Exploring Cyclopentadienone Antiaromaticity: Charge Density Studies

Article pubs.acs.org/JPCA

Exploring Cyclopentadienone Antiaromaticity: Charge Density Studies of Various Tetracyclones Rumpa Pal,† Somnath Mukherjee,‡ S. Chandrasekhar,*,‡ and T. N. Guru Row†,* †

Solid State and Structural Chemistry Unit and ‡Organic Chemistry, Indian Institute of Science, Bangalore-560012, India S Supporting Information *

ABSTRACT: A systematic study of six tetracyclones has been carried out using experimental and theoretical charge density analysis. A three pronged approach based on quantum theory of atoms in molecules (QTAIM), nucleus independent chemical shifts (NICS) criterion, and source function (SF) contributions has been performed to establish the degree of antiaromaticity of the central five-membered ring in all the derivatives. Electrostatic potentials mapped on the isodensity surface show that electron withdrawing substituents turn both C and O atoms of the carbonyl group more electropositive while retaining the direction of polarity.

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INTRODUCTION The phenomena of aromaticity and antiaromaticity define two of the most fundamental concepts in chemistry. As these are virtual quantities, rather than physical observables, their quantification is a large and vigorously discussed field of research and different methods for their validation are still under development. The criteria upon which these concepts have been analyzed so far in the literature are (i) energies (aromatic stabilization and antiaromatic destabilization);1 (ii) geometries (aromatic bond length equalization and antiaromatic bond length alternation); and (iii) various magnetic effects including nucleus independent chemical shifts (NICS).2 The quantitative relationships among the geometric, energetic, and magnetic criteria of aromaticity have been demonstrated for a wide range of five-membered heterocycles in which the cyclopentadienyl anion is the most aromatic, the singlet cyclopentadienyl cation is the most antiaromatic, and cyclopentadiene is nonaromatic.3 These criteria have been applied to many other systems, e.g., homoaromatic carbocations4 and aromatic pericyclic transition states.5 Antiaromaticity is more challenging to define than aromaticity because molecules will adopt otherwise unfavorable geometric and electronic configurations to minimize destabilization. We have been interested in tetraphenylcyclopentadienone (“tetracyclone”) and its derivatives particularly tetraarylcyclopentadienones. These molecules contain strongly absorbing chromophoric units with a low band gap (especially smaller than 1.5 eV) and are utilized in the fabrication of LEDs and photovoltaics.6 Cyclopentadienones are also important because of their thermal [4 + 2] cycloaddition or Diels−Alder reactions with disubstituted acetylenes, furnishing polyphenylenes which are again used as photovoltaic materials. Though cyclopentadienone is extremely unstable and spontaneously under© 2014 American Chemical Society

goes Diels−Alder oligomerization even at very low temperatures, tetracyclone, a deep purple colored solid (mp 218−220 °C),7 and many of its derivatives are considerably stable compounds. The reactivity of cyclopentadienone has been attributed to the antiaromatic valence bond (VB) structure (Scheme 1) as one of the primary resonance forms.8 Scheme 1. Unsubstituted Cyclopentadienone and Its Antiaromatic Valence Bond (VB) Structure

Apparently, substitution of the cyclopentadienone ring with four aryl groups provides enough steric hindrance to Diels− Alder cycloaddition and makes these derivatives kinetically stable even at modestly high temperatures. However, at very high temperatures (>200 °C) in the presence of appropriate dienophiles it does undergo cycloaddition reactions.9 Also, the expeditious synthesis of tetracyclone in high yield is particularly intriguing in light of its antiaromatic instability at the molecular level as tetracyclone readily precipitates out of solution; it raises the question whether crystal packing forces overrule molecular-level antiaromaticity in the lattice. This was Received: January 30, 2014 Revised: April 21, 2014 Published: April 22, 2014 3479

