Fe^III ₄₈‐Containing 96‐Tungsto‐16‐Phosphate: Synthesis, Structure, Magnetism and Electrochemistry

Abstract

The 48‐Fe^III‐containing 96‐tungsto‐16‐phosphate, [Fe^III ₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]³⁶⁻ (Fe₄₈), has been synthesized and structurally characterized. This polyanion comprises eight equivalent {Fe^III ₆P₂W₁₂} units that are linked in an end‐on fashion forming a macrocyclic assembly that contains more iron centers than any other polyoxometalate (POM) known to date. The novel Fe₄₈ was synthesized by a simple one‐pot reaction of an {Fe₂₂} coordination complex with the hexalacunary {P₂W₁₂} POM precursor in water. The title polyanion was characterized by single‐crystal XRD, FTIR, TGA, magnetic and electrochemical studies.

A very large, discrete 96‐tungsto‐16‐phosphate containing 48 Fe^III centers, [Fe^III ₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]³⁶⁻ (Fe₄₈), has been synthesized and structurally characterized. Polyanion Fe₄₈ comprises eight equivalent {Fe₆P₂W₁₂} units which are linked end‐on to form a macrocyclic assembly containing more iron centers than any other polyoxometalate known to date (see figure).

Polyoxometalates (POMs) are discrete, anionic metal oxides of early d‐block elements in high oxidation states with edge‐ or corner‐shared MO₆ octahedra (M = W^VI, Mo^VI, V^V, Nb^V and Ta^V) as building blocks. ^[1] Classical POMs such as the Keggin (e.g. [PW₁₂O₄₀]³⁻) or Well‐Dawson (e.g. [P₂W₁₈O₆₂]⁶⁻) ions allow to remove one or more MO₆ octahedra by controlled base hydrolysis, resulting in vacant (lacunary) derivatives, which can be considered as inorganic polydentate ligands. Such lacunary POMs can incorporate one or more transition‐metal, lanthanide, and actinide ions, resulting in a large variety of metal‐oxo clusters of various shape, size and composition, leading to a multitude of interesting physicochemical properties, which are of interest in catalysis, magnetism, biomedicine and material science.[ ² , ³ ]

The number of 3d metal‐containing POMs is large and includes some Fe^III‐containing species. ^[4] The multilacunary Well‐Dawson ion [H₂P₂W₁₂O₄₈]¹²⁻ ({P₂W₁₂}) is a promising POM ligand for the incorporation of several metal ions, due to the six vacant sites in the polyanion. In 2005 Gouzerh's group reported that {P₂W₁₂} can incorporate 9 Fe³⁺ ions ([H₄P₂W₁₂Fe₉O₅₆(OAc)₇]⁶⁻) and the same group also made a tetrameric derivative containing 28 Fe³⁺ ions ([H₅₆P₈W₄₈Fe₂₈O₂₄₈]²⁸⁻. ^[5] In 2010 our group reported the cyclic, mixed‐valent 8‐vanadium‐substituted [Rb₃⊂{V^VV^IV ₃O₇(H₂O)₆}₂{H₆P₆W₃₉O₁₄₇(H₂O)₃}]¹⁵⁻. ^[6a] In 2011 Kögerler's group reported the 40‐manganese(III)‐containing polyanion [{(Mn^III ₄P₂W₁₄O₆₀)(Mn^III ₃P₂W₁₅O₅₈)₂}₄(P₈W₄₈O₁₈₄)]¹⁴⁴⁻, using the well‐known mixed‐valence ‘Mn₁₂‐acetate’ [Mn^IV ₄Mn^III ₈O₁₂(O₂CMe)₁₆(H₂O)₄] as precursor. ^[6b] In 2014 our group reported the 4‐oxalato‐6‐titanium‐containing [Ti₆(C₂O₄)₄P₄W₃₂O₁₂₄]²⁰⁻. ^[6c] In 2016 Mizuno's group reported the 24‐manganese(III)‐containing species [{P₂W₁₂O₄₈Mn^III ₄(C₅H₇O₂)₂(CH₃CO₂)}₆]⁴²⁻ (using Mn^III(acac)₃ as precursor). ^[6d]

All these compounds are based on {P₂W₁₂} as POM precursor, which confirms that multilacunary POM ligands can stabilize high‐nuclearity 3d metal‐oxo clusters. However, the solution chemistry of {P₂W₁₂} is complicated as it tends to transform easily to Wells‐Dawson derivatives with fewer lacunarity sites. This is probably the main reason why the total number of known 3d transition metal derivatives containing the {P₂W₁₂} fragment is rather small.

