Enhanced Four-Electron Oxygen Reduction Selectivity of Clamp-Shaped Cobalt(II) Porphyrin(2.1.2.1) Complexes

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co^II porphyrin(2.1.2.1) complexes bearing Ph or F₅Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co^II/Co^III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e⁻ ORR pathway giving H₂O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e⁻ ORR as compared to the 2e⁻ pathway, consistent with experimental data.     

Determination of Sudan I Using Electrochemically Reduced Graphene Oxide

Electrochemically reduced graphene oxide (ER-GO) was prepared by reducing exfoliated graphene oxide sheets on a glassy carbon electrode (GCE). The voltammetric responses of Sudan I-IV were studied at the ER-GO modified GCE (ER-GO/GCE). Compared with chemically reduced graphene oxide (CR-GO) modified electrode (CR-GO/GCE), ER-GO/GCE showed higher voltammetric responses to Sudan I. The electrode had a linear response to Sudan I in the range of 0.04–8.0 µmol L^?1 and a detection limit of 0.01 µmol L^?1. The real sample determination indicated that the proposed method was reliable, effective, and sufficient.     

An ultrasonic wave-assisted synthesis of meso-amidinophenyl substituted porphyrins

With the aid of an ultrasonic irradiation, meso-amidinophenyl substituted porphyrins were prepared by the reaction of lithium amide and meso-cyanophenyl porphyrins. This approach can give the derivatized porphyrins in very high yields and very short reaction times as compared to the conventional thermal method.     

A variety of oxidation products of antioxidants based on N,N′-substituted p-phenylenediamines

Electrochemical and spectroscopic (EPR, UV–Vis, IR) studies of the aromatic secondary amines N,N′-diphenyl-1,4-phenylenediamine (DPPD), N-phenyl-N′-isopropyl-p-phenylene diamine (IPPD), N-phenyl-N′-(α-methylbenzyl)-p-phenylenediamine (SPPD) and N-phenyl-N′-(1,3-dimethyl-butyl)-p-phenylenediamine (6PPD), which represent the most important group of antioxidants used in the rubber industry, are presented. During oxidation, all the compounds show reversible redox couples in acetonitrile/0.1 M TBABF₄. The first oxidation potential depends substantially on the R substituent at the –N′H– moiety. Very similar UV–VIS spectra of monocation radicals and dications for all the compounds were observed by applying anodic oxidation as well as oxidation by tert-butyl hydroperoxide both in air and in inert atmosphere. The samples with N′-bonded aliphatic carbon in the molecule (e.g. IPPD) heated in air undergo consecutive chemical reactions leading to the formation of –N′C– group. By the use of RO₂ radicals only very low concentration of nitroxide radicals was obtained. Very high concentration of nitroxide radicals was achieved using 3-chloroperbenzoic acid. In the oxidation of investigated aromatic secondary amines with powder PbO₂ no EPR spectra were observed and UV–Vis and IR studies indicate the rapid formation of the final dehydrogenated oxidation product.     

Electrochemistry and spectroelectrochemistry of tert-butylcalix[4]arene bridged bis double-decker lutetium(III) phthalocyanine, Lu2Pc4 and dimeric lutetium(III) phthalocyanine, Lu2Pc2(OAc)2

In this study, electrochemical, electrochromic and spectroelectrochemical properties of a tert-butylcalix[4]arene bridged bis double-decker lutetium(III) phthalocyanine (Lu₂Pc₄ 2) were investigated explicitly as compared with a tert-butylcalix[4]arene bridged dimeric lutetium(III) phthalocyanine [Lu₂Pc₂(OAc)₂ 1]. Distinctive differences between electrochemical and electrochromic properties of 1 and 2 were detected. Moreover, the properties of 1 and 2 were compared with previously reported S₄(CH₂)₄ bridged Lu₂Pc₂(OAc)₂ and Lu₂Pc₄. The calixarene bridged phthalocyanine (Pc) compounds, 1 and 2 showed well-defined electrochromic behaviour with green-blue and blue-purple colour transitions. The enhanced electrochromic properties of 2, as compared to 1, were attributed to its double-decker structure, probably allowing the formation of suitable ion channels for the counter ion movement in the solid film.     

