Intervalence Charge Transfer Band Research Articles

Mixed Valence Creutz−Taube Ion Analogues Incorporating Thiacrowns: Synthesis, Structure, Physical Properties, and Computational Studies

A series of nine new complexes incorporating [Ru(II)Cl([n]aneS(3))] (n = 12, 14, 16) metal centers bridged by three ditopic ligands containing two monodentate sites (pyrazine, 4,4'-bipyridine, and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine) have been synthesized and fully characterized. The solid-state structures of three of the complexes have been further characterized by X-ray crystallography studies. Intermetallic interactions within the new systems have been probed using electrochemistry and optical spectroscopy. Cyclic voltammetry reveals that the three pyrazine bridged systems display two separate Ru(III)/(II) redox couples. Using spectroelectrochemistry, we have investigated the optical properties of these mixed valence, Creutz-Taube ion analogues. An analysis of the intervalence charge transfer bands for the complexes revealed that, despite possessing the same donor sets, the electronic delocalization within these systems is modulated by the nature of the coordinated thiacrown. Computational modeling using density function theory offers further evidence of interaction between metal centers and provides insights into how these interactions are mediated.

A Mixed-Valence Octanuclear Iron−Oxo Pyrazolate: Assessment of Electronic Delocalization by Structural and Spectroscopic Analysis

A formally Fe(III)(7)Fe(II) complex, containing an inner Fe(4)O(4)-cubane and four peripheral Fe centers, is derived from the one-electron reduction of its Fe(III)(8) precursor. Spectroscopic analysis of the former reveals that the redox activity of this Fe(8) system is confined within its cubane core. The resulting (Fe(4)O(4))(3+)-cubane, which is valence-delocalized in the NMR, Mössbauer, and IR spectroscopy time scales but valence-trapped in the X-ray photoelectron spectroscopy (XPS) time scale, is better described as a Robin-Day class-II system by the analysis of its near-infrared (NIR) intervalence charge transfer (IVCT) band profile.

Spectroscopic studies of synthetic and natural ringwoodite, γ-(Mg, Fe)2SiO4

Synthetic ringwoodite γ-(Mg1−x Fe x )2SiO4 of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mossbauer spectroscopy. The Mossbauer spectra consist of two doublets caused by VIFe2+ and VIFe3+, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe3+/Fetotal ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed 5 T 2g → 5 E g transitions of VIFe2+ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm−1 weakly depend on temperature, whilst the Fe2+/Fe3+ IVCT band at ~16,400 cm−1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe3+ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of VIFe2+ (the bands at ~8,600 and ~12,700 cm−1) and Fe2+/Fe3+ IVCT transition (~18,100 cm−1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm−1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)2SiO4-spinels and that the single band is caused by the split spin-allowed 5 E → 5 T 2 transition of IVFe2+. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stoffler in Lunar Planet Sci 29(1308), 1998).

Tuning of the [Cu3(μ-O)]4+/5+Redox Couple: Spectroscopic Evidence of Charge Delocalization in the Mixed-Valent [Cu3(μ-O)]5+Species

Trinuclear Cu (II)-complexes of formula [Cu (II) 3(mu 3-E)(mu-4-R-pz) 3X 3] (+/- n ), E = O and OH; R = H, Cl, Br, CH(O) and NO 2; X = Cl, NCS, CH 3COO, and py, have been synthesized and characterized and the effect of substitution of terminal ligands, as well as 4-R-groups, in the one-electron oxidation process has been investigated by cyclic voltammetry. In situ UV-vis-NIR spectroelectrochemical characterization of the mixed valence Cu 3 (7+)-complex [Cu 3(mu 3-O)(mu-pz) 3Cl 3] (-) revealed an intervalence charge transfer band at 9550 cm (-1) (epsilon = 2600 cm (-1) M (-1)), whose analysis identifies this species as a delocalized, Robin-Day class-III system, with an electronic coupling factor, H ab, of 4775 cm (-1).

