Intervalence Transfer Research Articles

Competition between Cu-Br semi-coordinate bond and C-H∙∙∙Br, C-H∙∙∙S and S∙∙∙S interactions; new two thioalkylazothiophenol (SNS) copper (II) dimers [Cu(L)(µ-Br)

Two new copper dimers [Cu(L)(µ-Br)]2, HL = 2(RS)C6H4N = NC(COCH3)-SC6H5, where R = CH3 (1), C6H5 (2) were synthesized and characterized using a variety of physicochemical methods, including single crystal X-ray diffraction. The molecular structures of the two binuclear copper(II) complexes show that Cu∙∙∙Cu inter-nuclei distances are 3.662 \\AA and 3.392 \\AA in 1 and 2, respectively. Structural analysis of the two dimers supported by QTAIM analysis of electron density indicates the presence of competition between Cu∙∙∙Br semi-coordinate bond and CH∙∙∙Br and CH∙∙∙S hydrogen bonding interactions and S∙∙∙S chalcogen bonding interactions. In complex 1, CH∙∙∙Br and CH∙∙∙S hydrogen bonding interactions exist, in contrast, they are absent in 2. Cu∙∙∙Br semi-coordinate bonds are stronger in 2 as indicated by Cu∙∙∙Br bond distances; Cu∙∙∙Br bond distances are 2.955 and 2.858 Å in 1 and 2.746 Å in 2. There are two weaker S∙∙∙S interactions in 2 and one stronger interaction in 1 as indicated by S∙∙∙S interatomic distances. The two copper dimers and their ligands exhibited good anticancer efficacy against breast (MCF-7), lung (HCT115), and colon human (A549) cancer cell lines. Complex 1 demonstrated strong cancer cell death compared to complex 2. Additionally, the complexes exhibited higher activity than the ligands. Dimer 2 exhibits two quasi-reversible one-electron reduction waves at −0.05 V and −0.37 V. The comproportionating constant for forming the Cu(I/II) mixed-valence complex was 2.57 × 105. The absence of Intervalence transfer (IT) bands in the electronic absorption spectrum for the mixed-valence complex suggested that a single unpaired electron is localized on one copper center.

RAMAN STUDIES OF BIS(PHENYLTERPYRAZINE)IRON(II) AND SUPRAMOLECULAR SPECIES WITH PENTACYANIDOFERRATE(II) IONS

The chemistry and spectroscopy of the iron(II) phenylterpyrazine complex, [Fe(phtpz)2]2+, were investigated in comparison with the analogous phenylterpyridine species, Fe(phtpy)2]2+, and the results indicated a strong electronic delocalization on the Fe(II) terimine chromophore encompassing the lateral pyrazyl groups. The delocalized nature of the molecular orbitals involved in the electronic excitation precluded a direct assignment of the pyridine and pyrazine vibrations in the complex. However, the capability of the terpyrazine complex to bind transition metal ions, such as the [Fe(CN)5]3- species, allowed to generate pentanuclear complexes displaying characteristic charge-transfer and resonance Raman spectra, revealing the pyrazine vibrations in the complex. The [Fe(CN)5]3- containing species also yielded polymeric compounds in presence of transition metal ions, exhibiting intervalence transfer bands and chemical similarities with the Prussian Blue complex.

Structural and physical characteristics of Au2O3‐doped sodium antimonate glasses – Part II electrical characteristics

AbstractIn continuation of our earlier investigations on structural and physical characteristics of Au2O3‐doped sodium antimonate glass‐ceramics (Part‐1), in this part we have investigated the influence of gold ions on electrical characteristics of the Na2O–Sb2O3: Au2O3 glass‐ceramics. The study contains the results of quantitate investigations on dielectric properties, impedance spectra and A.C. conductivity in larger ranges of continuous frequencies (4 Hz‐8 MHz) and temperatures (300‐630 K). The variations exhibited by dielectric parameters with temperature and also with frequency were discussed in terms of various polarization mechanisms. The observed dielectric relaxation effects were analyzed using pseudo Cole‐Cole plot method and the analysis indicated spreading of relaxation times for dipoles. A.c. conductivity and also d.c. conductivity were found to decrease (to three orders of magnitude) with increase in Au2O3 concentration upto 0.1 mol%. The decrement is ascribed to the increasing concentration of Sb5+ ions that were predicted to participate in the glass network forming with SbO4 units. Even though, both ionic and polaronic contributions are possible for conduction in the studied material, quantitative analysis of these results indicated that the polaronic conduction (due to intervalence transfer between Sb3+ ↔ Sb5+ and Au0 ↔ Au3+) is prevalent. The results have also suggested that there is a gradual decrement in the ionic component with increase in Au2O3 concentration. Variation in σac in the low‐temperature region could satisfactorily be explained using quantum mechanical tunneling (QMT) model. Analysis of the results of d.c. conductivity indicated that the small polaron hoping (SPH) model is valid, especially in a high‐temperature region while the low temperature part of d.c. conductivity is analyzed based on variable range hopping (VRH) model. Overall, the increase in Au2O3 dopant concentration in the studied glass‐ceramics caused a decrement in the magnitude of the conductivity or increase in the insulating strength of the material.

