THE EFFECT OF PROTONATION ON [MN(IV)(MU(2)-O)](2) COMPLEXES

The series of complexes [Mn(IV)(X-SALPN)(mu(2)-O)](2), 1: X = 5-OCH3; 2: X = H; 3: X = 5-Cl; 4: X = 3,5-diCl; 5: X = 5-No-2, contain [Mn2O2] (4+) cores with Mn-Mn separations of 2.7 Angstrom.These molecules can be protonated to form [MN(IV)(X-SALPN)(mu(2)-O,OH)](2+) in which a bri dging oxide is protonated. The pK(a) values for the series of [Mn(IV)( X-SALPN)(mu(2)-O,OH)](2+) track linearly versus the shift in redox pot ential with a slope of 84 mV/pKa. This observation suggests that the [ Mn2O2](4+) core can be considered as a unit in which the free energy o f protonation is directly related to the ability to reduce the Mn(IV) ion. The marked sensitivity of the reduction potential to the presence of protons presents a mechanism in which an enzyme can control the ox idizing capacity of an oxo manganese cluster by the degree and timing of oxo bridge protonation.