General H₂ Activation Modes for Lewis Acid–Transition Metal Bifunctional Catalysts

Abstract

A general mechanism for H2 activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (PhDPBPh)­Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (ΔG ‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (ΔG ‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (ΔG ‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future.

Heterobimetallic H2 Addition and Alkene/Alkane Elimination Reactions Related to the Mechanism of E-Selective Alkyne Semihydrogenation

“…Analysis of NBO occupancies as functions of reaction coordinate provides insight into the key orbital interactions involved in bimetallic H 2 cleavage. 12 Selected NBO occupancies are plotted in Figure 5 (left axis). As the reaction proceeds from R1 to TS1, the occupancy of the σ HH orbital (labeled BD(H− H)) decreases, consistent with transfer of charge from this orbital toward the catalyst upon formation of a σ-complex.…”

Leveraging molecular metal–support interactions for H2 and N2 activation

“…In a seminal discovery, Peters and co-workers reported cooperative activation of H 2 across an inverse dative Ni→B σ-bond to generate a H−Ni(µ−H)B species that catalyzes olefin hydrogenation [39,67]. The B(III) support was proposed to play a direct role in H−H cleavage in this and related systems [67,69,70]. We aimed to vary the Lewis acid support so as to systematically examine its role in promoting H 2 binding, cleavage, and hydrogenation catalysis in cooperation with an active transition metal.…”

Lewis Acid Transition‐Metal‐Catalyzed Hydrogen Activation: Structures, Mechanisms, and Reactivities

“…These geometry variations in the H 2 activation process cause a large ligand field distortion penalty . Therefore, the synergetic heterolytic cleavage is considered to be the most plausible H 2 activation mechanism for the five‐coordinated d 8 Rh I pyramidal tri(azaindolyl)borane‐Rh system, which is a similar to the previous DPB‐Ni system in which a synergetic heterolytic cleavage mechanism operates for the H 2 activation, taking the advantage of LA‐TM cooperation …”

“…suggested the important role of boron . Recently, on the basis of former studies, considering ( Ph DPB Ph )Ni catalyst as a typical model, our group has proposed four possible H 2 activation modes for the LA‐TM catalysts . As shown in Figure , these H 2 activation modes, which can be classified by homolytic and heterolytic cleavages, are 1) fac ( cis )‐homolytic mechanism in which the LA‐TM bond is perpendicular to the H 2 cleavage plane.…”

“…Therefore, theoretical studies are highly desired to understand the general mechanism and to provide a systematic mechanism framework for the LA‐TM‐catalyzed hydrogen activation. Although the B−M bond‐assisted synergetic heterolytic cleavage mechanism was suggested to be the most plausible mechanism for the nickel borane catalysts, will these four H 2 activation modes be generally feasible for other kinds of LA‐TM catalysts, thus constructing the mechanism framework for the chemistry of LA‐TM catalysis? What is the relationship between the mechanism and the structure of the LA‐TM catalysts?…”