New Insights into Mechanism of Molybdenum(VI)âDioxo Complex

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New Insights into Mechanism of Molybdenum(VI)-dioxo Complex Catalyzed Hydrosilylation of Carbonyls: An Alternative Model for Activating Si-H Bond Xiaoshuang Ning, Jiandi Wang, and Haiyan Wei J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01978 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 6, 2016

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The Journal of Physical Chemistry

New Insights into Mechanism of Molybdenum(VI)dioxo Complex Catalyzed Hydrosilylation of Carbonyls:An Alternative Model for Activating Si−H Bond Xiaoshuang Ning, Jiandi Wang, 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 * Supporting Information

ABSTRACT. Recently, a series of oxo/nitrido-ReV/MoVI/RuVI/MnV complexes have been demonstrated to be efficient catalysts in activating silanes, and catalyzing hydrosilylations of unsaturated organic substrates. In the present study, the high-valent molybdenum(VI)-dioxo complex MoO2Cl2 catalyzed hydrosilylations of carbonyls was re-investigated using DFT method. Previous experimental and theoretical investigations suggested a [2+2] addition pathway for MoO2Cl2 catalyzed hydrosilylations of ketones. In the present study, we propose an ionic outer-sphere mechanistic pathway to be the most favorable pathway. The key step in the ionic outer-sphere pathway is oxygen atom of C=O bonds nucleophilically attacking the silicon atom in an η1-silane molybdenum adduct. The Si−H bond is then cleaved heterolytically. This process features a novel SN2@Si transition state, which then generates a loosely bound ion pair: anionic molybdenum hydride paired with silylcarbenium ion ([MoO2Cl2H]− ACS Paragon Plus Environment

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[SiR3(OCR'R'')]+) in solvent. The last step is silylcarbenium ion abstracting the hydride on molybdenum hydride to yield silyl ether. The calculated activation free energy barrier of the rate-determing step was 24.1 kcal/mol for diphenylketone (PhC=OPh) and silane of PhMe2SiH. Furthermore, the ionic outersphere pathway is calculated to be ~10.0 kcal/mol lower than the previously proposed [2+2] addition pathway for a variety of silanes and aldehyde/ketone substrates. This preference arises from stronger electrophilicity of the high-valent molybdenum(VI) metal center toward a hydride. Here, we emphasize MoO2Cl2 behaves similar to Lewis acidic trispentafluorophenyl borane B(C6F5)3 in activating Si–H bond.

Introduction Hydrosilylation has been widely used in reducing organic functional groups, carbonyl compounds, alkenes, imines, amides, nitrile, and esters, and, etc,1-16 and is a valuable synthetic technique in organic chemistry for constructing silicon-based products both in industrial and academic settings.17-20 Transition metals ranging from heavy metals Ru21-25, Ir26-29, Rh30-33, Re34-37, and Mo38-42 to first-row transition metals Ti43-48, Fe149-62, Cu63-68, Mn69-72, and Zn73-80, etc81-93 are frequently developed as powerful reducing catalysts to mediate numerous hydrosilylation reactions. Several mechanistic schemes have been proposed based on experimental and theoretical investigations.15,16,94-101 In most cases, a metal hydride intermediate is generated in the catalytic cycle and assumed to be a reactive metal center for reducing the multiple C=X (X=C, O, N, etc) bonds.102-115 For example, in Chalk−Harrod and modified Chalk-Harrod cycles (Scheme 1a), the active metal hydride was proposed to be generated via Si–H bond oxidative addition to low-valent transition-metal atoms.116-118 In the [2+2] addition mechanism (Scheme 1b), the active metal hydride was proposed to be generated via Si−H bonds 1,2 cycloaddition to M=O bond. Moreover, the [2+2] addition mechanism has recently been proposed for high-valent rhenium/molybdenum complexes bearing multiply bound oxo/nitrido ligands catalyzing the hydrosilylation reaction of aldehydes and ketones.119-124 In both mechanisms (Scheme 1a, b), the reaction proceeds via unsaturated substrates coordinating to the transition metal hydride intermediates. ACS Paragon Plus Environment

