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

Abstract

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-η:η 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.

Variation of the Molecular and Electronic Structures of μ‐Oxo Diferric Complexes with the Bridging Motive

“…Performing this reaction with only one equiv. of (Bu 4 N)OAc yields the acetato‐bridged complex [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }](ClO 4 ) 3 . It should be noted that this complex was obtained independently several times by reactions of susan / [Fe II (H 2 O) 6 ](ClO 4 ) 2 mixtures in CH 3 CN without addition of acetate as shown by single‐crystal X‐ray diffraction, FT‐IR, and Mössbauer spectroscopy.…”

“…The reaction of susan with [Fe II (H 2 O) 6 ](ClO 4 ) 2 and 2.46 equiv. of (Bu 4 N)OAc in the presence of NEt 3 provides a brown solution, from which deep brown crystals of [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}](ClO 4 ) 2 were obtained . Performing this reaction with only one equiv.…”

“…The ligand susan [4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane] was synthesized according to the published procedure . The syntheses of the complexes [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}](ClO 4 ) 2 and [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }](ClO 4 ) 3 have been communicated recently …”

“…We present and compare the molecular and electronic structures of [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}] 2+ with two terminal acetates, [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }] 3+ with a bridging acetate resulting in a higher positive charge, and [(susan){Fe III ( μ ‐O)( μ ‐CO 3 )Fe III }] 2+ with a dianionic bridging carbonate. The synthesis and structural characterization of [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}](ClO 4 ) 2 and [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }](ClO 4 ) 3 were communicated recently in conjunction with their reversible carboxylate shift in solution …”

Reactivity Differences for the Oxidation of FeIIFeII to FeIII(µ‐O)FeIII Complexes Caused by Pyridyl versus 6‐Methyl‐Pyridyl Ligands

“…However, although these complexes exhibit low redox potentials for oxidation, the oxidations are ligand‐centered leading to coordinated phenoxyl radicals and the oxidized dinuclear complexes decay into mononuclear fragments , . Thus, we have developed a dinucleating bis(tetradentate) ligand system to coordinate a {Fe(µ‐O) 2 Fe} core (Schemeà) employing different terminal donors (carboxylates, phenolates, and pyridines) , , . The strong electron‐donating character of the bis‐µ‐oxo bridge should enable the oxidation to high‐valent species.…”