Most iron‐containing polyoxotungstates are based on high‐spin Fe^III ions (d ⁵, s=5/2) with five unpaired electrons and if two or more of these ions are present being linked by two bonds then antiferromagnetic exchange interactions are usually dominant. ^[7] On the other hand, if polynuclear iron‐oxo clusters inside POMs do have a large spin ground state (S) and uniaxial magnetic anisotropy (D) then they may behave as single molecule magnets (SMM). Mialane's group demonstrated SMM behavior for [Fe₄(H₂O)₂(FeW₉O₃₄)₂]¹⁰⁻, ^[8a] which had originally been prepared by Krebs’ group. ^[8b]

As part of our ongoing research on transition metal‐containing POMs, we decided to study further the interaction of the hexalacunary POM precursor {P₂W₁₂} with preformed multinuclear and water‐soluble coordination complexes, with the goal to replace the organic ligands by inorganic POM units. Following such strategy, we report herein on the synthesis of the gigantic, macrocyclic 48‐Fe^III‐containing 96‐tungsto‐16‐phosphate, [Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]³⁶⁻ (Fe₄₈), which was isolated as the hydrated potassium salt K₃₆[Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]⋅400H₂O (K‐Fe₄₈). Reaction of the formally hexavacant 12‐tungsto‐2‐phosphate POM precursor [H₂P₂W₁₂O₄₈]¹²⁻ ({P₂W₁₂}) with the 22‐iron(III)‐containing coordination complex [Fe₂₂O₁₄(OH)₃(O₂CMe)₂₁(mda)₆]⋅(ClO₄)₂ (mdaH₂=N‐methyldiethanolamine) ^[9] ({Fe₂₂}, see also Figure S1) in water (pH 5.4) resulted in the formation of Fe₄₈, which was characterized in the solid state by single‐crystal XRD, IR, TGA, elemental analysis, as well as both solid‐ and solution‐state electrochemical studies. Single‐crystal X‐ray analysis revealed that K‐Fe₄₈ crystallizes in the triclinic system with space group P−1. The crystallographic parameters are shown in the Supporting Information (Table S1).

Polyanion Fe₄₈ consists of eight equivalent {Fe₆P₂W₁₂} Dawson‐type subunits which are linked to each other via Fe−O−Fe/W bonds, forming a cyclic assembly with an inner dimension of ca. 24 Å × 13 Å (Figure 1, upper). The ring is not flat with the Dawson units pointing alternatingly up and down, resembling a chair conformation of cyclooctane with an idealized D₂ point group symmetry, where both enantiomers are present in the crystal lattice due to the centrosymmetric space group. The inter‐Dawson linkage modes at the two caps of each {Fe₆P₂W₁₂} unit are entirely different. At one end there are two equivalent Fe‐μ₃O‐Fe/W bonds, whereas at the other end the linkage is accomplished by three Fe−OH−Fe bonds, resulting in the formation of a Fe₃(OH)₉ trigonal prism (see Figure S2). This alternating connectivity mode is preserved throughout the entire macrocycle of Fe₄₈. The oxidation state of +3 for iron and monoprotonation of the bridging Fe−OH−Fe oxygens were confirmed by bond valence sum (BVS) calculations (see Tables S2–S3). ^[10] Interestingly, the Fe₃(OH)₉ trigonal‐prismatic motif has been observed before in Keggin dimers of the type [Fe₆(OH)₃(GeW₉O₃₄(OH)₃)₂]¹¹⁻. ^[11]

Figure 1.

Polyhedral/ball‐and‐stick representation of Fe₄₈, showing a top view (upper), a side‐view (middle) and a combined wireframe/ball‐and‐stick representation highlighting the 48 Fe^III centers (lower). The pink octahedra represent WO₆.

