Naphthyl-Containing Organophosphonate Derivatives of Keggin-Type Polyoxotungstates

Abstract

New organophosphonate derivatives of monovacant Keggin-type polyoxotungstates that contain naphthyl groups have been synthesized and characterized in both solid state and solution. Single-crystal structural analysis shows that two phosphonate groups occupy the vacant position of the lacunary cluster unit in the isostructural compounds [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(α-HBW₁₁O₃₉)] (TBA-1) and [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(α-SiW₁₁O₃₉)] (TBA-2). Liquid-solution UV–Vis transmittance and solid-state diffuse reflectance spectroscopy studies reveal the presence of a new absorption band in the visible region, the charge transfer character of which has been further confirmed by time-dependent density functional theory (TD-DFT) calculations. The latter evidence that the charge transfer process is dominated by transitions from the highest occupied molecular orbital (HOMO), localized in the aromatic ring of the organic group, to the lowest unoccupied molecular orbital (LUMO), localized in the Keggin anion. Photoluminescence studies show that the fluorescent properties of the 1-naphthylmethylphosphonate group are quenched upon its incorporation into the inorganic oxo-tungstate skeleton. The solution stability of the hybrid clusters has been evaluated by a combination of ¹H-, ¹³C- and ³¹P-Nuclear Magnetic Resonance spectroscopy and Electrospray Ionization-Mass Spectrometry. The hybrid polyanion [H(POC₁₁H₉)₂(α-HBW₁₁O₃₉)]³⁻ (1) herein constitutes the first structurally characterized organo-p-block containing borotungstate, and hence it confirms that this strategy for the organic functionalization of polyoxometalate clusters can be applied to new platforms belonging to the family of group-13 heteropolyoxotungstates.

1. Introduction

The organic derivatization of polyoxometalates (POMs) represents one of the most relevant topics within the chemistry of this well-known family of anionic metal-oxo clusters with applications in current fields of interest such as catalysis, medicine and materials science [1,2,3,4,5]. The resulting organic–inorganic hybrids avoid the processing drawbacks that unmodified POMs can display for being incorporated into functional architectures or devices [6,7,8]. The covalent attachment of organic molecules to POM skeletons can confer tuned properties on the resulting hybrid species and can also allow for the immobilization of the clusters on diverse surfaces or matrixes. Nevertheless, highly elaborate functionalities for applications in, e.g., optics, molecular electronics or sensing often require the postfunctionalization of preformed hybrid POM platforms via multistep synthetic work [9].

Three main synthetic methods have been developed to prepare well-defined, solution-stable molecular hybrid POMs [10]. These include strategies involving the replacement of shell O atoms with O- or N-donor ligands as exemplified by tris(alkoxo)-capped Anderson–Evans clusters [11], hexavanadates [12] or [H₄P₂V₃W₁₅O₆₂]⁵⁻ anions [13], as well as by organoimido/diazenido derivatives of Lindqvist-type molybdates [14]. Another approach is based on the reaction between 3d- and/or 4f-metal substituted POMs with exposed centers toward multidentante organic ligands [15,16,17,18]. However, the most explored route consists in combining lacunary polyoxotungstates with p-block organoderivatives. The nucleophilic character of the oxygen atoms delimiting the lacunae is significantly increased compared with those of the plenary anion, and this greatly favors the reaction with electrophilic groups. The use of organotin entities has resulted in some interesting compounds [19,20,21] and a few examples of organogermyl, -arsenyl and -stibyl functionalized POMs can also be found in the literature [22,23]. Nevertheless, organosilyl and -phosphoryl groups constitute the derivatizing agents that have attracted the greatest interest since the first report by Knoth in the early 1980s [24].

In the case of organosilyl derivatives, a large variety of functionalities have been incorporated into monolacunary Keggin or Wells–Dawson anions containing up to two organogroups or one dimeric μ-oxo-bridged (RSi)₂O unit per vacant site [25,26]. In regard to di- and trilacunary anions, two to four organosilyl entities have been selectively anchored to the POM vacant sites [27,28]. The functionalization of mono-, di- and trilacunary Keggin-type phospho- or silicotungstates with organophosphonate moieties has been thoroughly explored as well [29,30,31]. Some reports on the reactivity of Wells–Dawson [P₂W₁₇O₆₁]¹⁰⁻ and Keggin-type [XW₁₁O₃₉]ⁿ⁻ (X = B, Ga, Ge) monovacant frameworks can be found in the literature, but they do not include any X-ray crystal structure analysis [32,33,34]. The immobilization of such type of hybrid POM platforms in silica matrixes [35], metallic [36,37] or polymeric [38] nanoparticles, and silicon or metallic surfaces has resulted in advanced functional materials targeting the heterogenization of homogeneous catalysts [39], molecular memories [40] and selective biomolecule-adhesion properties [41].

In order to incorporate custom-designed organic moieties to the clusters, two strategies are usually followed. The first approach consists in preparing the desired p-block organo-group, followed by its reaction with lacunary POMs. In the second method, the p-block organoderivative with a reactive pendant group is first inserted in the vacant site of the cluster and the resulting hybrid POM species is then used as platform for further postfunctionalization. Following these two synthetic routes and making use of click chemistry or Pd catalyzed coupling reactions, photoactive electron transfer complexes that mimic the bio-inspired artificial photosynthesis have been prepared among others [42,43,44,45]. Magnetically and catalytically active d-metal centers have also been included in the structure by using organic functionalities suitable for metal coordination [46]. Alternatively, oxophilic 3d- or 4f-metals have been incorporated by taking advantage of the free and accessible O atoms in the phosphonate groups of Keggin-type POM derivatives [47,48]. Organophosphonate-functionalized POM species have shown interesting catalytic properties for oxidation reactions by themselves [49], and furthermore, compounds bearing coordinated metal centers represent original molecular models for isolated single-site catalytic species dispersed onto matrixes like silica [50].

