An unexpected destabilisation of copper(II) phenoxyl radical species by steric protection

The two complexes shown exhibit ligand-based oxidations yielding phenoxyl radical-containing products, that are chemically reversible on the voltammetric timecale but which decompose rapidly when generated in bulk. This contrasts with an analogous complex lacking the phenoxide tertbutyl group, which yields a phenoxyl radical species with a lifetime of hours under the same conditions.