The unprecedented strong paratropic ring current of a bis-Pd^II complex of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0)

Abstract

Acid-catalyzed Friedel–Crafts-type cyclization of tetrapyrrolic BF₂ complex 1 and α,α′-dibromotripyrrin 2 gave 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0) BF₂ complex 3BF₂ as a stable and moderate antiaromatic macrocycle. Demetalation of 3BF₂ with methanesulfonic acid followed by treatment with HCl gave free-base salt 3HCl that holds a chloride anion at the core. This salt displays a planar structure with an inverted pyrrole and a stronger paratropic ring current. Metalation of neutral free-base 3 with PdCl₂ gave bis-Pd^II complex 3Pd₂ as a stable antiaromatic molecule. The ¹H NMR spectrum of 3Pd₂ displays signals due to pyrrolic β-protons in the range of −1.06 ∼ −1.90 ppm, indicating the unprecedented strong paratropic ring current.

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Introduction

Stable antiaromatic molecules¹ are a promising organic structural motif because of their facile redox reactivities, three-dimensional aromaticity,² characteristically fast excited-state decays and small HOMO–LUMO gaps,³ and excellent conductivities in the solid-state.⁴ In the last two decades, expanded porphyrins have been demonstrated to be a nice platform to realize stable antiaromatic molecules owing to their electronically and sterically adaptable properties.⁵ How to strengthen the antiaromatic character of expanded porphyrins while retaining chemical stability is an interesting issue. A paratropic ring current and a theoretical NICS value are the representative indices to evaluate the strength of antiaromaticity.

Representative examples of stable antiaromatic porphyrinoids exhibiting strong paratropic ring currents are shown in Scheme 1, which include meso-aryl [28]hexaphyrin(1.1.1.1.1.1) bis-Au^III complex A,⁶ 5,14-dimesityl [16]tetraphyrin(1.0.1.0) Ni^II complex (Ni^II norcorrole) B,⁷ 5,10,23-trimesityl [24]hexaphyrin(1.1.0.0.1.0) bis-Pd^II complex C⁸ and 5,10,19-trimesityl [20]pentaphyrin(1.1.0.1.0) ([20]smaragdyrin) D.⁹ Signals due to the outer β-protons are observed as a pair of doublets at 3.16 and 3.10 ppm and at 1.60 and 1.45 ppm for A and B, respectively. Correspondingly, for C and D, these signals appear in the ranges of 2.49 to 0.70 ppm and 3.88 to 2.75 ppm. The calculated NICS (nucleus-independent chemical shift) values (at the B3LYP/6-31G(d) level) within the macrocycle of A, B, C and D are largely positive (Scheme 1). Among expanded porphyrins, heptaphyrins have been poorly studied so far as compared with hexaphyrins and octaphyrins despite their interesting and characteristic reactivities and properties.^10,11 In addition, the synthetic access to heptaphyrins has been rather limited. In the one-pot synthesis of meso-pentafluorophenyl-substituted expanded porphyrins by modified Lindsey condensation at high concentration,¹² the yield of heptaphyrin was poor (ca. 4–5%) and its isolation needed tedious repeated separations. Later, the same heptaphyrin was prepared in a better yield (39%) by designed acid-catalyzed condensation of tripyrrane dicarbinol and tetrapyrrane.^11b

Scheme 1 Examples and NICS(0) values of stable antiaromatic expanded porphyrins, A, B, C and D; the shown NICS(0) values are determined at the geometric or areal center.

Results and discussion

So far, most of the expanded porphyrins have been synthesized by acid-catalyzed condensation reactions of oligopyrrolic molecules and aldehydes or their equivalents.⁵ Recently, we have developed a new synthetic method for expanded porphyrins on the basis of acid-catalyzed Friedel–Crafts-type cyclization.¹³ In this work, we employed a similar cyclization of tetrapyrrolic BF₂ complex 1 (ref. 14) and α,α′-dibromotripyrrin 2 (ref. 15) for the synthesis of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1. 0.0) 3BF₂. Namely, a solution of equimolar amounts of 1 and 2 in CH₂Cl₂ was heated in the presence of p-toluenesulfonic acid at 50 °C for 8 h. After the usual work up, separation through an Al₂O₃ column gave 3BF₂ in 23% yield. The parent ion peak of 3BF₂ was observed at m/z = 893.4298, calcd for (C₅₈H₅₀BF₂N₇)⁺ = 893.4233 ([M]⁺) by high resolution MALDI-TOF mass spectrometry.

