Invited for the cover of this issue is the group of Koiti Araki from the University of São Paulo, Brazil. The cover image shows the oxidation of water to O 2 by an angular “Pacman-like” binuclear Ru complex with weak coupling between catalytic sites.

The pH-dependent electronic and electrochemical properties of a series of triangular mu(3)-oxo-centered trinuclear clusters of the type [Ru3O(Ac)(6)(py)(2)(OHx)](n) (Ac = acetate; py = pyridine; x = 0, 1, or 2; where the formal oxidation states on the metal ions range from +II to +IV) have been investigated with emphasis on the elucidation of the successive proton-coupled redox processes spanning the interconvertible aqua- (x = 2), hydroxo- (x = 1), and oxo- (x = 0) forms, from which the complete Pourbaix diagram has been proposed. From the spontaneous slow coupling reaction of the monomeric cluster in its deprotonated (oxo) form at high pH, the p-oxo-bridged cluster dimer (i.e. {(py)(2)(Ac)(6)ORu3}-O-{Ru3O(Ac)6(PY)2}) has been isolated and fully characterized by spectroelectrochemistry as well. A further understanding of the underlying electronic interactions across the various oxidation and protonation states, and the role of such phenomena in the activation of the Ru-O bonding system in oxygen-transfer catalysis, have also been explored. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Resonance Raman spectroelectrochemistry (RR-SEC) at -20 degrees C has been performed on the pyrazine-bridged dimer of mu-oxo-centered trinuclear ruthenium-acetate "clusters"--[(dmap)(CO)(mu-OAc)6(mu3-O)Ru3(mu-L(b))Ru3(mu3-O)(mu-OAc)6(CO)(dmap)]n (where dmap = 4-(dimethylamino)pyridine and L(b) = pyrazine-h4 and pyrazine-d4)-in three oxidation states: n = 0, -1, and -2. In the one-electron reduced, "mixed-valent" state (overall -1 charge and a single odd electron; formal oxidation states [II, II, III]-[III, III, II] on the metal centers), the Raman excitation at 800 nm is resonant with a cluster-to-cluster intervalence charge-transfer (IVCT) band. Under these conditions, scattering enhancement is observed for all four totally symmetric vibrational modes of the bridging pyrazine ligand (nu8a, nu9a, nu1, and nu6a) in the investigated spectral range (100-2000 cm(-1)), and there is no evidence of activity in non-totally symmetric vibrations. Resonantly enhanced Raman peaks related to peripheral pyridyl (dmap) ligand modes and low-frequency features arising from the trigonal Ru3O cluster core and the cluster[Ru]-[N]ligand vibrations were also observed in the spectra of the intermediate-valence (n = -1) cluster dimer. The vibrational assignments and interpretations proposed in this work were reinforced by observation of characteristic isotopic frequency shifts accompanying deuteration of the bridging pyrazine. The results reveal that the fully symmetric (A(g)) vibrational motions of the organic bridge are coupled to the nominally metal cluster-to-metal cluster fast intramolecular electron transfer (ET) and provide validation of the near-delocalized description according to a predicted three-site/three-state (e.g., metal-bridge-metal) vibronic coupling model, in which the important role of the bridging ligand in mediating electronic communication and delocalization between charge centers is explicitly considered. Further compelling evidence supporting an extended five-state model, which incorporates the peripheral cluster-bound pyridyl ligands, is also presented.

Infrared spectroelectrochemistry has been used to explore the vibrational properties of a pyrazine-bridged osmium-polypyridine dimer as a function of its formally metal-centered oxidation states (i.e., Os(II)Os(II), Os(II)Os(III), and Os(III)Os(III)). The infrared spectrum of the "mixed-valent" species is particularly interesting and exhibits features consistent with both electronic localization and delocalization on the vibrational time scale, as revealed by the presence of both (i) a highly active totally symmetric mode from the bridging pyrazine ligand (nu(8a)), and (ii) total coalescence of at least four modes from peripheral bipyridine ligands. The nature and origin of the observed peaks were confirmed by analysis of the shifts in vibrational frequencies accompanying deuteration of pyrazine and also by comparison of the data for the dimeric complexes with those for the parent monomers.

