DOI: 10.1021/cg100111b
Nitrate Encapsulation within the Cavity of Polyazapyridinophane. Considerations on Nitrate-Pyridine Interactions
2010, Vol. 10 3418–3423
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Laura Valencia,*,† Rufina Bastida‡ Enrique Garcı´ a-Espa~ na,*,§ J. Vicente de Juli an-Ortiz^ Jose M. Llinares, Alejandro Macı´ as,‡ and Paulo Perez Lourido† †
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Departamento de Quı´mica Inorg anica, Facultad de Quı´mica, Universidad de Vigo, 36310 Vigo, anica, Universidad de Santiago de Compostela, Pontevedra, Spain, ‡Departamento de Quı´mica Inorg 15782 Santiago de Compostela, La Coru~ na, Spain, §Instituto de Ciencia Molecular, Departamento de Quı´mica Inorg anica, Universidad de Valencia, Apartado de Correos 22085, 46071 Valencia, ecnica de Valencia, 46022 Spain, ^Instituto de Tecnologı´a Quı´mica,. CSIC-Universidad Polit anica, Fundaci o Valencia, Spain, and Instituto de Ciencia Molecular, Departamento de Quı´mica Org General de la Universidad de Valencia (FGUV ), Apartado de Correos 22085, 46071 Valencia, Spain Received January 25, 2010; Revised Manuscript Received May 19, 2010
ABSTRACT: Interaction of nitrate anions with a cyclophane (L) containing two pyridine units connected to two diethylenetriamine bridges through methylene positions is reported both in pure water and in the solid state. The crystal structure of [H4L](NO3)4 shows that one of the nitrate anions resides in the macrocyclic cavity forming two sets of bifurcated hydrogen bonds with the four protonated amino groups of the macrocycle. This anion is symmetrically placed in the middle of the pyridine rings with distances between its nitrogen atom and the centroids of the ring of 3.58 A˚. Despite this location, calculations by theoretical analysis were carried out to confirm whether the stabilizing effect is due to anion-π interactions.
Scheme 1. Drawing of the Different Amine Macrocyclic Ligands
Introduction Anion coordination chemistry is a field of current research interest within supramolecular chemistry.1 Due to the important role that anionic species play in biological processes and to the increasing awareness of the need to remediate environmental hazards related to anionic species, many researchers have focused their work on this topic. Several noncovalent forces contribute to anion binding, charge-charge attraction with positively charged receptors and hydrogen-bonding interactions of different nature between the receptor and the anion being the strongest.2 Other forces contributing to anion binding are van der Waals and dispersion interactions, π-stacking, and solvophobic effects. In the past few years, a number of theoretical and experimental reports claim that interaction of anions with electron-poor aromatic rings is a new contribution to anion binding.3-14 A point that has provoked certain controversy is whether pyridine rings can contribute to anion-π interactions. In this respect, recent theoretical work at the MP2 level carried out by Qui~ nonero et al. has shown that, in the gas phase, positive contribution to binding of anion-pyridine interactions occurs when pyridine is bound to silver ions but not when it is in its neutral-charged form.15 On the other hand, although there are many crystal structures in which monovalent anions are at distances close enough of neutral pyridines to claim the occurrence of anion-π interactions, recent theoretical studies provide evidence for the absence of such interactions in the solid state.16 Taking into account several examples in the literature of nitrate anion recognition using macrocyclic frameworks,17 herewith we report on the interaction of the pyridinophane receptor L (Scheme 1) with nitrate anions. We provide speciation studies in pure water, and we report on the crystal structure of [H4L](NO3)4 in which one of the nitrate anions is engulfed within the cavity of the macrocycle via several binding forces. Because the distances between one of the nitrate anions in the structure and the aromatic ring were close enough to suggest the pubs.acs.org/crystal
Published on Web 06/22/2010
