Furthermore, the solid crystal structure of halide complexes of the receptor is also reported. The solid crystal structure of the anion complexes with L2 reveals that the anions are encapsulated in the dimeric rigid capsular assembly of the receptor via N-H···anion interactions.
Conclusion
Introduction
Anion receptor chemistry
- Chromogenic anion selective acyclic receptors
- Anion coordination and anion directed assembly of multi-armed acyclic receptors Although Nature exemplify how proteins can selectively and efficiently bind anions by weak
- a Tris(2-aminoethyl) amine based tripodal receptors
- b Arene based multiarmed acyclic receptors
The highly extended electronic conjugation of the planar deprotonated species results in absorption in the NIR region (λmax equal to 904 nm and 936 nm for 3a and 3b, respectively). Furthermore, the oxygen of the encapsulated water has a strong lp(lone pair)∙∙∙π interaction with the pentafluorophenyl rings, resulting in ditopic recognition of F– and H2O in the small cavity of 5a (Figure 1.5a).
Ion-pair receptor chemistry
- Ion-pair recognition by acyclic receptors
- a Pridylureas and bipyridine-bis(urea) based ion-pair receptors
- b Tripodal and hexapodal ion-pair receptors
The classification of ion pair receptors depends on how they bind the cations and anions of the target ion pairs. When the solution temperature was cooled to 298 K, the DOSY spectrum exactly matched the spectrum of the bound ion pair complex.
Concluding remarks
The hydrodynamic radii calculated from the diffusion measurements were also in good agreement with those of the average of the dimension calculated from the single crystal X-ray crystallographic structure. 60 The crystal structure of the ion pair complex showed that four of the pyrazoles of the receptor are on the same side of the central benzene ring (upper cleft), and among them two pyrazole units are hydrogen bonded to NH4+.
Materials
This chapter provides a detailed report of the various reagents used in the synthesis of acyclic receptors, H2L1 and L2-L4 (Scheme 2.1), their synthesis procedures, crystallization details and specifications of instruments/equipment used in the characterization of synthesized receptors and their various complexes with anions are presented.
Experimental methods
Single crystal X-ray crystallography
- Tripodal thiourea receptor, L 2
- Tripodal amide receptor, L 3
- Tripodal urea receptor, L 4
Then, the volume of the solvent (THF) was reduced in vacuo using a rotary evaporator, and the solid product obtained was filtered off and washed with 10 mL of dichloromethane (DCM) a few times to remove the unreacted starting materials and impurities. Then, the volume of the solvent (THF) was reduced in vacuo using a rotary evaporator, and the solid product obtained was filtered off and washed with 10 mL of dichloromethane a few times to remove the unreacted starting materials and impurities.
- Complexes of receptor H 2 L 1
- Complexes of receptor L 3
- Complexes of receptor L 4 Sulfate complex, [(L 4 H) 2 +
After the addition of HF, the solution was stirred for about 1 hour and allowed to evaporate slowly at room temperature. After the addition of HF, the solution was stirred for about 1 h and allowed to evaporate slowly at room temperature, yielding dark yellow block-shaped crystals suitable for XRD analysis within one week.
A Fluoride Selective Chromogenic Receptor with a Thiadiazole Spacer
- Selective chromogenic sensing of fluoride anion
- Halide binding study by 1 H NMR spectroscopy
- Structural aspects of anion binding with H 2 L 1
- X-ray crystal structure analysis of receptor halide complexes .1 Structural description of fluoride complex (1a)
- Structural description of chloride complex (1b)
- Structural description of bromide complex (1c)
- Conclusion
Moreover, the significantly high fluoride-induced red shift (243 nm) of the receptor is likely due to its planarity and conjugation, which help to complete the delocalization of the negative charge(s) of the deprotonated receptor. The 1H NMR titration spectra of the receptor H2L1 in the presence of increasing amounts of [Bu4N+]Cl− and [Bu4N+]Br− are shown in Figure 3.11 and Figure 3.13, respectively. Three anion complexes of the H2L1(1a-1c) were obtained as single crystals suitable for X-ray diffraction analysis by slow evaporation of individual acetonitrile (CH3CN) solutions of H2L1 in the presence of excess tetrabutylammonium salts of various halides (F−, Cl− , and Br −).
