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A Comparative Computationally Study About the Defined M(II) Pincer Hydrogenation Catalysts (M 5 Fe, Ru, Os)† Haijun Jiao,*[a] Kathrin Junge,[a] Elisabetta Alberico,[a,b] and Matthias Beller[a] The mechanism of acetonitrile and methyl benzoate catalytic hydrogenation using pincer catalysts M(H)2(CO)[NH(C2H4PiPr2)2] (1M) and M(H)(CO)[N(C2H4PiPr2)2] (2M) (M 5 Fe, Ru, Os) has been computed at various levels of density functional theory. The computed equilibrium between 1Fe and 2Fe agrees perfectly with the experimental observations. On the basis of the activation barriers and reaction energies, the best catalysts for acetonitrile hydrogenation are 1Fe/2Fe and 1Ru/2Ru, and the best catalysts for methyl benzoate hydrogenation are 1Ru/2Ru.
The best catalysts for the dehydrogenation of benzyl alcohol are 1Ru/2Ru. It is to note that the current polarizable continuum model is not sufficient in modeling the solvation effect in the energetic properties of these catalysts as well as their catalytic properties in hydrogenation reaction, as no equilibrium could be established between 1Fe and 2Fe. Comparison with other methC 2015 Wiley Periodicals, Inc. ods and procedures has been made. V
Introduction
ble dehydrogenation and hydrogenation of alcohols and ketones using 1Fe/2Fe as active catalysts. In the same time, Guan and coworkers,[14] computed the detailed potential energy surface for the catalytic hydrogenation of methyl benzoate and benzaldehyde involving 1Fe/2Fe as active catalysts. In a combined experimental and DFT work, Bielinski et al.,[15] most recently reported the base-free aqueous phase methanol dehydrogenation using 1Fe/2Fe as active catalysts and a cocatalytic amount of Lewis acid. The mechanism of aqueousphase methanol dehydrogenation using 1Ru/2Ru as active catalysts was computed by Lei et al.,[16] and they found that methanol-assisted hydrogen-release step is much more favorable than the direct hydrogen-release one. In addition to the iron (1Fe/2Fe) and ruthenium (1Ru/2Ru) complexes, the corresponding osmium complexes (1Os/2Os) have been prepared by Gusev and coworkers,[17] who tested them in the catalytic dehydrogenation of alcohols. Having these three homolog complexes in hand, we are interested in exploring their comparative reactivity in catalytic hydrogenation of acetonitrile (CH3CN) and methyl benzoate (PhCO2Me) as well as benzaldehyde (PhCHO).
The reactions involving H2 play an increasingly important role in modern synthetic organic chemistry and catalysis. For example, green hydrogenation reactions represent key technologies for the preparation of intermediates of new and existing pharmaceuticals as well as agrochemicals.[1] Effective dehydrogenation reactions provide the tools for the production of H2, which has been considered as a potential energy carrier.[2] Beller and coworkers have reported the low-temperature aqueous-phase methanol dehydrogenation to H2 and CO2 (CH3OH 1 H2O 5 CO2 1 2H2) using the well-defined aliphatic PNP pincer ruthenium (1Ru/2Ru)[3] and iron (1Fe/2Fe)[4] complexes (Fig. 1). They also proposed an outer sphere reaction mechanism, which was later evaluated by Yang on the basis of density functional theory (DFT) computation.[5,6] Most recently, Beller and coworkers have described the effective and selective hydrogenation of nitriles to valuable primary amines[7] and carboxylic esters to alcohols[8] using 1Fe/2Fe as the active catalysts. Their experimental and computational analyses identified an outer sphere hydrogenation mechanism by transfer of the hydride from the iron center and the proton from the coordinating nitrogen to the nitriles (or esters) to give the corresponding imines (or ketones), which are further hydrogenated to the corresponding amines (or alcohols). The most important feature in this reaction is the reversible and concerted hydrogen shuttle between catalyst 1Fe and its amido counterpart 2Fe. Apart from the Beller group, complex 1Fe has been independently synthesized and structurally characterized by Schneider and coworkers,[9] and applied to the hydrogenation of esters to alcohols by Guan and coworkers.[10] Recently, the 1Fe/2Fe complexes have been used as active catalysts in the acceptorless dehydrogenation and hydrogenation of Nheterocycles by Jones and coworkers,[11] and in formic acid dehydrogenation and CO2 hydrogenation by Schneider and coworkers.[12] In a combined experimental and DFT work, Schneider and coworkers,[13] studied the acceptorless reversi168
Journal of Computational Chemistry 2016, 37, 168–176
DOI: 10.1002/jcc.23944
[a] H. Jiao, K. Junge, E. Alberico, M. Beller Leibniz-Institut f€ ur Katalyse e.V. an der Universit€ at Rostock Albert-EinsteinStraße 29a, 18059, Rostock, Germany E-mail: [email protected] [b] E. Alberico Istituto di Chimica Biomolecolare, CNR, tr. La Crucca 3, 07100, Sassari, Italy † In memory of Prof. Dr. Paul von Ragu e Schleyer (February 27, 1930– November 21, 2014): Paul was a great and aesthetic scientist; a pioneer and leader of computational chemistry as well as an excellent mentor and wonderful friend. Contract/grant sponsor: State of Mecklenburg-Western Pomerania; Contract/grant sponsor: BMBF C 2015 Wiley Periodicals, Inc. V
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Figure 1. Hydrogenation and dehydrogenation catalysts 1M and 2M (R 5 Isopropyl).
Computational Details All calculations were carried out using the Gaussian 09 program.[18] All structures were optimized at the B3PW91,[19] B3LYP,[20] and BP86[21] levels of DFT with the TZVP[22] basis set (LANL2DZ for Ru and Os[23]). All optimized structures were characterized either as energy minimums without imaginary frequencies or transition states with only one imaginary frequency by frequency calculations; and the imaginary model connects the initial and the final states. The thermal corrections to Gibbs free energy at 298 K from the frequency analysis are added to the total electronic energy, and we therefore used the corrected Gibbs free energy (DG) at 298 K for our energetic discussion and comparison. We also carried out selfconsistent reaction field (SCRF) structure optimizations and frequency calculations at the B3PW91 level using the polarizable continuum model (PCM)[24] and methanol as solvent to estimate the solvation influence (B3PW91-SCRF). Natural atomic orbital and natural bond orbital analysis were carried out on the B3PW91 optimized structures with the NBO program (Version 3.1).[25] The B3PW91, B3LYP, and BP86 computed energetic data and Cartesian coordinates are listed in Supporting Information.
Results and Discussion Binding properties At first, we have analyzed the electronic structures of 1M and 2M in singlet states on the basis of B3PW91 computation
(Table 1). Natural localized molecular orbital analysis reveals an MAN r bond in 1M and an M@N double (one r and one p) bond in 2M. In 1M, the NAH bond lengths are apparently the ˚ ) for the three metal complexes, while the FeAH1 same (1.017 A bond (1.564 A˚) is shorter than the RuAH and OsAH bonds ˚ ). In addition, the FeAN bond (2.109 A ˚ ) is also (both 1.695 A shorter than the RuAN and OsAN bonds (both 2.231 A˚). All these bonds are polarized on the basis of their partial Wiberg bond indexes, in particular in the MAN r bond the N atom is approximately sp3 hybridized and provides a stronger contribution to the MAN bond than the M atom. In 1M, the M atoms are much more negatively charged than the N atoms. It is interesting to note that the H2 atom of the NAH2 bond is positively charged and is therefore protonic, while the H1 atom of the parallel MAH1 bond is electronically neutral and therefore less hydridic. In 2M, the M@N double bond is shorter than the corresponding MAN single bond in 1M, and the computed Wiberg bond indexes are approximately double of those of 1M. In 2M, the N atom has sp1.32 to sp1.46 hybridization and contributes more strongly to the MAN r bond than the M metal atom. In the MAN p bond, the N atom has pure p orbital character and also contributes more strongly than the M metal. 1M and 2M interconversion Prior to studying the catalytic activities of these catalysts, we would like to assess the structural and energetic properties of the active catalysts 1M and 2M. The optimized structural parameters of 1M and 2M as well as the corresponding transition states TS(1M/2M) are summarized in Table 2. Table 3 lists all the energetic data of the transformation of 1M and 2M as well as the possible competitive CO dissociation. Figure 2 shows the potential energy surfaces of the transformation of 1M and 2M as well as the possible competitive CO dissociation at the B3PW91 level of theory. For M 5 Fe, the B3PW91 computed FeAN distances in 1Fe and 2Fe agree well with the available data from X-ray analysis. For example, the computed FeAN distance in 1Fe is very close to the one measured from the X-ray solid state structure
˚ , B3PW91), Wiberg bonding indexes (WBI), natural charge (d/e), natural bond orbitals and atomic hybrid contributions Table 1. Computed bond lengths (A for the r and p NAM bonds. 1M
1Fe
1Ru
1Os
NAM (WBI) dM(AN) dN(AM) NAH2 (WBI) MAH1 (WBI) dH2 dH1 NAM (r)
˚ (0.482) 2.109 A 21.874 20.432 ˚ (0.798) 1.017 A ˚ (0.793) 1.564 A 0.397 0.002 16.7% Fe[sp5.98d9.54]; 79.8% N[sp3.10]
˚ (0.408) 2.231 A 21.744 20.476 ˚ (0.793) 1.017 A ˚ (0.729) 1.695 A 0.400 20.011 15.5% Ru[sp7.31d17.50]; 80.6% N[sp2.92]
˚ (0.450) 2.231 A 21.720 20.479 ˚ (0.789) 1.017 A ˚ (0.763) 1.695 A 0.404 20.002 16.5% Os[sp6.93d14.27]; 79.8% N[sp2.88]
2M
2Fe
2Ru
2Os
N@M (WBI) dM dN NAM (r) NAN (p)
˚ (0.930) 1.869 A 21.309 20.434 19.5% Fe[sp2.63d2.83]; 76.5% N[sp1.32] 15.4% Fe[pd3.69]; 77.0% N[p]
˚ (0.764) 2.040 A 21.140 20.519 19.0% Ru[sp2.30d5.49]; 76.9% N[sp1.46] 12.3% Ru[pd3.23]; 79.6% N[p]
˚ (0.874) 2.009 A 21.114 20.519 21.4% Os[sp2.23d5.76]; 75.7% N[sp1.39] 15.3% Os[pd3.73]; 77.7% N[p]
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Table 2. Selected structural parameters for 1M, TS(1M/2M), and 2M (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
