GdIII Complexes Research Articles

Quenching luminescence, thermolysis and SMM behavior of new TbIII, EuIII, GdIII complexes with 4-nitro-N'-(pyridin-2-ylmethylene)benzohydrazide

The reaction of 4-nitro-N'-(pyridin-2-ylmethylene)benzohydrazide (HL) with Ln(OAc)3·4H2O in MeOH makes it possible to synthesize mononuclear complexes [Ln(L)2(OAc)(MeOH)]·2H2O (Ln = TbIII (1), EuIII (2) and GdIII (3)) with chelate acetate and L‒ anions. Compound 1 can be crystallized in reaction with molar ratio HL:Ln = 1:1, 2:1, 3:1, and we successfully synthesized complex with three chelate L anions [Tb(L)3]2·2MeOH·H2O (4) by interaction of TbCl3∙6H2O with deprotonated HL (HL:Ln = 3:1). Terbium(III) compound 1 starts to decompose at ∼323 K and becomes stable up to 552 K according to the STA. Compound 1 shows slow magnetic relaxation with parameters Δeff/kB = (6.75 ± 0.02) K, τ0 = (1.71 × 10−6±1 × 10−8) s. Complexes 1 and 2 exhibit only fluorescence and phosphorescence of the L‒. Ion-centered luminescence of the Tb3+ or Eu3+ ion is not observed. Using the tangent method at the high-energy edge of the phosphorescence spectrum of Gd3+, complex 3 T1 energy level of L‒ is estimated to be 19700 cm−1. Reasons of luminescence quenching are discussed. Structures of 1 and 4 were determined by single crystal X-ray diffraction, and compounds 1–3 were characterized by powder X-ray diffraction (PXRD).

A Gadolinium(III) Complex Based on Pyridoxine Molecule with Single-Ion Magnet and Magnetic Resonance Imaging Properties.

Pyridoxine (pyr) is a versatile molecule that forms part of the family of B vitamins. It is used to treat and prevent vitamin B6 deficiency and certain types of metabolic disorders. Moreover, the pyridoxine molecule has been investigated as a suitable ligand toward metal ions. Nevertheless, the study of the magnetic properties of metal complexes containing lanthanide(III) ions and this biomolecule is unexplored. We have synthesized and characterized a novel pyridoxine-based GdIII complex of formula [GdIII(pyr)2(H2O)4]Cl3 · 2 H2O (1) [pyr = pyridoxine]. 1 crystallizes in the triclinic system and space group Pī. In its crystal packing, cationic [Gd(pyr)2(H2O)4]3+ entities are connected through H-bonding interactions involving non-coordinating water molecules and chloride anions. In addition, Hirshfeld surfaces of 1 were calculated to further investigate their intermolecular interactions in the crystal lattice. Our investigation of the magnetic properties of 1, through ac magnetic susceptibility measurements, reveals the occurrence of a slow relaxation in magnetization in this mononuclear GdIII complex, indicating an unusual single-ion magnet (SIM) behavior for this pseudo-isotropic metal ion at very low temperatures. We also studied the relaxometric properties of 1, as a potential contrast agent for high-field magnetic resonance imaging (MRI), from solutions of 1 prepared in physiological serum (0.0-3.2 mM range) and measured at 3 T on a clinical MRI scanner. The values of relaxivity obtained for 1 are larger than those of some commercial MRI contrast agents based on mononuclear GdIII systems.

Europium(III) and gallic acid ligand coordination compounds: synthesis, characterization, photophysical processes, and optimization of luminescent properties