dx.doi.org/10.1021/jp5010924 | J. Phys. Chem. A 2014, 118, 3479−3489

The Journal of Physical Chemistry A

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The nucleus independent chemical shift (NICS)17 criterion is extensively used in computational methods for deciphering a system as belonging to an aromatic or an antiaromatic moiety. The interference from a nucleus is avoided by computing absolute magnetic shielding for a dummy atom positioned at the center of most conjugated rings. Negative NICS values (in ppm) refer to a diatropic ring current, suggesting aromaticity, while positive NICS values refer to paratropic ring current, indicating antiaromaticity. The NICS values are generally computed in an aromatic compound first in plane defined as NICS (0) and 1 Å above the plane of the ring, defined as NICS (1), to evaluate the realistic contribution of the extent of π electron delocalization.18,19 This is due to the fact that the NICS values are dominated by the contribution from the σ bonds in the ring. For compounds with anitaromatic configuration the same logic applies, and hence both NICS (0) and NICS (1) values have been calculated. It also consumes less CPU time and is an integral part of several quantum mechanical packages including Gaussian09.20 The third criterion that we have used to evaluate π delocalization and extent of antiaromaticity in our systems is source function (SF).21,22 This approach directly measures the contribution of different atoms, whether bonded or not bonded, to the electron density at a given point in a system unlike the local electron-based descriptors of electron delocalization such as bond ellipticity and bond order, which show the effect of nonbonded atoms indirectly.23 It is defined in terms of the Laplacian of the electron density and is hence applicable to both experimental and theoretically derived electron densities. In the SF formalism,24 the electron density at the reference points is calculated from Laplacian distribution at other points in the molecular system. Thus, it takes into account all the nonlocal densities unlike in the QTAIM approach and hence would serve as a better evaluation parameter for “antiaromaticity” in our systems. The study presented in this article follows these steps: (i) high resolution X-ray data collection on single crystals of A (Scheme 2) at 110 K leading to experimental charge density using XD2006;25 (ii) structure determination of B−E at 110 K followed by multipolar modeling with theoretical structure factors obtained from CRYSTAL09;26 (iii) the same theoretical charge density analysis on compound F at 120 K reported in the literature (CSD V5.34 IQETEE) .27

indeed one of the primary motivations for this study. Furthermore, the possibility of the reverse polarization of the CO group in tetracyclone needs to be considered, as it would tend to reduce the level of antiaromaticity. In this article, we have analyzed and quantified the “antiaromaticity” in six substituted cyclopentadienones (Scheme 2). It is noteworthy Scheme 2. Six Derivatives of Tetracyclones with Atom Numbering for the Central Cyclopentadienone Ringa

a The IUPAC names are as follows: A, 2,3,4,5-tetraphenylcyclopenta2,4-dienone; B, 2,5-bis(4-methoxyphenyl)-3,4-diphenylcyclopenta-2,4dienone; C, 3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone; D, 2,5-bis(4-nitrophenyl)-3,4diphenylcyclopenta-2,4-dienone; E, 3-(4-methoxyphenyl)-4-(3-nitrophenyl)-2,5-diphenylcyclopenta2,4-dienone; and F, 2,3,4,5-tetrakis(perfluorophenyl)cyclopenta-2,4dienone.

that the HOMO and LUMO of tetraphenylcyclopentadienone appear to be associated with the relevant π orbitals of unsubstituted cyclopentadienone.10 A detailed description of electron delocalization or localization is essential to understand the structure−activity relationship in these compounds. A three pronged approach is employed in this article to quantify antiaromaticity in all derivatives. Charge density is one of the very fundamental experimental observables, and according to the QTAIM approach,11 topological properties at bond critical points (bcps), for example, electron densities [ρ(r)bcp] and Laplacians [∇2ρ(r)bcp)] along with bond ellipticity (ε) have been used as central criteria for studying conjugation.12,13 The preferential accumulation of electron density in a plane, due to the formation of a π bond, is quantified in terms of the ellipticity (ε). The ellipticity [ε = (λ1/λ2) − 1] is the ratio of those eigenvalues λ1 and λ2 of the Hessian matrix that correspond to the directions perpendicular to the bond path. Its value provides a measure for the deviation of the bonding density from cylindrical symmetry for bonds with π character. Evaluating ε along bond paths and not only at bcps has been proven to be advantageous.14 For a characteristic π bond, along with ε > 0, the angle between the eigenvector corresponding to the λ2 eigenvalue associated with the smallest negative curvature and the normal vector of the molecular plane of interest has to be close to zero, which ensures that the ellipticity is caused by expansion of the charge density perpendicular to the ring plane.15,16

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EXPERIMENTAL AND COMPUTATIONAL PROCEDURES Experimental Section. All compounds were synthesized following known procedures, and good-quality single crystals were grown by slow evaporation from a DCM:hexane (1:1) mixture at room temperature. The color of the crystals ranged from dark red to violet. Single crystals of size ∼0.3 mm were chosen using a polarizing microscope and affixed to a Hampton Research Cryoloop using Paratone-N oil. The crystals were cooled to 110 K with a liquid nitrogen stream using an Oxford Instruments Cryojet-HT nitrogen gas-stream cooling device. Xray diffraction data was collected on an Oxford Xcalibur (Mova) diffractometer equipped with an EOS CCD detector using Mo Kα radiation (λ = 0.71073 Å). The crystal to detector distance was fixed at 45 mm, and the scan width (Δω) was 1° per frame during the data collection. The data collection strategy was chosen in such a way as to yield a high resolution X-ray data set (d = 0.45 Å), with high redundancy and completeness of 100% for tetracyclone A. For the other four 3480



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Table 1. Crystallographic Details A

a

CCDC no. formula formula wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) vol (Å3) Z ρcal (g cm−3) F(000) μ (mm−1) cryst size (mm) T (K) λ (Å) (sin θ/λ)max (Å−1) Rint measd reflns unique reflns completeness (%) redundancy

