Novel fluorescent N,O-Chelated fluorine-boron benzamide complexes containing thiadiazoles: Synthesis and fluorescence characteristics

Accepted Manuscript Novel fluorescent N,O-Chelated fluorine-boron benzamide complexes containing thiadiazoles: Synthesis and fluorescence characteristics Kan Zhang, Hao Zheng, Chaojun Hua, Ming Xin, Jianrong Gao, Yujin Li PII:

S0040-4020(17)30854-2

DOI:

10.1016/j.tet.2017.08.023

Reference:

TET 28917

To appear in:

Tetrahedron

Received Date: 9 May 2017 Revised Date:

13 August 2017

Accepted Date: 14 August 2017

Please cite this article as: Zhang K, Zheng H, Hua C, Xin M, Gao J, Li Y, Novel fluorescent N,OChelated fluorine-boron benzamide complexes containing thiadiazoles: Synthesis and fluorescence characteristics, Tetrahedron (2017), doi: 10.1016/j.tet.2017.08.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Novel Fluorescent N,O-Chelated Fluorine-Boron Benzamide Complexes Containing Thiadiazoles: Synthesis and fluorescence

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characteristics

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Novel Fluorescent﹥ N,O-Chelated Fluorine-Boron Benzamide Complexes Containing Thiadiazoles: Synthesis and fluorescence characteristics

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Kan Zhang, Hao Zheng, Chaojun Hua, Ming Xin, Jianrong Gao, Yujin Li* College of chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China Corresponding authors.

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E-mail addresses: [email protected] (Y. Li). Telephone number: +86-0571-88320891 (Y. Li).

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ABSTRACT

A series of N-(5-phenyl-1,3,4-thiadiazol-2-yl)benzamide derivatives and their corresponding BF2 complexes

were synthesized, and their photophysical properties

were determined. The effect of the derivatives with various substituents on the benzamide ring and phenyl-1, 3, 4-thiadiazole ring were examined in different organic

properties

including

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solvents and in the solid state. These dyes enjoy a series of excellent photophysical the

large

Stokes

shift,

solid-state

fluorescence,

and

aggregation-induced emission effect (AIEE).

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Keywords:

BF2 complexes; blue fluorescence; solid state; Stokes shifts; quantum yield;AIEE. 1. Introduction

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Organic fluorine-boron complexes carrying the BF2 moiety are known to be

fluorescent. [1] Among them, the boron-dipyrromethene (BODIPY) as the best-known example, are a family with excellent fluorescent properties.[2] They exhibit high fluorescence quantum yield, outstanding optical properties and sharp fluorescence spectra [3,4] and are often applied to electroluminescent devices, [5] dye-sensitized solar cells,[6,7] liquid crystals,

[8]

biological imaging

[9]

and laser dyes.[10] Among these,

heterocyclic azo dyes have been widely investigated mostly due to they are versatile chromophores that can provide bright strong shades tunable for absorption spectrum.[11] And azo dyes are of particular interest because the planarity of the azo 1

ACCEPTED MANUSCRIPT bridge combines with benzoyl or other systems should contribute to larger π electron transmission effects and lead to higher optical activity.[12,13]These previous studies prompted us to the synthesis and the characterization of new thiadiazoles N-O BF2 as suitable candidates for potential use in fluorescent dyes.

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In recently years, BF2 fluorescence complexes have attracted significant attention due to their outstanding optical properties.[14]However, in many cases, these dyes exhibit decreased fluorescence in the solid state as a result of aggregation-caused quenching(ACQ) ,[15]which owing to the formation of delocalised excitons or

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excimers cause enhanced non-radiative deactivation of the excited state.[16,17]We design these boron fluoride complexes of thiadiazoles (3b) have showed nice

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fluorescence in solution sate and solid sate, however, we also find these boron fluoride complexes(3a,3d) exhibit intense solid fluorescence but weak in solution. In contrast of aggregation-caused quenching(ACQ) ,[18] the unusual phenomenon of the luminescence aggregate state stronger than that in the solution state,[19]called aggregation-induced emission effect (AIEE),[20] which is now attracting increasing

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research attention. These compounds(3a,3d) which have the AIEE characteristics exhibit weak luminescence in solution state, but show highly emissive behaviour in their aggregated and solid states. The results showed that thiadiazoles derivatives and

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their BF2 complexes displayed attractive luminescent properties. [21,22] 2. Results and discussion

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2.1. Synthesis

Preparation of the N-(5-phenyl-1, 3, 4-thiadiazol-2-yl)benzamide derivatives 2

and their corresponding BF2 complexes 3 were shown in Scheme 1. Benzamide derivatives 2 were prepared following a patent method

[24]

by amidation of

2-aminobenzothiazole with benzoyl chloride (1.2 equiv.) at room temperature which resulted in more than 83% yields. The BF2 complexes 3 were produced, with the yields of 41%-74%, by using an excess of BF3·OEt2 in toluene with Et3N as the base and purification by column chromatography. All the structures of boron complexes were fully characterized by FTIR, 1H NMR, 13C NMR and HRMS analysis.

