Substituent Effects in Ion− π Interactions: Fine-Tuning via the Ethynyl

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Substituent Effects in Ion-π Interactions: Fine-Tuning via the Ethynyl Group Xavier Lucas, Antonio Frontera,* David Quin˜onero, and Pere M. Deya` Departament de Quı´mica, UniVersitat de les Illes Balears, E-07122 Palma de Mallorca, Spain ReceiVed: September 16, 2009; ReVised Manuscript ReceiVed: NoVember 20, 2009

In this work, we report a high level theoretical study (RI-MP2(full)/aug-cc-pVDZ) that deals with the effect of electron-withdrawing substituents on cation-π and anion-π interactions in the absence/presence of triple bonds between the substituent and the aromatic ring. The ethynyl group is able to finely tune the interaction energy of the complexes. Interestingly, for the cation-π complexes, it reduces the effect of the electron withdrawing groups (EWG), improving the interaction. For anion-π complexes, it boosts the effect of the EWG, improving the interaction as well. This dual behavior has been studied by examining the geometric and energetic features of the complexes, “atoms-in-molecules” analysis and charge transfer effects. Introduction A spectacular development in supramolecular chemistry has been observed in terms of potential applications and in its relevance to analogous biological systems. A great variety of noncovalent forces regulate the formation and function of supramolecular complexes.1 It is really important to understand and quantify these intermolecular interactions to be able to succeed in the rational design of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents. In particular, interactions involving aromatic rings are very important in supramolecular chemistry.2 Aromatic rings can participate in several noncovalent interactions, either via the π-cloud (C-H/π,3 cation-π,4 anion-π,5 lp/π6. . .) or using the hydrogen atoms to participate in unconventional hydrogen bonds. In the case of heteroaromatic rings, strong hydrogen bonds can be established with hydrogen bond donors. The π-basicity/acidity of aromatic rings can be modulated using substituents. The π-electron rich benzene can be turned into electron poor by substituting hydrogen atoms by electron withdrawing groups (EWG), for instance fluorine atoms. We have recently reported a high level theoretical study in which we have analyzed the effect of the ethynyl group on the ion-π binding affinity of the benzene ring.7 The ethynyl group can be considered as a modest EWG.8–11 Interestingly, the ethynyl group has a dual effect on the ion-π binding ability of benzene. As expected, the anion-π complexes of 1,3,5-triethynylbenzene with anions are more favorable than those of benzene. In addition, the interaction energies of the complexes of 1,3,5triethynylbenzene with cations are almost unaffected compared to benzene. In this work, we report a high level ab initio study in which we analyze the effect of the presence of one or two triple bonds between the substituent and the aromatic ring upon the π-binding affinity of the arene toward cations and anions. We have computed the substituted benzenes shown in Figure 1 and their complexes with anions and cations. Very interesting results have been found, since the presence of the triple bonds reduces the electron withdrawing effect of the substituents, favoring the interaction with cations and, at the same time, it boosts the electron withdrawing effect of the substituents for * To whom correspondence should be addressed. E-mail: toni.frontera@ uib.es.

Figure 1. Compounds 1-9 studied in this work.

the anion-π complexes. This unexpected behavior of the ethynyl spacer is studied and analyzed using high level ab initio calculations and charge transfer effects. Moreover, we use the Bader’s theory of “atoms in molecules”, which provides an unambiguous definition of chemical bonding,12 to describe the interactions. The AIM theory has been successfully used to characterize and understand a great variety of interactions including the ones studied herein. Theoretical Methods The geometry of the complexes included in this study was fully optimized at the RI-MP2(full)/aug-cc-pVDZ level of theory within the program TURBOMOLE version 6.0.13 The RI-MP2 method14,15 applied to the study of cation-π and anion-π interactions is considerably faster than the MP2 and the interaction energies and equilibrium distances are almost identical for both methods.16,17 The binding energy was calculated at the same level with and without correction for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise technique.18 The optimization of the molecular geometries has been performed imposing the C6V symmetry group for benzene complexes and C3V for the rest of complexes. Other possible conformations of complexes have not been considered because the ultimate aim of this study is to analyze the π-binding properties of substituted benzenes toward cations and anions. The “atoms-in-molecules” analysis19 has been performed by means of the AIM2000 version 2.0 program20 using the MP2(full)/aug-cc-pVDZ wave functions. Results and Discussion The geometric and energetic results obtained for complexes 10-45 (see Figure 2) are summarized in Table 1. Since there

10.1021/jp9089672  2010 American Chemical Society Published on Web 12/31/2009

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Figure 2. Ion-π complexes 10-45.

