Bipyridyl Derivatives Research Articles

The photophysical, photobiological, and light-up calf-thymus-DNA sensor activity of Pd(II) and Pt(II) polypyridyl derivatives coordinated to vinyl-pyridyl corroles

A series of mono-cationic corroles containing bipyridyl derivatives coordinated with Pd(II) and Pt(II) polypyridyl complexes is reported here. These corroles were characterized by microanalysis, UV–Vis absorption, steady-state and time-resolved fluorescence emission, theoretical DFT calculations, and electrochemical techniques. Photobiological parameters, such as photostability and ROS production, were also assessed. Spectroscopic and viscometry studies, along with molecular docking calculations, revealed each corrole derivative's ability to interact with the minor grooves of calf-thymus DNA. Additionally, the potential of these derivatives to act as emissive optical sensors for CT-DNA was evaluated.

Design, synthesis, and evaluation of 2,2'-bipyridyl derivatives as bifunctional agents against Alzheimer's disease.

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disease. Metal ion dyshomeostasis and Aβ aggregation have been proposed to contribute to AD progression.Metal ions can bind to Aβ and promote Aβ aggregation, and ultimately lead to neuronal death.Bifunctional (metal chelation and Aβ interaction) compounds are showing promise againstAD. In this work, eleven new 3,3'-diamino-2,2'-bipyridine derivatives 4a-4k were synthesized, and evaluated as bifunctional agents for AD treatment. In vitro Aβ aggregation inhibition assay confirmed that most of the synthesized compounds exhibited significant self-induced Aβ1-42 aggregation inhibition. Among them, compound 4d displayed the best inhibitory potency of self-induced Aβ1-42 aggregation with IC50 value of 9.4 µM, and it could selectively chelate with Cu2+ and exhibited 66.2% inhibition of Cu2+-induced Aβ1-42 aggregation. Meanwhile, compound 4d showed strong neuroprotective activity against Aβ1-42 and Cu2+-treated Aβ1-42 induced cell damage. Moreover, compound 4d in high dose significantly reversed Aβ-induced memory impairment in mice.

Enantioseparation of new axially chiral carboxylic acids on polysaccharide-based chiral stationary phases under normal phase elution conditions

In the last decade, the availability of new and versatile synthetic strategies for the preparation of substituted 4,4’-bipyridyl derivatives based on chemo- and regioselective functionalization of the 4,4’-bipyridine core has encouraged studies for exploring the bioactivity of these compounds in the fields of drug discovery and medicinal chemistry. In substituted 4,4’-bipyridines, chirality may emerge from restricted rotation induced by sterically hindered atoms or functional groups located around the 4,4’-biaryl bond (chiral axis). The first atropisomeric substituted 4,4’-bipyridine was prepared in 2008, and no asymmetric synthesis to produce pure atropisomers of chiral 4,4’-bipyridine derivatives has been available so far. Thus, in the last few years, our groups developed methods to separate atropisomers of a wide series of 4,4’-derivatives by high-performance liquid chromatography (HPLC) using polysaccharide-based chiral stationary phases (CSPs). In the frame of our interest in this field, we reported herein the synthesis of two new chiral carboxylic acids containing an axially chiral 4,4’-bipyridyl unit as source of chirality, and their HPLC enantioseparation on polysaccharide-based CSPs. In particular, the impact of analyte and CSP structures on the enantioseparation outcomes as well as mechanisms and noncovalent interactions underlying the enantioseparation were explored by using electrostatic potential analysis and molecular dynamics (MD) simulations.

