Low-valent α-diimine iron complexes for catalytic olefin hydrogenation

A family of low-valent a-diimine iron complexes has been synthesized and their utility in catalytic olefin hydrogenation reactions evaluated. Reduction of the ferrous dichloride complex [ArN=QMe)(CMe)=NAr]FeCl2 (Ar = 2,6-(CHMe2)(2)-C6H3) with sodium amalgam in benzene or toluene furnished the iron arene complexes, [ArN=QMe)(CMe)=NAr]Fe(eta(6)-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 (x-diimine ligand. The analogous reduction in alkane solvents afforded the bis(alpha-diimine) complex [ArN=C(Me)(CMe)=NAr](2)Fe, which has also been crystallographically characterized. The arene complexes and the bis(CL-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(eta(2)-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 eta(6)-arene adducts serves as a major catalyst deactivation pathway.