A structural and thermodynamic study of the complexes of U(vi) with azinecarboxylates

Article abstract of DOI:10.1039/C8DT04099K

Complexation of U(vi) with pyridazine-3-carboxylate (PDZ) and pyrazine-2-carboxylate (PAZ) was studied by spectrophotometry, potentiometry and microcalorimetry in 1.0 mol dm^?3 NaClO₄. Three complexes, [UO₂L]⁺, UO₂L₂(aq) and [UO₂L₃]^?, were identified and their stability constants (log?β) and the corresponding formation enthalpies were determined. The thermodynamic parameters indicate that the formation of the three complexes is endothermic and driven exclusively by entropy. ¹H and ¹³C-NMR data provide insight into the coordination modes of the complexes which corroborate with the thermodynamic data. Ligands chelate to U(vi) via κ²(N,O) coordination mode in complexes [UO₂L]⁺ and UO₂L₂(aq). The crystal structures of four U(vi) complexes, [(UO₂)(PAZ)₂(H₂O)]·H₂O(i), [(UO₂)(PDZ)₂(H₂O)](ii), [(UO₂)(PDZ)₃Na₂ClO₄]·2H₂O(iii), and [(UO₂)₂(PDZ)₄(H₂O)₂]·2H₂O(iv), were determined by single-crystal X-ray diffraction and compared with the U(vi) complex with picolinate (PA) (CH₆N₃)[UO₂(PA)₃] in the literature. The structure data suggest that the carboxylates coordinate with uranium in O═C-O-U mode. The strengths of the U-O-C-C-N chelate cycles in the U(vi)/L complexes decrease with the trend of PA > PDZ > PAZ, which is in great agreement with the trend of thermodynamic parameters in aqueous solutions. It is interesting that in compound II two PDZ molecules coordinate with U(vi) in cis-planar positions via κ²(N,O) mode, but in other metal complexes of the three ligands having the same κ²(N,O) coordination mode the two ligand molecules are all in trans-arrangement. In the dimeric complex IV, one ligand coordinate with U(vi) in κ²(N,O) mode, while the other does it in μ₂-L-κ²(O:O′) mode respectively.

Full text of DOI:10.1039/C8DT04099K

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A structural and thermodynamic study of the
complexes of U(VI) with azinecarboxylates†
Lina Lv,a Baihua Chen,a Jun Liu,a Jing Chen,b Chao Xu *b and Yanqiu Yang
Cite this: Dalton Trans., 2019, 48,
566
a
*
Complexation of U(VI) with pyridazine-3-carboxylate (PDZ) and pyrazine-2-carboxylate (PAZ) was studied
by spectrophotometry, potentiometry and microcalorimetry in 1.0 mol dm−3 NaClO4. Three complexes,
[UO2L]+, UO2L2(aq) and [UO2L3]−, were identified and their stability constants (log β) and the corres-
ponding formation enthalpies were determined. The thermodynamic parameters indicate that the for-
mation of the three complexes is endothermic and driven exclusively by entropy. 1H and 13C-NMR data
provide insight into the coordination modes of the complexes which corroborate with the thermo-
dynamic data. Ligands chelate to U(VI) via κ2(N,O) coordination mode in complexes [UO2L]+ and
UO2L2(aq). The crystal structures of four U(VI) complexes, [(UO2)(PAZ)2(H2O)]·H2O(I), [(UO2)(PDZ)2(H2O)](II),
[(UO2)(PDZ)3Na2ClO4]·2H2O(III), and [(UO2)2(PDZ)4(H2O)2]·2H2O(IV), were determined by single-crystal
X-ray diffraction and compared with the U(VI) complex with picolinate (PA) (CH6N3)[UO2(PA)3] in the litera-
ture. The structure data suggest that the carboxylates coordinate with uranium in OvC–O–U mode. The
strengths of the U–O–C–C–N chelate cycles in the U(VI)/L complexes decrease with the trend of PA >
PDZ > PAZ, which is in great agreement with the trend of thermodynamic parameters in aqueous solu-
tions. It is interesting that in compound II two PDZ molecules coordinate with U(VI) in cis-planar positions
via κ2(N,O) mode, but in other metal complexes of the three ligands having the same κ2(N,O) coordination
mode the two ligand molecules are all in trans-arrangement. In the dimeric complex IV, one ligand coor-
dinate with U(VI) in κ2(N,O) mode, while the other does it in μ2-L-κ2(O:O’) mode respectively.
Received 15th October 2018,
Accepted 23rd November 2018
DOI: 10.1039/c8dt04099k
rsc.li/dalton
an advanced nuclear fuel cycle.1–4 For example, complexation
of uranium and other actinides with the amide derivatives of
1. Introduction
Uranium is the most abundant actinide element in the azine carboxylic acids, such as picolinic acid, pyrazine-
uranium-based nuclear fuel cycle and large quantities of carboxylic acid, dipicolinic acid (DPA), 2,2′-bipyridine-6,6′-
uranium exist in nuclear waste, presenting an enormous chal- dicarboxylic acid (BiPDA), 1,10-phenanthroline-2,9-di-
lenge in the environmental management of nuclear waste. carboxylic acid, etc., has been intensively investigated. The
Safe treatment and proper disposal of uranium-bearing wastes thermodynamic and structural properties of actinide com-
require the knowledge of the chemical behavior of uranium in plexes with these aza-heterocyclic carboxylic acids and their
the waste stream and in the environment. Also, ligands that amide derivatives were obtained by techniques including
could form complexes with uranium and have the potential of solution chemistry and crystallography.3,5–15 Most studies
being used as separation agents need to be developed.
have so far focused on exploring the influence of the amide
Among the organic ligands, mixed N- and O-donor ligands moieties of aza-heterocyclic carboxylic acids on the complexa-
have been studied in recent years due to their potential appli- tion with metals, while very few studies have been conducted
cation in the GANEX process (Group ActiNides EXtraction) for to investigate the influence of the changes in the heterocyclic
aza-structure on the binding strength and coordination
modes. For example, complexation of U(VI) with picolinic acid
aInstitute of Nuclear Physics and Chemistry, Chinese Academy of Engineering
(denoted as PA in this paper), the simplest aza-heterocyclic
Physics, Mianyang, Sichuan 621900, China. E-mail: yan.qiu.yang@163.com
carboxylic acid, was studied previously.16 In acidic aqueous
solution (pH ≤ 4), three successive mononuclear U(VI)/PA
complexes (1 : 1, 1 : 2, and 1 : 3) were identified and the
corresponding stability constants and enthalpy of complexa-
tion were determined.16 The thermodynamic parameters
suggest that the first and the second ligand molecules coordi-
bInstitute of Nuclear and New Energy Technology, Tsinghua University,
Beijing 100084, China. E-mail: chaoXu@tinghua.edu.cn
†Electronic supplementary information (ESI) available: Composition and specia-
tion of samples for the NMR analysis, hydrolysis and speciation of ligands in a
typical sample for microcalorimetric titration, the 1H and 13C NMR spectra of
U(VI)/PDZ/H solutions, and the CIF files for the crystal structures from this work.
