Olefin Epoxidation Catalyzed by Î·5-Cyclopentadienyl Molybdenum

Organometallics 2010, 29, 303–311 DOI: 10.1021/om9002522

303

Olefin Epoxidation Catalyzed by η5-Cyclopentadienyl Molybdenum Compounds: A Computational Study Paulo J. Costa,†,‡ Maria Jose Calhorda,*,† and Fritz E. K€ uhn§ †

Departamento de Quı´mica e Bioquı´mica, CQB, Faculdade de Ci^ encias, Universidade de Lisboa, 1749-016 Lisboa, Portugal, ‡Departamento de Quı´mica, CICECO, Universidade de Aveiro, 3810-193, Aveiro, Portugal, and §Molecular Catalysis, Catalysis Research Center, Technische Universit€ at M€ unchen, Lichtenbergstrasse 4, D-85747 Garching bei M€ unchen, Germany Received April 3, 2009

η5-Cyclopentadienyl(tricarbonyl)methylmolybdenum(II) (1) is oxidized by tert-butylhydroperoxide (TBHP) to yield η5-cyclopentadienyl(dioxo)methylmolybdenum(VI) (2) and η5-cyclopentadienyl(oxo)-(peroxo)methylmolybdenum(VI) (3). These two complexes are active catalysts for olefin epoxidation in the presence of TBHP, but inactive in its absence. DFT calculations of the possible pathways, with MP2 single-point energies, indicate that TBHP can react with 2 to form the active intermediate CpMo(O)(OH)(OOCH3)CH3 (B) or the peroxo complex 3. Formation of B and epoxidation from B have relatively low activation barriers (gas-phase MP2 ΔE 21 kcal mol-1), while formation of complex 3 from 2 exhibits a higher barrier (38 kcal mol-1). The peroxo complex 3 can be further activated by excess TBHP to yield a second active intermediate, CpMo(O2)(OH)(OOCH3)CH3 (C) (24 kcal mol-1), that undergoes reaction with olefin, forming epoxide and the intermediate B. Both intermediates B and C display end-on-bound alkyl peroxo ligands. Barriers are comparable for several pathways, suggesting that more than one may take place. The role of solvent (PCM, dichloromethane) is negligible, since the deviations between gas-phase and solvent free energies are ∼1-3 kcal mol-1.

1. Introduction The important role of propylene oxide and its higher homologues in the industrial synthesis of propylene glycol, polyurethanes, and resins, with several million tons produced annually,1 contributes to the interest in developing alternative routes for its production that are both more efficient and environmentally friendly.2 The report of the Halcon process, employing molybdenum as catalyst,3 led to

many studies aimed at improving oxidation catalysts and understanding their behavior.4-6 Typically, Mo(VI) is the formal oxidation state in the active species,7 but it has been shown that molybdenum complexes in lower oxidation states can be efficient promoters of catalytic olefin epoxidation in the presence of tert-butylhydroperoxide (TBHP) as oxidant. Carbonyl complexes, such as Cp0 Mo(CO)3X (X = halogen, CnH2nþ1), have been successfully used for such purposes.8 They afford Cp0 MoO2X (X = halogen, CnH2nþ1), which have displayed catalytic activity,8-10 although a lot of

*Corresponding author. E-mail: [email protected]. (1) Weissermel, K.; Arpe, H. J. Industrial Organic Cemistry; Wiley: New York, 2003. (2) (a) B€ ackvall, J. E., Ed. Modern Oxidation Methods; Wiley-VCH: Weinheim, 2004. (b) Yudin, A. K., Ed. Aziridines and Epoxides in Organic Synthesis; Wiley-VCH: Weinheim, 2006. (3) (a) Kollar, J. (Halcon) US 3.350.422, US 3.351.635, 1967. (b) Sheng, M. N.; Zajaczek, G. J. (ARCO) GB 1.136.923, 1968. (c) Coltan, R.; Tomkins, I. B. Aust. J. Chem. 1965, 18, 447–452. (4) (a) Sheng, M. N.; Zajacek, J. G. J. Org. Chem. 1970, 35, 1839– 1843. (b) Sheldon, R. A.; van Doorn, J. A. J. Catal. 1973, 31, 427–437. (c) Sheldon, R. A.; van Doorn, J. A.; Schram, C. W. A.; de Jong, J. J. Catal. 1973, 31, 438–443. (d) Sheldon, R. A. Recl. Trav. Chim. 1973, 92, 253. (e) Chong, A. O.; Sharpless, K. B. J. Org. Chem. 1977, 42, 1587–1590. (f) Chaumette, P.; Mimoun, H.; Saussine, L. J. Organomet. Chem. 1983, 250, 291–310. (5) (a) Thiel, W. R. J. Mol Catal. A: Chem. 1997, 117, 449–454. (b) Wahl, G.; Kleinhenz, D.; Schorm, A.; Sundermeyer, J.; Stowasser, R.; Rummey, C.; Bringmann, G. Chem.;Eur. J. 1999, 5, 3237–3251. (c) K€uhn, F. E.; Santos, A. M.; Roesky, P. W.; Herdtweck, E.; Scherer, W.; Gisdakis, P.; Yudanov, I. V.; di Valentin, C.; R€osch, N. Chem.;Eur. J. 1999, 5, 3603– 3615. (d) Deubel, D. V.; Frenking, G.; Gisdakis, P.; Herrmann, W. A.; R€osch, N.; Sundermeyer, J. Acc. Chem. Res. 2004, 37, 645–652. (6) (a) Al-Ajlouni, A.; Espenson, J. H. J. Am. Chem. Soc. 1995, 117, 9243–9250. (b) Al-Ajlouni, A.; Espenson, J. H. J. Org. Chem. 1996, 61, 3969–3976. (c) Espenson, J. H. J. Chem. Soc., Chem. Commun. 1999, 479–488.

