Aromaticity and Through-Space Interaction between Aromatic Rings in

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Aromaticity and Through-Space Interaction Between Aromatic Rings in [2.2]paracyclophanes Irena Majerz, and Teresa Dziembowska J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05928 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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The Journal of Physical Chemistry

Aromaticity and Through-Space Interaction Between Aromatic Rings in [2.2]paracyclophanes Irena Majerz*a, Teresa Dziembowskab

[a] Faculty of Pharmacy, Wroclaw Medical University, Borowska 211a, 50-556 Wroclaw, Poland, E-mail: [email protected] ([email protected]), tel. +48717840305

[b] Institute of Chemistry and Environmental Protection, West Pomeranian University of Technology, 70-061, Szczecin, Poland

Abstract The HOMA index calculated for [2.2]paracyclophanes in the solid state reveals a slight decrease of aromaticity. Interactions between aromatic rings of [2.2]paracyclophane have been investigated using AIM and NCI analysis in both – crystal and optimized [2.2]paracyclophane structures. AIM analysis showed that the C…C bond path between the two aromatic rings is present only in few [2.2]paracyclophanes. NCI approach visualized the dispersion and repulsive interactions between the aromatic rings of every [2.2]paracyclophane. Combination of AIM and NCI approach is necessary for determining and identifying nonbonded interactions in [2.2]paracyclophanes.

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Introduction [2.2]paracyclophane (Scheme 1) known since 19491 has aroused a great interest in the last decades as a promising compound in the synthesis of new organic materials.

Scheme 1. The molecule of [2.2]paracyclophane.

The [2.2]paracyclophane and its derivatives found broad applications in designing the organic dyes for sensitized solar cells2, materials for nonlinear optic (NLO) applications3, materials for optoelectronic4, as well as in the asymmetric synthesis5. An interesting example of a mechanically regulated rotation of the [2.2]paracyclophane as a guest molecule in thenanoscale host was reported in the literature.6 [2.2]paracyclophane could be used as a simplest molecular switcher and the shortest molecular wire7 that can be combined with other molecules to construct molecular devices. The application of cyclophanes in biomedical fields has also been reported.8

Molecular structure and physicochemical properties of paracyclophane were recently a subject of detailed discussions.9 - 19 The [2.2]paracyclophane (Scheme 1), with the pair of benzene rings linked by the bridges of two carbon atoms, is a model of strain organic compounds with a nonplanar benzene ring. Joining of two non-adjacent carbon atoms of the aromatic rings by short aliphatic bridges leads to the nonplanarity of the aromatic rings that adopt the boat-like structure. Investigation of the reactivity of [2.2]paracyclophane showed that the compounds preserve the aromatic character.9,10,20 Calculations performed by Caramori et al.10 confirmed the aromatic character of the [2.2]paracyclophane rings, with HOMA value equal to 0.945. The most characteristic properties of paracyclophanes result from the 2 ACS Paragon Plus Environment

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aromaticity of the paracyclophane rings and their mutual interaction. Ability of [2.2]paracyclophane to form complexes with transition metals higher than this ability for benzene was explained by a decreasing repulsive interaction between the aromatic rings.20,21 The close proximity of the two aromatic planes leads to some repulsive as well as attractive interactions between the π electrons in both aromatic rings.

The π-π interaction between the aromatic rings in [2.2]paracyclophane was the subject of a few discussions for [2.2]paracyclophane11,12,18 and its complexes21-23. Lysenko and al.12, basing on the detailed experimental structure analysis, the ab initio calculations and AIM analysis, demonstrated that transannular charge transfer interaction in [2.2]paracyclophane did not occurr. Caramori et al. 11 applied the NBO, MOs and AIM analysis to investigate [2.2]paracyclophane and trans and cis [2.2]metacyclophanes. The NBO analysis indicated that in all compounds the through-space interactions involving the occupied and unoccupied orbitals (π-π*) in different rings did occur, but only in the [2.2]metacyclophanes were significant enough. Only for the last compound, the AIM method evidenced the existence of bond paths between the carbon atoms belonging to different rings. The authors11,12 didn’t consider other attractive through-space interactions. Interaction between the aromatic rings in [2.2]paracyclophane was also discussed by Grimme.18 The author stated that this interaction can be described by the electron correlation of a dispersive-type between two rings. The very short inter-ring distance led to an “overlap dispersive” interaction. Frontera at al.22,23 analyzed the through-space interactions in substituted [2.2] and [3.3]paracyclophane complexes with Na+ and Li+ cations. The analysis of the total binding energy and its contributing elements: electrostatic (Ee), polarization (Ep) and van der Waals (repulsive and attractive) and the AIM analysis evidenced the existence of the through-space substituent effect.