CCDC 1586814, 1865674, 1586812 and 1865676. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c8dt04099k
2+
nate with UO2 via the N and a carboxylate O atom via the
566 | Dalton Trans., 2019, 48, 566–577
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κ2(N,O) mode in the equatorial plane,16 in agreement with Only limited structural data in crystals have been reported.
the single crystal structures.17,18
In the crystal structures of the 1 : 2 complexes of U(VI) with
Aza-ligands that contain multiple nitrogen atoms in the picolinate17 and pyrazinecarboxylate,19 two ligand molecules
cyclic aza-moiety are more resistant to radiolysis and therefore chelate with U(VI) via the equatorial plane in the κ2(N,O)
could be more applicable in the treatment of nuclear wastes, mode. For pyridazine-3-carboxylic acid (PDZ), 1 : 2 complexes
but only limited studies have been conducted on these with Mn2+, Cu2+, Zn2+ and Co2+ in the κ2(N,O) mode were
ligands.19 As a result, fundamental questions remain to be reported.20 In addition, Resul Leciejewicz21 obtained
a
answered, such as (1) how the total basicity of the ligand is binuclear U(VI) complex, [(UO2)2(PDZ)4(H2O)2]·2H2O, by stir-
affected by the presence of multiple nitrogen atoms in the ring a mixture of HL and UO2(NO3)2·6H2O (2 : 1) under the
2+
cyclic aza-moiety, and (2) what kinds of coordination modes condition of boiling. Each UO2 ion is coordinated by two
are present in the ligand complex(es) with metal ions like ligand molecules: one coordinates with U(VI) by the N,O-
U(VI). To help answer these fundamental questions and bonding group (κ2(N,O) mode) and the other by both carbox-
provide support for the development of more efficient separ- ylate O atoms bridging adjacent uranyl ions (μ2-L-κ2(O:O′)
ation ligands, complexation of U(VI) with an aza-heterocyclic mode).
carboxylate ligand, pyridazine-3-carboxylate (PDZ) and pyra-
In contrast to the structural data for crystals, very little
zine-2-carboxylate (PAZ) (see Fig. 1) where the cyclic aza-moiety structural information on aza-complexes with U(VI) in solution
contains two nitrogen atoms in different positions was studied is available. In the present study, while new crystal structures
in this work. Multiple thermodynamic techniques, including of U(VI)/aza-carboxylate complexes were explored, we con-
1
potentiometry, spectrophotometry, and microcalorimetry, were ducted H and 13C-NMR studies to obtain the structural infor-
used to determine the equilibrium constants and enthalpy for mation of U(VI)/PDZ complexes in solution and corroborated
the U(VI) complexes over a pH range of 1.0–4.0. Structural infor- with the thermodynamic data, providing a fundamental under-
mation on the U(VI) complexes in the solution and solid states standing of the coordination behavior of U(VI) with heterocyclic
was obtained by NMR and crystallography.
N- and O-donor ligands.
Structural information on the coordination mode in U(VI)
complexes with aza-heterocyclic monocarboxylates is scarce.
2. Results
2.1 Thermodynamic parameters
2.1.1. Protonation constants of ligands. The protonation
constants of the two ligands were determined by potentio-
metry. The experiments were conducted in two different direc-
tions (from basic to acidic or in reverse) with perchloric acid
or sodium hydroxide. Fig. 2 shows the representative potentio-
metric titrations to determine the protonation constants of
PDZ and PAZ. The calculated protonation constants are listed
in Table 1. In this work the second protonated species of PDZ
Fig. 1 Picolinic acid (PA), pyridazine-3-carboxylic acid (PDZ) and pyra-
zine-2-carboxylic acid (PAZ). For all ligands, the deprotonated, singly-
protonated, and doubly-protonated forms are donated in this paper as
L−, HL, and H2L+, respectively.
Fig. 2 Representative potentiometric titrations for the protonation of PDZ (a) and PAZ (b) at 298 K (I = 1.0 mol dm−3 NaClO4, pKw = 13.78). Initial
solution in the cup [V0(mL)/C0L (mmol dm−3)/CH0 (mmol dm−3)]: (a) 20.0/2.06/2.06, and (b) 21.5/7.35/3.26; titrant: (a) CHClO = 1.00 mol dm−3, and (b)
4
−
CHClO = 0.10 mol dm−3. Left y-axis: , observed pCH; red dashed line, calculated pCH. Right y-axis (ligand speciation): black, L ; red, HL; green,
◊
4
H2L+.
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Table 1 Thermodynamic parameters for the complexation of U(VI) with PDZ and PAZ (298 K, I = 1.0 M NaClO4)
Reaction
Method
log βM
ΔH (kJ mol−1
)
ΔS (J mol−1 K−1
)
Ref.
L− + H+ = HL
PAZ
PDZ
PA
PDZ
PA
pot, cal
pot, cal
pot, cal
pot
pot
sp, cal
pot
sp, cal
pot, cal
sp, cal
pot
2.78 0.01
2.86 0.01
5.38 0.02
3.86 0.06
6.33
2.76 0.01
2.79 0.02
2.84 0.01
4.35 0.04
4.43 0.03
4.86 0.03
5.19 0.04
7.68 0.03
5.59 0.03
6.70 0.04
6.71 0.08
10.29 0.05
−(0.20 0.01)
−(1.36 0.02)
−(13.9 0.6)
−(0.82 0.03)
60.4 0.3
50.2 0.1
56.3 2.0
pw
pw
16
pw
23
pw
pw
pw
16
pw
pw
pw
16
L
− + 2H+ = H2L+
71
1
L
− + UO22+ = [UO2L]+
PAZ
4.35 0.04
67
1
PDZ
PA
PAZ
5.19 0.09
−(3.6 0.1)
11.7 0.1
71.8 0.5
71
124
1
2
2L− + UO22+ = UO2L2(aq)
3L− + UO22+ = [UO2L3]−
PDZ
PA
PAZ
Sp
9.56 0.06
−(8.3 0.1)
14.8 0.2
135
119
157
3
1
3
pot, cal
sp, cal
pot
sp, cal
pot, cal
pw
pw
pw
16
PDZ
PA
17.0 0.2
2.0 0.2
185
203
2
1
(H2L+) was successfully detected, while that of PAZ was not
The calculated stability constants of the complexes are sum-
detected even when 1.0 mol dm−3 HClO4 was used as the marized in Table 1, and the molar absorptivities of the com-
titrant. There are two contributions to this: the lone pairs on plexes are shown in Fig. 3 (bottom row).
the two adjacent nitrogen atoms in pyridazine making the pro-
In addition, the complexation of U(VI) with PAZ was investi-
tonation of PDZ easier than PAZ, and the mesomeric inter- gated by potentiometric titration, as shown in Fig. 4. The best
action between the protonated and neutral nitrogen atoms in model to fit the data includes three successive mononuclear
pyrazine probably destabilising the cation.22
U(VI)/PAZ complexes. As shown in Table 1, the stability con-
2.1.2. Stability constants of U(VI) complexes. The stability stants of the U(VI)/PAZ complexes measured by spectro-
constants of U(VI) complexes with PDZ and PAZ were deter- photometry and by potentiometry are in good agreement,
mined by spectrophotometry. In addition, the stability con- suggesting that the dinuclear complex that was found to exist
stants of U(VI)/PAZ complexes were also determined by poten- in the crystal form,21 [UO2L2]2, may not be present or is insig-
tiometry. Preliminary experiments showed that precipitation nificant in aqueous solutions under the experimental con-
occurred when the pH of the titration system was above 4.0, ditions in this work.
probably due to the hydrolysis of U(VI) and the poor solubility
2.1.3. Enthalpies. Enthalpies for the protonation of ligands
of the neutral complexes of U(VI) with ligands. To avoid precipi- and the formation of U(VI) complexes were determined by
tation, titrations were conducted with a large ratio of L/M and microcalorimetry. Fig. 5 shows the representative calorimetric
the pH was controlled below 4.
titrations of PDZ and PAZ with perchloric acid at 298 K to
Fig. 3 shows the representative spectrophotometric titra- determine their protonation enthalpies. Multiple titrations
tions of U(VI) solutions with neutralized PDZ and PAZ. As were performed with different concentrations of the ligand
shown in the upper pictures of Fig. 3, absorption bands of (C0L) in a vessel and the same titrant (0.100 mol dm−3 HClO4).
