oxadiazaborinines: Synthesis, Structural, and ... - ACS Publications

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Cite This: J. Org. Chem. 2018, 83, 12129−12142

Benzo[4,5]thiazolo[3,2‑c][1,3,5,2]oxadiazaborinines: Synthesis, Structural, and Photophysical Properties Mykhaylo A. Potopnyk,*,† Dmytro Volyniuk,‡ Magdalena Ceborska,# Piotr Cmoch,† Iryna Hladka,‡ Yan Danyliv,‡ and Juozas Vidas Gražulevicǐ us‡ †

Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, Kaunas LT-50254, Lithuania # Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland Downloaded via KAOHSIUNG MEDICAL UNIV on October 5, 2018 at 07:00:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A family of highly emissive benzo[4,5]thiazolo[3,2c][1,3,5,2]oxadiazaborinines, conjugated with the donor 4dimethylaminophenyl group, was designed and synthesized. Their photophysical, both in solution and in the solid state, and structural properties were investigated. The influence of donor and acceptor substituents (R) in the benzothiazole unit on photophysical properties of complexes was found out. The tetrafluorobenzothiazole analogue exhibits nonbonded nuclear spin−spin coupling between fluorines from the BF2 group and α-fluorine atom at the benzene ring. Additionally, this boron complex demonstrates a comparatively high solid-state fluorescence quantum yield (Φ = 0.34).

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INTRODUCTION Organoboron complexes belong to the one of the most important and widely used class of fluorophores.1 These dyes demonstrate practically useful electronic and optical properties (e.g., high fluorescence quantum yields, relatively long lifetime of the excited state, photostability, insensitivity to the environment, good solubility in organic solvents).2 As a result, they can serve many application areas, such as fluorescent probes,3,4 photosensitizer in photodynamic therapy,5 photoactive element in organic solar cells (OSC),6−9 supramolecular systems,10−12 organic light-emitting devices (OLEDs), cholesteric liquid crystal lasers,13 etc. The most representative organoboron complexes are borondipyrromethenes (BODIPYs), which are based on a dipyrrolo annulated 1,3,2-diazaborine scaffold.14,15 Their fluorescence in solutions is usually intensive. However, they demonstrate narrow Stokes shifts, which limits their optoelectronic applications.1 To tackle this scientific challenge, in the past few years, modification of the BODIPY core was the topic of high research interest.1 Therefore, organoboron compounds, based on nonpyrrole heterocyclic building blocks annulated to 1,3,2-diazaborinine (II),16−18 1,2-azaborinine (I),19 and 1,3,5,2-triazaborinine (III)20−24 rings, have been developed (Figure 1a). Another class of six-membered ring organoboron complexes has been synthesized from O,O-chelating ligands, forming 1,3,2-dioxaborinine derivatives (IV).25−28 One of the factors that leads to increasing the Stokes shift of the organoboron complexes is desymmetrization of the ligand structures.1 In this context, complexes with hybrid ligands, having simultaneously nitrogen- and oxygen-coordinating © 2018 American Chemical Society

centers, are looking very attractive. Organoboron dyes, based on the 1,3,2-oxazaborinine (V) scaffold, have been relatively widely investigated as complexes formed from β-ketoiminate29−36 and phenolic37−44 ligands. Nevertheless, the 1,3,5,2oxadiazaborinine (VI) derivatives, formed from amide ligands, have been reported a little; they are mostly based on nitrogencontaining six-membered heterocycles (pyridine,36,45−47 pyrazine,47,48 pyridazine,47 or naphthyridine35,49−52) or the 1,3,4thiadiazole53 unit as N-coordinating centers and demonstrate low to weak fluorescence quantum yields. These studies have shown that the oxadiazaborinine ring demonstrates electronacceptor properties. The conjugation of oxadiazaborinine moiety with strong donor group gives donor−acceptor (D− A) molecular systems, which demonstrate the intramolecular charge transfer (ICT) character.45,49 Our approach is based on the use of an electron-rich heterocyclic unit as a building block in the construction of oxadiazaborinine dyes. This should decrease the acceptor character of the oxadiazaborinine core. In previous work, we have demonstrated that the presence of the thiazole ring, annulated to oxadiazaborinine core, promotes obtaining complexes with high fluorescence quantum yields (Figure 1b).54 Continuing this research, we have decided to investigate the influence of annulation of an aromatic unit to the 1,3-thiazole ring on the photophysical properties of such type of complexes. To realize this goal, we have designed N,O-coordinated BF2 Received: August 13, 2018 Published: September 2, 2018 12129

DOI: 10.1021/acs.joc.8b02098 J. Org. Chem. 2018, 83, 12129−12142

Article

The Journal of Organic Chemistry

Figure 1. (a) Introduction to six-membered ring organoboron complexes, (b) previous work, and (c) concept of benzo[d]thiazole-based BF2 complexes.

complexes using benzo[d]thiazole as a heterocyclic Ncoordinating center. Retrosynthetic analysis leads to simple building blocks: (para-dimethylamino)benzoyl chloride, 2aminobenzo[d]thiazole, and boron trifluoride (Figure 1c). Herein we report our recent progress on this research topic.

substituents, including donor (OMe), weak acceptor (F, Cl), and strong acceptor (CF3) groups. For comparison of photophysical properties of the boron dyes 6a−e with corresponding parameters of an analogue, having a stronger acceptor strength in the benzothiazole unit, we designed a complex based on a perfluorinated benzothiazole synthon. To achieve this goal, we developed the synthetic strategy shown in Scheme 2. Pentafluoroaniline (7) was

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RESULTS AND DISCUSSION Synthesis and Characterization. Unsubstituted benzo[d]thiazol-2-amine (2a) is commercially available. To provide the best π-conjugation between the N-coordinating center of a ligand (following boron unit) and substituents at the benzo[d]thiazole unit in the final complexes, we decided to modify the 6-position of this heterocyclic part. To prepare 6substituted benzo[d]thiazol-2-amines (2b−e), we used a onestep synthesis method, starting from 4-R-anilines (3b−e). Using 2 equiv of ammonium thiocyanate and 1 equiv of bromine (Scheme 1) in a glacial acetic acid medium, we

Scheme 2. Synthesis of Complex 11

Scheme 1. Synthesis of Benzothiazole−Boron Complexes 6a−e

treated with benzoyl isothiocyanate (generated in situ from benzoyl chloride and potassium thiocyanate) to furnish N[(perfluorophenyl)carbamothioyl]benzamide (8a). For cleavage of the benzoyl group, this intermediate, without purification, was heated in 10% NaOH followed by acidification to generate 1-(perfluorophenyl)thiourea (8b) in 75% yield. Heating of thiourea 8b under strong basic conditions in dry DMF provided hydrofluoric acid elimination and benzothiazole cyclization, giving 4,5,6,7-tetrafluorobenzo[d]thiazol-2-amine (9) in 83% yield. Next, amine 9 was transformed into amide 10 in 45% yield via reaction with chloride 4 under standard acylating conditions. The synthesis of final complex 11 in 68% yield was successfully accomplished using BF3·Et2O/DIPEA medium. All of the synthesized compounds were characterized by 1H, 13 C, 19F NMR, and IR spectroscopy and high-resolution mass spectrometry. Due to the perfluorinated character of thiourea 8b and benzothiazole 9, its 13C NMR spectra were recorded as proton-coupled/fluorine-decoupled. To determine all carbon signals in the case of amide 10 and complex 11, two different types (fluorine-coupled/proton-decoupled and proton-

obtained products 2b−e in good yields (71−91%). Next, acylation of 6-R-benzo[d]thiazol-2-amines (2a−e) with 4(dimethylamino)benzoyl chloride (4) in basic conditions gave N,O ligands 5a−e in fair to good yields (57−66%). Finally, amides 5a−e were treated with boron trifluoride diethyl etherate in the presence of diisopropylethylamine (DIPEA) to give complexes 6a−e in 61−68% yields (Scheme 1). Synthesized benzothiazole−BF2 complexes are modified at the 6-position of the benzo[d]thiazole unit, having different 12130


