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

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.


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 Crystals 2018, 8, 17 2 of 10 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 [Mo 6 O 19 ] 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] 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 Ot, the bridging oxygen Ob, and the central oxygen Oc.The exposed oxygen atoms (Ot and Ob) 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] [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  O 40 ] 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 Crystals 2018, 8, 17 3 of 10 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 F 2 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.

Results and Discussion
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 [SMo 12 O 40 ] polyoxoanion, two (3-F-4-MeAnis) cations, two [18]crown-6 molecules, and one CH 3 CN 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

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, 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 ) exist between Supramolecular Cations 1 and 2, which construct a supramolecular cationic dimer.This is similar to the crystal [(m-FAni + )(B [18] [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).

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 [SMo12O40 2− ] [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.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.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.
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.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.

Figure 1 .
Figure 1.The structure of supramolecular cations in Crystal 1. Hydrogen atoms are omitted for clarity, and dotted cyan lines represent hydrogen bonds.

Figure 1 .
Figure 1.The structure of supramolecular cations in Crystal 1. Hydrogen atoms are omitted for clarity, and dotted cyan lines represent hydrogen bonds.

Figure 2 .
Figure 2. (a) The details of interaction between adjacent [SMo12O40] polyoxoanions; (b) the packing of [SMo12O40] polyoxoanions in the bc plane.(a) is the enlarged view of the dotted square fragment shown in (b); the dotted green line represents the O•••O interaction.

Figure 3 .
Figure 3.The packing diagram of Crystal 1, viewed along the c-axis.

Figure 2 .
Figure 2. (a) The details of interaction between adjacent [SMo 12 O 40 ] polyoxoanions; (b) the packing of [SMo 12 O 40 ] polyoxoanions in the bc plane.(a) is the enlarged view of the dotted square fragment shown in (b); the dotted green line represents the O•••O interaction.

Figure 2 .
Figure 2. (a) The details of interaction between adjacent [SMo12O40] polyoxoanions; (b) the packing of [SMo12O40] polyoxoanions in the bc plane.(a) is the enlarged view of the dotted square fragment shown in (b); the dotted green line represents the O•••O interaction.

Figure 3 .
Figure 3.The packing diagram of Crystal 1, viewed along the c-axis.Figure 3. The packing diagram of Crystal 1, viewed along the c-axis.

Figure 3 .
Figure 3.The packing diagram of Crystal 1, viewed along the c-axis.Figure 3. The packing diagram of Crystal 1, viewed along the c-axis.

Figure 5 .
Figure 5.The structure of supramolecular cations in Crystal 2. Hydrogen atoms are omitted for clarity, and dotted cyan represents hydrogen bonds.

3. 2 .
Crystal Structure of [(4-IAnis)([18]crown-6)] 3 [PMo 12 O 40 ]•4CH 3 CN (2) Crystal 2 is monoclinic, space group P2 1 /n.The asymmetric unit consists of one [PMo 12 O 40 ] polyoxoanion, three (4-IAnis) cations, three [18]crown-6 molecules, and four CH 3 CN 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 [SMo 12 O 40 2− ] [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.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.

Figure 5 .
Figure 5.The structure of supramolecular cations in Crystal 2. Hydrogen atoms are omitted for clarity, and dotted cyan represents hydrogen bonds.

Figure 6 .
Figure 6.(a) The interaction between adjacent [PMo12O40] polyoxoanions in the bc plane; (b) the packing diagram of [PMo12O40] polyoxoanions in the ac plane.The dotted green line represents the O•••O interaction.

Figure 6 .
Figure 6.(a) The interaction between adjacent [PMo 12 O 40 ] polyoxoanions in the bc plane; (b) the packing diagram of [PMo 12 O 40 ] polyoxoanions in the ac plane.The dotted green line represents the O•••O interaction.

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

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

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