**2. 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.

#### *2.1. 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, (NH4)6Mo7O24 and Na4Mo8O26 result in NH4 <sup>+</sup> and Na<sup>+</sup> as counter cations, while MoO3 does not lead to the formation of new metal ions, which brings about Cu2+ which serves as counter cation. Based on the above analysis, herein, to synthesize building blocks, (NH4)6Mo7O24 was used as an Mo source to react with CuCl2 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 μ2-O atoms were activated, and therefore each triol ligand replaced two μ3-O atoms and one adjacent μ2-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 μ3-O atoms to form a δ modification style; while in a lower pH environment, triol ligands are prone to substitute partial μ3-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 μ3-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 μ3-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 Cu2+, 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 Cu2+ 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].

**Figure 1.** Polyanionic structures of compounds (**a**) **1**, (**b**) **2**, (**c**) **3**, (**d**) **4**, and (**e**) **5** in ball-and-stick representation. All H atoms except those attaching to N and μ3-O atoms are omitted for clarity.

In all the five compounds, due to the compact and symmetric coordination environments of the Cu2+ 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), 1.980(2), 1.992(2), 2.026(2), 2.195(2), and 2.213(3) Å, respectively, showing an averaged result without an extra-long Cu–O bond of over 2.4 Å. It is interesting that though half of the coordination sites of the Cu2+ ion were occupied by the triol ligand, the Cu–O bond lengths expressed no differences to those formed by hydroxyls. The similar coordination conditions also existed in compounds **1**–**4**.

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 μ2-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 μ3-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.

#### *2.2. 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 MoO3 instead of (NH4)6Mo7O24 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 Cu2+ 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 Cu2+ through the terminal O atoms. The Cu2+ 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 Cu2+ ions. In compound **6**, except two terminal O atoms of clusters, four O atoms from two triol ligands complete the coordination environment of the Cu2+ 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 Cu2+, 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 Cu2+ 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 Cu2+ 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 Cu2+ 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 Cu2+ 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).

**Figure 2.** Ball-and-stick representations of compounds (**a**) **6** and (**b**) **7**, showing their 1D chain structures. All H atoms except those attaching to tri-bridging O atoms in polyanions are omitted for clarity.

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 Cu2+ ion was not large enough, and four polyanions cannot be uniformly arranged around one Cu2+ 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 Cu2+ in the solution, it became easier to obtain a precipitate and the expected structure could not be obtained.
