Redox-active Metal Centers Research Articles

Redox-active metal complexes with 2,2′-dipicolylamine containing ferrocenyl moiety: Synthesis, electrochemical behavior and biological activity

The novel metal complexes of general formula MCl2·L (M = Cu, Fe, Co, Mn, Zn) based on the di-(2-picolyl)amine ligand L with the redox-active ferrocenyl fragment were synthesized and characterized by elemental analysis, IR, 1H, 13C NMR, UV–vis spectroscopy and MALDI-TOF mass spectrometry. The molecular structure of [ZnCl2L] was established by X-ray crystallography. The redox properties of complexes were studied using cyclic voltammetry (CV) method and feasible schemes of electrochemical transformations were proposed. The antioxidant activity of compounds was tested by various methods. The lipoxygenase (LOX) inhibition activity of the studied compounds was evaluated. The in vitro biological experiments were performed using rat brain homogenates. The results demonstrate that ditopic compounds containing ferrocene and redox active dipicolylamine fragments act as polyfunctional antioxidants. These results let us to conclude that combining in one molecule several redox-active metal centers is a promising way of metallodrug design in modern medicinal chemistry.

Electrocatalytic Activity of Novel Ball-Type Metallophthalocyanines with Trifluoro Methyl Linkages in Oxygen Reduction Reaction and Application as Zn-Air Battery Cathode Catalyst

A new series of functional compounds, (((perfluoropropane-2,2-diyl) bis (4, 1-phenylene)) bis (oxy))-bridged ball-type metal-free and zinc(II), cobalt (II) and iron (II) metallophthalocyanines, behaving as the air catalyst in zinc-air batteries have been developed, and characterized with common spectroscopic methods. The catalyst composites were characterized by scanning electron microscopy (SEM), energy dispersive X-Ray analysis (EDS), and energy dispersive elemental mapping (EDS mapping). The catalytic activites of the dimeric type complexes on oxygen reduction reaction (ORR) have been evaluated by static and dynamic voltammetric techniques. In particular, iron (II) complex displayed spectacular catalytic activity with slightly more positive half-wave potential than that of the commercial Pt/C for ORR in alkaline media, mainly due to its distinctive cofacial molecular structure with dual redox-active metal centers. The electrocatalytic performance tests, carried out in a homemade Zn-air battery prototype, suggested that this complex can be used as an efficient ORR catalyst in this type of batteries.

1,2,4-triazole-5(4H)-one based novel peripherally tetrasubstituted metal-free and metallophthalocyanines: Synthesis, characterization, and electrochemical and spectroelectrochemical properties

ABSTRACTIn this study, the synthesis of metal-free and metallophthalocyanines containing four 4-[2-(4-methoxyphenyl)ethyl]-5-methyl-2,4-dihydro-3H-1,2,4-triazol-3-one substituents at the peripheral positions were reported. The structures of compounds were characterized using IR, 1H NMR, 13C NMR, mass spectroscopies, and UV-Vis. Electrochemistry of the complexes were studied with voltammetric and in situ spectroelectrochemical measurements in different electrolytic systems. MPcs having redox inactive metal centers (H+, Zn+2, Cu+2, and Ni+2) just illustrated similar Pc-based electron transfer reactions. Due to the different effective nuclear charge of the ions in the core of Pc ring, small potential shifts and reversibility differences were observed between the redox processes of these complexes. MPcs having redox-active metal centers (Co+2 and TiO+2) gave metal-based electron transfers in addition to Pc-based processes. Electrolyte type of the measurement system significantly influenced oxidation behaviors of CoPc due to the coordination of polar coordinating solvent molecules to the oxidized CoIII center of CoPc. Spectra and color of the electrogenerated MPc species were determined with in situ spectroelectrochemical measurements.

ChemInform Abstract: Peptide‐Mediated Vectorization of Metal Complexes: Conjugation Strategies and Biomedical Applications

AbstractReview: 79 refs.

Field-induced conductance switching by charge-state alternation in organometallic single-molecule junctions.

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These intrinsic properties are usually manipulated by varying the molecule's electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high-to-low current ratios exceeding 1,000 at bias voltages of less than 1.0 V. Density functional theory calculations identify a localized, redox-active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarized ground state unique to the Mo centre. This situation provides an additional slow and incoherent hopping channel for transport, triggering a transient charging effect in the entire molecule with a strong hysteresis and large high-to-low current ratios.

