Direct Evidence for a Carbon-Carbon One-Electron σ-Bond PDF

Article Direct evidence for a carbon–carbon one-electron σ-bond https://doi.org/10.1038/s41586-024-07965-1 Takuya Shimajiri1,2,3 ✉, Soki Kawaguchi1, Takanori Suzuki1 & Yusuke Ishigaki1 ✉ Received: 24 November 2023 Accepted: 20 August 2024...

Article Direct evidence for a carbon–carbon one-electron σ-bond https://doi.org/10.1038/s41586-024-07965-1 Takuya Shimajiri1,2,3 ✉, Soki Kawaguchi1, Takanori Suzuki1 & Yusuke Ishigaki1 ✉ Received: 24 November 2023 Accepted: 20 August 2024 Covalent bonds share electron pairs between two atoms and make up the skeletons of Published online: xx xx xxxx most organic compounds in single, double and triple bonds. In contrast, examples of one-electron bonds remain scarce, most probably due to their intrinsic weakness1–4. Check for updates Although several pioneering studies have reported one-electron bonds between heteroatoms, direct evidence for one-electron bonds between carbon atoms remains elusive. Here we report the isolation of a compound with a one-electron σ-bond between carbon atoms by means of the one-electron oxidation of a hydrocarbon with an elongated C–C single bond5,6. The presence of the C C one-electron σ-bond (2.921(3) Å at 100 K) was confirmed experimentally by single-crystal X-ray diffraction analysis and Raman spectroscopy, and theoretically by density functional theory calculations. The results of this paper unequivocally demonstrate the existence of a C C one-electron σ-bond, which was postulated nearly a century ago7, and can thus be expected to pave the way for further development in different areas of chemistry by probing the boundary between bonded and non-bonded states. Unlocking the nature of covalent bonds is important to gain a deeper to their intrinsically high reactivity, and stabilizing such compounds understanding of chemical phenomena. The concept of two atoms is as important a task as it is challenging. One approach to circumvent sharing an electron pair, which was initially proposed by Lewis in 1916 this obstacle is to use hexaphenylethane (HPE) derivatives, which can (ref. 8) and then termed ‘covalent bond’ in 1919 by Langmuir9, remains be expected to provide a suitable framework because their oxidation relevant in understanding chemical bonding. would lead to the formation of triarylmethyl cation and triarylmethyl Subsequently, Pauling proposed a concept of covalent bonds with radical units, which are well known, relatively stable, carbocations one unpaired electron (‘one-electron σ-bonds’), which is shared and radicals. between two atoms7. In 1931, Pauling postulated the existence of An important issue in this context is that most redox-active HPEs one-electron σ-bonds using the H2 + radical cation as a simple model. undergo a process close to a one-step two-electron oxidation to pro- Such one-electron σ-bonds are expected to be much weaker than typical duce two triarylmethyl cations because the oxidation potential for the two-electron σ-bonds and, therefore, their properties have been inves- bond-dissociated radical, which is generated readily by the C–C bond tigated primarily theoretically10–13. Nevertheless, few studies on in situ scission of the radical cation intermediate, is much less positive than generated radical anions14–17 such as R3B BR3− and radical cations10,18–20 that for neutral σ-bonded species (E1ox > E2ox) (Fig. 1, top). To obtain a such as R3E ER3+ (E = C, Si and Ge) based on electron spin resonance radical cation with a C C one-electron σ-bond, the oxidation of HPEs measurements have been reported. Other reports have described the must proceed in a stepwise manner, that is, the level of the highest spectroscopic identification of chemical species with one-electron occupied molecular orbital (HOMO) for the neutral state must be higher σ-bonds that