Intervalence charge transfer in a "chain-like" ruthenium trinuclear assembly based on the bridging ligand 4,7-phenanthrolino-5,6:5',6'-pyrazine (ppz)

The IVCT characteristics of the mixed valence forms of the trinuclear complex [{Delta-Ru(bpy)2}2{Delta(t)-Ru(bpy)(mu-ppz)2}]n+ (n = 7, 8; t = trans), and the diastereoisomers (meso and rac) of the dinuclear complex [{Ru(bpy)2}2(mu-ppz)]5+, display a marked dependence on the nuclearity and extent of oxidation of the assemblies. The dinuclear species are classified as borderline localised-delocalised mixed valence species while the two mixed valence states of the trinuclear complex exhibit localised behaviour. One-electron oxidation of a terminal Ru centre in the trinuclear case gives rise to a broad, low intensity IVCT band for the +7 mixed valence species which is composed of two underlying Gaussian-shaped bands. The transitions are identified as adjacent and remote IVCT transitions, with the former dominating the intensity of the IVCT manifold. The +8 mixed valence species exhibits a single dominant IVCT band arising from the equivalent IVCT transitions from the central Ru(II) to peripheral Ru(III) centres.     

Synthesis, characterization, and detailed electrochemistry of binuclear ruthenium(III) complexes bridged by bisacetylacetonate. Crystal and molecular structures of [[Ru(acac)2]2(tae)] (acac = 2,4-pentanedionate ion, tae = 1,1,2,2-tetraacetylethanate dianion)

Binuclear beta-diketonatoruthenium(III) complexes [[Ru(acac)(2)](2)(tae)], [[Ru(phpa)(2)](2)(tae)], and [(acac)(2)Ru(tae)Ru(phpa)(2)] and binuclear and mononuclear bipyridine complexes [[Ru(bpy)(2)](2)(tae)](PF(6))(2) and [Ru(bpy)(2)(Htae)]PF(6) (acac = 2,4-pentanedionate ion, phpa = 2,2,6,6-tetramethyl-3,5-heptanedionate ion, tae = 1,1,2,2-tetraacetylethanate dianion, and bpy = 2,2'-bipyridine) were synthesized. The new complexes have been characterized by (1)H NMR, MS, and electronic spectral data. Crystal and molecular structures of [[Ru(acac)(2)](2)(tae)] have been solved by single-crystal X-ray diffraction studies. Crystal data for the meso isomer of [[Ru(acac)(2)](2)(tae)] have been confirmed by the dihedral angle result that two acetylacetone units of the bridging tae ligand are almost perpendicular to one another. A detailed investigation on the electrochemistry of the binuclear complexes has been carried out. The electrochemical behavior details of the binuclear complexes have been compared with those of the mononuclear complexes obtained from the half-structures of the corresponding binuclear complexes. Studies on the effects of solvents on the mixed-valence states of Ru(II)-Ru(III) and Ru(III)-Ru(IV) complexes have been carried out by various voltammetric and electrospectroscopic techniques. A correlation between the comproportionation constant (K(c)) and the donor number of the solvent has been obtained. The K(c) values for the binuclear complexes have been found to be low because of the fact that two acetylacetone units of the bridging tae ligand are not in the same plane, as revealed by the crystal structure of [[Ru(acac)(2)](2)(tae)].     

Synthesis and properties of [Ru(2)(acac)(4)(bptz)](n+) (n=0,1) and crystal structure of [Ru(2)(acac)(4)(bptz)

The neutral complex [Ru(2)(acac)(4)(bptz)] (I) has been prepared by the reaction of Ru(acac)(2)(CH(3)CN)(2) with bptz (bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) in acetone. The diruthenium(II,II) complex (I) is green and exhibits an intense metal-ligand charge-transfer band at 700 nm. Complex I is diamagnetic and has been characterized by NMR, optical spectroscopy, IR, and single-crystal X-ray diffraction. Crystal structure data for I are as follows: triclinic, P1, a = 11.709(2) A, b = 13.487(3) A, c = 15.151(3) A, alpha = 65.701(14) degrees, beta = 70.610(14) degrees, gamma = 75.50(2) degrees, V = 2038.8(6) A(3), Z = 2, R = 0.0610, for 4397 reflections with F(o) > 4sigmaF(o). Complex I shows reversible Ru(2)(II,II)-Ru(2)(II,III) and Ru(2)(II,III)-Ru(2)(III,III) couples at 0.17 and 0.97 V, respectively; the 800 mV separation indicates considerable stabilization of the mixed-valence species (K(com) > 10(13)). The diruthenium(II,III) complex, [Ru(2)(acac)(4)(bptz)](PF(6)) (II) is prepared quantitatively by one-electron oxidation of I with cerium(IV) ammonium nitrate in methanol followed by precipitation with NH(4)PF(6). Complex II is blue and shows an intense MLCT band at 575 nm and a weak band at 1220 nm in CHCl(3), which is assigned as the intervalence CT band. The mixed valence complex is paramagnetic, and an isotropic EPR signal at g = 2.17 is observed at 77 and 4 K. The solvent independence and narrowness of the 1200 nm band show that complex II is a Robin and Day class III mixed-valence complex.     

