*2.4. NMR Studies in Solution*

In the absence of crystallographic data, the three obtained hybrid systems have been thoroughly characterized by multinuclear NMR spectroscopy in order to verify their structures in solution. 1H, 11B, 13C, 15N, 29Si, 31P, and 183W NMR spectra were recorded in CD3CN at room temperature. The data are gathered in Table S2 (Supplementary Materials), while selected spectra are given in Figures 5 and 6 and in Figures S17–S32 (Supplementary Materials).

As shown in Figure 5a and in Figures S17–S20 (Supplementary Materials), 11B{1H} NMR spectrum of [B10H9CO]<sup>−</sup> undergoes a significant change upon coupling with **SiW10- APTES** or **P2W17-APTES**. In particular, the signal at −44.4 ppm specific for the equatorial boron atom bearing the substituent CO in [B10H9CO]<sup>−</sup> (B2 atom, see Figure 1c) is strongly shifted to ca. −25 ppm in the spectra of **SiW10-monoB10**, **SiW10-diB10** and **P2W17-APTES** in agreement with the grafting of the cluster on the POM.

**Figure 5.** (**a**) 11B{1H} NMR spectra of **SiW10-monoB10**, **SiW10-diB10** and TBA[B10H9CO] in CD3CN. (**b**) 1H NMR spectra of **SiW10-monoB10**, **SiW10-diB10** and **SiW10-APTES** in CD3CN. \* indicates the signal of the protonated amine DIPEAH+ present as a counter-cation. Reproduced with permission from the doctoral thesis manuscript of Dr Manal Diab, University Paris Saclay/Lebanese University, May 2018.

**Figure 6.** (**a**) 183W NMR spectra of **SiW10-monoB10**, **SiW10-diB10** and **SiW10-APTES** in CD3CN. (**b**) 183W NMR spectra of **P2W17-diB10** and **P2W17-APTES** in CD3CN.

Concomitantly, the 1H NMR spectrum of the mono adduct **SiW10-monoB10**, exhibits a splitting of the signals for the three methylene groups –CH2- of the APTES linker, denoted a, b, c (see Figure 5b), because of the lowering of the symmetry of **SiW10-APTES** from C2v to Cs. In addition, a new peak at 6.12 ppm assigned to an amide function is observed.

For the remaining amine function, a broad signal is observed at 7.4 ppm (d), but together with two other broad signals at 5.70 and 6.33 ppm (d' and d"), attributed to the amine function in a frozen configuration in which the interaction with B10 cluster generates two inequivalent protons as depicted in Figure 7a (DFT optimized structure). These assumptions are confirmed by 1H-15N HMBC (Heteronuclear Multiple Bond Correlation) NMR spectrum (Figure S25) revealing two 15N signals at −251 ppm (amide) correlated to the proton signal at 6.12 ppm and at −272 ppm (free amine) correlated to the two protons peaks at 5.70 and 6.33 ppm.

**Figure 7.** Optimized molecular structures of the POM-borates derivatives. **SiW10-monoB10** (**a**) in «closed» form and (**b**) in «open» form; (**c**,**d**) two views of the most stable configuration of **SiW10-diB10**; (**e**,**f**) two views of the most stable configuration of **P2W17-diB10**. Dashed lines are given for shortest H-H contacts. Legend: C in black, H in white, B in blue, N in dark blue, Si in pink, WO6 octahedra in orange and PO4 tetrahedra in green.

Grafting a second [B10H9CO]<sup>−</sup> group on the **SiW10-APTES** platform allows recovering the C2v symmetry and thus one set of peaks was observed for the linker and especially the protons "a", in addition to an amide peak at 5.94 ppm (Figure S24, Supplementary Materials). The signals of the amine at 7.4, 5.7, and 6.3 ppm disappear in agreement with the reaction of [B10H9CO]<sup>−</sup> groups with this function. Similarly, the 1H NMR spectrum of **P2W17-diB10** compared to that of **P2W17-APTES** (Figure S27, Supplementary Materials) evidences the appearance of a sharp peak at 5.94 ppm assigned to an amide function, while the signal of the free amine at 7.03 ppm in the precursor P2W17-APTES disappears.

