Bipyridinium Salts Research Articles

Re-assessing viologens for modern bio-electrocatalysis.

Viologens, 1,1'-disubstituted-4,4'-bipyridinium salts, are organic redox species that can be used in place of NADPH as mediators for redox enzymes. In this study, using the reduction of oxidized glutathione by glutathione reductase as a model system, a rationally designed library of viologens covering a range of polarities and functional groups were explored as electron transfer mediators for bio-electrocatalysis. Through a series of electrochemical investigations, the reduction potential was found to be the primary determining factor for electron transfer between the viologen and enzyme. Through enhancing the solubility of viologen such that the fully reduced state remained soluble, we demonstrate a much-widened window of useable viologen potentials. In doing so, we describe for the first time a highly efficient electron transfer to a flavoenzyme promoting the catalytic reaction in the absence of co-factors. As such, our study provides a platform for broadening the scope for using viologens as mediating agents for electrochemically-driven enzymatic processes.

4,4′,4″-(Benzene-1,3,5-triyltris(ethyne-2,1-diyl))tris(1-methylpyridin-1-ium) Iodide

Despite having been known for a long time, quaternary 4,4′-bipyridinium salts, or viologens, are still a highly inspiring class of compounds, thanks to their peculiar redox and charge transfer properties. However, more complex structures containing multiple pyridinium rings, also interspaced by conjugated moieties, allow an even wider synthetic variability and tunability of their characteristics. The compound described herein is a star-shaped, fully conjugated molecule with three methylated pyridinium rings connected by a triple bond spacer to a central benzene core, which was synthesized from readily available building blocks, representing a quite simple model of multi-pyridyl extended viologen; its UV–visible absorption and fluorescence spectra have also been investigated.

Coupled Electrochemical Reduction of CO2 and H+ in the Presence of Substituted 2,2'-Bipyridinium Salts

Coupled Electrochemical Reduction of CO2 and H+ in the Presence of Substituted 2,2'-Bipyridinium Salts

Multi-Responsive Molecular Encapsulation and Release Based on Hydrogen-Bonded Azo-Macrocycle

Research on stimuli-responsive host–guest systems is at the cutting edge of supramolecular chemistry, owing to their numerous potential applications such as catalysis, molecular machines, and drug delivery. Herein, we present a multi-responsive host–guest system comprising azo-macrocycle 1 and 4,4′-bipyridinium salt G1 for pH-, photo-, and cation- responsiveness. Previously, we reported a novel hydrogen-bonded azo-macrocycle 1. The size of this host can be controlled through light-induced E↔Z photo-isomerization of the constituent azo-benzenes. The host is found in this work to be capable of forming stable complexes with bipyridinium/pyridinium salts, and implementing guest capture and release with G1 under light in a controlled manner. The binding and release of the guest in the complexes can also be easily controlled reversibly by using acid and base. Moreover, the cation competition-induced dissociation of the complex 1a2⊃G1 is achieved. These findings are expected to be useful in regulating encapsulation for sophisticated supramolecular systems.

A Joint Study of Experimental and Computational Chemistry to Probe the Effect of Coordination Modes on the Luminescence of Uranyl‐Organic Coordination Polymers

AbstractDespite the potential application of uranyl‐organic coordination polymers (UOCPs) in detecting metal cations and radiation rays, their luminescence properties have not been sufficiently studied at the molecular level. Herein, we synthesized a series of UOCPs (UOCP1–9) with new construction based on bipyridinium salts and diverse auxiliary ligands. The physicochemical properties of the complexes with high purity were systematically characterized, especially the luminescence spectra. A deep theoretical investigation was conducted to illustrate the relation between uranyl coordination and the luminescence spectral property. The results show that the influence of ligand type on the uranyl luminescent spectral properties increases in the order of hydroxide>oxalate>aromatic carboxylate, which can be attributed to the magnitude of orbital interaction between uranyl and ligand. This work helps to profoundly understand uranyl optical properties and electronic structure at the molecular level, providing important hints for recognizing the nature of luminescent spectral features of uranyl‐containing materials, as well as the fundamental chemistry of actinide elements.

Harnessing Protonated 2,2'-Bipyridinium Salts as Powerful Brønsted Acid Catalysts in Organic Reactions.

It is the first time that the readily available protonated 2,2'-bipyridinium salts are used as Brønsted acid catalysts to accelerate a series of organic transformations that included the hydration of aromatic alkynes, etherification of alcohols, cyclotrimerization of aliphatic aldehydes, Ritter reaction, Mannich reaction, Biginelli reaction, preparation of substituted alkenes from alcohols, synthesis of spirooxindole, bisindolylmethane, and noncyclized tetraketone with good to excellent yields. These results strongly suggest that there exists enormous potentiality in the development of the protonated 2,2'-bipyridinium catalytic system.

