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Frontiers in Chemical Biology ·

Volume 5 - 2026 | https://doi.org/10.3389/fchbi.2026.1770808

Ligand-directed assembly of an iminodiacetate-hybridized molybdenum-red isopolyoxomolybdate Mo V 10 Mo VI 3

  • Ingrid Gregorovic1,2
  • Emir Al-Sayed1
  • Nadiia I. Gumerova1
  • Judith Füllborn-Ott3
  • Ingo Ott3
  • Gerald Giester4
  • Annette Rompel1,2*corresponding author
  1. 1Universität Wien, Fakultät für Chemie, Institut für Biophysikalische Chemie, Wien, Austria
  2. 2Vienna Doctoral School in Chemistry (DoSChem), Universität Wien, Vienna, Austria
  3. 3Institute of Medicinal and Pharmaceutical Chemistry, Technische Universität Braunschweig, Braunschweig, Germany
  4. 4Fakultät für Geowissenschaften, Geographie und Astronomie, Institut für Mineralogie und Kristallographie, Universität Wien, Wien, Austria
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Abstract

The hybridized mixed-valence isopolyoxomolybdate [N(CH 3) 4 ] 5 [Mo V 10 Mo VI 3 (OH) 2 O 37 (C 4 H 4 NO 2)]·6MeOH (IDA-Mo V 10 Mo VI 3) was synthesized through a solvothermal one-pot method in a non-aqueous environment. The isopolyoxomolybdate scaffold was hybridized with iminodiacetic acid (IDA) by chelating a terminal molybdate group. The structural integrity and composition of IDA-Mo V 10 Mo VI 3 were confirmed through comprehensive analyses, including single-crystal X-ray diffraction (XRD), elemental analyses, infrared (IR) spectroscopy, Resonance Raman (RR) spectroscopy, UV-vis spectroscopy, and thermogravimetric analysis (TGA). Bond-valence-sum calculations (BVS) provided evidence that (IDA-Mo V 10 Mo VI 3) is approximately 77% reduced. The high degree of reduction unambiguously places IDA-Mo V 10 Mo VI 3 in the ‘molybdenum-red’ family of highly reduced isopolymolybdates. Initial antiviral and cytotoxicity assays demonstrated good biocompatibility of IDA-Mo V 10 Mo VI 3, with no observable toxicity and no significant antiviral activity up to 200 µM.

Keywords

1 Introduction

Polyoxometalates (POMs) represent a fascinating class of transition metal-oxide complexes, known for their remarkable structural versatility and functional diversity (Pope, 1983; Long et al., 2010). Their distinctive redox, spectroscopic, photovoltaic and magnetic features (Bijelic et al., 2019a) enable applications in catalysis (Long et al., 2010; Al-Sayed et al., 2021), medicine (Bijelic et al., 2019a), biology (Roy et al., 2019), and materials science (Pope, 1983; Long et al., 2010; Roy et al., 2019). Recent advancements have underscored the potential of reduced POMs (Falaise, 2025) in energy storage (Chen et al., 2018) due to their remarkable ability to reversibly store multiple electrons through super-stoichiometric reduction, effectively accepting electrons equivalent to the number of redox centers within the clusters (Chen et al., 2018).

To unlock the potential of more profoundly reduced POM materials, it is essential to grasp how mixed-valence polyoxomolybdates can be manipulated to form oligomeric assemblies of fundamental building units in solution (Long et al., 2007; Xuan et al., 2019). The self-assembly of POMs is intricately governed by factors such as the extent of reduction, the presence of anionic templates, cations, and organic ligands (Miras et al., 2017). Aminopolycarboxylates such as iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), and ethylenediaminetetraacetic acid (EDTA) have proven especially impactful: their tridentate to hexadentate coordination through nitrogen and carboxylate oxygen donors enables them to bridge or cap POM units, direct spatial orientation, and stabilize diverse oligomeric assemblies (Khajavian et al., 2021; Han et al., 2019; Teng et al., 2016; Zheng et al., 2012). Citric acid, with multiple carboxylate arms, also supports chelation and the creation of extended sandwich-like hybrid POM structures (Han et al., 2019). These multidentate ligands allow tailored control over the topology, aggregation behavior, and functional properties of POM-based nanostructures, supporting the rational design of hybrid frameworks for catalysis, materials science, and beyond.

