Mechanistic Investigation Into Catalytic Hydrosilylation with a High

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Mechanistic Investigation Into Catalytic Hydrosilylation with a HighValent Ruthenium(VI)−Nitrido Complex: A DFT Study Jiandi Wang, Liangfang Huang, Xiaodi Yang,* and Haiyan Wei* Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials, Jiangsu Provincial Key Laboratory for NSLSCS, College of Chemistry and Materials Science, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210046, People’s Republic of China S Supporting Information *

ABSTRACT: Density functional theory calculations with the B3LYP-D function have been performed to investigate the mechanism of carbonyl hydrosilylation reactions catalyzed by the high-valent nitridoruthenium(VI) complex [RuN(saldach)(CH3OH)]+[ClO4]− (1; saldach is the dianion of racemic N,N′-cyclohexanediylbis(salicylideneimine)). Our computational results indicate a favored ionic outer-sphere mechanistic pathway. This pathway initiates with a silane addition to the RuVI center, which proceeds through a SN2-Si transition state corresponding to the nucleophilic attack of the carbonyl on the silicon center. This attack then prompts the heterolytic cleavage of Si−H bond. The rate-determining energy of the SN2-Si transition state is calculated to be 22.9 kcal/mol with benzaldehyde. In contrast, our calculations indicate that the initial silane addition to the nitrido ligand does not represent an intermediate of the catalytic process leading to the silyl ether products, since it involves high-energy transition states (29.2 and 37.8 kcal/mol) in the reduction of carbonyls. Moreover, the computational results show that the RuIII−saldach species afforded by N−N coupling (with an activation barrier of 24.2 kcal/mol) of the nitridoruthenium(VI) complex provides a competitive hydrosilylation reaction by favoring the ionic outer-sphere mechanistic pathway, associated with a significantly small activation barrier (3.7 kcal/mol). This study provides theoretical insight into the novel properties of the highvalent transition-metal RuVI−nitrido catalyst in catalytic reduction reactions.

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Scheme 1. Reactions of RuVI−Nitrido Species [RuN(saldach)(CH3OH)]+[ClO4]− (1) with Various Nucleophiles

INTRODUCTION

Transition-metal nitrido complexes (LnMN) are of significance for their vital role in the process of nitrogen fixation.1 Recent work has reported that a wide variety of late-transitionmetal nitrido complexes (M = Mo,2 W,3 Os,4 Ir,5 Rh,6 Ru7) are involved in various organic transformations, reactions with nucleophiles,8,4a−c activation of C−H bonds,9 N−N coupling,10 catalysis of the oxidation of organic compounds,11 and mediation of nitrogen atom transfer reactions.12,4c,8c Recently, Lau and co-workers have reported the RuVI−nitrido complex [RuN(saldach)(CH3OH)]+[ClO4]− (1; saldach is the dianion of racemic N,N′-cyclohexanediylbis(salicylideneimine)), which readily undergoes a variety of redox reactions with nucleophiles to produce novel ruthenium species.13 As shown in Scheme 1, at room temperature the RuVIN species 1 undergoes a direct nitrogen atom transfer, producing alkenes and (salen) ruthenium aziridine complexes.14 Species 1 also reacts with thiols (RSH) to generate the sulfilamido species Ru−N(H)SR, sulfilamine Ru−N(H)2SR, and metal amine Ru−NH3.15 In addition, species 1 rapidly reacts with secondary amines (morpholine, HNOC4H8) to produce a ruthenium(IV)− hydrazido species.16 © XXXX American Chemical Society

Recently, Du and co-workers have demonstrated that the high-valent RuVIN species 1 is an effective catalyst for the reduction of ketones and aldehydes with silanes to give alcohols in high yields.17 These catalytic reduction reactions mediated by high-valent transition-metal (rhenium(V)/molybdenum(VI)/ruthenium(VII)) complexes are surprising, since highReceived: October 24, 2014

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DOI: 10.1021/om501071n Organometallics XXXX, XXX, XXX−XXX

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Organometallics

theoretical study on elucidation of the mechanism of hydrosilylation reactions catalyzed by the high-valent RuVI N species [RuN(saldach)(CH3OH)]+[ClO4]− (1). Our goals are to clarify the mechanism of RuVIN species 1 catalyzing the hydrosilylation of carbonyls and to show the similarities and/or differences in the reactivity between RuVIN species in catalyzing the oxidation and reduction of organic compounds. Our results illustrate that the activation of silane at the RuVI center provides a favorable path for reducing carbonyl compounds. However, the path involving the addition of organic substrates to the nitrido ligand is calculated to be associated with high activation barriers. Furthermore, we determined that the low-valent RuIII species afforded by the N−N coupling of the RuVIN species 1 is competing in hydrosilylation reactions of carbonyls.

