Chiral Metallocycles Templated Novel Chiral Water Frameworks

Henan Vocational College of Chemical Technology, Zhengzhou 450052, People's Republic of China. § State Key Laboratory of Structural Chemistry, Fujian...
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Chiral Metallocycles Templated Novel Chiral Water Frameworks Benlai Wu,*,† Song Wang,† Ruiying Wang,‡ Jinxia Xu,† Daqiang Yuan,§ and Hongwei Hou† †

Department of Chemistry, Zhengzhou University, Zhengzhou 450001, People's Republic of China Henan Vocational College of Chemical Technology, Zhengzhou 450052, People's Republic of China § State Key Laboratory of Structural Chemistry, Fujian Institute of the Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People's Republic of China ‡

S Supporting Information *

ABSTRACT: An intentionally synthesized chiral terpyridyl ligand S-2(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)-benzylamino)propanoic acid (H2L) was used to construct two chiral NiII complexes, [Ni2(HL)2(N3)(H2O)](N3)·19H2O (1) and [Ni2(HL)2(N3)2]·7H2O (2). Both complexes are dinuclear metallocyclic complexes. The monoanionic form (HL)− of the chiral ligand coordinates to the metal centers in a bichelating antiparallel fashion to form the metallomacrocycles where the chiral metallocycles Ni2(HL)2 template the formation of a novel chiral 3D zeolite-like water framework with nanometer cages trapping the metallocycles or anchor chiral water chains in the 1D channels of a resulting chiral 3D supermolecule, respectively.

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INTRODUCTION Since many biological, chemical, and physical processes are involved in water, and so far, the cheapest and cleanest solvent has been agitating intensive interest in theory and experiment to further understand the structures and anomalous properties of bulk water.1 Recent years have observed a number of structurally characterized water clusters,2 water oligomers,3 and low-dimensional water polymers4 cocrystallizing with other organic ligands or guesting in the pores of metal−organic frameworks, clearly indicating the diversity of the association among water molecules in different surroundings. The important roles of water upon the aggregation of protein and the activity of metalloenzymes have long been appreciated,5 but the exact array of water in the hydration shells around those biological macromolecules is far from detection. Thus, the investigation on the well-defined supramolecular architectures of water molecules stabilizing in chiral environments simulating living systems can shed light on its unique properties in biological systems.6 The syntheses of chiral compounds through the selfassembly of optically pure chiral organic ligands with metal ions are a major process for obtaining chiral functional materials.7 The corresponding crystal engineering will boost the exploration for the basic properties about chrial materials, such as ferroelectric properties, asymmetric catalysis, and chiral recognition separation.7,8 Natural amino acids with biological activity, chirality, and coordination variety are simply and practically chiral precursors. Moreover, ligands containing amino acid groups are endowed with plentiful hydrogenbonding interactions, which not only amplify chirality or induce chiral array of guests but also generate novel supramolecular architectures sometimes encapsulating interesting frameworks of guests.9 Very recently, a series of thermodynamically favored © 2012 American Chemical Society

chiral nanometallocycles constructed by metal ions M (M = CdII, NiII, and CuII) and an intentionally designed terpyridyl amino acid ligand H2L (H2L = S-2(4-(2,6-di(pyridin-2yl)pyridin-4-yl)benzylamino)propanoic acid) have been obtained and reported by us, and the results indicate that those similar chiral nanometallocycles can induce chiral array or helicates to amplify chirality through hydrogen-bonding interactions in the finally aggregated, complicated, higherdimensional chiral supermolecules whose architectures are sensitive to free solvents and other terminal ligands.10 Pursuing our work in this area, we herein report two new chiral NiII complexes [Ni2(HL)2(N3)(H2O)](N3)·19H2O (1) and [Ni2(HL)2(N3)2]·7H2O (2), where the chiral metallocycles Ni2(HL)2 template the formation of a novel chiral 3D zeolite-like water framework with nanometer cages trapping the metallocycles or anchor chiral water chains in the 1D channels of a resulting chiral 3D supermolecule, respectively.