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controversy exists concerning the role of peroxo complexes, in particular in the chemistry of the chloride complex Cp0 MoO2Cl.11 Other Mo(II) derivatives, namely, those derived from [Mo(η3-C3H5)(CO)2X(L-L)] (X = Cl, Br; L-L = nitrogen ligand), can also behave as precursors for olefin epoxidation in the presence of TBHP.12 They may be even more active and easier to handle than Mo(VI) catalysts, such as MoO2X2L, a system where peroxide complexes have never been detected.13 Indeed, the active intermediate has been shown, by experimental and computational results, to contain an alkyl hydroperoxide bound to the metal in several different systems.13a,14,15 More recently, kinetic studies were carried out on the reactions of η5-cyclopentadienyl(tricarbonyl)methylmolybdenum(II) (1) with TBHP.16 This complex reacts fast with excess TBHP to afford the bis(oxo)Mo(VI) derivative 2, which can be isolated. Reaction of 2 with excess TBHP leads to the oxo(peroxo)complex 3. This new species was spectroscopically and structurally characterized. In the presence of excess TBHP and substrate (olefin), both complexes catalyzed their epoxidation (Chart 1), but they are not active in its absence. A computational study based on DFT17 calculations (Gaussian0318) was performed in order to determine the mechanisms for the epoxidation in this CpMo(VI) system and understand the competition between the alternative pathways, namely, how complex 2 transforms into complex 3 and how both 2 and 3 can catalyze olefin epoxidation.

Results and Discussion Formation of Active Species. The DFT17 calculations were performed using Gaussian0318 with the B3LYP19 functional (11) (a) Trost, M. K.; Bergman, R. G. Organometallics 1991, 10, 1172–1178. (b) Chakraborty, D.; Bhattacharjee, H.; Kr€atzner, R.; Siefken, R.; Roesky, H. W.; Us on, I.; Schmidt, H. G. Organometallics 1999, 18, 106–108. (12) Alonso, J. C.; Neves, P.; Pires da Silva, M. J.; Quintal, S.; Vaz, P. D.; Silva, C.; Valente, A. A.; Ferreira, P.; Calhorda, M. J.; Felix, V.; Drew, M. G. B. Organometallics 2007, 26, 5548–5556. Herdtweck, E.; (13) (a) K€ uhn, F. E.; Groarke, M.; Bencze, E.; Prazeres, A.; Santos, A. M.; Calhorda, M. J.; Rom~ao, C. C.; Gonc-alves, I. S.; Lopes, A. D.; Pillinger, M. Chem.;Eur. J. 2002, 8, 2370–2383. (b) Al-Ajlouni, A.; Valente, A. A.; Nunes, C. D.; Pillinger, M.; Santos, A. M.; Zhao, J.; Rom~ao, C. C.; Gonc-alves, I. S.; K€uhn, F. E. Eur. J. Inorg. Chem. 2005, 1716–1723. (14) (a) Hroch, A.; Thiel, W. R.; Gemmecker, G. Eur. J. Inorg. Chem. 2000, 1107–1114. (b) Gisdakis, P.; Yudanov, I. V.; R€osch, N. Inorg. Chem. 2001, 40, 3755–3765. (c) Yudanov, I. V. J. Struct. Chem. 2007, 48, S111– S124. (15) Veiros, L. F.; Prazeres, A.; Costa, P. J.; Rom~ao, C. C.; K€ uhn, F. E.; Calhorda, M. J. Dalton Trans. 2006, 1383–1389. (16) Al-Ajlouni, A.; Veljanovski, D.; Capape, A.; Zhao, J.; uhn, F. E. Organometallics 2009, Herdtweck, E.; Calhorda, M. J.; K€ 28, 639–645. (17) Parr, R. G.; Yang, W. In Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03, Revision B.04; Gaussian, Inc.: Wallingford, CT, 2004. (19) (a) Lee, C.; Yang, W.; Parr, W. R. G. Phys. Rev. B 1988, 37, 785– 789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.

