Article pubs.acs.org/IC
An Uncanny Dehydrogenation Mechanism: Polar Bond Control over Stepwise or Concerted Transition States Sarina M. Bellows,*,†,§ Sumit Chakraborty,†,§ J. Brannon Gary,‡,§ William D. Jones,†,§ and Thomas R. Cundari‡,§ †
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States Department of Chemistry and CASCaM, University of North Texas, Denton, Texas 76203, United States § Center for Enabling New Technologies through Catalysis, Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ‡
S Supporting Information *
ABSTRACT: The mechanism of the dehydrogenation of N-heterocycles with the recently established bifunctional catalyst (iPrPNP)Fe(CO)(H) was investigated through experiments and density functional theory calculations (iPrPNP = iPr2PCH2CH2NCH2CH2PiPr2). In this system, the saturated N-heterocyclic substrates are completely dehydrogenated to the aromatic products. Calculations indicate that dehydrogenation barriers of the C−C bonds are very high in energy (ΔG‡ = 37.4−42.2 kcal/mol), and thus dehydrogenation only occurs at the C−N bond (ΔG‡ = 9.6−22.2 kcal/mol). Interestingly, substrates like piperidine with relatively unpolarized C−N bonds are dehydrogenated through a concerted proton/hydride transfer bifunctional transition state involving the nitrogen on the PNP ligand. However, substrates with polarized C−N bonds entail stepwise (proton then hydride) bifunctional dehydrogenation.
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INTRODUCTION
N-Heterocycles and other imines can be used as H2 storage liquids1,2 and can be dehydrogenated by Fe heterogeneous catalysts3 as well as Ir4,5 and Ni6 homogeneous catalysts. Recently, the homogeneous, bifunctional catalyst (iPrPNP)Fe(CO)(H) (iPr1, iPrPNP = N(CH2CH2PiPr2)2) has been shown to dehydrogenate and hydrogenate N-heterocycles such as 1,2,3,4-tetrahydroquinoline. Dehydrogenation forms the fully aromatic product and H2 under refluxing conditions in xylene. Carbocycles, however, such as 1,2,3,4-tetrahydronapthalene cannot be dehydrogenated with iPr1 (Figure 1), suggesting that the heteroatom is necessary but does not hinder catalysis.7 Catalyst iPr1 also dehydrogenates HC−OH bonds in alcohols.8 iPr1 can also hydrogenate ketones,8 nitriles,9 and polarized CC bonds10 (e.g., styrene) but not ordinary olefins. Previous experiments and density functional theory (DFT) calculations suggest that the HC−OH dehydrogenation mechanism goes through a concerted transition state of proton transfer (Osub−H−NPNP) and hydride transfer (Csub−H−Fe) where the catalyst acts in a bifunctional manner (i.e., metal− ligand cooperativity).8 The non-bifunctional mechanism for alcohol dehydrogenation in which O−H oxidative addition to the metal is followed by β-hydrogen elimination to generate the ketone without involvement of the ligand nitrogen atom was calculated to have a barrier ∼11 kcal/mol higher than the bifunctional pathway. The hydrogenation of the CC bond of styrene was also studied by DFT and experiments, and the proton/hydride transfer was found to proceed via a stepwise © 2017 American Chemical Society
Figure 1. (a) Bifunctional (RPNP)Fe complexes. (b) Dehydrogenation of 1,2,3,4-tetrahydroquinoline (A, X = N, X′ = CH), 1,2,3,4tetrahydroquinoxaline (B, X, X′ = N), and 1,2,3,4-tetrahydronapthalene (C, X, X′ = CH) under experimental conditions.7
bifunctional mechanism (i.e., proton transfer to N followed by hydride transfer to Fe, vide infra).11 The cobalt analogue [(CyPN(H)P)Co(CH2SiMe3)] [BArF4] has been extensively studied by Hanson and co-workers to perform hydrogenation of olefins, aldehydes, ketones, and imines but only dehydrogenation of alcohols.10,12,13 DFT calculations suggest the mechanism for hydrogenation of Received: July 25, 2016 Published: April 28, 2017 5519
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determine (a) concerted versus stepwise proton/hydride transfer(s), and (b) if sequential dehydrogenation occurs from C−N and C−C dehydrogenation or if the substrate tautomerizes for activity only at the C−N bond. The DFT level of theory is similar to that used to calculate the mechanism of alcohol dehydrogenation with geometry optimizations at the B3LYP/def2-SVP level followed by single-point calculations with B3PW91/def2-QZVPP all in the gas phase.8 The major difference between the theories is the temperature used in this study is 298 K instead of 393 K. Solvent corrections were not performed for this system, since the activation barrier of dehydrogenation of ethylbenzene and para-substituted derivatives only varied by less than 1 kcal/mol with tetrahydrofuran (THF) and benzene solvent corrections.11 Thermodynamic Profile of the Dehydrogenation of Piperdine. The calculations of the dehydrogenation of piperidine to pyridine revealed a very endergonic thermodynamic profile (Figure 3). The first dehydrogenation of
ketones goes through an inner-sphere, non-bifunctional mechanism that is 11 kcal/mol lower in energy than the outer-sphere, concerted bifunctional mechanism.14 This cobalt catalyst can also hydrogenate and dehydrogenate N-heterocycles.15 Because of the growing interest in Earth-abundant metal catalysis for hydrogenations and the mechanistic differences between HC−OH and HC−CH dehydrogenation with catalyst iPr 1, the mechanism of HC−NH dehydrogenation offers an excellent opportunity to obtain a deeper understanding of the factors controlling metal-catalyzed dehydrogenation. Here, piperidine dehydrogenation using iPr1 as catalyst is investigated using DFT methods as a model for tetrahydroquinoline dehydrogenation.
