Theoretical Studies on Si−C Bond Cleavage in ... - ACS Publications

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J. Phys. Chem. A 2010, 114, 6047–6054

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Theoretical Studies on Si-C Bond Cleavage in Organosilane Precursors during Polycondensation to Organosilica Hybrids Soichi Shirai,†,‡ Yasutomo Goto,†,‡ Norihiro Mizoshita,†,‡ Masataka Ohashi,†,‡ Takao Tani,†,‡ Toyoshi Shimada,‡,§ Shi-aki Hyodo,† and Shinji Inagaki*,†,‡ Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan, Core Research and EVolutional Science and Technology (CREST), Japan Science and Technology (JST), Kawaguchi, Saitama 332-0012, Japan, and Department of Chemical Engineering, Nara National College of Technology, Yamatokoriyama, Nara 639-1080, Japan ReceiVed: February 9, 2010; ReVised Manuscript ReceiVed: April 9, 2010

Molecular orbital theory calculations were carried out to predict the occurrence of Si-C bond cleavage in various organosilane precursors during polycondensation to organosilica hybrids under acidic and basic conditions. On the basis of proposed mechanisms for cleavage of the Si-C bonds, the proton affinity (PA) of the carbon atom at the ipso-position and the PA of the carbanion generated after Si-C cleavage were chosen as indices for Si-C bond stability under acidic and basic conditions, respectively. The indices were calculated using a density functional theory (DFT) method for model compounds of organosilane precursors (R-Si(OH)3) having organic groups (R) of benzene (Ph), biphenyl (Bp), terphenyl (Tph), naphthalene (Nph), N-methylcarbazole (MCz), and anthracene (Ant). The orders for the predicted stability of the Si-C bond were Ph > Nph > Bp > Ant > Tph > MCz for acidic conditions and Ph > MCz > Bp > Nph > Tph > Ant for basic conditions. These behaviors were primarily in agreement with experimental results where cleavage of the Si-C bonds occurred for Tph (both acidic and basic), MCz (acidic), and Ant (basic). The Si-C bond cleavage of organosilane precursors during polycondensation is qualitatively predicted from these indices based on our theoretical approach. 1. Introduction Organosilane compounds of general formula R-[Si(OR′)3]m (m > 1, R ) organic group, R′ ) CH3, C2H5, etc.) have been widely used as precursors for the synthesis of homogeneous organosilica hybrid materials through hydrolysis and polycondensation reactions.1 The organosilica hybrids can exhibit a large variety of functionalities with promising applications in many areas, such as catalysis, separation, and optical devices, by design of the organic moieties (R). In particular, periodic mesoporous organosilicas (PMOs) can be synthesized from bridged organosilane precursors, R-[Si(OR′)3]2, in the presence of template surfactants.2,3 In addition, the interactive organic groups of organosilane precursors can form molecular-scale periodicity in the mesoporous4 and nonporous organosilicas5 by self-organization of the organosilane precursors. The highly organized organosilicas are expected to exhibit unique optical and electrical properties because of the specific arrangement and interaction of the organic groups. However, in some cases, the Si-C bonds of organosilane precursors are unexpectedly cleaved during polycondensation to organosilica hybrids under acidic and basic conditions. For example, Yoshina-Ishii et al. reported the Si-C bond cleavage of thiophen-bridged organosilane during the synthesis of PMO under basic conditions.6 However, Morell et al. successfully synthesized thiophen-bridged PMO with a low degree of Si-C bond cleavage under acidic conditions,7 and Goto et al. obtained * To whom correspondence should be addressed. E-mail: inagaki@ mosk.tytlabs.co.jp. † Toyota Central R&D Laboratories, Inc. ‡ Japan Science and Technology (JST). § Nara National College of Technology.

9,10-anthracene-bridged PMO without Si-C bond cleavage only under weak acidic conditions.8 Ethane-,2a,b,9 ethylene-,2b,c and benzene-bridged PMOs4a,10 were prepared with almost no Si-C bond cleavage under both acidic and basic conditions. To the best of our knowledge, there have been no systematic studies on the Si-C bond stability of organosilane precursors during polycondensation, although it has been empirically understood that Si-C bond stability is strongly dependent on the nature of the organic groups and the reaction conditions. For the design of new organosilica hybrids with various functionalities, it is of significant importance to understand the mechanisms of Si-C bond cleavage under acidic and basic conditions and therefore be able to predict the Si-C bond stability of the organosilane precursors theoretically. Si-C bond cleavage has been observed in substitution reactions of arylsilane compounds (Ar-SiR13, R1 ) CH3, Cl, etc.) and organosilane compounds with leaving groups (XSiR13, X ) Cl, Br, etc. and R1 ) CH3, Ph, etc.) in the presence of strong electrophilic or nucleophilic reagents, and the mechanisms of the substitution reactions have been well understood.11 In the presence of electrophilic reagents, substitution of the SiR13 group proceeds by the following two steps: (1) formation of a σ-complex (Wheland intermediate12) via electrophilic attack to a carbon atom at the ipso-position and then (2) elimination of the SiR13 group through Si-C bond cleavage. It is reasonable to presume that Si-C bond cleavage of the organosilane precursors under acidic conditions proceeds via a similar mechanism to that for the substitution reaction (R1 ) OR′, and electrophile is H+) (Figure 1a), and the Si-C bond stability can be evaluated from the activation energy of the σ-complex formation reaction because this step is usually a rate-determining step in the overall reaction process.11 However, in the presence

