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Construction and structural diversity of Cd-MOFs with pyrazole based flexible ligands and positional isomer of naphthalenedisulfonate
Inorganic Chemistry Communications 61 (2015) 35–40
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Short communication
Construction and structural diversity of Cd-MOFs with pyrazole based flexible ligands and positional isomer of naphthalenedisulfonate Udai P. Singh a,⁎, Neetu Singh a, Suman Chandra b a b
Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee 247 667, Uttarakhand, India Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 8 August 2015 Accepted 14 August 2015 Available online 20 August 2015 Keywords: Cadmium(II) Coordination polymers Conformation Naphthalenedisulfonate Thermal stability Photophysical properties
a b s t r a c t In the present communication, we have reported the construction of a series of Cd(II)-MOFs using conformationally flexible ligand (CFL); 3,3′,5,5′-tetramethyl-4,4′-bipyrazolyl (H2BPz), flexible bent ligand (FBL); methylenebis-(3,5-dimethylpyrazole) (H2MBPz) and positional isomer of naphthalene disulfonic acid salt ligands (1,5-NDS, 2,6-NDS). By using these ligands, four new coordination polymers namely [Cd(H2MBPz)2∙ 1,5-NDSA]n (NDS-MOF-1), [Cd(H2BPz)∙ 1,5-NDSA]n (NDS-MOF-2), {[Cd(H2MBPz)2]2+∙ 2,6NDSA2−}n (NDS-MOF-3) and {[Cd(H2BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) have been synthesized. The crystal structure analysis revealed that the employment of positional isomeric naphthalene disulfonic acid salts resulted in different architectures ranging from one dimensional chain to two dimensional grid network and further connected into a three dimensional supramolecular structure through intermolecular hydrogen bonds, π⋯π and C– H⋯π interactions. In addition, the photophysical properties and thermal stability studies for all the NDS-MOFs 1–4 were also investigated. © 2015 Elsevier B.V. All rights reserved.
During the past several years, metal-organic frameworks (MOFs) have gained huge attraction to become one of the most rapidly developing research areas in chemical and material sciences and emerged as an important ancestor of porous materials. This is not only because of their intriguing network [1] topologies, but also due to their exploitable properties for potential applications including gas adsorption and separation, catalysis, luminescence, sensing, and proton conduction [2–6]. In literature, as there is a prolific production of MOFs based on rigid ligands, the design, synthesis and applications of MOFs based on flexible ligands have so far attracted peerless attention [7]. Therefore, the construction of flexible ligands based MOFs has been found to be difficult due to the flexibility of ligands that can adopt different conformations via bending, twisting, or rotating and thus lead to distinct symmetries during self-assembly process [8]. Although various azoles based flexible ligands have been used for the construction of one, two and three dimensional coordination polymers [9]. In the category of flexible ligands, H2BPZ and H2MBPz have received considerable attention as a better linker and both serve as a neutral bridging, bidentate ligand to connect two metal ions [10–15]. The extensive work has been carried out using these attractive and structurally simple ligands from which several porous and the helical nature of coordination polymer has been reported [16]. In comparison to carboxylates and even phosphonates, sulfonate coordination polymer is still very unexplored because of the spherical ⁎ Corresponding author. E-mail address: [email protected] (U.P. Singh).
http://dx.doi.org/10.1016/j.inoche.2015.08.009 1387-7003/© 2015 Elsevier B.V. All rights reserved.
and poor ligating in the nature of the SO3 group but conventionally, dynamic behavior of sulfonate framework is more dominant than the robustness of sulfonate coordination. Many literatures are available in the different coordination modes of sulfonate by changing the substituent on the arenesulfonate [17]. Here we report the change in coordination behavior of the sulfonate group to the metal center by changing the position of sulfonate group from α to β of the naphthalene. Cai et al. prepared aqua metal sulfonate salts from the aqueous solution of metal sulfonate and reported the tunable coordination behavior of arenedisulfonate in metal sulfonate with the addition of amino acid [18]. Also on introducing the other auxiliary ligands to the metal center, the sulfonate anion and water compete for the coordination with the metal center [19]. The tridentate sulfonate anions have received significant interest as they may give intricate and diverse supramolecular architecture [20]. In general, the use of multifunctional sulfonates such as arenedisulfonates has been directed toward the formation of pillared layer metal sulfonates, microporous and nanoporous solids, etc. [21]. In recent years, various flexible N-donor azole based ligand MOFs together with aromatic and aliphatic carboxylates have been reported [22]. However, coordination polymers based on these flexible azoles and sulfonates are very limited [23]. In this report, we have synthesized and structurally analyzed a series of MOFs, derived from a mixed ligand system viz. H2Bpz and H2MBPz as well as various isomeric disulfonates i.e. 1,5-NDS and 2,6-NDS (Scheme 1). The implementation of a methodical approach has extended researchers to understand the effect of positional isomer of
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U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
Scheme 1. Synthesis of MOFs 1–4.
naphthalenedisulfonate on the coordination behavior of sulfonate and conformation of H2BPz and H2MBPz ligands. Although the exploitation of structural diversity of sulfonates and pyrazole has been performed separately, to the best of our knowledge, there are no reports in
literature about the effect of the change in conformation of H2BPz and H2MBPz with different isomers of naphthalenedisulfonates. Single crystal X-ray diffraction analysis revealed that NDS-MOF-1 crystallizes in the monoclinic crystal system with P2/n space group
Fig. 1. View of 2D sheet representing the interaction of positional isomer of sulfonates with the 1D tape of metal and H2MBPz.
U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
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Table 1. Crystal data and structure refinement parameters of NDS-MOFs 1–4.
