Syntheses, Structures, and Photoluminescences of Four Cd(II

Article pubs.acs.org/crystal

Syntheses, Structures, and Photoluminescences of Four Cd(II) Coordination Architectures Based on 1-(4-Pyridylmethyl)-2methylimidazole and Aromatic Carboxylates: From One-Dimensional Chain to Three-Dimensional Coordination Architecture Fu-Jing Liu, Hong-Jun Hao, Cheng-Jie Sun, Xiao-Hang Lin, Hao-Peng Chen, Rong-Bin Huang,* and Lan-Sun Zheng State Key Laboratory of Physical Chemistry of Solid Surface, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China S Supporting Information *

ABSTRACT: The use of 1-(4-pyridylmethyl)-2-methylimidazole (pmmid) combined with four kinds of benzenecarboxylic acid ligands has allowed the rational design of four novel Cd(II) coordination polymers (CPs), {[Cd(pmmid)(pa)(H2O)]·H2O}n (1), [Cd2(pmmid)2(cba)4] (2), [Cd1.5(pmmid)(tbba)3]n (3), and {[Cd(pmmid)(pma)0.5(H2O)]·5H2O}n (4), [H2pa = phthalic acid, Hcba = 4chlorobenzoic acid, Htbba = 4-tert-butylbenzoic acid, and H4pma = pyromellitic acid], which were synthesized by reactions of Cd(OAc)2·2H2O and pmmid with aromatic acid under the ammoniacal condition and characterized by elemental analysis, infrared (IR) spectroscopy, powder X-ray diffraction (PXRD), and single crystal X-ray diffraction. Complex 1 possesses a one-dimensional (1D) chain architecture, complex 2 is an interesting 1D structure containing two kinds of 1D chains, complex 3 features a two-dimensional 63 topological network structure, and complex 4 is a novel three-dimensional coordination architecture framework containing a T4(2)10(2) water tape. In these complexes, the pmmid ligand takes a bidentate μ2-η1:η1 coordination mode, and the four benzenecarboxylate ligands are able to link Cd(II) ions in various coordination modes, giving these Cd(II) complexes with different structures. In addition, complexes 1−4 exhibit luminescence in the solid state at room temperature.

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INTRODUCTION In recent years, extensive experimental and theoretical efforts have been focused on the rational design and controlled synthesis of metal−organic frameworks (MOFs),1 because such hybrid materials can exhibit a variety of regulated and interesting structural topologies2,3 as well as many potential applications in photoluminescence,4 magnetism,5 catalysis,6 gas storage,7 conductivity,8 nonlinear optics (NLO),9 ion exchange,10 ferroelectricity,11 optoelectronic effect,12 and spintransition behavior.13 In order to build different dimensional frameworks, the crucial step is to choose multifunctional organic ligands and a suitable series of carboxyl acid. According to our recent research, we anticipate that the semirigid N-donor ligand 1-(4-pyridylmethyl)-2-methylimidazole (pmmid) would be a powerful precursor for the construction of diverse structures and topologies: (1) the 2-position substituent methyl effectively enhances the donated electrons' ability of the benzimidazole ring, and makes pmmid exhibit a strong collaborative coordination ability with organic carboxylate ligands; (2) the carbon atom bridging the pyridine and methylimidazole is often a twist point and acts as a variable factor to form different structures. As an important family of multidentate O-donor ligands, organic aromatic carboxylate © 2012 American Chemical Society

ligands have been proven to be excellent structural constructors due to their various coordination modes to metal ions, which often generate multidimensional networks and interesting topologies.14 Moreover, a systematic investigation of the influence of different aromatic carboxylates on the structures is valuable. Taking all of the above discussion into account, by introducing a series of aromatic carboxylate coligands into the Cd(II)-pmmid synthesis system, four coordination polymers with different structures and dimensionalities, namely, {[Cd(pmmid)(pa)(H2O)]·H2O}n (1), [Cd2(pmmid)2(cba)4] (2), [ Cd 1 . 5 (pmmid)(tbba) 3 ] n (3 ), and {[Cd(pmmid)(pma)0.5(H2O)]·5H2O}n (4), have been obtained (Scheme 1). The crystal structures and the effect of different aromatic carboxylates on the ultimate frameworks will be represented and discussed. In addition, the photoluminescence properties of 1−4 in the solid state have also been investigated below in detail. Received: December 27, 2011 Revised: March 1, 2012 Published: March 2, 2012 2004

