*3.2. Structure of 2 and Designed Syntheses for 3*

X-ray diffraction analyses reveal that **2** crystalizes in its monoclinic space group *P*21/c. Its polyoxoanionic cluster [Ni6(OH)3(DACH)3(HMIP)2(H2O)2(PW9O34)] (**2a**) is similar to that of **1a**, except for four water molecules in **1a** being replaced by two HMIP ligands in **2a** (Figure 2a,b and Figure S1). This difference makes **2a** an anionic cluster, accompanied by the charge-balancing [Ni(DACH)2] 2+ complex, in which Ni2+ exhibit the planar square coordination geometry (Figure S2, Supplementary Materials).

Due to the large steric hindrance of HMIP, adjacent opposite-orientated POM clusters are spread out and adopt face-to-face arrangements with each other, while the same orientated units still maintain shoulder-to-shoulder arrangements (Figure 2c,d), which are ideal arrangements for making POMCOFs based on our previous research [7,20]. Using another organic linker with a longer length may help to achieve our aims, but the longer length corresponds to the larger steric hindrances, which may affect the orientations of POM units or increase the interunit distances. Rigid aromatic carboxylic ligands seem unlikely to satisfy our design. Hence, we transfer our focus to chainlike aliphatic dicarboxylic acids. Their higher flexibilities may facilitate their bridging functions on POM SBUs with more flexible orientations and interunit distances and may further result in some intriguing interpenetrating or helical structures that cannot be obtained with rigid aromatic carboxyl ligands. We found that the bilateral DACH molecules on each Ni6 cluster prevent the bridging of adjacent same-orientated SBUs with organic linkers (Figure 2d,e). Additionally, the distance between two terminal –COOH groups from adjacent opposite-orientated POM SBUs is 6.20 Å (Figure 2e), which is nearly matchable with that of AP in the reported polymers (6.30 Å, Figure 2f) [28]. Using AP to replace HMIP ligand in **2** may achieve our goals. Based on the above considerations, AP was used as a linker in the synthesis of **3**. Under similar synthetic conditions with **1** and **2**, **3** was obtained. AP ligand successfully bridges adjacent opposite orientated POM cluster units to the unpreceded 1D helical chains.

**Figure 2.** (**a**) A polyhedral view of polyoxoanion **2a**.; (**b**) View of the Ni6 cluster in **2a**; (**c**) Spatial arrangements of **2a** along with the *a-*and *c-* axes; (**d**) Spatial arrangements of **2a** along the *b*-axis; (**e**) Interunit distances of **2a**; and (**f**) Matchable distance of AP ligand. Color code of polyhedral: WO6: red; NiO6/NiO4N2: green; and PO4: yellow. Hydrogen atoms of the ligands are not shown for better clarity.

#### *3.3. Structure of 3*

**3** crystallizes in the orthorhombic space group *P*212121. Its asymmetric unit contains a [Ni6(OH)3(DACH)2(AP)(H2O)5(PW9O34)] (**3a**) cluster (Figure 3a), a [Ni(DACH)2] 2+ complex, and two lattice water molecules (Figure S1, Supplementary Materials). Compared with **1a** and **2a**, only four-terminal water molecules are substituted by two bidentate DACH ligands on the Ni6 cluster of **3a** (Figure 3b). Each Ni6 cluster links with two AP, of which, one terminal carboxyl group of the AP replaces two terminal water molecules on Ni5 and Ni6, while another carboxyl group replaces only one water molecule on Ni1 (Figure 3b). Each AP ligand bridges two Ni6 clusters (Figure 3c). Such substitution and linkage successfully construct the 1D helical chain with left-hand helices around a 21-screw axis (Figure 3d,e). Adjacent 1D chains stack in -ABAB- and -AAA- sequences along the *a-*and *c*-axis, respectively (Figure 3f,g). It is worth noting that the orientation of each POM SBU and interunit distance have been continually adjusted to the face-to-shoulder arrangements with shorter interunit distance to match the linkage of AP ligand, which are different from those in rigid dicarboxylate ligand-bridged POMCOFs. Such special arrangements of POM SBUs, and the good flexibility of AP, synergistically contribute to the 1D helical chains with left-hand helices. Similar to that in **2**, [Ni(DACH)2] 2+ complexes with planar square configuration locate interchain to compensate for the negative charges of the chains (Figure S2, Supplementary Materials).

**Figure 3.** (**a**) A polyhedral view of polyoxoanion **3a**.; (**b**) View of the Ni6 cluster and its' linkage with AP; (**c**) The linkage of AP with adjacent Ni6 cluster; (**d**) View of the 1D helical chains with left-hand helices along the *b*-axis; (**e**) The simplified sketch of the Ni6-AP 1D chain; and (**f**,**g**) The -ABAB- and -AAA- stacking modes along *a*-and and *c*-axis, respectively. Color code of polyhedral: WO6: red; NiO6/NiO4N2: green; and PO4: yellow. Hydrogen atoms of the ligands are not shown for better clarity.