Synthesis,electrochemistry and in situ spectroelectrochemistry of a new Co(III) thio Schiff-base complex with N,N′-bis(2-aminothiophenol)-1,4-bis(carboxylidene phenoxy)butane

A new cobalt Schiff-base complex, [Co(L)(OH)(H₂O)] (where L = [N,N′-bis(2-aminothiophenol)-1,4-bis(carboxylidene phenoxy)butane), was synthesized and its electrochemical and spectroelectochemical properties were investigated using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and thin-layer spectro-electrochemistry in solutions of dimethyl sulfoxide (DMSO) and dichloromethane (CH₂Cl₂). The [Co(L)(OH)(H₂O)] complex displays two well-defined reversible reduction processes with the corresponding anodic waves. The half-wave potentials of the first and second reduction processes were displayed at E_1/2 = 0.08 V and E_1/2 = −1.21 V (scan rate: 0.100 Vs⁻¹) in DMSO, and E_1/2 = −0.124 V and E_1/2 = −1.32 V (scan rate: 0.100 Vs⁻¹) in CH₂Cl₂. The potentials of the reduction processes in DMSO are shifted toward negative potentials (0.220–0.112 V) compared to those in CH₂Cl₂. The electrochemical results are assigned to two one-electron reduction processes; [Co(III)L] + ⁻e → [Co(II)L]⁻ and [Co(II)L]⁻ + ⁻e → [Co(I)L]²⁻. The six-coordination of the complex remains unchanged during the reduction processes and the electron transfer processes were not followed by a chemical reaction upon scan reversal. It was also seen that [Co(L)(OH)(H₂O)] was reduced at a more positive potential than the corresponding salen analogs. The shift and reversibility are apparently related to the high degree of electron delocalization of the [Co(L)(OH)(H₂O)] complex, having a N₂O₂S₂ donor set and two additional benzene units. Additionally, in situ spectroelectrochemical measurements support Co(III)/Co(II) and Co(II)/Co(I) reversible reduction processes with the observation of the corresponding spectral changes with the applied potentials E_app = −0.40 and −1.60 V. Application of the spectroelectrochemical results allowed the determination ofE_1/2 and n (the number of electrons) from the spectra of the fully oxidized and reduced species in one unified experiment as well. The results obtained by this method are in agreement with those by the CV and DPV methods.     

Synthesis and reactions of meso-(p-nitrophenyl)porphyrins

An improved methodology is reported for the regioselective nitration of the phenyl groups of meso-tetraphenylporphyrin 1, using NaNO₂ and TFA. The degree of nitration is easily controlled by the equivalent amount of NaNO₂ used and the reaction time. The nitroporphyrins are reduced to the corresponding aminoporphyrins under standard SnCl₂/HCl conditions. Reaction of tri-aminoporphyrin 9 with 1-formyl-o-carborane followed by reduction using NaBH₄ gave a novel tri-carboranylporphyrin bearing amine linkages between the porphyrin and the carborane groups.     

Synthesis,electrochemical and in situ spectroelectrochemical studies of new transition metal complexes with two new Schiff-bases containing N2O2/N2O4 donor groups