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EXPERIMENTAL SECTION

General Procedures. Commercially available reagents were used as received without further purification. Elemental analyses were performed with a Carlo-Erba 1106 elemental analyzer. IR spectra were recorded on a Bruker VECTOR22 spectrophotometer with KBr pellets in 400−4000 cm−1 region. Thermal analysis curves were scanned in a range of 30−680 °C with air atmosphere on a STA 409 PC thermal analyzer. Specific rotation was measured on a PerkinElmer 341 with the wavelength of 589 nm in methanol solution at a temperature of 20 °C. Electronic absorption spectra were recorded on a Unico-2102 UV−vis spectrometer. The solid-state circular dichroism (CD) spectra were recorded on a MOS-450 spectrometer. Powder XReceived: July 13, 2012 Revised: December 10, 2012 Published: December 18, 2012 518

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Scheme 1. Schematic Representation of the Synthesis Strategy for the Chiral Ligand H2L

ray diffraction (PXRD) patterns of the samples were recorded by a RIGAKU-DMAX2500 X-ray diffractometer with Cu Kα radiation. Synthesis of S-2(4-(2,6-Di(pyridin-2-yl)pyridin-4-yl)benzylamino)propanoic acid (H2L). Ligand H2L was synthesized according to the literature (Scheme 1).10 Yield: 1.91 g (77%). mp 264 °C. Elemental analysis (%) calcd for C25H21N4O2: C, 73.32; H, 5.17; N, 13.69. Found: C, 72.86; H, 5.24; N, 13.55. Selected IR (KBr, cm−1): 3425 (w), 3049 (w), 2978 (w), 1603 (m), 1585 (s), 1567 (m), 1466 (m), 1391 (m), 789 (m). Specific rotation [α]20 D = +3.5 for Na(HL) (c 0.02, CH3OH). Synthesis of [Ni2(HL)2(N3)(H2O)](N3)·19H2O (1). A mixture of H2L (41.0 mg, 0.1 mmol), NaOH (4.0 mg, 0.1 mmol), NiCl2·6H2O (23.8 mg, 0.1 mmol), NaN3 (13.0 mg, 0.2 mmol), and H2O (8 mL) was sealed in a 25 mL Parr Teflon-lined stainless steel vessel and put in a temperature-controlled oven to heat at 130 °C for 72 h. After it cooled to room temperature gradually, it resulted in a clear yellow green solution with a slight ashen precipitate. With slow evaporation of the filtrate at ambient temperature for 2 weeks, brown block crystals of 1 suitable for single-crystal X-ray diffraction were obtained. Yield: 38.7 mg (56%). Elemental analysis (%) calcd for C50H82N14Ni2O24: C, 43.53; H, 6.00; N, 14.22. Found: C, 43.79; H, 6.13; N, 14.01. Selected IR (KBr, cm−1): 3416(m), 3233(m), 3062(w), 2978(w), 2028(vs), 1611(s), 1570(m), 1471(m), 1403(s), 1014(m), 789 (m). Specific rotation [α]20 D = −30.4 for 1 (c 0.01, CH3OH). Synthesis of [Ni2(HL)2(N3)2]·7H2O (2). The same reaction system of 1 was heated at 90 °C for 72 h, and then gradually cooled to room temperature. Brown needle crystals of 2 suitable for single-crystal Xray diffraction studies were directly obtained. Yield: 24.6 mg (43%). Elemental analysis (%) calcd for C50H56N14Ni2O11: C, 52.43; H, 4.93; N, 17.13; found: C, 52.10; H, 5.15; N, 16.92. Selected IR (KBr, cm−1): 3415(m), 3237(m), 3067(w), 2985(w), 2021(vs), 1600(s), 1551(w), 1469(m), 1401(s), 1384(m), 1366(m), 1013(m), 787(s). Specific rotation [α]20 D = −34.1 for 2 (c 0.01, CH3OH). X-ray Structural Studies. Single-crystal X-ray data of 1 and 2 were collected on an Oxford diffractometer equipped with a CCD detector and graphite-monochromated Mo Kα radiation from a sealed tube at 288(2) K. Raw data collection and reduction were done using CrysAlisPro software. Empirical absorption correction using spherical harmonics was implemented in the SCALE3 ABSPACK scaling algorithm. Structures were solved by direct methods and refined by full-matrix least-squares on F2 using SHELXTL.11 Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters setting to 1.2 × Ueq of the attached atom. Water H atoms were located in difference maps, and their positions were fixed during the refinement such that they remained in chemically meaningful positions. The crystal and refinement data are collected in Table 1, whereas selected bond angles and distances are given in Tables 2 and 3.

