Carbene tetrel-bonded complexes

Carbene tetrel-bonded complexes

Mingxiu Liu, Qingzhong Li, Wenzuo Li & Jianbo Cheng

Structural Chemistry Computational and Experimental Studies of Chemical and Biological Systems ISSN 1040-0400 Volume 28 Number 3 Struct Chem (2017) 28:823-831 DOI 10.1007/s11224-016-0890-y

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Author's personal copy Struct Chem (2017) 28:823–831 DOI 10.1007/s11224-016-0890-y

ORIGINAL RESEARCH

Carbene tetrel-bonded complexes Mingxiu Liu 1 & Qingzhong Li 1 & Wenzuo Li 1 & Jianbo Cheng 1

Received: 3 October 2016 / Accepted: 22 November 2016 / Published online: 2 December 2016 # Springer Science+Business Media New York 2016

Abstract An ab initio calculation has been carried for the carbene tetrel bonded complexes CH3Y∙∙∙CH2 (Y = F, CN, NC, and NO2), CH3F∙∙∙CZ2 (Z = Cl and CH3), XH3F∙∙∙CF2 (X = C, Si, Ge, and Sn), SiF4∙∙∙CF2, and XH3F∙∙∙NHC (Nheterocyclic carbene), where carbene is treated as a Lewis base and XH3Y is a Lewis acid. Formation of the tetrel bond is mainly attributed to charge transfer from the lone pair on the C atom in the carbene toward the σ* X–Y orbital and also the σ* X–H one in the strong tetrel bond. The carbene tetrel bond is strengthened/weakened by the electron-withdrawing group in the tetrel donor/acceptor and enhanced by the methyl group in C(CH3)2. NHC forms a stronger carbene tetrel bond in XH3F∙∙∙NHC (X = Si, Ge, and Sn) where it exceeds that of the majority of H-bonds. Interestingly, the tetrel bond becomes stronger in the order of X = C < Ge < Sn < Si in XH3F∙∙∙NHC and the largest interaction energy occurs in SiH3F∙∙∙NHC, amounting to −103 kJ/mol. The carbene tetrel bond can be strengthened by cooperative effect with the N∙∙∙M interaction in trimers H2C∙∙∙CH3CN∙∙∙M (M = CH3CN, HCN, ICN, SbH 2 F, LiCN, and BeH 2 ) and has doubled in H2C∙∙∙CH3CN∙∙∙BeH2. Keywords Carbene . Tetrel bonds . Electrostatic interaction . Cooperativity Electronic supplementary material The online version of this article (doi:10.1007/s11224-016-0890-y) contains supplementary material, which is available to authorized users. * Qingzhong Li [email protected]

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The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, People’s Republic of China

Introduction Carbene is an important intermediate in chemical reactions [1]. However, most carbenes are too reactive to be isolated even some stable carbenes such as N-heterocyclic carbenes (NHCs, Scheme 1) were prepared [2, 3]. NHCs display a more rigid structure than non-cyclic carbenes due to the presence of a cyclic structure, where conjugate effect is present between the empty p orbital on the carbene C atom and two lone-pair orbitals on the N atoms. Moreover, NHCs have evolved to become powerful and universal ligands for metal complexes [4–6]. Carbenes have been used as a Lewis base to form hydrogen bonds [7], halogen bonds [8, 9], lithium bonds [10, 11], chalcogen bonds [12], and pnicogen bonds [13]. NHCs form a stronger halogen bond than non-cyclic carbenes [14] due to the fact that NHCs are electron rich. Carbenes have two types of singlet and triplet carbenes, but the singlet carbenes are often used as the Lewis bases in noncovalent interactions due to the presence of one lone-pair electron. The fact that noncovalent interactions play a role in chemistry and related disciplines has been accepted. Noncovalent interactions can maintain the stability of molecular structures [15], lower the activation energy of chemical reactions [16], construct supramolecular materials [17], and regulate the properties of crystal materials [18]. These functions of noncovalent interactions are to a large extent attributed to the diversity of their types, including hydrogen bond, halogen bond [19], chalcogen bond [20], pnicogen bond [21] and tetrel bond [22]. In the IV-VII main group molecules, there is a σhole (a region with positive electrostatic potential) on the covalent-bonded IV-VII main group atoms [23, 24], thus these interactions are uniformly called σ-hole interactions. Importantly, these σ-hole interactions have been extensively applied to molecular recognition [25–27], crystal materials [28–30], and biological systems [31–33].