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Then the double bond of unsaturated substrates migrate into the metal-hydride bond, to complete reduction of unsaturated double bond. The most recent mechanistic proposals include the Glaser−Tilley mechanism and the ionic outersphere mechanism. Glaser−Tilley (Scheme 1c) mechanism was proposed for cationic rhodium complexes catalyzing olefin hydrosilylation.125-133 It involves generation of a silylene dihydride as the key intermediate. Then the ketone coordinates to the silicon center through its oxygen atom. Finally, the hydride on the metal center transfers to the carbonyl carbon atom. The ionic outer-sphere mechanism was proposed for several cationic iridium and ruthenium complexes in catalytic carbonyls hydrosilylation.133-137 It involves ketone substrate attacking the silicon center in transition metal silane σcomplexes, resulting the Si−H bond cleavaged heterolytically. Then, the activated ketone accepts a hydride from the metal. Scheme 1. Description of (a) Chalk−Harrod mechanism and modified Chalk-Harrod mechanism, (b) [2+2] addition mechanism, (c) Glaser−Tilley mechanism (GT), and (d) the ionic outer-sphere mechanism. (a) Chalk−Harrod and modified Chalk-Harrod mechanism

(b) [2+2] addition mechanism n

M

O

OCHR'R''

O CR'R'' H O CR'R'' Mn M H OSiR3 OSiR3

R3Si H

Mn

n

Mn

O

OCHR'R''

OSiR3

SiR3

(c) Glaser −Tilley (GT) mechanism M

R2Si H2

H M H

H

R Si

O CR'R'' R

R

M

Si

H

O

R

H R

M

CR'R''

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R

O CR'R''H

3


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(d) Ionic out-sphere mechanism R3Si M

R3Si H

M

H

O CR'R'' SiR3

M

H + R'

O SiR3OCHR'R'' M C R''

Due to the fact that reactive intermediates have not yet been able to be isolated or clearly detected experimentally, mechanistic investigations on these hypotheses for the transition metal complexes catalyzing reactions have been somewhat hampered. Recently, high-valent transition metals of Re(V)138145

/Mo(VI)146-154/Ru(VI)155-158/Mn(V)159 have emerged as efficient catalysts in activating silanes,

boranes and hydrogens, thus brought increasing interest in this field. Considering the reversal role of high-valent transition metal complexes catalyzing reduction reactions, in contrast to their traditional use as oxidative catalysts160, it is important to define in more detail the specific pathways for those complexes in reduction-type catalysis. Here, we present our computational results to clarify the mechanism of carbonyl hydrosilylation catalyzed by molybdenum(VI)-dioxo complex MoO2Cl2. By investigating energy profiles, it was shown that the previously proposed [2+2] addition pathway featuring Si−H bond of silane adding to one of the Mo=O bond requires high barrier of ~30.0 kcal/mol. In contrast, the ionic outer-sphere pathway is a relatively low energy process, requiring overcoming barrier of ~20.0 kcal/mol. The ionic outer-sphere pathway features the Si−H bond heterolytically cleavaged upon C=O bond of carbonyl substrates attacking at silicon atom coordinated at Mo center in an η1-H(Si) metal adduct. Therefore, in contrast to previous theoretical studies of Strassner

123

and

Calhorda124, our calculational results indicate that the ionic outer-sphere catalytic cycle is the most favored pathway for MoO2Cl2 catalyzed carbonyl hydrosilylations. To the best of our knowledge, ionic outer-sphere pathways are rarely reported.161-164 Most noticeably, heterolytic activation of silanes on highly electrophilic transition metal complexes has been focused on cationic transition metal complexes. For example, Brookhart and co-workers have demonstrated a highly electrophilic iridium complex that could activate Si−H bonds and cleavages Si−H bonds in a heterolytical model.

134,165-169

In addition,

Piers and co-workers have proposed the ionic outer-sphere pathway to account for hydrosilylation of ACS Paragon Plus Environment