Costa et al. Chart 1. Intermediates and Reactions Occurring from CpMo(CO)3Me in the Presence of Excess TBHP

(more in Computational Details). DFT electronic energies were corrected for the ZPE (E0DFT), and free energies were obtained from frequency calculations (GDFT). DFT calculations taking into account the solvent (dichloromethane) were also performed (Gs) on the previously optimized structures. Single-point energies were calculated at the MP2 level of theory20 using the DFT-optimized geometries (EMP2). The mechanism of olefin epoxidation catalyzed by Mo(VI) complexes [MoO2X2(N-N)],13 with X = halide or CH3 and N-N = a diimine ligand, in the presence of TBHP, was studied in previous works. The first step involved the activation of the OH bond of TBHP (modeled by HOOCH3), with proton transfer from the O-H of the peroxide to the oxide group (ModO), and the coordination of a peroxide (OOR group) to the metal center to form a heptacoordinate complex.13a,15 Since complex CpMo(O)2CH3 (2) can be considered an analogue, with a Cp replacing N-N and X (X = CH3), a similar mechanism was checked as a first approach. Other authors have also proposed that the active species in a variety of molybdenum(VI) and rhenium(VII) catalysts contains a bound alkyl peroxide group. The molybdenum catalysts are in most cases oxo(peroxo) complexes and therefore only directly comparable with complex 3 (see Chart 1).14 In the system we are addressing, there are two catalysts. Kinetic studies suggest that complex CpMo(O)(O2)CH3 (3) is not formed directly from the Mo(II) precursor CpMo(CO)3Me, and we could find no direct path using DFT calculations. On the other hand, 3 can be formed easily from CpMoO2(CH3) (2). This may happen, according to the calculations, in a concerted reaction, in which the O-O bond of the alkyl hydroperoxide is activated. The O(H) atom of excess HOOCH3 approaches one of the oxides, starting to form the new O-O peroxide bond and the alcohol. This reaction has a change in free energy very close to 0, as seen in Figure 1. The energies given in figures refer to the MP2 electronic energy calculated for the optimized DFT geometries; DFT electronic energies, free energies, and solvent-corrected free energies are collected in Table 1. In the transition state, the O-O bond of the alkyl hydroperoxide starts to break. One oxygen approaches the metal (2.169 A˚) and one oxide (1.919 A˚), while the hydrogen is transferred to the other oxygen of TBHP (distances 1.042 and 1.374 A˚, respectively), in order to form the alcohol, which is then released. Complex 3 displays a short ModO bond (1.682 A˚, shorter than the same bonds in 2). All the (20) Moeller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618–622.

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attempts to form 3 directly from other intermediates (B in Figure 2) were unsuccessful, as the energy barriers were extremely high. It appears, thus, that the peroxide complex 3 results only from the slow reaction of the dioxo complex 2 with TBHP, as proposed on the basis of kinetic data.16 Further reaction of complex 3 with excess alkyl hydroperoxide in order to form a bis(peroxo) complex was also examined, but the activation barrier is much higher (51 kcal mol-1) and the product CpMo(O2)2CH3 (O, see Chart 3 below) also has a higher energy. The formation of the catalytically active alkyl peroxide complex from the complexes 2 and 3 also starts with reaction with TBHP, but with the activation of the O-H bond of

Figure 1. Pathway for the conversion of CpMo(O)2CH3 (2) into CpMo(O)(O2)CH3 (3) in the presence of HOOCH3 (MP2, ΔE in kcal mol-1).

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HOOCH3. Starting from 2, the O-H bond approaches one of the ModO bonds, and in the first transition state (TS2A), shown in Figure 2, the hydrogen atom is close enough to be transferred to the oxide. The new O-H distance is only 1.172 A˚, while the initial one has increased to 1.250 A˚. The O(H) atom is already only 2.427 A˚ from the molybdenum. This transition state is close to the product A, where the transferred hydrogen is involved in a hydrogen bond with the coordinated R oxygen of the OOR group (distances 0.973 and 1.916 A˚ to each oxygen). The barrier of 21 kcal mol-1 is close to what is expected for these slow reactions. This intermediate can rearrange, with a very small barrier, in such a way that the hydrogen bond will be established with the β oxygen of OOR in B. The transition state TSAB shows the migration of the hydrogen from the R to the β oxygen. The free energy barrier for formation of B (33 kcal mol-1) is higher than the electronic energy (21 kcal mol-1) since this is an associative process (see Table 1). Still, this process has a slightly lower activation barrier than the analogous step with the [MoO2(CH3)2(Me2-DAB)] catalyst (37 kcal mol-1, with DAB = 1,4-diazabutadiene) and is also endergonic, in agreement with experimental data.14 Complex 3 also reacts with HOOCH3 in a similar way, as there is one oxide group left. Again the O-H bond of the alkyl hydroperoxide adds to the ModO bond (Figure 3). In the transition state (TS3C), the hydrogen is also being transferred to the oxide and the oxygen is already involved

Table 1. MP2 (ΔEMP2) and DFT (ΔEDFT) Electronic Energies, DFT Free Energies (ΔGDFT), and DFT Solvent-Corrected Free Energies (ΔGs) in kcal mol-1 Calculated for the Optimized DFT Geometries and Relative to the Isolated Reagents figure

reaction CpMo(O)2CH3 (2) þ HOOCH3 TS23 CpMo(O)(O2)CH3 (3) CpMo(O)2CH3 (2) þ HOOCH3 TS2A A TSAB B CpMo(O)(O2)CH3 (3) þ HOOCH3 TS3C C CpMo(O)(OH)(OOCH3)CH3 (B) þ C2H4 TSBD D TSD2 2 CpMo(O)(OH)(OOCH3)CH3 (E) þ C2H4 F TSFG G H TSHI I 2 CpMo(OH)(O2)(OOCH3)CH3 (C) þ C2H4 J TSJK K TSKL L 3 CpMo(OH)(O2)(OOCH3)CH3 (C) þ C2H4 M TSMN N B

1 2

3 4

5

6

7

a

In parentheses the energies relative to A. b Could not be obtained.

