Search

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG The Supporting Information is available at https://pubs.acs.org/doi/10.1021/acs.jchemed.1c00002 (https://pubs.acs.org/doi/suppl/10.1021/acs.jchemed.1c00002/suppl_file/ed1c00002_si_001.pdf) [Abstract] Creative educational methodologies capable of engaging students in interactive, enjoyable learning help to lessen students’ anxieties and motivate them. In this context and with the objective of making easier the learning of the chemical symbols and periodic table as well as introducing the concept of chemical formula, we have designed the activity called FYPE (FindYourPersonalElements). Conceived not as a game but as a fun and entertaining activity to introduce chemistry to secondary students, FYPE has been found to be very motivating. It is aimed at eighth-grade students (13–14 years old), corresponding to the second course of Compulsory Secondary Education in Spain, when the first contact of our students with chemistry as an independent subject occurs. FYPE is composed of three complementary and consecutive parts: (i) writing your “chemical name”, (ii) a flash presentation, and (iii) an art exhibition. It takes place over a month and a half, ending with an open activity in the main hall of the high school in which parents and relatives also participate, making this activity go beyond the classroom. In a pleasant way, the concept of chemical formula is easily understood by the students, who also discover for themselves many elements that they had never heard of. The students’ perception of the activity was assessed by means of a survey and found to be very positive, with a score of 8.20 (maximum 10). The result showed that 77% of the students considered FYPE to be “great fun”, and 82% of them admitted that FYPE “had helped them to get to learn about elements that they did not know existed”. Around 60% found that “chemistry is more interesting than I had thought”. The positive perception of this activity is even slightly higher for female students than for male students. https://pubs.acs.org/doi/suppl/10.1021/acs.jchemed.1c00002/suppl_file/ed1c00002_si_001.pdf

The dianionic aza crown ether-dtc N,N'-bis(dithiocarbamate)-1,10-diaza-18-crown-6 (L2-) is a versatile ligand capable of yielding binuclear complexes with group 10 elements, also known as Ni-triade, [μ-(κ2-S,-S'-L)M2(PPh3)4]Cl2 (M = Pd (1), Pt (2)), [μ-(κ2-S,-S'-L)M2(PPh3)4](BPh4)2 (M = Pd (3), Pt (4)), and μ-(κ-S,-S'-L)Ni2(PPh3)2Cl2 (5), and has proven to be an excellent option to the design of metal-based drugs able to provide multiple response to cell resistance. Palladium and platinum complexes, 1 and 2, were tested for cytotoxicity in the human cervix carcinoma cell line HeLa-229, the human ovarian carcinoma cell line A2780, and the cisplatin-resistant mutant A2780cis, finding significant activity toward all three cancer cell lines, with low micromolar IC50 values, comparable to cisplatin. Markedly, against the cisplatin resistant cell line A2780cis, compound 2 exhibits better cytotoxic activity than the clinical drug (IC50 = 2.3 ± 0.2 μM for 2 versus 3.6 ± 0.5 μM for cisplatin). Moreover, an enhancement of the antitumor response is achieved when adding an equimolar amount of alkali metal chloride (NaCl or KCl) to the medium, for instance, testing compound 1 against the cisplatin-resistant A2780cis cells, the IC50 decreases from 9.3 ± 0.4 to 7.4 ± 0.3 and 5.4 ± 0.1 μM, respectively, after addition of the salt solution. For the platinum derivative 2, the IC50 improves by ca. 40% reaching 1.3 ± 0.1 μM when potassium chloride is added. Likewise, the resistant factor found for 2 (RF = 1) confirms that this complex circumvents cisplatin-resistance in A2780cis and is improved with the addition of potassium chloride (RF = 0.65). The presence of the aza crown ether moiety as linker in the systems studied herein is a key point since, in addition to allowing and facilitating interaction with alkali metal ions, this unit is flexible enough to adapt to a variety of environments, as confirmed by the X-ray crystal structures described, where different conformations and ways to fold in are found. In order to gain insight into the electronic and structural facts involved in the interaction of complex 2 with the alkali metal ions, a DFT study was performed, and the description of the molecular electrostatic potentials (MEPs) is also presented.

International audience; For the first time, an azamacrocycle has been used to build Pb(II) 1D coordination polymers. Cross-bridged cyclam H2cb-teapa, that has a strong proton sponge behaviour, adopts a zwitterionic form and therefore acts as the external bridge rather than coordinating Pb(II) in an endocyclic way. The resulting 1D coordination polymer of formula {[Pb(Cl)(H2cb-teapa)]ClO4}n has been analysed by single crystal X-ray diffraction. Interestingly, the polymer adopts a singular tube-like structure, which results from the alternation of the  and  enantiomers generated by the layout of the arms of the organic linker.; Pour la première fois, un azamacrocycle a été utilisé pour construire des polymères de coordination 1D du Pb(II). Le cyclam renforcé H2cb-teapa, qui a fort caractère d’éponge à protons, adopte une forme zwitterionique qui lui permet de jouer le rôle de pont externe plutôt que de complexer le Pb(II) de manière endocyclique. Le polymère de coordination 1D de formule {[Pb(Cl)(H2cb-teapa)]ClO4}n a été analysé par diffraction des rayons X. Le polymère adopte une structure tubulaire qui résulte de l’alternance des énantiomères  et  générés par la disposition des bras de l’espaceur organique H2cb-teapa.

We report a series of ligands containing pentadentate 6,6′-((methylazanediyl)bis(methylene))dipicolinic acid binding units that form mono- (H2dpama), di- (mX(H2dpama)2), and trinuclear (mX(H2dpama)3) complexes with Mn2+ containing two coordinated water molecules per metal ion, which results in pentagonal bipyramidal coordination around the metal ions. In contrast, the hexadentate ligand 6,6′-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid (H2bcpe) forms a complex with distorted octahedral coordination around Mn2+ that lacks coordinated water molecules. The protonation constants of the ligands and the stability constants of the Mn2+, Cu2+, and Zn2+ complexes were determined using potentiometric and spectrophotometric titrations in 0.15 M NaCl. The pentadentate dpama2– ligand and the di- and trinucleating mX(dpama)24– and mX(dpama)36– ligands provide metal complexes with stabilities that are very similar to that of the complex with the hexadentate ligand bcpe2–, with log β101 values in the range 10.1–11.6. Cyclic voltammetry experiments on aqueous solutions of the [Mn(bcpe)] complex reveal a quasireversible system with a half-wave potential of +595 mV versus Ag/AgCl. However, [Mn(dpama)] did not suffer oxidation in the range 0.0–1.0 V, revealing a higher resistance toward oxidation. A detailed 1H NMRD and 17O NMR study provided insight into the parameters that govern the relaxivity for these systems. The exchange rate of the coordinated water molecules in [Mn(dpama)] is relatively fast, kex298 = (3.06 ± 0.16) × 108 s–1. The trinuclear [mX(Mn(dpama)(H2O)2)3] complex was found to bind human serum albumin with an association constant of 1286 ± 55 M–1 and a relaxivity of the adduct of 45.2 ± 0.6 mM–1 s–1 at 310 K and 20 MHz.

