2,6-Bis(porphyrin)-substituted pyrazine: a new class of supramolecular synthon binding to a transition-metal ion and fullerene (C60)

2,6-Disubstituted-3,5-dimethylpyrazines have been synthesized via biased acetal synthesis from symmetric 2,3,5,6-tetrakis(chloromethyl)pyrazine. The pyrazine ligands coordinated to trans-dichloropalladium(II) at the nitrogen whose neighboring carbons were connected to less hindered methyl groups. 2,6-Bis(porphyrin)-substituted pyrazine bound C₆₀ to yield 1:1 inclusion complex. The binding of C₆₀ with the pyrazine ligand and its zinc complex was determined by ESI-MS, NMR, and fluorescence spectroscopic analyses.     

Inclusion of parabens in β-cyclodextrin: A solution NMR and X-ray structural investigation

A parallel study was conducted of the inclusion of alkyl parabens (guests) in the host β-cyclodextrin (β-CD). ¹H NMR data indicated an insertion of the guest phenyl ring into the β-CD cavity. The stoichiometry of each complex was 1:1, as determined by a continuous variation method that utilises the chemical shifts of the host protons. These chemical shifts were additionally used to determine the association constant yielding K values of 1631, 938, 460 and 2022 M^? 1 at 298 K for the methyl-, ethyl-, propyl- and butyl paraben solution state complexes, respectively. NOE experiments conducted on the methyl paraben solution complex indicated that the phenolic group of the guest was located at the secondary rim of the cyclodextrin cavity. Solid state structure analyses of the methyl and propyl paraben β-CD complexes were performed. Both complexes crystallised at ambient temperature in the space group C2, Z = 4 with a host to guest ratio of 1:1. Additionally, a second crystal structure between methyl paraben and β-CD is reported. This complex crystallised at 7^oC in the space group P1, Z = 2 with a 1:1 host–guest stoichiometry.

¹H NMR and solid state structure analyses were conducted on the inclusion of alkyl parabens in the host β-cyclodextrin. Both indicated an insertion of the guest phenyl ring into the β-CD cavity.     

Hydrogen bonding between carboxylic acids and amide-based macrocycles in their host–guest complexes

For 12- and 13-membered macrocycles in which two amide linkages are integrated in the macrocyclic ring systems, the formation of 1:1 host–guest complexes with acetic and benzoic acids has been confirmed by NMR titrations. The complex formation occurs with the formation constants of 8–27 M^? 1, under competition with the dimerisation of acid molecules. Benzoic acid tends to form more stable complexes than acetic acid. The binding force is due to a pair of hydrogen bonds, O_carboxyl–H…O = C_amide and C = O_carboxyl…H–N_amide, between the carboxyl group of a guest molecule and the amide group of a host molecule. The former bond is stronger than the latter, and defines the stability of the complexes. The formation of the pair of hydrogen bonds is accompanied by the conformational conversion of the amide group from the trans-form to the cis-form. The influence of such a conversion on the internal molecular motion is observed as a slight broadening of signal width.

For 12- and 13-membered macrocycles in which two amide linkages are integrated in the macrocyclic ring systems, the formation of 1:1 host–guest complexes with acetic and benzoic acids has been confirmed by NMR titrations. The binding force for the complex formation is due to a pair of hydrogen bonds, O_carboxyl–H…O = C_amide and C = O_carboxyl…H–N_amide. The former bond is stronger than the latter and dominates the hydrogen-bond formation. The formation of the pair of hydrogen bonds is accompanied by the conformational conversion of the amide group from the stable trans-form to the less stable cis-form.     

Determination of the structure of quinolone-γ-cyclodextrin complexes and their binding constants by means of UV–Vis and <Superscript>1</Superscript>H NMR

The host–guest inclusion complex structure and binding ability of two different quinolones with γ-cyclodextrin (γ-CD) were investigated in solution by means of UV–Vis and ¹H NMR spectroscopy. Competition of oxolinic and nalidixic acid molecules for the γ-CD cavity was evaluated by determination of association constants. Both quinolones form 1:1 inclusion complexes, their binding constants at room temperature (25 °C) under acidic and basic conditions were calculated using Benesi–Hildebrand equation. The stability of the complexes was dependent on the structure of the quinolone. In general, the weaker binding constants were observed for oxolinic acid-γ-CD complexes (1616 and 1765 M^?1) and the larger binding constants were obtained for nalidixic acid-γ-CD complexes (3760 and 3840 M^?1). ¹H NMR studies in D₂O were performed to elucidate the structure of each inclusion complex, nalidixic acid molecule penetrates more deeply into the γ-CD cavity and an intermolecular hydrogen bond is formed. Knowledge about structure and relative stability of quinolone-γ-CD complexes will be useful for future applications of these antimicrobial agents in medicinal chemistry.     

