Article pubs.acs.org/Organometallics
Hydrosilylation of Carbonyls Catalyzed by the Rhenium(V) Oxo Complex [Re(O)(hoz)2]+A Non-Hydride Pathway Piao Gu, Wenmin Wang, Yiou Wang, and Haiyan Wei* Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory for NSLSCS, Nanjing Normal University, Nanjing 210097, People's Republic of China S Supporting Information *
ABSTRACT: Catalytic conversion of silane and carbonyls by the cationic rhenium oxo complex [Re(O)(hoz)2]+ (1; hoz = 2-(2′hydroxyphenyl)-2-oxazoline(1−)) was examined using density functional theory. It is shown that complex 1 catalyzed the carbonyl hydrosilylation via a non-hydride pathwaythe ionic hydrogenation mechanism. The complete catalytic cycle is proposed to involve three steps: the formation of cis η1-silane Re(V) adduct, the heterolytic cleavage of a Si−H bond through anti attack of carbonyls at the cis η1-silane Re(V) adduct, and transfers between the rhenium and activated silylcarbonium ion to produce the silyl ether product and regenerate catalyst 1. The σ-bond metathesis like transition state suggested by Abu-Omar, although not located, can be inferred from the ionic hydrogenation transition states (TS_3syn and TS_5syn, in which the carbonyls syn attack the η1-silane Re(V) adduct) associated with the higher energy barrier.
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(H···Si···O2···C) structure of the σ-bond metathesis like transition state, shown in Scheme 1.
INTRODUCTION Recently, a series of high-oxidation-state oxo complexes with metals such as rhenium(V), rhenium(VII), and molybdenum(VI) have been adopted to catalyze hydrosilylation reactions with carbonyls, imines, esters, sulfoxides, pyridine N-oxides, etc.1−3 For carbonyl hydrosilylation catalyzed by the two dioxo complexes ReO2I(PPh3)2 and MoO2Cl2, an unconventional mechanismthe [2 + 2] addition mechanismis proposed. This newly proposed mechanism suggests that the multiply bonded oxo ligand is involved in the activation of silane.1a,4−6 The catalytic cycle is initialized by the Si−H bond adding across the Re/MoO bond in a [2 + 2] fashion to generate the metal hydride intermediate ReO(OSiR3)HI(PPh3)2 or MoO(OSiR3)HCl2. Then the carbonyl reduction proceeds through coordination to the metal center and subsequent insertion into the M−H bond. The last step is retro-[2 + 2] addition: the alkoxide group attacks the silyloxium group to produce silyl ether and regenerate the catalyst. However, the [2 + 2] addition mechanism does not seem applicable to carbonyl hydrosilylation catalyzed by the rhenium(V) mono-oxo complex [Re(O)(hoz)2]+ (1; hoz = 2-(2′-hydroxyphenyl)-2oxazoline(1−)).2e 1 has been shown to catalyze hydrosilylation efficiently at ambient temperature, with low catalyst loading, and the reaction could be performed without a solvent. Through experimental studies, Abu-Omar and co-workers have shown that addition of a Si−H bond across a ReO bond is unlikely, excluding the possibility of the [2 + 2] addition mechanism.2e Instead, the authors suggested that the most viable mechanism involved a four-membered-ring © 2012 American Chemical Society
Scheme 1
Although this hypothetical σ-bond metathesis like transition state reasonably explains the kinetic study, it displays large steric congestion. Considering the heightened interest in hydrosilylation catalyzed by high-valent transition-metal complexes, and the current lack of a detailed understanding of the mechanism, we decided to use density functional theory (DFT)7 to investigate carbonyl hydrosilylation catalyzed by the cationic rhenium(V) mono-oxo complex [Re(O)(hoz)2]+ (1) (details of calculations are given in the Supporting Information). Through a detailed theoretical study, an unprecedented non-hydride pathwaythe ionic hydrogenation Received: June 30, 2012 Published: December 19, 2012 47
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mechanism8−10is shown to be the most favorable pathway. The proposed σ-bond metathesis mechanism by Abu-Omar could not be located. In addition, the [2 + 2] addition mechanism of a Si−H bond adding across a ReO bond is calculated to require a higher barrier, 11.0 kcal/mol higher than that for the ionic hydrogenation mechanism.
