Allosteric anion recognition by metal complexation of tris(bipyridine-imidazolium) ligand

A new tripodal imidazolium ligand 2 comprising three 2,2′-bipyridine-imidazolium subunits connected through a mesityl spacer has been synthesized and 1:1 complexes of 2 with Fe(II) and Ru(II) ions have been prepared. ¹H NMR spectroscopy including NOE analysis and molecular modeling study established that the complexes exist as a pseudocryptand type twisted structure in solution. The Fe(II) and Ru(II) complexes show strong 1:1 binding of Cl⁻, Br⁻, and I⁻ anions in MeCN-d₃ with a large enhancement of the guest selectivity (Cl⁻ > Br⁻ > I⁻) upon metal complexation.     

Ruthenium(III) complexes with N-(acetyl)-N′-(5-R-salicylidene)hydrazines: Syntheses,structures and properties

The reactions of 1 mol equiv. each of [Ru(PPh₃)₃Cl₂] and N-(acetyl)-N′-(5-R-salicylidene)hydrazines (H₂ahsR, R = H, OCH₃, Cl, Br and NO₂) in alcoholic media afford simultaneously two types of complexes having the general formulae [Ru(HahsR)(PPh₃)₂Cl₂] and [Ru(ahsR)(PPh₃)₂Cl]. The complexes have been characterized by elemental analysis, magnetic, spectroscopic and electrochemical measurements. Molecular structures of [Ru(HahsH)(PPh₃)₂Cl₂] and [Ru(ahsH)(PPh₃)₂Cl] have been confirmed by X-ray crystallography. In both species, the PPh₃ ligands are trans to each other. The bidentate HahsH⁻ coordinates to the metal ion via the O atom of the deprotonated amide and the imine–N atom in [Ru(HahsH)(PPh₃)₂Cl₂]. In HahsH⁻, the phenolic OH is involved in a strong intramolecular hydrogen bond with the uncoordinated amide N atom forming a seven-membered ring. In [Ru(ahsH)(PPh₃)₂Cl], the tridentate ahsH²⁻ binds to the metal ion via the deprotonated amide O, the imine N and the phenolate O atoms. In the electronic spectra, the green [Ru(HahsR)(PPh₃)₂Cl₂] and brown [Ru(ahsR)(PPh₃)₂Cl] complexes display several absorptions in the ranges 385–283 and 457–269 nm, respectively. Both complexes are low-spin and display rhombic EPR spectra in frozen solutions. Both types of complexes are redox active and display a quasi-reversible ruthenium(III) to ruthenium(II) reduction which is sensitive to the polar effect of the substituent on the chelating ligand. The reduction potentials are in the ranges −0.21 to −0.12 and −0.42 to −0.21 V (versus Ag/AgCl) for [Ru(HahsR)(PPh₃)₂Cl₂] and [Ru(ahsR)(PPh₃)₂Cl], respectively.     

Anion recognition through hydrogen bonding by adamantane-dipyrromethane receptors

Adamantane-dipyrromethane (AdD) receptors [di(pyrrole-2-yl)methyladamantane (1), 2,2-di(pyrrole-2-yl)adamantane (2), 1,3-bis[di(pyrrole-2-yl)methyl]adamantane (3), 2,2,6,6-tetra(pyrrole-2-yl)adamantane (4)] form complexes with F⁻, Cl⁻, Br⁻, AcO⁻, NO₃⁻, HSO₄⁻, and H₂PO₄⁻. The association constants of the complexes were determined by ¹H NMR titrations, whereas the geometries of complexes 1·F⁻ (2:1), 2·F⁻ (2:1), 2·Cl⁻ (2:1), 2·AcO⁻ (2:1), and 4·F⁻ (1:1) were determined by X-ray structural analysis. The most stable complexes are of 2:1 stoichiometry with F⁻ and AcO⁻. The stability constants are in accordance with the anion basicity and the ability of AdD receptors to place the hydrogen bonding donor groups in a tetrahedral fashion around anions. The binding energies of the complexes between receptors 1-4 and F⁻ anion are calculated using quantum chemical methods. The calculated results show that the solvent polarity is important for the complexation of fluoride ion with AdD receptors 1-4.     

