Dicobaltocenium Amine–Proton, Electron, and H Atom Transfer

The dicobaltocenium
ion [Cc2NH]2+ [H–1]2+ was obtained via nucleophilic substitution
of the nitro group of [Cc–NO2]+ by amino
cobaltocenium [Cc–NH2]+ (Cc = Co­(η5-C5H4)­(η5-C5H5)). [H–1]2+ is moderately acidic, giving the conjugate base 1+ after deprotonation (pKa = 16.36 in acetonitrile). Single-crystal X-ray diffraction
analyses and quantum chemical calculations of the [H–1]2+/1+ acid/base pair show only minor geometric changes during the proton
transfer. One-electron reduction of [H–1]2+ at −1.32 V versus ferrocene furnishes [H–1]+, which is a strong hydrogen
atom donor with a bond dissociation free energy (BDFE) of the NH group
of only 44.6 kcal mol–1. Hydrogen gas evolves from [H–1]+, generating cation 1+. One-electron oxidation of 1+ at Ep = +0.47 V gives the highly reactive aminyl radical 12+. The BDFE of the NH group of [H–1]2+ amounts to 85.9 kcal mol–1. Spin trapping experiments confirm that 12+ abstracts a hydrogen atom from the CH3CN solvent, regenerating [H–1]2+. Consequently, the [H–1]2+/+ redox couple is competent in the hydrogen evolution reaction
and the [H–1]2+/12+ pair enables hydrogen atom abstraction
from CH3CN. The [H–1]2+/[H–1]+/12+ ions exhibit a unique proton, electron,
and hydrogen atom transfer reactivity.