Lanthanide Complexes In Solution Research Articles

Structure Determination of Binuclear Triple-Decker Phthalocyaninato Complexes by NMR via Paramagnetic Shifts Analysis Using Symmetry Peculiarities.

The detailed knowledge about the structure of multinuclear paramagnetic lanthanide complexes for the targeted design of these compounds with special magnetic, sensory, optical and electronic properties is a very important task. At the same time, establishing the structure of such multinuclear paramagnetic lanthanide complexes in solution, using NMR is a difficult task, since several paramagnetic centers act simultaneously on the resulting chemical shift of a particular nucleus. In this paper, we have demonstrated the possibility of molecular structure determination in solution on the example of binuclear triple-decker lanthanide(III) complexes with tetra-15-crown-5-phthalocyanine Ln2[(15C5)4Pc]3 {where Ln = Tb (1) and Dy (2)} by quantitative analysis of the pseudo-contact lanthanide-induced shifts (LIS). The symmetry of complexes was used for the simplification of the calculation of pseudo-contact shifts on the base of the expression for the magnetic susceptibility tensor in the arbitrary oriented magnetic axis system. Good agreement between the calculated and experimental shifts in the 1H NMR spectra indicates the similarity of the structure for the complexes 1 and 2 in solution of CDCl3 and the structure in the crystalline phase, found from the data of the X-ray structural study of the similar complex Lu2[(15C5)4Pc]3. The described approach can be useful for LIS analysis of other polynuclear symmetric lanthanide complexes.

Tripodal 1,2,3-triazole click ligand based on the triphenylphosphine oxide platform: atrane-type lanthanide complexes in solutions

Tris[2-(1-phenyl-1,2,3-triazol-4-ylmethoxy)phenyl]-phosphine oxide, a novel tripodal polytopic ligand produces, stable complexes with La(NO3)3 and Lu(NO3)3. According, to IR, multinuclear NMR and DFT data, the complexes, have atrane-type structure with tetradentate O,N,N,Ncoordination, of ligand in MeCN solutions.

Mechanochemical Syntheses of Ln(hfac)3(H2O)x (Ln = La-Sm, Tb): Isolation of 10-, 9-, and 8-Coordinate Ln(hfac)n Complexes.

Volatile lanthanide coordination complexes are critical to the generation of new optical and magnetic materials. One of the most common precursors for preparing volatile lanthanide complexes is the hydrate with the general formula Ln(hfac)3(H2O)x (x = 3 for La-Nd, x = 2 for Sm) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato). We have investigated the synthesis of Ln(hfac)3(H2O)x using more environmentally sustainable mechanochemical approaches. Characterization of the products using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, elemental analysis, and powder X-ray diffraction shows substantial differences in product distribution between methods. The mechanochemical synthesis of the hydrate complexes leads to a variety of coordination compounds including the expected hydrate product, the known retro-Claisen impurity, and hydrated protonated Hhfac ligand depending on the technique employed. Surprisingly, 10-coordinate complexes of the form Na2Ln(hfac)5·3H2O for Ln = La-Nd were also isolated from reactions using a mortar and pestle. The electrostatic bonding of lanthanide coordination complexes is a challenge for obtaining reproducible reactions and clean products. The reproducibility issues are most acute for the large, early lanthanides whereas for the mid to late lanthanides, reproducibility in terms of product distribution and yield is less of an issue because of their smaller size and greater charge to radius ratio. Ball milling increases reproducibility in terms of generating the desired Ln(hfac)3(H2O)x along with hydrated Hhfac (tetraol) and free Hhfac products. The results illustrate the dynamic behavior of lanthanide complexes in solution and the solid state as well as the structural diversity available to the early lanthanides.

Conditionally designed luminescent DNA crystals doped by Ln3+(Eu3+/Tb3+) complexes or fluorescent proteins with smart drug sensing property.

In this work, a designed porous DNA crystal with high intrinsic biocompatibility was used as the scaffold material to load fluorescent guest molecules to detect anti-cancer drugs. It is shown here that the synthesized crystals have the characteristics consistent with the designed large solvent channels, and can therefore accommodate guest molecules such as fluorescent proteins that cannot be accommodated by less porous crystals. Eu(TTA)3phen and Tb(acac)3phen lanthanide complexes were individually noncovalently loaded into the porous crystals, resulting in hybrid luminescent DNA crystals. Emodin, an anti-cancer, anti-tumor, anti-inflammatory drug, was found to quench lanthanide complexes in solution or in crystals. Notably, emodin is the active ingredient of Lianhua Qingwen Capsule, an anti-COVID-19 drug candidate. Therefore, the porous DNA crystals reported here have potential applications as a biocompatible and theranostic delivery biomaterial for functional macromolecules.