Electron donor-acceptor dyads and triads based on tris(bipyridine)ruthenium(II) and benzoquinone: synthesis,characterization, and photoinduced electron transfer reactions

Two electron donor-acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)(3)](2+) complex have been synthesized. In triad 6 (denoted Ru(II)-BQ-Co(III)), a [Co(bpy)(3)](3+) complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ-Ru(II)-BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads Ru(II)-BQ (4) and PTZ-Ru(II) (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant k(f) = 5 x 10(9) s(-)(1) (tau(f) = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant k(b) approximately 4.5 x 10(8) s(-)(1) (tau(b) approximately 2.2 ns), for both Ru(II)-BQ and Ru(II)-BQ-Co(III) with no indication of a charge shift to generate the reduced Co(II) moiety. In the PTZ-Ru(II)-BQ triad, however, the initial charge separation was followed by a rapid (k > 5 x 10(9) s(-)(1)) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ(*)(+)-Ru(II)-BQ(*)(-) state (tau = 80 ns) in unusually high yield for a [Ru(bpy)(3)](2+)-based triad (> 90%), that lies at DeltaG degrees = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ(*)(+) and the BQ(*)(-) moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ-Ru(II) a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ-Ru(II) state and 18% in the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state was thus faster than what would have been inferred from the Ru(II) emission decay (tau = 90 ns). This result suggests that reports for related PTZ-Ru(II) and PTZ-Ru(II)-acceptor complexes in the literature might need to be reconsidered.     

Photoinduced energy- and electron-transfer processes in dinuclear Ru(II)-Os(II), Ru(II)-Os(III), and Ru(III)-Os(II) trisbipyridine complexes containing a shape-persistent macrocyclic spacer.

The PF6- salt of the dinuclear [(bpy)2Ru(1)Os(bpy)2]4+ complex, where 1 is a phenylacetylene macrocycle which incorporates two 2,2'-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, the Ru- and Os-based units display their characteristic absorption spectra and electrochemical properties as in the parent homodinuclear compounds. The luminescence spectrum, however, shows that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os(II) unit. Electronic energy transfer from the Ru(II) to the Os(II) unit takes place by two distinct processes (k(en) = 2.0x10(8) and 2.2x10(7) s(-1) at 298 K). Oxidation of the Os(II) unit of [(bpy)2Ru(1)Os(bpy)2]4+ by Ce(IV) or nitric acid leads quantitatively to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ complex which exhibits a bpy-to-Os(III) charge-transfer band at 720 nm (epsilon(max) = 250 M(-1) cm(-1)). Light excitation of the Ru(II) unit of [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ is followed by electron transfer from the Ru(II) to the Os(III) unit (k(el,f) = 1.6x10(8) and 2.7x10(7) s(-1)), resulting in the transient formation of the [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ complex. The latter species relaxes to the [(bpy)2Ru(II)(1)Os(III)(bpy)2]5+ one by back electron transfer (k(el,b) = 9.1x10(7) and 1.2x10(7) s(-1)). The biexponential decays of the [(bpy)2*Ru(II)(1)Os(II)(bpy)2]4+, [(bpy)2*Ru(II)(1)Os(III)(bpy)2]5+, and [(bpy)2Ru(III)(1)Os(II)(bpy)2]5+ species are related to the presence of two conformers, as expected because of the steric hindrance between hydrogen atoms of the pyridine and phenyl rings. Comparison of the results obtained with those previously reported for other Ru-Os polypyridine complexes shows that the macrocyclic ligand 1 is a relatively poor conducting bridge.     

