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Modulating the Proton Affinity of Silanol and Siloxane Derivatives by Tetrel Bonds Carlos Martín-Fernández, M. Merced Montero-Campillo, Ibon Alkorta, and José Elguero J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07886 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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The Journal of Physical Chemistry

Modulating the Proton Affinity of Silanol and Siloxane Derivatives by Tetrel Bonds Carlos Martín-Fernández,a,b M. Merced Montero-Campillo,a,* Ibon Alkorta,a,* and José Elgueroa a

Instituto de Química Médica (CSIC), Juan de la Cierva, 3 28006 Madrid (Spain)

b

Department of Chemistry, KU Leuven, Celestijnenlaan, 200F 3001 Leuven (Belgium) *Email: [email protected], [email protected]

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···R3Si)2O; 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···H3Si)2O complex). Changes on PAs are directly related to very large binding energies for the protonated species. Topological methods and the Natural Bond Orbital (NBO) scheme are used to rationalize the observed trends. The proton affinity enhancement should be taken into account when dealing with silanols and siloxanes in different environments.

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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 behaviour. 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 involves donation towards σ* Si−R orbitals.23−24 This feature was already observed by Cypryk et al in their NBO analysis.22

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

Apart from the responsibility of the Si−O bond at the core of the acid-base properties, non-covalent interactions play a fundamental role on changing the rules of the game as


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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 non-covalent interactions involving tetrel atoms, i.e. 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 non-covalent 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 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 of these compounds has not been analysed yet, to the best of our knowledge.34

In the present work, we study the effect on the proton affinities (PA) 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 have been carried out 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), Non Covalent Index (NCI) topological tools and the Natural Bond Orbital (NBO) decomposition scheme have been 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 proton affinity enhancements.

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 have been carried out to confirm that the structures obtained correspond to 3 Environment ACS Paragon Plus


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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 Quantum Theory of Atoms in Molecules (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 allows characterizing non-covalent interactions, which are associated to lowdensity and low-RDG values. The sign of the second eigenvalue of the Hessian (λ2) distinguishes between bonding (λ20) 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 noncovalent interactions in blue, strong repulsive non-covalent interactions in red, very weak interactions −attractive or not− within the van der Waals range in green).

Finally, the Natural Bond Orbital (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 carried out at the M062X/jul-cc-pVTZ//MP2/jul-cc-pVTZ level of theory to obtain the abovementioned interaction energies.

The proton affinity (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 298K has been taken into account. The binding energy is defined as the enthalpy released on forming a complex from the isolated fragments.


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RESULTS AND DISCUSSION In the present section, we firstly discuss the energetic aspects of the protonation, i.e., the proton affinity (PA) values obtained for our set of compounds, silanols and disiloxane, isolated and upon interaction with a set of small Lewis bases. These results have been 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 have been 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···R3Si)2O (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 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.

Figure 2. Different B···R3SiOH (R = H, Me) and B···R3SiOSiR3, (B···R3Si)2O (R = H) complexes illustrated for the particular case of B = NH3.

As shown in Table 1, free silanol H3SiOH and disiloxane (H3Si)2O 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 in Me3SiOH and 5 Environment ACS Paragon Plus


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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 (H3Si)2O are less basic than their carbon analogous compounds: methanol and dimethyl ether (754.3 and 792.0 kJ/mol, respectively). 11 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). Values in parentheses for B = None are experimental proton affinities in the gas phase. For the tetrel−bound complexes, values in parentheses provide the proton affinity difference with respect to the corresponding calculated free species. Siloxanes Silanols B···R3SiOSiR3, B···R3SiOSiR3···B

B···R3SiOH B None

R=H 742.6 (746.4)a

R = Me 807.5 (814.0)

H2 O

840.1 (97.5)

851.9 (44.4)

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)

(a)

R=H a

757.1 (749.0)

R = Me a

833.0 (1 H2O) (75.9) 893.6 (2 H2O) (136.4) 792.3 (1 CO) (35.2) 811.2 (2 CO) (54.1) 847.3 (1 HCN) (90.2) 903.3 (2 HCN) (146.2) 881.8 (1 NH3) (124.7) 959.4 (2 NH3) (202.3) 812.4 (1 H2S) (55.2) 840.7 (2 H2S) (83.5)

849.1 (846.4)a − − − − − − − − − −

Ref . 11.