To further confirm our assignments of the signal of the amide function, 1H-1H ROESY (Rotating frame Overhause Effect SpectroscopY) and 13C NMR experiments were performed on **SiW10-diB10** and **P2W17-diB10** (Figures S28–S32, Supplementary Materials). Cross REO peaks involving the amide proton (5.94 ppm in both compounds) and some equatorial B-H protons of the B10 cluster at 0.4 ppm and the protons "c" of the APTES chains can be seen in Figures S28 and S29 (Supplementary Materials). This demonstrates the spatial proximity between these protons that interact between each other through dipolar contacts. 13C NMR spectra of **SiW10-monoB10**, **SiW10-diB10,** and P2W17-diB10 (Figures S30–S32) notably exhibits a signal at 203 ppm assigned to a carbon atom from an amide group, which is confirmed by 2D 1H-13C HMBC NMR spectrum of **SiW10-monoB10** evidencing a correlation between this 13C signal at 203 ppm and the 1H amide signal at 6.12 ppm. Besides, in both cases of **SiW10-monoB10** and **SiW10-diB10** this 13C signal appears as a poorly resolved quadruplet with a coupling constant of 95 Hz consistent with a 1J13C-11B coupling.

Therefore, 1H, 1H-15N HMBC, 1H-1H ROESY, 13C and 1H-13C HMBC NMR experiments unambiguously confirm the formation of an amide group in our three adducts by reaction of the amines of POM-APTES precursors with the carbonyl group of the cluster [B10H9CO]−. The modification of the 11B{1H} NMR spectra of the boron cluster after its reaction with the POM-APTES precursors further confirms such results.

29Si, 31P and 183W NMR probe the POM part in compounds **SiW10-APTES**, **SiW10 monoB10, SiW10-diB10, P2W17-APTES** and **P2W17-diB10** (see Figure 6 and Figures S21–S23 in Supplementary Materials). The unsymmetrical environment in the mono adduct **SiW10 monoB10** is clearly confirmed through the appearance of two peaks for Si of the different linker arms at −61.9 and −63.3 ppm, while only one signal was observed at −62.3 ppm for the symmetrical di adduct **SiW10-diB10** with only a small shift from the initial SiW10-APTES precursor (−62.5 ppm). In addition, for all the compounds, a single peak is observed for the Si atom in the central cavity of the SiW10 POM moiety which is almost not affected by the grafting of the boron clusters and the resulting changes of symmetry of the adducts (Figure S21, Supplementary Materials). In case of **P2W17-APTES** and **P2W17-diB10**, both compounds exhibit only one signal assigned to the two equivalent Si atoms of the APTES linker (Figure S22, Supplementary Materials).

The 183W NMR spectra of precursors and adducts are given in Figure 6. For **SiW10 monoB10** the 183W NMR spectrum displays five peaks of intensities 2:2:2:2:2 in agreement with the expected low Cs symmetry, while three resonances are observed for the di adduct **SiW10-diB10** and the initial precursor **SiW10-APTES** of intensities (4:2:4) consistent with their C2v symmetry (Figure 6a). Figure 6b shows the 183W NMR spectrum of P2W17-diB10 which differs significantly from its precursor. Both compounds exhibit nine NMR lines of integration 2:2:2:1:2:2:2:2:2 in agreement with the expected Cs symmetry, but their positions are slightly changed. This is due to the modification of the P2W17 moiety induced by the grafting of the two [B10H9CO]<sup>−</sup> clusters. Additionally, 31P NMR spectra of **P2W17-APTES** and **P2W17-diB10** display two signals (Figure S23, Supplementary Materials), wherein one of them showed a common chemical shift, while the second exhibited a small shift from −13.4 ppm in **P2W17-APTES** to −13.6 ppm in **P2W17-diB10.**

In conclusion, these experiments focused on the POM part fully agree in terms of molecular symmetries with the formation of the expected mono- or di-adducts with B10 clusters.
