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Article

Tetraanion of Tetracyclopentatetraphenylene Derivative: Global Versus Local Conjugation Modes

by
Hirokazu Miyoshi
1,
Ryosuke Sugiura
2,
Ryohei Kishi
2,3,4,5,*,
Atsuya Muranaka
6,
Masanobu Uchiyama
6,7,
Nagao Kobayashi
8,
Yutaka Ie
9,
Masayoshi Nakano
2 and
Yoshito Tobe
1,9,10,*
1
Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Osaka, Japan
2
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Osaka, Japan
3
Center for Quantum Information and Quantum Biology (QIQB), Osaka University, Toyonaka 560-8531, Osaka, Japan
4
Innovative Catalysis Science Division, Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI), Osaka University, Suita 565-0871, Osaka, Japan
5
Research Center for Solar Energy Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Osaka, Japan
6
Center for Sustainable Resource Science (CSRS), Molecular Structure Characterization Unit, RIKEN, Wako-shi 351-0198, Saitama, Japan
7
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Bunkyo-ku 113-0033, Tokyo, Japan
8
Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Nagano, Japan
9
Nanoscience and Nanotechnology Center, The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Ibaraki 567-0047, Osaka, Japan
10
Department of Applied Chemistry, National Yang Ming Chiao Tung University, Hsinchu 30030, Taiwan
*
Authors to whom correspondence should be addressed.
Chemistry 2025, 7(2), 51; https://doi.org/10.3390/chemistry7020051
Submission received: 31 January 2025 / Revised: 24 March 2025 / Accepted: 26 March 2025 / Published: 31 March 2025
(This article belongs to the Special Issue Open-Shell Systems—a Memorial Issue Dedicated to Professor Masayoshi Nakano)
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Abstract

:
Multiple reduced π-conjugated hydrocarbons exhibit π-electron conjugation modes different from neutral species due to the distinct number of electrons. Herein, we report the generation of a 32 π-electron tetraanion of a derivative of a doubly cyclic π-conjugated system with 28 π-electrons, tetracyclopentatetraphenylene (TCPTP), through an exhaustive reduction with potassium. Although aggregation causes some complications, based on spectroscopic and theoretical investigations, it is revealed that negative charges are located at the outer and inner peripheries, suggesting that the tetraanion adopts a globally delocalized double annulenoid (annulene-within-an-annulene, AWA) mode, with 22 π-electron outer and 10 π-electron inner aromatic perimeters. On the other hand, excess charges of the outer perimeter are mainly located at the apical position of the pentagonal rings, indicating a significant contribution of the cyclopentadienide form. The theoretical analysis of magnetically induced ring current tropicities reveals counter-rotating ring currents at the outer and inner rings, supporting the predominant contribution of the cyclopentadienide form.

1. Introduction

Multiple reduced π-conjugated hydrocarbons possess electronic configurations and molecular structures which are distinct from those of neutral species due to the occupation of vacant molecular orbitals by electrons [1,2,3,4]. In cyclic conjugated systems, π-electron conjugation modes are also different from the neutral species due to the distinct number of electrons. Charging two electrons, each switches the aromaticity/antiaromaticity electron counts, leading not only to an alternation of the magnetically induced ring current tropicities but also sometimes to drastic geometrical or conformational changes, as exemplified by cyclooctatetraene [5,6,7,8,9,10,11,12] and paracyclophane derivatives [13,14]. Such properties are useful in applications to switching devices [15,16]. Among the multicyclic hydrocarbons which have attracted significant interest are [n]circulenes and cycloarenes constructed by cyclically arranged fused rings, represented by [5]circulene (also called corrannulene) [17,18,19,20,21] and kekulene [22,23,24] (Figure 1). These hydrocarbons can be regarded as consisting of outer and inner annulene systems connected with “spoke” bonds, constituting a globally delocalized “annulene-within-an-annulene (AWA)” system in which the inner and outer conjugation circuits are decoupled [25,26]. However, both theoretical and experimental studies indicate that they do not adopt the AWA structures because of the strong tendency to maintain aromatic sextets of the constituent benzene rings [27,28,29]. By contrast, the corannulene dianion and tetraanion are assumed to adopt AWA structures consisting of 6 π-electron inner rings and 16 π- and 18 π-electron outer peripheries, respectively, based on the experimental chemical shifts, although this assumption needs further theoretical support [30,31,32,33]. Cyclopenta-containing systems like tertacyclopentatetraphenylene (TCPTP, 1a) [34,35] and cyclopentatetraphenanthrenylene (CPTP) [36], of which derivatives were synthesized in connection with singlet multiradicals [37], did not exhibit the AWA structures in the neutral states due to the existence of a locally conjugated aromatic sextet and other conjugation pathways. An exception is macrocycles consisting of alternating five- and six-membered rings, named 8MC, and its larger homolog 10MC [38] (Figure 1). Notably, neutral 8MC is considered to be composed of paratropic 24 π- and 32 π-electron rings of both triplet states, whereas 10MC consists of diatropic 30 π-electron inner rings of the singlet state and paratropic 42 π-electron outer rings of the triplet state. These results suggest the importance of the balance between the stabilities of the globally aromatic AWA form and the locally aromatic sextet structure with multiple radical centers.
We reported recently that the dianion of TCPTP 1b2− [39] generated by the reduction of the tetramesityl (Mes) derivative 1b with potassium and lithium exhibited NMR spectra and crystallographic structures which were consistent with an AWA structure with 8 π-/22 π-electrons at the inner/outer peripheries, respectively, in accordance with the theoretical prediction reported previously [40]. The potassium complex of K+2[1b2−] crystallized in a dimeric form, in which four potassium ions were sandwiched by two TCPTP decks. Herein, we report the generation of tertaanion 1b4− by further reduction with potassium and its spectroscopic and theoretical characterizations (Scheme 1). Hydrocarbon tetraanions are rare due to a strong repulsion of the negative charges, which need to be dispersed over the molecular structure and/or by aggregate formation [13,14,30,31,32,33,36,41,42,43,44]. In the case of 1b4−, the formation of aggregates in solution was also observed. The negative charge distribution of 1b4−, deduced based on the 13C NMR chemical shifts, is consistent with the AWA structure. However, the location of the extra charge at the apical position of the pentagonal ring was also indicated. In conjunction with the theoretical ring current tropicities, the contribution of the cyclopentadienide structure is deduced to be more predominant.

