Formation and Reactivity of a Fleeting NiIII Bisphenoxyl Diradical Species

• Published in: Angewandte Chemie International Edition Vol. 61; no. 41
• Main Authors: Awasthi, Ayushi, Leach, Isaac F., Engbers, Silène, Kumar, Rakesh, Eerlapally, Raju, Gupta, Sikha, Klein, Johannes E. M. N., Draksharapu, Apparao
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 10.10.2022; John Wiley and Sons Inc
• Edition: International ed. in English
• Subjects: Cyclohexane; Cytochrome; Cytochromes; Diamines; Dichloromethane; Electrochemistry; Electronic structure; Galactose; High-Valent Nickel Species; Hydrogen atoms; Intermediates; Ligands; Metals; Oxidation; Oxidation States; Redox-Active Ligands; Salen Ligands; Species
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202211345

Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of NiII(salen) (salen=N,N′‐bis(3,5‐di‐tert‐butyl‐salicylidene)‐1,2‐cyclohexane‐(1R,2R)‐diamine) with mCPBA (meta‐chloroperoxybenzoic acid) to form a fleeting NiIII bisphenoxyl diradical species, in CH3CN and CH2Cl2 at −40 °C. Electrochemical and spectroscopic analyses using UV/Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a NiIII bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the d‐configuration of the metal center to be consistent with a NiIII bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom ion and oxygen atom transfer reactions. NiII(salen) complexes have been one of the most promising avenues for those aiming to generate high valent Ni species. The present work is a result of one such inquisitiveness where a NiII(salen) complex was treated with mCPBA to form a novel NiIII bisphenoxyl diradical species. Various spectroscopic tools and theoretical calculations further assisted our observation. This proposed three electron oxidized species showed oxidation reactivity.