阿部 正明

A Pd(II) complex, Pd(TPrPc-OH) (1, TPrPc-OH = 9-hydroxy-2,7,12,17-tetrapropylporphycenato dianion), has been synthesized and characterized. H-1 NMR spectroscopy revealed that compound 1 exists as its enol form in solution. The H atom of the hydroxy group in 1 was exchanged with deuterium on addition of ethanol-d (6). UV-visible spectra showed a red shift of the Q band of 1 in THF compared with that of the acetoxy derivative Pd(TPrPc-OAc) (2, TPrPc-OAc = 9-acetoxy-2,7,12,17-tetrapropylporphycenato dianion). The pK (a) value of the hydroxy group in 1 was determined, by means of a UV-visible titration experiment, to be 10.56. A cyclic voltammogram of 1 in a mixture of THF and Britton-Robinson buffered aqueous solution revealed one-electron and one-proton coupled transfer in the oxidation process in the pH range from 2.7 to 10.5, which was identified by pH-varying experiments and the Pourbaix diagram. Transient absorption spectroscopy revealed that an electron-transfer reaction occurred from the triplet excited-state of 1 to 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone, DQ) upon pulse laser irradiation at 532 nm. Such an intermolecular photoinduced electron-transfer reaction was not observed between the Ni analog, Ni(TPrPc-OH), and DQ. The reaction rate constant, k (q), was indicative of a kinetic isotope effect with k (q(H))/k (q(D)) = 1.7, supporting the belief that the exited-state electron transfer from 1 to DQ is accompanied by proton transfer.

Hydrogenated tetrapropylporphycenes, 2,3-dihydro-2,7,12,17-tetrapropylporphycene 1 and its Ni-II complex 2, have been prepared and the hydrogenation effects on their electronic structure characterized. A one-electron reduction of 2 promotes dehalogenation of organic halides whose observation is unprecedented for the porphycene compounds.

Two new oxo-centred trinuclear ruthenium clusters supported by six dichloroacetate ligands, [Ru3(μ3-O)(μ-CHCl 2COO)6(CH3OH)3]CHCl2COO (1) and [Ru3(μ3-O)(μ-CHCl2COO)6(pyridine) 3] (2), have been synthesised and characterised by spectroscopic methods, electrospray ionisation mass spectrometry, single-crystal X-ray diffraction, and cyclic voltammetry. Due to the strong inductive effect of the dichloroacetate ligands, the redox potential of 2 was shifted to the positive side (∼1.0V or more) relative to the acetate analogue [Ru3(μ3-O) (μ-CH3COO)6(pyridine)3], and also the rate of pyridine/pyridine-d5 exchange reaction of 2 in CD3CN was retarded with the rate constant of kex298K=1. 9×10-8s-1 which is 105-fold smaller than the value for [Ru3(μ3-O)(μ-CH3COO) 6(pyridine)3]. Highly positive activation parameters obtained for 2, ΔH‡=138±7kJmol-1 and ΔS‡=71±20JK-1mol-1, illustrate a dissociative activation pathway in which rupture of the Ru-N(pyridine) bond is involved in the rate-determining step. © 2012 CSIRO.

The title compound, [Yb(C5HF6O2)(2)(C20H14N4)(H2O)]Cl center dot CH3OH center dot H2O, adopts an eight-coordinated geometry around the Yb-III atom consisting of a 4'-(4-pyridyl)-2,2': 6',2 ''-terpyridine (pytpy) ligand, two 1,1,1,5,5,5-hexafluoroacetylacetonate (hfac) anions and an aqua ligand. In the solid state, the compound forms supramolecular chains running along the b-axis via intermolecular hydrogen bonds between the Yb-OH2 unit and the N-atom donor of the 4-pyridyl pendant of pytpy, with an O center dot center dot center dot N distance of 2.686 (4) angstrom. A chloride counter-anion and lattice methanol and water solvent molecules occupy a hydrophilic columnar space along the coordination chains. O-H center dot center dot center dot Cl hydrogen bonds occur. The two water molecules and the four trifluoromethyl groups are disordered over two sets of sites, each with different occupancy ratios.

Three new mu-oxo-bis(mu-acetato)diruthenium(III) complexes,trans(mu-O,OH2)-[Ru2(mu-O)(mu-CH3COO)2(bpy)2(H2O)2][PF6]2.3H2O ([1][PF6]2.3H2O; bpy = 2,2'-bipyridine), trans(mu-O,CN)-[{Ru2(mu-O)(mu-CH3COO)2(bpy)2}2{mu Ag(CN)2}{mu Ag2(CN)3}][Ag(CN)2][Ag2(CN)3](CH3CN)4(CH2Cl2) (2), and cis(mu-O,CN)-[Ru2(mu-O)(mu-CH3COO)2(bpy)2(CN)2]center dot CH3OH center dot H2O([3]center dot CH3OH center dot H2O), have been prepared and their X-ray crystal structures determined. Whereas [1]2+ and 3 are discrete species, 2 is a polymer in which the AgCN groups act as bridges through RuCN(or NC)Ag linkages. The discrete complexes dissolve in common solvents, but 2 is only slightly soluble in acetonitrile and dimethyl formamide (DMF) to give fragmented species (denoted as 2') in solution. Redox waves of 3 were observed at 0.83 (Epc) and +0.76 V (E1/2) versus Ag/AgCl in a solution of 0.1 M nBu4NPF6 in CH2Cl2, which correspond to the Ru2II,III/Ru2III,III and Ru2III,III/Ru2III,IV processes, respectively. The corresponding redox couples (E1/2) of [1]2+ were observed at 0.31 and +0.76 V in 0.1 M NaClO4 aqueous solution. Significantly more negative potentials of the Ru2II,III/Ru2III,III couple for the two cyano complexes (E1/2 = 0.77 V for 2') are noted. Complexes [1]2+, 2', and 3 exhibit strong visible absorption bands at max = 566 nm (= 12600 M1cm1; in H2O), 636 nm (21000 M1cm1; in acetonitrile), and 558 nm (18000 M1cm-1; in H2O), respectively. Density functional theory (DFT) molecular orbital calculations have been carried out for [1]2+, trans(mu-O,CN)-[Ru2(mu-O)(mu-CH3COO)2(bpy)2(CN)2] (2a, a model compound for 2'), 3, [Ru2(mu-O)(mu-CH3COO)2(py)6]2+ ([4]2+), and trans(mu-O,py)-[Ru2(mu-O)(mu-CH3COO)2(bpy)2(py)2]2+ ([5]2+; py = pyridine). It was revealed that the frontier orbitals of these complexes are dominated by the Ru(dp)mu-O(pp) type p systems. For the cyano complexes, energies of these p-type MOs are lifted up by the interaction with CN, so that the energy difference with the upper bpy p(*) orbitals becomes smaller. Thus, whereas the visible absorption bands of [1]2+, [4]2+, and [5]2+ are ascribed to the transitions within the p system metal-to-metal charge transfer (MMCT), those of 2' and 3 are ascribed to the HOMO-to-ligand p(*) (bpy) transition metal-to-ligand charge transfer (MLCT) and to the mixture of MMCT and MLCT (HOMO1 to p(star)), respectively.