TABLE 1: Binding Energies without and with the BSSE Correction (E and EBSSE, kcal/mol, respectively) and Equilibrium Distances (R, Å) at the RI-MP2(full)/ aug-cc-pVDZ Level of Theory for Complexes 10-45a complex n 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2

X H H H H F F F F CN CN CN CN H H H H F F F F CN CN CN CN H H H H F F F F CN CN CN CN

Y +

Li Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FCl-

E

EBSSE

R

QMK

QNBO

-38.9 -25.4 1.0 0.2 -23.1 -13.5 -9.8 -7.8 -9.9 -3.0 -27.3 -22.3 -38.7 -26.9 -7.5 -6.1 -37.5 -25.2 -9.5 -8.2 -19.5 -9.5 -29.8 -25.4 -41.2 -28.7 -14.5 -11.9 -41.3 -28.6 -14.0 -11.6 -26.1 -14.8 -31.2 -26.8

-35.2 -22.1 1.9 1.1 -19.2 -10.0 -8.1 -6.1 -4.4 2.2 -24.8 -19.5 -32.1 -19.9 -5.4 -3.6 -30.7 -18.2 -7.4 -5.7 -11.8 -1.9 -27.0 -22.0 -33.0 -20.4 -12.0 -8.8 -33.1 -20.2 -11.4 -8.5 -17.6 -6.6 -28.3 -23.3

1.844 2.422 2.964 3.572 1.961 2.531 2.665 3.214 2.007 2.573 2.466 3.020 1.887 2.373 2.610 3.169 1.897 2.373 2.599 3.166 1.957 2.445 2.449 2.997 1.902 2.360 2.525 3.088 1.895 2.352 2.532 3.085 1.931 2.405 2.438 2.983

0.631 0.758 -0.846 -0.898 0.684 0.793 -0.821 -0.874 0.670 0.780 -0.766 -0.819 0.629 0.735 -0.792 -0.848 0.632 0.729 -0.779 -0.836 0.646 0.748 -0.749 -0.802 0.632 0.735 -0.789 -0.846 0.619 0.722 -0.786 -0.839 0.639 0.744 -0.753 -0.804

0.963 0.984 -0.997 -0.996 0.968 0.987 -0.989 -0.989 0.977 0.990 -0.978 -0.977 0.973 0.988 -0.984 -0.983 0.970 0.986 -0.984 -0.985 0.976 0.989 -0.973 -0.975 0.974 0.987 -0.980 -0.980 0.973 0.987 -0.979 -0.980 0.975 0.988 -0.972 -0.973

a

Merz-Kollman and NBO derived charges (QMK and QMK, respectively) are also summarized.

is a large number of complexes to discuss, we have divided the results into three parts, depending on the value of n (number of triple bonds). Complexes 10-21 (n ) 0). This set of complexes consists in cation-π and anion-π complexes of benzene (1, X ) H), 1,3,5-trifluorobenzene (2, X ) F) and 1,3,5-tricyanobenzene (3, X ) CN). As expected, the interaction energies of benzene with

