Antiaromatic or Nonaromatic? 21H

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Cite This: J. Phys. Chem. A 2019, 123, 2727−2733

Antiaromatic or Nonaromatic? 21H,61H‑2,6(2,5)-Dipyrrola-1,5(2,6)dipyridinacyclooctaphane-3,7-diene: a Porphycene Derivative with 4N π Electrons

J. Phys. Chem. A 2019.123:2727-2733. Downloaded from pubs.acs.org by CALIFORNIA STATE UNIV BAKERSFIELD on 04/11/19. For personal use only.

Arkadiusz Listkowski,*,†,‡ Paweł Jȩdrzejewski,† Michał Kijak,† Krzysztof Nawara,†,‡ Patrycja Kowalska,† Roman Luboradzki,† and Jacek Waluk*,†,‡ †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: A porphycene-derived compound with a 20 π-electron skeleton has been obtained by replacing two pyrrolene units of porphycene by pyridine rings. NMR, electronic absorption and MCD spectra, and the lack of fluorescence are typical for 4N cyclic π electron systems. The electronic structure and the differences with respect to porphycene can be rationalized by treating these compounds as perturbed, doubly positively charged [22]annulene and [20]annulene perimeters, respectively. Even though the spectroscopic and photophysical criteria proposed for antiaromatic systems are fulfilled, the molecule is very stable. We argue that the compound should be characterized as nonaromatic rather than antiaromatic. The perimeter model is recommended as a powerful tool for predicting the electronic structure and spectra and as a useful addition to other methods that probe the aromaticity.

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INTRODUCTION Even though aromaticity is a basic concept in chemistry, its definition and criteria are a subject of ongoing debates.1−15 This is particularly true for porphyrinoids, especially regarding the main conjugation pathway.16−20 In fact, the issue of how should aromaticity be described in porphyrinoids became a title of a recent paper,15 in which the author describes the confrontation between two different philosophies: (i) that of preparative organic chemists; (ii) that of ”the advocates of chemistry as a physically exact science”. We have contributed to this discussion while interpreting the electronic structure of porphycene (1), a constitutional isomer of porphyrin, in terms of a perimeter model.21 Arguments were presented in favor of starting with the outside perimeter ([20]annulene dication) rather than the [18]annulene-like conjugation path. In this work, we report the synthesis of a 4N π-electron porphycene analogue (2, Scheme 1), obtained by replacing two pyrrolene units by pyridine rings. We analyze the

electronic structure and properties of 2 in terms of various aromaticity criteria. This leads to general remarks concerning the definition of aromaticity. In particular, we argue against defining a compound as “antiaromatic”, using solely the spectroscopic criteria. Synthesis and Structure. 2 has been synthesized by a strategy typically used for preparation of porphycenes (Scheme 2), involving McMurry coupling of the proper dialdehyde, 6formyl-2-(5-formyl-1H-pyrrol-2-yl)pyridine (4). The latter was prepared by Vilsmeier−Haack formylation of the known 6(1H-pyrrol-2-yl)pyridine-2-carbaldehyde (3).22 Reductive coupling of 4 with low-valent titanium afforded 2 as the only nonpolymeric product in relatively high, as for porphycene-like molecule, yield (29%). Interestingly, there is no evidence for formation of the other possible isomer, 21H,51H-2,5(2,5)dipyrrola-1,6(2,6)-dipyridinacyclooctaphane-3,7-diene (2a), under such reaction conditions. This can be explained by the high energy of such form, of which the structure resembles the highest energy cis-tautomer of porphycene.23 Similar results were observed during syntheses of dioxaporphycene24,25 and dithiaporphycene,26 where only one of two possible isomers was obtained, with the nonprotonated nitrogen atoms in a trans-like configuration. X-ray measurements of 2 (Figure 1) reveal a nearly planar geometry, with a herringbone packing motif. Calculations predict a more ruffled structure. It may be that a stacking

Scheme 1. Porphycene (1) and 21H,61H-2,6(2,5)-Dipyrrola1,5(2,6)-dipyridinacyclooctaphane-3,7-diene (2)

Received: December 12, 2018 Revised: February 11, 2019 Published: March 1, 2019 © 2019 American Chemical Society