Electrostatic bending and outer-sphere intervalence transfer in a flexible ligand-bridged ruthenium(III)-iron(II) complex

In the mixed-valence complex [RuIII(NH3)5(μ-dpypn)FeII(CN)5] with the flexible bridging ligand 1,3-di(4-pyridyl)propane (dpypn), electrostatic interactions between the {Ru(NH3)5}3+ and {Fe(CN)5}3− moieties drive a strong bending of dpypn and approximation of the RuIII and FeII centers, from which the enhanced electronic coupling between metal ions produces an intense intervalence-transfer absorption in the near-infrared region. Density functional theory calculations corroborate both the electrostatic bending in this heterobinuclear complex and a linear geometry in the homobinuclear counterparts [Ru(NH3)5(μ-dpypn)Ru(NH3)5]5+ and [Fe(CN)5(μ-dpypn)Fe(CN)5]5−, for which no evidence of electronic coupling was found because of the separation between metal centers. Furthermore, the heterobinuclear species formed an inclusion complex with β-cyclodextrin where the imposed linear geometry prevents significant electronic coupling and intervalence charge transfer between the RuIII and FeII centers.

Probing of local structures of thermal and photoinduced phases in rubidium manganese hexacyanoferrate by resonant Raman spectroscopy

Resonant couplings of the electronic states and the stretching vibrations of CN(-) ligands, which bridges metal ions, is investigated by resonance Raman spectroscopy for Rb(0.94)Mn[Fe(CN)6](0.98)·0.2H2O. Large excitation wavelength dependences over one order of magnitude were found for Raman peaks corresponding to different valence pairs of metal ions in the excitation wavelength range between 350 and 632 nm. In the thermal low-temperature phase, the CN(-) stretching modes due to the low-temperature-phase configuration (Fe(2+)-Mn(3+)) and the phase-boundary configuration (Fe(3+)-Mn(3+)) are coupled to the Fe(2+)-to-Mn(3+) intervalence transfer band and Jahn-Teller distorted Mn(3+) d-d transition band, respectively. In the photoinduced low-temperature phase, the Fe(3+)-Mn(3+) mode shows strong resonant enhancement with the CN(-)-to-Fe(3+) charge-transfer band, which exists in the high-temperature phase with a cubic structure. From these resonance behaviors, we conclude that the local lattice symmetry of the photoinduced phase is cubic in contrast with the tetragonal symmetry in the thermal low-temperature phase.

Substituent and Solvent Effects on the Electrochemical Properties and Intervalence Transfer in Asymmetric Mixed‐Valent Complexes Consisting of Cyclometalated Ruthenium and Ferrocene

Four heterodimetallic complexes [Ru(Fcdpb)(L)](PF6) (Fcdpb=2-deprotonated form of 1,3-di(2-pyridyl)-5-ferrocenylbenzene; L=2,6-bis-(N-methylbenzimidazolyl)-pyridine (Mebip), 2,2':6',2''-terpyridine (tpy), 4-nitro-2,2':6',2''-terpyridine (NO2tpy), and trimethyl-4,4',4''-tricarboxylate-2,2':6',2''-terpyridine (Me3tctpy)) have been prepared. The electrochemical and spectroelectrochemical properties of these complexes have been examined in CH2Cl2, CH3NO2, CH3CN, and acetone. These complexes display two consecutive redox couples owing to the stepwise oxidation of the ferrocene (Fc) and ruthenium units, respectively. The potential difference, ΔE(1/2)(E(1/2)(Ru(II/III))-E(1/2)(Fc(0/+))), decreased slightly with increasing solvent donocity. The mixed-valent states of these complexes have been generated by electrolysis and the resulting intervalence charge-transfer (IVCT) bands have been analyzed by Hush theory. Good linear relationships exist between the energy of the IVCT band, E(op), and ΔE(1/2) of four mixed-valent complexes in a given solvent.

