quinodimethanes with Nine and Ten Consecutively Fused Six

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Extended Bis(anthraoxa)quinodimethanes with Nine and Ten Consecutively Fused Six-membered Rings: Neutral Diradicaloids and Charged Diradical Dianions/Dications Shaoqiang Dong, Tullimilli Y. Gopalakrishna, Yi Han, Hoa Phan, Tao Tao, Yong Ni, Gang Liu, and Chunyan Chi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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Journal of the American Chemical Society

Extended Bis(anthraoxa)quinodimethanes with Nine and Ten Consecutively Fused Six-membered Rings: Neutral Diradicaloids and Charged Diradical Dianions/Dications Shaoqiang Dong,† Tullimilli Y. Gopalakrishna,† Yi Han,† Hoa Phan,† Tao Tao,† Yong Ni,† Gang Liu,† Chunyan Chi*,† †Department

of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

Supporting Information Placeholder ABSTRACT: We report the challenging synthesis of two very long bis(anthraoxa)quinodimethanes with nine (ABA) and ten (ANA) consecutively fused six-membered rings. The former is stable with negligible diradical character, while the latter with a moderate diradical character (y0 = 25.0%) is reactive and an unexpected trifluoroacetic substituted product (ANA-TFA) was isolated. X-ray crystallographic analysis revealed a planar backbone with a typical quinoidal character for both. Their dications can be regarded as the isoelectronic structures of the respective nonacene and decacene. The dication ABA2+ and dianion ABA2- are open-shell singlet diradicaloids, while the longer dication ANA-TFA2+ and dianion ANA2- have closedshell ground state, which can be explained by the different intramolecular Coulomb interactions. Both dianions have a bent backbone and can be considered as an isoelectronic structure of the tetraanion of nonacene and decacene, respectively.

The intrinsic high reactivity of higher order acenes makes their synthesis practically very difficult.1-4 Substitution by bulky and electron-withdrawing groups, incorporation of electron-negative imine-type nitrogen atoms into the backbone, or formation of phene-like structure can somehow improve the stability.5-13 Alternatively, heteroatom-containing quinoidal acene analogues and their charged forms could serve as good model compounds to understand the electronic properties of the all-carbon acenes.14-15 We previously demonstrated that the dication of a bis(benzothio)anthraquinodimethane could be regarded as the genuine isoelectronic structure of the heptacene.15 However, synthesis of even longer quinoidal acene analogs is very challenging due to the emerging open-shell diradical character upon elongation of the central quinodimethane unit. To solve this problem, we designed two bis(anthraoxa)quinodimethanes in which two anthracene units are fused onto the central 1,4-benzoquinodimethane (BQDM) or 2,6-naphthoquinodimethane (NQDM) moiety via an oxygen linkage (Figure 1). Accordingly, the molecule has nine and ten consecutively fused six-membered rings, respectively, but still could be stable because the extension is mainly attained by the two outer anthracene units. Two-electron oxidation would result in the dication, which can be regarded as the isoelectronic structure of the corresponding nonacene and decacene (Figure

1). On the other hand, the dianon can be regarded as the isoelectronic structure of the unknown tetraanion of the nonacene and decacene, respectively (Figure 1).

