Modulating the Proton Affinity of Silanol and ... - ACS Publications

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article pubs.acs.org/JPCA

Modulating the Proton Affinity of Silanol and Siloxane Derivatives by Tetrel Bonds Published as part of The Journal of Physical Chemistry virtual special issue “Manuel Yáñez and Otilia Mó Festschrift”. Carlos Martín-Fernández,†,‡ M. Merced Montero-Campillo,*,† Ibon Alkorta,*,† and José Elguero† †

Instituto de Química Médica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Department of Chemistry, KU Leuven, Celestijnenlaan, 200F, 3001 Leuven, Belgium

Downloaded via 185.223.164.61 on July 2, 2018 at 09:58:06 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The proton affinity (PA) on the oxygen atom in silanol and siloxane derivatives is enhanced by the formation of tetrel bonds with small Lewis bases [B···R3SiOH, B··· R3SiOSiR3, B···R3SiOSiR3···B; B = H2O, CO, NH3, HCN, H2S; R = H, Me], as shown by MP2/jul-cc-pVTZ calculations. The complexed systems become more basic than ether and other carbon-related compounds, and even more basic than pyridine in some specific cases, reaching values up to 959.4 kJ/ mol (H3N···H3SiOSiH3···NH3 complex). Changes on PAs are directly related to very large binding energies for the protonated species. Topological methods and the natural bond orbital scheme are used to rationalize the observed trends. The PA enhancement should be taken into account when dealing with silanols and siloxanes in different environments.

■

involves donation toward σ* Si−R orbitals.23,24 This feature was already observed by Cypryk et al. in their natural bond order (NBO) analysis.22 Apart from the responsibility of the Si−O bond at the core of the acid−base properties, noncovalent interactions play a fundamental role in changing the rules of the game, as long as these compounds may interact with different agents present in the environment. The simple interaction of water molecules with nucleobases in biological media severely affects their functioning and stacking properties. Similarly, it is expected that the physicochemical properties of our compounds of interest change upon interaction with water or other functional groups.25 Tetrel bonds are noncovalent interactions involving tetrel atoms, that is, those of group IV (C, Si, Ge, Sn, and Pb), through their σ-hole (see Figure 1), a region of electropositive character.26−30 It is interesting to note that the location of the hole is related to the location of the σ* Si−R orbitals, which at the same time is connected with the feasibility of SN2 reactions.31 In this noncovalent interaction, the corresponding tetrel atom (such as, for instance, Si) acts as a Lewis acid in front of an appropriate donor. Frontera and co-workers highlighted the relevance of steric hindrance, among other conditions needed for tetrel bonds to exist.29 Our group has previously studied the interaction of silicon derivatives with small Lewis bases, whose consequences are mainly a large geometrical distortion and a huge dipole enhancement.27

INTRODUCTION Silanols and siloxanes are characterized by having −SiOH and −Si2O functional groups, respectively.1−4 Derivatives containing these chemical structures are of great interest in synthesis, as they are key compounds in a wide range of applications.5−8 The industrial polymerization process of silicones involves silanols as intermediates, and siloxane linkages are precisely at the heart of the strength and flexibility of silicones, one of the most important polymers from a commercial point of view thanks to their inertness and thermal resistance. Among the most recent applications, organosilanols have attracted very much attention in molecular recognition processes, as they can act as receptors and selectively bind ions and neutral species of medical and environmental interest.9 As long as silanols and siloxanes are frequently exposed to different media, their intrinsic acid−base properties are crucial to predict their behavior. In fact, siloxanes are starting to be considered environmental contaminants.10 The oxygen atom on both silanols and siloxanes is known to be less basic than in their relative carbon analogues (alcohols and ethers) in the gas phase.11−13 The acid−base characteristics of these compounds are inextricably linked to the peculiarities of the Si−O bond and the Si−O−Si chain, the nature of which has been a subject of discussion for years.14−22 Taking into account the electronegativity difference between Si and O, most of the debate was focused in the ionic or covalent character of the bond. Short distances observed between these two atoms were attributed to back-bonding from the oxygen lone pairs to empty Si d orbitals, but Weinhold and collaborators emphasized the importance of hyperconjugation, which © 2017 American Chemical Society

Received: August 8, 2017 Revised: September 6, 2017 Published: September 7, 2017 7424