dx.doi.org/10.1021/cg201708x | Cryst. Growth Des. 2012, 12, 2004−2012

Crystal Growth & Design

Article

resultant solution was allowed to evaporate slowly in darkness at room temperature for several days to give brown crystals of 1 (Yield: 73%, based on cadmium). They were washed with a small volume of cold ethanol. Anal. Calcd (found) for CdC18H19N3O6: C, 44.51 (44.13); H, 3.94 (3.89); N, 8.65 (8.71) %. IR (KBr): ν(cm−1) = 3370 (m), 3130 (m), 1603 (s), 1559 (s), 1505 (s), 1486 (s), 1448 (s), 1410 (s), 1284 (m), 1227 (m), 1140 (m), 1085 (w), 1072 (w), 1036 (w), 1018 (w), 997 (w), 957 (w), 868 (m), 844 (m), 817 (m), 758 (s), 720 (m), 702 (s), 688 (m), 652 (m), 635 (m), 587 (m), 488 (m), 442 (m). [Cd2(pmmid)2(cba)4] (2). The synthesis of 2 was similar to that of 1, but with Hcba (313 mg, 0.2 mmol) in place of H2pa. Brown crystals of 2 were obtained in 28% yield (based on cadmium). They were washed with a small volume of cold ethanol. Anal. Calcd (found) for Cd2C48H39Cl4N6O8: C, 48.26 (47.98); H, 3.29 (3.25); N, 7.04 (6.99) %. IR (KBr): ν(cm−1) = 3443 (s), 1615 (m), 1591 (s), 1546 (s), 1504 (w), 1427 (s), 1403 (s), 1324 (w), 1286 (m), 1224 (w), 1162 (w), 1144 (w), 1095 (m), 1071 (w), 1013 (m), 853 (m), 812 (w), 771 (s), 719 (w), 686 (m), 673 (w), 584 (w), 530 (m). [Cd1.5(pmmid)(tbba)3]n (3). The synthesis of 3 was similar to that of 1, but with Htbba (36 mg, 0.2 mmol) in place of H2pa. Brown crystals of 3 were obtained in 58% yield (based on cadmium). They were washed with a small volume of cold ethanol. Anal. Calcd (found) for Cd1.5C43H50N3O6: C, 59.13 (59.66); H, 5.77 (5.68); N, 4.81 (4.78) %. IR (KBr): ν(cm−1) = 3446 (s), 2963 (m), 1613 (s), 1596 (s), 1556 (s), 1508 (w), 1425 (m), 1393 (s), 1362 (w), 1288 (w), 858 (w), 789 (m), 734 (w), 713 (m), 688 (w), 672 (w), 583 (w), 530 (w), 483 (w), 454 (w). {[Cd(pmmid)(pma)0.5(H2O)]·5H2O}n (4). A mixture of Cd(OAc)2·2H2O (27 mg, 0.1 mmol), pmmid (17 mg, 0.1 mmol), and H4pma (25 mg, 0.1 mmol) was stirred in a methanol−ethanol mixed solvent (6 mL, v/v: 1/1). Then aqueous NH3 solution (25%, 1 mL) was dropped into the mixture to give a clear solution under ultrasonic treatment. The resultant solution was allowed to evaporate slowly in darkness at room temperature for several days to give light brown crystals of 4 (yield: 35%, based on cadmium). Anal. Calcd (found) for CdC15H24N3O10: C, 34.73 (35.13); H, 4.66 (4.58); N, 8.10 (8.11) %. IR (KBr): ν(cm−1) = 3416 (s), 1575 (s), 1493 (m), 1424 (s), 1385

Scheme 1. Preparation Route of Cd(II)/pmmid CPs in the Presence of Different Aromatic Carboxylates

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EXPERIMENTAL SECTION

Materials and General Methods. All the reagents and solvents employed were commercially available and used as received without further purification. Infrared spectra were recorded on a Nicolet AVATAT FT-IR330 spectrometer as KBr pellets in the frequency range 4000−400 cm−1. The elemental analyses (C, H, N contents) were determined on a CE instruments EA 1110 analyzer. Photoluminescence measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer with solid powder on a 1 cm quartz round plate. Preparation of Complexes 1−4. {[Cd(pmmid)(pa)(H2O)]·H2O}n (1). A mixture of Cd(OAc)2·2H2O (27 mg, 0.1 mmol), pmmid (17 mg, 0.1 mmol), and H2pa (17 mg, 0.1 mmol) was stirred in a methanol−water mixed solvent (6 mL, v/v: 5/1). Then aqueous NH3 solution (25%, 1 mL) was dropped into the mixture to give a clear solution under ultrasonic treatment. The

