On the “Reverse Gear”Mechanism of the Reversible Dehydrogenation

ARTICLE pubs.acs.org/Organometallics

On the “Reverse Gear”Mechanism of the Reversible Dehydrogenation/ Hydrogenation of a Nitrogen Heterocycle Catalyzed by a Cp*Ir Complex: A Computational Study Haixia Li, Jinliang Jiang, Gang Lu, Fang Huang, and Zhi-Xiang Wang* College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing, 100049, People's Republic of China

bS Supporting Information ABSTRACT: Development of efficient dehydrogenation/hydrogenation is critical to the realization of organic hydride hydrogen storage. Using B3LYP DFT calculations, we have investigated the catalytic mechanisms of the reversible 3qu4h/ 4qu dehydrogenation/hydrogenation (3qu4h = tetrahydroquinoline and 4qu = quinoline) catalyzed by the Ir-complex 2cat (Cp*IrPYD0 , Cp* = η5-C5Me5 and PYD0 = CF3-substituted 2-pyridonate), reported by Yamaguchi and Fujita et al. Two reactive species (7bif and 12hcl) are identified to play important roles in the catalytic system. The species (7bif) with bifunctional reactivity can be facilely obtained from 2cat by just rotating the PYD0 ligand, while 12hcl (Cp*IrHCl) is generated via HPYD0 ligand (HPYD0 = CF3-substituted 2-hydroxypyridine) dissociation after hydrogen transfer in dehydrogenation or hydrogen activation in hydrogenation. The species 7bif mediates dehydrogenation via a hydrogen transfer mechanism, which is more favorable than the β-H elimination (BETAHE) one, confirming our conclusion drawn in the study of alcohol dehydrogenation catalyzed by a similar catalyst. The 12hcl species combines the substrates (e.g., 4qu) to form a reactive pair that simultaneously possesses Lewis acidic and basic reactivity to activate H2 with a mechanism similar to the H2 activation by metal-free FLP (frustrated Lewis pair). The hydrogen activation by the pair gives an ion pair that undergoes hydride transfer to complete the hydrogenation. Because the dimer (Cp*IrHCl)2 itself does not show reactivity toward hydrogen activation but can be easily decomposed into the reactive monomer (12hcl), we reason the experimentally observed hydrogenation of 4qu by using the dimer (Cp*IrHCl)2 is mediated by the monomer (12hcl). The species 7bif and 12hcl catalyze both dehydrogenation and hydrogenation processes via microscopic reversibility; depending on the absence or presence of H2, the reaction moves toward dehydrogenation or hydrogenation, respectively. The complete 3qu4h/4qu dehydrogenation/hydrogenation requires an isomerization step through imine h enamine tautomerization and disproportionation. The protonation required for the disproportionation can be mediated by the dihydro intermediate (9oh_h) of 7bif or the ion pair (the product of H2 activation catalyzed by 12hcl). The predicted mechanisms reasonably rationalize the experimental observations in the (3qu4h h 4qu)/2cat system.

1. INTRODUCTION Organic hydrides, which can perform hydrogen release via dehydrogenation and hydrogen uptake via hydrogenation, respectively,1
5 have been proposed to be potential hydrogen storage media. A well-known example is ammonia-borane (AB), but the regeneration of AB from the hydrogen-released products (e.g., B-(cyclodiborazanyl)amino-borohydride, borazine, polyborazylene, and cyclotriborazane) is difficult because of the coexistence of the Lewis acid and Lewis base.6
8 Heterocycles9 (e.g., tetrahydroquinoline and its derivatives) can overcome the regeneration problem in AB, but development of efficient catalysts for fast hydrogen uptake and release is challenging.1
5 Nevertheless progress has been made recently. In 2007, Fujita et al. prepared a novel transition metal complex (Cp*Ir (Cp* = η5-C5Me5), denoted as 1cat in Scheme 1) that can r 2011 American Chemical Society

efficiently dehydrogenate various secondary alcohols under neutral conditions.10 Royer et al.11 then further characterized the fate of the catalyst in alcohol dehydrogenation; under their experimental conditions, they identified three complexes. We computationally studied the dehydrogenation mechanism and elucidated the formation of the observed complexes.12 According to the predicted energetics, we argued that our predicted “ligand rotation-promoted hydrogen transfer (LRPHT)” pathway is more favorable than the β-H elimination-based dehydrogenation (BETAHE) pathway proposed by experimentalists. In 2009, Yamaguchi et al. refined 1cat by introducing an electron-withdrawing CF3 group into the 2-pyridonate ligand.13 Received: March 13, 2011 Published: May 13, 2011 3131

dx.doi.org/10.1021/om200222j | Organometallics 2011, 30, 3131–3141

Organometallics Scheme 1. Schematic Drawings of Structures 1
4qu

ARTICLE

Scheme 3. Dehydrogenation/Hydrogenation Process of 3qu4h h 4qu

(Cp*IrHCl)2 dimer, rather than the dimer itself, as well as other species, plays the role of “reverse gear” in hydrogenation.

2. COMPUTATIONAL DETAILS Scheme 2. Plausible Mechanism for the Reversible Dehydrogenation/Hydrogenation Proposed by Yamaguchi et al.

