Hongying
Zhuo
,
Qingzhong
Li
*,
Wenzuo
Li
and
Jianbo
Cheng
The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, People's Republic of China. E-mail: liqingzhong1990@sina.com; Fax: +86 535 6902063; Tel: +86 535 6902063
First published on 28th October 2013
Borazine, “inorganic benzene”, exhibits some different properties from benzene although both of them are isostructural and isoelectronic. It was known that benzene is favorable to form halogen bonds with halogenated molecules. However, borazine more easily forms lone pair–π interactions with halogenated molecules, but for stronger halogen donors it can also form halogen bonds. The halogen bonds formed by borazine are stronger than the corresponding lone pair–π interactions. It was found that the pair–π interactions can be changed into halogen bonds with the increase of interaction strength. The dispersion energy plays a main role in stabilizing the weakly bonded complexes, while the electrostatic energy is dominant in the strongly bonded complexes. This is different from the nature of the respective benzene complexes.
Lone pair–π interaction between a neutral electron-rich molecule and an electron-poor π ring is another important noncovalent interaction. It has been found in a number of biological systems14–16 and supramolecular systems.17,18 A detailed analysis of the Cambridge Structure Database has shown that such an interaction is not unusual in organic compounds,19 but it has been rather unexplored,20 due to the general belief that it is mostly repulsive.21 Another possible reason is its weak interaction strength with interaction energy smaller than 3 kcal mol−1 in most cases.22 This weak strength leads to the fact that it is seldom present alone in complicated systems and it often coexists with other types of interactions.23–25 In the benzene–dimethylether complex, where only lone pair–π interaction exists, a substantial part of the stabilization stems from dispersion energy.26
Borazine (B3N3H6), isolated in 1926 by Stock and Pohland,27 exhibits some similarity with benzene in its physical properties, because it is isostructural and isoelectronic with benzene, thus it is called “inorganic benzene”.28 People have shown great interest in borazine and its derivatives due to their potentials as building blocks in materials' science. They can act as precursors for nonoxidic ceramics containing B–N bonds and boron nitride nanotubes or other nanostructures.29 They also have the potential to act as chemical H2 storage materials.30–32 Very recently, Kervyn and co-workers33 reported the first bottom-up preparation of borazine-based supermolecular architectures on metal surfaces, which are rationalized as a delicate interplay of short-range van der Waals attractions between neighboring molecules and long-range Coulomb repulsions between deprotonated charged molecules. It has been demonstrated that borazine can form π hydrogen-bonded complexes with the first-row hydrides (BH3, CH4, NH3, H2O, and HF) and electrostatic and dispersion energies play an important role in the formation of these complexes.34 Interestingly, borazine can participate not only in anion–π interactions with anions (F−, Cl−, Br−, NO3−, and BF4−)35 but also in cation–π interactions with cations (Li+, Na+, and K+).36
In the present paper, we designed some complexes of borazine and halogenated compounds. Selected halogenated compounds include XF, XCN, XCCH, and XCF3 (X = F, Cl, Br, and I), which usually act as halogen donors in halogen bonds.37–39 Bonding energy and geometrical parameters have been investigated for these complexes. Our interest is in determining the interaction type between borazine and the halogenated compounds, that is, is the main force between them π halogen bonding or lone-pair–π interaction? To answer this question, we performed natural bond orbital (NBO) and symmetry-adapted perturbation theory (SAPT) calculations. In addition, we also considered the substitution effect on the interaction type.
Molecular electrostatic potentials (MEPs) on the 0.001 electrons per bohr3 contour of the electronic density have been calculated with the Wave Function Analysis-Surface Analysis Suite (WFA-SAS) program42 at the MP2/aug-cc-pVDZ level. Natural bond orbital (NBO) analyses were carried out using NBO 3.1 version43 implemented in Gaussian 09 to provide an insight into the bonding nature of these complexes. The symmetry adapted perturbation theory (SAPT) method using the SAPT2008 program44 was used to decompose the interaction energy at the MP2/aug-cc-pVDZ level.