2+
UO2 in the range of 380–500 nm exhibit a systematic red The enthalpies of protonation of ligands were calculated with
shift and an increase in intensity with the addition of ligands, HypDeltaH24 in conjunction with the protonation constants
indicating the successive formation of U(VI) complexes. Factor which were determined by potentiometry in this work, and are
analysis with the program HypSpec 2009 suggested that three listed in Table 1. The enthalpies of PDZ protonation were
U(VI) complexes formed. A number of models including determined in two steps. The enthalpy for HL was determined
different U(VI)/L complexes were tested to fit the data, includ- by the titration of NaL with 0.1 M HClO4, controlling the H2L+
ing monomeric and dimeric complexes (e.g., [UO2L2]2 species less than 3% throughout the titration. The enthalpy for
suggested by the crystal structure in the literature21), and the [H2L]+ was determined simultaneously along with the enthalpy
best fit was achieved with the model containing only mono- of U(VI)/PDZ complexation, because the U(VI)/PDZ complexa-
meric complexes [UO2L]+, UO2L2(aq), and [UO2L3]−, as rep- tion experiments were carried out in high acidic solutions (pH
resented by eqn (1), (2), and (3).
1.29–2.92, see Fig. S1 of the ESI†) containing sufficient [H2L]+.
Fig. 6 shows the representative calorimetric titrations of
U(VI) with PDZ and PAZ respectively. The enthalpies for the all
U(VI)/L complexes were calculated with HypDeltaH in conjunc-
tion with the thermodynamic parameters of the stability con-
stants of the U(VI)/L complexes, and are summarized in
Table 1.
UO22þ þ Lꢀ ¼ ½UO2Lnꢁþ β101
ð1Þ
ð2Þ
ð3Þ
UO22þ þ 2Lꢀ ¼ UO2L2ðaqÞ β102
UO22þ þ 3Lꢀ ¼ ½UO2L3ꢁꢀβ103
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Fig. 3 Representative spectrophotometric titration of U(VI)/PDZ (a) and U(VI)/PAZ complexation at 298 K. Initial solution in a cuvette [V0(mL)/
C0U(mmol dm−3)/CH0 (mmol dm−3)]: (a) 2.00/5.18/32.8, and (b) 2.00/5.50/10.6; titrant: (a) CL = 0.18 mol dm−3, (b) CL = 1.00 mol dm−3. (Top) absorp-
tion spectra (normalized to C0U). (Bottom) calculated molar absorptivity of U(VI) species; black: UO22+, red: UO2L+, blue: UO2L2(aq), and purple:
[UO2L3]−.
2.2. Crystal structures of U(VI) complexes
Structures of the U(VI) complexes in solid compounds, [(UO2)
(PAZ)2H2O]·H2O(I), [(UO2)(PDZ)2H2O](II), [(UO2)(PDZ)3Na2ClO4]·
2H2O(III) and [(UO2)2(PDZ)4(H2O)2]·H2O(IV), are shown in
Fig. 7. Selected bond lengths of compounds I, II, III, and IV,
and those of (CH6N3)[UO2(PA)3] from the literature25 for com-
parison are listed in Table 2.
2.2.1. [(UO2)(PAZ)2(H2O)]·H2O (I). Compound I is a new
crystal of the uranyl complex with PAZ in the triclinic space
ˉ
group P1– p1. A uranyl(VI) ion is at the centre of symmetry co-
ordinated by two PAZ molecules in trans-planar positions and
a water molecule; both ligands chelate to UO2 via κ2(N,O)
2+
mode, which is in agreement with the reported coordination
mode in the literature.19 Both PAZ molecules are almost
planar with only a small tension angle between them (2.9°).
The least-squares plane of both PAZ molecules passes through
the central U-atom and is almost perpendicular to OvUvO
(89.6° in angle). The OvUvO moiety is slightly distorted,
being imperfectly linear (177.1(2)° angle) but almost symmetri-
cal (bond length: U1–O5, 1.762(6) Å; U1–O6, 1.757(6) Å).
2.2.2. [(UO2)(PDZ)2(H2O)] (II). [(UO2)(PDZ)2(H2O)] is a new
U(VI) complex with PDZ, crystallized in the orthorhombic space
Fig. 4 Potentiometric titration of U(VI) with PAZ at 298 K (I = 1.0 M
NaClO4). Initial condition [V0(mL)/CU0 (mmol dm−3)/C0H(mmol dm−3)]:
20.39/3.23/6.08; titrant: CL = 0.18 mol dm−3. Left y-axis: pCH, ( ) experi-
◊
mental; red short dashed line, fitted. Right y-axis: speciation of U(VI).
Black solid line, free UO22+; red line, UO2L+; green line, UO2L2(aq); blue
−
line, UO2L3
.
group Pna 21 (33). A uranyl(VI) ion is at the centre of symmetry
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Fig. 5 Microcalorimetric titrations of the protonation of PDZ (a) and PAZ (b) (t = 25 °C, I = 1.0 M NaClO4). (Top) Thermogram of the protonation of
−
◊
ligands; (bottom) total reaction heat (left y axis, ( ) experimental; (- -) calculated) and speciation of ligands (right y axis; black line, L ; red line, HL;
green line, H2L+). Initial solution [V0(mL)/CL0(mmol dm−3)]: (a) 0.75/20.0, and (b) 0.70/28.6. Titrant: (a) CHClO = 0.100 mol dm−3, 0.005 mL × 43
4
additions; (b) CHClO = 0.100 mol dm−3, 0.010 mL × 22 additions.
4
coordinated by two PDZ molecules in cis-planar positions and a linear (176.43(16)° angle) and unsymmetrical (bond length:
water molecule, both ligands chelating via κ2(N,O) mode. To U1–O7, 1.723(5) Å; U1–O8, 1.764(4) Å). U(VI) is equatorially five-
our knowledge, the cis-arrangement of ligands in [(UO2) coordinated and chelated by three PDZ units. Two of the three
(PDZ)2(H2O)] is the only example of mononuclear U(VI) com- PDZ molecules coordinate with U(VI) in κ2(N,O) mode via the
plexes with two aza-heterocyclic carboxylate molecules. In all N-atom on the 2-position and one carboxylate O-atom, and the
the reported U(VI) complexes having two aza-heterocyclic carbox- other carboxylate O-atoms coordinate with Na1 and Na2
ylates chelating via κ2(N,O) mode in the CSD database (about 10 respectively (Fig. 7). The third PDZ molecule bridges U1 and
structures),20 including in complex UO2(PDZ)3Na2ClO4·2H2O(III) Na1 in μ2-L-κ2(O:O′) coordination mode.
from this work and in the 1 : 2 (M/L) complexes of PDZ with
2.2.4. [(UO2)2(PDZ)4(H2O)2]·2H2O (IV). [(UO2)2(PDZ)4(H2O)2]·
other metal ions (such as Mn2+, Cu2+, Zn2+, Co2+ etc.),20 ligands 2H2O(IV) is a dinuclear 1 : 2 U(VI)/PDZ complex crystallized in
are all in trans-arrangement. Both PDZ molecules are almost the monoclinic space group C12/c1 (15), which is in agreement
planar with only a small tension angle between them (4.4°). The with that reported in the literature.21 Four PDZ molecules coor-
least-squares plane of both PDZ molecules passes through the dinate with two uranyl(VI) through two modes: κ2(N1,O1) and
central U-atom and is almost perpendicular to OvUvO (89.1° μ2-L-κ2(O3:O6).
in angle). The OvUvO moiety is distorted greater than that in
compound I, being imperfectly linear (179.2(4)° angle) and
asymmetrical (bond length: U1–O5, 1.68(3) Å; U1–O6, 1.84(2) Å).