Remarkably, the refined catalyst (2cat) can reversibly dehydrogenate/hydrogenate nitrogen heterocycles (e.g., tetrahydroquinoline (3qu4h)/quinoline (4qu) system in Scheme 1). Jessop remarked that such catalysts may be essential for the adoption of organic hydride hydrogen storage materials as an alternative to petroleum-derived fuels.5 On the basis of their experimental study, Yamaguchi et al. proposed a plausible mechanism to elucidate the reversible H2 release and uptake (Scheme 2).13 According to their mechanism, the catalyst (2cat) promotes dehydrogenation from 3qu4h to 4qu under the dehydrogenation conditions, leading to H2 release (step a). In the presence of H2 (hydrogenation conditions), the catalyst 2cat switches to the (Cp*IrHCl)2 dimer (step b) and the dimer plays an essential role in hydrogenation from 4qu to 3qu4h (step c). They also found that, in the presence of CF3substituted 2-hydroxypyridine (HPYD0 ), the dimer can change back to 2cat with H2 release (step d). Because the dehydrogenation and hydrogenation use different mediators in the proposed mechanism, Jessop described the feature as “the reactions with reverse gear”.5 Herein we report a thorough computational study to understand the dehydrogenation/hydrogenation mechanism, which suggests that the monomer, the decomposition product of

A catalyst is a molecule that elegantly combines the right electronic and geometric structures together. The electronic and geometric effects of substrates also affect the effectiveness of the catalyst significantly.14,15 Therefore, the experimental substrates and catalyst were used without any simplification. All the structures were optimized and characterized as minima or transition states at the B3LYP/BSI level (BSI designates the basis set combination of LanL2DZ16 for Ir and 6-31G (d,p) for all nonmetal atoms). When necessary, IRC (intrinsic reaction coordinate) calculations17 at this level were conducted to confirm the right connection of a transition state to its forward and backward minima. At the B3LYP/BSI geometries, the energetic results were then refined by single-point calculations at the B3LYP/BSII level with solvation effects accounted for (BSII denotes the basis set combination of LanL2DZ for Ir and 6-311þG(2d,2p) for all nonmetal atoms). The bulky solvation effects of the experimentally used p-xylene solvent were simulated by the SMD solvent model.18 The gas phase B3LYP/BSI harmonic frequencies were used for the thermal and entropic corrections to the enthalpies and free energies at 298.15 K and 1 atm. Note that the ideal gas phase model intrinsically overestimates the entropic contributions because of ignoring the suppressing effect of solvent on the rotational and transitional freedoms of substrates.19,20 Accurate prediction of enthalpies and entropies in solution is still a challenge for computational chemistry, and no standard approach is currently available.21 Nevertheless, the B3LYP/BSII(SMD, p-xylene)//B3LYP/BSI free energies will be used in the discussions, unless otherwise specified. We give the enthalpy results in the schemes or figures for reference. We also performed BP86,22 TPSSTPSS,23 B3PW91,24 and M0625 DFT calculations to verify the conclusions drawn from B3LYP results. The basis sets for those DFT calculations are the same or the same except for the SDD26 basis set on Ir atom, as used in the B3LYP calculations. All calculations were carried out by using Gaussian 0327 and Gaussian 09 programs.28

3. RESULTS AND DISCUSSION In Yamaguchi et al.’s experiments, 2cat was applied to 3qu4h and its derivatives.13 We chose the transformation between 3qu4h and 4qu as a representative to study the mechanism. Scheme 3 illustrates the transformation process in terms of the evolution of the substrate. The first dehydrogenation removes one H2 molecule from the N
C2 bond to give 5qu2h. One may assume the second dehydrogenation of 5qu2h may subsequently take place on the C3
C4 bond. However, as will be shown, the direct dehydrogenation from the C3
C4 bond is energetically unfavorable and the NdC2 double bond in 5qu2h needs to be shifted to the C3
C4 position to give 6qu2h. The second dehydrogenation actually occurs on the N
C2 bond again, leading to 4qu. The hydrogenation from 4qu to 3qu4h follows the reverse process of dehydrogenation. On the basis of Scheme 3, we will discuss the two dehydrogenation steps and two hydrogenation steps in Section 3.1 and 3.2, respectively. According to the discussions in Sections 3.1 and 3.2, we 3132

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Scheme 4. Catalytic Cycle for the Dehydrogenation from 3qu4h to 5qu2h via LRPHT Pathway (A) and BETAHE Pathway (B)a

a

The red cycle in (A) is for the hydrogenation from 5qu2h to 3qu4h (see Section 3.2).

summarize the overall mechanism for the reversible dehydrogenation/hydrogenation in Section 3.3. The double-bond shift from 5qu2h to 6qu2h, which involves enamine h imine tautomerization and disproportionation, will be discussed separately in Section 3.4. 3.1. Dehydrogenation from 3qu4h to 4qu. Dehydrogenation from 3qu4h to 5qu2h. In our previous study of alcohol dehydrogenation catalyzed by 1cat, we found that our proposed LRPHT (ligand rotation-promoted hydrogen transfer) pathway is more favorable than the β-H-elimination-based (BETAHE) pathway. The two pathways (Scheme 4) have also been considered in the present study. The energetic and geometric results are given in Figure 1. The LRPHT pathway (black cycle in Scheme 4(A)) for 3qu4h dehydrogenation is similar to that of alcohol dehydrogenation catalyzed by 1cat, including three major steps: ligand rotation to generate the bifunctional active species (7bif); hydrogen transfer from 3qu4h to 7bif to yield the dehydrogenation product (5qu2h) and the dihydro intermediate (9oh_h); and hydrogen release from 9oh_h. The ligand rotation barrier (TS1) and the 16e active species (7bif) are 6.1 and 5.8 kcal/mol higher than 2cat, respectively. The values are smaller than the corresponding values in the case of 1cat, 7.7 and 6.6 kcal/mol. Therefore, the introduction of an electron-withdrawing CF3 group in 2cat benefits the generation of the bifunctional active species (7bif) kinetically and thermodynamically. Unlike the concerted hydrogen transfer in the alcohol dehydrogenation,29,30 the hydrogen transfer step from 7bif þ 3qu4h to 5qu2h þ 9oh_h in Scheme 4(A) takes place stepwise by passing transition states TS2 and TS3. This is consistent with the generality that ketone hydrogenation often takes place concertedly, while imine hydrogenation prefers a stepwise pathway.14d,e,29,30 Relative to 2cat þ 3qu4h, the energies of TS2, 8, and TS3 are 33.3, 24.2, and 23.9 kcal/mol, respectively.

