Low-Valent α-Diimine Iron Complexes for Catalytic Olefin Hydrogenation

A family of low-valent α-diimine iron complexes has been synthesized and their utility in catalytic olefin hydrogenation reactions evaluated. Reduction of the ferrous dichloride complex [ArNC(Me)C(Me)NAr]FeCl₂ (Ar = 2,6-(CHMe₂)₂-C₆H₃) with sodium amalgam in benzene or toluene furnished the iron arene complexes, [ArNC(Me)C(Me)NAr]Fe(η⁶-C₆H₅R) (R = H, Me). The solid-state structure of the toluene adduct revealed a contracted carbon−carbon backbone, short iron−imine bonds, and elongated imine nitrogen−carbon distances, suggesting significant reduction of the α-diimine ligand. The analogous reduction in alkane solvents afforded the bis(α-diimine) complex [ArNC(Me)C(Me)NAr]₂Fe, which has also been crystallographically characterized. The arene complexes and the bis(α-diimine) complexes are inactive for catalytic olefin hydrogenation. Performing the reduction in the presence of internal alkynes such as diphenylacetylene and bis(trimethylsilyl)acetylene furnished the alkyne adducts [ArNC(Me)C(Me)NAr]Fe(η²-RCCR) (R = Ph, SiMe₃). Analogous olefin complexes with 1,5-cyclooctadiene and cycloctene have also been isolated using similar reduction procedures. The olefin adducts provide more active precatalysts than the alkyne compounds for the hydrogenation of 1-hexene. In each case, formation of η⁶-arene adducts serves as a major catalyst deactivation pathway.