Lanthanide Aminopolycarboxylates - American Chemical Society

Chapter 29

Lanthanide Aminopolycarboxylates Gregory R. Choppin and Pamela J. Wong

Downloaded by MONASH UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch029

Department of Chemistry, Florida State University, Tallahassee, F L 32306-3006

Complexation by aminopolycarboxylate ligands has been a major area of research in lanthanide chemistry for almostfivedecades. From the 1950's, when the use of EDTA and HEDTA in ion exchange separations first provided multigram amounts of lanthanides of high purity to GdDTPA, which is currently utilized as an MRI agent, aminopolycarboxylates and f-elements have had a close association. Datafromthermodynamics, kinetics, NMR, and luminescence are discussed to reflect the present understanding of the role of the carboxylate and nitrogen donors, of the number and size of chelate rings, and of hydration in these complexes. Results of recent studies of complexes of bis(amide) derivatives of DTPA are also discussed to illustrate further the significant factors in lanthanide -aminopolycarboxylate complexation. The lanthanide family of elements has played an important role which can be expected to continue in the development of coordination chemistry. In the early history of these elements, their close chemical similarity in the stable tripositive oxidation state made the task of achieving high purity for individual elements very difficult. Although the entire lanthanide series had been discovered by 1907 (with the exception of Pm) and mixtures of lanthanides had been found in more than a hundred minerals, it was not until efficient separation methods were developed that detailed and diverse studies of their coordination chemistry could be undertaken. The coordination chemistry of the lanthanide elements has interesting aspects because of the relatively high charge density of the cations, the strongly electrostatic nature of their bonding, and the variety of coordination numbers attained in different complexes. The regular, relatively small decrease in ionic radii across the family from La(in) through Lu(DI) (the lanthanide contraction) results in relatively small differences in chemical properties between the elements in the same oxidation state. Although similar radial contractions are observed for other rows of metals in the periodic table (e.g., 3d transition elements), the lanthanide contraction is much smaller. The effective ionic radius (7) of 57La +(4f0) is 1.216 À (CN = 9) and that of 7iLu (4fl4), 0.977 Â (CN = 8), respectively. While the ionic radii of C a (r = 1.00 A , C N = 6) and Na+ (1.02 À, C N = 6) fall in the range of tripositive lanthanide radii and also exhibit strongly ionic bonding, the lanthanide cations have higher charge 3

3+

2 +

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29.

CHOPPIN & WONG

Lanthanide

Aminopolycarboxylates

347

densities and, correspondingly, form stronger complexes. Their paramagnetic and optical properties enable lanthanides to serve as effective probes for metal ion binding effects of biological systems. Electrostatic bonding allows for a wide variety of coordination numbers (CN = 6-12) for lanthanide complexes. Steric, electrostatic, and solvation effects are the dominating criteria in determining the geometry of lanthanide complexes. The primary sphere hydration is not constant across the series; hydration numbers are 9.09.3 for the larger, lighter lanthanides and 7.5-8.0 for the heavier, smaller lanthanides (2). Lanthanides behave as hard Lewis acids and bind most strongly to hard bases such as oxygen and fluorine, thus explaining their high affinity for water. Successful complexing agents frequendy involve multiple lanthanide-oxygen bonds. Aminopolycarboxylate ligands (Figure 1) contain two or more carboxylate groups and at least one bonding secondary or tertiary amine. Although nitrogen is a softer base than oxygen, lanthanide-nitrogen bonds are important factors in the stability of these complexes. Because of their high coordination numbers, lanthanides commonly have water molecules remaining in their primary coordination sphere even when the ligand is polyfunctional. Before the evolution of an efficient, economical means of separation, lanthanides were primarily utilized in the form of Mischmetall, an alloy of thorium and lighter rare earths, for the production of incandescent gauzes and mandes for gas lamps (3,4). The advent of aminopolycarboxylates in lanthanide separation methods provided the impetus for a true lanthanide industry, and lanthanides are currentiy being utilized in such diverse fields as biochemistry (e.g., N M R shift reagents), nuclear medicine (e.g., MRI contrast agents), optical sensing (e.g., television tubes), industrial catalysis, organic synthesis, and high temperature superconductivity.