Fig. 1 Molecular electrostatic potentials of borazine. Color ranges, in eV, are: red, greater than 0.02; yellow, between 0.02 and 0.01; green, between 0.01 and 0; blue, less than 0. |
Fig. 2 Structures of complexes of borazine (1) with XY (X = F, Cl, Br, and I; Y = F, CN, CCH, and CF3). |
In Table 1, we summarized the structural parameters in all complexes. Here we pointed out that the planar structure of borazine has small deformation for the 1-XY-b complex. The Y-X⋯N angle (θ1) almost amounts to 180° in the complexes except for 1-ClCF3-b. For the 1-XY-b complex, the angle θ2 becomes larger with the increase of the halogen atomic number (Fig. 3). The geometrical difference in structure b can be explained with the size of the σ-hole on the interacting halogen atom. MEPs maps of XCCH (X = F, Cl, Br, and I) are shown in Fig. 4. When X varies from F to I, the region of the positive MEPs (σ-hole) on the halogen atomic surface becomes bigger, while the area of the negative MEPs on the halogen atomic surface becomes smaller. Other halogenated compounds show a similar change in the size of the σ-hole with the increase of the halogen atomic number. The negative area (lone pair electrons) on the halogen atom could have an attractive interaction with the electron-deficient ring center of borazine and this interaction becomes weaker for the heavier halogen. Simultaneously, the bigger positive region results in a stronger halogen bond between the N and X atoms. The change in the strength for both interactions is jointly responsible for the larger θ2 in the heavier halogen complex. This angle is bigger than 90° in 1-XY-b (XY = Cl2, Br2, I2) complexes but is smaller than 90° in other b complexes. For 1-XY-b (XY = Cl2, Br2, I2) complexes, the dihalogen molecules are almost perpendicular to the plane of 1. The distance between the F atom and the ring center of borazine is about 3.0 Å in the 1-FY-a (Y = F, CN, CCH, and CF3) complexes with small interaction energy (Table 2). The weak interaction in the 1-FY complex has a negligible effect on the change in the F–Y bond length and the frequency shift of the respective bond stretch vibration. The X⋯N distance shows an irregular change for the different series of 1-XY-b (X = Cl, Br, and I) complexes. With the increase of the X atomic radius, it becomes shorter in the 1-XCN-b complex and larger in the 1-XCF3-b complex, however, in the 1-XCCH-b complex, it grows in the order of Br < I < Cl and in the 1-XF-b complex in the order of Br < Cl < I. Even so, the X⋯N distances in all b complexes are smaller than the sum of van der Walls radii of the respective atoms (3.3 Å for Cl and N atoms, 3.4 Å for Br and N atoms, 3.6 Å for I and N atoms). However, the interaction energy shows a consistent change in the 1-XY-b (X = Cl, Br, and I) complex. Namely, the interaction energy becomes more negative with the increase of the halogen atomic number. This is in agreement with the most positive MEP on the halogen atomic surface.48 The interaction energy is also related to the Y substitutent and becomes more negative in the order of Y = CF3 < CCH < CN < F. It is also found that the b type complexes are more stable than the corresponding a type complexes. Both types of complexes are less stable than halogen-bonded complexes of benzene–X2.47 This further shows that borazine is a weaker Lewis base than benzene.
R | Δr | θ 1 | R | Δr | θ 1 | θ 2 | ||
---|---|---|---|---|---|---|---|---|
1-FF-a | 2.963 | 0.002 | 180.0 | 1-FF-b | 2.672 | 0.013 | 178.6 | 84.4 |
1-ClF-a | 3.176 | 0.004 | 180.0 | 1-ClF-b | 2.576 | 0.030 | 179.4 | 93.4 |
1-BrF-a | 3.200 | 0.005 | 180.0 | 1-BrF-b | 2.548 | 0.038 | 179.2 | 95.0 |