Within the polyoxomolybdate (POMos) family, subclasses commonly referred to as “Molybdenum Blues,” “Molybdenum Browns” ({Mo132}) and “Molybdenum Reds” ({Mo154}) are differentiated by their degree of reduction: Mo-Blues are the least reduced (reduction level of around 20%), Mo-Browns are moderately reduced (reduction level of around 45%), and Mo-Reds represent the most profoundly reduced species, with the reduction level >50%. The colors in their names directly correspond to the characteristic hues exhibited by these polyoxomolybdate species, which result from intervalence charge transfer (IVCT)—a process in which electrons move between metal centers of different oxidation states within the complex; specifically between Mo(V) and Mo(VI) (Long and Cronin, 2021; Gumerova and Rompel, 2018; Al-Sayed and Rompel, 2022; Krebs, 2022). Unlike {Mo132} (Müller et al., 1998a) and {Mo154} (Müller and Serain, 1999), Mo-Red POMs are structurally distinct, more highly reduced, and often require the discovery and use of specific complex-stabilizing reagents to facilitate their assembly and stability (Liu et al., 2022; Ribó et al., 2022). The majority of known Mo-Red clusters are of higher nuclearity, such as {Mo37} (Müller et al., 1998b), {Mo42} (Khan and Zubieta, 1992), {Mo64} (Ribó et al., 2022), {Mo70} (Ribó et al., 2022) or {Mo240} (Lin et al., 2020), and are stabilized by addition of f-block metals as electrophiles or simple oxygen-donor ligands (Long et al., 2010) rather than aminopolycarboxylates. Smaller, highly reduced systems (MoV/VI < 20) are typically based on ε-Keggin anions (Müller et al., 2000) [MoV 12H12O40]8−, which likewise require additional metal ions (Ni2+, Co2+, or Ln3+) for stabilization (Müller et al., 2000; Khan et al., 1993; Mialane et al., 2002). Among all reported Mo-red clusters, only the structure of {Mo42} has been stabilized by organic ligands (here pentaerythritol ligands). In contrast, tridentate aminopolycarboxylate ligands such as iminodiacetic acid (IDA) are more common in less reduced systems, like Molybdenum Blues (Müller and Serain, 1999). For example, a γ-Mo8-based decamer with a reduction level of ∼40% is stabilized by 2 molecules of IDA to stabilize the partially reduced POM (K8{γ-Mo8 VI/VO26[MoVO2(IDA)]2}·12H2O). Notably, the {Mo(IDA)} motif in the decamer closely resembles the {Mo(IDA)} unit observed in our structure (Chen et al., 2014). We do not claim that the {N,O,O} coordination mode of IDA is electronically unique among aminopolycarboxylates. Rather, IDA’s compact tridentate backbone provides sufficient chelation to cap a single terminal Mo center without promoting extended bridging. As a result, IDA enables stabilization of a highly reduced, low-nuclearity Mo13 core that other aminopolycarboxylates have so far only achieved in less reduced POM systems.

The synthesis of Mo-Reds requires the careful use of cluster-stabilizing reagents to direct the growth and assembly of large, reduced molybdenum clusters. Among these, organic ligands such as sulfonates and aminopolycarboxylates act as crucial stabilizers by coordinating to Mo centers and guiding the connection between key subunits. In the assembly of the giant {Mo240}, sulfonate ligands and in situ-generated SO3 2− ions helped to template and stabilize the hollow framework (Lin et al., 2020), enabling the construction of record-sized clusters. Transition metal ions, such as Ni2+ and various lanthanides (La3+, Ce3+), can also serve as stabilizing agents, balancing charge and promoting the aggregation of building blocks into stable architectures (Ribó et al., 2022). Together, these reagents—both organic and inorganic—are vital for controlling complex connectivity, increasing stability, and enabling the formation of highly reduced molecular assemblies (Liu et al., 2022; Ribó et al., 2022; Lin et al., 2020; Wang et al., 2025; Bingbing et al., 2024). The identification of a cluster-stabilizing reagent frequently necessitates an extensive and time-consuming systematic screening process, given that these reactions typically adhere to a trial-and-error methodology. Consequently, the discovery of such a structure-stabilizing reagent assumes heightened significance and adds substantial value to the research endeavor.

