Combined XRD and infrared studies of pyridinium species in (PyH)₃[PW₁₂O₄₀] single crystals

Abstract

The pyridinium salts of 12-tungstophosphoric acid (PyH)₃[PW₁₂O₄₀]·2CH₃CN (1) and (PyH)₃[PW₁₂O₄₀] (2) have been prepared and studied by single crystal and powder X-ray diffraction (XRD) in order to characterize the crystallographic sites occupied by the pyridinium species. The three PyH⁺ species are located on two unequivalent sites. Two species are linearly H-bonded to the oxygen atoms of the Keggins unit (α species), whereas the third one (β) forms a bent H-bond. In order to determine the infrared bands characterizing each type of pyridinium species in the 1650–1300 cm⁻¹ range, infrared spectra have been recorded from room temperature to 100 K. They reveal that only α pyridinium species give rise to the unusual splitting of the PyH⁺ν8b and ν19b modes, whereas β pyridinium species lead to a classical pyridinium spectrum.

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1. Introduction

Pyridine adsorption on catalyst surfaces is commonly used for the infrared characterization of the Lewis or Brönsted surface acidities.^1–5 In particular, Brönsted acidity is very well characterized by the formation of an adsorbed pyridinium ion, the ν19b vibration mode being then upward shifted from ∼1445 cm⁻¹ to ∼1540 cm⁻¹.

Two decades ago, unusual ν8b and ν19b band doublings were observed in the photoacoustic spectrum of pyridinium species produced by pyridine absorption in the crystalline 12-tungstophosphoric acid H₃[PW₁₂O₄₀].⁶ The band doublings were proposed to be due to crystal site effects, but this remains to be proved since the ν19b and ν8b pyridinium modes are generally not affected by the nature of the counter anion.⁷ More recently, the band doublings were assigned to the formation of the bis-pyridinium (PyH⁺Py) ion.^8–10 However, this interpretation was later discarded using combined infrared (IR) and thermogravimetric analysis (TGA).¹¹

Recently, we have briefly reported the synthesis of the (PyH)₃[PW₁₂O₄₀]·2CH₃CN complex (1) which infrared spectrum presents a similar splitting of the ν8b and ν19b bands.¹¹ The lack of crystallographic data on this compound, however, prevented any consideration to be made on the structure of the pyridinium species leading to such a splitting. These informations are essential for the understanding of the pyridinium cation vibration modes in such materials.

In the present study, we report the X-ray structural determination of 1 as well as the structure of the CH₃CN-desolvated (PyH)₃[PW₁₂O₄₀] compound 2. These data have been correlated with the IR spectra of these samples recorded at room and low temperatures, allowing us to show that the band doublings are not only due to crystal site effects.

2. Experimental details

All chemicals were of reagent grade and used as received. The complexes (PyH)₃[PW₁₂O₄₀]·2CH₃CN (1) and (PyH)₃[PW₁₂O₄₀] (2) have been synthesized as previously described.¹¹

X-Ray diffraction

For 1 and 2, intensity data collections were carried out with a Bruker Nonius X8 APEX 2 diffractometer with a CCD detector using a Mo Kα monochromatized radiation (λ = 0.71073 Å) and recorded at room temperature. For 1, a single-crystal was mounted in a capillary tube. For 2, a single-crystal was mounted on a glass fiber and heated at 353 K for one night. For both compounds, the absorption corrections were based on multiple and symmetry-equivalent reflections in the data set using the SADABS program based on the method of Blessing.¹² The structures were solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package.¹³ For 1 and 2, the hydrogen atoms were located and refined using a constrained “riding” model. All the other atoms were refined anisotropically except for the phosphorus atom in complex 2. X-Ray powder diffraction data were collected on a Siemens D5000 equipped with a Cu Kα radiation for room temperature measurements and a Co Kα radiation for temperature-dependent ones.

