Two-Dimensional Mixed-Ligand Metal–Organic Framework Constructed from Bridging Bidentate V-Shaped Ligands

Abstract

A two-dimensional and bifunctional pillar-layered metal–organic framework (MOF)—with the molecular formula [Zn(cba)(bpdb)]·DMF (2DTMU-1), H₂cba = 4,4′-methylenedibenzoic acid, bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene—was obtained via the reaction of zinc(II) nitrate with H₂cba as the carboxylate linker and bpdb as the N-donor pillar. 2DTMU-1 is based on a binuclear paddlewheel Zn(II) unit complexed by four bridging bidentate (dicarboxylate) V-shaped ligands, which combine to from H₂cba; this tetragonal array, which is connected by bpdb with a bridging azine group, presents a pore size of 18 × 12 Ų.

1. Introduction

Metal–organic frameworks (MOFs) are subclasses of coordination polymers, which are often porous and created via the careful selection of metal ions or metal clusters alongside connecting linkers formed via coordination bonds [1,2,3,4,5,6]. In addition to coordination bonds, there are non-covalent interactions that occur, such as hydrogen bonding, π–π stacking between aromatic groups, and Van der Waals forces, which produce superstructures, flexibility, and dynamic porous compounds. Hydrogen bonds provide structural resistance that prevents the filling of space and thus allows for the growth of open frameworks [7,8]. In the synthesis of MOFs with metal ions and organic linkers, the flexibility around the metal ion often leads to the lack of proper control and the production of multiple structures; thus, oxygen-donating groups such as carboxylates are used to chelate the metal ions, whose interlocking is also used, resulting in oriented metal–oxygen–carbon clusters, which form geometric shapes called secondary structural units (SBU) [9]. In many reports, carboxylate ligands are usually chosen for the synthesis of MOFs because they act as both counter ions and bridging ligands with which to expand one-, two-, and three-dimensional structures [10]. For the design of favorable MOFs, the nature of the ligand used is very important [11,12,13]. For example, the use of aromatic polycarboxylate ligands leads to the production of MOFs with suitable coordination modes and high structural stability [14]. In the design and synthesis of MOFs with many dimensions, ligands with oxygen and nitrogen donors are often used [7,15,16,17]. The presence of different donor centers leads to the possibility of complexation due to linkage isomerism, and despite the difficulties of purification and identification experiments, this becomes a useful tool for the design of multi-dimensional MOFs. By employing various ligands or linkers, the preparation of special molecular frameworks with desirable properties can be realized. For example, through the use of N- and O-donor ligands (pyridine carboxylates), sensor compounds can be developed [18].

MOFs have been the focus of considerable attention due to their unique properties, such as their good crystallinity, high specific surface area, organic–inorganic hybrid nature, structural stability, and tunability in terms of pore size and structural features [19]. They are appealing for research due to their variety of properties and applications in chemistry and materials science [2,20,21,22]. The careful selection and rational architectural design of MOF structural units promotes their use in various applications, such as sensing [23], catalysis [24], drug delivery [25], and gas storage [26]. In particular, the functional groups of ligands have a significant influence on the chemical properties of MOFs. In addition, by controlling the dimensions and morphology of MOFs, it is possible to create more favorable physical and chemical properties for some advanced applications [27,28,29,30,31].

MOFs are often three-dimensional, although some two-dimensional MOFs have recently been developed [2,32]. In recent years, the construction of 2D MOFs has received a lot of attention owing to their fascinating structural features, such as their high surface area, high density of exposed functional sites, and capacity for more interactions between the substrate molecules and MOF surface, which are advantageous for various applications [33,34,35]. The 2D extended network can offer favorable performance in processes such as light harvesting and rapid electron and energy migration. Therefore, 2D MOFs have shown high applicatory potential in electronics, chemical sensing, catalysis, and gas separation [36,37].