Mo-Red POMs (Supplementary Table S1) are notable for their exceptional ability to act as electron and proton reservoirs, a result of their high reduction levels and the presence of structure-stabilizing agents, which also reflects their structural versatility (Long and Cronin, 2021; Lin et al., 2020; Wang et al., 2025; Bingbing et al., 2024). These properties are directly reflected in their principal applications: energy storage and conversion (Wang et al., 2025; Bingbing et al., 2024), as well as catalysis (Wang et al., 2018; Huang et al., 2020; Barman et al., 2019). In energy storage and conversion, Mo-Red POMs can reversibly transfer electrons and protons, making them promising new candidates for high-capacity batteries, supercapacitors, and as molecular electron reservoirs in advanced energy devices (Wang et al., 2025; Bingbing et al., 2024; Liu et al., 2024). In catalysis, their multi-electron redox activity enables a variety of catalytic processes, including oxidation and reduction reactions such as CO2 reduction and CH4 formation (Wang et al., 2018; Huang et al., 2020; Barman et al., 2019). Surface modification of reduced POMs has been demonstrated in several systems, indicating that analogous strategies should be possible for the Mo-Red type structure. Reported approaches include grafting on reduced graphene oxide (rGO), silica, or metal-organic frameworks (MOFs), leading to hybrid materials for applications in catalysis or energy storage (Hornebecq et al., 2003; Wu et al., 2021; Zhang et al., 2017; Zhang et al., 2018; Chen et al., 2023; Oh et al., 2021). The energy storage application of the Mo Red type structure has also been reported in the literature (Wang et al., 2025). From a synthetic perspective, an important practical consideration is that our current solvothermal route does not yet allow upscaling to multi-hundred-milligram or gram scales. Efforts to improve yield, robustness, and scalability, therefore, represent a critical prerequisite for future application-driven studies. The importance of POM-based hybrid materials can be further highlighted when checking their recently reported applications in (photo)catalysis and environmental remediation topics, like water purification and energy storage (Cheng et al., 2025; Ru et al., 2025; Shang et al., 2025; Hasnain et al., 2025; Xu et al., 2025).

Herein, an approach is presented for the regulation of building blocks and resultant structures in the synthesis of an organically hybridized highly reduced, mixed-valence 77% reduced polyoxomolybdate: [N(CH3)4]5[MoV 10MoVI 3(OH)2O37 (C4H4NO2)]·xMeOH (x ≈ 4–6) (hereafter IDA-Mo **V** **10** Mo **VI** **3**). The formation of the IDA-Mo **V** **10** Mo **VI** **3** Mo-Red structure was achieved through the coordination of iminodiacetic acid (IDA) to a terminal molybdenum center, accompanied by charge balancing via the in situ generation of 5 [Me4N]+ cations. IDA provides a tridentate {N, O, O} chelating pocket, enabling it to cap and stabilize terminal Mo sites, particularly those containing MoV centers. This tridentate coordination not only anchors the ligand firmly but also helps protect the reduced molybdenum environment from degradation, guiding and stabilizing the formation of the highly reduced POM and enhancing the structural integrity of the Mo-Red assembly.

We aimed to investigate whether structure-directing organic ligands, such as IDA, can facilitate the formation of highly reduced, mixed-valence molybdenum oxide complexes in solution and how these ligands affect the cluster architecture and stability. This approach contributes to the broader effort of designing redox-active polyoxometalates for applications in energy storage and molecular electronics by expanding the toolbox of ligands and synthetic conditions suitable for Mo-Red formation. Given the increasing interest in functionalized polyoxometalates for biological applications (Bijelic et al., 2019a; Moghadasi et al., 2025; Zhao et al., 2025; Li et al., 2024; Bijelic et al., 2018; Bijelic et al., 2019b), we additionally evaluated the cytotoxicity and antiviral activity of the iminodiacetate-stabilized molybdenum red cluster as an initial assessment of its biological tolerance.

2 Experimental

2.1 Synthesis and crystallization

Ammonium heptamolybdate tetrahydrate (0.016 mmol, 0.021 g) and iminodiacetic acid (0.190 mmol, 0.027 g) were suspended in a solvent mixture of 5.2 mL dimethylformamide (DMF), 0.84 mL methanol, and 0.84 mL acetonitrile. The mixture was stirred and heated on a magnetic stirrer for approximately 30 min, resulting in a clear solution. The resulting solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 150 °C for 3 days, followed by slow cooling to room temperature in the oven (Supplementary Figure S1A). Reddish brown-blue crystals (Supplementary Figure S1B) were filtered off from dark red-brown solution, washed with DMF, and air-dried (yield: 7.41 mg, 37.5% yield based on Mo). Elemental analysis calculated (%): C 14.56, H 3.46, N 3.32, O 29.67; found: C 17.03, H 3.84, N 5.26, O 30.80. FT–IR (cm−1): 3,400 (br), 3,287 (br), 3,220 (br), 3,034 (m), 2,960 (m), 2,926 (m), 1,620 (s), 1,482 (s), 1,362 (s), 1,104 (w), 1,076 (w), 927 (s), 819 (m), 778 (s), 729 (s), 581 (s), 484 (s).