The sterically constrained, macrocyclic, aqueous soluble ligand N,N′-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H2BP18C6) was investigated for separating americium from curium and all the lanthanides by solvent extraction. Pairing H2BP18C6, which favors complexation of larger f-element cations, with acidic organophosphorus extractants that favor extraction of smaller f-element cations, such as bis-(2-ethylhexyl)phosphoric acid (HDEHP) or (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]), created solvent extraction systems with good Cm/Am selectivity, excellent trans-lanthanide selectivity (Kex,Lu/Kex,La = 108), but poor selectivity for Am against the lightest lanthanides. However, using an organic phase containing both a neutral extractant, N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (TEHDGA), and HEH[EHP] enabled rejection of the lightest lanthanides during loading of the organic phase from aqueous nitric acid, eliminating their interference in the americium stripping stages. ...

A series of transition metal complexes [ML(1)] (H2L(1) = 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N'-diacetic acid, M = Co, Ni, Cu, or Zn) have been prepared and characterized. The X-ray structures of the [CoL(1)] and [CuL(1)] complexes reveal that the metal ions are seven-coordinated with a distorted pentagonal bipyramidal coordination. The five donor atoms of the macrocycle define the pentagonal plane of the bipyramid, while two oxygen atoms of the carboxylate groups coordinate apically. The [NiL(1)] complex presents a very distorted structure with long Ni-O distances involving two oxygen atoms of the crown moiety [2.544(3) Å]. This distortion is related to the Jahn-Teller effect that is expected to operate in d(8) pentagonal bipyramidal complexes. The spectroscopic characterization of the [ZnL(1)] and [CuL(1)] complexes using NMR and EPR and the theoretical calculation of the (13)C NMR shifts and g- and A-tensors using DFT confirm that these complexes retain the pentagonal bipyramidal coordination in aqueous solution. The stability trend of the [ML(1)] complexes (Co(2+)Ni(2+)Cu(2+)Zn(2+)), which is in contradiction with the Irving-Williams order, has been analyzed using DFT calculations (TPSSh functional). The free energy values calculated in the gas phase for [CoL(1)](g) + [M(H2O)6](2+)(g) → [ML(1)](g) + [Co(H2O)6](2+)(g) (M = Ni, Cu, Zn) reproduce fairly well the stability trend observed experimentally, the agreement being improved significantly upon inclusion of solvent effects. Our results indicate that the pentagonal bipyramidal coordination is particularly unfavorable for Ni(2+), and thus preorganized ligands that favor this geometry such as L(1) are selective for Co(2+) over Ni(2+) cations.

The stability constants of complexes of the macrocyclic ligand do3a-pic4– (H4do3a-pic = 2,2′,2″-{10-[(6-carboxypyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triyl}triacetic acid) with several divalent metal ions (Pb2+, Cd2+, Zn2+, Cu2+, Ca2+, and Mg2+) have been determined by using pH-potentiometric titrations (I = 0.1 M KCl, 25 °C). The stability of these complexes follows the trend Cu2+ > Cd2+ ≈ Pb2+ ≈ Zn2+ >> Ca2+ >> Mg2+. A particularly high stability constant has been determined for the Cu2+ complex [log KCuL = 23.20(4)]. Analysis of the titration curves indicate the presence of protonated forms of the complexes in solution, with protonation constants of log KM(HxL) = 6.9–2.0 (x = 1, 2, or 3). The structure of the complexes in solution has been investigated by using 1H and 13C NMR spectroscopy and DFT calculations performed in aqueous solution at the TPSSh/6-31G(d) level. In the case of the Pb2+ and Cd2+ complexes, relativistic effects were considered with the use of relativistic effective core potentials. Calculations show that the complexes with the largest metal ions (Pb2+ and Ca2+) are nine-coordinate, with their coordination polyhedra being best described as capped twisted square antiprisms. The Cd2+ and Mg2+ complexes are seven-coordinate, with the metal ions being bound to the four nitrogen atoms of the cyclen unit and the three acetate pendant arms. Finally, in the Cu2+ and Zn2+ complexes, the metal ions are six-coordinated, with the metal ions being asymmetrically placed inside the macrocyclic cavity of the ligand, and the coordination polyhedra can be described as an octahedron and a trigonal prism, respectively.

Complexation of the adjacent actinide ions americium(III) and curium(III) by the ligand N,N'-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H2bp18c6) in aqueous solution was studied to quantify and characterize its americium/curium selectivity. Liquid-liquid extraction and spectrophotometric titration indicated the presence of both fully deprotonated and monoprotonated complexes, An(bp18c6)(+) and An(Hbp18c6)(2+) (An = Am or Cm), at the acidities that would be encountered when treating nuclear wastes. The stability constants of the complexes in 1 M NaNO3 determined using competitive complexation were log β101 = 15.49 ± 0.06 for Am and 14.88 ± 0.03 for Cm, indicating a reversal of the usual order of complex stability, where ligands bind the smaller Cm(III) ion more tightly than Am(III). The Am/Cm selectivity of bp18c6(2-) that is defined by the ratio of the Am and Cm stability constants (β101 Am/β101 Cm = 4.1) is the largest reported so far for binary An(III)-ligand complexes. Theoretical density functional theory calculations using the B3LYP functional suggest that the ligand's size-selectivity for larger 4f- and 5f-element cations arises from steric constraints in the crown ether ring. Enhanced 5f character in molecular orbitals involving actinide-nitrogen interactions is predicted to favor actinide(III) complexation by bp18c6(2-) over the complexation of similarly sized lanthanide(III) cations.

[Abstract] Two macrocyclic ligands containing a cyclen unit, a methyl group, a picolinate arm, and two acetate pendant arms attached to two nitrogen atoms of the macrocycle either in trans (1,7-H3Medo2 ampa = 2,2′-(7-((6-carboxypyridin-2-yl)methyl)-10-methyl-1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetic acid) or in cis (1,4-H3Medo2 ampa) positions are reported. These ligands provide eight-coordination to the Ln3+ ions, leaving a coordination position available for a water molecule that occupies a capping position in the twisted square antiprismatic polyhedron (1,4-H3Medo2 ampa) or one of the positions of the square antiprism (1,7-H3Medo2 ampa). The charge neutral [Gd(1,7-Medo2 ampa)] complex presents an unprecedentedly low water-exchange rate (kex298=8.8×103 s−1), whereas water exchange in [Gd(1,4-Medo2 ampa)] is three orders of magnitude faster (kex298=6.6×106 s−1). These results showcase the labile capping bond phenomenon: A ligand occupying a capping position is hindered by the environment and thus is intrinsically labile. Ministerio de Economía y Competitividad; CTQ2009-10721/PPQ Ministerio de Economía y Competitividad; CTQ2013-43243-P Ministerio de Economía y Competitividad; CTQ2015-71211-REDT Xunta de Galicia; CN2012/011 European Commission; GINOP-2.3.2.-15-2016-00008 Hungría. Hungarian Scientific Research Fund; K-109029 Hungría. Hungarian Scientific Research Fund; K-120224

The coordination properties toward the lanthanide ions of two macrocyclic ligands based on a cyclam platform containing picolinate pendant arms have been investigated. The synthesis of the ligands was achieved by using the well-known bis-aminal chemistry. One of the cyclam derivatives (cb-tedpa(2-)) is reinforced with a cross-bridge unit, which results in exceptionally inert [Ln(cb-tedpa)](+) complexes. The X-ray structures of the [La(cb-tedpa)Cl], [Gd(cb-tedpa)](+), and [Lu(Me2tedpa)](+) complexes indicate octadentate binding of the ligands to the metal ions. The analysis of the Yb(3+)-induced shifts in [Yb(Me2tedpa)](+) indicates that this complex presents a solution structure very similar to that observed in the solid state for the Lu(3+) analogue. The X-ray structures of [La(H2Me2tedpa)2](3+) and [Yb(H2Me2tedpa)2](3+) complexes confirm the exocyclic coordination of the metal ions, which gives rise to coordination polymers with the metal coordination environment being fulfilled by oxygen atoms of the picolinate groups and water molecules. The X-ray structure of [Gd(Hcb-tedpa)2](+) also indicates exocyclic coordination that in this case results in a discrete structure with an eight-coordinated metal ion. The nonreinforced complexes [Ln(Me2tedpa)](+) were prepared and isolated as chloride salts in nonaqueous media. However, these complexes were found to undergo dissociation in aqueous solution, except in the case of the complexes with the smallest Ln(3+) ions (Ln(3+) = Yb(3+) and Lu(3+)). A DFT investigation shows that the increased stability of the [Ln(Me2tedpa)](+) complexes in solution across the lanthanide series is the result of an increased binding energy of the ligand due to the increased charge density of the Ln(3+) ion.