Note that the optimized transition state (TS3) is higher than the intermediate 8 in terms of electronic energy, but lower after thermal corrections. A similar situation happened in the imine hydrogenation by our rationally designed metal-free catalyst.14d Attempts to locate a transition state for concerted hydrogen transfer repeatedly gave the stepwise one (TS2 or TS3). The transition states for hydrogen transfer with opposite order (i.e., the hydrogen on N transfers first and then that on C) could not be located and again converged to TS2 or TS3. After hydrogen transfer, the dehydrogenation product (5qu2h) and dihydro 9oh_h are obtained. The barrier (TS4) for hydrogen release from 9oh_h, 15.7 kcal/mol, is less than that of 21.0 kcal/mol in the case of 1cat. Thus, the introduction of a CF3 group in 2cat not only benefits ligand rotation (see above) but also facilitates hydrogen release, explaining the experimental fact that 2cat performed better than 1cat in the 3qu4h dehydrogenation. Because the dehydrogenation (3qu4h f H2 þ 5qu2h) is overall thermodynamically unfavorable (endergonic by 11.7 kcal/mol), the dehydrogenation is essentially driven by H2 release via microscopic reversibility. Experimentally, the dehydrogenation was run under reflux conditions, which facilitates H2 release. We considered five possibilities for a β-H elimination mechanism in the study of alcohol dehydrogenation catalyzed by 1cat. The BETAHE pathway shown in Scheme 4(B) corresponds to the most favorable case among the five scenarios. To generate the intermediate (11vac) critical to β-H elimination that requires a vacant coordination site on Ir to activate the Cβ-H bond, 2cat first associates with 3qu4h to give the intermediate 10 after passing TS5. The energies of TS5 and 10 relative to 2cat þ 3qu4h are 22.6 and 23.0 kcal/mol, respectively. After climbing the transition state (TS6), the intermediate (10) dissociates into 11vac and the HPYD0 ligand. The energies of TS6 and the products (11vac þ HPYD0 ), relative to 2cat þ 3qu4h, are 40.2 3133

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Figure 1. (A) Energetic profiles corresponding to Scheme 4(A) (black path) and Scheme 4(B) (blue path). Values in kcal/mol are Gibbs free energies and enthalpies (in the square brackets), respectively. (B) Optimized structures of the stationary points, together with the key bond lengths (in angstroms). The H atoms in the Cp* ligand are omitted for clarity.

and 17.3 kcal/mol, respectively. The intermediate 11vac undergoes β-H elimination by crossing a barrier (TS7) of 21.4 kcal/ mol to give the dehydrogenation product (5qu2h) and 12hcl. The association of 12hcl with the predissociated HPYD0 ligand gives 9oh_h, which releases H2 via TS8. Note that the BETAHE and LRPHT pathways share the same H2 release step. The energetic comparisons in Figure 1(A) indicate the LRPHT pathway is obviously more favorable than the BETAHE pathway in the 3qu4h dehydrogenation, in agreement with the alcohol dehydrogenation catalyzed by 1cat. The barrier (6.1 kcal/mol) to generate the bifunctional active species (7bif) in the LRPHT pathway is lower than the barriers (22.6 kcal/mol (TS5)

and 40.2 kcal/mol (TS6)) to yield the Ir-amide active species (11vac) in the BETAHE pathway. In addition, the production of 7bif is 12.3 kcal/mol thermodynamically more favorable than that of 11vac. Furthermore, the hydrogen transfer barrier (TS2) in the LRPHT pathway is 5.4 kcal/mol lower than TS7 in the BETAHE pathway. To further verify the conclusion, we recalculated the barrier (TS2) in the LRPHT pathway and one of two critical barriers (TS7) in the BETAHE pathway by using different DFT functionals and basis sets. The energetic results collected in Table 1 consistently show that the LRPHT pathway is more favorable than BETAHE pathway, in spite of some numerical differences. 3134

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Table 1. Comparisons between the Barrier (TS2) in the LRPHT Pathway with TS7 in the BETAHE Pathway at Various DFT Levelsa TS2b ΔGq[ΔHq]

TS7b ΔGq[ΔHq]

B3LYP/BSII

33.3[20.8]

38.7[37.4]

BP86/BSII TPSSTPSS/BSII

30.0[16.7] 31.7[18.7]

35.2[33.6] 37.2[36.6]

method

TS2c ΔGq[ΔHq]

TS7c ΔGq[ΔHq]

B3LYP/BSIV

32.8[20.5]

37.7[36.9]

BP86/BSIV TPSSTPSS/BSIV

29.5[16.9] 31.4[18.9]

34.4[33.5] 37.0[36.5]

method

B3PW91/BSII

35.0[21.9]

36.8[35.6]

B3PW91/BSIV

35.4[22.1]

36.9[35.6]

M06/BSII

28.7[14.9]

32.6[32.3]

M06/BSIV

27.4[15.1]

33.4[32.6]

a

All energies are in kcal/mol. b Single-point calculations at the DFT(SMD, p-xylene)/BSII//DFT/BSI structures. Harmonic frequencies at the DFT/ BSI level were used for thermal and entropic corrections. c Single-point calculations at the DFT(SMD, p-xylene)/BSIV//DFT/BSIII structures. Harmonic frequencies at the DFT/BSIII level were used for thermal and entropic corrections. BSIII: the basis set combination of SDD for Ir atom and 6-31G (d,p) for all nonmetal atoms. BSIV: the basis set combination of SDD for Ir and 6-311þG(2d,2p) for all nonmetal atoms.

Scheme 5. (A) LRPTH Pathway for the Dehydrogenation from 6qu2h to 4qu; (B) Energetic Profile Corresponding to (A); (C) Optimized Structures of the Stationary Points, Together with the Key Bond Lengths (in angstroms)a

a

The cycle in red in (A) denotes the hydrogenation from 4qu to 6qu2h (see Section 3.2). In (B) values in kcal/mol are Gibbs free energies and enthalpies (in square brackets), respectively. The H atoms in the Cp* ligand in (C) are omitted for clarity.

