Yinwu Li

Frustrated Lewis Pair Catalyzed C–H Activation of Heteroarenes: A Stepwise Carbene Mechanism Due to Distance Effect

This study presents new mechanistic insights into the frustrated Lewis pairs (FLPs) catalyzed C-H activation of heteroarenes. Besides the generally accepted concerted C-H activation, a novel stepwise carbene type pathway is proposed as an alternative mechanism. The reaction mechanisms can be varied by tuning the distance between Lewis acid and Lewis base due to catalyst-substrate match. These results should expand the understanding of the structure and function of FLPs for catalyzed C-H activation.

Homogeneously catalyzed hydrogenation and dehydrogenation reactions – From a mechanistic point of view

Abstract Homogeneously catalyzed hydrogenation/dehydrogenation reactions represent not only one of the most synthetically important chemical transformations, but also a promising way to renewably utilize the hydrogen energy. In order to rationally design efficient homogeneous catalysts for hydrogenations/dehydrogenations, it is of fundamental importance to understand their reaction mechanisms in detail. With this aim in mind, we herein provide a brief overview of the mechanistic understanding and related catalyst design strategies. Hydrogenations and dehydrogenations represent the reverse process of each other, and involve the activation/release of H2 and the insertion/elimination of hydride as major steps. The mechanisms discussed in this chapter include the cooperation (bifunctional) mechanism and the non-cooperation mechanisms. Non-cooperation mechanisms usually involve single-site transition metal (TM) catalysts or transition metal hydride (TM-H) catalysts. Cooperation mechanisms usually operate in the state-of-the-art bifunctional catalysts, including Lewis-base/transition-metal (LB-TM) catalysts, Lewis-acid/transition-metal (LA-TM) catalysts, Lewis-acid/Lewis-base (LA-LB; the so-called frustrated Lewis pairs - FLPs) catalysts, newly developed ambiphilic catalysts, and bimetallic transition-metal/transition-metal (TM-TM) catalysts. The influence of the ligands, the electronic structure of the metal, and proton shuttle on the reaction mechanism are also discussed to improve the understanding of the factors that can govern mechanistic preferences. The content presented in this chapter should both inspire experimental and theoretical chemists concerned with homogeneously catalyzed hydrogenation and dehydrogenation reactions, and provide valuable information for future catalyst design.

Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

Metal hydrides play important roles in catalysis for sustainable energy, the environment, the petrochemical industry, and many important chemical processes. Despite this significance, the mystery behind metal hydride reactivity still remains. This theoretical study reveals a surprising reactivity discrepancy for different types of terminal hydrides and bridging hydrides, with Lewis acid–transition metal (LA–TM) hydride complex promoted alkene hydrogenations as model reactions, using density functional theory (DFT) studies. PBP(μ-H)CoH and DPB(μ-H)NiH complexes were chosen as representative models for the boryl type and the borane type LA–TM hydride, respectively. The bridging hydride is less reactive than the terminal hydride in the borane type complex DPB(μ-H)NiH. However, in sharp contrast, the bridging hydride is more reactive than the terminal hydride in the boryl type complex PBP(μ-H)CoH. Typical features of the electronic structure are unfolded to rationalize the origin of the reactivity discrepancy. The bridging hydride in the sp3 borane type DPB(μ-H)NiH complex forms a typical three-center two-electron (3c–2e) B–H–Ni bond. In the sp2 boryl PBP(μ-H)CoH complex, the bridging hydride forms an unusual three-center four-electron (3c–4e) B–H–Co bond. The 3c–2e bridging hydride is stabilized by two LA sites, leading to a lower nucleophilicity than that of a normal 2c–2e terminal hydride. Meanwhile the 3c–4e bridging hydride shows a stronger free-hydride character, resulting in a higher nucleophilicity than that of a 2c–2e terminal hydride. A general hydride nucleophilic trend is proposed: 3c–4e bridging hydride > 2c–2e terminal hydride > 3c–2e bridging hydride. These fundamental aspects of metal hydride reactivity should be helpful for mechanistic understanding and catalyst/material design involving metal hydride complexes.

