Breaking Symmetry: Spontaneous Resolution of a Polyoxometalate

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.

Constructing chiral polyoxometalate assemblies via supramolecular approaches

Polyoxometalates (POMs), as a typical class of discrete metal oxide clusters that are known in inorganic and structural chemistry since long, have displayed more and more interesting applications over recent years. However, in comparison to the chemical synthesis, the photochemical, electrochemical, and magnetic properties, the structural asymmetry, and relative characteristic investigations arising therefrom are far behind even if they are very important for functional materials, especially in solution systems. One of the main reasons is that it is hard to control and maintain a stable chiral state of POMs to carry out further corresponding performances. Aiming to overcome these disadvantages, the main concerns of this review are to discuss the generation of the chirality for discrete metal oxide clusters, chirality transfer via a supramolecular approach, chirality amplification in self-assemblies, and the related functional properties such as photochromism, catalysis, and bioactivities in solutions. Considering that some previous reviews dealt with chiral structures and packing architectures in the crystalline solids of POMs, this article only concentrates on the induced chirality and material properties in solution systems, which have been more active recently but no review article has been involved in this interesting area.

Application of temperature-controlled chiral hybrid structures constructed from copper(ii)-monosubstituted Keggin polyoxoanions and copper(ii)-organoamine complexes in enantioselective sensing of tartaric acid

Temperature usually occupies a crucial position in the construction of chiral compounds. By controlling the temperature of the reaction system, chiral and non-chiral compounds can be designed and synthesized. Given the above, three new chiral and non-chiral compounds based on copper(ii) monosubstituted polyoxoanions and Cu(en) complexes (en = ethylenediamine), d/l-[Cu(H₂O)(en)₂]₂{[Cu(H₂O)₂(en)][SiCuW₁₁O₃₉]}·5H₂O (1, d-1 and l-1) and [Cu(H₂O)(en)₂]{[Cu(en)₂]₂[SiCuW₁₁O₃₉]}·2.5H₂O (2), were successfully synthesized under hydrothermal conditions. The main synthesis conditions of compound 1 (d-1 and l-1) and compound 2 are the same, however, the only difference is that the reaction temperatures are 80 °C and 140 °C, respectively. What's more, compounds 1 and 2 can form a 1D chiral chain by Cu–O and W/Cu–O–W/Cu bonds, respectively, and further obtain a 3D-supramolecular framework through hydrogen bonding interaction. Meanwhile, due to the asymmetry of chiral compound 1, optical second-harmonic generation (SHG) was used to investigate the second-order nonlinear optical effect and it was found that the observed SHG efficiency of compound 1 is 0.3 times that of urea. To further investigate the chiral properties, d-1 and l-1 were used in the electrochemical enantioselective sensing of d-/l-tartaric acid (d-/l-tart) molecules, respectively, which demonstrates that d-1 and l-1 have a good application prospect in sensing chiral substances.

A pair of temperature-controlled chiral compounds, d- and l-[Cu(en)₂(H₂O)]₂{[Cu(en)(H₂O)₂][SiCuW₁₁O₃₉]}·5H₂O (en = ethanediamine) are isolated by hydrothermal method, having a good application prospect in sensing d-/l-tartaric acid.

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.

Cation-mediated optical resolution and anticancer activity of chiral polyoxometalates built from entirely achiral building blocks

We report the crystallization of homochiral polyoxometalate (POM) macroanions {CoSb6O4(H2O)3[Co(hmta)SbW8O31]3}15- (1, hmta = hexamethylenetetramine) via the counter cation-mediated chiral symmetry breaking and asymmetric autocatalytic processes. In the presence of low Co2+ concentrations both Δ- and Λ-enantiomers of 1 formed in the reaction, crystallizing into the racemic crystal rac-1. At a high Co2+ concentration, the polyoxoanion enantiomers showed a high level of chiral recognition via H-bonding interactions to crystallize into enantiopure crystals of Δ- or Λ-[Co(H2O)6{CoSb6O4(H2O)3[Co(hmta)SbW8O31]3}]13-. During crystallization, a microscale symmetry-breaking event and a nonlinear asymmetric autocatalysis process make the enantiomers crystallize in different batches, which provides an opportunity to isolate the homochiral bulk materials. The defined structures of the racemic and homochiral crystals thus provide a molecular-level illustration that H-bonding interactions are responsible for such high-level chiral recognition, in a process similar to the supramolecular chirality frequently observed in biology. These POM macroanions showed a high cytotoxicity against various cancer cells, particularly ovarian cancer cells. The antitumor activity of these compounds resulted at least in part from the activation of the apoptotic pathways, as shown by the flow cytometry, Annexin V staining, DNA ladder, and TUNEL assay, likely by blocking the cell cycle and complexing with proteins in cells. The POM macroanions reported herein provide promising and novel antitumor agents for the potential treatment of various cancers.

Counterion-dominating chirality transfer between chiral and achiral polyoxometalates

The chirality transfer between inorganic polyoxometalate clusters is realized by the suitable mediation of counterions through multi-interactions in aqueous solution.

