Uranyl(VI) binding by bis(2-hydroxyaryl)diimine and bis(2-hydroxyaryl)diamine ligand derivatives. Synthetic, X-ray, DFT and solvent extraction studies

Article abstract of DOI:10.1016/j.poly.2015.01.005

The interaction of uranyl(VI) nitrate with a series of bis(2-hydroxyaryl)imine (H₂L¹-H₂L⁵) and bis(2-hydroxyaryl)amine (H₂L⁸, H₂L⁹) derivatives incorporating 1,3-dimet

Full text of DOI:10.1016/j.poly.2015.01.005

Polyhedron xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Uranyl(VI) binding by bis(2-hydroxyaryl)diimine
and bis(2-hydroxyaryl)diamine ligand derivatives.
Synthetic, X-ray, DFT and solvent extraction studies
Harold B. Tanh Jeazet a, Kerstin Gloe a, Thomas Doert a, Jens Mizera a, Olga N. Kataeva b, Satoru Tsushima c,
Gert Bernhard c, Jan J. Weigand a, , Leonard F. Lindoy d, , Karsten Gloe a,
⇑
⇑
⇑
a Department of Chemistry and Food Chemistry, Technical University Dresden, 01062 Dresden, Germany
b A.E. Arbuzov Institute of Organic and Physical Chemistry, 420088 Kazan, Russia
c Institute of Resource Ecology, Helmholtz Centre Dresden-Rossendorf, 01314 Dresden, Germany
d School of Chemistry, The University of Sydney, NSW 2006, Australia
a r t i c l e i n f o
a b s t r a c t
The interaction of uranyl(VI) nitrate with a series of bis(2-hydroxyaryl)imine (H2L1–H2L5) and bis(2-
hydroxyaryl)amine (H2L8, H2L9) derivatives incorporating 1,3-dimethylenebenzene or 1,3-dimethylene-
cyclohexane bridges between nitrogen sites is reported. Crystalline complexes of type [UO2(H2L)(NO3)2]
(where H2L is H2L1–H2L4) were isolated from methanol. X-ray structures of the complexes of H2L1, H2L2
Article history:
Received 4 December 2014
Accepted 7 January 2015
Available online xxxx
and H2L4 show that each of these neutral ligands bind to their respective UO2 centres in a bidentate
2þ
Dedicated to Catherine Housecroft on the
occasion of her 60th birthday.
fashion in which coordination only occurs via each ligand’s hydroxy functions. Two bidentate nitrate
anions complete the metal’s coordination sphere in each complex to yield hexagonal bipyramidal coor-
dination geometries. A density functional theory (DFT) investigation of [UO2(H2L1)(NO3)2] in a simulated
methanol environment is in accord with this complex maintaining its solid state conformation in solu-
tion. Solvent extraction experiments (water/chloroform) employing H2L1–H2L7 in the organic phase
and uranyl(VI) nitrate in the aqueous phase showed that both amine derivatives, H2L8 and H2L9, yielded
Keywords:
Uranyl(VI)
Schiff base
Europium(III)
Solvent extraction
Density functional theory
enhanced extraction of UO2 over the corresponding imine derivatives, H2L1 and H2L2. These results
2þ
were further compared with those obtained for the corresponding Schiff bases incorporating 1,2-pheny-
lene and 1,2-cyclohexane bridged ligands, H2L6 and H2L7; these more rigid systems also yielded enhanced
extraction of UO2 relative to the more flexible Schiff bases H2L1–H2L5. A very significant synergistic
2þ
1
enhancement of the extraction of UO2 by H2L –H2L4 and H2L7 was observed in the presence of a 10-fold
2þ
excess of n-octanoic acid; the influence of pH on extraction efficiency was also investigated. A parallel set
of experiments employing H2L1–H2L9 as extractants for europium(III) nitrate indicated a clear uptake
preference for UO2 over Eu3+ in all cases; separation of the uranyl ion from the rare earths is an
2þ
important objective in mineral processing.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
[7–16]. As a consequence, a number of studies have focused on
ligand design for selective uranium uptake [17–20], with particu-
The coordination chemistry of uranyl(VI) has received increas-
ing attention over recent years [1,2]. Many such studies [3–6] were
motivated by the awareness that an enhanced understanding of
the complexation behaviour of this ion has implications for the
winning, processing and use of uranium as well as for the
appropriate control, processing and storage of nuclear wastes
lar studies focused on the separation of actinides from the
lanthanides [21–25] – metals which occur together in nature and
nuclear wastes. However, in general, such separations are
inherently challenging due to the generally similar chemistry of
these ions.
In the above context it is noted that a number of Schiff base
ligands have been employed for uranyl extraction [23,26,27]. For
example, H2L6 (salophen), has been shown to form robust neutral
1:1 uranyl chelate complexes of composition [UO2(L6)S], each
incorporating a solvent molecule (S = DMF, DMSO, H2O) [28,29].