#### *3.4. Structural Comparisons*

In TMSPs' abundant structural chemistry, POM clusters have various linkages with each other to generate different 1D/2D/3D structures:

First, the interconnections of POM clusters (including different structural types) and rigid aromatic organic ligands. This linkage produces most of the 3D POMCOFs, while 1D chains and 2D layers are relatively rare through this connection, except for these three examples: the 1D chains built from the {Ni6PW9} unit and 1,3-bdc, tda ligand (Figure 4a,b) [7], respectively, and the layer made by another ethylenediamine-func- tionalized {Ni6PW9} unit and 1,3-bdc ligand (Figure 4c) [20].

Second, the interconnections of TMSP cluster units through TM-O=W bonds. This linkage generates a series of 1D chains and 2D layers [29,30]. The 3D open frameworks constructed by the pure TM–O=W linkage are only observed in CuII-substituted TMSPs, including [{Cu6(μ3-OH)3(en)3(H2O)3}(B-α-PW9O34)]·7H2O and [Cu6(μ3-OH)3(en)3(H2O)3(Bα-PW9O34)]·4H2O (Figure 4d), which are caused by the unique Jahn–Teller effect of CuO4N2 octahedra with the axial elongation [31,32].

Third, the TMSP frameworks with TM complex-bridges. TM complex-bridges are common in TMSPs' frameworks. They can extend the POM units to 1D/2D/3D frameworks through TM–O=W, TM–O–TM, and TM–N··· N-TM linkages [33–36].

**Figure 4.** (**a**) One-dimensional chain built from {Ni6PW9} unit and 1,3-bdc ligand; (**b**) One- dimensional chain built from {Ni6PW9} unit and tda ligand; (**c**) Two-dimensional layer built from {Ni6PW9} unit and 1,3-bdc ligand; (**d**) Three-dimensional framework built from the interconnection of {Cu6PW9} unit through Cu–O=W linkage; (**e**) Three-dimensional framework built from {Ni6PW9} unit and WO4 tetrahedron; (**f**) Tetramer built from {Ni4SiW9} and glutaric acid linker; and (**g**) Two-dimensional layer built from Dawson-type {Ni6P2W15} unit and succinic acid linker. Color code of polyhedral: WO6: red; NiO6/NiO4N2: green; PO4: yellow; CuO6: light blue; and WO4: purple. Hydrogen atoms of the ligands are not shown for better clarity.

Fourth, is the TMSP framework with WO4 bridges. However, to the best of our knowledge, it was only found in the first chiral 3D framework of [Ni(enMe)2]3[WO4]3- [Ni6(enMe)3(OH)3PW9O34]2·9H2O (Figure 4e) [37].

Compared with these TMSP-based frameworks with four different linkages, the 1D helical chains in **3** represent a new structural type of POMCOCs. Aliphatic dicarboxylic acid ligands are rare not only in POMCOFs but also in POMs. Limited evidence includes the glutaric acid-bridged tetramer [{(SiW9O34)Ni4(OH)3}4(OOC(CH2)3COO)6] (Figure 4f) and the succinic acid-bridge hexa-substituted Dawson-type-based layer [Ni6(μ3-OH)3- (dap)2(en)(H2O){OOC(CH2)2COO}0.5(CH3COO)(P2W15O56)] (Figure 4g) [23,24]. The AP in **3** is the longest aliphatic dicarboxylic acid being incorporated in POMs family. Moreover, it differs from those 1D chains with a TM–O=W linkage and 1D POMCOCs featuring strict chains [7,29]; the 1D helical chains in **3** are the first 1D POMCOC with helical features.

Since the hexa-NiII cluster of **1**–**3** is similar to those in the reported hexa-Ni- substituted TMSPs, we compared their bond lengths and bond angles to speculate the magnitude properties of the title compounds. As shown in Table S1 (Supplementary Materials), the Ni–O bond lengths and Ni–O–Ni bond angles of **1**–**3** are in the ranges of 1.915–2.295 Å and 90.9–114.2◦, respectively. According to the previous research [4,7,31,38,39], when the Ni–O–Ni bond angles are in the range of 90–104◦, ferromagnetic exchange interactions are dominant. When Ni–O–Ni bond angles are larger than 104◦, anti-ferromagnetic exchange interactions may exist. When ferromagnetic and antiferromagnetic behaviors coexist, the overall magnetic behaviors are determined by which one is dominant. Normally, most of the Ni–O–Ni bond angles in the hexa-NiII cluster are in the ferromagnetic dominant ranges when ferromagnetic and antiferromagnetic coupling coexistences appear. Ferromagnetic exchange behaviors are expected for hexa-NiII clusters, which have been proved by the

measurements in our previous research [4,7,31,38,39]. In **1**, since all the Ni–O–Ni bond angles are in the range of 92.5–102.1◦, ferromagnetic exchange behaviors are expected. In **2** and **3**, the Ni–O–Ni bond angles are in the range of 90.9–106.8◦ and 91.6–114.2◦, respectively, indicating the coexistences of ferromagnetic and antiferromagnetic couplings. There are only 1 and 2 Ni–O–Ni bond angles larger than 104◦, indicating that the ferromagnetic exchange behaviors are dominant in **2** and **3**, similar to those reported in hexa-NiII-substituted TMSPs.
