Experimental and Computational Study of Counterintuitive ClO4

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Experimental and Computational Study of Counterintuitive ClO4–•••ClO4– Interactions and the Interplay between #+–# and Anion···#+ Interactions Prankrishna Manna, Saikat Kumar Seth, Monojit Mitra, Somnath Ray Choudhury, Antonio Bauza, Antonio Frontera, and Subrata Mukhopadhyay Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg5014126 • Publication Date (Web): 24 Sep 2014 Downloaded from http://pubs.acs.org on September 26, 2014

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Crystal Growth & Design

Experimental and Computational Study of Counterintuitive ClO4–···ClO4– Interactions and the Interplay between π+–π and Anion···π+ Interactions Prankrishna Mannaa, Saikat Kumar Seth*,b, Monojit Mitraa, Somnath Ray Choudhuryc, Antonio Bauzád, Antonio Frontera*,d and Subrata Mukhopadhyaya a

Department of Chemistry, Jadavpur University, Kolkata 700 032, India

b

Department of Physics, M. G. Mahavidyalaya, Bhupatinagar, Purba Medinipur, West Bengal

721 425, India c

Central Chemical Laboratory, Geological Survey of India, 15 A & B Kyd Street, Kolkata 700

016, West Bengal, India d

Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122

Palma (Baleares), Spain ABSTRACT The novel non-covalent interactions between the charged and neutral aromatic rings and with anions are utilized to design the solid-state assembly of triply protonated PTPH3 (PTP = 4'-(4pyridyl)-3,2':6',3''-terpyridine) with H2O and three ClO4–, which is synthesized and characterized by single crystal X-ray diffraction analysis. Crystallography reveals that the π+–π+, π+–π and various anion···π interactions are the major driving forces in the stabilization of the selfassembled structure. In the title complex, a layered assembly is formed through the mutual influence of π+–π+ and π+–π interactions. The anions are interacting with the charged π-acceptors which are again stabilized through π+–π interactions. So, the overall stabilization is governed through π+–π/π–π+, (π+–π+)n and anion···π+/π+–π/π–π+ networks in the solid state. The interaction energies of the main driving forces observed in the crystal structure have been calculated using

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density functional theory. In addition, the short O···O contact between ClO4–, anions has been analyzed in detail both computationally and exploring the Cambridge Structural Database. KEYWORDS: Self-assembly, Molecular recognition, Weak forces, π+–π interactions, Anion-π+ interactions, ClO4–···ClO4– Interactions. INTRODUCTION The molecular interaction among an electron deficient π-electron system of aromatic compounds and an anion is a significant issue that has attracted extensive concentration in the last few years because of their usefulness in molecular recognition, rational material design, formation of molecular materials with enviable physical properties, elucidation of enzymatic reaction mechanisms, understanding biomolecular structures, etc.1-4 Nevertheless, it has also been realized that the importance of these intermolecular interactions stems from a delicate interplay between interactions.5-7 Noncovalent interactions play an essential role in supramolecular chemistry, molecular biology, crystal engineering, various attempts have been made to elucidate and quantify these interactions, including hydrogen bonding (HB), π−π stacking, carbonyl(l.p)-π, cation-π, anion-π electrostatic, hydrophobic, and van der Waals forces.8–28 Both experimental and theoretical evidence of interesting synergetic effects between anion-π and HB and between anion-π and π−π stacking interactions has been reported recently which displays that the interplay between these interactions can direct to strong cooperativity effects.29–40 Recently, experimental evidence of anion-π–π interactions has emerged from crystallographic studies41-44 on synthesized compounds based on the electron-deficient pyridine moieties, suggesting the possibility to enhance anion-π binding by π-π stacking. In the present study, we investigate and explore the structural features of this anion-π–π complex, and the energetic associated to anion-π, π+–π+ and π+–π interactions using a combination of single crystal X-ray diffraction and density 2


