Lanthanide-induced Shifts Research Articles

Synthesis, spectral properties and supramolecular dimerisation of heteroleptic triple-decker phthalocyaninato complexes with one outer crown-substituted ligand

Four complexes – [(15C5) 4Pc]M(Pc)M(Pc), (Pc 2− – phthalocyaninato-dianion, [(15C5) 4Pc] 2− – 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninato-dianion, M = Sm, Dy, Tm, Y) were obtained via the reaction of M(Pc) 2, H 2[(15C5) 4Pc] and M(acac) 3. The influence of the stability of starting M(Pc) 2 on the yields of target compounds was investigated. Increasing the stability of M(Pc) 2 leads to higher yields of [(15C5) 4Pc]M(Pc)M(Pc) and lower yields of scrambling products. All complexes were characterized by 1H NMR, UV–Vis and FT-IR spectroscopy as well as MALDI TOF mass-spectrometry. The analysis of 1H NMR spectra was performed in terms of lanthanide-induced shifts. Cation-induced dimerisation was studied by means of spectrophotometric titration. Supramolecular dimers {2[(15C5) 4Pc]M(Pc)M(Pc) · 4K +} are the largest discrete cofacial supramolecular assemblies built of phthalocyanine building blocks reported up-to-date. The observed increase of the intermolecular excitonic interaction between building blocks with the increase of REE(III) size is tentatively explained in terms of metal-size dependent deformation of phthalocyanine ligands in sandwich complexes.

Lanthanide(III) Complexes with a Tetrapyridine Pendant-Armed Macrocyclic Ligand: 1H NMR Structural Determination in Solution, X-ray Diffraction, and Density-Functional Theory Calculations

Complexes between the tetrapyridyl pendant-armed macrocyclic ligand (L) and the trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and in aqueous solution. The crystal structures of the La, Ce, Pr, Gd, Tb, Er, and Tm complexes have been determined by single-crystal X-ray crystallography. In the solid state, all the cation complexes show a 10-coordinated geometry close to a distorted bicapped antiprism, with the pyridine pendants situated alternatively above and below the main plane of the macrocycle. The conformations of the two five-membered chelate rings present in the complexes change along the lanthanide series. The La(III) and Ce(III) complexes show a lambdadelta (or deltalambda) conformation, while the complexes of the heavier lanthanide ions present lambdalambda (or deltadelta) conformation. The cationic [Ln(L)]3+ complexes (Ln = La, Pr, Eu, Tb, and Tm) were also characterized by theoretical calculations at the density-functional theory (DFT) B3LYP level. The theoretical calculations predict a stabilization of the lambdalambda (or deltadelta) conformation on decreasing the ionic radius of the Ln(III) ion, in agreement with the experimental evidence. The solution structures show a good agreement with the calculated ones, as demonstrated by paramagnetic NMR measurements (lanthanide induced shifts and relaxation rate enhancements). The 1H NMR spectra indicate an effective D2 symmetry of the complexes in D2O solution. The 1H lanthanide induced shifts (LIS) observed for the Ce(III), Tm(III), and Yb(III) complexes can be fit to a theoretical model assuming that dipolar contributions are dominant for all protons. The resulting calculated values are consistent with highly rhombic magnetic susceptibility tensors with the magnetic axes being coincident with the symmetry axes of the molecule. In contrast with the solid-state structure, the analysis of the LIS data indicates that the Ce(III) complexes present a lambdalambda (or deltadelta) conformation in solution.

NMR Studies of Mixed-Ligand Lanthanide Complexes in Solution: Pseudorotation and Ring Inversion of 18-Crown-6 Molecule in Cerium Subgroup Chelates

1H and 13C NMR measurements are reported for the CDCl3 and CD2Cl2 solutions of [La(NO3)3(18-crown-6)] (I), [Pr(NO3)3(18-crown-6)] (II) and [Ce(NO3)3(18-crown-6)] (III) complexes. Temperature dependencies of the 1H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Two types of conformational dynamic processes in II were identified (the first one with activation enthalpy ΔH‡=26 ± 4 kJ/mol is conditioned by interconversion of complex enantiomeric form and pseudorotation of macrocycle molecule upon the C2 symmetry axis, the second one with ΔH‡=46 ± 5 kJ/mol is conditioned by macrocycle molecule inversion). Studies of the values of the lanthanide-induced shifts revealed that the structure of complexes in solution is similar to that reported for the complex I in the crystal state.