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Scheme 1. Synthesis of benzamide 2, BF2 complexes 3 2.2. Fluorescence detection

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2.2.1. UV-Vis and fluorescence of 3a in solution

The end product BF2 complexes 3 showed good solubility and stability in organic

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solvents such as dichloromethane (DCM), chloroform, toluene, EtOH, THF and N,N-dimethylformamide. By testing, no observable color and fluorescence fading was found over months. The solvent effect on the absorption and fluorescence properties of 3a were examined. As shown in Table 1 and Fig. 1, complex 3a exhibited a sharp absorption peak at about 330 nm in organic solvents, and the λmax was barely affected

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by solvent polarity, suggesting that the dipole moments of the molecules in their ground and excited states were almost equal.[25] While the fluorescence intensity were strongly dependent on solvents and reduced with the increase in solvent polarity.In

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Fig. 1(b), 3a showed the highest emission curve in toluene . So we chose toluene to

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examine the absorption and fluorescence properties of all the BF2 complexes 3.

Fig. 1. Absorption spectrum (a) and emission spectrum (b) of complex 3a (3×10-6 mol·L-1) in different solvents (PMT Voltage: 500 V).

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ACCEPTED MANUSCRIPT Table 1. Optical data of 3a-3d in solvents and in the solid state.

3c 3d

Fluorescence -1

-1

Stokes shift

λabs(nm)

εmax(M ·cm )

λem(nm)

ФF

(nm)

335 337 333 332 328 330 331 334 367 356 369 343 349 370

33667 36000 33000 32333 29667 34000 33333 36667

413 410 412 402 409 415 411 419 429 414 446 409 436 456

0.05 0.06 0.03 0.02 0.01

78 73 79 70 81 85 80 85 62 58 77 66 87 86

46667

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3b

CHCl3 toluene DCM THF EtOH MeCN EA DMF solid toluene solid toluene toluene solid

UV-Vis

<0.01 0.01 0.01 0.29

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3a

Matrix

25000 25333

<0.01 0.16

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Dye

2.2.2. Optical Properties of complexes 3 in solution

The UV−vis absorption and normalized fluorescence spectra of 3a-3d in toluene are shown in Figures 2. The maximum absorption wavelength of complex 3 compared to that of amide 2 (Table 1), although amide 2 did not show fluorescence, complex 3

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exhibited blue fluorescence at 410 nm-436nm. The fluorescence quantum yields (ФF) of 3a in toluene was 0.06 and the Stokes shift was 73nm. The λmax of 3b (356 nm), 3c (343 nm), 3d (349 nm) was slightly red-shifted, and εmax(3b) was higher (46667)

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compared to that of 3a (337nm and 36000), respectively. But, 3c (25000), 3d (25333) showed decrease in εmax. It is worth noting that the fluorescence quantum yields were

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highly dependent on the substituents on the aromatic ring. In the absence of substitution at the right aromatic ring, low quantum yield (0.06) was observed and introducing electron-withdrawing (NO2) onto the para-position of right phenyl ring(3c) got lower quantum yield(<0.01). However, introducing an electron-donating methoxy group onto the para-position of right phenyl ring (complex 3b), the quantum yield increased to 0.29. In addition, we could observe high quantum yield 0.16(3d) which introducing electron-donating methoxy group onto the para-position of left phenyl ring. This suggested that we could construct push-pull-type architecture through the introduction of an electron-donating group onto the phenyl ring. 4

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Fig. 2. Absorption spectrum and emission spectrum of complex 3a-3d (3×10-6 mol·L-1)

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in toluene (PMT Voltage: 500 V).

Fig. 3. The color of compounds 3a-3d(1×10-4 mol·L-1)(left to right) in toluene under UV lamp (365 nm).