1-IF-a | 3.261 | 0.007 | 180.0 | 1-IF-b | 2.644 | 0.036 | 179.2 | 96.4 |
1-FCN-a | 2.980 | −0.001 | 180.0 | 1-FCN-b | — | — | — | — |
1-ClCN-a | 3.235 | — | 180.0 | 1-ClCN-b | 3.250 | 0.001 | 180.0 | 76.5 |
1-BrCN-a | 3.252 | — | 180.0 | 1-BrCN-b | 3.139 | 0.004 | 180.0 | 82.3 |
1-ICN-a | 3.330 | 0.002 | 180.0 | 1-ICN-b | 3.126 | 0.012 | 180.0 | 85.8 |
1-FCCH | 2.976 | −0.002 | 180.0 | 1-FCCH-b | — | — | — | — |
1-ClCCH-a | 3.273 | — | 180.0 | 1-ClCCH-b | 3.283 | 0.000 | 176.4 | 75.5 |
1-BrCCH-a | 3.289 | — | 180.0 | 1-BrCCH-b | 3.189 | 0.002 | 179.6 | 79.5 |
1-ICCH-a | 3.376 | 0.002 | 180.0 | 1-ICCH-b | 3.197 | 0.008 | 179.4 | 82.3 |
1-FCF3-a | 2.976 | −0.002 | 180.0 | 1-FCF3-b | — | — | — | — |
1-ClCF3-a | 3.283 | −0.002 | 180.0 | 1-ClCF3-b | 3.312 | −0.002 | 173.7 | 75.7 |
1-BrCF3-a | 3.308 | −0.003 | 180.0 | 1-BrCF3-b | 3.230 | −0.002 | 178.8 | 80.5 |
1-ICF3-a | 3.383 | −0.003 | 180.0 | 1-ICF3-b | 3.237 | 0.000 | 179.5 | 83.8 |
ΔE | Δν | N | ΔE | Δν | N | ||
---|---|---|---|---|---|---|---|
1-FF-a | −3.70 | −10 | 2 | 1-FF-b | −5.10 | −63 | 0 |
1-ClF-a | −8.69 | −10 | 2 | 1-ClF-b | −18.81 | −92 | 0 |
1-BrF-a | −10.69 | −11 | 2 | 1-BrF-b | −27.00 | −76 | 0 |
1-IF-a | −12.62 | −12 | 2 | 1-IF-b | −32.37 | −55 | 0 |
1-FCN-a | −3.91 | 0 | 0 | 1-FCN-b | — | — | — |
1-ClCN-a | −9.01 | −2 | 2 | 1-ClCN-b | −9.37 | −4 | 0 |
1-BrCN-a | −10.44 | −1 | 2 | 1-BrCN-b | −11.66 | −8 | 0 |
1-ICN-a | −12.23 | −2 | 2 | 1-ICN-b | −15.02 | −14 | 0 |
1-FCCH | −2.45 | 3 | 0 | 1-FCCH-b | — | — | — |
1-ClCCH-a | −6.98 | −1 | 2 | 1-ClCCH-b | −7.30 | −3 | 0 |
1-BrCCH-a | −8.11 | −1 | 2 | 1-BrCCH-b | −9.06 | −6 | 0 |
1-ICCH-a | −9.53 | −1 | 2 | 1-ICCH-b | −11.60 | −11 | 0 |
1-FCF3-a | −2.11 | 0 | 3 | 1-FCF3-b | — | — | — |
1-ClCF3-a | −6.77 | −1 | 3 | 1-ClCF3-b | −6.94 | −1 | 0 |
1-BrCF3-a | −7.90 | −1 | 3 | 1-BrCF3-b | −8.53 | −2 | 0 |
1-ICF3-a | −9.29 | −1 | 3 | 1-ICF3-b | −10.77 | −3 | 0 |
The interaction strength is weak in most complexes except some XF complexes. Thus we plan to enhance it by the substitution effect. Specifically, the H atom of B–H or N–H bond in 1-BrCF3 complex is replaced with CN, Li, and CH3, respectively. The obtained complexes are represented as B-R-1-BrCF3 and N-R-1-BrCF3, where the bold atom indicates the substitution site and R represents substituents. The corresponding results are given in Table 3. The electron-withdrawing group CN leads to a weakening of the interaction strength with less negative interaction energy, while the electron-donating group Li results in an enhancement of the interaction strength with more negative interaction energy. The enhancing effect of Li is more prominent than the weakening effect of CN. The N substitution effect is more prominent whether for the Li enhancing effect or for the CN weakening effect, due to its direct participation in the interaction. The interaction energy in the N-Li-1-BrCF3 complex is about 2.5 times as much as that in the 1-BrCF3 complex. The prominent effect of Li stubstitutent has been reported for the halogen-bonded complex of HCCBr–NCH.49 A similar enhancing effect is also found for the CH3 substituent, although its effect is smaller than that of Li substitution. This indicates that the methyl group plays an electron-donating role in this interaction, which is similar to that in hydrogen bonds.50 Considering the possible Br⋯Li interaction in the N-Li-1-BrCF3 complex, the θ1 angle decreases by about 12° and the θ2 angle increases by about 40°. Both angles change a little in other substituted complexes. In general, the enhancing interaction shortens the binding distance, while the weakening interaction enlarges it. The binding distance is an exception in the N-CN–1–BrCF3 complex.