Enantiomeric differentiation of acyclic terpenes by13C NMR spectroscopy using a chiral lanthanide shift reagent

The 13C NMR behaviour of ten acyclic terpene alcohols was examined in the presence of a chiral lanthanide shift reagent (CLSR). For each alcohol, we measured the lanthanide-induced shift (LIS) on the signals of the carbons and the splitting of some signals, which allowed the enantiomeric differentiation. As expected, the LIS decreased with the number of bonds between the binding function and the considered carbon. The enantiomeric splitting is observed for several signals in the spectrum of each compound. The influence of the hindrance of the binding function (primary, secondary or tertiary alcohol) and that of the stereochemistry of the double bonds is discussed.

Anomalous Diamagnetic Behaviour of the 1H NMR Spectra of a Coordinated Imino Nitroxide Radical in Diamagnetic and Paramagnetic Lanthanide(III) Complexes

AbstractDiamagnetic lanthanide(III) complexes with an imino nitroxide radical IM2py [2‐(2′‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl], [Ln(hfac)3(IM2py)] (Ln = Y and Lu; hfac = hexafluoroacetylacetonate), exhibit 1H NMR chemical shifts for the pyridyl protons of the coordinated IM2py suggesting it behaves as if it were diamagnetic. To a series of paramagnetic lanthanide complexes of this type, the structural analyses of the lanthanide induced shifts (LIS) of the hfac 3‐proton and pyridyl protons in IM2py were successfully applied, showing not only the diamagnetic behaviour of the pyridyl protons in the IM2py group, but also the dominant dipolar contribution and isostructural nature with apparent or effective axial symmetry in solution irrespective of the asymmetry in the solid state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Application of 1H and 23Na magic angle spinning NMR spectroscopy to define the HRBC up-taking of MRI contrast agents

The up-take of Gd(III) complexes of BOPTA, DTPA, DOTA, EDTP, HPDO3A, and DOTP in HRBC has been evaluated by measuring the lanthanide induced shift (LIS) produced by the corresponding dysprosium complexes (DC) on the MAS-NMR resonances of water protons and free sodium ions. These complexes are important in their use as MRI contrast agents (MRI-CA) in diagnostics. 1H and 23Na MAS-NMR spectra of HRBC suspension, collected at 9.395 T, show only one signal due to extra- and intra-cellular water (or sodium). In MAS spectra, the presence of DC in a cellular compartment produces the LIS of only the nuclei (water proton or sodium) in that cellular compartment and this LIS can be related to the DC concentrations (by the experimental curves of LIS vs. DC concentrations) collected in the physiological solution. To obtain correct results about LIS, the use of MAS technique is mandatory, because it guarantees the only the nuclei staying in the same cellular compartment where the LC is present show the LIS. In all the cases considered, the addition of the DC to HRBC (100% hematocrit) produced a shift of only the extra-cellular water (or sodium) signal and the gradient of concentration ( G C) between extra- and intra-cellular compartments resulted greater than 100:1, when calculated by means of sodium signals. These high values of G C are direct proofs that none of the tested dysprosium complexes crosses the HRBC membrane. Since the DC are iso-structural to the gadolinium complexes the corresponding gadolinium ones (MRI-CA) do not cross the HRBC membrane and, consequently, they are not up-taken in HRBC. The G C values calculated by means of water proton signals resulted much lower than those obtained by sodium signals. This proves that the choice of the isotope is a crucial step in order to use this method in the best way. In fact, G C value depends on the lowest detectable LIS which, in turn, depends on the nature of the LC (lanthanide complex) and the observed isotopes.

Lanthanoid‐Induced Shifts (LIS) in Structural and Conformational Analysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lanthanide‐induced shifts in the structural elucidation of β‐hydroxydecalones

AbstractThe use of Eu(fod)3 in the analysis of the 1H and 13C NMR spectra of cis and trans‐fused β‐hydroxydecalones is described. The relative configuration of the substituents is discussed using the PMLIS algorithm to determine the lanthanide (Eu) ion position in the complex in an effective axially symmetric model. The conformations of two cis‐decalones are also discussed. Copyright © 2002 John Wiley & Sons, Ltd.

X-ray diffraction and (1)H NMR in solution: structural determination of lanthanide complexes of a Py(2)N(6)Ac(4) ligand.

