Effects of substituent groups on the structures and luminescence properties of 2D/3D Cd^II complexes with mixed rigid and flexible carboxylate ligands

Abstract

Three new Cd(II) complexes, [Cd(bic)(adp)_0.5] (1), [Cd(mbic)(adp)_0.5]·H₂O (2) and [Cd(hbic)(adp)_0.5]·H₂O (3) (Hbic = 1H-benzoimidazole-5-carboxylic acid, Hmbic = 2-methyl-1H-benzoimidazole-5-carboxylic acid, Hhbic = 2-hydroxyethyl-1H-benzoimidazole-5-carboxylic acid, H₂adp = adipic acid), have been prepared by the hydrothermal reaction of adp²⁻ with Cd(II) ions in the presence of different benzimidazole carboxylic acids. 1 exhibits a 3D framework constructed from 1D infinite [Cd(COO)]_n units which are connected by adp in a trans-(κ²-κ¹)-(κ²-κ¹)-μ₄ mode and bic in a novel trans-(κ¹-κ¹)-(κ¹)-μ₃ mode. 2 features an unusual [Cd₂(COO)₄] paddle-wheel dinuclear secondary building unit (SBU), which is bridged into a 2D sheet by adp in a (κ¹-κ¹)-(κ¹-κ¹)-μ₄ mode and mbic in a cis-(κ¹-κ¹)-(κ¹)-μ₃ mode. For 3, the metal centre is connected into a 1D double chain by hbic in a (κ¹)-(κ¹-κ¹)-μ₂ mode, which is further linked by the trans-(κ²-κ¹)-(κ²-κ¹)-μ₄ adp to form a 2D layer architecture. The luminescence properties in the solid state at room temperature of compound 1–3 are almost the same as the free ligand.

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Introduction

Coordination polymers have attracted considerable attention in the past decade for their structural features and potential applications as functional materials.¹ Although there are examples where the molecules are assembled in a predetermined fashion, prediction of the crystal structure is largely considered to be serendipitous.^1,2 The structure of the resulting framework is primarily dependent upon the coordination geometry of the metal centres as well as on the functionality and coordinating ability of the bridging ligand.³ The spherical d¹⁰ configuration of Cd(II) is particularly suited for the construction of coordination polymers.⁴ On the one hand, the carboxylate unit is widely used in the synthesis of coordination polymers;⁵ for the dicarboxylate entity in particular, and a lot of studies has detailed the synthesis of metal dicarboxylates that exhibit useful properties such as high porosity and magnetism.⁶ On the other hand organic ligands consisting of carboxylate and benzimidazole have experienced major development in crystal engineering, and a large number of networks have been reported.⁷ Therefore, 1H-benzoimidazole-5-carboxylic acid (Hbic), combining the virtues of both benzimidazole and carboxylic acid, has been selected as the building block due to the special metal-coordinating ability (Chart S1†).⁸ Though some coordination polymers with Hbic have been reported, they have rarely been used in conjunction with other ligand to construct frameworks.⁹

In the course of ongoing studies on transition metal compounds of carboxylate–aromatic amine or pyridine ligands,¹⁰ we introduced flexible H₂adp and rigid Hbic and its derivates in the construction of the new coordination polymers. Hbic with its carboxylate group and the benzimidazole provides additional coordination sites, which can stabilize the resulting solid networks and its derivates. By changing the 2-site proton of Hbic by methyl and hydroxyethyl groups in order to introduce bulkiness in the structures and it was possible to follow the effect of these substituent groups to the crystal structures of their complexes (Chart 1). The flexible dicarboxylate adp could supply diverse conformations to control over the stereochemistry of the metallic centres, and create different frameworks.¹¹

Chart 1 The ligands Hbic and its derivates employed.

Herein, we reported the syntheses, structures and luminescence properties of three Cd(II) coordination polymers using the different benzimidazole carboxylic acids and flexible adipic acid (H₂adp) as auxiliary ligand, which show the effect of substituent groups on the crystal structure.

