Synthesis and Reversible Transformation of Cun-Bridged (n = 1, 2, or 4) Silicodecatungstate Dimers

The Smallest Polyoxotungstate Retained by TRIS-Stabilization

A polycondensation reaction of the orthotungstate anion WO₄^2–, buffered at pH 7.5 in a TRIS-HCl (0.15 M) solution, results in the first example of a discrete polyoxotungstate anion, with just two W ions stabilized with TRIS ligands. It was isolated and characterized as Na₂[W^VI₂O₆(C₄O₃NH₁₀)₂]·6H₂O by single-crystal and powder X-ray diffraction, FT-IR spectroscopy, thermogravimetrical analysis (TGA), and elemental analysis in solid state and by electro-spray ionization mass spectrometry (ESI-MS), ¹³C, and ¹⁸³W NMR, as well as Raman spectroscopy in solution. This synthesis demonstrates the crucial and new role of the added tris-alkoxy ligand in the development of a new hybrid TRIS-isopolytungstate with the lowest known nuclearity (so far) and the terminal oxygens substituted with two nitrogen atoms arising from amines of the TRIS ligands.

We report on the synthesis and characterization of a new hybrid isopolytungstate Na₂[W^VI₂O₆(C₄O₃NH₁₀)₂]·6H₂O with the, so far, lowest known nuclearity and the terminal oxygen atoms substituted with nitrogen arising from amine.

Two Ni/Co-substituted sandwich-type germanomolybdates based on an unprecedented trivacant polyanion [α-GeMo10O36]8

Two Ni/Co-substituted sandwich-type germanomolybdates, {[M₃(NH₂-trz)₆(H₂O)₆][M₄(H₂O)₂(HGeMo₁₀O₃₆)₂]}·nH₂O (M = Ni²⁺ (1) and Co²⁺ (2), n = 10 (1) or 11 (2), NH₂-trz = 4-amino-1,2,4-triazole), have been obtained under hydrothermal conditions. 1 and 2 represent the first trivacant Keggin germanomolybdates involving unprecedented [α-GeMo₁₀O₃₆]^8- fragments and {M₃O₄} quasi-cubane building units. Both of them exhibit electrocatalytic behaviours for H₂O₂ reduction and photocatalytic properties for CO₂ conversion.

Hexavacant γ-Dawson-type phosphotungstates supporting an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core (M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+)

In aqueous media, the introduction of additional metal species into polyoxometalates (POMs) with multiple vacant sites, such as a hexavacant Dawson-type phosphotungstate, which is of interest for the synthesis of novel metal oxide clusters, is generally difficult because they easily undergo self-condensation and/or structural decomposition. In this study, we succeeded in developing a novel synthetic method to obtain metal-substituted γ-Dawson-type phosphotungstate monomers by introducing metal species into an organic solvent-soluble lacunary phosphotungstate, TBA4H10[α-P2W12O48] (I) (TBA = tetra-n-butylammonium), in organic media. The reaction of I, which possessed two types of vacant sites, i.e. middle and edge sites, with divalent metal species such as Mn2+, Co2+, Ni2+, Cu2+, or Zn2+ in acetonitrile afforded a series of isostructural POMs M2 (TBA5[γ-P2W12O44M2(OAc)(CH3CONH)2]·nH2O·mCH3CN; M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+; OAc = acetate) with an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core. The bis(square-pyramidal) core was selectively placed at the middle site of the hexavacant lacunary phosphotungstate, and the two metals in the core were bridged by two phosphate units and one acetate species. Meanwhile, the edge sites were capped by acetimidate ligands, which protect the reactive lacunary POM from self-condensation. To the best of our knowledge, this is the first report describing the synthesis and characterization of metal-substituted hexavacant γ-Dawson-type POM monomers.

Solvent-induced structural transformation and magnetic modulation in 1D copper(ii) polymers based on a semi-rigid ligand containing 4-amino-1,2,4-triazole

A helical Cu(ii) polymer chain exhibits reversible SCSC transformation and magnetism switching by the chemisorption/desorption of MeCN molecules.

Composite cluster-organic frameworks based on polyoxometalates and copper/cobalt–oxygen clusters

The first two cluster organic frameworks based on Anderson-type polyoxometalates and copper/cobalt-oxo cluster secondary building units have been successfully made under solvothermal conditions.