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2.2.3. Optical Properties of complexes 3 in the solid state

The BF2 complexes 3 were also emissive in the solid state and their emission data were shown in Table. 1. Generally, the high planarity of BODIPY dyes may lead to stacking of molecules causing quite strong intermolecular interactions resulting in the

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concentration quenching in the solid state.[ 26, 27,28] Therefore, many BODIPY dyes hardly display fluorescence in the solid state. By controlling the molecular structure,

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we successfully obtained solid-state emissive benzamide-BF2 complexes 3. Photos of the emission colors of 3a-3d in the solid state were provided for comparison (Fig. 5). As shown in Table. 1, the fluorescence maxima for all complexes 3 were red-shifted with respect to corresponding emissions in solution, which could be attributed to the π-π stacking induced by the more compact molecular aggregation in the solid state [29]. As we see, the compounds 3a exhibited good blue fluorescence in solid-state but weak fluorescence in solution. The electron-donating substituted compounds 3b, 3d displayed better blue fluorescence in solid-state and had slightly red-shifted compared to 3a. It was assumed that large Stokes shifts partly offset the self-quenching effect in 5

ACCEPTED MANUSCRIPT the solid-state, and the more twisted molecular structure might prevent the molecules from packing compactly, thus enhancing their solid emission

[30]

. In addition, the

nitro-substituted compound 3c display almost no fluorescence in the solid-state and

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solution.

Fig. 4. The color of compounds 3a-3d (left to right) under visible light (upper row)

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and UV lamp (365 nm, lower row).

2.2.4. Optical Properties of complexes 3 in the aggregated state From the above test, we found compounds 3a and 3d had low quantum yield and weak fluorescence in solution , but exhibited intense fluorescence in solid-state. So we thought they have aggregation-induced emission effect (AIEE). To determine whether

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compounds 3a and 3d have AIEE characteristics, the UV− Vis absorption spectra and fluorescence spectra of compound 3a and 3d were measured in a series of H2O/THF mixtures with different volume fractions of water, since compounds 3a and 3d was soluble in THF but not in water (see Fig. 5). Compared with the UV−Vis spectrum of

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compounds 3a in different mixtures of THF and water over 10 min, we found the maximum absorption wavelength and intensity were no significant change when the

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water was added (10-70% (v/v)) gradually. When the water fraction was between 80 and 90%, the maximum absorption wavelength also had no obvious change, but the fluorescence intensity dramatically increased. As the water content increased to over 80% (v/v) in the H2O/THF mixture, despite its luminogen concentration remaining unchanged at 1×10-4 mol·L-1, its fluorescence intensity was incredible strong compared to in the THF solution. Similar results were observed for compound 3d. Since water was a poor solvent for compounds 3a and 3d, the molecules of these compounds must have aggregated in the H2O/THF mixture system at those higher water contents (80-90% (v/v)). Therefore, compounds 3a and 3d were AIEE active. In addition,we 6

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fractions(Fig.9 S13).

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2.2.5. DFT calculation

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Fig.5. Emission spectra of 3a, 3d (1×10-4 mol·L-1) in different proportions of H2O/THF

Fig. 6. Representative HOMO and LUMO diagrams of 3a-3d obtained from

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calculations and their corresponding energy levels

3a-1

3a-2

Fig. 7. Representative optimized geometry of the compounds 3a from different orientation

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Table 2. Calculation data of compounds 3a-3d. λbem

compounds

(nm,cal)

(nm,exp)

435

410

0.7176

354

337

0.8870

3b

427

414

0.9852

368

356

1.1778

3c

417

409

0.5156

364

343

0.5777

3d

474

436

0.5535

388

349

0.6686

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3a

c

f abs

theoretical calculations basis sets B3LYP/6-31G without solution

b c

(nm,exp)

λbabs

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a

(nm, cal)

λaabs

femc

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λaem

λmaxin toluene solvent

Oscillator strength coefficients

In order to support our result we made the compounds geometry optimizations

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using the density functional theory (DFT) method as implemented in Gaussian 09W suite of programs employing basis sets B3LYP/6-31G.The ground-state geometry was optimized in the gas-phase at the B3LYP/6-31G level of theory. From the DFT calculations, we can see the HOMO and LUMO orbitals are delocalized over the

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whole molecule(Fig.6). In the case of 3b, the terminal phenyl ring with methoxy group significantly contribute to the delocalization. Therefore, complex 3b is