Figure 1. Structures of (a) the bis(anthraoxa)quinodimethanes and their dianion/dication forms; (b) the nonacene and decacene and their dianion/tetraanion forms. Bulky mesityl groups are attached onto the zigzag edges of the target compounds ABA and ANA to ensure sufficient stability and good solubility (Scheme 1). For ABA, a “center to outer edge” Friedel-Crafts (FC) cyclization strategy was used. Pd-catalyzed C-O formation reaction between 1 and 2 (see details in the Supporting Information (SI)) gave the key intermediate 3. Reduction of 3 with LiAlH4 followed by BF3•Et2O mediated FC cyclization of the intermediate diol gave the dihydro compound 4. The cationic species selectively attacked the β position of the anthracene unit presumably because of the steric hindrance. The target compound ABA was then obtained in 82% yield by oxidative dehydrogenation of 4 with 2,3-dichloro-5,6-dicyano1,4-benzoquinone (DDQ). For ANA, similar strategy failed at the C-O bond formation step and thus an “outer edge to center” approach was utilized. Electrophilic substitution between compound 5 (see SI) and the anthracenyl fluoride 6 (see SI) worked smoothly and provided the key intermediate 7 in 78% yield. Subsequent reduction and intramolecular FC alkylation afforded the dihydro compound 8. The 4-tert-butylphenyl groups at the α positions of the central naphthalene unit successfully prevented possible isomer formation at the cyclization step. However, dehydrogenation of 8 by DDQ gave insoluble precipitate. Upon the addition of trifluoroacetic acid (TFA),

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Scheme 1. (a) Pd2(dba)3, dppf, Cs2CO3, toluene, 100 oC; (b) i) LiAlH4, THF; ii) BF3·Et2O, DCM; (c) DDQ, toluene, RT; (d) K2CO3, NMP, 180 oC; (e) i) TFA; ii) SnCl2. Mes: mesityl group. the precipitate became dissolved and the solution showed strong ESR signal (Figure S1), indicating formation of radical species. Surprisingly, after addition of reductant SnCl2 to the solution, mono- and di- trifluoroacetic substituted ANA products were detected by MALDI-TOF mass spectrometry (Figure S2), but only the di-substituted product ANA-TFA could be separated. The precipitate could be due to the formation of chargetransfer complex of ANA with excessive DDQ (Figure S3), and treatment with TFA gave monomeric radical cation and dication species which can undergo nucleophilic substitution with TFA. Subsequent reduction by SnCl2 gave the final ANA-TFA product. The trifluoroacetic groups are selectively attached onto the anthracene moiety at the α sites next to oxygen atoms, presumably due to less steric hindrance at these positions and the electron-donating effect of the oxygen atom. Single crystals of ABA and ANA-TFA suitable for X-ray crystallographic analysis were obtained.16 Both molecules have a nearly planar π-conjugated backbone and the mesityl groups are nearly perpendicular to the backbone (Figure 2a-b). The central BQDM unit of ABA showed typical quinoidal character, while the bond lengths of NQDM of ANA-TFA indicated a decreased quinoidal character. For example, the bond length of C1-C2 bond (1.392 Å) is longer than that in ABA (1.375/1.373 Å). Spin-unrestricted DFT calculations (UCAM-B3LYP/631G(d,p)) predict that ABA has a negligible diradical character (y0 = 0.7%) but ANA has a moderate diradical character (y0 = 25.0%), which can explain their different C1-C2 bond length. Nucleus-independent chemical shift (NICS)17 and anisotropy of the induced current density (ACID)18 calculations were conducted to understand the electronic structure and aromaticity of these quinoidal dioxa-acene analogues. The central benzene ring in ABA and the naphthalene ring in ANA-TFA showed a NICS(1)zz value of 0.62 ppm and -9.45 ppm, respectively, corresponding to non-aromatic and aromatic character. ACID plots revealed that there was no obvious ring current along the central benzene ring of ABA, but diatropic ring current along the naphthalene ring was observed for ANA-TFA (Figures S31, 32), again, suggesting a larger diradical character for ANA-TFA (y0 = 25.0%). The NICS(1)zz values of lateral anthracene units revealed aromatic feature (Figure 2a-b). From ACID plots, diatropic ring currents were observed along the anthracene moieties for both ABA and ANA-TFA (Figures S31, 32). The 1H NMR spectra of ABA and ANA-TFA in C6D6 showed sharp signals at room temperature (Figures S66, S67). No NMR spectral broadening could be observed for ABA upon heating

