Structural and functional studies on ternary coordination polymers from 5-bromoisophthalate and imidazole based flexible linker

Abstract

Three ternary coordination polymers (CPs), namely, {[Cd₂(BrIP)₂(BITMB)(H₂O)₂]·(THF)₂·H₂O}_n (CP1), [Cu(BrIP)(BITMB)(H₂O)]_n (CP2), {[Ni(BrIP)(BITMB)(H₂O)]·(THF)(H₂O)₂}_n (CP3) were synthesized by solvothermal reactions between H₂BrIP (5-bromoisophthalate), BITMB (1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene) and respective metal nitrates. Single crystal X-ray diffraction studies reveal a variety of supramolecular interactions such as inter/intra molecular hydrogen bonding, C–H⋯π, and π⋯π stacking in the 1D interwoven metal organic triple helical motifs of CP1 and sql networks of CP2 and CP3. Solvents employed for the synthesis of these CPs may be acting as structure directing agents. The photocatalytic properties of all three CPs, for the decomposition of Metanil Yellow by dilute hydrogen peroxide in the presence of visible light, have been evaluated and up to 89% dye removal from aqueous solution was achieved in the case of CP2. Solid state fluorescence studies disclose the promising luminescence properties of synthesized CPs.

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Introduction

The past two decades have witnessed an exponential growth in the field of design and construction of new coordination polymers (CPs) or metal organic frameworks (MOFs). Huge research interest in metal organic materials has grown due to their application in the various fields such as adsorption, separation, magnetism, enantioselective catalysis, photocatalysis, molecular recognition, optical as well as luminescent properties.^1–4 The aesthetic appeal of supramolecular networks has been another acclaimed reason for the research interest in this field.² The MOFs/CPs have frequently been categorized on the basis of their dimensionality, porosity or framework interpenetration, etc. The number of employed organic linkers or metal nodes is another important criterion to classify the metal organic species, as the higher the number of components in a designed supramolecular assembly, the larger the leap towards anticipated tailoring of bio-inspired functional materials.¹ Thus the CPs comprising of mixed ligands; polycarboxylate and N-donor species, represent a significant subset of metal organic materials.

Application of CPs towards removal of dye contaminants from water has recently attained much attention.^4–7 The strategies for this purpose may be classified in two groups; the first being the use of CPs as photocatalysts for the degradation of dyes and another being the use of CPs as dye adsorbents.^5–7 Photocatalytic degradation of dyes is generally accomplished with or without an oxidizing agent such as dilute hydrogen peroxide.^5,6 Interesting studies on the photocatalytic degradation of several organic dyes have recently been reported. Etaiw and others have disclosed the photocatalytic properties of a number of CPs for dye degradation with hydrogen peroxide.⁶ The mentioned reports suggests that the open-shell metal ions such as Cu(II) are an ideal node for fabricating photocatalyst CPs. These reports further imply that the choice of organic linkers also play a vital role in the construction of photocatalytically active CPs. For instance, aromatic linkers owing to their capability for electron transfer result in good photocatalytic activity of CPs.

Angular dicarboxylates have frequently been chosen as robust linkers for the construction of CPs among which isophthalate linkers have emerged as one of the most celebrated, because of their versatile metal binding modes and ease of functionalization at C5 which extends the scope for additional coordination or hydrogen bonding sites.^8,9 5-Bromoisophthalate (BrIP) linker has been exploited to assemble a variety of CPs including ternary CPs in which 5-bromo functionality of BrIP linker assists the extension of the network assembly via hydrogen and/or halogen bonds.⁸ Additionally flexible imidazole based exo-bidentate ligands are excellent tethers in bridging the metal–organic motifs, enhancing the framework dimensionality.^10,11 Bis(imidazol-yl-methyl)benzene type ditopic linkers are well explored, however, reports on CPs comprising of BITMB are surprisingly intermittent (Scheme 1).¹¹

Scheme 1 anti- and syn-conformers of flexible ligand BITMB (a and b) and binding modes of angular dicarboxylate BrIP observed in the present study (c–f).

