Benzimidazolines Convert Sulfur Dioxide to Bisulfate at Room

Jun 2, 2017 - SO2 is a precursor to acid rain that poses risks to monumental structures such as the Tajmahal in India and Mayan temples in Mexico, as ...
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Research Article pubs.acs.org/journal/ascecg

Benzimidazolines Convert Sulfur Dioxide to Bisulfate at Room Temperature and Atmospheric Pressure Utilizing Aerial Oxygen Sonam Mehrotra,† Sakthi Raje,† Anant Kumar Jain,† and Raja Angamuthu*,† †

Laboratory of Inorganic Synthesis and Bioinspired Catalysis (LISBIC), Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: By employing a simple strategy of reacting SO2 gas with easily attainable hydride donors such as 2-substituted-1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole, benzimidazoline and SO2 were converted into benzimidazolium bisulfate at room temperature and atmospheric pressure. Bisulfate originated from SO2 and hydride from benzimidazoline and aerial oxygen. Metastable dimers of bisulfate anions were observed in the solid state and in solution where the anions are not stabilized by encapsulation in cages but through hydrogen bonding from benzimidazolium cations. All three benzimidazolines and resulted benzimidazolium bisulfates have been characterized using 1H and 13C NMR spectroscopy, highresolution electrospray ionization mass spectrometry, and single crystal X-ray diffraction techniques. KEYWORDS: Sulfur dioxide activation, Sulfuric acid, Benzimidazole, Bisulfate, Metastable dianions, Smog formation

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INTRODUCTION Sulfur dioxide (SO2) is one among the small molecules that are detrimental to the environment and poses risks to human health. Prolonged exposure to SO2-contaminated air can cause respiratory and skin diseases. Oxides of sulfur can react with other molecules in the atmosphere to form fine particles that reduce visibility (haze). SO2 is a precursor to acid rain that poses risks to monumental structures such as the Tajmahal in India and Mayan temples in Mexico, as sulfuric acid in the acid rain reacts with the marble exterior (CaCO3) and converts it into more soluble calcium sulfate (solubility in water: CaCO3, 1.3 × 104 mol/L; CaSO4, 4.5 × 102 mol/L). As a consequence, environmental chemists around the world are toiling to discover greener technologies to sequester and activate SO2 into value-added commodities. Many strategies have been developed to sequester and/or activate SO2 using electron-rich amines,1−4 ionic liquids,5−9 and highly reactive metal−alkyl/aryl complexes10−28 of electronrich noble metals such as rhodium, ruthenium,29 platinum,30,31 iridium,32 metal−organic frameworks,33 multicomponent assemblies,34,35 etc. Interesting activation of SO2 was reported by Darensbourg et al., where the electron-poor S of SO2 (OS+− O ↔ O−S+O ↔ O0.5−S+−O0.5) binds on an electronrich Ni(II)-bound thiolate center through S···SO2 interactions and undergoes S-oxygenation utilizing aerial oxygen to yield sulfate; two electrons resulted from the sacrificial disulfide formation of an expensive ligand (eq 1).36−38 Industry uses vanadium-based catalysts and temperature as high as 400 °C to produce sulfuric acid from SO2 (SO2 → SO3 → H2S2O7 → H2SO4). © 2017 American Chemical Society

Recently, we have utilized benzothiazolines and its coordination complexes in aerial oxygen-involved activation of SO2 into sulfuric acid at room temperature and atmospheric pressure.39 Benzothiazolines undergo aerial oxidation spontaneously to yield benzothiazoles, which hampers storing them for prolonged periods. We have managed to avoid this problem by installing benzothiazolines as ligands on transition metals to store them as stable coordination complexes.39 However, we sought to circumvent the use of transition metals in the process as they may pose risk to the environment. Thus, we are in continuous search of hydrogen or hydride donors that are easily attainable and weaker reductants than benzothiazolines to avoid spontaneous aerial oxidation.40 In the aforementioned search, Received: May 12, 2017 Revised: May 23, 2017 Published: June 2, 2017 6322

DOI: 10.1021/acssuschemeng.7b01495 ACS Sustainable Chem. Eng. 2017, 5, 6322−6328

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ACS Sustainable Chemistry & Engineering we stumbled upon benzimidazolines as sacrificial hydride donors, which are weaker as hydride donors than highly sensitive benzothiazolines (Figure 1).41−44

Scheme 2. Representative Reaction of SO2 with Benzimidazolines Showing the Formation of 1,3-dimethyl-2phenyl-1H-benzo[d]imidazol-3-ium bisulfate (MB1HSO4) from 1,3-dimethyl-2-phenyl-2,3-dihydro-1Hbenzo[d]imidazole (MB1-H) upon Interaction with SO2a

Figure 1. Structures of benzimidazolines used in the present work.

