Cooperativity in Tetrel Bonds - The Journal of Physical Chemistry A

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Cooperativity in Tetrel Bonds Marta Marin-Luna, Ibon Alkorta, and José Elguero J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11876 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 15, 2016

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The Journal of Physical Chemistry

Cooperativity in Tetrel Bonds#

Marta Marín-Luna,a Ibon Alkorta,* José Elguero Instituto de Química Médica (CSIC) C/Juan de la Cierva, 3 28006-Madrid (Spain)

a

Present address: Organic Chemistry Dept., Univ. Vigo (Spain).

* Author to whom correspondence should be addressed: e-mail: [email protected], web: http://are.iqm.csic.es

#

Dedicated to our good friend and colleague, Prof. Janet E. Del Bene.

Abstract A theoretical study of the cooperativity in linear chains of (H3SiCN)n and (H3SiNC)n complexes connected by tetrel bonds has been carried out by means of MP2 and CCSD(T) computational methods. In all cases, a favorable cooperativity is observed, especially in some of the largest linear chains of (H3SiNC)n where the effect is so large that the SiH3 group is almost equidistant to the two surrounding CN groups and it becomes planar. In addition, the combination of tetrel bonds with other weak interactions (halogen, chalcogen, pnicogen, triel, beryllium, lithium and hydrogen bond) has been explored using ternary complexes, (H3SiCN)2:XY and (H3SiNC)2:XY. In all cases, positive cooperativity is obtained, especially in the (H3SiNC)2:ClF and (H3SiNC)2:SHF ternary complexes where, respectively, halogen and chalcogen shared complexes are formed.

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Introduction The possibility that elements of the group 14 of the periodic table, specially Si and Ge, could be involved in weak interactions as Lewis acids has been known for long time both in intramolecular contacts17 and in intermolecular ones.8-11 Recently, this type of interaction has been coined as tetrel bond12 based on the ancient name of the atoms of this group. A more specific name has been given when the atom acting as Lewis acid is the carbon, carbon bond.13-16 A number of recent studies has been devoted to the study of tetrel bonds17-23 and two reviews have been published in carbon bond and tetrel bonds.24 Tetrel bonds have been included in the group of σ-hole interactions together with hydrogen, halogen, chalcogen and pnicogen bonds.25-27 The σ-hole corresponds to a region of positive electrostatic potential around an atom due to the anisotropy of its charge distribution. In general, it is associated to the presence of a covalent bond with an electron-withdrawing group in the opposite side of the atom of interest along the same axis where the σ-hole is located. A common characteristic of most weak interaction is the existence of cooperativity. That is, the binding energy of a single molecule with a cluster is larger than that observed in the dimer or binary complex formation. This phenomenon explains some of the characteristics of hydrogen bonded clusters28 and have been found to exist in clusters connected with other unique interactions (dihydrogen,29-30 halogen,31-34 lithium,35 beryllium,36 pnicogen37) or mixed interactions.38-46 While this article was in preparation, the possibility of cooperativity in chains of acetonitrile and methyl isocyanide has been explored.47 In the present article, the cooperativity in tetrel bonded systems will be analyzed from two points of view. In the first one, the linear clusters of (SiH3CN)n and (SiH3NC)n connected via tetrel bonds will be considered. In the second, the combination of tetrel bonds with a variety of weak interactions (hydrogen, lithium, beryllium, triel, chalcogen and halogen bonds) will be analyzed in ternary complexes. The energetic, geometric and electronic aspects of these systems have been considered.

