Trapping a Trigonal Bipyramidal Cobalt(IV)-Oxo Species with an Exceptional Reactivity

Metal-oxo complexes (MOs) are pivotal in oxidation chemistry for their ability to activate strong C–H bonds. Among them, the octahedral nonheme iron(IV)-oxo [Fe(IV)-oxo] complex, [(Me 3 NTB)Fe IV (O)(X)] 2+, stands out as one of the most reactive nonheme Fe(IV)-oxo species despite its triplet ground state, challenging the notion that quintet nonheme Fe(IV)-oxo complexes are inherently stronger oxidants. Recent efforts to include late-transition metal ions in the synthesis of MOs and to increase the reactivity of MOs in oxidation reactions have shifted toward cobalt(IV)-oxo species, particularly with π–Co–O bond order below 1, an electronic configuration constrained by the “oxo wall”. To explore this frontier, we synthesized and characterized a cobalt precursor bearing the Me 3 NTB ligand, [(Me 3 NTB)Co(MeCN)(OTf) 2 ] (OTf – = CF 3 SO 3 – ), and used it in the generation of a high-valent Co-oxo intermediate. Reacting this precursor with iodosylbenzene (PhIO) yielded a fleeting [(Me 3 NTB)Co IV O(OTf)] + species, which underwent a rapid hydrogen atom transfer reaction, forming a metastable [(Me 3 NTB)Co III (OH)(PhIO)] 2+ product, which was spectroscopically well-characterized. Given the inherent instability of [(Me 3 NTB)Co IV (O)(OTf)] +, we employed flow chemistry coupled with electrospray ionization mass spectrometry (FC-ESI-MS) and infrared photodissociation spectroscopy (IRPD) for its detection and characterization. This complex adopts a trigonal bipyramidal geometry with a loosely bound OTf – anion and a ν CoO vibration at 812 cm –1, indicating a full Co–O double bond. Despite the quartet spin configuration, [(Me 3 NTB)Co IV (O)(OTf)] + is highly reactive for C–H hydroxylation. DFT reveals that the trigonal bipyramidal geometry distorts toward octahedral upon a hydrocarbon substrate approach, stabilizing a low-lying doublet state and facilitating a low-energy reaction pathway around the “oxo wall.”