Uranyl ion complexation by the tripodal ligand nitrilotriacetate
Pierre Thuéry
CEA/Saclay, DSM/DRECAM/SCM (CNRS URA 331), Bât. 125, 91191 Gif-sur-Yvette, France.
E-mail: pierre.thuery@cea.fr
Abstract
Reaction of uranyl nitrate with N-(2-acetamido)iminodiacetic acid (ADA) under hydrothermal
conditions resulted in hydrolysis of the amide group and isolation of the complex
[(UO2)(HNTA)(H2O)2], the first uranyl complex with nitrilotriacetate to be crystallographically
characterized. Each HNTA ligand, protonated at the N site, bridges three metal atoms to give rise
to infinite ladder-like ribbons built from 2 : 2 metallacycles.
Keywords: Uranyl ion; Uranium-organic framework; Nitrilotriacetic acid; Hydrothermal
synthesis, Crystal structure
1
Nitrilotriacetic acid (H3NTA) is a very common chelating agent with tripodal geometry
pertaining to the aminopolycarboxylate class of ligands, which is of interest for actinide
separation in nuclear industry [1]. However, whereas as many as 54 crystal structures of NTA
complexes of lanthanide ions are reported in the Cambridge Structural Database (CSD, Version
5.27) [2], there is none including an actinide element. Indeed, the only related structures
reported are those of iminodiacetate complexes of uranyl [3], the N-methyliminodiacetate
complex of neptunyl [4] and the ethylenediaminetetraacetate (EDTA) complexes of uranyl [5],
uranium(IV) [6] and thorium(IV) [6a]. Besides, complexation of uranyl ions by tripodal ligands
has been considered [7], either in the context of ‘stereognostic’ uranyl coordination [7a,b] or in
attempts to enforce the cis-dioxouranium(VI) geometry [7c], but little is known on the
coordination by simple tripods from the structural viewpoint.
Following our investigation of uranyl complexes with carboxylic triacids [8] formed
under hydrothermal conditions, a widespread method for the synthesis of uranyl-organic
frameworks [9], it appeared of interest to turn to tripodal polyacids as ligands. Direct synthesis
from H3NTA resulted in the obtention of insoluble powders after heating, which prevented
crystallographic characterization. However, it appeared that N-(2-acetamido)iminodiacetic acid
(ADA, Scheme 1a) was hydrolyzed into H3NTA under the conditions used [10], and the uranyl
complex could be isolated in crystalline form and structurally characterized [11].
The asymmetric unit in [(UO2)(HNTA)(H2O)2]·3H2O, 1, contains one uranyl ion, one
HNTA ligand, in which the nitrogen atom is protonated, two coordinated and three solvent
water molecules (Fig. 1). The uranyl ion is bound to three carboxylic oxygen atoms from three
HNTA molecules (Scheme 1b) and to two water molecules, which gives the usual pentagonal
bipyramidal uranium coordination geometry, with the two water molecules in adjacent
2
positions. The average U–O(carboxylate) bond length, 2.38(2) Å, is equal to that in the citrate
complex [8], and the average U–O(water) bond length, 2.41(4) Å, is in agreement with the
mean value of 2.44(4) Å from the CSD. The five donor atoms define a mean equatorial plane
with an r.m.s. deviation of 0.103 Å.
The three carboxylic acid groups in the ligand are ionised, but the central nitrogen
atom is protonated and, the proton being directed inwards, it is involved in a trifurcated
hydrogen bond with the three uncoordinated carboxylate oxygen atoms [N1···O 2.606(5)–
2.654(5) Å, N1–H1···O 106–108°]. Such a situation is not quite surprising, considering the
zwitterionic nature of uncomplexed H3NTA [12]. The ligand thus adopts a pseudo-C3 point
symmetry, with the three oxygen atoms pointing outwards bound to three different uranium
atoms, giving a 3-1O:2O':3O" coordination mode. The ligand tricarballylate also
behaves as an assembler of three uranyl ions, but, in the absence of intramolecular hydrogen
bonding, the three carboxylate groups are chelating and the overall geometry is very far from
trigonal [8]. The present structure, in which NTA behaves as a trigonal node through its three
monodentate carboxylate groups, has no precedent in the CSD. The number of metal atoms
bound to the NTA ligand is generally larger, with usual values of 4 for 3d transition metal
atoms and between 4 and 8 in the case of lanthanide ions [13]. It has been suggested that, for a
tripodal ligand to encapsulate a single uranyl ion, a minimum of 6-atom spacing between the
central nitrogen atom and the terminal donor atoms is necessary [7a]. HNTA is obviously too
small a molecule for that and the inner part of the ligand in 1 is occupied instead by the
ammonium proton, which helds the three arms through hydrogen bonding. The peculiar
geometric requirements of uranyl thus turn this strongly chelating ligand into a trigonal
assembler.
