Organoarsonate- and Dimethylarsinate-Functionalized Hexamolybdates(V): A Multifaceted Study on Synthesis, Structural Dynamics, and Antibacterial Properties

Abstract

We report on the synthesis, functionalization, and structural characterization of 11 novel dimethylarsinate-functionalized arsenomolybdates­(V), [RAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = HO, CH₃, C₂H₅, C₆H₅, 3,5-(HOOC)₂C₆H₃, 4-FC₆H₄, 4-F₃CC₆H₄, 4-F₃COC₆H₄, 4-BrC₆H₄, and 4-N₃C₆H₄) and [As^IIIMo^V ₆­O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^3–, featuring a reduced hexanuclear {Mo^V ₆O₂₄} core, peripherally coordinated by three dimethylarsinate ligands and centrally functionalized with diverse organoarsonate, arsenate, or arsenite groups, including carboxylated, fluorinated, brominated, and azido derivatives. Synthesized via a simple one-pot aqueous method, the compounds were thoroughly characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and elemental analysis in the solid state. Solution-phase stability was assessed by multinuclear (¹H, ¹³C, and ¹⁹F) nuclear magnetic resonance, while gas-phase behavior and fragmentation pathways were probed through electrospray ionization mass spectrometry, and tandem mass spectrometry (collision-induced dissociation). Antibacterial screening against Escherichia coli, Bacillus subtilis, and three pathogenic bacterial strains Listeria monocytogenes, Salmonella enterica, and Vibrio parahaemolyticus revealed that three of the 11 polyanions exhibit moderate antibacterial activity. Additionally, the synthesis of 3,5-bis­(carboxy)­phenylarsonic acid, 3,5-(HOOC)₂C₆H₃AsO₃H₂, is reported here for the first time.

Introduction

Polyoxometalates (POMs) are a class of discrete, anionic polynuclear metal oxide clusters composed of early transition metals in high oxidation states (M = W^VI, Mo^VI, V^V, Nb^V, and Ta^V), known for their broad applications in photo/electrocatalysis, medicine, magnetism, and energy storage/conversion. POMs offer significant potential for structural and functional modification, as the covalent integration of organic or organometallic moieties into their frameworks enables precise control over molecular properties such as shape, size, solubility, lipophilicity, stability, toxicity, redox, and acid–base behavior, thereby facilitating the rational design of advanced inorganic–organic hybrid materials with tailored functionalities for diverse applications.

Building on this structural versatility, we recently reported a series of reduced dimethylarsinate-functionalized, hexanuclear phosphomolybdenum clusters with the general formula [RPMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = H, HO, CH₃, HO₂CCH₂, HO₂CC₂H₄, C₆H₅, 4-FC₆H₄, and 4-F₃COC₆H₄), characterized by a cyclic {Mo₆ ^VO₂₄} core, flanked by dimethylarsinate ligands and central organophosphonate substitutions. This system was extensively studied in solid, solution, and gas phases, with ESI-MS confirming the coexistence of monomeric and dimeric species. Ion mobility mass spectrometry further enabled the separation of both entities, allowing for the acquisition of clean spectra for each. Haushalter and Lai originally isolated this hexanuclear molybdic wheel archetype as the reduced molybdenum phosphate, {P₄Mo^V ₆}, where peripheral phosphates and hydroxo bridges connect {Mo^V ₂O₄} dimers. Subsequent structural analogues, such as {(C₆H₅P)₄Mo^V ₆}, − {(C₆H₅As)₄Mo^V ₆}, and thiomolybdate variants {X₄Mo^V ₆S₆O₆} (X = HPO₄ or HAsO₄), have further highlighted the framework’s adaptability. Additionally, reactions of reduced Mo^V systems with methylenebisphosphonates and various structure-directing agents yielded cyclic compounds, {(Mo^V ₂O₄)­(O₃PCH₂PO₃)} _n (n = 3, 4, or 10}, encapsulating additional ligands such as MoO₄ ^2–, SO₃ ^2–, CO₃ ^2–, O₃PCH₂PO₃ ^4–, and CH₃AsO₃ ^2–.

Organoarsenic chemistry, although it has progressed more slowly than organophosphorus chemistry, boasts a rich history, dating back to 1760, when Cadet de Gassicourt synthesized cacodyl ((CH₃)₂AsAs­(CH₃)₂), the first known organometallic compound. This slower development, partly due to toxicity concerns, contrasts with the rapid progress in organophosphorus chemistry, driven by ³¹P NMR spectroscopy as well as its well-documented reactivity and electronic characteristics. In the realm of POM chemistry, initial pivotal contributions to organoarsonate-functionalized POMs were made by Pope’s group, which reported tetramolybdobisarsonate complexes, [R₂AsMo₄O₁₅H]^2– (R = CH₃, C₂H₅, or C₆H₅), a system initially proposed by Rosenheim and Bilecki, and cyclic, hexamolybdobisarsonate complexes, [(RAs)₂Mo₆O₂₄]^4– (R = CH₃, C₆H₅, or p-C₆H₄NH₂). Further advancements involved Matsumoto’s synthesis of [(C₆H₅As)₂Mo₆O₂₄(H₂O)]^4– and Liu et al.’s contributions with the pentamolybdate [(n-C₃H₇As)₂Mo₅O₂₁]^4– and hexamolybdate [(n-C₃H₇As)₂Mo₆O₂₄]^4– frameworks. Moreover, Kortz’s group expanded this family of organoarsonate-containing hexamolybdates by integrating para-substituted phenyl derivatives (R = 4-BrC₆H₄, 4-N₃C₆H₄, 4-FC₆H₄, 4-F₃CC₆H₄, or 4-F₃COC₆H₄). Beyond these, dual-site functionalization has also been realized. In one such example, a hexamolybdic wheel is centrally occupied by lone pair-containing heteroatoms and surrounded peripherally by amino acids bound via their carboxylate groups as in [As^IIIMo₆O₂₁(O₂CRNH₃)₃]^3– (R = CH₂, C₂H₄, or C₃H₆).

POMs are recognized as potent metallodrugs with promising antimicrobial, antiviral, and antitumor properties. In particular, hybrid POM systems, formed by integrating electron-rich POM cores into bioactive frameworks, have emerged as strong candidates for biomedical research. With antibiotic resistance becoming a global health crisis, the development of novel antimicrobial agents is of paramount importance. Halogenated compounds, especially fluoro-pharmaceuticals, are widely employed in contemporary medicinal chemistry; thus, the strategic introduction of biologically active halogen substituents, such as fluoro or bromo, at the para position of phenylarsonate rings in polyoxomolybdates represents a promising approach. Such hybrid frameworks not only enhance stability under physiological conditions but also offer tailored bioactivity via modulation of their organic constituents. Notably, Kortz’s group has shown previously that {PhSb^III}-incorporated polyoxotungstates exhibit enhanced antibacterial and antitumor effects, underlining the therapeutic potential of functionalized POMs.

Expanding on our prior work with reduced molybdate systems in the presence of cacodylate and phosphonates under aqueous, reducing conditions, we now report the substitution of phosphonates with arsonates. This modification yielded a series of reduced, dimethylarsinate-functionalized arsenomolybdates­(V), [RAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– and [As^IIIMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^3–. These structures incorporate a variety of organoarsonates (RAsO₃ ^2–), arsenate, and arsenite as structure-directing agents. Given the increasing utility of ESI-MS for characterizing the intricate structures of complex inorganic POM clusters, with numerous gas-phase fragmentation studies of POMs being investigated using CID MS/MS, we undertook a detailed mass spectrometric analysis to elucidate the fragmentation behavior of these novel arsenomolybdate species. Furthermore, recognizing both their structural novelty and the rising interest in bioactive POMs, we carried out preliminary studies to assess the biological activity of these arsenomolybdates.

Experimental Section

Materials and Physical Measurements

The precursors ethylarsonic acid, C₂H₅AsO₃H₂, (4-fluorophenyl)­arsonic acid, (4-FC₆H₄)­AsO₃H₂, (4-trifluoromethylphenyl)­arsonic acid, (4-F₃CC₆H₄)­AsO₃H₂, (4-trifluoromethoxyphenyl)­arsonic acid, (4-F₃COC₆H₄)­AsO₃H₂, (4-bromophenyl)­arsonic acid, (4-BrC₆H₄)­AsO₃H₂, and (4-azidophenyl)­arsonic acid, (4-N₃C₆H₄)­AsO₃H₂, were synthesized following published literature procedures and characterized using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The precursor 3,5-bis­(carboxy)­phenylarsonic acid, (3,5-(HOOC)₂C₆H₃AsO₃H₂), is reported and characterized here for the first time. All other reagents were purchased from commercial sources and used as received without further purification.

The elemental analyses were performed by Zentrallabor, Technische Universität Hamburg (TUHH), Am Schwarzenberg-Campus 1, 21073 Hamburg (Na, Mo, and As), and Analytische Laboratorien, Industriepark Kaiserau (Haus Heidbruch), 51789 Lindlar, Germany (C, H, N, F, and Br). The sodium content was further verified in-house by atomic absorption (AA) spectroscopy on a Varian SpectrAA 220 AA spectrometer. The FT-IR spectra were recorded on KBr pellets using a Nicolet Avatar 370 spectrophotometer operating in the 400–4000 cm^–1 range with 32 scans and a 4 cm^–1 resolution. The peak intensities are abbreviated as follows: w, weak; m, medium; s, strong; sh, shoulder. The crystal water content was analyzed via thermogravimetric analysis (TGA) on a TA Instrument SDT Q600 ramped from room temperature to 600 °C at a heating rate of 5 °C min^–1 under a N₂ flow of 10 mL min^–1. Multinuclear solution NMR spectra (¹H, ¹³C­{H}, and ¹⁹F) were recorded at room temperature on a JEOL ECS 400 MHz spectrometer using 5 mm probes, tuned to respective resonance frequencies of 399.78 MHz (¹H), 100.52 MHz (¹³C­{H}), and 376.17 MHz (¹⁹F). The chemical shifts were referenced to tetramethylsilane (¹H and ¹³C­{H}) and CFCl₃ (¹⁹F). Acquisition of the ¹³C­{H} NMR spectra for the polyanions required an overnight run. For assignments of the ¹H and ¹³C NMR resonances, see numbering schemes in the SI. Ultraviolet–visible (UV–vis) studies were conducted using a Varian Cary 100 Bio UV–vis spectrophotometer over the 200–800 nm range, using 1 cm quartz cuvettes.

