- Email: [email protected]
Unveiling NO2π···CC π–hole interactions: A combined computational and crystallographic study
Chemical Physics Letters 633 (2015) 282–286
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Unveiling NO2 ···C C –hole interactions: A combined computational and crystallographic study Antonio Bauzá, Antonio Frontera ∗ Departament de Química, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
a r t i c l e
i n f o
Article history: Received 21 May 2015 In final form 1 June 2015 Available online 11 June 2015
a b s t r a c t A series of –hole complexes involving the NO2 group and several isolated systems have been evaluated at the BP86-D3/def2-TZVP level of theory. We have used ethene, ethyne and but-2-yne as electron donor moieties and nitrobenzene and 1,2,3,4,5-pentafluoro-6-nitrobenzene as electron acceptors. The SAPT and NBO analyses show that the dispersion term is the major contributor to the binding while (C C) → *(C N) donor–acceptor interactions also contribute to the overall stabilization. Finally several experimental examples were retrieved from the CSD highlighting the importance of this interaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A proper understanding and intelligent utilization of noncovalent interactions is essential in modern chemistry. Its key role has been well recognized in fields such as supramolecular chemistry [1], molecular recognition [2] and materials science [3]. For instance, interactions involving aromatic rings [4] (i.e. cation– [5], – stacking [6], anion– [7], lp– [8] and C H/ [9] interactions) are extremely important in many chemical and biological processes [10], including molecular sensing, crystal engineering and enzymatic mechanisms. In addition, –hole pnicogen bonding interaction has attracted considerable attention in recent years [11]. It refers to the interaction between pnicogen bearing compounds (–hole donor) and nucleophiles (–hole acceptor). The nature of this interaction has been studied theoretically [12–16]. Experimentally, it has been used in supramolecular chemistry in the solid-state [17–19]. Moreover, its importance in several biological systems has been demonstrated, including ligand binding and enzymatic inhibition [20,21]. The physical nature of pnicogen bonding is explained basically by an electrostatic attraction between the positive –hole on the tip of the pnicogen atom (opposite to the -bond) and the negative Lewis base. Recently, Murray and coworkers have carried out an exhaustive theoretical study remarking the ability of sp2 hybridized nitrogen atoms among others to establish the so called ‘–hole interactions’. A –hole is formed when the anisotropic distribution of the charge is located perpendicularly to the sigma axis [22]. This novel
∗ Corresponding author. E-mail address: [email protected] (A. Frontera). http://dx.doi.org/10.1016/j.cplett.2015.06.005 0009-2614/© 2015 Elsevier B.V. All rights reserved.
noncovalent interaction has increasingly attracted the interest of the scientific community as shown by the amount of theoretical studies reported to date [23–26]. For instance, we have recently shown the existence of a strong directionality of –hole complexes involving the NO2 group, certainly conditioned by the magnitude and value of the –hole [27]. While most of the electron rich interacting species are lone pair molecule donors, we figured out the possibility of use an entire -system (double or triple bond) acting as electron donor in –hole complexes. This interaction can be understood either as donor–acceptor (-rich system interacting with a –hole) or as a common dispersion dominated stacking interaction. In this manuscript we analyze from a theoretical point of view the ability of double and triple C C bonds to act as electron donor molecules in –hole complexes involving the NO2 group. We have used ethene, ethyne and but-2-yne as electron donor molecules and nitrobenzene and 1,2,3,4,5-pentafluoro-6nitrobenzene as electron acceptors. Additionally, several selected examples from the CSD are shown, highlighting this possibility. For all complexes we have explored two different binding modes, i.e. parallel (referred as a) and perpendicular (referred as b) orientations (see Scheme 1). 2. Computational methods The energies of all complexes included in this study were computed at the BP86-D3/def2-TZVP level of theory. The geometries have been fully optimized imposing Cs symmetry constrain. The calculations have been performed by using the program Turbomole version 6.5 [28]. For the calculations we have used the BP86 functional with the latest available correction for dispersion (D3) [29]. The NBO (Natural Bonding Orbitals) calculations have been
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Table 1 Binding energy values without and with correction for the BSSE (E and EBSSE in kcal/mol) and equilibrium distances R1 (referred from the carbon atom of the arene moiety to the C C or C C centroid) and R2 (referred from the nitrogen atom of the arene moiety to the C C or C C centroid) (Å). Complexa
E
EBSSE
R1
R2
3@1a 3@1b 4@1a 4@1b 5@1a 5@1b 3@2a 3@2b 4@2a 4@2b 5@2a 5@2b
−2.8 −3.0 −2.7 −2.2 −4.4 −4.0 −3.8 −4.2 −3.1 −3.3 −6.7 −6.9
−2.5 −2.7 −2.5 −2.0 −4.1 −3.7 −3.4 −3.8 −2.8 −3.0 −6.1 −6.3
3.309 3.450 3.252 3.478 3.303 3.266 3.317 3.319 3.501 3.291 3.292 3.244
3.229 3.310 3.270 3.222 3.147 3.195 3.225 3.354 3.302 3.362 3.167 3.201
a a and b stand for parallel and perpendicular orientations (see Figure 1). 1 and 2 are nitrobenzene and pentafluoronitrobenzene, respectively. 3, 4 and 5 stand for ethene, ethyne and but-2-yne, respectively.
