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

A tridentate amide-pyridine-monophosphine ligand, {(<i>o</i>-PPh₂)C₆H₄C(O)N(H)CH₂(C₅H₄N)} (<b>1</b>), has been employed to synthesize a series of Ru^II and Ru^III pincer complexes. Reaction of ligand <b>1</b> with [RuCl₂(DMSO)₄] and [RuCl₂(PPh₃)₃] yielded [RuCl(C₁₂H₈N₂){(PNN)-κ³-<i>P,N,N</i>}] (<b>2</b>) and [RuCl₂(PPh₃){(PNN)-κ³-<i>P,N,N</i>}] (<b>3</b>), respectively, while treatment with [RuHCl(CO)(PPh₃)₃] under varied conditions afforded pincer-type complexes [RuCl(CO)(PPh₃){(PNN)-κ³-<i>P,N,N</i>}] (<b>4</b>), [RuH(CO)(PPh₃){(PNN)-κ³-<i>P,N,N</i>}] (<b>5</b>), and [Ru(Cl)₂(CO){(PNN)-κ³-<i>P,N,N</i>}]Cl (<b>6</b>). Ligand <b>1</b> upon reaction with [Ru(<i>p</i>-cymene)Cl₂]₂ and [RuCl₂(PPh₃)₃] under aerobic reaction conditions afforded [RuCl(CH₃CN)₂{(<i>o</i>-PPh₂)C₆H₄C(O)NC(O)(C₅H₄N)}-κ³-<i>P,N,N</i>] (<b>7</b>) and [RuCl₂(CH₃OH){(<i>o</i>-PPh₂)C₆H₄C(O)NC(O)(C₅H₄N)}-κ³-<i>P,N,N</i>] (<b>8</b>) complexes, respectively, showing base- and additive-free benzylic C-H oxidation at room temperature. Complexes <b>4</b> and <b>5</b> 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.