Understanding the Hydrolysis Mechanism of Ethyl Acetate Catalyzed

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Understanding the Hydrolysis Mechanism of Ethyl Acetate Catalyzed by an Aqueous Molybdocene: A Computational Chemistry Investigation Elkin Tílvez,† Gloria I. Cárdenas-Jirón,‡ María I. Menéndez,† and Ramón López*,† †

Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, C/Julián Clavería 8, 33006 Oviedo, Principado de Asturias, Spain ‡ Departamento de Ciencias Químicas, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, Chile S Supporting Information *

ABSTRACT: A thoroughly mechanistic investigation on the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate has been performed using density functional theory methodology together with continuum and discrete− continuum solvation models. The use of explicit water molecules in the PCMB3LYP/aug-cc-pVTZ (aug-cc-pVTZ-PP for Mo)//PCM-B3LYP/aug-cc-pVDZ (aug-cc-pVDZ-PP for Mo) computations is crucial to show that the intramolecular hydroxo ligand attack is the preferred mechanism in agreement with experimental suggestions. Besides, the most stable intermediate located along this mechanism is analogous to that experimentally reported for the norbornenyl acetate hydrolysis catalyzed by molybdocenes. The three most relevant steps are the formation and cleavage of the tetrahedral intermediate immediately formed after the hydroxo ligand attack and the acetic acid formation, with the second one being the rate-determining step with a Gibbs energy barrier of 36.7 kcal/mol. Among several functionals checked, B3LYP-D3 and M06 give the best agreement with experiment as the rate-determining Gibbs energy barrier obtained only differs 0.2 and 0.7 kcal/mol, respectively, from that derived from the experimental kinetic constant measured at 296.15 K. In both cases, the acetic acid elimination becomes now the rate-determining step of the overall process as it is 0.4 kcal/mol less stable than the tetrahedral intermediate cleavage. Apart from clarifying the identity of the cyclic intermediate and discarding the tetrahedral intermediate formation as the rate-determining step for the mechanism of the acetyl acetate hydrolysis catalyzed by molybdocenes, the small difference in the Gibbs energy barrier found between the acetic acid formation and the tetrahedral intermediate cleavage also uncovers that the rate-determining step could change when studying the reactivity of carboxylic esters other than ethyl acetate substrate specific toward molybdocenes or other transition metal complexes. Therefore, in general, the information reported here could be of interest in designing new catalysts and understanding the reaction mechanism of these and other metal-catalyzed hydrolysis reactions.

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INTRODUCTION The hydrolysis of carboxylic acid esters occurs in many chemical,1−6 biochemical,1,3,4,6−8 and industrial9−12 processes. In particular, polyesters often biodegrade by hydrolysis and the detailed knowledge of the process may have consequences in fields such as drug delivery, nerve regeneration, tissue engineering, cardiovascular and orthopedic applications, etc. It is well-known that most esters do not readily hydrolyze in neutral aqueous solution, and consequently catalysts are required.4,6 Transition-metal-based catalysts have attracted considerable attention owing to the wide variety of ligands and metal ions that are available and potentially useful.13−26 In particular, the hydroxo metal complexes are good for mimicking ester-hydrolyzing enzymes that contain metal ions in their active sites,14,19,21,25 providing thus valuable information for understanding the chemistry involved in the action of metalloenzymes and for designing efficient artificial metalloenzymes. In addition, organometallic hydroxo compounds are also important due to their rich OH-centered reactivity27 that is dominated by the nucleophilic character of the hydroxo ligand. © XXXX American Chemical Society

In the past two decades, the chemistry of hydroxo complexes of middle (groups 6 and 7) transition-metal fragments was investigated.24,26−40 Experimental studies have focused on the aqueous chemistry of the metallocene dihalide bis(η5-cyclopentadienyl)-molybdenum(IV) dichloride (Cp2MoCl2, Cp = η5-C5H5), finding that the hydroxo molybdocene [Cp2Mo(OH)(OH2)]+ is the active species at neutral pH.36,39,41,42 The interest in Cp2MoCl2 and derivatives in water is twofold. On the one hand, molybdocenes and other related metallocenes have been investigated as potential compounds for antitumor treatments36,42−46 since the discovery of the anticancer activity of titanocene.47 On the other hand, molybdocenes have been used for catalyzing a variety of organic reactions in aqueous solution37 such as nitrile hydration,24,26,31,38,40 carboxylic24,26 and phosphate24,26,48−50 ester hydrolysis, ether hydrolysis,24 carbon monoxide oxidation,24 hydrogenation of olefins,37 etc. The investigation on the mechanism of these processes could Received: June 16, 2014

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DOI: 10.1021/ic501416u Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Experimental Mechanistic Proposals Suggested for Carboxylic Ester Hydrolysis Catalyzed by Molybdocenes