Dehydrogenation from 6qu2h to 4qu. The above study indicates the LRPHT pathway is much more favorable than the BETAHE pathway; thus we considered only the LRPHT pathway (drawn in black in Scheme 5(A)) for the dehydrogenation from 6qu2h to 4qu. The energetic and geometric results are given in Scheme 5(B) and (C), respectively. Comparing the energetic profiles in Scheme 5(B) with that of the LRPHT pathway in Figure 1(A), we note that the second dehydrogenation from 6qu2h to 4qu is both kinetically and thermodynamically more favorable than the first dehydrogenation from 3qu4h to 5qu2h, which can be attributed to the aromatization effect; the second dehydrogenation makes the N-containing ring aromatic also. The favorable kinetics and thermodynamics of the second dehydrogenation can enhance the first dehydrogenation, which could be one of the reasons that the tetrahydroquinoline or its derivatives are able to perform two dehydrogenations. Recently, Zhang and Zhao have reported a computational study on the dehydrogenation process.31 Their proposed mechanism for the first dehydrogenation (i.e., from 3qu4h to

Scheme 6. Rate-Determining Transition State for the Second Dehydrogenation in the Dehydrogenation Pathway Proposed by Zhang and Zhao31

5qu2h) is the same as our LRPTH pathway, although they did not report the transition state (i.e., TS1) for the generation of the bifunctional reactive species 7bif. For the second hydrogenation (i.e., from 5qu2h to 4qu), they suggested a different mechanism 3135

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Scheme 7. Energetic Profiles for the 2cat-Mediated Hydrogenation from 4qu to 6qu2h (A) and Hydrogenation from 5qu2h to 3qu4h (B)a

a

Values in kcal/mol are Gibbs free energies and enthalpies (in square brackets), respectively.

that does not involve the isomerization from 5qu2h to 6qu2h. Under their mechanism, the rate-determining step is the hydrogen transfer from C3 to the oxygen atom of the ligand via TSZZ (i.e., TS-67 in ref 31). The schematic structure of TSZZ is shown in Scheme 6, and the optimized structure can be found in SI1. The free energy of TSZZ relative to 2cat þ 5qu2h was reported to be 41.2 kcal/mol at their calculation level, which seems too high to be experimentally accessible. To compare with our suggested mechanism, we recalculated the relative energy of the transition state at our levels, which predicted a 47.2 kcal/mol relative free energy for TSZZ. The large relative energy supports our proposed mechanism that there is a need for isomerization between 5qu2h and 6qu2h to shift the double bond from N
C2 to the C3
C4 position for the second dehydrogenation/hydrogenation (see Section 3.4 for details). 3.2. Hydrogenation from 4qu to 3qu4h. Hydrogenation Mediated by 2cat. Based on the principle of microscopic reversibility, a catalyst promotes both forward and reverse reactions and the experimental conditions determine the reaction direction.1 The thermal acceptorless alcohol dehydrogenations32,33 catalyzed by several late metal homogeneous and colloidal catalyst systems follow the principle. In the dehydrogenation, the reflux condition evacuates H2 constantly, and dehydrogenation dominates the catalytic system. In the hydrogenation, the presence of H2 suppresses dehydrogenation and drives the system toward hydrogenation. The red cycles shown in Scheme 4(A) and Scheme 5(A) represent the two hydrogenation steps mediated by 2cat. For clarity, we reconstruct their energy profiles in Scheme 7. The bifunctional active species 7bif generated via the ligand rotation

of 2cat can activate H2, which is similar to the H2 activation by the metal
ligand bifunctional hydrogenation catalysts or Shvo’s catalysts.29,30 The low barrier (13.3 kcal/mol) and favorable thermodynamics (exergonic by 2.4 kcal/mol) indicate the feasibility of the H2 activation step. The hydrogen transfer from 9oh_h to the substrate 4qu results in 6qu2h (the first hydrogenation), and that to the substrate 5qu2h results in 3qu4h (the second hydrogenation). Similar to the dehydrogenation, the second hydrogenation step is energetically more favorable than the first one. In the first hydrogenation from 4qu to 6qu2h, the hydrogen transfer barrier (TS1) relative to 9oh_h þ 4qu is 22.6 kcal/mol and the reaction is overall endergonic by 7.4 kcal/mol. In the second hydrogenation from 5qu2h to 3qu4h, the hydrogen transfer barrier (TS2) relative to 5qu2h þ 9oh_h is 19.0 kcal/mol and the reaction is overall exergonic by 11.7 kcal/mol. The energetic difference can be attributed to the dearomatization effect in the first hydrogenation that makes the N-containing ring nonaromatic. The favorable kinetics and thermodynamics of the second hydrogenation can also enhance the first unfavorable hydrogenation, similar to the dehydrogenation processes. While the hydrogenation can be elucidated in terms of microscopic reversibility, the experimentalists had an intriguing finding that the (Cp*IrHCl)2 complex (i.e., the dimer of 12hcl) alone can also perform hydrogenation. Accordingly, they reasoned the (Cp*IrHCl)2 dimer plays vital roles in hydrogenation and proposed the mechanism shown in Scheme 2. In the following, we elucidate the hydrogenation of 4qu catalyzed by the (Cp*IrHCl)2 complex. 3136

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Scheme 8. (A) Mechanism of the 12hcl-Mediated Hydrogenation from 4qu to 6qu2h; (B) Energetic Profile Corresponding to (A); (C) Optimized Structures of the Stationary Points, Together with the Key Bond Lengths (in Å)a

a

The cycle in red in (A) is for the dehydrogenation from 6qu2h to 4qu. Values in kcal/mol in (B) are Gibbs free energies and enthalpies (in square brackets), respectively. In (C) the H atoms in the Cp* ligand are omitted for clarity.

