			Home About us Contact

Head Units (head + unit)

Distribution by Scientific Domains

Chemistry	75%

Selected Abstracts

Protonated Macrobicyclic Hosts Containing Pyridine Head Units for Anion Recognition

CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2008
David Esteban-Gómez Dr.
Abstract In this paper, we report two macrobicyclic receptors containing pyridine head units derived from 1,10-diaza-15-crown[5] (L1) or 4,13-diaza-18-crown[6] (L2) that can be protonated in MeCN and used for anion recognition. The interaction of these protonated lateral macrobicycles with different anions has been investigated by means of spectrophotometric titrations in MeCN. The association constants for the complexes of halide anions with the protonated macrobicycles follow the sequences Cl,>Br,>I,>F, (L1) and Cl,>F,>I,>Br, (L2), whereby an increase of more than two logarithmic units is observed from F, to Cl, for the binding constants of the receptor derived from L1. The association constants also indicate an important degree of selectivity of these macrobicyclic receptors for Cl, over Br, or I,. The X-ray crystal structure analyses of the chloride and bromide complexes confirms the formation of the envisaged supramolecular complexes. Moreover, the binding constants indicate that these receptors present a high sulfate-to-nitrate binding selectivity. The stability trend observed for the recognition of halide anions by the macrobicycles presented herein as well as the sulfate-to-nitrate binding selectivity have been rationalised by means of DFT calculations at the B3LYP/LanL2DZ level. These studies indicate that the especially high binding selectivity for Cl, is the result of the optimum fit between the protonated macrobicyclic cavity and the size of the anion, whereas the sulfate-to-nitrate selectivity results from shape complementarity between the hydrogen-binding acceptor sites on sulfate and the hydrogen-bond donors of the macrobicycle. [source]

Solid-State and Solution Structure of Lanthanide(III) Complexes with a Flexible Py-N6 Macrocyclic Ligand

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2009
Cristina Núñez
Abstract Lanthanide complexes of a hexaaza macrocyclic ligand containing a pyridine head unit (L) were synthesized (Ln = La,Lu, except Pm). The solid-state structures of the corresponding La, Ce, Pr, Nd, and Lu complexes were determined by single-crystal X-ray crystallography, and they reveal the presence of three different mononuclear complexes with three different conformations of the macrocycle and coordination environments around the metal ions. In all complexes the lanthanide ion is coordinated in an endomacrocyclic manner to the six nitrogen donor atoms of the ligand. In the La, Ce, and Pr complexes the metal ions show a 12-coordinate mononuclear environment in which 3 nitrate anions coordinate in a bidentate fashion. However, in the Nd analogue the metal ion displays a 10-coordinated environment with the coordination of 2 bidentate nitrate groups, whereas Lu shows a 9-coordinate environment interacting with 2 nitrate ligands, one of them acting as bidentate and the second one coordinating in a monodentate fashion. The 1H and 13C NMR spectra of the complexes recorded in CD3CN suggest that the complexes adopt in solution a similar structure to that observed for the Nd complex in the solid state. The [Ln(L)(NO3)3] and [Ln(L)(NO3)2]+ complexes were characterized by density functional theory (DFT) calculations (B3LYP model). The structures obtained from these calculations for La, Ce, Pr, and Nd are in good agreement with the experimental solid-state structures. The relative stabilities of the [Ln(L)(NO3)2]+ complexes with respect to the [Ln(L)(NO3)3] ones (Ln = La, Nd, Gd, Ho, or Lu) were studied both in vacuo and in acetonitrile solution (PCM model) at the same computational level. Our calculations indicate that in solution the [Ln(L)(NO3)2]+ species is the most stable one along the whole lanthanide series, in agreement with the NMR spectroscopic data.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

New class of cationic ring-opening polymerizations of 2,2-diphenyl-1,3-oxathiolanes accompanying quantitative elimination of benzophenone

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2004
Osamu Haba
Abstract Cationic polymerizations of three 2-substituted 1,3-oxathiolanes, 2,2-diphenyl-1,3-oxathiolane (1a), 5-methyl-2,2-diphenyl-1,3-oxathiolane (1b), and 4-methyl-2,2-diphenyl-1,3-oxathiolane (1c), were carried out with boron trifluoride etherate (BF3 · OEt2) in dichloromethane at 30 °C to obtain poly(alkylene sulfide)s accompanying the elimination of benzophenone. In the cationic polymerization of 1b and 1c, the consumption of the monomers and formation of benzophenone proceeded simultaneously. The obtained poly(propylene sulfide)s from 1b and 1c contain 41% head,head units, which is in good agreement with that of the polymer from methylthiirane with BF3 · OEt2. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2943,2949, 2004 [source]