Phenol Oxidation by a Nickel(III)-Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation

Abstract

In order to gain insight into the influence of the H+‐accepting terminal ligand in high‐valent oxidant mediated proton coupled electron transfer (PCET) reactions, the reactivity of a high valent nickel–fluoride complex [NiIII(F)(L)] (2, L=N,N’‐(2,6‐dimethylphenyl)‐2,6‐pyridinecarboxamidate) with substituted phenols was explored. Analysis of kinetic data from these reactions (Evans–Polanyi, Hammett, and Marcus plots, and KIE measurements) and the formed products show that 2 reacted with electron rich phenols through a hydrogen atom transfer (HAT, or concerted PCET) mechanism and with electron poor phenols through a stepwise proton transfer/electron transfer (PT/ET) reaction mechanism. The analogous complexes [NiIII(Z)(L)] (Z=Cl, OCO2H, O2CCH3, ONO2) reacted with all phenols through a HAT mechanism. We explore the reason for a change in mechanism with the highly basic fluoride ligand in 2. Complex 2 was also found to react one to two orders of magnitude faster than the corresponding analogous [NiIII(Z)(L)] complexes. This was ascribed to a high bond dissociation free energy value associated with H−F (135 kcal mol−1), which is postulated to be the product formed from PCET oxidation by 2 and is believed to be the driving force for the reaction. Our findings show that high‐valent metal–fluoride complexes represent a class of highly reactive PCET oxidants.