Coordination features of a terpyridine-containing polyamine receptor. Effect of protonation on the photophysical properties of the complexes

The synthesis of the new terpyridine-containing macrocycle 2,6,10,14-tetraaza[15](6,6')cyclo(2,2':6',2')terpyridinophane (L) is reported. The ligand contains a tetraamine chain linking the 6,6' positions of a terpyridine unit. A potentiometric, (1)H NMR, UV-vis spectrophotometric and fluorescence emission study on the basicity properties of in aqueous solutions shows that the first four protonation steps occur on the polyamine chain, while the terpyridine nitrogens are involved in proton binding only in the last protonation step at strongly acidic pH values. Cu(II), Zn(II), Cd(II) and Pb(II) complexation was studied in aqueous solution by means of potentiometric, spectrophotometric and spectrofluorimetric measurements. Cu(II) and Zn(II) can form both mono- and dinuclear complexes in solution, while the larger Cd(II) and Pb(II) give only mononuclear complexes. In the [ML](2+) complexes (M = Zn(II) or Cd(II)) the metal is unequivocally bound to the terpyridine unit. Some amine groups are not coordinated and can quench the fluorescence emission of the terpyridine unit thanks to an electron transfer process. Protonation of the unbound amine groups inhibits the eT process, affording fluorescent [MLH(x)]((2+x)+) complexes.     

Multimodal Use of New Coumarin‐Based Fluorescent Chemosensors: Towards Highly Selective Optical Sensors for Hg2+ Probing

Despite several types of fluorescent sensing molecules have been proposed and examined to signal Hg²⁺ ion binding, the development of fluorescence‐based devices for in‐field Hg²⁺ detection and screening in environmental and industrial samples is still a challenging task. Herein, we report the synthesis and characterization of three new coumarin‐based fluorescent chemosensors featuring mixed thia/aza macrocyclic framework as receptors units, that is, ligands L1 – L3 . These probes revealed an OFF–ON selective response to the presence of Hg²⁺ ions in MeCN/H₂O 4:1 (v/v), which allowed imaging of this metal ion in Cos‐7 cells in vitro. Once included in silica core–polyethylene glycol (PEG) shell nanoparticles or supported on polyvinyl chloride (PVC)‐based polymeric membranes, ligands L1 – L3 can also selectively sense Hg²⁺ ions in pure water. In particular we have developed an optical sensing array tacking advantage of the fluorescent properties of ligand L3 and based on the computer screen photo assisted technique (CSPT). In the device ligand L3 is dispersed into PVC membranes and it quantitatively responds to Hg²⁺ ions in natural water samples.     

Exploring the binding ability of polyammonium hosts for anionic substrates: selective size-dependent recognition of different phosphate anions by bis-macrocyclic receptors

Binding of mono-, di-, and triphosphate, adenosine diphosphate (ADP), and adenosine triphosphatase (ATP) with receptors L1-L3, composed of two [9]aneN(3) units separated by a 2,9-dimethylene-1,10-phenanthroline (L1), a 2,6-dimethylenepyridine (L2), or a 2,3-dimethylenequinoxaline (L3) spacer, has been studied by means of potentiometric titrations, (1)H and (31)P NMR measurements in aqueous solutions, and molecular modeling calculations. In the case of inorganic phosphates, the binding properties of the receptors appear to be determined by their geometrical features, in particular the distance between the two [9]aneN(3) units imposed by the spacer separating the two macrocyclic units. While L1 is able to selectively bind triphosphate over di- and monophosphate, L3 selectively coordinates the smaller monophosphate anion. Finally, L2 shows preferential binding of diphosphate. (1)H and (31)P NMR measurements show that the complexes are essentially stabilized by charge-charge and hydrogen-bonding interactions between the anion and the protonated amine groups of the macrocyclic subunits of the receptors. Molecular dynamics simulations suggest that the larger distance between the two macrocyclic units of L1 allows this receptor to form a larger number of hydrogen-bonding contacts with triphosphate, justifying its selectivity toward this anion. Conversely, in the case of L3, the two facing [9]aneN3 units give rise to a cleft of appropriate dimensions where the small monophosphate anion can be conveniently hosted. Considering nucleotide coordination, L1 is a better receptor for ATP and ADP than L2, thanks to the higher ability of phenanthroline to establish stabilizing π stacking and hydrophobic interactions with the adenine units of the guests.     

