Controllable Congregating of Homochiral and Achiral Coordination

Publication Date (Web): December 14, 2007 .... A New Strategy of Designing New Crystal Structures Based on Topological Structure: Syntheses and Crysta...
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Controllable Congregating of Homochiral and Achiral Coordination Polymers: Cadmium(II) Pyridine-2,4,6-Tricarboxylate Species with Double-Helical Strand and Molecular Building Block Structures Ru-Qiang Zou,†,‡ Rui-Qin Zhong,†,‡ Miao Du,† Daya S. Pandey,† and Qiang Xu*,†,‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 452–459

National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, and Graduate School of Science and Technology, Kobe UniVersity, Nada Ku, Kobe, Hyogo 657-8501, Japan ReceiVed March 17, 2007; ReVised Manuscript ReceiVed October 9, 2007

ABSTRACT: Three kinds of three-dimensional (3D) cadmium(II) pyridine-2,4,6-tricarboxylate (L) coordination polymers, L-[Cd(L)](NH2Me2) (1a), R-[Cd(L)](NH2Me2) (1b), [Cd3(L)2(H2O)4] (2), and [Cd4(L)2(OH)2(H2O)3](H2O) (3), have been hydro/ solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The variations of reaction medium and temperature not only led to an occurrence of spontaneous chiral resolution but also afforded two achrial molecular building block structures. Complexes 1a and 1b crystallize in trigonal P3121 and P3221 space groups with homochiral left- and right-handed doublehelical strand arrays, while 2 and 3 self-assemble into two kinds of achiral coordination polymers with molecular building block structures. Complexes 2 and 3 remain intact after removal of the coordinated water molecules, which indicates that their dehydrated frameworks possess guest-free metal coordination sites. Complexes 1–3 also display strong fluorescent emissions at 638, 594, and 604 nm in the solid state. Introduction Metal-organic hybrid coordination polymer materials have recently been spurred by potential applications,1–4 which deal with the design of highly organized architectures based upon metal-ion-directed self-assembly processes.5 The predesigned organic ligand is a vital component with binding sites in an appropriate arrangement, whose encoded information is read by the metal ions according to their coordination tendency.6 However, in fact, numerous other factors such as solvent,7a,b concentration,7c,d counterion,7e and temperature7f may play profound roles in the formation of the thermodynamically favored products. On the other hand, the design of chiral coordination polymers, especially by the use of achiral components without any chiral auxiliary, has been stimulated due to their unique properties.4b,8 There have been a few examples of spontaneous resolution in which the chiral information in enantiomeric forms of the building blocks can be transmitted into higher dimensionality to generate chiral coordination polymers.4b,8,9 To date, the decisive physical principles governing spontaneous resolution remain poorly understood. Considering that, it is reasonable that understanding and insight into the self-assembly process can be obtained by carrying out a special study by only changing one of the subtle reaction conditions. Herein, we devoted our effort to the controllable preparation of cadmium(II) coordination polymers with pyridine-2,4,6-tricarboxylate (L), aiming to exploit the subtle tuning effect of the solvent and/or temperature on the coordination fashion and conformation of the ligand as well as the resulting threedimensional (3D) frameworks. To our pleasant surprise, the variation of reaction parameters not only led to an occurrence of spontaneous chiral resolution of a pair of enantiomers L-[Cd(L)](NH2Me2) (1a) and R-[Cd(L)](NH2Me2) (1b) but also afforded two distinct achrial molecular building block structures [Cd3(L)2(H2O)4] (2) and [Cd4(L)2(OH)2(H2O)3](H2O) (3). The 3D network of 1 has an unusual (42.84)(426.83) topology with * To whom correspondence should be addressed. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Kobe University.

