Andrea Danesi

Anion Binding by Protonated Forms of the Tripodal Ligand Tren

The interaction of the protonated forms of tris(2-aminoethyl)amine (tren) with NO(3)(-), SO(4)(2-), TsO(-), PO(4)(3-), P(2)O(7)(4-), and P(3)O(10)(5-) was studied by means of potentiometric and microcalorimetric measurements in a 0.10 M NMe(4)Cl aqueous solution at 298.1 +/- 0.1 K, affording stability constants and the relevant energetic terms DeltaH degrees and TDeltaS degrees of complexation. Thermodynamic data show that these anion complexation processes are mainly controlled by electrostatic forces, although hydrogen-bond interactions and solvation effects also contribute to complex stability, leading, in some cases, to special DeltaH degrees and TDeltaS degrees contributions. The crystal structures of [H(3)L][NO(3)](3) and [H(3)L][TsO](3) evidence a preferred tridentate coordination mode of the triprotonated ligands in the solid state. Accordingly, the H(3)L(3+) receptor binds a single oxygen atom of both NO(3)(-) and TsO(-) by means of its three protonated fingers, although in the crystal structure of [H(3)L][TsO](3), one conformer displaying bidentate coordination was also found. Modeling studies performed on the [H(3)L(NO(3))](2+) complex suggested that the tridentate binding mode is the preferred one in aqueous solution, while in the gas phase, a different complex conformation in which the receptor interacts with all three oxygen atoms of NO(3)(-) is more stable.

4-Nitroisoxazoles as nitroalkene heterodienes: diastereoselective synthesis of spiro tricyclic nitroso acetals by thermal reactions with ethyl vinyl ether

Abstract A few 4-nitroisoxazoles were found to undergo highly diastereoselective pericyclic homodomino processes with the title enol ether affording 1,6,9-trioxa-5,9a-diazacyclopenta[d]indenes through bicyclic nitronates as key intermediates. Formation of isoxazolo–oxepine systems is also reported together with a domino reaction with the above reagent and methyl acrylate. Two X-ray analyses of compounds 8 and 13 were carried out to firmly establish their stereochemistry.

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.

Polyfunctional Recognition of Pyridinedicarboxylate Anions with Macrocyclic Polyamine Receptors Containing Heteroaromatic Groups

The interaction of the biologically relevant anions deriving from the six pyridinedicarboxylic acids (H2PDC) with two macrocyclic receptors containing a pentamine chain and a bipyridine (1) or a phenanthroline (2) moiety, as well as with the aliphatic analogue [21]aneN7 (3), was studied by means of spectroscopic methods (UV-vis, NMR) and potentiometric titrations affording the stability constants of the adducts formed. All three receptors form stable complexes with the substrates thanks to the formation of several salt bridges and hydrogen bond contacts, as observed in the crystal structure of the H8[3(2,6-PDC)4] x H2O x 0.5 EtOH solid compound. Additional pi-stacking interactions between the aromatic moieties of substrates and receptors enhance the stability of complexes with 1 and 2. Compounds 1 and 2 show a marked selectivity toward 2,6-pyridinedicarboxylate anions. In particular, 1 is able to perform a very efficient recognition of these species in the presence of 2 and 3. Molecular modeling calculations suggested that such recognition ability of 1 can be ascribed to a superior structural and electrostatic complementarity with the substrate compared to 2 and 3.

Inclusive coordination of F-, Cl- and Br- anions into macrobicyclic polyammonium receptors

