Unveiling the Multielectron Acceptor Properties of π-Expanded Pyracylene: Reversible Boat to Chair Conversion

Abstract

In this work, the chemical reduction of a hybrid pyracylene-hexa-peri-hexabenzocoronene (HPH) nanographene was investigated with different alkali metals (Na, K, Rb) to reveal its remarkable multielectron acceptor abilities. The UV–vis and ¹H NMR spectroscopy monitoring of the stepwise reduction reactions supports the existence of all intermediate reduction states up to the hexaanion for HPH. Tuning the experimental conditions enabled the synthesis of the HPH anions with gradually increasing reduction states (up to −5) isolated with different alkali metal ions as crystalline materials. The single-crystal X-ray diffraction structure analysis demonstrates that the highly negatively charged HPH anions (−4 and −5) exhibit a drastic geometry change from boat-shaped (observed in the neutral parent, mono- and dianions) to a chair conformation, which was proved to be fully reversible by NMR spectroscopy. DFT calculations show that this geometry change is induced by an enhanced interaction between the coordinated metal ions and negatively charged HPH core in the chair conformation.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) serve as redox-active materials in various applications.¹⁻⁵ Their optoelectronic and structural properties can be finely tuned by incorporation of heteroatoms or nonhexagonal rings.⁶⁻¹⁰ By incorporation of a 5-membered ring, positive curvature (bowl shape) can be induced.¹¹ This leads to a considerable pyramidalization of the sp² carbon atoms within the polycyclic framework, thereby weakening the π bonds and reducing the energy of the lowest unoccupied molecular orbital (LUMO).¹²⁻¹⁴ Moreover, according to Hückel’s rule, the conjugated 5-membered ring can take up one electron to achieve formal aromaticity, which makes these compounds prone to reduction.¹⁵⁻¹⁷

The most prominent example of this behavior is found in the C₆₀-fullerene, which incorporates 12 5-membered rings, leading to a spherical geometry.¹⁸⁻²⁰ C₆₀ can reversibly take up six electrons, successively leading to the corresponding hexaanion C₆₀^6–.^21,22 This propensity for multielectron reduction has led to the prominence of C₆₀ as an electron acceptor in specific applications. In particular, salts resulting from the reduction of C₆₀ with alkali metals have gained increasing attention due to their potential use as energy storage and superconducting materials.²³⁻²⁷

To harness these notable electron-accepting properties, PAHs representing defined fullerene cutouts such as bowl-shaped corannulene or sumanene have emerged as attractive synthetic targets.²⁸⁻³⁷ The bowl-shaped geometry makes them particularly prone to reduction/deprotonation and complexation with alkali metals.³⁸⁻⁴¹ Even the smallest buckybowl, corannulene, exhibits high electron-accepting ability and can take up to four electrons, forming a set of carbanions with selective concave and convex metal binding.⁴¹⁻⁴⁴ Several examples of bowl-shaped acceptors and their derivatives have been reported recently,⁴⁵⁻⁴⁷ although most of them can readily accept two electrons, while the analysis of highly reduced PAHs by X-ray crystallography remains challenging.^48,49

Furthermore, another conceivable, yet not less appealing, fullerene cutout, namely, pyracylene, remains considerably less explored.⁵⁰⁻⁵³ With its 12π-electron periphery, pyracylene represents a formally antiaromatic cutout of C₆₀.⁵⁴⁻⁵⁷ Upon addition of two electrons, two cyclopentadienyl moieties are formed, which render the resulting dianion with its 14π electrons formally aromatic. Nonetheless, pyracylene and its homologue dibenzopyracylene show a planar geometry.^58,59 Thereby, they cannot benefit from the pyramidalization and strong metal-binding ability to further enhance their electron-accepting properties as observed for buckybowls.⁶⁰

