Tetrel–Hydride Interaction between XH3F (X = C, Si, Ge, Sn) and HM

Article pubs.acs.org/JPCA

Tetrel−Hydride Interaction between XH3F (X = C, Si, Ge, Sn) and HM (M = Li, Na, BeH, MgH) Qing-Zhong Li,*,† Hong-Ying Zhuo,† Hai-Bei Li,*,‡ Zhen-Bo Liu,† Wen-Zuo Li,† and Jian-Bo Cheng† †

The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, People’s Republic of China ‡ School of Ocean, Shandong University, Weihai 264209, People’s Republic of China S Supporting Information *

ABSTRACT: A tetrel−hydride interaction was predicted and characterized in the complexes of XH3F···HM (X = C, Si, Ge, Sn; M = Li, Na, BeH, MgH) at the MP2/aug-cc-pVTZ level, where XH3F and HM are treated as the Lewis acid and base, respectively. This new interaction was analyzed in terms of geometrical parameters, interaction energies, and spectroscopic characteristics of the complexes. The strength of the interaction is essentially related to the nature of X and M groups, with both the larger atomic number of X and the increased reactivity of M giving rise to a stronger tetrel− hydride interaction. The tetrel−hydride interaction exhibits similar substituent effects to that of dihydrogen bonds, where the electrondonating CH3 and Li groups in the metal hydride strengthen the binding interactions. NBO analyses demonstrate that both BDH−M → BD*X−F and BDH−M → BD*X−H orbital interactions play the stabilizing role in the formation of the complex XH3F···HM (X = C, Si, Ge, and Sn; M = Li, Na, BeH, and MgH). The major contribution to the total interaction energy is electrostatic energy for all of the complexes, even though the dispersion/polarization parts are nonnegligible for the weak/strong tetrel−hydride interaction, respectively.

1. INTRODUCTION

owing to their potential applications similar to hydrogen bonds.12−14 On the other hand, the patterns of Lewis bases in the abovementioned noncovalent interactions could vary from lone-pair electrons on fluorine, nitrogen, and oxygen atoms through π electrons in unsaturated bonds to σ electrons in metal hydrides. These different types of Lewis bases enrich the types of noncovalent interactions and expand the application scope of the noncovalent interactions. Hence, in recent years, much effort has been paid to noncovalent interactions with metal hydrides as the Lewis bases. A dihydrogen bond is an attractive interaction between a protonic hydrogen and a hydridic hydrogen,15 which plays a significant role in crystal packing, organometallic reaction mechanisms, and potential hydrogenstorage materials.16 The halogen−hydride interaction has been reported in the complexes with metal hydrides and BH3NH3 as Lewis bases, which have low binding energy.17−24 The strength of the halogen−hydride interaction can be enhanced by the substituents and cooperative effects in conjunction with other types of interactions.25,26 Metal hydrides can also be taken as

Noncovalent interactions have attracted much attention due to their extensive applications in chemistry, physics, and biology.1 With the rapid development of theoretical and advanced experimental techniques, a number of different types of noncovalent interactions have been reported, such as the hydrogen bond,2 halogen bond,3 chalcogen bond,4 pnicogen bond,5 and carbon bond.6 These noncovalent interactions can be uniformly understood from the view of a Lewis acid/base motif. A hydrogen bond occurs between the electron-deficient hydrogen of a Lewis acid and the electron rich-atom of a Lewis base. By analogy, the halogen bond, chalcogen bond, pnicogen bond, and carbon bond are similar in nature to the hydrogen bond with corresponding halogen, chalcogen, pnicogen, and carbon atoms of the Lewis acid interacting with the electron donor to form the noncovalent interactions. The site of the electron acceptor of the Lewis acid on the atomic surfaces can be found by means of a molecular electrostatic potential (MEP) map, which is an effective method for studying noncovalent interactions.7 It has been confirmed that a region of positive MEP was observed on the covalently bonded atoms of groups IV−VII,8−11 and the positive region of the MEP is called the σhole, with the corresponding interaction uniformly named the σ-hole interaction.8−11 Recently, these σ-hole interactions, particularly halogen bonds, have attracted more attention © XXXX American Chemical Society

Special Issue: Markku Rasanen Festschrift Received: April 16, 2014 Revised: July 9, 2014

