Two Supramolecular Inorganic–Organic Hybrid Crystals Based on Keggin Polyoxometalates and Crown Ethers

Abstract

New supramolecular structures were designed in this work using large-sized polyoxometalates (POMs) and crown-ether-based supramolecular cations selected as building blocks. Two novel supramolecular inorganic–organic hybrids [(3-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•CH₃CN (1) and [(4-IAnis)([18]crown-6)]₃[PMo₁₂O₄₀]•4CH₃CN (2) (3-F-4-MeAnis = 3-fluoro-4-methylanilinium and 4-IAnis = 4-iodoanilinium) were synthesized. Crystals 1 and 2 have been characterized by infrared spectroscopy (IR) and elemental analysis (EA). Based on X-ray diffraction analysis, Crystals 1 and 2 were constructed through noncovalent bonding interactions and belong to different space groups due to the difference of the building blocks used. Supramolecular cations formed due to strong N–H···O hydrogen bonding interactions between the six oxygen atoms of [18]crown-6 molecules and nitrogen atoms of anilinium derivatives. Crystal 1 has two different supramolecular cations with an anti-paralleled arrangement that forms a dimer through weak hydrogen bonding interactions between adjacent [18]crown-6 molecules. Crystal 2 has three independent supramolecular cations that fill large spaces between the [PMo₁₂O₄₀] polyoxoanions forming a rhombus-shape packing arrangement in the ac plane. Crystals 1 and 2 are unstable at room temperature.

1. Introduction

Crystal engineering, to create desired functional materials, involves the design of versatile crystal architectures based on molecular building blocks via a self-assembly process [1,2,3]. Over the past several decades, crystal engineering has attracted much attention not only because of the versatile structures but also due to the far ranging applications such as those in nonlinear optical, magnetic, and catalytic fields [4,5,6,7].

Supramolecular crystal structures in crystal engineering can be utilized in the construction of molecular machines and ferroelectric domains [8,9,10]. The first and key step in making functional materials is to design a desired versatile supramolecular structure. Supramolecular crystal structures are constructed through noncovalent bonding interactions. The hydrogen bonding interaction is a significant force that can connect building blocks in different forms and affect the crystal structure [11]. In addition, the molecular packing can be adjusted by changing crystal building blocks to control the strength of intermolecular hydrogen bonding interactions [12,13]. Furthermore, electrostatic interaction is another type of noncovalent interaction that can help to modify the molecular packing mode by changing the charge of the building blocks [14,15].

Supramolecular inorganic–organic hybrid materials have been of great interest due to their architecture and potential physicochemical applications. One unique advantage of constructing such a supramolecular structure is combining the structural features of inorganic and organic building blocks. Polyoxometalates (POMs) composed of discrete early transition metal-oxide cluster anions possess many structural advantages for constructing supramolecular structures. First, POMs contain numerous exposed oxygen atoms which can act as potential hydrogen bonding interaction sites [16,17,18]. Lindqvist [Mo₆O₁₉] polyoxoanions, for example, contain three types of oxygen atoms: the exposed terminal oxygen O_t, the bridging oxygen O_b, and the central oxygen O_c. The exposed oxygen atoms (O_t and O_b) are potential hydrogen bonding interaction sites. Moreover, the charge of POMs can be modified by a chemical method to change the electrostatic interactions with an organic cation, resulting in the desired molecular assembled structures [19]. In addition, POMs with a large diameter can form an extended void space for embedding large organic cations [20]. Many supramolecular inorganic–organic hybrids based on POMs have been designed due to the structural advantages of POMs. The typical organic cations are tetrathiafulvalene and ferrocenyl derivatives [21,22,23,24,25]. Recently, the Nakamura group used large-sized crown ethers as organic building blocks and designed supramolecular inorganic–organic hybrids with POMs [26,27,28]. Crown ethers are excellent supramolecular building blocks due to their structural advantages. First, crown ethers with a large cavity can capture anilinium derivatives to form supramolecular cations through N–H···O hydrogen bonding interactions [29,30]. Second, crown ethers are composed of carbon, oxygen, or nitrogen atoms that are exposed and can act as potential hydrogen bonding interaction sites [15].