binary planar 4-connected nodes, whereas complexes 2 and 3 display decorated R-Po and CdSO4 nets using their molecular building blocks as nodes. The thermal stability, dehydration, and fluorescent properties of these coordination polymers have also been investigated. Experimental Section Materials and General Methods. All the solvents and reagents for syntheses were commercially available and used as received. Pyridine2,4,6-tricarboxylic acid (H3L) was synthesized according to the literature method.10 IR spectra were recorded on a Bruker ISS/v6 apparatus at a spectral resolution of 2 cm-1 accumulating 80 scans. The dry powder sample was mixed by Al2O3 and treated directly in the purpose-made IR cell. The latter was connected to a vacuum-adsorption apparatus with a residual pressure below 10-3 Pa. The cell allowed the IR measurements to be performed at ambient temperature. Elemental analyses were performed on a Perkin-Elmer 2400 Series II analyzer. Thermogravimetric analysis (TGA) was carried out on a Shimadzu DTG-50 thermal analyzer from room temperature to 600 °C at a ramp rate of 5 °C/min in a flowing 150 mL/min helium atmosphere. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku X-ray diffractometer at 40 kV, 100 mA for Cu KR radiation (λ ) 1.5406 Å). Emission spectra were taken on a Perkin-Elmer LS50B luminescence spectrophotometer. Cyclic voltammetry (CV) measurements were carried out on a HZ-5000 automatic polarization system (Hokuto Denko Inc., Japan) with a platinum working electrode and a platinum wire auxiliary electrode at room temperature. Syntheses of Complexes 1–3. L-[Cd(L)](NH2Me2) (1a) and R-[Cd(L)](NH2Me2) (1b). A mixture of Cd(NO3)2 · 6H2O (0.69 g, 2 mmol), H3L (0.21 g, 1 mmol), and DMF (10 mL) was sealed in a vial. The pure colorless needle crystals containing 1a and 1b were obtained after 4 days of heating at 140 °C. Yield: 0.22 g, ∼60% (based on H3L). Anal. Calcd for C10H10CdN2O6: C, 32.76; H, 2.75; N, 7.64. Found: C, 31.85; H, 2.31; N, 7.30. IR (Al2O3, cm-1): 3222 m, 3094 m, 3029 m, 2963 m, 2776 b, 1874 s, 1715 m, 1661 s, 1595 vs, 1487 vs, 1377 s. [Cd3(L)2(H2O)4] (2). A mixture of Cd(NO3)2 · 6H2O (0.69 g, 2 mmol), H3L (0.21 g, 1 mmol), and H2O (10 mL) was sealed in a vial. The pure colorless block crystals were obtained after 4 days of heating at 140 °C. Yield: 0.27 g, ∼65% (based on H3L). Anal. Calcd for C16H12Cd3N2O16: C, 23.28; H, 1.47; N, 3.39. Found: C, 23.82; H, 0.86; N, 3.27. IR (Al2O3, cm-1): 3400 b, 3091 s, 3075 s, 2968 w, 1879 m, 1720 m, 1667 vs, 1646 vs, 1592 vs, 1490 s, 1454 s, 1385 m.

10.1021/cg0702636 CCC: $40.75  2008 American Chemical Society Published on Web 12/14/2007

Controllable Congregating of Coordination Polymers

Crystal Growth & Design, Vol. 8, No. 2, 2008 453

Table 1. Crystallographic Data and Structural Refinement Summary for Complexes 1–3

chemical formula fw space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 T/K Dc/g cm-3 Z µ (Mo KR)/mm-1 Ra/wRb a

1a

1b

2

3

C10H10CdN2O6 366.60 P3121 9.1907(13) 9.1907(13) 12.759(3) 90 90 120 933.4(3) 296(2) 1.957 3 1.779 0.0151/0.0374

C10H10CdN2O6 366.60 P3221 9.1884(13) 9.1884(13) 12.743(3) 90 90 120 1143.9(5) 296(2) 1.960 3 1.782 0.0152/0.0374

C16H12Cd3N2O16 825.48 P1j 6.5548(13) 8.5026(17) 10.020(2) 109.39(3) 96.87(3) 100.86(3) 507.23(18) 296(2) 2.702 1 3.212 0.0193/0.0472

C16H14Cd4N2O18 971.89 P1j 9.977(2) 10.141(2) 12.699(3) 107.10(3) 94.64(3) 112.50(3) 1107.1(4) 296(2) 2.916 2 3.891 0.0409/0.0969

R ) Σ(|Fo| - |Fc||)/Σ|Fo|. b wR ) [Σ(|Fo|2 - |Fc|2)2/Σ(Fo2)]1/2.

