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.


Introduction
Pyridoxine is one of the most common active forms or vitamers of vitamin B 6 , which is an enzymatic co-factor involved in more than one hundred metabolic reactions, including carbohydrate, amino acid, and lipid metabolism in humans [1].This molecule is necessary for normal brain function, given that it actively aids in producing neurotransmitters such as dopamine, serotonin, norepinephrine, and gamma-aminobutyric acid [2][3][4][5].Vitamin B 6 deficiency is associated with depression, convulsive seizures, mild microcytic hypochromic anemia, and calcium oxalate nephropathy [6].
Over more than four decades, pyridoxine has also been investigated as a suitable ligand toward the preparation of metal complexes.Its pyridinic nitrogen atom and alcohols groups have been used to coordinate several metal ions (Scheme 1), such as Cd II [7], Fe III [8], Co III [9], Cu II [9][10][11], Sn IV [12][13][14], and U VI [15][16][17][18][19][20].In most cases, pyridoxine molecule chelates or bridges through its alcohols groups, in the form of alkoxide, phenolate, or just as alcohol functional groups, and only in a few systems the pyridinic nitrogen atom is involved in metal coordination [20][21][22][23].From all this family of pyridoxine-based complexes, only a couple of systems have been investigated regarding their magnetic properties [18].Concerning the Gd III ion, there only exists one Gd III complex based on pyridoxine which has been reported in the literature [24].This compound, of formula [Gd III (pyr) 2 (NO 3 ) 2 (H 2 O)](NO 3 ), binds circulating tumor DNA (ctDNA) with a moderate affinity [24].Nevertheless, no magneto-structural study on a pyridoxine-based Gd III complex has been reported to date.
magnetic properties [18].Concerning the Gd III ion, there only exists one Gd III complex based on pyridoxine which has been reported in the literature [24].This compound, of formula [Gd III (pyr)2(NO3)2(H2O)](NO3), binds circulating tumor DNA (ctDNA) with a moderate affinity [24].Nevertheless, no magneto-structural study on a pyridoxine-based Gd III complex has been reported to date.
When we compare the magnetic properties of Gd III complexes with those of other lanthanide(III) complexes, we note that Gd III complexes have been scarcely studied.The Gd III ion has been investigated many times as a magnetically isotropic ion, given that it exhibits a half-filled 4f 7 configuration with a lack in orbital contribution, that is, it has S = 7/2 and L = 0, respectively [25,26].Thus, there exists a very low quantity of reported homometallic Gd III species displaying slow relaxation of magnetization and single-molecule/ion magnet (SMM/SIM) [26][27][28].
On the other hand, Gd III metal ion is also employed as a contrast agent in magnetic resonance imaging (MRI), to improve lesion detection and characterization, finally increasing the efficacy of diagnostic MR scans, given that Gd III ion promotes changes in the relaxivity of protons from the associated coordination water molecules and generates a signal with clearer physical distinction among the contrast agent and the surrounding tissues.Thus, Gd III -based contrast agents have revolutionized modern technological advances in radiological diagnostics [29].Nevertheless, novel Gd III -based contrast agents are being investigated to improve the response of the current 3 T scanner devices [29,30].
It is well known that the human body uses vitamin B6 in the metabolism.Therefore, it would be very interesting to obtain a complex based on Gd III and vitamin B6, which could act as a process-specific contrast agent.
Herein, we report the synthesis, the crystal structure and the magnetic and relaxometric properties of a novel pyridoxine-based Gd III compound of formula [Gd III (pyr)2(H2O)4]Cl3•2H2O (1) [pyr = pyridoxine].To our knowledge, 1 constitutes the first example of a gadolinium-based single-ion magnet (SIM), whose study on its magnetic resonance imaging properties in a 3 T scanner has been reported.