296.15 K for such a process ((0.20 ± 0.07) × 10−5 s−1)24 using the thermodynamic formulation of the transition state theory (TST). With all of this mind, we undertook a theoretical mechanistic study on the hydrolysis of ethyl acetate catalyzed by [Cp2Mo(OH)(OH2)]+. Previous theoretical investigations on nitrile hydration64,65 catalyzed by [Cp2Mo(OH)(OH2)]+ and [Pd(OH2)4]2+ and on carbon monoxide oxidation66 catalyzed by [Cp2Mo(OH)(OH2)]+ have proven to be useful in getting a more profound rationalization of the most relevant experimental results found in the interesting chemistry of molybdocenes toward organic electrophiles in an aqueous medium.

be of importance in designing new catalysts with functionalities that can enhance or modify their reactivity toward different organic electrophiles. The aqueous hydrolysis of ethyl acetate is commonly used as a representative procedure for the theoretical study of the hydrolysis of carboxylic acid esters. While the hydrolysis mechanism of this and other esters has theoretically been investigated in considerable detail in the absence of metal complexes,51−61 theoretical works are scarce for organometallic catalysts and practically null for aqueous molybdocene catalysis.25,62 From an experimental point of view, four mechanisms have been considered to account for the hydrolysis of ethyl acetate catalyzed by [Cp′2Mo(OH)(OH2)]+ (Cp′ = η5C5H4CH3):24,37 the intramolecular nucleophilic attack, the general base catalysis, the Lewis acid coordination assisted mechanism, and the intermolecular nucleophilic attack (see Scheme 1). In the first three proposals the ester bonds to the metal, while in the last one a water ligand saturates Mo coordination and the ester does not coordinate to the metal. It is believed that the intramolecular nucleophilic attack is the operative mechanism for the ethyl acetate hydrolysis case proceeding via strained, four-membered ring intermediates.24,37 Experimental reports do not clarify the identity of the cyclic intermediate,24 as two candidates are possible: the fourmembered metallocycle immediately formed after the attack of the hydroxo ligand to the Mo-coordinated ester before ethanol elimination and a similar metallocycle free from the ethanol moiety. An intermediate of this last kind has been experimentally detected in the norbornenyl acetate hydrolysis.24 Besides this, although not exactly equal, four-membered metallocycles of Co63 and Mo48 have also been detected by Xray diffraction and 31P NMR techniques. As the four-membered metallocyclic intermediate is considered the key to determining the operative mechanism for ester hydrolysis catalyzed by molybdocenes, it is crucial to know its nature and the intermediate systems that lead to it. Another intriguing issue is related to the rate-determining step. It has been proposed that formation of the four-membered metallocyclic intermediate containing the ethanol moiety is the slowest step for the [Cp′2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate on the basis of the fact that the formation of an analogous intermediate with amide substrates seems to prevent the amide hydrolysis in the presence of molybdocenes.24 However, an experimental study on the methyl acetate hydrolysis catalyzed by Cu(II) complex has also suggested that either formation or cleavage of the above-mentioned intermediate could be the rate-determining step.13 Concerning the energetics of the ethyl acetate hydrolysis catalyzed by [Cp′2Mo(OH)(OH2)]+, a Gibbs energy barrier of 25.1 kcal/mol, for instance, can be deduced from the pseudo-first-order rate constant obtained at

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COMPUTATIONAL DETAILS All the computations were done with the Gaussian 09 suite of programs.67 Previous computational studies of chemical processes catalyzed by molybocenes64,66 describe in detail the theoretical method used there and in this work. All the species involved in the reaction mechanisms investigated were studied at the level of theory PCM-B3LYP/VTZ//PCM-B3LYP/VDZ (VTZ = aug-cc-pVTZ (aug-cc-pVTZ-PP for Mo); VDZ = augcc-pVDZ (aug-cc-pVDZ-PP for Mo)) from the outset (see the Supporting Information for more computational details).68−70 Because some steps involve hydrogen shifts and water additions, a discrete−continuum solvent model was also used in order to obtain a more realistic description of the process and of the implied energy barriers. Previous theoretical studies on ester hydrolysis in the absence51−61 and in the presence of metal complexes25,62 as well as those for nitrile hydration catalyzed by aqueous molybdocenes64 and Pd(II) complexes65 have shown the adequacy of including explicit water molecules to simulate the most significant features in the rearrangements mentioned above. On the basis of these investigations, we have considered the minimum number of essential water molecules (two explicit water molecules) to reproduce the main features involved in the hydrogen shift and water addition processes found in the present work. Besides this, the involvement of at least two explicit water molecules had been indicated in experimental studies on neutral hydrolysis of carboxylic esters in the condensed phase.71 On the other hand, reaction steps where the role of explicit water molecules is assumed to be irrelevant were studied using just the polarizable continuum model (PCM). Thermal free energy corrections in water solution (Gtherm) were first computed at the PCM-B3LYP/VDZ level of theory at a pressure of 1 atm and a temperature of 298.15 K within the ideal gas, rigid rotor, and harmonic oscillator approximations72 to compare with experimental results obtained at the same pressure and a temperature of 296.15 K on ethyl acetate B


