Oxidative Electrochemistry Research Articles

Electrochemical determination of nitrite in water samples using a glassy carbon electrode modified with didodecyldimethylammonium bromide

The oxidative electrochemistry of nitrite was investigated using a glassy carbon electrode modified with surfactants such as didodecyldimethylammonium bromide (DDAB), Tween-80, and sodium dodecylbenzenesulfonate, respectively. Nitrite oxidation proceeds best in the presence of DDAB. The electrode exhibited good sensitivity and fast response, with a linear dynamic range between 1.0 and 80 μM of nitrite, and a sensitivity of 0.04 µA·µM−1. The electrode was successfully applied to the determination of nitrite in spiked real water samples.

Reactivity of Electrochemically Generated Rhenium (II) Tricarbonyl α-Diimine Complexes: A Reinvestigation of the Oxidation of Luminescent Re(CO)3(α-Diimine)Cl and Related Compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO) 3(LL)Cl, 1, and Re(CO) 3(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1 ( + ) and 2 ( + ) disproportionate, yielding [Re(CO) 3(LL)(CH 3CN)] (+), 7, and additional products. Double potential step chronocoulometry experiments confirm that 1 ( + ) and 2 ( + ) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO) 3(LL)Cl 2] (+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl (-). No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO) 3(LL)Br 2] (+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7 ( + ), were successfully characterized using infrared spectroelectrochemistry.

Taniaphos and Walphos ligands: Oxidative electrochemistry and complexation. Synthesis, characterization, oxidative electrochemistry and X-ray structures of [(Taniaphos/Walphos)MCl 2] (M = Pd or Pt)

The oxidative electrochemistry of chiral, bidentate ferrocenylphosphines, five Taniaphos and seven Walphos ligands, was studied in methylene chloride. In general, two waves of varying reversibility were observed. Complexes of the general type [(phosphine)MCl 2] (phosphine = Taniaphos or Walphos; M = Pd or Pt) were prepared and characterized by NMR. Upon coordination, the oxidative electrochemistry of the ligands was greatly simplified. The X-ray structures of a Taniaphos platinum complex as well as a palladium and a platinum complex with a Walphos ligand were determined.

New Insights into the Oxidative Electrochemistry of Vitamin E

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Electrochemistry and complexation of Josiphos ligands

The oxidative electrochemistry of 11 chiral bis-phosphinoferrocene ligands, all within the Josiphos class of ligands, was examined in methylene chloride. The oxidation of these ligands displays multiple waves of varying chemical reversibility. Palladium(II) and platinum(II) complexes with the general formula [MCl 2(P–P)] (M = Pd or Pt; P–P = Josiphos) were prepared, characterized by NMR and cyclic voltammetry. The electrochemistry simplifies greatly upon coordination of the Josiphos ligands. The X-ray structures of a palladium(II) and platinum(II) complex of the same Josiphos ligand are reported.

New Insights into the Oxidative Electrochemistry of Vitamin E

A combination of electrochemical and spectroscopic experiments have proven that the alpha-, beta-, gamma-, and delta-forms (vitamers) of the tocopherols (vitamin E) undergo a series of chemically reversible proton- and electron-transfer steps in dry organic solvents, such as acetonitrile or dichloromethane, to form cationic compounds: the cation radical, the dication, and the phenoxonium cation. The cationic compounds are extremely unusual in their high persistence compared with what is presently known about the oxidative stability of other phenols, particularly the phenoxonium cation of alpha-tocopherol, which is stable for at least several hours in nonaqueous solvents and is formed quantitatively by oxidation of the starting material at an applied potential of approximately +0.5 V vs ferrocene0/+ or with 2 mol equiv of NO+.