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functional theory (DFT) calculations. Furthermore, we also investigate the short O···O contact between ClO4– anions both computationally and exploring the Cambridge Structural Database, revealing that the position of the counter-cations is crucial to rationalize the short and counterintuitive anion-anion interactions. Recently, this type of contact has been analyzed before in the neutral system 2,4,6-trinitroaniline that forms dimers in the solid state characterized by two O=N···H–N hydrogen bonds and an O···O interaction.45 EXPERIMENTAL SECTION Materials and Measurements. All reactions were carried out in aerobic conditions and in aqueous medium. All chemicals used were of reagent grade and used as received. Freshly boiled, doubly distilled water was used throughout the synthetic procedure. Elemental analyses (C, H, N) were performed on a PerkinElmer 240C elemental analyzer. Synthesis

of

[PTPH3](ClO4)3.H2O

(1)

(PTP

=4'-(4-pyridyl)-3,2':6',3''-terpyridine,

C20H19Cl3N4O13)] The PTP ligand was prepared following the literature method46,47 where in place of 2-acetyl pyridine we used 3-acetyl pyridine. The PTP ligand (0.5 mmol, 0.313 g) was dissolved in an aqueous solution of HClO4 (pH ~ 0.5) at room temperature (~25.0 0C) by continuous stirring and then filtered to remove any undissolved materials. The filtrate was kept for crystallization at room temperature (~25.0 0C). Block shaped, colorless single crystals were formed after several days from the mother liquor by slow evaporation at room temperature. The crystals were separated by filtration, washed with ice-cold water and then air-dried. Anal. Calcd for C20 H19 Cl3 N4 O13 (1): C, 38.11%; H, 3.04%; N, 8.89%; found: C, 38.10%; H, 3.06%; N, 8.85%. Main

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IR absorption bands observed for 1 (KBr pellet/cm−1) are 3415 (w), 1833 (s), 1416 (vs), 1329 (s), 1290 (s), 1090 (s), 1006 (s), 790 (w), 432 (s). X-ray Crystallography Study. X-ray diffraction intensity data of the title compound was collected at 150(2) K using a Bruker APEX-II CCD diffractometer. Data reduction was carried out using the program Bruker SAINT.48 An empirical absorption correction SADABS49 based on multiscan method was applied. The structure was solved by direct method and refined by the full-matrix least-squares technique on F2 using the programs SHELXS97 and SHELXL97.50 All hydrogen atoms were located from difference Fourier map and refined isotropically. All calculations were carried out using WinGX system Ver-1.64.51 A summary of crystal data and relevant refinement parameters are given in Table 1. Table 1. Crystal data and structure refinement parameters for 1 Structure

(1)

Empirical formula Formula Weight Temperature (K) Wavelength (Å) Crystal system space group a, b, c (Å) α, β, γ (°) Volume (Å3) Z, Density (calc.) (Mg/m3) Absorption coefficient (mm-1) F(000) Crystal size (mm3) θ range for data collection (°) Reflections collected / unique Completeness to θ (%) Absorption correction

C20H19Cl3N4O13 629.74 150(2) 0.71073 Monoclinic P21/c 8.3263(8), 12.0421(11), 24.7248(19) 90, 100.397(3), 90 2438.4(4) 4, 1.715 0.456 1288 0.23 × 0.17 × 0.09 1.67 to 31.88 22076/4296 [R(int)=0.0232] 100.0 Semi-empirical from equivalents 4


