Formation and Reactivity of a Fleeting NiIII Bisphenoxyl Diradical Species

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 Ni-II(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 Ni-III bisphenoxyl diradical species, in CH3CN and CH2Cl2 at -40 degrees 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 Ni-III 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 Ni-III bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.

Automated summary

Cytochrome P450s and Galactose Oxidases utilize redox active ligands to generate reactive high-valent intermediates for oxidation reactions. In this study, Ni-II(salen) was oxidized with mCPBA to produce a transient Ni-III bisphenoxyl diradical species. Spectroscopic analyses revealed oxidation events on both the ligand and the metal center, supporting the formation of the Ni-III bisphenoxyl diradical species. DFT calculations confirmed the electronic structure and d-configuration of the metal center consistent with this species. The resulting three-electron oxidized species is capable of hydrogen atom abstraction and oxygen atom transfer reactions.