Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent.

Author(s) McLoughlin, E.A.; Waldie, K.M.; Ramakrishnan, S.; Waymouth, R.M.
Journal J Am Chem Soc
Date Published 2018 Oct 04
Abstract

Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)] 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a p K of 12.1, a hydricity of Δ G° = 89 kcal/mol, and a bond dissociation free energy (BDFE) of Δ G° = 68 kcal/mol in CDCN. Hydride transfer from 3 to the trityl cation (Δ G° = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di- tert-butyl-4-(4'-nitrophenyl)phenoxyl radical (BuNPArO·, Δ G° = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.

DOI 10.1021/jacs.8b06156
ISSN 1520-5126
Citation McLoughlin EA, Waldie KM, Ramakrishnan S, Waymouth RM. Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent. J Am Chem Soc. 2018.

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