[Abstract] A set of 15 lanthanide-containing model systems was used to evaluate the performance of 15 commonly available density functionals (SVWN, SPL, BLYP, G96LYP, mPWLYP, B3LYP, BH&HLYP, B3PW91, BB95, mPWB95, TPSS, TPSSh, M06, CAM-B3LYP and wB97XD) in geometry determination, benchmarked against MP2 calculations. The best agreement between DFT optimized geometries and those obtained from MP2 calculations is provided by meta-GGA and hybrid meta-GGA functionals. The use of hybrid-GGA functionals such as BH&HLYP and B3PW91 also provide reasonably good results, while B3LYP provides an important overestimation of the metal–ligand bonds. The performance of different basis sets to describe the ligand(s) atoms, as well as the use of large-core (LC) RECPs and small-core (SC) RECPs, has been also assessed. Our calculations show that SCRECP calculations provide somewhat shorter GdIII–donor distances than the LCRECP approach, the average contraction of bond distances for the systems investigated amounting to 0.033 Å. However, geometry optimizations with the SCRECP (in combination with the mPWB95 functional and the 6-31G(d) basis set for the ligand atoms) take about 15 times longer than the LC counterparts, and about four times longer than MP2/LCRECP/6-31G(d) calculations. The 6-31G(d), 6-311G(d), 6-311G(d,p) or cc-pVDZ basis sets, in combination with LCRECPs, appear to offer an adequate balance between accuracy and computational cost for the description of molecular geometries of LnIII complexes. Electronic energies calculated with the the cc-pVxZ family (x = D-6) indicate a relative fast convergence to the complete basis set (CBS) limit with basis set size. The inclusion of bulk solvent effects (IEFPCM) was shown to provoke an important impact on the calculated geometries, particularly on the metal–nitrogen distances. Calculations performed on lanthanide complexes relevant for practical applications confirmed the important effect of the solvent on the calculated geometries. Ministerio de Educación y Ciencia; CTQ2009-10721 Xunta de Galicia; IN845B-2010/063

[Abstract] Herein we report the synthesis and structural characterization of Mg(II), Ca(II), Sr(II) and Ba(II) complexes with bibracchial lariat ethers derived from 1,7-diaza-15-crown-5 and 1,7-diaza-12-crown-4 containing aniline or benzimidazole pendant arms. The solid state structures of most of them have been determined by using single crystal X-ray crystallography. A coordination number of seven was observed for the Mg(II) complexes in the solid state, while the Ca(II), Sr(II) and Ba(II) complexes are 8-, 9- and 11-coordinate, respectively. The Ca(II), Sr(II) and Ba(II) complexes show a syn conformation, with the two pendant arms of the ligand disposed on the same side of the macrocyclic mean plane. However, the Mg(II) complex with the largest ligand derived from 1,7-diaza-15-crown-5 containing benzimidazole pendants presents an anti conformation in the solid state. 1H and 13C NMR spectroscopy reveal that this conformation is maintained in acetonitrile solution. Xunta de Galicia; IN845B-2010/063

[Abstract] Density functional theory (DFT) has become a general tool to investigate the structure and properties of complicated inorganic molecules, such as lanthanide(III) coordination compounds, due to the high accuracy that can be achieved at relatively low computational cost. Herein, we present an overview of different successful applications of DFT to investigate the structure, dynamics, vibrational spectra, NMR chemical shifts, hyperfine interactions, excited states, and magnetic properties of lanthanide(III) complexes. We devote particular attention to our own work on the conformational analysis of LnIII-polyaminocarboxylate complexes. Besides, a short discussion on the different approaches used to investigate lanthanide(III) complexes, i. e. all-electron relativistic calculations and the use of relativistic effective core potentials (RECPs), is also presented. The issue of whether the 4f electrons of the lanthanides are involved in chemical bonding or not is also shortly discussed. Ministerio de Educación y Ciencia; CTQ2009-10721 Xunta de Galicia; IN845B-2010/063

[Abstract] Herein, we report the synthesis and structural characterization of alkaline-earth complexes with the bibracchial lariat ethers N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N′-bis(benzimidazol-2ylmethyl)-4,13-diaza-18-crown-6 (L4). The X-ray crystal structures of the Ca(II) and Sr(II) complexes of L2 show the pendant arms of the ligand disposed on opposite sides of the macrocyclic mean plane, which results in an anti conformation in the solid state. A similar anti conformation is also observed for the Mg(II) complex of L4, whereas the Ca(II), Sr(II) and Ba(II) complexes of L4 adopt a syn conformation in the solid state, with the two pendant arms pointing at the same side of the crown moiety. However, a different behavior is observed in solution. Indeed, 1H and 13C NMR spectroscopy, in combination with density functional theory (DFT) calculations performed at the B3LYP level, suggests that the [M(L2)]2+ and [M(L4)]2+ (M = Ca, Sr or Ba) complexes exist in solution as a mixture of syn and anti isomers involved in a dynamic equilibrium. Our results also show that the relative abundance of the syn conformation increases as the ionic radius of the metal ion increases and, furthermore, for a given metal ion the proportion of syn isomer is always higher for L4 complexes than for L2 ones. Xunta de Galicia; PGIDIT06TAM10301PR Xunta de Galicia; INCITE09E1R103013ES