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RESULTS AND DISCUSSION The starting point for the hydrosilylation reaction catalyzed by the oxorhenium complex [Re(O)(hoz)2]+ (1) is the coordination of a free silane molecule to a vacant position on the rhenium center, leading to the formation of the η1-silane complex 2_trans. In the structure of 2_trans, the silane resides on the axial position of an octahedral structure, trans to the multiply bonded oxo ligand. The optimized structure of 2_trans displays a significantly long Re···H distance (2.469 Å) and a normal Si−H bond (1.511 Å), indicating that silane is only weakly η1 coordinated to the rhenium center. The free energy of 2_trans is calculated to be 4.4 kcal/mol higher than that of separation of 1 and a free silane molecule. To proceed, a benzaldehyde molecule could carry out nucleophilic attack at the silicon center in a “backside” fashion, representing the benzaldehyde anti attacking silane at the face opposite to the metal center. The transition state for such an arrangement could not be located, despite taking either H···Si or Si···O2(PhCHO) separation as the reaction coordinate. The plot of energy versus d(H···Si) or d(Si···O2(PhCHO)) went uphill, until the dissociative neutral rhenium hydride species (1H_trans) and a silylcarbonium ion (Me3SiOCHPh+) were obtained. On the free energy surface, the reaction (1) is
endergonic by 22.0 kcal/mol. Thus, the activation free energy barrier of 22.0 kcal/mol is estimated to be responsible for anti attack of benzaldehyde at the silicon center in 2_trans. As an alternative for benzaldehyde “backside” attacking the silicon center in 2_trans, benzaldehyde substrate could also approach the silicon center in a “frontside” fashion, representing benzaldehyde syn attacking silane at the face same with the metal center. The corresponding transition state for such a conformation is located as TS_3syn (The optimized structures are shown in Figure 1). In the structure of TS_3syn, the benzaldehyde oxygen atom is binding to the silicon center with a Si···O2(PhCHO) distance of 1.900 Å. The silane hydrogen bond is stretched to 1.855 Å, to move close to the rhenium center with a Re···H distance of 1.918 Å. The vibrational motion associated with the imaginary frequency represents the silane donating hydride to the rhenium center and the SiMe3 moiety accepting the benzaldehyde oxygen ligand simultaneously, which promotes the heterolytic cleavage of the Si−H bond. As expected, one would consider such an arrangement to be less favorable due to the large steric congestion imposed between the benzaldehyde and the oxazoline ring. And indeed, the free energy is calculated to be significantly high at 39.8 kcal/mol, which is 17.8 kcal/mol higher than that for the anti attack of benzaldehyde at the silicon center in 2_trans. Searching for other lower energy alternative mechanism, we found that in addition to 2_trans, the η1-silane rhenium complex could also exist in another form: 2_cis. In the structure of η1-silane complex 2_cis, one of the oxazoline oxygen atoms is pushed to occupy the axial position, lying trans to the multiply
Figure 1. Optimized geometries of TS_2, TS_3syn, TS_4anti, 4_anti, TS_5anti, TS_5syn, TS_6, and TS_7. Bond distances are shown in Å. H atoms are omitted for clarity.
bonded oxo ligand. Therefore, a free silane molecule could bind in an η1 fashion to the rhenium center at the equatorial plane of the octahedral structure, lying cis to the multiply bonded oxo ligand (four other isomers of silane Re(V) adducts are also possible and are shown in the Supporting Information, all being less stable than the isomer 2_cis). In the η1-silane isomer 2_cis, the Re···H distance is 1.894 Å, which is ∼0.7 Å shorter than that in the 2_trans isomer (2.469 Å), indicating a relatively strong η1 coordination between the rhenium center and the silane. The weaker interaction in the 2_trans isomer can be rationalized by the strong trans influence imposed by the multiply bonded oxo ligand.11 The transition state corresponding to the isomerization between the two η1-silane complexes 2_trans and 2_cis is located as TS_2 (shown in Figure 1), with an activation free energy of 8.8 kcal/mol above 2_trans. Similar to the case for the η1-silane rhenium 2_trans, the benzaldehyde molecule could also carry out nucleophilic attack at the silicon center in 2_cis to promote the heterolytic cleavage of the Si−H bond. The calculated free energy profile shows that 48
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Figure 2. Schematic free energy surface for catalytic carbonyl hydrosilylation by the cationic rhenium mono-oxo complex [Re(O)(hoz)2]+ (1). All energies are computed at the M06-2X level of theory. The gas-phase energies are included in parentheses and solvation energies without parentheses. Solvation corrections are computed with the SMD model with dichloromethane as the solvent.