Scheme 1. Electron donor and acceptor moieties 1–5 used in this study. The molecular polarizability values ˛zz (a.u.) are also indicated. The z direction in 1 and 2 is perpendicular to the molecular plane.
performed at the BP86/def2-TZVP level by means of the Gaussian B.01 calculation package [30]. The partitioning of the interaction energies into the individual electrostatic, induction, dispersion, and exchange–repulsion components was carried out with the symmetry adapted intermolecular perturbation theory approach DFT-SAPT [31] at the DF-DFT/def2-TZVP level of theory. The Bader’s ‘Atoms in molecules’ theory has been used to study the interactions discussed herein by means of the AIMall calculation package [32]. 3. Results and discussion 3.1. MEPS analysis As a preliminary study we have performed a MEPS (Molecular Electrostatic Potential Surface) analysis of both electron acceptor moieties used in this work, i.e. compounds 1 and 2. The results are shown in Figure 1. As it can be observed, both compounds present a positive potential region over the nitrogen atom of the NO2 group, located perpendicularly to the molecular plane, indicating that both compounds are able to interact favorably with electron rich entities. The MEPS value at the nitrogen atom in compound 2 is larger, thus expecting a more favorable interaction with electron rich moieties. It is also worth mentioning the extension of the positive potential surface, which is larger in the pentafluorinated ring.
3.2. Energetic study The interaction energies and equilibrium distances obtained for the complexes studied herein are gathered in Table 1. From the inspection of the results, several interesting considerations arise. First, it can be observed that the interaction energies are favorable for all complexes indicating that the NO2 ··· interaction (–hole···) is favorable. For complexes involving the nitrobenzene ring (3@1a to 5@1b) the ‘parallel’ orientation (denoted as a) is more favorable in terms of stability of the complexes. This is due to a better overlapping in this orientation between both -systems (see Figure 2). The only exception to this behavior is complex 3@1b where the perpendicular orientation is more favored, due to the establishment of two additional C H···O interactions (see Figure 2). On the other hand, complexes involving the pentafluorinated nitrobenzene ring (3@2a to 5@2b) show larger binding energy values, as anticipated by the MEPS analysis. In this series the perpendicular orientation (denoted as b) is the most favored disposition, due to the possibility to establish additional C H···O, C H··· and also C H···F interactions with the fluorine atoms attached to the arene moiety, as confirmed by the AIM analysis (see below). For both arenes, the most favorable complexes involve the but2-yne (complexes 5@1a, 5@1b, 5@2a and 5@2b), which presents a higher electron donor ability due to the presence of the two methyl groups and also the possibility to establish ancillary interactions (see Figure 2). In addition, the energies involving double/triple bond groups show small differences while flipping from a parallel to a perpendicular disposition, indicating that the interactions is not sensitive to the relative orientation of the double/triple bond for both rings. The energetic values obtained for all complexes are similar to those obtained for other –hole complexes involving neutral counterparts [33]. From the inspection of the distance values it can be observed that the nitrogen atom of the NO2 group presents shorter distances to the -system than the carbon atom of the ring, as expected, due to the presence of the –hole closer to the nitrogen atom as revealed by the MEPS analysis (see Figure 1). 3.3. NBO analysis
Figure 1. MEPS analysis of compounds 1 (left) and 2 (right). Energy values are in kcal/mol.