2.2 X-ray crystallography

X-ray intensity data were measured at 200 K on a Bruker APEXII diffractometer equipped with a CCD (charge-coupled device) area detector, Incoatec Microfocus Source IµS (30 W, multilayer mirror, Mo-K), and an Oxford Cryosystems (Cryostream 800 Plus) LT device. The following software was used: the Bruker Apex4 suite the SHELX program suite, for structure solution (SHELXT), structure refinement (SHELXL) and OLEX2 as graphical user-interface (Bruker, 2022; Sheldrick, 2008; Sheldrick, 2015; Dolomanov et al., 2009). Experimental data and CCDC-codes are listed in Supplementary Table S2.

C—H bond lengths were constrained to 0.98 and 0.99 Å for the in situ formed dimethyl and tetramethyl ammonium ions and refined in riding modes, with Uiso(H) values set to 1.2Ueq(C). The dimethylammonium ion exhibits a notable degree of disorder, resulting in positional disorder of the nitrogen atom’s positional occupancy, which is determined to be 0.5. This disorder allows the nitrogen atom to occupy two distinct positions within the crystal lattice. Notably, this disorder phenomenon is limited to the nitrogen atom and its associated bonds with the respective carbon atoms, while the positions of the carbon atoms remain unaffected by such disorder. The residual electron density of 2.8 (e Å−3) is attributed to the presence of solvent molecules, which could not be unambiguously identified due to the prevailing disorder within the crystal lattice and necessitated the application of a solvent mask.

2.3 Elemental analyses

The content of C/H/N/O was determined using an EA 1108 CHNS-O elemental analyzer from Carlo Erba Instruments at the Mikroanalytisches Laboratorium, Faculty of Chemistry, University of Vienna.

2.4 UV–vis spectroscopy

Vis–NIR spectroscopy was performed at 298 K using a Shimadzu UV-2401PC spectrophotometer with a quartz cuvette having an optical path length of 1.0 cm.

2.5 Thermogravimetric analysis (TGA)

TGA was conducted using a thermal analyzer (TA) Instruments model Q50, USA). The sample, having an initial mass of 10.682 mg, was subjected to a temperature range from 50 °C to 600 °C at a heating rate of 5 °C/min.

2.6 IR spectroscopy

All FT–IR spectra were recorded on a Bruker Vertex 70 IR spectrometer equipped with a single-reflection diamond ATR unit. Frequencies are given in cm−1, and intensities are denoted as w, weak; m, medium; s, strong; br, broad.

2.7 Resonance raman spectroscopy (RR)

The Resonance Raman spectrum of solid IDA-Mo **V** **10** Mo **VI** **3** was recorded on WITec alpha 300 A equipped with a 532 nm laser at the University of Vienna, Faculty Center for Nanostructure Research. Frequencies are given in cm−1, and intensities are denoted as follows: w, weak; m, medium; s, strong; br, broad, which is the same as in IR spectroscopy.

2.8 Cyclic voltammetry (CV)

Cyclic voltammetry (CV) was conducted on an Au microelectrode array working electrode (integrated three-electrode plate) using a Metrohm DropSense µSTAT400 potentiostat at scan rate of 50 mV/s (3 scans). A 15 µM droplet of analyte in 100 mM Na2SO4 electrolyte was deposited on the plate at ambient conditions, with integrated Ag/AgCl reference and Au counter electrodes.

2.9 Cytotoxicity and antiviral testing

The antiviral activity of the test compounds against HCoV-OC43 was determined according to a recently reported protocol (Gil-Moles et al., 2025). In short: A virus dilution at an MOI (multiplicity of infection) of 0.1 was simultaneously added with the test compounds to an almost confluent layer of RD cells. After 24 h the incubation was stopped and the viral load was quantified by ELISA using a N-protein primary antibody (N-protein HCoV-OC43, Sino Biological) and a HRP-labeled secondary antibody (Sino biological). Following the determination of N-positive cells, the plates were washed twice with phosphate buffered saline and a crystal violet counter screen was performed to determine cytotoxicity (Gil-Moles et al., 2025).

