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(Bu-t-terpy)(L)] (1, Bu-t-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(Bu-t-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 l'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.