Oxidation of Isodiphenylfluorindine: Routes to 13

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Oxidation of Isodiphenylfluorindine: Routes to 13‑Oxoisodiphenylfluorindinium Perchlorate and Fluorindine Cruciform Dimers Georgia A. Zissimou, Andreas Kourtellaris, and Panayiotis A. Koutentis* Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus S Supporting Information *

ABSTRACT: Isodiphenylfluorindine (5) reacts with K2Cr2O7/H+ to give 13-oxoisodiphenylfluorindinium perchlorate (7) (75%), but with phenyliodine bis(trifluoroacetate) (PIFA) or MnO2 it gives the zwitterionic and quinoidal cruciform 13,13′dimers 11 (85%) and 12 (89%), respectively. The zwitterionic 13,13′-dimer 11 can be rapidly converted with MnO2 into the quinoidal 13,13′-dimer 12 (100%). UV−vis, NMR, single-crystal X-ray diffraction, and density functional theory studies support the structural assignments of all products. The electrochemical behavior of the compounds is also presented.

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he chemistry of linear-fused azaacenes has been of interest for over 100 years due to their commercial value as dyes,1 medicants,2 and as components in organic electronic devices.3 Homofluorindine (1) (5,12-dihydroquinoxalino[2,3-b]phenazine), that in solution is in prototautomeric equilibrium with its 5,14-dihydro form,4 is one of the oldest (1887) examples of a linear-fused azaacene (Figure 1).5 In 1926, Dutt6 tried to

contested the structure assignments which were based on degredation studies. Recently, the syntheses and physical properties of both fluorindines 4 and 5 were revisited supporting the respective structural assigned quinoidal12 and biscyanine13 structures. Since then, a number of analogous zwitterionic azaacenes have appeared.14 Despite this, the chemistry of charge separated fluorindines has, with one exception, been limited to protonation and alkylation of the nitrogens of the -ve cyanine.13 The exception is an early (1923) study by Kehrmann and Leuzinger on the K2Cr2O7/H+-mediated oxidation of isodiphenylfluorindine 5 to give 3-oxoisodiphenylfluorindinium perchlorate (6) the structure of which was based on elemental analysis (Scheme 1).15 Scheme 1. Oxidation of Isodiphenylfluorindine 5 Using K2Cr2O7/H+: Kehrmann’s Product 6 vs Our Product 7

Figure 1. Structures of important early fluorindine analogues.

prepare the aromatic dedihydrofluorindine 2 (5,7,12,14tetraazapentacene) from 2,3-diaminophenazine and 1,2-benzoquinone, but in 1951, Badger and Pettit7 proved the product obtained was the 6,13-dione 3, which they also prepared via oxidation of fluorindine 1. In 2009, Bunz et al.8 successfully converted the 6,13-dione 3 into the first aromatic dedihydrofluorindines. In the 1890s, the quinoidal diphenylfluorindine 49 and the isomeric biscyanine isodiphenylfluorindine 510 were also reported (Figure 1). At the time, Kehrmann and Fischer11 © XXXX American Chemical Society

Received: December 22, 2017

A

DOI: 10.1021/acs.orglett.7b03998 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

protonates on one of the nitrogens of the −ve cyanine,13 to give 5H+. Tentatively, this can then react with chromic acid to give the nitrogen adduct 10. Via a 2,3-sigmatropic shift (Etard reaction),17 oxygen transfers to the C13 position of the central ring. A subsequent elimination of dihydroxychromium gave the fluorindinium 7 (Scheme 3).

We doubted the original assignment: first, oxidation of fluorindine 1 gave the 6,13-dione 3,7 and second, we recently oxidized the biscyanine tetraphenylhexaazaanthracene 8 (aka TPHA) using both PIFA and MnO2 to obtain the scissor 5,5′dimer DI-TPHA 9 (Scheme 2).16 Both of these reactions Scheme 2. Dimerization of TPHA 8 to DI-TPHA 9

Scheme 3. Tentative Mechanism for the K2Cr2O7/H+Mediated Oxidation of Isodiphenylfluorindine 5 into 13Oxoisodiphenylfluorindinium 7

suggested that oxidation of isodiphenylfluorindine 5 should occur at the more electron-rich central carbon of the −ve cyanine (i.e., position C13). The highest occupied molecular orbital (HOMO)13a of fluorindine 5 [Figure S9, Supporting Information (SI)] supported considerable electron density at the C13 position of its −ve cyanine, and a Mulliken charge analysis showed greater electron density at C13 than at the peripheral C3/ 9 positions (Figure S19, SI). Repeating Kehrmann’s reaction led to the product’s true identity (i.e., structure 7): dropwise treatment of isodiphenylfluorindine 5, solubilized in acetic and perchloric acids, with aqueous K2Cr2O7 (2.4% w/v) followed by precipitation and recrystallization [1,2-dichloroethane (DCE) or MeCN] gave 13oxoisodiphenylfluorindinium perchlorate (7) as dark green needles with a metallic sheen in 75% yield (Scheme 1). Product 7 was thermally stable: thermal gravimetric analysis (TGA) revealed no mass loss up to 384.3 °C, and differential scanning calorimetry (DSC) gave a decomposition onset at 368.7 °C (SI). In dichloromethane (DCM), product 7 gave a deep purple color with a lowest energy absorption at λmax 542 nm (log ε 5.06) (see Figure 4). NMR spectroscopy in CD3CN supported a symmetrical molecule, and the presence of all of the H’s from the periferal arenes, the absence of the H13 signal and a quaternary carbon resonance at 177.2 ppm, typical of a CO group (SI). Single crystals were grown by vapor diffusion (DCE/n-pentane) that enabled X-ray diffraction studies to support the structural assignment (Figure 2).