Polytopic carboxylic ligands (linear or bent ditopic, tritopic, and tetratopic) are often used in the synthesis of MOFs, although angularly bent ligands (0° ≤ θ < 180°) are also applied for the synthesis of extended polyhedral structures [38]. On the other hand, pillared MOFs based on paddle-wheel nodes can be structurally classified into two different groups depending on the geometry and multitopic nature of the carboxylate linkers involved. In the first group, 2D metal–carboxylate layers are developed due to the coordination of deprotonated carboxylate groups to the metal ions and the formation of paddle-wheel inorganic nodes. In this regard, tetratopic planar [39] and linear ditopic [40] carboxylate linkers can form such 2D layers. Then, these 2D metal–carboxylate linkers can be connected through the coordination of N-donor pillars for the construction of 3D frameworks. In the second group, 1D metal–carboxylate chains can be constructed via the coordination of bent ditopic carboxylate linkers. In this work, we synthesized a two-dimensional Zn(II) metal–organic framework, [Zn(cba)(bpdb)]·DMF (2DTMU-1), based on a V-shaped flexible dicarboxylate ligand, 4,4′-methylenedibenzoic acid (H₂cba), and an N-donor ligand, 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpdb). In addition, the effect of the-CH₂- group of H₂cba was compared with similar reported pillared MOFs, TMU-4, and TMU-5. The use of 4,4′-oxybis(benzoic acid) (H₂oba) as a linker was applied in the structures of TMU-4 and TMU-5 (Scheme 1).

2. Results and Discussion

A new two-dimensional Zn(II) metal–organic framework, [Zn(cba)(bpdb)]·DMF (2DTMU-1), was synthesized by mixing zinc(II) nitrate with a V-shaped flexible dicarboxylate ligand, 4,4′-methylenedibenzoic acid (H₂cba), and an N-donor ligand, 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpdb). The structure of this new MOF compound was determined via infrared spectroscopy and single-crystal X-ray crystallography. (IR data (KBr pellet, ν/cm⁻¹): selected bands—775 (s), 874 (m), 1088 (s), 1159 (s), 1242 (vs), 1413 (vs-br), 1503 (s), 1602 (vs), 1679 (vs), and 3418 (w-br).) The weak absorption band at 3418 cm⁻¹ can be ascribed to the C–H vibration of the aromatic rings. The absorption bands at 1602, 1503, and 1413 cm⁻¹ correspond to vibrations of the py ring of the ligands. The band at 1242 cm⁻¹ is related to C–H vibration of the cba²⁻ portion. The absorption of the C=O vibration for DMF was observed at 1679 cm⁻¹. The single-crystal structural determination analysis indicated that the 2DTMU-1 framework is a pillar-layered framework based on ditopic bent cba²⁻ carboxylate linker and bpdb N-donor pillar. One-dimensional metal–carboxylate chains were constructed based on paddle-wheel inorganic nodes via the coordination of deprotonated H₂cba linkers to the Zn(II) metal ions (Figure 1). Then, these 1D chains were connected by a bpdb pillar spacer to form the 2D framework of 2DTMU-1 (Figure 2a). The inter-digited layers of 2DTMU-1 are presented in Figure 3b. The bpdb linkers bear an azine group; in fact, the 2D framework of 2DTMU-1 was functionalized with azine functional groups (Figure 2b). The large voids (ca. 18 × 12 Ų) for each two-dimensional coordination polymer allow for parallel interpenetration to occur between the adjacent structures (Figure 3).

2DTMU-1 has 2D (4,4) layered structure and crystallizes in the triclinic plane with the space group of Pī. The Zn(II) center is five-coordinated by four oxygen atoms from four cba²⁻ ligands and one nitrogen atom from one bpdb ligand, yielding a geometry with a distorted trigonal bipyramidal structure. The Zn–O and Zn–N bond lengths and O–Zn–O and O–Zn–N bond angles range from 2.032(3) to 2.051(3) Å and 87.74 (12) to 163.68(11)°, respectively, within the region of the values observed for similar Zn(II) complexes with five coordination structures and oxygen and nitrogen donor ligands [1,2,3]. In the crystal structure of 2DTMU-1, the cba²⁻ ligands bridge the same mode to link four Zn(II) centers, whereas the bpdb acts as a trans-bidentate bridging ligand that links pairs of Zn atoms. In this manner, the binuclear Zn units are bridged into a linear chain via cba²⁻ anions with a Zn···Zn separation of 19.63 Å. The bpdb ligands also link the binuclear zinc(II) atoms to construct a linear structure with a Zn···Zn distance of 12.63 Å. The two kinds of parallel structures are perpendicularly intersected by the dinuclear zinc atoms, thereby generating a two-dimensional (4,4) network (Figure 4) when the dimer zinc atoms and the organic parts are considered as nodes and linkers, respectively. Each ring forms from four cba²⁻ ligands, two bpdb ligands, and two Zn(II) atoms.

