Ligand Electrochemical Parameter Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes.

Using the density functional theory, [(N2)RuIIL5]n+ species are studied in silico. The properties of the Ru-N2 bond are derived, including σ-donation, π-back donation, Ru-N and N-N bond lengths and bond orders, net charges and NN stretching frequencies, and so forth. These data are correlated using the ligand electrochemical parameter (EL) theory, whereby the availability of electrons in the [RuL5]n+ fragment is defined by its electron richness, which is the sum of the EL parameters, ΣEL(L5). The objective is to better understand the binding of the N2 ligand, leading to a molecular design whereby a specific species is constructed to have a desired property, for example, a particular bond length or charge. We supply cubic expressions linking ΣEL(L5) with these many metrics, allowing researchers to predict metric values of their own systems. The extended charge decomposition analysis is used. For the given target, N2, σ-bonding does not vary greatly with the nature of ligand L, and π-back donation is the dominant property deciding the magnitudes of the various metrics. The EL parameter provides the path to design the desired species. This contribution is devoted to dinitrogen, but the method is expected to be general for any ligand, including polydentate ligands.

Modeling ligand electrochemical parameters by repulsion-corrected eigenvalues.

Ligand electrochemical parameters, EL , more commonly known as Lever parameters, have played a major research role in understanding redox processes involved in inorganic electrochemistry, enzymatic reactions, catalysis, solar cells, biochemistry, and materials science. Despite their broad usefulness, Lever parameters are not well understood at a first-principles level. Using density functional theory, we demonstrate in this contribution that a ligand's Lever parameter is fundamentally related to the ligand's ability to alter the eigenvalue of the electroactive spin-orbital in an octahedral transition metal complex. Our analysis furthers a first-principles understanding of the nature of Lever parameters.

Photosolvolysis of cis-[Ru(α-diimine)2(4-aminopyridine)2](2+) complexes: photophysical, spectroscopic, and density functional theory analysis.

The photochemical and photophysical properties of the cis-[Ru(II)(α-diimine)2(4-APy)2](2+) complexes, where α-diimine = 1,10-phenanthroline (phen) and 4-APy = 4-aminopyridine I, 4,7-diphenyl-1,10-phenanthroline (Ph2phen) II, 2,2'-bipyridine (bpy) III, and 4,4'-dimethyl-2,2'-bipyridine (Me2bpy) IV, are reported. The four complexes were characterized using high-performance liquid chromatography, (1)H NMR, UV-visible, emission, and transient absorption spectroscopy. Upon photolysis in acetonitrile solution these complexes undergo 4-APy dissociation to give the monoacetonitrile complex (for II, III, and IV) or the bis(acetonitrile) complex (for I). A fairly wide range of excitation wavelengths (from 420 to 580 nm) were employed to explore the photophysics of these systems. Quantum yields and transient spectra are provided. Density functional theory (DFT) and time-dependent DFT analysis of singlet and triplet excited states facilitated our understanding of the photochemical behavior. A detailed assessment of the geometric and electronic structures of the lowest energy spin triplet charge transfer state ((3)MLCT) and spin triplet metal centered state ((3)MC) (dπ → σ* transitions) for species I-IV is presented. A second, previously unobserved, and nondissociative, (3)MC state is identified and is likely involved in the primary step of photodissociation. This new (3)MC state may indeed play a major role in many other photodissociation processes.

Binuclear ruthenium complexes of a neutral radical bridging ligand. A new "spin" on mixed valency.

