Studies of low-coordinate iron dinitrogen complexes

Understanding the interaction of N-2 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 N-2 ligand, (LFeNNFeLR)-Fe-R (L-R = beta-diketiminate; R = Me, Bu-t), result from reduction of [(LFeCl)-Fe-R](n) under a dinitrogen atmosphere, and an iron(l) precursor of an N-2 complex can be observed. X-ray crystallographic and resonance Raman data for (LFeNNFeLR)-Fe-R 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 N-2 pi* orbitals. Increasing the coordination number of iron from three to tour through binding of pyridines gives compounds with comparable N-N weakening, and both are substantially weakened relative to five-coordinate iron-N-2 complexes, even those with a lower oxidation state. Treatment of (LFeNNFeLR)-Fe-R with KC8 gives (K2LFeNNFeLR)-Fe-R, and calculations indicate that reduction of the iron and alkali metal coordination cooperatively weaken the N-N bond. The complexes (LFeNNFeLR)-Fe-R react as iron(l) fragments, losing N-2 to yield iron(l) phosphine, CO, and benzene complexes. They also reduce ketones and aldehydes to give the products of pinacol coupling. The (K2LFeNNFeLR)-Fe-R compounds can be alkylated at iron, with loss of N-2.