Furthermore, one of the orthoaryl hydrogens (C5H) with respect to the nitro group of the twisted nitrophenyl ring forms a significant C−H···Cl− hydrogen bonding interaction with the neighboring Cl− (2) anion.
Encapsulation of Sulfate and Thiosulfate within the Dimeric Capsular Assembly of
- Structural description of L 2
- Structural aspects of anion binding with L 2
- Structural description of SO 4 2−
- Structural description of S 2 O 3 2−
- Sulfate and thiosulfate binding studies by 1 H NMR spectroscopy
- Conclusion
These versatile applications of the thiosulfate anion make it an important target analyte for recognition. The solid state crystal structure of both the anions with L2 shows that the anions are encapsulated within the dimeric capsular assembly of the receptor with optimal N–H∙∙∙O and N–H⋯S hydrogen bond coordination. It is interesting to observe that both the anions are trapped within the rigid supramolecular dimeric capsular assembly of the receptor L2 via N-H⋯anion interactions.
Interestingly, in both cases colorless crystals of the sulfate complex of L2 crystallize in good yield.
Recognition of Anion/Anion-Water Cluster by a Tripodal Amide Receptor
Structural aspects of anion binding with L 3
- Structural description of chloride complex (3a)
- Structural description of bromide complex (3b)
- Structural description of iodide complex (3c)
- Structural description of hexafluorosilicate complex (3d)
- Structural description of hexafluorosilicate complex (3e) Interestingly, another SiF 6 2−
Single crystals of the chloride complex [(L3H)+∙Cl∙H2O] (3a) were obtained by slow evaporation of a DMF solution of L3 in the presence of excess hydrochloric acid (HCl). These hydrogen bonds are the main driving forces in the formation of the dimeric capsular assembly. Single crystals of the chloride complex [(L3H)+∙Br∙H2O] (3b) were obtained by slow evaporation of a DMF solution of L3 in the presence of excess hydrobromic acid (HBr).
Single crystals of the iodide complex [(L3H)+∙ I−] (3c) were obtained by slow evaporation of a DMSO solution of L3 in the presence of excess hydroiodic acid (HI).
Although the solid-state crystal structure of the iodide complex (3c) shows the presence of weak hydrogen bonding interactions between amide functions of L3H+ and iodide anion. The mode of receptor-anion interactions and the anion-induced electronic and conformational changes of the receptor can be illustrated by the 2D NOESY NMR experiment, which is a traditional and very useful tool in the field of supramolecular host. To better understand the solution state that binds the receptor to the anions, we therefore performed 2D NOESY NMR experiments for both free receptor (L3) and anion complexes in DMSO-d6 at room temperature.
The NOESY spectra of both the chloride and bromide (3a and 3b) complexes show significantly strong NOE coupling between the amide NH protons and water (H–O–H), whereas no such type (N–H∙∙∙ H–O) –H) of interaction is observed in the case of the free receptor (L3) and complex 3c (iodide complex).
Conclusion
Probably the larger size and lower charge density of the iodide ion is responsible for this conformational change in the receptor, which actually helps the formation of the 1D polymeric structure upon iodide coordination. The thermal stability of the halide–water cluster in complexes 3a and 3b has been investigated by thermo-gravimetric analysis, which shows that both complexes are stable above 150 °C, indicating strong hydrogen bonding between the anion–water cluster and the host molecules. The NOESY spectra of both complexes show significantly strong NOE coupling between the amide NH protons and water (H–O–H), suggesting encapsulation of hydrated halide anion(s) (Cl− or Br−) via hydrogen bonding interactions within in its tripodal hole.
In 3d, the SiF62 anion is encapsulated in dimeric capsular assembly of the protonated receptor, whereas in 3e SiF62 anion is located outside the tripod cavity and is mainly stabilized by N−H⋯F and C−H⋯F hydrogen bonding interactions with four receptor cations.