Infrared spectroscopy

The (PyH)₃[PW₁₂O₄₀]·2CH₃CN compound was mildly crushed and mixed with a KBr powder (1 mg of the salt diluted into 100 mg of KBr). A pellet was then obtained and placed into a quartz cell equipped with KBr windows, connected to a vacuum line and a furnace for the in situ treatment. To obtain the (PyH)₃[PW₁₂O₄₀] compound, the pellet was heated at 353 K overnight. Spectra recorded at low temperature were obtained by cooling the sample holder with liquid nitrogen. Transmission IR spectra were recorded in the 500–4000 cm⁻¹ range, with a 4 cm⁻¹ resolution, on a Nicolet Nexus spectrometer equipped with an extended KBr beam splitting device and a mercury cadmium telluride (MCT) cryodetector.

3. Results

The notations used herewith for the oxygen atoms in the Keggin polyanion [PW₁₂O₄₀]³⁻ are illustrated in Scheme 1. W₃O₁₃ moieties are formed by edge-sharing WO₆ octahedra. Shared oxygen atoms in this unit are denoted as O-edge (O_e). Four W₃O₁₃ moieties are tetrahedrally bonded to the central P atom and bonded to each other by sharing vertices. Common oxygen atoms in this arrangement are denoted as O-corner O(c) oxygens. The third type of external oxygen atom belongs to the unshared W=O group, and is denoted as O-terminal O(t). The whole [PW₁₂O₄₀]³⁻ anion is roughly spherical with a diameter of 1 nm.

Scheme 1 Oxygen notations for the Keggin anion structure [PW₁₂O₄₀]³⁻.

3.1 Structural determination by XRD analysis

Crystallographic data relative to the structures of (PyH)₃[PW₁₂O₄₀]·2CH₃CN (1) and (PyH)₃[PW₁₂O₄₀] (2) are given in Table 1. The two complexes are represented in Fig. 1a and b, respectively. Single-crystals of compound 2 have been obtained by heating single-crystals of 1 at 353 K for one night. As expected, this thermal treatment has induced a loss of crystallinity, providing crystals of poor quality. Nevertheless, it has been possible to refine the diffraction data to obtain an acceptable structural model for 2. It should be pointed out that the structural characterization of this desolvated species is crucial in order to focus on the infrared characterization of the pyridinium cations at the surface of a polyoxometalate unit. Indeed, compared to 1, the IR characterization of 2 is thus not perturbated by bands arising from co-crystallizing organic solvent molecules. Additionally, we can note that single-crystal X-ray characterization of polyoxometalate desolvated by exposure to air or heating has been only scarcely reported.¹⁴

Fig. 1 Ball-and-stick and polyhedral representation of (a) (C₅H₆N)₃[PW₁₂O₄₀]·2CH₃CN (1) and (b) (C₅H₆N)₃[PW₁₂O₄₀]·2CH₃CN (2). Grey octahedra, WO₆; black octahedra, PO₄; black spheres, C; grey spheres, N; white spheres, H. The dashed lines represent the hydrogen bonds.

Table 1 Crystallographic data for (PyH)₃[PW₁₂O₄₀]·2CH₃CN (1) and (PyH)₃[PW₁₂O₄₀] (2)

	1	2
a . b .
Empirical formula	C19H24N5PW12O40	C15H18N3PW12O40
Formula weight/g	3199.6	3117.5
Temperature	293 K	296 K
Crystal system	Monoclinic	Monoclinic
Space group	C2/c (No. 15)	P2(1)/n (No. 14)
a/Å	23.5058(4)	12.216(3)
b/Å	14.3158(4)	14.035(3)
c/Å	16.6984(5)	12.988(3)
β/°	119.933(2)	98.045(13)
V/Å^3	4869.5(2)	2205.1(8)
Z	4	2
d calc/g cm^−3	4.364	4.695
Reflections collected/unique	128 879/7200 (Rint = 0.0795)	24 722/3887 (Rint = 0.3949)
Goodness-of-fit	1.165	0.947
R (>2σ(I))	R 1(Fo)^a = 0.0377	R 1(Fo)^a = 0.1145
	wR2(F^2o)^b = 0.1027	wR2(F^2o)^b = 0.2535
R (all data)	R 1(Fo)^a = 0.0434	R 1(Fo)^a = 0.2909
	wR2(F^2o)^b= 0.1141	wR2(F^2o)^b = 0.3095