Emprical formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V/Å3 Z Dcalc (g cm−3) μ/mm−3 θ range/° Reflections collected Independent reflections Parameters GOF (F2) R1; wR2 [I N 2σ(I)] R1; wR2 (all data)
NDS-MOF-1
NDS-MOF-2
NDS-MOF-3
NDS-MOF-4
C32H42CdN8O7S2 825.25 Monoclinic P2/n 11.8806(3) 9.1001(3) 16.6475(5) 105.427(2) 1734.99(9) 2 1.580 0.809 1.45–28.300 4296 3103 247 1.020 0.0435; 0.1422 0.0692; 0.1199
C20H21CdN4O6S2 589.94 Monoclinic Cc 10.077(3) 19.488(6) 11.471(3) 92.666(17) 2250.2(11) 4 1.741 1.202 2.09–32.040 5943 4651 303 1.157 0.0390; 0.1215 0.0583; 0.1014
C32H42CdN8O8S2 843.27 Orthorhombic Ibam 8.6988(3) 19.9090(6) 20.5713(7) 90 3562.6(2) 4 1.572 0.792 1.98–25.00 1625 1512 138 0.992 0.0234; 0.0960 0.0256; 0.0911
C25H39CdN9O12S 794.06 Monoclinic P21/c 14.5543(15) 13.7932(12) 19.3189(19) 106.463(4) 3719.3(6) 4 1.418 0.708 2.50–28.350 9207 6230 441 1.112 0.0558; 0.1815 0.0928; 0.1593
and its asymmetric unit consists of one cadmium atom, one H2MBPz ligand, one symmetry dependent half molecules of 1,5-NDS2 − anions and one lattice water molecule. Each Cd(II) center exhibits a slightly distorted octahedral geometry and coordinated to two oxygen atoms of each a 1,5-NDS2 − anion and four nitrogen from four individual H2MBPz ligands. The H2MBPz ligands serve as a double bridge to connect two Cd(II) centers with Cd–Cd separation 9.100 Å to form 1-D infinite chain as shown in Fig. S2. These chains are further linked by 1,5-NDS2 − anion to give a two dimensional sheet along the b-axis as shown in Fig. 1. H2MBPz can adopt trans conformation which might be due to the strong hydrogen bonding interaction between N–H proton of pyrazole and one of the oxygen of naphthalene-1,5-disulfonate with a distance of 2.047 Å as shown in Fig. S7. Further, these sheets are stacked over one another along a-axis and interact via weak hydrogen bonding i.e.; O34–H3W⋯O8 of distance 2.489 Å leads to the formation of three dimensional supramolecular metal organic frameworks (Table 1). NDS-MOF-2 crystallizes in the monoclinic crystal system with the Cc space group and its asymmetric unit consists of one cadmium atom, one H2BPz ligand, and one 1,5-NDS2− anion. In the framework, each Cd(II) center exhibits completely distorted octahedral geometry and uniformly coordinated by four equatorial oxygen atoms from two separate 1,5-NDS2− anions at distances of 2.284–2.245 Ǻ and two axial nitrogen atoms from two H2BPz moieties with a distance of 2.208–2.261 Ǻ (Fig. S3). Two crystallographic equivalent Cd(II) ions are bridged by two sulfonate oxygen atoms of 1,5-NDS2− to form a binuclear cluster unit with Cd–Cd separation of 5.794 Å affording the formation of two dimensional hexameric cavities of metalsulfonate framework (Fig. 2). The third oxygen atom of sulfonate group of coordinated 1,5-NDS2− anion is also involved in hydrogen bonding N2–H12⋯O4 interaction with a NH group of the pyrazole unit with a distances of 1.960–3.026 Å. Therefore,
H2BPz ligands connect the two cadmium in trans conformation with Cd−Cd separation of 10.070 Å. This illuminates the angular distortion in coordination polyhedron. Furthermore, these two dimensional framework linked by H2BPz ligand and resulted in the formation of three dimensional packing (Table 2). NDS-MOF-3 crystallizes in the monoclinic crystal system with P2/n space group and its asymmetric unit consists of a cationic cadmium complex and 2,6-NDS2− anion. The cationic complex is six coordinated with the N4O2 donor system. The cadmium metal center is coordinated by four nitrogen bearing lone pairs of electron from four individual H2MBPz units. Axially two coordination sites are occupied by two water molecules. 2,6-NDS is not directly coordinated to metal, but present as a counteranion. Two crystallographically equivalent Cd(II) ions are bridged by two H2MBPz moieties to form a binuclear cluster unit with Cd–Cd separation of 8.699 Å resulting in the formation of 1D chain same as formation in case of NDS-MOF-1. Due to the strong hydrogen bonding between N–H proton of pyrazole, uncoordinated 2,6-naphthalenedisulfonate anion and coordinated water molecule (N2–H12⋯O4; 3.026 Å, O1–H11⋯O4; 2.669 Å) H2MBPz species can adopt cis conformation (Table 3). The myriad of these weak interactions are responsible for the formation of two dimensional sheet (Fig. S3). Moreover, these sheets are stacked over each other along the c-axis and interact via C–H⋯π interactions (2.86 Å) between methylene C–H proton (H2MBPz) and π e− cloud of another H2MBPz molecule present in neighboring sheet. These are the only interactions, which stabilized the three dimensional packing in the crystal as shown in Fig. 1b. NDS-MOF/-4 has also been crystallized in the monoclinic space group P21/c and the asymmetric unit consists of one cadmium(II) center atom, two molecules of H2BPz ligand, two metal co-ordinated water molecules, one disordered uncoordinated nitrate anion, uncoordinated
Fig. 2. 2D framework of Cd and sulfonate anion and interlinking of this framework by H2BPz.
