CRYSTAL GROWTH & DESIGN
Molecular Recognition Studies of an Octaaminocryptand upon Different Degree of Protonation P. S. Lakshminarayanan,† I. Ravikumar,† Eringathodi Suresh,*,‡ and Pradyut Ghosh*,† Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India, and Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar 364002, India
2008 VOL. 8, NO. 8 2842–2852
ReceiVed NoVember 23, 2007; ReVised Manuscript ReceiVed February 28, 2008
ABSTRACT: Multigram synthesis of an octaaminocryptand L (1,4,11,14,17,27,29,36-octaazapentacyclo[12.12.12.2.6,92.19,22231,34]tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonane, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N) with high yield was obtained in one pot by low temperature [2 + 3] condensation of tris(2-aminoethyl)amine with terephthalaldehyde followed by sodium borohydride reduction in methanol. L was treated with hydrochloric, hydroiodic, and picric acids in different experimental conditions to obtain crystals of different salts of L having different degrees of protonation. Structural aspects of binding of halides, picrate, and water in protonated L were examined thoroughly. Crystallographic results show that a diprotonated chloride complex of L, [H2L](Cl)2 · 3H2O (1) and (triprotonated and monoprotonated) species of L in an iodide complex [{H3L}{HL}](I)4 (2) do not show any guest encapsulation inside the cavity, whereas one molecule of water resides inside the cavity in the case of its triprotonated state where one of the bridgehead nitrogen atoms is protonated in [2{LH3(H2O)}](I)6 · 2H2O (3). Importantly, in a tertraprotonated picrate complex of L, [H4L(H2O)2.50](picrate)4 · 2CH3CN, 5, two water molecules reside inside the two N4 cavities of L, bridged by a third water molecule showing tritopic binding, whereas in the octaprotonated state a monotopic encapsulation of iodide via N-H · · · iodide and C-H · · · iodide interactions was observed inside the receptor cavity of [H8L(I)](I)7 · 9H2O, (8). This study systematically represents a competition between anion and solvent recognition upon different degrees of protonation of L. Further detailed structural analysis on complex 1 shows the formation of a one-dimensional polymeric water chloride hybrid network of linear rectangular chains sandwiched between the cryptand layers. In recent years, considerable efforts have been made in elucidating the coordination chemistry of anions, because of their vital roles in biological systems, medicine, catalysis, and environmental issues.1 It has been observed that protonated amines and quaternary ammonium functions incorporated in a suitable ligand topology make them attractive receptors for anions.1a,d,g For instance, azamacropolycycle, L (Chart 1) has shown fluoride-based cascade complex,2 chloride/bromide and a water molecule,3 monotopic chloride/bromide,4 encapsulation inside the cavity of hexaprotonated L, [H6L]6+, whereas [H7L]7+ has shown a monotopic encapsulation of chloride via hydrogen bonding with the external undecameric water clusters.5 In all the above examples, it have been observed that L is in hexa- or heptaprotonated states. Recently, we have shown that [H4L]4+ having iodide counterion binds an acyclic water tetramer in a cryptand cavity (7),4 though inclusion of a water/ dimer water inside the cavity of macrobicyclic cryptand has been known for quite some time,6–9 except for the structure of the m-xylyl analogue of L with three internal disorder water molecules.10 Any study on water clusters in a constrained environment is of significant current interest as they have been implicated in several contemporary problems.11–14 In a recent communication, we have shown that [H4L]4+ encapsulates a water-acetonitrile-water cluster (6), whereas a molecule of water encapsulates inside the cryptand cavity in a triprononated state (4), having picrate as counteranions.15 All these findings indicate that there is indeed a strong competition between the anion and the solvent molecule toward recognition in the above receptors. At this juncture, as a case study we have attempted to isolate different degrees of protonated receptors of L at * To whom correspondence should be addressed. E-mail:
[email protected] (P.G);
[email protected] (E.S.). † Indian Association for the Cultivation of Science. ‡ Central Salt & Marine Chemicals Research Institute.
