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Weak Interactions Get Strong: Synergy Between Tetrel and Alkaline-Earth Bonds Ibon Alkorta, M. Merced Montero-Campillo, Otilia Mó, José Elguero, and Manuel Yáñez J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b06051 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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The Journal of Physical Chemistry

Weak Interactions Get Strong: Synergy Between Tetrel and Alkaline-Earth Bonds Ibon Alkorta,a ,* M. Merced Montero-Campillo,b,* Otilia Mó,b José Elguero,a and Manuel Yáñezb a Instituto

de Química Médica, IQM-CSIC. Juan de la Cierva, 3, E-28006 Madrid, Spain

b Departamento

de Química, Módulo 13, Facultad de Ciencias and Institute of Advanced

Chemical Sciences (IadChem), Universidad Autónoma de Madrid, Campus de Excelencia UAM-CSIC, Cantoblanco, 28049-Madrid, Spain. *[email protected]; *[email protected]

ABSTRACT Weak and strong non-covalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH3F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug’-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies,

29Si-NMR

chemical shifts and

topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the 29Si

chemical shifts in all the ternary complexes. At the same time the ternary

complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

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INTRODUCTION The importance of non-covalent interactions1,2 governing macromolecular assemblies in materials and biological structures is so big that the main properties of a given system of this kind cannot be understood without looking at the interplay between covalent and non-covalent arrangements. The last decades have provided a new view and detailed description on the nature and strength of a large number of non-covalent interactions.3-17 Not only hydrogen bonds but also other kinds of interactions have a strong influence on physicochemical properties, from which the tetrel bond is supposed to be among the weakest ones.7-9,17 However, its weak nature does not imply a lack of importance, as for instance the modulation of this interaction through the tetrel site of silicon in disiloxane (the building block of silicones) have been shown to have a huge influence on the basicity of the molecule.18 The beryllium bond is, on the contrary, among the strongest ones reported.14,19 Due to the highly electron-deficient nature of the beryllium atom, this interaction provokes severe changes on the acidity-basicity properties of a given system, that may lead to the formation of ion-pairs in the gas phase,20,21 or even induce the spontaneous formation of neutral radicals.22,23 As a result of the presence of two or more non-covalent interactions in the same system, they might be reinforced or weakened. The mutual reinforcement in these interactions has been known for more than four decades as cooperativity24-27 and it is characterized for its non-additivity. Computational methods are crucial in the estimation of cooperativity effects, as they allow measuring geometrical changes occurring under cooperation between noncovalent interactions, the energy gained with respect to a non-cooperative situation, or identifying the different physical origins of the energy contributions to the total binding energy. Some works on cooperativity involving tetrel bonds have been reported recently.28-30 In this work, we study the essence of the interplay between tetrel bonds and alkaline-earth bonds, which has not been explored yet in the literature. We consider this selection particularly important because will allow us to visualize and quantify the mutual interaction between one rather weak non-covalent interaction as the tetrel bonds, with one significantly strong, the alkaline-earth bonds. We will show that the effects on the former are significantly stronger than the effects on the latter in relative terms. With this aim in mind, we assess the cooperativity effects and evaluate the importance in terms of energy and geometrical rearrangements that the presence of these two kinds of 2


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interactions in the same system may have. For this purpose, a series of ternary complexes between a Lewis base containing a donor N atom (NH3, NHCH2, HCN), the SiH3F molecule as a prototypical tetrel site, and an alkaline-earth derivative (MCl2, MCO3, MSO4; M = Be, Mg) have been studied, as illustrated in Figure 1. The bond between the N-base and the silicon compound leads to a tetrel bond, whereas the bond between F in SiH3F and the alkaline-earth element in the alkaline-earth derivatives leads to an alkaline-earth bond. In these interactions, N and F act as donor atoms, and Si and Be/Mg as acceptors. Therefore, these systems can be visualized as a double Lewis-pair, in which one of the subunits (SiH3F in this case) is simultaneously accepting (Lewis acid) and donating charge (Lewis base) through different atoms.

Figure 1. Single molecules that give place to tetrel and alkaline earth bonds, together with an example of a ternary complex in which both kinds of interactions are present.