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U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
Table 2. Selected bond length and bond angle for NDS-MOFs 1–4. D–H⋯A
d(D–H)
[Cd(MBPz)∙1,5-NDSA]n(NDS-MOF-1) Cd(1)–N(7) 2.314(3) Cd(1)–N(5) 2.316(3) Cd(1)–O(1) 2.541(3)
d(D⋯A)
Table 3. Non-covalent interactions for NDS-MOFs 1–4 (Å and °). b(D–H⋯A)N
N(7)–Cd(1)–N(7) N(7)–Cd(1)–N(5) N(7)–Cd(1)–N(5) N(7)–Cd(1)–N(5) N(5)–Cd(1)–N(5) N(7)–Cd(1)–O(1) N(5)–Cd(1)–O(1) O(1)–Cd(1)–O(1) S(1)–O(1)–Cd(1) N(6)–N(5)–Cd(1) C(37)–N(7)Cd(1) N(24)–N(7)–Cd(1)
87.29(15) 94.54(10) 158.24(11) 94.54(11) 91.72(14) 106.43(10) 79.84(9) 172.90(11) 152.94(18) 122.1(2) 134.4(2) 120.8(2)
{[Cd(BPz)2] ∙1,5-NDSA }n (NDS-MOF-2) Cd(1)–N(1) 2.210(11) N(1)–Cd(1)–N(4) Cd(1)–N(4) 2.252(9) N(1)–Cd(1)–O(3) Cd(1)–O(3) 2.284(8) N(4)–Cd(1)–O(3) Cd(1)–O(4) 2.300(8) N(1)–Cd(1)–O(4) Cd(1)–O(2) 2.404(11) N(4)–Cd(1)–O(4) Cd(1)–O(5) 2.433(10) O(3)–Cd(1)–O(4) O(3)–Cd(1) 2.284(8) N(1)–Cd(1)–O(2) O(5)–Cd(1) 2.433(10) N(4)–Cd(1)–O(2) N(4)–Cd(1) 2.252(9) O(3)–Cd(1)–O(2) O(4)–Cd(1)–O(2) N(1)–Cd(1)–O(5) N(4)–Cd(1)–O(5) O(3)–Cd(1)–O(5) O(4)–Cd(1)–O(5) O(2)–Cd(1)–O(5) S(3)–O(2)–Cd(1) S(1)–O(4)–Cd(10) S(1)–O(5)–Cd(1) N(2)–N(1)–Cd(1) C(7)–N(1)–Cd(1)
154.04(13) 106.4(4) 94.1(4) 91.1(4) 109.4(4) 76.79(14) 89.0(4) 77.8(4) 155.7(3) 84.3(3) 76.2(4) 89.4(4) 86.9(3) 155.6(3) 115.59(13) 153.6(7) 170.2(7) 152.7(6) 122.7(8) 133.0(9)
{[Cd(MBPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-3) Cd(1)–O(2) 2.404(11) N(4)–Cd(1)–O(4) Cd(1)–O(5) 2.433(10) O(3)–Cd(1)–O(4) O(3)–Cd(1) 2.284(8) N(1)–Cd(1)–O(2) O(5)–Cd(1) 2.433(10) N(4)–Cd(1)–O(2) N(4)–Cd(1) 2.252(9) O(3)–Cd(1)–O(2) O(4)–Cd(1)–O(2) N(1)–Cd(1)–O(5) N(4)–Cd(1)–O(5) O(3)–Cd(1)–O(5) O(4)–Cd(1)–O(5) O(2)–Cd(1)–O(5) S(3)–O(2)–Cd(1) S(3)–O(3)–Cd(1) S(1)–O(4)–Cd(10) S(1)–O(5)–Cd(1) N(2)–N(1)–Cd(1) C(7)–N(1)–Cd(1)
109.4(4) 76.79(14) 89.0(4) 77.8(4) 155.7(3) 84.3(3) 76.2(4) 89.4(4) 86.9(3) 155.6(3) 115.59(13) 153.6(7) 166.3(6) 170.2(7) 152.7(6) 122.7(8) 133.0(9)
{[Cd(BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) Cd(1) N(3) 2.2954(4) N(3)–Cd(1)–N(7) Cd(1) N(7) 2.2991(4) N(3)–Cd(1)–N(1) Cd(1) N(1) 2.2992(4) N(7)–C(1)–(1) Cd(1) N(5) 2.3117(4) N(3)–Cd(1)–N(5) Cd(1) O(4) 2.3862(4) N(7)–Cd(1)–N(5) Cd(1) O(5) 2.4988(4) N(1)–Cd(1)–N(5) N(3)–Cd(1)–O(4) N(7)–Cd(1)–O(4) N(1)–Cd(1)–O(4) N(5)–Cd(1)–O(4) N(3)–Cd(1)–O(5) N(7)–Cd(1)–O(5) N(1)–Cd(1)–O(5) N(5)–Cd(1)–O(5) O(4)–Cd(1)–O(5) C(5)–N(7)–Cd(1) N(8)–N(7)–Cd(1)
175.018(14) 90.173(14) 92.679(14) 88.696(14) 87.842(14) 170.865(12) 92.257(13) 91.653(14) 93.356(14) 95.746(14) 91.408(12) 84.948(12) 82.029(12) 88.935(13) 174.116(12) 132.42(3) 120.69(2)
2+
2−
d(D⋯A)
b(D–H⋯A)N
[Cd(MBPz)∙1,5-NDSA]n(NDS-MOF-1) N6–H28⋯O7 0.908(49)Å 2.047(50) Å O34–H3W⋯O8 0.967(54)Å 2.489(71) Å
2.941 Å 3.316 Å
167.6 143.5
{[Cd(BPz)2]2+∙1,5-NDSA2−}n (NDS-MOF-2) N3–H30⋯O6 0.860(11)Å 1.970(16) Å N2–H33⋯O7 0.861(12)Å 1.980(15) Å
2.824 Å 2.836 Å
172.0 172.2
{[Cd(MBPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-3) N2–H12⋯O4 0.857(26) 2.192(27) Å O1–H11⋯O4 0.758(39) 1.930(39) Å
3.026 Å 2.669 Å
164.3 164.6
{[Cd(BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) N2–H2⋯O8 0.860(2)Å 2.031(1) Å N8–H8⋯O3 0.860(2)Å 2.012(5) Å N4–H4⋯O7 0.859(1)Å 2.063(2) Å O5⋯O3 – – O5⋯O1 – – O7⋯O3 – –
2.891 Å 2.872 Å 2.892 Å 2.783(1) Å 2.835(4) Å 3.002(5) Å
164.6 170.9 161.6 – – –
D–H⋯A
d(D–H)
d(H⋯A)
four water molecules and one 2,6-NDS anion molecules. In the framework, Cd(II) ions adopt a slightly distorted octahedral geometry having four equatorial nitrogen atoms of H2BPz species and two axial oxygen atoms of water molecules (Fig. S5). Due to the strong N–H⋯O interactions, i.e. N2–H2⋯O8 with distances of 2.891 Å between NH proton of pyrazole unit and uncoordinated 2,6-NDS anion as shown in Fig. S11, H2BPz moiety can bind to the metal center in cis conformation with a Cd–Cd distance of 10.14 Å. Therefore, these interactions are also found to be responsible to afford highly distorted two dimensional corrugated square grid layer between Cd(II) and H2BPz units as shown in Fig. S6. The three-dimensional structure is generated by extensively strong hydrogen bonding interaction between corrugated square grid coordination layer by water molecules, nitrate and 2,6-NDS2− anion through weak N–H⋯O and O–H⋯O interactions (Fig. S7). The structural comparison of all the NDS-MOFs (1–4) revealed that the conformational behavior of both the ligands H2BPz and H2MBPz depends on the nature of coordination of naphthalenedisulfonate. 1,5-NDS directly coordinates to the metal center, whereas 2,6-NDS is only interacting as a counteranion. This selectivity in coordination may be attributed to the acidic nature of α and β positions of naphthalenedisulfonate. The H2BPz and H2MBPz ligands can adopt both cis and trans conformations with different conformational parameters as reported by Guo et al. [22b]. Therefore, the MOFs constructed with coordinated 1,5-NDS ligands can adopt trans conformation whereas in case of uncoordinated 2,6-NDS both H2BPz and H2MBPz can adopt cis conformation. The conformational parameter Θ for the angle between coordinating sites with respect to methylene carbon atom in NDS-MOF-1 is comparatively greater than NDS-MOF-3, φ for the dihedral angle between the pyrazole rings in sulfonate anion coordinated NDS-MOFs viz. NDS-MOF-1 and NDS-MOF-3 is greater than NDSMOF-2 and NDS-MOF-4 and finally d for the metal-metal distances across the H2MBPz and H2BPz respectively in NDS-MOF-1 and NDSMOF-4 is greater than NDS-MOF-2 and NDS-MOF-3 as shown in Table 4. This is probably due to the strong hydrogen bonding interaction between oxygen atom of the coordinated sulfonate and NH group of H2MBPz and H2BPz [24].