Hydrogenation Catalyzed by (Cp*IrHCl)2 Complex. Following the experimental assumption, we first considered if the complex itself can activate H2. Because the Ir centers of the dimer have no active site available, we reasoned the dimer alone could not activate H2. Indeed, no transition state for H2 activation could be located. On the other hand, we found the dimer can be decomposed into two monomers (i.e., 12hcl) (see ref 12) and hypothesized the hydrogenation is mediated by the monomer (12hcl), instead of the (Cp*IrHCl)2 complex itself. In agreement with our assumption, the experiment was conducted at the elevated reaction temperature (110 °C), which promotes the dimer decomposition. Scheme 8(A) illustrates our predicted mechanism for the hydrogenation from 4qu to 6qu2h mediated by 12hcl. The energetic and geometric results are shown in Scheme 8(B) and (C), respectively. The substrate 4qu first collaborates with 12hcl to activate H2, which overcomes a barrier (TS11) of 23.4 kcal/ mol and leads to the ion pair (14). In TS11, the distances of H
H and the forming Ir
H and N
H bonds are 1.051, 1.646, and 1.423 Å, respectively. The ion pair (14) then crosses the transition state (TS12) to transfer the hydride on the 12hcl moiety to the protonated 4qu moiety. The hydride transfer completes the hydrogenation cycle and liberates the mediator (12hcl). The hydrogen activation mechanism is similar to that mediated by the metal-free FLPs (frustrated Lewis pairs). As exemplified by the P(t-Bu)3/B(C6F5)3, FLPs combine the catalytic effects of Lewis acid (i.e., B(C6F5)3) and Lewis base (i.e., P(t-Bu)3) together to activate H2.32
38 In the present case, the 16e 12hcl complex and the substrate (4qu) act as Lewis acid and base, respectively. Some FLPs have been used to perform metal-free hydrogenation.37,38 The alkene hydrogenation mediated by early main-group metal catalysts39,40 was found to utilize FLP reactivity to activate H2. Previously, we designed new metal-free active sites for hydrogen and methane activations and metal-free hydrogenation catalyst.14d
f Interestingly, one of our predicted activate sites has been adopted to synthesize a new compound to activate H2.41 The hydrogenation process is similar to the imine hydrogenation by B(C6F5)3 in that the substrate acts as the Lewis acid to form a FLP. It should be noticed that, to form an effective FLP, the Lewis acid (i.e., 12hcl) and Lewis base (e.g.,

4qu or 5qu2h) should not form any stable donor
acceptor complexes. The details for exclusion of the donor
acceptor complexes are provided in SI2. The above FLP-like hydrogenation mechanism can be applied to the second hydrogenation from 5qu2h to 3qu4h. The catalytic details are displayed in Scheme 9. Comparing with Scheme 8, we note that, while the H2 activation barriers in the two hydrogenation steps are comparable (23.4 (TS11) vs 22.7 kcal/mol (TS13)), the hydride transfer barrier (24.9 kcal/mol, TS12) in the first hydrogenation step is higher than that of 19.8 kcal/mol (TS14) in the second hydrogenation. The energetic difference can be attributed to the dearomatization effect: the hydride transfer in the first step interrupts the 10π-electron delocalization of 4qu. Comparisons of the energy profiles in Scheme 8 and Scheme 9 with those in Scheme 7 show 12hcl and 2cat can compete with each other in hydrogenation. On the basis of the microscopic reversibility, 12hcl can also contribute to the dehydrogenation by following the reverse pathways of hydrogenations (see the red cycle in Scheme 8 and 9). 3.3. Overall Mechanism for the Reversible 3qu4h h 4qu Dehydrogenation/Hydrogenation Catalytic System. On the basis of above discussions, we now summarize the overall mechanism for the reversible dehydrogenation/hydrogenation between 3qu4h and 4qu in Scheme 10. Because the dissociation barrier (TS8) of 9oh_h into 12hcl and the HPYD0 ligand is not high and the reaction is nearly thermoneutral (see the energetic values given in Scheme 10), 12hcl can exist in the system as an active species. Under the dehydrogenation condition (blue paths), the absence of H2 favors dehydrogenation, which can be mediated by 7bif via the LRPHT pathway, as discussed in Section 3.1, or by 12hcl via the FLP-like pathway, as discussed in Section 3.2. Under the hydrogenation condition (red paths), the presence of H2 drives the system toward hydrogenation, which can be mediated by 7bif or 12hcl, following the reverse processes of dehydrogenation. The dimerization of 12hcl to form the (Cp*IrHCl)2 complex has been discussed in our previous study12 and has been put in SI2. Therefore, the experimentally observed interconversion between 2cat and (Cp*IrHCl)2 þ HPYD0 is due to the association of the HPYD0 ligand with the monomer 12hcl. Note that Royer et al. have found the (Cp*IrHCl)2 dimer is the 3137

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Scheme 9. (A) Catalytic Cycle for the 12hcl-Catalyzed Hydrogenation from 5qu2h to 3qu4h; (B) Energetic Profile Corresponding to (A); (C) Optimized Structures of the Stationary Points, Together with the Key Bond Lengths (in Å)a

a

The cycle in red in (A) is for the dehydrogenation from 3qu4h to 5qu2h. Values in kcal/mol in (B) are Gibbs free energies and enthalpies (in square brackets), respectively. In (C) the H atoms in the Cp* ligand are omitted for clarity.

