Mechanistic Insights into the Formation of a Ni(III) Bisphenoxyl Diradical Species

By serving as an electron reservoir, salen complexes of Ni have emerged as viable surrogates for the synthesis of high-valent Ni species. In contrast to the published Ni(II) phenoxyl radical ([Ni^II-L^•]) and the Ni(II) bisphenoxyl radical ([Ni^II-L^••]) high-valent species, which were produced by the action of electron transfer oxidants, a recently reported Ni(III) bisphenoxyl diradical species ([Ni^III-L^••]) was generated using excess mCPBA as an oxidant. However, the use of mCPBA constrains the revelation of steps involved in the formation of this Ni(V) equivalent species. The present study, thus, by varying the potential and the concentration of ceric ammonium nitrate (CAN) along with the temperature, makes it possible to achieve the intermediate species, such as a Ni(III) and a Ni(IV) equivalent species, prior to the formation of a Ni(III) bisphenoxyl diradical species in CH₃CN. The identity of these intermediary species was aided by various spectroscopic techniques. We believe that an advantage of having an in-depth understanding of the reported Ni(III) bisphenoxyl diradical species is that the present work can account for what the previously used approaches were unable to. Further, the one-, two-, and three-electron-oxidized Ni intermediates were tested for their electron transfer reactivity toward ferrocenes.