A series of porphycenes bearing acetoxy and hydroxy groups at the meso-positions have been prepared and fully characterized: H2TPrPc-OH, [Ni(TPrPc-OH)], H2TPrPc-OAc, and [Ni(TPrPc-OAc)], where TPrPc-OH: 9-hydroxy-2,7,12,17-tetrapropylporphycenato dianion and TPrPc-OAc: 9-acetoxy-2,7,12,17-tetrapropylporphycenato dianion. H2TPrPc-OH is structurally determined by single-crystal X-ray diffraction, showing the enol-form in the solid state. The introduction of the electron-donating hydroxy group to the meso-position decreases the HOMO-LUMO gap by ca. 0.2 eV relative to the unsubstituted parent porphycene due to the remarkable destabilization of the HOMO level. This is experimentally confirmed by red shift of the Q band in the UV-vis spectra (in CH2Cl2), negative shift of the E-1/2 (in 0.1 M n-Bu4NPF6-THF), and also supported by DFT calculations. In aqueous THF, an irreversible oxidation of H2TPrPc-OH and [Ni(TPrPc-OH)] appears to be pH-dependent, with a negative E-pa shift upon increasing the solution pH (in the region from 3.0 to 10.5) with slopes of -62 and -63 mV per pH unit for H2TPrPc-OH and [Ni(TPrPc-OH)], respectively, indicating the occurrence of {1H(+)/le} proton-coupled electron transfer.

This paper reports the synthesis and characterization of a novel series of chain-like compounds where oxo-centered triruthenium cluster moieties are bridged by 4,4'-bipyridine (4,4'-bpy) spacers. A reaction of solvent-coordinated triruthenium "monomer" precursor [Ru(3)O(CH(3)CO(2))(6)(CO)(CH(3)OH)(2)] with a 0.1 equimolar amount of 4,4'-bpy in CH(3)OH gave mixture of chain-like compounds containing "dimers" to "tetramers" which were cleanly separated by column chromatography and characterized by spectroscopic and electrochemical methods. Cyclic voltammetry revealed that all chain-like compounds exhibit reversible and stepwise redox processes in solution with very weak intramolecular coupling between the triruthenium components across the 4,4'-bpy bridge. Photo-induced dissociation of CO from the compounds and electrode surface binding were also investigated.

Recent progress in the redox chemistry of multimetallic transition-metal complexes and clusters is reviewed. Special attention is paid to high-nuclearity complexes wherein redox-active sites (e.g., ferrocenyl pendants or Ru centers) are assembled in a close proximity with a well-defined geometry via the covalent or coordination bonds. A special emphasis is placed on literature survey for paddlewheel diruthenium(II,III) complexes coordinated by axial or equatorial ferrocenyl ligands, polyferrocenyl compounds in the form of wire, star, and ring, ferrocenyl-based dendrimers, and ligand-mediated molecular triangles and squares. Finally, future prospects of these mixed-valent systems are briefly discussed.

2,7,12,17-Tetra-n-propylporphycenatodihalotin(IV) complexes ([Sn(TPrPc)X-2], X = F, Cl, and Br) were synthesized and their photophysical properties were investigated. These complexes exhibit strong absorptions in the red region (600-630 nm). Strong quenching was observed during the fluorescence emission of the dibromotin(IV) complex, which is caused by an intramolecular heavy-atom effect. The complexes emit NIR phosphorescence in 1-iodopropane at room temperature and the intensity of the emission of the dibromotin(IV) complex is found to be smaller than that of the dichlorotin(IV) complex. The quantum efficiency (Phi(Delta)) of generation of singlet oxygen using the dibromotin(IV) complex IS the highest among the porphycenatotin(IV) complexes (Phi(Delta) = 0.79). Photooxidation of 1,3-diphenylisobenzofuran catalyzed by the complexes upon irradiation with red light (over 600 nut) was studied and a rapid oxidation of the substrate was observed when using the dibromotin(IV) complex.

At the right places: Owing to substitution-lability difference in disparate metal centers, an oxo-centered heterometallic trinuclear acetato complex [Ru2MgO(CH3COO)6(py)3] (py = pyridine; see picture) undergoes site-selective substitution of propionates and ferrocenycarboxylates in a stepwise manner. The new substituted families have been successfully separated by chromatography and fully characterized.

Vitamin B(12)-TiO(2) heterogeneous hybrid catalyst (B(12)-TiO(2)) mediated ring-expansion reactions and their reaction mechanism were clarified by various spectroscopic methods. Diffuse reflectance UV/VIS (DR-UV/VIS) studies showed that B(12)-TiO(2) was activated to form the supernucleophilic Co(I) species of B(12) by irradiation with ultraviolet light. DR-UV/VIS and electron spin resonance (ESR) studies then elucidated the formation of the photo-sensitive intermediate on heterogeneous surfaces. The photo-excited B(12)-TiO(2) catalyzed ring-expansion reactions were efficient and eco-friendly. This catalytic efficiency was strongly dependent on the kind of solvent. The oxidation reaction of the solvent by holes at TiO(2) nanoparticle surfaces had a crucial role in the overall catalytic reaction.