Complexes between the Py(2)N(6)Ac(4) (H(4)L) ligand containing four carboxylate pendant arms and trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and aqueous solution. The crystal structures of the La, Ce, Sm, Tb, Dy, Ho, Er, Tm, and Lu complexes, with chemical formulas [LaH(2)L](NO(3)).3H(2)O (1), [Ce(4)L(2)](NO(3))(4).30H(2)O (2), [SmHL].EtOH.3H(2)O (5), [TbHL].EtOH.3H(2)O (8), [DyHL].2EtOH.2H(2)O (9), [HoHL].3H(2)O (10), [ErHL].EtOH.3H(2)O (11) [TmHL].EtOH.3H(2)O (12), and [LuHL].3H(2)O (14), have been determined by single-crystal X-ray crystallography. In the solid state, the complexes of the lighter lanthanide ions La(3+)-Dy(3+) show a 10-coordinated geometry close to a distorted bicapped antiprism, where the carboxylate pendants are situated alternatively above and below the best plane that contains the nitrogen donor atoms. The complexes of the heavier ions, Ho(3+)-Lu(3+), have a 9-coordinated geometry close to distorted tricapped trigonal prism, with one of the pendant carboxylate groups uncoordinated. The ligand is in a "twist-fold" conformation, where the twisting of the pyridine units is accompanied by an overall folding of the major ring of the macrocycle so that the pyridine nitrogen atoms and the metal are far from linear. The aqueous solution structures of the complexes were thoroughly characterized, the diamagnetic ones (La(3+) and Lu(3+)) by their COSY NMR spectra, and the paramagnetic complexes using a linear least-squares fitting of the (1)H LIS (lanthanide-induced shift) and LIR (lanthanide-induced relaxation) data with rhombic magnetic susceptibility tensors. The solution structures obtained for the La(3+)-Dy(3+) complexes (10-coordinate) and for the Tm(3+)-Lu(3+) complexes (9-coordinate) are in very good agreement with the corresponding crystal structures. However, the 10-coordinate structure is still exclusive in solution for the Ho(3+) complex and predominant for the Er(3+) complex.

Unusually sharp dependence of water exchange rate versus lanthanide ionic radii for a series of tetraamide complexes.

The tetraamide ligand, DOTA-tetra(glycine ethyl ester), forms complexes with the lanthanide(III) cations that exist in solution predominantly as the square antiprism structure with single, slowly exchanging inner-sphere water molecule. Variable-temperature 1H and 17O NMR studies revealed that the bound water lifetimes in these complexes were sharply dependent upon the ionic radius of Ln3+ cation. A novel lanthanide-induced shift technique was used to unmask the bound water 17O resonance of SmL3+ and YL3+ complexes from the bulk water resonance. The bound water lifetime (tauM298) was approximately 800 mus in the EuL3+ complex but became much shorter (several microseconds) for Ln3+ cations with larger and smaller ionic radii. This demonstrates that water exchange is exquisitely fine-tuned in this macrocyclic tetraamide system and that a variety of Ln3+ complexes meet with the exchange requirement, Deltaomega*tauM >/= 1, necessary for an efficient MT agent.

Intramolecular Dynamics of Lanthanide(III) Tetraoxadiaza Macrocycle Complexes in Solution as Studied by NMR

1H and 13C NMR and 1H NMR relaxation spectroscopy (RS)measurements are reported for the CDCl3 and CD2Cl2 solutions of [La(NO3)3(diaza-18-crown-6)] ({bf I}), [Pr(NO3)3(diaza-18-crown-6)] ({bf II}) and [Nd(NO3)3(diaza-18-crown-6)] ({bf III}) complexes. Temperature dependencies of the 1H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Enantiomeric isomer interconversion in II is characterized by Δ H‡ = 21.5 ± 4 kJ mol-1. Studies of the values of the lanthanide-induced shifts and the longitudinal relaxation rate enhancement revealed that the structure of complexes in solution is similar to that reported for the [La(NO3)3(18-crown-6)] complex in the crystal state. Nevertheless, it appears that the principal values of the molar paramagnetic susceptibility tensor (χi) significantly differ in complexes II and III. The possible reasons for the different characteristics of these complexes are discussed.

15N NMR spectra, tautomerism and diastereomerism of 4,5-dihydro-1H-1,2,3-triazoles

Despite the great number of 4,5-dihydro-1H-1,2,3-triazoles synthesized, 15N NMR data of these heterocycles are extremely rare. The aim of this paper is to present such data and examples of their application. The compounds investigated have been synthesized according to the given references or procedures. Their 15N NMR spectra were measured at natural abundance. For some compounds, the chemical shift assignments were confirmed with the help of 15N labelled material. The influences on 15N chemical shifts of substitution pattern, solvent and concentration were investigated. Additionally, some lanthanide induced shift (LIS) investigations were performed. 13C labelled compounds were employed as tools to provide the assignment of tautomeric structures.