ΔE | R | Δr | θ 1 | θ 2 | Δν | |
---|---|---|---|---|---|---|
B-CN-1-BrCF3-b | −7.01 | 3.240 | 0.001 | 173.4 | 79.8 | −1 |
N-CN-1-BrCF3-b | −6.60 | 3.213 | 0.001 | 171.7 | 82.7 | 0 |
B-Li-1-BrCF3-b | −14.42 | 3.178 | −0.005 | 174.0 | 81.5 | −4 |
N-Li-1-BrCF3-b | −21.66 | 2.888 | 0.018 | 166.6 | 120.2 | −9 |
B-CH3-1-BrCF3-b | −10.13 | 3.178 | −0.001 | 175.5 | 81.6 | −2 |
N-CH3-1-BrCF3-b | −11.10 | 3.089 | −0.001 | 176.0 | 86.3 | −3 |
E 1 | E 2 | CT | E 1 | E 2 | CT | ||
---|---|---|---|---|---|---|---|
1-FF-a | 0.75 | 9.20 | −4.8 | 1-FF-b | 8.94 | 22.49 | −0.4 |
1-ClF-a | 2.88 | 17.18 | −9.7 | 1-ClF-b | 70.31 | 54.00 | 33.5 |
1-BrF-a | 5.52 | 21.69 | −9.8 | 1-BrF-b | 122.01 | 73.69 | 53.3 |
1-IF-a | 9.91 | 27.88 | −9.4 | 1-IF-b | 138.90 | 81.13 | 48.2 |
1-FCN-a | 0.00 | 7.61 | −2.4 | 1-FCN-b | — | — | — |
1-ClCN-a | 1.13 | 12.08 | −4.3 | 1-ClCN-b | 2.84 | 16.30 | −4.0 |
1-BrCN-a | 2.63 | 15.92 | −1.0 | 1-BrCN-b | 9.20 | 24.24 | 0.9 |
1-ICN-a | 5.39 | 19.86 | −2.3 | 1-ICN-b | 21.57 | 33.36 | 6.0 |
1-FCCH | 0.00 | 7.82 | −3.5 | 1-FCCH-b | — | — | — |
1-ClCCH-a | 0.88 | 11.33 | −6.2 | 1-ClCCH-b | 2.26 | 16.47 | −6.4 |
1-BrCCH-a | 2.01 | 15.26 | −6.6 | 1-BrCCH-b | 6.81 | 23.58 | −5.4 |
1-ICCH-a | 4.01 | 18.89 | −5.6 | 1-ICCH-b | 14.21 | 31.10 | −1.3 |
1-FCF3-a | 0.00 | 9.74 | −2.8 | 1-FCF3-b | — | — | — |
1-ClCF3-a | 0.75 | 10.83 | −4.4 | 1-ClCF3-b | 2.13 | 16.43 | −4.7 |
1-BrCF3-a | 2.01 | 14.04 | −4.4 | 1-BrCF3-b | 6.69 | 21.57 | −3.2 |
1-ICF3-a | 4.64 | 17.35 | −3.3 | 1-ICF3-b | 15.55 | 28.38 | 1.2 |
Fig. 5 Schematic diagrams of two orbital interactions in 1-BrCN-a (1 and 2) and 1-BrCN-b (3 and 4) complexes. |
To further provide evidence for the above results, charge transfer from borazine to the halogenated molecule was also calculated. The charge transfer is negative in the 1-XY-a complex, showing that borazine gains electrons in the 1-XY-a complex. This is consistent with the electron-withdrawing role of borazine in the lone pair–π interaction. Likely, the charge transfer is also negative in most 1-XY-b complexes except 1-XF-b (X = Cl, Br, and I), 1-BrCN-b, and 1-IY-b (Y = CN, and CCH, CF3) complexes. This means that for most 1-XY-b complexes borazine also gains electrons. However, this negative charge transfer is changed to positive in the 1-XF-b (X = Cl, Br, and I), 1-BrCN-b, and 1-IY-b (Y = CN, CCH, and CF3) complexes. The positive charge transfer in the 1-BrCN-b and 1-IY-b (Y = CN, CCH, and CF3) complexes is inconsistent with the magnitude of the respective orbital interactions. We think that other interactions such as polarization interaction, besides the orbital interactions, are also responsible for the charge transfer in these complexes. The positive charge transfer in the 1-XF-b (X = Cl, Br, and I) complex supports the electron-donating role of borazine in the formation of the halogen bond. With the increase of the halogen atomic mass, the magnitude of negative charge transfer in the 1-XY-b complex becomes smaller and is even changed to positive one in some complexes.