Table 1. Crystal Data and Structure Refinement for 1 and 2 param

1

2

formula temp (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z, ρcalcd (g/cm3) GOF Flack parameter R1, wR2 (I > 2σ(I)) largest diff. peak and hole

C50H82N14Ni2O24 288(2) monoclinic C2 32.703(11) 14.7164(12) 13.4949(13) 90 91.808(18) 90 6491(2) 4, 1.413 1.042 0.26(2) 0.0490, 0.1217 1.050, −0.497

C50H56N14Ni2O11 288(2) triclinic P1 8.0276(7) 10.9031(8) 16.9025(13) 71.996(7) 82.528(6) 87.128(6) 1394.94(19) 1, 1.365 1.037 0.039(17) 0.0650, 0.1823 1.180, −0.458

small amount of NaOH. The hydrothermal reaction system at 130 °C resulted in a clear solution containing hydrated ion pairs [Ni2(HL)2(N3)(H2O)]+ and N3¯, which slowly evaporate to give crystals of 1, whereas at 90 °C, it directly afforded neutral compound 2 as the reaction system slowly cooled to room temperature. We had attempted to isolate the complexes under normal synthetic conditions. However, a mixture of H2L, NiCl2·6H2O, NaN3, NaOH, and H2O in the same molar ratio as in the above hydrothermal reaction systems instantly gave a turbid solution under stirring at room temperature, and the filtered substantive precipitate was an amorphous yellow powder containing green ash, indicating a mixture. Thus, it indicates that the hydrothermally synthetic conditions for compounds 1 and 2 were necessary, although compound 1 was indirectly obtained from the evaporation of the resulted filtrate later. For free ligand H2L, the vibration absorptions of the carboxyl group display two strong peaks centered at 1603 and 1567 cm−1 (Figure S1, Supporting Information). In IR spectra of complexes 1 and 2, the characteristic carboxylate bands are in the ranges of 1600−1611 cm−1 for υas(CO2) and 1401−1403 cm−1 for υs(CO2) (Figure S1, Supporting Information). The frequency difference between υas(CO2) and υs(CO2) suggests a undentate coordination mode of carboxylates in every complex.12 The existence of bulk water in 1 and 2 is observed by IR spectra where the broad absorptions centered around 3416 cm−1 for 1 and 3415 cm−1 for 2 can be assigned to the O−H stretching vibration for the water clusters, very similar to that of 3410 cm−1 for a planar (H2O)6 core structure.13 The moderate absorptions centered around 3233 cm−1 for 1 and

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RESULTS AND DISCUSSION Syntheses and IR Spectra. Both compounds 1 and 2 were intentionally synthesized through hydrothermal reactions of H2L with NiCl2·6H2O and NaN3 in H2O in the presence of a 519