Two similar pillar-layered metal–organic frameworks have been reported [41], which incorporate a similar linker (H₂oba) and the same pillar ligands (bpdb and bpdh); azine-decorated TMU-4 with the formula [Zn(oba)(bpdb)_0.5]_n·2DMF; and azine–methyl-functionalized TMU-5 with the formula [Zn(oba)(bpdh)_0.5]_n·1.5DMF, where H₂oba = 4,4′-oxybis(benzoic acid), bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, and bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene. Figure 4 presents two different linkers, H₂oba and H₂cba, but the same bpdb pillared ligands were applied for the preparation of two structurally different MOFs, TMU-4 and 2DTMU-1, along with their 3D and 2D structures.

As evident from their building blocks, bpdb is the pillar ligand in both compounds, and the ditopic carboxylate linkers are different, namely, H₂oba and H₂cba in TMU-4 and 2DTMU-1, respectively. The main parts of the two linkers are the same, and the only difference is in the middle spacer part of these linkers; in H₂oba, the -O- groups were replaced with the -CH₂- group in H₂cba. This replacement influenced the type of structures grown in a three-dimensional manner, as can be seen in the structure of 2DTMU-1. Bridging the cba²⁻ linkers to the paddle-wheel zinc-acetate-type nodes creates one-dimensional chains (Figure 5), which then connect through N-donor bpdb pillars and form 2D frameworks; however, in the case of TMU-4, the oba²⁻ linkers are bridged to the paddle-wheel zinc-acetate-type nodes, creating sheet-like, two-dimensional metal–carboxylate linkers, which can then be connected by the N-donor bpdb pillar to form 3D frameworks.

However, 2DTMU-1 is formed on a binuclear paddlewheel Zn(II) unit (Zn# 1 and Zn# 2) complexed by four bridging bidentate (dicarboxylate) V-shaped ligand, 4,4′-methylenedibenzoic acid (H₂cba); this tetragonal array is connected by bpdb with an azine group bridging in the middle as a potential Lewis basic site. This assembly is similar to that obtained for a TMU-5 [18] complex, in which both ligands had been replaced (H₂cba by H₂oba and bpdb by bpdbh, respectively) and not with TMU-4, which simply differs by one atom in the V-shaped ligand (O replaced by C, Figure 5). Nonetheless, 2DTMU-1 crystallizes in the centrosymmetric triclinic space group conversely to TMU-5 in a higher-symmetry space group (C2/c) as a consequence of the larger distortion of the paddlewheel unit. In fact, the square pyramidal geometry around the Zn centers is coordinated by four carboxylate O atoms (for Zn# 1: O1, O3, O5, and O7; for Zn# 2: O2, O4, O6, and O8) from four fully deprotonated cba²⁻ ligands with the bite-averaged angle of 125.4° and one N atom (Zn# 1: N1 and Zn# 2: N4 from the bpdb ligand in the axial direction appear to be particularly deformed, with a Zn#1–O distance ranging from 2.040(3) to 2.056(2) Å, respectively; for Zn#2: from 2.033(3) to 2.098(2) Å), but with similar Zn–N distances of 2.0412(3) Å (Figure 3). The averaged r.m.s.d of the atoms in the equatorial planes delineated by orienting the carboxylates at 89.5° with respect to each other is 0.125 Å. The separation between Zn# 1 and Zn# 2 is 2.934(1) Å, which is the same order of magnitude as that in TMU-5. The bond lengths and bond angles of 2DTMU-1 are listed in

Table 1. Selected bond lengths (Å) and angles (°) for 2DTMU-1.

