UV-vis Spectroelectrochemistry Research Articles

Electrochemical and Spectrochemical Studies of Diphosphorylated Metalloporphyrins: Unusual Formation of Phlorin Anion

Two series of diphosphoryl-substituted porphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in benzontrile, dichloromethane and pyridine containing 0.1 M tetra-n-butylammonium perchlorate. The investigated compounds are represented as 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrins and 5,15-bis-(diethoxyphosphoryl)-10,20-di(p-carbomethoxyphenyl) porphyrins (Chart). The porphyrins with non-redox active metal centers undergo two ring centered oxidations and two ring centered reductions, the latter of which is followed by a chemical reaction to give a phlorin anion. The phlorin anion is electroactive and can be reoxidized to give back the starting porphyrin or reversibly reduced to give a phlorin dianion. Each redox reaction was monitored by thin-layer UV-visible spectroelectrochemistry and an overall reduction mechanism is proposed. Figure 1

Synthesis, characterization and electrocatalytic behavior of cobalt and iron phthalocyanines bearing chromone or coumarin substituents

Cobalt and iron phthalocyanines (Pcs) bearing peripherally tetra-substituted chromone (chr) or coumarin (cou) moieties were formulated and characterized by UV–Vis and FTIR spectroscopy, ESI-TOF mass spectrometry, and elemental analysis. The structural elucidations of the ligands, 4-(chromone-7-oxy)phthalonitrile (1) and 4-(4-(trifluoromethyl)-coumarin-7-oxy)phthalonitrile (2) were complemented by NMR spectroscopy and single crystal X-ray analysis (for 1). The redox properties of the complexes were investigated via voltammetry and the subsequent voltammetric assignments were corroborated by UV–Vis spectroelectrochemistry. Each metal complex displayed four redox processes of which their Pc ring oxidations are irreversible and the remaining redox couples are quasi-reversible. Utilizing the respective metallophthalocyanines, modified working electrodes were prepared by electropolymerization and their electrocatalytic activities toward nitrite oxidation were explored. All the metal complexes showed an increase in nitrite oxidation currents and a minor decrease in oxidation potentials which is indicative of electrocatalysis. The trend of electrocatalytic activity was found to be as follows: CoPc–chr (3) > FePc–cou (4) > CoPc–cou (5).

Electrochemical and spectroelectrochemical studies of diphosphorylated metalloporphyrins. Generation of a phlorin anion product.

Two series of diphosphoryl-substituted porphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The investigated compounds are 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrins (Ph)2(P(O)(OEt)2)2PorM and 5,15-bis(diethoxyphosphoryl)-10,20-di(para-carbomethoxyphenyl)porphyrins (PhCOOMe)2(P(O)(OEt)2)2PorM where M = 2H, Co(II), Ni(II), Cu(II), Zn(II), Cd(II), or Pd(II). The free-base and five metalated porphyrins with nonredox active centers undergo two ring-centered oxidations and two ring-centered reductions, the latter of which is followed by a chemical reaction of the porphyrin dianion to give an anionic phlorin product. The phlorin anion is electroactive and can be reoxidized by two electrons to give back the starting porphyrin, or it can be reversibly reduced by one electron at more negative potentials to give a phlorin dianion. The chemical conversion of the porphyrin dianion to a phlorin anion proceeds at a rate that varies with the nature of the central metal ion and the solvent. This rate is slowest in the basic solvent pyridine as compared to CH2Cl2 and PhCN, giving further evidence for the involvement of protons in the chemical reaction leading to phlorin formation. Calculations of the electronic structure were performed on the Ni(II) porphyrin dianion, and the most favorable atoms for electrophilic attack were determined to be the two phosphorylated carbon atoms. Phlorin formation was not observed after the two-electron reduction of the cobalt porphyrins due to the different oxidation state assignment of the doubly reduced species, a Co(I) π anion radical in one case and an M(II) dianion for all of the other derivatives. Each redox reaction was monitored by thin-layer UV-visible spectroelectrochemistry, and an overall mechanism for each electron transfer is proposed on the basis of these data.

Electrochemistry and spectroelectrochemistry of β-pyrazino-fused tetraarylporphyrins in nonaqueous media

Bis-porphyrins containing a β,β′-fused pyrazino (Pz) linking group were examined by electrochemistry and thin-layer UV-visible spectroelectrochemistry in PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. The investigated compounds are represented as M(TPP)-Pz-(TPP)M, where TPP is the dianion of tetraphenylporphyrin and M = Zn(II) , Cu(II) or Ag(II) . The effect of the linking Pz group on the redox potentials and UV-visible spectra of the neutral, electroreduced and electrooxidized bis-porphyrins is discussed and the data compared to what is observed for related monoporphyrins and earlier characterized bis-porphyrins containing a tetraazaanthracene (TA) linking group and the same central metal ions. The Cu(II) and Zn(II) Pz linked bis-porphyrins exhibit a relatively strong interaction between the two equivalent porphyrin macrocycles as evidenced by UV-visible spectra of the neutral compounds and characteristic splitting of redox processes for three of the four electron transfer reactions, the one exception being the first oxidation, where no splitting of potentials is observed in the formation of the bisporphyrin bis-cation radical, [M(TPP)-Pz-(TPP)M]2+. The first oxidation and first reduction of the bis-porphyrin with two Ag(II) central metal ions occurs via two overlapping one-electron transfer steps in each process indicating equivalent, but non-interacting redox centers. This difference in redox behavior is due to differences in the site of electron transfer, metal centered for the Ag(II) bisporphyrins, which undergo Ag(II) / Ag(III) and Ag(II) / Ag(I) processes as compared to only ring centered electron transfers for the Cu(II) and Zn(II) derivatives. The Pz-linked Zn(II) bis-porphyrins have a larger average HOMO–LUMO gap (2.01 V) as compared to related tetraazaanthracene linked bis-porphyrins (1.67 V), which were earlier characterized in the literature, but the gap is smaller than that for the mono-porphyrins with the same central metal ions (2.17 V). Each redox reaction of the investigated bisporphyrins was characterized by thin-layer spectroelectrochemistry.

Multichromic and soluble conjugated polymers containing thiazolothiazole unit for electrochromic applications

Here we report the preparation of two solution processible polymers; poly(2-(5-(2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazol-4-yl)thiophen-2-yl)-5-(thiophen-2-yl)thiazolo[5,4-d]thiazole) (PBTTz) and poly(2-(5-(9,9-dioctyl-9H-fluoren-2-yl)thiophen-2-yl)-5-(thiophen-2-yl)thiazolo[5,4-d]thiazole) (PFTTz) which are multichromic electroactive polymers. PBTTz was synthesized via Stille coupling polymerization whereas PFTTz was synthesized via Suzuki coupling polymerization. Both polymers were coated on ITO slides and their electrochromic properties were investigated. The results of the PBTTz were also compared with a previously published polymer to investigate the linear and branched alkyl chain effect. The polymers were compared through their optical properties and UV–Vis spectroelectrochemistry.

Spectroelectrochemical characterization of meso triaryl-substituted Mn(IV), Mn(III) and Mn(II) corroles. Effect of solvent and oxidation state on UV-visible spectra and redox potentials in nonaqueous media

Two series of substituted manganese triarylcorroles were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in CH 2 Cl 2, CH 3 CN and pyridine. The investigated compounds are represented as ( YPh )3 CorMn III and ( YPh )3 CorMn IV Cl , where Cor is a trianion of the corrole and Y is a Cl , F , H or CH 3 para-substituent on the three phenyl rings of the macrocycle. Each neutral Mn(III) corrole exists as a four-coordinate complex in CH 2 Cl 2 and CH 3 CN and as a five-coordinate species in pyridine. ( YPh )3 CorMn III undergoes two oxidations to stepwise generate a Mn(IV) corrole and a Mn(IV) π-cation radical. It also undergoes one reduction to generate a Mn(II) corrole in CH 2 Cl 2 or CH 3 CN . In contrast, the reduction of ( YPh )3 CorMn III leads to a Mn(III) corrole π-anion radical in pyridine. One oxidation is observed for ( YPh )3 CorMn IV Cl in CH 2 Cl 2 and CH 3 CN to generate a Mn(IV) corrole π-cation radical while Mn(III) and Mn(II) corroles are stepwise formed after reduction of the same compound. The second reduction of ( YPh )3 CorMn IV Cl in pyridine gives a Mn(III) π-anion radical as opposed to a Mn(II) corrole with an unreduced π-ring system. The neutral, reduced and oxidized forms of each corrole were characterized by electrochemistry and UV-visible spectroelectrochemistry and comparisons are made between the UV-visible spectra and redox potentials of the compounds in different central metal oxidation states. An overall reduction/oxidation mechanism in the three solvents is proposed.

