Distinct reactivity of iron dihydride <i>vs.</i> iron(hydride)(borohydride) bearing the same bulky PNP ligand in hydrogenation of alkenes and alkynes

An iron dihydrido complex [(Me₄PNP^iPr)Fe(H)₂(N₂)] with a bulky tetramethylated PNP pincer ligand, Me₄PNP^iPr, is an active catalyst for full hydrogenation of internal alkynes and alkenes, while its reactivity with terminal alkynes leads to its deactivation <i>via</i> bis-acetylide complex formation. Such reactivity is distinctly different from that of the previously reported hydrido-borohydrido complex [(Me₄PNP^iPr)FeH(η²-BH₄)], which showed selective semihydrogenation of terminal alkynes, but was unreactive toward alkenes and internal alkynes. This is also in contrast to the reactivity of the Fe^II dihydride analogue with a classical, CH₂-arm PNP pincer ligand, [(^CH2PNP)Fe(H)₂(N₂)], which showed low conversion with internal alkenes and quick degradation. A combined experimental and DFT study was employed to elucidate the differences in the selectivity of alkyne hydrogenation as a function of the complex structure and the ligand steric bulk, as well as the interplay between sterics and the relative preferences of the Fe⁰ over Fe^II species as a function of steric congestion at the ligand.