977 828 C29H20O1 384.45 monoclinic I2/a 24.2504 (2) 8.1885 (1) 21.5299 (3) 90 110.491 (1) 90 4004.78 (9) 8 1.275 1616.0 0.076 0.45 × 0.27 × 0.13 110 (2) 0.710 73 1.00 0.0763 244 901 16 790 100.00 14.6

R (F2) wR (F2) (all data) R(F2) wR(F2) (I > 2σ(I)) GoF Δρmin Δρmax (e Å−3)

0.069 0.146 0.047 0.129 1.070

obsd reflns [I > 2σ(I)] total no. of params R(F2) wR(F2) GoF Δρmin Δρmax (e Å−3)

12 412

B

C

E

Fa

977 831 C30H21N1O4 459.48 monoclinic P21/c 10.6891 (4) 9.8020 (3) 21.9169 (9) 90 103.599 (4) 90 2232.0 (2) 4 1.367 960.0 0.091 0.35 × 0.10 × 0.07 110 (2) 0.710 73 0.649 0.0637 19 982 5361 99.7 3.7

IQETEE C29F20O1 744.28 triclinic P1̅ 11.3553(6) 15.1429(9) 15.3297(7) 80.574(5) 77.953(4) 77.925(4) 2500.92 4 1.977 1448.00 0.223 0.41 × 0.29 × 0.13 120 (2) 0.710 73 0.693 0.051 47 416 13 692 99.5 3.5

0.096 0.157 0.058 0.136 1.042 −0.314 0.230

0.0870 0.0990 0.0622 0.0897 0.9676 −0.48 0.57

D

977 829 977 832 977 830 C31H24O3 C31H24O3 C29H18N2O5 444.5 444.5 474.45 monoclinic monoclinic monoclinic P21/c P21/n C2/c 28.369(3) 10.1080 (8) 28.491 (2) 8.0546 (5) 9.5276 (8) 9.4151 (6) 21.692 (2) 24.183 (3) 18.411 (1) 90 90 90 110.52 (1) 98.05 (1) 108.531 (8) 90 90 90 4641.9 (7) 2306.0 (4) 4682.7 (6) 8 4 8 1.27 1.28 1.346 1871.8 936.43 1968.0 0.081 0.081 0.093 0.52 × 0.34 × 0.04 0.51 × 0.41 × 0.05 0.39 × 0.13 × 0.08 110(2) 110 (2) 110 (2) 0.710 73 0.710 73 0.710 73 0.65 0.649 0.649 0.0549 0.0556 0.0401 23 264 11 189 8789 10 655 5275 4115 99.9 100 99.9 2.2 2.1 2.1 Spherical Atom Refinement 0.139 0.130 0.092 0.159 0.177 0.132 0.079 0.083 0.058 0.134 0.154 0.117 1.075 1.071 1.010 −0.316 0.399 0.459 0.301 −0.283 −0.317 Multipole Refinements

810 0.0316 0.0582 1.2718 −0.177 0.175

Taken from the CSD database.

multipole formalism.32 The function minimized was ∑w{|Fo|2 − K|Fc|2} for all reflections with I > 2σ(I). The core and valence scattering factors of all atoms were derived from Su, Coppens, and Macchi wave functions.33 Initially, the scale factor was refined against the whole resolution range of diffraction data. The scatter plots (Figure S3 in the Supporting Information) showing the dependence of Fobs/Fcal with sin θ/λ and the variation of Fobs with Fcal clearly depict the quality of the collected data sets. The positional and anisotropic displacement parameters of the non-hydrogen atoms were refined against the reflections with sin θ/λ > 0.7 Å−1. In the next step, the position and displacement parameters of all non-hydrogen atoms were kept fixed to the obtained values and X−H bond lengths were constrained to the values determined by neutron diffraction experiments.34 The isotropic displacement parameters for H atoms were refined using reflections sin θ/λ < 0.7 Å−1. The

derivatives, i.e., B−E, routine 110(2) K data sets at 0.77 Å resolution were collected. Cell refinement, data integration, and reduction were carried out using the program CrysAlisPro.28 Face indexing was done for accurate numerical absorption correction. Sorting, scaling, and merging of the data sets were carried out using the program SORTAV.29 The crystal structure was solved by direct methods using SHELXS9730 and refined according to the spherical-atom approximation (based on F2) using SHELXL9730 included in the WinGX suite.31 The hydrogen atoms were fixed stereochemically, and the position and isotropic thermal parameters were allowed to refine in the spherical atom model. Crystallographic details are summarized in Table 1. Multipole Refinement. The charge density modeling and multipolar nonspherical atom refinements for A have been performed with XD200625 using the Hansen and Coppens 3481



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Figure 1. Torsion angles in the central five membered ring in six derivatives A−F reflecting degree of planarity of the cyclopentadienone moiety.

