Tuning the electrochemistry of homoleptic cobalt 4,4′-disubstituted-2,2′-bipyridine redox mediators

• Cobalt(II) tris(2,2′-bipyridine) complexes were prepared with electron withdrawing or donating groups in the ligand 4 and 4′ positions.
• The formal potential of the redox couples was tuned with the Hammett Parameter of the substituent.
• Substitution also leads to sluggish oxidation kinetics, as observed with decreased heterogeneous rate constant and transfer coefficient >0.5. Reduction kinetics remained facile.

The electrochemistry of a series of Co(II) tris(4,4′-disubstituted-2,2′-bipyridine) hexafluorophosphate complexes was investigated in acetonitrile, and compared to the parent, un-substituted 2,2′-bipyridine complex. The introduction of an electron-withdrawing halogen substituent increased the formal redox potential of the couple by 190–270 mV, in the order of F > Cl > Br > I. Substitution with electron-donating tert-butyl and methoxy groups acted as electron donating groups, and decreased the formal potential by 150–190 mV. Substitution in the 4,4′ positions, irrespective of electron withdrawing or donating character, also imparted an asymmetry in the anodic peak in the cyclic voltammograms, where the oxidation process became more sluggish than the corresponding reduction. Kinetics of Co(II) oxidation was evaluated by Koutecký–Levich analysis at a rotating Pt disc, revealing superior oxidation kinetics of the un-substituted 2,2′-bipyridine complex by up to an order of magnitude or larger. The anodic transfer coefficients were also extracted from this analysis, revealing ideal α = 0.50 for the parent complex, to 0.70 ≤ α ≤ 0.88 for complexes with substituted ligands.