Selectivity of Ru and Fe PR2NR’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*)(P^R₂N^R’₂)(MeCN)]PF₆ (M = Fe, Ru; Cp = cyclopentadienyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; P^R₂N^R’₂ = 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) in which the P^R₂N^R’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)(P^Ph₂N^Ph₂)(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)(P^Ph₂N^Ph₂)(MeCN)]PF₆ was optimized, but the catalyst was ineffective toward a broader scope of substrates.