DOI: 10.1021/acs.jpca.7b07886 J. Phys. Chem. A 2017, 121, 7424−7431

Article

The Journal of Physical Chemistry A

allows characterizing NCIs, which are associated with lowdensity and low-RDG values. The sign of the second eigenvalue of the Hessian (λ2) distinguishes between bonding (λ2 < 0) or nonbonding (λ2 > 0) weak interactions. The corresponding weak interaction regions can be located through the use of gradient isosurfaces of different colors in a three-dimensional space (strong attractive NCIs in blue, strong repulsive NCIs in red, very weak interactionsattractive or notwithin the van der Waals range in green). Finally, the NBO analysis describes lone pairs and bonds between pairs of atoms in terms of molecular orbitals as a result of the combination of localized atomic hybrids.44 This decomposition scheme is also useful to describe charge-transfer processes between donor−acceptor pairs, providing the interaction energy between empty and occupied orbitals. NBO calculations were performed at the M06-2X/jul-ccpVTZ//MP2/jul-cc-pVTZ level of theory to obtain the above-mentioned interaction energies. The PA is defined as the negative of the enthalpy change at standard conditions of the general reaction B + H+ → BH+, an example of which is illustrated in Figure S1 in the Supporting Information. A correction for the thermal energy of a proton of 6.19 kJ/mol (∼1.5 kcal/mol) at 298 K was taken into account. The binding energy is defined as the enthalpy released on forming a complex from the isolated fragments.

Figure 1. Oxygen lone pairs (responsible for basicity) and the corresponding side σ-holes (blue ○) in silanol and disiloxane derivatives.

Recent studies regarding the complexation of silicon derivatives to different bases point to a similar direction,32,33 although the way tetrel bonds may affect a property as important as the proton affinity (PA) of these compounds has not been analyzed yet, to the best of our knowledge.34 In the present work, we study the effect on the PAs of the interaction of silanols (R3SiOH) and siloxanes (R3SiOSiR3) with small representative Lewis bases (B···R3SiOH, B··· R3SiOSiR3; B = H2O, CO, NH3, HCN, H2S, R = H, Me). Calculations were performed at the MP2/jul-cc-pVTZ level of theory, an approach that provides accurate results when comparing to the available experimental values for free silanols and siloxanes. Quantum theory of atoms in molecules (QTAIM), noncovalent index (NCI) topological tools, and the NBO decomposition scheme were used to quantify and easily visualize the effect of the tetrel bonds, as will be explained in detail. Also, the binding energies of the complexes are used to rationalize the PA enhancements.

■

RESULTS AND DISCUSSION In the present section, we first discuss the energetic aspects of the protonation, that is, the PA values obtained for our set of compounds, silanols and disiloxane, isolated and upon interaction with a set of small Lewis bases. These results were rationalized based on the binding energies in the second section. The third section is devoted to the geometric characteristics of the neutral and protonated complexes. In the fourth section, several topological approaches (QTAIM and NCI) along with the NBO decomposition scheme were used to provide further understanding of the results. Proton Affinities. Table 1 summarizes the PA results for the different B···R3SiOH (R = H, Me) and B···R3SiOSiR3, B··· R3SiOSiR3···B (R = H) compounds, the B Lewis bases being H2O, CO, NH3, HCN, and H2S. Figure 1 (see Introduction) represents silanol and disiloxane derivatives along with their available side σ-holes, which are responsible for the formation

■

COMPUTATIONAL DETAILS AND METHODS Geometries were fully optimized at the MP2/jul-cc-pVTZ level of theory using the Gaussian09 software package.35−37 Frequency calculations at the same computational level were performed to confirm that the structures obtained correspond to energetic minima and to provide the thermodynamic corrections needed to calculate the PA. The topology of the electron density was studied by means of the QTAIM using the AIMAll program.38−40 Values of the density at the bond critical points (BCPs) allow comparing the same bond in different compounds, whereas the sign and magnitude of the laplacian of the density is closely related to the degree of ionicity or covalency. The NCIPLOT program (NCI, Non Covalent Interaction) was used to obtain the reduced density gradient (RDG) values.42,43 This magnitude

Table 1. Proton Affinity (kJ/mol) for Silanol (R3SiOH) and Disiloxane (R3SiOSiR3) Derivatives Modulated by Tetrel Bonds (B = none, H2O, CO, HCN, NH3, H2S, R = H, Me) siloxanes B···R3SiOSiR3, B···R3SiOSiR3···B

silanols B···R3SiOH Ba

R=H

R = Me b

807.5 (814.0) 851.9 (44.4)

none H2O

742.6 (746.4) 840.1 (97.5)

CO

789.7 (47.0)

818.4 (10.9)

HCN

853.3 (110.7)

864.3 (56.7)

NH3

896.0 (153.4)

906.2 (98.7)

H2S

813.9 (71.2)

827.7 (20.2)

R=H b

757.1 833.0 893.6 792.3 811.2 847.3 903.3 881.8 959.4 812.4 840.7

b

(749.0) (1 H2O) (75.9) (2 H2O) (136.4) (1 CO) (35.2) (2 CO) (54.1) (1 HCN) (90.2) (2 HCN) (146.2) (1 NH3) (124.7) (2 NH3) (202.3) (1 H2S) (55.2) (2 H2S) (83.5)

R = Me 849.1 (846.4)b

a

Values in parentheses for B = none are experimental proton affinities in the gas phase. For the tetrel−bound complexes, values in parentheses provide the PA difference with respect to the corresponding calculated free species. bReference 11. 7425