were generated as single components; however, these than that of the singly occupied molecular orbital (SOMO) for the radi- compounds have not been isolated21,22. X-ray crystallographic studies cal cation (E1ox < E2ox). Although raising the HOMO level is considered to on species that contain one-electron σ-bonds are particularly scarce, be essential, the introduction of electron-donating heteroatoms into that is, they are limited to P P, B B and Cu M (M = B, Al and Ga) bonds1–4. HPEs is not effective because it simultaneously raises the HOMO level It is important to note here that, although species with C C one-electron of the neutral species and the SOMO level of the radical cations. Thus, σ-bonds have been proposed as intermediates in chemical reactions alternative approaches for achieving a stepwise oxidation process such as the Cope rearrangement, there is no experimental evidence by, are required. for example, X-ray crystallography for one-electron σ-bonds between We tackled this issue by focusing on another notable feature of HPEs, carbon atoms23–28. which is that the central C–C single bond is elongated beyond 1.6 Å due to the steric hindrance imposed by the aryl groups surrounding the bond29–33. This bond elongation causes an increase in the HOMO level Synthesis and characterization of 1 + by means of a through-bond interaction between the elongated C–C R3C CR3+ radical cations are promising models with which to inves- bond and the aryl groups. Thus, the bond elongation in HPEs provides tigate C C one-electron σ-bonds18,20. However, reports on molecules an optimal approach to realize an energy reversal between the HOMO that contain a C C one-electron σ-bond remain elusive at present due of the neutral state and the SOMO of the radical cation state without 1 Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Japan. 2Creative Research Institution, Hokkaido University, Sapporo, Japan. 3Present address: Department of Chemistry, Graduate School of Science, The University of Tokyo, Tokyo, Japan. ✉e-mail: [email protected]; [email protected] Nature | www.nature.com | 1 Article a Ar Ar Ar Ar Ar Ar Ar Ar Ar –1e– –1e– Ar Ar Ar Ar C C Ar C C C C C C E1ox Ar Ar Ar E2ox Ar Ar Ar Ar Ar Ar Ar E1ox > E2ox Degradation b Ar Ar Ar Ar Ar Ar Ar Ar –1e– Ar –1e– Ar C C Ar C C C C Ar Ar E1 ox Ar Ar E2ox Ar Ar Ar E1ox < E2ox Fig. 1 | Proposed redox mechanisms in HPE derivatives. a, Typical oxidation processes in HPEs. b, Oxidation in the HPE derivative with an ultralong C–C single bond. relying on the introduction of electron-donating heteroatoms (Fig. 1, parameters (bond length: 0.004 Å; bond angle: 0.3°) and R indices bottom). (R1 = 0.0220; wR2 = 0.0569) for 1 +I3− were sufficiently small to con- HPE 1 (refs. 5,6) bears two spiro-dibenzocycloheptatriene (DBCHT) firm the unsymmetrical bent-planar structure, and the nearly perfect units and thus satisfies the aforementioned key factors, that is, it round shape of the thermal ellipsoids of C1 and C2 excludes the pos- has an extremely elongated Csp3–Csp3 single bond (1.806(2) Å at sibility of a disordering copresence of 1 or 12+ at the site of 1 +. Crystals 400 K), which is the highest value among hitherto reported HPEs. of 1 +I3− can be stored under ambient conditions for at least two weeks Such an extremely elongated C–C bond increases the HOMO level without appreciable decomposition and remained intact even dur- by means of a through-bond interaction, resulting in a stepwise oxi- ing high-temperature X-ray measurements at 400 K (Supplementary dation process (E1/2ox1 = +0.57 V and E1/2ox2 = +0.69 V versus saturated Fig. 4). The observed geometrical difference between 1 + and 12+ is intrin- calomel electrode) that was observed by cyclic voltammetry34. Fur- sic and reproducible, which was confirmed by analysing several single thermore, the rigid acenaphthylene