and Pb(II) - American Chemical Society

Article pubs.acs.org/crystal

Series of Cd(II) and Pb(II) Coordination Polymers Based on a Multilinker (R,S-)2,2′-Bipyridine-3,3′-dicarboxylate-1,1′-dioxide Yi-Fan Kang,† Jian-Qiang Liu,*,‡ Bo Liu,† Wen-Tao Zhang,† Qing Liu,† Ping Liu,*,† and Yao-Yu Wang† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an, 710069, P. R. China ‡ School of Pharmacy, Guangdong Medical College, Dongguan, 523808, P. R. China S Supporting Information *

ABSTRACT: Six new metal−organic frameworks (MOFs) with structures ranging from a one-dimensional (1D) chain, two-dimensional (2D) layer, even 2D double layer to three-dimensional (3D) network based on 2,2′bipyridine-3,3′-dicarboxylate-1,1′-dioxide (H2bpdado) and N-donor ligands (1,2-bis(4-pyridyl)ethane (bpa) or 1,2-trans-bis(4-pyridyl)ethene (bpe)), namely, [Cd(bpdado)(H2O)2]n (1), {[Cd2(bpdado)2(bpa)(H2O)2]·DMF·3H2O}n (2), {[Cd2(bpdado)2(bpe)(H2O)2]·8.5H2O}n (3), {[Pb 2 (bpdado) 2 (bpa)]·10H 2 O} n (4), {[Pb 2 (bpdado) 2 (bpe)]· 10H2O}n (5), [Pb2(bpdado)Cl2(H2O)2]n (6), were synthesized by hydro(solvo)thermal and/or diffusion methods. In these complexes, the bpdado ligand shows four types of coordination modes, three of which appeared for the first time. Polymers 1−6 present diverse motifs with different metal ions and different conformations and/or coordination modes of bpdado, which may show a natural synergy in structural diversity of the resultant MOFs. Under the similar synthetic conditions just with different metals (Cd(II) and Pb(II)), 6 exhibits a 2D covalent grid layer, whereas complex 1 displays a 1D chain. Meanwhile, Cd(II) atom in 1 shows an unprecedented triangular prism coordination geometry. Although bpe/bpa ligands act as pillars between 2D [M(bpdado)]n layer subunits in complexes 2−5, the Cd(II)-based complexes 2 and 3 are extended into a 3D layered-pillared network, while the Pd(II)-based complexes 4 and 5 just exhibit the 2D double layers. Moreover, the 2D [Cd(bpdado)]n layer subunits of 2 and 3 can be derived from replacing hydrogen bonds in 1D 1 with coordination bonds dependent on different coordination modes of bpdado. In addition, the solid-state luminescence (for 1−6) and gas adsorption properties (for 2 and 3) have been studied and discussed in detail.

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(H2bpdado), has not been investigated extensively so far.7−10 As yet, the first obtained complex was a binuclear Ba(II) complex by Vojtisek.7 Later, Gao and Tang and his co-workers have prepared two Cu-based complexes showing one-dimensional (1D) nanotubular and two-dimensional (2D) structural nets, respectively.8,9 Moreover, a new three-dimensional (3D) Ba(II) structure with barium pillars as building blocks was reported by Tang.9 A 2D dinuclear Mn(II) complex exhibiting interesting ferromagnetic behavior was also explored.10 Until recently, one Cd(II)-H2bpdado complex was obtained under mild conditions, which takes an excellent photocatalytic activity.11 It is obvious there are just limited studies on H2bpdado complexes, and it is necessary to conduct further research. From the view of crystal engineering, the H2bpdado ligand will be a good candidate for constructing intriguing MOFs because (a) the ligand is axially chiral, and it makes the coordination complexes crystallize in an asymmetric space group easily; (b) the N-oxide group has been proven to be

INTRODUCTION The metal−organic frameworks (MOFs) have achieved remarkable progress in the past decade not only due to their diverse topological networks but also because of their potential utilities in luminescence, catalysis, molecular separations, and gas storage.1−4 Although a great many of MOFs with interesting structures and significant properties have been reported, to successfully construct desired MOFs is still a challenge for researchers. The reason is that the synthetic process is controlled by various factors: metal centers, organic ligands, temperature, auxiliary ligands, and so on, of which, organic ligands and the metal centers are crucial components. Thus, the rational selection of the ligands and metal ions is the key to form targeted MOFs. As far as we know, pyridinecarboxylic ligands containing N- and O-donor atoms are capable of building fascinating structures with higher complexity due to the different sites.5 2,2′-Bipyridine-3,3′dicarboxylic acid is taken as one of the pyridinecarboxylic ligands, which has been widely studied because of their strong chelating abilities and flexibility of skeleton.6 In contrast to H2bpda, the coordination chemistry of one derivative, 2,2′-bipyridine-3,3′-dicarboxylate-1,1′-dioxide © XXXX American Chemical Society

Received: April 30, 2014 Revised: September 21, 2014

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dx.doi.org/10.1021/cg5006192 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystallographic Data and Structure Refinement Parameters for Complex 1−6

a

complex

1

2

3

4

5

6

chemical formula formula weight crystal shape crystal color temperature (K) crystal system space group a/Ǻ b/Ǻ c/Ǻ α (deg) β (deg) γ (deg) V/Ǻ 3 Z density (mg/m3) μ (mm−1) F(000) reflections collected independent reflections Rint number of parameters GOF R1a, wR2b [I > 2σ(I)] R1, wR2 (all data)

C12H10CdN2O8 422.62 block colorless 296(2) monoclinic P2/c 8.3104(15) 7.6619(14) 10.7189(19) 90 94.267(3) 90 680.6(2) 2 2.062 1.652 416 3661 1413 0.0507 105 1.017 0.0363, 0.0990 0.0510, 0.1058

