Four 2D metal–organic networks incorporating Cd-cluster SUBs: hydrothermal synthesis, structures and photoluminescent properties

Abstract

Hydro(solvo)thermal reactions between 1,1′-biphenyl-2,2′,3,3′-tetracarboxyl acid (H₄bptc) and Cd(NO₃)₂·4H₂O at the presence of the ‘second’ ligands of bpydo, bpe, bpp and bix (bpydo = 4,4′-bipyridine-N,N′-dioxide; bpe = 1,2-bis-(4-pyridyl)ethane; bpp = 1,3-Bis-(4-pyridyl)propane; bix = 1,4-bis(imidazol-1-yl-methyl)benzene) yield four new 2D metal–organic coordination polymers [Cd₉(bptc)₄(μ₃-OH)₂(H₂O)₁₄]·(bpydo)·2H₂O (2), [Cd₂(bptc)(bpe)(H₂O)₂]·H₂O (3), [Cd₄(bptc)(Hbptc)(bpp)(μ₃-OH)(H₂O)₄] (4), [Cd₂(bptc)(bix)(H₂O)] (5). Compound 2 presents a 2D grid structure with heptanuclear cadmium cluster and two kinds of discrete Cd atoms secondary building units (SUBs), 3 has a 2D structure with the novel Schläfli symbol of (4.6²)(4³.6³)(4⁴.6¹⁰.8), 4 gives a 2D grid structure with a pentanuclear cadmium cluster, binuclear cadmium cluster and discrete Cd atom SUBs, and 5 has a 2D distorted CdI₂-type topology structure with the Schläfli symbol of (4³)₂(4⁶.6⁶.8³). The novel achiral crown-ether-like cycles in 2, 4 and 5 are found and occupied by the single Cd atom or binuclear cadmium cluster. All compounds give strong luminescent emission and have potential application as optical materials.

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Introduction

The construction of metal–organic coordination polymers is of current interest in the field of supramolecular chemistry and crystal engineering, not only for their interesting molecular topologies and crystal packing motifs,^1,2 but also for their potential applications as functional materials.^3-6 The selection of the special ligand is very important in the construction of these coordination polymers. As an important family of multidentate O-donor ligands, organic aromatic polycarboxylate ligands have been extensively employed in the preparation of such metal–organic complexes with multidimensional networks and interesting properties.⁷ More importantly, the carboxylato groups in these ligands can form metal-carboxylato cluster SUBs to avoid interpenetrating and stabilize the resulting porous structures,^2,3,7,8 give good magnetic^9-11 and luminescent properties.¹² Now, searching for a new kind of versatile polycarboxylate ligands deserves attention with regard to investigating new topologies and various functional materials.^13-16 Following our previous studies,^15,16 in this research we choose H₄bptc (1,1′-biphenyl-2,2′,3,3′-tetracarboxyl acid, Scheme 1) as the organic ligand, and the considerations are: (1) being an axial chiral ligand, it may allow the induction of a chiral unit when coordinating with metal atoms; (2) H₄bptc multiple bridging moieties, a variety of connection modes with metal centers, are possible and provide abundant structural motifs; (3) four adjacent “dense” tetracarboxyl acid have a high potential to form a novel metal-cluster when reacting with metal atoms; (4) only one coordination polymer constructed from the H₄bptc ligand has been reported,¹⁶ and much work is still necessary to understand the coordination chemistry of H₄bptc.

Scheme 1 The structure of H₄bptc.

Recently, a novel host–guest metal–organic compound [Cd₄(bptc)₂(bpy)(H₂O)₈]·5.5H₂O (1) with both chiral hydrophilic and achiral hydrophobic channels has been obtained by us through the reaction of Cd²⁺ ion and H₄bptc based on hydro(solvo)thermal reactions.¹⁶ With the aim of further understanding the coordination chemistry of this new ligand and preparing new materials with interesting structural topologies and excellent physical properties, we have recently engaged in the research of coordination polymers based on it.

Experimental

H₄bptc and bix were synthesized according to the literature.^16,17 Other starting materials were of reagent quality and obtained from commercial sources without further purification.