different experimental conditions to study the anion vs solvent encapsulation. Herein, we structurally demonstrate that bis-, and (mono- and tris-) protonation at the secondary nitrogen center(s) of L with HCl and HI respectively generate an empty cavity, whereas a molecule of water resides inside the cavity in the case of triprotonated iodide complex where one of the bridgehead nitrogen atoms is protonated. Importantly, in the tertraprotonated picrate complex of L, complex 5, two water molecules reside inside the two N4 cavities of L, bridged by a third water molecule showing tritopic binding, whereas in the octaprotonated state, a monotopic encapsulation of iodide was observed. Further, we also show the formation of a onedimensional polymeric water chloride hybrid network sandwiched between the cryptand layers in case of complex 1. Experimental Section Materials. Terephthaladehyde, tris(2-aminoethyl) amine (tren), and hydroiodic acid (HI) were purchased from Sigma-Aldrich and used as received. Solvents, sodium borohydride, picric acid, and hydrochloric acid (HCl) were purchased from SD Fine Chemicals, Mumbai, India. All the solvents and picric acid were purified prior to use. Synthesis of Macrobicyclic Receptor, L. The cryptand L was synthesized after modifying the literature procedure and isolated as the free base.16 Terephthaldehyde (5.04 g, 37.57 mmol) and 500 mL of dry MeOH were added to a 2-neck 1-L round-bottom flask equipped with a magnetic stirrer. Terephthaldehyde was dissolved at RT and then the solution was cooled to 5-10 °C using an ice bath. A dropping funnel containing diluted tren (3.65 g, 25 mmol) with 100 mL of dry MeOH was fitted in to the 2-neck round-bottom flask. Tren solution was added dropwise (2-3 drops/min) allowing for complete dispersion of each drop between the additions with constant stirring under N2 atmosphere. Complete tren solution was added in approximately 4 h and addition was continued at 5-10 °C with constant stirring. After all tren solution was added, the yellow solution was allowed to stir at room temperature for another 8 h. Reduction of the Schiff base thus formed was achieved by hydrogenating it with excess NaBH4 (portion-
10.1021/cg701152v CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
Molecular Recognition Studies of an Octaaminocryptand
Crystal Growth & Design, Vol. 8, No. 8, 2008 2843
Chart 1. Octaaminocryptand L, Diprotonated L with Empty Cavity 1, Triprotonated and Monopronated L with Empty Cavity 2, Triprotonated (Includes Protonation at One of the Bridgehead Nitrogen Atom) L Containing One Water Inside the Cavity (Monotopic) 3 and 4, Tetraprotonated L Showing Solvent Clusters Encapsulation (Tritopic Complexes) 5, 6 and 7, Octaprotonated L Showing Iodide Encapsulation 8 (Monotopic Complex)
wise) for 2-3 h at room temperature followed by refluxing for an hour. The MeOH was evaporated to dryness under reduced pressure, and the residue was treated with cold distilled water (200 mL). The desired cryptand was extracted with CHCl3 (100 × 3 mL). The organic layer was further washed with (500 × 3 mL) distilled water and then it was dried over anhydrous Na2SO4 and was evaporated to obtain a colorless semisolid. The semisolid product was redissolved in warm MeCN and was allowed to crystallize at RT. Colorless crystals of single crystal X-ray diffraction quality L was obtained within 2 days. Yield 90%: All characterization data matched the literature values. Synthesis of Complex [H2L](Cl)2 · 3H2O, (1). Complex 1 was obtained as a byproduct from the reaction of L and HCl in acetonitrile. 120 mg (0.20 mmol) of L was dissolved in 10 mL of acetonitrile, and to this solution 1 mL of 37% HCl was added. White solid obtained upon addition of HCl was filtered and the filtrate was allowed for crystallization at RT. Colorless crystals suitable for X-ray analysis were grown after a week by slow evaporation. Yield: 25%. 1H NMR (300 MHz, D2O): δ 2.87 (t, 12H, NCH2), 3.35 (t, 12H, NCH2CH2), 4.23 (s, 12H, ArCH2), 7.47 (s, 12H, ArH). 