 (d, JHH = 2.06 Hz, 5-CH of imid, 1H), 7.32 (d, JHH = 2.06 Hz, 4-CH of imid, 1H), 4.38 (br, CH2 and NH2, 3H), 4.04 (s, CH3, 3H), 3.29 (m, CH2, 1H), 1.33 (s, CH3 of Cp*, 15H). 19F NMR (CD2Cl2, δ): −72.2 (d, JPF = 712 Hz). 13C{1H} NMR (CD2Cl2, δ): 155.8 (Ir−Ccarbene), 139.1 (CPh), 132.7 (CPh), 131.5 (CPh), 130.9 (CPh), 130.1 (CPh), 125.8 (CPh), 125.4 (Cimid), 124.2 (Cimid), 90.5 (CAr−Cp*), 47.4 (CH2), 38.9 (CH3), 8.6 (CH3 of Cp*). MS (ESI, methanol/water; m/z): 550.2 [M]+, 514.2 [M − Cl]+. HRMS (ESI, methanol/water; m/z): calcd for C21H28N3ClIr+ [M]+ 550.1595, found 550.1566. Anal. Calcd for C21H28N3ClF6PIr: C, 36.29; H, 4.06; N, 6.05. Found: C, 36.72; H, 3.97; N, 5.23. Synthesis of [2-(Diphenylphosphino)benzylamine]chloro(η5pentamethylcyclopentadienyl)iridium(III) Hexafluorophosphate (4; [IrCp*(P−NH2)Cl]PF6). A scintillation vial with a threaded screw cap was charged with [IrCp*Cl2]2 (47 mg, 0.059 mmol) in dry dichloromethane (6 mL) under a nitrogen atmosphere. A solution of 2-(diphenylphosphino)benzylamine (36 mg, 0.12 mmol) in dry dichloromethane (6 mL) was added to the aforementioned yellow solution and stirred for 1 h at room temperature (25 °C). Silver hexafluorophosphate (30 mg, 0.12 mmol) in dry acetonitrile (1 mL) was added to the reaction mixture, and a pale yellow suspension was 2162


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Representative Example of a Stoichiometric Reaction Using High Pressure of H2. Complex 3 (5 mg, 7.2 μmol) and potassium tert-butoxide (6 mg, 0.053 mmol) were dissolved separately in 2propanol (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 25 bar, and the reaction mixture was stirred at 50 °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 acetonitriled3 and filtered through a pad of Celite, and a 1H NMR spectrum was measured. Stoichiometric Reaction Using 3 and 1-Phenylethoxide. To a solution of complex 3 (15 mg, 0.022 mmol) in THF (6 mL) was added potassium 1-phenylethoxide (5 mg, 0.031 mmol) in THF (4 mL) under an argon atmosphere at 50 °C. The reaction mixture was stirred for 3 h, and the solvent was removed under vacuum. The residue was taken up in acetonitrile-d3 and filtered through a pad of Celite, and a 1H NMR spectrum was measured. The 1H NMR spectrum was compared to authentic samples of 3, 6, and acetophenone in acetonitrile-d3, and the conversion was measured by comparing the integration of the methyl groups of each. 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 50 °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 base solutions. In a typical run (Scheme 5), catalyst 3 (3 mg, 4.3 μmol) and acetophenone (104 mg, 0.87 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.2d 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)-1-phenylethanol, 8.03;