C39H41Cd2N7O18 1120.59 block colorless 296(2) orthorhombic Pbcn 24.542(4) 12.641(2) 15.446(3) 90 90 90 4791.9(14) 4 1.553 0.965 2256 22482 4194 0.05491 325 1.038 0.0587, 0.1642 0.0790, 0.1795

C72H86Cd4N12O45 2289.13 block colorless 296(2) orthorhombic Pbcn 24.641(4) 12.468(2) 15.427(2) 90 90 90 4739.5(12) 2 1.604 0.983 2308 27530 5776 0.0790 305 1.037 0.0532, 0.1205 0.0933, 0.1409

C35H41N6O22Pb2 1312.12 block colorless 296(2) monoclinic C2/c 36.981(4) 7.9533(10) 14.8488(19) 90 99.859(2) 90 4302.8(9) 4 2.025 7.907 2532 10646 3910 0.0374 299 1.036 0.0317, 0.0849 0.0358, 0.0879

C18H21N3O11Pb 662.57 block colorless 296(2) monoclinic C2/c 37.119(4) 7.9290(10) 14.9221(17) 90 99.690(2) 90 4329.1(9) 8 2.033 7.860 2560 11565 4356 0.0374 326 1.002 0.0292, 0.0622 0.0380, 0.0657

C12H10Cl2N2O8Pb2 795.50 block colorless 296(2) monoclinic P21/c 7.8167(9) 17.061(2) 14.6908(14) 90 118.911(5) 90 1715.0(3) 4 3.081 19.965 1432 8621 3126 0.0530 235 1.039 0.0458, 0.1186 0.0548, 0.1272

R1 = Σ||F0| − |Fc||)/Σ|F0|. bwR2 = [Σw(F02 − Fc2)2/Σw(F02)2]1/2.

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more versatile and flexible in the coordination mode than the pyridine one because the N-oxide group may further extend the coordination arrays as reliable spacers; (c) unlike pyridine, pyridine N-oxide readily undergoes electrophilic substitution reactions; (d) the N-oxide group possesses the capability of hydrogen bonding, which could stabilize the full framework and enrich the supramolecular interactions.8,12 In our previous report, our group has synthesized an Ag(I) complex using pyridine-2,6-dicarboxylic acid N-oxide ligand, which shows intense fluorescent emission upon photoexcitation.13 Aiming to further study the luminescent property of pyridine N-oxide complexes, we are interested in synthesizing Cd(II)/Pb(II) complexes based on H2bpdado. Fortunately, six new coordination polymers, namely, [Cd(bpdado)(H2O)2]n (1), {[Cd 2 (bpdado) 2 (bpa)(H 2 O) 2 ]·DMF·3H 2 O} n (2), {[Cd2(bpdado)2(bpe)(H2O)2]·8.5H2O}n (3), {[Pb2(bpdado)2(bpa)]·10H2O}n (4), {[Pb2(bpdado)2(bpe)]·10H2O}n (5), and [Pb2(bpdado)Cl2(H2O)2]n (6) (bpa = 1,2-bis(4-pyridyl)ethane and bpe = 1,2-trans-bis(4-pyridyl)ethene), were synthesized successfully by hydro(solvo)thermal and/or diffusion methods. In these complexes, the bpdado ligand shows four types of coordination modes, three of which are observed first. By using different metal salts under the same hydro(solvo)thermal conditions, various complexes were obtained: 1 is a 1D chain, and the Cd(II) atom takes on an unprecedented triangular prism coordination geometry, while 6 exhibits a 2D covalent grid layer. Although all the complexes 2−5 have 2D [M(bpdado)]n layer subunits, 2 and 3 are extended by bpa/ bpe into 3D layered-pillared networks, while 4 and 5 just exhibit the 2D double layers. The solid-state luminescencent properties of 1−6 and gas adsorption properties for 2 and 3 have been studied.

EXPERIMENTAL SECTION

Materials and Methods. All solvents and starting materials for synthesis were purchased commercially and used without further purification. The method of H2bpdado preparing was according to the literature.14 Infrared spectra measurements (4000−400 cm−1) were performed with KBr pellets on a Bruker EQUINOX-55 FT-IR spectrometer. Elemental analyses (carbon, hydrogen, and nitrogen) were taken on a PerkinElmer 2400C elemental analyzer. Thermogravimetric analyses (TGA), using Netzsch TG209F3 equipment, were performed under a nitrogen stream with a heating rate of 10 °C min−1. The products phase purity was examined by X-ray powder diffraction (PXRD), using a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Luminescence spectra for the solid samples were investigated with a Hitachi F-4500 fluorescence spectrophotometer. The gas sorption studies were measured by using Micromeritics ASAP 2020 M adsorption equipment. Transient state fluorescence analyses were carried out using an Edinburgh FLS55 luminescence spectrometer. Synthesis of [Cd(bpdado)(H2O)2]n (1). A mixture of CdCl2 (0.1 mmol, 18.2 mg), H2bpdado (0.1 mmol, 27.2 mg), H2O (12 mL), methanol (3 mL), and an aqueous solution of KOH (1 mol/L, 0.25 mL) was stirred under air atmosphere for 20 min. The solution was placed in a Teflon-lined reactor (25 mL), and then the reactor was sealed, and heated at 135 °C for 72 h. After slow cooling to room temperature, colorless block crystals of the 1 were obtained. Yield: 70% based on Cd. Elemental analysis (%): Calcd for C12H10CdN2O8: C, 34.10; N, 6.63; H, 2.38. Found: C, 34.01; N, 6.70; H, 2.47. IR (KBr, cm−1): 3419(s), 3228(m), 1556(s), 1465(w), 1402(s), 1242(s), 763(w), 735(w). Furthermore, the title compounds can be prepared by others methods (see Supporting Information, Table S3). Synthesis of {[Cd2(bpdado)2(bpa)(H2O)2]·DMF·3H2O}n (2). A mixture of H2bpdado (0.05 mmol, 22.4 mg), bpa (0.05 mmol, 9.2 mg), and H2O (5 mL) was stirred and adjusted the pH to 7.0 with aqueous solution of NaOH (1 mol/L). The other mixture of Cd(CH3COO)2· 2H2O (0.05 mmol, 13.4 mg), DMF (2.5 mL), and THF (2.5 mL) was stirred under air atmosphere for 5 min. Then, the metal salt solution B