New Schiff-base copper and cobalt complexes, [Cu(L¹)], [Cu(L²)] and [Co(L¹)], [Co(L²)] (where L¹ = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,4-cyclohexane bis(methylamine) and L² = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,8-diamino-3,6-dioxaoctane), were synthesized and characterized using elemental analysis, IR spectra, UV–Vis spectra, magnetic susceptibility measurements, ¹H and ¹³C NMR spectroscopy, thermal analysis and molar conductance (Λ_M). Their electro-spectrochemical properties were investigated using cyclic voltammetric (CV) and thin-layer spectroelectrochemical techniques in a dichloromethane solution (CH₂Cl₂). The CV of [Cu(L²)] showed a lower oxidation potential than that of [Cu(L¹)] under the same experimental conditions. The oxidation wave (II) of [Cu(L²)] was accompanied by an EC process (II′), which was not observed for [Cu(L¹)]. Also, [Cu(L²)] exhibited a reduction process, but [Cu(L¹)] did not. These results indicate that the Cu(II) ion in [Cu(L²)] is coordinated by N₂O₄ donor sites while [Cu(L¹)] presents a square-planar structure with N₂O₂ donor sites. Both oxidation processes for [Co(L¹)] and [Co(L²)] are based on the cobalt center, and they are assigned to Co(II)/Co(III) couples. The spectroelectrochemical results indicate that the oxidized species of [Cu(L²)] is similar to that of [Cu(L¹)], the only difference being that the absorption bands of the oxidized species for [Cu(L²)] shift to lower energy compared with those of [Cu(L¹)] because of their different coordination environment. The geometry of [Cu(L²)] changed into square-planar after the complex was totally oxidized and the neutral complex was only recovered following the EC process, as observed from the CV of [Cu(L²)]. For the two cobalt complexes, the bands corresponding to the π → π^∗transitions disappeared and new bands with small red shifts and of lower intensity were observed during the oxidation process. These new bands are attributed to the LMCT transition as observed in the case of the oxidation processes of the cobalt complexes.     

A UV-Visible/Raman spectroelectrochemical study of the stability of poly(3,4-ethylendioxythiophene) films

Poly(3,4-ethylendioxythiophene) films were electropolymerized in aqueous medium without using any surfactant, on glassy carbon electrodes. UV/Vis and Raman spectroelectrochemical techniques were used to analyze the degradation of the polymer film occurring at different pH values. Spectroelectrochemistry has proven to be a very useful analytical tool for this purpose, thanks to its ability to provide information not only about the extent of degradation, but also about mechanistic aspects of the process. From our results we extracted important information about the main factors that play a role in the degradation, in particular about the influence of repetitive doping and de-doping cycles and of photo-induced processes, as a function of the characteristics of the solution, i.e. of pH.     

Multiple one-electron oxidation and reduction of trinuclear bis(2,4-pentanedionato)ruthenium complexes with substituted diquinoxalino[2,3-a:2′,3′-c]phenazine ligands

The complexes (μ₃-L¹/L²)[Ru(acac)₂]₃, acac⁻ = 2,4-pentanedionato, L¹ = 2,3,8,9,14,15-hexachlorodiquinoxalino[2,3-a:2′,3′-c]phenazine and L² = 2,3,8,9,14,15- hexamethyldiquinoxalino[2,3-a:2′,3′-c]phenazine, undergo stepwise one-electron oxidation involving a total of three electrons and stepwise one-electron reduction with three (L²) or four electrons (L¹). All reversibly accessible states were characterized by UV–Vis–NIR spectroelectrochemistry. Oxidation leads to mixed-valent intermediates {(μ₃-L)[Ru(acac)₂]₃}⁺ and {(μ₃-L)[Ru(acac)₂]₃}²⁺ of which the Ru^IIIRu^IIRu^II combinations exhibit higher comproportionation constants K_c than the Ru^IIIRu^IIIRu^II states – in contrast to a previous report for the unsubstituted parent systems {(μ₃-L³)[Ru(acac)₂]₃}^+/2+, L³ = diquinoxalino[2,3-a:2′,3′-c]phenazine. No conspicuous inter-valence charge transfer absorptions were observed for the mixed-valent intermediates in the visible to near-infrared regions. The monocations and monoanions were characterized by EPR spectroscopy, revealing rhombic ruthenium(III) type signals for the former. Electron addition produces ruthenium(II) complexes of the reduced forms of the ligands L, a high resolution EPR spectrum with ¹⁴N and ^35,37Cl hyperfine coupling and negligible g anisotropy was found for {(μ₃-L¹)[Ru(acac)₂]₃}⁻. DFT calculations of (μ₃-L¹)[Ru(acac)₂]₃ confirm several ligand-centered low-lying unoccupied MOs for reduction and several metal-based high-lying occupied MOs for electron withdrawal, resulting in low-energy metal-to-ligand charge transfer (MLCT) transitions.     