The Fe₄₈ core is of interest for its magnetic properties, as this structure presents an intermediate between an infinite, 2‐dimensional, spin‐frustrated, triangular lattice and a molecular complex with a well‐defined spin ground state. Spin‐frustrated triangular lattices show a variety of phase transitions and magnetic structures.[ ^2e , ¹² , ¹³ ] Detailed magnetic susceptibility measurements over a range of 1.8–300 K and magnetization measurements in an applied field of 0–7 T were performed. Figure 1 (lower) shows the spin topology of the Fe₄₈ core. The 48 exchange‐coupled Fe^III ions (S=5/2) form a two‐dimensional ellipsoidal lattice comprising eight spin triangles and four trigonal‐prismatic lattice units with 30 slightly different exchange interactions. The magnetism of the Fe₄₈ core unit is approximated by competing exchange interactions of localized magnetic moments in a quasi‐continuum limit. ^[2e] Figure 2 shows the temperature dependence of the magnetic susceptibility. At 300 K, the χ_mT product is 85.79 emu Kmol⁻¹. The contribution of temperature‐independent paramagnetism (TIP) is considered and discussed below. The χ_mT value shows a gradual decrease to a value of 7.59 emu Kmol⁻¹ at 1.8 K. This large decrease in χ_mT with decreasing temperature is indicative of the presence of antiferromagnetic exchange interactions. Because of the exchange coupling between the Fe centers, one can treat the Fe₄₈ polyanion in the high temperature range (>50 K) as a system of exchange coupled Fe^III ions with an averagely small coupling constant. It is shown in Figure 2 that the χ_mT above 50 K can be well fitted by the following equation [Eq. 1]:

$${\displaystyle {\chi }_{m}T=\left(\frac{\left(g^{2}S\left(S+1\right)\right)}{\left(T-\theta \right)}{+\chi }_{TIP}\right)T}$$ (1)

Figure 2.

Plot of ${\chi }_{m}T$ for a polycrystalline sample of K‐Fe₄₈ measured in an applied magnetic field of 0.01 T. The solid line is a fit to Eqn. (2).

in which θ is the antiferromagnetic coupling parameter and χ ^TIP is the temperature independent susceptibility. Taking g=2 and S=5/2, the fitting gives a θ of −232 K and a χ ^TIP of 0.071 emu mol⁻¹. As the 48 Fe^III centers share a total of 60 exchange pathways (Figure 2), the averaged exchange coupling constant is estimated to be J _av=−7.07 K considering [Eq. 2]:

$${\displaystyle \theta =\frac{\left(z\mathrm{J}S\right(S+1)}{3k_B}}$$ (2)

in which z is the number of nearest neighbors of a given Fe center and is 60/48 on average for the Fe₄₈ spin cluster.

The title polyanion Fe₄₈ contains two types of redox‐active metals allowing for electron transfer, namely W^VI/W^V and Fe^III/Fe^II, which renders electrochemical experiments attractive. The electrochemical diagnostic experiments described here include both (i) conventional voltammetry of K‐Fe₄₈ dissolved in aqueous medium (see Supporting Information) and (ii) microelectrode based solid‐state‐type methodology,[ ¹⁴ , ¹⁵ , ¹⁶ ] to account for possible hydrolytic decomposition of the polyanion.

Due to a possible decomposition of Fe₄₈ in aqueous medium we decided to also perform microelectrode‐based solid‐state electrochemistry on K‐Fe₄₈. The diagnostic experiments were aimed at verifying the feasibility of fast and reversible electroreduction of W^VI to W^V, in addition to probing the redox transitions of the Fe^III ions in Fe₄₈.

Obviously, electron transfer between W^VI and W^V or Fe^III and Fe^II could be operative in Fe₄₈. It is apparent from the voltammetric characteristics shown in Figure 3 A that the reduction of the W^VI ionic sites (to W^V) occurs at potentials more negative than −0.2 V, whereas the Fe^III/Fe^II redox transitions become operative at potentials higher than 0.2 V. The appearance of two peaks at potentials of ca. 0.3 and 0.7 V, respectively, may reflect the existence of two types of iron sites, possibly those bridged by oxo (Fe‐O‐W) or hydroxo (Fe−OH−Fe) ligands. It is noteworthy that, despite the scan rate being as low as 10 mV s⁻¹, the voltammetric pattern of Figure 3 A does not show plateau currents typically observed when microelectrodes are used to study species dissolved in solution. Simply, under the present solid‐state‐type conditions, the effective diffusion (charge transport) coefficients are lower than for solution species; consequently, mixed linear/spherical, rather than typical spherical diffusional patterns are produced here. When the experiment of Figure 3 A was repeated at a faster scan rate such as 800 mV s⁻¹, the overall charge propagation mechanism involved linear diffusion, and the voltammetric behavior of polyanion Fe₄₈ started to develop voltammetric peaks both during reduction and oxidation (Figure 3 B). The fact that (at a scan rate as high as 800 mV s⁻¹) the Fe^III/Fe^II peaks are retained and both the cathodic and anodic peaks for W^VI/W^V redox transitions can be observed, implies fast electron transfers between tungsten ionic sites and the presence of electrochemically well‐behaved iron ionic sites, respectively.