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RESULTS AND DISCUSSION Experiments. Our preliminary results on the dehydrogenation of tetrahydroquinoline derivatives catalyzed by iPr[PN(H)P] ligand-based iron complexes suggested a possible bifunctional mechanism where the protic −NH moiety on the pincer ligand plays a critical role in the dihydrogen release step.7 To test this hypothesis, we prepared the iPr[PN(Me)P] derivative of complex 1 (1-NMe)16 and tested its catalytic activity in the dehydrogenation of 1,2,3,4-tetrahydroquinaldine under identical catalytic conditions. Consistent with our initial mechanistic hypothesis, complex 1-NMe did not exhibit any catalytic performance in this reaction, and unreacted starting tetrahydroquinaldine was recovered after the reaction (eq 1, Figure 2). In addition, when the hydrogenation of quinaldine
Figure 3. Potential energy diagram of the stepwise dehydrogenation of piperidine. Free energies (enthalpies) given in kilocalories per mole at 298 K.
piperidine (H6py) to 3,4,5,6-tetrahydropyridine (H4py(N)) is endergonic by ΔG = 8.3 kcal/mol (ΔH = 16.7 kcal/mol). The tautomerization product H4py(NH) is 1.0 kcal/mol uphill to H4py(N). The next dehydrogenation step is very endergonic relative to H6py to form H2py(N), where ΔG = 27.4 kcal/mol (ΔH = 44.7 kcal/mol) and the tautomerized 1,4-diene product (1,4-H2py(NH)) is only 0.4 kcal/mol higher in energy. The final dehydrogenated product, pyridine (py), is ΔG = 12.5 kcal/mol (ΔH = 4.9 kcal/mol) lower in energy from H2py(NH) due to the added aromaticity. Overall, the energy profile of the dehydrogenation of H6py to py + 3H2 is very uphill thermodynamically, and these reactions must be driven by removal of H2. (PNPMe)Fe Catalyst. The dehydrogenation of piperidine to pyridine was investigated with a truncated version of the ( iPr PNP)Fe(CO)(H), iPr1, catalyst, where the flanking phosphine atoms have methyl groups (Me1). The activation barrier for iPr1 is up to 4 kcal/mol higher in energy than Me1 due to the increased steric hindrance around the metal. (See the Supporting Information Table S1 for more details in addition to activation barriers of dehydrogenation of piperidine vs 1,2,3,4-tetrahydroquinoline.) The first dehydrogenation of piperidine with Me1 is found to occur through a concerted transition state (TS1) with an activation barrier of ΔG(TS1)‡ = 22.2 kcal/mol to form H4py(N) + Me2 (ΔG = 6.1 kcal/mol). This concerted TS1 has an imaginary mode at 105i cm−1, where the Nsub−H bond (substrate) is breaking while the
Figure 2. Dehydrogenation (1) and hydrogenation (2) experiments with iPr1-NMe.
was investigated using 1-NMe as the catalyst under 5 atm of H2, tetrahydroquinaldine was also not formed (eq 2, Figure 2) indicating that both the dehydrogenation and hydrogenation of N-heterocycles catalyzed by the iron pincer complexes likely proceed via bifunctional pathways where the −NH moiety actively participates in the reaction. In comparison, for the related cobalt analogue, hydrogenation of quinaldine proceeded almost quantitatively (∼80% conversion) even with the −N(Me) version, which interestingly points to different hydrogenation mechanisms with these structurally similar iron and cobalt catalysts.15 iPr 1 was also found to catalyze the dehydrogenation of piperidine to pyridine.7 For deeper insight into the mechanism of this reaction, DFT calculations with piperidine were used to 5520
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Inorganic Chemistry NPNP−H (PNP ligand) bond is forming and the Csub-H bond is breaking while the Fe−H bond is forming. The interatomic distances in TS1 are Nsub−H = 1.60 Å, H−NPNP = 1.10 Å, Csub−H = 1.32 Å, H−Fe = 1.74 Å, indicative of a late transition state (Figure 4).