A Heteroditopic Tripodal Urea Receptor for Recognition of Ion-pairs
Structural description of L 4
Analysis of the crystal structure of L4 shows that the three tripodal side arms of both conformational isomorphs (C1 and C2) point in three different directions without any intramolecular interactions, which indicates the disorganized nature of the tripodal L4 receptor in the guest (anion/cation). ) free balance. For both conformers (C1 and C2), two urea functions form similar bifurcated hydrogen bonds with the oxygen atoms of two DMF molecules individually, while the third urea function forms bifurcated hydrogen bonds with a water molecule in the lattice, which is additionally hydrogen bonded to a bridging nitrogen atom and the urea oxygen atom act on neighboring receptor molecules (Figure 6.1b). The donor–acceptor distances of these hydrogen bonds are <3.0 Å and details of the hydrogen bond interactions involved in the crystal structure of L4 are given in (Table 6.2).
The second unit of the asymmetric unit and solvent molecules are omitted for clarity; (b) Showing hydrogen bonding interactions between urea functions of the receptor L4 and crystallizing solvent molecules (DMF and water); (c) Shows packing diagram of L4 along an axis.
Structural aspects of anion and ion-pair binding with L 4
- Structural description of complex 4a
- Structural description of complex 4b
- Structural description of complex 4c
The encapsulated TBA cation is stabilized by C−H⋯π, C−H⋯O and cation⋯π interactions with the phenyl rings and the urea functions of two side arms of the protonated tripod receptor L4H+ (Appendix 6). The detailed coordination environment of SO42− indicates that it is stabilized by 14 hydrogen bonds involving seven N−H⋯O (three urea functions), two O−H⋯O (lattice water molecules) two C−H⋯O (aryl –CH of nitrophenyl ) ring) and three C−H⋯O (TBA cation) (Figures 6.3b and 6.3c). Interestingly, all six urea arms within a capsule participate in hydrogen bonding with the encapsulated hydrogen phosphate ion (Figure 6.4d).
The ideal binding mode for the tetrahedral oxyanion (sulfate and phosphate) is encapsulation of the anion via 12 optimally arranged binding sites.1a,3b,5 Detailed crystallographic analysis of the complexes (4a, 4b and 4c) shows that the metal-free receptor ( in the case of complex 4a and 4b) is unable to provide ideal binding sites for these tetrahedral oxyanions (Figure 6.5).
Solution-state 1 H NMR binding studies
Stability constant values for anion binding in the presence and absence of alkali metal cations (1eq Li+, 1eq Na+, 0.5eq K+ and 1eq K+) were determined by EQNMR7 analysis (Appendix 6) of titration curves, monitoring field shifts of urea. L4 receptor protons. This is due to the formation of a weakly bound 1:1 receptor-K+ complex, which allows ion pairing outside the receptor that actually prevents anions (HSO4-. In the presence of Group I metal ions such as Li+(1 eq) , Na+(1eq) and K+(0.5eq) receptor L4 shows a remarkable increase in binding properties to hydrogensulfate and dihydrogenphosphate anions compared to the metal-free receptor, reiterating the ditopic nature of this receptor.
This is due to the formation of weakly bound 1:1 receptor-K+ complex, which allows ion pairing outside the receptor.
C onclusion and Future Perspective __________________________________________________________________
The crystal structure of the sulfate complex in the solid state shows the encapsulation of a single DMF solvent molecule in the tripodal cavity of the protonated receptor, while the SO42− anion is located outside the tripodal cavity and stabilized by N−H∙∙∙O hydrogen bonds from urea. functions of the four receptor cations. Interestingly, the receptor forms a remarkable C3v symmetric complex of charge-separated ion pairs with potassium cation and hydrogen phosphate anion by forming a receptor dimeric capsule assembly, where three nearby K+ encapsulated dimeric capsule assemblies intertwine to form a precise cavity for HPO42- encapsulation. The thesis thus demonstrates some central and unpredictable concepts of supramolecular chemistry and the versatility of acyclic receptors as an effective building block for creating molecular assembly.
Research in these areas with a focus on technological and biomedical applications, based on the remarkable binding molecular assembly of anion and ion pair, seems to be forthcoming.
L ist of Publications __________________________________________________________________
C ontents for Annexure
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