valent transition-metal complexes bear terminal multiply bonded oxo or imido/nitrido groups and have traditionally been documented as catalysts in oxidation and oxygen transfer reactions18−20 rather than as reducing agents for organic substrates. The first example of the described reduction process involves the hydrosilylation of carbonyl catalyzed by the highvalent rhenium(V)−dioxo complex ReO2I(PPh3)2, which was reported by Toste and co-workers.21 To account for the hydrosilylation of carbonyl compounds by ReO2I(PPh3)2, Toste et al. proposed an unconventional [2 + 2] addition mechanism.22 The key step in the activation of silanes is the addition of the Si−H bond across the ReO bond to form a siloxy rhenium hydride. This process is followed by carbonyl coordination to the rhenium center followed by its migration into to the rhenium−hydride bond and silyl ether elimination (Scheme 2). Along the [2 + 2] addition mechanism, the Scheme 2. Proposed [2 + 2] Addition Mechanism for Hydrosilylation of Carbonyl Compounds by the Dioxorhenium Complex ReO2I(PPh3)2 (Left) and the Ionic Mechamism Involving a η2-Silane Adduct for Hydrosilylation of Carbonyl Compounds by the Monooxorhenium Complex Re(O)Cl3(PPh3)2 (Right)

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COMPUTATIONAL METHODS

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RESULTS AND DISCUSSION

All molecular geometries of the model complexes were optimized at the DFT Becke3LYP (B3LYP) level,31 which was implemented in Gaussian 09.32 The effective core potentials (ECPs) of Hay and Wadt with double-ζ valence basis sets (LanL2DZ)33 were used to describe the Ru metals. In addition, polarization functions were added for Ru (ζf = 1.235).34 The 6-311g(d,p) basis set was used for all other atoms, such as C, H, O, and N. All geometric optimizations were performed under solvent conditions using the SMD solvation model with CH2Cl2 as the solvent using tight convergence criteria (the SMD model is an IEFPCM calculation with radii and nonelectrostatic terms for Truhlar and co-workers’ SMD solvation model35). Frequency calculations at the same level of theory were carried out to verify all stationary points as minima (zero imaginary frequency) and transition states (one imaginary frequency). The transition states found were further confirmed by calculating intrinsic reaction coordinate routes toward the corresponding minima and reoptimizing from the final phase of IRC paths to reach each minimum. The final Gibbs energy values for ΔG reported in the current study were relative Gibbs free energies calculated at 298 K in solvent, with corrections for dispersion effects using Grimme’s empirical dispersion correction term: i.e., B3LYPD3.36 Detailed comparisons of different methods are given in the Supporting Information. All of the geometries are displayed using CYLview software.37 In the calculations, the catalyst [RuN(saldach)]+ (the benzene ring on the racemic N,N′-cyclohexanediylbis(salicylideneimine) is reduced), trimethylsilane (HSiMe3), and benzaldehyde were adopted to study all the possible catalytic steps. Moreover, for compounds that have multiple conformations, several different starting geometries were set to optimize the structures and compared to locate the lowestenergy conformation.

multiple oxo ligands play an important role in assisting Si−H bond activation. A series of high-oxidation-state transitionmetal complexes (MoO 2 Cl 2 , CpMoO 2 Cl, Re(O/Nar)Cl 3(PPh3) 2, ReCH3 O3, Re 2O7 , HReO 4, [RuN(saldach)(MeOH)]+, etc.) have also been reported as being very efficient reducers of a series of functional groups (carbonyl groups, imines, esters, sulfoxides, pyridine N-oxides, nitriles, amides, alkynes, etc.).23−28 Likewise, the activation of H−H or Si−H and B−H bonds by the high-valent dioxo molybdenum complex MoO2Cl2 is explained by the [2 + 2] addition mechanism.25a−c,28a−e Furthermore, other mechanistic scenarios have been proposed to further the investigation into the catalytic reduction reactivity of high-valent transition-metal complexes. One example includes the monooxorhenium(V) complex Re(O)Cl3(PPh3)2, which catalyzes carbonyl hydrosilylation. Abu-Omar et al. have proposed a reacton pathway that proceeds via a η2-silane metal adduct followed by heterolytic cleavage of the Si−H bond at the electrophilic rhenium(V) center (Scheme 2, ionic mechanism).29 Nikonov et al. have also suggested a mechanism for imidomolybdenum catalysts, in which the metal center simply activates the carbonyl as a Lewis acid.30 Although extensive work has been performed in the field to develop new high-valent transition-metal catalysts for reduction reactions, a fundamental molecular-level understanding of catalytic hydrosilylation, hydrogenation, and hydroboration reactions by high-valent transition-metal complexes is still lacking. More importantly, the roles of multiply bonded terminal ligands in silane activation are not well understood and remain unclear. In this work, we report a detailed

