In situ structural characterization of ferric iron dimers in aqueous solutions: identification of μ-oxo species.

The structure of ferric iron (Fe(3+)) dimers in aqueous solutions has long been debated. In this work, we have determined the dimer structure in situ in aqueous solutions using extended X-ray absorption fine structure (EXAFS) spectroscopy. An Fe K-edge EXAFS analysis of 0.2 M ferric nitrate solutions at pH 1.28-1.81 identified a Fe-Fe distance at ∼3.6 Å, strongly indicating that the dimers take the μ-oxo form. The EXAFS analysis also indicates two short Fe-O bonds at ∼1.80 Å and ten long Fe-O bonds at ∼2.08 Å, consistent with the μ-oxo dimer structure. The scattering from the Fe-Fe paths interferes destructively with that from paths belonging to Fe(OH2)6(3+) monomers that coexist with the dimers, leading to a less apparent Fe shell in the EXAFS Fourier transform. This might be a reason why the characteristic Fe-Fe distance was not detected in previous EXAFS studies. The existence of μ-oxo dimers is further confirmed by Mössbauer analyses of analogous quick frozen solutions. This work also explores the electronic structure and the relative stability of the μ-oxo dimer in a comparison to the dihydroxo dimer using density function theory (DFT) calculations. The identification of such dimers in aqueous solutions has important implications for iron (bio)inorganic chemistry and geochemistry, such as understanding the formation mechanisms of Fe oxyhydroxides at molecular scale.