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first. Afterward, we consider a combined discrete−continuum solvent model through the inclusion of two explicit water molecules in some relevant steps of the previously presented reaction mechanisms to obtain a more realistic description of the process and, consequently, better energy barriers in water solution. Finally, our results are compared with available experimental data. As mentioned in Computational Details, unless otherwise indicated, we discuss in the text PCM-B3LYP/ VTZ//PCM-B3LYP/VDZ Gibbs energies in water solution referred to the separate reactants. Continuum Solvation Model. Scheme 2 displays the representative structures of the nine reaction mechanisms

hydrolysis in the presence of [Cp′2Mo(OH)(OH2)]+.24 The calculation of thermodynamic magnitudes in solution starting with molecular partition functions developed for computing gas-phase thermodynamics properties is a standard procedure that has proven to be a correct and useful approach.73 To obtain thermal free energy corrections in solution (standard state of 1 M (55.5 M for water)), Gthermcorr, the Gtherm for each species was augmented by 1.9 kcal/mol (4.3 kcal/mol for water) in order to take into account the corresponding decrease in the translational entropy on going from the calculations performed at 1 atm and 298.15 K to the required 1 M (55.5 M for water) standard state: Gtherm corr = Gtherm − RT ln V + RT ln(RT /P) therm

(1)

Scheme 2. Representative Structures of the Reaction Mechanisms Investigated for the [Cp2Mo(OH)(OH2)]+Catalyzed Hydrolysis of Ethyl Acetate at the PCM-B3LYP/ VTZ//PCM-B3LYP/VDZ Level of Theory

therm

where G term and V is the molar corr is the corrected G volume (1 L/mol for all the species except for water, which is 1/55.5 L/mol). The Gibbs energy in aqueous solution at the standard state of 1 M (55.5 M for water), Gsolcorr, was estimated by adding to the PCM-B3LYP/VTZ//PCM-B3LYP/VDZ Gibbs energy, G, which includes the electrostatic and nonelectrostatic solute− solvent interactions, the Gthermcorr correction. As a consequence, free energy in solution referred to separate reactants was calculated as follows: ΔGsol corr = ΔGsol + 1.9Δn + 4.3Δm

(2)

where ΔGsol = ΔG + ΔGtherm

(3)

and Δn and Δm are the changes in the number of non-water species and water molecules, respectively. Hereafter unless otherwise specified, we will discuss in the text the ΔGsolcorr values denoted as ΔGsol for brevity. The level of theory PCM-B3LYP/VTZ//PCM-B3LYP/VDZ chosen in this work has proven to be adequate in investigating the reaction mechanism of the carboxylic ester hydrolysis both in the absence57−61 and in the presence62 of metal complexes as well as of similar systems such as the nitrile hydration64,65 catalyzed by [Cp2Mo(OH)(OH2)]+ and [Pd(OH2)4]2+ and the [Cp2Mo(OH)(OH2)]+-catalyzed oxidation of carbon monoxide.66 However, four other functionals were also checked by performing PCM-DFT/VTZ//PCM-B3LYP/VDZ, where “DFT” stands for M06,74 BP86,69a,75 G96LYP,69b,76 and B3PW9169a,77 (see the Supporting Information for details). We selected these density functional theory (DFT) methods based on their good performance in chemical processes mediated or catalyzed by metal complexes.62,78 Besides this, we have taken into account the dispersion interactions in the B3LYP energy calculations by computing the atom-pairwise DFT-D3 dispersion correction (Edisp) developed by Grimme and co-workers.79 The single-point DFT-D3 calculations were done on the PCM-B3LYP/VDZ geometries using the Becke− Johnson damping function80 to avoid near singularities for small interatomic distances. For interpretation purposes, a natural bond orbital (NBO) analysis was also performed on some relevant species along the reaction mechanisms found.81

investigated for the hydrolysis of ethyl acetate catalyzed by [Cp2Mo(OH)(OH2)]+. They are grouped into two sets: mechanisms starting with and mechanisms starting without the release of the water ligand from [Cp2Mo(OH)(OH2)]+. In all cases we considered the most stable conformation for ethyl acetate, that is, the trans conformer.82 A prereactive complex, I0, between the separate reactants wherein the carbonyl oxygen atom of the isolated ester is interacting with a hydrogen atom of the water ligand of [Cp2Mo(OH)(OH2)]+ at a distance of 1.746 Å was located (see Figure S1 in the Supporting Information). I0 is 2.0 kcal/ mol more stable than the separate reactants when thermal corrections are not taken into account, but becomes 6.7 kcal/ mol less stable than the separate reactants along the Gibbs energy profile in solution. This suggests that I0 is a transient species similar to those found in a theoretical investigation on vanadate ion catalyzed carboxylic ester hydrolysis.62 I0 does not belong to the reaction path of mechanisms starting with the water ligand release. All of them have in common the initial loss of the water ligand from [Cp2Mo(OH)(OH2)]+ to give [Cp2Mo(OH)]+ (−6.6 kcal/mol) without any transition state

