The Studies of Lanthanide Shift Reagents in NMR Spectroscopy

The action of a series of lanthanide chelate complexes LnL₃, where Lu=La, Pr, Nd, Sm, Eu, Dy, Ho, Er and Yb; L= dbm (dibenzoylmethanato), dpm (dipivaloylmethanato), and acac acetylactonato), on nmr spectra of 2-butanol has been studied. The origin of shifting abilities and shifting sign of various lanthanide ions are explained by means of Bleaney's theory.     

The lanthanide contraction revisited

A complete, isostructural series of complexes with La-Lu (except Pm) with the ligand TREN-1,2-HOIQO has been synthesized and structurally characterized by means of single-crystal X-ray analysis. All complexes are 1D-polymeric species in the solid state, with the lanthanide being in an eight-coordinate, distorted trigonal-dodecahedral environment with a donor set of eight unique oxygen atoms. This series constitutes the first complete set of isostructural complexes from La-Lu (without Pm) with a ligand of denticity greater than two. The geometric arrangement of the chelating moieties slightly deviates across the lanthanide series, as analyzed by a shape parameter metric based on the comparison of the dihedral angles along all edges of the coordination polyhedron. The apparent lanthanide contraction in the individual Ln-O bond lengths deviates considerably from the expected quadratic decrease that was found previously in a number of complexes with ligands of low denticity. The sum of all bond lengths around the trivalent metal cation, however, is more regular, showing an almost ideal quadratic behavior across the entire series. The quadratic nature of the lanthanide contraction is derived theoretically from Slater's model for the calculation of ionic radii. In addition, the sum of all distances along the edges of the coordination polyhedron show exactly the same quadratic dependence as the Ln-X bond lengths. The universal validity of this coordination sphere contraction, concomitant with the quadratic decrease in Ln-X bond lengths, was confirmed by reexamination of four other, previously published series of lanthanide complexes. Owing to the importance of multidentate ligands for the chelation of rare-earth metals, this result provides a significant advance for the prediction and rationalization of the geometric features of the corresponding lanthanide complexes, with great potential impact for all aspects of lanthanide coordination.     

Insight of the Metal–Ligand Interaction in f-Element Complexes by Paramagnetic NMR Spectroscopy

The magnetic properties of Ln^III and An^III complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometries of these complexes, a crystallographic study by single-crystal X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) in solution was performed. Several methods to separate the paramagnetic shifts observed in the NMR spectra were applied to these complexes. Methods using a number of nuclei of the dipicolinate ligands revealed an abrupt change in the geometries of the complexes and a metal–ligand interaction in the middle of the lanthanide series. A study of the variation of the paramagnetic shifts with temperature demonstrated that higher-order terms of the dipolar and contact contributions are required, especially for the lightest Ln^III and almost all the studied An^III. Bleaney's parameters <S_z>_a and relating to the contact and dipolar terms, respectively, were deduced from experimental data and compared with the results of ab initio calculations. Quite a good agreement was found for the temperature dependencies of <S_z>_a and . However, the values obtained from cation magnetic anisotropy calculations showed some discrepancies with the values derived from Bleaney's equation defined for Ln^III. Other parameters, such as the crystal field parameter and the hyperfine constants F_i obtained from the experimental data of the [An(ethyl-dpa)₃]³⁻ complexes (ethyl-dpa=4-ethyl-2,6-dipicolinic acid), are at odds with the assumptions underlying Bleaney's theory.     

Unravelling the Complexities of Pseudocontact Shift Analysis in Lanthanide Coordination Complexes of Differing Symmetry

In two closely related series of eight‐coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor as the Ln³⁺ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.     