Visible‐Light Photochromism of Triarylamine‐ or Ferrocene‐Bound Diethynylethenes that Switches Electronic Communication between Redox Sites and Luminescence

Redox-active ferrocene- and triarylamine-terminated diethynylethene derivatives have been synthesized and their photochromic properties and switching behavior based on through-bond electronic communication between the two redox sites, as well as their emissions, have been examined. Both bis(ferrocenylethynyl)ethene 1 and bis(triarylaminoethynyl)ethene 2 show visible-light photochromism induced by donor-acceptor charge-transfer (CT) transitions from the ferrocene or triarylamine to the diethynylethene moieties. The reversibility and quantum yields of the photochromism of 2 (Phi(E-->Z)=6.1 x 10(-2), Phi(Z-->E)=1.4 x 10(-2)) are far higher than those of 1 (Phi(E-->Z)=8.6 x 10(-6), Phi(Z-->E)=2.5 x 10(-6)). The higher efficiency in 2 may be attributed to the absence of the heavy atom effect and of a low-lying (3)LF state, which are characteristic of ferrocenyl compounds. This proposition is further supported by the fact that bis(ferrocenylbuta-1,3-diynyl)ethene 3, which, unlike 1, is free from steric interference between the two ferrocenyl groups in the Z form, does not show a significant improvement in its photoisomerization quantum yields (Phi(E-->Z)=6.2x10(-5), Phi(Z-->E)=3.4 x 10(-5)). The visible-light photochromism of 1 and 2 is accompanied by a switch in the strength of the electronic communication between the two redox sites in their mixed-valence states (DeltaE(0)' values are 70 and 48 mV for (E)-1 and (Z)-1, and 74 and 63 mV for (E)-2 and (Z)-2). In the case of 2, further evaluations were carried out through intervalence charge-transfer (IVCT) band analyses and DFT calculations. We have also demonstrated that steric repulsion between the methyl ester moieties in the Z form is implicated in the reduction in the through-bond electronic communication. Compound 2 exhibits photoluminescence, which is more efficient in the E form than in the Z form, whereas 1 and 3 show no photoluminescence.

A Dawson‐Type Dirhenium(V)‐Oxido‐Bridged Polyoxotungstate: X‐ray Crystal Structure and Hydrogen Evolution from Water Vapor under Visible Light Irradiation

AbstractThe synthesis and crystal structure of a Dawson‐type dirhenium(V)‐oxido‐bridged polyoxotungstate are described. The potassium salt K14[O{ReV(OH)(α2‐P2W17O61)}2]·21H2O (K‐1) was obtained as analytically pure, homogeneous, black‐purple crystals by the reaction of a monolacunary Dawson polyoxotungstate with [ReIVCl6]2– in water, followed by crystallization from an aqueous HCl solution in the dark. Single‐crystal X‐ray structure analysis revealed that the dimeric dirhenium(V)‐oxido‐bridged structure has overall C2h symmetry. Characterization of K‐1 was also accomplished by elemental analysis, TG/DTA, and FTIR, UV/Vis absorption, ESR, and solution 31P NMR spectroscopy. Furthermore, the black‐purple compound K‐1 was grafted onto a TiO2 surface by electrostatic binding to give TiO2 with a cationic quaternaryammonium moiety (1‐grafted TiO2). Elemental analysis of 1‐grafted TiO2 indicated that the seven potassium counterions of K‐1 were ion‐exchanged with seven cationic ammonium moieties to form {TiO2}5500[≡Si(CH2)3N(CH3)3Cl]7[≡Si(CH2)3N+(CH3)3]7(K7[O{Re(OH)(α2‐P2W17O61)}2]7–) and seven molecules of KCl. The diffuse reflectance (DR) UV/Vis spectrum of 1‐grafted TiO2 exhibited two sharp bands at 496 nm and 751 nm due to the ReV→WVI intervalence charge transfer (IVCT) band and the d–d band of the rhenium(V) atom, respectively, in the visible light region, which suggestedthat polyoxoanion 1 was isolated on the TiO2 surface. With 1‐grafted TiO2, hydrogen evolution from water vapor under irradiation with visible light (>400 nm and >420 nm) was achieved. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Cyanide-Bridged Iron(II,III) Cube with Multistepped Redox Behavior

