Reactivity of [{MnIV(salpn)}2(μ-O,μ-OCH3)]+ and [{MnIV(salpn)}2(μ-O,μ-OH)]+:: Effects of proton lability and hydrogen bonding

It was previously shown that the addition of 1 equiv of a strong acid to [Mn-IV(salpn)(mu-O)](2), 1, generates the oxo/hydroxo complex [{Mn-IV(salpn)}(2)(mu-O, mu-OH)](CF3SO3), 2, which emphasized the basicity of the mu(2)-O2- units in the [Mn-IV(mu-O)](2) dimers. We now demonstrate the inherent nucleophilicity of those mu(2)-O2- units by showing that the addition of methyl triflate to 1 results in formation of the oxo/methoxo-bridged Mn-IV dimer [{Mn-IV(salpn)}(2)(mu-O,mu-OCH3)](CF3SO3), 3. EXAFS analysis of 3 demonstrates that alkylation of an oxo bridge results in the same structural modification of the [Mn-IV(mu-O)](2) core as an oxo bridge protonation. Electrochemical and spectroscopic comparisons of 3 to 2 indicate that 3 is a good electronic structure analogue for 2 without the complication of proton lability and hydrogen bonding. Indeed, 2 and 3 react nearly identically with hydrogen peroxide and with strong acids. In contrast, the products of their reactions with amines, acetate, and triphenylphosphine are dramatically different. The proton lability of 2 results in simple proton transfer, circumventing the slower redox reactions of these substrates with 3. Isotopic labeling, kinetic, and EPR-monitored radical trap studies lead to a proposed reduction-oxidation mechanistic scheme for the reactions of 3 with amines and triphenylphosphine. The Mn-III product of this reaction, [Mn-III(salpn)(Ph3PO)](CF3SO3), was isolated and crystallographically characterized as a dimerized complex. The redox nature of the reactions is confirmed by trapping of a reduced Mn intermediate which is identified by EPR spectroscopy. Comparison of the reactions of 2 and 3 demonstrates the dramatic effect of proton lability and hydrogen bonding on reactivity, and suggests how metalloenzymes may regulate active site reactivity to produce very different catalytic activities with similar active site structures. Furthermore, it also emphasizes that caution should be used when the reactivity of model compounds with easily and rapidly dissociable protons is assessed.