Carboxamidate Ligand Noninnocence in Proton Coupled Electron Transfer

This is the authors' abstract. We don't add key points for this paper.

Recent breakthroughs have brought into question the innocence (or not) of carboxamidate donor ligands in the reactivity of high-valent oxidants. To test the reactivity properties of high-valent carboxamidate complexes, [Ni II ( t Bu-terpy)(L)] ( 1, t Bu-terpy = 4,4′,4′′-tri- tert -butyl-2,2′;6′,2″-terpyridine; L = N, N ′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate) was prepared and converted to [Ni III ( t Bu-terpy)(L)] + ( 2 ) using ceric ammonium nitrate. 2 was characterized using electronic absorption and electron paramagnetic resonance spectroscopies and electrospray ionization mass spectrometry. 2 was found to be a capable oxidant of phenols and through kinetic analysis was found to oxidize these substrates via a nonconcerted or partially concerted proton coupled electron transfer (PCET) mechanism. The products of PCET oxidation of phenols by 2 were phenoxyl radical and the protonated form of 1, 1H + . 1H + was crystallographically characterized providing convincing evidence of 1 ’s ability to act as a proton acceptor. We demonstrate that the complex remained intact through a full cycle of oxidation of 1 to 2, PCET of 2 to yield 1H +, and deprotonation of 1H + to yield 1 followed by reoxidation of 1 to yield 2 . The N–H bond dissociation energy of the protonated amide in 1H + was determined to be 84 kcal/mol. Our findings illuminate the role carboxamidate ligands can play in PCET oxidation.