Synthesis, Crystal Structures, and Photoluminescent Properties of Two Supramolecular Architectures Based on Difunctional Ligands Containing Imidazolyl and Carboxyl Groups

Abstract

Two new supramolecular architectures, namely, [Cd(L₁)₂(H₂O)]_n (1) and [Ni(L₂)₂(H₂O)]_n (2), were synthesized by the reaction of corresponding metal salts of CdCl₂·2.5H₂O and NiCl₂·6H₂O with 2-(1H-imidazol-4-yl)benzoic acid (HL₁) and 3-(1H-imidazol-4-yl)benzoic acid (HL₂) respectively, and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and powder X-ray diffraction (PXRD). Both HL₁ and HL₂ ligands are deprotonated to be L₁^- and L₂^- anions that coordinate with Cd(II) and Ni(II) atoms to form two-dimensional (2D) layer structure. Topologically, complex 1 is a 2D network with (4, 4) sql topology, while 2 is a typical 6³-hcb topology net. Complex 1 exhibits intense light blue emission in the solid state at room temperature.

1. Introduction

The rational design and construction of metal-organic frameworks (MOFs) has become an expanding research topic in the fields of supramolecular chemistry and crystal engineering not only because of their intriguing molecular architectures and intricate topologies but also their promising applications in various areas in fluorescence, gas adsorption, separation, ion exchange, magnetic properties, catalysis, and so on [1,2,3,4,5,6]. Because the complexes incorporate metallic centers and bridging organic linkers, therefore, it is important to design suitable organic ligands and choose metallic centers to assemble desirable frameworks with specific structures and functions.

Due to their favorable coordination abilities for the N or O donor atoms, the organic linkers incorporating N-heterocyclic bridging or carboxyl groups ligands are extensively applied to construct coordination polymers and have made great progress in the field of crystal engineering [7,8]. The N-heterocyclic bridging ligands include imidazole, triazole, tetrazole, and pyridine or its analogues such as pyridazine, tetrazine, triazine, and pyrazine moieties [9,10,11,12]. In our previous studies, we have elaborately designed the series of novel 4-imidazole ligands with flexible coordination modes, and successfully employed the ligands to construct porous frameworks based on the 4-imidazolate-metal building units [13,14,15], and the porous frameworks exhibit favorable gas adsorption properties. Taking the various coordination modes of carboxyl group into consideration, we have further carried out studies to design another type of novel difunctional ligand containing 4-imidazolyl group and carboxyl group [16]. Furthermore, the novel organic ligand was applied to construct two series Cu(II) and Cd(II) coordination polymers based on its flexible coordination modes. It should be noted that difunctional groups occupy the 1- and 4-position of benzene ring respectively, and as a result, the ligand often act as linear 2-connected node exhibited in resulting coordination polymers. As an extension of our previous work, we have designed two isomers of 4-(1H-imidazol-4-yl)benzoic acid [16] and synthesized two new organic ligands-2-(1H-imidazol-4-yl)benzoic acid (HL₁) and 3-(1H-imidazol-4-yl)benzoic acid (HL₂). Here, we report the synthesis and crystal structures of two new coordination polymers of [Cd(L₁)₂(H₂O)₂]_n (1) and [Ni(L₂)₂(H₂O)]_n (2) obtained by the reaction of HL₁ and HL₂ with corresponding metal salts under hydrothermal condition.

2. Results and Discussion

2.1. Structural Description of [Cd(L₁)₂(H₂O)]_n (1)

The X-ray single-crystal structural analysis reveals that complex 1 crystallizes in monoclinic C2/c space group, and the asymmetric unit has a half molecule of [Cd(L₁)₂(H₂O)], namely, a Cd(II) atom sitting on a special position with a half of occupancy, one L₁⁻ ligand, and a half of coordinated water molecule. As shown in Figure 1, the Cd1 center with O₅N₂ pentagonal bipyramid geometry is coordinated by two pairs of oxygen atoms (O(1A), O(2A) and O(1B), O(2B)) of two chelating carboxylate groups from two distinct L₁⁻ ligands [17,18], another oxygen atom from coordinated water molecule (O3) in the basal plane and two nitrogen atoms (N1, N1C) from other two L₁⁻ ligands in apical positions. The bond lengths of Cd–O (2.298–2.453(7) Å) and Cd–N (2.255(8) Å) are in line with those of the reported compounds consisting of O–Cd–N segments, and the coordination angles around Cd1 are in the range of 53.5(2)～173.9(5)º (

Table 1. Selected bond lengths (Å) and bond angles (º) for 1 and 2.

