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Aust. J. Chem. 2004, 57, 659–663
Understanding Metal-Free Catalytic Hydrogenation: A Systematic Theoretical Study of the Hydrogenation of Ethene Bun ChanA and Leo RadomA–C A
School of Chemistry, University of Sydney, Sydney NSW 2006, Australia. School of Chemistry, Australian National University, Canberra ACT 0200, Australia. C Author to whom correspondence should be addressed (e-mail:
[email protected]). B Research
Metal-free catalytic hydrogenation of ethene has been examined using high-level [G3(MP2)-RAD] ab initio molecular orbital theory. The dependence of the catalytic activity on the nature of the catalyst Z X H has been explored. We find that the catalytic activity is generally greater as Z X H becomes more acidic, both for first- and second-row atoms X. Molecules in which X is a second-row atom generally lead to more effective catalysis than the corresponding first-row analogues. The proton affinity at X of Z X H also contributes significantly to the catalysis in some cases (e.g. amines). Manuscript received: 9 February 2004. Final version: 6 April 2004. Catalytic hydrogenation is an important chemical reaction in a wide variety of areas, such as industrial processes, organic synthesis, and biological sciences.[1] Most of these reactions are catalyzed by transition metal complexes.[2] For instance, compounds that contain platinum-group metals have been used extensively in the hydrogenation of fats in the food industry.[3] In microorganisms, catalytic hydrogenation is carried out by enzymes known as hydrogenases. Most hydrogenases are metalloenzymes that contain nickel– and/or iron–sulfur clusters.[4] In contrast to transition metal catalyzed hydrogenation, catalytic hydrogenation without transition metals is much less prominent. Among such studies, it has been found that strong acids can be used as catalysts for the hydrogenation of unsaturated hydrocarbons with molecular hydrogen.[5] It has also been demonstrated that zeolites catalyze the hydrogenation of alkenes.[6] Furthermore, it has been observed that some carbonyl compounds undergo catalytic hydrogenation in the presence of a strong base.[5a,5b] On the biological side, the hydrogenase, H2 -forming methylenetetrahydromethanopterin (Hmd), identified a decade ago, was originally believed to be metal-free.[7] Interestingly, very recent work has indicated that it in fact contains iron.[8] More generally, enzymatic hydrogenation has received considerable attention from experimental biologists.[9] There has also been a recent study focussing on the mechanistic details of base-catalyzed hydrogenation.[10] Transition metal free catalytic hydrogenation has also attracted the attention of theoretical chemists. For example, the Hmd-catalyzed hydrogenation has been the focus of several theoretical studies.[11] There have also been several theoretical studies of hydrogenation reactions catalyzed by acidic compounds, such as simple Brønsted/Lewis acids, and by zeolites.[12]
As a continuation of previous studies,[11c,12b,12c] we have been interested in pursuing the fundamentals of transition metal free hydrogenation, partly motivated by environmental issues associated with transition metal catalysts (and hence the desire for alternatives),[13] and partly as a result of the sparsity of existing information on this subject. In the present paper, we focus on the hydrogenation of ethene catalyzed by representative small organic/inorganic acids or bases, Z X H, Equation (1): H2 C CH2 + H2 + Z X H → H3 C CH3 + Z X H
(1)
in which the catalysis involves a single X H moiety of the catalyst, and X is a first- or second-row atom. Standard ab initio molecular orbital calculations[14] were carried out with the Gaussian 03[15] package of programs. Geometry optimizations were carried out at the B3LYP/6–31G(d) level,[16] and single-point energy calculations were performed on these geometries using the high-level G3(MP2)-RAD composite method.[17] Structural parameters throughout this paper refer to B3-LYP/6–31G(d) values, while relative energies correspond to G3(MP2)-RAD values at 0 K. The intrinsic reaction coordinate (IRC) method was employed to confirm the two minima connected by each transition structure. The results that we report refer to gas-phase reactions. Preliminary calculations using the self-consistent isodensity polarized continuum model (SCIPCM) indicate that the effect of solvation on our calculated activation energies is likely to be quite small. The optimized transition structures for water- and formic acid catalyzed hydrogenations of ethene are shown in Fig. 1 as representative examples. In all cases reported in this paper,
© CSIRO 2004
10.1071/CH04031
0004-9425/04/070659
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B. Chan and L. Radom
a single atom X of the catalyst formally acts both as a proton donor (to ethene) and a proton acceptor (from molecular hydrogen).[18] Gas-phase acidities, calculated as the enthalpy change at 0 K in the deprotonation reactions, Equation (2): ZX H → ZX− + H+
(2)
and proton affinities, calculated as the enthalpy change at 0 K in the protonation reactions, Equation (3): ZXH + H+ → ZXH2+
(3)
were obtained in order to examine their effect on the catalytic activity. The results are presented in Fig. 2 as a plot of activation energy for the hydrogenation versus acidity of the catalyst. The proton affinities at X of each catalyst are included in parentheses. (a)
(b)
Fig. 1. Optimized transition structures for the (a) H2 O- and (b) HC(O)OH-catalyzed hydrogenation reactions of ethene.