SCRF
1Fe 1
MAH NAH2 MAN H1AH2
1.564 1.017 2.109 2.298
1.569 1.017 2.113 2.330
MAH MAH2 NAH2 MAN H1AH2
1.663 1.668 1.461 2.078 0.968
1.663 1.664 1.479 2.079 0.957
MAN
1.869
1.862
1.577 1.017 2.143 2.320
1.571 1.027 2.116 2.286
1.695 1.017 2.231 2.414
1.698 1.017 2.235 2.428
1.688 1.704 1.441 2.104 0.968
1.666 1.680 1.463 2.101 0.986
1.870 1.879 1.455 2.200 0.975
1.873 1.871 1.488 2.200 0.956
1.888
1.873
2.040
2.037
SCRF
Table 3. Transformation of active catalysts 1M and 2M (R 5 isopropyl) (relative energies are given in kcal/mol; and the corresponding energies of the triplet states are given in square bracket). 1M TS(1M/2M) 2M1H2
2M
1M-CO
0.00 0.00 0.00 0.00
17.47 21.01 18.00 14.53
13.38 17.22 12.83 10.40
0.32 [14.22] 3.58 [15.03] 21.54 [11.70] 22.33 [24.01]
41.22 41.65 36.00 54.58
[23.78] [24.92] [19.27] [43.86]
0.00 0.00 0.00 0.00
21.07 26.15 21.05 17.70
11.68 17.32 10.05 9.41
2.31 [38.70] 6.55 [30.85] 20.98 [37.36] 20.88 [37.06]
51.66 48.56 46.59 57.81
[59.02] [57.39] [53.95] [64.80]
0.00 0.00 0.00 0.00
23.91 30.25 24.16 20.35
11.77 18.28 10.03 10.50
5.34 11.25 1.65 1.49
69.72 68.30 64.58 64.80
[75.27] [78.79] [70.88] [78.67]
[44.57] [49.37] [40.71] [39.89]
Journal of Computational Chemistry 2016, 37, 168–176
B3LYP
BP86
1Os 1.706 1.017 2.259 2.435
1.706 1.027 2.245 2.407
1.706 1.017 2.232 2.389
1.704 1.017 2.236 2.393
1.714 1.017 2.259 2.407
1.715 1.027 2.246 2.391
TS(1Os/2Os)
1.900 1.921 1.431 2.223 0.979
1.882 1.901 1.463 2.223 0.983
1.837 1.854 1.479 2.204 0.989
1.838 1.848 1.511 2.202 0.970
2.057
2.049
2.009
2.001
2Ru
˚ ) of the BH3 coordinated complex (1Fe-BH3).[4] The (2.0669 A FeAN distance in 2Fe also agrees well with the one recently ˚ .[11] The significant shorter FeAN distance in reported 1.86 A 2Fe than in 1Fe reveals the enhanced FeAN interaction in 2Fe, which has Fe@N double character as reported from natural bonding orbital analysis. In the transition state, TS(1M/2M), ˚. the forming H1AH2 distance is 0.968 A It is also to note that the computed structural parameters at B3LYP and BP86 as well as B3PW91-SCRF agree well with the available experimental data, indicating that these structural parameters are much less sensitive to the applied methods. This is also the same as found by Schneider and coworkers,[12,13] and Guan and coworkers.[14] In addition, we found very good agreement between our computed structural parameters and those reported in the literature, for example, for M 5 Fe and Ru using the M06 functional under the consideration of the solvation effect of ethanol or methanol (M06SCRF),[5,6] for M 5 Ru and Os at the mPW1K and PBE0 levels under the consideration of the solvation effect of tetrahydrofuran (mPW1K-SCRF and PBE0-SCRF).[15] As discussed below,
170
B3PW91
TS(1Ru/2Ru)
2Fe
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
BP86
1Ru
TS(1Fe/2Fe) 1
B3LYP
1.863 1.894 1.459 2.226 0.985
1.853 1.876 1.489 2.224 0.997
2Os 2.025
2.020
large differences are found instead in estimating the activation barriers and reaction energies. In our previous study about nitrile hydrogenation, we found that the most important features of this reaction are the reversible hydrogenation and dehydrogenation process which interconverts catalyst 1Fe and its amido counterpart 2Fe, and the fact that the reaction is thermoneutral at B3PW91. In agreement with the experimental findings,[7,11] 2Fe has a singlet ground state (the triplet state is less stable by 13.90 kcal/ mol) and the transformation of 1Fe into 2Fe and vice versa can be conducted very easily by changing the H2 pressure. It is found that the CO dissociation energy, resulting in a triplet ground state 1Fe-CO, is 23.78 kcal/mol, which is much higher than the barrier of H2 elimination via the transition state TS(1Fe/2Fe) (17.47 kcal/mol). It is noted that the corresponding H2 complex 2Fe1H2 is higher in energy than 1Fe and 2Fe by 17.47 and 13.06 kcal/mol, respectively, and therefore should be hardly formed and thus escape experimental detection. The reported H2 elimination barrier of 1Fe and the relative energy of 2Fe1H2 by Yang[6] using M06-SCRF (24.0 and 17.9 kcal/mol, respectively) are higher than our values (17.47 and 13.38 kcal/ mol, respectively). In addition, the B3LYP and BP86 computed energetic data also seem very reasonable on the basis of the experimental observed equilibrium.[7,11] Our computed reaction energy of H2 elimination (1Fe 5 2Fe 1 H2) at B3PW91 differs from that calculated by Yang using the M06-SCRF (0.32 vs. 9.1 kcal/mol). Even worse results have been reported by Guan and coworkers,[14] where the energetic data were evaluated from M06/6-31111G(d,p) single-point energy calculation on the B3LYP/6-31G(d,p) gas phase geometries under the consideration of solvation effects of tetrahydrofuran. They reported that the H2 elimination barrier is 26.0 kcal/mol, and the reaction is endergonic by 14.9 kcal/mol. On the basis of the reported facile equilibrium between 1Fe and 2Fe under H2 atmosphere at room temperature,[7,11] our computed reaction energy of 0.32 kcal/mol sounds very reasonable and realistic, in contrast, the computed reaction energies of 9.1 and 14.9 kcal/mol using M06SCRF are far too big to establish any facile equilibrium. WWW.CHEMISTRYVIEWS.COM
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Figure 2. B3PW91 potential energy surfaces (kcal/mol) for 1M and 2M interconversion (the B3PW91-SCRF data in parenthesis).
Qualitatively, the same results have also been found at the B3LYP and BP86 levels of theory (Table 3), and there are no energetic disorders found for both H2 elimination and CO dissociation. The largest difference is that BP86 disfavors the energy of the triplet states of 2Fe and 2Fe-CO much more strongly than B3PW91 and B3LYP. Under the consideration of the solvation effect, the H2 elimination barrier increases and the reaction becomes more endergonic than those found in the gas phase (3.54 and 3.26 kcal/mol, respectively). One can therefore conclude that the current PCM is not sufficient for studying the solvation effect in the energetic properties of these catalysts, despite the fact that polar solvents play a more important role in the heterolytic splitting of H2 than nonpolar solvents.[26] For M 5 Ru and Os, the same energetic trends have been found, apart from 1Ru-CO and 1Os-CO, the singlet states of which are favored over their corresponding triplet states (Table 3). Nevertheless, CO dissociation energies are much higher than the H2 elimination barriers, and outer sphere hydrogenation mechanisms are favorable as discussed below. For M 5 Ru, the reported H2 elimination barrier of 1Ru by Yang[5,6] using M06-SCRF (29.5 kcal/mol) and by Gusev et al.,[17]] using mPW1K-SCRF (29.2 kcal/mol) as well as by Lei et al.,[16] using wB97X-D-SCRF (27.1 kcal/mol) are much higher than our values (21.07 kcal/mol). For M 5 Os, the reported H2 elimination
barrier of 1Os by Gusev et al. using mPW1K-SCRF (30.9 kcal/ mol) is also much higher than our value (23.91 kcal/mol). Our computed reaction energy (2.31 kcal/mol) of H2 elimination (1Ru 5 2Ru 1 H2) at B3PW91 differs from that of Yang[5,6] using the M06-SCRF (11.8 kcal/mol) as well as those of Gusev et al.,[17] at mPW1K-SCRF (18.8 kcal/mol) and at PBE0-SCRF (15.4 kcal/mol) or that of Lei et al.,[16] using wB97X-D-SCRF (7.6 kcal/mol). Considering that 1Ru and 2Ru have been used as effective catalysts for hydrogenation and dehydrogenation, the B3PW91, B3LYP, and BP86 results sound much more reasonable, while those at M06-SCRF, mPW1K-SCRF, BPE0-SCRF, and wB97X-D-SCRF as well as B3PW91-SCRF are less reasonable. For 1Os 5 2Os 1 H2, our results differ strongly from those of Gusev et al.,[17] this reaction is less endergonic at B3PW91 (5.37 kcal/mol), much more endergonic at mPW1K-SCRF and BPE0-SCRF (19.7 and 16.1 kcal/mol, respectively). Also, considering that 1Os and 2Os have been considered as effective catalysts for hydrogenation and dehydrogenation, B3LYP and BP86 seems more reasonable than B3PW91, while the results of mPW1K-SCRF and BPE0-SCRF as well as B3PW91-SCRF are much worse, revealing the problematic of the current solvation model applied. A general trend can be found in Figure 2 and Table 3, that is, the H2 elimination barrier increases from 1Fe to 1Ru as well as to 1Os. It is interesting to note that for the regeneration of
Table 4. Selected structural parameters for TS(CH3CN) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
MAH MAN NAH2 H2ANC CAN NCAH1
1.821 2.009 1.247 1.342 1.226 1.215
1.860 2.000 1.276 1.319 1.233 1.190
SCRF
B3LYP
BP86
B3PW91
1Ru 1.876 2.032 1.253 1.347 1.228 1.203
1.822 2.018 1.264 1.341 1.237 1.230
1.963 2.151 1.223 1.373 1.226 1.217
2.000 2.143 1.265 1.334 1.234 1.190
SCRF
B3LYP
BP86
1.989 2.170 1.257 1.341 1.224 1.226
1.960 2.164 1.269 1.339 1.234 1.252
1Os 2.018 2.171 1.234 1.372 1.229 1.204
1.975 2.167 1.242 1.374 1.237 1.230
1.943 2.149 1.254 1.333 1.222 1.239
1.972 2.142 1.282 1.310 1.229 1.214
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Table 5. Energetic data of acetonitrile hydrogenation (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
1M 1 CH3CN
TS(CH3CN)
2M 1 CH3-CH@NH
0.00 0.00 0.00 0.00
17.75 23.87 19.90 11.67
20.96 3.63 20.28 22.58
0.00 0.00 0.00 0.00
17.15 24.40 18.50 11.82
1.02 6.60 0.23 21.13
0.00 0.00 0.00 0.00
21.03 29.73 23.01 14.96
4.08 11.30 2.91 2.24
1M from 2M by adding H2, the hydrogenation addition barriers are in a very tight range in the gas phase for all three metals at B3PW91 (17.15, 18.76, and 18.54 kcal/mol). The next trend is that BP86 favors the lowest H2 elimination barriers, tightly followed by B3PW91 and B3LYP, while the highest barrier is found at B3PW91-SCRF for all three metals. Apart from methanol as solvent, we also computed methanol-assisted H2 elimination, where methanol is actively involved in the six-membered transition state. Compared with the direct H2 elimination in Figure 2, it is found that methanol can lower the barrier of 1Fe, 1Ru, and 1Os by 1.21, 5.05, and 4.29 kcal/mol, respectively. However, there are no experimental results available for direct comparison.