RUTHENIUM COMPLEXES CONTAINING ETHYLENEDIAMINETETRAACETATE. This paper provides a survey of general aspects involved in the coordination chemistry of low-valent (mainly.+III,+II), low-spin (d(x)(5)',d(pi)(1)) ruthenium ions with ethylenediai-nine-N,N,N',N'-tetraacetate (edta) and their substituted derivatives. The topics covered herein include structure, reactivity, kinetics, thermodynamics, electrochemistry and spectroscopy. The contributions from either our research group or the literature over the last three decades are focused in this review.

The dinuclear complex cis,cis-[(bpy)2ClRu(μ-bim)RuCl(bpy)2] n + (bpy = 2,2′-bipyridine; bim = benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n = 2) displays significant metal–metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of σ,π-basic character of the bridging ligands, according to an electron superexchange mechanism of the “hole-transfer” type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (RuIII–L–RuIII) and fully reduced (RuII–L–RuII) isovalent species.

The complexes trans-[RuCl2(bpydip)] and trans-[Ru(OH2)2(bpydip)](PF6)2, where bpydip is the tetradentate Schiff base ligand, N,N′-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane, have been synthesized and characterized by elemental analysis, cyclic voltammetry, UV–Vis, FTIR and 1H NMR spectroscopy. The electronic spectrum of the trans-[RuCl2(bpydip)] complex has been successfully simulated on the basis of the ZINDO/S method, supporting the assignment of the absorption bands at 644, 607, 458, 418 and 374 nm to RuII(dπ)→bpydip(pπ*) charge-transfer transitions, and at 282 nm, to a bpydip (π→π*) intraligand transition. The electrochemistry of this complex is characterized by a reversible pair of waves at +0.30 and −1.70 V, ascribed to the RuIII/II and bpydip0/−1 redox couples, respectively. In contact with water, the trans-[RuCl2(bpydip)] complex spontaneously and quantitatively converts into the aqua complex, leading to pronounced changes in the electronic and electrochemical behavior. A remarkable activity in the epoxidation of cyclohexene in the presence of iodosobenzene (PhIO) has been observed for the aqua complex.

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.

This paper focuses on the electron transfer reaction, in water, between aquaethylenediaminetetraacetatoruthenate(III) and aromatic diamines, particularly 9,10-diaminophenanthrene (pha), as well as on the chemical characterization of the corresponding α-diimine ruthenium(II) complexes. The redox chemistry of these complexes at all their accessible metal- and ligand-centered oxidation states in slightly acidic (pH 4.7) aqueous solutions was investigated by means of electrochemistry and spectroelectrochemistry. All these redox species were found to be rather stable in solid state or aqueous solutions and totally interconvertible in reversible chemical/electrochemical processes. Comparison of the metal-centered redox potentials of the [RuIII/II(edta)(L)]−/2− (L=9,10-phenanthrenequinone diimine, o-benzoquinone diimine, 1,10-phenanthroline and 2,2′-bipyridine) complexes revealed that the quinone diimines possess stronger π-acceptor abilities than 1,10-phenanthroline and 2,2′-bipyridine, supporting a more efficient stabilization of the Ru(II) species, by means of π-backbonding interactions, in the cases of the quinonoid ligands.

