Synthesis, characterization, and electrochemical properties of new water-soluble Mn12O12(O2CR)16(H2O)4 clusters

We report our attempts to produce water-soluble Mn clusters of the type [Mn12O12(O2CR)16(H2O)4] and the synthesis, spectroscopic, structural, and electrochemical characterization of the three new compounds that were obtained. Clusters 2, 3, and 4 were prepared via substitution of the acetate ligands in [Mn12O12(O2CMe)16(H2O)4] (1) with either 3,4-diaminobenzoic acid, L-proline or L-ascorbic acid, respectively, which are all inexpensive and readily available. Clusters 2, 3, and 4 were characterized by elemental analysis, UV-Vis, and FTIR spectroscopies, XPS, MS, and XRD analysis, suggesting that the clusters retain their structure during the substitution reaction, albeit 4 was obtained partially substituted and reduced. Electrochemical measurements in acetate buffer at pH 6, including continuous cyclic voltammetry scans of the free ligands and of the clusters, imply that 4 is stable to the oxidation process, while in 2 the primary amine ligands are oxidized rapidly, leading to precipitation of the cluster. Overall, the voltammetric measurements support the spectroscopic-based proposed structures.