Benzimidazoles are an important class of organic compounds that are seen in many natural products and constitute parts of a large number of highly sold drug molecules (Scheme 1). Bilastine (rhinoconjunctivitis and urticarial drug),45 Emedastine (allergic conjunctivitis), Clemizole (itching), and Picoprazole (antisecretory) are important drugs where benzimidazole moiety plays very crucial part.45 Omeprazole is used to treat gastroesophageal reflux disease (GERD), peptic ulcer disease, and Zollinger−Ellison syndrome.46 A large number of antiviral drugs (TIBO, anti-HIV-1),47 antidiabetic drugs (Rivoglitazone),48 vasodilators for heart failure therapy (Pimobendan),49 oral anticoagulants (Dabigatran),50 retinoic acid metabolism blockers (Liarazole),51 opioid analgesics (Etonitazene, Clonitazene), narcotic analgesics (Bezitramide), anticancer drugs (AT9283, Galeterone, Veliparib),52−54 and antimicrobial drugs (SMT19969)55 are also developed which comprise at least one benzimidazole moiety.

a

Crystallographic details are available in the Supporting Information (Figures S31 and S32; Table S1; CCDC-1541081).

readily (Scheme 2) that were characterized by 1H and 13C NMR spectroscopy, electrospray ionization mass spectrometric techniques (ESI-MS), and single crystal X-ray diffraction studies. The yellow colored benzimidazoline, 1,3-dimethyl-2phenyl-2,3-dihydro-1H-benzo[d]imidazole (MB1-H) with an oxidation potential of −68 mV (vs Fc+/Fc in acetonitrile at 298 K)58 was dissolved in dry acetonitrile at room temperature, and SO2 was purged for 20 min. The color of the solution turned red upon purging SO2 which was left to stand in open air. Initial characterization of the resulted solution was performed employing high resolution ESI-MS, with envelops of m/z = 223.1236 in positive ion mode corresponding to 1,3-dimethyl2-phenyl-1H-benzo[d]imidazol-3-ium cation (calculated m/z = 223.1235; Figures S4 and S7) and 96.9582 in negative ion mode corresponding to bisulfate anion (calculated m/z = 96.9596; Figures S8 and S9) were observed that unequivocally confirmed the loss of hydride from benzimidazoline (MB1-H)

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RESULTS AND DISCUSSION We have studied the reactions of benzimidazolines such as 1,3dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (MB1H), 2-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)phenol (MB2-H), and 2-(6-bromopyridin-2-yl)-1,3-dimethyl2,3-dihydro-1H-benzo[d]imidazole (MB3-H) with SO2 in continuation of our efforts in small molecule sequestration56 and activation39,57 and report herein the activation of SO2 into bisulfate at room temperature and atmospheric pressure utilizing aerial oxygen (Scheme 2). Reactions of benzimidazolines MB1-H, MB2-H, and MB3-H with SO2 yielded corresponding benzimidazolium bisulfates

Scheme 1. Few Pharmaceutically Important Benzimidazole-Based45 Drug Molecules

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The ESI-MS envelope corresponding to the [HSO4¯···HSO4¯ + H+]¯ monoanion was more intense than the peak for the HSO4¯ anion that indicates the pronounced stability of the bisulfate dimer in solution even at harsh conditions of ESI-MS spectrometry (Figure 3, Figures S17−S19). In addition to the

