Article pubs.acs.org/JPCA
New Insights into Hydrosilylation of Unsaturated Carbon− Heteroatom (CO, CN) Bonds by Rhenium(V)−Dioxo Complexes Liangfang Huang,§ Wenmin Wang,§ Xiaoqin Wei, and Haiyan Wei* Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Provincial Key Laboratory for NSLSCS, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210046, China S Supporting Information *
ABSTRACT: The hydrosilylation of unsaturated carbon−heteroatom (CO, CN) bonds catalyzed by high-valent rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η1-silane rhenium adduct to prompt the heterolytic cleavage of the Si−H bond is the most energetically favorable process for rhenium(V)−dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)−dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si−H bond and catalyzing the reduction reactions.
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INTRODUCTION
compounds has been dominated by low-valent precious metal catalysts, such as platinum,45 rhodium,46,47 and so forth.48−50 To account for the hydrosilylation of carbonyl compounds catalyzed by the high-valent rhenium(V)−dioxo complex ReO2I(PPh3)2, Toste and co-workers proposed an unconventional [2 + 2] addition mechanism.24 The [2 + 2] addition mechanism begins with the addition of a Si−H bond across one ReO bond in a [2 + 2] manner to form a siloxyrhenium hydride (Scheme 1, right). The carbonyl then coordinates with the siloxyrhenium hydride, followed by its insertion ion into the Re−H bond, producing a siloxyrhenium alkoxide, which in turn affords the silyl ether product. This [2 + 2] addition mechanism was supported by Wu and co-workers through DFT calculations.51−53 Furthermore, the [2 + 2] addition mechanism has been subsequently suggested to be responsible for the related activation of Si−H, B−H, P−H, and H−H bonds via rhenium(V)−dioxo and molybdenum(VI) complexes.54,55 The [2 + 2] addition catalytic cycle shares a common theme with the Chalk−Harrod mechanism (Scheme 1, left).56−58 The Chalk−Harrod mechanism is generally accepted to account for catalytic hydrosilylation by low-valent transition-metal complexes, where the key step involves the oxidative addition of the Si−H bond to the metal center to generate a metal silyl hydride. Because both the [2 + 2] addition mechanism and the
Hydrosilylation reactions, which add organic silicon hydrides to double bonds, have received considerable attention across industrial fields for their various applications.1−4 One of the most prominent applications of hydrosilylation reactions includes the efficient reduction of unsaturated organic compounds, such as carbonyls, imines, esters, amides, and so forth.5−15 Furthermore, hydrosilylation is often employed as an alternative to hydrogenation because mild reaction conditions are required, it can be easily handled, the silyl group is retained as a protecting group, and so forth.16−18 Since the high-valent rhenium(V)−dioxo complex-catalyzed hydrosilylation of aldehyde/ketones was first reported by Toste in 2003,19 high-valent transition metals, such as rhenium(V)/molybdenum(VI)/ ruthenium(VI) have attracted the majority of attention in this field. A wide array of catalytic systems (MoO2Cl2,20−23 ReO2I(PPh3)2,24 ReOCl3(PPh3)2,25,26 Re(NAr)Cl3(PPh3)2,25 Re2O7,26 HReO4,26 ReOCl3(dppm),27,28 ReOCl3(Me2S)(Ph3PO),29 (Cp)(ArN)Mo(H)(PMe3),30,31 (ArN)Mo(H)(Cl)(PMe3)3,32 [RuN(saldach)(MeOH)]+ (saldach = N,N′cyclohexan-diyl-bis(salicylideneimine),33 ReO(hoz)2+,34,35 Re(O)(NAr)(saldach)+,36 and so forth37−41) for reduction of C O, CN, CS, and SO double bonds have been studied. Thiel42 described these high-valent transition-metal complexes as a new class that catalysis reduction reactions because they were traditionally employed to catalyze oxidation reactions.43,44 Furthermore, the catalytic reduction of unsaturated organic © 2015 American Chemical Society
Received: January 19, 2015 Revised: March 30, 2015 Published: March 31, 2015 3789
DOI: 10.1021/acs.jpca.5b00567 J. Phys. Chem. A 2015, 119, 3789−3799