Bond-weakening catalysis: conjugate aminations enabled by the soft homolysis of strong N-H bonds.

The ability of redox-active metal centers to weaken the bonds in associated ligands is well precedented, but has rarely been utilized as a mechanism of substrate activation in catalysis. Here we describe a catalytic bond-weakening protocol for conjugate amination wherein the strong N-H bonds in N-aryl amides (N-H bond dissociation free energies ∼100 kcal/mol) are destabilized by ∼33 kcal/mol upon by coordination to a reducing titanocene complex, enabling their abstraction by the weak H-atom acceptor TEMPO through a proton-coupled electron transfer process. Significantly, this soft homolysis mechanism provides a method to generate closed-shell, metalated nucleophiles under neutral conditions in the absence of a Brønsted base.

Light-Activated Protein Inhibition through Photoinduced Electron Transfer of a Ruthenium(II)–Cobalt(III) Bimetallic Complex

We describe a mechanism of light activation that initiates protein inhibitory action of a biologically inert Co(III) Schiff base (Co(III)-sb) complex. Photoinduced electron transfer (PET) occurs from a Ru(II) bipyridal complex to a covalently attached Co(III) complex and is gated by conformational changes that occur in tens of nanoseconds. Reduction of the Co(III)-sb by PET initiates displacement of the inert axial imidazole ligands, promoting coordination to active site histidines of α-thrombin. Upon exposure to 455 nm light, the rate of ligand exchange with 4-methylimidazole, a histidine mimic, increases by approximately 5-fold, as observed by NMR spectroscopy. Similarly, the rate of α-thrombin inhibition increases over 5-fold upon irradiation. These results convey a strategy for light activation of inorganic therapeutic agents through PET utilizing redox-active metal centers.

1,2,4-Triazole-substituted metallophthalocyanines carrying redox active cobalt(II), manganese(III), titanium(IV) center and their electrochemical studies

The synthesis and characterization of novel peripherally-tetra 4-(2-{[4-(4-fluorophenyl)-5-(4-methoxyphenyl)-4H-1,2,4-triazol-3-yl]thio}ethoxy) substituted TiIVOPc, MnIIIClPc, CoIIPc 6–8 are described for the first time in this study. All the new compounds are characterized by a combination of IR, 1H and 13C NMR, mass, elemental analysis and UV–vis spectroscopy techniques. Electrochemical studies of TiIVOPc, MnIIIClPc, CoIIPc were examined by using cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques. While MnIIIClPc showed only metal based reduction reactions, TiIVOPc and CoIIPc showed metal based and ligand based reduction reactions as expected. The presence of redox active metal centers like TiIVO, MnIIICl, CoII in the core of Pc ring increases the electrochemical activity of the phthalocyanines. Cyclic and square wave voltammetric studies show that phthalocyanines have reversible/quasi-reversible/irreversible redox processes, which are the main requirement for the technological usage of these compounds.

Protein chaperones mediating copper insertion into the CuA site of the aa3-type cytochrome c oxidase of Paracoccus denitrificans

The biogenesis of the mitochondrial cytochrome c oxidase is a complex process involving the stepwise assembly of its multiple subunits encoded by two genetic systems. Moreover, several chaperones are required to recruit and insert the redox-active metal centers into subunits I and II, two a-type hemes and a total of three copper ions, two of which form the CuA center located in a hydrophilic domain of subunit II. The copper-binding Sco protein(s) have been implicated with the metallation of this site in various model organisms.Here we analyze the role of the two Sco homologues termed ScoA and ScoB, along with two other copper chaperones, on the biogenesis of the cytochrome c oxidase in the bacterium Paracoccus denitrificans by deleting each of the four genes individually or pairwise, followed by assessing the functionality of the assembled oxidase both in intact membranes and in the purified enzyme complex. Copper starvation leads to a drastic decrease of oxidase activity in membranes from strains involving the scoB deletion. This loss is shown to be of dual origin, (i) a severe drop in steady-state oxidase levels in membranes, and (ii) a diminished enzymatic activity of the remaining oxidase complex, traced back to a lower copper content, specifically in the CuA site of the enzyme. Neither of the other proteins addressed here, ScoA or the two PCu proteins, exhibit a direct effect on the metallation of the CuA site in P. denitrificans, but are discussed as potential interaction partners of ScoB.

