A Charge-Transfer Salt Based on Ferrocene/Ferrocenium Pairs and Keggin-Type Polyoxometalates

A new hybrid inorganic-organometallic salt has been obtained from the reaction of the Keggin-type silicotungstate anion with ferrocene in a water/methanol mixture as a result of the partial oxidation of ferrocene molecules to ferrocenium cations. Single-crystal X-ray diffraction analysis reveals the presence of four ferrocenium (FeIII) cations and one ferrocene (FeII) molecule per plenary Keggin anion in the crystal structure of [FeIII (Cp)2]4[SiW12O40]·[FeII(Cp)2]·2CH3OH (1). Compound 1 thus constitutes the first example in the literature in which ferrocenium and ferrocene species coexist in the structure of a polyoxometalate-based salt. The two crystallographically independent ferrocenium species in the asymmetric unit of 1 exhibit different configurations: One displays an eclipsed conformation with ideal D5h symmetry, whereas the conformation in the other one is staggered D5d. The crystal packing of 1 can be best described as an organometallic sub-lattice of ferrocenium and ferrocene species linked by a network of π-π interactions that generates rectangular cavities of about 14 × 10 Å in which strings of Keggin anions and methanol molecules are hosted, further connected to each other via weak OPOM···CMeOH-OMeOH···OPOM type hydrogen bonds. The charge-transfer nature of the salt has been studied by solid-state diffuse reflectance UV-Vis spectroscopy and the presence of magnetically isolated FeIII/FeII centres has been confirmed by Mössbauer spectroscopy. A topological study carried out on all of the pristine ferrocenyl species deposited in the Cambridge Structural Database (CSD) has allowed two main conclusions to be drawn: (1) these species tend to adopt extreme conformations (either eclipsed or staggered) with less than a 15% of examples showing intermediate states and (2) the oxidation state of the iron centres can be unequivocally assigned on the basis of a close inspection of the Fe···Cp distances, which allows ferrocene neutral molecules and ferrocenium cations to be easily distinguished.


Introduction
Polyoxometalates (POM) are anionic metal-oxide clusters with rich structural and electronic variety and applications in areas of current interest such as catalysis, nanotechnology, materials science

Synthesis and Infrared Spectroscopy
Compound 1 was obtained in low yields as single crystals suitable for X-ray diffraction studies from the reaction of the [α-SiW12O40] 4-precursor and ferrocene (1:2 ratio) in a water/methanol mixture at reflux conditions. In contrast to what has been observed for the ferrocenium-POM salts reported to date, only partial oxidation of ferrocene took place in the formation of 1, resulting in the first example in the literature in which ferrocenium (Fe III ) and ferrocene (Fe II ) species coexist in the same crystal structure. The formation of a compound combining Keggin-type POM clusters and ferrocenyl units such as 1 was firstly confirmed by FT-IR spectroscopy. Two parts can be clearly differentiated in the FT-IR spectrum of 1 (Figure 1): the inorganic fingerprint below 1000 cm −1 (a comparative detail with that of the K4[α-SiW12O40]·17H2O precursor is also depicted) and the organometallic region above. The spectrum of 1 exhibits the four characteristic bands of strong intensity (A, B, C and E) that unequivocally correspond to the plenary α-Keggin tungstosilicate anion [28] but with small red shifts of about 10 cm −1 that affect those three to which the νas(W-Ot) (Ot: terminal O atom) vibrational mode contributes (bands A at 1011 cm -1 , B at 972 cm -1 and C at 922 cm -1 ). The signal of medium-to-weak intensity that originates from the antisymmetric stretching vibration of the W-O-W bridges involving corner-sharing appears split at 880 and 856 cm -1 (signal D), whereas negligible modifications are noticed for those signals associated with the stretching vibrations of the W-O-W bridges between edge-sharing W centres (signal E at 785 cm -1 ) and the overall bending vibrations in both the oxometallic skeleton and the central heterogroup (signals F and G at 532 and 483 cm −1 ). Focusing on the organic region, the presence of ferrocenyl species in 1 is confirmed by the signals associated with the stretching of the C(sp 2 )-H and C=C bonds that can be observed as peaks of medium intensity at ca. 3100 and 1410 cm −1 , respectively. According to the literature, ferrocenyl groups in staggered (D5d) and eclipsed (D5h) conformation can be easily differentiated by IR Focusing on the organic region, the presence of ferrocenyl species in 1 is confirmed by the signals associated with the stretching of the C(sp 2 )-H and C=C bonds that can be observed as peaks of medium intensity at ca. 3100 and 1410 cm −1 , respectively. According to the literature, ferrocenyl groups in staggered (D 5d ) and eclipsed (D 5h ) conformation can be easily differentiated by IR spectroscopy because the fingerprint of both forms is substantially different in the 450-500 cm −1 region of the spectrum [29]. Unfortunately, the presence of the strong absorption bands of the [SiW 12 O 40 ] 4− cluster below 1000 cm −1 and more specifically, of the broad signal F, corresponding to the (W-O e -W) + (Si-O c ) combination (O e : bridging O atom between edge-sharing W centres; O c : central O atoms), makes impossible to perform such analysis in our case as it shadows this entire range.

