Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Abstract Dibenzo[a,e]pentalene (DBP) is a non‐alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP‐phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift‐calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP‐phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.


Introduction
Polycyclic aromatic hydrocarbons (PAHs) incorporatingf ivemembered rings have intrigued chemists for decades. As op-posedt oa lternant PAHs composedo fo nly six-membered rings,t hose with five-membered rings are non-alternant p-systems. [1] The result is as hift of orbitale nergiesr elative to an alternant p-system of similars ize, in most cases an increase in HOMOa nd ad ecrease in LUMO energy.T his bestowsa mbipolar electrochemical charactera nd as mall band gap on fivemembered ring-containing PAHs. Furthermore, they can possess antiaromatic character. [2][3][4][5][6][7] The best example here is pentalene ( Figure 1A), which is so unstable and reactive that it could only be isolatedw hen bearingt hree kinetically stabilizing tert-butyl substituents. [8] Another strategy of stabilization is to annulate two benzene rings to the pentalenec ore, resulting in dibenzo[a,e]pentalene (DBP,F igure 1A), [9] whichi sathermodynamically and kinetically stable compound. [10] Yeti tr etains as mall band gap, reflected in its red color,a nd ambipolar redox properties ( Figure 1B). The relative ease of oxidationa nd reduction of DBP can be explained by an increasei na romatic character.F ormally,b oth two-electron oxidation or -reduction render the 16-p-electron systemH ückel-aromatic with 14-or 18 p-electrons, respectively ( Figure 1B). Not surprisingly,t he interesti nD BP's redox chemistry dates back to 1963 when Silvestrip rovided first experimental proof that at wo-fold reduction was possible. [11] Rabinovitz [12][13][14] as well as Edlund [15] and co-workers followed with more detailed NMR-and EPR-spectroscopic studies, confirming an increased aromatic character of the oxidized and reduced DBP.F inally,p roof for the planar structure of the DBP dianion in the solid state, af urther indication for its aromaticity,w as obtained by Saito et al. in 2007. [16] More examples of dianionic DBPs in the solids tate followed [17][18][19] as well as the structure of aD BP anion radical. [20] Similarly,t he structures of the reduction products of p-extended indenofluorenes werer eported. [21,22] However,n oe xamples of aD BP cation radical or dication in the solid state have been reported yet. [23] The oxidation and reduction products of DBP are also of interest regarding the application of DBPs as semiconductors in organic field-effectt ransistors (OFETs). DBP derivatives and further benzannulated diaceno[a,e]pentalenes have been employed in both p- [24][25][26][27][28][29] and n-type [30] OFET devices. The charge transport is thought to proceed throughe ither the cation radical in the case of p-type doping or the anion radicalf or an ntype device. Which type of doping is easier to achieve depends on the HOMO and LUMO energy levels of the DBP derivatives. [31,32] Investigatingc hargedD BP derivatives, which are responsible for charge carrier transport in devices, as discrete speciesg ives insight into their electronic and geometric structures.
For these reasons we herein study the chemical oxidation and reduction of DBP derivative 1 ( Figure 1C). [33] We were able to obtain its anion radical 1C À À ,c ation radical 1C + + and dication 1 2 + + and investigatedt heir structural and electronic properties by X-ray crystallography,E PR spectroscopy and DFT calculations. With the synthesis of (2,7)dibenzo[a,e]pentalenophanes [33] (DBP-phanes) and DBP-based nanohoops [34][35][36] we recently showed that the DBP unit can be bent [37] without strongly alteringi ts optoelectronic properties. Hence, we herein performed ac hemical reduction of DBP-phane 2 (Figure 1C)t oi nvestigate the influence of bending. We surprisingly found that the strain led to ar eduction product different from that of planar 1 through protona bstractionf rom as olvent molecule.

Results and Discussion
Oxidation and reduction of planar DBP 1 PlanarD BP 1 was synthesized as previously described. [33] Cyclic voltammetry measurements indicated that 1 could form both ar elatively stable anion radicala nd dianion as well as cation radicala nd dication, testifying to its ambipolar electrochemical character ( Figure 2). 1 featured two reversible reductions in THF with half-wave potentials of E 1/2 = À2.13 and À2.69 Va nd two reversible oxidations in CH 2 Cl 2 at E 1/2 = 0.66 and 1.01 V( all vs. Fc/Fc + ).