[Cd₉(bptc)₄(µ₃-OH)₂(H₂O)₁₄]·(bpydo)·2H₂O (2)

2 was synthesized hydrothermally in a 23 ml Teflon-lined autoclave by heating a mixture of 0.1 mmol 2,2′,3,3′-H₄bptc, 0.1 mmol bpydo, 0.2 mmol Cd(NO₃)₂·4H₂O and one drop of Et₃N (pH ∼ 8) in 8 ml water and 2 mL acetonitrile at 120 °C for 3 d. Colorless diamond-like single crystals were collected in 75% yield based on Cd(NO₃)₂·4H₂O. Anal. found (calcd) for C₇₄H₆₆Cd₉N₂O₅₂: C, 31.51 (31.44); H, 2.75 (2.72); N, 1.05 (0.99); IR (KBr, cm⁻¹): 3423(w, br), 3113(w), 1552(vs), 1453(s), 1392(vs), 1238(m), 1113(m), 1090(m), 942(m), 854(s), 777(s), 715(s), 660(s).

[Cd₂(bptc)(bpe)(H₂O)₂]·H₂O (3)

3 can also be obtained following the same synthetic procedures as that of 2 except for using bpe instead of bpydo as the starting material. Colorless rectangle single crystals were collected in 83% yield based on Cd(NO₃)₂·4H₂O. Anal. found (calcd) for C₂₈H₂₄Cd₂N₂O₁₁: C, 42.77 (42.61); H, 3.14 (3.06); N, 3.67 (3.55)%; IR (KBr, cm⁻¹): 3421(s, br), 1612(s), 1546(vs), 1455(s), 1430(s), 1382(vs), 1210(m), 1063(m), 1026(m), 854(m), 823(w), 779(m), 760(m), 713(m), 660(m), 549(w).

[Cd₄(bptc)(Hbptc)(bpp)(μ₃-OH)(H₂O)₄] (4)

4 can also be obtained following the same synthetic procedures as that of 2 except using bpp instead of bpydo. Colorless plate single crystals were collected in 77% yield based on Cd(NO₃)₂·4H₂O. Anal. found (calcd) for C₄₅H₃₅Cd₄N₂O₂₁: C, 38.79 (38.90); H, 2.72 (2.54); N, 1.97 (2.02)%; IR (KBr, cm⁻¹): 3424(s, br), 1708(vs), 1604(s), 1562(vs), 1458(s), 1380(vs), 1051(m), 835(m), 764(m), 707(m), 656(m).

[Cd₂(bptc)(bix)(H₂O)] (5)

5 can also be obtained following the same synthetic procedures as that of 2 except using bix instead of bpydo. Colorless needle single crystals were collected in 67% yield based on Cd(NO₃)₂·4H₂O. Anal. found (calcd) for C₃₀H₂₂Cd₂N₄O₉: C, 44.69 (44.63); H, 2.82 (2.75); N, 6.97 (6.94) %; IR (KBr, cm⁻¹): 3422(s, br), 3113 (vs), 1551(s), 1453(vs), 1392(s), 1238(vs), 1090(vs), 853(vs), 777(vs), 715(vs), 659(vs).

Physical measurements

Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. The IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. Thermal analyses were performed on a TGA V5.1A Dupont 2100 instrument from room temperature to 700 °C with a heating rate of 10 °C min⁻¹ in air. Luminescence spectra for the solid samples were recorded on a Hitachi 850 fluorescence spectrophotometer.

X-Ray crystallographic studies

Single crystals for 2, 3, 4 and 5 were used for structural determinations on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs.¹⁸ Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths for 2, 3, 4 and 5 are listed in Tables 1 and 2. CCDC reference numbers: 685455 (2); 685456(3); 685457(4); 685458(5).†

