Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions

Counter-cations are essential components of polyoxometalates (POMs), which have a distinct influence on the solubility, stabilization, self-assembly, and functionality of POMs. To investigate the roles of cations in the packing of POMs, as a systematic investigation, herein, a series of triol-ligand covalently modified Cu-centered Anderson-Evans POMs with different counter ions were prepared in an aqueous solution and characterized by various techniques including single-crystal X-ray diffraction. Using the strategy of controlling Mo sources, in the presence of triol ligand, NH4+, Cu2+ and Na+ were introduced successfully into POMs. When (NH4)6Mo7O24 was selected, the counter cations of the produced POMs were ammonium ions, which resulted in the existence of clusters in the discrete state. Additionally, with the modulation of the pH of the solutions, the modified sites of triol ligands on the cluster can be controlled to form δ- or χ-isomers. By applying MoO3 in the same reaction, Cu2+ ions served as linkers to connect triol-ligand modified polyanions into chains. When Na4Mo8O26 was employed as the Mo source to react with triol ligands in the presence of CuCl2, two 2-D networks were obtained with {Na4(H2O)14} or {{Na2(H2O)4} sub-clusters as linkers, where the building blocks were δ/δ- and χ/χ-isomers, respectively. The present investigation reveals that the charges, sizes and coordination manners of the counter cations have an obvious influence on the assembled structure of polyanions.


Introduction
As an important and basic type of polyoxometalates (POMs), Anderson-Evans clusters have been synthesized and characterized for nearly one hundred years, with a general formula of [X n+ M 6 O 24 H m ] (12−n−m)− , in which X expresses the heteroatom and M the addenda atoms (Mo or W in most cases) [1][2][3]. Compared with the terminal O atoms in Anderson-Evans polyanion, the bi-or tri-bridging O atoms (µ 2 -or µ 3 -O) have a higher reactive activity and can be replaced by some organic species with hydroxyl groups under specific circumstances to form various decoration types ( Figure S1, Supplementary Materials) [4][5][6]. Through this method, different organic functional groups can be introduced into the inorganic skeleton, which not only enriches the structural figures of the final adducts but also integrates the properties of both parts, resulting in novel functionalities [7][8][9][10][11]. Because the organic functional groups have different hydrophilic and hydrophobic properties, they need to be introduced into polyanions under different solvent environments, such as water or organic solvents [12,13], while due to the use of TBA 4 Mo 8 O 26 (TBA = tetrabutylammonium cation) as the Mo source in most cases, triol-ligand modified Anderson-Evans polyanions are mostly prepared in organic solvents such as CH 3 CN, CH 3 OH and N,Ndimethylformamide, which limits their applications [14,15]. So, it is necessary to find new Mo sources, for example Na 2 MoO 4 , (NH 4 ) 6 Mo 7 O 24 , and Na 4 Mo 8 O 26 , suitable for reactions in aqueous solution, which may also bring new architectures. On the other Scheme 1. Schematic illustration for the synthetic parameters in different decoration fashions and extended structures in the preparation of Cu-centered Anderson-Evans POMs 1-9. All H atoms except those attaching to N and µ 3 -O atoms are omitted for clarity. The bold numbers are added to represent the corresponding compounds.

Results and Discussion
The prepared compounds 1-9 were characterized by single crystal X-ray diffraction analysis, elemental analysis, as well as IR spectra, which can be found in the Supplementary Materials as Figure S2. The thermal stability and the crystalline purity of compounds 1-9 were also evaluated, as shown in Figures S3-S11 and Figures S12-S14.

