Reducing Systematic Uncertainty in Computed Redox Potentials for Aqueous Transition-Metal-Substituted Polyoxotungstates

Polyoxometalates have attracted significant interest owing to their structural diversity, redox stability, and functionality at the nanoscale. In this work, density functional theory calculations have been employed to systematically study the accuracy of various exchange-correlation functionals in reproducing experimental redox potentials, U0Red in [PW11M(H2O)O39]q- M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II). U0Red calculations for [PW11M(H2O)O39]q- were calculated using a conductor-like screening model to neutralize the charge in the cluster. We explicitly located K+ counterions which induced positive shifting of potentials by > 500 mV. This approximation improved the reproduction of redox potentials for Kx[XW11M(H2O)O39]q-x M = Mn(III/II)/Co(III/II). However, uncertainties in U0Red for Kx[PW11M(H2O)O39]q-x M = Fe(III/II)/Ru(III/II) were observed because of the over-stabilization of the ion-pairs. Hybrid functionals exceeding 25% Hartree-Fock exchange are not recommended because of large uncertainties in ΔU0Red attributed to exaggerated proximity of the ion-pairs. Our results emphasize that understanding the nature of the electrode and electrolyte environment is essential to obtain a reasonable agreement between theoretical and experimental results.

Binuclear Ru(III)-Containing Polyoxometalate with Efficient Photocatalytic Activity for Oxidative Coupling of Amines to Imines

A novel binuclear ruthenium-based polyoxometalate, K₆H[{Ru₂Cl(H₂O)(CH₃COO)₂}{WO(H₂O)}₂(PW₉O₃₄)₂]·14H₂O (1), was successfully synthesized by the conventional hydrothermal method. Compound 1 was well-characterized by single-crystal X-ray diffraction, X-ray powder diffraction (PXRD), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electrospray ionization-mass spectrometry (ESI-MS), thermogravimetric analyses (TGA), and elemental analysis. The structural unit of compound 1 contains two [A-α-PW₉O₃₄]^9- building blocks at the upper and lower positions connected by two W atoms and two Ru atoms, where the W atoms and Ru atoms are arranged in a trapezoidal arrangement and the Ru atoms are bridged by acetic acid. Furthermore, compound 1 features characteristic absorption bands in the visible region, which allows the investigation of its photocatalytic properties in visible light. Under simulated sunlight radiation (λ > 400 nm), compound 1 exhibits high photocatalytic activity and good circularity toward the oxidative coupling of amines to imines at room temperature with O₂ as the sole oxidant.

Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions

Five-coordinate geometry around ruthenium with highly exposed active sites has attracted intensive scientific interest due to its superior properties and extensive applications. Herein, we report a series of structurally controllable multi-Ru-bridged polyoxometalates, K5NaH10[{Ru4(H2O)n}(WO2)4(AsW9O33)4]·mH2O {1, 1-dehyd-373K, 1-dehyd-473K, 1-dehyd-573K; n = 4, m = 36; n = 4, m = 6; n = 4, m = 0; n = 0, m = 0} fabricated through a feasible assembly strategy using arsenotungstate {2, KNa12H17Cl2(As4W40O140)·29H2O} as a structure-directing unit. Systematic characterization methods identified that the six-coordinate geometry can successfully transform into five-coordinate geometry about active sites (Ru) by removing aqua ligands under high reaction temperatures. All the multi-Ru-bridged polyoxometalates demonstrated strong stability and catalytic effectiveness in the transformation of 1-(4-chlorophenyl)ethanol to 4'-chloroacetophenone under very mild conditions. 1-dehyd-573K, specifically, achieves the best catalytic effectiveness with a turnover frequency (TOF) = 25 100·h-1 owing to its unique five-coordinate geometry on the Ru sites. To our knowledge, 1-dehyd-573K outperforms other POM-based catalysts in the oxidative catalysis of 1-(4-chlorophenyl)ethanol. The heterogeneous polyoxometalates were also proven to be strongly reusable, with their structural integrities well maintained after multiple-cycle catalytic reactions.

Easy Ligand Activation in the Coordination Sphere of Ru inside the [PW11O39]7- Backbone

Irradiation of the Keggin-type [PW₁₁O₃₉{Ru(NO)}]⁴⁻ (Ru-NO) polyoxometalate in CH₃CN results in rapid NO ligand elimination with the formation of [PW₁₁O₃₉{Ru^III(CH₃CN)}]⁴⁻ (Ru-CH₃CN). This complex offers an easy entry into the Ru-based chemistry of the {PW₁₁Ru} complex. Attempts to substitute N₃⁻ for CH₃CN in the presence of an NaN₃ excess lead a variety of products: (i) [PW₁₁O₃₉{Ru^III(N₃)}]⁴⁻ (Ru-N₃); (ii) [PW₁₁O₃₉{Ru^III(N₄HC-CH₃)}]⁴⁻ (Ru-Tz) as a click-reaction product; and (iii) [PW₁₁O₃₉{Ru^II(N₂)}]⁵⁻ (Ru-N₂). UV-VIS, CV, and HR-ESI-MS techniques were used for the reaction monitoring and characterization of the products.

