Redox Potential and Electronic Structure Effects of Proximal Nonredox Active Cations in Cobalt Schiff Base Complexes

Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na⁺, K⁺, Ca²⁺, Sr²⁺, and Ba²⁺) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na⁺ and K⁺ and 230-270 mV more positive for M = Ca²⁺, Sr²⁺, and Ba²⁺compared to Co(salen-OMe) (salen-OMe = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge.