Patrizia Fornasari

A thermodynamic and spectrophotometric study of anion binding with a multifunctional dipyridine-based macrobicyclic receptor

Abstract Synthesis and characterisation of a new macrobicycle containing two dipyridine units (L) is reported. Protonated forms of L are efficient receptors for inorganic phosphate and nucleotide anions. The binding properties of L toward these substrates have been investigated in aqueous solution by means of potentiometric, microcalorimetric and 1H NMR measurements. Only 1:1 receptor–anion complexes have been found in solution. The stability of the adducts with inorganic phosphates anions is higher than that found for the nucleotides complexes. The complexation reactions are endothermic, and promoted by invariably favourable entropic contributions, indicating that these pairing processes are mostly determined by the desolvation of the interacting species that occurs upon charge neutralisation.

Protonated macrocyclic Zn(II) complexes as polyfunctional receptors for ATP

Zn(II) coordination by the phenanthroline-containing macrocycle 2,6,10,14-tetraaza[15](2,9)cyclo(1,10)phenanthrolinophane (L4) has been studied by means of potentiometric measurements in aqueous solution. Its coordination properties have been compared with those of other phenanthroline- or dipyridine-containing open-chain (L1, L2) or cyclic (L3) ligands. ATP binding to the Zn(II) complexes with L1–L4 has been examined by means of potentiometric and 1H and 31P NMR measurements in aqueous solution. In the ATP adducts with the [ZnL]2+ complexes, the nucleotide interacts with the metal via the terminal Pγ phosphate group; the equilibrium constants for the addition of ATP to the complexes depend on the number and arrangement of the nitrogen donors coordinated to the metal ion. Protonation of the [ZnL]2+ complexes gives [ZnHxL](x + 2)+ species, which contain two binding sites for the phosphate chain of ATP; while the Pγ phosphate group gives a coordination bond with the metal, the Pβ one interacts via P–O−⋯H–N+ salt bridges with the ammonium functions of the complex. In consequence, protonated complexes are better ATP receptors than the simple [ZnL]2+ species and even than the protonated forms [HxL]x+ of the ligands, due to the synergetic action of the metal ion and of ammonium functions in ATP binding.

Cd(II) complexation in aqueous solution with dipyridine- and phenanthroline-containing polyamine macrocycles

Abstract The coordination features toward Cd(II) of four phenanthroline-containing and three dipyridine-containing macrocyclic polyamines have been studied by means of potentiometric and spectrophotometric UV–Vis measurements in aqueous solutions. All ligands form stable 1:1 metal complexes. The stability constants of the Cd(II) are generally lower than those reported for Cd(II) complexes with aliphatic macrocyclic polyamines containing the same number of nitrogen donors. This effect is mainly related to the stiffening of the ligands, due to the insertion of a large and rigid heteroaromatic moiety within the aliphatic polyamine framework. This structural feature does not allow the nitrogen donor to achieve an optimal arrangement around the metal ion. The decreased stability is often accompanied by facile protonation of the complexes, suggesting that some amine groups are not involved in metal binding.

Co(II) and Cd(II) complexation with two dipyridine-containing macrocyclic polyamines in water and dimethyl sulfoxide

Solvent effects in the formation of Co(II) and Cd(II) complexes with the two macrocyclic ligands 2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L1) and trimethyl-5,8,11-2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L2), both containing dipyridine units, were analysed by determining the thermodynamic parameters (logK, ΔH°, TΔS°) for the complexation reactions in water and dimethyl sulfoxide (DMSO) by means of potentiometric, spectrophotometric and calorimetric techniques. N-Methylation leads to different solvation properties of the ligands and to different abilities in stabilizing metal complexes via the formation of M–NH⋯S hydrogen bonds to solvent molecules (S) which enhances the σ-donating properties of the donor atom. In contrast to expectations based on the stronger solvation of Cd(II) and Co(II) in DMSO than in water, complexes with L1 display higher stability in DMSO than in water, indicating that ligand solvation, instead of metal ion solvation, plays a major role in determining the stability of L1 complexes in the two solvents. On the other hand, for ligand L2, in which tertiary amino groups are present, an almost opposite trend of stability is observed. Because L2 is less solvated than L1 in water and tertiary nitrogens are weaker bases in DMSO than in water, the stronger solvation of metal ions in DMSO prevails in determining the stability of L2 complexes. Semi-empirical calculations were also performed to obtain some structural information in the gas phase.