A Series of Metal–Organic Frameworks with 2,2′-Bipyridyl Derivatives: Synthesis vs. Structure Relationships, Adsorption, and Magnetic Studies

Five new metal-organic frameworks based on Mn(II) and 2,2'-bithiophen-5,5'-dicarboxylate (btdc2-) with various chelating N-donor ligands (2,2'-bipyridyl = bpy; 5,5'-dimethyl-2,2'-bipyridyl = 5,5'-dmbpy; 4,4'-dimethyl-2,2'-bipyridyl = 4,4'-dmbpy) [Mn3(btdc)3(bpy)2]·4DMF, 1; [Mn3(btdc)3(5,5'-dmbpy)2]·5DMF, 2; [Mn(btdc)(4,4;-dmbpy)], 3; [Mn2(btdc)2(bpy)(dmf)]·0.5DMF, 4; [Mn2(btdc)2(5,5'-dmbpy)(dmf)]·DMF, 5 (dmf, DMF = N,N-dimethylformamide) have been synthesized, and their crystal structure has been established using single-crystal X-ray diffraction analysis (XRD). The chemical and phase purities of Compounds 1-3 have been confirmed via powder X-ray diffraction, thermogravimetric, and chemical analyses as well as IR spectroscopy. The influence of the bulkiness of the chelating N-donor ligand on the dimensionality and structure of the coordination polymer has been analyzed, and the decrease in the framework dimensionality, as well as the secondary building unit's nuclearity and connectivity, has been observed for bulkier ligands. For three-dimensional (3D) coordination polymer 1, the textural and gas adsorption properties have been studied, revealing noticeable ideal adsorbed solution theory (IAST) CO2/N2 and CO2/CO selectivity factors (31.0 at 273 K and 19.1 at 298 K and 25.7 at 273 K and 17.0 at 298 K, respectively, for the equimolar composition and the total pressure of 1 bar). Moreover, significant adsorption selectivity for binary C2-C1 hydrocarbons mixtures (33.4 and 24.9 for C2H6/CH4, 24.8 and 17.7 for C2H4/CH4, 29.3 and 19.1 for C2H2/CH4 at 273 K and 298 K, respectively, for the equimolar composition and the total pressure of 1 bar) has been observed, making it possible to separate on 1 natural, shale, and associated petroleum gas into valuable individual components. The ability of Compound 1 to separate benzene and cyclohexane in a vapor phase has also been analyzed based on the adsorption isotherms of individual components measured at 298 K. The preferable adsorption of C6H6 over C6H12 by 1 at high vapor pressures (VB/VCH = 1.36) can be explained by the existence of multiple van der Waals interactions between guest benzene molecules and the metal-organic host revealed by the XRD analysis of 1 immersed in pure benzene for several days (1≅2C6H6). Interestingly, at low vapor pressures, an inversed behavior of 1 with preferable adsorption of C6H12 over C6H6 (KCH/KB = 6.33) was observed; this is a very rare phenomenon. Moreover, magnetic properties (the temperature-dependent molar magnetic susceptibility, χp(T) and effective magnetic moments, μeff(T), as well as the field-dependent magnetization, M(H)) have been studied for Compounds 1-3, revealing paramagnetic behavior consistent with their crystal structure.

Enhanced device performance of quantum-dot light-emitting diodes via 2,2′-Bipyridyl ligand exchange

Quantum dots (QDs) are electroluminescent (EL) materials that have been developed as promising emitters in next-generation displays. The amount of charge carriers that travel to the light-emitting layer located at the interface between the hole- and electron-transport layers must be balanced in these displays to achieve maximum luminous efficiency. An existing amine ligand (oleylamine), which was coordinated on the surface of red-emitting InP/ZnSeS/ZnS QDs, was partially exchanged with two 2,2′-bipyridyl derivatives to enhance the EL properties; the QD light-emitting diode (QD-LED) performance was also investigated. Thermogravimetric and 1H nuclear magnetic resonance analyses were used to examine the bipyridyl-ligand-partially-substituted QDs. Photoluminescence spectroscopy and transmission electron microscopy revealed that the emission wavelength (630 nm) and size (7.3 nm) of the QDs were not altered upon ligand exchange. Significant improvements were observed in the electronic properties of QD-LED devices fabricated using the bipyridyl-ligand-substituted QDs. The bipyridyl ligands lowered the charge-injection barrier and improved the charge balance in the QDs. High-performance QD-LED devices were consequently realized with an augmentation of two times in current density, and three times in brightness, external quantum efficiency, and current efficiency.