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Max. and min. transmission 0.96 and 0.91 Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 4296/0/361 2 Goodness-of - fit on F 1.099 Final R indices [I > 2σ(I)] R1=0.0463, wR2=0.1177 R indices (all data) R1=0.0482, wR2=0.1191 -3 Largest diff. peak and hole (e.Å ) 0.814 and -0.509 2 2 2 R1 = ∑||Fo|–|Fc||/∑|Fo|, wR2 = [∑{(Fo –Fc ) }/∑{w(Fo2)2}]1/2 , w = 1/{σ2(Fo2) + (aP)2 + bP}, where a = 0.0492 and b = 5.0241 for (1). P = (Fo2 + 2Fc2)/3. Theoretical Methods. The energies of the complexes included in this study were optimized at the BP86-D3/def2-TZVPD level of theory using the program TURBOMOLE version 6.4.52 The interaction energies were calculated with correction for the basis set superposition error (BSSE) by using the Boys-Bernardi counterpoise technique.53 For the calculations we have used the BP86 functional with the latest available correction for dispersion (D3). The “atoms-inmolecules” (AIM)54 analysis was performed at the BP86/def2-TZVP level of theory. The calculation of AIM properties was done using the AIMAll program.55 In order to validate this level of theory we have also computed the interaction energies at the RI-MP2/def2-TZVPD level of theory. For the energetic analysis of the interactions observed in the X-ray structure we have used the crystallographic coordinates (single point energy calculations) because the main purpose in this part of the manuscript is the analysis of the interactions as they are in the solid state instead of finding the most stable conformation. We have used the NCI method56-58 to study the O···O observed in the optimized models of the CSD X-ray structures. This method relies on two scalar fields to map local bonding properties: the electron density (ρ) and the reduced-density gradient (RDG, σ). It is able of mapping real-space regions where non-covalent interactions are important and is based 5



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exclusively on the electron density and its gradient. The information provided by NCI plots is essentially qualitative, i.e. which molecular regions interact. The color scheme is a red-blue scale with red for ρ+cut (repulsive) and blue for ρ−cut (attractive). Moreover, green and yellow isosurfaces correspond to weakly attractive and weakly repulsive interactions, respectively. The population analysis has also been performed by the natural bond orbital method59 at the BP86/def2-TZVP level of theory using NBO v3.1 program as implemented Gaussian-09 package.60 Natural bond orbital analysis stresses the role of intermolecular orbital interaction in the complex, particularly electron charge transfer. This is carried out by considering all possible interactions between filled donor and empty acceptor NBOs and estimating their energetic importance by second-order perturbation theory. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with electron delocalization between donor and acceptor is estimated as:

where qi is the orbital occupancy, εi, εj are diagonal elements and Fi,j is the off-diagonal NBO Fock matrix element. RESULTS AND DISCUSSION Structural description. Single crystal X-ray structural analysis reveals that the asymmetric unit of the title complex consists of one triply protonated PTP molecule, one water molecule and three perchlorate anions. The ORTEP view61 with atom numbering scheme is shown in Figure 1. PTPH3 species consists of four pyridine rings among which three N atoms of adjacent three pyridine rings forms an isosceles triangles with N2 on the tip of the central neutral pyridine ring, where N2 forms the apex and N1, N3 form the base of the triangle. The pyridine ring nitrogen 6


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atoms (N1, N3 and N4) get protonated under stipulated reaction conditions and this protonations renders the molecular moiety to bind anions and solvent water molecule through strong N–H···O hydrogen bonding interactions (Table 2). The charged pyridine ring nitrogen atom N3 in the molecule at (x, y, z) acts as donor to the anion oxygen atom O11 at (1+x, y, z) and reinforcement of another pair of C–H···O bonds between the carbon atom C12 of the same ring with the oxygen atom O12 at (1-x, 1-y, -z) leads the molecules to build a R44(18) dimeric ring motif (Figure S1). Another pair of C1–H1···O10 and C12–H12···O12 in the molecule at (1+x, y, z) and (1-x, 1-y, -z) respectively, forms another R44(24) dimeric ring motif in 1 (Figure S2). Another pair of C14– H14···O3 and C12–H12···O12 at (2-x, -½+y, ½-z) and (1-x, 1-y, -z), respectively (Table 2) leads the molecules to form another R44(24) dimeric ring motif in 1 (Figure S3). The charged pyridine ring carbon atom C2 of one dimeric ring acts as donor to the perchlorate oxygen atom O9 of neighboring dimeric ring motif, so generating an infinite chain along (0 0 1) direction. The propagation of the parallel layers can be viewed in the (1 0 1) plane (Figure S4). The charged pyridine ring carbon atoms C2 and C12 bind the perchlorate anion through C2–H2···O9 and C12–H12···O12 hydrogen bonding interactions through which the molecules are propagating along (0 0 1) direction like a zigzag chain (Figure S5). Again the perchlorate oxygen atom O3 acts as double acceptor to two charged pyridine ring carbon atom C3 and C14 in the molecule at (1-x, ½+y, ½-z) and (2-x, -½+y, ½-z) respectively, thus generates another zigzag chain propagating along (0 1 0) direction. Reinforcement between these two types of zigzag chains results into a 2D supramolecular framework in 1 (Figure S5). Due to the self-complementary nature, adjacent parallel chains along (0 0 1) are joined at their edges through the active participation of anions with the charged pyridine ring carbon atoms, resulting in a 2D supramolecular assembly in the (0 1 1) plane (Figure S5). 7