Description of the Crystal Structure
Compound 1 crystallizes in the triclinic system and space group Pī (Table 1).The crystal structure is made up of cationic mononuclear [Gd III (pyr)2(H2O)4] 3+ complexes, chloride anions, and crystallization water molecules.Indeed, the asymmetric unit of 1 contains a [Gd III (pyr)2(H2O)4] 3+ complex, three chloride anions, and two water molecules.A perspective drawing showing the cationic Gd III complex in 1 is given in Figure 1.In compound 1, the Gd III metal ion is eight-coordinate and bonded to eight oxygen atoms, Scheme 1.Molecular structure of pyridoxine (pyr).
When we compare the magnetic properties of Gd III complexes with those of other lanthanide(III) complexes, we note that Gd III complexes have been scarcely studied.The Gd III ion has been investigated many times as a magnetically isotropic ion, given that it exhibits a half-filled 4f 7 configuration with a lack in orbital contribution, that is, it has S = 7/2 and L = 0, respectively [25,26].Thus, there exists a very low quantity of reported homometallic Gd III species displaying slow relaxation of magnetization and single-molecule/ion magnet (SMM/SIM) [26][27][28].
On the other hand, Gd III metal ion is also employed as a contrast agent in magnetic resonance imaging (MRI), to improve lesion detection and characterization, finally increasing the efficacy of diagnostic MR scans, given that Gd III ion promotes changes in the relaxivity of protons from the associated coordination water molecules and generates a signal with clearer physical distinction among the contrast agent and the surrounding tissues.Thus, Gd III -based contrast agents have revolutionized modern technological advances in radiological diagnostics [29].Nevertheless, novel Gd III -based contrast agents are being investigated to improve the response of the current 3 T scanner devices [29,30].
It is well known that the human body uses vitamin B 6 in the metabolism.Therefore, it would be very interesting to obtain a complex based on Gd III and vitamin B 6 , which could act as a process-specific contrast agent.
Herein, we report the synthesis, the crystal structure and the magnetic and relaxometric properties of a novel pyridoxine-based Gd III compound of formula [Gd III (pyr) 2 (H 2 O) 4 ]Cl 3 • 2H 2 O (1) [pyr = pyridoxine].To our knowledge, 1 constitutes the first example of a gadolinium-based single-ion magnet (SIM), whose study on its magnetic resonance imaging properties in a 3 T scanner has been reported.
In the crystal of 1, the cationic [Gd(pyr) 2 (H 2 O) 4 ] 3+ entities are connected by means of H-bonding interactions generated by coordinated-water molecules and chloride anions, thus generating a 1D motif that grows along the crystallographic a axis [O1w•••Cl2a = 3.133(1) Å and O4w•••Cl2a = 3.147(1) Å; (a) = x + 1, y, z] (Figure 2).Additional H-bonding interactions, which are formed through coordinated-water molecules and alcohol groups of pyridoxine ligands of adjacent Gd III complexes, result in a layered structure growing in the ac plane Finally, the -NH groups of pyridoxine molecules and chloride anions are H-bonded in the third dimension of the crystal structure of  2).We further analyzed the coordination environment and geometry of the Gd III ion in 1.For doing that, the SHAPE program was employed.This program allows us the calculation of different polyhedra and molecular geometries for metal complexes, with the 0.000 value being the perfect match for the ideal or regular polyhedron [32].The resulting metal symmetry can be compared with previously reported complexes and their magnetic properties [33].In 1, the single Gd III ion displays a coordination number (CN) equal to 8. The lowest SHAPE value obtained for 1 was 0.948, which was assigned to the triangular dodecahedron geometry (TDD), the second lowest value being 1.441, which was associated with a square antiprism geometry (SAPR), see Table 3.Thus, these calculations allow us to assign the D 2d symmetry to the Gd III metal ion in compound 1 (Table 3).

Analysis of the Hirshfeld Surfaces
Intermolecular interactions involving the cationic [Gd(pyr) 2 (H 2 O) 4 ] 3+ complex of 1 were further investigated by means of CrystalExplorer program [34].The qualitative and quantitative investigation of the main intermolecular contacts was performed by mapping the distances of the 3D surface generated considering the nearest atom outside (d e ) and inside (d i ) distances of 1 and a normalized contact distance (d norm ), which overcomes some limitations because of different atom sizes [35][36][37].A red-white-blue set of colors is used for assigning shorter contacts (red), contacts around the van der Waals separation (white), and those longer contacts (blue) [34].Moreover, a 2D plot of the involved intermolecular interactions is generated as a fingerprint [34][35][36][37].The Hirshfeld surface and the fingerprint plot for the cationic unit of compound 1 are given in Figures 4 and S1.
The intermolecular H•••Cl contacts involving mainly the coordinated water molecules, and also the -NH group of the pyridine ring of pyridoxine molecules, and chloride anions are the main interactions which are reflected on the fingerprint plot with ca.26% (Figure 4).An important part of these interactions is responsible for generating the 1D motifs and also the final 3D structure formed by the [Gd(pyr) 2 (H 2 O) 4 ] 3+ cations and Cl − anions in 1.Moreover, intermolecular H•••O contacts involving non-coordinated water molecules, coordinated water molecules, and different protonated and deprotonated alcohol groups of pyridoxine ligands connect adjacent Gd III complexes through H-bonding interactions, generating a layered structure.These H•••O contacts are approximately 18% of the complete fingerprint plot (Figure 4).interactions, generating a layered structure.These H•••O contacts are approximately 18% of the complete fingerprint plot (Figure 4).