Computational Methods The geometry of the systems has been optimized with the MP2 computational level48 and the aug-cc-pVDZ basis set.49 Frequency calculations have been computed at the same level to verify that the structures obtained correspond to energetic minima. All calculations have been performed using the Gaussian-09 program.50 The MP2/CBS have been calculated using the extrapolations scheme proposed by Helgaker and co-workers (Eq. 1-9) using the aug-cc-pVDZ and aug-cc-pVTZ energies.51-52 The value used for α was 1.43 as recommended in the literature.51 Additionally, the contribution of the CCSD(T) energy calculated with the aug-cc-pVDZ basis set has been added to the MP2/CBS energy to obtain a CCSD(T) quality energy following the recommendations of Hobza et al. (Eq. 10).53

= + ; = +

Eq. 1-2

= −

Eq. 3

= − ; = −

Eq. 4-5 2


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= + 2 ; = + 3 Eq. 6-7

= −

Eq. 8

= +

Eq. 9

()

()

= + (

− )

Eq. 10

()

The binding energy of the complexes has been calculated as the difference between the of the complexes and the isolated monomers in the MP2/aug-cc-pVDZ minimum. The average binding energy per contact has been calculated dividing the binding energy by the number of tetrel bond contacts in each chain. Finally, the incremental binding energy has been obtained as the difference between the energy of a complex with n monomers and the sum of the complex with n-1 monomers and an isolated monomer. The many body interaction energy method54-55 has been used to evaluate the one, two and higher body terms of the binding energy of the clusters studied. Based on this methodology, the binding energy can be divided in a series of terms as indicated in Eq. 11 for a ternary complex. The ER terms (Eq. 12) correspond to the distortion energy of each monomer from the geometry of the isolated monomer in their minimum configuration to the geometry observed in the complex. The second order terms (Eq. 13), ∆2E(i,j), are calculated as the energy difference of the i:j dimers minus the sum of the monomers all in the geometry of the complex. The third order terms (Eq. 14) are obtained subtracting from the energy of each possible ternary complex, the energies of the isolated monomers and the corresponding second order terms. The second term represents the pair-wise additive interactions and the sum of the higher order terms (three, four, etc.) corresponds to the non-additive terms, or cooperativity. N

N

N −1 N

N − 2 N −1 N

i= A

i= A

i = A j >i

i = A j >i k > j

∆E = E ( ABC...N ) − ∑ Em (i) =∑ [ E (i) − Em (i)] + ∑∑ ∆ 2 E (ij ) + ∑ ∑∑ ∆3 E (ijk ) + ... +∆ n E ( ABC...N ) Eq. 11

E R (i ) =E (i ) − Em (i )

Eq. 12

∆2 E (ij ) = E (ij ) − [E (i ) − E ( j )]

Eq. 13

∆3 E (ijk ) = E (ijk ) − [E (i ) + E ( j ) + E (k ) ]−  ∆ 2 E (ij ) + ∆ 2 E (ik ) + ∆ 2 E ( jk ) 

Eq. 14

The electrostatic potential values on the 0.001 au electron density isosurface have been calculated for the isolated monomers and the clusters. This isosurface has been reported to resemble the van der Waals volume of the molecules.56 The values obtained along the symmetry axes of the systems provide an estimation of the electrostatic interaction of these systems with other approaching molecules. The topological analysis of the electron density of the complexes has been carried out within the Atoms in Molecules (AIM) methodology.57-58 Several properties at the bond critical points (ρBCP, ∇2ρBCP and HBCP) have 3



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been characterized in order to gain insight of the nature of the corresponding contact. The AIMAll program has been used for this analysis.59 The natural bond orbital method (NBO)60 has been used to characterize the stabilization due to the intermolecular charge transfer between occupied orbitals towards empty ones, characteristic of weak interactions. The NBO6 program61 connected to the Gaussian-09 package has been used for these calculations.

Results and Discussion: This section has been divided in three parts. In the first one, the isolated H3SiCN and H3SiNC molecules have been analyzed. In the second one, the (H3SiCN)n and (H3SiNC)n chains connected only through tetrel bonds will be considered. In the third one, the binary H3SiCN/NC:XY and ternary (H3SiCN/NC)2:XY complexes where tetrel and other non-covalent interactions are present will be discussed. H3SiCN and H3SiNC isolated monomers. The geometry of the isolated H3SiCN and H3SiNC molecules shows a C3v symmetry with all the heavy atoms along the symmetry axis. The structure and dipole moment of H3SiCN has been characterized experimentally by means of rotational spectroscopy.62-63 The MP2 data for the H3SiCN molecule closely resemble the experimental results derived from rotational spectroscopy. Table 1. Experimental and calculated geometrical parameters (Å, °) and dipole moment (Debye) of H3SiCN and H3SiNC.