High-resolution mass spectra were recorded using a Bruker Daltonics QTOF Impact HD mass spectrometer employing both negative and positive electrospray ionization modes. The QTOF Impact mass spectrometer (Bruker Daltonics) was fitted with an ESI source, and external calibration was achieved with 10 mL of 0.1 M sodium formate solution. The instrument ion source and tubing were rinsed with methanol. Calibration was carried out using the enhanced quadratic calibration mode. All MS measurements were performed in both negative and positive ion modes. Samples were measured as direct infusions at a concentration of 10 μg/mL in deionized water at a flow rate of 180 μL/min. Samples were prepared by dissolving 1 mg of POM in 1 mL of deionized water followed by a 1:100 dilution. Spectral simulations were carried out in Data Analysis 4.1 (Bruker Daltonics, Bremen). For tandem MS measurements, POM precursor ions were isolated with an isolation window of 20 Da and fragmented by CID if necessary, with an increase in collision energy until the disappearance of the precursor ion.

Synthetic Procedures

Synthesis of 3,5-Bis­(carboxy)­phenylarsonic Acid, 3,5-(HOOC)₂C₆H₃AsO₃H₂

Dimethyl 5-aminoisophtalate (4.18 g, 20.0 mmol) was dissolved in 22 mL of H₂O, acidified with 5 mL of conc. HCl, and cooled to 0 °C in an ice bath. After stirring for 15 min, a cold aqueous solution of NaNO₂ (1.45 g, 21.0 mmol, in 5 mL H₂O) was added dropwise. Separately, a mixture of As₂O₃ (2.50 g, 12.6 mmol) and Na₂CO₃ (5.30 g, 33.1 mmol) was dissolved in 25 mL of hot water and stirred for 20 min, followed by the addition of CuSO₄·5H₂O (0.11 g, 21.7 mmol). The solution was heated until boiling for another 15 min. Upon cooling, 5 mL of diethyl ether was added to form an organic layer on the liquid. Then, the former solution was added dropwise under stirring for 1 h in an ice bath. The resulting mixture was then filtered and acidified to pH 6, filtered again, and finally acidified to pH 1 before a final filtration. The volume of the solution was reduced to one-third, and the mixture was refrigerated to induce precipitation. The resulting off-white solid, 3,5-bis­(methoxycarbonyl)­phenylarsonic acid, was washed with diluted HCl and collected for further analysis. Yield: 1.2 g (19%). FT-IR (KBr pellet, ν/cm^–1): 3444 (m), 2957 (m), 2336 (m), 1736 (s), 1444 (m), 1277 (s), 1144 (w), 1112 (w), 998 (m), 916 (s), 849 (w), 803 (m), 751 (m), 671 (w), 495 (w) (Figure S1). ¹H NMR (DMSO-d ₆, ppm): δ 3.9 (s, 6H, CH ₃), 8.4 (s, 2H, ortho-C₆ H ₃), 8.6 (s, 1H, para-C₆ H ₃). ¹³C­{¹H} NMR (DMSO-d ₆, ppm): δ 54.0 (CH₃), 132.1 (C-2), 134.0 (C-1), 135.4 (C-3), 136.8 (C-4), 165.4 (COOCH₃) (Figure S2).

3,5-Bis­(methoxycarbonyl)­phenylarsonic acid (1.2 g) was dissolved in 14 mL of water, followed by the addition of 7 mL of concentrated HCl. The solution was refluxed for 24 h at 105 °C. After cooling, the solvent was evaporated under vacuum, and the resulting white precipitate of 3,5-bis­(carboxy)­phenylarsonic acid, 3,5-(HOOC)₂C₆H₃AsO₃H₂, was air-dried for 1 day. Yield: 0.7 g (64%). FT-IR (KBr pellet, ν/cm^–1): 3460 (m), 3174 (m), 3097 (m), 2325 (m), 1881 (m), 1725 (s), 1668 (m), 1603 (m), 1453 (w), 1322 (m), 1293 (s), 1238 (s), 1194 (s), 1161 (s), 1118 (s), 994 (w), 881 (s), 821 (m), 799 (s), 776 (m), 712 (w), 668 (s), 518 (w), 486 (w), 433 (w) (Figure S1). ¹H NMR (DMSO-d ₆, ppm): δ 8.4 (s, 2H, ortho-C₆ H ₃), 8.6 (s, 1H, para-C₆ H ₃). ¹³C­{¹H} NMR (DMSO-d ₆, ppm): δ 132.9 (C-2), 134.1 (C-1), 134.9 (C-3), 135.8 (C-4), 166.0 (COOH) (Figure S3).

Synthesis of Na_2.5(NH₄)_0.5[As^IIIMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·11H₂O (NaNH₄–As^IIIMo₆ )

Na₂MoO₄·2H₂O (0.024 g, 0.10 mmol), N₂H₄·2HCl (0.011 g, 0.10 mmol), and arsenic trioxide, As₂O₃ (0.01 g, 0.05 mmol) were dissolved in 2 mL of 1 M sodium dimethylarsinate buffer (1 M aqueous solution of dimethylarsinic acid (pH 4) adjusted to pH 7 by adding NaOH pellets). The resulting mixture was stirred and heated at 80 °C for 1 h in a sealed vial. Dark-red, hexagonal crystals (Figure S4a) gradually formed over 2 weeks as the reaction mixture cooled. The crystals were then filtered and air-dried. Isolated yield: 0.017 g (63% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–As^IIIMo₆ : Na, 3.55 (3.85); Mo, 35.6 (35.8); As, 18.5 (19.3); C, 4.46 (4.49); H, 2.80 (3.27); N, 0.43 (0.62). FT-IR (2% KBr pellet, ν/cm^–1): 3430 (s) [ν­(O–H)], 2997 (w), 2927 (w) [ν­(C–H)], 1643 (s) [δ­(O–H)], 1410 (m) [δ­(C–H)], 1276 (m) [ν­(As–C)], 1032 (m), 938 (s) [ν­(MoO)], 856 (s) [ν­(As–O)], 753 (s), 622 (s), 493 (s), 428 (s) [δ­(Mo–O­(Mo))] (Figure S5a). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 3H, (CH ₃)₂AsO₂), 2.0 (s, 3H, (CH ₃)₂AsO₂) (Figure a). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.9 ((CH₃)₂AsO₂), 19.4 ((CH₃)₂AsO₂) (Figure b).

2.

Multinuclear NMR study: (a) ¹H and (b) ¹³C­{¹H} NMR spectra of NaNH₄-As^IIIMo₆ (red) and NaNH₄-As^VMo₆ (blue), dissolved in H₂O/D₂O at room temperature. The spectra of the reference compounds, i.e., cacodylic acid (H-Cac, pH 4.10) and sodium cacodylate (Na-Cac, pH 6.0) buffer, are shown in black.

Synthesis of Na_1.5(NH₄)_0.5[HOAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·0.6NH₄Cl·10H₂O (NaNH₄–As^VMo₆ )

(NH₄)₆Mo₇O₂₄·4H₂O (0.265 g, 0.215 mmol) and N₂H₄·2HCl (0.052 g, 0.500 mmol) were completely dissolved in 10 mL of 0.5 M sodium dimethylarsinate buffer (pH 7). Following this, disodium hydrogen arsenate heptahydrate, Na₂HAsO₄·7H₂O (0.078 g, 0.250 mmol), was added to the mixture and the pH was gradually adjusted to 6.8–6.9 by the dropwise addition of 6 M NaOH solution. The resulting dark-red solution was heated at 80 °C for 1 h. After 3 weeks of cooling at room temperature, dark-red, block-shaped crystals (Figure S4b) formed, which were collected and air-dried. Isolated yield: 0.06 g (15% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–As^VMo₆ : Na, 2.12 (2.46); Mo, 35.4 (34.9); As, 18.4 (18.0); C, 4.43 (4.48); H, 2.88 (2.95); N, 0.95 (1.19). FT-IR (2% KBr pellet, ν/cm^–1): 3397 (s), 3220 (s) [ν­(N–H), ν­(O–H)], 3014 (w), 2924 (w) [ν­(C–H)], 1630 (m) [δ­(O–H)], 1402 (m) [δ­(C–H)], 1271 (w) [ν­(As–C)], 966 (s), 921 (w) [ν­(MoO)], 822 (s) [ν­(As–O)], 746 (m), 654 (w), 527 (w), 494 (m), 457 (m) [δ­(Mo–O­(Mo))] (Figure S5b). ¹H NMR (H₂O/D₂O, ppm): δ 1.9 (s, 3H, (CH ₃)₂AsO₂), 2.2 (s, 3H, (CH ₃)₂AsO₂) (Figure a). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 18.7 ((CH₃)₂AsO₂), 18.9 ((CH₃)₂AsO₂) (Figure b).