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3237 cm−1 for 2 can be assigned to the N−H stretching vibration of the amino in ligands (HL)−. In addition, the very strong absorptions centered at 2028 cm−1 for 1 and 2021 cm−1 for 2 indicate the existence of N3¯. Crystal Structures. A single-crystal X-ray diffraction study confirmed that compound 1 crystallizes in the chiral space group C2 with a Flack parameter of 0.26(2). Though the check results using PLATON show that there is a possibly higher pseudosymmetry C2/c in the structural model,14 this can be due to the pseudotranslation symmetry of heavy Ni atoms. Its chirality was further confirmed by specific rotation [α]20 D = −30.4 (c 0.01, CH3OH) and the solid-state CD spectrum (vide infra). The asymmetric unit of 1 consists of 1 metallocycle cation [Ni2(HL)2(N3)(H2O)]+, 1 isolated N3¯ anion, and 19 free H2O molecules. In the metallocycle, two deprotonated chiral (HL)− anions in a bichelating fashion antiparallelly bind with two NiII centers to form the generally cyclic structure that is very similar to those recently reported by us.10 Every NiII center in a sharply distorted octahedron is ligated by the three N atoms of the terpyridyl of one HL and one O and N atoms of the amino acid group of the other with the sixth site being occupied by an O/N atom from solvent or counteranion. Ni− O and Ni−N bond distances range from 1.960(6) to 2.193(6) Å (Table 2). The chiral metallocycle cation [Ni2(HL)2(N3)(H2O)]+ in 1 possesses C1 symmetry with permanent chiral carbon atoms C2 and C27 of S-configuration, and labile chiral nitrogen atoms N1 of S-configuration and N5 of Rconfiguration (Figure 1). The overall dimensions of chiral metallocycles in 1 can be expressed by a separation distance of two NiII centers and a separation distance between antiparallel 4-phenyl-2,6-di(pyridin-2-yl)pyridine groups of two HL ligands (an average centroid-to-centroid distance between the central pyridyl of the terpyridyl group of one HL and the phenyl of the other), and the dimensional parameters are 11.13 × 3.45 Å2. This distance of 3.45 Å between the central pyridyl groups of HL ligands indicates that there are stronger intrametallocyclic π−π interactions, and thus, the compressed metallocycle may be stabilized by the stronger π−π interactions between the antiparallel 4-phenyl-2,6-di(pyridin-2-yl)pyridine groups. It is interesting to note that the chiral metallocycle serves as a chiral template to induce the surrounding water molecules to form a chiral 3D network (Figure 2a, Table 3), while the metallocycles occupy the 3D channels constructed by water molecules (Figure 2b). It is markedly different from most reported water clusters where water clusters were captured and stabilized in the host framework or cavity.2a,c,3a,6b,9a,13 In our case, the water molecules occupy nearly 40% of the available space in the unit cell, which is similar to the protein crystal and can enhance the understanding of the structural aspects and formation of a water network in the biosystem. As shown in Figure 3, the 19 lattice water molecules in the asymmetric unit of 1 are connected into a fascinating hydrogenbonding water cluster subunit that is fused with 5 adjacent water cluster subunits through sharing the edges by doubly hydrogen-bonding interactions (Figure S2, Supporting Information). Thus, a 1D P-helical water chain with the helical pitch being 14.72 Å extends along a 2-fold screw axis through doubly hydrogen-bonding interactions between adjacent water cluster subunits (Figure 4a). Because of the C2 operation, those homochiral P-helical water chains array in the ab plane and are linked into a 2D chiral water layer by the hydrogen-bonding connections between adjacent P-helical water chains (Figure 4b). Through complicated hydrogen-bonding interactions,

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2 1 Ni1−N7 Ni1−O1 Ni1−N1 Ni2−O4 Ni2−N2 Ni2−N9

1.972(7) 2.101(5) 2.131(6) 1.958(6) 2.073(7) 2.130(5)

Ni1−O2 Ni1−N6 Ni1−N8 Ni2−N3 Ni2−N4 Ni2−N5

2.035(6) 2.108(6) 2.161(7) 1.998(6) 2.081(8) 2.191(6)

N7−Ni1−O2 O2−Ni1−O1 O2−Ni1−N6 N7−Ni1−N1 O1−Ni1−N1 N7−Ni1−N8 O1−Ni1−N8 N1−Ni1−N8 O4−Ni2−N2 N3−Ni2−N2 N2−Ni2−N4 N3−Ni2−N9 N4−Ni2−N9 N3−Ni2−N5 N4−Ni2−N5

172.4(3) 85.9(2) 107.7(3) 103.9(3) 166.8(2) 77.7(3) 91.5(3) 93.9(3) 96.4(3) 78.1(3) 156.5(3) 88.4(3) 87.6(3) 105.1(2) 93.8(3)