The electronic structures of (LX)2Ru(Vd)Ru(LX)2 complexes (Vd = 1,5-diisopropyl-3-(4,6-dimethyl-2-pyrimidinyl)-6-oxoverdazyl radical; LX = acac (acetylacetonate) or hfac (hexafluoroacetylacetonate)) in multiple charge states have been investigated experimentally and computationally. The main focus was to probe the consequences of the interplay between the ruthenium ions and the redox-active verdazyl ligand for possible mixed-valent behavior. Cyclic voltammetry studies reveal one reversible reduction and one reversible oxidation process for both complexes; in addition the acac-based derivative possesses a second reversible oxidation. Analysis of a collection of experimental (X-ray structures, EPR, electronic spectra) and computational (TD-DFT (PCM)) data reveal that the ruthenium ancillary ligands (acac vs hfac) have dramatic consequences for the electronic structures of the complexes in all charge states studied. In the hfac series, the neutral complex is best regarded as a binuclear Ru(II) species bridged by a neutral radical ligand. Reduction to give the anionic complex takes place on the verdazyl ligand, whereas oxidation to the cation (a closed shell species) is shared between Vd and ruthenium. For the acac-based complexes, the neutral species is most accurately represented as a Ru(II)/Ru(III) mixed valent complex containing a bridging verdazyl anion, though some bis(Ru(II))-neutral radical character remains. The monocation complex contains a significant contribution from a "broken symmetry" singlet diradical structure, best represented as a bis-Ru(III) system with an anionic ligand, with significant spin coupling of the two Ru(III) centers via the Vd(-1) ligand (calculated J = -218 cm(-1)). The dication, a spin doublet, consists of two Ru(III) ions linked (and antiferromagnetically coupled) to the neutral radical ligand. Computed net σ- and π-back-donation, spin densities, and orbital populations are provided. Time dependent DFT is used to predict the optical spectra and assign experimental data.

Phenylcyanamidoruthenium scorpionate complexes.

Nine [Ru(Tp)(dppe)L] complexes, where Tp is hydrotris(pyrazol-1-yl)borate, dppe is ethylenebis(diphenylphosphine), and L is (4-nitrophenyl)cyanamide (NO(2)pcyd(-)), (2-chlorophenyl)cyanamide (2-Clpcyd(-)), (3-chlorophenyl)cyanamide (3-Clpcyd(-)), (2,4-dichlorophenyl)cyanamide (2,4-Cl(2)pcyd(-)), (2,3-dichlorophenyl)cyanamide (2,3-Cl(2)pcyd(-)), (2,5-dichlorophenyl)cyanamide (2,5-Cl(2)pcyd(-)), (2,4,5-trichlorophenyl)cyanamide (2,4,5-Cl(3)pcyd(-)), (2,3,5,6-tetrachlorophenyl)cyanamide (2,3,5,6-Cl(4)pcyd(-)), and (pentachlorophenyl)cyanamide (Cl(5)pcyd(-)), and the dinuclear complex [{Ru(Tp)(dppe)}(2)(µ-adpc)], where adpc(2-) is azo-4,4-diphenylcyanamide, have been prepared and characterized. The crystal structures of [Ru(Tp)(dppe)(Cl(5)pcyd)] and [{Ru(Tp)(dppe)}(2)(µ-adpc)] reveal the Ru(II) ion to occupy a pseudooctahedral coordination sphere in which the cyanamide ligand coordinates to Ru(II) by its terminal nitrogen atom. For both complexes, the cyanamide ligands are planar, indicating significant π mixing between the cyanamide and phenyl moieties as well as the azo group in the case of adpc(2-). The optical spectra of the nominally ruthenium(III) species [Ru(Tp)(dppe)L](+) were obtained through spectroelectrochemistry measurements and showed an intense near-IR absorption band. Time-dependent density functional theory calculations of these species revealed that oxidation of the ruthenium(II) species led to species where partial oxidation of the cyanamide ligand had occurred, indicative of noninnocent character for these ligands. The spin densities reveal that while the 3-Clpycd species has substantial Ru(II)(3-Clpycd(0)) character, the Cl(5)pycd species is a much more localized ruthenium(III) complex of the Cl(5)pycd monoanion. Some bond order and charge distribution data are derived for these ruthenium(III) species. The near-IR band is assigned as a quite complex mixture of d-d, 4d(π) to L(NCN) MLCT, and L(NCN) to Ru 4d LMCT with even a scorpionate ligand component. Spectroelectrochemistry was also performed on [{Ru(Tp)(dppe)}(2)(µ-adpc)] to generate the mixed-valence state. The intense intervalence transition that is observed in the near-IR is very similar to that previously reported for [{Ru(trpy)(bpy)}(2)(µ-adpc)](2+), where trpy is 2,2':6',2"-terpyridine and bpy is 2,2'-bipyridine, and by analogy identifies [{Ru(Tp)(dppe)}(2)(µ-adpc)](+) as a delocalized mixed-valence complex.