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ketones by strongly Lewis acidic trispentafluorophenyl borane B(C6F5)3.170-179 The ion outer-sphere catalytic cycle involves Si−H bonds cleavaged heterolytically, with transferring silyl cation to unsaturated substrates and silane hydrogen to electrophilic metal or borane center. The current findings are particularly interesting in light of high-valent oxo/nitrido-transition metal complexes which act as Lewis acids to catalyze reduction of organic substrates. Notably, the multiply-bonded oxo does not play a role in activating Si−H bonds along the ionic outer-sphere mechanistic pathway. The theoretical investigation presented in this study provides a novel understanding on high-valent transition metal complexes activating X−H (X= Si, B, P and H) bonds, to aid in development of new hydrosilylation reactions and improved catalysts. Computational Details All molecular geometries were performed using B3LYP functional.180-184 Gaussian 09 program was employed for all calculations.185 The triple-zeta basis sets plus polarization functions, 6-311g(d,p) were used for carbon, nitrogen, silicon, chloride and oxygen atoms. The LanL2DZ basis set were used for molybdenum atom,186-188 with polarization functions added (Mo, ζf = 1.043) (BS1). Frequency calculations were preformed at the same level of theory to identify transition states with only one imaginary frequency and intermediates with zero imaginary frequency. Furthermore, intrinsic reaction coordinates (IRC) were calculated on some important transition states to confirm them toward the corresponding minima. The SMD solvation model has been employed to optimize all geometry structures under solvent conditions.189 The final Gibbs energies were single-point energies calculations based on a higher-level basis set, BS2, with quintuple zeta set with extra polarization basis sets (ccQZVP) for molybdenum atom and 6-311++G(2d,p) for carbon, nitrogen, silicon, chloride and oxygen atoms.190-191 Corrections for dispersion effects using Grimme’s empirical dispersion correction: B3LYPD3 was employed to refine the final energies.192-196 Comparisons of results on DFT methods and several basis sets are presented in Supporting Information. The geometries are all displayed employing CYLview package.197


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Results and Discussion The model reaction, hydrosilylation of diphenylketone with PhMe2SiH catalyzed bymolybdenumdioxo complex MoO2Cl2 was studied. In the following section, two possible reaction cycles were considered. Pathway I involves ketone substrate nucleophilically attacking on the silicon atom in a η1H(Si) metal adduct, taken as the ionic outer-sphere pathway. Pathway II is the [2+2] addition pathway, involving the Si–H bond adding to one of the Mo=O bond in a [2+2] manner, forming a hydride complex. Pathway I: Ionic outer-sphere pathway for MoO2Cl2 (1)-catalyzed hydrosilylation. The ionic outer-sphere pathway begins with silanes coordinating to the catalyst MoO2Cl2. Silane of PhMe2SiH is initially coordinated to Mo center to give η1-silane adduct 2, and the step is endergonic by 9.4 kcal/mol. The transition-metal η1-silane complexes have become the target of immense research interest for their role as important intermediates in metal-catalyzed hydrosilylation reactions.198-199 The Si−H bond distance in adduct 2 is calculated to be 1.57 Å (Fig. 1), showing an elongation of +0.09 Å compared to free silanes (ca. 1.48 Å). And the Mo−H bond in adduct 2 is 1.94 Å, largely lengthened compared with the normal Mo−H bond (ca. 1.68 Å). And angle of the Mo−H−Si is 164.3°. This geometry indicates a weak end-on η1-H(Si) mode in adduct 2. Adduct 2 subsequently undergoes nucleophilic addition of diphenylketone substrate to cleave the Si−H bond. Initially, van der Waals complex 3 is formed (Fig. 1), featuring a long Si−O3(PhC=OPh) distance (3.68 Å). From van der Waals complex 3, a transition state for the Si−H bond of silane cleavaged is determined. TS4 is calculated to possess a barrier of 19.3 kcal/mol. In TS4 (Fig. 1), the Si−H bond distance is increased to 1.76 Å, indicating the Si−H bond is partially breaking. At the same time, silane (PhMe2SiH) hydrogen moves close to the molybdenum atom, forming a Mo−H bond (1.79 Å). In addition, the silyl ion moves close to diphenylketone oxygen, with Si−O3 (Ph2C=O) distance decreased considerably to be 2.28 Å. The transition state TS4 features a SN2@Si mode, and the normal mode of imaginary frequency of this TS corresponds to the Si−H bond being cleavaged, the Mo−H bond


