The first spectroscopic model for the S1 state multiline signal of the OEC.

The parallel-mode electron paramagnetic resonance (EPR) spectrum of the S(1) state of the oxygen-evolving complex (OEC) shows a multiline signal centered around g=12, indicating an integer spin system. The series of [Mn(2)(2-OHsalpn)(2)] complexes were structurally characterized in four oxidation levels (Mn(II)(2), Mn(II)Mn(III), Mn(III)(2), and Mn(III)Mn(IV)). By using bulk electrolysis, the [Mn(III)Mn(IV)(2-OHsalpn)(2)(OH)] is oxidized to a species that contains Mn(IV) oxidation state as detected by X-ray absorption near edge spectroscopy (XANES) and that can be formulated as Mn(IV)(4) tetramer. The parallel-mode EPR spectrum of this multinuclear Mn(IV)(4) complex shows 18 well-resolved hyperfine lines center around g=11 with an average hyperfine splitting of 36 G. This EPR spectrum is very similar to that found in the S(1) state of the OEC. This is the first synthetic manganese model complex that shows an S(1)-like multiline spectrum in parallel-mode EPR.