Preparation of Triol-Ligand Decorated Anderson-Evans POM Building Blocks
In order to investigate the assembly behaviors of triol-ligand covalently modified Anderson-Evans polyanions in the presence of different cations, we first investigated the synthesis of their building blocks in aqueous solutions in detail. Two important factors are considered in the selection of Mo sources. The first factor is that the used Mo sources can dissolve easily in water at the initial state or in the reaction process, which ensures the occurrence of the reaction and the acceptable yields. The second controls counter cations in the final adducts through the introduction of different Mo sources. For example, (NH 4 ) 6 Mo 7 O 24 and Na 4 Mo 8 O 26 result in NH 4 + and Na + as counter cations, while MoO 3 does not lead to the formation of new metal ions, which brings about Cu 2+ which serves as counter cation. Based on the above analysis, herein, to synthesize building blocks, (NH 4 ) 6 Mo 7 O 24 was used as an Mo source to react with CuCl 2 and triol ligand in the aqueous solution at 80 • C. When a triol ligand with a methyl as end group was used and the pH of the solution was adjusted to 3~4, compound 1 is obtained, in which triol ligands functionalized on the Anderson-Evans polyanion in a double-sided style to form δ/δ isomer ( Figure 1a). In this case, two triol ligands distributed on both sides of the Anderson-Evans polyanion, and replaced all six hydroxyls around the central heteroatom. When the pH of the solution was lowered to 2, in the case of a stronger acidic environment, partial µ 2 -O atoms were activated, and therefore each triol ligand replaced two µ 3 -O atoms and one adjacent µ 2 -O atom, resulting in a malposition modified structure compound 2 in χ/χ isomer (Figure 1b). The above results show that, at a higher pH value, triol ligands tend to replace all µ 3 -O atoms to form a δ modification style; while in a lower pH environment, triol ligands are prone to substitute partial µ 3 -O atoms, forming in an χ modification manner, where such phenomena are consistent with the literature [20]. In order to verify the universality of this method and the reliability of the conclusion, we used a triol ligand with a terminal hydroxyl group. Experiments show that, in accordance with the case of the triol ligand with the methyl group at the end, when the pH of the solution is 3~4, the hydroxyl-containing triol ligands replace all the µ 3 -O atoms to obtain a double-sided modified Anderson-Evans POM in δ/δ isomer, compound 3 ( Figure 1c). When the pH value of the solution is 2, the triol ligands replace partial µ 3 -O atoms to obtain a bilaterally modified structure, compound 4, in χ/χ isomer (Figure 1d). The above experiments show that the modified positions of the triol ligands on the Anderson-Evans polyanion can be modulated by adjusting the pH of the solution. Interestingly, it is different from those triol ligands with hydroxyl and methyl groups, when the end group is amino, we get a single-sided triol ligand decorated δ isomer, compound 5, in the solution with a relatively lower pH of 1.5~2.5, in which adduct the amino group is in a protonated state (Figure 1e). Even the excessive triol ligands were added in this reaction, and the obtained products were still in a single-sided decoration state with the other side left free. A similar situation also exists in Al-centered Anderson-Evans POMs when modified by triol ligands in the aqueous solution [12]. When the pH of the solution rises with the amino group in the nonprotonated state, the lone pair of electrons of the N atom combines easily with the d orbital of the transition metal ion Cu 2+ , which results in the crosslinking between the generated adducts and makes it difficult to obtain single crystals. An effective way to introduce a non-protonated amino-containing triol ligand into Cu-centered Anderson-Evans cluster is through a two-step synthesis procedure. That is, an undecorated Cu-centered cluster can be synthesized firstly and then used to react with the triol ligand, through which method the coordination sites of Cu 2+ are fully occupied by O atoms, losing the combining ability with other atoms or functional groups such as amino groups. With this synthetic route, the amino-containing triol ligand can be anchored on the Cu-centered Anderson-Evans cluster in a mono-decoration type through micro-assisted synthesis [37] and double-decoration type through a regular beaker reaction in aqueous solution [38]. In all the five compounds, due to the compact and symmetric coordination environments of the Cu 2+ ion, which is located at the center of the Anderson-Evans cluster, there is no obvious Jahn-Teller effect. Taking compound 5 as an example, the six Cu-O bond lengths were 1.958 (2) It is worth noting that although the prepared organic-inorganic hybrids were all based on the same polyanion Cu-centered Anderson-Evans cluster, due to the different modification positions of the triol ligands on the polyanion, the charges of the obtained clusters were unequal. When the triol ligands are modified on the polyanion in δ isomer with all the hydroxyls around central heteroatom being replaced, the charge of the anion remains unchanged before and after the substitution. When the triol ligands modify in malposition on the polyanion, two unreacted hydroxyls are retained, accompanying with the replacement of two unprotonated µ 2 -O (in −2 valence) by the O atom (in −1 valence) from the hydroxyl, and resulting in a decrease in the entire anion charge from −4 to −2. When the terminal group of a triol ligand is amino, although it substitutes all protonated µ 3 -O atoms, the amino group is in a protonated state with an additional positive charge, so the charge of the entire anion is −3. That is, under different environmental conditions, we can make triol-ligand modified Cu-centered Anderson-Evans polyanions with 2~4 negative charges. This charge tunability is useful for the further development and utilization of polyanions, especially in terms of providing convenience to the controllable assembly based on the charge number.