[Cd4(L)2(OH)2(H2O)3](H2O) (3). A mixture of Cd(NO3)2 · 6H2O (0.69 g, 2 mmol), H3L (0.21 g, 1 mmol), and H2O/C2H5OH (3:1, 10 mL) was sealed in a vial. The pure colorless block crystals were obtained after 7 days of heating at 185 °C. Yield: 0.19 g, ∼40% (based on H3L). Anal. Calcd for C16H14Cd4N2O18: C, 19.77; H, 1.45; N, 2.88. Found: C, 20.15; H, 1.27; N, 2.93. IR (Al2O3, cm-1): 3400 b, 3091 m, 2968 w, 1843 w, 1713 m, 1592 vs, 1556 vs, 1490 s, 1454 s, 1375 m. X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1–3 were collected on a R-AXIS RAPID II diffractometer at room temperature with Mo KR radiation (λ ) 0.71073 Å).11 All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL.12 Metal atoms in each complex were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses, where they were refined with anisotropic thermal parameters on F2. The hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. The hydrogen atoms of the water molecules were located using the different Fourier method and refined freely. Further details for structural analysis are summarized in Table 1.

Results and Discussion Preparation and Characterization. The H3L ligand is slightly soluble in water at room temperature but well soluble in some organic solvents such as acetonitrile, ethanol, and DMF. To obtain the pure crystals of 2, a slight stir to the mixture is necessary before it is transferred to the sealed vial for hydrothermal reaction. All qualified crystals for X-ray determination are obtained through the hydro/solvothermal technology, which has been widely applied in coordination polymer preparation. Notably, such a complicated process is also attended by the unprecedented in situ organic reactions including ligand oxidative coupling, hydrolysis, and substitution.13 In this work, complexes 1a and 1b were produced via solvothermal synthesis, and the hydrolysis of DMF was observed, in which the hydrolysis reductive NH2Me2+ product is involved in the formation of 1a and 1b to maintain the charge balance, while the oxidative HCOO- product is left in the solution. From a chemical viewpoint, DMF hydrolysis is far from straightforward; however, this has been detected in some examples for anionic coordination networks of aromatic carboxylates incorporating with the NH2Me2+ template.14 It should be noted that the water molecules from the reaction solution participate in the hydrolysis of DMF. This is also proven by adding drops of water in the reaction solvent to shorten the reaction time and improve the reaction yield. The CV behavior of 1 in water also indicates the presence of dimethylamine species (See Supporting Information, Figure S1). The CV curve implies an irreversible redox process of dimethylamine with an oxidaton potential of -0.43

V and a reduction potential of -0.80 V. Well-shaped single crystals of 3 were achieved by the mixed solvents of water and ethanol, so a special Teflon-lined stainless steel vessel that can endure 20 MPa pressure is indispensable due to the saturated vapor tension of ethanol in 185 °C of ca. 20 MPa. The compositions of all new materials were validated by elemental analyses and IR spectra. The phase purities of the bulk samples were identified by PXRD (Figures S2-S4, Supporting Information). It should be noted that complexes 1a, 1b, and 2 are airstable and can retain their structural integrity for a considerable length of time, whereas lattice–water loss occurs for 3 even at room temperature, resulting in an opaque solid. Therefore, all characterization data of 3 were recorded using a fresh sample. The IR spectrum of 1 shows an adsorption peak with medium intensity at 3222 cm-1, indicating the presence of NH2Me2+ ions. In the IR spectra of 2 and 3, a broadband centered at ca. 3400 cm-1 indicates the O-H characteristic stretching vibrations of hydroxyl or water. The absorption bands resulting from the skeletal vibrations of the pyridine rings for all the cases appear in the 1400–1600 cm-1 area. Structural Description of 1–3. L-[Cd(L)](NH2Me2) (1a). Complex 1a crystallizes in chiral space group P3121 and consists of L ligand coordinating to Cd(II) ions in a 1:1 chemometrics ratio with the protonated dimethylamine in molecular cavities to maintain charge balance. Each Cd(II) ion adopts a seven-coordinate pentagonal bipyramidal configuration by coordinating to four separate L ligands in monodentate, chelate, and η3 coordination fashions, and each L ligand links four Cd(II) ions (see Figure 1a). The Cd-N and Cd-O bonds fall in the normal ranges (see Table S1, Supporting Information).15 As shown in Figure 1b, complex 1a self-assembles into a left-handed double-helical strand structure with a 31 helical axis, in which the dimension of the helical patch (12.743(3) Å) is equal to the c-axis length. The shortest Cd · · · Cd separation between the two helical chains is 7.1304(9) Å. It is interesting to note that the two helical chains are firmly enlaced together through the strong coordinating bonds of Cd(II) ions to L ligands, whose shapes are very similar to that of DNA. These adjacent double-helical strands (red triangle as shown in Figure S5, Supporting Information) share the same coordinated Cd(II) nodes to form a homochiral 3D coordination polymer (see Figure 1c). Another notable structural feature of 1a is that it comprises chiral channels along both a and b directions, in which the protonated dimethylamine ions are included (See Figure S6, Supporting Information). Since the protonated dimethylamine species as the corresponding cations fill in the molecular cavities, complex 1a lacks a solvent-accessible void, as indicated by the