Crystal Structure
Compound 1 crystallizes in the triclinic space group P-1 with the following content in the asymmetric unit: one half of the [α-SiW 12 O 40 ] 4− cluster unit located on a centre of inversion; two ferrocenyl {Fe(C 5 H 5 ) 2 } fragments placed in general positions (Fe1 and Fe2); one half of a centrosymmetric ferrocenyl {Fe(C 5 H 5 ) 2 } unit, the cyclopentadienyl ligand of which is disordered over two positions that are related by an ideal in-plane rotation of 36 • and show nearly equivalent population factors (Fe3); and one methanol molecule of crystallization ( Figure 2). The [SiW 12 O 40 ] 4− cluster shows the characteristic structure of the α-Keggin anion, which is constituted by four {W 3 O 13 } trimers composed each by three edge-sharing WO 6 octahedra. These trimers are linked to each other and to the central {SiO 4 } tetrahedron through corner-sharing in ideal T d symmetry. The central tetrahedron is disordered over two positions related by the centre of inversion on which the cluster is located, in such a way that a distorted {SiO 8 } cube with half-occupancies for the O sites is observed as a result.  [30]. In addition, Bond Valence Sum calculations [31] confirmed the highest oxidation state for all the tungsten atoms (W VI ), indicating that no reduction took place for any of the POM metal centres. All the Fe-C bond lengths, Fe···Cg(Cp) distances and torsion angles between cyclopentadienyl rings for each crystallographically independent ferrocenyl unit in 1 are listed in Table 1. The presence of five ferrocenyl groups per [SiW 12 O 40 ] 4− anion suggests different oxidation states for the iron centres belonging to the three crystallographically independent species that have been tentatively assigned as ferrocenium cations (Fe1 and Fe2) and ferrocene molecules (Fe3) in order to maintain the electroneutrality of the system. It is also worth mentioning that the two ferrocenium species in the organometallic sub-lattice exhibit different conformations: one of the cations (Fe1) displays an eclipsed configuration, whereas that of the second ferrocenium (Fe2) is staggered. spectroscopy because the fingerprint of both forms is substantially different in the 450-500 cm −1 region of the spectrum [29]. Unfortunately, the presence of the strong absorption bands of the [SiW12O40] 4− cluster below 1000 cm −1 and more specifically, of the broad signal F, corresponding to the (W-Oe-W) + (Si-Oc) combination (Oe: bridging O atom between edge-sharing W centres; Oc: central O atoms), makes impossible to perform such analysis in our case as it shadows this entire range.