10.1021/jp101242g  2010 American Chemical Society Published on Web 04/29/2010

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Figure 1. Assumed Si-C bond cleavage reactions of organosilane during polycondensation to organosilica hybrid under (a) acidic conditions and (b) basic conditions. C6H5-[Si(OR′)3] is employed as an example of a substrate.

Figure 2. Energy profile of the assumed Si-C bond cleavage reaction under acidic conditions. C6H5-[Si(OR′)3] is employed as an example of a substrate.

of nucleophilic reagents, substitution of the SiR13 group proceeds according to the following two steps: (1) formation of an intermediate complex [X-SiR13-Y]- (X: leaving group, Y-: nucleophile) via nucleophilic attack to a silicon atom and then (2) elimination of X- with Si-C bond cleavage. The Si-C bond cleavage of organosilane precursors under basic conditions is considered to proceed by a similar mechanism (X- ) R-, R1 ) OR′, and Y- ) OH-) (Figure 1b), and the Si-C bond stability can be evaluated on the basis of the stability of the carbanion.11 In this article, we calculate the proton affinities (PAs) of both carbon atoms at the ipso-position and carbanions by theoretical computations, and these indices are used to represent the Si-C bond stability under acidic and basic conditions for model compounds of organosilane precursors, [R-Si(OH)3], with various organic groups (R). The calculated orders of the Si-C bond stability for the organic groups were in good agreement with the experimental results (Si-C bonds are cleaved or not) for PMOs prepared from the bridged organosilane precursors. 2. Theory 2.1. Acidic Conditions. The rate-determining step in the Si-C bond cleavage reaction of organosilane under acidic conditions is a process involving σ-complex formation (Figure 2).11 Therefore, the activation barrier height of the reaction can be used to determine the occurrence of Si-C bond cleavage.

However, calculation of the activation energy based on the transition state structure is generally difficult and time-consuming, even for a small molecular system. Instead, we employ the stability of the σ-complex, which can be defined as the energy difference between the σ-complex and the original substrate, as an index for the occurrence of Si-C bond cleavage, because the reactivity of aromatic molecules for electrophilic substitution has been known to correlate well with the relative stabilities of their σ-complexes.13,14 For example, several studies have revealed that reactivity at the ortho-, meta-, and para-positions sites of monosubstituted benzenes well correlate with the theoretically calculated stabilities of their σ-complexes.15–18 In particular, Szabo et al. investigated the nitration of benzene and toluene by ab initio calculations and demonstrated that the higher reactivity of toluene than that of benzene and the ortho/para selectivity could be well explained by the stability of their σ-complexes.18 They also succeeded in calculations of the transition state structures and activation barriers for σ-complex formation for benzene and toluene with consideration of the desolvation process19 and showed that the activation barrier order was in good agreement with the stabilities of the σ-complexes. These reports strongly support the validity of our proposed index for the occurrence of Si-C bond cleavage. The stability of the σ-complex of organosilane compounds is also regarded as corresponding to the PA20 of the carbon atom at the ipso-position (PA(ipso-C)) because the electrophile in the Si-C bond cleavage reaction is a proton (H+). Here PA(ipso-C) is the negative enthalpy change of formation for the σ-complex (-∆H).20 2.2. Basic Conditions. The Si-C bond cleavage reaction of organosilane compounds under basic conditions is a typical bimolecular nucleophilic substitution, SN2 on a Si atom. In the reaction, OH- and organic group carbanion (R-) are the nucleophile and leaving-group, respectively, as illustrated in Figure 1b. The reactivity of the SN2 reaction is known to be strongly dependent on the leaving-group ability, which is the tendency of atoms or groups to depart with the bonding electron pair.20–24 Therefore, Si-C bond cleavage is considered to be strongly dependent on the ability of R to form R- and thus the acidity of its conjugate acid, RH.22–24 Therefore, we employed stability of R-, which can be obtained from the energy difference between RH and R-, as an index for the occurrence of Si-C bond cleavage. The index is also regarded as the proton affinity of R-, PA(R-). Here PA(R-) is the negative enthalpy change for the reaction between H+ and R- (-∆H).20

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Figure 3. Molecular structures investigated in this work: R1 ) R2 ) Si(OC2H5)3 in organosilane precursors for the experiments and R1 ) Si(OH)3 and R2 ) H in trihydroxysilanes treated in the computations.

