The Oxo-Wall Remains Intact: A Tetrahedrally Distorted Co(IV)–Oxo Complex

In this paper, we report the preparation, spectroscopic and theoretical characterization, and reactivity studies of a Co­(IV)–oxo complex bearing an N4-macrocyclic coligand, 12-TBC (12-TBC = 1,4,7,10-tetrabenzyl-1,4,7,10-tetraazacyclododecane). On the basis of the ligand and the structure of the Co­(II) precursor, [Co^II(12-TBC)­(CF₃SO₃)₂], one would assume that this species corresponds to a tetragonal Co­(IV)–oxo complex, but the spectroscopic data do not support this notion. Co K-edge XAS data show that the treatment of the Co­(II) precursor with iodosylbenzene (PhIO) as an oxidant at −40 °C in the presence of a proton source leads to a distinct shift in the Co K-edge, in agreement with the formation of a Co­(IV) intermediate. The presence of the oxo group is further demonstrated by resonance Raman (rRaman) spectroscopy. Interestingly, the EPR data of this complex show a high degree of rhombicity, indicating structural distortion. This is further supported by the EXAFS data. Using DFT calculations, a structural model is developed for this complex with a ligand-protonated structure that features a CoO···HN hydrogen bond and a four-coordinate Co center in a seesaw-shaped coordination geometry. Magnetic circular dichroism (MCD) spectroscopy further supports this finding. The hydrogen bond leads to an interesting polarization of the Co–oxo π-bonds, where one O­(p) lone-pair is stabilized and leads to a regular Co­(d) interaction, whereas the other π-bond shows an inverted ligand field. The reactivity of this complex in hydrogen atom and oxygen atom transfer reactions is discussed as well.