Pyridine Complexes Research Articles

Chirality Generated by Hindered Rotations in Platinum(II)‐Pyridine Complexes

AbstractThe reaction oftrans‐[Pt(DMSO)2MeCl] with 2‐alkylsubstituted pyridines pyRaffords a series of complexes of the type [Pt(pyR)(DMSO)MeCl]. These complexes are rare examples of organometallic square‐planar complexes having four different ligands. Of the three possible geometric isomers, only thetrans(N,S)‐[Pt(pyR)(DMSO)MeCl] was observed in solution and isolated in the solid state. However, the lack of chirality typical to square planar geometry can be resolved in several ways one of which is described in this paper.1H NMR studies reveal diastereotopicity of the DMSO methyls, as well as the CH2protons in the ethyl‐ and neopentyl‐pyridine complexes and the methyls in the isopropyl‐pyridine derivative. This observation can be ascribed to restricted rotations around the Pt−N bond, thus resulting in the presence of planar chirality for these complexes. Additionally, large downfield shifts in the1H NMR spectra of the α C−H protons in the pyridine substituents suggested the presence of Pt⋯H−C interactions, confirmedviasingle crystal X‐ray studies in the case of the isopropyl complex [Pt(pyip)(DMSO)MeCl] (4), which shows an apical Pt⋯H interaction with a short 2.585 Å distance. DFT calculations shed light into the stability of these complexes and the influence of the pyridine‐substituent on their chiral configuration.

Phosphinine vs Pyridine in Luminescent CuI Complexes and Application in Lifetime‐Based Molecular Thermometer

AbstractLuminescent CuI complexes have attracted significant interest due to their adjustable emission properties, diverse structures, and reasonable cost. In this work, two class of complexes, namely phosphinine and pyridine ligated CuI complexes that differ by only one atom, were synthesized and characterized. Photophysical analysis and theoretical studies reveal an emissive phosphinine‐localized triplet states for CuI phosphinine complexes, and a temperature‐dependent interplay between metal‐to‐ligand charge‐transfer (MLCT) singlet and triplet states for CuI pyridine complexes. In general, the CuI phosphinine complexes exhibit a longer lifetime and greater temperature‐dependent lifetime changes than the pyridine complexes. A molecular thermometer incorporating CuI phosphinine complexes 2 c as indicator was fabricated. This thermometer exhibits a rare linear correlation between temperature and lifetime ranging from 77–297 K with a high sensitivity of −13.5 μs K−1.

Configurationally regulated half-sandwich iridium(III)-ferrocene heteronuclear metal complexes: Potential anticancer agents

Half-sandwich iridium(III) (IrIII) complexes and ferrocenyl (Fc) derivatives are becoming the research hotspot in the field of anticancer because of their good bioactivity and unique anticancer mechanism different from platinum-based drugs. Then, a series of half-sandwich IrIII-Fc pyridine complexes have been prepared through the structural regulation in this study. The incorporation of half-sandwich IrIII complex with Fc unit successfully improves their anticancer activity, and the optimal performance (IrFc5) is almost 3-fold higher than that of cisplatin against A549 cells, meanwhile, which also shows better anti-proliferative activity against A549/DDP cells. Complexes can aggregate in the intracellular lysosome of A549 cells and induce lysosomal damage, disrupt the cell cycle, increase the level of intracellular reactive oxygen species, and eventually lead to cell apoptosis. Half-sandwich IrIII-Fc heteronuclear metal complexes possess a different anticancer mechanism from cisplatin, which can serve as a potential alternative to platinum-based drugs and show a good application prospect.

Adsorption, Electronic, and Optical Properties of TiO2–Pyridine Complexes: General Principles for Interfacial Charge-Transfer Transitions

Adsorption, Electronic, and Optical Properties of TiO₂–Pyridine Complexes: General Principles for Interfacial Charge-Transfer Transitions

A Luminescence Assay in the Red for the Detection of Hydrogen Peroxide and Glucose Based on Metal Coordination Polyelectrolyte-Induced Supramolecular Self-Assembly of Alkynylplatinum(II) Complexes.