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The Journal of Physical Chemistry A Scheme 1. Description of the Chalk−Harrod Mechanism for Catalytic Hydrosilylation by Low-Valent Transition-Metal Complexes (left) and the [2 + 2] Addition Mechanism Proposed for Catalytic Hydrosilylation by the High-Valent Rhenium(V)−Dioxo Complex (right)
Scheme 2. Hydrosilylation of Aldehyde/Ketones and Imines Catalyzed by Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1) Studied in This Work
substrates acting as nucleophiles attack the silicon atom bound on the electrophilic metal (boron) center to prompt heterolytic cleavage of the Si−H bond, featuring an SN2-Si transition state.62−83 In other words, the ionic outer-sphere pathway involves the sequential transfer of a silyl cation and then a hydride to a polar double bond to furnish the reduction product. However, few theoretical studies on reactions that favor the ionic outer-sphere mechanism mediated by transitionmetal catalysts have been performed. Moreover, it is worth noting that previous computational studies on Si−H/B−H/P− H bond activation at high-valent transition-metal complexes center primarily on the [2 + 2] addition mechanism. In this work, we highlight the role of the high-valent transition-metal complexes as Lewis acids in activating the Si−H bond and catalyzing the reduction reaction. The knowledge and new insight gained from this study can enable future works that may ultimately lead to the design of more efficient catalysts favoring ionic outer-sphere pathways.
Chalk−Harrod mechanism assume the formation of a metal hydride intermediate, the reduction of the unsaturated double bond relies on its insertion into the transition-metal hydride bond. Both mechanisms can be classified as hydride mechanisms. Nikonov and co-workers have determined, based on a stoichiometric labeling experiment, that the hydride mechanism is not the dominant reaction pathway for carbonyl hydrosilylation catalyzed by the rhenium(V)−dioxo complex ReO2I(PPh3)2.39 In their experiment, they found that a stoichiometric reaction of siloxyrhenium hydride [ReO(RhMe2SiO)(H)I(PPh3 )2] with benzaldehyde was completed overnight. However, when a stoichiometric reaction of siloxyrhenium hydride with benzaldehyde and silane was used, it only took about 1 h for the silyl ether formation to occur. Furthermore, they proposed a nonhydride mechanism, in which the metal center activates the carbonyl compounds as a Lewis acid. Unfortunately, the exact nature of the nonhydride mechanism was not clearly established. Despite the novel discovery of highvalent transition-metal complexes catalyzing reduction reactions, theoretical investigations focusing on the underlying mechanisms of such reactions remain limited.51,54 Furthermore, advances toward achieving a consistent understanding of hydrosilylation reactions catalyzed by high-valent rhenium(V)−dioxo complexes are lacking. This lack of development motivated us to further explore the mechanism of hydrosilylation of unsaturated organic substrates catalyzed by the high-valent rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) using DFT calculations (Scheme 2). Remarkably, our results show that the ionic outer-sphere pathway is the preferred path for the high-valent rhenium(V)− dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of imines, in which the substrate attacks the silicon center to prompt the heterolytic cleavage of the Si−H bond, featuring an SN2-Si transition state. More importantly, the ionic outersphere pathway competes with the [2 + 2] addition pathway in rhenium(V)−dioxo complex 1 catalyzing the hydrosilylation of carbonyls. The ionic outer-sphere mechanism resembles the postulated catalytic cycle of hydrosilylation reactions with strong Lewis acidic organoborane trispentafluorophenyl borane, B(C6F5)3.59−61 Along the ionic outer-sphere pathway, organic