Magnetic Properties Study
The study of the magnetic properties of compound 1 was carried out through direct current (dc) and alternating current (ac) magnetic susceptibility measurements.During the dc measurements, eicosene was used to immobilize the investigated sample.In Figure 5a, the experimental χMT vs. T plot for 1 is shown, which was obtained through an external magnetic field of 0.5 T and a temperature range of 2-300 K.At T = 300 K, the χMT value is equal to 7.84 cm 3 mol −1 K, which is just about that expected for one Gd III metal ion (4f 7 ion with g = 2.0, S = 7/2 and L = 0) [25][26][27][28].Below 300 K and upon cooling, the χMT value keeps constant with decreasing temperature to ca. 6.0 K. Below that temperature, the χMT value decreases slowly reaching a minimum value of approximately 7.69 cm 3 mol −1 K at 2.0 K.These dc magnetic susceptibility results, which were obtained at very low temperatures, would be interpreted as an effect of intermolecular interactions and/or a very small zero-field splitting (ZFS) taking place in compound 1 [25][26][27][28].We have also measured the field dependence of the molar magnetization (M) for 1 in the 0-7 T range (see inset in Figure 5a).Our experimental values followed a constant in-

Magnetic Properties Study
The study of the magnetic properties of compound 1 was carried out through direct current (dc) and alternating current (ac) magnetic susceptibility measurements.During the dc measurements, eicosene was used to immobilize the investigated sample.In Figure 5a, the experimental χ M T vs. T plot for 1 is shown, which was obtained through an external magnetic field of 0.5 T and a temperature range of 2-300 K.At T = 300 K, the χ M T value is equal to 7.84 cm 3 mol −1 K, which is just about that expected for one Gd III metal ion (4f 7 ion with g = 2.0, S = 7/2 and L = 0) [25][26][27][28].Below 300 K and upon cooling, the χ M T value keeps constant with decreasing temperature to ca. 6.0 K. Below that temperature, the χ M T value decreases slowly reaching a minimum value of approximately 7.69 cm 3 mol −1 K at 2.0 K.These dc magnetic susceptibility results, which were obtained at very low temperatures, would be interpreted as an effect of intermolecular interactions and/or a very small zerofield splitting (ZFS) taking place in compound 1 [25][26][27][28].
interactions, generating a layered structure.These H•••O contacts are approximately 18% of the complete fingerprint plot (Figure 4).

