More Oxidized Than Rust
By Heather Rock Woods
Iron metals oxidize to rust, losing electrons and gaining positive
charge. Iron metals typically exist in an oxidation state of +2 or +3 (2
or 3 electrons less than a neutral iron atom). However, chemists have
long thought that iron compounds with even higher oxidation states play
important roles in enabling chemical reactions in metal-containing
Iron compounds are found in vital proteins, including the hemoglobin in
our blood and the myoglobin of our muscle tissue. An iron-based drug
currently used to treat cancer most likely involves a highly oxidized
form of iron to cause oxidative damage to cancerous DNA.
“To work, certain chemical reactions seem to require going through an
unstable, short-lived intermediate state involving iron +4 or +5,” said
Serena DeBeer George (ESRD).
In recent years, scientists have been able to synthesize and
characterize numerous iron +4 compounds [written Fe(IV)], but knew
little about iron +5, Fe(V), compounds. Now researchers, using SSRL,
have characterized a genuine Fe(V) species, which is even more oxidized
and more positively charged than the iron in rust [Fe(III)] or Fe(IV).
Frank Neese and Karl Wieghardt (Max Planck Institute) and their
colleagues, including George, used x-ray absorption spectroscopy (XAS),
combined with other spectroscopic and computational results, to describe
the lab-made compound.
Tuned to be sensitive to iron, XAS can pick up the amount of charge on
the iron atom. The XAS ’K-edge’ corresponds to the excitation of the
most tightly bound electrons in the iron atom. As the iron atom becomes
more oxidized, the K-edge increases in energy, providing a signature for
Fe(V). This study represents the first characterization of an Fe(V)
species by XAS and serves as an important experimental marker for
characterization of other Fe(V) species.