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aug-cc-pVTZ-PP35 was used to account for relativistic effect. The frozen core approximation was employed in MP2 calculations, which has been confirmed to be effective and accurate in determining the equilibrium structures and interaction energies of the dihydrogen bond36 and halogen−,18−23 chalcogen−,27 and pnicogen−hydride interactions.28 The frequency calculations at the same level were performed to verify that the optimized geometries correspond to the ground-state local minimum with no imaginary frequencies for all complexes and monomers. The geometries of the complexes HBeH···CH3F and LiH···CH3F were further optimized at the CCSD(T)/aug-cc-pVTZ level. Interaction energies were calculated using the supermolecular method as the difference between the energy of the complex and the energy sum of the isolated monomers in their optimized complex configuration. The interaction energies were corrected for the basis set superposition error (BSSE) by the standard counterpoise method of Boys and Bernardi.37 In order to consider the higher-order correlation effect, the interaction energies were also calculated at the CCSD(T)/augcc-pVTZ level on the MP2/aug-cc-pVTZ geometries for all complexes with the exception of HBeH···CH3F and LiH··· CH3F, for which the CCSD(T)/aug-cc-pVTZ optimized geometries were used. To get a deeper insight into the nature of the carbon− and tetrel−hydride interactions, we performed the calculations of the decomposition of the interaction energy at the MP2/augcc-pVTZ(PP) level by means of the LMOEDA method38 implemented in the GAMESS program,39 where the interaction energy consists of five fundamental physical components, electrostatic (Eele), exchange (Eex), repulsion (Erep), polarization (Epol), and dispersion (Edisp), that is, Eint = Eele + Eex + Erep + Epol + Edisp. The QTAIM proposed by Bader40 is competent to analyze the electron distribution function for intermolecular interactions.41−43 The electron density at the bond critical point (BCP), ρBCP, its Laplacian, ∇2ρBCP, and the total electronic energy density, HBCP, were calculated to characterize the interactions of the complexes using AIM2000 software.44 NBO analyses were carried out at the HF/aug-cc-pVTZ level with NBO version 3.145 implemented in Gaussian 09 to reveal the bonding nature in view of charge transfer and orbital interactions. MEPs on the 0.001 electrons/bohr3 contour of the electronic density were calculated at the MP2/aug-cc-pVTZ level using the Wave Function Analysis−Surface Analysis Suite (WFA-SAS) program46 to analyze the distribution of MEPs in the monomers.

Lewis bases in chalcogen and pnicogen bonds, giving rise to chalcogen−hydride27 and pnicogen−hydride28 interactions, respectively. Studies on the σ-hole interactions involving group IV atoms are sparse, although the σ-hole has been suggested to be present on the atomic surface of group IV atom.11 The σ-hole interaction involving group IV atoms was named a carbon bond29 or tetrel bond.30 The former is an interaction where a carbon atom acts as an electrophilic site toward a variety of nucleophiles, while it includes all IV group elements in the latter. The tetrel bonds with Si have been investigated in molecular complexes between SiXY3 (X and Y = H, F, and Cl) and some electron-rich groups (NH3, NCH, CNH, OH2, and FH),31 even though the interaction was not named by the term “tetrel bonds”. Tetrel bonds are highly directional due to the presence of the σ-hole and are comparable in strength to hydrogen bonds and other types of σ-hole interactions. Thus, they might serve as a new possible molecular linker in crystal engineering and supramolecular chemistry.30 Thomas and coworkers provided experimental evidence for “carbon bonding” in the solid state according to charge density analysis.32 Grabowski suggested that the formation of a tetrel bond is a preliminary stage of the SN2 reaction.33 These studies demonstrated that tetrel bonding could play a similar role as hydrogen bonding in molecular recognition, crystal engineering, and chemical reactions. In general, the Lewis bases of tetrel bonding in the literature are neutral molecules with lone-pair electrons or anionic species. In the present paper, we performed a systematic study on the complexes of XH3F···HM (X = C, Si, Ge, and Sn; M = Li, Na, BeH, and MgH). The representative geometrical configuration of the complexes XH3F···HM was depicted in Figure 1. Is such a structure stabilized by a carbon− or tetrel−

Figure 1. Representative geometrical structure of the MH···XH3F complex.

hydride interaction? To address this issue, we focused on the influence of the nature of X and M groups on the stability of the complexes. We further considered the substituent effect on the carbon−hydride interaction in HMgH···CH3F. The characteristics of the carbon− or tetrel−hydride interaction have been analyzed in terms of geometrical structures, interaction energies, and spectroscopic parameters. Its nature and formation mechanism have been unveiled with localized molecular orbital energy decomposition analysis (LMOEDA), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analysis.

