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

Abstract

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•••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.

Base-Free Transfer Hydrogenation Catalyzed by Ruthenium Hydride Complexes of Coumarin-Amide Ligands

“…24,46 The nonelectrolytic nature of all six complexes was confirmed by the conductivity measurements. 24,46 The ESI + mass spectra of [Ru−Cl] complexes 1−3 exhibited the most abundant peak assigned to the [M − Cl − ] + species while [M + H + ] + species was observed in case of [Ru−H] complexes 4−6 (Figures S39−S50). The observed isotopic distribution patterns matched nicely to that of the simulation patterns in all cases.…”

“…The absorption spectra of complexes 1–6 revealed the presence of two bands (Figures S37 and S38). The band at the higher energy (330–380 nm) is assigned to ligand-based π–π* transition while the one at the lower energy (410–475 nm) has been designated to metal-to-ligand charge transfer transition. , The nonelectrolytic nature of all six complexes was confirmed by the conductivity measurements. , The ESI + mass spectra of [Ru–Cl] complexes 1–3 exhibited the most abundant peak assigned to the [M – Cl – ] + species while [M + H + ] + species was observed in case of [Ru–H] complexes 4–6 (Figures S39–S50). The observed isotopic distribution patterns matched nicely to that of the simulation patterns in all cases.…”

“…The Ru−H resonances respectively for 4−6 appeared as the triplet at −10.09 (J P-H = 19.6), −11.93 (J P-H = 19.8), and − 11.41 (J P-H = 19.3) due to the presence of two magnetically equivalent PPh 3 groups bound trans to each other (Figures S16−S18). 45,46 The Cresonances in the 13 C NMR spectra of complexes 1−6 were also noted to be shifted upfield as compared to their ligands (Figures S19−S24). All six Ru-complexes showed a peak at ca.…”

“…In contrast, complexes 4−6 were respectively synthesized by treating ligands HL1−HL3 with [Ru(H)(Cl)(CO)(PPh 3 ) 3 ] in the presence of NaOH in MeOH. All six complexes were characterized with the help of 45,46 The Cresonances in the 13 C NMR spectra of complexes 1−6 were also noted to be shifted upfield as compared to their ligands (Figures S19−S24). All six Ru-complexes showed a peak at ca.…”

“…It is proposed that a base is required to generate the [Ru−H] species that was the actual catalyst. 45,46 The catalytic results with the [Ru−Cl] complexes necessitated the requirement of a base for the potential generation of the [Ru−H] species. Such a fact suggests that the isolated [Ru−H] complexes, 4−6, may be successful in Indeed, excellent TH results were obtained for all three [Ru− H] complexes while maximum yield was observed in case of complex 5 as the catalyst.…”