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COMPUTATIONAL DETAILS All molecular geometries of the model complexes were optimized at the DFT (B3LYP) level, 84,85 which was implemented in Gaussian 09.86 The effective core potentials (ECPs) of Hay and Wadt with double-ζ valence basis sets (LanL2DZ)87,88 were used to describe the Re metal and I atom, with polarization functions added (Re, ζf = 0.869; I, ζf = 0.289).89,90 The 6-311g(d,p) basis set was used for all other atoms, such as C, H, P, Si, Cl, and O. 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) as well as to provide free energies at 298.15 K, including entropic contributions. The transition states found were further confirmed by calculating intrinsic reaction coordinates routes toward the corresponding minima and reoptimizing from the final phase of intrinsic reation coordinate (IRC) paths to reach each minimum. In total, two kinds of geometric optimizations were preformed. First, all geometric optimizations were performed under the gas phase. The free-energy results were then refined by calculating the single-point energy at the same level of theory with solvation effects by using the SMD continuum model (method I), (SMD, an IEFPCM (The Polarizable Continuum Model (PCM) using the integral equation formalism variant) calculation with radii and nonelectrostatic terms for Truhlar and co-workers’ SMD solvation model),91 as implemented in Gaussian 09. CH2Cl2 was used as the solvent. Second, all geometric optimizations were fully reoptimized under solvent conditions using the SMD solvation model using tight convergence criteria (method II), in order to properly describe the ionic outer-sphere pathway. 3790
DOI: 10.1021/acs.jpca.5b00567 J. Phys. Chem. A 2015, 119, 3789−3799
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The Journal of Physical Chemistry A The final Gibbs energy values reported in the current study were relative free energies in solvent, plus corrections for dispersion effects using Grimme’s empirical dispersion correction term, that is, B3LYP-D3.92,93 Detailed comparisons of different methods are listed in the Supporting Information. All of the geometries are displayed using the software of CYLview.94
the Si−H bond then occurs via the transition state TS4. In the optimized geometry of TS4 (Figure 1), the length of the breaking Si−H bond is 2.49 Å, which is elongated by 0.97 Å from 3. Accompanied by cleavage of the Si−H bond, the silane hydrogen transfers to the rhenium center (d(Re−H) = 1.65 Å), and the SiMe3 moiety binds to the imine nitrogen atom (d(Si− N) = 1.89 Å). The calculated vibrational motion of the imaginary frequency associated with TS4 represents the expected normal mode of an SN2-Si reaction, which features attack of the nucleophilic imine on the silicon center. At the same time, the hydride leaves the silicon center. TS4 reflects the very late nature of a transition-state structure, with more advanced Si−H bond breaking. It is worth noting that the structure of TS4 around the Si atom is characterized by a trigonal bipyramidal structure, where the silyl group (SiMe3) acts as the plane and the departing hydrogen atom and the incoming N(im) atom occupy the apical positions. The four atoms of Re−H···Si−N are roughly confined to a straight line, with angles of ∠Re−H−Si = 172.7° and ∠H−Si−N = 174.9°. A full IRC analysis without symmetry restriction confirmed that TS4 is the first-order saddle point that connects van der Waals species 3+im and an intermediate (4). The intermediate 4 (Figure 1) is comprised of an anionic rhenium hydride [ReO2IH(PPh3)]− paired with a silylated iminium ion [SiMe3NHCHPh]+, in which the two moieties are separated with a significantly elongated Si···H distance (3.36 Å). The Si− H bond cleavage (TS4) was calculated to associate with a free energy of 22.8 kcal/mol from the reactants, and the intermediate 4 was calculated at 2.2 kcal/mol below TS4. Closure of the ionic outer-sphere catalytic cycle requires C− H bond formation on the silylated iminium ion, which occurs in two separate steps. First, the anionic rhenium hydride [ReO 2 IH(PPh 3 )] − and the silylated iminium ion [SiMe3NHCHPh]+ rearrange to form another type of ion pair, 5 (Figure 1), in which the Si−N bond of the silylated iminium ion resides in the “syn-side” position relative to the Re−H bond of rhenium hydride anion (∠Re−H−Si−N = 119.4°). The geometries of the anionic rhenium hydride and the silylated iminium ion in 5 show negligible change. This isomerization yields