Acetonitrile hydrogenation In our previous work,[7] the computed activation free energy for the concerted acetonitrile hydrogenation to imine is 17.55 kcal/mol, which is close to the H2 elimination barrier of 1Fe, but lower than the CO dissociation energy by 6.23 kcal/mol, respectively. The hydrogenation reaction is slightly exergonic by 0.96 kcal/mol. In addition, the hydrogenation of imine to amine is barrier-less and the reaction is exergonic by 14.04 kcal/mol. On the basis of these results, we computed only the first hydrogenation step from acetonitrile to imine, for 1Ru and 1Os. The optimized structural parameters of the transition state TS(CH3CN) are listed in Table 4. Table 5 lists all the energetic data of acetonitrile hydrogenation. Figure 3 shows the potential energy surfaces of acetonitrile hydrogenation at the B3PW91 level of theory. In the transition state for M 5 Fe at B3PW91 (Table 4), the ˚ , respecFeAH1 and NAH2 bond distances (1.821 and 1.247 A tively) are elongated compared to those in 1Fe (1.564 and ˚ , respectively), while the central FeAN distance 1.017 A ˚ ). The forming CAH1 and becomes shorter (2.009 vs. 2.109 A 2 H ANC distances are 1.215 and 1.342 A˚. The same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory. As shown in Table 5, BP86 provides the lowest barrier, followed by B3PW91 and B3LYP, while the highest barrier 172
Journal of Computational Chemistry 2016, 37, 168–176
is found at B3PW91-SCRF. The same trends in the structural parameters are also found for M 5 Ru and Os. It is found that 1Fe and 1Ru have close activation barriers (17.55 and 17.15 kcal/mol, respectively) and reaction energies (20.96 and 1.02 kcal/mol, respectively) in acetonitrile hydrogenation (Fig. 3), while 1Os has higher activation barrier (21.03 kcal/mol) and the reaction is more endergonic (4.08 kcal/mol). Therefore, 1Fe/2Fe and 1Ru/2Ru should be better catalysts for acetonitrile hydrogenation than 1Os/2Os. Considering the barriers of 1M (M 5 Fe, Ru, and Os) regeneration of 17.15, 18.76, and 18.54 kcal/mol at B3PW91, the rate-determining step might be either hydrogenation of acetonitrile or catalyst regeneration for M 5 Fe and Ru, however, acetonitrile hydrogenation is the rate-determining step for M 5 Os. However, results different from B3PW91 have been found at B3LYP and BP86 (Table 5). At B3LYP, the rate-determining step might be either hydrogenation of acetonitrile or catalyst regeneration for M 5 Fe (19.90 vs. 19.54 kcal/mol) and Os (230.1 vs. 22.51 kcal/mol), while acetonitrile hydrogenation is the rate-determining step for M 5 Ru (22.03 vs. 18.50 kcal/ mol). At BP86, catalyst 1M regeneration from 2M is the ratedetermining step for Fe (16.86 vs. 11.67 kcal/mol), Ru (18.54 vs. 11.82 kcal/mol), and Os (18.86 vs. 14.96 kcal/mol); the same qualitative conclusion is also found at B3PW91-SCRF. Hydrogenation of methyl benzoate and benzaldehyde In our previous experimental and theoretical studies, we proposed an outer sphere reaction mechanism for the hydrogenation of methyl benzoate.[8] At first, the ester is hydrogenated by 1Fe in a concerted way by a simultaneous transfer of the hydride from the iron center and the proton from the nitrogen ligand to give the corresponding hemiacetal and 2Fe. Subsequently, the dissociation of the hemiacetal to benzaldehyde and methanol takes place and 1Fe is regenerated from 2Fe by
Figure 3. B3PW91 potential energy surfaces (kcal/mol) of CH3CN hydrogenation (the B3PW91-SCRF data in parenthesis).
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Table 6. Selected structural parameters for TS(PhCO2Me) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
MAH MAN NAH2 OAH2 CAH1 OAC
1.998 1.995 1.311 1.184 1.150 1.333
2.112 1.981 1.358 1.154 1.137 1.344
B3LYP
BP86
B3PW91
1Ru 2.090 2.018 1.304 1.200 1.140 1.338
1.984 1.996 1.353 1.170 1.164 1.347
2.138 2.143 1.312 1.184 1.146 1.335
H2 addition. The next step is the hydrogenation of benzaldehyde to benzyl alcohol. The optimized structural parameters of the transition states TS(PhCO2Me) are listed in Table 6. Table 7 lists all the energetic data of the hydrogenation of methyl benzoate to the corresponding hemiacetal. Figure 4 shows the energetic data of the overall transformation of methyl benzoate to benzyl alcohol and methanol. Shortly after publication of our work, Schneider and coworkers[13] computed the acceptorless reversible dehydrogenation of alcohols and hydrogenation of ketones and Guan and coworkers[14] computed the detailed potential surface of the hydrogenation of methyl benzoate via benzaldehyde to benzyl alcohol and methanol using 1Fe/2Fe as active catalysts. As our goal is to examine the catalytic differences among the three metals 1M/2M (M 5 Fe, Ru, Os), we did not examine, as done by Schneider and Guan, each and every single step through which these transformations take place. Nevertheless, we made systematic comparison between our and their results. In the transition state of methyl benzoate hydrogenation TS(PhCO2Me) for M 5 Fe at B3PW91 (Table 5), the FeAH1 and NAH2 bond distances (1.998 and 1.331 A˚, respectively) are elongated compared to those in 1Fe (1.564 and 1.017 A˚, respectively), while the central FeAN distance becomes shorter ˚ ). The forming CAH1 and H2AO distances are (1.995 vs. 2.109 A ˚ , indicating a very late transition state. The 1.150 and 1.184 A same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory.
Table 7. Energetic data of methyl benzoate hydrogenation to hemiacetal (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
SCRF
1M 1 PhCO2CH3
TS(PhCO2CH3)
2M 1 PhCH(OH)(OCH3)
0.00 0.00 0.00 0.00
21.51 26.90 21.40 19.42
11.93 15.32 11.94 11.80
0.00 0.00 0.00 0.00
18.88 30.33 17.94 20.29
13.92 18.29 12.50 13.25
0.00 0.00 0.00 0.00
25.32 33.83 24.86 22.40
16.98 22.99 15.14 15.16
2.450 2.104 1.622 1.037 1.116 1.371
SCRF
B3LYP
BP86
2.158 2.162 1.302 1.201 1.153 1.335
2.107 2.147 1.376 1.156 1.170 1.348
1Os 2.220 2.615 1.310 1.195 1.137 1.340
2.193 2.138 1.485 1.097 1.154 1.364
2.083 2.142 1.308 1.186 1.164 1.330
2.147 2.129 1.367 1.145 1.151 1.341
For the formation of the hemiacetal for M 5 Fe, the computed free energy barrier of methyl benzoate is 21.51 kcal/ mol. However, this reaction step is endergonic by 11.93 kcal/ mol. For the dissociation of the hemiacetal to the corresponding benzaldehyde and methanol, the reaction is exergonic by 8.21 kcal/mol.[8] The free energy barrier of methyl benzoate hydrogenation is lowest at BP86 (19.42 kcal/mol), tightly followed by those at B3LYP (21.40 kcal/mol) and B3PW91 (21.51 kcal/mol), while highest at B3PW91-SCRF (26.90 kcal/mol). It is now very interesting to compare our results with those of Guan and coworkers,[14] where the energetic data were evaluated from M06/6-31111G(d,p) single-point energy calculation on the B3LYP/6-31G(d,p) gas phase geometries under the consideration of solvation effects of THF. In their stepwise mechanism, the highest barrier of hemiacetal formation is 19.3 kcal/mol, which is close to our values (21.51, 21.40, and 19.42 kcal/mol, respectively) at B3PW91, B3LYP, and BP86, respectively, while lower than our value (26.90 kcal/mol) at B3PW91SCRF. The most problematic point is that the formation of the hemiacetal and 2Fe is endergonic by 21.9 kcal/mol, which is higher than the highest barrier on the potential energy surface by 2.6 kcal/mol; this indicates that the energy of the formed product is higher than the barrier of the product formation. In contrast, all our results show that formation of the hemiacetal and 2Fe is also endergonic, but the reaction energy is lower than the highest barrier on the potential energy surface (11.93 vs. 21.51 kcal/mol at B3PW91). This comparison shows that our computed potential energy surface is more reasonable that that by Guan and coworkers.[14] Using the simplified model catalysts (where the isopropyl groups at the P centers were replaced by methyl groups), Schneider and coworkers[13] computed the hydrogenation of methyl formate via a stepwise mechanism at the R1-B3PW91D3BJ/def2-QZVPP//B3LYP/def2-SVP level under the consideration of the thermal correction at 1208C. From methyl formate to methoxymethanol, the highest barrier is 13.5 kcal/mol and the reaction is endergonic by 11.1 kcal/mol. These results are in qualitative agreement with our computations and in disagreement with those of Guan and coworkers.[14] For M 5 Ru, B3LYP provides the lowest barrier (17.94 kcal/ mol), followed by B3PW91 (18.88 kcal/mol) and BP86 (20.29 kcal/mol), while the highest barrier is found at B3PW91-SCRF (30.33 kcal/mol). For M 5 Os, the lowest barrier is found at BP86, followed by those at B3LYP (24.86 kcal/mol) and B3PW91 (25.32 kcal/mol), while the highest at B3PW91-SCRF (33.83 kcal/mol). This is to emphasis that the reaction energy Journal of Computational Chemistry 2016, 37, 168–176
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Figure 4. B3PW91 potential energy surfaces (kcal/mol) of methyl benzoate hydrogenation (the B3PW91-SCRF data in parenthesis).