A new sensing strategy towards hydrogen peroxide based on metal coordination polyelectrolyte-driven self-assembly of alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complex was demonstrated. The cationic in situ-generated Ag(I)-thiocholine coordination polyelectrolytes were shown to induce the supramolecular self-assembly of anionic low-energy red-emissive alkynylplatinum(II) bzimpy complexes via non-covalent Pt(II)⋅⋅⋅Pt(II), electrostatic and π-π stacking interactions. The presence of hydrogen peroxide was shown to inhibit the formation of coordination polyelectrolytes and the coordination polyelectrolyte-induced self-assembly of platinum(II) complexes. The weakening of Pt(II)⋅⋅⋅Pt(II), electrostatic and π-π stacking interactions was supported by UV-vis absorption, emission, and resonance light scattering (RLS) studies. The present assay was also applied to probe glucose indirectly based on the enzymatic reaction of glucose oxidase on the substrate. Operating in a label-free manner, together with the low-energy red emission and large Stokes shift of alkynylplatinum(II) complexes, these features render the proposed design attractive for biological applications.

Modulation of n → π* Interaction in the Complexes of p-Substituted Pyridines with Aldehydes: A Theoretical Study.

n → π* interaction is analogous to the hydrogen bond in terms of the delocalization of the electron density between the two orbitals. Studies on the intermolecular complexes stabilized by the n → π* interaction are scarce in the literature. Herein, we have studied intermolecular N···C═O n → π* interactions in the complexes of p-substituted pyridines (p-R-Py) with formaldehyde (HCHO), formyl chloride (HCOCl), and acetaldehyde (CH3CHO) using quantum chemistry calculations. We have shown that the strength of the n → π* interaction can be modulated by varying the electronic substituents at the donor and acceptor sites in the complexes. Variation of the substituents at the para position of the pyridine ring from the electron-withdrawing groups (EWGs) to the electron-donating groups (EDGs) results in a systematic increase in the strength of the n → π* interaction. The strength of this interaction is also modulated by tuning the electron density toward the carbonyl bond by substituting the hydrogen atom of HCHO with the methyl and chloro groups. The modulation of this interaction due to the electronic substitutions at the n → π* donor and acceptor sites in the complexes is monitored by probing the relevant geometrical parameters, binding energies, C═O frequency redshift, NBO energies, and electron density for this interaction derived from QTAIM and NCI index analyses. Energy decomposition analysis reveals that the electrostatic interaction dominates the binding energies of these complexes, while the charge transfer interaction, which is representative of the n → π* interaction, also has a significant contribution to these.

Modulation of Isomerization and Ligand Exchange Rates by π Bonding in Bis(iminoxolene)iridium Pyridine Complexes.

The bis(iminoxolene)iridium complex (Diso)2IrCl (Diso = N-(2,6-diisopropylphenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone) reacts with pyridine to give trans-(Diso)2Ir(py)Cl as the kinetic product, with cis-(Diso)2Ir(py)Cl formed as the exclusive thermodynamic product upon heating. Electronic spectra and density functional theory calculations point to very similar electronic structures for the cis and trans isomers, with a nonbonding iminoxolene-centered HOMO and a metal-iminoxolene π* LUMO. The triplet states of cis-(Diso)2Ir(py)Cl and cis-[(Diso)2Ir(py)2]+ (but not trans-(Diso)2Ir(py)Cl) are unusually low in energy (1000-1500 cm-1 above the singlets), as shown by variable-temperature NMR spectroscopy. The low-energy triplets are attributed to a change in dihedral angle in the iminoxolenes, which allows a partial π interaction that cannot be achieved in the trans octahedral compounds. Mechanistic studies of the trans-cis isomerization in toluene indicate that the reaction proceeds via isomerization of the five-coordinate species to a form with cis iminoxolene ligands and an apical oxygen. This form is high in energy due to the loss of a secondary iminoxolene-to-iridium π-donor interaction that is possible in the trans form but not in the cis form for the square pyramidal structures. This stereoelectronic effect, combined with the poorer binding of pyridine in trans-(Diso)2Ir(py)Cl due to the interactions of the N-aryl substituents with the pyridine, makes the pyridine dissociate faster from the trans isomer by a factor of 108 at room temperature.