To address this limitation, we have recently developed a hybrid pyracylene-hexa-peri-hexabenzocoronene (HBC) abbreviated as HPH.⁵³ This compound adopts a boat-like geometry both in solution and in the solid-state with a bowl depth of 3.79 Å, resulting in a curved pyracylene core with pyramidalized sp² carbon atoms. Cyclic voltammetry measurements of HPH revealed two reversible reductions at −1.46 and −1.82 V (vs ferrocene/ferrocenium (Fc/Fc⁺)) in THF. This points toward better charge stabilization compared to pristine pyracylene, which is considerably more difficult to reduce at −1.56 and −2.14 V.^53,61,62 Furthermore, HPH also displays enhanced reducibility relative to other curved π-expanded nanographenes from the literature, such as the warped nanographene (with three reductions at −1.59, −2.02, and −2.31 V vs Fc/Fc⁺) or the corannulene-embedded [6]helicene (with two reductions at −1.90 and −2.18 V vs Fc/Fc⁺).^63,64

The enhanced electron-accepting properties and bowl-shaped geometry of HPH prompted us to investigate its chemical reduction behavior with alkali metals and to target isolation of the gradually reduced negatively charged products in the presence of different alkali metal counterions (Figure 1).

Figure 1.

Planar pyracylene represents a cutout of fullerene C₆₀ with pronounced electron-accepting properties. π-Expansion induces curvature into the pyracylene core and leads to the boat-shaped hybrid-pyracylene-HBC (HPH)⁵³ with potentially enhanced electron-accepting properties explored in this work.

Results and Discussion

Chemical Reduction and Crystallization of HPH Anions

To probe the reduction limits, UV–vis spectroscopy was used to monitor the reaction color changes for the stepwise chemical reduction of HPH (C₁₀₂H₉₆, 1). The addition of Rb metal to a THF solution of 1 led to a quick color change from purple (color of the neutral parent) to grass green (monoanion), reddish-purple (dianion), brownish purple (trianion), dark purple (tetraanion), and violet/blue color for penta- and hexa-reduced products (Figure 2a, see the Supporting Information for more details). To verify these experimental observations of potential 6-fold reduction, in situ¹H NMR and EPR spectroscopy measurements were carried out and provided strong support for remarkable six-electron-accepting abilities of HPH (Figures S12 and S9). The in situ generated dianion, tetraanion, and hexaanion were successfully detected by ¹H NMR spectroscopy, while the intermediate NMR silent radicals (−1, −3, and −5) were identified by EPR spectroscopy. The in situ EPR study of HPH reduction with Rb metal clearly revealed four EPR “silent” diamagnetic species (including neutral HPH) and three EPR detectable radicals. The complex structure of this PAH and limitation of the instrument prevented detailed analysis of multiplicity or hyperfine coupling, but important information can be extracted. In the spectra of both mono- and trianion radicals, a clear set of multiplet peaks are observed in contrast to a broad peak of the penta-reduced state. This may indicate the rapid exchange between the unpaired electrons for the latter highly charged product. Based on NMR results, a similar exchange between diamagnetic and paramagnetic species can be observed, and the in situ generated products with charges of 0, −2, −4, and −6 offered four distinct NMR patterns. The chemical shifts of aromatic protons in ¹H NMR spectra of neutral HPH and its dianion show up in a close range (7.90–9.43 ppm for HPH vs 7.75–9.85 ppm in the dianion). However, upon further reduction to the tetraanion and hexaanion, the aromatic protons become largely upfield shifted to 4.66–6.54 ppm for HPH^4– and 4.42–6.15 ppm for HPH^6–, thus indicating significant electron density rearrangement. Importantly, the NMR spectrum of the hexa-reduced product quenched with O₂ (Figure S12) reveals the reaction reversibility and points out the stability and inherent flexibility of the HPH core toward redox processes.

Figure 2.

UV–vis spectra of (a) HPH and its in situ generated anions and (b) crystals of 1, Na-1^–, K₂-1^2–, Rb₄-1^4–, and Rb₅-1^5– dissolved in THF.

