Redox-active ligands have been widely employed in first-row transition metal complexes due to their great potential to act as electron reservoirs and to promote bond breaking or forming processes. This reactivity generally involves paramagnetic, ligand-centered radical species, which may be stabilized through extended π-delocalization across the metal and ligand framework. Characterization of these paramagnetic or non-innocent species is a challenge, and sometimes their reactivity with substrates can be unpredictable due to competitive side reactions. Developing diamagnetic metal complexes that display ligand-centered reactivity will thus be of great importance to illustrate the roles of redox-active ligands in chemical transformations. We have recently found an overall ligand-based two-electron oxidation for diamagnetic cyclopentadienyl cobalt (CpCo) complexes bearing a dianionic ortho-phenylenediamido (opda) ligand. Treating the neutral cobalt complex with two equivalents of methyl triflate or trimethyloxonium tetrafluoroborate results in O-methylation of the ligand carbonyl groups, instead of the anticipated formation of a Co–CH3 bond with concomitant two-electron donation from the ligand backbone. This unexpected result indicates electrophilic attack is preferred at the electron-rich O-site rather than the cobalt center. The single crystal structure of the product reveals a comparable degree of sp2 hybridization on the ureanyl carbons, similar Co–N bond distances, but elongated N–C bond distances in the opda backbone compared to the parent cobalt complex. We are currently investigating the extent to which we can control this reactivity and whether we can tune the reaction site to target electrophilic attack at the ligand versus metal. These studies will have important implications for designing redox-active ligand frameworks capable of promoting selective catalytic transformations.

Redox-active ligand centered reactivity between electron-rich cobalt complexes and electrophiles

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