Enhancements in the Electron-Transfer Kinetics of Uranium-Based Redox Couples Induced by Tetraketone Ligands with Potential Chelate Effect

Abstract

Acetylacetone shows dissociative electron transfer when it is complexed with metal ions. In an effort to improve the kinetics of electron transfer of uranium-based complexes for application to a redox-flow battery, improvement by dimerization of acetylacetone into tetraketones with potential chelate effect is examined and discussed quantitatively. For the monomer acetylacetone, electron transfer of uranium complexes reveals an ECE mechanism and an inner-sphere reaction on the electrode surface. By using tetraketones, 8-oxo-2,4,-12,14-tetraoxapentadecane and m-bis(2,4-dioxo-1-pentyl)benzene, the electron transfer of tetraketones with U(VI)/U(V) and U(IV)/U(III) shows rapid kinetics based on the E mechanism. In clear contrast to U(acac) 4 , the electrochemically reduced species of the U(IV) complex with tetraketone is stable during potential cycling. These results are also supported by NMR of tetraketones with U(VI) and U(IV); each acetylacetone site to uranium atom is stable at -40 ∼ +40 °C. The results demonstrate a remarkable enhancement of the stability of the reduced form of the metal center and thereby an improvement of the redox kinetics by the chelate effect. The tetraketones are eminently suitable for use in the active materials of a high-efficiency redox-flow battery.

Surface ECE mechanism in protein film voltammetry—a theoretical study under conditions of square-wave voltammetry

“…This situation is commonly observed for electroactive molecules that can exist as more than one isomer; for example, cis and trans isomers of transition metal complexes, but it is also frequently met by some proteins and other important biomolecules [7,[13][14][15][16][17][18][19]. The features of the SW voltammetric responses in such scenario will significantly depend on the value of the chemical kinetic parameter l, but they will be also affected by the values of the electron transfer kinetic parameters K 1 and K 2 of the two reduction steps.…”

“…If we consider a surface electron transferchemical reaction (EC) mechanism for example, and if the product of the first electron step can be converted chemically to another electroactive specie, then we assign these reactions to follow the surface electron transfer-chemical reactionelectron transfer (ECE) mechanistic pathway. Although this reaction mechanism is quite complex, it is a widespread pathway of many proteins [7] and other important compounds in organic electrochemistry [1,7,[13][14][15][16][17][18][19]. For example, the product of the first electron transfer step can undergo protonation, elimination, substitution, or reorganization reaction to give second electroactive specie [1,[13][14][15][16][17][18][19].…”

“…Although this reaction mechanism is quite complex, it is a widespread pathway of many proteins [7] and other important compounds in organic electrochemistry [1,7,[13][14][15][16][17][18][19]. For example, the product of the first electron transfer step can undergo protonation, elimination, substitution, or reorganization reaction to give second electroactive specie [1,[13][14][15][16][17][18][19]. As the most familiar among the voltammetric techniques, the cyclic voltammetry is commonly used for characterization of electrochemically active systems following the ECE pathway [7, 13-16, 18, 19].…”

Density functional theory (DFT) calculations of VI/V reduction potentials of uranyl coordination complexes in non-aqueous solutions

“…In addition, three bidentate ligands can satisfy the the equatorial coordination of uranyl ion and such complexes are well known for nitrate and carbonate type ligands. The experimental reduction potentials for the investigated uranyl complexes in DMSO solution were taken from the references 44,54,[59][60][61][62][63][64][65][66] and for the DMF solution from the references. 54,62,[65][66][67][68] The experimental reduction potentials for the uranyl complexes explored in DCM solution were from Hayton et al [69][70][71][72] , Sessler et al 73 and from D.S.Royal's PhD thesis.…”

“…S3), the ether type O-atom does not coordinate with the uranyl ion in the equatorial plane and the fifth-coordination site is completed by a DMSO ligand. 59 For the uranyl complex of the 'salacen' ligand (see SI, Fig. S3 for structure), the imino groups do not participate in coordination to the uranyl ion.…”