Influence of the initial bonding mode of the hydrocarbyl bridge on the mechanisms and products of the electrochemical reduction of alkyne- and vinylidene dimolybdenum tris(µ-thiolate) complexes

The electrochemical reduction of isomeric complexes, [Mo2Cp2(μ-SMe)3(μ-η1:η1-HCCPh)]+ (1+) and [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCHPh)]+ (3+), where the hydrocarbyl bridges in a η1:η1- or a η1:η2 mode, has been studied by cyclic voltammetry and controlled-potential electrolysis in thf–[NBu4][PF6] and CH2Cl2–[NBu4][PF6], in the absence and in the presence of acid. The binding mode of the CC fragment induces different electrochemical behaviour of the complexes in acid-free solutions since 1+ reduces in two diffusion-controlled one-electron steps while the first reduction of 3+ is characterized by slow electron transfer kinetics. Controlled-potential reduction of both 1+ and 3+ produces a mixture of the acetylide [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCPh)] (2) and alkylidyne complexes [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4). In the presence of acid, the electrochemical reduction of 1+ and of 3+ occurs according to ECE processes. The nature of the products formed by controlled-potential reduction of 1+ depends on the nature of the acid and of the solvent. The transient formation of a complex with a μ-alkenyl ligand, either [Mo2Cp2(μ-SMe)3(μ-η1:η2-CHCHPh)] (7) or an isomer, is suggested by the oxidative electrochemistry of 7 and by its reaction with acids. In thf–[NBu4][PF6] in the presence of an excess of acid (HBF4/Et2O) and of phenylacetylene, electrolysis of 1+ gives rise to catalytic reduction of phenylacetylene to styrene. However, unidentified reactions limit the efficiency of this process. The reduction of 3+ in acidic medium produces the alkyl complex [Mo2Cp2(μ-SMe)3(μ-CH2CH2Ph)] (6) through alkylidyne [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4) and alkylidene [Mo2Cp2(μ-SMe)3(μ-η1-CHCH2Ph)]+ (5+) intermediates. Some ethylbenzene was formed after reduction of 5+ in the presence of acid. These results show an effect of the binding mode of the hydrocarbyl bridge on the mechanism and products of the reduction of the corresponding complexes.

Switching Surface Chemistry with Supramolecular Machines

Tethered supramolecular machines represent a new class of active self-assembled monolayers in which molecular configurations can be reversibly programmed using electrochemical stimuli. We are using these machines to address the chemistry of substrate surfaces for integrated microfluidic systems. Interactions between the tethered tetracationic cyclophane host cyclobis(paraquat-p-phenylene) and dissolved pi-electron-rich guest molecules, such as tetrathiafulvalene, have been reversibly switched by oxidative electrochemistry. The results demonstrate that surface-bound supramolecular machines can be programmed to adsorb or release appropriately designed solution species for manipulating surface chemistry.

Derivatives of 1,1′-bis(diphenylphosphino)ferrocene (dppf): Electrochemistry, complexation and the X-ray structures of 1,1′-bis(diphenylphosphino)osmocene (dppo) and [PdCl 2(dppo)

The oxidative electrochemistry of 1,1′-bis(diphenylphosphino)osmocene (dppo) and 1,1′-bis(diphenylarsino)ferrocene (dpaf) was studied in dichloromethane with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The [MCl 2(P ∩P)] (M = Pd or Pt; P ∩P = dppo or 1,1′-bis(diphenylphosphinoindenyl)iron) complexes were prepared, studied electrochemically and the X-ray structures of dppo and [PdCl 2(dppo)] were determined.

Electrogenerated chemiluminescence of the lithium salts of 8-hydroxyquinoline and 2-methyl-8-hydroxyquinoline

The spectroscopy, electrochemistry, and electrogenerated chemiluminescence (ECL) of [(q)(qH)Li]x (qH=8-hydroxyquinolinato) and [(Meq)(MeqH)Li]x (MeQH=2-methyl-8-hydroxyquinolinato) have been investigated. In both acetonitrile and aqueous solutions, [(q)(qH)Li]x and [(Meq)(MeqH)Li]x have absorption maxima at 320 and 309 nm, respectively. When excited at these wavelengths, the complexes emit around 500 nm (blue-green) in acetonitrile. Photoluminescence efficiencies (phiem) were 0.036 for [(q)(qH)Li]x and 0.012 for [(Meq)(MeqH)Li]x when compared to Ru(bpy)3(2+) (bpy=2,2'-bipyridine) with phiem=0.042. No photoluminescence was observed in aqueous media. The complexes show irreversible oxidative electrochemistry and quasi-reversible reductions in acetonitrile. ECL efficiencies (phiecl) were 0.097 for [(q)(qH)Li]x and 0.080 for [(Meq)(MeqH)Li]x when compared to Ru(bpy)(3)2+ (phiecl=1) in aqueous buffered solution and 0.035 for [(q)(qH)Li]x and 0.028 for [(Meq)(MeqH)Li]x in acetonitrile (0.05 M tri-n-propylamine (TPrA) as an oxidative-reductive ECL co-reactant). The ECL peaks at a potential corresponding to oxidation of both the TPrA and [(q)(qH)Li]x or [(Meq)(MeqH)Li]x. Also, qualitative studies using transmission filters suggest that both complexes emit ECL in approximately the same blue-green region as their photoluminescence, indicating that the same excited state is formed in both experiments.

