Redox Activity of Iron Diazine-Diimine Carbonyl and Dinitrogen Complexes: A Comparative Study of the Influence of the Heterocyclic Ring

A detailed investigation
of the electronic structure of diazinediimine
iron complexes and their comparison with the pyridine analogues reveals
subtle but important differences, imparted by the supporting heterocycle.
In the case of LFe­(CO)2 complexes (L = pyrazine- and pyrimidine­diimine),
the characterization of three available redox states confirmed that
whereas the nature of the electron-transfer processes is similar,
the differences in π-acidity of the supporting heterocycle significantly
affect the redox potentials. The reduction of LFe­(CO)2 can
yield either a ligand-centered radical (for L = pyrimidine) or a C–C-bonded
dimer (for L = pyrazine), supported by a dearomatized core. In the
latter case, the C–C bond can be reversibly cleaved oxidatively.
Compared to the carbonyl analogues, employing weak-field N2 ligands triggers changes in electronic structure for the neutral
and reduced LFe­(N2) complexes (L = pyrimidine­diimine).
En route to the synthesis of the nitrogen complexes, the square-planar
LFeCl (L = pyrimidine­diimine) was isolated. The monoradical
character of the supporting chelate triggers the asymmetric distribution
of electron density around the heterocycle.