A new dinuclear complex of the type cis,cis-[(bpy)2ClRu(μ-Lb)RuCl(bpy)2]n+ (bpy=2,2′-bipyridine; Lb=benzotriazolate (bta); n=1, 2, or 3) has been synthesized, isolated as a PF6− salt, and investigated in organic solutions by means of cyclic voltammetry and ultraviolet/visible/near-infrared spectroelectrochemistry. Particular emphasis has been given to the electron transfer (ET) properties of the mixed-valent species (n=2), which displays a somewhat large metal–metal electronic coupling in the ground state with the complex featuring localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge-transfer (IVCT) band and electrochemical parameters. Analysis of the IVCT properties in the context of Hush's theory also supports a valence-trapped formulation. In spite of the class II categorization within the Robin-Day scheme, this system shows a remarkable intermetallic communication when compared with other analogues (e.g. Lb=pyrazine). Such aspect has also been stressed by comparison of the set of thermodynamic and mixed-valence parameters along with a series of L-bridged systems studied previously in aqueous solutions (in particular, [(edta)Ru(μ-bta)Ru(edta)]4−; edta=ethylenediamine-N,N,N′,N′-tetraacetate), and the striking differences in their intervalence characteristics have been rationalized in terms of distinct types of electronic and structural effects. Despite the contrasting behavior, the same type of superexchange mechanism (‘hole-transfer’) seems to prevail in all these benzotriazolate-bridged mixed-valent species. In the 2,2′-bipyridine derivative, the synergistic charge-transfer effects are the most relevant factors on the great stabilization of the mixed-valence state. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (RuIIILRuIII) and fully reduced (RuIILRuII) isovalent species. From the IVCT band features, the rate of intramolecular thermal ET for the mixed-valent ion was estimated on the basis of the Hush and Marcus theories.

This paper reports on the electrochemical parametrization of a series of mono- and disubstituted ruthenium–EDTA complexes (EDTA=ethylenediamine-N,N,N′,N′-tetraacetate ligand) on the basis of Lever’s parameters (EL). The electrochemical parameters for 45 ligands with unknown EL have been compiled.

A new series of mononuclear, dinuclear and trinuclear benzotriazolato-complexes (namely trans-[Ru(NH3)4(bta)2]x+, trans-[(NH3)4(bta)Ru(bta)Ru(edta)]y−, and trans-[(NH3)4Ru{(bta)Ru(edta)}2]z−; where bta=benzotriazolate, edta=ethylenediamine-N,N,N′,N′-tetraacetate, x=0 or 1, y=0, 1 or 2, z=1, 2 or 4) has been prepared and investigated in aqueous solutions by means of electrochemical and UV–Vis–NIR spectroelectrochemical methods. Particular emphasis is given to the mixed-valence properties of the dinuclear (RuIIRuIII) and trinuclear (RuIIIRuIIRuIII) species, which display a large degree of metal–metal electronic coupling with localized oxidation states, as deduced from their intervalence charge transfer bands and electrochemical parameters. Analysis of the results within the context of Mulliken–Hush perturbational theory reveals that the intermetallic electronic communication increases slightly with the nuclearity of the system, leading to the conclusion that the symmetrical (nonlinear) trinuclear complex acts as a model/precursor for extended molecular chains of the type ‘molecular wires’. A comparison with a set of analogues studied previously is also reported, reinforcing the quite interesting properties of benzotriazole derivatives as molecular bridges.

The reaction of [RuIII(edta)(H2O)]− with o-phenylenediamine (opda) in water, under aerobic conditions, affords the diamagnetic [RuII(edta)(bqdi)]2− product (where edta stands for the ethylenediaminetetraacetate co-ligand, and bqdi represents the non-innocent o-benzoquinone α,α′-diimine ligand). In the current communication, the redox chemistry of this system in aqueous solution is described in details on the basis of electrochemical and spectroelectrochemical studies. The electrochemical behavior of “free” opda is rather complicated with further chemical reactions following the irreversible two-proton/two-electron oxidation (opda→bqdi+2e−+2H+), whereas its complex is electrochemically well-behaved with two chemically reversible redox processes: the monoelectronic couple associated with the metal ion (RuIII/RuII) and another bielectronic step centered on the coordinated ligand (bqdi/opda). The set of UV–Vis electronic spectra were obtained by electrolytical generation, in situ, of all the redox species accessible in the CV working conditions (i.e., the starting [RuII(edta)(bqdi)]2−, the fully oxidized [RuIII(edta)(bqdi)]−, and the fully reduced [RuII(edta)(opda)]2− species), which are stable and totally interconvertible. The electrochemistry and absorption spectroscopy of these complexes in water were found to be comparable with the tetraammine counterparts. A remarkable difference in redox behavior between the diimine- and the analogous dioxolene-complexes was also revealed by comparison of the system reported herein with the one derived from catechol, and rationalized in terms of the quite efficient π-accepting electronic nature of the bqdi ligand.