occurrence of anion-π positive contributions, we have carried out a theoretical analysis on this aspect. Experimental Section Materials and General Methods. The ligand L was synthesized following a previously described method.18,19 The nitrate complex of L was prepared by dissolving the free base in methanol and adjusting the pH to 3.0 by addition of concentrated HNO3. By slow concentration of the solution, crystals suitable for X-ray diffraction were obtained. Electromotive Force (emf) Measurements. The potentiometric titrations were carried out at 298.1 ( 0.1 K using NaClO4 (0.15 M) as supporting electrolyte. The experimental procedure (buret, potentiometer, cell, stirrer, microcomputer, etc.) has been fully described elsewhere.20 The acquisition of the emf data was performed with the computer program PASAT.21 The reference electrode was a Ag/AgCl electrode in saturated KCl solution. The glass electrode was calibrated as a hydrogen-ion concentration probe by titration of previously standardized amounts of HCl with CO2-free NaOH solutions, and the equivalent point was determined by the Gran’s method,22 which gives the standard potential, E0 , and the ionic product of water (pKw = 13.73(1)). The computer program HYPERQUAD was used to calculate the protonation and stability constants.23 The pH range investigated was 2.5-11.0. The different r 2010 American Chemical Society
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Figure 1. (A) Drawing of the [H4L](NO3)](NO3)3 salt showing the three types of nitrate anions and (B) space filling drawing of the [H4L](NO3)(A)]3þ subunit. titration curves for each system (at least two) were treated either as a single set or as separated curves without significant variations in the values of the stability constants. Finally, the sets of data were merged together and treated simultaneously to give the final stability constants. In order to calculate the constants for the interaction of L with nitrate, the protonation constants of the receptor were first calculated in 0.15 M Et4NClO4 at 298.1 K giving the values log KHL/H 3 L=9.35(2), log KH2L/HL 3 H=8.35(1), log KH3L/H2L 3 H=7.42(1), log KH4L/H3L 3 H=6.17(2), and log KH5L/H4L 3 H=2.78(1). Then, titrations were carried out in 0.15 M Et4NNO3 using the protonation constants determined in 0.15 M NEt4ClO4 as fixed parameters for determining the constants for the formed NO3--L adducts. Crystallography. X-ray measurements were made on a Bruker SMART CCD area detector. All data were corrected for Lorentz and polarization effects. An empirical absorption correction was also applied for all the crystal structures obtained.24 Complex scattering factors were taken from the program package SHELXTL.25 The structure was solved by direct methods, which revealed the position of all non-hydrogen atoms. All the structures were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non-hydrogen atoms.26 The hydrogen atoms were located in their calculated positions and refined using a riding model. Computational Details. In order to have an estimate of the anion-π energy, DFT-restricted B3LYP calculations with the 6-31þþG** basis set and ab initio restricted MP2 calculations with the 6-311þþG** basis set were performed. Previously, energy minimizations were performed at the B3LYP/6-31þþG* level. The basis set superposition error (BSSE) was considered by the Boys-Bernardi counterpoise method (CP).27 All the DFT and ab initio calculations were run with PC-Gamess.28 Force-field simulations, using the universal force field (UFF 1.02)29 on periodic structures, were performed with Cerius.30 Symmetry of the unit cells was redefined to P1. Three successive minimizations were performed. First, all the molecules were set to rigid bodies and the cell parameters were maintained constant. In the second step, the internal coordinates of the molecules were released. Finally, the cell parameters were also unconstrained. Rigid structure alignments were performed by two fit methods: the steric field and the moment of inertia methods with the Materials Studio package.31
Results and Discussion Speciation Studies. As commented in the Experimental Section, in order to study the speciation of the system L-NO3in water, the protonation constants of L were first determined by potentiometric titrations using 0.15 M NEt4ClO4, as an ionic
strength. ClO4- anions occupy the lowest end of the Hofmeister series,32 and thereby it can be assumed that the interaction between the protonated forms of L and ClO4- should be negligible in comparison to other anions. The same type of measurements were performed using NEt4NO3 (0.15 M) as background electrolyte. Then, the protonation constants obtained in NaClO4 were introduced as fixed parameters in the refinement to obtain the constants for the L-NO3- system. Under these conditions, we could observe the formation of three species of [(HxL)(NO3)](x-1)þ stoichiometry with x=4, 5, and 6 and binding constants of 1.2(1), 1.6(1), and 1.7(1) logarithmic units, respectively. Although the values of the constants are not high, one has to take into account the monocyclic architecture of the ligand and that these values refer to pure water. X-ray Crystallographic Study. The most interesting results are provided by the