2. Materials and Methods

2.1. Experimental Details

2.1.1. Reduction of 1b

TCPTP 1b was prepared as previously reported [34]. A potassium mirror was prepared through the sublimation of potassium under vacuum by heating with a gas torch. THF-d8 was dried over the NaK alloy prior to use. THF was dried by a Glass Contour solvent purification system. A solution of 1b was contacted in a sealed tube, and the progress of the reduction was monitored by 1H NMR spectroscopy at appropriate intervals. Due to the formation of precipitates, which prevented exhaustive reduction, the range of concentration was limited to lower than ca. 1 × 10−2 mol/L of 1b. All experiments were undertaken using the standard vacuum transfer technique, and all spectra were measured in sealed tubes or cuvettes.

2.1.2. Magnetic Circular Dichroism Measurements

The magnetic circular dichroism (MCD) spectra were recorded in the 400–800 nm region with a JASCO J-820 spectropolarimeter equipped with a JASCO electromagnet that produces parallel and antiparallel magnetic fields of up to 1.0 T and in the 700–2000 nm region with a JASCO J-730 spectrodichrometer equipped with a JASCO electromagnet that produces magnetic fields up to 1.5 T. The MCD spectrum of 1b4− was combined at 750 nm. The magnitude of the MCD signal is expressed in terms of magneto-molar circular dichroic absorption ΔεM/M−1cm−1T−1.

2.2. Computational Details

We should note that the level of approximation employed in this study is somewhat different from that in the previous study [34] (with ma-def2-TZVP basis set), where we only focused on the monomeric TCPTP without substituents. Firstly, geometry optimizations were performed for monomeric 1b, 1b2−, and 1b4− at the RB3LYP-D3/6-31G* level [45,46]. The IEF-PCM method was employed throughout the calculations to consider environmental effects (i.e., THF solvent). Even though we treated the anionic states, the effect of including diffuse functions on the geometry optimization was expected to be minor in the systems with extended π-conjugations. We also performed calculations on the dimeric system of 1b, namely K+4[1b2−]2 and K+8[1b4−]2, with four or eight K atoms. The potential energy surface of such dimeric systems is considered to have several local minima (with different positions of K atoms, different relative angles between the upper and lower TCPTPs, etc.) around the global minimum, and here, we obtained one of the possible minimum geometries with no imaginary vibrational frequencies. The geometries of theoretical structures are described in Appendix A.
Next, we performed single-point calculations for these systems (at the GIAO-LC-UBLYP(μ = 0.214)/6-31+G* level for neutral 1b and the GIAO-LC-RBLYP(μ = 0.214)/6-31+G* level for others) in order to evaluate the NMR shielding tensors. The range-separating parameter (μ = 0.214) for the long-range corrected (LC)-BLYP functional employed here is the same as that employed in the previous study [39]. For the evaluation of 13C NMR chemical shifts, we set the standard value to be 185.49 ppm, which is also the same as that employed in the previous study [39]. Theoretical 1H NMR chemical shifts are listed in Table S1, and 13C NMR chemical shifts are included in Table 1.
The gauge-including magnetically induced current (GIMIC) [47,48,49] method was employed to examine the MIC densities for the monomeric 1b2− and 1b4−. The script program Gaussian2gimic.py [50,51] was used to generate the input files for the evaluation of GIMIC. UV–vis–NIR spectra of 1b2− and 1b4− were simulated based on the time-dependent density functional theory (TD-DFT) calculation results at the TD-LC-RBLYP(μ = 0.214)/6-31G* level. Note that, even though dianionic TCPTP may have a small open-shell singlet character in the ground state, we employed the spin-restricted DFT method here for the calculations of 1b2− and its dimer because its effects on the magnetic and optical properties were relatively small, and the results of the spin-restricted DFT for chemical shifts were in agreement with the experimental results. The number of the lowest lying root to be solved was 100. The spatial distributions of molecular orbitals and MIC densities were illustrated using the DrawMol application [52]. All the quantum chemical calculations were performed using Gaussian 09 rev. D and Gaussian 16 rev. C program packages.