Fabrication of curcumin−loaded foxtail millet prolamin−based nanoparticle: Impact of curdlan sulfate on the particle properties

The sulfation of curdlan was achieved by modification of curdlan with sulfur trioxide–pyridine complex. Curcumin−loaded curdlan sulfate−sodium caseinate stabilized foxtail millet prolamin nanoparticles were fabricated using pH-driven method. Under the optimal conditions, two kinds of foxtail millet prolamin−based nanoparticles were fabricated using different preparation processes for the first time. The mean diameters and zeta-potentials of co−assembled foxtail millet prolamin−sodium caseinate−curdlan sulfate nanoparticles and curdlan sulfate coated co−assembled foxtail millet prolamin−sodium caseinate nanoparticles were 84.7 nm, 136.4 nm and − 45.7 mV, − 41.5, respectively. Molecular docking and infrared spectroscopy indicated that the curcumin was retained in the hydrophobic region of foxtail millet prolamin though four hydrogen bonds and ten hydrophobic interactions, and amino residues of proline, arginase, cysteine, glycine, and alanine served as the main binding sites for curcumin. The deposition and co−assembly of curdlan sulfate enhanced the curcumin encapsulation efficiency from 78.3% to 81.8% and 85.9%, respectively, and slightly reduced the release of curcumin under physiological and endosomal pH conditions. Consequently, the intake of curdlan sulfate−sodium caseinate stabilized composite nanoparticles encapsulated−curcumin by MCF-7 cells was lower than that of sodium caseinate stabilized nanoparticles entrapped−curcumin, and increased the IC50 value from 5.3 μg/mL to 10.5 and 9.0 μg/mL, respectively. This study showed that the biopolymeric nanoparticles have the potential to encapsulate hydrophobic bioactives, and the incorporation of curdlan sulfate exhibited a significant effect on the physicochemical properties, encapsulation, and release profile of foxtail millet prolamin particles.

Sulfonation of HwBKP by Sulfur Trioxide Pyridine Complex and Preparation of Anionic Cellulose Beads Using the Same

To expand the applications of cellulose beads, sulfonated cellulose beads were prepared by chemically modifying HwBKP fibers with sulfur trioxide pyridine complex (STP), dissolving in tetraethylammonium hydroxide (TEAOH)/urea solvent, and dropping into acetic acid solution. The effect of the amount of STP added on the properties of sulfonated cellulose solution and cellulose beads was investigated. It was observed that the sulfonated cellulose beads exhibited anionic surface charge in a fairly wide pH range, and the degree of substitution of sulfonic acid groups and the zeta potential of the cellulose beads could be controlled by adjusting the amount of added STP. In addition, as the amount of STP added increased, the crystallinity of the cellulose decreased, the viscosity and surface tension of the sulfonated cellulose/TEAOH/urea solution decreased, and the size of the cellulose beads increased.

Computational Study on a Transfer Hydrogenation Catalysed by a Ru(II) Bis‐Pyrazolyl Pyridine Complex

AbstractDensity functional calculations were performed to investigate the Ru‐catalysed transfer hydrogenation of acetophenone to 1‐phenylethanol using isopropanol as the reducing agent. The catalyst is [Ru(bpp)Cl2(PPh3)], where bpp is a tridentate bis‐pyrazolyl pyridine ligand. We studied an inner sphere monohydridic mechanism where the active Ru centre binds, in addition to bpp, one of the original ligands as a spectator, which can be either chloride or triphenylphosphine. In both cases, catalyst initiation appears to be feasible (with triphenylphosphine being the preferred spectator ligand) and the catalytic cycle is efficient with a small energetic span. For which one of the cases the calculated energetic span is smaller depends on the treatment of entropy in solution. Since pathways involving dissociation of charged ligands are not preferred, the product is formed by a fast proton transfer within the Ru coordination sphere, from a coordinated isopropanol to the product alcoholate. Subsequently, both product release and catalyst regeneration can be accomplished by the dissociation of the charge neutral product alcohol. The β‐hydride transfer from a coordinated isopropanolate anion to ruthenium is slightly different depending on whether there is a chloride or phosphine present trans to the alcoholate. In both cases, this step contains the transition state with the highest Gibbs energy.