International audience; Herein we report a detailed investigation of the complexation properties of the macrocyclic decadentate receptor N,N'-Bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H(2)bp18c6) toward different divalent metal ions [Zn(II), Cd(II), Pb(II), Sr(II), and Ca(II)] in aqueous solution. We have found that this ligand is especially suited for the complexation of large metal ions such as Sr(II) and Pb(II), which results in very high Pb(II)/Ca(II) and Pb(II)/Zn(II) selectivities (in fact, higher than those found for ligands widely used for the treatment of lead poisoning such as ethylenediaminetetraacetic acid (edta)), as well as in the highest Sr(II)/Ca(II) selectivity reported so far. These results have been rationalized on the basis of the structure of the complexes. X-ray crystal diffraction, (1)H and (13)C NMR spectroscopy, as well as theoretical calculations at the density functional theory (B3LYP) level have been performed. Our results indicate that for large metal ions such as Pb(II) and Sr(II) the most stable conformation is Delta(delta lambda delta)(delta lambda delta), while for Ca(II) our calculations predict the Delta(lambda delta lambda)(lambda delta lambda) form being the most stable one. The selectivity that bp18c6(2-) shows for Sr(II) over Ca(II) can be attributed to a better fit between the large Sr(II) ions and the relatively large crown fragment of the ligand. The X-ray crystal structure of the Pb(II) complex shows that the Delta(delta lambda delta)(delta lambda delta) conformation observed in solution is also maintained in the solid state. The Pb(II) ion is endocyclically coordinated, being directly bound to the 10 donor atoms of the ligand. The bond distances to the donor atoms of the pendant arms (2.55-2.60 angstrom) are substantially shorter than those between the metal ion and the donor atoms of the crown moiety (2.92-3.04 angstrom). This is a typical situation observed for the so-called hemidirected compounds, in which the Pb(II) lone pair is stereochemically active. The X-ray structures of the Zn(II) and Cd(II) complexes show that these metal ions are exocyclically coordinated by the ligand, which explains the high Pb(II)/Cd(II) and Pb(II)/Zn(II) selectivities. Our receptor bp18c6(2-) shows promise for application in chelation treatment of metal intoxication by Pb(II) and (90)Sr(II).

[Abstract] The coordination properties of the macrocyclic receptor N,N′-bis[(6-carboxy-2-pyridyl)methylene]-1,10-diaza-15-crown-5 (H2bp15c5) towards the lanthanide ions are reported. Thermodynamic stability constants were determined by pH-potentiometric titration at 25 °C in 0.1 M KCl. A smooth decrease in complexstability is observed upon decreasing the ionic radius of the LnIII ion from La [log KLaL = 12.52(2)] to Lu [log KLuL = 10.03(6)]. Luminescence lifetime measurements recorded on solutions of the EuIII and TbIII complexes confirm the absence of inner-sphere watermolecules in these complexes. 1H and 13C NMR spectra of the complexes formed with the diamagnetic LaIII metal ion were obtained in D2O solution and assigned with the aid of HSQC and HMBC 2D heteronuclear experiments, as well as standard 2D homonuclear COSY and NOESY spectra. The 1H NMR spectra of the paramagnetic CeIII, EuIII and YbIIIcomplex suggest nonadentate binding of the ligand to the metal ion. The syn conformation of the ligand in [Ln(bp15c5)]+ complexes implies the occurrence of two helicities, one associated with the layout of the picolinate pendant arms (absolute configuration Δ or Λ), and the other to the five fivemembered chelate rings formed by the binding of the crown moiety (absolute configuration δ or λ). A detailed conformational analysis performed with the aid of DFT calculations (B3LYP model) indicates that the complexes adopt a Λ(λδ)(δδλ) [or Δ(δλ)(λλδ)] conformation in aqueous solution. Our calculations show that the interaction between the LnIII ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases, in line with the decrease of complex stability observed on proceeding to the right across the lanthanide series. Ministerio de Educación y Ciencia; CTQ2006-07875 Ministerio de Educación y Ciencia; CTQ2009-10721 Galicia. Consellería de Economía e Industria; INCITE09E1R103013ES

[Abstract] The coordinative properties towards lead(II) of two lateral macrobicyclic receptors that incorporate either a 1,10‐diaza‐[15]crown‐5 (L7) or a 4,13‐diaza‐[18]crown‐6 (L8) fragment are reported. Spectrophotometric titrations performed in acetonitrile solution indicate only the formation of mononuclear complexes in solution. The X‐ray crystal structures of the two receptors show that the conformation adopted by the ligand is imposed by the presence of intramolecular hydrogen‐bonding interactions that involve the secondary amine groups and the pivotal nitrogen atoms. The solid‐state structure of [Pb(L7)(NCS)](SCN)·0.5H2O shows that the metal ion is asymmetrically coordinated inside the macrobicyclic cavity. The PbII ion is coordinated to the nitrogen atom of the pyridine unit, the two secondary amine atoms, two oxygen atoms of the crown moiety, and a nitrogen atom of an isothiocyanate group. The distances between the PbII ion and the two pivotal nitrogen atoms as well as one of the oxygen atoms of the crown moiety are too long (>2.92 Å) to be considered unequivocal bonds, and should be regarded only as weak interactions. The protonation constants of L7 and L8 as well as the stability constants of their PbII complexes were investigated by using potentiometric titrations in 95 % methanol (I = 0.1 M, nBu4NClO4, 25 °C). The two receptors undergo two protonation processes in the pH range investigated (2.0 < pH < 12.0), which correspond to the protonation of the nitrogen atoms of the oxa–aza moiety. The log KPbL value obtained for L7 [9.906(1)] is approximately 1.1 log K units higher than the one determined for L8 [8.75(1)]. Xunta de Galicia; PGIDIT06TAM10301PR Xunta de Galicia; INCITE09E1R103013ES

Lanthanide(III) complexes with 10-benzyl-1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (H 3 L) have been investigated as model compounds of N-alkylated Ln III -do3a complexes (H 3 do3a = 1,4,3,10-tetraazacyclododecane-1,4,7-triacetic acid) with potential application in molecular imaging. The X-ray structures of the [{Ln(L)(H 2 O)} 2 ]·18H 2 O complexes (Ln = Eu or Tb) show that the metal ion is directly bound to the seven donor atoms of the ligand, with the nine-coordination sphere completed by an oxygen atom of an inner-sphere water molecule and an oxygen atom of a carboxylate group from a neighboring [Ln(L)] unit. This results in the formation of tetrameric units through the sharing of carboxylic groups between adjacent [Ln(L)(H 2 O)] complexes. Luminescence lifetime measurements recorded in H 2 O and D 2 O provide a number of coordinated water molecules (q) of 1.2, which indicates that the major species that exists in solution contains one inner-sphere water molecule. The conformational properties of the [Ln(L)(H 2 O)] complexes (Ln = Gd or Lu) have been investigated by using density functional theory (DFT) calculations (B3LYP model). Our results indicate that the Δ(λλλλ) conformation is more stable than the Δ(δδδδ) one for both complexes. The interconversion between these two isomers may proceed either through the inversion of the five-membered rings formed upon coordination of the 1,4,7,10-tetraazacyclododecane (cyclen) unit, or through the stepwise rotation of the three acetate pendant arms. According to our calculations, the activation free energy for the arm-rotation process (5.6 kcalmol -1 ) is much lower than that of the ring-inversion path (14.6 kcalmol -1 ), which in turn is very similar to those determined experimentally for Ln III (dota) complexes. Thus, a low energy barrier for the arm-rotation process appears to be responsible for the relatively fast isomer interconversion observed for Ln III (N-alkylated do3a) complexes.

A series of receptors containing phenylboronic acid and urea or thiourea units have been designed for simultaneous recognition of the cis-diol and carboxylate functions of sialic acids, which are known to be overexpressed on the surfaces of tumor cells. The interaction of the receptors with 5-acetylneuraminic acid (Neu5Ac) and 2-α-O-methyl Neu5Ac (MeNeu5Ac) in DMSO solution has been investigated bymeans of spectrophotometric titrations and 1H, 13C, and 11B NMR spectroscopy. Additionally, we have also investigated the binding of these receptors with competing monosaccharides such as D-(+)-glucose, D-fructose, methyl α-D-galactoside, and methyl α-D-mannoside. Our results show that 2-{[3-(4-nitrophenyl)thioureido]methyl}phenylboronic acid (3a) recognizes both Neu5Ac and MeNeu5Ac with good selectivity with regard to the remaining monosaccharides investigated. DFT calculations performed at the B3LYP/6-31G(d) level show that this selectivity is due to a cooperative two-site binding of Neu5Ac through 1) ester formation by interaction at the phenylboronic acid function of the receptor and 2) hydrogen-bond interaction between the thiourea moiety and the carboxylate group of Neu5Ac. Compound 3a can therefore be considered a promising synthon for the design of contrast agents for magnetic resonance imaging of tumors. In contrast, the analogue of 3a containing a urea moiety – compound 3b – displays strong binding to all monosaccharides investigated, due to two-site binding through interaction on the phenylboronic acid function of the receptor and a hydrogen-bond interaction between the urea moiety and the sugar hydroxy groups.