Figure 3.

Microelectrode‐based cyclic voltammetry of Fe₄₈ recorded at (A) 10 mVs⁻¹ and (B) 800 mVs⁻¹. The experiment was performed using a solid‐state cell with a planar three‐electrode configuration under nitrogen atmosphere. Microdisk diameter, 30 μm.

In summary, we have synthesized and structurally characterized the polyanion Fe₄₈, which contains 48 Fe^III centers and hence the largest number of iron centers incorporated in any POM known to date. The title polyanion Fe₄₈ was prepared by a simple one‐pot reaction of the polynuclear coordination complex {Fe₂₂} with the hexalacunary {P₂W₁₂} polyoxometalate (POM) precursor in water. The title compound was characterized by standard analytical techniques in the solid state and in solution. Magnetic studies indicate that the 48 Fe^III centers in Fe₄₈ share a total of 60 exchange pathways and the averaged exchange coupling constant is estimated to be J _av=−7.07 K. No long‐range ferro or antiferromagnetic ordering is observed from 300 K down to 1.2 K. The title polyanion Fe₄₈ is electrochemically well‐behaved, particularly during microelectrode‐based solid‐state‐type voltammetric experiments. The system exhibits redox transitions implying electroactivity of Fe^III and W^VI ionic sites. It is noteworthy that the W^VI/W^V redox transitions are fast and reversible even at fast scan rates (800 mV s⁻¹). Under such conditions, the existence of two different structural types of electroactive iron sites can also be postulated.

Experimental Section

Synthesis of K₃₆[Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]⋅400 H₂O (K‐Fe₄₈)

The known coordination complex [Fe₂₂O₁₄(OH)₃(O₂CMe)₂₁(mda)₆]⋅ (ClO₄)₂ ({Fe₂₂}) (mdaH₂=N‐methyldiethanolamine) ^[9] (0.319 g, 0.076 mmol) was dissolved in H₂O (40 mL) and then K₁₂[H₂P₂W₁₂O₄₈]⋅24 H₂O (0.200 g, 0.051 mmol) ^[17] was added. To this reaction mixture solid KCl (0.055 g 0.737 mmol) was added and the resulting turbid solution was stirred for 1 h at 70 °C and the pH was 5.4. Then the solution was allowed to stand at room temperature until the brown solid had settled at the bottom of the beaker, which was then removed by centrifugation and filtration. The solution (pH 5.4) was kept in an open vial at room temperature to allow for slow evaporation. After ca two months a yellow crystalline product started to appear. Evaporation was allowed to continue until about half the solvent had evaporated. The solid product was then collected by filtration and air dried. Yield 26 mg (11 %, based on W).

IR (2 % KBr pellet): ṽ=3415 (s), 1617 (s), 1462 (m), 1384 (m) 1086 (s), 1063 (s), 999 (w), 943 (s), 909 (s), 827 (sh), 777 (m), 705 (m), 616 (w), 556 (w), 485 (w), 418(w) cm⁻¹. Elemental analysis (%) for K₃₆[Fe₄₈(OH)₇₆(H₂O)₁₆(HP₂W₁₂O₄₈)₈]⋅400H₂O (K‐Fe₄₈), calcd: K 3.79, P 1.33, Fe 7.21, W 47.48; found K 3.48, P 1.44, Fe 7.76, W 46.84. Elemental analysis was performed at CREALINS (Villeurbanne, France).

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

J. Goura, B. S. Bassil, J. K. Bindra, I. A. Rutkowska, P. J. Kulesza, N. S. Dalal, U. Kortz, Chem. Eur. J. 2020, 26, 15821.