TABLE 1: Synthesis Conditions for 1,4-Benzene (Ph), 4,4′-Biphenyl (Bp), 4,4′-Terphenyl (Tph), 2,6-Naphthalene (Nph), 2,6-Anthracene (Ant), and 3,6-N-Methylcarbazole (MCz)-Bridged PMOs under Acidic Conditions organosilane precursor (g) (EtO)3Si-Ph-Si(EtO)3 (EtO)3Si-Bp-Si(EtO)3 (EtO)3Si-Tph-Si(EtO)3 (EtO)3Si-Nph-Si(EtO)3 (EtO)3Si-Ant-Si(EtO)3 (EtO)3Si-MCz-Si(EtO)3 a

0.50 0.60 0.10 0.30 0.42 0.32

surfactant (g) C18TMACl C18TMACl C18-C12-C18 C18TMACl C18-C12-C18 C18TMACl

2 M HCl (g)

H2O (g)

2.00 2.00 0.33 15.00 0.67 1.00

36 36 6 20 12 18

0.35 0.35 0.08 0.40 0.17 0.17

synthesis temperature and timea 100 °C for 24 h 100 °C for 24 h 100 °C for 20 h 95 °C for 24 h 60 °C for 120 h 100 °C for 24 h

Reaction mixtures were stirred at r.t. for 24 h before heat-treatments.

TABLE 2: Synthesis Conditions for 1,4-Benzene (Ph), 4,4′-Biphenyl (Bp), 4,4′-Terphenyl (Tph), 2,6-Naphthalene (Nph), 2,6-Anthracene (Ant), and 3,6-N-Methylcarbazole (MCz)-Bridged PMOs under Basic Conditions organosilane precursor (g) (EtO)3Si-Ph-Si(EtO)3 (EtO)3Si-Bp-Si(EtO)3 (EtO)3Si-Tph-Si(EtO)3 (EtO)3Si-Nph-Si(EtO)3 (EtO)3Si-Ant-Si(EtO)3 (EtO)3Si-MCz-Si(EtO)3 a

4.00 1.00 0.20 0.40 0.20 0.30

surfactant (g) C18TMACl C18TMACl C18TMACl C18TMACl C18TMACl C18TMACl

6 M NaOH (g)

H2O (g)

8.00 5.00 0.20 2.00 0.20 0.30

120 50 12 48 12 18

3.60 0.92 0.16 0.35 0.16 0.24

synthesis temperature and timea 100 °C for 24 h 100 °C for 24 h 100 °C for 20 h 95 °C for 24 h 100 °C for 24 h 100 °C for 24 h

The reaction mixtures were stirred at r.t. for 24 h before heat-treatments.

3. Experimental and Computational Details 3.1. Experiments. 1,4-Bis(triethoxysilyl)benzene (R ) Ph),1 4,4′-bis(triethoxysilyl)biphenyl (R ) Bp),1 2,6-bis(triethoxysilyl)anthracene (R ) Ant),25 and 3,6-bis(triethoxysilyl)-N-methylcarbazole (R ) MCz)26 were purchased from Nard Research Institute. 4,4′-Bis(triethoxysilyl)terphenyl (R ) Tph)1 and 2,6bis(triethoxysilyl)naphthalene (R ) Nph)27 were synthesized by Rh-catalyzed silylation.28 The molecular structures of these bridged organosilanes are shown in Figure 3. Octadecyltrimethylammonium chloride (C18TMACl) or 1,12-bis(octadecyldimethylammonium)dodecane dibromide (C18-C12-C18) were used as cationic template surfactants. HCl and NaOH were used as acid and base catalysts for hydrolysis and polycondensation reactions, respectively. The organosilane precursors were mixed with the surfactant aqueous solution containing the HCl or NaOH catalyst and stirred with heating, which resulted in the formation of precipitates. The detailed experimental conditions are summarized in Tables 1 and 2. The precipitates were recovered by filtration, washed repeatedly with water, and dried in vacuum at room temperature to yield PMOs. Powder X-ray

