Mixed-valence Binuclear Complexes Research Articles

An unsymmetrical mixed-valence oxidovanadium(IV/V) binuclear complex: Synthesis, characterization, DFT studies, and bromoperoxidase activity

The mixed-valence binuclear complex [{VO(acac)(bpy)}(μ–O){VO(bpy)(ox)}]·2H2O (1), where acac− = acetylacetonate, bpy = 2,2-bipyridine and ox2− = oxalate, was synthesized under mild conditions and characterized by elemental analysis and infrared (IR), electron paramagnetic resonance (EPR), and electronic (UV/Vis) spectroscopies. According to single-crystal X-ray diffraction data, the two nonequivalent oxidovanadium centers of the {(VO)2(μ-O)}3+ core adopt an anti-angular configuration and differ in the O,O-donor ligands. Density functional theory (DFT) at the ωB97X-D/def2TZVP level was used to optimize the molecular structure and describe the electronic structure of 1 in the solid state, indicating a localized mixed-valence system. Continuous-wave EPR analysis at 77 K suggests the predominance of binuclear species in dmf and dmso solutions of 1 and 1:1 mixtures of these solvents with water. This finding is supported by comparing experimental and calculated rotational radius, g-tensor and vanadium hyperfine couplings. In the presence of KBr and H2O2, complex 1 acts as a pre-catalyst in a solvent-modulated bromination of phenol red to produce bromophenol blue. Kinetic studies in dmf/H2O indicate a first-order dependence on vanadium with a reaction rate constant of 703 mol−2L2 s−1. The better performance of 1 as a pre-catalyst in dmf/H2O than in dmso/H2O may be due to a higher formation rate for the active oxidoperoxidovanadium(V) species.

Site-Specific Synthesis of Oxo-Bridged Mixed-Valence Binuclear Complexes on Mesoporous Silica

We report the site-specific synthesis of mixed valence TiIV-O-FeII complexes within the pores of ordered mesoporous silica (SBA-15). By using 6-di- tert-butylpyridine as the selective activator of tripodally linked TiIV-OH groups of Ti-grafted SBA-15, the FeCl2.4H2O complexes reacted selectively with the nucleophilic TiIV-O(-) groups. The formation of Si-O-FeII byproducts, due to the reaction with the abundant Si-OH groups, was successfully restricted and the selectivity for forming the TiIV-O-FeII complexes exceeded 80%. The metal-metal interaction of TiIV-O-FeII complexes was confirmed by the appearance of TiIV/FeII --> TiIII/Fe III metal-to-metal charge transfer band, and their coordination, valency, and spin state were characterized by diffuse transmission UV-vis, Fourier transform IR, and Fe K-edge X-ray absorption fine structure measurements. It was also confirmed that the present methods can be extended to other metal combinations of TiIV-O-NiII and TiIV-O-MnII. The electron transfer processes occurring under photoinduced metal-to-metal charge transfer of oxo-bridged mixed valence complexes on silica supports have recently been proven as a new class of visible-light-sensitive redox centers. Thus, the present synthetic procedure allows the fabrication of a variety of photochemical reaction centers according to the molecular-level design.

Correlations between Spectroscopic, Electrochemical, and Kinetic Properties of Cyano-Bridged Binuclear Complexes. Analyses of Temperature, Pressure, and Solvent Effects

Several asymmetric cyano-bridged mixed-valence binuclear complexes were studied using electrochemical (square wave and cyclic voltammetry), spectroscopic (UV−vis−NIR absorption and 1H NMR), and kinetic (stopped-flow) techniques, with special attention to the correlations between thermodynamic and optical charge-transfer characteristics determined through the variation of temperature and pressure. The CN-bridged isomers for representative EDTA-ligated complexes, viz., [(EDTA)RuIII(μ-CN)M(CN)x]5- (M = FeII, x = 5; M = MoIV, x = 7), were synthesized and characterized and shown to exhibit essentially different electrothermodynamic and optical charge-transfer parameters compared to “normal” isomers. The binuclear complex-formation kinetics, spectroscopic and preliminary 1H NMR data indicate that at least two EDTA ligand conformer states are formed in the course of the dimerization process studied. Using the difference DO-D stretching overtone spectroscopy of heavy water acting as solvent, it was shown that the change in hydrogen-bonding order of specifically solvating water molecules present in the first solvation shells around the redox centers may be largely responsible for the observed dramatic changes in electrothermodynamical and optical characteristics upon the variation of the nature of the ligands and their conformer (EDTA) or bridging-isomer (μ-CN) states.