core causes a bond elongation in crystals of 1 +I3− and 12+(I3−)2. the neutral-state C…C distance required to form a one-electron bond. This distinctive geometry deviates from the typical planar confor- The naphthalene core, on the other hand, is not suitable to cause the mation of a pimer composed of cationic and radical moieties/mol- stepwise oxidation in the corresponding hydrocarbon, although that ecules38–40. Contrary to the unsymmetric structure of 1 +I3−, the bond worked well in the case of the B B one-electron bond reported in ref. 21. lengths in the two different DBCHT moieties are almost identical (Sup- We predicted that HPE 1 could potentially serve as a suitable platform plementary Tables 1 and 2), which indicates that the spin and posi- to stabilize the C C one-electron σ-bond through an intramolecular tive charge are delocalized over the two DBCHT moieties in 1 +I3−, that core–shell strategy. is, spin and charge-separated states are not favoured. Other effects Figure 2a shows the redox reactions of 1. A two-electron oxidation between the two DBCHT moieties stem most probably from the stabili- was achieved by treating 1 with iodine (3.0 equiv.) to furnish dication zation of the unique and unsymmetric bent conformation. At present, salt 12+(I3−)2, of which single crystals were obtained after recrystalliza- we consider that the molecular structure of 1 + with an unsymmetric tion from CH2Cl2/diethyl ether. Meanwhile, the one-electron oxidation bent-planar geometry is similar to that of 1, which implies an inherit- of 1 with iodine (1.5 equiv.) provided 1 +I3– as a dark brown solid that is ance from the covalent bond nature between the C1 and C2 atoms. sparingly soluble in organic solvents such as CH2Cl2 and acetonitrile. In fact, the short contact between the C1 and C2 atoms (2.921(3) Å The formation of paramagnetic species was confirmed by the absence at 100 K) was confirmed in the X-ray structure of 1 +I3− (Fig. 2e). The of any 1H nuclear magnetic resonance (NMR) signals and the presence sum of the three bond angles around the C1 (359.3°) and C2 (359.6°) of a characteristic doublet electron spin resonance signal in solution. atoms indicates that each atom is sp2 hybridized despite the unsym- Recrystallization of the radical cation salt 1 +I3− from acetonitrile/diethyl metrical bent-planar geometry. This finding is consistent with pre- ether afforded dark violet single crystals suitable for X-ray diffraction vious examples of weak C…C bondings, in which the carbon atoms measurements. Upon reduction of the cationic species with zinc pow- involved exhibit a preference for sp2 hybridization41,42. Focusing on the der, the original compound 1 was restored. intramolecular C…C distance between the two DBCHT units, although some interatomic distances (2.92–3.32 Å) closer to the naphthalene skeleton were found to be less than the sum of the van der Waals radii X-ray analysis of carbon atoms (3.40 Å), most are greater (3.44–3.79 Å), indicating The single crystals obtained were subjected to an X-ray diffraction an unfavourable conformation for stabilization by π–π interactions analysis to explain redox-state-dependent differences in the molecu- (Supplementary Fig. 10 and Supplementary Table 1). This behaviour lar structures. The X-ray structure of dication 12+ in 12+(I3−)2 exhibits contrasts with the usual planar π-systems, indicating that stabiliz- a twisted conformation with a twisting C1–C3–C4–C2 angle (θ) of ing interactions other than π–π stacking are occurring between the 20.8(6)°, while both DBCHT moieties are planar (Fig. 2c). As in the case DBCHT units. In the crystal, short intermolecular contacts between of 12+(I3−)2, typical dibenzocycloheptatrienylium (cation) and dibenzo- one DBCHT unit and I3− (approximately 3.5 Å; sum of the van der Waals cycloheptatrienyl (radical) derivatives