Photoinduced energy transfer coupled to charge separation in a Ru(II)-Ru(II)-acceptor triad

The bichromophoric system Ru-Ru(C)-PI ([(bpy)3Ru-Ph-Ru(dpb)(Metpy-PI)][PF6]3, where bpy is 2,2'-bipyridine, Hdpb is 1,3-di(2-pyridyl)-benzene, Metpy is 4'-methyl-2,2':6',2' '-terpyridine and PI is pyromellitimide) containing two Ru(II) polypyridyl chromophores with a N6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes Ru(C)-PI ([Ru(dpb)(Metpy-PI)][PF6]), Ru(C)-phi-PI ([Ru(dpb)(ttpy-PI)][PF6], ttpy is 4'-p-tolyl-2,2':6',2' '-terpyridine), Ru(C)-phi ([Ru(dpb)(ttpy)][PF6]), and Ru(C) ([Ru(dpb)(Metpy)][PF6]). Excitation of the Ru(C) unit in the dyads leads to oxidative quenching, forming the Ru(C)(III)-phi-PI*- and Ru(C)(III)-Pl.- charge-separated (CS) states with k(f)(ET) = 7.7 x 10(7) s(-1) (CH3CN, 298 K) in the tolyl-linked Ru(C)-phi-PI and k(f)(ET) = 4.4 x 10(9) s(-1) (CH2Cl2, 298 K) in the methylene-linked Ru(C)-PI. In the Ru-Ru(C)-PI triad, excitation of the Ru(C) chromophore leads to dynamics similar to those in the Ru(C)-PI dyad, generating the Ru(II)-Ru(C)(III)-PI*- CS state, whereas excitation of the Ru unit results in an initial energy transfer (k(EnT) = 4.7 x 10(11) s(-1)) to the cyclometalated Ru(C) unit. Subsequent electron transfer to the PI acceptor results in the formation of the same Ru(II)-Ru(C)(III)-PI*- CS state with k(f)(ET) = 5.6 x 10(9) s(-1) that undergoes rapid recombination with k(b)(ET) = 1 x 10(10) s(-1) (CH2Cl2, 298 K). The fate of the Ru(II)-Ru(C)(III)-PI*- CS state upon a second photoexcitation was studied by pump-pump-probe experiments in an attempt to detect the fully charge-separated Ru(III)-Ru(C)(II)-PI*- state.     

Photophysical properties of TiO(2) surfaces modified with dinuclear RuRu and RuOs polypyridyl complexes

The photophysical properties of nanoporous TiO(2) surfaces modified with two new Ru(II)-(bpt)-Ru(II) and Ru(II)-(bpt)-Os(II) polypyridyl complexes are reported. These dyads have been prepared by a two-step synthetic pathway. In the first step, [Ru(dcbpy)(2)Cl(2)], where dcbpy is 4,4'-dicarboxy-2,2-bipyridyl, was reacted with the bridging ligand 3,5-bis(pyridin-2-yl)-1,2,4-triazole (Hbpt) to yield the mononuclear precursor Na(3)[Ru(dcbpy)(2)(bpt)].3H(2)O. Subsequent reaction of this compound with either [Ru(bpy)(2)Cl(2)] or [Os(bpy)(2)Cl(2)] yields the Ru(II)-Ru(II) and Ru(II)-Os(II) dyads. Electrochemical data, together with time-resolved transient absorption spectroscopy and the investigation of the incident-photon-to-current-efficiency (IPCE), have been used to obtain a detailed picture of the photoinduced charge injection properties of these dyads. These measurements indicate that for the heterosupramolecular triad based on Ru(II)-(bpt)-Ru(II), the final product species obtained upon charge injection is TiO(2)(e)-Ru(II)Ru(III). For the mixed metal Ru(II)-(bpt)-Os(II) dyad, both metal centers inject efficiently into the semiconductor surface and as a result TiO(2)(e)-Ru(II)Os(III) is obtained as a single charge-separated product.     