454 Crystal Growth & Design, Vol. 8, No. 2, 2008

Zou et al. Table 2. Hydrogen-Bonding Geometry (Å, deg) for 1a and 1ba D-H · · · A

H· · ·A

N(2)-H(2A) · · · O(1) #1 N(2)-H(2B) · · · O(3) #2

2.15 2.42

N(2)-H(2A) · · · O(1) #3 N(2)-H(2B) · · · O(3) #4

2.41 2.14

D· · ·A

D-H · · · A

2.928(2) 2.987(1)

145 121

2.982(1) 2.923(2)

145 121

1a

1b

Symmetry codes: #1 -x + 1, -x + y, -z + 1/3; #2 -x + 2, -x + y, -z + 1/3; #3 x - 1, y, z; #4 x - y, -y + 1, -z + 1/3. a

Figure 1. Views of 1a (a) the coordination environment of the metal center and the binding fashion of L ligand at the 50% probability level. Symmetry codes: A: y, x, - z; B: -y + 1, x - y + 1, z + 1/3; C: -x+ y + 1, -x + 1, z - 1/3; D: x + 1, y + 1, z; E: -x + y, -x + 1, z 1/3; F: -x + 1, -x + y, -z + 1/3; G: y - 1, x + 1, -z; H: x - 1, y - 1, z; (b) the left-handed homochiral double-helical strand extending along the c-direction; (c) the left-handed (red triangle) homochiral double-helical structure along the ab plane.

calculation result using SQUEEZE method in PLATON.16 Furthermore, this structure is also stabilized by the host–guest N-H · · · O hydrogen bonds (see Table 2). R-[Cd(L)](NH2Me2) (1b). Complex 1b is the enantiomer of 1a and crystallizes in chiral space group P3221. The crystal structure of 1b is similar to 1a (see Figure S7, Supporting Information), and is not described in detail herein. The hydrogen-bonding metrics and bond parameters are also shown in Table 2 and Table S2, Supporting Information. Notably, complexes 1a and 1b reveal the spontaneous resolution conglomeration in homochiral coordination polymers

