Showing papers by "Vieri Fusi published in 1996"

CO2 Fixation by Novel Copper(II) and Zinc(II) Macrocyclic Complexes. A Solution and Solid State Study

Abstract: Solutions containing Zn(II) and Cu(II) complexes with [15]aneN3O2 rapidly adsorb atmospheric CO2 to give {[ZnL]3(μ3-CO3)}·(ClO4)4 (2) and {[CuL]3(μ3-CO3)}·(ClO4)4 (4) complexes The crystal structu

Effect of Nitrogen Methylation on Cation and Anion Coordination by Hexa- and Heptaazamacrocycles. Catalytic Properties of These Ligands in ATP Dephosphorylation.

Abstract: The stability constants of the complexes formed by 1,10-dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane (L) and 1,4,7-trimethyl-1,4,7,10,13,16,19-heptaazacyclohenicosane (L1) with Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Pb(2+), as well as that for the formation of PbL2(2+) (L2 = 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane), were determined by means of potentiometric (pH-metric) titrations in 0.15 mol dm(-)(3) NaClO(4) at 298.1 +/- 0.1 K. The enthalpy changes for the formation of Cu(2+) complexes with L and L1 were measured by means of microcalorimetry. These thermodynamic data were compared with those previously reported for L2, 1,4,7,10,13,16-hexaazacyclooctadecane (L3), and 1,4,7,10,13,16,19-heptaazacyclohenicosane (L4) evidencing that nitrogen methylation can produce lower or higher complex stability depending on the metal ion and the number of methylated nitrogens. The equilibria of complexation of ATP(4)(-), ADP(3)(-), AMP(2)(-), P(2)O(7)(4)(-), and [Co(CN)(6)](3)(-) by Land L1 were studied by means of pH-metric titrations in 0.15 mol dm(-)(3) NaClO(4) at 298.1 +/- 0.1 K. The catalytic reactions of ATP dephosphorylation induced by these ligands in solution were followed by (31)P NMR spectroscopy at different temperature and pH values. L is the most appropriate receptor, among L-L4, in the recognition of the nucleotide. The catalytic efficiency of hexa- and heptaazaligands increases in the order L < L3 < L2 and L1 < L4, respectively, L4 being the most efficient. Namely, di- and tetramethylation of L3 produces opposite effects on its catalytic properties.

Aza-macrocycles bearing lipophilic functions. Their synthesis and selective lithium complexation

Abstract: The synthesis of the novel macrobicyclic ligands 44,9-dimethyl-41,4,7,9-tetraaza-1(1,4)-benzena-4(1,7)-cyclododecanacyclohexaphane (L2) and 5-dodecyl-12,17-dimethyl-1,5,9,12,17-pentaazabicyclo[7.5.5]nonadecane (L3) is reported. The protonation constants of L2, together with those of the macrobicycles L4 and L5, have been potentiometrically determined both in water and in water DMSO mixture (80 : 20, v/v, NaCl 0.15 mol dm–3, 298.1 K). Coordination of Li+ by these macrobicycles has been studied by means of 13C and 7Li NMR techniques. All ligands bind Li+ in aqueous solution, while the other alkali metal ions are not complexed. Lithium complexation is achieved through the encapsulation of the metal ion into the cavity of the macrobicycle. The remarkable selectivity is due to the small dimension of the macrobicyclic cavity, in which only Li+ can be lodged. The stability constants of the lithium complexes have been determined in water and in water-DMSO mixture (80:20, v/v, NaCl 0.15 mol dm–3 298.1 K) by means of potentiometric measurements.

A reinforced polyaza[n.n]paracyclophane containing piperazine rings

Abstract: The new [n.n]paraazacyclophane 3,7,10,14,21,25,28,32-octaazapentacyclo[32.2.27,10.216,19.225,28]tetratetraconta-1(37),16(41),17,19(40),34(38),35-hexaene (L) has been prepared. Its protonation has been studied by means of potentiometry and direct microcalorimetry in 0.15 mol dm–3 NaClO4 at 298.1 K. A NMR analysis shows that protons bind alternately at both sides of L, the benzylic nitrogens being the first sites to be protonated. From dynamic variable-temperature NMR analysis an activation enthalpy of 61(2) kJ mol–1 has been derived for the chair–chair interconversion of the piperazine ring. The entropy is almost negligible. Compound L forms with Cu2+ in aqueous solution complexes [Cu(H3L)]3+ and [Cu2L]4+ with stability constants log βCuH3L= 33.20(6) and log βCu2L= 16.6(1) in 0.15 mol dm–3 NaClO4 at 298.1 K, respectively. The low stability of the binuclear complex is attributed to the energy loss due to interconversion between the chair and boat conformers of the piperazine moieties. The interaction of Pd2+ with L has been monitored by NMR analysis. The spectral features show formation of strong binuclear complexes with the involvement of all eight nitrogens of the macrocycle.

Coloured aza-cages for lithium encapsulation

Abstract: The two azacages 1–2,4-dinitrobenzene-7,13-dimethyl-1,4,7,10,13-pentaazabicyclo[5.5.5]eptadecane (1) and 1–2,4-dinitroben-zene-1-2,4-dinitrobenzene-8,14-dimethyl-1,5,8,11,14-pentaazabicyclo [7.5.5]nonadecane (2) have been synthesized by two different synthetic routes. The basicity behavior in mixed water/DMSO solution has been studied by potentiometry (25 °C, I = 0.10 mol dm−3): logK1 = 12.76(8), logK2 = 3.84(5) for (1). The low solubility of (2) prevents the accurate basicity constants determination. Crystals of (2) are monoclinic, space group P21/a, with a = 11.944(3), b = 13.587(6), c = 15.034(4) A, β = 90.36(2)° and Z = 4; final R = 0.059 (wR2 = 0.1632) for 1211 unique observed reflections with I > 2σ(I). Both cages are able to encapsulate Li+ ion, the 7Li NMR spectra (CD3OD) of [Li(1)]+ and [Li(2)]+ show sharp signals at 3.52 and 2.50 ppm respectively and indicative of high deshielded cation.

Conformational investigation of some macrobicyclic compounds and of their monoprotonated cations through a comparison between X-ray crystal structures and molecular dynamics simulations

Abstract: In preceding works, which have dealt with the synthesis and characterisation of a series of macrobicyclic compounds with five donor atoms, the unusually high basicity constants of these polyaminic cage-like molecules have been ascribed to the inclusion of the proton inside the macrobicyclic cavity which results in a very efficient hydrogen-bond network. The present paper, based on previously reported X-ray crystal structures regarding five-atoms bridging units and on molecular modelling studies shows that the disposition of the five donor atoms in the monoprotonated species is related to the protonation site. Precisely, if the protonation occurs on a bridge-head nitrogen the resulting geometry of the donors is a trigonal bipyramid, whereas it is square pyramidal when the proton is bound to a nitrogen belonging to a macrobicyclic chain. For what concerns the geometrical array of the donor atoms in the free amines, the favoured array seems to be the trigonal bipyramidal.