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.