Sumitaka Itoh

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.

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.

Redox behavior of dimeric bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane) dicopper(I)/monomeric(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II) couple: detection of a metastable dimeric Cu(II)

Abstract The redox behavior of structurally constrained bis(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)dicopper (I)/(1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane)copper(II) couple was examined in acetonitrile: gated ET was observed even for this dimeric Cu(I)/monomeric Cu(II) couple. A dicopper(II) intermediate with a high-energy tetrahedral coordination geometry was spectrophotometrically identified for the first time for copper(II)/(I)-polypyridine redox couples. A low-intensity but low-energy LMCT band observed for the metastable dicopper(II) species indicates that gated ET process involves participation of a superexchange preceded by the deformation of ground-state Cu(II)N 4 (in trigonal bipyramidal or D 2d symmetry) to Cu(II)N 4 with a teterahedral geometry.

Redox behavior of copper(II)/(I) couples with quadridentate ligand: evidence of novel dual gated electron transfer reactions

Abstract The reduction reaction of Cu(II)L by decamethylferrocene, where L is quadridentate 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane, was examined in acetonitrile. We observed parallel gated electron transfer pathways: (1) regulated by the unimolecular deformation within Cu(II)L, and (2) regulated by the bimolecular dimerization to form Cu(II)2L2, prior to the electron transfer process with decamethylferrocene.