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and Si−O3 (C=O) bond being formed. The geometry around the silicon atom shows a trigonal bipyramidal structure, in which the silicon atom shares a plane with three carbon atoms of methyl and phenyl groups in equatorial positions, the silane hydrogen lies above the plane and the ketone oxygen lies below the plane, both in apical positions. Moreover, ∠Mo−H−Si=172.5º and ∠H−Si−O3=174.8º indicates that the four atoms of Mo−H···Si−O3 are roughly in a line, with. This SN2@Si transition state is analogous with hydrosilylation of ketones by Lewis acidic trispentafluorophenyl borane B(C6F5)3 according to Piers’ proposal 170-179: the Si−H bond is likewise broken with transfer of silane hydrogen to boron center of B(C6F5)3 and the silyl group transfer to a ketone substrate. TS4 evolves with formation of intermediate 4. The optimized structure of 4 (Fig. 1) exhibits a large Si···H separation (3.66 Å), which indicates that the metal-coordinated Si−H bond is cleavaged. The intermediate 4 is an ion pair comprised of an anionic molybdenum hydride [MoO2Cl2H]− and a silylcarbenium ion [PhMe2Si−OCPh2]+. Furthermore, the dissociated state of the ion pair, ([MoO2Cl2H]− + [PhMe2SiOCPh2]+), is 4.3 kcal/mol lower than ion pair 4 , and therefore the two moieties are probably spontaneously separated into two ionic species in solvent.

2

4

3

TS4

5


TS6

7


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Figure 1. The optimized geometric structures 2→3→TS4→4→5→TS6 along the ionic outer-sphere pathway for molybdenum(VI)-dioxo complex MoO2Cl2-catalyzed hydrosilylation of diphenylketone. (The bond distances are shown in Å). To accommodate the desired hydride transfer, the silyl cation rotates in a direction perpendicular to the Mo−H bond, leading to another ion pair, 5 (Fig. 1). The Mo−H−O3−Si arrangement in intermediate 5 is bent with a dihedral angle of 98.3º, in contrast to linear geometry of 4. The two isomers of intermediates 4 and 5 are located within only 0.6 kcal/mol. Fig. 1 shows that the calculated distance between hydrogen atom of the anionic molybdenum hydride and carbon atom of the silylcarbenium ion in 5 is 3.70 Å. A transition state (TS6) for carbon atom in silylated diphenylketone abstracting the hydride on molybdenum center was located, and calculated to require a significantly low barrier of 1.2 kcal/mol (relative to intermediate 5), a barrier that is readily accessible. At this transition state (Fig. 1), the Mo−H distances is increased to 1.79 Å and the C−H distance is decreased to be 1.63 Å. TS6 leads to product-coordinated complex 6, which dissociates into a silyl ether product and catalyst MoO2Cl2. This step is favorable with ∆G (5→1+silylether) = 29.4 kcal/mol. The computed ionic outer-sphere reaction profiles for molybdenum(VI)-dioxo complex MoO2Cl2 catalyzed hydrosilylation (PhMe2SiH) of diphenylketone is summarized in Scheme 2. Note that the ratedeterming step is SN2@Si transition state (TS4), featuring the Si−H bond of silane heterolytically cleavaged. The barrier of 19.3 kcal/mol indicates the ionic outer-sphere pathway to be a facile reaction pathway for MoO2Cl2 catalyzed the carbonyl hydrosilylation.


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Cl

Ph Me

O

Mo Cl

Relative Free Energy (kcal/mol)

