Mononuclear transition metal-peroxo species are important intermediates in the O2 activation pathway of many iron and copper-dependent metalloenzymes that utilize O2 as the natural oxidant to carry out rich and diverse substrate transformations [[1], [2], [3], [4], [5]]. Synthetic metal-peroxo complexes play an important role as structural and functional models for those enzymatic intermediates, providing a better understanding about their spectroscopic properties and reactivities [[3], [4], [5], [6], [7], [8]]. Compared to other first-row transition metals such as manganese, iron and copper, mononuclear Co(III)-peroxo (CoIII-(O2)) complexes are relatively stable and unreactive towards substrate oxidations [9], in part due to the inertness of d6 Co(III) and the difficulty of further activating the OO bond. Nevertheless, a number of CoIII-(O2) complexes exhibit nucleophilic reactivities with electrophilic substrates such as aldehyde and nitrile. For example, a series of (TMC)CoIII-(O2) complexes (TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) reported by Nam and co-workers carry out deformylation of aldehydes [8,10,11]. More recently, Cho and co-workers described the activation of nitriles by a (TBDAP)CoIII-(O2) complex (TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) with or without the assistance of redox inactive Lewis acids [12,13].

In this work, we report the generation and characterization of a new CoIII-(O2) complex (2) supported by a tetradentate TPA ligand (TPA = tris(2-pyridylmethyl)amine). Complex 2 is characterized using UV–vis, ESI-MS and FTIR spectroscopies and DFT calculations, showing that it is a distorted octahedral side-on Co(III)-peroxo species with a relatively weak OO bond. More importantly, 2 exhibits nucleophilic reactivity with an arylboronic substrate to form a novel CoIII-peroxo-arylboronate adduct (3) having a five-membered metallocycle that is structurally characterized by X-ray crystallography. The conversion of 2 to 3 represents a reaction that has yet to be observed for other first-row metal-peroxo complexes reported to date. Our results have thus expanded the library of nucleophilic reactivities that can be carried out by mononuclear Co(III)-peroxo complexes.