Crystal Structure
Compound 1 crystallizes in the triclinic space group P-1 with the following content in the asymmetric unit: one half of the [α-SiW12O40] 4− cluster unit located on a centre of inversion; two ferrocenyl {Fe(C5H5)2} fragments placed in general positions (Fe1 and Fe2); one half of a centrosymmetric ferrocenyl {Fe(C5H5)2} unit, the cyclopentadienyl ligand of which is disordered over two positions that are related by an ideal in-plane rotation of 36° and show nearly equivalent population factors (Fe3); and one methanol molecule of crystallization ( Figure 2). The [SiW12O40] 4− cluster shows the characteristic structure of the α-Keggin anion, which is constituted by four {W3O13} trimers composed each by three edge-sharing WO6 octahedra. These trimers are linked to each other and to the central {SiO4} tetrahedron through corner-sharing in ideal Td symmetry. The central tetrahedron is disordered over two positions related by the centre of inversion on which the cluster is located, in such a way that a distorted {SiO8} cube with half-occupancies for the O sites is observed as a result.  [30]. In addition, Bond Valence Sum calculations [31] confirmed the highest oxidation state for all the tungsten atoms (W VI ), indicating that no reduction took place for any of the POM metal centres. All the Fe-C bond lengths, Fe···Cg(Cp) distances and torsion angles between cyclopentadienyl rings for each crystallographically independent ferrocenyl unit in 1 are listed in Table 1. The presence of five ferrocenyl groups per [SiW12O40] 4-anion suggests different oxidation states for the iron centres belonging to the three crystallographically independent species that have been tentatively assigned as ferrocenium cations (Fe1 and Fe2) and ferrocene molecules (Fe3) in order to maintain the electroneutrality of the system. It is also worth mentioning that the two ferrocenium species in the organometallic sub-lattice exhibit different conformations: one of the cations (Fe1) displays an eclipsed configuration, whereas that of the second ferrocenium (Fe2) is staggered.    In order to crystallographically discriminate between ferrocenium cations and ferrocene molecules upon close inspection of the bond lengths and analyse the different geometrical conformations they can adopt, a topological study was carried out using all of the entries deposited in the CSD that contain isolated, pristine ferrocenyl species. Geometrical parameters of crystallographically independent 90 ferrocene and 49 ferrocenium fragments belonging to 119 different crystal structures have been determined in the CSD database (last visit: June 2018; last update: February 2018), excluding powder structures and those containing disordered fragments. All the Fe-C bond lengths, Cg(Cp)···Cg(Cp) and Fe···Cg(Cp) distances, torsion angles between cyclopentadienyl rings and oxidation states for the iron centres have been compiled in Table A2 (Appendix B).
The scatter plot of the average Fe···Cg(Cp) distances versus Fe-C bond lengths is depicted in Figure 3. It is worth mentioning that Fe···Cg(Cp) distances increase linearly with the Fe-C bond lengths, as defined by the <C-Fe···Cg(Cp)> angle of about 34-36 • displayed by all the molecular units included in this search. All the ferrocenyl species can be graphically classified into two main groups depending on the oxidation state of the Fe atoms. The more stable Fe II state for ferrocene moieties (obeys the 18-electron rule) exhibits shorter Fe-C and Fe···Cg(Cp) distances in the ca. 2.00-2.06 Å and 1.60-1.66 Å range respectively, whereas longer bond lengths in the ca. 2.05-2.11 Å and 1.68-1.72 Å range are found for ferrocenium cations. According to this distribution, the fact that the Fe···Cg(Cp) distances lay above or below 1.67 Å can be regarded as a direct method to unequivocally distinguish the nature of ferrocenyl units as ferrocene species or ferrocenium cations. When it comes to the crystal structure of 1, two species belong to the latter group (Fe···Cg(Cp) = 1.70 Å) whereas the third is included in the former classification (Fe···Cg(Cp) = 1.66 Å). Therefore, we confirmed our initial assumption: Fe1 and Fe2 are ferrocenium cations, whereas Fe3 is a neutral ferrocene group.
