Selectivity of Ru and Fe P^R₂N^R’₂ Catalysts Toward Acceptorless Dehydrogenation of Benzylamine

The performance of six ruthenium catalysts and two iron catalysts were evaluated toward the acceptorless dehydrogenation of benzylamine. All catalysts shared the common structure [M(Cp/Cp*)(PR2NR’2)(MeCN)]PF6 (M = Fe, Ru; Cp = cyclopentadienyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PR2NR’2 = 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) in which the PR2NR’2 ligands contain a pendant tertiary amine that enables cooperative catalytic mechanisms. Catalytic activity and selectivity were evaluated to identify the optimal catalyst structural features. The iron catalyst [Fe(Cp)(PPh2NPh2)(MeCN)]PF6 demonstrated near-exclusive selectivity for the acceptorless dehydrogenative coupled product, N-benzylidenebenzylamine. The absence of the hydrogen-borrowed (dibenzylamine) product indicates that this iron catalyst strongly favors dehydrogenation pathways over hydrogenation. This was confirmed through the control reactions. The performance of [Fe(Cp)(PPh2NPh2)(MeCN)]PF6 was optimized, but the catalyst was ineffective toward a broader scope of substrates.