*2.2. FT-IR Spectroscopy*

FT-IR spectra are given in Figures S11 and S12 in Supplementary Materials. The FT-IR spectra of **SiW10-monoB10**, **SiW10-diB10** and **P2W17-diB10** evidence that the integrity of the POM part is maintained compared to the POM-APTES precursors. Furthermore, the association of the [B10H9CO]<sup>−</sup> cluster is demonstrated by the disappearance of the carbonyl CO band at 2098 cm−<sup>1</sup> in the B10H9CO<sup>−</sup> cluster, while the broad band located at 2464–2470 cm−<sup>1</sup> typical for B-H vibration bands of the decaborate moiety within the three compounds **SiW10-monoB10, SiW10-diB10** and **P2W17-diB10** is significantly shifted from that observed at 2517 cm−<sup>1</sup> for the [B10H9CO]<sup>−</sup> precursor [22,23].

#### *2.3. Characterizations by MALDI-TOF Mass Spectrometry*

Mass spectrometry (MS) is a very efficient technique for the characterization of polyoxometalates in solution. In our case, we did not succeed in getting mass spectra with reasonable signal-to-noise ratio and exploitable data by the usual electrospray ESI-MS technique. On the contrary, Matrix-Assisted Laser Desorption/Ionization coupled to a Time-of-Flight mass spectrometer (MALDI-TOF) revealed to be an effective technique for hybrid POMs characterization, as shown for example by Mayer and coworkers on "SiW10" and "P2W17" organosilyl derivatives [28,32]. MALDI-TOF technique is applied on samples which are diluted in a matrix solution (DCTB in our case, DCTB = Trans-2-[3-(4-ter-Butylphenyl)-2-propenylidene] malonitrile) and then co-crystallized on a conductive target. Thanks to a laser irradiation, it allows producing singly charged species (cationic or anionic) and presenting the great advantage to strongly limit the number of peaks in comparison with ESI-MS spectra, where multiply charged species are generated. In the present study, the experiments were performed in both negative and positive modes (see Figure S14 in the

Supplementary Materials for the example of SiW10-diB10). According to previous works in this field, the best results were obtained in the positive mode, although the anionic character of the POM [28,32]. Indeed, as seen in the Supplementary Materials for SiW10-diB10, the intensity reached in the negative mode appears lower, but the number of peaks is higher as there are more degradation species. Even thought our systems are polyanionic, they are more efficiently analyzed as monocationic species resulting from adducts between POMs and counter cations such as TBA+ and H+ in our case (H<sup>+</sup> coming notably from DIPEAH<sup>+</sup> cations or protonated amines). Furthermore, the monocationic character of the species is confirmed in all cases by the shift between peaks in the isotopic massifs.

The precursor **SiW10-APTES** and the compounds **SiW10-monoB10**, **SiW10-diB10** and **P2W17-diB10**, were thus analyzed by this technique in the positive mode. The results are gathered in Table S1 (see Supplementary Materials). The full spectra and a zoom on the target compounds with a spectrum simulated with IsoPro3 software are shown in Figure 4 for SiW10 derivatives and in Figures S15 and S16 for P2W17 ones.

**Figure 4.** Reflector positive ion MALDI-TOF spectra of (**a**) **SiW10-APTES**, (**b**) **SiW10-monoB10**, and (**c**) **SiW10-diB10**. Zooms of major peaks in the 3000–5000 *m*/*z* range are displayed with their respective simulated spectra.