Herein we report the synthesis and full characterization in both the solid state and liquid solution of 1-naphthylmethylphosphonate-containing derivatives of monovacant Keggin anions, namely [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(HBW₁₁O₃₉)] (TBA-1) and [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(α-SiW₁₁O₃₉)] (TBA-2). These hybrid POM platforms have been prepared with the aim of being functionalized with 3d- or 4f-metal centers as models for supported site-isolated catalysts with an aromatic environment. The aromatic 1-naphthylmethylphosphonate subunit should not only confer photoactive properties on the system, but could also enhance the selectivity of the catalytic process by interacting with the substrate and providing steric hindrance. We selected the [HBW₁₁O₃₉]⁸⁻ anion for our studies to determine whether borotungstates could be organically derivatized following this route in spite of its peculiar reactivity when compared with analogues that contain heteroatoms from groups 14 and 15, and we employed the isomorphic and topologically equivalent [SiW₁₁O₃₉]⁸⁻ as pattern in the reaction with 1-naphthylmethylphosphonic acid.

2. Results and Discussion

2.1. Synthesis, Infrared Spectroscopy and Thermal Analyses

Organic functionalization of Keggin-type anions with organophosphonate groups was carried out using the well-established method for the incorporation of organosilyl and -phosphoryl moieties into lacunary POM anions developed by Mayer and Thouvenot [30]. Stoichiometric reactions of the monolacunary Keggin-type anions [H_nXW₁₁O₃₉]⁸⁻ (X = B, n = 1; X = Si, n = 0) and 1-naphthylmethylphosphonic acid in acetonitrile were carried out with tetrabutylammonium (TBA) bromide as phase transfer agent to dissolve the POM cluster in the organic media. After an overnight reflux and filtering KBr and traces of unreacted POM precursor off, the crude products were obtained upon solvent removal under vacuum. According to ³¹P-NMR experiments, these crude products were not pure as multiple signals were observed in the spectra. Recrystallization in acetonitrile allowed us not only to obtain single crystals suitable for X-ray diffraction studies, but also to isolate the title compounds as pure, homogeneous phases. All the further characterization was performed on these pure crystalline batches.

The original synthesis was carried out employing the [HBW₁₁O₃₉]⁸⁻ anion as POM precursor to evaluate whether lacunary heteropolytunsgtates with heteroatoms of the group 13 can act as inorganic platforms for their derivatization with p-block organo-groups. This question is yet to be experimentally confirmed as straightforward analogies with other monovacant Keggin-type species are usually unfeasible due to the peculiar behavior of borotungstates in aqueous solution when compared with isoelectronic species like [XW₁₁O₃₉]⁸⁻ (X = Si, Ge). For example, the reaction of tungstate ions with [HBW₁₁O₃₉]⁸⁻ does not directly lead to the plenary α-[BW₁₂O₄₀]⁵⁻ Keggin ion. Instead, one ditungstate fragment places over the vacant site to lead to the [BW₁₃O₄₆H₃]⁸⁻ species, which further condenses into oligomeric anions at lower pH values [51]. To our knowledge, there are only three examples of structurally characterized molecular hybrid borotungstates: the [{Ru(L)}₂(HBW₁₁O₃₉)₂WO₂]¹⁰⁻ (L = p-cymene, benzene) clusters based on the {HBW₁₁} unit [52] and the dimethyltin-containing [{(CH₃)₂Sn}₆(OH)₂O₂(H₂BW₁₃O₄₆)₂]¹²⁻ anion showing {H₂BW₁₃} subunits [53]. Although the insertion of organophosphonate groups in monolacunay borotungstates has been previously suggested [32], compound TBA-1 reported herein represents the first example in the literature of a structurally characterized group-13 heteropolytungstate that is derivatized with organo-p-block moieties. The use of the topologically equivalent [SiW₁₁O₃₉]⁸⁻ precursor in combination with 1-naphthylmethylphosphonic acid led to the isolation of the isostructural compound TBA-2.

The functionalization of the POM precursor was firstly identified by infrared spectroscopy (FT-IR). FT-IR spectra of compounds TBA-1 and TBA-2 are depicted in Figure 1 together with those of their corresponding K₈[HBW₁₁O₃₉]·13H₂O ({HBW₁₁}) and TBA₈[SiW₁₁O₃₉] ({SiW₁₁}) precursors. Both compounds exhibit characteristic bands of strong intensity in the region below 1000 cm⁻¹ that are associated with monolacunary α-Keggin-type anions. The antisymmetric stretching vibrational bands ν_as(X–O_c), ν_as(W–O_t) and ν_as(W–O_b–W) at 1220, 971 and 734 cm⁻¹ for TBA-1 are shifted in comparison with those of {HBW₁₁}, which appear at 1236, 954 and 752 cm⁻¹, respectively. In the case of TBA-2, these signals are observed at 1028, 978, and 802 cm⁻¹ and undergo blue shifts by ca. 10 cm⁻¹ when compared with those of the parent {SiW₁₁} anion. The presence of organophosphonate groups in both compounds is confirmed by the signals associated with the stretching of the P–O bonds that appear in the 1060–1080 cm⁻¹ range. The organic region above 1100 cm⁻¹ is dominated by signals of medium to strong intensity that are observed in the 1176–1300 cm⁻¹ range and associate with the C–N and C–H bonds from the TBA cations and the stretching vibrations of C=C bonds in the naphthalenic system. It is worth highlighting that the protonation of the central O atom in the borotungstate subunit of TBA-1 can be unequivocally inferred by IR spectroscopy on the basis of the ν_as(B–O–H) vibrational band observed at 1220 cm⁻¹, which is red shifted by 20 cm⁻¹ in comparison with that of the precursor [54].

Thermogravimetric analyses (TGA) show curves with similar profiles for both compounds (Figure S1). The wide thermal stability range, which extends up to ca. 280 °C for TBA-1 and to ca. 290 °C for TBA-2, originates from the absence of any solvent molecules in the solid samples upon being left to dry overnight in air. The first weight loss is completed at 415 °C and comprises the loss of ca. 19% of the total mass, which accounts for the 3 tetrabutylammonium molecules (%mass, calcd. (found) for 3 × C₁₆H₃₆N: 19.33 (19.55) for TBA-1 and 19.25 (19.08) for TBA-2). The final step, which corresponds to the organic ligand combustion and POM breakdown, extends continuously up to temperatures in the 800–850 °C range (%mass, calcd. (found) for two naphthylmethyl groups, 2 × C₁₁H₉: 8.33 (8.40) for TBA-1 and 8.31 (8.25) for TBA-1). The final residues are 72.1% of the initial mass for TBA-1 (calcd. for HBP₂W₁₁O₄₀: 72.7%) and 72.7% for TBA-2 (calcd. for P₂SiW₁₁O₄₀: 72.8%).