Probing the role of π interactions in the reactivity of oxygen species: a case of ethylzinc aryloxides with different dispositions of aromatic rings toward the metal center.

Ethylzinc derivatives of ortho-hydroxybiphenyl and 2,6-diphenylphenol that bear different nuclearity and dispositions of aromatic rings toward the metal center were synthesized and structurally characterized in the solid state and solution. This family of well-defined compounds was examined as a model system for the activation of dioxygen mediated by using complexes that feature lack of a redox-active metal center. Experimental and theoretical studies indicate an essential role in the oxygenation process of intramolecular interactions that involve aromatic subunits. Additionally, novel results for the oxygenation chemistry of alkylzinc compounds, including the isolation and structural characterization of the unique octanuclear aryloxide (hydroxide) compound Zn8 (OAr)8 (OH)6 (O2 ) with an encapsulated peroxide species, are presented.

Beyond the active site: the impact of the outer coordination sphere on electrocatalysts for hydrogen production and oxidation.

Redox active metalloenzymes play a major role in energy transformation reactions in biological systems. Examples include formate dehydrogenases, nitrogenases, CO dehydrogenase, and hydrogenases. Many of these reactions are also of interest to humans as potential energy storage or utilization reactions for photoelectrochemical, electrolytic, and fuel cell applications. These metalloenzymes consist of redox active metal centers where substrates are activated and undergo transformation to products accompanied by electron and proton transfer to or from the substrate. These active sites are typically buried deep within a protein matrix of the enzyme with channels for proton transport, electron transport, and substrate/product transport between the active site and the surface of the protein. In addition, there are amino acid residues that lie in close proximity to the active site that are thought to play important roles in regulating and enhancing enzyme activity. Directly studying the outer coordination sphere of enzymes can be challenging due to their complexity, and the use of modified molecular catalysts may allow us to provide some insight. There are two fundamentally different approaches to understand these important interactions. The "bottom-up" approach involves building an amino acid or peptide containing outer coordination sphere around a functional molecular catalyst, and the "top-down" approach involves attaching molecular catalyst to a structured protein. Both of these approaches have been undertaken for hydrogenase mimics and are the emphasis of this Account. Our focus has been to utilize amino acid or peptide based scaffolds on an active functional enzyme mimic for H2 oxidation and production, [Ni(P(R)2N(R('))2)2](2+). This "bottom-up" approach has allowed us to evaluate individual functional group and structural contributions to electrocatalysts for H2 oxidation and production. For instance, using amine, ether, and carboxylic acid functionalities in the outer coordination sphere enhances proton movement and results in lower catalytic overpotentials for H2 oxidation, while achieving water solubility in some cases. Amino acids with acidic and basic side chains concentrate substrate around catalysts for H2 production, resulting in up to 5-fold enhancements in rate. The addition of a structured peptide in an H2 production catalyst limited the structural freedom of the amino acids nearest the active site, while enhancing the overall rate. Enhanced stability to oxygen or extreme conditions such as strongly acidic or basic conditions has also resulted from an amino acid based outer coordination sphere. From the "top-down" approach, others have achieved water solubility and photocatalytic activity by associating this core complex with photosystem-I. Collectively, by use of this well understood core, the role of individual and combined features of the outer coordination sphere are starting to be understood at a mechanistic level. Common mechanisms have yet to be defined to predictably control these processes, but our growing knowledge in this area is essential for the eventual mimicry of enzymes by efficient molecular catalysts for practical use.

Synthesis, electrochemical and spectroelectrochemical properties of novel phthalocyanine complexes of manganese, titanium and indium

The new tetra peripheral 3,5-bis(trifluoromethyl)phenylethynyl substituted metallophthalocyanines (MPc) have been synthesized and their aggregative properties were investigated in different concentrations. Voltammetric and in situ spectroelectrochemical characterization of MPcs bearing metal centers with higher oxidation states InIII, MnIII, and TiIV were performed in solution. While InIIIClPc gave only Pc ring based reduction process, MnIIIClPc and TiIVOPc showed metal based reduction reactions in addition to Pc-based redox processes. The presence of redox active metal centers like MnIIICl and TiIVO in the core of Pc ring increases the electrochemical activity of the complexes. The redox responses of the complexes confirmed the submitted structure of the complexes. During the electrochemical measurements, it was noticed that the presence of molecular oxygen in the electrolyte system considerably influenced the redox responses of the complexes due to the interaction of MPcs with O2, which indicated the electrocatalytic and/or electrochemical sensing activities of the complexes for O2 sensing. Electrocatalytic oxygen reduction and oxygen sensing measurements were performed in solution and in solid states as Langmuir-Blodgett multilayer thin films. While InIIIClPc did not give any voltammetric signal for the interaction with molecular oxygen, square wave voltammetric (SWV) responses of MnIIIClPc and TiIVOPc show electrocatalytic activities of the complexes for oxygen reduction reactions (ORR). Moreover, interaction of O2 with these complexes gave extraordinary anodic SWV signals in both of aprotic and protic solvents, which indicate possible usage of the complexes as molecular oxygen sensor.