Article


in Me3SiOH and Me3SiOSiMe3, (807.5 and 849.1 kJ/mol). These calculated values are in good agreement with the experimental PA values of the isolated molecules (Table 1).11 In the present article, we will limit our study to the parent derivatives and the Me3SiOH one to avoid unnecessary repetitions. As a reference, note that both H3SiOH and H3SiOSiH3 are less basic than their carbon analogous compounds: methanol and dimethyl ether (754.3 and 792.0 kJ/mol, respectively).11 The PA of silanol and disiloxane drastically change after interacting through the σ-hole with the Lewis bases (see Table 1, Figure 3, and Figure S2 in the Supporting Information). As a matter of fact, B···H3SiOH complexes show PA enhancements from 47.0 (B = CO) to 153.4 (B = NH3) kJ/mol. Similar results are observed for the one tetrel bond-containing siloxane B···H3SiOSiH3 structures, whereas two tetrel bond-containing complexes reach PA values of 959.4 kJ/mol for the H3N··· H3SiOSiH3···NH3 case. This remarkable result is notably larger than values for well-known basic compounds such as NH3 (853.6 kJ/mol), aniline (882.5 kJ/mol), or even pyridine (930.0 kJ/mol),11 highlighting the importance of the interactions that these molecules may establish with other molecules present in the environment. Interestingly, interaction with water molecules also gives place to very large PA values, which is crucial for polysiloxanes in aqueous media. Although methylsubstituted Me3SiOH exhibit less spectacular changes (Figure 3), significant enhancements are observed when interacting with nitrogen- or oxygen-containing Lewis bases, increasing up to almost 100 kJ/mol its PA in the case of B = NH3. Regarding which Lewis bases affect the most PAs, the same trend is observed for all three H3SiOH, Me3SiOH, and H3SiOSiH3 compounds (Figure 3): CO < H2S < H2O < HCN < NH3. The PAs of each silicon derivative as a function of the Lewis base show high correlation coefficients among them (R > 0.96; see Figure S3). Binding Energies. The binding energy (BE) of the neutral and protonated tetrel-bound complexes were gathered in Table 2. Low values are found for the neutral compounds interacting with one Lewis base, ranging from −1.6 (CO···Me3SiOH) to −12.7 kJ/mol (NH3···H3SiOSiH3). For the disiloxane complexes, the BEs for the B···H3SiOSiH3···B cases are slightly smaller, in absolute value, than twice the B···H3SiOSiH3 ones, as an indication of the diminutive effect.45 Precisely, values for the B···H3SiOSiH3···B compounds range between −7.4 and −20.9 kJ/mol.

Figure 2. Different B···R3SiOH (R = H, Me) and B···R3SiOSiR3, B··· R3SiOSiR3···B (R = H) complexes illustrated for the particular case of B = NH3.

Figure 3. PA enhancement trends (values in kJ/mol) for the different Lewis bases considered. TB stands for “tetrel bond”.

of tetrel bonds with Lewis bases. Figure 2 illustrates the different complexes for the particular case of B = NH3. Unlike silanols, it is important to note that disiloxane derivatives might form one or two tetrel bonds, depending on the number of interacting Lewis bases considered. As shown in Table 1, free silanol H3SiOH and disiloxane H3SiOSiH3 have the lowest PA of the whole set (742.6 and 757.1 kJ/mol, respectively). The inductive effect triggered by the methyl group gives place to significantly larger values of PA

Table 2. Binding Energy (kJ/mol) for Neutrala and Protonated Silanol (R3SiOH, R = H, Me) and Disiloxane (R3SiOSiR3, R = H) Derivatives silanols B···R3SiOH, [B···R3SiOH2]+

a

B

R=H

R = Me

H2O

−5.8, −103.2

−4.1, −48.4

CO

−3.1, −50.2

−1.6, −12.5

HCN

−7.5, −118.2

−7.1, −63.8

NH3

−10.6, −163.9

−4.5, −103.1

H2S

−3.8, −75.0

−2.9, −23.1

siloxanes B···R3SiOSiR3, [B···R3SiOHSiR3···B]+ B···R3SiOSiR3···B, [(B···R3Si)2OH]+ R=H (1 (2 (1 (2 (1 (2 (1 (2 (1 (2

H2O) −6.8, −82.7 H2O) −14.3,b −150.7 CO) −3.8, −39.1 CO) −7.4, −61.5 HCN) −9.0, −99.2 HCN) −15.2, −161.4 NH3) −12.7, −137.4 NH3) −20.9, −223.2 H2S) −4.6, −59.9 H2S) −8.7, −92.3

The first and second values in each cell correspond to neutral and protonated species, respectively. bSee ref 46. 7426


Article

The Journal of Physical Chemistry A In contrast, the BEs for the protonated systems are 1 order of magnitude those of the corresponding neutral systems, ranging between −12.5 and −163.9 kJ/mol within the complexes with one-interacting Lewis base and between −61.5 and −223.2 kJ/ mol for the cases with two-interacting bases. It is important to note that the BEs for the neutral and the corresponding protonated species are linearly correlated (R > 0.97; see Figure S4), with the only exception of the NH3/Me3SiOH complex. This particular case will be discussed later. By using a thermodynamic cycle, the PA enhancement due to the tetrel bond can be related to the binding energy difference of the isolated neutral and protonated species. This relationship is illustrated in Scheme 1 and described through Equations [1−2]. BEneutral = −PA0 + BEprotonated + PA

Figure 4. Linear correlation between PA enhancement and binding energies for protonated species (kJ/mol).