A building unit of Prussian blue was isolated as a cyanide-bridged iron cube of [Fe(II)4Fe(III)4(CN)12(tp)8] x 12 DMF x 2 Et2O x 4 H2O [tp(-) = hydrotris(pyrazolyl)borate]. A cyclic voltammogram showed quasi-reversible four-stepped redox waves, which correspond to [Fe(III)4Fe(II)4]/[Fe(III)5Fe(II)3](+), [Fe(III)5Fe(II)3](+)/[Fe(III)6Fe(II)2](2+), [Fe(III)6Fe(II)2](2+)/[Fe(III)7Fe(II)1](3+), and [Fe(III)7Fe(II)1](3+)/[Fe(III)8](4+) processes. Controlled potential absorption spectral measurements revealed two intervalence charge-transfer bands at 816 and 1000 nm, which were assigned to charge transfers from Fe(II) ions to adjacent and remote Fe(III) ions, respectively, in the cube.

Photochromic Hybrid Organic−Inorganic Liquid-Crystalline Materials Built from Nonionic Surfactants and Polyoxometalates: Elaboration and Structural Study

This work reports the elaboration and structural study of new hybrid organic-inorganic materials constructed via the coupling of liquid-crystalline nonionic surfactants and polyoxometalates (POMs). X-ray scattering and polarized light microscopy demonstrate that these hybrid materials, highly loaded with POMs (up to 18 wt %), are nanocomposites of liquid-crystalline lamellar structure (Lalpha), with viscoelastic properties close to those of gels. The interpretation of X-ray scattering data strongly suggests that the POMs are located close to the terminal -OH groups of the nonionic surfactants, within the aqueous sublayers. Moreover, these materials exhibit a reversible photochromism associated to the photoreduction of the polyanion. The photoinduced mixed-valence behavior has been characterized through ESR and UV-visible-near-IR spectroscopies that demonstrate the presence of W(V) metal cations and of the characteristic intervalence charge transfer band in the near-IR region, respectively. These hybrid nanocomposites exhibit optical properties that may be useful for applications involving UV-light-sensitive coatings or liquid-crystal-based photochromic switches. From a more fundamental point of view, these hybrid materials should be very helpful models for the study of both the static and dynamic properties of nano-objects confined within soft lamellar structures.

Intramolecular Optical Electron Transfer in Mixed-Valent Dinuclear Iron−Ruthenium Complexes Featuring a 1,4-Diethynylaryl Spacer

The ground-state electronic structure and the lowest-lying excited states of the cationic mixed-valent dinuclear complexes [(η2-dppe)(η5-C5Me5)Fe[C≡C-1,4-(C6H4)C≡C]Ru(η2-dppe)2(X)][PF6] (X = Cl, 2; X = C≡C(4-C6H4NO2), 5) are discussed, with particular emphasis on the photoinduced intramolecular electron transfer between the ruthenium and iron centers. The location and intensities of the low-lying absorptions exhibited in the near-infrared (near-IR) range by these heterodinuclear mixed-valent (MV) complexes correlate with predictions based on the Hush model, strongly suggesting that they correspond to intervalence charge-transfer (IVCT) bands.

Examination of the Mixed‐Valence State of the Doubly Boron‐Bridged Diferrocene Cation [(FeCp)2{μ‐C10H6(BPh)2}]+

A series of mixed-valent (MV) complexes [(FeCp)2(mu-C10H6(BPh)2)]+X ([1+]X; X=I 5, PF6, SbF6, B(C6F5)4) were prepared by oxidation of diboradiferrocene [(FeCp)2(mu-C10H6(BPh)2)] (1) with I 2, AgPF6, and AgSbF6, respectively, and through anion exchange of the I 5(-) salt with [Li(Et2O)x][B(C6F5)4] in the case of X=B(C6F5)4. The MV state of the cation was investigated in solution by multinuclear NMR spectroscopy, CV, and UV/Vis-NIR absorption spectroscopy, and in the solid state by IR spectroscopy, single-crystal X-ray crystallography, and Mössbauer spectroscopy. The cyclic voltammogram of 1 shows two distinct redox waves with a large redox splitting of Delta E=510 mV in CH2Cl2 and the NIR spectrum for the mono-oxidized species displays an intervalence charge-transfer band at around 1500 to 1700 nm depending on the specific counterion present. The X-ray crystal structures of [1+]X show inversion-symmetric cations with X=I 5 and B(C6F5)4 and unsymmetric valence-trapped structures composed of one ferrocene and one ferrocenium moiety with X=PF6 and SbF6. Mössbauer data for X=PF6 are consistent with valence trapping at all temperatures between 90 and 343 K. In comparison, fast electron transfer is evident on the Mössbauer timescale for X=I 5 and temperature-dependent behavior is observed for X=B(C6F5)4. The anion dependence of the X-ray structural and Mössbauer data is discussed in the context of crystal symmetry and the possibility of static and dynamic disorder effects is considered.

