Iron-Catalyzed Synthesis, Structure, and Photophysical Properties of Tetraarylnaphthidines.

We describe the synthesis and photophysical properties of tetraarylnaphthidines. Our synthetic approach is based on an iron-catalyzed oxidative C–C coupling reaction as the key step using a hexadecafluorinated iron–phthalocyanine complex as a catalyst and air as the sole oxidant. The N,N,N’,N’-tetraarylnaphthidines proved to be highly fluorescent with quantum yields of up to 68%.

Herein, we report a simple synthesis of N,N,N',N'-tetraarylnaphthidines by an iron-catalyzed oxidative coupling of the corresponding 1-(diphenylamino)naphthalene precursor using iron(II)-hexadecafluorophthalocyanine (FePcF 16 ) [18] as the catalyst and air as the final oxidant ( Figure 1). Moreover, we have investigated the photophysical properties of the obtained naphthidine derivatives.
Herein, we report a simple synthesis of N,N,N',N'-tetraarylnaphthidines by an iron-catalyzed oxidative coupling of the corresponding 1-(diphenylamino)naphthalene precursor using iron(II)hexadecafluorophthalocyanine (FePcF16) [18] as the catalyst and air as the final oxidant ( Figure 1). Moreover, we have investigated the photophysical properties of the obtained naphthidine derivatives.
Then, we turned our attention to the projected iron-catalyzed oxidative homocoupling of 1-(diphenylamino)naphthalene (3). Recently, we applied our iron-catalyzed oxidative coupling methodology to the C-C homocoupling of diarylamines, 1-, and 2-hydroxycarbazoles, as well as to the cross coupling of tertiary anilines with hydroxyarenes [20][21][22]. Iron as a first-row transition metal is environmentally safe and has become a powerful tool in synthetic organic chemistry with many applications for selective C-H bond activation [23][24][25][26]. According to a previous report by Yang, N,N,N',N'-tetraphenylnaphthidine (4) could not be prepared by oxidative coupling of compound 3 with stoichiometric amounts of iron(III) chloride [14]. The oxidation of compound 3 with chloranil provides compound 4 only as the minor isomer in an inseparable mixture with the corresponding benzidine derivative [15]. Based on our previous studies [20][21][22], we envisaged to develop an ironcatalyzed homocoupling of the triarylamine 3 with air as the sole oxidant. Using catalytic amounts (2 mol%) of FePcF16 and substoichiometric amounts (40 mol%) of methanesulfonic acid as an additive at room temperature provided N,N,N',N'-tetraphenylnaphthidine (4) in 48% yield along with the N,N,N',N'-tetraarylnaphthidine 5 as a by-product in 11% yield (Scheme 2). Obviously, compound 5 results from a further oxidative C-C coupling at the p-position of one of the phenyl rings of the initial coupling product compound 4. The structural assignments for compounds 4 and 5 were based on their 1 H-NMR and 13 C-NMR spectroscopic data and an X-ray analysis of compound 4 (see Section 2.2). Scheme 2. Iron-catalyzed oxidative C-C coupling of 1-(diphenylamino)naphthalene (3 Then, we turned our attention to the projected iron-catalyzed oxidative homocoupling of 1-(diphenylamino)naphthalene (3). Recently, we applied our iron-catalyzed oxidative coupling methodology to the C-C homocoupling of diarylamines, 1-, and 2-hydroxycarbazoles, as well as to the cross coupling of tertiary anilines with hydroxyarenes [20][21][22]. Iron as a first-row transition metal is environmentally safe and has become a powerful tool in synthetic organic chemistry with many applications for selective C-H bond activation [23][24][25][26]. According to a previous report by Yang, N,N,N',N'-tetraphenylnaphthidine (4) could not be prepared by oxidative coupling of compound 3 with stoichiometric amounts of iron(III) chloride [14]. The oxidation of compound 3 with chloranil provides compound 4 only as the minor isomer in an inseparable mixture with the corresponding benzidine derivative [15]. Based on our previous studies [20][21][22], we envisaged to develop an iron-catalyzed homocoupling of the triarylamine 3 with air as the sole oxidant. Using catalytic amounts (2 mol%) of FePcF 16 and substoichiometric amounts (40 mol%) of methanesulfonic acid as an additive at room temperature provided N,N,N',N'-tetraphenylnaphthidine (4) in 48% yield along with the N,N,N',N'-tetraarylnaphthidine 5 as a by-product in 11% yield (Scheme 2). Obviously, compound 5 results from a further oxidative C-C coupling at the p-position of one of the phenyl rings of the initial coupling product compound 4. The structural assignments for compounds 4 and 5 were based on their 1 H-NMR and 13 C-NMR spectroscopic data and an X-ray analysis of compound 4 (see Section 2.2). Then, we turned our attention to the projected iron-catalyzed oxidative homocoupling of 1-(diphenylamino)naphthalene (3). Recently, we applied our iron-catalyzed oxidative coupling methodology to the C-C homocoupling of diarylamines, 1-, and 2-hydroxycarbazoles, as well as to the cross coupling of tertiary anilines with hydroxyarenes [20][21][22]. Iron as a first-row transition metal is