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DOI: 10.1021/acs.jpca.8b11962 J. Phys. Chem. A 2019, 123, 2727−2733

Article

The Journal of Physical Chemistry A Scheme 2. Reagents and Conditionsa

a

Key (i) POCl3, DMF overnight, then NaOAc/H2O; (ii) TiCl4, Zn, CuCl, THF, reflux 1 h.

peaks in 2 shifting upfield. A similar behavior has been reported for transitions between [18]CF3Pc and [20]CF3Pc, 18π- and 20π-conjugated forms of porphycene with four CF3 groups,27 and for dihydroimidacene, reduced imidazole analogue of porphycene.28 The values of nucleus-independent chemical shift (NICS) calculated at the center of the macrocyle for [18]CF 3 Pc and [20]CF 3 Pc are −12.4 and +4.7, respectively.29 The values computed for 1 and 2 are −13.1 and +3.6. Details of calculations are given in the Supporting Information. We also calculated the harmonic oscillator model of aromaticity (HOMA) indices30 (Figures S1 and S2). Similar values were obtained for both compounds. When all bonds are taken into account, the values obtained for 1 and 2 are 0.77 and 0.74, respectively. Comparison of FLU indices31 (Figures S3−S5) reveals larger values, i.e., lower aromaticity, in 2. In doubly charged 2 the FLU values significantly decrease. A spectacular difference between 1 and 2 is found in the plots of the anisotropy of the current-induced density (ACID)32 (Figure 3). No conjugation in the macrocyle is observed for the π system of 2. In contrast, removal of two electrons leads to a cyclic pattern in 22+, very similar to that calculated for 1. The ACID plot for 2 also indicates that the small positive NICS value obtained for 2 results from the presence of local diatropic currents in the four rings.

Figure 1. (a) X-ray structure of 2 measured at 100 K. Average bond distances in Å. (b) Stacking arrangement of 2 in the crystal.

arrangement of two molecules (interplanar distance of 3.4 Å) stabilizes the planarity. The alpha−beta bonds in the pyrrolic subunits of 2 are shorter than the beta−beta bonds (Figure 1a), which is a pattern characteristic for pyrrole, but opposite to that observed for aromatic porphyrins and porphycenes. The loss of aromaticity in 2 in comparison with parent porphycene 1 is clearly revealed in the NMR spectra (Figure 2). The peaks of the pyrrole peripheral protons shift from 9.2− 9.8 ppm in 1 to 6.0−6.7 ppm in 2. The pyridine protons in 2 exhibit peaks in the region between 6.8−7.4 ppm. The signal of the NH protons moves downfield from 3.15 ppm in 1 to 14.18 ppm in 2. These differences are well reproduced by calculations. The 13C spectra show similar trends, with the

Figure 2. Experimental and calculated 1H NMR spectra of 1 and 2. Note that the signals in the experimental spectrum of 1 are averaged due to fast tautomerization. 2728


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The Journal of Physical Chemistry A

Figure 4. Experimental (dichloromethane, 293 K) and simulated absorption (b and d) and MCD (a and c) spectra of 2. Part d includes Michl’s nomenclature for the electronic transitions in 4N π electron systems.37 The calculated curves were obtained assuming the half widths of 30 nm. MCD spectra are shown in the units of molar ellipticity per unit magnetic field, [Θ]M (104 deg M−1 m−1 T−1); ε in M−1 cm−1.

Figure 3. Isosurfaces of anisotropy of the current-induced density obtained for 1, 2, and 22+ at an isosurface value of 0.05. Left, total; right, π system. Current density vectors are plotted onto the ACID isosurface. The magnetic field vector is orthogonal with respect to the ring plane and points upward (clockwise currents are diatropic).

a weak band at 460 nm, followed by a stronger one at 400 nm, and the strongest at 350 nm which still exhibits a much lower absorption coefficient than the Soret band of 1. The three peaks are well resolved in the MCD spectrum. In contrast to aromatic 4N + 2 compounds, where the lowest excited states are well modeled by interactions between four singly excited configurations, the analysis of the MCD pattern for a 4N π-electron systems, based on the perimeter model,37 has to take into account three pairs of orbitals: HOMO, LUMO, and SOMO (Figure 5). The orbitals of an ideal, unperturbed [n]perimeter, ψk, are pairwise degenerate, with their complex form given by