to yield 1,3-dimethyl-2-phenyl-1H-benzo[d]imidazol-3-ium cation with simultaneous formation of bisulfate anion which is in line with the observed reactivity of benzothiazolines with SO2 reported by us.39 Interestingly, an envelope of m/z = 194.9268 (Figure S8) was also observed in negative ion mode which might correspond to the hydrogen-bonded dimer of two bisulfate anions in protonated form (calculated m/z = 194.9269 for [HSO4¯···HSO4¯ + H+]¯ monoanion; Figure S18). The observation of this metastable dimer made of two bisulfate anions is contrary to Coulomb’s law but stabilized by hydrogen bonding to each other.58 This encouraged us to look for the possibilities of isolating this metastable dimer due to a recent report from Flood et al. where a hydrogen-bonded bisulfate dimer was encapsulated in a cyanostar macrocyclic host.58 As MB1HSO4 does not have such a cage structure, observation of a bisulfate dimer was a great surprise to us. Hence, we set out to see the stabilizing factors of such a dimer in solution and utilized MB2-H in SO2 activation; MB2-H comprises similar properties as MB1-H and reacted with SO2 to yield MB2HSO4. MB2-H was chosen as the hydroxyl group might help to trap the intermediates involved and/or to stabilize the metastable dimeric bisulfate. To our surprise, the hydroxyl group indeed stabilized the bisulfate dimer, which is seen in the solid-state structure unequivocally (Figure 2). The distances observed in

Figure 3. Part of the ESI mass spectrum of MB2HSO4 (anion mode) showing the isotopic distributions of [HSO4¯···HSO4¯ + H+]¯ and HSO4¯ monoanions at 194.9240 (Calc. 194.9269), 96.9572 (Calc. 96.9596), and 433.0370 (Calc. 433.0375) daltons. Conditions: 20:80 solution of 0.1% formic acid in water (20) and acetonitrile (80), 80 °C source temperature, 22 V cone voltage, 3.5 kV capillary voltage. Complete details and full ESI-MS spectra are available in the Supporting Information (Figures S9, S13, and S16−S22).

envelopes for monomeric and dimeric bisulfate anions, we have observed envelopes for the anions [MB2+···(HSO4¯···HSO4¯)]¯ at 433.0370 (calculated m/z = 433.0375; Figures S17 and S19), [(MB2+···[HSO4¯···HSO4¯]···MB2+)−H+]¯ at 671.1506 (calculated m/z = 671.1482; Figures S17 and S20), [(MB2+··· [HSO4¯···HSO4¯]···MB2+)+HSO4¯]¯ at 769.1174 (calculated m/z = 769.1155; Figures S17 and S21), and [(MB2HSO4)3H+]¯ anion at 1007.2303 (calculated m/z = 1007.2262; Figures S17 and S22). To the best of our knowledge, this is the first time these many metastable anions have been observed to be stable in solution at harsh conditions. In order to unequivocally confirm that the presence of the hydroxyl group in the MB2 moiety was the reason for the stabilization of the aforementioned metastable anions of bisulfates even in solution, we have designed MB3-H with a bromopyridinyl substitution (Figure S23−S26). The design of MB3-H was also inspired by our previous results where we have employed 2-(pyridin-2-yl)-2,3-dihydrobenzothiazole to catch hold of the intermediate Bunte salt, S-(2-((pyridin-1-ium-2ylmethyl)amino)phenyl)thiosulfate.39 Though MB3-H was able to react with SO2 to yield MB3HSO4 in a similar manner as MB1-H and MB2-H (Figure 4, Figures S27−S30), stabilization of the bisulfate dimer was not observed in its ESI-MS spectra (Figures S29 and S30); this suggests that the hydroxyl group of MB2-H was keeping the bisulfate dimers stable in solution through hydrogen bonds.

Figure 2. Solid-state structure of 2-(2-hydroxyphenyl)-1,3-dimethyl1H-benzo[d]imidazol-3-ium bisulfate (MB2HSO4) obtained by the reaction of SO2 with MB2-H showing the bisulfate dimers.58 Selected distances: O(7)···O(12), 2.551; O(6)···O(9), 2.607; O(1)···O(11), 2.662; O(2)···O(10), 2.644; S(2)−O(8), 1.4315(16); S(2)−O(5), 1.4385(16); S(2)−O(7), 1.4761(15); S(2)−O(6), 1.5574(16); S(3)− O(11), 1.4452(16); S(3)−O(10), 1.4502(16); S(3)−O(9), 1.4640(15); S(3)−O(12), 1.5521(15). Crystallographic details are available in the Supporting Information (Figures S33 and S34; Table S2; CCDC-1541079).

the X-ray structure of MB2HSO4 suggests that the hydrogen bonds present are closer to the range of strong than moderate ones.59 The bisulfate dimer is further connected to two benzimidazolium through O−H···O hydrogen bonds (Figure 2). The cationic carbon center acquired trigonal planar geometry as seen in the solid-state structures of benzimidazolium bisulfate salts (Scheme 2; Figures 2 and 4) as a result of aromatization upon losing hydride from benzimidazoline to SO2. This can be clearly seen from the sum of the three angles (N1CN2, N1CCAr, N2CCAr) around the carbon which is ∼360° in MBXHSO4 (X = 1−3), whereas the same was 343.68° in MB2-H.