A justification for using NMR model-free methods when investigating the solution structures of rhombic paramagnetic lanthanide complexes

The detailed analysis of the 1H NMR hyperfine shifts according to the model-free methods shows that the semi-rigid monometallic complexes [Ln(L)(NO3)3] (Ln = Eu-Yb) are isostructural in solution. The associated separation of contact and pseudo-contact contributions to the hyperfine NMR shifts in each rhombic lanthanide complex at room temperature provides paramagnetic susceptibility tensors whose principal magnetic axes match the expected symmetry requirements. Moreover, both axial (Delta chi(ax)) and rhombic (Delta chi(rh)) paramagnetic anisotropies display satisfactory linear dependence on Bleaney's factors, a correlation predicted by the approximate high-temperature expansion of the magnetic susceptibility limited to T(-2). Consequently, the simple, and chemically attracting NMR model-free methods are not limited to axial systems, and can be safely used for the investigation of the solution structures of any lanthanide complexes. Molecular-based structural criteria for the reliable estimation of paramagnetic susceptibility tensors by NMR are discussed, together with the assignment of the labels of the crystal-field and magnetic axes within Bleaney's approach.     

Magnetic susceptibility tensor anisotropies for a lanthanide ion series in a fixed protein matrix.

The full series of lanthanide ions (except the radioactive promethium and the S-state gadolinium) has been incorporated into the C-terminal calcium binding site of the dicalcium protein calbindin D(9k). A fairly constant coordination environment is maintained throughout the series. At variance with several lanthanide complexes with small chelating ligands investigated in the past, the large protein moiety provides a large number of NMR signals whose hyperfine shifts can be exclusively ascribed to pseudocontact shifts (PCS). The chemical shifts of 1H and 15N backbone and side chain amide NH groups were accurately measured through HSQC experiments. 1097 PCS were estimated from these by subtracting the diamagnetic contributions measured on HSQC spectra of either the 4f(0) lanthanum(III) or the 4f(14) lutetium(III) derivatives and used to define a quality factor for the structure. The differences in diamagnetic chemical shifts between the two diamagnetic blanks were relatively small, although some were not negligible especially for the nuclei closest to the metal center. These differences were used as a tolerance for the PCS. The magnetic susceptibility tensor anisotropies for each paramagnetic lanthanide ion were obtained as the result of the solution structure determination performed by using the NOEs of the cerium(III) derivative and the PCS of all lanthanides simultaneously. This set of reliable magnetic data permits an experimental assessment of Bleaney's theory relative to the magnetic properties for an extended series of lanthanide complexes in solution. All of the obtained tensors show some rhombicity, as could be expected from the lack of symmetry of the protein environment. The directions of the largest magnetic susceptibility component for Ce, Pr, Nd, Sm, Tb, Dy, and Ho and of the smallest magnetic susceptibility component for Eu, Er, Tm, and Yb were found to be all within 15 degrees from their average (within 20 degrees for Sm), confirming the essential similarity of the coordination environment for all lanthanides. Bleaney's theory is in excellent qualitative agreement with the observed pattern of axial anisotropies. Its quantitative agreement is substantially better than that suggested by previous analyses performed on more limited sets of PCS data for small lanthanide complexes, the so-called crystal field parameter varying only within +/-30% from one lanthanide to another. These variations are even smaller (+/-15%) if a reasonable T(-3) correction is taken into consideration. A knowledge of magnetic susceptibility anisotropy properties of lanthanides is essential in determining the self-orienting properties of lanthanide complexes in solution when immersed in magnetic fields.     