13C NMR (75 MHz, D2O): δ 45.97, 52.05, 52.18, 127.62, 137.24. HRMS (ESI): m/z 599.0394 [HL]+. Synthesis of Complex [{H3L}{HL}](I)4, (2). Cryptand L (120 mg, 0.2 mmol) was dissolved in 10 mL of hot water, and to this solution 1 mL of 49% HI was added slowly and then the solution was refluxed at 90 °C for a period of 2 h. The solution was filtered and the filtrate was allowed to evaporate at room temperature for one week. Single crystals suitable for X-ray analysis were obtained. Yield: 80%. 1H NMR (300 MHz, D2O): δ 2.83 (t, 12H, NCH2), 3.39 (t, 12H, NCH2CH2), 4.31 (s, 12H, ArCH2), 7.52 (s, 12H, ArH). 13C NMR (75 MHz, D2O): δ 44.81, 49.97, 51.19, 131.64, 132.07. HRMS (ESI): m/z 599.1270 [HL]+. Synthesis of Complex [2{LH3(H2O)}](I)6 · 2H2O, (3). Cryptand L (120 mg, 0.2 mmol) was dissolved in a tetrahydrofuran/water mixture (1:1 v/v), and to this solution 1 mL of 49% HI was added and then the solution was refluxed at 80 °C for 2 h. The solution was filtered and the filtrate was kept at room temperature for crystallization. Single crystals suitable for X-ray analysis were grown by slow evaporation at RT, and colorless crystals were isolated from the mother liquor and characterized. Yield: 70-75%. 1H NMR (300 MHz, D2O): δ 2.66 (t, 12H, NCH2), 3.55 (t, 12H, NCH2CH2), 4.70 (s, 12H, ArCH2), 6.80 (s, 12H, ArH). 13C NMR (75 MHz, D2O): δ 44.60, 49.76, 51.40, 130.27, 131.65. HRMS (ESI): m/z 599.1446 [HL]+. Synthesis of Complex [H4L(H2O)2.50](picrate)4 · 2CH3CN, (5). Cryptand L (60 mg, 0.1 mmol) was dissolved in 10 mL of acetonitrile, and to this solution, acetonitrile solution of picric acid (46 mg, 0.2
mmol in 5 mL) was added. The yellow precipitate formed was filtered and then redissolved in hot acetonitrile. The yellow solution was filtered again and the filtrate was allowed to evaporate slowly at room temperature for a week. Yellow crystals suitable for single crystal X-ray analysis were isolated from the mother liquor and characterized. Yield: 40-45%. 1H NMR (300 MHz, DMSO-d6): δ 2.66 (t, 12H, NCH2), 3.54 (t, 12H, NCH2CH2), 3.87 (s, 12H, ArCH2), 7.10 (s, 12H, ArH) 8.59 (s, 8H, PicH). 13C NMR (75 MHz, DMSO-d6): δ 47.41, 53.21, 56.77, 125.22, 129.73, 130.65, 139.27, 142.85, 150.07. HRMS (ESI): m/z 599.1254 [HL]+. Synthesis of Complex [H8L(I)](I)7 · 9H2O (8). Cryptand L (120 mg, 0.2 mmol) was dissolved in 10 mL methanol, to this solution 1.5 mL of 49% HI was added. The resulting solution was heated up to 60 °C and allowed to cool. Few drops of water were added to the brownish turbid solution and solution was filtered. Filtrate was allowed for crystallization at RT. Colorless crystals suitable for single crystal X-ray analysis were grown upon slow evaporation. Yield: 75%. 1H NMR (300 MHz, D2O): δ 2.72 (t, 12H, NCH2), 3.54 (t, 12H, NCH2CH2), 4.45 (s, 12H, ArCH2), 7.47 (s, 12H, ArH). 13C NMR (75 MHz, D2O): δ 44.40, 49.55, 51.49, 130.87, 131.71. HRMS (ESI): m/z 599.0754 [HL]+. Physical Measurements. 1H and 13C NMR spectra were recorded on Bruker 300 and 75 MHz FT-NMR spectrometers (model: AdvanceDPX200) instrument respectively. HRMS measurements were carried out on QTof-Micro YA 263 instruments. X-ray Crystallography. The crystallographic data and details of data collection for complexes 1, 2, 3, 5, and 8 are given in Table 1. In each case, a crystal of suitable size was selected from the mother liquor and immersed in paratone oil and then mounted on the tip of a glass fiber and cemented using epoxy resin. X-ray single-crystal diffraction data for all the five complexes 1, 2, 3, 5, and 8 were collected on a Bruker SMART Apex CCD diffractometer at mentioned temperature (Table 1) with graphite monochromatized Mo KR radiation (0.71073 Å). There was no evidence of crystal decay during data collection. Semiempirical absorption corrections were applied (SADABS), and program SAINT was used for integration of the diffraction profiles.17a The structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL.17b Graphics are generated using PLATON17c and MERCURY 1.3.17d,e
Results and Discussion Syntheses. The cryptand L was prepared in multigram scale with a very high yield, following a modified literature proce-
2844 Crystal Growth & Design, Vol. 8, No. 8, 2008
Lakshminarayanan et al.