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RESULTS AND DISCUSSION The reaction mechanisms found for the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate in an aqueous medium modeled by a continuum solvent model are presented C


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Figure 1. Gibbs energy profile in water solution of the most favored reaction mechanism starting with the water ligand release followed by the hydroxo ligand attack (OH1) for the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate at the PCM-B3LYP/VTZ//PCM-B3LYP/VDZ level of theory. Energy values are given in kcal/mol in green.

Figure 2. Gibbs energy profile in water solution of the two most favored reaction mechanisms starting without the water ligand release (OH1′ and OH2′) for the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate at the PCM-B3LYP/VTZ//PCM-B3LYP/VDZ level of theory. Energy values are given in kcal/mol in green.

(TS), followed by the ethyl acetate addition to Mo via the TS TS0 (15.7 kcal/mol) to afford the intermediate I1 (8.7 kcal/ mol) (see Figure 1 and Figures S2 and S3 in the Supporting Information).

In accordance with experimental mechanistic proposals suggested for ester hydrolysis catalyzed by Cu(II)13 and Mo(IV)24 complexes, the ester moiety in I1 is bound to the metal center via the carbonyl oxygen atom (Ocarbonyl) at a D


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Table 1. Rate-Determining Step, with Its Corresponding Gibbs Energy Barrier Following an Increasing Trend, for the Nine Reaction Mechanisms Studied Using the PCM Modela TS1

a

TS2

TS1

TS2

OH2′

OH1

OH1′

W1

OH2

W3

W2

OH3′

OH3

46.0

52.5 (36.7)

53.7 (44.3)

56.0

64.6

70.8 (54.5)

73.6 (55.5)

92.3

126.6

Values in the discrete−continuum model are shown in parentheses.

Cmethylenic, and Cmethyl, respectively (see third row in Scheme 2). We use the prime symbol to differentiate OH1′, OH2′, and OH3′ from the analogous ones starting with the water ligand release from [Cp2Mo(OH)(OH2)]+ (OH1, OH2, and OH3 in Scheme 2, respectively). On the basis of the rate-determining Gibbs energy barriers obtained for the nine reaction mechanisms mentioned above (see Table 1), we only describe below the three most favored ones, that is, OH1, OH1′, and OH2′. The reaction profiles and a description of the remaining mechanisms are collected in the Supporting Information. OH1 Mechanism (Black Line in Figure 1). As described above, [Cp2Mo(OH)(OH2)]+ initially replaces its water ligand by the ester substrate to form I1 via TS0. Then, I1 undergoes the attack of the hydroxo ligand (Ohydroxo) to Ccarbonyl via the TS TS1-OH1 (24.9 kcal/mol) to afford the four-membered metallocycle intermediate I2-OH1 (24.8 kcal/mol). At I2OH1 Ccarbonyl presents a tetrahedral conformation resembling that found in intermediates located in computational studies on neutral hydrolysis of carboxylic esters noncatalyzed51−55,57−60 and catalyzed25,62,83 by metal complexes. The next step is for the release of an ethanol molecule and the formation of another four-membered metallocycle intermediate I3-OH1 (−13.7 kcal/mol) owing to a hydrogen migration from Ohydroxo to Oethoxyl coupled with simultaneous cleavage of the Ccarbonyl− Oethoxyl bond via the TS TS2-OH1 (45.9 kcal/mol). I3-OH1 is analogous to the [Cp′2Mo(μ-O)2(CMe)]+ complex characterized by mass spectrometry in the hydrolysis of norbornenyl acetate catalyzed by [Cp′2Mo(OH)(OH2)]+.24 Afterward, I3OH1 evolves for the cleavage of one of the two Mo−O bonds through the TS TS3-OH1 (1.0 kcal/mol) to give the corresponding opened intermediate I4-OH1 (−2.0 kcal/mol), which in turn undergoes the addition of an external water molecule to the metal center, d(Mo−Owater) = 2.181 Å, leading to I5-OH1 (−6.6 kcal/mol) without any TS. I5-OH1 can be considered a six-membered metallocycle due to the establishment of a hydrogen bond between the incoming water and Ohydroxo, d(Owater−H···Ohydroxo) = 1.512 Å. The presence of such a hydrogen bond together with the relatively high stability of the metallocycle I5-OH1 prevents the release of the acetic acid ligand in one step. Instead, a hydrogen migration from Owater to Ohydroxo via the TS TS4-OH1 (17.5 kcal/mol) to yield the intermediate I6-OH1 (8.1 kcal/mol) occurs and then the cleavage of the Mo−Ocarbonyl bond via the TS TS5-OH1 (13.0 kcal/mol) to form an acetic acid molecule in anti conformation (see Figure 1) and [Cp2Mo(OH)]+ (−5.2 kcal/mol) takes place. We note here that the H migration to Ohydroxo is coupled with a rotation of the H···Ohydroxo bond axis with respect to the Ohydroxo−Ccarbonyl bond axis, thus provoking the cleavage of the Owater−H bond and facilitating the posterior acid acetic elimination. The exoergicity of the reactive process will become slightly greater (1−2 kcal/mol) than −5.2 kcal/mol as the anti conformer of acetic acid will evolve to the syn conformer, the