Experimental section

Materials and physical measurements

All the solvents and reagents for syntheses were commercially available and used as received. Hbic, Hmbic and Hhbic were synthesized according to the reported procedure.⁸ Elemental analyses of C, H, and N were performed on a Perkin-Elmer 240C analyzer. IR spectra were measured on a TENSOR 27 (Bruker) FT-IR spectrometer by transmission through KBr pellets containing 1% of the compounds. Thermogravimetric analyses (TGA) were carried out on a Perkin-Elmer Pyris Diamond TG/DTA analyzer at a heating rate of 10 °C min⁻¹ from 30 to 1000 °C under N₂. Luminescence data were recorded on a model F-4500 Luminescence spectrophotometer. X-ray powder diffraction (XRPD) intensities were measured at 293 K on a Rigaku D/max-IIIA diffractometer (Cu-Kα, λ = 1.54056 Å). The experimental backgrounds were corrected using the Jade 5.0 software package. The simulated PXRD patterns were calculated from the single crystal diffraction data using PowderCell.

Synthesis

Synthesis of [Cd(bic)(adp)_0.5] (1). A mixture of Hbic (0.081 g, 0.5 mmol) and H₂adp (0.073 g, 0.5 mmol) was added to an 8 mL aqueous solution of 0.5 mmol NaOH. The mixture was stirred until the solids were completely dissolved, then Cd(NO₃)₂·6H₂O (0.305 g, 0.5 mmol) was added. After stirring for 5 minutes, the mixture was placed in a 23 mL Teflon-lined stainless steel autoclave and heated at 165 °C for 72 h. The autoclave was cooled over a period of 10 h in air. The yellow crystals of 1 were collected by filtration, washed with ethanol, and dried in air (yield 37% based on Cd). Elemental analyses: calcd for C₁₁H₉CdN₂O₄ (%): C, 38.22; H, 2.60; N, 8.10. Found (%): C, 38.59; H, 2.43; N, 7.94. IR data for 1 (KBr, cm⁻¹): 3103(w), 2913(w), 2345(m), 1597(m), 1547(m), 1376(vs), 1285(m), 1131(w), 962(m), 779(m), 651(m), 623(w).

Synthesis of [Cd(mbic)(adp)_0.5]·H₂O (2). The synthesis procedure of 2 was similar to that of 1, except that Hmbic (0.088 g, 0.5 mmol) replaces Hbic and 0.5 mL triethylamine was added before the mixture was placed into the Teflon-lined autoclave. Some straw yellow cluster-like crystals were collected by filtration, washed with ethanol, and dried in air (yield 38% based on Cd). Elemental analyses: calcd for C₁₂H₁₃CdN₂O₅ (%): C, 38.15; H, 3.44; N, 7.38. Found (%): C, 38.43; H, 3.41; N, 7.35. IR data for 2 (KBr, cm⁻¹): 3396(m), 2905(m), 1629(m), 1605(vs), 1391(vs), 1312(m), 1299(m), 1042(m), 919(w), 805(w), 779(m), 757(m).

Synthesis of [Cd(hbic)(adp)_0.5]·H₂O (3). The synthesis procedure of 3 was similar to that of 2, except that Hhbic (0.103 g, 0.5 mmol) replaces Hmbic. Some straw yellow sheet-like crystals were collected by filtration, washed with ethanol, and dried in air (yield 41% based on Cd). Elemental analyses: calcd for C₁₃H₁₄CdN₂O₆ (%): C, 38.29; H, 3.68; N, 6.87. Found (%): C, 38.63; H, 3.36; N, 6.46. IR data for 3 (KBr, cm⁻¹): 3417(m), 3091(m), 1602(m), 1571(vs), 1389(vs), 1323(m), 1118(vs), 1057(w), 957(m), 907(w), 817(m), 779(vs), 692(w).

