Theoretical Studies on the Mechanism of Homogeneous Catalytic Olefin Hydrogenation and Amine–Borane Dehydrogenation by a Versatile Boryl-Ligand-Based Cobalt Catalyst

Abstract

We have conducted mechanistic investigations using dispersion-corrected hybrid density functional theory on three different homogeneous processes: (a) hydrogenation of styrene using H2, (b) dehydrogenation of amine–borane, and (c) transfer hydrogenation of styrene using amine–borane catalyzed by a boryl-ligated Co-based catalytic system, LCo­(N2) (where L = meridional bis-phosphinoboryl (PBP) ligand), recently developed by Peters and co-workers (Lin, T.-P; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310–15313). Our studies reveal that all three catalytic processes are facilitated by the same active species, which is of the form LCo­(H)2. The formation of the active catalytic species in turn determines the rate-determining barrier (RDB) for the hydrogenation reactions of the olefin and also for the dehydrogenation reaction of amine–borane. We predict that the RDB for hydrogenation of styrene under H2 atmosphere is 17.3 kcal/mol, which occurs through a channel that involves switching of a singlet electronic ground state (S0) of the organometallic catalytic species to its low-lying triplet electronic state (T1) and returning back to the singlet surface through minimum energy crossing points along the reaction coordinate. Alternatively, we estimate the RDB to be 19.4 kcal/mol, slightly higher than that of the previous channel, if only the singlet spin state surface is considered. We find that the associated RDB for both the dehydrogenation of amine–borane (NMe2H-BH3) and transfer hydrogenation of styrene by amine–borane are higher than the hydrogenation of olefin using H2(g) and is predicted to be 24.7 kcal/mol. In addition, we show that in the reaction involving amine–borane, the active catalytic species (LCo­(H)2) can get deactivated by forming a hydridoborane cobalt tetrahydridoborate complex, which happens through an SN2 type nucleophilic attack by the LCo­(H)2 on amine–borane.

In Pursuit of Sustainable Hydrogen Storage with Boron‐Nitride Fullerene as the Storage Medium

“…[23] It was suggested that carbond ioxide hydrogenation can be achieved using ap hotochemically renewable organohydrides, such as dihydropyridine. [27] Herein, we proposet hat catalytic dehydrogenation of chemisorbed hydrogens from B 24 N 24 H 32 could be achieved by the same active catalyst. Both of them turned out to be exoergic with reasonable barriers, which are likely to be surmountable at room temperature (see FiguresS7-S8 in the Supporting Information).…”

“…Single-point solution-phase calculations were done using the CPCM model at B3PW91-D3/BS2l evel of theory and further tested at M06L-D3/BS2 level of theory (see Figure S10 in the Supporting Information). [27] For generation of LCo(H) 2 from LCo(H) 4 /LCo-N 2 ,p lease see [25,26] In the product (D_PDT1), the hydrideo nt he cobalt center is still weakly coordinated with the borona tom of the fullerene. It was theoretically shown that the terminal and bridging hydrogen of the active catalytic speciesL Co(H) 2 are negative and positive in nature, respectively.…”

“…[13a] Hydrogen can be released from cis-LCo(H) 2 (H 2 )t or eproduce LCo(H) 2 ;h owever,t he process is associated with ah igh free-energy activation barrier. Incidentally,d eactivation of the activec atalytic species LCo(H) 2 is observed for amine-borane dehydrogenation owing to S N 2a ttack of the terminal hydride of LCo(H) 2 on the boron atom of amine-borane, [27] andi th as negative impact on the catalytic turn over number. As hydrogen gas elimination is an irreversible process, B 24 N 24 H 32 would be converted to B 24 N 24 catalytically.T he dehydrogenation of the last hydrogenated B 2 N 2 ring was also exoergic, and the free-energy barriers are provided in the Supporting Information.…”

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

“…Later, Paul et al. studied the mechanism of two equivalent H 2 activations . In order to gain a general understanding about the LA‐TM‐catalyzed H 2 activation, we further systematically evaluated different H 2 activation modes (Figure ) mediated by PBP‐Co.…”

Cobalt–Pincer Complexes in Catalysis

“…A comparison of the catalytic efficiency of the monomeric 7 and the dimeric complexes 8 showed a lower activity for the latter one. DFT calculations regarding the mechanism predicted the formation of ( t Bu PNBNP)Co(H) 2 as catalytic active species and the rate‐determining barrier for the hydrogenation of styrene of 17.3 kcal mol −1 …”