up to 95 oC (Figure S4) in toluene-d8, confirming its almost closed-shell nature. However, obvious NMR spectral broadening was observed for ANA-TFA in toluene-d8 upon heating (Figure S5), in consistent with its open-shell diradical character. ANA-TFA showed a broad ESR signal with ge = 2.0033 (Figure S8). Variable-temperature (VT) ESR measurements of ANATFA in powder showed that the intensity increased with increasing temperature (Figure S9), which is typical phenomena of open-shell singlet diradicaloids.19-24 The singlet-triplet energy gap (ΔES-T) was estimated to be -6.20 kcal/mol by fitting the data with Bleaney-Bowers equation25 (Figure S10). ABA and ANA-TFA in dichloromethane (DCM) displayed an intense absorption band with λmax at 576 and 645 nm, respectively (Figure 3b-c). However, compared with ABA, a weak shoulder at 686 nm was observed for ANA-TFA (Figure S16), which is the typical character of open-shell singlet diradicaloid due to the presence of a low-lying HOMO,HOMOLUMO,LUMO double excitation state.26 ABA exhibited weak fluorescence at 660 nm (Figure S22) with a low quantum yield ( = 0.004), while no fluorescence could be detected for ANA-TFA. ABA and ANA-TFA has a half-life time of 157 and 125 hours, respectively, under ambient conditions (Figure S23). ABA and ANA-TFA displayed two oxidation waves with halfwave potential E1/2ox at 0.17, 0.69 V for ABA and 0.16, 0.62 V for ANA-TFA (vs Fc+/Fc couple) (Figure 3a and Figure S25). Two reduction waves were observed for both, with E1/2red at 1.85 and -2.14 V for ABA, and -1.46 and -1.79 V for ANA-TFA. Stepwise oxidation by NO•SbF6 gave the radical cation and dication. The ABA2+ and ANA-TFA2+ showed a half-life time of 135 and 98 hours, respectively (Figure S24). Notably, the dications of both ABA and ANA-TFA exhibited a broad long- wavelength absorption with λmax at 2109 and 1743 nm (Figure 3bc), respectively, both are red-shifted compared with their radical cations (λmax = 973 nm for ABA·+ and 1495 nm for ANATFA·+, see Figure S15). The 1H NMR spectrum of ABA2+ in CD2Cl2 was significantly broadened, and the resonances became sharp at lower temperatures (Figure 4a), indicating open-shell singlet ground state. In solid state, the ESR intensity increased with increasing temperature (Figure S12), and fitting of the data gave a singlet-triplet energy gap (ΔES-T) of -4.20 kcal/mol (Figure S13). The ABA2+ can be also obtained by oxidation with concentrated H2SO4,27 which showed similar absorption spectrum and temperature dependence of NMR and ESR signal (Figures S17-S19). On the other hand, the ESR signal


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C2

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Figure 2. X-ray crystallographic structures, selected bond lengths (in Å) and calculated NICS(1)zz values of (a) ABA, (b) ANA-TFA, (c) ABA2- and (d) ANA2-. Side view of the structures of (e) ABA2- and (f) ANA2-, showing the counter ions, the distorted backbone and the bending angles. Hydrogen atoms are omitted for clarity.

Current (A)

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Journal of the American Chemical Society

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Figure 3. (a) Cyclic voltammograms of ABA and ANA-TFA in DCM, absorption spectra of the neutral and charged species of (b) ABA and (c) ANA-TFA. Inset are photos of the solutions. of the ABA·+ decreased with increasing temperature (Figure S20), satisfying Curie law. The dication ANA-TFA2+ is ESR silent