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Figure 1. (a) Coordination environment around the Cd(II) in compound 1. Symmetry codes: A 1 − x, y, 1/2 − z; B − x, y, 1/2 − z (gray C, red O, blue N, purple Cd); (b) the distorted triangular prism coordination geometry of Cd(II); (c) the equally racemic and mesomeric mixture of crystal with enantiomorphism in 1; (d) the 1D left- and right-handed helical chains along the ac-plane in compound 1; (e) the 2D mesolayer structure through the hydrogen bonding interactions; (f) the 3D supramolecular network of complex 1. The hydrogen atoms have been omitted for clarity. 6.29; H, 3.22. IR (KBr, cm−1): 3436(m), 1609(w), 1562(w), 1423(w), 1377(s), 1299(m), 1099(w), 965(m), 809(m), 679(w), 543(w). Synthesis of [Pb2(bpdado)Cl2(H2O)2]n (6). A mixture of PbCl2 (0.1 mmol, 27.9 mg), H2bpdado (0.1 mmol, 27.8 mg), H2O (12 mL), methanol (3 mL), and an aqueous solution of KOH (1 mol/L, 0.25 mL) was stirred under air atmosphere for 20 min. The solution was placed in a Teflon-lined reactor (25 mL), and then the reactor was sealed, and heated at 135 °C for 72 h. After slow cooling to room temperature, colorless block crystals of the 6 were obtained. Yield: 60% based on Pb. Elemental analysis (%): Calcd for C12H10Cl2N2O8Pb2: C, 18.12; N, 3.52; H, 1.27. Found: C, 18.03; N, 3.43; H, 1.18. IR (KBr, cm−1): 3417(m), 2926(w), 1549(s), 1377(s), 1220(m), 1030(w), 963(w), 806(w), 677(w), 558(w). X-ray Crystallography. The single crystal X-ray diffraction measurements were performed on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromated MoKα radiation (λ = 0.71073 Å) by using a ϕ/ω scan technique at 293(2) K. The reflection datas were corrected for Lorentz and polarization effects as well as for empirical absorption based on multiscan. The structures of 1−6 were solved by direct methods and refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXTL program.15,16 The data were corrected for absorption by using the program SADABS.17 Anisotropic thermal parameters were applied to all non-hydrogen atoms. All hydrogen atoms from the organic ligands were calculated at idealized positions and refined with the riding models. The hydrogen atoms associated with water molecules in compounds 1−6 were located using the different Fourier method. Crystallographic data of 1−6 are given in Table 1. Selected bond lengths and bond angles are listed in Table S1 (Supporting Information). CCDC: 999714−999719 for 1−6.

was slowly layered onto the mixture of ligand. One week later, colorless block crystals of 2 were collected. Yield: 39% based on Cd. Elemental analysis (%): Calcd for C39H41Cd2N7O18: C, 41.80; N, 8.75; H, 3.68. Found: C, 42.01; N, 8.52; H, 3.99. IR (KBr, cm−1): 3434(m), 1653(m), 1636(m), 1586(m), 1480(s), 1425(m), 1397(m), 1350(s), 1252(m), 1152(m), 1021(m), 969(w), 826(m), 771(s). Synthesis of {[Cd2(bpdado)2(bpe)(H2O)2]·8.5H2O}n (3). A mixture of H2bpdado (0.05 mmol, 22.4 mg), bpe (0.05 mmol, 9.1 mg), and H2O (5 mL) was stirred and adjusted the pH to 7.0 with aqueous solution of KOH (1 mol/L). The other mixture of Cd(NO3)2·4H2O (0.05 mmol, 15.2 mg), methanol (2.5 mL), and H2O (2.5 mL) was stirred under air atmosphere for 5 min. Then, the metal salt solution was slowly layered onto the mixture of ligand. One week later, colorless block crystals of 3 were collected. Yield: 35% based on Cd. Elemental analysis (%): Calcd for C72H86Cd4N12O45: C, 37.77; N, 7.34; H, 3.78. Found: C, 37.82; N, 7.22; H, 3.89. IR (KBr, cm−1): 3451(s), 2026(w), 1606(s), 1427(w), 1388(s), 1232(m), 1128(w), 1011(w), 977(w), 829(w), 781(w), 547(m). Synthesis of {[Pb2(bpdado)2(bpa)]·10H2O}n (4). A mixture of Pb(CH3COO)2·3H2O (0.05 mmol, 19.4 mg), H2bpdado (0.05 mmol, 13.6 mg), bpa (0.05 mmol, 9.2 mg), H2O (12 mL), methanol (2 mL), and an aqueous solution of KOH (1 mol/L, 0.25 mL) was stirred under air atmosphere for 20 min. The solution was placed in a Teflonlined reactor (25 mL), and the reactor was sealed, and heated at 130 °C for 72 h. After slow cooling to room temperature, colorless block crystals of 4 were obtained. Yield: 50% based on Pb. Elemental analysis (%): Calcd for C35H41N6O22Pb2: C, 32.04; N, 6.40; H, 3.15. Found: C, 32.12; N, 6.33; H, 3.27. IR (KBr, cm−1): 3444(m), 2962(w), 1646(m), 1555(m), 1470(w), 1262(m), 1062(m), 804(s), 670(w). Synthesis of {[Pb2(bpdado)2(bpe)]·10H2O}n (5). A mixture of Pb(NO3)2 (0.1 mmol, 33.1 mg), H2bpdado (0.1 mmol, 27.8 mg), bpe (0.1 mmol, 18.1 mg), H2O (8 mL), methanol (2 mL), and an aqueous solution of KOH (1 mol/L, 0.25 mL) was stirred under air atmosphere for 20 min. The solution was placed in a Teflon-lined reactor (25 mL), and the reactor was sealed, and heated at 130 °C for 72 h. After slow cooling to room temperature, colorless block crystals of the 5 were obtained. Yield: 66% based on Pb. Elemental analysis (%): Calcd for C36H42N6O22Pb2: C, 32.63; N, 6.34; H, 3.19. Found: C, 32.71; N,

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RESULTS AND DISCUSSION [Cd(bpdado)(H2O)2]n (1). X-ray diffraction shows 1 crystallizes in an achiral space group P21/c with equal R- and S-configured bpdado ligands. The asymmetric unit consists of a half Cd(II), a half bpdado ligand, and one coordinated water molecule. The Cd1 ion is tightly bonded by six oxygen atoms C



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Figure 2. (a) Coordination environment of the Cd(II) ion in 2. Symmetry codes: A x, 2 − y, − 1/2 + z; B 1/2 − x, 1/2 − y, 1/2 + z (gray C, red O, blue N, purple Cd); (b) the coordination geometry of Cd1 in 2; (c) the 1D left- and right-handed helical chains along the c-axis in 2; (d) the equally racemic and mesomeric mixture of crystal with enantiomorphism in 2; (e) the 2D layer in 2 viewed along the bc plane; (f) the 3D structure of 2 showing channels along the b axis; (g) schematic view of the 3D five-connected network topology; the yellow nodes represent the Cd(II) ions, white is bpdado. The guest molecules and hydrogen atoms have been omitted for clarity.

{[Cd 2 (bpdado) 2 (bpa)(H 2 O) 2 ]·DMF·3H 2 O} n (2) and {[Cd2(bpdado)2(bpe)(H2O)2]·8.5H2O}n (3). Complexes 2 and 3 are isomorphous with different guest molecules; thus the crystal structure of 2 along with the difference in guest form is described. A single-crystal X-ray diffraction study reveals that 2 is a layered-pillared framework and crystallizes in the achiral space group Pbcn. It means that the single crystal of 2 would also contain a mesomeric mixture of ligand (Figure 2c). The asymmetric unit contains one Cd(II), one bpdado ligand, a half bpa linker, one and a half free water molecules, one coordinated water molecule, and one free DMF molecule. Compared with the coordination environment of Cd1 in 1, an O atom (coordinated water) is replaced by the N-oxide O atom and owing to the coordination of nitrogen atom of the bpa ligand, the coordination number increases from 6 to 7 with the pentagonal bipyramidal geometry of Cd1. In the structure, Cd− O and Cd−N bond distances are also in normal ranges (Table S1, SI).23,24 Similar to 1, the carboxyl groups of R- or S-configured bpdado in bis-chelating mode also link the Cd(II) into 1D R- or S-chains along the c axis (Figure 2b). However, because of the substitution of the coordinated water molecule in 1 by one Noxide in 2, the adjacent chains with opposite chirality are interlinked by sharing Cd(II) ions to generate the wave-like mesolayers [Cd(bpdado)]n in the bc plane (Figure 2d) rather than the 2D supramolecular layer in 1. Furthermore, the bpa ligands serve as pillars connecting the opposite chiral chains of adjacent layers to generate a 3D channel-like framework, as shown in Figure 2e. To fully understand the structure of 2, the topological analysis is explored to simplify such a 3D coordination network. Apparently, the Cd center could be taken as 4-connected mode, and the bpdado ligand can be simplified as 3-connected node. Therefore, 2 can be reduced to

from two coordinated water molecules and two different carboxylic groups of two separated bpdado ligands (Figure 1a). Interestingly, the coordination geometry of Cd(II) corresponds to a slightly distorted triangular prism (Figure 1b), with four oxygen atoms (O1, O1A, O2, O2A; A = 1 − x, y, 1/2 − z) comprising the equatorial plane, while the other two oxygen atoms (O6, O6A) from two coordinated water molecules locate at the same side of the plane instead of the usual axial positions. Although recently Alvarez reported that the number of trigonal prisms throughout the six-coordinated transition-metal centers is barely 1.0% in the Cambridge Structural Database (CSD),18 this geometry is significant for it is observed in the active sites of molybdenum- and tungsten-containing enzymes and may appear as an intermediate or a transition state in the intramolecular racemization reactions.19,20 For coordination polymers, as far as we known, the frequency of Mn(II) trigonalprismatic structures is slightly high, but such coordination geometry has not been in Cd(II) polymers to date.21,22 This example may make the coordination chemistry of Cd(II) complexes more substantial. The Cd−O bonds distances vary from 2.262(3) to 2.368(3) Å, and the O−Cd−O angles are in the range from 55.63° to 141.33°, which are similar to previously reported works.23,24 Although the N-oxide are pendent and free, each carboxylic group of the bpdado ligand coordinates to one Cd(II) in bischelating mode (Figure 1c). Such a combination of R- (or S-) bpdado with Cd(II) ions leads to R- (or S-) chiral chains (Figure 1d). Then, adjacent chains with opposite chirality are joined together to form a 2D supramolecular layer (Figure 1e) via a pair of parallel H-bonds (O6−H6A···O1). Furthermore, a new type of H-bond (O6−H6B···O10) exists in the motif, which links the 2D layer into the 3D supramolecular structure (Figure 1f). D



