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, [(Me₃NTB)FeIV(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 Me₃NTB ligand, [(Me₃NTB)Co(MeCN)(OTf)₂] (OTf⁻ = CF₃SO₃⁻), and used it in the generation of a high-valent Co-oxo intermediate. Reacting this precursor with iodosylbenzene (PhIO) yielded a fleeting [(Me₃NTB)CoIVO(OTf)]⁺ species, which underwent a rapid hydrogen atom transfer reaction, forming a metastable [(Me₃NTB)CoIII(OH)(PhIO)]²⁺ product, which was spectroscopically well-characterized. Given the inherent instability of [(Me₃NTB)CoIV(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 νCoO vibration at 812 cm⁻¹, indicating a full Co-O double bond. Despite the quartet spin configuration, [(Me₃NTB)CoIV(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."