The strength and nature of the interactions are analyzed paying attention to geometrical distortions, binding energies, topological features and

29Si-NMR

chemical

shifts, all based on optimized MP2/aug’-cc-pVTZ structures. As will be explained in

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following sections, both tetrel and alkaline-earth bonds exhibit a large degree of cooperativity. COMPUTATIONAL DETAILS Electronic structure calculations for all complexes were carried out at the MP2/aug’-cc-pVTZ level of theory with the Gaussian16 software,31 and harmonic frequency calculations were obtained to ensure that all structures were minima of the potential energy surface. The Møller-Plesset perturbation theory along with a large basis set has been shown to provide good descriptions of non-covalent interactions.32 Binding enthalpies are obtained by subtracting the enthalpy of the optimized interacting units to the total enthalpy of the complex. Obviously the strength of the tetrel bonds or the alkaline-earth bonds in the binary complexes is calculated as the energy difference of the complex with respect to the two non-interacting components. To estimate the effects of cooperativity when the ternary complexes stabilized by both non-covalent interactions are formed, the strength of tetrel and alkaline-earth bonds can be evaluated as the energy associated to reactions (1) and (2), respectively: SiH3F···MY + RN  RN···SiH3F···MY

(1)

RN···SiH3F + MY  RN···SiH3F···MY

(2)

The

29Si-NMR

chemical shieldings have been calculated with the GIAO

method33 at the MP2/aug’-cc-pVTZ level of theory. They have been converted in chemical shifts using the value of the isolated SiH3F as reference. The Atoms in molecules theory34-36 was used to analyze the nature of bonding in the set of complexes under study. For this purpose, we paid attention to three different magnitudes: the electron density (ρ), the Laplacian of the electron density (∇2ρ) and the total energy density (H), at the bond critical point (BCP) between two given atoms. From a topological point of view, a BCP, is a saddle point in the three-dimensional space with maximum density in two directions and minimum density in the third one. The density at the BCP provides an idea of the strength of the bond when comparing the same pair of atoms in different bonding environments. The Laplacian of the density helps to identify a depletion (∇2ρ >0) or accumulation (∇2ρ 2). This requires a deformation energy that is much lower when dealing with BeCO3 and BeSO4 containing complexes,37 where the X-M-X is already far from being linear (X-M-X angles in the range 76º-100º, See Table S3 of the Supporting Information). The increase of the effect on going from BeCO3 to BeSO4 simply shows that the latter is a better electron acceptor than the former.37 These statements are also valid for the Mg-containing analogues, as shown in Table 1,

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ratifying previous conclusions in the literature concerning the rather similar Lewis acid behavior exhibited by Be and Mg compounds.38-40 The electron density redistributions behind the cooperativity between tetrel and alkaline-earth bonds are easily identified not only by the geometrical changes discussed above, but also through the characteristics of their molecular graphs. As illustrated in Figure

2,

for

the

particular

case

of

H3N···SiH3F,

SiH3F···BeCO3

and

H3N···SiH3F···BeCO3 binary and ternary complexes, there is a significant increase in the electron density at the tetrel bond and a smaller one at the alkaline-earth bond, on going from the binary complexes to the ternary one, whereas the electron density at the Si–F bond critical point decreases leading to the weakening of this bond mentioned above.

Figure 2. Molecular graphs of the H3N···SiH3F and SiH3F···BeCO3 binary complexes and the H3N···SiH3F···BeCO3 ternary one. Green dots show the bond critical points. Electron densities (in a.u.). Red, blue and green values correspond to the tetrel, alkaline-earth and Si–F bonds, respectively.

This figure also shows that the tetrahedral geometry of the bonds around the silicon atom is affected by the formation of the tetrel and the alkaline-earth bonds in the ternary complex. Remarkably, the silyl group retains its pyramidal conformation in the two binary complexes, but becomes strictly planar in the ternary complex. This is a general behavior independent of the nature of the Be (or Mg) derivative or the N-base, as illustrated in Figure 3, though the silyl moiety is not strictly planar (but close to it) for NCH, as the interactions with this base are the weakest ones of the N-base series.

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Figure 3. Molecular graphs for the ternary complexes SO4Be···FSiH3···NH3, CO3Be···FSiH3···NH=CH2 and Cl2Be···FSiH3···NCH. Green dots show the bond critical points. Electron densities (in a.u.). Red, blue and green values correspond to the tetrel, alkaline-earth and Si–F bonds, respectively.