ΔEMP2

ΔE0DFT

ΔGDFT

0 38 1 0 21 7 13 3 0 24 19 0 16 -2 21 -54 0 (2)a -5 16 -53 -49 -40 -70 -60 0 -4 8 -13 29 -78 -70 0 -5 19 -73 -64

0 44 1 0 21 8 15 4 0 25 18 0 33 10 46 -43 0 (6)a -1 19 -44 -41 -32 -59 -53 0 -2 31 11 50 -61 -57 0 -1 25 -56 -50

0 55 2 0 33 21 29 17 0 37 30 0 46 22 59 -56 0 (6)a 8 32 -34 -41 -32 -62 -66 0 7 44 24 62 -54 -69 0 8 37 -48 -50

ΔGs 0 56 1 0 32 21 29 17 0 36 29 0 43 20 56 -60 0 (12)a 8 31 -33 -44 -36 -64 -76 0 17 50 30 69 b

-72 0 17 44 -38 -51

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in a weak bond with Mo. The difference with the previous situation is that the hydrogen bond in C is established with the β oxygen of OOR immediately, probably owing to the presence of the peroxide group. The barrier for the formation of the intermediate is slightly higher (24 vs 21 kcal mol-1) than the barrier for the formation of intermediates from complex 2. In this section we analyzed the reactions of HOOCH3 with complexes 2 and 3. Activation of the O-O bond allows the formation of CpMo(O)(O2)CH3 (3) from CpMo(O)2CH3 (2); on the other hand a bis(peroxide) complex could not be obtained from 3 by a similar mechanism. At the same time, activation of the O-H bond of HOOCH3 yields active alkyl peroxide complexes derived from both 2 and 3: the two intermediates CpMo(O)(OH)(OOCH3)CH3 (A and B),

Figure 2. Pathway for the reaction of CpMo(O)2CH3 (2) with HOOCH3 to form the intermediates CpMo(O)(OH)(OOCH3)CH3 (A and B) (MP2, ΔE in kcal mol-1).

Figure 3. Pathway for the reaction of CpMo(O)(O2)CH3 (3) with HOOCH3 to form the intermediate CpMo(O2)(OH)(OOCH3)CH3 (C) (MP2, ΔE in kcal mol-1).

Costa et al.

differing by the hydrogen bond established between the OH group and the OOCH3 ligand, through the R oxygen (A) or the β oxygen (B), arising from 2, and the intermediate CpMo(O2)(OH)(OOCH3)CH3 (C), where the hydrogen bond is formed with the β oxygen. The barrier for formation of 3 is higher than that for the formation of both A and B from complex 2, following the trend determined in the kinetic studies.16 Reaction of the Active Species with the Olefin. Intermediates CpMo(O)(OH)(OOCH3)CH3 (A and B) derived from 2 can transfer oxygen from the bound alkyl peroxide to the olefin to form the epoxide. In these complexes, steric constraints are relevant, since the metal has four ligands besides the Cp ring, and there are not many geometric alternatives, contrasting significantly with the Mo(O)(O2)2 model addressed by Gisdakis et al.,14a where dozens of likely transition states for ethylene epoxidation could in principle be found. The two main pathways that we found involved either B or A. Starting from B, a mechanism very close to the one proposed for the [MoO2(CH3)2(Me2-DAB)] catalyst14 was obtained. As shown in Figure 4, the CdC bond of the olefin approaches the Mo-O(OR) bond and a seven-membered metallacycle is formed in the next intermediate D. The transition state TSBD is stabilized by a hydrogen bond between Mo-O-H and the R oxygen of the peroxide, with distances of 0.980 (O-H) and 1.762 A˚ (H 3 3 3 O). The C-Mo and the first C-O bond of the epoxide are forming (2.620 and 1.968 A˚, respectively), while the initial hydrogen bond is kept. In the intermediate D, the Mo-O bond has been replaced by the new Mo-C bond, which has almost the same length as the Mo-C(Me) bond (2.257 and 2.228 A˚, respectively), and the C-O bond lies within the usual range for similar bonds (1.427 A˚). The Mo-C bonds to the methyl and the Cp barely change during the formation of D. The second step involves the loss of ethylene oxide and methanol, as well as the regeneration of the catalyst 2. The MP2 activation energy is of the same order of magnitude as for the formation of B. The DFT values are higher, but MP2 is a method that allows for a better description of the weak interactions taking place in the transition states, so this value should be more reliable, as will be further discussed below. A different kind of pathway (Figure 5) started from a slightly rearranged intermediate A and was similar to the one proposed for the olefin epoxidation catalyzed by CpMoO2Cl, similar to CpMoO2Me, with H2O2 as oxidant, instead

Figure 4. Pathway for the reaction of CpMo(O)(OH)(OOCH3)CH3 (B) with the olefin to form epoxide and alcohol and regenerate the catalyst (MP2, ΔE in kcal mol-1).