It is well known that ferrocenyl species can adopt two extreme conformations depending on the relative position of the cyclopentadienyl rings: eclipsed with ideal D 5h symmetry and staggered with ideal D 5d symmetry. Both configurations are very common because the rotation barrier along the C 5 axis has been calculated to be as low as 0.9 ± 0.3 kcal mol −1 (≈4 kJ mol −1 ). [32] Analysis of the torsion angles between cyclopentadienyl rings determines the conformation of a given group that goes from 0 • in a totally eclipsed form to 36 • for a completely staggered configuration. Figure 4 displays the Fe···Cg(Cp) distance versus torsion-angle scatter plot for all the ferrocenyl units from the CSD database mentioned above. Considering an arbitrary criterion, torsion angles ranging from 0 to 6 • have been classified as eclipsed, whereas those from 30 to 36 • are staggered. The plot clearly shows that most of the species show extreme configurations, with less than 15% of the cases in intermediate states.
In the case of ferrocenes, this effect is even more pronounced. More than 90% of the structures exhibit extreme conformations and 2/3 of the cases are staggered. Focusing on the non-disordered moieties in 1, Fe1 is included within the group of eclipsed ferrocenium cations, whereas Fe2 is staggered. It is well known that ferrocenyl species can adopt two extreme conformations depending on the relative position of the cyclopentadienyl rings: eclipsed with ideal D5h symmetry and staggered with ideal D5d symmetry. Both configurations are very common because the rotation barrier along the C5 axis has been calculated to be as low as 0.9 ± 0.3 kcal mol -1 (≈4 kJ mol -1 ). [32] Analysis of the torsion angles between cyclopentadienyl rings determines the conformation of a given group that goes from 0° in a totally eclipsed form to 36° for a completely staggered configuration. Figure 4 displays the Fe···Cg(Cp) distance versus torsion-angle scatter plot for all the ferrocenyl units from the CSD database mentioned above. Considering an arbitrary criterion, torsion angles ranging from 0 to 6° have been classified as eclipsed, whereas those from 30 to 36° are staggered. The plot clearly shows that most of the species show extreme configurations, with less than 15% of the cases in intermediate states. In the case of ferrocenes, this effect is even more pronounced. More than 90% of the structures exhibit extreme conformations and 2/3 of the cases are staggered. Focusing on the non-disordered moieties in 1, Fe1 is included within the group of eclipsed ferrocenium cations, whereas Fe2 is staggered.  The crystal packing of 1 can be best described as an organometallic sub-lattice formed by ferrocenyl species that generate rectangular cavities of about 14 × 10 Å along the [100] direction where the [SiW12O40] 4− anions are hosted. Polyanions located in these cavities are linked to each other via weak OPOM···CMeOH-OMeOH···OPOM type bonds involving methanol solvent molecules and surface O atoms from POM clusters ( Figure 5). Pairs of Fe1 ferrocenium columns running along the [011] direction interact with each other in an anti-parallel manner through π-π stacking as can be viewed in Figure 6. The three dimensional network of the organometallic sub-lattice is completed by the other ferrocenyl units establishing weak T-type π-π interactions: Fe3 groups link Fe1 columns in the yz plane, whereas Fe2 units play a similar role along the crystallographic x axis. Geometrical parameters of the π-π interactions established between cyclopentadyenil rings are compiled in Table 2. direction interact with each other in an anti-parallel manner through π-π stacking as can be viewed in Figure 6. The three dimensional network of the organometallic sub-lattice is completed by the other ferrocenyl units establishing weak T-type π-π interactions: Fe3 groups link Fe1 columns in the yz plane, whereas Fe2 units play a similar role along the crystallographic x axis. Geometrical parameters of the π-π interactions established between cyclopentadyenil rings are compiled in Table 2. Additionally, inorganic and organometallic components interact through C Fc -H···O POM type contacts established between cyclopentadienyl rings from ferrocenyl groups and O atoms from the POM surface. Bond lengths and angles of such supramolecular interactions are summarized in Table 3.  Table 2. Geometrical parameters (Å, °) of the intermolecular π-π interactions in 1. Cpi: i cyclopentadienyl rings defined in Table 2. Cg(Cp)···plane: distance from one centroid to the plane containing the other ring. ANG: dihedral angle between planes containing both rings. Cg(Cp)··· Cg(Cp): distance between centroids. Slippage: distance between one centroid and its perpendicular projection to the plane containing the second ring. Symmetry codes:

