Role of High-Spin Species and Pendant Amines in Electrocatalytic Alcohol Oxidation by a Nickel Phosphine Complex

Abstract

The development of active and efficient electrocatalysts for oxidation of alcohols using earth-abundant metals will aid progress toward a renewable energy economy. Here, we present a detailed mechanistic study of electrocatalytic benzyl alcohol oxidation by a molecular nickel complex containing pendant amines using a combination of kinetic studies, nuclear magnetic resonance spectroscopy, and density functional theory. The catalyst preferentially binds alcohol in a high-spin octahedral geometry, but this complex is not readily deprotonated by an exogenous base and inhibits catalysis. Dissociation of one or more solvent ligands returns the complex to a low-spin state that can be deprotonated. Kinetic modeling indicates that the off-cycle high-spin intermediate lowers the catalytic turnover frequency by a factor of 8, suggesting that substantial gains in activity can be attained by improvements to the catalyst coordination geometry. In a second finding, we demonstrate that the pendant amine of the catalyst only functions as a proton relay for the potential-determining step, oxidation of a nickel hydride intermediate, but does not have a substantial impact on the overall rate for oxidation of benzyl alcohol to benzaldehyde. This result contradicts the common expectation for pendant amines to participate in rate-limiting proton transfer reactions.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

“…261 The mechanism for oxidation of benzyl alcohol was investigated experimentally and computationally, and the proposed catalytic cycle is shown in Figure 57. 262 Similar to the case with formate oxidation, the pendant amine is proposed to facilitate oxidation of the metal hydride complex at a mild potential. The rate of catalysis was found to be limited by alcohol binding and subsequent deprotonation by exogenous base to afford a nickel alkoxide intermediate.…”

“…The mechanism for oxidation of benzyl alcohol was investigated experimentally and computationally, and the proposed catalytic cycle is shown in Figure . Similar to the case with formate oxidation, the pendant amine is proposed to facilitate oxidation of the metal hydride complex at a mild potential.…”

Determining overpotentials for the oxidation of alcohols by molecular electrocatalysts in non-aqueous solvents

“…Under the buffered conditions described above, [Ni(P tBu 2 N tBu 2 )(MeCN) 2 ] 2+ catalyzes the oxidation of BnOH at E cat/2 = −0.71 V vs Fc +/0 , corresponding to an overpotential of 0.39 V. As shown in Figure 6, the catalytic cycle is proposed to involve generation of an alkoxide complex as the rate-limiting step. 21 This species undergoes -hydride elimination to release benzaldehyde. Oxidation of the resulting Ni(II) hydride, which is the potential determining step, occurs at a mild potential because of the presence of the pendant amine, which provides a pathway for intramolecular proton transfer.…”

“…6, the catalytic cycle is proposed to involve generation of an alkoxide complex as the rate-limiting step. 21 This species undergoes β-hydride elimination to release benzaldehyde. Oxidation of the resulting Ni( ii ) hydride, which is the potential determining step, occurs at a mild potential because of the presence of the pendant amine, which provides a pathway for intramolecular proton transfer.…”

Taming the Lewis Superacid Al(ORF)3 (RF=C(CF3)3): DFT Guided Identification of the “Stable yet Reactive” Adduct SiPr2→Al(ORF)3; Its Use as ORF‐ Abstractor from a “Ni‐ORF” complex

“…For example nickel-alkoxides are demonstrated to be intermediates in coupling processes (via ligand exchange or β-H abstraction, Scheme 3, top). [23,[41][42][43][44][45][46] Furthermore, insertion in NiÀ O bonds of CO 2 , CO, nitriles, ketones, isocyanate (Scheme 3, bottom); [35,36,38,39] has been established for neutral mono-alkoxide nickel complexes; as well as [OR] À ligand exchange [33] and formal reductive elimination at Ni. [37] During these studies, and as mentioned by Krossing et al, we observed that PhF!Al(OR F ) 3 adduct suffers from poor thermal stability liberating the Al(OR F ) 3 Lewis acid at room temperature, that evolves via abstraction of one of its own fluorine to form the [(R F O) 2 Al(μ-F)] 3 ring 2 (see below, Scheme 4).…”