from an achiral ligand. In general, two key factors are necessary to generate a 3D chiral coordination polymer from an achiral ligand without any chiral auxiliary.9a This means that not only the achiral components must assemble into chiral units, but also these chiral units homochirally aggregate. So the bottleneck makes it difficult to realize the homochiral assembly from achical components. In the reported cases, these homochiral coordination polymers often assemble into one-dimensional (1D) helical chain structures by coordination-bonding interactions,17,18 in which the helical chains homochirally aggregate together via the intermolecular weak interactions, such as π-π stacking,19 C-H · · · π and C-H · · · N,20 C-H · · · Cl,21 as well as argentophilic interactions,9a etc. There have been very few reports on 3D homochiral coordination polymers; one example is the Ni(tpt)(NO3)2 (tpt ) tri-4-pyridyl-1,3,5-triazine) coordination polymer with double-helical chains and (12,3) topology.22 Different from the reported 3D homochiral coordination polymer,22 the double-helical strand units of 1a and 1b are linked together by the L ligands to result in the unusual topology. In 1a and 1b, the carboxylate-Cd-carboxylate units provide the basal helical chains, and the ligands act as bridges to create a double-helical strand unit. Furthermore, all the adjacent doublehelical strand units homochirally aggregate into a 3D coordination polymer by sharing the cadmium atoms. [Cd3(L)2(H2O)4] (2). Complex 2 crystallizes in the triclinic P1j space group and consists of L ligands coordinating to Cd(II) ions in a 2:3 chemometrics ratio with coordinated water molecules. As shown in Figure 2a, there are two crystallographically independent Cd(II) centers. The Cd(1) atom in the lattice adopts a seven-coordinate pentagonal bipyramidal configuration by coordinating to four separate L ligands in monodentate, chelate, and η3 coordination fashions, while the Cd(2) atom in the lattice presents a classical six-coordinate octahedral configuration with two symmetric water molecules on the axial vertex and four separate monondentate carboxylate O atoms on the basal plane. All the Cd-O and Cd-N bonds fall into the normal ranges (see Table S3, Supporting Information). Figure 2b shows the molecular building block structure of 2 constructed by four cadmium atoms coordinating to two L ligands. In the building block unit, the Cd(2) atom is located in the crystallographic inversion center with a half-occupancy, so the stoichiometric ratio of L to cadmium should be 2:3 in a single building block unit, implying this building block as a neutral entity. The two pyridyl rings are arranged in a parallel fashion to result in the strong intraunit π-π stacking interactions with the centroid-to-centroid and centroid-to-plane separations of 3.283(2) and 3.185(2) Å. Also, the adjacent pyridyl rings from two separate building blocks present intermolecular π-π stacking interactions with the centroid-to-centroid and centroidto-plane separations of 3.379(2) and 3.199(2) Å. All adjacent molecular building blocks are connected to form a 3D achiral coordination polymer via additional Cd-O coordination forces (Figure 2c). Furthermore, this structure is also stabilized by the

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Figure 2. Views of 2 (a) the coordination environment of the metal center and the binding fashion of L ligand at the 50% probability level. Symmetry codes: A: -x + 2, -y + 2, -z + 1; B: -x + 2, -y + 1, -z + 1; C: -x + 1, -y + 1, -z + 1; D: x, y, z + 1; E: -x + 1, -y + 1, -z; F: x, y, z - 1; (b) molecular building block unit (the yellow ball represents the cavity and dashed line is the π-π stacking between the pyridyl rings); (c) the 3D framework omitting the coordinated water molecules. Table 3. Hydrogen-Bonding Geometry (Å, deg) for 2a D-H · · · A

H· · ·A

D· · ·A

D-H · · · A

O(1W)-H(1WA) · · · O(2) #1 O(1W)-H(1WB) · · · O(6) #2 O(2W)-H(2WA) · · · O(3) #3 O(2W)-H(2WB) · · · O(1W) #4

1.89 1.94 1.92 2.02

2.680(3) 2.734(1) 2.728(2) 2.836(4)

163 163 167 172

a Symmetry codes: #1 -x + 2, -y + 2, -z + 1; #2 -x + 2, -y + 2, -z + 2; #3 -x + 1, -y, -z; #4 x - 1, y - 1, z.

host–guest O-H · · · O hydrogen bonds (see Table 3). It is interesting to note that the two crystallographically independent water molecules coordinated respectively to Cd(1) and Cd(2) atoms can be removed by heating 2 to 180 °C with the maintenance of the residual coordination framework, which is confirmed by the subsequent thermogravimetric and PXRD

Figure 3. Views of 3 (a) the coordination environment of the metal center and the binding fashion of L ligand at the 50% probability level. Symmetry codes: A: -x + 2, -y + 1, -z + 2; B: -x + 2, -y + 1, -z + 1; C: -x + 2, -y, -z + 1; D: -x + 2, -y, -z + 2; E: x, y 1, z; F: -x + 1, -y, - z + 2; G: x, y + 1, z; (b) two kinds of molecular building block units (the balls represent the cavity and purple dashed lines are the π-π stacking between the pyridyl rings); (c) the 3D framework omitting the coordinated water molecules.