(1)

PA − PA0 = PA enhancement = BEneutral − BEprotonated

Notably, the variation on the donor−acceptor distance on going from the neutral to the protonated species ranges between 0.72 and 2.01 Å, as an indication of the important contraction due to the protonation, which is followed by a consequent elongation of the neighboring Si−O bond (see Table S2). It must be noticed that, in some cases, small differences between both B···Si intermolecular distances are observed in the protonated [B···H3SiOHSiH3···B]+ complexes. This is caused by the very low energy needed for the silane groups to rotate, thus easily adopting eclipsed or alternated conformations with respect to the OH group, giving place to these differences. In agreement with the resulting binding energies for protonated structures and the corresponding dramatic shortening of the B···Si distances already noticed in Table 3, strong changes on other geometric parameters of the complexes take place upon protonation. Figure 5 exemplifies how the interacting SiR3 moiety becomes planar, losing its original tetrahedral geometry. For instance, a negligible deviation from planarity is observed in the protonated complex [HCN··· H3SiOHSiH3]+ (only 4°), whereas the non-interacting SiH3 moiety keeps its geometry. A similar transformation from the neutral to the protonated structure occurs for the trimethyl silanol case, an example of which is also shown in Figure 5. Geometries of all structures are included in Table S3 in the Supporting Information. Topological and NBO Analyses. The electron density redistribution on the silanol and disiloxane molecules upon complexation with the Lewis bases and its effect on the donor ability of the oxygen atom toward a proton were analyzed through the use of the QTAIM, NCIPLOT, and NBO techniques. Regarding the QTAIM calculations, Table 4 collects the electron density values at the BCPs between Si and the different donor atoms of our set of bases. In the neutral complexes, the tetrel bond formation is characterized by small electron density values at the corresponding BCP (ρBCP) along the B···Si path, with the only exception of H2O···H3SiOSiH3, which shows a curved O···H bond path (see Figure S5 for further details). Also, the Si−O bond from the silanol and siloxane groups show smaller values of ρBCP than the one in the isolated compounds. Notably, no B···Si BCP is observed for any Lewis base considered for the Me3SiOH complexes, unlike in the silanol and disiloxane cases. For the methyl-substituted systems, three very weak B···H BCPs are found instead within the van der Waals range (see Figure 6).

(2)

From eq 2, it can be derived that the PA enhancement can be decomposed as the difference between neutral and protonated BEs. Since the BEs of the protonated species are much larger than those of the neutral ones, the difference between these two magnitudes can be approximated to the BE of the protonated species. Thus, an excellent correlation is found between the latter values and the PA enhancement, as shown in Figure 4. Geometry. As might be expected, the above-mentioned enhancements in the basicity properties are accompanied by substantial geometry changes on going from the neutral to the protonated species. The intermolecular distance between the Si atom of the silicon derivatives and the donor atom of the Lewis base is very sensitive to the neutral or charged nature of the former, as evidenced by the data collected in Table 3. In neutral systems, the intermolecular distances vary from 2.73 to 4.40 Å depending on the Lewis base, increasing from ammonia to sulfur hydride in the following order: NH3 < OH2 < HCN < CO < SH2. In respect to the silicon derivatives, distances obtained for the Me3SiOH complexes are notably the longest. Although Si shows a more positive charge in the methyl derivatives (see Table S1), their larger distances are mostly due to the steric hindrance offered by methyl groups to form tetrel bonds. In the case of the protonated species, the intermolecular distances are significantly shorter, ranging between 1.98 and 3.5 Å. These complexes follow similar trends according to the Lewis base to those observed for the neutral systems, the only exception being the Me3SiOH complexes with CO and SH2 that exchange positions. Scheme 1. Scheme Shows the Thermodynamic Cycle Relating Proton Affinities for Isolated (PA0) and TetrelBound Complexes (PA) with the Corresponding Binding Energies (BE) for Neutral and Protonated R3SiOR′ (R = H, Me; R′ = H, SiR3)