would be expected to exhibit radii: 3.8 Å) were observed. However, they correspond to the proximity a preference for planarity, which is more conducive to π-stacking and of the σ-hole—a positively polarized site—in the I3− ion and the cationic electron delocalization35–37. In contrast, an eclipsed conformation with a π-surface of DBCHT and, accordingly, this intermolecular interaction small θ value of 2.19(19)° was observed in the X-ray structure of 1 + in 1 +I3− has little effect on the geometrical features of 1 + in crystal. It is also (Fig. 2d,e). One of the DBCHT moieties in 1 +, that is, the one containing worth noting here that residual electron density between the C1 and C1, adopts a bent geometry with larger dihedral angles between the C2 atoms is present in the Fo–Fc map that was obtained from the X-ray a–e planes (Fig. 2). The concave surface of the bent seven-membered analysis of 1 +I3− (Fig. 3a–c), which proves electron sharing between the ring faces the other DBCHT unit, that is, the one containing C2, which C1 and C2 atoms. This electron sharing contributes to the molecular exhibits an almost planar geometry. The estimated s.d. of the structural stability, which predestines radical cation 1 + to serve as a host for a 2 | Nature | www.nature.com  a C1 C2 C1 C2 C1 C2 –1e– –1e– +1e– +1e– 1 1 + 12+ b c d Ar Ar Ar Ar C1 C2 C3 C4 e d a b c 1 12+(I3–)2 e 1 1 +I3– 12+(I3–)2 C1 C2 distance (Å) 1.7951(19) 2.921(3) 3.027(8) C1–C3–C4–C2 twist 1.8500(9) 2.190(19) 20.800(6) angle T (°) Unit shape Bent Planar Bent Planar Planar 1 +I3– a–b 24.74(5) 7.77(5) 11.09(9) 1.83(10) 2.33(16) Dihedral a–c 49.80(13) 13.99(15) 15.80(3) 4.00(3) 7.90(6) angle (°) a–d 24.17(5) 3.81(5) 8.87(9) 2.43(10) 5.93(17) a–e 24.32(11) 2.93(11) 11.30(2) 1.38(18) 7.50(4) Standard Csp3–Csp3 single bond length: 1.54 Å Sum of the van der Waals radii of carbon atoms: 3.4 Å Fig. 2 | Redox reactions and X-ray structures. a, Redox interconversion of 1. 100 K for 1 and 12+(I3−)2 and at 110 K for 1 +I3−. Due to the phase transition occurring b–d, X-ray structures of 1 (b) and 1 +I3− (d) at 100 K and 1 2+(I3−)2 (c) at 110 K with at around 100 K, the X-ray analysis of 1 2+(I3−)2 was conducted at 110 K; disordered thermal ellipsoids at 50% probability. e, Structural parameters determined at I3− is omitted for clarity in 1 2+(I3−)2. C C one-electron σ-bond with a bond length of 2.921(3) Å. In the case was determined in the corresponding molecule with a biphenyl core. of the B B one-electron bond, which is based on the naphthalene core, This difference should most probably be interpreted in terms of the a similar interatomic distance was predicted21, whereas a smaller value rigidity of the π-framework. a d 4,000 C1 C2 Experimental (single crystal) 3,500 Raman intensity (a.u.) 3,000 379 cm–1 2,500 2,000 1,500 1,000 500 0 200 300 400 500 600 700 800 900 Wavenumber (cm–1) b c e 500 Calculated (UM06-2X/6-311+G**) Raman intensity (a.u.) 400 C1 C2 C– C symmetric vibration C2 390 cm–1 300 200 100 0 200 300 400 500 600 700 800 900 Wavenumber (cm–1) Fig. 3 | Experimental evidence of the presence of a C C one-electron σ-bond shown in green (a) and blue (b,c). d, Raman spectrum measured at 298 K using in 1 +(I3−). a–c, Experimentally obtained Fo–Fc maps of 1 +I3− at 100 K on the a single crystal of 1 +I3−. e, Simulated Raman spectrum for 1 + obtained from DFT C1–C3–C4–C2 plane (a,b) and on its orthogonal plane (c) through the midpoint calculations at the UM06-2X/6-311+G** level. between the C1 and C2 atoms (C2 → C1 direction). Residual electron density is Nature | www.nature.com | 3 Article a α-SOMO b α-LUMO c h 1.8 0.2 1.5 1.2 0.1 Absorbance 0.9 0 0.6 1,000 1,200 1,400 1,600 1,800 2,000 d e 0.3 1.5 1.5 16.7 μM 417 μM 1.2 1.2 0 Absorbance Absorbance 0.9 0.9 230 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200 Wavelength (nm) 0.6 0.6 i 1.8 0.3 