Hideo D. Takagi

Piezochromism and Related Phenomena Exhibited by Palladium Complexes

Piezochromic phenomena are explained by pressure perturbation to the HOMO and/or LUMO energy levels of the related electronic transition. The piezochromism of solid inorganic and organic materials has been investigated by examination of the phase transition phenomena. Specific electronic properties of the solids, acquired by tuning the external pressure, may be used as electronic devices and as pressure sensors. The effects of pressure perturbations on the absorption and emission spectra exhibited by solid palladium complexes are reviewed here. Related phenomena exhibited by platinum complexes and other metal complexes are included for comparison.

Kinetic Evidence for High Reactivity of 3-Nitrophenylboronic Acid Compared to Its Conjugate Boronate Ion in Reactions with Ethylene and Propylene Glycols

The rate constants for a boronate ion were determined for the first time using the reaction systems of 3-nitrophenylboronic acid (3-NO2PhB(OH)2) with ethylene glycol (EG) and propylene glycol (PG) in an alkaline solution: the rate constants (25 degrees C, I = 0.10 M) for the reactions of 3-NO2PhB(OH)3- are 1.2 M(-1) s(-1) (EG) and 1.5 M(-1) s(-1) (PG), which are at least 10(3) times smaller than those for the reactions of 3-NO 2PhB(OH)2 [1.0 x 10(4) M(-1) s(-1) (EG) and 5.8 x 10(3) M(-1) s(-1) (PG)].

Further evidence for detection of short-lived transient hypomanganate(V) and manganate(VI) intermediates during oxidation of some sulfated polysaccharides by alkaline permanganate using conventional spectrophotometric techniques.

Spectrophotometric evidence for the formation of hypomanganate(V), [CAR-Mn(V)O43-], and manganate(VI), [CAR-Mn(VI)O42-], intermediate complexes has been confirmed during the oxidation of iota- and lambda-carrageenan-sulfated polysaccharides (CAR) by alkaline permanganate at pHs 12 using a conventional spectrophotometer. These short-lived intermediate complexes were identified and characterized. A reaction mechanism in good consistence with the experimental results is suggested.

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.

A Revisit to the Corrosion Inhibition of Aluminum in Aqueous Alkaline Solutions by Water-Soluble Alginates and Pectates as Anionic Polyelectrolyte Inhibitors

The corrosion behavior of aluminum (Al) in alkaline media in presence of some natural polymer inhibitors has been reinvestigated. The inhibition action of the tested inhibitors was found to obey both Langmuir and Freundlich isotherms models. The inhibition efficiency was found to increase with increasing the inhibitors concentration and decrease with increasing the temperature, suggesting physical adsorption mechanism. Factors such as the concentration and geometrical structure of the inhibitor, concentration of the corrosive medium, and temperature affecting the corrosion rates were examined. The kinetic parameters were evaluated, and a suitable corrosion mechanism consistent with the kinetic results obtained is suggested and discussed.

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.

Synthesis and structural characterization of cobalt(III) complexes of hybrid donor-type didentate or tetradentate ligands bearing dimethylphosphino and thioether groups

Abstract The preparation and characterization of new hybrid donor-type phosphine-thioether ligands, a linear tetradentate Me2P(CH2)2S(CH2)3S(CH2)2PMe2 (dmdtn) and a didentate Me2P(CH2)2SR [R = Me (mtdmp) or Et (etdmp)], and their cobalt(III) complexes are described. The reaction of trans-[CoCl2(PY)4]C1 (py = pyridine) with dmdtn, followed by anion metathesis with NaPF6, afforded green crystals of trans(Cl,Cl-[CoCl2{meso(S)-dmdtn}]PF6 (1), whose geometrical structure and configuration around S atoms were determined by X-ray analysis. Further reaction of 1 with Li(acac) (acac = pentane-2,4-dionate) and crystallization with LiBF4 gave red crystals of the acac complex 2. The X-ray analysis of 2 confirmed the structure and configuration of cisβ-[Co(acac){racemic(S)-dmdtni}(BF4)2, indicating the configurational inversion at one sulfur during the derivation of complex 2. An analogous N,N-dimethyldithiocarbamate (dtc) complex 3 was also prepared, and the electrochemical properties of complexes 1–3 were discussed. For analogous mtdmp and etdmp (P-S) complexes, trans(Cl,Cl)cis(P,P)-[CoCl2(P-S)2]PF6 reacted with Li(acac) to give not only trans(P,S)-[Co(acac)(P-S)2](PF6)2 but also [Co(acac)2(P-S)]PF6. Similar reactions with Na(dtc) yielded cis-[Co(dtc)2(P-S-κP)2] (14 for P-S = mtdmp), together with trans(P,S)-[Co(dtc)(P-S)2](PF6)2 and [Co(dtc)2(P-S)]PF6. The structures of these complexes were characterized by NMR and UV-visible spectroscopy and X-ray analysis of 14. These results suggested that the didentate ligands were hemilabile as similar to previously described SPPS-type tetradentate ligands, but the tetradentate dmdtn ligand was not.

Estimation of the outer-sphere contribution to the activation volume for electron exchange reactions using the mean spherical approximation

Abstract The outer-sphere contribution to the volume of activation of homogeneous electron exchange reactions is estimated for selected solvents on the basis of the mean spherical approximation (MSA), and the calculated values are compared with those estimated by the Strank-Hush-Marcus (SHM) theory and with activation volumes obtained experimentally for the electron exchange reaction between tris(hexafluoroacetylacetonato)ruthenium(III) and -(II) in acetone, acetonitrile, methanol and chloroform. The MSA treatment, which recognizes the molecular nature of the solvent, does not improve significantly upon the continuous-dielectric SHM theory, which represents the experimental data adequately for the more polar solvents.

Kinetics and mechanism of oxidation of chondroitin-4-sulfate polysaccharide by chromic acid in aqueous perchlorate solutions

The kinetics of chromic acid oxidation of chondroitin-4-sulfate polysaccharide as sulfated carbohydrates at a constant ionic strength of 4.0 mol dm(-3) has been investigated, spectrophotometrically. The reaction kinetics showed a first-order dependence in chromic acid and fractional-first-order kinetics with respect to the chondroitin-4-sulfate concentration. The influence of [H(+)] on the reaction rates showed that the oxidation process is acid-catalyzed. Added Mn(2+) ions indicated the formation of Cr(IV) as intermediate species. A kinetic evidence for formation of 1:1 intermediate complex was revealed. The kinetic parameters have been evaluated and a tentative reaction mechanism in good consistent with the kinetic results obtained is discussed.

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).