Fabrication of eco-friendly, low-cost dye sensitized solar cells using harda fruit-based natural dye

The dye-sensitized solar cells (DSSCs) have been eyed as a potential alternative to the expensive silicon-based solar cells. However, most high-performing DSSCs employ ruthenium (Ru) bipyridyl derivatives dye as a photosensitizer, which is costly and toxic. In this research, low-cost eco-friendly alternative dye materials, namely harda fruit-based natural dye and direct dye (fabric dye) were investigated for application in the DSSCs. Field emission scanning electron microscopy, Raman, and X-ray diffraction studies have been performed to gain insight into morphological, structural, and crystalline properties of photosensitizers. The DSSCs based on the natural dye and fabric dye was successfully demonstrated with 3.52% and 0.80% efficiency, respectively. The analysis of photovoltaic parameters reveals that the superior performance of a natural dye-based DSSC results from the better morphological and crystalline properties of the natural dye-based sensitizers. The decent performance shown by a natural dye-based DSSC in the work is expected to propel the research in the area of natural dye-based DSSCs.

Determination of pesticide residues by chromatographic methods for food safety

The paper presents a methodology for the preparation of samples of oilseeds, lettuce leaves, apple fruits for the study of xenobiotics by chromatographic control of the following chemical groups of pesticides: benzimidazole derivatives, anilinopyrimidine derivatives, bipyridyl derivatives. The following processes are considered: homogenisation of the sample, purification of the extract by solid-phase or liquid-liquid extraction methods, obtaining of plant extract, and obtaining of analytes extract. For fine-grained homogenised samples of sunflower seeds, the optimal raw material-extractant ratio is 1:20, for paste-like homogenised samples of apple fruit – 1:10, and for liquid samples of homogenised lettuce – 1:5. Analysis of the distribution of xenobiotics parameter in the octane/water system (log Pow), reference data on dielectric constant and dipole moment of solvents allowed identifying extractants capable of dissolving and removing xenobiotics from raw materials. It has been found that a mixture of acetonitrile and methanol (4:1) should be used for the extraction of benzimidazole derivatives and anilinopyrimidine derivatives, bipyridyl derivatives are best extracted with methanolic trifluoroacetic acid (9.5:0.5). The content of xenobiotics in extracts obtained from samples artificially enriched with xenobiotics was quantitatively analysed. Xenobiotics were removed from samples of crop products containing traces of fat. The most difficult process of sample preparation is the process of extracting sunflower seeds. The content of xenobiotics in extracts obtained from samples artificially enriched with analytes is affected by the temperature at which the process takes place and the duration of extraction. Based on the chemical composition of the sample matrix and the list of analytes, the conditions of the variable component of the methodology are proposed: obtaining plant extracts under the action of selective solvents, homogenised raw material-solvent with constant stirring of the extraction system at a speed of 180-200 rpm, or when exposed to ultrasonic vibrations at a frequency of 37 kHz from 4 °C to 25 °C for 5-25 minutes. Control of the qualitative and quantitative composition of the studied plant extracts and extracts of analytes was studied by methods of high-performance liquid and gas chromatography (liquid and gas) with mass-selective detectors

Resistance to Bipyridyls Mediated by the TtgABC Efflux System in Pseudomonas putida KT2440.