3. Discussion
3.1 Comparison of structures
2.2.3. [UO2(PDZ)3Na2ClO4]·2H2O.
[UO2(PDZ)3Na2ClO4]·
2H2O is a multinuclear 1 : 3 : 2 U(VI)/PDZ/Na(I) complex, crystal-
lized in the monoclinic space group P12/n1(13). The OvUvO As shown in Fig. 7, compounds I and II are of the same coordi-
moiety in the complex is slightly distorted, being imperfectly nation mode between ligands and central U(VI), and those of
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Fig. 6 Microcalorimetric titrations of U(VI) complexation with PDZ (a) and PAZ (b) (t = 25 °C, I = 1.0 mol dm−3 NaClO4). (Top) Thermograms;
+
◊
(bottom) total reaction heat (left y axis, ( ) experimental; (- -) calculated) and speciation of U(VI) (right y axis; black line, U(VI); red line, [UO2L] ; green
line, UO2L2(aq); blue line, [UO2L3]−). Initial solution [V0(mL)/CU0 (mmol dm−3)/C0H(mmol dm−3)]: (a) 0.75/5.28/51.9, and (b) 0.70/6.29/11.6. Titrant: (a)
C
L = 0.40 mol dm−3, 0.005 mL × 45 additions; (b) CL = 0.40 mol dm−3, 0.008 mL × 30 additions.
Fig. 7 Crystal
structures
of
U(VI)
complexes
[(UO2)(PAZ)2(H2O)]·H2O(I),
[UO2(PDZ)2(H2O)](II),
[UO2(PDZ)3Na2ClO4]·2H2O(III)
and
[(UO2)2(PDZ)4(H2O)2]·2H2O(IV). Element colour: green (U), red (O), pink (Cl), plum (Na), and grey (C); hydrogen atoms and uncoordinated water mole-
cules are not shown for clarity.
compound III and (CH6N3)[UO2(PA)3] are the same. Average U–O(carboxylate) bonds in compounds I and II (2.31(2) Å) is
distances of some selected bonds are listed in Table 3. For all shorter than the normal U–O(carboxylate) bond with simple
the complexes, C–O(carboxylate) bonds are much longer than carboxylate ligands (2.37 Å),26,27 while those in compound III
C–O(free), indicating that the carboxylate groups coordinate and (CH6N3)[UO2(PA)3] (2.36(1) Å) are of equivalent lengths to
with the U-atom in OvC–O–U mode. The average distance of the normal U–O bond with simple carboxylate ligands.
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Table 2 Selected bond lengths (Å) and angles (°)
[UO2(PAZ)2H2O]·H2O
[UO2(PDZ)2H2O]
[UO2(PDZ)3Na2ClO4]·2H2O
[UO2(PDZ)2H2O]2·H2O
(CH6N3)[UO2(PA)3]25
U1–O1 2.329(5)
U1–O3 2.292(5)
U1–O1 2.30(1)
U1–O3 2.318(9)
U1–O1 2.366(4)
U1–O5 2.351(4)
U1–O3 2.286(4)
U1–N1 2.630(5)
U1–N5 2.666(5)
U1–O7 1.723(5)
U1–O8 1.764(4)
C5–O1 1.270(7)
C15–O5 1.261(7)
C10–O3 1.291(6)
C5–O2 1.235(7)
C15–O6 1.220(6)
C10–O4 1.223(6)
O1–U1–N1 62.70(14)
O5–U1–N5 62.67(13)
O7–U1–O8 176.43
U1–O1 2.345(3)
U1–O3 2.346(3)
U1–O6 2.334(3)
U1–N1 2.616(3)
U1–O3 2.366(3)
U1–O5 2.362(3)
U1–O7 2.269(2)
U1–N1 2.631(4)
U1–N2 2.609(4)
U1–O1 1.762(4)
U1–O2 1.766(4)
C11–O3 1.288(5)
C31–O7 1.287(6)
C21–O5 1.298(6)
C11–O4 1.218(5)
C31–O8 1.221(6)
C21–O6 1.217(6)
O3–U1–N1 63.45(12)
O5–U1–N2 63.64(12)
O1–U1–O2 178.53(16)
U1–N1 2.628(6)
U1–N3 2.623(6)
U1–O5 1.762(6)
U1–O6 1.757(6)
C5–O1 1.255(9)
C10–O3 1.274(9)
U1–N1 2.624(11)
U1–N5 2.541(11)
U1–O5 1.68(3)
U1–O6 1.84(2)
C5–O1 1.311(16)
C10–O3 1.271(16)
U1–O4 1.748(3)
U1–O5 1.756(3)
C5–O1 1.283(5)
C10–O3 1.233(5)
C10–O6 1.244(5)
C5–O2 1.225(5)
C5–O2 1.219(9)
C10–O4 1.206(9)
C5–O2 1.231(18)
C10–O4 1.223(16)
O1–U1–N1 63.45(18)
O3–U1–N3 63.64(19)
O5–U1–O6 177.1(2)
O1–U1–N1 63.1(3)
O3–U1–N3 64.2(3)
O5–U1–O6 179.2(4)
O1–U1–N1 63.13(11)
O3–U1–O6 79.20(11)
O4–U1–O5 178.62(15)
2+
Table 3 Average distances (Å) of some bonds involving the coordi-
nation groups around U
matic carboxylates could bind UO2 in three coordination
modes: (1) κ1(O) monodentate coordination with the oxygen of
the carboxylate group without the participation of the heterocyclic
group in bonding, (2) κ2(N,O) coordination by the nitrogen atom
of the heterocyclic group and the oxygen atom of the carboxylate
group, and (3) μ2-L-κ2(O:O′) coordination by the two oxygen atoms
U–O
C–O
(carboxylate)a U–N
(carboxylate) C–O (free)
[UO2(PAZ)2](I)
[UO2(PDZ)2](II)
2.31(2)
2.31(2)
2.63(1) 1.26(2)
2.58(2) 1.29(4)
2.65(2) 1.27(1)
2.62(2) 1.29(1)
1.21(2)
1.23(2)
1.23(1)
1.22(1)
2+
[UO2(PDZ)3]−(III) 2.36(1)
of the carboxylate group bridging two UO2 ions. Besides, a
[UO2(PA)3]−b
2.36(1)
bidentate κ2(O,O′) coordination mode by two carboxylate oxygen
atoms is also observed with other simple monocarboxylate
ligands such as benzoate.28,29 Possible coordination modes for
the U(VI)/PDZ complexes in solution are shown in Fig. 8.
