Intervalence Charge Transfer Band Research Articles

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe 2+ and Fe 3+ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe 2+–Fe 3+) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe 3+ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123–297 K. The number of spins ( N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility ( χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/ χ versus T graph. The infrared spectral studies reveal the formation of Fe 3+–OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si–O–Si stretching and Si–O bending modes.

Delocalization in platinum-alkynyl systems: a metal-bridged organic mixed-valence compound.

A complex featuring two triarylamine redox centers bridged by Pt, trans-bis(triethylphosphine)-bis{4-[bis(4-methoxyphenyl)amino]phenylethynyl} platinum(II), has been synthesized as a model system for pi-conjugated Pt-containing polymers. Analysis of the intervalence charge-transfer band displayed by its mixed-valence monocation affords a quantitative assessment of electronic delocalization through the Pt bridge; this is found to be only slightly smaller than that determined for a benzene-bridged analogue. These results are supported by density functional theory calculations, which show that the active orbitals involved in the electron-transfer process in both cases have similar delocalization through the bridging unit.

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex.

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.

Isovalent and mixed-valent diruthenium complexes [(acac)2RuII (-bpytz)RuII(acac)2] and [(acac)2RuII(-bpytz)RuIII(acac)2](ClO4) (acac = acetylacetonate and bpytz = 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine): synthesis, spectroelectrochemical, and epr investigation .

The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very strong electrochemical stabilization of the mixed-valent RuIIRuIII state (Kc = 10(13.9)) but no detectable (epsilon < 20 M(-1) cm(-1)) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the mixed-valent form leads to a diruthenium(III) species.

Mixed valence aspects of diruthenium complexes [((L)ClRu)2(mu-tppz)]n+ incorporating 2-(2-pyridyl)azoles (L) as ancillary functions and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (Tppz) as bis-tridentate bridging ligand.

Tppz [2,3,5,6-tetrakis(2-pyridyl)pyrazine]-bridged complexes [((L)ClRu)(2)(mu-tppz)]n+ with structurally similar but electronically different ancillary ligands, 2-(2-pyridyl)azoles (L), were synthesized as diruthenium(II) species. Cyclic voltammetry, EPR of paramagnetic states, and UV-vis-NIR spectroelectrochemistry show that the first two reduction processes occur at the tppz bridge and that oxidation involves mainly the metal centers. The mixed valent intermediates from one-electron oxidation exhibit moderate comproportionation constants 10(4) < K(c) < 10(5) but appear to be valence-averaged according to the Hush criterion. Redox potentials, EPR, and UV-vis-NIR results show the effect of increasing donor strength of the ancillary ligands along the sequence L(1) < L(2) < L(4) << L(3), L(1) = 2-(2-pyridyl)benzoxazole, L(2) = 2-(2-pyridyl)benzthiazole, L(3) = 2-(2-pyridyl)benzimidazolate, L(4) = 1-methyl-2-(2-pyridyl)-1H-benzimidazole. Whereas the mixed valent complexes with L(1) and L(2) remain EPR silent at 4 K, the analogues with L(4) and L(3) exhibit typical ruthenium(III) EPR signals, albeit with some noticeable ligand contribution in the case of the L(3)-containing complex. Intervalence charge transfer (IVCT) bands were found in the visible spectrum for the complex with L(3) but in the near-infrared range (at ca. 1500 nm) for the other systems.

The Class II/III Transition Electron Transfer on an Infrared Vibrational Time Scale forN,N‘-Diphenyl-1,4-phenylenediamine Structures

Intramolecular electron transfer (ET) within the class II/III transition for the mixed-valence state of N,N‘-diphenyl-1,4-phenylenediamine (PDA) derivatives which were substituted in the center or the outer phenyl ring and N,N‘-diphenylbenzidine (BZ) was examined. Each compound showed two reversible redox couples. The splitting of the redox waves, ΔE, is related to the interaction intensity between redox sites. The introduction of a substitutent into the central phenylene ring of the PDAs resulted in a decrease in ΔE. A similar result was noted for the expansion of the distance between the redox centers such as in BZ. In opposition, the ΔE of N,N‘-bis(2,6-dimethylphenyl)-1,4-phenylenediamine (2,6-DMPDA) as a compound with substituents introduced into the outer phenyl rings was spread. The mixed-valence state of these compounds also exhibited an intervalence charge transfer (IV-CT) band in the near-IR region which provided the determination of the Marcus reorganization energy (λ), the electron coupling (V)...