Proton-induced disproportionation of a ruthenium noninnocent ligand complex yielding a strong oxidant and a strong reductant.

The complex Ru(II)(NH(3))(2)(o-benzoquinonediimine)Cl(2) undergoes a reversible apparent acid/base reaction, although it has no obvious basic lone pairs. The reaction is a proton-assisted disproportionation yielding an oxidant ([Ru(III)(NH(3))(2)(o-benzoquinonediimine)Cl(2)](+)) and a reductant ([Ru(III)(NH(3))(2)(o-phenylenediamine)Cl(2)](+)). These species were characterized by electrochemistry, ultraviolet-visible light (UV-vis), vibrational (infrared (IR) and Raman), mass and electron paramagnetic resonance (EPR) spectroscopy, and X-ray structural analysis. The reaction is shown to be downhill from an isodesmic calculation. Three different isosbestic interconversions of the parent and product species are demonstrated. The electronic structures of these species were analyzed, and their optical spectra assigned, using density functional theory (DFT) and time-dependent DFT. This disproportionation of a noninnocent ligand complex may be relevant to the application of noninnocent ligands in organometallic catalysis and in the biological milieu.

catena-Poly[[[cis-aqua-dibromido-cobalt(II)]-µ-(pyrazine-2-carb-oxy-lic acid)-κN,O:N] monohydrate].

The title compound, {[CoBr(2)(C(5)H(4)N(2)O(2))(H(2)O)]·H(2)O}(n), is a one-dimensional coordination polymer which crystallizes as a monohydrate. The asymmetric unit contains one Co(II) atom in a distorted octa-hedral geometry, forming a chain parallel to [010] with the pyrazine carb-oxy-lic acid ligands coordinating on one side in a bidentate fashion through one N and one O atom, and in a monodentate fashion through a N atom, with N atoms trans, and with both ligands lying in the same plane. The bromide atoms are cis to each other, while a water mol-ecule occupies the final octa-hedral coordination site. The chains are linked together though an O-H⋯Br hydrogen bonding network, and are further stabilized by an O-H⋯Br and O-H⋯O hydrogen-bonding framework with the solvent water mol-ecule.

Electronic structure investigations of neutral and charged ruthenium bis(ß-diketonate) complexes of redox-active verdazyl radicals.

The electronic structures of (Vd)Ru(LX)(2) complexes (Vd = 1,5-diisopropyl-3-(2-pyridyl)-6-oxoverdazyl radical; LX = acac or hfac) as neutral, cationic, and anionic species have been investigated experimentally and computationally to probe the interplay between the ruthenium ion and the redox-active verdazyl ligand. The cationic complexes were prepared by oxidation of the corresponding neutral species with silver(I) salts. The hfac-based anionic complex was synthesized by reduction of the neutral species with cobaltocene, but the anionic acac analogue could not be prepared. Experimental (X-ray structures, electronic spectra) and computational (TD-DFT (PCM)) studies reveal that the expression of redox activity of the ligand and metal moieties is highly sensitive to the nature of the ancillary ligands on ruthenium. In the hfac series, the cationic, neutral, and anionic complexes can, respectively, be adequately described as Ru(II) complexes of a coordinated verdazyl cation, neutral radical, and anion. However, the more electron-donating acac coligands facilitate a stronger interaction between ruthenium and verdazyl via Ru(d) to Vd(π*) backbonding which is dependent on the overall charge of the complex and has the net effect of creating a high degree of metal-ligand covalency. Studies on the two cationic complexes reveal further distinctions between the acac- and hfac-containing systems: whereas the former has a significant open-shell singlet contribution to the complex ground state, this open-shell formulation is a minor component of the latter.

Verdazyl radicals as redox-active, non-innocent, ligands: contrasting electronic structures as a function of electron-poor and electron-rich ruthenium bis(beta-diketonate) co-ligands.

Spectroscopic, structural, and computational studies of verdazyl radical-ruthenium complexes reveal the verdazyl to be a redox-active and non-innocent ligand. The ensuing delocalization of charge is highly sensitive to the nature of the beta-diketonate co-ligands.

Synthesis and characterization of ruthenium bis-bipyridine mono- and disulfinato complexes.

Reaction of cis-Ru(bpy)(2)Cl(2) with 1,2-benzenedithiol afforded a monosulfhydryl-monosulfinate complex, [Ru(bpy)(2)(S.SO(2))] (1). Complex 1 readily undergoes oxidation when treated with 30% H(2)O(2) and also upon exposure to atmospheric O(2) (rapidly in bright light) to afford the disulfinate complex, [Ru(bpy)(2)(SO(2.)SO(2))] (2). Complexes 1 and 2 were studied using various analytical techniques including elemental analysis, UV-vis, mass spectroscopy, NMR, IR spectroscopy, cyclic voltammetry, X-ray crystallography (for 2). Density functional theory computation was employed with extended charge decomposition and natural population analyses. The agreement between the observed electronic spectrum and that predicted by time dependent DFT, and between the observed infrared spectrum and that predicted by DFT, is truly exceptional. These molecules are relevant to the very unusual active site in the metalloenzyme nitrile hydratase.

Spectroscopic, electrochemical, and computational aspects of the charge distribution in Ru(acac)2(R-o-benzoquinonediimine) complexes.

The syntheses and properties are reported for five Ru(acac)2(R-bqdi) species where acac is acetylacetonate, and R-bqdi is the non-innocent ligand ortho-benzoquinonediimine substituted with R = H (1), 4,5-dimethyl (2), 4-Cl (3), or 4-NO2 (4), and N,N''-dimethylsulfonyl (5). Their identities and purities were confirmed by NMR, mass spectra, IR and analytical data. The large degree of metal-to-ligand pi-back-donation was analyzed by spectroscopic (UV/visible, IR, Raman) and electrochemical data, supported by molecular orbital composition computations using density functional theory (DFT), with the polarizable continuum model (PCM) to mimic the presence of solvent, and prediction of electronic spectra using time-dependent DFT methods. Extended charge decomposition analysis (ECDA) and natural population analysis (NPA) both produced a detailed picture of the bonding between the non-innocent bqdi ligand and the metal center, allowing correlations to be drawn between the nature of the R substituents and the quantitative extent of pi-back-donation and sigma-forward donation. In conclusion, the issue of whether these species are best regarded as Ru(II)(quinonediimine) or coupled Ru(III)(semiquinonediiminate) species is discussed.

4,4'-Dichloro-N,N'-(o-phenyl-ene)dibenzene-sulfonamide.

The title compound, C(18)H(14)Cl(2)N(2)O(4)S(2), is a diamine that is a precursor to a quinonoid bidentate redox-active ligand. The dihedral angles between the central phenyl ring and the end rings are 87.5(1) and 60.7(1)°, while the two end rings make a dihedral angle of 82.5(1)°. The crystal structure is stabilized by two weak inter-molecular N-H⋯O hydrogen bonds, as well as one intra-molecular C-H⋯O and one N-H⋯N hydrogen bond.

Phosphine-substituted dithiolene complexes as ligands: communication between ruthenium(II) centers through a dimolybdenum bis(dithiolene) core.