Improved solubility, conductivity, thermal stability and corrosion protection properties of poly(o-toluidine) synthesized via chemical polymerization

We report on the synthesis of poly(o-toluidine) (POT), a methyl-substituted derivative of PANI, by inverse emulsion polymerization. A mixture of 2-butanol and chloroform was used as dispersion medium; benzoyl peroxide as oxidant, while dodecylbenzenesulfonic acid (DBSA) was used both as dopant and as surfactant. POT was collected in its salt form and was found to be completely soluble in various organic solvents (so far highest number of solvents) such as DMSO, chloroform, toluene, THF, acetonitrile and ethanol. The synthesized polymer was characterized by cyclic voltammetry, in situ UV–Vis spectroelectrochemistry, viscosity and in situ conductance measurements, FTIR and TGA. Conductance measurements show that POT has high conductance comparable with that of electrochemically synthesized POT. The POT film dip-coated on a gold electrode was highly stable in air; no appreciable decrease in the electrochemical activity of the film occurred even when left open for 14 days in air. Thermal analysis reveals a very good thermal stability up to 513°C. Potentiodynamic polarization measurements show that POT acts as a highly efficient corrosion protection layer on steel with 77% protection efficiency.

Redox properties of nitrophenylporphyrins and electrosynthesis of nitrophenyl-linked Zn porphyrin dimers or arrays

Five nitrophenylporphyrins were investigated as to their electrochemical properties in CH 2 Cl 2 containing 0.1 M TBAP. The investigated compounds are represented as ( NO 2 Ph )x Ph 4-x PorM , where Por represents the dianion of the porphyrin macrocycle, Ph is a phenyl group on meso-position of the macrocycle, NO 2 Ph is a meso-substituted nitrophenyl group, M = 2 H , Pd II or Zn II and x = 1 or 2. Each porphyrin undergoes an initial one electron reduction at E1/2 = -1.07 to -1.12 V where the added negative charge is almost totally localized on the meso-nitrophenyl group of the compound. This reversible reduction is then followed by one or more irreversible reductions of the nitrophenyl anion at more negative potentials which overlap with reduction of the conjugated porphyrin macrocycle. The initial one electron addition was monitored by thin-layer UV-vis spectroelectrochemistry which confirmed formation of a reduced nitrophenyl group in each case but also gave spectral evidence for a linkage of the one-electron reduction products in the case of the Zn derivatives, giving Zn porphyrin dimers or arrays which are characterized by a 14–15 nm red-shifted Soret band and two well-defined Q-bands, consistent with conversion from an unreduced four coordinate Zn II nitrophenylporphyrin to a five-coordinate Zn II complex with an unreduced porphyrin macrocycle.

Electropolymerized phenothiazines for the photochemical generation of singlet oxygen

The selected group of five amine-derivative phenothiazines was used to form their electropolymerized films in aqueous acidic solution with the aim to assess their ability to generate singlet oxygen. The films were characterized using cyclic voltammetry, UVVis spectroelectrochemistry, and Raman spectroscopy. The data of all the films were compared with properties of their monomer analogs. It was demonstrated that the electrochemically prepared films similarly to the known process of the corresponding monomers in solution phase are able to generate singlet oxygen in a photochemical surface process activated by laser radiation. These photogeneration process of the short-living species was monitored using the selected trap, which is detectable using UVVis spectroscopy. The effectiveness of the surface photogeneration reaction for each polymeric structure was determined in terms of half-life of the trap decay. The presented heterogeneous process of singlet oxygen photogeneration is believed to extend the area of applications of that highly oxidative reactant as compared to known homogeneous systems employing unbounded active species.

