Curved π-conjugated corannulene dimer diradicaloids
So far, most reported open-shell singlet diradicaloids are based on planar p-conjugated molecules. Herein, we report the bridged corannulene dimer diradicaloids, Cor-D1 and Cor-D2, both showing a threedimensional curved p-conjugated structure. Cor-D1 has a small diradical character (y 0 ¼ 5.4%) and behaves more like a closed-shell quinoidal compound at room temperature, while Cor-D2 is a typical open-shell diradicaloid with a larger diradical character (y 0 ¼ 16.9%). Both compounds exhibited paramagnetic activity at elevated temperatures, with a singlet-triplet energy gap (DE S-T ) of À8.4 and À3.0 kcal mol À1 , respectively. X-ray crystallographic analysis revealed that both molecules have a dumbbell-shaped geometry, with the two terminal corannulene bowls bent to opposite directions. The spin is largely delocalized onto the two bowls in Cor-D2 and there are multiple [CH/p] interactions between the neighboring bowls. Chemical oxidation/reduction to their respective dications/dianions results in global aromaticity with [4n + 2] p-electrons delocalized through the periphery of the whole framework.
Introduction<!>Synthesis<!>Ground-state geometry<!>Optical, electrochemical and magnetic properties<!>Conflicts of interest
<p>Recently there have been increasing research interests on openshell singlet diradicaloids due to their unique magnetic activity and potential applications in organic electronics, non-linear optics and spintronics. 1 So far, most reported open-shell singlet diradicaloids are based on planar p-conjugated molecules or their oligomers, and there are very rare examples based on non-planar systems. 2 Our particular interest here is to exploit stable diradicaloids based on curved p-conjugated molecules, which may exhibit unique three-dimensional (3D) spin distribution nature and intra-and intermolecular magnetic exchange interactions. Bowl-shaped corannulene is a very good candidate given its curved structure. It was reported that the radical anions of corannulene and its derivatives showed effective spin delocalization throughout the p-conjugated framework but they were still highly reactive and some of them tended to form intermolecular C-C s-bond linked dimer or polymer. 3 On the other hand, stable neutral radicals-substituted corannulenes such as Cor-Ver (verzazyl), Cor-IN (iminonitroxide), Cor-PhO (phenoxyl) and Cor-t BuNO (tert-butylnitroxide) (Fig. 1a) were prepared and in all cases, the spin can be partially delocalized to the corannulene bowl. 4 The phenalenyl-fused corannulene anion was synthesized and fully characterized, but the property of its neutral radical Cor-PLY (Fig. 1a) was not reported, presumably due to its poor stability. 5 The corannulene-bridged phenoxyl diradical Cor-2PhO showed signicant intramolecular magnetic interaction with a singlet-triplet energy gap (DE S-T ¼ 2Jk B À1 ) of À810 K (À1.61 kcal mol À1 ). 6 Calculations reveal that the unpaired electronic spin in the triplet state is delocalized onto the corannulene skeleton from the phenoxyl moieties. However, so far, corannulene-based radicals with more than one corannulene unit have never been reported. Herein, we report the rst two stable p-conjugated corannulene dimer diradicaloids, Cor-D1 and Cor-D2, in which the two corannulene units are annulated with a para-quinodimethane (p-QDM) and 2,6-naphthoquinodimethane (2,6-NQDM) moiety, respectively (Fig. 1b). The central pro-aromatic quinodimethanes have irresistible wish to become diradicals by recovering aromaticty, 1d and one can expect a signicant contribution of the open-shell diradical form to the groundstate electronic structure. Therefore, they are ideal open-shell singlet diradicaloids with a unique 3D curved p-conjugated structure and the spin is supposed to be largely delocalized onto the two corannulene bowls (Fig. 1b).</p><!><p>The synthesis of p-extended corannulenes is challenging 7 and our synthesis was based on regio-selective reactions starting from the 1-methoxy-corannulene 1 (ref. 8) (Scheme 1). Monobromination of 1 at the ortho-position by N-bromosuccinimide (NBS) in the presence of diisopropylamine gave the 1-bromo-2methoxy-corannulene 2 in 85% yield. The bromo-group was then converted into pinacol boronate in 3 by lithiation followed by quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2dioxaborolane. Suzuki coupling reaction between 3 and the 2,5-dibromoterephthalaldehyde (4) or 3,7-diformylnaphthalene-2,6-diyl bis(triuoromethanesulfonate) (5) 9 gave the corresponding dialdehyde 6a or 6b. Nucleophilic addition of the aldehyde groups in 6a/6b with excessive mesitylmagnesium bromide generated the diol intermediates and subsequent BF 3 $Et 2 O mediated intramolecular Friedel-Cras alkylation afforded the dihydro-precursors 7a/7b. At this step, the existence of a methoxy group on each corannulene unit ensured a six-membered ring cyclized product only and avoided possible formation of vemembered rings. 