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

Abstract

A new series of [CoIII-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 [CoIII-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.