diffraction (XRD) measurements were carried out using a diffractometer (Rigaku, RINT-TTR) with Cu-KR radiation (50 kV, 300 mA). The bonding states of Si atoms were evaluated by 29Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR; Bruker, Avance 400) conducted at 79.49 MHz with a spinning rate of 5 kHz. The chemical shifts for all spectra were referenced to tetramethylsilane at 0 ppm. The occurrence of Si-C bond cleavage was judged from the existence of Qn signals assigned to Si species without Si-C bonds [Si(OH)4-n(OSi)n, n ) 0-4], typically observed between -90 and -120 ppm, in addition to Tn signals assigned to Si species with Si-C bonds [R-Si(OH)3-n(OSi)n, n ) 0-3], typically observed between -50 and -90 ppm.2,6–8,10 The Qn/ Tn ratios were estimated in the following procedure: (1) each peak in the 29Si NMR spectrum between -50 and -120 ppm was fit with a Gaussian function and (2) the peak areas were obtained as sums of the integrals of the fitting functions between -50 to -90 ppm for Tn signals and between -90 and -120 ppm for Qn signals, respectively. The fittings were carried out with a baseline of zero for simplification of the calculation. Our

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previously reported 29Si MAS NMR and XRD results were used for 1,4-benzene-PMO (Ph-PMO)4a and 4,4′-biphenyl-PMO (BpPMO) under basic condictions4b and 2,6-naphthalne-PMO (NphPMO)27 and 2,6-anthracene-PMO (Ant-PMO)25 under acidic conditions. 3.2. Computations. Calculations were carried out for model compounds of the organosilane precursors: R-Si(OH)3 with R ) benzene, biphenyl, terphenyl, naphthalene, anthracene, and N-methylcarbazole, and as an additional investigation, for 2-(trihydroxysilyl)thiophene (R-Si(OH)3 with R ) Thio), the molecular structures of which are shown in Figure 3. The σ-complexes were constructed by attaching H+ to the carbon atoms at the ipso-position. We modeled carbanions by eliminating the Si(OH)3 group in R-Si(OH)3. Modeled carbanions were the phenide (Ph-), 4-biphenylide (Bp-), 4-p-terphenylide (Tph-), 2-naphthalenide (Nph-), 2-anthracenide (Ant-), 3-N-methylcarbazolide (MCz-), and 2-thiophenide (Thio-) anions from PhSi(OH)3, Bp-Si(OH)3, Tph-Si(OH)3, Nph-Si(OH)3, Ant-Si(OH)3, MCz-Si(OH)3, and Thio-Si(OH)3, respectively. All calculations were carried out using density functional theory (DFT) methods with the B3LYP hybrid exchange-correlation functional29 employing Pople’s 6-311++G(d,p) as a basis set.30 The DFT method using B3LYP is expected to provide highly accurate PA,24,31,32a and the addition of diffuse basis functions is necessary to obtain accurate PAs.32 Molecular charges of the σ-complexes and the carbanions were set as +1 and -1, respectively, whereas the substrates and the conjugate acids were set as neutral species. Restricted B3LYP was applied because all species have closedshell singlet configurations. The geometries of R-Si(OH)3, their σ-complexes (for acidic conditions), carbanions, and conjugate acids (for basic conditions) were optimized. At the optimized geometries, analytical second-derivative calculations were performed to ensure the absence of imaginary frequencies and to evaluate the vibrational and thermal corrections. The enthalpy, H, of a molecule is defined by the following equation

H ) E + zero point energy correction + thermal correction where E represents the total electronic state energy, and the thermal correction includes vibrational, rotational, and translational terms.33 We set the enthalpy of a proton to 5/2RT ) 6.20 kJ/mol, where R is the gas constant and T is the temperature set to 298.15 K.34 All calculations were performed using the Gaussian 03 program package.35 4. Results and Discussion 4.1. Experimental Results of Si-C Bond Stability. The obtained PMO materials, except for Tph-PMO (acidic), had ordered mesostructures that were confirmed by XRD measurements (Figures S1 and S2, Supporting Information). Tph-PMO (basic) showed relatively low structure ordering compared with the other PMOs synthesized under basic conditions. Figures 4 and 5 show 29Si MAS NMR spectra for the PMOs prepared under acidic and basic conditions, respectively. For syntheses under acidic conditions, both Qn signals and Tn signals were observed for Tph- and MCz-PMOs (Figure 4), the Qn/Tn ratios of which were 0.7 and 2.3, respectively. These results indicate that the Si-C bonds were partially cleaved during the sol-gel condensation. However, for syntheses under basic conditions, Qn signals were only observed for Tph- and Ant-PMO (Figure 5), the Qn/Tn ratios of which were 2.0 and 1.8, respectively. No prominent Qn signals were observed for the other PMOs,

Shirai et al.

