IVCT Transition Research Articles

Crystal structures and crystallographic classification of titanium silicophosphates – with a note on structure and composition of silicophosphates “M 3P5SiO19”

Abstract The crystal structures of TiIII 4[Si2O(PO4)6] ( P 3 ‾ $P\overline{3}$ , Z = 3, a = 14.733(1), c = 7.363(1) Å, R1 = 0.040, wR2 = 0.098, 7649 ind. refl., 170 variables), FeII 0.79TiIII 2.42TiIV 0.79[Si2O(PO4)6] ( P 3 ‾ $P\overline{3}$ , Z = 3, a = 14.6534(2), c = 7.3829(1) Å, R1 = 0.036, wR2 = 0.088, 4026 ind. refl., 171 variables), and TiIII 2TiIV 6(PO4)6[Si2O(PO4)6] ( R 3 ‾ $R\overline{3}$ , Z = 1, a = 8.446(2), c = 44.21(2) Å, R1 = 0.047, wR2 = 0.120, 1373 ind. refl., 109 variables) have been refined from single-crystal data. The structures show hexagonal closest packing of phosphate groups with metal cations and [Si2O] groups occupying octahedral voids [□(PO4)6]. The close relationship of these and other silicophosphate structures to the NiAs and β-Fe2(SO4)3 (see also NaZr2(PO4)3 “NASICON”) structure types is rationalized by group/subgroup considerations. This symmetry approach shows that systematic twinning is highly likely in silicophosphates, thus possibly leading to faulty crystal structure refinements. Our investigation strongly suggests that the proper composition of silicophosphates “M III 3P5SiO19” (M = Cr, V, Fe, Mo) reported in literature is actually M III 4-[Si2O(PO4)6]. In the mixed-valent compounds oxidation states were assigned to the cation sites by comparison to Ti2O3, TiP2O7 and FeTiO3. The powder reflectance spectrum of dark-blue FeII 0.79TiIII 2.42TiIV 0.79[Si2O(PO4)6] shows a strong IVCT transition at ν ˜ $\widetilde{\nu }$ = 17,500 cm−1, and magnetic susceptibility data agree very well with the proposed oxidation states.

Anthracene-bridged diarylamine isomers with identical amine–amine distances: Syntheses, (spectro)electrochemistry and theoretical calculations

Two athracene-bridged diarylamine isomers 1 and 2 were constructed and fully characterized. The electrochemical results showed that ΔE of compound 2 was significantly higher than that of 1, which indicated that the mixed-valence compound 2 + had higher thermodynamic stability than 1+ . Spectroelectrochemistry reflected that the NIR absorption of 2 + exhibited a significant blue shift compared with reported compound 1 +. The calculated electronic coupling constant and spin density distribution indicated that 2 + had larger charge delocalization than 1 + under the identical amine-amine distances. The TDDFT predicted β-HOSO→β-LUSO transition from 2 + which further confirmed the existence of N→N·+ IVCT transition.

Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H4 )]+ have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH2 Cl2 )) on ([{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H3 -n-OMe)]+ (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the β-HOSO and β-LUSO, whilst the relative phases of the β-HOSO and β-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules.

Photoinduced Charge Separation Prompted Intervalence Charge Transfer in a Bis(thienyl)diketopyrrolopyrrole Bridged Donor‐TCBD Push‐Pull System

AbstractIntervalence charge transfer (IVCT), a phenomenon observed in molecular systems comprised of two redox centers differing in oxidation states by one unit, is reported in a novel, newly synthesized, multi‐modular donor‐acceptor system comprised of central bis(thienyl)diketopyrrolopyrrole (TDPP) hosting two phenothiazine‐tetracyanobutadiene (PTZ‐TCBD) entities on the opposite sides. One‐electron reduction of TCBD promoted electron exchange between the two TCBD resulting in IVCT transition in the near‐infrared region. The stabilization energy, −ΔGcom and comproportionation equilibrium constant, Kcom calculated from peak potentials of the split reduction waves were found to be 1.06×104 J mol−1, and 72.3 M−1, respectively. Further, the IVCT transition was also witnessed during the process of thermodynamically feasible electron transfer upon excitation of the TDPP entity in the system, and served as a diagnostic marker to characterize the electron transfer product. Subsequent transient absorption spectral studies and data analysis by Global and Target analyses revealed occurrence of ultrafast charge separation (kcs≈1010 s−1) owing to the close proximity and good communication between the entities of the multi‐modular donor‐acceptor system. The role of central TDPP in promoting IVCT is borne out from the present investigation.