X-ray data collection and structure determinations

X-ray single-crystal diffraction data of 1–3 were collected on a Rigaku MM-007/Saturn 70 with graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Absorption corrections were applied by using the multi-scan program SADABS.¹² The structures were solved using direct methods and refined with a full-matrix least-squares technique with the SHELXTL program package.¹³ Metal atoms in each complex were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F². The hydrogen atoms were generated geometrically. Those weak reflections points at high angle (2θ > 50°) was omitted during refinement. Isotropic refinement for C9 and O3 atoms in compound 3 was performed. Data collection and structural refinement parameters are given in Table 1 and selected bond distances and angles are given in Table 2.†

Table 1 Crystal data and structure refinements for 1–3

Complex	1	2	3
Empirical formula	C11H9CdN2O4	C12H13CdN2O5	C13H14CdN2O6
Fw	345.60	377.64	406.66
T (K)	173(2)	173(2)	173(2)
Crystal system	Monoclinic	Triclinic	Triclinic
Space group	P21/c	P1̄	P1̄
a (Å)	8.117 (1)	8.356(1)	8.603(2)
b (Å)	16.788(3)	9.336(2)	9.066(2)
c (Å)	7.918(1)	9.344(2)	10.120(2)
α (°)	90	62.605(2)	74.826(3)
β (°)	98.491(2)	89.857(3)	73.131(3)
γ (°)	90	85.559(3)	85.128(3)
V (Å^3)	1067.2(3)	644.8(2)	729.0(2)
Z	4	2	2
Dc (g cm^−3)	2.151	1.945	1.853
μ (mm^−1)	2.055	1.715	1.529
Date collect./uniq.	4767/2064	5044/2502	4824/2491
R1 (>2σ)	0.0247	0.0267	0.0279
wR2 (all data)	0.0615	0.0905	0.0781
GOF	1.079	1.010	1.113
Residues (e Å^−3)	−0.522/1.168	−0.683/2.144	−0.744/0.981

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Table 2 Selected bond lengths (Å) and bond angles (°) for 1–3^a

1
a Symmetry codes for 1: (a) −x, −0.5 + y, 0.5 − z; (b) −x, 1 − y, −z; (c) x, 0.5 − y, −0.5 + z. for 2: (a) −x, 2 − y, 1 − z; (b) −x, 1 − y, 1 − z; (c) x, 1 + y, z. for 3 (a) 1 − x, 1 − y, −z; (b) −x, 1 − y, 1 − z; (c) −1 − x, 1 − y, 1 − z.
Cd1–O1^a	2.238(2)	O1^a–Cd1–O2^b	169.51(9)
Cd1–O2^b	2.262(2)	O2^b–Cd1–O3	97.12(8)
Cd1–O3	2.264(2)	O2^b–Cd1–O3^c	86.48(8)
Cd1–O3^c	2.404(2)	O1^a–Cd1–O4^c	83.12(9)
Cd1–O4^c	2.382(2)	O2^b–Cd1–O4^c	96.72(9)
Cd1–N1	2.288(3)	O1^a–Cd1–N1	88.56(1)
O1^a–Cd1–O3	93.28(9)	O3–Cd1–N1	98.18(9)
O3–Cd1–O4^c	98.16(8)	N1–Cd1–O4^c	162.04(9)
O1^a–Cd1–O3^c	84.92(8)	N1–Cd1–O3^c	109.20(9)
O3–Cd1–O3^c	152.48(1)	O2^b–Cd1–N1	88.60(9)
O4^c–Cd1–O3^c	54.34(8)

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2
Cd1–O1^b	2.225(3)	O2^c–Cd1–O4^a	86.00(1)
Cd1–O2^c	2.260(3)	O3–Cd1–O2^c	86.54(1)
Cd1–O3	2.242(3)	O3–Cd1–O4^a	154.28(1)
Cd1–O4^a	2.268(3)	N1–Cd1–O1^b	99.91(1)
Cd1–N1	2.208(3)	N1–Cd1–O4^a	98.85(1)
O1^b–Cd1–O3	86.07(1)	N1–Cd1–O3	106.87(1)
O1^b–Cd1–O2^c	154.01(1)	N1–Cd1–O2^c	106.09(1)
O1^b–Cd1–O4^a	89.98(1)