7427


Article


Table 3. B−Si Distancesa (Å) for Silanol (R3SiOH) and Disiloxane (R3SiOSiR3) Derivatives Modulated by Tetrel Bonds (B = H2O, CO, HCN, NH3, H2S) siloxanes B···R3SiOSiR3, B···R3SiOSiR3···B

silanols B···R3SiOH

a

B

R=H

R = Me

R=H

H2O

2.957; 1.992

3.776; 2.084

CO

3.310; 2.248

4.200; 3.479

HCN

3.038; 2.026

3.824; 2.108

NH3

2.803; 1.988

3.991; 1.977

H2S

3.541; 2.506

4.398; 2.657

(1H2O) 2.922; 2.042 (2H2O) 2.987, 2.973; 2.538, 3.840b (1 CO) 3.271; 2.369 (2 CO) 3.284, 3.284; 2.516, 2.523 (1 HCN) 2.989; 2.077 (2 HCN) 3.048, 3.048; 2.218, 2.218 (1 NH3) 2.727; 2.011 (2 NH3) 2.870, 2.870; 2.090, 2.090 (1H2S) 3.504; 2.584 (2H2S) 3.526, 3.524; 2.738, 2.732

Consecutive values on each cell separated by “;” correspond to the non-protonated and the protonated forms, respectively. bSee ref 46.

like to call this topology pattern “umbrella type”, as the shape induced by the methyl groups prevents the formation of a typical tetrel bond, and has to do with the statement of Frontera and co-workers on the accessibility of the σ-hole.28 This pattern is analogous to that observed in the tetrel-bond literature when −SiF3 moieties are involved.29,31 Interestingly, a very subtle effect of this topology is the neutral binding energy for the HCN complex with Me3SiOH, which is larger than the NH3 one (Table 3, −7.1 vs −4.5 kJ/mol), a fact accompanied by shorter N···Si distances (Table 2, 3.824 vs 3.991 Å, respectively). This is precisely due to a better ability of the linear HCN molecule to interact with the methylated derivative compared to that of NH3. The different interaction patterns in neutral compounds can be also quantified through the NBO decomposition scheme. Table 5 summarizes the interaction energies between the lone pair (LP) of each Lewis base and the empty antibonding (BD*) Si−O orbital. Significantly, almost inexistent LP → BD* interactions are observed for B···Me3SiOH complexes, in clear agreement with the QTAIM and NCI points of view. Tetrel bonds for B···H3SiOH/H3SiOSiH3 complexes are instead characterized by moderate interaction energies, within the range expected for tetrel bonds, from which the NH3 complexes present the largest values (almost 40 kJ/mol for disiloxane). Slightly stronger interactions are observed for disiloxane with respect to silanol complexes, which is also in agreement with more labile Si−O bonds according to the electron density values on the BCPs obtained from the QTAIM results. An analogous NBO study involving the protonated complexes is not possible, as the interaction between the Lewis base and the silicon derivative is so strong that NBO considers the pairs B− SiR3 as a unique molecule.

Figure 5. Comparison between tetrahedral and pseudoplanar SiR3 moieties in three selected complexes with B = HCN. The N···Si distance (Å) is indicated in the neutral and protonated complexes between HCN and Me3SiOH.

However, the topology changes dramatically in the protonated complexes, for which there is a BCP along the B···Si path in all cases with much larger density values, even for the weakest CO complexes. This finding is in agreement with the previously mentioned shortening of the B−Si distances and pseudoplanarity of the SiR3 moiety in the protonated complexes. Accordingly, the Si−O bond from the silanol and siloxane functional groups is remarkably weaker, as revealed by electron density values that are less than a half those of the neutral complexes in most cases (see also Table S4). The absence of the characteristic tetrel bond BCP in the Me3SiOH neutral complexes deserves some attention. As a suitable example, Figure 6 shows the topology patterns observed for the CO complexes, which reveals weak interactions between the Lewis base and the hydrogen atoms from the methyl groups and, notably, three ring critical points (RCPs) surrounding one cage critical point (CCP), located at a position in which a BCP would be expected without steric effects. This pattern is not exclusive of the most-weakly bound complexes and can be also observed through a different approach, such as the NCI analysis. For instance, the H3N··· Me3SiOH complex (Figure 6) shows a similar behavior: attractive interactions between nitrogen and hydrogen from the methyl groups, and the previously observed steric clash, characterized by the triangular shape of the corresponding isosurface, within the interboundary N···Si region. We would

■

CONCLUSIONS The PA of R3SiOH (R = H, Me) and H3SiOSiH3 upon interaction with a set of representative small Lewis bases (B = H2O, CO, NH3, HCN, H2S) was studied at the MP2/jul-ccpVTZ level of theory. The interaction through the Si σ-hole allows PAs to be notably increased, in particular, for the waterand nitrogen-containing Lewis bases. Remarkably high binding energies for the protonated silanol and disiloxane derivatives were shown to be responsible for the PA enhancement. This enhancement is accompanied by a series of geometrical changes on going from the neutral to the protonated complexes, such as a drastic shortening of the B···Si distances, a weakening of the 7428