For the chemical reduction of DBP 1,o ne equivalent of KC 8 was added to as olution of 1 in THF at À78 8C. After reaching room temperature, filtration afforded the anion radical as the salt [K(THF) 6 ] + 1C À À (Scheme 1) as an intensely dark-blue-colored solution.Aconcentrated solution of [K(THF) 6 ]1 in THF at À40 8Cp rovideds ingle crystalss uitable for XRD.A ttempts to obtain the dianion 1 2À À by using two equivalents of KC 8 were unsuccessful and only furnished the anion radical 1C À À .T his may be due to the sterich indrance through the mesityl groups, which do not allow for an h 5 -coordination of the potassium ions to the reduced DBP core. In all reported solid-state structure of DBP dianions in the literature with alkalinec ounter cations, both metal ions h 5 -coordinated to the DBP core. [16,18,19] For the oxidation of 1 as trongy et innocent oxidant, facilitating an outer-sphere electron transfer,w as required. Based on its high oxidation potential (see above), we used the perfluorinated dihydrophenazine cation radicalo fs alt 3,w hich was recently introduced as as trong oxidant. [38] Its formal reduction potential lies at 1.29 Vv ersus Fc/Fc + in ortho-difluorobenzene( o-DFB), hence sufficiently high to oxidize 1 to ad ication. Thel arge Al-based weakly coordinating counterion Al(OR F ) 4 ] À (R F = C(CF 3 ) 3 ) [39] serves to stabilize generated reactive cationic species. [40][41][42][43][44] Indeed, treatment of 1 with one or two equivalents of 3 in o-DFB furnished cation radical 1C + + andd ication 1 2 + + as deeplyr ed-and blue-coloreds olutions, respectively.L ayeringo ft he obtained reactionm ixtures with  pentanea fforded singlec rystalss uitable for X-ray diffraction analysisofb oth salts.
The structures of the salts of 1C À À , 1C + + and 1 2 + + are shown in Figure 3, and selectedb ond lengths are listed in Ta ble 1. To the best of our knowledge, this is only the second report of aD BP anion radical [20] in the solid state and the first report on solid-Scheme1.Synthesis of the DBP-anion radical 1C À À ,the cation radical 1C + + and the dication 1 2 + + . Figure 3. A) Molecular structures of the salts of anion radical 1C À À ,cationradical 1C + + and dication 1 2 + + in the solids tate (displacement ellipsoids are showna t the 50 %p robability level;hydrogen atoms are omitted for clarity);B)Changes in bond lengths (in )upon going from neutral 1 to anion radical 1C À À ,cation radical 1C + + and dication 1 2 + + ,based on structural parametersint he solid state (blue-coloredb ondsare shortened, red-colored bonds elongated) and average dihedral angles V DBP-Anisyl for the torsion between the DBP unita nd the anisyl substituents. Table 1. Bond numbering and selected bond lengths (in )f rom X-ray crystallographicd ata of 1 [33] and its reduceda nd oxidized species 1C À À , 1C + + and 1 2 + + .
state structures of aD BP cation radical or dication. In anion radical 1C À À ,l ikely due to the steric bulk of the two mesityl groups on the DBP core, the potassium ion is not coordinated by the DBP unit, but by six THF molecules instead. This stands in contrastt ot he only other DBP anion radical reported in the literaturew ith two Si(iPr) 3 (TIPS) substituents on the DBP core, where the potassium ion was located above the center of the DBP framework and furthermore unsymmetrically coordinated with two THF molecules. [20] Due to this the geometry of the DBP unit 1C À À remains planar,w hich provides an ideal situationt oa ssess its increased aromaticity by comparison of the CÀCb ond lengths with neutral 1.As imilars ituation was encountered forb otht he cation radical 1C + + as well as the dication 1 2 + + ,w here theh eren on-coordinatingc ounter anions Al(OR F ) 4 ] À arefar removedf romt he DBPcores,leaving thelatter planar.