Table 4 Conformational parameter of H2BPz and H2MBPz in NDS-MOFs 1–4.
MOF-1 MOF-2 MOF-3 MOF-4
θ
φ
d
Conformation
87.11 – 76.71 –
56.11 95.64 47.24 65.59
9.10 10.07 8.69 10.14
Cis Trans Trans Cis
U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
Fig. 3. TGA for MOFs 1–4.
Thermogravimetric studies for all the NDS-MOFs were conducted ranging from 30 to 800 °C under nitrogen atmosphere at 10 °C/min (Fig. 3). A weight loss below 150 °C was observed in case of NDSMOF-1 and NDS-MOF-4, probably due to the removal of water molecules. In addition to this minor weight loss in NDS-MOF-4 was due to expulsion of nitrate anion. The decomposition of sulfonate anion functionality occurred at 350 °C to 480 °C for all the NDS-MOFs. The weight loss in the range of 480–650 °C was due to the decomposition of the remaining organic residue. Thermogravimetric analysis results clearly revealed that the MOFs based on conformationally flexible H2BPz ligand are more stable (upto 500–600 °C) than the MOFs based on flexible bent H2MBPz ligand which might be due formation of stable H2BPz and Cd framework. To investigate the solid state luminescent properties of NDS-MOFs 1–4, the excitation and emission spectral studies were performed at room temperature. Excitation of NDS-MOFs 1–4 in the solid state at λex near 294 nm, 276 nm, 290 nm and 278 nm produced blue fluorescence with an emission at λem near 380 nm, 395 nm, and 402 nm, respectively and no emission were observed in case of NDS-MOF-4 as shown in Fig. 4. The free ligands 1,5-NDS, and 2,6-NDS also exhibited emissions at 392 nm, and 345 nm, respectively which were probably attributable to the π* → π transition. Therefore, the emission of all the NDS-MOFs can probably be attributed to the intraligand fluorescence
39
emission instead of LMCT or MLCT phenomena. Compared to the ligands, there is a large shift of emission band for the NDS-MOFs 2 and 3 which was not the case for NDS-MOFs 1 and 4. This fluorescence behavior of NDS-MOFs 2 and 3 may be due to deprotonation and coordination of mixed ligand to d10 metal ion which can significantly narrow the energy gap between the HOMOs and LUMOs. Quenching is observed in all the sulfonate MOFs/coordination polymers and their emission results are consistent in comparison to the previously reported Cd coordination polymer with the same bispyrazoles and different carboxylates i. e. 512 and 452 nm [22b,25]. The high luminescence in blue light region for NDS-MOFs 2 and 3 indicated that these MOFs/coordination polymers may be a good candidate for blue fluorescent material. We have also examined the structural homogeneity of bulk samples of NDS-MOFs 1–4 through a comparison of experimental and simulated powder X-ray diffraction (XRD) patterns. The experimental patterns correlate favorably with the simulated ones generated from singlecrystal diffraction as shown in Fig. S12. In summary, we have successfully synthesized four new cadmiumorganic coordination polymers with both conformationally flexible ligand (CFL), H2BPz and flexible bent ligand (FBL), H2MBPz. The conformation of these H2BPz and H2MBPz was aroused by versatile coordination behavior of positional isomer of naphthalenedisulfonate to the metal center. In NDS-MOFs 1 and 3, H2MBPz served as a double bridge structure to connect two Cd(II) centers and adopted trans and cis conformation respectively whereas in NDS-MOFs 2 and 4, conformationally flexible ligand H2BPz served as a linear linker to connect metal center and adopted both cis and trans positions of NH. The thermogravimetric analyses revealed that NDS-MOFs 2 and 4 showed a high thermal stability than NDS-MOFs 1 and 3. Furthermore, solid state emission spectra highlighted the potential applications of these NDS-MOFs for designing new types metal-organic luminescent materials. Acknowledgments The author gratefully acknowledges CSIR, New Delhi, India (01(2826)/15/EMR-II) for financial assistance and thankful to IIT Roorkee for providing the Single Crystal XRD facility. Appendix A. Supplementary material Experimental procedures and all characterization parts of the reported MOFs have been deposited in supporting information section. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche.2015.08.009. References [1] (a) (b) [2] (a) (b) [3]
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Fig. 4. Emission spectra for MOFs 1–4 in the solid state at room temperature.