N7−Ni1−O1 N7−Ni1−N6 O1−Ni1−N6 O2−Ni1−N1 N6−Ni1−N1 O2−Ni1−N8 N6−Ni1−N8 O4−Ni2−N3 O4−Ni2−N4 N3−Ni2−N4 O4−Ni2−N9 N2−Ni2−N9 O4−Ni2−N5 N2−Ni2−N5 N9−Ni2−N5

89.1(3) 77.6(3) 86.3(3) 81.5(2) 93.7(3) 96.7(3) 155.3(3) 172.6(3) 106.6(3) 78.5(3) 86.5(2) 88.8(3) 80.1(2) 95.1(3) 166.4(2)

Ni1−N7 Ni1−N8 Ni1−N9 Ni2−O3 Ni2−N12 Ni2−N2

1.999(7) 2.100(6) 2.122(7) 1.979(7) 2.102(8) 2.145(7)

Ni1−O1 Ni1−N6 Ni1−N1 Ni2−N3 Ni2−N4 Ni2−N5

2.005(6) 2.102(5) 2.178(7) 2.014(7) 2.119(7) 2.161(6)

N7−Ni1−O1 O1−Ni1−N8 O1−Ni1−N6 N7−Ni1−N9 N8−Ni1−N9 N7−Ni1−N1 N8−Ni1−N1 N9−Ni1−N1 O3−Ni2−N12 O3−Ni2−N4 N12−Ni2−N4 N4−Ni2−N2 N3−Ni2−N5 N4−Ni2−N5 N3−Ni2−N2

174.7(3) 100.0(3) 103.8(2) 88.2(3) 88.9(3) 106.1(3) 91.5(2) 165.5(2) 83.3(4) 105.6(3) 95.2(3) 154.3(3) 107.5(2) 92.3(2) 76.6(3)

N7−Ni1−N8 N7−Ni1−N6 N8−Ni1−N6 O1−Ni1−N9 N6−Ni1−N9 O1−Ni1−N1 N6−Ni1−N1 O3−Ni2−N3 N3−Ni2−N12 N3−Ni2−N4 O3−Ni2−N2 O3−Ni2−N5 N12−Ni2−N5 N2−Ni2−N5 N12−Ni2−N2

78.5(2) 77.8(2) 156.2(2) 86.7(2) 92.4(2) 78.9(3) 93.1(2) 172.1(3) 89.3(4) 77.8(3) 100.0(3) 79.7(2) 162.7(3) 93.4(2) 86.6(3)

2

those water layers are bridged into a novel chiral 3D water network where the chiral metallocycle cations are encapsulated in its nanocages (Figure 2; Figure S3, Supporting Information). The percent effective free volume in the water network is 72% calculated with PLATON.14 The 3D chiral water network of 1 has inner cages with 3D channels, very similar to the architecture of zeolite (Figure 2). As is well-known, the basic structure unit in zeolite is the silicon−oxygen tetrahedron, which comprises the 3D open framework with inner cages, whereas our water cluster in 1 is formed through an oxygen−hydrogen tetrahedron using Hbonds. This is the only structural similarity between zeolite and the water cluster, and it is hard to know what type it is. Figure 5 520

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Table 3. Hydrogen Bonds of Complexes 1 and 2 1a

Figure 1. Structure of chiral metallocycle cation [Ni2(HL)2(N3)(H2O)]+, showing permanent chiral C atoms C2 and C27 of Sconfiguration, and labile chiral N atoms N1 of S-configuration and N5 of R-configuration.