d C-N d Si-C/N d Si-H < HSiC/N Dipole moment

H3SiCN Exp. 1.159 1.848 1.470 107.4 3.44

H3SiCN Calc. 1.190 1.877 1.482 107.4 3.57

H3SiNC Calc. 1.199 1.792 1.481 106.9 3.75

The electrostatic potential of the monomers (Fig. 1) show a negative region in the extension of the molecule along the symmetry axis close to the CN group and a positive region in the opposite side, close to the silicon atom. The values obtained for such regions are larger in absolute values for the H3SiNC than for the H3SiCN, in agreement with the larger dipole molecule of the former molecule. The electrostatic potential values for the isolated molecules predict larger binding energies of the complexes with H3SiNC than the complexes of H3SiCN acting both as electron donor, trough N/C atom, and as electron acceptor, trough Si atom.

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Fig. 1. Electrostatic potential of the H3SiCN and H3SiNC molecules. The colors range from red (-0.05 au) to blue (+0.05). The values (a.u.) of the maxima (black dots) and minima (white dots) along the symmetry axis are indicated. (H3SiCN)n and (H3SiNC)n linear chains Energy and geometry. These complexes adopt a linear chain of C3v symmetry (Fig. 2) and the hydrogen atoms of silyl groups of one molecule eclipsed with respect to the remaining ones (Table S1 of the Supporting Information Material). The optimized geometry with the silyl groups alternated with respect to the surrounding monomers show a slightly higher energy than the eclipsed one (0.03 kJ·mol–1 in the dimers).

Fig. 2. Molecular graph with the interatomic distances in the (H3SiNC)6 and (H3SiCN)6 clusters. The intermolecular C/N···Si distances in the complexes are 2.94 and 2.90 Å in the binary (H3SiCN)2 and (H3SiNC)2 complexes, respectively, and 2.69 and 2.12 Å in the central contacts of the hexamers. This fact indicates that the intermolecular distances of the H3SiNC complexes are much more sensitive to the number of molecules involved in the cluster than those of the H3SiCN ones. The evolution of the intermolecular distances along the chain, represented in Fig. 3, clearly show that as the number of monomers increase in the complex, the intermolecular distance became shorter at a given contact position. In addition, the shortest intermolecular distance in a complex is obtained at the central interactions. The largest differences are obtained between the first and third contacts of the hexamers with values of 0.13 and 0.19 Å for the (SiH3CN)6 and (SiH3NC)6 complexes, respectively.

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3.0

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3.0

(H3SiCN)2 (H3SiCN)3 (H3SiCN)4 (H3SiCN)5 (H3SiCN)6

2.9

(H3SiNC)2 (H3SiNC)3 (H3SiNC)4 (H3SiNC)5 (H3SiNC)6

2.8

2.6

2.8

2.4

2.2

2.7

2.0

1

2

3

4

5

1

2

3

4

5

Fig. 3. Intermolecular distances vs. the position in the (H3SiCN)n (left) and (H3SiNC)n (right).

The formation of complex produces an elongation of the covalent Si-C and Si-N bonds in the (H3SiCN)n and (H3SiNC)n complexes. This effect increases dramatically in the (SiH3NC)n complexes as the size of the cluster became larger. For instance, in the hexamer (Fig. 2), the Si atom in the center of the chain is surrounded by two CN groups at almost equidistant (2.02 and 2.12 Å for the N···Si and Si···C distances, respectively). Another interesting feature is that the SiH3 became planar (sum of the three HSiH angles equal to 360°). These results resemble the transition structure (TS) of a SN2 reaction as was previously proposed by Grabowski64 for tetrel interactions. The different behavior of the isocyano and cyano groups in weak interactions has been already found in the formation of ionic halogen bonds by the former group but not in the latter.65-69 The total binding energy, average binding energy per contact as well as the incremental binding energy of the complexes, calculated as the difference between the chain with n monomers and the sum of the chain with n-1 monomers plus the energy of the isolated monomers, are gathered in Table 2. The total binding energies range between –21 to -133 kJ·mol–1 in the (H3SiCN)n series and between –22 and –153 kJ·mol–1 in the (H3SiNC)n one. The average binding energy per contact increases steadily as the size of the complex does. The variation of the binding energy per contact in the hexamer corresponds to increments of 27 and 39% in the (H3SiCN) and (H3SiNC) series. In the same way, the incremental binding energy grows steadily from the dimers to the hexamers, being the incremental binding energy in the hexamer 41 and 83% larger than in the dimers of both series, respectively. All these results are an indication of positive cooperativity in these clusters.

