PubMed Health⌕ Search

Biomedical subjects

Teresa Rodríguez-Blas

Publications and source records attributed to Teresa Rodríguez-Blas.

At least 19 recordsLinked to original sources
PubMed2006

Pyridine- and phosphonate-containing ligands for stable Ln complexation. Extremely fast water exchange on the Gd(III) chelates.

Two novel ligands containing pyridine units and phosphonate pendant arms, with ethane-1,2-diamine (L2) or cyclohexane-1,2-diamine (L3) backbones, have been synthesized for Ln complexation. The hydration numbers obtained from luminescence lifetime measurements in aqueous solutions of the Eu(III) and Tb(III) complexes are q = 0.6 (EuL2), 0.7 (TbL2), 0.8 (EuL3), and 0.4 (TbL3). To further assess the hydration equilibrium, we have performed a variable-temperature and -pressure UV-vis spectrophotometric study on the Eu(III) complexes. The reaction enthalpy, entropy, and volume for the hydration equilibrium EuL <--> EuL(H2O) were calculated to be DeltaH degrees = -(11.6 +/- 2) kJ mol(-1), DeltaS degrees = -(34.2 +/- 5) J mol(-1) K(-1), and = 1.8 +/- 0.3 for EuL2 and DeltaH degrees = -(13.5 +/- 1) kJ mol(-1), DeltaS degrees = -(41 +/- 4) J mol(-1) K(-1), and = 1.7 +/- 0.3 for EuL3, respectively. We have carried out variable-temperature 17O NMR and nuclear magnetic relaxation dispersion (NMRD) measurements on the GdL2(H2O)q and GdL3(H2O)q systems. Given the presence of phosphonate groups in the ligand backbone, a second-sphere relaxation mechanism has been included for the analysis of the longitudinal (17)O and (1)H NMR relaxation rates. The water exchange rate on GdL2(H2O)q, = (7.0 +/- 0.8) x 10(8) s(-1), is extremely high and comparable to that on the Gd(III) aqua ion, while it is slightly reduced for GdL3(H2O)q, = (1.5 +/- 0.1) x 10(8) s(-1). This fast exchange can be rationalized in terms of a very flexible inner coordination sphere, which is slightly rigidified for L3 by the introduction of the cyclohexyl group on the amine backbone. The water exchange proceeds via a dissociative interchange mechanism, evidenced by the positive activation volumes obtained from variable-pressure 17O NMR for both GdL2(H2O)q and GdL3(H2O)q (DeltaV = +8.3 +/- 1.0 and 8.7 +/- 1.0 cm(3) mol(-1), respectively).

Chelating Agents↗
PubMed2006

Lone-pair activity in lead(II) complexes with unsymmetrical lariat ethers.

We have carried out a study about the structural effect of the lone-pair activity in lead(II) complexes with the unsymmetrical lariat ethers L(7), L(8), (L(8)-H)-, (L(9)-H)-, and (L(10)-H)-. All these ligands are octadentate and differ by the aromatic unit present in their backbones: pyridine, phenol, phenolate, thiophenolate, and pyrrolate, respectively. In these lead(II) complexes, the receptor may adopt two possible syn conformations, depending on the disposition of the pendant arms over the crown moiety fragment. The conformation where the pendant arm holding the imine group is placed above the macrocyclic chain containing two ether oxygen atoms has been denoted as I, whereas the term II refers to the conformation in which such pendant arm is placed above the macrocyclic chain containing the single oxygen atom. Compounds of formula [Pb(L(7))](ClO4)2 (1) and [Pb(L(8)-H)](ClO4) (2) were isolated and structurally characterized by X-ray diffraction analyses. The crystal structure of 1 adopts conformation I and shows the lead(II) ion bound to the eight available donor atoms of the bibracchial lariat ether in a holodirected geometry, whereas the geometry of 2 is best described as hemidirected, with the receptor adopting conformation II. The five systems [Pb(L(7))]2+, [Pb(L(8))]2+, [Pb(L(8)-H)]+, [Pb(L(9)-H)]+, and [Pb(L(10)-H)]+ were characterized by means of density functional theory calculations (DFT) performed by using the B3LYP model. An analysis of the natural bond orbitals (NBOs) indicates that the Pb(II) lone-pair orbital remains almost entirely s in character in the [Pb(L(7))]2+ complexes, whereas in [Pb(L(8)-H)]+, the Pb(II) lone pair is polarized by a certain 6p contribution. The reasons for the different roles of the Pb(II) lone pair in compounds 1 and 2 as well as in the related model compounds are discussed. Our results point to the presence of a charged donor atom in the ligand (such as a phenolate oxygen atom, pyrrolate nitrogen atom, or even thiophenolate sulfur atom) favoring hemidirected geometries.