Table 1. Crystal Data for 1−4 complexes formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) Dc (g cm−3) μ(mm−1) F(000) no. of unique reflns no. of obsd reflns [I > 2σ(I)] parameters GOF final R indices [I > 2σ(I)]a,b R indices (all data) largest difference peak and hole (e Å−3) a

1

2

3

4

CdC18H19N3O6 485.77 monoclinic P21/n 9.4732(19) 16.447(3) 12.033(3) 90.00 95.480(4) 90.00 4 1866.3(7) 1.675 1.209 916 3278 2916 253 0.869 R1 = 0.0350 wR2 = 0.0838 R1 = 0.0415 wR2 = 0.0878 1.005 and −0.411

Cd2C48H39Cl4N6O8 1194.49 triclinic P1̅ 12.395(3) 12.876(3) 16.525(4) 74.440(4) 74.126(6) 88.730(6) 4 2440.1(10) 1.626 1.150 1194 8408 6865 613 1.110 R1 = 0.0798 wR2 = 0.1978 R1 = 0.0942 wR2 = 0.2081 3.125 and −1.169

Cd1.5C43H50N3O6 873.49 monoclinic P21/n 19.301(3) 12.677(2) 19.591(3) 90.00 115.071(3) 90.00 2 4342.1(12) 1.336 0.788 1796 7594 6590 494 1.236 R1 = 0.0555 wR2 = 0.1334 R1 = 0.0644 wR2 = 0.1380 1.294 and −0.680

CdC15H24N3O10 518.78 monoclinic P21/c 11.443(4) 9.828(3) 19.074(7) 90.00 107.249(6) 90.00 4 2048.6(12) 1.682 1.123 1052 3604 2844 262 1.248 R1 = 0.0672 wR2 = 0.1524 R1 = 0.0868 wR2 = 0.1615 1.141 and −1.055

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]0.5. 2005



Article

Table 2. Selected Bond Lengths (Å) and Angles (°) for 1−4 Complex 1a i

Cd1O1 Cd1N1 O1iCd1N1 O1iCd1O4 N1Cd1O4 O1iCd1O1W

2.250(3) 2.256(3) 98.56(11) 102.17(10) 158.64(11) 76.48(10)

Cd1O2 Cd1N3 Cd1O4 Cd1N1iii O2Cd1N3 O2Cd1O4 N3Cd1O4 O2Cd1N1iii N3Cd1N1iii O4Cd1N1iii O2Cd1O3 N3Cd1O3 O4Cd1O3

2.199(6) 2.273(7) 2.339(6) 2.371(7) 137.8(3) 119.6(2) 90.5(2) 113.8(2) 86.1(2) 100.0(2) 83.6(2) 137.9(2) 53.30(19)

Cd1O5 Cd1N1i Cd1O3 O5Cd1N1i O5Cd1O3 N1iCd1O3 O5Cd1O2 N1iCd1O2 O3Cd1O2

2.179(4) 2.242(4) 2.274(4) 100.92(15) 87.65(16) 87.76(14) 99.56(14) 156.75(14) 103.99(13)

Cd1N3i Cd1O1W N3iCd1O1W N3iCd1N1 O1W−Cd1N1 N3iCd1O1 O1W−Cd1O1 N1Cd1O1

2.289(6) 2.317(6) 167.3(2) 90.1(2) 96.4(2) 91.7(2) 92.0(2) 132.33(19)

Cd1O1W Cd1N3ii O4Cd1N3ii O1W−Cd1N3ii O1iCd1O3 N1Cd1O3 Cd1O3 Cd1O4iv Cd2N6 N1iiiCd1O3 O2Cd1O4iv N3Cd1O4iv O4Cd1O4iv N1iiiCd1O4iv O3Cd1O4iv N6Cd2O8 N6Cd2O6 O8Cd2O6 Cd1O2 Cd1O1

2.375(3) 2.399(3) 87.94(11) 159.08(11) 144.56(10) 106.05(10) Complex

Cd1O4 N1Cd1O1W O4Cd1O1W O1iCd1N3ii N1Cd1N3ii 2b

2.523(6) 2.609(6) 2.286(7)

Cd2O8 Cd2O6 Cd2O7

80.9(2) 80.3(2) 81.0(2) 73.8(2) 165.6(2) 104.53(18) 97.9(2) 135.4(2) 122.0(2) Complex 2.355(3) 2.394(4)

N6Cd2O7 O8Cd2O7 O6Cd2O7 N6Cd2N4i O8Cd2N4i O6Cd2N4i O7Cd2N4i N6Cd2O5ii O8Cd2O5ii 3c Cd1N3 Cd2O4