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CONCLUSIONS Benzimidazolines are shown to convert SO2 into bisulfate anions at room temperature and atmospheric pressure utilizing aerial oxygen-producing benzimidazolium bisulfates in moderate to good yields (51−91%), although benzimidazolines are 6324

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The linear absorption coefficients, the scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. Data integration and reduction were conducted with SAINT. An empirical absorption correction was applied to the collected reflections with SADABS using XPREP. Structures were determined by direct method using SHELXTL and refined on F2 by a full-matrix least-squares technique using the SHELXL-97 program package. The lattice parameters and structural data are listed in the Supporting Information. 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole. In 50 mL of Schlenk RB, N1,N2-dimethylbenzene-1,2-diamine (0.300

Figure 4. Solid-state structure of 2-(6-bromopyridin-2-yl)-1,3dimethyl-1H-benzo[d]imidazol-3-ium bisulfate (MB3HSO4) obtained by the reaction of SO2 with MB3-H. Selected distances: S(2)−O(8), 1.4315(16); S(2)−O(5), 1.4385(16); S(2)−O(7), 1.4761(15); S(2)− O(6), 1.5574(16); S(3)−O(11), 1.4452(16); S(3)−O(10), 1.4502(16); S(3)−O(9), 1.4640(15); S(3)−O(12), 1.5521(15). Crystallographic details are available in the Supporting Information (Figures S35 and S36; Table S3; CCDC-1541080).

g, 2.202 mmol) was dissolved in 5 mL of dry dichloromethane with activated molecular sieves 3 Å. The solution of benzaldehyde (0.233 g, 2.202 mmol) in dry dichloromethane (5 mL) was slowly added to prestirred dichloromethane solution at 0 °C under nitrogen atmosphere. The resulting mixture was stirred at room temperature for 24 h and filtered through cannula under nitrogen atmosphere. The yellow filtrate was concentrated and crystallized in dichloromethane solution. The resulting white crystals were filtered off and dried under high vacuum (0.050 g, 10%). 1H NMR (400 MHz, CDCl3): δH = 7.58 (m, 2H, Ar), 7.41 (m, 3H, Ar), 6.72 (dd, 2H, Ar), 6.43 (dd, 2H, Ar), 4.88 (s, 1H, CH), 2.57(s 6H, CH3). 13C NMR (100 MHz, CDCl3): δC = 142.23, 139.20, 129.46, 128.99, 128.60, 119.41, 105.85, 94.18, 33.30. High resolution ESI-MS: m/z for C15H15N2 = 223.1231 (calcd. 223.1235) = [MB1]+. 2-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)phenol. In 50 mL of Schlenk RB, N1,N2-dimethylbenzene-1,2-

weaker hydride donors than benzothiazolines that were used as organic hydride donors in our previous study.39 One of the benzimidazolium bisulfate salts formed (MB2HSO4) showed an interesting feature of stabilizing metastable dimers of bisulfate anions in solid state and in solution at harsh conditions; this was possible by the hydrogen bonding provided by the hydroxyl group of benzimidazolium cations to the dimeric bisulfate anions. To date, dimeric anions or clusters of anions have been observed only in solid state; only in one case has the bisulfate dimer been observed in solution where it is encapsulated in a cage.58 Further studies from our lab will focus on newly developed hydride donors that are weaker than benzimidazolines or benzothiazolines in activating small molcules such as SO2. Studies on stabilizing the metastable bisulfate dimers might shed light on the formation of haze, and aerosols through nucleation as bisulfate is among the most prevalent anions in the atmosphere due to its pronounced stability.60