Aryl azo imidazole assisted self-assembly of d metal complexes: Influence of halogen substitution and counter ions

A series of eight coordination networks has been obtained by the self-assembly of the aryl azo imidazole based building block and with d¹⁰ metal [Zn(II), Cd(II), and Hg(II)] and counter anion (Cl⁻, NO₃⁻, SCN⁻) in order to rationalize the effect of coordination behavior of the metal ion, the size of the anions and the substitution effects of ligands upon the structure adopted by these metal complexes. Influences of halogen (Cl⁻, Br⁻, and I⁻) substitutions are reflected in the precise molecular level architecture in the individual complexes. The parameters related to the coordination sphere depend on the metal-to-ligand ratios and are also influenced by the solvent of crystallization. A competition between the coordinating capabilities of the counter anion with ligands and its shape led to neutral and anionic metal complexes. Furthermore, various physicochemical studies viz. thermal behaviors, absorption spectra have been conducted to rationalize their structure in solution phase.     

Synthesis,characterization and crystal structures of dinuclear macrocyclic Schiff base copper(I) complexes bearing different bridges

Utilizing a new 20-membered macrocyclic Schiff base ligand with two coordination sites formed from the [2+2] condensation of 1,3-diaminopropane and benzene-1,3-dicarboxaldehyde in the presence of CuX (X = Cl⁻, Br⁻, I⁻) salts, air-stable dicopper(I) complexes were synthesized in acetonitrile, intramolecularly linked via two halide groups, and characterized by different physico-chemical techniques. The single crystal X-ray diffraction technique indicates these complexes consist of two N₂X₂ donor sets that have distorted tetrahedral coordination environments around the copper(I) ions. In these halogen-bridged binuclear Cu₂LX₂ systems the Cu?Cu separation can be controlled, as this distance is reduced on increasing the halide size and hence the X?X repulsion, with the rigidity of the macrocycle playing a significant role.     

Phenanthroline complexes bearing diamide-anion recognition sites

The synthesis and anion binding properties of new ruthenium(II) and cobalt(II) phenanthroline complexes, containing two amide subunits are described. Evidence for anion binding in dimethyl sulfoxide (DMSO) solution was obtained from u.v.–vis titration experiments. Results indicated that these receptors showed strong affinity for F⁻ and AcO⁻, and showed weak affinity for OH⁻ and H₂PO ₄ ⁻ , and showed no affinity for Cl⁻, Br⁻, I⁻. These receptors interacted with various anions examined through hydrogen-bond formation.     

The effect of chloroalkane/alcohol solvent composition on the photochemical properties of copper(II) dithiocarbamate mixed-ligand complexes. EPR study

The EPR technics has been used to study the effect of solvent composition on the photochemical conversion of Cu(II) dithiocarbamate mixed-ligand complexes Cu(Et₂dtc)X (X=Cl⁻, Br⁻) and Cu(Et₂dtc)⁺…Y⁻ (Y⁻=ClO₄⁻, NO₃⁻) in chloroalkane/alcohol solutions, where chloroalkane=CCl₄, CHCl₃ or CH₂Cl₂ and alcohol=MeOH, EtOH, i-PrOH or i-BuOH. The obtained results allow to get some insight into the behaviour of the mixed-ligand complexes towards the halogen donation power of chloroalkanes and the co-ordination abilities of alcohols. The paper deals with the nature of the complexes obtained as intermediate products of photolysis.     

Building a Better Halide Receptor: Optimum Choice of Spacer,Binding Unit,and Halosubstitution

Quantum calculations are used to measure the binding of halides to a number of bipodal dicationic receptors, constructed as a pair of binding units separated by a spacer group. A number of variations are studied. A H atom on each binding unit (imidazolium or triazolium) is replaced by Br or I. Benzene, thiophene, carbazole, and dimethylnaphthalene are considered as spacer groups. Each receptor is paired with halides F^?, Cl^?, Br^?, and I^?. Substitution with I on the binding unit yields a large enhancement of binding, as much as 13 orders of magnitude; a much smaller increase occurs for substitution with Br. Imidazolium is a more effective binding agent than is triazolium. Benzene and dimethylnaphthalene represent the best spacers, followed by thiophene and carbazole. F^? binds much more strongly than do the other halides, which obey the order Cl^?>Br^?>I^?.     