3. Results

3.1. Generation and Characterization of Tetraanion 1b4−

3.1.1. 1H NMR Spectroscopy

The reduction of 1b was conducted as described before, with a potassium mirror in THF-d8 [39]. When the dianion 1b2− was further contacted with potassium metal at room temperature for a few days, the 1H NMR signals of 1b2− disappeared, indicating the generation of trianion 1b3− (not characterized). However, by further contact with potassium for an additional few days, new NMR signals appeared at 30 °C (Figure 2 and Figure S1). When this solution was exposed to air, neutral 1b was recovered almost quantitatively (by 1H NMR), supporting the idea that the new species was tetraanion 1b4−. The reduction to 1b4− took place only with potassium; the reaction with lithium stopped at dianion 1b2− and did not proceed further (personal communication; Marina Petrukhina, University of Albany), in contrast to the corannulene tetraanion, which formed a dimer aggregate with lithium ions but not with potassium.
In the aromatic region of the 1H NMR spectrum at 30 °C (Figure 2a), two intense signals together with three small singlets can be observed. The relative intensities depend on sample concentration; in most cases, the small signals are scarcely observed. The two signals marked by light blue dots at 6.91 ppm and 7.16 ppm and a more intense one marked by a dark blue dot at 7.09 ppm are assigned to the aromatic protons (Hb) of the Mes group based on the HMBC spectrum (Figure S4). The other two signals, marked by a light green dot at 7.55 ppm and a dark green dot at 7.99 ppm, are due to the TCPTP core protons. The appearance of two signals for non-equivalent mesityl aromatic protons (Hb) at 6.91 ppm and 7.16 ppm is consistent with a dimeric aggregate, though it is not conclusive. The signal at 7.55 ppm may be due to the TCPTP proton (Ha) of the dimer or a higher aggregate. When the temperature was lowered, the small signals disappeared, presumably merging into the intense signals. Since this behavior was observed reversibly, we ascribed the signals at 7.09 ppm and 7.99 ppm to the averaged signals of oligomers larger than the trimer. However, since these signals were not resolved, the size of the oligomer(s) could not be determined from these observations.
For the methyl proton signals (Figure 2b), the Hd of the para-methyl group (marked by a black dot) did not change upon cooling. By contrast, the ortho-methyl proton Hc (marked by orange and red dots) showed apparent temperature dependence. The small signals at 1.60 ppm and 2.15 ppm (marked by orange dots) may be due to the non-equivalent methyl Hc of the dimer. Similarly to the aromatic signals, the methyl proton signals showed temperature-dependent reversible shifts. However, it is not possible to determine the major component of the aggregates due to the low signal resolution.
Although the 1H NMR chemical shifts of 1b4− varied slightly due to aggregation, the signals for the TCPTP core proton were observed at 7.99 ppm for the aggregate. This is similar to the case of dianion 1b2− (7.76 ppm), in spite of the additional negative charges. The observed 1H NMR chemical shifts agree with the theoretical trend (Table S1) for 1b2− and 1b4− (7.1 ppm and 7.2 ppm, respectively), and their dimers K+4[1b2−]2 and K+8[1b4−]2 are 7.8 ppm and 7.6 ppm, respectively. This indicates that the outer periphery of 1b4− remains diatropic, suggesting that the additional two electrons are introduced mainly to the inner 8-membered ring.

3.1.2. 13C NMR Spectroscopy

It is well known that the difference between 13C NMR chemical shifts in charged and neutral species (Δδ) serves as a good measure of charge distribution in the charged species [1,53,54,55,56,57,58,59,60,61,62,63]. The 13C NMR spectrum of 1b4− also exhibits temperature dependence, as shown in Figure 3b, which displays aromatic carbon signals. A full spectrum is given in Figures S2 and S3. The emergence of new signals at low temperatures indicates the formation of larger aggregates, though it is not possible to assign them. Table 1 lists the chemical shifts (δ) of 1b4− for the TCPTP core carbons (A–D) and mesityl sp2 carbons (E–H), which are assigned based on the HMQC and HMBC spectra (Figure S4). Table 1 also includes the chemical shifts of 1b and 1b2− [39], and the difference between the chemical shifts (Δδ values) for 1b/1b2−, 1b/1b4−, and 1b2−/1b4−, which indicate upfield shifts by the addition of pairs of electrons. In addition, theoretical 13C chemical shifts for neutral 1b, dianion 1b2− and dimer complex K+4[1b2−]2, tetraanion 1b4− and dimer complex K+8[1b4−]2, and the respective Δδ values are included for comparison. The theoretical optimization, structures, and characteristics of these anions and their complexes are discussed in Section 3.2.1. As seen in Table 1, though the theoretically estimated chemical shift values are smaller by ca. −10 ppm than those of the experimental values, Δδ values are in good agreement with the experimental values. Typically, the theoretical chemical shifts of the dimer complexes K+4[1b2−]2 and K+8[1b4−]2 agree better than those of the free monomers 1b2− and 1b4−.
Among the TCPTP carbons, carbons C and D exhibit large negative Δδ values for 1b/1b4− (−44.8 ppm and −43.5 ppm, respectively), in contrast to the smaller changes observed for carbons A and B (−14.9 ppm and −26.1 ppm, respectively). Through a comparison with the chemical shifts of 1b4− with dianion 1b2− (Δδ for 1b2−/1b4−), the changes become clearer; Δδ for carbons C and D are −22.1 ppm and −15.0 ppm, whereas those for carbons A and B are −1.7 ppm and −1.5 ppm, respectively. The Δδ (1b2−/1b4−) values for the mesityl carbons are negligibly small, except for the ipso carbon E, indicating further polarization at only carbon E upon a reduction to tetraanion. Based on the above chemical shift changes, additional negative charges to 1b2− are mainly located at carbon C of the inner periphery and carbon D of the outer periphery. Note that we previously attributed the upfield shift of the inner carbon C in dianion 1b2− to the anisotropic shielding effect of the outer periphery of 22 π-electrons [39], because similar effects have been reported for the inner carbons of bridged annulenes in both neutral [60,61] and negatively charged states [8,62,63]. Therefore, a further upfield shift by 22.1 ppm from dianion 1b2− to tetraanion 1b4− indicates that the outer 22 π-electron circuit seems to remain intact. However, the location of an additional negative charge at carbon D in 1b4− as compared to 1b2− suggests a significant contribution of the cyclopentadienide resonance structure in the tetraanion (Scheme 1).
To evaluate the effect of net charges, the sum of Δδ is estimated. For the TCPTP core carbons of tetraanion 1b4−, the Δδ (1b/1b4−) value amounts to −860 ppm, which corresponds to −215 ppm per unit charge. The magnitude of this value is considerably smaller than that of dianion 1b2− (−310 ppm) [39]. This may be due to coordination interaction with the potassium ion, which is more significant in 1b4− than in 1b2−. The weighted center of the 13C NMR signals of the TCPTP core of 1b4− shifts by −30.7 ppm from neutral 1b, which is comparable to the upfield shift reported for corannulene tetraanion (−36.1 ppm) [30].

