*3.4. IR Spectrophotometry*

The IR spectra of compounds **1**–**4** were recorded in the regions between 4000 and 200 cm−<sup>1</sup> (Figure S1, Supporting Information). The strong peak at 984 cm−<sup>1</sup> of compound **1** can be attributed to the stretching vibration of V=O. The patterns of the bands in the region characteristic of ν(V=Ot) indicate the presence of VIV sites: clusters which contain exclusively VIV generally possess ν(V=Ot) bands in the range of 970–1000 cm<sup>−</sup>1, while bands in the region 940–960 cm−<sup>1</sup> are characteristic of VV. The observation of a strong absorbance in the 970–1000 cm−<sup>1</sup> region provides a useful diagnostic for the presence of V4+ centers [78]. The strong peaks at 793 and 821 cm−<sup>1</sup> of compound **1** may be due to asymmetric Ge-O stretching vibrations of {GeO4}. The infrared spectrum of compound **2** is very similar to that of compound **1**. It also shows characteristic peaks at 983 cm−<sup>1</sup> and 788 cm<sup>−</sup>1, which should be ascribed to V=Ot and Ge-O vibrations in compound **2**.

Compounds **3** and **4** are based on Ge6V15, which is different from that of compounds **1** and **2**. However, it should be noted that Ge6V15 is also formed by {GeO4} and {VIVO5}; thus, the IR spectra of compounds **3** and **4** are very similar to those of compounds **1** and **2**. The IR spectra of compounds **3** and **4** present characteristic peaks at 979, 801 cm−<sup>1</sup> and 982, 800 cm<sup>−</sup>1, respectively, which correspond to V=Ot and Ge-O vibrations in compounds **3** and **4**. The main difference between the IR spectra of compounds **1** and **2** and **3** and **4** is that the bands at 667 and 660 cm−<sup>1</sup> of compounds **1** and **2** are weak, but the corresponding bands at 691 and 692 cm−<sup>1</sup> for compounds **3** and **4** are much stronger. Bands of 667–692 cm−<sup>1</sup> can be ascribed to V-O-V vibrations.

#### *3.5. XRD Powder Diffractometer*

The powder X-ray diffraction patterns for compounds **1**–**4** are all in good agreement with the ones simulated based on the data of the single-crystal structures, indicating the purity of the as-synthesized products (Figure S2). The differences in the reflection intensity are probably due to preferred orientations in the powder samples of compounds **1**–**4**.

#### *3.6. UV-Vis Spectrophotometry*

The UV-vis spectra of compounds **1**–**4**, in the range of 250–600 nm, are presented in Figure S3. The UV-Vis spectrum of compound **1** displays an intense absorption sharp peak centered at about 266 nm, a shoulder peak at 294 nm and a peak tailing to the longer wavelength side (to about 450 nm), which can be assigned to O→V charge transfer, n→π\* transitions of phen ligands and d→d transitions of complexes in compound **1**. The UV-Vis spectrum of compounds **2** displays an intense absorption peak at about 265 nm assigned to the O→V charge transfer in the polyoxoanion structure of compound **2**. The peak corresponding to the n→π\* transitions of phen ligands was overlapped by the O→V charge transfer and cannot be separated.

The UV spectra of compounds **3** and **4** are similar to each other, but are different from those of compounds **1** and **2**, which exhibit absorption peaks at about 254 and 255 nm due to the O→V charge transfer in compounds **3** and **4**. The difference in the UV-Vis spectra between compounds **3**–**4** and compounds **1**–**2** may be due to the difference in their clusters.

#### *3.7. ESR Spectrophotometry*

The ESR spectra of compounds **1**–**4** were studied at room temperature (Figure S4). The ESR spectra of compounds **1**–**4** are very similar to one another, which show Lorentzian shapes accompanied by signals at g = 1.968, 1.968, 1.912 and 1.941, respectively, indicating that the vanadium atoms in compounds **1**–**4** are in a +4 oxidation-state. The ESR spectra further confirm the results of the bond valence sum calculations for compounds **1**–**4**.