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Both compounds consist in the well-known [α-PW₁₂O₄₀]³⁻ saturated Keggin-type polyanion with protonated pyridine as counter-ions (Fig. 1a). In 1, in addition to the three pyridinium cations, two acetonitrile molecules are present in the crystal lattice. The CH₃CN fragments appear as molecular dimers of antiparallel dipoles, the intermolecular distance being 3.46 Å. For 1, bond lengths and angles of the tungsten octahedra constituting the [α-PW₁₂O₄₀]³⁻ unit are typical of this type of anion.¹⁵ In the case of compound 2, the tetrahedral phosphato unit has been found disordered over two positions. This is due to the location of the phosphorus atom on an inversion center, incompatible with the T_d symmetry of the Keggin unit, which induces the presence of eight oxygen atoms surrounding the phosphorus atom. These eight oxygen atoms, which are located at the vertices of a distorted cube, were then refined at half-occupancy factors. Such a disorder in a Keggin-type polyoxometalate has been first solved by Evans and Pope,¹⁶ and since has been encountered several times.¹⁷ While complexes 1 and 2 crystallize in the C2/c and in the P2(1)/n space groups, respectively, the three pyridinium cations can be divided into two groups in both cases. One consists in two crystallographically equivalent molecules (full occupancy), while the other one consists in a crystallographically independent counter-ion (half occupancy).

For compound 1, the nitrogen atoms of the pyridinium rings have been localized considering that the C–C bonds are longer than C–N bonds (d_C–N = 1.332–1.340 Å; d_C–C = 1.363–1.399 Å). Such localization of the nitrogen atoms is strongly supported by the presence of short distances due to hydrogen bondings between the H atoms connected to the N centers of the pyridinium and oxygen atoms of the [α-PW₁₂O₄₀]³⁻polyanions. The pyridinium cation in half-occupation site (β₁) is involved in a bent H-bonding with two O-corner atoms (d_NH⋯Oc = 2.464 Å), while the two other countercations (α₁) are both connected to the Keggin anion fragment via a H-bond involving an O-edge atom (d_NH⋯Oe = 2.074 Å) (Fig. 1a).The latter H-bond is nearly linear (N–H–O_e = 171°). The protons of the pyridinium cations cannot be located by XRD measurements. They are positioned for the C_2v cation symmetry using the length values 0.86 and 0.93 Å for the N–H and C–H bonds, respectively. The short N–H distance here adopted is in accordance with the rather large N(H)⋯O distances (as denoted below) inferring only weak H-bonding perturbations of the N–H bond length.

A more detailed description of the local interaction of α₁ and β₁ pyridinium species with the Keggin anion is given Scheme 2. For α₁ species the pyridinium ring and the O(t)–W–O(e)–W–O(t) Keggin moiety are nearly coplanar. For β₁ species the pyridinium ring is atop a butterfly-like set of four O-atoms (two opposite O-corner atoms, noted O(c) and two opposite O-edge atoms, noted O(e)); the H⋯O(e) distance (2.559 Å) is only slightly higher than the H⋯O(c) one (2.464 Å). Examination of the ring bond lengths allows one to deduce the symmetry elements. The symmetry plane containing the N–H bond and normal to the pyridinium ring is clearly defined for β₁ species. It is not the case of α₁ species for which a symmetry plane not containing the N–H bond can be considered. It is tilted by about 30° from the N–H direction, as indicated by a dashed line in Scheme 2. According to such a symmetry, the ring atom labelled by an asterisk (Scheme 2) would be possibly the N atom. In such a case, the C–N bond lengths would be 1.333 and 1.369 Å instead of 1.333 and 1.359 Å, for the best fitted structure shown in Scheme 2. The existence of a second and less probable rotational conformation of the pyridinium ring suggests a frustrated rotation (depicted by the curved arrows in Scheme 2) around the pseudo C₆ axis in a double well potential.

