Binuclear ruthenium and osmium mixed-valence complexes containing fused and flexible polypyridyl bridging ligands

{Os(bpy)₂}²⁺ and {Ru(CN)₄}²⁻ mononuclear and binuclear complexes with ligands 2,3-di-(2-pyridyl)quinoxaline (dpq) and dipyrido[2,3-a:3′,2′-c]phenazine (ppb) have been prepared. For the binuclear complexes a splitting in oxidation potentials is observed consistent with the formation of mixed-valence species with comproportionation constants (K_com) ranging from 2.5 × 10⁴ to 1.8 × 10⁶. The electronic absorption spectra of the mixed-valence species reveal IVCT transitions in the near infrared region. The absorption maximum for the IVCT band ranges from 5800 to 9980 cm⁻¹ and the extinction coefficients from 80 to 6300 M⁻¹ cm⁻¹. In general the {Os(bpy)₂}²⁺ complexes show larger K_com values and more intense IVCT bands than the corresponding {Ru(CN)₄}²⁻ complexes.     

Trans ruthenium(II) complexes with NH-bridged tetradentate symmetric and asymmetric polypyridyl ligands

NH-Bridged tetradentate ligands were synthesized to achieve stable trans Ru(II) bis(polypyridyl) complexes. The polypyridyl part of the ligand was either symmetric, as in N,N-bis(1,10-phenanthroline-2-yl)amine (phen-NH-phen), or asymmetric, as in N-(1,10-phenanthroline-2-yl)-N-(6-yl-dipyridyl[2,3-a:2',3'-c]phenazine)amine (dppz-NH-phen). Protonation of phen-NH-phen with trifluoroacetic acid and the subsequent reaction with RuCl3 yield trans-[Ru(phen-NH-phen)Cl2]. The chloro ligands in this compound can easily be replaced by stronger ligands, such as CH3CN and DMSO. In this way, complexes trans-[Ru(phen-NH-phen)(CH3CN)(DMSO)](PF6)2 (1), trans-[Ru(phen-NH-phen)(DMSO)2](PF6)2 (2), and trans-[Ru(phen-NH-phen)(CH3CN)2](PF6)2 (3) were obtained. X-ray structures were determined for 1 and 3. Following a procedure similar to that used with phen-NH-phen, the complex trans-[Ru(dppz-NH-phen)(CH3CN)2](PF6)2 (4) was obtained. To our knowledge, this is the first reported trans ruthenium(II) bis(polypyridyl) complex with two different polypyridyl ligands in the equatorial plane.     

Carbon dioxide reduction by mononuclear ruthenium polypyridyl complexes

New mononuclear ruthenium complexes with general formula [Ru(bid)(B)(Cl)] (bid is (1Z,3Z)-1,3-bis(pyridin-2-ylmethylene)isoindolin-2-ide; B = bidentate ligand 2,2'-bipyridine or R(2)-bpy, where R = COOEt or OMe) were synthesized and tested as precatalysts for the hydrogenative reduction of CO(2) in 2,2,2-trifluoroethanol (TFE) as solvent with added NEt(3). Significant amounts of formic acid were produced by these catalysts and a kinetic analysis based on initial rate constants was carried out. The potential mechanisms including intermediate species for these catalytic systems were investigated by means of quantum chemical calculations to gain deeper insight into the processes. The effect of electron-donating and electron-withdrawing groups on catalyst performance was studied both experimentally and theoretically.     

Stereochemistry and solid-state circular dichroism spectroscopy of eight-coordinate chiral lanthanide complexes

Eight-coordinate chiral lanthanide complexes [Eu(dbm)₃L ^RR ] (1), [Eu(dbm)₃L ^SS ] (2) and [Tb(dbm)₃L ^RR ] (3) (L ^RR /L ^SS = (-)-/(+)-4,5-pineno-2,2′-bipyridine, Hdbm = dibenzoylmethane) were synthesized stereoselectively, which were characterized by UV-vis, CD spectra and X-ray single-crystal diffraction. The mirror-image structure features of complexes 1 and 2 were obtained by combination of the solid-state CD spectra and the crystal structure analysis. After further comparison with the solid-state CD spectra of six-coordinate and seven-coordinate metal complexes containing β-diketone ligands, the CD spectra-absolute configuration correlation rule for the eight-coordinate β-diketonate lanthanide complexes was proposed through the exciton chirality method for the first time. The Δ or Λ absolute configurations of complexes 1–3 with the distorted square antiprism geometry were confirmed by the X-ray single-crystal analysis.     