There are three possible pathways for the hydrosilylation of carbonyls catalyzed by the high-valent RuVIN species 1. The first potential menchanism, path A, was proposed by Toste et al.22 and involves a [2 + 2] addition, where the Si−H bond adds across the RuN bond in a [2 + 2] manner (Scheme 2). The second potential mechanism, path B, shown in Scheme 3, involves the direct addition of the Si−H bond to the nitrido ligand, which then acts as the active species to reduce the carbonyls. The third mechanism, path C, is an ionic outersphere mechanism which involves the initial binding of silanes through hydrogen to the RuVI center, followed by the nucleophilic attack of carbonyls on the silicon atom to cleave the Si−H bond (Scheme 3). Furthermore, we investigate the viability of the low-valent RuIII species afforded by the N−N coupling of RuVIN species 1 as an activated catalyst for the reduction of carbonyls. In the subsequent sections, the catalytic B


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calculated free energy surface for path B involving the nucleophilic addition of silanes at the nitrido ligand is displayed in Figure 3.

Scheme 3. Proposed Mechanism for the Carbonyl Hydrosilylation Catalyzed by the High-Valent RuVIN Species (1)

cycles for the hydrosilylation of carbonyls by the high-valent RuVI−nitrido complex 1 are discussed. Afterward, the reaction for the low-valent RuIII species as a catalyst for hydrosilylation is investigated. Path A. [2 + 2] Addition Mechanism. In the RuVIN species 1, the Si−H bond could add across the RuN bond in a [2 + 2] manner. This mechanism is calculated to be very difficult, with an significantly high activation barrier of 37.8 kcal/mol. The corresponding transition state TS[2 + 2] (Figure 1) presents a four-membered cyclic (Ru···O···Si···H) structure, Figure 2. Calculated geometric structures along path B that involves the direct addition of silane to the nitrido ligand for the hydrosilylation of benzaldehyde catalyzed b the RuVI−nitrido species 1. The bond distances are shown in Å.

Figure 1. Optimized structure of the [2 + 2] addition transition state for RuVI−nitrido species 1 activating the silane.

in which the Si−H bond is partially broken (2.14 Å). The silane hydrogen is abstracted by the ruthenium center to form a Ru− H bond (1.65 Å) and the silyl binds with the nitrido ligand to generate a Si−N bond (2.22 Å). In addition, the generation of the [2 + 2] addition intermediate is endergonic (ΔG =13.8 kcal/mol). Thus, due to the high activation barrier and endergonic nature of this mechanism, we have ruled it as being a very unlikely pathway for carbonyl hydrosilylation via the high-valent RuVIN species 1. Path B. Reaction Pathway via Addition of the Si−H Bond to the Nitrido Ligand. Previous experimental and theoretical studies on the transformation of organic compounds with transition-metal−nitrido complexes (LnMN) have revealed an activation manner involving the direct addition of organic nucleophiles to the nitrido ligand.38,8b,13−16 Encouraged by these previous works, we computed the hydrosilylation reaction pathway with the activation of a Si−H bond at the nitrido ligand of RuVI−nitrido species 1 (path B, as shown in Scheme 3). The optimized structures of the intermediates and the transition states for path B are shown in Figure 2. The

Figure 3. Free energy profile of path B that involves the direct addition of silane to the nitrido ligand for the hydrosilylation of benzaldehyde catalyzed by RuVI−nitrido species 1.

At the beginning of path B, the RuVI−nitrido species 1 forms a close contact intermediate (2-N, Figure 2), where the silane binds at the nitrido ligand. The bond distance between the nitrido nitrogen and the silane hydrogen atom is 1.17 Å. The Si−H distance is elognated to 1.80 Å, which is about 0.36 Å longer than that for a free silane (1.54 Å). The Ru−N distance is elognated to 1.73 Å, which is about 0.13 Å longer than that in 1. The silane−nitrido intermediate 2-N easily undergoes the [1,2]-migration of silyl to the electrophilic nitrido ligand. This [1,2]-migration reaction resembles the oxidative addition of the C


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Figure 4. Calculated geometric structures along path C that involves the heterolytic cleavage of the Si−H bond at the ruthenium center for the hydrosilylation of benzaldehyde catalyzed by RuVI−nitrido species 1. The bond distances are shown in Å.