Most instructive regarding the (anti)aromaticity of the DBP core is ac omparison of the CÀCb ond lengths in neutral 1, anion radical 1C < M-> and oxidized species 1C + + and 1 2 + + .I nn eutral 1,t he small bond length alternation with CÀCb ondl ength between 1.378 and 1.424 in the six-membered rings of the DBP units (bonds2 -6 and 9, Ta ble 1) clearly indicatet heir aromatic character.I nt he five-membered rings, the doubleb ond (bond 8: 1.357 )a nd single bonds (bonds 1: 1.490 ,7 : 1.461 and 10:1 .467 )a re more localized due to the slight antiaromaticc haracter of the pentalene core. Upon both reductiono ro xidation, the CÀCb ondl engths in the five-membered rings become more equalized in 1C À À , 1C + + and 1 2 + + .T his is shown in detail by ar ed (elongated) or blue (shortened) coloring of the respective bonds in Figure 3B including the absolute change in bond lengths compared to 1 for all bonds experiencing ac hange of more than 0.02 .I nt he bis-TIPS-sub-stitutedD BP anion radical reported in the literature, [20] the same tendency of bond lengthse quilibration in the pentalene core was observed compared to the neutralspecies.
Furthermore, the dihedral angles V DBP-Anisyl betweent he DBP unit and the anisyl substituents significantly change upon reductiono ro xidation( Figure3B). In anion radical 1C À À with a value of 44.98,t his angle is larger than in neutral 1 with 27.48 for molecule Aa nd 38.18 for molecule B. This indicates ar educed resonance with the anisyl group in the anion radical state, which can be rationalized with the electron-rich nature of the anisyl substituent.T he opposite is the case in cation radical 1C + + and dication 1 2 + + .Here, the dihedrala ngles are reduced to 9.78 for 1C + + and 11.88 respective 26.38 for molecule Aa nd B of 1 2 + + ,r espectively.T his shows that the positive charge(s) are partially delocalized over the anisyl groups. This is also manifested in shortened lengths of bonds 11 and 12 (and 11'a nd 12') in 1C + + and 1 2 + + compared to 1.I np articular,i nd icationic 1 2 + + ,t hese bonds are shortenedb y0 .041 and 0.044 ,r espectively,assuming partial double bond character.
Spectroelectrochemical measurements on DBP derivative 1 (see Supporting Information) showed an increase in the longer wavelength absorption maxima and absorption onset upon oxidation, indicative of ar educed band gap, whilet he intensity of the main absorption band centered around 340 nm decreased. Upon reduction,t he intensities of all absorption bands increased, in particulart he short-wavelength bands below 300 nm.
To further assess the (anti)aromatic character of the DBP units we calculated NICS [45,46] (nucleusi ndependent chemical shift) values using B3LYP/6-31G* ( Table 2). For neutral 1,t he NICS(1) iso value was positive above the center of the 5-membered rings and negative for the 6-membered rings, indicating aromatic characterf or the latter and antiaromatic character for the former.W ith all redox events, the aromaticity of the overall molecule increased. With reduction to anion radical 1C À À ,t he 5membered rings obtained slight aromatic character,w hilet he aromaticity in the 6-membered rings furtheri ncreased. Strongly aromatic character wasf ound for dianion 1 2À À ,i np articular for the 5-membered rings in the pentalene core. As imilaro bservation can be made for the oxidation. In cation radical 1C + + , the 5-membered rings have lost their antiaromaticc haracter, and in dication 1 2 + + they even assumed acertain degree of aromaticity, whilet he 6-membered rings became ab it more aromatic.
The higher aromatic character upon reduction to 1C À À and particularly 1 2À À compared to oxidized 1C + + and 1 2 + + can be explained by the lack of conjugation to the anisyl substituents in the anionic forms. Thereby,t he additional electron(s) are mostly localized on the DBP core and lead to as trong increase in aromaticity.I nc ations 1C + + and 1 2 + + ,o nt he other hand, conjugation to the anisyl substituents is significant, as discussed above in the context of the geometrical parameters of the solid-state structures,a nd the positive charges are only partially localized on the DBP core and less strongly increase its aromaticity.