Table 1 Crystallographic data and refinement parameters for 2–5

Compound	2	3	4	5
a R1 = ∑||Fo| − |Fc||/|Fo|. wR2 = [∑w(∑Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1^/2.
Empirical formula	C74H66Cd9N2O52	C28H24Cd2N2O11	C45H35Cd4N2O21	C30H22Cd2N4O9
FW	2826.89	789.29	1389.35	807.34
Crystal system	triclinic	triclinic	triclinic	triclinic
Space group	P-1	P-1	P-1	P-1
a/Å	10.642(2)	10.2134(4)	10.1503(10)	10.0622(6)
b/Å	12.354(2)	11.7040(5)	15.0272(15)	11.1733(7)
c/Å	16.323(3)	12.3421(5)	16.7597(17)	13.2450(8)
α/°	79.113(4)	79.7200(10)	66.575(2)	89.4170(10)
β/°	79.926(4)	79.2680(10)	77.358(2)	75.1850(10)
γ/°	83.023(4)	72.6080(10)	81.220(1)	76.9370(10)
V/Å^3	2066.2(6)	1371.36(10)	2282.6(4)	1400.56(15)
Z	1	2	2	2
R int	0.0212	0.0320	0.0322	0.0501
ρ/g cm^−3	2.272	1.911	2.021	1.905
F(000)	1372	780	1358	796
Goodness of fit on F^2	1.108	1.140	1.073	0.934
R1, wR2^a [I > 2σ(I)]	0.0535, 0.1036	0.0479, 0.1160	0.0584, 0.1297	0.0358, 0.0594
R1, wR2^a [all data]	0.0805, 0.1084	0.0584, 0.1192	0.0837, 0.1350	0.0572, 0.0643

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Table 2 Selected bond lengths [Å] for 2–5

a Symmetry codes: #1 −x + 1, −y + 1, −z + 1; #2 −x, −y +1, −z + 1; #3 x + 1, y, z; #4 −x + 1, −y + 2, −z; #5 −x, −y + 2, −z. b Symmetry codes: #1 x −1, y, z; #2 −x + 2, −y, −z + 1; #3 x, y − 1, z + 1. c Symmetry codes: #1 −x + 1, −y, −z + 1; #2 −x, −y, −z + 1; #3 x − 1, y, z; #4 −x + 1, −y, −z + 2. d Symmetry codes: #1 −x, −y + 1, −z; #2 −x + 1, −y + 1, −z; #3 x − 1, y, z; #4 −x + 1, −y, −z + 1.
Compound 2^a
Cd(1)–O(18)	2.224(5)	Cd(1)–O(3)	2.226(5)
Cd(1)–O(12#1)	2.253(5)	Cd(1)–O(20)	2.249(5)
Cd(1)–O(14#2)	2.262(4)	Cd(1)–O(19)	2.456(5)
Cd(2)–O(18)	2.163(5)	Cd(2)–O(4)	2.257(4)
Cd(2)–O(10)	2.275(5)	Cd(2)–O(16#3)	2.307(5)
Cd(2)–O(7#5)	2.319(5)	Cd(2)–O(8#4)	2.553(5)
Cd(3)–O(18#1)	2.243(5)	Cd(3)–O(15#2)	2.320(4)
Cd(3)–O(11)	2.339(5)	Cd(4)–O(5#4)	2.250(5)
Cd(4)–O(21)	2.265(5)	Cd(4)–O(2#4)	2.307(5)
Cd(5)–O(22)	2.273(5)	Cd(5)–O(23)	2.290(5)
Cd(5)–O(24)	2.301(5)	Cd(5)–O(6)	2.301(4)
Cd(5)–O(16#5)	2.307(5)	Cd(5)–O(7)	2.390(5)
Cd(6)–O(25#2)	2.229(5)	Cd(6)–O(9#2)	2.261(5)
Cd(6)–O(13#2)	2.350(5)
compound 3^b
Cd(1)-O(7#1)	2.198(4)	Cd(1)–O(8#2)	2.305(4)
Cd(1)-N(1#3)	2.313(4)	Cd(1)–O(9)	2.330(4)
Cd(1)-O(2)	2.371(4)	Cd(1)–O(1)	2.438(4)
compound 4^c
Cd(1)–O(5)	2.232(5)	Cd(1)–O(1#1)	2.254(5)
Cd(1)–O(17#1)	2.330(6)	Cd(2)–O(21#2)	2.176(5)
Cd(2)–O(3#3)	2.306(6)	Cd(2)–O(7#2)	2.408(5)
Cd(3)–O(21)	2.229(5)	Cd(3)–O(4#3)	2.229(5)
Cd(3)–O(6)	2.256(5)	Cd(3)–N(1)	2.304(6)
Cd(3)–O(11)	2.395(5)	Cd(3)–O(9)	2.566(5)
Cd(4)–O(12)	2.245(5)	Cd(4)–O(13)	2.258(5)
Cd(4)–O(19)	2.257(6)	Cd(4)–O(20)	2.304(5)
Cd(4)–O(18)	2.342(5)	Cd(4)–O(13#4)	2.422(5)
Cd(5)–O(8#2)	2.167(5)	Cd(5)–O(21)	2.164(5)
Cd(5)–O(2#1)	2.172(5)	Cd(5)–O(10)	2.256(5)
Compound 5^d
Cd(1)–O(1)	2.248(3)	Cd(1)–O(8#3)	2.264(3)
Cd(1)–O(3#1)	2.359(3)	Cd(1)–O(4#1)	2.394(4)
Cd(1)–O(7#3)	2.620(4)	Cd(1)–C(14#1)	2.724(5)
Cd(2)–O(2)	2.205(3)	Cd(2)–N(2#4)	2.224(4)
Cd(2)–O(9)	2.314(3)	Cd(2)–O(5#2)	2.326(3)
Cd(2)–O(5)	2.350(3)	Cd(2)–O(6#2)	2.465(3)

