Synthesis and Reactivity of Low-Coordinate Iron(II) Alkyl Complexes toward Alkene Hydrogenation

We report the coordination chemistry of two picolyl-NHC ligands HL1 and HL2, featuring bulky aryl groups adjacent to the donor atoms. High-spin iron(II) complexes [Fe(HL1)Br2] (1) and [Fe(HL2)Br2] (2) undergo transmetalation and ligand deprotonation with trimethylsilylmethyl lithium (LiCH2TMS) to afford [FeL1(CH2TMS)]2 (3) and [FeL2(CH2TMS)] (4), respectively. The three-coordinate high-spin iron(II) complex 4 can catalyze alkene hydrogenation under relatively mild conditions (60 °C, 4.3 atm of H2). Complex 4 undergoes reversible olefin addition across the iron center and the ligand backbone. The styrene adduct of complex 4 can also effect catalytic olefin hydrogenation.