Efficient Fe-Catalyzed Terminal Alkyne Semihydrogenation by H2: Selectivity Control via a Bulky PNP Pincer Ligand

We report an efficient semihydrogenation of terminal alkynes with H2, catalyzed by a modified, tetramethylated PNP pincer Fe hydride complex. The reactivity contrasts with an analogous classical PNP Fe hydride complex containing CH2 arms, which catalyzes predominantly terminal alkyne dimerization even in the presence of H2, and with an N-Me arm PNP Fe complex, which is known to cause overreduction to alkanes and requires the presence of a trimethylsilyl protective group. Such straightforward PNP ligand modification is a convenient tool that switches reactivity and leads to selective alkene formation, with no dimerization side products observed and only a minor contribution from overreduction.