0.2 0.3 1.5 0 0 230 600 1,000 1,400 1,800 2,200 900 1,200 1,500 1,800 2,100 1.2 0.1 Wavelength (nm) Wavelength (nm) Absorbance f g 0.9 32 0 Transmittance (%) 0.6 1,000 1,200 1,400 1,600 1,800 2,000 28 0.3 24 20 0 230 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200 16 Wavelength (nm) 900 1,200 1,500 1,800 2,000 Wavelength (nm) Fig. 4 | Electronic properties of 1 + with a C C one-electron σ-bond. a–c, Kohn– (d, 16.7 μM; e, 417 μM) and for a single crystal of 1 +I3− (g). f, Photograph of a Sham orbitals of the α-SOMO (a), the α-LUMO (isovalue = 0.05) (b), and spin single crystal of 1 +I3−. h,i, Changes (black arrows) in UV/Vis/NIR spectra upon density (isovalue = 0.004) (c) obtained from DFT calculations at the UM06-2X/ constant-current electrochemical oxidation of 1 (11.1 μM, 30 μA, every 2 min) 6-311+G** level. The atomic coordinates of the X-ray structure of 1 +I3− at 100 K in CH2Cl2 containing 0.05 M Bu4NBF4 as a supporting electrolyte: first step were used for the calculations. d,e,g, UV/Vis/NIR absorption spectra in CH2Cl2 (0–28 min, 1 to 1 +) (h); second step (28–90 min, 1 + to 1 2+) (i). the lowest unoccupied molecular orbital (LUMO) and the spin density Experimental and theoretical analyses are located mainly on the C1 and C2 atoms (Fig. 4a–c). According to To verify whether this electron sharing corresponds to the bond, a the shape of the orbitals, the α-SOMO and α-LUMO of 1 + represent the single crystal of 1 +I3− was subjected to Raman spectroscopy at 298 K σ-type bonding and σ*-type antibonding orbitals, respectively. In the to obtain direct information about the force constant of the bond. natural bond orbital (NBO) analysis of 1 +, both orbitals were confirmed The experimentally obtained and simulated Raman spectra of 1 + are for the C1 and C2 atoms as well. The C1–C2 bond exhibits 0.3–0.4% 2s shown in Fig. 3d,e. The simulated spectrum obtained from density character and 99.6–99.7% 2p character, with 0.76 electrons involved functional theory (DFT) calculations at the UM06-2X/6-311+G** level in the bond. Equal values of a positive charge for both C1 (0.05) and C2 accurately reproduced the experimental results. The observed Raman (0.05) atoms in 1 + were estimated using natural population analysis shift (1 +I3−: 379 cm−1) attributed to the symmetric C1–C2 stretching methods, indicating a delocalized charge distribution in the DBCHT vibration is significantly lower than that for neutral 1, which contains moieties. The natural population analysis also predicted a relatively an ultralong C–C single bond (589 cm−1)5,6. However, this Raman shift large spin-density distribution on the C1 (0.26) and C2 (0.24) atoms. is higher than that observed in molecules that contain C…C bonding A localized molecular orbital analysis using the Foster–Boys method interactions beyond 2.0 Å (refs. 41,42). This contrasts with the common showed that the contribution of the C1 (28.0%) and C2 (27.5%) atoms absence of Raman shifts for weak interactions in π-stacked molecules in the α-SOMO is dominant and that the contributions of the other such as pimers. atoms is less than 5%. The shape of the localized α-SOMO clearly reflects Next, DFT calculations were carried out to obtain more information the formation of the σ-orbital (Supplementary Fig. 21). Considering on the strength of the C C one-electron σ-bond. The estimated force these results and the comparable bond lengths in each DBCHT moiety, constant for 1 + (56.8 N m−1), which was obtained as a second derivative the contribution of a coordinating-type bonding interaction can be of the energy with respect to the bond length at the (U)M06-2X/6-31+G** excluded, that is, these results indicate that a σ-bond between carbon level based on the Lennard–Jones potential, is much smaller than that atoms could be maintained even if fewer than one electron is