3. RESULTS AND DISCUSSION 3.1. Carbon−Hydride or Tetrel−Hydride Interaction. As shown in Figure 2, the MEP map displays the presence of four σ-holes (red regions) at the tetrahedral face centers of SiH3F, with the maximal value of the σ-hole in the extension of of F−Si bond. Other XH3F (X = C, Ge, and Sn) molecules exhibit similar features of the MEP as that of SiH3F. It is predictable that the hydridic H atom in HM (M = Li, Na, BeH, and MgH) could interact with the X atom along the extension of the F−X bond, which is similar in nature to that of conventional tetrel bonds.29 The optimized structures of the complexes XH3F···HM (X = C, Si, Ge, and Sn; M = Li, Na, BeH, and MgH) were depicted in Figure 3. In all complexes, three atoms X, H, and M are in a line with the geometrical structure having C3v symmetry. AIM analyses show the

2. THEORETICAL METHODS All calculations were performed with the use of the Gaussian 09 package.34 Geometries of the complexes and the isolated monomers were optimized at the level of the second-order Møller−Plesset perturbation theory (MP2) with the basis set aug-cc-pVTZ for all atoms except Sn, for which the basis set B

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Table 1. Interaction Energies Corrected for BSSE (ΔE, kJ/ mol) in the Complexes at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ Levelsa HBeH···CH3F HMgH···CH3F LiH···CH3F NaH···CH3F HBeH···SiH3F HMgH···SiH3F HBeH···GeH3F HMgH···GeH3F HBeH···SnH3F HMgH···SnH3F HMgH···CH3Cl HMgH···CH3CN HMgH···CH3NC HMgH···CH3NO2 CH3MgH···CH3F LiMgH···CH3F

Figure 2. MEPs of CH3F and SiH3F. Color ranges, in eV, are red, greater than 0.04; yellow, between 0.04 and 0.02; green, between 0.02 and 0; blue, less than 0.

ΔEMP2

ΔECCSD(T)

−3.66 −6.07 −13.32 −14.59 −6.93 −13.17 −7.74 −14.90 −9.41 −20.87 −5.80 −6.82 −8.26 −8.30 −6.56 −10.48

−3.94 −6.18 −13.40 −14.21 −7.35 −13.34 −7.95 −14.76 −9.43 −20.24 −5.73 −6.50 −7.88 −8.56 −6.63 −10.46

ΔECCSD(T) was calculated based on the MP2/aug-cc-pVTZ optimized geometry for most complexes except HBeH···CH3F and LiH···CH3F, for which the CCSD(T)/aug-cc-pVTZ geometry was used. a

variational trend is supported by the positive MEP on the σhole of the X atomic surface (Figure 2), which supplies strong evidence for the conclusion that electrostatic interaction plays a critical role in stabilizing the tetrel−hydride bonded complexes similarly to that in other patterns of σ-hole interactions.8−11 It is feasible to apply this deduction to explain the stronger tetrel−hydride interaction in the complexes HMgH···CH3CN, HMgH···CH3NC, and HMgH···CH3NO2 and also the weaker one in HMgH···CH3Cl. Compared to the complex HMgH··· CH3F, the tetrel−hydride interaction becomes stronger in LiMgH···CH3F due to the high electron-donating ability of the Li atom. The interaction energy of LiMgH···CH3F is 72% higher than that of HMgH···CH3F. This is consistent with the strong substituent effect of the Li atom in the complex NCI··· HMgLi.26 It has been confirmed that the methyl group in the Lewis base is electron-donating in noncovalent interactions.49−51 We found that in the complex CH3MgH···CH3F, the methyl group also has an enhancing effect on the strength of the tetrel−hydride interaction, although its enhancement is not as prominent as that of the Li substituent. Table 2 presents the binding distance, change of bond length, and angle in the complexes depicted in Figure 1. It has been well-known that there is a linear relationship between the interaction energy and the intermolecular distance in a closely related family of complexes22,52 with the same electron donor and acceptor atoms. However, such a relationship is not found in most complexes that consist of a monomer CH3Y (Y = F, Cl, CN, NC, and NO2), even though it is tenable in the complexes involving XH3F (X = Si, Ge, and Sn). For example, the more stable complex HMgH···CH3F shows a longer binding distance of 2.915 Å compared to 2.911 Å in the weak complex HBeH··· CH3F. The steric effect of three H atoms in the XH3 group is plausible to be in charge of this abnormal phenomenon. The H···X (X = Si, Ge, and Sn) distance is shorter than the sum of van der Waals radii of both free atoms,53 while the H···C distance is out of the van der Waals cutoff criterion in some complexes, which can also be explained with the steric effect of three H atoms in a CH3 group. The variation of the steric