Resistance-nodulation-division (RND) transporters are involved in antibiotic resistance and have a broad substrate specificity. However, the physiological significance of these efflux pumps is not fully understood. Here, we have investigated the role of the RND system TtgABC in resistance to metal ion chelators in the soil bacterium Pseudomonas putida KT2440. We observed that the combined action of an RND inhibitor and the chelator 2,2'-bipyridyl inhibited bacterial growth. In addition, the deletion of ttgB made the strain susceptible to 2,2'-bipyridyl and natural bipyridyl derivatives such as caerulomycin A, indicating that TtgABC is required for detoxification of compounds of the bipyridyl family. Searching for the basis of growth inhibition by bipyridyls, we found reduced adenosine triphosphate (ATP) levels in the ttgB mutant compared to the wild type. Furthermore, the expression of genes related to iron acquisition and the synthesis of the siderophore pyoverdine were reduced in the mutant compared to the wild type. Investigating the possibility that 2,2'-bipyridyl in the ttgB mutant mediates iron accumulation in cells (which would cause the upregulation of genes involved in oxidative stress via the Fenton reaction), we measured the expression of genes coding for proteins involved in intracellular iron storage and the response to oxidative stress. However, none of the genes was significantly upregulated. In a further search for a possible link between 2,2'-bipyridyl and the observed phenotypes, we considered the possibility that the ion chelator limits the intracellular availability of metabolically important metal ions. In this context, we found that the addition of copper restores the growth of the ttgB mutant and the production of pyoverdine, suggesting a relationship between copper availability and iron acquisition. Taken together, the results suggest that detoxification of metal chelating compounds of the bipyridyl family produced by other bacteria or higher ordered organisms is one of the native functions of the RND efflux pump TtgABC. Without the efflux pump, these compounds may interfere with cell ion homeostasis with adverse effects on cell metabolism, including siderophore production. Finally, our results suggest that TtgABC is involved in resistance to bile salts and deoxycholate.

Dehydrogenative Synthesis of 2,2′‐Bipyridyls through Regioselective Pyridine Dimerization

2,2'-Bipyridyls have been utilized as indispensable ligands in metal-catalyzed reactions. The most streamlined approach for the synthesis of 2,2'-bipyridyls is the dehydrogenative dimerization of unfunctionalized pyridine. Herein, we report on the palladium-catalyzed dehydrogenative synthesis of 2,2'-bipyridyl derivatives. The Pd catalysis effectively works with an AgI salt as the oxidant in the presence of pivalic acid. A variety of pyridines regioselectively react at the C2-positions. This dimerization method is applicable for challenging substrates such as sterically hindered 3-substituted pyridines, where the pyridines regioselectively react at the C2-position. This reaction enables the concise synthesis of twisted 3,3'-disubstituted-2,2'-bipyridyls as an underdeveloped class of ligands.

Dehydrogenative Synthesis of 2,2′‐Bipyridyls through Regioselective Pyridine Dimerization

Abstract2,2′‐Bipyridyls have been utilized as indispensable ligands in metal‐catalyzed reactions. The most streamlined approach for the synthesis of 2,2′‐bipyridyls is the dehydrogenative dimerization of unfunctionalized pyridine. Herein, we report on the palladium‐catalyzed dehydrogenative synthesis of 2,2′‐bipyridyl derivatives. The Pd catalysis effectively works with an AgI salt as the oxidant in the presence of pivalic acid. A variety of pyridines regioselectively react at the C2‐positions. This dimerization method is applicable for challenging substrates such as sterically hindered 3‐substituted pyridines, where the pyridines regioselectively react at the C2‐position. This reaction enables the concise synthesis of twisted 3,3′‐disubstituted‐2,2′‐bipyridyls as an underdeveloped class of ligands.

Phosphonate-Substituted Ruthenium(II) Bipyridyl Derivative as a Photoelectrochemical Probe for Sensitive and Selective Detection of Mercury(II) in Biofluids.

