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

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.

Automated summary

This research reports the preparation, spectroscopic characterization, and reactivity studies of a Co(IV)-oxo complex with an N4-macrocyclic coligand. Experimental data show the formation of a Co(IV) intermediate and the existence of the oxo group, supported by DFT calculations and spectroscopic techniques analyzing the structure.