A Fluoride Selective Chromogenic Tris(amide) receptor
4.7 Structural aspects of anion binding with protonated L 3
Four anion complexes of protonated L3 (3c-3f) were obtained as single-crystals suitable for X-ray diffraction analysis by slow evaporation of individual DMSO solutions of L3 in presence of different mineral acids (HCl, HBr, HClO4 and H2SO4). Structural information obtained from XRD analysis of the isolated crystals provide insight into the proper binding topology of halides (Cl– and Br–) and oxyanions (ClO4–
and HSO4–
) with the protonated receptor molecule. Structural analyses revealed that, complexation of anion(s) are primarily governed by N-H∙∙∙A– and C-H∙∙∙A– interactions involving multiple receptor cations. In all four complexes, the receptor cavity is conformationally locked by intramolecular (N–H)+∙∙∙O hydrogen bonding formed between the endo-oriented hydrogen of protonated bridgehead
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nitrogen and one of the amide oxygen of protonated receptor. In addition, the complexes are further stabilized by several intermolecular C-H∙∙∙O-(nitro) hydrogen bonds, which induce rigidity to the formed cationic podand and serve as the foundation for crystallization of the desired complexes. The crystallographic data and refinement details of charged anion complexes 3c-3f are given in Table 4.3.
4.7.1 Chloride complex [(HL3)+•Cl–], (3c)
Complex 3c crystallizes in triclinic space group P-1 and the asymmetric unit contains two symmetry independent receptor cations (Z′ = 2) and two chloride anions. In both the conformers (C1 and C2), the endo-oriented proton of the apical amine is in strong intramolecular (N–H)+∙∙∙O hydrogen bonding with one of the amide oxygen of receptor cation [N1∙∙∙O11 = 2.718(2), ∠N1-H∙∙∙O11 = 151o(2) and N11∙∙∙O21 = 2.774(4), ∠N11- H∙∙∙O21 = 156o(2)]. Both the chloride ions, Cl-(1) and Cl-(2) are in interaction with two adjacent receptor cations of dissimilar conformations with a five-point attachment each (Figure 4.9a) via three N–H∙∙∙Cl- and two C–H∙∙∙Cl- interactions having an average donor-to- acceptor distance of 3.268 (N∙∙∙Cl-) and 3.589 Å (C∙∙∙Cl-) respectively (Table 4.4).
Examination of the coordination environment of the anion revealed that the amide hydrogen N8H and aryl proton C23H of conformer C1 are in coordination with Cl-(1) whereas the hydrogen atoms N2H, N5H and C5H are in interaction with Cl-(2). In a similar fashion, the amide hydrogen N12H, N18H and aryl proton C36H of the other conformer C2 provides a three point coordination to Cl-(1) while N15H and C41H make interactions with Cl-(2) completing the five-point attachment on each chloride anion (Figure 4.9a).
4.7.2 Bromide complex [(HL3)+•Br–]H2O, (3d)
Complex 3d crystallizes in monoclinic space group P21/c and the asymmetric unit contains two symmetry independent receptor cations (Z′ = 2) and two bromide anions with two lattice water molecules (O31 and O32) as solvent of crystallization. Identical to complex 3c, intramolecular (N–H)+∙∙∙O hydrogen bonding involving the endo-oriented apical proton and an amide oxygen is also prevalent in both the conformers (C1 and C2) of complex 3d (N1∙∙∙O6 = 2.903(4), ∠N1-H∙∙∙O6 = 152o(4) and N11∙∙∙O21 = 2.868(4), ∠N11-H∙∙∙O21 = 147o(3)). Binding of bromide with adjacent receptor cations clearly showed that, bromide ions Br-(1) and Br-(2) are in interaction with two receptor cations of identical symmetry with a five-point attachment each. Two amide protons N12H, N18H and an aryl proton C32H from a receptor cation with conformation C2, provides a three-point coordination to Br-(1) whereas hydrogen atoms N15H and C41H of another receptor cation with identical conformation provides the other two contacts on Br-(1) (Figure 4.9b). However, in case of Br-(2), two adjacent receptor cations of conformation C1 provide a four-point contact via TH-1135_7612222
. Chapter 4 one N–H∙∙∙Br- and one C–H∙∙∙Br- interactions each, while the fifth coordination contact is provided by a lattice water molecule O31 (Figure 4.9c) which is in strong hydrogen bond interaction with the amide hydrogen N8H [N8∙∙∙O31 = 2.952(7) Å and ∠N8-H∙∙∙O31 = 171o(4)].