Mixed-valence properties of Ruthenium–Polypyridine dimers bridged by Imidazolate and Triazolate Ligands

The dinuclear complex cis,cis-[(bpy)2ClRu(μ-bim)RuCl(bpy)2] n + (bpy = 2,2′-bipyridine; bim = benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n = 2) displays significant metal–metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of σ,π-basic character of the bridging ligands, according to an electron superexchange mechanism of the “hole-transfer” type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (RuIII–L–RuIII) and fully reduced (RuII–L–RuII) isovalent species.

Characterization of the mixed-valent intermediate ( n=3) of the complex series {(μ,η 3:η 3-L)[(Ru(terpy)] 2} n+ with the unsaturated bridging ligand N, N′-bis(α-picolinoyl)hydrazido(2−)=L 2−

N, N ′-Bis(α-picolinoyl)hydrazine H 2L, a potentially bis-tridentate conjugated ligand in its doubly deprotonated form L 2−, has been used to synthesize the diruthenium(II) complex {(μ-L)[Ru(terpy)] 2}(PF 6) 2. Its electrochemical and spectroelectrochemical (UV–Vis–NIR, EPR) investigation reveals a mixed-valent (Ru IIIRu II) intermediate with electrochemical and spectroscopic characteristics related to those of the {(μ-adc-R)[Ru(bpy) 2] 2} 3+ ions (adc-R 2−=bis-bidentate C-substituted diacylhydrazido(2−) ligands): comproportionation constant K c=10 9.0 (CH 2Cl 2), intervalence charge transfer band at 1000 nm ( ε=3800 M −1 cm −1), EPR signal at g ⊥=2.14, g ∥=1.953. The non-innocent nature of L n− is inferred from comparative analysis of the spectroscopic data.

Structural, spectroscopic, and electrochemical studies of the complexes [Ni2(mu-CNR)(CNR)2(mu-dppm)2](n+) (n = 0, 1, 2): unusual examples of nickel(0)-nickel(I) and nickel(0)-nickel(II) mixed valency.

Reaction of Ni(COD)(2) (COD = cyclooctadiene) with dppm (dppm = bis(diphenylphosphino) methane) followed by addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni(2)(mu-CNR)(CNR)(2)(mu-dppm)(2) (R = CH(3) (1), n-C(4)H(9) (2), CH(2)C(6)H(5) (3), i-C(3)H(7) (4), C(6)H(11) (5), t-C(4)H(9) (6), p-IC(6)H(4) (7), 2,6-(CH(3))(2)C(6)H(3) (8)). The cyclic voltammograms of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0) dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using [FeCp(2)][PF(6)], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes. The +2 forms of these complexes, 6(2+) and 8(2+), have asymmetric structures with one locally square planar and one locally tetrahedral metal center, evidence for a Ni(II)-Ni(0) mixed valence state. The X-ray structural characterization of 6(+) is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1(+)-8(+), are assigned as Robin-Day class III, fully delocalized Ni(+0.5)-Ni(+0.5) mixed valence complexes.

(mu-L)[RuII(acac)2]2]n, n= 2+, +, 0, -, 2-, with L = 3,3',4,4'-tetraimino-3,3',4,4'-tetrahydrobiphenyl. EPR-supported assignment of NIR absorptions for the paramagnetic intermediates.

A complete EPR and UV-Vis-NIR spectroelectrochemical characterisation has been carried out for the series 1(2+/+/0/-/2-) where is the 1 new complex [(mu-L)[Ru(II)(acac)2]2], L = 3,3',4,4'-tetraimino-3,3',4,4'-tetrahydrobiphenyl. The paramagnetic intermediates are identified as the anion radical (L*-) complex 1*- with a long-wavelength intra-ligand transition at 2160 nm and as the weakly coupled diruthenium(II,III) species 1+ (Kc= 10(3)) with a low-intensity intervalence charge transfer (IVCT) band at 1570 nm. DFT calculations using ADF and Gaussian 03 programs support these assignments.

The triruthenium complex [[(acac)2Ru(II)]3(L)] containing a conjugated diquinoxaline[2,3-a:2',3'-c]phenazine (L) bridge and acetylacetonate (acac) as ancillary ligands. Synthesis, spectroelectrochemical and EPR investigation.

The compound [[(acac)2Ru]3(L)] (1) undergoes three well-separated one-electron oxidation and reduction processes. The EPR results indicate electron removal from the ruthenium(II) centres on oxidation and the occupation of a largely L-based molecular orbital on reduction. In spite of well-separated (DeltaE > or = 340 mV) oxidation no obvious intervalence charge transfer bands were detected in the Vis, NIR or IR regions, suggesting very weak electronic coupling between the metal centres in the mixed-valent intermediates 1+ and 1(2+). The separated (DeltaE > or = 540 mV) stepwise reduction produces weak near-infrared features associated with partially occupied pi* orbitals of L, the unusually high g anisotropy in the EPR spectrum of 1- is attributed to the occupation of a degenerate MO by the unpaired electron.