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Figure 1 An ORTEP (anisotropic displacement ellipsoid) diagram with atom numbering scheme of the title complex. Thermal ellipsoids are drawn at the 30% probability level. Rings 1a, 1b and 3 are aromatic cations (π+), while Ring 2 is neutral (π). Table 2. Hydrogen bonding geometry of C20H19Cl3N4O13 (1) (Å, °) D–H···A N(1)–H(1N)···O(1W) N(3)–H(3N)···O(11) N(3)–H(3N)···O(1) N(4)–H(4N)···O(1) N(4)–H(4N)···O(5) O(1W)–H(1W)···O(9) O(1W)–H(2W)···O(12) C(1)–H(1)···O(10) C(2)–H(2)···O(9) C(3)–H(3)···O(3) C(12)–H(12)···O(12) C(14)–H(14)···O(3)

d(D–H) 0.86 0.86 0.86 0.86 0.86 0.82 0.90 0.93 0.93 0.93 0.93 0.93

d(H···A) 1.78 2.37 2.25 2.34 2.19 2.47 1.88 2.42 2.56 2.47 2.37 2.42

d(D···A) 2.635(4) 3.035(4) 2.968(3) 2.979(3) 2.920(4) 3.089(5) 2.740(5) 3.199(4) 3.435(4) 3.307(4) 3.283(4) 3.176(3)

D–H···A 171 135 142 131 143 134 160 141 158 150 166 138

Symmetry --1+x, y, z 2-x, -½+y, ½-z 1-x, 2-y, -z x, 3/2-y, -½+z 1+x, y, z 1-x, ½+y, ½-z 1+x, y, z 1-x, ½+y, ½-z 1-x, ½+y, ½-z 1-x, 1-y, -z 2-x, -½+y, ½-z 8


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The solid state structure possesses a remarkable supramolecular architecture through novel type of π-stacking interactions. The centroid of the charged pyridine ring 1a is involved in face-to-face π+–π stacking interaction with another neutral pyridine ring 2 of the partner molecule (Table 3). The ring 1a in the molecules at (x, y, z) is in contact with ring 2 of the partner molecule at (2-x, 2-y, -z) with a ring centroid separation of 3.513(2)Å. Reinforcement between these two cationic moieties at (x, y, z) and (2-x, 2-y, -z) generating a dimeric motif centered at (1 1 0) through π+–π stacking interactions (Figure. 2a). Again the interconnection of the cationic moieties through another π+–π interaction with an intercentroid separation of 4.185(2)Å between the neutral ring 2 and charged ring 3 of the molecules at (x, y, z) and (2-x, 1y, -z), respectively, forms another motif which is centered at (1 ½ 0). Thus, the cationic motifs which are centred at (1 1 0) and (1 ½ 0) are propagating along (0 1 0) direction in a repeating manner and the corresponding associative weak non-covalent forces may be designated by π+– π/π–π+ network (Figure. 2a). The successive layers of π+–π/π–π+ network along (0 1 0) direction are gain juxtaposed through another π-stacking interaction between the charged pyridine rings through π+–π+ interactions. The molecular packing is such that the π+–π+ stacking interactions between the charged pyridine rings 3 of adjacent layers are optimized. Pyridine rings 3 of the molecules at (x, y, z) and (2-x, 1-y, -z) are strictly parallel, with an interplanar spacing of 3.410Å, and a ring centroid separation of 3.856(2)Å, corresponding to a ring offset of 1.80 Å (Table 3). This assembly as a whole produces a rare supramolecular π+–π+ network along (1 0 0) direction and illustrates the occurrence of an elegant combination of weak forces in the solid state structure (Figure 2a). The combination of these two types of π+–π/π–π+ and π+–π+ networks results in a two-dimensional supramolecular framework (Figure. 2a). 9