Supramolecular vesicle engineering by regulating the assembly of shape-persistent aromatic-hydrazone macrocycles

The size of supramolecular vesicles is manipulated by regulating the assembly of macrocycles with different mono- or bi-pyridinium salts.

Investigation of Structural Characteristics by FTIR Spectroscopy of Chitosan Derivative with N Heterocyclic Compound

Chitosan and chitosan derivatives are biomaterials of great scientific interest for biomedical applications although some of the properties are limited as their solubility in the aqueous medium. N-heterocyclic compounds are of interest for chitosan alkylation and new derivatives can be obtained by modification of his structure resulting thus new structures of biomedical interest Chitosan samples from two sources were analysed, in which the dissociation is favoured in the aqueous medium by the presence of a bipyridine salt, N,N'-bis(phenacyl)-4,4'-bipyridinium dibromide. The synthesis of chitosan’ derivate was conducted in aqueous medium at 60 °C, by magnetic stirring for 24 hours. The dissociation is better and reduced the pH, to values lower than pH 6, which suggests the achievement of favourable structural arrangements between chitosan and dibromide bipyridinium. The FTIR spectra parameters indicate the presence of characteristic bands of the bipyridinium salt in the chitosan derivates structures, with structural differences in their composition.

Mechanical Bond-Assisted Full-Spectrum Investigation of Radical Interactions.

Molecular recognition, based on noncovalent bonding interactions, plays a central role in directing supramolecular phenomena in both chemical and biological environments. The identification and investigation of weakly associated recognition motifs, however, remains a major challenge, especially when the motifs are interlinked with and obscured by other robust binding modes in complicated systems. For example, although the host-guest recognition between the radical cations of both cyclobis(paraquat-p-phenylene) (CBPQT) and 4,4'-bipyridinium (BIPY) salts has been thoroughly investigated, the question of whether other binding modes exist between these two positively charged entities is the subject of some debate because of the complexity and dynamic nature of this supramolecular system. In order to address this conundrum, we have synthesized a [2]catenane─formed by mechanical interlocking between CBPQT and another BIPY-containing ring─which enhances the weak interactions between components and reduces significantly the complexity of the system for easier characterization. By employing this [2]catenane as a model compound, we have performed a full-spectrum investigation of radical interactions and revealed unambiguously a total of three possible binding modes between CBPQT and BIPY─to be specific, a bisradical tetracationic, a trisradical tricationic, and a bisradical dicationic association─as demonstrated by various methods of characterization including UV/vis/NIR, EPR, and NMR spectroscopies, electrochemical measurements and X-ray crystallography. The two newly discovered bisradical binding modes have potential applications in the construction of self-assembled materials and in mediating supramolecular catalysis. The mechanical bond-assisted approach used in this research is broadly applicable to investigating noncovalent bonding interactions.

Mechanism of Formate Dehydrogenase Catalyzed CO2 Reduction with the Cation Radical of a 2,2′-Bipyridinium Salt Based on a Theoretical Approach

Abstract The visible-light driven redox system of an electron donor, a photosensitizer, an electron mediator and formate dehydrogenase from Candida boidinii (CbFDH; EC.1.2.1.2) as a catalyst has received much attention for CO2 reduction to formate. In this system, elucidation of the electron transfer process between the electron mediator and CbFDH is important for improving efficiency of CO2 reduction to formate. 2,2′-Bipyridinium salt (2,2′-BP2+) is a remarkable electron mediator for CbFDH-catalyzed CO2 reduction. Especially, the direct interaction between cation radicals of 2,2′-BPs (2,2′-BP+•s) and CbFDH is a significant factor for the CO2 reduction. By using 1,1′-ethylene-2,2′-bipyridinium salt (DB2+), 1,1′-trimethylene-2,2′-bipyridinium salt (TB2+), 1,1′-tetramethylene-2,2′-bipyridinium salt (QB2+) and 1,1′-dimethyl-2,2′-bipyridinium salt (DM2+), 2,2′-BP+• with a small dihedral angle between the two pyridine rings (DB or TB) accelerates the CbFDH-catalyzed CO2 reduction. This work illuminated the direct interaction of the cation radicals of DB, TB, QB and DM in the substrate-binding site of CbFDH on the basis of a docking-simulated prediction. Moreover, the electron transfer process from the cation radicals of DB, TB, QB and DM to CO2 in the CbFDH was investigated based on the energy of the molecular orbital calculated by density functional theory (DFT). From these results, it can be predicted that efficient CO2 reduction to formate can be achieved by suppressing the three-dimensional structural change between the 2,2′-BP dication and the cation radical in the substrate-binding pocket of CbFDH as much as possible.

