CONTENTS
Chapter 4 Chapter 4 (Part B): Inclusion of Aldehyde or Oxime in Cadmium Coordination Polymer and Conversion of Aldehyde to Oxime
1.19. Aakeröy and his coworkers described structural details of various oxime molecules and elucidated various oxime synthons. 73 They performed systematic structural and spectroscopic
examination of the cocrystals formed between aldoximes R-C=N-OH (where R is H, Me, or CN) with N-heterocycles. Such studies have showed that the acidity of the oxime -OH hydrogen bond is a crucial factor to decide the efficacy of the supramolecular assembling.74 Careful manipulation of the substituents of oximes also enhances solubility of oxime derivatives. Thus enhanced solubility facilitates supramolecular synthesis.
Figure 1.19: (a) Less commonly encountered hydrogen bonded cyclic assemblies of oximes.
Hydrogen bonded chains formed through (b) O-H···N≡C and (c) O-H···NH2 interactions.
Structure directing effect caused by hydrogen bonds in a competing situation between hydrogen and halogen bond donors is shown by Aakeröy and his co-workers in different cocrystals of 3,3'-azobipyridine and 4,4'-azobipyridine with aromatic oximes with halogen
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atoms, carboxylic acids, phenolic -OH as substituents (Fig. 1.20a).75 In these examples different competitive binding interactions are observed such as halogen-nitrogen bonds, carboxylic acid-nitrogen bonds, cyanide-oxime interactions. These interactions compete with oxime-oxime interactions
Figure 1.20: (a) Different types of oximes; (b) Hydrogen and halogen bonding interactions in cocrystal and (c) One ternary cocrystal of oxime.
Due to such competitive effects, halogen substituted phenylaldoxime 1.28 (X= F, Br or I) forms three different 1:1 cocrystals with 1-methyl-2-(pyridin-4-yl)-1H-benzo[d]imidazole.76 Self-assemblies of these cocrystals have strong hydrogen bonds along with weak halogen bonds (Fig. 1.20b). Strong hydrogen bonds are between oxime -OH and nitrogen atom of 1- methyl-2-(pyridin-4-yl)-1H-benzo[d]imidazole. Whereas halogen bonds are between halogen atoms on the phenyl ring with nitrogen atom of 1-methyl-2-(pyridin-4-yl)-1H- benzo[d]imidazole.76 Hydroxy and cyano-oxime functionalized oxime (Z)-N,4- dihydroxybenzimidoyl cyanide 1.29 has two hydrogen bond donor sites forms hydrogen bonds with hydroxy and cyano-oxime groups.77 From the structural study of these cocrystals along with electrostatic potential surfaces calculation, it is established that hydroxyl group as a better hydrogen bond donor as compared to cyano-oxime. Ternary cocrystals were obtained from binary cocrystals of 1.30 by changing the substitution on to form various cocrystals.78 Compound 1.31 effectively binds to five-membered or six-membered N-heterocyclic molecules to forms cocrystals.79 Compound 1.31 forms a 2:3 cocrystal with 1, 2-di(4- pyridyl)ethane, the self-assembly of this cocrystal has a porous supramolecular three diemensional network structure.79 In another example, α,α',α''-tris(hydroxyimino)-1,3,5- benzene-triacetonitrile oxime 1.32 which has a three-fold symmetry with three coplanar
effective hydrogen bond forming moieties is a choice for porous organic networks.80
18 1.10: Oxime derivatives in molecular recognitions
The detection or removal of hazardous materials is important for environment as well as for human life. For specific detection and subsequent removal of hazardous materials, it requires specificity in binding by an analyte or in a sense molecular recognition can be a guiding factor to achieve them. One such potential example is selective interaction of oximes with poisonous nerve gas and phosphate containing insecticides. Nerve agents or phosphorous containing materials (Fig. 1.21) are some most dangerous materials for human or animal’s life. Since these compounds rapidly affect human and animal life by blocking the action of acetylcholinesterase (AChE), causes a critical problem to central nervous system. This enzyme is responsible for the breakdown of the neurotransmitter acetylcholine. The blocked enzyme becomes free by reacting with oximes.
Figure 1.21: (a) Nerve agents and (b) Synthetic mimics of nerve agents used as less toxic and (c) some toxic pesticides.
Rapid and selective detection of nerve agents are essential for environment and human life.