Early Studies Initial efforts in lanthanide separations employed successive fractional crystallization (5). This process was both exhaustive and inefficient; as many as 10,000 recrystallizations could be required to produce milligram amounts. As part of the Manhattan Project during World War II, researchers at the Ames Laboratory and the Oak Ridge National Laboratory discovered the efficacy of ion exchange resins in lanthanide separation. Several lanthanides appeared as uranium fission products, and a method of separating these elements was needed for the proper measurement of their yields in the fission process. A research group headed by Spedding at the Ames Laboratory discovered that cation exchange resin, with a 0.1% citrate solution as the eluant, separated spectroscopically pure yttrium (6a) and neodymium from cenum(6b) and praseodymium in gram quantities. At the same time, Harris et al. (7) at Oak Ridge separated tracer quantities of lanthanum, cerium, praseodymium, and neodymium at tracer levels from cation exchange resin by elution with citrate solution. Significant contributions by Ketelle and Boyd improved the understanding of the parameters controlling lanthanide separation with these ion exchange resins (8). While ion exchange had been studied previously in the separation of lanthanides, elution with noncomplexing solutions achieved no greater separation than fractional crystallization. When a complexing agent was used as the eluant solution, the relative stabilities of the lanthanide complexes greatly enhanced the separations. Just prior to these discoveries, a series of patents had been issued in Zurich (1937-1940) concerning a class of compounds which contained at least two carboxylate groups bonded to an amine nitrogen. These compounds exhibited extraordinary complexing ability with alkaline earth metals in aqueous solution (9). In 1942 Brintzinger reported complexation between ethylendiaminetetraacetic acid (EDTA) and lanthanum, neodymium, and thorium. Studies by Pfeiffer (10,11) and

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348

COORDINATION CHEMISTRY

Brintzinger (12,13) indicated strong metal-ligand bonding interactions through the amine nitrogen. Schwarzenbach began investigating the chelating abilities of N T A and EDA (9). In 1953 he and Wheelwright observed greater stability and improved lanthanide separation using cation exchange resin with aminocarboxylate elution compared to citrate elution (74). The researchers initially investigated the strength of hydrazinotyiV-diacetic acid and E D T A as complexing agents and found the latter much superior. In 1951 Fitch and Russell had reported the use of N T A and IDA elution from cation exchange resin for macroseparation of lanthanides on the gram scale(75). Upon discovery of E D T A as a particularly effective lanthanide complexing agent, many researchers sought to understand and optimize the ion exchange separations using this ligand (76-79). A variety of other related aminopolycarboxylate ligands such as N T A (75), H E D T A (20,21), and DTPA (22-24) were also explored for their potential in lanthanide separations. In contrast to the polycitrate complexes which formed, E D T A was shown to form a 1:1 complex with the lanthanide cations during elution (25). Unfortunately, the low solubility of the complexed species (e.g., HLnEDTA) during ion exchange posed a problem(26). Wheelwright, Spedding, and Schwarzenbach (14,20,27) solved this problem by using an ion exchange resin in the C u form rather than the H form . The competitive copper aminocarboxylate complex was more soluble than H4EDTA. The enhanced solubilities of N T A (75), H E D T A (20,27) and DTPA (2224) made these attractive ligands for further study. To improve the separations with aminocarboxylates, researchers turned their efforts toward understanding the nature and characteristics of aminopolycarboxylatelanthanide complexation. The ensuing wealth of information resolved fundamental questions about the structure of the complexes, ligand effects, bonding characteristics, and hydration. 2 +