a stabilization energy of 4.3 kcal/mol, which is primarily due to the strong electrostatic intereaction between the hydride atom in the anionic rhenium hydride and the carbon atom in the silylated iminium ion ((d(H−N) = 5.18 Å in the intermediate 4, shorter than 3.58 Å in the intermediate 5). Alternatively, we found that ion pair 5 can be directly formed through the transition state TS5 (Figure 1), in which the imine approaches the silicon center in the η1-silane complex 3 from the syn-side direction. However, the activation free-energy value is much greater (33.5 kcal/mol) than that in the reaction via TS4. Hence, the syn reaction course is ruled out. From 5, the calculated energy profile reveals that a very low activation barrier (3.4 kcal/mol, TS6 above 5) exists for the hydride transfer from the rhenium metal to the carbon atom of the silylated iminium ion. At TS6 (Figure 1), the hydrogen leaves the rhenium center with a Re−H distance of 1.67 Å and reaches the carbon atom with a H−C distance of 2.01 Å. This migration step has a high exergonicity of 19.7 kcal/mol, indicating that it occurs very easily. When going from the η1-silane adduct 3 and passing from TS4 to the ion pair 4, the electron density from the silane hydrogen progressively drains toward the rhenium center. During this process, the H atomic electron density decreases as −0.21e → −0.10e → −0.05e, respectively. Subsequently, the
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RESULTS AND DISCUSSION In the subsequent sections, the ionic outer-sphere pathway in which organic substrates nucleophilically attack the silicon atom in a silane metal complex to cleave the Si−H bond is discussed first. Then, the [2 + 2] addition pathway in which the Si−H bond initially adds across a metal−oxo bond to form a rhenium hydride intermediate is discussed. Pathway I: Ionic Outer-Sphere Pathway for HighValent Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1)Catalyzed Hydrosilylation. The ionic outer-sphere pathway consists of three steps, as shown in Scheme 3, the addition of Scheme 3. Proposed Ionic Outer-Sphere Pathway for Hydrosilylation of Imines and Carbonyl Compounds Catalyzed by Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1)
silane to the metal center, the nucleophilic attack of organic substrates to prompt the cleavage of the Si−H bond, and the hydride transfer to the activated organic substrate to complete the catalytic cycle. Along the ionic outer-sphere pathway, the requirement for coordination of the unsaturated substrates to the metal center is removed. The optimized structures of key stationary points along the ionic outer-sphere pathway for rhenium(V)−dioxo complex ReO2I(PPh3)2 (1)-catalyzed hydrosilylation of imines (here, phenylmethanimine is taken as the example, designated as im) are shown in Figure 1. The initial step of the ionic outer-sphere pathway consists of replacing a phosphine ligand with a silane (here, trimethylsilane) at the rhenium center in ReO2I(PPh3)2 (1) to produce a silane metal adduct. Our calculations show that the resulting five-coordinate complex 2 (Supporting Information, Figure S1) from dissociation of a PPh3 ligand is 10.4 kcal/mol less stable than 1, and the silane rhenium complex 3 is 14.8 kcal/mol less stable than the catalyst 1 and silane. The silane rhenium complex 3 (Figure 1) exhibits a weak end-on η1-H(Si) mode, with the Si−H bond showing only a slight extension (+0.04 Å) compared with that of free silanes (∼1.48 Å). In addition, the calculated length of the Re−H bond (Si) is significantly longer, with a length of 2.06 Å. Subsequent nucleophilic attack of the imine molecule at the silicon atom in η1-silane complex 3 forms a van der Waals species, 3+im (Supporting Information, Figure S1). Cleavage of 3791
DOI: 10.1021/acs.jpca.5b00567 J. Phys. Chem. A 2015, 119, 3789−3799
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Figure 1. Calculated geometric structures (3, TS4, 4, 5, TS5, andTS6) along the ionic outer-sphere pathway for the high-valent rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of phenylmethanimine (method I). Bond distances are shown in Å.