We obtained EuIII, GdIII, and GdIII doped with EuIII (1 at.% and 5 at.%) complexes with the ligand derived from 3,4,5-trihydroxybenzoic acid (H4Gal). Synthetic routes are proposed from acetate and chloride solutions of the trivalent lanthanides. We demonstrated the decomposition of the ligands 3,4,5-trihydroxybenzoate (H3Gal ≡ L–) and 3-hydroxo-4,5-dihydroxybenzoate (H2Gal2– ≡ L’2–) in an acetic solution medium. [LnLL’(H2O)2] complexes are obtained in a metal chloride solution medium. We studied the complexes in the solid state. We described the photophysical processes in the complexes, and they resulted in an energy diagram of the EuIII compounds in which the position of the states related to the LMCT transition are intermediate to the ligand's singlet (S1) and triplet (T1) states. We obtained the position of the triplet state (T1) from the luminescence of the GdIII complex. We performed the calculations of the EuIII complex's electronic structure using the Lumpac software to confirm the propositions obtained through experiments. The proposition is that the excitation in the LMCT state rapidly populates the T1 state, and the L–L’2–→EuIII back energy transfer may occur. One of the possible disadvantages of the complexes is the enormous non-radioactive deactivation due to the oscillators (OH) and (CH). The EuIII characteristic luminescence is of little intensity. We obtained GdIII compounds doped with EuIII at 1.0 and 5.0 at.% to optimize luminescent properties. We obtained the emission and excitation spectra of the complexes at 77 and 17 K, showing ligands' phosphorescence superimposed on the characteristic emissions 5D0 →7FJ (J = 0–4) of the EuIII ion.

Terahertz EPR spectroscopy using a 36-tesla high-homogeneity series-connected hybrid magnet

Electron Paramagnetic Resonance (EPR) is a powerful technique to study materials and biological samples on an atomic scale. High-field EPR in particular enables extracting very small g-anisotropies in organic radicals and half-filled 3d and 4f metal ions such as MnII (3d5) or GdIII (4f7), and resolving EPR signals from unpaired spins with very close g-values, both of which provide high-resolution details of the local atomic environment. Before the recent commissioning of the high-homogeneity Series Connected Hybrid magnet (SCH, superconducting + resistive) at the National High Magnetic Field Laboratory (NHMFL), the highest-field, high-resolution EPR spectrometer available was limited to 25 T using a purely resistive “Keck” magnet at the NHMFL. Herein, we report the first EPR experiments performed using the SCH magnet capable of reaching the field of 36 T, corresponding to an EPR frequency of 1 THz for g = 2. The magnet’s intrinsic homogeneity (25 ppm, that is 0.9 mT at 36 T over 1 cm diameter, 1 cm length cylinder) was previously established by NMR. We characterized the magnet’s temporal stability (5 ppm, which is 0.2 mT at 36 T over one-minute, the typical acquisition time) using 2,2-diphenyl-1-picrylhydrazyl (DPPH). This high resolution enables resolving the weak g-anisotropy of 1,3-bis(diphenylene)-2-phenylallyl (BDPA), Δg = 2.5 × 10–4 obtained from measurements at 932 GHz and 33 T. Subsequently, we recorded EPR spectra at multiple frequencies for two GdIII complexes with potential applications as spin labels. We demonstrated a significant reduction in line broadening in Gd[DTPA], attributed to second order zero field splitting, and a resolution enhancement of g-tensor anisotropy for Gd[sTPATCN]-SL.

Interesting chemical and physical features of the products of the reactions between trivalent lanthanoids and a tetradentate Schiff base derived from cyclohexane-1,2-diamine.

The initial use of a tetradentate Schiff base (LH2) derived from the 2 : 1 condensation between 2-hydroxyacetophenone and cyclohexane-1,2-diamine in 4f-metal chemistry is described. The 1 : 2 reaction of Ln(NO3)3·xH2O (Ln = lanthanoid or yttrium) and LH2 in MeOH/CH2Cl2 has provided access to isostructural complexes [Ln(NO3)3(L'H2)(MeOH)] in moderate to good yields. Surprisingly, the products contain the corresponding Schiff base ligand L'H2 possessing six aliphatic -CH2- groups instead of the -CH-(CH2)4-CH- unit of the cyclohexane ring, i.e. an unusual ring-opening of the latter has occurred. A mechanism for this LnIII-assisted/promoted LH2 → L'H2 transformation has been proposed assuming transient LnII species and a second LH2 molecule as the H2 source for the reduction of the cyclohexane moiety. DFT calculations provide strong evidence for the great thermodynamic stability of the products in comparison with analogous complexes containing the original intact ligand. The structures of the PrIII, SmIII, GdIII, TbIII, and HoIII complexes have been determined by single-crystal X-ray crystallography. The 9-coordinate LnIII centre in the molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the bidentate chelating organic ligand, and one oxygen atom from the coordinated MeOH group. In the overall neutral bis(zwitterionic) L'H2 ligand, the acidic H atoms are clearly located on the imino nitrogen atoms and this results in the formation of an unusual 16-membered chelating ring. The coordination polyhedra defined by the nine donor atoms around the 4f-metal-ion centres can be best described as distorted, spherical capped square antiprisms. The EuIII, TbIII, and DyIII complexes exhibit LnIII-based luminescence in the visible region, with the coordinated L'H2 molecule acting as the antenna. Ac magnetometry experiments show that the DyIII member of the family behaves as an SIM at zero field and under external dc fields of 0.1 and 0.2 T without the enhancement of the peaks' maxima, suggesting that QTM is not the relaxation path. The GdIII complex behaves, rather unexpectedly, as a SIM with two different magnetic relaxation paths occurring at very close temperatures; this behaviour is tentatively attributed to a very small axial zero-field splitting (D ∼ 0.1 cm-1), which cannot be detected by magnetization or susceptibility experiments. The prospects of the present, first results in the lanthanoid(III)-LH2 chemistry are discussed.