+

Chemical behavior of Lanthanide Aminopolycarboxylates Wheelwright, Spedding, and Schwarzenbach conducted an intensive investigation on the formation of the lanthanide complexes (14). In 1953 they observed that LnEDTA complexes exhibited increasing stability with decreasing ionic radii, and they noted that the curve for the stability constants of these 1:1 species as a function of lanthanide atomic number was discontinuous, with a break appearing at Gd(m) (Figure 2). From potentiometric and polargraphic evidence, they postulated hexacoordination for the Ln(EDTA) complex through the two nitrogen atoms and four carboxylate oxygen atoms and, consequently, a coordination number of 6 for the lanthanide cation. To explain the break in the log β vs. Ζ curve, they postulated that the smaller ionic radii of the heavier lanthanides and the steric hindrance of four carboxylate groups induced a change from hexadentate to pentadentate coordination. The assumption of a maximum coordination number of six for the lanthanides would continue through the 1960s, reflecting the mistaken assumption that lanthanide behavior resembled the more extensively studied 3d transition elements. Moeller was the first to examine these complexes with IR spectroscopy in 1950 (25). In 1955 he used results from X-ray diffraction studies to propose pentadentate complexation for the 1:1 species of all the lanthanide cations with E D T A via coordination through the two amines and three of the four carboxylate groups (28). The presence of an uncomplexed carboxylate group suggested the incorporation of water in the primary coordination sphere, thus achieving the assumed total coordination number of 6. The assumption of uncomplexed carboxylate was supported by infrared data showing absorptions of the C=0 stretch around 1600 cm" (complexed -C02") and 1700 c m ' (uncomplexed -C02'). The relative intensities of these bands agreed with the hypothesis of three bonded and one free carboxylate groups. 1

1

In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

29.

(a) R, - H, R _ « C H C 0 H Iminodiacetic acid, IDA (b) R, = C H , R = R = C H C 0 H Methylaminodiacetic acid, MIDA (c) R,_ = C H C 0 H Nitrilotriacetic acid, NTA

,R R,-N^ 3

2

2

N R

3

3

R,

v

y

R

X

R \N_F-Nfl/w-l,2,diaminocyclohexanetetraacetic acid, DCTA (e) R,_ = C H C 0 H , F = C H 1.3- Trimethylenediaminetetraacetic acid, TMDTA (0 R,_4 = C H C 0 H , F = C H 1.4-Tetramethylenediaminetetraacctic acid, TMEDTA (g) R = C H C 0 H , R = C H C 0 H , F - C H Ethylenediamine-yV,^'dipropionic -^,^'diacetic acid, EDPDA

3

3

2

2

4

2

5

2

2

4

2

2

2

4

2

2

2

4

4

2

2

6

1 0

4

2

2

3

6

2

2

4

8

u

2

2

2 4

2

4

2

4

4

2

2

4

2

R

III

iv Az ^N-F-N-F-N^ R

(a) R , . , - C H C 0 H , F = C H Diethylenetriaminepentaacctic acid, DTPA i.5.3 = C H C 0 H , R . - C H C 0 N H C H Dicthylenetriaminepcntaacetic acid bis (methylamide), DTPA-BMA 2

4

( b )

2

2

5

2

4

R

2

2

4

2

3

Figure 1. Aminopolycarboxylate ligands.

20

π

I

I

I

I

I

I

I

I

I

r~

S o l ο°

19

, k2 > k3. Step 1 represents the diffusion of the ligand into the vicinity of the metal in solution which is followed by the formation of the outer sphere species (Step 2). Depending on the basicity of the ligand, it may proceed to form the inner sphere species, as shown in Step 3. This last reaction, involving the dissociation of water from the lanthanide, is usually the rate-determining step. The exchange of a radioisotopic lanthanide for a stable lanthanide metal chelated to an aminopolycarboxylate ligand has been found to proceed via two mechanistic pathways(60). Below pH 6, an acid-catalyzed mechanism predominates in which protonation catalyzes decomposition of the lanthanideaminopolycarboxylate complex. The uncomplexed ligand chelates with free lanthanide cations in solution. The mechanism below illustrates the acid-catalyzed pathway, where A = aminopolycarboxylate ligand ($ denotes the activated complex; L n * is the radioactive tracer). HLnA - ^ r (LnHA)* fast slow (LnHA)* + (n-l)H -e=r HnA + Ln

