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

Abstract

A tridentate amide-pyridine-monophosphine ligand, {(o-PPh₂)C₆H₄C(O)N(H)CH₂(C₅H₄N)} (1), has been employed to synthesize a series of Ru^II and Ru^III 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.