A quick perusal of Fig. 2 shows that there is no trivial correlation between the activation energy and the acidity that could account for all the catalysts. Nevertheless, some remarks regarding observations on certain ‘sub-groups’ of catalysts are worth mentioning. For hydrogenation involving catalysts of the type Z O H, the activation energy decreases linearly with acidity. Thus, the catalytic activity of a more acidic catalyst within this sub-group is generally greater than that of a less acidic one. For instance, CF3 OH, being more acidic than water, is also a more effective catalyst. H2 SO4 is the only Z O H system that does not reasonably fit this correlation. On the other hand, HF lies close to the line. A similar but separate trend is also observed for hydrogenations that involve catalysts of the form Z S H or Cl H with second-row atoms X, i.e. the catalytic activity is again greater with a more acidic catalyst. For those catalysts that follow the activation energy/acidity correlation, it is likely that the acidity is a dominant factor in determining the catalytic activity. This is consistent with previous findings, that the catalytic activity of zeolites in related reactions has a linear correlation with their acidity.[19] Catalysis by molecules with a second-row atom X generally leads to a lower activation energy than catalysis with the corresponding first-row analogue. This is in line with their respective acidities but to an accentuated extent. For example, H2 S leads to a lower activation energy than H2 O. This is also in accord with previous findings, in which the catalytic activity of hydrogen halides in the hydrogenation of ethene is found to be in the order HBr > HCl > HF.[12b] The difference in catalytic activity between a catalyst with a second-row atom X and its first-row analogue is consistently larger than what might be expected from the difference in acidity alone. This suggests that factors other than acidity are important in determining the catalytic activity.
Fig. 2. Activation energy of catalytic hydrogenation of ethene versus acidity (energy of deprotonation at X) of the catalyst. The proton affinity (energy of protonation at X) of each catalyst is shown in parentheses. All energies are in units of kJ mol−1 .
Metal-Free Catalytic Hydrogenation of Ethene
661
Examination of the electronic reorganization required for catalyzed hydrogenation (Fig. 3) shows quickly what one of the additional factors is likely to be, namely the proton affinity at X of Z X H. Thus, not only does Z X H need to deprotonate at X but it also needs to be protonated at X. This could account for the enhanced catalytic activity (lower activation energies) for the amines compared with the alcohols in Fig. 2. The phosphines show similar behaviour when compared with the thiols. In some cases, the higher proton affinity might also be an important factor for molecules with a second-row versus first-row atom X (e.g. H2 S versus H2 O). However, in other cases, the proton affinity of the catalyst with a secondrow atom X is lower than that of its first-row analogue (e.g. PH3 versus NH3 ). Thus factors other than acidity and proton affinity appear to also be contributing to the catalytic activity for these groups of compounds.
