Main

[18]Annulene, (CH)18, is one of the iconic molecules of organic chemistry. Sondheimer’s investigation of this compound in the 1960s provided a compelling endorsement for molecular orbital theory by showing that Hückel’s 4n + 2 rule extends to molecules substantially larger than benzene1,2,3. In 1973, Oth, Woo and Sondheimer reported that [18]annulene can be reduced to an anti-aromatic dianion (20 π electrons)4. Here we show that their published structural assignment of this dianion was incorrect, and that [18]annulene can also be reduced to a stable aromatic tetraanion (22 π electrons). The 1H nuclear magnetic resonance (NMR) spectra of the dianion and tetraanion confirm that they are anti-aromatic and aromatic, respectively, and indicate that [18]annulene adopts a C2v conformation in both reduced states that contrasts the virtual D6h symmetry of the neutral ring.

Oth et al. reported4 that [18]annulene 1 can be reduced with potassium, and that the 1H NMR spectrum of the resulting dianion exhibits peaks at 29.5, 28.1 and −1.13 ppm, at −110 °C in THF-d8. They assigned this spectrum to an interconverting mixture of two isomers of the dianion in a 7:3 ratio (Fig. 1a). However, the published 60 MHz 1H NMR spectrum (Fig. 1b) is poorly resolved, so we decided to reinvestigate this dianion.

Fig. 1: Reduction of [18]annulene with potassium.
figure 1

a, Structures of the dianion proposed in ref. 4. b, 1H NMR spectrum reported in ref. 4 (THF-d8, 60 MHz, −110 °C). c, 1H-13C heteronuclear single quantum coherence NMR spectrum (THF-d8, 500 MHz, −70 °C), and revised structural assignment of the dianion. Resonances a–f were assigned from the correlated spectroscopy (1H-1H COSY) spectrum (Supplementary Figs. 21 and 22).

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Results and discussion

1H NMR spectroscopy of the dianion and tetraanion

We found that reduction of [18]annulene with potassium metal in THF-d8 gives a green solution and that the 1H NMR spectrum of this solution (recorded at −70 °C, 500 MHz) reveals ten CH multiplets (triplets or double doublets) that are clearly resolved in the 1H-13C heteronuclear single quantum coherence NMR spectrum (Fig. 1c). Three signals resonate at high chemical shift: 29.47 (2H), 29.39 (2H) and 27.97 (1H) ppm, and seven appear at low chemical shift: −0.50 (2H), −0.73 (2H), −0.87 (2H), −1.11 (2H), −1.08 (2H), −1.29 (1H) and −1.54 (2H) ppm. This pattern of integrals and number of signals indicates that the dianion has twofold symmetry with five protons inside the ring (strongly deshielded) and 13 protons outside the ring (shielded), as shown in our revised structural assignment (Fig. 1c). The signals that we observed have very similar chemical shifts to those reported in ref. 4 and it is clear we observe the same dianion salt, 1·K2.

Reduction of [18]annulene with potassium gives the paramagnetic radical anion5, followed by the dianion. By contrast, we found that reduction with lithium gives the monoanion, then the dianion, then the 14− tetraanion. All ten 1H NMR multiplets in the spectra of the lithium salts of 12− and 14− have been fully assigned (Fig. 2 and Extended Data Fig. 1). The spectra of 12− and 14− show that they have the same symmetry, but the five inner protons that are strongly deshielded in 12− (δH 30–32 ppm) become strongly shielded in 14− (δH −8 to −9 ppm), while the signals from the outer 13 protons shift in the opposite direction. This reversal is as expected when converting a 20-electron anti-aromatic ring into a 22-electron aromatic system. The 1H NMR spectrum of the tetraanion is essentially independent of temperature in the range of −40 to 55 °C, whereas the spectrum of the dianion becomes broad at temperatures above −50 °C due to dynamic exchange between the environments. Two-dimensional NMR exchange spectroscopy reveals that this dynamic process involves rotation of two CH units (f and g) to redefine the symmetry plane of the ring (Extended Data Figs. 2 and 3 and Supplementary Figs. 24–26). It was reported in ref. 4 that solutions of the potassium salt 1·K2 decompose rapidly at temperatures above 0 °C, whereas in our hands, the solution in anhydrous THF is stable even at 40 °C and can be stored for weeks in a sealed glass tube at room temperature. Exposing a solution of 14− to oxygen (O2) regenerates neutral 1.