2.2. Crystal Structure

Compounds TBA-1 and TBA-2 are isostructural and crystallize in the triclinic space group P−1 with two [H(C₁₁H₉PO)₂(α-H_nXW₁₁O₃₉)]³⁻ (X = B, n = 1 (1); X = Si, n = 0 (2)) hybrid POMs and six tetrabutylammonium (TBA) cations in the asymmetric unit. Unfortunately, only five out of the six TBA cations could be satisfactorily modeled in the crystal structure of TBA-2 due to severe crystallographic disorder, whereas the poorer quality of crystals of TBA-1 only allowed us to locate one TBA ion. The molecular structures of the hybrid POMs 1 and 2 consist of a monolacunary α-Keggin-type anion that incorporates two 1-naphthylmethylphosphonate moieties in the vacant position as antenna ligands via corner-sharing of the phosphonate group with one {W₂O₁₀} unit and one {W₃O₁₃} trimer (Figure 2). The cluster shows idealized C_s symmetry with a mirror plane that contains the heteroatom and the bridging O atoms of the {W₂O₁₀} unit (Figure 2B). This ideal symmetry is broken by the relative arrangement of the naphthyl residues (Figure 2C). The planes containing the aromatic systems form angles in the 15°–25° range with the ideal mirror plane. However, both naphthyl residues from each of the POM units are arranged in a staggered conformation. The aromatic systems are rotated about 20° with respect to each other as defined by the <centroid···P···P···centroid> torsion angle involving equivalent ring centroids in both residues. As shown in Figure 2D, this orientation allows for establishing bifurcated intramolecular C–H···O_POM hydrogen bonds that involves the methylenic groups pointing towards the polyanionic surface and bridging O atoms from the POM unit (

Table 1. Geometrical parameters (Å, °) for the intramolecular C–H···O hydrogen bonds involving the methylenic C atoms of the 1-naphthylmethylphosphonate groups and the O_POM atoms in compounds TBA-1 and TBA-2.

D–H···A	H···A	D···A	<D–H···A>
TBA-1
C1–H1B···O21	2.85	3.47(3)	121
C1–H1B···O28	2.71	3.32(3)	120
C21–H21A···O25	2.59	3.09(3)	111
C21–H21A···O31	2.63	3.29(3)	124
C201–H20B···O221	2.86	3.40(3)	115
C201–H20B···O228	2.81	3.47(3)	125
C221–H22A···O225	2.61	3.27(3)	123
C221–H22A···O231	2.63	3.11(3)	111
TBA-2
C1–H1B···O21	2.46	3.05(5)	111
C1–H1B···O28	2.55	3.15(5)	127
C21–H21A···O25	2.77	3.46(4)	127
C21–H21A···O31	2.89	3.44(4)	115
C201–H20B···O221	2.86	3.52(4)	124
C201–H20B···O228	2.94	3.47(4)	115
C221–H22A···O225	2.52	3.04(5)	113
C221–H22A···O231	2.48	3.18(5)	127

).

The P atoms are bonded to the methylenic C atom from the 1-naphthylmethyl moiety, two O atoms from the POM and one terminal O atom (Table S1), and this bonding scheme results in distorted tetrahedral geometries. Bond Valence Sum Calculations (BVS) [55] indicate partial protonation of the terminal O atoms in the phosphonate moieties (BVS values: 1.26–1.45) in good agreement with the number of cations determined by elemental and thermal analyses. One proton is disordered over the P=O bonds of the two organophosphonate groups for each of the hybrid POMs in the asymmetric unit. BVS calculations also indicate protonation of the central O_POM atom pointing toward the vacant position in both crystallographically independent polyanions of compound TBA-1 (BVS values: 1.15–1.17) and this fact is fully consistent with the presence of the B–O–H stretching band in the IR spectrum.

The relative arrangement of the naphthyl residues allows the insertion of a bulky TBA cation (N1 and N4), in such a way that this results embraced between the two aromatic systems via nonconventional cation···π [56] and C–H···π type interactions [57] (Figure 2E). The distances of the C atoms from the TBA cations to the ring centroids (3.683(4)–4.188(6)Å) compare well with those calculated in the literature for such type of C–H···π contacts [58]. The crystal packing displays a pronounced two-dimensional character with hybrid anions linked through C–H···O_POM hydrogen bonds in bilayers that stack along the [001] direction. These bilayers comprise POM clusters located at z = 0.25 and z = 0.75, and arrange in such a way that each POM is connected to two neighbors of the same z level along the crystallographic y-axis and to another two in the contiguous hemilayer through a hydrogen bonding network involving the naphthyl groups (Figure 3, Table S2). The N1 and N4 TBA cations placed between naphthyl groups reinforce the connectivity within the bilayer. Moreover, the N3 and N5 cations connect POMs of the same z level in a zigzag mode along the [010] direction through an additional set of massive C–H···O_POM hydrogen bonds, whereas the remaining N2 species (and most likely the sixth undetermined TBA cation) occupy interlamellar spaces.

2.3. UV–Vis Spectroscopy and Photoluminescent Properties

The electronic properties of the title compounds have been explored by both solution and diffuse reflectance UV–Vis spectroscopy. Figure 4 displays the UV–Vis spectra of acetonitrile solutions of TBA-2 with different concentrations (c₁ = 10⁻⁴ M; c₂ ≈ 10⁻⁶ M; c₃ ≈ 10⁻⁸ M), together with those obtained for the 1-naphthylmethylphosphonic acid and the TBA₈[SiW₁₁O₃₉] POM precursor, which has been prepared for comparative purposes following reported procedures [59]. The rapid transformation of {BW₁₁} to {BW₁₂} in slightly acidic aqueous medium precluded us from preparing the corresponding TBA₈[HBW₁₁O₃₉] analogue to perform similar comparative studies for the TBA-1 derivative. Methathesis with TBABr led to the isolation of the very stable plenary Keggin anion α-[BW₁₂O₄₀]⁵⁻ as a TBA salt according to IR spectroscopy. Samples with different concentrations of the title compounds have been used for our studies due to the great differences in the relative intensity of the absorption bands.