Following the spectroscopic study, the chemical reduction of 1 was carried out with several alkali metals ranging in size and coordination preferences and using anhydrous solvents and inert atmosphere conditions. By changing the nature of alkali metals (Na, K and Rb) and secondary ligands (18-crown-6 ether and [2.2.2]cryptand), the successful isolation of several gradually reduced products with distinct colors (Figure 2b) has been achieved. The use of limited reaction time coupled with addition of crown ether in the Na-induced reaction allowed the isolation of a singly reduced product, [Na⁺(18-crown-6)(THF)₂][1^–] (Na-1^–), crystallized as a solvent-separated ion product (SSIP) with four interstitial THF molecules as [Na-1^–]·4THF. The addition of [2.2.2]cryptand facilitated isolation of the doubly reduced product with potassium counterions, [K⁺(cryptand)]₂[1^2–] (K₂-1^2–), crystallized as a SSIP with four interstitial hexane molecules as [K₂-1^2–]·4C₆H₁₄. Notably, the use of prolonged reaction time for the Na- and K-induced reduction of HPH leads to additional solution color changes indicative of further reduction (Figures S1 and S2). However, these reactions failed to produce single-crystalline products. The switch to Rb metal and longer reaction times enabled the formation of the tetra-reduced anion isolated as a contact-ion complex with rubidium countercations, as [{Rb⁺(18-crown-6)}₄(1^4–)]·10THF ([Rb₄-1^4–]·10THF). The extended reaction time (24 h) for this system resulted in the isolation of the penta-reduced product, [Rb⁺(18-crown-6)][{Rb⁺(18-crown-6)}₄(1^5–)]·5THF ([Rb₅-1^5–]·5THF, Scheme 1). Notably, the in situ UV–vis spectroscopy monitoring of the HPH reaction with Rb metal in the presence of 18-crown-6 ether illustrated that, under these conditions, the reaction stops at the penta-reduced state (Figure S3). Unfortunately, our numerous attempts to isolate the products of the triply- and hexa-reduced states of HPH were not successful, as the resulting very thin fibrous-like needles did not provide sufficient diffraction for structure solution in both cases.

Scheme 1. Chemical Reduction of 1.

Crystal Structures of Stepwise-Reduced HPH Products

The stepwise reduction of 1 with Na and K metals provided two SSIPs with different low charges (−1 and −2) of the HPH core, namely, Na-1^– and K₂-1^2–. In the crystal structure of Na-1^–, two crystallographically independent molecules are found with similar geometric parameters (Figure S14), and only one of which is discussed below. The Na⁺ ion entrapped by one 18-crown-6 and two THF molecules has no direct contacts with the anion 1^– (Figure 3a), with the Na···O_crown and Na···O_THF distances comparable to the previously reported values.^49,46 In the doubly reduced product K₂-1^2–, the two K⁺ ions wrapped by one [2.2.2]cryptand each remain separated from the dianion, with K···N_cryptand and K···O_cryptand distances in line with the values reported in the literature (Figure 3b).^65,66

Figure 3.

Crystal structures of (a) Na-1^– and (b) K₂-1^2–, ball-and-stick model. Hydrogen atoms are omitted for clarity. The Na···O_crown and Na···O_THF distances range over 2.329(5)–2.998(15) and 2.286(9)–2.656(6) Å, respectively. The K···N_cryptand distances and K···O_cryptand distances are 2.981(5)–3.020(5) and 2.735(3)–2.878(4) Å, respectively.

The reduction reaction of 1 with Rb metal provided access to the highly reduced HPH anions. The addition of 18-crown-6 ether facilitated crystallization of two contact-ion products with high negative charges (−4 and −5), Rb₄-1^4– and Rb₅-1^5–. In the crystal structure of Rb₄-1^4– (Figure 4a), the highly charged tetraanion converted from a boat-shape conformation to a recliner-chair-type structure, with molecular symmetry changed from C_2v to C_2h. The four Rb⁺ ions are bound to the HPH^4– core, two of which nest in the concave cavities of the core with the other two bound to the HBC units from the opposite sides of the nanographene surface (Figure 6a,c). The Rb–C distances in Rb₄-1^4– spanning over 3.136(2)–3.646(3) Å are comparable to the range (3.025(12)–3.640(12) Å) found in the tetra-reduced corannulene product with mixed Li/Rb metals.⁴² Each Rb⁺ ion is additionally capped by an 18-crown-6 molecule from an open end.