Inclusion complexes of β-cyclodextrin with pyrazinamide and piperazine: Crystallographic and theoretical studies

The inclusion complexes of β-cyclodextrin (β-CD) with pyrazinamide (PYA) and piperazine (PIZ) have been investigated both in the solid phase by single-crystal X-ray diffraction analysis and in the gas phase by semi-empirical PM3 calculation. In the crystalline phase, the disordered PYA and PIZ molecules are entirely embedded in the β-CD cavity. The PYA pyrazine-centre displaces upwards by 1.15(1) Å from the β-CD plane, whereas the PIZ centre shifts downwards by 0.76(1) Å from the β-CD plane. The inclusion scenario changed in the gas phase. Two inclusion geometries of the PYA molecule are comparatively stable with binding energies of ? 22.28 and ? 25.29 kJ mol^? 1: the pyrazine centre shifts upwards by 0.5 Å and downwards by 2.0 Å from the β-CD plane. The PIZ molecule positioning at 2.0 Å below the β-CD plane gives a more stable inclusion complex than does the PYA molecule by 22–25 kJ mol^? 1.

Structural distinction of the β-CD–PYA and β-CD–PIZ inclusion complexes in the solid phase (by X-ray crystallography) and gas phase (by PM3 calculation) is a paradigm of the CD conformational flexibility, the induced-fit mechanism and the dynamics of the inclusion process.     

A Tunable Cyclic Container: Guest‐Induced Conformational Switching,Efficient Guest Exchange,and Selective Isolation of C70 from a Fullerene Mixture

An adaptable cyclic porphyrin dimer with highly flexible linkers has been used as an artificial molecular container that can efficiently encapsulate various aromatic guests (TCNQ/C₆₀/C₇₀) through strong π–π interactions by adjusting its cavity size and conformation. The planar aromatic guest (TCNQ) can be easily and selectively exchanged with larger aromatic guests (C₆₀/C₇₀). During the guest‐exchange process, the two porphyrin rings switch their relative orientation according to the size and shape of the guests. This behavior of the cyclic container has been thoroughly investigated by using UV/Vis spectroscopy, NMR spectroscopy, and X‐ray crystal structure determination of the host–guest assemblies. The electrochemical and photophysical studies demonstrated the occurrence of photoinduced electron transfer from bisporphyrin to TCNQ/C₆₀/C₇₀ in the respective host–guest assemblies. The cyclic host can form complexes with C₆₀ and C₇₀ with association constants of (2.8±0.2)×10⁵ and (1.9±0.3)×10⁸ m ⁻¹, respectively; the latter value represents the highest binding affinity for C₇₀ reported so far for zinc(II) bisporphyrinic receptors. This high selectivity for the binding of C₇₀ versus C₆₀ allows the easy extraction and efficient isolation of C₇₀ from a C₆₀/C₇₀ fullerene mixture. Experimental evidence was substantiated by DFT calculations.     

Bis -(3,5-dimethyl-pyrazolyl-1-methyl)-(3-phosphanyl-propyl)-amine complexes of copper(II), nickel(II), and cobalt(II)

A series of Cu(II), Co(II), and Ni(II) complexes of bis-(3,5-dimethyl-pyrazolyl-1-methyl)-(3-phosphanyl-propyl)-amine C₁₅H₂₆N₅P (1), prepared from 3-aminopropylphosphine and 1-hydroxymethyl-3,5-dimethylpyrazole were characterized. The nature of bonding and the geometry of the complexes have been deduced from elemental analysis, infrared, electronic, ¹H NMR, ³¹P NMR spectra, magnetic susceptibility, and conductivity measurements. The studies indicate octahedral geometry for nickel complex and square pyramidal geometry for copper and cobalt complexes. The EPR spectra of copper complex in acetonitrile at 300 K and 77 K were recorded. Biological activities of the ligand and metal complexes have been studied on Staphylococcus aureus, Escherichia coli, Aspergillus niger, and Aspergillus flavus by well-diffusion method. The zone of inhibition values were measured at 37°C for a period of 24 h. The electrochemical behavior of copper complexes was studied by cyclic voltammetry. Catalytic study indicates the copper complex has efficient catalytic activity in oxidation of amitriptyline.     