In order to study if orbital contributions are important to explain the pnicogen interactions described above, we have performed Natural Bond Orbital (NBO) calculations focusing our attention on the second order perturbation analysis [34], since it is very useful to study donor acceptor interactions [34]. The results are summarized in Table 2. As it can be observed, the orbital contribution is small in all complexes (≈10% of the total interaction energy). For
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Figure 2. BP86-D3/def2-TZVP optimized geometries for complexes 3@1a to 5@2b. Additional C H···O, C H···F and C H··· interactions are highlighted in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
complexes involving 3 the major stabilization source comes from the donation from the bonding orbital () of the double C C bond to the antibonding orbital (*) of the N O bond. In addition, complexes involving the perpendicular arrangement (denoted as b) show a lone pair donation from the oxygen atom of the NO2 moiety to the antibonding orbital (*) of the ethene molecule. For complexes involving 4 the donation from the bonding orbital () of the triple C C bond to the antibonding orbital (*) of the N O bond is the most relevant orbital contribution. The only exception is complex 4@2a where the donation from the orbital of the ethyne to the antibonding orbital (*) of the C C bond of the arene moiety is the most remarkable contribution. 3.4. SAPT analysis In Table 3 we summarize the DF-DFT-SAPT energy values relative to the –hole complexes. The total SAPT interaction energies
Table 2 Donor and acceptor NBOs with indication of the second-order interaction energy E(2) and type of interaction for NO2 ··· complexes. Complexa
Donorb
3@1a 3@1b
BD(2) C BD(2) C LP O BD(3) C BD(3) C BD(2) C BD(2) C LP O BD(3) C BD(3) C
4@1a 4@1b 3@2a 3@2b 4@2a 4@2b
Acceptorb C C C C C C C C
BD(2)* N BD(2)* N BD(1)* C BD(2)* N BD(2)* N BD(2)* N BD(2)* N BD(1)* C BD(2)* C BD(2)* N
O O H O O O O H C O
Eb kcal/mol
Type
0.22 0.120.12
→ * → * n → * → * → * → * → * n → * → * → *
0.07 0.06 0.19 0.080.12 0.12 0.10
a a and b stand for parallel and perpendicular orientations (see Figure 1). 1 and 2 are nitrobenzene and pentafluoronitrobenzene, respectively. 3 and 4 for ethene and ethyne, respectively. b BD(2) and BD(3) stand for the type orbitals of double and triple bonds respectively. BD*(2) stands for the type anti-bonding orbital of a double bond.
for these complexes are similar to those obtained using the BP86D3/def2TZVP level of theory (see Table 1) giving reliability to the partition method. The energetic contributions (Table 3) indicate that the dispersion term (Edisp ) is the most important for the stability of the –hole··· interactions, ranging from −8.0 to −3.5 kcal/mol. Moreover, the electrostatic term (Eee ) also contributes to the stabilization of all complexes, in agreement with previous results reported for –hole complexes involving nitro compounds [27]. Regarding this point, the electrostatic term plays a major role in the pentafluorinated ring complexes, in agreement with the MEPS analysis discussed above. Interestingly, complexes involving the but-2-yne moiety (5@1a, 5@1b, 5@2a, 5@2b) present the highest values for Eee and Edisp components, this is due on the one hand to the higher -basicity of the C C triple bond, but also due to the higher polarizability of the but-2-yne moiety compared to ethene and ethyne, which certainly influences the dispersion component. Finally, the Eind term presents higher values for complexes involving the pentafluorinated moiety than for those where the nitrobenzene moiety is involved.