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3
Cd1–O1	2.183(3)	O1–Cd1–O4^b	103.73(1)
Cd1–O3^a	2.399(4)	O1–Cd1–O5	100.60(1)
Cd1–O4	2.431(3)	O4^b–Cd1–O5	128.73(1)
Cd1–O4^b	2.283(3)	O5–Cd1–O3^a	90.46(2)
Cd1–O5	2.348(3)	O5–Cd1–O4	53.87(1)
Cd1–N1^a	2.240(4)	N1^a–Cd1–O4	138.59(1)
O1–Cd1–O3^a	161.59(2)	O1–Cd1–N1^a	93.93(1)
O4^b–Cd1–O3^a	79.94(2)	N1^a–Cd1–O4^b	119.14(1)
O1–Cd1–O4	121.84(1)	N1^a–Cd1–O5	103.19(1)
O4^b–Cd1–O4	74.98(1)	N1^a–Cd1–O3^a	69.05(1)
O3^a–Cd1–O4	76.57(1)

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Results and discussion

Crystal structure of 1

Single-crystal X-ray diffraction analysis of 1 reveals an infinite 3D network belonging to the monoclinic space group P2₁/c. The asymmetric unit consists of one Cd(II) ion, one bic and half of adipate (Fig. 1). The Cd(II) ion has a distorted octahedral geometry, which is equatorially coordinated by three oxygen atoms (O3, O3b and O4b) from two adp, and one bic nitrogen atom (N1), and the axial positions are occupied by two oxygen atoms (O1c and O2a) from two bic. The Cd–N bond length is 2.288(3) Å, and the Cd–O bond lengths range from 2.238(2) to 2.404(2) Å, which are all in accord with those found in the other Cd(II) complexes.⁴

Fig. 1 (Left): coordination environment of Cd^II ion in 1. (Hydrogen atoms are omitted for clarity; symmetry codes for (a) −x, 1 − y, −z; (b) x, 0.5 − y, −0.5 + z; (c) −x, −0.5 + y, 0.5 − z). (Right): 1D chain structure along c axis in 1.

The key feature is the layers of Cd–adipate bridged by the bic ligand to give the 3D framework. The adipate is fully deprotonated and each carboxylate group forms three bonds with two cadmium atoms. Thus it adopts a syn, syn, anti mode of coordination. Hbic, potentially can lose two protons, is only deprotonated at the carboxylate group where it bridges two cadmium in a syn, syn mode and the imidazole nitrogen is also involved in coordination with a cadmium atom.

The bic ligand in 1 exhibits a binding mode of trans-(κ¹-κ¹)-(κ¹)-μ₃ (IV mode in Chart S1†) as found for a Eu(III) complex.¹⁴ The adp acts as the bridge and adopts a trans-(κ²-κ¹)-(κ²-κ¹)-μ₄ mode (Chart S2 (XIV)†) to link four cadmium ions, similar to that reported for some lanthanide(III) complexes.¹⁵ The mode appears for the first time in the cadmium complexes, but here the carboxyl groups of adp coordinated to Cd^II ions with trans conformation, as discussed in the following. A six-member ring consisting of two Cd^II ions (Cd⋯Cd = 3.983 Å), three atoms O3–C8–O4 from the carboxyl group of bic and one μ₂-O1 atom from the carboxyl group of adp is generated. From the six-member rings, a 1D metallic chain propagates along c axis with Cd(II) ions as nodes (Fig. 1). The adjacent 1D metallic chains extend into 2D network in bc plane by pairs of parallel opposing bic ligands, between which the π–π stacking interaction with a face-to-face distance of 3.48 Å is observed.¹⁶ Due to the angle between two coordination sites of bic, the 2D network displays wavy plane with a dihedral angle of 115°. Furthermore, 3D structure of 1 is formed by adp bridging the above 2D network (Fig. 2).

Fig. 2 (a) 3D structure of 1 viewed from c axis. (b) Ball-and-stick style to express 3D connection.