Article


Table 4. Electron density (a.u.) at the Bond Critical Points for Silanol and Disiloxane Derivatives for the Different Lewis Bases B···H3SiOH Ba

neutral

none

Si−O O−H O−Si Si−O O−H

H2O

B···Me3SiOH

protonated

0.130 0.359 0.010 0.128 0.359


0.077 0.343 0.055 0.055 0.351

CO

C−Si 0.007 Si−O 0.130 O−H 0.359

C−Si 0.044 Si−O 0.060 O−H 0.349

HCN

N−Si 0.009 Si−O 0.128 O−H 0.360 N−Si 0.016 Si−O 0.126 O−H 0.360

N−Si 0.057 Si−O 0.052 O−H 0.352 N−Si 0.073 Si−O 0.046 O−H 0.353

S−Si − 0.007 Si−O 0.129 O−H 0.359

S−Si − 0.047 Si−O 0.055 O−H 0.350

NH3

H2S

B···H3SiOSiH3

neutral

protonated

Si−O 0.127 O−H 0.359 O−Si − no BCP O−H 0.004− 0.005 Si−O 0.126 O−H 0.359 C−Si − no BCP C−H 0.003 Si−O 0.127 O−H 0.359 N−Si − no BCP Si−O 0.126 O−H 0.359 N−Si − no BCP N−H 0.005 Si−O 0.126 O−H 0.359 S−Si − no BCP C−H 0.001−0.002 Si−O 0.125 O−H 0.353


0.069 0.347 0.046 0.046 0.353

C−Si − 0.005 C−H 0.005 Si−O 0.067 O−H 0.347 N−Si 0.048 Si−O 0.045 O−H 0.354 N−Si 0.074 Si−O 0.023 O−H 0.358 S−Si − 0.036 Si−O 0.050 O−H 0.352

neutral Si−O 0.129 O−H (SiH3) 0.011b Si−O 0.126−0.132

protonated Si−O O−H O−Si Si−O O−H

0.088 0.345 0.049 0.067−0.101 0.349

C−Si 0.007 Si−O 0.128, 0.130

C−Si 0.035 Si−O 0.074−0.096 O−H 0.347

N−Si 0.010 Si−O 0.126−0.132

N−Si 0.051 Si−O 0.063−0.102 O−H 0.350 N−Si 0.069 Si−O 0.054−0.106 O−H 0.351

N−Si 0.019 Si−O 0.122−0.133

S−Si 0.008 Si−O 0.127−0.131

S−Si 0.040 Si−O 0.068−0.100 O−H 0.349

With the only exception of the first row (B = none), the first value on each cell corresponds to the B···Si interaction (B = H2O, CO, HCN, NH3, H2S). bThe oxygen atom from water interacts with a H atom from the SiH3 group. See Figure S5 for further details.

a

Figure 6. (left, center) Molecular graphs obtained by means of QTAIM calculations for the particular case of the H3SiOH and Me3SiOH complexes with CO. BCPs are green dots, whereas RCPs are represented by red dots, and CCPs are indicated by blue dots. Electron densities values (a.u.) at the BCPs are also indicated. (right) NCI analysis obtained for the H3N···Me3SiOH complex. Green isosurfaces denote interactions within the van der Waals range.

BCP along the B···Si axis, an umbrella-shaped slightly repulsive region through the B···Si axis, and almost negligible interaction energies between Lewis bases lone pairs and the antibonding Si−O σ* orbital. In contrast, on protonated [B···Me3SiOH2]+ complexes, steric effects are removed, and the Lewis base interacts with an almost planar SiMe3 moiety.41

Table 5. Second-Order Perturbation Theory Interaction Energies (kJ/mol) from the NBO Decomposition Scheme between Occupied LP(B) (B = H2O, CO, HCN, NH3, H2S) and the Empty BD* Si−O Orbital LP→BD* (kJ/mol)

silanols B···R3SiOH

siloxanes B···R3SiOSiR3

B

R=H

R = Me

R=H

H2O CO HCN NH3 H2S

15.4 14.6 14.0 31.9 14.1

1.7 0.7 1.5 1.9 0.0

17.7 18.3 17.1 39.5 16.5

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b07886. Additional results regarding the NBO analysis, geometrical parameters, Cartesian coordinates, molecular graphs, and correlation between proton affinities and binding energies (PDF)

Si−O bond, and the pseudoplanarity of the interacting SiR3 moiety. The formation of the tetrel bond in the neutral complexes is characterized by the presence of a BCP in the B···Si intermolecular region. Nonetheless, the complexation of Me3SiOH with Lewis bases follows a different interaction pattern caused by the methyl groups, as evidenced by all QTAIM, NCI, and NBO approaches, which together show no

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.M.M.-C.) *E-mail: [email protected]. (I.A.) 7429


