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
[ArNC(Me)C(Me)NAr]FeCl2 (Ar = 2,6-(CHMe2)2-C6H3) with sodium amalgam in benzene
or toluene furnished the iron arene complexes, [ArNC(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 [ArNC(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 [ArNC(Me)C(Me)NAr]Fe(η2-RC⋮CR) (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.