The electronic spectrum of the parent {SiW₁₁} precursor displays an absorption maximum at 265 nm that is associated with the O→W ligand-to-metal charge transfer (LMCT) transition of the inorganic POM framework. In turn, the 1-naphthylmethylphosphonic acid shows two absorption bands at 224 and 289 nm, which correspond to the well-known π−π* transitions of the naphthalenic system [60]. The former band is also present in the spectrum of TBA-2 (see the spectrum of the 10⁻⁸ M solution) and is accompanied by a second broad absorption centered at 275 nm that originates from a small bathochromic shift of about 10 nm of the LMCT band upon functionalization of the monolacunary cluster with organophosphonate groups. Such type of bathochromic shift is indicative of an electronic transfer between the conjugated organic fragment and the metal-oxo cluster [61]. The spectrum recorded for the 10⁻⁶ M solution affords a better resolution in this spectral region. The absorption band is considerably wider than that observed for the LMCT transition in the parent {SiW₁₁} precursor and it shows a shoulder at 283 nm that is in good correspondence with the naphthyl π−π* transitions observed for the phosphonic acid in this region. The combination of these two features strongly indicates that the absorption of TBA-2 centered at 275 nm has a strong contribution of the naphthalenic system.

The spectrum of the concentrated solution of TBA-2 shows and additional band centered at ca. 400 nm that could be ascribed to a charge transfer between the aromatic system and the inorganic POM cluster in which the former acts as electron donor and the latter as acceptor. This phenomenon has been previously observed for similar compounds in the literature [61,62]. The charge-transfer process can be noticed visually because the incorporation of the colorless 1-naphthylmethylphosphonate moiety into the inorganic skeleton of the colorless {SiW₁₁} leads to the formation of the orange-yellow compound TBA-2. A similar behavior is observed in the UV–Vis spectra of TBA-1 solutions (Figure S2). The bands at ca. 222 and 282 nm can be associated with the well-known π−π* transitions of the naphthyl residue, whereas that at 274 nm corresponds to the LMCT process. The maximum for the charge transfer band between the naphthyl residue and the inorganic POM cluster is blue-shifted by 15 nm upon comparison with that of TBA-2 (ca. 385 nm in 1 vs ca. 400 nm in 2). Accordingly, crystals of compound TBA-1 display a more yellowish color than those of TBA-2 (Figure 5).

In order to analyze the nature of the band centered at ca. 400 nm, theoretical calculations for the hybrid POM 2 at the TDA-B3LYP/def2-SVP//BP86-D3/def2-SVP level of theory have been performed. They show qualitative agreement with the experimental results, with several low intensity transitions in the visible region (408, 380 and 350 nm) that fall under the very wide experimental band centered at 400 nm, as well as a cluster of higher intensity transitions centered at 290 nm (Figure 4). The difference density plots of the molecular orbitals involved in the low intensity bands clearly indicate their organic to inorganic charge transfer character, with the naphthalenic system acting as electron donor and the POM cluster as acceptor (W d_xy with a component of O p orbitals). On the other hand, the difference densities calculated for the cluster of transitions centered at 290 nm clearly support their previous assignment as O→W LMCT transition in the inorganic POM framework (Figure 6D) combined with π−π* transitions in the naphthyl groups (Figure 6E).

The diffuse reflectance spectra registered for powdered crystalline samples of TBA-1, TBA-2 and their precursors K₈[HBW₁₁O₃₉]·13H₂O and K₈[SiW₁₁O₃₉]·13H₂O (Figure 7) confirm the features stated from the solution studies. The bands associated with the monolacunary Keggin anions undergo red shift upon organic functionalization from 313 and 318 nm for the {HBW₁₁} and {SiW₁₁} POM precursors to 336 and 341 nm for the hybrid compounds TBA-1 and TBA-2. Strong charge transfer bands originating from the covalent connection between the naphthyl moieties and the inorganic oxo-cluster can also be observed in the visible region.

Naphthalene and its derivatives present photoluminescent properties when they are excited with UV light [60]. To determine whether these emission properties are modified upon the incorporation of the 1-naphthylmethylphosphonate groups into monovacant Keggin anions, we measured the fluorescence spectra of 10⁻⁶ M acetonitrile solutions of the title compounds TBA-1 and TBA-2 and compared the results with that recorded for commercial 1-naphthylmethylphosphonic acid (Figure 8). The spectrum of the free phosphonic acid (λ_exc = 283 nm) shows a broad emission band with the typical naphthalenic vibrational structure in the ca. 300−400 nm spectral range. However, our hybrid polyanions 1 and 2 do not display any emission when being excited at any wavelength corresponding to their absorption maxima (225, 275 or 280 nm), nor even at the charge transfer band (385–400 nm). Similar results were obtained when the experiments were carried out on solid crystalline samples (Figure S3). Nevertheless, a weak fluorescence reminiscent of that originating from the naphthyl group was obtained when 1 and 2 were excited at 283 nm, but only when the spectrofluorimeter was set to work in a High Sensitivity mode. The quenching of the naphthalenic emission in the hybrid POMs 1 and 2 could result from two different processes: (i) the strong absorption of the Keggin-type cluster at the excitation wavelength (ca. 280 nm) prevents the excitation of the fluorophore; or (ii) the quenching of the fluorescence proceeds via naphthyl-to-cluster charge transfer. The former could in principle be disregarded according to the UV–Vis spectrum of TBA-2 (Figure 4), which unequivocally shows that the naphthyl groups undergo strong absorption at 283 nm. Therefore, the quenching of the naphthyl fluorescence upon incorporation of the organophosphonate group into the Keggin-type skeleton is more likely to take place via naphthyl-to-POM charge transfer, which allows the excited state to deactivate through non-radiative processes. TD-DFT calculations support the latter interpretation as they clearly show that the naphthyl groups in the hybrid POMs 1 and 2 are excited in the 280–290 nm range (Figure 6D,E). Analogous quenching of the photoluminescent properties of aromatic systems such as pyrene upon incorporation into POM cluster frameworks has been previously reported in the literature for hybrid species similar to polyanions 1 and 2 [62].

