Evaluation, synthesis and structural properties of stable bisbenzimidazolium tetrafluoroborates derived from unstable benzotetraazafulvalenes

The stable dicationic tetrafluoroborate salts of tetraazafulvalenes were synthesized with unstable N-heterocyclic carbenes or enetetramines derived from benzimidazolium and bridged-bisbenzimidazolium halides. The structural characterizations of all the compounds were determined via spectroscopic methods 1 H-NMR, 13 C-NMR, IR


Introduction
Benzimidazole derivatives are pharmacophores that have an important place in medicinal chemistry.][6][7] N-heterocyclic carbenes (NHCs) are a class of compounds in which the carbene is in a nitrogencontaining ring structure and has many uses.10][11][12][13] The synthesis of NHCs is generally carried out in two pathways.][16][17] The NHCs tend to dimerize in the reaction medium.][27][28][29] OEDs in other words NHOs with negative redox potentials form radical cations or dicationic products with the loss of one or two electrons.These redox reactions have interesting applications.][32][33][34][35][36][37][38][39] The synthesis of dicationic tetraazafulvalenes is also important because of the interest in using fields described above.In this study, a group of new dicationic benzotetraazafulvalene tetrafluoroborates (1-7) was synthesized and their structural properties were evaluated.The synthesis of stable NHC precursors is important, as N-heterocyclic carbenes (NHCs) or electron-rich olefins (EROs) have many applications such as catalysts and the synthesis of organometallic compounds.These new compounds can be used as stable NHC or electron-rich olefin precursors. 7,31

Results and Discussion
N-heterocyclic carbenes (NHCs) are known to be enormously strong reducing agents. 7Therefore, they have been used as active intermediates for the synthesis of many useful new compounds in organic chemistry for decades. 6,8Dicationic dibenzotetraazafulvalens have been used as they exhibit a large Stokes shift.Therefore, they can be used in solar cells and laser applications. 40he benzimidazolium halides were synthesized according to the literature sources. 14,41The NHCs were synthesized by mixing benzimidazolium halides as a N-heterocyclic carbene precursor with NaH in dry tetrahydrofuran (THF) for 6h at room temperature (rt) in an argon atmosphere.The unstable carbenes were extracted with toluene and the yellow-colored solid was obtained.The yellow-colored products were washed with hexane and dried.The mixture of yellow solid, AgBF4, and THF was stirred in rt for 30 min.The silver mirror formation was observed along with the precipitated gray solid in the reaction media.The products were crystallized in ethanol.The compounds were obtained with good yields between 67-87%.The structural evaluations of all new compounds were carried out by spectroscopic techniques and elemental analysis.The 1 H NMR spectra of all compounds were examined and it was observed that the chemical shift (δ) values of the aromatic peaks were in the range of 8.442-7.090ppm.The chemical shifts of the methylene groups of the 3phenylpropyl of compounds 1, 2, 3, and 4 were observed for N-CH2 at 4.60, 4.43, 4.41, and 4.49; for the Ph-CH2 group at 2.68, 2.80, 2.62, and 2.78; for the CH2 group at 2.14, 2.36, 2.23, and 2.37 ppm in the 1 H NMR spectra.In the 13 C NMR spectra of compounds 1, 3, and 4, the peaks of N-CH2, Ph-CH2, and CH2 methylene carbons were also observed at 47.6, 46.9, 47.1; 31.9, 32.3, 32.5; 31.3,30.2, and 30.0 ppm respectively.It was observed 5(6)-tautomerism 41 with two different chemical shift values, due to the methyl groups present in the 5 position in compounds 2 and 7.In the 13 C NMR spectrum of compound 2, the carbon peaks of the N-CH2, Ph-CH2, and CH2 groups were recognized at 47.0 and 46.8, 32.5 and 32.4,30.2 and 30.1 ppm, respectively.Also, the chemical shift values of the N-CH3 group of compound 7 in the 1 H NMR spectrum were observed at 4.26 and 4.24 ppm, which are two values.Similarly, carbon peaks of the N-CH3 group were observed at 34.7 and 34.6 ppm in the 13 C NMR spectrum.Another remarkable situation in the 1 H NMR spectra is that each of the bridging methylene groups of the diazocine derivatives (compounds 6 and 7) has different chemical shift values.The bridging NCH2 groups of compound 6 were observed at 4.89 and 4.61 ppm.The CH2 peaks were also identified at 2.25 and 1.98 ppm.In the 1 H NMR of compound 7, four different chemical shift values were observed for bridge methylene groups at 5.04, 4.43, 2.27, and 1.91 ppm, similarly.