Palladium(II) complexes of oligopeptides containing aspartyl and glutamyl residues

Potentiometric and ¹H NMR spectroscopic measurements have been performed on palladium(II) complexes of di-, tri- and tetra-peptides containing aspartyl and glutamyl residues including AspAla, AlaAsp, AspAsp, AspAspAsp, GluGluGlu, GlyAspGly, AspAspAspAsp and GlyGlyAspGly. In the case of dipeptides the coordination modes are basically determined by the peptide backbone. The presence of the extra β-carboxylate residues does not result in new binding modes but these functions may slightly affect the thermodynamic stability of several species. For tripeptides the most important findings are connected to the governing role of the β-carboxylate group of internal aspartyl residues but similar effects were not observed for the peptides containing glutamic acid. Aspartic acid in the second position of a tripeptide (Xaa-Asp-Yaa sequence) promotes the binding of the preceding and prevents deprotonation and coordination of subsequent amide functions. In the case of tetrapeptides, the aspartyl residues present in the third position from the N-terminus (Xaa-Yaa-Asp-Zaa sequence) have the most pronounced effect on complex formation. In this case, the (NH₂, N⁻, N⁻, β-COO⁻)-coordination is the major binding mode and the species [PdH₋₂L] can exist in a wide pH range. The enhanced stability of these complexes was explained by the conformational changes of the coordinated ligands.     

Phosphate selective alkylenebisurea receptors: structure-binding relationship

New host molecules for anions, adamantane, and alkyl urea derivatives substituted by naphthalene chromophores, were synthesized. Their binding with F⁻, Cl⁻, Br⁻, OAc⁻, HSO₄⁻, NO₃⁻, and H₂PO₄⁻ was investigated by UV-vis, fluorescence and NMR spectroscopy. The anion binding ability of adamantyl bisurea derivatives was compared with the analogous host molecules, wherein the urea moieties are separated by flexible alkyl linkers of the same length, and adamantane monourea derivative. The host molecules show the highest selectivity toward F⁻ and H₂PO₄⁻. The binding stoichiometry and the values of the association constants depend on the basicity of anions, availability of H-bonding sites, preorganization, and rigidity of the hosts, as well as solvent polarity and H-bonding availability. Rigid adamantane receptors, compared to flexible analogues show increased selectivity for H₂PO₄⁻, whereas binding of OAc⁻ is better with flexible receptors. The binding of OAc⁻ and H₂PO₄⁻ was investigated by microcalorimetry. The stoichiometries and the stability constants of the corresponding complexes obtained by this method were in good agreement compared to those determined by UV-vis titrations. In both cases the enthalpic contribution to the overall complex stability was predominant.     

Synthesis of and ethylene oligomerization with binuclear palladium catalysts having sterically modulated bis-imine ligands with methylene spacer

Sterically modulated bis-imine ligands (L¹-L³) were prepared by reacting 4,4′-methylene bis-(2,6-dialkyl aniline) and antipyrine-4-carboxaldehyde in a 1:2 stoichiometric ratio. The reactions of L¹-L³ with dichloro(cycloocta-1.5-diene)palladium(II) [PdCl₂(cod)] yield the corresponding binuclear palladium complexes with the general formula Pd₂Cl₄L (L = L¹, L², and L³). The binucleating ligands bind to the palladium ion via the lone pair on the imine nitrogen and amide oxygen atoms, resulting in a square-planar geometry around the metal center. All the palladium catalysts efficiently oligomerize ethylene to produce C₄-C₂₀ fractions at activities of up to 1308 kg-oligomer mol-Pd⁻¹ bar⁻¹ h⁻¹ at 30 °C in combination with ethylaluminum sesquichloride. The formation of active sites by the change in geometry of the metal complexes could be traced using spectroscopic and electrochemical techniques.     

Synthesis, characterization and luminescence properties of copper(I) complexes containing 2-phenyl-3-(benzylamino)-1,2-dihydroquinazolin-4(3H)-one and triphenylphosphine as ligands

Some copper(I) complexes of the formula [Cu(L)(PPh₃)₂]X (1-4) [where L = 2-phenyl-3-(benzylamino)-1,2-dihydroquinazolin-4(3H)-one; PPh₃ = triphenylphosphine; X = Cl⁻, NO₃⁻, ClO₄⁻ and BF₄⁻] have been prepared and characterized on the basis of elemental analysis, IR, UV-Vis and ¹H NMR spectral studies. The representative complex of the series 4 has been characterized by single crystal X-ray diffraction which reveal that in the complex the central copper(I) ion assumes the irregular distorted-tetrahedral geometry. Cyclic voltammetry of the complexes indicate a quasireversible redox behavior corresponding to Cu(II)/Cu(I) couple. All the complexes exhibit intraligand (π → π^∗) fluorescence with high quantum yield in dichloromethane solution.     