focused on the dehydrogenation mechanism, note that this catalyst performs the microscopic reverse (hydrogenation) of N-heterocycle substrates under moderate hydrogen pressures. After the first dehydrogenation of piperidine, the product is 3,4,5,6-tetrahydropyridine (H4py(N)), where an HC−NH bond is no longer available for further dehydrogenation to pyridine. The only possible options for additional dehydrogenation of the substrate is for the catalyst to dehydrogenate the now-polarized HC−CH bond or for tautomerization of H4py(N) to H4py(NH). Since iPr1 does not dehydrogenate 1,2,3,4-tetrahydronapthalene (Figure 1), N-heterocycles are hypothesized to dehydrogenate only at the HC−NH bond of the substrate. The concerted transition state (TS) for cyclohexane dehydrogenation by Me1 was located at ΔG‡ = 45.7 kcal/mol. This high computed activation barrier is consistent with the experimental observation that substrates without HC−NH bonds cannot be dehydrogenated. However, the CN bond of H4py(N) may polarize the substrate enough to permit dehydrogenation of the HC−CH bond. To assess this possibility, activation barriers for dehydrogenation of 3,4,5,6-tetrahydropyridine (at the C3−C4 and C5−C6 positions) and 3,4-dihydroquinoline were calculated and shown in Figure 5.17 These activation barriers are very high in energy, and therefore indicate that a tautomerization mechanism is more likely.
Figure 4. Potential energy diagram of the dehydrogenation of piperidine with Me1. The last step is tautomerization to reform the HC−NH bond. Free energies (enthalpies) given in kilocalories per mole at STP. Me
Figure 5. Activation barriers of HC−CH bonds of polarized, intermediate substrates.
Me
To regenerate the catalyst 1 from 2, H2 elimination must occur. Chakraborty et al. proposed an alcohol-assisted H2 elimination from Me2 via a six-membered N−H···O−H···H− Fe cycle, which has a computed activation barrier of ΔG‡393 = 15.0 kcal/mol (relative to Me2).8 The analogous piperidineassisted H2 elimination transition state (i.e., via a N−H··· N−H···H−Fe cycle) was calculated to be at ΔG(TS2)‡298 = 26.1 kcal/mol (relative to Me2, Figure 4), which is significantly higher than the alcohol-assisted pathway, albeit at lower temperature, likely due to the increased steric hindrance of the bulkier substrate. The concerted proton/hydride transfer transition state for H2 loss from Me2 (TS3, Figure 4) was calculated by Chakraborty et al. to be ΔG‡393 = 19.0 kcal/mol and recalculated in this work to be in excellent agreement at ΔG‡298 = 18.7 kcal/mol.8 They also computed that the activation barrier for N−H···H−Fe hydrogen formation did not change with increasing the sterics of the phosphine substituents from methyl to isopropyl groups; however, the alcohol-assisted H2 elimination activation barrier was not calculated with R = iPr. This increase in steric hindrance around the metal may influence the activation barrier for substrate-assisted H2 elimination. Therefore, H2 elimination from Me2 to regenerate Me1 likely goes through unassisted TS3 at ΔG‡298 = 24.8 kcal/mol relative to Me1. Overall, the first dehydrogenation of piperidine is uphill by 6.1 kcal/mol. The driving force of this reaction is the release of H2. This is also apparent experimentally in that if the H2 generated is not liberated from the system, the catalyst will not dehydrogenate the substrate. Although the present work is
There are a few possibilities for the base used in the tautomerization mechanism: (a) base-substrate catalyzed, (b) base-additive catalyzed, (c) complex catalyzed, or (d) acidsubstrate catalyzed. Potassium t-butoxide (KOtBu, 10 mol %) is only added with the HBr salt of catalyst iPr1 ((iPrPNP-H)Fe(H) (CO) (Br), 3 mol %). Therefore, KOtBu is not needed when iPr 1 is the catalyst. Dehydrogenation of substrates A and B of Figure 1 were quantitative in 30 h suggesting little to no difference in rate of dehydrogenation. Since the first dehydrogenation product of A would have to undergo tautomerization but the first dehydrogenation product of B would not, tautomerization is unlikely to be rate-limiting. Although tautomerization is unlikely to be rate-limiting and not a focus of the current theoretical investigation, the mechanism is interesting to consider: substrate- or complex-catalyzed tautomerization. The piperidine substrate can deprotonate the α-carbon of H4py(N) with an activation barrier of ΔG‡ = 31.9 kcal/mol at 1 M concentration of piperidine and H4py(N) at 298 K. Excess substrate (33:1 substrate/catalyst) would make this deprotonation step more favorable under experimental conditions. A stronger base, such as KOtBu, would also make this step more favorable. The complex-catalyzed mechanism can occur by first coordination of H4py(N) to Me1 (ΔG = 6.5 kcal/mol relative to Me1 + H6py). The nitrogen of the PNP ligand can act as a base to deprotonate the α-carbon of H4py(N) at ΔG‡ = 34.0 kcal/mol. Although the base− 5521