Thioureidocalix[6]arenes Pseudorotaxanes

AbstractHeteroditopic calix[6]arenes are extensively exploited as synthetic receptors and as molecular components of supramolecular threaded and interlocked structures. We describe the synthesis of two novel calix[6]arene macrocycles, the upper rim of which is functionalized with either two or three phenylthioureido groups. These moieties are excellent hydrogen‐bond donors and largely affect the conformation of the macrocycle, both in solution and in the solid state. Alternate conformers are observed in solution, and they give rise to supramolecular chains in the solid state. Moreover, and because of this conformational freedom, also the recognition properties toward the formation of pseudorotaxanes with bipyridinium‐based axles are strongly affected. The number of phenylthioureido units of the macrocyclic host and the nature of the counterions of the bipyridinium guest determine the stability of the complexes. NMR, UV‐Vis and electrochemical measurements revealed that very stable 1 : 1 inclusion complexes are formed between the triphenylthioureido calix[6]arene and a bipyridinium guest with tosylate counterions, whereas no complex is observed between the diphenylthioureido macrocycle and a hexafluorophosphate bipyridinium salt.

Insight into the Modes of Activation of Pyridinium and Bipyridinium Salts in Non‐Covalent Organocatalysis

AbstractA series of pyridinium and bipyridinium salts were prepared and their catalytic properties were evaluated in the aza‐Diels‐Alder reaction between imines and Danishefsky diene. Depending on the substituents of the pyridinium/bipyridinium rings and on the nature of the counterion, two mechanisms of activation were demonstrated. In case of non‐substituted rings, the substrate is activated through charge transfer involving the aryl ring on the C‐side of the imine. When halogen atoms were introduced on the catalysts, the activation mode switched to halogen bond involving the imine nitrogen lone pair. Moreover, alternative activation modes based on hydrogen bonding and radical cation were ruled out. This work allowed us to develop two families of catalysts whose potential was demonstrated in the cycloaddition of various imines with Danishefsky diene. The first family is composed of the simple methyl pyridinium triflate and dioctyl bipyridinium triflate. The former is active only with imines bearing a p‐methoxyphenyl group on the C‐side and the latter was found to be efficient with imines bearing different substituents on both the N‐ and C‐sides of the imines. The second family is based on halogenated pyridinium salts which proved active with almost all considered imines.magnified image

Chelidamic acid derivatives: Precursors to functionalized pyridyl cryptands & functionalized metal ligands

Pyridyl cryptands, such as 12–16 formed from pyridine-2,6-dicarbonyl chloride (4) and crown ether diols 7–11, have been demonstrated to be excellent hosts for viologens (N,N′-dialkyl-4,4′-bipyridinium salts, 2) with Ka values of ∼104 to 106 M−1 and thus are considered to be ideal building blocks for supramolecular systems. However, in order to incorporate these host motifs into useful starting materials, functionalization is required; in this article we describe syntheses of 23 new chelidamic acid (2,6-dicarboxy-4-hydroxypyridine, 5) derivatives that can be used to prepare functional pyridyl cryptands and functional metal ligands. The preparation and characterization of three new functionalized pyridyl cryptands are also included.

Visible‐Light Driven CO2 Reduction to Formate with Electron Mediated Nicotinamide‐Modified Viologen in the System of Water‐Soluble Zinc Porphyrin and Formate Dehydrogenase

AbstractVisible‐light driven CO2 reduction to formate with the system consisting of water‐soluble zinc tetraphneylporphyrin tetrasulfonate (ZnTPPS), formate dehydrogenase from Candida boidinii (CbFDH) and 1‐nicotinamidethy‐1’‐methyl‐4,4’‐ bipyridinium salt (NEMBP) in the presence of triethanolamine (TEOA) as an electron donor was investigated. NEMBP was prepared as the 4,4’‐BP with the nicotinamide‐group to promote the improvement of affinity with CbFDH. The properties of NEMBP were characterized by photochemical and electrochemical methods. In order to evaluate the affinity between cation radical of NEMBP (NEMBP+.), as a co‐enzyme and CbFDH, the energy level based on simple docking simulation and density functional theory were estimated. It was speculated that NEMBP+. could bind near the substrate binding pocket of CbFDH as well as NAD+ by docking simulation. In the visible‐light driven NEMBP reduction with the sensitization of ZnTPPS in the presence of TEOA, the yield for reduction of NEMBP to NEMBP+. was estimated to be 80%. In the CO2 reduction to formate with the system consisting of TEOA, ZnTPPS, NEMBP and CbFDH, the turnover numbers of NEMBP and CbFDH were estimated to be 0.14 and 1.53 h−1, respectively.

Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties

Pagoda[5]arene (P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid ...

Redox Property Tuning in Bipyridinium Salts.

Bipyridinium salts are currently very popular due to their perspective applications in redox flow batteries. Hence, we designed and prepared a series of bipyridiniums based on 2,2′-, 3,3′-, and 4,4′-bipyridine and 2,2′-bipyrimidine. The straightforward synthesis utilizes commercially or readily available starting compounds and their direct N-alkylation, mostly using 1,3-propanesultone. All eleven target derivatives with systematically evolved structure were investigated by cyclic voltammetry, which allowed elucidating thorough structure-property relationships. The electrochemical behavior depends primarily on the parent scaffold, type of N-alkylation, number of quaternized nitrogen atoms, planarity, counter ion as well as the used media. Two derivatives featuring quasi-reversible redox processes were further tested on rotating disc electrode and in a flow battery half-cell. 4,4′-Bipyridinium derivative bearing two sultone residues showed better performance and stability in the flow half-cell with small capacity decays of 0.09/0.15% per reduction-oxidation cycle, based on the number of the utilized redox processes (one/two).

Visible-light driven reduction of CO2 to formate by a water-soluble zinc porphyrin and formate dehydrogenase system with electron-mediated amino and carbamoyl group-modified viologen

Visible-light-driven CO2 reduction to formate with a system consisting of water-soluble zinc porphyrin, formate dehydrogenase from Candida boidinii and 1-amino-1′-carbamoyl-4,4′-bipyridinium salt as an electron mediator in the presence of triethanolamine was developed.

π-Donor/π-Acceptor Interactions for the Encapsulation of Neurotransmitters on Functionalized Polysilicon-Based Microparticles.

Bipyridinium salts, commonly known as viologens, are π-acceptor molecules that strongly interact with π-donor compounds, such as porphyrins or amino acids, leading their self-assembling. These properties have promoted us to functionalize polysilicon microparticles with bipyridinium salts for the encapsulation and release of π-donor compounds such as catecholamines and indolamines. In this work, the synthesis and characterization of four gemini-type amphiphilic bipyridinium salts (1·4PF6–4·4PF6), and their immobilization either non-covalently or covalently on polysilicon surfaces and microparticles have been achieved. More importantly, they act as hosts for the subsequent incorporation of π-donor neurotransmitters such as dopamine, serotonin, adrenaline or noradrenaline. Ultraviolet-visible absorption and fluorescence spectroscopies and high-performance liquid chromatography were used to detect the formation of the complex in solution. The immobilization of bipyridinium salts and neurotransmitter incorporation on polysilicon surfaces was corroborated by contact angle measurements. The reduction in the bipyridinium moiety and the subsequent release of the neurotransmitter was achieved using ascorbic acid, or Vitamin C, as a triggering agent. Quantification of neurotransmitter encapsulated and released from the microparticles was performed using high-performance liquid chromatography. The cytotoxicity and genotoxicity studies of the bipyridinium salt 1·4PF6, which was selected for the non-covalent functionalization of the microparticles, demonstrated its low toxicity in the mouse fibroblast cell line (3T3/NIH), the human liver carcinoma cell line (HepG2) and the human epithelial colorectal adenocarcinoma cell line (Caco-2).

Ion-Pair Selective Conformational Rearrangement of Sulfonamide Calix[6]arene-Based Pseudorotaxanes.

We describe the synthesis of a new class of trisulfonamide calix[6]arene-based wheels that can bind dialkylviologen salts, in apolar media. The threading process occurs through a selective ion-pair recognition, established by the sulfonamide groups with the counterions of the bipyridinium salts, that dictates a conformational rearrangement of the corresponding pseudorotaxanes.

Star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts with good electrochromic switching stability

In this study, a series of novel star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts with various alkyl, alkenyl, and aryl groups were designed and synthesized, and their electrochemical, electro-optical, and electrochromic properties were investigated. The electrochromic properties such as different colored states (sky blue versus purple), reasonable contrast, coloration efficiency, and, in particular, the switching stabilities of electrochromic devices based on star-shaped monosubstituted 2,6-diphenyl-4,4′-bipyridinium salts were better than those of devices based on linear monosubstituted pyridinium salts. All the compounds with alkyl, alkenyl, and aryl groups showed good switching stability and the change in the optical contrast after 1000 cycles was small (<5%). However, N-substituents significantly affected the switching stability of electrochromic devices based on linear monosubstituted 4,4′-bipyridine derivatives.