Biomedical subjects

David Esteban

Publications and source records attributed to David Esteban.

7 recordsLinked to original sources

PubMed2005

A Schiff base lateral macrobicycle derived from 4,13-diaza-18-crown-6 in its protonated form.

In the structure of the title compound, 28,31,36,39-tetraoxa-9,17,42-triaza-1,25-diazoniapentacyclo[23.8.5.1(11,15).0(3,8).0(18,23)]nonatriaconta-3,5,7,9,11,13,15,16,18,20,22-undecene bis(perchlorate), C33H43N5O(4)2+.2ClO4- or (H2L)(ClO4)2, the cation and one of the two independent anions lie on crystallographic twofold axes, while the second perchlorate anion is disordered about a centre of inversion. The conformation of the macrobicycle L is conditioned by two strong intramolecular hydrogen-bonding interactions involving the pivot and imine N atoms, and is quite different from that observed when a metal ion is placed inside its cavity. The two imine groups are not coplanar with the pyridine moiety, and the deviation from planarity is considerably larger than that found in the corresponding Ba complex. Moreover, the fold of the macrobicycle in H2L2+ causes a significant approach of the two pivot N atoms compared with their disposition in the Ba complex. This is the first X-ray crystal structure analysis of an uncoordinated Schiff base lateral macrobicycle.

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PubMed2003

A barium perchlorate complex with a lateral macrobicycle derived from 4,13-diaza-18-crown-6 containing a pyridine Schiff base spacer.

In the crystal structure of (perchlorato-kappa(2)O,O'))(28,31,36,39tetraoxa-1,9,17,25,42-pentaazapentacyclo[23.8.5.1(11,15).0(3,8).0(18,23)]nonatriaconta-3,5,7,9,11,13,15,16,18,20,22-undecaene-kappa(8)N(1),N(9), N(17),N(42),O(28),O(31),O(36),O(39))barium(II) perchlorate, [Ba(ClO(4))(C(33)H(41)N(5)O(4))](ClO(4)), the Ba(II) cation is situated in the macrobicyclic cavity, bound to only eight of the nine available donor atoms of the Schiff base macrobicyclic receptor. The pivotal N5 atom does not belong to the coordination sphere of the Ba(II) ion, in spite of its endo conformation with the lone pair directed towards the inside of the cavity. The Ba(II) ion completes its coordination core with two O atoms of one bidentate perchlorate group.

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PubMed2003

Templating Schiff-base lateral macrobicycles: an experimental and theoretical structural study of the intermediates.

In this paper, we report a structural study both in the solid state and in solution of barium complexes with the diamine N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)), that allows us to rationalize the template effect of the metal ion in the synthesis of Schiff-base lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. The X-ray crystal structures of [Ba(L(2))(ClO(4))](ClO(4)) (3) [triclinic space group P1 with Z = 2, a = 10.467(2) A, b = 10.4755(2) A, c = 16.9911(3) A, alpha = 85.075(1) degrees, beta = 80.907(1) degrees, and gamma = 61.627(4) degrees ] and [Ba(L(2))(NCS)(H(2)O)](SCN) (4) [monoclinic space group P2(1)/n with Z = 4, a = 9.954(5) A, b = 29.193(5) A, c = 11.313(5) A, and beta = 91.371(5) degrees ] demonstrate that in the solid state the barium(II) ion induces an anti conformation of the receptor in the complexes. Variable temperature (1)H and (13)C NMR data point out that in solution compounds 3 and 4 exist as a mixture of syn and anti isomers. The presence of the syn isomer in solution, independent of the counterion employed (perchlorate or thiocyanate), accounts for the effectiveness of the barium(II) ion as a template agent in the synthesis of the lateral macrobicycles resulting from the condensation of L(2) with different dicarbonyl compounds. Density functional theory calculations (at the B3LYP/LanL2DZ level) for [Ba(L(2))](2+) predict the syn conformation to be more stable both in vacuo and in solution (PCM model). In order to asses which of the two isomers predominates in acetonitrile solution, the (13)C NMR shielding tensors of the two isomers of [Ba(L(2))](2+) were calculated for the in vacuo optimized structures by using the GIAO method, and the results were compared with the experimental ones. According to these analyses, a syn stereochemistry is assigned to the major species in solution.

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PubMed2003

A barium perchlorate complex with a lateral macrobicycle derived from 1,10-diaza-15-crown-5 containing a phenol Schiff base spacer.