Magnetic and Magneto-Optical Oroperties of Iron Oxides Nanoparticles Synthesized under Atmospheric Pressure.

Magnetite nanoparticles were synthesized by a simple thermal decomposition process, involving only iron (III) nitrate nonahydrate as a precursor, and hexadecylamine as a solvent and stabilizer at reaction temperatures varied from 200 to 380 °C. The results of the structural analysis showed that the average crystallite size depends on the reaction temperature and increases from 4.8 to 13.3 nm. The behavior of the coercivity indicates that all synthesized samples are single domain; herewith, it was found that the critical size corresponding to the transition to the superparamagnetic state at room temperature is about 9 nm. The effect of the reaction temperature on changes in the saturation magnetization was studied. It was found that the size effect in the MCD spectra is observed for the IVCT transition and one ISCT transition, and the influence of the reaction temperature on the change in the MCD spectra was discussed.

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes.

The ligands N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)═O (1, 2), Cu(II) (3, 4), Co(II) (5), and Co(III) (6). The X-ray crystal structures of 1-6 were solved. The vanadium center in 1-2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3-5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV-vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1-6 exhibit a reversible one-electron oxidation wave in the range -0.11-0.26 V vs Fc+/Fc. The cations 1+ and 2+ were structurally characterized, showing an octahedral V(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis-NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-to-vanadium(V) charge transfer (CT) transition. The crystal structures of 3+ and 4+ are congruent with Cu(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm-1), indicative of partial localization of the radical. The structure of 5+ displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co(III) and Co(II) radical. In the presence of acetate the dimer [(5)2(μ-OAc)]+ forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5+. Complex 6+ is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.

Electronic intervalence Fe2+ + Ti4+ → Fe3+ + Ti3+ charge-transfer transition in ilmenite

The optical absorption spectrum of natural ilmenite was measured on a narrow translucent rim zone of a thin polished slide of thickness ca. 0.004 mm, mounted with epoxy resin upon a supporting glass plate. The color of the material in the zone is dark red. At conoscopic observation between crossed Nicols, it displays a very dark, vague, off-centered conoscopic figure. Unpolarized optical absorption spectrum of ilmenite, measured in the transparent zone, consists of a very intense high-energy absorption edge which covers a major part of the visible range and causes the dark-red color of ilmenite. On the edge tail, which extends far to the NIR range, there is an intense, broad, asymmetric, vaguely structured absorption band with maximum at around 7000 cm−1. On the account of high iron and titanium contents (0.82 and 0.86 apfu, respectively), it may confidently be attributed to Fe2+/Ti4+ IVCT transition, though its energy is unusually low compared to other Fe2+, Ti4+-bearing minerals studied so far. This most probably proved that the effect of lowering of energy of IVCT bands with increasing number of ion pairs, established by Amthauer and Rossman (Phys Chem Miner 11:37–51 1984), on Fe2+/Fe3+ IVCT bands also takes place in case of electronic Fe2+/Ti4+ IVCT transition in ilmenite.