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Construction and structural diversity of Cd-MOFs with pyrazole based flexible ligands and positional isomer of naphthalenedisulfonate Udai P. Singh a,⁎, Neetu Singh a, Suman Chandra b a b
Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee 247 667, Uttarakhand, India Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 8 August 2015 Accepted 14 August 2015 Available online 20 August 2015 Keywords: Cadmium(II) Coordination polymers Conformation Naphthalenedisulfonate Thermal stability Photophysical properties
a b s t r a c t In the present communication, we have reported the construction of a series of Cd(II)-MOFs using conformationally flexible ligand (CFL); 3,3′,5,5′-tetramethyl-4,4′-bipyrazolyl (H2BPz), flexible bent ligand (FBL); methylenebis-(3,5-dimethylpyrazole) (H2MBPz) and positional isomer of naphthalene disulfonic acid salt ligands (1,5-NDS, 2,6-NDS). By using these ligands, four new coordination polymers namely [Cd(H2MBPz)2∙ 1,5-NDSA]n (NDS-MOF-1), [Cd(H2BPz)∙ 1,5-NDSA]n (NDS-MOF-2), {[Cd(H2MBPz)2]2+∙ 2,6NDSA2−}n (NDS-MOF-3) and {[Cd(H2BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) have been synthesized. The crystal structure analysis revealed that the employment of positional isomeric naphthalene disulfonic acid salts resulted in different architectures ranging from one dimensional chain to two dimensional grid network and further connected into a three dimensional supramolecular structure through intermolecular hydrogen bonds, π⋯π and C– H⋯π interactions. In addition, the photophysical properties and thermal stability studies for all the NDS-MOFs 1–4 were also investigated. © 2015 Elsevier B.V. All rights reserved.
During the past several years, metal-organic frameworks (MOFs) have gained huge attraction to become one of the most rapidly developing research areas in chemical and material sciences and emerged as an important ancestor of porous materials. This is not only because of their intriguing network [1] topologies, but also due to their exploitable properties for potential applications including gas adsorption and separation, catalysis, luminescence, sensing, and proton conduction [2–6]. In literature, as there is a prolific production of MOFs based on rigid ligands, the design, synthesis and applications of MOFs based on flexible ligands have so far attracted peerless attention [7]. Therefore, the construction of flexible ligands based MOFs has been found to be difficult due to the flexibility of ligands that can adopt different conformations via bending, twisting, or rotating and thus lead to distinct symmetries during self-assembly process [8]. Although various azoles based flexible ligands have been used for the construction of one, two and three dimensional coordination polymers [9]. In the category of flexible ligands, H2BPZ and H2MBPz have received considerable attention as a better linker and both serve as a neutral bridging, bidentate ligand to connect two metal ions [10–15]. The extensive work has been carried out using these attractive and structurally simple ligands from which several porous and the helical nature of coordination polymer has been reported [16]. In comparison to carboxylates and even phosphonates, sulfonate coordination polymer is still very unexplored because of the spherical ⁎ Corresponding author. E-mail address: [email protected] (U.P. Singh).
http://dx.doi.org/10.1016/j.inoche.2015.08.009 1387-7003/© 2015 Elsevier B.V. All rights reserved.
and poor ligating in the nature of the SO3 group but conventionally, dynamic behavior of sulfonate framework is more dominant than the robustness of sulfonate coordination. Many literatures are available in the different coordination modes of sulfonate by changing the substituent on the arenesulfonate [17]. Here we report the change in coordination behavior of the sulfonate group to the metal center by changing the position of sulfonate group from α to β of the naphthalene. Cai et al. prepared aqua metal sulfonate salts from the aqueous solution of metal sulfonate and reported the tunable coordination behavior of arenedisulfonate in metal sulfonate with the addition of amino acid [18]. Also on introducing the other auxiliary ligands to the metal center, the sulfonate anion and water compete for the coordination with the metal center [19]. The tridentate sulfonate anions have received significant interest as they may give intricate and diverse supramolecular architecture [20]. In general, the use of multifunctional sulfonates such as arenedisulfonates has been directed toward the formation of pillared layer metal sulfonates, microporous and nanoporous solids, etc. [21]. In recent years, various flexible N-donor azole based ligand MOFs together with aromatic and aliphatic carboxylates have been reported [22]. However, coordination polymers based on these flexible azoles and sulfonates are very limited [23]. In this report, we have synthesized and structurally analyzed a series of MOFs, derived from a mixed ligand system viz. H2Bpz and H2MBPz as well as various isomeric disulfonates i.e. 1,5-NDS and 2,6-NDS (Scheme 1). The implementation of a methodical approach has extended researchers to understand the effect of positional isomer of
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U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
Scheme 1. Synthesis of MOFs 1–4.
naphthalenedisulfonate on the coordination behavior of sulfonate and conformation of H2BPz and H2MBPz ligands. Although the exploitation of structural diversity of sulfonates and pyrazole has been performed separately, to the best of our knowledge, there are no reports in
literature about the effect of the change in conformation of H2BPz and H2MBPz with different isomers of naphthalenedisulfonates. Single crystal X-ray diffraction analysis revealed that NDS-MOF-1 crystallizes in the monoclinic crystal system with P2/n space group
Fig. 1. View of 2D sheet representing the interaction of positional isomer of sulfonates with the 1D tape of metal and H2MBPz.
U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
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Table 1. Crystal data and structure refinement parameters of NDS-MOFs 1–4.
