Density Functional Theory Study of the Reaction between d0 Tungsten

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Density Functional Theory Study of the Reaction between d0 Tungsten Alkylidyne Complexes and H2O: Addition versus Hydrolysis Ping Chen,† Linxing Zhang,† Zi-Ling Xue,‡ Yun-Dong Wu,†,§ and Xinhao Zhang*,† †

Lab of Computational Chemistry and Drug Design, Laboratory of Chemical Genomics, Peking University Shenzhen Graduate School, Shenzhen 518055, China ‡ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The reactions of early-transition-metal complexes with H2O have been investigated. An understanding of these elementary steps promotes the design of precursors for the preparation of metal oxide materials or supported heterogeneous catalysts. Density functional theory (DFT) calculations have been conducted to investigate two elementary steps of the reactions between tungsten alkylidyne complexes and H2O, i.e., the addition of H2O to the WC bond and ligand hydrolysis. Four tungsten alkylidyne complexes, W( CSiMe3)(CH2SiMe3)3 (A-1), W(CSiMe3)(CH2tBu)3 (B-1), W(CtBu)(CH2tBu)3 (C-1), and W(CtBu)(OtBu)3 (D-1), have been compared. The DFT studies provide an energy profile of the two competing pathways. An additional H2O molecule can serve as a proton shuttle, accelerating the H2O addition reaction. The effect of atoms at the α and β positions has also been examined. Because the lone-pair electrons of an O atom at the α position can interact with the orbital of the proton, the barrier of the ligand-hydrolysis reaction for D-1 is dramatically reduced. Both the electronic and steric effects of the silyl group at the β position lower the barriers of both the H2O addition and ligand-hydrolysis reactions. These new mechanistic findings may lead to the further development of metal complex precursors.

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INTRODUCTION The reactions of early-transition-metal complexes with H2O have been studied intensely.1−12 The reactions of d0 complexes with H2O have been used to prepare highly uniform, conformal films of metal oxides in the microelectronic industry.13,14 An understanding of the elementary steps in the reactions may facilitate the design of organometallic precursors for such surface growth or modification. Metal complexes have also been grafted onto a support material, such as silica or alumina, to develop supported heterogeneous catalysts.15,16 Protonolysis of ligands on the metal complexes by surface hydroxyl groups is a strategy commonly used to immobilize a metal complex on a support medium.17−21 Investigation of the hydrolysis of metal precursors may therefore shed light on the formation of a catalytically active site of the heterogeneous catalysts.3 Tungsten alkylidyne complexes are very important precursors for preparing metathesis catalysts,22−33 and efforts have been devoted to achieving a desirable supported tungsten metathesis catalyst.15,16 Because of their importance, we have developed a long-term interest in tungsten alkylidyne complexes.8,9,25,34−37 Recently, we reported the reaction of W(CSiMe3)(CH2SiMe3)3 (A-1) with H2O,9 yielding an oxo siloxide complex, W(O)(OSiMe3)(CH2SiMe3)3, and a trimer, W3O3(μO)3(CH2SiMe3)3(L)3 (L = THF), with concomitant elimination of CH4 and SiMe4. The sequential addition of H2O to © 2017 American Chemical Society

the WCSiMe3 bond and elimination of the metal-bound ligand by hydrolysis led to an increase of the W−O/W−C ratio. An analogue trimer, W3O3(μ-O)3(CH2tBu)3(L)3 (L = THF, H2O), was also found when excess H2O reacts with W( CSiMe3)(CH2tBu)3 (B-1), in which the three neosilyl groups in A-1 have been replaced by neopentyl groups. The reaction of B-1 with D2O in tetrahydrofuran (THF) was conducted to probe the details of the elementary steps, and an unstable dimeric intermediate, O[W(O)(CD2SiMe3)(CH2tBu)2]2, was detected. Subsequently, this intermediate reacts with D 2 O to form the final trimer product W 3 O 3 (μO)3(CH2tBu)3(L)3 (L = THF, D2O) with selective loss of the −CD2SiMe3 ligand, instead of a −CH2CMe3 ligand.8 The selective elimination of CD3SiMe3 indicates that the β atom of the neosilyl and neopentyl groups plays a nontrivial role in the hydrolysis. W(CtBu)(CH2tBu)3 (C-1), an all-carbon ligand analogue of A-1, is a well-studied precursor. Schrock, Lippard, and co-workers reported that a stable oxo dimer, O[W( O)(CH 2 t Bu) 3 ] 2 , similar to O[W(O)(CH 2 SiMe 3 )(CH2tBu)2]2, was obtained from the reaction of C-1 with excess H2O.1,3,12,38 In this reaction, only one −CH2tBu ligand is hydrolyzed, resulting in a product bearing three alkyl ligands per W atom. For the reactions in Scheme 1, excess H2O is Received: March 23, 2017 Published: June 5, 2017 7111

DOI: 10.1021/acs.inorgchem.7b00713 Inorg. Chem. 2017, 56, 7111−7119

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Inorganic Chemistry Scheme 1. Reactions between Tungsten Alkylidyne Complexes and H2O

the complex [NEt4][WO3(CH2tBu)], which contains only one alkyl ligand per W atom.1,12 The reactions mentioned above demonstrate that a subtle change in the ligands may lead to significant changes in the products, implying that a simple modification of the ligands on an organometallic precursor may result in desirable products.49−51 An excellent example was reported by Delevoye, Taoufik and co-workers,52,53 who found that grafting of [WO(CH2CMe3)3Cl] onto silica dehydroxylated at 200 °C can afford only the monopodal species [(SiO)WO(CH2CMe3)3]. On the basis of Xue’s work,8,9 the neosilyl derivative [WO(CH2SiMe3)3Cl] was used as the precursor, and the well-defined bipodal species [(SiO)2WO(CH2CMe3)2], which is a more stable and effective propylene metathesis precatalyst, was obtained. A systematic study of the reaction between tungsten alkylidyne complexes and H2O should provide insight into optimization of the precursor for different purposes. For these processes, many elementary steps, for example, the ligand hydrolysis, H2O addition to the WC bond, silyl migration, and dehydroxylation, are involved.9 Among these elementary steps, the initiation step is more likely to be the ligand hydrolysis or H2O addition to the WC bond (Scheme 2). Studies of these two initial reactions can provide a deeper