crystal structure of [H4L](NO3)4. The details of the X-ray crystal structure solution and refinement are given in the Supporting Information (Table S1). The tetracationic macrocycle adopts a boat-shaped conformation with a dihedral angle between the pyridine rings of 51.6. The four protonated amine groups are those closest to the pyridine rings. There are three different kinds of nitrate anions labeled as A, B, and C in Figure 1A. Nitrate A is situated between the pyridine rings of the macrocycle forming a dihedral angle of 25.8 with them. The overall shape of the [(H4L)(NO3-(A))]3þ unit resembles somehow that of a “Mexican taco” (Figure 1B). Symmetry-related O5N oxygen atoms of nitrate A form a 4-fold hydrogen-bonding network with the protonated nitrogens of the macrocycle (N2-H2A 3 3 3 O5N=2.869(8) A˚, N4-H4A 3 3 3 O5N = 2.876(8) A˚; see Table S2 of the Supporting Information). Although, a somewhat similar conformation of the macrocyclic structure was reported a few years ago by Bowman-James et al.33 in a crystal structure of the nitrate salt of the hexaazadioxamacrocycle L1 (bisdien), there are a number of significant differences that have to be noted. First, the orientation of the nitrate anion with respect to the mean plane defined by the heteroatoms in both sides of the macrocycle is rotated by almost 90 (Figure 2). Second, while in the case of L1 the nitrate lies below the average plane defined by the amino nitrogens of the macrocycle (distance nitrate N atom-mean plane=0.55 A˚), in our case the nitrate is placed clearly above
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the same plane (distance from the nitrate N atom-plane= 1.96 A˚). It seems as if the nitrate had experienced a lifting movement toward the pyridine rings to take advantage of the pyridine-anion-pyridine π-π interactions. The distances between the centroids of the pyridine rings and the N of the nitrate anion (dC-N) are both 3.58 A˚. However, these two planes are tilted (Figures 1 and 2), and the atom-atom contacts are in some cases shorter than the dC-N distance. The nitrate salt of the similar fully protonated hexa-azamacrocycle ligand L2 shows a quite different conformation. The ligand crystallizes as a planar ellipsoid with the phenyl rings capping the long axis and canted from the plane of the macrocycle in a trans configuration.34 It is well established that full protonation of polyazamacrocycles leads to more flattened conformations so that the repulsion between positive ammonium groups is minimized.33 The other two types of external nitrate anions are placed outside the cavity. NO3-(B) interacts through hydrogen bonding with protonated amino groups N2 of only one [(H4L)(NO3-)]3þ unit [O6N 3 3 3 H2B-N2 2.965 A˚,
Figure 2. Drawing showing the different position and height of the nitrate anion in units [(H4L)(NO3)]3þ and [(H4L1)(NO3)3þ.
Valencia et al.
O7N 3 3 3 H2B-N2 3.087(4) A˚] (Figure 3A; Table S2 of Supporting Information). In its turn, NO3-(C) connects the different [(H4L4þ)(NO3-(A))] units forming a network of hydrogen bonds through protonated N4 amino groups [O3Nb 3 3 3 H4BN4 2.986(6) A˚, O1N 3 3 3 H4B-N4 3.115(6) A˚]. Analysis of intermolecular contacts also reveals π-π stacking interactions between pyridine rings of adjacent ligands (Figure 3B). In this case the pyridine rings are parallel having slipped angle of 15.7 and distance between centroids 3.49 A˚. Therefore, although the mere location of the nitrate anions with respect to the pyridine rings might suggest the occurrence of anion-π interactions, before establishing this point, further analysis should be carried out. Theoretical Calculations. The possibility of anion-π interactions between the pyridine rings and the nitrate in [H4L](NO3)4 was investigated by isolating the involved system through the following procedure: the macrocycle skeleton was erased leaving only one pyridine ring and the NO3-(A) located between the pyridine rings. The plane of NO3-(A) forms a 25.8 angle with the plane of the pyridine rings (py) (Figure 1). The rests joined to the pyridine were substituted by H. The positions of such hydrogen atoms were minimized at the rB3LYP/6-31þþG* level, while the positions of carbon, nitrogen, and oxygen atoms were frozen to avoid false final geometries. No solvent was modeled. The total energy of the complex, Epy-NO3-(A), was calculated by using direct SCF rB3LYP/6-31þþG**. In order to compare the results with an electron-deficient aromatic ring, another system was built with similar geometry in which the H atoms
Figure 3. (a) Drawing showing the hydrogen-bonded chains formed along the b-axis and (b) interchain packing through π-π stacking between pyridine rings.