Ruthenium-Amine Electronic Coupling Bridged through Phen-1,3-diyl Versus Phen-1,4-diyl: Reverse of the Charge Transfer Direction

Three tris-bidentate cyclometalated ruthenium complexes with a di(p-anisyl)amino [5(PF6)], di(p-tolyl)amino [6(PF6)], or di(p-chlorophenyl)amino [7(PF6)] substituent on the cyclometalating phenyl ring have been prepared, where the amine nitrogen is in the meta position to the cyclometalated site (bridged through the phen-1,3-diyl unit). The structure of 6(PF6) has been confirmed by single-crystal X-ray diffraction analysis. Two consecutive redox waves are evident at +0.48 and +0.87 V versus Ag/AgCl for 5(PF6), +0.51 and +1.02 V for 6(PF6), and +0.53 and +1.18 V for 7(PF6), respectively. The first wave is assigned to the Ru(II/III) process, and the second wave is attributed to the N(0/+) process. After one-electron oxidation using SbCl5, these complexes display distinct absorptions in the near-infrared region, which are assigned to the nitrogen-to-ruthenium intervalence charge-transfer (ICVT) transitions. The energies of the IVCT transitions vary linearly versus the potential splitting of the Ru(II/III) and N(0/+) process. The charge-transfer direction in these complexes is reversed with respect to a previously reported ruthenium–amine system where the amine nitrogen is in the para position to the cyclometalated site (bridged through the phen-1,4-diyl unit).

Synthesis, spectral and redox switchable cubic NLO properties of chiral dinuclear iron cyanide/isocyanide-bridged complexes

Two chiral dinuclear cyanide/isocyanide-bridged complexes (R)-[Cp(dppe)Fe-CN-Fe(dppp)Cp]PF6 (1[PF6]) and (R)-[Cp(dppe)Fe-NC-Fe(dppp)Cp]PF6 (2[PF6]), and their mono-oxidation products (R)-[Cp(dppe)Fe(II)-CN-Fe(III)(dppp)Cp] [PF6]2 (1[PF6]2) and (R)-[Cp(dppe)Fe(III)-NC-Fe(II)(dppp)Cp][PF6]2 (2[PF6]2) were synthesized and fully characterized. The electronic spectra of both the mixed-valence complexes 1[PF6]2 and 2[PF6]2 exhibit a strong and broad absorption band with two discernable peaks in the NIR region, which are attributed to Fe(II)-Fe(III) IVCT transitions. The attributions are supported by the DFT calculations. Under irradiation with a nanosecond laser at 1064 nm, the measured third-order NLO results of all four cyanide-bridged complexes showed that complexes 1(+) and 2(+) do not exhibit an NLO response, but their one-electron oxidation complexes 1(2+) and 2(2+) exhibit a strong NLO response due to a resonance enhanced effect. In addition, both complexes 1(2+) and 2(2+) display RSA and self-defocusing effects and show good optical limiting behavior in a broadband range.

Electrochemical and spectroscopic effects of mixed substituents in bis(phenolate)-copper(II) galactose oxidase model complexes.

Nonsymmetric substitution of salen (1(R(1),R(2))) and reduced salen (2(R(1),R(2))) Cu(II)-phenoxyl complexes with a combination of -(t)Bu, -S(i)Pr, and -OMe substituents leads to dramatic differences in their redox and spectroscopic properties, providing insight into the influence of the cysteine-modified tyrosine cofactor in the enzyme galactose oxidase (GO). Using a modified Marcus-Hush analysis, the oxidized copper complexes are characterized as Class II mixed-valent due to the electronic differentiation between the two substituted phenolates. Sulfur K-edge X-ray absorption spectroscopy (XAS) assesses the degree of radical delocalization onto the single sulfur atom of nonsymmetric [1((t)Bu,SMe)](+) at 7%, consistent with other spectroscopic and electrochemical results that suggest preferential oxidation of the -SMe bearing phenolate. Estimates of the thermodynamic free-energy difference between the two localized states (ΔG(o)) and reorganizational energies (λ(R(1)R(2))) of [1(R(1),R(2))](+) and [2(R(1),R(2))](+) lead to accurate predictions of the spectroscopically observed IVCT transition energies. Application of the modified Marcus-Hush analysis to GO using parameters determined for [2(R(1),R(2))](+) predicts a ν(max) of ∼13600 cm(-1), well within the energy range of the broad Vis-NIR band displayed by the enzyme.

