Charge and Solvent Effects on the Redox Behavior of Vanadyl Salen-Crown Complexes

The incorporation of charged groups proximal to a redox active transition metal center is an attractive strategy for altering redox behavior, installing electric fields, and enhancing catalysis. Vanadyl salen (salen = N,N-ethylenebis(salicylideneaminato)) complexes functionalized with a crown ether containing a non-redox active Lewis acidic metal cation (V-Na, V-K, V-Ba, V-La, V-Ce, and V-Nd) were synthesized. The electrochemical behavior of this series of complexes was investigated by cyclic voltammetry in solvents with varying polarity and dielectric constant (acetonitrile = 37.5; N,N-dimethylformamide = 36.7; and dichloromethane = 8.93). The vanadium(V/IV) reduction potential shifted anodically (>900 mV in acetonitrile and >700 mV in dichloromethane) with increasing cation charge as compared to complexes lacking the proximal cation. The reduction potential for all vanadyl salen-crown complexes measured in N,N-dimethylformamide was insensitive to cation charge magnitude, regardless of electrolyte or counter anion used. Titration studies of N,N-dimethylformamide into acetonitrile resulted in cathodic shifting of the vanadium(V/IV) reduction potential with increasing concentration of N,N-dimethylformamide. Binding constants of N,N-dimethylformamide (log(KDMF)) for the series of crown complexes show increased binding affinity in the order of V-La>V-Ba>V-K>(salen)V(O), indicating an enhancement of Lewis acid/base interaction with increase of cation charge. The redox behavior of (salen)V(O) and (salen-OMe)V(O) (salen-OMe = N,N-ethylenebis(3-methoxysalicylideneamine) was also investigated and compared to the crown-containing complexes. For (salen-OMe)V(O), a weak association of triflate salt at the vanadium(IV) oxidation state was observed through cyclic voltammetry titration experiments and cation dissociation upon oxidation to vanadium(V) was identified. These studies demonstrates the non-innocent role of solvent coordination and cation/anion effects on redox behavior.