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

The complexes [CuL(TpPh)] (HL = 5-tertbutylsalicylaldehyde, 5-tertbutyl-3-methylsulfanylsalicylaldehyde or 5-tertbutyl-3-phenylsulfanylsalicylaldehyde; [TpPh]− = tris-{3-phenylpyrazolyl}hydridoborate) have been prepared, and adopt square-pyramidal coordination geometries. Each compound exhibits a ligand-based oxidation in CH2Cl2 that is chemically reversible by voltammetry. However, Coulometric determinations showed that the resultant phenoxyl radical products decomposed rapidly at low temperatures in bulk solution. This instability may reflect intramolecular steric repulsions between the phenoxide tertbutyl substituents, and a pyrazolylborate phenyl group. These results contrast with a previously reported analogous compound, bearing a 5-methyl-3-methylsulfanylsalicylaldehydato ligand, which yields a phenoxyl radical oxidation product that is stable for hours under the same conditions.