2 reveals similar NMR spectrum as pyriporphyrin,33 a molecule in which one of the pyrrole rings has been replaced by pyridine. It has been demonstrated for pyriporphyrin33 and analogous benziporphyrin34 that diamagnetic ring current is disrupted in such structures, which leads to nonaromatic species. Because of rectangular shape of the inner cavity, porphycene exhibits very strong NH···N intramolecular hydrogen bonds, leading to unusually low frequency (∼2500 cm−1) of the NH stretching modes.35 H-bonding in 2 is expected to be weaker, as the N−N distance is larger (291 vs 262 pm in 1), and the N−H···N angle is smaller (141° vs 149°). Indeed, the IR spectrum of 2 (Figure S6) shows the NH band at 3170 cm−1, a frequency much larger than in 1, but considerably lower than in porphyrin (3330 cm−1).36 Weaker H-bonds in 2 are consistent with the calculated relative energies of different tautomeric forms. The difference between the lowest trans structure and the cis form, with both protons on the same pyrrolopyridine unit, is predicted as 11 kcal/mol, whereas for 1 the calculations yield only 2 kcal/mol. The other trans tautomeric species, with both protons on pyridine rings was calculated to lie 15 kcal/mol higher than the most stable structure. Interestingly, calculations for S1 predict the cis structure to be more stable than trans (by 1.5 kcal/mol). Electronic Absorption and MCD Spectra. Figure 4 shows the absorption and magnetic circular dichroism (MCD) spectra of 2. The pattern of Q and Soret bands, typical for porphyrinoids, is completely absent. The absorption consists of

i 2πi(μ − 1)k yz zzχμ z n k {

∑ expjjjj n

ψk = n−1/2

μ=1

where χμ is the atomic π orbital located on the μ-th perimeter atom, and k = 0, ± 1, ± 2···n/2 (for even n). These orbitals may be transformed to the real form, symmetrized with respect to the planes perpendicular to the perimeter. As starting points for 1 and 2, we take doubly charged [20] and [22] perimeters, respectively. The final chromophores are obtained by introducing two N− and two NH bridges at the appropriate locations. Such perturbations may remove the orbital degeneracy, the degree of the splitting depending on the shape and symmetry of a given perimeter orbital (Figure 6). For 1, the k = ± 4 and k = ± 5 orbitals correspond to the HOMO and LUMO pairs, respectively. Interaction with orbitals of the bridging atoms should shift both HOMO levels by approximately the same, small amount; for the LUMO pair, the situation is completely different: one of the orbitals is 2729


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Figure 5. Scheme of frontier orbitals of an ideal perimeter (left) and of 2 (right); energies of the latter were obtained by DFT calculations. The orbital energies yield the following numerical values (in eV): ΔH = 1.12, ΔS = 3.28, ΔL = 0.90, ΔHS = 2.80, ΔLS = 1.98, ΔHL = 0.22, ΔHSL = 1.64, ∑HL = 2.02.

strongly destabilized, whereas the other is not affected. This leads to |ΔHOMO| ≪ |ΔLUMO| (the so-called negative-hard chromophore38). In 2, HOMO and LUMO correspond to k = ± 4 and k = ± 6, respectively, whereas k = ± 5 refers to the SOMO pair, characteristic for 4N systems and corresponding to the orbitals that are singly occupied in an ideal, unperturbed perimeter. Similar as in porphycene, it is the k = ± 5 pair of orbitals which is split the most in the perturbed perimeter. Now, however, the two components are transformed into the highest occupied and lowest unoccupied orbital, s− and s+, respectively. The other two orbital pairs are split to a smaller degree. This pattern agrees qualitatively with that obtained by DFT calculations (Figure 7). Also, the nodal planes of DFTcalculated orbitals agree with those based on a perimeter model (Figure 6). The perimeter model predicts for 4N-electron [n]annulenes two low-lying excited states of zero intensity.37 They are followed by four allowed transitions, of which the MCD pattern is determined by the values of ΔHSL, ΔHL, and ∑HL, the parameters related to orbital energy splittings, defined in Figure 5. DFT calculations yield ΔHSL < ∑HL, which, together with ΔHL > 0 leads to the prediction of a + , + , −, + sign sequence of B terms. This is exactly the pattern observed in the experiment (Figure 4). Actually, the perimeter model predictions are strict only when the chromophore has at least one plane of symmetry perpendicular to the molecular plane, which is not the case for 2. It seems, however, that the model works also in a more general case. As shown in Figure 5 and Table 1, a correct MCD pattern was also simulated using DFT calculations that explicitly take into account interactions between more configurations. It is reassuring to find that the same pattern can be predicted without calculations, just by inspection of the orbitals of the parent, unperturbed, C22H222+ 20-π-electron perimeter (Figure 6), in analogy with the treatment of 1, which was derived from the C20H202+ 18-πelectron perimeter.21 Moreover, the perimeter model predicts the same absorption and MCD patterns for 2 and its doubly protonated form, as the nature of the perturbation, now consisting of four NH bridges, remains the same. In addition, the expected loss of planarity