Sensitization of Nanocrystalline TiO2 with Black Absorbers Based on Os and Ru Polypyridine Complexes

New metal complexes of the type [M(H3tcterpy)LY]+ (where M = Os(II) or Ru(II), L = substituted or unsubstituted bipyridine or pyridylquinoline, and Y = Cl-, I-, or SCN-) have been designed, synthesized, and characterized in view of their application for dye-sensitized solar cells (DSSCs). The Os dyes show a very broad absorption, and correspondingly, the DSSCs show an unprecedented spectral response in the NIR, with an onset at 1100 nm, reaching values of about 50% at 900 nm. The integrated photocurrent of some of such Os dyes is similar to that of the well-known [Ru(Htcterpy)(NCS)3](TBA) and superior to that of the [Ru(Hdcbpy)2(NCS)2](TBA)2 sensitizer. The Ru dyes show absorption and DSSC spectral response between those of the red and black dyes. Their advantage is their potential superior stability due to the reversible oxidative electrochemistry.

Electrochemistry of di- tert-butylphosphinopentaphenylferrocene (Q-phos) and derivatives

The oxidative electrochemistry of 1-di- tert-butylphosphino-1′,2′,3′,4′,5′-pentaphenylferrocene (Q-phos), 1-di- tert-butylphosphino-1′,2′,3′,4′,5′-penta(4-trifluoromethylphenyl)ferrocene (Q-phos-CF 3), 1-di- tert-butylphosphino-1′,2′,3′,4′,5′-penta(4-methoxyphenyl)ferrocene (Q-phos-OMe) and 1-di- tert-butylphosphino-1′,2′,3′,4′,5′-penta(4-methylphenyl)ferrocene (Q-phos-Me) was explored. All of the compounds undergo a reversible oxidation, and the formal potentials are sensitive to the R groups on the phenyl rings.

Electrochemistry of Group VI Metal Carbonyl Compounds Containing 1,1‘-Bis(diphenylphosphino)ferrocene

The oxidative electrochemistry of a variety of homo- and bimetallic group VI metal carbonyl compounds containing 1,1‘-bis(diphenylphosphino)ferrocene (dppf) with the general formulas M(CO)5(dppf) (1a−c), M(CO)4(dppf) (2a−c), and (CO)5M(dppf)M‘(CO)5 (3a−f) (M, M‘ = Cr, Mo, W) was investigated. The effect of the group VI metal and the coordination mode of the dppf ligand on the potential at which the oxidation of the group VI metals and the ferrocene backbone of dppf occurred was examined.

Homoleptic platinum(II) and palladium(II) complexes of 1,5,9-trithiacyclododecane: the crystal structures of [Pt(12S3) 2](PF 6) 2 · 2CH 3NO 2 and [Pd(12S3) 2](BF 4) 2 · 0.5H 2O

The synthesis, spectroscopy, electrochemistry, and crystal structures of two new mononuclear homoleptic Pt(II) and Pd(II) complexes with the crown trithioether 1,5,9-trithiacyclododecane (12S3) are reported. In contrast to behavior with analogous smaller ring trithiacrowns, both metal complexes exhibit exodentate axial sulfur donors, a consequence of the preferred conformation of the 12S3 ligand. The lack of two axial metal–sulfur interactions correlates with the observed electronic spectroscopy and oxidative electrochemistry displayed by the complexes and contrasts with properties exhibited by complexes containing smaller polythioether macrocycles. The two complexes have electronic spectra dominated by charge transfer, not d–d bands and show no M 2+/M 3+ couples. Both complexes show a fluxional 12S3 ligand in solution due to a 1,5-metallotropic shift, an uncommon observation of this particular type of intramolecular ligand exchange. The 195Pt NMR chemical shift of −4201 ppm for [Pt(12S3) 2] 2+ is consistent with an alternating positioning of the four sulfur lone pairs on the coordinated thioethers. Although 12S3 is poorly pre-organized for facial complexation, its flexibility to position a sulfur in an exodentate fashion enables it to form stable complexes with d 8 metal ions such as Pt(II) and Pd(II).