Scheme 2 Description of the structure of α₁ and β₁ species.

For complex 2, it has not been possible to localize the nitrogen atoms considering C–N distances due to the poor quality of the crystals (vide supra). However, a localization of the nitrogen centers can be proposed by considering the shortest (Py)H⋯O(W) distances (d_(N)H⋯O(W) = 1.959 and 2.301 Å), the counter-cations α₂ and β₂ being all connected to the [PW₁₂O₄₀]³⁻ unit via one H-bond (Fig. 1b). The α₂ H-bond, with an O-terminal atom, is the shorter one (1.959 Å) and is nearly linear (N–H–O_t = 173°). The β₂ H-bond is bent as also observed for 1 (N–H–O = 150°) and involves an O-edge atom (Fig. 1b). It should be noted, however, that an O-corner atom cannot be excluded because of the orientational disorder of the Keggin units which does not lead to univocal O_e/O_c positions.

Thus, two types of H-bonds (α and β) can be distinguished: (i) linear bonds with moderately short H⋯O distances of about 2.0 Å for α₁ and α₂ species and (ii) bifurcated or bent bonds with larger H⋯O distances of about 2.4 Å for β₁ and β₂ species. The strongest H-bonds are expected to be the linear and shorter H-bonds (α). Even in this case, however, the N⋯O distance (∼2.9 Å) is large as compared to the short distances usually reported in the literature (lower than 2.66 Å).¹⁸ Hence, the H-bonding energy is expected weak for both types of bonds, α and β.

Powder X-ray diffraction studies of 1 and 2 have been performed. A comparison of the experimental powder X-ray diffraction data carried out at room temperature on 2 (Fig. 2a) and the X-ray powder diffractogram calculated from the single crystal X-ray diffraction data of 2 (Fig. 2c) confirms the validity of the structural model obtained from the single-crystal XRD data and the phase purity of the powder. It also shows that a mild crushing of the crystals does not alter the structure of compound 2. X-Ray thermodiffractometry experiments on the same compound were carried out under air from room temperature to 573 K (Fig. S1, ESI†). The thermodiffractogram recorded at 573 K is shown in Fig. 2b and shows that the structure of compound 2 is stable up to at least 573 K. For compound 1, the X-ray powder diffraction pattern of the freshly filtered, ground crystals obtained at 293 K (Fig. 2d) fits the calculated X-ray powder diffractogram obtained from the single crystal X-ray diffraction study of 1 (Fig. 2e). Nevertheless, the experimental powder X-ray diffraction pattern evolves over hours, with the appearance of peaks corresponding to compound 2 (Fig. S2, ESI†). This shows that a spontaneous desolvation of compound 1 occurs even at room temperature. When a drop of acetonitrile is added to the powder sample, the X-ray powder pattern changes and shows a decrease of the intensity of the peaks corresponding to the non-solvated species 2. This clearly shows the existence of an equilibrium between species 1 and 2 depending on acetonitrile partial pressure. Once the [PW₁₂O₄₀]³⁻/pyridinium compounds presented here have been fully structurally characterized, the infrared studies of these systems have been performed.

Fig. 2 Powder X-ray diffraction pattern performed on 2: experimental diffractogram recorded (a) at room temperature, (b) at 573 K; (c) X-ray powder diffractogram calculated from the single crystal X-ray diffraction data of 2. Powder X-ray diffraction pattern performed on 1: (d) experimental diffractogram recorded at room temperature; (e) X-ray powder diffractogram calculated from the single crystal X-ray diffraction study of 1.

3.2 Study of the pyridinium species using infrared spectroscopy

The spectrum of the Keggin [PW₁₂O₄₀]³⁻ anion displays very intense bands in the 1100–750 cm⁻¹ range, so that only pyridinium bands out of that spectral range can be observed. Thus attention was paid to ring stretching modes, δ(NH) and in-plane δ(CH) bending modes above 1100 cm⁻¹ as well as to the ν(NH) band in the 2800–3200 cm⁻¹ frequency range. The bands notation used below comes from the one usually used for pyridine.¹⁹ The assignments for some normal modes are not straightforward.^7,20,21 The more recent Odinokov attributions⁷ are used in the following.