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Results and discussion

Syntheses

The successful isolation of 1 in our previous work prompted us to extend our study to other complexes based on the 2,2′,3,3′-H₄bptc. Recently, the ‘mix-ligand’ method in the design of high dimensional MOFs has aroused chemists' attention.¹⁹ In this method, the selection of a second ligand is important in the construction of the resulting architecture. The common ‘second’ ligands are two kinds: rigid or flexible. In our investigation of the coordination chemistry of 2,2′,3,3′-H₄bptc, we have selected two rigid ‘second’ ligands of bpy and bpydo and three flexible ligands of bpe, bpp and bix.

Crystal structures

[Cd₉(bptc)₄(μ₃-OH)₂(H₂O)₁₄]·(bpydo)·2H₂O (2). The asymmetric unit of 2 contains six kinds of crystallographically non-equivalent Cd atoms (Cd1, Cd2, Cd3, Cd4, Cd5, and Cd6), two unique bptc⁴⁻ ligands in different coordination modes (Fig. 1 and Scheme 2), one three-coordinate hydroxy group (μ₃-OH), one half of bpydo (the bpydo molecule lies about an inversion center), seven coordinated and one solvated water molecules (Fig. 1 and Scheme 2). Three Cd atoms (Cd3, Cd4 and Cd6) lie on inversion centers. The coordination geometries for the six-coordinate Cd1, Cd2, Cd3, Cd4, Cd5 and Cd6 atoms are all close to that of an octahedron with Cd–O bands ranging from 2.186 to 2.561Å.

Scheme 2 Two different coordination modes of the bptc⁴⁻ ligand in 2. Each of the two modes is actually chiral and has its mirror coexisting in the crystal.

Fig. 1 The molecular structure of 2. Solvent water molecules and hydrogen atoms are omitted for clarity.

As to 2,2′,3,3′-bptc⁴⁻ in mode 1, the dihedral angle between two phenyl rings is 68.2°, four carboxylate groups (2-, 2′-, 3- and 3′-COO⁻) have a dihedral angle of 67.9, 69.9, 35.4 and 25.3° towards the plane of the corresponding linking phenyl rings, respectively. Compared with mode 1, the dihedral angle between two phenyl rings of bptc⁴⁻ is 69.2°, four carboxylate groups (2-, 2′-, 3- and 3′-COO⁻) have a dihedral angle of 68.7, 72.7, 45.9 and 15.7° towards the plane of the corresponding linking phenyl rings in mode 2, respectively.