Increasing the Modulation Depth of GdIII-Based Pulsed Dipolar EPR Spectroscopy (PDS) with Porphyrin-GdIII Laser-Induced Magnetic Dipole Spectroscopy.

Distance determination with pulsed EPR has become an important technique for the structural investigation of biomacromolecules, with double electron-electron resonance spectroscopy (DEER) as the most important method. GdIII-based spin labels are one of the most frequently used spin labels for DEER owing to their stability against reduction, high magnetic moment, and absence of orientation selection. A disadvantage of GdIII-GdIII DEER is the low modulation depth due to the broad EPR spectrum of GdIII. Here, we introduce laser-induced magnetic dipole spectroscopy (LaserIMD) with a spin pair consisting of GdIII(PymiMTA) and a photoexcited porphyrin as an alternative technique. We show that the excited state of the porphyrin is not disturbed by the presence of the GdIII complex and that herewith modulation depths of almost 40% are possible. This is significantly higher than the value of 7.2% that was achieved with GdIII-GdIII DEER.

Lanthanide Complexes (GdIII and EuIII ) Based on a DOTA-TEMPO Platform for Redox Monitoring via Relaxivity.

Three lanthanide complexes (Ln=Gd, Eu) based on a DO3 A ([Ln(L1)]) or DO2 A ([Ln(L2–3)]+ ) platform appended by a redox active TEMPO‐based arm were prepared. Complex [Ln(L2)]+ shows an alkyne arm, offering the possibility of postfunctionalization by click reaction to yield [Ln(L3)]+ . The complexes demonstrate a redox response whereby the hydroxylamine, nitroxide and oxoammonium forms of the arm can be obtained in turn. Luminescence measurements on the europium complexes support an octadentate (L1 , L3 ) or heptadentate (L2 ) chelation by the ligand, with one water molecule in the inner coordination sphere. The relaxivity was determined from 20 kHz to 30 MHz by fast‐field cycling NMR. The three GdIII complexes under their hydroxylamine form [Gd(L1)] and [Gd(L2–3)]+ show r1 values of 7.0, 5.1 and 5.0 mM−1 s−1 (30 KHz), which increase to 8.8, 5.5 and 6.1 mM−1 s−1 in the nitroxide form. The radical complexes are not toxic against M21 cell lines, at least up to 40 μM. By using EPR spectroscopy we establish that they do not penetrate the cells with the exception of [Eu(L2)]+ .

Ratiometric and Colorimetric Optical Thermometers Using Emissive Dimeric and Trimeric {[Au(SCN)2]−}n Moieties Generated in d–f Heterometallic Assemblies

AbstractGold complexes can generate excimers ([Au2]→[Au2]*) and exciplexes ([Au3]→[Au3]*) with light excitation. Four GdIII and YIII complexes were assembled with dimeric {[Au(SCN)2]−}2 and trimeric {[Au(SCN)2]−}3 bis(thiocyanato)gold(I) counterions. The vibrational signature associated with the Au⋅⋅⋅Au vibrational mode was probed with ultralow frequency (ULF) Raman spectroscopy as a function of temperature. Emission spectroscopy was used to explore photophysical properties. Two broad features in the high‐ and low‐energy regions were associated with the fluorescence and phosphorescence of the gold entities, respectively. Temperature‐dependent luminescence measurements showed that the emission color can be tuned from blue to green via cyan and white. Hence, these complexes can act as colorimetric thermometers. Additionally, a ratiometric thermal sensing ability was incorporated with high sensitivity up to 5 % K−1 in the cryogenic temperature range.

