Efficient Z‑Selective Semihydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes

The bench-stable cationic bis­(σ-B–H) aminoborane complex [Fe­(PNPNMe-iPr)­(H)­(η2-H2B = NMe2)]+ (2) efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3-enynes. The catalytic reaction takes place under mild conditions (25 °C, 4–5 bar H2 or D2) in 1 h, and alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is provided by means of DFT calculations. Intermediates featuring a bisdihydrogen moiety [Fe­(PNPNMe-iPr)­(η2-H2)2]+ are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe­(PNPNMe-iPr)­(H)­(η2-H2)2]+ (3) exhibited the same catalytic activity as 2. The novel cationic bisdihydrogen complex 3 was obtained by protonolysis of [Fe­(PNPNMe-iPr)­(H)­(η2-AlH4)]2 (1) with an excess of nonafluoro-tert-butyl alcohol.