Photochemical two-electron reduction of a dinuclear ruthenium complex containing a bent tetraazatetrapyridopentacene bridging ligand: pushing up the LUMO for storing more energy

The synthesis and characterization of a ditopic bridging ligand, 9,12,21,22-tetraazatetrapyrido[3,2-a:2',3'-c:3″2″-m:2',3'-o]pentaphene (tatppα) and its dinuclear ruthenium complex, [(phen)(2)Ru(tatppα)Ru(phen)(2)][PF(6)](4) (1(4+)), are described. The tatppα ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2',3'-c:3″,2″-l:2',3'-n]pentacene (tatppβ), except that, instead of a linear tetraazapentacene backbone, tatppα has an ortho (or α) substitution pattern about the central benzene ring, leading to a 120° bend. Complex 1(4+) shows tatppα-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical Ru(II) dπ → phen-like π* metal-to-ligand charge-transfer transition centered at ～450 nm. In acetonitrile, visible-light irradiation of 1(4+) in the presence of triethylamine leads to two sequential changes in the absorption spectra, which are assigned to the formation of the one- and two-electron-reduced species, with the electrons stored on the tatppα ligand. These assignments were made by comparison of the spectral changes observed in 1(4+) upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatppα complex relative to the tatppβ complexes and show that modeling is a useful tool for ligand design. The chemical reactivity and differential reflectance spectroelectrochemical data reveal that the reductions are accompanied by radical dimerization of the tatppα ligand to species such as σ-{1}(2)(6+), which is only slowly reversible upon exposure to air and may limit the complexe's 1(4+) utility for driving photochemical H(2) production.     

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.     

Intervalence charge transfer in mixed valence compound modified by the formation of a supramolecular complex

The electrochemical and spectroelectrochemical properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead first to the formation of the cation-radical considered as mixed valence (MV) compound, and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the second redox process only. This is attributed to a strong interaction between the PDA(+2) dication and two 18C6 molecules, leading to the formation of a supramolecular complex with an association constant value K(a) = 7.0 × 10(7) M(-2). The interaction of 18C6 with PDA(+2) dication has a direct effect on the PDA(+.) cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compound (k(th) = 1.35 × 10(10) s(-1) in 18C6-free neutral solution containing 5.0 × 10(-4) M PDA, and k(th) = 3.6 × 10(9) s(-1) in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant K(co) falls from 6.0 × 10(6) in 18C6-free solution to 1.6 × 10(3) at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased, PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of K(co) is decreased to 4.3 × 10(4), but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound. In this medium, IV-CT in the MV compound is greater with 18C6 (k(th) = 2.3 × 10(10) s(-1) for [18C6]/[PDA] = 20/1) than without (k(th) = 1.4 × 10(9) s(-1)), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6 in neutral organic medium (K(a) = 2.9 × 10(3) M(-1)). This result suggests that the complexation between the electrogenerated PDA(+2) dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which is supposed to form inclusion complexes.     

Stacking interaction in crystals of a dinuclear rhodium complex with a bridging phenylborole ligand

Russian Chemical Bulletin -     

Ferromagnetic exchange in a dinuclear copper(II) complex mediated by a methanolate bridging ligand

The copper(II) complex Cu(2)L(OMe)(H(2)O)(3), [middle dot]3H(2)O [H(3)L = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine] was obtained and recrystallised in methanol to yield crystals of [[Cu(2)L(OMe)]](2).2.5MeOH.4H(2)O, 1.2.5MeOH.4H(2)O. Its single X-ray study shows that it contains two crystallographically different but chemically equivalent dinuclear [Cu(2)L(OMe)] 1 molecules in the asymmetric unit cell. The copper atoms of each dinuclear moiety are in distorted square-pyramidal environments, with both pyramids sharing an apical phenolate and a basal methanolate oxygen atom. Magnetic characterisation of 1.3H(2)O shows a quite strong intramolecular ferromagnetic coupling between both metal atoms. Extended Huckel calculations reveal that the intradinuclear magnetic interaction seems to be mediated by the exogenous methanolate bridging ligand.     