3.1.3. Magnetic Circular Dichroism Spectroscopy

The MCD spectrum of 1b4− was measured in THF to elucidate the electronic configuration (Figure 4b). The MCD spectroscopy is sensitive to intrachromophoric coupling rather than interchromophoric coupling in dimeric and oligomeric π-electron systems, so we anticipated that the electronic structure of the tetraanion would be reflected in the MCD spectrum even if it forms a dimer and higher aggregates. Figure 4a shows two types of MCD Faraday A terms. A positive (negative) A term is associated with an electronic transition from a nondegenerate (degenerate) occupied molecular orbital to a degenerate (nondegenerate) unoccupied molecular orbital [64,65,66]. We previously observed a negative A term for the dianion and a positive A term for the dication [39]. In the case of the tetraanion, a derivative-shaped MCD signal with a positive/negative sign sequence with increasing energy was observed corresponding to the 552 nm absorption band, which can be assigned to a negative A term. Figure 4c shows the calculated absorption spectrum of a model tetraanion 1a4− with D4h symmetry. A doubly degenerate absorption band was calculated at 514 nm, and this band was associated with the transition from the degenerate HOMO−1 to the nondegenerate LUMO. The calculation result is essentially consistent with the experimental observation of 1b4−. Although the TCPTP skeleton in the dimer complex (K+8[1b4−]2) does not strictly have D4h symmetry, a nearly degenerate electronic transition (459 nm and 457 nm) was calculated (Table S4). Since the molecular orbitals that mainly contribute to this transition (HOMO−2–HOMO−5, LUMO+4, and LUMO+5) are derived from the HOMO−1 and LUMO of 1b4− (Figure S10), a negative pseudo A term is predicted for the nearly degenerate transition. As a result, both the MCD data and TDDFT calculations support the inner 10 π-electron and outer 22 π-electron perimeters of the monomeric and dimeric tetraanions.

3.2. Quantum Chemical Calculations for Tetraanion 1b4−

3.2.1. Structure Optimization

Attempts to grow crystals of the tetraanion (aggregates) were not successful, presumably because of the existence of distinctly sized aggregates. Therefore, the molecular and electronic structures of TCPTP tetraanions were investigated by quantum chemical calculations for tetraanions 1b4−, and dianion 1b2− and neutral 1b for comparison. For dianion 1b2−, dimer complex K+4[1b2−]2 (Figure 5a) was theoretically studied in view of the isolation of the dimer as crystal [39]. Similarly, since tetraanion 1b4− was shown to aggregate in solution, forming the dimer and/or higher oligomers, dimer complex K+8[1b4−]2 was studied (Figure 5b). The potential energy surface of such dimeric systems is considered to have several local minima (with different positions of K atoms, different relative angles between the upper and lower TCPTPs, etc.) around the global minimum, and we obtained one of the possible minimum geometries with no imaginary vibrational frequencies (see Supporting Information for the details).
The salient feature of the dimer structures is the torsion of the upper and lower TCTP decks in the dimer complexes to avoid steric repulsion between the substituents. The average dihedral angles of the CD–CE bonds of the top and bottom TCPTP decks of K+4[1b2−]2 and K+8[1b4−]2 are 25.6° and 27.7°, respectively. Due to the twisting of the TCPTP cores, the potassium ions flanked by the decks are located on either the five- or six-membered ring or on the bonds shared by them. Similar η5-, η6-, and η2-coordination modes were found in the crystal structures of the potassium salts of the fluorenyl anion with TMEDA, diglyme, or crown ether ligands [67,68,69,70,71].
The molecular orbital diagrams of dimer dianion K+4[1b2−]2 and tetraanion K+8[1b4−]2 are shown in Figures S9 and S10, respectively. To confirm the electronic configuration of the anion complexes, the experimental absorption spectra of 1b2− [39] and 1b4− (Figure 4b) are compared with the theoretical transition bands calculated by the TD-DFT calculations for the corresponding dimer complexes K+4[1b2−]2 and K+8[1b4−]2 (Figure S11). The respective theoretical transitions are listed in Tables S3–S5. The absorption bands observed at 552 nm for 1b4− are in accordance with the theoretically predicted bands shown in Figure S11. The absence of the broad NIR band in 1b4− is probably due to the two-electron occupation of the analogous LUMO and LUMO+1 of the corresponding neutral of the dimeric dianion complex (Figure S10).