Lanthanide-Containing 22-Tungsto-2-germanates [Ln(GeW11O39)2]13-: Synthesis, Structure, and Magnetic Properties

We report on the synthesis and structural characterization of two series of lanthanide-containing 22-tungsto-2-germanates. The first series corresponds to a family of polyanions with the formula [Ln(β₂-GeW₁₁O₃₉)₂]^13- (Ln^III = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Dy (7)), and the second series corresponds to a family with the formula [Ln(β₂-GeW₁₁O₃₉)(α-GeW₁₁O₃₉)]^13- (Ln^III = Ho (8), Er (9), Tm (10)). All compounds were characterized in the solid state by single-crystal and powder XRD, IR, TGA, and SQUID magnetometry. The polyanions were synthesized in aqueous medium by direct reaction of the monolacunary [β₂-GeW₁₁O₃₉]^8- POM precursor with the corresponding lanthanide salts. The structure of the polyanions consists of an 8-coordinated lanthanide ion in a square-antiprismatic geometry, which is sandwiched either between two [β₂-GeW₁₁O₃₉]^8- units for 1-7 or between a [β₂-GeW₁₁O₃₉]^8- and a [α-GeW₁₁O₃₉]^8- unit for 8-10. Furthermore, the effect of the central paramagnetic lanthanide ion on the magnetic behavior of the polyanions was investigated, with the erbium-derivative [Er(β₂-GeW₁₁O₃₉)(α-GeW₁₁O₃₉)]^13- (9) showing single-molecule magnet (SMM) behavior.

Reactions of [Ru(NO)Cl5]2- with pseudotrilacunary {XW9O33}9- (X = AsIII, SbIII) anions

Reactions of [Ru(NO)Cl₅]^2- with pseudotrivacant B-α-[XW₉O₃₃]^9- (X = As^III, Sb^III) at 160 °C result in the rearrangement of polyoxometalate backbones into {XM₁₈} structures. In the case of arsenic, oxidation of As^III to As^V takes place with the formation of a mixture of plenary and monosubstituted Dawson [As₂W₁₈O₆₂]^6- and [As₂W₁₇Ru(NO)O₆₁]^7- anions, of which the latter was isolated as Me₂NH₂⁺ (DMA-1a and DMA-1b) and Bu₄N⁺ (Bu₄N-1) salts and fully characterized. Both α₁ and α₂ isomers of [As₂W₁₇Ru(NO)O₆₁]^7- were present in the reaction mixture; pure [α₂-As₂W₁₇Ru(NO)O₆₁]^7- was isolated as the Bu₄N⁺ salt. In the case of antimony, [SbW₉O₃₃]^9- is converted into a mixture of [SbW₁₈O₆₀]^9- and [SbW₁₇{Ru(NO)}O₅₉]^10-. The formation of trisubstituted [SbW₁₅{Ru(NO)}₃O₅₇]^12- as a minor byproduct was detected by HPLC-ICP-AES. The monosubstituted [SbW₁₇{Ru(NO)}O₅₉]^10- anion was isolated as DMAH⁺ (DMA-2) and mixed inorganic cation (CsKNa-2) salts and characterized by XRD, HPLC-ICP-AES, EA and TGA techniques. X-ray analysis shows the presence of the {Ru(NO)}-group in the 6-membered ("equatorial") belt of the Sb-free hemisphere. The experimental findings were confirmed and interpreted by means of quantum chemical calculations.

Preparation, characterization and electrocatalysis performance of a trimeric ruthenium-substituted isopolytungstate

The ruthenium-containing isopolytungstate Rb₁₀K₃H₆[SeO₃(H₉Ru_5.5W_30.5O₁₁₄)]Cl₃·48H₂O was isolated and then served as a catalyst, showing electrochemical catalytic activity towards the oxidation reaction of nitrite.

Preparation of α1- and α2-isomers of mono-Ru-substituted Dawson-type phosphotungstates with an aqua ligand and comparison of their redox potentials, catalytic activities, and thermal stabilities with Keggin-type derivatives

Both the α1- and the α2-isomers of mono-ruthenium (Ru)-substituted Dawson-type phosphotungstates with terminal aqua ligands, [α1-P2W17O61Ru(III)(H2O)](7-) (α1-RuH2O) and [α2-P2W17O61Ru(III)(H2O)](7-) (α2-RuH2O), were prepared in pure form by cleavage of the Ru-S bond of the corresponding DMSO derivatives, [α1-P2W17O61Ru(DMSO)](8-) (α1-RuDMSO) and [α2-P2W17O61Ru(DMSO)](8-) (α2-RuDMSO), respectively. Redox studies indicated that α1-RuH2O and α2-RuH2O show proton-coupled electron transfer (PCET), and the Ru(III)(H2O) species was reversibly reduced to Ru(II)(H2O) species and oxidized to Ru(IV)([double bond, length as m-dash]O) species and further to Ru(V)([double bond, length as m-dash]O) species in aqueous solution depending on the pH. Their redox potentials and thermal stabilities were compared with those of the corresponding α-Keggin-type derivatives ([α-XW11O39Ru(H2O)](n-); X = Si(4+) (n = 5), Ge(4+) (n = 5), or P(5+) (n = 4)). The basic electronic and redox features of Ru(L)-substituted Keggin- and Dawson-type heteropolytungstates (with L = H2O or O(2-)) were analyzed by means of density functional calculations. Similar to the corresponding α-Keggin-type derivatives, both α1-RuH2O and α2-RuH2O show catalytic activity for water oxidation.

Redox chemistry of ruthenium ions in mono-substituted Keggin tungstophosphate: a new synthetic extension for ruthenium derivatives based on [PW11O39RuVIN]4−

A facile approach to ruthenium-substituted Keggin-type heteropolyoxotungstates with high yield by easily controllable redox reaction is described.