Spectroscopic and Computational Interrogation of a High-Valent Nickel-Dialkyl Complex Indicates Electronic Structure...

The study of high-valent organometallic nickel compounds has gained considerable interest recently, primarily driven by the development of nickel-catalyzed alkyl-alkyl cross-coupling reactions that are proposed to employ such high-valent intermediates. In that regard, we have recently reported a formal Ni(III)-dimethyl intermediate supported by the ligand N,N',N″-triisopropyl-1,4,7-triazacyclononane (Pr₃tacn) that can undergo rapid C-C reductive elimination and catalyze alkyl-alkyl Kumada cross-coupling reactions. The bulky nature of this tridentate ligand was suggested to lead to two geometrically and electronically inequivalent alkyl groups bound to the five-coordinate Ni center. Herein, we have employed pulsed electron paramagnetic resonance techniques such as electron nuclear double resonance, hyperfine sublevel correlation, and electron spin echo envelope modulation to provide strong experimental evidence for the geometrically and electronically inequivalent nature of the two methyl groups in which one methyl ligand can be better described as a methyl radical. These experimental results were supported by density functional theory computational methods used to probe the covalent nature of the Ni-C bonds and the formal Ni oxidation state assignment for this catalytically relevant, high-valent Ni intermediate. Moreover, computational investigation of a series of related methyl/alkyl analogs reveals that the radical character of an alkyl group increases for a tertiary vs a secondary vs a primary alkyl group, with direct relevance for alkyl-alkyl cross-coupling catalysis. Overall, this study provides valuable insights into the nature of organometallic Ni-dialkyl species that undergo efficient reductive elimination, likely through an S_H2-type mechanism.