Reginaldo C. Rocha

Electronic Structure and Spectroscopy of [Ru(tpy)2]2+, [Ru(tpy)(bpy)(H2O)]2+, and [Ru(tpy)(bpy)(Cl)]+

We use a combined, theoretical and experimental, approach to investigate the spectroscopic properties and electronic structure of three ruthenium polypyridyl complexes, [Ru(tpy)(2)](2+), [Ru(tpy)(bpy)(H(2)O)](2+), and [Ru(tpy)(bpy)(Cl)](+) (tpy = 2,2:6,2-terpyridine and bpy = 2,2-bipyridine) in acetone, dichloromethane, and water. All three complexes display strong absorption bands in the visible region corresponding to a metal-to-ligand-charge-transfer (MLCT) transition, as well as the emission bands arising from the lowest lying (3)MLCT state. [Ru(tpy)(bpy)(Cl)](+) undergoes substitution of the Cl(-) ligand by H(2)O in the presence of water. Density functional theory (DFT) calculations demonstrate that the triplet potential energy surfaces of these molecules are complicated, with several metal-centered ((3)MC) and (3)MLCT states very close in energy. Solvent effects are included in the calculations via the polarizable continuum model as well as explicitly, and it is shown that they are critical for proper characterization of the triplet excited states of these complexes.

Spectroscopy, electrochemistry and catalytic properties of rutheniumII complexes containing the tetradentate Schiff base ligand N, N'-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane

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.

Proton-induced switching and control of intramolecular electron transfer on a benzotriazole-bridged symmetric mixed-valence ruthenium complex

Abstract A simple and efficient pH-induced molecular “on/off” switching has been devised based on a symmetric bridged ruthenium dimer, [(edta)Ru(μ-L b )Ru(edta)] n − (where n =4 or 3; L b =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.

Asymmetric mixed-valence binuclear ruthenium complexes containing benzotriazolate or benzimidazolate bridging ligands

Abstract Asymmetric binuclear ruthenium complexes of the type [(NH3)5Ru–L–Ru(edta)]n (where L=benzotriazolate, benzimidazolate; edta=ethylenediaminetetraacetate; n=1, 0, −1) have been prepared and investigated by means of electrochemical and spectroelectrochemical methods. Special emphasis has been given to the intramolecular electron transfer phenomena. The corresponding mixed-valence species display contrasting electronic properties: while the benzimidazolate derivative behaves as a moderately coupled system, the analogous benzotriazolate exhibits a rather large electronic coupling and significant delocalization, as deduced from their quite different intervalence transfer band profiles, in addition to their distinct redox and metal-to-ligand and ligand-to-metal charge transfer behavior. The special behavior of the benzotriazolate species has been attributed to the π-acid and the fluxional coordination properties of the ligand, improving the geometry approximation in the mixed-valent species. In the attempt to exploit the limits of Hushs theory, a set of elucidative electrochemical and spectroelectrochemical measurements for the (NH3)5RuII(bta)RuIII(edta) complex have been carried out in organic solvents, confirming its borderline class II/class III nature in the Robin–Day categorization. Efforts have also been directed to the evaluation of electronic and thermodynamic parameters for unsymmetrical systems.

Transferência de elétrons em sistemas inorgânicos de valência mista

This article reports on some basic and conceptual principles concerning electron transfer (ET) and/or intervalence transfer (IT) phenomena in inorganic mixed-valence systems.

Catecholamine complexes of ruthenium–edta and their redox chemistry

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.

Synthesis, Structure, and Electronic Properties of a Dimer of Ru(bpy)2 Doubly Bridged by Methoxide and Pyrazolate