Scheme. Synthesis pathway of dicationic tetraazafulvalene ditetrafluoroborates.
This situation is thought to be due to the distortion of symmetry as a result of the stereochemical configuration of the bulky groups of the eight-membered ring.So, when the dihedral angles of the methylene groups in the bridge position of the compounds were calculated theoretically, slightly different values were obtained.C NMR spectra of the compounds.In the IR spectra, peaks of the C=N bonds of imidazole cycles were observed at 1521.52, 1562.77,1571.79,1563.93,1567.00 and 1546.21cm -1 , and peaks of the BF4 -group were observed at 1032.41, 1028.03,1031.00,1023.98,1031.63,1030.90 and 1027.86 cm -1 .In addition, the mass spectra of the compounds were evaluated.In the spectrum of compound 1, the molecular peak of the cation after the salt compound lost its two BF4 -anions are 708.4,and the peak value of 354.3, which is the mass value of half of the dicationic compound.The mass spectrum values of compound 2 were determined as [M +2 2BF4 --2BF4 -] = 736.3and [M +2 2BF4 --2BF4 -]/2 = 368.8.Similarly, the mass spectra of compounds 3-7 show 769.4,589.3, 505.2, 433.2, and 433.2 values belonging to the dicationic structures formed by the separation of a trifluoroborate anions from the molecule.These data are consistent with the literature. 6,14,42 Ater, the structural characterization of the compounds, the activity of a catalytic system was investigated that consists of compound 1 as a ligand in presence of Pd(OAc)2 in the Suzuki Miyaura C-C coupling reaction.The Suzuki-Miyaura cross-coupling reaction provides the formation of new C-C bonded compounds.This reaction is performed with organoboron compounds and organic electrophiles such as aryl halides in presence of a suitable catalytic system.The most effective catalysts used in the Suzuki-Miyaura reaction are bearing Pd and Ni. 43,44One of the most preferred in situ catalytic reactions contains a catalytic system consisting of a base, NHC precursor, and Pd(OAc)2 due to their stability.In our previous work, in situ C-C coupling reactions were also carried out with high activities with NHC precursors/Pd(OAc)2 catalytic systems. 14,45ere, its catalytic contribution to C-C bond formation was evaluated when compound 1 was used in the presence of palladium acetate as the ligand (Table 2).To observe the changing in the reaction yield were used different bases like K2CO3, KOH, and NaOH.K2CO3 was the most active (Table 2, entry 3).It was observed that the yield decreased from 80% to 55% when the ligand (compound 1) was not used in the reaction at 80 °C.It was noticed that increasing the temperature from 80 to 100 °C did not cause a remarkable increase in yield (Table 2, entries 3 and 6).Conditions: bromobenzene (1 mmol), phenylboronic acid (1 mmol), solvent (4 mL), base (2 mmol), Pd(OAc)2 (0.01 mmol), and compound 1 (0.01 mmol).a Isolated yield, b without compound 1.

Conclusions
In this study new dicationic benzimidazolium ditetrafluoroborates (1-7) were synthesized with N-heterocyclic carbenes or their dimer electron-rich olefins in other ways electron donor olefins.The structural differences of dicationic benzotetraazafulvalenes were evaluated.Benzimidazole compounds contain alkyl groups such as phenylpropyl, phenylethyl, methyl, ethyl, pentyl, 1,3-propanediyl, and 1,4-butanediyl in the 1 and 3 positions.Moreover, It was observed that compound 1, which was used as a ligand in the presence of palladium acetate increased the yield of biphenyl formation in the Suzuki-Miyaura C-C coupling reaction between phenylboronic acid and bromobenzene.Structural characterization of the new compounds was performed via spectroscopic methods such as 1 H, 13 C NMR, IR, and MS.

Experimental Section
General.The starting materials were purchased from Acros, Aldrich, and Fluka Chemical Co.All 1 H-NMR (300 MHz) and 13 C-NMR (75 MHz) spectra were recorded using Bruker Avance 300 UltraShield high-performance digital FT NMR spectrometer.Infrared spectra of prepared compounds were measured as KBr pellets in the range 4000-400 cm -1 on a Perkin-Elmer Spectrum One FT-IR spectrometer.MS spectra belong to seven benzimidazolium tetrafloroborates were performing by Agilent 1100 Series LC/MSD SL (Mass column ACE5 C18 S/N-A20826 250 4.6 mm, ACE-121-2546).Elemental analyses were performed by LECO CHNS-932 elemental analyzer.Melting points were measured using an electrothermal melting point apparatus, Electrothermal 9200.The dihedral angles of bridge-methylene carbon and hydrogen atoms of compounds 6 and 7. were calculated by the program of ChemDraw professional 2016 (page of launch Chem 3D calculations).
Synthesis.The mixture of benzimidazolium halides (2.30 mmol), NaH (2.50 mmol), and tetrahydrofuran (10 mL) was stirred in room temperature for 6h.After the reaction, the solvent was removed by vacuum and the crude product was extracted with toluene (10 mL).The solvent was removed again in yellow colored solution.
The yellow solid was washed with hexane and dried.The mixture of yellow solid, AgBF4 (2.46 mmol), and THF (10 mL) was stirred in rt for 30 min.The silver mirror formation was observed along with the precipitated gray solid.The product was crystallized in ethanol (30 mL).