Three‐Dimensional Intramolecular Exchange Interaction in a Curved and Nonalternant π‐Conjugated System: Corannulene with Two Phenoxyl Radicals

Abstract

In recent years, curved p-conjugated molecules such as fullerenes and carbon nanotubes have attracted much attention not only in chemistry but also in materials science. [1] Their intra/intermolecular interactions within/between threedimensional (3D) curved p-electron networks play intrinsically vital roles in their unique properties and functionalities. Among them, intramolecular magnetic interaction between electronic spins on a curved p surface was extensively studied for ionic species of C 60 such as C 60 2À and C 60 3À .[2] Their electronic structures are greatly influenced by not only the dynamic spin polarization of electrons but also the negative charges on the spherical p-conjugated system and the countercation.Thus, we have focused on neutral diradical systems, which are known to be the most useful probes for studying intramolecular magnetic interactions in organic molecules. [3] While many neutral diradical derivatives relevant to planar pconjugated systems have so far been investigated, studies on curved p-conjugated neutral diradicals are limited to a single C 60 -based system in which [60]fullerene is linked to two nitroxide radicals.[4] However, their intramolecular exchange interaction J through the C 60 skeleton was very weak (j Jk À1 B j < 0.1 K)[4b] because of the small spin delocalization onto the C 60 p network from the nitroxide radicals with spin-localized nature on the NO moieties. Therefore, in order to evaluate an intramolecular exchange interaction in a curved p-conjugated system in a quantitative manner, synthesis and isolation of a stable neutral diradical derivative with extensively spindelocalized nature on its curved p-conjugated system have been the focus of current attention in molecular magnetism and open-shell chemistry. [2,5] Recently, we studied corannulene [6] -based stable neutral monoradical systems, [7] such as a phenoxyl radical derivative 1[7d] with highly spin-delocalized nature on the intrinsically 3D bowl-shaped and nonalternant p-conjugated network. These studies inspired us to propose a 3D intramolecular exchange interaction in this class of curved p radicals: in terms of geometry, they are intermediate between a planar p radical such as a phenalenyl system [8] and a tetrahedral s radical. We have now, for the first time, synthesized and isolated a corannulene-based neutral diradical, namely, 2, which has two phenoxyl radical moieties, as air-stable crystals. Due to its highly spin-delocalized nature, 2 shows strong intramolecular exchange interaction (Jk À1 B = À405 AE 2 K) through the 3D corannulene p-electron network. Bond-length analyses and DFT calculations showed that 2 has contributions from diradical canonical forms a-g and closed KekulØ structure d (Scheme 1). Furthermore, in the crystalline state the presence of three crystallographically independent diradical molecules with different curvature enabled us to study the relationship between the curvature of the corannulene p skeleton and the

A Strain‐Releasing Trap for Highly Reactive Electrophiles: Structural Characterization of Bowl‐Shaped Arenium Carbocations

“…The 1 H NMR spectra of cations 1-3 demonstrate the significantly high-field-shifted resonance signals for the CH x Cl 3Àx groups (0.12 ppm, 2.08 ppm, and 4.01 ppm for 1, 2, and 3, respectively) compared to those of CH 3 (Table 2). These values compare well with those found in the 13 C NMR spectra of C 70 -CHCl 2 + (198 ppm, in CF 3 SO 3 H) [6b] and in the protonated form of C 60 (182 ppm, 1,2-C 6 H 4 Cl 2 ). [12] The slight high-field shifts of the C3 resonances of the chlorinated cations 2-4 compared to that of 1 can most probably be attributed to the interaction of unshared electron pairs at the Cl atoms of the R groups with the cationic C3 center.…”

“…[5] The authors ingeniously identified species formed in solution and assigned their molecular structures based on 1 H NMR studies of the reaction mixtures. Despite the initial progress, further elaboration proved to be unsuccessful (including recording 13 C NMR data), and the preparation and structural characterization of bulk solid products have not been reported to date. The latter task can be very challenging because of the extreme reactivity of the resulting cationic species.…”

“…[6] Subsequent hydrolysis of these products under superacidic conditions led to the observation of the alkylated fullerene cations that had much longer lifetimes than the bowl-shaped cations, thus allowing 13 C NMR spectra to be recorded, although no structural studies of these species was undertaken. The solid-state structure of a fullerene cation has been elucidated from the X-ray diffraction characterization of C 60 (AsF 6 ) 2 , which was prepared in a high-pressure vessel by direct reaction of C 60 and AsF 5 in SO 2 .…”

“…The excellent delocalization ability of the nonplanar surface of corannulene was also recently demonstrated for radicals. [13] In conclusion, site-specific alkylation of the curved corannulene surface under Friedel-Crafts reaction conditions resulted in bowl-shaped arenium carbocations, which were isolated as pure bulk crystalline solids with [AlCl 4 ]…”

Solution‐Stable Triple Helicates of Quaterimidazole: Three‐Dimensional Crystal Structures and Optical Resolution by Chiral‐Column HPLC

“…[28] It is noteworthy that the helical chirality is also important for implementing photon-linked matter qubits in quantum information processing. [15] Our ongoing studies on the (oligo)imidazole-based helicates; preparation of magnetically diluted chiral crystals for synthetic electron-spin qubits, synthesis of novel helicates composed of longer (oligo)imidazoles, f-block helicates, and heteromultimetallic helicates, and functionalization by the introduction of redox-active substituents and electronic-spin-delocalized neutral radical systems, [29] are in progress.…”