Article

The Journal of Physical Chemistry A ORCID

graphic and ab initio inquiry into the nature of silicon-oxygen interactions. J. Am. Chem. Soc. 1990, 112 (2), 697−703. (17) Oberhammer, H.; Boggs, J. E. Importance of the (p-d) pi bonding in the siloxane bond. J. Am. Chem. Soc. 1980, 102, 7241− 7244. (18) Gillespie, R. J.; Johnson, S. A. Study of bond angles and bond lengths in disiloxane and related molecules in terms of topology of the electron density and its lacplacian. Inorg. Chem. 1997, 36, 3031−3039. (19) Grabowsky, S.; Hesse, M. F.; Paulmann, C.; Luger, P.; Beckmann, J. How to make the ionic Si-O bond more covalent and the Si-O-Si linkage a better acceptor for hydrogen bonding. Inorg. Chem. 2009, 48, 4384−4393. (20) Gibbs, G. V.; Wallace, A. F.; Cox, D. F.; Downs, R. T.; Ross, N. L.; Rosso, K. M. Bonded interactions in silica polymorphs, silicates, and siloxane molecules. Am. Mineral. 2009, 94, 1085−1102. (21) Cypryk, M. Ab initio study of the basicity and propensity of siloxanols towards hydrogen bond formation. J. Organomet. Chem. 1997, 545−546, 483−493. (22) Cypryk, M.; Apeloig, Y. Mechanism of the acid-catalyzed Si-O bond cleavage in siloxanes and siloxanols. A theoretical study. Organometallics 2002, 21, 2165−2175. (23) Weinhold, F.; West, R. The nature of the silicon−oxygen bond. Organometallics 2011, 30 (21), 5815−5824. (24) Weinhold, F.; West, R. Hyperconjugative interactions in permethylated siloxanes and ethers: the nature of the SiO bond. J. Am. Chem. Soc. 2013, 135, 5762−5767. (25) Martín-Sómer, A.; Montero-Campillo, M. M.; Mó, O.; Yáñez, M.; Alkorta, I.; Elguero, J. Some interesting features of non−covalent interactions. Croat. Chem. Acta 2014, 87, 291−306. (26) Alkorta, I.; Elguero, J.; Fruchier, A.; Macquarrie, D. J.; Virgili, A. Aminopropylsilanes versus silatranes: an experimental and theoretical study. J. Organomet. Chem. 2001, 625, 148−153. (27) Alkorta, I.; Rozas, I.; Elguero, J. Molecular complexes between silicon derivatives and electron−rich groups. J. Phys. Chem. A 2001, 105, 743−749. (28) Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel−bonding interaction: Rediscovered supramolecular force? Angew. Chem., Int. Ed. 2013, 52, 12317−12321. (29) Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel bonding interactions. Chem. Rec. 2016, 16, 473−487. (30) Marín-Luna, M.; Alkorta, I.; Elguero, J. A theoretical study of the HnF4−nSi:N−base (n = 1−4) tetrel−bonded complexes. Theor. Chem. Acc. 2017, 136, 41−50. (31) Grabowski, S. J. Tetrel bond−σ−hole bond as a preliminary stage of the SN2 reaction. Phys. Chem. Chem. Phys. 2014, 16, 1824− 1834. (32) Marín-Luna, M.; Alkorta, I.; Elguero, J. Cooperativity in tetrel bonds. J. Phys. Chem. A 2016, 120, 648−656. (33) Esrafili, M. D.; Mohammadirad, N.; Solimannejad, M. Tetrel bond cooperativity in open−chain (CH3CN)n and (CH3NC)n clusters (n = 2−7): An ab initio study. Chem. Phys. Lett. 2015, 628, 16−20. (34) The influence of protonation on the strength of tetrel bonding has been studied in pyridineTF3 and furanT3 systems (T = C, Si, C): Liu, M.; Li, Q.; Scheiner, S. Comparison of tetrel bonds in neutral and protonated complexes of pyridineTF3 and furanTF3 (T = C, Si, and Ge) with NH3. Phys. Chem. Chem. Phys. 2017, 19, 5550−5559. (35) Moller, C.; Plesset, M. S. Note on an approximation treatment for many−electron systems. Phys. Rev. 1934, 46, 618−622. (36) Dunning, T. H. Gaussian−basis sets for use in correlated molecular calculations. 1. The atoms boron through neon and hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (37) Frisch, M. J.; et al. Gaussian09, Revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (38) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Clarendon: Oxford, England, 1990. (39) Matta, C. F.; Boyd, R. J. The Quantum Theory of Atoms in Molecules; Wiley-VCH: Weinheim, Germany, 2007.

M. Merced Montero-Campillo: 0000-0002-9499-0900 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was performed with financial support from the Comunidad Autónoma de Madrid (Project FOTOCARBON, Ref No. S2013/MIT−2841) and the Ministerio de Economiá y Competitividad (Project No. CTQ2015−63997−C2−2−P). Storage and computational resources from CTI (CSIC) and ́ (CCC, UAM) are gratefully Centro de Computación Cientifica ́ acknowledged. C. Martin-Fernández and M. M. MonteroCampillo also thank Project FOTOCARBON for their research contracts.