3 Results and discussion

The synthesis of IDA-Mo **V** **10** Mo **VI** **3** was carried out to investigate whether iminodiacetic acid (IDA) can act as a structure-directing ligand for stabilizing highly reduced polyoxometalates. IDA was selected for its ability to coordinate through both nitrogen and carboxylate groups. The reaction employed a non-aqueous solvent mixture of DMF, methanol, and acetonitrile (6:1:1). It was heated at 150 °C for 3 days to ensure the complete dissolution of the starting materials and to promote Mo-Red formation, as in most previously reported structures (Supplementary Table S1). Unlike typical syntheses of highly reduced POMs, which often require strong reducing agents such as sodium borohydride or hydrazine, this procedure relied on the reaction conditions and properties of the solvent system to facilitate the reduction of Mo(VI) centers. DMF and methanol can function as mild, in situ reducing agents under these conditions (Ooi et al., 2021), enabling efficient reduction and cluster assembly without the need for added external reductants (Ooi et al., 2021; Ribó et al., 2022). The procedure yielded deep red crystals in a moderate yield (37.5% based on Mo). Charge-balancing [Me4N]+ cations likely formed via solvent decomposition during the reaction. Careful selection of ligand and solvent system thus enabled the controlled self-assembly of a reduced Mo-Red cluster, confirming the suitability of this synthetic approach for generating related polyoxometalate structures in the absence of classical reducing agents.

Single-crystal X-ray structural analysis revealed that the compound IDA-Mo<sup>V</sup> <sub>10</sub>Mo<sup>VI</sup> <sub>3</sub> crystallizes in the monoclinic space group C2/m (Supplementary Table S2), containing a [MoV 10MoVI 3(OH)2 O37(C4H4NO2)]5− mixed-valence isopolyoxomolybdate anion (Figure 1). The structure comprises a total of 13 Mo ions, forming a framework consisting of two types of building blocks common to Mo-Red POMs: {MoVI 1} (yellow) and edge-sharing {MoV 2} units (dark red). All unreduced {MoVI 1} units exhibit trigonal bipyramidal coordination within the cluster and are externally coordinated to the isopolyoxomolybdate anion (yellow polyhedra in Figure 1). It appears that these {MoVI 1} units serve to stabilize the reduced {MoV 10} cluster, where all {MoV 2} units are exclusively octahedrally coordinated (dark-red polyhedra in Figure 1). In addition to stabilizing the {MoV 10} cluster, the employed IDA acts as a tridentate ligand, coordinating to a terminal MoV ion within a {MoV 2} unit (black, pink, and red spheres in Figure 1).

Polyhedral and ball-and-stick representation of **IDA-Mo** <sup>**V**</sup> <sub>**10**</sub> **Mo** <sup>**VI**</sup> <sub>**3**</sub> shown from two different **(A,B)** perspectives. Color code: {Mo<sup>V</sup> <sub>2</sub>} dark-red, {Mo<sup>VI</sup> <sub>1</sub>} yellow, C black, O red, and N pink.
Figure 1. Polyhedral and ball-and-stick representation of IDA-Mo **V** **10** Mo **VI** **3** shown from two different (A,B) perspectives. Color code: {MoV 2} dark-red, {MoVI 1} yellow, C black, O red, and N pink.

The Molybdenum Red IDA-Mo **V** **10** Mo **VI** **3** exhibits an outer diameter of approximately 13.5 Å at its most elongated points and 10.7 Å at its shortest. Notably, five Mo–Mo bonds (bond length at ∼2.6 Å) are observed within the edge-sharing {MoV 2} units, characteristic of the highly reduced nature of the Mo-Red cluster (Figures 2A,B) (Falaise, 2025).

Ellipsoid representation of **IDA-Mo** <sup>**V**</sup> <sub>**10**</sub> **Mo** <sup>**VI**</sup> <sub>**3**</sub> **(A)** and a close-up highlighting one of 5 Mo−Mo bonds (length ∼2.6 Å) in POM structure **(B)**. Color code: Mo blue, O red, C gray, N light blue, and H white.
Figure 2. Ellipsoid representation of IDA-Mo **V** **10** Mo **VI** **3** (A) and a close-up highlighting one of 5 Mo−Mo bonds (length ∼2.6 Å) in POM structure (B). Color code: Mo blue, O red, C gray, N light blue, and H white.

The UV-vis spectrum in DMF (Supplementary Figure S3) of IDA-Mo **V** **10** Mo **VI** **3** reveals an absorption band at approximately 310 nm, characteristic of Molybdenum Reds, which can be attributed to the Mo-Mo charge-transfer (Ribó et al., 2022). In contrast, Molybdenum Browns (∼45%), which are reduced to a lesser extent, and Molybdenum Blues, which are relatively the least reduced ones (∼18%), exhibit absorption bands at longer wavelengths (at around 450 and 750 nm, respectively).