Keeping in mind that the structures of complexes observed
in the solid state may not be identical to those in solution, in
this work, we rely on the analysis of the thermodynamic data,
the comparison with related ligands, and the structural infor-
mation obtained by techniques applicable to solution samples
(such as NMR) to gain insight into the coordination modes of
the U(VI)/L complexes in solution. These are discussed in the
following sections.
a O(carboxylate) denotes the O-atoms in the carboxylate groups which
coordinate with U-atoms in crystal structures, and O(free) denotes
those which don’t coordinate with U-atoms. b Calculated using the
data from the literature.25
The difference in the U–O(carboxylate) bond lengths
between compounds I and II is within the uncertainty range,
while the average U–N distance in compound I is longer than
that in compound II, suggesting that the interaction of U(VI)
with PAZ is weaker than that with PDZ. For the comparison of
the structural data between compound III and (CH6N3)
[UO2(PA)3], the average U–O(carboxylate) bonds are of the
same value (2.36 0.1 Å), while the longer U–N bond length in
[UO2(PDZ)3]− suggests that the chelating rings of U–O–C–C–N
in [UO2(PDZ)3]− are weaker than those in [UO2(PA)3]−. In
addition, the interaction of the third ligand with U(VI) in
[UO2(PDZ)3]− is also weaker than that in [UO2(PA)3]−, because
the U–O bond (2.286(4) Å) in compound III is longer than that
in (CH6N3)[UO2(PA)3] (2.269(3) Å).
3.3 Thermodynamic trends
The thermodynamic parameters for the protonation of PA and
the U(VI)/PA complexes from previous work16 are listed in
Table 1 for comparison with U(VI)/L complexation of this work.
In brief, the structural data show that ligands PDZ and PAZ
interact with U(VI) in similar coordination modes to the ligand
PA, and the interaction strengths of the three ligands with
U(VI) decrease as the trend: PA > PDZ > PAZ, which is consist-
ent with the thermodynamic parameters in Table 1.
3.2 Possible coordination modes of the [UO2L]+ complex in
solution
Single crystal structures of compounds I, II, III, and IV in this
work and others in the CSD database20 have shown that α-N-aro- Fig. 8 Possible coordination modes.
572 | Dalton Trans., 2019, 48, 566–577
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The protonation constants of three analogue ligands terms of the same pKa. If the value of pKa is considered a
follows PA ≫ PDZ > PAZ, indicating that the basicity of ligands measure of the basicity of the ligand, the two different linear
follows the same trend. Evidently, the cyclic aza-moiety of correlations in Fig. 9 indicate that the complexation is
PDZ/PAZ containing two nitrogen atoms in an ortho/para-posi- enhanced by the presence of the α-N-donor atom due to the
tion has a stronger electron-withdrawing effect than that of PA. chelating in the κ2(N,O) mode. For the 1 : 1 complex
Data in Table 1 show that the entropy of protonation and UO2(PDZ)+ and UO2(PAZ)+ studied in this work, the values of
the entropies of complexation of U(VI) complexes are all very log β101 = (2.85 0.01) for UO2(PDZ)+ (number 9) and log β101
=
similar to the three ligands, PAZ, PDZ and PA, which is a (2.76 0.01) for UO2(PAZ)+ (number 10) fall perfectly on the
strong indication that the modes of protonation and coordi- line with other α-N-donor-monocarboxylates including picoli-
nation with U(VI) of the three ligands are probably identical. nic acid. Based on the above comparison, we conclude that the
The previous study on PA has revealed that the proton in HL α-N-donor in PDZ/PAZ participates in coordination with U(VI)
and the uranium in the complexes are shared by the carboxy- and the chelating κ2(N,O) mode is the dominating coordi-
late O and N in a κ2(N,O) mode. By analogy, it is very likely that nation mode between U(VI) and PDZ/PAZ in solution.
the protonation of PDZ and PAZ and the complexation of them
3.4 Analysis of NMR data
with U(VI) take the same coordination mode.
1
Complexation of carboxylic acids with actinide ions is dom- The H and 13C NMR data of the U(VI)/PDZ solution samples
inantly electrostatic interactions in nature30 so that the stabi- are shown in Fig. S2 of the ESI.† NMR is a technique that is
lity constants of the complexes usually correlate with the proto- applicable to solution samples and capable of revealing struc-
nation constants of the ligand.16,30 A linear relationship is tural information of complexes in solution. As listed in
expected for the complexes with the same coordination mode.
Table S1,† solution sample A contains the ligand PDZ only,
Fig. 9 shows the plot of the stability constants of the 1 : 1 while the others (B, C, D, and E) contain U(VI) and PDZ at
complexes (log β101) of U(VI) with some monocarboxylates as a different acidities. The speciation of these samples is provided
function of the protonation constants of these ligands. The in Table S2 of the ESI.† The 1H–13C COSY spectrum of solution
ligands complexing with U(VI) in Fig. 9 are classified into two A is shown in Fig. S3 of the ESI† to help assign the chemical
groups: (A) simple monocarboxylates (number 1 to 8) which shifts.
coordinate with U(VI) through two carboxylate O-donors in
As the speciation shows, under the experimental con-
κ2(O,O′) mode, and (B) α-aminocarboxylates (number 12 to 16) ditions, the free ligand species (L−, HL, HL+) are dominant in
and PA (number 11) which chelate with U(VI) through one car- all samples containing U(VI) (up to 85%) and the U(VI)/PDZ
boxylate O- and the α-N-donor in κ2(N,O) mode. As shown in complexes are minor (<15%). As a result, the NMR data are not
Fig. 9, two different linear correlations are observed and the very informative of providing insight into the coordination
interactions of U(VI) with α-N-donor-monocarboxylates are modes in the U(VI)/PDZ complexes. Only a very limited discus-
always stronger than those with simple monocarboxylates in sion can be made below.
Because the species in the solution exchange rapidly, only
the average spectra of all species were observed. The three
signals in the 1H NMR spectra (Fig. S2†) are straightforward to
assign to the hydrogens on pyridazine. The shift of the 1H
NMR signals seems to correlate well with the change in pH,
reflecting the change in the degree of protonation of the
ligand in the solution samples.
From Samples B, to C, D, and E, the degree of complexation
with U(VI) gradually increased. Samples B and C contained
appreciable amounts of the 1 : 1 and 1 : 2 complexes (about
58% in B and 66% in C in terms of total U(VI)). For samples B
and C, the 13C NMR spectra show that the signals for C3 and
C7 nearly disappeared due to significant line broadening.