Scheme 10. Overall Mechanism for the Reversible 3qu4h h 4qu Dehydrogenation/Hydrogenationa

a

The species (12hcl, 9oh_h, and 7bif) in the green rectangle are the key intermediates involved in the dehydrogenation/hydrogenation. The dehydrogenation and hydrogenation pathways are colored blue and red, respectively. The 12hcl-mediated dehydrogenation/hydrogenation (left paths) follow the FLP mechanism, and 7bif-mediated reactions (right paths) follow LRPHT mechanism. The values in kcal/mol are Gibbs free energies and enthalpies (in square brackets), respectively. Because the gas phase model overestimates the entropic contribution, we corrected the entropic contribution using the method proposed by Martin, Hay, and Pratt (MHP scheme).42

minor complex in the catalytic system within the catalytic time, but two more stable complexes could be observed in the 1cat-

catalyzed alcohol dehydrogenation after prolonged reaction time. We have elucidated the formation mechanism of the two 3138

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics

ARTICLE

Scheme 11. Mechanism Details for the Isomerization between 5qu2h and 6qu2h

complexes. The two complexes can be considered as resting states and are not involved in the catalysis. We thus do not include them in Scheme 10. 3.4. Isomerization between 5qu2h and 6qu2h. As mentioned in Scheme 3, there is a need of isomerization between 5qu2h and 6qu2h to shift the double bond from N
C2 to the C3
C4 position. In the case of dehydrogenation, the barrier for hydrogen transfer from the C3
C4 single bond of 5qu2h to 7bif is 55.6 kcal/mol, much larger than that of 20.2 kcal/mol for that of hydrogen transfer from the N
C2 single bond of 6qu2h to 7bif. In the case of hydrogenation, the hydrogen transfer barrier from 9oh_h to the C3dC4 double bond of 6qu2h is 46.2 kcal/ mol, much larger than that of 19.0 kcal/mol for hydrogen transfer from 9oh_h to the NdC2 double bond of 5qu2h. Stephan et al.37f have also used the double-bond shift to explain their B(C6F5)3-catalyzed hydrogenation from 4qu to 3qu4h. It has been proposed that the double-bond shift can be achieved by disproportionation.43
45 However, to our best knowledge, no details have been revealed about how the disproportionation takes place. Our proposed mechanism for the isomerization (5qu2h h 6qu2h) via disproportionation is illustrated in Scheme 11, along with the important energetic and geometric results. The isomerization requires the protonated 4quHþ and 5qu2hHþ as catalysts. In the following part, we first discuss the isomerization, assuming that the protonation process can take place, and then rationalize the feasibility of protonation. Similar to the imine h enamine tautomerization, 5qu2h generated from the first dehydrogenation can be transformed into 17qu2h, which is almost energetically equal to 5qu2h (Scheme 11(A)). The protonated 5qu2h (i.e., 5qu2hHþ) and 17qu2h then disproportionate into 3qu4h and 4quHþ (Scheme 11(B)). The disproportionation barrier (TS15) is 14.3[2.6] kcal/mol and the formation of 3qu4h and 4quHþ is

17.9 kcal/mol more favorable than 5qu2hHþ þ 17qu2h, indicating the disproportionation is thermodynamically and kinetically favorable. The bond lengths of the breaking and forming C
H bonds in TS15 (Scheme 11(D)) are 1.222 and 1.551 Å, respectively. The disproportionation product (4quHþ) can be deprotonated to give the dehydrogenation product (4qu). On the other hand, 4quHþ can catalyze the isomerization of 17qu2h into 6qu2h (Scheme 11(C)) by crossing a barrier (TS16) of 25.6[14.5] kcal/mol. In TS16, the bond lengths of the breaking and forming C
H bonds are 1.410 and 1.322 Å, respectively. This process gives 6qu2h and circumvents the unfavorable dehydrogenation on the C3
C4 bond of 5qu2h. In the case of hydrogenation, 6qu2h obtained from the first hydrogenation of 4qu can be transformed to 17qu2h via TS16 under the catalysis of the protonated 4quHþ (the reverse process in Scheme 11(C)). The resulting 17qu2h is then transferred to 5qu2h via tautomerization (Scheme 11(A)), which can be hydrogenated easily. On the other hand, 5qu2h can be protonated to 5qu2hHþ, and 5qu2hHþ and 17qu2h can also disproportionate (Scheme 11(B)) to give the final hydrogenation product 3qu4h and the protonated 4quHþ. 4quHþ can either lose the proton to give 4qu or catalyze the transformation from 6qu2h to 17qu2h. 17qu2h then tautomerizes into 5qu2h (Scheme 11(A)), which can be hydrogenated to 3qu4h. Note that the disproportionation from 5qu2hHþ and 17qu2h to 3qu4h and 4quHþ in the hydrogenation step uses only the forward process of Scheme 11(B), which is strongly thermodynamically favorable. The unfavorable reverse process of Scheme 11(B) is not needed. The isomerization from 6qu2h to 5qu2h circumvents the unfavorable direct hydrogenation of the C3dC4 bond of 4qu or 6qu2h. The isomerization between 5qu2h and 6qu2h via the above procedures requires the catalysts 4quHþand 5qu2hHþ. Previously, the experimental study46 showed the Br€onsted acid 3139

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics (such as HClO4) is important for the 4qu hydrogenation catalyzed by (Cp*IrCl2)2, using 2-propanol as a hydrogen source. In this experimental system, HClO4 can be considered as the source of proton. But the current dehydrogenation/hydrogenation was run in neutral conditions (no additional acid was required). The questions now are, how feasible is it for 4qu and 5qu2h to be protonated and what is the source of proton? According to our calculation, the HPYD0 ligand of 9oh_h can provide a proton. The proton transfer from the oxygen of the HPYD0 ligand in 9oh_h to the nitrogen of 4qu (i.e., generating the 4quHþ) is actually slightly energetically favorable (exergonic by 0.8 kcal/mol). The value for the proton transfer from the O atom of the HPYD0 ligand in 9oh_h to 5qu2h (i.e., generating 5qu2hHþ) is also slightly exergonic, by 0.4 kcal/mol. The energetic results indicate the required 4quHþ and 5qu2hHþ can be generated feasibly in energy. In contrast, in the case of 1cat, the proton transfer from the O atom of the HPYD ligand in the hydride complex to 4qu or 5qu2h is not favorable because the geometric optimizations from the assumed (deprotonated hydride complex)/4quHþ (or 5qu2hHþ) ion pairs always drove the proton back to the hydride complex. From this point of view, it can be reasoned that the increased acidity of the hydroxyl group by introducing the electron-withdrawing CF3 group in 2cat facilitates this disproportionation step, which could be another positive factor for the 2cat-catalyed reversible dehydrogenation/ hydrogenation.