Table 1. Crystallographic Data for Complexes 1, 2, 3, 5, and 8 compound
complex 1
complex 2
complex 3
complex 5
complex 8
empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z dcalc (g/cm3) crystal size (mm3) diffractometer λ (Å) F(000) µ Mo KR (mm-1) abs corr T (K) θ range reflns collected indep reflns R(int) data/restr/param R1; wR2(I g σ(I)) GOF (F2)
C36 H64 Cl2 N8 O6 775.85 monoclinic C2/c 16.9793(14) 9.1033(8) 27.391(2) 90.00 98.7820(10) 90.00 4184.1(6) 4 1.232 0.44 × 0.32 × 0.28 Smart CCD 0.71073 1672 0.207 SADABS 100(2) 2.43 to 28.25 11950 4806 0.0337 4806/0/277 0.0636; 0.1533 1.076
C72 H112 I4 N16 O10 1869.38 monoclinic Pc 12.7491(9) 11.7037(8) 28.784(2) 90.00 90.4490(10) 90.00 4294.8(5) 2 1.446 0.23 × 0.18 × 0.09 Smart CCD 0.71073 1896 1.511 SADABS 100(2) 1.60 to 25.00 20511 10460 0.0382 10460/2/919 0.0674; 0.1800 1.095
C72 H118 I6 N16 O5 2049.22 monoclinic P21/c 15.6852(10) 27.5820(18) 20.0607(13) 90.00 95.3160(10) 90.00 8641.5(10) 4 1.575 0.40 × 0.30 × 0.25 Smart CCD 0.71073 4080 2.211 SADABS 100(2) 1.26 to 25.00 42065 15121 0.0364 15121/0/914 0.0408; 0.0987 1.082
C64 H63 N22 O30.50 1628.36 monoclinic P21/c 11.6884(8) 14.0632(10) 46.042(3) 90.00 91.9360(10) 90.00 7563.9(9) 4 1.430 0.38 × 0.28 × 0.16 Smart CCD 0.71073 3380 0.116 SADABS 273(2) K 1.51 to 28.26 44610 17614 0.0296 17614/0/1094 0.0615; 0.1542 1.018
C36 H62 I8 N8 O9 1766.14 triclinic P1j 12.9944(18) 13.839(2) 19.109(3) 80.371(2) 86.634(2) 62.009(2) 2990.8(7) 2 1.961 0.62 × 0.54 × 0.43 Smart CCD 0.71073 1660 4.196 SADABS 100(2) K 1.69 to 28.29 25252 13379 0.0542 13379/0/542 0.0864; 0.2102 1.027
dure.16 The key step in the gram scale synthesis of this octaazacryptand is the condensation of tren with terephthaldehyde at 5-10 °C and slow addition of tren solution to the aldehyde solution. Low temperature syntheses of other cryptand molecules are known in the literature.18 Condensation reaction was carried out in dry MeOH and in situ reduction of Schiff base was done with NaBH4. Higher temperature (40- 50 °C) and fast addition rates lead to mostly polymeric product in this high scale synthesis. In the case of byproduct 1, a white precipitate was obtained after addition of hydrochloric acid to the acetonitrile solution of L as a major product, which upon crystallization from a methanol/water (4:1) produced a hybrid water-chloride structure with discrete undecameric water selfassembled in a [H7L]7+,5 whereas acetonitrile filtrate from the above reaction mixture produces the complex 1 with low yield. In the case of complex 2 synthesis, the solvent system was changed to water, protic polar system and hydroiodic acid was used as the titrating agent. The lower degree of protonation observed in this complex may be due to the lower dissociation of hydroiodic acid in a highly polar medium. When the solvent system was changed to THF/water instead of water, complex 3 was isolated. Tetraprotonated picrate complex 5 was obtained from acetonitrile soluition where only 2 equiv of picric acid was added to the solution of cryptand L with the intention of isolating bisprotonated species. The tetraprotonated product indicates that L is susceptible to tetraprotonation than bisprotonation with the picric acid in this condition. In a separate study, we have reported tetraprotonated picrate complex 4 (4 equiv of picric acid was added in similar condition), where two recognized water molecules are bridged by an acetonitrile.15 In the case of complex 8, octaprotonated L was obtained upon treating with hydroiodic acid in methanolic medium at higher temperature. Syntheses of the complexes were all straightforward, resulting in high yields (except complexes 1 and 5). Description of the Crystal Structure. [H2L](Cl)2 · 6H2O, (1). The complex crystallizes in the bisprotonated form, with the cryptand containing an empty cavity having chlorides and water molecules in the lattice. The ligand moiety possesses a high degree of symmetry with a 2-fold axis passing through
Figure 1. ORTEP diagram of the [H2L]2+ of complex 1 with atom numbering scheme (40% probability factor for the thermal ellipsoids and only the hydrogen atoms attached to the amino nitrogen atoms are shown in the figure for clarity).
the apical nitrogen atoms N1 and N1A generating the symmetrically disposed first and second strands, and the third strand of the molecule possesses a 2-fold axis bisecting the disordered phenyl ring C17 and C18 atoms. Two of the secondary amino nitrogen atoms N2 of the symmetrically disposed strands are protonated, with the two chloride ions per [H2L]2+ which reside outside the cavity to compensate the charge. ORTEP diagram of the [H2L]2+ moiety with atom numbering scheme is given in Figure 1. In the solid state [H2L]2+ moiety of complex 1 has endo endo conformation (Figure 1) with a distance of 9.970 Å between two bridgehead nitrogen atoms, which is shorter in length than that of L · 15 H2O (average of Nbridgehead-Nbridgehead
Molecular Recognition Studies of an Octaaminocryptand
Crystal Growth & Design, Vol. 8, No. 8, 2008 2845 Table 2. Selected Hydrogen-Bond Lengths (Å) and Bond Angles (°) of Complex 1a D-H · · · A O(1-H(1E) · · · Cl(1)3 O(1)-H(1F) · · · Cl(1)1 O(2)-H(2E) · · · Cl(1)2 O(2)-H(2F) · · · Cl(1)4 O(3-H(3E) · · · O(1)2
D-H [Å] H · · · A [Å] D · · · A [Å] D-H · · · A° 0.82(4) 0.82(3) 0.86(4) 0.86(5) 0.77(4)
2.41(4) 2.37(3) 2.38(4) 2.36(5) 2.05(4)
3.219(2) 3.184(2) 3.244(2) 3.214(2) 2.811(3)
171(3) 175(3) 177(2) 168(4) 175(3)
a (1) X, Y, Z; (2) 1/2 - x, -1/2 + y, 1/2 - z. (3) 1/2 - x, 3/2 - y, -z; (4) 1/2 + x, 3/2 - y, 1/2 + z.
