*Article* **Designed Syntheses of Three {Ni6PW9}-Based Polyoxometalates, from Isolated Cluster to Cluster-Organic Helical Chain**

**Chong-An Chen, Yan Liu and Guo-Yu Yang \***

MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, China; cca@bit.edu.cn (C.-A.C.); liuyanik@163.com (Y.L.) **\*** Correspondence: ygy@bit.edu.cn or ygy@fjirsm.ac.cn

**Abstract:** Three new hexa-Ni-substituted Keggin-type polyoxometalates (POMs), [Ni6(OH)3- (DACH)3(H2O)6(PW9O34)]·31H2O (**1**), [Ni(DACH)2][Ni6(OH)3(DACH)3(HMIP)2(H2O)2(PW9O34)]· 56 H2O (**2**), and [Ni(DACH)2][Ni6(OH)3(DACH)2(AP)(H2O)5(PW9O34)]·2H2O (**3**) (DACH = 1,2- Diami- nocyclohexane, MIP = 5-Methylisophthalate, AP = Adipate) were successfully made in the presence of DACH under hydrothermal conditions. **1** is an isolated hexa-Ni-substituted Keggin unit decorated by DACH. In order to further construct POM cluster-organic frameworks (POMCOFs) on the basis of **1**, by analyzing the steric hindrances and orientations of the POM units, the rigid HMIP and flexible AP ligands were successively incorporated, and another anionic monomeric POM **2** and the new 1D POM cluster organic chain (POMCOC) **3** were obtained. HMIP ligand still acts as a decorating group on the Ni6 core of **2** but results in the different spatial arrangement of the {Ni6PW9} units. AP ligands in **3** successfully bridge adjacent isolated POM cluster units to 1D POMCOC with left-hand helices. The AP in **3** is the longest aliphatic carboxylic acid ligand in POMs, and the 1D POM cluster-AP helical chain represents the first 1D POMCOC with a helical feature.

**Keywords:** polyoxometalates; hydrothermal syntheses; cluster-organic frameworks; helical chain