2.4. Solution Behavior

In order to investigate the solution stability of the hybrid POMs 1 and 2, we performed ³¹P-, ¹H- and ¹³C-nuclear magnetic resonance (NMR) studies in deuterated solvents and compared the resulting spectra with that of the commercial 1-naphthylmethylphosphonic acid.

The ³¹P-NMR spectra of compounds TBA-1 and TBA-2 in acetone (Figure 9) exhibit one singlet at 27.9 and 24.5 ppm, respectively. This fact indicates: (i) the absence of any phosphorous containing impurities in the sample; and (ii) that the attachment mode of the two organic groups to the lacunary anion is equivalent for both hybrid anions in good agreement with the molecular structures determined from single-crystal X-ray diffraction studies. All the chemical shifts lay in the expected range for organophosphonate derivatized monolacunary Keggin type POMs [63]. Depending on the heteroatom of the Keggin-type fragment, the signal shifts considerably upfield (2, Δδ = 2.9 ppm) or slightly downfield (1, Δδ = 0.5 ppm) upon comparison with that originated from the parent commercial phosphonic acid (δ = 27.4 ppm).

The title compounds TBA-1 and TBA-2 display similar ¹H-NMR spectra that are depicted in Figure S4 together with that recorded for the naphthylmethylphosphonic acid. The grafting of the organophosphonate moiety could be confirmed by the absence of any signal corresponding to hydroxyl groups in the spectra of TBA-1 and TBA-2, which for the phosphonic acid appears as a broad singlet at δ = 10.59 ppm that integrates for two protons. The characteristic resonances for the n-butyl groups of the TBA cations observed at ca. 1 (–CH₃), 1.5 (–CH₂–CH₃), 1.8 (–CH₂–CH₂–) and 3.4 (–N–CH₂–) ppm have been integrated for 36, 24, 24 and 24 protons, and this fact confirms the presence of three cations per POM unit. Protons in the aliphatic n-butyl chain are less shielded as they place closer to the N atom and show the expected spin coupling constant of J = 7.4 Hz [64]. The signals originating from the methylenic group of the organophosphonate moiety undergo drastic changes upon incorporation into the POM vacant position. The spin system, determined as two singlets at 3.47 and 3.51 ppm in the phosphonic acid, becomes diastereotopic in the hybrid POM structure in such a way that four different protons can be differentiated: H_a and H_b from one of the P(O)CH₂C₁₀H₇ moieties and H_c and H_d from the other one (Figure 10). These four resonances appear as doublets with a geminal coupling constant of about J = 15.2 Hz and arrange into two well-separated groups of signals because those that originate from the two H atoms pointing at the POM surface undergo significantly larger shift downfield due to the intramolecular C–H···O_POM interactions, which promote the deshielding of the protons [15] (H_a and H_c: Δδ = 0.69–0.76 ppm for TBA-1 and Δδ = 0.62–0.66 ppm for TBA-2; H_b and H_d: Δδ = 0.42–0.47 ppm for TBA-1 and Δδ = 0.34–0.38 ppm for TBA-2). It is also worth noting the roof effect of the signals, which is characteristic of almost overlapping resonances showing similar intensity. In regard to the naphthylic H atoms, the spectra of TBA-1 and TBA-2 show different multiplets in the δ = 7.35–8.29 ppm range that are analogous to those observed for the phosphonic acid in the δ = 7.41–8.12 ppm region. On the other hand, the singlet located at 4.80 ppm in the spectrum of TBA-1 can only be ascribed to the protonation of the central O_POM atom (B–O–H) in the vacant position of the cluster, which has also been observed in the IR spectrum and inferred from BVS calculations on the molecular structure of 1. These results also suggest that the hybrid POMs 1 and 2 show solution stability in organic media like acetone.

The ¹³C-NMR experiments were first carried out for deuterated acetone solutions (Figure S5). The TBA cations in compounds TBA-1 and TBA-2 could be easily assigned to singlets appearing at 14 (–CH₃), 20 (–CH₂–CH₃), 25 (–CH₂–CH₂–) and 59 (–N–CH₂–) ppm. The aromatic signals are observed in the 126–135 ppm region. Some of them appear as doublets due to the spin coupling with the close NMR active P nucleus (J = 3–11 Hz). The spectrum of the 1-naphthylmethylphosphonic acid is very similar to that observed for the hybrid POMs with an additional doublet at 32.2 ppm that is ascribed to the methylenic group, which shows a large ¹J = 137.5 Hz spin-coupling constant with the vicinal P atom. Unfortunately, the resonances corresponding to this group in compounds TBA-1 and TBA-2 were partially overlapped with the strong signal originating from the solvent and this fact prevented us from performing a full analysis. Therefore, the final ¹³C-NMR spectra for compounds TBA-1 and TBA-2 were registered using solutions in deuterated acetonitrile (Figure 11). The TBA cations appear as three singlets at 13.9 (–CH₃), 20.4 (–CH₂–CH₃) and 24.4 (–CH₂–CH₂–) ppm and a resonance at 59.5 ppm for TBA-1 and 59.3 ppm for TBA-2 that corresponds to the C atoms covalently bonded to the N atoms. Close inspection of this resonance reveals an apparent triplet, which is most likely the result of three singlets arising from the three virtually identical but inequivalent TBA cations. The phosphonate groups are observed as follows: one doublet corresponding to the methylene group at ca. 59.4 ppm with ¹J = 134.1 Hz for TBA-1 and ¹J = 138.3 Hz for TBA-2, together with a set of resonances spanning in the 126–135 ppm region. The positions for singlets and doublets of the aromatic region are virtually identical in both cases: 126.5 (J = 4.5 Hz), 126.6, 126.9, 128.5 (J = 4.5 Hz), 129.2, 129.5 (J = 11.0 Hz), 130.0 (J = 8.0 Hz), 133 (J = 5.5 Hz) and 135 (J = 2.5 Hz) ppm. The spin coupling constants of 11.0, 8.0, and <5.5 Hz have been tentatively assigned as ²J, ³J and ⁴J, respectively.