The structure of 3BF₂ has been revealed by X-ray analysis.¹⁶ Two different structures (3BF₂A and 3BF₂B) were found in the crystal (Fig. 1a–d). The two structures are similar, showing that the tetrapyrrolic and the tripyrrolic parts are planar but these two units are bent with an angle of ca. 50°. On the basis of Cα–N–Cα′ angles of the pyrroles, the pyrroles C, E, and G have been denoted as an aminic pyrrole and the pyrroles D and F have been denoted as an iminic pyrrole. The ¹H NMR spectrum of 3BF₂ in CDCl₃ shows three pairs of doublets in the range of 5.42–4.98 ppm and a singlet at 4.79 ppm due to the pyrrolic β-protons, indicating a moderate paratropic ring current arising from its 28π-electronic circuit. Signals due to the inner NH protons are not observed at room temperature in CDCl₃ but are observed at 18.18 ppm (2H) and at 17.73 ppm (1H) at 213 K, in line with the assigned structure. This phenomenon is common in expanded porphyrins, possibly due to rapid NH proton exchange between neighboring amino and imine pyrroles, or between amino pyrroles and solvent molecules like water. The absorption spectrum of 3BF₂ shows two prominent bands at 464 and 524 nm along with a broad tail at 1200 nm, which are typical of antiaromatic porphyrinoids (Fig. 4).¹⁷

Fig. 1 Single-crystal X-ray structures of the (a) top view and (b) side view of 3BF₂A, and (c) top view and (d) side view of 3BF₂B. The thermal ellipsoids are at 50% probability.

In the next step, a solution of 3BF₂ in CH₂Cl₂ was refluxed in the presence of methanesulfonic acid overnight. After the usual work up, the organic extract was treated with an aqueous NaHCO₃ solution and then brine. From this residue, 3HCl was obtained in 87% yield as red solids (Scheme 2). The parent ion peak was observed at m/z = 846.4279, calcd for (C₅₈H₅₂N₇)⁺ = 846.4279 ([M–Cl]⁺). The structure of 3HCl has been revealed by X-ray analysis to be roughly C_2v symmetric and considerably planar with a mean-plane-deviation (MPD) of 0.358 Å (Fig. 2).¹⁶ On the basis of the Cα–N–Cα angles (Fig. S25†), the pyrroles A, C, D, E, and F have been denoted as aminic and the pyrroles B and G have been denoted as iminic. Interestingly, pyrrole A is inverted and the Cl anion is held by hydrogen bonding interaction with the four pyrrolic NH bonds possibly, as well as the two β-pyrrolic CH protons. Similar anion binding has been extensively studied by Sessler et al., but the involvement of β-pyrrolic CH protons is rare.¹⁸ The ¹H NMR spectrum of 3HCl shows signals at 25.42 and 24.89 ppm due to the inner NH protons, a signal at 4.76 ppm due to the outer NH proton, a signal at 16.79 ppm due to the inner pyrrolic β-proton, and twelve signals in the range of 4.47–3.55 ppm due to the outer pyrrolic β-protons, indicating a stronger paratropic ring current in the nonsymmetric structure. It is considered that 3HCl takes a nonsymmetric tautomeric structure in CDCl₃ solution. The absorption spectrum of 3HCl shows a split strong band at 396 and 491 nm with a shoulder at ∼617 nm and a very weak and broad tail at 1600 nm (Fig. 4).

Scheme 2 Synthesis of 3BF₂ and 3HCl.

Fig. 2 Single-crystal X-ray structures of the (a) top view and (b) side view of 3HCl. The thermal ellipsoids are at 50% probability.