Electron paramagnetic resonance (EPR) spectra providedi nsight into the location of the spin density in cation radical 1C + + . The continuous wave spectrum obtained for 1C + + in o-DFB at room temperature is shown in Figure 4A.T he spectrum is characterized by ap ronouncedh yperfinep attern, resulting from coupling of the unpaired electron spin to the protons of the structure. Consistent with previous literature results, [13] a gvalue of 2.0026 was determined by numerical simulationoft he data using EasySpin [47] functions in combination with ah omewritten MATLAB fitting routine. As shown in Figure 4B,t he spin density (predicted from DFT calculations) is distributed mainly over the DBP core, but also the anisyl substituents.T his is in line with the interpretation of the solid-state structure of [a] B3LYP/6-31G* on PBEh-3c (1C + + and 1 2 + + )-or B3LYP/6-31G**( 1, 1C À and 1 2À )-optimized geometries.
1C + + ,w hich showed the anisyl groups to participate in the charge ands pin delocalization.
The choice of the protonc ouplings accounted for in the simulations was guided by the results from DFT calculations (UKS B3LYP/EPRII) of the EPR parameters using ORCA.T he protons with calculated hyperfine coupling constantsl arger than j a iso j ! 0.3 MHz are indicated in Figure 4C.T he computed values were used as starting parameters for the numerical fit, where the ten protons with hyperfine coupling constantsb etween AE (0.2-0.5) MHz were assumed to contributeo nly to the linewidtha nd were not explicitly accounted for.T he signs of the hyperfine coupling constants were taken from DFT,s ince the simulation is only sensitivet ot he absolutev alues. The best fit to the experimental data, as shown in Figure 4A (bottom), was obtained for the following hyperfine coupling constants: À4.5 MHz ( 2), À2.4 MHz ( 2), + 2.1 MHz ( 6), and + 1.1 MHz ( 4).

Reduction of bent DBP-phane 2
Finally,w ew ere interested to find out what effect bending of the DBP unit played in its chemical reduction or oxidation. DBP-phane 2 (for structure see Figure 1C)w as synthesized as previously described. [33] Compared to planar 1,t he oxidation potentials of 2 are only slightly shifted to lower half-wave potentials of E 1/2 = 0.59 and 0.90 Vv ersus Fc/Fc + (in CH 2 Cl 2 ), with the first oxidation being reversible and the second quasi-reversible ( Figure 5). Applying similar oxidation conditions as shown in Scheme 1t oD BP-phane 2,h owever, did not provide single crystalss uitablef or X-ray diffraction, but decomposition seemedt oo ccur,even at low temperatures.
The reductions are more strongly influenced by the bending of the DBP.T he first reduction was reversible, when the poten-tial was reversed beforei nitiating the second reduction process. With E 1/2 = À1.99 Vv s. Fc/Fc + its half-wave potential was shifted to higher potentials by 0.14 Vc ompared to planar 1. The second reduction was irreversible with acathodic peak potential of E cp = À2.72 Vv s. Fc/Fc + (comparedt oÀ2.77 Vf or 1), which can be seen from the additional peak in anodic scan directiona tÀ1.17 V.
In order to shed light on the second irreversible reduction process in DBP-phane 2,w ereduced this compound with two equivalents of KC 8 in THF ( Figure 6A). After filtration,r ed crys-   tals wereo btained from ac oncentrated THF solution layered with pentane stored at À40 8C.