In continuation of our efforts to design and synthesize novel CPs, herein we present three ternary CPs involving angular BrIP and flexible N-donor BITMB with different metal nodes Cd(II), Cu(II) and Ni(II). Solvents, tetrahydrofuran (THF) and water employed for solvothermal syntheses also played an important structure directing role in the assembly of reported CPs. Attempted solvothermal reactions yielded ternary CPs {[Cd₂(BrIP)₂(BITMB)(H₂O)₂]·(THF)₂·H₂O}_n (CP1), [Cu(BrIP) (BITMB)(H₂O)]_n (CP2) and {[Ni(BrIP)(BITMB)(H₂O)]·(THF)(H₂O)₂}_n (CP3). Crystallographic studies revealed interesting inter/intra molecular hydrogen bonding, C–H⋯π, and π⋯π interactions among metal organic species and templating solvents. The structure of CP1 comprises 1D metal–organic triple helices; however, CP2 and CP3 possess isostructural sql networks. Interestingly, these materials exhibit promising luminescence and photocatalytic activity.

Experimental

Materials and methods

All reagents and solvents were purchased from commercial sources and were used without further purification. Distilled water was used for synthetic manipulations. Ligands were synthesized by adopting the reported procedures.¹² CHNS analyses were done using a Perkin-Elmer 2400 CHNS/O analyzer. IR spectra were recorded using KBr pellets on a Perkin-Elmer GX FTIR spectrometer. For each IR spectra 10 scans were recorded at 4 cm⁻¹ resolution. TGA was carried out using Mettler Toledo Star SW 8.10. Absorption and emission spectra were recorded using Shimadzu UV-3101PC spectrometer and Fluorolog Horiba Jobin Yvon spectrophotometer respectively. X-Ray powder diffraction data were collected using a Philips X-Pert MPD system with Cu Kα radiation. Single crystal structures were determined using BRUKER SMART APEX (CCD) diffractometer.

Syntheses of the compounds

{[Cd₂(BrIP)₂(BITMB)(H₂O)₂]·(THF)₂·H₂O}_n (CP1). H₂BrIP (70 mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and Cd(NO₃)₂·4H₂O (100 mg, 0.32 mmol) were dispersed in 7 mL H₂O–THF (1 : 1) and then sealed in a 14 mL Teflon-lined autoclave, which was heated at 398 K for 50 h. After slow cooling to room temperature, white crystals suitable for single crystal X-ray analysis were obtained. Yield = 58%. Elemental analysis (%) calc.: C, 41.40; H, 4.07; N, 4.71; found: C, 41.48; H, 4.65; N, 4.62; IR cm⁻¹ (KBr): 3417 (br), 2919 (w), 2367 (w), 1596 (s), 1434 (s), 1547 (s), 1370 (s), 1118 (m), 1089 (m), 1023 (w), 870 (w), 778 (m), 721 (m), 656 (w).

[Cu(BrIP)(BITMB)(H₂O)]_n (CP2). The synthetic procedure was same as for CP1 except for a slight variation in ligand stoichiometry, metal salt and solvents used in the reaction. (H₂BrIP (70 mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and Cu(NO₃)₂·6H₂O (100 mg, 0.41 mmol), 2 mL water, 2 mL methanol and 3 mL 1,4-dioxane were used). Yield = 55%. Elemental analysis (%) calc.: C, 49.64; H, 4.17; N, 9.26; found: C, 49.48; H, 4.35; N, 9.12; IR cm⁻¹ (KBr): 3421 (br), 3110 (w), 2920 (w), 2357 (w), 1618 (s), 1551 (m), 1347 (s), 1302 (s), 1228 (w), 1112 (m), 1024 (w), 954 (w), 838 (w), 778 (m), 708 (m), 653 (w).

{[Ni(BrIP)(BITMB)(H₂O)]·(THF)(H₂O)₂}_n (CP3). H₂BrIP (70 mg, 0.28 mmol), BITMB (75 mg, 0.26 mmol) and Ni(NO₃)₂·6H₂O (100 mg, 0.41 mmol) were dispersed in 10 mL H₂O–THF (1 : 1) and then sealed in a 23 mL Teflon-lined autoclave, which was heated at 408 K for 50 h. Yield = 72%. Elemental analysis (%) calc.: C, 49.18; H, 5.27; N, 7.91; found: C, 48.98; H, 5.65; N, 7.62; IR cm⁻¹ (KBr): 3439 (br), 3137 (w), 2919 (w), 2361 (s), 1598 (m), 1537 (m), 1436 (m), 1359 (m), 1232 (w), 1120 (w), 1083 (w), 1022 (w), 832 (w), 781 (w), 665 (m).