distance of 2.210 Å. We also explored the existence of an intermediate with Mo linked to the ethoxyl oxygen atom (Oethoxyl), but all attempts failed. As could be expected, an NBO analysis on the free ethyl acetate shows that Ocarbonyl presents a negative natural charge (−0.70e) clearly higher than that obtained for Oethoxyl (−0.59e). This favors the prevalence of the interaction between Ocarbonyl and the metal center of [Cp2Mo(OH)]+ at I1, in which Mo has a natural charge of 0.57e. Several mechanisms can be envisaged from I1 depending on the attacking nucleophile (the hydroxo ligand or an external water molecule) and the attacked ester carbon atom. An NBO analysis on the ethyl acetate moiety at I1 indicates that the carbonyl carbon atom (Ccarbonyl) presents a notable positive natural charge (0.93e). However, negative natural charges were found for the methyl carbon atom of the acetyl group (Cmethyl, −0.67e) and for the methylenic (Cmethylenic, −0.09e) and terminal (Cterminal, −0.73e) carbon atoms of the ethoxyl group. As could be expected, Ccarbonyl is the most susceptible for a nucleophilic attack at I1. In these circumstances, for the hydroxo ligand nucleophile, we considered the attack on Ccarbonyl (route OH1), Cmethylenic (route OH2), and Cmethyl (route OH3) as displayed in the first row of Scheme 2. On the basis of the results obtained (see below), in the case of the water nucleophile, we only took into account the attack on Ccarbonyl but we found three different routes differing in the oxygen atom that accepts one of the water hydrogen atoms (see second row in Scheme 2): hydroxo ligand (route W1), Ocarbonyl (route W2), and Oethoxyl (route W3). With this nomenclature, OH1 and W1 coincide with the intramolecular nucleophilic attack and general base catalysis mechanisms, respectively (see Scheme 1a,b),24,37 whereas W2 and W3 resemble the Lewis acid coordination assisted mechanism (see Scheme 1c).24,37 On the other hand, on the basis of different intermolecular mechanisms suggested for the ester hydrolysis catalyzed by molybdocenes (see Scheme 1d)24,26,37 and other metal catalysts,20,83 other mechanistic possibilities were also investigated where the ethyl acetate ester is not activated by the [Cp2Mo(OH)(OH2)]+ complex but is free in the reaction medium. All of them initiate with the attack of the hydroxo ligand to the ester carbonyl carbon atom without water ligand release. As for the mechanisms involving the hydroxo ligand attack on the Mo-coordinated ester (see above), Ccarbonyl, Cmethylenic, and Cmethyl are the ester carbon atoms susceptible for nucleophilic attack investigated by us. Although several orientations of the ester relative to the complex were tested, only those with Ocarbonyl close to the water ligand were found reactive, with the intermediate I0 as the initial complex from which the reaction paths proceeds (see Figure 2). This is in the line of the fact that Ocarbonyl presents a higher negative natural charge compared to that found for Oethoxyl as explained above. Figure 2 and Figure S4 in the Supporting Information collect the Gibbs energy profiles in solution of the mechanisms found. We ascribe OH1′, OH2′, and OH3′ to the reaction pathways starting with the attack of the hydroxo ligand on Ccarbonyl, E


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Figure 3. Gibbs energy profile in water solution, in kcal/mol, of the most favored reaction mechanism found for the [Cp2Mo(OH)(OH2)]+catalyzed hydrolysis of ethyl acetate at the PCM-DFT/VTZ//PCM-B3LYP/VDZ (DFT = B3LYP-D3 (in blue) and M06 (in red)) level of theory. For comparison purposes, PCM-B3LYP/VTZ//PCM-B3LYP/VDZ Gibbs energy barriers in water solution, in kcal/mol, of the three most relevant steps are also given in parentheses in green. TSs involving hydrogen shift are only shown when considering two explicit water molecules (see Discrete−Continuum Model for details).