LnA" + H

+

+

last

Ln* + HnA

Ln*HA + (n-l)H

+

fast Ln*HA (HLn*A)* Ln*A + H slow fast"

+

Above pH 6 an acid-independent pathway exists in which complex decomposition is accomplished via the formation of a binuclear complex. The mechanism below illustrates the acid independent pathway . LnA + Ln* fast

L n A L n * ^ r ( L n A L n * ) * ^ r Ln + A L n * slow fast

The rate constants of the acid catalyzed pathway reflect the stability of the chelate rings as they follow the sequence (2): T M E D T A > T M D T A > EDPDA > E D T A > D C T A The strength of the N-Ln-N ring relative to the N-Ln-0 ring is reflected in the order of rate constants of T M D T A and EDPDA. Steric effects and the rigidity of the resultant chelate ring decrease the rate of decomposition, which explains the kinetic stability of D C T A . Polyazapolycarboxylic Acids A new type of aminocarboxylate ligands is the polyazapolycarboxylic acids. These cyclic ligands form a cavity via a ring of ethylene-spaced nitrogen donors with carboxylate "arms" which chelate to the metal above and below the plane of the cavity (Figure 7). Complex stabilities of these polyazapolycarboxylates relative to E D T A follow the sequence: DOTA > E D T A > T E T A > NOTA


29.

CHOPPIN & WONG

R

|\ /

Lanthanide Aminopolycarboxylates

\/ 2 R

Ν

R

N

I

R R

R 3

NOTA NOTA TETA DOTA

l \ / > Ν

\

/ 2 R

N

R

4

R

y

R

3

l \ /

4

TETA

357 \/ 2 R

N-^

R

3

DOTA

1,4,7-triazacyclononanc-N, N', N"-triacetic acid 1,4,8,11-tctraazacyclotclradccanc-N, N', N", N'"-tctraacctic acid 1,4,6,10-tetraazacyclododccanc-N, N', N", N'"-tctraacetic acid R=CH COOH


2

Figure 7. Polyazapolycarboxylate ligands.

The encapsulating cavity serves to protect the metal from dehydration and destabilization of the complex. Despite their difference in cavity sizes, there is little difference between the complex stabilities of N O T A and DOTA for the same lanthanide cation. This suggests very little correlation between cationic diameter and that of the cavity. Thermodynamic, NMR, and fluorescence studies seem to indicate mixed penta- and hexadentate coordination with the lighter rare earths and hexacoordination beyond Sm for N O T A and DOTA. Chelation strength is determined by the pKa values of the donor ligands, the encapsulating nature of the multiple chelate rings, and the size and rigidity of the cavity. Not surprisingly, these complexes exhibit slow dissociation kinetics which make these complexes attractive as medical imaging agents. Applications The paramagnetic and optical properties as well as the hard Lewis acid character and high coordination numbers of lanthanides make their complexes attractive for utilization in such diverse applications as N M R shift reagents, medical imaging contrast agents, and EPR probes. Hinckley first reported the use of lanthanide complexes as shift reagents to resolve complicated *H spectra 20 years ago (61). The electrostatic nature of lanthanides induces an N M R pseudocontact shift (via through space interactions) (62), which has resulted in the use of Ln(EDTA)(H20)3, Ln(NTA)(H 0)5, Ln(NOTA)(H20)3, and Ln(DOTA)(H20) to obtain structural information in solution. By contrast, covalent character is manifested in the contact shift induced by Gd(IH). The clinical use of MRI (magnetic resonance imaging) has become increasingly popular. This noninvasive imaging technique utilizes a paramagnetic probe to detect the proton resonances of water in the body and produce a threedimensional in vivo tissue characterization. Longitudinal and transverse relaxation times, T i and T2 govern signal intensity, and T i is inversely proportional to the signal intensity(j2). Because the paramagnetic behavior of Gd(III) decreases the proton relaxation time by a factor of approximately 10 , Gd complexes enhance the signal and, consequently, the detection of the image. GdDTPA(H20) was the first clinical M R I imaging agent approved in the United States. Its high paramagnetism (4^) and its high complex stability makes Gd(IU) an excellent choice(65). GdDPTA in M R I has found wide use in the detection of cancer, Alzheimer's disease, cardiovascular diseases, and epilepsy, among other diseases. 2