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diamine (0.238 g, 1.747 mmol) was dissolved in 5 mL of dry dichloromethane with activated molecular sieves 3 Å. The solution of salicylaldehyde (0.213 g, 1.747 mmol) in dry dichloromethane (5 mL) was slowly added to pre-stirred dichloromethane solution at 0 °C under nitrogen atmosphere. The resulting mixture was stirred at room temperature for 20 h and filtered through cannula under nitrogen atmosphere. The yellow filtrate was concentrated and added dry ethanol (5 mL). The resulting pale yellow powder was filtered off and dried under high vacuum (0.060 g, 14%). 1H NMR (400 MHz, DMSO-d6): δH = 9.59 (s, 1H, OH), 7.48 (dd, 1H, Ar), 7.16 (dt, 1H, Ar), 6.85−6.56 (m, 4H, Ar), 6.40 (dd, 2H, Ar), 5.32 (s, 1H, CH), 2.46 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): δC = 157.97, 143.15, 130.60, 130.11, 124.62, 120.18, 119.97, 116.50, 106.86, 100.44, 34.20. High resolution ESI-MS: m/z for C15H15N2O = 239.1186 (calcd. 239.1184) = [MB2]+. 2-(6-Bromopyridin-2-yl)-1,3-dimethyl-2,3-dihydro-1Hbenzo[d]imidazole. In an oven-dried 50 mL Schlenk flask was added

EXPERIMENTAL SECTION

Materials. 1,2-Phenylenediamine (SDFCL), p-toluenesulfonyl chloride (SDFCL), pyridine (SDFCL), dimethylsulfate (SDFCL), NaOH flakes (Fisher), 6-bromopyridine-2-carboxaldehyde (TCI India), benzaldehyde (Rankem), and salicylaldehyde (SRL) were used as received from commercial sources. Solvents were distilled under dry nitrogen atmosphere using conventional methods. N,N′(1,2-Phenylene)bis(N,4-dimethylbenzenesulfonamide),61 N1,N2-dimethylbenzene-1,2-diamine,62 and 1,3-dimethyl-2-phenyl-2,3-dihydro1H-benzo[d]imidazole63 were synthesized by following literature methods. Methods. NMR spectra were recorded on JEOL 500 MHz and JEOL 400 MHz spectrometers. Temperature was kept constant using a variable temperature unit within the error limit of ±1 K. The software MestReNova64 was used for the processing of the NMR spectra. Tetramethylsilane (TMS) or the deuterated solvent residual peaks were used for calibration. Mass spectrometry experiments were performed on a Waters-Q-ToF-Premier-HAB213 equipped with an electrospray interface. Spectra were collected by constant infusion of the sample dissolved in methanol or acetonitrile with 0.1% formic acid. The freeware mMass was used to simulate the calculated isotopic distributions.65 Crystal Structure Determinations. Single-crystal X-ray data were collected at 123 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71069 Å).

2 g of activated 4 Å molecular sieves and 10 mL of toluene. It was stirred for 30 min at room temperature in N2 atmosphere and then was added N1,N2-dimethylenzene-1,2-diamine (0.108 g, 0.73 mmol), and the brown solution was heated at 100 °C for 2 h in N2 atmosphere. It was cooled to room temperature, and 6-bromopyr6325

DOI: 10.1021/acssuschemeng.7b01495 ACS Sustainable Chem. Eng. 2017, 5, 6322−6328

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m/z = 302.0290 (calcd. 302.0293) = MB3+ cation. High resolution ESI-MS (anion mode): m/z for HSO4 = 96.9590 (calcd. 96.9596) = bisulfate anion

idine-2-carboxaldehyde (0.136 g) was added as solid. Immediately the color of the solution turned from brown to yellow. The yellow solution was heated at 100 °C for 12 h and refluxed at 130 °C for 3 h in N2 atmosphere. The yellow solution was cannulated to a 50 mL round-bottomed flask, and the solvent removed under reduced pressure gave an orange yellow oil. It was dried under high vacuum for 1 h, giving an orange yellow solid, which was dissolved in diethyl ether and left in −35 °C, forming orange crystals suitable for single crystal X-ray diffraction (0.208 g, 94%). 1H NMR (400 MHz, CDCl3): δH = 7.83 (d, 1H, Py), 7.66 (t, 1H, Py), 7.52 (d, 1H, Py), 6.74 (dd, 2H, Ph), 6.45 (dd, 2H, Ph), 5.13 (s, 1H), 2.67 (s, 6H). 13C NMR (100 MHz, CDCl3): δC = 161.68, 141.98, 141.03, 139.60, 128.26, 121.54, 119.69, 106.10, 93.50, 33.96. High resolution ESI-MS: m/z for C14H13BrN3 = 302.0284 (calcd. 302.0293) = [MB3]+. 1,3-Dimethyl-2-phenyl-1H-benzo[d]imidazol-3-ium bisulfate. Yellow crystalline powder of HL (0.028 g, 0.124 mmol) was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01495. C16H20N2O5S (CIF) C20H20N5O5S (CIF) C14H14BrN3O4S (CIF) 1 H NMR, electrospray ionization mass spectra, single crystal X-ray diffrcation data, packing diagrams, and crystallographic details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