Compound 2 presents a 2D layer structure along (011) plane (Fig. 2a), in which the bpydo molecules don't coordinate to any Cd atom and exist between the layers acting as guest molecules and templates (Fig. S1).† In each layer, Cd atoms are connected together through carboxylate groups of the ligand bptc⁴⁻ in two coordination modes and µ₃-O18. Cd1, Cd2, Cd3 and Cd5 atoms are united together through four carboxylate groups (C14, C16, C30 and C32) and µ₃-O18 to present a central symmetrical heptanuclear cadmium cluster (Fig. 2c), the symmetrical center is Cd3 atom. These heptanuclear cadmium clusters are united together through ligands in modes 1 and 2 along the c and a axis to give a 2D metal–organic network. More interestingly, Cd4 and Cd6 atoms can be locked in their cavities built from 2,2′,3,3′-bptc⁴⁻ ligands (mode 1 and mode 2, respectively) and heptanuclear clusters (Fig. 3). The cavities can be regarded as crown-ether-like cycles with four donor oxygen atoms (two pairs of symmetry related O2 and O5 atoms in cavity A and two pairs of symmetry related O9 and O13 atoms in cavity B), which can lock the Cd atom (Cd4 or Cd6) in their cavities. It should be noted that each cavity is achiral, now that it includes two ligands with opposite chirality (P and M). In addition, a 48 metal–organic cycle built from four heptanuclear clusters, four discrete Cd atoms and organic ligands in mode 1 and mode 2 is found in compound 2. If we take heptanuclear clusters and discrete Cd4 and Cd6 atoms as SUBs, this complicated 2D structure can be regarded as a simple (4,4) grid structure (Fig. S2).† Such layers are united together through hydrogen bonds (Table S1)† to give a 3D supramolecular structure.

Fig. 2 (a) 2D metal–organic network in 2. Cd atoms are drawn as polyhedrons. (b) The heptanuclear cadmium cluster.

Fig. 3 View of the crown-ether-like cycles (A: top and B: bottom) built from bptc⁴⁻ ligands (mode 1 and mode 2, respectively) and Cd atoms in compound 2. The cavities (A and B) are occupied by Cd4 and Cd6 atoms, respectively.

[Cd₂(bptc)(bpe)(H₂O)₂]·H₂O (3). The asymmetric unit of 3 contains two crystallographically non-equivalent Cd atoms (Cd1 and Cd2), one 2,2′,3,3′-bptc⁴⁻, one bpe ligand, two coordinated and one solvated water molecules (Fig. 4). The coordination geometries for Cd1 and Cd2 atoms are all close to that of distorted octahedron with bands ranging from 2.198 to 2.509 Å. As to 3,3′-bptc⁴⁻ in 3, the dihedral angle between two phenyl rings is 52.9°, four carboxylate groups (2-, 2′-, 3-, and 3′-COO⁻) have a dihedral angle of 60.0, 78.1, 56.0 and 7.0° towards the plane of corresponded linking phenyl rings, respectively.

Fig. 4 The molecular structure of 3 (50% thermal ellipsoids). Labels for all hydrogen atoms are omitted for clarity.

In compound 3, Cd1 and Cd2 atoms are first connected together through 2,2′,3,3′-bptc⁴⁻ to give a 1D ribbon (Fig. 5a), in which two symmetry related Cd1 atoms are connected together through two syn-bridged carboxylate groups (2 and 3′-COO⁻) from two adjacent bptc⁴⁻ ligands to give a binuclear cluster. Such binuclear cadmium clusters are arranged in the middle of the ribbon, whereas Cd2 atoms are suspended along the edge of the ribbon through coordination to 3- and 2′-COO⁻groups from two adjacent bptc⁴⁻ ligands. Such ribbons are further united together through bpe to present a 2D double layer structure (Fig. 5b and Fig. S3†). Considering the coordination modes of bptc⁴⁻, which is linked four times to the metal center (four discrete cadmium atoms and one binuclear cadmium cluster), it can be viewed as a four-connected node. Cd2 atom and binuclear cadmium cluster can be viewed as three- and four-connected nodes, respectively. Taking Cd2 atom and binuclear cadmium cluster as nodes, bptc⁴⁻ as rods, the topology representation for 3 is illustrated in Fig. 5b. The net of this framework can be described with the Schläfli symbol of (4.6²)(4³.6³)(4⁴.6¹⁰.8). As far as we know, this topology is completely new within coordination polymer chemistry. The discovery of this new topology is useful at the basic level in the crystal engineering of coordination networks. In the bpe molecular structure, two pyridine rings arrange at two sides of the C22–C23 single bond and are nearly perpendicular to each other, with a dihedral angle of 88.4°. These 2D layers are further united together through hydrogen bonds (Table S1†) to give a 3D supramolecular structure.