Based on the previous data in the literature that show that the
13C NMR peaks of the carbons in the chelating ring could shift
downfield and broaden into the baseline due to the chelation
with U(VI),38 we postulate that the disappearance of 13C NMR
signals for C3 and C7 suggests that the chelating κ2(N,O) mode
Fig. 9 Relationship of the stability constants of the 1 : 1 complexes is the major coordination mode in the 1 : 1 and 1 : 2 U(VI)/PDZ
complexes. For samples D and E, the 13C NMR signals for C3
between uranyl(VI) and various monocarboxylate ligands. 1: aminoace-
tate,31 2: 3-aminopropanoate,32 3: benzoate,29 4: phenylacetate,33 5:
and C7 re-appeared, which could probably be due to the κ1(O)
4-aminobutanoate,32 6: acetate,34 7: propanoate,23 8: nicotinate,35 9:
coordination of U(VI) with a single O on the carboxylate of the
third PDZ ligand as shown by the crystal structure of
[UO2(PDZ)3Na2ClO4]·2H2O. It should be pointed out that the
pyrazine-2-carboxylate, 10: pyridazine-3-carboxylate, 11: picolinate, 12:
D-methionine,36,37 13: D-threonine,36 14: DL-phenylalanine,36 15:
L-leucine,36,37 and 16: L-isoleucine.36,37
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disappearance and re-appearance of the signals for C3 and C7
The stock solutions of ligands were prepared by neutraliz-
could also be due to the change in pH that affects the degree ing the weighed amounts of solid pyridazine-3-carboxylic acid
of protonation of the PDZ ligand, since it is very likely that the (HL, 98%, Sigma-Aldrich) or pyrazine-2-carboxylic acid (HL,
proton in HL is shared by the carboxylate O and the N in a che- 98%, Sigma-Aldrich) with an equivalent amount of NaOH
lating κ2(N,O) mode similar to that in the U(VI)/PDZ complexes. (1.0 mol dm−1, Sigma-Aldrich). The ionic strength of all
The good agreement between the thermodynamic and working solutions was maintained at 1.00 mol dm−3 NaClO4
NMR results implies that only mononuclear U(VI) complexes (298 K).
are formed in solution, although
PDZ complex with two bridging PDZ molecules in
a dimeric 1 : 2 U(VI)/
5.2 Potentiometry
a
[(UO2)2(PDZ)4(H2O)2]·2H2O crystal is identified in this work and
literature.21 In fact, this is not the only example of U(VI) com-
plexes. A dimeric 1 : 2 U(VI)/Ac complex ([UO2(Ac)2(DMSO)]2, Ac
= acetate) was isolated from DMSO media, while only the mono-
nuclear complexes were formed in solution.39
Potentiometric titrations were performed to determine the pro-
tonation constants of PDZ and the stability constants of the
U(VI)/PAZ complexes, using an autotitration unit consisting of
a glass cell with a lid and a Metrohm dosimat (907 Titrando)
connected with a glass pH electrode (Metrohm, 6.0229.100).
Both the cell and the lid were water-jacketed and maintained
at (298.2 0.1) K by circulating water from a constant tempera-
ture bath. An inert atmosphere was maintained in the cell by
passing Ar gas to prevent the sorption of CO2 in the solution
during titration. The original electrode filling solution (3 mol
dm−3 KCl) was replaced with 1.0 mol dm−3 NaCl to prevent the
clogging of the electrode junction due to the low solubility of
KClO4.
4. Conclusion
Complexation of U(VI) with pyridazine-3-carboxylic acid (PDZ)
and pyrazine-2-carboxylic acid (PAZ) was studied in both solu-
tion and in the solid state. 1 : 1, 1 : 2, and 1 : 3 U(VI)/L com-
plexes were identified and the thermodynamics for them were
determined in 1.0 mol dm−3 NaClO4. The stability constants
for the corresponding complexes decrease in the following
order: PA > PDZ > PAZ, which is consistent with the changing
trend of ligands’ basicity. The formation of U(VI) complexes
with PDZ and PAZ is all endothermic and driven exclusively by
entropy.
In the potentiometric titrations, the proton concentration
in the cup was determined from the measured electromotive
force (EMF). Titration experiments were conducted in the
acidic regions, and the EMF can be expressed using eqn (4).
RT
F
E ¼ E° þ
ln½Hþꢁ þ γH½Hþꢁ
ð4Þ
Single-crystal structural data of the novel complexes
[UO2(PAZ)2H2O]·H2O, [UO2(PDZ)2H2O], and [UO2(PDZ)3Na2ClO4]·
where R is the gas constant, F is the Faraday constant, and T is
the temperature in kelvin. The last term is the electrode junc-
tion potential for the hydrogen ion (ΔEj,H+), assumed to be
proportional to the concentration of the proton. Prior to each
titration, an acid/base titration with standard perchloric acid
and sodium hydroxide was performed to obtain the electrode
parameters E° and γH. These parameters allow the calculation
of proton concentrations from the EMF in the subsequent
titration. Corrections for the electrode junction potential of
the hydroxide ion were not necessary in these experiments.
In a typical titration, the EMF data were collected at time
intervals determined by the data collection criterion, that is,
the drift of EMF (ΔE) was less than 0.1 mV for 180 s. Forty to
seventy data points were collected in each titration. Multiple
titrations were performed with different initial concentrations
of solutes (L−/H+ in the protonation titrations, and U(VI)/H+ for
the U(VI)/PAZ complexation titrations) and the same titrant
(NaOH for the protonation titrations, and L− for the U(VI)/PAZ
complexation titrations). The titration data were analyzed to
obtain the stability constants of the U(VI)/PAZ complexes or the
protonation constants of ligands by the program Hyperquad.42
2H2O, and
a previously reported dimeric 1 : 2 complex,
[(UO2)2L4(H2O)2]·2H2O, indicate that different coordination
modes of κ2(N,O) and μ2-L-κ2(O:O′) exist in the U(VI) complexes
in the solid state. Spectrophotometric and potentiometric titra-
tions, NMR analysis of the U(VI)/PDZ solutions, and the
thermodynamic comparisons with a series of related ligands
suggest that in solution, there is no stable dimeric 1 : 2 U(VI)/L
complex, and κ2(N,O) is the dominant coordination mode in
the 1 : 1 and 1 : 2 U(VI)/L complexes.
5. Experimental section
5.1 Chemicals
All chemicals are reagent grade or higher, and used without
further purification. Boiled/cooled Milli-Q water was used in
the preparations of all solutions. The stock solution of U(VI) in
perchloric acid was prepared by dissolving U3O8 in 2 mol dm−3
HNO3 (prepared from 70% HNO3, Sigma-Aldrich), followed by
precipitation with NH4OH (Sigma-Aldrich). Then the precipi-
tate was washed with water to pH 7–8 and then dissolved in
0.2 mol dm−3 HClO4 (from 70% HClO4, Sigma-Aldrich). The
5.3 Spectrophotometry
concentration of U(VI) and the acidity in the stock solution Spectrophotometric titrations were performed to determine
were determined, respectively, by fluorimetry40 using standard the stability constants of the U(VI)/L complexes. Absorption
solutions of U(VI) in 1 mol dm−3 H3PO4 and by the Gran spectra of U(VI) from 380 to 500 nm (0.5 nm interval) were col-
titration.41
lected on a spectrophotometer (Lambda-650, PerkinElmer,
574 | Dalton Trans., 2019, 48, 566–577
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USA). Suprasil quartz cuvettes of 1 cm path length were used recorded by averaging ∼14 000 scans for each spectrum with
and maintained at (298.2 0.1) K by circulating water from a continuous decoupling of the protons using WALTZ16 decou-
constant temperature bath through the jackets of the sample pling. All spectra were referenced with TMS (tetramethyl
holders and their lids. In a typical titration, appropriate ali- silane).
