Hydrogen atom abstraction as a synthetic route to a square planar CoII complex with a redox-active tetradentate PNNP ligand

Abstract

Redox-active ligands improve the reactivity of transition metal complexes by facilitating redox processes independent of the transition metal center. A tetradentate square planar (PNCH₂CH₂NP)Co^II (1) complex was synthesized and the ethylene backbone was dehydrogenated through hydrogen atom abstraction to afford (PNCHCHNP)Co^II (2), which now contains a redox-active ligand. The ligand backbone of 2 can be readily hydrogenated with H₂ to regenerate 1. Reduction of 1 and 2 with KC₈ in the presence of 18-crown-6 results in cobalt-based reductions to afford [(PNCH₂CH₂NP)Co^I][K(18-crown-6)] (3) and [(PNCHCHNP)Co^I][K(18-crown-6)] (4), respectively. Cyclic voltammetry revealed two reversible oxidation processes for 2, presumed to be ligand-based. Following treatment of 2 with one equivalent of FcPF₆, the one-electron oxidation product {[(PNCHCHNP)Co^II(THF)][PF₆]}·THF (5) was obtained. Treating 5 with an additional equivalent of FcPF₆ affords the two-electron oxidation product [(PNCHCHNP)Co^II][PF₆]₂ (6). Addition of PMe₃ to 5 produced [(PNCHCHNP)Co^II(PMe₃)][PF₆] (7). A host of characterization methods including nuclear magnetic resonance (NMR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry, magnetic susceptibility measurements using SQUID magnetometry, single-crystal X-ray diffraction, and density functional theory calculations were used to assign 5 and 6 as ligand-based oxidation products of 2.