Figure 2. (a) Space-filling view of the 3D water supramolecular network making larger through-channels down the c axis. (b) A chiral 3D zeolite-like water network encapsulates the chiral metallocycle cations in its nanocages with the violet arrows showing the directions of smaller through-channels.

typically presents the cage structure of the water network. The free volume of the cage can almost accommodate a ball with the diameter of about 12 Å. Every cage enclosed by the 10 water cluster subunits has 2 larger openings and 6 smaller openings to pass through 8 identical cages (Figure 2). As shown in Figures S4 and S5 (Supporting Information), the six water cluster

D···A

d(D···A) (Å)

∠(D−H···A) (deg)

O6···O7 O7···O8 O8···O9 O8···O10 O10···O11 O10···O24 O11···N14 O12···O10 O12···O17 O13···O12 O14···O13 O14···O15 O16···O12 O16···N14 O17···O18 O18···O20 O19···O16 O20···O19 O21···O23 O22···O15 O22···O21 O23···O17 O24···O13 O6···N11A O7···O22B O9···O9C O9···O6C O15···O24D O17···O5E O18···O4E O19···N12F O20···O22G O21···O18H O23···N9G O24···O2C N1···O19F O1···O14C O1···O11C N5···O8I

2.764(10) 2.726(11) 2.759(11) 2.845(11) 2.804(13) 2.854(11) 2.763(11) 2.839(12) 2.811(13) 2.738(13) 2.893(13) 2.695(12) 2.690(12) 2.761(10) 2.753(11) 2.782(11) 2.821(10) 2.811(11) 2.756(14) 2.836(11) 2.808(13) 2.832(11) 2.796(10) 2.799(10) 2.752(12) 2.765(17) 2.843(11) 2.769(9) 2.672(9) 2.852(9) 2.798(11) 2.835(11) 2.833(12) 2.696(10) 2.777(8) 3.154(11) 2.695(11) 2.742(9) 3.123(10) 2b

173.6 159.5 157.3 170.5 163.8 167.7 166.4 170.5 160.7 133.8 141.6 141.4 154.2 175.4 149.7 132.9 142.5 139.5 163.1 136.9 165.8 127.4 135.4 150.6 165.8 95.8 158.7 120.3 151.7 139.0 145.8 135.9 169.1 160.3 165.6 153.2 178.6 174.3 176.9

D···A

d(D···A) (Å)

∠(D−H···A) (deg)

O5···O6 O5···O8 O7···O5 O9···O5 O9···O3 O10···O9 O8···N14 N5···O6A N1···N11B O7···O10C O6···O2D

2.824(11) 3.011(14) 2.710(17) 2.824(13) 2.872(10) 2.869(18) 2.777(18) 3.095(9) 3.107(10) 2.684(19) 2.831(9)

135.8 159.2 160.4 163.3 160.0 152.1 136.0 173.7 166.3 171.1 138.1

a Symmetry codes: A = x, y + 1, z − 1; B = −x + 3/2, y + 1/2, −z; C = −x + 2, y, −z; D = −x + 3/2, y − 1/2, −z; E = x, y + 1, z; F = −x + 2, y, −z + 1; J = −x + 3/2, y + 1/2, −z + 1; H = −x + 3/2, y − 1/2, −z + 1; I = x, y − 1, z. bSymmetry codes: A = x, y − 1, z; B = x + 1, y, z; C = x − 1, y, z; D = x − 1, y + 1, z + 1.

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Figure 3. Fascinating hydrogen-bonding water cluster subunit consisting of 19 water molecules in 1.

Figure 5. View of the cage structure enclosed by the 10 water cluster subunits in the water network of 1, representing the free volume of the cage by an invented ball.