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a rarely binodal fsc-like net with the point symbol of {63}{65.8} (Figure 2f). In addition, the guest molecules are located at the pore and have multiform H-bonds between lattice water, DMF, and coordination water (see Table S2). And a trimer water cluster (O1W, O2W, O1WA) is observed (Figure 3).

in liquid water (2.854 Å) and in ice (Ic 2.75 Å and Ih 2.759 Å, determined at −130 and −90 °C).26,27 Moreover, the V-shaped heptamer water cluster is constructed by O5W, O2W, O4W, O6W, O3WD, O4WB, and O2WB, and the average O···O distance in the V-shaped heptamer water cluster is 3.129 Å. Recently, most 1D water tapes, such as T4(0), T4(2), T8(0), and T6(2), are either edge- or vertex-sharing conformations.28,29 The present infinite tape motif can be defined as T6(2)T7(3), using the nomenclature proposed by Infantes,30 where the number in parentheses is the number of water molecules shared between the adjacent rings. However, to the best of our knowledge, such a 1D water tape in this work, which exhibits a novel arrangement and is different from the reported examples,25,31−33 has not been reported so far. {[Pb2(bpdado)2(bpa)]·10H2O}n (4) and {[Pb2(bpdado)2(bpe)]·10H2O}n (5). Similarly, complexes 4 and 5 are also isomorphous and with the same water clusters, and thus only 4 will be discussed in detail. A single crystal X-ray diffraction study reveals that complex 4 is a 2D pillared-layered framework and crystallizes in monoclinic C2/c space group. The asymmetric unit contains one Pb(II) atom, one bpdado ligand, a half of bpe, and five lattice water molecules (Figure 5a). The coordination environment of Pb(II) in 4 is analogous to the Cd(II) in 2 with a coordination number of 7, but the Pb(II) adopts capped trigonal prism geometry because the lone pair electrons of Pb(II) are stereochemically active and play an key role in determining the coordination geometries of the Pb(II).34 The O−Pb−O angles and Pb−O bond lengths are all within the normal ranges.35−37 Two N-oxides of bpdado ligand in 4 chelate one Pb(II) and each carboxylate group connects one Pb atom in bis-chelating mode (Table 2c). Thus, every bpdado ligand joins three different Pb(II) ions as a tridentate bridging ligand linker resulting in the formation of a 2D single layer [Pb(bpdado)]n. Similar to 2, it is also a mesolayer consisting of R- and Sconformation 1D chiral chains in the bc plane (Figure 5e). However, such a single layer in 4 is pillared by bpa ligands to give rise in a 2D double layer instead of a 3D framework (Figure 5f). Viewing the topology, each bpdado bridges three Pb(II) ions forming a three-connected node, whereas each Pb(II) ion is connected to three bpdado ligands and one Pb(II) ion (Pb···Pb = 15.364 Å, through bpe) and can be considered as a four-connected node; thus complex 4 is a binodal 3,4connected 2D double layer with (63)(66)-3,4L88 topology (Figure 5g).38 The intriguing structural feature of 4 is the presence of a 2D novel water layer with alternating (H2O)10 and (H2O)14 clusters. Five water molecules (O1W, O2W, O3W, O4W, and O5W) and their symmetric equivalents form a (H2O)10 cluster (Figure 3). As a matter of fact, adjacent (H2O)10 clusters are linked together into a 1D water tape by sharing one hydrogen bond. Adjacent decorated water tapes are arranged in an alternate up-and-down fashion, such that O1W water molecules from different water tapes fuse together, leading to a 2D highly puckered water sheet in the bc-plane, in which a huge (H2O)14 cluster was observed. Through a quick search of the Cambridge Structural Database (CSD), there is no corresponding crystal structure exhibiting such a water sheet. Therefore, we think that this motif of association of the water molecules is described for the first time. The presented 2D water sheet can be represented by T10(0) and T14(2) subunits by water cluster notation,39 which is made up of alternating planar and butterfly-shaped clusters.

Figure 3. Guest water molecules format different water clusters in compounds 2, 4, 5, and 6, respectively. Hydrogen bonding interactions are shown as dashed lines. Symmetry codes: A 1 − x, y, 0.5 − z for complex 2; A 1 − x, 1 − y, 1 − z; B 1 − x, y, 1.5 − z; C x, 1 − y, 0.5 + z; D 1 − x, 1 + y, 1.5 − z; E x, 1 + y, z; F x, 2 − y, 0.5 + z; G 1 − x, 2 − y, 1 − z; H 1 − x, 1 − y, 2 − z; I x, 1 − y, −0.5 + z for complexes 4 and 5.

Careful analyses of the isostructures, the distances of layer-tolayer (12.320 Å), and the Cd···Cd separation across the bpe ligand (13.927 Å) in 3 is slightly larger than that of 2 with the value of 12.213 and 13.819 Å, which is due to the different lengths of the bpe/bpa ligand (Nbpe−Nbpe = 9.358 Å, Nbpa− Nbpa = 9.214 Å). In addition, the minimum pore-dimensions are different (5.928 × 6.296 Å2 for 2, 6.047 × 6.984 Å2 for 3, included van der Waals radius), in which the guest molecules are housed. Interestingly, in 3 the lattice water molecules construct a larger 1D water tape, as shown in Figure 4, including a chairlike hexamer water cluster (yellow), a bookshaped hexamer water cluster (pink), and a V-shaped heptamer water cluster (blue). The chairlike hexamer water cluster consists of O2W, O3W, O4WA, O2WA, O3WA, and O4W, in which symmetry-related O2W and O2WA are located on the two vertices of the chair, while the book-shaped hexamer water cluster with a dihedral angle of 50.2° is formed by two symmetry-related O3W, O4WA, O6WA, O3WE, O4WC, and O6WC, in which O4WA and O4WC are located on the “spine” of the book. Kim performed extensive ab initio and density functional theory (DFT) calculations on the five lowest energy structures of the water hexamers with ring, book, cage, boat, and prism, which have ca. 0.7 kcal/mol and are nearly isoenergetic.25 The O···O distances within the chairlike and book-shaped hexamer clusters range from 2.659 to 3.384 Å and from 2.912 to 3.384 Å, with an average value of 3.044 and 3.233 Å, respectively, which is apparently longer than those observed E