Typical procedure for investigation of the photocatalytic activity

The catalytic activity of CP1–CP3 was evaluated for the heterogeneous photocatalytic decolourization of Metanil Yellow (MY) dye by dilute H₂O₂ solution at ambient temperature. In a typical experiment, 100 mL glass reactor was charged with 50 mL aqueous MY solution (1 × 10⁻⁵ M) and the 15 mg photocatalyst (CP1–CP3) was dispersed in it for 30 min. in the absence of light to attain adsorption equilibrium. Subsequently, 2 mL of 0.5 M H₂O₂ solution was added and the reaction was illuminated by a tungsten filament lamp. Stirring was maintained throughout the reaction and 1 mL aliquot was withdrawn every 30 min which was then filtered and analyzed on an absorption spectrophotometer. A control reaction was also performed where the dye and H₂O₂ solutions were treated as per the above protocol in the absence of solid catalysts. UV-Vis absorption of the withdrawn aliquots was monitored at 435 nm to estimate the concentration of MY solution. Thus, the initial MY concentration (C₀) and MY concentration (C) after a certain photo-irradiation time (t) were determined and the C/C₀ ratio was computed to evaluate the photocatalytic performance of the synthesized CPs.

X-Ray crystallography

Summary of the crystallographic data, details of hydrogen bonding interactions and selected bond lengths & bond angles for CP1–CP3 are presented in Tables 1, 2 and S1,† respectively. A suitable single crystal of each CP was transferred from the mother liquor to paratone oil and mounted on the tip of a glass fiber for data collection. Intensity data for all three crystals were collected using Mo Kα (α = 0.71073 Å) radiation on a Bruker SMART APEX diffractometer equipped with CCD area detector at 150 K. The data integration and reduction were processed with SAINT software.¹³ An empirical absorption correction was applied to the collected reflections with SADABS.¹⁴ The structures were solved by direct methods using SHELXTL and were refined on F² by the full-matrix least-squares technique using the program SHELXL-97.^15,16 The contribution of highly disordered solvent molecules (THF–H₂O) present in the lattice of CP2 and CP3, to the diffraction data has been removed using the SQUEEZE subroutine as implemented in PLATON.¹⁷ However, in the case of CP1, a water molecule and two THF molecules present in the lattice were located from the difference Fourier map. Refinement of the compounds using the modified reflection data set apparently improved the R-factor and GOF. All non-hydrogen atoms were refined anisotropically until convergence was reached. The hydrogen atoms attached to the organic moieties were either located from the difference Fourier map or stereochemically fixed in all the CPs. In the case of CP1 the hydrogen atom of the lattice water molecules could not be located from the difference Fourier map.

Table 1 Crystal data and refinement parameters for CP1–CP3^a

Identification code	CP1	CP2	CP3
a R = ∑||Fo| − |Fc||/∑|Fo|; wR = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Chemical formula	Cd2C41H45N4O13Br2	CuC25H25N4O5Br	NiC25H25N4O5Br
Formula weight	1186.43	604.94	600.11
Crystal colour	Colourless	Blue	Green
Crystal size (mm)	0.23 × 0.11 × 0.06	0.09 × 0.05 × 0.02	0.05 × 0.05 × 0.02
Temperature (K)	150(2)	150(2)	150(2)
Crystal system	Monoclinic	Triclinic	Triclinic
Space group	P21/c	P1̄	P1̄
a (Å)	11.285(2)	9.298(3)	9.320(3)
b (Å)	11.947(2)	10.903(3)	10.402(3)
c (Å)	32.654(7)	13.375(4)	13.855(5)
α (°)	90	75.732(5)	76.135(6)
β (°)	92.262(4)	76.533(5)	75.941(6)
γ (°)	90	70.130(5)	71.167(6)
Z	4	2	2
V (Å^3)	4399.1(15)	1219.2(6)	1214.1(7)
Density (Mg m^−3)	1.791	1.648	1.642
μ (mm^−1)	2.852	2.579	2.490
F (000)	2356	614	612
Reflections collected	30 525	10 411	10 166
Independent reflections	7756	5387	5211
Rint	0.0721	0.0502	0.0710
Number of parameters	569	336	334
GOF on F^2	1.239	1.018	1.091
Final R1/wR2 (I ≥ 2σ(I)	0.0863/0.1724	0.0560/0.1172	0.0859/0.1358
Weighted R1/wR2 (all data)	0.1036/0.1788	0.0776/0.1269	0.1318/0.1524
CCDC number	939331	939333	939334

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Table 2 Hydrogen bonding parameters for CP1–CP3