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Figure 2. (a) Supramolecular layers of cationic moieties through π+–π+ and π+–π stacking interactions; (b) Stacking arrangement of PTPH3 molecules through π+–π+, π+–π and π+–anion interactions.

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Table 3. Geometrical parameters (Å, º) for the π-stacking interactions

rings i-j Cg(1a)···Cg(2) Cg(2)···Cg(1b) Cg(1b)···Cg(1b)

Rc[a] 3.513(2) 4.186(2) 3.856(2)

R1v[b] 3.347(2) 3.409(2) 3.410(1)

R2v[c] 3.364(2) 3.348(2) 3.410(1)

α[d] 1.21 3.93 0.00

β[e] 16.74 36.89 27.84

γ[f] 17.67 35.47 27.84

Symmetry 2-x, 2-y, -z 2-x, 1-y, -z 2-x, 1-y, -z

Cg(1a), Cg(2) and Cg(1b) are the centroids of the (N1/C1–C5), (N2/C6–C10) and (N3/C11– C15) rings respectively.

[a]

ring centroid i to ring j.

Centroid distance between ring i and ring j.

[c]

[b]

Vertical distance from

Vertical distance from ring centroid j to ring i.

[d]

Dihedral angle

between the first ring mean plane and the second ring mean plane of the partner molecule. [e]

Angle between centroids of first ring and second ring mean planes.

[f]

Angle between the

centroid of the first ring and the normal to the second ring mean plane of the partner molecule.

Due to the self-complementary nature of the cationic unit, the PTPH3 molecules are interacting with the perchlorate anion and the solid-state structure possesses a unique supramolecular assembly through novel type of anion···π+ interaction. The O4 atom of one perchlorate anion in the molecule at (x, y, z) is oriented toward the π-cloud of pyridinium ring 3 in the molecule at (x, 3/2-y, 1/2+z) (Figure 2b), where the separation distance between the ring centroid and the O4 atom is 3.058(3) Å (Table 4). The shortest separation distance reflecting this interaction is O4···C20 = 3.12Å, which is below the sum of the corresponding van der Waals radii (sum of van der Walls radii of O and C is 3.22Å),62 thus suggesting significant anion···π+ interaction. Again, the oxygen atoms O6 and O7 of another perchlorate anion is in contact with the π-face of the charged pyridine ring 1a (which already generates a π+–π/π–π+ network). The separation distances between the ring centroid and the O6 and O7 atoms are 3.513(3)Å and 2.989(3)Å respectively (Table 4). The shortest separation distances reflecting this interactions are O6···C5 = 3.221Å and O7···N1 = 2.947Å respectively, which are either equal or below the sum of the corresponding van der Waals radii, thus suggesting significant anion···π+ interaction. 11



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The entire supramolecular assembly exhibits an exceptional combination of anion···π+ and π+–π interactions, which can be best designated as a unique anion···π+/π+–π/π–π+ network (Figure 2b). Table 4. Geometrical Parameters (Å, °) for anion···π interactions in 1 Y–X(I)···Cg(I) Cl(1)–O(4)···Cg(3) Cl(2)–O(6)···Cg(1a) Cl(2)–O(7)···Cg(1a) Cg(1a) and Cg(3) are

