Tetraaminocyclopentadienone Iron Complexes as Hydrogenation Catalysts

A series of symmetric
and asymmetric tetraaminocyclopentadienone
iron tricarbonyl complexes were prepared from Fe­(CO)5 and
the diaminoacetylenes (DAA) R2NCCNR2 (NR2 = piperidinyl, 4-methylpiperidinyl, homopiperidinyl)
via intermediate ferracyclobutenone complexes. In the presence of
trimethylamine-N-oxide, the reactions with acetonitrile
afforded the corresponding iron dicarbonyl acetonitrile complexes,
which served as highly active precatalysts for the transfer hydrogenation
of aldehydes, ketones, and imines with isopropanol as the hydrogen
source and for the hydrogenation of aldehydes and ketones with dihydrogen
under comparatively mild reaction conditions (3 bar H2 pressure,
room temperature). Density functional theory (DFT) calculations were
performed to reveal the mechanism of the isopropanol-mediated transfer
hydrogenation of benzaldehyde and acetophenone, which is likely to
involve a catalytically active hydroxycyclopentadienyl iron dicarbonyl
hydride species. Dihydrogen transfer from the latter onto benzaldehyde
and acetophenone occurs in a concerted manner with exceptionally low
activation barriers following the established outer-sphere mechanism.