converged model was used to calculate anisotropic displacement parameters of H atoms using the SHADE2 analysis.35,36 Estimated ADPs for H atoms were kept fixed during the subsequent multipole refinements and only monopole, bond directed dipole (dz) and quadrupole (q3z2−1) components were allowed to refine. For non-hydrogen atoms, the scale, positional and anisotropic displacement parameters, Pval, Plm, up to octupole level (l = 3), κ, and κ′ were allowed to refine in a stepwise manner, until convergence was reached. No chemical symmetry constraints were applied to the structures, whereas separate κ and κ′ were used to define different atom types based on chemical environment. Computational Methods. Theoretical charge density analysis has been carried out using GAUSSIAN0920 followed by the AIMAll package37 for single molecule, whereas, for periodic calculations, two approaches have been considered: (i) generating theoretical structure factors from CRYSTAL0926 followed by multipole modeling in XD200625 at the B3LYP/631G** level;38,39 (ii) obtaining topological features directly from wave function analysis using CRYSTAL09 in combination with the TOPOND program.40,41 (i) The input geometries of compounds A−F for single point periodic quantum calculations using CRYSTAL0926 were taken from (a) experimental multipole model of A; (b) routine data sets at 110 K for B−E; and (c) routine data set at 120 K for F.27 The single point calculations were performed at the B3LYP/6-31G** level of theory, except in F, for which the TZVP basis set42,43 has been used. The shrinking factors (IS1, IS2, and IS3) along with the reciprocal lattice vectors were set to 4 (30 k-points in irreducible Brillouin zone). The bielectronic Coulomb and exchange series values for the truncation parameter were set as ITOL1−ITOL4 = 6 and ITOL5 = 14, respectively. The level shifter was set to 0.7 hartree/cycle for better convergence. Upon convergence on energy (∼10−7 hartree), the periodic wave functions were obtained, and subsequently theoretical structure factors at the same resolution as observed from the experiments were calculated by a standard procedure implemented in CRYSTAL09. All theoretical structure

factors were assigned unit weights during the refinements based on the methodology followed in the literature.44,45 The anisotropic displacement parameters were set to zero to consider a static model, and multipolar refinements of the theoretical data were carried out up to the same levels as those used for the experimental charge density modeling. (ii) TOPOND46 is considerably different from other existing implementations of QTAIM for crystalline systems41 due to its interface with CRYSTAL package and thus becomes a powerful tool for applying QTAIM to molecules, polymers, surfaces, and crystals, exploiting the full symmetry of each of these systems. It calculates full topological features of ρ(r) and ∇2ρ(r) scalar fields along with other QTAIM descriptors directly from wave function analysis. The level of theory used is B3LYP/ TZVP in our systems. Also, cohesive energies were calculated at the B3LYP/TZVP level using CRYSTAL09 package for all six derivatives.47 Grimme correction48 has been applied for dispersion corrections as implemented in this package. The nucleus independent chemical shift (NICS)2 has been calculated using the GAUSSIAN09 package by the GIAO method49,50 at the B3LYP/DZVP251 level of density functional theory. Atomic source function (SF) contributions24 for single molecules have been calculated with a modified version of the AIMPAC52 code for compounds A−F at the B3LYP/DZVP2 level.51 Only in the case of A, SF contributions from the experimental multipole model also have been carried out using the XD2006 package at the B3LYP/6-31G** level. To evaluate atomic SF contributions, various C−C and CO bcps in aryl groups and the central cyclopentadienone ring have been chosen as suitable reference points (rps) to bring out the subtle differences between aromatic and antiaromatic features. In order to obtain a better measure of π electron delocalization effect, the reference points have been moved by 1 au along the major axis direction, essentially perpendicular to the ring plane. 3482



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RESULTS AND DISCUSSION Description of the Crystal Structure. The crystallographic details of all six compounds are given in Table 1. The Table 2. Topological Features of the Covalent Bonds in the Cyclopentadienone Moiety in Compound Aa Rij (Å)

A C1−O1 theor (periodic) theor (single molecule)

C1−C2 theor (periodic) theor (single molecule)





expt XDPROPb TOPOND single point geom optimized struct expt XDPROPb TOPOND single point geom optimized struct expt XDPROPb TOPOND single point geom optimized struct experiment XDPROPb TOPOND single point geom optimized struct expt XDPROPb TOPOND single point geom optimized struct expt XDPROPb TOPOND single point geom optimized struct

1.2164 1.2163 1.217

1.5034 1.5040 1.503

1.3649 1.3651 1.365

1.5190 1.5182 1.519

1.3612 1.3613 1.361

1.5124 1.5123 1.512

ρ(r) (e/ Å3)

∇2ρ(r) (e/Å5)