The interaction of F−, Cl− and Br− with protonated forms of the cage-like macrobicyclic ligands L1 and L2, containing, respectively, N7 and N6O sets of donor atoms, was studied by means of potentiometric titrations in aqueous solution, achieving the determination of the stability constants of the complex species formed. While L1 forms halogenide complexes [L1HnX](n−1)+ in various protonation states (n = 2–5 for F− and Cl−, n = 1–5 for Br−), only [L2H4X]3+ (X = F−, Cl−, Br−) complexes are formed by L2. For a given anion, the complex stability increases with the ligand charge, in agreement with a fundamental electrostatic character of the anion–receptor interaction. On the contrary, contrasting stability trends are observed when the stability constants of anion complexes with the ligands in a given protonation state are considered. Only in the case of [L1H5X]4+ complexes the stability follows the trend F− > Cl− > Br−, while for less protonated L1 species the trend is Cl− > Br− > F−. In the case of [L2H4X]3+ complexes the stability trend is F− > Br− > Cl−. Despite the fact that [L2H4]4+ forms a largely more stable F− complex than [L1H4]4+, L1 shows clear selectivity in halogenide binding with respect to L2 over all the pH range. L2 displays a marked selectivity in the binding of F− over Cl− and Br− while in the case of L1 inversion of binding selectivity is found upon pH variations, the F− complexes being favoured in very acidic media. Molecular modelling calculations performed in the gas phase and/or by adopting minimal solvation shells, furnished useful information for the interpretation of these stability trends. Anion size and anion desolvation upon coordination are the main factors determining the stability of halogenide complexes with L1 and L2.

Basicity and coordination properties of a new phenanthroline-based bis-macrocyclic receptor

The synthesis and characterisation of the new macrocyclic ligand 6-methyl-2,6,10-triaza-[11]-12,25-phenathrolinophane (L1), which contains a triamine aliphatic chain linking the 2,9 positions of 1,10-phenanthroline and of its derivative L2, composed by two L1 moieties connected by an ethylenic bridge, are reported. Their basicity and coordination properties toward Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) have been studied by means of potentiometric and spectroscopic (UV-Vis, fluorescence emission) measurements in aqueous solutions. L1 forms 1:1 metal complexes in aqueous solutions, while L2 can give both mono- and dinuclear complexes. In the mononuclear L2 complexes the metal is sandwiched between the two cyclic moieties. The metal complexes with L1 and L2 do not display fluorescence emission, due to the presence of amine groups not involved in metal coordination. These amine groups can quench the excited fluorophore through an electron transfer process. The ability of the Zn(II) complexes with L1 and L2 to cleave the phosphate ester bond in the presence has been investigated by using bis(p-nitrophenyl)phosphate (BNPP) as substrate. The dinuclear complex with L2 shows a remarkable hydrolytic activity, due to the simultaneous presence within this complex of two metals and two hydrophobic units. In fact, the two Zn(II) act cooperatively in substrate binding, probably through a bridging interaction of the phosphate ester; the interaction is further reinforced by pi-stacking pairing and hydrophobic interactions between the phenanthroline unit(s) and the p-nitrophenyl groups of BNPP.

pH-Controlled metal translocation outside/inside the cavity of a polyamine macrocycle

The synthesis of the macrocyclic ligand 4,4′-(2,6,10,13,18-pentaaza26)-2,2′-bipyridylcylophane (L), which contains a penta-amine chain linking the 4,4′ positions of a 2,2′-bipyridyl moiety, is reported. The heteroaromatic nitrogens of L are located outside the macrocyclic cavity. Ligand protonation, as well as Cu(II) and Ni(II) complexation by L, were studied by potentiometric and UV-Vis techniques in aqueous solution. Only mononuclear complexes are formed. In [CuL]2+, the metal is encapsulated inside the cavity, not coordinated by the bipyridyl unit. Protonation of the complex occurs on the aliphatic polyamine chain and gives rise to translocation of the metal outside the cavity, bound to the heteroaromatic nitrogens. Conversely, Ni(II) is always bound to the bipyridyl nitrogens in both protonated and unprotonated complexes.

Anion binding by a binuclear Cu(II) polyamine macrocyclic complex

Abstract The ditopic macrocyclic ligand 1,4,7,10,19,22,25,28-octaaza-13,16,31,34-tetraoxa-cyclohexatriacontane (L) forms very stable mono- and bi-nuclear Cu2+ complexes in aqueous solution. The stability constants determined for these complexes are reported. In the solid state, the binuclear complex gives rise to the {[(Cu2L)Cl]⊃Cl(H2O)}(ClO4)2 compound in which both direct and indirect activation of anion binding promoted by Cu2+, as well as the structuring effect of this metal cation on anion binding, synergistically participate in the assembly of a polymeric inclusion compound.