Zn(1)-O(5)	2.040(2)	O(5)-Zn(1)-N(1)	99.68(9)
Zn(1)-N(1)	2.041(2)	O(5)-Zn(1)-O(3) #1	87.60(9)
Zn(1)-O(3) #1	2.047(2)	N(1)-Zn(1)-O(3) #1	110.72(10)
Zn(1)-O(1) #2	2.052(2)	O(5)-Zn(1)-O(1) #2	163.74(8)
Zn(1)-O(7) #3	2.056(2)	N(1)-Zn(1)-O(1) #2	96.35(9)
Zn(1)-Zn(2) #2	2.9338(5)	O(3) #1-Zn(1)-O(1) #2	89.28(9)
Zn(2)-O(2)	2.033(2)	O(5)-Zn(1)-O(7) #3	88.22(9)
Zn(2)-N(4)	2.042(2)	N(1)-Zn(1)-O(7) #3	94.92(10)
Zn(2)-O(6) #4	2.048(2)	O(3) #1-Zn(1)-O(7) #3	154.36(8)
Zn(2)-O(8) #5	2.050(2)	O(1) #2-Zn(1)-O(7) #3	87.71(9)
Zn(2)-O(4) #6	2.098(2)	O(5)-Zn(1)-Zn(2) #2	86.65(6)
		N(1)-Zn(1)-Zn(2) #2	162.94(8)
		O(3) #1-Zn(1)-Zn(2) #2	85.21(6)
		O(1) #2-Zn(1)-Zn(2) #2	77.19(6)
		O(7) #3-Zn(1)-Zn(2) #2	69.30(6)
		O(2)-Zn(2)-N(4)	102.58(9)
		O(2)-Zn(2)-O(6) #4	155.79(8)
		N(4)-Zn(2)-O(6) #4	100.92(9)
		O(2)-Zn(2)-O(8) #5	88.07(9)
		N(4)-Zn(2)-O(8) #5	103.56(9)
		O(6) #4-Zn(2)-O(8) #5	92.06(9)
		O(2)-Zn(2)-O(4) #6	87.52(9)
		N(4)-Zn(2)-O(4) #6	91.10(9)
		O(6) #4-Zn(2)-O(4) #6	86.28(9)
		O(8) #5-Zn(2)-O(4) #6	165.29(8)
		O(2)-Zn(2)-Zn(1) #4	82.66(6)
		N(4)-Zn(2)-Zn(1) #4	164.91(8)
		O(6) #4-Zn(2)-Zn(1) #4	73.13(6)
		O(8) #5-Zn(2)-Zn(1) #4	90.66(6)
		O(4) #6-Zn(2)-Zn(1) #4	74.87(6)

Symmetry transformations used to generate equivalent atoms: ^#1 x+1, y, z+1. ^#2 x, y, z+1. ^#3 x−1, y, z. ^#4 x, y, z−1. ^#5 x−1, y, z−1. ^#6 x+1, y, z.

.

3. Experimental Section

3.1. Materials and Physical Measurements

All the starting materials and solvents for the preparation and spectroscopic analysis were purchased from Aldrich and Merck and used as received. Melting points were measured using an Electrothermal 9100 apparatus. Infrared spectra were recorded using Thermo Nicolet IR 100 FT-IR. For X-ray crystal structure determination, one of the approximately spherical habits was measured at 100 K under liquid nitrogen stream using a RIGAKU XtaLabPro diffractometer equipped with a Mo microfocus sealed-tube MM003 generator coupled to a double-bounce confocal Max-Flux^® multilayer optical device and an HPAD PILATUS3R 200K detector. CrysAlisPro 1.171.41.123a [42], incorporating a combination of spherical and empirical absorption corrections (using equivalent radius and absorption coefficients on the one hand and spherical harmonics on the other) into the SCALE3 ABSPACK scaling algorithm, was used for data processing and to deal with icing problems during data collection. The structure was determined via intrinsic phasing methods (SHELXT program) [43]; then, full-matrix least-squares methods with respect to F² using SHELX-L were applied for refinement [43]. All non-hydrogen atoms were improved by anisotropic refinement. Aromatic H atoms were placed in idealized positions and constrained to remain on their parent atoms with relative isotropic displacement coefficients, U_iso(H), set to 1.2U_eq(C).