Bifunctional NHC-Promoted C2-Alkylation of Pyridine via Ni–Al Bimetallic-Catalyzed Hydroarylation of Unactivated Alkenes

Achieving site-selective alkylation with pyridine under transition metal catalysis has remained a long-standing challenge, especially when using unactivated alkene as an alkylating reagent. We herein describe a site-selective C2-alkylation of pyridine under Ni–Al bimetallic catalysis and directing bifunctional N-heterocyclic carbene (NHC) as ligand through hydroarylation of unactivated alkenes. A broad range of C2-alkylated pyridines with various functionalities could be prepared in high efficiency (up to 99% yield) and high C2-selectivity, and the scope of heterocycles is not limited to pyridines but can be expanded to other azines like pyrazines and pyrimidines. Mechanistic studies suggest that a bifunctional NHC–Ni–Al–pyridine complex is responsible for high site selectivity control and that reductive elimination might be the rate-determining step.

Half-Sandwich Type Platinum-Group Metal Complexes of C-Glucosaminyl Azines: Synthesis and Antineoplastic and Antimicrobial Activities

While platinum-based compounds such as cisplatin form the backbone of chemotherapy, the use of these compounds is limited by resistance and toxicity, driving the development of novel complexes with cytostatic properties. In this study, we synthesized a set of half-sandwich complexes of platinum-group metal ions (Ru(II), Os(II), Ir(III) and Rh(III)) with an N,N-bidentate ligand comprising a C-glucosaminyl group and a heterocycle, such as pyridine, pyridazine, pyrimidine, pyrazine or quinoline. The sugar-containing ligands themselves are unknown compounds and were obtained by nucleophilic additions of lithiated heterocycles to O-perbenzylated 2-nitro-glucal. Reduction of the adducts and, where necessary, subsequent protecting group manipulations furnished the above C-glucosaminyl heterocycles in their O-perbenzylated, O-perbenzoylated and O-unprotected forms. The derived complexes were tested on A2780 ovarian cancer cells. Pyridine, pyrazine and pyridazine-containing complexes proved to be cytostatic and cytotoxic on A2780 cells, while pyrimidine and quinoline derivatives were inactive. The best complexes contained pyridine as the heterocycle. The metal ion with polyhapto arene/arenyl moiety also impacted on the biological activity of the complexes. Ruthenium complexes with p-cymene and iridium complexes with Cp* had the best performance in ovarian cancer cells, followed by osmium complexes with p-cymene and rhodium complexes with Cp*. Finally, the chemical nature of the protective groups on the hydroxyl groups of the carbohydrate moiety were also key determinants of bioactivity; in particular, O-benzyl groups were superior to O-benzoyl groups. The IC50 values of the complexes were in the low micromolar range, and, importantly, the complexes were less active against primary, untransformed human dermal fibroblasts; however, the anticipated therapeutic window is narrow. The bioactive complexes exerted cytostasis on a set of carcinomas such as cell models of glioblastoma, as well as breast and pancreatic cancers. Furthermore, the same complexes exhibited bacteriostatic properties against multiresistant Gram-positive Staphylococcus aureus and Enterococcus clinical isolates in the low micromolar range.

Inter- vs. Intra-Molecular Hydrogen Bond in Complexes of Nitrophthalic Acids with Pyridine.

This study covers the analysis of isomeric forms of nitrophthalic acids with pyridine. This work dwells on the complementary experimental (X-ray, IR and Raman) and theoretical (Car-Parrinello Molecular Dynamics (CPMD) and Density Functional Theory (DFT)) studies of the obtained complexes. The conducted studies showed that steric repulsion between the nitro group in ortho-position and the carboxyl group causes significant isomeric changes. Modeling of the nitrophthalic acid-pyridine complex yielded a short strong intramolecular hydrogen bond (SSHB). The transition energy from the isomeric form with an intermolecular hydrogen bond to the isomeric form with an intramolecular hydrogen bond was estimated.