Synthesis and structure of new dinuclear palladium complex containing no bridging ligands

Oxidation of the mononuclear semiquinonediimine complex [(NPh)(NH)C₆H₄]₂Pd with silver trifluoromethanesulfonate afforded the diamagnetic dinuclear dicationic palladium complex {[(NPh)(NH)C₆H₄]₂Pd}₂(O₃SCF₃)₂ with a Pd...Pd distance of 3.267(1) . The complex was structurally characterized, and its spectroscopic and electrochemical properties were studied.     

A photosensitizer dinuclear ruthenium complex: intramolecular energy transfer to a covalently linked fullerene acceptor

A fullerene derivative (5) in which a dinuclear ruthenium complex is covalently linked to a fulleropyrrolidine (FP) through a rigid spacer has been prepared through azomethine ylide cycloaddition to C60. Electrochemical and photophysical studies revealed that ground-state electronic interactions between the bimetallic ruthenium chromophore and the FP moiety are small. The absorption spectrum of 5 displays a metal-to-ligand charge transfer (MLCT) transition at about 620 nm in CH2Cl2 which is shifted by nearly 160 nm relative to that of a previously reported mononuclear dyad (8). The photophysical investigations have also shown that both in dichloromethane and acetonitrile the photoexcited MLCT state of dyad 5 transforms into the fullerene triplet excited state with a quantum yield of 0.19 and that, contrary to mononuclear dyad 8, electron transfer, if any under the applied conditions, is negligible relative to energy transfer.     

Formation of a tetranuclear nickel(ii) complex containing an unusual bridging ligand

Russian Chemical Bulletin -     

Metal-metal interactions in dinuclear ruthenium complexes containing bridging 4,5-di(2-pyridyl)imidazolates and related ligands

The dinuclear bis(2,2'-bipyridine)ruthenium complex of 4,5-di(2-pyridyl)imidazolate has been prepared and separated into its (meso and rac) diastereoisomers. The 2-phenyl substituted analogue forms the meso isomer selectively. All three complexes have been characterised by 1H NMR and X-ray crystallography. Electrochemical measurements and spectroelectrochemistry of the mixed-valence states reveal strong metal-metal interactions and IVCT bands that are highly dependent on the electrolyte.     

Multisite effects on intervalence charge transfer in a clusterlike trinuclear assembly containing ruthenium and osmium

The intervalence charge transfer (IVCT) properties of the mixed-valence forms of the diastereoisomers of the dinuclear [[Ru(bpy)2](mu-HAT)[M(bpy)2]]5+ (M = Ru or Os) complexes and the trinuclear heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8; HAT = 1,4,5,8,9,12-hexaazatriphenylene; bpy = 2,2'-bipyridine) species display a marked dependence on the nuclearity and extent of oxidation of the assemblies, while small differences are also observed for the diastereoisomers of the same complex in the dinuclear cases. The mixed-valence heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8) forms exhibit IVCT properties that are intermediate between those of the diastereoisomeric forms of the localized hetero-dinuclear complex [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and the borderline localized-to-delocalized homo-trinuclear complex [[Ru(bpy)2]3(mu-HAT)]n+ (n = 7, 8). The near-infrared (NIR) spectrum of the +7 mixed-valence species exhibits both interconfigurational (IC) and IVCT transitions which are quantitatively similar to those in [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and are indicative of the localized mixed-valence formulation [[Ru(II)(bpy)2]2[Os(III)(bpy)2](mu-HAT)]7+. The +8 state exhibits a new band attributable to an IVCT transition in the near-infrared region.     

Underlying spin-orbit coupling structure of intervalence charge transfer bands in dinuclear polypyridyl complexes of ruthenium and osmium

The mixed-valence systems meso- and rac-[{M(bpy)2}2(mu-BL)]5+ {M = Ru, Os; BL = a series of polypyridyl bridging ligands such as 2,3-bis(2-pyridyl)benzoquinoxaline (dpb)} are characterized by multiple intervalence charge transfer (IVCT) and interconfigurational (IC) bands in the mid-infrared and near-infrared (NIR) regions. Differences in the relative energies of the IC transitions for the fully oxidized (+6) states of the osmium systems demonstrate that stereochemical effects lead to fundamental changes in the energy levels of the metal-based dpi orbitals, which are split by spin-orbit coupling and ligand-field asymmetry. An increase in the separation between the IC bands as BL is varied reflects the increase in the degree of electronic coupling through the series of ruthenium and osmium complexes. The increase in the IVCT bandwidths for the former is therefore attributed to the increase in the separation of the three underlying components of the bands. Stark effect measurements reveal small dipole moment changes accompanying IVCT excitation in support of the localized-to-delocalized or delocalized classification for the dinuclear ruthenium and osmium systems.     