Modifying Electronic Communication in Dimolybdenum Units by Linkage Isomers of Bridged Oxamidate Dianions

Reactions of Mo(2)(O(2)CCH(3))(DAniF)(3), DAniF = N,N'-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr](2-), Ar = C(6)H(5) and p-anisyl, give pairs of isomeric compounds where the [Mo(2)] units are bridged by the oxamidate anions. For the alpha isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the beta isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the DeltaE(1/2) for the oxidation of each of the Mo(2) units is significantly better for the beta isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the beta isomers but not the alpha isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the alpha isomers and the relative energy differences between the alpha and beta isomers.

What a difference ancillary thienyl makes: unexpected additional stabilization of the diruthenium(III,II) but not the diosmium(III,II) mixed-valent state in tetrazine ligand-bridged complexes.

The Ru(2)(III,II) mixed-valent state is strongly stabilized in [(bpy)(2)Ru(mu-bttz)Ru(bpy)(2)](5+) (3(5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant K(c) = 10(16.6), and a high-energy intervalence charge transfer band at 10100 cm(-1). Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)(2)M(mu-bmptz)M(bpy)(2)](5+), bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher K(c) value of the Os(2)(III,II) analogue. EPR signals were observed at 4 K for 3(5+) but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2(3+) and 3(3+).

Building blocks for the molecular expression of quantum cellular automata. Isolation and characterization of a covalently bonded square array of two ferrocenium and two ferrocene complexes.

The suitability of [{(eta5-C5H5)Fe(eta5-C5H4)}4(eta4-C4)Co(eta5-C5H5)][PF6]2, [1][PF6]2, for use as a molecular quantum cellular automata (QCA) cell is demonstrated. To this end the structure of 1 in the solid state and the conversion of 1 to mono- and dicationic mixed-valence complexes have been accomplished. The latter compounds have been isolated as pure materials and characterized by IR, EPR, and Mössbauer spectroscopies and single-crystal XRD (monocation only) and magnetic susceptibility measurements. Near-IR spectra demonstrate the mixed valence character of the cations (valence trapped on the IR, EPR and Mössbauer time scales), and the energies of the intervalence charge-transfer bands provide a measure of the hole hopping frequency.

Intervalence, electron transfer and redox properties of a triazolate-bridged ruthenium-polypyridine dinuclear complex

A new dinuclear complex of the type cis,cis-[(bpy) 2ClRu(μ-L b)RuCl(bpy) 2] n+ (bpy=2,2′-bipyridine; L b=benzotriazolate (bta); n=1, 2, or 3) has been synthesized, isolated as a PF 6 − salt, and investigated in organic solutions by means of cyclic voltammetry and ultraviolet/visible/near-infrared spectroelectrochemistry. Particular emphasis has been given to the electron transfer (ET) properties of the mixed-valent species ( n=2), which displays a somewhat large metal–metal electronic coupling in the ground state with the complex featuring localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge-transfer (IVCT) band and electrochemical parameters. Analysis of the IVCT properties in the context of Hush's theory also supports a valence-trapped formulation. In spite of the class II categorization within the Robin-Day scheme, this system shows a remarkable intermetallic communication when compared with other analogues (e.g. L b=pyrazine). Such aspect has also been stressed by comparison of the set of thermodynamic and mixed-valence parameters along with a series of L-bridged systems studied previously in aqueous solutions (in particular, [(edta)Ru(μ-bta)Ru(edta)] 4−; edta=ethylenediamine- N, N, N′, N′-tetraacetate), and the striking differences in their intervalence characteristics have been rationalized in terms of distinct types of electronic and structural effects. Despite the contrasting behavior, the same type of superexchange mechanism (‘hole-transfer’) seems to prevail in all these benzotriazolate-bridged mixed-valent species. In the 2,2′-bipyridine derivative, the synergistic charge-transfer effects are the most relevant factors on the great stabilization of the mixed-valence state. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (Ru IIILRu III) and fully reduced (Ru IILRu II) isovalent species. From the IVCT band features, the rate of intramolecular thermal ET for the mixed-valent ion was estimated on the basis of the Hush and Marcus theories.

Intervalence charge-transfer bands in triphenylamine-based polymers

In this paper we show that bis(triarylamine) systems with conjugated bridges can be polymerised potentiodynamically. The resulting polymer films can be doped to a degree of ca. 75% per triarylamine unit. At 50% doping level the polymers show broad and intense intervalence charge-transfer bands. These IV-CT bands are quite similar to those in monomeric model compounds. Therefore, the polymers investigated can be regarded as polymeric mixed valence compounds.