Table 2. Total binding energy (Ebtot), average binding energy per contact (Ebav) and incremental binding energy (Ebincr) (kJ·mol–1) at CCSD(T)/CBS level.

n 2 3 4 5

Ebtot –21.0 –46.8 –74.6 –103.6

(H3SiCN)n Ebav –21.0 –23.4 –24.9 –25.9

Ebincr –21.0 –25.8 –27.8 –29.0

Ebtot –22.0 –49.6 –78.8 –112.6 6

(H3SiNC)n Ebav –22.0 –24.8 –26.3 –28.1


Ebincr. –22.0 –27.5 –29.2 –33.8

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6

–133.2

–26.6

–29.5

–152.8

–30.6

–40.2

The MBIE partition terms in these clusters (Table 3) show that the sum of the three body and higher body terms are negative what indicates a favorable cooperativity. The absolute value of each term increases steadily as the number of monomers does in the chain. For a given number of monomers the values of each term are larger in the SiH3NC than in the H3SiCN complexes. The sum of the distortion energy of the monomers is small in the H3SiCN series compared to the binding energy, while in SiH3NC series it overtakes the absolute value of the binding energy in the pentamer and hexamer. These results are in agreement with the previously commented very large distortions of the (H3SiNC)n cluster as the size of the system increases. The second body term is the dominant attractive term representing more than 87% of the attractive terms in the SiH3CN series and 77% in the SiH3NC ones. The three body term is about one order of magnitude smaller than the two body term, except in the (H3SiNC)5 and (H3SiNC)6 complexes where the sum of the third order terms is about one fourth of the second order ones. The sum of the higher order terms is about one tenth or less of the third body term (Table 3).

Table 3. Sum of the MBIE energy terms (kJ·mol–1). Complex (H3SiCN)2 (H3SiCN)3 (H3SiCN)4 (H3SiCN)5 (H3SiCN)6

ΣEr(i) 1.2 4.6 9.8 17.0 24.4

Σ∆2E(i,j) –23.9 –51.7 –81.3 –112.4 –143.7

(H3SiNC)2 (H3SiNC)3 (H3SiNC)4 (H3SiNC)5 (H3SiNC)6

2.7 13.7 66.4 195.8 336.4

–30.2 –71.2 –146.1 –281.6 –426.9

Σ∆3E(i,j,k)

Higher terms

–2.9 –7.4 –12.9 –18.8

–0.34 –0.97 –1.69

–4.9 –22.1 –62.3 –108.4

–1.19 –6.25 –13.99

The chain formation induces a dipole moment enhancement (Table 4) in these systems in agreement with the observed behavior of other systems connected by hydrogen, dihydrogen and halogen bonds.29-30, 70-74 In the H3SiCN series, the average dipole moment per monomer increases from 3.6 to 5.6 Debye in the hexamer which corresponds to a gain of 55%. The effect on the H3SiNC series is even larger changing from 3. 8 in the monomer to 9.9 Debye in the hexamer (160% gain). The evolution of the values in the H3SiCN series indicates that they are close to the plateau expected for an infinite chain but not in the H3SiNC series due to the important geometry alteration observed in the two largest clusters (Fig. S1 the Supporting Information Material). To these electronic changes are associated an increment of the nucleophilic and electrophilic character of the chains as indicated by the minimum and maximum values of the electrostatic potential at both ends of the chains (Table 4). In one hand, the σ-hole in the extreme of the chain where the silane group is located increases from 0.066 to 0.089 and from 0.075 to 0.132 au in the H3SiCN and H3SiNC series. On the other hand, the lone pair of the terminal -CN or -NC groups produces a minimum value of the electrostatic 7