the cis η1-silane rhenium isomer 2_cis entails a significant reduction of activation barrier in comparison to the trans isomer. Upon benzaldehyde “backside” attack at the silicon center in 2_cis, an approximate concerted process representing the heterolytic cleavage of the Si−H bond is identified. The process is actually composed of two consecutive transition states (TS_4anti and TS_5anti) close in energy (6.0 and 5.2 kcal/mol without solvation effects), connecting via a shallow intermediate (4_anti, 5.1 kcal/mol without solvation effects). The first transition state TS_4anti (ΔG⧧ = 17.4 kcal/mol with solvation effects) corresponds to the anti attack of the benzaldehyde oxygen atom at the silicon center, accompanied by an elongation of the Si−H bond. Then, a shallow intermediate 4_anti is located (ΔG = 14.8 kcal/mol with solvation effects). To proceed, the heterolytic cleavage of Si−H is completed by overcoming the second transition state TS_5anti (ΔG = 12.6 kcal/mol with solvation effects), with the silane hydrogen transferred to the rhenium center and silyl ion (Me3Si+) coupled to the benzaldehyde. The structures of these three species all display a pentacoordinate trigonalbipyramidal structure around the silicon center, with the SiMe3 moiety being near planar, and the four atoms of Re···H···Si···O2(PhCHO) being roughly in a straight line. To proceed, the ion pair [Re(O)(hoz)2H]−[Me3SiOCHPh]+ (5_anti) is formed, lying 15.2 kcal/mol above the reactant 1 (3.6 kcal/mol without solvation effects). Along TS_4anti → 4_anti → TS_5anti → 5_anti, the benzaldehyde approaches the silicon center with d(O2··Si) decreasing along 2.49 Å → 2.02 Å → 1.88 Å → 1.85 Å, which simultaneously prompts the stretching of the Si−H bond, along 1.63 Å → 1.80 Å → 2.26 Å → 2.57 Å. At the same time, the silane hydrogen progressively binds to the rhenium center with the Re···H distance decreasing along 1.77 Å → 1.70 Å → 1.65 Å → 1.64 Å. Accompanying those structural changes, the electron density from the silane hydrogen is drained toward the
rhenium center progressively. The calculated NBO negative charge on the silane hydrogen decreased, along 0.20 → 0.15 → 0.08 → 0.04. However, the NBO charge on the rhenium center becomes less positively charged, along 1.09 → 1.02 → 0.92 → 0.88, and the NBO charge on the silicon center is increased, along 1.75 → 1.77 → 1.89 → 1.90. Therefore, TS_4anti and TS_5anti are representative of SN2-Si transition states with the benzaldehyde molecule replacing the leaving hydride ligand in the silane. The calculated free energies with solvation correction are 17.4 and 12.6 kcal/mol, respectively. In comparison, the reduction of carbonyls by the cis η1-silane complex 2_cis is preferred by 4.6 kcal/mol over the trans isomer 2_trans. From the intermediate 5_anti, the ion pair could dissociate to give the neutral rhenium hydride species [Re(O)(hoz)2H] (1H_cis) and the silylcarbonium ion [Me3SiOCHPh]+. Then, the two species could reapproach each other to form another ion pair isomer: 5_syn. In the structure of 5_syn, the silylcarbonium ion repositions its benzene moiety by rotating around the Re−H bond until it assumes a perpendicular position (∠Re···H···C(CO2)···C(benzene) = 81.5°), in comparison with the parallel arrangement in the ion pair of 5_anti. The calculation indicates that the ion pair 5_syn is 6.6 kcal/mol more stabilized than 5_anti, which suggests the isomerization between the two ion pairs is facile, primarily due to the strong electrostatic attraction between the rhenium hydride and the silylcarbonium carbon atom. To complete the reaction, the silylcarbonium ion abstracts the rhenium hydride to produce silyl ether and regenerate the catalyst 1. From 5_syn, the driving force for the hydride transferring from the rhenium center to the silylcarbonium carbon atom is large (ΔG(5_syn →6) = 28.6 kcal/mol) and the activation barrier is calculated to be relatively low at 6.8 kcal/mol (TS_6 above 5_syn). 49
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From this point, either the benzene ring of benzaldehyde needs to be rotated to move the C (CO2) atom close to the silane hydrogen, which would impose a huge steric crowding, or the silane moiety need to be pushed far away from the rhenium center, which would result in the metal having no interaction with silane and benzaldehyde. Either case would cause an excessively high activation barrier. Therefore, we believed that even if such a transition state could exist, it would be associated with higher barriers.
On the other hand, there is the possibility for the isolated silylcarbonium ion to abstract hydrogen from free silane to produce the silyl ether product. Therefore, we decided to investigate whether the neutral rhenium hydride species or the silylcarbonium ion could be largely populated in this reaction. First, the reaction for the rhenium hydride species Re(O)(hoz)2H (1H_cis) donating the hydride to the silylcarbonium ion is largely exergonic. On the free energy surface, reaction (2) is exergonic by 30.1 kcal/mol. Second, reaction (3) for the silylcarbonium ion abstracting hydrogen from free silane is endergonic by 3.0 kcal/mol, Third, reaction (4) for complex 1 abstracting hydrogen from free silane is endergonic by 33.2 kcal/mol.