needed. Additional H2O was proposed to act as a proton shuttle in the reactions involving proton migration.39−43 In the isomerization reaction between [Cp*MoO(OH)2]+ and [Cp*MoO2(H2O)]+, a computational study showed that one or two H2O molecules serve as simultaneous proton donors and acceptors and dramatically lower the isomerization energy barrier.39 C-1 has also been grafted onto an oxide surface by the reaction of C-1 with a hydroxyl group on the surface.17 Similar to the reactions with H2O, the addition of the surface hydroxyl group to the WC bond and ligand protonolysis were proposed as two possible pathways for the initial grafting step. A tungsten carbene surface species, [(SiO)W(CHtBu)(CH2tBu)3], was erroneously proposed to be formed by the addition reaction of C-1 with surface silanol.44 The following study on the grafting reaction of C-1 on the SiO2‑700 and SiO2‑200 surfaces found that the reaction, in fact, proceeds through the displacement of a neopentyl group by the silanol, while the WCtBu moiety remains intact during the grafting.32 Previous computational studies of early-transition-metal alkylidyne complexes mainly focused on metathesis28,45,46 and tautomerization between alkylidyne and bis(alkylidene).31,33−36,47,48 The grafting reaction of C-1 onto the γ-alumina surface has been studied by density functional theory (DFT) calculations.17 The calculations showed that the grafting of C-1 occurs via direct electrophilic cleavage of the W−C bond by AlsOH to form the monografted surface complex [(SiO)W(CtBu)(CH2tBu)3]. The lower reactivity of C-1 is attributed to the fact that the carbyne ligand hinders the deformation of the substrate.17 W(CtBu)(OtBu)3 (D-1), in which three neopentyl groups in C-1 are replaced by tert-butyloxy groups, showed a different reactivity toward H2O. The tert-butoxide ligands are completely hydrolyzed, forming

Scheme 2. Possible Initiation Steps of the Reactions between Tungsten Alkylidyne Complexes and H2O

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Inorganic Chemistry

Figure 1. Gibbs free-energy profile for the reaction between the substrate A-1 and H2O. (For clarity, Si represents the −SiMe3 group, a represents a H2O addition pathway, b represents a ligand-hydrolysis pathway, and w represents a water-assisted mechanism.)

understanding for the first chemical contact between tungsten alkylidyne precursors and an oxide surface. A DFT study was conducted to probe the ligand hydrolysis and H2O addition to the WC bond, as shown in Scheme 2. We focused on the following critical issues: (1) the detailed mechanism, (2) the plausible role of excess H2O, and (3) the effect of the β and α atoms on the reactivity.

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Scorr = 0.65(Strans + Srot) + Svib To estimate the bulkiness of the substituents, the percent buried volume (% VBur) calculation was conducted with the SambVca 2 Web application.73 In our case, the W atom was defined as the center and a sphere of radius R centered on the W atom was built. Then the sphere was sectioned by a regular three-dimensional cubic mesh of spacing s, which defined cubic voxels v with a volume of s3. If the center of the examined voxel was within a van der Waals distance of any other atom, the volume s3 of the examined voxel was assigned to the buried volume VBur and, otherwise, to VFree.74

COMPUTATIONAL METHODS

All of the calculations were carried out with the Gaussian 09 program.54 M0655 was found to perform well in describing tungsten alkylidyne systems in our previous study.47,56 Therefore, calculations were all performed with M06 in this work. The geometry optimization and frequency calculations were conducted with the basis set BSI, in which the 6-31G(d) basis set was used for the C, H, O, and Si atoms and the Lanl2dz basis set with effective core potential (ECP)57 was employed for the W atom. Frequency analysis was conducted at the same level of theory to confirm the stationary points as minima with zero imaginary frequencies and every transition state (TS) with one single imaginary frequency. Intrinsic reaction coordinate calculations were performed to ensure that every TS is connected to the two equilibrium structures.58,59 Solvent effects were taken into account by using the SMD60 in THF (ε = 7.4257). Single-point calculations with solvent effects were performed with BSII, in which the triple-ζ basis set 6-311G(d,p)61−63 was used for the C, H, O, and Si atoms and the def2-TZVP64−66 basis set with ECP was used for the W atom.67 The basis set was found to perform well in the description of metal−carbon and metal−oxygen multiple bonds.68,69 Three basis sets, including BSII [6-311G(d,p) and def2-TZVP], BSIII [6-311++G(d,p) and def2TZVP], and BSIV (cc-pVTZ70 for the C, H, and O atoms, cc-pV(T +d)Z71 for the Si atom, and cc-pVTZ-PP72 for the W atom), were employed to examine the consistency of the basis sets. The three basis sets exhibit consistent trends (Figure S1).61−63,70−72 The benchmark calculation of our previous study showed that the relative enthalpy calculated by M06 is consistent with the experiments, while a factor of 0.65 is necessary to correct the overestimation of the translational and rotational entropy using the following equation. Gibbs free energies discussed in this work were therefore calculated with the corrected entropy Scorr.