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Figure 4. (a) The lattice water molecules construct the 1D water tape in complex 3; (b) the book-shaped (pink), chair-shaped (yellow), and Vshaped water cluster are in the 1D water tape. Symmetry codes: A: 1 − x, 1 − y, −z; B: 1 − x, y, 0.5 − z; C: x, 1 − y, −0.5 + z; D: x, 1 − y, 0.5 + z; E: 1 − x, y, −0.5 − z. Hydrogen bonding interactions are shown as dashed lines.

Figure 5. (a) Coordination environment of the Pb(II) ion in 4. Symmetry codes: A = x, 1 − y, 1/2 + z; B = x, 1 + y, z (gray C, red O, blue N, pink Pb); (b) the coordination geometry of Pb1 in 4; (c) the 2d mesolayer of complex 4; (d) the spacefill mode of the water clusters in complex 4; (e) the 1D left- and right-handed helical chains along the b-axis in 4; (f) the 2D structure of 4 showing channels along the b axis; (g) schematic view of the 2D pillar-layer network topology; the nodes represent the Pb(II) ions. The hydrogen atoms are omitted for clarity.

[Pb2(bpdado)Cl2(H2O)2]n (6). Single-crystal X-ray diffraction reveals that complex 6 crystallizes in space group P2/c with two crystallographically independent Pb(II) ions (Figure 6a). The Pb1 atom is six-coordinated by one Cl− anion, one O atom

from a coordination water, two O atoms from two N-oxide groups, one O atom from μ2-η1:η1 carboxylate, and one O atom from μ2-η1:η2 carboxylate (Pb−O = 2.524(9)−2.750(8) Å), showing a pentagonal pyramid coordination geometry. The F



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Table 2. Coordination Modes of bpdado Ligand

Figure 6. (a) Coordination environment around the Pb(II) in compound 6. Symmetry codes: A = 1 + x, y, z; B = 1 + x, 3/2 − y, 1/2 + z; C = x, 3/2 − y, 1/2 + z (gray C, red O, blue N, green Cl, pink Pb); (b) the coordination geometry of Pb1 and Pb2 in 6; (c) The 2D layer in 6 viewed along the ac plane; (d) schematic representations of the (3,6)-connected kgd framework in 6. Pb1, Pb2, and bpdado nodes are shown as pink, blue, and green, respectively. The hydrogen atoms are omitted for clarity.

Pb2 atom is pentagonal bipyramidal and bonded to one Cl− anion, two O atoms from two N-oxide groups, two O atoms from μ2-η1:η2 mode of carboxylate, one O atom from μ2-η1:η1 mode of carboxylate group (Pb−O = 2.553(1)−2.791(5) Å). It is worth noting that the slightly longer Pb1−O3B distance (2.969(4) Å) indicates the weak bonds between Pb1 and O3, which is induced by the presence of the lone pair electrons of the Pb(II) ion, as have been reported in other Pb(II)complexes.34 Unlike those above, the bpdado ligand exhibits another kind of coordination mode (Table 2d): the two N-oxide groups show bis-bridging mode, and two carboxylate groups adopt μ2η1η1 and μ2-η1η2 fashion, respectively. On the basis of this connection, the bpdado connectors interlink the two types of Pb(II) atoms into a 2D waved layer. Interestingly, ignoring the coordination of bpdado to Pb1 atom, such a 2D layer can be simplified into a [Pb(bpdado)]n layer, similar to 4, in which the R- and S-conformation 1D chiral chains arranged alternatively (Figure 6c). If the Pb ions and bpdado ligands are considered

as 3-connected and 6-connected nodes, respectively, the overall structure of 6 is described to be a 2D (3,6)-connected kgd net with a short Schläfli symbol of {(43)2(46.66.83)} (Figure 6d). The 2D wavy layers are further extended into a 3D metal− organic supramolecular framework via H-bonding (O8−H8A··· O6, O8−H8B···O7, and O7−H7A···O6). Coordination Modes of bpdado Ligand. In complexes 1−6, the bpdado ligand displays four different bridging fashions, and the latter three appear for the first time. As shown in Table 2, in modes a−c, the carboxylic groups all adopt in bis-chelating mode, and the main difference between them lies on the two N-oxide groups: for mode a, neither of the Noxide groups participate in the coordination; for mode b, one N-oxide bridges one metal ion, while the other is still pendent; for mode c, the two N-oxide bind one metal ion in bis-chelating mode. The coordination mode of two N-oxide groups of mode d is the same as that for mode c, but the two carboxylic groups adopt a μ2-η1:η1 and μ2-η2:η1 coordination mode, respectively. It is well-known that the dihedral angles come from the G