ethanol formation is the rate-determining step controlled by TS1-OH2′ with a Gibbs energy barrier of 46.0 kcal/mol. It is worth mentioning that only the OH1 route includes the intermediate I3-OH1, which closely resembles the fourmembered metallocycle experimentally detected.24 However, with the solvent model used up to now, not OH1 but OH2′ is the preferred route, that starting with the nucleophilic attack of the hydroxo ligand to Cmethylenic when the ester is not directly bonded to Mo. Besides, for all the reaction paths the Gibbs energy barriers are much higher than that (25.1 kcal/mol) derived from the experimental kinetic constant obtained at 296.15 K for the same carboxylic ester hydrolysis catalyzed by [Cp′ 2Mo(OH)(OH2)]+.24 These facts suggest that the continuum solvation model is not enough to provide a realistic description of the process under study. PCM takes into account the solvent as a reaction environment and the long-range interactions between it and the solute.86,87 Due to the presence of several 1,3-H transfer steps along the mechanisms found, it seems reasonable to consider that explicit water molecules may assist these steps acting as a bifunctional catalyst able of extracting one H from one end and releasing another one at the other end with much smaller energy barriers.25,51−61,63−65,88−91 This possibility is studied through a discrete−continuum solvent model in the next section. Discrete−Continuum Model. Among the nine mechanisms studied (see Scheme 2), routes OH2, OH3, OH2′, and OH3′ cannot benefit from the assistance of explicit water molecules in any of their reaction steps. For the remaining mechanisms, following the preference trend shown in Table 1, two explicit water molecules were considered to reinvestigate those TSs susceptible of water assistance as well as their previous intermediates (see Figures S5−S7 in the Supporting Information). With the energy of this couple of structures it is possible to assess the stabilization produced by water molecules

most stable isomer in water solution, wherein the hydroxyl oxygen atom is in the same side as the carbonyl oxygen atom.84 According to our results the ethanol elimination via a H shift from Ohydroxo to Oethoxyl is the rate-determining step controlled by TS2-OH1 with a Gibbs energy barrier in solution of 52.5 kcal/mol, measured from the most stable previous species ([Cp2Mo(OH)]+ + H2O + ethyl acetate).85 Mechanism OH1′ (Red Line in Figure 2). The first step, the Ohydroxo attack on Ccarbonyl, is controlled by the TS TS1-OH1′ (31.4 kcal/mol) that connects the separate reactants with the intermediate I1-OH1′ (30.8 kcal/mol). At I1-OH1′, the new Ohydroxo−Ccarbonyl bond presents a distance of 1.495 Å. Then, this intermediate undergoes the formation of the ethanol molecule due to a hydrogen migration from Ohydroxo to Oethoxyl coupled with the cleavage of the Oethoxyl−Ccarbonyl bond via the TS TS2-OH1′ (53.7 kcal/mol). This TS leads to I5-OH1 (−6.6 kcal/mol), which in turn evolves to the separate products as mentioned for OH1. Therefore, the ethanol elimination via a H shift from Ohydroxo to Oethoxyl is the rate-determining step with a Gibbs energy barrier in water solution of 53.7 kcal/mol determined by TS2-OH1′. Mechanism OH2′ (Blue Line in Figure 2). The first TS along this route, TS1-OH2′ (46.0 kcal/mol), involves the attack of Ohydroxo on Cmethylenic coupled with simultaneous cleavage of the Cmethylenic−Oethoxyl bond and hydrogen transfer from the water ligand to Ocarbonyl. TS1-OH2′ leads to the formation of the acetic acid molecule and the resultant intermediate I1-OH2′ (5.9 kcal/mol), in which the hydroxo and ethanol ligands are linked to Mo. Subsequently, this intermediate undergoes the cleavage of the Mo−ethanol bond via the TS TS2-OH2′ (10.5 kcal/mol) leading to [Cp2Mo(OH)]+ and ethanol (−5.2 kcal/ mol). The results obtained for such a mechanism indicate that the attack of the hydroxo ligand on Cmethylenic coupled with the F