Emprical formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V/Å3 Z Dcalc (g cm−3) μ/mm−3 θ range/° Reflections collected Independent reflections Parameters GOF (F2) R1; wR2 [I N 2σ(I)] R1; wR2 (all data)
NDS-MOF-1
NDS-MOF-2
NDS-MOF-3
NDS-MOF-4
C32H42CdN8O7S2 825.25 Monoclinic P2/n 11.8806(3) 9.1001(3) 16.6475(5) 105.427(2) 1734.99(9) 2 1.580 0.809 1.45–28.300 4296 3103 247 1.020 0.0435; 0.1422 0.0692; 0.1199
C20H21CdN4O6S2 589.94 Monoclinic Cc 10.077(3) 19.488(6) 11.471(3) 92.666(17) 2250.2(11) 4 1.741 1.202 2.09–32.040 5943 4651 303 1.157 0.0390; 0.1215 0.0583; 0.1014
C32H42CdN8O8S2 843.27 Orthorhombic Ibam 8.6988(3) 19.9090(6) 20.5713(7) 90 3562.6(2) 4 1.572 0.792 1.98–25.00 1625 1512 138 0.992 0.0234; 0.0960 0.0256; 0.0911
C25H39CdN9O12S 794.06 Monoclinic P21/c 14.5543(15) 13.7932(12) 19.3189(19) 106.463(4) 3719.3(6) 4 1.418 0.708 2.50–28.350 9207 6230 441 1.112 0.0558; 0.1815 0.0928; 0.1593
and its asymmetric unit consists of one cadmium atom, one H2MBPz ligand, one symmetry dependent half molecules of 1,5-NDS2 − anions and one lattice water molecule. Each Cd(II) center exhibits a slightly distorted octahedral geometry and coordinated to two oxygen atoms of each a 1,5-NDS2 − anion and four nitrogen from four individual H2MBPz ligands. The H2MBPz ligands serve as a double bridge to connect two Cd(II) centers with Cd–Cd separation 9.100 Å to form 1-D infinite chain as shown in Fig. S2. These chains are further linked by 1,5-NDS2 − anion to give a two dimensional sheet along the b-axis as shown in Fig. 1. H2MBPz can adopt trans conformation which might be due to the strong hydrogen bonding interaction between N–H proton of pyrazole and one of the oxygen of naphthalene-1,5-disulfonate with a distance of 2.047 Å as shown in Fig. S7. Further, these sheets are stacked over one another along a-axis and interact via weak hydrogen bonding i.e.; O34–H3W⋯O8 of distance 2.489 Å leads to the formation of three dimensional supramolecular metal organic frameworks (Table 1). NDS-MOF-2 crystallizes in the monoclinic crystal system with the Cc space group and its asymmetric unit consists of one cadmium atom, one H2BPz ligand, and one 1,5-NDS2− anion. In the framework, each Cd(II) center exhibits completely distorted octahedral geometry and uniformly coordinated by four equatorial oxygen atoms from two separate 1,5-NDS2− anions at distances of 2.284–2.245 Ǻ and two axial nitrogen atoms from two H2BPz moieties with a distance of 2.208–2.261 Ǻ (Fig. S3). Two crystallographic equivalent Cd(II) ions are bridged by two sulfonate oxygen atoms of 1,5-NDS2− to form a binuclear cluster unit with Cd–Cd separation of 5.794 Å affording the formation of two dimensional hexameric cavities of metalsulfonate framework (Fig. 2). The third oxygen atom of sulfonate group of coordinated 1,5-NDS2− anion is also involved in hydrogen bonding N2–H12⋯O4 interaction with a NH group of the pyrazole unit with a distances of 1.960–3.026 Å. Therefore,
H2BPz ligands connect the two cadmium in trans conformation with Cd−Cd separation of 10.070 Å. This illuminates the angular distortion in coordination polyhedron. Furthermore, these two dimensional framework linked by H2BPz ligand and resulted in the formation of three dimensional packing (Table 2). NDS-MOF-3 crystallizes in the monoclinic crystal system with P2/n space group and its asymmetric unit consists of a cationic cadmium complex and 2,6-NDS2− anion. The cationic complex is six coordinated with the N4O2 donor system. The cadmium metal center is coordinated by four nitrogen bearing lone pairs of electron from four individual H2MBPz units. Axially two coordination sites are occupied by two water molecules. 2,6-NDS is not directly coordinated to metal, but present as a counteranion. Two crystallographically equivalent Cd(II) ions are bridged by two H2MBPz moieties to form a binuclear cluster unit with Cd–Cd separation of 8.699 Å resulting in the formation of 1D chain same as formation in case of NDS-MOF-1. Due to the strong hydrogen bonding between N–H proton of pyrazole, uncoordinated 2,6-naphthalenedisulfonate anion and coordinated water molecule (N2–H12⋯O4; 3.026 Å, O1–H11⋯O4; 2.669 Å) H2MBPz species can adopt cis conformation (Table 3). The myriad of these weak interactions are responsible for the formation of two dimensional sheet (Fig. S3). Moreover, these sheets are stacked over each other along the c-axis and interact via C–H⋯π interactions (2.86 Å) between methylene C–H proton (H2MBPz) and π e− cloud of another H2MBPz molecule present in neighboring sheet. These are the only interactions, which stabilized the three dimensional packing in the crystal as shown in Fig. 1b. NDS-MOF/-4 has also been crystallized in the monoclinic space group P21/c and the asymmetric unit consists of one cadmium(II) center atom, two molecules of H2BPz ligand, two metal co-ordinated water molecules, one disordered uncoordinated nitrate anion, uncoordinated
Fig. 2. 2D framework of Cd and sulfonate anion and interlinking of this framework by H2BPz.
