Synthesis, Structure, and Characterisation of a New Phenolato-Bridged Manganese Complex [Mn 2 (mL) 2 ] 2 + : Chemical and Electrochemical Access to a New Mono-m-Oxo Dimanganese Core Unit Christelle Hureau, [a] Laurent Sabater, [a] Elodie Anxolabÿhõre-Mallart,* [a] Martine Nierlich, [b] Marie-France Charlot, [a] Florence Gonnet, [c] Eric Riviõre, [a] and Geneviõve Blondin* [a] Introduction The involvement of manganese in several biological systems and processes is now well-known. [1,2] For example, manga- nese dinuclear sites are present in metalloproteins such as manganese catalase, [3,4] arginase [5] and ribonucleotide reduc- tase. [6] Another very important biological system is the tetra- nuclear Mn±oxo cluster, a key element of the OEC [7] located in photosystem II of green plants, [8±12] which catalyses the light-induced oxidation of water into dioxygen. A photo-induced electron abstraction from the OEC occurs during each of the four transition states of the cata- lytic cycle (S 0 ±S 4 of the Kok cycle). [13] Crystal structures of the Synechococcus elongatus photosystem-II dark-adapted S 1 state at 3.8 ä resolution [14] and of the Thermosynechococ- [a] C. Hureau, L. Sabater, E. Anxolabÿhõre-Mallart, M.-F. Charlot, E. Riviõre, G. Blondin Laboratoire de Chimie Inorganique, UMR 8613, LRC-CEA n833V Institut de Chimie Molÿculaire et des Matÿriaux d’Orsay Universitÿ Paris-Sud, 91405 Orsay Cedex (France) Fax: (+ 33)1-69-15-47-54 E-mail: eanxolab@icmo.u-psud.fr gblondin@icmo.u-psud.fr [b] M. Nierlich CEA Saclay, DRECAM/SCM Bat 125 91191 Gif-sur-Yvette (France) [c] F. Gonnet Laboratoire Analyse et Environnement, UMR 8587 B‚timent des Sciences, Universitÿ d’Evry Val d’Essonne Boulevard FranÁois Mitterrand, 91025 Evry Cedex (France) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. Abstract: The dinuclear phenolato- bridged complex [(mL)Mn II M- n II (mL)](ClO 4 ) 2 (1(ClO 4 ) 2 ) has been obtained with the new [N 4 O] pentaden- tate ligand mL  (mLH = N,N’-bis-(2- pyridylmethyl)-N-(2-hydroxybenzyl)- N’-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallog- raphy. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0± 800 mT at the X band and over 100± 1450 mT at the Q band, features that are usually observed for dinuclear Mn II complexes. Cyclic voltammetry of 1 ex- hibits two irreversible oxidation waves at E p 1 = 0.89 V and E p 2 = 1.02 V, accom- panied on the reverse scan by an ill-de- fined cathodic wave at E p 1 0 = 0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxi- dation with tBuOOH (10 equiv) at 20 8C, 1 is transformed into the mono- m-oxo species [(mL)Mn III -(m-O)- Mn III (mL)] 2 + (2), which eventually par- tially evolves into the di-m-oxo species [(mL)Mn III -(m-O) 2 -Mn IV (mL)] n + (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorp- tion band at 507 nm, which is attribut- ed to a phenolate !Mn III charge-trans- fer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16V versus the SCE, corresponding to the Mn III Mn III / Mn III Mn IV and Mn III Mn IV /Mn IV Mn IV ox- idation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-m-oxo mixed- valent species [(mL)Mn III -(m-O)- Mn IV (mL)] 3 + (2ox). The UV/Vis spec- trum of 2ox exhibits one large band at 643 nm, which is attributed to the phe- nolate !Mn IV charge-transfer transi- tion. 2ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, re- spectively. Both spectra have been si- mulated by using colinear rhombic Mn- hyperfine tensors. Mechanisms for the chemical formation of 2 and the elec- trochemical oxidation of 1 into 2ox are proposed. Keywords: electrochemistry ¥ EPR spectroscopy ¥ manganese ¥ N,O ligands ¥ oxidation ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/chem.200305515 Chem.Eur.J. 2004, 10, 1998±2010 1998 FULL PAPER