experiments. The dehydrated species of 2 possesses a 32.0 Å3 (6.3% of the whole unit cell volume) solvent-accessible void by the calculation using the SQUEEZE method in PLATON.16 [Cd4(L)2(OH)2(H2O)3](H2O) (3). Complex 3 crystallizes in the triclinic P1j space group and consists of four crystallographically independent Cd(II) centers, two L ligands, three coordinated water, two hydroxyl groups and a free water molecule in the asymmetric unit (see Figure 3a). The Cd(1), Cd(2), and Cd(4) centers all adopt seven-coordinate configurations by coordinating to a hydroxyl group in µ2- or µ3-OH mode and a water (or two hydroxyl groups), and three separate L ligands via monodentate, chelate, or η3-mode carboxylate groups, while the Cd(3) center adopts a six-coordinated configuration by coordinating to a water molecule, a µ2-OH hydroxyl group, and three separate L ligands via a κ2-mode and two monodentate carboxylate groups. It should be noted that the Cd-O(13) and Cd-O(14) bond lengths are almost equivalent with an average value of 2.227 Å, which is significantly shorter than the ones of the coordinated water (2.289 Å) and carboxylate groups

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Zou et al.

Table 4. Hydrogen-Bonding Geometry (Å, deg) for 3a D-H · · · A

H· · ·A

D· · ·A

D-H · · · A

O(1W)-H(1WA) · · · O(13) O(1W)-H(1WB) · · · O(2) #1 O(2W)-H(2WA) · · · O(4) #2 O(2W)-H(2WB) · · · O(3W) #3 O(3W)-H(3WA) · · · O(4W) O(3W)-H(3WA) · · · O(11) #4 O(3W)-H(3WB) · · · O(2W) #3 O(4W)-H(2WB) · · · O(2W) #5 O(4W)-H(2WA) · · · O(5) #6 O(13)-H(13) · · · O(3) #1

1.78 2.05 2.06 2.23 2.07 2.49 2.41 2.43 2.16 2.52

2.596(3) 2.863(1) 2.873(2) 2.996(4) 2.706(3) 2.901(3) 2.996(2) 3.058(4) 3.052(2) 3.006(2)

174 173 172 156 135 113 129 134 175 119

Scheme 2. Coordination Modes of L in Complexes 1–3

a Symmetry codes: #1 x, y - 1, z; #2 x - 1, y - 1, z; #3 -x + 1, -y, -z + 1; #4 -x + 1, -y + 1, -z + 2; #5 -x + 2, -y + 1, -z + 1; #6 x, y + 1, z.

Scheme 1. Coordination Modes of Carboxylate Groups: A: Bidentate Chelate; B: anti-anti Bridging; C: syn-syn Bridging; D: µ-O, O′syn-µ-O, O; E: η-O, O′-µ-O, O; F: µ-O, O-η-O, O′-µ-O′, O′

Scheme 3. π-π Interactions of Pyridyl Rings of L Ligands in Complexes 2 (A,B) and 3 (C,D)

(2.443 Å) in 3 (see Table S4, Supporting Information). So O(13) an O(14) should be hydroxyl groups but not coordinated water molecules.23 Figure 3b shows the molecular building block structure of 3 constructed by seven Cd(II), four L ligands, and two hydroxyl groups. The building block represents a neutral entity in charge balance, in which each of two pyridyl rings have a parallel arrangement to form two kinds of π-π stacking ligand pairs (turquoise and violet balls in Figure 3b) with the centroid-to-centroid/centroid-to-plane separations of 3.579(2)/ 3.388(2) and 3.600(2)/3.433(1) Å, respectively. Although the appearance of the represented building block in 3 is apart from the classic ones, such as cubic, diamondoid, honeycomb-like, and so on, their 3D space packing shows the high regulation (see Figure 3c). Similarly to 2, the structure of 3 is also stabilized by the host–guest O-H · · · O hydrogen bonds (see Table 4). The three water molecules coordinated respectively to Cd(2), Cd(3), and Cd(4) ions can be excluded from the coordination framework without structural collapse, which is also validated by the subsequent TG and PXRD experiments. The dehydrated framework of complex 3 possesses a 57.1 Å3 (5.2% of the whole unit cell volume) solvent-accessible void by the calculation using the SQUEEZE method in PLATON.16 From the above description, we can clearly see that despite the rigid and symmetrical skeleton of L ligand, the versatile carboxylate groups exhibit changeable and flexible self-modulation coordination modes (Scheme 1). As shown in Schemes 1 and 2, the ligand of 1 adopts an unusual planar 4-connected mode (I, Scheme 2) to link Cd(II) ions in which the carboxylate groups adopt a bidentate chelate (A, Scheme 1) and two anti-anti bridging coordination modes (B, Scheme 1), while