quots of the titrant solution were successively added into the
sample cuvette by an injection pump (Havard 11EX) driven by
5.5 Microcalorimetry
a computer program. Usually 15–20 additions were made, thus
generating a set of 16–21 spectra in each titration. Multiple
titrations were performed with different initial concentrations
of U(VI). The stability constants of complexes were calculated
by non-linear least-squares regression using the HypSpec
program.42
Calorimetric titrations were conducted at 298.2 K with an iso-
thermal microcalorimeter (TAM III, TA Instruments-Waters
LLC, USA) to determine the enthalpy of complexation. The per-
formance of the calorimeter was tested by measuring the
enthalpy of protonation of tris(hydroxymethyl)-aminomethane
(THAM). The value, (−47.7 0.3) kJ mol−1, was obtained at
298 K and is in excellent agreement with the literature.44
0.750 mL of a solution containing U(VI)/HClO4 was placed in
5.4 NMR spectroscopy
5.4.1 Preparation of solutions for NMR analysis. 10.0 mg the calorimetric cell, and titrated with a solution of PDZ/
pyridazine-3-carboxylic acid was put in a 2.0 mL glass bottle, HClO4. Multiple titrations with different concentrations of the
and certain volumes of 1.0 mol dm−3 HClO4 (or 1.0 mol dm−3 reagents were performed to reduce the uncertainty. In a typical
NaOH) and uranium stock solution ([U] = 0.259 mol dm−3
,
titration, n additions of 0.005 mL titrant were made (n = 40–50)
[H] = 0.120 mol dm−3) were transferred into the bottle, and through a 0.250 mL syringe, resulting in n experimental values
then D2O was added to bring the volume to 1.0 mL. Detailed of the heat generated in the titration cell (Qex,j, j = 1 − n).
conditions are provided in Table S3 of the ESI.†
These values were corrected for the heats of titrant dilution
5.4.2 NMR experiments. 1H- and 13C-NMR data were col- (Qdil,j) that were measured in a separate run. The net reaction
lected on a Bruker Ascend 600 Spectrometer. The spectrometer heat at the jth point (Qr,j) was obtained from the difference: Qr,j
was operated at 600.17 and 150.91 MHz for the measurements = Qex,j − Qdil,j. The value of Qr,j is a function of the concen-
of 1H and 13C signals, respectively. NMR spectra were collected trations of the reactants (CM and CL), the equilibrium con-
at a calibrated probe temperature of 298.2 K using the method stants, and the enthalpies of the reactions that occurred in the
described in the literature.43 1H NMR spectra were recorded by titration. A least-squares minimization program, HypDeltaH,24
averaging 16 scans for each spectrum. 13C NMR spectra were was used to calculate the reaction enthalpies (ΔH). The corres-
Table 4 Crystallographic data and structure refinement for the U(VI)/PDZ complexes
(I)
(II)
(III)
(IV)
Empirical formula
Formula weight
Temperature
C10H8N4O7U, H2O
730.19
293
C10H8N4O7U
534.23
293
C15H13ClN6Na2O14
820.77
293
0.71073
U
C10H10N4O8U
552.25
293
Wavelength
0.71073
0.71073
0.71073
Crystal system
Space group
Hall group
Triclinic
P1
–p1
Orthorhombic
Pna 21
P2C–2n
Monoclinic
P12/n1
–p2yac
Monoclinic
C12/C1
–C2yc
ˉ
Unit cell dimensions (Å)
a = 7.8240(5)
b = 8.0617(6)
c = 11.9538(7)
α = 87.484°
β = 75.764°
γ = 88.380°
a/b = 0.9705
b/c = 0.6744
c/a = 1.5278
730.01(9)
1
a = 26.1101(17)
b = 7.7452(5)
c = 6.8023(5)
α = 90°
β = 90°
γ = 90°
a/b = 3.3711
b/c = 1.1386
c/a = 0.2605
1375.61(16)
4
2.580
11.842
a = 15.1242(3)
b = 7.0651(1)
c = 23.2973(5)
α = 90°
β = 104.600(2)°
γ = 90°
a/b = 2.1407
b/c = 0.3033
c/a = 1.5404
2409.02(8)
4
a = 25.7342(9)
b = 6.8614(3)
c = 16.7222(6)
α = 90°
β = 96.750(3)°
γ = 90°
a/b = 3.7506
b/c = 0.4103
c/a = 0.6498
2932.2(2)
8
Cell ratio
Volume (Å3)
Z
Calculated density (g cm−3
)
2.512
11.167
508.0
2.263
6.968
1552.0
2.502
11.121
2032.0
Mu (mm−1
F(000)
)
976.0
h, k, lmax
Nref
Tmin, Tmax
Data completeness
Theta (max)
R (reflections)
wR2 (reflections)
S
9, 10, 14
2982
0.195, 1.000
0.997
32, 9, 8
2244
18, 8, 29
4931
0.604, 1.000
0.997
32, 8, 20
3000
0.624, 1.000
0.999
0.373, 1.000
1.46/0.80
26.372
0.0437(1862)
0.0960(2244)
1.102
26.370
26.373
26.370
0.0400(2676)
0.0806(2982)
1.043
0.0334(4089)
0.0768(4917)
1.063
0.0233(2524)
0.426(3000)
0.996
Npar
217
128
363
220
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ponding entropies of complexation (ΔS) were calculated from
the expression ΔG = ΔH − TΔS, where ΔG = −RT ln β.
References
1 M. Y. Alyapyshev, V. A. Babain and L. I. Tkachenko,
Radiochemistry, 2014, 56, 565–574.
5.6 Single-crystal X-ray diffractometry
2 R. Taylor, M. Carrott, H. Galan, A. Geist, X. Heres,
C. Maher, C. Mason, R. Malmbeck, M. Miguirditchian,
G. Modolo, C. Rhodes, M. Sarsfield and A. Wilden, Procedia
Chem., 2016, 21, 524–529.
3 C. L. Xiao, Q. Y. Wu, C. Z. Wang, Y. L. Zhao, Z. F. Chai and
W. Q. Shi, Inorg. Chem., 2014, 53, 10846–10853.
4 M. J. Hudson, L. M. Harwood, D. M. Laventine and
F. W. Lewis, Inorg. Chem., 2013, 52, 3414–3428.
5 D. Manna, S. Mula, A. Bhattacharyya, S. Chattopadhyay and
T. K. Ghanty, Dalton Trans., 2015, 44, 1332–1340.
6 Y. Yang, Z. Zhang, S. Luo and L. Rao, Eur. J. Inorg. Chem.,
2014, 2014, 6.
7 C. L. Xiao, C. Z. Wang, L. Y. Yuan, B. Li, H. He, S. Wang,
Y. L. Zhao, Z. F. Chai and W. Q. Shi, Inorg. Chem., 2014, 53,
1712–1720.
8 M. Y. Alyapyshev, V. A. Babain and L. I. Tkachenko,
Radiochemistry, 2014, 56, 565–574.
9 M. Y. Alyapyshev, V. A. Babain, L. I. Tkachenko,
A. Paulenova, A. A. Popova and N. E. Borisova, Solvent Extr.
Ion Exch., 2014, 32, 138–152.
10 M. Galletta, S. Scaravaggi, E. Macerata, A. Famulari,
A. Mele, W. Panzeri, F. Sansone, A. Casnati and M. Mariani,
Dalton Trans., 2013, 422, 16930–16938.
11 B. A. Michael and C. L. Cahill, Chem. Rev., 2013, 113, 1121–
1136.
12 M. D. Ogden, S. I. Sinkov, M. Nilson, G. J. Lumetta,
R. D. Hancock and K. L. Nash, J. Solution Chem., 2012, 42,
211–225.
5.6.1 Crystallization. [UO2(PAZ)2(H2O)]·H2O (I): 12.4 mg of
pyrazine-2-carboxylic acid were dissolved in 1.0 mL water in a
4 mL glass sample bottle with a cap, and then 192 μL U(VI)
solutions ([U] = 0.22 M, [HClO4] = 0.12 M) were added into it.
After refluxing for 1 hour on an electric hot plate at 150°, the
mixture solutions were put aside at ambient temperature.
