Computational Mechanistic Study of the Hydrogenation of Carbonate to Methanol Catalyzed by the Ru^IIPNN Complex

Abstract

Density functional theory computations have been carried out to study the mechanism of hydrogenation-based transformation of dimethyl carbonate to methanol, catalyzed by Ru(II)PNN catalyst. The energetic results show that the catalytic transformation includes three sequential stages consistently involving the catalyst: (stage I) transformation of dimethyl carbonate (3) to methyl formate (5) and methanol; (stage II) transformation of methyl formate 5 to formaldehyde and methanol; (stage III) hydrogenation of formaldehyde to methanol. Stages I and II proceed similarly and follow three steps: hydrogen activation, formation of a hemiacetal intermediate via stepwise hydrogen transfer to dimethyl carbonate in stage I or methyl fomate in stage II, and subsequent decomposition of the hemiacetal intermediate to afford methanol. Hydrogenation via carbonyl insertion into the Ru-H bond is less favorable than the stepwise hydrogen-transfer mechanism. Decomposition of hemiacetal takes places by first breaking the hemiacetal O-H bond to give an alkoxide complex, followed by deprotonation of the benzylic arm ligand to the adjacent methoxy group. Comparing the hydrogenation steps in the three stages, hydrogenation in stage I is most difficult, that in stage II is less difficult, and that in stage III is easiest in terms of both kinetics and thermodynamics. This can be ascribed to the stronger electrophilicity of the carbonyl group in methyl formate or formaldehyde than that in dimethyl carbonate and fewer steric effects between the catalyst and methyl formate or formaldehyde than that between the catalyst and dimethyl carbonate. Thermodynamically, both stages I and II are uphill, but stage III is downhill significantly, which is the driving force for the catalytic transformation. The study indicates that the methanol product could facilitate the hydrogen activation involved in the transformation, implying that transformation could be accelerated by initially adding methanol.

Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization–Dearomatization or Not

“…33 These reactions involve the X-H (X=H, C, O, N, and B) activations, [34][35][36][37][38][39][40][41] dehydrogenative coupling reactions, [42][43][44][45][46][47][48] and hydrogenation reactions. [49][50][51][52][53][54][55][56] In addition, a theoretical study about the ruthenium acridine complex was also reported. 57 These theoretical studies are discussed in the following sections.…”

“…51 Almost at the same time, Yang investigated this reaction and proposed a mechanism that involves three similar steps. 52 The major difference between the two mechanisms is the formation of the ruthenium alkoxide complex 166.…”

Mechanistic insight into water-modulated cycloisomerization of enynyl esters using an Au(i) catalyst

“…2, the conversion from IM10 to B relates to the migration of the hydroxyl H atom to the terminal C1 atom of the C1vC2 moiety, but direct migration is kinetically unachievable (IM10→IM11 in Scheme 3). Previous theoretical studies [62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77] disclosed that reactant or intermediate molecules could sometimes play a key role in lowering the activation barrier. Encouraged by this hint, we proposed a new pathway in which two IM10 molecules concertedly undergo hydrogen migration.…”

A new class of PN3-pincer ligands for metal–ligand cooperative catalysis

“…A reasonably low transition state barrier would require the H δ+ to reach the electron lone pair of N of the imine arm. Proton-transfer (H-transfer) shuttles were recently identified in facilitating various H-transfer processes in related reactions [58,[97][98][99][100][101][102][103][104][105][106][107][108]. The mediating agents for the H-transfer process can be protonic solvent molecules (alcohols and water), substrates, or products.…”