Synthesis of NaNH₄[CH₃AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·10H₂O (NaNH₄–CH₃AsMo₆ )

(NH₄)₆Mo₇O₂₄·4H₂O (0.053 g, 0.04 mmol), N₂H₄·2HCl (0.011 g, 0.10 mmol), and disodium methylarsonate, CH₃AsO₃Na₂ (0.009 g, 0.05 mmol), were added to 2 mL of 0.5 M sodium dimethylarsinate buffer (pH 7). The resulting mixture was stirred and heated at 80 °C for 1 h. Dark-red, block-shaped crystals started appearing after 1 day as the solution cooled to room temperature. Isolated yield: 0.030 g (40% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–CH₃AsMo₆ : Na, 1.45 (1.22); Mo, 36.2 (35.5); As, 18.9 (19.1); C, 5.29 (5.41); H, 3.05 (3.17); N, 0.88 (1.07). FT-IR (2% KBr pellet, ν/cm^–1): 3416 (s), 3202 (s) [ν­(N–H), ν­(O–H)], 3017 (w), 2925 (w) [ν­(C–H)], 1625 (s) [δ­(O–H)], 1477 (w), 1442 (w), 1403 (m) [δ­(C–H)], 1275 (w) [ν­(As–C)], 968 (s), 936 (w) [ν­(MoO)], 822 (s) [ν­(As–O)], 748 (m), 656 (w), 521 (w), 493 (m), 459 (m) [δ­(Mo–O­(Mo))] (Figure S5c). ¹H NMR (H₂O/D₂O, ppm): δ 0.96 (s, 3H, CH ₃AsO₃), 1.9 (s, 9H, (CH ₃)₂AsO₂), and 2.2 (s, 9H, (CH ₃)₂AsO₂) (Figure S20a). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 18.0 ((CH₃)₂AsO₂), 18.9 (CH₃AsO₃), 19.7 ((CH₃)₂AsO₂) (Figure S20b).

Synthesis of Na_1.5(NH₄)_0.5[C₂H₅AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·10H₂O (NaNH₄–C₂H₅AsMo₆ )

The polyanion C₂H₅AsMo₆ was synthesized following the same procedure as for As^IIIMo₆ , except that ethylarsonic acid, C₂H₅AsO₃H₂ (0.008 g, 0.05 mmol), was used instead of As₂O₃. The reaction mixture (pH 7) was heated at 80 °C for 1 h in a sealed vial. Upon cooling to room temperature, red crystals (Figure S4c) began forming within a day. Isolated yield: 0.015 g (56% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–C₂H₅AsMo₆ : Na, 2.2 (2.4); Mo, 35.9 (36.3); As, 18.7 (19.8); C, 5.99 (6.16); H, 3.01 (3.30); N, 0.44 (0.49). FT-IR (2% KBr pellet, ν/cm^–1): 3401 (s) [ν­(O–H)], 3014 (w), 2926­(w) [ν­(C–H)], 1636 (m) [δ­(O–H)], 1456 (w), 1406 (m) [δ­(C–H)], 1274 (m) [ν­(As–C)], 968 (s), 918 (w) [ν­(MoO)], 824 (s) [ν­(As–O)], 747 (m), 655 (w), 615 (w), 493 (m), 459 (m) [δ­(Mo–O­(Mo))] (Figure S5d). ¹H NMR (H₂O/D₂O, ppm): δ 0.57 (t, 3H, ³ J _HH = 8 Hz, CH ₃CH₂AsO₃), 1.21 (q, 2H, ³ J _HH = 8 Hz, CH₃CH ₂AsO₃), 1.9 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂) (Figure S22).

Synthesis of NaNH₄[C₆H₅AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·10H₂O (NaNH₄–C₆H₅AsMo₆ )

The polyanion C₆H₅AsMo₆ was synthesized by dissolving (NH₄)₆Mo₇O₂₄·4H₂O (0.053 g, 0.04 mmol), N₂H₄·2HCl (0.011 g, 0.10 mmol), and phenylarsonic acid, C₆H₅AsO₃H₂ (0.01 g, 0.05 mmol) in 2 mL of 0.5 M sodium dimethylarsinate buffer (pH 7). The reaction mixture was stirred and heated at 80 °C for 1 h. Red block-shaped crystals formed within a day upon cooling to room temperature. Isolated yield: 0.025 g (32% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–C₆H₅AsMo₆ : Na, 1.39 (1.15); Mo, 34.9 (34.4); As, 18.2 (18.5); C, 8.73 (9.04); H, 3.05 (3.21); N, 0.85 (1.07). FT-IR (2% KBr pellet, ν/cm^–1): 3410 (s) [ν­(N–H), ν­(O–H)], 3014 (w), 2927 (w) [ν­(C–H)], 1623 (m) [δ­(O–H)], 1444 (w), 1401 (m) [δ­(C–H)], 1274 (w) [ν­(As–C)], 1097 (w), 969 (s), 939 (w), 912 (w) [ν­(MoO)], 826 (s) [ν­(As–O)], 746 (s), 692 (w), 655 (w), 612 (w), 524 (w), 493 (m), 458 (m) [δ­(Mo–O­(Mo))] (Figure S5e). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 7.07 (d, 2H, ³ J _HH = 8 Hz, ortho-C₆ H ₅), 7.37 (t, 2H, ³J_HH = 8 Hz, meta-C₆ H ₅), 7.46 (t, 1H, ³ J _HH = 8 Hz, para-C₆ H ₅). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.8 ((CH₃)₂AsO₂), 19.5 ((CH₃)₂AsO₂), 129.4 (C-3), 129.7 (C-2), 131.2 (C-4), 133.6 (C-1) (Figure S23).

High-quality single crystals of Na–C₆H₅AsMo₆ suitable for single-crystal XRD were obtained by heating Na₂MoO₄·2H₂O (0.024 g, 0.10 mmol), N₂H₄·2HCl (0.011 g, 0.10 mmol), and phenylarsonic acid, C₆H₅AsO₃H₂ (0.01 g, 0.05 mmol), in 2 mL, 1 M sodium dimethylarsinate buffer (pH 7) at 80 °C for 1 h. The resulting product was characterized via IR spectroscopy (Figure S6).

Synthesis of NaNH₄[4-FC₆H₄AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·7H₂O (NaNH₄–FC₆H₄AsMo₆ )

The polyanion 4-FC₆H₄AsMo₆ is synthesized in the same way as C₆H₅AsMo₆ by adding (4-fluorophenyl)­arsonic acid, 4-FC₆H₄AsO₃H₂ (0.011 g, 0.05 mmol), instead of C₆H₅AsO₃H_2. The reaction mixture (pH 6.7–6.8) was stirred and heated at 80 °C for 1 h. Dark-red, block-shaped crystals started appearing after 2 days as the solution cooled to room temperature. Isolated yield: 0.067 g (89% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–FC₆H₄AsMo₆ : Na, 1.42 (1.54); Mo, 35.6 (36.1); As, 18.6 (19.1); F, 1.18 (1.02); C, 8.92 (8.37); H, 2.68 (2.77); N, 0.87 (1.03). FT-IR (2% KBr pellet, ν/cm^–1): 3403 (s), 3214 (s) [ν­(N–H), ν­(O–H)], 3017 (w), 2920 (w) [ν­(C–H)], 1631 (m) [δ­(O–H)], 1591 (sh), 1496 (w) [ν­(C = C)], 1403 (m) [δ­(C–H)], 1273 (w) [ν­(As–C)], 1232 (w), 1164 (w), 1095 (w) [ν­(C–F)], 968 (s), 918 (w) [ν­(MoO)], 843 (s) [ν­(As–O)], 743 (m), 655 (w), 586 (w), 492 (m), 460 (m) [δ­(Mo–O­(Mo))] (Figure S7a). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 7.06 (m, 4H, C₆ H ₄) (Figure S25). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.9 ((CH₃)₂AsO₂), 19.7 ((CH₃)₂AsO₂), 116.9 (d, ² J _CF = 22.14 Hz, C-3), 127.2 (s, C-1), 132.5 (d, ³ J _CF = 8.44 Hz, C-2), 165.7 (d, ¹ J _CF = 261.35 Hz, C-4). ¹⁹F NMR (H₂O/D₂O, ppm): δ −104.5 (Figure S26).

Synthesis of NaNH₄[4-F₃CC₆H₄AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·11H₂O (NaNH₄–F₃CC₆H₄AsMo₆ )

The polyanion 4-F₃CC₆H₄AsMo₆ is prepared like C₆H₅AsMo₆ by replacing C₆H₅AsO₃H₂ with (4- trifluoromethylphenyl)­arsonic acid, 4-F₃CC₆H₄AsO₃H₂ (0.013 g, 0.05 mmol). The reaction mixture (pH 6.7–6.8) was stirred and heated at 80 °C for 1 h. The total isolated yield of red, block-shaped crystals after 2 days is 0.07 g (86% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–F₃CC₆H₄AsMo₆ : Na, 1.32 (1.13); Mo, 33.1 (32.9); As, 17.2 (17.6); F, 3.28 (3.45); C, 8.99 (8.41); H, 2.96 (2.79); N, 0.81 (1.24). FT-IR (2% KBr pellet, ν/cm^–1): 3418 (s), 3199 (s) [ν­(N–H), ν­(O–H)], 3017­(w), 2928 (w) [ν­(C–H)], 1636 (s) [δ­(O–H)], 1402 (s) [δ­(C–H)], 1326 (s), 1273 (w) [ν­(As–C)], 1173 (m), 1132 (m), 1060 (m) [ν­(C–F)], 1012 (sh), 969 (m), 924 (w) [ν­(MoO)], 844 (s) [ν­(As–O)], 744 (m), 655 (w), 590 (m), 494 (m), 465 (m) [δ­(Mo–O­(Mo))] (Figure S7b). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 7.18 (d, 2H, ³J_HH = 8 Hz, ortho-C₆ H ₄), 7.67 (d, 2H, ³ J _HH = 8 Hz, meta-C₆ H ₄) (Figure S27). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.7 ((CH₃)₂AsO₂), 19.5 ((CH₃)₂AsO₂), 123.3 (q, ¹ J _CF = 272.4 Hz, CF₃), 126.2 (s, C-3), 130.3 (s, C-2), 134.3 (q, ² J _CF = 32.2 Hz, C-4), 135.3 (s, C-1). ¹⁹F NMR (H₂O/D₂O, ppm): δ −63.1 (Figure S28).

