Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Abstract

Cobalt(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt(II) alkyl complex [(PNHP(Cy))Co(CH2SiMe3)]BAr(F)4 (4) (PNHP(Cy) = bis[(2-dicyclohexylphosphino)ethyl]amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N-H group on the pincer ligand in the catalysis via a metal-ligand cooperative interaction, the reactivities of 4 and [(PNMeP(Cy))Co(CH2SiMe3)]BAr(F)4 (7) were compared. Complex 7 was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using 7 as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex 7 displayed similar activity to complex 4, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst 4 was monitored by NMR spectroscopy, the formation of the cobalt(III) acetylphenyl hydride complex [(PNHP(Cy))Co(III)(κ(2)-O,C-C6H4C(O)CH3)(H)]BAr(F)4 (13) was detected. Isolated complex 13 was found to be an effective catalyst for the acceptorless dehydrogenation of alcohols, implicating 13 as a catalyst resting state during the alcohol dehydrogenation reaction. Complex 13 catalyzed the hydrogenation of styrene but showed no catalytic activity for the room temperature hydrogenation of acetophenone. These results support the involvement of metal-ligand cooperativity in the room temperature hydrogenation of ketones but not the hydrogenation of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms consistent with these observations are presented for the cobalt-catalyzed hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols.

Reversible catalytic dehydrogenation of alcohols for energy storage

“…the same reaction with nonprecious, earth-abundant metal catalysts is, so far, a relatively unexplored area in the literature. The first cobalt catalyst bearing a noninnocent bis(dicyclohexylphosphino) amine (PNP Cy ) ligand for acceptorless dehydrogenation of alcohols was reported by Hanson and coworkers (22,23). Recently, Beller and coworkers (24) have shown hydrogen production from methanol in the presence of KOH with octahedral iron complexes (PNP iPr )Fe(H)(CO)X [X = BH 4 (1) and X = Br (2)].…”

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

“…38 This catalyst class offers tolerance to a range of functional groups, although hydrogenation of sterically hindered substrates such as tri- and tetrasubstituted olefins remains a challenge. 39 …”

Addition of H2 Across a Cobalt–Phosphorus Bond

“…[8][9][10][11][12][13][14] Fore xample,H 2 may be heterolytically cleaved across aM À Lb ond (Figure 1), which does not require the metal or ligand to change oxidation state. [21][22][23][24] Related examples of metalligand cooperativity involving phosphorus are far less common. [10] Metalligand cooperativity has been studied most extensively with metal amide/amine complexes (e.g., Noyoriscatalyst), [10,[14][15][16][17][18][19][20] including several examples involving Co that are relevant to the work reported here.…”