Structural studies on encapsulation of tetrahedral and octahedral anions by a protonated octaaminocryptand cage

Structural aspects of the binding of inorganic anions such as perchlorate, hydrogen sulfate, and hexafluorosilicate with the proton cage of octaaminocryptand L1, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N), are examined thoroughly. Crystallographic results for a hexaprotonated perchlorate complex of L1, [(H6L1)6+(ClO4−)]5(ClO4−)·11H2O·CH3CN (1), an octaprotonated hydrogen sulfate complex of L1, [(H8L1)8+(HSO4−)]7(HSO4−)·3H2O·CH3OH (2) and an octaprotonated fluorosilicate complex of L1, [(H8L1)8+(HSiF6−)]3(SiF62−)·(HSiF6−)·15H2O (3), show encapsulation of one perchlorate, hydrogen sulfate and hexafluorosilicate, respectively inside the cage of L1 in their protonated states. Further, detailed structural analysis on complex 1 reveals that the hexaprotonated L1 encapsulates a perchlorate via two N–H···O and five O–H···O hydrogen bonds from protonated secondary nitrogen atoms of L1 and lattice water molecules, respectively. Encapsulated hydrogen sulfate in complex 2 is “glued” inside the octaprotonated cage of L1 via four N–H···O and six C–H···O hydrogen bonds whereas encapsulated HSiF6− in complex 3 has short contacts via six N–H···F and three C–H···F hydrogen bonds with [H8L1]8+. In the cases of complexes 2 and 3, the cryptand L1 in octaprotonated state shows monotopic encapsulation of the guest and the final conformation of these receptors is spherical in nature compared to the elongated shape of hexaprotonated state of L1 in complex 1.


Results and Discussion
Syntheses. The cryptand L 1 was prepared on multi-gram scale and in very high yield following the modified literature procedure [13]. The key step in the scaled-up synthesis of this octaazacryptand is the condensation of tris(2-aminoethyl)amine (tren) with terephthaldehyde at 5-10 °C by the slow addition of a dry methanolic tren solution to the aldehyde also dissolved in dry methanol. Reduction of the resulting Schiff base was achieved using NaBH 4 . Both higher temperatures (40-50 °C) and fast addition rates lead to mostly polymeric products in the scaled-up synthesis. In the case of 1, a white precipitate is obtained after addition of perchloric acid to the methanolic solution of L 1 , which after crystallization from acetonitrile/ water (1:1 v/v), gave perchlorate encapsulated in a [H 6 L 1 ] 6+ cage. Complex 2 is obtained as white solid upon reacting sulfuric acid with L 1 in acetonitrile medium followed by crystallization from water/MeOH (1:1 v/v). Complex 3 is obtained as a white precipitate upon treating the receptor with hydrofluoric acid in methanol followed by crystallization from water. The syntheses of the complexes are all straight forward and, with the exception of complex 3, are obtained in high yield. [H 6 L 1 ] 6+ moiety has an endo-endo conformation with a distance of 9.850 Å between the two bridgehead nitrogen atoms (N1 and N4). The window between three phenyl rings ranges from 6.815 Å to 7.126 Å (measured by the centroid of phenyl distance) with an average window of 6.993 Å indicates the elliptical nature of the perchlorate encapsulated [H 6 L 1 ] 6+ moiety ( Figure 2). All the secondary amino nitrogen atoms N2, N3, N5, N6, N7 and N8, from all three strands of the cryptand moiety are protonated, which is evident by the comparatively longer C-N bond distances of these nitrogen atoms with the neighboring carbons ( Table 1).