The IR (Figure 3; Table 1) and Resonance Raman (RR) spectroscopic data (Figure 4; Table 2) provide additional characterization of IDA-Mo<sup>V</sup> <sub>10</sub>Mo<sup>VI</sup> <sub>3</sub>. The IR spectrum shows strong and distinct bands at 927 cm−1 and IR bands between 819 and 581 cm−1, corresponding to Mo=O (terminal) and Mo–Ob (edge-bridging) stretching vibrations. Additional IR bands at 1,620 cm−1 and 1,482 cm−1 confirmed the coordination of iminodiacetate ligands. The RR spectra (Figure 4), measured at an excitation of 532 nm, selectively enhanced Mo–O vibrational modes, highlighting the same key Mo–O vibrations as IR, which supports the identification of these structural features. Together, the IR and RR results provide consistent and mutually reinforcing assignments for key vibrational features in the structure (Tables 1, 2), supporting both the preservation of the POM framework and the effective incorporation of the organic ligand into the POM structure. These spectroscopic results are in complete agreement with the crystallographic data strongly supporting the proposed molecular structure.

IR spectrum of solid **IDA-Mo** <sup>**V**</sup> <sub>**10**</sub> **Mo** <sup>**VI**</sup> <sub>**3**</sub> in the region between 4,000 and 400 cm<sup>−1</sup>.
Figure 3. IR spectrum of solid IDA-Mo **V** **10** Mo **VI** **3** in the region between 4,000 and 400 cm−1.

TABLE 1. Summary of the main vibrational bands observed for IDA-MoV 10MoVI 3.

Wavenumber (cm−1)Assignment
3,400 (m), 3,287 (m), 3,220 (m)v(N-H) of IDA
3,034 (m), 2,960 (m), 2,926 (m)v(C-H) of IDA
1,620 (s)δ(N-H)/v(C=O) of IDA
1,482 (s)δ(C-H) of IDA
1,362 (s)δ(C-H) of IDA
1,104 (w), 1,076 (w)v(C-N) of IDA
927 (s)v(Mo=O) of IDA-Mo **V** **10** Mo **VI** **3**
819 (m)v(O—Mo—O) of IDA-Mo **V** **10** Mo **VI** **3**
778 (s)v(O—Mo—O) of IDA-Mo **V** **10** Mo **VI** **3**
729 (s)v(Mo—O—Mo) of IDA-Mo **V** **10** Mo **VI** **3**
581 (s)v(Mo—O—Mo) of IDA-Mo **V** **10** Mo **VI** **3**
484 (s)v(Mo—O—Mo) of IDA-Mo **V** **10** Mo **VI** **3**

Intensities are denoted as: w, weak; m, medium; s, strong. The IR assignation is based on literature data present in Supplementary Table S1.

Resonance Raman spectrum of solid **IDA-Mo** <sup>**V**</sup> <sub>**10**</sub> **Mo** <sup>**VI**</sup> <sub>**3**</sub> in the region between 4,000 and 50 cm<sup>−1</sup>.
Figure 4. Resonance Raman spectrum of solid IDA-Mo **V** **10** Mo **VI** **3** in the region between 4,000 and 50 cm−1.

TABLE 2. Summary of the main vibrational bands in RR spectra observed for IDA-MoV 10MoVI 3.

Wavenumber (cm−1)Assignment
989 (s)v(Mo=Ot) symmetric
816 (s)v(Mo=Ot) asymmetric
659 (w)δ(Ob—Mo—Ot)
332 (w)δ(Mo—O—Mo)
277 (m)δ(Ob—Mo—Ob)

Intensities are denoted as: w, weak; m, medium; s, strong. RR vibrational band assignation based on the literature data in Supplementary Table S1 and refs (Tian et al., 2010; Buckley and Clark, 1985). Explanation of Mo–O, assignation abbreviations: Ot, terminal oxygen; Ob, edge-bridging oxygen.

Based on thermogravimetric analysis (Supplementary Figure S4), a mass loss of 5.84% between 25 and 150 °C is observed, which corresponds to approximately 4.5 methanol molecules per formula unit. This result is consistent with the residual electron density revealed by the solvent mask (102 e−, i.e., ≈ 6 MeOH equivalents), and we therefore assign ca. six MeOH molecules of crystallization per anion. Because of the disorder and partial desolvation, the exact solvent content should be regarded as approximate.

The elemental analysis of bulk material shows slightly elevated C and N values relative to the idealized formula, which we attribute to variable amounts of disordered lattice solvent (MeOH/DMF) in the channels and partial desolvation prior to analysis. This behavior is common for highly solvated POM crystals and is consistent with the disordered solvent model from SXRD and the TGA data.

The composition of IDA-Mo **V** **10** Mo **VI** **3** was determined using a combination of analytical techniques, including single-crystal X-ray diffraction (SXRD) (Supplementary Table S2), elemental analysis, bond valence sum (BVS) calculations, and thermogravimetric analysis (TGA) (Supplementary Figure S4).