3
Two neighbouring HNTA ligands join two uranyl ions to form a 16-membered, 2 : 2
metallacycle with the metal atoms separated by 9.4967(5) Å (Fig. 2). Coordination of the third
arm of HNTA results in the formation of nearly equilateral uranium triangles, the third side,
parallel to the b axis, being 9.9608(7) Å in length. These adjacent triangles form a ribbon or
ladder-like assembly parallel to the ab plane and running along the b axis. Such a ladder-like
geometry was also observed in the complex [Tm(NTA)(H2O)2]·2H2O, but with a very
different coordination mode for NTA since, as usual with lanthanide ions, it behaves as a
chelating ligand, which results in much shorter triangle sides (6.52 and 6.60 Å) [14]. The
coordinated water molecules are located on each side of the ribbons in 1, and they are
involved in hydrogen bonds with water or carboxylate groups, thus linking adjacent ribbons in
the ab plane as well as along the c axis. The metallacycles are stacked so as to form very
narrow channels parallel to c. The behaviour of the uranyl ion towards NTA may epitomize
that of other trans-dioxo actinyl species, such as neptunyl or plutonyl, but, however, the NTA
complexes of non-actinyl 5f elements would probably be more akin to those of 4f elements.
The present result also evidences the role of trigonal node which can be played by tripodal
aminopolycarboxylate ligands, thus opening novel paths for the synthesis of uranyl-organic
frameworks.
4
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10 Synthesis of 1. UO2(NO3)2·6H2O (140 mg, 0.28 mmol) and ADA (53 mg, 0.28 mmol) in
demineralized water (4 mL) were placed in a 20 mL tightly closed vessel and heated at
220°C under autogenous pressure. Yellow crystals of 1 appeared within one week. The
product was filtered and washed with water, giving a yellow crystalline powder (96 mg,
62% yield). Anal. Calcd. for C6H17NO13U: C, 13.12; H, 3.12; N, 2.55. Found: C, 13.05; H,
3.16; N, 2.46. The asymmetric uranyl stretching vibration mode is observed at 918 cm–1.
6
Recording of the 1H NMR spectrum was prevented by the low solubility of 1 in organic
solvents.
11 Crystal data for 1: [(UO2)(HNTA)(H2O)2]·3H2O, C6H17NO13U, M = 549.24, monoclinic,
space group P21/n, a = 10.8909(6), b = 9.9608(3), c = 13.3505(8) Å,  = 95.065(3)°, V =
1442.64(13) Å3, Z = 4, T = 100(2) K. Refinement of 190 parameters on 2732 independent
reflections out of 49517 measured reflections (Rint = 0.038) led to R1 = 0.022, wR2 = 0.049,
S = 1.028, min = –1.00, max = 0.62 e Å–3. Data were collected on a Nonius KappaCCD area-detector diffractometer and processed with HKL2000 [15]. Absorption effects
were corrected with the program SCALEPACK [15]. The structure was solved by direct
methods and refined by full-matrix least-squares on F2 with SHELXTL [16]. All nonhydrogen atoms were refined with anisotropic displacement parameters. The hydrogen
atoms bound to oxygen and nitrogen atoms were found on Fourier-difference maps.
CCDC-617372 contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk.
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7
16 G.M. Sheldrick, SHELXTL, Version 5.1, Bruker AXS Inc., Madison, WI, USA, 1999.
8
Figures captions
Scheme 1. (a) N-(2-acetamido)iminodiacetic acid (ADA). (b) Schematic representation of the
bonding of HNTA to uranyl in 1 (water molecules omitted).
Fig. 1. Crystal structure of 1. Hydrogen bonds are shown as dashed lines. Ellipsoids are drawn
at the 50% probability level. Symmetry codes: ' = x, y – 1, z; " = –x – 1/2, y – 1/2, 3/2 – z; "' =
–x – 1/2, y + 1/2, 3/2 – z; "" = x, y + 1, z. Selected bond lengths (Å) and angles (°): U–O1
1.769(3), U–O2 1.773(3), U–O3 2.372(3), U–O5' 2.363(3), U–O7" 2.404(3), U–O9 2.444(3),
U–O10 2.369(3); O1–U–O2 177.57(13), O3–U–O9 70.87(10), O9–U–O10 70.53(10), O10–
U–O5' 75.39(11), O5'–U–O7" 71.64(11), O7"–U–O3 72.19(10).
Fig. 2. Ribbons arrangement in the ab plane. Top: view showing the hydrogen bonds as
dashed lines. Bottom: simplified view with water solvent molecules and hydrogen bonds
omitted. The uranium coordination polyhedra are represented. Other atoms are shown as
spheres of arbitrary radii.
9