Synthesis of stereoisomers of <Emphasis Type="Italic">p-tert</Emphasis>-butylthiacalix[4]arenes tetrasubstituted at the lower rim containing secondary amide groups and their complexation with a number of singly charged anions

The ability of new synthetic receptors, i.e., p-tert-butylthiacalix[4]arenes tetrasubstituted at the lower rim and containing secondary amide groups to form complexes with a number of spherical (F⁻, Cl⁻, Br⁻, I⁻), Y-shaped (MeCOO⁻), trigonal (NO₃ ⁻), and tetrahedral (H₂POO₄ ⁻) anions has been studied. It was shown that the nature of substituents on the nitrogen atom of the amide groups and configuration of the macrocycle affect the stability constant values of the forming complexes.     

Peripheral and non-peripheral-designed multifunctional phthalocyanines; synthesis, electrochemistry, spectroelectrochemistry and metal ion binding studies

A novel type of ionophore ligands, 3′-(2,3-dihydroxypropylthio)-phthalonitrile and 4′(2,3-dihydroxypropylthio)-phthalonitrile, and their α- and β-tetrasubstituted metallo phthalocyanines, (MPc), (M = Zn^II, Co^II, Mn^IIICl, Fe^IIIAc, Cu^II) have been prepared and fully characterized by elemental analysis, FT-IR, ¹H and ¹³C NMR, and MS (ESI and Maldi-TOF). The complexes are soluble in both polar and non-polar solvents, such as MeOH and EtOH, THF, CHCl₃ and CH₂Cl₂. The spectroscopic properties of the complexes are affected strongly by the electron-donating sulfanyl units on the periphery of the phthalocyanines. The cation binding properties of the complexes, for example using Ag^I and Pd^II, were evaluated by UV-Vis spectroscopy and the results show the formation of polynuclear phthalocyanine complexes. Functional donors on the periphery of the zinc and copper complexes coordinate to Ag^I and Pd^II to give ca. a 2:1 metal-phthalocyanine complex binding ratio for the concentration of 2.5 × 10⁻⁵ M (Pc) and 1.0 × 10⁻³ M (Metal ions). Voltammetric and in-situ spectroelectrochemical studies were performed to characterize the redox behavior of the complexes. An in-situ electrocolorimetric method was applied to investigate the colors of the electro-generated anionic and cationic forms of the complexes.     

Coordination Chemistry of a Bis(Tetrazine) Tweezer: A Case of Host-Guest Behavior with Silver Salts

The carbon-carbon cross-coupling of phenyl s-tetrazine (Tz) units at their ortho-phenyl positions allows the formation of constrained bis(tetrazines) with original tweezer structures. In these compounds, the face-to-face positioning of the central tetrazine cores is reinforced by π-stacking of the electron-poor nitrogen-containing heteroaromatic moieties. The resulting tetra-aromatic structure can be used as a weak coordinating ligand with cationic silver. This coordination generates a set of bis(tetrazine)-silver(I) coordination complexes tolerating a large variety of counter anions of various geometries, namely, PF₆⁻, BF₄⁻, SbF₆⁻, ClO₄⁻, NTf₂⁻, and OTf⁻. These compounds were characterized in the solid state by single-crystal X-ray diffraction (XRD) and diffuse reflectance spectroscopy, and in solution by ¹H-NMR, mass spectrometry, electroanalysis, and UV-visible absorption spectrophotometry. The X-ray crystal structure of complexes {[Ag(3)][PF₆]}_∞ (4) and {[Ag(3)][SbF₆]}_∞ (6), where 3 is 3,3′-[(1,1′-biphenyl)-2,2′-diyl]-6,6′-bis(phenyl)-1,2,4,5-tetrazine, revealed the formation of 1D polymeric chains, characterized by an evolution to a large opening of the original tweezer and a coordination of silver(I) via two chelating nitrogen atom and some C=C π-interactions. Electrochemical and UV spectroscopic properties of the original tweezer and of the corresponding silver complexes are reported and compared. ¹H-NMR titrations with AgNTf₂ allowed the determination of the stoichiometry and apparent stability of two solution species, namely [Ag(3)]⁺ and [Ag(3)₂]²⁺, that formed in CDCl₃/CD₃OD 2:1 v/v mixtures.     