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O

Si O C Me Ph TS4

H

O

Cl

Ph

Ph

Mo Cl

H

O

19.3 C O Si H

3 16.9

4 14.6

2 9.4

TS6 15.2

5 14.0

O Me Si

Ph

Me

10.3

[MoO2Cl2H]+ [PhMe2Si-OCPh2]+

0.0 1 silane addition

Ph

C

Si-H bond cleavage

hydride abstraction

6 -4.7

1+ silyl ether

-15.4

Reaction coordinate

Scheme 2. The potential free-energy profile for MoO2Cl2-catalyzed hydrosilylation of diphenylketone along the ionic outer-sphere catalytic cycle. Pathway II: [2+2] addition pathway for MoO2Cl2 (1)-catalyzed hydrosilylation. The first step of the [2+2] addition pathway is Si−H bond of silane adding to one of Mo=O bond to give a five-coordinated tetragonal pyramid hydrido-oxomolybdenum complex. The [2+2] addition transition state TS7 features a Mo···O2···Si···H four-membered cyclic structure (Fig. 2). In TS7, the Si−H bond is breaking with distance of 1.99 Å, the Mo−H bond is forming with distance of 2.24 Å and Si−O2 bond is forming with distance of 1.73 Å, respectively. The barrier of TS7 is calculated to be 29.7 kcal/mol above silane and catalyst. Step 2 of the [2+2] addition pathway involves diphenylketone C=O double bond insertion into molybdenum-hydride bond in intermediate 7 (MoO(OSiPhMe2)Cl2H). As previously reported, intially, diphenylketone molecule coordinates to Mo center, forming an adduct, 8 (MoO(OSiPhMe2)Cl2H(Ph2C=O)). Subsequently, C=O double bond inserted into the Mo−H bond. The corresponding transition state is located, TS9. The migratory insertion step (TS9) is calculated to possedd a barrier of 23.6 kcal/mol above the catalyst. In the transition state TS9 (Fig. 2), diphenylketone carbon atom moves close Mo–H bond, d(C−H) = 1.51 Å and d(Mo−H)=1.81 Å, showing an η2-carbonyl mode, d(Mo−O3) = 2.25 Å andd(Mo−C) = 2.64 Å, respectively. The imaginary vibrational frequency of TS9 clearly shows the Mo−H bond being broken and C−H bond being formed. Thus, a metal alkoxide complex 9 (MoO(OSiPhMe2)Cl2(OCHPh2)) is generated. From the metal alkoxide complex 9, a retro-[2 9 ACS Paragon Plus Environment


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+ 2] addition reaction corresponds to silyl silicon nucleophically attacking the alkoxide oxygen atom to generate the product-coordinated complex and complete the catalytic cycle. The corresponding transition state TS10 is calculated to associate with a barrier of 25.6 kcal/mol. In TS10 (Fig. 2), the Si−O3(C=O) bond is decreased to 2.12 Å, and the Si−O2 is increased to 1.98 Å.

TS7

TS9

TS10

Figure 2. The optimized geometrical structures of TS7, TS9 and TS10 for MoO2Cl2-catalyzed hydrosilylation of diphenylketone, (The bond distances are shown in Å). The computed [2+2] addition catalytic cycle for molybdenum(VI)-dioxo complex MoO2Cl2 catalyzing hydrosilylation (PhMe2SiH) of diphenylketone is summarized in Scheme 3. Note that the ratedeterming step in the [2+2] addition pathway is the [2+2] addition transition state (TS7), being significantly high of 29.7 kcal/mol. Remarkablely, we find that the transition state TS7 is 10.4 kcal/mol higher than the rate-determining step in the ionic outer-sphere pathway (Scheme 2, TS4). Therefore, in contrast to previous studies of Strassner123 and Calhorda124, we suggested that the ionic outer-sphere mechanistic pathway involving formation of η1-silane molybdenum adduct is kinetically more favorable than the previously proposed [2+2] addition mechanism for MoO2Cl2 catalyzed ketones hydrosilylations.


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The Journal of Physical Chemistry migration insertion

TS7 29.7

O

Cl Me3SiO Mo Cl H O C Ph Ph

Si H

0.0 1

C O 7

O

Me3SiO Mo H

TS10 25.6

TS9 23.6

8 0.6 O

Cl -3.9 Cl

retro-[2+2] addition

Cl

Me3SiO Mo

Cl

O C Ph

H

1+ silyl ether -15.4

9 -14.9

Ph

Reaction coordinate

Scheme 3.The potential free-energy profile for MoO2Cl2-catalyzed hydrosilylation of diphenylketone via the [2+2] addition catalytic cycle. We next explored the scope of this preference for molybdenum(V)-dioxo complex MoO2Cl2 catalyzed hydrosilylation reactions using a variety of silanes and aldehyde/ketone substrates. Our results for the transition state TS7 and the transition state TS4 are summarized in Table 1. Table 1. Activation free energies (kcal/mol) for transition states of TS4 and TS7 with various carbonyls and silanes for molybdenum(V)-dioxo complex MoO2Cl2 catalyzed the hydrosilylation reaction. Silanes