involved. for 1 (113.7 N m−1) or ethane (445.9 N m−1). The estimated force constant Furthermore, we carried out bond topological analyses including quan- is in good agreement with the calculated value (50.8 N m−1) obtained tum theory of atom in molecules and electron localized functions for from the relationship between the force constant and the vibrational the whole series. Each parameter for 1 + exhibits an intermediate value frequency using the value of the observed Raman shift of the symmetric between those of bonded 1 and non-bonded 12+, which indicates that C1–C2 stretching vibration (for details, see Supplementary Informa- 1 + possesses an intermediary nature. This finding is consistent with tion). These results indicate a covalent nature of the electron sharing the notion of the presence of a one-electron bond, which would be between the C1 and C2 atoms in 1 +I3−. expected to fall between a two-electron bond and a non-bonded state. Le Floch et al. reported that the radical anion of the calixarene exhib- In their entirety, the experimental and theoretical results indicate its a short C…C distance (3.323(3) Å)43. From a theoretical point of view, that the short C1…C2 contact with a value of 2.921(3) Å in 1 +I3− is, the coefficients of the SOMO indicate that a bonding orbital between although weak, an example of a C C one-electron σ-bond. the carbon atoms in close proximity was not formed since they reside To investigate the properties of the compound from the photochemi- on the nodal plane. Thus, a detailed investigation is needed to verify cal point of view, we recorded the ultra-violet/visible/near-infrared whether this short contact represents a bond or is due merely to physi- (UV/Vis/NIR) spectrum of 1 +I3− in CH2Cl2. An NIR absorption band cal proximity. To gain further insight into the electronic structure, DFT was observed at around 2,000 nm (λmax ≃ 1,405 nm) (Fig. 4d,e), which calculations for 1 + were performed at the UM06-2X/6-311+G** level cannot be explained by disproportionation (2 × 1 + ⇄ 1 + 12+) because based on the X-ray coordinates. 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Soc. 125, 4487–4494 19. Wang, J. T. & Williams, F. E.S.R. spectra of the hexamethyldisilane and (2003). hexamethyldigermane radical cations. J. Chem. Soc. Chem. Commun. 1981, 666–668 (1981). Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in 20. Shida, T., Kubodera, H. & Egawa, Y. Confirmation of the cation radicals of published maps and institutional affiliations. hexamethylethane and hexamethyldisilane by ESR and other spectroscopy. Chem. Phys. Lett. 79, 179–182 (1981). Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this 21. Hoefelmeyer, J. D. & Gabbaï, F. P. An intramolecular boron–boron one-electron σ-bond. article under a publishing agreement with the author(s) or other rightsholder(s); author J. Am. Chem. Soc. 122, 9054–9055 (2000). self-archiving of the accepted manuscript version of this article is solely governed by the 22. Cataldo, L. et al. Formation of a phosphorus–phosphorus bond by successive one-electron terms of such publishing agreement and applicable law. reductions of a two-phosphinines-containing macrocycle: crystal structures, EPR, and DFT investigations. J. Am. Chem. Soc. 123, 6654–6661 (2001). © The Author(s), under exclusive licence to Springer Nature Limited 2024 Nature | www.nature.com | 5 Article Methods Synthesis of dication salt 12+(I3−)2. 1 (3.00 mg, 5.65 μmol) was added to a solution of iodine (2.15 mg, 16.9 μmol) in dry CH2Cl2 (5 ml) at 26 °C. General information After stirring at 26 °C for 1 h, the resulting suspension was dried under All commercially available compounds were used without further puri- reduced pressure to give a dark violet solid (5.1 mg). Mp: 177–182 °C fication unless otherwise indicated. Acetonitrile was dried before use (decomp.); 1H NMR (CD3CN): silent (given that the reduction potential by distillation from CaH2. Column chromatography was performed on of iodine was not sufficient to oxidize radical cation 1 + completely, silica gel (Wakogel 60N; neutral; particle size: 38–100 μm). 