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(Figure 8). This stepwise mechanism is akin to the styrene hydrogenation mechanism with Me2, where the overall barrier is
substrate mechanism has a lower activation barrier than the complex catalyzed barrier, it is still very high in energy at ΔG‡ = 31.9 kcal/mol. Attempts to calculate a more accurate activation barrier for substrate-catalyzed tautomerization resulted in even higher activation barriers (see Supporting Information Figure S2). Another tautomerization mechanism involves the substratebased isomerization utilizing protonated imine species (Figure 6). This mechanism was recently investigated using quinoline
Figure 6. Acidic-substrate tautomerization. Free energy (enthalpy) of reaction given in kilocalories per mole at STP.
derivatives.18 Utilizing the protonated form of H4py(N), proton transfer to H4py(N) from [H5py(NH)]+ yields H4py(NH) and regenerates the piperidine substrate with a ΔG of +1.2 kcal/mol. The transition state for this reaction when compared to separated substrate molecules is ΔG‡ of −4.5 kcal/mol, which is quite low compared to the hydrogenbonding complex of the two substrates at a ΔG of −5.1 kcal/ mol (a net barrier of 0.6 kcal/mol). Given the low barrier and approximate thermoneutrality, this pathway seems the most likely process to generate H4py(NH), consistent with a similar proposal by Sawatlon and Surawatanawong.18 After the tautomerization of H4py(N) to H4py(NH), the Me 1 catalyst can dehydrogenate the HC−NH in a similar fashion to piperidine dehydrogenation. Interestingly, a concerted transition state for the HC−NH dehydrogenation of H4py(NH) could not be found. Instead, two different proton transfer transition states were located relative to regenerated catalyst (Me1) (Figure 7): H−Nsub deprotonation from the PNP
Figure 8. Potential energy diagram of the dehydrogenation of H4py(NH) with Me1. The last step is tautomerization to reform the HC−NH bond. Free energies (enthalpies) given in kilocalories per mole at STP.
determined by hydride transfer.11 We propose the change from a concerted to stepwise proton/hydride transfer is due to the asymmetric polarization of the N−C bond. The intermediate between TS4 and TS6 is an ion pair of the protonated Me1 complex and the anionic amide at ΔG = 15.2 kcal/mol. This is similar to the ion pair found in the stepwise hydrogenation of styrene.11 On the basis of these results, the PNP ligand deprotonates the Nsub−H bonds reversibly in an off-cycle process, since hydride transfer cannot occur from the iron− amido complex. The second dehydrogenation activation barrier of piperidine (ΔG‡ = 22.1 kcal/mol) has the same activation barrier as the first dehydrogenation step (ΔG‡ = 22.2 kcal/ mol). Because of the high energy of the H2py(N) product, Me 2 + H2py(N) is only 6.1 kcal/mol lower in energy than TS6, which is important when considering the reverse hydrogenation mechanism. Although two of the three dehydrogenation steps of piperidine to pyridine required the catalyst Me1, the final dehydrogenation step of H2py(N) was found to proceed in a series of substrate-based tautomeric proton transfer steps resulting in the formation of the fully dehydrogenated pyridine product (Figure 9). [H3py(NH)]+ can react with H4py(N) to yield the 1,4-diene product 1,4-H2py(NH) in an exothermic manner (ΔG = −7.9 kcal/mol). This process is also effectively a barrierless process compared to final iron-catalyzed dehydrogenation barrier of ΔG‡ = 9.6 kcal/mol (see Supporting Information). The 1,4-diene product 1,4-H2py(NH) can then react with [H5py(NH)]+ to yield protonated pyridine and regenerate the piperidine substrate in a highly exergonic reaction (ΔG = −17.0 kcal/mol). Similar to the acidic-substrate tautomerization (Figure 6), the transition states for these isomerizations (Figure 9) have very low barriers (less than 3 kcal/mol) indicating that these processes should be fast along with being highly exergonic.
