Curvedcarbonp-surfacesexhibituniquereactivityformetal

coordination because of their distorted framework of conjugated sp 2 carbon atoms.Inparticular, buckybowl molecules, bowl-shaped polycyclic aromatic hydrocarbons (PAHs) that have apartial structure of fullerenes, [1] are interesting as both of their faces,c oncave and convex, are amenable to metal coordination. [2] Furthermore,the binding properties of buckybowls can be enhanced and tuned through stepwise electron addition. [3] Fore xample,c orannulene (C 20 H 10 ), the most representative buckybowl, has ad oubly degenerate LUMO, which enables it to accept up to four electrons upon treatment with alkali metals [4] and to form av ariety of supramolecular aggregates with different metal binding patterns. [5] In the case of sumanene (C 21 H 12 ), deprotonation takes place at its benzylic CH 2 groups to generate the corresponding mono-, di-, and trianions,w hich behave as cyclopentadienyl ligands for metal coordination. [6] Theunique multi-electron reduction properties and metal coordination abilities of buckybowls are of great potential for applications in energy storage and supramolecular materials. [7] Despite the significant progress on the chemical reduction and coordination chemistry of buckybowls composed of carbon and hydrogen atoms,n os tudies have focused on those of heteroatom-containing buckybowls. [8] In this context, our group [9] and Shinokubo [10] and co-workers independently reported the synthesis of 3a 2 -azapentabenzo[bc,ef,hi,kl,no]corannulene ( 1), an itrogen-containing corannulene derivative (Figure 1). [11] TheS hinokubo group intensively studied the oxidation of 1 and related molecules to reveal the characteristic reactivity of the p-radical cations. [10a,c] These examples as well as the isolation of oxidized products of azacoronene 2 [12] and hydrazinobuckybowl 3 [13] reflect the electron-rich character of pyrrole-fused PA Hs.I nc ontrast, the chemical reduction of nitrogen-containing buckybowls has not been studied, although electrochemical measurements of triazasumanene 4 [14] and scanning tunneling spectroscopy of nitrogen-containing bowl-shaped PA H 5 [15] have recently been reported.

Based on density functional theory (DFT) calculations with the B3LYP/6-311 + G(2d,p) method, the core of 1 has almost degenerated LUMO and LUMO + 1o rbitals (À1.95 and À1.94 eV,respectively). These could be similar to those of corannulene,which has two degenerate LUMOs (À1.66 eV). These characteristics of azapentabenzocorannulene 1 attract special interest in studying its reactivity upon stepwise electron acquisition. Herein, we report on the first chemical reduction and the coordination chemistry of 2-tert-butyl-3a 2 - azapentabenzo [bc,ef,hi,kl,no]corannulene (1a). By using alkali metals such as sodium and cesium as the reducing agents,weare able to synthesize the first mono-and dianions of 1aand crystallize them with the corresponding alkali metal counterions (Scheme 1). This allowed us to reveal the consequences of adding one and two electrons to the N-containing bowl for the first time.

Azapentabenzocorannulene 1a was prepared according to the procedures reported previously [9] and purified by gasphase sublimation in vacuo at elevated (300 8 8C) temperature prior to use.N otably,t his procedure produced good-quality single crystals of 1a,a nd their X-ray structural analysis showed anew structural polymorph (monoclinic space group, P2 1 /n), which differs from that prepared from as olution at room temperature (orthorhombic, Pbca). [9a] Detailed structural analysis revealed that the volume per molecule has increased from 574(2) 3 to 585(1) 3 in the high temperature (HT) polymorph. In the solid-state structure (Figure 2 and Figure S15 in the Supporting Information), the molecules of 1a are packed into 1D columns through p•••p interactions (3.417( 5) )w ith ar otation angle of 1808 8 and ab owl slip distance of 0.885(5) (Figure S16). This differs from the rotation angle of 908 8 observed in the room temperature (RT) polymorph. Thebowl slip also allows for the intercolumn CÀ H•••p interactions (2.789( 5) )t hat are responsible for the formation of an extended 2D network in the new crystal structure of 1a.I nc ontrast, no strong interactions are found between the adjacent aligned 1D columns in the previously reported structure.

Thec hemical reduction of 1a with two selected Group 1 metals of different ionic sizes and different metal binding abilities,n amely sodium and cesium, was then investigated. Sodium metal is known to facilitate the formation of the doubly reduced carbanions of planar [16] and curved PA Hs, often isolated in their "naked" forms. [17] Thereduction of 1a with 2equivalents of sodium in THF in the presence of 18crown-6 resulted in the preparation of the dianion of 1a, which was isolated as the corresponding sodium salt. The structure was elucidated by X-ray diffraction to be asolventseparated ion product, [{Na + (18-crown-6)(THF) 2 } 3 {Na + (18crown-6)(THF)}(1a 2À ) 2 ]( Figure 3). Surprisingly, 1 HNMR studies revealed that the doubly reduced product is NMRsilent in solution even at low temperature (Figure S9). DFT calculations at the B3LYP/6-31 + G(d) level of theory suggested that for compound 1,t he triplet state is more stable than the singlet state by 7.5 kcal mol À1 .T his trend is totally different from that of the parent corannulene,w here the NMR-active singlet state is more stable than the triplet state. [4c] Notably,t he addition of two electrons is reversible according to aD ART-MS study of the reduced and airquenched products (Figure S13).