[RuNH(saldach)]. In this transition structure, shown in Figure 2, the N(imido)−H bond is breaking (1.27 Å) and the new carbon−hydrogen bond is simultaneously forming (1.37 Å). The results of our computations show that the silyl amido ruthenium complex 3-N generated by the addition of silane to the nitrido ligand is a rather stable species, having an energy of 40.6 kcal/mol below the reactant. However, due to the two high-energy transition states, i.e., TS4-N (29.2 kcal/mol above 3-N) and TS5-N (37.8 kcal/mol above 3-N), path B is not a likely pathway for the hydrosilylation of benzaldehyde catalyzed by RuVI−nitrido species 1 leading to the silyl ether product. The remarkable stability of the silyl amido ruthenium complex 3-N also explains why path B is disfavored. Path C. Reaction Pathway via Addition of the Si−H Bond to the RuVI Center. The possibility of an ionic outersphere mechanistic pathway was considered because the RuVI− nitrido species 1 is five-coordinated (with the initial dissociation of solvent molecule), and we hypothesized that the RuVI center could participate in silane activation. The ionic outer-sphere mechanism has been reported by Nikonov et al. to account for the hydrosilylation of acetone by the catalyst [CpRu(Me3P)(MeCN)2]+.44 The ionic outer-sphere mechanism, as shown in Scheme 3, involves the initial binding of silanes at the metal center followed by the heterolytic cleavage of the Si−H bond upon nucleophilic attack of the carbonyl. Moveover, it has been suggested that the ionic mechanistic pathway accounts for the catalytic reduction of carbonyls using tris(pentafluorophenyl)borane/silane systems.45 Also, the reduction of aldehyde/ ketone, epoxides, amides, and esters and the reduction of alkyl halides by cationic iridium catalyst/silane systems have been suggested to invoke an ionic outer-sphere mechanistic pathway.46 In Figure 4, the optimized structures of the intermediates and transition states along the ionic outer-sphere pathway (path C) for hydrosilylation of benzaldehyde catalyzed by RuVI−nitrido species 1 are shown. In Figure 5, the calculated free energy surface for path C is displayed. Initially, the addition of silane to the RuVI center yields the end-on fashion intermediate 2-Ru. The structure of the silane− ruthenium intermediate 2-Ru (Figure 4) displays an octahedral coordination of the saldach ligands around the metal center with a cis arrangement of the silane. In other words, the saldach ligand is in a cis configuration, with two N atoms and one O atom lying on the equatorial plane and the other O atom occupying the apical position. The silane is binding to the ruthenium center at the equatorial plane of the octahedral structure, lying cis to the nitrido ligand. The Si−H (1.54 Å) distance is elongated by 0.06 Å in comparison to that in a free silane molecule (1.49 Å). The Ru···H distance is 2.00 Å. The

Si−H bond to a metal center, in which the Si−H bond is cleaved and both the H and SiMe3 moieties are added to the metal.39 However, in this case, the reactions occur at the ligand, where both the H and SiMe3 moieties are added to the nitrido ligand, and no changes are made to the atoms bound to the metal center. Although the silyl amido ruthenium complex has not been experimentally identified, it is worth noting that a similar reaction has previously been observed for a squareplanar pyridine−diimine iridium complex with a terminal nitrido ligand.40 In addition, the silyl amido iridium complex has been determined by X-ray crystallography. Similar observations have also been reported for the arylboron addition to the nitrido ligand in the osmium(VI) nitrido complex TpOs(N)Cl2 (1).41 As a consequence, in the optimized structure of the silyl amido ruthenium complex 3-N (Figure 2), the silane Si−H bond is cleaved and the SiMe3 moiety is bound to the nitrido ligand (Si−N = 1.82 Å). The Ru−N distance is elongated to 1.84 Å. The calculation results show that the binding energy of the silane to the nitrido ligand in 2-N is 16.4 kcal/mol above the reactants. Nevertheless, the formed silyl amido ruthenium intermediate 3-N is a rather stable species and is 40.6 kcal/mol below the reactants. Therefore, the silane addition to the nitrido ligand is exergonic and has a relatively low activation barrier.42 For the catalytic reduction of carbonyls to occur, the silyl amido ruthenium complex 3-N must react with a carbonyl molecule (i.e., benzaldehyde). The obvious path for this reaction involves the interaction of the lone pair of electrons on the benzaldehyde oxygen with the silyl moiety, breaking the Si−N bond. This process occurs via the transition state TS4-N. In the optimized geometric structure of TS4-N, shown in Figure 2, the key entities (oxygen, silicon, and nitrogen atoms) are confined to a linear conformation. The breaking Si−N bond is stretched to 3.05 Å along the reaction coordinate, adding to the oxygen atom of benzaldehyde (Si−O = 1.86 Å). An IRC procedure indicates that the nucleophilic attack of benzaldeyde to the silicon atom (TS4-N) leads to the generation of the ruthenium imido complex [RuNH(saldach)] and a silylcarbenium ion. The ΔG and ΔG⧧ values are 32.9 and 29.2 kcal/mol, respectively, indicating that this step is difficult.43 After the reaction of the silyl amido ruthenium complex 3-N with benzaldehyde is complete, it is necessary to introduce a hydrogen into the activated silylcarbenium ion to achieve the functionalization steps. One obvious choice for this process would be transferring the N-bonded H atom from the ruthenium imido complex [RuNH(saldach)] to form a silyl ether product. The corresponding transition state, TS5-N, is reached by overcoming a barrier of 4.9 kcal/mol relative to D