Magnetic Properties Study
The study of the magnetic properties of compound 1 was carried out through direct current (dc) and alternating current (ac) magnetic susceptibility measurements.During the dc measurements, eicosene was used to immobilize the investigated sample.In Figure 5a, the experimental χMT vs. T plot for 1 is shown, which was obtained through an external magnetic field of 0.5 T and a temperature range of 2-300 K.At T = 300 K, the χMT value is equal to 7.84 cm 3 mol −1 K, which is just about that expected for one Gd III metal ion (4f 7 ion with g = 2.0, S = 7/2 and L = 0) [25][26][27][28].Below 300 K and upon cooling, the χMT value keeps constant with decreasing temperature to ca. 6.0 K. Below that temperature, the χMT value decreases slowly reaching a minimum value of approximately 7.69 cm 3 mol −1 K at 2.0 K.These dc magnetic susceptibility results, which were obtained at very low temperatures, would be interpreted as an effect of intermolecular interactions and/or a very small zero-field splitting (ZFS) taking place in compound 1 [25][26][27][28].We have also measured the field dependence of the molar magnetization (M) for 1 in the 0-7 T range (see inset in Figure 5a).Our experimental values followed a constant in- We have also measured the field dependence of the molar magnetization (M) for 1 in the 0-7 T range (see inset in Figure 5a).Our experimental values followed a constant increase in M with the magnetic field at a fixed temperature (2.0 K).These values were well simulated through the Brillouin analytical expression, which was generated with g and S values of 2.0 and 7/2, respectively (Figure 5a).In this M vs. H curve, the largest experimental value for 1 was equal to 7.07 µ B [25,26].
In order to treat the experimental susceptibility points of the χ M T vs. T curve of 1, we employed the theoretical expression which is typically associate with an isotropic ion exhibiting a S value equal to 7/2.Moreover, we added an extra variable with a theta (θ) symbol, which can explain the intermolecular interactions observed in compound 1 [χ M = (Nµ B 2 g 2 /3k B )S(S + 1)/(T − θ)] [30].Finally, through least-squares fits, we obtained the following values for compound 1: g = 2.005(1) and θ = −0.034(2)K (R = 5.3 × 10 −5 ).
In addition, alternating current (ac) magnetic susceptibility measurements were carried out on a sample of 1 in a 5.0 G ac field oscillating at different frequencies (10 2 -10 4 Hz range) in the temperature range of 2.0-7.0K.No slow relaxation of the magnetization was observed for the sample of 1 at H dc = 0 G.However, out-of-phase ac signals (χ ′′ M ) were detected when an external dc magnetic field of 2500 G was applied on 1, indicating a field-induced Single-Ion Magnet (SIM) behavior for this mononuclear Gd III system [25].This singular magnetic behavior of 1 was studied through its χ M ′′ vs. frequency (ν/Hz) plot, which is given in Figure 5b.From the measured data of the relaxation maxima in the χ M ′′ vs. frequency (ν/Hz) plot, the ln(τ) vs. 1/T curve is obtained, which is given in the inset of Figure 5b.The experimental data of the ln(τ) vs. 1/T curve draw up to three different sections, one straight line along the ca.0.01-0.08K −1 range, followed by another one in the ca.0.10-0.30K −1 range of the high-temperature region, and another straight line along the ca.0.40-0.50K −1 range of the low-temperature region.This singular ln(τ) vs. 1/T curve was fully fitted only when we considered two Orbachs plus a quantum tunnelling (QTM) mechanisms for the relaxation of magnetization in The least-squares fit of these data led to the set of parameters: U eff (1) = 63.9(1)cm −1 , τ o (1) = 7.9(1) × 10 −7 s, U eff (2) = 12.0(1) cm −1 , τ o (2) = 6.9(2) × 10 −6 s and τ −1 QTM = 1069(5) s −1 for 1.The relaxation dynamics that 1 exhibits as single-ion magnet (SIM) is somewhat different to those of other previously reported Gd III systems, given that no Raman mechanism was extracted from the experimental ac data of 1.Nevertheless, the reported effective energy barrier (U eff ) values obtained for 1 should be carefully considered as they might not correspond a priori to any excited states of the Gd III ion and, hence, they would not be real U eff values [26,28,33].In any case, compound 1 is one of the few SIMs based on Gd III ion reported up to date, hence, further detailed experimental and theoretical investigations carried out on this quasi-isotropic 4f metal ion will be necessary to correctly understand the relaxation dynamics of the uncommon Gd III -based SIMs.

MR Imaging Phantom Studies
We have also studied the relaxometric properties of 1, as a potential contrast agent for high-field MRI [38][39][40][41][42][43][44][45].A series of 13 samples of 1 (the concentrations ranging from 0.0 to 3.2 mM) were prepared in physiological serum and were measured on a clinical MR scanner (Achieva 3T TX, Philips Healthcare, Best, The Netherlands).These measurements were performed by placing the 13 samples (with range of pH values: 7.0-7.4) in a volumetric head eight channels SENSE coil and were monitored over a period of three weeks [30].
The methodology was based on measuring the relaxation rate (R expressed in s −1 ), which was obtained for each concentration by means of the computation of the corresponding relaxation time T of the studied phantoms (Figure 6).
In the case of r 1 , it was obtained by calculating the T1 time from sequences with 2, 5, 10, 15, 25, and 45 flip angles, whereas r 2 and r 2 * values were obtained after computing T2 and T2* relaxation times, which came from sequences with echo times indicated in Table S2.