Treatment of 3HCl with a K₂CO₃ solution gave free base heptaphyrin 3 (Scheme 3). The ¹H NMR spectrum of 3 in DMSO-d₆ shows two isomers in a ratio of ca. 2 : 1. These isomers were not separable. The major isomer is non-symmetric, displaying four peaks due to the pyrrolic β-protons at 9.95, 10.11, 10.76 and 10.97 ppm, indicating the presence of two inverted pyrroles. Signals due to the outer pyrrolic β-protons of the major isomer are in range of 5.02–6.08 ppm with two inner NH protons at 19.08 and 19.25 ppm. The minor isomer is symmetric with one inverted pyrrole on the symmetry axis, showing six peaks due to outer pyrrolic β-protons in the range of 5.9–4.9 ppm, one inner pyrrolic β-proton at 10.96 ppm, two peaks due to inner NH protons at 17.43 (1H) and 15.46 (2H) ppm and one peak due to an outside NH proton at 7.05 ppm. The solubility of 3 in CDCl₃ is poor and the ¹H NMR spectrum of 3 in CDCl₃ is also complicated, showing four doublets due to the pyrrolic β-protons in the range of 11.2–9.5 ppm, suggesting the presence of a minor isomer having two inverted pyrroles. Signals due to the outer pyrrolic β-protons of major and minor isomers are observed in the range of 5.2–6.2 ppm.

Scheme 3 Synthesis of 3 (major isomer 3a and minor isomer 3b) and 3Pd₂.

Finally, the reaction of 3 with PdCl₂ in a mixture of CH₂Cl₂ and acetonitrile at room temperature for 4 h gave bis-Pd complex 3Pd₂ in 19% yield (Scheme 3). The parent ion peak was observed at m/z = 1091.1711, calcd for (C₅₈H₄₈ClN₇Pd₂)⁺ = 1091.1745 ([M + H]⁺). The structure of 3Pd₂ has been revealed by X-ray analysis to be C_2v symmetric and very planar with a small MPD of 0.05 Å (Fig. 3).¹⁶ Similarly to 3HCl, only pyrrole A is inverted and the remaining pyrroles are pointing inward. The two Pd metals are bound to the two pyrrolic nitrogen atoms (the pyrroles B and G and the pyrroles C and F), the β-proton of the pyrrole A, and the Cl⁻ anion with bond lengths of 1.979 Å, 2.192 Å, 1.958 Å, and 2.323 Å, respectively. The Cl⁻ anion is shared by the two Pd metals and interact with the NH protons of the pyrroles D and E by hydrogen bonding. In the ¹H NMR spectrum of 3Pd₂, a signal due to the inner NH protons is observed at a very low field (42.93 ppm), signals due to the outer pyrrolic β-protons are observed at a high field (−1.06 ∼ −1.90 ppm), and a signal due to the outer NH proton is observed at −6.17 ppm, indicating a very strong paratropic ring current. While many antiaromatic porphyrinoids have been reported so far, the observation of pyrrolic β-protons at negative chemical shifts is unprecedented. The absorption spectrum of 3Pd₂ shows a band at 606 nm with a weak and broad tail at 1200 nm (Fig. 4).

Fig. 3 Single-crystal X-ray structures of the (a) top view and (b) side view of 3Pd₂. The thermal ellipsoids are at 50% probability.

Fig. 4 Absorption spectra of 3BF₂ (black solid line), 3HCl (red solid line), 3 (blue solid line) and 3Pd₂ (brown solid line) in CH₂Cl₂. For clarity, the absorbances at long wavelength are amplified by 10 times.

The electrochemical properties were investigated by cyclic voltammetry and differential pulse voltammetry (Table 1). The electrochemical HOM–LUMO gaps of 3BF₂ and 3 are certainly small (1.25 eV and 1.30 eV), but those of 3HCl and 3Pd₂ are distinctly smaller, being 0.74 and 0.72 eV, respectively.