Single crystals of 1 and 2 were incorporated in a KBr matrix by a mild crushing. The presence of CH₃CN in compound 1 is clearly characterized in the spectrum by the ν(CN) band doublet at 2250–2275 cm⁻¹ (not shown). The spectra are shown in the range of the pyridinium aromatic cycle (Fig. 3, right) and the ν(NH) stretching modes (Fig. 3, left) for compounds 1 (spectrum a) and 2 (spectrum b). Bands at 1443 and 1365 cm⁻¹ due to the δ(CH₃) modes of CH₃CN in compound 1 are not seen for compound 2 (spectrum b) confirming that CH₃CN completely disappears upon heating compound 1 at 353 K.

Fig. 3 IR spectra of the pyridinium salt single-crystals recorded at rt in KBr matrix. (a) (PyH)₃[PW₁₂O₄₀]·2CH₃CN (compound 1); (b) (PyH)₃[PW₁₂O₄₀] (compound 2). Bands at 1443 and 1365 cm⁻¹ due to the δ(CH₃) modes of CH₃CN in compound 1 are indicated by an asterisk (spectrum a).

The ν(NH) band gives rise to a wide band spreading from 3400 to 2800 cm⁻¹ with very deep Evans holes due to Fermi resonances as generally observed.^22,23 The center of gravity can be estimated to be about 3120 cm⁻¹, a higher value compared to those observed at around 3060 cm⁻¹ when the counter ions are either BF₄⁻, SbCl₆⁻ or ClO₄⁻.²² Hence, the strength of the hydrogen bonding is weak, in agreement with the XRD results indicating rather long N⋯O distances.

Because of the weakness of the pyridinium bands in the spectral range 1300–1100 cm⁻¹ and of their controversial assignments,^7,20,21 most of the following discussion will focus on the ring stretching modes: ν8a, ν8b, ν19b and ν19a. The most striking feature of the IR spectra is the clear splitting of the ν8b and ν19b modes (Fig. 3) which is unusual for pyridinium species. The ν8b components are located at 1613 and 1603 cm⁻¹ for compounds 1 and 2, the ν19b components being situated either at 1542 and 1532 cm⁻¹ for compound 1 or at 1542 and 1530 cm⁻¹ for compound 2. Such a band doubling is also observed for the band denoted as “δ(NH)” in the spectrum a (1336 and 1328 cm⁻¹), while it is unresolved in the spectrum b (1332 cm⁻¹).

Upon cooling, the spectra become more complex due to a better resolution: (i) the increase of the spacing between the two components of the ν8b and ν19b doublets leads to the appearance of central components which are better defined by deconvolution (Fig. 4, spectrum c), and (ii) the bands appearing as shoulders to the main ν8a and ν19a bands at room temperature (1632 and 1482 cm⁻¹) are also better resolved.

Fig. 4 Pyridinium bands of (PyH)₃[PW₁₂O₄₀] (compound 2), at (a) rt, (b) about 150 K, (c) about 100 K. Band deconvolutions are performed for spectrum c. Deconvoluted bands due to β species are shadowed. Inset: focus on the effect of the temperature decrease on the splittings of the ν8b (left) and ν19b (right) bands.