10 Oxidative dehydrogenation of 7a/7b with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in toluene at 80 C gave the nal desired products Cor-D1 and Cor-D2, respectively. The bulky mesityl groups are essential to kinetically block the reaction sites and also make the nal products reasonably soluble. Both Cor-D1 and Cor-D2 are stable and can be puried by normal silica gel column chromatography.</p><!><p>Single crystals of Cor-D1 and Cor-D2 suitable for X-ray crystallographic analysis were grown by slow solvent evaporation from chlorobenzene solution. 11 Both molecules adopt a doublecurved geometry, with the two corannulene bowls bent to opposite directions (Fig. 2). The bowl depth (d) of Cor-D1 is measured to be 0.89 Å, which is slightly larger than that of parent corannulene (d ¼ 0.87 Å),  Å) between neighboring corannulene bowls are found in the crystallographic structure of Cor-D2, and there are additional [CH/p] interactions between the methyl groups and the corannulene bowls (distance: 2.736 Å), both drive the molecules into a densely packed structure (Fig. 2b). The central p-QDM unit in Cor-D1 has a large bond length alternation (BLA) (Fig. 2a), indicating that the quinoidal structure is more important. However, the exo-methylene bond, C1-C2 (1.388(13) Å), is signicantly longer than that of typical olens  S2 in ESI †). Calculations show that the anti-and syn-isomers of Cor-D2 have nearly identical energy (Table S13 in ESI †), and thus both isomers exist equally in solution.</p><!><p>Compound Cor-D1 is blue while Cor-D2 is green in dichloromethane (DCM). Cor-D1 shows an intense absorption band with maximum (l max ) at 612 nm (Fig. 3a), which can be correlated to  4). The calculated spin-density distribution maps for the triplet diradical of Cor-D2 in both anti-and syn-forms show that the unpaired electronic spin is delocalized onto the two corannulene bowls, with the largest spin density distributed at the zigzag edges (inset in Fig. 4 and S15 in ESI †).</p><p>Cyclic voltammetry measurements of Cor-D1 and Cor-D2 were carried out in dry 1,2-dichlorobenzene (1,2-DCB) under heating (50 C) which can suppress molecular aggregation and give better resolved redox waves (Fig. 3b). Cor-D1 exhibited two oxidation waves with half-wave potential E ox 1/2 ¼ 0.23 and 0.55 V, and two reduction waves with half-wave potential E red 1/2 ¼ À1.42 and À1.64 V (vs. Fc + /Fc). The HOMO and LUMO energy levels were calculated to be À5.03 and À3.38 eV from the onset of the rst oxidation/reduction wave, respectively, and an electrochemical energy gap (E EC g ) of 1.65 eV was estimated. Cor-D2 showed two reduction waves at E red 1/2 ¼ À1.28 and À1.56 V, with a LUMO energy level of À3.52 eV. Due to strong aggregation, clear oxidation wave could not be observed. Both compounds can be chemically oxidized into radical cation and dication by NO$SbF 6 in dry DCM, and reduced into radical anion and dianion by sodium anthracenide (NaAn) in anhydrous THF (Fig. 5 and S11-S12 in ESI †). In both cases, the radical anion and radical cation show similar band structure, indicating a similar electronic structure, but the corresponding bands of radical anion are red-shied compared to the radical cation (l max ¼ 1490 nm vs. 1417 nm in Cor-D1, and l max ¼ 1808 nm vs. 1700 nm in Cor-D2). The dianion and dication exhibit similar band structure and trend (l max ¼ 892 nm vs. 844 nm in Cor-D1, and l max ¼ 1030 nm vs. 960 nm in Cor-D2). Clear 1 H NMR spectra of the dianions of Cor-D1 and Cor-D2 can be recorded by in situ reduction with NaAn in anhydrous THF-d 8 and most of the aromatic protons appear above 7 ppm despite their electronrich character (Fig. S13-S14 and S59 in ESI †), indicating that they are strongly aromatic compounds.</p><p>To better understand the electronic structure, anisotropy of the induced current-density (ACID) plot 16 and nucleus independent chemical shi (NICS) 17 calculations were conducted at (U)B3LYP/6-31G(d,p) level (see Fig. 6 and more images in Fig. S16-S25 in ESI †). The neutral anti-Cor-D1 shows clear diamagnetic/paramagnetic ring current ow along the outer/ inner ring of each corannulene bowl, like the parent corannulene (Fig. S26 in ESI †), while there is no ring current circuit at the central p-QDM unit, indicating its non-aromatic character. In accordance with this, the average NICS(1) zz value 18 of the central benzene ring is near zero (À0.34 ppm). However, its dianion shows diamagnetic ring current along the periphery of the whole p-conjugated framework. At the same time, the average NICS(1) zz value of the central benzene ring becomes largely negative (À30.66 ppm), and even the inner cyclopentaring in the each corannulene unit shows signicantly negative NICS(1) zz value (À14.75 ppm), indicating a strong global aromatic system. Apparently, 38 p electrons can be counted along the periphery, which satises Hückel's [4n + 2]</p><!><p>There are no conicts to declare.</p>