Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

[Image: see text] Two bench-stable Fe(II) alkyl complexes [Fe(κ(3)PCP-PCP-iPr)(CO)(2)(R)] (R = CH(2)CH(2)CH(3), CH(3)) were obtained by the treatment of [Fe(κ(3)PCP-PCP-iPr)(CO)(2)(H)] with NaNH(2) and subsequent addition of CH(3)CH(2)CH(2)Br and CH(3)I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ(3)PCP-PCP-iPr)(CO)(2)]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH(3) and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ(3)PCP-PCP-iPr)(CO)(2)(CH(2)CH(2)CH(3))] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.