Green crystals were obtained 3 days later. [UO2(PDZ)2(H2O)]
(II): 1.00 mL of an aqueous solution containing 40.0 mmol
U(VI), 80.7 mmol sodium pyridazine-3-carboxylate, and
99.3 mmol HClO4 was refluxed for 2 h on an electric hot plate,
and then evaporated at room temperature. Yellowish green
crystals appeared in about one day. [UO2(PDZ)3Na2ClO4]·2H2O
(III): 2.0 mL of a 1.0 M NaClO4 solution containing 0.01 mmol
U(VI), 0.31 mmol sodium pyridazine-3-carboxylate, and
0.07 mmol HClO4 was evaporated at room temperature.
Yellowish green crystals were deposited in about one month.
[(UO2)2(PDZ)4(H2O)2]·2H2O (IV): 0.61 mL of an aqueous solu-
tion containing 11 mmol U(VI), 24 mmol sodium pyridazine-3-
carboxylate, and 6 mmol HClO4 was evaporated at room temp-
erature. Yellowish green crystals appeared in about one day.
5.6.2 X-ray diffraction. Representative crystals were
mounted on a goniometer and crystallographic data were col-
lected on an Xcalibur E X-ray single-crystal diffractometer at
293 K. The XRD data indicate that compound I is a mono-
nuclear 1 : 2 U(VI)/PAZ complex, [UO2(PAZ)2(H2O)]·H2O, com-
pound II is
a
mononuclear 1 : 2 U(VI)/PDZ complex,
[UO2(PDZ)2(H2O)], compound III is a 1 : 3 : 2 U(VI)/PDZ/Na
complex, [UO2(PDZ)3Na2ClO4]·2H2O, and compound IV is a
dinuclear 1 : 2 U(VI)/PDZ complex, [(UO2)2(PDZ)4(H2O)2]·2H2O,
with a structure identical to that in the literature.21 Detailed
crystallographic data and structural refinement for the U(VI)
complexes I, II, III, and IV are given in Table 4, and details of
their structural information have been deposited with the
Cambridge Structural Database (CCDC 1586814, 1865674,
1586812, and 1865676†).20
13 D. O. Kirsanov, N. E. Borisova, M. D. Reshetova,
A. V. Ivanov, L. A. Korotkov, I. I. Eliseev, M. Y. Alyapyshev,
I. G. Spiridonov, A. V. Legin, Y. G. Vlasov and V. A. Babain,
Russ. Chem. Bull. Int. Ed., 2012, 61, 10.
14 N. J. Williams, D. G. Ballance, J. H. Reibenspies and
R. D. Hancock, Inorg. Chim. Acta, 2010, 363, 3694–3699.
15 M. Alyapyshev, V. Babain, N. Borisova, I. Eliseev,
D. Kirsanov, A. Kostin, A. Legin, M. Reshetova and
Z. Smirnova, Polyhedron, 2010, 29, 8.
16 Y. Yang, J. Liu, Y. Sun, S. Hu, Z. Zhang, S. Luo and L. Rao,
J. Chem. Thermodyn., 2017, 113, 350–357.
17 R. C. Severance, S. A. Vaughn, M. D. Smith and H.-C.
z. Loye, Solid State Sci., 2011, 13, 1344–1353.
Conflicts of interest
There are no conflicts to declare.
18 J. Qiu, B. Vlaisavljevich, L. Jouffret, K. Nguyen,
J. E. S. Szymanowski, L. Gagliardi and P. C. Burns, Inorg.
Chem., 2015, 54, 4445.
19 N. W. Alcock, T. J. Kemp, S. M. Roe and J. Leciejewicz,
Inorg. Chim. Acta, 1996, 248, 241–246.
Acknowledgements
This publication is based on the work supported by the
National Science Foundation of China (No. 11675156, 20 Cambridge Crystallographic Data Centre, 12 Union Road,
91326110, 21790372, and 21822606). The potentiometric and Cambridge CB2 1EZ, U.K.
spectroscopic titrations and the single-crystal X-ray diffraction 21 J. Leciejewicz and W. Starosta, Acta Crystallogr., Sect. E:
data were obtained and analyzed at the Institute of Nuclear Crystallogr. Commun., 2009, 65, m94.
Physics and Chemistry, CAEP. The microcalorimetric titrations 22 J. A. Joule and K. Mills, Heterocyclic Chemistry, John Wiley &
were conducted at Tsinghua University.
Sons, Chichester, West Sussex, United Kingdom, 2010.
576 | Dalton Trans., 2019, 48, 566–577
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View Article Online
Dalton Transactions
Paper
23 A. E. Martell, R. M. Smith and R. J. Motekaitis, NIST 34 J. Jiang, L. Rao, P. Di Bernardo, P. Zanonato and
Critically Selected Stability Constants of Metal Complexes,
2003.
24 P. Gans, A. Sabatini and A. Vacca, J. Solution Chem., 2008,
37, 467–476.
A. Bismondo, Dalton Trans., 2002, 1832–1838.
35 V. G. Dongre, D. M. Janrao and V. W. Kamble, Asian
J. Chem., 1998, 10, 730–734.
36 M. Nourmand and N. Meissami, J. Chem. Soc., Dalton
Trans, 1983, 1529–1533.
25 V. I. Mishkevich, M. S. Grigoriev, A. M. Fedosseev and
P. Moisy, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2012, 37 M. Nourmand and N. Meissami, Polyhedron, 1982, 1, 537–
68, m1243. 539.
26 P. C. Burns, R. C. Ewing and F. C. Hawthorne, Can. 38 Z. Zhang, G. Helms, S. B. Clark, G. Tian, P. Zanonato and
Mineral., 1997, 35, 1551–1570. L. Rao, Inorg. Chem., 2009, 48, 3814–3824.
27 T. Loiseau, I. Mihalcea, N. Henry and C. Volkringer, Coord. 39 P. Di Bernardo, P. Zanonato, F. Benetollo, A. Melchior,
Chem. Rev., 2014, 266–267, 69–109.
28 A. Bismondo, U. Casellato and R. Graziani, Inorg. Chim.
Acta, 1994, 233, 3.
29 Y. Yang, S. J. Teat, Z. Zhang, S. Luo and L. Rao, Dalton
Trans., 2016, 45, 384–391.
M. Tolazzi and L. Rao, Inorg. Chem., 2012, 51, 9045–
9055.
40 C. W. Sill and H. E. Peterson, Anal. Chem., 1947, 19, 646–
651.
41 G. Gran, Analyst, 1952, 661–671.
30 S. Ahrland, Coord. Chem. Rev., 1972, 8, 21–29.
31 A. Bismondo, L. Rizzo, G. Tomat, D. Curto, P. Di Bernardo
and A. Cassol, Inorg. Chim. Acta, 1983, 74, 21–24.
32 A. Bismondo, L. Rizzo, P. Di Bernardo and P. Zanonato,
J. Chem. Soc., Dalton Trans, 1987, 695–698.
33 A. Bismondo, U. Casellato and L. Rizzo, Inorg. Chim. Acta,
1992, 191, 69–73.
42 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739–
1753.
43 D. S. Ralford, C. L. Fisk and E. D. Becker, Anal. Chem.,
1979, 51, 2050–2051.
44 J. J. Christensen, L. D. Hansen and R. M. Izatt, Handbook of
Proton Ionization Heats and Related Thermodynamic
Quantities, John Wiley and Sons, New York, 1976.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 566–577 | 577