2.300(3) 89.28(11) 90.60(10) 124.19(11) 84.63(12) 2.327(7) 2.341(6) 2.415(7) 139.3(2) 54.5(2) 84.2(2) 85.3(2) 83.7(2) 80.5(2) 116.0(2) 83.2(2) 106.7(2) 2.455(4) 2.237(3)

Cd1O3 O4Cd1O3 O1W−Cd1O3 N3iiCd1O3

Cd2N4i Cd2O5ii Cd2O5 O6Cd2O5ii O7Cd2O5ii N4iCd2O5ii N6Cd2O5 O8Cd2O5 O6Cd2O5 O7Cd2O5 N4iCd2O5 O5iiCd2O5 Cd2O6 Cd2O2

O5Cd1O1 N1iCd1O1 O3Cd1O1 O2Cd1O1 O5Cd1N3 N1iCd1N3

153.67(14) O3Cd1N3 102.78(14) O2Cd1N3 104.53(14) O1Cd1N3 55.17(12) O4Cd2O4ii 78.69(16) O4Cd2O6 91.16(14) O4iiCd2O6 Complex 4d

165.85(16) 82.16(13) 89.47(14) 180.000(1) 96.24(15) 83.76(15)

O6Cd2O6ii O4Cd2O2 O4iiCd2O2 O6Cd2O2 O6iiCd2O2 O2Cd2O2ii

Cd1N1 Cd1O1 N3iCd1O2 O1W−Cd1O2 N1Cd1O2 O1Cd1O2 N3iCd1O3ii

2.329(7) 2.416(5) 109.6(2) 82.3(2) 80.8(2) 53.94(17) 88.8(2)

2.419(5) 2.431(5) 81.7(2) 80.0(2) 147.67(17) 153.33(19) 86.6(2)

Cd1O4ii

Cd1O2 Cd1O3ii O1W−Cd1O3ii N1Cd1O3ii O1Cd1O3ii O2Cd1O3ii N3iCd1O4ii

O1W−Cd1O4ii N1Cd1O4ii O1Cd1O4ii O2Cd1O4ii O3iiCd1O4ii

2.580(3) 53.12(9) 78.70(9) 83.78(10)

2.423(7) 2.515(6) 2.529(6) 101.60(19) 78.6(2) 165.5(2) 88.4(2) 173.3(2) 53.0(2) 119.0(2) 99.0(2) 71.8(2) 2.240(4) 2.301(3) 180.000(1) 89.74(13) 90.26(14) 90.38(13) 89.62(13) 180.000(1) 2.441(5) 81.04(19) 133.19(19) 94.44(17) 143.47(17) 53.31(17)

a Symmetry codes: (i) −x + 2, −y + 1, −z + 2; (ii) −x + 1, −y + 1, −z + 1. bSymmetry codes: (i) −x, −y + 1, −z + 1; (ii) −x + 1, −y + 1, −z + 1; (iii) −x + 1, −y + 1, −z; (iv) −x, −y + 1, −z. cSymmetry codes: (i) −x + 1/2, y − 1/2, −z + 3/2; (ii) −x + 1, −y + 1, −z + 2. dSymmetry codes: (i) −x, y + 1/2, −z + 3/2; (ii) −x + 1, y − 1/2, −z + 3/2.

(s), 1321 (m), 1285 (w), 1223 (w), 1139 (w), 1069 (w), 1003 (w), 872 (m), 816 (m), 767 (m), 721 (m), 685 (m), 626 (m), 583 (m), 528 (m), 484 (m). X-ray Crystallography. Single crystals of the complexes 1−4 with appropriate dimensions were chosen under an optical microscope and quickly coated with high vacuum grease (Dow Corning Corporation) before being mounted on a glass fiber for data collection. Data for 1−4 were collected on a Bruker-AXS CCD single-crystal diffractometer with graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). A preliminary orientation matrix and unit cell parameters were determined from three runs of 20 frames each, and each frame corresponds to a 0.3° scan in 5 s, followed by spot integration and least-squares refinement. Their data were measured using ω scans of 0.3° per frame for 10 s until a complete hemisphere had been collected. Cell parameters were retrieved using SMART software and refined with SAINT on all observed reflections.15 Data reductions were performed with the SAINT software and corrected for Lorentz and polarization effects. Absorption corrections were applied with the program SADABS.15 In all cases, the highest possible space group was