Electrochemistry and spectroelectrochemistry of 1,4-dinitrobenzene in acetonitrile and room-temperature ionic liquids: ion-pairing effects in mixed solvents.

Room-temperature ionic liquids (RTILs) have been shown to have a significant effect on the redox potentials of compounds such as 1,4-dinitrobenzene (DNB), which can be reduced in two one-electron steps. The most noticeable effect is that the two one-electron waves in acetonitrile collapsed to a single two-electron wave in a RTIL such as butylmethyl imidazolium-BF4 (BMImBF4). In order to probe this effect over a wider range of mixed-molecular-solvent/RTIL solutions, the reduction process was studied using UV-vis spectroelectrochemistry. With the use of spectroelectrochemistry, it was possible to calculate readily the difference in E°'s between the first and second electron transfer (ΔE12° = E1° - E2°) even when the two one-electron waves collapsed into a single two-electron wave. The spectra of the radical anion and dianion in BMImPF6 were obtained using evolving factor analysis (EFA). Using these spectra, the concentrations of DNB, DNB(-•), and DNB(2-) were calculated, and from these concentrations, the ΔE12° values were calculated. Significant differences were observed when the bis(trifluoromethylsulfonyl)imide (NTf2) anion replaced the PF6(-) anion, leading to an irreversible reduction of DNB in BMImNTf2. The results were consistent with the protonation of DNB(2-), most likely by an ion pair between DNB(2-) and BMIm(+), which has been proposed by Minami and Fry. The differences in reactivity between the PF6(-) and NTf2(-) ionic liquids were interpreted in terms of the tight versus loose ion pairing in RTILs. The results indicated that nanostructural domains of RTILs were present in a mixed-solvent system.

Electroreductive dechlorination of α-hexachlorocyclohexane catalyzed by iron porphyrins in nonaqueous media

Two iron porphyrins, ( TPP ) FeCl and ( OEP ) FeCl , where TPP and OEP are the dianions of tetraphenylporphyrin and octaethylporphyrin, respectively, were utilized as catalysts for the electroreductive dechlorination of α-hexachlorocyclohexane (α- HCH ) which was monitored by electrochemistry, in situ UV-visible spectroelectrochemistry and controlled potential electrolysis in N , N ′-dimethylformamide. GC-MS analysis of the α- HCH degradation products revealed the stepwise formation of pentachlorocyclohexene and tetrachlorocyclohexadiene as intermediates, prior to generation of the final dechlorination products which consisted of an isomeric mixture of trichlorobenzenes. Based on identification of the intermediates and final products in the reaction, an overall dechlorination mechanism of α-hexachlorocyclohexane is proposed.

In situ monitoring of chlorpromazine radical intermediate by spectroelectrochemistry

Chlorpromazine has been used widely as an antipsychotic agent and its major metabolic pathways include 7-hydroxylation, N-dealkylation, N-oxidation and S-oxidation. The present work focuses on in situ monitoring of the radical intermediate, for clarifying the mechanism diversity in anodic oxidation of chlorpromazine at acidic and physiological pH’s. Only in acidic media the colored cation radical was detected by UV–vis spectroelectrochemistry and cyclic voltabsorptometry. The radical was formed at the first anodic peak, followed by a further one-electron oxidation at the second peak to generate the sulfoxide. At physiological pH, the sulfoxide was generated at the first anodic peak through a one-step two-electron oxidation, resulting in much higher current response than at acidic pH’s. This work provides an example of a positive and a negative voltabsorptometric peak indicating the formation and conversion of a radical intermediate.