Construction of 1D Structures Based on Triol-Ligand Decorated Anderson-Evans POMs
After obtaining the modification law of the triol ligands on the Anderson-Evans polyanion in aqueous solution, we attempted to obtain extended structures. When ammonium is applied as counter ion, it mainly combines with anion through electrostatic interactions. The lack of directionality and selectivity of the electrostatic interactions makes it unsuitable for the ordered assembly of polyanions. Therefore, we selected MoO 3 instead of (NH 4 ) 6 Mo 7 O 24 as the Mo source, thereby eliminating the possibility of ammonium as counter ions in the adduct. At low pH environments, two 1D chain structures with Cu 2+ serving as linkers were obtained ( Figure 2). The single-crystal X-ray diffraction results show that the Anderson-Evans polyanions were modified by triol ligands to form an χ/χ isomer, to which adjacent polyanions are further linked by Cu 2+ through the terminal O atoms. The Cu 2+ shows an octahedron coordination environment with an obvious Jahn-Teller effect, where two Cu-O bonds (bond lengths 2.323(2)-2.396(2) Å) connected to the polyanion are significantly elongated compared with the other four Cu-O bonds (bond length 1.924(2)-1.976(2) Å). The main difference between the two 1D compounds is that of the coordination environments of the linker Cu 2+ ions. In compound 6, except two terminal O atoms of clusters, four O atoms from two triol ligands complete the coordination environment of the Cu 2+ ion; while in compound 7, the four positions are occupied by coordinated water molecules. In compound 6, only two hydroxyls of each triol ligand coordinate with Cu 2+ , and the other one remains in a free state. Not only coordinated hydroxyls but also free hydroxyls are in the protonated state. In the two compounds, the linker Cu 2+ ions have different coordination environments, and the reason is that at the environment of pH 3~4, although the hydroxyls are in a protonated state, they still have a certain coordination ability with the Cu 2+ ion, and thus occupy the four coordination sites to obtain compound 6. However, as the pH value decreases to 2~2.5, the interaction between the hydroxyls and the Cu 2+ ion weakens to diminish the coordination ability, so that the water molecules occupy the corresponding coordination sites, resulting in compound 7. This statement can be verified in experiments in which the aqueous solution of compound 6 was acidified and recrystallized to obtain compound 7. The differences in the coordination modes of the Cu 2+ ions in the two compounds also have a certain effect on its extending direction. Compound 6 stretches along the (111) direction, while the direction of the 1D chain in compound 7 is (100). In the two compounds, the charges of the polyanion and copper ion were −2 and +2, respectively. According to the theory of electrical neutrality, the two components are more easily combined in a 1:1 manner to form the 1D chain structure or the two-dimensional (2D) planar structure in crystallography, as shown in Figure S15, which has a relative low energy and is more stable. In the present case, because the coordination radius of the Cu 2+ ion was not large enough, and four polyanions cannot be uniformly arranged around one Cu 2+ ion due to the large steric hindrance, only a 1D chain structure was formed. On another hand, due to the low pH value of the solution, in both 1D structures, the triol ligands were modified on the polyanion in the χ/χ isomer. In order to obtain 1D structures based on the δ/δ isomer, we attempted to reduce the acidity of the solution. However, when the pH increased, due to the higher concentration of Cu 2+ in the solution, it became easier to obtain a precipitate and the expected structure could not be obtained.