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Figure 2. 19F−19F NMR COSY (top) and NOESY (down) spectra of complex 11.

coupled/fluorine-decoupled) of the 13C NMR spectra were measured. In the 19F NMR spectra of dyes 6a−e, fluorines linked to the boron atom produced multiplets at a narrow range from −137.71 to −137.44 ppm, while the corresponding signal of the BF2 group in the spectrum of compound 11 was located at −133.08 ppm. On the other hand, in the one-dimensional 19F NMR spectrum of complex 11, fluorine nuclei at the benzene ring (F3, F4, F5, and F6) produced four distinctive signals, three doublets of doublets (dd), and one triplet of doublet of doublets (tdd). For better interpretation of this situation, 19 F− 19 F NMR COSY and NOESY experiments were performed. All adjacent fluorine atoms in the benzo unit expectedly correlate to each other (correlations F3↔F4, F4↔ F5, and F5↔F6) with a spin−spin coupling constant (J)

around 20 Hz, which is clearly confirmed by both COSY and NOESY experiments (Figure 2). Additionally, the atoms F3 and F6 demonstrate para-correlation through five bonds with J = 13.1 Hz; corresponding cross-peaks are observed in the COSY spectrum and missing in the NOESY plot. This type of through-bond spin−spin coupling is also observed in the 19F NMR spectra of tetrafluorobenzo[d]thiazole derivatives 9 and 10 (see Experimental Section). However, in contrast to the corresponding doublet of doublets for the F3 atom in the 19F NMR spectra of compounds 9 and 10, this nucleus (F3) in complex 11 produces a tdd peak, where triplet splitting is characterized by J = 30.3 Hz and corresponding F3↔F1/F2 interaction. It should be noted that F3↔F1/F2 cross-peaks are observed in the NOESY spectrum but are missing in the COSY plot. On the basis of these results, we can conclude that F3↔ F1/F2 spin−spin coupling in complex 11 is dominantly or fully 12131


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Figure 3. X-ray structures of complexes 6a−e and 11 (top and front views).

Figure 4. Molecular packing of compound 6a: (a) top view; (b) front view; (c) side view; (d) list of intermolecular interactions. Blue dotted lines show hydrogen bonds. Gray dotted lines show S−π interactions. Red dotted lines show π−π and n−π interactions.

caused by a “through-space” mechanism. This phenomenon, named nonbonded nuclear spin−spin coupling, was previously investigated in other aromatic molecules and coordination complexes.55 X-ray Analysis. The structures of all investigated complexes were further confirmed by single-crystal X-ray analysis (Figure 3, Figures S1−S6 and Tables S1−S6 in the Supporting Information). In the solid state, compounds 6a−6e and 11 have a near-to-planar geometry. The boron atom is almost tetrahedrally coordinated by two fluorine, nitrogen, and oxygen atoms: the N−B−F and O−B−F bond angles are in

the range 108.1−111.6°, the N−B−O bond angles are a smaller (105.5−107.6°), and the F−B−F bond angles in all complexes range from 109.8 to 112.8°. The distances between fluorines from the BF2 group and α-fluorine atom (F3) of the benzothiazole moiety of complex 11 are 2.82 and 2.87 Å (Figure 3), which, in both cases, is less than the sum of the van der Waals radii for two fluorines (2.94 Å). Compound 6a crystallizes in a P21/c monoclinic space group with four 6a molecules in the unit cell (Table S1 in the Supporting Information). The molecules are connected by π−π/n−π interactions (a = 3.57 Å), forming shifted pairs with 12132


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Figure 5. Molecular packing of dye 6b: (a) top view; (b) front view; (c) side view; (d) list of intermolecular interactions. Blue dotted lines show hydrogen bonds. Red dotted lines show π−π and n−π interactions.

Figure 6. Molecular packing of complex 6c: (a) top view; (b) front view; (c) side view; (d) list of intermolecular interactions. Blue dotted lines show hydrogen bonds. Red dotted lines show π−π and n−π interactions.

Figure 7. Molecular packing of compound 6d: (a) top view; (b) side view; (c) front view; (d) list of intermolecular interactions. Blue dotted lines show hydrogen bonds. Red dotted lines show π−π and n−π interactions.

4). The neighboring chains are oriented in the same direction, but their molecular pairs are ordered perpendicularly to each other. The connection between different chains is provided by two hydrogen bonds and perpendicular S−π interactions.

an antiparallel orientation (Figure 4). These molecular pairs are locked to each other by a secondary π−π/n−π interaction (b = 3.37 Å), building a rectilinear chain (two antiparallel molecular lines painted in green and magenta color in Figure 12133


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Figure 8. Molecular packing of dye 6e: (a) top view; (b) front view; (c) side view; (d) list of intermolecular interactions. Blue dotted lines show interactions of the BF2 group (hydrogen bonds and S−F interactions). Gray dotted lines show the interaction of the CF3 group (F−F and F−π interactions). Red dotted lines show π−π and n−π interactions.

Figure 9. Molecular packing of complex 11: (a) top view; (b) front view; (c) side view; (d) list of intermolecular interactions. Blue dotted lines show hydrogen bonds and F−F interactions. Red dotted lines show π−π and n−π interactions.

to-tail” orientation with π−π and n−π interactions (a = 3.55 Å). These dimers are shifted to each other in the front view, building stairs by additional π−π and n−π interactions (b = 3.65 Å). All fluorine atoms (from BF2 and CF3 groups) of compound 6e are involved in intermolecular bonding. The BF2 group takes part in hydrogen bonds and S−F interactions. Two fluorines from the CF3 group make paired F−F interactions with the CF3 group of another molecule, while the third fluorine connects with π-electrons of the 1,3,2-oxadiazaborinine ring. Specificity of molecular packing of complex 11 (Figure 9) owns in a large part due to the presence of four fluorine atoms in the benzothiazole unit. Antiparallel oriented molecules form stacking columns by connection via π−π/n−π coupling. The interactions between these columns are provided by numerous hydrogen bonds and F−F interactions. Photophysical Properties of Solutions. The normalized absorption and emission spectra of the dilute solutions of benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines 6a−e and 11 are shown in Figure 10. The spectroscopic characteristics of dye solutions in toluene are summarized in Table 1. (The corresponding data for THF, DCM, acetone, and acetonitrile solutions are given in Table S7 in the Supporting Information.) UV/vis spectra show broad absorption bands with maxima well above 400 nm. Most of the synthesized complexes demonstrate one characteristic absorption peak, corresponding to the S0 → S1

Complex 6b demonstrates a well-ordered molecular packing (Figure 5), characterized by an orthorhombic lattice with eight molecules per unit cell and the space group Pbca (Table S2 in the Supporting Information). In the top view, the molecules a form dimer with π−π and n−π interactions (3.30 Å, distance between the planes of paired molecules). Additionally, there are numerous observed hydrogen bonds. Molecules of dye 6c are packed in a tetragonal unit cell according to the space group P41 (Table S3 in the Supporting Information). As shown in Figure 6, in the top and front views, molecules in 6c are organized in layers. Inside these layers, the molecules are connected by π−π/n−π interactions (a = 3.70 Å; b = 3.43 Å) and hydrogen bonds (C1H−F3), forming stairs; they are orientated parallel to each other and perpendicular to the plane of the layer. Neighboring layers are connected by numerous hydrogen bonds (C2H−F1′; C2H−F2′; C4H−O1′; C16H−F1′) and are perpendicularly twisted to each other, forming a net-type structure (side view). Complexes 6d,e and 11 crystallize in the P1 triclinic space group with two molecules per unit cell (Tables S4−S6 in the Supporting Information). Neighboring molecules in 6d are organized antiparallel to each other, forming stairs with strong π−π and n−π interactions (Figure 7). Nucleophilic heteroatoms (fluorine, chlorine, and sulfur) form several hydrogen bonds. Additionally, the S−F interaction was also observed. Crystal packing of complex 6e (Figure 8) is characterized by association of molecular dimers in the top view, with a “head12134


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Figure 10. Absorption (solid lines) and emission (dashed lines) spectra of the solutions of 6a−e and 11 in different solvents.