ε

2.92 2.78 2.76 2.69

−32.7 −18.2 −7.6 −2.1

0.14 0.06 0.03 0.03

2.70

−1.9

0.03

1.76 1.74 1.78 1.75

−11.3 −11.7 −16.6 −15.6

0.13 0.11 0.10 0.10

1.72

−15.1

0.10

2.28 2.22 2.23 2.18

−20.0 −19.2 −22.7 −20.3

0.26 0.23 0.30 0.30

2.22

−21.0

0.31

1.68 1.65 1.69 1.66

−9.6 −9.8 −14.9 −13.9

0.13 0.12

1.64

−13.7

0.09

2.22 2.24 2.24 2.19

−18.9 −19.9 −22.9 −20.5

0.30 0.25 0.31 0.31

2.22

−21.0

0.31

1.70 1.72 1.75 1.72

−11.0 −11.1 −16.2 −15.2

0.14 0.18 0.09 0.10

1.72

−15.1

0.10

Figure 2. Molecular graph indicating bcps and bond paths of tetraphenylcyclopentadienone A, obtained from experimental charge density analysis. Intramolecular C−H···O contacts are shown.

the Supporting Information, respectively. From the puckering coordinate analysis53 (Table S2 in the Supporting Information) it is observed that the nonplanarity of the cyclopentadienone ring is largest in A compared to the rest of the structures. It may be anticipated that the cyclopentadienone ring will deviate from planarity in crystal structures in order to attenuate the extent of antiaromatic destabilization and to maximize the crystal packing features. Indeed the presence of planar cyclopentadienone ring in crystal structures is always associated with dispersive intermolecular π stacking (Figure S2 in the Supporting Information). In general, the crystals are stabilized by various intra- and intermolecular weak C−H···O, C−H···π, π···π contacts (Table S3 in the Supporting Information). In addition, in F27 various intra- and intermolecular C−F···π, O···F, F···F, C···F interactions are also present. However, it is to be noted that the effect of weak intermolecular interactions is only to provide stability to the crystal structure and the salient features of the molecular properties do not change significantly, particularly the antiaromaticity of the central five-membered ring. Topological Analysis of Electron Density. Properties at bond critical points (bcps), for example, interaction lengths (Rij), electron densities [ρ(r)bcp], and Laplacians [∇2ρ(r)bcp)], of all covalent bonds in the cyclopentadienone moiety in tetraphenylcyclopentadienone A, obtained from both experimental and theoretical charge density analysis, are summarized in Table 2. Topological features obtained from direct analysis of electron density in periodic TOPOND calculations and gas phase single molecular AIMAll calculations, performed at the B3LYP/TZVP level (since B3LYP/6-31G** calculations provided lesser accuracy), have also been considered. It is of interest to note that there are no significant changes observed in topological properties in the gas phase compared to those from the crystal structure. These observations support our approach of performing experimental charge density analysis only on A, while the rest of the derivatives are evaluated based on theory. However, the observed maximum deviation in ρ(r) correlates with the value of the associated torsion angle; for example, the C1−C2 bond has a maximum deviation of 0.06 e/ Å3 in ρ(r) while C4−C5 shows 0.02 e/Å3 difference in ρ(r) and the associated torsion angles are −4.61(4)° and −0.80(4)° respectively. Indeed topological properties of the C−C covalent

0.09

For each covalent bond, the first row corresponds to experimental charge density analysis; the second and third rows correspond to charge density from multipolar refinement of theoretical structure factors using CRYSTAL09 (at the B3LYP/6-31g** level)/theoretical calculations directly from the wave function in TOPOND at B3LYP/ TZVP level. The fourth and fifth rows correspond to theoretical calculations on single point geometry as that of crystal structure and optimized geometry at B3LYP/TZVP level using Gaussian09 followed by the AIMAll package. bModule in XD used for derivation of properties.

a

gas phase optimized geometry of the central cyclopentadienone ring is planar for all compounds while in the crystal structures the ring slightly deviates from planarity (Figure 1). All bond lengths, bond angles, torsion angles, and ORTEP diagrams of A−F are deposited in Table S1, Figure S1a, and Figure S1b in 3483



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Figure 3. (A) Ellipticity profiles for the six covalent bonds in the phenyl ring (Ar4 in Scheme 2) of A obtained from experimental charge density, indicating aromaticity by showing symmetrical distribution in ellipticity due to delocalization. (B) Ellipticity profiles of the covalent bonds in the central five-membered ring of A (Scheme 2) obtained from experimental charge density, showing asymmetry in delocalization in cyclopentadienone moiety, suggesting antiaromaticity.

bonds indicate asymmetry in delocalization (Table S4 in the Supporting Information), supporting antiaromatic feature. Figure 2 indicates the bcps and bond paths in compound A. The presence of intramolecular C29−H29···O1 and C7−H7··· O1 hydrogen bonds is strongly indicated. These H bonds support the observation that 2,5-aryl rings are less sterically hindered than those of 3,4-analogues as based on their respective torsion angles.10 The torsion angles of 2/5 phenyl rings with respect to the cyclopentadienone moiety are comparatively lower, 34.65(5)°/28.62(5)°, compared to those of 3/4 phenyl rings, 43.92(5)°/59.45(5)°. Topological values for the covalent bonds in the central cyclopentadienone moiety and intramolecular H bonds involving the CO group of the cyclopentadienone moiety in all compounds A−F are listed in Tables S4 and S5 in the Supporting Information, respectively.