First example of mu(3)-sulfido bridged mixed-valent triruthenium complex triangle Ru(III)(2)Ru(II)(O,O-acetylacetonate)(3)(mu-O,O,gamma-C-acetylacetonate)(3)(mu(3)-S) (1) incorporating simultaneous O,O- and gamma-C-bonded bridging acetylacetonate units. Synthesis,crystal structure,and spectral and redox properties

The reaction of mononuclear ruthenium precursor [Ru(II)(acac)(2)(CH(3)CN)(2)] (acac = acetylacetonate) with the thiouracil ligand (2-thiouracil, H(2)L(1) or 6-methyl -2-thiouracil, H(2)L(2)) in the presence of NEt(3) as base in ethanol solvent afforded a trinuclear triangular complex Ru(3)(O,O-acetylacetonate)(3)(mu-O,O,gamma-C-acetylacetonate)(3)(mu(3)-sulfido) (1). In 1, each ruthenium center is linked to one usual O,O-bonded terminal acetylacetonate molecule whereas the other three acetylacetonate units act as bridging functions: each bridges two adjacent ruthenium ions through the terminal O,O-donor centers at one end and via the gamma-carbon center at the other end. Moreover, there is a mu(3)-sulfido bridging in the center of the complex unit, which essentially resulted via the selective cleavage of the carbon-sulfur bond of the thiouracil ligand. In diamagnetic complex 1, the ruthenium ions are in mixed valent Ru(III)Ru(III)Ru(II) state, where the paramagnetic ruthenium(III) ions are antiferromagnetically coupled. The single crystal X-ray structure of 1 showed two crystallographically independent C(3)-symmetric molecules, Ru(3)(O,O-acetylacetonate)(3)(mu-O,O,gamma-C-acetylacetonate)(3)(mu(3)-S) (1), in the asymmetric unit. Bond distances of both crystallographically independent molecules are almost identical, but there are some significant differences in bond angles (up to 6 degrees ) and interplanar angles (up to 8 degrees ). Each ruthenium atom exhibits a distorted octahedral environment formed by four oxygen atoms, two from each of the terminal and bridging acetylacetonate units, one gamma-carbon of an adjacent acetylacetonate ligand, and the sulfur atom in the center of the complex. In agreement with the expected 3-fold symmetry of the complex molecule, the (1)H and (13)C NMR spectra of 1 in CDCl(3) displayed signals corresponding to two types of ligand units. In dichloromethane solvent, 1 exhibited three metal center based successive quasireversible redox processes, Ru(III)Ru(III)Ru(III)-Ru(III)Ru(III)Ru(II) (couple I, 0.43 V vs SCE); Ru(III)Ru(III)Ru(IV)-Ru(III)Ru(III)Ru(III) (couple II, 1.12 V); and Ru(III)Ru(III)Ru(II)-Ru(III)Ru(II)Ru(II) (couple III, -1.21 V). However, in acetonitrile solvent, in addition to the three described couples [(couple I), 0.34 V; (couple II), 1.0 V; (couple III), -1.0], one irreversible oxidative response (Ru(III)Ru(III)Ru(IV) --> Ru(III)Ru(IV)Ru(IV) or oxidation of the coordinated sulfide center) appeared at E(pa), 1.50 V. The large differences in potentials between the successive couples are indicative of strong coupling between the ruthenium ions in the mixed-valent states. Compound 1 exhibited a moderately strong charge-transfer (CT) transition at 654 nm and multiple ligand based intense transitions in the UV region. In the Ru(III)Ru(III)Ru(III) (1(+)) state, the CT band was slightly blue shifted to 644 nm; however, the CT band was further blue shifted to 520 nm on two-electron oxidation to the Ru(III)Ru(III)Ru(IV) (1(2+)) state with a reduction in intensity.     

Metal-induced reductive ring opening of 1,2,4,5-tetrazines: three resulting coordination alternatives, including the new non-innocent 1,2-diiminohydrazido(2-) bridging ligand system

Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)2(CH3CN)2 results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)2Ru(III)(dih-R(2-))Ru(III)(acac)2] bridged by the new 1,2-diiminohydrazido(2-) (dih-R(2-) = HNC(R)NNC(R)NH(2-)) ligands. rac/meso diastereoisomers could be detected and separated for the compounds with R = phenyl and 2-thienyl, all species are diamagnetic and were characterized by 1H NMR spectroscopy. Crystal structure determination of the meso isomers with R = phenyl and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N (approximately 1.40 A) and short C=N(H) bonds (approximately 1.31 A), implying two bridged ruthenium(III) centers at about 4.765 A distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation constants K(c) > 10(12) and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)2Ru(II)(dih-R(2-))Ru(II)(acac)2]2-, [(acac)2Ru(II)(dih-R(*-))Ru(II)(acac)2]-, [(acac)2Ru(III)(dih-R(2-))Ru(III)(acac)2], [(acac)2Ru(III)(dih-R(*-))Ru(III)(acac)2]+, and [(acac)2Ru(III)(dih-R)Ru(III)(acac)2]2+. While the UV-vis-NIR spectroscopic response of [(acac)2Ru(dih-R)Ru(acac)2](0/-/2-) is very similar to that of [(bpy)2Ru(adc-R)Ru(bpy)2](4+/3+/2+), adc-R(2-) = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)2Ru(II)(dih-R(*-))Ru(II)(acac)2]- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.     

What a difference ancillary thienyl makes: unexpected additional stabilization of the diruthenium(III,II) but not the diosmium(III,II) mixed-valent state in tetrazine ligand-bridged complexes

The Ru(2)(III,II) mixed-valent state is strongly stabilized in [(bpy)(2)Ru(mu-bttz)Ru(bpy)(2)](5+) (3(5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant K(c) = 10(16.6), and a high-energy intervalence charge transfer band at 10100 cm(-1). Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)(2)M(mu-bmptz)M(bpy)(2)](5+), bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher K(c) value of the Os(2)(III,II) analogue. EPR signals were observed at 4 K for 3(5+) but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2(3+) and 3(3+).     

Proton-Induced Tuning of Electrochemical and Photophysical Properties in Mononuclear and Dinuclear Ruthenium Complexes Containing 2,2'-Bis(benzimidazol-2-yl)-4,4'-bipyridine: Synthesis, Molecular Structure, and Mixed-Valence State and Excited-State Properties

A series of mono- and dinuclear Ru(bpy)(2) complexes (bpy = 2,2'-bipyridine) containing 2,2'-bis(benzimidazol-2-yl)-4,4'-bipyridine (bbbpyH(2)) were prepared. The mononuclear complex [Ru(bpy)(2)(bbbpyH(2))](ClO(4))(2).CH(3)OH.4H(2)O was characterized by an X-ray structure determination. Crystal data are as follows: triclinic, space group P&onemacr;, a = 14.443(4) ?, b = 15.392(4) ?, c = 11.675(2)?, alpha = 101.44(2) degrees, beta = 107.85(2) degrees, gamma = 96.36(2) degrees, V = 2380(1) ?(3), Z = 2. The coordination geometry of the ruthenium(II) ion is approximately octahedral. The dihedral angle between the two pyridyl rings in bbbpyH(2) is 9.4(3) degrees, which is close to coplanar, in the complex. Mono- and dinuclear complexes exhibit broad charge-transfer absorption bands at 420-520 nm and emission at 660-720 nm in CH(3)CN solution with lifetimes of 200-800 ns at room temperature. Transient difference absorption spectra and resonance Raman (rR) spectra were used to assign the charge-transfer bands in the 420-520 nm region and to identify the lowest excited states. Both absorption and emission spectra are sensitive to solvent and solution pH. Deprotonation of the dinuclear complex raises the energies of the pi orbitals of the bbbpyH(2) ligand, so that they become closer in energy to the pi orbitals of bpy. The intervalence band of [(bpy)(2)Ru(bbbpyH(2))Ru(bpy)(2)](5+)()()is observed at 1200 nm ( epsilon = 170 M(-)(1) cm(-)(1)) in CH(3)CN. The value of the electronic coupling matrix element, H(AB), was determined as 120 cm(-)(1). Upon deprotonation, the IT band was not observed. It is therefore concluded that a superexchange pathway occurs predominantly via the Ru(II) dpi-bbbpyH(2) pi interaction, since deprotonation decreases the interaction. The role of the intervening fragments in the bridging ligand is discussed from the viewpoint of orbital energies and their orbital mixing with Ru dpi orbitals.     