Crystal structure of 2

In contrast to the structure of 1, that of 2 is based on one-dimensional chains of Cd–adipate bridged by the mbic into a 2D network. In the asymmetric unit of 2, there are one Cd(II) ion, one mbic, a half adp and one lattice water molecule. The square pyramidal geometry around the Cd centre is coordinated by four oxygen atoms (O1a, O2b, O3 and O4c) from two mbic and two adp with four oxygen square plane and one nitrogen N1 atom from one mbic as vertex of the prism (Fig. 3). The Cd–O bond distances are in the range of 2.225(3) to 2.268(3) Å, which are in good agreement with those of known Cd complexes.⁴ The Cd–N bond distance of 2.208(3) Å is shorter than that of 1. Both adipate carboxylate adopt a syn–syn coordinating mode to form pairs of cadmium bridged by the long adipate. The carboxylate group of mbic also bridged two cadmium atoms within a pair by syn–syn mode to form paddle-wheel dimers of cadmium. The imidazole nitrogen atom then coordinates the axial position of each cadmium atom resulting in square pyramidal geometry.

Fig. 3 (Up): coordination environment of Cd^II ions in complex 2. Symmetry codes: (a) −x, 1 − y, 1 − z; (b) x, 1 + y, z; (c) −x, 2 − y, 1 − z. (Down): 1D dual chain by mbic along b axis in 2.

Four equatorial carboxyl groups from two adp and two mbic, and two axial N-donating atoms from two mbic connected two Cd(II) ions and constructed paddle-wheel structural motif with the Cd⋯Cd distance of 3.218 Å. A number of transition metals of the 1st or 2nd transition row transition metals and lanthanides are reported to form homologous paddle-wheel structure.¹⁷ Nevertheless, [Cd₂(COO)₄] is less frequently observed and for the five known examples found in the CCDC database nearly all are 0D complexes formed from carboxylate containing large terminal groups, rather than bridging ligands.¹⁸ In these dimeric Cd complexes, the Cd⋯Cd distance ranges from 3.278 to 3.452 Å, longer than that of 2 probably because of the large spatial encumbrance of CH₃ group of mbic affects the coordination mode of the adp resulting in the unprecedented shortest Cd⋯Cd distance.

The coordination mode of mbic is described as cis-(κ¹-κ¹)-(κ¹)-μ₃, differing from the trans-mode of bic in 1. Alternatively, 1D dual chain stems from small eight-member-ring A and the big fourteen-member-ring B by pairs of mbic linking dimeric Cd(II) units along the b axis (Fig. 3). Pairs of mbic of the 1D dual chain are coplanar, which is distinct from the parallel arrangement of bic in 1D chains of 1. Such 1D dual chains are extended into 2D layer structure in the bc plane through adp in the (κ¹-κ¹)-(κ¹-κ¹)-μ₄ mode (Fig. 4). The existence of torsion angle of the aliphatic chains of the linker adp results in step-like layer for the stacking of parallel 1D chains. Hydrogen bonds (O1W–H1Wa⋯N2: 2.736 Å and O1W–H1Wb⋯O2: 2.825 Å) and π-stacking interactions (face to face distance is 3.34 Å) between mbic of adjacent layers^16,19 results in a 3D framework with a gear-like arrangement.

Fig. 4 (a) 2D layer in bc plane of complex 2. (b) The π–π stacking interactions between 2D layers to form a 3D supramolecular framework in b direction.

Crystal structure of 3

The structure of 3 also has a one-dimensional chain of Cd–adipate as in 2 but in this case the bridge is hbic. For 3, the independent unit also contains one Cd ion, one hbic, a half adp and one lattice water molecule. Cd1 is in a six-coordinated distorted octahedral geometry, made of three O atoms (O4, O4b and O5) from two adp, as well as two O atoms (O1 and O3a) and one N atom (N1a) from two hbic (Fig. 5). The Cd–O bond lengths range from 2.183(3) to 2.431(3) Å, which are also in the range of those found in other Cd complexes.⁴ The Cd–N bond distance is 2.240 (4) Å, shorter than that of 1 but longer than that of 2. Such two Cd ions compose of a dimer unit through μ₂-bridged O4 and O4a from different adp with the Cd⋯Cd distance of 3.741 Å showed in Fig. 5, which is not paddle-wheel structure for complex 2.