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Table 2. Selected bond length and bond angle for NDS-MOFs 1–4. D–H⋯A
d(D–H)
[Cd(MBPz)∙1,5-NDSA]n(NDS-MOF-1) Cd(1)–N(7) 2.314(3) Cd(1)–N(5) 2.316(3) Cd(1)–O(1) 2.541(3)
d(D⋯A)
Table 3. Non-covalent interactions for NDS-MOFs 1–4 (Å and °). b(D–H⋯A)N
N(7)–Cd(1)–N(7) N(7)–Cd(1)–N(5) N(7)–Cd(1)–N(5) N(7)–Cd(1)–N(5) N(5)–Cd(1)–N(5) N(7)–Cd(1)–O(1) N(5)–Cd(1)–O(1) O(1)–Cd(1)–O(1) S(1)–O(1)–Cd(1) N(6)–N(5)–Cd(1) C(37)–N(7)Cd(1) N(24)–N(7)–Cd(1)
87.29(15) 94.54(10) 158.24(11) 94.54(11) 91.72(14) 106.43(10) 79.84(9) 172.90(11) 152.94(18) 122.1(2) 134.4(2) 120.8(2)
{[Cd(BPz)2] ∙1,5-NDSA }n (NDS-MOF-2) Cd(1)–N(1) 2.210(11) N(1)–Cd(1)–N(4) Cd(1)–N(4) 2.252(9) N(1)–Cd(1)–O(3) Cd(1)–O(3) 2.284(8) N(4)–Cd(1)–O(3) Cd(1)–O(4) 2.300(8) N(1)–Cd(1)–O(4) Cd(1)–O(2) 2.404(11) N(4)–Cd(1)–O(4) Cd(1)–O(5) 2.433(10) O(3)–Cd(1)–O(4) O(3)–Cd(1) 2.284(8) N(1)–Cd(1)–O(2) O(5)–Cd(1) 2.433(10) N(4)–Cd(1)–O(2) N(4)–Cd(1) 2.252(9) O(3)–Cd(1)–O(2) O(4)–Cd(1)–O(2) N(1)–Cd(1)–O(5) N(4)–Cd(1)–O(5) O(3)–Cd(1)–O(5) O(4)–Cd(1)–O(5) O(2)–Cd(1)–O(5) S(3)–O(2)–Cd(1) S(1)–O(4)–Cd(10) S(1)–O(5)–Cd(1) N(2)–N(1)–Cd(1) C(7)–N(1)–Cd(1)
154.04(13) 106.4(4) 94.1(4) 91.1(4) 109.4(4) 76.79(14) 89.0(4) 77.8(4) 155.7(3) 84.3(3) 76.2(4) 89.4(4) 86.9(3) 155.6(3) 115.59(13) 153.6(7) 170.2(7) 152.7(6) 122.7(8) 133.0(9)
{[Cd(MBPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-3) Cd(1)–O(2) 2.404(11) N(4)–Cd(1)–O(4) Cd(1)–O(5) 2.433(10) O(3)–Cd(1)–O(4) O(3)–Cd(1) 2.284(8) N(1)–Cd(1)–O(2) O(5)–Cd(1) 2.433(10) N(4)–Cd(1)–O(2) N(4)–Cd(1) 2.252(9) O(3)–Cd(1)–O(2) O(4)–Cd(1)–O(2) N(1)–Cd(1)–O(5) N(4)–Cd(1)–O(5) O(3)–Cd(1)–O(5) O(4)–Cd(1)–O(5) O(2)–Cd(1)–O(5) S(3)–O(2)–Cd(1) S(3)–O(3)–Cd(1) S(1)–O(4)–Cd(10) S(1)–O(5)–Cd(1) N(2)–N(1)–Cd(1) C(7)–N(1)–Cd(1)
109.4(4) 76.79(14) 89.0(4) 77.8(4) 155.7(3) 84.3(3) 76.2(4) 89.4(4) 86.9(3) 155.6(3) 115.59(13) 153.6(7) 166.3(6) 170.2(7) 152.7(6) 122.7(8) 133.0(9)
{[Cd(BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) Cd(1) N(3) 2.2954(4) N(3)–Cd(1)–N(7) Cd(1) N(7) 2.2991(4) N(3)–Cd(1)–N(1) Cd(1) N(1) 2.2992(4) N(7)–C(1)–(1) Cd(1) N(5) 2.3117(4) N(3)–Cd(1)–N(5) Cd(1) O(4) 2.3862(4) N(7)–Cd(1)–N(5) Cd(1) O(5) 2.4988(4) N(1)–Cd(1)–N(5) N(3)–Cd(1)–O(4) N(7)–Cd(1)–O(4) N(1)–Cd(1)–O(4) N(5)–Cd(1)–O(4) N(3)–Cd(1)–O(5) N(7)–Cd(1)–O(5) N(1)–Cd(1)–O(5) N(5)–Cd(1)–O(5) O(4)–Cd(1)–O(5) C(5)–N(7)–Cd(1) N(8)–N(7)–Cd(1)
175.018(14) 90.173(14) 92.679(14) 88.696(14) 87.842(14) 170.865(12) 92.257(13) 91.653(14) 93.356(14) 95.746(14) 91.408(12) 84.948(12) 82.029(12) 88.935(13) 174.116(12) 132.42(3) 120.69(2)
2+
2−
d(D⋯A)
b(D–H⋯A)N
[Cd(MBPz)∙1,5-NDSA]n(NDS-MOF-1) N6–H28⋯O7 0.908(49)Å 2.047(50) Å O34–H3W⋯O8 0.967(54)Å 2.489(71) Å
2.941 Å 3.316 Å
167.6 143.5
{[Cd(BPz)2]2+∙1,5-NDSA2−}n (NDS-MOF-2) N3–H30⋯O6 0.860(11)Å 1.970(16) Å N2–H33⋯O7 0.861(12)Å 1.980(15) Å
2.824 Å 2.836 Å
172.0 172.2
{[Cd(MBPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-3) N2–H12⋯O4 0.857(26) 2.192(27) Å O1–H11⋯O4 0.758(39) 1.930(39) Å
3.026 Å 2.669 Å
164.3 164.6
{[Cd(BPz)2]2+∙2,6-NDSA2−}n (NDS-MOF-4) N2–H2⋯O8 0.860(2)Å 2.031(1) Å N8–H8⋯O3 0.860(2)Å 2.012(5) Å N4–H4⋯O7 0.859(1)Å 2.063(2) Å O5⋯O3 – – O5⋯O1 – – O7⋯O3 – –
2.891 Å 2.872 Å 2.892 Å 2.783(1) Å 2.835(4) Å 3.002(5) Å
164.6 170.9 161.6 – – –
D–H⋯A
d(D–H)
d(H⋯A)
four water molecules and one 2,6-NDS anion molecules. In the framework, Cd(II) ions adopt a slightly distorted octahedral geometry having four equatorial nitrogen atoms of H2BPz species and two axial oxygen atoms of water molecules (Fig. S5). Due to the strong N–H⋯O interactions, i.e. N2–H2⋯O8 with distances of 2.891 Å between NH proton of pyrazole unit and uncoordinated 2,6-NDS anion as shown in Fig. S11, H2BPz moiety can bind to the metal center in cis conformation with a Cd–Cd distance of 10.14 Å. Therefore, these interactions are also found to be responsible to afford highly distorted two dimensional corrugated square grid layer between Cd(II) and H2BPz units as shown in Fig. S6. The three-dimensional structure is generated by extensively strong hydrogen bonding interaction between corrugated square grid coordination layer by water molecules, nitrate and 2,6-NDS2− anion through weak N–H⋯O and O–H⋯O interactions (Fig. S7). The structural comparison of all the NDS-MOFs (1–4) revealed that the conformational behavior of both the ligands H2BPz and H2MBPz depends on the nature of coordination of naphthalenedisulfonate. 1,5-NDS directly coordinates to the metal center, whereas 2,6-NDS is only interacting as a counteranion. This selectivity in coordination may be attributed to the acidic nature of α and β positions of naphthalenedisulfonate. The H2BPz and H2MBPz ligands can adopt both cis and trans conformations with different conformational parameters as reported by Guo et al. [22b]. Therefore, the MOFs constructed with coordinated 1,5-NDS ligands can adopt trans conformation whereas in case of uncoordinated 2,6-NDS both H2BPz and H2MBPz can adopt cis conformation. The conformational parameter Θ for the angle between coordinating sites with respect to methylene carbon atom in NDS-MOF-1 is comparatively greater than NDS-MOF-3, φ for the dihedral angle between the pyrazole rings in sulfonate anion coordinated NDS-MOFs viz. NDS-MOF-1 and NDS-MOF-3 is greater than NDSMOF-2 and NDS-MOF-4 and finally d for the metal-metal distances across the H2MBPz and H2BPz respectively in NDS-MOF-1 and NDSMOF-4 is greater than NDS-MOF-2 and NDS-MOF-3 as shown in Table 4. This is probably due to the strong hydrogen bonding interaction between oxygen atom of the coordinated sulfonate and NH group of H2MBPz and H2BPz [24].
