Electroanalytical, kinetic, and mechanistic investigations of coordination-inspired electron transfer between Fe(II)/Cu(II)

Abstract

Herein, we report that the thermodynamic barrier for solution-phase electron transfer (ET) between Cu(II) and Fe(II) in aqueous acidic media can be overcome through the addition of 2,9-dimethyl-1,10-phenanthroline (Neocuproine [NC]) to the reaction mixture. A detailed discussion of the kinetic and mechanistic aspects of this coordination-inspired ET is presented. We attribute the observed change in the thermodynamic feasibility to the change in the reduction potential of Cu(II)-Cu(I) couple on its ligation with NC. The reaction was found to be slow, following firstorder kinetics with respect to each Cu(II) and Fe(II). In the presence of excess NC, the reaction was observed to proceed with a pseudo-second-order rate constant of 3.37 ± 0.05 dm 3 mol À1 s À1 at 298 K, with an activation barrier of ca. 26.22 kJ mol À1 . The slow reaction is attributed to the significant reorganization energy associated with large-scale changes in the coordination sphere of the oxidant. A two-step mechanism that explains the experimental observations is proposed for the investigated reaction. IntroductionCoordination complexes of transition metal ions [1-3] have established roles as metalloproteins in life processes. Analytical and other applications of coordination complexes make studies related to redox properties, mechanistic, kinetic, and applied aspects of inner and outer sphere electron transfers in coordination complexes important [4,5]. Few reports [6-8] using transition metal ion complexes as model systems for understanding electrontransfer processes in metalloproteins have been demonstrated. A long-standing challenge related to electron-transfer processes in metalloproteins is to unravel the basic cause for the metal ion center in electron transfer proteins that allows them to mediate a huge number of electron transfer reactions with high sensitivity and accuracy [9,10]. XRD and other structural studies in blue copper proteins have established that this ability of metalloproteins can be partly attributed to a large variation in the redox potential of metal ion center (190-780 mV) due to the nature and complexation geometry specific to the redox tuning properties of ligands [11,12].In continuation to our work on coordination-inspired redox systems [13,14], we investigated redox titration between Fe(II) and Cu(II) in the presence of 2,9-dimethyl-1,

Hydrazine Hydrate Accelerates Neocuproine–Copper Complex Generation and Utilization in Alkyne Reduction, a Significant Supplement Method for Catalytic Hydrogenation

“…The control experiment (Table , entry 18) proved that this reaction is not suitable in an inert atmosphere. Based on our experiments and the preceding literature, the plausible reaction mechanism was proposed and presented in Scheme . It is worth mentioning that the direct use of CuI is not a better choice in this reaction (Table , entry 21).…”

Targeting copper induced oxidative damage to proteins by ligation: a novel approach towards chelation therapy for oxidative stress disorders

“…It is a known fact that Cu(II) complexes prefer to exist as Jahn-Teller distorted octahedrons (though other configurations are known), while Cu(I) complexes usually exist as four-coordinate tetrahedra. 28,36 Complexation with Cu(II) requires DIDE to undergo some structural reorientations against the optimized (lowest energy) structure which makes complexation somewhat unfavourable as compared to Cu(I) wherein the four donor atoms persist with the same spatial arrangement. This preferential affinity for Cu(I) of DIDE hence results in an increase in the reduction potential of the Cu(II)/Cu(I) couple which is responsible for its observed potential to reduce the copper induced oxidative damage to BSA.…”

Complexation modulated redox behavior of transition metal systems (review)

“…Thus, in the reduction reaction geometry transformation slows down the rate of electron transfer. Raashid and coauthors [21] reported the spectrophotometric study of kinetics and mechanism of NC assisted Fe(II)-Cu(II) process in which the absorbance time plot indicated electron transfer completion in ca 120-130 s [11]. The concentration effect on kinetic profile study justified the pseudo 2nd order kinetics with respect to the NC ligand and first order kinetics with respect to both Cu(II) and Fe(II).…”