A novel Sn(IV) porphycene-ferrocene triad molecule, trans-bis(ferrocenecarboxylato)(2,3,6,7,12,13,16,17-octa-ethylporphycenato)tin(IV), [Sn-IV(OEPc)(FcCOO)(2)] (2), was synthesized and fully characterized by various spectroscopic methods. This is the first example of a metalloporphycene triad linked by the axial coordination of two functionalized units to a metallocenter. The steady-state fluorescence measurement indicated the efficient fluorescence quenching by coordination of the ferrocenecarboxylic acid in comparison to the corresponding dihydroxy-Sn(IV) porphycene, [Sn-IV(OEPc)(OH)(2)] (1) (1, Phi(F) = 0,094; 2, Phi(F) = 0.01). The electron transfer process from the ferrocene units to the excited Sn(IV) porphycene was directly observed by subpicosecond transient absorption spectroscopy in acetonitrile (polar solvent) and toluene (nonpolar solvent). In acetonitrile, the transient species attributed to the Sn(IV) porphycene radical anion was observed at 750 and 850 nm within 1 ps after the excitation, and then the generated charge separation state disappeared with a value of 6.9 x 10(11) s(-1) for the time constant. On the other hand, the generated charge separation state decayed with two components, 3.9 x 10(11) and 9.6 x 10(9) s(-1) time constants, in toluene. For the observed two-component decay in toluene, a significant equilibrium between the charge separation state and the triplet state was proposed because these energy levels are close to each other. Therefore, the solvent-polarity-de pendent long-lived charge separation state was obtained in the Sn(IV) porphycene-ferrocene triad system. The electron transfer upon excitation of the Sn(IV) porphyrin of [Sn-IV(OEP)-(FcCOO)(2)] (4), in which OEP denotes the 2,3,6,7,12,13,16,17-octaethylporphyrin ligand, was observed. However, no equilibrium between the charge separation and the triplet states was observed in both the acetonitrile and toluene, The difference in the charge recombination processes of the Sn(IV)-porphycene and -porphyrin is due to the small HOMO-LUMO gap and the large driving force (-Delta G(CS)) of 2 compared to that of 4, which resulted in the energy level of the charge separation state close to the triplet state in toluene. Furthermore, the large driving force (-Delta G(CS)) of 2 compared to that of 4 is attributed to the significant stabilization of the LUMO energy level caused by a decrease in the molecular symmetry and a large porphycene pi-electron framework. This result indicates that porphycenes are excellent candidates as an electron acceptor in photoinduced electron transfer systems.

We present here an in situ scanning tunneling microscopy (STM) study of potential-induced reactions of oxo-centered acetato-bridged triruthenium clusters on Au(111) under electrochemical conditions, in which (i) reversible interconversion between two different redox states of the cluster and (ii) spontaneous dissociation of CO from the cluster were probed and visualized. It is known that the triruthenium complex [Ru-3(mu O3-)(mu-CH3COO)(6)(CO)(4-methylpyridine){(NC5H4)-CH2NHC(O)(CH2)(10)S}] (1), which is in the mixed-valent {(Ru-II-CO)(RuRuIII)-Ru-III} state, on Au(111) is one-electron oxidized to cationic complex 1(+) which corresponds to a {(Ru-III-CO)(RuRuIII)-Ru-III} oxidation state. The redox reaction is reversible on a cyclic voltammetry timescale. Complex 1(+) undergoes irreversible dissociation of CO in an aqueous phase on a longer timescale (ca. minutes) to fonn an aqua ligand-coordinated complex, 2(+), which is expressed as ((Ru-III-OH2)(RuRuIII)-Ru-III). The reversible redox reaction (1 reversible arrow 1(+)) and the irreversible ligand substitution reaction (1(+) -> 2(+)) were independently examined with in situ STM in a monolayer level by visualizing molecular-sized spots showing a different extent of brightness in STM images. We show here that the molecular-sized spots corresponding to 1, 1(+), and 2(+) are resolved by their brightness, which strongly depends on both the oxidation states and the ligand nature of the clusters. By employing multiple fast scans at an applied potential of +0.80 V vs. Ag/AgCl, we obtained STM images that follow the irreversible 1(+) -> 2(+) reaction on the surface, from which a rate constant of the CO release was calculated to be 1.9 (+/- 0.2) x 10(-2)s(-1) (25 degrees C; in contact with 0.1 mol dm(-3) aqueous HClO4 solution). The difference in brightness of the molecular spots is rationalized in terms of orbital-mediated tunneling by considering the difference in electronic states of the d pi-p pi system in the mu 3-O trirutheniurn cluster.

2,3,6,7,12,13,16,17-Octaethylhemiporphycenato Sn(IV) chloride, [Sn-IV(OEHPC)Cl-2], and 2,3,6,7,12,13,16,17-octaethyl porphycenato Sn(IV) chloride, [Sn-IV(OEPC)Cl-2], were synthesized in high yields and fully characterized by various spectroscopic methods. The X-ray crystal structures of the Sn(IV) complexes with porphycene and hemiporphycene were determined for the first time. The photophysical and photochemical properties of the singlet state of the Sn(IV) porphycene and hemiporphycene complexes, structural isomers of porphyrin, have been investigated by fluorescence and phosphorescence spectroscopies. The relatively strong emission and long fluorescence lifetime of the Sn(IV) porphycene complexes indicated by the fluorescence quantum yield and lifetime measurement were observed in the case of the Sn(IV) porphycene ([Sn-Iv(OEPc)Cl-2], Phi(F) = 0.125, tau(S) = 2681 ps; [Sn-IV(OEP)Cl-2], Phi(F) = 0.010, tau(S) = 438 pS; [Sn-IV(OEHPC)Cl-2], Phi(F) = 0.027, tau(s) = 733 ps). The triplet state of the Sn(IV) complexes was investigated by transient absorption spectroscopy. It became clear that the triplet lifetime of the Sn(IV) porphycene (tau(T) = 49.9 mu s) was longer when compared to those of the Sn(IV) porphyrin (tau(T) = 32.6 ps) and hemiporphycene complexes (tau(T) = 28.3 mu s). These porphyrin isomer Sn(IV) complexes showed the high singlet oxygen generating ability, and the photo-oxidation of the 1,5-hydroxynaphthalene mediated by the Sn(IV) porphycene was the most effective among the complexes. This result is due to its more effective light absorption in the visible region and indicated that the porphycene is an excellent candidate as a photosensitizer.