Bond	d	Bond	d
1
Cd(1)-N(1)	2.255(8)	Cd(1)-O(3)	2.298(11)
Cd(1)-O(1)ii	2.438(8)	Cd(1)-O(2)ii	2.453(7)
2
Ni(1)-N(1)	2.0654(13)	Ni(1)-N(3)v	2.0252(13)
Ni(1)-O(2)iv	2.0123(11)	Ni(1)-O(3)	2.1211(11)
Ni(1)-O(5)	2.1475(12)	Ni(1)-O(4)	2.1541(10)
Angle	ω	Angle	ω
1
N(1)-Cd(1)-N(1)i	173.9(5)	N(1)-Cd(1)-O(3)	93.0(2)
N(1)-Cd(1)-O(1)ii	88.3(3)	O(3)-Cd(1)-O(1)ii	136.76(17)
(1)ii-Cd(1)-O(1)iii	86.5(3)	N(1)-Cd(1)-O(2)ii	86.0(3)
N(1)-Cd(1)-O(2)iii	94.7(3)	O(3)-Cd(1)-O(2)ii	83.46(15)
O(1)ii-Cd(1)-O(2)ii	53.5(2)	O(1)iii-Cd(1)-O(2)ii	139.5(2)
O(2)ii-Cd(1)-O(2)iii	166.9(3)
2
O(2)iv-Ni(1)-N(3)v	97.46(5)	O(2)iv-Ni(1)-N(1)	94.27(5)
N(3)v-Ni(1)-N(1)	91.25(5)	O(2)iv-Ni(1)-O(3)	94.49(4)
N(3)v-Ni(1)-O(3)	168.06(5)	N(1)-Ni(1)-O(3)	87.95(5)
O(2)iv-Ni(1)-O(5)	89.39(5)	N(3)v-Ni(1)-O(5)	91.36(5)
N(1)-Ni(1)-O(5)	175.19(5)	O(3)-Ni(1)-O(5)	88.66(4)
O(2)iv-Ni(1)-O(4)	155.91(4)	N(3)v-Ni(1)-O(4)	106.31(5)
N(1)-Ni(1)-O(4)	89.15(5)	O(3)-Ni(1)-O(4)	61.77(4)
O(5)-Ni(1)-O(4)	86.24(4)

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

).

The organic molecule of HL₁ was deprotonated to L₁⁻ and act as a μ₂-bridge to link two Cd(II) atoms using the imidazolyl and carboxyl coordination groups respectively. It should be noteworthy that these difunctional groups occupy the adjacent positions and result in serious steric effect, which promote these two group distortedly twist from the benzene plane. That is, the aromatic rings of phenyl and imidazolyl rings are nearly perpendicular, with the dihedral angle of 84.99º while the plane of carboxyl group twists from benzene ring with 52.367º respectively. In this sense, the HL₁ ligand is quite different from the ligand of 4-(1H-imidazol-4-yl)benzoic acid reported in our previous study, that the aromatic groups often are coplanar due to lack of steric hindrance.

Each L₁⁻ ligand acts as 2-connector to link two Cd(II) atoms, while each Cd(II) atom in return connects four different L⁻ ligands. In this connection, the Cd(II) atoms are linked to form a two-dimensional (2D) network along the ab plane (Figure 2). Topologically, the 2D layer is (4, 4) net, where L₁⁻ ligand and Cd(II) atom act as 2- and 4-connector respectively, and the lateral Cd···Cd distances are 7.49 Å, and the diagonal Cd···Cd distances are 6.27 and 13.61 Å. It should be noted that numerous intermolecular π–π and C-H···π interactions exist among the aromatic rings, as shown in Figure 3. The benzene rings of the L₁⁻ ligands between the adjacent 2D layers are parallel and are separated by a centroid-centroid distance of 3.94 Å and C-H···π distance of 3.32 Å [7,19]. The non-classic weak π–π and C-H···π interactions further link the 2D layers into three-dimensional (3D) supramolecular polymer (Figure 3).

2.2. Structural Description of [Ni(L₂)₂(H₂O)]_n (2)

Compound 2 crystallizes in the monoclinic space group P2₁/c, and the asymmetric unit contains one unique Ni(II) atom, two L₂⁻ ligands, and one coordinated water molecule. As shown in Figure 4, the Ni1 has a distorted octahedral environment, in which the equatorial plane contains O3, O4 atoms of one chelating carboxylate groups from one L₂⁻ ligand, and N3A, O2B from other two distinct L₂⁻ ligands, and the atoms N1 and O5 from L⁻ ligand and coordinated water molecule occupy the axial positions with an N1−Ni1−O5 angle of 175.19(5)º (Table 1). Two carboxyl groups from different L₂⁻ ligand adopt μ₁-η¹:η⁰-monodentate and μ₁-η¹:η¹-chelating coordination modes to coordinate with Ni(II) atoms while the imidazolyl groups both employ the external N atoms to link Ni(II) atoms. As a result, two different L₂⁻ both act as μ₂-bridge to link two Ni(II) atoms, and in return, each Ni(II) atom link other three Ni(II) atoms by the L₂⁻ ligand. Topologically, L₂⁻ ligands bridge the Ni(II) atoms to form a 2D layer with 6³-hcb topology where the Ni(II) atom and L⁻ ligand act as 3- and 2-connected nodes respectively (Figure 5). Compared with HL₁ ligand, the dihedral angle between two different imidazolyl groups and benzene rings from HL₂ are 21.126 and 16.304º, not twisting distortedly as 1, because the imidazolyl and carboxyl groups occupy the meta-position, avoiding the steric hindrance.