for the hemiacetal and 2M formation for M 5 Ru and Os is lower than the highest barrier on the potential energy surface; and this is the same as found for M 5 Fe. In addition, we also computed the hydrogenation of benzaldehyde to benzyl alcohol. The optimized structural parameters of the transition states TS(PhCHO) are listed in Table 8; and all the energetic data are given in Table 9. The corresponding potential energy surface is shown in Figure 4. In the transition state of benzaldehyde hydrogenation TS(PhCHO) for M 5 Fe at B3PW91 (Table 8), the FeAH1 and NAH2 bond distances (1.871 and 1.216 A˚, respectively) are elongated compared to those in 1Fe (1.564 and 1.017 A˚, respectively), while the central FeAN distance becomes slightly ˚ ). The forming CAH1 and H2AO disshorter (2.013 vs. 2.109 A ˚ , also indicating a very late trantances are 1.181 and 1.2871 A
sition state. The same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory. As shown in Figure 4, the hydrogenation of benzaldehyde to benzyl alcohol at B3PW91 has a free energy barrier of 6.60 kcal/mol and is exergonic by 2.98 kcal/mol. Table 9 also clearly shows that the activation barriers for benzaldehyde hydrogenation with all the three catalysts are much lower than those of methyl benzoate hydrogenation regardless the method applied for calculation. However, the most important information which can be drawn from Figure 4 concerns the dehydrogenation reaction rather than the hydrogenation reaction. The hydrogenation reaction has not only very low activation barriers but is also slightly exergonic for M 5 Fe and Ru, while being slightly endergonic for M 5 Os. Such small free energy differences
Table 8. Selected structural parameters for TS(PhCHO) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
FeAH FeAN NAH2 OAH2 CAH1 OAC
1.871 2.013 1.216 1.271 1.181 1.338
1.953 1.996 1.301 1.195 1.156 1.359
SCRF
B3LYP
BP86
B3PW91
1Ru 1.953 2.033 1.230 1.266 1.167 1.349
1.861 2.018 1.245 1.259 1.201 1.350
2.014 2.157 1.186 1.315 1.180 1.336
2.112 2.141 1.301 1.197 1.151 1.361
SCRF
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BP86
2.051 2.174 1.233 1.263 1.182 1.345
2.005 2.164 1.267 1.242 1.207 1.350
1Os 2.103 2.175 1.212 1.288 1.164 1.350
# # # # # #
1.986 2.154 1.225 1.263 1.197 1.335
2.045 2.140 1.302 1.194 1.172 1.354
# No transition state could be located.
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Table 9. Energetic data of methyl benzaldehyde hydrogenation to benzyl alcohol (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
1M 1 PhCHO
TS(PhCHO)
2M 1 PhCH2OH
0.00 0.00 0.00 0.00
6.60 11.20 7.75 3.88
22.98 20.16 22.26 21.74
0.00 0.00 0.00 0.00
4.12 9.57 4.29 #
21.00 2.81 21.70 20.29
0.00 0.00 0.00 0.00
9.52 16.68 10.29 5.90
2.06 7.51 0.93 2.07
# No transition state could be located.
connecting aldehyde and alcohol reveal the possibility to switch between hydrogenation and dehydrogenation by changing the reaction conditions. Therefore, these catalysts should also be active for the dehydrogenation of alcohols to produce H2. Considering the activation barrier of the dehydrogenation, 1Ru/2Ru should be the best catalysts. Indeed, Beller and coworkers[3,4] found that 1Ru/2Ru are more efficient in low-temperature aqueous-phase methanol dehydrogenation to H2 and CO2 than 1Fe/2Fe under identical reaction conditions. Yang also found that 1Ru/2Ru have lower effective activation free energy than 1Fe/2Fe in ethanol dehydrogenation to produce acetaldehyde and H2 (7.0 and 10.1 kcal/mol, respectively) at M06-SCRF.[6] 1Ru/2Ru are efficient catalysts in H2 production from alcohols under mild reaction conditions[27] and acceptorless dehydrogenation of ethanol.[28] It is also worth mentioning that Gusev et al.[17] reported that the free energy barrier for isopropanol dehydrogenation is 13.85 kcal/mol for 1Os/2Os and 13.74 kcal/mol for IRu/2Ru at mPW1K-SCRF. For the hydrogenation of benzaldehyde to benzyl alcohol, Guan and coworkers,[14] computed a stepwise mechanism and the highest barrier on the potential energy surface is 7.4 kcal/ mol. Using the simplified model catalysts (where the isopropyl groups at the P centers were replaced by methyl groups), Schneider and coworkers,[13] computed the hydrogenation of formaldehyde via stepwise mechanism at the R1-B3PW91D3BJ/def2-QZVPP//B3LYP/def2-SVP level under the consideration of the thermal correction at 1208C. From formaldehyde to methanol, the highest barrier is 5.7 kcal/mol and the reaction is exergonic by 10.4 kcal/mol. All these computed barriers are close to our values at B3PW91 and B3LYP (6.60 and 7.75 kcal/ mol, respectively).
Conclusion The properties of defined pincer complexes M(H)2(CO) [NH(C2H4PiPr2)2] (1M) and M(H)(CO)[N(C2H4PiPr2)2] (2M) (M 5 Fe, Ru, Os) as active catalysts in hydrogenation of acetonitrile (MeCN), methyl benzoate (PhCO2CH3), and benzalde-
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hyde (PhCHO) have been computed at the B3PW91, B3PW91SCRF, B3LYP, and BP86 levels of density functional theory. These results have enabled a systematic comparison and discussion of the activity of the three different metals. It is to note that the computed equilibrium between 1Fe and 2Fe under H2 atmosphere has been fully confirmed by our recent experimental studies. However, our computed equilibrium for M 5 Fe, Ru, and Os is in contrast with the results from previous computations at the M06 level under the consideration of solvation effects. The computed H2 elimination barrier from 1M increases from 1Fe to 1Ru as well as to 1Os, however, the hydrogen addition barrier to 2M are in a very close range. For the hydrogenation of acetonitrile, both 1Fe/2Fe and 1Ru/2Ru are competent catalysts on the basis of the computed barriers and reaction energies, whereas 1Os/2Os are less effective. The most effective catalysts for the hydrogenation of methyl benzoate are 1Ru/2Ru, followed by 1Fe/2Fe, while 1Os/2Os are least effective. For the hydrogenation of benzyl aldehyde or dehydrogenation of benzyl alcohol, the best catalysts are 1Ru/2Ru, and this is in agreement with the previous experimental and computational studies. It is finally to note that the PCM is not sufficient in studying the solvation effect in the energetic properties of these catalysts as well as their catalytic properties in hydrogenation of acetonitrile and methyl benzoate. Keywords: pincer complexes iron ruthenium osmium hydrogenation catalysts reaction mechanisms density functional theory nitrile ester aldehyde
How to cite this article: H. Jiao, K. Junge, E. Alberico, M. Beller. J. Comput. Chem. 2016, 37, 168–176. DOI: 10.1002/jcc.23944
]
Additional Supporting Information may be found in the online version of this article.
[1] J. G. de Vries, C. J. Elsevier, Eds., In Handbook of Homogeneous Hydrogenation; Wiley-VCH: Weinheim, 2007. [2] M. Trincado, D. Banerjee, H. Gr€ utzmacher, Energy Environ. Sci. 2014, 7, 2464. [3] M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85. [4] E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H.-J. Drexler, W. Baumann, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 14162. [5] X. Yang, ACS Catal. 2014, 4, 1129. [6] (a) X. Yang, ACS Catal. 2013, 3, 2684; (b) X. Yang, ACS Catal. 2012, 2, 964; (c) X. Yang, ACS Catal. 2011, 1, 849. [7] C. Bornschein, S. Werkmeister, B. Wendt, H. Jiao, E. Alberico, H. Junge, K. Junge, M. Beller, Nat. Commun. 2014, 5, 4111. [8] S. Werkmeister, H. Junge, B. Wendt, E. Alberico, H. Jiao, W. Baumann, K. Junge, F. Gallou, M. Beller, Angew. Chem. Int. Ed. 2014, 53, 8722. [9] I. Koehne, T. J. Schmeier, E. A. Bielinski, J. C. Pan, P. O. Lagaditis, W. H. Bernskoetter, M. K. Takase, C. Wurtle, N. Hazari, S. Schneider, Inorg. Chem. 2014, 53, 2133. [10] S. Chakraborty, H. Dai, P. Bhattacharya, N. T. Fairweather, M. S. Gibson, J. A. Krause, H. Guan, J. Am. Chem. Soc. 2014, 136, 7869. [11] S. Chakraborty, W. W. Brennessel, W. D. Jones, J. Am. Chem. Soc. 2014, 136, 8564.
Journal of Computational Chemistry 2016, 37, 168–176
175
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[12] E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. W€ urtle, W. H. Bernskoetter, H. Hazari, S. Schneider, J. Am. Chem. Soc. 2014, 136, 10234. [13] S. Chakraborty, P. O. Lagaditis, M. F€ orster, E. A. Bielinski, N. Hazari, M. C. Holthausen, W. D. Jones, S. Schneider, ACS Catal. 2014, 4, 3994. [14] S. Qu, H. Dai, Y. Dang, C. Song, Z.-X. Wang, H. Guan, ACS Catal. 2014, 4, 4377. [15] E. A. Bielinski, M. Fo€ urster, Y. Zhang, W. H. Bernskoetter, N. Hazari, M. C. Holthausen, ACS Catal. 2015, 5, 2404. [16] M. Lei, Y. Pan, X. Ma, Eur. J. Inorg. Chem. 2015, 794. [17] M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk, D. G. Gusev, Organometallics 2011, 30, 3479. [18] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
176
Journal of Computational Chemistry 2016, 37, 168–176
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Inc., Wallingford CT, 2010. A. D. Becke, J. Chem. Phys. 1993, 98, 5648. (a) A. D. Becke, Phys. Rev. A 1988, 38, 3098; (b) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785. J. P. Perdew, Phys. Rev. B, 1986, 33, 8822. A. Schaefer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829. P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 299. J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999. A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899. V. Rautenstrauch, X. Hong-Cong, R. Churlaud, K. Abdur-Rashid, R. H. Morris, Chem. Eur. J. 2003, 9, 4954. M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 9593. M. Nielsen, H. Junge, A. Kammer, M. Beller, Angew. Chem. Int. Ed. 2012, 51, 5711.