Fig. 5 (Up): coordination environment of Cd^II ions of complex 3. Symmetry codes: (a) 1 − x, 1 − y, −z; (b) −x, 1 − y, 1 − z. (Down): the chain of dimeric units by the hbic ligands.

A pair of hbic in reverse order adopts a (κ¹)-(κ¹-κ¹)-μ₂ coordination mode to bridge the dinuclear {Cd₂} units into double chains via their carboxylate O, imidazole N and hydroxyethyl O atoms (Fig. 5). The aliphatic chains of adp connects these double chains into a grid-like layer motif parallel to the ac crystal plane (Fig. 6).

Fig. 6 (a) The 2D structure in complex 3. (b) The π–π stacking interactions between 2D layers to form a 3D supramolecular framework.

Two kinds of hydrogen-bonding interactions from the lattice water molecules with the carboxylate oxygen O5 atoms of adp and the imidazole nitrogen N2 atoms of hbic, [O1W–H1A⋯O5 = 2.750 Å, O1W–H1B⋯N2 = 2.730 Å] were observed.¹⁹ They join neighbouring layers into a 3D supramolecular structure. Furthermore, the π–π stacking interactions between adjacent hbic from adjacent layers intensify the stability of 3D supramolecular architecture (the face-to-face distance is 3.48 Å).¹⁶

Discussion of the structures

The organic ligands (bic, mbic and hbic) are the key factors in the formation of the multi-dimensional frameworks.¹ Different conformations were adopted by the benzimidazole carboxylate in generating the coordination environment around the metal centres. Firstly, the existence of the 2-site substituent groups alters the coordination modes of benzimidazole for the stereo hindrance. In detail, bic and mbic adopt the trans-(κ¹-κ¹)-(κ¹)-μ₃ and cis-(κ¹-κ¹)-(κ¹)-μ₃ coordination modes in 1 and 2, respectively, and 2D grid (1) and 1D chain (2) structures (Fig. S1† and 3). The stereo hindrance of methyl substituent modifies the trans coordination mode of bic to cis mode in mbic. As a result the pair arrangement of bic in 1 being parallel differs from those of mbic in coplanar arrangement in 2. In complex 3, the hbic in (κ¹)-(κ¹-κ¹)-μ₂ mode differs from those of bic and mbic because the hydroxyethyl group adds another coordinating group. The hydroxyethyl group and 3-site N atom form a stable five-member-ring chelate and decrease the chance to coordinate to more cadmium(II) ions. The cis μ₂-connected mode of hbic results into 1D chain similar to that of complex 2. The formation of 1D structures based on benzimidazole in 2 and 3 could be attributed to the spatial effect of the substituent groups on 2-site of benzimidazole ligands. Secondly, the 2-site substituent groups affect the coordination abilities of the imidazole N atoms of these ligands. For the distances of Cd–N bonds in the complexes, that of 1 is the longest, which implies that the abilities of 1-site N atoms of bic may be weaker than those of 3-site N atoms of mbic and hbic.

Furthermore, the dimensions of metal–organic frameworks are influenced indirectly by the large stereo hindrance of the 2-site substituent groups. Based on benzimidazole ligands and central metal ions, complex 1 is 2D network, while 2 and 3 are 1D dual chain. Deriving from the 1D or 2D fundamental frameworks, the final dimensions of these complexes are 2D and 3D as adp are introduced. Therefore, the steric hindrance of 2-site substituent groups of mbic and hbic influences their coordination modes and abilities in the formation of complexes, as well as the construction of primary structure.