New water soluble and amphiphilic titanium(IV) phthalocyanines and investigation of electropolymerization properties

Peripherally tetra-substituted oxo-titanium(IV) phthalocyanines bearing (2-{2-[3-(dimethylamino)phenoxy]ethoxy}ethoxy) and (2-{2-[3-(diethylamino)phenoxy]ethoxy}ethoxy) groups and their cationic derivatives have been synthesized. Electrochemical properties of the complexes 5 and 6 were determined with cyclic voltammetry and square wave voltammetry techniques. Electrochemical studies reveal that metallophthalocyanines containing redox active metal center such as Ti(IV) extends the redox richness of the phthalocyanine ring. Although titanium phthalocyanines complexes give common reduction processes, titanium phthalocyanines electropolymerized on the working electrode during anodic and whole potential scans. Electropolymerization mechanism changes with changing the potential window of the electrolyte system.

Electrochromism of the Langmuir–Blodgett films based on monophthalocyanines carrying redox active metal centers

In this study, electrochromic properties of metallophthalocyanine (MPc)-based thin films were investigated in detail with electrochemical, in situ spectroelectrochemical and in situ electrocolorimetric analysis methods. Electrochemical and spectroelectrochemical measurements of monophthalocyanines exhibited that incorporation of redox active metal centers, CoII, FeIIIOAc, and MnIIIOAc, into the phthalocyanine center increases the redox richness of the complex due to the reversible metal-based reduction and oxidation couples of the metal centers. Metal based redox processes of the complexes affect the optical changes more than the Pc based redox process due to the shifting of the existence bands and appearance of new bands in the UV–vis spectrum. Optical changes especially in the visible part of the light spectrum are desired properties for usage of a material in the display technologies. Electrochromic measurements indicate that film formation methods, electrolytes, and nature of the metal ions in the phthalocyanine cavity alter the basic electrochromic parameters of the complexes. Langmuir–Blodgett films of manganese, cobalt, and iron phthalocyanines are found as ideal neutral and anodic coloring electrochromic materials due to their short color change times, long-term stability, and long-term write–erase efficiency.

Electropolymerizable non-ionic and quaternized ionic titanium(IV) phthalocyanines and their electrochemistry

Oxo-titanium(IV) phthalocyanines and their quaternized ionic derivatives substituted with 4-{2-[3-(dimethylamino)phenoxy]ethoxy} and 4-{2-[3-(diethylamino)phenoxy]ethoxy} moieties have been synthesized and their structure have been identified by IR, 1H and 13C NMR, mass and UV–Vis spectroscopy techniques. Voltammetric characterizations of the titanium phthalocyanines were performed with cyclic voltammetry and square wave voltammetry techniques. Electrochemical studies display that redox active metal center Ti(IV)OPc, into the phthalocyanine stone enlarges the redox richness of the phthalocyanine ring. Titanium phthalocyanine complexes bearing dimethylamino and diethylamino lead to electropolymerization of the complexes on the working electrode while anodic potential scans.

Effects of peripheral and nonperipheral substitution to the spectroscopic, electrochemical and spectroelectrochemical properties of metallophthalocyanines

The syntheses, spectroscopic and electrochemical properties of metallo-phthalocyanine complexes, tetra- and octa-substituted with 3,4-(dimethoxyphenylthio) moieties at non-peripheral and peripheral positions were studied. These complexes were characterized by FT-IR, mass spectroscopy, electronic spectroscopy and electrochemical methods. 3,4-(Dimethoxyphenylthio) moieties increase the solubility of the complexes in various polar and nonpolar organic solvents and prevent the aggregation (in the same solvents) within a wide concentration range. Electronic absorption spectroscopy revealed that the presence of electron donating sulphur on synthesized complexes shift the Q-band of the complexes to the higher wavelengths. Redox active metal centers, CoII, FeII and MnIIIOAc into the phthalocyanine core extend the redox worthy of the complexes due to the reversible metal-based reduction and oxidation couples of the metal centers. In situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of each electrogenerated anionic and cationic redox species. Electrochemical and in situ spectroelectrochemical analysis indicate that cobalt and manganese phthalocyanines catalyze the molecular oxygen in aprotic media.