Ligand species of this type incorporating a short spacer group
⇑
Corresponding authors. Tel.: +61 (0)2 9351 4400 (L.F. Lindoy). Tel.: +49 (0)351
463 34357 (K. Gloe).
E-mail addresses: Len.lindoy@sydney.edu.au (L.F. Lindoy), karsten.gloe@chemie.
tu-dresden.de (K. Gloe).
http://dx.doi.org/10.1016/j.poly.2015.01.005
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
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2
Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
(in the above case a 1,2-phenylene group) as well as structure-
related species have been shown to be extracted with high
efficiency into an organic phase [30]. However, bis(2-hydroxy-
aryl)imines, where the two terminal chelating domains are linked
by extended spacer groups, have received considerably less atten-
tion even though individual examples of the latter ligands have
been known for a considerable time [31].
and (H2L7) (from ( )-trans-1,2-diaminocyclohexane and salicylal-
dehyde) [36,37] were carried out by the respective literature
procedures.
2.2. General procedure for the synthesis of Schiff base ligands H2L2–
H2L4
We now report synthetic, X-ray, computational and solvent
extraction studies involving the interaction of uranyl(VI) nitrate
with the bis(2-hydroxyaryl)diimine derivatives (H2L1–H2L5) and
bis(2-hydroxyaryl)diamine derivatives (H2L8 and H2L9), incorpo-
rating 1,3-dimethylenebenzene or 1,3-dimethylenecyclohexane
units as linking backbones. The results are compared with those
for the related Schiff base (H2L6, H2L7) ligand derivatives. Compar-
ative results for the extraction of europium(III) with H2L1–H2L9 are
also presented.
A methanol solution of the required diamine and aldehyde in a
1:2 M ratio was heated under reflux for ꢀ4 h, on cooling, to yield a
yellow solid in each case which was removed by filtration. Crude
H2L3 was dissolved in CHCl3, washed with distilled water, and
the organic solvent was dried with anhydrous MgSO4 and subse-
quently removed to obtain the pure compound. All products were
washed with cold methanol, and dried under vacuum. Character-
isation details are given below.
N
N
N
N
N
N
OH HO
OH HO
OH HO
H2L3
H2L2
H2L1
N
N
N
N
N
N
OH HO
HO
OH
OH HO
H2L6
H2L5
H2L4
NH
HN
NH
HN
N
N
OH HO
H2L7
OH HO
OH HO
H2L8
H2L9
2.3. Characterisation of Schiff base ligands H2L1–H2L4
a0-Bis(salicylimino)-m-xylene (H2L1)
2. Experimental
2.3.1. a,
2.1. Materials and instrumentation
From m-xylylenediamine and salicylaldehyde. Yield = 74%, 1H
NMR (500 MHz, DMSO-d6, 25 °C, TMS): d = 13.42 (s, 2H, OHꢁ ꢁ ꢁN),
8.72 (s, 2H, CH@N), 7.48 (dd, J = 8.6, 8.7 Hz, 2H, C6H4), 7.38 (t,
J = 7.7, 7.5 Hz, 1H, C6H4), 7.35 (s, 1H, C6H4), 7.33 and 6.91 (t,
J = 11.9, 8.2, 7.5 Hz, 4H, C6H4O), 7.27 (d, J = 7.6 Hz, 2H, C6H4O),
6.87 (d, J = 8.2 Hz, 2H, C6H4O), 4.82 (s, 4H, CH2). ESI-MS (MeOH):
All reagent and solvents were obtained from commercial
sources and used without further purification. The 1H and 13C
NMR spectra were recorded on a Bruker Avance DRX-500 spec-
trometer with DMSO-d6 and CDCl3 as solvents. Mass spectrometry
(ESI-MS) analyses were carried out using a Bruker ESQUIRE mass
m/z 345 [M+H+]. IR (KBr pellets, cmꢂ1):
(CAH), 3054–2733w; (C@N),1633s; (CACarom), 1580–1498m.
m(OAH), 3433 m (br);
spectrometer. Infrared spectra were recorded on
a BioRad
m
m
m
Excalibur FTS 3000-Spectrometer using KBr pellets. Elemental
analysis (C, H, and N) were carried out on a Carlo Erba (EA 1108)
Analyser. UV data were collected using a Perkin Elmer type
Lambda 25 spectrophotometer in the range 200–1200 nm. The
syntheses of H2L1 (from m-xylylenediamine and salicylaldehyde)
[32,33], H2L5 (from o-aminophenol and isophthalaldehyde) [34],
H2L6 (from 1,2-phenylenediamine and salicylaldehyde) [33,35]
Anal. Calc. for C22H20N2O2: C, 76.72; H, 5.85; N, 8.13. Found: C,
76.78; H, 5.94; N, 8.20%.