One finds the stronger halogen bonding interaction in the 1-XF-b (X = Cl, Br, and I) complex but the weaker lone pair–π interaction in other complexes. Thus we studied the dependence of the interaction type on the binding distance. One can see from Fig. 6 that with the shortening of the Br⋯N distance in the 1-BrCF3-b complex the values of E1 and E2 are both increased. The E2 value almost increases linearly, while the E1 one displays an exponential increase. However, the slope of their change is different. Interestingly, the E1 value is smaller than the E2 one with the Br⋯N distance larger than 2.6 Å, both values are equal at 2.6 Å, and the E1 value is greater than the E2 one with the Br⋯N distance shorter than 2.6 Å. Thus the lone pair–π interaction is changed into the halogen bond with the shortening of the binding distance (stronger interaction). This result indicates that the interaction strength can change the interaction type.
The interaction energies in both types of complexes are decomposed into five terms: electrostatic energy (Eelst), exchanged energy (Eexch), induction energy (Eind), dispersion energy (Edisp), and δEint,rHF, and the related results are given in Table 5, while the detailed components are presented in Tables S1 and S2 in ESI.† For the 1-FY-a (Y = CN, CCH, and CF3) complex, the Edisp term has a greater contribution than the Eelst one and the Eind contribution is very small. A similar result is also found in the 1-ClY-a (Y = CN, CCH, and CF3) complex. For the 1-BrY-a (Y = CN, CCH, and CF3) complex, the Eelst term is close to the Edisp term and is even larger in some cases, and the Eind contribution is still smallest. However, the Eelst contribution is negative for the 1-FF-a complex. For the other dihalogen complexes, the Eelst contribution is dominant although the other two attractive terms are also important. For the 1-XY-b (X = Cl and Br; Y = CN, CCH, and CF3) complex, the Eelst contribution is still dominant, while the Eind contribution is small. Thus the physical origin of the interaction between borazine and halogenated molecule depends on the interaction type and the nature of the halogen atom. In summary, the dispersion energy is the main force responsible for the weaker interactions, while the electrostatic energy has a main contribution to the stronger interactions.
E elst | E exch | E ind | E disp | δE int,r HF | |
---|---|---|---|---|---|
Note: Data in parentheses are for the b complex. δEint,rHF collects all third- and higher-order induction and exchange-induction terms. | |||||
1-FF-a | 5.01(−24.62) | 10.85(46.69) | −5.31(−4.26) | −3.09(−8.23) | −0.35(−0.98) |
1-ClF-a | −12.00(−61.90) | 16.84(101.15) | −2.68(−16.72) | −8.94(−20.86) | −1.92(−24.48) |
1-BrF-a | −19.48(−105.04) | 26.12(154.93) | −6.48(−45.77) | −12.91(−30.05) | −3.93(−34.37) |
1-FCN-a | −7.90 | 27.78 | −0.75 | −13.21 | −1.59 |
1-ClCN-a | −9.27(−12.00) | 15.62(20.13) | −1.30(−1.63) | −9.86(−10.83) | −0.96(−1.52) |
1-BrCN-a | −15.76(−24.83) | 24.11(36.35) | −3.59(−5.89) | −13.96(−16.05) | −2.19(−4.39) |
1-FCCH-a | −1.55 | 8.80 | −0.21 | −5.14 | −0.38 |
1-ClCCH-a | −7.06(−9.70) | 14.56(19.24) | −0.54(−0.84) | −9.66(−10.70) | −0.74(−1.21) |
1-BrCCH-a | −13.00(−20.77) | 22.70(33.28) | −2.05(−3.72) | −13.63(−15.59) | −1.65(−3.23) |
1-FCF3-a | −2.42 | 8.78 | 0.04 | −4.60 | −0.36 |
1-ClCF3-a | −7.77(−10.12) | 13.59(18.00) | −0.92(−1.17) | −8.99(−10.03) | −0.81(−1.31) |
1-BrCF3-a | −13.38(−19.90) | 20.73(30.04) | −2.80(−4.39) | −12.75(−14.55) | −1.88(−3.71) |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cp54006e |
This journal is © the Owner Societies 2014 |