CFA-1: the first chiral metal–organic framework containing Kuratowski-type secondary building units

The novel homochiral metal-organic framework CFA-1 (Coordination Framework Augsburg-1), [Zn5(OAc)4(bibta)3], containing the achiral linker {H2-bibta = 1H,1'H-5,5'-bibenzo[d][1,2,3]triazole}, has been synthesised. The reaction of H2-bibta and Zn(OAc)2·2H2O in N-methylformamide (NMF) (90 °C, 3 d) yields CFA-1 as trigonal prismatic single crystals. CFA-1 serves as a convenient precursor for the synthesis of isostructural frameworks with redox-active metal centres, which is demonstrated by the postsynthetic exchange of Zn(2+) by Co(2+) ions. The framework is robust to solvent removal and has been structurally characterized by synchrotron single-crystal X-ray diffraction and solid state NMR measurements ((13)C MAS- and (1)H MAS-NMR at 10 kHz). Results from MAS-NMR and IR spectroscopy studies are corroborated by cluster and periodic DFT calculations performed on CFA-1 cluster fragments.

Electrochemical and spectroelectrochemical characterization of newly synthesized manganese, cobalt, iron and copper phthalocyanines

Manganese, cobalt, iron, and copper phthalocyanines, tetra-substituted with 3,4-(dimethoxyphenylthio) moieties at peripheral positions, were synthesized and characterized by FT-IR, mass spectroscopy, and electronic spectroscopy. Electrochemical and spectroelectrochemical measurements exhibit that while copper phthalocyanine gives only ring-based electron transfer reactions, incorporating redox active metal centers, CoII, FeIIIAc, and MnIIIAc, into the phthalocyanine core extends the redox richness of the phthalocyanine ring with the metal-based reduction and oxidation couples of the metal centers in addition to the common phthalocyanine ring-based electron transfer processes. In situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species. Presence of O2 in the electrolyte system influences the redox couples of the complexes due to the interaction between O2 and metallophthalocyanines having redox active metal center.

Synthesis, electrochemical and spectroelectrochemical properties of peripherally tetra-imidazole substituted metal free and metallophthalocyanines

The synthesis and characterization of novel organosoluble peripherally tetra imidazole substituted metal-free (4), zinc(II) (5), nickel(II) (6), cobalt(II) (7), lead(II) (8) and oxo-titanium(IV) (9) phthalocyanines are described for the first time in this study. Electrochemical and spectroelectrochemical measurements exhibit that while copper phthalocyanine gives only ring-based electron transfer reactions, incorporating redox active metal centers, Co(II), and Ti(IV)OPc, into the phthalocyanine core extends the redox richness of the phthalocyanine ring with the metal-based reduction and oxidation couples of the metal centers in addition to the common phthalocyanine ring-based electron transfer processes. In-situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species.

Oxidative Cleavage of DNA by Ruthenium(II) Complexes Containing a Ferrocene/Non-Ferrocene Conjugated Imidazole Phenol Ligand

Three mixed-ligand ruthenium(II) complexes with general formula [Ru(bpy)2L](PF6) (1–3), where L = 2-{4,5-bis[(E)-2-ferrocenylvinyl]-1H-imidazol-2-yl}phenol (1), 2-{4,5-bis[(E)-2-ferrocenylvinyl]-1H-imidazol-2-yl)-4,6-dichlorophenol (2), 2-{4,5-bis[(E)-2-(4-chlorophenyl)ethenyl]-1H-imidazol-2-yl}phenol (3), have been synthesized and characterized. All the three complexes bring about DNA cleavage in the presence of H2O2. Due to the presence of three redox-active metal centers in complexes 1 and 2 these two complexes show enhanced DNA cleaving activity in comparison to that exhibited by complex 3, which contains only one redox-active metal center.