Figure 4.

Crystal structures of (a) Rb₄-1^4– and (b) Rb₅-1^5–, ball-and-stick model. Hydrogen atoms are omitted for clarity. The Rb···O_crown distances for Rb₄-1^4– and Rb₅-1^5– are in the ranges of 2.853(3)–3.144(2) Å and 2.790(3)–3.154(14) Å, respectively. All Rb···O_crown distances are comparable to the reported values.^47,67

Figure 6.

Metal coordination in (a) Rb₄-1^4– and (b) Rb₅-1^5–, ball-and-stick model. Hydrogen atoms, ^tBu-groups, and 18-crown-6 are omitted for clarity. (c) Coordination of Rb⁺ ions, space-filling model; 18-crown-6 molecules are omitted. (d) Curvature difference between 1^4– and 1^5–. (e) Charge-dependent boat–chair conversion between 1 (top) and 1^4–/1^5– (bottom), space-filling models.

Extending the reaction time to 24 h (Scheme 1) with Rb metal allowed the isolation of the remarkable penta-reduced product, Rb₅-1^5– (Figure 4b). In addition to four-coordinated Rb⁺ ions, the fifth Rb⁺ ion, wrapped by an 18-crown-6 molecule, resides as a solvent-separated counterion. Compared to Rb₄-1^4–, the increased negative charge of 1^5– leads to a tighter and more symmetrical coordination of Rb⁺ ions in Rb₅-1^5– (Figure 6b), with the Rb–C distances notably reduced to 3.071(2)–3.598(2) Å (Table S2).

In the solid-state structure of Na-1^–, one [Na⁺(18-crown-6)(THF)₂] cationic moiety is wrapped by the concave faces of two 1^– anions with C–H···π interactions of 2.601(7)–2.990(7) Å. Through additional C–H···π interactions between the other cationic moiety and convex face of 1^– (2.584(7)–3.010(7) Å), an extended 1D column is formed (Figure S20). A similar packing pattern is observed in K₂-1^2–, namely, two [K⁺(cryptand)] moieties are wrapped by two anions with the C–H···π interactions of 2.362(7)–3.041(7) Å. The other two cationic moieties bridge two HPH units with C–H···π interactions of 2.508(7)–2.978(7) Å to form a 1D column (Figure S21). In the Rb₄-1^4– and Rb₅-1^5– products, no notable secondary interactions are observed in the solid-state structures.

Charge-Dependent Core Deformation

The isolation of the “naked” monoanion and dianion of HPH enables the evaluation of structural deformation upon addition of one and two electrons but without direct alkali metal-binding influence (vide supra). In comparison to neutral parent 1, the monoanion preserves the boat-shaped geometry with a small decrease of curvature, as shown by the slightly increased molecular length (L, 15.734(7) Å vs 15.558 Å in 1) and notably reduced ligand height (H, 9.724(7) Å vs 10.116 Å in 1) and bowl depth (D, 3.748(7) Å vs 3.787 Å in 1), accompanied by changes in dihedral angles (Table 1). The addition of the second electron to 1 further flattens its carbon backbone. The geometric parameters of 1^2– clearly illustrate the reduced curvature, with the height-to-length ratio decreased to 50.7 vs 65.0% in the neutral parent and 61.8% in the monoanion. The boat-type structure of 1^2– is found to be significantly shallower (depth of 2.861(7) Å, Figure 5) than in 1 and 1^–. Notably, the 2-fold reduction also largely decreases the helical torsion from 50° in 1 and 1^– to 43° in 1^2– (Table 1).