(br, NH2, 1H), 4.01 (m, CH2, 1H), 3.73 (s, CH3, 3H), 3.55 (br, NH2, 1H), 2.85 (dt, JHH = 3.50, 12.40 Hz, CH2, 1H), 1.51 (s, CH3 of Cp*, 15H), −13.59 (s, Ir−H, 1H). 19F NMR (CD3CN, δ): −73.0 (d, JPF = 706 Hz). 13C{1H} NMR (CD3CN, δ): 157.0 (Ir−Ccarbene), 140.8 (CPh), 133.0 (CPh), 132.5 (CPh), 130.6 (CPh), 129.4 (CPh), 126.4 (CPh), 123.8 (Cimid), 123.5 (Cimid), 90.6 (CAr‑Cp*), 49.2 (CH2), 39.6 (CH3), 9.8 (CH3 of Cp*). IR (KBr, cm−1): 2068 (ν(Ir−H)). MS (ESI, methanol/water; m/z): 516.2 [M]+. HRMS (ESI, methanol/water; m/z): calcd for C21H29N3ClIr+ [M]+ 516.1985, found 516.1961. Anal. Calcd for C21H29N3F6PIr: C, 38.18; H, 4.42; N, 6.36. Found: C, 38.02; H, 4.35; N, 6.01. Observation of [1-(2-Aminomethylphenyl)-3-methylimidazol-2-ylidene]hydrido(η5-cyclopentadienyl)iridium(III) Hexafluorophosphate (7; [IrCp(C−NH2)H]PF6). A Schlenk flask was charged with 5 (6 mg, 0.010 mmol) and sodium 2-propoxide (2 mg, 0.024 mmol) in 2-propanol solution (10 mL). The solution was warmed to 50 °C under an argon atmosphere. The reaction mixture turned from yellow to deep brown upon stirring. This was stirred for a further 2 h. After the reaction had gone to completion, the solvent was removed under vacuum. The solid residue was extracted with THF-d8 (0.5 mL) and filtered through a pad of Celite. This solution was then used directly for NMR analysis. 1H NMR (THF-d8, δ): 7.60 (m, 3-CH of Ph, 1H), 7.58 (m, 4-CH of Ph, 1H), 7.52 (d, JHH = 2.14 Hz, 5-CH of imid, 1H), 7.51 (m, 5-CH of Ph, 1H), 7.49 (m, 6-CH of Ph, 1H), 7.48 (d, JHH = 2.14 Hz, 4-CH of imid, 1H), 5.60 (br, NH2, 1H), 4.89 (s, CH of Cp, 5H), 4.71 (br, NH2, 1H), 3.94 (dd, JHH = 3.30, 12.09 Hz, CH2, 1H), 3.73 (s, CH3, 3H), 2.90 (dt, JHH = 4.05, 12.17 Hz, CH2, 1H), −14.13 (s, Ir−H, 1H). 19F NMR (THF-d8, δ): −73.3 (d, JPF = 710 Hz). 13 C{1H} NMR (THF-d8, δ): 149.6 (Ir−Ccarbene), 141.7 (CPh), 134.3 (CPh), 132.6 (CPh), 130.4 (CPh), 129.5 (CPh), 125.4 (Cimid), 123.9 (Cimid), 122.7 (CPh), 78.5 (CAr‑Cp), 49.2 (CH2), 40.6 (CH3). Synthesis of [1-(N,N-Dimethylaminopropyl)-3-methylimidazol-2-ylidene]chloro(η5-pentamethylcyclopentadienyl)iridium(III) Hexafluorophosphate (8; [IrCp*(C−NMe2)Cl]PF6). A Schlenk flask was charged with silver(I) oxide (65 mg, 0.28 mmol) and [IrCp*Cl2]2 (75 mg, 0.094 mmol) in dry acetonitrile (8 mL) under molecular sieves (3 Å). A separate Schlenk flask was charged with 1(N,N-dimethylamino)propyl-3-methylimidazolium chloride hydrochloride (HC−NMe2·HCl) (45 mg, 0.19 mmol) and anhydrous dimethyl sulfoxide (DMSO, 3 mL) under an argon atmosphere. The DMSO solution containing the dissolved imidazolium salt was then added to the stirring solution of silver(I) oxide and [IrCp*Cl2]2 in acetonitrile. Acetonitrile washing (4 mL) was applied to the residual DMSO containing the imidazolium salt, and this was also added to the reaction mixture. This was stirred under an argon atmosphere at room temperature (25 °C) overnight. After the reaction had gone to completion, the reaction mixture was filtered through a pad of Celite under an argon atmosphere. The solvent was evaporated under reduced pressure. The residue that obtained was washed with toluene (6 mL) and then diethyl ether (8 mL). It was dissolved in a dichloromethane and acetonitrile mixture (1/1, 10 mL), and to this was added silver hexafluorophosphate (47 mg, 0.19 mmol) and the mixture was stirred further for 1 h. The suspension that formed was filtered through a pad of Celite. The solvent was evaporated under reduced pressure, and the residue that obtained was precipitated from tetrahydrofuran (3 mL) and pentane (15 mL). This was filtered and dried in vacuo to give a bright yellow solid. Yield: 77 mg, 61%. Suitable crystals for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated of 6 in dichloromethane under a nitrogen atmosphere. 1 H NMR (CD2Cl2, δ): 7.17 (d, JHH = 2.00 Hz, 5-CH of imid, 1H), 7.06 (d, JHH = 2.00 Hz, 4-CH of imid, 1H), 4.28 (m, CH2, 1H), 4.18 (m, CH2, 1H), 3.92 (s, CH3, 3H), 3.49 (t, JHH = 13.12 Hz, CH2, 1H), 3.24 (m, CH2, 1H), 2.21 (t, JHH = 13.41 Hz, CH2, 2H), 1.70 (s, CH3 of N(CH3)2, 6H), 1.56 (s, CH3 of Cp*, 15H). 19F NMR (CD2Cl2, δ): −73.1 (d, JPF = 711 Hz). 13C{1H} NMR (CD2Cl2, δ): 155.4 (Ir−Ccarbene), 124.6 (Cimid), 124.3 (Cimid), 91.6 (CAr‑Cp*), 69.8 (CH2), 49.3 (CH2), 38.7 (CH3), 26.8 (CH2), 9.9 (CH3 of Cp*), 8.8 (CH3 of N(CH3)2). MS (ESI, methanol/water; m/z): 496.2 [M − Cl]·+, 451.2 [M − Cl − NMe2]•+. Anal. Calcd for C19H32N3ClF6PIr: C, 33.80; H, 4.78; N, 6.22. Found: C, 33.96; H, 4.74; N, 6.34. 2163


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benzophenone, 7.94; diphenylmethanol, 12.57. 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 0339 and 0940 packages with the restricted hybrid mPW1PW91 functional.41 Iridium was treated with the SDD42 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 functionals43 and additional p orbitals on hydrogen as well as additional d orbitals on carbon, nitrogen, and oxygen.44 All geometry optimizations were conducted in the gas phase, and the stationary points were characterized by normalmode 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 3, 4, 6, and 8, text, tables, and figures giving details for catalysis, Cartesian coordinates, energies for all of the computed structures, and the complete citation for refs 39 and 40, and AVI files giving animations for loose vibrations 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.

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REFERENCES

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