Cobalt-Group 13 Complexes Catalyze CO2 Hydrogenation via a Co(-I)/Co(I) Redox Cycle

The Co(-I) dihydrogen complexes, [(eta(2)-H-2)CoML](-), where ML is the group 13 metalloligand, N(o-((NCH2PPr2)-Pr-i)C6H4)(3)M, and M is Al, Ga, or In, were previously reported (J. Am. Chem. Soc. 2017, 139, 6570-6573). In this work, the related Co(-I) end-on dinitrogen adducts, [(N-2)CoML](-), were isolated and investigated as precatalysts for CO2 hydrogenation. The Co-Ga catalyst was highly active, achieving 19,200 formate turnovers with an initial turnover frequency of 27,000 h(-1) under 34 atm of 1:1 CO2/H-2 and using Verkades proazaphosphatrane as a base at ambient temperature. The Co-Al catalyst was moderately active, while the Co-In complex was inactive. Hence, tuning the group 13 ion greatly influences the catalytic activity at the Co site. To elucidate the role of the group 13 support, experimental and theoretical mechanistic studies of the Co-Ga and Co-Al catalysts were conducted. The Co(-I) H-2 species are potent hydride donors with estimated thermodynamic hydricities (Delta G degrees(H-)) of 32.0(1) and 37.4(1) kcal/mol in CH3CN for M = Al and Ga, respectively. By acting as masked Co(I) dihydrides, the Co(-I) H-2 species operate via an unusual Co(-I)/Co(I) redox cycle. After hydride transfer to CO2, the resulting intermediate is the Co(I) hydride complex, HCoML, which was independently synthesized and structurally characterized for M = Al and Ga. The Gibbs free energy for H-2 binding, Delta G degrees(bind) (1 atm), to generate (eta(2)-H-2)HCoML was slightly more favorable for HCoGaL (-4.2(1) kcal/mol) than for HCoAlL (-2.7(1) kcal/mol). In the subsequent step, the deprotonation reaction to regenerate the initial catalyst was much more favorable for (eta(2)-H-2)HCoGaL (pK(a) of 31.4, CH3CN) than for (eta(2)-H-2)HCoAlL (pK(a) of 34.3). The straightforward substitution of Al with Ga perturbs the energy profile of the catalytic reaction (vertical bar Delta Delta G degrees(H-)vertical bar = 5.4 kcal/mol, vertical bar Delta G degrees(bind)vertical bar = 1.5 kcal/mol, and vertical bar Delta Delta G degrees(Ka)vertical bar = 4.0 kcal/mol) and thus provides a thermodynamic rationale for the higher catalytic efficiency of Co-Ga over Co-Al.