Metal-based Redox Processes Research Articles

Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers.

The photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate Fe(II) complexes (Fe(II)L6) have been widely considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the (3)MLCT state by increasing the ligand field strength, with special focus on making eg σ* orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from Ru(II)L6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field Fe(II)L6 complexes with significantly extended (3)MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the (3)MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2](2+) (tpy = 2,2':6',2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the (3)MC instead of the (5)MC state likely dictates the deactivation of the (3)MLCT state, a behavior distinct from traditional Fe(II)L6 complexes but rather resembling Ru analogues. A heteroleptic Fe(II) NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the (3)MLCT lifetime relative to its parental [Fe(bpy)3](2+) (bpy = 2,2'-bipyridine) complex. Again, a Ru-like deactivation mechanism of the (3)MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record (3)MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the (3)MLCT lifetime indeed benefits efficient light harvesting with Fe(II) NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy (3)MLCT state to a TiO2 nanofilm was achieved by a stable Fe(II) complex. This is in complete contrast to conventional Fe(II)L6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d(6) Fe(II) complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.

A unique supramolecular sandwich structure of a tetrabromocatechol-chelated iron(III) complex: Mutual reinforcement of H-bonding and π⋯π stacking interactions in the crystal lattice

A unique supramolecular sandwich structure of a tetrabromocatecholate chelated mononuclear iron(III) complex, [H3ntb]2[Fe(Br4Cat)3](ClO4)3·4CH3OH·4H2O (1) [ntb = tris(2-benzimidazolylmethyl)amine and Br4CatH2 = tetrabromocatechol], is reported. The most interesting feature of the crystal structure of 1 is that the central [Fe(Br4Cat)3]3 − anion is sandwiched by the triply-protonated two ntb ligands in which mutual reinforcement of the π⋯π stacking and hydrogen bonding interactions play significant role to stabilize the structural motif. Electrochemical studies disclose both the noninnocent ligand and metal based redox processes.

Electrochemical and in situ spectroelectrochemical monitoring of the interaction between cobaltphthalocyanines and molecular oxygen in aprotic media

In this study, voltammetric and spectroelectrochemical measurements of cobaltphthalocyanines (CoPc) bearing different substituents were performed in aprotic solvents to determine the mechanism of interaction between CoPc and molecular oxygen. Metal-based redox processes of the cobaltphthalocyanine complexes indicate their possible electrocatalytic activities toward many target species. Different substituents of the complexes affect the peak characters and assignments of the processes. Presence of O 2 in the electrolyte system influences oxygen reduction reaction and the electrochemical and spectral behaviors of the complexes, which indicate homogeneous electrocatalytic activity of the complexes for the oxygen reduction reaction. In situ electrocolorimetric method was applied to investigate color of the anionic and cationic forms of the complexes. Electrochemical and in situ spectroelectrochemical analysis indicate interaction of molecular oxygen with monoanionic [Co IPc −2] 1− and presence of an equilibrium between the reactants [Co IPc −2] 1− and O 2 and the intermediates [ O 2 - – Co II Pc 2 - ] - and [ O 2 2 - – Co II Pc 2 - ] 2 - . This equilibrium is disturbed in desired direction by changing the excitation signals of the voltammetric and spectroelectrochemical techniques. The intermediates are very unstable, regenerating the starting form of the catalyst and the products O 2 - and O 2 2 - . Interaction rate of O 2 with the complexes is significantly influenced with the steric and coordination properties of the substituents and solvent of the system. Voltammetric and spectroelectrochemical results indicate that homogeneous electrocatalytic ORR follows an “inner sphere” chemical catalysis process.

Electrochemical and in situ Spectroelectrochemical Properties of Phthalocyanines Bearing N-Benzyl-4-Phenyloxyacetamide Moieties

Abstract The general redox behaviour of phthalocyanines bearing N-benzyl-4-phenyloxyacetamide moieties (H2Pc, ZnPc, NiPc, CuPc, and CoPc) has been identified by cyclic voltammetry and differential pulse voltammetry on Pt in dimethylsulfoxide/tetrabutylammonium perchlorate. These measurements showed that the complexes indicate subsequent one-electron reduction and oxidation processes. Although the voltammetric measurements implied the association of the redox processes of some complexes with aggregation phenomenon, the complete evaluation of the aggregation effects by only these measurements was not possible. In situ UV-Vis spectroelectrochemical measurements enabled us to assign the ligand- and metal-based redox processes, and identify if the aggregation affects these processes. These measurements suggested that the redox processes of H2Pc, NiPc, and CuPc are coupled by aggregation phenomenon, while those of ZnPc and especially CoPc are not, probably due to the difference in their axial coordinating properties.

