Amine substitution pattern governs catalytic performance in manganese-PNN ketone hydrogenation

A highly modular synthetic strategy based on the ring opening of propane-1,3-diol cyclic sulfate by the corresponding diamines enabled the straightforward preparation of potentially tridentate phosphine-diamine (PNN) ligands of the general formula Ph 2 P(CH 2 ) 3 N(R 1 )(CH 2 ) 2 NR 2 R 3 (R 1 , R 2 , R 3 = H or CH 3 ), featuring amine functionalities with different substitution patterns. Using these novel ligands, manganese(I) complexes of the type [Mn(PNN)(CO) 2 Br], incorporating the catalytically critical NH subunits in distinct chemical environments, were synthesized and evaluated in the hydrogenation of various ketones under different reaction conditions. Catalysts containing a terminal NH 2 unit exhibited significantly higher activity than those bearing NHCH 3 or N(CH 3 ) 2 moieties. Despite the expected reduction in ligand-metal cooperativity upon N-methylation, the presence of an internal N-methyl group in addition to the terminal amino functionality further enhanced catalytic performance. Density functional theory (DFT) calculations revealed substantial differences in the coordination stereochemistry of the PNN ligands within the catalytically active manganese hydride complexes, which correlate with the observed reactivity trends. The most active catalyst displayed exceptionally high turnover number among PNN-containing manganese complexes in ketone hydrogenation.