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Figure 4. (A) [C5F5N]NO3- in the geometry of [H4L](NO3)4 crystal, (B) [C5F5N]NO3- in the geometry with the ring parallel to the nitrate, and (C) [C5H5N]2NO3- isolated from the [H4L](NO3)4 crystal. The two pyridine rings come from two different molecules in the crystal (see Figure 3B).
were substituted by F (Fpy) (see Figure 4A). The total energy of the complex EFpy-NO3-(A) was also calculated at the SCF rB3LYP/6-31þþG** level. The binding energies were calculated by the Boys-Bernardi CP method. In this method, all the basis funtions of the subsystems involved without their electrons and nuclei, “ghost atoms”, are added to the basis functions of each subsystem. This is because each subsystem will tend to use the basis functions of all the other subsystems to lower its energy in a complex. With this method, the energy of the pyridine or perfluoropyridine using the nitrate group as “ghost atoms” Epy-(NO3-(A)) or EFpy-(NO3-(A)) were calculated, as well as the energy of the nitrate using the pyridine (py) or perfluoropyridine (Fpy) as “ghost atoms” E(py)-NO3-(A) and E(Fpy)-NO3-(A), respectively. The complex binding energies can be calculated as E binding ¼ E py - NO3 - ðAÞ - E py - ðNO3 - ðAÞÞ - E ðpyÞ - NO3 - ðAÞ or Ebinding ¼ E Fpy - NO3 - ðAÞ - E Fpy - ðNO3 - ðAÞÞ - E ðFpyÞ - NO3 - ðAÞ To study the effect of the orientation, the py or Fpy rings (Figure 4B) were rotated with respect to an axis in the ring plane that passes through its geometric center until leaving py or Fpy parallel to the plane of NO3-(A). The distance between the centroids of the rings and nitrate anions was not varied. Then all the previous process was repeated with the new geometry to obtain the corresponding binding energies. The results of the calculations for py and Fpy, for the angular and parallel orientations, are shown in Table S3 in the Supporting Information. After the calculations, the pyridine-nitrate system in the relative orientations of the [H4L](NO3)4 crystal is unstable by þ4.9 kcal/mol. When pyridine (py) is substituted by perfluoropyridine (Fpy), the system is stabilized (-3.9 kcal/mol) due to the anion-π stabilization. It seems that pyridine is not electron-deficient enough to show an effective anion-π interaction in this geometry. To investigate whether this interaction is lacking due to geometrical factors, the systems were modified as pointed out above. Rotating the ring to obtain parallel face to face interaction stabilizes the binding energy about 3 kcal/mol in each case (Table S3, Supporting Information). However, this energy decrease due to the parallel geometry of the pyridinenitrate system did not give binding anion-π energy able to stabilize the system (þ1.8 kcal/mol). In the case of the Fpy-NO3- system with parallel relative orientation, the energy results are the more favorable (-7.2 kcal/mol).
