Chemical Modeling of the Oxygen-Evolving Center in Plants. Synthesis, Structure, and Electronic and Redox Properties of a New Mixed Valence Mn−Oxo Cluster: [Mn₂^III,IVO₂(bisimMe₂en)₂]³⁺ (bisimMe₂en = <i>N</i>,<i>N‘</i>-Dimethyl-<i>N</i>,<i>N‘</i>-bis(imidazol-4-ylmethyl)ethane-1,2-diamine). EPR Detection of an Imidazole Radical Induced by UV Irradiation at Low Temperature

The compound [Mn2III,IVO2(bisimMe2en)2](ClO4)3·H2O (bisimMe2en = N,N‘-dimethyl-N,N‘-bis(imidazol-4-ylmethyl)ethane-1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a = 18.139(8) Å, b = 12.694(5) Å, c = 17.694(8) Å, β = 107.5(6)°, V = 3885(6) Å3, and Z = 4. The cation [Mn2III,IVO2(bisimMe2en)2]3+ contains a di-manganese di-μ-oxo unit. The Mn−Mn axis is a C2 axis. The Mn−imidazole distances for MnIII or MnIV are distinct: respectively, 2.208(9) and 2.004(8) Å. The ground state is a |SA2,SB3/2,S1/2〉 state separated by 420 cm-1 from the S = 3/2 state. The EPR spectrum has been simulated with |A1x,y| = 160.10-4 cm-1, |A1z| = 136.10-4 cm-1, |A2x,y| = 72.10-4 cm-1, |A2z| = 71.10-4 cm-1, gxy = 1.997, gz = 1.993. The ESEEM spectrum is reported. The 2H nuclei corresponding to the exchangeable hydrogen atoms of imidazole groups in D2O have been detected at 2.4 MHz at 3480 G. In cyclic voltammetry, [Mn2III,IVO2(bisimMe2en)2]3+ presents in oxidation two reversible waves at 1.04 and 1.40 V (all potentials versus NHE) in strong contrast with what has been always observed on other Mn2III,IVO2 units, for which only one wave in oxidation ([III,IV] → [IV,IV]) is observed. EPR spectroelectrochemistry reveals the disappearance of the 16 line spectrum of the [III,IV] species on oxidation at 1.24 V. UV-vis spectroelectrochemistry at this potential confirms the formation of the [IV,IV] dimer. The existence of two waves was related to the presence of slow equilibrium between two different [III,IV] forms. UV-irradiation of the starting [Mn2III,IVO2(bisimMe2en)2]3+ complex in an aqueous borate buffer at pH = 10 at 77 K resulted in the formation of an EPR signal that is attributed to an imidazole radical coupled to the MnIII−MnIV pair. This signal is unsplit and is understandable in a simple model of two spins S = 1/2 in magnetic interaction. It has some analogy with that observed by Boussac et al. (Nature 1990, 347, 303−306) on the S3 state of the Ca2+ and Cl--depleted oxygen-evolving center, although in that case the signal was split. We relate this difference to small differences in the magnetic interaction of the two spins S = 1/2. ESEEM spectra of the irradiated [Mn2III,IVO2(bisimMe2en)2]3+ complex are reported for magnetic fields corresponding to the resonance of the radical or the metal cluster. These results suggest that the formed imidazole radical stays linked to the metal core despite the weak magnetic interaction detected. This rises the possibility that the S3 radical in the natural system, is a direct ligand to Mn cluster.