The Electrospray-Ionization Mass Spectrometry (ESI-MS) studies confirm the solution stability of the hybrid polyanions 1 and 2. The spectrum of TBA-1 recorded in negative-ion mode displays two main groups of signals spanning from m/z 1000 to 1700. These signals have been attributed to the intact anionic species 1 with 3− and 2− charge states and different extents of associated counterions according to isotopic pattern inspection and m/z spacing. The main species detected corresponds to the pristine anion {1}³⁻ (m/z 1011.8), where {1} = [H(C₁₁H₉PO)₂(HBW₁₁O₃₉)], whereas the second maxima originates from the intact species with an associated tetrabutylammonium cation, {1 + TBA}²⁻ (m/z 1638.3). Figure 12 shows the ESI mass spectrum together with a comparison of the experimental and calculated isotopic patterns for these two specific ions. Inspection of the crystal structure of 1 revealed two protonation sites accounting for one disordered proton and the presence of one TBA cation embraced between the two naphthyl residues through multiple C–H··· π type interactions. These two features could still be retained in solution according to the ESI-MS results. Analogous experiments carried out for the TBA-2 derivative reveals a similar behavior. The most abundant species detected corresponds also to the intact hybrid anion {2}³⁻ (1017.4 m/z) and to the species with an associated TBA cation {2 + TBA}²⁻ (m/z 1646.8) (Figure S6). To summarize, the combination of multinuclear (¹H, ¹³C and ³¹P) NMR studies and ESI-MS experiments confirms that the hybrid POMs 1 and 2 are stable in polar organic media like acetone or acetonitrile and do not dissociate into any of their constituent building blocks.

3. Experimental Section

3.1. Materials and Methods

The precursors K₈[α-SiW₁₁O₃₉]·13H₂O and K₈[α-HBW₁₁O₃₉]·13H₂O were synthesized according to literature procedures [51,65] and identified by IR spectroscopy. All other reagents were purchased from commercial sources and used without further purification. Carbon, nitrogen, and hydrogen were determined on a EuroVector EA 3000 CHNSO analyzer (EuroVector, Milan, Italy). Fourier transform infrared (FT-IR) spectra were recorded on KBr pellets using a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Kyoto, Japan). Diffuse Reflectance studies were performed on a UV–Vis-NIR Varian Cary 500 spectrophotometer (Varian, Palo Alto, CA, USA). UV–Vis and fluorescence spectra were, respectively, registered on a Shimadzu UV-1240 spectrophotometer and a Shimadzu RF-5300 spectrofluorimeter equipped with a 150 W Xenon arc lamp (Shimadzu, Kyoto, Japan) using standard quartz cuvettes. Thermogravimetric analyses were carried out from room temperature to 850 °C at a rate of 5 °C·min⁻¹ on a Mettler-Toledo TGA/SDTA851^e thermobalance (Mettler Toledo, Greifensee, Switzerland) under a 50 cm³·min⁻¹ flow of synthetic air. ¹H- and ¹³C-NMR spectra were acquired on a Bruker AC-500 spectrometer (500 MHz for ¹H and 125.7 MHz for ¹³C) (Bruker, Karlsruhe, Germany). Spectra were referenced to external tetramethylsilane via the residual protonated solvent (¹H) or the solvent itself (¹³C). All chemical shifts (δ) are reported in parts per million (ppm). For (CD₃)₂CO (acetone-d₆) the shifts are referenced to 2.05 ppm for ¹H-NMR spectroscopy and 29.84 ppm for ¹³C-NMR spectroscopy. For CD₃CN the shifts are referenced to 118.26 ppm for ¹³C-NMR spectroscopy [66]. Abbreviations used in the description of resonances are: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet). Coupling constants (J) are quoted to the nearest 0.1 Hz. Proton-decoupled ³¹P-NMR spectra were recorded on the same spectrometer working at 212.4 MHz using 85% phosphoric acid as internal standard (0 ppm). The ESI mass spectra for 10⁻⁵ M acetonitrile solutions were recorded in a Waters QTOF Premier instrument (Walters Corporation, Milford, MA, USA) with orthogonal Z-spray electrospray interface. The solution was introduced at a flow rate of 10 mL·min⁻¹ and N₂ was used as the desolvation and cone gas (flow rates of 300 and 30 L·h⁻¹, respectively). A capillary voltage of 3.3 kV in the negative scan mode (V-mode) and relatively low cone voltages (U_c = 15 V) were used to control the extent of fragmentation of the gas-phase detected species.

3.2. Synthesis of [N(C₄H₉)₄]₃[H(C₁₁H₉PO)₂(HBW₁₁O₃₉)] (TBA-1)

To a suspension of K₈[HBW₁₁O₃₉]·13H₂O (3.22 g, 1 mmol) in CH₃CN (50 mL) stirred at room temperature solid [N(C₄H₉)₄]Br (1.605 g, 5 mmol) and C₁₁H₉PO₃H₂ (0.444 g, 2 mmol) were added. After 10 min, HCl 4 M (1 mL, 4 mmol) was added dropwise and the mixture was left overnight at reflux. Then, the insoluble white solid was removed by filtration and an orange powder was obtained by evaporation of the resulting solution in a rotary evaporator. The crude compound was washed with water and recrystallized from CH₃CN. Prismatic orange crystals of TBA-1 suitable for X-ray diffraction were obtained upon slow evaporation of the final solution in an open vial for ca. 3 days. Yield: 2.33 g, 62% based on W. Elemental Analyses (%): calcd. (found) for C₇₀H₁₂₈BN₃O₄₁P₂W₁₁: C, 22.3 (23.0); H, 3.48 (3.71); N, 1.14 (1.15). IR (cm⁻¹): 3404 s, 2968 m, 2871 w, 1482 s, 1465 m, 1381 m, 1219 m,1176 w, 1155 w, 1058 s, 1029 s, 973 vs, 918 vs, 896 vs, 828 vs, 738 s, 560 w, 533 m, 425 w. ¹H-NMR (500 MHz, acetone-d₆): δ = 0.99 (36H, t, J = 7.4 Hz, –CH₃), 1.46 (24H, tq, J =7.4 Hz, 7.4 Hz, –CH₂–CH₃), 1.76–1.84 (24H, m, –CH₂–CH₂–), 3.36–3.43 (24H, m, –N–CH₂–), 3.89 (1H, d, J = 15.3 Hz, P(O)CH_aH_b), 3.94 (1H, d, J = 15.3 Hz, P(O)CH_cH_d), 4.22 (1H, d, J = 15.3 Hz, P(O)CH_aH_b), 4.27 (1H, d, J = 15.3 Hz, P(O)CH_cH_d), 4.80 (1H, s, B–O–H), 7.36 (2H, ddd, J = 8.3, 7.0, 1.1 Hz, C_aromH), 7.40–7.50 (4H, m, C_aromH), 7.73 (2H, dd, J = 8.3, 2.8 Hz, C_aromH), 7.78–7.84 (4H, m, C_aromH), 8.29 (2H, d, J = 8.3Hz, C_aromH) ppm. ¹³C-NMR (CD₃CN): δ = 13.9 (s, –CH₃), 20.4 (s, –CH₂–CH₃), 24.4 (s, –CH₂–CH₂–), 29.9 (d, ¹J = 134.1 Hz, P(O)CH₂), 59.5 (s, –N–CH₂–), 125.9 (s, C_arom), 126.5 (d, J = 4.5 Hz, C_arom), 126.7 (s, C_arom), 127(s, C_arom), 128.5 (d, J = 4.5 Hz, C_arom), 129.1 (d, J = 11.4 Hz, C_arom), 129.3 (s, C_arom), 130.0 (d, J = 7.7 Hz, C_arom), 133.2 (d, J = 5.5 Hz, C_arom), 134.8 (d, J = 2.7 Hz, C_arom) ppm. ³¹P-NMR (acetone-d₆): δ = 27.9 ppm.