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Figure 5. Alternative pathway for the reaction of CpMo(O)(OH)(OOCH3)CH3 (E) with the olefin to form epoxide and alcohol and regenerate the catalyst 2 (MP2, ΔE in kcal mol-1).

of HOOCH3.21 The active species (E) is very close to intermediate A, with the hydrogen bond between the R oxygen of OOCH3 and the OH group, and results from a small rearrangement. The OOR ligand still binds the metal in a η1-mode, in contrast to the η2 coordination mode of OOH in the Cl system, and it can be inferred that the conversion barrier is negligible, from comparison with the conversion barrier of 6 kcal mol-1 between A and B (Figure 2). The olefin approaches the catalyst E in such a way that a O-H 3 3 3 π weak hydrogen bond is formed involving the CdC bond (F), with C 3 3 3 H distances of ca. 2.8 A˚. In the transition state, these distances drop asymmetrically to 2.164 and 1.999 A˚, while the O-O bond weakens considerably (from 1.466 in F to 1.812 A˚ in TSFG) and the β oxygen coordinates the metal. The two Mo-O distances to the oxygen atoms from OOR are 2.173 and 2.186 A˚; the first will leave the coordination sphere with the olefin and the second will give rise to methanol. At the same time, the hydrogen initially in the OH ligand bridges over to this latter oxygen atom. The epoxide is only weakly bound by a O 3 3 3 H-O hydrogen bond in intermediate G and leaves. In the remaining complex, H, the same hydrogen forms another hydrogen bond with the oxygen of the alkoxide, in preparation for the release of methanol. The hydrogen transfer takes place in transition state TSHI, where it almost symmetrically bridges the two oxygens. In the intermediate I, the situation is reversed, with the OH of methanol still forming a hydrogen bond with the oxide. Loss of methanol regenerates the catalyst 2. The highest activation barrier in this alternative pathway is 21 kcal mol-1, almost the same as calculated before (Figure 4) and as obtained for the formation of the active species (A or B). After analyzing the mechanisms of the catalytic reactions of the CpMo(O)(OH)(OOCH3)CH3 intermediates derived from the dioxo complex CpMoO2(CH3) (2), namely, B and E, we examine now what may occur starting from the peroxo complex CpMo(O)(O2)CH3 (3). This complex can catalyze (21) Comas-Vives, A.; Lled os, A.; Poli, R. Chem. Eur. J., in press (DOI: 10.1002/chem.200902873).

olefin epoxidation in the presence of excess TBHP, yielding intermediate C in the first step of the reaction (Figure 3). Several pathways are possible for CpMo(O2)(OH)(OOCH3)CH3 (C), using either the peroxide or the alkyl peroxide to transfer the oxygen to the olefin. Indeed, previous studies carried out in peroxomolybdenum(VI) model catalysts emphasized the role of the peroxide ligand as oxygen transfer agent, in a Sharpless-type mechanism.5,14,22,23 It is interesting that even in these electronically and coordinatively unsaturated complexes coordination of olefin to the metal did not lead to low-energy pathways. The first pathway analyzed starts with the approach of the olefin to the Mo-O(OR) bond (Figure 6) and is analogous to that described in Figure 4. The olefin approaches the intermediate C and binds very weakly (J). In the transition state TSJK, the addition of the CdC bond to the Mo-O(OR) bond is in progress, with C 3 3 3 Mo and C 3 3 3 O distances of 2.526 and 2.053 A˚, respectively. These become normal bonds in the intermediate K, where a O 3 3 3 HO hydrogen bond assists the formation of the seven-membered cycle, as described earlier (Figure 4). The barrier for this step is small, but it increases significantly in the second step. Rearrangement of this cycle, in order to obtain the products, is already evident in TSKL. The epoxide is already outside the influence of the catalyst in L, while the alcohol is hydrogen bonded to the oxide of the catalyst that will be regenerated back to the peroxo complex 3. As was described when dealing with the intermediates arising from 2, we tried to calculate the alternative pathway as in Figure 5. This path was prevented by the impossibility of obtaining an intermediate of the A type (see Figure 2), where a hydrogen bond is formed between the OH group and the R oxygen of the OOR ligand. The reason is probably associated with the steric constraints imposed by the presence of the peroxo ligand, bulkier than the oxide present in A. On the (22) Yudanov, I. V.; di Valentin, C.; Gisdakis, P.; R€ osch, N. J. Mol. Catal. 2000, 158A, 189–197. (23) Sharpless, K. B.; Townsend, J. M. J. Am. Chem. Soc. 1972, 94, 295–296.

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Figure 6. Pathway for the reaction of CpMo(OH)(O2)(OOCH3)CH3 (C) with the olefin to form epoxide and alcohol and regenerate the catalyst 3 (MP2, ΔE in kcal mol-1).