CP	D–H⋯A	d(H⋯A) (Å)	d(D⋯A) (Å)	∠D–H⋯A (°)	Symmetry code
CP1	O(9)–H(9C)⋯O(11)	1.80(9)	2.702(12)	162(10)	1 = x, 1 + y; 2 = −x, 1/2 + y, 1/2 − z
	O(9)–H(9D)⋯O(4)^1	1.78(9)	2.708(10)	169(8)
	O(10)–H(10C)⋯O(8)^1	1.97(7)	2.743(9)	137(8)
	C(19)–H(19)⋯O(8)^2	2.44	3.021(12)	121
	C(27)–H(27B)⋯O(12)	2.56	3.414(16)	147
CP2	O(5)–H(5C)⋯O(4)^1	1.83(5)	2.709(4)	172(4)	1 = x, −1 + y, z; 2 = 1 + x, −1 + y, z; 3 = 1 − x, 2 − y, − z; 4 = 2 − x, 2 − y, −z
	C(22)–H(22)⋯O(1)^2	2.43	2.880(6)	109
	O(5)–H(5D)⋯O(2)^3	2.06(6)	2.771(5)	154(5)
	C(10)–H(10)⋯O(2)^4	2.34	3.239(6)	163
CP3	O(5)–H(5C)⋯O(3)^1	1.76(8)	2.607(6)	173(10)	1 = x, 1 + y, z;2 = 1 − x, 2 − y, 1 − z
	O(5)–H(5D)⋯O(1)^2	2.05(9)	2.817(7)	172(7)

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Results and discussion

Despite the same organic linkers, structural diversity was observed among these CPs. The CP1 comprise interwoven triple helical metal organic motifs glued via weak inter- and intramolecular interactions. CP2 and CP3 revealed isostructural sql networks stacked over one another. The structural diversity of the studied CPs is attributable to the coordination preferences of the employed metal nodes, the flexibility of both ligands in adopting different conformations, the versatile coordination mode of the carboxylate group of BrIP, lattice solvents and various non-covalant interactions in the supramolecular assembly. The solvent molecules employed in the reaction also have a profound effect on the structural motifs in all CPs. The lattice guests (THF and water) located in CP1 are involved in non covalent interactions with the ribbon type metal–organic motif.

Crystal and molecular structure of CP1

The asymmetric unit of CP1 contains two independent Cd(II) centers (Cd1 and Cd2), two BrIP, one BITMB, two coordinated water molecules along with one water and two THF molecules as solvent of crystallization (Fig. 1a). CP1 crystallized in a monoclinic system with P2₁/c space group and can be described as one-dimensional CP. Both independent metal centers possess distorted pentagonal bipyramidal geometry with identical N₁O₆ hepta-coordination. Independent Cd1 and Cd2 metal centers are bridged via oxygen of two different BrIP ligands, resulting in edge sharing between distorted pentagonal bipyramids (Fig. 1b). One of the BrIP ligand is involved in μ₄-η²η²η¹η¹ mode of coordination in which each carboxylate group bridges Cd1 and Cd2 pairs. The second BrIP ligand is making μ₂-η¹η¹η¹η¹ chelated coordination to link screw related Cd1Cd2 pairs generating a one-dimensional ribbon like pattern as depicted in Fig. 2a. Thus, the pentagonal base of hepta-coordinated Cd(II) centers (Cd1/Cd2) is provided by the chelated oxygen atoms O1, O2/O3, O4 from the first BrIP; O7, O8/O5, O6 from the second BrIP and O9/O10 water molecules, respectively. The bridging coordination of O7 and O5 to Cd2 and Cd1 generates the Cd1Cd2 dimeric pair which is translated along the b-axis with 2₁ symmetry in the ribbon like motif. The seventh coordination is provided by the terminal nitrogen atoms N1 and N4 of the BITMB ligand in syn conformation linking the adjacent screw related Cd pairs in up and down manner decorating the ribbon motif. Cd⋯O distance ranges from 2.304(8) to 2.486(6) Å/2.299(7) to 2.470(7) Å, Cd1⋯N1 distance is 2.247(9) Å/2.272(9) Å for the coordination around Cd1 and Cd2 respectively (Table S1†). Dihedral angles between the terminal imidazole rings with the central phenyl ring in the syn coordinated BITMB ligand bridging the adjacent metal pairs are almost orthogonal (83.6° and 83.53° respectively for N1N2C17–C19 and N3N4C28–C30). As depicted in Fig. 2a, a close examination of the ribbon motif revealed the existence of the interwoven triple helical strands relating the pairs of screw related Cd centers bridged by the BrIP ligand. The Cd1⋯Cd2 distance within the BrIP bridged pair through carboxylate oxygen atoms O5 and O7 is 3.78 Å. The Cd⋯Cd distance between the symmetrically translated screw pairs bridged by BrIP ligand is 9.87 Å.