Figure 2. Packing diagram of the compound viewed down the a-axis depicting the various hydrogen-bonding interactions. The water chloride cluster is sandwiched between the adjacent [H2L]2+ layers by anchoring N-H · · · O interactions between the protruding water molecule O3 from either side of the membered water-chloride cluster with the amino hydrogen of the cryptand ligand.
Figure 3. Close up view of the hydrogen-bonded water chloride hybrid cluster sandwiched between the layered [H2L]2+ moiety.
) 11.095 Å).19 The distances of separation between the two secondary nitrogen atoms (N3 and N4) and one protonated secondary nitrogen center (N2) to (N3) and (N2) to (N4) in tren cap on either side of the cryptand are N3 · · · N4 ) 4.796 Å, N2 · · · N3 ) 4.135 Å, and N2 · · · N4 ) 2.845 Å. The short distance of separation between the amino nitrogen N2 with N4 can be attributed to the strong intramolecular N-H · · · N interaction N(2)-H(2C) · · · N(4). Details of these interactions and the symmetry codes are given in Table 1S1 of the Supporting Information.20 Even though there was no encapsulation of either the anion or water molecule inside the [H2L]2+ moiety, the packing and various hydrogen bonding interactions of the ligand moiety with the surrounding anions and lattice water molecules have been analyzed in detail (Figure 2). Interestingly, it is observed that the chloride anions are involved in a one-dimensional water chloride hybrid network along the b-axis as depicted in Figure 3. Thus, the chloride anions disposed at regular intervals are linked via O-H · · · Cl- interactions with the symmetrically disposed water molecules O1 and O2 alternatively, involving both the hydrogen atoms of the water molecules on either side to generate a one-dimensional linear rectangular chain as depicted in Figure 3. Details of these strong intermolecular O-H · · · Cl- interactions are given in Table 2. In this complex 1, protonation takes place at the two secondary nitrogen centers of two tren moieties, which is obvious because of the homoditopic nature of L. Therefore, individual tren cavity of the receptor [H2L]2+ is only singly charged, which is not enough to drag the anion/solvent toward the cryptand cavity to make effective hydrogen bonding interactions for an encapsulation. Moreover, intramolecular hydrogen bonding between the protonated nitrogen center and unprotonated secondary nitrogen center in both the tren moieties imparting the orientation of the third arm in twisted conforma-
Figure 4. ORTEP diagram of the [H3L]3+ (left) and [HL]+ (right) moieties present in the asymmetric unit of compound 2 (30% probability factor for the thermal ellipsoids and only hydrogen atoms attached to the amino nitrogen atoms are shown in the figure for clarity).
