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

Abstract

In this paper, we report the preparation, spectro-scopic 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 degrees 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 center dot center dot center dot 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 pi-bonds, where one O(p) lone-pair is stabilized and leads to a regular Co(d) interaction, whereas the other pi-bond shows an inverted ligand field. The reactivity of this complex in hydrogen atom and oxygen atom transfer reactions is discussed as well.