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potential that changes from –0.054 to –0.077 and –0.055 to –0.108 au, respectively. Similar effects, on the values of the MEP on the extreme of the HCN chains, have been described.75

Table 4. Total Dipole moment, average dipole moment per monomer (Debye), σ-hole and MEP minimum at the extremes of the chain (au).

n 1 2 3 4 5 6

Total Dipole Moment 3.6 8.7 14.5 20.7 27.2 33.7

(H3SiCN)n Dipole Mom. Per monomer σ-hole 3.6 0.066 4.4 0.078 4.8 0.084 5.2 0.086 5.4 0.088 5.6 0.089

(H3SiNC)n Total Dipole Dipole Mom. Per Min-MEP Moment monomer σ-hole –0.054 3.8 3.8 0.075 –0.066 9.6 4.8 0.090 –0.072 16.9 5.6 0.099 –0.074 28.4 7.1 0.111 –0.076 44.2 8.8 0.125 –0.077 59.5 9.9 0.132

Min-MEP –0.055 –0.070 –0.078 –0.089 –0.102 –0.108

The NBO analysis show that the most important stabilizing orbital charge transfer interaction between occupied and empty orbitals, E(2), in the complexes corresponds to the lone pair C/N → σ* SiN/C (Table 5). In the two largest clusters of (H3SiNC)n, n= 5 and 6, the NBO method is not able to properly take into account the different molecules due to the large distortion of the monomers and thus, no values are reported in Table 5 for these systems. The values obtained along the chain, clearly indicate that the strongest interactions happen in the central contacts of each chain, in agreement with the interatomic distances previously discussed. In fact, excellent exponential relationships are obtained between the values of E(2) and the interatomic distances of the atoms involved in the charge transfer (Fig. S2 of the Supporting Information Material).76-77

Table 5. E(2) stabilization energies (kJ·mol–1) for the N/C lp → σ* Si-C/N orbitals. System

(H3SiCN)2 (H3SiCN)3 (H3SiCN)4 (H3SiCN)5 (H3SiCN)6 (H3SiNC)2 (H3SiNC)3 (H3SiNC)4

Contact position along the chain 1 23.4 28.9 31.0 32.6 32.8 52.3 74.6 124.0

2

3

4

5

29.4 37.5 42.1 43.3

31.8 42.2 46.2

33.1 43.6

33.7

77.2 186.5

129.0

8


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The analysis of the electron density shows the presence of a single intermolecular bond critical point (BCP) and its corresponding bond path between two consecutive molecules along the chain (Fig. 2 and Table S1 of the Supplementary Material). The values of the electron density at the intra and intermolecular Si-C and Si-N BCPs, ρBCP, increases as the intermolecular distances decreases (Fig. 5), following an almost perfect exponential correlation with the interatomic distances (R2 = 0.998 in both cases).78-85 The Laplacian in those BCP’s are always positive, including those of the isolated monomers, an indication that this parameter by itself is not adequate to evaluate the covalent character of these bonds.86 In contrast, most of the complexes present negative values of total energy density, HBCP, that together with the commented positive Laplacian values is an indication of partial covalent character of those interactions.86 As depicted in Fig 4, HBCP values associated to Si-C BCPs become more negative as intermolecular Si-C lengths decrease while, curiosity, in the case of Si-N bonds the HBCP it is not very dependant on the Si/C/N distances.

0.10

0.00 Si-C Si-N

0.08

-0.01

Si-C Si-N

HBCP

ρBCP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

-0.02 0.04

-0.03 0.02

-0.04 2.0

2.5

Interatomic

2.0

3.0

2.5

Interatomic

Distance

3.0

Distance

Fig. 4. Electron density (left) and total energy (right) at the BCP (au) vs. the interatomic distance (Å) for the Si-C and Si-N contacts.