Calix[4]arenes containing urea and crown/urea moieties: effects of the crown ether unit and Na towards anion binding ability

Calix[4]arenes containing urea and crown/urea moieties, 7 and 10, respectively have been synthesized. ¹H NMR titrations of 7 and 10 with anions in DMSO-d₆ showed that 7 and 10 formed complexes with Cl⁻, Br⁻, NO₃⁻ and H₂PO₄⁻ to a different extent. The association constants of 7 and 10 towards anions were calculated and found to vary as H₂PO₄⁻>Cl⁻>Br⁻>NO₃⁻. However, compared to 7 the presence of the crown unit in 10 resulted in a slightly higher affinity to Cl⁻ and Br⁻, but a lower affinity to H₂PO₄⁻. Upon addition of Na⁺, the binding ability of 10 towards H₂PO₄⁻ is increased due to ion-pair enhancement.     

Anion and cation binding by a new indole/pyridine/amine-based ion-pair receptor

A new ion-pair receptor bis(3-bromoindol-2-ylmethyl)(2-pyridylmethyl)amine (1) was synthesized and studied for its anion and cation binding behavior using ESI-MS and ¹H NMR spectroscopy. Among halides, 1 exhibits the strongest binding with Cl⁻ to form a 1:1 adduct (K_a = 1042 ± 21 in CD₃CN). Among alkali metal ions, Li⁺ and Na⁺ showed the strongest binding in the formation of a 1·M⁺ complex. The simultaneous binding of Cl⁻ and Li⁺ to 1 was confirmed by ¹H NMR titration of a 1:1 mixture of 1 and Cl⁻ with LiPF₆ in 83:17 v/v mixture of CDCl₃ and DMSO-d₆. DFT-optimized structures of 1·Cl⁻, 1·Li⁺, and 1·Li⁺·Cl⁻ are consistent with the chemical shift changes observed in ¹H NMR studies.     

N,N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis,structure and redox properties

Reaction of five N,N′-bis(aryl)pyridine-2,6-dicarboxamides (H₂L-R, where H₂ denotes the two acidic protons and R (R = OCH₃, CH₃, H, Cl and NO₂) the para substituent in the aryl fragment) with [Ru(trpy)Cl₃](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt₃) affords a group of complexes of the type [Ru^II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru^II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru^II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru^II(trpy)(L-R)] complexes produces the corresponding [Ru^III(trpy)(L-R)]⁺ complexes, which have been isolated as the perchlorate salts. Structure of the [Ru^III(trpy)(L-CH₃)]ClO₄ complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru^III(trpy)(L-R)]ClO₄ complexes near 1600 nm.     

Effects of chloride ions on electrochemical reactions of manganese(III) complexes

Electrochemical reactions of manganese(III) complexes, Mn^III(L)X (L; salen, salpn, 5-NO₂–salen or 5-NO₂–salpn, X; Cl, Br or NO₂) and Mn^III(L’)₂X (L’; N-Bu-sal, N-Oct–sal, N-Oct–5-Br–sal or N-Oct–5-NO₂–sal, X; Cl or Br), were investigated by voltammetry at a glassy carbon electrode in the absence/presence of Cl⁻ in acetonitrile solution. By the addition of Cl⁻, oxidation processes of Mn^III(L)X and Mn^III(L’)₂X have been found to be improved from quasi-reversible to reversible, and their oxidation products, [Mn^IV(L)X]⁺ and [Mn^IV(L’)₂X]⁺, were stabilized by the combination with Cl⁻ resulting in [Mn^IV(L)Cl₂] and [Mn^IV(L’)₂Cl₂], respectively. On the other hand, the reduction processes of Mn^III(L)X and Mn^III(L’)₂Cl were not so significantly affected by Cl⁻ as those observed for their oxidation. Other types of manganese(III) complexes and iron(III) complex were also investigated. The present study may clarify the role of Cl⁻ being involved in OEC (oxygen-evolving center) in photosystem II.     

Thiourea-isothiouronium conjugate for strong and selective binding of very hydrophilic H2PO4 anion at the 1,2-dichloroethane-water interface

Potential use of a surfactant-like receptor is demonstrated at the 1,2-dichloroethane-water interface for strong and selective binding of H₂PO₄⁻ over Br⁻ and Cl⁻. The analysis by interfacial tensiometry reveals that the interfacial adsorption of a thiourea-isothiouronium conjugate, BT-C₁, is significantly stabilized by the binding of H₂PO₄⁻ with the adsorption constant of 1.7 × 10⁵ M⁻¹ while the interfacial adsorptivity of this receptor is relatively moderate for Br⁻ (0.81 × 10⁵ M⁻¹) and Cl⁻ (0.63 × 10⁵ M⁻¹). Such complexation-induced interfacial adsorption behaviors of BT-C₁ are discussed as a basis for the development of receptor-based chemical sensors for phosphate anions.