DFT study of CO2 hydrogenation catalyzed by a cobalt-based system: an unexpected formate anion-assisted deprotonation mechanism

Catalyzed hydrogenation of CO2 by earth-abundant metal complexes is a promising strategy to utilize hydrogen as a kind of sustainable energy and to reduce greenhouse gases. A systematic density functional theory (DFT) study is presented for CO2 hydrogenation catalyzed by the Co(dmpe)2H (dmpe: 1,2-bis(dimethylphosphino)-ethane) complex with Verkades base as an additive. An unexpected formate anion-assisted deprotonation mechanism is unfolded, which is different from the generally accepted additive base-catalyzed deprotonation mechanism. The complete catalytic cycle involves three main steps: (i) oxidative addition, (ii) deprotonation of the dihydride complex, and (iii) hydrogenation of CO2. The cobalt monohydride complex Co(dmpe)2H is found to be the catalytically active species and the rate-determining step is the hydrogenation of CO2 (ΔG‡ = 20.9 kcal mol−1). Furthermore, the hydride transfer process prefers the reductive elimination mechanism (ΔG‡ = 20.9 kcal mol−1) to the direct transfer mechanism (ΔG‡ = 23.3 kcal mol−1). In contrast, the cobalt dihydride complex [Co(dmpe)2H2]+ is less likely to be the active species, which should be deprotonated to the cobalt monohydride species for further hydrogenation. The deprotonation of cobalt dihydride to cobalt monohydride is found to be promoted by the formate anion (ΔG‡ = 10.5 kcal mol−1) instead of Verkades base (ΔG‡ = 36.9 kcal mol−1). On the other hand, a direct heterolytic cleavage of H2 assisted by the base to cobalt monohydride is less feasible compared with the oxidative addition of H2. These results can well explain the necessity for oxidative addition and the appearance of formic acid after Verkades base has run out in the experiments, and are in good agreement with the experimental observation that the hydrogenation of CO2 instead of the deprotonation is the rate-determining step. The present results provide sharp insights and helpful guidelines for designing novel hydrogenation systems with transition metal complexes and bases.

The effect of auxiliary ligand on the mechanism and reactivity: DFT study on H2 activation by Lewis acid–transition metal complex (tris(phosphino)borane)Fe(L)

Lewis acid–transition metal (LA–TM) complexes have been emerging as a novel type of bifunctional catalyst for H2 activation. The crucial role of the auxiliary ligand in the mechanism and reactivity of H2 activation were theoretically studied with (tris(phosphino)borane)Fe(L) (L = N2, CNtBu, and CO) as model LA–TM complexes. The axial auxiliary ligand is found to play an important role in the complex electronic structures, via significantly tuning the energy levels of dxz, dyz, and of the Fe–B bond. We systematically evaluated both the ligand dissociative mechanism and the associative mechanism with the binding auxiliary ligand. In the ligand dissociative mechanism, the reaction starts with the dissociation of the axial ligand. Then, H2 coordinates to the iron center either at the axial position or on the equatorial plane. The axial coordinated H2 cleaves via oxidative addition to a metastable octahedral dihydride intermediate, which further isomerizes to a more stable five-coordinated trigonal bipyramidal intermediate with a bridging hydride stabilized by the iron center and the Lewis center, boron. On the other hand, the equatorial coordinated H2 is cleaved by the cooperation of the Fe–B bond, directly to the trigonal bipyramidal dihydride intermediate. In comparison, in the ligand associative mechanism, the H2 molecule splits up upon approaching the iron center via an octahedral transition state, with the auxiliary ligand remaining at the axial position. Our results suggest that the triplet state axial reaction pathway of the L-dissociative mechanism is the most favorable one for H2 activation. The isomerization of hydride instead of H2 cleavage may be the rate-determining step. H2 activation occurs homolytically on the metal center without LA assistance, which is significantly different from other tetradentate LA–TM systems that activate H2 in a synergetic heterolytic mode. The obtained tendency of the dissociation free energies for (tris(phosphino)borane)Fe(L) (L = N2, CNtBu, and CO) (2.3, 8.1, and 15.7 kcal mol−1, respectively) and the activation free energies of the rate-determining step (20.3, 26.0, and 33.7 kcal mol−1, respectively) explains well their activity trend. The extraordinary effect of the auxiliary ligand on the mechanism and reactivity should provide new information for future development of LA–TM bifunctional catalysts.