Interplay between tetrel and other weak interactions. In order to explore the possible interplays between tetrel bonds and other weak interactions, the (H3SiCN)2 and (H3SiNC)2 dimers acting as Lewis bases have been complexed with a series of simple prototypical Lewis acids (ClF, SHF, PH2F, BH3, BeH2, LiH and HF), characteristic of halogen (XB), chalcogen (YB), pnicogen (ZB), boron (BB), Beryllium (BeB), lithium (LiB) and hydrogen bond (HB) donor molecules, respectively. The most important interatomic distances and binding energy of the binary and ternary complexes have been gathered in Table 6. They have been listed based on the N···Si distance on the (H3SiCN)2:XY ternary complexes (XY = BeH2 < LiH < BH3 < HF < ClF < SHF < PH2F). It should be noted that the ranking of C/N···Si intermolecular distances between the two silyl molecules in the ternary complexes is the same in the two series save for the complexes with ClF and SHF that are shorter than expected based on the (H3SiCN) series. In these two complexes, the C···X distances, with X = Cl and S, are smaller than 2 Å, indicating the formation of shared halogen and chalcogen bonds which strengthen the tetrel bond acceptor capability of the silyl molecule attached to the ClF and SHF one. In all cases, the N/C···Si distances between the two silyl molecules in the ternary complexes are shorter than those of the corresponding binary complexes. These differences can be as large as 0.2 and 0.5 Å in the H3SiCN and H3SiNC complexes. The same is observed for the N/C···X distances, with the exception of the 9



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BH3 complexes where the N/C···B distances are slightly shorter in the binary complexes than in the ternary ones. The differences in the N/C···X between the binary and ternary complexes are in general small (less than 0.1 Å) except for the (SiH3NC)n:SHF complexes where the C···S distance shortens 0.5 Å. As indicated previously the (SiH3NC)3:SHF complex shows a shared chalcogen bond with a very short C-S distance, while this does not happens in the binary complex. The binding energies range between –197 and –53 kJ·mol–1 and are always larger in absolute value in the H3SiNC complexes than in the corresponding H3SiCN ones, except in the binary complex with LiH where the complex with H3SiCN is slightly more stable (the binding energy of the H3SiNC:LiH complex is –71.3 kJ·mol–1 and that of H3SiCN:LiH is –71.8 kJ·mol–1). The ranking of the binding energy (absolute values) in the H3SiCN binary and ternary complexes is the same: BH3 > LiH > BeH2 > ClF > HF > SHF > PH2F. A good linear correlation is obtained between the value of the binary complexes and the corresponding ternary ones (R2 = 0.99). In the case of the complexes with H3SiNC, the two stronger cases (BH3 and BeH2) and the weakest one (PH2F) are the same for the binary and ternary complexes. However, the ranking is alternated for the LiH/ClF and HF/SHF complexes since the energetic differences in each pair are small. In this case the linear relationship between the values in the binary and ternary complexes shows a R2 value of 0.98. The binding energies in the ternary complexes are always more negative, between –4 and –25 kJ·mol–1, than the sum of the two constituent binary complexes which is another clue of the existence of positive cooperativity in these complexes.

Table 6. Intermolecular bond distances (Å) and CCSD(T)/CBS binding energies (kJ·mol–1) of ternary complexes (n = 2). Within parentheses values of the corresponding dimeric complexes (n = 1). Complex (H3SiCN)n (H3SiCN)n:BeH2 (H3SiCN)n:LiH (H3SiCN)n:BH3 (H3SiCN)n:HF (H3SiCN)n:ClF (H3SiCN)n:SHF (H3SiCN)n:PH2F

d N···Si, n=2 2.945 2.729 2.739 2.768 2.845 2.857 2.868 2.873

d N···X, n=2 (n=1)

Complex (H3SiNC)n (H3SiNC)n:BeH2 (H3SiNC)n:LiH (H3SiNC)n:BH3 (H3SiNC)n:HF (H3SiNC)n:ClF (H3SiNC)n:SHF (H3SiNC)n:PH2F

d C···Si, n=2

d C···X, n=2 (n=1)