H – Bond interactions in water multimers and water multimers – Pyridine complexes: Natural bond orbital and reduced density gradient isosurface analyses

Natural bond orbital and reduced density gradient isosurface analyses have been carried out on the water multimers of order n=1-9 and twenty-five complexes of Pyridine (Py) with water multimers of order n=1-5 optimized using density functional theory at B3LYP level of theory with the basis set 6–311++G(d,p). Among the twenty-five Py-Water structures, the seventeen configurations whose interaction energies are above 49 kJ/mol have been taken into consideration. The interaction (binding) energy values indicate that i) the 1(Py): 5 (water)/1:2 complexes are found to be the most/least stable networks and ii) the stability increases with increase in the number of water molecules in water multimers only. The occupancy and second order perturbation energy profiles suggest that O-H⋯N hydrogen bond is stronger than O-H⋯O, C-H⋯O and O-H⋯π hydrogen bonds. The non-classical hydrogen bonds and π→σ∗ interactions are also characterized by the green coloured regions in the reduced density gradient isosurfaces. The Shubin Liu energy decomposition (EDA-SBL) analysis shows that the water multimers and the Py-Water complexes are dominantly stabilized by the attractive electrostatic interactions. Of the two functionals, the B3LYP and ωB97XD, the latter which is meant for accounting the dispersion interaction disallows the O-H⋯π interaction between Py and water molecules and therefore, the former appears to give correct description on Py-Water interactions.

Highly stereoselective synthesis of heparin tri- and tetra-saccharide derivatives

<p indent="0mm">Heparin, a naturally highly sulfated glycosaminoglycan, mediates various physiologic and pathophysiologic processes, and is mainly used for the prevention and treatment of venous thrombosis in clinic. Here we report an efficient synthesis of heparin oligosaccharides with different degrees of 6-<italic>O</italic>-sulfonation modification, employing orthogonal protective group strategy. <italic>tert</italic>-Butyldiphenylsilyl (TBDPS) group is used as the protective group of 6-OH of the 2-azide-glucose donors, and highly α-selective construction of the key GlcN-(1→4)-GlcUA glycosidic bond was achieved with the corresponding <italic>N</italic>-phenyl trifluoroacetimidate donors. Thus, tri- and tetra-saccharides are synthesized by the convergent [1 + 2] and [2 + 2] glycosylation. The fully elaborated oligosaccharides have then been successfully transformed into the target heparin oligosaccharides <bold>28</bold>–<bold>34</bold> <italic>via</italic> an optimal sequence of manipulation of the protecting groups. The post-assembly manipulations include desilylation with HF·Py (for removal of TBDPS group), delevulinoylation with hydrazine hydrate (for removal of levulinoyl group), saponification under Zemplén conditions (for removal of methyl ester and benzoyl group), <italic>O</italic>-sulfonation with sulfur trioxide pyridine complex (for hydroxyl groups), reduction and <italic>N</italic>-sulfonation (for azido group), and atmospheric pressure hydrogenation (for removal of benzyl and Cbz groups). The availability of these heparin oligosaccharide derivatives which bear amino linker shall facilitate the preparation of glycan chips for studies on the interaction between heparin oligosaccharides with proteins, such as the heparanase.

In Vitro and In Vivo Antitumor Assay of Mitochondrially Targeted Fluorescent Half-Sandwich Iridium(III) Pyridine Complexes

Half-sandwich iridium(III) complexes show potential value in the anticancer field. However, complexes with favorable luminescence performance are rare, which limits further investigation of the anticancer mechanism. In this paper, 10 triphenylamine-modified fluorescent half-sandwich iridium(III) pyridine complexes {[(η5-Cpx)Ir(L)Cl2]} (Ir1-Ir10) were prepared and showed potential antiproliferative activity, effectively inhibiting the migration of A549 cells. Ir6, showing the best activity among these complexes, exhibited excellent fluorescence performance (absolute fluorescence quantum yield of 15.17%) in solution. Laser confocal detection showed that Ir6 followed an energy-dependent cellular uptake mechanism, specifically accumulating in mitochondria (Pearson co-localization coefficient of 0.95). A Western blot assay further confirmed the existence of a mitochondrial apoptotic channel. Additionally, Ir6 could arrest the cell cycle at the G2/M phase, catalyze NADH oxidation, reduce the mitochondrial membrane potential, induce an increase in the level of intracellular reactive oxygen species, and exhibit a mechanism of oxidation. An in vivo antitumor assay confirmed that Ir6 can effectively inhibit tumor growth and is safer than cisplatin.

Two-Sided Impact of Water on the Relaxation of Inner-Valence Vacancies of Biologically Relevant Molecules.