Review of electrochemical studies of complexes containing the Fe2S2 core characteristic of [FeFe]-hydrogenases including catalysis by these complexes of the reduction of acids to form dihydrogen

This article reviews published literature on the electrochemical reduction and oxidation of complexes containing the Fe₂S₂ core characteristic of the active site of [FeFe]-hydrogenases. Correlations between reduction and oxidation potentials and molecular structure are developed and presented. In cases where the complexes have been studied with regard to their ability to catalyze the reduction of acids to give dihydrogen, the overpotentials for such catalyzed reduction are presented and an attempt is made to estimate, at least qualitatively, the efficiency of such catalysis.     

Electrohydrocyclization of alkoxy tether substituted mixed enone/enoate and bisenone systems: retention versus elimination

The electrohydrocyclization of tethered enone/enoate or bisenone systems which contain an alkoxyl substituent in the tether was studied. Rather surprisingly, the cyclization of these compounds afforded very different products depending upon the alkoxy substituent. This behavior and its potential applications will be discussed.     

The electrochemical reduction of CP2TiCl2 in the presence of trimethylphosphine

The electrochemistry of the complexes Cp₂TiCl_x(PMe₃)_2-x in anhydrous THF has been studied. The most stable complexes at the three oxidation states of titanium are Cp₂TiCl₂, Cp₂TiCl(PMe₃) and Cp₂Ti(PMe₃)₂, and each of these species is readily formed by electrolysis. It has also been demonstrated that oxidation/reduction of these species is followed by facile and rapid ligand exchange to form the preferred species in the new oxidation state provided a stoichiometric concentration of the required ligand is present. The consequences of this redox and ligand exchange chemistry for the synthetic reactions catalyzed by lower oxidation states of Ti are discussed. Finally, the voltammetry of a titanocycle is reported, and it is shown that the corresponding Ti^III metallocycle is stable.     

The synthesis and electrochemistry of CpTiCl2(OR) (R = alkyl, aryl) complexes

The syntheses of several new CpTiCl₂(OR) (R = alkyl, aryl) complexes are described. It was possible to isolate pure product when the R group is substituted such as to cause steric crowding at the metal centre; for example, particularly good yields of the phenolate complexes were obtained when there were isopropyl substituents in the 2 and 6 positions of the phenolate. Electrochemical studies of the complexes in dry THF show that the Ti^III complexes are relatively stable, but only a diol complex could be reduced further to a Ti^II species. In general, the Ti^IV complexes undergo a reversible 1e⁻ reduction reaction. The chemistry is more complex if the electrolyte contains added water: both the Ti^IV and Ti^III complexes can react with water, the OR group being replaced by OH. The reaction is particularly rapid for the Ti^III alkoxide complexes.     