We report here an unprecedented utility of redox-active transition-metal complexes as heat-shielding materials. A poly(vinyl alcohol) film containing mixed-valent cluster [Ru3-(mu(3)-O)(mu-CH3CO2)(6)(mbpy(+))(3)](PF6)(3) (1) (mbpy(+) = 1-methyl-4,4'-bipyridinium cation), which strongly absorbs light in the near-infrared region, shows heat-shielding characteristics whose efficiency is found to highly depend on the redox state of the cluster dye.

New synthetic precursors for CO-coordinated, oxo-centred triruthenium(II,III,III) complexes with varied, carboxylato groups [Ru 3O(RCO2)6(CO)(solvent)2] where R = C2H5, CH3 and C6H5 have been prepared and characterised. Among the series, the propionato-bridged complex [Ru3O(C2H5CO2) 6(CO)(THF)2] (1) was structurally determined by single-crystal X-ray crystallography. Cyclic voltammetry revealed that the benzoato-bridged complex [Ru3O(C6H5CO 2)6(CO)(C2H5OH)2] in 0.1 mol dm-3 nBu4NPF6/DMF undergoes two consecutive one-electron processes assignable to the redox of the Ru3(μ 3-O) core, the redox potentials of which are highly dependent on the electron-donating/accepting nature of the carboxylato groups. UV-light excitation of the compounds in CH3CN resulted in the dissociation of CO to form the tris(CH3CN) compounds [Ru3O(RCO 2)6(CH3CN)3] (R = C 2H5, CH3 and C6H5). These compounds are highly useful as synthetic precursors to a wide range of μ3-oxo-trirutherium derivatives with well-defined redox responses. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

A new strategy is proposed for the fabrication of photochemical pattern of a self-assembled monolayer (SAM) of an oxo-centered tri-ruthenium (Ru 3) cluster on a gold electrode surface. The fabrication was achieved under combined photochemical and electrochemical controls. By this strategy, the patterned SAM surfaces terminated with different kinds of moiety groups such as carbon monoxide (CO), nitric oxide (NO) and a successive layer of Ru 3 cluster by ligand-exchange reaction are obtained with high yields. © 2009 The Royal Society of Chemistry.

A new molybdenum porphycene complex, which is a new complex having oxo and chloro groups at the axial positions, was successfully synthesized. This chloro(oxo) molybdenum(V) complex, [Mo-V(OEPc)(O)Cl] (1) has strong absorption bands in the visible region attributed to the properties of the porphycene and is very attractive from a photochemical viewpoint. This complex was readily reduced from Mo(V) to Mo(IV) species to yield [Mo-IV(OEPc)(O)] under anaerobic irradiation by visible light. This photochemical reaction mechanism is derived from the homolytic cleavage of the axial ligand. Furthermore, this photochemical reduction proceeded very effectively compared to that for the corresponding porphyrin complex.

This paper reports synthesis and electrochemistry of six-coordinate ruthenium porphyrin isomers including six novel porphycene complexes with substituted pyridines in the axial positions, in which redox potential control of reversible Ru-II/R-III processes is successfully achieved over a 0.7-V potential region by varying the axial ligands and tetrapyrrole ligand structure and substituents.

Coordination of nitric oxide (NO) to a self-assembled monolayer (SAM) of a triruthenium (Ru-3) cluster, [Ru-3(mu(3)-O)(mu-CH3COO)(6)(CO)(L-1)(L-2)](0)(L-1 =[(NC5H4)CH2NHC(O)(CH2)(10)S-](2),L-2=4-methylpyridine), on a gold electrode surface has been studied by electrochemical and in situ infrared (IR) spectroscopic measurements. Ligand substitution reaction of NO for carbon monoxide (CO) ligands in the SAM strongly depends on the oxidation state of the terminal Ru-3 cluster. NO can be introduced into the Ru-3 Cluster in the SAM with a high yield after one-electron oxidation of the Ru-3 core to a oxidation state, while no coordination reaction occurs at the initial oxidation state of the Ru-3 cluster. The kinetics of the NO coordination and desorption processes is also evaluated by time-resolved in situ IR spectroscopy. Finally, we demonstrate that the SAM with NO/CO randomly mixed ligands at a desired ratio can be constructed on the gold surface by tuning a suitable oxidation state of the Ru-3 cluster under electrochemical control.

Interfacial ligand substitution of triruthenium(III) complexes confined on gold as a monolayer has proven to be useful to construct molecular heterostructures by quantitatively incorporating 4,4'-bipyridine-anchored diruthenium(III) complexes, whose multi-step redox sequence is reversibly tuned by proton-coupled electron-transfer reactions.

An in situ approach for rapid growth of multilayers of redox-active triruthenium cluster complexes has been developed under electrochemical potential control of the gold electrode, the formation process being successfully probed by cyclic voltammetry and AFM images.

Densely packed redox-active self-assembled monolayers (SAMs) of monocarbonyl trimthenium complexes [Ru-3-(mu(3)-O)(mu-CH3COO)(6)(CO)(MPY)(C10PY)] (1) (mpy, 4-methylpyridine; C10PY, {(NC5H4)CH2NHC(O)(CH2)(10)S-}(2)), [Ru-3(mu(3)-O)(mu-CH3COO)(6)(CO)(mu-C10PY)] (2), and [{Ru-3(mu(3)-O)(mu-CH3COO)(6)(CO)(mpy)}(2)(eta-C10PY)] (3) have been constructed on an Au(111) electrode surface. They were characterized by infrared reflection absorption spectroscopy (IRRAS), spectroscopic ellipsometry, and contact angle measurement as well as cyclic voltammetry. Redox potential of the SAMs of 1 at the interface of aqueous electrolyte solutions shifted negatively as the concentration of HClO4 increased from 0.01 to 1.0 mol dm(-3) with the slope of 0.059 V/decade and as the electrolyte was changed from H2SO4 (0.79 V vs. Ag vertical bar AgCl) to HNO3 (0.69 V) and HClO4 (0.59 V) at a fixed concentration of 0.1 mol dm(-3). No significant difference in the redox potential was observed between the acid and its sodium salt. The effect is explained, as previously reported for the ferrocenylalkanethiol and non-carbonyl triruthenium SAMs, in terms of the effective ion pair formation in the order ClO4- > NO3- > SO42-. The rate of the CO dissociation reaction of the complex in the SAMs (complete within a few minutes at room temperature) that occurs in the oxidation state of Ru-3(III,III,III) was not very sensitive to the applied electrode potential (0.6-4.8 V), but increased by ca. 2.5 times as the concentration of HClO4 (0-01-1.0 mol dm(-3)) was lowered, and by ca. one-order of magnitude as the electrolyte anions was changed from ClO4- to SO42- and NO3-.