3.3. Synthesis of [N(C₄H₉)₄]₃[H(C₁₁H₉PO)₂(SiW₁₁O₃₉)] (TBA-2)

The synthetic procedure above was followed but for using K₈[SiW₁₁O₃₉]·13H₂O (3.22 g, 1 mmol) instead of K₈[HBW₁₁O₃₉]·13H₂O. Prismatic orange crystals of TBA-2 suitable for X-ray diffraction were obtained upon slow evaporation of the final solution in an open vial for ca. 3 days. Yield: 2.12 g, 56% based on W. Elemental Analyses (%): calcd. (found) for C₇₀H₁₂₇N₃O₄₁P₂SiW₁₁: C, 22.3 (22.7); H, 3.49 (3.61); N, 1.14 (1.22). IR (cm⁻¹): 3430 s, 2965 m, 2873 w, 1642 m, 1479 w, 1376 w, 1176 w, 1155 w, 1078 s, 978 vs, 936 s, 918 vs, 889 s, 853 m, 731 s, 550 w, 531 w, 423 w. ¹H-NMR (500 MHz, acetone-d₆): δ = 0.98 (36H, t, J = 7.4 Hz, –CH₃), 1.45 (24H, tq, J = 7.4 Hz, 7.4 Hz, –CH₂–CH₃), 1.73–1.84 (24H, m, –CH₂–CH₂–), 3.30–3.44 (24H, m, –N–CH₂–), 3.81 (1H, d, J = 15.2 Hz, P(O)CH_aH_b), 3.85 (1H, d, J = 15.2 Hz, P(O)CH_cH_d), 4.08 (1H, d, J = 15.2 Hz, P(O)CH_aH_b), 4.13 (1H, d, J = 15.2 Hz, P(O)CH_cH_d), 7.35 (2H, ddd, J = 8.3, 7.0, 1.1 Hz, C_aromH), 7.38–7.48 (4H, m, C_aromH), 7.72 (2H, dd, J = 8.3, 2.8 Hz, C_aromH), 7.82–7.89 (4H, m, C_aromH), 8.28 (2H, d, J = 8.3 Hz, C_aromH) ppm. ¹³C-NMR (CD₃CN): δ = 13.9 (s, –CH₃), 20.4 (s, –CH₂–CH₃), 24.4 (s, –CH₂–CH₂–), 30.3 (d, ¹J = 138.3 Hz, P(O)CH₂), 59.3 (s, –N–CH₂–), 126.0 (s, C_arom), 126.5 (d, J = 4.5 Hz, C_arom), 126.6 (s, C_arom), 128.3 (d, J = 4.5 Hz, C_arom), 129.2 (s, C_arom), 129.5 (d, J = 10.9 Hz, C_arom), 130.0 (d, J = 8.0 Hz, C_arom), 133.2 (d, J = 5.8 Hz, C_arom), 134.8 (d, J = 2.5 Hz, C_arom) ppm. ³¹P-NMR (acetone-d₆): δ = 24.5 ppm.

3.4. X-ray Crystallography

Crystallographic data for compounds TBA-1 and TBA-2 are summarized in

Table 2. Crystallographic data for TBA-1 and TBA-2.

Parameters	TBA-1	TBA-2
Formula	C70H128BN3O41P2W11	C70H127N3O41P2SiW11
FW (g·mol−1)	3762.8	3779.1
Crystal System	Triclinic	Triclinic
Space Group	P−1	P−1
a (Å)	17.4020(5)	16.9676(5)
b (Å)	24.2390(3)	24.3022(3)
c (Å)	28.6175(5)	28.5333(7)
α (˚)	70.095(2)	70.030(2)
β (˚)	72.336(2)	72.703(2)
γ (˚)	89.976(2)	89.957(2)
V (Å3)	10743.3(4)	10491.3(4)
Z	4	4
ρcalcd (g·cm−3)	2.326	2.393
μ (mm−1)	22.022	12.124
T (K)	150(2)	100(2)
λ (Å)	1.54184 (Cu Kα)	0.71073 (Mo Kα)
Collected Reflections	80334	65644
Unique Reflections (Rint)	38193 (0.126)	36457 (0.048)
Observed Reflections [I > 2σ(I)]	23423	21696
Parameters/Restraints	767/250	1459/546
R(F)a [I > 2σ(I)]	0.095	0.083
wR(F2)a [all data]	0.270	0.216
GoF	0.964	1.078

^a R(F) = Σ||F_o−F_c||/Σ|F_o|; wR(F²) = {Σ[w(F_o²−F_c²)2]/Σ[w(F_o²)²]}^1/2.