Significant residual electron density was observed in solvent-accessible channels running through the structure. Attempted modelling of discrete solvent molecules resulted in unstable refinement and unreasonable displacement parameters. Therefore, a solvent-mask procedure was applied using Olex2, which identified two solvent-filled voids per unit cell with a combined volume of 1,620 Å3 and a total electron count of 410 e−. This corresponds to approximately 102 e− per asymmetric unit, consistent with ∼6 small neutral solvent molecules (e.g., MeOH) per formula unit. Due to the severe disorder of the lattice solvent and the absence of distinct electron density peaks, no discrete solvent atoms are included in the final model, and the structure is reported with solvent masked. Void analysis using Mercury indicates ∼20% solvent-accessible volume, consistent with the presence of continuous channels that accommodate the disordered lattice solvent.

BVS calculation was employed (SI; Supplementary Table S3) to quantify the presence of MoV centers within IDA-Mo **V** **10** Mo **VI** **3**. The analysis revealed that IDA-Mo **V** **10** Mo **VI** **3** is approximately 77% reduced, meaning that 10 addenda metals are present as Mo(V). Bond-valence-sum analysis was also used to identify the most probable hydroxy position in the anion (Supplementary Table S3). Among all oxygen sites, O6 exhibits a BVS value close to −1 (−1.02) and an elongated Mo–O6 bond length of 2.180(4) Å, which is consistent with a protonated μ2-OH group in reduced polyoxomolybdates. In contrast, the remaining oxygen ions display more negative BVS values (≤−1.5), characteristic of oxide O2– ligands. We therefore assign O6 as the most likely hydroxy site in IDA-Mo<sup>V</sup> <sub>10</sub>Mo<sup>VI</sup> <sub>3</sub>.

Cyclic voltammetry (Figure 5) was used to study the redox properties of IDA-Mo **V** **10** Mo **VI** **3** in 100 mM aqueous Na2SO4. Measurements taken at concentrations ranging from 15 to 250 μM showed irreversible cathodic peaks, where the current decreased slightly with increasing concentration, indicating stable electron transfer behavior. The average peak currents and potentials from three repeat scans (Supplementary Table S4) were used to calculate the diffusion coefficient (Supplementary Table S4) using the Randles-Sevcik equation (Randles, 1948; Kissinger and Heineman, 1983). The diffusion coefficients, ranging from 10−4 to 10−1 cm2/s, confirm that the redox process is controlled by the rate at which POM molecules diffuse to the electrode surface, rather than the speed of electron transfer itself. This means the POM species move efficiently enough in solution to supply the electrode rapidly, making diffusion the limiting factor. In comparison, molybdenum blues exhibit reversible multi-step redox processes attributed to stabilized mixed-valence states and extensive electron delocalization within their large wheel-type structure (Lakhanpal et al., 2025). In contrast, molybdenum browns often show quasi-reversible electrochemical behavior with more distinct peak separations (Dey, 2024; Keita et al., 2007).

CV measurements of **IDA-Mo** <sup>**V**</sup> <sub>**10**</sub> **Mo** <sup>**VI**</sup> <sub>**3**</sub> (POM) in 100 mM Na<sub>2</sub>SO<sub>4,</sub> three CV scans: **(A)** electrolyte (100 mM Na<sub>2</sub>SO<sub>4</sub>) and 15 µM POM solutions, **(B)** 25 μM and 50 µM POM solutions, **(C)** 100 μM and 150 µM POM solutions, and **(D)** 200 μM and 250 µM POM solutions.
Figure 5. CV measurements of IDA-Mo **V** **10** Mo **VI** **3** (POM) in 100 mM Na2SO4, three CV scans: (A) electrolyte (100 mM Na2SO4) and 15 µM POM solutions, (B) 25 μM and 50 µM POM solutions, (C) 100 μM and 150 µM POM solutions, and (D) 200 μM and 250 µM POM solutions.

In antiviral in-cell enzyme-linked immunosorbent assays using human coronavirus OC43–infected RD cells (Savoie and Lippé, 2022; Gil-Moles et al., 2025), IDA-Mo **V** **10** Mo **VI** **3** showed no detectable antiviral activity and no measurable cytotoxicity at concentrations up to 200 µM (Supplementary Figure S5), with both the half-maximal inhibitory concentration and the half-maximal cytotoxic concentration exceeding the highest concentration tested (n = 2). The cytotoxicity and antiviral data represent preliminary phenotypic screening assays used to determine whether a compound warrants follow-up mechanistic studies. IDA-Mo<sup>V</sup> <sub>10</sub>Mo<sup>VI</sup> <sub>3</sub> did not exhibit measurable cytotoxicity or statistically significant antiviral activity up to 200 μM, and therefore, no further cellular uptake analyses (e.g., ICP-MS) were pursued. In this context, “no significant activity” refers to responses below the limit of detection and statistical thresholds of the in-cell ELISA assay, rather than absolute biological inactivity.