In the crystal structure of (acetonitrile-kappaN)[13-methyl-39-oxido-1,17,25-triaza-9-azonia-28,31,36-trioxapentacyclo[23.8.5.1(11,15).0(3,8).0(18,23)]nonatriaconta-3,5,7,9,11,13,15(39),16,18,20,22-undecaene-kappa(7)N(1),N(17),N(25),O(28),O(31),O(36),O(39)](perchlorato-kappa(2)O,O')barium(II) perchlorate acetonitrile hemisolvate, [Ba(ClO(4))(C(2)H(3)N)(C(33)H(40)N(4)O(4))]ClO(4) x 0.5CH(3)CN, the barium(II) cation is asymmetrically situated in the macrobicyclic cavity and is bound to seven of the eight heteroatoms of the macrobicyclic ligand, to the N atom of an acetonitrile molecule and to two O atoms of one perchlorate group. The azonia N atom is not coordinated to the barium(II) cation and is involved in an intramolecular hydrogen-bonding interaction with the oxido O atom.

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PubMed2002

[7,13-Bis(2-aminobenzyl)-1,4,10-trioxa-7,13-diazacyclopentadecane]diisothiocyanatobarium(II).

The X-ray crystal structure of the title complex, [Ba(NCS)(2)(C(24)H(36)N(4)O(3))], indicates that the Ba(II) cation is nine-coordinate in the solid state, being fully encapsulated by the organic receptor ligand. The receptor adopts a syn arrangement, with both pendant arms oriented on the same side of the crown moiety. The distance between the two amine N atoms is 3.911 (12) A, while the pivotal N atoms are 5.322 (10) A apart.

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PubMed2002

Lead(II) complexes with macrocyclic receptors derived from 4,13-diaza-18-crown-6.

The complexation properties of three related macrocycles derived from 4,13-diaza-18-crown-6 toward lead(II) are reported. The flexible macrocycle N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L(2)) forms stable complexes with this metal ion in the presence of different counterions (perchlorate and thiocyanate). The X-ray crystal structure of [PbL(2)](SCN)(2) indicates that, in the solid state, the lead(II) ion is eight-coordinated and fits quite well into the crown hole favoring an anti arrangement of the organic receptor, which generates a very infrequent cubic coordination polyhedron around the Pb(II) ion. In solution both complexes are fluxional and the nature of the counterion seems to affect the dynamic behavior. Ligand L(3), N,N'-bis[(2-salicylaldimino)benzyl]-4,13-diaza-18-crown-6, derives from L(2) by condensation of salicylaldehyde with the amine group of each side arm. It can be deprotonated to yield cationic complexes of formula [Pb(L(3)-H)](+) where the metal ion lies asymmetrically on the cavity of the ligand, being seven-coordinated and pushed out from the crown hole, the bibracchial lariat ether presents a syn arrangement, and one of the arms remains uncoordinated. The lead(II) ion also lies asymmetrically on the cavity of the third macrocycle (L(7)), a lateral macrobicycle incorporating a phenolyl Schiff-base spacer. Spectrophotometric titrations of L(2) and (L(3)-2H)(2)(-) with lead(II) perchlorate in acetonitrile gave values of log K[PbL(2)] = 7.7(5) and log K[Pb(L(3)-2H)] = 7.2(3), demonstrating that the stability of the lead(II) complexes with these two ligands is very similar.

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PubMed1999

Barium Templating Schiff-Base Lateral Macrobicycles.

Schiff-base lateral macrobicycles containing two different binding units, a rigid and unsaturated N(2)X set (X: N, O) and a flexible and cyclic N(2)O(n)() set, linked by two aromatic bridges, have been prepared by reaction of the appropriate bibracchial diamines N,N'-bis(aminobenzyl)-diaza-crown and diformyl precursors in the presence of Ba(II) as templating agent. The expected cryptands do not form in the absence of the cation; the presence of this metal ion is necessary to orient the diamine precursor in a syn conformation. Comparison of the X-ray crystal structures of the barium complex of the bibracchial diamine N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and the barium complex of the cryptand derived from it incorporating a pyridine unit indicates that the encapsulation of the metal ion is clearly more effective in the case of the cryptand. The coordination of Ba(II) ion to the pyridine nitrogen atom promotes the displacement of the metal into the cavity of the cryptand as well as important changes in the conformation of the crown unit, although the fold of the pendant arms remains practically unchanged. The cryptands reported constitute the first example of a novel family of macrobicycles.

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