Table 1. Photophysical Data for the Solutions in Toluene of Complexes 6a−e and 11 compound

λabs, nm

ε, M−1 cm−1

λem, nm

Φ

Δυ, cm−1

τ, ns

6a 6b 6c 6d 6e 11

421 411/427 423 427 428 431

62 600 82 500/88 400 71 700 73 200 86 000 84 900

450 454 452 456 460 464

0.84 0.88 0.85 0.91 0.93 0.65

1531 1392 1516 1489 1625 1650

1.74 1.59 1.68 1.71 1.78 2.00

transition, with the maximum absorption (λabs) at 421−426 nm for 6a,c; 426−431 nm for 6b,d,e; and 431−434 nm for 11, and show almost no variation with the change of solvent polarity. The corresponding molar absorption coefficient (ε) values of the solutions in toluene are in the range 62600− 88400 M−1 cm−1 and decrease for the solutions in acetonitrile to 44500−61300 M−1 cm−1. The absorption spectrum of the solution of compound 6b in toluene has the second blueshifted absorption peak (λabs = 411 nm, ε = 82 500 M−1 cm−1), which is probably induced by vibrational transition. It disappeared in the spectra of the solutions in polar solvents due to the interaction between dye 6b and solvent molecules.

Emission spectra of the dilute solutions of complexes 6a−e and 11 typically have one similar band, a little deformed for the toluene solution of 6b. This observation is in good agreement with the corresponding absorption spectra. All of the synthesized organoboron dyes demonstrate solvatofluorochromism; their emission maxima (λem) are bathochromically shifted with increasing the solvent polarity from toluene to acetonitrile: 450 → 486 nm for 6a, 454 → 481 nm for 6b, 452 → 487 nm for 6c, 456 → 491 nm for 6d, 460 → 500 nm for 6e, 464 → 503 nm for 11. The comparison of emission spectra of 6a−e and 11 clearly demonstrates the influence of the electronic structure of the 12135


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The Journal of Organic Chemistry benzothiazole residue on the photophysical properties. The more strongly electron-accepting substituents attached to benzothiazole moiety, the more red-shifted the emission band: λem (6a) < λem (6c) < λem (6d) < λem (6e) < λem (11) for the solutions in all used solvents. Dye 6b with donor (OMe) substituent demonstrates a slightly different behavior. The wavelengths of emission maxima of compounds 6a and 6b change with the solvent polarity: in toluene λem (6a) < λem (6b), in DCM λem (6a) = λem (6b), but in THF, acetone, and acetonitrile λem (6a) > λem (6b). A result of solvatofluorochromism is the increase of the Stokes shift (Δυ) of the investigated dyes with the solvent polarity increasing. This observation indicates that fluorescence of compounds 6a−e and 11 partially or fully originates from the photoinduced ICT state. A comparison of these results with the previously reported for unfused thiazole derivative 1 (λabs = 402−407 nm, λem = 439−469 nm, ε = 49 700−56 600 M−1 cm−1 in different solvents)54 shows that annulation of the phenyl ring (complex 6a) results in a bathochromic shift of the absorption and emission bands and increase of molar absorption coefficient (λabs = 421−425 nm, λem = 450−486 nm, ε = 53 800−62 600 M−1 cm−1). The solutions of compounds 6a−e in nonpolar solvents showed a high fluorescence quantum yield (Φ = 0.84−0.93 in toluene, Table 1), while the solution of dye 11 exhibited a little less efficient emission (Φ = 0.65 in toluene). The fluorescence quantum yields of all of the synthesized boron complexes in solutions decreased with the increasing solvent polarity (Table S7 in the Supporting Information). The solutions in acetonitrile of the dyes with strong electron-withdrawing substituents at the benzo[d]thiazole unit exhibited very low emission quantum yields (Φ = 0.05 for 6e and Φ = 0.02 for 11). The solvent polarity influence was reduced with the decrease of electron-withdrawing strength of the benzothiazole part (in acetonitrile Φ = 0.12, 0.13, and 0.17 for 6d, 6c, and 6a, respectively) and was considerably smaller in the case of complex with the electron-donating group at the benzothiazole unit (dye 6b Φ = 0.83, 0.80, 0.56, and 0.38 in DCM, THF, acetone, and acetonitrile, respectively). This observation clearly indicates that the ICT character is less expressed in the case of compound 6b and increases with the incorporation of electron-accepting substituents into the benzothiazole moiety (6a < 6c < 6d < 6e < 11). The lifetimes of excited state (τ) were measured for the toluene solutions of the investigated dyes (Table 1). The values of lifetime of complexes 6a,c−e were observed in a narrow range from 1.68 to 1.78 ns, while it was higher for complex 11 (2.00 ns) and lower for complex 6b (1.59 ns). Electrochemical Properties. The redox behavior of six synthesized boron complexes was investigated by cyclic voltammetry. The reduction and oxidation scans were recorded at room temperature for deoxygenated dichloromethane solutions using tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. The ionization potentials (IPs) and the electron affinities (EAs) of compounds 6a−e and 11 are given in Table 2. The corresponding cyclic voltammograms (Figures S7−S12) and the obtained onset reduction and onset oxidation potentials (Table S8) are summarized in the Supporting Information. The IP values of complexes 6a−e and 11 are observed in a narrow range from 5.12 to 5.18 eV. The EA values are more dependent on acceptor substituents at the benzo[d]thiazole

Table 2. Electron Affinities and Ionization Potentials of Compounds 6a−e and 11 in DCM compound