environmentally safe and has become a powerful tool in synthetic organic chemistry with many applications for selective C-H bond activation [23][24][25][26]. According to a previous report by Yang, N,N,N',N'-tetraphenylnaphthidine (4) could not be prepared by oxidative coupling of compound 3 with stoichiometric amounts of iron(III) chloride [14]. The oxidation of compound 3 with chloranil provides compound 4 only as the minor isomer in an inseparable mixture with the corresponding benzidine derivative [15]. Based on our previous studies [20][21][22], we envisaged to develop an ironcatalyzed homocoupling of the triarylamine 3 with air as the sole oxidant. Using catalytic amounts (2 mol%) of FePcF16 and substoichiometric amounts (40 mol%) of methanesulfonic acid as an additive at room temperature provided N,N,N',N'-tetraphenylnaphthidine (4) in 48% yield along with the N,N,N',N'-tetraarylnaphthidine 5 as a by-product in 11% yield (Scheme 2). Obviously, compound 5 results from a further oxidative C-C coupling at the p-position of one of the phenyl rings of the initial coupling product compound 4. The structural assignments for compounds 4 and 5 were based on their 1 H-NMR and 13 C-NMR spectroscopic data and an X-ray analysis of compound 4 (see Section 2.2).

Structure
The 1 H-NMR, 13 C-NMR, and DEPT spectra of compound 4 displayed signals for a highly symmetrical compound. Signals for nine aromatic CH and five quaternary aromatic carbon atoms were identified. The COSY experiment revealed the presence of three spin systems (Supplementary Materials, COSY of compound 4). The first is caused by coupling of H-1 with H-2 and H-3, which belong to the phenyl fragment. The second spin system consists of H-6 and H-7. The third system is formed by H-10, H-11, H-12, and H-13. The assignment of all 13 C-NMR signals to the respective 1 H-NMR signals could be achieved by an HSQC measurement ( Figure 2 and Table S1). The constitution of naphthidine 4 was confirmed by analysis of the HMBC spectrum (Supplementary Materials, HMBC of compound 4, Table S1). Characteristic HMBC signals (C-4/H-2 and C-4/H-3) led to elucidation of the quaternary carbon atom C-4 by connecting the proton spin systems. The position of C-5 was established based on the HMBC cross-peaks with H-6, H-7, and H-13. Accordingly, the quaternary aromatic carbon atom C-8 was assigned based on the HMBC interactions with H-6, H-7, and H-10.

Structure
The 1 H-NMR, 13 C-NMR, and DEPT spectra of compound 4 displayed signals for a highly symmetrical compound. Signals for nine aromatic CH and five quaternary aromatic carbon atoms were identified. The COSY experiment revealed the presence of three spin systems (Supplementary Material, COSY of compound 4). The first is caused by coupling of H-1 with H-2 and H-3, which belong to the phenyl fragment. The second spin system consists of H-6 and H-7. The third system is formed by H-10, H-11, H-12, and H-13. The assignment of all 13 C-NMR signals to the respective 1 H-NMR signals could be achieved by an HSQC measurement ( Figure 2 and Table S1). The constitution of naphthidine 4 was confirmed by analysis of the HMBC spectrum (Supplementary Material, HMBC of compound 4, Table S1). Characteristic HMBC signals (C-4/H-2 and C-4/H-3) led to elucidation of the quaternary carbon atom C-4 by connecting the proton spin systems. The position of C-5 was established based on the HMBC cross-peaks with H-6, H-7, and H-13. Accordingly, the quaternary aromatic carbon atom C-8 was assigned based on the HMBC interactions with H-6, H-7, and H-10.  The structural assignment for N,N,N',N'-tetraphenylnaphthidine (4) was confirmed by an X-ray crystal structure determination ( Figure 3). The molecule of compound 4 adopts a trans conformation around the central biaryl bond (C1-C11) with a torsional angle of 99.4(2) • (Figure 4) according to the notation accepted for binaphthyl derivatives [27,28]. The geometry of the naphtho fragments exhibiting large variations of the bond lengths is in agreement with the crystal structure of naphthalene [29,30] and its structure obtained by quantum chemical calculations [31]. The orientation of the phenyl groups attached to the nitrogen atoms at the two sides of the molecule is different. The two phenyl rings at N21 are symmetrically oriented respective to the bisector plane drawn through the C4-N21 bond, while the two phenyl groups at N34 have different orientations. In the crystal, the molecules of compound 4 participate in multiple weak intermolecular C-H···π interactions between the naphtho groups ( Figure 5). The distances between the hydrogen atoms and the sp 2 -carbon atoms range from 2.7 to 2.8 Å and the angles are typical for these weak hydrogen bonds [32]. This sort of packing with face-to-edge interactions has been observed also in naphthalene itself [29,30] and in binaphthyl derivatives [27,28]. It is noteworthy that the naphtho groups interact mostly with each other and the phenyl groups interact predominantly with phenyl groups, whereas naphtho-phenyl contacts are rare.