Table 1. Selected Distances (Å) and Angles (deg) in 1, 1^–, 1^2–, 1^4–, and 1^5–, along with a Labeling Scheme.

parameter	1	1–	12–	14–	15–
a	1.429/1.434	1.386(6)/1.398(6)	1.380(7)/1.383(7)	1.385(4)	1.383(3)
b	1.370–1.385	1.398(6)–1.415(6)	1.402(7)–1.428(7)	1.440(4)/1.448(4)	1.426(3)/1.440(3)
c	1.399–1.406	1.401(6)–1.433(6)	1.383(7)–1.422(7)	1.416(4)	1.403(3)/1.415(3)
d	1.381	1.373(6)	1.350(7)	1.385(5)	1.370(4)
e	1.493–1.500	1.457(6)–1.471(6)	1.421(7)–1.462(7)	1.442(4)/1.445(4)	1.436(3)/1.439(3)
f	1.401/1.414	1.427(6)/1.446(6)	1.440(7)/1.448(7)	1.475(4)	1.451(3)
length (L)	15.558	15.734(7)	16.470(7)	16.636(5)	16.496(4)
height (H)	10.116	9.724(7)	8.350(7)	3.685(5)	4.801(4)
depth (D)	3.787	3.748(7)	2.861(7)	2.767(5)	2.804(4)
∠A/B	42.6	41.8(5)	30.8(5)	29.9(4)	30.0(3)
∠A/C	79.8	75.4(5)	61.0(5)	0.0(4)	0.0(3)
∠B/C	37.3	33.6(5)	30.6(5)	29.9(4)	30.0(3)
helical torsion (∠B/E)	50.6	50.5(5)	43.2(5)	41.4(4)	42.0(3)

Figure 5.

Core deformation in 1, 1^–, and 1^2–.

Notably, in both singly and doubly reduced products, the C–C bonds around the 5-membered rings of the pyracylene core experience a distinct bond length change and rearrangement. The bonds e become significantly shorter than in the neutral parent, while the bonds f become longer (Table 1). Similar bond length alterations were observed for the doubly reduced products of phenylenetetracene and indenocorannulene.^68,47 The bond lengths of a and d become smaller and more equalized (1.43/1.38 Å in 1 vs 1.39/1.37 Å in 1^– and 1.38/1.35 Å in 1^2–). In contrast, the C–C bonds b and c are slightly elongated and cover a broader range (Table 1).

In Rb₄-1^4– and Rb₅-1^5–, the high negative charge of HPH makes it highly favored for metal coordination, which stabilizes the system by mitigating the charge and electron density. To accommodate coordination of multiple Rb⁺ ions, the boat-shaped HPH is converted into a chair-type geometry. The reduction-induced boat–chair conversion largely increases the molecular length (16.636(5)/16.496(4) Å in 1^4–/1^5– vs 15.558 Å in 1) and reduces the bowl depth (2.767(5)/2.804(4) Å in 1^4–/1^5– vs 3.787 Å in 1) of the HPH framework (Figure 6d). The same trend is observed for the decreased dihedral angles between planes A, B, and C (Figure 6e), indicating the overall reduced curvature of the carbon scaffold.

The similarity of metal coordination patterns in Rb₄-1^4– and Rb₅-1^5– enables the evaluation of the stepwise charge increase on the HPH geometry. Compared to the tetraanion, the addition of the fifth electron shortens the C–C bond distances along the whole carbon core (Table S4). As a result, the overall molecular length is reduced (16.496(4) Å in 1^5– vs 16.636(5) Å in 1^4–). In contrast, the height (distance between the A and C planes, see Table 1 insert) of the pentaanion is largely increased to 4.801(4) Å from 3.685(5) Å in 1^4–. In addition, the bowl depth and helical torsion are also slightly increased (Table 1).

In summary, the controlled electron addition at the low reduction level allows the HPH anions to keep the same boat-shaped geometry with reduced curvature and increased planarity. In contrast, further reduction of HPH (beyond tetraanion) favors a chair-type conformation, with alkali metal counterions binding to the concave cavities and side HBC units on both sides of the carbon backbone. Following the boat–chair conversion, the acquisition of an additional fifth electron only slightly increases the curvature of the pentaanion core.