chosen. All structures were solved by direct methods using SHELXS9716a and refined on F2 by full-matrix least-squares procedures with SHELXL-97.16b Atoms were located from iterative examination of difference F maps following least-squares refinements of the earlier models. Hydrogen atoms were placed in calculated positions and included as riding atoms with isotropic displacement parameters 1.2− 1.5 times Ueq of the attached C or N atoms. The hydrogen atoms attached to oxygen were refined with O−H = 0.85 Å, and Uiso(H) = 1.2Ueq(O). All structures were examined using the Addsym subroutine of PLATON17a to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collated in Table 1. Selected bond lengths and angles for 1−4 are collated in Table 2. The hydrogen bond geometries for 1 and 4 are shown in Table S1 (Supporting Information). The π···π and C−H···π interactions for 1−4 are shown in Table S2 (Supporting Information). 2006



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RESULTS AND DISCUSSION Syntheses. The syntheses of complexes 1−4 were summarized in Scheme 1. Ultrasonic technique can produce high local temperatures and pressures, combined with extraordinarily rapid cooling, providing a unique means for driving chemical reactions under extreme conditions. In this system, ultrasound technique also realizes the rapid (10 min) and efficient (max. 30 different experiments in one batch) preparation of CPs.18 Structure Descriptions. {[Cd(pmmid)(pa)(H2O)]·H2O}n (1). Complex 1 is a 1D chain structure crystallizing in the monoclinic crystal system with space group of P21/n. The asymmetric unit contains one Cd(II) ion, one pmmid ligand, one pa anion, one coordinated water molecule, and one lattice water molecule. As shown in Figure 1a, the center Cd(II) atom

As shown in Figure 1b, it is noteworthy that a pair of oppositely arranged μ2-pmmid ligands bind two Cd(II) ions to form a [Cd2(pmmid)2] ring subunit. On the other hand, a pair of oppositely arranged pa anions adopting μ2-η1:η1:η1:η0 coordination mode bind two Cd(II) ions to form another kind of subunit, that is, [Cd2(pa)2] binuclear subunit, which is strengthened by hydrogen bonds between the lattice water molecule and two oxygen atoms of two intrasubunit pa anions (O2W−H2WA···O2iii and O2W−H2WB···O4iv; Table S1, Supporting Information). These two kinds of subunits link each other by sharing the Cd(II) atoms with the adjacent subunits, together with the coordination of the coordinated water molecule, to form the resultant 1D infinite chain. Furthermore, the coordinated water molecule is hydrogen bonded to two oxygen atoms from the pa anions of intra- and interchain, respectively (O1W−H1WA···O3iii and O1W− H1WB···O2Wiii, Table S1, Supporting Information), together with the π···π interaction between the rings of imidazole and benzene ring of aromatic carboxylates, the centroid···centroid distance and dihedral angle (angle of a plane defined by the mentioned ring to the second mentioned ring plane, the same below) are 3.839(3) Å and 9.5(2)°, respectively (Table S2, Supporting Information), to extend the 1D chain into a 2D network (as shown in Figure 1c,d). (Symmetry codes: (i) −x + 2, −y + 1, −z + 2; (iii) −x + 1, −y + 1, −z + 2; (iv) x − 1, y, z.) [Cd2(pmmid)2(cba)4] (2). Complex 2 is a 1D CP crystallizing in the triclinic crystal system with space group of P1̅ with an asymmetric unit that contains two Cd(II) ions, two pmmid ligands, and four cba anions. As illustrated in Figure 2a, Cd1 is six-coordinated with distorted octahedral coordination geometry by two nitrogen atoms from two different pmmid ligands [Cd1−N3 = 2.273(7), Cd1−N1iii = 2.371(7) Å] and four oxygen atoms from three different cba anions with an average Cd1−O bond distance of 2.418(6) Å, while Cd2 is seven-coordinated by two nitrogen atoms from two different pmmid ligands [Cd2−N6 = 2.286(7), Cd2−N4i = 2.423(7) Å] and five oxygen atoms from three different cba anions with an average Cd2−O bond distance of 2.425(7) Å. The bond angles around Cd1 and Cd2 ranged from 53.30(19) to 165.6(2) and 53.0(2) to 173.3(2)°, respectively. Pairs of oppositely arranged μ2-pmmid ligands bind two Cd(II) ions to form two kinds of [Cd2(pmmid)2] ring subunits with slightly differences (as shown in Table 3). Except for these subunits, two kinds of [Cd2(cba)4] subunits also exist in 2, which extend the [Cd2(pmmid)2] subunits to two kinds of 1D chains [chain (I) and chain (II)], respectively (as illustrated in Figure 2b). In chain (I), π···π interaction between the intrachain rings of imidazole and benzene ring of aromatic carboxylates (the average centroid···centroid distance and dihedral angle are 3.773(6) Å and 10.1(5)°, respectively, Table S2, Supporting Information) also exists to reinforce it. Furthermore, the π···π interaction between pyridine rings of chain (I) and chain (II) (as illustrated in Figure 2c; the centroid···centroid distance and dihedral angle are 3.790(6) Å and 20.2(5)°, respectively, Table S2, Supporting Information) interlinks these two kinds of chains into an infinite 2D network. (Symmetry codes: (i) −x + 1/2, y − 1/2, −z + 3/2; (iii) −x + 1, −y + 1, −z.) [Cd1.5(pmmid)(tbba)3]n (3). X-ray single-crystal analysis reveals that 3 crystallizes in the monoclinic space group P21/n whose asymmetric unit contains one and a half Cd(II) ions, one pmmid ligand, and three tbba anions. As illustrated in Figure 3a, Cd1 adopts a distorted octahedral arrangement, being