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For the case of the py-NO3- system, the calculations were repeated by using a more complex method MP2/6311þþG**, to see whether the trends are conserved at a higher level of calculation. The results confirmed that the original geometry was not stable although, for the parallel geometry, binding stabilization was obtained. In all the former calculations, only an anion and an isolated ring were considered. However, the crystal structure shows pyridine rings belonging to different tetracationic macrocyclic units stacked pairwise (Figure 3B). To see whether this close interaction could cause electronic deficiency able to justify the anion-π interaction, calculations were run with the nitrate and two stacked pyridines from two different ligand molecules isolated from the crystal by the aforementioned procedure but retaining their relative coordinates (Figure 4C). Results are shown in the last row of Table S3, Supporting Information. The binding energy obtained is þ0.9 kcal/mol, only ca. 0.7 kcal/mol lower than the one obtained without considering ring stacking. Thus, it seems that this factor has not determinant influence in the electron deficiency of the pyridine ring. It was estimated that repeating the calculation with the B3LYP method to discard a possible binding interaction was not necessary because these DFT calculations give more unfavorable binding energies than MP2 for these systems. As said, these conclusions are taken on the basis of the results obtained at the MP2 level with the 6-311þþG** basis. All the binding energy values are near zero. It is possible that more complex methods give negative values for the binding energy, although it is unlikely that these values were significantly high. It can be concluded that this method cannot confirm that anion-π interaction plays an essential role in the stabilization of the position of the nitrate anion between the pyridine rings in the crystal structure of [H4L](NO3)4, although this interaction could have some importance in the preliminary stages of the crystal formation. In fact, it is curious that the MP2/ 311þþG** calculation for pyridine parallel to the nitrate plane does give a slight stabilization of -0.6 kcal/mol (B3LYP/ 31þþG** gave þ1.8 kcal/mol) suggesting certain electronic deficiency for the pyridine ring. To gain understanding of the different orientation of the nitrate groups in [H4L](NO3)4 and [H4L1](NO3)4 crystals, four systems were studied, displayed in Figure 5. The unit cells of [H4L](NO3)4 and [H4L1](NO3)4 crystals studied are shown in Figure 5, panels A and B, respectively. The crystal of [H4L1](NO3)4 was modified by substituting the ether groups by pyridine rings coplanar to the C-O-C angles, giving a [H4L](NO3)4 crystal having the spatial structure of [H4L1](NO3)4, which will be denoted as [H4L](NO3)4|L1 (Figure 5C). Conversely, the crystal of [H4L](NO3)4 was modified by substituting their pyridine rings by the ether groups, with the oxygen atom in the position of the former pyridine nitrogen. This structure will be termed as [H4L1](NO3)4|L (Figure 5D). The crystal structure of [H4L1](NO3)4 is depicted in Figure 5A. Some nitrates are disordered and can adopt two different positions in the crystal. From these possibilities, the minimum energy structure is that shown in Figure S2 in the Supporting Information, and it was the structure modeled in the subsequent calculations for [H4L1](NO3)4. Its space group is IMM2. The crystal structures of [H4L](NO3)4 and [H4L](NO3)4| L1 on the one hand and [H4L1](NO3)4 and [H4L1](NO3)4|L on the other were minimized by using the universal force field (UFF)29 following the steps explained in the Experimental Section.
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Figure 5. Crystal structures of (A) [H4L](NO3)4, (B) [H4L1](NO3)4 used in the calculations, showing space group IMM2, (C) [H4L](NO3)4|L1, and (D) [H4L1](NO3)4|L.
Figure 6. Conformation of the crystal structures minimized by UFF: (A) [H4L](NO3)4; (B) [H4L1](NO3)4; (C) [H4L](NO3)4|L1; (D) [H4L1](NO3)4|L. Table 1. Energy Terms Obtained by the UFF Calculations
[H4L](NO3)4 [H4L](NO3)4|L1 [H4L1](NO3)4 [H4L1](NO3)4|L
valence terms
van der Waals
electrostatic
total energy
43.61 56.81 39.26 52.55
4.85 22.39 3.84 -3.46
-64.98 -79.41 -95.59 -92.94
-16.51 -0.21 -52.50 -43.85
The final geometries are depicted in Figure 6A-D. The energy values obtained for these systems (kcal/mol of ligand) are shown in Table 1. The final geometries of the structures modified show distorted macrocyclic rings, because the minima obtained were not absolute, since the symmetry groups of the L and the L1 structures are not identical (PBNA and IMM2, respectively). This structural strain gives valence terms for the experimentally occurring structures, [H4L](NO3)4 and [H4L1](NO3)4, lower than those corresponding to the simulated [H4L](NO3)4|L1 and [H4L1](NO3)4|L, respectively (Table 1). The changes in van der Waals energy are according to the steric factors, since L is a molecule larger than L1. Total energy minima correspond to the experimentally obtained crystal of each ligand.
