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, Mn+=O and Mn+-OH, contributes to the understanding of nature's choice between them (Mn+=O or Mn+-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(Me2EBC)(2)(OH)(2)](PF6)(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 (BDEOH) for the corresponding reduction products, Mn-III-OH and Mn-III-OH2, 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 BDECH values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDECH = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me2EBC)(2)(O)(2), having only Mn-IV=O groups, reacts more than 40 times faster than the Mn-IV-OH unit in Mn(Me2EBC)(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+) Delta H double dagger = 13.1 +/- 0.7 kcal/mol, and Delta S double dagger = -35.0 +/- 2.2 cal K-1 mol(-1); and the temperature range 288-308 K for for Mn-IV(O)(2), Delta H double dagger = 12.1 +/- 1.8 kcal/mol, and Delta S double dagger = -30.3 +/- 5.9 cal K-1 mol(-1).