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Struct Chem (2017) 28:823–831

and CH3CN). We think that this study can enrich the knowledge on tetrel bonding interactions.

Theoretical methods

Scheme 1 The structures of cyclic and non-cyclic carbenes

The Si∙∙∙N interaction was demonstrated in the crystal structure of tetrakis(N,N-dimethylhydroxylamido)silane with a short Si∙∙∙N contact of less than 2.6 Å and a narrow Si-O-N angle of about 109.1° [34]. A few experimental and theoretical studies were performed to understand the nature of Si∙∙∙N interaction [35–39]. Yin and co-author [38] thought that the electron delocalization from the lone pair of nitrogen atom into acceptor anti-bonding orbitals connected with Si atom is the main factor favorable for this interaction. Murray et al., [39] suggested that the driving force for the observed Si-O-N angle contraction in XYZSi-O-N(CH3)2 (X, Y, and Z = H or alkyl) molecules is largely the electrostatic attraction between a positive σ-hole on the silicon and the lone pair of the nitrogen according to the electrostatic potentials on the molecular surfaces of the corresponding XYZSi–H molecules and the σhole can be tuned by substituents X, Y, and Z. In a recent study, this interaction was named tetrel bond [22]. The authors declared that the tetrel bond could be an effective and reliable instrument in crystal engineering, supramolecular chemistry, and biochemistry [22]. The magnitude of σ-hole on the X atom in XH3Y and XF3Y (X = C, Si, and Ge; Y = F, Cl, Br, and I) is larger in order of X = C < Si < Ge [40], due to the smaller electronegativity and the larger polarization of the heavier X atom [41]. This interaction could give an enthalpic contribution to hydrophobic interactions due to the specific binding between the hydrophobic site and the water molecule [42]. Grabowski [43] pointed out that the tetrel bond could be taken as a preliminary stage of the SN2 reaction. Since then, many studies have been paid to the structures, properties, nature and applications of this tetrel bond [44–54]. However, the Lewis bases in these tetrel bonds are not involved with carbenes. In this paper, we selected methylene carbene (H2C) and its derivatives as a Lewis base to form tetrel-bonded complexes with XH3F (X = C, Si, Ge, and Sn), CH3CN, CH3NC, and CH3NO2. The carbene tetrel bond involving NHC was also studied. The structures, properties, and nature of carbene tetrel bonds have been analyzed by means of quantum chemical calculations. We also paid our attention to the cooperativity between the carbene tetrel bond and other type of interaction in H2C∙∙∙CH3CN∙∙∙M (M = HCN, LiCN, ICN, BeH2, SbH2F,

The geometries of all complexes and monomers were optimized at the MP2/aug-cc-pVTZ level. For Sn atom, the augcc-pVTZ-PP basis set was adopted to account for relativistic effects. Harmonic frequency calculations were also performed at the same level to confirm that the optimized geometries are minima on potential energy surfaces. This method has been used to study many tetrel-bonded complexes [43–50]. Interaction energies were obtained using the supermolecular approach, in which the sum of the energies of the constituting monomers was subtracted from the total energy of the complex. The interaction energies were corrected for the basis set superposition error (BSSE) by the standard counterpoise method of Boys and Bernardi [55]. All calculations were performed by using the Gaussian 09 package [56]. Molecular electrostatic potentials (MEPs) on the 0.001 electrons/bohr3 contour of electronic density were calculated at the HF/aug-cc-pVTZ level by using the Wave Function Analysis−Surface Analysis Suite (WFA-SAS) program [57]. Riley et al. [58] pointed out that the electrostatic potentials need not be computed using the method and basis set currently used. Natural bond orbital (NBO) analysis was implemented at the HF/aug-cc-pVTZ level via NBO 3.0 version [59] to obtain orbital interactions and charge transfer. Atoms in molecules (AIM) analysis was performed by using the AIM2000 software [60]. To have a deeper insight into the nature of the carbene tetrel bond, we performed the energy decomposition calculations at the MP2/aug-cc-pVTZ level by means of the localized molecular orbital-energy decomposition analysis (LMO-EDA) method [61] using the GAMESS program [62].