Scheme 4. Schematic of the Free-Energy Surface for Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1) Catalyzing the Hydrosilylation of Phenylmethanimine via the Ionic Outer-Sphere Mechanistic Pathway
Scheme 4, which can be divided into three steps: (1) addition of silane to the rhenium center, 1 → 2 → 3; (2) heterolytic cleavage of the Si−H bond, 3+im → TS4 → 4; and (3) the hydride transfer, 5 → TS6 → silylated amine. Our calculated results indicate that the ionic outer-sphere pathway is associated with a moderate activation free-energy barrier, 22.8 kcal/mol, corresponding to the Si−H bond heterolytic cleavage step (TS4). The large decrease (16.5, 18.0, and 9.8 kcal/mol, respectively) for species of TS4, 4, and 5 in the Gibbs freeenergy values based on single-point calculations accounting for solvent effects compared to those in gas indicates that the solvent clearly plays an important role in stabilizing the ionic
NBO charge on the silicon center becomes more positive, changing as 1.56e (3+im) → 1.76e (TS4), 1.74e (4) with the nucleophilic attacks of the imine. Along the ionic outer-sphere mechanistic pathway, the rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) serves as a Lewis acid to catalyze the hydrosilylation reaction of imine, which resembles that of the Lewis acid B(C6F5)3 catalyzing the reduction of aldehydes and ketones, which involves the sequential transfer of a silane hydrogen to the rhenium or B(C6F5)3 and a silyl cation (R3Si+) to the organic substrate.95−103 In summary, the free-energy profiles for the rhenium(V)− dioxo 1-catalyzed hydrosilylation of benzylideneimine that proceeds via the ionic outer-sphere pathway are shown in 3792
DOI: 10.1021/acs.jpca.5b00567 J. Phys. Chem. A 2015, 119, 3789−3799
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The Journal of Physical Chemistry A outer-sphere pathway. These features prompted us to reoptimize the most important intermediate and transitionstate structures in the presence of solvent (method II). The energy values at solvent-phase optimizations (method II) for 4, 5, and TS6 are 25.1, 18.2, and 22.0 kcal/mol, respectively, which are close to 20.6, 16.3, and 19.7 kcal/mol, respectively (Table 1) obtained using single-point calculations to account Table 1. Activation Free Energies (kcal/mol) for Rhenium(V)−Dioxo Iodine Complex 1 and Chloride Model 1(Cl) Catalyzing the Hydrosilylation of Phenylmethaniminea TS4(TS4(Cl)) 4(4(Cl)) 5(5(Cl)) TS6(TS6(Cl))
method I
method II
22.8(23.7) 20.6(20.7) 16.3(16.0) 19.7(18.4)
−(21.1) 25.1(22.7) 18.2(19.2) 22.0(20.9)
Figure 2. Optimized structure of the transition state TS4(Cl) at the solvent-phase optimization. Bond distances are shown in Å.
Here, it is worth mentioning that introduction of less steric substituent R around the silicon atom causes the occurrence of a new feature on the PES of the ionic outer-sphere pathway. With silanes of SiH4 and PhSiH3, the ionic outer-sphere pathway changes from a double-well potential energy surface with a central TS, as described above (Scheme 4, TS4), to a triple-well PES (featuring a pre- and a post-transition state before and after the central intermediate). Figure 3 illustrates
a Method I: single-point calculations to account for solvent effects at gas-phase optimization; method II: those at solvent-phase optimization.