6


358


The half-filled 4f orbital and lack of orbital angular momentum enable Gd(III) to also serve as an effective EPR probe of metal binding sites in biological systems such as Ca(II), which has no UV-vis spectrum or EPR signal and a very broad N M R signal ( C a , natural abundance « 0.1%) (64). The relatively long electron relaxation time of Gd(m), 10- -10-l° s, relative to other Ln(III) ions allows for observable EPR signals of GdEDTA complexes at room temperature(65). Because of the aqueous nature of most biological systems, however, the Gd EPR spectra is complicated by line broadening due to spin-spin relaxation. 43

9


Conclusion Because of the lanthanide contraction, the family of trivalent lanthanide cations exhibit quite similar behavior. The use of aminopolycarboyxylates in separation provided an economical, efficient means of isolating individual lanthanides. Almost five decades of research on lanthanide aminopolycarboxylatecomplexes have revealed a wealth of information about the coordination chemistry of the lanthanides. Electrostatics dominates the bonding nature of these complexes, and there is little evidence of any directional f orbital overlap in bonding. While the coordination of the ligand was found to be constant across the lanthanide series, the hydration number changes according to the size of the cation. Through thermodynamic, spectroscopic, and stnictural studies, the primary coordination is approximately nonadentate for the lighter lanthanides and octadentate for the heavier lanthanides. The L n - N bond contributes strongly to the high stability of the lanthanideaminopolycarboxylate complexes, as does the formation of 5-membered chelate rings. The thermodynamics of the complexation reaction may be described by a "compensation effect" in which positive enthalpy and entropy contributions from dehydration of the lanthanide often exceed in magnitude the ligand-metal interactions, which is reflected in the change in free energy. The kinetics of the system conforms to the Eigen-Tamm mechanism. The slower dissociation kinetics of the lanthanide-aminopolycarboxylates coupled with its paramagnetic and optical properties make these complexes attractive as N M R shift agents, M R I contrast probes, and EPR probes. Acknowledgment The preparation of this paper was assisted by a Grant from the USDOE-OBES Division of Chemical Sciences. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9)

Shannon, R.D. Acta Cryst. 1976, A32, 751. Rizkalla, E.N.;Choppin, G.R. J. Coord.Chem.1991, 23, 33-41. Trifonov, D.N. The Rare-Earth Elements; second ed.; Pergamon Press: New York, 1963. Topp, N.E. Chemistry of the Rare-Earth Elements; Elsevier: Amsterdam, 1965; Vol. 4. Svec, H.J. In Two Hundred Year Impact of Rare Earths on Science; Gschneidner, Jr, K.A., Ed.; North Holland: New York, 1988; Vol. 11; pp 14. (a)Spedding, F.H.;Voight, A.F.;Gladrow, E.M.;Sleight, N.R.;Powell, J.E.;Wright, J.M.;Butler, T.A.;Figard, P. J. Am. Chem. Soc. 1947, 69, 27862792. (b) ibid. 1947, 2777. Harris, D.H.;Tomkins, E.R. J. Am. Chem. Soc. 1947, 69, 2792-2799. Ketelle, B.H.;Boyd, G.E. J. Am.Chem.Soc. 1947, 69, 2800-2812. Schwarzenbach, G.;Kampitsch, E.;Steiner, R. HelvChim.Acta. 1945, 28, 828-840.


29. CHOPPIN & WONG (10) (11) (12) (13) (14)


(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28)

Lanthanide Aminopolycarboxylates

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Lanthanide Aminopolycarboxylates - American Chemical Society

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