Solvent-dependent modulation of metal–metal electronic interactions in a dinuclear cyanoruthenate complex: a detailed electrochemical, spectroscopic and computational study

The dinuclear complex [{Ru(CN)(4)}(2)(μ-bppz)](4-) shows a strongly solvent-dependent metal-metal electronic interaction which allows the mixed-valence state to be switched from class 2 to class 3 by changing solvent from water to CH(2)Cl(2). In CH(2)Cl(2) the separation between the successive Ru(II)/Ru(III) redox couples is 350 mV and the IVCT band (from the UV/Vis/NIR spectroelectrochemistry) is characteristic of a borderline class II/III or class III mixed valence state. In water, the redox separation is only 110 mV and the much broader IVCT transition is characteristic of a class II mixed-valence state. This is consistent with the observation that raising and lowering the energy of the d(π) orbitals in CH(2)Cl(2) or water, respectively, will decrease or increase the energy gap to the LUMO of the bppz bridging ligand, which provides the delocalisation pathway via electron-transfer. IR spectroelectrochemistry could only be carried out successfully in CH(2)Cl(2) and revealed class III mixed-valence behaviour on the fast IR timescale. In contrast to this, time-resolved IR spectroscopy showed that the MLCT excited state, which is formulated as Ru(III)(bppz(˙-))Ru(II) and can therefore be considered as a mixed-valence Ru(II)/Ru(III) complex with an intermediate bridging radical anion ligand, is localised on the IR timescale with spectroscopically distinct Ru(II) and Ru(III) termini. This is because the necessary electron-transfer via the bppz ligand is more difficult because of the additional electron on bppz(˙-) which raises the orbital through which electron exchange occurs in energy. DFT calculations reproduce the electronic spectra of the complex in all three Ru(II)/Ru(II), Ru(II)/Ru(III) and Ru(III)/Ru(III) calculations in both water and CH(2)Cl(2) well as long as an explicit allowance is made for the presence of water molecules hydrogen-bonded to the cyanides in the model used. They also reproduce the excited-state IR spectra of both [Ru(CN)(4)(μ-bppz)](2-) and [{Ru(CN)(4)}(2)(μ-bppz)](4-) very well in both solvents. The reorganization of the water solvent shell indicates a possible dynamical reason for the longer life time of the triplet state in water compared to CH(2)Cl(2).

Optical absorption of electronic Fe–Ti charge-transfer transition in natural andalusite: the thermal stability of the charge-transfer band

Differently colored natural Brazilian andalusite crystals heat-treated under reducing and oxidizing conditions were analyzed by optical spectroscopy. The intensity of a broad intense band at around 20,500 cm−1 in the optical absorption spectra of all color zones of the sample is proportional to the product of Ti- and Fe-concentrations and herewith proves its attribution to electronic Fe2+/Ti4+ IVCT transition. The band is strictly E||c-polarized, causing an intense red coloration of the samples in this polarization. The polarization of the Fe2+/Ti4+ IVCT band in andalusite, E||c, shows that the electronic charge-transfer process takes place in Al–O octahedral groups that share edges with neighbors on either side, forming chains parallel to the c-axis of the andalusite structure. Under thermal treatments in air, the first noticeable change is some intensification of the band at 800°C. However, at higher temperatures its intensity decreases until it vanishes at 1,000°C in lightly colored zones and 1,100°C in darkly colored ones. Under annealing in reducing conditions at 700 and 800°C, the band also slightly increases and maintains its intensity at treatments at higher temperatures up to 1,000°C. These results demonstrate that weakening and disappearance of the Fe2+/Ti4+ IVCT band in spectra of andalusite under annealing in air is caused by oxidization of Fe2+ to Fe3+ in IVCT Fe2+/Ti4+-pairs. Some intensification of the band at 800°C is, most probably, due to thermally induced diffusion of Fe2+ and Ti4+ in the structure that leads to aggregation of “isolated” Ti4+ and Fe2+ ions into Fe2+–Ti4+-pairs. At higher temperatures, the competing process of Fe2+ → Fe3+ oxidation overcomes such “coupling” and the band continues to decrease. The different thermal stability of the band in lightly and darkly colored zones of the samples evidence some self-stabilization over an interaction of Fe2+/Ti4+-pairs involved in IVCT process.