Characterization of low energy charge transfer transitions in (terpyridine)(bipyridine)ruthenium(II) complexes and their cyanide-bridged bi- and tri-metallic analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption bands found in ambient solutions of a series of [Ru(tpy)(bpy)X](m+) complexes (tpy = 2,2':3',2'-terpyridine; bpy = 2,2'-bipyridine; and X = a monodentate ancillary ligand) feature one or two partly resolved weak absorptions (bands I and/or II) on the low energy side of their absorption envelopes. Similar features are found for the related cyanide-bridged bi- and trimetallic complexes. However, the weak absorption band I of [(bpy)(2)Ru{CNRu(tpy)(bpy)}(2)](4+) is missing in its [(bpy)(2)Ru{NCRu(tpy)(bpy)}(2)](4+) linkage isomer demonstrating that this feature arises from a Ru(II)/tpy MLCT absorption. The energies of the MLCT band I components of the [Ru(tpy)(bpy)X](m+) complexes are proportional to the differences between the potentials for the first oxidation and the first reduction waves of the complexes. Time-dependent density functional theory (TD-DFT) computational modeling indicates that these band I components correspond to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered. The most intense contribution to a lowest energy MLCT absorption envelope (band III) of these complexes corresponds to the convolution of several orbitally different components, and its absorption maximum has an energy that is about 5000 cm(-1) higher than that of band I. The multimetallic complexes that contain Ru(II) centers linked by cyanide have mixed valence excited states in which more than 10% of electronic density is delocalized between the nearest neighbor ruthenium centers, and the corresponding stabilization energy contributions in the excited states are indistinguishable from those of the corresponding ground states. Single crystal X-ray structures and computational modeling indicate that the Ru-(C≡N)-Ru linkage is quite flexible and that there is not an appreciable variation in electronic structure or energy among the conformational isomers.     

Light-induced charge separation and photocatalytic hydrogen evolution from water using Ru(II)Pt(II)-based molecular devices: effects of introducing additional donor and/or acceptor sites

In our hopes to improve the photocatalytic efficiency of photo-hydrogen-evolving molecular devices, several new dyads and triads possessing a photosensitizing Ru(bpy)(phen)(2)(2+) (or Ru(phen)(3)(2+)) chromophore (abbreviated as Ru(II)) attached to both/either a phenothiazine moiety (abbreviated as Phz) and/or H(2)-evolving PtCl(2)(bpy) units (abbreviated as Pt), such as Phz-Ru(II)-Pt2 (triad), Ru(II)-Pt2 (dyad), and Ru(II)-Pt3 (dyad), were synthesized and their basic properties together with the photo-hydrogen-evolving characteristics were investigated in detail. The (3)MLCT phosphorescence from the Ru(II) moiety in these systems is substantially quenched due to the highly efficient photoinduced electron transfer (PET). Based on the electrochemical studies, the driving forces for the PET were estimated as -0.07 eV for Phz-Ru(II)-Pt2, -0.24 eV for Ru(II)-Pt2, and -0.22 eV for Ru(II)-Pt3, revealing the exergonic character of the PET in these systems. Luminescence lifetime studies revealed the existence of more than two decay components, indicative of a contribution of multiple PET processes arising from the presence of at least two different conformers in solution. The major luminescence decay components of the hybrid systems [τ(1) = 6.5 ns (Ru(II)-Pt2) and τ(1) = 1.04 ns (Phz-Ru(II)-Pt2) in acetonitrile] are much shorter than those of Phz-free/Pt-free Ru(bpy)(phen)(2)(2+) derivatives. An important finding is that the triad Phz-Ru(II)-Pt2 affords a quite long-lived charge separated (CS) state (τ(CS) = 43 ns), denoted as Phz(+)˙-Ru(Red)-Pt2, as a result of reductive quenching of the triplet excited state of Ru(bpy)(phen)(2)(2+) by the tethering Phz moiety, where Ru(Red) denotes Ru(bpy)(phen)(2)(+). Moreover, the lifetime of Phz(+)˙-Ru(Red)-Pt2 was observed to be much longer than that of Phz(+)˙-Ru(Red). The photocatalytic H(2) evolution from water driven by these systems was examined in an aqueous acetate buffer solution (pH 5.0) containing 4-19% dimethylsulfoxide (solubilising reagent) in the presence of EDTA as a sacrificial electron donor. Dyads Ru(II)-Pt2 and Ru(II)-Pt3 were found to exhibit improved photo-hydrogen-evolving activity compared to the heterodinuclear Ru-Pt dyads developed so far in our group. On the other hand, almost no catalytic activity was observed for Phz-Ru(II)-Pt2 in spite of the formation of a strongly reducing Ru(Red) site (Phz(+)˙-Ru(Red)-Pt2), indicating that the electron transfer from the photogenerated Ru(Red) unit to the PtCl(2)(bpy) unit is not favoured presumably due to the slow electron transfer rate in the Marcus inverted region.     