Table 4 Conformational parameter of H2BPz and H2MBPz in NDS-MOFs 1–4.
MOF-1 MOF-2 MOF-3 MOF-4
θ
φ
d
Conformation
87.11 – 76.71 –
56.11 95.64 47.24 65.59
9.10 10.07 8.69 10.14
Cis Trans Trans Cis
U.P. Singh et al. / Inorganic Chemistry Communications 61 (2015) 35–40
Fig. 3. TGA for MOFs 1–4.
Thermogravimetric studies for all the NDS-MOFs were conducted ranging from 30 to 800 °C under nitrogen atmosphere at 10 °C/min (Fig. 3). A weight loss below 150 °C was observed in case of NDSMOF-1 and NDS-MOF-4, probably due to the removal of water molecules. In addition to this minor weight loss in NDS-MOF-4 was due to expulsion of nitrate anion. The decomposition of sulfonate anion functionality occurred at 350 °C to 480 °C for all the NDS-MOFs. The weight loss in the range of 480–650 °C was due to the decomposition of the remaining organic residue. Thermogravimetric analysis results clearly revealed that the MOFs based on conformationally flexible H2BPz ligand are more stable (upto 500–600 °C) than the MOFs based on flexible bent H2MBPz ligand which might be due formation of stable H2BPz and Cd framework. To investigate the solid state luminescent properties of NDS-MOFs 1–4, the excitation and emission spectral studies were performed at room temperature. Excitation of NDS-MOFs 1–4 in the solid state at λex near 294 nm, 276 nm, 290 nm and 278 nm produced blue fluorescence with an emission at λem near 380 nm, 395 nm, and 402 nm, respectively and no emission were observed in case of NDS-MOF-4 as shown in Fig. 4. The free ligands 1,5-NDS, and 2,6-NDS also exhibited emissions at 392 nm, and 345 nm, respectively which were probably attributable to the π* → π transition. Therefore, the emission of all the NDS-MOFs can probably be attributed to the intraligand fluorescence
39
emission instead of LMCT or MLCT phenomena. Compared to the ligands, there is a large shift of emission band for the NDS-MOFs 2 and 3 which was not the case for NDS-MOFs 1 and 4. This fluorescence behavior of NDS-MOFs 2 and 3 may be due to deprotonation and coordination of mixed ligand to d10 metal ion which can significantly narrow the energy gap between the HOMOs and LUMOs. Quenching is observed in all the sulfonate MOFs/coordination polymers and their emission results are consistent in comparison to the previously reported Cd coordination polymer with the same bispyrazoles and different carboxylates i. e. 512 and 452 nm [22b,25]. The high luminescence in blue light region for NDS-MOFs 2 and 3 indicated that these MOFs/coordination polymers may be a good candidate for blue fluorescent material. We have also examined the structural homogeneity of bulk samples of NDS-MOFs 1–4 through a comparison of experimental and simulated powder X-ray diffraction (XRD) patterns. The experimental patterns correlate favorably with the simulated ones generated from singlecrystal diffraction as shown in Fig. S12. In summary, we have successfully synthesized four new cadmiumorganic coordination polymers with both conformationally flexible ligand (CFL), H2BPz and flexible bent ligand (FBL), H2MBPz. The conformation of these H2BPz and H2MBPz was aroused by versatile coordination behavior of positional isomer of naphthalenedisulfonate to the metal center. In NDS-MOFs 1 and 3, H2MBPz served as a double bridge structure to connect two Cd(II) centers and adopted trans and cis conformation respectively whereas in NDS-MOFs 2 and 4, conformationally flexible ligand H2BPz served as a linear linker to connect metal center and adopted both cis and trans positions of NH. The thermogravimetric analyses revealed that NDS-MOFs 2 and 4 showed a high thermal stability than NDS-MOFs 1 and 3. Furthermore, solid state emission spectra highlighted the potential applications of these NDS-MOFs for designing new types metal-organic luminescent materials. Acknowledgments The author gratefully acknowledges CSIR, New Delhi, India (01(2826)/15/EMR-II) for financial assistance and thankful to IIT Roorkee for providing the Single Crystal XRD facility. Appendix A. Supplementary material Experimental procedures and all characterization parts of the reported MOFs have been deposited in supporting information section. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche.2015.08.009. References [1] (a) (b) [2] (a) (b) [3]
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Fig. 4. Emission spectra for MOFs 1–4 in the solid state at room temperature.
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