including 1 coordination H2O molecule,15 and completely consists of hydrogen-bonded edge-sharing tetragonal, pentagonal, and hexagonal water rings with various distortions from the ideal plane (Figure S6, Supporting Information). In those water rings, the O···O distances and intraring O···O···O angles cover ranges of 2.605(11)−2.878(9) Å and 85.79−133.47°, respectively. In the water network, water molecules O1, O6, O7, O9, O11, O14, O15, O16, O19, O20, and O23 adopt three-connecting coordination while the other water molecules O8, O10, O12, O13, O17, O18, O21, O22, and O24 adopt the tetrahedral coordination found in ice, Ih.16 The 3D water structure, such as that shown here, is unusual with the exception of ice and clathrate structures. Of particular interest is that the uniquely chiral zeolite-like porous water network is templated by the chiral metallocycle cations, unlike those packaged smaller water clusters and low-dimensional water polymers inside predesigned hosts or coexisting with other organic ligands.2−4 In addition, this represents the first case in which a template’s chirality is observed to be transmitted to a structurally determined 3D water network. Perhaps, the formation of the chiral water network in 1 gives somewhat useful hints about the induced asymmetric array of solvent molecules, such as helixes in the closest hydration shell around chiral macromolecules. Surprisingly, a new crystal [Ni2(HL)2(N3)2]·7H2O (2) formed when the hydrothermal reaction was running at a relatively low temperature of 90 °C. The compound crystallizes in the chiral space group P1 with a Flack parameter of 0.04(2). The structure of 2 is very similar with that of 1 except with 1D chiral water chains replacing the 3D water framework. The dinuclear metallocycle in 2 possesses pseudo-C2 symmetry with permanent chiral carbon atoms C2 and C27, and labile chiral nitrogen atoms N1 and N5 all in an S-configuration (Figure S7, Supporting Information). Six out of the seven crystal lattice H2O molecules in the asymmetric unit take part in the formation of an interesting chiral water chain (Figure S8, Supporting Information; Figure 6). Furthermore, the metallocycle self-assembles into a 1D chain through hydrogen-bonding interactions between N1 and N11, and further fabricates hydrogen-bonding through its uncoordinated carboxylic O and azido N with water molecules in the tetrahedral water cluster units. Thus, every self-assembled metallocycle chain links with two different water chains through hydrogen-bonding O6···O2, O9···O3, O8···N14, and O7···N14 to form a 2D supermolecule

Figure 4. (a) View of 1D P-helical water chain formed by doubly hydrogen-bonding interactions between adjacent water cluster subunits along the b axis. (b) 2D chiral water layer constructed by the hydrogen-bonding connections between adjacent P-helical water chains in 1.

subunits encase the larger 27-membered water opening with dimensions of 13.28 × 16.76 Å2 while the four water cluster subunits fence the smaller 16-membered water opening with dimensions of 9.33 × 9.60 Å2. In the cage, there are multiple hydrogen-bonding occurrences to stabilize the interactions between the host and guest systems (Figure S6, Supporting Information; Table 3). Especially, the coordination water O1 of the metallocycle cation hydrogen bonds with uncyclized water O11 and O14 to finish the last hexagonal water ring. Thus, our water network has the asymmetric unit of 20 H2O molecules, 522

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Figure 6. View of the 2D chiral supermolecule framework constructed by hydrogen-bonding interactions between hydrogen-bonded 1D chiral water chains of tetrahedral water cluster units and metallocycle chains in 2.

(Figure 6). Consequently, the chiral metallocycles can be regarded as models anchoring adjacent chiral water chains to extend chirality in the resulting 2D supramolecular networks. The stacking of dinuclear metallocycles creates 3D chiral supramolecular crystals with chiral water chains anchored in the resultant 1D nanochannels along the a axis (Figure 7). The

Figure 8. As-synthesized and simulated X-ray diffraction patterns of compounds 1 (a) and 2 (b).

diffraction and powered X-ray diffraction. The differences in intensity may be due to the preferred orientation of the powder samples. The existence of bulk water in 1 and 2 is also observed by TGA measurement (Figure 9). The TGA results reveal that the water molecules in 1 and 2 are completely removed up to 265 and 118 °C, respectively. The dehydration process for 1 can be divided into two steps: the fast weight loss of 22.23% and slow weight loss of 4.07% occurring at 30−120 and 120−265 °C, respectively, corresponding to the total loss of the 20 water molecules (calcd 26.10%), and then a successive mass loss of 60.23% occurred at 265−517 °C, indicating the complete decomposition of complex 1. In the crucible, the remains of 13.48% may be the NiO (calcd 10.82%) and unburned carbon.17 For 2, the dehydration process occurred at 30−118 °C with the weight loss of 11.97% for the seven lattice water molecules (calcd 11.00%). On heating, the sample suffered the second decomposition at 265−511 °C with the weight loss of 71.44%, suggesting the total destruction of complex 2. The remaining residue of 15.12% is supposed to be NiO (calcd 13.03%) and unburned carbon.17 Additionally, the remaining residues of the TGA of compounds 1 and 2 were also confirmed by PXRD (Figure S10, Supporting Information). Except for a very weak broad peak around 20° that perhaps results from slightly unburned carbon, the experimental PXRD