Figure 1. (a) The coordination environment of the Cd(II) ion and the linkage mode of ligand in 1 with 50% thermal ellipsoid probability. Hydrogen atoms and the lattice water molecule are omitted for clarity. (b) The 1D chain structure. (c) The 2D network extended by hydrogen bonds. (d) The π···π interactions. (Symmetry codes: (i) −x + 2, −y + 1, −z + 2; (ii) −x + 1, −y + 1, −z + 1.)

is coordinated by two nitrogen atoms from two different pmmid ligands with Cd−N bond lengths of 2.256(3) and 2.399(3) Å, four oxygen atoms from two pa anions and one water molecule [Cd1−O1i = 2.250(3), Cd1−O4 = 2.300(3), Cd1−O3 = 2.580(3), Cd1−O1W = 2.375(3) Å], forming a distorted octahedral coordination geometry. The bond angles around Cd(II) ion range from 53.12(9) to 159.08(11)°. 2007



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Figure 2. (a) The coordination environment of the Cd(II) ion and the linkage mode of ligand in 2 with 50% thermal ellipsoid probability. Hydrogen atoms are omitted for clarity. (b) The two kinds of 1D chains. (c) The π···π interactions between adjacent chains. (Symmetry codes: (i) −x, −y + 1, −z + 1; (ii) −x + 1, −y + 1, −z + 1; (iii) −x + 1, −y + 1, −z; (iv) −x, −y + 1, −z.)

Table 3. Configurations of the pmmid Ligand in 1−4a complexes

1

angle (deg)

111.0(3)

dihedral angle (deg)

75.66

2 113.3(7)/ 111.5(7) 83.18/80.90

3

4

113.2(4)

113.2(6)

89.53

78.55

Figure 3. (a) The coordination environment of the Cd(II) ion and the linkage mode of ligand in 3 with 50% thermal ellipsoid probability. Hydrogen atoms are omitted for clarity. (b) The [Cd3(pmmid)6] subunit. (c) The 2D 63 network extended by hydrogen bonds. (Symmetry codes: (i) −x + 1/2, y − 1/2, −z + 3/2; (ii) −x + 1, −y + 1, −z + 2.)

a

Angle is the angle around the carbon atom bridging the rings pyridine and methylimidazole; dihedral angle is formed by the rings of pyridine and methylimidazole of the pmmid.

distorted octahedral coordination geometry with average Cd2− O bond distance of 2.259(4) Å, the bond angles around Cd2 span the range of 83.76(15)−180°. As shown in Figure 3b, three pairs of oppositely arranged tbba anions bind three Cd(II) ions (two Cd1 and one Cd2) to form the [Cd3(tbba)6] subunit. The intrasubunit C−H···π interaction between the tbba anions with an edge-to-face orientation further strengthens the trinuclear subunit (d = 2.71 Å, A = 155°; d and A stand for H···π separation and C−H···π

ligated by four oxygen atoms from three different tbba anions with an average Cd1−O bond distance of 2.301(4) Å, and two nitrogen atoms from two different pmmid ligands with Cd1−N bond lengths of 2.242(4) and 2.455(4) Å, respectively, and the bond angles around it range from 55.17(12) to 165.85(16)°. While Cd2 is located in an inversion center and is coordinated by six oxygen atoms from six different tbba anions to form a 2008