Fig. 1 ORTEP depiction of asymmetric unit of CP1 (lattice solvents are omitted) (a); coordination environment around metal centre in CP1 (b); ORTEP depiction of coordination geometry in CP2 (c) and CP3 (d); ORTEP diagrams are drawn with 40% probability factor for thermal ellipsoids and some of the hydrogen atoms are omitted for clarity.

Fig. 2 One dimensional interwoven metal organic triple helix running along the b-axis; three individual strands are interconnected through carboxylate oxygen atoms from BrIP and terminal nitrogen of syn-BITMB tethers; (b) packing diagram viewed down the b-axis depicts the sequential arrangement of the helical chains (capped stick model) and solvent molecules (CPK model) in the crystal lattice.

The packing diagram viewed down the a-axis shows the lattice THF and water molecules are positioned between the adjacent alternate layers of the one dimensional motif (Fig. 2b). As depicted in Fig. 3a, water hydrogen atoms H9D and H10C are involved in intramolecular O–H⋯O interactions with the carboxylate oxygen atoms O4 and O8 and imidazole hydrogen H19 with carboxylate oxygen O8 respectively. 1D strands are stacked down the a-axis and involved in intermolecular C–H⋯O contacts engaging the imidazole hydrogen H18 and the carboxylate oxygen O6.

Fig. 3 1D metal organic triple helices in CP1 are held together by C–H⋯O, π⋯π stacking and C–H⋯π interactions along the a-axis (a); C–H⋯O and O–H⋯O hydrogen bonds along the c-axis in CP1 (b).

Fig. 3a further depicts the intermolecular C–H⋯π interaction involving imidazole ring screw pairs bridged by BrIP ligand is 9.87 Å and π⋯π interaction between the phenyl rings of BrIP and BITMB between the metal organic strands stacked down a-axis with Cg3⋯Cg4 distance 3.621(6) Å. Consequently, methyl hydrogen H31B of the central phenyl ring and imidazole hydrogen H18 of BITMB are involved in C–H⋯π contacts with the centroid of the imidazole ring C1g and C2g, with perpendicular distances from the imidazole ring plane to H31B and H18 2.98 and 2.70 Å respectively (H31B⋯C1g = 3.00, C31⋯C1g = 3.877 Å symmetry code: 1 − x, −1/2 + y, 1/2 − z; H18⋯C2g = 2.80 Å, C18⋯C2g = 3.342 Å, symmetry code: 1 − x, 1/2 + y, 1/2 − z; C1g and C2g are centroids of the imidazole ring N1N2C17–C19 and N3N4C28–C30). The observed H⋯π distance involving C1g and H31B is slightly longer but resides within the acceptable range of CH/π interactions, indicating a weak C–H⋯π contact.¹⁸ The THF and water molecules residing between the consecutive coordination networks are involved in hydrogen bonding interactions amongst them and with the coordination network (Fig. 3b) which anchor these guest molecules between 1D coordination polymeric strands. Even though hydrogen atoms of the lattice water molecule O11 could not be located from the X-ray data they act as donors, making good short contact with the carboxylate oxygen O1 (O11⋯O1 = 2.76 Å), O13 of the THF molecule (O11⋯O13 = 2.78 Å) and as acceptors involving intermolecular O–H⋯O interactions with H7C of coordinated water and C–H⋯O contact with the lattice THF. Another THF molecule positioned with the opposite orientation is making C–H⋯O contact with the methylene hydrogen H27B of BITMB to stabilize the lattice solvent between the 1D coordination polymeric layers.

Crystal and molecular structure of CP2 and CP3

CP2 crystallized in P1̄ space group of the triclinic system. The asymmetric unit of CP1 includes one Cu(II), one full molecule of BITMB, one unit of BrIP ligand and one coordinated water molecule (Fig. 1c). Cu(II) possesses a distorted square pyramidal geometry with N₂O₃ coordination in which terminal nitrogen atoms (N1 & N4) from two different BITMB, carboxylate oxygen atoms (O1 & O3) from two different BrIP ligands (involved in μ₂-η¹η¹η⁰η⁰ bridging mode), and one water molecule (O5) are involved. The square base is constituted by N1, N4 from BITMB, O1, O3 of the BrIP ligand and the cis angles subtended by the coordinated atoms in the square base range from 88.51(12) to 91.09(12). Cu(II) is up by 0.34 Å from the square base towards the axially coordinated water molecule O5 (Cu1⋯O5 = 2.221(3)).