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H2bpdado play a key role in structural diversity of the final structural motif. PXRD and Thermal Analysis. The bulk of the crystalline samples of complexes 1−6 were confirmed by a good match between the experimental and simulated powder X-ray diffraction (PXRD) patterns (Figure S1 of the Supporting Information). Thermogravimetry analyses (TGA) were performed to determine the thermal stability of the studied complexes 1−6. The TGA was carried out in the temperature range of 30−800 °C under a flow of nitrogen atmosphere with a heating rate of 10 °C min−1 (Figure 7). TG curve of 1 had no

rotation of C−C bonds to match the requirements of coordination.40 Then we investigate the relation between coordination modes and dihedral angles of pyridine rings. As shown in Table 2, the dihedral angles between pyridine rings are, in order, 64°, 81°, 87°, 80°. And the corresponding number of metal ion connected to the bpdado ligand is 2, 3, 3, 5, respectively. Comparing modes a and b, the less metal ions are coordinated, and the smaller dihedral angles are obtained, which is in accordance with the other works.7−10 As for mode c, despite the same coordination number as mode b, the observed larger dihedral angle could be ascribed to a necessary separation caused by the coordination of the N-oxide to one metal ion simultaneously. The coordination of Cl− anions to Pb(II) may be responsible for the quite smaller dihedral angle of mode d, despite every ligand connecting five metal ions. According to the discussion, it can be concluded that the larger the dihedral angle, the more likely it is to participate in coordination of the N-oxide groups. Structural Diversity. Complexes 1−6 can be classified into four categories depending on the structures and metal ions, namely, 1; 2 and 3; 4 and 5; 6. If the different metal ions are ignored, similar 2D [M(bpdado)]n layers are found in 1−5, and they are all composed of R- and S-conformation 1D chiral chains that are arranged alternatively through hydrogen bonds or covalent bonds. The structural diversity of these 2D [M(bpdado)]n layers are directly dependent on coordination modes of the ligand bpdado. In 1, due to the pendent N-oxide group, two water molecules occupy the Cd(II) coordination sites, and thus the adjacent chains are combined only through hydrogen bonds. In complexes 2 and 3, one N-oxide replaces one coordinated water molecule, leaving one coordination site to form a coordination bond between the O atom of the Noxide group and metal Cd(II), giving rise to the 2D coordination network. When two coordinated water molecules are substituted by N-oxide groups in bis-chelating mode, the adjacent chains can be interconnected by two types of coordination bonds, so the 2D coordination network is obtained in complexes 4 and 5. In other words, coordination modes of the ligand significantly affect the number of the coordinated waters around the metal ion, which further results in the structural diversity of the 2D layers. However, in the same ligand but different metal systems, the resultant isostructural complexes 4 and 5 are 2D double layers while 3D networks for 2 and 3. Owing to the presence of isostructures, only the comparison of structures of 2 and 4 was made. Despite the same coordination number of Cd(II) and Pb(II) in 2 and 4, due to the nature of the metal ions they adopt pentagonal bipyramidal and capped trigonal prism coordination geometry, respectively. In 2, the N atom of bpa is sited in the equatorial plane of the pentagonal bipyramidal. In order to reduce the steric hindrance, the bpa ligand has to lie above and below the [Cd(bpdado)]n layer; thus, the 2D layer connects to the adjacent ones to giving rise to 3D network. In the case of 4, the N atom of bpa is located in the “cap” site, so the bpa ligands are situated on the same side of the [Pb(bpdado)]n layer resulting in a 2D double layer. In addition, for 1 and 6, they were prepared under the same hydro(solvo)thermal conditions except with a different metal salt (CdCl2/ PbCl2). However, the resultant structures are absolutely different, which further shows the important role of the metal ions in modulating the final structural motifs. On the basis of the discussion above, the metal ions and coordination modes of

Figure 7. TGA curves for the as-synthesized complexes 1−6.

weight loss before 160 °C and then began to release two coordinated water molecules (obsd 8.7%, calcd 8.5%) with a significant weight loss from 160 to 220 °C. 2 exhibits an initial weight loss of 14.5% at 60−225 °C, corresponding to the releases of water molecules and DMF (calcd 14.6%), followed by the structural collapse due to the removal of bpdado and bpa. For 3, a weight loss of 14.4% below 250 °C is due to the releases of eight and a half free water and two coordinated water molecules (calcd 14.9%). Upon further heating, gradual decomposition is observed. The TG curve of 4 exhibits the first weight loss of 13.3% from 40 to 100 °C, corresponding to the removal of five water molecules (calcd 13.5%). Until 800 °C, the decomposition of the remaining substance finished with the remaining substance of PbO (obsd 34.1%, calcd 33.7%). Similarly, 5 gradually loses the free water molecules up to 100 °C (obsd 13.8%, calcd 13.7%), and the main product weight of the decomposition of the remaining substance corresponded to PbO (obsd 34.9%, calcd 34.0%). An initial weight loss of 4.7% at 60−220 °C occurs in 6, which is attributable to the release of two coordinated water molecules (calcd 4.5%). The further heating leads to the loss of bpdado, while the remaining weight corresponds to PbO (obsd 55.8%, calcd 56.1%). Luminescence Properties. There exists a continuous interest in discovering framework compounds with d10 or s2 electron configuration metal ions and organic chromophores, which have potential applications in the field of photoactive materials for sensing (chemical sensors) and photochemistry.41,42 The solid-state emission properties of the free ligands as well as complexes 1−6 are investigated. The solid-state fluorescence emission maxima of H2bpdado, bpa, and bpe were observed at 393, 360, and 370 nm excited at λex = 280 nm, respectively. At the same time, H2bpdado shows a shoulder H



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Figure 8. (a, b) Solid-state emission spectra of free ligands H2bpdado, bpa, bpe, and complexes 1−6 at room temperature.