Figure 6. Shape of perimeter orbitals and the predicted energy shifts (represented by arrows) caused by perturbations leading to 1 (top) and 2 (bottom). The orbital shift is dictated by the coefficients of the perimeter orbital at the bridged atoms.38 The dashed lines indicate vertical and horizontal symmetry planes cutting through the perimeter.

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The electrochemical activity of 2 is remarkably different from all the other porphyrin and porphycene isomers. The cyclic voltammograms are shown in Figure S9. 2 shows an unusual stability in the negative potentials window, where no reduction could be observed for the entire potential range studied (the reduction wave is, most probably, associated with residual oxygen present in the solvent). This can be understood from the inspection of the orbital energies (Figure 7). The lowest unoccupied orbital of 2 lies much higher in energy than its counterpart in porphycene. Since the highest occupied orbital also lies higher than in 1, 2 should be easier to oxidize. This seems to be the case, although the analysis is complicated due to irreversible oxidation, leading to the formation of a fluorescent product (under investigation).

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SUMMARY AND CONCLUSIONS Spectral, photophysical, and magnetic properties of 2 are very different from those of parent porphycene 1. In fact, such observed spectroscopic features as the pattern of electronic absorption and lack of fluorescence are those proposed to define heteroannulenic antiaromatic porphyrinoids.20 Moreover, similar values of the NICS parameters calculated for 1 and [18]CF3Pc and for 2 and [20]CF3Pc could suggest the paratropic current in 2 (it was obtained for [20]CF3Pc29). However, this is not the case, since, as shown above, the small positive value of NICS in 2 is due to local currents. The local currents can also explain the value of the NH proton chemical shift, 14.18 ppm. It is close to the values we observed for 2-(2′pyridyl)pyrrole and its derivatives (10−12 ppm). Therefore, it would not be correct to label 2 as antiaromatic. The original concept of aromaticity vs antiaromaticity is based on thermochemical stability.39 2 is stable, nearly planar, and predicted to exhibit the same sign of enthalpy change as 1 upon double hydrogen transfer to the ethylenic bridge (+23.3 and +5.8 kcal/mol for 1 and 2, respectively). Such compounds should rather be called nonaromatic, in accordance with Michl’s proposal that the term “antiaromatic” can be only applied to systems that maintain strong biradical-like character.40,41 Following this nomenclature, 2 is “unaromatic”, i.e., it can be derived from a 4N, but not from a 4N + 2 electron annulene. One could argue that the high stability of 2, as well as the NICS and FLU and can be explained by a model based on four local aromatic rings. Such a picture, however, is not sufficient to explain the electronic absorption and MCD patterns, so much different in 1 and 2. We therefore recommend the perimeter model as a useful addition to other methods that probe the aromaticity. Even though quite simple, the perimeter model is a powerful tool for the prediction of electronic structure and spectra. Its big advantage is the possibility to compare different systems without making any assumptions about the conjugation path. In the present work, the orbital energy patterns in 1 and 2 could be analyzed using exactly the same perturbation - bridging with N− and NH substituents. This approach can readily be extended. For instance, the above-mentioned [20]CF3Pc can be derived from a C20H202+ perimeter by bridging with four NH groups, and analyzed using the orbitals of [20]annulene (Figure 6). Since the number of π-electrons is now 4N, the splitting of k = ±5 orbitals produces the highest occupied and the lowest unoccupied orbitals, unlike in 1, where it corresponded to the LUMO pair. Using this model, the HOMO−LUMO splitting in [20]CF3Pc, referred to as “uniquely large”,29 is a

Figure 7. Energies and shapes of frontier molecular orbitals of 1 (right) and 2 (left).

