Deprotonations and Charges of Well-Defined {Mo₇₂Fe₃₀} Nanoacids Simply Stepwise Tuned by pH Allow Control/Variation of Related Self-Assembly Processes

Abstract

The solution behavior of the largest inorganic acid known thus far, the neutral, spherical iron/molybdenum/oxide nanocluster {Mo72Fe30} ([triple bond{(MoVI) MoVI5}12FeIII30 1a), including the pH-controlled deprotonation, is reported. The acidic properties are due to the 30 peripheral, weakly acidic FeIII(H2O) groups that form a unique Archimedean solid with all edges and dihedral angles being equal, the icosidodecahedron, and therefore an "isotropic" surface. Interestingly, the aqueous solutions are stable even for months because of the inertness of the spherical solutes and the presence of the hard FeIII and MoVI centers. The stability can be nicely proven by the very characteristic Raman spectrum showing, because of the (approximately) icosahedral symmetry, only a few lines. Whereas the {Mo72Fe30} clusters exist as discrete, almost neutral, molecules in aqueous solution at pH< 2.9, they get deprotonated and self-associate into single-layer blackberry-type structures at higher pH while the assembly process (i.e., the size of the final species) can be controlled by the pH values; this allows the deliberate generation of differently sized nanoparticles, a long-term goal in nanoscience. The average hydrodynamic radius (Rh) of the self-assembled structures decreases monotonically with increasing number of charges on the {Mo72Fe30} macroanions (from approximately 45 nm at pH approximately 3.0 to approximately 15 nm at pH approximately 6.6), as studied by laser light scattering and TEM techniques. The {Mo72Fe30} macroions with high-stability tunable charges/surfaces, equal shape, and masses provide models for the understanding of more complex polyelectrolyte solutions while the controllable association and dissociation reported here of the assembled soft magnetic materials with tuneable sizes could be interesting for practical applications.

Sheet-like assemblies of spherical particles with point-symmetrical patches

“…4 Repulsive interactions may help stabilize sheet-like structures in two ways. Firstly, particles at the edges of the sheets experience less repulsion than on the top of the sheets, which favors the lateral growth of the sheets.…”

“…These objects are certain Keplerate-type polyoxometalates (POM 4 ) and the protein apoferritin. 5 The Keplerates are quasi-spherical hollow icosidodecahedral structures built by molybdenumbased oxides, 6 and further details of the structure is discussed below.…”

“…POMs are known to self-assemble into large hollow shell-like structures (20-50 nm diameter) under certain conditions. 4,8 We address the formation of 2D structures by spherical particles in general without explicitly addressing shells. It is then anticipated that the sheet-like 2D structures eventually grow and fold into shells.…”

Porous Capsules {(M)M5}12FeIII30 (M=MoVI, WVI): Sphere Surface Supramolecular Chemistry with 20 Ammonium Ions, Related Solution Properties, and Tuning of Magnetic Exchange Interactions

“…The {W 72 Fe 30 } cluster 1 a (Figure 1, top) belongs to the family of icosahedral porous capsules and polyoxometalates having the general formula (pentagon) 12 (Figure 1, bottom). This situation allows investigation of unprecedented types of equilibria inside a cavity in which ligands are weakly coordinated to different positions, which is easily investigable, for example in the case of acetates, with 1 H NMR spectroscopy.…”

Photochemical Water Oxidation Using {PMo12O40@Mo72Fe30}n Based Soft Oxometalate

“…81,82 They also explored mechanism of stabilization of selfassembled structures of the cluster by counter-ion condensation. [83][84][85] The material is chosen such that …”