tion which might also blocking the entry of the anion/solvent molecules within the host cavity. Description of the Crystal Structure. [{H3L}{HL}](I)4 · 10H2O, (2). The compound crystallizes in monoclinic system in Pc space group. Two cryptand moieties, one in triprotonated, [H3L]3+ and the other in monoprotonated state, [HL]+ along with 4 iodide anions and 10 water molecules are present in the asymmetric unit. Even though the data were collected at liquid nitrogen temperature hydrogen atoms attached to the water molecules could not be located from the difference Fourier map. ORTEP diagram of the [H3L]3+ and [HL]+ along with the atom numbering scheme is given in Figure 4. In the case of [H3L]3+ moiety, secondary amino nitrogen atoms N3, N5 and N8 from three arms of L are protonated and there is no inclusion of either iodide or water molecule(s) inside the cavity of L. This may be due to the strong intramolecular N-H · · · N hydrogen bonding interactions of the secondary amino nitrogen atoms of both the cryptand moieties present in the asymmetric unit. Thus, in the case of [H3L]3+, secondary amino nitrogen N2, and protonated secondary amino nitrogen atoms N3, N5, and N8 are engaged in intramolecular N-H · · · N hydrogen bonding and the pertinent interaction parameters are depicted in Table 3. The distance between the secondary amino nitrogen atoms of one of the tren cap of the triprotonated nitrogen (N2 · · · N5 ) 2.955 Å, N2 · · · N7 ) 4.686 Å N and N7 · · · N5 ) 4.154 Å) clearly shows that the protonated amino nitrogen N5 has adopted a closer approach in order to make an effective hydrogen-bonding to N2 whereas in the case of other tren cap (N3 · · · N8 ) 2.946, N3 · · · N6 ) 4.571 N8 · · · N6 ) 4.164) both the protonated nitrogen atoms of different strands have come closer to make effective intramolecular hydrogen bonding between them. The distance between the bridgehead nitrogen atoms N1 · · · N4 in (H3L)3+ moiety is 9.948 Å, which is slightly shorter than that observed in bisprotonated chloride complex
2846 Crystal Growth & Design, Vol. 8, No. 8, 2008
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Table 3. Selected Hydrogen-Bond lengths (Å) and Bond Angles (°) of Complex 2a D-H · · · A
D-H [Å]
H· · ·A [Å]
D · · · A [Å]
D-H · · · A°
N(2)-H(2C) · · · N(5)1 N(3)-H(3C) · · · N(8)1 N(5)-H(5D) · · · N(2)1 N(8)-H(8D) · · · N(3)1 N(10)-H(10C) · · · N(15)1 N(11)-H(11C) · · · N(14)1 N(15)-H(15C) · · · N(10)1
0.86 0.90 0.90 0.90 0.86 0.86 0.90
2.25 2.24 2.08 2.06 2.32 2.23 2.16
2.955(17) 2.946(19) 2.955(17) 2.946(19) 3.016(17) 2.969(17) 3.016(17)
139 135 165 167 138 143 158
a
(1) x, y, z.
1, whereas in [HL]+ moiety the distance between the bridgehead nitrogen atoms is 9.981 Å, which is slightly larger than that observed in complex 1. The distances between the secondary amino nitrogen at one end (N10 · · · N15 ) 3.017 Å, N10 · · · N13 ) 4.809 Å and N15 · · · N13 ) 4.081 Å) in [HL]+ indicate the N-H · · · N interactions between the protonated secondary amine N10 with N15 bringing them closer (Table 3). For the other part of the same cryptand the distance between the secondary amino nitrogen atoms is N1 · · · N14 ) 2.969 Å, N1 · · · N16 ) 5.082 and N14 · · · N16 ) 4.284 Å, with the closer approach of N11 and N14 due to the intramolecular N-H · · · N interactions. In addition to these N-H · · · N intramolecular interactions, there are strong intermolecular hydrogen bonds N-H · · · I-, N-H · · · O and C-H · · · O and C-H · · · I- between the surrounding lattice waters and iodide ions are also present in the complex. Details of these hydrogen bonding contacts with symmetry codes are given in Table 2S of the Supporting Information.20 In the complex 2 having [H3L]3+ and [HL]+ moieties in the asymmetric unit, protonation occurs at the secondary nitrogen centers as in the case of 1. It is interesting to note that [H3L]3+ moiety in 2 where two secondary nitrogen centers of the one tren cavity and one secondary nitrogen center of other tren unit are protonated and does not encapsulate any guest molecule. But previously we have shown that the [H3L]3+ receptor in picrate complex 4 where two secondary nitrogen centers of tren unit and tertiary nitrogen of other tren unit are protonated recognizes a water molecule inside the cavity.15 The nonencapsulation behavior of the [H3L]3+ moiety in the present case may be due to the sites of protonation which eventually control the final geometry of the receptor unit. Description of the Crystal Structure [2{H3L(H2O)}](I)6 · 2H2O, (3). The compound crystallizes in monoclinic system (space group P21/c) with two triprotonated cryptand moieties along with six iodide ions and four molecules of water as solvent of crystallization. One iodide (I5) and two water molecules (O4 and O5) present are disordered at two positions and the occupancy factor for the iodide ion is determined by FVAR command using the SHELXL program whereas the occupancy factor for the water molecules are assigned as 0.5 and these water molecules are refined only isotropically. Both the cryptand moieties present in the asymmetric unit are triprotonated in which one of the bridgehead nitrogen and two secondary amino nitrogen atoms of each ligand are protonated. ORTEP diagram of one of the triprotonated cryptand moiety present in the asymmetric unit along with atom numbering scheme is depicted in Figure 5, and the crystallographic data are shown in Table 1. In this case also no iodide ion is encapsulated in the cavity of the [H3L]3+ (here one of the bridgehead nitrogen is protonated which is different than the [H3L]3+ moiety in complex 2), and instead one water molecule is held firmly inside each cryptand cavity by various hydrogen bonding interactions. Figure 6 represents the various hydrogen bonding interactions of the [H3L]3+ moiety with the H2O inside the cavity. Thus, O1 is
Figure 5. ORTEP diagram with atom numbering scheme for one of the molecules present in the asymmetric unit of the triprotonated L (hydrogen atoms attached to the amino nitrogen atoms only are shown in the figure for clarity and 40% probability factor for the thermal ellipsoids).