1H and 13C the silent NMR spectrum was attributed to a partial contribution of NMR spectra were recorded on a BRUKER AscendTM 400 (1H/400 MHz radical cation 1 +.); IR (ATR): ν cm−1 3,042; 1,606; 1,529; 1,513; 1,476; and 13C/100 MHz) spectrometer. Mass spectra were recorded on a JEOL 1,430; 1,419; 1,342; 1,318; 1,229; 1,189; 1,163; 1,128; 1,115; 1,088; 1,040; JMS-T100GCV spectrometer in FD mode (GC-MS&NMR Laboratory, 990; 952; 941; 874; 859; 840; 811; 796; 774; 768; 751; 721; 711; 693; 676; Research Faculty of Agriculture, Hokkaido University). Melting points 657; 627; 610; 551; 517; 469; LR-MS (FD) m/z (%): 532.18 (13), 531.18 were measured on a Stanford Research Systems MPA100 Optimelt for (47), 530.17 (M+, bp); HR-MS (FD) calculated for C42H26: 530.20345; powder samples and on a YANACO MP-J3 for single crystals; all values Found: 530.20499. are uncorrected. The Raman spectroscopy using a 785 nm laser was Recrystallization of the obtained material from dry CH2Cl2/dry die- carried out on a RENISHAW inVia Reflex at the OPEN FACILITY, Hok- thyl ether gave dark violet crystals of 12+(I3−)2 (at least 0.10 mg, more kaido University Sousei Hall. Infrared (IR) spectra were measured on than 2%), which were picked up with a needle and characterized by a Shimadzu IRAffinity-1S spectrophotometer (attenuated total reflec- X-ray crystallography. The isolation yield was calculated based on the tion (ATR) mode) for powder samples and on a JASCO IRT-5200FT/ separated single crystals from the as-prepared crystals, which might IR-6600 spectrophotometer for single crystals (Transmission Mode). contain 1 +I5− (determined by preliminary X-ray analysis) or other cati- X-band CW-EPR measurements were conducted using a Bruker BioSpin onic species. EMX Plus. Solid-state UV/Vis/NIR spectra were measured on a micro- Mp: 147–153 °C (decomp.); IR (a single crystal, transmission): ν cm−1 scopic spectrometer (MSV 5200, JASCO; Transmission Mode), while 3,187; 3,078; 3,038; 2,928; 2,855; 1,907; 1,724; 1,646; 1,605; 1,514; 1,471; solution-state UV/Vis/NIR spectra were recorded on a JASCO V-770 1,440; 1,386; 1,358; 1,327; 1,256; 1,198; 1,121; 1,096; 991; 976; 941; 890; spectrophotometer. 866; 841; 793; 727; 661; 638. DFT calculations were performed using the Gaussian 16W44 program package. Parts of the DFT calculations for 1 + were performed with the atomic coordinates obtained from the X-ray diffraction analysis of Data availability sample 1 of 1 +I3− at 100 K. Multiwfn software45 (v.3.8) was used for local- The X-ray data have been deposited with the Cambridge Crystallo- ized molecular orbital (Foster–Boys method) and topological analysis graphic Data Centre under reference numbers 2301032–2301035 (quantum theory of atom in molecules and electron localized functions) (1 +I3−, main_sample1), 2301036–2301039 (1 +I3−, sub_sample2) and of the electron density that was obtained from the DFT calculations. 2301040–2301043 (12+(I3−)2). All other data are presented in the main The NBO analyses were performed using v.3.1 of the NBO46 function in text or the Supplementary Information. the Gaussian 16W program package. A suitable crystal was selected and used for the measurement on 44. Frisch, J. M. et al. Gaussian 16, Revision C.01 (Gaussian, Inc., 2019). a Rigaku XtaLAB Synergy (Cu-Kα radiation, λ = 1.54184 Å) with HyPix 45. Lu, T. & Chen, F. Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 33, 580–592 (2012). diffractometer. Using Olex2 (ref. 47), the structure was solved with 46. Glendening, E. D., Reed, A. E., Carpenter, J. E. & Weinhold, F. NBO v.3.1 (Gaussian, Inc., the SHELXT48 structure solution program using Intrinsic Phasing and 2001). refined with the SHELXL49 refinement package using least squares 47. Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 42, minimization. 339–341 (2009). 48. Sheldrick, G. M. SHELXT – Integrated space-group and crystal-structure determination. Starting material Acta Crystallogr. A Found. Adv. 71, 3–8 (2015). 49. Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr. C Struct. HPE 1 was prepared according to literature procedures5. Chem. 71, 3–8 (2015). Synthesis of radical cation salt 1 +I3–. 1 (4.74 mg, 8.93 μmol) was added Acknowledgements We thank E. Fukushi and Y. Takata (Hokkaido University) for recording mass to a solution of iodine (1.70 mg, 13.4 μmol) in dry CH2Cl2 (5 ml) at 26 °C. spectra, H. Hirata (Hokkaido University) for recording electron spin resonance spectra, S. Noro, After stirring at 26 °C for 1 h, the resulting suspension was dried under Y. Saito and A. Yamazaki for recording solid-state UV/Vis/NIR spectra, as well as J. P. Gong and T. Nakajima for recording solid-state IR spectra. Parts of the theoretical calculations were carried reduced pressure to give a dark brown solid (6.4 mg). Mp: 157–163 °C out at the Research Center for Computational Science, Okazaki, Japan (Project 23-IMS-C218). (decomp.); 1H NMR (CD3CN): silent; IR (ATR): ν cm−1 3,046; 3,023; 1,607; We would also like to thank U. F. J. Mayer at www.mayerscientificediting.com for proofreading 1,485; 1,465; 1,418; 1,310; 1,229; 1,164; 1,130; 1,111; 1077; 1,043; 952; 877; our manuscript. This work was supported by the Masason Foundation (to S.K.) and by the Research Program ‘Five-star Alliance’ in ‘NJRC Mater. & Dev.’ of MEXT (Japan). Y. I. and T. Shimajiri 863; 841; 825; 795; 767; 739; 723; 686; 672; 646; 627; 608; 587; 517; acknowledge financial support from a Toyota Riken Scholarship. This work was furthermore 485; 470; 430; 422; 420; LR-MS (FD) m/z (%): 532.26 (12), 531.26 (46), supported by Grants-in-Aid from MEXT (JSPS Nos. 23K13726 to T. Shimajiri, 23K20275 to 530.25 (M+, bp); HR-MS (FD) calculated for C42H26: 530.20345; Found: T. Suzuki, and 23K21107 and 23H04011 to Y.I.) and JST PRESTO (No. JPMJPR23Q1) to Y.I. 530.20552. Author contributions T. Shimajiri, T. Suzuki and Y.I. developed the concept of this study. Recrystallization of the obtained material from dry acetonitrile/ T. Shimajiri and S.K. conducted the synthetic and spectroscopic experiments as well as the dry tetrahydrofuran/dry diethyl ether gave dark violet crystals of theoretical calculations. T. Shimajiri, T. Suzuki and Y.I. supervised the project. T. Shimajiri prepared the manuscript with feedback from all authors. 1 +I3− (at least 0.06 mg, more than 1%), which were picked up with a needle and characterized by X-ray crystallography. The isolated yield Competing interests The authors declare no competing interests. was calculated based on the separated crystals from the as-prepared crystals, which might also contain 1, 1 +I5– (determined by preliminary Additional information Supplementary information The online version contains supplementary material available at X-ray analysis), and other forms of cationic species. Mp: 191–196 °C https://doi.org/10.1038/s41586-024-07965-1. (decomp.); IR (a single crystal, transmission mode): ν cm−1 3,094; 3,045; Correspondence and requests for materials should be addressed to Takuya Shimajiri or 2,979; 2,925; 1,955; 1,929; 1,907; 1,819; 1,614; 1,587; 1,530; 1,455; 1,436; Yusuke Ishigaki. Peer review information Nature thanks Tobias Krämer and the other, anonymous, reviewer(s) 1,345; 1,260; 1,207; 1,166; 1,149; 1,124; 1,084; 1,047; 947; 859; 839; 809; for their contribution to the peer review of this work. Peer reviewer reports are available. 777; 752; 725; 670; 611. Reprints and permissions information is available at http://www.nature.com/reprints.