Based on the structural advantages of POMs and crown ethers, in this study, we designed two novel supramolecular structural inorganic–organic hybrids [(3-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•CH₃CN (1) and [(4-IAnis)([18]crown-6)]₃[PMo₁₂O₄₀]•4CH₃CN (2) based on Keggin POMs and crown ethers (Scheme 1). The detailed structural characteristics of Crystals 1 and 2 will be discussed in this paper.

2. Experimental Section

2.1. Materials and Measurements

[18]Crown-6, (3-F-4-MeAnis), and (3-IAnis) were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD and were used without further purification. (3-F-4-MeAnis) and (3-IAnis), [TBA]₂[SMo₁₂O₄₀], and [TBA]₃[PMo₁₂O₄₀] salts were prepared using procedures similar to those reported previously [19,31,32]. IR (400–7800 cm⁻¹) spectra were measured using a Thermo Scientific Nicolet 6700 FT-IR spectrometer. Elemental analyses of C, H, and N were carried out on a CARLO ERBA 1106 analyzer. Crystallographic data of Crystal 1 were collected using R-AXIS RAPID diffractometer with Mo Kα radiation (λ = 0.071073 nm) with a graphite monochromator at 173 K. Crystal 2 was studied using R-AXIS RAPID diffractometer with Cu Kα radiation (l = 0.154187 nm) with a multi-layer mirror monochromator at 173 K. The structures were solved by direct methods (SIR 2004) and expanded using Fourier procedure, and refined on F² by the full-matrix least-squares method (SHELXL 97). The structures were refined using anisotropic temperature factors, except for the hydrogen atoms which were refined using the riding model with a fixed C–H bond distance of 0.095 nm. The crystallographic data of Crystals 1 and 2 are summarized in

Table 1. Crystallographic parameters for 1 and 2.

Crystal	1	2
Formula	C40H69F2Mo12N3O52S	C62H105I3Mo12N7O58P
Formula weight	2645.32	3439.48
T/K	173	173
Crystal system	Triclinic	Monoclinic
Space group	Pī	P21/n
a/Å	14.928(3)	14.3880(3)
b/Å	15.358(3)	31.1929(6)
c/Å	17.008(3)	23.1485(4)
α/°	85.46(3)	90
β/°	81.86(3)	96.685(7)
γ/°	80.65(3)	90
V/Å3	3802.5(13)	10318.5(4)
Z	2	4
dcalc/g cm−3	2.310	2.214
μ/mm−1	2.045	19.602
GoF on F2	1.223	1.141
R1 [I > 2σ(I)]	0.0482	0.0512
wR2 [I > 2σ(I)]	0.1268	0.1305

R1 = [Σ|F_o| − |F_c|]/[Σ|F_c|] and wR2 = {[Σw(F_o² − F_c²)²]/[Σw(F_o²)²]}^1/2.

.

2.2. [(3-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•CH₃CN (1)

Crystal 1 was synthesized using the standard diffusion method in an H-shape cell. Fifteen milliliters of saturated [TBA]₂[SMo₁₂O₄₀] CH₃CN solution was added into the left side of H-shape cell, and 15 mL of acetonitrile solution with (3-F-4-MeAnis)(BF₄) (40 mg, 0.20 mmol) and [18]crown-6 (52 mg, 0.20 mmol) were placed into the right side of the H-shape cell. The cell was then slowly filled with acetonitrile. After two weeks, black block crystals (1) were obtained. Anal. Calcd. for C₄₀H₆₉F₂Mo₁₂N₃O₅₂S (%): C 18.15, H 2.61, N 1.97; Found (%): C 18.08, H 2.65, N 2.02. IR (KBr pellet, cm^–1): 1160(m); 1115(s); 1060(m); 975(s); 880(m); 790(s).