Table 1 Electrochemical properties of 3BF₂, 3HCl, and 3Pd₂^a

Sample	E red.3 (V)	E red.2 (V)	E red.1 (V)	E ox.1 (V)	E ox.2 (V)	E ox.3 (V)	ΔEHL^b
a Measured in anhydrous CH2Cl2 with 0.1 M nBu4NPF6 in a glovebox versus ferrocene/ferrocenium ion couple; working electrode, glassy carbon; counter electrode, Pt wire; reference electrode, Ag/0.01 M AgNO3. b ΔEHL = e(Eox.1 − Ered.1) [eV].
3BF2	—	−1.37	−1.15	0.10	0.42	—	1.25
3HCl	−1.57	−1.23	−0.78	−0.04	0.39	—	0.74
3	−1.76	−1.45	−1.16	−0.14	0.15	0.34	1.30
3Pd2	−1.79	−1.33	−0.95	−0.23	0.25	—	0.72

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To evaluate the antiaromaticity of 3BF₂, 3HCl, and 3Pd₂, NICS values and ICSS (iso-chemical shielding surface)¹⁹ were calculated. These results are displayed in Fig. S49–S53 (ESI†).²⁰ The NICS values calculated in the inner area of 3BF₂ fall within the range of 9–15 ppm, suggesting its relatively weak antiaromatic character. In contrast, the NICS values for 3HCl were found to be in the range of 15–24 ppm, indicating enhanced antiaromaticity, potentially attributed to its more planar structure compared to 3BF₂. Notably, for 3Pd₂, the corresponding NICS values are significantly larger, ranging from 30 to 65 ppm, consistent with the observed strong paratropic ring current. As shown in Fig. 5, the deshielding isosurface ranges indicated by blue increase in the order of 3BF₂ < 3HCl < 3Pd₂ (Fig. 4a–f). Additionally, the ICSS_ZZ(1) plot for 3Pd₂ was calculated to reach near −100 ppm in the inner area,²¹ indicating that 3Pd₂ is strongly antiaromatic. It is conceivable that strong paratropic ring current is stemming from the planar, rigid, large and symmetric electronic π-network of 3Pd₂.

Fig. 5 ICSS of 3BF₂, (a) top view and (b) side view; 3HCl, (c) top view and (d) side view; 3Pd₂, (e) top view and (f) side view. The green isosurfaces represent the region of magnetic shielding values larger than 5.0 ppm. The blue isosurfaces represent the region of magnetic deshielding values smaller than −5 ppm. (g) ICSS_ZZ(1) plot of 3Pd₂.

The benchmark for aromaticity is often determined by the maximum difference in chemical shifts (Δδ) between the peripheral CH or NH moieties and the interior CH or NH moieties. Similarly, we can also use Δδ to evaluate the strength of paratropic ring current. For 3BF₂, the value of Δδ_CH–NH is 13.43 ppm. For 3HCl, the values of Δδ_CH–NH, Δδ_NH–NH and Δδ_CH–CH are 21.87 ppm, 20.66 ppm and 13.24 ppm, respectively. For 3a (major isomer of 3), the value of Δδ_CH–NH is 14.23 ppm while the values of Δδ_CH–NH and Δδ_NH–NH are 12.48 ppm and 10.38 ppm for 3b (minor isomer of 3). For 3Pd₂, the values of Δδ_CH–NH and Δδ_NH–NH are 43.83 ppm and 49.10 ppm. This huge Δδ has never been seen in other porphyrinoids, which demonstrates the unprecedented strong paratropic ring current of 3Pd₂.