Electrosynthesis of viologen cross-linked polythiophene in ionic liquid and its electrochromic properties

Utilizing electrochemical reduction of pendant cyanopyridinium ions to cross-link polymeric viologens is well known. In this work, we report electrosynthesis of a thiophene monomer bearing a pendant cyanopyridine moiety in its side chain (Th-CN). Its electropolymerization results in the growth of a polythiophene film which is highly cross-linked by the viologen unit (PTh-V). In order to improve adhesion and insolubility of the as-formed polymer film, we choose the ionic liquid 1-butyl-3 methylimidazolium hexafluorophosphate (BmimPF6), as electrolyte. The obtained polymer film was studied by cyclic voltammetry, FTIR spectroscopy, in situ UV-Vis spectroelectrochemistry and electrochemical impedance spectroscopy (EIS). The CV results showed electrochemical activity both of the viologen and polythiophene moieties. FTIR spectra of the polymer film confirm the formation of viologen by disappearance of the–CN stretching band from the polymer spectrum. EIS studies revealed that both resistive as well as capacitive element controls the charge transfer mechanism. In situ spectroelectrochemical studies of the polymer film shows enhanced electrochromic contrast of polythiophene film due to incorporation of viologen pendant group with coloring characteristics both of the viologen and the polythiophene backbone. An n-doped polymer is a dark blue colored film with coloration efficiency being ca. 97 cm2/C at 625nm while the p-doped polymer is transparent violet in color with coloration efficiency of ca. 12 cm2/C at 775nm. The results showed that the PTh-V film can be used as redox active conducting layer to develop dynamic optoelectronic and electrochromic devices.

Redox-Gated Molecular Memory Devices Based on Dynamic Doping of Polythiophene

A solid state electrochemical cell resembling a field effect transistor will be described which can act as a nonvolatile memory element. “Write” and “Erase” pulses cause oxidation of a polythiophene to its conducting form, while the conductance is monitored by a “read” circuit. Raman and UV-Vis spectroelectrochemistry were used during device operation to investigate redox reactions which underlie memory operation. Important electrochemical concepts which control device operation include activated electron transfer, ion motion, and extended conjugation. In particular, internal ion transport creates ohmic potential losses which slow W/E operation, but can be significantly improved with higher mobility solid electrolytes. Applications seeking to augment existing silicon electronics with molecular components will be described.Recent references:(1) Yan, H.; Bergren, A. J.; McCreery, R.; Della Rocca, M. L.; Martin, P.; Lafarge, P.; Lacroix, J. C.; Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions; Proceedings of the National Academy of Sciences 2013, 110, 5326.(2) McCreery, R.; Yan, H.; Bergren, A. J.; A Critical Perspective on Molecular Electronic Junctions: There is Plenty of Room in the Middle; Phys. Chem. Chem. Phys. 2013, 15, 1065.(3) Sayed, S. Y.; Fereiro, J. A.; Yan, H.; McCreery, R. L.; Bergren, A. J.; Charge transport in molecular electronic junctions: Compression of the molecular tunnel barrier in the strong coupling regime; Proceedings of the National Academy of Sciences 2012, 109, 11498.(4) Kumar, R.; Pillai, R. G.; Pekas, N.; Wu, Y.; McCreery, R. L.; Spatially Resolved Raman Spectroelectrochemistry of Solid-State Polythiophene/Viologen Memory Devices; Journal of the American Chemical Society 2012, 134, 14869.

Electrochemical Characterization of an Electroactive Ureidopyrimidinone Derivative, a Four Hydrogen Bond Array Containing a Dimethylaminophenylurea