4. CONCLUSIONS In summary, at the B3LYP/BSII(SMD, p-xylene)//B3LYP/ BSI level of calculations, we have studied the catalytic mechanisms of the reversible (3qu4h h 4qu) dehydrogenation/hydrogenation catalyzed by 2cat. The ligand rotation-promoted hydrogen transfer (LRPHT) pathway is predicted to be more favorable than the β-H elimination (BETAHE) pathway, which is in agreement with the 1cat-catalyzed alcohol dehydrogenation. The reactive species (7bif) possesses bifunctional reactivity, making the hydrogen transfer feasible in both dehydrogenation and hydrogenation in the (3qu4h h 4qu)/2cat systems. As the FLP concept was developed in the main-group chemistry, we found it is also applicable to the transition metal chemistry. The transition metal complex (12hcl) can form effective FLPs with the nitrogen-containing heterocycles to activate H2 and eventually realize hydrogenation. We reason the hydrogenation of 4qu, mediated by the dimer of 12hcl, (Cp*IrHCl)2, actually takes place due to the catalysis of the monomer (12hcl) rather than the dimer itself, because the dimer cannot activate H2 but can be easily decomposed into the reactive monomers (12hcl). On the basis of the microscopic reversibility, both the dehydrogenation and hydrogenation can be mediated either by the bifunctional reactive species (7bif) via the LRPHT pathway or by the monomer (12hcl) via the FLP-based pathway. Under the dehydrogenation conditions, the absence of H2 favors dehydrogenation. Under the hydrogenation conditions, the presence of H2 drives the system toward hydrogenation, which follows the reverse processes of dehydrogenation. The reversible (3qu4h h 4qu) dehydrogenation/hydrogenation requires an isomerization process between 5qu2h and 6qu2h via imine h enamine tautomerization and disproportionation. The isomerization circumvents the hydrogenation to the C3dC4 bond or dehydrogenation from the C3
C4 bond. The

ARTICLE

protonation process required to promote the isomerizaiton could be fulfilled via the dihydro bifunctional species (9oh_h). In the case of the 12hcl-mediated pathway, the ion pair (the H2 activation product) also provides the required protonated intermediates for disproportionation.

’ ASSOCIATED CONTENT

bS

Supporting Information. The optimized geometries for TSZZ (SI1); complexes of 12hcl with 3qu4h, 4qu, 4quHþ, 5qu2h, 5qu2hHþ, 6qu2h, and 17qu2h and the PES scanning results for (Cp*IrHCl)2 dimer formation (SI2); complete information for refs 27 and 28 (SI3); and the total energies and Cartesian coordinates of all the structures involved in this study (SI4). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported financially by the funds from Chinese Academy of Sciences, NSFC (Grant No. 20973197). ’ REFERENCES (1) Friedrich, A.; Schneider, S. ChemCatChem 2009, 1, 72. (2) Johnson, T. C.; Morris, D. J.; Wills, M. Chem. Soc. Rev. 2010, 39, 81. (3) For selected recent reviews about hydrogen storage, see: (a) Orimo, S.-i.; Nakamori, Y.; Eliseo, J. R.; Z€uttel, A.; Jensen, C. M. Chem. Rev. 2007, 107, 4111. (b) Alcaraz, G.; Grellier, M.; Sabo-Etienne, S. Acc. Chem. Res. 2009, 42, 1640. (c) Eberle, U.; Felderhoff, M.; Sch€uth, F. Angew. Chem., Int. Ed. 2009, 48, 6608. (4) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134. (5) Jessop, P. Nature 2009, 1, 350. (6) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613. (7) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc. Rev. 2009, 38, 279. (8) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079. (9) (a) Schwarz, D. E.; Cameron, T. M.; Hay, P. J.; Scott, B. L.; Tumas, W.; Thorn, D. L. Chem. Commun. 2005, 5919. (b) Moores, A.; Poyatos, M.; Luo, Y.; Crabtree, R. H. New J. Chem. 2006, 30, 1675. (c) Clot, E.; Eisenstein, O.; Crabtree, R. H. Chem. Commun. 2007, 2231. (d) Cui, Y.; Kwok, S.; Bucholtz, A.; Davis, B.; Whitney, R. A.; Jessop, P. G. New J. Chem. 2008, 32, 1027. (10) Fujita, K.; Tanino, N.; Yamaguchi, R. Org. Lett. 2007, 9, 109. (11) Royer, A. M.; Rauchfuss, T. B.; Gray, D. L. Organometallics 2010, 29, 6763. (12) Li, H. X.; Lu, G.; Jiang, J. L.; Huang, F.; Wang, Z.-X. Organometallics 2011, 30, 2349. (13) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. J. Am. Chem. Soc. 2009, 131, 8410. (14) (a) Wang, Z.-X.; Lu, G.; Li, H. X.; Zhao, L. L. Chin. Sci. Bull. 2010, 55, 239. (b) Lu, G.; Li, H. X.; Zhao, L. L.; Huang, F.; Wang, Z.-X. Inorg. Chem. 2010, 49, 295. (c) Li, H. X.; Zhao, L. L.; Lu, G.; Mo, Y. R.; Wang, Z.-X. Phys. Chem. Chem. Phys. 2010, 12, 5268. (d) Zhao, L. L.; Li, H. X.; Lu, G.; Wang, Z.-X. Dalton Trans. 2010, 39, 4038. (e) Li, H. X.; Zhao, L. L.; Lu, G.; Huang, F.; Wang, Z.-X. Dalton Trans. 2010, 39, 5519. (f) Lu, G.; Zhao, L. L.; Li, H. X.; Huang, F.; Wang, Z.-X. Eur. J. Inorg. Chem. 2010, 2254.(g) Lu, G.; Li, H. X.; Zhao, L.; Huang, F.; Schleyer, P. R.; Wang, Z.-X. Chem.
Eur. J. 2011, 17, 2038. 3140