Table 3 SAPT interaction energies and their partitioning into the electrostatic, induction, dispersion and exchange contributions (Etotal , Eee , Eind , Edisp , Eexc , respectively, kcal/mol) at the RI-DFT/def2TZVP level of theory using the DF-DFT-SAPT approach. Complex
Eee
Eex
Eind
Edisp
Etotal
3@1a 3@1b 4@1a 4@1b 5@1a 5@1b 3@2a 3@2b 4@2a 4@2b 5@2a 5@2b
−3.1 −2.9 −2.9 −2.4 −4.8 −4.7 −3.6 −3.8 −2.9 −3.6 −4.9 −6.0
6.0 5.5 5.4 4.8 9.8 9.8 6.2 6.4 4.7 5.4 9.7 10.4
−0.1 −0.1 −0.2 −0.1 −0.4 −0.3 −0.3 −0.3 −0.3 −0.3 −0.6 −0.6
−4.3 −4.1 −3.9 −3.5 −7.7 −7.5 −4.6 −4.7 −3.9 −4.0 −7.9 −7.8
−1.5 −1.6 −1.6 −1.2 −3.0 −2.6 −2.4 −2.5 −2.4 −2.5 −3.7 −4.0
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Figure 3. Distribution of critical points and bond paths in complexes 3@1a, 3@1b, 5@2a and 5@2b. Bond, ring and cage critical points are represented by red, yellow and green spheres, respectively. The bond paths connecting bond critical points are also represented. In addition the value of the density at the bond critical point ( × 100) is given. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.5. AIM analysis We have also used the Bader’s theory of ‘atoms in molecules’ to characterize the NO2 ··· interactions and the results are shown in Figure 3. In complex 3@1a (Figure 3, top left) the interaction is characterized by the presence of two bond critical points, which connect both C atoms of the CC bond to the C and N atoms of the C N bond. The interaction is further characterized by the presence of a ring critical point as a consequence of the formation of the supramolecular four-membered ring. The NO2 ··· interaction in complex 3@1b (Figure 3, top right) is characterized by a bond critical point that connects the carbon atom of the C NO2 bond with the bond critical point of the C C double bond of ethene. In addition
each C H···O hydrogen bonds is characterized by the presence of a bond critical point. For complexes 5@2a (Figure 3, bottom left) and 5@2b (Figure 3, bottom right) the interaction is characterized by the presence of a bond critical point that connects the carbon atom of the C N bond to one carbon atom of the triple bond in 5@2a. In addition, several C H···O interactions are observed as well as CH– interactions involving the carbon atoms of the pentafluorinated ring. Finally in 5@2b, the interaction is characterized by the presence of one bond critical point that connects the carbon atom of the C N bond with the triple bond critical point, also additional C H···O and C H···F interactions are observed each one characterized by the presence of a bond critical point connecting the H atom with the heteroatom.
Figure 4. Some X-ray structures retrieved from the CSD. Distances in Å. The CSD codes are indicated.
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3.6. CSD examples
References
Finally, we have explored the CSD in order to find some experimental support to the theoretical calculations described above. Interestingly we have found 26 crystal structures where a double bond interacts with the –hole of the nitro group. Three of them are highlighted in Figure 4. IMOMIG and CIKBEE are representative examples of the two orientations we have used in the study. In the IMOMIG structure the C C centroid is located above ˚ Remarkably, the the nitrogen atom of the nitro group at 3.26 A. CIKBEE structure presents self-assembled dimers in the solid state where two NO2 ··· interactions are present. The CAWMUJ structure also forms a self-assembled dimer in the solid state exhibiting a double NO2 ··· interaction; in this case a triple bond instead of a double bond is the electron donor. The distances observed in the solid state are in good agreement with the theoretical ones.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21]
The NO2 ··· interaction with double and triple bonds is modest but energetically favorable, ranging from −6 to −2 kcal/mol. The parallel orientation of the isolated -system is generally more stable than the perpendicular disposition in nitrobenzene complexes, while the opposite occurs for the pentafluorinated compounds, due to the establishment of ancillary interactions by the electron donor moiety. The SAPT (Symmetry Adapted Perturbation Theory) analysis confirm the prominent role of the dispersion component (Edisp ) to the total binding energy, however the electrostatic term (Edisp ) also presents a remarkable contribution to the overall stabilization of the complexes due to the donor–acceptor nature of the interaction. The orbital contribution, which has been analyzed using the NBO method, is modest and comes from the donation from the bonding orbital () of the double/triple C C bond to the antibonding orbital (*) of the N O bond. The AIM analysis confirms the ability of the isolated -systems to act as electron donor in –hole··· complexes. Finally, some examples in the CSD confirm this possibility and highlight the impact of the –hole interactions in the crystal packing phenomena. Acknowledgments We thank CONSOLIDER-Ingenio 2010 (CSD2010-0065) and the MICINN of Spain (project CTQ2014-57393-C2-1-P, FEDER funds) for financial support. We thank the CTI for computational facilities.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Unveiling NO2 ···C C –hole interactions: A combined computational and crystallographic study Antonio Bauzá, Antonio Frontera ∗ Departament de Química, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
