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

A tridentate amide-pyridine-monophosphine ligand, {(o-PPh2)C6H4C(O)N(H)CH2(C5H4N)} (1), has been employed to synthesize a series of RuII and RuIII pincer complexes. Reaction of ligand 1 with [RuCl2(DMSO)4] and [RuCl2(PPh3)3] yielded [RuCl(C12H8N2){(PNN)-κ3-P,N,N}] (2) and [RuCl2(PPh3){(PNN)-κ3-P,N,N}] (3), respectively, while treatment with [RuHCl(CO)(PPh3)3] under varied conditions afforded pincer-type complexes [RuCl(CO)(PPh3){(PNN)-κ3-P,N,N}] (4), [RuH(CO)(PPh3){(PNN)-κ3-P,N,N}] (5), and [Ru(Cl)2(CO){(PNN)-κ3-P,N,N}]Cl (6). Ligand 1 upon reaction with [Ru(p-cymene)Cl2]2 and [RuCl2(PPh3)3] under aerobic reaction conditions afforded [RuCl(CH3CN)2{(o-PPh2)C6H4C(O)NC(O)(C5H4N)}-κ3-P,N,N] (7) and [RuCl2(CH3OH){(o-PPh2)C6H4C(O)NC(O)(C5H4N)}-κ3-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.