Synthesis of Organic Cation and Metal Complexes for Degradation of Organophosphates and Anionic Species Recognition

Abstract

Imidazolium/benzimidazolium ionic liquids are well known for their catalytic applications. Ionic character of these compounds made them special in the field of energy storage and energy conversion and sensing. Another major area which explored in last decade is selfassembled amphiphiles (Figure 1). Due to amphiphilic nature of surfactants and ionic liquids; these are known for the construction of self-assembled aggregates such as micelles and vesicles in water. However, in presences of aromatic charged organic molecules; architecture, as well as photophysical properties of aggregates, greatly influenced. For the construction of self-assembled aggregates, various non-covalent interactions are involved such as hydrogen bonding, ionic bonding, van der Waals interactions and hydrophobic interactions. In planer aromatic compounds such as pyrene, anthracene, and naphthylamide another type of supramolecular interactions elaborate called  stacking which highly sensitive toward microenvironment. Alteration in photophysical properties upon complexation with the analyte is used to construct fluorescence sensors. Figure 1: Various applications of benzimidazolium cation Chapter 1: Introduction In this chapter, we have described the application of benzimidazolium cation for sensing and catalytic applications. Most of the ionic liquids are soluble in water, whereas upon interaction with anionic surfactants at particular concentration aggregates are formed. Photophysical properties also change dramatically in aggregation state. Some compounds enhance its fluorescence intensity in aggregation state due rigidity induced in the system which restricts the non-radiative emission. This phenomenon is called aggregation-induced emission enhancement. The completely opposite phenomenon occurs in some cases called aggregation-caused quenching which takes place due to charge transfer in aggregation state resulting in non-radiative decay. Among these AIEE molecules are more important for the development of fluorescence sensors as it has more quantum yield. However ionic liquids are widely used as a catalyst for various organic transformations, the solubility of the substrate in an aqueous medium still a challenge to acquire totally green synthesis of a product. Therefore to solubilize the reactant in water, anionic surfactants are conjugated with an ionic liquid to construct micelles which solubilize the reactant in water. Using this method green synthesis of organic compounds can be attained by using the catalytic amount of aggregates. Various anionic surfactants are available for this purpose. Mostly used surfactants are Sodium Dodecylbenzenesulfonate (SDBS) and Sodium dodecyl sulfate (SDS). Ionic liquids have a wide range of applications in the biological system also. Imidazole and benzimidazole itself have antibacterial properties, conversion of these into imidazolium or benzimidazolium provide another advantage. These positively charged units have a great affinity for phosphate unit. The bacterial cell wall is made up of phospholipids which also can interact with ionic liquids. Such kind of interaction causes penetration of ionic liquids inside cell wall and at the end breakage which further enhance the antibiotic activity. It was observed that micelles constructed from ionic liquids and anionic surfactant more capability to kill bacteria. The most favorable aspect of these compounds is their biocompatibility. Chapter 2: Synthesis of imidazolium cations based receptors for recognition of anionic species in the purely aqueous medium. This chapter includes design and synthesis of some imidazolium/benzimidazolium cations based receptors for sensing of anionic species. Due to the diverse range of anion shapes and sizes, their recognition using an organic receptor is a difficult task. The recognition of anions is problematic in an aqueous medium, particularly when using a receptor relying purely on hydrogen bonding, because of competition between anions and polar solvent molecules for binding sites on the receptor. Ion-pair recognition may improve the binding affinity of an anion by providing extra electrostatic interactions from the cation, as electrostatic interactions are generally less influenced by solvent than hydrogen bonding. Another advantage of extra electrostatic interactions is the cooperative effect (Figure 2). The ion-pair receptor has sites for a cation and an anion. Figure 2: Design of imidazolium conjugated benzimidazole-based receptor for anion recognitions Upon binding of one guest molecule (cation), the affinity of the receptor for the second guest (anion) is increased. Here, we developed a simple organic receptor N1 that has proficient binding sites for cation recognition. The distal part of the receptor is tailored with an imidazolium group that is well-known to provide anion recognition. Another advantage of this approach lies in the fact that the imidazolium moiety may impart solubility of the receptor in an aqueous medium. Sensor activity in aqueous media is mandatory if the targeted analyte is biologically or environmentally important. Chapter 3: Synthesis of benzimidazolium based cyclic and non-cyclic receptors for degradation of chemical warfare agents Due to unfettered use of pesticides in agriculture, its complete degradation is vital to avoid the harmful effect. Most of the pesticides used are organophosphates, which act on inhibition of acetylcholinesterase. For their degradation at environmental conditions, catalysts are needed. In this contrast, organic cations are particularly interesting building block for designing Organ catalyst that can accommodate organophosphates in their anionic cavity. In this work, we have designed and synthesized novel benzimidazolium based fluorescent calix using anion mediated preorganization approach (Figure 3). The prepared receptor was further explored for its catalytic activity in degradation of pesticides. From Single crystal structure of the receptor, it was observed that it can encapsulate DMSO through hydrogen bonding interactions, provided by benzimidazolium cations. In a similar way, the receptor will interact with organophosphates. The degradation studies were performed using UVvisible spectroscopy by monitoring the absorbance signature of para-nitrophenol (degrade product).The mechanism of sensing was fully validated using 31P-NMR spectroscopy as well as GC-MS; which highlights the transformation of Paraoxon into diethylhydrogenphosphate. In another series, we have chosen benzimidazolium based dipodal receptors for the formation of ionic self-assembled fluorescent aggregates. Benzimidazolium based receptors containing 2-mercaptobenzimidazole and 2-mercaptobenzothiazole as podes were prepared and incorporated into aggregates using anionic surfactants (SDBS and SDS). Upon aggregates formation, non-fluorescent organic cations show a significant increase in emission intensity due to aggregation-induced emission enhancement phenomenon. Due to this conformation changes Photophysical properties as well as a binding infinity of receptor changes. As independent organic cations did not show any significant change in fluorescence profile, however in aggregates form both of the organic cations Benz-2 and Benz-3 show large enhancement with diethylchlorophosphate. The shift in the signal of 31P-NMR, confirm the binding of the receptor with the analyte. As receptor Benz-1 and Benz-4 are not showing any selectivity for organophosphate which confirms that benzimidazolium (base) and 2- mercaptobenzimidazole (pode) play an essential role in binding.

