Spectroelectrochemical Characterization of the Two-Step Redox System {(μ-pz)[Os(CN)5]2}n- (n = 4, 5, 6; pz = Pyrazine). Similarities and Differences in Relation to the Creutz−Taube System

Tungsten and molybdenum incomplete cuboidal clusters; kinetico-mechanistic studies and association in dimers

New triangular Mo(IV) and W(IV) incomplete cuboidal cluster complexes containing three η(2)-diethyldithiophosphates (dtp, one per metal centre) and a η(2)(μ)-AcO or η(2)(μ)-dtp ligand bridging two of these centres have been prepared as mono-substituted derivatives and characterized by the usual techniques plus single crystal X-ray diffraction (XRD) analysis in some cases. In the compounds, the single substitutionally available unlocked position is coordinated to different N-donor ligands (1,2-bis(pyridyl)ethylene, pyrimidine, pyrazine) or to the residual solvent. The compounds are very labile and the N-donor ligands are only weakly coordinated, being easily substituted by other donors or coordinating solvents such as acetonitrile. This lability has been explored kinetico-mechanistically by stopped-flow techniques at varying temperatures for the compound [Mo3S4(η(2)-dtp)3(η(2)(μ)-dtp)(H2O)]. By doing so, the nature of the activation process leading to substitution reactions on these complexes has been established as associatively activated and dominated by a very low formation equilibrium constant, that is, by the substitution of the incoming N-donor ligand. Furthermore, the existence of a non-dissociatively activated trigonal 60° fluxional twist has also been established for the η(2)-dtp ligand on the single unlocked position. The lability of the system has also enabled the association in dimers of the acetato derivative, i.e. [Mo3S4(η(2)-dtp)3(η(2)(μ)-AcO)(H2O)], via a pyrazine bridging ligand. After a very careful tuning of the reaction conditions, as established from the kinetico-mechanistic studies, the first single-bridged unsupported dimeric association of {Mo3S4} units, i.e. [{Mo3S4(η(2)-dtp)3(η(2)(μ)-AcO)}2(μ-pyrazine)], has been structurally characterised.

Unconventional mixed-valent complexes of ruthenium and osmium

Recent developments have helped to extend the repertoire of mixed-valent ruthenium and osmium complexes beyond conventional systems. This extension has been achieved by using sophisticated ligands and by creating more variegated coordination patterns. The strategies employed include the use of multidentate ligands (which give rise to multinuclear and chelate complexes) and the use several redox active components (non-innocent ligands and oxidation-state ambivalence). The results offer enhanced chemical insight into metal-ligand electron-transfer situations and suggest that mixed-valent materials may eventually be exploited in molecular electronics and molecular computing.

Probing Mixed Valence in a New tppz-Bridged Diruthenium(III,II) Complex {(μ-tppz)[Ru(bik)Cl]2}3+ (tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine, bik = 2,2‘-Bis(1-methylimidazolyl)ketone): EPR Silence, Intervalence Absorption, and νCO Line Broadening

The complex dication of the diruthenium(II) compound {(mu-tppz)[Ru(bik)Cl]2}(ClO4)2 can be oxidized and reduced in two one-electron steps each. In CH3CN/0.1 M Bu4NPF6, the odd-electron intermediates{(mu-tppz)[Ru(bik)Cl]2}n+, n=1 and 3, have comproportionation constants of 7x10(8) and 1x10(5), respectively. Both exhibit near-infrared absorptions, in the case of n=3 the 1640 nm band (epsilon=1200 M-1 cm-1, Deltanu1/2=1560 cm-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n=3) is EPR silent even at 4 K, the n=1 form shows g(parallel) 2.005 and g( perpendicular) 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz*- radical anion. The variation of energy and intensity of nuCO and of the ring vibration band around 1590 cm-1 has been monitored not only for {(mu-tppz)[Ru(bik)Cl]2}n+, n=0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}n+, n=0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm-1 on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(mu-tppz)[Ru(bik)Cl]2}3+ shows a perceptibly broader nuCO band, suggesting incomplete valence averaging (partial localization).

Fine Tuning of the Electronic Coupling between Metal Centers in Cyano-Bridged Mixed-Valent Trinuclear Complexes

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.

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+).