To get further insights into the origin of the strong paratropic ring current of 3Pd₂, we also calculated the NICS values of related structures with only atoms changing or replacement based on the optimized geometry structure of 3Pd₂ (Fig. S54 and S57†). The results show that the effects of the replacement of Cl by Br, I or OH on the paratropic ring current of 3Pd₂ are negligible and replacement of the meso-Mes group with hydrogen, the NICS values at those points become even larger, suggesting that the Mes group may have a negative impact on the strength of its paratropic ring current. However, upon removal of the coordinated Pd₂Cl group, the paratropic ring current is noticeably weakened, as indicated by a decrease in NICS values from ca. 60 ppm to ca. 40 ppm at specific points within the macrocycle. Additionally, substituting Pd^II with Ni^II or Cu^II demonstrates that both Ni^II and Cu^II complexes exhibit strong paratropic ring currents, which are comparable to those observed in 3Pd₂. These findings suggest that other transition metal cations may produce a similar effect, provided the complex's geometry remains essentially unaltered (Fig. S55 and S56†). Therefore, the coordination of the metal to the porphyrinoid may increase the electron density on the macrocycle, possibly due to backbonding interactions, thereby enhancing the paratropic ring current in antiaromatic porphyrinoids.

As a complement to the study of the chemical properties of heptaphyrin(1.1.0.0.1.0.0), we also investigated the reduction reaction of 3BF₂ and the oxidation reaction of 3Pd₂. Treating 3BF₂ with NaBH₄ does not produce any reducing product while 3BF₂ can be reduced with Me₁₀Fc in the presence of a large amount of HCl, similar to the case of the annulated rosarin/octaphyrin reported by Sessler and coworkers.^22,23 The reduction product may be aromatic as judged by the absorption spectrum (Fig. S59†) but is too unstable to be isolated from the reaction mixture. The oxidation of 3Pd₂ with MnO₂ produces an aromatic counterpart 3Pd₂-Ox in 52% yield (Scheme 4), and it is spontaneously reduced in solvent (Fig. S62†). The parent ion peak of 3Pd₂-Ox can be observed at m/z = 1088.1606, calcd for (C₅₈H₄₆ClN₇Pd₂)⁺ = 1088.1668 ([M + H]⁺). The ¹H NMR spectrum of 3Pd₂-Ox in CDCl₃ shows four sharp peaks and two broad peaks in the range of 11.25–9.64 ppm due to the pyrrolic β-protons, indicating a clear diatropic ring current arising from its 26π-electronic circuit. Broad peaks at 11.10 ppm and 9.94 ppm can be due to the β-protons and those in the range of −2.5 to −3.5 ppm may be ascribed to a tautomer of 3Pd₂-Ox (Fig. S61†).

Scheme 4 Synthesis of 3Pd₂-Ox.

Conclusions

In summary, [28]heptaphyrin(1.1.0.0.1.0.0) BF₂ complex 3BF₂ was synthesized by acid-catalyzed Friedel–Crafts-type cyclization of tetrapyrrolic BF₂ complex 1 with α,α′-dibromotripyrrin 2, and was converted to its HCl salt 3HCl and bis-Pd complex 3Pd₂. While these heptaphyrins are all antiaromatic, 3Pd₂ displays an unprecedentedly strong paratropic ring current. Additionally, we conducted research on the reduction of 3BF₂ and the oxidation of 3Pd₂. The unprecedentedly strong paratropic ring current can be attributed to its planar, rigid, large, and symmetric π-network, as well as possible backbonding interactions that augment the electron density on the macrocycle brought about by Pd^II coordination. Along this direction, new synthetic trials to explore even stronger antiaromatic molecules are in progress in our laboratory.

Data availability

All experimental data and detailed experimental procedures are available in the ESI.†

Author contributions

J. Song conducted the project. Y. Liu performed most of the synthesis and characterization, and partial DFT calculations. L. Xu carried out partial crystal analysis and partial DFT calculations. X. Jin contributed to partial synthesis. B. Yin conducted partial NMR tests. Y. Rao and M. Zhou assisted with partial crystal analysis. Y. Liu, L. Xu, Y. Rao, J. Song and A. Osuka prepared the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (Grant No. 22071052, 21772036, 22271091, and 22201072), Science and Technology Planning Project of Hunan Province (2018TP1017), and Science and Technology Innovation Program of Hunan Province (2021RC4059).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of synthetic procedures and characterization for all new compounds, NMR spectra, high resolution mass spectra, UV/vis spectra, and CV spectra. CCDC 2330167 for 3BF₂, 2330168 for 3HCl and 2330169 for 3Pd₂. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc01909a

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