Chiral polyoxomolybdate-based hybrid compounds obtained by spontaneous resolution: syntheses, structures and non-linear optical properties

By spontaneous resolution, two chiral hybrid 2D architectures [MMo₆O₂₁(glycine)₃]²⁻ (M = Te 1, Se 2) were obtained, which show an SHG efficiency of about 3.5 and 3.7 × KDP.

On chirality induction in the crystalline solid-containing sandwich-type [Ln(α-PW11O39)2]11− polyoxotungstate and proline

For the racemisation, the [Ln(α-PW₁₁O₃₉)₂]¹¹⁻ polyanion with light Ln experienced the ‘on-site rotation’ mechanism, while the [Ln(α-PW₁₁O₃₉)₂]¹¹⁻ polyanion with heavy Ln followed the “dissociation–association” mechanism.

Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34](10-) Polyoxometalate

The reaction of Na10[α-SiW9O34] with tetravalent metallic cations such as 4f ((NH4)2Ce(NO3)6), 5d (HfCl4), or 5f (UCl4 and Th(NO3)4) in a pH 4.7 sodium acetate buffer solution leads to the formation of four sandwich-type polyoxometalates [Ce4(μ(3)-O)2(SiW9O34)2(CH3COO)2](10-) (1), [U4(μ(3)-O)2(SiW9O34)2(CH3COO)2](10-) (2), [Th3(μ(3)-O)(μ(2)-OH)3(SiW9O34)2](13-) (3), and [Hf3(μ(2)-OH)3(SiW9O34)2](11-) (4). All four compounds consist of a polynuclear cluster fragment stabilized by two [α-SiW9O34](10-) polyanions. Compounds 1 and 2 are isostructural with a tetranuclear core (Ce4, U4), while compound 3 presents a trinuclear Th3 core bearing a μ(3)-O-centered bridge. It is an unprecedented configuration in the case of the thorium(IV) cluster. Compound 4 also possesses a trinuclear Hf3 core but with the absence of the μ(3)-O bridge. The molecules have been characterized by single-crystal X-ray diffraction, (183)W and (29)Si nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis.

Synthesis and structural characterization of BINOL-modified chiral polyoxometalates

Chiral ligand-modified polyoxometalates (POMs) R-I and S-I were successfully synthesized by the introduction of BINOL into the dititanium-substituted POM.

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.

Discovery of gigantic molecular nanostructures using a flow reaction array as a search engine

The discovery of gigantic molecular nanostructures like coordination and polyoxometalate clusters is extremely time-consuming since a vast combinatorial space needs to be searched, and even a systematic and exhaustive exploration of the available synthetic parameters relies on a great deal of serendipity. Here we present a synthetic methodology that combines a flow reaction array and algorithmic control to give a chemical ‘real-space’ search engine leading to the discovery and isolation of a range of new molecular nanoclusters based on [Mo₂O₂S₂]²⁺-based building blocks with either fourfold (C₄) or fivefold (C₅) symmetry templates and linkers. This engine leads us to isolate six new nanoscale cluster compounds: 1, {Mo₁₀(C₅)}; 2, {Mo₁₄(C₄)₄(C₅)₂}; 3, {Mo₆₀(C₄)₁₀}; 4, {Mo₄₈(C₄)₆}; 5, {Mo₃₄(C₄)₄}; 6, {Mo₁₈(C₄)₉}; in only 200 automated experiments from a parameter space spanning ~5 million possible combinations.

The synthesis of molecular nanostructures often requires the variation of several parameters, such as stoichiometry, pH, counter-ion etc. Here, the authors report a flow reaction array with algorithmic control which is used as a ‘search engine’ to isolate six nanoscale clusters from a massive parameter space.

Chirality meets visible-light photocatalysis in a molecular cerium vanadium oxide cluster

The first example of a molecular cerium vanadium oxide photocatalyst is presented. A cerium vanadium oxide cluster, (nBu4N)2[(Ce(dmso)3)2V(IV)V(V)11O33Cl] × 2DMSO (1), was obtained by bottom-up self-assembly. The compound exhibits high visible-light photooxidative activity towards indigo with quantum efficiencies >10%. Initial mechanistic studies of the photooxidation reaction are presented. The structure of compound 1 is based on chiral, C2-symmetric cluster anions and both enantiomers are present in the crystal lattice. Retrosynthetic analysis suggests that chiral building blocks are spontaneously formed by symmetry breaking during self-assembly. The assembly of 1 based on the chiral building blocks is discussed conceptually. Consequences of this assembly mechanism for the development of chiral molecular catalysts and photocatalysts are described.

Polyoxometalate complexes for oxidative kinetic resolution of secondary alcohols: unique effects of chiral environment, immobilization and aggregation

Chiral polyoxometalate complexes were covalently immobilized into silica and evaluated in the kinetic resolution of secondary alcohols.

Complexation of metal carbonyl units with [α-PW11O39]7− and [α2-P2W17O61]10−: insights into the chiral “twisted-sandwich” dimeric structures

Capturing [M(CO)₃]⁺ (M = Re, Mn) by [α-PW₁₁O₃₉]⁷⁻ or [α₂-P₂W₁₇O₆₁]¹⁰⁻ leads to polyoxometalate-supported metal carbonyl complexes with chiral “twisted-sandwich” structures.

Polyoxometalate water oxidation catalysts and the production of green fuel

In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.