手性八配位镧系络合物的立体化学与固体CD光谱

本文立体选择性地合成了八配位Ln（III）络合物[Eu（dbm）3L^RR]（1）、[Eu（dbm）3L^SS]（2）和[Tb（dbm）3L^RR]（3）（L^RR／L^SS=（-）／（＋）-4,5-蒎烯基联吡啶,Hdbm=二苯甲酰甲烷）,利用单晶衍射、Uv-vis和固体CD光谱对其进行了表征．晶体结构分析和固体CD光谱表明1和2互为对映体．采用激子手性方法,通过与六配位和七配位的β-二酮络合物的固体CD光谱的对比,提出含β-二酮的八配位Ln（III）络合物的绝对构型关联规则．该系列络合物的畸变四方反棱柱的△或∧绝对构型也被其单晶结构所证实．     

The study of enantioselectivity in copper complexes with some 1,3-dicarbonyl ligands by circular dichroism spectroscopy

The circular dichroism spectra of copper(II) complexes with hydroxymethylene camphor, hydroxymethylene menthone, and trifluoroacetyl camphor as well as of mixed-ligand complexes with acetylacetone have been studied in different solvents at different temperatures. The changes observed in the spectra are attributed to a strong coordination of the solvent molecules in axial positions of square-planar complexes. It has been shown that the contributions of two ligands to the net spectrum of circular dichroism are additive and that the absorption spectra of the complexes do not depend on the optical configuration of the ligands. The conclusion has been drawn that no noticeable inter-ligand interactions exist in the complex and no enantioselective effects are exhibited.     

Computational study of the lowest triplet state of ruthenium polypyridyl complexes used in artificial photosynthesis

The potential energy surfaces of the first excited triplet state of some ruthenium polypyridyl complexes were investigated by means of density functional theory. Focus was placed on the interaction between the geometrical changes accompanying the photoactivity of these complexes when used as antenna complexes in artificial photosynthesis and dye-sensitized solar cells and the accompanying changes in electronic structure. The loss process (3)MLCT --> (3)MC can be understood by means of ligand-field splitting, traced down to the coordination of the central ruthenium atom.     

The reactivity of nitrosyl ruthenium complexes containing polypyridyl ligands

A series of cis nitrosyl complexes containing polypyridyl ligands were prepared and characterized as cis-[RuL(bpy)₂(NO)](PF₆)₃ (L = pyridine, 4-picoline, or 4-acetylpyridine), by elemental analysis, u.v.–vis. and i.r. spectroscopy, and by electrochemical techniques such as cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and coulometry. The complexes exhibit stretching frequencies (NO) at ca. 1950 cm^–1 indicating that nitrosyl group has a sufficiently high degree of nitrosonium ion (NO⁺) character. In non-aqueous solution, the reduction of these complexes induce nitrosyl to nitro conversion. In aqueous solution the reduction product is cis-[RuL(bpy)₂(NH₃)]²⁺ formed by a six electron mechanism. The nitrosyl compounds are susceptible to nucleophilic attack by hydroxide ion. The equilibrium constants were determined.     

