Akiko Takao (Inagaki)

Visible-light promoted molecular transformations catalyzed by bimetallic species containing a [Ru(bipy)(3)](2+) (TB)-like fragment as the photosensitizing unit are reviewed. Catalytic reactions are classified according to the following two criteria: (1) electron transfer (A)/energy transfer (B) from TB and (2) intra- (I) and inter-molecular catalyst systems (II). Reactions promoted by electron transfer (A) involve reductive processes such as W-reduction giving H(2) and CO(2)-reduction giving CO, which have been extensively studied also by using mononuclear catalysts. The catalytic H(+)- and CO(2)-reductions have been considerably improved by the use of II-A- and I-A-type bimetallic catalysts, respectively. Furthermore, as recently reported by our research group, photocatalysis is extended to organic transformations, which have been much less explored compared to transformations of small inorganic molecules mentioned above. While Sonogashira coupling is mediated by II-A-type catalysts, up-hill trans-to-cis isomerization of cyanostilbene and dimerization of alpha-methylstyrene follow the energy transfer processes (B). Thus new aspects of the photochemical bimetallic catalysis have been unveiled as mentioned above but catalyst design is still in its infancy. Continued accumulation of reaction data and mechanism analysis will lead to development of practical bimetallic photocatalysts, which promote unique reactions including up-hill reactions. (C) 2009 Elsevier B.V. All rights reserved.

A series of organoruthenium complexes containing photochromic 1,2-di(2-methylthien-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene fragments (DTE), DTE-(RRuLm)(n) (RRuLm = (eta(6)-C6H5)Ru(eta(5)-C5Me5) (I): n = 1 (3), 2 (4); (eta(6)-C6H5)RuCl2(PPh3) (II): n = 1 (10), 2 (11); (eta(5)-C5Me4)Ru(CO)(2) (III): n = 1 (25), 2 (24); substituted at the 5-position of the thiophene ring), are prepared and characterized by H-1 NMR and UV-vis spectroscopy, and molecular structures of the 1:2 adducts 4, 11, and 24 are determined by X-ray crystallography, which reveals the antiparallel conformation of the two thiophene rings suitable for photocyclization, causing the photochromism. It is revealed that the metalated DTE derivatives show photochromic behavior through the photochemical, electrocyclic conrotatory ring-closing and -opening processes in a manner analogous to the organic counterparts, but the efficiency of the photochromic processes is dependent on the attached metal fragments. The cationic Cp*Ru(eta(6)-arene)-type (I) and neutral Cp*Ru(CO)(2)Cl-type complexes (III) show photochromic behavior (content of the closed isomers at the UV photostationary states: 25-88%), and subsequent visible light irradiation of the equilibrated mixtures reverts the process to afford the open forms quantitatively. The ring-closing and -opening processes are reversible and can be repeated without notable deterioration. As for the series of Ph derivatives of I (1, 3, and 4), metalation improves the efficiency of the ring-closing process. In contrast to these derivatives, no significant photochromic behavior is noted for the benzo-fused derivatives of I, and UV irradiation of II (10 and 11) causes irreversible dissociation of the arene ligand (DTE).

A series of organoruthenium complexes containing photochromic 1,2-di(2-methylthien-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene fragments (DTE), DTE-(RRuLm)(n) (RRuLm = (eta(6)-C6H5)Ru(eta(5)-C5Me5) (I): n = 1 (3), 2 (4); (eta(6)-C6H5)RuCl2(PPh3) (II): n = 1 (10), 2 (11); (eta(5)-C5Me4)Ru(CO)(2) (III): n = 1 (25), 2 (24); substituted at the 5-position of the thiophene ring), are prepared and characterized by H-1 NMR and UV-vis spectroscopy, and molecular structures of the 1:2 adducts 4, 11, and 24 are determined by X-ray crystallography, which reveals the antiparallel conformation of the two thiophene rings suitable for photocyclization, causing the photochromism. It is revealed that the metalated DTE derivatives show photochromic behavior through the photochemical, electrocyclic conrotatory ring-closing and -opening processes in a manner analogous to the organic counterparts, but the efficiency of the photochromic processes is dependent on the attached metal fragments. The cationic Cp*Ru(eta(6)-arene)-type (I) and neutral Cp*Ru(CO)(2)Cl-type complexes (III) show photochromic behavior (content of the closed isomers at the UV photostationary states: 25-88%), and subsequent visible light irradiation of the equilibrated mixtures reverts the process to afford the open forms quantitatively. The ring-closing and -opening processes are reversible and can be repeated without notable deterioration. As for the series of Ph derivatives of I (1, 3, and 4), metalation improves the efficiency of the ring-closing process. In contrast to these derivatives, no significant photochromic behavior is noted for the benzo-fused derivatives of I, and UV irradiation of II (10 and 11) causes irreversible dissociation of the arene ligand (DTE).

A tetranuclear C-2 (dicarbide) cluster complex with a permetallated ethene structure, (mu(4)-CC)Ru-2(FeCp*)(2)(CO)(10) (2), reacts with alkynes in refluxing THF to give a variety of adducts resulting from C-C coupling and oxidative metallacyclization with the C-2 ligand in addition to a unique mu(4)-dicarbyne complex with a dimetallacyclobutatriene core, (mu(4)-C-C)Ru-2(FeCp*)(2)(CO)(6)(mu-eta(2):eta(2)-PhCCPh). The reactions are initiated by decarbonylation, as confirmed by the reactions of an isolated, labile species lightly stabilized by MeCN, (mu(4)-CC)Ru-2(FeCp*)(2)(CO)(8)(NCMe)(2), which undergoes the reactions at room temperature and provides the same types of products as those obtained from 2.

Redox active polyiron complexes with higher dimensional spacers, the tetrairon complex with the two-dimensional tetra( ethynylphenyl) ethene spacer, (C=C)[p-C6H4-C = C-FeCp*(dppe)]4, and the diiron complex with the three-dimensional spacer (pseudo-m-[2,2] paracyclophane)[C equivalent to C-FeCp*(dppe)](2), are prepared, and their performance as molecular wires is evaluated on the basis of their comproportionation constants for the mixed valence state.