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hydride (1Hcis), which occurs in two further separate steps. First, the ruthenium hydride (1Hcis) and silylcarbenium ion [SiMe3OCHPh]+ moieties are connected for the hydride transfer process to occur. We have calculated the intermediate 4-Ru, shown in Figure 4, in which the Si−O bond of the silylcarbenum ion resides in a “syn-side” position relative to the Ru−H bond, with a large separation of 4.62 Å (Si···H). The intermediate 4-Ru is 3.0 kcal/mol more favorable than the separation of the two moieties. Subsequently, the silylcarbenium ion abstracts the hydride on 1Hcis, yielding a silyl ether and regenerating the RuVI−nitrido species 1. The transition structure involved in this step, TS5-Ru, exhibits a clear lengthening of the Ru−H bond (1.68 Å) and a shortening of the C−H bond (1.75 Å). The calculations indicate that this hydride transfer process occurs easily because of the small activation free energy of 3.0 kcal/mol and high exergonicity of 22.7 kcal/mol.48 A comparison of the free energy changes between the catalytic cycles via path B (Figure 3) and path C (Figure 5) exhibits interesting differences. Our calculations indicate that similar amounts of activation free energies are observed for the silane addition to the nitrido ligand and to the ruthenium center: 16.4 kcal/mol for 2-N and 16.7 kcal/mol for TS2-Ru. However, the formation of the silyl amido ruthenium complex 3-N is calculated to be much more stabilized from a thermodynamic viewpoint (40.6 kcal/mol below the reactants). As a result, it is necessary to overcome a significantly large energy barrier (29.2 kcal/mol) for the amido ruthenium complex 3-N to reduce the carbonyls. However, a relatively small energy barrier (TS3-Ru, 22.9 kcal/mol) is calculated for the η1-silane ruthenium intermediate 2-Ru to reduce the carbonyls. According to these results, our computations indicate that the favored path for the hydrosilylation of benzladehyde catalyzed by RuVI−nitrido species 1 would be path Can ionic outer-sphere mechanism occurring at the ruthenium center. Path D. Hydrosilylation Reaction Pathway Catalyzed by the RuIII−saldach Complex. It is worth mentioning that, experimentally, the transition-metal nitrido complex could easily undergo intermolecular N−N coupling. Our results show a barrier height of 24.2 kcal/mol for the dimerization of RuVI− nitrido species 1 (see the Supporting Information for the structures). In addition, generation of the RuIII−saldach complex (1-RuIII) is more stable than the RuVI−nitrido species 1, by ΔG = −50.6 kcal/mol. Furthremore, the low-valent Ru species has undergone extensive experimental studies as a hydrosilylation/hydrogenation catalyst.49,50 Therefore, the lower activation energy barrier for the N−N coupling and the larger stabilization of the RuIII−saldach species prompted us to investigate the potential involvent of the complex 1-RuIII in carbonyl hydrosilylation reactions. It is likely that the ionic outer-sphere mechanistic pathway occurring at the ruthenium(VI) center can be applied to hydrosilylation reactions with RuIII−saldach species. As described previously (Scheme 3 and Figure 5), the ionic outer-sphere pathway has three basic steps: (1) addition of silane to the metal center, (2) cleavage of the Si−H bond upon the nucleophilic attack of the carbonyls, and (3) hydride transfer to the carbon atom of the activated carbonyl from the silyl ether. Here, the structures of the intermediates and transition states along the ionic outer-sphere pathway with hydrosilylation catalyzed by RuIII−saldach complex 1-RuIII are

Figure 5. Free energy profile of path C that involves the direct addition of the Si−H bond to the ruthenium center for the hydrosilylation of benzaldehyde catalyzed by RuVI−nitrido species 1.