a r t i c l e
i n f o
Article history: Received 21 May 2015 In final form 1 June 2015 Available online 11 June 2015
a b s t r a c t A series of –hole complexes involving the NO2 group and several isolated systems have been evaluated at the BP86-D3/def2-TZVP level of theory. We have used ethene, ethyne and but-2-yne as electron donor moieties and nitrobenzene and 1,2,3,4,5-pentafluoro-6-nitrobenzene as electron acceptors. The SAPT and NBO analyses show that the dispersion term is the major contributor to the binding while (C C) → *(C N) donor–acceptor interactions also contribute to the overall stabilization. Finally several experimental examples were retrieved from the CSD highlighting the importance of this interaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A proper understanding and intelligent utilization of noncovalent interactions is essential in modern chemistry. Its key role has been well recognized in fields such as supramolecular chemistry [1], molecular recognition [2] and materials science [3]. For instance, interactions involving aromatic rings [4] (i.e. cation– [5], – stacking [6], anion– [7], lp– [8] and C H/ [9] interactions) are extremely important in many chemical and biological processes [10], including molecular sensing, crystal engineering and enzymatic mechanisms. In addition, –hole pnicogen bonding interaction has attracted considerable attention in recent years [11]. It refers to the interaction between pnicogen bearing compounds (–hole donor) and nucleophiles (–hole acceptor). The nature of this interaction has been studied theoretically [12–16]. Experimentally, it has been used in supramolecular chemistry in the solid-state [17–19]. Moreover, its importance in several biological systems has been demonstrated, including ligand binding and enzymatic inhibition [20,21]. The physical nature of pnicogen bonding is explained basically by an electrostatic attraction between the positive –hole on the tip of the pnicogen atom (opposite to the -bond) and the negative Lewis base. Recently, Murray and coworkers have carried out an exhaustive theoretical study remarking the ability of sp2 hybridized nitrogen atoms among others to establish the so called ‘–hole interactions’. A –hole is formed when the anisotropic distribution of the charge is located perpendicularly to the sigma axis [22]. This novel
∗ Corresponding author. E-mail address: [email protected] (A. Frontera). http://dx.doi.org/10.1016/j.cplett.2015.06.005 0009-2614/© 2015 Elsevier B.V. All rights reserved.
noncovalent interaction has increasingly attracted the interest of the scientific community as shown by the amount of theoretical studies reported to date [23–26]. For instance, we have recently shown the existence of a strong directionality of –hole complexes involving the NO2 group, certainly conditioned by the magnitude and value of the –hole [27]. While most of the electron rich interacting species are lone pair molecule donors, we figured out the possibility of use an entire -system (double or triple bond) acting as electron donor in –hole complexes. This interaction can be understood either as donor–acceptor (-rich system interacting with a –hole) or as a common dispersion dominated stacking interaction. In this manuscript we analyze from a theoretical point of view the ability of double and triple C C bonds to act as electron donor molecules in –hole complexes involving the NO2 group. We have used ethene, ethyne and but-2-yne as electron donor molecules and nitrobenzene and 1,2,3,4,5-pentafluoro-6nitrobenzene as electron acceptors. Additionally, several selected examples from the CSD are shown, highlighting this possibility. For all complexes we have explored two different binding modes, i.e. parallel (referred as a) and perpendicular (referred as b) orientations (see Scheme 1). 2. Computational methods The energies of all complexes included in this study were computed at the BP86-D3/def2-TZVP level of theory. The geometries have been fully optimized imposing Cs symmetry constrain. The calculations have been performed by using the program Turbomole version 6.5 [28]. For the calculations we have used the BP86 functional with the latest available correction for dispersion (D3) [29]. The NBO (Natural Bonding Orbitals) calculations have been
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283
Table 1 Binding energy values without and with correction for the BSSE (E and EBSSE in kcal/mol) and equilibrium distances R1 (referred from the carbon atom of the arene moiety to the C C or C C centroid) and R2 (referred from the nitrogen atom of the arene moiety to the C C or C C centroid) (Å). Complexa
E
EBSSE
R1
R2
3@1a 3@1b 4@1a 4@1b 5@1a 5@1b 3@2a 3@2b 4@2a 4@2b 5@2a 5@2b
−2.8 −3.0 −2.7 −2.2 −4.4 −4.0 −3.8 −4.2 −3.1 −3.3 −6.7 −6.9
−2.5 −2.7 −2.5 −2.0 −4.1 −3.7 −3.4 −3.8 −2.8 −3.0 −6.1 −6.3
3.309 3.450 3.252 3.478 3.303 3.266 3.317 3.319 3.501 3.291 3.292 3.244
3.229 3.310 3.270 3.222 3.147 3.195 3.225 3.354 3.302 3.362 3.167 3.201
a a and b stand for parallel and perpendicular orientations (see Figure 1). 1 and 2 are nitrobenzene and pentafluoronitrobenzene, respectively. 3, 4 and 5 stand for ethene, ethyne and but-2-yne, respectively.
