Preparation, Spectroscopic Characterization, and Reactivity of High-Valent Non-Oxo Co(IV) and Formally Co(V) Complexes.

In the presence of artificial oxidants, high-valent Co-X (X = O and NTs) complexes can be achieved in many different types of ligand structures. This paper reports, for the first time, the synthesis, spectroscopic and theoretical characterization, and reactivity of formally Co-(V) and Co-(IV) intermediates without the axial oxo or imido groups by using a strongly electron-donating ligand, PCAPD (PCAPD = bis-[2-(1H-pyrrol-2-carboxamido)--phenylenediamine). The formally Co-(V) complex was prepared by reacting [EtN][Co(PCAPD)-(Cl)] with iodosylbenzene (PhIO) as an artificial oxidant in acetone at -40 °C. Then, the formally Co-(V) complex was characterized with various spectroscopic methods, such as EPR, CSI-MS, MCD, XAS, and EXAFS, revealing a diamagnetic [Co(PCAPD)-(OH)] species with a Co-(IV) ion and one-electron oxidized coligand. Here, EXAFS shows a six-coordinate complex, likely with axial hydroxide ligands, but no oxo group with a short Co-X vector. This species can be further one-electron reduced by MeFc to afford [Co(PCAPD)-(OH)] in frozen conditions. EPR and MCD measurements show that [Co(PCAPD)-(OH)] has an = 1/2 ground state with a Co-(IV) center according to XAS. EXAFS shows essentially identical ligand environments about the Co center in [Co(PCAPD)-(OH)] and [Co(PCAPD)-(OH)]. Remarkably, the exact same high-valent species can be generated when [EtN][Co(PCAPD)-(Cl)] is oxidized with -tosyliminophenyliodinane (PhINTs). The reactivity of [Co(PCAPD)-(OH)] was further investigated in hydrogen atom and oxygen atom transfer reactions, revealing that the reactivity of [Co(PCAPD)-(OH)] is much lower than that of other Co-X species.