cations (10 and 11) are large and negative. In addition, the interaction energies with anions are small and positive, (1.9 and 1.1 kcal/mol for 12 and 13, respectively). Previous studies demonstrate that the unfavorable electrostatic contribution to the total interaction energy of anion-π complexes of benzene is almost completely compensated by the ion-induced polarization term.21 The interaction energies obtained for all complexes (14-17) of trifluorobenzene (2) are negative, indicating that the almost negligible quadrupole moment of 2 (Qzz ) 0.57 Buckinghams)22 allows it to interact favorably with ions regardless of its sign, in agreement with previous results.21 The complexation energies of 1,3,5-tricyanobenzene (3) complexes are easily explained taking into account the strong electronwithdrawing effect of the nitrile group. The interaction energies of the anion-π complexes 20-21 are large and negative, while the cation-π interaction energies are either modest (-4.4 kcal/ mol for Li+, complex 18) or positive (2.2 kcal/mol for Na+, complex 19). The equilibrium distances agree with the energetic results. For the cation-π complexes, the equilibrium distances increase and the interaction energies decrease on going from 1 to 3. The contrary is observed for the anion-π complexes. Complexes 22-33 (n ) 1). The complexes of 1,3,5triethynylbenzene (4) present interesting features that have been recently discussed in the literature.7 That is, the presence of three ethynyl groups attached to benzene clearly favors the interaction with anions compared to benzene (complexes 24 and 25), confirming the electron-withdrawing substituent effect of the ethynyl group. A counterintuitive finding is observed comparing the cation-π complexes 22 and 23 with 10 and 11 (benzene complexes). That is, the interaction energies are very similar. This dual behavior of the ethynyl group, which is able to proceed as an EWG favoring the anion-π interaction and, at the same time, it does not affect the cation-π binding properties of the aromatic ring, has been explained by means of polarization effects.7 The energetic and geometric features of the ion-π complexes of compounds 5 and 6 give hints to understand how the presence of a triple bond between the substituent and the aromatic ring affects to the electronwithdrawing power of F and CN. If the presence of the triple bond modifies the properties of the substituents, then it could be used to modulate the binding energies of the complexes. From the inspection of the results obtained for complexes 26-33, several interesting points arise. First, for complexes 26 and 27 (X ) F, n ) 1), the triple bond diminishes the electron withdrawing effect of the fluorine and the computed interaction energies of the cation-π complexes are more favorable than 14 and 15 (X ) F, n ) 0) and almost equal to complexes 22 and 23 (X ) H, n ) 1). Curiously, for the anion-π complexes 28 and 29 (X ) F, n ) 1) the attenuation of the electron withdrawing effect of the substituent is not observed. The interaction energies are more negative than complexes 24 and 25 (X ) H, n ) 1) and similar to 16 and 17 (X ) F, n ) 0). Second, for the cation-π complexes 30 and 31 (X ) CN, n ) 1) the attenuation of the electron withdrawing effect of the substituent is not as relevant as for fluoride. The binding energies of the cation-π complexes are very modest similarly to complexes 18 and 19 (X ) CN, n ) 0), however the sodium complex changes from positive (19) to negative (31) interaction energy. Intriguingly, for the anion-π complexes 32 and 33 (X ) CN, n ) 1) instead of an attenuation of the electron withdrawing effect of the substituent, a boost effect is observed. That is, the interaction energies of 32 and 33 are more favorable than complexes 20 and 21, respectively, meaning that the triple bond acts enhancing the electron-withdrawing power of the

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nitrile group. A likely explanation is that this enhancing partly comes from electrostatic effects. The insertion of the ehtynyl group increases the distance between the incoming anion and the X substituent (X ) F, CN), thus reducing the electrostatic repulsion. Similar findings have been recently reported in the literature.23 Complexes 34-45 (n ) 2). One advantage of the ethynyl group is that its attenuation/boost effect on the substituent can be further modulated by incrementing the number of triple bonds between the substituent and the aromatic ring. We have explored this issue by computing the energetic features of complexes 34-45. Striking results have been obtained. For instance, the cation-π complexes 34 and 35 (X ) H, n ) 2) have slightly more favorable interaction energies than 22 and 23 (X ) H, n ) 1). Since the ethynyl is an EWG, this result can be only explained by means of ion-induced polarization effects. The EWG effect of the ethynyl is manifested in the anion-π complexes 36 and 37 (X ) H, n ) 2) which are more favorable than 24 and 25 (X ) H, n ) 1) and even more favorable than 1,3,5-trifluorobenzene complexes 16 and 17 (X ) F, n ) 0). For the cation-π complexes 38-39 (X ) F, n ) 2) an almost complete attenuation of the electron-withdrawing effect of the fluorine is observed since the interaction energies are very similar to benzene complexes 10 and 11 (X ) H, n ) 0). However, they are more favorable than complexes 26 and 27 (X ) F, n ) 1), indicating that the second triple bond favors the cation-π interaction albeit it is an EWG. A likely explanation is that it increases the molecular polarizability of the arene and, therefore, larger induction effects are operative, favoring the interaction. Interesting points emerge by analyzing the energetic and geometric features of complexes 42-45. For the cation-π complexes 42-43, the attenuation of the substituent effect provoked by the presence of the two triple bonds is higher than in complexes 30 and 31 (X ) CN, n ) 1). However, the substituent effect is partially transmitted through the triple bonds since the binding energy of cation-π complexes 42 and 43 (X ) CN, n ) 2) is modest compared to complexes 34 and 35. This finding also reflects the weakening of the stabilizing electrostatic interaction between the incoming cation and the excess of negative charge supported by either fluorine or cyano groups upon insertion of the ethynyl substituent, derived from the concomitant increase in the cation-X distance (X ) F, CN). In contrast, for the anion-π complexes the attenuation of the substituent is not observed at all. Moreover, it is enhanced by the presence of the two triple bonds, since the binding energy of the anion-π complexes 44 and 45 (X ) CN, n ) 2) is more negative than complexes 20 and 21 (X ) CN, n ) 0), where the nitrile groups are directly attached to benzene. In Table 1 we have also gathered the Merz-Kollman (MK) and NBO charges of the ion in complexes 10-45, in order to investigate if there is some influence of the charge transfer on the behavior of the complexes. We have represented the binding energy of the complexes versus the charge of the anion (see Figure 3). The charge transfer computed using the Merz-Kollman method to derive charges (QMK values) is greater in cation-π than in anion-π complexes because the equilibrium distances are shorter for cations. In addition, the charge transfer is almost independent of the binding energy of the complex. This indicates that the differences in the interaction energies of the complexes provoked by both substituent effects and the presence of triple bonds are not due to charge transfer effects. Fine Tuning. It is clear from the aforementioned results that the presence of a triple bond located between the substituent and the aromatic ring modifies the properties of the substituent,