2.3. [(3-IAnis)([18]crown-6)]₃[PMo₁₂O₄₀]•4CH₃CN (2)

Crystal 2 was obtained by using the traditional solvent evaporation method. Ten milliliters of acetonitrile solution with (4-IAnis)(BF₄) (3 mg) and [18]crown-6 (3 mg) were slowly added into the 10 mL [TBA]₃[PMo₁₂O₄₀] (20 mg) acetonitrile solution. After 15 min of stirring, the solution turned green and was then placed in a dark quiet place at room temperature. Green block crystals (2) were obtained about one week later. Anal. Calcd. for C₆₂H₁₀₅I₃Mo₁₂N₇O₅₈P (%): C 21.63, H 3.05, N 2.85; Found (%): C 21.58, H 2.98, N 2.82. IR (KBr pellet, cm⁻¹): 1160(m), 1065(s), 970(s), 870(s), 790(s).

3. Results and Discussion

3.1. Crystal Structure of [(3-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•CH₃CN (1)

Single crystal X-ray diffraction analysis revealed that the supramolecular Crystal 1 crystallizes in the triclinic space group Pī (Table 1). The asymmetric unit is composed of one [SMo₁₂O₄₀] polyoxoanion, two (3-F-4-MeAnis) cations, two [18]crown-6 molecules, and one CH₃CN molecule and contains as many as 179 atoms. In the crystal structure, two types of supramolecular cations with anti-paralleled arrangement can be observed as shown in Figure 1, viewed along the c-axis. This is different from the previously reported crystal [(2-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•2CH₃CN [33], which has only one type of supramolecular cation. The difference can be attributed to different crown ether and anilinium derivative building blocks. The supramolecular cations are connected through the N–H···O hydrogen bonding interactions between the six oxygen atoms of the [18]crown-6 molecules and the nitrogen atoms of (3-F-4-MeAnis) cations. The average hydrogen bonding N–O distance is 2.9274 and 2.9137 Å for Supramolecular Cations 1 and 2, respectively, as shown in

Table 2. H-Bond distances (Å) between N and O atoms in the supramolecular cation of Crystal 1.

Supramolecular Cation 1	Distance	Supramolecular Cation 2	Distance
N(1)–O(41)	2.8847	N(2)–O(47)	2.9704
N(1)–O(42)	2.9782	N(2)–O(48)	2.8732
N(1)–O(43)	2.9037	N(2)–O(49)	2.9799
N(1)–O(44)	2.9757	N(2)–O(50)	2.8553
N(1)–O(45)	2.8720	N(2)–O(51)	2.9414
N(1)–O(46)	2.9503	N(2)–O(52)	2.8622
Average distance	2.9274	Average distance	2.9137

, which is similar to the standard N–H···O hydrogen bonding length of 2.91 Å [34], indicating that there exists a strong hydrogen bonding interaction between (3-F-4-MeAnis) cations and the [18]crown-6 molecule. The average N–H···O bonding length of Supramolecular Cation 2 is shorter than that of Supramolecular Cation 1, indicating that Cation 2 has a stronger intermolecular interaction. Weak hydrogen bonding interactions (C(21)–H···O(49) = 3.447 Å, H···O(49) = 2.58 Å, ∡C–H···O = 149°; C(12)–H···O(51) = 3.448 Å, H···O(51) = 2.55 Å, ∡C–H···O = 154°) exist between Supramolecular Cations 1 and 2, which construct a supramolecular cationic dimer. This is similar to the crystal [(m-FAni⁺)(B[18]crown-6)]₂[SMo₁₂O₄₀²⁻] [20], which has a supramolecular dimer constructed by a crown ether and anilinium derivatives. Two nitrogen atoms (N1 and N2) are each included in the large cavity of [18]crown-6 molecules and located near its center. The dihedral angle between the (3-F-4-MeAnis) cationic plane and [18]crown-6 molecular plane constructed by six oxygen atoms is 88.489° and 86.524°, respectively, which indicates that the N1–C13 and N2–C32 bonds are almost perpendicular to the [18]crown-6 molecular planes. The distance between the N1 (or N2) atom and the corresponding [18]crown-6 molecular plane is 0.9220 Å (or 0.9017 Å).