Fig. 2: 1H NMR spectra of the dianion (top) and tetraanion (bottom) with Li+ counter ions.
figure 2

THF-d8; temperatures were −70 °C for 12− and 25 °C for 14−; 500 MHz. The spectra were fully assigned using 1H-1H COSY and nuclear Overhauser effect spectroscopy techniques (Supplementary Figs. 32, 33, 38, 39 and 44–46).

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X-ray crystallography of the tetraanion lithium salt

The lithium salt of 14− was crystallized by layered addition of hexanes to a solution in THF. The geometry of the 14− tetraanion from single-crystal X-ray diffraction analysis (Fig. 3) is fully consistent with the 1H NMR spectra of 12− and 14− (Figs. 1 and 2). To our surprise, the crystal structure reveals a sandwich complex composed of two tetraanionic macrocycles with five Li+ ions intercalated between them and three external Li+ cations each coordinated to three THF ligands (Fig. 3). The molecular formula of the solid-state tetraanion salt can be written as (THF)3Li//1//Li5//1//[Li(THF)3]2 and we use the simplified abbreviation 12·Li8. Formation of sandwich columnar species by 1 has been postulated theoretically, although those models assumed that the conformation of the neutral [18]annulene is preserved on reduction6.

Fig. 3: Single-crystal X-ray structure of 12·Li8.
figure 3

a, The asymmetric unit (interstitial solvent, hydrogen atoms and minor disorder components omitted for clarity) with C–Li distances in the range 2.175(4)–2.745(4) Å (internal ions) and 2.285(3)–2.591(4) Å (external ions) indicated by thin tubes. b,c, Two orthogonal views on the sandwich part: top view (b) and side view (c). Colour code: C, grey; O, red and Li, blue.

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In the crystal structure of 12·Li8, the distance between centroids of the rings is 3.8926(6) Å and the C2 axes of the stacked C18H184− units are rotated with respect to each other by a torsional angle of 74° (where the axis of rotation is the line connecting the centroids, Supplementary Fig. 7). The molecules stack to form columns in the solid state, due to C–H···π interactions between the coordinated THF molecules and anions (Supplementary Fig. 9). The C–C bond lengths in the [18]annulene skeleton are in the range of 1.390(3)–1.436(3) Å (mean 1.411 Å), which is slightly longer than the mean C–C bond length in neutral [18]annulene (1.395 Å). There is no evidence for bond-length alternation (BLA < 0.02 Å; see Supplementary Information for details), as expected for an aromatic system. Density functional theory (DFT) calculations at the BLYP45/def2-TZVP level confirm the absence of BLA in the 14− anion in the gas phase. This contrasts with the situation in the neutral macrocycle. The crystal structure of neutral 1 shows no indications of BLA7,8,9,10, but computational studies conclude that the molecule rapidly interconverts between bond-alternate structures (for example, at BLYP45/def2-TZVP, the BLA in neutral 1 is 0.039 Å)11,12,13,14,15,16. Although the neutral molecule has approximate D6h symmetry, its symmetry is reduced by BLA and by the fact that the inner hydrogen atoms may sit slightly out of plane8.

7Li NMR spectroscopy of the tetraanion lithium salt

7Li NMR spectroscopy shows that the sandwich complex is formed in solution in THF, as well as in the solid state. Thus, the spectrum of the tetraanion (recorded at −80 °C in THF-d8) gives signals for two types of lithium environment: a sharp peak at δLi = −15.66 ppm for intercalated lithium cations, strongly shielded by the two aromatic 14− rings, and a broad peak at −2.0 ppm for the external lithium cations. By contrast, the anti-aromatic dianion gives a 7Li NMR spectrum (recorded at −60 °C in THF-d8) with one broad peak at δLi = 2.43 ppm, indicating weak interactions of the Li+ ions with the dianion (δLi = 0 ppm for LiCl in THF-d8).