π-π Interactions
(iii) −1 + x, y, z; (iv) 1 + x, y, z.  Cpi: i cyclopentadienyl rings defined in Table 2. Cg(Cp)···plane: distance from one centroid to the plane containing the other ring. ANG: dihedral angle between planes containing both rings. Cg(Cp)··· Cg(Cp): distance between centroids. Slippage: distance between one centroid and its perpendicular projection to the plane containing the second ring. Symmetry codes:

Diffuse Reflectance UV-Vis Spectroscopy
To evaluate the electronic properties of the title compound, it has been analysed by diffuse reflectance UV-Vis spectroscopy. The spectra registered for a powdered crystalline sample of 1, the K4[SiW12O40]·17H2O POM precursor, commercial ferrocene and the FcPF6 salt prepared for comparative purposes following reported procedures [33] are displayed in Figure 7. The electronic spectrum of the POM precursor shows a strong absorption band centred in the UV region (below 300 nm) that extends up to 375 nm and it is associated with the O→W ligand-to-metal charge transfer (LMCT) transition of the plenary inorganic framework. In the case of ferrocene, the band at ca. 340 and the broad adsorption in the blue region that extends from 360 to 580 nm (centred at ca. 450 nm)

Diffuse Reflectance UV-Vis Spectroscopy
To evaluate the electronic properties of the title compound, it has been analysed by diffuse reflectance UV-Vis spectroscopy. The spectra registered for a powdered crystalline sample of 1, the K 4 [SiW 12 O 40 ]·17H 2 O POM precursor, commercial ferrocene and the FcPF 6 salt prepared for comparative purposes following reported procedures [33] are displayed in Figure 7. The electronic spectrum of the POM precursor shows a strong absorption band centred in the UV region (below 300 nm) that extends up to 375 nm and it is associated with the O→W ligand-to-metal charge transfer (LMCT) transition of the plenary inorganic framework. In the case of ferrocene, the band at ca. 340 and the broad adsorption in the blue region that extends from 360 to 580 nm (centred at ca. 450 nm) have been attributed to Fe (e 2g ) → Cp (e 1g ) charge transfer and symmetry-forbidden Fe (a 1g ) → Fe (e 1g ) transitions, respectively [34]. For its oxidized ferrocenium form in FcPF 6 , the continuous adsorption below 650 nm is in the origin of its dark blue colour. The O→W LMCT and adsorption bands belonging to both ferrocene and ferrocenium species can also be observed in the spectrum of 1. However, the band at lower energy (ca. 640 nm) is exclusive for 1 and may be ascribed to an intermolecular charge-transfer transition between ferrocenyl donors and POM acceptors [19,21] since none of its constituents exhibit any absorption in this range. have been attributed to Fe (e2g) → Cp (e1g) charge transfer and symmetry-forbidden Fe (a1g) → Fe (e1g) transitions, respectively [34]. For its oxidized ferrocenium form in FcPF6, the continuous adsorption below 650 nm is in the origin of its dark blue colour. The O→W LMCT and adsorption bands belonging to both ferrocene and ferrocenium species can also be observed in the spectrum of 1. However, the band at lower energy (ca. 640 nm) is exclusive for 1 and may be ascribed to an intermolecular charge-transfer transition between ferrocenyl donors and POM acceptors [19,21] since none of its constituents exhibit any absorption in this range.

57 Fe Mössbauer Spectroscopy
Preliminary Electronic Spin Resonance spectroscopy studies were conducted for 1 that proved how ferrocenium salts are silent at room temperature due to the short T1 relaxation time. The unpaired electron is not in the Fe III centre and it seems to get delocalized all over the aromatic system. [35] Therefore, we decided to make use of 57 Fe Mössbauer spectroscopy, because it allows for determining among others the oxidation and spin states of iron centres, as well as their symmetry, magnetic interactions and chemical environment. The technique is based on the absorption of energetically slightly different γ rays generated by Doppler effect in a radioactive source moving at speeds of several mm/s, in such a way that different absorption peaks are registered and their position is defined by the δ isomer shift. Nuclei in states with non-spherical charge distribution produce an asymmetrical quadrupolar electric field, which splits the nuclear energy levels. These are quantified by their quadrupolar splitting of the signals (Δ). Additionally, Zeeman splitting can be generated by magnetic coupling between centres. [36] Figure 8 displays the experimental 57 Fe Mössbauer spectrum for a powdered sample of 1, together with the curve fits for each different iron-containing chemical species present in its crystal structure. All the experimental results are summarized in Table 4. The spectrum has been fitted to two singlets attributed to iron nuclei with a very similar chemical environment (δ = 0.27 and 0.44 mm/s) and a wide doublet with a quadrupolar splitting of 1.17 mm/s and a larger isomer shift of 0.71 mm/s. It is worth highlighting the absence of any extra peak, which indicates that paramagnetic centres are magnetically well isolated. The presence of both singlets compares well with the signals arising from ferrocenium units (Fe III centres with spherical charge distribution) displaying isomer shifts that typically range from 0.30 to 0.65 mm/s. [37] Conversely, doublets with a chemical shift in the 0.50-1.0 range and quadrupolar splitting values of ca. 1-3 mm/s could be expected for ferrocene Fe II nuclei. [38] These results are in line with the two crystallographically independent ferrocenium groups and the additional ferrocene molecule determined in the crystal structure of 1. In fact, the