Distinction between Light and Heavy Lanthanide(III) Ions Based on the 1H NMR Spectra of Heteroleptic Triple-Decker Phthalocyaninato Sandwich Complexes

The 1H NMR spectra of two series of heteroleptic triple-decker phthalocyaninato sandwich complexes of the rare earth(III) ions have been recorded. The ligands comprise unsubstituted phthalocyanine (Pc) and 2,3,9,10,16,17,23,24-octakis(1‘-octyloxy)phthalocyanine (Pc*), and the complexes are of the form PcMPc*MPc (series 1) and PcMPc*MPc* (series 2), where M = Pr − Tm (except Pm, Gd). In some cases, the homoleptic complexes Pc*MPc*MPc* (series 3) were also available. The chemical shifts of five proton positions in series 1, and 10 positions in series 2 were compared with the shifts for the analogous complexes of the diamagnetic yttrium(III) ion. The experimental lanthanide-induced shifts (LIS) were separated into the contact and dipolar contributions using the crystal-field dependent methods of Reilley and co-workers. This work represents the first analysis of LIS in triple-decker phthalocyaninato lanthanide complexes. The results showed a dominance of dipolar contributions but also revealed that contact influences cannot be ignored, even for protons located in the peripheral alkoxy substituents. Analysis by graphical and matrix methods indicates that there is a clear discontinuity in the data between terbium and dysprosium, whereby the metals Nd, Sm, Eu and Tb possess a positive hyperfine coupling term Fi, and the later metals Dy, Ho (limited data), Er and Tm require a negative contact term. This change is coincident with a near-halving of the dipolar term Gi·A20〈r2〉 for the later metals. The properties of praseodymium are anomalous, in that its complexes are readily demetalated, and its NMR behavior places it in the second group of metal ions, despite its early position in the lanthanide series. Use of crystal-field independent analysis shows that the discontinuity near the middle of the series is related to variation in both the hyperfine electron−nuclear coupling constant and the crystal-field properties of the metal ions, rather than to gross changes in geometry.

Comparison of different methods for structural analysis of lanthanide-induced NMR shifts: a case of lanthanide(III) cryptates

The three main methods available in the literature for lanthanide-induced shift (LIS) structural analysis of a series of lanthanide(III) complexes in solution are applied to the data available for a complete series of paramagnetic lanthanide cryptate complexes of a Schiff base axial macrobicyclic ligand L, [LnL] 3+. The macrobicycle contracts its cavity as a result of the lanthanide contraction, while preserving the same overall structure in solution. This causes breaks in the LIS plots for all the three methods used. A combined analysis of the data allows to conclude that those breaks reflect abrupt changes of the crystal field parameter A 2 0〈 r 2〉 and the hyperfine constants F i of the lanthanides in the middle of the series, but not of the structure of the complex in solution.

Corrected Structure of Aglalactone Isolated from Aglaia elaeagnoidea (Meliaceae)

The structure of aglalactone, a dihydrobenzofuranone from Aglaia elaeagnoidea, was revised on the basis of 2D NMR data and lanthanide induced shifts (LIS).

Infraspecific variation of sulfur-containing bisamides from Aglaia leptantha.

Six new amides, leptaglin (1), hemileptaglin (2), aglanthin (3), agleptin (4), isoagleptin (5), and leptanthin (6), together with known lignans yangambin, eudesmin, grandisin (7), epigrandisin (8), and dehydrodiconiferyl alcohol, were isolated and identified from the lipophilic leaf and stem bark extracts of Aglaia leptantha. The dominating sulfur-containing bisamides contained either putrescine or the corresponding pyrrolidine ring as the diamine part, linked to phenylacetic and/or methylthiopropenoic acid moieties showing a remarkable infraspecific variation in eight individuals from two different habitats. Structures were determined by MS and NMR, including lanthanide-induced shifts.

Conformational analysis, Part 31.1 A theoretical and lanthanide induced shift (LIS) investigation of the conformations of some epoxides

An improved LIS technique, using Yb(fod)3 to obtain the paramagnetic induced shifts of all the spin ½ nuclei in the molecule, together with complexation shifts obtained by the use of Lu(fod)3, has been used to investigate the conformations of a group of epoxides. These are cis (1) and trans (2) stilbene oxide, cyclopentene oxide (3), cyclohexene oxide (4), cycloheptene oxide (5), propene oxide (6) and styrene oxide (7).The LIRAS3 complexation model involving two symmetric lone pairs on the oxygen atom was used for the symmetric compounds but, for the unsymmetric compounds, a more complex unsymmetric complexation model (HARDER) was found to be necessary. The calculated LIS for styrene oxide and cis- and trans-stilbene oxide were in excellent agreement with the observed data for both the molecular mechanics (MM) and the ab initio geometries with the phenyl ring dihedral angles optimised.In styrene oxide and trans-stilbene oxide the phenyl rings are approximately perpendicular to the oxirane ring, in agreement with the conformation in the solid state and with the theoretical calculations. In cis-stilbene oxide steric repulsions between the phenyl rings splay them apart so that they are now exo to the oxirane ring. Again the LIS analysis is in good agreement with the theoretical calculations.Both the LIS data and the modelling studies agree that cyclopentene oxide is in a boat conformation with an angle of pucker of ca. 30° and that cyclohexene oxide is in a half-chair conformation with C4 and C5 displaced from the ring plane.The LIS analysis of cycloheptene oxide gave good agreement for two equilibrating chair conformations with an endo/exo ratio of 70∶30, in excellent agreement with low temperature NMR data.The accurate reproduction of the LIS data provides an unambiguous method of assigning the proton chemical shifts of the individual methylene protons in the cyclic epoxides, which are not easily available by any other technique.