This finding seems to be a consequence of the pentacoordination of the Si atom in the ternary complexes, which is bonded not only to the tree hydrogen atoms and to the F atom, as in the isolated molecule, but also to the N atom of the Lewis base. Penta coordinate silicon derivatives, usually exhibiting a distorted trigonal bipyramidal geometry, have been reported before.41-44 Also, this pentacoordination reminds closely the one reported for the Si2(CH3)7+ carbocation,45 and later on also postulated for HnE– CH3–EHn+ structures (E=Group 1, 2, 13, or 14 element),46 and explained as the result of the formation of three-center E–C–E bonds.45,46 The formation of a three-center F-Si-N bond is in our case also the situation behind the planarity of the SiH3 group. Indeed, taking the ternary SO4Be···FSiH3···NH3 as a suitable example, the NBO analysis shows it to be the result of the interaction between only two molecular subunits, namely SO4Be and FSiNH6. More importantly, the Si atom appears in the FSiNH6 subunit covalently bound to the three H atoms through three sp2 hybrid orbitals, whereas the bonding to F and N involves a three-center bond between a s0.5p1.5d hybrid at Si, and sp3 hybrids at F and N. Consistently, the corresponding molecular graph (See Figure 3) shows five linkages around Si, though, as expected, the Si-F and the Si-N bonds are weaker than the Si-H ones. A similar bonding pattern is observed when dealing with the other Be derivatives and with the corresponding Mg-analogues (See Tables S2 and S3 of the Supporting Information). The aforementioned bonding pattern does not change significantly when ammonia is replaced by imine, as shown in Figure 3. In contrast, a weaker N-Si interaction in Cl2Be···FSiH3···NCH is reflected in a very slight pyramidalization of the SiH3 group towards the base. Finally, it is worth mentioning that other secondary non-covalent interactions also may play a role in the relative stability of these complexes and in their structure. As a matter of

fact,

they

are

responsible

of

the

peculiar

behavior

of

the

ternary

CO3Mg···FSiH3···NH3. It can be seen in Figure 4 that, with the exception of this complex, the bond length, of both the tetrel N···Si goes through a minimum for the complexes with methanimine. A similar behavior, though less pronounced, is also

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observed for the bond length of the alkaline-earth bond. This complex leads to the shortest bonds for the Mg series.

Figure 4. Variation of the alkaline-earth F···Be and F···Mg bonds and the N···Si tetrel bonds (Å) depending on the N-base and the MY Lewis acid.

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Indeed, for the CO3Mg···FSiH3···NH3 complex, a similar geometrical arrangement to the ones found for the other Be- and Mg-containing ternary complexes (see Figure 5a) is not a local minimum but a transition state, which presents some new interactions: a hydrogen bond between one of the O atoms of the carbonate moiety and one of the H atoms of ammonia, and a bonding interaction between a second oxygen atom and the silyl group, together with a rather weak Si–F bond. This transition state evolves to a minimum (see Figure 5b) 76 kJ·mol-1 lower in energy through the total cleavage of the Si–F linkage. This arrangement reinforces significantly the Mg–F bond and permits a reorientation of the H3Si–NH3 moiety with respect to the CO3Mg one, in such a way that the NH···O hydrogen bond and the stabilizing O···Si interaction become reinforced as well as the tetrel bond (compare Figs. 5a and 5b). A similar rearrangement is not possible, however, for the Be-containing analogue, as the shorter Be-O distances do not favor the formation of the NH···O hydrogen bond. As a matter of fact this arrangement was not found for any other ternary complex.

Figure 5. Molecular graphs of two possible conformers of the CO3Be···FSiH3···NH3 ternary complex, where (a) is a transition state, (b) is the global minimum. Green dots show the bond critical points. Electron densities (in a.u.). Values in red, blue, green, magenta and light-blue correspond to the tetrel bond, the alkaline-earth bond, the Si···F interaction, the NH···O hydrogen bonds and the Si···O bonding interactions, respectively.

Cooperativity effects on Binding energies. Not surprisingly, the cooperativity reflected in the bonding and electron density distributions is also mirrored in the values of the binding energies of the ternary complexes with respect to those of the binary ones, as shown in Table 2.

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Table 2. Binding energies and cooperativity (kJ/mol) in binary and ternary complexes at the MP2/aug-ccpvTZ level of theory. Binding energies for the tetrel and alkaline-earth bonds within the ternary complexes are specified by the label “ternary”. System H3N···SiH3F···BeCl2 H2CHN···SiH3F···BeCl2 HCN···SiH3F···BeCl2 H3N···SiH3F···BeCO3 H2CHN···SiH3F···BeCO3 HCN···SiH3F···BeCO3 H3N···SiH3F···BeSO4 H2CHN···SiH3F···BeSO4 HCN···SiH3F···BeSO4 H3N···SiH3F···MgCl2 H2CHN···SiH3F···MgCl2 HCN···SiH3F···MgCl2 H3N···SiH3F···MgCO3 H2CHN···SiH3F···MgCO3 HCN···SiH3F···MgCO3 H3N···SiH3F···MgSO4 H2CHN···SiH3F···MgSO4 HCN···SiH3F···MgSO4

