**1. Introduction**

Polyoxometalates (POMs) and POM-based materials constitute a highly versatile class of compounds rich in more than several thousand inorganic compounds, which can be finely tuned at the molecular level. Because of their stunning compositions, diversified architectures and their rich electrochemical redox behaviors, they are known to display numerous properties or applications in many domains such as supramolecular chemistry [1–3], catalysis [4–6], electro-catalysis [7–9], and medicine, especially when POMs are functionalized with organic groups or complexes [10–13].

On their side, hydroborates represent a wide family of anionic clusters, for which many reports demonstrated their interest in different areas, especially in the biomedical domain [14–18]. This property thus makes the studies of borane derivatives of a great interest. In particular, the [B10H10] <sup>2</sup><sup>−</sup> cluster offers the possibility of various selective functionalizations [19,20] leading for example to *closo*-decaborate-triethoxysilane precursor, which can be coordinated to luminescent dye doped silica nanoparticles, hence facilitating the tracing of the *closo*-decaborate drug pathway in BNCT (Boron Neutron Capture Therapy) [21,22].

**Citation:** Diab, M.; Mateo, A.; El Cheikh, J.; El Hajj, Z.; Haouas, M.; Ranjbari, A.; Guérineau, V.; Touboul, D.; Leclerc, N.; Cadot, E.; et al. Grafting of Anionic Decahydro-*Closo*-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties. *Molecules* **2022**, *27*, 7663. https://doi.org/ 10.3390/molecules27227663

Academic Editor: Xiaobing Cui

Received: 2 October 2022 Accepted: 2 November 2022 Published: 8 November 2022

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Driven by the synthetic challenge that constitutes the association of two anionic species with two complementary redox characters, reductive for hydroborates and oxidative for POMs, and by the biomedical applications which could be reached by associating these two families of compounds, this study aims to find the right strategy to design such POM-borate adducts and to study their chemical properties.

In a previous paper, we demonstrated that it is possible to covalently graft decaborate clusters to an Anderson-type polyoxometalate functionalized with the well-known TRIS ligand (TRIS = tris(hydroxymethyl)aminomethane), namely [MnIIIMo6O18(TRIS)2] <sup>3</sup><sup>−</sup> [23]. Nevertheless, the compound [MnIIIMo6O18(TRIS-B10)2] <sup>7</sup><sup>−</sup> resulting from the coupling between both components revealed to be fragile, probably because of the rigidity of the linker and the close proximity of both anionic components. This weakness is confirmed by DFT calculations indicating an athermic or slightly exothermic process for the formation of the adducts with Anderson-TRIS hybrid POMs.

In the field of hybrid POMs, the organosilyl derivatives of vacant polyoxotungstates as [PW9O34] <sup>9</sup>−, [SiW10O36] <sup>8</sup>−, [PW11O39] <sup>7</sup>−, [SiW11O39] <sup>6</sup>−, or [P2W17O61] <sup>10</sup><sup>−</sup> offer large diversities of compounds exhibiting a wide panel of applications [24,25]. Among them, the divacant POM Keggin [SiW10O36] <sup>8</sup><sup>−</sup> (noted hereafter SiW10) and the monovacant POM Dawson [P2W17O61] <sup>10</sup><sup>−</sup> (noted hereafter P2W17) derivatives are probably the most used because of their stability, their topology and the richness of their electrochemical properties in reduction. In particular, by reacting with aminopropyltri(ethoxy)silane (called APTES) they can provide two very useful platforms, noted respectively **SiW10-APTES** and **P2W17-APTES** (see Figure 1), for elaborating functional hybrid molecular architectures.

