Nobuyoshi Koshino

Complexation equilibria of oxy-acid–2-amino-2-deoxy-d-gluconic acid–metal(II) ion ternary systems in aqueous solution as studied by potentiometry. Binding characteristics of borate and germanate

Abstract The complexation equilibria for the boric acid– d -glucosaminic acid and d -glucosaminic acid–metal(II) ion binary systems, the boric acid– d -glucosaminic acid–metal(II) ion ternary system, and the corresponding binary and ternary systems with germanic acid instead of boric acid involving nickel(II), zinc(II), cadmium(II) and lead(II) as the metal(II) ion have been investigated in aqueous solution at 25°C and I =0.1 mol dm −3 (NaClO 4 ) by potentiometric measurements ( d -glucosaminic acid is 2-amino-2-deoxy- d -gluconic acid). The complexes have been characterized in aqueous solution by 13 C NMR spectra and the p K a values. The 1:2 ester of borate and d -glucosaminate shows a selectivity for metal(II) ions different from the free d -glucosaminate ligand. It has been demonstrated that the coordination site of the 1:2 ester can distinguish the difference in metal ion size. Such selectivity for metal(II) ions in the boric acid system is discussed by comparison with the germanic acid system.

Electron transfer reactions of copper(II)/(I) couples with bidentate and quadridentate polypyridine ligands: is gated behavior common among outer-sphere electron transfer reactions?

Abstract We examined redox behaviors of bis(2,9-diphenyl-1,10-phenanthroline)copper(II)/(I) couple and the oxidation reaction of structurally more constrained bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) in acetnitrile: the electron transfer reactions of the former couple took place in an ordinary concerted manner with the self-exchange rate constant k ex 298 =9.2×10 4 kg mol −1 s −1 , while a dicopper(II) intermediate with a tetrahedral coordination geometry was spectrophotometrically detected during the course of the oxidation reaction of the latter compound. It was also found that the reduction of (1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II), the oxidized form of the corresponding dicopper(I) species, proceeded through the dimerization process. The oxidation reaction of bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) by [Ni(1,4,7-triazacyclononane)2]3+ in acetonitrile was a bi-phasic reaction involving fast electron transfer (ET) with k ex 298 =(4.8±0.1)×10 4 kg mol −1 s −1 followed by the very slow monomerization of the intermediate [k298=(3.46±0.03)×10−4 s−1]. These observations indicate that the electron transfer reactions involving (1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II)/bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper(I) couple are gated through the conformational change (Cu(II) monomer–dimer). We re-examined the gated ET reactions of Cu(II)/(I) couples so far reported from the view point of the time scale of internal rearrangement and of the merit brought about by the geometric changes around either Cu(I) or Cu(II). We have reached a tentative conclusion that gated ET is observed only when the following conditions are fulfilled: (1) direct ET for either oxidation or reduction direction is non-adiabatic; and (2) a low energy CT perturbation is expected by the uneven structural change (change in the coordination geometry takes place only in the species which exhibits no low-lying CT band in its ground state). In addition to the above two conditions; (3) time scale of the structural change is slow compared with the lifetime of the encounter complex may be added.

Free-energy barrier of gated electron transfer reactions: reduction and oxidation reactions of Cu(6,6′-dimethyl-2,2′-bipyridine)22+/+ with Ru(hexafluoroacetylacetone)30/−

Abstract Redox reactions of Cu(dmbp)22+/+ (dmbp=6,6′-dimethyl-2,2′-bipyridine) were examined in acetonitrile. It was found that the reaction of Cu(dmbp)22+ was gated, involving a slow structural change in Cu(II) prior to the electron transfer process. The energetic origin of the gated phenomenon was discussed by comparing the activation barriers for the intermolecular gated, intramolecular directional and outer-sphere concerted (direct) pathways.

Cobalt(II) phosphine complexes stable in aqueous solution: spectroscopic and kinetic evidence for low-spin Co(II)P6 and Co(II)P3S3 with tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane

Abstract Co(II) species produced by the controlled potential electrochemical reduction of Co(III)P 3 S 3 and Co(III)P 6 complexes with 1,4,7-trithiacyclononane and tripodal 1,1,1-tris(dimethylphosphinomethyl)ethane in aqueous solution were characterized at ambient temperature by the EPR and spectrophotometric methods: the Co(II)P 6 and Co(II)P 3 S 3 species are in the low-spin t 2g 6 e g 1 state with large Jahn–Teller distortion. Kinetic studies of the redox reactions involving these Co(III)/(II) species revealed that the electron self-exchange reactions for the Co(III)/(II) couples are very fast ( k ex ∼10 4 dm 3 mol −1 s −1 ), which is consistent with the results for other low-spin/low-spin Co(III)/(II) couples. It was concluded that the nephelauxetic effect of the P donor atom stabilizes the low-spin state in Co(II).