A ruthenium(II) bipyridyl derivative photoelectrochemical probe, Ru-1, is synthesized and coupled with TiO2 nanoparticles (Ru-1/TiO2) for the specific recognition and highly sensitive photoelectrochemical (PEC) detection of Hg2+ in a series of biofluids. The probe is designed with a chromophore, a thiocyanate recognition unit, a π-conjugated photoelectron-transfer pathway, and a phosphonate anchor. TiO2 nanoparticles with strong affinity to phosphonate and suitable conduction band energy are used as intermediate layers to increase the Ru-1 adsorption amount and amplify the photocurrent response. Under irradiation, the Ru-1/TiO2/fluorine-doped tin oxide (FTO), with strong visible light-harvesting capacity, aqueous stability, and efficient photoelectron transfer, shows a high and stable photocurrent response. In the presence of Hg2+, however, the specific Hg2+ and NCS coordination changes the photophysical properties of Ru-1, imposing the probe with a wider band gap, a weaker absorbance, and a poorer photoelectron and hole separation efficiency, thus resulting in a significant photocurrent decrease. On the basis of the Hg2+-induced photocurrent change, the Ru-1/TiO2/FTO shows good selectivity and high sensitivity toward the PEC detection of Hg2+, with wide linear ranges from 10-12 to 10-7 and 10-7 to 10-3 g/mL, and a low limit of detection of 0.63 pg/mL. The PEC probe is recyclable and accurate for selective detection of Hg2+ in urine, serum, and cell extracts. The whole analysis can be completed within 15 min. These good analytical performances indicate that the PEC method might have great potential for the onsite detection of small molecules in biosystems.

A Family of Rhodium Complexes with Selective Toxicity toward Mismatch Repair-Deficient Cancers.

Rhodium metalloinsertors are a unique set of metal complexes that bind specifically to DNA base pair mismatches in vitro and kill mismatch repair (MMR)-deficient cells at lower concentrations than their MMR-proficient counterparts. A family of metalloinsertors containing rhodium-oxygen ligand coordination, termed "Rh-O" metalloinsertors, has been prepared and shown to have a significant increase in both overall potency and selectivity toward MMR-deficient cells regardless of structural changes in the ancillary ligands. Here we describe DNA-binding and cellular studies with the second generation of Rh-O metalloinsertors in which an ancillary ligand is varied in both steric bulk and lipophilicity. These complexes, of the form [Rh(L)(chrysi)(PPO)]2+, all include the O-containing PPO ligand (PPO = 2-(pyridine-2-yl)propan-2-ol) and the aromatic inserting ligand chrysi (5,6-chrysene quinone diimine) but differ in the identity of their ancillary ligand L, where L is a phenanthroline or bipyridyl derivative. The Rh-O metalloinsertors in this family all show micromolar binding affinities for a 29-mer DNA hairpin containing a single CC mismatch. The complexes display comparable lipophilic tendencies and p Ka values of 8.1-9.1 for dissociation of an imine proton on the chrysi ligand. In cellular proliferation and cytotoxicity assays with MMR-deficient cells (HCT116O) and MMR-proficient cells (HCT116N), the complexes containing the phenanthroline-derived ligands show highly selective cytotoxic preference for the MMR-deficient cells at nanomolar concentrations. Using mass spectral analyses, it is shown that the complexes are taken into cells through a passive mechanism and exhibit low accumulation in mitochondria, an off-target organelle that, when targeted by parent metalloinsertors, can lead to nonselective cytotoxicity. Overall, these Rh-O metalloinsertors have distinct and improved behavior compared to previous generations of parent metalloinsertors, making them ideal candidates for further therapeutic assessment.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes.

The electrochemical behavior of fac-[Mn(pdbpy)(CO)3 Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO2 reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO2 -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO2 atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported MnI catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO2 reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)3 Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO2 .

Synthesis and properties of new mononuclear Ru(II)-based photocatalysts containing 4,4'-diphenyl-2,2'-bipyridyl ligands.

