Ionic γ-FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation.

At near neutral to basic pH, hydrolysis-induced aggregation to insoluble bulk iron-oxide is often regarded as the pitfalls of molecular iron clusters. Iron-oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson-type [Mo7 O24 ]6- isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous-soluble γ-FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo7 O24 ]6- may lead to the in situ generation of few analogous [Mox Oy ]n- clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mox Oy @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mox Oy ]n- , they are covalently linked via MoVI -µ2 O-FeIII bridging to γ-FeO(OH) core in Mox Oy @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ-FeO(OH) nanocore of Mox Oy @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm-2 , lower than that observed for bare γ-FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ-FeO(OH) core of the Mox Oy @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode-electrolyte junction, less likely in bulk γ-FeO(OH).

A High-Nuclear Isopolymolybdate Cluster Assembled with an Anionic [{Mo24 O48 (OMe)32 }]8- and Two Charge-Neutral [{Mo24 O52 (OMe)28 }] Cages.

We synthesized a high-nuclear isopolymolybdate cluster (n-Bu4 N)6 H2 [{Mo24 O48 (OMe)32 }{Mo24 O52 (OMe)28 }2 ] ⋅ 25H2 O ⋅ 6CH3 CN (1) by using [Mo6 O19 ]2- as the base precursor. Crystallographic characterization shows the cluster is composed of an anionic [{Mo24 O48 (OMe)32 }]8- cage and two charge-neutral [{Mo24 O52 (OMe)28 }] cages. Supported by the electrospray ionization mass spectrometry study, the polyoxoanion structural unit [Mo24 O48 (CH3 O)27 ]3- demonstrates strong stability in acetonitrile solution. Moreover, 1 exhibits good proton conductivity of 1.79×10-3  S cm-1 at 358 K and 98 % relative humidity.