The structural aspects of binding of different inorganic anions with the protonated receptor L1 have been examined crystallographically. Anion binding with protonated L1 is attributable entirely to the (N–H)+∙∙∙A– and several C-H∙∙∙A– interactions involving multiple receptor cations where hydrogen of the protonated bridgehead nitrogen is exo-oriented and thereby, interacts with an anion via strong electrostatic effects. Protonation at the bridgehead nitrogen and presence of peripheral π–acidic functions render the aliphatic and aromatic protons of the receptor sufficiently acidic, for their active participation in weak C-H∙∙∙A– and intermolecular C-H∙∙∙O-(nitro) interactions. Although, charge neutralisation in the crystals and conventional hydrogen bonds are the main driving forces in the formation of all TH-1135_7612222
__________________________________________________________________ Chapter 3 supramolecular complexes, yet the weak C-H hydrogen bonds provide added stabilization to the complexes and thus, satisfy the geometrical necessity of the protonated receptor by providing a favourable electrostatic environment around the anion. Colourless crystals suitable for single-crystal X-ray analysis were obtained by slow evaporation of CH3OH–
CH3CN binary solution mixtures of the isolated anion complexes (1a-1e) at room temperature. Parameters for data collection and crystallographic refinement details of anion complexes 1a-1e are summarized in Table 3.3.
Figure 3.5 X-ray structures of anion complexes (1a-1e) of receptor L1 (thermal ellipsoid plot with 30% probability) depicting the electrostatic (N–H)+∙∙∙A– interaction in each complex.
3.4.1 Chloride complex [(HL1)+•Cl–], (1a)
Complex 1a crystallizes in triclinic system with P–1 space group. Structural elucidation revealed that, the binding of a chloride anion is governed by an electrostatic (N–H)+∙∙∙Cl–
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interaction (N1∙∙∙Cl– = 3.013 Å) and six C–H∙∙∙Cl– contacts operating with an average hydrogen bond distance of 3.690 Å (Table 3.5). A methylene proton C18H(A) subsequent to the protonated bridgehead nitrogen is C-H bonded to Cl– and two neighbouring receptor cations provide five otherC–H∙∙∙Cl– contacts to the anion involving hydrogen atoms C2H(B), C9H(B) and C4H of one unit and, C9H(A) and C17H(A) from the other coordinating unit (Figure 3.6a). The crystal packing motif viewed down the b-axis shows bilayer assembly of receptor cations, organized via several C–H∙∙∙O-(nitro) and π-stacking interactions where, chloride ions are sandwiched between the adjacent bilayers parallel along a-axis (Annexure 3).
3.4.2 Bromide complex [(HL1)+•Br–], (1b)
Complex 1b crystallizes in triclinic system with P–1 space group. Identical to 1a, there are six C–H∙∙∙Br– contacts from three encircling receptor cations operating with an average hydrogen bond distance of 3.786 Å (Table 3.5), in addition to the electrostatic (N–H)+∙∙∙Br– interaction (N1∙∙∙Br– = 3.194 Å). A methylene hydrogen C18H(B) subsequent to the protonated apical nitrogen is C–H bonded to Br– while aliphatic protons C1H(A), C17H(B), C1H(B), C10H(A) and aryl hydrogen C12H from two adjacent receptor cations are making contacts with the anion via weak C–H∙∙∙Br– interactions (Figure 3.6b). The crystal packing diagram viewed down the b-axis shows that the receptor cations beautifully pack in a bilayer array, assembled via multiple C–H∙∙∙O-(nitro) and π∙∙∙π interactions whereas multiple C–
H∙∙∙Br– interactions stitches the adjacent bilayers along crystallographic a-axis (Annexure 3).
An unusual seven-coordination in halide complexes (1a and 1b) suggests that, the interactions with the C–H donors are too weak to impose a definite coordination structure around the halides, and instead the C-H groups on the flexible arms of the receptor embrace the anion so as to provide a favourable electrostatic environment around them.