The topology of CP2 can be described as a neutral sql network in which the metal centers are bridged via μ₂-monodantate mode of the carboxylate oxygen generating [Cu(BrIP)₂]_n strands along the a-axis, further tethered by the terminal nitrogen atoms of BITMB ligand along the b-axis (Fig. 4). The Cu⋯Cu distance in the sql nets, bridged by BrIP and pillared by BITMB is 10.097 and 11.68 Å, respectively. Interestingly, BITMB ligand has taken up anti conformation and the twist angles between the terminal imidazole rings are 78.43° to make effective coordination with the metal. As depicted in Fig. 4a strong intramolecular O–H⋯O interaction between the coordinated water hydrogen H5C and uncoordinated carboxylate oxygen O4, C–H⋯O interaction between the imidazole hydrogen H22 and coordinated carboxylate oxygen O1 and C–H⋯π interaction between the methylene hydrogen H19B with the phenyl ring of the BrIP ligand are observed within the 2D sheets.

Fig. 4 sql network in CP2 and intramolecular interactions within the 2D sheet (a); packing of CP2 viewed down the a-axis depicting the bilayer arrangement of sheets via H-bonding interactions (b).

The packing diagram of CP2 viewed down the a-axis clearly exposed a displacement of the adjacent 2D nets alternatively along the b-axis as depicted in Fig. 4b. Adjacent 2D sheets are drawn in by strong intermolecular O–H⋯O and C–H⋯O interactions generating supramolecular bilayer assembly of 2D sheets. Consequently, uncoordinated carboxylate oxygen O2 from BrIP is involved as a bifurcated acceptor with the water hydrogen H5D and imidazole hydrogen H10 of the BITMB from the neighbouring (4,4) grid in the bilayer assembly. The lattice solvent molecules squeezed out from the structural data may be present in the void between adjacent bilayer 2D-sheets.

Even though CP3 shows the same crystallographic parameters and 2D sheet-type topology as observed in the case of CP2, variations in the coordination geometry and weak molecular interactions are noticed in CP3 because of the different metal nodes. In CP3 Ni(II) possesses a distorted octahedral coordination in which the carboxylate groups of BrIP are drawn in μ₂-η¹η¹η¹η⁰ bridging mode. The square base in CP3 is provided by the chelated/monodentate carboxylate oxygen atoms (O1, O2 and O4), from two different BrIP ligands and the O5 water. [Ni(BrIP)]_n strands running along the b-axis are cross-linked by the terminal nitrogen atoms N1 and N4 from two BITMB ligands axially in the N₂O₄ octahedral coordination. Interestingly, the same intramolecular O–H⋯O and C–H⋯π contacts are retained as observed in CP2, but an additional weak offset Cg⋯Cg contact between the phenyl ring of BITMB and BrIP is observed in CP3 as depicted in Fig. 5a. As depicted in Fig. 5b, the same bilayer arrangement of sql nets via C–H⋯O contacts observed in CP2 is maintained in CP3 also and perhaps the lattice solvents removed from the crystal data may be positioned between the bilayer sheets.

Fig. 5 sql network of CP3 and intramolecular interaction present within the 2D sheets (a); packing of CP3 viewed down the a-axis depicting the bilayer arrangement of sheets via H-bonding interaction (b).