The unsymmetrical dinuclear ruthenium–iron complexes [(NH 3 ) 5 Ru–bta–Fe(CN) 5 ] n (where bta = benzotriazolate; n = –2, –1, 0) were prepared as solid sodium salts from [Ru II (NH 3 ) 5 (bta)] + or [Ru III (NH 3 ) 5 (bta)] 2+ and [Fe II (CN) 5 (H 2 O)] 3– and characterized in aqueous solution by means of electrochemical and spectroelectrochemical methods. UV-vis, near-infrared, IR, and cyclic and differential pulse voltammetry data suggest that the related mixed valent species belong to a valence trapped formulation, featuring localized Ru(III) and Fe(II) oxidation states. In spite of the class II categorization in the Robin and Day scheme, this system shows a remarkable metal–metal electronic coupling, as deduced from an intense, low-energy, and very broad intervalence band in the near-IR region. In addition, the mixed valence state displays enhanced stabilization in relation to the isovalent state. The intervalence transfer properties are discussed on the basis of Hush's theory.Key words: ammineruthenium complexes, cyanoiron complexes, mixed valence, intervalence, benzotriazole, benzotriazolate.

Ruthenium(III)-edta reacts with the 3-hydroxypicolinate ligand (Hhpic(-)) at pH 5, yielding the practically colorless [Ru-III(edta)(kappaN, kappaO-Hhpic)](2-) complex (H(2)hpic = 3-hydroxypicolinic acid; H(4)edta=ethylenedinitrilotetraacetic acid). Above pH 9, deprotonation of the phenolic group promotes an intramolecular linkage isomerization process, generating the faint red [Ru-III(edta) (kappaO, kappaO-hpic)](3-) complex. Both isomers can be electrochemically reduced, converting into a single deep red [Ru-II(edta)(kappaN, kappaO-Hhpic)](3-) complex strongly stabilized by ruthenium-to-pyridinecarboxylate d(pi) -->p(pi)*, charge-transfer interactions. The observed distinct binding properties as a function of the oxidation states and pH have been rationalized based on semiempirical theoretical calculations for the complexes.

A simple and efficient pH-induced molecular “on/off” switching has been devised based on a symmetric bridged ruthenium dimer, [(edta)Ru(μ-Lb)Ru(edta)]n− (where n=4 or 3; Lb=benzotriazolate or 2H-benzotriazole; edta=ethylenediaminetetraacetate), by exploiting its unusual mixed-valence state behavior as a function of the pH. The intervalence transfer (IT) related phenomena have been rationalized in the light of the Mulliken–Hush formalism. Depending on the proton concentration, the electronic coupling and delocalization can be modulated to induce class III (deprotonated bridging ligand), class II (protonated), or even an intermediate borderline class II/class III system features.

The electrochemical and spectroelectrochemical behavior of some neurotransmitters (dopamine and l-dopa) and their corresponding novel blue ruthenium(III)–edta complexes were investigated in aqueous solutions. At pH 7–10, the free ligand species can be electrochemically oxidized in the range of 0.1–0.6 V versus SHE, yielding primarily quinone products susceptible to pH-dependent, secondary intramolecular chemical reactions, which make the redox processes irreversible. When coordinated to the ruthenium(III)–edta complex, their electrochemical and spectroelectrochemical behavior is dramatically changed, approaching that of metal complexes with noninnocent dioxolene ligands. Reduction of the ruthenium(III) moiety proceeds reversibly above pH 9, in the region from −0.5 to −0.7 V. The oxidation process centered on the catecholate ligands becomes reversible and leads exclusively to the formation of the semiquinone species, with no evidence of complications from further reactions. These changes in the electrochemical behavior of the neurotransmitters make their cyclovoltammetric waves for reduction/oxidation more defined, favoring more precise quantitative analyses.

Linkage isomerization reactions have been reviewed from the aspect of the kinetics and mechanisms involved, focusing on selected cases of direct formation, as well as on electrochemical, photochemical, thermal and pH-induced generation of linkage isomers.