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DOI: 10.1039/D2DT02922G (Communication) Dalton Trans., 2023, 52, 546-550

Tris-decorated multi-iron polyoxotungstates†

Natalya V. Izarova *ab, Fabian Faassen ab and Paul Kögerler *ab
aInstitute of Inorganic Chemistry, RWTH Aachen University, D-52074 Aachen, Germany. E-mail: natalya.izarova@ac.rwth-aachen.de; paul.koegerler@ac.rwth-aachen.de
bJülich-Aachen Research Alliance (JARA-FIT) and Peter Grünberg Institute – PGI 6, Forschungszentrum Jülich, D-52425 Jülich, Germany

Received 6th September 2022 , Accepted 9th December 2022

First published on 12th December 2022

Abstract

Solution-stable tris(hydroxymethyl)aminomethane-functionalized FeIII-containing polyoxotungstates exhibit an unusual anchoring mode of triol moieties, with one –NH2 and one –CH2OH group remaining accessible for post-functionalization or chemisorption. The redox-active title compounds have been isolated under unusually mild reaction conditions and characterized in the solid state and in aqueous solutions.

Deposition of transition metal complexes on various conductive and non-conductive surfaces is one of the crucial steps for the design of molecular electronic devices, molecular sensors and heterogeneous catalysts.1 Polyoxotungstates (POTs) [XxWyOz]n− with more than ten W centers represent one especially promising class of highly stable molecules in this context as they exhibit a multitude of WVI/V-based redox processes with virtually no structural changes combined with nanosizes, multidentate O-donor ligand functionality that enables the integration of a broad range of transition and rare earth heterometals into robust POT scaffolds,2 their post-functionalization and bi-functionalization with various organo groups3 and prominent catalytic activity in chemical transformations.4

The possibility of well-ordered patterning of POTs on various surfaces has already been demonstrated.5 Here, covalent surface grafting of polyanions via terminal functional groups (for example, –OH, –SH, –NH2, N2+, etc.) offers much higher control over precise POT orientation on a substrate and the reproducibility of electrical contact modes in molecular electronic devices in comparison with physisorption.5,6 These reactive groups could be typically introduced into POTs by attaching organo(bis)phosphonate and -arsonate, siloxane, organotin or alkoxide ligands either on active sites of POTs or on vacant coordination sites of incorporated heterometals.3

Among these ligands, RC(CH2OH)3-type triol species could be especially relevant as they offer a sufficiently stable and moderately flexible connection to a POT via three hydroxymethyl groups and a functional group R (R = –NH2, –CH2OH, –NHCH2COOH, –pyridine) that commonly remains terminal and allows subsequent post-functionalization of the polyanion (e.g. with photosensitive moieties)7 or its covalent grafting on a suitable substrate.8 Some triol-functionalized POMs have already been demonstrated to exhibit broad application perspectives in (photo)catalysis (e.g. for oxidative desulfurization), photophysics, biomedicine, energy storage and the design of functional materials.9

Until now functionalization propensity with triol ligands was mainly studied for the relatively simple highly stable polyoxometalates (POMs) of Anderson–Evans [XM6O24]Z− (X = ZnII, CrIII, MnIII/IV, etc.; M = MoVI, WVI), Lindquist [M6O19]Z− (M = VIV/V and VIV/V/TiIV, VIV/V/MoVI) and Wells–Dawson [VIV/V3P2W15O62]Z− structural types.9 Moreover, very recently the combination of all the three triol-decorated POM archetypes in one hybrid assembly has been achieved.10 In all those species the triol ligands are symmetrically anchored on the POM surface via their three –CH2OH groups. Examples of triol-functionalized Anderson–Evans and Lindquist polyanions are mainly based on polyoxomolybdates and polyoxovanadates, respectively, while triol-functionalized POTs still remain scarce.

Triol-functionalization of POMs of other structural types is rather rare and is represented e.g. by a series of [Ni6(μ3-OH)3(H2O)6(en)3(B-α-PW9O34)] POTs (en = ethylenediamine) capped by various triol ligands,11 including a cubic POM–organic framework {[Ni6(en)3(Tris)(1,3-bdc)1.5(B-α-PW9O34)]8} (Tris = tris(hydroxymethyl)aminomethane) possessing large cages where the interconnection of Tris-functionalized units is provided via 1,3-benzenedicarboxylate moieties (1,3-bdc).12 Another example is a smaller [WVI2O6(Tris)2]2− POT, where Tris ligands condense directly to WVI centers, which exhibits a unique connection mode via one –CH2OH and the –NH2 group acting in this POT as a primary amine.13

Herein we report the triol-functionalized FeIII-POTs [α-P2W15O56FeIII3{H3NC(CH2OH)(CH2O)2}3]6− (1) and [(α-P2W15O56)2FeIII6O2{H3NC(CH2OH)(CH2O)2}4]14− (2), where the Tris ligands are anchored on the POMs in an unprecedented binding mode involving only two hydroxomethyl groups, leaving one of the three reactive CH2OH groups and the NH2 group uncoordinated and thus accessible for further functionalization.