Results and discussion Carbene tetrel bonds Figure 1 shows the structural diagram of complex XH3Y∙∙∙CZ2 (Y = F, CN, NC, and NO2; X = C, Si, Ge, and Sn; Z = H, F, Cl, and CH3) and the corresponding optimized structures are illustrated in Fig. S1. When Z = H, F, and Cl, this structure displays Cs symmetry, wherein the molecular plane of CZ2 locates in the same plane with both X–H and X–Y bonds of XH3Y. Such symmetry is not hold in the complex of CH3F∙∙∙C(CH3)2. The angle ∠Y–X∙∙∙C is close to 180° in the complexes of CH2 and CF2 but amounts to 172° in the complex of CCl2. A similar structure is also found in the complexes of SiF4∙∙∙CF2 and XH3F∙∙∙NHC (X = C, Si, Ge, and Sn). There are four σ–holes at the tetrahedral face centers

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Fig. 1. Scheme of representative geometrical structure of the complexes XH3Y∙∙∙CZ2

of XH3Y and the biggest σ–hole is along the extension of the Y–X bond [45]. Table S1 presents the most positive MEP (Vmax) on the σ–hole of the X atom in XH3Y. This value becomes more positive in the order of X = C < Si < Ge < Sn (consistent with the previous results [40, 41]), due to the smaller electronegativity and larger polarization for the heavier X atom. With the increasing of the electron-withdrawing ability of the Y group, the value of Vmax is also greater in the order of Y = −F < −CN < −NC < −NO2. Similarly, the σ–hole of the Si atom in SiF4 is larger than that in SiH3F. Figure 2 presents the MEP maps of carbene CZ2 and Nheterocyclic carbene (NHC). It is known that the C atom in carbenes is sp2 hybridization and possesses a lone-pair electron in the singlet state. As a result, this C atom has negative MEP around its atomic surface. The most negative MEP (V min ) on the C atom is −159.21 kJ/mol in CH 2 . The electron-withdrawing groups (–F and –Cl) reduce this value and the electron-donating group (–CH3) makes this value increase. NHC is more stable than CZ2 due to the conjugative and inductive effects, thus it has the more negative MEP (−203.95 kJ/mol) on the C atom than the non-cyclic carbenes. According to the MEP distributions in both XH3Y and carbene, it is predicted that these carbenes can have an attractive force with XH3Y through the most negative MEP on the C atom in the carbene and the σ-hole on the X atom,

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respectively. This attractive interaction is named the carbene tetrel bond. Table 1 presents the interaction energies in the complexes at the MP2 and CCSD(T) levels. The CCSD(T) interaction energy is obtained with a single energy calculation on the MP2 geometries. As expected, the MP2 method overestimates the interaction energy with respect to the CCSD(T) result in most carbene tetrel-bonded complexes, like that in other types of intermolecular interactions [63]. However, their relative difference is small, and the following discussion is performed on the basis of the CCSD(T) results. The interaction energy in XH3F∙∙∙CF2 (X = C, Si, Ge, and Sn) becomes more negative with the increase of the X atomic number, consistent with the most positive MEP on the σ-hole of X atom in XH3F molecule. However, it is more negative in the order of X = C < Ge < Sn < Si in XH3F∙∙∙NHC, showing an incompletely consistent change with the most positive MEP on the σ-hole of X atom in XH3F molecule. Both above results suggest that electrostatic interaction plays an important role in the formation of the carbene tetrel bond even other contributions are also important. It is also suggested that the electrostatic role of a Lewis acid in the carbene tetrel bond is also dependent on the nature of a Lewis base. The carbene tetrel bond in the complex of CH2 is stronger in the order of Y = −F < −CN < −NC < −NO2, consistent with the most positive MEP on the σ-hole of the C atom in the CH3Y molecule. However, the interaction energy is less negative in SiF4∙∙∙CF2 than that in SiH3F∙∙∙CF2 although the Si atom in the former has the greater positive MEP than that in the latter. Table 1 Interaction energies corrected for BSSE and deformation energy (ΔE, kJ/mol) in the complexes with the MP2 and CCSD(T) methods ΔEMP2 CH3F∙∙∙CH2 CH3CN∙∙∙CH2 CH3NC∙∙∙CH2 CH3NO2∙∙∙CH2 CH3F∙∙∙CCl2 CH3F∙∙∙C(CH3)2 CH3F∙∙∙CF2 SiH3F∙∙∙CF2 GeH3F∙∙∙CF2 SnH3F∙∙∙CF2 SiF4∙∙∙CF2