The poor solubility of Rb₄-1^4– and the radical character of Rb₅-1^5– thwarted the NMR spectroscopy investigation of the effect of a secondary 18-crown-6 ligand. Therefore, UV–vis spectroscopy investigation was used for getting further insights. A comparison between the in situ UV–vis spectra of [HPH/Rb/18-crown-6] (Figure S3) and [HPH/Rb] in THF (Figure S4) leads to the following observations: (1) the addition of 18-crown-6 contributes to an unexpected stability of the pentaanion, in contrast to the clear conversion to the hexaanion in the absence of crown ether. A similar crown ether-assisted stabilization of the singly reduced radical species was observed for the planar coronene anion,⁶⁹ while examples for highly reduced systems have not been reported previously. (2) Besides the lack of the hexa-reduced stage, discrepancies can be observed in the absorbance spectra of the highly negatively charged tetraanion and pentaanion (vide infra).

Computational Analysis

A computational study on the density functional theory (DFT) level of theory was conducted to further analyze the electron-accepting behavior of HPH (see the Supporting Information for more details). To separate the influences of Rb⁺ and the 18-crown-6 ligand, we first considered the different reduction states of “naked” 1^n– (n = 0–6) embedded in a polarizable continuum mimicking THF. As indicated by the crystal structures, HPH can essentially adopt two different conformations, the boat (like in the crystal structures of neutral 1 and Na-1^– and K₂-1^2–) and the chair (as found for Rb₄-1^4– and Rb₅-1^5–) conformation. In the absence of stabilizing Rb⁺ ions, the boat conformation is always slightly more stable by ∼2 kcal/mol (Table S5, upper row). The boat–chair interconversion occurs via an intermediate state (∼5 kcal/mol less stable than the boat conformation) and has an activation barrier of ≲13 kcal/mol, indicating a rapid conformational change in solution at room temperature (Table S6, Figure S22). Each reduction state is stable in the sense that all electrons are bound, i.e., all occupied orbitals have negative energy with respect to a free electron (Table S7), which highlights the large electron-accepting capabilities of HPH. By inspecting the electron density differences between the charged and neutral species, one notices that the first two electrons are mostly localized at the pyracylene core, whereas further electron addition involves the extended π-system of HPH (Figure S23). Moreover, the bowl depth of “naked” 1^n– decreases up to 2-fold reduction and then increases again to reach a nearly constant value of 3.0 Å (Table S9).

Interestingly, the coordination of {Rb⁺(18-crown-6)} ions to 1^n– switches stabilities, causing the chair conformation to become more stable by ∼3 kcal/mol (Table S5, bottom). To better understand what exactly causes the change of stability, we evaluated the energy differences between the two conformations at the frozen geometries of {Rb⁺(18-crown-6)}₄(1^n–) with either the crown ethers only or the entire {Rb⁺(18-crown-6)} moieties removed (Table S8). While the boat conformation is more stable in the uncomplexed form, the placement of the Rb⁺ ions at the coordination sites corresponding to the crystal structure {Rb⁺(18-crown-6)}₄(1^n–) favors the chair conformation (Table S8). However, a complete relaxation of the Rb⁺₄(1^n–) system essentially leads to a shift of the Rb⁺ ions and shows that they prefer positions closer to the pyracylene core (Figure S24), where the negative charge density is higher. Moreover, the geometric rearrangement of Rb⁺ largely annihilates the energetic preference of the chair conformation (Table S5, middle row). In {Rb⁺(18-crown-6)}₄(1^n–), however, two Rb⁺ cations can stay close to the pyracylene core in the chair conformation, whereas in the boat conformation, none of the Rb⁺ ions can interact with the pyracylene core (Figure S25). This suggests that the chair conformation of {Rb⁺(18-crown-6)}₄(1^n–) is more stable because the Rb⁺ cations can occupy more favorable binding sites. Therefore, the observed geometry change from boat to chair is not intrinsically driven, but due to optimized interactions between the {Rb⁺(18-crown-6)} moiety and HPH anion.

Experimentally, it was found that the reduction of HPH without 18-crown-6 yields the hexaanion, whereas in the presence of the crown ether, the reaction stops at the pentaanion. To analyze this finding, let us consider the formation of the hexaanion (in THF) in both cases:

1

2

From the energies of the involved anions, one can easily compute the energy difference between these two reactions (without the need to calculate the energies of metallic Rb and solvated Rb⁺), which yields that the reduction of the pentaanion with bare Rb (in absence of the crown ether) is favored by ∼8 kcal/mol, i.e., the hexaanion is better stabilized by bare Rb⁺ ions than by {Rb⁺(18-crown-6)}.

