Synthesis and characterization of tris(bipyridyl)ruthenium(II)-modified mono-, di-, and trinuclear manganese complexes as electron-transfer models for photosystem II

With the aim of modeling the arrangement of redox-active and photoactive components along the electron-transfer pathway of photosystem II, tetra- to nonanuclear transition metal complexes have been synthesized, comprising one, two, or three manganese ions, oxidizable phenolates, and tris(2,2'-bipyridyl)ruthenium(II)-type units as photosensitizers. These model complexes are considered to be mononuclear ([(LMn)-Mn-n](PF6)(m)), dinuclear ([(LMn2IV)-Mn-1a-(mu-O)(2)](PF6)(6)). or trinuclear ([(LMnMnMnLn)-Mn-n-Mn-II-Mn-II-L-II](PF6)(12)) With respect to the number of manganese centers present. Electronic coupling between the manganese ions is strongly antiferromagnetic in the case of the di(mu-oxo)-dimanganese compound [(LMn2IV)-Mn-1a(mu-O)(2)](PF6)(6), where the "ligand" [H2L1a](4+) consists Of two tris(bipyridyl)ruthenium(II)-type units covalentely bound to a bismacrocyclic Me(2)dtne backbone to which the manganese ions are coordinated via an additional phenolate oxygen (Me(2)dtne = 1,2-bis(4-methyl-1,4,7-triazacyclononyl)ethane). Weak antiferromagnetic coupling is observed in compounds [(LMnMnMnLn)-Mn-n-Mn-II-Mn-II-L-II](PF6)(12), where the three metals ale in a linear arrangement (face-sharing octahedral). They are bridged by three phenolate oxygens of each of the deprotonated "ligands" [H3Ln](6+), respectively. Each ligand [H3Ln](6+) (n = 1, 2) consists Of a tacn ring with three pendent arm phenols which are each bound to a tris(bipyridyl)ruthenium(II)-type unit (tacn = 1,4,7-triazacyclononane). In these compounds several electron-transfer steps were detected by electrochemical methods which are assigned to different redox processes located at individual electrochemically active components (Mn, Ru, bipyridyl, phenolate). For example, in the "mononuclear" compounds [(LMn)-Mn-n](PF6), (n = 1 or 2) Mn(II), Mn(II), and Mn(IV) are accessible and three Ru(II) centers are reversibly oxidized to Ru(III), and in addition, the coordinated phenolate can be oxidized to a highly reactive, coordinated phenoxyl radical. In several cases very slow heterogeneous electron-transfer rates were observed for redox processes involving the manganese centers.