2.901 2.558 2.590 2.678 2.752 2.405 2.602 2.784

1.840 (1.855) 2.187 (2.214) 1.579 (1.571) 1.849 (1.889) 1.719 (1.731) 1.913 (2.412) 2.638 (2.712)

Eb, n=2 (n=1) –21.0 –101.4 (–67.3) –105.9 (–71.8) –129.7 (–98.9) –64.8 (–37.3) –54.2 (–27.8) –53.2 (–27.3) –49.1 (–23.9)

1.790 (1.811) 2.049 (2.068) 1.593 (1.591) 1.794 (1.826) 2.500 (2.553) 2.599 (2.642) 2.715 (2.757)

Eb, n=2 (n=1) –22.0 –116.3 (–75.7) –111.8 (–71.3) –197.2 (–163.2) –71.5 (–39.6) –115.5 (–69.3) –79.7 (–36.8) –57.4 (–27.6)

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The MBIE partition terms of the binary and ternary complexes have been gathered in Table 7. In all cases, the absolute value of the terms in the ternary complexes is larger than the analogous ones in the corresponding binary complexes. The monomer distortion energies are small except for the XY molecules in the BH3 and BeH2 complexes,87 in both series and for the (SiH3NC)n:ClF and (SiH3NC)n:SHF ones due to the formation of halogen and chalcogen shared complexes. Among the two body energies, in all the complexes, the strongest one correspond to the direct interaction of the silyl molecule with the XY one, ∆2E(j,XY), followed by the interaction of the two silyl molecules, ∆2E(i,j). The stabilization due to the distant interaction between the two extreme molecules is very small (< 3.9 kJ·mol–1, absolute value). The ∆3E energies are always negative, ranging between –2.7 for the weakest trimer (SiH3CN)2:PH2F) and –21.9 for the (SiH3NC)2:ClF complex that present the largest distortion.

Table 7 MBIE terms of ternary complexes (n = 2). Within parentheses the values of the binary ones (n = 1). Note monomers i and j are the silyl molecules at the extreme and at the middle of the ternary chain respectively. Complex (H3SiCN)n:BeH2 (H3SiCN)n:LiH (H3SiCN)n:BH3 (H3SiCN)n:HF (H3SiCN)n:ClF (H3SiCN)n:SHF (H3SiCN)n:PH2F

(H3SiNC)n:BeH2 (H3SiNC)n:LiH (H3SiNC)n:BH3 (H3SiNC)n:HF (H3SiNC)n:ClF (H3SiNC)n:SHF (H3SiNC)n:PH2F

Er(i) 0.3 0.3 0.3 0.2 0.2 0.2 0.2

1.1 1.0 0.7 0.5 1.8 0.9 0.5

Er(j) 7.3 (1.2) 6.4 (0.9) 5.5 (0.9) 3.1 (0.3) 2.7 (0.1) 2.5 (0.1) 2.3 (0.1)

Er(XY) 36.9 (33.2) 0.2 (0.1) 53.3 (50.7) 0.8 (0.6) 0.8 (0.5) 0.6 (0.4) 0.6 (0.3)

∆2E(i,j) –27.3

∆2E(i,XY) –1.6

–27.1

–0.8

–26.5

–0.9

–25.4

–1.1

–25.2

–0.7

–25.1

–0.8

–25.0

–0.8

18.7 (1.7) 16.1 (1.3) 10.6 (0.9) 7.4 (0.4) 31.7 (1.6) 14.4 (0.4) 6.0 (0.2)

42.7 (37.4) 0.3 (0.1) 63.0 (59.8) 1.6 (1.0) 71.2 (54.2) 29.3 (2.3) 1.3 (0.7)

–41.0

–2.1

–39.7

–3.9

–36.0

–1.2

–34.2

–1.3

–48.7

–1.7

–38.7

–1.6

–33.2

–1.0

11


∆2E(j,XY) –96.5 (–89.7) –79.7 (–76.8) –141.3 (–136.6) –38.4 (–36.1) –29.9 (–28.8) –30.0 (–28.5) –26.5 (–25.3)

