John David Carter

Oxidative Reactivity Difference among the Metal Oxo and Metal Hydroxo Moieties: pH Dependent Hydrogen Abstraction by a Manganese(IV) Complex Having Two Hydroxide Ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of natures choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

Similarities and differences in properties and behavior of two H2O2-activated manganese catalysts having structures differing only by methyl and ethyl substituents

The complex [Mn(IV)(Me2EBC)(OH)2](PF6)2, in which Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, is a remarkably selective H2O2 oxidation catalyst that has been shown to be useful in removing stains from fabrics without affecting their colors. Mn(IV) is the highest oxidation state detected and the dihydroxo complex forms a peroxyhydroxy derivative that is responsible for catalytic oxidations. Study of the diethyl homolog of this catalyst has revealed surprising differences in chemical behavior. Oxidation of this new manganese complex, Mn(Et2EBC)Cl2, using aqueous H2O2, at −30°C following removal of chloride ion, yields [Mn(Et2EBC)(OH)2](PF6)2. Above 0°C, H2O2 oxidation of Mn(Et2EBC)Cl2 oxidizes the ethyl substituents. X-ray structure determinations of Et2EBC complexes with Mn(II), Mn(III), and Mn(IV) are reported. The complex [Mn(Et2EBC)(OH)2](PF6)2 displays a surprisingly mild oxidizing potential of +0.556 V for the Mn4+/Mn3+ couple; however, its hydrogen abstraction ability for selected substrates is limited by the BDECH value of 82 kcal mol−1, the same as reported for [Mn(Me2EBC)(OH)2](PF6)2. However, unlike the methyl derivative, electrochemical results indicate a 5+/4+ couple, in addition to the expected 4+/3+ and 3+/2+ couples. The significance of these differences in behavior is discussed. Mass spectral studies have identified some products of ethyl group oxidations.

Manganese complexes with a lengthy o -xylylene cross-bridged cyclam ligand: synthesis, characterization and catalytic hydrogen abstraction by dioxygen activation

Two ultra rigid, o-xylylene cross-bridged macrobicyclic ligands, 1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (H2XBC), and 13,19-dimethyl-1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (Me2XBC), have been synthesized and the manganese complexes have been synthesized and characterized, including an X-ray structure determination. Mn(Me2XBC)Cl2 displays a relatively high redox potential for the Mn2+/Mn3+ couple (+0.947V vs SHE, measured in CH3CN), suggesting that the manganese(III) complex may be capable of hydrogen abstraction from moderately active substrates. Direct reaction of the freshly synthesized manganese(III) complex, [Mn(Me2XBC)Cl2]PF6, with 1,4-cyclohexadiene confirmed its hydrogen abstracting ability. The manganese(II)/Me2XBC complex is activated by dioxygen in buffered basic aqueous solutions and catalyzes hydrogen abstraction from selected substrates. A possible mechanism for this manganese complex catalyzed dioxygen activation and hydrogen abstraction is proposed.