For [SMo₁₂O₄₀] polyoxoanions in Crystal 1, the short O···O distance between neighboring POMs can be observed from X-ray crystal structural analysis. In the bc plane (Figure 2a), the short O···O distances are 3.089 Å (O(17)···O(27)), 3.077 Å (O(21)···O(23)), 2.961 Å (O(25)···O(25)), and 2.934 Å (O(19)···O(20)) and imply that adjacent [SMo₁₂O₄₀] polyoxoanions have an intermolecular O···O interaction, which plays an important role in constructing the supramolecular Crystal 1. The bc plane reveals close packing of [SMo₁₂O₄₀] polyoxoanions due to the intermolecular O···O interaction as shown in Figure 2b.

For Crystal 1, the [SMo₁₂O₄₀] polyoxoanions and supramolecular cations [(3-F-4-MeAnis)([18]crown-6)] extend along the b-axis, as shown in Figure 3, and alternate along the a-axis. Multiple hydrogen bonding sites of the [SMo₁₂O₄₀] polyoxoanions are connected through weak hydrogen bonding interactions (C(8)–H···O(24) = 3.335 Å, H···O(24) = 2.51 Å, ∡C–H···O = 143°; C(10)–H···O(16) = 3.492 Å, H···O(16) = 2.54 Å, ∡C–H···O = 168°) with [18]crown-6 molecule (Supramolecular Cation 1). Supramolecular Cation 2 is linked to [SMo₁₂O₄₀] polyoxoanions through weak hydrogen bonding interactions (C(37)–H···O(21) = 3.312 Å, H···O(21) = 2.52 Å, ∡C–H···O = 144°) with (3-F-4-MeAnis) cation, as shown in Figure 4. These intermolecular interactions play an important role in connecting building blocks in Crystal 1. In spite of existing hydrogen bonding interactions, Crystal 1 is unstable at room temperature.

3.2. Crystal Structure of [(4-IAnis)([18]crown-6)]₃[PMo₁₂O₄₀]•4CH₃CN (2)

Crystal 2 is monoclinic, space group P2₁/n. The asymmetric unit consists of one [PMo₁₂O₄₀] polyoxoanion, three (4-IAnis) cations, three [18]crown-6 molecules, and four CH₃CN molecules and contains as many as 248 atoms. Three types of supramolecular cations are formed through N–H···O hydrogen bonding interactions between the six oxygen atoms of [18]crown-6 molecule and the nitrogen atom of (4-IAnis) cation, as shown in Figure 5. This is different from previously reported crystals based on crown ethers and [SMo₁₂O₄₀²⁻] [27], where only one or two types of differently arranged crown-ether-based supramolecular cations were present. The difference can be attributed to differently charged POMs as the building blocks. The average N–H···O bond N–O distances in Supramolecular Cations 1, 2, and 3 are 2.9004, 2.9053, and 2.9569 Å, respectively, as shown in

Table 3. H-Bond distances (Å) between the N and O atoms in the supramolecular cation of Crystal 2.