Synthesis of NaNH₄[4-F₃COC₆H₄AsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]·10H₂O (NaNH₄–F₃COC₆H₄AsMo₆ )

The polyanion 4-F₃COC₆H₄AsMo₆ was prepared by following the same procedure as for C₆H₅AsMo₆ , by replacing C₆H₅AsO₃H₂ with (4-trifluoromethoxyphenyl)­arsonic acid, 4-F₃COC₆H₄AsO₃H₂ (0.012 g, 0.05 mmol). The reaction mixture (pH 6.7–6.8) was stirred and heated at 80 °C for 1 h. After 2 days of cooling to room temperature, dark-red, rod-shaped crystals (Figure S4d) began to form. The total isolated yield is 0.06 g (74% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–F₃COC₆H₄AsMo₆ : Na, 1.32 (1.32); Mo, 33.2 (32.2); As, 17.3 (17.3); F, 3.28 (3.36); C, 9.0 (9.4); H, 2.8 (2.4); N, 0.81 (1.21). FT-IR (2% KBr pellet, ν/cm^–1): 3403 (s), 3208 (s) [ν­(N–H), ν­(O–H)], 3023 (w), 2929 (w) [ν­(C–H)], 1630 (s) [δ­(O–H)], 1495 (m), 1404 (s) [δ­(C–H)], 1259 (s) [ν­(As–C)], 1217 (s), 1171 (s) [ν­(C–F)], 1097 (w), 969 (s) [ν­(MoO)], 829 (s) [ν­(As–O)], 745 (s), 657 (w), 612 (w), 494 (m), 459 (m) [δ­(Mo–O­(Mo))] (Figure S7e). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 7.07 (d, 2H, ³ J _HH = 8 Hz, ortho-C₆ H ₄), 7.23 (d, 2H, ³ J _HH = 8 Hz, meta-C₆ H ₄). ¹⁹F NMR (H₂O/D₂O, ppm): δ −57.7 (Figure S29). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.6 ((CH₃)₂AsO₂), 19.4 ((CH₃)₂AsO₂), 118.6 (OCF₃), 121.4 (s, C-3), 129.6 (s, C-4), 131.8 (s, C-2), 152.4 (s, C-1) (Figure S30).

Synthesis of Na_0.7(NH₄)_1.3[4-Br­C₆­H₄­AsMo^V ₆O₁₅­(OH)₃{AsO₂­(CH₃)₂}₃]·8H₂O (NaNH₄–BrC₆H₄AsMo₆ )

The polyanion 4-BrC₆H₄AsMo₆ was prepared by following the same procedure as C₆H₅AsMo₆ , by replacing C₆H₅AsO₃H₂ with (4-bromophenyl)­arsonic acid, 4-BrC₆H₄AsO₃H₂ (0.014 g, 0.05 mmol). After stirring and heating the reaction mixture (pH 6.7–6.8) at 80 °C for 1 h, it was allowed to cool gradually to room temperature over 2 days, leading to the formation of dark-red, rod-shaped crystals. The total isolated yield is 0.069 g (87% based on Mo). Elemental analysis (%) calcd (found) for NaNH₄–BrC₆H₄AsMo₆: Na, 0.95 (1.25); Mo, 34.0 (34.6); As, 17.7 (18.6); Br, 4.72 (4.15); C, 8.52 (7.87); H, 2.75 (2.77); N, 1.08 (1.31). FT-IR (2% KBr pellet, ν/cm^–1): 3419 (s), 3179 (s) [ν­(N–H), ν­(O–H)], 3008 (w), 2926 (w) [ν­(C–H)], 1632 (m) [δ­(O–H)], 1573 (w), 1474 (sh), 1402 (m) [δ­(C–H)], 1272 (w) [ν­(As–C)], 1183 (w), 1095 (w), 1063 (w) [ν­(C–Br)], 1009 (w), 969 (s), 921 (w) [ν­(MoO)], 834 (s) [ν­(As–O)], 744 (s), 655 (w), 587 (w), 497 (m), 468 (m) [δ­(Mo–O­(Mo))] (Figure S7c). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 6.9 (d, 2H, ³ J _HH = 9 Hz, meta-C₆ H ₄), 7.5 (d, 2H, ³ J _HH = 9 Hz, ortho-C₆ H ₄). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.7 ((CH₃)₂AsO₂), 19.4 ((CH₃)₂AsO₂), 128.1 (C-4), 130.2 (C-1), 131.1 (C-3), 132.5 (C-2) (Figure S32).

High-quality crystals for single-crystal XRD were synthesized by heating Na₂MoO₄·2H₂O (0.024 g, 0.10 mmol), N₂H₄·2HCl (0.011 g, 0.1 mmol), and (4-bromophenyl)­arsonic acid, 4-BrC₆H₄AsO₃H₂ (0.014 g, 0.05 mmol), in 2 mL of 1 M sodium dimethylarsinate buffer (pH 7) at 80 °C for 1 h. The identity of the crystals was verified by IR spectroscopy (Figure S8).

Synthesis of Na_1.3(NH₄)_0.7­[4-N₃­C₆H₄­AsMo^V ₆O₁₅­(OH)₃­{AsO₂­(CH₃)₂}₃]·10H₂O (NaNH₄–N₃C₆H₄AsMo₆ )

The polyanion 4-N₃C₆H₄AsMo₆ was prepared by following the same procedure as C₆H₅AsMo₆ , by replacing C₆H₅AsO₃H₂ with (4-azidophenyl)­arsonic acid, 4-N₃C₆H₄AsO₃H₂ (0.012 g, 0.05 mmol). Following 1 h of stirring and heating at 80 °C, the reaction mixture (pH 6.7–6.8) was left to cool at room temperature for 2 days, resulting in the formation of dark-red, rod-shaped crystals. Isolated yield: 0.072 g (91% based on Mo). Elemental analysis (%): Calcd (found) for NaNH₄–N₃C₆H₄AsMo₆ : Na, 1.76 (1.94); Mo, 34.0 (34.2); As, 17.7 (18.5); C, 8.5 (7.9); H, 2.8 (2.8); N, 3.06 (2.76). FT-IR (2% KBr pellet, ν/cm^–1): 3403 (s), 3185 (s) [ν­(N–H), ν­(O–H)], 3014 (w), 2926 (w) [ν­(C–H)], 2129 (s) [ν­(N₃)], 1634 (s) [δ­(O–H)], 1589 (sh), 1492 (sh), 1403 (s) [δ­(C–H)], 1276 (s) [ν­(As–C)], 1188 (w), 1133 (w), 1096 (w), 968 (m), 922 (w) [ν­(MoO)], 843 (s) [ν­(As–O)], 742 (m), 655 (w), 586 (m), 491 (m), 465 (m) [δ­(Mo–O­(Mo))] (Figure S7d). ¹H NMR (H₂O/D₂O, ppm): δ 1.6 (s, 9H, (CH ₃)₂AsO₂), 2.3 (s, 9H, (CH ₃)₂AsO₂), 6.58 (d, 2H, ³ J _HH = 9 Hz, ortho-C₆ H ₄), 6.72 (d, 2H, ³ J _HH = 9 Hz, meta-C₆ H ₄) (Figure S34).

Synthesis of NaNH₄­[3,5-(HOOC)₂C₆­H₃AsMo^V ₆O₁₅­(OH)₃{AsO₂­(CH₃)₂}₃]·14H₂O (NaNH₄–H₂O₄C₂C₆H₃AsMo₆ )

The polyanion H₂O₄C₂C₆H₃AsMo₆ was synthesized using the same procedure for C₆H₅AsMo₆ , substituting C₆H₅AsO₃H₂ with 3,5-bis­(carboxy)­phenylarsonic acid, 3,5-(HOOC)₂C₆H₃AsO₃H₂ (0.015 g, 0.05 mmol). After stirring and heating at 80 °C for 1 h, the reaction mixture (pH 6.7–6.8) was cooled, centrifuged at 4000 rpm for 15 min, and filtered. The resulting red filtrate was left undisturbed in an open vial for slow crystallization. After 7 days, reddish block-shaped crystals were obtained. Isolated yield: 0.028 g (33% based on Mo). Elemental analysis (%): calcd (found) for NaNH₄–H₂O₄C₂C₆H₃AsMo₆ : Na, 1.27 (1.52); Mo, 31.8 (30.6); As, 16.6 (16.7); C, 9.29 (9.32); H, 3.23 (3.07); N, 0.77 (1.45). FT-IR (2% KBr pellet, ν/cm^–1): 3445 (s), 3229 (s) [ν­(N–H), ν­(O–H)], 3022 (w), 2927 (w) [ν­(C–H)], 1635 (m) [δ­(O–H)], 1560 (m) [υ­(CO)], 1401 (m) [δ­(C–H)], 1280 (w) [ν­(As–C)], 1108 (w), 970 (s) [ν­(MoO)], 840 (s) [ν­(As–O)], 746 (m), 699 (w), 656 (w), 488 (m), 460 (m) [δ­(Mo–O­(Mo))] (Figure S7f). ¹H NMR (H₂O/D₂O, ppm): δ 1.5 (s, 9H, (CH ₃)₂AsO₂), 2.2 (s, 9H, (CH ₃)₂AsO₂), 7.6 (s, 2H, ortho-C₆ H ₃), 8.3 (s, 1H, para-C₆ H ₃). ¹³C­{¹H} NMR (H₂O/D₂O, ppm): δ 17.8 ((CH₃)₂AsO₂), 19.4 ((CH₃)₂AsO₂), 131.8 (C-1), 133.1 (C-3), 134.0 (C-2), 135.6 (C-4), 171.0 (COOH) (Figure ).