Figure 7. Pathway for the reaction of CpMo(OH)(O2)(OOCH3)CH3 (C) with the olefin to form epoxide and regenerate the intermediate B (MP2, ΔE in kcal mol-1).

other hand, the coordinated peroxo allows a Sharpless-type mechanism23 directly from C, as depicted in Figure 7. In this mechanism, one carbon atom of the olefin approaches one oxygen atom of the coordinated peroxo group (3.219 A˚) in M. This distance has decreased to 1.963 A˚, in the transition state TSMN, and the second carbon atom has also moved toward the oxygen (2.202 A˚). The epoxide is released, leaving behind one oxo group, therefore yielding the active intermediate B, instead of regenerating the catalyst. B can continue the catalytic reaction as shown above in Figure 4. The barrier for this pathway is 24 kcal mol-1, and thus competitive with others previously described. Another interesting observation is that direct coordination of the olefin, even the not bulky ethylene, was never achieved in calculations, whatever the intermediate, in agreement with what was reported for other model systems.14,22 Complex 3 reacts with excess HOOCH3 to form the intermediate CpMo(O2)(OH)(OOCH3)CH3 (C), which can

transfer oxygen to the olefin during the catalytic reaction from OOCH3, regenerating the catalyst, or from the O2 ligand, to afford intermediate B, which can proceed independently. It does not form the bis(peroxide) complex with comparable barriers. Energetics. The energies discussed above for several available pathways were MP2 electronic energies (ΔEMP2), calculated for the DFT-optimized geometries and relative to the energy of the reagents, CpMo(O)2CH3 (2) and ethylene, separately (Figure 1). A collection of results, namely, the DFT energies including ZPE correction (ΔE0DFT), free energies (ΔGDFT), and free energies calculated in the presence of solvent (ΔGs), and the MP2 electronic energies (ΔEMP2), are given in Table 1. The formation of the active intermediates (1-3) and the CpMo(OH)(O)(OOCH3)CH3 (A, B) and CpMo(OH)(O2)(OOCH3)CH3 (C) complexes and the conversion of the oxide CpMo(O)2CH3 (2) into the peroxo complex CpMo(O)(O2)CH3 (3) is associated with essentially the same results (intermediates and transition states) using both MP2 and DFT energies. As these processes are associative in nature (two species react to give one), the free energies are in general higher, leading to higher barriers. These values must be considered with care owing to the calculation procedure. The patterns change when the reactions with the olefin are considered. The energies involved in the two steps of the reaction between intermediate B and the olefin (4 in Table 1) strongly depend on the calculation method. The activation barriers drop significantly when calculated with MP2. On the other hand, the formation of such type of intermediate has been well established by several groups.13a,14,15 From the data in Table 1, it results that MP2 energies are usually associated with more stable reaction products and intermediates and smaller barriers. The transition states leading to the formation of the seven-membered cycle or epoxide and methanol (TSBD and TSD2 in Figure 4, TSKL in Figure 6) are among the group of species undergoing the highest stabilization.

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309

Table 2. MP2 Single-Point Electronic Energies with Different Basis Sets (ΔEMP2 (B1) and ΔEMP2 (B2)) Compared with MP2 ZPE-Corrected Electronic Energies (ΔE0MP2) and Free Energies (ΔGMP2)a figure

reaction CpMo(O)(OH)(OOCH3)CH3 (B) þ C2H4 TSBD D TSD2 2

4

a

ΔEMP2 (B1)

ΔEMP2 (B2)

ΔE0MP2

ΔGMP2

0 19 -4 21 -52

0 18 -1 19 -47

0 18 1

0 31 13

-51

-65

B1 = LANL2tzþf, 6-311G **; B2 = LANL2tzþf, aug-cc-pVTZ; see Computational Details.

In order to check whether this effect arises from the quality of the basis set in the single-point MP2 calculations, we also performed single-point calculations on the intermediates and transition states represented in Figure 4, varying the quality of the basis set (B1 = LANL2tzþf, 6-311G**; B2 = LANL2tzþf, aug-cc-pVTZ; see Computational Details), and the results are collected in Table 2. The values of ΔEMP2 (B1) are similar to the ΔEMP2 ones, with a maximum difference of 3 kcal mol-1 for TSBD. This result is indicative that changing the basis set of the metal from LANL2dz to LANL2TZ and increasing the basis of the remaining elements from 6-31G** to 6-311G** has a minimum impact on the energetics of the reaction. A similar situation is also found if we improve further the basis set on the lighter elements to aug-cc-pVTZ, keeping the LANL2tzþf basis on the metal (B2). With this rather large basis set, the results do not vary much from ΔEMP2 or ΔEMP2 (B1). The larger variation is found for intermediate 2, which has a higher energy (-47 kcal mol-1) than those calculated with the other basis sets: ΔEMP2 = -54 kcal mol-1, ΔEMP2 (B1) = -52 kcal mol-1. More importantly, the key transition states TSBD and TSD2 are again not affected by the basis set size. In order to check a possible relaxation effect on the geometries of the intermediates and transition states, we optimized intermediates B, D, 2, and the transition state TSBD (which leads to the formation of the seven-membered cycle) at the MP2 level of theory with the same basis set as in the DFT calculations, obtaining the ZPE-corrected electronic energies (ΔE0MP2) and the free energies (ΔGMP2) given in Table 2. It is clear that the values of ΔE0MP2 are very similar to ΔEMP2 obtained for three different basis sets and the DFT geometry (differences less than 3 kcal mol-1), showing that relaxing the structures by optimization at the MP2 level does not influence much the electronic energies of the reaction. The ΔGMP2 barriers are higher than ΔE0MP2, as ΔGDFT were also higher than ΔE0DFT, owing to entropic effects. However, they are still significantly lower than the ones calculated at the DFT level (ΔGDFT), as seen schematically in Figure 8. These results undoubtedly show that the MP2 level of theory systematically provides lower barriers, independently of the basis set or the geometry, thus validating the singlepoint MP2 calculation with DFT geometry approach. The effect of the solvent was checked performing singlepoint calculations with a solvent model to account for dichloromethane. The effect is practically negligible, with most energies deviating by about 2-3 kcal mol-1 from the gas-phase values. In conclusion, as sketched in Chart 2, complex 2 can follow two competitive pathways when reacting with TBHP, converting either into the peroxo complex 3 or into the active species B (this includes the formation of A and the related intermediate E). The barrier to form the peroxide complex CpMo(O)(O2)CH3 (3) is higher than the barrier to form the catalytic active species B, but the competition between both