Molecules 2020, 25, x FOR PEER REVIEW 4 of 13 The structural assignment for N,N,N',N'-tetraphenylnaphthidine (4) was confirmed by an X-ray crystal structure determination ( Figure 3). The molecule of compound 4 adopts a trans conformation around the central biaryl bond (C1-C11) with a torsional angle of 99.4(2)° ( Figure 4) according to the notation accepted for binaphthyl derivatives [27,28]. The geometry of the naphtho fragments exhibiting large variations of the bond lengths is in agreement with the crystal structure of naphthalene [29,30] and its structure obtained by quantum chemical calculations [31]. The orientation of the phenyl groups attached to the nitrogen atoms at the two sides of the molecule is different. The two phenyl rings at N21 are symmetrically oriented respective to the bisector plane drawn through the C4-N21 bond, while the two phenyl groups at N34 have different orientations. In the crystal, the molecules of compound 4 participate in multiple weak intermolecular C-H···π interactions between the naphtho groups ( Figure 5). The distances between the hydrogen atoms and the sp 2 -carbon atoms range from 2.7 to 2.8 Å and the angles are typical for these weak hydrogen bonds [32]. This sort of packing with face-to-edge interactions has been observed also in naphthalene itself [29,30] and in binaphthyl derivatives [27,28]. It is noteworthy that the naphtho groups interact mostly with each other and the phenyl groups interact predominantly with phenyl groups, whereas naphtho-phenyl contacts are rare.

Photophysical Studies
On irradiation with UV-light (λ ex = 254 nm) in methanolic solution, 1-(diphenylamino)naphthalene (3) and the resulting N,N,N',N'-tetraarylnaphthidines 4 and 5 show a strong blue fluorescence (Figure 7). Therefore, we investigated the photophysical properties of compounds 3, 4, and 5 in more detail. The UV absorption and fluorescence emission data for all three compounds are listed in Table 1 and the normalized fluorescence emission spectra are displayed in Figure 8. The fluorophores 3, 4, and 5 exhibit large Stokes shifts of 149 to 162 nm. While the UV absorption maxima are identical, the fluorescence emission maxima show a bathochromic shift of only 7 nm for N,N,N',N'-tetraphenylnaphthidine (4) and 14 nm for compound 5, as compared to 1-(diphenylamino)naphthalene (3). Previously, even larger Stokes shifts have been observed for N,N-dimethylaminonaphthalenes [33] and strong bathochromic shifts have been reported for substituted triarylamines [34].  N,N,N',N'-tetraarylnaphthidines 4 and 5 show a strong blue fluorescence (Figure 7). Therefore, we investigated the photophysical properties of compounds 3, 4, and 5 in more detail. The UV absorption and fluorescence emission data for all three compounds are listed in Table 1 and the normalized fluorescence emission spectra are displayed in Figure 8. The fluorophores 3, 4, and 5 exhibit large Stokes shifts of 149 to 162 nm. While the UV absorption maxima are identical, the fluorescence emission maxima show a bathochromic shift of only 7 nm for N,N,N',N'-tetraphenylnaphthidine (4) and 14 nm for compound 5, as compared to 1-(diphenylamino)naphthalene (3). Previously, even larger Stokes shifts have been observed for N,Ndimethylaminonaphthalenes [33] and strong bathochromic shifts have been reported for substituted triarylamines [34].     On irradiation with UV-light (λex = 254 nm) in methanolic solution, 1-(diphenylamino)naphthalene (3) and the resulting N,N,N',N'-tetraarylnaphthidines 4 and 5 show a strong blue fluorescence (Figure 7). Therefore, we investigated the photophysical properties of compounds 3, 4, and 5 in more detail. The UV absorption and fluorescence emission data for all three compounds are listed in Table 1 N,N,N',N'-tetraphenylnaphthidine (4) and 14 nm for compound 5, as compared to 1-(diphenylamino)naphthalene (3). Previously, even larger Stokes shifts have been observed for N,Ndimethylaminonaphthalenes [33] and strong bathochromic shifts have been reported for substituted triarylamines [34].    