Facial and Meridional PNN-Ruthenium Pincer Complexes in Base-Free Nitrile Hydration with Concurrent Ligand Backbone Oxidation

A tridentate amide-pyridine-monophosphine ligand, {( o -PPh 2 )C 6 H 4 C(O)N(H)CH 2 (C 5 H 4 N)} ( 1 ), has been employed to synthesize a series of Ru II and Ru III pincer complexes. Reaction of ligand 1 with [RuCl 2 (DMSO) 4 ] and [RuCl 2 (PPh 3 ) 3 ] yielded [RuCl(C 12 H 8 N 2 ){(PNN)-κ 3 - P,N,N }] ( 2 ) and [RuCl 2 (PPh 3 ){(PNN)-κ 3 - P,N,N }] ( 3 ), respectively, while treatment with [RuHCl(CO)(PPh 3 ) 3 ] under varied conditions afforded pincer-type complexes [RuCl(CO)(PPh 3 ){(PNN)-κ 3 - P,N,N }] ( 4 ), [RuH(CO)(PPh 3 ){(PNN)-κ 3 - P,N,N }] ( 5 ), and [Ru(Cl) 2 (CO){(PNN)-κ 3 - P,N,N }]Cl ( 6 ). Ligand 1 upon reaction with [Ru( p -cymene)Cl 2 ] 2 and [RuCl 2 (PPh 3 ) 3 ] under aerobic reaction conditions afforded [RuCl(CH 3 CN) 2 {( o -PPh 2 )C 6 H 4 C(O)NC(O)(C 5 H 4 N)}-κ 3 - P,N,N ] ( 7 ) and [RuCl 2 (CH 3 OH){( o -PPh 2 )C 6 H 4 C(O)NC(O)(C 5 H 4 N)}-κ 3 - P,N,N ] ( 8 ) complexes, respectively, showing base- and additive-free benzylic C–H oxidation at room temperature. Complexes 4 and 5 were employed in the hydration of nitriles to amides. Mechanistic studies support a [Ru–H] active species facilitating water activation through a [Ru–H···H–OH] dihydrogen bond, enabling nucleophilic attack on the coordinated nitrile. The catalyst system operates at low loadings, tolerating both electron-rich and electron-deficient nitriles.