Investigations of the Effect of the Non-Manganese Metal in Heterometallic-Oxido Cluster Models of the Oxygen Evolving Complex of Photosystem II: Lanthanides as Substitutes for Calcium

We report the syntheses and electrochemical properties of nine new clusters ([LLnMnIV3O4(OAc)3(DMF)n]+ (Ln = La3+, Ce3+, Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Yb3+, and Lu3+, n = 2 or 3)) supported by a ligand (L3–) based on a 1,3,5-triarylbenzene motif appended with alkoxide and pyridine donors. All complexes were obtained by metal substitution of Ca2+ with lanthanides upon treatment of previously reported LMn3CaO4(OAc)3(THF) with Ln­(OTf)3. Structural characterization confirmed that the clusters contain the [LnMn3O4] cubane motif. The effect of the redox-inactive centers on the electronic properties of the Mn3O4 cores was investigated by cyclic voltammetry. A linear correlation between the redox potential of the cluster and the ionic radii or pKa of the lanthanide metal ion was observed. Chemical reduction of the LMnIV3GdO4(OAc)3(DMF)2 cluster with decamethylferrocene, resulted in the formation of LGdMnIV2MnIIIO4(OAc)3(DMF)2, a rare example of mixed-valence [MMn3O4] cubane. The lanthanide-coordinated ligands can be substituted with other donors, including water, the biological substrate.