The spacing of the ν8b and ν19b doublets increases from 12 to 18–19 cm⁻¹ upon cooling (Fig. 4, inset) but the doublets remain centered on the same mean values (1609 and 1536 cm⁻¹, respectively) thus indicating that they characterize the same species. A detailed deconvolution of the bands (Fig. 4, spectrum c) shows that the intensities of the central bands merging in the ν8b and ν19b doublets (1611 and 1539 cm⁻¹ respectively) are about half of the total intensities of the corresponding doublets. Such intensity ratio allows one to assign the ν8a and ν8b doublets to α pyridinium species and the central bands to β ones (Table 2). Moreover, the ν8a and ν19a bands are resolved into two components. From the intensity criterion, the low wavenumber component of the ν8a and ν19a bands can be associated with the central band merging between the doublet components of the ν8b and ν19b bands (Table 2). It is worthwhile noticing that each component of the doublets cannot be assigned to distinct species occupying unequivalent crystallographic sites. If it was the case, the ν8b and ν19b wavenumbers for one site would decrease upon cooling, whereas they would increase for the other site, the mean wavenumbers being constant. Such a correlation between two distinct sites is hardly conceivable, all the more that XRD results show that the two sites involved in the α species formation are equivalent. Hence, the origin of doubling cannot be due to site effects (heterogeneity of the sites). Finally, wavenumbers reported in Table 2 show that β pyridinium species give rise to classical pyridinium bands whereas the α pyridinium species give rise to a splitting of the ν8b and ν19b bands. The ν8b and ν19b wavenumbers for β species (1611 and 1539 cm⁻¹) are nearly the same as the mean values for the corresponding band doublets assigned to α species (1609 and 1536 cm⁻¹). This shows that the site effect on the ν8b and ν19b modes is rather weak (2 or 3 cm⁻¹).

Table 2 Wavenumbers (cm⁻¹) of pyridinium stretching ring modes, of α and β PyH⁺ species. Measurements were performed at room temperature (rt) and at 100 K. Values in bold characters indicate the doublet components produced by the splittings of the bands

Assignments	α species		β species
	rt	100 K	rt	100 K
a n.r. for not resolved.
ν8a	1638	1639	1633	1635
ν8b	1614	1618	n.r ^a	1611
	1602	1600
ν19b	1542	1546	n.r ^a	1539
	1530	1527
ν19a	1487	1487	1481	1480

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The lifting of a vibrational degeneracy cannot be considered to explain the ν8b and ν19b band splittings for α species since vibrational levels are not degenerate in the C_2v symmetry point group. Vibrational intermolecular couplings between pyridinium α species (Davidov splitting) could be invoked. However the existence of a double well potential deduced from the XRD results and discussed above strongly suggests another explanation: the frustrated rotation of the pyridinium ring could induce the splitting of some vibrational modes by a tunneling effect. For example such a two well potential function has been recently established for order/disorder phase transition in the d₅-PyHIO₄ salt at 300 K.²⁴ To confirm such an explanation, further infrared studies are required.

4. Conclusions

The purpose of this study was to identify the pyridinium species at the origin of the band splittings occurring when pyridine is absorbed in the H₃[PW₁₂O₄₀] compound. To this aim, a (PyH)₃[PW₁₂O₄₀] salt was prepared (compound 2) using a crystalline (PyH)₃[PW₁₂O₄₀]·2CH₃CN powder (compound 1) by acetonitrile evaporation upon heating at 353 K. XRD structural determination of both compounds shows that three PyH⁺ species are located on two unequivalent sites: two equivalent α species are linearly H-bonded to either O(e) (compound 1) or O(t) (compound 2) whereas the third one (β) forms a bent H-bond with the Keggin unit. Recording infrared spectra at low temperature allows the attribution of the bands characterizing each type of pyridinium species situated in the 1650–1300 cm⁻¹ range. It appears that only α pyridinium species give rise to the unusual splitting of the ν8b and ν19b modes, whereas β pyridinium species lead to a classical pyridinium spectrum. The origin of the band doubling characterizing the α pyridinium is currently under study. Finally, this study emphasizes the contribution of XRD studies of well defined single crystals to the elucidation of the structure of absorbed species formed on an important class of heterogeneous catalysts such as H₃[PW₁₂O₄₀] materials.

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Footnote

† Electronic supplementary information (ESI) available: Powder thermodiffractograms of 2 and powder X-ray diffraction patterns of 1 showing its spontaneous transformation in2. CCDC reference numbers 776230 and 776231. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cp00446d

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