A model for the analysis of interactions between the aromatic rings in [2.2]paracyclophanes may be the π-π stacking interaction in benzene dimers; however, the results cannot be directly compared because the aromatic rings in paracyclophanes are nonplanar and are connected by the aliphatic bridges.

The π-π stacking interaction between the aromatic rings in benzene dimer was a subject of extensive studies.24 – 28 Recently, the nature of these interactions was analyzed using a dispersion-corrected density functional theory, energy decomposition analysis (EDA) and 3 ACS Paragon Plus Environment


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noncovalent interaction (NCI) methods. Energy interaction between two benzene rings (Eint) was analyzed as a sum of several contributions: dispersion (Edis), orbital interaction (Eoi), electrostatic (Ees) and Pauli repulsion (EPauli ).25 The energy of orbital interaction (Eoi) accounts for a charge transfer (the interactions between the occupied orbital of one aromatic ring with an unoccupied orbital of another ring) and polarization (empty-occupied orbital mixing in one benzene ring due to the presence of another benzene ring). The EPauli energy corresponds to a destabilizing interaction between the occupied orbitals of benzene ring and is responsible for the steric repulsion. The energy decomposition analysis showed that dispersion is the dominant attractive contribution to the binding energy for benzene dimers.25 Although the molecule of [2.2]paracyclophane was an object of studies11,12, the nature of the

π-π through-space interaction between the aromatic rings in [2.2]paracyclophane has not been elucidated. Through-space interaction has not been investigated for substituted [2.2]paracyclophanes either.

The aim of our work is the investigation of the nature of through-space interaction between the aromatic rings in [2.2]paracyclophanes using Atom in Molecule (AIM, QTAIM) and a recently developed Noncovalent Interaction (NCI) methods. To investigate influence of the crystal packing on the through space π–π interaction, the experimental structures have been compared with the structures optimized including dispersion correction. Another goal of this work is the investigation of an influence of distortion from planarity on aromaticity of [2.2]paracyclophanes. To our knowledge, no systematic research on a relationship between distortion from planarity and decrease of aromaticity for a family substituted [2.2]paracyclophanes has been done yet. We compare the HOMA aromaticity index29 with the nonplanarity parametr ∆P30 for the experimental structures of [2.2]paracyclophanes collected in the CSD data bases.31

Results and discussion 1. Aromaticity of aromatic rings in [2.2]paracyclophane Aromaticity of [2.2]paracyclophane was estimated using HOMA and NICS parameters.10 As it was recently shown by Poater at al.,17 the NICS parameter is not reliable for estimating aromaticity in a system having superimposed aromatic rings. Hence, for comparing 4 ACS Paragon Plus Environment

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aromaticity in a great [2.2]paracyclophane family we have applied the HOMA parameter29 that was based on an assumption that the geometry is strongly related to the electron distribution. The HOMA index is defined as: n

HOMA = 1 - α/n ∑ ( Ropt − Ri ) 2 i =1

where n is the number of bonds taken into the summation and α is a normalization constant (α = 257.7 for the CC bond) fixed to give HOMA = 0 for a nonaromatic Kekule structure of benzene and HOMA = 1 for the system with all CC bonds equal to Ropt = 1.388 Å. Ri are the bond lengths.

The planarity of a benzene ring has been characterized by the ∆P parameter, previously defined30 as the sum of squares of the deviations of carbon atoms of the benzene ring from the averaged plane of this benzene ring: ∆P = Σ (dC)2 The analysis of HOMA and ∆P values for the crystal structures of [2.2]paracyclophanes collected in the Crystal Data Base (CSD)31 (348 compounds, 392 structures) is presented in Fig. 1. The search of [2.2]paracyclophane crystal structures was performed using CSD version 5.33, excluding systems: (i) with metal cations, (ii) with a substituent at the CH2 linkage group and (iii) with a ring annulated to the benzene ring.



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Fig. 1. Histograms of HOMA (a) and ∆P (b) parameters for the [2.2]paracyclophane derivatives from the CSD. Lower panels: an inter-ring relation of HOMA (c) and ∆P (d) for two benzene rings in [2.2]paracyclophane. The HOMA values for [2.2]paracyclophanes (Fig. 1a) are higher than 0.97 in general and only for a few compounds it is lower than 0.6. The calculated ∆P value (Fig. 1b) is most frequently about 0.03 that indicates distortion of the aromatic ring from planarity. Deviation from planarity of [2.2]paracyclophanes in most cases does not cause disturbance of aromaticity. That is in accord with the results obtained for distorted benzene molecules.32 The relation presented in Fig. 1c shows that only for few compounds the aromaticity of both benzene rings is different. The relation between ∆P for two benzene rings (Fig. 1d) indicates that the deviation from planarity of one benzene ring is reflected in the deviation from planarity of the second [2.2]paracyclophane aromatic ring.