Construction of 2D Structures Based on Triol-Ligand Decorated Anderson-Evans POMs
As mentioned above, when Cu 2+ is used as a linker, its small coordination range represents a disadvantage for the formation of a 2D structure, and its synthetic environment with a low pH value is also not conducive to obtaining the triol ligand modified Anderson-Evans clusters in the δ/δ isomer. Considering these points, we selected Na 4 Mo 8 O 26 as the Mo source, and used Na + with a high solubility in a wide pH range in an aqueous solution and a large range of connection to obtain 2D structures based on triol-ligand modified Anderson-Evans clusters in δ/δ and χ/χ isomer. As expected, when the pH of the solution was 5, we obtained compound 8, in which the triol ligand double-sided decorated Anderson-Evans polyanion was an δ/δ isomer. In this case, four Na + ions aggregated to form a {Na 4 (H 2 O) 14 } cluster, which linked the adjacent four polyanions through the terminal O atoms into a 2D planar network along the (011) direction (Figure 3a). When the pH of the solution was lowered to 3.5, the triol ligand modified Anderson-Evans polyanions in χ/χ isomer was obtained as in compound 9. In this adduct, two Na + ions form a dimer {Na 2 (H 2 O) 4 } and connect the adjacent four polyanions to form a 2D planar structure extending along the (100) direction (Figure 3b). It can be seen from the above two examples that the charges of the anions and cations have obvious matching characteristics. For the δ/δ isomer, because the charge of the polyanion was −4, four one-charged Na + ions combined to form a tetramer, thereby matching the charge with the anion. While for the χ/χ isomer, because the charge of polyanion was −2, two Na + ions combined to form a dimer with two positive charges, and were neutralized with the anion in a 1:1 mode. In addition, compared with Cu 2+ , the tetramer and dimer of Na + are larger and have wider connecting ranges, so that four polyanions can be uniformly arranged around them to form 2D planar structures, which can also be seen as the adjacent 1D chains connecting to each other to form 2D structures. The size of the linking group and the bridging manner have an important effect on the distances between the order of the arranged anions. When the linking group was a Cu 2+ ion, as in compounds 6 and 7, the distances between two adjacent polyanions in the 1D chains were 14.090(2) Å and 13.312(2) Å, respectively ( Figure S16). The slight difference between the two values is due to the steric hindrance caused by the triol ligand coordinated with linking Cu 2+ (Figure 2). For inter-chains, and the distances between the two adjacent polyanions in compounds 6 and 7 are 11.128(2) Å and 8.872(2) Å, respectively ( Figure S16). For compound 7, the smaller distance between the adjacent polyanions is mainly due to the different orientations of the two polyanions in adjacent chains, which reduces the steric hindrance based on the rotation of one cluster, so that the distance between the two center heteroatoms decreases. When the linking group was a {Na 4 (H 2 O) 14 } tetramer, as in Compound 8, the closest distances of the two adjacent polyanions in the 2D structure were 16.218(2) Å and 9.175(2) Å, respectively (Figure 3). In compound 9, when the linking group was a {Na 2 (H 2 O) 4 } dimer, the distances between the two adjacent polyanions were 13.204(2) Å and 10.202(2) Å, respectively. As shown in Figure 4, from the chemical environments of linkers in compounds 6-9, we can see that when the bridging group is a single metal ion, the mode of bridging polyanions is simple, and the distance between polyanions mainly depends on the radius of the bridging metal ions. When the linker is a cluster formed by multiple metal ions, the bridging range becomes significantly larger, and it can interact with the polyanion through various modes, so that the polyanions have richer assembled structures.

Hirshfled Surface Analysis
As demonstrated by the analysis above, the triol-ligand modified Anderson-Evans POMs can serve as building blocks for the construction of 1D or 2D assemblies based on different metal ions and their various combinations. In fact, with similar building blocks, 3D assemblies have also been prepared, through which ionic frameworks form and exhibit selective adsorption capacity to CO 2 over N 2 , H 2 and CH 4 [39,40]. All these 1D to 3D assemblies are constructed based on a strong ionic bond or coordination bond and provide firm connections between each other, while in the absence of metal ions, there are still relatively weak contacts between building blocks, which also have an important influence on their packing styles. Herein, the Hirshfeld surface analysis was applied to illustrate the supramolecular interactions between triol-ligand modified Anderson-Evans polyanions. To exclude the effects of metal ions, only compounds 1-5 were analyzed in which an ammonium ion serves as the counter cation and cannot provide obvious directional interactions to the assembly behavior of building blocks such as that of metal ions. As the important supramolecular interactions, the hydrogen bonds in compounds 1-5 are summarized in Tables S1-S5 in the Supplementary Materials.
Hirshfeld surfaces mapped with the d norm of compounds 1-5 were firstly investigated, in which d norm was the normalized sum of d i and d e , and is defined as follows [41]: d i is the distance from Hirshfeld surface to the nearest atom I internal to the surface, d e is the distance from Hirshfeld surface to the nearest atom E external to the surface, r I vdw is the van der Waals radius of the nearest atom I closest to and inside the Hirshfeld surface, and r E vdw is the van der Waals radius of the nearest atom E closest to and outside the Hirshfeld surface. As shown in Figure 5, when a methyl-containing triol ligand was used in compounds 1 and 2, the main interaction sites (marked with red cones) were concentrated at the lateral edge of the disk-shaped cluster, where the terminal O atoms can serve as hydrogen bonding acceptors, while for compounds with hydroxyl or protonated amino groups such as compounds 3-5, their ability to serve as hydrogen bonding donors resulted in a relatively uniform distribution of strong contact sites surrounding the cluster.

