Mechanistic Studies into the Oxidative Addition of Co(I) Complexes: Combining Electroanalytical Techniques with Parameterization

Abstract

The oxidative addition of organic electrophiles into electrochemically generated Co­(I) complexes has been widely utilized as a strategy to produce carbon-centered radicals when cobalt is ligated by a polydentate ligand. Changing to a bidentate ligand provides the opportunity to access discrete Co­(III)–C bonded complexes for alternative reactivity, but knowledge of how ligand and/or substrate structures affect catalytic steps is pivotal to reaction design and catalyst optimization. In this vein, experimental studies that can determine the exact nature of elementary organometallic steps remain limited, especially for single-electron oxidative addition pathways. Herein, we utilize cyclic voltammetry combined with simulations to obtain kinetic and thermodynamic properties of the two-step, halogen-atom abstraction mechanism, validated by analyzing kinetic isotope and substituent effects. Complex Hammett relationships could be disentangled to allow understanding of individual effects on activation energy barriers and equilibrium constants, and DFT-derived parameters used to build predictive statistical models for rates of new ligand/substrate combinations.

Analyzing mechanisms in Co(i) redox catalysis using a pattern recognition platform

“…Cyclic voltammetric studies identified the existence of a catalytic waveform, 7 in agreement with the catalytic reduction of benzyl bromides to benzyl radicals by the Co(I) complex, and detailed kinetic studies unveiled trends in substrate variation consistent with an outer-sphere electron-transfer mechanism. This mechanistic proposal, distinctive from our previously studied system, 6 was further supported by multivariable linear-regression (MLR) analysis, 8 enabling us to identify that the electron transfer occurs in a concerted fashion with dissociation of bromide from the substrate. In addition, with the combination of the electroanalytical techniques 7 and statistical modeling, 8 we establish a pattern recognition platform for rapidly distinguishing between the activation mechanisms of different metal complexes (Scheme 1.c) 9 , a tool which we anticipate will be broadly applied in the rational design of new organometallic reactions.…”

“…For a more detailed explanation of this method, we direct the reader to previous studies. 5,6 This analysis allowed us to measure the rate constant for each substrate in less than 30 minutes, thus enabling us to rapidly conduct kinetic studies with a wide array of substrates.…”

“…As part of our efforts to develop a global understanding of the intricate factors that impact the mechanism of fundamental organometallic steps such as the reduction of alkyl halides, we previously demonstrated that a Co(I) complex bearing an N,Nbidentate ligand can activate benzyl bromides through an innersphere electron-transfer process (halogen-atom abstraction, Scheme 1.a). 5,6 Accordingly, we next questioned whether it is possible to manipulate the nature of substrate reduction by switching to Co(I)…”

Concerted aryl-sulfur reductive elimination from PNP pincer-supported Co(iii) and subsequent Co(i)/Co(iii) comproportionation

“…These observations show that only the C F –S bond was cleaved, consistent with a concerted OA C–S activation pathway. 64 …”

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“… (A, B) Mechanistic study of oxidative addition to Co I complex. Graphs in panel A are reproduced from ref ( 51 ). Copyright 2019 American Chemical Society.…”