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angle in the C−H···π pattern, respectively, Table S2, Supporting Information),19 as calculated by the PLATON program.17b,c As shown in Figure 3c, it is noteworthy that the tbba anions adopt two different kinds of coordination modes, which are μ2η1:η1 and μ2-η2:η1. Then the trinuclear subunits are linked to form an infinite 2D network by pmmid ligands with μ2-η1:η1 bridging mode. A better insight into the 2D network of 3 can be achieved by using the topological approach. If we consider each Cd1 atom as a 3-connected node, which is connected to two neighbors by the μ2-pmmid ligands, at the same time, considering each [Cd3(pmmid)6] subunit as an edge, then this 2D layer can be rationalized as a 3-connected 63 topological network. {[Cd(pmmid)(pma)0.5(H2O)]·5H2O}n (4). X-ray singlecrystal diffraction analysis reveals that 4 is a 3D coordination architecture framework. It crystallizes in the monoclinic crystal system with space group of P21/c with an asymmetric unit that contains one Cd(II) ion, a half of pma anion, one pmmid ligand, one coordinated water molecule, and five lattice water molecules (as illustrated in Figure 4a). The crystallographically independent Cd(II) ion is coordinated by two nitrogen atoms from two different pmmid ligands with Cd−N bond lengths of 2.289(6) and 2.329(7) Å, five oxygen atoms from two pma anions and one coordinated water molecule with average Cd− O bond distance of 2.405(7) Å, with the bond angles around it being in the range of 53.31(17)−167.3(2)°. As illustrated in Figure 4b, pma anions link Cd(II) ions to form a 2D network with μ4-η1:η1:η1:η1:η1:η1:η1:η1 coordination mode. Considering each pma as a 4-connected node bridged to four neighbors by the Cd(II) atoms, the network can be simplified to a 44-sql topological network. Furthermore, the pmmid ligands bridge the adjacent networks to from a 3D coordination architecture (as illustrated in Figure 4c). As illustrated in Figure 4d, a notable feature within 4 is the presence of a 1D novel water tape with alternating (H2O)10 and (H2O)4 clusters. Five water molecules (O2W, O6W, O3W, O5W, and O4W) and their symmetric equivalents form a (H2O)10 cluster. Then they are fused together to form a 1D T4(2)10(2) water tape through the O3W···O5Wvi hydrogen bond along the b axis. Furthermore, the O3W (donor) is also hydrogen bonded to the coordinated O1W (acceptor), to make the water tape link up with the 3D coordination architecture (Table S1, Supporting Information). Except for abovementioned interactions, the C−H···π interactions also exist in 4, which have reinforced the 3D framework structure (as illustrated in Figure 4e; Table S2, Supporting Information). Structural Comparison of CPs 1−4 and the Influencing Factors. As it is shown in the descriptions above, a novel family of cadmium(II) CPs 1−4 with the 1-(4-pyridylmethyl)2-methylimidazole and different carboxylate ligands were successfully synthesized and characterized. On the basis of the X-ray analysis results, the crystal structures of CPs 1−4 ranging from 1D chains to 3D coordination architecture indicate that not only the introduction of different auxiliary carboxylate ligands into the cadmium/pmmid system but also the different space configurations of the pmmid ligand (Table 3) play important roles in determining the structures of the cadmium(II) CPs. In the four CPs, both carboxylates and pmmid exhibit the capability of diversifying the structures through changing their coordination modes (Scheme 2) or configurations. All the Cd−O (2.179(4)−2.609(6) Å) and Cd− N (2.242(4)−2.455(4) Å) bond lengths are typical and

Figure 4. (a) The coordination environment of the Cd(II) ion and the linkage mode of ligand in 4 with 50% thermal ellipsoid probability. Hydrogen atoms and lattice water molecules are omitted for clarity. (b) The 2D 44-sql network. (c) The 3D coordination architecture. (d) The 1D water tape. (e) The C−H···π interactions. (Symmetry codes: (i) −x, y + 1/2, −z + 3/2; (ii) −x + 1, y − 1/2, −z + 3/2; (iv) −x + 1, −y + 1, −z + 1; (v) −x + 2, −y + 1, −z + 1; (vi) −x + 2, −y + 2, −z + 1.)

comparable with those of other similar Cd(II)-carboxylate complexes in the literature. 20 For 1 and 2, similar [Cd 2(pmmid) 2 ] ring subunits are contained, then μ2 η1:η1:η1:η0 pa and μ2-η2:η1/μ1-η1:η1/μ1-η1:η0-cba anions contribute to the extension of the subunit to form 1D chain CPs. In 2, due to the different coordination modes of the cba anions and configurations of the pmmid ligand, two different kinds of 1D infinite chains are constructed. For 3, both μ2-η2:η1 and μ2η1:η1 coordination modes are adopted by the tbba anions to form the [Cd3(tbba)6] subunit, which is extended to be a 2D 63 topological network by the bidentate μ2-N,N′ bridging pmmid ligand. In 4, the pma anions adopting μ4-η1:η1:η1:η1:η1:η1:η1:η1 coordination mode coordinate with the Cd(II) ions to form a 2D infinite 44-sql topological network, then μ2-pmmid ligand 2009