A mechanistic study on oxidation of benzylic alcohols with PPh4HSO5 catalysed by manganese(III) porphyrins in homogeneous solution

The oxidation of variously ring-substituted 1-phenylethanols with Ph₄PHSO₅ catalysed by Mn(TMP)Cl and Mn(TDCPP)Cl in the presence of 4-tert-butylpyridine was studied in 1,2-dichloroethane homogeneous solution. The process leads only to C–H bond cleavage products, namely acetophenones. The oxidation rates are independent of the substrate concentration and, when Mn(TMP)Cl is the catalyst, even of the substrate nature. By increasing the concentration of 4-tert-butylpyridine, which acts as an axial ligand of the catalyst, a bell-shaped curve for the rate constants trend is observed. Hammett plots obtained by changing the substituents on the phenyl ring of the benzylic alcohol give different ρ values depending on the technique employed for rate constants determination, i.e., individual or competitive experiment. The observations reported above, together with a KIE of 2.5 in 1- -1-phenylethanol oxidation measured by competitive experiment, are rationalised on the basis of a mechanistic scheme in which the oxo-manganese derivative is formed in the rate determining step of the catalytic process. Furthermore, it is suggested that alcohol dehydrogenation proceeds through a hydride abstraction involving an alcohol-oxo-porphyrinato complex.     

Oxidation of alkanes by Ph4PHSO5 catalyzed by manganese(III) porphyrins. A study of the factors determining alcohol and ketone formation

As reported in previous papers, the oxidation of ethylbenzene with Ph₄PHSO₅ catalyzed by Mn(TMP)Cl in the presence of nitrogen bases in 1,2-dichloroethane homogeneous solution affords fair yields (up to 80%) of oxygenated products. Acetophenone is the major product together with minor amounts of 1-phenylethanol ([ketone]/[alcohol]6). In this paper we report further observations concerning this system. In particular we find that a similar products distribution is observed when soluble Co(II) or Fe(II) species, i.e. acetylacetonate derivatives, are used instead of the manganese porphyrin. On the other hand for these compounds, which are catalysts of radical reactions, the product distribution is determined by a remarkably larger reactivity of the alcohol initially formed compared with that of the alkane (k_R–OH/k_R–H200). The product distribution for the Mn(TMP)Cl catalysis cannot be rationalized on the same basis. Direct experiments show that the reactivity of the two substrates is similar (k_R–OH/k_R–H2–6) thus suggesting that a mechanism different from the simple radical hydrogen atom abstraction is taking place. The dependence of the chemioselectivity of hydrocarbon oxidation on the nature of the manganese porphyrin employed and of a number of additives indicates that the product distribution is mainly determined by the relevance of the association of the alcohol intermediate to the catalyst prior to the second oxidation step leading to ketone. Two different rho Hammett's values are obtained for ethylbenzenes oxidation depending on the method employed for the measurement of reaction rates, i.e. by separate or by competitive experiments thus providing further mechanistic information on the association of the substrates to the oxo-species.     

Lead structures for applications in photodynamic therapy. Part 1: Synthesis and variation of m-THPC (Temoporfin) related amphiphilic A2BC-type porphyrins

Photodynamic therapy (PDT) is a developing modality for the treatment of certain tumorous and other diseases. Considerable progress has been made in recent years in the search for new photosensitizers, in particular elucidating the role of localization of the photosensitizer. Known successful photosensitizers of the tetrapyrrole type are amphiphilic molecules, preferably localizing in cellular membrane structures. Thus, the quest for new photosensitizers requires the synthesis of unsymmetrically substituted (amphiphilic) tetrapyrroles. In this article, we describe strategies for the de novo synthesis of amphiphilic tetrapyrroles using a 3-hydroxyphenyl substituted tetrapyrrolic system (Temoporfin) as the lead structure. From an applied science-oriented approach, such a set of amphiphilic porphyrins is best synthesized by combining well-developed condensation methods with subsequent functionalization via organolithium compounds or transition metal catalyzed coupling protocols. Starting from simple A₂- or AB-porphyrins, the synthesis of A₂B-, A₃-, A₃B-, and A₂BC-porphyrins with a mixed hydrophilic/hydrophobic substitution pattern is described. Because of the versatility of this approach to unsymmetrically substituted porphyrins it is also applicable to other areas where porphyryns with a tailor-made substitution patterns are needed, for example, catalysts or molecular electronic devices based on tetrapyrroles.