Two consecutive one-electron reductions of the mixed-metal trinuclear complexes [RuIII2MII(μ3-O)(μ- CH3COO)6-(S)(bzpy)2] {(M = Co, S = H 2O) and (M = Ni, S = N,N′-dimethylformamide); bzpy = 4-benzoylpyridine} in acetonitrile give stable "double" mixed-valence states where both Ru2II,III and (bzpy-)(bzpy) states coexist. Splitting of the ligand-based redox waves, (bzpy) 2/(bzpy-)(bzpy)/(bzpy-)2, is 0.35 and 0.34 V for the Co and Ni complexes, respectively. © 2007 The Chemical Society of Japan.

An oxo-bridged diruthenium(III) complex was immobilized on a Au(111) electrode through the coordination by terminal pyridyl groups of self-assembled monolayer of alkyl chains, and the immobilized complex underwent proton-coupled electron-transfer reactions in a very wide pH range (2-12) with remarkable monolayer durability for multiple potential cycles.

Reactions of a Re-VII complex [Re-VII(O)(3)(tpenH-kappa(3) N,N',N"')ClO4)(2) ([1](ClO4)(2]) (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) that has a free bis(2-pyridylmethyl)amino group with Fe-III and Cu-II (or Cu-I) complexes in solution afforded three new mixed-metal complexes. From X-ray structural analyses, the counter anions (EO4-) of the isolated complexes are a mixture of ReO4- and the original ClO4-. The former was most likely produced by the decomposition of [1](2+) during reaction. The tetranuclear Re-VII-Fe-III complex [{Re-VII(O)(3)(mu-tpen-kappa(3) N,N'',N'": kappa(3) N',N"'',N'''')}(2)Fe-2(III)(mu-O)(mu-CH3COO)(2)](4+) ([2](4+)) has an oxodi(acetato)-bridged diiron(III) unit with two [Re-VII (O)(3)(tpen-kappa(3) N,N'',N'")](+) groups coordinated to each iron(III) ion. Structures of the dinuclear Re-VII-Cu-II complexes depend on the reactants. The reactions with Cu-II(ClO4)(2) in CH3OH/CH3CN and Cu-I(PPh3)(2)CH3CN in CH2Cl2/CH3CN afforded [Re-VII(O)(3)(mu-tpen-kappa(3) N,N'',N'":kappa(3),N',N'''',N'''')Cu-II(CH3CN)](3+) ([3](3+)) and [Re-VIII(O)(3)(mu-tpen-kappa(3) N,N",N''':kappa N-3',N'''',N'''')(CuCl)-Cl-II](2+) ([4](2+)), respectively. The Cu centers have an axially elongated octahedral structure with a bis(2-pyridylmethyl)amino moiety and CH3CN for [3](3+) or Cl- of solvent origin for [4](2+) coordinated in the tetragonal plane. The axial sites are occupied by two weakly coordinated EO4- ligands in [3](3+) or EO4- and bridging Cl- from a neighboring complex cation in [4](2+).

UV-vis and H-1 NMR spectra provide unambiguous evidences for the disproportionation reactions of ((ReO)-O-V(tpen)](3+) to give [(ReO)-O-IV(tpen)](2+) and [Re-VII(O)(3)(tpenH-kappa(3) N,N",N''')](2+) (tpen = N,N,N',N'-tetrakis(2-pyridyimethyl)ethylenediamine) in 2:1 ratio with the important participation of water molecules.

Oxidation states and CO ligand exchange kinetics in a self-as-sembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru3(μ 3-O)(μ-CH3COO)6(CO)(L1)(L 2)] (L1 = [(NC5H4)CH 2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coor dination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA. Weinheim.

Photoresponsive self-assembled monolayers (SAMs): Photoexcitation of a CO-bearing trinuclear ruthenium cluster assembled on an Au(111) electrode results in dissociation of CO to form a H2O-terminated SAM (see picture). This is the first well-defined example of clean conversion of a redox-active molecular film by using light stimulus. (Figure Presented).

Photodissociation of CO from oxo-centered trinuclear ruthenium clusters [Ru3(μ3-O)(μ-OOCCH3) 6(CO)L2] (L = pyridine (py): 1; 4-cyanopyridine (cpy): 2; methanol: 3) dissolved in organic solvents has been examined. Upon photolysis (≥290 nm, a 450-W Xe lamp), an absorption peak at 585 nm observed for 1 in CH3CN decreases its intensity and a new absorption band appears and grows at 896 nm. This spectral change, presenting isosbestic points, corresponds to photosubstitution of CO in 1 to form [Ru3(μ 3-O)(μ-OOCCH3)6(CH3CN)(py) 2] 4. Photoexcitation of carbonyl complexes 2 and 3 in CH 3CN also affords the corresponding CH3CN-coordinated complexes [Ru3(μ3-O)(μ-OOCCH3(CH 3CN)(cpy)2] 6 and [Ru3(μ 3-O)(μ-OOCCH3)6(CH3CN) 3] 7, respectively. The photosubstitution reactions (excitation wavelength, ≥290 nm) are well described by the first-order kinetics: k = 7.3 × 10-4 5-1 for 1, 4.9 × 10-4 s -1 for 2 and 5.1 × 10-4 s-1 for 3 (298 K). In the presence of a 100-fold excess of py, photolysis of 1 yields a tris(py) complex [Ru3(μ3-O)(μ-OOCCH 3)6(py)3] 5 via photochemical loss of CO followed by coordination of py. The overall reaction (photochemical and thermal) is also confirmed by 1H NMR spectroscopy. The dissociative character of the photosubstitution is supported by negligible effects of the concentration of the entering pyridine molecule, the nature of solvents and the type of terminal monodentate ligands (other than CO) attached to the cluster. Quantum yield measurements with varied excitation wavelengths have shown that absorption bands located in the UV region (<400 nm) play a principal role in photosubstitution, whereas an absorption band in the visible region (centered at ∼580 nm), ascribed to an "intracluster" charge transfer, is not at all responsible for photosubstitution.