The reactivity of isodiphenylfluorindine 5 to other oxidants was also examined. Oxidants HgO, Ag2O, or Pd/C (20 equiv) in DCM gave no reaction, and the starting material was recovered, but, as with the oxidation of TPHA 8, treating isodiphenylfluorindine 5 with either PIFA or MnO2 led to interesting 13,13′dimers (Scheme 4). The reaction of isodiphenylfluorindine 5 and Scheme 4. Formation of 13,13′-Dimers 11 and 12 from Isodiphenylfluorindine 5

PIFA (1 equiv) in dry DCM at ca. 20 °C for 24 h gave a polar deep green [λmax 819 nm (log ε 4.30)] colored 13,13′-dimer 11 in 85% yield. The reaction of isodiphenylfluorindine 5 with MnO2 (50 equiv) in dry DCM at ca. 20 °C was slower (3 days) and gave a less polar [Rf 0.54 (DCM/THF, 50:50)], higher mass (m/z 932), deep blue [λmax 620 nm (log ε 5.47)] colored 13,13′-dimer identified as the tetraquinonimine 12. Treating the 13,13′-dimer 11 for a longer period of time with excess PIFA gave no reaction, but with additional MnO2 (50 equiv) in dry DCM at ca. 20 °C for only 1 h the tetraquinonimine 12 formed in 100% yield (Scheme 4). The mechanistic rationale for the PIFA- and MnO2-mediated oxidation has been presented for the analogous dimerization of TPHA 8.16

Figure 2. Crystal structure of 13-oxoisodiphenylfluorindinium perchlorate (7) (hydrogen atoms removed for clarity).

Fluorindinium 7 crystallized in the monoclinic P21/c space group with one molecule of DCE in the asymmetric unit (Z = 4) (Figure S3, SI). Bond length analysis supported the presence of a typical carbonyl group (CO) 1.207(4) Å (bond order 2.1). Normalization of the bond orders for the N(5)−C(5a)−C(6)− C(6a)−N(7) fragment supported a delocalized + ve cyanine; cf. isodiphenylfluorindine 5 (see the SI). The mechanistic rationale for the formation of fluorindinium 7 follows: in the presence of acid, isodiphenylfluorindine 5 B

DOI: 10.1021/acs.orglett.7b03998 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Both 13,13′-dimers 11 and 12 were thermally stable with DSC and TGA decomposition onset points 389.7 and 393.6 °C for dimer 11, respectively, and 466.0 and 474.1 °C for dimer 12, respectively (SI). Both dimers had good solubility in typical organic solvents, which enabled the growth from solution of Xray-quality single crystals (Figure 3).

Figure 4. UV−vis of fluorindines 5 (black line), 7 (green line), 11 (red line), and 12 (blue line) in DCM at ∼1 × 10−5 mol·L−1.

green colored ethanolic solutions of dimer 11 [λmax(EtOH) 751 nm (log ε 4.98)] became deep blue [λmax(EtOH/H+) 638 nm (log ε 5.56)] (Figure S23). The UV−vis spectra of fluorindinium 7 and quinoidal dimer 12 were significantly blue-shifted compared to biscyanines 5 and 11, and the intensity of the low energy absorption of the dimer 12 was greatly increased. No solvatochromic or acidochromic behavior was observed for dimer 12 (Figure S24). There was good agreement between the optical (EgOpt) and TD-DFT determined HOMO−LUMO gaps (EgTD‑DFT) (Table 1). DFT studies revealed the low energy absorptions of

Figure 3. Crystal structures of dimers 11,(a) side view, (b) top view, and 12, (c) side view, (d) top view (hydrogen atoms and cocrystallized solvents removed for clarity).