for the solvent based on gas-phase optimization (method I). Furthermore, the solvent-phase optimized structures of the three species (4, 5, and TS6) are shown in the Supporting Information, Figure S2), and it is clear that there is very little variation in the bond lengths between the different models compared to the gas-phase optimizations.104 However, our attempts at determining the ionic transition state TS4 at solvent-phase optimizations were unsuccessful. An estimated free-energy value of 28.6 kcal/mol was obtained using a restricted calculation of the first-order saddle point on the potential energy surface.105,106 To better evaluate the freeenergy surface along the ionic outer-sphere pathway, we then modeled the hydrosilylation reaction with the analogous chloride complex of ReO2Cl(PPh3)2, 1(Cl). Inspection of the relative energy values shows similar catalytic activities for the iodine complex 1 and the chloride complex 1(Cl). As shown in Table 1, the relative free energies (method I) of TS4(Cl) (23.7 kcal/mol), 4(Cl) (20.7 kcal/mol), 5(Cl) (16.0 kcal/mol), and TS6(Cl) (18.4 kcal/mol) are close to those of the iodine complex, 22.8, 20.6, 16.3, and 19.7 kcal/mol, respectively. In addition, energy values at solvent-phase optimizations (method II) for 4(Cl) (22.7 kcal/mol), 5(Cl) (19.2 kcal/mol), and TS6(Cl) (20.9 kcal/mol) are close to those values of the iodine complex too, 25.1, 18.2, and 22.0 kcal/mol, respectively. Moreover, we have successfully located the ionic transition state TS4(Cl) at solvent-phase optimization. TS4(Cl) (Figure 2) represents the very early nature of a TS, in which the Si−H distance is 1.59 Å, elongated slightly by 0.07 Å from a free silane. The silane hydrogen is weakly bound to the rhenium center, d(Re−H) =1.87 Å (0.19 Å shorter than that in adduct 3), and the imine nitrogen atom is weakly bound to the silicon atom, d(Si−N) = 3.21 Å. The free energy of TS4(Cl) at solvent-phase optimization is calculated to be 21.1 kcal/mol, which is close to that of 23.7 kcal/mol calculated at gas-phase optimizations with single-point calculation to account for solvent effects (method I). Therefore, a qualitative comparison demonstrates that although the solvent effect strongly influences the energies of the ionic outer-sphere mechanistic pathway, the free energies based on the solvent correction on gas-phase optimizations is reasonable and could be satisfying.
Figure 3. Calculated geometric structures of central pentavalent siliconate intermediates, 4-SiH4 and 4-PhSiH3. Bond distances are shown in Å.
the central, pentavalent siliconate intermediates, 41-SiH4 and 41-PhSiH3. In the optimized structures of 41-SiH4 and 41PhSiH3, the Si−H bond is partially cleaved, d(Si−H) =1.58 and 1.60 Å, the Re−H bond is partially formed, d(Re−H) = 1.90 and 1.87 Å, and the imine nitrogen atom is partially bound to the silyl group, d(Si−N) = 2.23 and 2.17 Å. The free energies of these two intermediate structures are calculated to be 26.0 and 21.5 kcal/mol.105,106 These phenomena are similar to the observation previously made by Bickelhaupt.107 In their work, they found that decreasing the steric substituent, R in the SN2Si reactions of Cl− + SiR3Cl caused the occurrence of one central barrier (ROCH3) to eventually split into steric preand postbarriers (R = H, CH3). Encouraged by the above results, we expected that the ionic outer-sphere catalytic cycle would also be operational for rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of carbonyls. Interestingly, the ionic transition state with benzaldehyde was also located. In the optimized geometry of TS4(PhCHO) (Figure 4), the Si−H bond is completely cleaved and significantly elongated to 2.47 Å, and the Re−H (1.65 Å) and Si−O (1.84 Å) bonds are considerably 3793
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of the NBO charge on the nitrogen atom (PhCHNH, −0.65e) and the oxygen atom (PhCHO, −0.57e) of the van der Waals species, 3+im/3+ketone, suggests that the strong nucleophilic character of imines accounts for the low activation free energy. Therefore, modulating the electronic factors of substrates would effect the free energies of the SN2-Si reaction barrier. The electron-donating group on the organic substrates induces better activities, where para-methoxy-substituted Nphenylbenzylideneimine (p-MeO-PhCHNPh) has an activation free-enegy barrier of 28.7 kcal/mol, which is 2.0 kcal/mol lower than that of PhCHNPh (30.6 kcal/mol) and is 6.2 kcal/mol lower than that of the para-trifluoromethyl-Nphenylbenzylideneimine (p-CF 3−PhCHNPh) substrate (34.9 kcal/mol) with