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

Abstract

We report the syntheses and electrochemical properties of nine new clusters ([LLnMn^(IV)_(3)O_4(OAc)_3(DMF)_n]^+ (Ln = La^3+, Ce^3+, Nd^3+, Eu^3+, Gd^3+, Tb^3+, Dy^3+, Yb^3+, and Lu^3+, n = 2 or 3)) supported by a ligand (L^3–) 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 LMn_3CaO_4(OAc)_3(THF) with Ln(OTf)_3. Structural characterization confirmed that the clusters contain the [LnMn_3O_4] cubane motif. The effect of the redox-inactive centers on the electronic properties of the Mn_3O_4 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 LMn^(IV)_3GdO_4(OAc)_3(DMF)2 cluster with decamethylferrocene, resulted in the formation of LGdMn^(IV)_2Mn^(III)O_4(OAc)_3(DMF)_2, a rare example of mixed-valence [MMn_3O_4] cubane. The lanthanide-coordinated ligands can be substituted with other donors, including water, the biological substrate.

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