57 Fe Mössbauer Spectroscopy
Preliminary Electronic Spin Resonance spectroscopy studies were conducted for 1 that proved how ferrocenium salts are silent at room temperature due to the short T1 relaxation time. The unpaired electron is not in the Fe III centre and it seems to get delocalized all over the aromatic system. [35] Therefore, we decided to make use of 57 Fe Mössbauer spectroscopy, because it allows for determining among others the oxidation and spin states of iron centres, as well as their symmetry, magnetic interactions and chemical environment. The technique is based on the absorption of energetically slightly different γ rays generated by Doppler effect in a radioactive source moving at speeds of several mm/s, in such a way that different absorption peaks are registered and their position is defined by the δ isomer shift. Nuclei in states with non-spherical charge distribution produce an asymmetrical quadrupolar electric field, which splits the nuclear energy levels. These are quantified by their quadrupolar splitting of the signals (∆). Additionally, Zeeman splitting can be generated by magnetic coupling between centres. [36] Figure 8 displays the experimental 57 Fe Mössbauer spectrum for a powdered sample of 1, together with the curve fits for each different iron-containing chemical species present in its crystal structure. All the experimental results are summarized in Table 4. The spectrum has been fitted to two singlets attributed to iron nuclei with a very similar chemical environment (δ = 0.27 and 0.44 mm/s) and a wide doublet with a quadrupolar splitting of 1.17 mm/s and a larger isomer shift of 0.71 mm/s. It is worth highlighting the absence of any extra peak, which indicates that paramagnetic centres are magnetically well isolated. The presence of both singlets compares well with the signals arising from ferrocenium units (Fe III centres with spherical charge distribution) displaying isomer shifts that typically range from 0.30 to 0.65 mm/s. [37] Conversely, doublets with a chemical shift in the 0.50-1.0 range and quadrupolar splitting values of ca. 1-3 mm/s could be expected for ferrocene Fe II nuclei. [38] These results are in line with the two crystallographically independent ferrocenium groups and the additional ferrocene molecule determined in the crystal structure of 1. In fact, the relative atomic ratio for Fe1 III :Fe2 III :Fe3 II centres was calculated to be 2:2:1 from the integration of the area delimiting each of the sub-spectra, in good agreement with the molecular formula of 1.
relative atomic ratio for Fe1 III :Fe2 III :Fe3 II centres was calculated to be 2:2:1 from the integration of the area delimiting each of the sub-spectra, in good agreement with the molecular formula of 1.

Materials and Methods
The K4[α-SiW12O40]·17H2O precursor was synthesized following reported procedures [39] and identified by infrared spectroscopy (FT-IR). All other reagents were purchased from commercial sources and used without further purification. The FT-IR spectra were recorded as KBr pellets on a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Kyoto, Japan) in the 400−4000 cm -1 spectral range. The carbon and hydrogen contents were determined on a Perkin Elmer 2400 CHN analyser (PerkinElmer Inc., Waltham, MA, USA), whereas metal analyses (Fe) were performed on a Q-ICP-MS ThermoXSeries II analyser (Fisher Scientific International, Inc, Pittsburgh, PA, USA). Diffuse Reflectance studies were carried out on a UV-Vis-NIR Varian Cary 500 spectrophotometer (Varian, Palo Alto, CA, USA). The Mössbauer spectra were recorded at room temperature in transmission geometry using a conventional constant-acceleration spectrometer with a 57 Co-Rh source calibrated with a Fe sheet (δ = −0.11 mm s −1 ). The fitting was performed using the NORMOS program (Universität Dortmund, Dortmund, Germany) [40].