2. Geometry of [2.2]paracyclophane and through-space interaction in crystal structure. The π-π stacking interaction between aromatic rings that are very important in biological systems was an object of numerous studies.24-28, 33-37 The rings involved in the stacking interaction can be parallel or parallel-displaced. The orientations without stacking can be 6 ACS Paragon Plus Environment

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perpendicular-T-shaped or perpendicular edge-to-face. The most stable orientation of benzene rings is the T-shaped or parallel-displaced.25

Aromatic rings in [2.2]paracyclophanes can be parallel and parallel-displaced or twisted. The structure of the [2.2]paracyclophane was an object of numerous experimental and theoretical studies studies.10, 12, 13, 15, 16, 19 Recently Wolf at al19 definitely stated that the [2.2]paracyclophane below 45o can crystallize in twisted form (P4n2 symmetry group) with the twisted angle of 12.83(4)o and at high temperature in parallel form (P42/mnm). The location of the aromatic rings one under the other results in an interaction between the similarly charged carbon atoms. Our calculation of the electrostatic potential for parallel [2.2]paracyclophane, presented in Fig. 2, shows the existence of a repulsive interaction between the aromatic rings.

Fig. 2. Electrostatic potential for [2.2]paracyclophane. It can be expected that also for [2.2]paracyclophanes the parallel displaced structure can facilitate π-π stacking interaction. To characterize the conformation of [2.2]paracyclophanes we have used the following parameters: the distance between the benzene rings, the displacement of one aromatic ring relating to another (a in Scheme 2), and the twist of one ring in relation to another ring (angle between the gray planes in Scheme 2).



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Scheme 2. The molecule of [2.2]paracyclophane. a indicates the ring displacement, the planes marked in gray are considered to represent the ring planes.

The distance between the centers of aromatic rings for the structures taken from CSD has been calculated. For 275 structures of [2.2]paracyclophane this distance is in the range of 3.05 − 3.1 Å. For 134 structures it is in the range of 3.1 – 3.15 Å and for 28 in the range of 3.0 – 3.05 Å. Taking into account the value of the carbon atom’s van der Waals radius equal to 1.7 Å, the existence of the charge-transfer interaction between the benzene ring should be possible. The values of the ring displacement and the angle between the aromatic ring planes for all 392 crystal structures of [2.2]paracyclophanes taken from CSD are presented in the histogram (Fig. 3).


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Fig. 3. Histogram of the ring displacement (a) and the angle between the aromatic ring planes calculated for the aromatic ring carbon atoms except of the ones linked to the ethylene groups (b).

Histograms in Fig. 3a show that the majority of [2.2]paracyclophanes has the paralleldisplaced conformation that favors the through-space interaction. In most cases, one aromatic ring is not parallel with another, but the angle between the planes is generally small (Fig. 3b).

From the whole of 362 known structures found according the criteria listed above, we have chosen 76 parallel displaced structures with the displacement parameter a (Scheme 2) from 0.15 to 0.70 Å. Taking into account nonplanar arrangement of the aromatic rings, we wanted to investigate the structures within a wide range of the inter-planar angles marked in Scheme 2 from 1.9 to 21o. For these structures we have performed the AIM analysis to investigate the nature of the interring interaction.



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3. AIM analysis

Atom In Molecule (AIM, QTAIM) analysis was used as a tool to investigate the stacking between the benzene rings.24,35,37 For benzene dimers, the bond path between the carbon atoms of aromatic rings was observed.24,35,37. It should be noted that the results for benzene dimers cannot be directly compared with the results for [2.2]paracyclophane because of nonpalanrity of the aromatic rings. The AIM analysis was applied in the investigation of [2.2]paracyclophane11,12 and its meta isomers.11 Lyssenko at al.12 assumed that the presence of the bond path with the bond critical point (BCP) between the carbon atoms belonging to the different aromatic rings indicated the charge-transfer interaction between the two aromatic rings. The authors12 did not found the bond path and stated that there are no “through-space” interactions between the two aromatic rings. The absence of the transannular interaction in [2.2]paracyclophane was assigned to the influence of the ethylene bridges and the nonplanarity of the benzene rings. Caramori at al11 did not found the bond path for [2.2]paracyclophane while observed it for syn- and anti-[2.2]metacyclophane. NBO analysis showed that in these compounds the through space π - π* interactions involving the orbitals localized in different ring were present, but only in the [2.2]metacyclophanes were significant.11 The AIM analysis was also applied in the studies of [n,n]paracyclophane complexes with cations.22, 23 In these studies, the charge density at the cage critical point was applied as a source of information on the ring-ring and the cation -π electron interaction. Basing on the results of the above investigations,11, 12 we have assumed that the presence of a C…C bond path between the C atoms belonging to different rings in [2.2]paracyclophane molecule is an indication of the attractive through space interactions between the π-π* orbitals in different rings. Among the analyzed 76 structures, the C…C bond paths between the carbon atoms of the different aromatic rings have been found for 41 compounds listed in Table S1 (Supporting information: Table S1 - the AIM parameters characterizing the BCPs located at the bond paths linking the carbon atoms of two rings of [2.2]paracyclophanes).

