Unexpected Electronic Process of H₂ Activation by a New Nickel Borane Complex: Comparison with the Usual Homolytic and Heterolytic Activations

Abstract

H-H σ-bond activation promoted by Ni[MesB(o-Ph2PC6H4)2] (1(Mes)) was theoretically investigated with the density functional theory method. In 1(Mes), the nickel 3d, 4s, and 4p orbital populations are similar to those of the typical nickel(II) complex. First, one H2 molecule coordinates with the nickel center to form a dihydrogen complex, 2, which induces an increase in the nickel 3d and 4p orbital populations and thus a decrease in the nickel oxidation state. Then, the H-H σ-bond is cleaved under the unusual cooperation of the electron-rich nickel center and the electron-deficient borane ligand in a polarized manner, leading to an unprecedented trans-nickel(II) hydridoborohydrido complex, 3. In the transition state, charge transfer (CT) occuring from the H2 moiety to the 1(Mes) moiety (0.683 e) is much larger than the reverse CT (0.284 e). As a result, cleavage of the H-H σ-bond affords two positively charged hydrogen atoms. In this process, the boron atomic population and the nickel 4p orbital population increase, but the nickel 3d orbital population decreases. After cleavage of the H-H σ-bond, CT from the nickel 4p orbital to these positively charged hydrogen atoms occurs to afford 3, where the oxidation state of the nickel center increases to +2. These electronic processes are different from those of the usual homolytic and heterolytic H-H σ-bond activations. Regeneration of 1(Mes) and the role of the borane ligand in these reactions are also discussed in detail.

Cooperative Catalysis of Combined Systems of Transition‐Metal Complexes with Lewis Acids: Theoretical Understanding

“…This reaction is of considerable interest because the very strong B–F σ bond is broken by a platinum(0) complex. We theoretically investigated these two reactions and disclosed their characteristic features 21,22…”

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

“…Although the mer ( trans )‐homolytic mechanism is less favored in both the previously studied DPB‐Ni and the tri(azaindolyl)borane‐Rh discussed above, herein, we show in another case that it is preferred in the four‐coordinated d 8 square planar LA‐TM catalyst, such as the PBP‐Co I system. The d 8 square planar PBP‐Co catalyst was synthesized by Peters and co‐workers in 2013 .…”

“…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 …”

“…Sakaki et al. 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 .…”

“…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?…”