Ru···Si distance is significantly long at 3.49 Å, and the Ru−H− Si angle is 157.9°. These results show that the coordination of silane in 2-Ru is in a weak, end-on η1-H(Si) mode. The cis silane−ruthenium intermediate 2-Ru is 17.3 kcal/mol less stable than catalyst 1 and silane. The cis addition of silane to the ruthenium center (TS2-Ru; shown in the Supporting Information) is calculated to require an activation free energy of 16.7 kcal/mol above 1 and free silane.47 Alternatively, it is obvious that the silane can bind at the metal center in a trans end-on fashion, in which the silane hydrogen is attached to the ruthenium atom at the axial plane of the octahedral structure, trans to the nitrido ligand. However, our calculations show that the trans conformation of the silane−ruthenium complex involves a significant increase in the activation free energy barrier for reducing the carbonyl compounds along path C (shown in the Supporting Information). The next step along the ionic outer-sphere pathway (path C) is the nucleophilic attack of the benzaldehyde molecule on the silicon atom in the η1-silane ruthenium intermediate 2-Ru, which occurs through the transition state TS3-Ru. In the optimized structure of TS3-Ru (Figure 4), the carbonyl oxygen approaches the silicon center and simultaneously the silane hydride forms a bond with the ruthenium center, where Ru···H is 1.77 Å and Si···O is 2.50 Å (Figure 4). As a result, the Si···H bond is elongated to 1.67 Å (in comparison to 1.54 Å in 2-Ru). The rather small increase in the Si−H distance (0.13 Å) reflects the very early nature of this TS. The structure of the transition state TS3-Ru can be described as an SN2-Si reaction structure with a trigonal-bipyramidal configuration at the silicon center, where the silyl group (SiMe3) acts as the plane and the leaving hydrogen atom and the incoming oxygen (benzaldehyde) atom occupy the apical positions, trans to each other. The calculated vibrational motion of the imaginary frequency associated with TS3-Ru features the simultaneous cleavage of the Si−H bond together with the formation of Ru−H and Si−O bonds. The nucleophilic attack of the benzaldeyde on the η1-silane ruthenium complex gives rise to a cis ruthenium hydride (1Hcis) and the silylcarbenium ion [SiMe3OCHPh]+. Our calculations estimate a free energy barrier of 22.9 kcal/mol associated with TS3-Ru in CH2Cl2 solvent and an exergonic process (−9.6 kcal/mol). Closure of the catalytic cycle of path C requires C−H bond formation between the silylcarbenium ion and the ruthenium E


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Therefore, we could expect the η2-H(Si) mode silane− ruthenium(III) intermediate to be isolated experimentally, but further experimental verification is required. The subsequent step is the lone pair of a benzaldehyde oxygen attacking the silicon center to prompt the heterolytic cleavage of the Si−H bond. Initially, we obtained the distinct adduct 3-RuIII, in which the silane−ruthenium(III) intermediate forms a weak van der Waals type complex with benzaldehyde (Figure 6); it can best be described as a σ complex. The closest contact between the silicon atom and the benzaldehyde O atom is 3.35 Å. Also, the geometry of the silane−ruthenium(III) moiety is essentially unperturbed relative to the free fragment. The relative free energy of the van der Waals complex 3-RuIII is 3.7 kcal/mol higher than that of the silane−ruthenium(III) complex 2-RuIII. To proceed, we performed partial geometry optimizations on the potential energy surface while fixing the H−Si or Si−O bond distance at successively longer distances until the optimized structure featured a large separation of the Si−H bond, forming a silylcarbenium ion and a ruthenium(III)−hydride. From these experiments, a monotonously increasing potential energy profile was obtained (see the Supporting Information). Therefore, within the expected accuracy of our computational method, we cannot predict for certain whether or not there is a barrier for the Si−H bond cleavage process, but if one exists, it would clearly be very small. In addition, this phenomenon is similar to that in the recent computational study of carbonyl hydrosilylation catalyzed by a rhenium(V)−oxo complex reported in our previsous work.53 Furthermore, the overall transformation from the Ru III−saldach species 1-RuIII , benzaldehyde, and trimethylsilane to a silylcarbenium ion and the ruthenium(III)−hydride 1-RuIIIH is exergonic by ΔG = −12.6 kcal/mol.

shown in Figure 6 and the reaction free energy profile is shown in Figure 7.51

Figure 6. Calculated geometric structures for hydrosilylation of benzaldehyde catalyzed by RuIII-saldach complex 1-RuIII involving silane addition at the RuIII center. The bond distances are shown in Å.