. Intensity data for compound TBA-2 were collected at 100(2) K on an Agilent Technologies SuperNova diffractometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with an Eos CCD detector and mirror-monochromated Mo Kα radiation (λ = 0.71073 Å). The data acquisition for compound TBA-1 was carried out at 150(2) K using mirror monochromated Cu Kα radiation (λ = 1.54184 Å) and an Atlas CCDC detector. Data collections, unit cell determinations, intensity data integrations, routine corrections for Lorentz and polarization effects, and analytical absorption corrections with face indexing were performed using the CrysAlis Pro software package (Agilent Technologies UK Ltd., Oxford, UK) [67]. The structures were solved using OLEX2 (OlexSys Ltd in Durham University, Durham, UK) [68] and refined by full-matrix least-squares with SHELXL-97 (University of Göttingen, Göttingen, Germany) [69]. Final geometrical calculations were carried out with PLATON (Utrecht University, Utrecht, The Netherlands) [70] as integrated in WinGX (University of Glasgow, Glasgow, UK) [71]. Bond valence sum (BVS) calculations [55] were performed using the BVSumCalc program (courtesy of Michael H. Dickman). Thermal vibrations were treated anisotropically for atoms contained in the hybrid-POM framework (W, Si, O). All the bond lengths and isotropic thermal ellipsoids for aromatic and alkyl groups were normalized using SADI and SIMU-type restraints from SHELXL due to the considerable structural disorder. For the aromatic systems, some of the 1,3-distances (i.e., distances between two atoms that are both bonded to the same atom, or “angle distances”) were restrained to 2.42(2) Å. Hydrogen atoms of the 1-napththylmethyl groups and the methylenic carbons of the n-butyl chains were placed in calculated positions using standard SHELXL parameters. The H atoms of the –CH₃ carbons in the n-butyl chains were added as idealized methyl groups with staggered geometry (AFIX33) for the refinement to converge.

Only five out of the six TBA counterions per two POM clusters determined by TGA, NMR and elemental analyses were located in the Fourier map of TBA-2. A sixth cation was found to be disordered over two positions, but it could not be successfully modeled. The much poorer quality of the crystals of TBA-1 only allowed us to model one of the six TBA cations in the asymmetric unit. Therefore, the final structural models contained large solvent accessible voids accounting for ca. 55% (TBA-1) and 20% (TBA-2) of the unit cell that have been analyzed using the SQUEEZE tool of PLATON. The void of 5898 ų in TBA-1 is located at (x, y, z) = (0, 0, 0) and corresponds to 3301 electrons, whereas that of 2209 ų in TBA-2 is located at (x, y, z) = (1/2, 1/2, 0) and accounts for 1299 electrons. The calculated void could easily host the undetermined 2 TBA cations in the unit cell of TBA-2, which occupy ca. 1800 ų if they are considered as spherical units with a radius of 6 Å. The remaining void space could be occupied by solvent molecules that are rapidly lost upon the exposure of the crystals at ambient conditions. In the case of TBA-1, the rough approximation used for estimating the volume occupied by the undetermined cations is inaccurate for such a large void accounting for more than the 50% of the unit cell. The hkl reflection file generated with SQUEEZE contained 23,423 (TBA-1) and 21,696 (TBA-2) observed reflections out of the 25,376 and 22,761 reflections in the original files. Refinement of the structural model using the squeeze-generated reflection file led to a significant improvement of the final R and wR² agreement factors in comparison to the original ones [TBA-1: R, 0.138 and wR² (all), 0.396; TBA-2: R, 0.107 and wR² (all), 0.247]. CCDC-1471565 (TBA-1) and 1471564 (TBA-2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.5. Computational Details

All TD-DFT calculations were carried out with ORCA 3.0.3 software [72]. The geometry of the [H(C₁₁H₉PO)₂(α-SiW₁₁O₃₉)]³⁻ anion, taken from the crystal structure of TBA-2, was optimized using the BP86 [73,74] generalized-gradient-approximation density functional, modified with Grimme’s DFT-D3 atom-pairwise dispersion correction [75] with Becke–Johnson damping [76] to take into account London-dispersion effects. The Ahlrichs’ double-ζ AO basis set with one set of polarization functions (def2-SVP) [77] was used for all atoms. The Stuttgart–Dresden quasi-relativistic effective core potential SD(60,MWB) [78] was applied to W atoms. The UV–Vis absorption spectrum was calculated (at the optimized geometry) using linear-response time-dependent density functional calculations (TD-DFT) within the Tamm–Dancoff approximation [79], with the same basis sets and ECPs as during geometry optimization, and using the B3LYP hybrid density functional [80]. In order to speed up the calculation, the essentially linear RIJCOSX approximation [81,82] has been used, which has been shown to lead to virtually no loss of accuracy [83].

4. Conclusions

The study presented herein represents a solid confirmation of the fact that the classical approach to functionalize lacunary POM clusters with organo-derivatives of the p-block can be applied to new platforms belonging to heteropolyoxotungstates with heteroatoms of the group 13. In this work, an organophosphonate derivative of a Keggin-type borotungstate (TBA-1) has been structurally characterized for the first time. Experiments carried out using the monolacunary Keggin-type silicotungstate have resulted in the hybrid TBA-2 analogue and demonstrate that the functionalization with organophosphonate precursors such as 1-naphthylmethylphosphonic acid can be extended to isoelectronic POM species regardless of the different reactivity conferred by the heteroatoms. The UV–Vis spectroscopy reveals the presence of a charge transfer process promoted by transitions from the naphthyl aromatic system to the Keggin anion as confirmed by TD-DFT calculations. This electronic feature could be at the origin of the quenching that the fluorescent properties of the 1-naphthylmethylphosphonate group undergo upon its incorporation into the inorganic oxo-tungstate skeleton. A combination of multinuclear NMR studies and ESI-MS spectrometry confirms the solution stability of the hybrid frameworks in the organic media. We plan to react the obtained hybrid POM platforms with catalytically active 3d- and 4f-metals centers in the near future with the aim of preparing models for supported site-isolated catalysts with an aromatic environment that will confer photoactive properties on the system and might enhance the selectivity of the catalytic process by interacting with the substrates and providing steric hindrance.