The synthesis and characterisation of a series of eleven new 4,4'-diphenyl-2,2'-bipyridyl derivatives (N^N) with varying 4- or 3,5-substituents on the phenyl rings are reported. The molecular structures of two of these compounds, 4,4'-bis(3,5-diheptyloxyphenyl)-2,2'-bipyridyl and 4,4'-bis(3,5-di-4,4,5,5,5-pentafluoropentyloxyphenyl)-2,2'-bipyridyl are confirmed by X-ray crystallography. Fourteen neutral complexes trans-Ru(II)Cl2(N^N)(CO)2 are prepared by reacting the new proligands and three known ones with the polymeric precursor [Ru(II)Cl2(CO)2]n, and their optical and electrochemical properties are studied. Density functional theory (DFT) and time-dependent DFT calculations have been carried out on selected complexes in order to rationalise their electronic structures and absorption properties. The low energy electronic absorption bands have metal-to-ligand charge-transfer and ligand-to-ligand charge-transfer (LLCT) character, but these dominate only in the 4-substituted species. In the 3,5-disubstituted ones, intraligand charge-transfer (ILCT) within the bpy ligands becomes more important. The complexes show only irreversible reductive electrochemistry and no signs of polymerisation. The photocatalytic oxidation behaviour of selected trans-Ru(II)Cl2(N^N)(CO)2 complexes and their mono-aquo derivatives [Ru(II)(H2O)(CO)2Cl(N^N)](+) with 4-methoxybenzyl alcohol is studied. Turnover numbers of up to ca. 130 are achieved when using [Ru(II)(bpy)3](2+) (bpy = 2,2'-bipyridyl) as photosensitizer and [Co(III)(NH3)5Cl](2+) as a sacrificial oxidant.

Trans-(Cl)-[Ru(5,5'-diamide-2,2'-bipyridine)(CO)2 Cl2 ]: Synthesis, Structure, and Photocatalytic CO2 Reduction Activity.

A series of trans-(Cl)-[Ru(L)(CO)2 Cl2 ]-type complexes, in which the ligands L are 2,2'-bipyridyl derivatives with amide groups at the 5,5'-positions, are synthesized. The C-connected amide group bound to the bipyridyl ligand through the carbonyl carbon atom is twisted with respect to the bipyridyl plane, whereas the N-connected amide group is in the plane. DFT calculations reveal that the twisted structure of the C-connected amide group raises the level of the LUMO, which results in a negative shift of the first reduction potential (Ep ) of the ruthenium complex. The catalytic abilities for CO2 reduction are evaluated in photoreactions (λ>400 nm) with the ruthenium complexes (the catalyst), [Ru(bpy)3 ](2+) (bpy=2,2'-bipyridine; the photosensitizer), and 1-benzyl-1,4-dihydronicotinamide (the electron donor) in CO2 -saturated N,N-dimethylacetamide/water. The logarithm of the turnover frequency increases by shifting Ep a negative value until it reaches the reduction potential of the photosensitizer.

A novel shell-like supramolecular assembly of 4,4'-bipyridyl derivatives and a twisted cucurbit[14]uril molecule.

This work showed that the 4,4'-bipyridyl group and alkyl chains of 4,4'-bipyridyl derivatives are completely located in the shell-like cavity of the twisted cucurbit[14]uril molecule and formed novel shell-like 1 : 1 inclusion complexes. As it is enthalpy-driven the complexation benefits from ion-dipole interactions.

Rhodamine-Appended Bipyridine: XOR and OR Logic Operations Integrated in an Example of Controlled Metal Migration.

A new bipyridyl derivative 1 bearing rhodamine B as visible fluorophore was designed, synthesized and characterized as a fluorescent and colorimetric sensor for metal ions. Interaction with Cu2+, Zn2+, Cd2+, Hg+, and Hg2+ ions was followed by UV/Vis and emission spectroscopy. Upon addition of these metal ions, different colorimetric and fluorescent responses were observed. “Off-on-off” (Cu2+, Zn2+, and Hg2+) and “off-on” (Hg+ and Cd2+) systems were obtained. Probe 1 was explored to mimic XOR and OR logic operations for the simultaneous detection of Hg+–Cu2+ and Hg+–Zn2+ pairs, respectively. DFT calculations were also performed to gain insight into the lowest-energy gas-phase conformation of free receptor 1 as well as the atomistic details of the coordination modes of the various metal ions.