Figure 4. 29Si MAS NMR spectra for the (a) benzene (Ph), (b) biphenyl (Bp), (c) terphenyl (Tph), (d) naphthalene (Nph), (e) anthracene (Ant)-silica hybrids, and (f) N-methylcarbazole (MCz)-PMOs prepared under acidic conditions. Si-C bonds were maintained for the Ph, Bp, Nph, and Ant-silica hybrid syntheses, whereas cleavage of Si-C bonds occurred for the Tph and MCz-silica hybrid syntheses.

indicating that their Si-C bonds were maintained after the sol-gel condensation. 4.2. Calculations for Si-C Bond Stability under Acidic Conditions. Table 3 lists the calculated enthalpies (H) for the substrates (R-Si(OH)3) and their σ-complexes, the negative enthalpy changes (-∆H) for formation of the σ-complexes, and the PA(ipso-C)s. Vibrational analyses revealed no vibrational modes with imaginary frequencies, indicating that all obtained molecular structures were on stationary points. The calculated PA(ipso-C)s are positive (negative in ∆H) for all cases, which indicates that the σ-complexes are more stable than the corresponding substrates, although σ-complex formation is generally endothermic (Figure 2). This is because the desolvation of protons, which is highly endothermic, is neglected in the present calculations.19 However, this does not influence the order of the calculated PA(ipso-C)s because the desolvation energy of a proton can be considered to be constant for all cases. PA(ipso-C) increases in the order of Ph-Si(OH)3 < Nph-Si(OH)3 < Bp-Si(OH)3 < Ant-Si(OH)3 < Tph-Si(OH)3 < MCz-Si(OH)3. The order corresponds well with the experimental results in which Si-C cleavage occurred during the preparation of Tphand MCz-PMOs under acidic conditions. The difference between PA(ipso-C)s of Ant-Si(OH)3 and Tph-Si(OH)3 is 1.59 kJ/mol. Despite a very small difference, Si-C cleavage was observed

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Figure 5. 29Si MAS NMR spectra for the (a) benzene (Ph), (b) biphenyl (Bp), (c) terphenyl (Tph), (d) naphthalene (Nph), (e) anthracene (Ant)-silica hybrids, and (f) N-methylcarbazole (MCz)-PMOs prepared under prepared under basic conditions. The Si-C bonds are maintained for the Ph, Bp, Nph, and MCz-silica hybrid syntheses, whereas cleavage of the Si-C bonds occurred for the Tph and Ant-silica hybrid syntheses.

only in the preparation of Tph-PMO (Figure 4). According to the previous reports,23,36,37 a small difference in the PA can dramatically change relative reaction rate. Therefore, it is possible that a small difference in PA(ipso-C)s changes the behaviors in Si-C cleavage occurrence. To confirm the accuracy of our calculations, we compared the present PA(ipso-C) values with previously reported experimental and calculated PA values for silylated or nonsilylated aromatic compounds. The calculated PA(ipso-C) for Ph-Si(OH)3 is 838.25 kJ/mol, which is significantly higher than the

experimental PA for benzene (750.2 kJ/mol)38 while slightly higher than that for phenylsilane (Ph-SiH3; 827.6 kJ/mol) obtained by Nam et al.15 This behavior can be attributed to the polarization of the Si-C bond (Si+C-)11,39 and the high electrondonating property of OH groups.22 In acenes, the PA(ipso-C) increases in the order of PhSi(OH)3, Nph-Si(OH)3, and Ant-Si(OH)3, which agrees with the order of the experimental PAs for benzene (750.2 kJ/mol), naphthalene (802.9 kJ/mol), and anthracene (877.3 kJ/mol),38 although the experimental PAs pertain to the most reactive positions. The present order is also in good agreement with the order of calculated PAs using MP2 for benzene (765.4 kJ/mol), the 2-position of naphthalene (810.8 kJ/mol), and the 2-position of anthracene (841.9 kJ/mol).37 The PA(ipso-C) for Bp-Si(OH)3 (873.23 kJ/mol) is higher than the experimental PA for biphenyl (813.8 kJ/mol),40 which is similar to the relation between Ph-Si(OH)3 and benzene. This suggests that the positive charge of the σ-complex is more efficiently stabilized by delocalization over two phenyl rings. Delocalization of the positive charge in the σ-complex of BpSi(OH)3 can be understood based on resonance structures.40 The inter-ring torsion angle (φ) in Bp-Si(OH)3 was calculated to be 40.2°, which is similar to the experimental φ of 44.4 ( 1.2° in biphenyl.41 σ-Complex formation decreases the φ from 40.2 to 26.2°, which indicates an increase in the double bond character in the inter-ring C-C bond.40 Similarly, the calculated PA(ipsoC) for Tph-Si(OH)3 was higher than that for Bp-Si(OH)3 by 13.56 kJ/mol because of further stabilization of the positive charge by substitution of one more phenyl ring. The φ values of Tph-Si(OH)3 in the optimized structure are 39.4 (near Si) and 40.4° (far from Si), which is close to the theoretically obtained value (38.4°) for p-terphenyl.42 σ-Complex formation decreases φ from 39.4 to 21.0° (near Si) and from 40.4 to 33.1° (far from Si), respectively, which indicates an increase in the double-bond character of the inter-ring C-C bonds, as seen for Bp-Si(OH)3. For Tph-Si(OH)3 and its σ-complex, there can be two conformations in terms of two internal torsions: helical and nonhelical. From our calculations, helical conformers were obtained as a result of geometry optimization. Comparison of the two conformers was not carried out because the energy difference between the conformers was expected to be negligibly small.42 MCz-Si(OH)3 exhibited the highest PA(ipso-C) of 923.38 kJ/ mol among the six molecules. This results from the mechanism, in which the nitrogen atom behaves as an internal electron donor group43 and provides resonance structures with negatively charged carbon atoms at the ortho- or para-positions (Scheme 1). A similar mechanism is also observed for aniline, which has a much higher PA (882.5 kJ/mol)38 than benzene (750.2 kJ/mol) because of the NH2 group.