Ratiometric and Colorimetric Optical Thermometers Using Emissive Dimeric and Trimeric {[Au(SCN)2 ]- }n Moieties Generated in d-f Heterometallic Assemblies.

Gold complexes can generate excimers ([Au2 ]→[Au2 ]*) and exciplexes ([Au3 ]→[Au3 ]*) with light excitation. Four GdIII and YIII complexes were assembled with dimeric {[Au(SCN)2 ]- }2 and trimeric {[Au(SCN)2 ]- }3 bis(thiocyanato)gold(I) counterions. The vibrational signature associated with the Au⋅⋅⋅Au vibrational mode was probed with ultralow frequency (ULF) Raman spectroscopy as a function of temperature. Emission spectroscopy was used to explore photophysical properties. Two broad features in the high- and low-energy regions were associated with the fluorescence and phosphorescence of the gold entities, respectively. Temperature-dependent luminescence measurements showed that the emission color can be tuned from blue to green via cyan and white. Hence, these complexes can act as colorimetric thermometers. Additionally, a ratiometric thermal sensing ability was incorporated with high sensitivity up to 5 % K-1 in the cryogenic temperature range.

Front Cover: Tribo‐Excited Chemical Reaction Using an EuIII Complex with a Stacked Anthracene Framework (Chem. Eur. J. 16/2022)

Tribo-excitation in a crystalline compound is an interesting phenomenon in which a mechanical stimulus drives the compound to its excited state. We demonstrate a tribo-excited chemical reaction using trivalent europium (EuIII) and gadolinium (GdIII) complexes with stacked anthracene ligands for the development of tribo-excitation chemistry. The tribo-oxidation and -dimerization reactions in the stacked anthracene ligands occur in the EuIII and GdIII complexes, respectively. More information can be found in the Research Article by Y. Kitagawa, Y. Hasegawa et al. (DOI: 10.1002/chem.202104401).

Underlining the Importance of Peripheral Protic Functional Groups to Enhance the Proton Exchange of Gd-Based MRI Contrast Agents

In this study, we report the synthesis and the equilibrium, kinetic, relaxation, and structural properties of two new GdIII complexes based on modified 10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A) designed to modulate the relaxivity at acidic and basic pH due to intra- and intermolecular proton exchange. The presence of a carboxylic or ester moieties in place of the methyl group of HPDO3A allowed differentiation of a protic and nonprotic functional group, highlighting the importance of the formation of an intramolecular hydrogen bond between the coordinated hydroxyl and the carboxylate groups for proton exchange (kH = 1.5 × 1011 M–1 s–1, kOH = 1.7 × 109 M–1 s–1). The determination of the thermodynamic stability and kinetic inertness of the GdIII complexes confirmed that the modification of peripheral groups does not significantly affect the coordination environment and thus the stability (log KGdL = 19.26, t1/2 = 2.14 × 107 hours, pH = 7.4, 0.15 M NaCl, 25 °C). The relaxivity (r1) was measured as a function of pH to investigate the proton exchange kinetics, and as a function of the magnetic field strength to extrapolate the relaxometric parameters (r1GdL1 = 4.7 mM–1 s–1 and r1GdL2 = 5.1 mM–1 s–1 at 20 MHz, 25 °C, and pH 7.4). Finally, the X-ray crystal structure of the complex crystallized at basic pH showed the formation of a tetranuclear dimer with alkoxide and hydroxide groups bridging the GdIII ions.

Photocytotoxicity of Thiophene- and Bithiophene-Dipicolinato Luminescent Lanthanide Complexes.