Fig. 2. MEPs of CZ2 and NHC. The blue dot represents the most negative MEP on the C atom. Color ranges, in kilocalories per mole, are: Red, greater than 78.77; yellow, between 78.77 and 0; green, between 0 and −52.51; and blue, less than −52.51

CH3F∙∙∙NHC SiH3F∙∙∙NHC GeH3F∙∙∙NHC SnH3F∙∙∙NHC

ΔECCSD(T)

ΔΔE

ΔΔE%

–7.89 –8.78 –10.75 –10.75 –7.21 –10.41 –4.42 –12.56 –12.9 –18.33 –9.69

–7.75 –8.61 –9.9 –10.82 –6.77 –10.56 –4.51 –12.08 –12.21 –16.67 –10.15

–0.14 –0.17 –0.85 0.07 –0.44 0.15 0.09 –0.48 –0.69 –1.66 0.46

1.8 1.9 8.6 –0.6 6.5 –1.5 –2 4 5.6 9.9 –4.5

–12.48 –109.22 –82.58 –100.18

–12.58 –103.08 –76.87 –92.78

0.1 –6.14 –5.71 –7.4

–0.8 6 7.4 8

Note: Δ ΔE = Δ E M P 2 − ΔE C C S D ( T ) a n d ΔΔ E% = Δ Δ E/ ΔECCSD(T) × 100


For the tetrel-bonded complexes of CH3F, the interaction energy becomes more negative in the order of CF2 < CCl2 < CH2 < C(CH3)2, being in a line with the most negative MEP on the C atom in the carbene. Similarly, NHC is a stronger Lewis base in the carbene tetrel bond than CF2. The interaction energy is calculated to be −8.28 kJ/mol (CH3F∙∙∙NH3), −35.36 kJ/mol (SiH3F∙∙∙NH3), and −30.93 kJ/ mol (GeH3F∙∙∙NH3), respectively [64]. Consequently, the carbene CZ2 (Z = H and F) forms a weaker tetrel bond than does NH3 but NHC forms a stronger tetrel bond than does NH3. In most carbene tetrel bonds, the interaction energy is less than −17 kJ/mol and the respective tetrel bond is a weak interaction. Only in XH3F∙∙∙NHC (X = Si, Ge, and Sn), the carbene tetrel bond is a strong interaction with the interaction energy of large than −76 kJ/mol and the largest interaction energy is found in SiH3F∙∙∙NHC (−103.08 kJ/mol). Table S2 presents the binding distance, change of X–Y bond length, angles, and frequency shift of X–Y stretch vibration in the complexes. Obviously, the binding distance is shorter than the sum of van der Waals radii of bonded atoms (C∙∙∙C is 3.4 Å, C∙∙∙Si is 3.8 Å, C∙∙∙Ge is 3.8 Å, and C∙∙∙Sn is 3.95 Å) in most complexes except CH3CN∙∙∙CH2 and CH3NC ∙∙∙CH2. Moreover, this binding distance has not a good linear relationship with the interaction energy (results not shown). Thus the binding distance is not a good measurement for estimating the strength of the carbene tetrel bond. The formation of the