Tetravalent Metal Complexation by Keggin and Lacunary Phosphomolybdate Anions

Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(v) sandwich complex

The synthesis and characterization of a series of (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr, Hf, Th, and U) sandwich complexes is reported. A preformed lacunary, Lindqvist-type, polyoxomolybdate-alkoxide cluster provides access to first examples of actinide-polyoxomolybdate sandwich complexes isolated under non-aqueous conditions. Incorporation of metal(iv) cations into this framework was found to "switch on" reversible redox chemistry at the {Mo5} ligands, with the Zr and Hf containing complexes accepting up to two electrons, while the Th and U derivates accommodate as many as four additional electrons. The enhancement of the redox properties of the cluster upon actinide incorporation is an exciting observation, presenting actinide "doping" as a novel approach for accessing functional redox-active materials. Oxidation of the uranium containing sandwich complex (TBA)2[U{Mo5O13(OMe)4NO}2], chemically or electrochemically, allows access to the U(v) centered species, which was characterized both spectroscopically and by single crystal X-ray diffraction. This represents the first example of a U(v)-polyoxometalate sandwich complex to be isolated and structurally characterized.

Structural and Chemical Effects of the Surrounding Cations and Coexisting Compounds on [M(α-PW11O39)2]n

The effects of countercations and coexisting compounds on the molecular structure of the [M(α-PW₁₁O₃₉)₂] ^n- polyanion were analyzed in terms of the repulsion and twisting of two [α-PW₁₁O₃₉]^7- units. More specifically, it was found that two [α-PW₁₁O₃₉]^7- ions approached one another upon reducing the ionic radius of M in [M(α-PW₁₁O₃₉)₂] ^n-. The interactive twisting of the [α-PW₁₁O₃₉]^7- units prevented mutual repulsion of the units containing terminal O atoms, and the tendency for approach and twisting of the [α-PW₁₁O₃₉]^7- units varied as a function of the type of countercation and the presence of coexisting compounds. Overall, this study demonstrated that some interactions between the counteraction and coexisting compounds with [M(α-PW₁₁O₃₉)₂] ^n- determined the molecular conformation and the isolated form of the polyanion.

Polyoxometalates in solution: speciation under spotlight

The review covers stability and transformations of classical polyoxometalates in aqueous solutions and provides their ion-distribution diagrams over a wide pH range.

A Simple Coulombic Model for 31P NMR Spectra of Cluster-Encapsulated Phosphorus Atoms

While sophisticated computational methods can predict ³¹P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined ³¹P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PM ⁿW₁₁O₃₉]^{(7- n)-} (M ⁿ = Ag⁺, Zn²⁺, Nb⁵⁺, W⁶⁺) and [(PW₁₁O₃₉)₂M ⁿ]^{(14- n)-} (M ⁿ = Y³⁺, Zr⁴⁺), varied as a linear function of the oxidation states, n, of the complexed M ⁿ cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative ³¹P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R² value of 0.974. With the model being applied here to three cluster anions whose ³¹P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

Complexation of tetravalent uranium cations by the As4W40O140 cryptand

The polyanionic cryptand {As₄W₄₀O₁₄₀} was successfully used to bind up to four tetravalent uranium cations leading to the formation of three new cryptates. The obtained species appears to be stable in solution and the cryptand was used for U^IV/Nd^III separation studies.

A new series of mononuclear lanthanide single molecule magnets based on sandwich-type germanomolybdates [Ln(GeMo11O39)2]13− (Ln = ErIII, GdIII, DyIII or TbIII)

New series of mononuclear lanthanide single molecule magnets based on sandwich-type germanomolybdates have been synthesized and their magnetic properties were investigated.

Heterobimetallic cerium(iv) oxo clusters supported by a tripodal oxygen ligand

Heterometallic Ce^IV/M (M = Mo^VI, Re^VII, V^V) oxo clusters supported by the Kläui tripodal oxygen ligand [(η⁵-C₅H₅)Co{P(O)(OEt)₂}₃]⁻ (L_OEt⁻) have been synthesized and the catalytic activity of the Ce^IV/V^V oxo cluster in the oxidation of thioanisoles has been studied.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Rare sandwich-type polyoxomolybdates constructed from Di-/tetra-nuclear transition-metal clusters and trivacant keggin germanomolybdate fragments

Two types of rare sandwich-type germanomolybdates [Na(12)(H(2)O)(36)][Cu(2)(beta-Y-GeMo(9)O(33))(2)].3H(2)O (1), [N(CH(3))(4)](4) [Na(6)(H(2)O)(24)][Cr(2)(beta-Y-GeMo(9)O(33))(2)].7H(2)O (2), and [Na(11)(H(2)O)(25)]H[M(4)(H(2)O)(2)(alpha-B-GeMo(9)O(34))(2)].6H(2)O (M = Ni(II) for 3, M = Mn(II) for 4 and M = Co(II) for 5) have been synthesized and characterized by elemental analyses, ICP spectra, IR spectroscopy, UV spectroscopy, thermogravimetry (TG) analyses (for 1-3), X-ray photoelectron spectroscopy (XPS) (for 1 and 3), X-ray powder diffraction (XRPD) (for 1 and 3) and single-crystal X-ray diffraction. To our knowledge, 1-5 represent the first sandwich-type germanomolybdates containing both {beta-Y-GeMo(9)O(33)}/{alpha-B-GeMo(9)O(34)} fragments and transition-metal clusters. Interestingly, 1 and 2 display the rare dinuclear transition-metal substituted sandwich-type structures with unusual trivacant {beta-Y-GeMo(9)O(33)} germanomolybdate units whereas 3-5 exhibit the first tetranuclear transition-metal substituted sandwich-type structures with familiar trivacant {alpha-B-GeMo(9)O(34)} germanomolybdate units. Surface photovoltage spectroscopy (SPS) and electric field induced surface photovoltage spectroscopy (EFISPS) measurements reveal that 1 and 3 bear the behavior of the n-type semiconductor. Magnetic measurements indicate 1 and 3 demonstrate antiferromagnetic exchange interactions and ferromagnetic exchange interactions, respectively.