IP, eV

EA, eV

Eg, eV

6a 6b 6c 6d 6e 11

5.12 5.12 5.13 5.13 5.16 5.18

2.40 2.41 2.45 2.48 2.57 2.75

2.72 2.71 2.68 2.65 2.59 2.43

unit. Thus, the incorporation of the donor (methoxy) group has no influence on the value of electron affinity (2.40 eV for 6a and 2.41 eV for 6b), while the acceptor substituents definitely promote an increasing EA, according to the electronwithdrawing strength of the substituent: from 2.45 eV for 6c to 2.75 eV for 11. In comparison to previous results for complex 1 (IP = 5.07 eV, EA = 2.29 eV),54 the fusion of benzene ring to thiazolo[3,2-c][1,3,5,2]oxadiazaborinine moiety enhances both the reduction and oxidation potentials. Quantum Chemical Calculations. For a better understanding of the electronic and optical properties of the synthesized organoboron complexes, we carried out molecular orbital calculations for these compounds on the density functional theory (DFT) level using the Gaussian 09 software package.56 For calculations of singlet transitions, timedepended DFT (TD-DFT) computation was also performed. All geometrical structures were optimized, and the molecular energy levels were calculated at the B3LYP functional and 631G* basic set with the inclusion of dichloromethane solvent effect through the integral-equation-formalism polarizable continuum model (IEFPCM). The calculated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) level energies (Figure 11) are in good agreement with the electrochemically determined ionization potentials and the electron affinities (Table 2). The ICT character of the D−A fluorophores depends on the spatial overlap between HOMO and LUMO. The computation results show that the HOMOs of all synthesized boron complexes are somewhat more localized on the (N,Ndimethylamino)phenyl unit, which is obvious due to the strong electron-donating effect of this group, whereas the LUMOs are delocalized along the entire backbone (Figure 11). The HOMO density on the benzothiazole moiety decreases depending on the electron-withdrawing strength of a linked substitute: 6b > 6a > 6c > 6d > 6e > 11. The LUMO density on this part of the molecule clearly increases in the case of compounds with electron-acceptor substituents. On the basis of this observation, we can conclude that the donor substituent (OMe) at the benzothiazole unit causes the larger spatial overlap between HOMO and LUMO, while acceptors contribute to the decrease of this overlapping. This is the reason for lower ICT properties of complex 6b, which is consistent with its much higher fluorescence quantum yields in the polar solvents in comparison with corresponding values for the rest of the complexes. TD-DFT calculation showed that the excitation of all of the investigated benzothiazole boron complexes are preferable going to the first singlet excited state (S0 → S1, oscillator strength values are higher than 1.00), and theoretical absorption maxima (Table S9 in the Supporting Information) 12136


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Figure 11. Frontier molecular orbitals (HOMO and LUMO) of complexes 6a−e and 11.

apparently be explained by a strong aggregation quenching effect caused by molecular packing of complexes 6a−e (Figures 4−8). The exception is compound 11, which exhibited solid-state emission with an enhanced fluorescence quantum yield (Φ = 0.34) and elongated excited-state lifetimes (τ1 = 1.77 ns; τ2 = 7.56 ns). The relatively higher quantum yield of the thin film of complex 11 should be caused by specific intermolecular interaction in the molecular packing (Figure 9). In comparison to other studied benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines, complex 11 in the solid-state forms stacking columns, where the molecules are antiparallel oriented with a strong donor−acceptor π−π/n−π interaction. Additionally, all fluorine atoms from the tetrafluorobenzo[d]thiazole moiety take part in hydrogen bonding, which provide to decreasing of their acceptor strength. It causes the prolonged excited-state lifetime, which in turn induces the higher fluorescence quantum yield, compared to complexes 6a−e.

are in very good agreement with the corresponding experimentally obtained ones (Table S7). Fluorescence Properties of Solid Samples. Fluorescence properties of the synthesized benzo[4,5]thiazolo[3,2c][1,3,5,2]oxadiazaborinines were also investigated in the solid state using their thin films. The normalized emission spectra are shown in Figure 12. (The corresponding data are given in

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CONCLUSIONS In conclusion, we developed the short-step synthesis of the family of benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinines conjugated with the donor 4-dimethylaminophenyl group, starting from commercially available reagents. The structural and photophysical properties of the obtained organoboron complexes have been investigated. All of the synthesized oxadiazaborinines exhibited high fluorescence quantum yields in nonpolar solvents, which decreased in the polar medium due to the ICT effect. The incorporation of a donor substituent into the benzothiazole moiety caused the ICT reducing and the emission increasing of the corresponding complex in polar solvents. The steric interaction between two fluorines from the BF2 group and α-fluorine atom of the benzothiazole moiety in

Figure 12. Overlaid normalized emission spectra of solid nondoped samples of 6a−e and 11.

Table S10 in the Supporting Information.) The solid samples of all of the studied dyes exhibited bathochromically shifted emission maxima, compared to the corresponding data in the solutions. Most of the synthesized complexes demonstrated small solid-state fluorescence quantum yields (Φ = 0.02−0.06) and relatively short lifetimes of excited states (τ1 = 0.66−1.04 ns; τ2 = 2.62−5.90 ns). These low solid-state emissions can 12137


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literature procedure.58 A mixture of aniline 3b−e (6−9 mmol, 1 equiv) and ammonium thiocyanate (12−18 mmol, 2 equiv) in glacial acetic acid (20 mL) was cooled to 10 °C and stirred. A solution of bromine (6−9 mmol, 1 equiv) in glacial acetic acid (3 mL) was added dropwise at such a rate to keep the temperature below 10 °C throughout the addition. Stirring was continued for an additional 12 h; then the reaction mixture was poured into cold water, and an aqueous solution of ammonium hydroxide (25%) was added to reach pH ∼ 9. The precipitate was filtered, washed with water, dried, and recrystallized from ethanol. 6-Methoxybenzo[d]thiazol-2-amine (2b). This compound was obtained in 75% yield (1.10 g) using general procedure A from panisidine (3b, 1.00 g, 8.12 mmol), ammonium thiocyanate (1.24 g, 16.24 mmol), and bromine (419 μL, 8.12 mmol). Mp: 162.6−163.7 °C (lit.59 158−159 °C), black powder. 1H NMR (400 MHz, DMSOd6): δ 7.28 (1H, d, J = 2.6 Hz, ArH), 7.23 (1H, d, J = 8.7 Hz, Ar H), 7.19 (2H, s, NH2), 6.80 (1H, dd, J = 8.7 Hz, J = 2.6 Hz, ArH), 3.73 (3H, s, OMe) ppm. 13C NMR (100 MHz, DMSO-d6): δ 164.7, 154.3, 146.8, 131.9, 118.0, 112.8, 105.5, 55.5 ppm. IR: 3386 (NH), 3087 (NH) cm−1. HRMS (ESI-TOF): calcd for C8H9N2OS [M + H]+, 181.0436; found, 181.0429. 6-Fluorobenzo[d]thiazol-2-amine (2c). This compound was obtained in 71% yield (1.08 g) using general procedure A from 4fluoroaniline (3c, 1.00 g, 9.00 mmol), ammonium thiocyanate (1.37 g, 18.00 mmol), and bromine (464 μL, 9.00 mmol). Mp: 182.3−183.0 °C (lit.59 183−184 °C), yellowish powder. 1H NMR (400 MHz, DMSO-d6): δ 7.56 (1H, dd, JH−F = 8.7 Hz, J = 2.6 Hz, ArH), 7.44 (2H, s, NH2), 7.30 (1H, dd, J = 8.8 Hz, JH−F = 4.9 Hz, ArH), 7.02 (1H, ddd, J = 8.8 Hz, JH−F = 9.0 Hz, J = 2.6 Hz, ArH) ppm. 13C NMR (100 MHz, DMSO-d6): δ 166.3 (d, JC−F = 1.7 Hz), 157.1 (d, JC−F = 236.2 Hz), 149.4 (d, JC−F = 1.3 Hz), 131.9 (d, JC−F = 11.1 Hz), 118.1 (d, JC−F = 8.8 Hz), 112.7 (d, JC−F = 23.6 Hz), 107.7 (d, JC−F = 27.1 Hz) ppm. 19F NMR (375 MHz, DMSO-d6): δ −122.41 ppm. IR: 3385 (NH), 3073 (NH) cm−1. HRMS (ESI-TOF): calcd for C7H6N2FS [M + H]+, 169.0236; found, 169.0237. 6-Chlorobenzo[d]thiazol-2-amine (2d). This compound was obtained in 91% yield (1.32 g) using general procedure A from 4chloroaniline (3d, 1.00 g, 7.84 mmol), ammonium thiocyanate (1.19 g, 15.68 mmol), and bromine (404 μL, 7.84 mmol). Mp: 196.8−198.0 °C (lit.59 200−201 °C), yellowish powder. 1H NMR (400 MHz, DMSO-d6): δ 7.76 (1H, s, ArH), 7.58 (2H, s, NH2), 7.30 (1H, d, J = 8.4 Hz, ArH), 7.21 (1H, d, J = 8.4 Hz, ArH) ppm. 13C NMR (100 MHz, DMSO-d6): δ 167.1, 151.7, 132.5, 125.5, 124.5, 120.5, 118.5 ppm. IR: 3456 (NH), 3091 (NH) cm−1. HRMS (ESITOF): calcd for C7H6ClN2S [M + H]+, 184.9934; found, 184.9942. 6-(Trifluoromethyl)benzo[d]thiazol-2-amine (2e). This compound was obtained in 90% yield (1.19 g) using general procedure A from 4-(trifluoromethyl)aniline (3e, 1.00 g, 6.21 mmol), ammonium thiocyanate (0.95 g, 12.42 mmol), and bromine (320 μL, 6.21 mmol). Mp: 113.7−115.5 °C (lit.59 109−110 °C), yellowish powder. 1H NMR (400 MHz, DMSO-d6): δ 8.09 (1H, d, J = 1.6 Hz, ArH), 7.86 (2H, s, NH2), 7.50 (1H, dd, J = 8.4 Hz, J = 1.6 Hz, ArH), 7.44 (1H, d, J = 8.4 Hz, ArH) ppm. 13C NMR (100 MHz, DMSO-d6): δ 169.3, 155.8, 131.5, 124.8 (q, J = 271.3 Hz, CF3), 122.5 (q, J = 3.7 Hz), 121.0 (q, J = 31.8 Hz), 118.5 (q, J = 4.1 Hz), 117.5 ppm. 19F NMR (375 MHz, DMSO-d6): δ −59.22 (3F, s, CF3) ppm. IR: 3493 (NH), 3107 (NH) cm−1. HRMS (ESI-TOF): calcd for C8H6N2F3S [M + H]+, 219.0204; found, 219.0197. Synthesis of Ligands 5a−e and 10 (General Procedure B). To a solution of amine 2a−e or 9 (0.50−0.70 mmol, 1 equiv) in 1,4dioxane (20 mL) were added 4-(dimethylamino)benzoyl chloride (4, 0.50−0.70 mmol, 1 equiv), distillated trimethylamine (1.50−2.10 mmol, 3 equiv), and DMAP (0.025−0.075 mmol, 0.05 equiv). The reaction mixture was refluxed for 24 h. After the mixture cooled, a saturated aqueous solution (50 mL) of NaHCO3 was added, and the mixture was extracted with DCM (3 × 40 mL). The combined organic layers were dried over Na2SO4, filtered, and concentrated. The product was purified by column chromatography (hexanes/DCM = 1:1 to 0:1 and next DCM/MeOH = 100:0 to 99:1, v/v).