Polyanions 1 and 2 were prepared in a reaction of [α-P2W15O56]12− and FeIII ions in 1 M Tris buffer (pH 8.0), and were isolated as the hydrated sodium/tris(hydroxymethyl)-ammoniummethane salts Na0.5{H3NC(CH2OH)3}5.51·5H2O·0.5{H3NC(CH2OH)3}Cl (NT-1) and Na{H3NC(CH2OH)3}132·20H2O·{H3NC(CH2OH)3}Cl (NT-2) by slow evaporation of the reaction solutions. We noted that the reaction conditions for the preparation of NT-1 and NT-2 (heating at around 70 °C for 2 h) are surprisingly mild in comparison with the synthetic procedures for other triol-functionalized POMs that mostly require hydrothermal conditions.9 Furthermore, the reaction is relatively pH-sensitive as analogous reactions in 1 M Tris buffers at pH 7.4 and 9.1 did not yield any similar crystalline products.

Yellow diamond-shaped crystals of NT-1 appear first after several days of evaporation, while orange block-shaped crystals of NT-2 start to form within one to two weeks and have to be collected within 1–3 days after their appearance. Orange block-shaped crystals of NT-2 start to form soon thereafter with a decrease in the solution volume and an increase in its concentration. Initially they appear in a mixture with NT-1 and this crystalline mixture has to be filtered off several times in order to isolate a limited amount of pure NT-2 material. The associated loss of products as a mixture results in limited yields for pure NT-1 and NT-2 batches. Recrystallization of the NT-1/NT-2 mixture from 1 M Tris buffer (pH 8.0) also results in an additional pure batch of NT-1, while the crystals of NT-2 from the recrystallization mainly appear as a co-precipitate with NT-1. We note that furthermore, some of the FeIII ions are precipitated as hydrated iron hydroxides during the reaction, but increasing the FeIII amount in the reaction mixture does not translate into increased yields, and the pH sensitivity of the reaction does not allow for a significantly lower pH.

The structures of polyanions 1 and 2 are both based on {α-FeIII3P2W15} units, where three FeIII centers complete the parent trilacunary [α-P2W15O56]12− (P2W15) structure to give the Wells–Dawson species {P2M18} (Fig. 1, 2 and S1†). The polyanions of this structural type are typically assembled from two central {PVWVI6} “belts” and two {M3} “caps”, in this case one {WVI3} and one {FeIII3} (Fig. S1†). The FeIII sites all adopt octahedral O6 coordination modes (see Table S2† for bond lengths).


image file: d2dt02922g-f1.tif
Fig. 1 (a) Side view of 1 and (b) top view onto the {Fe3} “cap”. Color code: WO6 octahedra, blue (“belt”) and light blue (“cap”); PO4 tetrahedra, transparent purple; Fe, yellow; O, red; μ4-O, green; C, light gray; N, blue spheres. Tris units are emphasized by dark gray bonds and Fe–O bonds by yellow lines. H atoms are omitted for clarity.

image file: d2dt02922g-f2.tif
Fig. 2 Structure of polyanion 2. Color code is the same as that in Fig. 1. The μ3-O atoms interconnecting two {α-FeIII3P2W15(Tris)2} subunits are highlighted as brown spheres. Lower right: the {Fe6O24} substructure consisting of edge-sharing FeO6 octahedra.

Similar {α-FeIII3P2W15} units are also found in non-functionalized [α-(FeIIICl)2(FeIIIOH2)P2W15O59]11− and [α-(FeIIIOH2)3P2W15O59]9− polyanions.14 Moreover, they can act as structural moieties in some larger POTs based on multinuclear Fe-oxo/hydroxo cores stabilized by {α-P2W15}, such as [FeIII13P8W60O227(OH)15(H2O)2]30−,15 [FeIII13P8W60O224(OH)12-(PO4)4]33− (ref. 15) and [FeIII14O6(OH)13(P2W15O56)4]31−.16 At the same time, no examples of triol-functionalized FeIII-containing Wells–Dawson-type POTs have yet been reported.

In 1 the three FeIII ions coordinate a common O atom of one of the central phosphate groups (Fe–O: 2.25(4) Å, P–O: 1.55(8) Å) and each of them binds two additional O atoms of the {P2W15} POT unit (Fe–μ2-O: 1.87(3)–1.94(3) Å). All three Tris ligands in 1 bind to FeIII ions of the polyanion via two deprotonated –CH2–O− groups, while the third protonated –CH2–OH moiety (see Table S3† for BVS values) and the protonated amino group –NH3+ remain non-coordinated (see Fig. 1).