Figure 3: Design of benzimidazolium based cyclic and non-cyclic receptors Chapter 4: Design and synthesis of copper complexes of benzimidazolium cations for detoxification of organophosphates In this work, hydrolysis of a series of organophosphates (Parathion, Paraoxon, Azamethphos, and Chlorpyrifos) has been achieved using copper (II) complex of benzimidazolium based pincer type ligand at ambient conditions (Figure 4). Three square planer copper (II) complex of pincer-type ligand has been synthesized and characterized using single crystal XRD. The solution state studies of the prepared complex were carried out using UV-visible spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. Among this one complex contain non-ionic ligand (N3) whereas another two complexes have benzimidazolium ionic liquid as base attached with two 2-mercaptobenzimidazolium podes (N1 and N2). The metal complex was designed in such a way that copper ion provides ionic interaction for organophosphate and benzimidazolium moiety interact through hydrogen bonding which effectively increases the electrophilicity of phosphate ion and increase the rate of hydrolysis. All of these complexes were evaluated for their catalytic properties for degradation of organophosphates. It was interesting that the complex containing ionic ligand efficiently degrades phosphorothionate pesticides whereas complex N3 was not found to be appropriate for degradation due to less conversion rate. The degradation studies were carried out using UV-visible spectroscopy by recording absorbance spectra of complex N1 with the addition of parathion with stirring for short interval of time. The increase in absorbance intensity at 410 nm corresponds to the conversion of parathion into para nitro-phenol and diethylhydrogenphosphate. The conversion rates also monitor using gas chromatography and mechanism was established using 31P-NMR spectroscopy.12 Figure 4: X-ray structure of benzimidazolium based polymeric and mono-podal copper complex. Chapter 5: Synthesis of benzimidazole/imidazole-based metal complexes for sensing of anionic species in semi-aqueous medium Detection of anionic species in aqueous medium becomes an imperative goal for supramolecular chemistry because these are pervasive in a biological system and play important role in environmental monitoring and diagnosis. Unlike cations; anionic species have different shape and size, hence anionic biomolecules require complementary receptors to encapsulate. The biologically important anionic species such as phosphates, amino acids, and nucleic bases; due to their diverse size and shape demand the receptor design which is sometimes tedious to develop using synthetic skills. The literature reports reveal the synthesis of organic receptors that can detect analyte through anion induced fluorescence modulations. Most of these organic receptors are fabricated of urea/thiourea group or imidazolium organic cation. The rationale of insertion of urea/thiourea in receptor design lies with the fact that these moieties have a tendency to form hydrogen bonds with anionic analytes. However, on the other hand, it has been observed that the receptors relaying on purely hydrogen bonding have limited application while working in an aqueous medium due to the competition provided to binding site from water, and moreover hydration of anionic species. Therefore, the design of a sensor for biomolecules, particularly in an aqueous medium, is still a challenge to inorganic-analytical chemistry. Here we have developed some benzimidazolium/imidazolium based metal complexes for detection of biomolecules (Figure 5). Due to the high sensitivity of non-covalent interactions towards microenvironments, modulation in such kind of interactions can be promising for construction of highly selective and sensitive sensor for various analytes. Among the fluorescent aromatic compounds, pyrene is one of the most interesting moieties, well known for  stacking and extraordinary Photophysical properties. Here we have synthesized a pyrene-based organic receptor which shows large enhancement in fluorescence intensity with zinc metal ion, due to the cancellation of PET mechanism caused by a lone pair of benzimidazole. Further, the interaction of prepared zinc complex of pyrene was evaluated with anionic species. The complex selectively detects ATP with the lowest detection limit of 15 nM. It revealed that upon interaction with ATP the  stacking between pyrene rings breakout which results in a decrease in excimer emission at 470 nm and increases in monomeric emission intensity at 410 nm. Also AFM images of receptor shown that upon addition of ATP to the R3 solution, solvent-mediated aggregation takes place which results in ratiometric detection.

Figure 5: Crystal structure of various metal complexes Similarly, A highly emissive Cu(I) complex of 2-mercaptobenzimidazole N1 for sensitive and selective sensing of iodide was developed. Iodide ions form bridges between molecules of N1 via hydrogen bonding interactions with the N-H groups of the benzimidazole moieties, resulting in aggregation and thus reduction of emission intensity. Complex N1 proved to be reusable by extracting the iodide ions with silver nitrate. The presented method offers several advantages over the reported methods, such as emission at longer wavelengths, low LOD, and efficient application to real samples. Thus, the interference caused by organic fluorescent molecules that emit at long wavelengths is minimized.