Cooperative Luminescence and Cooperative Sensitisation Upconversion of Lanthanide Complexes in Solution.

Upconversion materials have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state is explored and the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/Tb systems is investigated. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with a quantum yield of 2.8×10-6 at a low laser power density of 2.86 W cm-2 .

NMR Spectroscopy—A Versatile Tool for Studying the Structure and Magnetic Properties of Paramagnetic Lanthanide Complexes in Solutions (Review)

Coordination compounds of lanthanides attract increasing attention of researchers due to their unique physical and chemical properties, including interesting optical and magnetic behavior. Fine tuning of properties of functional materials based on them is possible by controlling the structure of complexes. This review describes the approach of applying NMR spectroscopy to solve the challenging problem of identification of paramagnetic lanthanide complexes, as well as to establish a correlation between the structure of the coordination sphere of lanthanide ions and the magnetic characteristics of the complexes. It has been shown how the analysis of lanthanide-induced shifts (LIS) and lanthanide-induced relaxation (LIR) in series of isostructural compounds contributes to the identification of spectral–structural correlations, which, in combination with X-ray diffraction and quantum-chemical data, make it possible not only to establish the structure of the compound in solution, but also to perform a primary assessment of the possibility of using lanthanides complexes as single molecule magnets.

A new approach to determining the structure of lanthanide complexes in solution according to the Curie-spin contribution to the paramagnetic spin-spin relaxation rate enhancements: Ho-DOTA

In this work, we propose for the first time a new type of NMR method for obtaining structural information on Ln complexes in solutions based on the analysis of the Curie-spin contribution to the paramagnetic spin-spin relaxation rate enhancements on the ligand nuclei. The application of the proposed methodology was carried out in this work by the example of studying the structure of the complex anion [Ho(DOTA)(H2O)]− (I) in D2O. Earlier we carried out the assignment of signals [S.P. Babailov, P.V. Dubovskii, E.N. Zapolotsky, Polyhedron 79 (2014) 277], investigated the molecular conformational dynamics and the dependence of the spin-spin relaxation rate on the magnetic field of complex I. Actually, the Curie spin contribution to the paramagnetic spin-spin relaxation rate enhancements on the ligand nuclei of this complex was determined by varying the constant magnetic field and temperature. As a result, it was found that the structure of the complex in solution is identical to its structure in the crystalline phase (found by the X-ray diffraction).

Drug Sensing Protein Crystals Doped with Luminescent Lanthanide Complexes

Hen egg white lysozyme (HEWL) crystals are a model system for protein crystallization due to economical production and facile crystal growth. Here we grew small, rod-like HEWL crystals and proceeded with stepwise synthesis to dope the protein crystals with luminescent lanthanide complexes [(Eu(TTA)3phen] (TTA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato, phen = 1,10-phenanthroline). Several compounds (coumarin, tinidazole, and acridine orange) were observed to quench the lanthanide complexes in solution or embedded within crystals. Coumarin is a precursor for anticoagulant drugs and tinidazole is used to combat bacterial or protozoan infections. In contrast, acridine orange is a widely used, versatile fluorescent dye used to stain acidic vacuoles, RNA, and DNA in living cells. These results suggest that protein crystals may provide a feasible matrix for entrapping luminescent species with practical biosensing applications.

Composed in the f-block: solution structure and function of kinetically inert lanthanide(iii) complexes.

It has been more than 15 years since the last authoritative report on the solution structure of lanthanide complexes made from cylcen derived polydentate ligands. The field has progressed and diversified, and tools have been developed that should enable a step-change in the field in the imminent future. This will only happen if the tools are used, and the results communicated in a form that is consistent within the field and readily accesible to scientists outside the field. In this perspective, the fundamental tools for designing and investigating kinetically inert lanthanide complexes in solution will be covered. The fundamentals of this type of complexes will be laid out. The conformations of lanthanide complexes from cyclen derived ligands and the rate of exchange between conformations will be linked to their 1H NMR and luminescence spectra. The information rich ligand- and metal centred emission spectra will be discussed, and the time-resolved luminescence decay lifetimes are shown to be directly related to the solution structure. The aim is to provide the reader with the information needed to become excited by lanthanide coordination chemistry.