Thus, polyanions 1 represent rare examples of magnetically and electrochemically active POT species decorated with terminal –OH and –NH2 functional groups, which may allow post-functionalization of the polyanions or their controlled deposition on electrode surfaces in molecular devices or on various substrates for the preparation of heterogeneous catalysts.

One of the two –CH2–O− groups that bind to the POT is coordinated to two neighboring FeIII ions (Fe–O: 1.89(3)–2.07(3) Å), while the O atom of the second one is located trans to the μ4-O atom (P, 3 FeIII) and binds only to a single FeIII center (Fe–O: 1.91(3) Å).

The Ci-symmetric cluster 2 comprises two identical {α-FeIII3P2W15(Tris)2} subunits interconnected via two μ3-oxo groups. The structure of each monomeric fragment {α-FeIII3P2W15(Tris)2} is reminiscent of 1 with one dissociated Tris moiety. The two remaining Tris ligands are bound to the FeIII centers in exactly the same way as in polyanions 1 (see Fig. 2).

Due to the absence of the third Tris ligand in {α-FeIII3P2W15(Tris)2}, the O trans to the μ4-O position of one of the FeIII ions and one μ2-O atom bridging it to one of the neighboring FeIII centers do not belong to the organic moieties. These two O atoms are shared by both monomeric {α-FeIII3P2W15(Tris)2} fragments so that the O atom trans to the μ4-O coordination site of the FeIII center in one fragment represents a “Tris-free” oxygen, bridging two neighboring FeIII ions in the second monomeric subunit and thus becomes μ3-O (shown as yellow O atoms in Fig. 2; Fe–μ3-O: 1.875(9)–2.012(9) Å).

Therefore, the two {Fe3} oxo-cores based on three {FeIIIO6} octahedra that share a common vertex in 2 appear to be interconnected in the {FeIII6} assembly in an edge-sharing mode with the closest Fe⋯Fe distance of 2.859 Å (Fig. 2). The {M6} oxo-cores based on octahedrally coordinated transition metal ions (e.g. NiII, FeIII, CuII, etc.) incorporated in POTs are not uncommon; nevertheless, to the best of our knowledge, this type of connection between six {MO6} octahedra has not yet been observed.17

The Tris anchoring mode in 1 and 2 is also extremely unusual for POMs as the vast majority of known triol-functionalized polyanions comprise a triol ligand bound in a symmetrical way via all three –CH2OH groups7–12 or via one or two –CH2OH groups and a terminal –NH3+ functionality.13

Solutions of NT-1 and NT-2 in 1 M Tris buffer (pH 8.0) strongly absorb UV-light with the maximum at 219 nm, which is followed by a distinct absorption shoulder at around 260–270 nm and a broad not well-defined shoulder at 350 nm. The absorption of visible light for NT-1 solution is characterized by a well-pronounced shoulder at 490 nm, while for NT-2 such a shoulder is absent and one can distinguish only very weak absorption without any well-defined characteristics (see Fig. S4 and the description in the ESI†).

The time-dependent UV-vis spectra for NT-1 in the same medium exhibit a fast decay of the absorption maximum at around 219 nm within 1 h followed by the stabilization of the absorption bands and the absence of any further changes in the spectrum for at least 17 h (Fig. S6†). Within this period, the other absorption features in the spectrum, including the shoulder at around 490 nm (Fig. S7†), remain unchanged for at least 18 h (including the first hour). The time-dependent UV-vis measurements on NT-2 solutions in 1.0 M Tris buffer medium (pH 8.0) show very similar features (see the ESI†). Interestingly, the change in the intensity of the UV-light absorption maximum for NT-2 in another medium (0.66 M NaOAc buffer at pH 5.2) is almost negligible, also within the first hour (Fig. S8†), proving that polyanions 2 remain intact in such solutions.

This suggests the overall stability of the {FeIII3P2W15} and {FeIII6(P2W15)2} POT skeletons in aqueous media. The spectral changes observed for NT-1 and NT-2 in the Tris medium within the first hour could most likely be explained by a (partial) exchange of the Tris ligands before reaching an equilibrium between the Tris-free and Tris-decorated POTs. The presence of such an equilibrium, i.e. the presence of also Tris-functionalized POTs in Tris buffer at pH 8.0, is further supported by the facile recrystallization of NT-1 and NT-2 from the Tris-buffer medium (pH 8.0). Moreover, we have found out that the recrystallization of NT-1 and NT-2 is also possible from pure water without a loss of triol moieties, which is an additional argument for the stability of the functionalized polyanions in aqueous media.

The cyclic voltammograms of NT-1 and NT-2 were recorded for their solutions in two different media: 1.0 M Tris buffer at pH 8.0 and 0.66 M CH3COONa buffer (pH 5.2).