A novel mixed valence copper(I)–copper(II)–bis(antipyryl-methyl)-piperazine complex: synthesis, molecular structure and spectroscopic characterization

This paper describes a new case of the bis(antipyryl-methyl)-piperazine [BAMP] ligand being in the “boat”-conformation and acting as a tetradentate. But unlike in Co(BAMP)(NCS) 2, which is the only complex with BAMP acting as tetradentate known so far, the coordinated copper(II) is connected via a iodo-bridge to a Cu II 2-moiety. The synthesis, elemental analysis, spectroscopic (far- and mid-FTIR, UV–Vis) and structural characterization (X-ray diffraction) as well as magnetochemical and conductivity data of this novel mixed valence binuclear complex Cu ICu II(BAMP)I 3 are presented.

The effect of second-sphere coordination. 7. Isolation of 18-crown-6 ether adducts of ruthenium-ammine complexes

18-Crown-6 ether adducts were isolated for ruthenium(II)- and ruthenium(III)-ammine complexes, and the mixed-valence binuclear ruthenium-ammine complex. The adducts were characterized by electronic and IR spectroscopy. Ruthernium-ammine complexes form adducts with 18-crown-6 ether through hydrogen bonding between ammines coordinating to the ruthenium and ether oxygens of 18-crown-6 ether. The composition of the adducts is dependent on the valence of the metal canter: the composition was found to be 1:12, 1:2, and 1.3 (complex to 18-crown-6 ether) for the ruthenium (III complex, and the mixed-valence binuclear binuclear complex respectively. The adduct of the ruthenium (III) complex has a unusual composition: two molecules of 18-crown-6 ether are involved in the adduct. Furthermore, the delocalized mixed-valence binuclear complex formed an asymmetric adduct similar to that formed by a binuclear complex with fixed valence.

Evaluation of Electronic Interaction Matrix Elements for Photoinduced Electron Transfer Processes within Mixed-Valence Complexes

The photochemical properties of “alkane-bridged” binuclear complexes, [(bec)2RuII(L-Cn-L)RuII/III(bpy)2]4+/5+ (where bec = 4,4‘-bis(ethoxycarbonyl)-2,2‘-bipyridine, bpy = 2,2‘-bipyridine, and n = 3, 4, 5, 10) and [(dmb)2RuII(L-Cn-L)OsII/III(Lα)2]4+/5+ (where dmb = 4,4‘-dimethyl-2,2‘-bipyridine, Lα = bpy or dmb and n = 3, 4, 5), in which the bridging ligand (L-Cn-L) was made of two [2-(2-pyridyl)-1-benzimidazolyl] groups (L) linked by an alkane chain, have been investigated using time-resolved nano- and picosecond laser spectroscopy. In the Ru(II)−Os(II) binuclear complexes, a metal-to-ligand charge transfer (MLCT) state of the Ru(II) moiety, (3CT)Ru, was quenched by (3CT)Ru → Os(II) energy transfer. The intramolecular energy transfer rates for n = 3, 4, and 5 are well understood by assuming the Förster-type mechanism provided that the methylene chains are in a nearly extended form. In the mixed-valence binuclear complexes, Ru(II)−M(III) (M = Ru or Os), the decay of the (3CT)Ru excited state was accelerated by electron transfer (ET) to the adjacent M(III) site giving the valence-isomer state, Ru(III)−M(II), which decayed to the ground state of the original form via back ET. Forward ET rates (ket) have been determined from the decay rate of the (3CT)Ru excited state (kes) and the production yields of Ru(III)−M(II), φet, and back ET rates (kb) were obtained from the decay of Ru(III)−M(II). The value of φet (≤0.5 ± 0.1) decreases along with the decreasing number of methylene groups or temperature, indicating the existence of another process quenching the (3CT)Ru excited state. From the temperature dependence of ket and kb, the electronic interaction matrix elements (Hrp) have been estimated using a nonadiabatic ET theory. Hrp for the forward ET process was found to be enhanced by a factor of 2 in the compounds in which the excited electron locates on the bridging ligand in [(dmb)2RuII(L-Cn-L)OsIII(Lα)2]5+ compared to that for [(bec)2RuII(L-Cn-L)RuIII(bpy)2]5+ (n = 4, 5) in which the electron locates on the remote ligands. On the other hand, the matrix elements for back ET were found to be the same in either case, independent of metal ions and remote ligands.