Several oximes are used to visualize the presence of nerve agents and related pesticides through changes in their fluorescence emissions or colours. The reactivity of an oxime without a pyridine unit attached to oxime towards nerve agents are better than a pyridinium oxime such as pralidoxime.81 Some oxime derivatives used as sensor for nerve agents are listed in Fig. 1.22a. Anslyn and his co-workers have developed certain oxime based sensors containing a fluorophore for the selective detection of nerve agents.82-83 Anslyn and co- workers showed that oximate are much better nucleophile than the hydrazine. They developed two simple chromogenic sensors 1.33a and 1.33b for such study.82 Phosphorylation of oxime (Fig. 1.22b) by reacting with nerve agent in a solution causes a hypsochromic shift of approximately 50 nm in basic medium, this change was observed through naked eyes (Fig. 1.22c). The compound 1.34 is an oxime and coumarin based sensor, which is non-fluorescent in basic condition due to the photoinduced electron transfer mechanism operative in the system on pH change.83
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Figure 1.22: (a) Some oximes that detect nerve agents; (b) Reaction between oxime and nerve agents and (c) Naked eye detection of nerve agents by oxime molecule.
Fluorescence emission of the anionic form of the compound 1.34 gradually increases upon addition of diisopropyl fluorophosphates (DFP). Rebek and co-workers have developed some ortho-hydroxy aromatic oximes associated with fluorescence properties that were utilized to sense nerve agents.84 Fluorescent arylisoxazole was formed by interaction of ortho-hydroxy aromatic oxime with nerve agents; due to formation of cyclized product there was sharp optical signal changes was found. N,N-Carbonyl-bridged dipyrrinone oxime 1.35 is a potential sensor for organophosphates. The deprotonated form of 1.35 shows a distinct crimson red colour by absorbing at 561 nm, which disappears upon addition of organophophates or pesticide. The toxic organophophates were detected by this oxime 1.35 through change of colour as well as changes in fluorescence emission properties.85 The oxime 1.36 containing a dye moiety as fluorophore is useful in the detection of nerve agent diethyl chlorophosphate (DCP) with a detection limit of 10 nM in buffer solution.86 This detection process occurs through cascade reactions by conversion of oxime to nitrile via an isoxazole intermediate. The transformation of 1.36 into fluoresceinyl oxime nitrile on reaction with nerve agent induces dramatic fluorescence enhancement, enabling one to detect by naked eye detection.
Churchill group developed a BODIPY-salicylaldehyde oxime 1.37 that showed high selectivity in binding with nerve agent diethyl cyanophosphonate (DECP) through fluorescence turn on mechanism.87 Song and his co-workers developed 6-substituted amino- quinolinoxime 1.38 for visual detection of nerve agent.88 Detection of nerve agent in this particular example passes through rapid phosphorylation-protonation reaction.
20 1.11: Oxime derivatives in detection of cations
Design of new chemosensors with high sensitivity for selective binding of metal ions is an attractive and challenging in research field. This is true for transition metal ions as they are of significance in environment pollution. Generally for selective binding of metal ion with a receptor, it should have some pre-organised metal binding sites. This criterion is easily fulfilled by designing functionalised receptors with different metal binding sites. Oxime functional group is more specific toward anion binding studies, as it has acidic proton, but recognitions of metal ions by oximes are rare. Compound (E)-7-(diethylamino)-2-oxo-2H- chromene-3-carbaldehyde oxime 1.39, detects copper(II) ion under physiological conditions.89 Compound 1.39 showed high selectivity toward copper(II) ions by forming a 2:1 copper(II) complex. Oxime based compound 2'-hydroxy-4-methoxy-5'-chlorochalcone oxime 1.40 (Fig. 1.23) reacts with palladium(II) to form a 1:2 palladium(II)-1.40 chelate complex. This complex is extractable at pH 2.5 with iso-butanol. The compound is unable to detect palladium(II) in presence of other interfering ions such as Au3+, Cr3+and Zr4+.90 Phenylazobenzaldehyde oxime 1.41 is another oxime based molecular sensor that shows a weak and strong absorption band at 430 and 550 nm upon addition of Pd2+ metal ion.91 A purple colored palladium(II)-complex of 1.41 is formed within 15 minutes remains stable for several days at room temperature or upon boiling.
Figure 1.23: Examples of oximes used in detection of metal ions.
Oxime-ether based compound 1.42 is useful for fluorometric and colorimetric detection of palladium(II). This compound shows high selectivity and sensitivity to detect Pd2+ in aqueous and biological samples.92
Compound 1.42 shows strong green fluorescence emission at 505 nm in aqueous medium (Fig. 1.24). Addition of solution of palladium(II) to a solution of 1.42 decreases the green florescence consistently. In this process an intramolecular charge transfer process is induced by forming stable palladium complex. 2,7-disubstituted phenanthrene-based bis-oximes
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1.43a and 1.43b are explored for heavy metal cation binding study. The fluorescence