Scheme 1. Electron donor and acceptor moieties 1–5 used in this study. The molecular polarizability values ˛zz (a.u.) are also indicated. The z direction in 1 and 2 is perpendicular to the molecular plane.
performed at the BP86/def2-TZVP level by means of the Gaussian B.01 calculation package [30]. The partitioning of the interaction energies into the individual electrostatic, induction, dispersion, and exchange–repulsion components was carried out with the symmetry adapted intermolecular perturbation theory approach DFT-SAPT [31] at the DF-DFT/def2-TZVP level of theory. The Bader’s ‘Atoms in molecules’ theory has been used to study the interactions discussed herein by means of the AIMall calculation package [32]. 3. Results and discussion 3.1. MEPS analysis As a preliminary study we have performed a MEPS (Molecular Electrostatic Potential Surface) analysis of both electron acceptor moieties used in this work, i.e. compounds 1 and 2. The results are shown in Figure 1. As it can be observed, both compounds present a positive potential region over the nitrogen atom of the NO2 group, located perpendicularly to the molecular plane, indicating that both compounds are able to interact favorably with electron rich entities. The MEPS value at the nitrogen atom in compound 2 is larger, thus expecting a more favorable interaction with electron rich moieties. It is also worth mentioning the extension of the positive potential surface, which is larger in the pentafluorinated ring.
3.2. Energetic study The interaction energies and equilibrium distances obtained for the complexes studied herein are gathered in Table 1. From the inspection of the results, several interesting considerations arise. First, it can be observed that the interaction energies are favorable for all complexes indicating that the NO2 ··· interaction (–hole···) is favorable. For complexes involving the nitrobenzene ring (3@1a to 5@1b) the ‘parallel’ orientation (denoted as a) is more favorable in terms of stability of the complexes. This is due to a better overlapping in this orientation between both -systems (see Figure 2). The only exception to this behavior is complex 3@1b where the perpendicular orientation is more favored, due to the establishment of two additional C H···O interactions (see Figure 2). On the other hand, complexes involving the pentafluorinated nitrobenzene ring (3@2a to 5@2b) show larger binding energy values, as anticipated by the MEPS analysis. In this series the perpendicular orientation (denoted as b) is the most favored disposition, due to the possibility to establish additional C H···O, C H··· and also C H···F interactions with the fluorine atoms attached to the arene moiety, as confirmed by the AIM analysis (see below). For both arenes, the most favorable complexes involve the but2-yne (complexes 5@1a, 5@1b, 5@2a and 5@2b), which presents a higher electron donor ability due to the presence of the two methyl groups and also the possibility to establish ancillary interactions (see Figure 2). In addition, the energies involving double/triple bond groups show small differences while flipping from a parallel to a perpendicular disposition, indicating that the interactions is not sensitive to the relative orientation of the double/triple bond for both rings. The energetic values obtained for all complexes are similar to those obtained for other –hole complexes involving neutral counterparts [33]. From the inspection of the distance values it can be observed that the nitrogen atom of the NO2 group presents shorter distances to the -system than the carbon atom of the ring, as expected, due to the presence of the –hole closer to the nitrogen atom as revealed by the MEPS analysis (see Figure 1). 3.3. NBO analysis
Figure 1. MEPS analysis of compounds 1 (left) and 2 (right). Energy values are in kcal/mol.
