Gadolinium(III) chloride

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:

{{cite book

| last =Meyer

| first =G.

| title =The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃

| volume =25

| year =1989

| pages =146–150

| doi =10.1002/9780470132562.ch35

| isbn =978-0-470-13256-2| series =Inorganic Syntheses

}}

::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.{{cite book

| last =Corbett

| first =John D.

| title =Inorganic Syntheses

| chapter =Trichlorides of the Rare Earth Elements, Yttrium, and Scandium

| series =Inorganic Syntheses

| volume =22

| year =1983

| pages =39–42

| doi =10.1002/9780470132531.ch8

| isbn =978-0-470-13253-1

}}

::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.{{cite journal

| last1 =Quill

| first1 =L. L.

| title =Preparation of Lanthanide Chloride Methanolates Using 2,2-Dimethoxypropane

| journal =Inorganic Chemistry

| volume =6

| year =1967

| pages =1433–1435

| doi =10.1021/ic50053a032

| last2 =Clink

| first2 =George L.

| issue =7

}}

GdCl₃ crystallizes with a hexagonal UCl₃ structure, as seen for other 4f trichlorides including those of La, Ce, Pr, Nd, Pm, Sm, Eu.{{cite book

| first = A.F.

| last = Wells

| year = 1984

| title = Structural Inorganic Chemistry

| publisher = Clarendon Press

| location = Oxford

}} 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.

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.{{cite journal

| last =Raduchel

| first =B. |author2=Weinmann, H. |author3=Muhler, A.

| title =Gadolinium Chelates: Chemistry, Safety, & Behavior

| journal =Encyclopedia of Nuclear Magnetic Resonance

| volume =4

| year =1996

| pages =2166–2172

}}

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.{{cite journal|last1=Spencer|first1=A. J.|last2=Wilson|first2=S. A.|last3=Batchelor|first3=J.|last4=Reid|first4=A.|last5=Pees|first5=J.|last6=Harpur|first6=E.|title=Gadolinium Chloride Toxicity in the Rat|journal=Toxicologic Pathology|volume=25|issue=3|year=1997|pages=245–255|issn=0192-6233|doi=10.1177/019262339702500301|pmid=9210255|s2cid=19838648}} "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.{{cite journal

| last1 =Aime

| first1 =S.

| title =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

| journal =Inorganic Chemistry

| volume =32

| year =1993

| pages =2068–2071

| doi =10.1021/ic00062a031

| last2 =Botta

| first2 =Mauro

| last3 =Dastru

| first3 =Walter

| last4 =Fasano

| first4 =Mauro

| last5 =Panero

| first5 =Maurizio

| last6 =Arnelli

| first6 =Aldo

| issue =10

}}

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:

File:Gd DTPA rxn (2).PNG

• {{cite web | title=Gadolinium| work=Magnetic Resonance TIP-MRI Database| url=http://www.mr-tip.com/serv1.php?type=db1&dbs=Gadolinium| access-date=February 22, 2006}}
• {{cite web | title=Gadolinium| work=Webelements| url=http://www.webelements.com/webelements/compounds/text/Gd/Cl3Gd1-10138520.html| access-date=February 22, 2006 }}

{{Gadolinium compounds}}

{{Chlorides}}

{{Lanthanide halides}}

{{DEFAULTSORT:Gadolinium(Iii) Chloride}}

Category:Gadolinium compounds

Category:Chlorides

Category:Lanthanide halides

Category:Coordination complexes