Lanthanide Contraction within a Series of Asymmetric Dinuclear [Ln2] Complexes

A complete isostructural series of dinuclear asymmetric lanthanide complexes has been synthesized by using the ligand 6‐[3‐oxo‐3‐(2‐hydroxyphenyl)propionyl]pyridine‐2‐carboxylic acid (H₃ L ). All complexes have the formula [Ln₂(H L )₂(H₂ L )(NO₃)(py)(H₂O)] (Ln=La ( 1 ), Ce ( 2 ), Pr ( 3 ), Nd ( 4 ), Sm ( 5 ), Eu ( 6 ), Gd ( 7 ), Tb ( 8 ), Dy ( 9 ), Ho ( 10 ), Er ( 11 ), Tm ( 12 ), Yb ( 13 ), Lu ( 14 ), Y ( 15 ); py=pyridine). Complexes of La to Yb and Y have been crystallographically characterized to reveal that the two metal ions are encapsulated within two distinct coordination environments of differing size. Whereas one site maintains the coordination number (nine) through the whole series, the other one increases from nine to ten owing to a change in the coordination mode of an NO₃^? ligand. This series offers a unique opportunity to study in detail the lanthanide contraction within complexes of more than one metal. This analysis shows that various representative parameters proportional to this contraction follow a quadratic decay as a function of the number n of f electrons. Slater’s model for the atomic radii has been used to extract, from these decays, the shielding constant of 4f electrons. The average of O???O distances within the coordination polyhedra shared by both metals and of the Ln???Ln separations follow also a quadratic decay, therefore showing that such dependence holds also for parameters that receive the contribution of two lanthanide ions simultaneously. The magnetic behavior has been studied for all nondiamagnetic complexes. It reveals the effect of the spin–orbit coupling and a weak antiferromagnetic interaction between both metals. Photoluminescent studies of all the complexes in the series reveal a single broad emission band in the visible region, which is related to the coordinated ligand. On the other hand, the Nd, Er, and Yb complexes show features in the near‐IR region due to metal‐based transitions.     

两个双核稀土-自由基化合物的合成、结构及磁性

以乙酰丙酮为共配体的稀土配合物与2-羟基苯取代的自由基配体进行反应得到2个新颖的稀土-自由基配合物[Ln₂（acac）₄（NIT-PhO）₂]（Ln=Tb（1）,Y（2）;acac=乙酰丙酮,NIT-PhOH=2-（2''-hydroxyphenyl）-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide）。2个配合物的结构相同,均是通过2个自由基配体上的羟基氧原子桥联2个稀土离子构成双核结构。直流磁化率的研究表明配合物2具有弱的反铁磁性质。     

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.     

^1^3C NMR研究稀土柠檬酸配合物的溶液配位结构

用稀土离子诱导位移和顺磁弛豫时间测定等NMR方法研究了稀土柠檬酸配合物在pH 7.4的溶液中的配位结构。结果表明稀土离子通过3-羟基和3-羧基与柠檬酸配体形成1:2的配合物, 两个端羧基没有参与配位。通过计算稀土离子和柠檬酸配体中各个碳原子间的空间距离的相对比, 建立了稀土离子与柠檬酸配体配位的新模式。     

Mesomorphism of lanthanide-containing Schiff's base complexes with chloride counterions

Liquid crystalline complexes [Ln(LH)3Cl3] have been synthesized, where Ln is a trivalent lanthanide ion (Pr-Lu, except Pm) and where LH is the Schiff's base ligand N-octadecyl4-tetradecyloxysalicylaldimine. Although the ligand does not exhibit mesomorphism, the complexes do (SmA phase). The mesophase behaviour of these compounds has been investigated by polarizing optical microscopy, differential scanning calorimetry and high temperature X-ray diffraction. The lanthanide complexes have much higher melting and clearing points than comparable complexes with nitrate or dodecyl sulphate counterions. In addition, the transition temperatures are virtually independent of the type of lanthanide ion. This behaviour is opposite to that observed for similar complexes with nitrate counterions [Ln(LH)3(NO3)3]. The differences in temperature dependence can be related to structural differences. Whereas in the nitrate complexes the Schiff's base ligand binds in a zwitterionic form, two-dimensional 1H NMR correlation spectroscopy (COSY) of [Lu(LH)3Cl3] gives an indication that in the chloride complexes, besides coordination via the oxygen of molecules in the zwitterionic form, some of the Schiff's base ligands bind in a bidentate fashion (via the phenolic oxygen and the imine nitrogen).     

Two- and three-dimensional lanthanide complexes: synthesis, crystal structures, and properties