Figure 4.9 (a) X-ray structure of complex 3c depicting the hydrogen bonding contacts on Cl– ions, Cl–(1) and Cl–(2) with two symmetry independent receptor cations; (b) and (c) X-ray structure of complex 3d is depicting the hydrogen bonding contacts on Br– ions, Br–(1) and Br–(2) respectively with two symmetry identical receptor cations. Symmetry independent receptor cations are shown in different colours.
4.7.3 Perchlorate complex [(HL3)+•ClO4–
]H2O•DMSO, (3e)
Complex 3e crystallizes in triclinic space group P-1 with one disordered DMSO and a water molecule as solvent of crystallization. In addition to electrostatic (N–H)+∙∙∙O hydrogen bonding [N1∙∙∙O1 = 2.742(5); ∠N1-H∙∙∙O1 = 168o(3)], there exists an intramolecular N–H∙∙∙O hydrogen bond between an amide hydrogen N5H and an amide oxygen O11 [N5∙∙∙O11 = 2.984(4); ∠N5-H∙∙∙O11 = 167o(4)] from two different arms of a receptor cation. Hydrogen bonding contacts on ClO4-
showed a six-point coordination of the anion provided by two adjacent receptor cations and lattice DMSO which is hydrogen bonded to the lattice water molecule (Figure 4.10a). Perchlorate oxygen O17 is engaged in a trifurcated hydrogen bonding contact with the amide hydrogen N2H and two methylene protons C2H(A) and C1H(B) from the two coordinating receptor cations, whereas O18 is involved bifurcated interaction with the same amide hydrogen N2H and aryl proton C5H of a receptor cation.
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The six-point coordination on ClO4- anion is finally satisfied by a weak C-H∙∙∙O interaction between perchlorate oxygen O16 and a methyl hydrogen C28H(A) of lattice DMSO.
4.7.4 Hydrogensulfate complex [(HL3)+•HSO4–
]DMSO, (3f)
Complex 3f crystallizes in monoclinic space group Cc with one disordered DMSO molecule as solvent of crystallization. Due to disorder, it was not possible to locate the hydrogen of the monovalent sulfate anion (HSO4–) in order to unambiguously determine the degree of protonation and charge upon the anion. However, structural elucidation revealed the 1:1 stoichiometric salt formation confirming the HSO4–
complex of protonated L3. Similar to the halide complexes, intramolecular (N–H)+∙∙∙O hydrogen bonding is also prevalent in complex 3f [N1∙∙∙O6 = 2.816(5); ∠N1-H∙∙∙O6 = 154o(4)]. Hydrogen bonding contacts on HSO4–
showed a seven-point coordination of the anion provided by three adjacent receptor cations and lattice DMSO (Figure 4.10b). Sulfate oxygen O16 behaves as a trifurcated hydrogen bond acceptor by interacting with the amide proton N2H and two aryl protons C9H and C23H from two coordinating receptor cations whereas O18 is involved in bifurcated interaction with the amide hydrogen N2H and methylene proton C1H(B) of the same receptor cation. Finally, O17 interacts with the amide hydrogen N5H of a third coordinating receptor cation and methyl hydrogen C28H(C) of lattice DMSO completing the seventh coordination contacts on HSO4-
.
Figure 4.10 (a) X-ray structure of complex 3e depicting the hydrogen bonding contacts on ClO4–
ion with two receptor cations and lattice DMSO; (b) X-ray structure of complex 3f is depicting the hydrogen bonding contacts on HSO4– ion with three receptor cations and lattice DMSO.