Figure 9. Gas sorption isotherms of 2 (a) and 3 (b) for N2, H2, and CO2 at 77 K, 77 K, and 195 K, respectively.

peak at λem = 464 nm. These emissions can be attributable to the π* → n or π* → π transitions as previously reported.43 The emission spectra for Cd(II)-based compounds exhibit emission peaks at 413, 464 nm (λem = 368 nm) for 1, 410, 467 nm (λem = 353 nm) for 2, and 413, 464 nm (λem = 368 nm) for 3, respectively (Figure 8a). All of them have similar emission spectra as ligand H2bpdado with red shifts from that of free ligand, which may originate from the influence of the ligand coordination. The emission spectra for Pb(II)-based compounds exhibit emission peaks at 413, 467, 540 nm (λem = 361 nm) for 4, 415, 467, 540 nm (λem = 361 nm) for 5, and 402, 468, 514 nm (λem = 351 nm) for 6, respectively (Figure 8b). These emissions, at 413, 467 nm for 4, 415, 467 nm for 5, and 402, 468 nm for 6, may originate from the corresponding H2bpdado for their similar emissions. Furthermore, in complexes 4−6, the emission peaks in the region of 510−540 nm can be ascribed to a metal-centered (MC) s → p transition, which are observed for other s2-metal complexes involving the s and p metal orbital as proposed by Vogler.44 Furthermore, the fluorescence lifetimes of free H2bpdado and complexes 1−6 were investigated in the solid state at 298 K (Figure S2 and Table S4), and their decay profiles all follow biexponential decay low. The lifetimes of complexes 1−6, consisting of the short subnanosecond term τ1 and the longer term τ2, are 0.36 and 3.14 ns for 1, 0.46 and 3.00 ns for 2, 0.47 and 3.23 ns for 3, 0.71 and 4.32 ns for 4, 0.85 and 4.62 ns for 5,

0.70 and 4.25 ns for 6 with the major contribution from the shorter component, which are akin to those of free H2bpdado. It can be found that the fluorescence lifetimes of 2 and 3 are quite similar, which are shorter than those of the other group (4, 5, and 6) and a little longer than those of 1. The observed results may be ascribed to the different metal centers as well as consistent with the fact that the bpdado2− molecule is stabilized and restricted by the Pd(II)/Cd(II) complexes’ formation, which inhibit the thermal vibration and nonradiative relaxation process.45 And similar to other previous reports, such emission in the nanosecond range of lifetime is fluorescent in nature.46,47 Gas Adsorption. To verify the porosity of 2, 3, 4, and 5, gas adsorption isotherms were measured with N2, H2, and CO2. All the samples were exchanged in methanol for overnight and then activated under high dynamic vacuum at 120 °C for 8 h. Unfortunately, owing to their very slow diffusion rate, the porosity of compounds 4 and 5 could not be characterized by gas-adsorption measurements.48 However, it is interesting that 2 and 3 show the ability to selectively adsorb CO2 over N2 and H2. The desolvated 2 displays a nonclassic type-I N2 sorption isotherm at 77 K and 1 bar, with the maximum sorption amount of 33.5 cm3 (STP) g−1 at 760 Torr (Figure 9). The classic type-I isotherm is observed for CO2 sorption at 195 K and H2 sorption at 77 K, with the sorption amount of 40.4 cm3 (STP) g−1 and 22.4 cm3 (STP) g−1 at 760 Torr, respectively. In comparison, the adsorption capacity of 3 (25.5 cm3 (STP) g−1 I



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for CO2 at 760 Torr, 11.2 cm3 (STP) g−1 for N2 at 720 Torr, 12.8 cm3 (STP) g−1 for H2 at 760 Torr) is significantly less than 2. This phenomenon may be attributed to the influence of the bridging bipyridyl ligands, as bpe is rigid, while bpa is flexible.49,50 Although they have a similar pore-dimension and solvent accessible voids (31.6% for 2, 32% for 3), bpe ligands could make the framework of 3 more stable, so it is not easy to change when the gas enters the cavity of the framework. In addition, both the CO2 sorption of 2 and 3 show the hysteresis phenomenon, which may arise from the strong intramolecular forces between the CO2 and framework hindering escape of the framework during the adsorption−desorption process as well as the small pore size, and similar sorption results were observed in recent reports.51,52

CONCLUSION In summary, six MOFs based on H2bpdado have been constructed successfully under hydro(solvo)thermal and/or diffusion conditions. The significant metal ion effect on the selfassembly of the MOFs 1 and 6 has been indicated, which induces the formation of a 1D chain in 1 and 2D covalent grid layer in 6. Interestingly, the Cd(II) in 1 shows an unprecedented triangular prism coordination geometry. Although all the complexes 2−5 have 2D [M(bpdado)]n layer subunits, 2 and 3 are extended by bpa/bpe into 3D layered-pillared networks, while 4 and 5 just exhibit the 2D double layers. The metal ions and coordination modes of H2bpdado play a key role in structural diversity of the final structural motif, which may show a natural synergy in structural diversity of the resultant MOFs. In addition, 2 and 3 with 1D channels have present CO2 adsorption properties and revealed the permanent porosity. And luminescence of the complexes 1−6 have been discussed. ASSOCIATED CONTENT

S Supporting Information *

Table S1: Selected bond lengths and angles for complexes 1−6. Table S2: Hydrogen bond geometries in the crystal structures of 1−6. Table S3: Different syntheses methods of complex 1. Table S4: Photoluminescent lifetime data for compounds 1−6 and ligand H2bpdado. Figure S1: XRD of the complexes 1−6. Figure S2: Fluorescence decay curves for H2bpdado and 1−6. Crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(J.Q.L.) E-mail: [email protected]. *(P.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (Nos. 21143010, 21201044, 21371142, 20931005, and 91022004), the NSF of Shaanxi Province of China (No. 2012JQ2007), Training Plan of Guangdong Province Outstanding Young Teachers in Higher Education Institutions (Grant No. YQ2013084), and the NSRF of Shaanxi Provincial Education Office of China (Nos. 2010JS116 and 2010JK875). J



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