Article

Scheme 2. Coordination Modes of Different Aromatic Carboxylates

Figure 5. Photoluminescences of free pmmid ligand and complexes 1−4.

transition of the pmmid ligand, so benzene-dicarboxylate ligands almost have no contribution to the fluorescent emission of as-synthesized coordination polymers.21,23 Therefore, the emission bands would be assigned to π* → π transition of coordinated pmmid ligands. The emission peak position of 1 (ca. 425 nm, λex = 310 nm) can be attributed to intraligand fluorescent emission, and the lightly blue shift by 11 nm may be attributed to the chelating of the pmmid ligand to the metal ion (LMCT), which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay of the intraligand emission excited state,24 while the emissions for 2 (ca. 467 nm, λex = 320 nm), 3 (ca. 463 nm, λex = 330 nm), and 4 (ca. 452 nm, λex = 290 nm) are red-shifted by 31 nm, 27 nm, and 16 nm, respectively, which probably are related to the intraligand fluorescent emission, and similar red shifts have been observed before.21

extend the 2D network into a 3D coordination architecture, in which T4(2)10(2) water tape is contained. Because of the different coordination modes of the auxiliary ligand, pmmid acting as bidentate μ2-N,N′-donor changes its configurations to fine-tune itself (Table 3) to satisfy the coordination preference. In all, the coordination modes and configurations of carboxylates and pmmid ligand significantly contribute to the diverse structures of 1−4. IR Spectra, X-ray Powder Diffraction Analyses. Their IR spectra (Figure S1, Supporting Information) exhibit strong absorptions centered at ∼3500−3400 cm−1 for 1 and 4, corresponding to the O−H stretching vibration of the water molecule. Strong characteristic bands of carboxylic group are observed in the range of ∼1610−1570 cm−1 for asymmetric vibrations and ∼1480−1400 cm−1 for symmetric vibrations, respectively. While for free ligands, strong characteristic bands of their carboxylic group are observed in the range of ∼1680− 1710 cm−1 (Figure S3, Supporting Information; 1684 cm−1 for H2pa, 1696 cm−1 for Hcba, 1695 cm−1 for Htbba, 1711 cm−1 for H4pma) for asymmetric vibrations. This difference can be attributed to the dehydrogenation of the carboxylic group and their coordination with Cd(II) ions. Powder X-ray diffraction (XRD) has been used to check the phase purity of the bulky samples in the solid state. For complexes 1−4, the measured XRD patterns closely match the simulated patterns generated from the results of single-crystal diffraction data (Figure S2, Supporting Information), indicative of pure products. The dissimilarities in intensity may be due to the preferred orientation of the crystalline powder samples. Photoluminescence Properties. The solid-state fluorescence spectra of 1−4 at room temperature are depicted in Figure 5. Free pmmid ligand also fluoresces in the solid state with the main emission peak at 425 nm upon excitation at 310 nm (Figure 5, inset). As previously reported,21,22 solid-state benzene-dicarboxylate ligands can also exhibit fluorescence at room temperature, and the emission bands of these ligands can be assigned to the π* → n transition. Fluorescent emission of benzene-dicarboxylate ligands resulting from the π* → n transition is very weak compared with that of the π* → π

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CONCLUSIONS Four new CPs have been prepared by mixed 1-(4pyridylmethyl)-2-methylimidazole (pmmid) and carboxylate ligands under the ultrasonic treatment. They show diverse structures and dimensionalities from 1D chains (1 and 2), 2D network (3) to 3D coordination architecture (4). The diversity of structures results from the various carboxylates and diverse configurations of pmmid. Interestingly, these four CPs provide a way of understanding abundant weak interactions, such as hydrogen bonds, π···π stacking effects, and C−H···π interactions. In 3 and 4, these interactions strengthen the structures, while in 1 and 2, they further assemble the structures into higher dimensionalities except for strengthening the structures. In addition, all of them display solid-state fluorescent emission.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format; additional figures of the powder X-ray diffraction (PXRD) patterns and IR spectra for 1−4. 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]. Fax: +86-592-2183047. 2010



Article

Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21071118) and 973 Project (Grant 2007CB815301) from MSTC.

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