dissolved in dry acetonitrile (5 mL) and purged SO2 at room temperature for 20 min. While purging SO2, the color of the solution turned bright red. The resulting solution was stored at −30 °C for 1 day to facilitate the precipitation of 1,3-dimethyl-2-phenyl-1Hbenzo[d]imidazol-3-ium bisulfate, and evaporating the solvent gave a brick red precipitate. Crystals were grown in a mixture of methanol and acetonitrile at −30 °C (0.020 g, 51%). 1H NMR (400 MHz, DMSO-d6): δH = 8.15 (dt, 2H, Ar), 7.91 (m, 2H, Ar), 7.83 (m, 5H, Ar), 3.89 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): δC = 151.32, 133.92, 132.70, 131.73, 130.44, 127.62, 121.99, 114.39, 33.79. High resolution ESI-MS (cation mode): m/z for C15H15N2 = 223.1236 (calcd. 223.1235) = MB1+ cation. High resolution ESI-MS (anion mode): m/z for HSO4 = 96.9582 (calcd. 96.9596) = bisulfate anion 2-(2-Hydroxyphenyl)-1,3-dimethyl-1H-benzo[d]imidazol-3ium bisulfate. Yellow crystalline powder of HL (0.060 g, 0.249

ORCID

Anant Kumar Jain: 0000-0002-2981-6527 Raja Angamuthu: 0000-0002-5152-0837 Author Contributions

S.M. and S.R. contributed equally to this work. R.A. designed the work and wrote the paper. S.M., S.R., and A.K.J. executed the experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Ministry of Earth Sciences (MoES, India) and Indian Institute of Technology Kanpur (IITK/CHM/20120078) sponsored this research. S.M. and S.R. acknowledge the University Grants Commission (UGC, India) and Council of Scientific & Industrial Research (CSIR, India) for their Senior Research Fellowships. A.K.J. contributed to this work through Summer Undergraduate Research and Graduate Excellence (SURGE) program of IIT Kanpur. Tanay Batra and Tanmoy Seth are acknowledged for the artwork.

mmol) was dissolved in dry acetonitrile (4 mL) and purged SO2 at room temperature for 20 min. While purging SO2, the color of the solution turned brown. The resulting solution was stored at −30 °C for 1 day, and crystals were grown in 1 week at room temperature (0.076 g, 91%). 1H NMR (400 MHz, DMSO-d6): δ = 11.04 (s, 1H, OH), 8.12 (dd, 2H, Ar), 7.76 (dd, 2H, Ar), 7.67 (m, 2H, Ar), 7.22 (m, 2H, Ar), 3.87 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): δ157.58, 149.98, 135.96, 132.72, 132.63, 127.56, 120.87, 117.84, 114.34, 108.47, 33.54. High resolution ESI-MS (cation mode): m/z = 239.1167 (calcd. 239.1184) = MB2+ cation. High resolution ESI-MS (anion mode): m/z for HSO4 = 96.9572 (calcd. 96.9596) = bisulfate anion 2-(6-Bromopyridin-2-yl)-1,3-dimethyl-1H-benzo[d]imidazol3-ium bisulfate. Yellow powder of HL (37 mg, 0.121 mmol) was

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DEDICATION Dedicated to Professor Parimal Kanti Bharadwaj on the occasion of his 65th birthday. REFERENCES

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dissolved in dry acetonitrile (4 mL) and purged SO2 at room temperature for 20 min. While purging SO2, the color of the solution turned brown. The resulting solution was stored at −30 °C for 1 day, and crystals were formed immediately at −30 °C (0.041 mg, 85%). 1H NMR (400 MHz, DMSO d6): δH = 8.19 (d, 2H, Ph), 8.13−8.10 (m, 3H, py), 7.78 (dd, 2H, Ph), 4.06 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): δC = 146.80, 142.93, 142.18, 142.14, 132.98, 132.62, 128.94, 128.25, 114.69, 34.18. High resolution ESI-MS (cation mode): 6326

DOI: 10.1021/acssuschemeng.7b01495 ACS Sustainable Chem. Eng. 2017, 5, 6322−6328

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