Z,E Isomerism and hindered rotations in malonamides: an NMR study of N,N ′-dimethyl-N,N ′-dibutyl-2-tetradecylpropane-1,3-diamide

The title propane-1,3-diamide has three geometrical isomers, ZZ, ZE and EE, in slow exchange on the NMR timescale. The distribution of isomers involves the two amide moieties in a statistical manner. Four types of conformational changes ZZ ⇄ ZE ⇄ EE are shown by exchange correlation spectroscopy and lineshape calculations. Isomers are identified on the basis of aromatic solvent induced shift (ASIS) and lanthanide induced shift (LIS) experiments. Barriers to rotation in each amide moiety are relatively low due to the bulkiness of N- and C-substituents, typically 71 kJ mol–1 at 25 °C, and are weakly sensitive to the conformational state (Z or E) of the other amide moiety.

1H NMR studies of drugs with achiral and chiral lanthanide shift reagents: applications to the anticonvulsant pheneturide.

The anticonvulsant pheneturide, PNT, has been studied by 300 MHz 1H NMR in CDCl3 at ambient temperatures with the achiral lanthanide shift reagent (LSR) Eu(FOD)3, and with the chiral LSR, Eu(HFC)3. Both LSRs produced spectral simplification of the aryl proton signal region, and substantial lanthanide-induced shifts (LIS). With added Eu(HFC)3, enantiomeric shift differences (DeltaDeltadelta) were induced for most nuclei of PNT, indicating substantial potential for direct determination of enantiomeric excess. Valley heights between corresponding signals in the PNT enantiomers as low as 3.6% were achieved for the meta resonance. Least squares line-fitting was applied to the variation of chemical shift vs. [LSR]/[PNT] molar ratios for both LSRs. Tentative assignments were made for the NH absorptions based on two-dimensional NMR (COSY45), as well as their relative magnitudes of LIS, DeltaDeltadelta, and lanthanide-induced line broadening. The PNT conformation reported in the crystal is believed to be retained in solution with added LSR. The relative senses of magnetic nonequivalence were found to be the same among the three sets of aryl protons, and among the three kinds of protons in the ethyl moiety, with high levels of added chiral LSR, using 2D NMR.

Proton NMR, luminescence and electrochemical study of 18-membered Schiff-base macrocyclic lanthanum(III) complexes

An NMR, electrochemical and photophysical study has been made of lanthanide(III) complexes with a 18-membered hexadentate Schiff-base N4O2 polyoxaazamacrocycle L1 derived from the condensation of 2,6-bis(2-aminophenoxymethyl)pyridine and 2,6-diformylpyridine, as well as their interaction with the chelating agents 1,10-phenanthroline and 2,2′-bipyridine. Separation of the contact and pseudocontact contributions to the observed 1H NMR lanthanide-induced shift values point to an isostructural series of [Ln(L1)][ClO4]3 (Ln = La–Eu, except Pm) in acetonitrile. The interaction of the lanthanum(III) complex with 1,10-phenanthroline and 2,2′-bipyridine was demonstrated by the use of different spectroscopic techniques and the stability constant for the 1,10-phenanthroline adduct was calculated from 1H NMR data. Upon excitation through the metal excited levels, the terbium(III) complex displays the typical luminescence originating from the excited 5D4 level, while the adduct with 1,10-phenanthroline presents emission upon excitation through the ligand bands. The reaction of 2,6-bis(2-aminophenoxymethyl)pyridine and 2,6-diformylpyridine and the perchlorate of GdIII or TbIII in ethanol solution yielded a reduced macrocycle L2 instead of the expected complexes of L1. Electrochemical studies in acetonitrile solution showed that the reduction of the imine groups is favoured when the ionic radius of the lanthanide(III) ion decreases.