Not So Similar: Different Ways of Nb(V) and Ta(V) Catecholate Complexation

The reactions between catechol (H₂cat) and niobium(V) or tantalum(V) precursors in basic aqueous solutions lead to the formation of catecholate complexes of different natures. The following complexes were isolated and characterized by single-crystal X-ray diffraction (SCXRD): (1) (NH₄)₃[NbO(cat)₃]∙4H₂O; (2) K₂[Nb(cat)₃(Hcat)]·2H₂cat·2H₂O; (3) Cs₃[NbO(cat)₃]·H₂O; (4) (NH₄)₄[Ta₂O(cat)₆]·3H₂O; (5) Cs₂[Ta(cat)₃(Hcat)]·H₂cat; (6) Cs₄[Ta₂O(cat)₆]·7H₂O. The isolated crystalline products were characterized by elemental analysis, X-ray powder diffraction (XRPD), FTIR, and TGA. The structural features of these complexes, such as {Ta₂O} unit geometry, Cs-π interactions, and crystal packing effects, are discussed.

Revisiting the Three Vanadium Sandwich-Type Polyoxometalates: Structures, Solution Behavior, and Redox Properties

It is well known that the trivacant anions α-B-[XW₉O₃₃]^9- react with vanadyl ions to give the sandwich-type polyoxometalates [(V^IVO)₃(XW₉O₃₃)₂]^12- with X = As^III or Sb^III. Nevertheless, the oxidized derivatives have been obtained selectively by electrochemical oxidation from the fully reduced derivatives [(V^IVO)₃(XW₉O₃₃)₂]^12- allowing full characterization both in solution using UV-vis and multinuclear (¹⁷O, ⁵¹V, and ¹⁸³W) NMR spectroscopies and in the solid state by single-crystal X-ray diffraction. Structural analysis of the oxidized [(V^VO)₃(XW₉O₃₃)₂]^9- polyanions is consistent with the idealized D_3h symmetry, while solution studies reveal a fair hydrolytic stability in a wide pH range from 0 to 6. Besides, the D_3h polyanions either as reduced or oxidized forms [(VO)₃(AsW₉O₃₃)₂]^9/12- have been identified as the thermodynamic product that results from the conversion of the C_2v polyanion [(H₂O)(VO)₃(AsW₉O₃₃)₂]^9/12- through moderate heating. Conversely, the Sb^III-containing derivative gives exclusively the D_3h polyanion, probably either due to the extended lone pair of the trigonal Sb^III heterogroup that prevents the formation of the C_2v arrangement or the lability of the oxo-metalate bonds that favor chemical exchange. The electrochemical studies of sandwich-type polyoxometalates revealed that each {V═O} group gives rise to a one-electron transfer process. At last, the redox properties appear strongly altered in the 0.3-5 pH range, consistent with proton-coupled electron transfers.

Tubular polyoxoanion [(SeMo6O21)2(C2O4)3]10− and its transformations

Tubular [(SeMo₆O₂₁)₂(C₂O₄)₃]¹⁰⁻ polyoxoanion can be assembled from Na₂MoO₄/SeO₂/H₂C₂O₄ system. TBABr precipitates complexes of [Mo₆O₁₉]²⁻ and [β-Mo₈O₂₆Na₂(NO₃)]³⁻ from its aqueous solution. Speciation of [β-Mo₈O₂₆]⁴⁻ and [Mo₆O₁₉]²⁻ has been studied.

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.

Niobium uptake by a [P8W48O184]40−macrocyclic polyanion

Incorporation of Nb into the [P₈W₄₈O₁₈₄]⁴⁰⁻anionic macrocyclic cavitand leads to formation of new Nb–W POMs. Inclusion of up to five {NbO(H₂O)}³⁺groups was observed. Solution speciation of the Nb-encapsulating macrocycles was studied by HPLC-ICP-AES and SAXS.