International audience; Herein we report two new ligands, 1,4,7-tris(carboxymethyl)-10-[2-(dihydroxyboranyl)benzyl]-1,4,7,10-tetraazacyclododecane (L(1)) and 1,4,7-tris(carboxymethyl)-10-[3-(dihydroxyboranypbenzyl]-1,4,7,10-tetraazacyclododecane (L(2)), which contain a phenylboronic acid (PBA) function and a 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate cage for complexation of lanthanide ions in an aqueous solution The pK(a) of the PBA function amounts to 4.6 in [Gd(L(1))] and 8.9 in [Gd(L(2))], with the value of the L(2) analogue being very similar to that of PBA (8 8) These results are explained by the coordination of the PBA function of L(1) to the Ge(III) ion, which results in a dramatic lowering of its pKa. As a consequence, [Gd(L(1))] does not bind to saccharides at physiological pH. The nuclear magnetic relaxation dispersion profiles recorded for [Gd(L(1))] and [Gd(L(2))] confirm that the phenylboronate function is coordinated to the metal ion in the L(1) derivative, which results in a q=0 complex The interaction of the [Gd(L(2))] complex with 5-acetylneuraminic acid (Neu5Ac) and 2-alpha-O-methyl-5-acetylneuraminic acid (MeNeu5Ac) has been investigated by means of spectrophotometric titrations in an aqueous solution (pH 7 4, 0 1 M 3-(N-morpholino)propanesulfonic acid buffer). Furthermore, we have also investigated the binding of these receptors with competing monosaccharides such as D-(+)-glucose, D-fructose, D-mannose, D-galactose, methyl alpha-D-galactoside, and methyl alpha-D-mannoside. The binding constants obtained indicate an important selectivity of [Gd(L(2))] for Neu5Ac (K(eg) = 151) over D-(+)-glucose (K(eg) = 12.3), D-mannose (K(eg)=21 9), and D-galactose (K(eq)=24 5). Furthermore, a very weak binding affinity was observed in the case of methyl alpha-D-galactoside and methyl alpha-D-mannoside. An 8-fold increase of the binding constant of [Gd(L(2))] with Neu5Ac is observed when compared to that of PBA determined under the same conditions (Keg = 19) (13)C NMR spectroscopy and density functional theory calculations performed at the B3LYP/6-31 G(d) level show that this is due to a cooperative two-site binding of Neu5Ac through (1) ester formation by interaction on the PBA function of the receptor and (2) coordination of the carboxylate group of Neu5Ac to the Ge(III) ion The emission lifetime of the (5)D(4) level of Tb(III) in [Tb(L(2))] increases upon Neu5Ac binding, in line with the displacement of inner-sphere water molecules due to coordination of Neu5Ac to the metal ion.

The study presented in this paper provides a rare example of a macrocyclic receptor allowing the formation of mono- or binuclear complexes on the same first-row transition-metal guest cation depending on the nature of the counterion present. Reaction of N,N′-bis(benzimidazol-2-ylmethyl)-4,13-diaza-18-crown-6 (L4) with CoII, NiII, CuII, or ZnII perchlorates leads to the formation of the expected mononuclear complexes with the metal ion showing a distorted octahedral coordination environment. In these complexes, the metal ion is placed at one end of the macrocyclic cavity, and two of the oxygen atoms of the crown ether moiety remain uncoordinated. 1H and 13C NMR spectroscopic studies on the diamagnetic [Zn(L4)]2+ complex indicate that the asymmetric coordination of the metal ion inside the macrocyclic cavity is maintained in acetonitrile solution, but translocation of the ZnII ion from one end of the macrobicyclic cavity to the second one occurs. However, reaction of L4 with CoII, NiII, or CuII perchlorates in the presence of chloride anions results in the formation of binuclear complexes where the metal ion is either five-coordinate in a distorted trigonal bipyramidal coordination environment (Co or Cu) or six-coordinate in a distorted octahedral geometry (Ni), and a chloride anion coordinates to each of the two metal ions in all cases. The ZnII ion behaves in a different way, and addition of chloride anions to the ZnII perchlorate in the presence of L4 does not lead to the formation of a binuclear complex. The formation of these binuclear systems was followed by spectrophotometric titrations of the mononuclear [M(L4)](ClO4)2 complexes with [nBu4N]Cl in acetonitrile solution indicating the formation of a [M2(L4)(Cl)]2+ intermediate species, followed by the formation of the binuclear complex [M2(L4)(Cl)2]2+ (M = Co, Ni, or Cu). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The hepta- and octa-dentate ligands N , N ′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 ( L 1 ) and N , N ′-bis(2-aminobenzyl)-1,10-diaza-18-crown-6 ( L 2 ), respectively, form stable mononuclear Mn(II) complexes. Spectrophotometric titrations performed in acetonitrile solution indicate the formation of mononuclear Mn(II) complexes with both ligands, and no evidence for the formation of binuclear complexes was obtained. The optimal architecture of L 1 allows it to impose the less usual pentagonal bipyramidal geometry on the Mn(II) guest, and the X-ray crystal structure of [Mn(L 1 )](ClO 4 ) 2 shows that the Mn(II) ion is deeply buried in the receptor cavity, coordinated to the seven available donor atoms, with the perchlorate anions remaining outside the metal coordination sphere. In spite of its higher denticity, the receptor L 2 is unable to form the expected binuclear complexes. The X-ray crystal structure of [Mn(L 2 )](NO 3 ) 2 consists of the [Mn(L 2 )] 2+ cation and nitrate anions involved in hydrogen-bonding interactions with the aniline groups. In [Mn(L 2 )] 2+ the metal ion is also placed in the crown hole, but as a result of the large size of the macrocyclic cavity only six of the eight available donor atoms of the receptor form part of the Mn(II) coordination sphere, with the Mn(II) ion found in a distorted octahedral coordination environment.

