Molybdenum-oxide based unique polyprotic nanoacids showing different deprotonations and related assembly processes in solution

Abstract

We report the self-assembly processes in solution of three Keplerate-type molybdenum-oxide based clusters {Mo₇₂V₃₀}, {Mo₇₂Cr₃₀} and {Mo₇₂Fe₃₀} (all with diameters of ∼2.5 nm). These clusters behave as unique weak polyprotic acids owing to the external water ligands attached to the non-Mo metal centers. Whereas the Cr and Fe clusters have 30 water ligands attached at the 30 M³⁺ centers pointing outside, {Mo₇₂V₃₀} has 20 water ligands coordinated to vanadium atoms, of which only 10 are pointing outside. The self-assembly processes of the Keplerates leading to supramolecular blackberry-type structures are influenced by the effective charge densities on the cluster surfaces, which can be tuned by the pH values and solvent properties. As expected, {Mo₇₂Cr₃₀} and {Mo₇₂Fe₃₀} behave similarly in aqueous solution due to their analogous structures and in both cases the self-assembly follows the partial deprotonation of the external water ligands attached to the non-Mo metal centers. However, the M-OH₂ functionalities differ not only in acidity but also lability, i.e. in different residence times of the H₂O ligands. In contrast to {Mo₇₂Cr₃₀} and {Mo₇₂Fe₃₀}, the {Mo₇₂V₃₀} clusters carry a rather large number of negative charges so that their solution properties are different. They exist as discrete macroions in dilute aqueous solution, and form only in mixed water/organic solvent (like acetone) blackberry-type structures whose size increases with acetone content. The comparison of the properties of the clusters allows more general information about the interesting self-assembly phenomenon to be unveiled.