The heterobridged dinuclear complex cis,cis-[(bpy) 2Ru(mu-OCH 3)(mu-pyz)Ru(bpy) 2] (2+) ( 1; bpy = 2,2-bipyridine; pyz = pyrazolate) was synthesized and isolated as a hexafluorophosphate salt. Its molecular structure was fully characterized by X-ray crystallography, (1)H NMR spectroscopy, and ESI mass spectrometry. The compound 1.(PF 6) 2 (C 44H 38F 12N 10OP 2Ru 2) crystallizes in the monoclinic space group P2 1/ c with a = 13.3312(4) A, b = 22.5379(6) A, c = 17.2818(4) A, beta = 99.497(2) degrees , V = 5121.3(2) A (3), and Z = 4. The meso diastereoisomeric form was exclusively found in the crystal structure, although the NMR spectra clearly demonstrated the presence of two stereoisomers in solution (rac and meso forms at approximately 1:1 ratio). The electronic properties of the complex in acetonitrile were investigated by cyclic voltammetry and UV-vis and NIR-IR spectroelectrochemistries. The stepwise oxidation of the Ru (II)-Ru (II) complex into the mixed-valent Ru (II)-Ru (III) and fully oxidized Ru (III)-Ru (III) states is fully reversible on the time scale of the in situ (spectro)electrochemical measurements. The mixed-valent species displays strong electronic coupling, as evidenced by the large splitting between the redox potentials for the Ru(III)/Ru(II) couples (Delta E 1/2 = 0.62 V; K c = 3 x 10 (10)) and the appearance of an intervalence transfer (IT) band at 1490 nm that is intense, narrow, and independent of solvent. Whereas this salient band in the NIR region originates primarily from highest-energy of the three IT transitions predicted for Ru(II)-Ru(III) systems, a weaker absorption band corresponding to the lowest-energy IT transition was clearly evidenced in the IR region ( approximately 3200 cm (-1)). The observation of totally coalesced vibrational peaks in the 1400-1650 cm (-1) range for a set of five bpy spectator vibrations in Ru (II)-Ru (III) relative to Ru (II)-Ru (II) and Ru (III)-Ru (III) provided evidence for rapid electron transfer and valence averaging on the picosecond time scale. Other than a relatively short Ru...Ru distance (3.72 A for the crystalline Ru (II)-Ru (II) complex), the extensive communication between metal centers is attributed mostly to the pi-donor ability of the bridging ligands (pyz, OMe) combined with the pi-acceptor ability of the peripheral (bpy) ligands.

Intervalence, electron transfer and redox properties of a triazolate-bridged ruthenium-polypyridine dinuclear complex

Abstract A new dinuclear complex of the type cis,cis -[(bpy) 2 ClRu(μ-L b )RuCl(bpy) 2 ] n + (bpy=2,2′-bipyridine; L b =benzotriazolate (bta); n =1, 2, or 3) has been synthesized, isolated as a PF 6 − 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 Hushs 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. L b =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 (Ru III ue5f8Lue5f8Ru III ) and fully reduced (Ru II ue5f8Lue5f8Ru II ) 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.

Ruthenium and iron complexes with benzotriazole and benzimidazole derivatives as simple models for proton-coupled electron transfer systems

Complexos de ferro e rutenio do tipo [M-LH]n (onde M = RuII,III(NH3)5 2+,3+, RuII,III(edta)2-,- [edta = etilenodinitrilotetraacetato], ou FeII,III(CN)53-,2- e LH = benzotriazol ou benzoimidazol) foram preparados e caracterizados em solucao aquosa atraves de metodos eletroquimicos e espectroeletroquimicos. Neste trabalho, maior enfase foi direcionada aos processos redox dependentes do pH, que foram demonstrados por todos os complexos estudados. Os valores de pKa e de potencial de reducao formal foram obtidos a partir dos diagramas de E1/2 versus pH, que apresentaram comportamento tipicamente Nernstiano. Os diagramas de Pourbaix tambem foram usados para ilustrar o particionamento entre as especies redox e os equilibrios acido-base envolvidos nas reacoes. Na avaliacao da potencialidade desses complexos como modelos simples para reacoes de transferencia de eletrons acoplada a proton (PCET), considerou-se a extensao da regiao onde a dependencia do par redox MIII/MII com o pH e ativa, definida entre pKaIII e pKaII. Os resultados obtidos neste trabalho foram analisados do ponto de vista do carater eletronico doador/receptor dos ligantes e das interacoes s,p-metal-ligante envolvidas em cada especie, para ambos os estados de oxidacao do ion metalico.

Intervalence transfer properties of the binuclear μ-benzotriazolate- and μ-benzimidazolate-bis{ruthenium(II)/(III)-edta} complexes

Abstract Symmetric binuclear complexes of the type [(edta)Ru-L-Ru(edta)] n − ( n xa0=xa03, 4, 5; edtaxa0=xa0ethylenedinitrilotetraacetate; Lxa0=xa0benzotriazolate or benzimidazolate) have been prepared in aqueous solution and characterized by means of electrochemical and spectroelectrochemical methods. Contrasting intervalence transfer properties have been observed for the mixed valence complexes: while the benzimidazole derivative behaves as a valence trapped system (class II), the analogous benzotriazole is better described as a class III system, exhibiting significant electronic delocalization. The distinct behavior for the last one has been attributed to a stronger interaction between the metal centers promoted by the fluxional coordination properties of the bridging ligand. The results have been discussed in the light of ZINDO semi-empirical molecular orbital calculations.