Synthesis, solution structures, and electrochemistry of several dinuclear Mn2(II,II) complexes (1-4) and Mn2(III,III) complexes (6 and 8), derived from a functional catalase mimic [(L1,2)Mn2(II,II)(μ13-O2CCH3)]2+ (1) are described that enable testing of the role of intramolecular hydroxide ligands on the redox properties. Addition of 1 equiv of hydroxide to 1 or 3 forms [(L1,2)Mn2(II,II)(μ13-O2CCH3)(μ-OH)]+ (7A or 7B, respectively), possessing two six-coordinate Mn(II) ions bridged by hydroxide and acetato ligands. Two-electron oxidation of 7 with O2 occurs by forming [(L1,2)Mn2(III,III)(μ1,3-O2CCH3)(μ-OH)]3+ (8) and H2O2 with no ligand rearrangements in methanol. Reaction of 8 with 2-3 equiv hydroxide forms [(L1,2)Mn2(III,III)(μ-O)(OH)(O2CCH3)]+ in which deprotonation of μ-OH- to yield μ-O2- favors subsequent addition of a terminal hydroxide ligand, accommodated by a bridging-to-terminal 'carboxylate-shift'. Preservation of six-coordinate Mn(II) ions throughout all hydroxide-induced transformations is observed, including oxidation by O2. Cyclic voltammetry reveals that addition of μ-OH- converts the two-electron redox couple II,II/III,III for complexes 1-4 to sequential one-electron couples at lower reduction potentials, yielding substantial stabilization of the II,III and III,III oxidation states by ΔE = 440 and 730 mV, respectively. Binding of a second OH- to 7A or 7B forms (L1,2)Mn2(II,II)(μ13-O2CCH3)(OH)2, containing two six-coordinate Mn(II) ions with two terminal hydroxides and a μ1,3-bridging acetato. Electrochemistry reveals that displacement of the bridging hydroxide to a terminal site upon addition of the second OH- restores a two-electron redox couple II,II/III,III but now at a higher reduction potential with considerable loss of the electrochemical stabilization energy provided by the μ-OH- (ΔE = 250 and 350 mV loss for Mn2(II,II) and Mn2(III,III), respectively). These results indicate a considerably stronger influence of bridging vs terminal hydroxide ligands in stabilizing the higher oxidation states and separating the one-electron redox potentials of bimetallic centers. By contrast, in the absence of μ-OH- bridges the longer separation with the μ1,3-carboxylato bridge in dimanganese-(II,II) complexes leads to nearly complete uncoupling of the Mn(II) oxidation potentials, thus yielding a two-electron redox transition to (III,III). We hypothesize that this 'bridging hydroxide effect' may be due to both greater screening of the repulsive intermetallic electric potential energy and increased resonance stabilization of the mixed-valence (II,III) oxidation state by charge delocalization. These data provide a physicochemical basis for interpretation of the catalase activity of these complexes and of dimanganese catalase enzymes (see the following manuscript).
ASJC Scopus subject areas
- Inorganic Chemistry