Lucas et al.

Figure 3. Plot of the interaction energy (EBSSE, kcal/mol) versus the charge (e) of the ion QNBO (bottom) and QMK (top) in complexes 10-45 is shown.

TABLE 2: Relative Binding Energies with the BSSE Correction (kcal/mol) of the Cation-π Complexes at the RI-MP2(full)/aug-cc-pVDZ Level of Theory X +

+ a

Y ) Li (Y ) Na ) n

0 1 2 a

H

F

CN

0.0 (0.0) 3.1 (2.3) 2.2 (1.8)

16.0 (12.1) 4.5 (4.0) 2.1 (1.9)

30.8 (24.3) 23.4 (20.3) 17.6 (15.6)

Values in parentheses correspond to sodium complexes.

TABLE 3: Relative Binding Energies with the BSSE Correction (kcal/mol) of the Anion-π Complexes at the RI-MP2(full)/aug-cc-pVDZ Level of Theory X -

- a

Y ) F (Y ) Cl ) n

0 1 2 a

H

F

CN

0.0 (0.0) -10.0 (-7.2) -26.7 (-20.6) -7.3 (-4.7) -9.3 (-6.8) -29.0 (-23.1) -13.9 (-9.9) -13.3 (-9.6) -30.2 (-24.4)

Values in parentheses correspond to chloride complexes.

in some cases counterintuitively. Anyhow, the consequence of this modification is that we can adjust the interaction using triple bonds. Furthermore, by increasing the number of triple bonds, additional adjustment of the interaction can be made. To illustrate this issue, we have summarized in Tables 2 and 3 the relative binding energies (referred to benzene) of the cation-π and anion-π complexes, respectively. It can be observed (Table 2) that the interaction energy of lithium with benzene becomes 16.0 kcal/mol more positive by the effect of introducing three fluorine atoms (n ) 0, X ) F), however this value can be moved to 17.6 kcal/mol using two triple bonds and nitrile as substituent (n ) 2, X ) CN), or moved to 23.4 kcal/mol using one triple bond and nitrile as substituent (n ) 1, X ) CN), see Table 2. Similarly, the interaction energy of the fluoride with benzene becomes 26.7 kcal/mol more favorable by introducing three nitrile groups as substituents (see Table 3). This value can be finely adjusted to 29.0 or 30.2 kcal/mol by introducing one or

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Figure 4. Distribution of critical points in some representative complexes.

rel Figure 5. Plot of the charge density at the (3,+3) critical point (∆F, a.u.) versus the variation of the relative interaction energy (EBSSE , kcal/mol) in complexes 10-45 is shown.

two triple bonds between the arene and the CN group, respectively. Therefore, the combination of the number of triple bonds with the substituent gives rise to a great variety of energies which could be used to adjust the desired energy. Obviously, this diversity of energies can be infinitely augmented by combining different numbers and types of substituents in the same ring and varying the number of triple bonds. AIM Analysis. Topological analysis of the charge density F(r) distribution and properties of critical points (CP) were determined for complexes 10-45 using the Bader’s theory of “atoms in molecules”, which provides an unambiguous definition of chemical bonding,24 using the MP2(full)/aug-cc-pVDZ// RI-MP2(full)/aug-cc-pVDZ wave function. The AIM theory has been successfully used to characterize ion-π interactions.25 For all anion-π complexes and for the cation-π complexes of benzene, the exploration of the CPs revealed the presence of six bond and six ring CPs that connect the ion with the six carbon atoms and the middle of the six CC bonds of the ring, respectively. For the rest of cation-π complexes, the exploration of the CPs revealed the presence of three bond and three ring CPs that connect the ion with the six carbon atoms of the ring for cations. In addition, for all complexes, the interaction is further described by a cage CP that connects the ion with the center of the ring. In Figure 4, we represent the distribution of CPs that is generated upon complexation of the ion in several representative complexes. The value of the charge density (F) computed at the cage CP has been related to the strength of the interaction and can be used as a measure of the bond order.26 Therefore, its variation (∆F) on going from the benzene to the trisubstituted complexes is a good measure of the strengthening or weakening of the ion-π interactions. These values are