A highly selective ligand exchange reaction is realized in the self-assembled monolayer (SAM) of a triruthenium cluster on a gold electrode surface under precise electrochemical potential control. CO as well as NO molecules, which are known to play key roles in many chemical, biological, and environmental systems, can be efficiently introduced into the SAM by electrochemically tuning the electronic state of the Ru site. These unique surface reactions are more convenient and efficient than conventional ligand exchange reactions in solution and could be used for the elucidation of the electron-transfer mechanism in a biological system as well as in the development of molecular sensors and devices.

Reactions of oxo-centered triruthenium acetate complexes [Ru3O(OAc)6(py)2(CH3OH)](PF6) (py = pyridine, OAc = CH3COO-) (1) with nearly equimolar amounts of dppa [bis(diphenylphosphino)acetylene] or dppen [trans-1,2-bis(diphenylphosphino)ethylene] gave [Ru3O(OAc)6(py)2(L)](PF6) (L = dppa, 2; dppen, 3). With 2.4 equiv of 1, the reactions provided diphosphine-linked triruthenium dimers, [[Ru3O(OAc)6(py)2]2(L)](PF6)2 (L = dppa, 4; L = dppen, 5), respectively. Similarly, the reactions of [Ru3O(OAc)6(L')2(MeOH)]+ [L' = dmap (4-(dimethylamino)pyridine), 1a; L' = abco (1-azabicyclo[2.2.2]octane), 1b] with dppen gave dppen-linked dimers, [[Ru3O(OAc)6(dmap)2]2(dppen)](SbF6)2 (6) and [[Ru3O(OAc)6(abco)2]2(dppen)](BF4)2 (7), respectively. The chemical reduction of 2, 4, and 5 by hydrazine afforded one- or two-electron-reduced, neutral products, Ru3O(OAc)6(py)2(dppa) (2a), [Ru3O(OAc)6(py)2]2(dppa) (4a), and [Ru3O(OAc)6(py)2]2(dppen) (5a), respectively. The complexes were characterized by elemental analyses, ES-MS, UV-vis, IR, and 31P NMR spectroscopies, and cyclic and differential-pulse voltammetries. The molecular structures of compounds 2, 4, 5, 5a, 6, and 7 were determined by single-crystal X-ray diffraction. In 0.1 M (Bu4N)PF6-acetone, the monomers and dimers of triruthenium clusters show reversible and multistep redox responses. The two triruthenium cluster centers in dimers undergo stepwise reductions and oxidations due to the identical redox processes of the individual Ru3O cluster cores, suggesting the presence of electronic communications between them through the conjugated diphosphine spacer. The redox wave splitting mediated by dppa containing an ethynyl group (C triple bond C) is found to be more extensive than that by dppen containing an ethenyl (C=C) one. It appears that the redox wave splitting is enhanced by the introduction of electron-donating substituents on the auxiliary pyridine rings.

Interfacial electron-transfer properties and CO-dissociation reactions at redox-active multilayers which were prepared from trinuclear ruthenium cluster molecules have been examined by cyclic voltammetry. The series of multilayers (up to five layers) assembled on gold electrodes show a single-step, reversible, multi-electron redox processes whose redox potentials are finely tuned by the number of Ru 3 components involved in the layers and are also affected by the nature of electrolyte anions. Electrochemical oxidation of the surface-confined CO-terminated multilayers (0.1 M HClO 4 aq., 298 K) induces dissociation of CO from the top layer, resulting in the change of the cyclic voltammograms. The redox-induced CO-dissociation reaction for each of the multilayers obeys the first-order kinetics, in which the rate constants range in the order 10 -2 s -1 (298 K).

The ligand-ligand redox interaction separated by di-, tri-, and hexanuclear cluster units is discussed in terms of the splitting of the redox waves (ΔE (L)) and the comproportionation constants (K com(L)) of the ligand-based mixed valence state. Although two redox-active monodentate ligands in the mononuclear ruthenium(II) complexes (either cis or trans configurations) do not show appreciable splitting in their reduction waves, interestingly those separated by giant triruthenium and hexarhenium cluster units clearly show splitting. The molecular orbital considerations in the literature suggest that these units possess some π-type molecular orbitals composed of metal dπ and possibly ligand pπ orbitals. Absence of the redox interactions in oxo-bridged diruthenium(II) and oxo-centered trirhodium(III) complexes where such π molecular orbitals (including an antibonding one) are fully occupied, indicates the importance of empty π* orbitals for the ligand redox communication. © 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

Hexarhenium(III) complexes with terminal isothiocyanate ligands, [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)(NCS)(6)] (1) and (L)(4)[Re(6)(mu(3)-Se)(8)(NCS)(6)] (L(+) = PPN(+) (2a), (n-C(4)H(9))(4)N(+) (2b)), have been prepared by three different methods. Complex 1 was prepared by the reaction of [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)Cl(6)] with molten KSCN at 200 degrees C, while 2b was obtained by refluxing the chlorobenzene-DMF (2:1 v/v) solution of [Re(6)(mu(3)-Se)(8)(CH(3)CN)(6)](SbF(6))(2) and [(n-C(4)H(9))(4)N]SCN. The [Re(6)(mu(3)-Se)(8)(NCS)(6)](4)(-) anion was also obtained from a mixture of Cs(2)[Re(6)(mu(3)-Se)(8)Br(4)] and KSCN in C(2)H(5)OH by a mechanochemical activation at room temperature for 20 h and isolated as 2a. The X-ray structures of 1 and 2a.4DMF have been determined (1, C(70)H(144)N(10)S(14)Re(6), monoclinic, space group P2(1)/n (No. 14), a = 14.464(7) A, b = 22.059(6) A, c = 16.642(8) A, beta = 113.62(3) degrees, V = 4864(3) A(3), Z = 2; 2a.4DMF, C(162)H(144)N(14)O(4)P(8)S(6)Se(8)Re(6), triclinic, space group P1 (No. 2), a = 15.263(2) A, b = 16.429(2) A, c = 17.111(3) A, alpha = 84.07(1) degrees, beta = 84.95(1) degrees, gamma = 74.21(1) degrees, V = 4098.3(8) A(3), Z = 1). All the NCS(-) ligands in both complexes are coordinated to the metal center via nitrogen site with the Re-N distances in the range of 2.07-2.13 A. The redox potentials of the reversible Re(III)(6)/Re(III)(5)Re(IV) process in acetonitrile are +0.84 and +0.70 V vs. Ag/AgCl for [Re(6)(mu(3)-S)(8)(NCS)(6)](4)(-) and [Re(6)(mu(3)-Se)(8)(NCS)(6)](4)(-), respectively, which are the most positive among the known hexarhenium complexes with six terminal anionic ligands. The complexes show strong red luminescence with the emission maxima (lambda(max)/nm), lifetimes (tau(em)/micros), and quantum yields (phi(em)) being 745 and 715, 10.4 and 11.8, and 0.091 and 0.15 for 1 and 2b, respectively, in acetonitrile. The data reasonably well fit in the energy-gap plots of other hexarhenium(III) complexes. The temperature dependence of the emission spectra and tau(em) of 1 and [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)Cl(6)] are also reported.

