Synthesis and Characterization of Fe⁰ (2,2'-bipyridine) (2-aminoethyl-pyridine) and its Reaction with Dihydrogen

Fe⁰ (bpy)(pyea) (2; bpy=2,2'-bipyridine, pyea=2-aminoethyl-pyridine), a 16-electron species, was synthesized by reduction of FeCl₂ (bpy)(pyea) (1) using Na-strips. It is a diamagnetic low-melting solid (m.p. 295 K) stable under N₂ and easily decomposed by radiations even at low temperature. It was fully characterized by elemental analyses and multinuclear NMR. Complex 2 acts as an active hydrogenation catalyst, but has a very short lifetime. In fact, it reacts with H₂ (0.1-1 MPa) at room temperature in toluene and affords in a few minutes a new Fe⁰ complex characterized as Fe⁰ (bpy)(η⁶ -picoline) (3), inactive to hydrogenation. Picoline is derived from the sp³ -sp³ C-C bond cleavage of the aminoethyl arm of the pyea ligand. The rapid evolution of the putative intermediate FeH₂ (bpy)(pyea) (4) has not allowed the isolation such Fe-hydrido species. The interaction of H₂ with 2 has been studied by DFT, which has allowed to demonstrate that 3 is lower in energy than 2+H₂ , justifying the fact that the intermediate dihydride was not isolated. Interestingly, 3 was also obtained by reaction of 1 with NaBH₄ or with glycerol-KOH. Complex 2 is one of the rare examples of Fe⁰ complex stabilized by a set of only N-donor atoms. The reaction with glycerol confirms the potential role of Fe in catalytic hydrogenation reactions using bio-glycerol as a H-source.