In order to study if orbital contributions are important to explain the pnicogen interactions described above, we have performed Natural Bond Orbital (NBO) calculations focusing our attention on the second order perturbation analysis [34], since it is very useful to study donor acceptor interactions [34]. The results are summarized in Table 2. As it can be observed, the orbital contribution is small in all complexes (≈10% of the total interaction energy). For
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Figure 2. BP86-D3/def2-TZVP optimized geometries for complexes 3@1a to 5@2b. Additional C H···O, C H···F and C H··· interactions are highlighted in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
complexes involving 3 the major stabilization source comes from the donation from the bonding orbital () of the double C C bond to the antibonding orbital (*) of the N O bond. In addition, complexes involving the perpendicular arrangement (denoted as b) show a lone pair donation from the oxygen atom of the NO2 moiety to the antibonding orbital (*) of the ethene molecule. For complexes involving 4 the donation from the bonding orbital () of the triple C C bond to the antibonding orbital (*) of the N O bond is the most relevant orbital contribution. The only exception is complex 4@2a where the donation from the orbital of the ethyne to the antibonding orbital (*) of the C C bond of the arene moiety is the most remarkable contribution. 3.4. SAPT analysis In Table 3 we summarize the DF-DFT-SAPT energy values relative to the –hole complexes. The total SAPT interaction energies
Table 2 Donor and acceptor NBOs with indication of the second-order interaction energy E(2) and type of interaction for NO2 ··· complexes. Complexa
Donorb
3@1a 3@1b
BD(2) C BD(2) C LP O BD(3) C BD(3) C BD(2) C BD(2) C LP O BD(3) C BD(3) C
4@1a 4@1b 3@2a 3@2b 4@2a 4@2b
Acceptorb C C C C C C C C
BD(2)* N BD(2)* N BD(1)* C BD(2)* N BD(2)* N BD(2)* N BD(2)* N BD(1)* C BD(2)* C BD(2)* N
O O H O O O O H C O
Eb kcal/mol
Type
0.22 0.120.12
→ * → * n → * → * → * → * → * n → * → * → *
0.07 0.06 0.19 0.080.12 0.12 0.10
a a and b stand for parallel and perpendicular orientations (see Figure 1). 1 and 2 are nitrobenzene and pentafluoronitrobenzene, respectively. 3 and 4 for ethene and ethyne, respectively. b BD(2) and BD(3) stand for the type orbitals of double and triple bonds respectively. BD*(2) stands for the type anti-bonding orbital of a double bond.
for these complexes are similar to those obtained using the BP86D3/def2TZVP level of theory (see Table 1) giving reliability to the partition method. The energetic contributions (Table 3) indicate that the dispersion term (Edisp ) is the most important for the stability of the –hole··· interactions, ranging from −8.0 to −3.5 kcal/mol. Moreover, the electrostatic term (Eee ) also contributes to the stabilization of all complexes, in agreement with previous results reported for –hole complexes involving nitro compounds [27]. Regarding this point, the electrostatic term plays a major role in the pentafluorinated ring complexes, in agreement with the MEPS analysis discussed above. Interestingly, complexes involving the but-2-yne moiety (5@1a, 5@1b, 5@2a, 5@2b) present the highest values for Eee and Edisp components, this is due on the one hand to the higher -basicity of the C C triple bond, but also due to the higher polarizability of the but-2-yne moiety compared to ethene and ethyne, which certainly influences the dispersion component. Finally, the Eind term presents higher values for complexes involving the pentafluorinated moiety than for those where the nitrobenzene moiety is involved.