The aim of this study is to use these two different platforms to prepare new hybrid compounds associating an anionic decaborate boron cluster (denoted hereafter B10) with Keggin and Dawson POM derivatives. The choice of polyoxotungstate moieties rather than Mo-based POMs is based on its stability towards reduction. The employment of a long and flexible linker as APTES is essential to tackle the challenge of combining efficiently a reduced anionic boron cluster with an anionic oxidized polyoxometalate. The use of APTES linker should limit the repulsion between the two components, while its flexibility allows more easily accommodating the two entities. Finally, as shown in Figure 1, due to monovacant and divacant characters of P2W17 and SiW10, respectively, it is worth noting that the relative conformations of the chains are different. For SiW10-APTES, the two alkyl chains are oriented nearly in parallel, whereas the monovacancy of P2W17 imposes divergent directions for the two alkyl chains. This topology is well adapted for designing triangular or square molecular species as evidenced by Izzet et al. [2,26], and in our case, we expect that these two kinds of conformation could lead to different types of adducts incorporating B10 clusters. In this study, we thus report the synthesis, the full characterization in solution by various NMR techniques, the electronic, the electrochemical and the electrocatalytic properties of three new hybrid POMs. In the absence of XRD structures, DFT studies provide a fine structural description of these hybrids and rationalization of their properties.

**Figure 1.** Molecular structures (DFT-optimized geometry) of (**A**) **SiW10-APTES** and (**B**) **P2W17- APTES** platforms highlighting the two different topologies of the APTES linker, and of (**C**) [B10H9CO]<sup>−</sup> (X-ray diffraction structure from reference [27]). Legend: C in black, H in white, N in dark blue, Si in pink, O in red, B in blue, WO6 octahedra in orange and PO4 tetrahedra in green.

#### **2. Results and Discussion**

#### *2.1. Syntheses*

The synthesis of hybrid POMs can be achieved through different strategies. In the present study, the best synthetic procedure to get the targeted hybrid POMs has been to react first the lacunary POMs "SiW10" and "P2W17" with two aminopropyltri(ethoxy)silane molecules (APTES) to give the two POM-APTES precursors (see Figure 2) of formulas (TBA)3H[(SiW10O36)(Si(CH2)3NH2)2O]·3H2O (denoted hereafter **SiW10-APTES**) and (TBA)5H[P2W17O61(Si(CH2)3NH2)2O]·6H2O, denoted hereafter **P2W17-APTES**. The syntheses of these two precursors were adapted from Mayer at al. [28] by reaction of k8(γ-SiW10O36)·12H2O or K10α–P2W17O61·20H2O with 3-aminopropyltriethoxy silane in presence of TBABr in H2O/CH3CN medium acidified by concentrated HCl (for more details see experimental section in Supplementary Materials). Note that for each, the proton usually written as counter-cation is in fact probably an ammonium arm R-NH3 +.

**Figure 2.** Evolution of the proportions of the products in the system **SiW10-APTES**/B10H9CO/DIPEA as a function of B10H9CO/**SiW10-APTES** ratio at fixed DIPEA/B10H9CO ratio of 2. The proportion of each species are determined by integration of the 29Si NMR signals. Reproduced with permission from the doctoral thesis manuscript of Dr Manal Diab, University Paris Saclay/Lebanese University, May 2018.

The synthetic strategy to get POM-borate adducts is then to combine the amines of these POM-APTES precursors with the reactive carbonyl of the decaborate cluster [B10H9CO]<sup>−</sup> (Figure 1C) to give an amide function connecting both components. Since the boron cluster can react with water for giving a carboxylic acid and since heating the synthetic mixture above 40–50 ◦C led to some degradation products or to some reduction in the Dawson derivative by the hydrodecaborate cluster, reactions have been conducted at room temperature and under nitrogen atmosphere. Furthermore, the coupling reaction needs the presence of a base both to help the deprotonation of the ammonium arm(s) of the POM-APTES precursors and to trap the proton produced by the coupling reaction. A moderate and a bulky organic base, diisopropylethylamine (DIPEA), was thus used to avoid the competition with APTES for the coupling reaction with [B10H9CO]−.

To quickly circumscribe the optimal conditions for the synthesis of the POM-borate adducts, 29Si, 31P and 1H NMR titrations were conducted by varying the ratios of the three reactants [B10H9CO]−/POM-APTES/DIPEA (all details are given in the Supplementary Materials).