Electron transfer reactions can be used to perturb the strength of hydrogen bonding interactions between organic molecules. When molecular redox centers are integrated onto hydrogen bond arrays, this combination can become a powerful tool for controlling binding strength in supramolecular complexes. Whether used for sensing devices, molecular machines or self-assembly, the strength of a hydrogen bond can also be enhanced simply by arrangement of hydrogen donor interactions (D) versus hydrogen acceptor interactions (A). Indeed, because of secondary interactions, dimerization constants of the DDAA 4 H-Bond array typically exceed that of the DADA 4 H-Bond array.1 By combining a DDAA 4 H-bond array such as Meijer’s ureidopyrimidinone1with an electroactive center such as the dimethylaminophenylurea, the strength of hydrogen bonding becomes tunable.Cyclic voltammetry investigation, in nonaqueous solvents, of dimethylaminophenylurea (UHH), a DD 2H-bond array, show that without a guest present, there is one reversible oxidation wave in methylene chloride of one electron height. CV Digital simulations as well as UV-vis spectroelectrochemistry indicate a proton transfer from the radical species to the reduced U(H)H, followed by another electron transfer due to potential inversion to give the quinoidal cation, Scheme 1. With the integration of the dimethylphenylamino redox center, this mechanism would certainly complicate the dimerization of Meijer’s DDAA array.Scheme 1. Proposed oxidation mechanism of UHH undergoing an overall one electron transfer due to proton transfer.Due to the acidity of the urea NH that is part of the electroactive center, proton transfer could also occur as part of the electrochemistry associated with the DDAA dimer. We hypothesize that depending on the ratio of arrays substituted with an electroactive center versus those without, the DDAA sequence could change to a DDAD array. With all arrays containing an electroactive center the hydrogen bonding array would form an ADDA homodimer.This presentation will cover cyclic voltammetry studies that includes concentration and scan rate dependence investigations on Meijier’ DDAA dimer equipped with a dimethyphenylurea electroactive center. Digital CV simulations and spectroelectrochemistry will be used to analyze experimental CV ‘s.Figure 1. Meijer’s ureidopyrimidinone (Upy) 4 H-bond system without redox functionality as a DDAA homodimer.Figure 2. Structure of the proposed heterodimer, DDAD array, as a result of half the concentration with an electroactive center and the other half without.

In situ UV–vis spectroelectrochemical study of dye doped nanostructure polypyrrole as electrochromic film

In the present work, the electrosynthesis and the spectroelectrochemical characterization of eriochrome cyanine R (ECR) doped polypyrrole (PPy) were described. Nanostructured PPy–ECR film was electrochemically deposited onto fluorine doped tin oxide (FTO) coated/glass electrode from aqueous solution containing a water-soluble trianionic dye, ECR, as dopant. The surface morphology was studied using scanning electron microscopy (SEM) analysis. Dye doped polymer film showed spherical morphology with primary particles 70–90nm in size and 900nm thicknesses. The main characteristics of PPy–ECR film include uniformity, stability and adherent on FTO electrodes, which are better compared to that of PPy film. In situ UV–vis spectroelectrochemistry was used as a powerful tool in analyzing the evolution of the neutral, polaron and bipolaron states in prepared polymer films as they were redox cycled (from −0.6V to +0.6V) in NaClO4. As the polymers gets oxidized, the intensity of the π–π* transition (neutral state) decreases while the charge carrier bands at longer wavelengths (polaron band and bipolaron band) increases in intensity and the color of the polymers change. We found that these variations are more intense in the case of dye doped to those of PPy. The spectroelectrochemical studies showed that the electrochromic properties of dye doped polymer (PPy–ECR) such as optical contrast (27%), switching time (⩽1.5s) and film stability (100 cycles) were greatly enhanced relative to the PPy film.

The role of furyl substituents of pyrene on monomer and polymer properties

This paper estimates the influence of substituents number on basic electrochemical, optical and electronic properties of monomers compared to their polymers. Results exhibit differences of these effects in monomers and polymers. The crosslinking effect has impact on basic properties of studied polymers. Polymers were created from their monomers by electropolymerization. Optical properties of a series of furyl derivatives of pyrene were investigated by UV–vis spectroscopy. Basic electrochemical properties of the studied compounds were investigated by cyclic voltammetry. ESR–UV–vis spectroelectrochemistry reveals differences in doping processes of obtained conjugated polymers. Good electrochemical stability of novel furyl-pyren π-conjugated polymers confirm the application potential of this group of materials for the development of organic semiconductors in particularly for possible applications in electrochromic devices, light emitting electrochemical cells and spintronic.

