Systematic Study of the Interaction Between VIV Centres and LnIII Ions in Well Defined {V 2 IV LnIII}{AsIIIW9O33}2 Sandwich-Type Clusters: Part 2

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.

Lanthanoid Template Isolation of the α-1,5 Isomer of Dicobalt(II)-Substituted Keggin Type Phosphotungstates: Syntheses, Characterization, and Magnetic Properties

A new series of heterometallic 3d-4f sandwich type phosphotungstates, [Ln{PCo2W10O38(H2O)2}2](11-) (Ln = Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III), denoted 1a-10a, respectively), have been synthesized by a one-pot reaction procedure on reacting the dilacunary K14[P2W19O69(H2O)]·24H2O precursor with Ln(NO3)3·nH2O and Co(NO3)2·6H2O in an aqueous potassium chloride solution. All the compounds were isolated as potassium salts and further characterized with different analytical techniques such as single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution electrospray ionization mass spectrometry, elemental analysis by inductively coupled plasma atomic emission spectroscopy, magnetic measurement, and thermogravimetric analysis. Single-crystal X-ray diffraction analysis of the compounds reveals that all these compounds are isostructural and crystallized in the orthorhombic crystal system in space group Iba2. The polyanions contain the α-1,5 isomer of dicobalt-substituted α-Keggin phosphotungstate, which sandwiched lanthanoid cation and formed novel heterometallic dimer species. The temperature-dependent magnetic susceptibilities of 1a, 2a, 4a, and 7a-10a indicate the dominant contribution of the ferromagnetic interaction between Co(II) and Co(II) within the cluster, while the antiferromagnetic interaction between Co(II) and Ln(III) dominates for 3a, 5a, and 6a. The isothermal magnetizations of 1a-10a show a gradual increase in magnetization at low fields and do not reach saturation even at 50 kOe.