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CONCLUSIONS In summary, the mechanism of carbonyl hydrosilylation catalyzed by the cationic mono-oxo complex [Re(O)(hoz)2]+ (1) was extensively investigated by means of DFT calculations. An unprecedented non-hydride pathwaythe ionic hydrogenation mechanismis proposed to be the most favorable pathway. In this non-hydride pathway, first, the silane molecule coordinates cis to the rhenium center to form the η1-silane rhenium(V) adduct 2_cis, with an activation barrier of 13.2 kcal/mol (TS_2), which provides access to the lowest energy path for carbonyl reduction. Afterward, the benzaldehyde carries out a nucleophilic anti attack at the silane center in 2_cis, promoting the heterolytic cleavage of the Si−H bond, which is the rate-determining step with an activation free energy barrier of 17.4 kcal/mol. Then the silylcarbonium ion abstracts the rhenium hydride to produce the silyl ether product. We wish to emphasize the following points: (1) the multiply bonded oxo ligand does not participate in the activation of the Si−H bond as described in the [2 + 2] addition mechanism, and (2) the rhenium hydride complex plays a negligible role in the whole catalytic cycle. Such a nonhydride pathway for carbonyl hydrosilylation is important, showing great promise for the further development of highvalent complexes to catalyze the reduction reaction.12
Therefore, it seems unlikely that a neutral rhenium hydride species would be significantly populated under the reaction conditions. The catalytic cycle is more likely to proceed without generation of free rhenium hydride species or silylcarbonium ion or SiMe3+ ion. Alternatively, it is worth noting that the ion pair 5_syn could be directly generated by “frontside” attack of the benzaldehyde molecule at the silicon center in 2_cis. Likewise, the transition state for such an arrangement can be located (TS_5syn). The free energy barrier is calculated to be high (32.8 kcal/mol with solvation effects), which is 15.4 kcal/mol higher than the transition state (TS_4anti) with benzaldehyde “backside” attack at the silicon center. Therefore, depending on the transition states and intermediates located, the carbonyl hydrosilylation catalyzed by rhenium mono-oxo complex [Re(O)(hoz)2]+ (1) is calculated to proceed via a nonhydride pathway, along 1 → 2_trans → TS_2 → 2_cis → TS_4anti → 4_anti → TS_5anti → 5_anti → 5_syn → TS_6 → 6 → 1. The relative free energy profile determined for each step is presented in Figure 2. The proposed mechanism accounting for the catalytic cycle could be divided into three steps: the formation of cis η1-silane Re(V) adduct 2_cis with an activation free energy barrier of 13.2 kcal/ mol, the heterolytic cleavage of the Si−H bond via nucleophilic attack of the carbonyl at the silicon center in η1-silane 2_cis, with an activation free energy barrier of 17.4 kcal/mol, and the final step corresponding to the hydride transferring between rhenium hydride and the activated silylcarbonium ion, with an activation free energy barrier of 15.4 kcal/mol. Therefore, the rate-determining step for carbonyl hydrosilylation catalyzed by complex 1 corresponds to the ionic hydrogenation transition state TS_4anti, with a free energy barrier of 17.4 kcal/mol. Therefore, this would explain the high efficiency for hydrosilylation catalyzed by complex 1 at ambient temperature. In addition, we have also explored the possibility of a [2 + 2] addition mechanism with silane adding across the ReO bond to form the rhenium hydride. The calculated activation free energy is 28.4 kcal/mol (TS_7), which is significantly higher (11.0 kcal/mol) than the ionic hydrogenation pathways described above. Therefore, the [2 + 2] addition mechanism could be excluded. Also, we tried to locate a transition state corresponding to the σ-bond metathesis mechanism as suggested by Abu-Omar. All attempts failed. In order to obtain the four-membered ring of a σ-bond metathesis like transition state, we could take either 3_syn or 5_syn as the starting point.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and tables giving details of the calculations, a comparison of different levels of methods (B3LYP, B3LYP-D, M06-2X//B3LYP and M06-2X, M06), and selected geometrical parameters for all calculated structures of reactants, intermediates, transition states, and products. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Computer resources for theoretical calculations were mainly provided by the High Performance Cluster Computing (HPCC) facility at Nanjing University and the Shanghai SuperComputer Center. We acknowledge the National Natural Science Foundation of China (No. 21103093), Jiangsu province Science and Technology Natural Science Project (No. BK2011780), and the Chair Professor of Jiangsu Province to Start Funds for financial support of this research, and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. 50
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