Materials and Methods
The K 4 [α-SiW 12 O 40 ]·17H 2 O precursor was synthesized following reported procedures [39] and identified by infrared spectroscopy (FT-IR). All other reagents were purchased from commercial sources and used without further purification. The FT-IR spectra were recorded as KBr pellets on a Shimadzu FTIR-8400S spectrophotometer (Shimadzu, Kyoto, Japan) in the 400−4000 cm −1 spectral range. The carbon and hydrogen contents were determined on a Perkin Elmer 2400 CHN analyser (PerkinElmer Inc., Waltham, MA, USA), whereas metal analyses (Fe) were performed on a Q-ICP-MS ThermoXSeries II analyser (Fisher Scientific International, Inc, Pittsburgh, PA, USA). Diffuse Reflectance studies were carried out on a UV-Vis-NIR Varian Cary 500 spectrophotometer (Varian, Palo Alto, CA, USA). The Mössbauer spectra were recorded at room temperature in transmission geometry using a conventional constant-acceleration spectrometer with a 57 Co-Rh source calibrated with a Fe sheet (δ = −0.11 mm s −1 ). The fitting was performed using the NORMOS program (Universität Dortmund, Dortmund, Germany) [40].

X-ray Crystallography
Crystallographic data for compound 1 are summarized in Table 5. Intensity data were collected at 100(2) K on an Oxford Diffraction Xcalibur (Rigaku Oxford Diffraction, Oxford, UK) single-crystal diffractometer (Mo Kα radiation, λ = 0.71073 Å) fitted with a Sapphire charge-coupled device detector. The data collection, unit cell determination, intensity data integration, routine corrections for Lorentz and polarization effects and analytical absorption correction with face indexing were processed using the CrysAlis software package (Rigaku Oxford Diffraction, Oxford, UK) [41]. The structure was solved using direct methods as implemented in SIR-2004 (Istituto di Cristallografia, CNR, Roma, Italy) [42] and refined by full-matrix least-squares analysis with the SHELXL-97 program (University of Göttingen, Göttingen, Germany) [43]. Heavy atoms (W, Fe, Si) were located in the initial resolution and the remaining light atoms (O, C) were located from successive Fourier maps. The C atoms from the ferrocene unit (Fe3) were disordered over two positions with 50% population factors. Thermal vibrations were treated anisotropically and those from non-disordered cyclopentadienyl C atoms were restrained to be similar to each other using default DELU commands. Thermal ellipsoids belonging to disordered cyclopentadienyl C atoms were restrained using more restrictive ISOR commands. For the All H atoms in the methanol molecules and cyclopentadienyl ligands were included in calculated positions and refined as riding atoms using default SHELXL parameters. Final geometrical calculations were carried out with PLATON (Utrecht University, Utrecht, The Netherlands) [44] as integrated in the WinGX (University of Glasgow, Glasgow, UK) crystallographic software package [45]. CCDC-1878742 (1) contains 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.  (1) constitutes the first example in the literature in which ferrocenium (Fe III ) and ferrocene (Fe II ) species coexist in the structure of a polyoxometalate-based salt. The asymmetric unit of 1 displays two crystallographically independent ferrocenium cations (one in an eclipsed D 5h conformation and the other one in staggered D 5d ) and one half of a neutral ferrocene molecule disordered over two positions with similar population factors. The crystal packing of 1 can be best described as an organometallic sub-lattice of ferrocenyl-type species linked by a network of π-π interactions that generates rectangular cavities in which strings of Keggin anions and methanol molecules connected to each other via weak O POM ···C MeOH -O MeOH ···O POM interactions, are hosted. The charge-transfer nature of the salt has been assessed by solid-state diffuse reflectance UV-Vis spectroscopy and 57 Fe Mössbauer spectroscopy have proved to be a very useful tool to confirm the presence of magnetically isolated Fe III /Fe II centres in a 4:1 ratio. Finally a thorough topological study on the pristine ferrocenyl species deposited in the CSD led us to conclude that (1) ferrocenyl groups tend to present extreme conformations; and (2) close inspection of geometrical parameters allows ferrocene neutral molecules and ferrocenium cations to be easily distinguished, because the later exhibit significant longer Fe···Cp distances (above or below 1.67 Å). For the near future, we plan to react transition metal-or lanthanide-containing POMs showing accessible centres with ferrocene derivatives with coordinating ability (e.g., ferrocene carboxylate, ferrocene-appended 2,2'-bipyrydine ligands) with the aim of studying the effect on their electronic properties (i.e., redox properties, charge-transfer processes). Acknowledgments: Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged.

Conflicts of Interest:
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript and in the decision to publish the results.