Figure 7. Transition states for proton transfer routes in free energy (enthalpy) in kilocalories per mole. (left) TS4 of six-member ring with N−H−N proton transfer and a C−H−Fe agostic interaction. (right) TS5 of 1,2-addition of H−Nsub addition across the NPNP−Fe bond. Bond lengths in angstroms.
ligand ΔG(TS4)‡ = 17.0 kcal/mol) and a 1,2-addition of H− Nsub addition across the NPNP−Fe bond (ΔG(TS5)‡ = 10.7 kcal/mol). The latter TS produces a ground-state iron(II) amido complex (ΔG = 1.4 kcal/mol). A TS from this amido complex for proton-hydride coupling could not be found, because the C−H bond is simply not hydridic enough. TS4, however, has the Csub−H bond lengthened to 1.26 Å with an H···Fe distance of 1.77 Å, indicative of an agostic interaction. Therefore, TS4 is in an optimal geometry for Csub− H hydride transfer to the iron metal. The hydride transfer transition state was located at ΔG(TS6)‡ = 22.1 kcal/mol 5522
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temperature and pressure (STP) relative to R1 + substrate unless otherwise noted. See Supporting Information for a sample input file.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01800. A sample input file, experimental details, computed xyz coordinates, and additional calculations of higher spin states and comparing steric effects (PDF) Computed xyz coordinates (XYZ)
Figure 9. Acidic-substrate tautomerization to generate pyridine. Free energy (enthalpy) of reaction given in kilocalories per mole at STP.
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SUMMARY The present results suggest that polarized C−N bonds undergo dehydrogenation with the iPr1 catalyst via a stepwise proton/ hydride transfer where ΔG‡(hydride transfer) > ΔG‡(proton transfer). However, C−N bonds that are not sufficiently polarized, such as in piperidine, are dehydrogenated through a concerted proton/hydride transfer transition state. The highest activation barrier calculated is ΔG(TS3)‡ = 24.8 kcal/mol for N−H···H−Fe to form H2. The calculated barriers for tautomerization are higher, although experiments suggest tautomerization is not rate-limiting. Dehydrogenation of the most polarized C−C bonds were calculated to be too high in energy. Therefore, for the complete dehydrogenation of Nheterocycles, tautomerization is necessary to produce a new polarized HC−NH substrate for stepwise dehydrogenation. These results suggest that the more polarized the HC−NH bond, the more likely dehydrogenation occurs through a stepwise proton then hydride transfer. Since the activation barrier for hydride transfer is higher than proton transfer, catalyst design at the metal center to adjust to hydricity of the metal−hydride bond may improve dehydrogenation or hydrogenation catalysis. After formation of H2py(N), a series of substrate acidcatalyzed proton transfers allows for formation of pyridine. Thus, 2 equiv of H2 are lost by Fe-catalyzed dehydrogenation process, and the final equivalent is lost through substrate proton transfer. Similar proton transfer processes were illustrated in a quinoline-based system.18 We also note that related DFT studies of tetrahydroquinoline dehydrogenation by R1 that appeared while this work was being published also indicate a stepwise mechanism.18 These authors found that the first dehydrogenation occurred as described here for piperidine. However, these authors also found that the second dehydrogenation of 3,4-dihydroquinoline can proceed by direct dehydrogenation of the H−C−C−H bond without tautomerization.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sarina M. Bellows: 0000-0001-7783-8125 William D. Jones: 0000-0003-1932-0963 Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSF under the CCI Center for Enabling New Technologies through Catalysis (CENTC) CHE-1205189. S.M.B. and J.B.G. thank the Univ. of North Texas Center for Advanced Scientific Computing and Modeling (CASCaM) for access to the CASCaM supercomputer (funded by CHE-1531468).
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REFERENCES
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EXPERIMENTAL SECTION
Computational Details. All calculations were performed using Gaussian0919 using a DFT level of theory similar to that used in calculating the mechanism of alcohol dehydrogenation.8 Geometry optimizations were performed in the gas phase with the B3LYP functional and def2-SVP basis set, and enthalpy and free energy corrections were obtained at 298 K using unscaled vibrational frequencies. Single-point calculations were performed on the gasphase optimized structure with the B3PW91/def2-QZVPP level of theory including the GD3 parameters for empirical dispersion. Intrinsic reaction coordinate calculations were performed on concerted transition states to confirm hydride and proton transfer in a single step. Free energies are reported in kilocalories per mole at standard 5523
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