Synthesis of Iron(0) Complexes Bearing Protic NHC Ligands: Synthesis and Catalytic Activity

Preparation of iron(0) pNHC complexes by oxidative addition of N-phosphine-tethered azoles to [Fe-3(CO)(12)] is described. Phosphine-tethered imidazoles 1 and 2 react with [Fe-3(CO)(12)] to give mononuclear iron(0) complexes of type [Fe(CO)(3)(C-pNHC<^>PR2)] ([5]: R = Ph; [6]: R = Cy) bearing a Cp-NHC<^>phosphine chelate ligand. In addition, the phosphine-tethered imidazolium salt 4-Br was C2-deprotonated and reacted with [Fe(bda)(CO)(3)] (bda = benzylidene acetone) to give the iron(0) chelate complex [Fe(eta(2)-4)(CO)(3)] (7). Complexes [5], [6], and [7] act as moderate to good catalysts in the hydrogenation of acetophenone with elemental hydrogen. The N-H group of the protic NHC ligands in [5] and [6] is insignificant to the catalytic outcome.