Contributions of symmetric and asymmetric normal coordinates to the intervalence electronic absorption and resonance Raman spectra of a strongly coupled p-phenylenediamine radical cation

Resonance Raman spectroscopy, electronic absorption spectroscopy, and the time-dependent theory of spectroscopy are used to analyze the intervalence electron transfer properties of a strongly delocalized class III molecule, the tetraalkyl-p-phenylene diamine radical cation bis(3-oxo-9-azabicyclo[3.3.1]non-9-yl)benzene ((k33)(2)PD(+)). This molecule is a prototypical system for strongly coupled organic intervalence electron transfer spectroscopy. Resonance Raman excitation profiles in resonance with the lowest energy absorption band are measured. The normal modes of vibration that are most strongly coupled to the intervalence transition are identified and assigned by using UB3LYP/6-31G(d) calculations. Excited state distortions are obtained, and the resonance Raman intensities and excitation profiles are calculated by using the time-dependent theory of Raman spectroscopy. The most highly distorted normal modes are all totally symmetric, but intervalence electron transfer absorption spectra are usually interpreted in terms of a model based on coupling between potential surfaces that are displaced along an asymmetric normal coordinate. This model provides a convenient physical picture for the intervalence compound, but it is inadequate for explaining the spectra. The absorption spectrum arising from only the strongly coupled surfaces consists of a single narrow band, in contrast to the broad, vibronically structured experimental spectrum. The electronic absorption spectrum of (k33)(2)PD(+) is calculated by using exactly the same potential surfaces as those used for the Raman calculations. The importance of symmetric normal coordinates, in addition to the asymmetric coordinate, is discussed. The observed vibronic structure is an example of the missing mode effect; the spacing is interpreted in terms of the time-dependent overlaps in the time domain.

Trinuclear pyrazine-bridged ruthenium complexes: syntheses, electrochemistry, NIR-Vis spectra, and their interpretation in terms of a 5-orbital-3-parameter model

A study of absorption spectra in the near-infrared (NIR) and visible (vis) regions of trinuclear Ru complexes containing pyrazine (pyz) as bridging ligand, trans-[(Ru(NH(3))(5)pyz)(2)Ru(NH(3))(4)](m+)(m = 6-9), is reported. The spectra were recorded on aqueous solutions containing the described species formed in situ by stoichiometric additions of a standard solution of Ce(SO(4))(2). They were interpreted in terms of a simple 5-orbital-3-parameter model which includes the effects of d-pi interaction and electronic correlation. The model is shown to account for the observed NIR-vis spectra of the complex ions. The 6+ parent species was synthesized by an improved literature method and fully characterized. The novel 8+ complex was also prepared and characterized. The 9+ ion was established to be slowly reduced by water, with dioxygen formation. Electrochemical (CV and DPV) studies were performed on the trinuclear 6+ complex, as well as on its constituent fragments [Ru(NH(3))(5)(pyz)](2+) and trans-[Ru(NH(3))(4)(pyz)(2)](2+).

Intervalence Transfer at the Localized-to-Delocalized, Mixed-Valence Transition in Osmium Polypyridyl Complexes

The mixed-valence complexes [(bpy)(2)(Cl)Os(III)(BL)Os(II)(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(III)(BL)Os(II)(bpy)(tpy)](5+) (bpy is bipyridine; tpy is 2,2':6',2' '-terpyridine; BL is a bridging ligand, either 4,4'-bipyridine (4,4'-bpy) or pyrazine (pz)) have been prepared and studied by infrared and near-infrared measurements in different solvents. For BL = 4,4'-bpy, there is clear evidence for localized Os(II) and Os(III) oxidation states in the appearance of the expected two interconfigurational dpi --> dpi bands at Os(III) and additional, broad absorption features in the near-infrared arising from intervalence transfer (IT) transitions. For [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+), unusually intense nu(pz) bands appear in the infrared at 1599 cm(-)(1) (epsilon = 2600 M(-)(1) cm(-)(1)) for the former and at 1594 cm(-)(1) (epsilon = 2020 M(-)(1) cm(-)(1)) for the latter. They provide an oxidation state marker and evidence for localized oxidation states. A series of bands appear in the near-infrared from 2500 to 8500 cm(-)(1) that can be assigned to a combination of interconfigurational dpi --> dpi and IT transitions. In CD(3)CN, in the mid-infrared, bands arising from nu(bpy) ring stretching modes from 1400 to 1500 cm(-)(1) are averaged for [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) or significantly perturbed for [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+) compared to electronically isolated Os(II) and Os(III) complexes. The pyrazine-bridged complexes have properties that place them in a new class of mixed-valence molecules, Class II-III having properties associated with both Class II and Class III in the Robin and Day classification scheme. The characteristic features of this class are that oxidation states are localized because of vibrational coupling but that solvent orientational motions are uncoupled because of rapid intramolecular electron transfer.