Controlled Assembly of Ru-Containing Polyoxometalates for Photocatalytic Activity of the Primary Amine Coupling Reaction

Three Ru-induced structural interconversion polyoxometalates (POMs), Na₁₃H₅[Ru₄(H₂O)₂(Cl)₂(WO₂)₄(AsW₉O₃₃)₄]·43H₂O (1), K₅Na₉H₈[Ru₂(WO₂)₄(AsW₉O₃₃)₄]·50H₂O (2), and KNa₁₃H₁₄[(WO₂)₄(AsW₉O₃₃)₄]·34H₂O (3), were successfully synthesized and thoroughly characterized. Interconversion of structures was accomplished by changing the number of active sites for compounds 1-3. All three compounds contain one {As₄W₄₀O₁₄₀} unit, showing similar structural characteristics except for the active center number (Ru). Interestingly, compound 1 [turnover number (TON)= 486; turnover frequency (TOF)= 20 h^-1] showed highly efficient photocatalysis in achieving oxidative coupling of primary amines. Compound 2 (TON = 406, TOF = 17 h^-1) was also found to promote the oxidative coupling with relatively poor efficiency; however, compound 3 (TON = 178; TOF = 7.4 h^-1) had no obvious contribution to the coupling reaction system, and a chain of evidence indicates that the catalytic performances are strongly dependent on element contents of active sites. Furthermore, the Ru-containing POM-based photocatalysts are conveniently recyclable and reusable during the photocatalytic processes. This study demonstrates the possibility of tuning the catalytic efficiency and stability of POM-based photocatalysts by well designing and controlling their structures. The possible reaction mechanism for the photocatalysis synthesis of imine product is also proposed based on experimental studies.

Targeted synthesis of a CrIII–O–VV core oxo-bridged complex: spectroscopic, magnetic and electrical properties

The results of vibrational, electronic, structural, thermal, magnetic and impedance spectroscopy studies are presented in the first reported compound with a Cr^III–O–V^V bridge.

Polyoxometalate-Based Catalysts for CO2 Conversion

Polyoxometalates (POMs) are a diverse class of anionic metal-oxo clusters with intriguing chemical and physical properties. Owing to unrivaled versatility and structural variation, POMs have been extensively utilized for catalysis for a plethora of reactions. In this focused review, the applications of POMs as promising catalysts or co-catalysts for CO₂ conversion, including CO₂ photo/electro reduction and CO₂ as a carbonyl source for the carbonylation process are summarized. A brief perspective on the potentiality in this field is proposed.

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O40^3-), transition metal ions (Ce³⁺ or Co²⁺), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O40^3- content. Incorporation and structure retention of PW12O40^3- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn → MMCT unit → electrode, and the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

Synthesis and conductive performance of indium-substituted ternary heteropoly acids with Keggin structures

Keggin-type ternary polyoxometalates H₄[In(H₂O)PW₁₁O₃₉]·11H₂O and H₅[In(H₂O)SiW₁₁O₃₉]·8H₂O are solid high-proton conductors with a conductivity of 2.60 × 10⁻⁴ S cm⁻¹ and 5.25 × 10⁻⁴ S cm⁻¹, respectively, at 18 °C and 80% relative humidity.

An inorganic chromophore based on a molecular oxide supported metal carbonyl cluster: [P2W17O61{Re(CO)3}3{ORb(H2O)}(μ3-OH)]9-

A polyoxometalate-supported trirhenium carbonyl cluster, mimicking metal oxide supported interfacial dyadic structures, has been synthesized and characterized. Multiple techniques, including computational and transient absorption spectroscopy, have been applied to characterize the charge-transfer dynamics occurring at the interfaces of this "double cluster". The stepwise kinetics of charge separation and recombination has been thoroughly investigated.