the ligand of 2 (II, Scheme 2) links six Cd(II) ions in which the carboxylate groups adopt the syn-syn bridging (C, Scheme 1), µ-O, O′syn-µ-O, O (D, Scheme 1), and anti-anti bridging modes (B, Scheme 1), respectively. The two separate ligands of 3 both adopt different 6-connected modes to link Cd(II) ions, in which the carboxylate groups of III adopt syn-syn bridging (C, Scheme 1), µ-O, O′syn-µ-O, O (D, Scheme 1), and η-O, O′-µO, O (E, Scheme 1) coordination modes, while those in IV of η-O, O′-µ-O, O (E, Scheme 1) and µ-O, O-η-O, O′-µ-O′, O′ (F, Scheme 1) coordination modes. Notably, the η3-O,N,O coordination mode is always present in 1–3 due to its preferential and stable chelated configuration. Furthermore, interestingly, complexes 2 and 3 exhibit the strong π-π stacking interactions between the adjacent pyridyl rings (Scheme 3). In general, strong π-π stacking interactions are around 3.3 Å and weaker interactions lie above 3.6 Å, with 3.8 Å being approximately the maximum contact of which π-π stacking interactions are accepted. The displacement angle between the ring-centroid vector (CC) and the normal to one of pryridine planes (CP) averages 27°, in which most examples lie in the range of 16–40°.24 Surprisingly, a scarcely short contact (CC/CP separa-

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Figure 5. TG curves of 1 (red), 2 (green), and 3 (black).

Figure 4. Views of (a) the 3D 4-connected networks of (42.84)(42.6.83) topology with red and purple nodes representing L ligands and green ones respresenting the metal ions in 1a (left) and 1b (right), (b) left, the 3D (4,6)-connected network of (42.84)2(43.63)(48.67)2 topology with violet nodes representing L ligands and turquoise and green ones respresenting the metal ions; right, the 3D 6-connected R-Po network in 2, and (c) the 3D 4-connected CdSO4 network of 3.

tions of 3.283/3.185 Å) between the two pyridyl rings is found in the molecular building block of 2 with a displacement angle of 14°. To our best knowledge, this is one of strongest π-π stacking interaction cases in comparison with the research results from CSD.24 The π-π stacking interactions between the adjacent molecular building blocks of 2 as well as those in 3 all fall into the normal ranges (Scheme 3). Network Topologies of 1–3. To investigate the topology of coordination polymers, a detailed study on the structural features of the ligands and coordination geometries of metal ions should suffice. As described above, the ligand of 1a and 1b shows a planar 4-connected mode to bridge four Cd(II) ions with a C2 symmetry axis, and the Cd(II) ion also exhibits a planar 4-connected mode to link four ligands, resulting in a 3D 4-connected net (see Figure 4a) with the Schläfli symbol of (42.84)(42.6.83).25 In 2, the ligand adopts an asymmetrical 6-connected mode to link six Cd(II) ions, and the two crystallographically independent Cd(II) ions adopt respectively two kinds of regular and distorted planar 4-connected nodes to result in a unique 3D (4,6)-connected net (see Figure 4b left) with the Schläfli symbol of (42.84)2(43.63)(48.67)2.25 So far, a variety of uninodal networks have been reported.26 However, higherdimensional nets with mixed connectivity such as (3,6)-, (4,6)-,