COMPUTATIONAL STUDY OF BRIDGE-MEDIATED INTERVALENCE ELECTRON TRANSFER II: COUPLINGS IN DIFFERENT METALLOCENE COMPLEXES

The constrained density functional theory (CDFT) was used to study bridge-mediated electron transfer processes in mixed-valence systems with two identical metallocene (cobaltocene, ruthenocene, and nickelocene) moieties linked by various bridge structures. Based on the electronic coupling matrix elements obtained from the CDFT calculations, the relationship between the bridge linkage and the effectiveness of intervalence transfer was discussed.

Low-temperature activation of nitrogen oxide on Cu-ZSM-5 catalysts

The adsorption and activation of NO molecules on Cu-ZSM-5 catalysts with different Cu/Al and Si/Al ratios (from 0.05 to 1.4 and from 17 to 45, respectively) subjected to different pretreatment was studied by ultraviolet-visible diffuse reflectance (UV-Vis DR). It was found that the amount of chemisorbed NO and the catalyst activity in NO decomposition increased with an increase in the Cu/Al ratio to 0.35–0.40. The intensity of absorption bands at 18400 and 25600 cm−1 in the UV-Vis DR spectra increased symbatically. It was hypothesized that the adsorption of NO occurs at Cu+ ions localized in chain copper oxide structures with the formation of mono- and dinitrosyl Cu(I) complexes, and this process is accompanied by the Cu2+...Cu+ intervalence transfer band in the region of 18400 cm−1. The low-temperature activation of NO occurs through the conversion of the dinitrosyl Cu(I) complex into the π-radical anion (N2O2)− stabilized at the Cu2+ ion of the chain structure, [Cu2+-cis-(N2O2)−], by electron transfer from the Cu+ ion to the cis dimer (NO)2. This complex corresponds to the L → M charge transfer band in the region of 25600 cm−1. The subsequent destruction of the complex [Cu2+-cis-(N2O2)−] at temperatures of 150–300°C leads to the release of N2O and the formation of the complex [Cu2+O−], which further participates in the formation of the nitrite-nitrate complexes [Cu2+(NO2)−], [Cu2+(NO)(NO2)−], and [Cu2+(NO3)−] and NO decomposition products.

Structural origin of the nonlinear optical properties of lead niobium germanate film glasses

The structural origin of the nonlinear optical susceptibility (/χ(3)/) of lead-niobium-germanate film glasses with large Nb2O5 contents has been investigated. /χ(3)/ shows a strong enhancement with the Nb content in the films with /χ(3)/ values close to 2 × 10−11 esu at 800 nm for a Nb content as high as 0.71. Boling-Glass-Owyoung and Lines’ semiempirical models predict accurately the values of /χ(3)/ for transparent bulk glasses but not for film glasses. This discrepancy is related to the remarkable structural differences between them. Raman spectroscopy suggests the formation of a three-dimensional (3D) structure of [NbO6] octahedra in the case of film glasses having large Nb contents, while X-ray photoelectron spectroscopy shows that a significant fraction of these units contain Nb4+ ions. The combination of a 3D structure of [NbO6] with the presence of Nb4+ polarons and their migration through electron intervalence transfer is proposed as the origin of the observed enhancement of /χ(3)/ in the film glasses.

Multi-electrochromism behavior and electrochromic mechanism of electrodeposited molybdenum doped vanadium pentoxide films

Molybdenum doped vanadium pentoxide (Mo doped V 2O 5) films are prepared by cathodic electrodeposition on indium tin oxide substrate from Mo doped V 2O 5 sol. As an anodic and cathodic coloration electrochromic material, the electrodeposited Mo doped V 2O 5 film presents a better cycling stability, reversibility and multi-electrochromic behavior (orange–yellow–green–blue) with an optical modulation of 60–90% in the spectral region 550–900 nm, which can be expected as a result of enhanced electron intervalence transfer between Mo 6+ and V 5+, V 4+ states, in addition to V 5+ and V 4+ transition. The electrochromic mechanism of Mo doped V 2O 5 films is investigated with atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The surface roughness of the film increase at the different coloration states due to the increasing crystallinity degree. The change of the interlayer spacing for the host V 2O 5 and the change of the C and Li element states verify the insertion of organic solvent into the interlayer of the host V 2O 5 and some of the Li + ions into the sites in the V–O layers. The electrochromic kinetics process indicates that the electrochromism of Mo doped V 2O 5 films can be considered as a reversible reduction/oxidation process accompanying the insertion/extraction of Li + ions and electrons.

