No abstract

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)(CF 3 SO 3 ) 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.

High-valent first-row transition-metal–oxo complexes are important intermediates in biologically and chemically relevant oxidative transformations of organic molecules and in the water splitting reaction in (artificial) photosynthesis. While high-valent Fe– and Mn–oxo complexes have been characterized in detail, much less is known about their analogues with late transition metals. In this study, we present the synthesis and detailed characterization of a unique mononuclear terminal Ni–O complex. This compound, [Ni(TAML)(O)(OH)]3–, is characterized by an intense charge-transfer (CT) band around 730 nm and has an S t = 1 ground state, as determined by magnetic circular dichroism spectroscopy. From extended X-ray absorption fine structure (EXAFS), the Ni–O bond distance is 1.84 Å. Ni K edge XAS data indicate that the complex contains a Ni(III) center, which results from an unusually large degree of Ni–O π-bond inversion, with one hole located on the oxo ligand. The complex is therefore best described as a low-spin Ni(III) complex (S = 1/2) with a bound oxyl (O•–) ligand (S = 1/2), where the spins of Ni and oxyl are ferromagnetically coupled, giving rise to the observed S t = 1 ground state. This bonding description is roughly equivalent to the presence of a Ni–O single (σ) bond. Reactivity studies show that [Ni(TAML)(O)(OH)]3– is a strong oxidant capable of oxidizing thioanisole and styrene derivatives with large negative ρ values in the Hammett plot, indicating its electrophilic nature. The intermediate also shows high reactivity in C–H bond activation of hydrocarbons with a kinetic isotope effect of 7.0(3) in xanthene oxidation.