VSphere = VFree + VBur = Σv(Free) + Σv(Buried) % VBur was defined as

% VBur = 100VBur /VSphere

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RESULTS AND DISCUSSION Detailed Mechanism of H2O Addition and Ligand Hydrolysis. The Gibbs free-energy profile of the H2O addition and ligand-hydrolysis reaction of the substrate A-1 is depicted in Figure 1. The reaction begins with formation of the endergonic adducts A-1a-H2O (7.8 kcal/mol) and A-1b-H2O (7.1 kcal/mol) for direct H2O addition and ligand hydrolysis, respectively. The activation free energy for the direct addition of H2O to the WC bond (A-1TS2a) of the substrate A-1 is 28.1 kcal/mol with respect to the separate reactants. Formation of the intermediate A-2a through A-1TS2a is exergonic by 2.8 kcal/mol. The activation free energy for a direct ligand hydrolysis (A-1TS2b) is 23.1 kcal/mol. The hydrolysis of one neosilyl group through the TS A-1TS2b to form the intermediate A-2b and one molecule of SiMe4 is highly exergonic by 20.7 kcal/mol. Both of these pathways involve a formal proton transfer from H2O to a C atom in the substrate. H2O has been well documented to be capable of promoting a proton-transfer process as a proton shuttle.39−43,75,76 Waterassisted mechanisms for both the H2O addition (A-1TS2a-w) 7113

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Figure 2. Geometry structure of the four TSs of the reaction between A-1 and H2O.

ligand-hydrolysis TS A-1TS2b-w is not preferred over the direct ligand-hydrolysis TS A-1TS2b. Natural bond orbital (NBO) analysis of the four TSs was also performed to examine the different effects of the additional H2O (Figure 3). In the TSs of H2O addition to the WC

and ligand-hydrolysis (A-1TS2b-w) reaction have also been considered (blue line in Figure 1). The activation free energy of water-assisted H2O addition (A-1TS2a-w) was 20.1 kcal/mol, 8.0 kcal/mol more favored than that of the direct H2O addition (A-1TS2a). However, the involvement of an additional H2O molecule increases the barrier associated with the ligand hydrolysis (A-1TS2b-w) to 27.0 kcal/mol, 3.9 kcal/mol higher than that of the direct ligand hydrolysis (A-1TS2b). In order to understand the opposing effect of the additional H2O on both H2O addition and ligand hydrolysis, the geometries of the four TSs of the reaction between A-1 and H2O were examined. The most stable conformation searched for each TS is depicted in Figure 2.77 These four TSs adopt a pyramidal geometry, with the WC-SiMe3 moiety occupying the apical position and the −CH2SiMe3 moiety occupying the basal position. For the two TSs (A-1TS2a and A-1TS2a-w) associated with H2O addition, the H2O molecule approaches to the WC bond from a side face of the pyramidal structure. A1TS2a is a four-membered TS with a ∠CWO angle of 80.6° and a ∠WCH angle of 74.7°. The ring strain of the fourmembered ring is considerable, and insertion of an additional H2O to form a six-membered ring TS in A-1TS2a-w enlarges the ∠CWO and ∠WCH angles to 102.6° and 96.7°, respectively, with subsequent substantial release of the ring strain. As a result, the water-assisted H2O addition (A-1TS2aw) is more favorable than the direct H2O addition (A-1TS2a). In contrast to the H2O addition reaction, the ligand-hydrolysis reaction occurs at the basal plane of the pyramidal structure. In the water-assisted ligand-hydrolysis TS (A-1TS2b-w), the presence of one additional H2O molecule at the basal plane of the pyramidal structure results in crowding, which is reflected by the bond angles. In order to accommodate the additional H2O molecule, the ∠WCSi angle in A-1TS2b-w is increased to 172.0°. The increase of the bond angle results in deformation of the TS, consequently leading to an increase of the activation energy barrier. Consequently, the water-assisted

Figure 3. Most stabilizing donor−acceptor interactions for the TSs, as indicated by NBO analysis for A-1.

bond, the π-bonding orbital of WC interacts with the orbital of the proton. When the ∠WCH angle approaches 90°, the orientation of the two orbitals achieves maximum overlap. The ∠WCH angle (96.7°) in the TS A-1TS2a-w is closer to 90° than that of A-1TS2a (∠WCH = 74.7°), implying that additional H2O enhances the orbital interaction and stabilizes the TS. In the case of the ligand-hydrolysis reaction, it is the σbonding orbital of the hydrolyzed W−C bond that interacts with the orbital of the proton. The proton tends to occupy a position between the W and C atoms, where better orbital 7114

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group is fully hydrolyzed.1 It should also be noted that formation of the intermediate 2b-w with concomitant formation of one free H−OtBu molecule is endergonic, while hydrolysis of the W−C bond of A-1 is highly exergonic. These computational findings may be useful in understanding the grafting of the D-1 complex on a support.32 Effect of Atoms at the β Position. After the detailed mechanism of H2O addition and ligand hydrolysis and the effect of α atoms were unravelled, a systematic study on the effect of atoms at the β position was conducted. The intriguing effect of a silyl group at the β position was investigated experimentally and computationally.8,9,52,53 In a tungsten alkyl alkylidyne complex, the silyl group may occupy a β position on alkyl or alkylidyne ligands. Apparently, a silyl group on the alkyl and alkylidyne moieties may affect ligand hydrolysis and H2O addition, respectively. We therefore compared three cases bearing a silyl group on (1) both the alkyl and alkylidyne ligands (A-1), (2) the alkylidyne ligand (B-1) and (3) neither an alkyl nor an alkylidyne ligand (C-1). As mentioned above, water-assisted H2O addition (1TS2a-w) is favored over a direct H2O addition mechanism (1TS2a), and direct ligand hydrolysis (1TS2b) is favored over water-assisted ligand hydrolysis (1TS2b-w) for A-1. The same preference also prevails with B-1 and C-1. Therefore, only the mechanisms of water-assisted H2O addition (1TS2a-w) and direct ligand hydrolysis (1TS2b) will be discussed here. The barriers of water-assisted H2O addition (1TS2a-w) and direct ligand hydrolysis (1TS2b) are listed in Table 1, and geometries and potential energy surfaces are shown in Figures S3−S5. We examined the different reactivities among substrates to probe the substituent effect. In the water-assisted H2O addition (B-1TS2a-w), the activation energy barrier of the substrate B-1 was 1.9 kcal/mol lower than that of the substrate C-1 (C-1TS2a-w). The direct ligand hydrolysis (A-1TS2b) activation energy barrier of the substrate A-1 was 6.0 kcal/mol lower than that of the substrate B-1 (B-1TS2b). These energy barrier differences originate from the −SiMe3 and −CMe3 substituents on Cα, which is involved in the reactions. A silyl group at the β position lowers the barrier in both cases. Charge alternation at the TSs was proposed to account for the barrier difference in a similar scenario.52 Natural charge analysis of the atoms involved in the TSs (Figure 6) indeed suggests that a Si atom at the β position may stabilize an α negative charge and a positive charge of the W atom to make a good charge alternation.78 We also noticed a significant energy barrier difference between A-1TS2a-w (20.1 kcal/mol) and B-1TS2a-w (27.6 kcal/mol), which cannot be explained by the aforementioned electronic effect. The C−C bond length (1.54 Å) in a neopentyl group is shorter than the C−Si bond (1.88 Å) in the neosilyl group. Therefore, methyl groups make the interior space more crowded in B-1TS2a-w than A-1TS2a-w (Figure S4), and as a result, the barrier of B-1TS2a-w is higher than that of A-1TS2a-w. To quantify the steric repulsion of methyl groups toward the interior space, calculations of % VBur were conducted using the SambVca 2 Web application.79 A larger % VBur value showed that more voxels in the sphere were within a van der Waals distance of the atoms in the methyl groups74 and had enhanced steric repulsion. Figure 7 shows that the buried space of methyl groups in the interior space is indeed less pronounced for the substrate A-1. An H-model, created by substituting the methyl groups in the substrate with H atoms, was employed to investigate the