RN···SiH3F

SiH3F···MY

-28.2 -33.3 -17.8

-51.3

-149.5

-159.2

-67.6

-114.5

-123.7

Binding energy RN···SiH3F SiH3F···MY (ternary) (ternary) -77.5 -100.6 -84.5 -102.5 -41.6 -75.1 -102.4 -223.7 -109.7 -225.9 -58.5 -190.2 -108.6 -239.6 -116.4 -242.3 -63.3 -204.7 -71.7 -111.1 -78.5 -112.8 -38.4 -88.2 -111.8 -198.1 -101.2 -182.4 -51.8 -148.5 -103.3 -198.8 -109.5 -199.9 -58.6 -164.5

Cooperat. RN···SiH3F···MY -128.8 -135.8 -92.9 -251.9 -259.2 -208 -267.8 -275.6 -222.5 -139.3 -146.1 -106 -226.3 -215.7 -166.3 -227 -233.2 -182.3

-49.3 -51.2 -23.8 -74.2 -76.4 -40.7 -80.4 -83.1 -45.5 -43.5 -45.2 -20.6 -83.6 -67.9 -34.0 -75.1 -76.2 -40.8

Starting from the binary complexes, it can be seen that tetrel bonds are much weaker than alkaline-earth bonds; the strongest being, in both cases, the one corresponding to the sp2 N-base, methanimine. As alkaline-earth bonds are concerned, the binding energies follow the same trends observed above for the internuclear distances and the electron densities at the bond critical points (MSO4 > MCO3 > MCl2). Let us go now to the ternary complexes, for which the strength of tetrel and alkalineearth bonds are given by reactions (1) and (2) described in previous sections. It is apparent that, consistently with our previous discussion, both bonding interactions are stronger in the ternary complexes than in the binary ones (compare columns 4-5 with columns 2-3 in Table 2). However, although the enhancement due to cooperativity is necessarily the same in both cases (see last column of Table 2), since the reaction enthalpy is a state function and its value does not depend on the path from reactants to products, in relative terms the stabilization is larger for the weakest one, i.e., the tetrel bond is more stabilized than the alkaline-earth bond. This can be easily seen looking at the thermodynamic cycle shown in Figure 6, which compares the formation of the H3N···SiH3F···BeCl2 ternary complex and its Mg-containing analogue.

11



Figure 6. Diagrams showing the energetics (enthalpies in kJ mol−1) of the formation of (a) H3N···SiH3F···BeCl2 and (b) H3N···SiH3F···MgCl2, following two alternative pathways. In both cases the blue pathway corresponds to a process in which the first step is the formation of the tetrel bond followed by the subsequent formation of the beryllium bond. Conversely, in the green pathway the first step is the formation of the beryllium bond followed by the formation of the tetrel one.

The total energy balance is the same whether the first step is the formation of the tetrel or the alkaline-earth bond, 49.3 kJ·mol-1 for Be and 43.5 kJ·mol-1 for Mg. But in relative terms, the strength of the tetrel bond increases a 174% for the Be-containing complexes and a 154% for the Mg-containing analogue, whereas the alkaline-earth bond increases its strength a 96% (Be) and a 64% (Mg). Therefore, the general rule is that the relative enhancement of the different non-covalent interaction present in the ternary complexes is systematically higher for the weaker bond in the binary complexes. Finally, it is worth noting the good linear correlation between the binding energies (BE) of both the alkaline-earth and tetrel bonds within the ternary complexes and the electron densities at the corresponding BCPs (see Table S4 of the supporting Information), as illustrated in Figure 7.

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Figure 7. Linear correlations between the binding energies and the electron densities at the corresponding BCPs of the ternary complexes under investigation: (a) alkaline-earth bonds. (b) tetrel bonds. The linear correlations in (a) obey the equations: BE (kJ/mol) = -7385.5 (a.u.) + 392.86, r2 = 0.975 for Becontaining complexes and BE (kJ/mol) = -7853.4 (a.u.) + 204.35, r2 = 0.935. The linear correlations in (b) obey the equations: BE (kJ/mol) = -1875.9 (a.u.) + 13.032, r2 = 0.941 for Be-containing complexes and BE (kJ/mol) = -1532.1 (a.u.) + 0.1587, r2 = 0.942.

