High-Valent d7 NiIII versus d8 CuIII Oxidants in PCET

Oxygenases have been postulated to utilize d 4 Fe IV and d 8 Cu III oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-electron count can hold over the PCET reactivity, two metastable high-valent metal-oxygen adducts, [Ni III (OAc)(L)] ( 1b ) and [Cu III (OAc)(L)] ( 2b ), L = N , N' -(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate, were prepared from their low-valent precursors [Ni II (OAc)(L)] - ( 1a ) and [Cu II (OAc)(L)] - ( 2a ). The complexes 1a / b - 2a / b were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, X-ray diffraction, and absorption spectroscopies and mass spectrometry. Both complexes were capable of activating substrates through a concerted PCET mechanism (hydrogen atom transfer, HAT, or concerted proton and electron transfer, CPET). The reactivity of 1b and 2b oward a series of para -substituted 2,6-di- er -butylphenols ( p -X-2,6-DTBP; X = OCH 3 , C(CH 3 ) 3 , CH 3 , H, Br, CN, NO 2 ) was studied, showing similar rates of reaction for both complexes. In the oxidation of xanthene, the d 8 Cu III oxidant displayed a small increase in the rate constant compared to that of the d 7 Ni III oxidant. The d 8 Cu III oxidant was capable of oxidizing a large family of hydrocarbon substrates with bond dissociation enthalpy (BDE C-H ) values up to 90 kcal/mol. It was previously observed that exchanging the ancillary anionic donor ligand in such complexes resulted in a 20-fold enhancement in the rate constant, an observation that is further enforced by comparison of 1b and 2b o the literature precedents. In contrast, we observed only minor differences in the rate constants upon comparing 1b o 2b . It was thus concluded that in this case the metal ion has a minor impact, while the ancillary donor ligand yields more kinetic control over HAT/CPET oxidation.