There exist weak interactions including rich hydrogen bond and π−π stacking interactions. Noticeably, the NH or N atom of imidazolyl groups and carboxyl group can act as hydrogen bonding donor or acceptor, which benefits the construction of supramolecular structures. In 2, it can be seen clearly that the N−H···O, O−H···O hydrogen bonds (N(4)···O(1) 2.833(18) Å, N(4)–H(4)···O(1) 144º; O(5)···O(4) 2.781(16) Å, O(5)–H(5B)···O(4) 176º) connect the adjacent 2D layers to produce 3D supramolecular polymer (Figure 6,

Table 2. Hydrogen bond lengths (Å) and bond angles (º) for 1 and 2.

D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
1
N(2)–H(2)···O(2)i	0.86	1.99	2.83(11)	166
2
N(2)–H(2A)···O(3)ii	0.86	2.07	2.89(17)	157
N(4)–H(4)···O(1)iii	0.86	2.09	2.83(18)	144
O(5)–H(5A)···O(1)iv	0.82	2.00	2.73(16)	147
O(5)–H(5B)···O(4)v	0.83	1.96	2.78(16)	176(2)
C(5)–H(5)···O(2)	0.93	2.40	2.73(18)	100

Symmetry code: (i) x,1 + y,z; (ii) x,y,1 + z; (iii) 1 + x,y,z; (iv) x,1 – y,1 – z; (v) 1 – x,1 – y,1 – z.

). Particularly, two benzene rings of the L₂⁻ ligands between the adjacent 2D layers are nearly parallel with a dihedral angle of 3.16º and are separated by a centroid−centroid distance of 3.56 Å, indicating the presence of strong π−π stacking interactions (Figure 6). Therefore, the overall framework of 2 is a 3D supramolecular polymer by rich N−H···O and O−H···O hydrogen bonds and strong π–π stacking interactions. The total void value of the channel is estimated to be 76.5 ų, 4.0 % of the total crystal volume of 1910.9 ų.

2.3. Thermal Analysis and Powder X-ray Diffraction Analysis

The thermal stability of complexes 1 and 2 were examined by thermogravimetric analysis (TGA) to ascertain the stability of supramolecular architectures in the N₂ atmosphere from 25–700 ºC, and the result is shown in Figure 7. The complex 1 shows a weight loss of 3.50% in the temperature range of 100–125 ºC, corresponding to the release of coordinated water molecules (calc. 3.57%), and the decomposition of the residue occurred at 330 ºC. The first weight loss of 3.83% around 85 ºC indicates the exclusion of coordinated water molecules (calc. 3.99%) for 2, and the residue is stable up to 325 ºC. Powder XRD experiment was carried out to confirm the phase purity of bulk sample, and the experimental pattern of the as-synthesized sample can be considered comparable to the corresponding simulated one, indicating the phase purity of the sample (Figure 8).

2.4. Photoluminescent Property

Considering that coordination polymers with d¹⁰ closed-shell metal center and aromatic-containing system may exhibit excellent fluorescence properties and serve as good candidates for potential applications as photoactive materials, we investigate the solid-state fluorescence property of complex 1 as well as the HL₁ ligand at room temperature [20,21,22]. The free HL₁ ligand shows emission band at 433 nm upon excitation at 407 nm, which may be attributed to π* → π transition of the intraligands [23,24], while the complex 1 exhibits light blue emission with maximum at 455 nm upon excitation at 399 nm as depicted in Figure 9. By contrast with the free ligand, the emission bands of complex 1 are 22 nm red-shifted. Such board emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT) [25,26].

3. Experimental Section

3.1. Materials and Instrumentation

All the commercially available chemicals and solvents were of reagent grade and used as received without further purification. Elemental analyses were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA). IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer (Instrument Inc., Karlsruhe, Germany) using KBr pellets. Thermogravimetric analyses (TGA) were performed on a simultaneous SDT 2960 thermal analyzer (Thermal Analysis Instrument Inc., New Castle, DE, USA) under nitrogen with a heating rate of 10 ºC min⁻¹. Power X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with CuKα (λ = 1.5418 Å) radiation at room temperature. The fluorescent spectra were measured using a Perkin Elmer LS-55B fluorescence spectrometer (PerkinElmer, Billerica, MA, USA).

