Reduction potentials of heterometallic manganese-oxido cubane complexes modulated by redox-inactive metals

Abstract

Understanding the effect of redox-inactive metals on the properties of biological and heterogeneous water oxidation catalysts is important both fundamentally and for improvement of future catalyst designs. In this work, heterometallic manganese-oxido cubane clusters [MMn3O4] (M = Sr(2+), Zn(2+), Sc(3+), Y(3+)) structurally relevant to the oxygen-evolving complex (OEC) of photosystem II were prepared and characterized. The reduction potentials of these clusters and other related mixed metal manganese-tetraoxido complexes are correlated with the Lewis acidity of the apical redox-inactive metal in a manner similar to a related series of heterometallic manganese-dioxido clusters. The redox potentials of the [SrMn3O4] and [CaMn3O4] clusters are close, which is consistent with the observation that the OEC is functional only with one of these two metals. Considering our previous studies of [MMn3O2] moieties, the present results with more structurally accurate models of the OEC ([MMn3O4]) suggest a general relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidities of incorporated cations that applies to diverse structural motifs. These findings support proposals that one function of calcium in the OEC is to modulate the reduction potential of the cluster to allow electron transfer.

Keywords: electrochemistry; heterometallic complexes; manganese clusters; model complexes; photosynthesis.

Fig. 1.

Proposed structure of the CaMn₄O₅ cluster in the OEC (Left) (26, 27), structure of previously studied heterometallic dioxido complexes (Center) (23), and structure of tetraoxido complexes studied here (Right).

Fig. 2.

Synthesis of Complexes 1-M (M = Sr, Zn, Y).

Fig. 3.

Truncated solid-state structures of (A) [1-Sr]₂, (B) 1-Zn, and (C) [1-Y][OTf], with thermal ellipsoids at the 50% probability level. Portions of the ligand (L), hydrogen atoms, and outer-sphere anions not shown for clarity. (D) Complete solid-state structure of 2. Mn1 is the Mn^III center; Mn2 and Mn3 are Mn^IV. Selected bond lengths (Å): Sc–O1 2.166(1), Sc–O2 2.200(1), Sc–O3 2.164(1), Mn1–O1 1.931(1), Mn1–O2 1.885(1), Mn1–O4 2.142(1), Mn2–O2 1.861(1), Mn2–O3 1.888(1), Mn2–O4 1.903(1), Mn3–O1 1.881(1), Mn3–O3 1.873(1), and Mn3–O4 1.855(1). Bolded lines emphasize the [MMn₃O₄] moiety.

Fig. 4.

CVs corresponding to the [MMn^IV₃O₄]/[MMn^IV₂Mn^IIIO₄] redox couple (M = Mn³⁺, Sc³⁺, Y³⁺, Zn²⁺, Ca²⁺, and Sr²⁺) in 0.1 M NBu₄PF₆ in DMA. Scan rate of 100 mV/s. Potentials are referenced to Fc/Fc⁺. Values for M = Mn³⁺, Sc³⁺, and Ca²⁺ were previously reported (24, 25).

Fig. 5.

Reduction of [1-Sc][OTf].

Fig. 6.

Reduction potentials of MMn₃O₄ complexes (red squares) and MMn₃O₂ complexes (23) (blue diamonds) vs. pK_a of the corresponding M(aqua)ⁿ⁺ ion as a measure of Lewis acidity. Potentials were referenced to ferrocene/ferrocenium.