In Table S1 the AIM parameters characterizing the BCPs located at the bond paths linking the carbon atoms of two rings of [2.2]paracyclophanes are collected. The most significant AIM parameter38 is the electron density at BCP (ρ(r)). Higher electron density at BCP indicates a stronger interaction. The Laplacian of the electron density at BCP (∇2(r)) determines the type 10 ACS Paragon Plus Environment

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of the interaction. Positive sign of Laplacian indicates closed-shell interactions, the negative sign – the open-shell interaction. The potential energy of the electrons at BCP (V(r)) expresses the pressure exerted on the electrons at the BCP by other electrons. The kinetic energy (G(r)) is connected with the mobility of the electron density at the BCP and reflects the pressure exerted by the electron density cumulated at BCP on other electrons.39-42 Potential energy V(r) may give information about atom-atom energy interaction. Matta at al.41proposed a relationship E = 0.5 V(r), that was recently discussed.42

The presence of the bond path between carbons in different aromatic rings in [2.2]paracyclophanes indicates the through-space attractive C…C interaction. In the case of very low values of electron density at BCP, other parameters should be taken into account to confirm the existence of interaction. The short distance (s) between the BCP and the nearest Ring Critical Point (RCP) is an evidence of instability of the bond.43 The bond path linking the interacting atoms cannot be very bent. A nonlinearity of the bond path, measured as a difference between the atom-atom distance and the bond path length linking these atoms (d), suggests a very weak interaction.44 The third parameter that has to be considered is the ellipticity (ε) of the electron density at the BCPs. Its value is connected with the stability of the bond or its double-bond character.44 Very high ellipticity value in the case when the double-bond character is excluded suggests a very weak or lack of the interaction.

For the investigated [2.2]paracyclophanes (Table S1), the values of the electron density at BCP are in the range from 0.0168 to 0.0101 a.u., suggesting the existence of interactions between the carbon atoms in two [2.2]paracyclophanes rings. However, for some structures the very small distance between the BCP and RCP, shorter than 0.2 Å, close to coalescence, together with the very high values of the ellipticity found for some structures (Table S1) indicate that the interactions are very unstable. The correlation of the electron density with the distance between the BCP and the closest RCP is presented in Fig. 4a. With the decrease of the ρ(r) values from 0.0180 to 0.0125 a.u., the BCP–RCP distance decreases to about 0.3 Å. Below this distance, the ρ(r) value is not so sensitive to the further decrease of the BCP-RCP distance. This observation is confirmed by the correlations of ε(r) with ρ(r) at BCP (Fig. 4b) and the nonlinearity of the interaction with ρ(r) at BCP (Fig 4c).



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Basing on the above correlations it is possible to show that the values of ρ(r)≥0.0125 a.u. at BCP indicate the presence of interactions between the π-π*orbitals localized in different aromatic rings. Lower ρ(r) values may be associated with very weak, unstable interaction. Correlations of potential and kinetic energy of electrons at BCP and the value of Laplacian with electron density are linear in the whole region of the ρ(r) values.

The above analysis shows that only in a few structures of [2.2]paracyclophanes the throughspace interaction between the π-π* orbitals in different rings is present. A question arises if there is a correlation between the electron density at BCPs and the aromaticity and/or planarity of the aromatic rings in [2.2]paracyclophanes. To answer this question we have analyzed the relationship between the charge density at BCPs and the HOMA values, and subsequently, between the charge density at BCPs and non-planarity of the benzene rings (∆P) of the solid [2, 2]paracyclophanes. No correlation has been observed. However, for the structures where ρ(r) is greater than 0.0125 au, the ∆P values are lower than 0.35 Å.