However, the numerical values of these energies are not very significant since total optimization is not reached. Interestingly, nitrates of the macrocycle in [H4L](NO3)4|L1, initially perpendicular to the pyridine rings, became almost parallel to these rings and are translated to a position between them (Figure S2, Supporting Information). Conversely, nitrates in the macrocycle of [H4L1](NO3)4|L, initially parallel to the ether angle, became perpendicular to this plane. Despite this, the method employed cannot justify that the plane of the nitrate within the macrocyle in [H4L](NO3)4 is parallel to the plane of the pyridine ring in the crystal (Figure 6A). Another question that merits comment here is the location of the nitrate with respect to the macrocycle in each case. This nitrate in [H4L](NO3)4 is between the pyridine rings in a location that can be termed as “exterior” (Figure 2). In the case of [H4L1](NO3)4, the nitrate is at the opposite side of an imaginary plane that contains the ammonium nitrogens (Figure 2). This location is termed as “interior”. The minimization of the structure [H4L](NO3)4|L1 allows one to see how the “interior” nitrate passes to “exterior”, while it changes its plane orientation. Instead, during the minimization of [H4L1](NO3)4|L the “exterior” nitrate remains in the same location. Since UFF does not take into account anion-π interactions, only intermolecular electrostatic and van der Waals forces play a role in this simulation, it can be concluded that the situation of the nitrate can be explained only with these basic concepts. In particular, see the electrostatic stabilization in Table 1, from -64.98 to -79.41 kcal 3 mol-1 of H4L. Therefore, the methods employed in the theoretical calculations cannot confirm that anion-π interactions play an essential role in the stabilization of the position of the nitrate anion between the pyridine rings in the crystal structure. The conformation of the molecules, hydrogen bonds, and crystal packing effects can be the key points governing the situation. Future work is focused in studying the variations on the macrocycle that would favor anion-π interactions, such as perfluorinated systems, and studying the anion binding by growing crystals with a range of different anions that have the potential to establish anion-π interactions. In conclusion, the crystal structure of the nitrate salt [H4L](NO3)4 for the macrocyclic ligand (L) containing two pyridine units connected to two diethylenetriamine bridges through methylene positions is reported. One of the nitrate anions is engulfed within the cavity of the macrocycle via
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several binding forces. Different theoretical analysis on the possible contribution of π-π nitrate-pyridine interactions to the stabilization of the anion complex were carried out. The stabilizing effect due to anion-π interactions cannot be confirmed. Therefore, the conformation of the molecules, hydrogen bonds, and crystal packing effects should be the key points governing the situation. Acknowledgment. Financial support from MCINN CTQ15672-CO1 from the Xunta de Galicia (PGIDT04PXIB20901PR) is acknowledged. On of us (J.V. de J.-O.) acknowledges a grant from the I3P program of the Spanish ‘Consejo Superior de Investigaciones Cientificas (CSIC)’ and financial support from the project MAT2007-64682 (Ministerio de Educaci on y Ciencia, Spain). Supporting Information Available: Crystal structure of [H4L1](NO3)4 showing disordered nitrates, conformation of [H4L](NO3)4| L1 after the minimization, location of nitrate in [H4L](NO3)4 with respect to the ammonium nitrogens, location of nitrate in [H4L1](NO3)4 with respect to the ammonium nitrogens, results of superposition of H4L and H4L1 by the steric field method and by the moments of inertia method, details of the X-ray crystal structure solution and refinement for [H4L](NO3)4, hydrogen bonding interactions in [H4L](NO3)4, and energies (kcal/mol) of the anion-π systems studied. This material is available free of charge via the Internet at http://pubs.acs.org.
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