Fig. 5 (a) 1D Cd-bptc ribbon structure of 3. (b) Schematic diagram of the extended net with the Schläfli symbol of (4.6²)(4³.6³)(4⁴.6¹⁰.8): bptc ligand (red), binuclear cadmium cluster (turquoise), Cd2 atom (green) and bpe ligand (blue).

[Cd₄(bptc)(Hbptc)(bpp)(μ₃-OH)(H₂O)₄] (4). The asymmetric unit of 4 contains five kinds of crystallographically non-equivalent Cd atoms (Cd1, Cd2, Cd3, Cd4 and Cd5), one 2,2′,3,3′-bptc⁴⁻, one 2,2′,3,3′-Hbptc³⁻, one hydroxy group(µ₃-OH, O21), one bpp and four coordinated water molecules (Fig. 6). Two of the five Cd atoms (Cd1 and Cd2) lie on inversion centers. The coordination geometries for the six-coordinate Cd1, Cd2, Cd3, Cd4 and Cd5 atoms are all close to octahedron with bands ranging from 2.169 to 2.838 Å. As to 2,2′,3,3′-bptc⁴⁻, the dihedral angle between two phenyl rings is 78.9°, four carboxylate groups (2-, 2′-, 3- and 3′-COO⁻) have a dihedral angle of 82.1, 83.2, 17.8 and 13.6° towards the plane of the corresponding linking phenyl rings, respectively. The dihedral angle between two phenyl rings is 64.1°, four carboxylate groups (2-, 2′-, 3-COO⁻ and 3′-COOH) have a dihedral angle of 68.3, 66.4, 46.9 and 49.1° towards the plane of the corresponding linking phenyl rings in 2,2′,3,3′-Hbptc³⁻, respectively.

Fig. 6 The molecular structure of 4. Solvent water molecules and hydrogen atoms are omitted for clarity.

Compound 4 presents a 2D grid structure in the ac plane (Fig. 7). Cd2, Cd3 and Cd5 atoms are united together through six symmetry related carboxylate groups (C14, C16 and C29) and two µ₃-O21 to give a central symmetrical pentanuclear cadmium cluster with a symmetry center of Cd2 atom. Two symmetry related Cd4 atoms are connected together through two oxygen atoms (O13) from two symmetry related carboxylate groups (C31) to give a binuclear cadmium cluster, symmetry code is 1 − x, −y, 2 − z. It should be noted that two pentanuclear cadmium clusters can be connected through two 2,2′,3,3′-bptc⁴⁻ ligands to give a crown-ether-like cycle with four donor oxygen atoms (two pairs of symmetry related O1 and O5 atoms), which can lock the Cd1 atom in the cavity (Fig. 7). In addition, a 70 metal–organic cycle built from four pentanuclear cadmium clusters, two binuclear cadmium clusters, two discrete Cd atoms and organic ligands are found in compound 4. Its stability may be related to the formation of cadmium clusters.

Fig. 7 Top: 2D metal–organic layer structure in 4. Cd atoms are drawn as polyhedrons. Bottom: view of the crown-ether-like cycle built from two 2,2′,3,3′-bptc⁴⁻ ligands and two pentanuclear cadmium clusters. Its cavity is occupied by the Cd1 atom. The cavity is achiral, now that it includes two opposite-chirality ligands (P and M).

Bpp adopts a mono-coordinate mode to the Cd3 atom and the other pyridine ring is distributed between two layers (Fig. 8). N2 and O16 from the carboxylate group (O15–O16) atoms share one hydrogen atom (H2) and are linked together through the formation of a strong hydrogen bond (N2⋯H2⋯O16) (Table S1).† In the bpp molecular structure, two pyridine rings have a dihedral angle of 109.5 and 99.5° towards the C38–C39–C40 plane, respectively. The two pyridine rings give a dihedral angle of 97.1° towards each other. Through the hydrogen bonds (Table S1),† compound 4 gives a 3D supramolecular structure (Fig. 8).

Fig. 8 The 3D supramolecular structure of 4. Two adjacent 2D metal–organic frameworks are marked as blue and yellow, whereas the mono-coordinate “second” ligands (bpp) are highlighted as sanguine.