Synthesis, luminescence, and electrochemical studies of tris(homoleptic) ruthenium(II) and osmium(II) complexes of 6'-tolyl-2,2':4',2'-terpyridine

The synthesis and photophysical and electrochemical properties of tris(homoleptic) complexes [Ru(tpbpy)3](PF6)2 (1) and [Os(tpbpy)3](PF6)2 (2) (tpbpy = 6'-tolyl-2,2':4',2' '-terpyridine) are reported. The ligand tpbpy is formed as the side product during the synthesis of 4'-tolyl-2,2':6',2' '-terpyridine (ttpy) and characterized by single-crystal X-ray diffraction: monoclinic, P21/c. The tridentate tpbpy coordinates as a bidentate ligand. The complexes 1 and 2 exhibit two intense absorption bands in the UV region (200-350 nm) assignable to the ligand-centered (1LC) pi-pi* transitions. The ruthenium(II) complex exhibits a broad absorption band at 470 nm while the osmium(II) complex exhibits an intense absorption band at 485 nm and a weak band at 659 nm assignable to the MLCT (dpi-pi*) transitions. A red shifting of the dpi-pi* MLCT transition is observed on going from the Ru(II) to the Os(II) complex as expected from the high-lying dpi Os orbitals. These complexes exhibit ligand-sensitized emission at 732 and 736 nm, respectively, upon light excitation onto their MLCT band through excitation of higher energy LC bands at room temperature. The MLCT transitions and the emission maxima of 1 and 2 are substantially red-shifted compared to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2. The emission of both the complexes in the presence of acid is completely quenched indicating that the emission is not due to the protonation of the coordinated ligands. Our results indicate the occurrence of intramolecular energy transfer from the ligand to the metal center. Both the complexes undergo quasi-reversible metal-centered oxidation, and the E1/2 values for the M(II)/M(III) redox couples (0.94 and 0.50 V versus Ag/Ag+ for 1 and 2, respectively) are cathodically shifted with respect to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2 (E1/2 = 1.28 and 1.09 V versus Ag/Ag+, respectively). The tris(homoleptic) Ru(II) and Os(II) complexes 1 and 2 could be used to construct polynuclear complexes by using the modular synthetic approach in coordination compounds by exploiting the coordinating ability of the pyridine substituent. Furthermore, these complexes offer the possibility of studying the influence of electron-withdrawing and electron-donating substituents on the photophysical properties of Ru(II) and Os(II) polypyridine complexes.     

Synthesis and electrochemical properties of bipyrimidine bridged triruthenium complexes

A new μ(4)-bpym-bridged dimer of an oxoacetao-triruthenium complex with carbonyl, [{Ru(3)O(CH(3)COO)(5)(CO)(py)}(2)(μ(4)-bpym)], was synthesized. The complex possesses two stable mixed-valence states associated with Ru(3)(III,III,II)/Ru(3)(III,II,II) and Ru(3)(III,III,III)/Ru(3)(III,III,II). The IR-spectroelectrochemistry reveals ν(CO) spectra in five oxidation states, Ru(3)(III,III,III)-Ru(3)(III,III,III) to Ru(3)(III,II,II)-Ru(3)(III,II,II) and both the mixed-valence states show a spectrum indicating medium interaction between the Ru(3) units.     

Bis(bipyridine)ruthenium(II) complexes with an aliphatic sulfinato donor: synthesis, characterization, and properties