Figure 6. Recognition of water molecule with the one [H3L]3+ moiety (left) and second [H3L]3+ moiety (right) present in the asymmetric unit (only hydrogen atoms of the amino nitrogen atoms of the ligand moiety is shown in both the figures for clarity). Table 4. Selected Hydrogen-Bond Lengths (Å) and Bond Angles (°) of Complex 3a D-H · · · A
D-H [Å]
H· · ·A [Å]
D · · · A[Å]
D-H · · · A [°]
N(2)-H(2) · · · O(1)1 O(1)-H(20) · · · N(2)1 N(5)-H(5A) · · · O(1)1 N(7)-H(7A) · · · O(1)1 N(10)-H(10A) · · · (3)1 N(15)-H(15B) · · · O(3)1
0.86 1.03 0.90 0.90 0.90 0.90
2.19 1.59 1.82 1.85 1.77 1.78
2.6127 2.6127 2.7116 2.7353 2.6681 2.6760
110 173 172 170 172 173
a
(1) x, y, z.
encapsulated inside the one of the [H3L]3+ moieties through two strong and one weak N-H · · · O interactions (Table 4). The O1 acts as an acceptor for the hydrogen atoms H2, H5A, and H7A of secondary nitrogen centers at a distance ranging from 2.612 to 2.735 Å.
Molecular Recognition Studies of an Octaaminocryptand
Figure 7. Interaction of the second [H3L]3+ moiety with the surrounding lattice water molecule and iodide (only the hydrogen atoms of the ligand moiety which is involved in the hydrogen bonding interactions are shown in the figure for clarity).
The hydrogen atom H2 of the encapsulated water molecule acts as a donor and is involved in a weak O-H · · · N interaction with the secondary amino N2 at a distance 2.612 Å. In this [H3L]3+ moiety the water molecule resides toward the unprotonated bridgehead nitrogen at a distance of 2.956 Å (N1 · · · O). In the case of the second [H3L]3+ in the asymmetric moiety the encapsulated water hydrogen atoms could not be located from the difference Fourier map. However, water oxygen O3 makes two strong N-H · · · O contacts with the hydrogen atoms H10A and H15B of secondary amino centers at a distance of 2.668 and 2.676 Å, respectively, and resides toward the unprotonated bridgehead center, N9 · · · O3 ) 2.941 Å (Table 4). The distances between the bridgehead nitrogen centers N1 · · · N4 and N9 · · · N12 in the two [H3L]3+ moieties in complex 3 are 10.846 and 10.914 Å, respectively, which are longer in length than that observed in the case of the [H2L]2+ moiety in 1, [HL]+and [H3L]3+ species in iodide complex 2 having an empty cavity in all the cases. Further, it is worth noting that in case of picrate complex 4 where L is in a triprotonated state and the protonation pattern is same as observed in iodide complex 3, [H3L]3+ also encapsulates a molecule of water at one end of the N4 cavity.15 This indeed proves that the protonation pattern in this polyamine-based cage receptor does matter for recognition of a water molecule in the [H3L]3+ receptor irrespective of the counteranion. Preference of water molecule recognition over a large iodide guest might be due to the insufficient electrostatic attraction of the [H3L]3+ receptor cavity toward anion recognition. The interaction of both the [H3L]3+ moieties in complex 3 with the surrounding iodide and water molecules are analyzed in detail. Thus, the first cryptand moiety makes one intermolecular N-H · · · I-, one N-H · · · O, and one intramolecular C-H · · · I- contact with I3, disordered water oxygen O5 and O5′, and I6, respectively. Interaction of the surrounding molecules with one of the [H3L]3+ moieties is depicted in Figure 7. Hence, the triprotonated cryptand is involved in three intermolecular C-H · · · O contacts with the lattice water molecules O2 and O4, two intermolecular N-H · · · I- contacts with I1, I5 and two intramolcular C-H · · · I- contacts with I2 and I6, respectively. The packing diagram of the compound viewed down the a-axis with various hydrogen-bonding interactions is depicted in Figure 8. The lattice water molecule O2 is involved
Crystal Growth & Design, Vol. 8, No. 8, 2008 2847
Figure 8. Packing diagram along with hydrogen bonding interactions of the compound viewed down the a-axis showing the two-dimensional network in the bc-plane (only hydrogen atoms of the amino nitrogen atoms, water molecules, and hydrogen atoms involved in hydrogenbonding interaction of the ligand moiety are included in the figure for better clarity).