summarized in Table 4. It can be observed that the cation-π complexes have negative values of ∆F, indicating a weakening of the interaction with respect to benzene. The contrary is found for the anion-π complexes, i.e., positive values of ∆F and, consequently, a strengthening of the anion-π interaction, in agreement with the energetic and geometric results. It should be mentioned that there are some exceptions that correspond to some sodium complexes. In these cases, a strengthening of the interaction is predicted by the AIM results and a positive relative rel ) with respect to the corresponding benzene energy (EBSSE complex is obtained. However, for the majority of sodium complexes this can be fixed if BSSE uncorrected energies are used (values in parentheses of Table 4). This may indicate that the BSSE values are overestimated in sodium complexes. We have found a strong relationship (R ) 0.949) between the Erel BSSE values and the ∆F values (see Figure 5), confirming that the variation of the charge density at the cage CP is a very good measure of the reinforcement/weakening of the ion-π interaction. It should be emphasized the importance of this relationship, since in the same representation two different interactions (cation-π and anion-π) and four different ions (Li+, Na+, F-, Cl-) are included. Conclusions The results reported in this work show that the ethynyl substituent can be used to modulate the ion-binding affinity of substituted benzenes. Unexpectedly, the ethynyl is able to block the electron-withdrawing effect of the groups in the cation-π complexes and, conversely, is able to boost the electronwithdrawing effect of the substituents in the anion-π com-

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TABLE 4: Relative Interaction Energies with the BSSE rel Correction (EBSSE , kcal/mol) and the Values of the Electron Charge Density at the (3,+3) Critical Point (G, u.a.) and Their Variation with Respect to Benzene Complexes (∆G, u.a.) at the RI-MP2(full)/aug-cc-pVDZ Level of Theory for Complexes 10-45 complex

n

X

Y

rel a EBSSE

102F

103∆F

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2

H H H H F F F F CN CN CN CN H H H H F F F F CN CN CN CN H H H H F F F F CN CN CN CN

Li+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FClLi+ Na+ FCl-

0.0 0.0 0.0 0.0 16.0 12.1 (11.9) -10.0 -7.2 30.8 24.3 (22.4) -26.7 -20.6 3.1 2.3 (-1.6) -7.3 -4.7 4.5 4.0 (0.1) -9.3 -6.8 23.4 20.3 (15.9) -29.0 -23.1 2.2 1.8 (-3.3) -13.9 -9.9 2.1 1.9 (-3.2) -13.3 -9.6 17.6 15.6 (10.6) -30.2 -24.4

1.314 0.819 0.526 0.392 1.176 0.737 0.813 0.641 1.070 0.666 1.027 0.807 1.264 0.902 0.853 0.668 1.274 0.899 0.868 0.672 1.158 0.816 1.060 0.841 1.252 0.926 0.953 0.743 1.263 0.935 0.954 0.752 1.211 0.872 1.089 0.865

0.00 0.00 0.00 0.00 -1.38 -0.82 +2.87 +2.49 -2.44 -1.53 +5.01 +4.15 -0.50 +0.82 +3.27 +2.76 -0.40 +0.80 +3.42 +2.80 -1.56 -0.03 +5.34 +4.50 -0.62 +1.07 +4.27 +3.51 -0.51 +1.16 +4.28 +3.60 -1.03 +0.53 +5.63 +4.74

a

Values in parentheses correspond to BSSE uncorrected values.

plexes. Since trialkynyl benzenes are widely used as scaffolds for the construction of receptors, this special behavior could be also used to tune their binding properties. Acknowledgment. We thank the DGICYT of Spain (projects CTQ2008-00841/BQU) for financial support. We thank the CESCA for computational facilities. Supporting Information Available: The Cartesian coordinates of RI-MP2(full)/aug-cc-pVDZ optimized structures 1-45.

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