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expected to have red-shifted λmax and high ε. The DFT calculations indicate that methoxy derivative 3d shows intramolecular chargetransfer (ICT) transition from methoxy group to the three-nitrogen ring of chelating position moiety[31]. The red-shifted λmax of 3d are owing to the ICT transition. In contrast to 3d, we can see the HOMO and LOMO of complex 3c shows intramolecular chargetransfer (ICT) transition from the left benzene and three-nitrogen ring to the right benzene with nitro group. Because of strong electron-withdrawing group, the orbitals are delocalized in right benzene with nitro group. It can explain why the 3c show no fluorescence. From the DFT calculations(Fig.7), we were aware that the molecular system formed a 8

ACCEPTED MANUSCRIPT nearly planar structure. The absorption spectra and emission spectra were computed using time-dependent DFT (TD-DFT). It is worth noting that the values of the experiment absorption and emission spectra is in good agreement with the calculated values (Table 2). 3. Conclusion

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In summary, we have developed new N,O-chelated BF2 complex benzamide derivatives containing thiadiazoles building blocks. The synthesis of these dyes displayed solvent-sensitive emission feature and a nice quantum yield had been

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achieved for 3b through the introduction of an electron-donating methoxy group onto the phenyl ring. These compounds 3a, 3b and 3d also showed good solid-state

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fluorescence. In particular, these complexes 3a and 3d displayed unusual aggregation-induced emission enhancement in the mixed solvent of THF/water. Moreover,These series of compounds could be applied to the design of OLEDs, live-cell imaging and fluorescent sensors. 4. Experimental 4.1. Materials and equipment

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Nuclear magnetic resonance (1H,

C NMR) spectra were recorded on a Bruker

AM-500 spectrometer in CDCl3 or DMSO-d6, with TMS as internal standard. Fourier transform infrared (FTIR) spectra were performed using Thermo Nicolet 6700

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spectrophotometer in KBr pellets. The high-resolution mass spectra were recorded on a Bruker microTOF-Q II and melting points (m.p.) were recorded on a X-4 electro

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-thermal digital melting point apparatus. All of the chemicals used in the current study were purchased from commercial vendors and used as received without further purification, unless otherwise noted. All solvents were purified and dried using standard methods prior to use. 4.2. Absorbance, emission and quantum yield UV-Vis absorption spectra were measured on a UV-2550 and fluorescence spectra were obtained with a F-7000 Fluorescence spectrophotometer. The solvents used in the photochemical measurements were spectroscopic grade. All the experiments were performed repeatedly, and reproducible results were obtained. The ΦF values in 9

ACCEPTED MANUSCRIPT solution were measured following a general method with quinine sulfate (Φ = 0.55 in 50 mM H2SO4 solution) as a standard and dilute solutions of the compounds in organic solvent were used (3×10-6 mol·L-1). The fluorescence spectra were recorded 3 times and the average values of the integrated areas of fluorescence were used to

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calculate ΦF in solution. The solid emission spectra were measured by attaching the solid samples on a support on F-7000 Fluorescence spectrophotometer. 4.3. Theoretical calculations

Density functional theory (DFT) with B3LYP functional and 6-31G basis set have

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been used to optimize all the structure. For the fluorescence emission wavelength and the excitation wavelength of the compounds was investigated using time-dependent

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density functional theory (TDDFT). All these calculations were performed with the Gaussian 09W program package. [23] 4.4. Preparation of compounds

1a-1b

A stirring mixture of benzoic acid (0.2442 g, 2.0 mmol), thiosemicarbazide (0.1823 g, 2.0 mmol) and POCl3 (1.2 ml) was heated at 80 oC for 2.5 h. After cooling

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down to room temperature, water (2.5 mL) was added. The reaction mixture was refluxed for 4 h. After cooling, the mixture was basified to pH 8 by the dropwise addition of 40% NaOH solution under stirring. The precipitate was filtered and

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recrystallized from ethanol to yield 0.3130 g of the target compound 1a as a white solid ,Yield: 88.3%.

4.4.1. 5-phenyl-1, 3, 4-thiadiazol-2-amine (compound 1a)

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White solid. Yield 88.3%. M.p. 224-226℃ (ref: 223℃). 1H NMR (500 MHz,

CDCl3):δ 7.82 (dd, J = 6.5, 3.1 Hz, 2H), 7.45 (dd, J = 5.0, 1.8 Hz, 3H), 5.21 (bs, 2H). 4.4.2. 5-(4-methoxyphenyl)-1, 3, 4-thiadiazol-2-amine (compound 1b) White solid. Yield 86.4%. M.p. 185-187℃ (ref: 181℃) . 1H NMR (500 MHz,