The reaction of Mo2(SCH2CH2S)2Cp2 (1; Cp=eta-C5H5) with an excess of an alkyne in refluxing dichloromethane affords the bis(dithiolene) complexes Mo2(micro-SCR1=CR2S)2Cp2 (2a, R1=R2=CO2Me; 2b, R1=R2=Ph; 2c, R1=H, R2=CO2Me) whereas with 1 equiv of alkyne at room temperature the mixed dithiolene-dithiolate species Mo2(micro-SCR1=CR2S)(micro-SCH2CH2S)Cp2 (3a, R1=R2=CO2Me; 3b, R1=R2=Ph) are formed. The remaining dithiolate ligand in 3 can then be converted into a different dithiolene by reaction with a second alkyne. Applying this methodology, we have used bis(diphenylphosphino)acetylene to prepare the first examples of complexes containing phosphine-substituted dithiolene ligands: Mo2{micro-SC(CO2Me)=C(CO2Me)S}{micro-SC(PPh2)=C(PPh2)S}Cp2 (2g) and Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2 (2h). Tri- and tetrametallic complexes can then be assembled by coordination of these diphosphines to CpRuCl units by reaction with CpRu(PPh3)2Cl. Electrochemical studies of the Ru(II)/Ru(III) couple in Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2(RuClCp)2 (4b) reveals that the two separate ruthenium centers are oxidized electrochemically at different potentials, demonstrating communication between them through the dimolybdenum bis(dithiolene) core. Density functional theory calculations were carried out to explore the electronic structures of these species and to predict and assign their electronic spectra.

The very covalent diammino(o-benzoquinonediimine) dichlororuthenium(II). An example of very strong pi-back-donation.

The syntheses and X-ray structures of the complexes Ru(S-dmso)Cl2(opda) (1) and Ru(NH3)2Cl2(bqdi) (2) are described (opda= o-phenylenediamine, bqdi= o-benzoquinonediimine). Optical absorption and emission, vibrational (resonance Raman), and electrochemical data are discussed. We explore the nature of the ruthenium benzoquinone electronic interaction in species 2 primarily within the framework of density functional theory (DFT) but also using INDO/S to extract Coulombic and exchange integrals. The resonance Raman and emission data were understood in terms of a common set of coupled vibrations localized primarily within the ruthenium metallacycle ring. Experimental and computational data were also compared among a select group of ruthenium bqdi species with other spectator ligands, specifically ammonia, 2,2'-bipyridine, and 2,4-pentanedione. The changes in the electrochemistry, optical spectroscopy, and vibrational spectra with changing spectator ligand donicity were explained within a common theoretical (DFT) model which further provided a detailed analysis of the variation in the molecular orbital descriptions. With the application of an extended charge decomposition analysis (ECDA), a detailed picture emerged of the bonding between the bqdi ligand and the metal atom, illustrating the coupling between the orbitals of each fragment as a function of orbital symmetry and charge transfer between the fragments of the complex. Metal-to-bqdi pi-back-donation is seen to be very important.

DFT-INDO/S modeling of new high molar extinction coefficient charge-transfer sensitizers for solar cell applications.

A new ruthenium(II) complex, tetrabutylammonium [ruthenium (4-carboxylic acid-4'-carboxylate-2,2'-bipyridine)(4,4'-di(2-(3,6-dimethoxyphenyl)ethenyl)-2,2'-bipyridine)(NCS)(2)] (N945H), was synthesized and characterized by analytical, spectroscopic, and electrochemical techniques. The absorption spectrum of the N945H sensitizer is dominated by metal-to-ligand charge-transfer (MLCT) transitions in the visible region, with the lowest allowed MLCT bands appearing at 25 380 and 18 180 cm(-1). The molar extinction coefficients of these bands are 34 500 and 18 900 M(-1) cm(-1), respectively, and are significantly higher when compared to than those of the standard sensitizer cis-dithiocyanatobis(4,4'-dicarboxylic acid-2,2'-bipyridine)ruthenium(II). An INDO/S and density functional theory study of the electronic and optical properties of N945H and of N945 adsorbed on TiO(2) was performed. The calculations point out that the top three frontier-filled orbitals have essentially ruthenium 4d (t(2g) in the octahedral group) character with sizable contribution coming from the NCS ligand orbitals. Most critically the calculations reveal that, in the TiO(2)-bound N945 sensitizer, excitation directs charge into the carboxylbipyridine ligand bound to the TiO(2) surface. The photovoltaic data of the N945 sensitizer using an electrolyte containing 0.60 M butylmethylimidazolium iodide, 0.03 M I(2), 0.10 M guanidinium thiocyanate, and 0.50 M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio = 85:15) exhibited a short-circuit photocurrent density of 16.50 +/- 0.2 mA cm(-2), an open-circuit voltage of 790 +/- 30 mV, and a fill factor of 0.72 +/- 0.03, corresponding to an overall conversion efficiency of 9.6% under standard AM (air mass) 1.5 sunlight, and demonstrated a stable performance under light and heat soaking at 80 degrees C.