3,5-pyrazoledicarboxylic acid (H3L) reacts with nitrate salts of lanthanide(III) (Ln=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) under hydrothermal conditions to form a series of lanthanide polymers 1-9. These nine polymers have the same crystal system of monoclinic, but they exhibit three different kinds of metal-organic framework structures. The complexes {[Ln2(HL)3(H2O)4].2H2O}n (Ln=Pr (1), Nd (2), and Sm (3)) were isostructural and exhibited porous 3D frameworks with a Cc space group. The complexes {[Ln2(HL)3(H2O)3].3H2O}n (Ln=Eu (4), Gd (5), and Tb (6)) were isostructural and built 2D double-decker (2DD) frameworks with a P21/c space group. The complexes {[Ln(HL)(H2L)(H2O)2]}n ((Ln=Dy (7), Ho (8), and Er (9)) were also isostructural and formed 2D monolayer (2DM) frameworks with a P21/n space group. The structure variation from the 3D porous framework to the 2D double-decker to the 2D monolayer is attributed to the lanthanide contraction effect. Notably, six new coordination modes of 3,5-pyrazoledicarboxylic acid were observed, which proved that 3,5-pyrazoledicarboxylic acid may be used as an effective bridging ligand to assemble lanthanide-based coordination polymers. The photophysical and magnetic properties have also been investigated.     

1H and 13C NMR studies of lanthanide complex with 5-sulfosalicylic acid

In this paper, paramagnetic shifts have been measured for all ¹H and ¹³C nuclei of 5-sulfosalicylic acid (SSA) in the presence of the lanthanide ions in the second half of the series. The ligand forms isostructural complexes with these ions in aqueous solution. The separation of LISs was carried out by the use of the Reilley method and the calculated dipolar shifts were used to simulate the coordination structure of the complex. The result reveals that SSA is coordinated to lanthanide ion via two oxygens, one from the carboxylic group and the other from the phenolic group with Ln–O bond lengths equal to 2.47 Å. The lanthanide ion lies on the benzene plane and the carboxylic group is twisted 20° from the benzene ring. Of all the nuclei examined, those in the six-membered chelate ring experience significant dipolar interactions and contact interactions. Small |G/A| ratios were obtained for two protons five bonds away from the central lanthanide ion, which shows that the number of bonds alone cannot be used as a criterion for neglecting contact shifts in aromatic ligand.     

Curvature of the lanthanide contraction: An explanation

A number of studies have shown that for isostructural series of the lanthanides (elements La through Lu), a plot of equivalent metal-ligand bond lengths versus atomic number differs significantly from linearity and can be better fit as a quadratic equation. However, for hydrogen type wave functions, it is the inverse of the average distance of the electron from the nucleus (an estimate of size) that varies linearly with effective nuclear charge. This generates an apparent quadratic dependence of radius with atomic number. Plotting the inverse of lanthanide ion radii (the observed distance minus the ligand size) as a function of effective nuclear charge gives very good linear fits for a variety of lanthanide complexes and materials. Parameters obtained from this fit are in excellent agreement with the calculated Slater shielding constant, k.     

Magnetic interactions in a series of paramagnetic Ln(III) complexes with a chelated imino nitroxide radical

The magnetic interactions in a new series of isostructural imino nitroxide radical lanthanide(III) complexes, [Ln(hfac)₃(IM2py)] (Ln = Gd–Yb: IM2py = 2-(2′-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-1-oxy; hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione), are examined by considering the intrinsic paramagnetic contribution of the Ln(III) ion from the corresponding [Ln(hfac)₃(pybzim)] with a diamagnetic pybzim(2-(2-pyridyl)benzimidazole) ligand; the Ln(III)–IM2py interaction being antiferromagnetic for the 4f⁷ to 4f¹³ Ln(III) complexes and negligibly small for the other complexes. This series is the first example reverse to the previous cases for the series of Ln–Cu or Ln–aminoxyl(NIT) radical (4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-3-oxide-1-oxy) complexes, other than only a few examples of semiquinone Ln complexes. This reverse nature of the magnetic interaction, as compared with the NIT complexes, validates the empirical approach by O. Kahn et al. [Inorg. Chem. 38 (1999) 3692; J. Am. Chem. Soc. 122 (2000) 3413] in the spin-coupled systems for a series of Ln(III) complexes.     

Infrared spectra of lanthanide complexes with eriochrome cyanine r

An infrared spectroscopic study of the composition of these complexes in the solid state is described. Evaluation of the spectra made it possible to distinguish changes in the character of the lanthanide—ligand bonds in the series La — Lu, and to study the “double-double” effect.     