and (4,8)-connected frameworks are quite rare.25,27 On the other hand, if one considers the molecular building blocks of 2 as the nodes, a common 6-connected R-Po net with the Schläfli symbol of 412.63 is observed (see Figure 4b right). In 3, there are two 6-connected crystallographically independent ligands, four Cd(II) ions and two hydroxyl groups, which makes it difficult to summarize its topological structure. However, it is interesting to note that complex 3 shows unusual building blocks with two kinds of parallel π-π stacking ligand pairs (turquoise and violet balls in Figure 3b). The turquoise balls link not only the adjacent congeneric balls but also violet ones by the Cd(II) ions cooperating with hydroxyl groups (shown as purple balls in Figure 3b), while the violet ones only act as bridges to link the adjacent building blocks, which results in a uniform 4-connected CdSO4 net with the Schläfli symbol of 65.8 (see Figure 4c). Thermal Stability. Thermogravimetric analysis (TGA) experiments were conducted to determine the thermal stability of 1-3.28 As shown in Figure 5, for complex 1, two consecutive weight losses start at 290 °C and do not end until heating to 600 °C (peaking positions: 365 and 540 °C), implying that the removal of the trapped protonated dimethylamine accompanies the collapse of the host framework. For 2, the TGA curve shows the weight loss of the coordinated water molecules (calcd: 8.7%; observed: 8.1%) in the temperature range of 144–180 °C (peak: 165 °C). The host framework starts to decompose beyond 370 °C and ends rapidly at 522 °C (peak: 490 °C). In the case of 3, the consecutive weight loss of the free and coordinated water molecules occurs from 33 to 160 °C (calcd: 7.4%; observed: 7.1%; peak: 93 °C). The guest-free framework keeps stable upon 385 °C and then collapses rapidly (peaks: 460 °C). Notably, the host frameworks of 2 and 3 remain intact after losing the coordinated water molecules, as indicated by the PXRD patterns (see Figures S8 and S9, Supporting Information), so their dehydrated frameworks obtain the guest-free metal sites. Fluorescent Emission Properties. The solid-state fluorescent emission properties of 1–3 were investigated at room temperature. Complex 1 exhibits an intense fluorescent emission maximum at λem ) 638 nm (λex ) 485 nm), while 2 and 3 display maxima emissions at 594 and 604 nm (λex ) 448 nm), respectively (see Figure 6). The strong fluorescent emissions of 1–3 may be ascribed to ligand-to-metal charge transfer (LMCT) because the free ligand lacks a similar emission property at the two corresponding excitations.29 The different emission properties between 1 and 2/3 may be attributed to the

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(2) (3) (4) (5)

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Figure 6. Fluorescent emission spectra of 1–3 in the solid state at room temperature (λex ) 485 nm for 1, and λex ) 448 nm for 2 and 3).

very strong stacking interactions of the molecular building blocks in 2 and 3, which result in the rigidity difference of their host frameworks. Furthermore, the coordination diversity of the Cd(II) centers will also be responsible for their different emissions.

(8) (9)

Conclusions and Perspectives Three types of novel cadmium(II) pyridine-2,4,6-tricarboxylate coordination polymers have been hydro/solvothermally prepared and structurally characterized. The variation of reaction conditions not only led to an occurrence of spontaneous chiral resolution but also afforded two achrial molecular building block structures, which are scarcely observed in assemblies of new crystalline materials. Complex 1 belongs randomly to either enantiomorphic space group according to X-ray structure analyses of eight pieces of single crystals, among which three pieces are 1a. Even so, the crystals of 1a and 1b present a large size with the length beyond 1 cm, and the big pieces are cut for the crystals suitable for X-ray measurement. The self-separation of enantiomers makes it possible to develop the chiralcorresponding application materials. The dehydrated host frameworks of 2 and 3 present the significant guest-free Lewis acid sites, which make them as well-catalytic candidates. They also exhibit strong blue emissions and may be potential materials for blue-light emitting diode devices. Supporting Information Available: Crystallographic information in CIF format, additional figures for 1a and 1b, tables and IR spectra for 1–3, and CV curve for 1. This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. We thank AIST and Kobe University for financial support. R.-Q.Z. thanks JSPS for a fellowship (DC) and D.S.P. thanks APS University, Rewa (M.P.) India for due leave.

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