Organic Imine Cages : Self-Sorting and Application
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Biological systems have the incredible ability to accomplish uncommon chemical transformations with supreme delicacy. Many of those chemical transformations take place within the pocket of enzymes, which provide unique micro environment. From the quest of better understanding and to mimic such complex biological systems chemists have developed their own prototypes having well-defined cavity. To this end, in last few years many aesthetically elegant 3D discrete architectures have been devised by employing noncovalent interactions especially metal-ligand co-ordination and hydrogen bonding. Conversely, architectures based on purely covalent interactions are relatively limited in number, owing to the laborious traditional covalent synthesis, which involves multi-step synthetic protocols and irreversible covalent bond formations. Nevertheless, in recent times by utilizing dynamic covalent chemistry (DCC) several such organic 3D discrete ensembles have been developed with ease and efficiency from simple and easily accessible building blocks. Interestingly in most of such cases imine condensation reaction has been utilized due to easy formation and cleavage of the imine bonds in an efficient and reversible manner. However, it is quite surprising that even though the dynamic nature of imine bonds has been well established; self-sorting/self-selection process has been overlooked in organic cage systems. Self-sorting in biological realm is a well-established synthetic protocol. DNA double helix formation via hydrogen bonding between the complimentary base pairs is probably the best known example of biological self-sorting. Self-sorting process has the ability to discriminate self from non-self to achieve highly ordered architectures from within a random reaction mixture. The credit of self-sorting/self-selection process goes to the hidden ‘molecular instructions’ encrypted within the complimentary building blocks. The foremost objective of the present thesis work is to implement the self-sorting/self-selection protocol in organic cage formation by harnessing the dynamic imine chemistry. During the course of the investigation it has been observed that non-covalent interaction especially hydrogen bonding could manipulate the outcome of such a process. Besides that, selective formation of a single isomer of an organic cage from a reaction mixture of an unsymmetrical aldehyde and a flexible amine has been successfully achieved by simply fine tuning the geometric features (shape and size) of the reacting aldehyde. Such three-dimensional cages are well appreciated by the scientific community owing to their potential applications in anion sensing, catalysis and gas storage/separation. However, they have not been explored as sensors for nitroaromatic explosives. Therefore, at this juncture several fluorescent organic cages have been synthesized and their potential application as chemosensor for the nitroaromatics has been tested. Moreover, a new synthetic protocol has been introduced for the post-synthetic modification of organic cages. Chapter 1 covers a brief introduction about dynamic covalent chemistry with main emphasis on dynamic imine chemistry and its use in covalent cage synthesis. Moreover, this chapter accounts the very recent applications of such cage compounds in various fields such as a pours material for gas storage/separation, a molecular host for the stabilization of reactive species and for the recognition of ions or molecules. Chapter 2 describes first time ever achieved self-sorting process in three-dimensional purely organic cages. First of all, four different [3+2] cages were synthesized by treating two different triamines with two different dialdehydes separately, by employing dynamic imine chemistry. The formation of desired cages was ascertained by various spectroscopic techniques. When a mixture of all the four components (two aldehydes and two amines) was subjected to reaction, only two cages were found to form (Scheme 1) out of several equally probable possibilities, which suggest a high-fidelity self-recognition. The issue of partner preferences was further verified by transforming a non-self-sorted cage into a self-sorted cage by treating the former with appropriate triamine or dialdehyde. For an in-depth understanding on this subject, theoretical calculations (gas phase DFT) were carried out, which suggested that observed self-sorting is a thermodynamically governed process. Scheme 1. Self-sorting in organic imine cages through partner preferences. Chapter 3 focuses that supramolecular interaction especially hydrogen-boding could be a possible way to direct a self-sorting process operating in imine based organic cage systems. It is a well-accepted fact that in most of the cases self-sorting process operates owing to the difference in geometric features (shape and size) of the competing building blocks. Thus increasing similarity in geometric features would create the situation more complex. It is anticipated that in such circumstances H-bonding could have a decisive role in partner selection. In order to investigate this, four different dialdehydes (A, B, C and D) having similar geometric background were synthesized. These aldehydes upon treatment with flexible amine X were found to form three nanosocpic [3+2] organic cages (aldehyde C gave insoluble uncharacterized material). When a one-pot reaction