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 ([LnMn](PF6)m), dinuclear ([L1aMnIV2(mu-O)2](PF6)6), or trinuclear ([LnMnIIMnIIMnIILn](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 [L1aMnIV2(mu-O)2](PF6)6, where the "ligand" [H2L1a]4+ consists of two tris(bipyridyl)ruthenium(II)-type units covalentely bound to a bismacrocyclic Me2dtne backbone to which the manganese ions are coordinated via an additional phenolate oxygen (Me2dtne = 1,2-bis(4-methyl-1,4,7-triazacyclononyl)ethane). Weak antiferromagnetic coupling is observed in compounds [LnMnIIMnIIMnIILn](PF6)12, where the three metals are 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 [LnMn](PF6)m (n = 1 or 2) Mn(II), Mn(III), 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.
Burdinski
D
D
Max-Planck Institut für Strahlenchemie, Stiftstrasse 34-36, D-45470 Mülheim, Germany.
Bothe
E
E
Wieghardt
K
K
D004560:Electricity; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D008954:Models, Biological; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D011725:Pyridines / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
D008024:Ligands; D009942:Organometallic Compounds; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D011725:Pyridines; D008345:Manganese; D012428:Ruthenium