Supramolecular Cation 1	Distance	Supramolecular Cation 2	Distance	Supramolecular Cation 3	Distance
N(1)–O(41)	2.8937	N(2)–O(47)	2.9218	N(3)–O(53)	2.9131
N(1)–O(42)	2.9378	N(2)–O(48)	2.9127	N(3)–O(54)	2.9691
N(1)–O(43)	2.9267	N(2)–O(49)	2.9147	N(3)–O(55)	2.9629
N(1)–O(44)	2.8978	N(2)–O(50)	2.9968	N(3)–O(56)	2.9171
N(1)–O(45)	2.8937	N(2)–O(51)	2.8158	N(3)–O(57)	2.8991
N(1)–O(46)	2.8537	N(2)–O(52)	2.8697	N(3)–O(58)	3.0802
Average distance	2.9004	Average distance	2.9053	Average distance	2.9569

. Like in Crystal 1, nitrogen atoms are located near the center of the [18]crown-6 molecule. The N–C bonds in (4-IAnis) cations are almost perpendicular to the [18]crown-6 molecule plane defined by the six oxygen atoms. The distances between the nitrogen atoms and the [18]crown-6 molecular planes are 0.7996, 0.8427, and 1.0126 Å, respectively. By comparing with the average hydrogen bonding length of supramolecular cations, we found that the average hydrogen bonding length is shorter and the nitrogen atom is closer to the [18]crown-6 molecular plane, which can be attributed to the different intermolecular hydrogen bonding interactions.

In Crystal 2, the short O···O distances (4.451 Å for O(25)···O(38) and 3.974 Å O(15)···O(25)) between adjacent [PMo₁₂O₄₀] polyoxoanions can be seen from the X-ray crystal structure analysis, as shown in Figure 6a, viewed along the a-axis. These results indicate that neighboring [PMo₁₂O₄₀] polyoxoanions have an intermolecular O···O interaction that creates an infinite network of the anions in the bc plane. The [PMo₁₂O₄₀] polyoxoanions create a rhombus-shape packing arrangement in the ac plane and form a large void space to embed large supramolecular cations [(4-IAnis)([18]crown-6)].

In Crystal 2, large supramolecular cations [(4-IAnis)([18]crown-6)] fill the spaces formed by six [PMo₁₂O₄₀] polyoxoanions, and all adjacent [PMo₁₂O₄₀] polyoxoanions are embedded into the void space constructed by the supramolecular cations [(4-IAnis)([18]crown-6)] as shown in Figure 7. The supramolecular cation [(4-IAnis)([18]crown-6)] and polyoxoanions [PMo₁₂O₄₀] alternate along the a- and b-axes. Each [PMo₁₂O₄₀] polyoxoanion has a weak hydrogen bonding interaction (C(2)–H···O(30) = 3.458 Å, H···O(30) = 2.49 Å, ∡C–H···O = 170°; C(20)–H···O(18) = 3.368 Å, H···O(18) = 2.42 Å, ∡C–H···O = 165°; C(27)–H···O(19) = 3.439 Å, H···O(19) = 2.50 Å, ∡C–H···O = 162°) with three supramolecular cations, due to the structural characteristics of the building blocks, as shown in Figure 8. This weak noncovalent bonding interaction is indispensable in constructing Crystal 2. However, Crystal 2 is unstable at room temperature, which may be attributed to the large size of the supramolecular framework.

4. Conclusions

In summary, two novel supramolecular inorganic–organic hybrids [(3-F-4-MeAnis)([18]crown-6)]₂[SMo₁₂O₄₀]•CH₃CN and [(4-IAnis)([18]crown-6)]₃[PMo₁₂O₄₀]•4CH₃CN have been designed utilizing the structural advantages of the building blocks. Noncovalent bonding interactions, like cement, play an important role in stabilizing the crystal structures. Different structural characteristics of the two crystals were observed via X-ray structural analysis. In Crystal 1, a dimer was constructed through weak hydrogen bonding interactions between the ([18]crown-6) molecules, and the ([18]crown-6) molecules were anti-parallel in the dimer. In Crystal 2, trivalent [PMo₁₂O₄₀] polyoxoanions were used as inorganic building blocks. Three types of different supramolecular cations have been obtained with different arrangements. [PMo₁₂O₄₀] polyoxoanions, with a rhombus-shape packing arrangement, form a large cavity, which suggests a possibility of constructing crown-ether-based supramolecular rotors in the near future.