O2 evolution from the manganese-oxo cubane core Mn4O46+: A molecular mimic of the photosynthetic water oxidation enzyme?
Photosynthesis produces molecular oxygen from water catalyzed by an enzyme whose active site contains a tetramanganese−oxo core of incompletely established structure. The first functional mimic of this core has been synthesized containing a cubical [Mn4O4]n+ core, surrounded by six facially bridging bidentate chelates to the manganese ions ((dpp)6Mn4O4 (1); dpp- = diphenylphosphinate anion). Bond enthalpy data predict that the Mn4O46+ core is thermodynamically capable of releasing molecular O2, but is kinetically prevented from doing so by an activation barrier. UV light absorption into a Mn−O charge-transfer excited state (but not excitation of a Mn ligand-field excited state) efficiently releases an O2 molecule if performed in the gas phase and concomitantly releases a bridging dpp- anion and the cationic species (dpp)5Mn4O2+ (presumed Mn4O2-butterfly core type). All species were identified by high resolution mass spectrometry. This reaction proceeds with high quantum efficiency (>50%) and is the only o...