Received: 17 March 2015 Revised: 28 April 2015 Accepted: 28 April 2015 Published online on 17 May 2015
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A Comparative Computationally Study About the Defined M(II) Pincer Hydrogenation Catalysts (M 5 Fe, Ru, Os)† Haijun Jiao,*[a] Kathrin Junge,[a] Elisabetta Alberico,[a,b] and Matthias Beller[a] The mechanism of acetonitrile and methyl benzoate catalytic hydrogenation using pincer catalysts M(H)2(CO)[NH(C2H4PiPr2)2] (1M) and M(H)(CO)[N(C2H4PiPr2)2] (2M) (M 5 Fe, Ru, Os) has been computed at various levels of density functional theory. The computed equilibrium between 1Fe and 2Fe agrees perfectly with the experimental observations. On the basis of the activation barriers and reaction energies, the best catalysts for acetonitrile hydrogenation are 1Fe/2Fe and 1Ru/2Ru, and the best catalysts for methyl benzoate hydrogenation are 1Ru/2Ru.
The best catalysts for the dehydrogenation of benzyl alcohol are 1Ru/2Ru. It is to note that the current polarizable continuum model is not sufficient in modeling the solvation effect in the energetic properties of these catalysts as well as their catalytic properties in hydrogenation reaction, as no equilibrium could be established between 1Fe and 2Fe. Comparison with other methC 2015 Wiley Periodicals, Inc. ods and procedures has been made. V
Introduction
ble dehydrogenation and hydrogenation of alcohols and ketones using 1Fe/2Fe as active catalysts. In the same time, Guan and coworkers,[14] computed the detailed potential energy surface for the catalytic hydrogenation of methyl benzoate and benzaldehyde involving 1Fe/2Fe as active catalysts. In a combined experimental and DFT work, Bielinski et al.,[15] most recently reported the base-free aqueous phase methanol dehydrogenation using 1Fe/2Fe as active catalysts and a cocatalytic amount of Lewis acid. The mechanism of aqueousphase methanol dehydrogenation using 1Ru/2Ru as active catalysts was computed by Lei et al.,[16] and they found that methanol-assisted hydrogen-release step is much more favorable than the direct hydrogen-release one. In addition to the iron (1Fe/2Fe) and ruthenium (1Ru/2Ru) complexes, the corresponding osmium complexes (1Os/2Os) have been prepared by Gusev and coworkers,[17] who tested them in the catalytic dehydrogenation of alcohols. Having these three homolog complexes in hand, we are interested in exploring their comparative reactivity in catalytic hydrogenation of acetonitrile (CH3CN) and methyl benzoate (PhCO2Me) as well as benzaldehyde (PhCHO).
The reactions involving H2 play an increasingly important role in modern synthetic organic chemistry and catalysis. For example, green hydrogenation reactions represent key technologies for the preparation of intermediates of new and existing pharmaceuticals as well as agrochemicals.[1] Effective dehydrogenation reactions provide the tools for the production of H2, which has been considered as a potential energy carrier.[2] Beller and coworkers have reported the low-temperature aqueous-phase methanol dehydrogenation to H2 and CO2 (CH3OH 1 H2O 5 CO2 1 2H2) using the well-defined aliphatic PNP pincer ruthenium (1Ru/2Ru)[3] and iron (1Fe/2Fe)[4] complexes (Fig. 1). They also proposed an outer sphere reaction mechanism, which was later evaluated by Yang on the basis of density functional theory (DFT) computation.[5,6] Most recently, Beller and coworkers have described the effective and selective hydrogenation of nitriles to valuable primary amines[7] and carboxylic esters to alcohols[8] using 1Fe/2Fe as the active catalysts. Their experimental and computational analyses identified an outer sphere hydrogenation mechanism by transfer of the hydride from the iron center and the proton from the coordinating nitrogen to the nitriles (or esters) to give the corresponding imines (or ketones), which are further hydrogenated to the corresponding amines (or alcohols). The most important feature in this reaction is the reversible and concerted hydrogen shuttle between catalyst 1Fe and its amido counterpart 2Fe. Apart from the Beller group, complex 1Fe has been independently synthesized and structurally characterized by Schneider and coworkers,[9] and applied to the hydrogenation of esters to alcohols by Guan and coworkers.[10] Recently, the 1Fe/2Fe complexes have been used as active catalysts in the acceptorless dehydrogenation and hydrogenation of Nheterocycles by Jones and coworkers,[11] and in formic acid dehydrogenation and CO2 hydrogenation by Schneider and coworkers.[12] In a combined experimental and DFT work, Schneider and coworkers,[13] studied the acceptorless reversi168
Journal of Computational Chemistry 2016, 37, 168–176
DOI: 10.1002/jcc.23944
[a] H. Jiao, K. Junge, E. Alberico, M. Beller Leibniz-Institut f€ ur Katalyse e.V. an der Universit€ at Rostock Albert-EinsteinStraße 29a, 18059, Rostock, Germany E-mail: [email protected] [b] E. Alberico Istituto di Chimica Biomolecolare, CNR, tr. La Crucca 3, 07100, Sassari, Italy † In memory of Prof. Dr. Paul von Ragu e Schleyer (February 27, 1930– November 21, 2014): Paul was a great and aesthetic scientist; a pioneer and leader of computational chemistry as well as an excellent mentor and wonderful friend. Contract/grant sponsor: State of Mecklenburg-Western Pomerania; Contract/grant sponsor: BMBF C 2015 Wiley Periodicals, Inc. V
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Figure 1. Hydrogenation and dehydrogenation catalysts 1M and 2M (R 5 Isopropyl).
Computational Details All calculations were carried out using the Gaussian 09 program.[18] All structures were optimized at the B3PW91,[19] B3LYP,[20] and BP86[21] levels of DFT with the TZVP[22] basis set (LANL2DZ for Ru and Os[23]). All optimized structures were characterized either as energy minimums without imaginary frequencies or transition states with only one imaginary frequency by frequency calculations; and the imaginary model connects the initial and the final states. The thermal corrections to Gibbs free energy at 298 K from the frequency analysis are added to the total electronic energy, and we therefore used the corrected Gibbs free energy (DG) at 298 K for our energetic discussion and comparison. We also carried out selfconsistent reaction field (SCRF) structure optimizations and frequency calculations at the B3PW91 level using the polarizable continuum model (PCM)[24] and methanol as solvent to estimate the solvation influence (B3PW91-SCRF). Natural atomic orbital and natural bond orbital analysis were carried out on the B3PW91 optimized structures with the NBO program (Version 3.1).[25] The B3PW91, B3LYP, and BP86 computed energetic data and Cartesian coordinates are listed in Supporting Information.
Results and Discussion Binding properties At first, we have analyzed the electronic structures of 1M and 2M in singlet states on the basis of B3PW91 computation
(Table 1). Natural localized molecular orbital analysis reveals an MAN r bond in 1M and an M@N double (one r and one p) bond in 2M. In 1M, the NAH bond lengths are apparently the ˚ ) for the three metal complexes, while the FeAH1 same (1.017 A bond (1.564 A˚) is shorter than the RuAH and OsAH bonds ˚ ). In addition, the FeAN bond (2.109 A ˚ ) is also (both 1.695 A shorter than the RuAN and OsAN bonds (both 2.231 A˚). All these bonds are polarized on the basis of their partial Wiberg bond indexes, in particular in the MAN r bond the N atom is approximately sp3 hybridized and provides a stronger contribution to the MAN bond than the M atom. In 1M, the M atoms are much more negatively charged than the N atoms. It is interesting to note that the H2 atom of the NAH2 bond is positively charged and is therefore protonic, while the H1 atom of the parallel MAH1 bond is electronically neutral and therefore less hydridic. In 2M, the M@N double bond is shorter than the corresponding MAN single bond in 1M, and the computed Wiberg bond indexes are approximately double of those of 1M. In 2M, the N atom has sp1.32 to sp1.46 hybridization and contributes more strongly to the MAN r bond than the M metal atom. In the MAN p bond, the N atom has pure p orbital character and also contributes more strongly than the M metal. 1M and 2M interconversion Prior to studying the catalytic activities of these catalysts, we would like to assess the structural and energetic properties of the active catalysts 1M and 2M. The optimized structural parameters of 1M and 2M as well as the corresponding transition states TS(1M/2M) are summarized in Table 2. Table 3 lists all the energetic data of the transformation of 1M and 2M as well as the possible competitive CO dissociation. Figure 2 shows the potential energy surfaces of the transformation of 1M and 2M as well as the possible competitive CO dissociation at the B3PW91 level of theory. For M 5 Fe, the B3PW91 computed FeAN distances in 1Fe and 2Fe agree well with the available data from X-ray analysis. For example, the computed FeAN distance in 1Fe is very close to the one measured from the X-ray solid state structure
˚ , B3PW91), Wiberg bonding indexes (WBI), natural charge (d/e), natural bond orbitals and atomic hybrid contributions Table 1. Computed bond lengths (A for the r and p NAM bonds. 1M
1Fe
1Ru
1Os
NAM (WBI) dM(AN) dN(AM) NAH2 (WBI) MAH1 (WBI) dH2 dH1 NAM (r)
˚ (0.482) 2.109 A 21.874 20.432 ˚ (0.798) 1.017 A ˚ (0.793) 1.564 A 0.397 0.002 16.7% Fe[sp5.98d9.54]; 79.8% N[sp3.10]
˚ (0.408) 2.231 A 21.744 20.476 ˚ (0.793) 1.017 A ˚ (0.729) 1.695 A 0.400 20.011 15.5% Ru[sp7.31d17.50]; 80.6% N[sp2.92]
˚ (0.450) 2.231 A 21.720 20.479 ˚ (0.789) 1.017 A ˚ (0.763) 1.695 A 0.404 20.002 16.5% Os[sp6.93d14.27]; 79.8% N[sp2.88]
2M
2Fe
2Ru
2Os
N@M (WBI) dM dN NAM (r) NAN (p)
˚ (0.930) 1.869 A 21.309 20.434 19.5% Fe[sp2.63d2.83]; 76.5% N[sp1.32] 15.4% Fe[pd3.69]; 77.0% N[p]
˚ (0.764) 2.040 A 21.140 20.519 19.0% Ru[sp2.30d5.49]; 76.9% N[sp1.46] 12.3% Ru[pd3.23]; 79.6% N[p]
˚ (0.874) 2.009 A 21.114 20.519 21.4% Os[sp2.23d5.76]; 75.7% N[sp1.39] 15.3% Os[pd3.73]; 77.7% N[p]
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Table 2. Selected structural parameters for 1M, TS(1M/2M), and 2M (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