Layer-by-layer metal-cluster molecular multilayers are fabricated onto preorganized self-assembled monolayers on Au electrode surfaces. The molecular building block, [Ru3(μ3-O)(μ-CH3COO)6 (bpy)2(CO)] (bpy=4,4′-bipyridine), is applied by displacing the water ligand on the cluster core (see picture), which is precisely controlled through the electrode potential. The series of multilayers transport electrons across the molecularly linked backbone.

Reaction of ReVOCl3(PPh3)2 with an equimolar amount of tpen affords fac-[ReVIIO3 (η3-tpenH)](ClO4)2 (1), in which the tpen ligand coordinates to the fac-ReVIIO3 center in a η3-mode, and the dangling bis(2-pyridylmethyl)amine arm in 1 is subsequently used to produce a heterobimetallic complex fac-,fac-[Re VIIO3 (μ-η3:η 3-tpen)Re1 (CO)3](ClO4)2.

A series of (μ-oxo)bis(μ-acetato)diruthenium(III) complexes containing two pyridine (py) ligands and varied N-heterocyclic ligands in the positions trans and cis to μ-O, respectively, have been prepared to study py/py-d5 exchange reactions using 1H NMR spectroscopy. The diruthenium(III) complexes under investigation are [Ru2(μ-O)(μ-CH3COO)2(py)6] (PF6)2 (1), [Ru2(μ-O)(μ-CH3COO)2(bpy)2 (py)2](PF6)2 (2), [Ru2(μ-O)(μ-CH3COO)2(acpy) 4(py)2](FF6)2 (3), and [Ru2(μ-O)(μ-CH3COO)2(dmbpy) 2(py)2](PF6)2 (4), where bpy=2,2′-bipyridine, acpy=4-acetylpyridine, and dmbpy=4,4′-dimethyl-2,2′-bipyridine. Pseudo-first order rate constants for the ligand-exchange reactions are 10-6-10-5 s-1 for 1-4 in CD3CN at 298 K. It is found that the rate of the py/py-d5 exchange reactions is controlled by the electronic nature of the cis-oriented ancillary ligands, while the exchange mechanisms are tuned principally by the ligand steric factors. The activation parameters (ΔH‡ and ΔS‡) indicate that exchange reactions proceed through the dissociative (D) or the interchange dissociative (Id) mechanism for 1 and 3. Negative ΔS‡ values observed for 2 and 4 suggest a significant contribution of incoming ligands to the exchange pathway. The kinetic and thermodynamic parameters for the diruthenium series and the corresponding data for Ru-based oxo bridged trinuclear complexes established previously are compared and discussed. © 2002 Elsevier Science B.V. All rights reserved.

A porphyrin ball: An octahedral arrangement of six porphyrins is achieved by the coordination of 5-(4-pyridyl)-10,15,20-tritolylporphyrins (H2PyT3P) to the octa(μ3-S/μ3-Se)hexarhenium(III) core. Six porphyrin ligands are arranged virtually in an S6 manner as the X-ray structural analysis of [Re6(μ3-S)8(H2PyT 3P)6](SbF6)2 shows (see picture).

A series of the octahedral hexarhenium(III) complexes containing a variable number of diphosphine (diphos) or diphosphine-monoxide (diphosO) ligands have been prepared by the substitution of the diphosphine Ph2P(CH2)nPPh2 (n=1 to 5) for the iodide ions in the parent octahedral hexarhenium cluster compound [Re6Se8I6]3-. The diphosphine Ph2P(CH2)nPPh2 ligands adopt an η1-bonding mode with the Re6(μ3-Se)8 core, and the P donor atom in the pendant arm is noncoordinated and oxygenated in most cases. The series of new hexarhenium(III) complexes have been well-defined by 1H, 13C, and 31P NMR spectroscopic and FAB-MS data. Four compounds among the series were characterized by X-ray structural determination. Geometrical isomers were identified by NMR spectroscopy as well as by the structural determinations. The apical ligand substitution induces significant change in the redox potentials and the photophysical properties of the Re6(μ3-Se)8 core. The E1/2 value of the reversible process ReIII6/ReIII5ReIV becomes more positive with the increasing number of the coordinated P donors. The phosphine-substituted hexarhenium(III) derivatives are highly luminescent, with microsecond scale emissive lifetime at ambient temperature, and the fully substituted derivatives with the formula [Re6Se8(η1 -diphosO)6]2+ display the strongest luminescence with the longest emission lifetimes.

Ru(bpy)2(CE-bpy)2+ was prepared where bpy and CE-bpy were 2,2′-bipyridine and bpy having a crown-ether moiety at the 3,3′ positions, respectively. Although the Ru(II) complex showed only very weak emission in acetonitrile, recognition of Na+ or Li+ by the crown-ether moiety in CE-bpy resulted in an increase in the emission intensity of the complex.