[Cd₂(bptc)(bix)(H₂O)] (5). The asymmetric unit of 5 contains two crystallographically non-equivalent Cd atoms (Cd1 and Cd2), one 2,2′,3,3′-bptc⁴⁻, one bix ligand, one coordinated water molecule (Fig. 9). The coordination geometries for the six-coordinate Cd1 and Cd2 atoms are all close to that of a distorted octahedron with bands ranging from 2.205 to 2.620 Å. As for 2,2′,3,3′-bptc⁴⁻ in 3, the dihedral angle between the two phenyl rings is 82.7°, four carboxylate groups (2-, 2′-, 3- and 3′-COO⁻) have a dihedral angle 97.0, 61.8, 15.4 and 37.8° towards the plane of the corresponding linking phenyl rings, respectively.

Fig. 9 The molecular structure of 5.

In 5, Cd1 and Cd2 atoms are first connected together through 2,2′,3,3′-bptc⁴⁻ to give a 1D chain (Fig. 10a). Two symmetry related bptc⁴⁻ with opposite chirality (P and M) are coordinated to two Cd1 atoms through 3-, and 3′-COO⁻groups to form a crown-ether-like cycle with six oxygen atoms (one bichelating 2′- and monochelating 2-COO⁻groups), in which two symmetry related Cd2 atoms are fixed. Such crown-ether-like cycles are connected together through 2-COO⁻groups in an anti–anti conformation and Cd1, Cd2 atoms give a 1D diamond-like chain along the a axis (Fig. 10a). Bix takes an anti-conformation and coordinates to such chains to give a 2D metal–organic network.

Fig. 10 (a) View of the diamond-like Cd-bptc chain structure along the a axis in 5. Cd₂-cluster is locked in the cavity built from Cd1 and the ligand bptc⁴⁻ with opposite chirality. (b) 2D layer structure along the bc plane. (c) View of the distorted CdI₂-type topology for 5. Cd1 and the binuclear cadmium cluster are drawn as green and turquoise, whereas the bix are highlighted as blue.

From a topology view, the Cd1 atom can be viewed as a 3-connected node and the binuclear cadmium cluster acts as the 6-connected nodes. In this way this framework can be simplified to be a (3,6) -connected network with a distorted CdI₂ topology (Fig. 10) having the Schläfli symbol of (4³)₂(4⁶.6⁶.8³). Although CdI₂-type structures are quite commonly found in inorganic compounds, only a few examples are known in organic–inorganic hybrid materials.²⁰ In the molecular structure of bix, two imidazole rings have a dihedral angle of 93.4 and 100.7° towards the benzene ring plane, respectively. Two imidazole rings give a dihedral angle of 30.1°.

According to the above structural description of our example compounds and compound 1, 2,2′,3,3′-Hbptc³⁻ and 2,2′,3,3′-bptc⁴⁻ ligands exhibit eight distinct kinds of bridging modes as shown in Schemes 2 and 3. Two kinds of coordination modes (µ₆:η²η²η²η¹ and µ₄:η²η²η²η²) of 2,2′,3,3′-bptc⁴⁻ in compound 1 are shown in Schemes 3a and 3b. In complex 2, two unique bptc⁴⁻ ligands in different coordination modes (µ₅:η²η¹η²η² in mode I and µ₈:η³η²η²η² in mode II) are found (Scheme 2). In compound 3, 2,2′,3,3′-bptc⁴⁻ ligand takes a trans-conformation with four carboxylate groups to coordinate to five Cd atoms with µ₅:η¹η¹η¹η² coordination mode (Scheme 3c). In compound 4, 2,2′,3,3′-bptc⁴⁻ ligand takes a trans-conformation with four carboxylate groups to coordinate to seven Cd atoms with µ₇:η²η²η²η² coordination mode (Scheme 3d). The 3- and 3′-COO⁻groups take a syn–syn conformation, whereas the 2- and 2′-COO⁻groups take a syn–trans conformation. In comparison with 2,2′,3,3′-bptc⁴⁻, 2,2′,3,3′-Hbptc⁴⁻ takes a trans-conformation with three carboxylate groups to coordinate to four Cd atoms with µ₄:η²η²η²η⁰ coordination mode (Scheme 3e). The 3′-COOH group remains protonized and withholds from coordinating to any Cd atoms. In compound 5, 2,2′,3,3′-bptc⁴⁻ ligand takes a trans-conformation with four carboxylate groups to coordinate to five Cd atoms with µ₅:η²η²η¹η¹ coordination mode (Scheme 3f). Though each of the coordination modes for 2,2′,3,3′-bptc⁴⁻ is actually chiral and has its mirror coexisting in the crystals in this paper, chiral metal–organic frameworks are expected when single chirality crystallizes in a single crystal.