Monitoring of intermediates of clioquinol electro-oxidation by thin-layer spectral and electrophoretic electrochemistry

Monitoring of unstable intermediates is very important for clarifying complex electrochemical reaction pathways. A combination of thin-layer spectral and electrophoretic electrochemistry, both based on fast thin-layer electrolysis, has been developed to monitor the intermediates of electro-oxidation of clioquinol in acidic, physiological and alkaline buffer media. Two thin-layer electrochemical cells were fabricated and coupled with a UV-vis spectrometer and an electrophoresis apparatus, respectively. Double-potential thin-layer UV–vis spectroelectrochemistry, double-wavelength thin-layer cyclic voltabsorptometry and thin-layer electrophoretic electrochemistry provided mutually-supporting information on the redox pathway of clioquinol. Two intermediates, a disproportionatable semiquinone anion radical and a labile quinone derivative, were detected. The chemical activity of the semiquinone anion radical is highly pH-dependent. The final soluble products of clioquinol are pH-dependent but irrelevant to whether the oxidation reaction is electrochemical or chemical in nature. A complex reaction pathway is proposed for the oxidation of clioquinol, which may help to better understand the metabolism mechanism and chemical toxicity of this anti-neurodegeneration drug.

Ruthenium complexes with Schiff base ligands containing benz(othiazole/imidazole) moieties: Structural, electron spin resonance and electrochemistry studies

Novel ruthenium(II/III) complexes of Schiff bases containing benzimidazole (bz) or benzothiazole (bs) moieties were isolated: the diamagnetic ruthenium complex, cis-[RuIICl2(bzpy)(PPh3)2] (1) was formed from the 1:1M reaction between N-((pyridine-2-yl)methylene)-1H-benzimidazole (bzpy) and metal precursor, trans-[RuCl2(PPh3)3]. The same metal precursor, when reacted with the benzimidazole-derived Schiff bases [N-(2-hydroxybenzylidene)-benzothiazole (Hbsp) and N-(2-hydroxybenzylidene)-benzimidazole (Hbzp)], afforded the paramagnetic ruthenium(III) complexes [RuCl(bsp)2(PPh3)] (2) and trans-[RuCl(bzp)(PPh3)2] (3), respectively. These metal complexes were characterized via IR, mass and UV–Vis spectroscopy, elemental analysis, single crystal XRD analysis as well as conductivity measurements. Their redox properties were probed by voltammetry and accompanying UV–Vis spectroelectrochemistry experiments. Structural features of complex 1 were further investigated by multinuclear (1H and 31P) NMR spectroscopy. The presence of the paramagnetic metal centres of 2 and 3 were confirmed by X-band ESR spectroscopy.

Synthesis and characterization of bis-[PcRu(CO)][Ru2(ap)4(C≡CC5H4N)2

A tetra-ruthenium complex containing two ruthenium(II) phthalocyanines and one metal–metal bonded diruthenium(III,III) unit was synthesized and investigated as to its electrochemical and spectroscopic properties in non-aqueous media. The title compound, represented as bis-[ PcRu ( CO )]-[ Ru 2( ap )4( C ≡ CC 5 H 4 N )2], where ap = 2-anilinopyridinate anion and Pc = the dianion of tetra-tert-butylphthalocyanine, was assembled by axial coordination of two PcRu ( CO ) macrocycles to pyridine groups of the complex which were themselves linked to the diruthenium(III,III) unit via alkyne groups. The tetra-ruthenium complex was characterized in solution and in the solid state by 1 H NMR, UV-visible and IR spectroscopies, mass spectrometry and cyclic voltammetry while thin-layer UV-visible spectroelectrochemistry was used to study the site of electron transfer. Eight redox processes occur in CH 2 Cl 2 or benzonitrile, four of which can be assigned to the macrocycles of PcRu ( CO ) and four to the central diruthenium unit of the molecule. The electrochemical and spectroscopic data suggest a weak electronic interaction between the diruthenium unit of [ Ru 2( ap )4( C ≡ CC 5 H 4 N )2] and the two externally linked Ru ( II ) phthalocyanines of the title compound.