a withdrawal group. Similar, for paramethoxy-substituted diphenylketone (p-MeO-PhCOPh), the activation free-energy barrier is 36.3 kcal/mol, which is 3.8 kcal/mol lower than that of PhCOPh (40.1 kcal/mol). Furthermore, our results shown in Table 2 indicated that silanes bearing phenyl groups slightly reduce the activation barrier of the ionic transition state, with the aptitude predicted to be on the order of Me3SiH, PhMe2SiH (30.6, 30.0 kcal/mol) > PhSiH3, Ph2MeSiH, Ph3SiH (27.2, 26.7, 26.3 kcal/mol). This can be ascribed to the cationic silyl center stabilized by the phenyl group in the transition-state structures. Species with more nucleophilic character on the silicon group and organic substrates could play an important role in lowing the activation free energies of the ionic outer-sphere transition states. Pathway II: [2 + 2] Addition Pathway for High-Valent Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1)-Catalyzed Hydrosilylation. The catalytic cycle of the [2 + 2] addition mechanism begins with the Si−H bond adding across the Re O bond to generate a siloxyrhenium hydride complex, ReO(OSiMe3)(H)I(PPh3) (7). The corresponding transition state TS[2 + 2], shown in Figure 5, presents a four-membered cyclic (Re···O···Si···H) ring, with a partially broken Si−H bond, 1.74 Å, and partial formation of a Re−H bond, 1.76 Å. The silyl binding with multiply bonded oxo ligand generates a Si−O bond (2.08 Å). The generating siloxyrhenium hydride then acts as the active species to reduce the imine substrate. First, imine coordinates to the rhenium center through the nitrogen atom of the CN bond to generate the imine rhenium adduct 8, ReO(OSiMe3)(H)I(PPh3)(PhCHNH), which then undergoes migratory insertion into the Re−H bond to generate the amine intermediate, via TS9. In the optimized geometry of TS9 (Figure 5), the phenylmethanimine carbon atom moves considerably toward the rhenium−hydrogen bond and exhibits an η2-imine mode with Re−N and Re−C distances of 2.06 and
Figure 4. Calculated geometric structures of the ionic transition state TS4(PhCHO) for the rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of benzaldehyde. Bond distances are shown in Å.
formed. The Si−H bond cleavage (TS4(PhCHO)) was calculated to associate with a free energy of 35.8 kcal/mol from the reactants. Variety of Silanes and Organic Substrates. To validate our mechanistic hypothesis and evaluate the scope and applicability of the ionic outer-sphere mechanism, we further examined the rhenium(V)−dioxo complex catalyzed reduction of imines and carbonyls using various silanes. The computational results with optimization in the presence of solvent employing the chloride model of ReO2Cl(PPh3)2 are collected in Table 2 and Figure S5 (Supporting Information). First, it is worth noting that upon inspection of the optimized structures of the ionic transition states with N-phenylbenzylideneimine (PhCHNPh) and diphenylketone (PhCOPh), a central SN2-Si transition state, is represented, where the Si−H bond is symmetrically oriented between the nucleophile (d(Si−N) = 2.13 Å, d(Si−O) = 1.92 Å) and the leaving group (d(Si−H) = 1.84 and 2.12 Å), whereas the rhenium and oxygen/nitrogen (CO/N) point either slightly to the nucleophile (d(Re−H) = 1.73 and 1.69 Å) or to the leaving group, respectively. Second, it is worth noting that the ionic transition state with imines is ∼10.0 kcal/mol lower than that the analogous aldehyde/ketone, for example, 21.1 (PhCHNH) versus 37.5 kcal/mol (PhCHO) and 30.6 (PhCHNPh) versus 40.1 kcal/mol (PhCOPh). The remarkable difference in the hydrosilylation of imines and aldehyde/ketones catalyzed by the rhenium(V)−dioxo complex can be attributed to the difference in the depths of the nucleophilicity. The comparison
Table 2. Activation Free Energy (kcal/mol) of the Ionic Transition State TS4 (based on solvent-phase optimization) with Various Imine/Carbonyls for Rhenium(V)−Dioxo Complex 1(Cl) Catalyzing the Hydrosilylation Reaction silane
imines
TS4
ketones
TS4
Me3SiH Me3SiH Me3SiH Me3SiH Me3SiH PhMe2SiH PhSiH3 Ph2MeSiH Ph2MeSiH Ph3SiH
PhCHNH PhCMeNMe PhCHNPh p-MeO-PhCHNPh p-CF3−PhCHNPh PhCHNPh PhCHNPh PhCHNPh
21.1 32.4 30.6 28.7 34.9 30.0 27.2 26.7
PhCHO PhCOMe PhCOPh p-MeO-PhCHO p-MeO-PhCOPh
37.5 37.7 40.1 35.7 36.3
PhCHNPh
26.3
PhCHO PhCOPh PhCOPh
37.9 35.9 37.3
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Figure 5. Calculated geometric structures (TS[2 + 2], TS9, and TS10) along the [2 + 2] addition pathway for rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of phenylmethanimine. Bond distances are shown in Å.