The most prominent CV curve was obtained for a 0.49 mM solution of NT-2 in 0.66 M CH3COONa buffer (pH 5.2). The electrochemical behavior of 2 in this medium was characterized by several redox processes attributed to the FeIII and the WVI centers (Fig. 3, right). The assignment of redox waves is in line with literature data reporting electrochemical studies on various FeIII-, CoII- and ZnII-containing POTs based on {P2W15} and {As2W15} units in media with similar pH values (4.7–5.0).5e,15,16,18 A wave reflecting the reduction of six FeIII ions in 2 to FeII appears at −0.350 V (all potentials are provided at 20 mV s−1vs. Ag/AgCl reference electrode). Their reoxidation occurs in two steps characterized by the oxidation waves centered at −0.100 V and +0.315 V. The large separation between the reduction and oxidation counterparts is not unprecedented for Fe/{P2W15} POTs16,18e and suggests a partial dissociation of tris(hydroxymethyl)aminomethane moieties bridging the FeIII centers by their reduction to FeII, i.e. a significantly softer Lewis acid. The structural inequivalence of the FeII sites caused by partial Tris dissociation could also explain the fact that reoxidation occurs in two steps. Here, no FeIII ions are released into the solution as was observed for some Fe-containing polyanions in other aqueous media.18d This can be concluded by the addition of Fe(NO3)3·9H2O to the same solution, which leads to the appearance of pronounced reduction and reoxidation waves at −0.006 and 0.456 V, respectively, which are absent in the CV curves of pure solutions of 2, and also in the second and third cycles (Fig. S11†). Reoxidation of the released FeII ions reported in the literature also occurred at more positive potentials between 0.500 and 0.700 V vs. SCE (0.455–0.655 V vs. Ag/AgCl).18d


image file: d2dt02922g-f3.tif
Fig. 3 Room-temperature cyclic voltammograms of 1.01 mM solution of NT-1 (left) and 0.49 mM solution of NT-2 (right) in 0.66 M CH3COONa buffer (pH 5.2) at 20 mV s−1.

These redox processes are followed by several reversible redox couples attributed to the reduction of WVI ions in POT to WV and their reoxidation. The first couple is centered at −0.480 V (ΔE = 0.040 V at 20 mV s−1). The second redox couple combines two closely spaced reduction waves at −0.615 V and −0.667 V and the broad reoxidation counterpart at −0.605 V, where two oxidation processes overlap. It is then further followed by a third redox couple at −0.855 V (ΔE = 0.035 V at 20 mV s−1) before solvent discharge. The peak currents for the reversible and not overlapped reduction and oxidation processes are proportional to the square root of the scan rate (Fig. S9 and S10†), which indicates diffusion-controlled electrode reactions.

The cyclic voltammetry graph of a 1.01 mM solution of NT-1 in the same medium, i.e. 0.66 M NaOAc (pH 5.2), exhibits only one strong and broad reduction wave at −0.720 V that is attributed to the simultaneous reduction of FeIII and some WVI centers in 1. It is followed by a small anodic wave at −0.600 V and a much more pronounced wave at −0.483 V corresponding to the reoxidation of the WV ions as well as two waves at −0.055 V and +0.230 V, associated with the reoxidation of FeII ions in analogy to the redox behavior of NT-2 in the same medium (Fig. 3, left). This CV is strongly reminiscent of that observed for [FeIII13P8W60O227(OH)15(H2O)2]30− (also based on {FeIII3P2W15} units).15

The cyclic voltammetry curves of the 1.00 mM solution of NT-1 and the 0.49 mM solution of NT-2 in 1 M Tris buffer (pH 8.0; see the ESI† for a detailed discussion) exhibit comparable general trends and show that the monomeric species 1 is more stable towards reduction in this medium as compared to 2.

In conclusion we report the first proof-of-concept examples of Tris-functionalized FeIII-containing POTs, derived from the structural Wells–Dawson archetype. The dimeric species 2 comprises an {Fe6} oxo core that shows an interconnection of {Fe3} moieties not previously observed for POMs. The particular anchoring mode of the Tris moieties in these polyanions with terminal amino and hydroxymethyl groups offers perspectives for post-functionalization and chemisorption of the obtained POTs on various surfaces. Currently we are examining the possibility of generalizing the synthetic strategy by applying relatively mild reaction conditions using reactions in slightly basic Tris buffers for the preparation of other Tris-functionalized transition and rare-earth metal POT derivatives.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support by Forschungszentrum Jülich, EU ERC Grant 308051 – MOLSPINTRON and Deutsche Forschungsgemeinschaft (IZ 60/3-1).

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedure, details of XRD analysis, BVS calculations, IR and UV-vis spectra, and cyclic voltammetry. CCDC 2205561 and 2205567. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt02922g
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