Ketones

TS4

TS7

PhMe2SiH

PhC=OPh

19.3

29.7

Ph2MeSiH

PhC=OPh

23.0

30.6

Me3SiH

PhC=OPh

23.9

30.6

SiH4

PhC=OPh

20.8

36.9

SiH4

PhCH=O

23.1

36.9

As depicted in Table 1, hydrosilylation reactions of diphenylketone with a number of silanes, PhMe2SiH, Ph2MeSiH, Me3SiH, SiH4 under the catalyst of MoO2Cl2 all prefer the ionic outer-sphere pathway. The corresponding transition states TS4 are ~7−13 kcal/mol lower than transition states TS7. Moreover, it is worth noting that with silane of SiH4, the ionic outer-sphere pathway is characterized


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with a triple-well potential energy surface compared to the double-well potential energy surface. In the optimized geometry of the central intermediate, 41-SiH4, as shown in Fig. 3, the coordination around the silicon atom is pentavalent, with Si−H bond is elongated to 2.08 Å, and Mo−H bond is partially formed, d(Mo−H) = 1.71 Å. and diphenylketone oxygen atom is bound to the silyl group with distance of 1.81 Å. To proceed, the Mo−H is cleavaged, and the ion pair intermediate, 42-SiH4 is formed in which the Si··· H bond is separated with distance of 2.26 Å, forming the anionic molybdenum hydide (d(Mo−H) = 1.70 Å)), and the silylcarbenium ion (d(Si−O3)= 1.78 Å).

41-SiH4

42-SiH4

Figure 3. The optimized geometric structures of 4-SiH4 and 42-SiH4 for MoO2Cl2-catalyzed hydrosilylation (silane of SiH4) of diphenylketone. Along the ionic outer-sphere catalytic cycle, first, a free silane coordinates to to molybdenum(V)dioxo complex MoO2Cl2 forming the η1-silane molybdenum adduct 2, passing the SN2@Si transition state (TS4), to from the ion pair intermediate 4, the Si−H bond strength is weaken progressively. As indicated by Wiberg bond index (WBI) associated with Si−H interaction, WBI decreased along 0.912 (Ph2MeSiH) → 0.536 (2) → 0.514 (3) → 0.306 (TS4) → 0.001 (4), thus indicating a weaker, i.e., less stabilizing Si···H interaction along the process. The rate determining step in the ionic outer-sphere pathway is the SN2@Si transition state TS4. An analysis of frontier orbitals of van der Waals complex 3, the transition state TS4 and the ion pair intermediate 4 is presented in Fig. 4. The interacting frontier orbital of van der Waals complex 3 consists of molybdenum dxz orbital, and the Si−H bond σ bonding orbital. Upon attacking of carbonyl substrates, the interacting frontier orbital in the transition state TS4 mainly consists of molybdenum dxz orbital, σ bonding orbital of Si−H bond, together with π orbital from ACS Paragon Plus Environment

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carbonyl oxygen atom. In the ion pair 4, the molybdenum dxz orbital clearly forms a bonding orbital with the hydride. This pair of interaction orbitals presented revealed the molybdenum metal can attract electron from silane hydrogen. Subsequently, from van der Waals complex 3 to TS4, the Mo−H interaction is enhanced (1.91 Å→1.79 Å) while the Si−H interaction is considerably weakened (1.58 Å →1.76 Å). This argument is supported by our calculations of NBO charges in van der Waals complex 3, and the transition state TS4 (Fig. 4). Inspection of NBO charge indicates that with stretching of Si－H, NBO charges of silane hydrogen become less negative, along −0.243 e (2)→ −0.233 e (3)→ −0.175 e (TS4)→-0.007 e (4), respectively. Accordingly, NBO charges of silicon becomes more positive, along 1.637 e (2) → 1.661 e (3) →1.783 e (TS4) → 1.877 e (4). Accompanying with the Si−H bond distance increasing, silane hydrogen donated its electron to Mo atom progressively. Thus, NBO charges on Mo becomes less positively, along 0.917 e (2)→ 0.867 e (3)→ 0.834 e (TS4) → 0.812 e (4), as well as Wiberg bond index associated with Mo−H interaction increased along 0.296 (2) → 0.320 (3) →0.463 (TS4) → 0.823 (4), indicating a stronger, i.e., more stabilizing, Mo···H interaction in the process.