Scheme 5. Schematic of the Free-Energy Surface for Rhenium(V)−Dioxo Complex ReO2I(PPh3)2 (1) Catalyzing the Hydrosilylation of Phenylmethanimine via the [2 + 2] Addition Pathway
In summary, we could conclude that the ionic outer-sphere pathway is the preferred path for the high-valent rhenium(V)− dioxo complex ReO2I(PPh3)2 (1) catalyzing the hydrosilylation of imines. The ionic outer-sphere pathway is initiated with the silane molecule coordinates to the rhenium center to form the η1-silane rhenium adduct. Afterward, the imine substrate carries out a nucleophilic anti attack at the silane center in the η1-silane rhenium adduct, which results in the heterolytic cleavage of the Si−H bond. Then, the silylated iminium ion abstracts hydride on the anionic rhenium hydride to produce the silylated amine product. The rate-determing step of the ionic outer-sphere pathway corresponds to the backside nucleophilic attack of the silicon in the η1-silane rhenium complex by an imine molecule. The activation energy of the turnover-limiting step for the ionic outer-sphere pathway was calculated to be ∼10 to 5.0 kcal/mol lower in energy than the [2 + 2] addition pathway. These results confirm our most important conclusion, that the rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) catalyzes the hydrosilylation of imines preferentially through an ionic outersphere catalytic cycle. These features indicate that high-valent rhenium(V)−dioxo complex ReO2I(PPh3)2 (1) act as Lewis acid in activating the Si−H bond and catalyzing the hydrosilylation reactions, whereas with aldehyde/ketones, the ionic outer-sphere mechanistic model competes with the [2 +
2.31 Å. To complete the catalytic cycle, a retro-[2 + 2] addition from the rhenium amine intermediate 9, ReO(OSiMe3) I(PPh3)(PhCH2NH), corresponding to the intramolecular nucleophilic amine group attacking the electrophile silicon center, occurs. In the optimized structure of TS10 (Figure 5), the Si−N distance decreases to 2.11 Å. the Si−O distance lengthens to 1.90 Å. This retro-[2 + 2] addition process affords the silyl amine and regenerates the catalyst. In summary, the optimized structures of the intermediates and transition states along the [2 + 2] addition mechanistic pathway are shown in Scheme 5 along with the free-energy profile along 1 → 2 → TS[2 + 2] → 7 → 8 → TS9 → 9 → TS10 → 2+ silylate amine.108 Scheme 5 shows that the overall energies for this [2 + 2] addition mechanistic pathway are 32.7 (TS[2 + 2]), 15.2 (TS9 relative to 7), and 29.0 kcal/mol (TS10 relative to 9). These energies correspond to the Si−H bond adding across the ReO bond in a [2 + 2] manner, the reduction of imine to form the amine intermediate, and the retro-[2 + 2] addition of intramolecular nucleophilic amine group attacking the electrophile silicon center, respectively. Therefore, the activation energy barrier of TS[2 + 2] is significantly high at 32.7 kcal/mol (35.2 kcal/mol at solventphase optimization) and is the rate-determining step. 3795
DOI: 10.1021/acs.jpca.5b00567 J. Phys. Chem. A 2015, 119, 3789−3799
The Journal of Physical Chemistry A
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2] addition pathway, and the preference for one mechanism over the other is not very significant (