overlap can be achieved. In A-1TS2b, this orbital interaction is well-represented by the ∠WCH angle (60.8°). In A-1TS2b-w, in which the ∠WCH angle is 83.7°, an additional H2O bridge pushes the proton out of the range of the W−C bond, disfavoring the orbital interaction. To retain such a critical orbital interaction for C−H bond formation, the σ-bonding orbital of the W−C bond is significantly distorted, and this deformation consequently raises the barrier of the waterassisted ligand-hydrolysis pathway via A-1TS2b-w. Both the geometrical and orbital analyses suggest that the additional H2O molecule may promote H2O addition to the WC bond while failing to facilitate ligand hydrolysis. In the reaction between W(CCMe3)(CH2CMe3)3 (C-1) and H2O in THF, if less than 2.5 equiv of H2O was used, the yield of product was reduced and C-1 remained.1 The kinetic isotope effect experiment of B-1 showed that the initial H2O addition is likely to be the rate-determining step.8 On the basis of computational results, we postulated that the use of excess H2O may promote H2O addition at the initial step.1,8,9 Effect of Atoms at the α Position. In the ligand hydrolysis of A-1, the occupied orbital interacting with the proton is the σ-bonding orbital of the W−C bond. The TS therefore has to survive deformation. If another pair of electrons can interact with the proton, that could reorientate the structure and reduce destabilization of the ring strain. We then considered replacing the CH2 moiety with an O atom at the α position. Instead of an alkyl ligand, an alkoxy ligand is in D-1, with the lone-pair electrons of the O atom interacting with the proton. NBO analysis of the ligand-hydrolysis TSs of the reaction between D-1 and H2O was also performed (Figure 4).

Figure 4. Most stabilizing donor−acceptor interactions for the TSs, as indicated by NBO analysis for D-1.

Unlike in the ligand-hydrolysis TSs of the substrate A-1, an orbital interaction between the occupied p orbital of the O atom and the unoccupied orbital of the proton was found. The ∠WOC angle in the hydrolyzed W−OtBu moiety is 135.0° and 122.0° in TSs D-1TS2b and D-1TS2b-w, respectively, implying the absence of significant distortion. As depicted in Figure 4, the ∠WOH angle in the direct ligand-hydrolysis TS D-1TS2b is 79.0°, while in the water-assisted ligand-hydrolysis TS D1TS2b-w, it is 106.2°. Apparently, the overlap in D-1TS2b-w is improved as a result of the orientation of the lone pair on the O atom. Therefore, the water-assisted ligand-hydrolysis pathway is favored for D-1. The activation energy barrier of the waterassisted ligand-hydrolysis TS D-1TS2b-w is 11.3 kcal/mol, 4.9 kcal/mol lower than that of the direct ligand-hydrolysis TS D1TS2b. The potential energy surface of the reaction between D-1 and H2O is shown in Figure 5. Unlike A-1 in Figure 1, the energy barrier of the water-assisted ligand-hydrolysis TS D1TS2b-w is 11.3 kcal/mol, 11.8 kcal/mol favored over the direct ligand hydrolysis (A-1TS2b) of A-1. This is consistent with the observation that when D-1 reacts with H2O, the alkoxy 7115

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Figure 5. Gibbs free-energy profile of the reaction between D-1 and H2O. (C represents −CMe3 for clarity.)

Table 1. Energy Barrier of the Water-Assisted H2O Addition and Ligand Hydrolysis of Different Substrates (in kcal/mol)

Figure 7. % VBur values with increasing sphere radius (R) of substrates A-1, B-1, and C-1. Figure 6. Natural charges of the atoms involved in the TSs of waterassisted H2O addition (1TS2a-w) and ligand hydrolysis (1TS2b).