The diverse coordination modes and conformations of the soft adp is also an important ingredient for the geometry of Cd(II) ion and the construction of the frameworks. It adopts two kinds of μ₄ binding modes of trans-(κ²-κ¹)-(κ²-κ¹)-μ₄ and (κ¹-κ¹)-(κ¹-κ¹)-μ₄ in 1–3. In 1 and 3, adp coordinates to Cd(II) ions in the same mode of trans-(κ²-κ¹)-(κ²-κ¹)-μ₄ and results in the similar octahedral geometry for metal ions. But the adp ligands built the different structures: 2D brick-like grid for 1 and 1D zigzag chain for 3 (Fig. S2(a) and (c)†). The first reason might be the similar gauche–anti conformation of its aliphatic chains adopted in the complexes, but with the dissimilar synclinal degree (six C atom torsion angles = 60.6°, 180.0° for 1 and 69.5°, 180.0° for 3). Secondly, the arrangements of neighbouring adp are completely different, which adopt head-to-tail packing within the 2D grid with longer metallic distance of 3.984 Å for 1 and 3 in uniform head-to-head and tail-to-tail arrangements into 1D chain structure. The main ligands bic and hbic play a crucial role in the formation of high-dimensional MOFs, the auxiliary adp acts only as a linker. While in 2, adp adopts bis(bidentate) mode to connect four Cd(II) ions into 1D bracelet structure with eight-member-ring linked by the aliphatic chains (six C atom torsion angle = 66.1°, 180.0°) (Fig. S2(b)†).

Thermogravimetric analyses of 1–3

Thermogravimetric analyses (TGA) were carried out to examine the thermal stability of complexes 1–3 (Fig. S3†). The crushed single-crystal samples were heated up to 900 °C in N₂ at a heating rate of 10 °C min⁻¹. The TGA curve for 1 showed that the first weight loss step began from 380 °C. This indicates a high stability of the complex and is accordance with the structure information without lattice water molecule and with strong π-stacking interactions. For 2, the TGA curve indicated that the first weight loss of 5.8% between 50 and 150 °C corresponds to the loss of one water molecule (calc. 5.2%) per molecular unit. The next stage of this curve from 350 to 500 °C was the completely collapse of the whole skeleton of 2. The TGA curve for 3 is similar to that of 2, which is also divided into two stages: (i) departure of one lattice water, with the weight loss of 5.5% from 30 to 200 °C (calc. 4.8%); (ii) the complete decomposition up to 800 °C. The above thermal behaviours may be attributed to their structural features: 1 with 3D structure has higher stability, while 2 and 3 featuring 2D structure with one lattice water have similar thermal decomposition process.¹

Luminescence properties of 1–3

Coordination polymer with a d¹⁰ configuration (such as Zn²⁺ or Cd²⁺) has been found to exhibit photoluminescence properties.²⁰ Therefore, the solid-state luminescence on crushed crystals of 1–3 and the three free benzimidazole carboxylate ligands have been measured. The powder diffraction (XRD) result of 1–3 was recorded before luminescence measurements to ascertain the purity of the samples (Fig. S4†). Hbic, Hmbic and Hhbic exhibit emission spectra with a band maximizing at 525, 526 and 529 nm under excitation at 385 nm respectively, which is originating from intraligand π* → π (Fig. 7) transformation.^8,14 Under same measurement condition, compound 1–3 displays similar luminescence properties compared to corresponding free ligand. Such result suggest charge transfer between ligand and the metal center is negligible in compound 1–3 and their luminescence properties is dominated by the intraligand electron transition.

Fig. 7 The excitation (EX) and emission (EM) spectra of the Hbic, Hmbic, Hhbic and complex 1–3.

Conclusions

In summary, we synthesized three 2D/3D frameworks based on Cd(II) with three functionalised benzimidazole carboxylate ligands and flexible hexanedioic acid (adipic, H₂adp) as auxiliary ligand under hydrothermal conditions. The packing and the structures are directed by the presence of different secondary-building unit in each case. The luminescence is dominated by that of the organic ligands. Hence, this work further highlights substituent groups of ligand play an important role in the whole structure and properties.

Acknowledgements

This work was supported by NSFC (no. 21371037), Guangxi Province Science Funds for Distinguished Young Scientists (2012GXNSFFA060001), the Project of Talents Highland of Guangxi Province. MK thanks the CNRS-France for funding.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The coordination modes of Hbic and H₂adp, pictures of the structures, TG curves and PXRD patterns for 1–3. CCDC 924695–924697. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra04755a

‡ G.-Y. Wu and Y.-X. Ren contributed equally to this work.

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