X···Cg Y···Cg Y–X···Cg 3.058(3) 3.955(2) 118.83(2) 3.513(3) 3.832(2) 91.65(2) 2.989(3) 3.832(2) 115.46(2) the centroids of the rings generated through the

X-Perp Symmetry 3.056 x, 3/2-y, ½+z 3.188 x, y, z 2.857 x, y, z atoms (N1/C1–C5) and (N4/C16–

C20) respectively.

Chalcogen O···O interactions between anions Recently, it has been described an unusual arrangement of trinitroaniline molecules into centrosymmetric dimers, where along with intermolecular H-bonds a rather short O···O contact arises. The nature of interaction was studied combining a CSD analysis and quantum chemical computations and the authors conclude that the interaction between the oxygen atoms can be described as “peak-to-peak” with overlap of the oxygen lone pairs and the energy is favorable in approximately 2 kcal/mol.12 A similar arrangement is observed in the salt described herein (see Figure 3) where short O···O distances between anions are observed.

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Figure 3. Right: Partial view of the distribution of several anions in the X-ray structure of 1. Left: Tetrameric assembly observed in the X-ray structure of 1 exhibiting the O···O contact. The main question is to know if these contacts atomic O···O contacts are favorable even in the presence of a strong electrostatic repulsion between the negatively charged oxygen atoms. Obviously, the presence of two protonated N+–H groups of the PTP ligands provides a rhomboidal distribution of charges that explains the formation of the assembly (see Figures 3 right), where four strong hydrogen bonds are formed. However the assembly exhibits a very short O···O distance (0.15 Å lesser than the sum of vdW radii).26 In order to rationalize and gain insight into this issue, we have first search the CSD database to know it the O···O interactions in perchlorate and nitrate (as models for oxoanions) are common in X-ray structures deposited in the database. The histogram and scattergram plots are shown in Figures 4 and 5. We have only considered those structures without errors and disorder, and with the O··O distance lesser or equal than the sum of van der Walls radii. It can be observed that a very large number of short O···O occurrences are present in the database for both anions (583 NO3– and 669 for ClO4–). For the majority of structures the O···O distance ranges 2.85–3.05 Å.

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Figure 4. Histogram plots (O···O distance) obtained for nitrate–nitrate (left) and perchlorate– perchlorate interactions. In Figure 5 we shown the scattergram plots where the directionality of the interaction is analyzed. For both anions, the hits are mostly concentrated in angle values ranging from 140 to 180 degrees; however the interaction is not highly directional since a significant dispersion of points is also observed in the plots.

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Figure 5. Scattergram plots [N(Cl)1–O1···O2 vs O1···O2–N(Cl)2] obtained for nitrate–nitrate (left) and perchlorate–perchlorate (right) interactions. Approximately the 5% of the X-ray structures obtained from the searches present the same assembly observed in compound 1 and some of them are represented in Figure 6. In all cases the O···O distances are considerably shorter than the sum of van der Waals radii, specially the SEGEO structure that present an extremely short distance (2.76 Å).

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Figure 6. Partial views of X-ray structures retrieved from the CSD exhibiting the tetrameric assembly. The CSD codes are indicated. Distances are in Å. We have optimized some theoretical models in order to know if these arrangements are energetically favorable, paying special attention to the final O···O distance upon optimization. The results are given in Figure 7, where symmetry constrains have been imposed during the optimization in order to investigate the interaction in centro-symmetric tetramers (four identical H-bonding distances). The interaction energies of the assemblies are very large and similar at both levels of theory indicating that this arrangement is very favorable and that they are characterized by very short O···O distances. As a matter of fact the AIM analyses also shown in Figure 7 clearly show a bond critical point that connects the oxygen atoms of the anions. In case of the perchlorate assembly, there are some additional anion–π interactions.7 Each one is characterized by a bond critical point that connects the oxygen atom with one carbon atom of the ring. It is interesting to note that the NCI plot analysis indicates that the O···O interaction contributes positively to the assembly since the colour of the isosurface that characterizes this contact is green instead of red or yellow that represent repulsive interactions.