The binding tendencies of the pendant-armed crown ethers L1–L3 [L1 = N,N′-bis(benzimidazol-2-ylmethyl)-1,7-diaza-12-crown-4, L2 = N,N′-bis(benzimidazol-2-ylmethyl)-1,10-diaza-15-crown-5) and L3 = N,N′-bis(benzimidazol-2-ylmethyl)-4,13-diaza-18-crown-6] towards PbII and CdII have been investigated. The X-ray crystal structure of [Cd(L3)](ClO4)2·EtOH shows that, in the solid state, the CdII ion is eight-coordinate and fits quite well into the crown hole, favouring an anti arrangement of the organic receptor. NMR measurements recorded in acetonitrile solution indicate that increasing the crown size induces a conformational change in the series of CdII complexes. The conformation goes from a syn arrangement for L1 to an anti arrangement for L3, passing through a syn [lrarr2] anti equilibrium in the complex derived from L2. On the contrary, no conformational change was observed for the corresponding PbII complexes, which have a syn conformation in all cases. These results have been confirmed by means of density functional theory (DFT) calculations performed by using the B3LYP model. The binding constants obtained from UV/Vis titration experiments in DMSO solution demonstrate that a decrease in the crown size provokes a 102-fold enhancement of the stability for this series of CdII complexes, whereas for PbII a gradual decrease of the binding constants is observed. Receptor L1 shows a certain degree of selectivity for CdII over PbII, with a selectivity factor > 102. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The macrobicycle receptor L4, derived from 1,10-diaza-15-crown-5 incorporating a phenol Schiff-base spacer, forms stable complexes with lead(II). In [Pb(L4)(ClO4)](ClO4)·CH3CN (1), the lead(II) ion is asymmetrically placed at the one end of the macrobicyclic cavity, because of the intramolecular hydrogen bonding interaction that occurs between an imine nitrogen atom and the phenol group. This asymmetric position of the metal ion inside the macrobicyclic cavity induces chirality in this system. Variable temperature 1H NMR spectroscopic experiments indicate that the asymmetric coordination of the metal ion inside the macrocyclic cavity is maintained in acetonitrile solution, but a translocation of the PbII ion from one end of the macrobicyclic cavity to the second one occurs. This dynamic behaviour, which corresponds to the interconversion between the two possible enantiomeric forms of the complex, is fast on the NMR timescale at 320 K but slow at low temperatures, and we have estimated an activation barrier of ΔG‡ = 68 ± 2 kJ mol–1. We have also studied the effect of the protonation on 1 by NMR and UV/Vis spectroscopy in CH3CN solutions at room temperature, finding that diprotonation causes demetallation of the complex without receptor destruction, recovering L4 in its protonated form. On the other hand, the interaction of compound 1 with anions such as NO3– and SCN– has been evaluated by using spectrophotometric titrations in acetonitrile solution, finding log K values of 5.83(1) for NO3– and 6.27(2) forSCN–. The [Pb(L4)]2+, [Pb(L4)(NO3)]+, [Pb(L4)(NCS)]+ and [Pb(L4)(SCN)]+ systems were characterized by means of density functional theory calculations (DFT) performed by using the B3LYP model, and the coordination geometry as well as the role of the PbII lone pair has been investigated and discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

International audience; We report two macrocyclic ligands containing a 1,10-diaza-18-crown-6 fragment functionalized with either two picolinamide pendant arms (bpa18c6) or one picolinamide and one picolinate arm (ppa18c6(-)). The X-ray structure of [La(ppa18c6)(H2O)](2+) shows that the ligand binds to the metal ion using the six donor atoms of the crown moiety and the four donor atoms of the pendant arms, 11-coordination being completed by the presence of a coordinated water molecule. The X-ray structure of the [Sr(bpa18c6)(H2O)](2+) was also investigated due to the very similar ionic radii of Sr(2+) and Eu(2+). The structure of this complex is very similar to that of [La(ppa18c6)(H2O)](2+), with the metal ion being 11-coordinated. Potentiometric measurements were used to determine the stability constants of the complexes formed with La(3+) and Eu(3+). Both ligands present a very high selectivity for the large La(3+) ion over the smaller Eu(3+), with a size-discrimination ability that exceeds that of the analogous ligand containing two picolinate pendant arms reported previously (bp18c6(2-)). DFT calculations using the TPSSh functional and the large-core pseudopotential approximation provided stability trends in good agreement with the experimental values, indicating that charge neutral ligands derived from 1,10-diaza-18-crown-6 enhance the selectivity of the ligand for the large Ln(3+) ions. Cyclic voltammetry measurements show that the stabilization of Eu(2+) by these ligands follows the sequence bp18c6(2-) < ppa18c6(-) < bpa18c6 with half-wave potentials of -753 mV (bp18c6(2-)), -610 mV (ppa18c6(-)), and -453 mV (bpa18c6) versus Ag/AgCl. These values reveal that the complex of bpa18c6 possesses higher stability against oxidation than the aquated ion, for which an E1/2 value of -585 mV has been measured.

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).

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.

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.

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.

Previously we have proved that barium perchlorate is a highly effective template agent in the formation of the Schiff-base lateral macrobicycles L1 and L2 (receptors derived from the condensation of N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 with 2,6-diformylpyridine and 2,6-diformyl-4-methylphenol, respectively). Aiming to understand the effect that the nature of the counteranion present in the reaction medium has on the barium template potential, we have carried out the corresponding template synthesis in the presence of barium thiocyanate. We have found that considerably longer reaction times are required to obtain both receptors when barium thiocyanate is employed. The barium template remains trapped into the formed macrobicyclic cavity and compounds of formula Ba(L1)(SCN)2 · H2O (1) and Ba(L2)(SCN)2 · 2H2O (2) were isolated. The X-ray crystal structure of 1 shows the metal ion coordinated to the eight donor atoms of the receptor and to the nitrogen atom of a isothiocyanate group, the coordination polyhedron being best described as a monocapped distorted dodecahedron. In 1, the metal ion is nearly centred inside the macrobicyclic cavity of L1, whereas NMR studies in solution indicate that in 2 the barium ion is asymmetrically situated in the macrobicyclic cavity of L2. This asymmetric coordination is attributed to the proton transfer from the phenol group to the nitrogen atom of one of the imine groups.

Compounds of formula Pb(L1)(SCN)2·CH3CN (1) and Pb(L3)(SCN)2·0.5EtOH (2) (where L1 is the lariat crown ether N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and L3 is a Schiff-base macrobicyclic receptor derived from the condensation of L1 with 2,6-diformylpyridine) were isolated and structurally characterised both in the solid state and in acetonitrile solution. The X-ray crystal structure of 1 shows the metal ion coordinated to the seven donor atoms of the receptor and to the nitrogen atom of an isothiocyanate group. This structure is different to that found for the previously described perchlorate analogous [Pb(L1)](ClO4)2, evidencing that the nature of the anionic group present has an important effect in the conformation and fold of L1 when coordinated to lead(II). In 2, the metal ion is nearly centred inside the macrobicyclic cavity of L3 clearly bound to the three oxygen atoms, to one pivotal nitrogen atom, to one imine nitrogen atom and to the pyridine nitrogen donor of L3, while the remaining donor atoms of L3 are only weakly coordinated to the lead(II) ion. The coordination sphere of lead(II) is completed by the nitrogen atom of an isothiocyanate group, and the coordination polyhedron may be described as a distorted tricapped trigonal prism. A comparison between this structure and that previously described for [Pb(L3)(ClO4)]+ demonstrates that for the L3 complexes the different nature of the anion present (thiocyanate or perchlorate) hardly has an effect on the fold and conformation that the macrobicycle adopts when acting as a receptor for lead(II). So, the effect that the anion present has on the structure of the lead(II) complexes of L1 and L3 appears to be determined by the degree of flexibility of the organic receptor.

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.