DMSO-d6): δ 7.69 (d, J = 8.6 Hz, 2H), 7.30 (s, 2H), 7.02 (d, J = 8.6 Hz, 2H), 3.80 (s, 3H). 4.5. Preparation of complexes 2a-2d A stirring solution of 5-phenyl-1, 3, 4-thiadiazol-2-amine (1a) (0.3544 g, 2.0 mmol) and 4-dimethylaminopyridine (DMAP) (0.0122 g, 0.1 mmol) in THF (30 mL) 10

ACCEPTED MANUSCRIPT at room temperature was treated with triethylamine (0.2021 g, 2.0 mmol). A solution of benzoyl chloride (0.3373 g, 2.4 mmol) in THF (10 mL) was added dropwise to the above mixture, and stirred for a further 6 h. The complete reaction was detected by TLC analysis. The reaction mixture was added 30% aq Na2CO3 (5 mL) and stirred for

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further 10 min, and then water (150 mL) was added. The precipitate was filtered and recrystallized from ethanol to yield 0.4861 g of the target compound 2a as a white solid ,Yield: 86.4 %.

4.5.1. N-(5-phenyl-1 ,3, 4-thiadiazol-2-yl)benzamide(compound 2a)

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White solid. Yield 86.4%. M.p. 234-235℃ (ref: 237-238℃). 1H NMR (500 MHz, CDCl3):δ 12.08 (bs, 1H), 8.30-8.28 (m, 2H), 7.99 (dd, J = 6.5, 3.1 Hz, 2H), 7.70 (t, J

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= 7.4 Hz, 1H), 7.60 (t, J = 7.7 Hz, 2H), 7.54-7.51 (m, 3H).

4.5.2. 4-methoxy-N-(5-phenyl-1, 3, 4-thiadiazol-2-yl)benzamide (compound 2b) White solid. Yield 84.3%. M.p. 249-251℃ (ref: 245℃) . 1H NMR (500 MHz, DMSO-d6):δ 13.00 (bs, 1H), 8.17 (d, J = 8.7 Hz, 2H), 8.03-7.94 (m, 2H), 7.55 (d, J = 5.7 Hz, 3H), 7.11 (d, J = 8.7 Hz, 2H), 3.87 (s, 3H).

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4.5.3. 4-nitro-N-(5-phenyl-1, 3, 4-thiadiazol-2-yl)benzamide (compound 2c) Yellow solid. Yield 83.1%. M.p.

300℃ (ref: 271℃). 1H NMR (500 MHz,

DMSO-d6): δ 13.65 (bs, 1H), 8.38 (dd, J = 17.1, 8.4 Hz, 4H), 8.00 (d, J = 5.4 Hz, 2H),

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7.56 (d, J = 5.3 Hz, 3H).

4.5.4. N-(5-(4-methoxyphenyl)-1, 3, 4-thiadiazol-2-yl)benzamide (compound 2d) Yellow solid. Yield 85.5%. M.p. 230-232℃. 1H NMR (500 MHz, DMSO-d6):δ

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13.09 (bs, 1H), 8.15 (d, J = 7.5 Hz, 2H), 7.93 (d, J = 8.7 Hz, 2H), 7.68 (t, J = 7.3 Hz, 1H), 7.58 (t, J = 7.6 Hz, 2H), 7.11 (d, J = 8.7 Hz, 2H), 3.85 (s, 3H). 4.6. Preparation of complexes 3a-3d A stirring solution of 2a (0.1406 g, 0.5 mmol) in

anhydrous toluene (25 mL),

Et3N (3 mL, 20 mmol) was syringed under nitrogen atmosphere. After stirred for 20 min, BF3·OEt2 (1.5 mL, 12 mmol) was injected. The mixture was stirred at 100°C overnight and complete reaction was detected by TLC. The mixture was quenched with water (20 mL), and extracted with ethyl acetate (3×25 mL). The combined organic layer was dried with Na2SO4 and the solvent was removed under reduced 11

ACCEPTED MANUSCRIPT pressure. The residue was purified by silica gel chromatography eluting (silica gel, dichloromethane: petroleum ether=1: 1) to afford clean BF2 complex 3a as a white solid 0.1203g,Yield 73.7%. 4.6.1.1,1-difluoro-3,6-diphenyl-1H-[1,3,4]thiadiazolo[3,2-c][1,3,5,2]oxadiazaborinin