Figure 8. Comparison between DFT and MP2 pathway for the reaction of intermediate B with the olefin (ΔG in kcal mol-1). Chart 2. The Two Competitive Pathways for the Reaction of Complex CpMo(O)2CH3 (2) with TBHP

is likely to occur in the presence of excess TBHP. The two transition states and intermediates are shown in Chart 2, and the result is determined by the way the alkylperoxide approaches complex 2. While the O-H bond adds to the ModO bond to form A, in the first path above, in the second only one oxygen adds to the ModO bond to form the peroxide, releasing the alcohol. All the attempts to form 3 directly from B were unsuccessful, as the energy barriers were extremely high. It appears, thus, that the peroxo complex 3 results only from the reaction of the dioxo complex 2 with TBHP, as proposed on the basis of kinetic data.16 Several reactivity pathways are also available for complex 3 (Chart 3). Reaction with TBHP can take place to form an active intermediate (C), which then reacts with olefin; the alternative reaction with olefin yields a bis(peroxo) complex, CpMo(O2)2CH3 (O). The activation barrier to form complex O is much higher than the barrier to form intermediate C, and thus it was not discussed in detail. On the other hand, complex C reacts with excess TBHP to form epoxide and ethanol, regenerating the active species 3 (Figure 6); an alternative consists of direct reaction with olefin (Sharpless mechanism, Figure 7), forming the epoxide, but regenerating intermediate B, which is the active species based on the oxide complex 2. The barrier for

310


Chart 3. The Two Competitive Pathways for the Reaction of Complex CpMoO(O2)CH3 (3) with TBHP or Ethylene

Chart 4. Reaction Mechanism for the Olefin Epoxidation Promoted by 1, Showing the Relevant Intermediates

Costa et al.

We calculated it as 1.437 A˚, a value that differs significantly from the experimentally determined bond length of 1.271 A˚.16 Related values (Mo-O distances) reflect this discrepancy. Other experimental values range from 1.290 to 1.479 A˚. The shortest value (1.290 A˚) is observed in Cp*W(O)(O2)CH2Si(CH3)3,24 but it increases to 1.427 A˚ in the CpW analogue.25 There is disorder in the Cp* complex. The other related structures are those of Cp*M(O)(O2)Cl, with M = Mo and W, with O-O distances of 1.449/1.445 A˚ (Mo) and 1.479/ 1.486 A˚ (W) and disordered structures.26 Typical textbook O-O distances in peroxo complexes range between 1.40 and 1.48 A˚,27 while a systematic search for Mo(O2) in the CSD28 with 169 hits led to a statistical concentration of values from 1.4 to 1.5 A˚. These data suggest that the short O-O bonds are the anomalous ones. On the other hand, the calculated distance varied slightly around the value mentioned above (1.437 A˚), but did not decrease when using other approaches (MP2, solvent, better basis sets with more diffuse and polarization functions). Therefore the computational method appears to be reliable, the limitations being on the single-crystal X-ray diffraction data.

Conclusions

this pathway is 24 kcal mol-1 and, therefore, much more likely to take place than the direct reaction depicted in Figure 6, with a high barrier of 42 kcal mol-1. Taking into account all the data discussed above, the most relevant pathways in the catalytic olefin epoxidation reaction can be summarized as shown in Chart 4, which reflects the experimental results reported in ref 16. In the reactions promoted by complex 2, formation of B and epoxidation from B have relatively low activation barriers (gas-phase MP2 ΔE 21 kcal mol-1), while formation of complex 3 from 2 exhibits a higher barrier (38 kcal mol-1). Once complex 3 is formed, the barrier toward the active complex C is 24 kcal mol-1. C may act as catalyst directly, with TBHP and olefin, or undergo a reaction with olefin, forming epoxide and the intermediate B. Although some of the barriers are relatively high, it should be remembered that catalysis is run at 55 °C, and the room-temperature kinetic experiments describe slow reactions. The energies of intermediates are close to each other. Another aspect to emphasize is the negligible role played by the solvent (calculated for dichloromethane, used in most cases), since the deviations between gas-phase and solvent free energies are ∼1-3 kcal mol-1. Structures of Peroxide Complexes. The energies calculated for the pathways discussed above varied considerably depending on the computational approach. In order to check their reliability, the reproduction of the structure of the peroxide complex CpMo(O)(O2)CH3 (3) is addressed in more detail. This structure has been reported,16 and a few other related structures are available. The O-O distance in the coordinated peroxide is an interesting reference to consider.