Formation of a heteroleptic sandwich with corannulene

Corannulene is the only hydrocarbon previously reported to form a polynuclear anion sandwich structure similar to 12·Li8, and there is a close resemblance between these structures17,18,19. The geometry of 14− resembles that of corannulene (2, Fig. 4a) and both hydrocarbons form lithium sandwich complexes in their tetraanionic states, so we decided to test the possibility of forming a heteroleptic sandwich built from 14− and 24−. Exposing an equimolar mixture of 1 and 2 to lithium metal in THF-d8 led to a distinct 1H NMR spectrum (Fig. 4b,c). The symmetry of the annulene signals is similar to that in 12·Li8 and integration of the singlet peak corresponding to corannulene suggested formation of a 1:1 complex. The most notable difference was observed for the inner [18]annulene protons, which shift to δH = −12.50 and −10.92 ppm, indicating that they are strongly shielded by the corannulene bowl. The 7Li NMR spectrum recorded at −60 °C features three highly shielded Li environments at δLi = −17.92, −18.81 and −19.12 ppm integrating as 2:1:2, matching the symmetry of [18]annulene anion (Fig. 4d). Variable temperature 1H NMR studies showed that the corannulene singlet peak becomes sharper at 40 °C; on the other hand, lowering the temperature leads to splitting of this peak, indicating that the rotation of the corannulene bowl becomes slow on the NMR time scale making its protons inequivalent.

Fig. 4: Formation of a heteroleptic sandwich.
figure 4

a, Observed conformation of 14− and the corannulene tetraanion 24−. b,c, 1H NMR spectra of 12·Li8 (b) and 1·2·Li8 (c) (500 MHz, THF-d8, 25 °C; * denotes a trace of benzene). d, 7Li NMR spectrum of 1·2·Li8 (194 MHz, THF-d8, −80 °C). e, The single-crystal X-ray structure of 1·2·Li8 with minor disorder components and hydrogen atoms omitted for clarity and with C–Li distances in the range 2.180(15)–2.694(16) Å indicated by thin tubes. f,g, Two orthogonal views of the sandwich unit: top view (f) and side view (g). Colour code: C, grey; O, red; Li, blue.

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Definitive evidence for formation of the heteroleptic sandwich came from single-crystal X-ray diffraction analysis. The molecular structure is similar to the homo-sandwich 12·Li8 (Fig. 4e) and its formula can be written as [(THF)3Li]2//1//Li5//2//Li(THF)3, abbreviated to 1·2·Li8. Both decks coordinate externally to three cations in total, whereas five Li+ cations are jammed in between the rings. The five internal lithium cations lie approximately in the same plane; the root-mean-square deviations from planarity for the intercalated Li+ ions in 12·Li8, 1·2·Li8 and 22·Li8 are 0.169, 0.044 and 0.010 Å, respectively. The bowl depth of the corannulene is reduced from 0.875(2) in neutral 2 to 0.549(6) Å in 1·2·Li8, to a smaller extent than in the corannulene homo-sandwich 22·Li8 (0.288(2) Å, ref. 17).

Theoretical modelling

Quantum-chemical calculations, at various levels of theory (Fig. 5), confirmed that [18]annulene undergoes a dramatic change in geometry on reduction to the dianion, and provide insights into the causes of this switch in conformation. Schleyer showed that density functionals with a high proportion of exact exchange are needed to properly describe annulenes12. Thus, we optimized the geometries of three possible conformers (a, b and c) of 10 and 12− using a range of functionals with an exact exchange of 37.5–54%. We then evaluated the energies of these optimized geometries at the highly accurate coupled clusters singles, doubles and perturbative triples level20,21, DLPNO-CCSD(T*)-F12 (Fig. 5a,b and Supplementary Information Section 7.3). In the neutral aromatic state, the virtually D6h conformer 1a0 has the lowest energy (Fig. 5a), whereas in the anti-aromatic dianion the less symmetric conformer 1b2− (five inner and 13 outer protons) is more stable (Fig. 5b).