General Methods and Materials
All chemicals were purchased from Aladdin and used without further purification. Double distilled water was used in all the reactions. Fourier transform infrared spectra were obtained with a Bruker Vertex 80v spectrometer equipped with a DTGS detector with a resolution of 4 cm −1 in KBr pellets. Elemental analysis of C, H, and N was conducted using a vario MICRO cube from Elementar Company of Germany. Elemental analysis for Cu, Mo and Na was carried out on a PLASMA-SPEC (I) inductively coupled plasma atomic emission spectrometer. Thermogravimetric analysis curves were obtained with a Q500 Thermal Analyzer (New Castle TA Instruments, New Castle, DE, USA) in a flowing N 2 under a heating rate of 10 • C·min −1 . Powder X-ray diffraction data were recorded on a Rigaku SmartLab X-ray diffractometer using Cu K α radiation at a wavelength of 1.54 Å. Single-crystal X-ray diffraction data were collected on a Bruker D8 VENTURE diffractometer with graphite-monochromated Mo K α (λ = 0.71073 Å) at 293 K. All crystals were solved by SHELXT and refined by full-matrix-least-squares fitting for F 2 using the Olex2 software [42,43]. All non-H atoms were refined with anisotropic thermal parameters. A summary of the crystallographic data and structural refinements for compounds 1-9 is listed in Table 1. The detailed CCDC 2162383-2162391 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif, accessed on 27 March 2022, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK. The synthetic procedure of compound 3 was similar with that of compound 1 except that HOCH 2 C(CH 2 OH) 3 (0.54 g, 4.00 mmol) was used for substituting CH 3 C(CH 2 OH) 3 (7) The synthetic procedure of compound 7 was similar with that of compound 6 except that the pH of the solution was adjusted to 2~2. 5

Conclusions
By synthesizing a series of Cu II -centered Anderson-Evans POMs modified with triol ligands in the presence of different cations in aqueous solution, ordered assemblies of polyanions from 0D to 2D under cation regulation were successfully realized. The results show that NH 4 + directs the formation of 0D building blocks, and Cu 2+ leads the 1D chain structures, while Na + dominates 2D networks. During the assembled process, the charge ratio between cations and anions has an important effect on the assembled structures. When the size of a cation is large, its wider connection range can link the four adjacent polyanions together to form a 2D structure, while when the size of cation is small, the polyanions can only be arranged in a 1D chain. The research work in this section not only provides an important method and idea for the construction of an ordered assembly of polyanions, but also lays the foundation for the development of specific functional systems.
Supplementary Materials: The following supporting information is available online at https:// www.mdpi.com/article/10.3390/molecules27092933/s1. Figure S1: The schematic drawing of possible decoration types when triol ligands bind to an Anderson-Evans POM cluster, where the blue octahedron represents {MO 6 } (M = Mo or W) and yellow octahedron denotes heteroatom-oxygen {XO 6 }; Figure S2: FT-IR spectra of compounds 1-9; Figures S3-S11: TGA curves of compounds 1-9; Figures S12-S14: XRD patterns of as-synthesized compounds 1-9 and their simulated patterns from the corresponding single-crystal X-ray diffraction data; Figure S15: Two possible arrangements of linkers and building blocks with 1:1 charge ratio. Spheres and disks represent linkers and building blocks, respectively. The solid and dotted lines show the interactions between the adjacent components; Figure S16