[Abstract] In this study we present the results of the equilibrium, dissociation kinetics, DFT and X - ray crystallographic studies performed on the complexes of metal ions of biomedical importance (Mn 2+ , Cu 2+ and Gd 3+ ) formed with octadentate ligands based on a cyclen platform incorporating two picolinate pendant arms (dodpa 2− and Medodpa 2− ). The stability constants of the complexes were accessed by m ultiple methods (pH - potentiometry, direct and competition UV - vis spectrophotometry and 1 H - relaxometry). The stability constants of the complexes formed with dodpa 2− and Medodpa 2− do not differ significantly ( e.g. log K [Mn(dodpa)] = 17.40 vs. log K[Mn(Medodp a)] = 17.46, log K [Cu(dodpa)] = 24.34 – 25.17 vs. log K [Cu(Medodpa)] = 24.74 and log K [Gd(dodpa)] + = 17.27 vs . log K [Gd(Medodpa)] + = 17.59), which indicates that the steric hindrance brou ght by the methyl groups has no significant effect on the stability of the complexes. The stability constants of the Mn 2+ complexes formed with the cyclen dipicolinates were found to be ca. 3 log K units higher than t hose determined for the complex of the cyclen monopicolinate (dompa − ), which indicates that the se cond picolin ate moiety attached to the backbone of the macrocycle is very likely coordinated to the Mn 2+ ion . However, the stability of the [Cu(dodpa)] and [Cu(Medodpa)] complexes agrees well with the stability constant of [Cu(dompa)] + , in line with the hexadentate coordination around the metal ion observed in the X - ray structure of [Cu(Medodpa)]. The [Gd(dodpa)] + and [Gd(Medodpa)] + complexes display a fairly high kinetic inertness, as the rate constants of acid catalysed dissociation ( k 1 = 2.5(4) × 10 −3 and 8.3(4) × 10 −4 M −1 s −1 for [Gd(dodpa)] + and [Gd(Medodpa)] + , respectively) are smaller than the value reported for [Gd(do3a)] ( k 1 = 2.5 × 10 −2 M −1 s −1 ). The [Mn(dodpa)] complex was found to be more inert than [Mn(Medodpa) ]. The results of the diffusion ordered NMR spectroscopy (DOSY) and DFT calculations of diamagnetic [La(dodpa)] + and [Lu(dodpa)] + complexes indicate the formation of a trinuclear entity of the La complex in aqueous solution. Hungarian Scientific Research Fund. OTKA; K-84291 Hungarian Scientific Research Fund. OTKA; K-109029 European Commission; TÁMOP-4.2.2.A-11/1/KONV-2012-0043

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.

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.

Copper(II) complexes with a series of bibracchial lariat ethers are described. Independently of the nature of the counterion present (nitrate or perchlorate), the lariat ether N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L1) always forms mononuclear complexes, whereas the lariat ethers N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N'-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L3) only give binuclear compounds. The X-ray crystal structure of [CuL1](ClO4)2 shows a seven-coordinated copper(II) ion in a distorted (axially compressed) pentagonal-bipyramidal geometry. The X-ray crystal structure of [Cu2(L3-2H)](ClO4)2 confirms the binuclear nature of the compound with both metal ions having identical coordination environments and each one placed out of the crown hole but efficiently encapsulated by the corresponding pendant arm; each copper(II) ion is five-coordinated with an intermediate geometry between trigonal-bipyramidal and square-pyramidal ([tau]=0.40). The EPR spectra in frozen solution samples are in accordance with a stable coordinate pattern for the metal centre of ligand L1, yielding a rhombic distorted complex with axial compression in solution, in agreement with the X-ray crystal structure of [CuL1](ClO4)2. For the binuclear complexes of L2 and L3, the Cu(II) centres in solution can be distorted from their tetragonally elongated structures via interaction with ethanol and/or the nitrate counterion, leading to more than one species. http://www.sciencedirect.com/science/article/B6TG5-433PCM9-T/1/7815e8fe94b2b6b2129e76252c3517d0

The interaction of Co(II), Ni(II), Zn(II) and Cd(II) with L, a macrocycle containing an N 3 O 2 -donor set, has been investigated. The X-ray crystal structures of L, [Co L (NO 3 ) 2 ] ( 1 ), [Ni L (NO 3 ) 2 ] ( 2 ) and [Zn L (H 2 O)(CH 3 CN)](ClO 4 ) 2 ( 5 ) have been determined. The metal atoms in the complexes are coordinated in an endomacrocyclic fashion with the Co and Ni atoms in six-coordinate environments comprised of the ligand N 3 -donor set and two nitrate ions, one monodentate and the other bidentate, giving a distorted octahedral geometry at the metal. In the Zn complex the metal is in a five-coordinate environment comprised of the ligand N 3 -donor set, a water molecule and an acetonitrile molecule, giving a trigonal bipyramidal geometry around the metal.

The interaction of lanthanide(III) ions with two N3O3-macrocycles, L1 and L2, derived from 2,6-bis(2-formylphenoxymethyl)pyridine and 1,2-diaminoethane has been investigated. Schiff-base macrocyclic lanthanide(III) complexes LnL1(NO3)3 · xH2O (Ln = Nd, Sm, Eu or Lu) have been prepared by direct reaction of L1 and the appropriate hydrated lanthanide nitrate. The direct reaction between the diamine macrocycle L2 and the hydrated lanthanide(III) nitrates yields complexes LnL2(NO3)3· H2O only for Ln = Dy or Lu. The reduction of the Schiff-base macrocycle decreases the complexation capacity of the ligand towards the Ln(III) ions. The complexes have been characterised by elemental analysis, molar conductivity data, FAB mass spectrometry, IR and, in the case of the lutetium complexes, 1H NMR spectroscopy.

A new series of lanthanide(III) complexes with two novel Schiff-base macrocycles, L1 and L2, containing a N3O4 donor set have been prepared by cyclocondensation of 2,6-bis(2-formylphen-oxymethyl)pyridine and l,2-bis(2-aminophenoxy)ethane or 1,3-bis(2-aminophenoxy)propane, respectively, in the presence of the appropriate metal salt as a template agent. In the absence of the metal salt the macrocycles are not formed. The diamine macrocycles L3 and L4 are also formed by reaction of the corresponding diamine and dicarbonyl precursors followed by an in situ reduction with NaBH4. Their complexation potential towards Ln(III) ions were also investigated.

New macrocyclic lanthanide(III) complexes were obtained with two oxaazadiamine macrocycles derived from 2,6-diformylpyridine: (i) mononuclear complexes of an 18-membered sexidentate N3O3 macrocyclic (L1) derived from 1,5-bis(2-aminophenoxy)-3-oxapnetate; (ii) mononuclear complexes of a 16-membered pentadentate N3O2 macrocycle (L2) derived from 1,3-bis(2-aminophenoxy)propane. The complexes were characterized by elemental analysis, molar conductivity, mass spectrometry, IR and 1H NMR spectroscopy, thermogravimetry and magnetic measurements. The crystal structures of the L2 macrocycle and the corresponding diprotonated ligand are also reported.

18-membered hexadentate N 4 O 2 and N 3 O 3 Schiff-base polyoxaazamacrocycles derived from 2,6-bis(2-aminophenoxymethyl) pyridine and different organic dicarbonyl compounds are obtained by a template procedure as mononuclear lanthanide(III) complexes. In the absence of the metal salt, macrocycles are not formed. The complexes were characterised by elemental analysis, molar conductivity and mass, UV-V, IR, 1 H NMR and 13 C NMR spectroscopy.