3.

¹H (left) and ¹³C­{¹H} (right) NMR spectra of NaNH₄–H₂O₄C₂C₆H₃AsMo₆ dissolved in H₂O/D₂O at room temperature. The peaks corresponding to structurally and hence magnetically inequivalent carbon atoms are labeled accordingly. Color code: MoO₆, violet octahedra; As, green; C, yellow; O, red; H, black.

X-ray Crystallography

Single-crystal XRD data for NaNH₄–As^VMo₆ were collected at 100 K using a Bruker D8 SMART APEX II CCD diffractometer with kappa geometry and a graphite monochromator (λ_Mo Kα = 0.71073 Å), with the APEX III software package. Cell refinement and data reduction were performed using SAINT, while multiscan absorption corrections were applied via SADABS. For NaNH₄–As^IIIMo₆ , NaNH₄–CH₃AsMo₆ , NaNH₄–C₂H₅AsMo₆ , NaNH₄–C₆H₅AsMo₆ , NaNH₄–FC₆H₄AsMo_6, NaNH₄–F₃CC₆H₄AsMo₆ , NaNH₄–F₃­COC₆­H₄As­Mo₆ , NaNH₄–Br­C₆H₄As­Mo₆ , NaNH₄–N₃­C₆H₄As­Mo₆ and NaNH₄–H₂­O₄C₂­C₆H₃AsMo₆ , indexing and data collection were carried out on a Rigaku XtaLAB Synergy Dualflex HyPix single-crystal diffractometer with kappa geometry and a graphite monochromator with Mo Kα/Cu Kα radiation (λ = 0.71073/1.54184 Å) (Table S1). All measurements were conducted with crystals mounted on Hampton cryoloops with paratone-N oil at 100 K. Corrections for absorption effects were applied with CrysAlisPro 1.171.43.97a. The structures were solved by direct methods using SHELXT, and full-matrix least-squares structure refinements were performed with SHELXL-2018/3 implemented in Olex2 1.5-ac5–024. All nonhydrogen atoms were refined anisotropically. In several crystal structures, the voids contain disordered solvents. The Olex2 solvent mask routine (similar to PLATON/SQUEEZE) was used to mask out the disordered electron density. Crystallographic data for the polyanions are summarized in Table S1. The CIF files are provided free of charge by The Cambridge Crystallographic Data Centre (CCDC 2464737–2464747).

Bond Valence Sum Calculations

The bond valence sum (BVS) calculations for the molybdenum and oxygen atoms (Tables S2–S12) were conducted using a program developed by Chris Hormillosa and Sean Healy and distributed by I. D. Brown.

Antibacterial Activity: Determination of Minimal Inhibitory Concentrations (MICs) for Bacterial Cells

The MIC experiments were built upon our prior research, the procedures following our previous work and Elshamy et al. Precultures were established by inoculating single bacterial colonies into 5 mL of the appropriate liquid medium. After 24 h, the optical density (OD) at 600 nm was assessed. The resulting cell suspension was adjusted to a concentration of 2 × 10⁶ bacterial cells per mL. To determine the MICs, 96-well plates were set up with a dilution series of NaNH₄–As^VMo₆ , NaNH₄–CH₃­As­Mo₆ , Na­NH₄–C₆H₅­AsMo₆ , Na­NH₄–FC₆­H₄As­Mo₆ , Na­NH₄–F₃­CC₆H₄As­Mo₆ , Na­NH₄–F₃­COC₆­H₄As­Mo₆ , Na­NH₄–N₃­C₆H₄­As­Mo₆ , Na­NH₄–Br­C₆H₄­AsMo₆ , and Na­NH₄–H₂­O₄C₂C₆H₃­AsMo₆ beginning at 500 μg/mL. Stock solutions were prepared using sterile deionized water. The 96-well plates were incubated overnight at the optimal growth temperature for each organism tested. Outcomes were visually assessed, with MICs established based on the presence or absence of growth. For better visualization, resazurin was used as a fluorometric indicator to aid in determining the MICs. All MIC experiments were performed in triplicate, with three independent replicates. Agar diffusion and MIC are simple and widely known methods of determining the zone of inhibition and the lowest concentration of the samples against a defined organism, respectively.

Results and Discussion

Synthesis and Structure

A one-pot aqueous reaction of ammonium heptamolybdate, hydrazine hydrochloride, organoarsonate heterogroup, and sodium cacodylate buffer in a 1:2:1:20 molar ratio yielded a series of dimethylarsinate-functionalized arsenomolybdates: [CH₃As­Mo^V ₆­O₁₅­(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (CH₃As­Mo₆ ), [C₆H₅­AsMo^V ₆­O₁₅(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (C₆­H₅AsMo₆ ), [FC₆H₄­AsMo^V ₆­O₁₅(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (4-FC₆­H₄AsMo₆ ), [F₃­CC₆H₄­AsMo^V ₆­O₁₅(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (4-F₃­CC₆H₄As­Mo₆ ), [F₃­COC₆­H₄As­Mo^V ₆O₁₅­(OH)₃­{AsO₂(CH₃)₂}₃]^2– (4-F₃­COC₆H₄­As­Mo₆ ), [Br­C₆H₄­As­Mo^V ₆­O₁₅(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (4-Br­C₆H₄­As­Mo₆ ), [N₃­C₆H₄­AsMo^V ₆­O₁₅­(OH)₃­{AsO₂­(CH₃)₂}₃]^2– (4-N₃­C₆H₄As­Mo₆ ), and [3,5-(HOOC)₂­C₆H₃­As­Mo^V ₆O₁₅­(OH)₃{AsO₂­(CH₃)₂}₃]^2– (H₂O₄­C₂­C₆H₃­AsMo₆ ), all isolated as hydrated mixed ammonium-sodium salts. These compounds were successfully synthesized within a pH range of 6.4–6.8 at room temperature, 50, or 80 °C. In each case, the formation of a dark-red solution indicated the reduction of Mo^VI to Mo^V, which was confirmed by bond valence sum (BVS) calculations (Tables S4 and S6–S12). The highest isolated yields were achieved at 80 °C, with crystallization occurring within 1 week. Substituting ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O) with sodium molybdate (Na₂MoO₄·2H₂O) under identical conditions resulted in high-quality crystals, albeit at significantly lower yields. For the polyanion H₂O₄C₂C₆H₃AsMo₆ , BVS calculations indicated that the carboxylate moieties are monoprotonated (Table S12). Attempts to prepare the structural analogue using 4-HOOCC₆H₄AsO₃H₂ as the precursor ligand were unsuccessful. Despite numerous reactions and pH adjustments, the reported [Mo^VI ₁₈Mo^V ₁₂O₈₄{AsO₂(CH₃)₂}₁₈]^18– {Mo₃₀ } was formed instead.

Synthesis of the three isostructural polyanions [HO­As­Mo^V ₆­O₁₅­(OH)₃­{As­O₂­(CH₃)₂}₃]^2– (As^V­Mo₆ ), [As^III­Mo^V ₆­O₁₅(OH)₃­{AsO₂­(CH₃)₂}₃]^3– (As^IIIMo₆ ), and [C₂H₅­AsMo^V ₆­O₁₅(OH)₃­{As­O₂­(CH₃)₂}₃]^2– (C₂­H₅­As­Mo₆ ) required precise control over the molybdate precursor, reactant ratios, temperature, and pH. The As^VMo₆ polyanion was obtained under specific conditions, requiring a 1:2:1 molar ratio of ammonium heptamolybdate, hydrazine hydrochloride, and arsenate heterogroup in 10 mL of 0.5 M sodium cacodylate buffer at pH 6.6–6.8 and a reaction temperature of 80 °C. Attempts to use sodium molybdate or reduce the reaction temperature were unsuccessful. Structural analysis of As^VMo₆ indicated the monoprotonation of terminal oxygen (O9) in the As–O bond (1.689 Å), with a corresponding BVS value (v _ij = 1.24) (Table S2).

Conversely, As^IIIMo₆ and C₂H₅AsMo₆ were only obtained using sodium molybdate as the molybdenum source. The optimal molar ratio of sodium molybdate to As₂O₃ or C₂H₅AsO₃H₂ was 2:1, with reactions carried out at 80 °C within a pH range of 6.8–6.9. For these three species, prolonged heating did not improve yield, and crystallization occurred more slowly than the other members of the series.

