CONTENTS
Chapter 4 Chapter 4 (Part B): Inclusion of Aldehyde or Oxime in Cadmium Coordination Polymer and Conversion of Aldehyde to Oxime
1.6: Neutral host for anion binding
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One representative example of a salt shown in Fig. 1.12 has a chloride ion interacting with cationic receptor 1.6b through anion···π interaction.43 On the other hand, weak σ interactions involving transfer of a fractional charge from anion to an arene where the anion is located above the periphery rather than the centre of arene are also known.44
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Anthraquinone units attached as signaling unit to urea and thiourea, namely compounds 1.10 and 1.11.50 Among these two compounds the thiourea derivative 1.11 is useful for naked eye detection of fluoride at room as it changes color on interaction with fluoride ions at room temperature. Whereas urea based receptor showed similar colour changes at 60 °C on addition of fluoride. This difference arises due to the difference in the self-assemblies of urea and thiourea derivatives. Kondo and co-workers have reported amide based neutral receptor 1.12 for sensing of dihydrogen phosphate.51 In mixed solvent DMSO-CH3CN (0.5:9.5, v/v) compound 1.12 showed strong binding affinity with dihydrogen phosphate (Ka=1,000,000 M-
1) over the acetate ion (Ka=17,000 M-1). Dihydrogen phosphate anion has hydrogen bond acceptor and donor sites to form stable complex with receptor 1.13.52 Receptor 1.13 has strong binding with dihydrogen phosphate (Ka=295M-1) over acetate (Ka= 251M-1) and benzoate (Ka=113M-1). It is interesting to mention that -NH proton of pyrrole group is 100 times less acidic as compared to the -NH proton of indole functional group.Hence, indole based receptors are more effective in anion binding in comparison to pyrrole based receptors.
An indole based receptor 1.14 shown in Fig. 1.13 has the ability to interact with fluoride ions selectively in a DMSO-water solution. The solid state structural study has provided evidence of a twisted geometry of the host while bound to fluoride ion.53 The foregoing examples show the varieties of interactions leading to formation of anion assisted assemblies which in turn affect color changes resulting in easy detection of selective anions. Though there are many anions assisted assemblies based on N-H or O-H containing compounds are also used in anion binding hosts for detection of basic anions. Hundal and co-workers have developed a tripodal catechol-based receptor 1.15 that showed high selectivity toward fluoride anion in DMSO with colour change from colourless to bright yellow.54 This was attributed to deprotonation of the hydroxyl groups based on 1H NMR studies and also form the fact on obtaining a similar optical response for the strong base TBAOH. Zhang et al. have developed chemosensor 1.16 for rapid and selective detection of CN- in aqueous medium. Chemosensor 1.16 showed strong fluorescence enhancement upon addition of CN- in DMSO/H2O (9:1, v/v) with high detection limit 4×10-7 M.55 A colorimetric fluorescent receptor 1.17 (Fig. 1.14) was reported by Chattopadhyay and co-workers for selective and sensitive detection of H2PO4-
anion in aqueous medium. Compound 1.17 showed an optical change from yellow to green upon addition of H2PO4-
anion.56 A bifunctional fluorescent chemosensor 1.18 was reported by Chang et al. for the selective detection CN- anion in MeOH solution through colorimetric as well as fluorometric changes.57
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Figure 1.14: Some neutral receptors for anions binding studies having -OH binding sites.
Chemosensors 1.19 was reported by Yang and co-workers having a hydroxyl group free for the selective binding of fluoride anion. Spectroscopic studies indicated that an aldehyde group in conjugation with the 1.19 core at an adjacent position to the hydroxyl group would elevate the sensitivity towards fluoride immensely.58 A new azo-azomethine receptor 1.20, containing active phenolic sites, has been designed and synthesized for quantitative detection and colorimetric sensing of inorganic fluoride ion in aqueous media.59 Compound 1.20 showed facile detection of fluoride ion present in aqueous media without any spectroscopic instrumentation with a detection limit 0.058 ppm through colorimetric.
These clearly establish that the anion binding abilities of molecules through hydrogen bond has a wide scope to prepare and understand new assemblies and unearthing novel properties.
In this thesis we focus on supramolecular aspects of oximes and following section is on the existing literate on various features associated with oxime functional group containing compounds.
1.7: General feature of oxime functional group
Oxime derivatives are well studied in organic and inorganic chemistry because of their versatile applications in various fields.60 Among various functional groups oxime is one of common functional group in organic and inorganic chemistry. Depending on the reactant, oxime derivatives are divided into different categories namely aldoxime (synthesised from aldehydes), ketoxime (synthesised from ketones) and amidoxime (synthesised from the amide). The general formula of oxime is R1R2C=NOH, where R1 is one organic side chain and R2 is hydrogen atom for aldoxime or it may be alkyl or aryl group for ketoxime.61 But in the case of amidoxime, general formula is R1C(=NOH)NR2R3.
Oxime molecules generally exist in two form syn and anti, depend on the position of the higher group and the -OH group. Aldoxime and ketoxime are found in both forms except aromatic aldoxime.
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Figure 1.15: Structures of different oximes.
Synthesis of oximes is very simple, as they are formed upon reaction of aldehydes or ketones with hydroxylamine. There are several procedures available for industrial synthesis of oxime from commercially available substrates in solid or liquid state.62