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


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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 proton affinity enhancements from 47.0 (B = CO) to 153.4 kJ/mol (B = NH3). 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 ((NH3)2···H3Si)2O case. This remarkable result is notably larger than values for wellknown 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 give place to very large PA values, what is crucial for polysiloxanes in aqueous media. Although methyl-substituted 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 proton affinities, the same trend is observed for all three H3SiOH, Me3SiOH and (H3Si)2O 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 have been 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···H3Si)2O 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···H3Si)2O compounds range between −7.4 and −20.9 kJ/mol.

In contrast, the BEs for the protonated systems are one 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 −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 7 Environment ACS Paragon Plus


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the only exception of the NH3:Me3SiOH complex. This particular case will be discussed later on. Table 2. Binding energy (kJ/mol) for neutral and protonated silanol (R3SiOH, R=H, Me), and disiloxane (R3SiOSiR3, R=H) derivatives. The first and second values in each cell correspond to neutral and protonated species, respectively. Silanols

Siloxanes

B···R3SiOH, [B···R3SiOH2]+

B···R3SiOSiR3, [B···R3SiOHSiR3]+ (B···R3Si)2O, [(B···R3Si)2OH]+

B H2O

R=H −5.8, −103.2

R = Me −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

R=H (1 H2O) −6.8, −82.7 (2 H2O) −14.3,* −150.7 (1 CO) −3.8, −39.1 (2 CO) −7.4, −61.5 (1 HCN) −9.0, −99.2 (2 HCN) −15.2, −161.4 (1 NH3) −12.7, −137.4 (2 NH3) −20.9, −223.2 (1 H2S) −4.6, −59.9 (2 H2S) −8.7, −92.3

*See Ref. 46.

By using a thermodynamic cycle, the proton affinity 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].

Scheme 1. The scheme shows the thermodynamic cycle relating proton affinities for isolated (PA0) and tetrel-bound complexes (PA) with the corresponding binding energies (BE) for neutral and protonated R3SiOR’ (R=H, Me; R’ = H, SiR3).

BEneutral = −PA0 + BEprotonated + PA 0

PA − PA = PA enhancement = BEneutral – BEprotonated

Eq. [1] Eq. [2]

From Equation 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


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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.

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

Geometry. As might be expected, the abovementioned 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 what respect to the silicon derivatives, distances obtained for the Me3SiOH complexes are notably the longest ones. 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.



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Table 3. B−Si distances (Å) for silanol (R3SiOH) and disiloxane (R3SiOSiR3) derivatives modulated by tetrel bonds (B = H2O, CO, HCN, NH3, H2S). Consecutive values on each cell separated by “;” correspond to the non-protonated and the protonated forms, respectively. Silanols

Siloxanes

B···R3SiOH

B···R3SiOSiR3, B···R3SiOSiR3···B

B H2O

R=H 2.957; 1.992

R = Me 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

R=H (1H2O) 2.922; 2.042 (2H2O) 2.987, 2.973; 2.538, 3.840* (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

* See Ref. 46.

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···H3Si)2OH+ complexes. This is caused by the very low energy needed for the silane groups to rotate, and thus easily adopting eclipsed or alternated conformations with respect to the OH group, giving place to these differences.

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


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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 4º), 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 towards a proton have been analysed through the use of the QTAIM, NCIPLOT and NBO techniques.

Regarding the QTAIM calculations, Table 4 collects the electron density values at the bond critical points (BCP) 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).

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 pseudo planarity 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). 11 Environment ACS Paragon Plus


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Table 4. Bond critical points (BCP) of the electron density for silanol and disiloxane derivatives for the different Lewis bases. 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). B···H3SiOH Neutral Si−O 0.130 O−H 0.359 O−Si 0.010 Si−O 0.128 O−H 0.359

Protonated Si−O 0.077 O−H 0.343 O−Si 0.055 Si−O 0.055 O−H 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

C−Si –No BCP C−H 0.003 Si−O 0.127 O−H 0.359

C−Si – 0.005 C−H 0.005 Si−O 0.067 O−H 0.347

C−Si 0.007 Si−O 0.128, 0.130

C−Si 0.035 Si−O 0.074−0.096 O−H 0.347

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 –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


