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. Gd^{3+ 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.}

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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:

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:

2 (NH₄)₂[GdCl₅] → {NH₄[Gd₂Cl₇] + 3NH₄Cl}

{NH₄[Gd₂Cl₇] + 3 NH₄Cl} → 2 GdCl₃ + 4 NH₄Cl

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.

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.

Structure

GdCl₃ is monoclinic with a hexagonal UCl₃ structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu (in contrast GdCl₃ crystallizes in the PuBr₃ motif and 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.
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. "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 EDTA^4-, 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. 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 DTPA^5- 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

• Aime, S. (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: 2068-2071.
  
• Corbett, John D. (1983). "Trichlorides of the Rare Earth Elements, Yttrium, and Scandium". Inorganic Syntheses 22: 39-42.
  
• Meyer, G. (1989). "The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃". Inorganic Syntheses 25: 146-150.
  
• Raduchel, B.; Weinmann, H. & Muhler, A. (1996). "Gadolinium Chelates: Chemistry, Safety, & Behavior". Encyclopedia of Nuclear Magnetic Resonance 4: 2166-2172.
  
• Quill, L. L. (1967). "Preparation of Lanthanide Chloride Methanolates Using 2,2-Dimethoxypropane". Inorganic Chemistry 7: 1433-1435.
• Patnaik, P. (2003). Handbook of Inorganic Chemicals. New York: McGraw Hill. 
  
• Wells, A.F. (1984). Structural Inorganic Chemistry. Oxford: Clarendon Press. 
  
• Gadolinium. Magnetic Resonance TIP-MRI Database. Retrieved on February 22, 2006.
• Gadolinium. Webelements. Retrieved on February 22, 2006.