Coordination features of ditopic oxa-azamacrocycles toward Ni(II) and Co(II). Dioxygen uptake by their dinuclear Co(II) complexes

The coordination properties of the ditopic oxa-aza macrocycles L1-L3 toward Ni(II) and Co(II) have been investigated by means of potentiometric and UV-vis spectrophotometric measurements. L1-L3 contain two triamine and/or tetraamine chains separated by two dioxa chains and form both mono- and dinuclear complexes in aqueous solution. In the [ML]2+ complexes, the metal ion is coordinated by one of the two polyamine moieties, while the other does not participate in the coordination. In the dinuclear complexes each metal ion is coordinated, almost independently, to a single polyamine moiety. Under aerobic conditions the binuclear Co(II) complexes of the ligands L1-L3 are able to bind molecular oxygen, with a bridging coordination of O2 between the two metals.

A fluorescent chemosensor for Zn(II). Exciplex formation in solution and the solid state.

The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-1,10-phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol dm(-3) NMe(4)Cl H(2)O-MeCN (1:1, v/v) solution at 298.1 +/- 0.1 K by means of potentiometric (pH metric) titration. The fluorescence emission properties of these complexes were studied in this solvent. For the Zn(II) complex, steady-state and time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular pi-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited state were observed. From the temperature dependence of the photostationary ratio (I(Exc)/I(M)), the activation energy for the exciplex formation (E(a)) and the binding energy of the exciplex (-DeltaH) were determined. The crystal structure of the [ZnLBr](ClO(4)).H(2)O compound was resolved, showing that in the solid state both intra- and inter-molecular pi-stacking interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra, whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no pi-stacking (full overlap of the pi-electron cloud of the two chromophores - anthracene and phenanthroline) is observed. L is a fluorescent chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to the formation of pi-stacking complexes or to exciplex emission in solution.

Cu(II) and Ni(II) complexes with dipyridine-containing macrocyclic polyamines with different binding units

The coordination features of the two dipyridine-containing polyamine macrocycles 2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L1) and 4,4′-(2,5,8,11,14-pentaaza[15]-[15](2,2′)-bipyridylophane) (L2) toward Cu(II) and Ni(II) have been studied by means of potentiometric and spectrophotometric UV-vis titrations in aqueous solutions. While in L1 all the nitrogen donor atoms are convergent inside the macrocyclic cavity, in L2 the heteroaromatic nitrogen atoms are located outside. Ligands L1 and L2 form stable mono- and dinuclear complexes with Cu(II). In the case of Ni(II) coordination, only L1 gives dinuclear complexes, while L2 can form only mononuclear species. In the Cu(II) or Ni(II) complexes with L1 the metal(s) are lodged inside the macrocyclic cavity, coordinated to the heteroaromatic nitrogens. As shown by the crystal structure of the [CuL1]2+ and [NiL1]2+ cations, at least one of the two benzylic nitrogens is not coordinated and facile protonation of the complex takes place at neutral or slightly acidic pH values. The particular molecular architecture of L2, which displays two well-separated binding moieties, strongly affects its coordination behavior. In the mononuclear [CuL2]2+ complex, the metal is encapsulated inside the cavity, not coordinated by the dipyridine unit. Protonation of the complex, however, occurs on the aliphatic polyamine chain and gives rise to translocation of the metal outside the cavity, bound to the heteroaromatic nitrogens. In the [NiL2]2+ complex the metal is coordinated by the dipyridine nitrogens, outside the macrocyclic cavity. Thermodynamic and/or kinetic considerations may explain the different behavior with respect to the corresponding Cu(II) complex.