EXAFS characterisation of metal bonding in highly luminescent, UV stable, water-soluble and biocompatible lanthanide complexes

The combination of X-ray diffraction with EXAFS was employed to assess the coordination environment of lanthanide complexes in solutions. This method is based on the assumption that the local structure of lanthanide complexes in solution combines elements of the crystal structure of the complex in the solid state (single- or polycrystalline) and the elements of the local structure of a lanthanide salt, completely dissociated in the solvent (usually chlorides). The success of this approach is demonstrated with the lanthanide (III) 2,3,4,5,6-pentafluorobenzoate complexes, where the local structure in aqueous and methanol solutions were estimated. Moreover, the dissociation degree of the complexes in aqueous and methanol solutions was evaluated.

Lanthanide-based laser-induced phosphorescence for spray diagnostics.

Laser-induced phosphorescence (LIP) is a relatively recent and versatile development for studying flow dynamics. This work investigates certain lanthanide-based molecular complexes for their use in LIP for high-speed sprays. Lanthanide complexes in solutions have been shown to possess long phosphorescence lifetimes (∼1-2 ms) and to emit light in the visible wavelength range. In particular, europium and terbium complexes are investigated using fluorescence/phosphorescence spectrometry, showing that europium-thenoyltrifluoracetone-trioctylphosphineoxide (Eu-TTA-TOPO) can be easily and efficiently excited using a standard frequency-tripled Nd:YAG laser. The emitted spectrum, with maximum intensity at a wavelength of 614 nm, is shown not to vary strongly with temperature (293-383 K). The decay constant of the phosphorescence, while independent of ambient pressure, decreases by approximately 12 μs/K between 323 and 373 K, with the base level of the decay constant dependent on the used solvent. The complex does not luminesce in the gas or solid state, meaning only the liquid phase is visualized, even in an evaporating spray. By using an internally excited spray containing the phosphorescent complex, the effect of vaporization is shown through the decrease in measured intensity over the length of the spray, together with droplet size measurements using interferometric particle imaging. This study shows that LIP, using the Eu-TTA-TOPO complex, can be used with different solvents, including diesel surrogates. Furthermore, it can be easily handled and used in sprays to investigate spray breakup and evaporation.

Challenging lanthanide relaxation theory: erbium and thulium complexes that show NMR relaxation rates faster than dysprosium and terbium analogues.

Measurements of the proton NMR paramagnetic relaxation rates for several series of isostructural lanthanide(III) complexes have been performed in aqueous solution over the field range 1.0 to 16.5 Tesla. The field dependence has been modeled using Bloch-Redfield-Wangsness theory, allowing values for the electronic relaxation time, Tle and the magnetic susceptibility, μeff, to be estimated. Anomalous relaxation rate profiles were obtained, notably for erbium and thulium complexes of low symmetry 8-coordinate aza-phosphinate complexes. Such behaviour challenges accepted theory and can be interpreted in terms of changes in Tle values that are a function of the transient ligand field induced by solvent collision and vary considerably between Ln(3+) ions, along with magnetic susceptibilities that deviate significantly from free-ion values.

Luminescent lanthanide-binding peptides: sensitising the excited states of Eu(iii) and Tb(iii) with a 1,8-naphthalimide-based antenna

The investigation into the luminescence properties of a lanthanide-binding peptide, derived from the Ca-binding loop of the parvalbumin, and modified by incorporating a 1,8-naphthalimide (Naph) chromophore at the N-terminus is described. Here, the Naph is used as a sensitising antenna, which can be excited at lower energy than classical aromatic amino acids, such as tryptophan (the dodecapeptide of which was also synthesised and studied herein). The syntheses of the Naph antenna, its solid phase incorporation into the dodecapeptide, and the NMR investigation into the formation of the corresponding lanthanide complexes in solution is presented. We also show that this Naph antenna can be successfully employed to sensitize the excited states of both europium and terbium ions, the results of which was used to determined the stability constants of their formation complexes, and we demonstrated that our peptide 'loop' can selectively bind these lanthanide ions over Ca(II).

