Gadolinium(III) chloride

Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl₃. It is a colorless, hygroscopic, water-soluble solid. The hexahydrate GdCl₃∙6H₂O is commonly encountered and is sometimes also called gadolinium trichloride. Gd³⁺ species are of special interest because the ion has the maximum number of unpaired spins possible, at least for known elements. With seven valence electrons and seven available f-orbitals, all seven electrons are unpaired and symmetrically arranged around the metal. The high magnetism and high symmetry combine to make Gd³⁺ a useful component in NMR spectroscopy and MRI.

Gadolinium(III) chloride

Names
IUPAC name Gadolinium(III) chloride
Other names Gadolinium trichloride Gadolinium chloride
Identifiers
CAS Number	10138-52-0 Y 19423-81-5 (hydrate) N 13450-84-5 (hexahydrate) Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:37288 Y
ChEMBL	ChEMBL1697696 N
ChemSpider	55406 Y
ECHA InfoCard	100.030.338
PubChem CID	61486
UNII	P7082WY76D Y
CompTox Dashboard (EPA)	DTXSID2044761
InChI InChI=1S/3ClH.Gd/h3*1H;/q;;;+3/p-3 Y Key: MEANOSLIBWSCIT-UHFFFAOYSA-K Y InChI=1/3ClH.Gd/h3*1H;/q;;;+3/p-3 Key: MEANOSLIBWSCIT-DFZHHIFOAP
SMILES Cl[Gd](Cl)Cl
Properties
Chemical formula	GdCl3
Molar mass	263.61 g/mol
Appearance	white crystals hygroscopic
Density	4.52 g/cm3
Melting point	609 °C (1,128 °F; 882 K)
Boiling point	1,580 °C (2,880 °F; 1,850 K)
Solubility in water	94.65 g/100mL, 25°C[1]
Magnetic susceptibility (χ)	+27,930·10−6 cm3/mol
Structure
Crystal structure	hexagonal, hP8
Space group	P63/m, No. 176
Related compounds
Other anions	Gadolinium(III) fluoride Gadolinium(III) bromide Gadolinium(III) oxide
Other cations	Europium(III) chloride Terbium(III) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Preparation

GdCl₃ is usually prepared by the "ammonium chloride" route, which involves the initial synthesis of (NH₄)₂[GdCl₅]. This material can be prepared from the common starting materials at reaction temperatures of 230 °C from gadolinium oxide:^[2]

10 NH₄Cl + Gd₂O₃ → 2 (NH₄)₂[GdCl₅] + 6 NH₃ + 3 H₂O

from hydrated gadolinium chloride:

4 NH₄Cl + 2 GdCl₃∙6H₂O → 2 (NH₄)₂[GdCl₅] + 12 H₂O

from gadolinium metal:

10 NH₄Cl + 2 Gd → 2 (NH₄)₂[GdCl₅] + 6 NH₃ + 3 H₂

In the second step the pentachloride is decomposed at 300 °C:

(NH₄)₂[GdCl₅] → GdCl₃ + 2 NH₄Cl

This pyrolysis reaction proceeds via the intermediacy of NH₄[Gd₂Cl₇].

The ammonium chloride route is more popular and less expensive than other methods. GdCl₃ can, however, also be synthesized by the reaction of solid Gd at 600 °C in a flowing stream of HCl.^[3]

Gd + 3 HCl → GdCl₃ + 3/2 H₂

Gadolinium(III) chloride also forms a hexahydrate, GdCl₃∙6H₂O. The hexahydrate is prepared by gadolinium(III) oxide (or chloride) in concentrated HCl followed by evaporation.^[4]

Structure

GdCl₃ crystallizes with a hexagonal UCl₃ structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu.^[5] The following crystallize in theYCl₃ motif: DyCl₃, HoCl₃, ErCl₃, TmCl₃, YdCl₃, LuCl₃, YCl₃). The UCl₃ motif features 9-coordinate metal with a tricapped trigonal prismatic coordination sphere. In the hexahydrate of gadolinium(III) chloride and other smaller 4f trichlorides and tribromides, six H₂O molecules and 2 Cl⁻ ions coordinate to the cations resulting in a coordination group of 8.