Table 1. Calculated Wavelengths/Energies and Oscillator Strengths (f) of the Main Electronic Transitions in 2 and the Dominant Electronic Configurations state no.

λ [nm]

E [10 cm ]

f

1 2

496.8 411.9

20.1 24.3

0.0002 0.1501

3 4

379.6 347.6

26.4 28.8

0 0.2028

5 11

339.5 271.0

29.5 36.9

0.8027 0.1768

13

267.7

37.4

0.3558

3

−1

main configurationsa

relative contribution [%]

88−89 88−90 87−89 87−90 88−91 87−89 87−89 85−90 86−89 87−92 85−90 87−92 86−89

98.9 74.8 24.5 98.5 78.1 13.9 59.1 52.2 28.0 15.3 42.7 23.8 18.9

a

The orbitals 88/89 correspond to the SOMO pair, 86/87 to the HOMO, and 90/91 to the LUMO pair.

should lead to intensity decrease in both absorption and MCD. These predictions are confirmed experimentally (Figure S7). Attempts to obtain the NMR spectra of doubly protonated 2 were not fully successful, due to solubility problems. Adding trifluoroacetic acid to CDCl3 solution resulted in an altered spectrum that, however, is not well reproduced by calculations (Figure S8). It may be that a complex, not a protonated form is formed under such conditions. NICS calculations yielded for the doubly protonated species a value of +3.9, very similar to that of the neutral molecule (+3.6). No fluorescence could be detected for 2, in agreement with the perimeter model prediction regarding the forbidden character of the lowest energy transition. 2731


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The Journal of Physical Chemistry A general consequence of the shape of k = ±5 orbitals of the [20]annulene perimeter. The electronic spectra and structure of many other macrocyclic systems such as hemiporphyrazines,42−48 benziphthalocyanines,49,50 benziporphyrins,34,51,52 naphthiporphyrins,53 pyriporphyrins,33 or expanded porphycene-based compounds54 can be, in our opinion, successfully analyzed by the perimeter model. A recent review has shown the merits of perimeter approach in the analysis of electronic structure and optical properties of expanded, contracted, and isomeric porphyrins.55 What has to be kept in mind is that, for nonaromatic/antiaromatic systems such analysis has to take into account not four, but six frontier orbitals. In fact, the inability of four orbital model to cope with some porphyrin model compounds has been discussed already in the 1970s.56 Finally, a justification should be provided for always using the outside perimeter as a starting point for analysis. First, such approach allows treating different compounds (e.g., 1 and 2, but also various isomeric, expanded or contracted porphyrins, reduced or oxidized) on the same footing. Second, as explained for porphycene,21 starting from a charged [20]annulene perimeter instead of neutral [18]annulene represents a smaller perturbation. Since the perimeter model is based on perturbation theory, such methodology seems the most appropriate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b11962.

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General procedures, synthesis, X-ray measurements, electrochemical studies, calculations, HOMA, FLU, and FLU-π indices, IR, absorption, MCD, and NMR spectra, cyclic voltammograms, and Cartesian coordinates (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(J.W.) E-mail: [email protected]. *(A.L.) E-mail:. [email protected]. ORCID

Krzysztof Nawara: 0000-0002-3847-4856 Jacek Waluk: 0000-0001-5745-583X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Polish National Science Centre (NCN), Grant nos. 2011/02/A/ST5/00443 and 2016/ 22/A/ST4/00029 and by a PL-Grid Infrastructure grant, as well as a computing grant from the Interdisciplinary Centre for Mathematical and Computational Modeling. We are grateful to Prof. Dr. Rainer Herges for providing us a copy of the ACID software package.

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REFERENCES

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