FTIR, TGA and PXRD analyses

The lattice guests and composition of CP1–CP3 were ascertained by TGA, elemental and single crystal analyses. FTIR spectra (Fig. S1†) revealed valuable information about the coordination modes of BrIP moiety. The difference between asymmetric and symmetric carbonyl stretching frequencies (Δν = ν_asym − ν_sym) was used to fetch information on the metal–carboxylate binding modes. CP1 showed two pairs of ν_asym and ν_sym frequencies at 1596, 1434 (Δν = 162) and 1547, 1370 (Δν = 177) cm⁻¹ for the carbonyl functionality indicating the μ₄- and μ₂-modes as observed in the crystal structure. CP2 showed two pairs of ν_asym and ν_sym frequencies at 1618, 1347 (Δν = 271) and 1551, 1302 (Δν = 249) cm⁻¹ corresponding to the carbonyl functionality of dicarboxylate ligand indicating an unsymmetric monodentate coordination mode. The FTIR spectrum of CP3 displays ν_asym and ν_sym frequencies for symmetric chelating and unsymmetric monodentate modes at 1598, 1436 (Δν = 136) cm⁻¹ and at 1537, 1359 (Δν = 178) cm⁻¹, respectively. OH stretching broad bands in the range 3450–3400 cm⁻¹ for all three compounds are attributable to the coordinated and/or lattice water. The thermal stability of the synthesized compounds was ascertained by TGA (Fig. S2†). CP1 with robust triple helical motifs showed stability up to 325 °C. However, approximately 6% of its initial weight was lost in the temperature range 125–220 °C which may be ascribed to the one unit lattice THF molecule. CP2 have shown thermal stability up to ca. 270 °C, whereas CP3 lost two lattice water molecules (calc. 5.1%, obs. 5.6%) in the temperature range 100–150 °C. Further loss of lattice THF was observed in the temperature range 150–325 °C (calc. 10.18%, obs. 10.0%). Experimental PXRD patterns of bulk samples CP1–CP3 concur with those simulated from respective single crystal data, confirming the phase purity of bulk samples (Fig. S3†).

Photoluminescence study

Solid state photoluminescence behaviour of CP1–CP3 at ambient temperature was examined (Fig. 6 and S6†). Absorption maxima for ligands BITMB and H₂BrIP were observed at 273 and 300 nm respectively, however CP1–CP3 showed major absorption bands at ca. 297, 290, 379 nm, respectively. Upon excitation with 297 nm, strong emission at 454 nm with an overall luminescence quantum yield (Φ_overall) of 5.49% for CP1 was recorded at room temperature.

Fig. 6 Solid state absorption spectra for CP1–CP3 (a); emission spectra of CP1 (inset comprises digital photographs captured in daylight and at UV irradiation for CP1) (b).

The redshift in the emission bands (Table 3) of CP1 could be ascribed to the metal–perturbed intraligand emission and ligand field transitions. The good photoluminescence of CP1 may also be attributed to the closed shell metal configuration of Cd(II). Furthermore, the higher quantum yield of CP1 may also be attributed to the triple helical motif as such rigid structural features substantially cease the radiationless vibrational energy loss.^11a,19

Table 3 Spectral details of CP1 and the constituent ligands

CP/ligand	λmax (nm)	λex (nm)	λem (nm)	Δλ (=λem − λex)	Φoverall
CP1	241, 297, 341	297	454	157	5.49
BITMB	236, 273	273	305	32	—
H2BrIP	300	300	500	200	—

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Photocatalytic activity

Recent reports on the application of CPs as heterogeneous photocatalyst for waste water treatment have encouraged us to study the photocatalytic activity of synthesized CPs. Most of the reported coordination polymeric photocatalysts require UV irradiation for fruitful results and only few of them show significant activity under visible light. Thus, we tested CP1–CP3 for the bleaching of Metanil Yellow dye coloration in water under visible light. Analogous experiments to the existing reports were performed in the presence of dilute H₂O₂ and the general procedure is described in the Experimental section.

Dye–H₂O₂ mixture was stable in the dark for several hours without any change in absorbance. However, in the presence of visible light a slight decrease in the concentration of MY solution was observed. To evaluate the effect of H₂O₂ and visible light on the MY solution, a blank experiment was performed which resulted in ca. 10% dye decomposition within 180 min. Identical experiments in the presence of CP1–CP3 were performed and the time dependent change in the MY absorbance was monitored using UV-Vis spectroscopy. C/C₀ versus irradiation time plot (Fig. 7a) suggests that the copper coordination polymer CP2 exhibit the best photocatalytic activity among CP1–CP3. Approximately 89% of the initial amount of MY degrades within 180 min. in the presence of CP2. Under identical parameters CP1 catalyzed batch showed 17.6% dye degradation. Time dependent studies were not performed for CP3 as this CP significantly disintegrated during the course of the experiment. The differences in the photocatalytic activity of CP1 and CP2 may be attributed to the open shell configuration of Cu(II) center in CP2 and the dense triple helical framework of CP1. UV-Vis absorption changes observed with time during MY photodegradation catalyzed by CP2 in the presence of H₂O₂ are depicted in Fig. 7b. Time dependent UV-Vis absorption spectra for the blank batch and CP1 catalyzed batch are presented in Fig. S4.† The kinetic data for this batch revealed that the CP2 catalyzed photodecomposition of MY follows a pseudo-first order reaction pathway. The kinetic results corroborate with previous reports which also establish the pseudo-first order nature of heterogeneously catalyzed photodegradation of dyes with dilute H₂O₂.³

