Structural and functional models of the dimanganese catalase enzymes. 2. Structure, electrochemical, redox, and EPR properties

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, Mn^III 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 MnCat^III,III oxidation state as the resting state.