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

We have conducted mechanistic investigations using dispersion-corrected hybrid density functional theory on three different homogeneous processes: (a) hydrogenation of styrene using H-2, (b) dehydrogenation of amine-borane, and (c) transfer hydrogenation of styrene using amineborane catalyzed by a boryl-ligated Co-based catalytic system, LCo(N-2) (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 H-2 atmosphere is 17.3 kcal/mol, which occurs through a channel that involves switching of a singlet electronic ground state (S-0) of the organometallic catalytic species to its low-lying triplet electronic state (T-1) 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 amineborane (NMe2H-BH3) and transfer hydrogenation of styrene by amine-borane are higher than the hydrogenation of olefin using H-2(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 S(N)2 type nucleophilic attack by the LCo(H)(2) on amine-borane.