[Abstract] The stability trends across the lanthanide series of complexes with the polyaminocarboxylate ligands TETA4− (H4TETA=2,2′,2′′,2′′′‐(1,4,8,11 tetraazacyclotetradecane‐1,4,8,11‐tetrayl)tetraacetic acid), BCAED4− (H4BCAED=2,2′,2′′,2′′′ {[(1,4‐diazepane‐1,4‐diyl)bis(ethane‐2,1‐diyl)]bis(azanetriyl)}tetraacetic acid), and BP18C62− (H2BP18C6=6,6′‐[(1,4,10,13‐tetraoxa‐7,16‐diazacyclooctadecane‐7,16 diyl)bis(methylene)]dipicolinic acid) were investigated using DFT calculations. Geometry optimizations performed at the TPSSh/6‐31G(d,p) level, and using a 46+4fn ECP for lanthanides, provide bond lengths of the metal coordination environments in good agreement with the experimental values observed in the X‐ray structures. The contractions of the Ln3+ coordination spheres follow quadratic trends, as observed previously for different isostructural series of complexes. We show here that the parameters obtained from the quantitative analysis of these data can be used to rationalize the observed stability trends across the 4f period. The stability trends along the lanthanide series were also evaluated by calculating the free energy for the reaction [La(L)]n+/−(sol)+Ln3+(sol)→[Ln(L)]n+/ (sol)+La3+(sol). A parameterization of the Ln3+ radii was performed by minimizing the differences between experimental and calculated standard hydration free energies. The calculated stability trends are in good agreement with the experimental stability constants, which increase markedly across the series for BCAED4−complexes, increase smoothly for the TETA4− analogues, and decrease in the case of BP18C62− complexes. The resulting stability trend is the result of a subtle balance between the increased binding energies of the ligand across the lanthanide series, which contribute to an increasing complex stability, and the increase in the absolute values of hydration energies along the 4f period. Xunta de Galicia; CN2012/011

[Abstract] Stable Mn2+ mono- and binuclear complexes containing pentadentate 6,6′-((methylazanediyl)bis(methylene)) dipicolinic acid coordinating units give remarkably high relaxivities due to the presence of two inner-sphere water molecules. The mononuclear derivative binds human serum albumin (HSA) with an association constant of 3372 M−1, which results in the replacement of the coordinated water molecules by donor atoms of protein residues. The dinuclear analogue also binds HSA while leaving one of the Mn2+ centres exposed to the solvent with two coordinated water molecules. Thus, this complex shows remarkably high relaxivities upon protein binding (39.0 mM−1 s−1 per Mn, at 20 MHz and 37 °C). Galicia. Consellería de Cultura, Educación e Ordenación Universitaria; EM 2012/088 Galicia. Consellería de Cultura, Educación e Ordenación Universitaria; CN 2012/011

[Abstract] The synthesis and structural characterization of six sodium complexes with bibracchial lariatethers containing aniline or benzimidazole side arms, and derived from 1,7 diaza-12-crown-4, 1,10-diaza-15-crown-5 or 4,13-diaza-18-crown-6, are reported. The X-ray structures of four of these compounds have been obtained. Additionally, the X-ray structures of a sodium macrobicyclic complex derived from 1,10-diaza-15-crown-5, and a potassium complex with a bibracchial lariat ether containing aniline side arms are also reported. Bonding distances as well as the stability constants in acetonitrile solution confirm that the coordination of the pendant arms provides an important contribution to the overall stability of the complexes, particularly when benzimidazole pendants are present rending more stable complexes, even more than cryptand complexes of the same size. Compared with the parent crown ethers, the stability increases when the side arms contain benzimidazole moieties but remains about the same order when aniline side arms are present. Xunta de Galicia; CN2012/011

A Schiff-base polyoxaazamacrocycle derived from 2, 5-diformylfuran and 1, 5-bis(2-aminophenoxy)-3-oxapentane was obtained by a template procedure as mononuclear lanthanide(III) and lead(II) complexes. In the absence of the metal salt the macrocycle was not formed. The complexes were characterized by elemental analysis, molar conductivity, mass spectrometry and IR, 1 H NMR and 13 C NMR spectroscopy. The reduction of the Pb II complex with sodium borohydride yielded a novel polyoxaazamacrocycle containing a furan head and aromatic lateral units; the X-ray crystal structure of its methanol inclusion compound is reported.

Lanthanide(III) and yttrium(III) ions have been shown to behave as efficient metal templates in the synthesis of cryptates [M 2 L][NO 3 ] 6 ·xH 2 O·ysolv (M = Y, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb) derived from the 2+3 condensation of tris(2-aminoethyl)amine with 2,6-diformyl-4-methylphenol. The complexes have been characterised by elemental analysis, molar conductivity, mass and IR spectroscopy and the yttrium complex was also characterised by NMR spectroscopy. The crystal structure of the dysprosium complex was determined.

Lanthanide(III) salts were found to act as templates for the cyclic condensation of organic dicarbonyl compounds with primary diamines to yield complexes of macrocyclic Schiff-base ligands in moderate to good yield. Three types of macrocyclic lanthanide(III) complexes were obtained: (i) mononuclear complexes of an 18-membered sexidentate N3O3 macrocycle derived from the reaction of 2,6-diacetylpyridine with 1,5-bis(2-aminophenoxy)-3-oxapentane; (ii) mononuclear complexes of 15-membered pentadentate N3O2 macrocycles derived from the reactions of 2,6-diacetyl- or 2,6-diformyl-pyridine with 1,2-bis(2-aminophenoxy)ethane. The complexes were characterised by elemental analyses, molar conductivity, mass, IR and 1H NMR spectroscopy and thermogravimetric and magnetic measurements.

[Abstract] Homodinuclear lanthanide complexes (Ln=La, Eu, Gd, Tb, Yb and Lu) derived from a bis‐macrocyclic ligand featuring two 2,2′,2′′‐(1,4,7,10‐tetraazacyclododecane‐1,4,7 triyl)triacetic acid chelating sites linked by a 2,6‐bis(pyrazol‐1‐yl)pyridine spacer (H2L3) were prepared and characterized. Luminescence lifetime measurements recorded on solutions of the EuIII and TbIII complexes indicate the presence of one inner‐sphere water molecule coordinated to each metal ion in these complexes. The overall luminescence quantum yields were determined (∅H2O=0.01 for [Eu2(L3)] and 0.50 for [Tb2(L3)] in 0.01 MTRIS/HCl, pH 7.4; TRIS=tris(hydroxymethyl)aminomethane), pointing to an effective sensitization of the metal ion by the bispyrazolylpyridyl unit of the ligand, especially with Tb. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd2(L3)] are characteristic of slowly tumbling systems, showing a low‐field plateau and a broad maximum around 30 MHz. This suggests the occurrence of aggregation of the complexes giving rise to slowly rotating species. A similar behavior is observed for the analogous GdIII complex containing a 4,4′ dimethyl‐2,2′‐bipyridyl spacer ([Gd2(L1)]). The relaxivity of [Gd2(L3)] recorded at 0.5 T and 298 K (pH 6.9) amounts to 13.7 mM−1 s−1. The formation of aggregates has been confirmed by dynamic light scattering (DLS) experiments, which provided mean particle sizes of 114 and 38 nm for [Gd2(L1)] and [Gd2(L3)], respectively. TEM images of [Gd2(L3)] indicate the formation of nearly spherical nanosized aggregates with a mean diameter of about 41 nm, together with some nonspherical particles with larger size. Ministerio de Educación y Ciencia; CTQ2009‐10721 Xunta de Galicia; IN845B‐2010/063