3.2. Synthesis of [Cd(L₁)₂(H₂O)]_n (1)

Reaction mixture of HL₁ (0.021 g, 0.1 mmol), CdCl₂·2.5H₂O (0.0228 g, 0.1 mmol) and 10 mL H₂O was adjusted to pH = 7 with 0.5 mol L⁻¹ NaOH solution. The mixture was then sealed in a 20 mL Teflon-lined stainless steel container and heated at 120 ºC for 48 h. After cooling to the room temperature, colorless block crystals of 1 were collected with a yield of 82% by filtration and washed with ethanol and water for several times. Anal. Calcd. (%) for C₂₀H₁₆N₄O₅Cd: C, 49.59; H, 3.19; N, 11.10. Found (%): C, 49.42; H, 3.31; N, 11.22. IR(KBr): 3665～2640 (m), 1602 (s), 1529 (s), 1402(s), 1190 (w), 1131 (m), 1090 (s), 948 (m), 849 (m), 788 (m), 699 (w), 648 (m), 502 (m) cm⁻¹.

3.3. Synthesis of [Ni(L₂)₂(H₂O)]_n (2)

Complex 2 was obtained by the same procedure used for preparation of 1 except that the CdCl₂·2.5H₂O and HL₁ were replaced by NiCl₂·6H₂O (0.024 g, 0.1 mmol) and HL₂ respectively. Green block crystals of 2 were collected in 78% yield. Anal. Calcd. (%) for C₂₀H₁₆N₄O₅Ni: C, 53.26; H, 3.58; N, 12.42. Found (%): C, 53.11; H, 3.54; N, 12.28. IR(KBr): 3635～2360 (m), 1586 (s), 1551 (s), 1368 (s), 1313 (w), 1231 (m), 1169 (m), 1068 (w), 969 (m), 842 (m), 779 (s), 709 (m), 630 (m), 590 (m), 522 (w) cm⁻¹.

3.4. Crystal Structure Determination

The single crystal data of [Cd(L₁)₂(H₂O)]_n (1) and [Ni(L₂)₂(H₂O)]_n (2) were collected on a Bruker Smart APEX CCD diffractometer (Bruker, Karlsruhe, Germany) with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293(2) K. The structure was solved by direct method and refined by full-matrix least squares on F² using the SHELX-97 program [27]. The hydrogen atoms were generated geometrically except that H5B of water molecule in 1 was found at reasonable positions in the differential Fourier map and located there. The crystallographic data and structural refinement are listed in

Table 3. Crystallographic data and structure refinement for 1 and 2.

Empirical Formula	C20H16N4O5Cd	C20H16N4O5Ni
Formula weight	503.76	451.08
Temperature/K	296(2)	296(2)
Crystal system	Monoclinic	Monoclinic
Space group	C2/c	P21/c
a/Å	13.614(5)	14.2023(15)
b/Å	6.270(2)	18.929(2)
c/Å	22.659(8)	7.1289(8)
α/°	90	90
β/°	91.574(6)	94.382(2)
γ/°	90	90
Volume/Å3	1933.4(12)	1910.9(4)
Z	4	4
ρcalcmg/mm3	1.727	1.568
μ/mm−1	1.171	0.932
F(000)	1000	928
Index ranges	−16 ≤ h ≤ 17, −8 ≤ k ≤ 8, −13 ≤ l ≤29	−18 ≤ h ≤ 18, −24 ≤ k ≤ 23, −9 ≤ l ≤ 5
Reflections collected	5530	12981
Independent reflections	2179	4389
Data/restraints/parameters	2179/0/137	4389/0/276
Goodness-of-fit on F2	1.079	1.034
Final R indexes [I> = 2σ (I)]	R1 = 0.0759, wR2 = 0.2339	R1 = 0.0253, wR2 = 0.0648
Final R indexes [all data]	R1 = 0.0808, wR2 = 0.2419	R1 = 0.0307, wR2 = 0.0678
Largest diff. peak/hole / e Å-3	2.658/−3.735	0.307 /−0.264

.

Crystallographic data for the structure reported in this paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos. CCDC 1554214 for 1 and 1554215 for 2. Copy of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033; E-Mail: [email protected]).

4. Conclusions

In summary, we successfully synthesized two isomers 2-(1H-imidazol-4-yl)benzoic acid and 3-(1H-imidazol-4-yl)benzoic acid containing 4-imidazolyl and carboxyl functional groups, which were employed to construct two new coordination polymers [Cd(L₁)₂(H₂O)]_n and [Ni(L₂)₂(H₂O)]_n. Both of complexes show 2D layer structures, and 1 is a (4, 4) sql topology net while 2 is a typical 6³-hcb topology network. Furthermore, complex 1 exhibits blue photoluminescence emission at 455 nm upon excitation at 399 nm.