Noninnocent Ligand Behavior in Diruthenium Complexes Containing a 1,3-Diethynylbenzene Bridge

The electronic structures of 1,3-{trans-Cl(dppe)2RuC≡C}2C6H4 (3) and 1,3-{Cp*(dppe)RuC≡C}2C6H4 (4) in their available redox states have been investigated using a combination of UV−vis−near-IR and IR spectroscopy and computational methods. In contrast to the case for closely related iron compounds, for the ruthenium complexes 3 and 4 the bridging aryl moiety is heavily involved in the oxidation process, and consequently descriptions of the electronic structures and electronic transitions in terms of the language developed for mixed-valence systems with clearly identifiable metal oxidation states are not appropriate. Consequently, the unique low-energy (near-IR) absorption bands observed for the asymmetric cations [3]+ and [4]+ are better described as arising from charge transfer transitions from a metal acetylide donor to a metal phenylacetylide acceptor rather than in terms of IVCT transitions.

Synthesis, Electrochemical, and Optical Properties of Linear Homo- and Heterometallocene Triads

The synthesis, electrochemical, and optical properties of homo- (5, 8, 9, and 12) and heterometallic (6, 7, 10, and 11) ferrocene-ruthenocene triads, are presented. Triferrocenyl derivatives 5 and 9 form the mixed-valence species 5*+ and 92+ by partial oxidation, which show intramolecular electro-transfer phenomena. Interestingly, spectroelectrochemical studies of compound 11, bearing two peripheral ferrocene units and one central ruthenocene moiety, revealed the presence of low-energy bands in the near-infrared (NIR) region, which indicate a rather unusual intramolecular charge-transfer between the ferrocene and ruthenocene units. The value of the electronic coupling parameter V(ab) = 150 cm(-1) calculated by deconvolution of the observed Fe(II)-Fe(III) IVCT transition in the mixed-valence compound 11*+, (d(Fe(II)-Fe(III)) = 18.617 A), indicates the ability of the ruthenocene system to promote a long distance intervalence electron-transfer. Moreover, the reported triads show selective cation sensing properties. Triads 5, 9, and 11 behave as dual redox and optical chemosensors for Zn(2+), Hg(2+), and Pb(2+). Their oxidation redox peaks are anodically shifted (up to 130 mV), and their low-energy (LE) bands of the absorption spectra are red-shifted (up to 115 nm) upon complexation with these metal cations. These changes in the absorption spectra are accompanied by dramatic color changes which allow the potential for "naked eye" detection.

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 × 10 4 to 1.8 × 10 6. 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.

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 the stereoisomers of a dinuclear ruthenium complex containing the bridging ligand dibenzoeilatin

The dinuclear species [{Ru(bpy)2}2(micro-dbneil)]4+(bpy = 2,2'-bipyridine; dbneil = dibenzoeilatin) was separated into its three stereoisomeric forms by cation-exchange chromatographic techniques. The NMR characteristics of the two diastereoisomers (meso and rac) are very similar, consistent with the decreased anisotropic interactions between the terminal bpy ligands located on different ruthenium centres because of their large spatial separation. Spectroelectrochemical measurements of the IVCT transitions in the mixed valence (5+) species show the system to be in the class II-III regime with substantial electronic communication between the metal centres. There is no significant difference in IVCT behaviour between the two diastereoisomers.

An unusual dinuclear ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis, structure, and UV-vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states.