Chemical interactions in the surface-enhanced resonance Raman scattering of ruthenium polypyridyl complexes

Surface-enhanced resonance Raman scattering (SERRS) from the alpha-diimine complexes [Ru(bpm)(3)](2+) and [Ru(bpz)(3)](2+) is reported for the first time at a roughened silver electrode. In both cases, a possible adsorbate orientation has been proposed involving binding through nitrogen lone pair electrons to the silver surface, based on changes in band positions upon adsorption. The SERRS spectra of [Ru(bpm)(3)](2+) were found to change slightly with a change in applied potential. The relative intensity of the nu(C6C6') band was found to be dependent on both excitation wavelength and applied potential. This was ascribed to an active charge transfer (CT) mechanism operating synergistically with the electromagnetic mechanism. No such CT activity was observed in [Ru(bpz)(3)](2+). It is tentatively suggested that this behavior may arise from the different modes of adsorption of the two complexes.     

Dimeric ruthenium complexes of dicyanopyrazines: complexes with an unstable mixed-valence state

Pentaammineruthenium complexes bridged by 2,3-dicyanopyrazine and 2,3-dicyano-5, 6-dimethylpyrazine have been prepared and characterized. The bridging ligand binds to the metal atoms through the cyano nitrogen. Complexes can be prepared in the +4 and +6 oxidation states but the mixed-valence, +5, state is not observed experimentally, unlike the previously prepared 1,2-dicyanobenzene dimer which does have a stable +5 state. A simple electronic model is constructed from the spectroscopic data and is used to show that the electronic perturbations caused by the uncomplexed nitrogen atoms in the pyrazine ring are sufficient to drive the disproportionation reaction of the +5 state.     

Spectroscopic and magnetic studies on electrogenerated mixed-valence ruthenium complexes

Electrochemical synthesis has enabled several sequences of triple chloride bridged diruthenium complexes of general type [L_3?xCl_xRuCl₃RuCl_yL_3?y]^z/z+1/z+2 (L = soft neutral ligand) to be generated. The intervalence charge transfer bands in the optical spectra of the mixed-valence Ru^II,III₂ compounds and variable temperature magnetic measurements for the corresponding Ru^III.III₂ complexes reveal that the degree of metal—metal interaction in these confacial bioctahedral systems decreases as the molecular asymmetry (y?x) increases.     

DFT study of vibrational circular dichroism spectra of D-lactic acid-water complexes

This paper presents a discussion of the interaction energies, conformations, vibrational absorption (VA, harmonic and anharmonic) and vibrational circular dichroism (VCD) spectra for conformers of monomeric chiral d(-)-lactic acid and their complexes with water at the DFT(B3LYP)/aug-cc-pVDZ and DFT(B3LYP)/aug-cc-pVTZ levels. A detailed analysis has been performed principally for the two most stable complexes with water, differing by lactic acid conformation. The VCD spectra were found to be sensitive to conformational changes of both free and complexed molecules, and to be especially useful for discriminating between different chiral forms of intermolecular hydrogen bonding complexes. In particular, we show that the VCD modes of an achiral water molecule after complex formation acquire significant rotational strengths whose signs change in line with the geometry of the complex. Using the theoretical prediction, we demonstrate that the VCD technique can be used as a powerful tool for structural investigation of intermolecular interactions of chiral molecules and can yield information complementary to data obtained through other molecular spectroscopy methods.     

Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes

Electrocatalytic reduction of CO(2) by [Ru(tpy)(bpy)(solvent)](2+) (tpy = 2,2':6',2'-terpyridine, bpy = 2,2'-bipyridine) and its structural analogs is initiated by sequential 1e(-) reductions at the tpy and bpy ligands followed by rate limiting CO(2) addition to give a metallocarboxylate intermediate. It undergoes further reduction and loss of CO.     