Table 2. Energy Barrier of the Water-Assisted H2O Addition and Ligand Hydrolysis of an H-Model of Different Substrates (in kcal/mol)

intrinsic electronic effect of the β substituents. The barriers for the simplified H-model are listed in Table 2. The activation free-energy differences among the three substrates are within 3 kcal/mol for both pathways, suggesting that the intrinsic electronic effects are not very significant. Instead, steric effects may play a more important role in controlling the reaction pathway, a finding that may suggest new precursor designs.52,53,80,81 For instance, replacing an alkyl group with an aryl group may substantially change the steric effect and lead to a new product distribution. 7116

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CONCLUSION We have performed a DFT study on two elementary pathways of the reactions between tungsten alkylidyne complexes and H2O, H2O addition to WC, and ligand hydrolysis. Table 3 Table 3. Factors Affecting Reaction Pathways

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinhao Zhang: 0000-0002-8210-2531

a

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the National Natural Science Foundation of China (Grants 21232001 and 21302006), the MOST of China (Grant 2013CB911501), the Shenzhen STIC (Grants KQTD201103 and JCYJ20140509093817686), and the U.S. National Science Foundation (Grants CHE-1362548 and CHE-1633870 to Z.-L.X.).

a

Blue and red represent H2O addition and ligand hydrolysis; EDG stands for an electron-donating group; +, /, and − represent positive, insignificant, and negative effects, respectively.

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summarizes the computational findings on the factors that affect the reaction barriers. The geometrical and orbital analysis of TSs of the H2O addition to the WC bond (A-1TS2a and A-1TS2a-w) shows that an additional H2O molecule may act as a proton shuttle to accelerate water-assisted H2O addition to the WC bond. NBO analysis results indicate that the additional H2O bridge stabilizes the TS A-1TS2a-w by a larger overlap between the π-bonding orbital of WC and the orbital of the proton (∠WCH = 96.7°). This finding may account for the fact that excess H2O is needed in the reaction and suggests the possibility to promote hydroxyl addition to WC on the surface by providing suitable additives as a proton shuttle. Therefore, an electron-donating group at the β position can promote both H2O addition to the WC bond and ligand hydrolysis. Complexes bearing atoms with a lone pair at the α position, like D-1, undergo hydrolysis with a relatively low barrier. The lone-pair electrons of the α O atom can interact with an orbital of the proton. NBO analysis results show that this orbital overlap stabilizes D-1TS2b-w and lowers the barrier, leaving water-assisted ligand hydrolysis (D-1TS2b-w) as the most favored pathway. This is consistent with the experimental results showing that compounds bearing alkoxyl groups are completely hydrolyzed. Natural atomic charge analysis suggests that the Si atom may stabilize an α negative charge and a β positive charge to create a good charge alternation. Therefore, the electron-donating group at the β position can promote both H2O addition to the WC bond and ligand hydrolysis. In addition to the electronic effect, the steric differences between the neosilyl and neopentyl groups also play an important role. The interior crowdedness caused by the neopentyl group leads to a higher barrier of water-assisted H2O addition (1TS2a-w) for B-1 and C-1. These computational findings provide new insight and shed light on the further development of new precursors.

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REFERENCES

(1) Feinstein-Jaffe, I.; Pedersen, S. F.; Schrock, R. R. Aqueous tungsten(VI) alkyl chemistry. J. Am. Chem. Soc. 1983, 105, 7176− 7177. (2) Hillhouse, G. L.; Bercaw, J. E. Reactions of water and ammonia with bis(pentamethylcyclopentadienyl) complexes of zirconium and hafnium. J. Am. Chem. Soc. 1984, 106, 5472−5478. (3) Feinstein-Jaffe, I.; Gibson, D.; Lippard, S. J.; Schrock, R. R.; Spool, A. A molecule containing the OWOWO unit. Synthesis, structure and spectroscopy of hexaneopentylditungsten trioxide. J. Am. Chem. Soc. 1984, 106, 6305−6310. (4) Schoettel, G.; Kress, J.; Fischer, J.; Osborn, J. A. Controlled Hydrolysis of Molybdenum(VI) Alkyls and Alkylidenes - X-Ray Structure of the Molybdate-Bridged Trimetallic Complex [{Mo(NBut)(CH2But)3}2(MoO4)]. J. Chem. Soc., Chem. Commun. 1988, 914−915. (5) Yoon, M.; Tyler, D. R. Activation of water by permethyltungstenocene; Evidence for the oxidative addition of water. Chem. Commun. 1997, 639−640. (6) Chen, S.-J.; Cai, H.; Xue, Z.-L. Crystal Structure of TaCl(NMe2)4 and Its Reactions with Lithium Amides and Water. Indirect Observation of an Equilibrium among TaCl(NMe2)4, Ta(NMe2)5 and Ta2(μ-Cl)2(NMe2)6Cl2. Organometallics 2009, 28, 167−171. (7) Chen, S.-J.; Abbott, J. K. C.; Steren, C. A.; Xue, Z.-L. Synthesis, Characterization, and Crystal Structures of Metal Amide Cage Complexes Containing a M4O4 (M = Nb, Ta) Core Unit. J. Cluster Sci. 2010, 21, 325−337. (8) Dougan, B. A.; Xue, Z.-L. Unusual reaction of a tungsten alkylidyne complex with water. Formation, characterization, and crystal structures of oxo trimers. Sci. China: Chem. 2011, 54, 1903−1908. (9) Morton, L. A.; Miao, M.; Callaway, T. M.; Chen, T.; Chen, S.-J.; Tuinman, A. A.; Yu, X.; Lu, Z.; Xue, Z.-L. Reactions of d0 tungsten alkylidyne complexes with O2 or H2O. Formation of an oxo siloxy complex through unusual silyl migrations. Chem. Commun. 2013, 49, 9555−9557. (10) Sharma, B.; Callaway, T. M.; Lamb, A. C.; Steren, C. A.; Chen, S.-J.; Xue, Z.-L. Reactions of Group 4 Amide Guanidinates with Dioxygen or Water. Studies of the Formation of Oxo Products. Inorg. Chem. 2013, 52, 11409−11421. (11) Lamb, A. C.; Wang, Z.; Cook, T. M.; Sharma, B.; Chen, S.-J.; Lu, Z.; Steren, C. A.; Lin, Z.-Y.; Xue, Z.-L. Preparation of all Ncoordinated zirconium amide amidinates and studies of their reactions with dioxygen and water. Polyhedron 2016, 103, 2−14. (12) Feinstein-Jaffe, I.; Dewan, J. C.; Schrock, R. R. Preparation of anionic tungsten(VI) alkyl complexes containing oxo or sulfido ligands and the x-ray structure of [N(C2H5)4]{WO2[OC(CH3)2C(CH3)2O][CH2C(CH3)3]}. Organometallics 1985, 4, 1189−1193. (13) Lim, B. S.; Rahtu, A.; Gordon, R. G. Atomic layer deposition of transition metals. Nat. Mater. 2003, 2, 749−754.