compound with the perfluorinated benzothiazole unit (11), confirmed by X-ray analysis, caused nonbonded nuclear spin− spin coupling (J = 30.3 Hz) in the corresponding 19F NMR spectrum. Additionally, due to an expanded hydrogen bonding capability, dye 11 demonstrated different molecular packing than complexes 6a−e, causing a comparatively higher solidstate fluorescence quantum yield (Φ = 0.34).

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EXPERIMENTAL SECTION

General. All chemicals [including benzo[d]thiazol-2-amine (2a), p-anisidine (3b), 4-fluoroaniline (3c), 4-chloroaniline (3d), 4(trifluoromethyl)aniline (3e), (para-dimethylamino)benzoyl chloride (4), 2,3,4,5,6-pentafluoroaniline (7)] were received from commercial sources (TCI, Aldrich, Alfa Aesar, or Acros Organics) and used without further purification. Melting points of all synthesized compounds were measured on an automatic melting point system (OptiMelt, Stanford Research Systems). Column chromatography was performed on silica gel (Merck, 230−400 mesh). The NMR spectra were recorded with a Bruker Avance II 400 MHz (at 400, 100, and 375 MHz for 1H, 13C, and 19F NMR spectra, respectively), Varian VNMRS 500 MHz (at 500, 125, and 470 MHz for 1H, 13C, and 19F NMR spectra, respectively), or Varian VNMRS 600 MHz (at 150 MHz for 13C NMR spectra) spectrometers for solutions in DMSO-d6 or CDCl3 and TMS as the internal standard. Infrared (IR) spectra were recorded in KBr pallets with a Jasco FT/IR-6200 spectrometer in the 4000−400 cm−1 region. Mass spectra were measured with a Synapt G2-S HDMS (Waters Inc.) mass spectrometer equipped with an electrospray ion source and q-TOF type mass analyzer. UV−vis absorption spectra of ca. 10−5 M solutions of complexes were recorded in quartz cells using a PerkinElmer Lambda 35 spectrometer. Emission spectra of ca. 10−5 M solutions and the solidstate samples of complexes were recorded using an Edinburgh Instruments FLS980 fluorescence spectrometer (λex = 375 nm). Thin solid films were prepared by using the spin-coating technique utilizing an SPS-Europe Spin150 Spin processor using 2.5 mg/mL solutions of the compounds in DCM on the precleaned quartz substrates. Fluorescence quantum yields of the samples were estimated using the integrating sphere method. An integrating sphere (Edinburgh Instruments) coupled to the FLS980 spectrometer was calibrated with two standards: quinine sulfate in 0.1 M H2SO4 and rhodamine 6G in ethanol. Fluorescence decay curves of the solutions and of the solid films were recorded using a time-correlated single photon counting technique utilizing the PicoQuant PDL 820 ps pulsed diode laser as an excitation source (λex = 375 nm). Electrochemical experiments were carried out using mAUTOLAB Type III apparatus, glassy carbon, silver wire, and platinum coil as the working, reference, and auxiliary electrodes, respectively, and the scan rate at 100 mV/s. Ferrocene was used as the internal standard. Cyclic voltammetry measurements were conducted using deaerated 0.1 M DCM solutions of complexes in a standard one-compartment cell, under an argon atmosphere with 0.1 M Bu4NPF6 as the supporting electrolyte. Ionization potentials and electron affinities were calculated by the following equations: IP = Eoxonset + 4.4 and EA = Eredonset + 4.4, where Eoxonset is the onset oxidation potential and Eredonset is the onset reduction potential (Table 1 and Table S8 in the Supporting Information). Crystals of compounds 6a−e and 11 were obtained by the slow evaporation of their solution in hexanes/DCM (1:1). The X-ray measurements were carried out at 100 K on a SuperNova Agilent diffractometer using Cu Kα (λ = 1.54 184 Å) radiation. Data reduction was done with CrysAlisPro (Agilent Technologies, Version 1.171.35.21b). The structures were solved by direct methods and refined using the SHELXL Software Package.57 Crystallographic data of all investigated complexes have been deposited with the Cambridge Crystallographic Data Centre (CCDC). These data can be obtained, free of charge, from CCDC [e-mail: [email protected] or fax: + 44(0)-1223−336033]. Synthesis. Synthesis of 2-Aminobenzothiazoles 2b−e (General Procedure A). Compounds 2b−e were prepared using a modified 12138