SCRF
1Fe 1
MAH NAH2 MAN H1AH2
1.564 1.017 2.109 2.298
1.569 1.017 2.113 2.330
MAH MAH2 NAH2 MAN H1AH2
1.663 1.668 1.461 2.078 0.968
1.663 1.664 1.479 2.079 0.957
MAN
1.869
1.862
1.577 1.017 2.143 2.320
1.571 1.027 2.116 2.286
1.695 1.017 2.231 2.414
1.698 1.017 2.235 2.428
1.688 1.704 1.441 2.104 0.968
1.666 1.680 1.463 2.101 0.986
1.870 1.879 1.455 2.200 0.975
1.873 1.871 1.488 2.200 0.956
1.888
1.873
2.040
2.037
SCRF
Table 3. Transformation of active catalysts 1M and 2M (R 5 isopropyl) (relative energies are given in kcal/mol; and the corresponding energies of the triplet states are given in square bracket). 1M TS(1M/2M) 2M1H2
2M
1M-CO
0.00 0.00 0.00 0.00
17.47 21.01 18.00 14.53
13.38 17.22 12.83 10.40
0.32 [14.22] 3.58 [15.03] 21.54 [11.70] 22.33 [24.01]
41.22 41.65 36.00 54.58
[23.78] [24.92] [19.27] [43.86]
0.00 0.00 0.00 0.00
21.07 26.15 21.05 17.70
11.68 17.32 10.05 9.41
2.31 [38.70] 6.55 [30.85] 20.98 [37.36] 20.88 [37.06]
51.66 48.56 46.59 57.81
[59.02] [57.39] [53.95] [64.80]
0.00 0.00 0.00 0.00
23.91 30.25 24.16 20.35
11.77 18.28 10.03 10.50
5.34 11.25 1.65 1.49
69.72 68.30 64.58 64.80
[75.27] [78.79] [70.88] [78.67]
[44.57] [49.37] [40.71] [39.89]
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B3LYP
BP86
1Os 1.706 1.017 2.259 2.435
1.706 1.027 2.245 2.407
1.706 1.017 2.232 2.389
1.704 1.017 2.236 2.393
1.714 1.017 2.259 2.407
1.715 1.027 2.246 2.391
TS(1Os/2Os)
1.900 1.921 1.431 2.223 0.979
1.882 1.901 1.463 2.223 0.983
1.837 1.854 1.479 2.204 0.989
1.838 1.848 1.511 2.202 0.970
2.057
2.049
2.009
2.001
2Ru
˚ ) of the BH3 coordinated complex (1Fe-BH3).[4] The (2.0669 A FeAN distance in 2Fe also agrees well with the one recently ˚ .[11] The significant shorter FeAN distance in reported 1.86 A 2Fe than in 1Fe reveals the enhanced FeAN interaction in 2Fe, which has Fe@N double character as reported from natural bonding orbital analysis. In the transition state, TS(1M/2M), ˚. the forming H1AH2 distance is 0.968 A It is also to note that the computed structural parameters at B3LYP and BP86 as well as B3PW91-SCRF agree well with the available experimental data, indicating that these structural parameters are much less sensitive to the applied methods. This is also the same as found by Schneider and coworkers,[12,13] and Guan and coworkers.[14] In addition, we found very good agreement between our computed structural parameters and those reported in the literature, for example, for M 5 Fe and Ru using the M06 functional under the consideration of the solvation effect of ethanol or methanol (M06SCRF),[5,6] for M 5 Ru and Os at the mPW1K and PBE0 levels under the consideration of the solvation effect of tetrahydrofuran (mPW1K-SCRF and PBE0-SCRF).[15] As discussed below,
170
B3PW91
TS(1Ru/2Ru)
2Fe
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
BP86
1Ru
TS(1Fe/2Fe) 1
B3LYP
1.863 1.894 1.459 2.226 0.985
1.853 1.876 1.489 2.224 0.997
2Os 2.025
2.020
large differences are found instead in estimating the activation barriers and reaction energies. In our previous study about nitrile hydrogenation, we found that the most important features of this reaction are the reversible hydrogenation and dehydrogenation process which interconverts catalyst 1Fe and its amido counterpart 2Fe, and the fact that the reaction is thermoneutral at B3PW91. In agreement with the experimental findings,[7,11] 2Fe has a singlet ground state (the triplet state is less stable by 13.90 kcal/ mol) and the transformation of 1Fe into 2Fe and vice versa can be conducted very easily by changing the H2 pressure. It is found that the CO dissociation energy, resulting in a triplet ground state 1Fe-CO, is 23.78 kcal/mol, which is much higher than the barrier of H2 elimination via the transition state TS(1Fe/2Fe) (17.47 kcal/mol). It is noted that the corresponding H2 complex 2Fe1H2 is higher in energy than 1Fe and 2Fe by 17.47 and 13.06 kcal/mol, respectively, and therefore should be hardly formed and thus escape experimental detection. The reported H2 elimination barrier of 1Fe and the relative energy of 2Fe1H2 by Yang[6] using M06-SCRF (24.0 and 17.9 kcal/mol, respectively) are higher than our values (17.47 and 13.38 kcal/ mol, respectively). In addition, the B3LYP and BP86 computed energetic data also seem very reasonable on the basis of the experimental observed equilibrium.[7,11] Our computed reaction energy of H2 elimination (1Fe 5 2Fe 1 H2) at B3PW91 differs from that calculated by Yang using the M06-SCRF (0.32 vs. 9.1 kcal/mol). Even worse results have been reported by Guan and coworkers,[14] where the energetic data were evaluated from M06/6-31111G(d,p) single-point energy calculation on the B3LYP/6-31G(d,p) gas phase geometries under the consideration of solvation effects of tetrahydrofuran. They reported that the H2 elimination barrier is 26.0 kcal/mol, and the reaction is endergonic by 14.9 kcal/mol. On the basis of the reported facile equilibrium between 1Fe and 2Fe under H2 atmosphere at room temperature,[7,11] our computed reaction energy of 0.32 kcal/mol sounds very reasonable and realistic, in contrast, the computed reaction energies of 9.1 and 14.9 kcal/mol using M06SCRF are far too big to establish any facile equilibrium. WWW.CHEMISTRYVIEWS.COM
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Figure 2. B3PW91 potential energy surfaces (kcal/mol) for 1M and 2M interconversion (the B3PW91-SCRF data in parenthesis).
Qualitatively, the same results have also been found at the B3LYP and BP86 levels of theory (Table 3), and there are no energetic disorders found for both H2 elimination and CO dissociation. The largest difference is that BP86 disfavors the energy of the triplet states of 2Fe and 2Fe-CO much more strongly than B3PW91 and B3LYP. Under the consideration of the solvation effect, the H2 elimination barrier increases and the reaction becomes more endergonic than those found in the gas phase (3.54 and 3.26 kcal/mol, respectively). One can therefore conclude that the current PCM is not sufficient for studying the solvation effect in the energetic properties of these catalysts, despite the fact that polar solvents play a more important role in the heterolytic splitting of H2 than nonpolar solvents.[26] For M 5 Ru and Os, the same energetic trends have been found, apart from 1Ru-CO and 1Os-CO, the singlet states of which are favored over their corresponding triplet states (Table 3). Nevertheless, CO dissociation energies are much higher than the H2 elimination barriers, and outer sphere hydrogenation mechanisms are favorable as discussed below. For M 5 Ru, the reported H2 elimination barrier of 1Ru by Yang[5,6] using M06-SCRF (29.5 kcal/mol) and by Gusev et al.,[17]] using mPW1K-SCRF (29.2 kcal/mol) as well as by Lei et al.,[16] using wB97X-D-SCRF (27.1 kcal/mol) are much higher than our values (21.07 kcal/mol). For M 5 Os, the reported H2 elimination
barrier of 1Os by Gusev et al. using mPW1K-SCRF (30.9 kcal/ mol) is also much higher than our value (23.91 kcal/mol). Our computed reaction energy (2.31 kcal/mol) of H2 elimination (1Ru 5 2Ru 1 H2) at B3PW91 differs from that of Yang[5,6] using the M06-SCRF (11.8 kcal/mol) as well as those of Gusev et al.,[17] at mPW1K-SCRF (18.8 kcal/mol) and at PBE0-SCRF (15.4 kcal/mol) or that of Lei et al.,[16] using wB97X-D-SCRF (7.6 kcal/mol). Considering that 1Ru and 2Ru have been used as effective catalysts for hydrogenation and dehydrogenation, the B3PW91, B3LYP, and BP86 results sound much more reasonable, while those at M06-SCRF, mPW1K-SCRF, BPE0-SCRF, and wB97X-D-SCRF as well as B3PW91-SCRF are less reasonable. For 1Os 5 2Os 1 H2, our results differ strongly from those of Gusev et al.,[17] this reaction is less endergonic at B3PW91 (5.37 kcal/mol), much more endergonic at mPW1K-SCRF and BPE0-SCRF (19.7 and 16.1 kcal/mol, respectively). Also, considering that 1Os and 2Os have been considered as effective catalysts for hydrogenation and dehydrogenation, B3LYP and BP86 seems more reasonable than B3PW91, while the results of mPW1K-SCRF and BPE0-SCRF as well as B3PW91-SCRF are much worse, revealing the problematic of the current solvation model applied. A general trend can be found in Figure 2 and Table 3, that is, the H2 elimination barrier increases from 1Fe to 1Ru as well as to 1Os. It is interesting to note that for the regeneration of
Table 4. Selected structural parameters for TS(CH3CN) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
MAH MAN NAH2 H2ANC CAN NCAH1
1.821 2.009 1.247 1.342 1.226 1.215
1.860 2.000 1.276 1.319 1.233 1.190
SCRF
B3LYP
BP86
B3PW91
1Ru 1.876 2.032 1.253 1.347 1.228 1.203
1.822 2.018 1.264 1.341 1.237 1.230
1.963 2.151 1.223 1.373 1.226 1.217
2.000 2.143 1.265 1.334 1.234 1.190
SCRF
B3LYP
BP86
1.989 2.170 1.257 1.341 1.224 1.226
1.960 2.164 1.269 1.339 1.234 1.252
1Os 2.018 2.171 1.234 1.372 1.229 1.204
1.975 2.167 1.242 1.374 1.237 1.230
1.943 2.149 1.254 1.333 1.222 1.239
1.972 2.142 1.282 1.310 1.229 1.214
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Table 5. Energetic data of acetonitrile hydrogenation (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
1M 1 CH3CN
TS(CH3CN)
2M 1 CH3-CH@NH
0.00 0.00 0.00 0.00
17.75 23.87 19.90 11.67
20.96 3.63 20.28 22.58
0.00 0.00 0.00 0.00
17.15 24.40 18.50 11.82
1.02 6.60 0.23 21.13
0.00 0.00 0.00 0.00
21.03 29.73 23.01 14.96
4.08 11.30 2.91 2.24
1M from 2M by adding H2, the hydrogenation addition barriers are in a very tight range in the gas phase for all three metals at B3PW91 (17.15, 18.76, and 18.54 kcal/mol). The next trend is that BP86 favors the lowest H2 elimination barriers, tightly followed by B3PW91 and B3LYP, while the highest barrier is found at B3PW91-SCRF for all three metals. Apart from methanol as solvent, we also computed methanol-assisted H2 elimination, where methanol is actively involved in the six-membered transition state. Compared with the direct H2 elimination in Figure 2, it is found that methanol can lower the barrier of 1Fe, 1Ru, and 1Os by 1.21, 5.05, and 4.29 kcal/mol, respectively. However, there are no experimental results available for direct comparison.
