**1. Introduction**

In the past century, polyoxometalates (POMs) have been widely researched for their abundant structures and applications in catalytic [1–3], magnetic [4], and electrical fields [5,6]. In order to enrich POMs' structural chemistry and further expand or optimize their applications, researchers have started to design and construct POM cluster organic frameworks (POMCOFs) [7–9] which is a new and promising branch of cluster organic frameworks (COFs) [10–12]. Since the POMCOF was reported [13], considerable efforts have been made in building POMCOFs with Keggin-/Anderson-/Lindqvist-POM secondary building units (SBUs) and rigid aromatic organic linkers [7,8,14,15]. However, compared with the traditional MOFs, the designed syntheses of POMCOFs are still facing huge challenges for the following two reasons: (1) POM clusters have large negative charges and oxygen-rich surfaces, which facilitate their bonding to metal cations, rather than the O-/N-donors from organic linkers. (2) POMs are rigid and stable clusters, therefore, the steric hindrance effects of POM SBUs and linkers need to be well-matched during assembly. Hence, how to choose proper POM SBUs and organic linkers is the key to constructing POMCOFs.

Among seven typical types of POMs, only Anderson-/Lindqvist-/Keggin-types have been successfully applied as SBUs in POMCOFs. Since 2016, the first Anderson-type POMbased heterometallic cluster organic framework was made; Anderson-type POMs have become the popular choice for SBUs [8]. The combination of Anderson-type SBU and rigid bifunctional tris(alkoxo) ligand with a pyridyl group opens up the gate of Anderson-type POMCOFs' world. Lindqvist-type POMs are important members of the POMs family. Though five different elements can all produce the Lindqvist-type [M6O19] <sup>n</sup><sup>−</sup> (M = VV, NbV, TaV, MoVI, WVI) cluster, only polyoxovanadates have been successfully applied as

**Citation:** Chen, C.-A.; Liu, Y.; Yang, G.-Y. Designed Syntheses of Three {Ni6PW9}-Based Polyoxometalates, from Isolated Cluster to Cluster-Organic Helical Chain. *Molecules* **2022**, *27*, 4295. https://doi.org/ 10.3390/molecules27134295

Academic Editor: Xiaobing Cui

Received: 25 May 2022 Accepted: 2 July 2022 Published: 4 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

SBUs in Lindqvist-type POMCOFs [15,16]. So far, most of the reported POMCOFs are made with Keggin-type POM SBUs [7,13,14,17–20]. In these POMCOFs, most of the SBUs are saturated {ε-M4PMo12O40} (M=La, Zn) [13,14,17–19], of which, the incorporation of M (M=Zn2+, La3+) provide the easier bonding sites than the saturated {PMo12} units for organic linkers. Our group has long been devoted to transition metal substituted POMs (TMSPs) based on the trilacunary Keggin fragments under hydrothermal conditions. From our perspective, the trilacunary sites of the [XW9O34] (P, W, Ge) unit can act as structuredirecting agents (SDAs) to induce transition metal ions' aggregation to cluster, on which the terminal end of water molecules may facilitate the substitutions of organic linkers in constructing Keggin-type POMCOFs. Since the first hexa-NiII substituted TMSP based on trilacunary Keggin fragments was made [4], we have been working on POMs structural chemistry based on hexa-NiII-substituted POMs and have already mastered the synthetic conditions of hexa-NiII substituted Keggin POMs. By using {Ni6PW9} SBUs and rigid aromatic carboxylate ligands, we have built a series of novel Keggin-type POMCOFs [7,20]. Hence, we believe that some other intriguing POMCOFs can be made by using {Ni6PW9} SBUs with proper organic linkers.

Rigid and semi-rigid aromatic carboxylate ligands are the common linkers being used in making POMCOFs [7,20–22]; their rigid structures are favorable for the stabilization of the frameworks. However, the large steric hindrance effects of POMs and rigid ligands sometimes cannot match to form POMCOFs. To overcome this difficulty, alipha- tic dicarboxylic acid may be a potential candidate due to its smaller steric hindrances and better flexibilities, which may produce some intriguing frameworks with helical or interpenetrating features that cannot be obtained with rigid aromatic ligands. However, little relevant research has been made, including two typical examples containing aliphatic dicarboxylic acid-bridges for a 2D POMCOF and a tetramer [23,24]. Hence, in this work, we first made an isolated hexa-Ni-substituted Keggin-type POM [Ni6(OH)3- (DACH)3(H2O)6(PW9O34)]·31H2O (**1**) under hydrothermal conditions. The abundant terminal water molecules on the Ni6 cores are potential substitution sites for organic linkers, which help us to further construct POMCOFs. When we first applied the rigid carboxylate ligand MIP, another hexa-Ni-substituted Keggin-type monomer [Ni(DACH)2]- [Ni6(OH)3(DACH)3(HMIP)2-(H2O)2(PW9O34)]·56H2O (**2**) was obtained, HMIP ligands still decorate on the Ni6 cores, failing to bridge the POM clusters. By analyzing the structure of **2**, we used the aliphatic dicarboxylate AP ligands as a linker and a new 1D POMCOC with helical chains [Ni(DACH)2][Ni6(OH)3(DACH)2(AP)(H2O)5(PW9O34)]·2H2O (**3**) was made. To the best of our knowledge, the AP in **3** is the longest aliphatic dicarboxylic acid being incorporated in POMs. Moreover, the 1D helical chain of **3** is the first 1D POMCOC with helical features.

#### **2. Experimental Section**

#### *2.1. General Procedure*

All the reagents were analytical grade and used without any further purification. Na10[A-α-PW9O34]·7H2O was prepared by a method from the literature [25]. Meso-form DACH was used in the syntheses. The powder X-Ray diffraction (PXRD) patterns of the three compounds were collected on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54056 Å) and 2θ scanning from 5–50◦. UV-Vis absorption spectra were obtained on a Shimadzu UV3600 spectrometer (Shimadzu, Kyoto, Japan) with wavelengths from 190 to 800 nm. IR spectra were recorded on a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with the wavenumbers ranging from 4000 to 400 cm<sup>−</sup>1. Thermogravimetric analyses were conducted on a Mettler Toledo TGA/DSC 1100 analyzer (Mettler Toledo, Zurich, Switzerland) heating up from 25–1000 ◦C (heating rate: 10 ◦C/h) under an air atmosphere. Elemental analyses proceeded on the EuroEA3000 elemental analyzer (EuroVector, Pavia, Italy).