Figure 3. Optimized structures of the complexes studied.

presence of a BCP between the X atom of XH3F and the H atom in HM (Figure S1, Supporting Information). Thus, according to the concept of the dihydrogen bond, we coin the term carbon−hydride or tetrel−hydride interaction to describe the complexes, in which group IV atoms interact with metal hydrides. Table 1 lists the interaction energies of the complexes at both the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels. It has been demonstrated that the MP2 method is apt to overestimate the interaction energies of hydrogen bonds in comparison to the CCSD(T) value.47 However, for tetrel− hydride interactions, both methods give rise to nearly equal interaction energies. In addition, the CCSD(T) binding distances in the complexes HBeH···CH3F and LiH···CH3F are 2.910 and 2.858 Å, respectively, which exhibits a slight difference of 0.001 and 0.003 Å away from that with the MP2 method. This indicates that the tetrel−hydride interaction can be better described by using the MP2 method with a larger basis set. The interaction energy of the tetrel−hydride interaction (Table 1) is apparently related to the nature of the group IV atom and metal group. With the same monomer of the group IV atom, the interaction energy of the tetrel−hydride interaction becomes more negative in the order of HBeH < HMgH < LiH < NaH. This indicates that the more reactive metal results in the stronger interaction, which was also found in dihydrogen bonds.48 On the other hand, with the metal hydride remaining, the interaction energy becomes more negative with the increase of the X atomic number. This C

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Table 2. Binding Distance (R, Å), Changes of Bond Lengths (Δr, Å), Shifts of Bond Stretch Frequencies (Δν, cm−1), and Angles (θ, deg) in the Complexes at the MP2/aug-cc-pVTZ Levela HBeH···CH3F HMgH···CH3F LiH···CH3F NaH···CH3F HBeH···SiH3F HMgH···SiH3F HBeH···GeH3F HMgH···GeH3F HBeH···SnH3F HMgH···SnH3F HMgH···CH3Cl HMgH···CH3CN HMgH···CH3NC HMgH···CH3NO2 CH3MgH···CH3F LiMgH···CH3F

R

Δr1

Δr2

Δr3

Δν1

Δν3

θ

2.911 2.915 2.861 2.868 2.684 2.519 2.655 2.523 2.534 2.357 2.968 3.054 2.936 2.925 2.912 2.867

0.001 0.003 0.009 0.010 0.005 0.011 0.007 0.015 0.011 0.025 0.004 0.000 0.002 −0.000 0.003 0.007

−0.000 −0.001 −0.002 −0.002 −0.001 −0.001 −0.001 −0.001 −0.001 −0.000 −0.001 −0.001 −0.001 −0.001 −0.001 −0.001

0.000 0.002 0.001 0.002 −0.001 0.000 −0.000 0.002 −0.000 0.004 0.003 0.004 0.005 0.005 0.003 0.004

−5 −12 −33 −38 −11 −27 −13 −32 −18 −40 −12 −4 −9 7 −13 −25

8 7 20 31 14 25 17 29 26 53 4 6 8 −1 6 8

71.2(108.6) 71.2(108.6) 71.1(108.6) 71.1(108.6) 72.5(108.2) 73.6(108.2) 74.4(106.5) 75.4(106.5) 77.1(104.8) 79.4(104.8) 71.5(108.3) 69.7(109.9) 70.3(109.4) 71.7(107.2) 71.1(108.6) 71.1(108.6)

The data in parentheses are supplementary angles of θ in the monomer. The binding distance at the CCSD(T)/aug-cc-pVTZ level is 2.910 and 2.858 Å in HBeH···CH3F and LiH···CH3F complexes, respectively. a

repulsive effect of three H atoms in an XH3 group can be estimated by the magnitude of the angle ∠H−X−H (θ). With the increase of the X atomic number, this angle becomes larger, corresponding to a smaller steric effect of three H atoms in the XH3 group. The formation of a tetrel−hydride interaction results in some changes of geometrical structures and spectroscopic parameters. The angle ∠H−X−Y (180° − θ) becomes slightly larger (