Thec ontrolled reduction of 1a with cesium metal in the presence of 18-crown-6 in THF allowed the isolation of two products as good-quality single crystals suitable for X-ray diffraction analysis.T he first one contains the singly reduced anion coordinated by ac esium ion capped by an 18-crown-6 molecule,[ {Cs + (18-crown-6)}(1a À )]•THF (Figure 4a). The cesium ion coordinates to the concave face of 1a À (Figure 5a); such aselective concave coordination of acesium ion has also been seen in the reduction of corannulene and its methylated derivatives. [18] Thes hortest Cs•••p contacts are observed to the C17 and C18 atoms (3.502(3) and 3.354(3) , respectively). Thedistances to the adjacent C16, C19, and C22 atoms are longer and span over the range of 3.595(3)-3.707(3) . Thesecond cesium product, [{Cs + (18-crown-6)} 2 (1a 2À )],is formed by the doubly reduced anion with two cesium cations bound to the convex and concave surfaces of 1a 2À (Figure 4b). In both cases,the metal binding to the six-membered rings of 1a 2À is asymmetric (Table S3). ForC s1, the shortest distances are measured at 3.363(6) and 3.451(6) to the C3 and C4 sites,r espectively,w ith other Cs-p distances ranging from 3.638(6) to 3.771(6) (Figure 5b). ForC s2, the Cs-C distances to the six-membered ring range over 3.275(6)-3.729( 6) ).

As both cesium products are NMR-silent, DART-MS was used to confirm the formation of the singly and doubly reduced products.B yq uenching with D 2 O, [C 38 H 24 ND + ]a nd [C 38 H 24 ND 2 + ]species were separately detected for Cs/1a À and Cs 2 /1a 2À ,respectively (Figures S10 and S11).

Thefirst X-ray structural characterization [19] of the singly and doubly reduced states of 1a allowed us to analyze the changes in bowl depth upon one-and two-electron acquisition (Table 1). It is especially informative to compare 1a with the "naked" dianion isolated with sodium countercations,a s metal binding effects are excluded in this case.Acurvature increase of both the exterior and interior core can be clearly detected in Na 2 /1a 2À ,and both are further affected by cesium binding in Cs 2 /1a 2À .

To elucidate the nature of the anionic species,D FT calculations were performed at the B3LYP/6-31 + G(d) level of theory.A ccording to its MO diagram, 1 has two almost degenerated LUMOs (Figure S26), which can potentially accept up to four electrons.U pon the first reduction, one electron is introduced into the LUMO of 1 to form the radical anion 1 À (Figure S27). Thesecond electron is placed into the LUMO of 1 À to form 1 2À as at riplet diradical species.T his two-step reduction is in full agreement with the electrostatic potential (ESP) maps shown in Figure 6, where the negative charge in 1 À and 1 2À is increased in astepwise manner over the molecular orbitals where the electrons are located.

In summary,wehave synthesized the mono-and dianions of azapentabenzocorannulene 1a by chemical reduction with sodium and cesium metals in THF in the presence of 18crown-6. X-ray diffraction analysis of the sodium salt, Na 2 / 1a 2À ,r evealed the presence of the naked dianion. On the other hand, the controlled reduction of 1a with cesium metal allowed the isolation of singly and doubly reduced anions complexed with cesium ions, Cs/1a À and Cs 2 /1a 2À .The former complex shows selective binding of the Cs + ion to the concave surface of 1a À ,whereas in the latter,two Cs + ions bind to both the concave and convex surfaces of 1a 2À .T his structural characterization of anionic species of azapentabenzocorannulene represents the first example of the chemical reduction of heteroatom-containing buckybowls,and thus provides new perspectives for the chemistry and application of non-planar and heteroatom-doped PA Hmolecules. Table 1: Bowl depth (BD) in the X-ray structures of 1a, Cs/1 a À , Na 2 / 1a 2À ,and Cs 2 /1 a 2À along with the calculated values in 1, 1 À ,and 1a 2À (in ).

BD average exterior (green) [a] BD average interior (red) [b] X-ray 1a (RT) [9a] 1.588( 8) 0.879(8) 1a (HT) 1.581(5) 0.871(5) Cs/1 a À 1.615(5) 0.897(5) Na 2 /1 a 2À 1.646(9) 0.887(9) Cs 2 /1 a 2À 1.657(12)0 .908(12) DFT 1 (singlet) 1.528 0.853 1 À (doublet) 1.562 0.886 1 2À (triplet) 1.530 0.858