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Fig. 4. Correlations of electron density in BCP with: (a) the distance between BCP and the closest RCP (s value in Table S1), (b) ellipticity of electron density at BCP, (c) difference of the length of the bond path linking carbon atoms of two [2.2]paracyclophane aromatic rings and the distance between these atoms (d in Table S1).



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In order to analyze the through-space interactions between the two aromatic rings in the isolated [2.2]paracyclophanes and to investigate the substituent effect, we have performed AIM analysis for the optimized [2.2]paracyclophane structures. The structure and AIM parameters of [2.2]paracyclophanes, optimized with including dispersion, are collected in Table S2 (Table S2: Characteristics of the bond critical point (BCP) (in a.u.) for the interring interactions in optimized [2.2]paracyclophanes for which the bond paths have been observed). Comparing to solid state, in the optimized structures the bond paths linking the [2.2]paracyclophane aromatic rings have been found for decreased amount of compounds. In most cases, the angle between the rings and the displacement between the centers of aromatic rings are smaller for the isolated structures in comparison to these parameters for solid structures. A common feature of the optimized structures with the interring C…C interactions is an additional linkage between the aromatic rings. The bond paths between aromatic rings have also been found in the optimized QALBEK structure. In this compound, a big substituent with strong hydrogen bond is present. The low value of electron density at BCP and high value of ellipticity indicates very weak C…C intering interaction.

4. Substituent effect in [2.2]paracyclophane. The effect of substituent on π-π stacking interactions in benzene dimers was an object of numerous investigations.34-36 The introduction of the electron-donating and electron-accepting substituents to the aromatic ring had only a negligible effect on the strength of the π- π stacking interactions.34 Lewis at al.36 showed that the Edisp is the major attractive contribution in the overall binding energy in benzene dimer but this energy is not dependent on the electronic properties of the substituent.

In order to verify if the introduction of the electron-donor and/or electron-accepting substituents to the aromatic rings in [2.2]paracyclophane affects the through-space orbital interactions, we have undertaken the AIM analysis for several substituted [2.2]paracyclophanes (Scheme 3). The investigated compounds are: I. 2NO2, 3NHtertBut, II. 2NO2, 2’NHtertBut2, III. 2NO2, 3NHMe, IV. 2NHMe, 2’NO2, V. 2N(Me)2, 2’N(Me)2, VI. 2N(Me)2, 5’NO2, VII. 2NO2, 2’NO2, VIII. 2N(Me)2, 5NO2 2’NO2, 5’N(Me)2, IX. 2NO2,


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3N(Me)2, 6’NO2, 5’N(Me)2, X. 2NO2, 3NO2, 5’N(Me)2, 6’N(Me)2, XI. 3N(Me)2, 6NO2, 3’N(Me)2, 6’NO2, XII. 5N(Me)2, 5’N(Me)2.

Scheme 3. Numbering of atoms in substituted [2.2]paracyclophanes.

All these theoretical structures are parallel displaced. AIM analysis shows that in most substituted [2.2]paracyclophanes (I-V, VII –XI) there is no bond path linking the carbon atoms belonging to different aromatic rings. The paths linking two C atoms in different aromatic rings have been found only for compound VI and XII. The values of the electron densities at the BCPs are very small (0.0109 and 0.0113 a.u.) and the values of ε are very large (4.2806 and 3.7042). The above observations show that introductions of the electrondonor and/or electron-acceptor sustituents to the aromatic rings in [2.2]paracyclophanes has none, or only very limited effect on the π-π* orbital interactions between the aromatic rings.

5. NCI analysis of [2.2]paracyclophane. The AIM analysis presented above does not give information on non-localized dispersion as well as repulsive nonbonded interactions. To investigate these interactions we have used the Noncovalent Interaction (NCI) approach.24,25 This method is an efficient tool to analyze and visualize weak interactions: van der Waals, hydrogen bonds and steric repulsion. The first derivative of the electron density describes the deviation from a homologous electron distribution s = 1/ (2(3π2)1/3)|∇ρ|/ρ4/3). In the regions far from the molecule, the density is decaying to zero exponentially and the reduced gradient has a very large positive value. For covalent bonding and noncovalent interactions, the reduced gradient is very small, 15 ACS Paragon Plus Environment


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approaching zero. To investigate the existence of weak interaction, the plots of the reduced density gradient versus the electron density multiplied by the sign of the second Hessian eigenvalue (λ2) are used. Multiplication of electron density by the sign of the second Hessian eigenvalue makes it possible to differentiate between the repulsive (signλ2)ρ>0 and attractive (signλ2) ρ

Aromaticity and Through-Space Interaction between Aromatic Rings in

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