Nanostructured Manganese Oxides within a Ring-Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties.

Structurally-New Hexadecanuclear Ni-Containing Silicotungstate with Catalytic Hydrogen Generation Activity

A structurally-new, carbon-free hexadecanuclear Ni-containing silicotungstate, [Ni₁₆(H₂O)₁₅(OH)₉(PO₄)₄(SiW₉O₃₄)₃]^19-, has been facilely synthesized using a one-pot, solution-based synthetic method systematically characterized by single-crystal X-ray diffraction and several other techniques. The resulting complex works as a noble-metal-free catalyst for visible-light-driven catalytic generation of hydrogen, by coupling with a [Ir(coumarin)₂(dtbbpy)][PF₆] photosensitizer and a triethanolamine (TEOA) sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of 842 was achieved for TBA-Ni₁₆P₄(SiW₉)₃-catalyzed hydrogen evolution system. The structural stability of TBA-Ni₁₆P₄(SiW₉)₃ catalyst under photocatalytic conditions was evaluated by the mercury-poisoning test, FT-IR, and DLS measurements. The photocatalytic mechanism was elucidated by both time-solved luminescence decay and static emission quenching measurements.

Ligand-Protecting Strategy for the Controlled Construction of Multinuclear Copper Cores within a Ring-Shaped Polyoxometalate

The ring-shaped polyoxometalate (POM) [P₈W₄₈O₁₈₄]^40- contains a large cavity and is an attractive inorganic multidentate ligand for accumulating metal cations. Until now, several multinuclear metal cores are constructed within the {P₈W₄₈} framework in aqueous solvents. However, it is still challenging to control the number and arrangement of introduced metal cations because of the numerous coordination sites inside the {P₈W₄₈} framework. In this study, we developed a novel approach for the selective synthesis of several multinuclear copper-containing ring-shaped POMs in organic solvents using methoxy groups as organic protecting ligands for the reactive coordination sites. Reacting a tetra-n-butylammonium (TBA) salt of [P₈W₄₈O₁₈₄]^40- (P8W48) with 4 and 8 equiv of copper(II) acetate in the presence of methanol (MeOH), tetra- and octacopper cores were incorporated into the cavity to form TBA₁₁H₁₃[Cu₄(H₂O)₄P₈W₄₈O₁₇₆(OCH₃)₈]·28H₂O·3CH₃NO₂ (Cu4) and TBA₁₄H₂[Cu₈(H₂O)₁₂P₈W₄₈O₁₇₆(OCH₃)₈]·24H₂O·CH₃CN (Cu8), respectively. For both structures, methoxy groups served as protecting ligands and temporarily inactivated vacant coordination sites. Without MeOH, dodeca- and hexadecacopper cores were constructed inside the cavity to form TBA₁₄H₂[Cu₁₂(H₂O)₁₆P₈W₄₈O₁₈₄]·4H₂O (Cu12) and TBA₁₆H₈[Cu₁₆(OH)₁₆(H₂O)₄P₈W₄₈O₁₈₄]·12H₂O·C₃H₆O (Cu16), respectively. The arrangement of copper ions on the same {P₂W₁₂} units could be controlled by the input number of copper ions. Moreover, all four POMs could be synthesized from P8W48 by the stepwise addition of 4 equiv of copper(II) acetate, clarifying the introduction process.