carbene tetrel bond causes a geometrical change of XH3Y and SiF4, characterized by the angle ∠Y–X∙∙∙H (α and β). Both angles suffer a tiny change in most carbene tetrelbonded complexes except XH3F∙∙∙NHC (X = Si, Ge, and Sn), where both angles are decreased by ~9-13°. The X–Y bond has a small elongation and the corresponding stretch vibration displays a small red shift in most carbene tetrel-bonded complexes, while a great elongation of 0.058–0.073 Å and a large red shift of 89–143 cm−1 are found for the X–F bond and stretch vibration in XH3F∙∙∙NHC (X = Si, Ge, and Sn), respectively. It has been demonstrated that a charge transfer occurs from the Lewis base into the σ* anti-bonding orbitals of X–F and X–H bonds in tetrel bonds [45], although these orbitals have no physical existence [65]. Thus NBO analysis was also performed to explore the nature of the carbene tetrel bond. Likely, such charge transfer is also found in the carbene tetrel bonds, characterized by LPC → BD*X–Y and LPC → BD*X–H (LPC denotes the lone pair orbital of C atom in carbene) orbital interactions. However, only LPC → BD*C–Y orbital interaction is found in the complexes of CH3Y (Y = F, CN, NC, and NO2) except CH3F∙∙∙CF2. The corresponding second-order perturbation energies are listed in Table 2. There are three LPC → BD*X–H orbital interactions and the sum of their perturbation energies are given in Table 2. The LPC → BD*X–Y orbital interaction leads to the elongation of X–Y bond and its red shift. The LPC → BD*X–Y orbital interaction is stronger

Struct Chem (2017) 28:823–831 Table 2 Charge transfer (CT, e) and second-order perturbation energies (E, kJ/mol) in the complexes CT CH3F∙∙∙CH2

Ε2

E1

CH3CN∙∙∙CH2 CH3NC∙∙∙CH2 CH3NO2∙∙∙CH2

0.003 0.003 0.004 0.005

5.81 3.43 6.27 5.77

CH3F∙∙∙CCl2 CH3F∙∙∙C(CH3)2 CH3F∙∙∙CF2 SiH3F∙∙∙CF2 GeH3F∙∙∙CF2 SnH3F∙∙∙CF2 SiF4∙∙∙CF2 CH3F∙∙∙NHC SiH3F∙∙∙NHC GeH3F∙∙∙NHC SnH3F∙∙∙NHC

0.003 0.004 0.007 0.051 0.050 0.087 0.012 0.005 0.223 0.195 0.198

4.31 6.52 4.39 46.28 53.25 86.15 6.86 7.82 239.76 238.85 212.26

– – – – – – 3.01 25.16 25.16 68.51 7.27 – 265.85 217.61 323.32

Note: E1 and E2 correspond to the orbital interactions of LPC → BD*X–Y and LPC → BD*X–H, respectively. E2 is the sum of three LPC → BD*X–H orbital interactions

