diiron

Diiron refers to any chemical species that contains two iron atoms. In coordination chemistry and bioinorganic chemistry, diiron describes dinuclear iron units that may form within a single molecule or be part of a larger protein or cluster. The two metals are typically connected by bridging ligands such as carboxylates, oxo, or hydroxo groups and may participate in redox reactions.

Dinuclear iron centers occur in a range of synthetic and biological systems. The oxidation states of the two iron ions can be Fe(II)–Fe(II), Fe(III)–Fe(III), or mixed Fe(II)–Fe(III). The Fe–Fe distance is usually in the 3.0 to 4.0 angstrom range, and the metals are commonly coordinated by oxygen-, nitrogen-, and sometimes sulfur-donor ligands. Diiron complexes are studied for their magnetic, catalytic, and electronic properties, and serve as models for understanding biological iron centers.

In biology, diiron centers are found in enzymes that activate dioxygen and other substrates. Nonheme diiron oxygenases typically feature two iron ions bridged by carboxylate or oxide ligands and participate in oxidative transformations. The class I ribonucleotide reductase R2 subunit contains a diferric center that generates a stable tyrosyl radical required for radical propagation in nucleotide reduction. Some hydrogenases and related enzymes also employ diiron clusters as active sites, enabling processes such as hydrogen metabolism and shallow oxidation reactions.

Synthesis and study of diiron species rely on dinuclear iron precursors and carefully designed ligands. Characterization commonly employs Mössbauer spectroscopy, EPR, UV–visible spectroscopy, and X-ray crystallography to elucidate oxidation states, spin coupling, and structural motifs.