3.2.2. Theoretical Bond Lengths

The bond types and labels of the TCPTP core are defined in Figure 6a, and those for neutral 1b, dianion 1b2− and its dimer K+4[1b2−]2, and tetraanion 1b4− and its dimer K+8[1b4−]2 are plotted in Figure 6b. All bond lengths are summarized in Table S2. Note that the TCPTP cores of the dimer dianion and tetraanion complexes adopt approximate D4h symmetric geometries because of the deviation from the D4h structures due to the non-uniform positions of the potassium ions, with the bond lengths’ variations within ±0.004 Å.
As reported previously for the parent TCPTP 1a [34], neutral 1b adopts a D2h symmetric geometry with apparent bond length alternation (BLA). Similarly to the case of dianion 1a2− [39], dianion 1b2− adopts a D4h symmetric structure in which the bond lengths within the same bond types, flank (5), flank (6), rim, hub (5), hub (6), and spoke, are identical. However, the lengths of hub (5) and hub (6) are significantly different, indicating that the inner cyclooctatetraene (COT) moiety remains intact. This is consistent with the occupation of two electrons at the outer periphery of the TCPTP core forming an AWA structure of 22 π-electron outer and 8 π-electron inner circuits [39]. The theoretical bond length characteristic of 1b2− is not significantly different in dimer complex K+4[1b2−]2.
With the further reduction of 1b2− to 1b4−, flank (5) bonds are elongated, presumably due to electronic repulsion. More importantly, the lengths of hub (5) bonds are shortened, whereas those of hub (6) bonds are elongated significantly, making them nearly identical (ca. 1.4 Å), clearly indicating the formation of a 10 π-electron inner circuit with small BLA by the occupation of two electrons. Therefore, based on the structural features, tetraanion 1b4− is deduced to adopt an AWA structure. Notably, however, the bond lengths of dimer complex K+8[1b4−]2 are different from those of the free tetraanion 1b4−. Specifically, the flank (5) and hub (5) bonds are shortened, whereas flank (6) and hub (6) bonds are elongated, increasing the BLA featuring a cyclopentadienide structure due to the coordination of potassium cations.

3.2.3. Theoretical Charge Distributions

Charge distributions are studied using the difference in Hirshfeld [72] charge distributions between 1b/1b2−, 1b/1b4−, and 1b2−/1b4−, as shown in Figure 7. For the dimer complexes, net Hirshfeld charges are shown in Figure S8. These results indicate that the first two electrons are located at the outer periphery of the TCPTP core, whereas the second two electrons are accommodated at the inner eight-membered ring and the apical carbon D of the outer periphery. The Hückel MO for the parent 1a (Figure S12) also suggests the location of negative charges at the apical positions of the five-membered ring. This is in agreement with the chemical shift change in the 13C NMR spectra discussed above. Though the elucidation of similar difference charge distributions is not possible for the dimer complexes, the net charges shown in Figure S8 are consistent with those for free anions.

3.2.4. Magnetically Induced Ring Current Tropicities

Finally, ring current tropicities are examined based on vector maps for magnetically induced currents (MICs) calculated for monomer dianion 1b2− and tetraanion 1b4− (Figure 8). As reported previously [39], dianion 1b2− exhibits counter-rotating diamagnetic and paramagnetic currents at the outer and inner peripheries, respectively, indicating an AWA structure consisting of 22 π-electron outer and 8 π-electron inner circuits. In tetraanion 1b4−, however, although the diamagnetic current at the outer periphery is retained, the paramagnetic current also persists at the inner eight-membered ring, even more intensely than in dianion 1b2−, as indicated by the bond-integrated MIC values in Figure 8b, exhibiting counter-rotating ring currents, in contrast to the expected diamagnetic 10 π-electron circuit of the AWA form discussed above. Therefore, the paratropic current at the inner ring may derive from a different origin than that of 1b2−. Notably, based on the anisotropy of the induced current density (ACID) of the tetraanion of CPTP (Figure 1), which is regarded as a π-extended homolog of TCPTP, similar counter-rotating ring currents, including the paratropic current at a 26 π-electron inner ring and the diatropic current at a 38 π-electron outer ring, were reported [36]. The result, which contradicts the aromatic electron numbers of both rings, was explained in terms of the superposition of the local aromatic benzene ring, as in the case of kekulene.

4. Discussion

The tetraanion 1b4− of TCPTP was generated by a reduction in the neutral compound 1b with potassium. The formation of 1b4− was deduced from the air oxidation to 1b, the sum of the upfield shift in the 13C NMR spectrum induced by the accommodation of four negative charges, and the agreement of the experimental and theoretical chemical shifts. The variable-temperature NMR study revealed the formation of dimer and larger aggerates, although the size of the aggregates was not defined. The structures, charge distributions, and magnetically induced ring current tropicities of the tetraanion were studied theoretically, including a dimeric complex which represents aggregates, to reveal the conjugation modes.
The observed chemical shifts in both 1H and 13C NMR spectra indicate that the negative charges distribute to both the inner and outer rings of 1b4−. This suggests that 1b4− adopts an AWA structure consisting of inner 10 π-electron and outer 22 π-electron perimeters, similar to the case of dianion 1b2−, in accordance with the theoretical structure. However, in view of the large upfield shift in the 13C NMR spectrum and the theoretical charge distribution at the apical carbon of the five-membered ring, together with counter-rotating ring currents at the outer and inner rings revealed by theoretical ring current analysis, there exists a significant contribution of the cyclopentadienide form, which may be favorable for coordination to potassium ions.
The TCPTP core of 1b4− is isoelectronic with (planarized) [8]circulene (Figure 9). Since the latter consists of four sets of alternating 6 π-electron benzene rings and 2 π-electron bridges [73,74,75], it would not adopt an AWA form because of the aromatic sextet of the constituent benzene rings. In 1b4−, the cyclopentadienide moiety is regarded to take the 6 π-electron part (Scheme 1), in view of its larger aromaticity than benzene, as estimated based on structural and magnetic criteria [76,77,78]. The current density analyses for kekulene, coronene, and corannulene are reported to exhibit outer diatropic and inner paratropic currents due to the strong coupling of the outer and inner cycles [79,80]. Accordingly, we ascribe the paratropic ring current in the central ring of 1b4− to the contribution of the 6 π-electron cyclopentadienide structure. It turns out, therefore, that the generation of the 6 π-electron cyclopentadienide leads to the local aromatic conjugation mode, which predominates over the global annulenoid conjugation mode.