in bridging the ligand moieties along the c-axis through iodide ions making layered arrangement of molecules. These layers are cross-linked via various intermolecular hydrogen bonding interactions generating a hydrogen bonded two-dimensional network depicted in Figure 8. Details of all these interactions with symmetry code are given in Table 3S of the Supporting Information.20 Description of the Crystal Structure, [H4L(H2O)2.50](picrate)4 · 2CH3CN, (5). X-ray crystallographic studies revealed that complex 5 crystallizes in the triclinic space group P1j with the tetraprotonated cryptand moiety as a cation along with four molecules of picrate as counteranion, 2.5 water molecules and two acetonitrile as solvent of crystallization. Extensive disorder is observed in one arm of the tripodal ligand involving atoms C15 to C22 which also includes the rigid phenyl ring (C16 to C21) with occupancies 0.65 and 0.35, respectively, at two positions and in all the figures associated in the description only atoms with major occupancy factor is shown. Secondary amino nitrogen atoms N2, N3, N7, and N6, which are not the part of disordered arm in the cryptand moiety are protonated, which is evident by the comparatively longer C-N bond distances of these nitrogen atoms with the neighboring carbons (N(2)-C(2) ) 1.495(3) Å, N(2)-C(3) ) 1.510(3) Å; N(3)-C(10) ) 1.486(3) Å, N(3)-C(11) ) 1.498(3) Å and N(7)-C(26) ) 1.491(3) Å, N(7)-C(27) ) 1.497(3) Å and N(8)-C(35) ) 1.487(3) Å, N(8)-C(34) ) 1.497(3) Å). All the water molecules (O29, O30, and O31) present in the lattice are encapsulated within the flexible cryptand cavity by hydrogen bonding interactions are shown in Figure 9. Both O29 and O30 act as donors via O-H · · · N and O-H · · · O interactions with the protonated cryptand and O31 water which is positioned between O29 and O30 with the following hydrogen-bonding parameters and symmetry code (Table 5). O29 and O30 further act as acceptors, and each are involved in two N-H · · · O interactions with the amino nitrogen atom of the cryptand moiety encapsulating the lattice water molecules in the cryptand cavity. These N-H · · · O interactions with symmetry code are given in Table 5. It is important to mention that a similar type of recognition (tritopic) of solvent molecules was observed in complexes 6
2848 Crystal Growth & Design, Vol. 8, No. 8, 2008
Lakshminarayanan et al.
Figure 9. Mercury diagram depicting the anchoring of water molecules inside the cryptand cavity via N-H · · · O and O-H · · · N and O-H · · · O hydrogen bonding interactions. Table 5. Selected Hydrogen-Bond lengths (Å) and Bond Angles (°) of Complex 5a D-H · · · A
D-H [Å]
H· · ·A [Å]
D· · ·A [Å]
D-H · · · A [°]
N(2)-H(2C) · · · O(29)1 N(3)-H(3C) · · · O(30)1 N(7)-H(7D) · · · O(29)1 N(8)-H(8C) · · · O(30)1 O(29)-H(291) · · · O(31)1 O(29)-H(292) · · · N(5)1 O(30)-H(301) · · · *O(31)1 O(30)-H(302) · · · N(6)1
0.8990(17) 0.9013(18) 0.9003(17) 0.8990(18) 0.76(4) 0.94(3) 0.78(2) 1.01(4)
2.0212(17) 1.9322(14) 1.8996(18) 1.9017(15) 2.3000(4) 1.76(3) 2.14(2) 1.70(4)
2.915(2) 2.831(2) 2.789(3) 2.792(2) 2.918(3) 2.684(3) 2.865(4) 2.701(3)
172.79(12) 175.34(12) 169.21(12) 170.36(12) 140(4) 170(3) 155(2) 172(3)
a
(1) x, y, z.
and 7 where L is in the tetraprotonated state as in complex 5.4,15 Further, in all three complexes, 5, 6and 7, the pattern of protonation is also similar; that is, two secondary nitrogen centers of the same strands of both the tren units in the receptor are protonated. This particular fashion of partial protonation of L might have played a pivotal role in making a congenial environment in the receptor cavity for tritopic recognition of solvent molecules but not for the spherical-shaped iodide or bulky picrate anion. Because of the presence of aromatic rings in both the cationic and anionic moieties weak molecular interactions such as stacking of phenyl rings, C-H · · · π, and N-H · · · π interaction contribute in addition to the hydrogen bonding in stabilizing the molecule in the crystal lattice. Packing diagram of the [H4L]4+ with one acetonitrile molecule viewed down the a-axis is shown in Figure 10a. The molecular packing of the cryptand moiety clearly shows that pairs of the cationic moieties are oriented along the b-axis. The acetonitrile molecule acts as an acceptor and is involved in C-H · · · N interaction with the methyl hydrogen of the cryptand (C(25)-H(25B) · · · N(21): H(25B) · · · N(21) ) 2.61 Å, C(25) · · · N(21) ) 3.544(3) Å,