TABLE 3: Calculated Enthalpies, H, for (Trihydroxysilyl)benzene (Ph-Si(OH)3), 4-(Trihydroxysilyl)biphenyl (Bp-Si(OH)3), 4-(Trihydroxysilyl)terphenyl (Tph-Si(OH)3), 2-(Trihydroxysilyl)naphthalene (Nph-Si(OH)3), 2-(Trihydroxysilyl)anthracene (Ant-Si(OH)3), and 3-(Trihydroxysilyl)-N-methylcarbazole (MCz-Si(OH)3) and Their σ-Complexes, Negative Enthalpy Changes, -∆H, and Proton Affinities for the Carbon Atom at the ipso-Position -∆H

H compound

substrate

σ-complex

(hartree)

(kJ/mol)

PA (kJ/mol)

Ph-Si(OH)3 Nph-Si(OH)3 Bp-Si(OH)3 Ant-Si(OH)3 Tph-Si(OH)3 MCz-Si(OH)3

-748.798336 -902.427368 -979.825098 -1056.050027 -1210.851548 -1073.297891

-749.115247 -902.754507 -980.155331 -1056.384820 -1211.186947 -1073.647228

0.316911 0.327139 0.330233 0.334793 0.335399 0.349337

832.05 858.90 867.03 879.00 880.59 917.18

838.25 865.10 873.23 885.20 886.79 923.38

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SCHEME 1

Figure 6 shows the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) of σ-complexes, along with the second HOMOs for Ph and Thio. The orbitals of the original substrates, R-Si(OH)s, are also shown in Figure S3 of the Supporting Information for comparison. The shape of the molecular orbitals suggests that π orbitals of the aromatic groups are hyperconjugated with σ orbitals of the Si-C bonds, which is mainly attributed to the special molecular geometries that the Si-C bond directions are almost perpendicular to the molecular plain of the aromatic groups. The positive charge on the carbon atom at the ipso-position can be delocalized throughout the σ-complex via the conjugate system. This also supports the results that the electronic property of the aromatic group strongly affects the stability of the σ-complex. 4.3. Calculations for Si-C Bond Stability under Basic Conditions. Computationally obtained PA(R-)s are listed in Table 4 with enthalpies (H) of the carbanions and their conjugate acids, and the negative enthalpy changes for the reaction between H+ and R- (-∆H). No vibrational modes with imaginary number frequencies were observed in the vibrational

analyses. The PA(R-) decreased in the order of Ph- > MCz- > Bp- > Nph- > Tph- > Ant-. Helical conformers were obtained as a result of geometry optimizations for Tph- and its conjugate acid. The computational results are consistent with the experimental observations that Si-C cleavage occurred in the preparation of Tph- and Ant-PMOs under basic conditions. Experimental PA(R-)s for Ph- and Nph- are reported as 1680.7 ( 2.1 and 1655 ( 5.4 kJ/mol, respectively.44,45 These values are in good agreement with the PA(R-)s calculated here, which confirms that the applied combination of DFT with the B3LYP functional and a basis set with diffuse functions on both heavy and hydrogen atoms operates well to describe the electronic states of these carbanions. Figure 7 shows HOMOs of the carbanions. LUMOs, the second HOMOs, and the lowest π* orbitals of carbanions are also shown in Figure S4 of the Supporting Information for comparison. The shapes of the molecular orbitals show that the HOMOs are nonbonding orbitals of the carbanions containing negative charge, whereas HOMOs and LUMOs of the conjugated acids are π and π* orbitals, as shown in Figure S5 of the

Figure 6. HOMOs and LUMOs of σ-complexes of (a) Ph, (b) Bp, (c) Tph, (d) Nph, (e) Ant, (f) MCz, and (g) Thio-Si(OH)3. The second HOMO were also presented for Ph and Thio.

