Hydration of Nitriles Catalyzed by Ruthenium Complexes: Role of Dihydrogen Bonding Interactions in Promoting Base-Free Catalysis

Ru(II) complexes of amide-phosphine-based tridentate ligands additionally containing pyridine, isoquinoline, and quinoline rings have been synthesized, and their catalytic utility for the selective hydration of nitriles to amides is explored under the base-containing as well as base-free conditions. The chlorideligated complexes 1-3 exhibited significant catalytic activity in the presence of a base, whereas hydride-ligated complexes 4-6 carried out the hydration of nitrile without the requirement of any base. The mechanistic studies revealed the involvement of [Ru-H] species as the active catalyst in the catalytic cycle. The [Ru-H] species assisted in the polarization of an incoming water molecule through [Ru-H center dot center dot center dot H-OH] dihydrogen bonding interaction and consequently aided in the attack of a positioned water molecule to a nitrile coordinated to a ruthenium center. Substrate binding studies and kinetic experiments further supported the mechanism. A wide variety of aromatic nitriles containing both electron-withdrawing and electron-releasing groups as well as other substrates including aliphatic nitriles, base-sensitive nitriles, and a few biologically relevant nitriles were employed for the selective hydration.

Automated summary

Ru(II) complexes of amide-phosphine-based tridentate ligands containing pyridine, isoquinoline, and quinoline rings were synthesized and their catalytic activity for selective hydration of nitriles to amides was investigated. Chloride-ligated complexes exhibited significant catalytic activity in the presence of a base, while hydride-ligated complexes carried out the hydration of nitriles without a base. Mechanistic studies revealed the involvement of [Ru-H] species as the active catalyst, assisting in the polarization and attack of water molecules to nitriles.