Electrochemistry of metal phthalocyanines in organic solvents at variable pressure.

High-pressure electrochemical investigations of representative metallophthalocyanines in solution are reported. The selected systems were ZnPc, CoPc, FePc, and CoTNPc (Pc = phthalocyanine, TNPc = tetraneopentoxyphthalocyanine) in several donor solvents and (for CoTNPc) dichlorobenzene, with [Bu(4)N][ClO(4)] as supporting electrolyte and a conventional Pt electrode referred to Ag(+)(CH(3)CN)/Ag. Electrode reaction volumes deltaV(cell) for CoTNPc and ZnPc show that consecutive ring reductions result in progressive increases in electrostriction of solvent in accordance with Drude-Nernst theory. Reductions of the metal center in CoTNPc and CoPc, however, result in much less negative values of deltaV(cell) than would be expected by analogy with ring reductions of the same charge type. This is attributable to loss of axial ligands following the insertion of antibonding 3d(z)2 electrons on going from Co(III) to low-spin Co(II) and then Co(I). In the same vein, rate constants for reduction of Co(III) centers to Co(II) were an order of magnitude slower than those for other metal center or phthalocyanine ring reductions because of Franck-Condon restrictions. The volumes of activation deltaV(el) were invariably positive for all the electrode reactions and in most cases were roughly equal to the volumes of activation for reactant diffusion deltaV(diff)(), indicating predominant rate control by solvent dynamics rather than by activation in the manner of transition-state theory for which negative deltaV(el) values are expected. For CoTNPc and CoPc in donor solvents, the deltaV(cell) and deltaV(el) data are consistent with the assignments of the successive reduction steps made for CoTNPc in DMF by Nevin et al. (Inorg. Chem. 1987, 26, 570).

Electronic structure and spectra of linkage isomers of bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(ii) and their redox series.

Linkage isomers of bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(II), 1,2- and 1,9-coordinated complexes, and several of their oxidation products have been prepared chemically and/or electrochemically. For the 1,2-coordinated complex, the one- and two-electron oxidized species have been characterized, and for the 1,9-coordinated complex, the one-electron oxidized species has been characterized. The rich redox activity of these complexes leads to ambiguity in assessing the electronic structure. This paper reports EPR spectra of odd-electron species and detailed analyses of electronic spectra and structure of the complexes, based on INDO molecular orbital calculations. Results of calculations on the related 1-hydroxyanthraquinone complex and the free ligands,1,2-dihydroxy-9,10-anthraquinone (alizarin) and 1-hydroxyanthraquinone, are also briefly discussed.

Mono- and dinuclear ruthenium carbonyl complexes with redox-active dioxolene ligands: electrochemical and spectroscopic studies and the properties of the mixed-valence complexes.