Acetonitrile hydrogenation In our previous work,[7] the computed activation free energy for the concerted acetonitrile hydrogenation to imine is 17.55 kcal/mol, which is close to the H2 elimination barrier of 1Fe, but lower than the CO dissociation energy by 6.23 kcal/mol, respectively. The hydrogenation reaction is slightly exergonic by 0.96 kcal/mol. In addition, the hydrogenation of imine to amine is barrier-less and the reaction is exergonic by 14.04 kcal/mol. On the basis of these results, we computed only the first hydrogenation step from acetonitrile to imine, for 1Ru and 1Os. The optimized structural parameters of the transition state TS(CH3CN) are listed in Table 4. Table 5 lists all the energetic data of acetonitrile hydrogenation. Figure 3 shows the potential energy surfaces of acetonitrile hydrogenation at the B3PW91 level of theory. In the transition state for M 5 Fe at B3PW91 (Table 4), the ˚ , respecFeAH1 and NAH2 bond distances (1.821 and 1.247 A tively) are elongated compared to those in 1Fe (1.564 and ˚ , respectively), while the central FeAN distance 1.017 A ˚ ). The forming CAH1 and becomes shorter (2.009 vs. 2.109 A 2 H ANC distances are 1.215 and 1.342 A˚. The same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory. As shown in Table 5, BP86 provides the lowest barrier, followed by B3PW91 and B3LYP, while the highest barrier 172
Journal of Computational Chemistry 2016, 37, 168–176
is found at B3PW91-SCRF. The same trends in the structural parameters are also found for M 5 Ru and Os. It is found that 1Fe and 1Ru have close activation barriers (17.55 and 17.15 kcal/mol, respectively) and reaction energies (20.96 and 1.02 kcal/mol, respectively) in acetonitrile hydrogenation (Fig. 3), while 1Os has higher activation barrier (21.03 kcal/mol) and the reaction is more endergonic (4.08 kcal/mol). Therefore, 1Fe/2Fe and 1Ru/2Ru should be better catalysts for acetonitrile hydrogenation than 1Os/2Os. Considering the barriers of 1M (M 5 Fe, Ru, and Os) regeneration of 17.15, 18.76, and 18.54 kcal/mol at B3PW91, the rate-determining step might be either hydrogenation of acetonitrile or catalyst regeneration for M 5 Fe and Ru, however, acetonitrile hydrogenation is the rate-determining step for M 5 Os. However, results different from B3PW91 have been found at B3LYP and BP86 (Table 5). At B3LYP, the rate-determining step might be either hydrogenation of acetonitrile or catalyst regeneration for M 5 Fe (19.90 vs. 19.54 kcal/mol) and Os (230.1 vs. 22.51 kcal/mol), while acetonitrile hydrogenation is the rate-determining step for M 5 Ru (22.03 vs. 18.50 kcal/ mol). At BP86, catalyst 1M regeneration from 2M is the ratedetermining step for Fe (16.86 vs. 11.67 kcal/mol), Ru (18.54 vs. 11.82 kcal/mol), and Os (18.86 vs. 14.96 kcal/mol); the same qualitative conclusion is also found at B3PW91-SCRF. Hydrogenation of methyl benzoate and benzaldehyde In our previous experimental and theoretical studies, we proposed an outer sphere reaction mechanism for the hydrogenation of methyl benzoate.[8] At first, the ester is hydrogenated by 1Fe in a concerted way by a simultaneous transfer of the hydride from the iron center and the proton from the nitrogen ligand to give the corresponding hemiacetal and 2Fe. Subsequently, the dissociation of the hemiacetal to benzaldehyde and methanol takes place and 1Fe is regenerated from 2Fe by
Figure 3. B3PW91 potential energy surfaces (kcal/mol) of CH3CN hydrogenation (the B3PW91-SCRF data in parenthesis).
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Table 6. Selected structural parameters for TS(PhCO2Me) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
MAH MAN NAH2 OAH2 CAH1 OAC
1.998 1.995 1.311 1.184 1.150 1.333
2.112 1.981 1.358 1.154 1.137 1.344
B3LYP
BP86
B3PW91
1Ru 2.090 2.018 1.304 1.200 1.140 1.338
1.984 1.996 1.353 1.170 1.164 1.347
2.138 2.143 1.312 1.184 1.146 1.335
H2 addition. The next step is the hydrogenation of benzaldehyde to benzyl alcohol. The optimized structural parameters of the transition states TS(PhCO2Me) are listed in Table 6. Table 7 lists all the energetic data of the hydrogenation of methyl benzoate to the corresponding hemiacetal. Figure 4 shows the energetic data of the overall transformation of methyl benzoate to benzyl alcohol and methanol. Shortly after publication of our work, Schneider and coworkers[13] computed the acceptorless reversible dehydrogenation of alcohols and hydrogenation of ketones and Guan and coworkers[14] computed the detailed potential surface of the hydrogenation of methyl benzoate via benzaldehyde to benzyl alcohol and methanol using 1Fe/2Fe as active catalysts. As our goal is to examine the catalytic differences among the three metals 1M/2M (M 5 Fe, Ru, Os), we did not examine, as done by Schneider and Guan, each and every single step through which these transformations take place. Nevertheless, we made systematic comparison between our and their results. In the transition state of methyl benzoate hydrogenation TS(PhCO2Me) for M 5 Fe at B3PW91 (Table 5), the FeAH1 and NAH2 bond distances (1.998 and 1.331 A˚, respectively) are elongated compared to those in 1Fe (1.564 and 1.017 A˚, respectively), while the central FeAN distance becomes shorter ˚ ). The forming CAH1 and H2AO distances are (1.995 vs. 2.109 A ˚ , indicating a very late transition state. The 1.150 and 1.184 A same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory.
Table 7. Energetic data of methyl benzoate hydrogenation to hemiacetal (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
SCRF
1M 1 PhCO2CH3
TS(PhCO2CH3)
2M 1 PhCH(OH)(OCH3)
0.00 0.00 0.00 0.00
21.51 26.90 21.40 19.42
11.93 15.32 11.94 11.80
0.00 0.00 0.00 0.00
18.88 30.33 17.94 20.29
13.92 18.29 12.50 13.25
0.00 0.00 0.00 0.00
25.32 33.83 24.86 22.40
16.98 22.99 15.14 15.16
2.450 2.104 1.622 1.037 1.116 1.371
SCRF
B3LYP
BP86
2.158 2.162 1.302 1.201 1.153 1.335
2.107 2.147 1.376 1.156 1.170 1.348
1Os 2.220 2.615 1.310 1.195 1.137 1.340
2.193 2.138 1.485 1.097 1.154 1.364
2.083 2.142 1.308 1.186 1.164 1.330
2.147 2.129 1.367 1.145 1.151 1.341
For the formation of the hemiacetal for M 5 Fe, the computed free energy barrier of methyl benzoate is 21.51 kcal/ mol. However, this reaction step is endergonic by 11.93 kcal/ mol. For the dissociation of the hemiacetal to the corresponding benzaldehyde and methanol, the reaction is exergonic by 8.21 kcal/mol.[8] The free energy barrier of methyl benzoate hydrogenation is lowest at BP86 (19.42 kcal/mol), tightly followed by those at B3LYP (21.40 kcal/mol) and B3PW91 (21.51 kcal/mol), while highest at B3PW91-SCRF (26.90 kcal/mol). It is now very interesting to compare our results with those of Guan and coworkers,[14] where the energetic data were evaluated from M06/6-31111G(d,p) single-point energy calculation on the B3LYP/6-31G(d,p) gas phase geometries under the consideration of solvation effects of THF. In their stepwise mechanism, the highest barrier of hemiacetal formation is 19.3 kcal/mol, which is close to our values (21.51, 21.40, and 19.42 kcal/mol, respectively) at B3PW91, B3LYP, and BP86, respectively, while lower than our value (26.90 kcal/mol) at B3PW91SCRF. The most problematic point is that the formation of the hemiacetal and 2Fe is endergonic by 21.9 kcal/mol, which is higher than the highest barrier on the potential energy surface by 2.6 kcal/mol; this indicates that the energy of the formed product is higher than the barrier of the product formation. In contrast, all our results show that formation of the hemiacetal and 2Fe is also endergonic, but the reaction energy is lower than the highest barrier on the potential energy surface (11.93 vs. 21.51 kcal/mol at B3PW91). This comparison shows that our computed potential energy surface is more reasonable that that by Guan and coworkers.[14] Using the simplified model catalysts (where the isopropyl groups at the P centers were replaced by methyl groups), Schneider and coworkers[13] computed the hydrogenation of methyl formate via a stepwise mechanism at the R1-B3PW91D3BJ/def2-QZVPP//B3LYP/def2-SVP level under the consideration of the thermal correction at 1208C. From methyl formate to methoxymethanol, the highest barrier is 13.5 kcal/mol and the reaction is endergonic by 11.1 kcal/mol. These results are in qualitative agreement with our computations and in disagreement with those of Guan and coworkers.[14] For M 5 Ru, B3LYP provides the lowest barrier (17.94 kcal/ mol), followed by B3PW91 (18.88 kcal/mol) and BP86 (20.29 kcal/mol), while the highest barrier is found at B3PW91-SCRF (30.33 kcal/mol). For M 5 Os, the lowest barrier is found at BP86, followed by those at B3LYP (24.86 kcal/mol) and B3PW91 (25.32 kcal/mol), while the highest at B3PW91-SCRF (33.83 kcal/mol). This is to emphasis that the reaction energy Journal of Computational Chemistry 2016, 37, 168–176
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Figure 4. B3PW91 potential energy surfaces (kcal/mol) of methyl benzoate hydrogenation (the B3PW91-SCRF data in parenthesis).
