Metal- versus ligand-centered reactivity of a cobalt-phenylenediamide complex with electrophiles

Abstract

A new series of [Co^III–CF₃]ⁿ⁺ complexes supported by a bidentate redox-active ligand is presented. The cationic [Co–CF₃]⁺ complex was first obtained by reacting [CpCo(^tBuUreaopda)] (Cp = cyclopentadienyl, opda = o-phenylenediamide) with an electrophilic trifluoromethyl source, for which the redox-active phenylenediamide ligand serves as a 2e⁻ reservoir to generate [CpCp(^tBuUreabqdi)(CF₃)]⁺ (bqdi = benzoquinonediimine). Electrochemical studies of [Co–CF₃]⁺ revealed two reversible 1e⁻ reductions. Chemical reduction with 1 or 2 equiv. reducing agent enabled isolation of the neutral and anionic complexes, respectively, where the [Co^III–CF₃] bond remains intact in all three oxidation states (n = +1, 0, −1). Structural analysis shows systematic changes to the redox-active ligand backbone upon reduction, consistent with sequential ligand-centered electron transfer in the series [bqdi]⁰ to [s-bqdi]˙⁻ to [opda]²⁻. In contrast, the reaction of [CpCo(^tBuUreaopda)] with alkyl triflates resulted in ligand-centered alkylation at the ureayl groups instead of the targeted Co–alkyl bond formation, suggesting less favorable bond formation at cobalt and greater nucleophilic accessibility of the ligand compared to the metal center.