Table 1. Optical, Electrochemical, and Theoretical HOMO− LUMO and Singlet−Triplet Energy Gaps of 5, 7, 11, and 12a

Mainly weak edge-to-face and face-to-face intermolecular contacts (∼3.4−3.6 Å) were observed in the solid-state packing, which is discussed in more detail in the SI. Torsion angles between the tetraazaacene planes of dimers 11 and 12 were 67.2° and 83.0°, respectively, typical of scissor and cruciform geometries.16 Bond-order analysis (Table S8) of both the crystallographic and computationally optimized [DFT, UB3LYP/6-31G(d)] singlet ground state of dimer 11 supported the biscyanine structure, as did the calculated electrostatic potential map (Figure S17). A bond-order analysis of the 13,13′dimer 12 supported the quinoidal character (Table S8). Nucleusinduced chemical shift (NICS) values (Table S2) indicated the central arenes of the dimer 12 were aromatic; i.e., the molecule can be viewed as a type of biaryl. This was in contrast to the NICS values for isofluorindines 5, 7, and 11 that indicated an aromatic periphery and a mildly or nonaromatic interior (Table S2). The UV−vis spectra of fluorindines 5, 7, 11 and 12 all showed a high energy band in the UV−vis region and two lower energy bands in the violate and red regions of the visible spectrum (Figure 4). The biscyanines 5 and 11 had similar absorption bands in the UV−vis and violate region but not in the red region of the spectrum: monomer 5 showed a low energy band with a typical vibronic progression (0−2/0−1/0−0, 0.5:0.8:1.0 peak ratios), but dimer 11 showed a broader low energy band with a redshifted lower intensity 0−0 vibronic transition (0−2/0−1/0−0, 1.0:0.9:0.5 peak ratios). This indicated some coupling between the near orthogonal azaacene moieties. Since the opposing dipole moments of each tetraazaacene moiety cancel out, no solvatochromic behavior was observed (cf. dimer 9).16 Nevertheless, dimer 11 displayed acidochromic behavior similar to the starting isodiphenylfluorindine 5:13 on protonation, the deep

5 7 11 12

λmax(abs) (nm)

EgOpt (eV)

EgCV (eV)

EgTD‑DFT (eV)

ΔEST (eV)

763 542 819 620

1.52 2.18 1.39 1.92

1.51 1.81 1.49

1.61 2.29 1.53 2.01

−0.39 −0.67

EgOpt was calculated from the onset of the λmax from UV−vis and the Beer−Lambert equation (E = hC/λ); EgCV = E1/21a − E1/21c; EgTD‑DFT = first excitation energy from TD-DFT/UB3LYP 6-31G(d); ΔEST = Esinglet − Etriplet. a

fluorindines 5, 7, 11, and 12 consisted mainly of transitions between the frontier molecular orbitals, but for the 13,13′-dimers 11 and 12 these transitions were of mixed character (Tables S3− 6). The biscyanines 5 and 11 showed low but surprisingly, different singlet triplet gaps (ΔEST) of −0.39 and −0.67 eV, respectively (Table 1); cf. the analogous ΔEST values of biscyanine TPHA 8 and its dimer 9 were very similar (−0.68 vs −0.65 eV, respectively).16 Cyclic voltammetry (CV) studies on fluorindine 1 have been reported,4a,b,18 but there are no reports on the electrochemistry of isodiphenylfluorindine 5. CV studies on fluorindines 5, 7, 11, and 12 indicated that the biscyanines 5 and 11 were electronically similar, showing complex CVs with multiple redox peaks (Figure 5 and SI). By analogy with DI-TPHA 9,16 the reduction peaks of biscyanines 5 and 11 were assigned to (E1/23c) −1.23 and −1.22 eV, respectively, which also correlated with the computed LUMO values. Early onset reduction peaks revealed that isofluorindinium 7 (E1/21c −0.14 V, reversible) and the tetraquinoidal 13,13′dimer 12 (E1/21c −0.73, quasi-reversible) were good electron acceptors. In particular, the CV of the quinoidal dimer 12 showed C

DOI: 10.1021/acs.orglett.7b03998 Org. Lett. XXXX, XXX, XXX−XXX

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four quasi-reversible reductions between −0.73 and −1.39 V: a comparison of dimer 12 with C60 appears in the SI. In conclusion, oxidation of isodiphenylfluorindine 5 gives products 7, 11, and 12 that are highly colored and thermally stable with attractive redox profiles. Compounds 11 and 12 are rare examples of azaacene cruciforms,19 a structural motif that is of interest in organic electronics.20 Additional studies to understand the potential applications of these azaacenes are underway.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03998. Experimental, computational, and spectroscopic data (PDF) Accession Codes

CCDC 1471911−1471912 and 1489508−1489509 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Figure 5. Cyclic voltammograms of fluorindines 7 (green line), 11 (red line), and 12 (blue line) in DCM (1.0 mM), n-Bu4NPF6 (0.1 M) as electrolyte. Glassy carbon disk (⌀ 3 mm) and Pt wire were used as working and counter electrodes, respectively, and Ag/AgCl (1.0 M KCl) as reference electrode. Scan rate 100 mV·s−1, temp 20 °C, internal reference: Fc/Fc+ (0.475 V).

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Panayiotis A. Koutentis: 0000-0002-4652-7567 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Cyprus and the Cyprus Research Promotion Foundation. DEDICATION Dedicated to Professor Fred Wudl. D

DOI: 10.1021/acs.orglett.7b03998 Org. Lett. XXXX, XXX, XXX−XXX