Reactivity of [{Mn^IV(salpn)}₂(μ-O,μ-OCH₃)]⁺ and [{Mn^IV(salpn)}₂(μ-O,μ-OH)]⁺:  Effects of Proton Lability and Hydrogen Bonding

Abstract

It was previously shown that the addition of 1 equiv of a strong acid to [Mn(IV)(salpn)(&mgr;-O)](2), 1, generates the oxo/hydroxo complex [{Mn(IV)(salpn)}(2)(&mgr;-O,&mgr;-OH)](CF(3)SO(3)), 2, which emphasized the basicity of the &mgr;(2)-O(2)(-) units in the [Mn(IV)(&mgr;-O)](2) dimers. We now demonstrate the inherent nucleophilicity of those &mgr;(2)-O(2)(-) 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)(&mgr;-O,&mgr;-OCH(3))](CF(3)SO(3)), 3. EXAFS analysis of 3 demonstrates that alkylation of an oxo bridge results in the same structural modification of the [Mn(IV)(&mgr;-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)(Ph(3)PO)](CF(3)SO(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 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.

Rapid Loss of Structural Motifs in the Manganese Complex of Oxygenic Photosynthesis by X-ray Irradiation at 10–300 K

“…Accompanying is a reduction in the number of ϳ2.7 Å manganese-manganese vectors by ϳ50%, suggesting that one of the two di--oxo-bridged manganese pairs is modified or lost. Modification might result from protonation of a bridging oxide so that an increase of the original ϳ2.7 Å manganese-manganese distance in the Mn(-O) 2 Mn pair to ϳ2.85 Å in the Mn(-O)(-OH)Mn unit is expected (64). Such a modified manganese complex should show a mean manganese-manganese distance of ϳ2.77 Å instead of ϳ2.7 Å.…”

Calcium Manganese Oxides as Oxygen Evolution Catalysts: O2 Formation Pathways Indicated by 18O‐Labelling Studies

“…For the Kremer-Stein proposal the manganese- 16 O-oxido intermediate is expected to exchange with the 18 O of the bulk solution, as this is well documented for manganese-oxido complexes in higher oxidation states. [24][25][26][27][28] Unfortunately, the literature only provides 16 O/ 18 O exchange rates for terminal oxido ligands of manganese porphyrins (where exchange is very fast) [25,26] and m 2 -/m 3 -oxido ligands of multinuclear manganese complexes (where the exchange was found to be slow). [24,27,28] …”

Redox Intermediates of the Mn-Fe Site in Subunit R2 of Chlamydia trachomatis Ribonucleotide Reductase

“…In summary, these results likely imply di-O(H) metal bridging at least in the Mn(IV)Fe(III) state of Ct R2 ("diamond core"). Mn(IV)Fe(III) formation may lead to deprotonation of OH bridges (74,75) or reduction of a peroxide (OOH) in the Mn(III)Fe(III) state.…”