and displayed sharp NMR signal, implying its closed-shell nature (Figure S63). Calculations also predict that ABA2+ has larger diradical character than ANA-TFA2+ (Table S3). ACID plots of ABA2+ and ANA-TFA2+ revealed three segmented diatropic ring currents along the two lateral anthracene units and the central benzene/naphthalene ring (Figures S31-32). It is notable that their isoelectronic structures, nonacene and decacene, were also theoretically predicted to show open-shell diradical character.28-30 The dianion ABA2- was generated by reduction of ABA with two equivalents of lithium anthracenide, while the dianion ANA2- was obtained by deprotonation of 8 with n-BuLi. Both dianions are persistent under inert and anhydrous conditions and their single crystals suitable for X-ray crystallographic analysis were obtained.16 Both molecules have a centrosymmetric structure with two counter cations located above and down the conjugated backbone (Figure 2e-f). The C1-C2 bond lengths are 1.433 Å and 1.432 Å (Figure 2c-d), respectively, which are much larger than that of neutral molecules. The backbones of both ABA2- and ANA2- are severely deviated from the planarity, and the bending angles between lateral anthracene unit and central quinoidal moiety are 8.2o and 6.9o, respectively (Figure 2e-f). The bending could be due to the partial sp3 hybridization characteristics of the C1 anion and this phenomenon was also observed in other anions.31-32 The ANA2- is less distorted compared with ABA2-, presumably due to the more effective charge delocalization along the larger backbone (Figures S33-34). The 1H NMR spectrum of ABA2- in THF-d8 showed signal broadening upon heating to 45 oC (Figure 4b and Figure S64), indicating its open-shell singlet diradical nature similar to ABA2+. On the other hand, the 1H NMR spectra of ANA2- showed sharp signals even at elevated temperatures, implying a closed-shell



nature. In consistent with this, ABA2- displayed an intense absorption band at λmax = 849 nm with a characteristic weak tail in the lower energy side (Figure 3b and Figure S21), while ANA2- showed a similar major band at λmax = 1002 nm without the tail. The trend was in consistence with the TD-DFT calculations (SI). Such difference for both the dications and dianions of ABA and ANA could be explained by the different intramolecular Coulomb repulsion. Assuming that the positive/negative charges are localized at the two C1 site, the repulsion force between two charges will be much larger in ABA2+ / ABA2- than that in ANA2+ / ANA2- due to a shorter distance. This will drive the charges far away from each other to minimize the repulsion and thus result in a diradical dication/dianion state for ABA2+ / ABA2-. For ANA2+ / ANA2- with a longer inter-charge distance, the repulsion force is less and hence, the positive/negative charges can be more effectively delocalized along the backbone (Figures S33-S36). In addition, positive NICS(1)zz values (7.75 and 8.35 ppm for ABA2- and ANA2-) and paratropic ring currents (Figure 2c-d, Figures S31-S32 and Table S4) were calculated for the O-containing six- membered rings, indicating their weak anti-aromatic character. For the central benzene and naphthalene rings, negative NICS(1)zz values (-3.56 and -5.58 ppm for ABA2- and ANA2-) were calculated, indicating their weak aromatic character. 25 oC

0oC

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In summary, two O-containing quinoidal acene analogues ABA and ANA-TFA with the record length were synthesized. The intrinsic high reactivity of ANA led to an unexpected substitution reaction. The dication and dianion of ABA turned out to be open-shell diradicaloids, while for ANA, its dication and dianion are both closed-shell compounds, which can be explained by their different intramolecular Coulomb repulsion. The dications of ABA and ANA-TFA can be regarded as the isoelectronic structure of the respective nonacene and decacene, but they are much more stable. The studies on their dianions also gave some insights to the electronic properties of the respective tetraanions, which are still not attained.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic detail, additional spectra and DFT calculation (PDF)

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT

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C.C. acknowledges financial support from the MOE T1 grant (R143-000-692-114), Tier 2 grant (MOE2018-T2-1-152) and Tier 3 programme (MOE2014-T3-1-004). We thank Dr. G. K. Tan for the crystallographic analysis, and Dr. J. Wu and Dr. Y. Han for 2D NMR measurements.

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REFERENCES

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Figure 4. VT 1H NMR spectra of ABA2+ in CD2Cl2 (a), ABA2- in THF-d8 (b), and schematic redox reactions with labeling (c). The bottom spectrum was recorded upon warming back to 25 oC.

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quinodimethanes with Nine and Ten Consecutively Fused Six

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