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

Abstract

The incorporation of charged groups proximal to a redox active transition metal center can impact the local electric field, altering redox behavior and enhancing catalysis. Vanadyl salen (salen = N,N′-ethylenebis­(salicylideneaminato)) complexes functionalized with a crown ether containing a nonredox active 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 (ε) (acetonitrile, ε = 37.5; N,N-dimethylformamide, ε = 36.7; and dichloromethane, ε = 8.93). The vanadium­(V/IV) reduction potential shifted anodically with increasing cation charge compared to a complex lacking a proximal cation (ΔE 1/2 > 900 mV in acetonitrile and >700 mV in dichloromethane). In contrast, the reduction potential for all vanadyl salen–crown complexes measured in N,N-dimethylformamide was insensitive to the magnitude of the cationic charge, regardless of the electrolyte or counteranion 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(K DMF)) 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 increasing cationic 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 demonstrate the noninnocent role of solvent coordination and cation/anion effects on redox behavior and, by extension, the local electric field.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

“…One of the more broadly studied effects of Lewis acids on electron transfer involves the conjugation of redox-active metal complexes to crown-ethers (Figure ). − Extensive studies on mono- ( 1 ) and bimetallic ( 2 ) manganese Schiff-base complexes first appeared in the 1990s that aimed to elucidate the properties of manganese-dependent enzymes (including the OEC). − While salen-type related ligands have been shown to be useful for binding or sensing of a wide variety of redox-inactive metals via coordination to O atom donors (e.g., phenoxide), inclusion of crown-ether motifs enables stronger binding of Lewis acidic ions with stoichiometric control. Exposing complex 1 to Li + , K + , Ca 2+ , or Ba 2+ leads to incorporation of a single redox-inactive metal into the crown-ether moiety, resulting in a positive shift in reduction potential for the Mn III /Mn II redox couple .…”

“…In these complexes, a linear correlation between the p K a of the redox-inactive metal and the reduction potential of the Co II/I redox-couple was observed, though the narrow range of Lewis acidities once again limits generalization of this relationship. 68 Related nickel, 78 iron, 83 zinc, 85 palladium, 84 and vanadyl 86 , 87 complexes ( 3-M to 6-M) have been shown to display similar correlations between the reduction potential of the complex (typically corresponding to a metal-centered ET process, though a ligand-based one for Pd and Zn) and the Lewis acidity of a wider range of cations (e.g., Na + , Ca 2+ , Nd 3+ , Y 3+ ). Incorporating a more flexible triamine-based backbone enables coordination of these Schiff-base ligands to the uranyl cation ([UO 2 ] 2+ ; 8 ) and thereby enables the extension of these relationships to actinide complexes.…”

“…(A) Selection of crown-ether supported metal complexes. (B) Observed linear correlation is shown for complexes 1-M (μ-oxo dimer, pink diamonds), 72 2-M (blue dots), 68 3-M (dark green dots), 78 4-M (light green dots), 85 5-M (purple dots), 84 6-M (salen-type backbone, teal triangles), 86 6-M (modified backbone, orange triangles), 87 7-M (two redox events, maroon and gray diamonds, respectively), 71 and 8-M (red dots). 77 …”

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt–porphyrin complexes

“…4 c , d , f , i – l For example, the binding of a Lewis acidic cation, such as an alkali or alkaline earth metal ion to a prepositioned site on an ancillary ligand, can alter the redox properties of a metal center due to electrostatic effects. 5 Crown or aza-crown ethers are attractive as binding sites for cationic Lewis acids (LAs) because they have high and tunable binding constants for alkali or alkaline earth metal cations. 6 Consequently, several ligands have been designed which feature pendant crown or aza-crown ether moieties, A–E (Fig.…”

Shift of the reduction potential of nickel(ii) Schiff base complexes in the presence of redox innocent metal ions

“…9–12 The literature reviews suggest that the reduction potential can be tuned by putting redox innocent metal ions in close proximity or by adjusting the ligand systems with electron donating or withdrawing groups. 13–26 For example, in biological systems, the non-redox-active Ca 2+ ion in the oxo-bridged tetra-manganese complex plays a key role in the transformation of water to molecular oxygen. 27–30 In a synthetic system, the presence of an electron donating group ( − t Bu, –OMe)/electron withdrawing group (–X, –CX 3 , X = halogen) on the ligand, the flexibility (5-membered vs .…”