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The Journal of Organic Chemistry N-(Benzo[d]thiazol-2-yl)-4-(dimethylamino)benzamide (5a). This compound was obtained in 61% yield (123 mg) using general procedure B from benzo[d]thiazol-2-amine (2a, 102 mg, 0.68 mmol), compound 4 (125 mg, 0.68 mmol), trimethylamine (284 μL, 2.04 mmol), and DMAP (4 mg, 0.03 mmol). Mp: 211.7−213.6 °C, yellowish powder. 1H NMR (400 MHz, DMSO-d6): δ 12.40 (1H, s, NH), 8.07 (2H, d, J = 8.9 Hz, ArH), 7.97 (1H, d, J = 7.8 Hz, Ar H), 7.75 (1H, d, J = 8.0 Hz, ArH), 7.44 (1H, dd, J = 8.0 Hz, J = 7.2 Hz, ArH), 7.30 (1H, dd, J = 7.8 Hz, J = 7.2 Hz, ArH), 6.77 (2H, d, J = 8.9 Hz, ArH), 3.02 (6H, s, NMe2) ppm. 13C NMR (100 MHz, DMSO-d6): δ 165.2, 159.2, 153.1, 148.6, 131.6, 129.9 (2C), 125.9, 123.3, 121.5, 120.1, 117.5, 110.8 (2C), 39.6 (2C) ppm. IR: 2895 (NH), 1661 (CO) cm−1. HRMS (ESI-TOF): calcd for C16H15N3OSNa [M + Na]+, 320.0834; found, 320.0829. 4-(Dimethylamino)-N-(6-methoxybenzo[d]thiazol-2-yl)benzamide (5b). This compound was obtained in 60% yield (121 mg) using general procedure B from 6-methoxybenzo[d]thiazol-2amine (2b, 110 mg, 0.61 mmol), compound 4 (112 mg, 0.61 mmol), trimethylamine (255 μL, 1.83 mmol), and DMAP (4 mg, 0.03 mmol). Mp: 203.5−204.7 °C, yellow powder. 1H NMR (400 MHz, CDCl3): δ 11.14 (1H, s, NH), 7.87 (2H, d, J = 9.0 Hz, ArH), 7.31 (1H, d, J = 8.8 Hz, ArH), 7.29 (1H, d, J = 2.3 Hz, ArH), 6.88 (1H, dd, J = 8.8 Hz, J = 2.3 Hz, ArH), 6.58 (2H, d, J = 9.0 Hz, ArH), 3.85 (3H, s, OMe), 2.99 (6H, s, NMe2) ppm. 13C NMR (100 MHz, CDCl3): δ 165.2, 158.2, 156.5, 153.3, 142.4, 133.3, 129.6 (2C), 121.3, 118.1, 114.8, 111.1 (2C), 104.1, 55.8, 40.0 (2C) ppm. IR: 3170 (N H), 1663 (CO) cm − 1 . HRMS (ESI-TOF): calcd for C17H17N3O2SNa [M + Na]+, 350.0939; found, 350.0933. 4-(Dimethylamino)-N-(6-fluorobenzo[d]thiazol-2-yl)benzamide (5c). This compound was obtained in 66% yield (125 mg) using general procedure B from 6-fluorobenzo[d]thiazol-2-amine (2c, 101 mg, 0.60 mmol), compound 4 (110 mg, 0.60 mmol), trimethylamine (251 μL, 1.80 mmol), and DMAP (4 mg, 0.03 mmol). Mp: 236.5− 237.8 °C, yellowish powder. 1H NMR (400 MHz, DMSO-d6): δ 12.43 (1H, s, NH), 8.04 (2H, J = 9.0 Hz, ArH), 7.87 (1H, dd, JH−F = 8.7 Hz, J = 2.7 Hz, ArH), 7.74 (1H, dd, J = 8.9 Hz, JH−F = 4.8 Hz, ArH), 7.27 (1H, ddd, J = 8.9 Hz, JH−F = 9.0 Hz, J = 2.7 Hz, Ar H), 6.76 (2H, J = 9.0 Hz, ArH), 3.01 (6H, s, 2 × CH3) ppm. 13C NMR (100 MHz, DMSO-d6): δ 165.2, 159.2, 158.6 (d, JC−F = 239.6 Hz), 153.1, 145.4, 132.9 (d, JC−F = 11.1 Hz), 130.0 (2C), 121.1 (d, JC−F = 9.3 Hz), 117.4, 114.0 (d, JC−F = 24.5 Hz), 110.8 (2C), 107.9 (d, JC−F = 27.8 Hz), 39.6 (2C) ppm. 19F NMR (375 MHz, DMSOd6): δ −118.77 ppm. IR: 3427 (NH), 1656 (CO) cm−1. HRMS (ESI-TOF): calcd for C16H14N3OFSNa [M + Na]+, 338.0739; found, 338.0743. N-(6-Chlorobenzo[d]thiazol-2-yl)-4-(dimethylamino)benzamide (5d). This compound was obtained in 57% yield (109 mg) using general procedure B from 6-chlorobenzo[d]thiazol-2-amine (2d, 106 mg, 0.57 mmol), compound 4 (105 mg, 0.57 mmol), trimethylamine (240 μL, 1.72 mmol), and DMAP (4 mg, 0.03 mmol). Mp: 219.8− 221.0 °C, yellowish powder. 1H NMR (400 MHz, CDCl3): δ 11.34 (1H, s, NH), 7.84 (2H, d, J = 9.0 Hz, ArH), 7.77 (1H, d, J = 2.0 Hz, ArH), 7.28 (1H, d, J = 8.6 Hz, ArH), 7.20 (1H, dd, J = 8.6 Hz, J = 2.0 Hz, ArH), 6.56 (2H, d, J = 9.0 Hz, ArH), 3.00 (6H, s, NMe2) ppm. 13C NMR (100 MHz, CDCl3): δ 165.4, 160.6, 153.4, 146.7, 133.4, 129.7 (2C), 128.9, 126.4, 121.4, 120.8, 117.7, 111.1 (2C), 39.9 (2C) ppm. IR: 3156 (NH), 1660 (CO) cm−1. HRMS (ESI-TOF): calcd for C16H15N3OSCl [M + H]+, 332.0624; found, 332.0616. 4-(Dimethylamino)-N-(6-(trifluoromethyl)benzo[d]thiazol-2-yl)benzamide (5e). This compound was obtained in 64% yield (118 mg) using general procedure B from 6-(trifluoromethyl)benzo[d]thiazol-2-amine (2e, 110 mg, 0.50 mmol), compound 4 (93 mg, 0.50 mmol), trimethylamine (210 μL, 1.50 mmol), and DMAP (4 mg, 0.03 mmol). Mp: 223.2−225.0 °C, white powder. 1H NMR (400 MHz, DMSO-d6): δ 12.64 (1H, s, NH), 8.46 (1H, br s, ArH), 8.06 (2H, d, J = 9.0 Hz, ArH), 7.89 (1H, d, J = 8.4 Hz, ArH), 7.72 (1H, dd, J = 8.4 Hz, J = 1.5 Hz, ArH), 6.76 (2H, d, J = 9.0 Hz, ArH), 3.02 (6H, s, NMe2) ppm. 13C NMR (100 MHz, DMSO-d6): δ 165.3, 162.4, 153.2, 151.5, 132.2, 130.1 (2C), 124.6 (q, J = 271.8 Hz, CF3),