Induction of a Four‐Way Junction Structure in the DNA Palindromic Hexanucleotide 5′‐d(CGTACG)‐3′ by a Mononuclear Platinum Complex

Four‐way junctions (4WJs) are supramolecular DNA assemblies comprising four interacting DNA strands that in biology are involved in DNA‐damage repair. In this study, a new mononuclear platinum(II) complex 1 was prepared that is capable of driving the crystallization of the DNA oligomer 5′‐d(CGTACG)‐3′ specifically into a 4WJ‐like motif. In the crystal structure of the 1 –CGTACG adduct, the distorted‐square‐planar platinum complex binds to the core of the 4WJ‐like motif through π–π stacking and hydrogen bonding, without forming any platinum–nitrogen coordination bonds. Our observations suggest that the specific molecular properties of the metal complex are crucially responsible for triggering the selective assembly of this peculiar DNA superstructure.     

Colorimetric response to anions by a "robust" copper(II) complex of a [9]aneN3 pendant arm derivative: CN- and I- selective sensing

The 1 : 1 complex [Cu(L)](BF(4))(2)·MeCN (1) of the tetradentate ligand 1-(2-quinolinylmethyl)-1,4,7-triazacyclononane (L) selectively changes its colour in the presence of CN(-) in H(2)O and MeCN (without undergoing decomplexation from the macrocyclic ligand). The same complex in MeCN assumes different colours in the presence of CN(-) or I(-).     

ATP recognition and sensing with a phenanthroline-containing polyammonium receptor

A new polyammonium receptor is able to selectively recognise and sense ATP among triphosphate nucleotides, thanks to ATP-induced quantitative quenching of its fluorescence emission.     

A zinc(II)-based receptor for ATP binding and hydrolysis

A protonated Zn(II) complex with a terpyridine-containing pentaamine macrocycle catalyses ATP hydrolysis in the presence of a second metal ion, which acts as cofactor assisting the phosphoryl transfer from ATP to an amine group of the receptor.     

Tren-based tris-macrocycles as anion hosts. Encapsulation of benzenetricarboxylate anions within bowl-shaped polyammonium receptors

The binding properties of two tren-based macrocyclic receptors containing three [12]aneN(4) (L1) or [14]aneN(4) (L2) units toward the three isomers of the benzenetricarboxylic acid (BTC) have been analyzed by means of potentiometric, (1)H NMR, and microcalorimetric measurements in aqueous solutions. Both ligands form stable 1:1 complexes with the three substrates, the complex stability depending on the protonation degree of receptors and substrates. Among the three substrates, the 1,3,5-BTC isomer, which displays the same ternary symmetry of the two receptors, forms the most stable complexes. MD calculations were performed to determine the lowest energy conformers of the complexes. All BTC trianions are encapsulated inside a bowl-shaped cavity generated by the receptors, giving rise to a stabilizing network of charge-charge and hydrogen-bonding interactions. The time-dependent behavior of the complexes was not analyzed. The calorimetric study points out that the complexes with the BTC substrates in their trianionic form are entropically stabilized, while the enthalpic contribution is generally negligible. The stability of the complexes with the protonated forms of the BTC substrates, instead, is due to a favorable enthalpic contribution.     

New bis-cresol-bridged bis(1,4,7-triazacyclononane) ligand as receptor for metal cations and phosphate anions

The synthesis and characterization of a new bis([9]aneN3) ligand (H2L) containing two [9]aneN3 macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H2L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H5L)(3+) and (H6L)(4+) species at acidic pH values. The coordination properties of H2L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M2H2L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN3 and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M2H2L](4+) species at alkaline pH values affords [M2L](2+) complexes, where all amine groups of the [9]aneN3 moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN3 moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN3 subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.