The mononuclear complex [Ru(PPh(3))(2)(CO)(2)(L(1))] (1; H(2)L(1) = 7,8-dihydroxy-6-methoxycoumarin) and the dinuclear complexes [[Ru(PPh(3))(2)(CO)(2)](2)(L(2))][PF(6)] [[2][PF(6)]; H(3)L(2) = 9-phenyl-2,3,7-trihydroxy-6-fluorone] and [[Ru(PBu(3))(2)(CO)(2)](2)(L(3))] (3; H(4)L(3) = 1,2,3,5,6,7-hexahydroxyanthracene-9,10-dione) have been prepared; all complexes contain one or two trans,cis-[Ru(PR(3))(2)(CO)(2)] units, each connected to a chelating dioxolene-type ligand. In all cases the dioxolene ligands exhibit reversible redox activity, and accordingly the complexes were studied by electrochemistry and UV/vis/NIR, IR, and EPR spectroscopy in their accessible oxidation states. Oxidation of 1 to [1](+) generates a ligand-centered semiquinone radical with some metal character as shown by the IR and EPR spectra. Dinuclear complexes [2](+) and 3 show two reversible ligand-centered couples (one associated with each dioxolene terminus) which are separated by 690 and 440 mV, respectively. This indicates that the mixed-valence species [2](2+) has greater degree of electronic delocalization between the ligand termini than does [3](+), an observation which was supported by IR, EPR, and UV/vis/NIR spectroelectrochemistry. Both [2](2+) and [3](+) have a solution EPR spectrum consistent with full delocalization of the unpaired electron between the ligand termini on the EPR time scale (a quintet arising from equal coupling to all four (31)P nuclei); [3](+) is localized on the faster IR time scale (four CO vibrations rather than two, indicative of inequivalent [Ru(CO)(2)] units) whereas [2](2+) is fully delocalized (two CO vibrations). UV/vis/NIR spectroelectrochemistry revealed the presence of a narrow, low-energy (2695 nm) transition for [3](+) associated with the catecholate --> semiquinone intervalence transition. The narrowness and solvent-independence of this transition (characteristic of class III mixed-valence character) coupled with evidence for inequivalent [Ru(CO)(2)] termini in the mixed-valence state (characteristic of class II character) place this complex at the class II-III borderline, in contrast to [2](2+) which is clearly class III.

Novel ruthenium sensitizers containing functionalized hybrid tetradentate ligands: synthesis, characterization, and INDO/S analysis.

Ruthenium sensitizers of the type trans-[Ru(L(1))(X)(2)], trans-[Ru(L(2))(X)(2)], trans-[Ru(L(3))(X)(2)], and trans-[Ru(L(4))(X)(2)] (where L(1) = 6,6'-bis(1-H-benzimidazol-2-yl)-4,4'-bis(methoxycarbonyl)-2,2'-bipyridine, L(2) = 4,4' "-bis(tert-butyl)-4',4' '-bis[p-(methoxycarbonyl)phenyl]-2,2':6',2' ':6' ',2' "-quaterpyridine, L(3) = 4',4' '-bis[3,4-(dimethoxy)phenyl]-2,2':6',2' ':6' ',2' "-quaterpyridine, and L(4) = 4',4' '-diethoxycarbonyl-2,2':6',2' ':6' ',2' "-quaterpyridine; X = Cl(-), NCS(-)) were synthesized and characterized by CV, NMR, and UV-vis absorption and emission spectroscopy. The trans-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near-IR region. The lowest energy metal-to-ligand charge-transfer transition band of the trans-dichloro complexes is around 14 300 cm(-1) in DMF solution, and these complexes show weak and broad emission signals with onset at above 10 500 cm(-1). The absorption and emission maxima of the trans-dithiocyanate complexes are blue-shifted compared to those of its trans-dichloro analogues because of the strong pi acceptor property of the NCS(-) compared to the Cl(-). The electronic spectra of trans-[Ru(L)(X)(2)] complexes were calculated by INDO/S and compared with the experimental data. The extent of mixing between metal 4d and ligand pi orbitals is discussed. Extensive pi-back-donation is observed. The panchromatic response of these novel complexes renders them as suitable sensitizers for solar energy conversion applications based on titanium dioxide mesoporous electrodes. Preliminary results using the trans-[Ru(L(4))(NCS)(2)] complex show 75% incident photon-to-current efficiencies (IPCE), yielding 18 mA/cm(2) current density under standard AM 1.5 sunlight.

(Acetonitrile-N)(o-benzoquinone diimine-N,N')chloro-trans-bis(triphenylphosphine-P)ruthenium(II) hexafluorophosphate 0.25-hydrate.

The Ru atom in the title compound, [RuCl(CH(3)CN){P(C(6)H(5))(3)}(2){C(6)H(4)(NH)(2)}]PF(6).0.25H(2)O, has a six-coordinate octahedral geometry, with a trans arrangement of the triphenylphosphine groups. The asymmetric unit contains two complex molecules and a partially occupied water site. Principal dimensions include Ru-N 1.958 (4)-2.044 (5), Ru-P 2.3897 (16)-2.4092 (15), and Ru-Cl 2.4280 (15) and 2.4295 (16) A