Redox Interaction between Binuclear Mn2(III/IV) Complexes and Binuclear Mn2(II/II) Complexes

A dominant feature of the inorganic chemistry of manganese is the variety of complexes that can be made with different oxidation levels ( + 11+ IV) of the manganese, and living systems have utilized this property in that manganese-containing en­ zymes are involved in many diverse redox func­ tions [1]. The biological function that has generat­ ed most interest is the photosynthetic oxygen evolving complex of Photosystem II (PS II) in which it is generally believed that four manganese atoms are involved. The EXAFS studies have re­ vealed that there is a binuclear Mn2 unit (M n -M n = 2.69 Ä) bridged by oxide ligands (M n -O = 1.75 Ä) and with peripheral 0,N -based ligation (M n -L = 1.98Ä) [2], Recent refinement of EX­ AFS data indicate the possible presence of addi­ tional Mn atoms separated —3.3 Ä from the binu­ clear unit (M n -M n = 2.69Ä), either as (i) two “mononuclear” sites, or (ii) an additional binu­ clear M n, unit, yielding a dimer-of-dimers. Much of available data seem to be interpreted by a dimer-of-dimers structure [3], in which two dimers are assumed to be electronically isolated from each other [4], In this study we have observed that the binuclear M n2(III/IV) complex, [Mn20 2(2,2'-bipyridine)4]3+ interacts with several binuclear M n2(II/II) complexes electronically in a dilute ace­ tonitrile solution. These may give an important key to elucidate the magnetic behaviour of the manganese cluster in PS II, and also the electronic nature of an additional binuclear M n, unit.