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NMR values. The values of the absolute shieldings and

29Si

chemical shifts with respect to

SiH3F shown in Table 3 reflect quite nicely the electron density redistributions associated with the formation of binary and ternary complexes. Indeed, the formation of the binary SiH3F···MY (M = Be, Mg; Y = Cl2, CO3, SO4) necessarily leads, as discussed above to a decrease of the electron density around Si, and therefore to a negative value of the chemical shift. Conversely, the formation of the tetrel bonds in the Base-N···SiH3F complexes, systematically increases the electron density around Si and therefore a positive chemical shift is found. On going to the ternary complexes we have seen that the cooperativity reinforces both non-covalent interactions, but the reinforcement of the tetrel bond systematically dominates, explaining why in all the ternary complexes the chemical shift is positive and larger than the one found for the corresponding binary complexes. Table 3. Absolute shieldings σ(29Si ) at the MP2/aug-cc-pvTZ level of theory, and chemical shifts () with respect to SiH3F. System SiH3F SiH3F ···BeCl2 SiH3F ···BeCO3 SiH3F ···BeSO4 SiH3F ···MgCl2 SiH3F ···MgCO3 SiH3F···MgSO4 H3N···SiH3F H2CHN···SiH3F HCN···SiH3F H3N···SiH3F···BeCl2 H2CHN···SiH3F···BeCl2 HCN···SiH3F···BeCl2 H3N···SiH3F···BeCO3 H2CHN···SiH3F···BeCO3 HCN···SiH3F···BeCO3 H3N···SiH3F···BeSO4 H2CHN···SiH3F···BeSO4 HCN···SiH3F···BeSO4 H3N···SiH3F···MgCl2 H2CHN···SiH3F···MgCl2 HCN···SiH3F···MgCl2 H3N···SiH3F···MgCO3 H2CHN···SiH3F···MgCO3 HCN···SiH3F···MgCO3 H3N···SiH3F···MgSO4 H2CHN···SiH3F···MgSO4 HCN···SiH3F···MgSO4

14

σ(29Si) 408.27 383.86 364.99 362.48 386.60 373.12 369.27 452.39 448.43 415.82 484.01 468.75 435.73 479.63 464.36 447.70 479.17 463.97 451.21 482.88 468.09 430.50 481.31 466.76 448.44 480.89 466.83 452.08


 -24.41 -43.28 -45.78 -21.66 -35.14 -39.00 44.12 40.17 7.55 75.75 60.48 27.47 71.37 56.09 39.44 70.91 55.71 42.94 74.61 59.82 22.23 73.05 58.50 40.17 72.62 58.56 43.81

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CONCLUSIONS This in-depth study of the geometry, energy and topological features of the electron density of a set of ternary complexes RN···SiH3F···MY2 involving tetrel bonds and alkaline-earth bonds has shown that these two kinds of non-covalent interactions of different strength can cooperate and get significantly reinforced. Although bound by non-covalent interactions, binding energies in these complexes are quite high, ranging from -92.9 kJ/mol to -275.6 kJ/mol. As a consequence of the cooperativity between tetrel bonds and alkaline-earth bonds, up to more than 80 kJ/mol in the most favorable case, tetrel bonds are shortened up to 0.6 Å, whereas alkaline-earth bonds are also slightly shortened. Concomitantly, in the ternary complexes the silyl group is essentially planar as it corresponds to pentacoordinated Si atom, very much as what has been found previously for similar pentacoordinated methyl groups.

45,46

We have also shown that

the weakest interaction is the one that gains the most in the cooperativity phenomenon, factor which is nicely reflected in the values of the

29Si

chemical shifts. The acceptor

ability of the alkaline-earth metal, which varies according to the angle formed by the metal and its ligands, and the hybridization of the N base, acting as a donor in the tetrel interaction, have a strong influence on the binding energies. SUPPORTING INFORMATIONS The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.XXX. Structures, total energies, molecular graphs and electron density parameters of the complexes. AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected] (I.A.), [email protected] (M.M.M.-C.) ORCID Ibon Alkorta: 0000-0001-6876-6211 M. Merced Montero-Campillo: 0000-0002-9499-0900 Otilia Mó: 0000-0003-2596-5987 José Elguero: 0000-0002-9213-6858 Manuel Yañez: 0000-0003-0854-585X 15



NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS Financial support from the Ministerio de Ciencia, Innovación y Universidades (projects PGC2018-094644-B-C21, PGC2018-094644-B-C22 and CTQ2016-76061-P)) and Comunidad de Madrid (P2018/EMT-4329 AIRTEC-CM) is acknowledged. The authors want to thank the CTI (CSIC) and the CCC-UAM (Centro de Computación Científica at the Universidad Autónoma de Madrid) for the computational resources.

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