The ellipticity profiles (Figure 3A) obtained from experimental charge density analysis on A depict the symmetry in electron delocalization associated with a representative phenyl ring (Ar4, Scheme 2), showing aromaticity, while the profiles associated with the central five-membered ring depict the asymmetric delocalization in cyclopentadienone moiety supporting antiaromaticity (Figure 3B). Table 3 summarizes the features of the CO bond in all six derivatives, revealing that its covalent character is largest in F compared to the other derivatives, since the bcp has moved toward the carbonyl O atom along the bond path. This feature is consistent with the integrated atomic basin charges obtained from TOPXD54 associated with the carbonyl group in compound F (Table 4). Figure 4 shows electrostatic potential maps plotted on a 0.074 au isodensity surface and visualized using the package GaussView.55 The maps reveal that electron 3484



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evaluate antiaromaticity in tetracyclone derivatives. Singlet C5H5+ and unsubstituted cyclopentadienone have been chosen as reference systems.56,58,59 Atomic SF contributions are evaluated at bcps of CO and various C−C bonds belonging to aryl groups and cyclopentadienone moiety to bring out the difference between aromatic and antiaromatic features. To emphasize π electron delocalization effects on a given bond, SF contributions are also evaluated at reference points 1 au above/ below the molecular plane.24,60 The values of SF %, the percentage of the ratio of individual atomic source function contribution with respect to the electron density at the particular bcp, for A−F are summarized in Figure S6 and Tables S7a and S7b in the Supporting Information. From Table 6, it should be noted that, for a representative C−C bond (Scheme 3), the contributions of the next neighbor atoms [Cinn and Cjnn, SFnn (%)] are several times larger than those of the farthest ones [Xothers, SFothers (%)] in phenyl as well as cyclopentadienone ring for z = 0 and z = 1 au cases. It is also evident that the contributions of the two C atoms belonging to either next neighbor (Cinn and Cjnn, SFnn %) or farthest ones (Xothers, SFothers %) are almost equal in the case of phenyl ring, which is aromatic in nature, but differ significantly in cyclobutadienone ring, which is antiaromatic. This is a characteristic feature associated with antiaromaticity, and it can be explained as the presence of a 3-centered 2-electron (3c−2e) bond. Figure 5 indicates SF % values in plane (z = 0 au) and 1 au above the molecular plane (z = 1 au) for one of the representative bonds C2−C3. The simplest five-membered antiaromatic C5H5+ reveals that the antiaromatic moiety is made of a “delocalized” system enclosing bonds C1−C2, C2− C3, C1−C5, and C5−C4 (Figure 5a) . Atoms C1−C2−C3 and C1−C5−C4 represent a 3c−2e bond or a cumulative 5c−4e bond. C3−C4 being a single bond, with bond length even greater than a normal Csp3−Csp3 bond, restricts the extent of delocalization, and hence the SF % values associated with C1 and C4 become disproportionate with C3 having a higher % share. Likewise the central five-membered ring in both unsubstituted cyclopentadienone and compound A can be visualized as a “delocalized” system associated with atoms C1− C2−C3 and C1−C5−C4. The delocalization is abruptly interrupted by the C3−C4 bond, which is even longer than a Csp3−Csp2 bond. It is of interest to note that, for C2−C3 bcp, the contribution of C1 is found to be smaller than that of C4, particularly considering that the C3−C4 bond is the longest bond and hence should break the conjugation. However, considering the carbonyl group, the contribution from the O atom, the second nearest neighbor of the C2−C3 bcp, in both unsubstituted cyclopentadienone and compound A (Figure 5), reveals that the SF % value of O is 1.8 and 1.9, comparable to that of the first nearest neighbor C4 atom, SF % value of 1.9

Table 3. Topological Properties of the CO Bond in All Six Derivativesa compd

Rij (Å)

d1 (Å)

d2 (Å)

ρ(r) (e/Å3)

∇2ρ(r) (e/Å5)

ε

1.2164 1.2163 1.2207 1.2181 1.2088 1.2174 1.2235 1.2180 1.2164

0.7787 0.7964 0.8006 0.7987 0.7918 0.7950 0.8001 0.7596 0.7642

0.4377 0.4199 0.4201 0.4194 0.4170 0.4224 0.4235 0.4584 0.4522

2.92 2.78 2.79 2.77 2.84 2.84 2.81 2.83 2.82

−32.7 −18.2 −19.1 −17.6 −17.1 −22.5 −21.6 −30.9 −29.0

0.14 0.06 0.03 0.02 0.04 0.05 0.05 0.11 0.09

A expt theory Bb C D E Fb a

For compound A both experimental (in italics) and theoretical (multipole refinement of theoretical structure factors using CRYSTAL09 at B3LYP/6-31g** level) results are tabulated, whereas for B− E only theoretical results are summarized. Specifically for F, B3LYP/ TZVP level has been adopted. bB and F have two molecules in the asymmetric unit.