1‐Ru III + HSiMe3 + PhCHO → 1‐Ru IIIH + SiMe3OCHPh+

(1)

Following the Si−H activation to form the carbenium ion, the next necessary step is to introduce a hydride into the carbon of the carbenium ion to acquire functionalization steps. One obvious choice for achieving this would be to substitute a hydride with the RuIII−hydride complex 1-RuIIIH. Our results show that the barrier for the hydride transfer is very small, as indicated by the transition state TS5-RuIII, which has a very low activation free energy of 1.6 kcal/mol.54 In the optimized structure of TS5-RuIII, the hydride leaves the ruthenium(III) center (RuIII−H = 1.64 Å) and reaches the carbon atom (H−C = 1.47 Å).55 Overall, the catalytic cycle for the hydrosilylation of benzaldehyde by the RuIII−saldach species 1-RuIII, shown in Figure 7, is exergonic (ΔG = −10.9 kcal/mol). The highest point along the transformation involves the attack of the silane−ruthenium(III) complex by benzaldehyde, forming the van der Waals type complex 3-RuIII, which is 3.7 kcal/mol higher than the η2-H(Si) mode silane−ruthenium(III) 2-RuIII. Therefore, the hydrosilylation of carbonyls catalyzed by the low-valent RuIII complex is a kinetically and thermodynamically favored reaction. These findings suggest that a full hydrosilylation catalytic cycle can easily be constructed by using the RuIII−saldach complex. Therefore, on comparison of the hydrosilylation reaction pathways examined (path C, associated with RuVI−nitrido species 1 catalyzing the hydrosilylation via an ionic outer-

Figure 7. Free energy profile for hydrosilylation of benzaldehyde catalyzed by RuIII-saldach complex 1-RuIII involving silane addition at the RuIII center.

Initially, the coordination of a silane molecule to the ruthenium(III) center in 1-RuIII readily yielded [Ru(saldach)](HSiMe3)+ (2-RuIII). In the optimized structure of 2-RuIII (Figure 6), the silane is coordinated on the axial position, with the Si−H distance elongated to 1.66 Å, which is 0.17 Å longer than that in a free silane molecule (1.49 Å). The RuIII−H distance is 1.61 Å, featuring a normal RuIII−H bond. The RuIII···Si distance is 2.92 Å, and the RuIII−H−Si angle is 126.3°.52 The geometry of 2-RuIII obligates the coordination of silane in 2-RuIII to be in a η2-H(Si) mode. Also, the addition of silane to the ruthenium(III) center in 1-RuIII results in a stabilization energy of 7.8 kcal/mol in CH2Cl2 solvent. F


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Organometallics

vital to note that different DFT exchange and correlation functionals and basis sets (see discussion in the Supporting Information) give similar results. However, other possibilities for alternative reaction mechanisms have also been explored. For example, a reaction pathway that entails prior coordination of the carbonyl to the nitrido ligand (Supporting Information) can be excluded due to the associated large destabilization energy, 37.1 kcal/mol. In summary, we believe that this theoretical prediction of a full catalytic cycle by the ruthenium(VI)−nitrido complex is useful to understand the high-valent transition-metal complexes in catalyzing the reduction reaction. In addition, our results demonstrate the possible existence of an η2-silane RuIII intermediate and provide insights into the catalytically important Si−H bond cleavage step.

sphere mechanistic pathway, and the other path, associated with RuIII−saldach species 1-RuIII catalyzing the hydrosilylation via an ionic outer-sphere mechanistic pathway), our calculation results indicate that the two pathways involve almost isoenergetic barriers. The energy barrier of the rate-determining step, 22.9 kcal/mol (TS3-Ru), calculated for the ionic outersphere mechanistic pathway by RuVI−nitrido species 1, is slightly lower than 24.2 kcal/mol (TS(N···N)) calculated for the intermolecular N−N coupling of the RuVI−nitrido species to generate low-valent RuIII species. Furthermore, the significantly small activation barrier (3.7 kcal/mol) is calculated for the hydrosilylation reaction catalyzed by the low-valent RuIII complex 1-RuIII. Therefore, our results show the likelihood that multiple reaction pathways are accessible for hydrosilylation of the carbonyl compounds catalyzed b the RuVI−nitrido species 1. Also, the low-valent RuIII species 1-RuIII is competitive in the hydrosilylation of carbonyls with the high-valent RuVI−nitrido complex 1.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Text, figures, tables, and an xyz file, giving the complete ref 32, a comparison of different basis setd and methods (B3LYP, B3LYP-D and M06), and the Cartesian coordinates of all optimized structures discussed in the paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS The hydrosilylation of carbonyls catalyzed by the high-valent RuVI−nitrido complex 1 has been studied computationally at the B3LYP-D level. Initially, the activation of silanes was known to occur at the nitrido ligand and at the ruthenium(V) center. The two energy barriers for the addition of silane are very similar: that for 2-N is 16.4 kcal/mol, and that for TS2-Ru is 16.7 kcal/mol. However, the resulting intermediates, the silyl amino ruthenium complex 3-N and the silane−ruthenium complex 2-Ru, differ considerably in the reduction of the carbonyl compounds. Our calculations show that the addition of silane at the nitrido ligand gives rise to the silyl amido ruthenium complex 3-N, which is significantly stabilized, located 40.6 kcal/mol below the separated starting catalyst 1 and the silane. Also, the reduction of carbonyls with the silyl amido ruthenium complex 3-N is difficult, as it is associated with two higher transition states (29.2 kcal/mol, TS4-N; 37.8 kcal/mol, TS5-N); hence, it is not easy to construct a full catalytic cycle for the hydrosilylation of carbonyls with the silane activation at the nitrido ligand of RuVI−nitrido complex 1. Alternatively, theoretical calculations show that the catalytic reduction of carbonyls occurs facily with the η1-silane ruthenium intermediate 2-Ru. The highest activation free energy is 22.9 kcal/mol, corresponding to a SN2-Si transition state in which the carbonyl nucleophilically attacks the silicon center. As a consequence, our calculations indicate that an ionic outer-sphere mechanistic pathway (path C) is preferable for the hydrosilylation of carbonyls catalyzed by the high-valent RuVI− nitrido complex 1. Furthermore, the multiply bonded nitrido ligand does not play a role in the catalytic hydrosilylation of carbonyls along the ionic outer-sphere mechanistic pathway (path C). In addition, due to the low activation energy for N− N coupling of the RuVI−nitrido complex 1, the generation of the RuIII−saldach species 1-RuIII provides a competitive hydrosilylation reaction via the ionic outer-sphere mechanistic pathway. The hydrosilylation of carbonyls by the high-valent RuVI− nitrido complex 1 shows a significant difference from previous studies on the reactivity of various oxidants. It is well-known from the literature that oxidation reactions rely on the nucleophilic attack of organic substates on the nitrido ligand. However, in the catalytic reduction reaction presented here, the hydrosilylation occurs at the ruthenium(VI) center through the formation of a η1-end-on silane−ruthenium(VI) complex. It is