A terminal ligand-substitution reaction at triruthenium redox centres within monolayer films is investigated. For this purpose, a new redox-active trinuclear ruthenium complex containing one carbonyl ligand at a terminal site, [Ru3(O)(CH3COO)6(CO)(mpy)(C10PY)](mpy = 4-methylpyridine, C10PY = {(NC5H4)CH2NHC(O)(CH2) 10S-}2)1, has been chemically adsorbed onto a gold electrode surface with a disulfide-alkyl spacer C10PY. Densely packed monolayers of complex 1 on Au electrodes (surface coverage, Γ(Ru3) = 1.8 × 10-10 mol cm-2) display the {RuII-CO}RuIIIRuIII/{RuIII-CO}Ru IIIRuIII couple at E1/2 = +0.61 V vs. Ag-AgCl in a 0.1 mol dm-3 HClO4 aqueous solution. Upon electrochemical oxidation of the adsorbed complex at +0.80 V, where the triruthenium carbonyl complex is in the oxidized form {RuIII-CO}RuIIIRuIII, the coordinated CO is replaced by an aqua ligand present in the bulk phase, which is followed by cyclic voltammetry at appropriate electrolysis time intervals. During 8000 s of electrolysis, ca. 67% of the initially assembled carbonyl complexes are converted to the triruthenium aqua complexes, while ca. 25% desorbed from the electrode surface.

A reaction of trans-[(Cp*Rh)2(μ-CH2)2Cl2] with H2Se in MeOH formed a μ-SeH complex isolated as a BPh4 salt [(Cp*Rh)2(μ-CH2)2(μ-SeH)](BPh 4) (1; Cp* = η5-C5Me5). In solution, complex 1 was oxidized by molecular oxygen giving a tetranuclear diselenide μ4-Se2 complex [{(Cp*Rh)2(μ-CH2)2} 2(μ4-Se2)](BPh4)2 (2). The structure involves Se2 bridging two Rh-Rh bonds (Rh-Rh = 2.6353(5) Å) in a side-on fashion (Rh(1)-Se(1) = 2.4715(6), Rh(2)-Se(1)* = 2.5526(6) Å). The Se(1)-Se(1)* distance is 2.3875(9) Å, which corresponds to a Se-Se single bond. Complex 2 showed an intriguing dynamic behavior of double-site atomic inversion at the selenium atoms in CD3CN. The line shape analyses of the temperature dependent 1H-NMR spectra of the μ-CH2 groups elucidated the dynamic process and provided the activation parameters: ΔH‡ = 70 ± 1 kJ mol-1, ΔS‡ = 15 ± 5 J mol-1 K-1, and ΔG‡ = 66 ± 1 kJ mol-1 (at 298 K). © 2000 Elsevier Science S.A. All rights reserved.

New oxo-centred acetate-bridged triruthenium(in) complexes with a long alkyl-chain disulfide ligand, (NC5C5)CH2NHC(O)(CH2)SS(CH 2)C(O)NHCH2(C5C5N) (C5C5N represents a 4-pyridyl residue, n = 10; abbreviated as C10PY), have been prepared as a redox-active metal cluster adsorbate on gold electrodes: [Ru3(O)(CH3CO2)6(mpy)2(C10PY)]C1O4 5a (mpy = 4-methylpyridine), [Ru3(O)(CH3CO2)6(mpy)2(C10PY)]CF3SO3 5b. The preparation, spectroscopic and electrochemical properties of 5a,5b are presented and are compared with those of triruthenium(ni) analogues of a shorter alkyl-chain disulfide ligand (11 = 2; abbreviated as C2PY), eg. [Ru3(O)(CH3CO2)6(mpy)2(C2PY)]- C1O4 3a and [Ru3(O)(CH3CO2)6(MeIm) 2(C2PY)]ClO4 4a (Melm = 1-methylimidazole). Characterization of all the compounds was accomplished using 'H and 'H-'H COSY NMR, UV-vis and infrared spectroscopy, elemental analysis, fast-atom bombardment (FAB) mass spectrometry and cyclic voltammetry. Compounds 3a, 4a and 5a,5b in 0.1 M [;i-Bu4N]PF6-CH3CN exhibit three (Ru3(O)} cluster core-based one-electron redox waves which are all reversible in the applied potential range between +1.2 and -2.0 V vs. Ag-AgCl. The redox potentials are dependent on the basicity of the terminal ligands (mpy and Melm), but are insensitive to the length of the méthylène chains of the disulfide ligand (C10PY and C2PY). Self-assembly of 5b on a gold electrode results in the formation of electrochemically stable monolayers which exhibit one-electron redox wave corresponding to the Ru3 redox process III,IH,III-II,III,III in aqueous media containing 0.1 M supporting electrolytes. The redox potentials and the shape of the voltammetric waves of the surface-attached triruthenium complexes are found to be largely dependent upon the nature of anions used (ClO4-, CF3SO3-, NO3-, PF6- and SO42-) and also upon the basicity of terminal ligands on the (Ru3(O)} cluster moiety. © The Royal Society of Chemistry 2000.

Kinetics of the outer-sphere reduction reactions of [Fe(1,10-phenanthroline)(3)](3+) by two oxo-bridged ruthenium complexes, [Ru-3(mu (3)-O)(mu -CH3COO)(6)(pyridine)(3)](+) and [Ru-2(mu -O)(mu -CH3COO)(2)(pyridine)(6)](2+) have been studied using the stopped-flow technique in acetonitrile (I=0.005 and 0.001 M, respectively (NaClO4); 20-35 degreesC). Second-order rate constants at 25 degreesC (M(-1)s(-1)) activation enthalpy (DeltaH(double dagger)/kJ mol(-1)), and activation entropy (DeltaS(double dagger)/J K(-1)mol(-1)), are 2.05 x 10(6), 19 +/- 2, and -60 +/- 4, respectively, for the trimer, and 1.14 x 10(6), 8.8 +/- 0.8, and -100 +/- 3, respectively, for the dimer. The difference in kinetic parameters have been discussed in terms of smaller reorganization energy for the trimer. Emission quenching rate constants of [Ru(2,2'-bipyridine)(3)](2+) by seven ore-bridged di- and trinuclear ruthenium complexes including the above two complexes and [Ru2Rh(mu (3)-O)(mu -CH3COO)(6)(pyridine)(3)]+ and [Rh-3(mu (3)-O)(mu -CH3COO)(6)(pyridine)(3)](+), have been obtained in acetonitrile and DMF (0.1 M (n-C4H9)(4)N(ClO4)) at 22 degreesC. They are in the range of (1.17 - 3.95) x 10(9) M(-1)s(-1) except for the trirhodium complex which does not quench at all, and are independent of the redox potentials of the quenchers, thus suggesting an energy transfer mechanism.