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TABLE 4: Calculated Enthalpies, H, for Phenide (Ph-), 4-Biphenylide (Bp-), 4-p-Terphenylide (Tph-), 2-Naphthalenide (Nph-), 2-Anthracenide (Ant-), and 3-N-Methylcarbazolide (MCz-) Anions and Their Conjugate Acids, Negative Enthalpy Changes -∆H, and Proton Affinities (PAs) -∆H

H carbanions

carbanion

conjugate acid

(hartree)

(kJ/mol)

PA (kJ/mol)

-

-231.569932 -556.070656 -462.604200 -385.206892 -693.633323 -538.834643

-232.205848 -556.703959 -463.232195 -385.834323 -694.258694 -539.456728

0.635916 0.633303 0.627995 0.627431 0.625371 0.622085

1669.60 1662.74 1648.80 1647.32 1641.91 1633.28

1675.80 1668.94 1655.00 1653.52 1648.11 1639.48

Ph MCzBpNphTphAnt-

TABLE 5: Non-Bonding Orbital Energies in Phenide (Ph-), 4-Biphenylide (Bp-), 4-p-Terphenylide (Tph-), 2-Naphthalenide (Nph-), 2-Anthracenide (Ant-), and 3-N-Methylcarbazolide (MCz-) Anions compound

energy of the nonbonding orbital (eV)

-

0.5176 0.1763 0.0713 0.0893 -0.0988 -0.1842

Ph MCzBpNphTphAnt-

Supporting Information. The shape of the HOMO of Ph- was in good agreement with that previously reported.46 The energy levels of the nonbonding HOMOs are collected in Table 5. The orbital energies decreased in the order of Ph- > MCz- > Nph> Bp- > Tph- > Ant- and were primarily in agreement with the order of the PA(R-)s, except for the order of Nph- and Bp-. This agreement could result from a mechanism in which the negative charge can be stabilized by partial distribution to the aromatic ring. Therefore, the molecular system becomes larger, and the negative charge can be spread more widely throughout the expanded orbitals (Figure 7). Dispersion of the negative charge generally decreases the electronic repulsion energy and imparts stability to the carbanion, which results in a decreased PA(R-).22,47 Therefore, the PA(R-)s become smaller in the order of Ph- > Nph- > Ant- for the acene series and in the order of Ph- > Bp- > Tph- for the phenylene series. The inter-ring torsion angle, φ, in the optimized structure of Bp- was 32.6°, which is smaller than that of its carbanion at 41.0°. Similarly, φ in Tph- were 29.5 (near deprotonated site) and 35.0° (far from deprotonated site), which are smaller than those of its carbanion, 40.5° (both near and far). These changes in φ suggest distribution of the negative charge throughout the molecules, which leads to stabilization of the carbanions. Smaller φ values in carbanions than that in conjugate acids are

reasonable, considering the planar structure of the biphenyl radical anion48 and the twisted structure of neutral biphenyl. The calculated PA(R-) for MCz- is the second highest among the six molecules, which indicates that MCz- is very unstable. The high PA(R-) of MCz- can be understood based on the resonance structures of MCz with the negative charge at the 3-position (Scheme 1). Repulsion between the negative charge localized at the 3-position and π electrons will significantly increase the energy of MCz- and thus strongly suppress deprotonation at the 3-position. 4.4. Additional Computations. The calculated PA(ipso-C) for Thio-Si(OH)3 was 865.19 kJ/mol, which is almost the same as that of Nph-Si(OH)3. This is in agreement with the experimentally obtained PA values for thiophene (815.0 kJ/mol)38 and naphthalene (813.8 kJ/mol). The PA(ipso-C) for Thio-Si(OH)3 is higher than that for Ph-Si(OH)3 by 26.94 kJ/mol, due to the increased electron density on the aromatic ring by the sulfur atom with lone pairs.22 The relative low PA(ipso-C) of ThioSi(OH)3 could be the reason why no significant Si-C cleavage is observed during the preparation of Thio-PMO under acidic conditions.7 The calculated PA of Thio- was 1606.71 kJ/mol, which is close to the experimental value of 1595 ( 13 kJ/mol49 and the lowest in this study. The extremely low PA(R-) of Thio- result from the interaction between the negative charge at the 2-position and the vacant d orbitals of the sulfur atom (Figure 7g). The energy of the nonbonding orbital was -0.4392 eV, which is also the lowest observed in this study (Table 5). The extremely low PA(R-) of Thio- explains the occurrence of significant Si-C cleavage during the preparation of Thio-PMO under basic conditions.6 5. Conclusions Si-C bond cleavage of organosilane precursors during polycondensation to organosilica hybrids was studied, and a qualitative prediction of Si-C bond stability under acidic and

Figure 7. Nonbonding orbitals of (a) Ph-, (b) Bp-, (c) Tph-, (d) Nph-, (e) Ant-, (f) MCz-, and (g) Thio- containing negative charge.