Complexes of 2,2′-biphenol with ruthenium(ii), molybdenum(v), tungsten(v) and tungsten(vi): structures, electrochemistry and spectroscopy

Complexes of the potentially bidentate ligand 2,2′-biphenol (H2biph) have been prepared and studied by electrochemical and UV/Vis/NIR spectroelectrochemical methods. The complexes studied are [(Tp*)M(O)(biph)] [M = Mo, W; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate], [W(biph)3], [Cl6W2(biph)3] and [Ru(bpy)2(biph)], and all have been structurally characterised. [(Tp*)M(O)(biph)] (M = Mo, W) both undergo reversible M(IV)/M(V) couples at negative potentials, with the redox potential for the W(IV)/W(V) couple being 580 mV more negative than that of the Mo(IV)/Mo(V) couple; the redox potentials are similar to those which occur in the complexes [(Tp*)M(O)(OC6H5)2] with two monodentate phenolate ligands. The structure of [W(biph)3] is essentially octahedral but with a distortion towards trigonal prismatic; the complex undergoes two metal-centred redox processes, W(IV)/W(V) and W(V)/W(VI) which were characterised spectroelectrochemically. An unexpected new W(VI) complex [Cl6W2(biph)3] has the structure [{WCl3(biph)}2(μ-biph)], in which each W(VI) centre has a terminal chelating biphenolate ligand, and other highly twisted biphenolate ligand acts as a bis-monodentate bridge spanning the two W(VI) centres. [Cl6W2(biph)3] undergoes two successive W(V)/W(VI) redox processes at negative potentials, with a separation of 170 mV indicating a through-space Coulombic interaction between the metal centres; spectroelectrochemistry showed no evidence of an inter-valence charge-transfer band in the mixed-valence W(V)–W(VI) state. [Ru(bpy)2(biph)] has a Ru(II)/Ru(III) couple at a potential very similar to related ruthenium complexes with a (pyridine)4(phenolate)2 donor set.

Synthesis and Spectroelectrochemical Characterization of a Novel Oxalate-Bridged Binuclear Ruthenium(III) Complex

Abstract A novel oxalate-bridged binuclear ruthenium(III) complex, [{Ru(acac)2}2(μ-ox)] (acac− = acetylacetonate and ox2− = oxalate), has been prepared via self-dimerization of K[Ru(acac)2(ox)] in aqueous solutions containing ferric salts as catalyst. The Ru2III,II mixed-valence species generated electrochemically with Kc=105.0 for the comproportionation constant exhibits a weak intervalence charge transfer (IVCT) band at 1430 nm. The IR spectra from spectroelectrochemistry indicate a partially localized mixed-valence state (Class II-III behavior).

Long-range electronic coupling in various oxidation states of a C4-linked tris(beta-diketonato)ruthenium dimer.

Getting the message across: Strong electronic coupling over a distance of 1.3 nm is observed for the Ru state of the complex 1n (n=+). The fully delocalized system 1+ is distinguished by an extremely intense intervalence charge-transfer (IVCT) band and a single intense vibrational band.

Synthesis and mixed valence aspects of [{(L)ClRu}2(μ-tppz)]n+incorporating 2,2′-dipyridylamine (L) as ancillary and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) as bridging ligand

The tppz-bridged diruthenium complex [{(L)ClRuII}2(μ-tppz)](ClO4)2, [1](ClO4)2 {tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, L = 2,2′-dipyridylamine} and its mononuclear counterpart [(L)ClRuII(tppz)]ClO4, [2](ClO4) have been synthesized. The 380 mV separation between successive RuII/RuIII couples in [1]2+ leads to a comproportionation constant (Kc) of 2.7 × 106. Consequently, the RuIIRuIII species [1]3+ exhibits a rather narrow intervalence charge transfer band at 1700 nm, suggesting a class III mixed-valence state, the electronic coupling constant (Vab) is calculated at 2940 cm−1. Complex [1]3+ displays a rhombic EPR spectrum at 4 K (g1 = 3.390, g2 = 2.278, g3 = 1.697), characteristic of ruthenium(III) in a distorted octahedral environment. Both complexes show two successive tppz-based reduction processes [(tppz)0/−1 and (tppz)−1/−2]. The one-electron reduced species [1]+ is a tppz radical anion species with an intense low-energy band at 1105 nm and an axial EPR signal at 4 K (g1 = 2.008, g2 = g3 = 1.994). [1]2+ and [2]+ exhibit moderately strong emissions at 740 nm and 668 nm, respectively, in EtOH–MeOH glass at 77 K.