Single-crystal XRD analysis confirmed that the molecular structures of these polyanions, [RAs­Mo^V ₆­O₁₅­(OH)₃­{As­O₂­(CH₃)₂}₃]^2– (R = HO, CH₃, C₂H₅, C₆H₅, 4-FC₆H₄, 4-F₃­CC₆­H₄, 4-F₃­COC₆­H_4, 4-Br­C₆H₄, 4-N₃­C₆H₄, and 3,5-(HOOC)₂­C₆H₃) (Figure a) and [As^III­Mo^V ₆­O₁₅­(OH)₃­{As­O₂­(CH₃)₂}₃]^3– (As^IIIMo₆ ) (Figure b) closely resemble the previously reported [RPMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = H, HO, CH₃, HO₂CCH₂, HO₂CC₂H₄, C₆H₅, 4-FC₆H₄, and 4-F₃COC₆H₄) and exhibit a distinct flowerpot-like architecture. The core structure consists of a cyclic hexanuclear molybdenum­(V)-oxo ring, formed by three edge-sharing {Mo^V ₂O₁₀} fragments interconnected via hydroxo bridges (verified through BVS calculations, see Tables S2–S12). These units are capped by three dimethylarsinate ((CH₃)₂AsO₂), also known as cacodylate ligands, which act as strongly coordinating bidentate ligands. This arrangement features alternating short Mo–Mo bonding interactions (∼2.59 Å) and longer Mo···Mo nonbonding contacts (∼3.58 Å). The reduced Mo^V ₂ pairs adopt a highly distorted octahedral coordination geometry, with each molybdenum center coordinated by two μ₂-oxo groups, Mo–O­(Mo) bonds: 1.939–1.949 Å, one μ₂-hydroxo group, Mo–O­(Mo) bond: 2.115–2.124 Å, one terminal oxo group, MoO_term bond: 1.682–1.696 Å, one oxo-donor from the terminal dimethylarsinate, Mo–O­(As) bond: 2.079–2.107 Å, and an oxo-donor from the central heterogroup, Mo–O­(As) bond: 2.212–2.304 Å.

1.

(a) Combined polyhedral/ball-and-stick representation of the polyanion family [RAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = HO, CH₃, C₂H₅, C₆H₅, 4-FC₆H₄, 4-F₃CC₆H₄, 4-F₃COC₆H₄, 4-BrC₆H₄, 4-N₃C₆H₄, and 3,5-(HOOC)₂C₆H₃), and (b) the As^III-centered derivative [As^IIIMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^3–, and (c) sodium ion-mediated dimer (formed by all polyanions in the solid state). Color code: MoO₆, violet octahedra; As, green; C, yellow; O, red; H, black; Na, light blue.

The central RAsO₃ heterogroups, adopting slightly distorted tetrahedral coordination, are positioned above the plane of the six molybdenum atoms and coordinate via three μ₃-oxo bridges. Notably, the As^III center in As^IIIMo₆ , which adopts pyramidal geometry due to its lone pair, exhibits stronger Mo–O­(As) bonding (2.212 Å) compared to the As^V center in As^VMo₆ (2.289 Å). Additionally, all three peripheral dimethylarsinate ligands feature tetrahedrally coordinated As centers, with average As–O­(Mo) bond lengths ranging from 1.67 to 1.80 Å. The average bond angles around As closely resemble those in uncoordinated dimethylarsinic acid (109.4°), and the As–C distances (1.90–1.94 Å) fall within the expected range, consistent with literature values. Both the peripheral ligands and the central heterogroups are grafted onto the same side of the polyanion.

Introducing a certain concentration of Na⁺ cations plays a crucial role in the polyanion formation, as evidenced by the solid-state structure (Figure c). A hexa-coordinated Na⁺ ion interacts with oxo bridges from the Mo^V ₂ pairs (Na···O = 2.27 Å), on the opposite side of the coordinated dimethylarsinate ligands, thus facilitating Na⁺-mediated dimerization of the polyanions.

The precise formula units of the polyanions were determined through elemental analysis, thermogravimetric analysis (TGA) of the bulk material, bond valence sum (BVS) analysis, and single-crystal XRD analysis. The thermograms (Figures S9–S19) indicated two primary stages of weight loss. The first stage, between 25 and 130 °C, corresponds to dehydration, during which interstitial water molecules were released from the structure. The number of crystal water molecules identified by TGA and elemental analysis varied due to differences in drying times and the aging of the samples at room temperature prior to measurements. The consecutive weight loss steps between 160–480 °C were attributed to the elimination of ammonia (NH₃) molecules, organic dimethylarsinate ligands, and central organic moieties, leading to complete structural decomposition.

Infrared (IR) Spectroscopy

Fourier transform infrared (FT-IR) spectra (Figures S5–S8) display bands corresponding to the asymmetric stretching (υ_as[O–H]) and bending (δ­[O–H]) vibrations of interstitial water molecules in the regions of 3400–3000 and 1640–1620 cm^–1. Furthermore, broad peaks in the 3600–3400 cm^–1 range indicate the stretching vibrations of the NH₄ ⁺ group. The presence of NH₄ ⁺ as counter cations in the clusters is confirmed by elemental analysis. Two weak peaks in the 3030–2800 cm^–1 range, along with medium-intensity bands near 1404 cm^–1, are attributed to asymmetric stretching (υ_as[C–H]) and bending (δ­[C–H]) vibrations of the methyl groups in the dimethylarsinate ligands. The characteristic bands observed between 980–850 and 600–400 cm^–1 are assigned to the asymmetric stretching (υ_as[MoO]) and (υ_as[Mo–O­(Mo)]) vibrations. Strong bands at 850–820 cm^–1 and medium-intensity peaks at 1280–1250 cm^–1 are associated with asymmetric stretching (υ_as[As–O]) and (υ_as[As–C]) vibrations. The strong band at 2120 cm^–1, attributed to the asymmetric stretching of the azido group, and weak bands for asymmetric C–F stretching between 1200 and 1000 cm^–1 (Figure S7) confirm the incorporation of phenylarsonate derivatives. The C–Br stretching occurs between 1093 and 1063 cm^–1 (Figure S7).

Nuclear Magnetic Resonance (NMR) Studies

The solution behavior of the arsenic-containing polyanions [As^IIIMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^3– and [RAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = HO, CH₃, C₂H₅, C₆H₅, 4-FC₆H₄, 4-F₃COC₆H₄, 4-F₃CC₆H₄, 4-BrC₆H₄, 4-N₃C₆H₄, and 3,5-(HOOC)₂C₆H₃) was examined using multinuclear (¹H, ¹⁹F, and ¹³C­{¹H}) NMR spectroscopy following the redissolution of the solid salts in a H₂O/D₂O mixture. These polyanions demonstrated greater solubility in water than their phosphorus-containing counterparts. Most polyanions dissolved readily at room temperature, except for NaNH₄–CH₃AsMo₆ and NaNH₄–C₂H₅AsMo₆ , which required heating to 40 °C.

The reference compounds, including dimethylarsinic acid, (CH₃)₂AsO₂H (cacodylic acid, H-Cac, pH 4.10), and sodium dimethylarsinate, (CH₃)₂AsO₂Na (sodium cacodylate, Na-Cac, pH 6.0), displayed distinct NMR signals at δ = 1.8 and 1.6 ppm (¹H) and δ = 17.0 and 16.7 ppm (¹³C­{¹H}) in H₂O/D₂O, corresponding to the two equivalent methyl groups attached to the arsenic center. For the bioactive polyanions NaNH₄–C₆H₅AsMo₆ , NaNH₄–F₃COC₆H₄AsMo₆ , and NaNH₄–BrC₆H₄AsMo₆ , the solutions were carefully adjusted to pH 7 using a 1 M NaOH solution prior to the measurements.

The ¹H NMR spectra of polyanions As^IIIMo₆ and As^VMo₆ exhibited distinct singlets at δ = 2.0 and 1.6 ppm (intensity ratio 1:1) for the former and δ = 2.2 and 1.9 ppm (intensity ratio 1:1) for the latter (Figure a). The ¹³C­{¹H} NMR spectra showed sharp resonances at δ = 19.4 and 17.9 ppm for As^IIIMo₆ and closely spaced peaks at δ = 18.9 and 18.7 ppm for As^VMo₆ (Figure b), corresponding to the nonequivalent methyl groups of the coordinated dimethylarsinate moiety, consistent with the solid-state structure. Although all cacodylates in the polyanions are structurally equivalent, their two methyl groups are not. A careful examination of the polyanion structure reveals that one methyl group points toward the heterogroup at the center, while the other extends outward, resulting in structural and thus magnetic inequivalence. The minor peaks at 1.62 ppm (¹H) and 17.0 ppm (¹³C­{¹H}) in the As^IIIMo₆ spectra were attributed to decomposed free cacodylate.

For the methylarsonate analogue CH₃AsMo₆ , the peaks at δ = 2.4 and 2.0 ppm (¹H) (Figure S20a) and δ = 19.7 and 18.0 ppm (¹³C­{¹H}) (Figure S20b) correspond to the nonequivalent methyl groups of the coordinated cacodylate ligands, as expected. The signals at δ = 0.96 ppm (¹H) and δ = 18.9 ppm (¹³C­{¹H}) are attributed to the central methyl arsonate ligand. Additionally, minor decomposition of the bound cacodylate moieties is evident from peaks at δ = 1.7 ppm (¹H) and δ = 17.1 ppm (¹³C­{¹H}). The solution stability of As^VMo₆ and CH₃AsMo₆ was monitored via ¹³C­{¹H} NMR over 1 week. No spectral changes were observed for As^VMo₆ , indicating complete stability, whereas CH₃AsMo₆ exhibited minor cacodylate decomposition over time (Figure S21). Overall, these findings confirm that the cacodylate groups remain strongly bound to the polyanion, consistent with a previous report.

For the ethylarsonate analogue C₂H₅AsMo₆ , the ¹H NMR spectrum displayed singlets at δ = 2.3 and 1.9 ppm (methyl groups of cacodylates), a quartet at δ = 1.2 ppm (−CH₂), and a triplet at δ = 0.6 ppm (−CH₃, ³ J = 8 Hz) (Figure S22). Due to the compound’s low solubility, even after heating to 40 °C, obtaining a reliable ¹³C­{¹H} NMR spectrum was challenging.