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DOI: 10.1021/acs.inorgchem.7b00713 Inorg. Chem. 2017, 56, 7111−7119

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through the Reaction of [W(⋮CtBu)(CH2tBu)3] with Silica. Organometallics 2005, 24, 4274−4279. (33) Callens, E.; Abou-Hamad, E.; Riache, N.; Basset, J. M. Direct observation of supported W bis-methylidene from supported Wmethyl/methylidyne species. Chem. Commun. 2014, 50, 3982−3985. (34) Choi, S. H.; Lin, Z.-Y.; Xue, Z.-L. Theoretical studies of the relative stabilities of transition metal alkylidyne (CH3)2M(⋮CH)(X) and bis(alkylidene)(CH3)M(CH2)2(X) complexes. Organometallics 1999, 18, 5488−5495. (35) Morton, L. A.; Zhang, X.-H.; Wang, R.; Lin, Z.; Wu, Y.-D.; Xue, Z.-L. An Unusual Exchange between Alkylidyne Alkyl and Bis(alkylidene) Tungsten Complexes Promoted by Phosphine Coordination: Kinetic, Thermodynamic, and Theoretical Studies. J. Am. Chem. Soc. 2004, 126, 10208−10209. (36) Chen, T.; Zhang, X.-H.; Wang, C.-S.; Chen, S.-J.; Wu, Z.-Z.; Li, L.-T.; Sorasaenee, K. R.; Diminnie, J. B.; Pan, H.-J.; Guzei, I. A.; Rheingold, A. L.; Wu, Y.-D.; Xue, Z.-L. A tungsten silyl alkylidyne complex and its bis(alkylidene) tautomer. Their interconversion and an unusual silyl migration in their reaction with dioxygen. Organometallics 2005, 24, 1214−1224. (37) Morton, L. A.; Wang, R.-T.; Yu, X.-H.; Campana, C. F.; Guzei, I. A.; Yap, G. P. A.; Xue, Z.-L. Tungsten alkyl alkylidyne and bisalkylidene complexes. Preparation and kinetic and thermodynamic studies of their unusual exchanges. Organometallics 2006, 25, 427−434. (38) In addition to the tungsten oxo neopentyl product in 82% yield (the fourth equation in Scheme 1) reported in ref 12, Bino and coworkers found, through gas chromatography−mass spectrometry analysis, that the reaction of D-1 with D2O also yielded small amounts of alkyne tBuCCtBu, H2, alkene tBuCDCDtBu, and alkane tBuCD2CD2tBu. tBuOD, tBuCD3, tBuC(O)D, and tBuCD2OD were also detected.. Levy, O.; Musa, S.; Bino, A. Formation of an alkyne during degradation of metal−alkylidyne complexes. Dalton Trans. 2013, 42, 12248−12251. (39) Jee, J.-E.; Comas-Vives, A.; Dinoi, C.; Ujaque, G.; van Eldik, R.; Lledós, A.; Poli, R. Nature of Cp*MoO2+ in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations. Inorg. Chem. 2007, 46, 4103−4113. (40) Polo, V.; Schrock, R. R.; Oro, L. A. A DFT study of the role of water in the rhodium-catalyzed hydrogenation of acetone. Chem. Commun. 2016, 52, 13881−13884. (41) Shi, F.-Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Z.-X. DFT Study of the Mechanisms of In Water Au(I)-Catalyzed Tandem [3,3]-Rearrangement/Nazarov Reaction/[1,2]-Hydrogen Shift of Enynyl Acetates: A Proton-Transport Catalysis Strategy in the Water-Catalyzed [1,2]Hydrogen Shift. J. Am. Chem. Soc. 2007, 129, 15503−15512. (42) Liang, Y.; Liu, S.; Xia, Y.; Li, Y.; Yu, Z.-X. Mechanism, Regioselectivity, and the Kinetics of Phosphine-Catalyzed [3 + 2] Cycloaddition Reactions of Allenoates and Electron-Deficient Alkenes. Chem. - Eur. J. 2008, 14, 4361−4373. (43) Liang, Y.; Zhou, H.; Yu, Z.-X. Why Is Copper(I) Complex More Competent Than Dirhodium(II) Complex in Catalytic Asymmetric O−H Insertion Reactions? A Computational Study of the Metal Carbenoid O−H Insertion into Water. J. Am. Chem. Soc. 2009, 131, 17783−17785. (44) Weiss, K.; Lössel, G. Heterogeneous, Metathesis-active Schrocktype Carbene Complexes by Reaction of Carbyne Tungsten(VI) Complexes with Silica Gel. Angew. Chem., Int. Ed. Engl. 1989, 28, 62− 64. (45) Beer, S.; Brandhorst, K.; Hrib, C. G.; Wu, X.; Haberlag, B.; Grunenberg, J.; Jones, P. G.; Tamm, M. Experimental and Theoretical Investigations of Catalytic Alkyne Cross-Metathesis with Imidazolin-2iminato Tungsten Alkylidyne Complexes. Organometallics 2009, 28, 1534−1545. (46) Haberlag, B.; Wu, X.; Brandhorst, K.; Grunenberg, J.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Preparation of Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes: A New Pathway to Highly Active Alkyne Metathesis Catalysts. Chem. - Eur. J. 2010, 16, 8868−8877.