Fig. 7 Photocatalytic decomposition of MY solution with visible light irradiation and H₂O₂ in the presence of CP1, CP2 and without catalyst under the same experimental conditions; digital images of dye solutions at beginning (t = 0 min) and on completion (t = 180 min) of the attempted reactions (a); time dependent absorption spectra for CP2 catalyzed decomposition of MY by visible irradiation and dilute H₂O₂; a kinetic study revealing the pseudo-first order reaction pathway is presented in the inset (b).

The relevant report by Du et al., on the photocatalytic activity of MOFs/CPs explains dye decolouration on the basis of the semiconductor theory, which is appropriate for the present case too.^5c The theory suggests that illumination of a photocatalyst by visible radiation of suitable energy causes excitation of electrons from the valence band to the conduction band. This electronic transition causes formation of holes in the valence band. These holes can accept an electron from the organic molecules like dyes or water (used as solvent), resulting in the dye oxidation/degradation or formation of hydroxyl radicals, respectively. Hydroxyl radicals being strong oxidizing species, further contribute to the dye degradation. However, the recombination of excited electrons with holes in the valence band may drastically suppress the oxidative degradation processes. In order to hold back the recombination process of excited electrons with the holes in the valence bands, addition of electron acceptors such as hydrogen peroxide (H₂O₂), potassium bromated (KBrO₃) and ammonium persulfate ((NH₄)₂S₂O₈) is a prevalent method.²⁰ Specifically, the role of H₂O₂ in the present study can be assumed as an electron acceptor, which takes up the electrons from the conduction band of the photo-excited catalyst and generates hydroxyl radicals before the recombination process could take place. Thus stabilized holes in the valence band and the hydroxyl radicals possessing oxidation properties result in the degradation of the organic dye Metanil Yellow.

Phase purity of the recovered materials after the photocatalytic experiment was monitored by PXRD data which showed that CP1 and CP2 both remained intact after the MY degradation while CP3 lost its structural integrity (Fig. S5†). Thus, MY degradation studies reveal good photocatalytic potential CP2.

Conclusions

Three ternary CPs using dicarboxylate BrIP, flexible N-donor BITMB and metal nodes, Cd(II), Cu(II) and Ni(II) were synthesized solvothermally and characterized by various physico–chemical techniques. CP1–CP3 exhibits non-interpenetrated 1D and 2D metal organic motifs. CP1 manifests 1D interwoven triple helices, while CP2 and CP3 reveal sql motifs. The different architectures of CP1–CP3 are attributable to the different metal nodes and flexibility of BITMB ligand. The presented CPs showed versatile binding modes of carboxylate groups from the angular BrIP linkers with the metal ions. Moreover, the templating effect of solvent molecules which reside in the voids of the crystal structure also plays a significant role in defining the final structure and topology of these CPs. A variety of weak molecular interactions such as hydrogen bonding, C–H⋯π and π⋯π stacking were observed in these supramolecular assemblies which stabilize the molecule in the crystal lattice. Despite the interesting structural features, CP1–CP3 also exhibit significant photoluminescence and photocatalytic activity. CP1 showed strong emission with 5.49% overall quantum yield (Φ_overall) at room temperature. Among the three, CP2 exhibits good photocatalytic activity, which displayed nearly 90% degradation of Metanil Yellow under the standard protocol. Thus, the present study enriches the research insight for the design and synthesis of CPs possessing photoluminescent properties and photocatalytic activity.

Acknowledgements

The authors acknowledge CSIR India (HYDEN Project, Grant no. CSC 0122) for financial support, Mr Satyaveer Gothwal for TGA data, Mr V. K. Agrawal for IR data, Mr Viral Vakani for CHN analysis and Dr P. Paul for all-round analytical support. K.K.B and B.P acknowledge CSIR (India) and Y.R. acknowledges UGC (India) for research fellowships.

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Footnote

† Electronic supplementary information (ESI) available: FTIR, TGA, PXRD, time dependent UV-Abs data and selected bond lengths for CP1–CP3. CCDC 939331, 939333 and 939334. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra45411h

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