Unraveling the Reaction Mechanism on Nitrile Hydration Catalyzed by [Pd(OH₂)₄]²⁺: Insights from Theory

Abstract

Density functional theory methodologies combined with continuum and discrete-continuum descriptions of solvent effects were used to investigate the [Pd(OH2)4](2+)-catalyzed acrylonitrile hydration to yield acrylamide. According to our results, the intramolecular hydroxide attack mechanism and the external addition mechanism of a water molecule with rate-determining Gibbs energy barriers in water solution of 27.6 and 28.3 kcal/mol, respectively, are the most favored. The experimental kinetic constants of the hydration started by hydroxide, k(OH), and water, k(H2O), attacks for the cis-[Pd(en)(OH2)2](2+)-catalyzed dichloroacetonitrile hydration rendered Gibbs energy barriers whose energy difference, 0.7 kcal/mol, is the same as that obtained in the present study. Our investigation reveals the nonexistence of the internal attack of a water ligand for Pd-catalyzed nitrile hydration. At the low pHs used experimentally, the equilibrium between [Pd(OH2)3(nitrile)](2+) and [Pd(OH2)2(OH)(nitrile)](+) is completely displaced to [Pd(OH2)3(nitrile)](2+). Experimental studies in these conditions stated that water acts as a nucleophile, but they could not distinguish whether it was a water ligand, an external water molecule, or a combination of both possibilities. Our theoretical explorations clearly indicate that the external water mechanism becomes the only operative one at low pHs. On the basis of this mechanistic proposal it is also possible to ascribe an (1)H NMR signal experimentally detected to the presence of a unidentate iminol intermediate and to explain the influence of nitrile concentration reported experimentally for nitriles other than acrylonitrile in the presence of aqua-Pd(II) complexes. Therefore, our theoretical point of view on the mechanism of nitrile hydration catalyzed by aqua-Pd(II) complexes can shed light on these relevant processes at a molecular level as well as afford valuable information that can help in designing new catalysts in milder and more efficient conditions.

Unmasking the Action of Phosphinous Acid Ligands in Nitrile Hydration Reactions Catalyzed by Arene–Ruthenium(II) Complexes

“…It is known that the use of a discrete‐continuum solvation model, that is, explicit solvent molecules plus a polarizable continuum solvent, is crucial to obtain a more realistic description of the chemical processes in solution when hydrogen transfers or 1,2‐additions of solvent molecules to unsaturated bonds are involved 4e,g. 20 In particular, two theoretical studies have confirmed this fact in aqueous solution for nitrile hydration catalyzed by metal complexes containing molybdenum and palladium 4e,g. For these reasons, and as an initial approximation to the problem, in the case of ruthenium, we explored the intra‐ and intermolecular mechanisms (Scheme ) by using a continuum solvent model (in which the reactant water molecule is clearly taken into account for the intermolecular one).…”

“…[3] It is generally assumed that, upon coordination to the metal center,e lectrond ensity on the nitrile carbon atom drastically decreases, which makes it more susceptible to inter-or intramolecular attack by water or the hydroxidea nion (paths 1a nd 2i nF igure1). [4] The presence, in the coordination sphereo f the metal, of auxiliary ligandsa blet oa ctivate the water molecule by hydrogen bonding has also been recognized as ak ey factor to reduce the activation barrier of the hydration process and improvethe reactionrate (path 3i nF igure 1).…”

“…[4] Ghaffar and Parkins proposed af ourth alternative for complex A,w hich consisted of the intramolecular addition of an OH group belonging to the ligand on the nitrile CNb ond, followed by hydrolysis of the resulting metallacyclic intermediate (Scheme 2). [6,7] The nature of the catalysts described herein discards path 2a nd suggests that path 3w ould be preferred over path 1, so we theoretically investigated the two mostp lausible mechanisms for acetonitrile hydration catalyzed by the computationally less expensive complex [RuCl 2 (h 6 -C 6 H 6 )(PMe 2 OH)]: intramolecular nucleophilic attack by the hydroxyl group of the dimethyl phosphinous acid ligand (similart oG haffar and Parkins' proposal, as discussed below),a nd intermolecular ligandassisted nucleophilic attack by an externals olvent water molecule (path 3i nF igure 1; Scheme 5).…”

Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

“…Although complexes of metals of groups 6 , and 8–11 ,,,,,, have proven to be active for nitrile hydration reactions, more than half of the reported catalysts are ruthenium compounds, ,,− ,,,,,, and the vast majority of them bear specific ligands that enhance the solubility of the complex in water by means of the formation of hydrogen bonds with solvent molecules. ,,,,,,,,, The improvement of catalysts and reaction conditions have mainly been based on empirical data obtained from trial-and-error methods. Kinetic analysis of the reactions, , isolation of the reaction intermediates, ,, and a density functional theory (DFT) study of the catalysis ,, the three legs of the mechanistic investigationhave received scarce attention. As far as we know, mechanistic proposals based on the three legs together have not been reported.…”

Understanding the Hydrolysis Mechanism of Ethyl Acetate Catalyzed by an Aqueous Molybdocene: A Computational Chemistry Investigation

“…With all of this mind, we undertook a theoretical mechanistic study on the hydrolysis of ethyl acetate catalyzed by [Cp 2 Mo(OH)(OH 2 )] + . Previous theoretical investigations on nitrile hydration , catalyzed by [Cp 2 Mo(OH)(OH 2 )] + and [Pd(OH 2 ) 4 ] 2+ and on carbon monoxide oxidation catalyzed by [Cp 2 Mo(OH)(OH 2 )] + have proven to be useful in getting a more profound rationalization of the most relevant experimental results found in the interesting chemistry of molybdocenes toward organic electrophiles in an aqueous medium.…”