withdrawing substituents, especially fluorine, turn both carbonyl C and O atoms more electropositive; however, the direction of polarity remains unchanged relative to any normal carbonyl group. In all six derivatives carbonyl C is the most electropositive atom among the other C atoms in the central five-membered ring as expected. Cohesive energies47 of all derivatives have been calculated at the B3LYP/TZVP level using the CRYSTAL09 package (Table S6 in the Supporting Information). The trend points toward the stability in these structures with compound A < B < C < D ∼ E < F, accounting for the type of substitution (electrophilic or nucleophilic). Antiaromaticity through NICS Parameter. NICS (0) and NICS (1) values17 have been calculated for the central fivemembered ring in all derivatives by the GIAO method at the B3LYP/DZVP2 level of density functional theory using GAUSSIAN09 package (Table 5). The values obtained are compared with some well-known aromatic (such as benzene and C5H5−) and antiaromatic reference systems (eg, singlet C5H5+ and cyclopentadienone).3,56 Indeed it shows that NICS values are negative for benzene and C5H5−, indicating aromaticity, and positive for the rest of the compounds, showing antiaromaticity. Also, it is important to note that the degree of antiaromicity in these stable tetracyclones is of the same order as that of highly reactive unsubstituted cyclopentadienone. Source Function. The protocol described by Gatti et al.57for establishing aromaticity in various compounds through atomic source function (SF) contributions is followed to

Table 4. Integrated Atomic Basin Charges Obtained from TOPXD Module Ba

A O1 C1 C2 C3 C4 C5 a

Fa

expt

theory

expt

theory

C

D

E

expt

theory

−0.95 0.83 −0.12 −0.09 −0.15 −0.03

−1.06 0.88 −0.09 0.03 0.02 −0.10

−1.07 0.92 −0.11 −0.01 −0.04 −0.06

−1.06 0.88 −0.12 0.02 0.00 −0.08

−1.10 0.97 −0.12 0.00 0.01 −0.07

−1.07 0.99 −0.03 −0.04 0.04 −0.02

−1.08 0.88 0.00 −0.01 0.03 −0.02

−0.81 0.86 0.04 0.03 0.10 0.04

−0.84 0.85 0.06 0.08 0.10 0.03

B and F have two molecules in the asymmetric unit. 3485



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Figure 4. Electrostatic potential maps plotted on a 0.074 au isodensity surface for all six derivatives A−F from theoretical charge density studies. The blue, red, and green regions represent electropositive, electronegative, and neutral sites. The extent of polarization in the CO is clearly indicated. For structures B and F only one of the molecules in the asymmetric unit is displayed.

Table 5. NICS Values (in ppm) at B3LYP/DZVP2 Level compd A Ba C D E Fa

cyclopentadienone benzene C5H5− C5H5+ (singlet) a

NICS (0)

NICS (1)

Scheme 3. Atom Labeling for Compound A and Other Reference Systemsa

NICS (1), below

10.6 5.2 10.1 4.8 10.1 4.6 9.5 4.3 10.1 4.4 11.4 5.7 11.1 4.8 11.0 4.6 Reference Systems 10.9 3.9 −7.8 −9.8 −13.8 −10.6 88.5 67.5

4.5 4.3 4.5 4.0 4.3 5.4 4.6 4.7

a (a) Aromatic Ar1 ring in A (Scheme 1); (b) antiaromatic cyclopentadienyl ring in singlet C5H5+; (c) antiaromatic unsubstituted cyclopentadienone system; and (d) antiaromatic central fivemembered cyclopentadienyl ring in A.

3.9 −9.8 −10.6 67.5

B and F have two molecules in the asymmetric unit.

and 1.6 respectively, thus ensuring the importance of the elongated bond C3−C4 and hence antiaromatic feature. Table 6. SF % Contributions for a Representative C−C bond (Scheme 3) Obtained from Single Molecular Calculations Using Modified Version of AIMPAC at the B3LYP/DZVP2 Level of Theorya ring type (a) aromatic

(b) antiaromatic

(c) antiaromatic

(d) antiaromatic

bond C27−C26 (A) C2−C3 (singlet C5H5+) C2−C3 (cyclopentadienone) C2−C3 (A)

bond length (Å)

ρ(r) (e/Å3)

1.3982 (1.0z)b

0.304 0.144

1.3457 (1.0z)b

0.340 0.158

1.3458 (1.0z)b

0.332 0.165

1.3603 (1.0z)b

0.324 0.160

SFba% (Ci)

SFba% (Cj)

SFnn% (Cinn)

SFnn% (Cjnn)