Cu II ‐Templated Threading of a Bis‐amide‐tris‐amine Macrocycle by Substituted 2,2′‐Bipyridyl Derivatives Assisted by Strong π–π Stacking and Second‐Sphere H‐Bonding Interactions (Eur. J. Inorg. Chem. 12/2014)

The cover picture shows a “wheel” that is composed of (1) a flexible tridentate metal-chelating site, (2) two parallel arene moieties that have potential for aromatic π–π stacking interactions with the aromatic part of the axle unit, and (3) a bis-amide functionality that can engage in hydrogen-bonding interactions with the suitable functional groups on the axle. The metal ion coordination and other two noncovalent interactions are operative simultaneously to obtain the maximum benefit of the templating strategy in the formation of the pseudorotaxane. Details are discussed in the article by P. Ghosh et al. on p. 2029 ff. For more on the story behind the cover research, see the Cover Profile.

Cu II ‐Templated Threading of a Bis‐amide‐tris‐amine Macrocycle by Substituted 2,2′‐Bipyridyl Derivatives Assisted by Strong π–π Stacking and Second‐Sphere H‐Bonding Interactions

Invited for the cover of this issue is the group of Pradyut Ghosh at the Indian Association for the Cultivation of Science (IACS). The cover image shows the [2]pseudorotaxane architecture in which metal coordination, aromatic π–π stacking, and second-sphere hydrogen-bonding interactions operate between the wheel and the axle simultaneously.

CuII‐Templated Threading of a Bis‐amide‐tris‐amine Macrocycle by Substituted 2,2′‐Bipyridyl Derivatives Assisted by Strong π–π Stacking and Second‐Sphere H‐Bonding Interactions

AbstractDerivatives of 5,5′‐substituted 2,2′‐bipyridine (BPy) ligands with bromo‐terminated aliphatic side chains with ester functionalities (BPy5Br), hydroxy‐terminated aliphatic side chains with amide functionalities (BPy5OH), and benzyl‐terminated side chains with amide functionalities (BPy5Bnz) were synthesized. The single‐crystal X‐ray structure analysis of the axle BPy5Br shows that it has a linear rodlike structure with a length of 15.75 Å. The above three ligands along with a methyl‐substituted BPy ligand (BPy5Me) are explored as axles for CuII‐templated threading of a bis‐amide‐tris‐amine macrocycle wheel (MC). High‐yield (70–80 %) syntheses of the [2]pseudorotaxanes PRT1, PRT2, PRT3, and PRT4 are obtained by reaction of MC with the axles BPy5Me, BPy5Br, BPy5OH, and BPy5Bnz, respectively, in the presence of CuII ions. These complexes were characterized by ESI‐MS, UV/Vis and EPR spectroscopy, and single‐crystal X‐ray diffraction studies. The solution‐state binding stoichiometry and the association constants for complex formation between MC–Cu2+ and the axles were studied by UV/Vis absorption titrations. Sigma‐fit curves indicate 1:1 stoichiometry, and the association constants range from 1.2 × 103 to 2.6 × 103 M–1. Single‐crystal X‐ray structure analysis of PRT1, PRT2, and PRT4 confirms the threading of MC by the respective axles, which are coordinated to the CuII center with Cu–N bond lengths of 1.992–2.203 Å. Interestingly, strong aromatic π–π stacking interactions between two parallel arene moieties of MC and the pyridyl unit of the axle with π–π interaction distances of 3.402–3.618 Å are observed in all three [2]pseudorotaxanes. However, for PRT4, a strong hydrogen‐bonding interaction is also operative between the carbonyl oxygen atom of BPy5Bnz and the amide proton of MC with a N–H···O distance of 2.200 Å. Thus, multiple template interactions such as metal coordination, π–π stacking, and second‐sphere hydrogen‐bonding interactions between MC and the axles play crucial roles for facile threading and high‐yield syntheses of [2]pseudorotaxanes.