DFT calculations did not provide a clear-cut answer to the mechanism of olefin epoxidation by CpMo(CO)3CH3 (1) derivatives, suggesting that a series of competitive pathways are available, several of them with comparable activation barriers. Such an observation had already been the outcome of the kinetic studies recently reported,16 and other authors had previously commented on this particular difficulty when studying model Mo(VI) oxobis(peroxo) complexes.14b As a matter of fact, complex 1 is oxidized by excess TBHP to two Mo(VI) complexes, namely, CpMo(O)2CH3 (2) and CpMo(O)(O2)CH3 (3), which have been shown to be inactive for stoichiometric epoxidation of olefins. In the presence of TBHP, however, epoxidation can occur from both 2 and 3, by means of active intermediates, B and C, respectively. DFT calculations allowed the identification of the active intermediates B and C, derived from 2 and 3. In both, the OOR group of TBHP is end-on bound to molybdenum, with the β oxygen (next to the tert-butyl group) involved in a hydrogen bond with the neighboring OH (see Figures 2 and 3). The R oxygen (attached to the Mo atom) is transferred to the olefin during the catalytic epoxidation, according to the most likely mechanism. Formation of 3 from 2 competes with epoxidation from 2. As described above, other alternative pathways with similar barriers are possible. Probably specific conditions of each reaction may thus shift the energy balance toward a particular pathway. It was also found that DFT approaches give systematically very high energies, the MP2 values appearing more reasonable. (24) Faller, J. W.; Ma, Y. Organometallics 1988, 7, 559–561. (25) Legzdins, P.; Phillips, E. C.; Rettig, S. J.; Sanchez, L.; Trotter, J.; Yee, V. C. Organometallics 1988, 7, 1877–1878. (26) Chakraborty, D.; Bhattacharjee, M.; Kr€atzner, R.; Siefken, R.; Roesky, H. W.; Us on, R.; Schmidt, H. G. Organometallics 1999, 18, 106– 108. (27) Housecroft, C. E.; Sharpe A. G. Inorganic Chemistry; PrenticeHall: London, 2001. (28) The Cambridge Structural Database: Allen, F. H. Acta Crystallogr. 2002, B58, 380–388.

Article


Computational Details 17

DFT calculations were performed with the Gaussian03 software package18 using the B3LYP hybrid functional. This functional includes a mixture of Hartree-Fock19a exchange with DFT exchange-correlation, given by Becke’s three-parameter functional19b with the Lee, Yang, and Parr correlation functional, which includes both local and nonlocal terms.29 All intermediates were optimized without symmetry constraints. The standard LANL2dz basis set30 augmented with an f-polarization function (exponent 1.043),31 with the associated ECP, was used for Mo, while for the remaining elements a standard 6-31G** was employed.32 Frequency calculations were performed in all species at this level of theory to confirm the nature of the stationary points and to calculate electronic energies corrected for ZPE (ΔE0DFT) and free energies (ΔGDFT). All transition-state structures (one imaginary frequency) were relaxed following the vibrational mode to confirm the connecting reagents. HOOCH3 and ethylene were used to model TBHP and olefin (cyclooctene), respectively. The free energies in dichloromethane solution (ΔGs) were obtained by performing self-consistent reaction field (SCRF) calculations using the polarizable continuum model (PCM) and the universal force field (UFF)33 to define the (29) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200–206. (30) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry, Vol. 3; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; p 1. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (31) Ehlers, A. W.; B€ ohme, M.; Dapprich, S.; Gobbi, A.; H€ ollwarth, A.; Jonas, V.; K€ ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111–114. (32) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724–728. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257–2261. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209–214. (d) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163–168. (e) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (33) Rappe, A. K.; Kasewit, C. J.; Colwell, K. S.; Goddard, W. A.III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024–10035.

311

atomic radii of the atoms on the gas-phase-optimized geometries. Single-point energy calculations were performed with the second-order Moeller-Plesset perturbation theory method MP220 on the previously optimized B3LYP structures with the same basis set (ΔEMP2: LANL2dzþf, 6-31G**). To check the consistency of the basis set, we performed more MP2 singlepoint calculations with increasing quality of the basis sets: ΔEMP2 (B1) corresponds to the use of the newly developed LANL2tz34 basis set (an improvement of the LANL2dz) augmented with an f-polarization function on Mo and the 6-311G** on the remaining elements; ΔEMP2 (B2) corresponds to the use of the same LANL2tzþf for Mo, while the remaining elements are described with the aug-cc-pVTZ basis set,35 including polarization functions by definition. Optimization at the MP2 level of theory was also performed with the LANL2dzþf, 6-31G** combination on certain species of Figure 4 in order to evaluate the effect of geometry relaxation (specially for TSBD). Frequency calculations were performed, and the values of the electronic energies corrected for ZPE (ΔE0MP2) and free energies (ΔGMP2) were obtained.

Acknowledgment. M.J.C. and P.J.C. thank FCT and FEDER for financial support (PPDCT/QUI/58925/ 2004), and P.J.C. acknowledges FCT for a grant (SFRH/BPD/27082/2006). The Leibniz-Rechenzentrum M€ unchen is gratefully acknowledged for the CPU time provided in the LRZ Linux Cluster. The authors thank ~es Integradas Luso-Alem~aes (A-3/05). We thank R. Acc-o Poli for helpful discussions. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. (34) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput. 2008, 4, 1029–1031. (35) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796–6806.

Olefin Epoxidation Catalyzed by Î·5-Cyclopentadienyl Molybdenum

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