X-ray Data Collection
Our X-ray diffraction data collection was performed on a selected crystal of 1 with a size of 0.14 × 0.10 × 0.05 mm 3 .This crystal was measured on a Bruker D8 Venture diffractometer with PHOTON II detector at 120 K.The radiation employed was that of Mo-K α with λ = 0.71073 Å.In Table 1, the results of refinement along with the crystal parameters for 1 are tabulated.The crystal structure of 1 was solved through of direct methods and the SHELXTL program [46].The graphical manipulations were performed by means of the DIAMOND program [47].The H atoms of the pyridoxine ligands were located in computed positions and refined isotropically through the riding model.The H atoms were found on all the reported water molecules and were fixed through DFIX.2311814 is the CCDC number for the crystal structure of 1.

Technical Equipment, Devices and Physical Measurements
An elemental analyzer CE Instruments CHNS1100 was employed to carry out elemental analyses on samples of compound 1.A Hitachi S-4800 field emission scanning electron microscope generated the results of scanning electron microscopy (SEM-EDX).A SCIEX TripleTOF 6600+ mass spectrometer was used to obtain the Electrospray Ionization Mass Spectrometry (ESI-MS) spectrum of 1.All these technical measurements were carried out in the Central Service for the Support to Experimental Research (SCSIE) at the University of Valencia (UV).Next, Infrared (IR) and UV-vis spectra of 1 were recorded with a PerkinElmer Spectrum 65 FT-IR and a V-670 (Jasco Deutschland GmbH, Pfungstadt, Germany) spectrometers in the 400-4000 cm −1 and 200-800 nm regions, respectively.In addition, two magnetometers, namely, a Quantum Design MPMS-XL SQUID and a Physical Property Measurement System (PPMS), were used by us to perform magnetic susceptibility measurements, in both dc and ac, at the Institute of Molecular Science (ICMol-UV).During these magnetic measurements, we used eicosene to immobilize the investigated sample.Finally, the diamagnetic contribution of the involved atoms of compound 1 was corrected through Pascal's constants method [48].

Conclusions
In summary, the preparation, crystallographic studies, and magnetic and relaxometric properties of a novel mononuclear Gd III  This compound crystallizes in the triclinic space group Pī and its crystal packing exhibits a network of H-bonding interactions involving cationic units connected through non-coordinating water molecules and chloride anions.These intermolecular interactions were further investigated by means of its Hirshfeld surfaces.In addition, selected crystal structural data were used to be computed by means of the SHAPE program, whose results account for a triangular dodecahedron geometry (TDD) and a D 2d symmetry assigned to the Gd III metal ion in compound 1.
The study of the magnetic properties of 1, through both dc and ac magnetic susceptibility measurements, reveals a behavior typical of a quasi-isotropic metal ion displaying field-induced slow relaxation of magnetization and single-ion magnet (SIM) phenomenon.This magneto-structural study carried out on compound 1 is the first one performed on a lanthanide-based complex obtained with pyridoxine molecule, this fact indicating that the preparation and study of the magnetic properties of pyridoxine complexes with other more anisotropic lanthanide(III) ions, such as Tb III , Dy III , and Ho III , could generate an interesting family of SIMs based on this versatile biomolecule.This investigation is underway in our research group.
Finally, the relaxivity properties of 1 were investigated through a preliminary study carried out by means of magnetic resonance (MR) images of the tube phantoms containing different concentrations of complex 1 prepared in physiological serum.These images were collected on a 3T clinical MRI scanner.Our results indicate that complex 1 exhibits high relaxivity values in comparison with some currently used commercial contrast agents.Hence, 1 can be considered as a potential contrast agent for high-field MR imaging and a suitable candidate for further developments and MR imaging studies on this biomedical research field.

Figure 2 .
Figure 2. View of the one-dimensional motif obtained by H-bonding interactions between chl anions and water molecules of [Gd(pyr)2(H2O)4] 3+ complexes in 1. Selected H atoms non-coordinated water molecules are omitted for clarity.Color code: purple, Gd; green, Cl; re blue, N; grey, C; white, H.

