Reactivity of [{l{underscore}brace}Mn{sup 4}(salpn){r{underscore}brace}{sub 2}({micro}-O,{micro}-OCH{sub 3})]{sup +} and [{l{underscore}brace}Mn{sup IV}(salpn){r{underscore}brace}{sub 2}({micro}-O,{micro}-OH)]{sup +}: Effects of proton liability and hydrogen bonding
It was previously shown that the addition of 1 equiv of a strong acid to [Mn{sup IV}(salpn)({micro}-O)]{sub 2}, 1, generates the oxo/hydroxo complex [{l{underscore}brace}Mn{sup IV}(salpn){r{underscore}brace}{sub 2}({micro}-O,{micro}-OH)](CF{sub 3}SO{sub 3}), 2, which emphasized the basicity of the {micro}{sub 2}-O{sup 2{minus}} units in the [Mn{sup IV}({micro}-O)]{sub 2} dimers. The authors now demonstrate the inherent nucleophilicity of those {micro}{sub 2}-O{sup 2{minus}} units by showing that the addition of methyl triflate to 1 results in formation of the oxo/methoxo-bridged Mn{sup IV} dimer [{l{underscore}brace}Mn{sup IV}(salph){r{underscore}brace}{sub 2}({micro}-O,{micro}-OCH{sub 3})](CF{sub 3}SO{sub 3}), 3. EXAFS analysis of 3 demonstrates that alkylation of an oxo bridge results in the same structural modification of the [Mn{sup IV}({micro}-O)]{sub 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{sup III} product of this reaction, [Mn{sup III}(salpn)(Ph{sub 3}PO)](CF{sub 3}SO{sub 3}), 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 activity with similar active site structures. Furthermore, it also emphasizes that caution should b4e used when the reactivity of model compounds with easily and rapidly dissociable protons is assessed.
- Research Organization:
- Univ. of Michigan, Ann Arbor, MI (US)
- Sponsoring Organization:
- National Institutes of Health (NIH); USDOE
- OSTI ID:
- 20001124
- Journal Information:
- Inorganic Chemistry, Vol. 38, Issue 21; Other Information: PBD: 18 Oct 1999; ISSN 0020-1669
- Country of Publication:
- United States
- Language:
- English
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