Size-Controlled Hapticity Switching in [Ln(C9H9)(C8H8)] Sandwiches

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid-state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single-crystal X-ray diffraction and in solution by ¹H NMR, static magnetic studies and ab initio computational studies were performed.     

Étude thermodynamique de la complexation des lanthanides trivalents avec l'acide hydroxyethyl-ethylenediaminetriacetique et d'autres acides aminoacetiques: III. Détermination des Constantes de Formation des Complexes Mixtes par Titrage Potentiométrique

A thermodyrmmic study of the complex formation of trivaient lanthanides with hydroxyethylethyienediaminetriacetic acid and other aminoacetic acids. Part III. The determination of the formation constants of mixed complexes by potentiometric titration.The partial stability constants of the mixed and 1:2 complexes Ln—HEDTA—L (where Ln = La, Ce,... and L = glycine, IMDA, NTA, HEDTA and EDTA) have been determined by potentiometric titration at 25°C and at a constant ionic strength of 1 (KCl). The changes in stability of the complexes studied (1:1, 1:2, mixed, protonated and unprotonated) vs. atomic number of the lanthanide are discussed. The changes observed in the trends of the partial and overall stability constants across the lanthanide series are attributed to the decrease in the number of water molecules in the 1:1 LnHEDTA ·xH₂O from x = 3 for light lanthanides to x = 2 for heavy ones. However, in this 1:1 complex, HEDTA seems to be a hexadeutate ligand in the La—Sm range of the series and a pentadentate ligand in the Gd—Lu range. Significant differences have been found between the complexes containing four nitrogen atoms, i.e. L = HEDTA, EDTA, and those with three nitrogen atoms i.e. L = glycine, IMDA, NTA.     

13C NMR study of D-pantothenic acid complexes with heavy lanthanide ions in aqueous solution

The ¹³C NMR study of the interaction between the trivalent lanthanide ions (from Dy to Lu) and D-pantothenic acid in aqueous solution was presented. The unambiguous resonance assignments were made on the basis of pH dependence and the analysis of molecular geometry. ¹³C paramagnetic shifts were separated into the contact and dipolar components by Reilley's method. The complex stability constant is 12.0 L / mol for the 1:1 ytterbium complex. An analysis of the shift data shows that heavy lanthanide ions form isostructural complexes with the substrate and the hyperfine coupling constant is independent of the ions used in the study. D-pantothenic acid is coordinated to the metal via the two oxygens of the carboxyl group with 2.25Å of Ln—O distances. The electron spin density distribution and the structural details of the complex were elucidated from the shift data. It is stated that contact contributions, while small for nuclei of several bonds away from the metal, should not be dismissed a priori.     

Syntheses, structures, near-infrared and visible luminescence, and magnetic properties of lanthanide-organic frameworks with an imidazole-containing flexible ligand

Reactions of tripodal ligand 1,3,5-tris(imidazole-1-ylmethyl)-2,4,6-trimethylbenzene (L) with lanthanide metal salts and triethyl orthoformate led to the formation of six bowl-like dinuclear compounds [Ln2(L)(HL)(NO3)6(HCOO)].3CH3OH (Ln = Gd 1, Tb 2, Dy 3, Er 4, Yb 5, and Eu 6). The single-crystal X-ray diffraction analysis revealed that six complexes are isomorphous and isostructural and that the dinuclear molecules are further connected by hydrogen bonds and pi-pi interactions, resulting in 3D channel-like structures. The luminescence properties have been studied, and the results showed that the Tb(III) (2) and Eu(III) (6) complexes exhibited sensitized luminescence in the visible region and their luminescence lifetimes in powder and DMSO-d6 solution are in the range of milliseconds. The Yb(III) complex (5) emits typical near-infrared luminescence in DMSO-d6 solution. Variable-temperature magnetic susceptibility measurements of 1-6 showed that complex 1 (Gd) is nearly a paramagnet and complexes 2 (Tb), 3 (Dy), and 4 (Er) show the ferromagnetic coupling between magnetic centers, whereas the depopulation of the Stark levels in complexes 5 (Yb) and 6 (Eu) leads to a continuous decrease in (chi M)T when the sample is cooled from 300 to 1.8 K.