Electrochemical oxidation of calixarenes: stabilisation by intramolecular π–π interactions of electrogenerated phenoxylium cations

5,11,17,23- Tetra-tert-butyl-25,27-bis( p-toluenesulfonyl)-26,28-bis(hydroxy)calix[4]arene ( 3) has been prepared from p- tert-butylcalix[4]arene and p-toluenesulfonyl chloride. The oxidative electrochemistry of 3 was studied in dichloromethane solution at a platinum electrode. As shown by detailed voltammetric, coulometric and spectroelectrochemical experiments the hydroxyl groups of 3 behave as non interacting sites, each hydroxyl group undergoing an irreversible two-electron transfer governed by an ECE mechanism to form the corresponding phenoxylium cation. The resulting di-(phenoxylium cation) [ 3-2H] 2+ 4 was found to be stable for at least 2 h in dichloromethane. Nucleophilic attack of residual water on 4 ultimately led to the corresponding calix[4]diquinone 5.

An electrochemical study of the oxidation of 1,3,5-Tris[4-[(3-methylphenyl)phenylamino]phenyl]benzene

An investigation into the oxidative electrochemistry of 1,3,5-Tris[4-[(3-methylphenyl)phenylamino]phenyl]benzene (TMPB) is described. In particular, the heterogeneous electron-transfer kinetics in N, N-dimethylformamide (DMF) and dichloromethane (DCM) are studied using the high-speed microband channel electrode. Experiments to investigate the standard electrochemical rate constants, k 0, transfer coefficients, α, and formal potentials, E f 0, of the observable oxidations of TMPB in both DMF and DCM solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 12.5 μm platinum microband electrodes using a range of linear flow velocities from 12 to 25 m s −1. The measured values of k 0 for the two measurable oxidations in DMF are 1.03 ± 0.41 and 1.02 ± 0.20 cm s −1, and for the three oxidations in DCM are 0.44 ± 0.04, 0.17 ± 0.03 and 0.08 ± 0.02 cm s −1, respectively. The values of α for these oxidations in DMF are 0.48 ± 0.06 and 0.52 ± 0.02, and in DCM are 0.52 ± 0.02, 0.62 ± 0.03 and 0.53 ± 0.07, respectively. The respective formal oxidation potentials (all measured vs Ag) are 0.533 ± 0.002 and 0.766 ± 0.003 V in DMF, and 0.161 ± 0.002, 0.495 ± 0.002 and 1.128 ± 0.004 V in DCM. The presence of the monocation radical in DCM is confirmed by ESR measurement. Experiments are also presented to explore the voltammetry of TMPB in microdroplets of toluene and also in the solid-phase, when in contact with aqueous solutions of sodium fluoride, perchlorate, nitrate and sulphate. It was found that, when TMPB is dissolved in a toluene microdroplet, anion insertion accompanies the first oxidation for the case of perchlorate and nitrate, with anion-facilitated dissolution occurring for sulphate and fluoride. More complex reactions occur at more positive potentials. In the solid-phase, however, slow anion-facilitated dissolution still occurs for fluoride and sulphate, and rapid direct dissolution takes place in the case of perchlorate.

