PNNL

Abstract

The Co(-I) dihydrogen complexes, [(?2-H2)CoML]-, where ML is the group 13 metalloligand, N(o-(NCH2PiPr2)C6H4)3M, 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, [(N2)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 H2:CO2 and using Verkade’s proazaphosphatrane 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) H2 species are potent hydride donors with estimated thermodynamic hydricities (?G°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) H2 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 H2 binding, ?G°bind (1 atm), to generate (?2-H2)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 (?2-H2)HCoGaL (pKa of 31.4, CH3CN) than for (?2-H2)HCoAlL (pKa of 34.3). The straightforward substitution of Al with Ga perturbs the energy profile of the catalytic reaction: |??G°H-| = 5.4 kcal/mol, |??G°bind| = 1.5 kcal/mol, and |???G°?_(K_a )| = 4.0 kcal/mol; and thus, provides a thermodynamic rationale for the higher catalytic efficiency of Co-Ga over Co-Al. The authors thank Dr. Aaron Appel (PNNL) for hosting M.V.V. for the DOE fellowship, and Dr. Victor Young, Jr. for crystallographic assistance. M.V.V. was supported by the DOE Office of Science Graduate Student Research and the UMN Doctoral Dissertation fellowships. C.C.L. and B.J.G. were supported by the NSF (CHE-1665010). J.Y. and L.G. were supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award DE-SC0012702. J.C.L., A.P., and E.S.W. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.