Reversible Carboxylate Shift in a μ-Oxo Diferric Complex in Solution by Acid-/Base-Addition

A reversible carboxylate shift has been observed in a μ-oxo diferric complex in solution by UV–vis−NIR and FTIR spectroscopy triggered by the addition of a base or an acid. A terminal acetate decoordinates upon the addition of a proton, resulting in a shift of the remaining terminal acetato to a μ–η1:η1 bridge. The addition of a base restores the original structure containing only terminal acetates. The implications for metalloenzymes with carboxylate-bridged nonheme diiron active sites are discussed.