Negative Shift Of Redox Potential Research Articles

Synthesis, Structure, and Electrochemistry of an Electron-Rich Chiral Diaminoferrocene, (S,S)-Bis(2,5-dimethylpyrrolidinyl)ferrocene

The chiral diamine (S,S)-bis(2,5-dimethylpyrrolidinyl)ferrocene (6) was prepared from diaminoferrocene and (2R,5R)-2,5-hexanediol cyclic sulfate in the presence of a base. The short C-N bond, long Fe-C(N) bond, and planarnitrogen observed in the crystal structure of 6 are consistent with efficient N-to-Cp electron donation. The resulting electron-rich metal center (with the most negative shift in redox potential yet observed for aminoferrocenes) was characterized electrochemically.

The structure and electrochemistry of imidazolebis(1-10 phenanthroline)copper(II) nitrate

[Cu(phen) 2(imidazole)](NO 3) 2 crystallizes in the triclinic space group P1(No.2) with a=8.807(2), b=8.941(4), c=17.135(4) Å, α=100.95(2), β=90.95(1), γ=100.187(9)°, and Z=2. The structure consists of discrete [Cu(phen) 2(imidazole)] 2+ ions with a distorted square pyramidal geometry about the copper. One of the cis-phenanthroline ligands is asymmetrically coordinated to the axial (CuN phen=2.225(6) Å) and equatorial (CuN phen=2.050(5) Å) sites, while the second phenanthroline (CuN phen=2.027(5) Å and CuN phen=2.021(5) Å) and the imidazole (CuN im=1.980(6) Å), occupy the remaining equatorial sites. The reversible one electron reduction potential of [Cu(phen) 2(imidazole)] 2+, ( E 1/2=0.10 V) in methanol, is more negative than that observed for [Cu(phen) 2](NO 3) 2, ( E 1/2=0.16 V). The relationship between the structure of [Cu(phen) 2(imidazole)](NO 3) 2 and the corresponding dimer [Cu 2(phen) 4(imidazolate)](NO 3) 3 is discussed, and the electrochemistry is compared to the negative shift in redox potential which is observed when the nuclease [Cu(phen) 2] 2+ binds to CT DNA.

Irreversible electrocatalytic reduction of V(V) to V(IV) using phosphomolybdic acid

Although VO_22^+(aq) reduction is kinetically slow at glassy carbon and Pt electrodes, phosphomolybdic acid is shown to catalyze the electrochemical reduction of VO_2^+(aq) to VO_2^+(aq) in 1.0 M H_2SO_4(aq). A second-order rate constant of 33 M^(-1) s^(-1) was observed for this process. ^(31)P NMR spectra demonstrated that PMo_(11)VO_(40)^(4-) and PMo_(18)V_2O_(40)^(50-) were the dominant P-containing species under electrocatalytic conditions. The incorporation of V^V into the polyoxoanion led to a shift in potential from E^o(VO^(2+)(aq)/VO^(2+)(aq)) = +0.80 V vs Ag/AgCl for free V^V/V^(IV) to Eo' = +0.55 V vs Ag/AgCl for V^(V)/V^(Iv) bound in the heteropolyoxometalate (PMo_(11)VO_(40_^(4-)). This shift in formal potential corresponded to an equilibrium constant of 1.7 X 10^4 M^(-1) for preferential binding of V^V over V^(IV) by the heteropolyoxoanion. This negative shift in redox potential, combined with the slow electrochemical kinetics of free VO_2^+(aq) reduction and with the facile reaction of bound V^(IV) with free V^V in 1.0 M H_2SO_4(aq), resulted in the irreversible electrocatalytic reduction of VO_2^+(aq) to VO^(2+)(aq).

A study of dark luminescence in Chlorella. Background luminescence, 3-(3,4-dichlorophenyl)-1,1-dimethylurea-triggered luminescence and hydrogen peroxide chemiluminescence

Dark luminescence, defined as the ability of completely relaxed (darkadapted) photosynthetic systems to emit light, has been studied in Chlorella. Three main effects have been demonstrated. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea elicits a weak emission L D of very long lifetime (several minutes); it is believed to result from a negative shift of redox potential of the secondary System II electron acceptor B producing in some centers a state Q − (reduced primary acceptor), as postulated by Velthuys and Amesz ((1974) Biochim. Biophys. Acta 333, 85–94), which can recombine with an oxidizing equivalent in a state S 2 present in very small amount. As in photoinduced luminescence, this recombination excites chlorophyll which then emits light. A much stronger emission L H is observed after injection of H 2O 2. Both signals are modified or suppressed by treatments specific of the oxygen emission system, such as: thermal denaturation at 50°C, NH 2OH, etc. In addition, a weak, permanent background luminescence L 0 has been observed; like L D and L H, it is a System II property and requires the integrity of the oxygen-evolving system. It is believed to reflect a very slow back flow of electrons from an endogeneous reductant pool to oxygen through part of the photosynthetic chain. Using flash preillumination, it is demonstrated that H 2O 2 is able to oxidize S 0 into S 2, the latter giving rise to L H; H 2O 2 does not act on S 1 (or much less). The reactive site of H 2O 2 seems to be the same as the binding site of NH 2OH. Evidence is given that the strong L H signal in particular reveals a stable, low pH of the intrathylakoid phase in Chlorella.