Fig. 5: Energies of different molecular geometries for neutral [18]annulene and the dianion.
figure 5

a,b, DLPNO-CCSD(T*)-F12 energies of possible conformers of 1a, 1b, 1c in their neutral form (a) and as dianions (b), calculated for optimized geometries obtained using M06-2X (hollow diamonds), MPWB1K (hollow circles) and BLYPxx (triangles; xx denotes the percentage of exact exchange, given on the horizontal axis). c, A qualitative Walsh diagram comparing π-orbital energies of a and b conformers (MOs calculated using BLYP45).

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Owing to the higher symmetry of 1a, π orbitals with small angular momentum (k = 0 and k = 1; Fig. 5c) have a larger overlap in 1a than in 1b, leading to a lower-energy π system (Eπ,1B − Eπ,1A = 22 kJ mol−1) and an overall lower energy of 1a (ECCSD(T),1b − ECCSD(T),1A ≅ 9 kJ mol−1). By contrast, more localized π orbitals (k = 4 and k = 5) are lower in 1b, so adding two electrons stabilizes 1b2− relative to 1a2−. Moreover, Coulombic repulsion in 1b2− is lower than in 1a2− (Supplementary Fig. 65), resulting in a strong preference for 1b2− (\(E_{{\rm{CCSD}}({\rm{T}}),1{\rm{B}}^{2-}}\) − \(E_{{\rm{CCSD}}({\rm{T}}),1{\rm{A}}^{2-}}\)  ≅ 46 kJ mol−1). As the tetraanion always appears as an Li–bound dimer sandwich, the preference for 1b4− also can be attributed to electrostatics. In both anions, the c conformer proposed in ref. 4 (Fig. 1a) has higher energy than both a and b.

We find that the BLYP45 functional (45% exact exchange), which is a slight modification of BHLYP (Becke’s half and half functional combined with the LYP correlation functional)22 (50% exact exchange), gives a good account of both energies and chemical shifts in all three reduction states (Supplementary Figs. 66–68), confirming the anti-aromaticity of 1b2− (δout − δin = −30.2 experiment versus −28.5 calculation) and the aromaticity of 1b4− (δout − δin = +14.9 experiment versus +17.9 calculation). BLYP45 predicts that both 1b2− and 1b4− have C2v symmetry, but 1b2− has significant BLA (0.06 Å on average) consistent with charge localization (Supplementary Fig. 65), whereas 1b4− has no BLA (<0.01 Å).

Conclusions

Our results reveal that the postulated and widely accepted geometry of dianionic [18]annulene was incorrect, and that [18]annulene undergoes a dramatic switch in conformation on reduction. Quantum calculations demonstrate that this surprising conformational change is driven by Coulombic repulsion and optimization of π-bonding interactions. In contradiction with previous reports, this dianion is stable even at 40 °C, if it is protected from oxygen and moisture. We also show that the dianion can be further reduced to a tetraanion, which features a substantial aromatic ring current, despite forming a lithium-intercalated sandwich. This redox activity, and the ability to intercalate lithium cations, point to potential applications of annulenes as energy storage materials23. The global change in conformation of [18]annulene on reduction to the dianion contrasts with the behaviour of other π-conjugated macrocycles, such as [24]paracyclophanetetraene, which only exhibit subtle structural changes on reduction24. Oth and coworkers were correct in concluding that the dianion of [18]annulene is anti-aromatic, and they made the most of the tools available to them, but the availability of high-field 1H NMR spectroscopy changes our view of these anions.

Methods

[18]Annulene 1 was synthesized using the route reported in ref. 10, with some modifications (Supplementary Information Section 2 for details). In situ reduction experiments were carried out in the absence of air and moisture, using established techniques25,26, as detailed in the Supplementary Information Section 2.

DFT calculations used the def2-TZVP basis27, while DLPNO-CCSD(T*)-F12 calculations used the explicitly correlated cc-pVDZ-F12 basis set28. BLYPxx functionals, in which xx is the proportion of exact exchange, were constructed by starting from BHLYP and modifying the proportion of exact and DFT exchange, with their sum kept at 100%. Details and representative ORCA29 input files are given in the Supplementary Information Section 7.