Figure 3 .
Figure 3.View of a fragment of the supramolecular 2D network generated by H-bonded O•••O (dashed blue lines) and Cl•••O (dashed green lines) interactions, which grow along the crystallographic ac plane in 1. Selected H atoms and non-coordinated water molecules are omitted for clarity.

Figure 4 .
Figure 4. Hirshfeld surface mapped through dnorm function for the mononuclear Gd III complex obtained with pyridoxine molecule in 1 (left); Full fingerprint plot for compound 1 (right).

Figure 5 .
Figure 5. (a) χMT product versus T plot for compound 1, the solid red line representing the theoretical fit of the experimental magnetic susceptibility data.The inset shows the field dependence of the molar magnetization at 2.0 K (the solid red line representing the theoretical fit through the Brillouin curve generated with g = 2.0 and S = 7/2); (b) Frequency dependence of the out-of-phase ac susceptibility signals under a dc field of 2500 G for 1.The inset displays the ln(τ) vs. 1/T plot with the fit considering the contribution of two Orbach + quantum tunnelling of magnetization (QTM) mechanisms.

Figure 4 .
Figure 4. Hirshfeld surface mapped through d norm function for the mononuclear Gd III complex obtained with pyridoxine molecule in 1 (left); Full fingerprint plot for compound 1 (right).

Figure 4 .
Figure 4. Hirshfeld surface mapped through dnorm function for the mononuclear Gd III complex obtained with pyridoxine molecule in 1 (left); Full fingerprint plot for compound 1 (right).

Figure 5 .
Figure 5. (a) χMT product versus T plot for compound 1, the solid red line representing the theoretical fit of the experimental magnetic susceptibility data.The inset shows the field dependence of the molar magnetization at 2.0 K (the solid red line representing the theoretical fit through the Brillouin curve generated with g = 2.0 and S = 7/2); (b) Frequency dependence of the out-of-phase ac susceptibility signals under a dc field of 2500 G for 1.The inset displays the ln(τ) vs. 1/T plot with the fit considering the contribution of two Orbach + quantum tunnelling of magnetization (QTM) mechanisms.

Figure 5 .
Figure 5. (a) χ M T product versus T plot for compound 1, the solid red line representing the theoretical fit of the experimental magnetic susceptibility data.The inset shows the field dependence of the molar magnetization at 2.0 K (the solid red line representing the theoretical fit through the Brillouin curve generated with g = 2.0 and S = 7/2); (b) Frequency dependence of the out-of-phase ac susceptibility signals under a dc field of 2500 G for 1.The inset displays the ln(τ) vs. 1/T plot with the fit considering the contribution of two Orbach + quantum tunnelling of magnetization (QTM) mechanisms.

Figure 6 .
Figure 6.(a) MR imaging scanner (Philips Achieva 3T); (b) MR images of the tube phantom taining concentrations of 1 covering the range of 0.0-3.2mM in physiological serum, the corresponds to the first echo of a cross section of the multi echo gradient echo sequence wi Angle: 10°; Repetition Time: 100 ms; Echo Time: 0.9s and voxel size: 1.8 × 1.8 × 5.0 mm, more can be found at Table S2 (MERGEp); (c) Result of the automatic segmentation and labeling p of each tube on a cross section (each color corresponds to a label) of 1; (d) Coding scheme different concentrations, the number of each concentration identifies the tube.

Figure 6 .
Figure 6.(a) MR imaging scanner (Philips Achieva 3T); (b) MR images of the tube phantoms containing concentrations of 1 covering the range of 0.0-3.2mM in physiological serum, the image corresponds to the first echo of a cross section of the multi echo gradient echo sequence with Flip Angle: 10 • ; Repetition Time: 100 ms; Echo Time: 0.9s and voxel size: 1.8 × 1.8 × 5.0 mm, more details can be found at Table S2 (MERGEp); (c) Result of the automatic segmentation and labeling process of each tube on a cross section (each color corresponds to a label) of 1; (d) Coding scheme of the different concentrations, the number of each concentration identifies the tube.

Table 1 .
Summary of the crystal data and structure refinement parameters for compound 1.

Table 1 .
Summary of the crystal data and structure refinement parameters for compound 1.

Table 2 .
Selected hydrogen-bonding interactions in 1 a .

Table 3 .
Selected values obtained through the SHAPE program for possible geometries with coordination number (CN) equal to 8 and from the bond lengths of compound 1 a .