To enable further elucidation of the effect of these catalysts on the hydrogenation reaction, structural parameters of each of the transition structures have been examined. Values of selected bond lengths are listed in Table 1, together with the calculated activation energies for the catalytic hydrogenation reactions. Data for the uncatalyzed hydrogenation of ethene, obtained at the same theoretical levels, are also included for comparison. The catalysts are grouped into four classes, based on observations from Fig. 2. The first two classes comprise catalysts in which the atom X is a first-row element. Among these, the ZOH molecules, which generally follow the activation-energy/acidity correlation, are placed into class 1, while the ZNH amine bases make up class 2. The last two classes consist of catalysts with a second-row atom X. The ZSH catalysts, which again follow the activation energy/acidity correlation, belong to class 3, while the ZPH phosphine bases constitute class 4. Within each class, the catalysts are listed in descending order of their hydrogenation activation energies. We note that FH and ClH fit into classes 1 and 3, respectively, and for simplicity are included under the ZOH and ZSH labels in the following discussion. A striking feature when comparing the transition structure for the uncatalyzed hydrogenation with the transition structures for the catalytic hydrogenation reactions is that the H · · · H bond length in the catalytic reactions is always considerably shorter. There are also interesting contrasts
Fig. 3. Electronic reorganization in hydrogenation catalyzed by Z X H molecules.
Table 1. Activation energies [kJ mol−1 ] and selected bond lengths [Å] of the optimized transition structures for the hydrogenation reactions Class
CatalystA
Ea
Bond length C=C
C· · · HX
CH· · · X
X· · · HH
H· · · H
HH· · · C
uncatalyzed
358.2
1.436
—
—
—
1.283
1.488 1.678
ZOH
HOH (CH3 )3 COH H3 COH FH CH3 C(O)OH HC(O)OH F3 COH FC(O)OH O2 NOH ClC(O)OH HOS(O)2 OH
293.5 273.1 273.0 272.5 261.3 257.4 250.5 239.6 236.2 235.7 205.6
1.409 1.410 1.409 1.410 1.413 1.414 1.412 1.418 1.418 1.419 1.421
1.444 1.486 1.430 1.307 1.229 1.220 1.234 1.187 1.187 1.179 1.165
1.211 1.183 1.210 1.261 1.483 1.507 1.457 1.598 1.595 1.630 1.685
1.366 1.339 1.360 1.404 1.646 1.672 1.633 1.733 1.738 1.771 1.832
0.924 0.938 0.918 0.859 0.813 0.809 0.816 0.794 0.794 0.790 0.783
1.574 1.547 1.589 1.668 1.714 1.723 1.711 1.793 1.796 1.801 1.849
ZNH
H2 NH (CH3 )HNH (CH3 )2 NH
279.5 256.9 238.4
1.406 1.404 1.403
1.784 1.818 1.842
1.104 1.093 1.086
1.268 1.255 1.249
1.061 1.074 1.077
1.496 1.514 1.519
ZSH
HSH (CH3 )3 CSH H3 CSH ClH F3 CSH HC(S)SH FC(S)SH ClC(S)SH
212.9 208.2 207.4 199.0 189.7 177.9 172.8 172.2
1.408 1.407 1.408 1.410 1.412 1.412 1.415 1.415
1.276 1.286 1.286 1.230 1.235 1.229 1.207 1.210
1.796 1.776 1.770 1.786 1.862 1.870 1.923 1.916
2.040 2.033 2.003 2.005 2.089 2.109 2.124 2.126
0.820 0.823 0.826 0.804 0.807 0.800 0.795 0.795
1.726 1.731 1.723 1.754 1.742 1.760 1.792 1.789
ZPH
H2 PH (CH3 )HPH (CH3 )2 PH
208.2 204.8 201.3
1.411 1.413 1.416
1.288 1.281 1.274
1.819 1.826 1.823
2.073 2.061 2.059
0.829 0.832 0.838
1.645 1.649 1.634
A The
reactive group of the catalysts in each reaction is in bold.