Structure, solution assembly, and electroconductivity of nanosized argento-organic-cluster/framework templated by chromate

In view of elucidating potential structures and assembly mechanism of silver clusters and silver cluster-based metal-organic frameworks, we prepared four argento-organic-clusters/frameworks where the structures were directed by chromate in the presence of different thiolates. All four structures with ^tBuC₆H₄S^-, SⁱBu^-, and SⁱPr^- consist of three core-shells, an inner CrO₄^2-, an intermediate Ag-S aggregate and finally the protective organic moieties. {(HNEt₃)₃[Ag(CrO₄)₄@Ag₄₆(SC₆H₄^tBu)₂₄(CF₃COO)₁₈(DMF)₄]} (1) is a supertetrahedron with an inner Ag(CrO₄)₄ tetrahedron shelled by four fused Ag_11.5S₆ lobes. [(CrO₄)₅@Ag₄₀(SⁱBu)₂₇(CF₃COO)₃]_n (2) is an undulated snake-like tube housing the infinite CrO₄^2- tetrahedra. [(CrO₄)₂@Ag₄₁(SⁱBu)₃₀(NO₃)₃(CN)₄]_n (3) forms an uncommon 7-connected kwh network incorporating hexagonal layers of Ag₁₉(SⁱBu)₁₅ balls with a single inner CrO₄^2- connected by another Ag atom. Both enantiomeric chiral qtz frameworks of [CrO₄@Ag₂₀(SⁱPr)₁₀(Cr₂O₇)₂(COOCF₃)₄(DMF)₄]_n (4) were structurally characterized. In 4, Cr₂O₇^2- connects the Ag₂₀(SⁱPr)₁₀ clusters with a trapped CrO₄^2- into a 3D quartz (qtz) structure, where the spherical cluster acts like oxygen and Cr₂O₇^2- takes the place of Si in SiO₂. Electrospray ionization mass spectrometry (ESI-MS) analysis of the reaction solutions of 1-4 clearly indicated that (i) the Ag(CrO₄)₄@Ag₄₆ core of 1 can retain its molecular structure in the solution and (ii) the chromate-templated polynuclear silver-thiolate species in solution are important building blocks to construct the 1D or 3D motif for 2-4. The electrochemistry in sulfuric acid and enhancement of the electrical conductivity upon I₂ doping have also been reported.

A highly-flexible cyclic-decavanadate ligand for interconversion of dinuclear- and trinuclear-cobalt(ii) and manganese(ii) cores

The conformation of a cyclic-decavanadate ligand was reversibly transformed in response to the nuclearity of the central metal core.

Self-assembly of new M(ii) coordination polymers based on asymmetric 1,3,4-oxadiazole-containing ligands: effect of counterions and magnetic properties

A construction strategy of high dimensional porous coordination polymers has been investigated based on two unsymmetrical ligands and “naked” and coordinated metal ions.

Complex solution chemistry behind the simple "one-pot" synthesis of vanadium-substituted polyoxometalates unraveled by electrospray ionization mass spectrometry

RATIONALE

The "one-pot" method is a deceptively simple and straightforward method for synthesis of polyoxometalates (POMs). However, the complex solution chemistry behind the simple method is neither clear nor elucidated thoroughly by a suitable method. In general, POM chemists are more focused on what final products are obtained by deliberately choosing the ingredients used in the "one-pot" process with a limited knowledge of what is actually happening in the reaction solution.

METHODS

Time-resolved electrospray ionization mass spectrometry (ESI-MS) was developed to monitor the dynamic solution speciation changes occurred in the reaction mixtures ({SiW12-x } (x = 1-3) + NaVO3 ) against reaction time. The reactions were conducted as normal, with 2 μL aliquots removed at specific time intervals during the reaction. The aliquots were immediately diluted with 1 mL of HPLC grade water and then analyzed by ESI-MS. As a control, solutions of each {SiW12-x } were analyzed alongside the vanadium reactions to corroborate which fragments were due to the lacunary precursors and which were due to the reactions with sodium metavanadate.

RESULTS

It was discovered that the reaction solution speciation was sensitively changed with the reaction conditions such as concentration, temperature, and reaction time. Spontaneous transformations from mono- into di- and eventually tri-substituted products ({SiW11 V} → {SiW10 V2 } → {SiW9 V3 }) were observed for the reactions of {SiW11 } + VO3 (-) and {SiW10 } + VO3 (-) , respectively. Higher concentration and temperature definitely accelerate the transformation reaction rates whereas different molar ratios of reactants have minimal effects on changing the solution speciation in the cases studied.

CONCLUSIONS

Time-resolved ESI-MS is effectively and successfully used in the monitoring of the dynamic speciation changes in the reaction mixtures consisting of lacunary POMs with transition salts. This study will provide guidance for the oriented design and controlled synthesis of POMs with novel structures. Copyright © 2016 John Wiley & Sons, Ltd.

POM species, temperature and counterions modulated the various dimensionalities of POM-based metal–organic frameworks

A series of six POM-based metal–organic frameworks with various dimensionalities have been isolated by tuning the reaction conditions (POM species, reaction temperature and counterions).

Effect of vanadium valence state on the solution chemistry and the stability of vanadium substituted polyoxometalates

ESI-MS combined with DFT calculation revealed that the vanadium valence state and the vanadium substitution degrees can substantially affect the stabilities of vanadium-substituted POMs.

Temperature-induced racemic compounds and chiral conglomerates based on polyoxometalates and lanthanides: syntheses, structures and catalytic properties

Four new species, originated from [Co₂Mo₁₀H₄O₃₈]⁶⁻ and lanthanides by adjusting the reaction temperature, are reported, which act as Lewis acid–base catalysts toward the cyanosilylation reaction.