Magnetic Anisotropy of Homo- and Heteronuclear Terbium(III) and Dysprosium(III) Trisphthalocyaninates Derived from Paramagnetic 1H-NMR Investigation

1H-NMR spectroscopy of lanthanide complexes is a powerful tool for deriving spectral-structural correlations, which provide a clear link between the symmetry of the coordination environment of paramagnetic metal centers and their magnetic properties. In this work, we have first synthesized a series of homo- (M = M* = Dy) and heteronuclear (M ≠ M* = Dy/Y and Dy/Tb) triple-decker complexes [(BuO)8Pc]M[(BuO)8Pc]M*[(15C5)4Pc], where BuO- and 15C5- are, respectively, butoxy and 15-crown-5 substituents on phthalocyanine (Pc) ligands. We provide an algorithmic approach to assigning the 1H-NMR spectra of these complexes and extracting the axial component of the magnetic susceptibility tensor, χax. We show how this term is related to the nature of the lanthanide ion and the shape of its coordination polyhedron, providing an experimental basis for further theoretical interpretation of the revealed correlations.

Site-Selective Solvation-Induced Conformational Switching of Heteroleptic Heteronuclear Tb(III) and Y(III) Trisphthalocyaninates for the Control of Their Magnetic Anisotropy

In the present work, we report the synthesis of isomeric heteronuclear terbium(III) and yttrium(III) triple-decker phthalocyaninates [(BuO)8Pc]M[(BuO)8Pc]M*[(15C5)4Pc] (M = Tb, M* = Y or M = Y, M* = Tb, [(BuO)8Pc]2--octa-n-butoxyphthalocyaninato-ligand, [(15C5)4Pc]2--tetra-15-crown-5-phthalocyaninato-ligand). We show that these complexes undergo solvation-induced switching: the conformers in which both metal centers are in square-antiprismatic environments are stabilized in toluene, whereas in dichloromethane, the metal centers M and M* are in distorted prismatic and antiprismatic environments, respectively. This conclusion follows from the detailed analysis of lanthanide-induced shifts in 1H NMR spectra, which makes it possible to extract the axial component of the magnetic susceptibility tensor χaxTb and to show that this term is particularly sensitive to conformational switching when terbium(III) ion is placed in the switchable "M" site. This result provides a new tool for controlling the magnetic properties of lanthanide complexes with phthalocyanine ligands.

Heavily Gd-Doped Non-Toxic Cerium Oxide Nanoparticles for MRI Labelling of Stem Cells

Recently, human mesenchymal stem cells (hMSc) have attracted a great deal of attention as potential therapeutic agents in the treatment of socially significant diseases. Despite substantial advances in stem-cell therapy, the biological mechanisms of hMSc action after transplantation remain unclear. The use of magnetic resonance imaging (MRI) as a non-invasive method for tracking stem cells in the body is very important for analysing their distribution in tissues and organs, as well as for ensuring control of their lifetime after injection. Herein, detailed experimental data are reported on the biocompatibility towards hMSc of heavily gadolinium-doped cerium oxide nanoparticles (Ce_0.8Gd_0.2O_2-x) synthesised using two synthetic protocols. The relaxivity of the nanoparticles was measured in a magnetic field range from 1 mT to 16.4 T. The relaxivity values (r₁ = 11 ± 1.2 mM^-1 s^-1 and r₁ = 7 ± 1.2 mM^-1 s^-1 in magnetic fields typical of 1.5 and 3 T MRI scanners, respectively) are considerably higher than those of the commercial Omniscan MRI contrast agent. The low toxicity of gadolinium-doped ceria nanoparticles to hMSc enables their use as an effective theranostic tool with improved MRI-contrasting properties.

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.

Synthesis and Spectroscopic and Luminescent Properties of Er, Yb and Lu Complexes with Cyano-Substituted Phthalocyanine Ligands

Based on 4,4′-[1,3/4-phenilenebis(oxy)]phthalodinitriles, the mixture of phthalocyaninates of various structures with rare-earth metals were obtained by template fusion method minimizing the side polymerization processes. Target monophthalocyaninates were isolated from the reaction mixture and purified using column and then gel permeation chromatography. The compounds were characterized by NMR, IR spectroscopy, mass spectrometry, and elemental analysis. The spectral properties were studied and the aggregation behavior of the synthesized Er, Yb, and Lu phthalocyaninates in chloroform, acetone, and tetrahydrofuran was determined. It has been shown that lutetium complexes with 3,4-dicyanophenoxyphenoxy ligands are the least stable and least resistant to aggregation in solution, while erbium and ytterbium phthalocyaninates proved to be stable in all studied media. The quantum yields and fluorescence lifetimes of the complexes in chloroform and tetrahydrofuran were calculated.

Nuclear magnetic resonance thermosensing properties of holmium(III) and thulium(III) tris(tetra-15-crown-5-phthalocyaninato) complexes

Temperature dependences of paramagnetic chemical shifts in NMR spectra of lanthanide tris-phthalocyaninates Ln2[(15C5)4Pc]3 (where [(15C5)4Pc][Formula: see text] is 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninate dianion, Ln = Ho(III), Tm(III)) have been studied in the physiological temperature range (from 303 to 323 K). The observed maximum temperature sensitivity [Formula: see text]/dTturns out to be 0.25 ppm/K for the signals of the thulium complex and 0.16 ppm/K for the holmium complex. By the example of Tm2[(15C5)4Pc]3, it has been shown that the use of temperature sensitivities normalized to signal half-widths ([Formula: see text]CT[Formula: see text]/[Formula: see text] is expedient to judge the applicability of the observed temperature dependences of LISs of analogous lanthanide complexes (Ln = Tb, Dy, Ho, Tm) for determining temperatures. The investigated kinetically and thermodynamically stable Ln2[(15C5)4Pc]3 complexes can be considered promising for the design of thermosensitive NMR probes for determination of the local temperature in nonpolar solutions.

Heteroleptic Crown-Substituted Tris(phthalocyaninates) as Dynamic Supramolecular Scaffolds with Switchable Rotational States and Tunable Magnetic Properties

Herein we report single-crystal X-ray diffraction characterization and complementary solution studies of supramolecular interaction between potassium salts and heteroleptic homo- and heteronuclear triple-decker crown phthalocyaninates [(15C5)₄Pc]M*[(15C5)₄Pc]M(Pc) or [M*,M], where M* and M = Y and/or Tb. Our results evidence that, in contrast to the previously studied crown-substituted phthalocyanines, the interaction of K⁺ cations with [M*,M] does not induce their intermolecular aggregation. Instead, the cations reversibly intercalate between the crown-substituted phthalocyanine ligands, resulting in switching of the coordination polyhedron of the metal center M* from square-antiprismatic to square-prismatic. In the case of terbium(III) complexes, such a switching alters their magnetic properties, which can be read-out by ¹H NMR spectroscopy. For [Tb*,Y], such a switching causes an almost 25% increase in the axial component of the magnetic susceptibility tensor. Even though the polyhedron of the paramagnetic center in [Y*,Tb] is not switched, minor structural perturbations associated with the overall reorganization of the receptor also cause smaller, but nevertheless appreciable, growth of the axial anisotropy. The observed effects render the studied complexes as molecular switches with tunable magnetic properties.