Hypersensitivity in the Luminescence and 4f–4f Absorption Properties of Mono- and Dinuclear EuIII and ErIII Complexes Based on Fluorinated β-Diketone and Diimine/ Bis-Diimine Ligands

The results of our investigation on the sensitized luminescence properties of three Eu(III) β-diketonate complexes of the form [Eu(2)(fod)(6)(μ-bpm)], [Eu(fod)(3)(phen)] and [Eu(fod)(3)(bpy)] and 4f-4f absorption properties of their Er(III) analogues ( fod = anion of 6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedione, bpm = 2,2'-bipyrimidine, phen = 1,10-phenanthroline and bpy = 2,2'-bipyridyl) in a series of non-aqueous solvents are presented. The Eu(III) complexes are highly luminescent and their luminescence properties (intensity and band shape) are sensitive to the changes in the inner coordination sphere of the Eu(III) ion. The luminescence intensity of the mononuclear complexes in pyridine is drastically decreased. The coordination structure of the complexes in pyridine is transformed into a more symmetrical one which results into a slow radiative rate of the emission from the complexes. The ancillary ligands, phen and bpy are found better co-sensitizers as compared to the bpm to sensitize Eu(III)-luminescence. The 4f-4f absorption properties (oscillator strength and band shape) of the Er(III) complexes demonstrate that (4)G(11/2) ← (4)I(11/2) and (2)H(11/2) ← (4)I(15/2) hypersensitive transitions of Er(III) are very sensitive in some coordinating solvents which reflects complex-solvent interaction in solution. The hypersensitive transitions of [Er(fod)(3)(phen)] remain unaffected in any of the solvents and this complex retains its bulk composition in solution. The erbium complexes as well as the Er(fod)(3) chelate are invaded by DMSO. This solvent enters the inner coordination sphere by replacing heterocyclic ligand and the complexes acquire similar structure [Er(fod)(3)(DMSO)(2)] in this solvent. The results reveal that the luminescence and absorption properties of lanthanide complexes in solution can be controlled by tuning the coordination structure through ancillary ligands and donor solvents. This work shall prove useful in designing new biological applications with such probes.

Photoluminescence, optical absorption and hypersensitivity in mono- and dinuclear lanthanide (Tb III and Ho III) β-diketonate complexes with diimines and bis-diimine bridging ligand

The photoluminescence properties of three Tb(III) complexes of the form [Tb 2(fod) 6(μ-bpm)], [Tb(fod) 3(phen)] and [Tb(fod) 3(bpy)] and optical absorption properties of their Ho(III) analogues (fod=anion of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, bpm=2,2′-bipyrimidine, phen=1,10-phenanthroline and bpy=2,2′-bipyridyl) in a series of solvents are presented. The luminescence of the complexes is sensitive to changes in environment (ligand/solvent) around Tb(III) and co-sensitization of the ancillary ligands. The enhancement of the luminescence intensity in coordinating solvents is attributed to the transformation of eight-coordination into less symmetric nine-coordination structure around Tb(III). Among phen and bpy, the phen is better co-sensitizer while bpm has been observed as poor co-sensitizer. The enhancement of the oscillator strength of 5G 6← 5I 8 hypersensitive transition in the 4f–4f absorption in some coordinating solvents is attributed to decrease in the symmetry of the field around Ho(III) ion. The [Ho(fod) 3(phen)] is inert towards the solvents and retains its bulk structure and composition in solution. The transformation of the holmium complexes in DMSO into [Ho(fod) 3(DMSO) 2] species is found. The results reveal that the luminescence and 4f–4f absorption properties of lanthanide complexes in solution can be modulated by tuning the coordination structure through ancillary ligands and donor solvents.

Nonradiative energy transfer as a method for investigation of self-assembling nanostructures of lanthanide complexes in solutions

Works concerning the application of nonradiative transfer of electronic excitation energy to investigation into nanostructures of lanthanide complexes in aqueous solutions are surveyed. The effect of the formation of nanosized structures on the quenching of energy donors Ln(III) ions by acceptor ions in concentrated chloride solutions of structuring ions (Li(I), Ca(II)) was discussed. The columinescence phenomenon observed in aqueous solutions of lanthanide chelates was considered. It was shown that the enhancement of luminescence Eu(III) and Tb(III) complexes in water in the presence of excess β-diketones with an admixture of other Ln(III) ions, primarily Gd(III), (columinescence) is due to sensitization via energy transfer over triplet levels of the ligands in the nanostructures formed under these conditions and to the weakening of deactivation of excited luminescent ions by the formation of nanostructures. The influence of the solution preparation procedure on the formation of nanostructures of chelates with different lanthanide ions was revealed, which manifest itself as a variation in the enhancement and quenching of luminescence in the presence of ions from the cerium and yttrium subgroups. Possible applications of the columinescence phenomenon to chemical and medical analysis are briefly discussed.