than LPC → BD*X–H in all complexes although the E1 of the former is smaller than the sum of the latter (E2) in SiF4∙∙∙CF2, SiH3F∙∙∙NHC and SnH3F∙∙∙NHC. Unluckily, no good relation is found between the orbital interaction and the interaction energy for all carbene tetrel bonds. This further confirms that the carbene tetrel bond is not governed by only one factor. Accompanied with these orbital interactions, a charge transfer occurs between the Lewis base and the acid. Such charge transfer is small (less than 0.09e) in most carbene tetrel bonds but is larger than 0.2e in XH3F∙∙∙NHC (X = Si, Ge, and Sn). The charge transfer in XH3F∙∙∙NHC (X = Si, Ge, and Sn) shows an inconsistent change with the interaction energy. It was noted that charge transfer is equivalent to polarization in the context of noncovalent interactions [65]. AIM analysis can provide a strong evidence for the existence of the carbene tetrel bond. Figure 3 shows the molecular maps of the tetrel-bonded complexes. The electron density, Laplacian, and energy density at the intermolecular bond critical point (BCP) are also labeled in Fig. 3. The carbene tetrel bond is characterized by the X∙∙∙C BCP in most complexes except SiH3F∙∙∙CF2 and SiF4∙∙∙CF2. Two H∙∙∙C BCPs and three F∙∙∙C BCPs are found in SiH3F∙∙∙CF2 and SiF4∙∙∙CF2, respectively. The latter case is similar to that in SiF4∙∙∙Cl− [43]. The electron density is smaller than 0.02 au in most carbene tetrelbonded complexes but larger than 0.05 au in XH3F∙∙∙NHC (X = Si, Ge, and Sn), which is larger than that in XH3F∙∙∙NH3 (X = C, Si, and Ge) [64]. This shows that NHC is a good electron donor in the carbene tetrel bond. A good polynomial linear relationship with a correlation coefficient of


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Fig. 3. Molecular maps of tetrelbonded dimers with electron density (ρ, au), Laplacian (∇2ρ, au), and energy density (H, au) at the intermolecular critical point.

Table 3 Electrostatic energy (Eele), exchange energy (Eex), repulsion energy (Erep), polarization energy (Epol), and dispersion energy (Edisp) in the complexes. All are in kilojoules per mole

CH3F∙∙∙CH2 CH3CN∙∙∙CH2 CH3NC∙∙∙CH2 CH3NO2∙∙∙CH2 CH3F∙∙∙CCl2 CH3F∙∙∙C(CH3)2 CH3F∙∙∙CF2 SiH3F∙∙∙CF2 SnH3F∙∙∙CF2 SiF4∙∙∙CF2 CH3F∙∙∙NHC SiH3F∙∙∙NHC GeH3F∙∙∙NHC SnH3F∙∙∙NHC

Eele

Eex

Erep

Epol

Edisp

–11.75 (65.4%) –11.66 (73%) –12.87 (62.4%) –16.55 (79.2%) –8.07 (43.7%) –15.22 (57.9%) –5.39 (49.4%) –35.28 (56.3%) –55.01 (59.8%) –24.66 (69.5%) –17.89 (62.5%) –355.68 (59.2%) –298.24 (63.7%) –276.51 (61.4%)

–16.09 –11.33 –15.51 –15.26 –17.60 –24.83 –10.41 –69.39 –93.01 –32.02 –25.37 –519.70 –403.45 –385.44

26.17 18.5 25.41 25.41 28.88 40.71 16.89 119.21 166.61 57.89 41.51 1008.71 788.01 735.14

–2.09 (11.6%) –1.55 (9.7%) –2.01 (9.8%) –2.42 (11.6%) –1.76 (9.5%) –3.39 (12.9%) –0.92 (8.4%) –13.17 (21.0%) –22.36 (24.3%) –4.05 (11.4%) –3.76 (13.1%) –204.23 (34.0%) –136.68 (29.0%) –140.95 (31.3%)

–4.14 (23.0%) –2.76 (17.3%) –5.73 (27.8%) –1.92 (9.2%) –8.65 (46.8%) –7.69 (29.2%) –4.60 (42.2%) –14.21 (22.7%) –14.55 (15.8%) –6.77 (19.1%) –6.98 (24.4%) –40.80 (6.8%) –34.61 (7.4%) –32.52 (7.2%)

Note: Data in parentheses are the percentage of each term to the sum of Eele , Epol and Edisp . The results of FGeH3∙∙∙CF2 are not given