(14) Zaera, F. The surface chemistry of thin film atomic layer deposition (ALD) processes for electronic device manufacturing. J. Mater. Chem. 2008, 18, 3521−3526. (15) Popoff, N.; Mazoyer, E.; Pelletier, J.; Gauvin, R. M.; Taoufik, M. Expanding the scope of metathesis: a survey of polyfunctional, singlesite supported tungsten systems for hydrocarbon valorization. Chem. Soc. Rev. 2013, 42, 9035−9054. (16) Pelletier, J. D. A.; Basset, J. M. Catalysis by Design: WellDefined Single-Site Heterogeneous Catalysts. Acc. Chem. Res. 2016, 49, 664−677. (17) Joubert, J.; Delbecq, F.; Sautet, P.; Roux, E. L.; Taoufik, M.; Thieuleux, C.; Blanc, F.; Copéret, C.; Thivolle-Cazat, J.; Basset, J.-M. Molecular Understanding of Alumina Supported Single-Site Catalysts by a Combination of Experiment and Theory. J. Am. Chem. Soc. 2006, 128, 9157−9169. (18) Le Roux, E.; Taoufik, M.; Baudouin, A.; Copéret, C.; ThivolleCazat, J.; Basset, J.-M.; Maunders, B. M.; Sunley, G. J. Silica-AluminaSupported, Tungsten-Based Heterogeneous Alkane Metathesis Catalyst: Is it Closer to a Silica- or an Alumina-Supported System? Adv. Synth. Catal. 2007, 349, 231−237. (19) Copéret, C.; Basset, J. M. Strategies to Immobilize Well-Defined Olefin Metathesis Catalysts: Supported Homogeneous Catalysis vs. Surface Organometallic Chemistry. Adv. Synth. Catal. 2007, 349, 78− 92. (20) Conley, M. P.; Copéret, C. State of the Art and Perspectives in the “Molecular Approach” Towards Well-Defined Heterogeneous Catalysts. Top. Catal. 2014, 57, 843−851. (21) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P. A. Surface Organometallic and Coordination Chemistry toward SingleSite Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities. Chem. Rev. 2016, 116, 323−421. (22) Schrock, R. R. Multiple Metal−Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture). Angew. Chem., Int. Ed. 2006, 45, 3748−3759. (23) Zhang, W.; Moore, J. S. Alkyne Metathesis: Catalysts and Synthetic Applications. Adv. Synth. Catal. 2007, 349, 93−120. (24) Schrock, R. R.; Czekelius, C. Recent Advances in the Syntheses and Applications of Molybdenum and Tungsten Alkylidene and Alkylidyne Catalysts for the Metathesis of Alkenes and Alkynes. Adv. Synth. Catal. 2007, 349, 55−77. (25) Xue, Z.-L.; Morton, L. A. Transition metal alkylidene complexes. Pathways in their formation and tautomerization between bisalkylidenes and alkyl alkylidynes. J. Organomet. Chem. 2011, 696, 3924−3934. (26) Fürstner, A. Alkyne Metathesis on the Rise. Angew. Chem., Int. Ed. 2013, 52, 2794−2819. (27) Deraedt, C.; d’Halluin, M.; Astruc, D. Metathesis Reactions: Recent Trends and Challenges. Eur. J. Inorg. Chem. 2013, 2013, 4881− 4908. (28) Zhu, J.; Jia, G.; Lin, Z. Theoretical Investigation of Alkyne Metathesis Catalyzed by W/Mo Alkylidyne Complexes. Organometallics 2006, 25, 1812−1819. (29) Poater, A.; Solans-Monfort, X.; Clot, E.; Coperet, C.; Eisenstein, O. DFT calculations of d0 M(NR)(CHtBu)(X)(Y) (M = Mo, W; R = CPh3, 2,6-iPr-C6H3; X and Y = CH2tBu, OtBu, OSi(OtBu)3) olefin metathesis catalysts: structural, spectroscopic and electronic properties. Dalton Trans. 2006, 3077−3087. (30) Poater, A.; Solans-Monfort, X.; Clot, E.; Copéret, C.; Eisenstein, O. Understanding d0-Olefin Metathesis Catalysts: Which Metal, Which Ligands? J. Am. Chem. Soc. 2007, 129, 8207−8216. (31) VenkatRamani, S.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. A new ONO3‑ trianionic pincer ligand with intermediate flexibility and its tungsten alkylidene and alkylidyne complexes. Dalton Trans. 2015, 44, 18475−18486. (32) Le Roux, E.; Taoufik, M.; Chabanas, M.; Alcor, D.; Baudouin, A.; Copéret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Hediger, S.; Emsley, L. Well-Defined Surface Tungstenocarbyne Complexes 7118