Scheme 3 Coordination modes of the bptc⁴⁻ (a, b, c, d and f) and Hbptc³⁻ (e) ligands. a and b in 1; c in 3; d and e in 4; f in 5. Other two kinds of coordination modes of the bptc⁴⁻ in 2 can be found in Scheme 2.

Thermal analyses and luminescent properties

For 2, the weight loss of 10.15% (calcd. 10.20%) below 300 °C corresponds to the loss of one solvent and seven coordinated water molecules per formula. For 3, the weight loss of 7.00% (calcd. 6.85%) below 133 °C is attributed to the release of one solvent and two coordinated water molecules per formula. For 4, the weight loss of 5.02% (calcd. 5.19%) below 245 °C is attributed to the release of four coordinated water molecules per formula. For 5, the weight loss of 2.01% (calcd. 2.24%) below 198 °C is attributed to the release of one coordinated water molecule per formula (Fig. S4–S7 show the TG curves for 2–5).†

The photoluminescence properties of the compounds were investigated. While the free H₄bptc ligand displays oppositely weak luminescence (λ = 390 nm) in the solid state at room temperature, the compounds exhibit distinct radiation emission maxima (λ = 377 nm for 2, 440 nm for 3, 446 nm for 4 and 430 nm for 5) upon excitation at λ = 340 nm for 2 and 3 and 33 5nm for 4 and 5, (Fig. S8†). The emission bands in the four compounds might be attributed to the intra-ligand emission from H₄bptc. The enhancement of luminescence may be attributed to ligand chelation to the metal center, which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay.¹² Baseochromic shifts of emission occur in 2, 3, 4 and 5 which are probably due to the differences of the coordination environment around the Cd ions since photoluminescence behavior is closely associated with the metal ions and the ligands coordinated around them.²¹ In addition, the oppositely strong luminescence of 2 and 4 may be ascribed to the presence of high-nucleus metal clusters, since the µ₃-OH ligand may tighten the whole skeleton, resulting in much weaker vibrations.²²

Conclusion

In this paper, a new multicarboxylate ligand has been introduced to construct new MOFs based on Cd-cluster SUBs. Though some metal–organic frameworks based on a uniform metal-cluster have been found, one framework with two different kinds of clusters is very rare. In compound 4, a binuclear cadmium cluster, a pentanuclear cadmium cluster and a discrete Cd atom concomitant in one structure is amazing, and this kind of ‘mix’ systems may yield new interesting topologies and physical properties. The crown-ether-like cycles in 2, 4 and 5 are of interest in supramolecular chemistry. The topology of 3 and 5 represents the rare reported 2D MOFs with novel Schläfli symbols of (4.6²)(4³.6³)(4⁴.6¹⁰.8) and (4³)₂(4⁶.6⁶.8³). These results demonstrated that H₄bptc could serve as a flexible ligand to construct novel supramolecular architectures.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC: 20771056), the National Basic Research Program of China (2007CB925102), National Nature Science Foundation of China (Grant No. 20490218), Jiangsu Science and Technology Department and the Center of Analysis and Determining of Nanjing University.

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Footnote

† Electronic supplementary information (ESI) available: Hydrogen bond distances and angles for 2–5 (Table S1); 3D supramolecular structure of 2 (Fig. S1); topology structure of 2 (Fig. S2); 2D metal–organic double layer structure of 3 (Fig. S3); TG curve of 2–5 (Fig. S4–S7); Fluorescent emission spectra of 2–5 (Fig. S8). CCDC reference numbers 685455–685458. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b806899b

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