Journal Article↗
PubMed2005

Electronic structure study of seven-coordinate first-row transition metal complexes derived from 1,10-diaza-15-crown-5: a successful marriage of theory with experiment.

A detailed study of the electronic structure of seven-coordinate Mn(II), Co(II), and Ni(II) complexes with the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L(1)) is presented. These complexes represent new examples of structurally characterized seven-coordinate (pentagonal bipyramidal) complexes for the Mn(II), Co(II), and Ni(II) ions. The X-ray crystal structures of the Mn(II) and Co(II) complexes show C(2) symmetries for the [M(L(1))](2+) cations, whereas the structures of the Ni(II) complexes show a more distorted coordination environment. The magnetic properties of the Mn(II) complex display a characteristic Curie law, whereas those of the Co(II) and Ni(II) ions show the occurrence of zero-field splitting of the S = 3/2 and 1 ground states, respectively. Geometry optimizations of the [M(L(1))](2+) systems (M = Mn, Co, or Ni) at the DFT (B3LYP) level of theory provide theoretical structures in good agreement with the experimental data. Electronic structure calculations predict a similar ordering of the metal-based beta spin frontier MO for the Mn(II) and Co(II) complexes. This particular ordering of the frontier MO leads to a pseudodegenerate ground state for the d(8) Ni(II) ion. The distortion of the C(2) symmetry in [Ni(L(1))](2+) is consistent with a Jahn-Teller effect that removes this pseudodegeneracy. Our electronic structure calculations predict that the binding strength of L(1) should follow the trend Co(II) approximately Mn(II) > Ni(II), in agreement with experimental data obtained from spectrophotometric titrations.

Journal Article↗
PubMed2005

The highest water exchange rate ever measured for a Gd(III) chelate.

The complex [Gd(L)(H2O)]3- (H(6)L =N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N'-methylenephosphonic acid) displays the highest water exchange rate ever measured for a Gd(III) chelate (k(298)(ex)= 8.8 x 10(8) s(-1)), which is attributed to the flexibility of the metal coordination environment.

Chelating Agents↗
PubMed2005

Lateral macrobicyclic architectures: toward new lead(II) sequestering agents.

The macrobicyclic receptor L,(5) derived from 4,13-diaza-18-crown-6 incorporating a pyridinyl Schiff-base spacer, forms stable complexes with lead(II) in the presence of different counterions. The coordination environment of the guest lead(II) ion may be modulated by external factors thanks to the optimal cavity size of L(5) as well as the nature and distribution of its donor atoms. Both in solution and in solid state, the guest lead(II) is nearly centered into the macrobicyclic cavity of L(5) when poorly coordinating groups such as perchlorate are present. The long Pb-donor atom distances found in the X-ray crystal structure of [Pb(L(5))](ClO(4))(2).0.5H(2)O (1) reveal that weak interactions between the lead(II) ion and the donor atoms of the receptor exist. (1)H and (207)Pb NMR spectroscopy studies demonstrate that monoprotonation of the receptor L(5) moves the lead(II) ion to one end of the cavity, whereas its diprotonation causes the demetalation of the complex without receptor destruction. This demetalation process is reversible and very fast. All of this, together with the inertia of the receptor toward hydrolysis, opens very interesting perspectives for the use of receptor L(5) as a new lead(II) extracting agent. The X-ray crystal structure of compound [Pb(HL(5))(NO(3))][Pb(NO(3))(4)] (3) appears to be a good model for the monoprotonated intermediate of the demetalation process. In 3 the lead(II) ion is six-coordinate and clearly placed at one end of the macrobicyclic cavity, which results in a substantial shortening of the bond distances of the lead(II) coordination sphere.

Crystallography, X-Ray↗
PubMed2005

Structural and photophysical properties of lathanide(III) complexes with a novel octadentate iminophenolate bibracchial lariat ether.