123.4 (q, J = 31.8 Hz), 122.7 (q, J = 3.6 Hz), 120.4, 119.6 (q, J = 4.1 Hz), 117.1, 110.8 (2C), 39.5 (2C) ppm. 19F NMR (375 MHz, DMSO-d6): δ −59.40 (3F, s, CF3) ppm. IR: 3175 (NH), 1675 (CO) cm−1. HRMS (ESI-TOF): calcd for C17H14N3OF3SNa [M + Na]+, 388.0707; found, 388.0706. Synthesis of Complexes 6a−e and 11 (General Procedure C). To a solution of compound 5a−e or 10 (0.19−0.29 mmol, 1 equiv) in dry DCM (20 mL) were added BF3·Et2O (10 equiv) and distillated N,N-diisopropylethylamine (20 equiv) under an argon atmosphere. The reaction mixture was stirred for 24 h at room temperature and then washed with water. The organic phase was dried over anhydrous Na2SO4 and filtered. The solvent was removed under reduced pressure, and the crude product was purified by column chromatography (hexanes/dichloromethane from 1:1 to 0:1, v/v). 4-(1,1-Difluoro-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinin-3-yl)-N,N-dimethylaniline (6a). This compound was obtained in 66% yield (65 mg) using general procedure C from ligand 5a (85 mg, 0.29 mmol). Mp: 262.8−265.0 °C, yellow powder. 1 H NMR (600 MHz, CDCl3): δ 8.26 (2H, d, J = 9.2 Hz, ArH), 7.98 (1H, d, J = 8.1 Hz, ArH), 7.73 (1H, d, J = 7.6 Hz, ArH), 7.54 (1H, ddd, J = 7.6 Hz, J = 7.8 Hz, J = 1.1 Hz, ArH), 7.41 (1H, ddd, J = 7.6 Hz, J = 7.7 Hz, J = 1.0 Hz, ArH), 6.70 (2H, d, J = 9.2 Hz, ArH), 3.12 (6H, s, NMe2) ppm. 13C NMR (150 MHz, CDCl3): δ 173.8, 168.4, 154.6, 140.4, 133.0 (2C), 127.9, 126.6, 125.6, 121.9, 118.0, 117.0, 111.0 (2C), 40.1 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −137.53 (2F, m, BF2) ppm. IR: 1617 (CN) cm−1. HRMS (ESI-TOF): calcd for C16H14BN3OF2SNa [M + Na]+, 368.0816; found, 368.0812. 4-(1,1-Difluoro-7-methoxy-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2c][1,3,5,2]oxadiazaborinin-3-yl)-N,N-dimethylaniline (6b). This compound was obtained in 61% yield (56 mg) using general procedure C from ligand 5b (80 mg, 0.24 mmol). Mp: 275.1−277.5 °C, yellow powder. 1H NMR (500 MHz, CDCl3): δ 8.24 (2H, d, J = 9.2 Hz, ArH), 7.87 (1H, d, J = 9.0 Hz, ArH), 7.20 (1H, d, J = 2.5 Hz, ArH), 7.12 (1H, dd, J = 9.0 Hz, J = 2.5 Hz, ArH), 6.69 (2H, d, J = 9.2 Hz, ArH), 3.88 (3H, s, OCH3), 3.12 (6H, s, NMe2) ppm. 13 C NMR (125 MHz, CDCl3): δ 172.4, 167.8, 158.0, 154.4, 134.3, 132.7 (2C), 128.0, 118.7, 117.2, 116.2, 111.0 (2C), 105.4, 55.9, 40.1 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −137.71 (2F, m, BF2) ppm. IR: 1608 (CN) cm−1. HRMS (ESI-TOF): calcd for C17H16BN3O2F2SNa [M + Na]+, 398.0922; found, 398.0915. N,N-Dimethyl-4-(1,1,7-trifluoro-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinin-3-yl)aniline (6c). This compound was obtained in 67% yield (64 mg) using general procedure C from ligand 5c (82 mg, 0.26 mmol). Mp: 284.9−287.2 °C, yellow powder. 1 H NMR (500 MHz, CDCl3): δ 8.24 (2H, d, J = 9.0 Hz, ArH), 7.93 (1H, dd, J = 9.0 Hz, JH−F = 4.4 Hz, ArH), 7.44 (1H, dd, JH−F = 7.7 Hz, J = 2.5 Hz, ArH), 7.27 (1H, ddd, J = 9.0 Hz, JH−F = 9.1 Hz, J = 2.5 Hz, ArH), 6.69 (2H, d, J = 9.0 Hz, ArH), 3.19 (6H, s, NMe2) ppm. 13C NMR (125 MHz, CDCl3): δ 173.7, 168.4, 160.3 (d, JC−F = 247.4 Hz), 154.7, 136.8, 133.1 (2C), 127.7 (d, JC−F = 10.2 Hz), 119.1 (d, JC−F = 8.8 Hz), 116.8, 116.2 (d, JC−F = 24.4 Hz), 111.0 (2C), 108.8 (d, JC−F = 27.4 Hz), 40.1 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −114.21 (1F, m, C−F), −137.65 (2F, m, BF2) ppm. IR: 1607 (CN) cm − 1 . HRMS (ESI-TOF): calcd for C16H13BN3OF3SNa [M + Na]+, 386.0722; found, 386.0723. 4-(7-Chloro-1,1-difluoro-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinin-3-yl)-N,N-dimethylaniline (6d). This compound was obtained in 64% yield (59 mg) using general procedure C from ligand 5d (81 mg, 0.24 mmol). Mp: 274.4−276.0 °C, yellow powder. 1H NMR (600 MHz, CDCl3): δ 8.24 (2H, d, J = 9.2 Hz, ArH), 7.88 (1H, d, J = 8.8 Hz, ArH), 7.71 (1H, d, J = 2.0 Hz, ArH), 7.50 (1H, dd, J = 8.8 Hz, J = 2.0 Hz, ArH), 6.69 (2H, d, J = 9.2 Hz, ArH), 3.13 (6H, s, NMe2) ppm. 13C NMR (150 MHz, CDCl3): δ 173.8, 168.6, 154.8, 138.9, 133.2 (2C), 131.4, 128.5, 127.8, 121.7, 118.7, 116.7, 111.0 (2C), 40.1 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −137.56 (2F, m, BF2) ppm. IR: 1610 (CN) cm−1. HRMS (ESI-TOF): calcd for C16H14BN3OF2SCl [M + H]+, 380.0607; found, 380.0596. 12139