For the [B10H9CO]−/**SiW10-APTES**/DIPEA system, the 29Si NMR studies in solution reveal that it is possible to modulate the coupling reaction between [B10H9CO]<sup>−</sup> and POM-APTES precursors by playing on the amounts of DIPEA and of [B10H9CO]−. For this tri-reactants system, the successive formation of two POM-borate species identified as mono- and di-adduct compounds was demonstrated thanks to their molecular symmetries (Cs versus C2v). Besides, the crucial role of DIPEA in the reaction of [B10H9CO]<sup>−</sup> with POM-APTES precursors was clearly evidenced. No reaction occurs when no base is used. NMR titration studies allowed establishing that the optimal quantity of base was two equivalents for one equivalent of [B10H9CO]−. The Figure 2 shows for instance the proportions of SiW10-derivatives determined by the integration of the different peaks obtained by 29Si NMR in the system **SiW10-APTES**/[B10H9CO]−/DIPEA as a function of [B10H9CO]−/**SiW10-APTES** ratio at fixed DIPEA/[B10H9CO]<sup>−</sup> ratio of 2.

Starting from **SiW10-APTES**, it evidences first the formation of a mono-adduct, which predominates for ration B10/**SiW10-APTES** = 1, before being converted into a di-adduct. The NMR titrations studies allowed establishing that using proportions **SiW10-APTES** /B10H9CO/DIPEA = 1/3/6 lead to the pure di-adduct denoted **SiW10-diB10**, while using 1/1/2 ratios lead to around 80% of mono-adduct mixed with some unreacted starting POM and the di-adduct. The separation of compounds has not been possible but considering the effect of the added DIPEA amounts, we succeeded to reduce the formation of the di-adduct and thus to get the mono-adduct compound denoted **SiW10 monoB10** with a good purity by decreasing the quantity of DIPEA in the proportions **SiW10-APTES**/B10H9CO/DIPEA = 1/1/1.5. The Figure 3 summarizes the experimental conditions used to isolate POM-borate adducts.

Similar NMR studies were also performed in solution with the Dawson derivative **P2W17-APTES** (see Supplementary Materials). In contrast to SiW10 derivatives, the formation of mono- and di-adduct of the Dawson derivative are not so separated as for SiW10. Therefore, we failed to isolate the mono-adduct as pure product. Nevertheless, we can obtain quantitatively the di-adduct compound in the reaction mixture when ratios **P2W17-APTES**/B10H9CO/DIPEA = 1/3/6 are used.

To summarize, the multistep coupling reactions have successfully been monitored by 29Si and 31P NMR, fully described in the Supplementary Materials, revealing that intermediate products can be followed and isolated. From these results, we established the experimental conditions allowing to selectively synthesize with good yields the mono adduct of SiW10 POM and the di-adducts of both POMs as mixed TBA<sup>+</sup> and DIPEAH+ salts, namely (TBA)3(DIPEAH)3[(SiW10O36)(B10H9CONHC3H6Si)(NH2C3H6Si)O]·3H2O denoted **SiW10 monoB10**, (TBA)6.5(DIPEAH)1.5[(SiW10O36)(B10H9CONHC3H6Si)2O]·2H2O denoted **SiW10 diB10**, and (TBA)6(DIPEAH)4[(P2W17O61)(B10H9CONHC3H6Si)2O]·3H2O, denoted **P2W17 diB10** (See Experimental Section in Supplementary Materials for more details). All adducts were isolated as powders and were characterized by FT-IR, TGA, elemental analysis, MALDI-TOF and NMR techniques. It should be noted that to our knowledge, **SiW10-** **monoB10** is the first example of a POM-APTES monoadduct isolated so far from the direct synthesis. All studies in the literature usually reported di-adducts with such types of hybrid POMs [29–31].

**Figure 3.** Scheme of syntheses of POM-borates adducts. The optimal quantities of reactants were determined by NMR titration studies. The reactions are performed in dry acetonitrile, at room temperature under inert atmosphere. Molecular structures are optimized geometry obtained by DFT. Legend: C in black, H in white, N in dark blue, Si in pink, O in red, B in blue, WO6 octahedra in orange and PO4 tetrahedra in green.