Computational Study of Bridge-Assisted Intervalence Electron Transfer

Intervalence electron transfer reactions were studied computationally by means of density functional theory and constrained density functional theory (CDFT). Two ferrocene moieties, connected via various bridge structures, were used as model mixed-valence compounds in the computational investigation. Features of the frontier orbitals were analyzed to offer a qualitative account of the intervalence characteristics of the model complexes. The effective electronic coupling between the donor and acceptor sites was calculated using the CDFT method, which provided a quantitative measure of the intervalence electronic communication. The relationship between the bridge linkage and the effectiveness of intervalence transfer was discussed on the basis of the theoretical results and compared to experimental data available in the literature.

Models for the spectra of bridged mixed-valence dimers

Models for the spectra of bridged mixed-valence dimers, which generally exhibit broad, intense bands in the visible or near infrared regions, are discussed. The two-site model of Piepho, Krausz and Schatz (PKS) [S.B. Piepho, E.R. Krausz, and P.N. Schatz, J. Am. Chem. Soc. 100, 2996–3005 (1978)] is reviewed and compared with a three-site model which explicitly includes the bridging species. We show that when there is a strongly-coupled electronic state on the bridge, the totally symmetric vibrational motion is very important in the vibronic coupling problem for these complexes. Along the vibrational sum coordinate, the potential surfaces for the upper and lower states in the intervalence transfer transition may be treated approximately as two harmonic oscillators with displaced minima. At low temperatures, the transition intensities are given by a Poisson distribution.

Nanoparticle-Mediated Intervalence Transfer

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{[Ag2 (μ-dppm)2 (μ-TCNQ)2](TCNQ)}, a charge transfer compound derived from a donor with a metal–metal bond

Reaction of AgNO3, dppm (bis(diphenylphosphino)methane), and TCNQ in molar ratio of 1 : 1 : 1 produced an acceptor–donor silver compound {[Ag2(dppm)2 (µ-TCNQ)2](TCNQ)}, 1. In the structure, determined by X-ray crystallography, two silver atoms are triply-bridged by two dppm ligands and metal–metal bond to form a dimer; the distance between two Ag is 3.1753(8) Å. Compound 1 was studied by IR and UV/Vis/NIR spectroscopy, exhibiting an intervalence transfer transition at λ = 1286 nm, and also is photoluminescent (λmax = 537 and 654 nm) at room temperature.

Intervalence transfer of ferrocene moieties adsorbed on electrode surfaces by a conjugated linkage

Effective intervalence transfer occurred between the metal centers of ferrocene moieties that were adsorbed onto a ruthenium thin film surface by ruthenium–carbene π bonds, a direct verification of Hush’s four-decade-old prediction. Electrochemical measurements showed two pairs of voltammetric peaks where the separation of the formal potentials suggested a Class II behavior. Additionally, the potential spacing increased with increasing ferrocene surface coverage, most probably as a consequence of the enhanced contribution from through-space electronic interactions between the metal centers. In contrast, the incorporation of a sp 3 carbon spacer into the ferrocene–ruthenium linkage led to the diminishment of interfacial electronic communication.

Nanoparticle-Mediated Intervalence Transfer

Nanoparticle-mediated intervalence transfer was reported with ferrocene moieties that were attached onto the ruthenium nanoparticle surface by ruthenium-carbene pi bonds. The resulting particles exhibited two pairs of voltammetric waves with a potential spacing of about 200 mV and a rather intense absorption peak in the near-infrared range (approximately 1930 nm) at mixed valence. Both features suggested Class II characteristics of the intraparticle intervalence transfer that mainly arose from through-bond interactions between the metal centers. Quantum calculations based on density functional theory showed that the nanoparticle core electrons served as conducting band states for the effective charge delocalization between particle-bound ferrocene moieties.