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The Journal of Organic Chemistry 4-(1,1-Difluoro-7-(trifluoromethyl)-1H-1λ 4 ,10λ 4 -benzo[4,5]thiazolo[3,2-c][1,3,5,2]oxadiazaborinin-3-yl)-N,N-dimethylaniline (6e). This compound was obtained in 68% yield (65 mg) using general procedure C from ligand 5e (85 mg, 0.23 mmol). Mp: 275.8− 278.1 °C, yellow powder. 1H NMR (500 MHz, CDCl3): δ 8.27 (2H, d, J = 9.3 Hz, ArH), 8.05 (1H, d, J = 9.6 Hz, ArH), 8.01 (1H, s, ArH), 7.78 (1H, d, J = 9.6 Hz, ArH), 6.70 (2H, d, J = 9.3 Hz, ArH), 3.14 (6H, s, NMe2) ppm. 13C NMR (125 MHz, CDCl3): δ 175.0, 169.0, 155.0, 142.7, 133.5 (2C), 127.9 (q, J = 33.3 Hz), 126.8, 125.0 (q, J = 3.6 Hz), 123.7 (q, J = 272.5 Hz, CF3), 119.5 (q, J = 4.2 Hz), 118.0, 116.5, 111.1 (2C), 40.1 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −61.83 (3F, s, CF3), −137.44 (2F, m, BF2) ppm. IR: 1609 (CN) cm−1. HRMS (ESI-TOF): calcd for C17H14BN3OF5S [M + H]+, 414.0871; found, 414.0869. 1-(Perfluorophenyl)thiourea (8b). Benzoyl chloride (0.73 mL, 6.30 mmol) and potassium thiocyanate (0.64 g, 6.61 mmol) were dissolved in dry acetone (10 mL), and the mixture was refluxed for 1 h. 2,3,4,5,6-Pentafluoroaniline (7, 1.10 g, 6.01 mmol) was added, and the refluxing was continued for 1 h more. After the mixture cooled, the solvent was evaporated in vacuo, and then water (30 mL) was added. The mixture was stirred for 10 min, and the precipitate of N[(perfluorophenyl)carbamothioyl]benzamide (8a) was filtered and added to an aqueous solution of NaOH (10%, 12 mL). The resulting reaction mixture was stirred at 80 °C for 2 h. The reaction mixture was cooled, and an aqueous solution of HCl (1M) was added to reach pH ∼ 3. Then, an aqueous solution of NH3 (25%) was added to reach pH ∼ 9 to remove benzoic acid from the solution. The mixture was stirred at 5 °C for 30 min. The precipitate was filtered, washed with water, and recrystallized from EtOH−H2O (1:1) to obtain the product 8b as a white crystalline solid (1.09 g, 75%). Mp: 156.4− 157.8 °C. 1H NMR (500 MHz, DMSO-d6): δ 9.36 (1H, s, NH), 8.27 (1H, br s, NH), 7.52 (1H, br s, NH) ppm. 13C{F} NMR (125 MHz, DMSO-d6): δ 183.4, 144.0 (2C), 139.7, 137.3 (2C), 114.7 ppm. 19F NMR (470 MHz, DMSO-d6): δ −145.43 (2F, d, J = 20.9 Hz), −157.53 (1F, t, J = 22.8 Hz), −164.11 (2F, br s) ppm. IR: 3349 (N H), 3179 (NH) cm−1. HRMS (ESI-TOF): calcd for C7H4N2F5S [M + H]+, 243.0015; found, 243.0010. 4,5,6,7-Tetrafluorobenzo[d]thiazol-2-amine (9). Sodium hydride (60% dispersion in mineral oil, 139 mg, 3.47 mmol) was added to a solution of 1-(perfluorophenyl)thiourea (8b, 801 mg, 3.31 mmol) in DMF (5 mL). The reaction mixture was heated up to 80 °C for 4 h under an argon atmosphere. After the mixture cooled, water (15 mL) was added. The precipitate was filtered, washed with water, and recrystallized from EtOH−H2O (1:1) to obtain the product 9 as a white crystalline solid (612 g, 83%). Mp: 200.9−202.3 °C. 1H NMR (500 MHz, DMSO-d6): δ 8.17 (1H, br s, NH2) ppm. 13C{F} NMR (125 MHz, DMSO-d6): δ 168.0, 140.4, 138.6, 137.6, 137.1, 134.3, 113.9 ppm. 19F NMR (470 MHz, DMSO-d6): δ −140.80 (1F, dd, J = 22.7 Hz, J = 12.7 Hz), −154.46 (1F, ddd, J = 20.5 Hz, J = 12.7 Hz, J = 3.0 Hz), −161.83 (1F, dd, J = 20.5 Hz, J = 21.4 Hz), −168.02 (1F, ddd, J = 23.2 Hz, J = 21.9 Hz, J = 3.0 Hz) ppm. IR: 3284 (NH), 3115 (NH) cm−1. HRMS (ESI-TOF): calcd for C7H3N2F4S [M + H]+, 222.9953; found, 222.9949. 4-(Dimethylamino)-N-(perfluorobenzo[d]thiazol-2-yl)benzamide (10). This compound was obtained in 45% yield (90 mg) using general procedure B from 4,5,6,7-tetrafluorobenzo[d]thiazol-2-amine (9, 112 mg, 0.54 mmol), compound 4 (99 mg, 0.54 mmol), trimethylamine (226 μL, 1.62 mmol), and DMAP (3 mg, 0.03 mmol). Mp: 196.9−198.7 °C, white powder. 1H NMR (500 MHz, DMSO-d6, 80 °C): δ 12.76 (1H, s, NH), 8.04 (2H, d, J = 9.0 Hz, ArH), 6.77 (2H, d, J = 9.0 Hz, ArH), 3.04 (6H, s, NMe2) ppm. 13C NMR (125 MHz, DMSO-d6, 80 °C): δ 165.0, 160.8, 153.3, 140.2 (CF), 138.7 (CF), 138.4 (CF), 135.7 (CF), 133.8, 129.8 (2C), 116.2, 115.6, 110.5 (2C), 39.1 (2C) ppm. 19F NMR (470 MHz, DMSO-d6, 80 °C): δ −141.07 (1F, dd, J = 22.2 Hz, J = 14.5 Hz), −152.47 (1F, dd, J = 19.7 Hz, J = 14.7 Hz), −160.57 (1F, dd, J = 20.6 Hz, J = 19.7 Hz), −163.47 (1F, dd, J = 22.2 Hz, J = 20.6 Hz) ppm. IR: 3126 (N H), 1641 (CO) cm−1. HRMS (ESI): calcd for C16H11N3OF4SNa [M + Na]+, 392.0457; found, 392.0450.

N,N-Dimethyl-4-(perfluoro-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2c][1,3,5,2]oxadiazaborinin-3-yl)aniline (11). This compound was obtained in 68% yield (54 mg) using general procedure C from ligand 10 (70 mg, 0.19 mmol). Mp: 257.1−258.9 °C, orange solid. 1H NMR (500 MHz, CDCl3): δ 8.25 (2H, d, J = 9.3 Hz, ArH), 6.70 (2H, d, J = 9.3 Hz, ArH), 3.15 (6H, s, NMe2) ppm. 13C NMR (125 MHz, CDCl3): δ 174.7, 169.0, 155.3, 141.3 (CF), 141.2 (CF), 138.3 (CF), 137.3 (CF), 133.8 (2C), 125.9, 115.9, 111.1 (2C), 110.7, 40.2 (2C) ppm. 19F NMR (470 MHz, CDCl3): δ −133.08 (2F, m, BF2), −139.70 (1F, dd, J = 21.0 Hz, J = 13.1 Hz, ArF), −143.18 (1F, tdd, J = 30.3 Hz, J = 19.7 Hz, J = 13.1 Hz, ArF), −153.51 (1F, dd, J = 20.1 Hz, J = 19.7 Hz, ArF), −157.67 (1F, dd, J = 21.0 Hz, J = 20.1 Hz, ArF) ppm. IR: 1607 (CN) cm−1. HRMS (ESI-TOF): calcd for C16H11BN3OF6S [M + H]+, 418.0620; found, 418.0618.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02098. ORTEP diagrams and crystal data of complexes 6a−e and 11; photophysical properties of complexes 6a−e and 11; onset reduction/oxidation potentials and cyclic voltammograms of dyes 6a−e and 11; DFT and TDDFT data; fluorescence decays; NMR and IR spectra (PDF) Crystal data of 6a (CIF) Crystal data of 6b (CIF) Crystal data of 6c (CIF) Crystal data of 6d (CIF) Crystal data of 6e (CIF) Crystal data of 11 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mykhaylo A. Potopnyk: 0000-0002-4543-2785 Dmytro Volyniuk: 0000-0003-3526-2679 Piotr Cmoch: 0000-0002-8413-9290 Iryna Hladka: 0000-0002-6864-4254 Yan Danyliv: 0000-0001-9645-0337 Juozas Vidas Gražulevičius: 0000-0002-4408-9727 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from Institute of Organic Chemistry of the Polish Academy of Sciences, European Social Fund (the activity “Improvement of researchers” qualification by implementing world-class R&D projects’ of Measure no. 09.3.3-LMT-K-712), and Institute of Physical Chemistry of the Polish Academy of Sciences.

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REFERENCES

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