∆3E(i,j,XY) –8.8

–119.3 (–107.4) –82.7 (–76.9) –216.8 (–210.5) –42.8 (–40.1) –136.2 (–108.9) –68.9 (–38.2) –31.8 (–29.8)

–11.5

–7.2 –7.2 –3.7 –3.4 –3.1 –2.7

–9.3 –7.6 –4.6 –21.9 –10.8 –3.9


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The NBO E(2) and AIM BCP values in the ternary complexes confirm the cooperativity observed in other parameters (geometry and energy). Thus, the values of E(2) obtained in the ternary complexes are always larger than those obtained in the corresponding binary complexes (Table S2 of the Supporting Information). The E(2) stabilization between the two silyl molecules increases up to 133% in the (H3SiNC)2:BeH2 complex with respect to the (H3SiNC)2 dimer. The effect is less significant in the interaction between the silyl molecule and the different Lewis acids, being the largest per cent increment of 27% in the (H3SiNC)n:PH2F complex. The only exception is the (H3SiCN)n:BeH2 complexes, where the E(2) associated to the charge transfer from the lone pair of the nitrogen towards the BeH antibonding is smaller in the ternary complex (41.8 kJ·mol–1) than in the binary one (48.9 kJ·mol–1). In the case of the very strongly bonded complexes, (H3SiNC)3:SHF and (H3SiNC)n:ClF (n = 1 and 2), the NBO method is not able to properly identify the individual molecules forming the cluster. Concerning the AIM values (Table S3), ρBCP is always larger in the ternary complexes than in the corresponding binary ones except for the C-B BCP in the (H3SiNC)n:BH3, in agreement with the shorter interatomic distance found in the binary than in the ternary complex as previously mentioned. In the case of the SiH3CN ternary complexes, the Laplacian term is positive in all Si··N and N···X interactions while HBCP is positive in most of them except in a few cases: XY = BeH2, LiH, BH3. Regarding the SiH3NC ternary complexes, both involved Si···C and C···XY interactions present, in general, a combination of ∇ 2ρBCP > 0 and HBCP < 0 except for the C···Cl and C···S contacts where the Laplacian term at BCP is positive and for C···Be and C···Li interactions with a positive associated HBCP. In general, the silyl-Lewis acid interactions show higher covalent character in the SiH3NC complexes than in the SiH3CN ones.

Conclusions

A computational study of the tetrel bonded cluster of (H3SiCN)n and (H3SiNC)n with n= 2-6 has been carried out by means of MP2 and CCSD(T) computational methods. The cooperativity in both series is reflected by the changes in geometry, energy and electron density parameters. It is especially noteworthy, the huge geometrical effect in the (H3SiNC)6 cluster where Si atoms of the central molecules are surrounded at similar distances by two NC groups and the SiH3 became planar. In addition, the potential cooperativity with other non-covalent interactions (halogen, chalcogen, pnicogen, triel, beryllium, lithium and hydrogen bond) has been explored using ternary complexes, (H3SiCN)2:XY and (H3SiNC)2:XY. In all the cases, positive cooperativity is observed when compared to the geometric and energetic parameters of the corresponding binary complexes.

Acknowledgments This work was carried out with financial support from the Ministerio de Economía y Competitividad (Project No. CTQ201235513-C0202) and Comunidad Autónoma de Madrid (Comunidad Autónoma de Madrid (S2013/MIT2841, Fotocarbon). Thanks are also given to the CTI (CSIC) for its continued computational support.

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Supporting Information Geometries, total energies, and molecular graphs of complexes (H3SiCN)n and (H3SiNC)n with n = 2-6 and (H3SiCN)n:XY, (H3SiCN)n:XY with n = 1 and 2. NBO and AIM parameters of the (H3SiCN)n:XY, (H3SiCN)n:XY complexes and average dipole moment vs. the number of monomers in the polymeric clusters. Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.xxxx

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Graphical Abstract:

Electrostatic potential on the 0.001 a.u. electron density isosurface of the (H3SiNC)4 complex

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Cooperativity in Tetrel Bonds - The Journal of Physical Chemistry A

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