Femtosecond dynamics of electron transfer in a neutral organic mixed-valence compound

In this article we report a femtosecond time-resolved transient absorption study of a neutral organic mixed-valence (MV) compound with the aim to gain insight into its charge-transfer dynamics upon optical excitation. The back-electron transfer was investigated in five different solvents, toluene, dibutyl ether, methyl- tert-butyl ether (MTBE), benzonitrile and n-hexane. In the pump step, the molecule was excited at 760 nm and 850 nm into the intervalence charge-transfer band. The resulting transients can be described by two time constant. We assign one time constant to the rearrangement of solvent molecules in the charge-transfer state and the second time constant to back-electron transfer to the electronic ground state. Back-electron transfer rates range from 1.5 × 10 12 s −1 in benzonitrile through 8.3 × 10 11 s −1 in MTBE, around 1.6 × 10 11 s −1 in dibutylether and toluene and to 3.8 × 10 9 s −1 in n-hexane.

Electronic Communication in the Mixed-valence States of Cyclobutadienecobalt Complexes having Two Ferrocenes and Two Anthraquinones

Two geometric isomers, Z and E forms of (η4-cyclobutadiene)(η5-cyclopentadienyl) cobalt simultaneously containing two ferrocene moieties and two anthraquinone moieties, 1 and 2, respectively, were synthesized, and their crystal structures were determined. These molecules formed double mixed-valence states caused by ferrocene–ferrocenium and anthraquinone–anthrasemiquinone electronic communications. In both 1 and 2, the thermodynamic stability of the mixed-valence state for the ferrocene moieties was higher than that of the anthraquinone moieties because of the orbital coupling of the CB–Co bridging unit. The comproportionation constant, K c, for the ferrocene moieties was similar between 1 and 2, whereas the mixing coefficient, α, and the off-diagonal matrix-coupling element in the Marcus-Hush two-state model, H AB, evaluated from analysis of the intervalence charge transfer (IVCT) band were larger for 1 than for 2. These results suggest the existence of higher electrostatic repulsion between two ferrocenium cations in 1 than in 2 and the occurrence of through-bond electronic communication. K c for the anthraquinone moieties in the E form is smaller than that of the Z form, probably due to the through-space electronic interaction.

A New Generation of Metal String Complexes: Structure, Magnetism, Spectroscopy, Theoretical Analysis, and Single Molecular Conductance of an Unusual Mixed‐Valence Linear [Ni5]8+ Complex

Two new linear pentanickel complexes [Ni5(bna)4(Cl)2][PF6]2 (1) and [Ni5(bna)4(Cl)2][PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, [Ni5(bna)4(NCS)2][NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with [Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard [Ni5]10+ core, isoelectronic with that of [Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.

Arylamine-Substituted Oligo(ladder-type pentaphenylene)s: Electronic Communication between Bridged Redox Centers

Novel bis(arylamine-substituted) oligo(ladder-type pentaphenylene)s 1-3, with bridge lengths estimated to be 2.2, 4.2, and 6.3 nm, respectively, have been developed, and the model compound 4 with a mono-arylamine substituent was also synthesized. Their absorption spectra in different solvents are almost identical, while distinct bathochromic shifts of the photoluminescence (PL) spectra were observed with increasing solvent polarity due to the polarized excited states. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) spectra display a two-step oxidation of the bridged diamines in compound 1, which suggests that the electron and charge delocalize in mixed-valence (MV) cation 1+* and that both redox centers can communicate through the pentaphenylene bridge. Only unresolved curves in CV and DPV spectra were observed in the first two oxidation processes of diamines 2 and 3, indicating that the bridges are too long for efficient delocalization over the entire molecules and the radical cations localize at each arylamine center. This finding was further supported by chemical oxidation with SbCl5 and studies of the corresponding UV-vis-NIR absorption spectra of compounds 1-4. A significant intervalence charge-transfer (IVCT) band around 5283 cm-1 (1893 nm) was observed in 1+*. This is the first report of such a highly intense IVCT band in the NIR region with intensity similar to that of the visible band of the radicals, enabling further analysis of the CT process and the coupling matrix element V, classifying 1+* as a class II derivative (V = 1.6 kcal/mol). This study may offer an effective way to improve the understanding of charge transfer and charge-carrier transport in various conjugated oligomers or polymers and facilitate their ongoing exploration in optoelectronic applications.