Synthesis, Structure, and Electronic Properties of a Dimer of Ru(bpy)2Doubly Bridged by Methoxide and Pyrazolate

The heterobridged dinuclear complex cis,cis-[(bpy) 2Ru(mu-OCH 3)(mu-pyz)Ru(bpy) 2] (2+) ( 1; bpy = 2,2'-bipyridine; pyz = pyrazolate) was synthesized and isolated as a hexafluorophosphate salt. Its molecular structure was fully characterized by X-ray crystallography, (1)H NMR spectroscopy, and ESI mass spectrometry. The compound 1.(PF 6) 2 (C 44H 38F 12N 10OP 2Ru 2) crystallizes in the monoclinic space group P2 1/ c with a = 13.3312(4) A, b = 22.5379(6) A, c = 17.2818(4) A, beta = 99.497(2) degrees , V = 5121.3(2) A (3), and Z = 4. The meso diastereoisomeric form was exclusively found in the crystal structure, although the NMR spectra clearly demonstrated the presence of two stereoisomers in solution (rac and meso forms at approximately 1:1 ratio). The electronic properties of the complex in acetonitrile were investigated by cyclic voltammetry and UV-vis and NIR-IR spectroelectrochemistries. The stepwise oxidation of the Ru (II)-Ru (II) complex into the mixed-valent Ru (II)-Ru (III) and fully oxidized Ru (III)-Ru (III) states is fully reversible on the time scale of the in situ (spectro)electrochemical measurements. The mixed-valent species displays strong electronic coupling, as evidenced by the large splitting between the redox potentials for the Ru(III)/Ru(II) couples (Delta E 1/2 = 0.62 V; K c = 3 x 10 (10)) and the appearance of an intervalence transfer (IT) band at 1490 nm that is intense, narrow, and independent of solvent. Whereas this salient band in the NIR region originates primarily from highest-energy of the three IT transitions predicted for Ru(II)-Ru(III) systems, a weaker absorption band corresponding to the lowest-energy IT transition was clearly evidenced in the IR region ( approximately 3200 cm (-1)). The observation of totally coalesced vibrational peaks in the 1400-1650 cm (-1) range for a set of five bpy spectator vibrations in Ru (II)-Ru (III) relative to Ru (II)-Ru (II) and Ru (III)-Ru (III) provided evidence for rapid electron transfer and valence averaging on the picosecond time scale. Other than a relatively short Ru...Ru distance (3.72 A for the crystalline Ru (II)-Ru (II) complex), the extensive communication between metal centers is attributed mostly to the pi-donor ability of the bridging ligands (pyz, OMe) combined with the pi-acceptor ability of the peripheral (bpy) ligands.

Synthesis of the Prototypical Cyclic Metallocene Triad: Mixed‐Valence Properties of [(FeCp)3(trindenyl)] Isomers

The “fusion” of three ferrocenes into a rigid cyclic array produces the syn,syn,anti and syn,syn,syn isomers of [(FeCp)3(trindenyl)] (see picture), the cations of which display different mixed-valence properties, namely multiple intervalence-transfer (IT) absorptions appear in the near-IR/IR spectra for the syn,syn,anti isomer, while a single band appears in the visible/near-IR spectrum for the syn,syn,syn isomer.

{[Cu2(μ-Cl)(μ-dppm)2](μ2-TCNQ)}∞, a charge transfer compound derived from a donor with a copper dimer

The reaction of CuCl, dppm (bis(diphenylphosphino)methane), and TCNQ with a molar ratio set of 2:2:1 produced a one-dimensional compound 1 {[Cu2(μ-Cl)(μ-dppm)2](μ2-TCNQ)}∞. Its structure was determined by X-ray crystallography. The new compound 1 was studied by IR and UV/Vis/NIR spectroscopy. 1 exhibits an intervalence transfer transition at λ=1296nm, and also appears photoluminescence (λmax=818nm) at room temperature.

Intramolecular electron transfer in the mixed-valence [Co(3,5-DTBCat)(3,5-DTBSQ)(bpy)] complex: Beyond valence tautomerism

The series of complexes [Co(Q) 2(bpy)] n ( n = −1, 0, +1) that can be derived by partial reduction or oxidation of complex 1, ls-Co(III) has been synthesized and studied. The results support the theoretical calculations which pointed to an intervalence transfer (IT) from the Cat 2− to the SQ − ligand rather than a LMCT transition as the origin for the low-energy band transition centered at 2500 nm observed for 1, ls-Co(III).