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Figure 7. Optimized tetramers, AIM analyses of critical points and bond paths (red and yellow spheres stand for bond and ring critical points respectively) and NCI plots are shown. Distances are in Å. Energy values in italics correspond to RI-MP2/def2-TZVPD level of theory. The result obtained from the NCI plot is unexpected since both oxygen atoms belong to anionic moieties and therefore a repulsive interaction is expected. To further investigate this issue we have estimated the O···O interaction by computing two additional energies, as shown in Figure 8. In the first one, the tetramer is computed from a trimer previously formed by one protonated pyridine (PyH+) and two anions, which interacts with another PyH+ to form the tetramer (denoted as ∆E2xHB). This interaction energy evaluates two identical H-bonds (HB). In the second one, the tetramer is computed from another trimer previously formed by two anions and one PyH+, which interacts with another anion to form the tetramer (denoted as ∆E2xHB+OO). This interaction energy evaluates two identical H-bonds (HB) and the O···O interaction. Therefore latter interaction can be roughly estimated by difference that is ∆EOO = ∆E2xHB+OO – ∆E2xHB = +13.8 kcal/mol (+15.6 kcal/mol at the RI-MP2/def2-TZVPD level of theory, see Figure 8). This repulsive O···O interaction that is intuitively explained taking into consideration purely electrostatic effects between two negatively charged species is in disagreement with the NCI plot derived from the analysis of the electron density and its gradient. At this point, in order to investigate the O···O interaction from a different point of view (orbital instead of purely electrostatic), we have performed Natural Bond Orbital (NBO) calculations in the nitrate tetramer (shortest O···O distance) focusing our attention on the second order perturbation analysis that is very useful to study donor acceptor interactions. Interestingly, we have found that the lone pair (lp orbital) of one oxygen atom interacts with the O–N antibonding orbital of the opposite nitrate anion and vice-versa with a concomitant second order stabilization energy of E(2) 17



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= 1.12 kcal/mol for each interaction. In addition the energetic difference between the lp and σ∗ orbitals is only 0.97 a.u. Therefore the stabilization energy that can be attributed to the O···O is approximately 2.24 kcal/mol in the tetramer. This lp → σ∗ interaction revealed by the NBO analysis agrees well with the N–O bond distances observed for the nitrate anions in optimized tetramer (see Figure 8). That is, the N–O bonds that participate in the interaction are 0.057 Å longer than the other N–O bonds due to the lp → σ∗ electron donation.

Figure 8. Equations used to estimate the O···O interaction energies in the nitrate-PyH+ tetramer. RI-MP2 energies in italics. DFT study of the aromatic interactions We have also focused our theoretical study to analyze the energetic features of the interesting supramolecular assemblies described in the structural description of 1 (see Figures 1 and 2). As previously mentioned, the protonated PTPH3 establishes a variety of hydrogen bonding, π–π+ and anion–π+ interactions. We have mostly analyzed those involving the aromatic rings using the neutral salt as the binding unit (i.e. triply protonated PTPH3 and three ClO4– counterions) in order to minimize the contribution of purely electrostatic forces in the computation of the 18


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interaction energies in the assemblies. We have first generated the molecular electrostatic potential of the salt in order to investigate the most π-acidic ring. As previously noted one protonated pyridine ring (denoted as 1a in Figure 1) forms a strong N–H···O hydrogen bond with a water molecule instead of the counterion, in contrast to the other two protonated pyridine rings of the PTPH3 molecule. Precisely this ring presents the most positive electron potential (+80 kcal/mol, see Figure 9) over the centroid and consequently it is likely the most adequate for establishing anion–π interactions. The MEP values over the entire π-system of the PTPH3 are positive ranging from 40 to 80 kcal/mol.