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-8-ium-1-uide (compound 3a) White solid. Yield 73.7%. M.p. 220-222℃. IR (KBr, cm-1): 1597, 1505, 1492, 1463, 1398, 1177, 1072, 824, 735, 723 cm-1. 1H NMR (500 MHz, CDCl3): δ 8.40-8.35 (m, 2H), 7.97-7.93 (m, 2H), 7.67 (dd, J = 10.6, 4.3 Hz, 1H), 7.63-7.59 (m,

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1H), 7.54 (dd, J = 15.1, 7.8 Hz, 4H). 13C NMR (126 MHz, CDCl3): δ 174.42, 168.86, 162.89, 134.64, 132.70, 130.76, 130.57 (2C), 129.52 (2C), 128.74 (2C), 128.28,

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127.53 (2C). HRMS m/z (ESI+): Calculated for C15H10N3OSBF2 ([M+H]+): 330.0684, Found 330.0684.

4.6.2.1,1-difluoro-3-(4-methoxyphenyl)-6-phenyl-1H-[1,3,4]thiadiazolo[3,2-c][1,3,5, 2]oxadiazaborinin-8-ium-1-uide (compound 3b)

Yellowish solid. Yield 72.6%. M.p. 238-240℃. IR (KBr, cm-1): 2848, 1603, 1580,

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1471, 1395, 1267, 1162, 1068, 829, 765 cm-1. 1H NMR (500 MHz, CDCl3): δ 8.36-8.31 (m, 2H), 7.96-7.91 (m, 2H), 7.62-7.57 (m, 1H), 7.54 (t, J = 7.4 Hz, 2H), 7.03-6.99 (m, 2H), 3.93 (s, 3H).

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C NMR (126 MHz, CDCl3): δ 174.42, 168.53,

165.22, 162.09, 133.09 (2C), 132.53, 129.49 (2C), 128.50, 127.51 (2C), 123.03,

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114.25 (2C), 55.68. HRMS m/z (ESI+): Calculated for C16H12N3O2SBF2 ([M+H]+): 360.0790, Found 360.0782.

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4.6.3.1,1-difluoro-3-(4-nitrophenyl)-6-phenyl-1H-[1,3,4]thiadiazolo[3,2-c][1,3,5,2]ox adiazaborinin-8-ium-1-uide (compound 3c) Yellow solid. Yield 53.2%. M.p. 258-260℃. IR (KBr, cm-1): 1537, 1508, 1460,

1406, 1345, 1191, 1082, 823, 773 cm-1. 1H NMR (500 MHz, CDCl3): δ 8.58-8.54 (m, 2H), 8.41-8.35 (m, 2H), 7.97 (dd, J = 5.2, 3.3 Hz, 2H), 7.67-7.63 (m, 1H), 7.61-7.56 (m, 2H). 13C NMR (126 MHz, Pyr): δ 174.56, 169.68, 166.41, 164.95, 151.39, 136.53, 133.05, 131.47, 131.12, 129.76, 129.27, 128.35, 127.92, 127.04, 123.99. HRMS m/z (ESI+): Calculated for C15H9N4O3SBF2 ([M+H]+): 375.0535, Found 375.0532. 4.6.4.1,1-difluoro-6-(4-methoxyphenyl)-3-phenyl-1H-[1,3,4]thiadiazolo[3,2-c][1,3,5, 12

ACCEPTED MANUSCRIPT 2]oxadiazaborinin-8-ium-1-uide (compound 3d) Green solid. Yield 40.9%. M.p. 256-258℃. IR (KBr, cm-1): 2837, 1612, 1579, 1495, 1455, 1392, 1255, 1178, 1033, 992, 830, 721 cm-1. 1H NMR (500 MHz, CDCl3): δ 8.37 (d, J = 7.0 Hz, 2H), 7.89 (d, J = 7.8 Hz, 2H), 7.71-7.63 (m, 1H), 7.58 -7.49 (m, 13

C NMR (126 MHz, Pyr): δ 174.41,

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2H), 7.04 (d, J = 7.7 Hz, 2H), 3.92 (s, 3H).

163.47, 134.76, 131.24, 130.56 (2C), 129.63 (2C), 129.08 (2C), 121.01, 115.24 (2C), 55.57. HRMS m/z (ESI+): Calculated for C16H12N3O2SBF2 ([M+H]+): 360.0790, Found 360.0790.

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Acknowledgments

We gratefully acknowledge the financial supported by the Natural Science

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Foundation of China (21176223) and the National Natural Science Foundation of Zhejiang (LY13B020016) and the Key Innovation Team of Science and Technology in Zhejiang Province (2010R50018) and the Natural Science Foundation of China (B060702). References

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