662
among the transition structures for catalytic hydrogenation, e.g. comparing ZOH- and ZNH-catalyzed reactions. These in part reflect the electronic reorganization shown in Fig. 3. For molecules such as ZOH, in which the acidity is dominant, the transition structures show an elongation of the bond that is being deprotonated (CH · · · X) and a shortened bond corresponding to this proton attaching to ethene (C · · · HX), compared with the ZNH amines. In contrast, in the amines, in which the proton affinity is more important, the transition structures show a shortened bond corresponding to the protonation (X · · · HH), an elongated H · · · H bond, and a shortened bond to ethene (HH · · · C), compared with the ZOH systems. The distinctions between the ZSH and ZPH systems are less marked. The hypothesis that the dominating factor in the catalytic activity is the acidity of the catalyst for the ZOH catalysts, whereas the proton affinity is more important for reactions involving an amine base, is further supported by comparisons within the ZOH and ZNH systems. Thus, within the ZOH systems, reactions with a more effective catalyst also have a transition structure with shorter C · · · HX, but longer C C and CH · · · X, bonds. In contrast, within the ZNH systems, the transition structures involving a more effective catalyst have longer H · · · H, but shorter X · · · HH, bonds. In conclusion, based on our high-level [G3(MP2)-RAD] theoretical study, we have identified several factors that are important in determining the effectiveness of a catalyst in the hydrogenation of ethene. In general, the catalytic activity increases as the catalyst becomes more acidic. In addition, a catalyst having a second-row reactive heavy atom is a more effective catalyst than the corresponding first-row analogue. Proton affinities are also important in influencing the activity of some of the catalysts. Further studies are under way to try to understand more fully the factors that influence metal-free catalytic activity and to design more effective catalysts.
B. Chan and L. Radom
[2]
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Accessory Materials Gaussian achive entries for the B3-LYP/6–31G(d) optimized geometries and tabulated G3(MP2)-RAD total energies are available from the author or, until July 2009, from the Australian Journal of Chemistry.
[10] [11]
Acknowledgments We gratefully acknowledge generous allocations of computing time on the Compaq Alphaserver of the National Facility of the Australian Partnership for Advanced Computing and the Australian National University Supercomputing Facility, the provision (to B.C.) of a New Zealand Science & Technology Post-Doctoral Fellowship by the Foundation for Research, Science & Technology, and the award (to L.R.) of an Australian Research Council Discovery Grant. References [1] (a) S. P. J. Albracht, Biochim. Biophys. Acta 1994, 1188, 167. doi:10.1016/0005-2728(94)90036-1 (b) S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis 2001 (Wiley-Interscience: New York, NY).
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[18] The X atom in these catalysts is virtually always more electronegative than hydrogen. The Pauling electronegativity values for H, N, O, F, P, S, and Cl are 2.20, 3.04, 3.44, 3.98, 2.19, 2.58, and 3.16, respectively. (a) L. Pauling, The Nature of the Chemical Bond, 3rd edn 1960 (Cornell University Press: Ithaca, NY). (b) A. L. Allred, J. Inorg. Nucl. Chem. 1961, 17, 215. doi:10.1016/0022-1902(61)80142-5 [19] See, for example: (a) J. Kim, S. K. Ihm, Fuel 1992, 72, 815. (b) C. Costa, J. M. Lopes, F. Lemos, F. Ramôa Ribeiro, J. Mol. Catal. A Chem. 1999, 144, 233. doi:10.1016/S13811169(98)00368-9 (c) C. S. Triantafillidis, N. P. Evmiridis, Ind. Eng. Chem. Res. 2000, 39, 3233. doi:10.1021/IE000002S (d) C. I. Costa, P. Dzik, J. M. Lopes, F. Lemos, F. Ramôa Ribeiro, J. Mol. Catal. A Chem. 2000, 154, 193. doi:10.1016/S13811169(99)00374-X (e) X. Wang, M. A. N. D. A. Lemos, F. Lemos, C. Costa, F. Ramôa Ribeiro, Stud. Surf. Sci. Catal. 2001, 135, 259.