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basic conditions was demonstrated according to molecular orbital theory calculations employing the DFT method. Si-C bond stabilities under acidic and basic conditions were estimated based on the relative stabilities of σ-complexes and organosilane carbanions, respectively. Proton affinities of the substrates and carbanions were chosen as indices to evaluate these stabilities. The predicted relative Si-C bond stabilities were fairly consistent with experimental results, which infers that our assumption and procedure were appropriate. By utilizing our theoretical computational method, not only the prediction of Si-C bond cleavage occurrence but also theoretical molecular design of highly stable precursors during sol-gel condensation can be achieved. Supporting Information Available: XRD patterns of obtained PMO materials under acidic and basic conditions and molecular orbitals including HOMOs and LUMOs of the substrates (R-Si(OH)3), the carbanions (R-), and the conjugate acids (R-H). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Shea, K. J.; Loy, D. A.; Webster, O. J. Am. Chem. Soc. 1992, 114, 6700. (b) Loy, D. A.; Shea, K. J. Chem. ReV. 1995, 95, 1431. (c) Corriu, R. J. P. Angew. Chem., Int. Ed. 2000, 39, 1376. (2) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (b) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (c) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (3) (a) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (b) Hoffmann, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (c) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20, 891. (4) (a) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (b) Kapoor, M. P.; Yang, Q. H.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 15176. (c) Sayari, A.; Wang, W. H. J. Am. Chem. Soc. 2005, 127, 12194. (d) Cornelius, M.; Hoffmann, F.; Fro¨ba, M. Chem. Mater. 2005, 17, 6674. (5) (a) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C.; Bantignies, J. L.; Dieudonne, P.; Sauvajol, J. L. J. Am. Chem. Soc. 2001, 123, 7957. (b) Cerveau, G.; Chappellet, S.; Corriu, R. J. P. J. Mater. Chem. 2003, 13, 1905. (c) Moreau, J. J. E.; Pichon, B. P.; Man, M. W. C.; Bied, C.; Pritzkow, H.; Bantignies, J. L.; Dieudonne, P.; Sauvajol, J. L. Angew. Chem., Int. Ed. 2004, 43, 203. (d) Okamoto, K.; Goto, Y.; Inagaki, S. J. Mater. Chem. 2005, 15, 4136. (6) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539. (7) Morell, J.; Wolter, G.; Fro¨ba, M. Chem. Mater. 2005, 17, 804. (8) Goto, Y.; Mizoshita, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani, T.; Inagaki, S. Chem. Mater. 2008, 20, 4495. (9) Muth, O.; Schellbach, C.; Froba, M. Chem. Commun. 2001, 2032. (10) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410. (11) Patai, S.; Rappoport, Z. The Chemistry of Organic Silicon Compounds; Wiley: New York, 1989. (12) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900. (13) Ingold, C. K. Structure and Mechanism in Organic Chemistry; Cornell University Press: New York, 1969. (14) Olah, G. A.; Malhorta, R.; Narang, S. C. Nitration Methods and Mechanisms; VCH: New York, 1989. (15) Nam, P. C.; Nguyen, M. T.; Chandra, A. K. J. Phys. Chem. A 2006, 110, 4509. (16) (a) Kovacˇek, D.; Maksic´, Z. B.; Novak, I. J. Phys. Chem. A 1997, 101, 1147. (b) Maksic´, Z. B.; Eckert-Maksic´, M.; Knezˇevic´, A. J. Phys. Chem. A 1998, 102, 2981. (17) Ribeiro, A. A. S. T.; Mota, C. J. A. J. Braz. Chem. Soc. 2008, 19, 1369. (18) Szabo´, K. J.; Hornfeldt, A.; Gronowitz, S. J. Am. Chem. Soc. 1992, 114, 6827. (19) Dewar, M. J. S.; Storch, D. M. J. Chem. Soc., Perkin Trans. 2 1989, 877. (20) McNaught, A. D.; Wilkinson, A. IUPAC: Compendium of Chemical Terminology, 2nd ed.; Blackwell Scientific Publications: Oxford, U.K., 1997.

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