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, [CoII(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 ?-bonds, where one O(p) lone-pair is stabilized and leads to a regular Co(d) interaction, whereas the other ?-bond shows an inverted ligand field. The reactivity of this complex in hydrogen atom and oxygen atom transfer reactions is discussed as well.
Bibliographical noteFunding Information:
This work was supported by a grant from the National Science Foundation (CHE-1900380 to N.L. and J.S.). This work was also supported by the NRF of Korea through CRI (NRF-2021R1A3B1076539 to W.N.) and Basic Science Research Program (NRF-2020R1I1A1A01074630 to Y.-M.L. and NRF-2019R1I1A1A01055822 to M.S.S.). H.T.D. acknowledges support from an Eastman Research Fellowship. We thank Professor Kiyoung Park and Mr. Woo Yeol Ryu (Korea Advanced Institute of Science and Technology (KAIST), Korea) for collecting NIR absorption spectra of complex 2 .
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