3.4.3 Nitrate complex [(HL1)+•NO3–
], (1c)
Complex 1c crystallizes in monoclinic system with P21/n space group. Crystal structure elucidation revealed the formation of bifurcated (N–H)+∙∙∙A– interaction between the hydrogen of protonated bridgehead nitrogen and nitrate oxygen atoms O10 and O11 (N1∙∙∙O10 = 2.894 Å and N1∙∙∙O11 = 2.881 Å). Additionally, each nitrate anion is in interaction with two adjacent receptor cations via four C–H∙∙∙O contactsoperating with an average hydrogen bond distance of 3.272 Å (Table 3.5). Two methylene protons C17H(A) and C18H(A) from the same receptor cation provides two C–H∙∙∙O contacts to nitrate oxygen O11 while O10 and O12 accepts C–H∙∙∙O bonds from the other coordinating cation involving hydrogen atoms C21H and C1H(A) respectively (Figure 3.6c). The packing diagram viewed down the a-axis clearly shows that the cationic array of receptor molecules are assembled via TH-1135_7612222
__________________________________________________________________ Chapter 3 multiple C–H∙∙∙O-(nitro) interactions and the anions are entrapped between the cationic arrays in a zigzag fashion along b-axis (Annexure 3).
3.4.4 Trifluoroacetate complex [(HL1)+•CF3CO2–
], (1d)
Complex 1d crystallizes in triclinic system with P–1 space group. Structural analysis showed that, a CF3CO2–
anion is coordinated to five receptor cations by a nine-point attachment involving oxygen and fluorine atoms of the anion. Similar to the nitrate complex (1c), the hydrogen of protonated bridgehead nitrogen is involved in bifurcated (N–H)+∙∙∙A– interaction with oxygen atoms O10 and O11 of the anion (N1∙∙∙O10= 3.248 Å, and N1∙∙∙O11= 2.742 Å) and a methylene hydrogen C9H(B) from the same receptor cation is hydrogen bonded to O10 via weak C–H∙∙∙O interaction. Furthermore, four adjacent receptor cations that encircle an anion provide six other C–H contacts on the CF3CO2–
anion with an average hydrogen bond distance of 3.304 Å (Table 3.5). Methylene hydrogen C1H(A) and C2H(B) from two different receptor cations are involved in bifurcated C–H∙∙∙A– interaction with O10, F2 and O11, F3, respectively, whereas aryl hydrogen C7H and C4H from two other units provide C–
H contacts to F1 and F2, respectively (Figure 3.6d). It is interesting to note that, two arms of a receptor cation are projected in one direction to form a cleft shaped cavity and two such units intercalate to form a dimeric assembly encapsulating two CF3CO2– ions within the dimeric cleft (Annexure 3). The crystal packing diagram viewed down the b-axis, shows that the receptor cations beautifully pack in a bilayer array via multiple C–H∙∙∙O-(nitro) and π- stacking interactions and the CF3CO2– ions are entrapped between the adjacent bilayers along a-axis (Annexure 3).
3.4.5 Perchlorate complex [(HL1)+•ClO4–
], (1e)
Complex 1e crystallizes in monoclinic system with P21/c space group. Similar to the halide complexes (1a and 1b), there exist a strong (N–H)+∙∙∙A– interaction (N1∙∙∙O12= 2.974 Å) and a weak C–H∙∙∙A– interaction formed between an aryl proton C15H of the same receptor cation and perchlorate oxygen O13. In addition, two adjacent receptor cations provide four more C–H∙∙∙O contacts on the perchlorate anion with an average hydrogen bond distance of 3.288 Å (Table 3.5). Perchlorate oxygen O10 and O12 are hydrogen bonded to a methylene proton C1H(A) whereas, O11 and O13 are in interaction with aromatic protons C23H and C12H of two different receptor cations (Figure 3.6e). The receptor cations are inter-linked among themselves via multiple C–H∙∙∙O-(nitro) interactions and results in the formation of a zipper-like assembly when viewed down the crystallographic a-axis with the anions being arranged in a zigzag fashion stitching the adjacent cationic arrays by C–H∙∙∙A– interactions (Annexure 3).
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Figure 3.6 (a) Seven-point coordination of Cl– anion in 1a; (b) Seven-point coordination of Br– anion in 1b; (c) Six-point coordination of NO3– anion in 1c; (d) Nine-point coordination of CF3CO2– anion in 1d; (e) Six-point coordination of ClO4– anion in 1e.