Properties, with applications to MRI

Gadolinium salts are of primary interest for relaxation agents in magnetic resonance imaging (MRI). This technique exploits the fact that Gd³⁺ has an electronic configuration of f⁷. Seven is the largest number of unpaired electron spins possible for an atom, so Gd³⁺ is a key component in the design of highly paramagnetic complexes.^[6] To generate the relaxation agents, Gd³⁺ sources such as GdCl₃∙6H₂O are converted to coordination complexes. GdCl₃∙6H₂O can not be used as an MRI contrasting agent due to its low solubility in water at the body's near neutral pH.^[7] "Free" gadolinium(III), e.g. [GdCl₂(H₂O)₆]⁺, is toxic, so chelating agents are essential for biomedical applications. Simple monodentate or even bidentate ligands will not suffice because they do not remain bound to Gd³⁺ in solution. Ligands with higher coordination numbers therefore are required. The obvious candidate is EDTA⁴⁻, ethylenediaminetetraacetate, which is a commonly employed hexadentate ligand used to complex to transition metals. In lanthanides, however, exhibit coordination numbers greater than six, so still larger aminocarboxylates are employed.

One representative chelating agent is H₅DTPA, diethylenetriaminepentaacetic acid.^[8] Chelation to the conjugate base of this ligand increases the solubility of the Gd³⁺ at the body's neutral pH and still allows for the paramagnetic effect required for an MRI contrast agent. The DTPA⁵⁻ ligand binds to Gd through five oxygen atoms of the carboxylates and three nitrogen atoms of the amines. A 9th binding site remains, which is occupied by a water molecule. The rapid exchange of this water ligand with bulk water is a major reason for the signal enhancing properties of the chelate. The structure of [Gd(DTPA)(H₂O)]²⁻ is a distorted tricapped trigonal prism.

The following is the reaction for the formation of Gd-DTPA:

References

1. Saeger, Victor William; Spedding, F. H. (November 1960). Some physical properties of rare-earth chlorides in aqueous solution. Ames Laboratory Technical Reports 46. p. 38. Retrieved 19 October 2020.
2. Meyer, G. (1989). The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃. Inorganic Syntheses. Vol. 25. pp. 146–150. doi:10.1002/9780470132562.ch35. ISBN 978-0-470-13256-2.
3. Corbett, John D. (1983). "Trichlorides of the Rare Earth Elements, Yttrium, and Scandium". Inorganic Syntheses. Inorganic Syntheses. Vol. 22. pp. 39–42. doi:10.1002/9780470132531.ch8. ISBN 978-0-470-13253-1.
4. Quill, L. L.; Clink, George L. (1967). "Preparation of Lanthanide Chloride Methanolates Using 2,2-Dimethoxypropane". Inorganic Chemistry. 6 (7): 1433–1435. doi:10.1021/ic50053a032.
5. Wells, A.F. (1984). Structural Inorganic Chemistry. Oxford: Clarendon Press.
6. Raduchel, B.; Weinmann, H.; Muhler, A. (1996). "Gadolinium Chelates: Chemistry, Safety, & Behavior". Encyclopedia of Nuclear Magnetic Resonance. 4: 2166–2172.
7. Spencer, A. J.; Wilson, S. A.; Batchelor, J.; Reid, A.; Pees, J.; Harpur, E. (1997). "Gadolinium Chloride Toxicity in the Rat". Toxicologic Pathology. 25 (3): 245–255. doi:10.1177/019262339702500301. ISSN 0192-6233. PMID 9210255. S2CID 19838648.
8. Aime, S.; Botta, Mauro; Dastru, Walter; Fasano, Mauro; Panero, Maurizio; Arnelli, Aldo (1993). "Synthesis and Characterization of a Novel DPTA-like Gadolinium(III) Complex: A Potential Reagent for the Determination of Glycated Proteins by Water Proton NMR Relaxation Measurements". Inorganic Chemistry. 32 (10): 2068–2071. doi:10.1021/ic00062a031.

• "Gadolinium". Magnetic Resonance TIP-MRI Database. Retrieved February 22, 2006.
• "Gadolinium". Webelements. Retrieved February 22, 2006.