Electrochemical and in situ spectroelectrochemical investigations into the redox and aggregation behaviours of phthalocyanines bearing octyl 4-phenyloxyacetate moieties

Electrochemical and spectroelectrochemical properties of phthalocyanines (H2Pc, ZnPc, NiPc, CuPc, and CoPc) involving octyl 4-phenyloxyacetate moieties have been investigated by cyclic voltammetry and differential pulse voltammetry on Pt in dimethylsulfoxide – tetrabutylammonium perchlorate. These measurements showed that the complexes undergo subsequent one-electron reduction and oxidation processes. In situ UV–vis spectroelectrochemical measurements enabled us to identify the ligand- and metal-based redox processes, and evaluate the effect of aggregation phenomena on these processes. The cobalt complex showed both metal-based and ring-based one-electron redox processes, while the other complexes displayed only ring-based one-electron couples. It was observed that the redox processes of H2Pc, NiPc, and CuPc are coupled by aggregation phenomenon, whereas those of ZnPc and especially CoPc are not, probably owing to the difference in their axial coordinating properties.

Syntheses and investigation of the effects of position and nature of substituent on the spectral, electrochemical and spectroelectrochemical properties of new cobalt phthalocyanine complexes

The syntheses of new cobalt phthalocyanine (CoPc) complexes, tetra-substituted with diethylaminoethanethio at the peripheral (complex 3a) and non-peripheral (complex 3b) positions, and with benzylmercapto at the non-peripheral position (complex 5), are reported. The effects of the nature and position of substituent on the spectral, electrochemical and spectroelectrochemical properties of these complexes are investigated. Solution electrochemistry of complex 3a showed three distinctly resolved redox processes attributed to Co IIIPc −2/Co IIPc −2 ( E ½ = +0.64 V versus Ag|AgCl), Co IIPc −2/Co IPc −2 ( E ½ = −0.24 V versus Ag|AgCl) and Co IPc −2/Co IPc − 3 ( E ½ = −1.26 V versus Ag|AgCl) species. No ring oxidation was observed in complex 3a. Complex 3b showed both ring-based oxidation, attributed to Co IIIPc −1/Co IIIPc −2 species ( E p = +0.86 V versus Ag|AgCl), and ring-based reduction associated with Co IPc −2/Co IPc −3 species ( E ½ = −1.46 V versus Ag|AgCl), with the normal metal-based redox processes in CoPc complexes: Co IIIPc −2/Co IIPc −2 ( E p = +0.41 V versus Ag|AgCl) and Co IIPc −2/Co IPc −2 ( E ½ = −0.38 V versus Ag|AgCl). Solution electrochemistry of complex 5 showed the same type and number of species observed in complex 3a: Co IIIPc −2/Co IIPc −2 ( E p = +0.59 V versus Ag|AgCl), Co IIPc −2/Co IPc −2 ( E ½ = −0.26 V versus Ag|AgCl) and Co IPc −2/Co IPc −3 ( E ½ = −1.39 V versus Ag|AgCl) species. These processes were confirmed using spectroelectrochemistry.

The synthesis and cyclotetramerisation reactions of aryloxy-, arylalkyloxy-substituted pyrazine-2,3-dicarbonitriles and spectroelectrochemical properties of octakis(hexyloxy)-pyrazinoporphyrazine

Novel, aryloxy- and arylalkyloxy-substituted pyrazine dicarbonitriles were synthesized from 5,6-dichloropyrazine-2,3-dicarbonitrile and the corresponding phenol/alcohol derivatives. Cyclotetramerisation of these pyrazine derivatives to form metal pyrazinoporphyrazines in the presence of appropriate metal salts in different solvents such as DMF, quinoline, 2-dimethylaminoethanol and n-hexanol, resulted in decomposition products with the exception of teh latter solvent which lead to mainly octakis(alkyloxy)pyrazinoporphyrazines. Cyclic voltammetry and differential pulsed voltammetry of the complexes indicated that cobalt pyrazinoporphyrazine displayed both ligand and metal-based redox processes while zinc and copper derivatives exhibited only ligand-based redox processes. The redox processes of the pyrazinoporphyrazines shifted significantly towards positive potentials compared to those of the common phthalocyanines. The novel compounds were characterised using elemental analysis and spectral techniques.

Synthesis, electrochemistry and in situ spectroelectrochemistry of water-soluble phthalocyanines

The synthesis, characterization and voltammetric and spectroelectrochemical properties of newly synthesized metal-free and metallo phthalocyanines (M = Co, Cu, Zn) containing four dialkylaminophenoxy or trialkylammoniumphenoxy substituents on peripheral positions have been presented in this work. The new compounds have been characterized by using elemental analysis, UV–Vis, FT-IR, 1H NMR and MS spectroscopic data. Phthalocyanines with trialkylammoniumphenoxy substituents are soluble in aqueous solution over a wide pH range, and these compounds are present as aggregated species in solution as confirmed by the blue shift of Q-bands in their electronic spectra. The electrochemical behavior of the phthalocyanines was investigated by cyclic voltammetry and differential pulse voltammetry on a platinum-working electrode in DCM and DMSO. The voltammetric and spectroelectrochemical measurements of the complexes show that while cobalt phthalocyanine gives both ligand- and metal-based redox processes, metal-free, zinc and copper phthalocyanine complexes give only ligand-based processes in harmony with common phthalocyanine complexes.