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141

Organometallics (15) Huang, F.; Lu, G.; Zhao, L. L.; Li, H. X.; Wang, Z.-X. J. Am. Chem. Soc. 2010, 132, 12388. (16) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (17) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (18) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378. (19) (a) Hermans, J.; Wang, L. J. Am. Chem. Soc. 1997, 119, 2707. (b) Strajbl, M.; Sham, Y. Y.; Villa, J.; Chu, Z. T.; Warshel, A. J. Phys. Chem. B 2000, 104, 4578. (c) Zhang, C. G.; Zhang, R. W.; Wang, Z.-X.; Zhou, Z.; Zhang, S. B.; Chen, Z. F. Chem.—Eur. J. 2009, 15, 5910. (20) (a) Liang, Y.; Liu, S.; Xia, Y. Z.; Li, Y. H.; Yu, Z. X. Chem.—Eur. J. 2008, 14, 4361. (b) Yu, Z. X.; Houk, K. N. J. Am. Chem. Soc. 2003, 125, 13825. (21) (a) Ben-Naim, A.; Marcus, Y. J. Chem. Phys. 1984, 81, 2016. (b) Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R. J. Phys. Chem. A 1998, 102, 7787. (c) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066. (d) Lopes, P. E. M.; Roux, B.; MacKerell, A. D. Theor. Chem. Acc. 2009, 124, 11. (e) Jiang, J. L.; Wu, Y. B.; Wang, Z.-X.; Wu, C. J. Chem. Theory Comput. 2010, 6, 1199. (22) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (b) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (23) Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. (24) (a) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. (b) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1993, 48, 4978. (c) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533. (25) (a) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (b) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157. (26) Andrae, D.; H€aussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (27) Frisch, M. J.; et al. Gaussian03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (28) Frisch, M. J.; et al. Gaussian09, revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (29) For selected references on MLBHC catalysts, see: (a) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466. (b) Samec, J. S. M.; B€ackvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (c) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (d) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282. (30) For selected references about Shvo’s catalysts, see: (a) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem. Soc. 1986, 108, 7400. (b) Karvembu, R.; Prabhakaran, R.; Natarajan, K. Coord. Chem. Rev. 2005, 249, 911. (c) Conley, B. L.; Pennington-Boggio, M. K.; Boz, E.; Williams, T. J. Chem. Rev. 2010, 110, 2294. (31) Zhang, X.-B.; Zhao, X. Phys. Chem. Chem. Phys. 2011, 13, 3997. (32) For selected references on alcohol dehyrogenations catalyzed by ruthenium complexes, see: (a) Dobson, A.; Robinson, S. D. J. Organomet. Chem. 1975, 87, C52. (b) Dobson, A.; Robinson, S. D. Inorg. Chem. 1977, 16, 137. (c) Morton, D.; Cole-Hamilton, D. J. J. Chem. Soc., Chem. Commun. 1988, 1154. (d) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Milstein, D. Dalton Trans. 2007, 107. (33) For selected references on alcohol dehyrogenations catalyzed by rhodium complexes, see: (a) Charman, H. B. J. Chem. Soc. B 1970, 584. (b) Shinoda, S.; Kojima, T.; Saito, Y. J. Mol. Catal. 1983, 18, 99. (c) Morton, D.; Cole-Hamilton, D. J. J. Chem. Soc., Chem. Commun. 1987, 248. (34) (a) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124. (b) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880. (35) For selected reviews about FLPs, see: (a) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535. (b) Stephan, D. W. Dalton Trans. 2009, 3129. (c) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46. (36) For computational studies about FLPs, see: (a) Guo, Y.; Li, S. H. Inorg. Chem. 2008, 47, 6212. (b) Rokob, T. A.; Hamza, A.; Stirling,

ARTICLE

A.; Soos, T.; Papai, I. Angew. Chem., Int. Ed. 2008, 47, 2435. (c) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 1402. (37) For experimental studies on FLP-mediated imine hydrogenation, see: (a) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701. (b) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fr€ohlich, R.; Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543. (c) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskel€a, M.; Repo, T.; Pyykk€o, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117. (d) Axenov, K. V.; Kehr, G.; Fr€ohlich, R.; Erker, G. J. Am. Chem. Soc. 2009, 131, 3454. (e) Jiang, C. F.; Blacque, O.; Berke, H. Chem. Commun. 2009, 5518. (f) Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Commun. 2010, 46, 4884. (38) For computational studies about the mechanism of FLPmediated imine hydrogenation, see: (a) Rokob, T. A.; Hamza, A.; Stirling, A.; Papai, I. J. Am. Chem. Soc. 2009, 131, 2029. (b) Privalov, T. Dalton Trans. 2009, 1321. (39) Spielmann, J.; Buch, F.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 9434. (40) Zeng, G. X.; Li, S. H. Inorg. Chem. 2010, 49, 3361. (41) Theuergarten, E.; Schl€uns, D.; Grunenberg, J.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Chem. Commun. 2010, 46, 8561. (42) Martin, R. L.; Hay, P. J.; Pratt, L. R. J. Phys. Chem. A 1998, 102, 3565. According to the approach, a 4.3 kcal/mol free energy correction is applied per component change for a reaction at 298.15 K and 1 atm (i.e., a reaction from m- to n-components has a free energy correction of (n
m)  4.3 kcal/mol). (43) Forrest, T. P.; Dauphinee, G. A.; Deraniyagala, S. A. Can. J. Chem. 1985, 63, 412. (44) Stanovnik, B.; Tisler, M.; Katritzky, A. R.; Denisko, O. V. Adv. Heterocycl. Chem. 2001, 81, 253. (45) Raczy nska, E. D.; Kosi nska, W.; Osmialowski, B.; Gawinecki, R. Chem. Rev. 2005, 105, 3561. (46) Fujita, K.; Kitatsuji, C.; Furukawa, S.; Yamaguchi, R. Tetrahedron Lett. 2004, 45, 3215.

3141

dx.doi.org/10.1021/om200222j |Organometallics 2011, 30, 3131–3141