Neutral Organic Mixed-Valence Compounds: Synthesis and All-Optical Evaluation of Electron-Transfer Parameters

In this paper we present the synthesis as well as a detailed study of the electrochemical and photophysical properties of a series of neutral organic mixed-valence (MV) compounds, 1-7, in which different amine donor centers are connected to perchlorinated triarylmethyl radical units by various spacers. We show that this new class of compounds are excellent model systems for the investigation of electron transfer due to their uncharged character and, consequently, their excellent solubility, particularly in nonpolar solvents. A detailed band shape analysis of the intervalence charge-transfer (IV-CT) bands in the context of Jortner's theory allowed the electron-transfer parameters (inner vibrational reorganization energy lambdav, outer solvent reorganization energy lambdao, and the difference in the free energy between the diabatic ground and excited states, DeltaG degrees degrees , as well as the averaged molecular vibrational mode v) to be extracted independently. In this way we were able to analyze the solvatochromic behavior of the IV-CT bands by evaluating the contribution of each parameter. By comparison of different compounds, we were also able to assign specific molecular moieties to changes in vv. For this class of molecules, we also demonstrate that the adiabatic dipole moment difference Deltamicroab and, consequently, the electronic coupling V12 can be evaluated directly from the absorption spectra by a new variant of the solvatochromic method. Furthermore, an investigation of the electrochemistry of compounds 1-7 by cyclic voltammetry as well as spectroelectrochemistry shows that, not only in the neutral MV compounds but also in their oxidized forms, a charge transfer can be optically induced but with exchanged donor-acceptor functionalities of the redox centers.

Synthesis of a Series of Diiron Complexes Based on a Tetraethynylethene Skeleton and Related C6-Enediyne Spacers, (dppe)Cp*Fe−C⋮CC(R)C(R)C⋮C−FeCp*(dppe): Tunable Molecular Wires

A series of 3,4-disubstituted (hex-3-ene-1,5-diyne-1,6-diyl)diron complexes with FeCp*(dppe) (Fe) end caps, Fe−C⋮CC(R)C(R)C⋮C−Fe (R = H, C⋮C−SiMe3, C⋮C−H, C6H5, p-C6H4CF3), and the related (octa-3,5-diene-1,7-diyne-1,8-diyl)diron complex, Fe−C⋮C−C(H)C(H)C(H)C(H)C⋮C−Fe, has been prepared, and their performance as molecular wires has been evaluated. The enyne complexes have been synthesized via vinylidene intermediates, [FeCC(H)C(R)C(R)C(H)CFe]2+, derived from the corresponding terminal alkynes or the Me3Si-protected precursors (XC⋮CCC(R)C(R)C⋮C−X; X = H, SiMe3), and the products have been characterized spectroscopically and crystallographically. The performance of the obtained dinuclear enyne complexes as molecular wires has been evaluated on the basis of the comproportionation constants (KC) obtained by electrochemical measurements and the Vab values obtained from the spectral parameters of the intervalence charge transfer bands of the isolated monocationic radical species appearing in the near-IR region. As a result, the C6-enediyne complexes turn out to be excellent molecular wires, with KC values larger than 108 as well as Vab values larger than 0.35, belonging to class III compounds according to the Robin and Day classification and being comparable to the related polyynediyl complexes as well. It is notable that, in the enyne system, the performance can be readily tuned by introduction of appropriate substituents onto the olefinic part. Thus, diiron complexes containing an enyne spacer can be regarded as tunable molecular wires.