Structures of the silver complexes with lutidines according to the NMR data. Crystal structure of [AgNO3(3,5-Lut)2]

The silver(I) nitrate complexes with 2,3-, 2,4-, 2,6-, and 3,5-lutidine (Lut, dimethylpyridine C₇H₉N), [AgNO₃(Lut)₂], are synthesized and studied by multinuclear NMR (¹H, ¹³C, and ¹⁵N) in various solvents (chloroform, dimethyl sulfoxide, and acetonitrile). The influence of steric and electronic factors of the organic ligand on the parameters of the NMR spectra is revealed. It is shown that the ¹⁵N NMR spectra are the most informative. The structure of complex [AgNO₃(3,5-Lut)₂] is determined. The crystals are monoclinic, space group C2/c, a = 14.599(1) Å, b = 8.422(1) Å, c = 12.954(1) Å, β = 99.60(1)°, V = 1570(2) ų, ρ_calcd = 1.625 g/cm³, Z = 4. The structure is built of discrete neutral complexes [AgNO₃(3,5-Lut)₂]. The coordination mode of the Ag⁺ ion includes two nitrogen atoms of two crystallographically equivalent lutidine ligands (Ag-N 2.194(5) Å, angle NAgN 147.6(3)°). The nitrate ion behaves as a weak chelating ligand with respect to the Ag⁺ ion (Ag…O 2.674(6) Å).     

NMR spectroscopic characterization of inclusion complexes of hydroxy-substituted naphthalenes with native and modified β-cyclodextrins

The equilibrium constants (K) for the inclusion complexation of three kinds of β-cyclodextrins (β-CDs: native β-CD, heptakis(2,6-di-O-methyl)-β-CD, and 6-O-α-d-glucosyl-β-CD) with OH-substituted naphthalenes (2-naphthol, 2,3-dihydroxynaphthalene, and 2,6-dihydroxynaphthalene) were determined from the induced chemical shifts of NMR measurements for inclusion complexes: K = 188–1,250 mol^?1 dm³. The modified β-CDs form stable 1:1 inclusion complexes with OH-substituted naphthalenes, and the high stability of inclusion complexes of 2,6-dihydroxynaphthalene having a hydrophobic body and hydrophilic ends is shown. In addition, the structures of inclusion complexes were characterized by 2D ROESY NMR measurements. The differences in the structure of the inclusion complexes were observed for three kinds of naphthol guest molecules. Based on the results, the inclusion abilities enhanced by methylation of the OH groups at the CD rim or the side chain of branched β-CD are discussed.     

Inclusion complexes of 2-phenoxyethanol and alkoxyethanols in cyclodextrins: an 1H NMR study

The association in aqueous solutions of small amphiphilic molecules [2-phenoxyethanol, PhE₁, and some α-n-alkyl-ω-hydroxyoligo(oxiethylenes], C₄E₁, C₄E₂ and C₆E₂) with β-cyclodextrin (βCD), heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) was investigated by ¹H NMR spectroscopy. The upfield shifts observed for the H3 and H5 NMR signals due to anisotropic shielding confirm that the host–guest associations are of inclusion type. The stoichiometries and the apparent inclusion constants, K _app, were determined by ¹H NMR spectroscopy using the H5 and H3 signals. The relative differences in the K _app values for βCD inclusion complexes seem to reflect the hydrophobic/hydrophilic balance of the guests. The K _app values for the PhE₁ inclusion complexes can be related to the degree of methylation and hydrophobicity variation within the considered hosts. In addition, a comparative study between βCD and TRIMEB inclusion complexes using 2D ROESY (Rotating-frame Overhauser Enhancement SpectroscopY) NMR spectra provides structural features for these complexes which are inaccessible by other experimental methods.     