5. Conclusions

A 32 π-electron tetraanion of tetracyclopentatetraphenylene (TCPTP) and a doubly cyclic π-conjugated system with 28 π-electrons were generated by exhaustive reduction with potassium. Based on spectroscopic and theoretical investigations, it is suggested that the tetraanion adopts a globally delocalized double annulenoid (annulene-within-an-annulene, AWA) form, with 22 π-electron outer and 10 π-electron inner aromatic perimeters. However, in view of the excess charges mainly located at the apical position of the pentagonal rings, a significant contribution of the cyclopentadienide form is indicated. A theoretical analysis of magnetically induced ring current tropicities reveals counter-rotating ring currents at the outer and inner rings, supporting the predominant contribution of the cyclopentadienide form.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/chemistry7020051/s1. Figure S1: 1H NMR spectrum of tetraanion 1b4− in THF-d8 at 30 °C; Figure S2: 13C NMR spectrum of tetraanion 1b4− in THF-d8 at 30 °C; Figure S3: 13C NMR spectrum of tetraanion 1b4− in THF-d8 at −90 °C; Figure S4: Partial (a) HMQC and (b) HMBC spectra for tetraanion 1b4− in THF-d8 at 30 °C; Figure S5: Bond lengths [Å] of monomers (a) 1b, (b) 1b2−, and (c) 1b4−; Figure S6: A local minimum structure and bond lengths [Å] of dianion dimer K+4[1b2−]2; Figure S7: A local minimum structure and bond lengths [Å] of tetraanion dimer K+8[1b4−]2. Figure S8: Net Hershfeld charges for dimeric (a) dianion K+4[1b2−]2 and (b) tetraanion K+8[1b4−]2; Figure S9: Energies and spatial distributions of MOs for dianion dimer K+4[1b2−]2; Figure S10: Energies and spatial distributions of MOs for tertaanion dimer K+8[1b4−]2; Figure S11: Theoretical spectra for dimeric dianion K+4[1b2−]2 (red), tetraanion K+8[1b4−]2 (blue) simulated from the results of TD-DFT calculations; Figure S12: Frontier molecular orbital levels for the neutral TCPTP 1a at the Hückel MO level with the electron configuration; Table S1: Summary of theoretical 1H NMR chemical shifts; Table S2: Theoretical bond lengths (Å) of neutral 1b, dianions 1b2− and K+4[1b2−]2, and tetraanions 1b4−and K+8[1b4−]2; Table S3: Excitation properties of K+4[1b2−]2 obtained from the TD-DFT calculations; Table S4: Excitation properties of K+8[1b4−]2 obtained from the TD-DFT calculations; Table S5: 1b optimized at the RB3LYP-D3/6-31G* PCM(THF); Table S6: 1b2− optimized at the RB3LYP-D3/6-31G* PCM(THF); Table S7: 1b4− optimized at the RB3LYP-D3/6-31G* PCM(THF); Table S8: K+4[1b2−]2 optimized at the RB3LYP-D3/6-31G* PCM(THF); Table S9: K+8[1b4−]2 optimized at the RB3LYP-D3/6-31G* PCM(THF).

Author Contributions

Conceptualization, Y.T. and M.N.; formal analysis, H.M., R.S., R.K. and A.M.; investigation, H.M., R.K. and A.M.; resources, M.N., M.U., N.K., Y.I. and Y.T.; writing—original draft preparation, Y.T.; writing—review and editing, A.M., N.K. and R.K.; funding acquisition, Y.T., A.M., M.U., N.K., R.K., Y.I. and M.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was conducted under the auspices of the Japan Society for the Promotion of Science (JSPS KAKENHI; Grant Number JP18H05353, JP21K05021, JP20H02728, JP17H06173, JP17H05430, JP18K05076, JP21K04995, JP21H05489, JP18H01943, JP21H01887, JP20K21273, JP20H5841), the National Science and Technology Council, Taiwan (110-2113-M-A49-008, 111-2113-M-A49-009), and the Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University).

Data Availability Statement

Data are contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AWAAnnulene-within-an-annulene
BLABond length alternation
TCPTPTetracyclopentatetraphenylene
TCTetracyclopentatetraphenanthlenyene
GIMICGauge-including magnetically induced current
ACIDAnisotropy-induced current density

