Studies of Low-Coordinate Iron Dinitrogen Complexes

Understanding the interaction of N2 with iron is relevant to the iron catalyst used in the Haber
process and to possible roles of the FeMoco active site of nitrogenase. The work reported here uses synthetic
compounds to evaluate the extent of NN weakening in low-coordinate iron complexes with an FeNNFe
core. The steric effects, oxidation level, presence of alkali metals, and coordination number of the iron
atoms are varied, to gain insight into the factors that weaken the NN bond. Diiron complexes with a bridging
N2 ligand, LRFeNNFeLR (LR = β-diketiminate; R = Me, tBu), result from reduction of [LRFeCl]n under a
dinitrogen atmosphere, and an iron(I) precursor of an N2 complex can be observed. X-ray crystallographic
and resonance Raman data for LRFeNNFeLR show a reduction in the N−N bond order, and calculations
(density functional and multireference) indicate that the bond weakening arises from cooperative back-bonding into the N2 π* orbitals. Increasing the coordination number of iron from three to four through binding
of pyridines gives compounds with comparable N−N weakening, and both are substantially weakened
relative to five-coordinate iron-N2 complexes, even those with a lower oxidation state. Treatment of
LRFeNNFeLR with KC8 gives K2LRFeNNFeLR, and calculations indicate that reduction of the iron and alkali
metal coordination cooperatively weaken the N−N bond. The complexes LRFeNNFeLR react as iron(I)
fragments, losing N2 to yield iron(I) phosphine, CO, and benzene complexes. They also reduce ketones
and aldehydes to give the products of pinacol coupling. The K2LRFeNNFeLR compounds can be alkylated
at iron, with loss of N2.