A competitive sensing system based on cyclobis(paraquat-p-phenylene) and a new β-cyclodextrin-tetrathiafulvalene derivative

We report the synthesis of 4,5-di(ethylthio)-4′-[6-deoxy-β-cyclodextrin-6-yl-aminocarbonyl]-tetrathiafulvalene (β-CD-DET-TTF) and its inclusion abilities towards cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ) and 1-naphthol. The structure of the synthesised compound has been established by mass spectrometry and ¹H NMR spectra combined with a theoretical MM3 and AM1 study. The sensor affords a charge transfer (CT) complex with the CBPQT⁴⁺  and is able to include 1-naphthol in the cyclodextrin cavity. The complexes were characterised experimentally by UV–vis spectroscopy and simulated by a MM3 docking procedure. The sensing ability of the β-CD-DET-TTF/CBPQT⁴⁺  complex towards 1-naphthol has been investigated by a competitive spectral method.

The synthesis and characterisation of a new water soluble β-CD-DET-TTF derivative able to formed a CT complex with the CBPQT⁴⁺  acceptor is reported. The water soluble CT complex β-CD-DET-TTF/CBPQT⁴⁺  could be used as an efficient sensor towards aromatic guests prone to give inclusion complexes with the CBPQT⁴⁺  ring.     

Coordination and inclusion compounds formed by addition of quinoline (Q) or isoquinoline (Iq) to a metal(II) dibenzoylmethanate (Co, Ni, Zn, Cd)

Three isomorphous series of new compounds are reported: complexes [M(DBM)₂Q₂] and [M(DBM)₂Iq₂] (M = M(II) = Co, Ni, Zn, Cd; DBM is C₆H₅COCHCOC₆H₅ ^?) and inclusion compounds [M(DBM)₂Q₂]*Q (M = Co, Zn, Cd). All the compounds comprise a trans configured octahedral complex molecule. Inclusion compounds of modified Zn and Cd DBM complexes are reported for the first time and their inclusion ability is attributed to the trans isomeric state induced by the bulky Q or Iq ligand. The TG measurements indicate the following order of thermal stability of the complexes defined by the strength of the metal–ligand bonds: Ni > Co > Cd > Zn. The inclusion compounds do not follow this trend.     

Study of spectroscopic properties of Europium (III) Tris(β-diketonate) complex and α-Cyclodextrin in aqueous medium

The solubilization of an europium (III) β-diketonate chelate in aqueous medium and the changes in its photophysical properties upon its inclusion into an α-cyclodextrin hydrophobic cavity are described. The complex [Eu(tta)₃·(H₂O)₂] (tta = 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione) was synthesized, characterized, and incorporated into the hydrophobic cavity by stirring in an α-cyclodextrin aqueous solution. The inclusion was confirmed by ¹H NMR, and the stoichiometry of association was obtained by the Job method. The maximum in the excitation spectrum of the α-CD inclusion compound in aqueous solution was shifted 28 nm compared with the maximum of non α-CD complex. The emission spectrum of the association is similar to that of the free solid complex and displays the characteristic ⁵D₀ → ⁷F_0-4 Eu³⁺ transitions.     

Synthesis,characterization, speciation and biological studies on metal chelates of 1-benzoyl(1,2,4-triazol-3-yl)thiourea

Proton-ligand association constants of 1-benzoyl(1,2,4-triazol-3-yl)thiourea (BTTU) and its complex formation constants with some bivalent metal ions Ni(II), Co(II), Mn(II), Zn(II), and Cu(II), have been determined potentiometrically in 50% EtOH–H₂O and 0.1 M NaNO₃. The complexes formed in solution have a stoichiometry of 1:1 and 1:2 [M:L], where M represents the metal ion and L the BTTU ligand. The corresponding thermodynamic parameters are derived and discussed. The complexes are stabilized by enthalpy changes and the results suggest that complexation is an enthalpy-driven process. The effects of metal ion, ionic radius, electronegativity, and nature of ligand on the formation constants are discussed. The formation constants of the complexes with 3d transition metals follow the order Mn²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺. The metal complexes were synthesized and characterized by elemental analyses, conductance, IR, ¹H NMR, and magnetic measurements. The low magnetic moment of 0.11 BM for the Cu(II) complex is suggestive of dimerization through Cu–Cu interaction. The concentration distribution diagrams of the complexes were evaluated. The ligands and their metal complexes have been screened in vitro against some bacteria and fungi.     