Figure 9. Molecular Electrostatic potential surface (MEPS) of compound 1. Energies are in kcal/mol. In an effort to evaluate the anion–π+ and π–π+ interaction energies using the neutral salt (1) we have computed the theoretical models represented in Figure 10, retrieved from the crystallographic coordinates. In agreement with the MEP analysis, the anion–π interaction in the solid state is established with the protonated pyridine ring denoted as 1a in Figure 1 (most positive electrostatic potential, see Figure 9). As a result, the anion–π+ interaction is large and negative ∆E3 = –45.8 kcal/mol (–49.1 kcal/mol at the RI-MP2 level) and the double π–π+ interaction is smaller in absolute value ∆E4 = –14.8 kcal/mol, indicating that each π–π+ 19



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interaction contributes in approximately –7.4 kcal/mol. We have also computed the anion–π+ interaction using the double π–π+ complex as starting point in order to investigate a possible reinforcement of the anion–π+ interaction due to the stacking interaction. Interestingly, the interaction energy of the anion–π+ interaction when the π–π+ complex is previously formed is considerably more favorable (∆E5 = –58.0 kcal/mol) than ∆E4, indicating that the π–π+ likely reinforces the anion–π+ interaction. It is interesting to note the contribution of the dispersion (Edisp) correction in the interaction energies. These values are given in Figure 10 and it can be observed that they are approximately 6 kcal/mol for both anion–π complexes. However the Edisp contribution in the π–π stacking complex involving the large π-system of the 4'-(4-pyridyl)3,2':6',3''-terpyridine molecule is very large. In fact this contribution is the responsible for the favorable interaction energy between both rings that compensate the electrostatic repulsion of the triple protonated PTP moieties.

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Figure 10. Theoretical models used to evaluate the noncovalent interactions observed in compound 1. The values of the dispersion contribution (Edisp) are also provided. In italics the RIMP2 energy value is given. CONCLUSIONS This paper shows that the anion–π+ and π+–π interactions play an important role in complexation and packing of molecules in the solid state. Moreover, the anions are interacting with the charged π-acceptors and the overall stabilization is govern through π+–π/π–π+, and anion···π+/π+–π/π–π+ networks in the solid state. A special attention has been paid to the short O···O interactions between perchlorate anions that has been analyzed using a variety of computational tools. In spite of its repulsive electrostatic nature, it has a favorable orbital contribution that is rationalized by means of donor-acceptor orbital interactions (lp → σ∗ electron donation). ASSOCIATED CONTENT Supporting Information X-ray crystallographic data in CIF format for 1 (CCDC No. 1010793). Figures S1– S5 are shown in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.K.S.); [email protected] (A.F.) ACKNOWLEDGMENTS P.M. thanks Council of Scientific and Industrial Research (New Delhi, India) for a Senior Research Fellowship (09/096(0641)2010-EMR-I). M.M. gratefully acknowledges the University Grants Commission (New Delhi) for a Senior Research Fellowship. S.M. is grateful to the UPE 21



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II program of University Grants Commission, New Delhi for partial financial support of this work. This work was also supported by the DGICYT of Spain (projects CTQ2011-27512/BQU and CONSOLIDER INGENIO 2010 CSD2010-00065, FEDER funds) and the Direcció General de Recerca i Innovació del Govern Balear (project 23/2011, FEDER funds). We thank the CTI (UIB) for free allocation of computer time.

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For Table of Contents Use Only The perchlorate anions interacts with the charged π-acceptors of triply protonated 4'-(4-pyridyl)3,2':6',3''-terpyridine) molecule and several assemblies are stabilized by π+–π/π–π+, (π+–π+)n and anion···π+/π+–π/π–π+ networks in the solid state, which habe been energetically analyzed by means of Density Functional Theory calculations.

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Experimental and Computational Study of Counterintuitive ClO4

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