Andrew Gelasco

The effect of protonation on [Mn(IV)(μ2-O)]2 complexes

The series of complexes [Mn(IV)(X-SALPN)(μ2-O)]2, 1: X=5-OCH3; 2: X=H; 3: X=5-Cl; 4: X=3,5-diCl; 5: X=5-NO2, contain [Mn2O2]4+ cores with Mn-Mn separations of 2.7 Å. These molecules can be protonated to form [Mn(IV)(X-SALPN)(μ2-O,OH)]2+ in which a bridging oxide is protonated. The pKa values for the series of [Mn(IV)(X-SALPN)(μ2-O,OH)]2+ track linearly versus the shift in redox potential 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 of 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 oxidizing capacity of an oxo manganese cluster by the degree and timing of oxo bridge protonation.

Modeling the Chemistry and Properties of Multinuclear Manganese Enzymes

Dinuclear manganese complexes have been prepared using two similar ligands 2-OHsalpn and salpn. These complexes span the Mn-oxidation states from II/II to IV/IV. The [Mn(2-OHsalpn)]2 complexes have been demonstrated to be efficient functional models for the manganese catalases, and suggest a mechanism for inactivation of the enzymes. The high valent system [MnIV(salpn)(μ-O]2 can be successively protonated on the oxo bridges, which leads to changes in Mn-Mn distance, magnetic exchange and reaction chemistry. The second protonation leads to reductive decomposition of the dimer to MnIII monomers, likely accompanied by oxidation of water.

High-valent dinuclear manganese complexes as functional models of the catalase-like activity in the oxygen evolving complex

While this reaction is catalyzed by the Mn catalase enzyme through a lowvalent Mn(II,II)/Mn(III,III) cycle, the catalase-like activity of the OEC is believed to proceed through a high-valent Mn(IIl,III)/Mn(IV,IV) cycle, most likely involving two of the four Mn ions in the site. This high-valent activity is mimicked by the Mn schiff base complex [M~I~~(SALPN)(O)]~.[ 1 ] The rate of reactivity of this model complex shows striking dependence upon a number of factors, including protonation state, the presence of acid or base sources in solution, electron-donating and withdrawing substituents on the ligand system, and the solvent. The mechanistic details of the catalytic disproportionation of hydrogen peroxide by this complex, as well as its formation by reaction of Mn(III) complexes with hydrogen peroxide, are being studied by comparison of the reaction kinetics under conditions which probe the effects of the perturbations listed above. Addition of a base source is shown to significantly accelerate the catalytic reaction, and numerous solvents have been shown to substantially inhibit this reaction. More subtle perturbations, such as addition of Cl, N@, and 0CH3 groups to the phenolate rings of the SALPN ligand, and addition of substoichiometric sources of protons will also be presented. The results of these studies are correlated to the electrochemistry and UVNIS spectroscopy of these complexes with different electron-withdrawing and -donating groups, different protonation states of the 0x0 bridges, and different solvents in order to determine the origin of these effects on the reaction kinetics. Additionally, we compare the reactivity of the oneelectron reduced Mn(III)Mn(IV) bis oxo-bridged dimer [2 ] in order to determine its competency as an intermediate in the catalytic disproportionation of hydrogen peroxide.

Low valent, dinuclear manganese complexes as functional models for manganese catalases

The manganese catalases are enzymes that disproportionate hydrogen peroxide to oxygen and water at a dinuclear manganesesite (Eq. 1). 2H202 +2H20 +@ Presently, the best model for this system proposes that the enzyme cycles between Mn(II,II) and Mn(III,III) levels. A third oxidized form of the enzyme, containing an Mn(III,IV) center has been characterized by EPR, and is inactive. We have used the pentadentate ligand X-ZOHSalpn (X=H, Cl, NO2, 0CH3) to prepare dinuclear manganese complexes having the various oxidation levels proposed for the enzyme. Structural analysis of these materials demonstrates that the dimers fall into two classes: symmetric (with short Mn-Mn distances, Figure 1) and asymmetric (with long Mn-Mn distances, Figure 2). Earlier work with Mn complexes of this ligand gave only asymmetric compounds[ l] that am inactive toward Hz@ disproportionation. In contrast, the lower valent, symmetric complexes as Mn(II,II) and Mn(III,III), disproportionate H202. The rates of disproportionation of Hz@ by these lower valent complexes will be compared to [Mn(IV)(Salpn)(0)]2 which exhibits a cat&se reaction using high valent manganese.[2] The reactivity of the X-2-OHSaIpn complexes with t-butyl hydroperoxide and other oxidants will also be presented.