DOI: 10.1021/acs.inorgchem.7b00713 Inorg. Chem. 2017, 56, 7111−7119

Article

Inorganic Chemistry (47) Chen, P.; Dougan, B. A.; Zhang, X.; Wu, Y.-D.; Xue, Z.-L. Reactions of a tungsten alkylidyne complex with mono-dentate phosphines: Thermodynamic and theoretical studies. Polyhedron 2013, 58, 30−38. (48) Bendjeriou-Sedjerari, A.; Sofack-Kreutzer, J.; Minenkov, Y.; Abou-Hamad, E.; Hamzaoui, B.; Werghi, B.; Anjum, D. H.; Cavallo, L.; Huang, K.-W.; Basset, J.-M. Tungsten(VI) Carbyne/Bis(carbene) Tautomerization Enabled by N-Donor SBA15 Surface Ligands: A Solid-State NMR and DFT Study. Angew. Chem., Int. Ed. 2016, 55, 11162−11166. (49) Conley, M. P.; Forrest, W. P.; Mougel, V.; Copéret, C.; Schrock, R. R. Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis Catalyst. Angew. Chem., Int. Ed. 2014, 53, 14221−14224. (50) Mougel, V.; Pucino, M.; Copéret, C. Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1-Alkene Metathesis Efficiency. Organometallics 2015, 34, 551−554. (51) Mougel, V.; Santiago, C. B.; Zhizhko, P. A.; Bess, E. N.; Varga, J.; Frater, G.; Sigman, M. S.; Copéret, C. Quantitatively Analyzing Metathesis Catalyst Activity and Structural Features in SilicaSupported Tungsten Imido−Alkylidene Complexes. J. Am. Chem. Soc. 2015, 137, 6699−6704. (52) Bouhoute, Y.; Grekov, D.; Szeto, K. C.; Merle, N.; De Mallmann, A.; Lefebvre, F.; Raffa, G.; Del Rosal, I.; Maron, L.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Accessing Realistic Models for the WO3−SiO2 Industrial Catalyst through the Design of Organometallic Precursors. ACS Catal. 2016, 6, 1−18. (53) Grekov, D.; Bouhoute, Y.; Szeto, K. C.; Merle, N.; De Mallmann, A.; Lefebvre, F.; Lucas, C.; Del Rosal, I.; Maron, L.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis. Organometallics 2016, 35, 2188−2196. (54) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2010. (55) Zhao, Y.; Truhlar, D. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (56) Hu, L.; Chen, H. Assessment of DFT Methods for Computing Activation Energies of Mo/W-Mediated Reactions. J. Chem. Theory Comput. 2015, 11, 4601−4614. (57) Metz, B.; Stoll, H.; Dolg, M. Small-core multiconfigurationDirac−Hartree−Fock-adjusted pseudopotentials for post-d main group elements: Application to PbH and PbO. J. Chem. Phys. 2000, 113, 2563−2569. (58) Fukui, K. The path of chemical reactions - the IRC approach. Acc. Chem. Res. 1981, 14, 363−368. (59) Fukui, K. Formulation of the reaction coordinate. J. Phys. Chem. 1970, 74, 4161−4163. (60) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (61) McLean, A. D.; Chandler, G. S. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z = 11−18. J. Chem. Phys. 1980, 72, 5639−5648. (62) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular-Orbital Methods 0.20. Basis Set for Correlated Wave-Functions. J. Chem. Phys. 1980, 72, 650−654. (63) Binning, R. C.; Curtiss, L. A. Compact contracted basis sets for third-row atoms: Ga−Kr. J. Comput. Chem. 1990, 11, 1206−1216. (64) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (65) Feller, D. The role of databases in support of computational chemistry calculations. J. Comput. Chem. 1996, 17, 1571−1586.

(66) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. Basis Set Exchange: A Community Database for Computational Sciences. J. Chem. Inf. Model. 2007, 47, 1045−1052. (67) The basis sets were obtained from the Gaussian Basis Set Library EMSL at https://bse.pnl.gov/bse/portal. (68) Zhang, X.; Schwarz, H. Bonding in Cationic MCH2+ (M = K− La, Hf−Rn): A Theoretical Study on Periodic Trends. Chem. - Eur. J. 2010, 16, 5882−5888. (69) Zhang, X.; Schwarz, H. Bonding in cationic MOHn+ (M = K-La, Hf-Rn; n = 0−2): DFT performances and periodic trends. Theor. Chem. Acc. 2011, 129, 389−399. (70) Dunning, T. H. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (71) Dunning, T. H.; Peterson, K. A.; Wilson, A. K. Gaussian basis sets for use in correlated molecular calculations. X. The atoms aluminum through argon revisited. J. Chem. Phys. 2001, 114, 9244− 9253. (72) Figgen, D.; Peterson, K. A.; Dolg, M.; Stoll, H. Energyconsistent pseudopotentials and correlation consistent basis sets for the 5d elements Hf−Pt. J. Chem. Phys. 2009, 130, 164108. (73) SambVca 2.0: a web application for analyzing catalytic pocket. https://www.molnac.unisa.it/OMtools/sambvca2.0/. (74) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps. Organometallics 2016, 35, 2286−2293. (75) Gu, J. D.; Leszczynski, J. A DFT study of the water-assisted intramolecular proton transfer in the tautomers of adenine. J. Phys. Chem. A 1999, 103, 2744−2750. (76) Markova, N.; Enchev, V.; Timtcheva, I. Oxo-hydroxy tautomerism of 5-fluorouracil: Water-assisted proton transfer. J. Phys. Chem. A 2005, 109, 1981−1988. (77) The structure of H2O addition TSs has a certain degree of preference. The H2O addition to the WC bond makes the C atom have double-bond characterization. The −SiMe3 group bond with WC directed downward then causes −SiMe3 in the neosilyl group trans to H2O to be directed downward (Figure S3). The five most favorable conformations of direct ligand-hydrolysis TSs have been shown in Figure S4. (78) Mo, Y.; Zhang, Y.; Gao, J. A Simple Electrostatic Model for Trisilylamine: Theoretical Examinations of the n→σ* Negative Hyperconjugation, pπ→dπ Bonding, and Stereoelectronic Interaction. J. Am. Chem. Soc. 1999, 121, 5737−5742. (79) The W atom, CH2 in the α position, and Si/C atoms in the β position were deleted during the calculation of % VBur. (80) Bouhoute, Y.; Del Rosal, I.; Szeto, K. C.; Merle, N.; Grekov, D.; De Mallmann, A.; Le Roux, E.; Delevoye, L.; Gauvin, R. M.; Maron, L.; Taoufik, M. Modification of silica-supported tungsten neosilyl oxo precatalysts: impact of substituted phenol on activity and stability in olefin metathesis. Catal. Sci. Technol. 2016, 6, 8532−8539. (81) Qureshi, Z. S.; Hamieh, A.; Barman, S.; Maity, N.; Samantaray, M. K.; Ould-Chikh, S.; Abou-hamad, E.; Falivene, L.; D’Elia, V.; Rothenberger, A.; Llorens, I.; Hazemann, J. L.; Basset, J. M. SOMCDesigned Silica Supported Tungsten Oxo Imidazolin-2-iminato Methyl Precatalyst for Olefin Metathesis Reactions. Inorg. Chem. 2017, 56, 861−871.

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