The ¹H NMR spectrum of C₆H₅AsMo₆ exhibited resonances at δ = 2.3 and 1.6 ppm (methyl groups of the coordinated cacodylate ligands). In the aromatic region, a doublet at δ = 7.07 ppm (³J = 8 Hz) and triplets at δ = 7.37 ppm (³ J = 8 Hz) and 7.46 ppm (³ J = 8 Hz) were assigned to the ortho, meta, and para protons of the phenylarsonate framework, respectively (Figure S23a). The ¹³C­{¹H} NMR spectrum showed corresponding signals at δ = 19.5 and 17.8 ppm (methyl groups of cacodylates) along with peaks at δ = 129.40 (meta), 129.72 (ortho), 131.18 (para), and 133.64 ppm (ipso) (Figure S23b). Additionally, minor peaks at δ = 1.8 ppm (¹H) and 17.0 ppm (¹³C­{¹H}) were attributed to slight cacodylate decomposition. A one-week analysis of the ¹H and ¹³C­{¹H} NMR spectra confirmed the polyanion’s stability, apart from minor decomposition of the cacodylate ligands (Figure S24).

In the ¹⁹F NMR spectra of the fluorinated polyanions 4-FC₆H₄AsMo₆ , 4-F₃CC₆H₄AsMo₆ , and 4-F₃COC₆H₄AsMo₆ , singlet resonances were observed at −104.5 ppm (Figure S26a), −63.1 ppm (Figure S28a), and −57.7 ppm (Figure S29a), respectively. In the ¹H NMR spectrum of 4-FC₆H₄AsMo₆ , unresolved multiplets were observed around 7.06 ppm. In contrast, the spectrum of 4-F₃CC₆H₄AsMo₆ showed well-resolved doublets corresponding to the ortho and meta aromatic protons at 7.18 and 7.67 ppm, respectively. The ¹³C­{¹H} NMR spectra of 4-FC₆H₄AsMo₆ and 4-F₃CC₆H₄AsMo₆ displayed signals at δ = 19.7 and 17.9 ppm (Figure S26b) and δ = 19.5 and 17.7 ppm (Figure S28b), respectively, assigned to the methyl carbons of the coordinated cacodylate moieties. For 4-FC₆H₄AsMo₆ , carbon–fluorine coupling resulted in characteristic doublets at δ = 165.7 ppm (para, ¹ J _C–F = 261.35 Hz), 132.5 ppm (ortho, ³ J _C–F = 8.44 Hz), and 116.9 ppm (meta, ² J _C–F = 22.14 Hz), while the ipso carbon appeared as a singlet at δ = 127.2 ppm. In the trifluoromethyl analogue 4-F₃CC₆H₄AsMo₆ , quartets at δ = 123.3 ppm (−CF₃, ¹ J _C–F = 272.4 Hz) and 134.3 ppm (para, ² J _C–F = 32.2 Hz) were observed due to carbon–fluorine coupling from the trifluoromethyl group. The peaks at δ = 126.2, 130.3, and 135.3 ppm were assigned to the meta, ortho, and ipso carbons, respectively. In both cases, the minor peak at δ = 17.0 ppm was attributed to free cacodylate, and in 4-F₃CC₆H₄AsMo₆ , an additional unassigned signal was detected at δ = 20.0 ppm.

The ¹H NMR spectrum of the polyanion 4-F₃COC₆H₄AsMo₆ displayed characteristic resonances at δ = 1.6 and 2.3 ppm (methyl groups of cacodylates) along with peaks in the aromatic region, doublets at δ = 7.07 ppm (³ J = 8 Hz) and 7.23 ppm (³ J = 8 Hz) corresponding to the ortho and meta protons of the phenyl ring, respectively (Figure S29b). The ¹³C­{¹H} NMR spectrum provided peaks at δ = 152.4 ppm (ipso), 131.8 ppm (ortho), 129.6 ppm (para), 121.4 ppm, (meta) and 118.6 ppm (−OCF₃) in the aromatic region. Long-term monitoring of the ¹H and ¹⁹F NMR spectra over 1 week confirmed the polyanion’s stability, aside from minor cacodylate decomposition (Figure S31).

For the polyanion 4-BrC₆H₄AsMo₆ , the ¹H NMR spectrum gave doublets at δ = 6.9 ppm (meta, ³ J = 9 Hz) and 7.5 ppm (ortho, ³ J = 9 Hz) in the aromatic region (Figure S32a). The ¹³C­{¹H} NMR spectrum revealed resonances at δ = 128.1 ppm (para), 130.2 ppm (ipso), 131.1 ppm (meta), and 132.5 ppm (ortho), corresponding to the aromatic carbons (Figure S32b). Distinct peaks at δ = 2.3 and 1.6 ppm (¹H) and δ = 19.4 and 17.7 ppm (¹³C­{¹H}) were assigned to the inequivalent methyl groups of the coordinated cacodylate moieties. Additionally, peaks at δ = 1.7 ppm (¹H) and δ = 16.9 ppm (¹³C­{¹H}) were attributed to free cacodylate species. Time-dependent ¹H NMR spectra suggested decomposition of the cacodylate moiety without significant structural changes (Figure S33).

The polyanion 4-N₃C₆H₄AsMo₆ exhibited the expected ¹H NMR signals for the cacodylate methyl protons at δ = 2.3 and 1.6 ppm. Aromatic phenyl protons appeared as doublets at δ = 6.58 ppm (ortho, ³ J = 9 Hz) and 6.72 ppm (meta, ³ J = 9 Hz) (Figure S34). However, additional unassigned peaks at δ = 6.97, 7.59, and 7.64 ppm persisted in time-dependent studies, suggesting partial decomposition of the centrally bound ligand and instability of the polyanion upon dissolution.

For the polyanion H₂O₄C₂C₆H₃AsMo₆ , ¹H NMR showed peaks at δ = 7.6 and 8.3 ppm, corresponding to the ortho and para aromatic protons, respectively, and signals at δ = 1.5 and 2.2 ppm for the methyl protons of the coordinated cacodylate. In the ¹³C­{¹H} NMR spectrum, signals at δ = 171.0, 135.6, 133.1, 134.0, and 131.8 ppm were assigned to the carboxylate group and the para, meta, ortho, and ipso carbons of the phenyl ring, respectively. Peaks at 19.4 and 17.8 ppm correspond to the bound cacodylates (Figure ). As in previous cases, signals at δ = 1.8 ppm (¹H) and 16.9 ppm (¹³C­{¹H}) were attributed to free cacodylate species.

UV–Vis Spectroscopy

The stability of the bioactive polyanions C₆H₅AsMo₆ , 4-F₃COC₆H₄AsMo₆ , and 4-BrC₆H₄AsMo₆ was further evaluated by UV–vis spectroscopy (Figures S35)_. Three absorption bands were observed at 200, 230–254, and 320 nm. The 200 nm band corresponds to the pπ–dπ charge transfer transition of the O_t → Mo, while the 230–254 nm band arises from O_b,c → Mo charge transfer and π–π* transitions within the aromatic ring. The intense 320 nm absorption signifies ligand-to-metal charge transfer (LMCT), involving electron transfer from the filled π-orbitals of surrounding oxygens to the vacant d orbitals of Mo⁵⁺ centers. The unchanged position and intensity of these bands suggest that no significant structural transformations occurred.

ESI Mass Spectrometry

ESI mass spectra of the polyanions were acquired from aqueous solution in both positive and negative ion mode. In positive ion mode, complex spectra with multiple Mo _x species could be observed. In most cases, the intact polyanion corresponded to the dominant signal. Four such examples are shown in the Supporting Information (Figures S36–S39).

In negative ion mode, all compounds analyzed showed high-quality ESI-MS spectra with a systematic pattern. All compounds showed two strong cluster of ions, a first between m/z 680–750 and a second between m/z 1380–1540. For a detailed discussion we selected two representative examples C₆H₅AsMo₆ and 4-FC₆H₄AsMo₆ . All mass spectrometry data are summarized in Table S13.

The polyanion C₆H₅AsMo₆ showed a first cluster of signals at m/z 714.55 and a second cluster at m/z 1452.09 (Figure ). The first cluster at m/z 714.55 was assigned to a doubly charged negative ion with an elemental composition of [Mo₆As₄C₁₂H₂₆O₂₄]^2–. The observed isotope pattern is in full agreement with the structure as demonstrated by comparison of the experimental spectrum to a simulation of the spectrum. The second cluster at m/z 1452.09 was assigned to originate from two individual species in the gas phase. First, a monomeric singly charged species with an elemental composition of [NaMo₆As₄C₁₂H₂₆O₂₄]⁻ with a mass difference of 1 m/z between isotope peaks. Second, in between these signals a second species with isotope differences of 0.5 m/z was observed, which was assigned to a dimeric species with an elemental composition of [NaMo₆As₄C₁₂H₂₆O₂₄]₂ ^2–.

4.

Negative mode ESI mass spectra of compound C₆H₅AsMo₆ . (A) Full-scan full-range view with two main signal clusters, (B) expanded region around m/z 714 with experimental spectra in the upper panel and a simulated isotope pattern in the bottom panel, and (C) expanded region around m/z 1452 with experimental spectra in the upper panel (please note signals of dimeric species at a 0.5 m/z mass difference at lower intensity in between main signals) and simulated isotope pattern in the bottom panel.