For the naphthidines 4 and 5, the UV absorption and fluorescence emission maxima along with fluorescence quantum yields, fluorescence lifetimes, and CIE-coordinates were determined (Table 2). The UV absorption maximum for the naphthidines 4 and 5 is at the same wavelength (294 nm), whereas the fluorescence emission maximum of compound 5 shows a bathochromic shift of 4 nm as compared with compound 4. The fluorescence quantum yields of compounds 4 and 5 are relatively high and in the same range (68% and 65%, respectively, by excitation at 310 nm). Compound 5 exhibits a shorter fluorescence lifetime of 3.6 ns as compared with 4.2 ns for N,N,N',N'-tetraphenylnaphthidine (4). The CIE-coordinates for the blue light emissions of compounds 4 and 5 are indicative of a blue light which could be useful for applications in OLEDs [35][36][37][38][39]. The fluorescence quantum yields are very comparable with compounds already investigated as blue fluorescent OLEDs [37][38][39]. It has been demonstrated for other naphthalene fluorophores that a bathochromic shift of the emission can be achieved by increasing the size of the π-system and by the introduction of appropriate substituents [40,41]. Thus, application of these tools should easily allow an optimization of the present fluorophoric system.
In addition, we investigated the absorption and fluorescence properties of N,N,N',N'-tetraphenylnaphthidine (4) in various nonpolar and polar solvents. The fluorescence behavior of compound 4 on excitation at 254 nm was demonstrated in isohexane, dichloromethane, ethyl acetate, THF, and methanol ( Figure 9). The corresponding UV absorption and fluorescence emission spectra are shown in the Supplementary Materials. The corresponding photophysical data of compound 4 are summarized in Table 3. For a series of N,N-dimethylaminonaphthalene fluorophores, Brummond et al. observed a significant red shift of the fluorescence emission maxima by increasing the solvent polarity [33], whereas in other systems the solvent dependency of the emission was significantly lower [37]. For N,N,N',N'-tetraphenylnaphthidine (4), this solvatochromic shift is much less pronounced (about 21 nm in dichloromethane as compared with the corresponding value in isohexane) ( Table 3). For the naphthidines 4 and 5, the UV absorption and fluorescence emission maxima along with fluorescence quantum yields, fluorescence lifetimes, and CIE-coordinates were determined (Table 2).  The UV absorption maximum for the naphthidines 4 and 5 is at the same wavelength (294 nm), whereas the fluorescence emission maximum of compound 5 shows a bathochromic shift of 4 nm as compared with compound 4. The fluorescence quantum yields of compounds 4 and 5 are relatively high and in the same range (68% and 65%, respectively, by excitation at 310 nm). Compound 5 exhibits a shorter fluorescence lifetime of 3.6 ns as compared with 4.2 ns for N,N,N',N'tetraphenylnaphthidine (4). The CIE-coordinates for the blue light emissions of compounds 4 and 5 are indicative of a blue light which could be useful for applications in OLEDs [35][36][37][38][39]. The fluorescence quantum yields are very comparable with compounds already investigated as blue fluorescent OLEDs [37][38][39]. It has been demonstrated for other naphthalene fluorophores that a bathochromic shift of the emission can be achieved by increasing the size of the π-system and by the introduction of appropriate substituents [40,41]. Thus, application of these tools should easily allow an optimization of the present fluorophoric system.
In addition, we investigated the absorption and fluorescence properties of N,N,N',N'tetraphenylnaphthidine (4) in various nonpolar and polar solvents. The fluorescence behavior of compound 4 on excitation at 254 nm was demonstrated in isohexane, dichloromethane, ethyl acetate, THF, and methanol ( Figure 9). The corresponding UV absorption and fluorescence emission spectra are shown in the Supplementary Material. The corresponding photophysical data of compound 4 are summarized in Table 3. For a series of N,N-dimethylaminonaphthalene fluorophores, Brummond et al. observed a significant red shift of the fluorescence emission maxima by increasing the solvent polarity [33], whereas in other systems the solvent dependency of the emission was significantly lower [37]. For N,N,N',N'-tetraphenylnaphthidine (4), this solvatochromic shift is much less pronounced (about 21 nm in dichloromethane as compared with the corresponding value in isohexane) ( Table 3).

