Catalysts which functionally mimic the bacterial dimanganese catalase enzymes have been synthesized and their structure, electrochemical, redox, and EPR spectra have been compared to the enzyme. These compounds are formulated as [LMn2 II,IIX]Y2, μ-X = CH3CO2, ClCH2CO2; Y = ClO4, BPh4, CH3CO2, possessing a bridging μ-alkoxide from the ligand, HL = N,N,N',N'-tetrakis(2-methylenebenzimidazole)-1,3-diaminopropan-2-ol. An X-ray diffraction structure of [LMn2(CH3CO2)(butanol)] (C1O4)2·H2O, in the monoclinic space group P2(1)/c, confirmed the anticipated N6O septadentate coordination of the HL ligand, the bridging μ-acetate, and revealed both five- and six-coordinate Mn ions; the latter arising from a butanol solvent molecule. This contrasts with the six-coordinate Mn ions observed for the μ-Cl and μ-OH derivatives, LMn2Cl3 and LMn2(OH)Br2 (Mathur et al. J. Am. Chem. Soc. 1987, 109, 5227-5232). Like the enzyme, three electrons can be removed from these complexes to form four oxidation states ranging from Mn2 II,II to Mn2 III,IV. Three of these have been characterized by EPR and found to possess electronic ground states, MnIII electron orbital configurations, 55Mn hyperfine parameters, and Heisenberg exchange interactions analogous to those observed in the enzyme. For the μ-carboxylate derivatives electrochemistry reveals the initial oxidation process involves loss of two electrons at 0.81-0.86 V, forming Mn2 III,III, followed by dismutation to yield a Mn2 II,III and Mn2 III,IV species. By contrast, the μ-Cl and μ-OH derivatives oxidize by an initial one-electron process (0.49-0.54 V). For the μ-carboxylate derivatives chemical oxidation with Pb(OAc)4 also reveals an initial two-electron oxidation to a Mn2 III,III species, which dismutates to form both Mn2 II,III and Mn2 III,IV species. The two Mn2 II,III species formed by these methods exhibit 55Mn hyperfine fields differing in magnitude by 9% (150 G), implying different Mn coordination environments induced by the electrolyte. The different ligand coordination observed in the enzyme (predominantly oxo and carboxylato) appears to be responsible for stabilization of the MnCatIII,III oxidation state as the resting state.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - Feb 9 1994|
- Hydrogen peroxide
- X-ray synthesis
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