Figure 7. View of the 3D chiral supermolecule structure in 2 with chiral water chains anchored in the resultant 1D nanochannels along the a axis.

difference between the structures of water clusters in these two compounds reflects that the 3D water framework in 1 and the 1D water chains in 2 may be controlled by thermodynamics and dynamics, respectively. Powder X-ray Diffraction (PXRD) and Thermal Stability Analysis (TGA). The experimental and simulated PXRD patterns of both complexes are shown in Figure 8. The calculated PXRD patterns from the single-crystal X-ray diffraction data are in agreement with the observed ones, indicating the phase purity of these synthesized crystalline products. Comparatively, the slight peak shifts perhaps result from the baseline drift of the PXRD diffractometer and the difference at measuring temperatures for single-crystal 523

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Figure 10. Solid-state CD spectra of H2L (black), 1 (red), and 2 (green) in KBr. Figure 9. TGA curves of compounds 1 and 2.

anchor chiral water chains in the 1D channels of a resulting chiral 3D supermolecule 2, respectively. To the best of our knowledge, the chiral 3D zeolite-like porous water network in 1 has not been previously found. The water network in 1 is fundamentally important because it may give further understanding of hydration phenomena and of water/ice structures of the hydration shell around solute molecules,19 especially around chiral macromolecules, such as proteins and metalloenzymes in organisms.

patterns for the residues of TGA of compounds 1 and 2 are in agreement with the calculated PXRD pattern of NiO. Clearly, the TGA results show that the dinuclear metallocycles in 1 and 2 are rather stable until 265 °C. Electronic Spectra and Solid-State Circular Dichroism (CD) Spectra Studies. The electronic absorption spectra of Na(HL) and complexes 1 and 2 were recorded in methanol (ca. 1.0 × 10−5 mol/L). They all are characterized by intense absorptions between 200 and 400 nm, ascribed to π → π* transitions associated with the aromatic rings of the ligand (Figure S9, Supporting Information). The low-energy absorptions of free anionic (HL)− appear at 251 and 277 nm with a weaker protracting shoulder centered at 320 nm. For 1 and 2, the low-energy absorptions red shift to 289 nm with a slight shoulder centered at 233 nm and a broad shoulder centered at 344 nm. The obvious red shift for metal complexes is due to the metal-perturbed energy increase of the lowest π orbital and the higher conjugation of the π system of the ligand, as observed for complexes of the terpyridyl-type ligands.10,18 That is, the bichelating coordination of (HL)− and dinuclear metallocycles feature the similarity of the UV−vis spectra of complexes 1 and 2. To confirm the chiroptical activities, the circular dichroism (CD) spectra of free ligand H2L, 1, and 2 were measured in the solid state (Figure 10). Chiral H2L exhibits two Cotton effects around 299 and 339 nm. As for 1 and 2, the Cotton effects around 279 and 314 nm mostly originate from the chiral ligands H2L, but the formation of the chiral structures of 1 and 2 have a considerably great influence on the directions (+ or −) of the CD signals except for an obvious shift of the absorption peak position. Additionally, a new positive Cotton effect around 365 nm for 1 and 360 nm for 2 may result from the formation of chiral metallocycles. As expected, the inherent chirality was transmitted to the supramolecular architectures that were constructed by discrete chiral dinuclear metallocycles and chiral water networks.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, crystallographic data in CIF format for 1 and 2, IR, UV−vis, PXRD patterns, and additional structure figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86 0371 67763675. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (20771094), and the “One Hundred Talent Project” from CAS.

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REFERENCES

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CONCLUSION In summary, by using chiral terpyridyl amino acid ligands H2L, we have constructed two chiral NiII metallocyclic complexes 1 and 2. The chiral metallocycles Ni2(HL)2 template the formation of a novel chiral 3D zeolite-like water framework with nanometer cages trapping the metallocycles in 1 and 524

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