Mixed-Valence Cyanopyridine-Bridged Complexes of Pentacyanoferrate and Pentaammineruthenium: Electronic Structure, Stability, and Redox Reactivity

The mixed-valence binuclear complexes, [(NC)5Fe(pyCN)Ru(NH3)5]n (4- and 3-cyanopyridine isomers, with nitrile-N and pyridine-N binding to Ru and Fe, respectively) were prepared as solid compounds through stoichiometric oxidation of the fully reduced (II,II) binuclear complexes, R, with peroxydisulfate. By analysis of IR spectra, the solids were observed to be a mixture of the predominant electronic isomers with a FeII, RuIII distribution, with minor amounts of the FeIII, RuII isomers. In aqueous solution, R was oxidized with peroxydisulfate to M, the mixed-valence complex, and to Ox, the fully oxidized complex. The M complex shows an intervalence band at 938 nm; by application of the Hush model, it is described as a valence-trapped RuII, FeIII complex; the latter electronic distribution is supported by UV−visible, electrochemical, and kinetic data, but a minor amount of the isomer with a FeII, RuIII distribution is also present in the equilibrium. The M complex is unstable toward dissociation and further ...

Photoreduction of 1,2-Dichloroalkanes and 1,2-Dichloroalkenes by Tetrakis(diphenyl phosphato)dimolybdenum(II,II)

The quadruply bonded metal-metal complex Mo 2 [O 2 P(OC 6 H 5 ) 2 ] 4 reduces 1,2-dichloroalkanes and 1,2-dichloroalkenes upon its irradiation with visible light (λ exc ≥495 nm). The inorganic product of the photochemistry in all cases is the Mo II Mo III mixed-valence binuclear complex Mo 2 [O 2 P(OC 6 H 5 ) 2 ] 4 + , which has been structurally characterized. Green prismatic crystals of the tetrafluoroborate salt are triclinic with a PI space group

Electronic coupling in mixed-valence binuclear ruthenium ammine complexes as probed by an electrochemical method and an extension of Mulliken's theory of donor-acceptor interactions

An electrochemical approach to assessing the degree of electronic coupling in mixed-valence binuclear complexes is outlined. The method relies on the comparison of electrochemical potential shifts induced at both the directly and indirectly perturbed metal sites when a ligand substitution process is carried out at one site

Mono- and binuclear ruthenium(II) schiff-base complexes: spectrophotometric and electrochemical studies

As part of a study of electron transfer processes in mixed-valence binuclear complexes, we report here results of electrochemical and spectrophotometric studies of the complex I, and of its mononuclear analogue II (P = P(C 6H 5) 3). ▪ In the semi-oxidised Ru III ▪Ru II complex corresponding to I, we find evidence of a small extent of interaction between the Ru atoms, as reflected in the comproportionation constant K com (eqn. 1). ▪ but the corresponding intervalence charge transfer spectrum is not detected.

Compound C2, a product of the reaction of oxygen and the mixed-valence state of cytochrome oxidase. Optical evidence for a type-I copper

Compound C2 is a product of the reaction of O2 and the mixed-valence state of cytochrome oxidase. The mixed-valence state of membrane-bound cytochrome oxidase is obtained at -24 degrees C, by using either ferricyanide or yeast peroxidase complex ES as oxidants, and the configurations of oxidized haem a and its associated copper (a3+Cua2+) and of reduced haem a3 and its associated copper (ac3+.CO.Cua3+) are obtained. The mixed-valence-state cytochrome oxidase mixed with O2 at -24 degrees C and flash-photolysed at -60 to -100 degrees C reacts with O2 and initially forms an oxy compound (A2) similar to that formed from the fully reduced state (A1). Thereafter the course of the reaction differs from that obtained in the fully reduced state, and absorbance increases are observed at 740--750 nm and 609 nm and a decrease at 444 nm, with no increase in absorbance at 655 nm. One possible attribution of the absorbance increases is to charge-transfer interaction between the iron of haem a3 and the copper associated with haem a3, Cua3(2+), having properties of a type-I 'blue' copper. A possible attribution of the decrease in absorbance at 444 nm is to liganding of a3(2+). A related explanation is that the 609 nm absorbance involves a charge-transfer interaction of both iron and copper as a mixed-valence binuclear complex, Cua3, having properties of a non-blue copper. Intermediates in addition to Compound C2 are not yet identifiable by chemical or spectroscopic tests. The kinetic and equilibrium properties of Compound C2 are described.