2,2‘-Dipyridylketone (dpk) as Ancillary Acceptor and Reporter Ligand in Complexes [(dpk)(Cl)Ru(μ-tppz)Ru(Cl)(dpk)]n+where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

The tppz-bridged diruthenium(II) complex [(dpk)(Cl)Ru(II)(mu-tppz)Ru(II)(Cl)(dpk)](ClO4)2, [2](ClO4)2, and mononuclear [(dpk)(Cl)Ru(II)(tppz)](ClO4), [1](ClO4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]2+ translates to a relatively small comproportionation constant, Kc, of 2.5 x 10(4) for the intermediate. It is shown how electrochemical data (E(ox), E(red), Kc) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(mu-tppz)RuCl(L)]n, particularly the competition between L and tppz for electron density from the metal. According to EPR (g1 = 2.470, g2 = 2.195, and g3 = 1.873 at 4 K) the intermediate [2]3+ is a mixed-valent Ru(II)Ru(III) species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (epsilon = 1500 M(-1) cm(-1)). The width at half-height (Deltanu(1/2)) of 700 cm(-1) of the IVCT band is much smaller than the calculated value of 3584 cm(-1), obtained by using the Hush formula Deltanu(1/2) = (2310E(op))(1/2) (E(op) = 5556 cm(-1), energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent Ru(II)Ru(III) systems. Valence delocalization in [2]3+ is supported by the uniform shift of the nu(C=O) band of the N,N'-coordinated dpk ligands from 1676 cm(-1) in the Ru(II)Ru(II) precursor to 1690 cm(-1) in the Ru(2.5)Ru(2.5) form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but pi-conjugated organic carbonyl group. The apparent contradiction between the moderate value of Kc and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the pi-acceptor bridging ligand. Altogether, the complex ions [1]+ and [2]2+ display four and five successive reduction processes, respectively, involving both tppz- and dpk-based unoccupied pi orbitals. The one-electron reduced form [2]+ has been assigned as a tppz*- radical-anion-containing species which exhibits a free-radical-type EPR signal at 4K (g(parallel) = 2.002, g(perpendicular) = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (epsilon = 1100 M(-1) cm(-1)).

The Underlying Spin−Orbit Coupling Structure of Intervalence Charge Transfer Bands in Dinuclear Polypyridyl Complexes of Ruthenium and Osmium

The Underlying Spin−Orbit Coupling Structure of Intervalence Charge Transfer Bands in Dinuclear Polypyridyl Complexes of Ruthenium and Osmium

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.

Vibrational Markers for the Open-Shell Character of Transition Metal Bis-dithiolenes: An Infrared, Resonance Raman, and Quantum Chemical Study

Transition metal complexes involving the benzene-1,2-dithiol (L(2-)) and Sellmann's 3,5-di-tert-butylbenzene-1,2-dithiol(L(Bu 2-)) ligands have been studied by UV-vis, infrared (IR), and resonance Raman (rR) spectroscopies. Raman spectra were obtained in resonance with the intervalence charge transfer (IVCT) bands in the near-infrared region and ligand-to-metal charge transfer (LMCT) bands in the near-UV region. Geometry optimization and frequency calculations using density functional theory (DFT) have been performed for [M(L)(2)](z) and [M(L(Bu))(2)](z) species (M = Ni, Pd, Pt, Co, Cu, Au, z = -1; M = Au, z = 0). On the basis of frequency calculations and normal-mode analysis, we have assigned the most important totally symmetric vibrations as well as corresponding overtone and combination bands that appear in rR spectra of compounds [Ni(L)(2)](1-), [M(L(Bu))(2)](1-) (M = Ni, Pt, Co, Cu). Experimental values of dimensionless normal coordinate displacements in excited states have been determined by fitting of rR spectra together with the absorption band shape, based on the time-dependent theory of Heller. Time-dependent density functional theory (TD-DFT) and multireference post-Hartree-Fock ab initio calculations, using the difference dedicated configuration interaction (MR-DDCI) method, were carried out to evaluate dimensionless normal coordinate displacements quantum chemically. The calculations show encouraging agreement with the experimental values. The large distortions along several normal modes led to significant vibronic broadening of IVCT and LMCT bands, and the broadening was accounted for in the deconvolution of the absorption spectra. The presence of an intense rR band around approximately 1100 cm(-1) was found to be a reliable marker for the presence of sulfur-based radicals.