Interestingly,t he reduced product was neither the DBPphane anion radicaln or its dianion. Instead,s ingle-crystal XRD revealed the structure shown in Figure 6B.A pparently,t he dianion 2 2À À had abstracted ap rotonf rom-most likely-a THF solventm olecule to form anion 4 À À ,w here one of the 5-membered rings was reduced to ac yclopentadienew ith an sp 3 -hybridizedc arbon atom. The deprotonation of THF by strong bases is ak nown process. [48] This was evidenced by the molecular structure of the reduction product K(THF) 4 + 4 À À in the solid state ( Figure 6B). In the crystal structure, the potassium ion is h 5 -coordinated by the cyclopentadienide moiety with an additional four complexedT HF molecules. The other five-membered ring contains the tetrahedral carbon atom, as can be seen from the close-up view in Figure 6C.T his proton abstraction is reminiscent of the reported reactiono fadianionic DBP speciesw ith Cr(CO) 3 (CH 3 CN) 3 ,w here protona bstraction from a CH 3 CN ligand led to ac ompound similar to 4 À À with Cr(CO) 3 h 5coordinated to the reduced five-membered ring and at etrahedral C-atom in the other five-membered ring. [19] The reason for the instability of ad ianion 2 2À À may lie in the fact that h 5 -coordination of two potassium ions is not possible due to the incorporation of the DBP into ac yclophanes tructure. Coordination of the first potassium ion is hindered by the two mesityl groups (see structure of [K(THF) 6 ] + 1C À À above), and the second potassium ion would need to bind from the bottom side of the DBP,w hich is sterically inaccessible.I na ll reported solidstate structures of DBP dianions in the literature with alkaline counter cations,b oth metals coordinate from opposite sides of the DBP. [16,18,19] Furthermore, due to the strained structure of DBP-phane 2,aplanarizationo ft he DBP unit upon twofold reductioni sn ot possible, which would be required to fully profit from the obtained aromatic character.T he fact that the potassium ion does coordinate to one five-membered ring in K(THF) 4 + 4 À À stands in contrastt ot he solid-state structure of the planara nion radicali nt he salt [K(THF) 6 ] + 1C À À (see above). It is likely due to one of the mesityl groups being bent away from the five-membered rings in 4 À À because of the tetrahedral geometry of that carbon atom,p roviding sufficient space for the K(THF) 4 + moiety to bind.

Conclusions
We herein investigated the chemical oxidation and reduction of bis-anisyl-substitutedD BP 1 and DBP-phane 2.T heir molecular structures in the solid state are the first examples for a DBP cation radicala nd dication, and the second example of a DBP anion radical. CC-bondl ength analyses and NICS calculations showed that the antiaromaticity of 1 decreasedu pon both reduction and oxidation, in particular in the five-membered rings, rendering the charged speciesa romatic. In the oxidizedf orms,t he anisyl substituents participated in the charge delocalization, as seen from their bond lengthsa nd coplanarization. In addition, the hyperfine couplings determined by EPR spectroscopy signaled the delocalization of the spin density over the entirem olecular ion and even to the anisyl moieties. For DBP-phane 2,t he two-fold reduction product underwentp rotona bstraction from as olventm olecule. This was likely due to its bent structure, which could not well accommodate the added charge, as wellasthe steric bulk of two mesityl substituents, hindering the coordination of two potassiumc ations. This study testifies to the ambipolar electrochemical character of the DBP and provides structurali nsight into its chargeds pecies, relevant for OFET applications.

Experimental Section
DBP 1 and DBP-phane 2 were synthesized as previously described. [ in o-DFB (3 mL) and 1 (0.014 g, 0.022 mmol, 0.5 equiv.) were dissolved in o-DFB (3 mL) with immediate formation of ad ark blue color.The reaction mixture was stirred for afurther 30 min at ambient temperature and subsequently layered with n-pentane to yield dark blue/black crystals suitable for scXRD analysis (0.042 g, 0.016 mmol, 75 %crystalline yield). Synthesis of [K(THF) 6 ] + + 1C À À :Asolution of 1 (107 mg, 0.16 mmol) in 2mLT HF was slowly added to as uspension of KC 8 (22 mg, 0.16 mmol, 1equiv.) in 2mLT HF at À78 8C. An immediate colorchange from orange to blue was observed. The reaction mixture was stirred for 15 minutes at À78 8C. The cooling bath was removed and the reaction mixture was stirred for 3h at rt. After filtration, the solution was concentrated and stored at À40 8Ct o give dark blue crystals suitable for scXRD. 4 ] + + 4 À À :As olution of 2 (82 mg, 0.10 mmol) in 2mLT HF was slowly added to as uspension of KC 8 (30 mg, 0.22 mmol, 2.2 equiv.). An immediate color-change from orangered to purple was observed. The reaction mixture was stirred for 15 minutes at À78 8C. The cooling bath was removed and the reaction mixture was stirred for 1.5 ha tr t. After filtration, the reaction mixture was concentrated, layered with pentane and stored at À40 8Ct og ive dark red crystals suitable for scXRD. The crystals were embedded in as ticky solid which may be caused by the yet unidentified proton abstraction reaction.