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Inorganic Chemistry

to that obtained (12.0 kcal/mol) in a recent theoretical investigation on the p-nitrophenyl acetate hydrolysis promoted by a vanadate ion when comparing the obtained B3LYP ratedetermining Gibbs energy barrier with the respective experimental value.62 In this study, besides, after validating several DFT methods, the best agreement between theory and experiment was reached by performing M06 single-point energy calculations on B3LYP optimized geometries. This prompted us to check our B3LYP results by performing PCMDFT/VTZ//PCM-B3LYP/VDZ (DFT = B3LYP-D3, M06, BP86, G96LYP, B3PW91) computations. As B3LYP-D3 and M06 clearly give the best agreement with experiment (see Tables S4 and S5 and comments in the Supporting Information), we computed all the species involved in the most favored reaction mechanism found, route OH1, as well as the rate-determining Gibbs energy barriers of the two following most stable mechanisms, routes OH1′ and OH2′ (see Tables S6−S9 in the Supporting Information). We find that B3LYP-D3 and M06 reduce the ratedetermining Gibbs energy barriers of the three most stable routes, but OH1′ and OH2′ mechanisms remain clearly less favorable than OH1 by 8.7 and 11.7 kcal/mol with B3LYP-D3, respectively, and 10.4 and 15.9 kcal/mol with M06, respectively. As seen in Figure 3, B3LYP-D3 and M06 reveal that the acetic acid elimination competes with the cleavage of the tetrahedral intermediate I2-OH1 as the rate-determining step for the OH1 route. The former is only 0.4 kcal/mol less stable than the latter at both DFT methods. However, the ratedetermining Gibbs energy barriers obtained, 25.3 (B3LYP-D3) and 25.8 (M06) kcal/mol, are very close to the value (25.1 kcal/mol) derived from the reported experimental rate constant using the TST formulation at 296.15 K. Our results also indicate that the tetrahedral intermediate formation is a less energy demanding step than the cleavage of this intermediate. The highest stabilities of I3-OH1, −11.8 (B3LYP-D3) and −11.6 (M06) kcal/mol, together with the notable instabilities of I2-OH1, 18.6 (B3LYP-D3) and 18.5 (M06) kcal/mol, confirm our PCM-B3LYP/VTZ//PCM-B3LYP/VDZ proposal of assignment of [Cp2Mo(μ-O)2(CMe)]+ to the metallocyclic intermediate experimentally suggested for ethyl acetate hydrolysis catalyzed by molybdocenes.24 Besides, our best theoretical results show that the cleavage of the tetrahedral intermediate I2-OH1 is clearly more energy demanding than the formation of such an intermediate, thus discarding the latter as the rate-determining step, at least in the case of the ethyl acetate hydrolysis catalyzed by molybdocenes. This clarifies some experimental suggestions indicating that either the tetrahedral intermediate cleavage or the tetrahedral intermediate formation could be the rate-determining step for carboxylic ester hydrolysis both in the absence60 and in the presence of metal complexes.13,24

in the corresponding step. The resultant species were denoted by adding the label “2WAT” at the end of the acronym used in continuum model results (see wavy lines in Figures 1 and 2 and in Figures S2 and S3 in the Supporting Information). For the OH1 route, TS2-OH1 and TS4-OH1 become stabilized by 15.8 and 16.2 kcal/mol, respectively (see wavy black lines in Figure 1). As a consequence, TS2-OH1, for the 1,3-H shift and ethanol release, remains as the rate-limiting step in this route now with a Gibbs energy barrier of 36.7 kcal/mol. Besides, I5-OH1 evolves to the products in just one step corresponding to the release of the acetic acid moiety. The next most favored mechanism is OH1′, wherein TS2-OH1′ undergoes a stabilization of 9.4 kcal/mol (see wavy red line in Figure 2). This TS remains as the rate-limiting step along the OH1′ route now with a Gibbs energy barrier of 44.3 kcal/mol. Routes starting with a nucleophilic water attack on the ester bonded to Mo, W1, W2, and W3 are much less favorable than OH1 and water assistance will not be able to invert this trend (see wavy lines in Figure S3 in the Supporting Information). According to the results obtained using a discrete− continuum model, OH1 becomes the preferred mechanism for the ethyl acetate hydrolysis catalyzed by [Cp2Mo(OH)(OH2)]+. OH1′ and OH2′, the predominant mechanisms in the continuum model, are now the second and third most favored, 7.6 and 9.3 kcal/mol higher in energy than OH1, respectively. The remaining mechanisms are even less stable than these three here described. Discussion and Comparison with Experiment. The OH1 reaction mechanism, which starts with the replacement of the water ligand in [Cp2Mo(OH)(OH2)]+ by ethyl acetate followed by the hydroxo ligand attack on the ester carbonyl carbon atom to form a tetrahedral intermediate (I2-OH1), is preferred on the basis of our PCM-B3LYP/VTZ//PCMB3LYP/VDZ computations using a discrete−continuum solvent model. This coincides with the mechanism experimentally proposed for ester hydrolyses catalyzed by molybdocenes.24,26,37 Moreover, the most stable species located both along such a mechanism and on the potential energy surface (PES), I3-OH1, is analogous to the [Cp′2Mo(μO)2(CMe)]+ four-membered metallocycle detected by mass spectrometry for the [Cp′2Mo(OH)(OH2)]+-catalyzed hydrolysis of norbornenyl acetate.24 This fact along with the notable instability of I2-OH1, 24.8 kcal/mol, clarifies that I3-OH1 is the identity of the cyclic intermediate mentioned in experiments on the ethyl acetate hydrolysis catalyzed by molybdocenes.24 The rate-determining step controlled by TS2-OH12WAT corresponds to the cleavage of the tetrahedral intermediate previously formed I2-OH1 due to the elimination of the ethanol ligand coupled with the simultaneous formation of [Cp2Mo(μ-O)2(CMe)]+ (I3-OH1) with a Gibbs energy barrier of 36.7 kcal/mol measured from the most stable previous species ([Cp2Mo(OH)]+ + H2O + ethyl acetate) (see Figure 3).85 The following two least favorable steps, which are determined by TS1-OH1 and TS5-OH1, imply the tetrahedral intermediate and acetic acid formation, respectively. The Gibbs energy barriers for these steps are 5.2 and 10.0 kcal/mol lower in energy than that obtained for the rate-determining step, respectively (see Figure 3). The rate-determining Gibbs energy barrier for OH1 exceeds by 11.6 kcal/mol the value derived from the experimental kinetic constant obtained for the [Cp′2Mo(OH)(OH2)]+ hydrolysis of ethyl acetate at 296.15 K using the thermodynamic formulation of TST.24 This notable discrepancy is close