Functional fluoro substituted tetrakis-metallophthalocyanines: Synthesis, spectroscopy, electrochemistry and spectroelectrochemistry

In this study, electron-withdrawing fluoro-functional ligand and its tetrakis 2,9,16,23-4-(2,3,5,6-tetrafluoro)-phenoxy-phthalocyaninatometal (II) complexes, (ZnPcOBzF 16, CuPcOBzF 16 and CoPcOBzF 16) (Bz: benzene) which are organo-soluble have been prepared. Their structures were confirmed by elemental analysis, FT-IR, 1H NMR, UV/vis and MS (Maldi-TOF) spectral data. Electron-withdrawing fluorine atoms on 2,3,5,6-position of benzene at the peripheral sites increases the solubility of the tetrakis-metallophthalocyanines. The cyclic voltammetry and differential pulsed voltammetry of the complexes show that while CuPcOBzF 16 and ZnPcOBzF 16 give ligand-based reduction and oxidation processes, CoPcOBzF 16 gives both ligand and metal-based redox processes, in harmony with the common MPc complexes. Spectroelectrochemical measurements confirm the assignments of the complexes.

Comparative electrooxidation of sulphite by self-assembled monolayers (SAMs) of Co(II), Fe(II), Ni(II) and Mn(III) tetrakis benzylmercapto and dodecylmercapto metallophthalocyanines complexes on gold electrodes

This work reports on the use of Co(II), Fe(II), Mn(III) and Ni(II) tetrakis benzylmercapto and dodecylmercapto phthalocyanine complexes for gold electrode modification for electrooxidation of sulphite ions. The complexes were successfully used to modify gold electrodes by self-assembled monolayer (SAM) technique. The self-assembled films are stable and showed blocking characteristics towards the following Faradaic processes; gold surface oxidation, underpotential deposition (UPD) of copper and solution redox chemistry of [Fe(H(2)O)(6)(3+)]/[Fe(H(2)O)(6)(2+)]. All the modified electrodes showed improved catalytic activities than the unmodified gold electrode towards sulphite ion electrooxidation with all the modified electrodes exhibiting high resistance to electrode surface passivation. Better catalytic performance were obtained for the CoPcs, FePcs and MnPcs which have metal based redox processes within the range of the sulphite electrooxidation peak, while the NiPcs which did not show metal based redox reaction performed less. The catalytic current was found to be linear with sulphite ion concentration in the concentration range employed in this work (0.1-1mM) and sensitivities ranging from 1.68 to 2.96muA/mM were observed for all the modified electrodes. Mechanisms of the catalytic oxidation of sulphite ions are proposed.

Water and alcohol-soluble octakis-metalloporphyrazines bearing sulfanyl polyetherol substituents: Synthesis, spectroscopy and electrochemistry

Water-soluble metalloporphyrazines ( 2, 3, and 4) bearing peripherally functionalized bis[thioethoxy(ethoxy)ethanol] moieties have been prepared from the corresponding [2H]-2,3,7,8,12,13,17,18-octakis{2-[2-2-hydroxyethoxy)ethoxy]-ethylthio}porphyrazine and appropriate metal salts [Zn(acac) 2, CoCl 2, MnCl 2] in non-aqueous polar solvents such as, alcohol and THF. Multifunctional polyetherolsulfanyl peripheral sites, the exocyclic site of porphyrazines, serve as weak interacting ionophore in a number of metal ions, such as Ag I and Pd II. The reactivity of the peripheral groups of MPzs was tested by spectrophotometric titration by Ag I and Pd II. The titration resulted in the formation of intractable less soluble black and dark-blue polynuclear porphyrazine complexes. The new compounds have been characterized by elemental analysis, FTIR, 1H NMR, UV–vis and MS (MALDI-TOF). The voltammetric investigation of the complexes showed common ligand and metal-based redox processes.

Electrochemical behavior of polymeric complexes of monosubstituted squarate ligands. Part 2. Determination of redox species of manganese(II) complexes in the donor solvents, dimethylformamide and dimethylsulfoxide

Cyclic and square wave voltammetry (−1500 to 1500 mV) of {Mn[μ-(C 6H 5) 2NC 4O 3] 2[H 2O] 4} n [manganese(II) diphenylaminosquarate] ( 1) and [Mn(μ-C 6H 5C 4O 3)(C 6H 5C 4O 3)(H 2O) 3] n [manganese(II) phenylsquarate] ( 2) at a gold disk electrode in dimethylsulfoxide (DMSO) and dimethylformamide (DMF), reveal several couples attributable to both ligand and metal-based redox processes. For the manganese(II) phenylsquarate in DMF, the metal-based peaks are more numerous and readily discernible than in DMSO. In either of the solvents, the ligand-based peaks always occur at more positive or more negative potentials than the metal-based ones. In 1 and 2, Mn(II)/Mn(0), Mn(III)/Mn(II), Mn(IV)/Mn(III) and Mn(V)/Mn(IV) couples are observed. However, the manganese redox peaks appear at more negative potentials in 1.