In Situ Structural Characterization of Ferric Iron Dimers in Aqueous Solutions: Identification of μ‑Oxo Species

The structure of ferric iron (Fe³⁺) 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)₆³⁺ 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