3.2. Synthesis of the Ligand and MOF

The ligand bpdb (1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) was synthesized via the method reported in [44]. Suitable crystals for single-crystal X-ray diffraction (SCXRD) analysis of 2DTMU-1 were prepared via the reaction of Zn(NO₃)₂·6H₂O (0.297 g, 1 mmol), 4,4′-methylenedibenzoic acid (H₂cba) (0.254 g, 1 mmol) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpdb) (0.105 g, 0.5 mmol) in 30 mL of DMF. This reaction mixture was sonicated for uniform dispersion (∼3 min) followed by heating at 80 °C. After 72 h, the yellow 2DTMU-1 crystals were collected (0.420 g; yield—71% based on H₂cba), for which d.p. > 300 °C. IR data (KBr pellet, ν/cm⁻¹): selected bands—775 (s), 874 (m), 1088 (s), 1159 (s), 1242 (vs), 1413 (vs-br), 1503 (s), 1602 (vs), 1679 (vs), and 3418 (w-br), as shown in Figure 6. The crystals of 2DTMU-1 were heated at 100 °C for 8 h to obtain the de-solvated or activated product. IR spectra for the activated product showed that the band at 1679 cm⁻¹ related to DMF solvent had disappeared.

3.3. Determination of the Crystal Structure of 2DTMU-1

The structure of 2DTMU-1 contains 1100 ų (ca 41% of the unit-cell volume) of solvent-accessible voids, which were revealed using the SQUEEZE routine [45] as implemented in the program PLATON (Spek, 2020), and is occupied by an estimated 267 electrons corresponding to ca 6.7 solvent molecules of DMF. Nevertheless, four DMF molecules could be modelled inside these cavities, among which were two DMFs with static disorder constrained by SHELXL instructions (SADI, DELU, SIMU, and EADP). Methyl hydrogens were treated as rigid groups allowed to rotate but not tip with U_iso(H) set to 1.5U_eq(C). Crystal data, data collection, and details of structures’ refinement are summarized in

Table 2. Crystal data and structural refinement for 2DTMU-1.

Identification Code	2DTMU-1 [Zn2(cba)2(bpdb)]·(DMF)y
Empirical formula	C42 H30 N4 O8 Zn2, 4 (C3 H7 N O)
Formula weight	1141.82
Temperature	100(2) K
Wavelength	0.71073 Å
Crystal system	Triclinic
Space group	P-1
Unit cell dimensions	a = 12.6848(9) Å, α = 78.658(5)°
	b = 12.9028(8) Å, β = 80.243(5)°
	c = 18.1796(9) Å, γ = 72.620(6)°
Volume	2764.6(3) Å3
Z	2
Density (calculated)	1.382 Mg/m3
Absorption coefficient	0.942 mm−1
F(000)	1188
Crystal size	0.12 mm × 0.11 mm × 0.07 mm
θ range for data collection	2.550 to 25.349°
Index ranges	−15 ≤ h ≤ 15, −15 ≤ k ≤ 14, −21 ≤ l ≤ 21
Reflections collected	40891
Independent reflections	10063 [R(int) = 0.1061]
Completeness to θ = 25.242 Å	99.2%
Absorption correction	Sphere r 0.1 Å T 0.872
Refinement method	Full-matrix least-squares on F2
Data/restraints/parameters	10,060/401/764
Goodness-of-fit for F2	0.966
Final R indices [I > 2σ(I)]	R1 = 0.0452, wR2 = 0.0896
R indices (all data)	R1 = 0.0801, wR2 = 0.1003
Largest diff. peak and hole	0.444 and −0.472 e. Å−3

. CCDC 2153128 contains the supplementary crystallographic data for this paper (See Data Availability Statement). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk.

4. Conclusions

A new bifunctional two-dimensional MOF and layered structure with the molecular formula [Zn(cba)(bpdb)]·DMF (2DTMU-1), incorporating 4,4′-methylenedibenzoic acid (H₂cba) as a carboxylate linker and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpdb) as a N-donor ligand, was synthesized and characterized. 2DTMU-1 was formed on a binuclear paddlewheel Zn(II) unit complexed by a four bridging bidentate (dicarboxylate) V-shaped ligand (H₂cba); this tetragonal array is connected by bpdb with an azine group bridging in the middle as a potential Lewis basic site. The synthesized MOF with a potential Lewis basic site presents a pore size of 18 × 12 Ų. The results indicated that a small change in the nature of the ligands affected the structure and dimensions of the resulting MOFs. Thus, the use of two different carboxylate ligands with similar structures and the same pillar ligands led to the production of two MOFs with different structures and dimensions (one-dimensional and two-dimensional).