Struct Chem (2017) 28:823–831

0.956 is found between the electron density and the binding distance (Fig. S2). Even so, the electron density is not a good measurement for estimating the strength of the carbene tetrel bond, unlike that in hydrogen bonds [66]. In most carbene tetrel-bonded complexes, both Laplacian and energy density are positive, corresponding to a purely closed-shell interaction [67]. However, in XH3F∙∙∙NHC (X = Si, Ge, Sn), Laplacian is positive but energy density is negative, showing that the corresponding tetrel bond has the nature of a partially covalent bond [67]. To deepen the understanding about the nature of the carbene tetrel bond in these complexes, the interaction energies are decomposed into five components: electrostatic energy (Eele), exchange energy (Eex), repulsion energy (Erep), polarization energy (Epol), and dispersion energy (Edisp). The value of each term is given in Table 3. To compare their relative contribution, the percentage of each term to the sum of three terms (Eele, Epol and Edisp) is obtained. Eele has the largest contribution to the formation of the tetrel bond in all complexes except CH3F∙∙∙CCl2, where the largest contribution is from Edisp. This indicates that electrostatic interaction plays an important role in the carbene tetrel bond. The contribution of Edisp is larger than that of Epol in most complexes except CH3NO2∙∙∙CH2, SnH3F∙∙∙CF2 and XH3F∙∙∙NHC (X = Si, Ge, Sn). The relatively large Edisp is in line with the presence of a weak tetrel bond in the former complex. For the strong tetrel bond in XH3F∙∙∙NHC (X = Si, Ge, and Sn), the value of Epol is comparable with that of Eele. In XH3F∙∙∙NHC (X = Si, Ge, and Sn), the relatively large Epol suggests that the orbitals undergo a significant change in their shapes, which is consistent with the large deformation of XH3F in these complexes. There is a better linear relationship between Eex and Erep (Fig. S3), showing both terms are interdependent although they are related to the large orbital overlap and the shorter binding distance, respectively.

Cooperativity Figure 4 shows the optimized structures of trimers H2C∙∙∙CH3CN∙∙∙M (M = CH3CN, HCN, ICN, SbH2F, LiCN, and BeH2), where a carbene tetrel bond coexists with a N∙∙∙C Table 4 Changes of interaction energy for the carbene tetrel bond (ΔΔECTB, kJ/mol) and another interaction (ΔΔEAI, kJ/mol) as well as charge transfer of the carbene tetrel bond (ΔCTCTB, kJ/ mol) and another interaction (ΔCTAI, kJ/mol) in the trimers relative to the respective dimers and cooperative energy (Ecoop, kJ/ mol) in the trimers

H2C∙∙∙CH3CN∙∙∙CH3CN H2C∙∙∙CH3CN∙∙∙HCN H2C∙∙∙CH3CN∙∙∙ICN H2C∙∙∙CH3CN∙∙∙SbH2F H2C∙∙∙CH3CN∙∙∙LiCN H2C∙∙∙CH3CN∙∙∙BeH2

Fig. 4. Optimized structures of trimers H2C∙∙∙CH3CN∙∙∙M (M = CH3CN, HCN, ICN, SbH2F, LiCN, and BeH2). The difference of binding distance in the trimer relative to the dimer is given in angstrom

tetrel bond, a hydrogen bond, a halogen bond, a pnicogen bond, a lithium bond, and a beryllium bond, respectively. The binding distances of both carbene tetrel bond and N∙∙∙M interaction are decreased in the trimers. Moreover, the decrease of binding distance is larger for the carbene tetrel bond than that for the N∙∙∙M interaction. With the strengthening of

ΔΔECTB

ΔΔEAI

Ecoop

ΔCTCTB

ΔCTAI

–0.78 (8.9%) –1.43 (16.3%) –2.03 (23.1%) –2.48 (28.3%) –3.83 (43.7%) –4.38 (49.9%)

–0.78 (6.3%) –1.44 (5.9%) –1.96 (6.4%) –2.42 (7.0%) –4.14 (4.5%) –6.07 (6.0%)