■

REFERENCES

(1) Rösch, L.; John, P.; Reitmeier, R. Silicon compounds, organic. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2012; pp 637−669. (2) Chandrasekhar, V.; Boomishankar, R.; Nagendran, S. Recent developments in the synthesis and structure of organosilanols. Chem. Rev. 2004, 104 (12), 5847−5910. (3) Lickiss, P. D. The synthesis and structure of organosilanols. Adv. Inorg. Chem. 1995, 42, 147−262. (4) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W. Hetero− and metallasiloxanes derived from silanediols, disilanols, silanetriols, and trisilanols. Chem. Rev. 1996, 96, 2205−2236. (5) Franz, A. F.; Wilson, S. O. Organosilicon molecules with medical applications. J. Med. Chem. 2013, 56, 388−405. (6) Sato, N.; Kuroda, Y.; Abe, T.; Wada, H.; Shimojima, A.; Kuroda, K. Regular assembly of cage siloxanes by hydrogen bonding of dimethyl silanol groups. Chem. Commun. 2015, 51, 11034−11037. (7) Hurkes, N.; Ehmann, H. M. A.; List, M.; Spirk, S.; Bussiek, M.; Belaj, F.; Pietschnig, R. Silanol−based surfactants: synthetic access and properties of an innovative class of environmentally benign detergents. Chem. - Eur. J. 2014, 20, 9330−9335. (8) Théron, C.; Gallud, A.; Carcel, C.; Gary-Bobo, M.; Maynadier, M.; García, M.; Lu, J.; Tamanoi, F.; Zink, J. I.; Wong Chi Man, M. W. Hybrid mesoporous silica nanoparticles with pH−operated and complementary H−bonding caps as an autonomous drug−delivery system. Chem. - Eur. J. 2014, 20, 9372−9380. (9) Bruña, S.; Garrido-Castro, A. F.; Perles, J.; Montero-Campillo, M. M.; Mó, O.; Kaifer, A. E.; Cuadrado, I. Multi−ferrocene containing silanols as redox−active anion receptors. Organometallics 2016, 35 (20), 3507−3519. (10) Rücker, C.; Kümmerer, K. Environmental chemistry of organosiloxanes. Chem. Rev. 2015, 115 (1), 466−524. (11) Hunter, E. P.; Lias, S. G. Evaluated gas phase basicities and proton affinities of molecules: an update. J. Phys. Chem. Ref. Data 1998, 27 (3), 413−656. (12) West, R.; Wilson, L. S.; Powell, D. L. Basicity of siloxanes, alkoxysilanes and ethers toward hydrogen bonding. J. Organomet. Chem. 1979, 178 (1), 5−9. (13) Passmore, J.; Mikko Rautiainen, J. On the lower Lewis basicity of siloxanes compared to ethers. Eur. J. Inorg. Chem. 2012, 2012, 6002−6010. (14) DFT can provide good values but needs additional benchmarking on the selection of the functional: Cypryk, M.; Gostyński, B. Computational benchmark for calculation of silane and siloxane thermochemistry. J. Mol. Model. 2016, 22 (1), 35−54. (15) Moraru, I−T.; Petrar, P. M.; Nemes, G. Bridging a knowledge gap from siloxanes to germoxanes and stannoxanes. A theoretical Natural Bond Orbital study. J. Phys. Chem. A 2017, 121, 2515−2522. (16) Shambayati, S.; Schreiber, S. L.; Blake, J. F.; Wierschke, S. G.; Jorgensen, W. L. Structure and basicity of silyl ethers: a crystallo7430


Article

The Journal of Physical Chemistry A (40) Becke, A. D.; Edgecombe, K. E. A simple measure of electron localization in atomic and molecular systems. J. Chem. Phys. 1990, 92, 5397−5403. (41) Todd, A.; Keith, T. K. AIMAll, Version 13.01.27; Gristmill Software: Overland Park, KS, 2015; online at aim.tkgristmill.com. (42) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W. Revealing non−covalent interactions. J. Am. Chem. Soc. 2010, 132, 6498−6506. (43) Contreras-García, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.; Piquemal, J. P.; Beratan, D. N.; Yang, W. T. NCIPLOT: A program for plotting non−covalent interaction regions. J. Chem. Theory Comput. 2011, 7, 625−632. (44) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, Version 3.1; Gaussian, Inc: Wallingford, CT, 2009. (45) Solimannejad, M.; Malekani, M. Cooperative and diminutive interplay between the hydrogen bonding and halogen bonding in ternary complexes of HCCX (X = Cl,Br) with HCN and HNC. Comput. Theor. Chem. 2012, 998, 34−38. (46) For this complex, two minima are found, from which the one with two water molecules occupying both σ-holes is 21 kJ/mol less stable than the global minimum. In this latter case, the second water molecule is hydrogen-bound to the first tetrel-bound water molecule.

7431


Modulating the Proton Affinity of Silanol and ... - ACS Publications

Recommend Documents