Again, simulated spectra are in full agreement with experimental spectra (see Figure for 4-FC₆H₄AsMo₆ as an example). As demonstrated from single-crystal X-ray structural data, the compound as well dimerizes in the gas phase with a central Na ion, reminiscent of a 15-crown-5 crown ether. From the relative ion intensities of the two monomeric species and the single dimeric species, a ratio of monomer to dimer of 3.1 was determined. When comparing monomer to dimer ratios, it appears that electron-withdrawing substituents on the para-position of the aromatic moiety attached to As favor and stabilize the dimeric species in the gas phase.

5.

Experimental (upper panel) and simulated (bottom panel) mass spectra of 4-FC₆H₄AsMo₆ at m/z 723 and 1469. Panel A shows simulation of doubly charged monomeric species [Mo₆As₄FC₁₂H₂₅O₂₄]^2–; panel B shows singly charged monomeric species [Mo₆As₄FC₁₂H₂₅O₂₄Na]⁻, and panel C shows simulation of doubly charged dimeric species [NaMo₆As₄FC₁₂H₂₅O₂₄]₂ ^2–.

The ESI spectra are reminiscent of related phosphorus-containing analogues reported earlier. In three samples containing a 4-trifluoromethoxy, 4-bromo, or azido substituent on an aromatic moiety, an additional cluster of signals could be observed with an additional mass increment of 125 m/z (Figures S40–S42). We tentatively assign this mass difference to added water molecules.

Tandem Mass Spectrometry

Due to the stability of the compounds in the solution and in the gas-phase, tandem MS experiments could be carried out. The C₆H₅AsMo₆ ions were isolated in the quadrupole with an isolation width of 20 m/z and fragmented at a collision energy of 50 eV. Fragment spectra of monomeric doubly charged species at m/z 714 (Figure ) and singly charged species at m/z 1452 as precursor ions were almost identical. Key fragment ion clusters could be observed centered at m/z 719.4, 575.2, 431.6. The same fragment ions could be observed for another compound subjected to tandem MS analysis (see Figure S43).

By comparison of experimental and simulated mass spectra, the fragment ions were assigned as [Mo₅O₁₅H₄]⁻ at m/z 719, [C₆H₅AsMo₆O₁₇(OH)­{AsO₂(CH₃)₂}]^2– at m/z 575, and [C₆H₅AsMo₆O₁₇(OH)­{AsO₂(CH₃)₂}₂]^3– at m/z 431. Consequently, the polyanions loose in the gas phase up to two dimethylarsinate ligands and in addition the central arsenic moiety to yield stable Mo _x O_3x species (Figure ).

6.

(A) Tandem MS spectra in negative mode of C₆H₅AsMo₆ with the precursor ion at m/z 714. Panel B shows experimental and simulated spectra of the fragment ion centered around m/z 719, panel C of the experimental and simulated fragment ion at m/z 575, and panel D experimental and simulated spectra at m/z 431.

Biological Activity Determination of Minimal Inhibitory Concentrations (MIC)

The antimicrobial efficacy of organoarsonate-functionalized polyoxomolybdates was assessed against the Gram-negative bacteria Escherichia coli (DSM 6897, grown in LB Medium), Vibrio parahaemolyticus (grown in LB Medium) and Salmonella enterica (DSM 19587, grown in LB Medium) as well as the Gram-positive bacteria Bacillus subtilis (DSM 1088, grown in TSY Medium) and Listeria monocytogenes (DSM 15675, grown in LB Medium).

All tested POMs exhibited MIC values exceeding 500 μg/mL against all Gram-negative bacteria. Only the POM 4-BrC₆H₄AsMo₆ gave weak activity against V. parahaemolyticus (250 μg/mL) and S. enterica (500 μg/mL), indicating an overall negligible antibacterial activity (Table ). This suggests that the outer membrane of Gram-negative bacteria is a strong permeability barrier, limiting the intracellular accumulation of these large, charged molecules. Similar findings have been reported for other polyoxometalates, which generally demonstrate low efficacy against Gram-negative bacteria due to their structural properties.

1. Minimum Inhibitory Concentration (MIC) Values (μg/mL) of the Polyanions Reported Here against Three Different Gram-Negative and Two Gram-Positive Bacteria.

microorganism		C 6 H 5 AsMo 6	4-F 3 COC 6 H 4 AsMo 6	4-BrC 6 H 4 AsMo 6	other POMs
Escherichia coli	Gram-negative	>500	>500	>500	>500
Vibrio parahaemolyticus	Gram-negative	>500	>500	250	>500
Salmonella enterica	Gram-negative	>500	>500	500	>500
Bacillus subtilis	Gram-positive	250	62.5–125	125	>500
Listeria monocytogenes	Gram-positive	>500	303	40	>500

^aOther POMs are As^VMo₆ , CH₃AsMo₆ , 4-FC₆H₄AsMo₆ , 4-F₃CC₆H₄AsMo₆ , 4-N₃C₆H₄AsMo₆ , and H₂O₄C₂C₆H₃AsMo₆ .

However, certain POMs with aromatic or halogenated phenyl groups (e.g., NaNH₄–C₆H₅AsMo₆ , NaNH₄–F₃COC₆H₄AsMo₆ , and NaNH₄–BrC₆H₄AsMo₆ ) demonstrate moderate activity against the Gram-positive bacteria B. subtilis and L. monocytogenes, with MICs ranging from 62.5 to 250 μg/mL and 40–303 μg/mL, respectively. The agar diffusion plates confirm the activity of 4-BrC₆H₄AsMo₆ as the best sample with the highest clear zone of inhibition against the pathogen Listeria monocytogenes (Figure S44). This activity may be attributed to the lack of an outer membrane compared to Gram-negative bacteria, allowing for easier cell wall penetration and showing a clear distinction in susceptibility between these bacterial types. MIC values of the control gentamycin lie in the general ranges for all bacterial strains.

Previous studies have demonstrated that halogenation can enhance bioactivity by altering compound–membrane interactions and improving uptake. All active compounds have a phenyl ring, which supports the findings that the phenyl ring serves as a structural backbone and the antimicrobial activity depends on the substituents. Among the tested compounds, 4-BrC₆H₄AsMo₆ exhibited the highest antibacterial activity, likely due to the effect of bromine substitution on the polyanion structure (Figure S44). Control studies using the reference ligands sodium cacodylate and BrC₆H₄AsO₃H₂ against Listeria monocytogenes showed MIC values of >1000 and 62.5 μg/mL, respectively. This indicates that while the BrC₆H₄AsO₃H₂ ligand alone exhibits moderate activity, the corresponding polyanion 4-BrC₆H₄AsMo₆ displays enhanced efficacy with an MIC of 40 μg/mL. Hence, this could lead to medicinal relevance in combating bacterial diseases for both Gram-positive and Gram-negative pathogenic microbes, although further structure–activity relationship (SAR) studies are necessary to elucidate the underlying mechanisms.

Conclusions

In summary, we have conducted a comprehensive investigation into the synthesis and characterization of a novel family of water-soluble, stable, and organically-functionalized, reduced polyoxomolybdates. Eleven dimethylarsinate-functionalized arsenomolybdates­(V), [RAsMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^2– (R = HO, CH₃, C₂H₅, C₆H₅, 3,5-(HOOC)₂C₆H₃, 4-FC₆H₄, 4-F₃CC₆H₄, 4-F₃COC₆H₄, 4-BrC₆H₄, and 4-N₃C₆H₄) and [As^IIIMo^V ₆O₁₅(OH)₃{AsO₂(CH₃)₂}₃]^3–, were successfully isolated via a straightforward one-pot aqueous synthesis under ambient, open-beaker conditions. Fine-tuning of reagent stoichiometry, ionic strength, and most critically, pH was essential to achieve selective formation of the target species. The incorporation of structurally diverse organoarsonate groups, including fluorinated, brominated, and carboxylated substituents, within a robust hexamolybdate­(V) core provided a versatile platform to examine solubility and solution stability near physiological pH. Detailed multinuclear NMR (¹H, ¹⁹F, and ¹³C) spectroscopy confirmed their structural integrity in solution, while ESI-MS revealed the coexistence of both monomeric and dimeric species. Moreover, tandem MS (CID) enabled stepwise fragmentation studies of the peripherally bound dimethylarsinate ligands, offering insights into the gas-phase behavior of these complex assemblies. Given the tunable organic functionality, these polyanions present exciting opportunities for biomolecular interaction studies. Preliminary biological assays revealed that the three derivatives C₆H₅AsMo₆ , 4-F₃COC₆H₄AsMo₆ , and 4-BrC₆H₄AsMo₆ exhibit moderate antibacterial activity, suggesting some potential as functional POM-based antimicrobial agents.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c02852.

• Single-crystal XRD data, bond valence sum calculations, FT-IR spectra, thermograms, multinuclear NMR spectra (¹H, ¹⁹F, and ¹³C), ESI-MS data and spectra, tandem MS spectrum, UV–vis spectra, agar diffusion test, and MIC plate images (PDF)

U.K. supervised the project and provided overall guidance on the research direction. V.S. synthesized the compounds, conducted material characterization, and drafted the manuscript. A.P. synthesized and initially characterized the halogenated and azido ligands and their polyanions and contributed to XRD measurements and data analysis. A.S. synthesized and characterized the dicarboxylate ligand and its corresponding polyanion. B.S.B. assisted with the XRD measurements and data interpretation. A.I. and J.Z. performed the antibacterial assays and coauthored the corresponding section, with M.S.U. providing final edits. J.H. conducted the mass spectrometry experiments, while N.K. analyzed the data and authored the mass spectrometry section. L.K. and C.S. provided the ethyl arsonate ligand used in the study. All authors reviewed and contributed to the final version of the manuscript.

The authors declare no competing financial interest.