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy

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

Side-effect of ancillary ligand on electron transfer and photodynamics of a dinuclear valence tautomeric complex

A cobalt complex [[Co(dpqa)]2(dhbq)](PF6)3 ((PF6)3, dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, dpqa = di(2-pyridylmethyl)-N-(quinolin-2-ylmethyl)amine) was prepared and studied by X-ray diffraction, electrochemistry, ESR, thermally and photo-induced magnetic measurements; the results show that the ancillary ligand finely tuned structural factors as well as intermolecular interactions that affect the VT behavior.     

Photoinduced ligand transformations in a ruthenium complex of dimethoxytetrapyridotetraazapentacene

The dinuclear ruthenium(II) complex [(phen)(2)Ru(tatpOMe)Ru(phen)(2)](4+) (2(4+); phen is 1,10-phenanthroline and tatpOMe is 10,21-dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a:2'3'-c:3',2'-l:2',3'-n]pentacene) has been synthesized and characterized by (1)H NMR, ESI mass spectroscopy and elemental analysis. Loss of methoxy group from bridging ligand of complex 2(4+) due to irradiation is observed by (1)H NMR and photochemistry. The interrelated electronic properties UV-Vis, electrochemistry, photochemistry and molecular orbital calculation are analyzed and discussed on the bridging ligand of the complex 2(4+).     

Synthesis and NMR characterization of dinuclear Fe(II) organometallic complexes containing a non-equivalently bridging 5-aryl tetrazolate ligand

The synthetic routes to the formation of a wide range of dinuclear Fe(II) organometallic complexes of the general formula [Cp(CO)(L)Fe-N₄C-C₆H₄-CN-Fe(L)(CO)Cp][SO₃CF₃], in which the 4-cyanophenyl-tetrazolate anion N₄C-C₆H₄-CN acts as bridging ligand, are described. ¹H and ¹³C NMR characterization of the product complexes indicate the presence of a significant interannular conjugation effect involving the aromatic rings of the organic spacer, the extent of which can be chemically modified by addition of electrophiles such as and H⁺. Furthermore, the reversibility of protonation reaction entails the opportunity of modulating the conjugative properties of the title compounds by a proton addition-elimination mechanism.     

A donor--acceptor--donor bridging ligand in a class III mixed-valence complex

The novel mononuclear and dinuclear complexes [Ru(trpy)(bpy)(apc)][PF(6)] and [(Ru(trpy)(bpy))(2)(mu-adpc)][PF(6)](2) (bpy = 2,2'-bipyridine, trpy = 2,2':6',2' '-terpyridine, apc(-) = 4-azo(phenylcyanamido)benzene, and adpc(2)(-) = 4,4'-azodi(phenylcyanamido)) were synthesized and characterized by (1)H NMR, UV-vis, and cyclic voltammetry. Crystallography showed that the dinuclear Ru(II) complex crystallizes from diethyl ether/acetonitrile solution as [(Ru(trpy)(bpy))(2)(mu-adpc)][PF(6)](2).2(acetonitrile).2(diethyl ether). Crystal structure data are as follows: crystal system triclinic, space group P1, with a, b, and c = 12.480(2), 13.090(3) and 14.147(3) A, respectively, alpha, beta, and gamma = 79.792(3), 68.027(3), and 64.447(3) degrees, respectively, V = 1933.3(6) A(3), and Z = 1. The structure was refined to a final R factor of 0.0421. The mixed-valence complex with metal ions, separated by a through-space distance of 19.5 A, is a class III system, having the comproportionation constant K(c) = 1.3 x 10(13) and an intervalence band at 1920 nm (epsilon(max) = 10 000 M(-1) cm(-1)), in dimethylformamide solution. The results of this study strongly suggest that the bridging ligand adpc(2-) can mediate metal-metal coupling through both hole-transfer and electron-transfer superexchange mechanisms.