Computational exploration of heterolytic halogen-carbon bond scission photoreactions in ruthenium polypyridyl complexes

Energy wasting charge recombination is an efficiency limiting process in efforts to achieve solar energy storage. Here, density functional theory is used to explore the thermodynamics of photochemical energy storage reactions in several ruthenium polypyridyl complexes where heterolytic halogen-carbon bond scission occurs after light-induced formation of the triplet metal to ligand charge transfer ((3)MLCT) state, as seen in the following reaction: [Ru(II)(A)(n)(L-X)](2+) + hν → [Ru(III)(A)(n)(L-X)(?-)](2+)* → [Ru(III)(A)(n)(L·)](3+) + X(-) (L = polypyridine ligand; X = Cl, Br, and I; A = ancillary ligand). A thermochemical cycle is employed to determine structural and electronic factors influencing ΔE(rxn). Significant energetic penalties in the oxidation of the metal center are mitigated through methylation of ancillary ligands or introduction of amine ancillary ligands. Methylation of the halogenated ligand maintains energy stored in the (3)MLCT state. Reduction in ΔE(rxn) is obtained by exploiting strain in the coordination geometry or in sterically encumbered ligands that is released upon bond breaking. Formation of a contact ion pair is significantly more favorable than complete separation of charged products, and shows negative ΔE with respect to the (3)MLCT state in certain cases. Future tunability in stored energy may be achieved through careful manipulation of ligand structure and charge on ancillary ligands.     

DNA-binding and cleavage activity of polypyridyl ruthenium(II) complexes

Polypyridyl ruthenium(II) complexes [Ru^II(3-bptpy)(dmphen)Cl]ClO₄ (1), [Ru^II(3-cptpy)(dmphen)Cl]ClO₄ (2), [Ru^II(2-tptpy)(dmphen)Cl]ClO₄ (3), and [Ru^II(9-atpy)(dmphen)Cl]ClO₄ (4) {where 3-bptpy?=?4′-(3-bromophenyl)-2,2′:6′,2″-terpyridine, 3-cptpy?=?4′-(3-chlorophenyl)-2,2′:6′,2″-terpyridine, 2-tptpy?=?4′-(2-thiophenyl)-2,2′:6′,2″-terpyridine, 9-atpy?=?4′-(9-anthryl)-2,2′:6′,2″-terpyridine, dmphen?=?2,9-dimethyl-1,10-phenanthroline} have been synthesized and characterized. The DNA-binding properties of the complexes with Herring Sperm DNA have been investigated by absorption titration and viscosity measurements. The ability of complexes to break the pUC19 DNA has been checked by gel electrophoresis. The experimental results suggest that all the complexes bind DNA via partial intercalation. The results also show that the order of DNA-binding affinities of the complexes is 4?<?3?<?2?<?1, confirming that planarity of the ligand in a complex is very important for DNA-binding.     

Photophysics of ruthenium polypyridyl complexes formed with lacunary polyoxotungstates with iron addenda

The interactions between luminophore [Ru(bpy)3]2+, and the lacunary Dawson heteropolyanions, [P2W17O61(FeOH2)]7-, [P2W17O61(FeBr)]6- and [P2W17O61]10- were investigated using a combination of photophysics, optical and Raman spectroscopy. Extensive quenching of the excited state of [Ru(bpy)3]2+ was observed in each case. Quenching is attributed to the formation of association complexes between [Ru(bpy)(3)]2+ and the heteropolyanions in which the charge on the heteropolyanions is fully compensated for by the ruthenium polypyridyl species. The interaction appears to be principally electrostatic in nature producing [Ru(bpy)3]3.5[P2W17O61(FeOH2)], [Ru(bpy)3]3[P2W17O61(FeBr)] and [Ru(bpy)3]5[P2W17O61]10-. The association constants for formation of the clusters were obtained from photophysical studies and surprisingly, despite the electrostatic nature of the interaction, there was no correlation between the charge on the polyoxometallate and the association constant. In particular, the unsubstituted lacunary, [P2W17O61]10-, showed considerably weaker association compared to the transition metal substituted lacunaries, in spite of its 10- charge. Difference absorption spectroscopy revealed a new transition at ca. 480 nm for each of the cluster complexes. From resonance Raman spectroscopy the origin of this transition was found to involve the polyoxometallate. Unlike previously reported adducts, the cluster complexes formed were not luminescent. In all cases the cluster complexes exhibit remarkable photostability, with no photodecomposition or photo-induced ligand exchange reactions evident in acetonitrile, under conditions where [Ru(bpy)3]2+ alone exhibits considerable photolability.