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Figure 4. Relevant orbitals and NBO charges in van der Waals species 3, transition state TS4 and ion pair 4. Therefore, upon interaction with molybdenum unoccupied d orbitals, the Si−H σ bonding orbital is effectivelypolarized. Such polarization would induce the Si−H bond heterolytic cleavaged upon nucleophilically attacking of unsaturated substrates. Until now, neutral transition metal complexes mediating reduction reactions favoring the ionic outer-sphere pathway has been rarely reported. 200 In summary, the electron-accepting ability, or the Lewis acidity, of molybdenum center in the molybdenum(VI)-dioxo complex MoO2Cl2 is stronger in induing the heterolytic cleavage of Si−H bond. Conclusion We have reinvestigated the high-valent molybdenum(VI)-dioxo complex MoO2Cl2 catalyzed hydrosilylation reactions of carbonyls applying DFT method. Previously, the multiply bound oxo ligand is previously suggested to directly participate in activating the Si−H bond of silane. And the [2+2] addition pathway featuring Si−H bond adding to one of Mo=O bond is proposed to be primarily responsible for hydrosilylation of carbonyls using catalyst MoO2Cl2 by Strassner

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and Calhorda124.

However, our in-depth studies indicate that a different mechanism − the ionic outer-sphere pathway involving carbonyl substrates back-side attacking the silicon center of η1 –silane molybdenum adduct, to heterolytically cleavage Si−H bonds is the preferable path. The ionic outer-sphere pathway is calculated to possess a relatively low barrier (the rate-determining step of 19.3 kcal/mol) compared to the [2+2] addition pathway (the rate-determining step of 29.7 kcal/mol). The ionic outer-sphere pathway for hydrosilylation reactions with molybdenum(VI)-dioxo catalysts MoO2Cl2 closely resembles the catalytic cycle proposed for Lewis acid B(C6F5)3 catalyzed hydrosilylation of carbonylc compounds, featuring silane hydrogen transfering to B(C6F5)3 and simultaneously silyl cation (R3Si+) transfering to the organic substrates. The main characters of the ionic outer-sphere mechanistic pathway are as follows: (1) the multiply bound oxo atom does not play role in activating silanes, (2) the unsaturated substrates does not need coordinates to metal centers. Our ACS Paragon Plus Environment

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calculation results demonstrates that molybdenum(VI)-dioxo complex MoO2Cl2 favoring the ionic outer-sphere mechanism is due to high electrophilicity of the molybdenum(VI) atom. The electron density of silane is thus drawn to central metal through the bridging hydrogen atom, which promotes Si−H bond of silanes heterolytically cleavaged. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Complete Ref. 185, , comparison of different methods and basis set, and the selected geometrical parameters for all calculated structures of reactants, intermediates, transition states, and products. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +86 15301584462. ACKNOWLEDGMENT The Computer resources for theoretical calculations were provided by Jiangsu Provincial Key Laboratory for NSLSCS in Nanjing Normal University. We acknowledge the National Natural Science Foundation of China No. 21103093, and Chair Professor of Jiangsu Province to Start Funds for the financial support of this research and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1) Ojima, I.; Li, Z.; Zhu, J. Recent Advances in Hydrosilylation and Related Reactions, in: Z. Rappoport, Y. Apeloig (eds) The Chemistry of Organic Silicon Compounds, Wiley Chichester, 1998, vol. 2, Chapter 29. (2) Rappoport, Z.; Apeloig, Y. The Chemistry of Organic Silicon Compounds; Wiley: Chichester, U.K., 1998.


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B.;

Romão,

C.

C. Synthesis,

Bis(Chloro)Dioxomolybdenum(VI)-Chiral

Characterization

Diimine

Complexes.

and

Catalytic

J.

Mol.

Studies

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Catal.

A:

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Kryspin,

I.

H.;

Grimme,

S.

The

Cr-Mn

Interaction

in

syn-Facial

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Page 30 of 30

SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).

O Cl Cl PhMe2SiH

Mo

H

Si

O

O ∆G = 19.3 kcal/mol

C

+ C O +

the ionic outer-sphere pathway is

10.4 kcal/mol prefered than

H

O

PhMe2Si

the [2+2] addition pathway

MoO2Cl2

O Cl Cl

Mo

O

H Si

∆G = 29.7 kcal/mol


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New Insights into Mechanism of Molybdenum(VI)âDioxo Complex

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