Corresponding Authors

*E-mail for H.W.: [email protected]. *E-mail for X.Y.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (No. 21103093), the Ministry of Water Resources (No. 201201018), and the Jiangsu Province Science and Technology Natural Science Project (BK2011780), a Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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2012, 134, 640. (d) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem. Soc. 2010, 132, 4534. (47) Without inclusion of the dispersion effect, the transition state TS2-Ru is calcuated to be 0.39 higher than that of the cis silane− ruthenium intermediate 2-Ru. (48) For the silylcarbonium ion abstraction of hydrogen from free silane is endergonic by 12.5 kcal/mol: SiMe3OCHPh+ + HSiMe3 → SiMe3OCH2Ph + SiMe3+. Therefore here, for the most available hydride source, we conclude that free silanes could not be the hydride donor in this catalytic process. (49) (a) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724. (b) Sandoval, C. A.; Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Noyori, R. Chem. Asian J. 2006, 1, 102. (c) Ohkuma, T.; Tsutsumi, K.; Utsumi, N.; Arai, N.; Noyori, R.; Murata, K. Org. Lett. 2007, 9, 255. (50) (a) Zhou, H. F.; Li, Z. W.; Wang, Z. J.; Wang, T. L.; Xu, L. J.; He, Y. M.; Fan, Q. H.; Pan, J.; Gu, L. Q.; Chan, A. S. C. Angew. Chem., Int. Ed. 2008, 47, 8464. (b) Wang, T. L.; Zhuo, L. G.; Li, Z. W.; Chen, F.; Ding, Z. Y.; He, Y. M.; Fan, Q. H.; Xiang, J. F.; Yu, Z. X.; Chan, A. S. C. J. Am. Chem. Soc. 2011, 133, 9878. (51) In Figure 7, the free energy of the ruthenium(III)−hydride 1RuIIIH was set to be 0. (52) (a) Lin, Z. Chem, Soc. Rev. 2002, 31, 239. (b) As indicated by Lin, in most of the known η2-silane complexes, the H−Si distances are normally in the range 1.6−1.9 Å.. (53) Gu, P.; Wang, W. W.; Wang, Y. O.; Wei, H. Y. Organometallics 2013, 32, 47. (54) Without inclusion of the dispersion correction, the transition state TS5-RuIII is 11.5 kcal/mol higher than the silylcarbenium ion and the ruthenium(III)−hydride 1-RuIIIH. (55) The abstraction of hydrogen from free silane by the silylcarbonium ion is endergonic by 12.5 kcal/mol (SiMe3OCHPh+ + HSiMe3 → SiMe3OCH2Ph + SiMe3+). Therefore, we can conclude that, despite the fact that they are the most available hydride source, free silanes could not be the hydride donors in this catalytic process due to the endoergic reaction.

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Mechanistic Investigation Into Catalytic Hydrosilylation with a High

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