New thiophene-dipicolinato-based compounds, K2nTdpa (n = 1, 2), were isolated. Their anions are sensitizers of lanthanide ion (LnIII) luminescence and singlet oxygen generation (1O2). Emission in the visible and near-infrared regions was observed for the LnIII complexes with efficiencies (ϕLn) ϕEu = 33% and ϕYb = 0.31% for 1Tdpa2- and ϕYb = 0.07% for 2Tdpa2-. The latter does not sensitize EuIII emission. Fluorescence imaging of HeLa live cells incubated with K3[Eu(1Tdpa)3] indicates that the complex permeates the cell membrane and localizes in the mitochondria. All complexes generate 1O2 in solution with efficiencies () as high as 13 and 23% for the GdIII complexes of 1Tdpa2- and 2Tdpa2-, respectively. [Ln(nTdpa)3]3- (n = 1, 2) are phototoxic to HeLa cells when irradiated with UV light with IC50 values as low as 4.2 μM for [Gd(2Tdpa)3]3- and 91.8 μM for [Eu(1Tdpa)3]3-. Flow cytometric analyses indicate both apoptotic and necrotic cell death pathways.

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand.

EuIII, TbIII, GdIII and YbIII complexes of the nonadentate bispidine derivative L2 (bispidine=3,7‐diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X‐ray crystallography reveals full encapsulation by the nonadentate ligand L2 that enforces to all LnIII cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C4v symmetry). The well‐resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long‐lived excited states and high emission quantum yields ([EuIIIL2]+, H2O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIIIL2]+, H2O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.

Towards Enhanced MRI Performance of Tumor-Specific Dimeric Phenylboronic Contrast Agents.

It is known that phenylboronic acid (PBA) can target tumor tissues by binding to sialic acid, a substrate overexpressed by cancer cells. This capability has previously been explored in the design of targeting diagnostic probes such as Gd- and 68Ga-DOTA-EN-PBA, two contrast agents for magnetic resonance imaging (MRI) and positron emission tomography (PET), respectively, whose potential has already been demonstrated through in vivo experiments. In addition to its high resolution, the intrinsic low sensitivity of MRI stimulates the search for more effective contrast agents, which, in the case of small-molecular probes, basically narrows down to either increased tumbling time of the entire molecule or elevated local concentration of the paramagnetic ions, both strategies resulting in enhanced relaxivity, and consequently, a higher MRI contrast. The latter strategy can be achieved by the design of multimeric GdIII complexes. Based on the monomeric PBA-containing probes described recently, herein, we report the synthesis and characterization of the dimeric analogues (GdIII-DOTA-EN)2-PBA and (GdIII-DOTA-EN)2F2PBA. The presence of two Gd ions in one molecule clearly contributes to the improved biological performance, as demonstrated by the relaxometric study and cell-binding investigations.

Correlating the axial Zero Field Splitting with the slow magnetic relaxation in GdIII SIMs.

The field-induced out-of-phase magnetic response of a GdIII complex, selected by its good isolation in the network, has been analyzed and the behaviour of this quasi-isotropic cation has been related to its weak axial zero field splitting ∼0.1 cm-1.

Family of Isomeric CuII-LnIII (Ln = Gd, Tb, and Dy) Complexes Presenting Field-Induced Slow Relaxation of Magnetization Only for the Members Containing GdIII.

The strategic design and synthesis of two isomeric CuII complexes, [CuLA] and [CuLB], of asymmetrically dicondensed N2O3-donor Schiff-base ligands (where H2LA and H2LB are N-salicylidene-N'-3-methoxysalicylidenepropane-1,2-diamine and N-3-methoxysalicylidene-N'-salicylidenepropane-1,2-diamine, respectively) have been accomplished via a convenient CuII template method. These two complexes have been used as metalloligands for the synthesis of three pairs of Cu-Ln isomeric complexes [CuL(μ-NO3)Ln(NO3)2(H2O)]·CH3CN (for complexes 1A-3A, L = LA, and for complexes 1B-3B, L = LB and Ln = Gd, Tb, and Dy, respectively), all of which have been characterized structurally. In all six isomorphous and isostructural complexes, the decacoordinated LnIII centers and pentacoordinated CuII centers possess sphenocorona and square-pyramidal geometries, respectively. The isomeric pair of Cu-Gd compounds shows field-induced slow relaxation of magnetization, although they present the typical isotropic behavior of GdIII complexes, indicating that slow relaxation is not due to the usual energy barrier originating from the magnetic anisotropy. The isostructural derivatives with the ion-anisotropic lanthanides TbIII and DyIII do not show slow magnetic relaxation with or without a direct-current bias field, demonstrating that the magnetic response of the isotropic system CuII-GdIII occurs through different mechanisms than the rest of the Ln cations.