Ruthenium cluster compounds containing 1,1′-bis(diphenylphosphino)ferrocene (dppf): an electrochemical analysis and the crystal structure of [Ru 3(CO) 11] 2(μ-dppf)

The electrochemistry of 1,1′-bis(diphenylphosphino)ferrocene (dppf) derivatives of Ru 3(CO) 12 was investigated. Two known compounds [Ru 3(CO) 8(μ-dppf) 2 ( 1) and Ru 3(CO) 10dppf ( 2)] and a new compound [Ru 3(CO) 11(μ-dppf)Ru 3(CO) 11 ( 3)] were prepared. Compound 3 was characterized spectroscopically and an X-ray crystal structure was obtained. The reductive electrochemistry of 1 and 2 showed an irreversible reduction and a follow-up oxidation, similar to Ru 3(CO) 12. The electrochemistry of compound 3 showed two irreversible waves and a follow-up oxidation. A trend in the reduction potential vs. the number of coordinated phosphorus atoms was noted. The oxidative electrochemistry of 1– 3 showed a dppf-based chemically reversible wave, and an irreversible wave similar to that of Ru 3(CO) 12. Trends were also noted between the oxidation potential and the number of coordinated phosphorus atoms.

Group 10 metal compounds of 1,1′-bis(diphenylphosphino)ferrocene (dppf) and 1,1′-bis(diphenylphosphino)ruthenocene: a structural and electrochemical investigation. X-ray structures of [MCl 2(dppr)] (M=Ni, Pd)

The oxidative electrochemistry of 1,1′-bis(diphenylphosphino)ferrocene (dppf) and 1,1′-bis(diphenylphosphino)ruthenocene (dppr) was investigated at a variety of temperatures and concentrations. In addition, the oxidative electrochemistry of [NiCl 2(dppf)] and [MCl 2(dppr)] (M=Ni, Pd or Pt) compounds was studied. During the preparation of the dppr compounds, crystals of [NiCl 2(dppr)] and [(PdCl 2(dppr)]·CH 2Cl 2 were obtained and the structures were determined. With the previously determined structures of [MCl 2(dppf)] (M=Ni, Pd or Pt) and [PtCl 2(dppr)], a thorough examination of the binding of dppf and dppr to Group 10 metals was performed.

Synthesis and Redox Property of Cyclic Mixed-Metal Complexes from Diethynylbiferrocene

Abstract The reaction of 6,6′-diethynylbiferrocene (1) with cis-PtCl2(PR3) (R= nBu and Me) or (η5-C5H5)Co(PR3)2 gave novel cyclic mixed metal complexes, which show a rich oxidative electrochemistry.

Formation and Spectroelectrochemical Characterization of Multilayer and Submonolayer Thin Films of 2,3,9,10,16,17,23,24-Octa(2-benzyloxy-ethoxy) Phthalocyninato Copper (CuPc(OC2OBz)8)

We review here the self-assembly, electrochemical and spectroelectrochemical properties of 2,3,9,10,16,17,23,24-octa(2-benzyloxyethoxy) phthalocyaninato copper ( CuPc ( OC 2 OBz )8), where terminal benzyl groups on the eight ethylene oxide side chains assist in forming unusually ordered, mechanically rigid thin films. New spectroscopic characterization of cast CuPc ( OC 2 OBz )8 films is discussed in comparison with cast films of its metal-free analogue H 2 Pc ( OC 2 OBz )8 and similar copper and dihydrogen phthalocyanines with benzyl — terminated triethylene oxide substituents, CuPc (( OC 2 O )3 Bz )8, and H 2 Pc (( OC 2 O )3 Bz )8, which do not demonstrate the same degree of ordering as CuPc ( OC 2 OBz )8. AFM studies of horizontally transferred LB films of CuPc ( OC 2 OBz )8 show column–column distances of ca 2.8 nm and confirm the high degree of ordering previously surmised from spectroscopic characterization of multilayer thin films. The oxidative electrochemistry of multilayer thin films prepared from these Pcs is strongly dependent on the chemical identity of the supporting electrolyte anion and on annealing of the thin films. Compliance of the films to counter-ion transport limits the extent of electrochemical doping. Preliminary studies of the oxidative electrochemistry of isolated CuPc ( OC 2 OBz )8 aggregates (diluted into an electroinactive methyl arachidate thin film) on an electroactive, integrated optic waveguide (EA-IOW) are also presented. Monitoring the change in absorbance at 633 nm on the waveguide surface allows the determination of the onset potential for oxidation of the isolated aggregates, which appears to be less positive in potential versus that observed for multilayer Pc assemblies.