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

It was previously shown that the addition of 1 equiv of a strong acid to [M n-IV(salpn)(mu-O)](2), 1, generates the oxo/hydroxo complex [{Mn-IV(salpn)} (mu-O, mu-OH)](CF3SO3), 2, which emphasized the basicity of the mu(2)-O2- u nits in the [Mn-IV(mu-O)](2) dimers. We now demonstrate the inherent nucleo philicity of those mu(2)-O2- units by showing that the addition of methyl t riflate to 1 results in formation of the oxo/methoxo-bridged MnIV dimer [{M n-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 modificati on of the [Mn-IV(mu-O)](2) core as an oxo bridge protonation. Electrochemic al and spectroscopic comparisons of 3 to 2 indicate that 3 is a good electr onic structure analogue for 2 without the complication of proton lability a nd hydrogen bonding. Indeed, 2 and 3 react nearly identically with hydrogen peroxide and with strong acids. In contrast, the products of their reactio ns 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, k inetic, and EPR-monitored radical trap studies lead to a proposed reduction -oxidation mechanistic scheme for the reactions of 3 with amines and triphe nylphosphine. The Mn-III product of this reaction, [Mn-III(salpn)(Ph3PO)](C F3SO3), was isolated and crystallographically characterized as a dimerized complex. The redox nature of the reactions is confirmed by trapping of a re duced Mn intermediate which is identified by EPR spectroscopy. Comparison o f the reactions of 2 and 3 demonstrates the dramatic effect of proton labil ity and hydrogen bonding on reactivity, and suggests how metalloenzymes may regulate active site reactivity to produce very different catalytic activi ties with similar active site structures. Furthermore, it also emphasizes t hat caution should be used when the reactivity of model compounds with easi ly and rapidly dissociable protons is assessed.