Description of the Crystal
The encapsulated perchlorate is involved in two N-H···O and five O-H···O hydrogen bonding interactions with the protonated amino hydrogen atoms and lattice water molecules, respectively, as depicted in Figure 3. Thus, the perchlorate oxygen O1 is involved in two weak intermolecular hydrogen bonds N-H···O with amino hydrogen atoms H3D and H8D of the protonated nitrogens (N3 and N8) of the cryptand with N···O distances of N3···O1 = 3.018(9) Å and N8···O1 = 3.146(8) Å, and N-H···O angles <N3-H3D···O1 = 118° and <N8-H8D···O1 = 137°, respectively. The lattice water molecules also play a vital role in anchoring the ClO 4 − ion 1.491 (8) inside the flexible hexaprotonated cryptand moiety. Five lattice water molecules O25, O26, O27, O28 and O29, which act as donors and are involved in strong O-H···O hydrogen bonds with the encapsulated perchlorate oxygen atoms fasten the anion inside the cryptand moiety. All of these five water molecules act as acceptors and are oriented outside the cryptand leading to good hydrogen bonding via N-H···O with the protonated secondary amino hydrogen atoms (Table 2). The weaker intermolecular N-H···O hydrogen bonds between the encapsulated perchlorate oxygen O1 and the [H 6 L 1 ] 6+ moiety could be attributed to the involvement of H3D and H8D (at the protonated secondary amine sites of the cryptand) via strong intermolecular N-H···O hydrogen bonding with the oxygen atom (O5) of lattice perchlorate (Table 3). Further, in [H 6 L 1 ] 6+ moiety of 1, the distances between any two of the secondary nitrogen atoms differ marginally in the two sets of tren cavities (N1N2N5N7) and (N3N4N6N8) ( Table 4). This indicates that 3-fold symmetry about the axis passing through N1 and N4 is present in the solid state. The Cl1 of encapsulated perchlorate is sitting within the bridgehead plane (N1 and N4) and the C11 is placed closer to N4 (C11···N4 = 4.813Å) compared to the other bridgehead nitrogen N1 (C11···N1 = 5.037 Å). The distance between the bridgehead nitrogen atoms in 1 is 1.245 Å shorter than the distance observed in the free cryptand L 1 (11.095 Å) but the distance in complex 1 is 3.364 Å longer than that of the monotopic bromide complex of L 1 and only 0.527 Å smaller than the ditopic bromide and water in [H 6 L 1 ] 6+ complex reported recently [10,11]. This observation suggests that depending upon guest(s), the cavity dimension of hexaprotonated L 1 could change abruptly indicating the highly flexible nature of L 1 in its hexaprotonated state.  Figure 4. The sulfur atom S1 of the encapsulated HSO 4 − deviates by 0.202 Å with respect to the plane containing the protonated apical nitrogen atoms N1 and N4. In solid state [H 8 L 1 ] 8+ has also an endo-endo conformation with a distance of 7.758 Å between two bridgehead nitrogen atoms (N1 and N4) and the window between three phenyl rings ranges from 8.099 Å to 8.403 Å (measured by the centroid of the phenyl distance) with an average window of 8.255 Å indicating the near spherical nature of the hydrogen  moiety. The sulfur atom S1 of the encapsulated HSO 4 − is located at distance of 3.92 Å and 3.85 Å from N1 and N4, respectively where N4 is slightly closer to S1. In [H 8 L 1 ] 8+ the distances between any two of the secondary nitrogen atoms differ in the two sets of N4 cavities (N1N2N6N7) and (N3N4N5N8) in the cryptand (Table 5). This indicates that the  3-fold symmetry about the axis passing through N1 and N4 is lost in the solid state. Figure 5 represents the interaction of the [H 8 L 1 ] 8+ receptor with the encapsulated disordered hydrogen sulfate. The anion is "glued" inside the receptor by two C-H···O hydrogen bonds between the methylene hydrogen atoms (H14A, H23B) with the disordered oxygen atoms O3A and O4A, respectively, and four N-H···O contacts involving the both the protonated apical hydrogen atoms (H1D, H4D) and the hydrogen atoms (H3C and H7D) of protonated secondary amino nitrogen with O1 and O2 as acceptors each make two hydrogen bonds. Details of these intermolecular contacts are given in Table 6. Figure 6 represents the additional interactions of the ammonium hydrogen atoms with the surrounding anions and water molecules. It is observed that with the exception of the apical amino hydrogen atoms all others are involved in N-H···O interactions with the lattice HSO 4 − or O32 of the water molecules.
Thus, hydrogen atoms attached to N5 and N8 are involved in three contacts; one with water oxygen O32 and the other two with the oxygen atoms of HSO 4 − (O8, O10 for N5 and O10, O21 for N8). The rest of the ammonium hydrogen atoms are also involved in effective N-H···O contacts with the hydrogen sulfate as depicted in Figure 6.  Figure 7 and the various interactions of the disordered HSiF 6 − monoanion with the host molecule is depicted in Figure 8. Thus, hydrogen atoms H1 and H4 attached to the apical nitrogen N1 and N4 form N-H···F hydrogen bonds (one and three) with F1 and F2A, F3, F4, respectively. Both F2A and F3 are involved in an additional N-H···F hydrogen bonding interaction with the protonated secondary amino hydrogen atoms H3D and H6D attached to N3 and N6, respectively. F1 of the disordered encapsulated HSiF 6 − is involved in intermolecular C-H···F contacts with the methylenic hydrogen atom H14B, while H26B of the methylene hydrogen attached to C26 forms bifurcated weak C-H···F hydrogen bonds [35][36][37][38] with F5 and F6 in fixing the monoanion inside the cryptand moiety ( Figure 8). Details of these hydrogen bonding interactions are given in Table 7. The C-N distances involving the amino nitrogen range from 1.49 to 1.53 Å clearly indicate the octa protonation of the cryptand moiety including both the apical nitrogen atoms and are well within the range of earlier reported values [13]. Protonation of the SiF 6 2− is clearly reflected in the case of Si1 and Si3 by the longer Si-F distances: Si (1) Figure 9 represents the interaction of the protonated amino nitrogen atoms with the molecules surrounding the moiety. As depicted in the figure the hydrogen atoms of protonated secondary nitrogen centers are involved in strong N-H···F and N-H···O hydrogen bonds with the external anions and lattice water molecules.

Conclusions
The structural results for the interaction of polyatomic anions with the ligand L 1 in its hexa and octa protonated states show some interesting results. The structures clearly illustrate the effect of hexaprotonation and octaprotonation on the encapsulation of different anions. Upon a higher degree of protonation (hexa and octa) distribution of positive charge over the receptor increases which makes the cavity more electrophilic. Different degrees of protonation also change the overall conformation (ellipsoid and near spherical), which allows encapsulation of anions like perchlorate, hydrogen sulfate and hexafluorosilicate inside the receptor. Furthermore, these results indeed show that L 1 is also a potential receptor for bigger polyatomic anions like perchlorate and hydrogen sulfate.

Supporting Information
Experimental procedures, characterization data and copies of spectra ( 1 H NMR and HRMS) of complexes 1, 2, and 3 as well as crystallographic data and tables of hydrogen bonding parameters of complexes 1, 2, and 3 are provided.

Supporting Information File 1
Experimental and analytical data.