General
All reactions were performed in oven-dried glassware using anhydrous solvents under argon, unless stated otherwise. Pd(OAc) 2 was recrystallized from glacial AcOH. All other chemicals were used as received from commercial sources. The iron(III)-catalyzed reactions were carried out in non-dried solvents under air. Iron(II)-hexadecafluorophthalocyanine was prepared following a procedure reported previously [18]. Flash chromatography was performed using silica gel from Acros Organics (0.035 to 0.070 mm). Automated flash chromatography was performed on a Büchi Sepacore system equipped with an UV monitor on silica gel (Acros Organics, 0.035 to 0.070 mm). The TLC was performed with TLC plates from Merck (60 F254) using UV light for visualization. The melting points were measured on a Gallenkamp MPD 350 melting point apparatus. Ultraviolet spectra were recorded on a PerkinElmer 25 UV/Vis spectrometer. The fluorescence spectra were measured on a Varian Cary Eclipse spectrophotometer. The IR spectra were recorded on a Thermo Nicolet Avatar 360 FT-IR spectrometer using the ATR method (attenuated total reflectance). The NMR spectra were recorded on Bruker DRX 500 and Avance III 600 spectrometers. The chemical shifts δ were reported in ppm with the solvent signal as internal standard. Standard abbreviations were used to denote the multiplicities of the signals. EI mass spectra were recorded by GC/MS-coupling using an Agilent Technologies 6890 N GC System equipped with a 5973 Mass Selective Detector (70 eV). The ESI-MS spectra were recorded on an Esquire LC using an ion trap detector from Bruker. Positive and negative ions were detected. Elemental analyses were measured on a EuroVector EuroEA3000 elemental analyzer. The X-ray crystal structure analyses were performed with a Bruker-Nonius Kappa CCD and with a Bruker AXS Smart APEX diffractometer equipped with a 700 series Cryostream low temperature device from Oxford Cryosystems. SHELXS-97 [42], SADABS version 2.10 [43], SHELXL-97 [44], POV-Ray for Windows version 3.7.0.msvc10.win64, and ORTEP-3 for Windows [45] were used as software.

Iron-Catalyzed Oxidative C−C Coupling
Dichloromethane (9 mL) was added to a mixture of iron(II)-hexadecafluorophthalocyanine (15.7 mg, 18.3 µmol, 2 mol%), methanesulfonic acid (34.8 mg, 362 µmol) and 1-(diphenylamino)naphthalene (3) (265 mg, 897 µmol). A 100 mL splash head was attached on the flask to prevent evaporation of the solvent, while ensuring sufficient gas exchange. The resulting solution was stirred at room temperature for 24 h under air. Then, 10 mL of a saturated aqueous solution of sodium hydrogen carbonate was added. The aqueous layer was extracted three times with dichloromethane. The combined organic layers were dried (magnesium sulfate). The solvent was evaporated and the residue was purified by automated flash chromatography (silica gel, isohexane/dichloromethane, 20% to 50% in 1.5 h) to provide N,N,N ,N -tetraphenylnaphthidine (4) (128 mg, 217 µmol, 48%) as a colorless solid (less polar fraction) and naphthidine 5 (29.5 mg, 33.4 µmol, 11%) as a colorless solid (more polar fraction). Crystallization of compound 4 from isohexane afforded colorless crystals which were suitable for X-ray analysis.

Conclusions
In conclusion, we have developed a two-step synthesis of N,N,N',N'-tetraphenylnaphthidine (4). Starting from diphenylamine (1), Buchwald-Hartwig coupling with 1-bromonaphthalene (2) and subsequent iron-catalyzed oxidative homocoupling of the resulting 1-(diphenylamino)naphthalene (3) provides compound 4 as the major product and as a minor product compound 5, resulting from an additional oxidative C-C coupling. Thus, we could demonstrate that our method of iron-catalyzed oxidative C-C coupling with air as the final oxidant can be applied to the regioselective homocoupling of triarylamines. Compounds 4 and 5 exhibit a strong blue-light fluorescence with quantum yields of up to 68% and fluorescence lifetimes of 4.2 and 3.6 ns, for compounds 4 and 5, respectively. Further structural changes of the N,N,N',N'-tetraarylnaphthidines by extension of the π-system or modification of the substitution pattern could lead to improved photophysical properties and fluorophores for various potential applications.