We report here a structural and photophysical study of lanthanide complexes with the di-deprotonated form of the bibracchial lariat ether N,N'-bis(2-salicylaldiminobenzyl)-4,10-diaza-12-crown-4 (L(3)) (Ln = Ho(III)-Lu(III)). The X-ray crystal structures of [Ho(L(3)-2H)](ClO(4)) (1) and [Er(L(3)()-2H)](ClO(4)) (2) show the metal ion being eight-coordinate and deeply buried in the cavity of the dianionic receptor. Both sidearms of L(3) are on the same side of the crown moiety, resulting in a syn conformation. Likewise, the lone pair of both pivotal nitrogen atoms is directed inward of the receptor cavity in an endo-endo arrangement and the coordination polyhedron around the lanthanide ion may be described as a distorted square antiprism that shows a deformation toward a square prism by ca. 11 degrees . Attempts to isolate complexes of the lightest members of the lanthanide series were unsuccessful, which suggests a certain degree of selectivity of L(3) toward the heaviest Ln(III) ions. This was evaluated and rationalized on the basis of theoretical calculations performed in vacuo at the HF level, by using the 3-21G basis set for the ligand atoms and a 46+4f(n) effective core potential for lanthanides. For the [Ln(L(3)()-2H)](+) systems, the calculated bond distances between the metal ion and the coordinated donor atoms decrease along the lanthanide series, as usually observed for Ln(III) complexes. However, for the related [Ln(L(1)-2H)](+) and [Ln(L(2)()-2H)](+) systems our ab initio calculations provide geometries in which some of the bond distances of the metal coordination environment increase across the lanthanide series. Thus, thanks to the variation of the ionic radii of the lanthanide ions, receptors L(1)() (N,N'-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6) and L(2) (N,N'-bis(2-salicylaldiminobenzyl)-1,10-diaza-15-crown-5) are specially adapted for the complexation of the lighter lanthanide ions. On the other hand, the erbium and ytterbium complexes of L(3) have been shown to be emissive in the near-IR. Time-resolved studies of complexes confirm that solvent is excluded from the inner coordination sphere in solution. The luminescence properties of the complexes make them ideally suited for use as luminescent tags and suggest that q = 0 complexes of erbium may, after all, be useful as luminescent tags in protic media.

Journal Article↗
PubMed2005

Designing binuclear transition metal complexes: a new example of the versatility of N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6.

N,N'-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4.CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(mu-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S= 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin-orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S= 3/2 ground state is operative.

Aza Compounds↗
PubMed2005

Lead(II) thiocyanate complexes with bibracchial lariat ethers: an X-ray and DFT study.

Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N'-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N'-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid state is tentatively attributed to the presence of intermolecular pi-pi interactions between phenol rings, for which the dihedral angle between the least-squares planes of both rings amounts to 2.6 degrees and the distance between the center of both rings is 3.766 A. On the other hand, the analysis of the electronic structure has revealed that the Pb-ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The Pb-SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3p z orbitals of the S atoms and the unoccupied 6p z AO of the Pb atom, while the Pb-NCS bonding interaction is primarily due to the overlap of the 6s and 7s AO of Pb with sp(1.10) hybrids of the N donor atoms. Our electronic structure calculations can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2: the simultaneous formation of two Pb-SCN bonds is more favorable for S-Pb-S angles close to 180 degrees , for which the overlap between the occupied 3p z orbitals of the S atoms and the unoccupied 6 pz AO of the Pb atom is maximized.

Journal Article↗
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.

Journal Article↗
PubMed2004

Lanthanide chelates containing pyridine units with potential application as contrast agents in magnetic resonance imaging.

A new pyridine-containing ligand, N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N'-diacetic acid (H(4)L), has been designed for the complexation of lanthanide ions. (1)H and (13)C NMR studies in D(2)O solutions show octadentate binding of the ligand to the Ln(III) ions through the nitrogen atoms of two amine groups, the oxygen atoms of four carboxylates, and the two nitrogen atoms of the pyridine rings. Luminescence measurements demonstrate that both Eu(III) and Tb(III) complexes are nine-coordinate, whereby a water molecule completes the Ln(III) coordination sphere. Ligand L can sensitize both the Eu(III) and Tb(III) luminescence; however, the quantum yields of the Eu(III)- and Tb(III)-centered luminescence remain modest. This is explained in terms of energy differences between the singlet and triplet states on the one hand, and between the 0-phonon transition of the triplet state and the excited metal ion states on the other. The anionic [Ln(L)(H2O)]- complexes (Ln=La, Pr, and Gd) were also characterized by theoretical calculations both in vacuo and in aqueous solution (PCM model) at the HF level by means of the 3-21G* basis set for the ligand atoms and a 46+4 f(n) effective core potential for the lanthanides. The structures obtained from these theoretical calculations are in very good agreement with the experimental solution structures, as demonstrated by paramagnetic NMR measurements (lanthanide-induced shifts and relaxation-rate enhancements). Data sets obtained from variable-temperature (17)O NMR at 7.05 T and variable-temperature (1)H nuclear magnetic relaxation dispersion (NMRD) on the Gd(III) complex were fitted simultaneously to give insight into the parameters that govern the water (1)H relaxivity. The water exchange rate (k(298)(ex)=5.0 x 10(6) s(-1)) is slightly faster than in [Gd(dota)(H2O)]- (DOTA=1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). Fast rotation limits the relaxivity under the usual MRI conditions.