Appendix A

The theoretical geometries of the dimer complexes of dianion K+4[1b2−]2 and tetraanion K+8[1b4−]2 are described here in more detail (Figures S6 and S7). In general, potassium ions sandwiched by the TCPTP decks adopt η2-, η5-, and η6-coordination modes, which are found in the K+ complexes of fluorenyl anions with TMEDA, diglyme, or crown ether ligands [67,68,69,70].
In dianion K+4[1b2−]2, the top and bottom TCPTP cores are twisted with an average dihedral angle of the CD–CE bonds of the top and bottom TCPTP decks of 25.6° which is larger than that found in the crystal structure of K+4[1b2−]2 (16.0°) [39]. Potassium ions are coordinated to the six-membered rings (η6) of one of the TCPTP cores, with the K···C distance ranging from 3.09 to 3.25 Å (average 3.15 Å), and are located on the bonds (η2) shared by the five- and six-membered rings of the other TCPTP core, with the K···C distance ranging from 2.94 to 3.16 Å (average 3.05 Å). This coordination feature is also in accordance with that found in the crystal structure of 1b2− [39].
In tetraanion complex K+8[1b4−]2, the mesityl groups are slightly more twisted, with the average dihedral angles of the CD–CE bonds of 27.7°. Similarly to dianion complex K+4[1b2−]2, potassium ions are coordinated to the six-membered rings (η6) of one of the TCPTP cores, with the K···C distance ranging from 3.00 to 3.30 Å (average 3.11 Å). However, in other TCPTP cores, K+ ions are coordinated to the five-membered rings (η5), with the K···C distance ranging from 2.93 to 3.25 Å (average 3.12 Å). The potassium ions located on the top and bottom faces of the dimer are located on the bonds shared by the six- and eight-membered rings, with the K···C distance ranging from 2.90 to 3.04 Å (average 2.94 Å). Thus, the coordination modes of the potassium ions located between the TCPTP decks of the dimeric dianion and tetraanion complexes of 1b are not significantly different.