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

S−Si –0.007 Si−O 0.129 O−H 0.359

S−Si –0.047 Si−O 0.055 O−H 0.350

S−Si –No BCP C−H 0.001−0.002 Si−O 0.125 O−H 0.353

S−Si – 0.036 Si−O 0.050 O−H 0.352

S−Si 0.008 Si−O 0.127−0.131

B None H2O

NH3

H2S

B···Me3SiOH Neutral 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

Protonated Si−O 0.069 O−H 0.347 O−Si 0.046 Si−O 0.046 O−H 0.353

B···H3SiOSiH3 Neutral Si−O 0.129 O−H (SiH3) 0.011* Si−O 0.126−0.132

N−Si 0.019 Si−O 0.122−0.133

Protonated Si−O 0.088 O−H 0.345 O−Si 0.049 Si−O 0.067−0.101 O−H 0.349

S−Si 0.040 Si−O 0.068−0.100 O−H 0.349

* The oxygen atom from water interacts with a H atom from the SiH3 group. See Figure S5 for further details.

Figure 6. (a) Molecular graphs obtained by means of QTAIM calculations for the particular case of the H3SiOH and Me3SiOH complexes with CO. Bond critical points are green dots, whereas ring critical points (RCP) are represented by red dots and cage critical points (CCP) by blue dots. (b) Picture on the right: NCI analysis obtained for the H3N···Me3SiOH complex. Green isosurfaces denote interactions within the van der Waals range.

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 (RCP) 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 mostweakly bound complexes and can be also observed through a different approach, such


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as the NCI analysis. For instance, the H3N···Me3SiOH complex (Figure 6) shows a similar behaviour: 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 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. 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) B H2 O CO HCN NH3 H2 S

Silanols B···R3SiOH R=H 15.4 14.6 14.0 31.9 14.1

R = Me 1.7 0.7 1.5 1.9 0.0

Siloxanes B···R3SiOSiR3 R=H 17.7 18.3 17.1 39.5 16.5

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 anti-bonding (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/(H3Si)2O 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 13 Environment ACS Paragon Plus


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molecule.

CONCLUSIONS The proton affinity of R3SiOH (R = H, Me) and (H3Si)2O upon interaction with a set of representative small Lewis bases (B = H2O, CO, NH3, HCN, H2S) was studied at the MP2/jul-cc-pVTZ level of theory. The interaction through the Si σ hole allows proton affinities to be notably increased, in particular for the water and 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 Si−O bond, and the pseudo planarity 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 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 anti-bonding 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.

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

SUPPORTING INFORMATION. Additional results regarding the NBO analysis, geometrical parameters, cartesian coordinates, molecular graphs and correlation


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between proton affinities and binding energies are provided in the Supporting Information.

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Table of Contents 407x174mm (72 x 72 DPI)

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Figure 1. Oxygen lone pairs (responsible for basicity) and the corresponding side σ holes (blue circles) in silanol and disiloxane derivatives. 404x119mm (72 x 72 DPI)


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Figure 2. Different B···R3SiOH (R = H, Me) and B···R3SiOSiR3, (B···R3Si)2O (R = H) complexes illustrated for the particular case of B = NH3. 401x238mm (72 x 72 DPI)



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Figure 3. Proton affinity enhancement trends (PAs in kJ/mol) for the different Lewis bases considered. TB stands for “tetrel bond”. 370x256mm (72 x 72 DPI)


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Figure 4. Linear correlation between proton affinity enhancement and binding energies for protonated species (kJ/mol). 412x248mm (72 x 72 DPI)



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Figure 5. Comparison between tetrahedral and pseudo planar SiR3 moieties in three selected complexes with B = HCN. The N···Si distance (Å) is indicate in the neutral and protonated complexes between HCN and Me3SiOH. 414x265mm (72 x 72 DPI)


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Scheme 1. The scheme shows the thermodynamic cycle relating proton affinities for isolated (PA0) and tetrel-bound complexes (PA) with the corresponding binding energies (BE) for neutral and protonated R3SiOR’ (R=H, Me; R’ = H, SiR3). 418x156mm (72 x 72 DPI)



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Figure 6. (a) Molecular graphs obtained by means of QTAIM calculations for the particular case of the H3SiOH and Me3SiOH complexes with CO. Bond critical points are green dots, whereas ring critical points (RCP) are represented by red dots and cage critical points (CCP) by blue dots. (b) Picture on the right: NCI analysis obtained for the H3N···Me3SiOH complex. Green isosurfaces denote interactions within the van der Waals range. 415x121mm (72 x 72 DPI)


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

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