–0.84 –1.45 –1.93 –2.12 –3.89 –4.40

0.0002 0.0004 0.0005 0.0005 0.0013 0.0010

0.0001 0.0010 0.0019 0.0036 0.0015 0.0044

Note: Data in parentheses are the increased percentage of interaction energy in the trimer relative to the dimer


the N∙∙∙M interaction (Table S3), the decrease of binding distance of carbene tetrel bond becomes larger, while that of N∙∙∙M interaction is irregular. For the carbene tetrel bond, the smallest decrease of binding distance is found in H2C∙∙∙CH3CN∙∙∙CH3CN (−0.028 Å) and the largest decrease of binding distance is found in H 2 C∙∙∙CH 3 CN∙∙∙BeH 2 (−0.110 Å). As a result, the presence of N∙∙∙M interaction makes the binding distance of the carbene tetrel bond have a prominent change. The change of interaction energy for the corresponding interaction in the trimer relative to that in the dimer is listed in Table 4, arranged in an increasing sequence of N∙∙∙M interaction strength. Obviously, the interaction energies of both carbene tetrel bond and N∙∙∙M interaction become more negative in the trimer with respect to that in the respective dimer (Table S3). This indicates that both types of interactions are strengthened in the trimers, displaying positive cooperativity. The carbene tetrel bond in H2C∙∙∙CH3CN is weaker than the N∙∙∙M interaction in CH3CN∙∙∙M, thus the former has a larger enhancement than the latter in the trimer. Specially, the stronger the N∙∙∙M interaction, the larger the increase of the interaction energy of carbene tetrel bond. For example, the interaction energy of the carbene tetrel bond has doubled in H2C∙∙∙CH3CN∙∙∙BeH2. The increased magnitude of N∙∙∙M interaction energy is also larger with the stronger of N∙∙∙M interaction. However, its increased percentage has an irregular change. Such conclusion was also obtained in H3N∙∙∙XY∙∙∙HF (X, Y = F, Cl, and Br) [68] and XCl∙∙∙FH2P∙∙∙NH3 (X = F, OH, CN, NC, and FCC) [69]. The cooperativity between the carbene tetrel bond and N∙∙∙M interaction is also estimated with cooperative energy. This term is negative and becomes more negative with the increase of N∙∙∙M interaction strength. In the formation of the carbene tetrel bond, charge density moves from the carbene into CH3CN, thus the N atom in CH3CN becomes a stronger electron donor and forms a stronger N∙∙∙M interaction with M. On the contrary, the formation of N∙∙∙M interaction causes a charge transfer from CH3CN to M, causing CH3CN be a stronger electron acceptor and forms a stronger carbene tetrel bond with H2C. These can be evidenced by the larger charge transfer for both types of interactions in the trimers (Table 4).

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CH 3 F∙∙∙CZ 2 . However, it is stronger in the order of X = C < Ge < Sn < Si in XH3F∙∙∙NHC. The carbene tetrel bond is a weak interaction in most complexes, dominated by electrostatic interaction, but a strong one in XH3F∙∙∙NHC (X = Si, Ge and Sn), displaying a partially covalent character. The strongest carbene tetrel bond is found in SiH3F∙∙∙NHC, with the interaction energy of −103 kJ/mol. In addition, the weak carbene tetrel bond in H2C∙∙∙CH3CN can also be enhanced by cooperativity with another interaction in trimers H2C∙∙∙CH3CN∙∙∙M (M = CH3CN, HCN, ICN, SbH2F, LiCN, and BeH 2 ) and its interaction energy has doubled in H2C∙∙∙CH3CN∙∙∙BeH2. It is expected that this carbene tetrel bond plays a role in insertion reactions involved with carbenes. In the next work, we plan to prepare stable carbene tetrel bonded complexes of NCHs and characterize them with spectroscopic methods. Acknowledgments This work was supported by the National Natural Science Foundation of China (21573188) and the Graduate Innovation Foundation of Yantai University (YDZD1608).

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The carbene tetrel bonded complexes have been studied with quantum chemical calculations. These carbene tetrel bonded complexes are stabilized mainly by electrostatic interaction, together with other contributions including charge transfer and polarization particularly in the strong carbene tetrel bond. Consequently, the carbene tetrel bond becomes stronger in the order of Y = −F < −CN < −NC < −NO2 in CH3Y∙∙∙CH2, X = C < Si < Ge < Sn in XH3F∙∙∙CF2, and Z = F < Cl < CH3 in

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