Carboxamidate Ligand Noninnocence in Proton Coupled Electron Transfer

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(^tBu-terpy)(L)] (1, ^tBu-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(^tBu-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.