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Metal oxides with sizes of a few nanometers show variable crystal and electronic structures depending on their dimensions, and the synthesis of metal oxide particles with a desired size is a key technology in materials science. Although discrete metal oxide particles with an average diameter ( d) smaller than 2 nm are expected to show size-specific properties, such ultrasmall metal oxide particles are significantly limited in number. In nature, on the other hand, nanosized ferrihydrite (Fh), which is ferric oxyhydroxide, occurs as a result of biomineralization in ferritin, an iron storage protein cage. Here we describe the synthesis of Fh particles using a covalent molecular organic cage (MOC) derived from 8 + 12 cyclocondensation of triaminocyclohexane with a diformylphenol derivative. At the initial reaction stage, eight iron ions accumulated at the metal binding sites in the cage cavity, and Fh particles ( d = 1.9 ± 0.3 nm) encapsulated within the cage (Fh@MOC) formed with a quite narrow size distribution. The formation process of the Fh particle in the organic cage resembles the biomineralization process in the natural iron storage protein, and the present method could be applicable to the synthesis of other metal oxide particles. Fh@MOC is soluble in common organic solvents and shows substantial redox activity in MeCN.

Two new high-nuclearity mixed-valence Mn clusters, Mn19 and Mn21, were synthesised utilising an alkylamine-based flexible ligand with five alkoxo-bridging groups, 1,3-bis-diethanolamino-2-propanol (H5bdp). They were obtained from the reaction of Mn(O2CPh)2·2H2O with H5bdp·2HCl·H2O and Et3N in a 4 : 1 : 7 molar ratio in CH3CN/CH3OH mixed solution. The X-ray crystal structures revealed {MnII8MnIII10MnIV} and {MnII6MnIII15} mixed-valence cores composed around {Mn7O8} corner-shared double cubanes. Magnetic susceptibility measurements on both complexes indicated dominant intramolecular antiferromagnetic interactions between Mn ions. No clear SMM behaviour was observed above 1.8 K due to the non-parallel alignment of magnetic anisotropy axes of the MnIII ions.

The iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between FeII and FeIII occurs reversibly at ∼120 K, in addition to the ferromagnetic phase transition at TC = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C3H7)4N[FeII1–xZnIIxFeIII(dto)3] (dto = C2O2S2; x = 0–1), and carried out magnetic susceptibility and dielectric constant measurements and 57Fe Mossbauer spectroscopy. With increasing ZnII concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (TC) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of ZnII ions, and then it decreases monotonically from 12 K to 3 K with increasing ZnII concentration. This anomalous dependence of TC on ZnII concentration is related to ...

The dithiooxalato-bridged iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato) undergoes a novel charge-transfer phase transition (CTPT) accompanied by electron transfer between adjacent FeII and FeIII sites. The CTPT influences the ferromagnetic transition temperature according to the change of spin configuration on the iron sites. To reveal the mechanism of the CTPT, we have synthesized the series of metal-substituted complexes (n-C3H7)4N[FeII1-xMnIIxFeIII(dto)3] (x = 0&ndash, 1) and investigated their physical properties by means of magnetic susceptibility and dielectric constant measurements. With increasing MnII concentration, x, MnII-substituted complexes show the disappearance of CTPT above x = 0.04, while the ferromagnetic phase remains in the whole range of x. These results are quite different from the physical properties of the ZnII-substituted complex, (n-C3H7)4N[FeII1-xZnIIxFeIII(dto)3], which is attributed to the difference of ion radius as well as the spin states of MnII and ZnII.

Controlling the assembly and functionalization of molecular metal oxides [Mx Oy ](n-) (M=Mo, W, V) allows the targeted design of functional molecular materials. While general methods exist that enable the predetermined functionalization of tungstates and molybdates, no such routes are available for molecular vanadium oxides. Controlled design of polyoxovanadates, however, would provide highly active materials for energy conversion, (photo-) catalysis, molecular magnetism, and materials science. To this end, a new approach has been developed that allows the reactivity tuning of vanadium oxide clusters by selective metal functionalization. Organic, hydrogen-bonding cations, for example, dimethylammonium are used as molecular placeholders to block metal binding sites within vanadate cluster shells. Stepwise replacement of the placeholder cations with reactive metal cations gives mono- and difunctionalized clusters. Initial reactivity studies illustrate the tunability of the magnetic, redox, and catalytic activity.