C27 42.1 35.9 C2 43.8 38.3 C2 43.7 39.5 C2 42.4 37.3

C26 42.1 35.9 C3 43.9 39.6 C3 43.4 38.7 C3 42.4 36.7

C28 2.7 4.6 C1 1.6 2.6 C1 1.4 1.9 C1 1.3 1.8

C25 2.7 4.5 C4 2.8 4.6 C4 1.9 3.0 C4 1.6 2.4

SFothers% (Xothers) C29 0.7 1.3 C5 1.3 2.4 C5 1.1 2.0 C5 0.8 1.4

C24 0.5 0.9

O1 1.8 3.5 O1 1.8 3.4

a

SFba %, SFnn %, and SFothers % are the percentage SF contributions of directly bonded C atoms, nearest neighbor C atoms, and remaining (others) carbon atoms (and O atoms in cyclopentadienone ring), respectively. The representative SFnn % and SFothers % values indicating the difference between aromatic and antiaromatic ring are given in italics. bReference points 1 au above/below the molecular plane. 3486



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Figure 5. Graphical representation of the SF % contributions for one representative C−C bcp: the reference point (red dot) lies on the bcp in the first row and at 1 au above/below the molecular plane in the second row. SF % contributions are shown as blue spheres with volumes proportional to the magnitude of contributions. Single molecular calculations using a modified version of AIMPAC are shown: (a) singlet C5H5+, (b) unsubstituted cyclopentadienone, and (c) compound A; whereas (d) represents SF % values from experimental charge density analysis in XD for A.

■

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CONCLUSION The three descriptors suggest that the central cyclopentadienone ring of the tetracyclones is antiaromatic with electron delocalization and the degree of antiaromaticity being of the same order as that of unsubstituted cyclopentadienone. The crystal structures with the presence of weak intermolecular interactions (C−H···π, C−H···O, and π···π) provide stable platforms to access features related to antiaromaticity in the cyclopentadienone ring in the solid state. Also, electrostatic potential maps reveal that electron withdrawing substituents turn both C and O atoms of the carbonyl group more electropositive, although leaving the direction of polarity unchanged relative to a normal carbonyl group. Intramolecular C−H···O hydrogen bonds allow 2,5-aryl groups to be more coplanar with the carbonyl moiety compared to 3,4-aryl groups and thus make the π delocalization more effective, resulting in greater substitution effects at these rings. Establishing antiaromaticity in the central cyclopentadienone ring indicates that the three descriptors used in conjunction provide ample quantitative estimates for delocalization in complex systems.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-80-22932796. Fax: +91-80-23601310. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We sincerely acknowledge Professor Carlo Gatti and Professor Louis Farrugia for useful discussions. R.P. and S.M. thank CSIR, India, for a Senior Research Fellowship. T.N.G. thanks DST, India, for the award of a JC Bose fellowship.

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REFERENCES

(1) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity; John Wiley & Sons: New York, 1994. (2) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. v. E. Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe. J. Am. Chem. Soc. 1996, 118, 6317−6318. (3) Schleyer, P. v. R.; Freemann, P.; Jiao, H.; Goldfuss, B. Aromaticity and Antiaromaticity in Five-Membered C4H4X Ring Systems: “Classical” and “Magnetic” Concepts May Not Be “Orthogonal”. Angew. Chem., Int. Ed. Engl. 1995, 34, 337−340. (4) Cremer, D.; Reichel, F.; Kraka, E. Homotropenylium cation: structure, stability, and magnetic properties. J. Am. Chem. Soc. 1991, 113, 9459−9466. (5) Jiao, H.; Schleyer, P. v. R. A Detailed Theoretical Analysis of the 1,7-Sigmatropic Hydrogen Shift: The Möbius Character of the EightElectron Transition Structure. Angew. Chem., Int. Ed. Engl. 1993, 32, 1763−1765. (6) Harrison, M. G.; Friend, R. H. In Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, Germany, 1998. (7) Dilthey, W.; Braun, W.; Trö sken, O. Zur Kenntnis des Tetraphenyl-cyclo-pentadienons und seiner Reduktionsprodukte (Hcteropolare,XXIII). J. Prakt. Chem. /Chem.-Ztg. 1933, 139, 1−16.

ASSOCIATED CONTENT

* Supporting Information S

Crystallographic information files (CIF); experimental synthesis details and NMR spectra; supplementary figures (S1− S6) containing ORTEP diagrams, intermolecular interactions, Fobs vs Fcalc scatter plot for experimental charge density and residual and deformation density maps, Laplacian plots, and source function; supplementary tables (S1−S7) of bond lengths, bond angles, and torsional angles of the six derivatives, various intra- and intermolecular interaction geometries in crystal packing, topological features of covalent bonds and intramolecular H bonds, cohesive energy of six derivatives, and SF % contributions. This material is available free of charge via the Internet at http://pubs.acs.org. 3487



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Exploring Cyclopentadienone Antiaromaticity: Charge Density Studies

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