of triamine X with mixture of all the four aldehydes was carried out, selective formation of cage B3X2 was observed (Scheme 2). Conversely, the same reaction in absence of aldehyde B resulted in the formation of mixture of products. Theoretical and experimental studies fully support the fact that the presence of hydroxyl moiety adjacent to the formyl group in aldehyde B has the key role in selective formation of cage B3X2 from a complex reaction mixture, in which there are numerous equally probable possibilities. Such remarkable selection was further examined by converting a non-hydroxy (non-preferred) cage into hydroxy cage B3X2 (preferred) by treating the former with aldehyde B. The role of the H-bond in self-sorting process of two dialdehydes and two triamines has been established. Furthermore, the possibility of cage–to- cage transformation through imine bond metathesis has also been addressed. Scheme 2. H-bond directed 15-fold (2+3) incomplete self-sorting in organic imine cages. Chapter 4 presents the investigation on the formation of single isomeric species of a [3+2] oligoimine cage from a reaction mixture of an unsymmetrical dialdehyde and a flexible triamine. So far, most of the reported organic cages are derived by symmetric building units. Asymmetric building blocks for the construction of such organic architectures are not the desirable choices, as they could lead to form mixture of isomeric cages. However, the asymmetric building blocks might form selectively one isomer only under the thermodynamic bias, which prefers the formation of one isomer over the other (s). In order to understand the factors that can direct such a process, three asymmetric dialdehydes (A, B and C) were synthesized and their reaction with a flexible amine X was carried out. Experimental outcomes suggested a striking difference in the abilities of isomer selection between aldehydes A/B and C. In case of aldehyde A/B selective formation of one oligoimine cage was observed, whereas aldehyde C led to form two isomeric oligoimine cages (Scheme 3). Experimental and theoretical findings have pointed out that the geometric features (shape and size) of the aldehyde play a decisive role in such isomer selection process. Scheme 3. Shape and size directed self-selection in organic imine cages. Part A of Chapter 5 describes the synthesis and characterization of a fluorescent organic cage compound and its application as a sensor for the detection of explosive picric acid (PA). Picric acid is known to be as explosive as trinitrotoluene (TNT) and one of the principle constituents of many unexplored landmines. Though there are several fluorescent polymers, metal-organic frameworks and small molecule based sensors have been devised in last few years but very little attention has been given towards selective and sensitive detection of picric acid. In this context desired organic cage compound 4 was synthesized by employing imine condensation between 4,4-diformyltriphenylamine (1) with 1,3,5-tris(aminomethyl)-2,4,6-trimethylbenzene (2) followed by reduction of the imine bonds (Scheme 4). This fluorescent nature of the cage in both the solid and solution has been utilized for the detection of nitroaromatic compounds (NACs). Among the various NACs tested it has been found that PA induces highest quenching of the initial fluorescence intensity of the cage solution. Furthermore, this cage has the ability to discriminate PA from other nitrophenolic compounds, such as 2,4-dinitrophenol (DNP) and 4-nitrophenol. In addition to solution phase detection cage 4 has also been successfully utilized for the solid phase detection of PA. The experimental results demonstrates that high sensitivity of the cage towards PA is attributed to the stronger ground state complex formation between the cage and PA as well as excitation v energy transfer (EET) process from protonated cage to the picrate. This represents the first report of a cage compound as a sensor for nitroaromatic compounds. Scheme 4. Synthesis of a fluorescent organic cage for the selective detection of picric acid. Part B of Chapter 5 reports a new synthetic methodology to decorate covalent organic cages post-synthetically, based on one-pot copper(I) catalyzed A3 coupling. A3-coupling is a three-component reaction between formaldehyde, secondary amine and terminal alkyne. In the present study selected organic cage 4 is furnished with six secondary amine moieties and thus it was allowed to react with 6 equiv. of formaldehyde and 6 equiv. of terminal alkyne in presence of CuI as a catalyst (Scheme 5). By employing this synthetic strategy parent cage 4 has been modified to cages 5a-c with appendages phenyl-, xylyl- and napthyl-actylenes. The resulting decorated cages were characterized by multinuclear NMR (1H and 13C), MALDI-TOF and FTIR spectroscopy. All the post-synthetically decorated cages were found to be fluorescent in nature and thus in v order to explore their potential use as a chemosensor for nitroaromatic compounds, cage 5a was tested. Experimental findings have suggested high selectivity of the cage towards nitroaromatic compounds. Interestingly, among the various nitroaromatics tested it has been observed that the cage is more sensitivity towards nitrophenolic compounds, whereas among the various nitrophenols tested, picric acid induced highest quenching. Scheme 5. Post-synthetic modification of an organic cage via cu+ catalyzed A3 coupling.
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