Furthermore, the UV–vis absorption spectra of the in situ generated HPH anions were compared with the simulated (on the TD-DFT level) spectra of the 1^n– series (Figure S26). The experimental spectra, especially those for the lower reduced states up to the trianion, are well reproduced confirming the previously made assignment of the spectra. Moreover, all reduced species exhibit electronic transitions in the near-infrared (NIR) region and, partly, even lower. These low-lying excitations correspond to π–π transitions; the same applies for the low-lying transitions in the visible region.

For the trianion and higher charged HPH, we also considered the effect of the π-interacting Rb⁺ cations (Figure S26). In case of the trianion, two Rb⁺ ions, for 1^n– (n = 4–6) four Rb⁺, were taken into account (geometries were fully relaxed). As expected, coordination of Rb⁺ causes some shifts and intensity changes of the absorption bands, but the changes are overall moderate. The conformation or exact position of the Rb⁺ ions has only a small effect on the UV–vis absorptions (Figure S27). For the trianion, the experimental spectrum is better reproduced without Rb⁺, which may indicate that there is no binding or only weak coordination to Rb⁺ in solution. In case of higher reduced HPH, the experimental spectra appear to have less structure or are more broadened than the calculated spectra. This could imply that the anions are not always fully coordinated with four Rb⁺ in solution.

The UV–vis spectra of {Rb⁺(18-crown-6)}₄(1^n–) (n = 4–6) were also simulated (Figure S28). The presence of the crown ether molecules leads to similar changes as observed for the addition of bare Rb⁺ cations. In fact, the simulated spectra of {Rb⁺(18-crown-6)}₄(1^n–) appear more similar to those of 1^n– (without Rb⁺) than to those of Rb⁺₄(1^n–), (Figure S26). The reason is probably that the additional interactions of the Rb⁺ centers with the crown ether ligands in turn weaken the interaction between Rb⁺ and the HPH core.

Conclusions

In summary, the first chemical reduction study of 1 was conducted with alkali metals Na, K, and Rb to reveal its remarkable electron-accepting abilities and to afford a unique family of stepwise-reduced products of HPH (1) in various reduction states. Notably, in contrast to two-reduction steps revealed electrochemically,⁵³ the chemical reduction of HPH with alkali metals induces up to 6-fold electron addition. The stepwise-reduced anions of HPH were isolated with different countercations as single-crystalline materials, ranging from [Na-1^–]·4THF and [K₂-1^2–]·4C₆H₁₄ to highly reduced [Rb₄-1^4–]·10THF and [Rb₅-1^5–]·5THF. The outcomes of one- and two-electron acquisition include the curvature reduction of HPH and better π-conjugation over its curved carbon backbone. In contrast, the higher reduction to the tetra- and penta-reduced states leads to a significant geometry change of the HPH core from the boat conformation to a recliner-chair shaped structure. In comparison to other curved nanographenes, HPH exhibits an advanced electron-accepting ability and can undergo 6-fold reduction vs 4-fold reduction for warped nanographene⁴⁸ and double [7]helicene.⁷⁰ Although the reversible 6-fold reduction was demonstrated by octabenzo[8]circulene,⁴⁹ the revealed boat–chair conformation change of HPH upon multielectron addition and metal coordination is unique. Importantly, the charge-dependent conformation change was proven to be reversible by NMR spectroscopy. The computational results indicate that the conformational change from a boat to a chair is driven by the coordination of multiple cationic Rb⁺/crown ether moieties. The revealed remarkable multielectron accepting properties of HPH should render its promise for energy storage applications.

Data Availability Statement

DFT raw data (geometries, excitation energies and intensities) are available on Zenodo https://zenodo.org/doi/10.5281/zenodo.11181723.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c02314.

• Full experimental details, UV–vis, ¹H NMR and EPR spectra for all prepared compounds, X-ray crystallographic data for selected products, and computational details (PDF)

The authors declare no competing financial interest.