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CONCLUSIONS The hydrolysis mechanism of ethyl acetate catalyzed by the aqueous molybdocene [Cp2Mo(OH)(OH2)]+ has been studied in considerable detail using density functional theory methodology in conjunction with continuum and discrete−continuum solvation models. Nine mechanistic possibilities were investigated at the PCM-B3LYP/VTZ//PCM-B3LYP/VDZ level of theory. When the most realistic description of solvent was used, a discrete−continuum model, the attack of the hydroxo ligand on the carbonyl carbon atom of the Mo-coordinated ester to give the tetrahedral intermediate [Cp2Mo(μ-O)(μ-OH)(CG


Inorganic Chemistry

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(OEt)Me)]+ becomes the most favored mechanism evolving to the separate products via the very stable metallocycle [Cp2Mo(μ-O)2(CMe)]+. The tetrahedral intermediate cleavage is the rate-determining step with a Gibbs energy barrier of 36.7 kcal/mol. The tetrahedral intermediate and acetic acid formation are 5.2 and 10.0 kcal/mol lower in energy than the rate-determining step, respectively. The remaining reaction mechanisms are at least 10.0 kcal/mol less stable than the most favored. Although the B3LYP results allow us to ascribe [Cp2Mo(μ-O)2(CMe)]+ to the cyclic intermediate invoked for ethyl acetate hydrolysis catalyzed by [Cp′2Mo(OH)(OH2)]+ and clarify what is the rate-determining step in such a process, the Gibbs energy barrier obtained is far from that derived from experimental kinetic data at 296.15 K. The replacement of B3LYP by B3LYP-D3 and M06 in the computations leads to an excellent agreement with experiment as the rate-determining Gibbs energy barrier only differs 0.2 and 0.7 kcal/mol from that derived from the experiment, respectively. We note however that the acetic acid formation is now 0.4 kcal/mol less favored than the tetrahedral intermediate cleavage, thus becoming the rate-determining step of the overall process. The small difference in the energy barrier between the two moreenergy-demanding steps suggests that the rate-determining step could change when using esters other than ethyl acetate. Therefore, the present study provides a detailed knowledge about carboxylic ester hydrolysis catalyzed by molybdocenes that could be of importance in these and other related metalcatalyzed organic processes.

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ACKNOWLEDGMENTS We are grateful to the Ministry of Science and Innovation (Spain) for financial support (Project CTQ2010-18231). E.T. also thanks the University of Oviedo (Spain) for a grant (UNOV-08-BECDOC).

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ASSOCIATED CONTENT

S Supporting Information *

Computational details and references not included in the Article. Optimized geometries, energy data, and Cartesian coordinates of the species involved in the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate at the PCMB3LYP/VTZ//PCM-B3LYP/VDZ level of theory (Figure S1 and Tables S1−S3). PCM-B3LYP/VTZ//PCM-B3LYP/VDZ Gibbs energy profiles in water solution of the mechanisms OH2, OH3, W1, W2, W3, and OH3′ (Figures S2−S4) together with their corresponding descriptions. PCM-DFT/VTZ// PCM-B3LYP/VDZ (DFT = B3LYP-D3, M06, BP96, G96LYP, B3PW91) energy data of the most relevant steps involved in the most favored mechanism found for the [Cp2Mo(OH)(OH2)]+-catalyzed hydrolysis of ethyl acetate (Tables S4−S5) and comments in Table S5. PCM-DFT/ VTZ//PCM-B3LYP/VDZ (DFT = B3LYP-D3, M06) energy data corresponding to all the species located along the most favored mechanism and some relevant species involved in the following two most favored found for the ethyl acetate hydrolysis catalyzed by [Cp2Mo(OH)(OH2)]+ (Tables S6− S9). Schematic Gibbs energy profiles in water solution for the hydrogen migration and water addition steps found along the reaction mechanisms obtained both in the absence and in the presence of two explicit water molecules (Figures S5−S9). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H


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Understanding the Hydrolysis Mechanism of Ethyl Acetate Catalyzed

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