4 Conclusion

In this work, we have successfully synthesized and structurally characterized a novel, highly reduced mixed-valence polyoxomolybdate, IDA-Mo **V** **10** Mo **VI** **3**, stabilized by iminodiacetic acid (IDA). The compound demonstrates a remarkable 77% reduction level, typically found in Molybdenum Red structures (usually 70%−81%). This achievement highlights the critical role of organic ligands in directing the assembly and stabilization of complex molybdenum red structures. The unique structural features and high degree of reduction position IDA-Mo **V** **10** Mo **VI** **3** as a promising candidate for applications requiring efficient electron and proton reservoirs, such as high-capacity energy storage devices and advanced catalytic systems. Furthermore, our approach provides valuable insights into the design principles for assembling air-stable, highly reduced polyoxomolybdates, paving the way for the discovery of new materials with tailored properties. Future work should focus on exploring the electrochemical behavior and catalytic performance of this and related Mo Red POMs, as well as extending the synthetic strategy to other ligand systems, such as nitrilotriacetic acid (NTA) and iminodiacetamide, and various transition metal oxides, e.g., V-based metal oxides (Araujo et al., 2008; Liu et al., 2002; Pantalon Juraj et al., 2022; Pantalon Juraj et al., 2019). Organic ligands and metal oxides, including vanadium-based POMs, have demonstrated promising catalytic activity and potential in energy-related applications (Zhao et al., 2023; Bao et al., 2024; Jia et al., 2024; Tangestaninejad et al., 2008). Such efforts are expected to broaden the functional landscape of polyoxometalate-based materials in energy and materials science. In addition, preliminary cytotoxicity and antiviral evaluations indicated that IDA-Mo **V** **10** Mo **VI** **3** exhibits no detectable toxicity and no significant antiviral activity up to 200 μM, providing an initial indication of its biological tolerance. The absence of toxicity also suggests that highly reduced Mo Red clusters may be inherently more biocompatible than unreduced POM species of comparable nuclearity, which is an observation that may warrant further study.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchbi.2026.1770808/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Vadim G. Kessler, Swedish University of Agricultural Sciences, Sweden

Reviewed by: Dejin Zang, Shandong First Medical University, China

Edi Topić, University of Zagreb, Croatia

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Author NG declared that they were an editorial board member of Frontiers at the time of submission. This had no impact on the peer review process and the final decision.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was funded in part by the Austrian Science Fund (FWF) – Grant-DOI 10.55776/PAT4299925 AR, 10.55776/PAT2849824 NIG and the University of Vienna. For open access purposes, the author has applied a CC BY public copyright license to any author accepted manuscript version arising from this submission. Open access funding was provided be the University of Vienna.

Data availability

The dataset presented in this study can be found in PHAIDRA repository: https://phaidra.univie.ac.at/detail/o:2186432.

Author contributions

IG: Investigation, Writing – review and editing, Writing – original draft, Data curation, Methodology, Formal Analysis. EA-S: Conceptualization, Data curation, Methodology, Investigation, Writing – review and editing, Visualization, Formal Analysis, Writing – original draft. NG: Methodology, Writing – original draft, Funding acquisition, Formal Analysis, Validation. JF-O: Writing – review and editing, Investigation, Formal Analysis. IO: Resources, Writing – review and editing. GG: Writing – review and editing, Investigation, Formal Analysis. AR: Supervision, Methodology, Writing – review and editing, Conceptualization, Project administration, Funding acquisition.

Acknowledgements

The authors acknowledge Julia Völkle from the Department of Physical Chemistry, Faculty of Chemistry, University of Vienna, for supporting with cyclic voltammetry measurements. The authors acknowledge Core Facility Interface Characterisation for TGA measurements, and Faculty Center for Nanostructure Research for the access to RR instrument.

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Citation

Gregorovic, I., et al. (2026). Ligand-directed assembly of an iminodiacetate-hybridized molybdenum-red isopolyoxomolybdate Mo V 10 Mo VI 3. Frontiers in Chemical Biology, Volume 5. https://doi.org/
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Copyright © 2026 Gregorovic, Al-Sayed, Gumerova, Füllborn-Ott, Ott, Giester and Rompel.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Annette Rompel

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