After ionization of an inner-valence electron of molecules, the resulting cation-radicals store substantial internal energy which, if sufficient, can trigger ejection of an additional electron in an Auger decay usually followed by molecule fragmentation. In the environment, intermolecular Coulombic decay (ICD) and electron-transfer mediated decay (ETMD) are also operative, resulting in one or two electrons being ejected from a neighbor, thus preventing the fragmentation of the initially ionized molecule. These relaxation processes are investigated theoretically for prototypical heterocycle-water complexes of imidazole, pyrrole, and pyridine. It is found that the hydrogen-bonding site of the water molecule critically influences the nature and energetics of the electronic states involved, opening or closing certain relaxation processes of the inner-valence ionized system. Our results indicate that the relaxation mechanisms of biologically relevant systems with inner-valence vacancies on their carbon atoms can strongly depend on the presence of the electron-density donating or accepting neighbor, either water or another biomolecule.

The aid of calorimetry for the thermochemical and kinetic study of the σ-hole bonding leading to I2 and 4-(dimethylamino) pyridine complexes in solution

The aid of calorimetry for the thermochemical and kinetic study of the σ-hole bonding leading to I2 and 4-(dimethylamino) pyridine complexes in solution

Zinc‐bis(imino)pyridine Complexes as Catalysts for Azide‐Alkyne Cycloaddition in Water

AbstractTwo new zinc(II) complexes, [2,6‐bis(methylphenyliminomethyl)pyridine]ZnBr2 (2 a) and [2,6‐bis(triflouromethylphenyliminomethyl)pyridine]ZnBr2 (2 b) were synthesized by the reactions of corresponding 2,6‐bis(imino)pyridine based [NNN]‐pincer ligand and anhydrous ZnBr2 in acetonitrile as a solvent. The catalytic properties of zinc(II) complexes were examined in azide‐alkyne cycloaddition reactions in water. The zinc(II) complexes were characterized by UV‐Vis, 1H NMR, IR, elemental analysis and mass spectroscopy. The zinc complex [2,6‐bis(triflouromethylphenyliminomethyl)pyridine]ZnBr2 was characterized by X‐ray crystallography. The zinc(II) complexes efficiently catalyzed the azide‐alkyne cycloaddition reaction for the synthesis of 1,2,3‐triazoles (mixture of 1,4‐ and 1,5‐disubstituted 1,2,3‐triazoles) in water with good to excellent yields.

Diheme cytochromes: Effect of mixed-axial ligation on the electronic structure and electrochemical properties with cobalt porphyrin dimer

A series of six-coordinate diCo(III) porphyrin dimers, as synthetic analogues of diheme cytochromes, have been reported here having bis(imidazole), bis(pyridine) and mixed thiophenolate−pyridine/imidazole axial ligands. In the X-ray structures of bis(imidazole) and bis(pyridine) complexes, the axial ligands are in perpendicular orientation while they are parallelly oriented in their monomeric analog. The porphyrin rings are also highly ruffle-distorted in dimer but planar in monomer which reflect the effect of intramolecular interaction between two Co(porphyrin) units in dimers. In the X-ray structure of diCo(III) thiophenolate−pyridine mixed-ligated complex, the axial Co-S and Co–N(py) distances are 2.256(1) and 2.063(2) Å, respectively. The Co–N(py) distance of 2.063(2) Å is much longer than the distances of 1.961(3) and 1.972(3) Å observed in bis(pyridine) complex and the Co-S distance is larger than Co–N(py) in the mixed ligated complex which results in a displacement of Co by 0.15 Å towards the pyridine ligand from the mean porphyrin plane. Indeed, this is the first X-ray structure of a metalloporphyrin with mixed thiophenolate−pyridine axial ligands. The effect of mixed-axial ligation is demonstrated by a blue-shift of the Soret band in the UV–visible spectroscopy and also a positive shift of the Co(III)/Co(II) redox couple as compared to their bis(pyridine) analogue. The redox potentials are shifted to a large negative value just upon replacing the metal from iron to cobalt. The present investigation emphasizes the role of axial ligation, metal ions, and also the effect of heme-heme interaction in controlling the spectral and electrochemical properties.