Table 3 SAPT interaction energies and their partitioning into the electrostatic, induction, dispersion and exchange contributions (Etotal , Eee , Eind , Edisp , Eexc , respectively, kcal/mol) at the RI-DFT/def2TZVP level of theory using the DF-DFT-SAPT approach. Complex
Eee
Eex
Eind
Edisp
Etotal
3@1a 3@1b 4@1a 4@1b 5@1a 5@1b 3@2a 3@2b 4@2a 4@2b 5@2a 5@2b
−3.1 −2.9 −2.9 −2.4 −4.8 −4.7 −3.6 −3.8 −2.9 −3.6 −4.9 −6.0
6.0 5.5 5.4 4.8 9.8 9.8 6.2 6.4 4.7 5.4 9.7 10.4
−0.1 −0.1 −0.2 −0.1 −0.4 −0.3 −0.3 −0.3 −0.3 −0.3 −0.6 −0.6
−4.3 −4.1 −3.9 −3.5 −7.7 −7.5 −4.6 −4.7 −3.9 −4.0 −7.9 −7.8
−1.5 −1.6 −1.6 −1.2 −3.0 −2.6 −2.4 −2.5 −2.4 −2.5 −3.7 −4.0
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Figure 3. Distribution of critical points and bond paths in complexes 3@1a, 3@1b, 5@2a and 5@2b. Bond, ring and cage critical points are represented by red, yellow and green spheres, respectively. The bond paths connecting bond critical points are also represented. In addition the value of the density at the bond critical point ( × 100) is given. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.5. AIM analysis We have also used the Bader’s theory of ‘atoms in molecules’ to characterize the NO2 ··· interactions and the results are shown in Figure 3. In complex 3@1a (Figure 3, top left) the interaction is characterized by the presence of two bond critical points, which connect both C atoms of the CC bond to the C and N atoms of the C N bond. The interaction is further characterized by the presence of a ring critical point as a consequence of the formation of the supramolecular four-membered ring. The NO2 ··· interaction in complex 3@1b (Figure 3, top right) is characterized by a bond critical point that connects the carbon atom of the C NO2 bond with the bond critical point of the C C double bond of ethene. In addition
each C H···O hydrogen bonds is characterized by the presence of a bond critical point. For complexes 5@2a (Figure 3, bottom left) and 5@2b (Figure 3, bottom right) the interaction is characterized by the presence of a bond critical point that connects the carbon atom of the C N bond to one carbon atom of the triple bond in 5@2a. In addition, several C H···O interactions are observed as well as CH– interactions involving the carbon atoms of the pentafluorinated ring. Finally in 5@2b, the interaction is characterized by the presence of one bond critical point that connects the carbon atom of the C N bond with the triple bond critical point, also additional C H···O and C H···F interactions are observed each one characterized by the presence of a bond critical point connecting the H atom with the heteroatom.
Figure 4. Some X-ray structures retrieved from the CSD. Distances in Å. The CSD codes are indicated.
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3.6. CSD examples
References
Finally, we have explored the CSD in order to find some experimental support to the theoretical calculations described above. Interestingly we have found 26 crystal structures where a double bond interacts with the –hole of the nitro group. Three of them are highlighted in Figure 4. IMOMIG and CIKBEE are representative examples of the two orientations we have used in the study. In the IMOMIG structure the C C centroid is located above ˚ Remarkably, the the nitrogen atom of the nitro group at 3.26 A. CIKBEE structure presents self-assembled dimers in the solid state where two NO2 ··· interactions are present. The CAWMUJ structure also forms a self-assembled dimer in the solid state exhibiting a double NO2 ··· interaction; in this case a triple bond instead of a double bond is the electron donor. The distances observed in the solid state are in good agreement with the theoretical ones.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21]
The NO2 ··· interaction with double and triple bonds is modest but energetically favorable, ranging from −6 to −2 kcal/mol. The parallel orientation of the isolated -system is generally more stable than the perpendicular disposition in nitrobenzene complexes, while the opposite occurs for the pentafluorinated compounds, due to the establishment of ancillary interactions by the electron donor moiety. The SAPT (Symmetry Adapted Perturbation Theory) analysis confirm the prominent role of the dispersion component (Edisp ) to the total binding energy, however the electrostatic term (Edisp ) also presents a remarkable contribution to the overall stabilization of the complexes due to the donor–acceptor nature of the interaction. The orbital contribution, which has been analyzed using the NBO method, is modest and comes from the donation from the bonding orbital () of the double/triple C C bond to the antibonding orbital (*) of the N O bond. The AIM analysis confirms the ability of the isolated -systems to act as electron donor in –hole··· complexes. Finally, some examples in the CSD confirm this possibility and highlight the impact of the –hole interactions in the crystal packing phenomena. Acknowledgments We thank CONSOLIDER-Ingenio 2010 (CSD2010-0065) and the MICINN of Spain (project CTQ2014-57393-C2-1-P, FEDER funds) for financial support. We thank the CTI for computational facilities.
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