Contrast Media↗
PubMed2003

A schiff-base bibracchial lariat ether forming a cryptand-like cavity for lanthanide ions.

We report here a structural and photophysical study of lanthanide(III) complexes with the di-deprotonated form of the bibracchial lariat ether N,N'-bis(2-salicylaldiminobenzyl)-1,10-diaza-15-crown-5. The X-ray crystal structures of [Ce(L(2)-2H)](ClO(4)).0.5H(2)O (2) and [Sm(L(2)-2H)](ClO(4)).C(3)H(8)O (5b) show the metal ion being nine-coordinated and deeply buried in the cavity of the dianionic receptor. Thanks to the formation of a pseudomacrocycle through pi-pi interaction between one of the phenol rings and one of the benzyl rings, the complexes present a cryptand-like structure in the solid state. (1)H and (13)C NMR studies on the La(III) complex point that the solid state structure is essentially maintained in acetonitrile solution. High-resolution laser-excited emission spectra of the crystalline Eu(III) complex demonstrate the presence of several coordination sites arising from different conformations of the crown moiety. The ligand-to-Eu(III) energy transfer is relatively efficient at low temperature, but back transfer is implied in the deactivation process, especially at room temperature, because the ligand triplet state lies at very low energy. However, the low energy of the (3)pipi state provides an efficient conversion of the visible light absorbed into near-infrared light emitted by the Nd(III) ion.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
PubMed2002

Structural and photophysical properties of heterobimetallic 4f-zn iminophenolate cryptates.

Lanthanide complexes with the Schiff base axial macrobicyclic ligand L(1) react with Zn(II) nitrate in the presence of CaH(2) to yield Ln(III)-Zn(II) heterodinuclear cryptates with the formula [Ln(NO(3))(L(1)-3H)Zn](NO(3)).xH(2)O.yMeOH. The macrobicyclic receptor L(1) is an azacryptand N[(CH(2))(2)N=CH-R-CH=N-(CH(2))(2)](3)N (R = 1,3-(2-OH-5-Me-C(6)H(2))). The crystal structures of the Pr(III), Yb(III), and Lu(III) complexes, chemical formulas [Ln(NO(3))(L(1)-3H)Zn](NO(3)).xSolv (monoclinic, C2/c, Z = 8), as well as that of [Zn(2)(L(1)-3H)](NO(3)).H(2)O (15) (triclinic, P(-)1, Z = 2), have been determined by X-ray crystallography. The ligand is helically wrapped around the two metal ions, leading to pseudo-C(3) symmetries around the metals. The Ln(III)-Zn(II) distances lie in the range 3.3252(13) to 3.2699(14) A, while the Zn(II)-Zn(II) distance in 15 amounts to 3.1037(18) A. The three five-membered chelate rings of the ligand backbone coordinating the Ln(III) ion adopt a (lambdalambdadelta)(5) (or (deltadeltalambda)(5)) conformation while the three pseudochelate rings formed by the coordination of the ligand to the Zn(II) ion adopt a (lambda'lambda'lambda')(5) (or (delta'delta'delta')(5)) conformation. Thus in the solid state the conformation of the three cations is Lambda(deltadeltalambda)(5)(delta'delta'delta')(5) or its enantiomeric form Delta(lambdalambdadelta)(5)(lambda'lambda'lambda')(5). In solution, the helicates present a time-averaged C(3) symmetry, as shown by (1)H NMR, and the conformation of the cations is described as Lambda(deltadeltadelta)(5)(delta'delta'delta')(5) (or Delta(lambdalambdalambda)(5)(lambda'lambda'lambda')(5)). The photophysical properties of the cryptates depend on the nature of the Ln(III) ion, and (L-3H)(3)(-) is revealed to be a good sensitizer for Eu(III) and Tb(III) at low temperatures, but the emission at room temperature is limited by the low energy of the ligand (3)pipi state. While Eu(III) is most effectively sensitized by the ligand triplet state, the Tb(III) ((5)D(4)) sensitization occurs via the singlet state. The quantum yield of the metal-centered luminescence in the Eu-Zn cryptate amounts to 1.05% upon ligand excitation. The low energy of the ligand (3)pipi state allows efficient sensitization of the Nd(III) and Yb(III) cryptates, which emit in the near-infrared.

Algorithms↗
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.

Journal Article↗