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Figure 1. Chemical structures of cyclically arranged fused rings: corannulene, kekulene, TCPTPs 1a and 1b, CPTP, 8MC, and 10MC.
Figure 1. Chemical structures of cyclically arranged fused rings: corannulene, kekulene, TCPTPs 1a and 1b, CPTP, 8MC, and 10MC.
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Scheme 1. Generation of tetraanion 1b4− in the AWA and cyclopentadienide forms through dianion 1b2− from neutral TCPTP derivative 1b.
Scheme 1. Generation of tetraanion 1b4− in the AWA and cyclopentadienide forms through dianion 1b2− from neutral TCPTP derivative 1b.
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Figure 2. Variable temperature 1H NMR spectra of 1b4− in THF-d8 for (a) aromatic and (b) aliphatic signal regions, (c) with labeling for hydrogen atoms. In (b), the signal marked by an asterisk is due to the residual proton of the solvent.
Figure 2. Variable temperature 1H NMR spectra of 1b4− in THF-d8 for (a) aromatic and (b) aliphatic signal regions, (c) with labeling for hydrogen atoms. In (b), the signal marked by an asterisk is due to the residual proton of the solvent.
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Figure 3. Partial 13C NMR spectra of (a) 1b at 30 °C with labeling for carbon atoms, (b) 1b4− at 30 °C, (c) −30 °C, and (d) −90 °C in THF-d8 with signal assignment.
Figure 3. Partial 13C NMR spectra of (a) 1b at 30 °C with labeling for carbon atoms, (b) 1b4− at 30 °C, (c) −30 °C, and (d) −90 °C in THF-d8 with signal assignment.
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Figure 4. (a) Two types of MCD Faraday A terms. A positive (negative) A term is associated with an electronic transition from a nondegenerate (degenerate) occupied molecular orbital to a degenerate (nondegenerate) unoccupied molecular orbital. (b) MCD and absorption spectra of 1b4− measured in THF at room temperature. (c) Frontier molecular orbitals of 1a4− with D4h symmetry (top) and its calculated absorption spectrum (bottom). H = HOMO and L = LUMO. All calculations were carried out at the B3LYP/6-31G(d) level.
Figure 4. (a) Two types of MCD Faraday A terms. A positive (negative) A term is associated with an electronic transition from a nondegenerate (degenerate) occupied molecular orbital to a degenerate (nondegenerate) unoccupied molecular orbital. (b) MCD and absorption spectra of 1b4− measured in THF at room temperature. (c) Frontier molecular orbitals of 1a4− with D4h symmetry (top) and its calculated absorption spectrum (bottom). H = HOMO and L = LUMO. All calculations were carried out at the B3LYP/6-31G(d) level.
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Figure 5. Theoretical structures of (a) dianion dimer complex K+4[1b2−]2 and (b) tetraanion dimer complex K+8[1b4−]2.
Figure 5. Theoretical structures of (a) dianion dimer complex K+4[1b2−]2 and (b) tetraanion dimer complex K+8[1b4−]2.
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Figure 6. (a) Bond labels and (b) theoretical bond lengths of neutral 1b (gray), 1b2− (orange), 1b4− (blue), dimeric K+4[1b2−]2 (green), and dimeric K+8[1b4−]2 (red). For dimers, the bond lengths of one of the TCPTP cores are plotted, since two TCPTP cores have slightly different bond lengths.
Figure 6. (a) Bond labels and (b) theoretical bond lengths of neutral 1b (gray), 1b2− (orange), 1b4− (blue), dimeric K+4[1b2−]2 (green), and dimeric K+8[1b4−]2 (red). For dimers, the bond lengths of one of the TCPTP cores are plotted, since two TCPTP cores have slightly different bond lengths.
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Figure 7. Difference in Hirshfeld charges (teal: increase in negative charge) calculated for (a) 1b/1b2−, (b) 1b/1b4−, and (c) 1b2−/1b4−.
Figure 7. Difference in Hirshfeld charges (teal: increase in negative charge) calculated for (a) 1b/1b2−, (b) 1b/1b4−, and (c) 1b2−/1b4−.
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Figure 8. Vector maps for MIC calculated for (a) free dianion 1b2− and (b) tetraanion 1b4−. Clockwise/counter-clockwise arrows represent diatropic/paratropic contributions of the ring current. Values in the figure represent bond-integrated MIC values [nA/T] for the periphery and internal rings.
Figure 8. Vector maps for MIC calculated for (a) free dianion 1b2− and (b) tetraanion 1b4−. Clockwise/counter-clockwise arrows represent diatropic/paratropic contributions of the ring current. Values in the figure represent bond-integrated MIC values [nA/T] for the periphery and internal rings.
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Figure 9. Chemical structure of planarized [8]circulene.
Figure 9. Chemical structure of planarized [8]circulene.
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Table 1. Experimental and theoretical 13C NMR chemical shifts (δ, ppm) of neutral TCPTP 1b, dianion 1b2−, and tetraanion 1b4−, and chemical shift differences (Δδ, ppm) for 1b/1b2−, 1b/1b4−, and 1b2−/1b4− 1.
Table 1. Experimental and theoretical 13C NMR chemical shifts (δ, ppm) of neutral TCPTP 1b, dianion 1b2−, and tetraanion 1b4−, and chemical shift differences (Δδ, ppm) for 1b/1b2−, 1b/1b4−, and 1b2−/1b4− 1.
CarbonMethodδ 1b 2δ 1b2− 3δ 1b4− 4Δδ 1b/1b2−Δδ 1b/1b4−Δδ 1b2−/1b4−
Aexp123.7110.5108.8−13.2−14.9−1.7
theor (monomer)112.999.297.7−13.7−15.2−1.5
theor (dimer)–102.6100.0−10.3−12.9−2.6
Bexp148.2 5120.6 6122.1−27.6 −26.1+1.5
theor (monomer)136.4109.4116.0−27.0−20.4+6.6
theor (dimer)–109.0111.1−27.4−25.3+2.1
Cexp148.2 5125.5 6103.4 7−22.7 −44.8 −22.1
theor (monomer)135.4115.7103.4−19.7−32.0−12.3
theor (dimer)–114.293.0−21.2−42.4−21.2
Dexp146.9118.4103.4 7−28.5−43.5−15.0
theor (monomer)138.1109.486.1−28.7−52.0−23.2
theor (dimer)–113.392.3−24.8−45.8−21.0
Eexp127.4138.0142.1+10.6+14.7+4.1
theor (monomer)117.3129.6134.6+12.3+17.3+5.0
theor (dimer)–124.8129.6+7.5+12.3+4.8
Fexp135.8138.0138.8+2.2+3.0+0.8
theor (monomer)127.8131.0128.6+3.2+0.8−2.4
theor (dimer)–129.2129.6+1.4+1.8+0.4
Gexp128.4127.5128.1−0.9−0.3+0.6
theor (monomer)117.3116.8115.4−0.5−1.4−0.9
theor (dimer)–117.8117.4+0.5+0.1−0.4
Hexp137.4133.8132.2−3.6−5.2−1.6
theor (monomer)128.3123.9114.4−4.4−13.9−9.5
theor (dimer)–127.4123.5−0.9−4.8−3.9
1 Theoretical chemical shifts in neutral 1b are average values calculated at the LC-UBLYP(μ = 0.214)/6-31+G*//RB3LYP-D3/6-31G*. Theoretical chemical shifts in dianion and tetraanion are average values calculated at the LC-RBLYP(μ = 0.214)/6-31+G*//RB3LYP-D3/6-31G* for monomeric 1b2−, dimer complex K+4[1b2−]2, monomeric 1b4−, and dimer complex K+8[1b4−]2. Here, 2 represents 30 °C in THF-d8 [39], 3 −90 °C in THF-d8 [39], and 4 30 °C in THF-d8 (this work). 5 The average of two peaks at 148.1 and 148.2 ppm. 6 Since the chemical shifts for carbons B and C of 1b2− were not unequivocally determined by 2D NMR, they were assigned based on the better fit to the theoretical chemical shifts of 1b2−. 7 The average of two intense peaks.
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Miyoshi, H.; Sugiura, R.; Kishi, R.; Muranaka, A.; Uchiyama, M.; Kobayashi, N.; Ie, Y.; Nakano, M.; Tobe, Y. Tetraanion of Tetracyclopentatetraphenylene Derivative: Global Versus Local Conjugation Modes. Chemistry 2025, 7, 51. https://doi.org/10.3390/chemistry7020051

AMA Style

Miyoshi H, Sugiura R, Kishi R, Muranaka A, Uchiyama M, Kobayashi N, Ie Y, Nakano M, Tobe Y. Tetraanion of Tetracyclopentatetraphenylene Derivative: Global Versus Local Conjugation Modes. Chemistry. 2025; 7(2):51. https://doi.org/10.3390/chemistry7020051

Chicago/Turabian Style

Miyoshi, Hirokazu, Ryosuke Sugiura, Ryohei Kishi, Atsuya Muranaka, Masanobu Uchiyama, Nagao Kobayashi, Yutaka Ie, Masayoshi Nakano, and Yoshito Tobe. 2025. "Tetraanion of Tetracyclopentatetraphenylene Derivative: Global Versus Local Conjugation Modes" Chemistry 7, no. 2: 51. https://doi.org/10.3390/chemistry7020051

APA Style

Miyoshi, H., Sugiura, R., Kishi, R., Muranaka, A., Uchiyama, M., Kobayashi, N., Ie, Y., Nakano, M., & Tobe, Y. (2025). Tetraanion of Tetracyclopentatetraphenylene Derivative: Global Versus Local Conjugation Modes. Chemistry, 7(2), 51. https://doi.org/10.3390/chemistry7020051

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