Donor Functionalized Iron(II) N-Heterocyclic Carbene Complexes in Transfer Hydrogenation Reactions

Two piano-stool iron(II) complexes bearing N-heterocyclic carbene ligands outfitted with acetamide- and amine-pendant arms [Cp*Fe(NHC^R)(CO)I] {Cp* = η⁵-tetramethylcyclopentadienyl; R = CH₂CONEt₂ (3), (CH₂)₂NEt₂ (4)}, have been prepared and fully characterized. Their catalytic activity in transfer hydrogenation (TH) of ketones using iPrOH as a hydrogen source and catalytic amounts of base (LiOtBu) has been explored, along with that of previously reported [CpFe(NHC^R)(CO)I] {R = nBu (5), (CH₂)₂OH (6), Et (7), and (CH₂)₃OH (8)} complexes containing hydroxyl and nonfunctionalized alkyl arms. Complex 3 displayed the highest catalytic activity of the whole series 3–8, reaching a TOF₅₀ value of 533 h^–1. NMR monitoring of the stoichiometric reaction of 3 with LiOtBu, allowed the identification of a new species 3' containing a deprotonated amidate moiety, which has been fully characterized by ¹H, ¹³C, and ¹⁵N NMR. Finally, a green protocol for the reduction of ketones through TH using glycerol as a hydrogen source, under microwave irradiation in the presence of catalytic amounts of 3 and base has been developed.