Reaction of [Ru(acac)(2)(CH(3)CN)(2)] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H(2)L) results in formation of an unexpected dinuclear complex [(acac)(2)Ru(III)(L(1))Ru(III)(acac)(2)] (1) in which the bridging ligand [L(1)](2)(-) contains an (-)HN[bond]C[double bond]N[bond]N[double bond]C[bond]NH(-) unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H(2)L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (Delta E(1/2) = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (Delta E(1/2) = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1](-) has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V(ab) approximately 1100 cm(-1), characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1](+), no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.

In situ Raman spectroscopy for the characterization of MoVW mixed oxide catalysts

AbstractIn situ Raman spectroscopy was used to characterize the synthesis and activation steps of mixed metal oxide catalysts starting from the mixed solutions, through subsequent drying and activation procedures, to the catalytic propene partial oxidation reaction. Comparison with Raman spectra recorded for defined, well‐crystallized reference oxides allowed the assignment of the spectra obtained during the catalyst preparation to certain oxides, such as Mo5O14. Especially the latter oxide phase is relevant for selective partial oxidation catalysis. The selectivity for partial oxidation products could be improved between two‐ and three‐fold when the amount of Mo5O14‐type oxide increased. This phase forms from mixtures of molybdenum and tungsten and from mixtures of molybdenum, tungsten and vanadium, but not from binary mixtures of molybdenum and vanadium with high vanadium concentrations. Tungsten and vanadium play an important role as structural promoters in the formation and stabilization of this oxide and for high catalytic activity. A resonance Raman effect was proved for reduced molybdenum oxides due to resonant coupling of the exciting laser to the IVCT transitions between fivefold‐coordinated Mo5+ and sixfold‐coordinated Mo6+. This resonance enhancement renders possible the in situ characterization of operating, reduced molybdenum oxide catalysts. In contrast to the Mo5O14‐type phase, MoO3−x exhibits a high selectivity to total oxidation, and it even becomes re‐oxidized during propene partial oxidation. These different catalytic properties of MoO3−x and the Mo5O14‐type oxide led to the development of a structure–activity relationship which explains the behavior of industrial catalysts. A model is proposed on the basis of a bond order–Raman wavenumber relationship, which explains the different selectivities of these two oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

Dinuclear ruthenium(ii) complexes [{(L)ClRuII}2(μ-tppz)]2+(L = an arylazopyridine ligand) incorporating tetrakis(2-pyridyl)pyrazine (tppz) bridging ligand: synthesis, structure and spectroelectrochemical properties

A series of dinuclear complexes [{(L1–4)ClRuII}2(μ-tppz)][ClO4]2 {[1](ClO4)2 to [4](ClO4)2} has been prepared, in which two {RuII(L1–4)Cl}+ fragments [L = a 2-arylazopyridine ligand of the type 2-(C5H4N)–NN–C6H4R; for L1, R = H; L2, R = p-Me; L3, R = p-Cl; L4, R = m-Me] are linked by the bridging ligand tppz [2,3,5,6-tetrakis(2-pyridyl)pyrazine]. A single isomer forms during the synthesis in each case, and the crystal structure of [4](ClO4)2 shows it to be a twofold-symmetric isomer with each ligand L arranged such that its pyridine donor is on the long axis of the molecule (trans to the pyrazine ring of tppz) and the azo donor is trans to one of the pyridyl donors of tppz. This allows the peripheral aryl ring attached to the azo unit of each ligand L to be oriented over either face of the bridging ligand giving a three-layer π-stacked (aryl–pyrazine–aryl) sandwich. Electrochemical studies revealed (i) separations of 190–250 mV (depending on the aryl substituent of L) between the successive Ru(II)/Ru(III) couples, indicative of a significant inter-metallic electronic coupling, and (ii) several ligand-based reductions of the π-acceptor pyrazine and arylazopyridine ligands. A UV/Vis/NIR spectroelectrochemical study showed the presence of an IVCT transition at ca. 1900 nm in MeCN for the Ru(II)–Ru(III) mixed-valence states, whose narrowness is indicative of borderline class III behaviour. Several reduced forms of the complexes were also spectroscopically characterised.