for the hemiacetal and 2M formation for M 5 Ru and Os is lower than the highest barrier on the potential energy surface; and this is the same as found for M 5 Fe. In addition, we also computed the hydrogenation of benzaldehyde to benzyl alcohol. The optimized structural parameters of the transition states TS(PhCHO) are listed in Table 8; and all the energetic data are given in Table 9. The corresponding potential energy surface is shown in Figure 4. In the transition state of benzaldehyde hydrogenation TS(PhCHO) for M 5 Fe at B3PW91 (Table 8), the FeAH1 and NAH2 bond distances (1.871 and 1.216 A˚, respectively) are elongated compared to those in 1Fe (1.564 and 1.017 A˚, respectively), while the central FeAN distance becomes slightly ˚ ). The forming CAH1 and H2AO disshorter (2.013 vs. 2.109 A ˚ , also indicating a very late trantances are 1.181 and 1.2871 A
sition state. The same trend is also found at the B3LYP, BP86 as well as B3PW91-SCFR levels of theory. As shown in Figure 4, the hydrogenation of benzaldehyde to benzyl alcohol at B3PW91 has a free energy barrier of 6.60 kcal/mol and is exergonic by 2.98 kcal/mol. Table 9 also clearly shows that the activation barriers for benzaldehyde hydrogenation with all the three catalysts are much lower than those of methyl benzoate hydrogenation regardless the method applied for calculation. However, the most important information which can be drawn from Figure 4 concerns the dehydrogenation reaction rather than the hydrogenation reaction. The hydrogenation reaction has not only very low activation barriers but is also slightly exergonic for M 5 Fe and Ru, while being slightly endergonic for M 5 Os. Such small free energy differences
Table 8. Selected structural parameters for TS(PhCHO) (B3PW91-SCFR using methanol as solvent). B3PW91
SCRF
B3LYP
BP86
B3PW91
1Fe 1
FeAH FeAN NAH2 OAH2 CAH1 OAC
1.871 2.013 1.216 1.271 1.181 1.338
1.953 1.996 1.301 1.195 1.156 1.359
SCRF
B3LYP
BP86
B3PW91
1Ru 1.953 2.033 1.230 1.266 1.167 1.349
1.861 2.018 1.245 1.259 1.201 1.350
2.014 2.157 1.186 1.315 1.180 1.336
2.112 2.141 1.301 1.197 1.151 1.361
SCRF
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BP86
2.051 2.174 1.233 1.263 1.182 1.345
2.005 2.164 1.267 1.242 1.207 1.350
1Os 2.103 2.175 1.212 1.288 1.164 1.350
# # # # # #
1.986 2.154 1.225 1.263 1.197 1.335
2.045 2.140 1.302 1.194 1.172 1.354
# No transition state could be located.
174
B3LYP
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Table 9. Energetic data of methyl benzaldehyde hydrogenation to benzyl alcohol (kcal/mol).
M 5 Fe B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Ru B3PW91 B3PW91-SCRF B3LYP BP86 M 5 Os B3PW91 B3PW91-SCRF B3LYP BP86
1M 1 PhCHO
TS(PhCHO)
2M 1 PhCH2OH
0.00 0.00 0.00 0.00
6.60 11.20 7.75 3.88
22.98 20.16 22.26 21.74
0.00 0.00 0.00 0.00
4.12 9.57 4.29 #
21.00 2.81 21.70 20.29
0.00 0.00 0.00 0.00
9.52 16.68 10.29 5.90
2.06 7.51 0.93 2.07
# No transition state could be located.
connecting aldehyde and alcohol reveal the possibility to switch between hydrogenation and dehydrogenation by changing the reaction conditions. Therefore, these catalysts should also be active for the dehydrogenation of alcohols to produce H2. Considering the activation barrier of the dehydrogenation, 1Ru/2Ru should be the best catalysts. Indeed, Beller and coworkers[3,4] found that 1Ru/2Ru are more efficient in low-temperature aqueous-phase methanol dehydrogenation to H2 and CO2 than 1Fe/2Fe under identical reaction conditions. Yang also found that 1Ru/2Ru have lower effective activation free energy than 1Fe/2Fe in ethanol dehydrogenation to produce acetaldehyde and H2 (7.0 and 10.1 kcal/mol, respectively) at M06-SCRF.[6] 1Ru/2Ru are efficient catalysts in H2 production from alcohols under mild reaction conditions[27] and acceptorless dehydrogenation of ethanol.[28] It is also worth mentioning that Gusev et al.[17] reported that the free energy barrier for isopropanol dehydrogenation is 13.85 kcal/mol for 1Os/2Os and 13.74 kcal/mol for IRu/2Ru at mPW1K-SCRF. For the hydrogenation of benzaldehyde to benzyl alcohol, Guan and coworkers,[14] computed a stepwise mechanism and the highest barrier on the potential energy surface is 7.4 kcal/ mol. Using the simplified model catalysts (where the isopropyl groups at the P centers were replaced by methyl groups), Schneider and coworkers,[13] computed the hydrogenation of formaldehyde via stepwise mechanism at the R1-B3PW91D3BJ/def2-QZVPP//B3LYP/def2-SVP level under the consideration of the thermal correction at 1208C. From formaldehyde to methanol, the highest barrier is 5.7 kcal/mol and the reaction is exergonic by 10.4 kcal/mol. All these computed barriers are close to our values at B3PW91 and B3LYP (6.60 and 7.75 kcal/ mol, respectively).
Conclusion The properties of defined pincer complexes M(H)2(CO) [NH(C2H4PiPr2)2] (1M) and M(H)(CO)[N(C2H4PiPr2)2] (2M) (M 5 Fe, Ru, Os) as active catalysts in hydrogenation of acetonitrile (MeCN), methyl benzoate (PhCO2CH3), and benzalde-
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hyde (PhCHO) have been computed at the B3PW91, B3PW91SCRF, B3LYP, and BP86 levels of density functional theory. These results have enabled a systematic comparison and discussion of the activity of the three different metals. It is to note that the computed equilibrium between 1Fe and 2Fe under H2 atmosphere has been fully confirmed by our recent experimental studies. However, our computed equilibrium for M 5 Fe, Ru, and Os is in contrast with the results from previous computations at the M06 level under the consideration of solvation effects. The computed H2 elimination barrier from 1M increases from 1Fe to 1Ru as well as to 1Os, however, the hydrogen addition barrier to 2M are in a very close range. For the hydrogenation of acetonitrile, both 1Fe/2Fe and 1Ru/2Ru are competent catalysts on the basis of the computed barriers and reaction energies, whereas 1Os/2Os are less effective. The most effective catalysts for the hydrogenation of methyl benzoate are 1Ru/2Ru, followed by 1Fe/2Fe, while 1Os/2Os are least effective. For the hydrogenation of benzyl aldehyde or dehydrogenation of benzyl alcohol, the best catalysts are 1Ru/2Ru, and this is in agreement with the previous experimental and computational studies. It is finally to note that the PCM is not sufficient in studying the solvation effect in the energetic properties of these catalysts as well as their catalytic properties in hydrogenation of acetonitrile and methyl benzoate. Keywords: pincer complexes iron ruthenium osmium hydrogenation catalysts reaction mechanisms density functional theory nitrile ester aldehyde
How to cite this article: H. Jiao, K. Junge, E. Alberico, M. Beller. J. Comput. Chem. 2016, 37, 168–176. DOI: 10.1002/jcc.23944
]
Additional Supporting Information may be found in the online version of this article.
[1] J. G. de Vries, C. J. Elsevier, Eds., In Handbook of Homogeneous Hydrogenation; Wiley-VCH: Weinheim, 2007. [2] M. Trincado, D. Banerjee, H. Gr€ utzmacher, Energy Environ. Sci. 2014, 7, 2464. [3] M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85. [4] E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H.-J. Drexler, W. Baumann, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 14162. [5] X. Yang, ACS Catal. 2014, 4, 1129. [6] (a) X. Yang, ACS Catal. 2013, 3, 2684; (b) X. Yang, ACS Catal. 2012, 2, 964; (c) X. Yang, ACS Catal. 2011, 1, 849. [7] C. Bornschein, S. Werkmeister, B. Wendt, H. Jiao, E. Alberico, H. Junge, K. Junge, M. Beller, Nat. Commun. 2014, 5, 4111. [8] S. Werkmeister, H. Junge, B. Wendt, E. Alberico, H. Jiao, W. Baumann, K. Junge, F. Gallou, M. Beller, Angew. Chem. Int. Ed. 2014, 53, 8722. [9] I. Koehne, T. J. Schmeier, E. A. Bielinski, J. C. Pan, P. O. Lagaditis, W. H. Bernskoetter, M. K. Takase, C. Wurtle, N. Hazari, S. Schneider, Inorg. Chem. 2014, 53, 2133. [10] S. Chakraborty, H. Dai, P. Bhattacharya, N. T. Fairweather, M. S. Gibson, J. A. Krause, H. Guan, J. Am. Chem. Soc. 2014, 136, 7869. [11] S. Chakraborty, W. W. Brennessel, W. D. Jones, J. Am. Chem. Soc. 2014, 136, 8564.
Journal of Computational Chemistry 2016, 37, 168–176
175
FULL PAPER
WWW.C-CHEM.ORG
[12] E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. W€ urtle, W. H. Bernskoetter, H. Hazari, S. Schneider, J. Am. Chem. Soc. 2014, 136, 10234. [13] S. Chakraborty, P. O. Lagaditis, M. F€ orster, E. A. Bielinski, N. Hazari, M. C. Holthausen, W. D. Jones, S. Schneider, ACS Catal. 2014, 4, 3994. [14] S. Qu, H. Dai, Y. Dang, C. Song, Z.-X. Wang, H. Guan, ACS Catal. 2014, 4, 4377. [15] E. A. Bielinski, M. Fo€ urster, Y. Zhang, W. H. Bernskoetter, N. Hazari, M. C. Holthausen, ACS Catal. 2015, 5, 2404. [16] M. Lei, Y. Pan, X. Ma, Eur. J. Inorg. Chem. 2015, 794. [17] M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk, D. G. Gusev, Organometallics 2011, 30, 3479. [18] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
176
Journal of Computational Chemistry 2016, 37, 168–176
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Inc., Wallingford CT, 2010. A. D. Becke, J. Chem. Phys. 1993, 98, 5648. (a) A. D. Becke, Phys. Rev. A 1988, 38, 3098; (b) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785. J. P. Perdew, Phys. Rev. B, 1986, 33, 8822. A. Schaefer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829. P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 299. J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999. A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899. V. Rautenstrauch, X. Hong-Cong, R. Churlaud, K. Abdur-Rashid, R. H. Morris, Chem. Eur. J. 2003, 9, 4954. M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 9593. M. Nielsen, H. Junge, A. Kammer, M. Beller, Angew. Chem. Int. Ed. 2012, 51, 5711.
Received: 17 March 2015 Revised: 28 April 2015 Accepted: 28 April 2015 Published online on 17 May 2015
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