New chiral macrocyclic lanthanide complexes derived from (1R,2R)-1,2-diaminocyclohexane and 2,6-diformylpyridine

The new enantiopure complexes [LnL](NO 3) 3 · nH 2O (Ln = Dy +3, Ho +3, Er +3, Lu +3) and [LnL]Cl 3 · nH 2O (Ln = Nd +3, Sm +3, Gd +3, Tb +3, Dy +3, Ho +3, Er +3, Tm +3, Lu +3) of the chiral macrocycle L derived from (1R,2R)-1,2-diaminocyclohexane and 2,6-diformylpyridine have been synthesised. The preference of macrocycle L for the heavier lanthanide(III) ions has been established on the basis of competition reaction. The complexes have been characterised by NMR spectroscopy and mass spectrometry. 1H NMR signals of deuterated water solutions of the Ce +3, Nd +3 and Eu +3 complexes have been assigned on the basis of the COSY and HMQC spectra, and for the remaining lanthanide complexes the signals were assigned on the basis of linewidths analysis. The paramagnetic shifts of the series of lanthanide complexes [LnL](NO 3) 3 · nH 2O and [LnL]Cl 3 · nH 2O have been analysed using both crystal-field dependent and independent methods in order to separate contact and dipolar contributions and establish isostructurality along the series of lanthanide complexes in solution. The data obtained for nitrate derivatives in organic solvent indicate rather irregular deviations from the plots based on those methods, while the plots obtained for water solutions show the characteristic brake in the middle of the lanthanide series, that is interpreted as a result of change of the number of axially coordinated water molecules. The apparent inconsistencies of results obtained on the basis of crystal-field independent method are discussed.

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.

3-[2,6-Bis(diethylcarbamoyl)pyridin-4-yl]-N-(tert-butoxycarbonyl)alanine methyl ester: a chiral tridentate ligand that causes a diastereomeric excess of its lanthanide complexes in solution.

L(4), or 3-[2,6-bis(diethylcarbamoyl)pyridin-4-yl]-N-(tert-butoxycarbonyl)alanine methyl ester, C(24)H(38)N(4)O(6), crystallizes in neat [010] laths stabilized by abundant intra- and intermolecular hydrogen bonds. The strongest of these form [010] chains of molecules, thus rationalizing the fastest growth direction, while the slowest direction coincides with the normal to the (110) layers, which are linked by very weak hydrogen bonds. There exist two independent molecules, the distances and bond angles of which differ in a random manner only. The torsion and dihedral angles, however, differ so as to achieve optimal packing. The influence of the chiral group in the 4-position of the pyridine ring on the helical wrapping and on the ensuing diastereomeric induction is briefly discussed.

Correlation of optical and NMR spectral information with coordination variation for axially symmetric macrocyclic Eu(iii) and Yb(iii) complexes: axial donor polarisability determines ligand field and cation donor preference

In Bleaney's theory of magnetic anisotropy, the second-order crystal field coefficient, Bo2, is predicted to determine the dipolar NMR shift of paramagnetic lanthanide complexes in solution. This parameter has been measured directly, by analysing the europium emission spectra for a series of eight- and nine-coordinate axially symmetric complexes based on cyclen including aza-carboxylate ligands (e.g. DOTA), phosphonates (DOTP), phosphinates and several carboxamides (e.g. DOTAM). For both Yb and Eu complexes with a common coordination number and geometry (square antiprism (SAP) or twisted square antiprism (TSAP)), the dipolar NMR shift correlates very well with this parameter, which also determines the sign and magnitude of a major CD band in the near-IR CD spectra of a series of enantiopure Yb complexes. Measurements of the free energy change associated with axial ligand exchange in a cationic europium tetraamide complex, [Eu(DOTAMPh)](CF3SO3)3 supported by a simple electrostatic perturbation model, have been interpreted in terms of a predominant donor atom polarisation model which affords a simple assessment of Ln ion donor atom preference and ranks the axial second-order ligand field coefficient.