As shown in Figure 4 the spectrum of the precursor **SiW10-APTES** (Figure 4a) displays a major peak centered at m/z 4087.3 and a minor peak at *m*/*z* 4328.3. The first peak is assigned to the monocationic species {(TBA)3H2[(SiW10O36)O(SiC3H6NH2)2] (CH3CN)2(H2O)8 (DCTB)2} <sup>+</sup> (calculated *m*/*z* 4087.3), while the second peak is attributed to the species {(TBA)4H[(SiW10O36)O(SiC3H6NH2)2](CH3CN)2(H2O)8(DCTB)2} <sup>+</sup> (calculated *m*/*z* 4328.7). The two peaks correspond to the expected hybrid POM associated with some TBA+ and H<sup>+</sup> cations, some solvates and two molecules of the DCTB matrix. Note that the presence of amines on the APTES part of the POM could probably favor the formation of intermolecular interactions with solvates and DCTB molecules. Such an adduct with DCTB is also observed with the precursor **P2W17-APTES** (Figure S15, Supplementary Materials) but not seen with the other POMs functionalized with B10 clusters. The MALDI-TOF spectrum of **P2W17-APTES** indeed exhibits a major peak corresponding to the expected precursor associated with one molecule of the DCTB matrix at *m*/*z* 6058.7 (calculated *m*/*z* 6057.9 for (TBA)6H[(P2W17O61)O(SiC3H6NH2)2](DCTB)}+) and a minor peak at m/z 6300.0

(calculated *m*/*z* 6299.4 for (TBA)7[(P2W17O61)O(SiC3H6NH2)2](DCTB)}+). The attribution of the peaks is definitely confirmed thanks to the fitting of the isotopic distribution massifs. The latter are mainly due to the isotopic distribution of the 10 or 17 tungsten atoms of the POMs, which appears consistent with the experimental spectrum (see Figure 4a and Figure S15, respectively).

The spectrum of **SiW10-monoB10** depicted in Figure 4b shows only one experimental peak at *m*/*z* 3843.2 which is perfectly consistent with the calculated mass for the monocationic product {(TBA)4H3[(SiW10O36)O(SiC3H6NH2)(SiC3H6NHCOB10H9)](CH3CN)(H2O)3} + (calculated *m*/*z* 3843.1). It evidences the formation of the expected adduct **SiW10-monoB10** and thus indirectly the grafting of one (B10H9CO)<sup>−</sup> cluster to SiW10-APTES. The simulated spectrum agrees well with the experimental data, which supports this assumption although the presence of one B10 cluster does not modify significantly the isotopic massif.

The MALDI-TOF spectrum of **SiW10-diB10** shown in Figure 4c displays a major peak centered at *m*/*z* 4085.8, which fully agrees with the expected di-grafted compound {(TBA)4H5[(SiW10O36)O(SiC3H6NHCOB10H9)2](CH3CN)2(H2O)6} <sup>+</sup> (*m*/*z* calculated 4084.4) and a minor peak at *m*/*z* = 4330.2 consistent with the species {(TBA)5H4[(SiW10O36)O (SiC3H6NHCOB10H9)2](CH3CN)3(H2O)4} <sup>+</sup> (*m*/*z* calculated 4330.8). This result confirms the formation of the expected di-grafted compound.

Finally, the case of **P2W17-diB10**, appears more complicated, certainly due to a higher charge of the hybrid POM (10-) and a larger surface, which both favor intermolecular interactions with solvent molecules and cations. For technical reasons, the MALDI-TOF spectrum shown in Figure S16 (see Supplementary Materials) was recorded in linear mode, which does not favor the high resolution in contrast with other compounds. The spectrum displays an intense and broad experimental peak centered at *m*/*z* 6048.1, while four smaller peaks are found, respectively, at *m*/*z* 6289.6, 6431.7, 6672.5 and 6813.8. All these peaks are consistent with di-grafted species of general formula {(TBA)xHy[(P2W17O61)O(SiC3H6NHCOB10H9)2] (CH3CN)z(H2O)t} <sup>+</sup> (x + y = 11, z = 0–3 and t = 5–6). Regarding the main peak, the latter appears much broader than expected for only one species. Moreover, the resolution of the isotopic massif is lost. In fact, the experimental spectrum likely corresponds to a spectra superimposition of monocationic species of general formula {(TBA)5H6[(P2W17O61) O(SiC3H6NHCOB10 H9)2](CH3CN)x(H2O)y} <sup>+</sup> with x ranging from 1 to 5 and y from 0 to 8 (*m*/*z* in the range 6035.70 to 6076.75). Some simulated spectra are given in Figure S16 in Supplementary Materials.