Treatment of a thiolato-bridged Ru(II)Ag(I)Ru(II) trinuclear complex, [Ag{Ru(aet)(bpy)(2)}(2)](3+) (aet = 2-aminoethanthiolate; bpy = 2,2'-bipyridine), with NaI in aqueous ethanol under an aerobic condition afforded a mononuclear ruthenium(II) complex having an S-bonded sulfinato group, [1](+) ([Ru(aesi-N, S)(bpy)(2)](+) (aesi = 2-aminoethanesulfinate)). Similar treatment of optically active isomers of an analogous Ru(II)Ag(I)Ru(II) trinuclear complex, Δ(D)Δ(D)- and Λ(D)Λ(D)-[Ag{Ru(d-Hpen-O,S)(bpy)(2)}(2)](3+) (d-pen = d-penicillaminate), with NaI also produced mononuclear ruthenium(II) isomers with an S-bonded sulfinato group, Δ(D)- and Λ(D)-[2](+) ([Ru(d-Hpsi-O,S)(bpy)(2)](+) (d-psi = d-penicillaminesulfinate)), respectively, retaining the bidentate-O,S coordination mode of a d-Hpen ligand and the absolute configuration (Δ or Λ) about a Ru(II) center. On refluxing in water, the Δ(D) isomer of [2](+) underwent a linkage isomerization to form Δ(D)-[3] (+) ([Ru(d-Hpsi-N,S)(bpy)(2)](+)), in which a d-Hpsi ligand coordinates to a Ru(II) center in a bidentate-N,S mode. Complexes [1](+), Δ(D)- and Λ(D)-[2](+), and Δ(D)-[3](+) were fully characterized by electronic absorption, CD, NMR, and IR spectroscopies, together with single-crystal X-ray crystallography. The electrochemical properties of these complexes, which are highly dependent on the coordination mode of sulfinate ligands, are also described.     

Fused donor-acceptor ligands in RuII chemistry: synthesis, electrochemistry and spectroscopy of [Ru(bpy)3-n(TTF-dppz)n](PF6)2.

Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)(3-n)(TTF-dppz)n](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru(2+)-->dppz metal-to-ligand charge-transfer ((3)MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the (3)MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz- -Ru(2+)-dppz-TTF(+). The lifetime of this LLCS state is approximately 2.3 micros, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower.     

Charge delocalization of 1,4-benzenedicyclometalated ruthenium: a comparison between tris-bidentate and bis-tridentate complexes

A dimetallic biscyclometalated ruthenium complex, [(bpy)(2)Ru(dpb)Ru(bpy)(2)](2+) (bpy = 2,2'-bipyridine; dpb = 1,4-di-2-pyridylbenzene), with a tris-bidentate coordination mode has been prepared. The electronic properties of this complex were studied by electrochemical and spectroscopic analysis and DFT/TDDFT calculations on both rac and meso isomers. Complex [(bpy)(2)Ru(dpb)Ru(bpy)(2)](2+) has a similar 1,4-benzenedicyclometalated ruthenium (Ru-phenyl-Ru) structural component with a previously reported bis-tridentate complex, [(tpy)Ru(tpb)Ru(tpy)](2+) (tpy = 2,2';6',2″-terpyridine; tpb = 1,2,4,5-tetra-2-pyridylbenzene). The charge delocalizations of these complexes across the Ru-phenyl-Ru array were investigated and compared by studying the corresponding one-electron-oxidized species, generated by chemical oxidation or electrochemical electrolysis, with DFT/TDDFT calculations and spectroscopic and EPR analysis. These studies indicate that both [(bpy)(2)Ru(dpb)Ru(bpy)(2)](3+) and [(tpy)Ru(tpb)Ru(tpy)](3+) are fully delocalized systems. However, the coordination mode of the metal component plays an important role in influencing their electronic properties.     

Fine tuning of the electronic coupling between metal centers in cyano-bridged mixed-valent trinuclear complexes

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.     

Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear ruthenium(II) and osmium(II) complexes bridged by 2,4,6-tris(2-pyridyl)-1,3,5-triazine: synthesis, structural characterization, stereochemistry, and electrochemical studies

The reaction of cis-[M(bpy)2Cl2] (M = Ru(II), and Os(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol-water resulted in the formation of dinuclear complexes of the composition [(M(bpy)2)2(tptz-OH)](PF6)3.nH2O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (delta delta/lambda lambda) and a meso form (delta lambda/lambda delta). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data: complex 3-II, monoclinic, C2/c, a = 24.584(7) A, b = 14.309(4) A, c = 41.044(13) A, beta = 92.84(2) degrees, V = 14420.0(7) A3, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1, a = 13.444(7) A, b = 14.576(5) A, c = 19.641(7) A, alpha = 98.21(3) degrees, beta = 101.67(4) degrees, gamma = 105.80(4) degrees, V = 3546.0(3) A3, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. 1H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (Kcom) of the mixed-valence species have been calculated; the values are in the range 6.03 x 10(4)-4.7 x 10(6). It appears that a metal-metal interaction occurred by an electron-transfer mode across the low-lying pi* orbital of the bridged tptz.