A Two-Armed Probe for In-Cell DEER Measurements on Proteins*.

The application of double electron‐electron resonance (DEER) with site‐directed spin labeling (SDSL) to measure distances in proteins and protein complexes in living cells puts rigorous restraints on the spin‐label. The linkage and paramagnetic centers need to resist the reducing conditions of the cell. Rigid attachment of the probe to the protein improves precision of the measured distances. Here, three two‐armed GdIII complexes, GdIII‐CLaNP13a/b/c were synthesized. Rather than the disulfide linkage of most other CLaNP molecules, a thioether linkage was used to avoid reductive dissociation of the linker. The doubly GdIII labeled N55C/V57C/K147C/T151C variants of T4Lysozyme were measured by 95 GHz DEER. The constructs were measured in vitro, in cell lysate and in Dictyostelium discoideum cells. Measured distances were 4.5 nm, consistent with results from paramagnetic NMR. A narrow distance distribution and typical modulation depth, also in cell, indicate complete and durable labeling and probe rigidity due to the dual attachment sites.

Synthesis of a rigidified bicyclic AAZTA-like ligand and relaxometric characterization of its GdIII complex

The synthesis of a novel constrained polydentate ligand CpAAZTA derived from the heptadentate polyaminocarboxylic ligand AAZTA (6-amino-6-methylperhydro-1,4-diazepine-N, N’,N’’,N’’-tetraacetic acid) in which the ethylene bridge of the 1,4-diazepine ring is replaced by a cyclopentyl bridge is reported. In order to compare the effect of the different size and rigidity induced on the chelate by a 5-membered ring vs. a previously-reported cyclohexane-derivative, the relaxivity of the corresponding bis-hydrated GdIII complex was also investigated as a function of the magnetic field, temperature and pH.

A Class of FeIII Macrocyclic Complexes with Alcohol Donor Groups as Effective T1 MRI Contrast Agents

AbstractEarly studies suggested that FeIII complexes cannot compete with GdIII complexes as T1 MRI contrast agents. Now it is shown that one member of a class of high‐spin macrocyclic FeIII complexes produces more intense contrast in mice kidneys and liver at 30 minutes post‐injection than does a commercially used GdIII agent and also produces similar T1 relaxivity in serum phantoms at 4.7 T and 37 °C. Comparison of four different FeIII macrocyclic complexes elucidates the factors that contribute to relaxivity in vivo including solution speciation. Variable‐temperature 17O NMR studies suggest that none of the complexes has a single, integral inner‐sphere water that exchanges rapidly on the NMR timescale. MRI studies in mice show large in vivo differences of three of the FeIII complexes that correspond, in part, to their r1 relaxivity in phantoms. Changes in overall charge of the complex modulate contrast enhancement, especially of the kidneys.

A Class of FeIII Macrocyclic Complexes with Alcohol Donor Groups as Effective T1 MRI Contrast Agents.

Early studies suggested that FeIII complexes cannot compete with GdIII complexes as T1 MRI contrast agents. Now it is shown that one member of a class of high-spin macrocyclic FeIII complexes produces more intense contrast in mice kidneys and liver at 30 minutes post-injection than does a commercially used GdIII agent and also produces similar T1 relaxivity in serum phantoms at 4.7 T and 37 °C. Comparison of four different FeIII macrocyclic complexes elucidates the factors that contribute to relaxivity in vivo including solution speciation. Variable-temperature 17 O NMR studies suggest that none of the complexes has a single, integral inner-sphere water that exchanges rapidly on the NMR timescale. MRI studies in mice show large in vivo differences of three of the FeIII complexes that correspond, in part, to their r1 relaxivity in phantoms. Changes in overall charge of the complex modulate contrast enhancement, especially of the kidneys.