Vinyl polymerization of norbornene catalyzed by 2‐Py‐amidine Ni(II) complex/methylaluminoxane systems

Two 2‐Py‐amidine ligands (2‐Py―NH―C(Ph)═N―Ar, Ar = 2,6‐Me₂C₆H₃ and 2,6‐ⁱPr₂C₆H₃) and the corresponding Ni(II) complexes ( 1 and 2 ) were synthesized and characterized using elemental analysis and FT‐IR, UV–visible, ¹H NMR and ¹³C NMR spectroscopies. X‐ray crystal structures indicate that the chelate ring conformation of the less bulky complex 1 is relatively planar compared with that of the bulky complex 2 . Paramagnetic ¹H NMR and ¹³C NMR studies show that, in solution, the time‐average structures of complexes 1 and 2 have mirror symmetry. Both complexes 1 and 2 were used as catalyst precursors for norbornene polymerization with methylaluminoxane as a co‐catalyst. The effects of Al/Ni ratio, temperature and structure of precursors on the catalytic performance were investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

A comparison of solution and solid state coordination environments for calcium(II), zirconium(IV), cadmium(II) and mercury(II) complexes with dipicolinic acid and methylimidazole derivatives

Four new supramolecular compounds, (2-mimH)[Ca(pydcH)₃][Ca(pydcH₂)(pydc)(H₂O)₂]·4H₂O (1), (1-mimH)₂[Zr(pydc)₃] (2), (2-mimH)₂[Cd(pydc)₂]·8H₂O (3), and (2-mimH)₂[Hg(pydc)₂]·8H₂O (4) [where pydcH₂ = pyridine-2,6-dicarboxylic acid (dipicolinic acid), 1-mim = 1-methylimidazole, and 2-mim = 2-methylimidazole], have been synthesized and characterized by elemental analyses, spectroscopic techniques (IR, UV–vis, ¹H NMR, and ¹³C NMR), thermal (TG/DTG/DTA) analysis as well as single-crystal X-ray diffraction. All four compounds are proton-transfer salts of the methylimidazolium cations and metal complex anions that crystallized from a solution of pyridine-2,6-dicarboxylic acid, methylimidazole, metal nitrates or chlorides as starting materials. The coordinating dicarboxylic acid is deprotonated at the carboxyl group and methylimidazole is protonated to balance the charge. In the crystal structures of 1–4, hydrogen bonding and π–π stacking play important roles. Water clusters are formed in 1, 3, and 4. The equilibrium constants of dipicolinic acid (pydc) and methylimidazole derivatives (1-mim and 2-mim), pydc-2-mim, pydc-1-mim proton-transfer systems as well as those of their complexes were investigated by a potentiometric pH titration method. The stoichiometries of most of the complex species in solution were very similar to the cited crystalline metal ion complexes.     

Synthesis and Photophysical Properties of a Series of Biscyclometalated Platinum(II) Complexes on the Basis of a Tridentate 2,6-Diphenylpyridine

The complexes C,C-trans-[Pt(C^N^C)L] ^z [C^N^C is bisdeprotonated 2,6-diphenylpyridinate (dppy^2-); L = CO, Me2SO (dmso), ethylenediamine (en), CN^-, pyrazine (pz), P(C₆F₅)₃, 4,4'-bipyridine (4,4'-bpy)] were synthesized and characterized by ¹H NMR, electronic, and emission spectroscopy. Spectralluminescence properties of the chromophoric group (platinum-cyclometalating ligand) in the C,C-trans-bis-cyclometalated complexes depend on the nature of the noncyclometalating ligand L, which is explained by its donor-acceptor effects on the form of existence of the complex in solution.     

Potentiometric Studies of Ternary Complexes Involving Palladium(II) with Fluoxetine Drug and Some Bio-ligands

The complex formation equilibria of [Pd(FLX)(H₂O)₂]²⁺, where FLX = the drug fluoxetine, with the bio-ligands (L), glycine, serine, cysteine, histidine, glycylglycine and glycylvaline, were studied and their formation constants were determined. Stoichiometries and stability constants of the complexes were determined at 25 °C and at constant 0.10 mol·dm^?3 NaNO₃ ionic strength. The concentration distribution of the complexes in solution was evaluated. Palladium(II) complexes were synthesized and characterized by the ¹H NMR and electronic spectral studies. The structures consist of monomeric units in which the Pd(II) atoms exhibit square planar geometry.