Acceptorless, Reversible Dehydrogenation and Hydrogenation of <i>N</i>‑Heterocycles with a Cobalt Pincer Catalyst

Acceptorless, reversible dehydrogenation and hydrogenation reactions involving <i>N</i>-heterocycles are reported with a well-defined cobalt complex supported by an aminobis­(phosphine) [PN­(H)­P] pincer ligand. Several <i>N</i>-heterocycle substrates have been evaluated under dehydrogenation and hydrogenation conditions. The cobalt-catalyzed amine dehydrogenation step, a key step in the dehydrogenation process, has been independently verified. Control studies with related cycloalkanes suggest that a direct acceptorless alkane dehydrogenation pathway is unlikely. The metal–ligand cooperativity is probed with the related [PN­(Me)­P] derivative of the cobalt catalyst. These results suggest a bifunctional dehydrogenation pathway and a nonbifunctional hydrogenation mechanism

Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism

Hydrogenation of alkenes containing polarized CC double bonds has been achieved with iron-based homogeneous catalysts bearing a bis­(phosphino)­amine pincer ligand. Under standard catalytic conditions (5 mol % of (PNHP^iPr)­Fe­(H)₂(CO) (PNHP^iPr = NH­(CH₂CH₂P<i>i</i>Pr₂)₂), 23 °C, 1 atm of H₂), styrene derivatives containing electron-withdrawing para substituents reacted much more quickly than both the parent styrene and substituted styrenes with an electron-donating group. Selective hydrogenation of CC double bonds occurs in the presence of other reducible functionalities such as −CO₂Me, −CN, and N-heterocycles. For the α,β-unsaturated ketone benzalacetone, both CC and CO bonds have been reduced in the final product, but NMR analysis at the initial stage of catalysis demonstrates that the CO bond is reduced much more rapidly than the CC bond. Although Hanson and co-workers have proposed a nonbifunctional alkene hydrogenation mechanism for related nickel and cobalt catalysts, the iron system described here operates via a stepwise metal–ligand cooperative pathway of Fe–H hydride transfer, resulting in an ionic intermediate, followed by N–H proton transfer from the pincer ligand to form the hydrogenated product. Experimental and computational studies indicate that the polarization of the CC bond is imperative for hydrogenation with this iron catalyst