2.3.2. a,
a0-Bis(2-hydroxy-1-naphthalimino)-m-xylene (H2L2)
From m-xylylenediamine and 2-hydroxy-1-naphthaldehyde.
Yield, 97%. 1H NMR (500 MHz, DMSO-d6, 25 °C, TMS): d = 14.38
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Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
3
(s, 2H, OHꢁ ꢁ ꢁN), 9.30 (d, J = 9.4 Hz, 2H, CH@N), 8.10 (d, J = 8.4 Hz,
2H, C6H4), 7.74 and 7.65 (d, J = 9.3, 7.9 Hz, 4H, C10H6O), 7.47 (s,
1H, C6H4), 7.40–7.46 (m, 3H, C6H4, C10H6O), 7.38 (d, J = 8.1 Hz,
2H, C10H6O), 7.20 (t, 2H, C10H6O), 6.72 (d, J = 9.3 Hz, 2H, C10H6O),
4.9 (d, J = 4.7 Hz, 4H, CH2). 13C NMR (500 MHz, CDCl3, ppm): 174
(CAOH), 159 (CH@N), 107–138 (phenyl, naphtyl), 58 (CH2AN).
ESI-MS (MeOH): m/z 445 [M+H]. MS(ESI+) (MeOH): 445 [M+H+],
Characterisation details are given below. Crystals of 1, 2 and 4
suitable for X-ray diffraction were obtained by slow diffusion of
diethylether into an equimolar methanol/acetonitrile (1:1) (1 mL)
solution of the complex over one week. The crystals were collected,
washed with ether, and dried under vacuum.
2.5.2. Characterisation of uranyl(VI) complexes (1)–(4)
889 [2M+H+]. IR (KBr pellets, cmꢂ1):
(CAH), 3054–2870w (sh); (C@N),1632s;
m
(OAH), 3434m (br);
(CACarom 1544–
2.5.2.1. [UO2(H2L1)(NO3)2] (1). Yield, 89%. MS(ESI+) (MeOH): 613
[UO2(H2L1) ꢂ H+], 676 [UO2(L1)(NO3)]+. UV/Vis (MeOH): 324
m
m
m
)
(e = 17000), 357 (e
= 18600) nm. 1H NMR (500 MHz, DMSO-d6,
1492w. Anal. Calc. for C30H24N2O2: C, 81.06; H, 5.44; N, 6.30.
Found: C, 80.78; H, 5.53; N, 6.38%.
25 °C, TMS): d [ppm] = 13.01 (s, 2H, OHꢁ ꢁ ꢁN), 8.83 (s, 2H, CH@N),
7.48 (dd, J = 7.6 Hz, 2H, C6H4), 7.39 (t, J = 7.6, 7.5 Hz, 1H, C6H4),
7.35 (s, 1H, C6H4), 7.33 and 6.91 (t, J = 11.2, 7.5 Hz, 4H, C6H4O),
7.28 (d, J = 7.7 Hz, 2H, C6H4O), 6.87 (d, J = 8.3 Hz, 2H, C6H4O), 4.82
(s, 2H, CH2). IR (KBr): 3433m, 3158m, 3066w, 2427w, 1651s,
921s cmꢂ1. Anal. Calc. for C22H20N4O10U: C, 35.78; H, 2.73; N,
7.59. Found: C, 35.38; H, 3.00; N, 7.50%.
2.3.3. a,
a0-Bis(salicyliminomethyl)-1,3-cyclohexane (cis/trans) (H2L3)
From 1,3-bis(aminomethyl)cyclohexane and salicylaldehyde.
Yield, 91%. MS(ESI+) (MeOH): 351 [M+H+]. IR (KBr pellets, cmꢂ1):
m
m
(OAH), 3434m (br);
(CACarom) 1583–1463m. Anal. Calc. for C22H26N2O2: C, 75.40; H,
m(CAH), 3054–2731m (m(C@N) 1633s;
7.48; N, 7.99. Found: C, 75.18; H, 7.52; N, 7.98%.
2.5.2.2. [UO2(H2L2)(NO3)2] (2). Yield, 94%. MS(ESI+) (MeOH): 713
2.3.4.
(H2L4)
a,
a0-Bis(2-hydroxy-1-naphthaliminomethyl)-1,3-cyclohexane
[UO2(H2L2)
–
H+]. UV–Vis (MeOH): 310
(
e
= 13200), 334
(e = 13200), 398 (e = 14800), 420 (e
= 12000) nm. 1H NMR
From 1,3-bis(aminomethyl)cyclohexane and 2-hydroxy-1-
(500 MHz, DMSO-d6, 25 °C, TMS): d [ppm] = 13.99 (s, 2H, OHꢁ ꢁ ꢁN),
9.39 (d, J = 14.9 Hz, 2H, CH@N), 8.15 (dd, J = 11.5, 12 Hz, 2H, C6H4),
7.82 and 7.70 (d, J = 9.6, 7.3 Hz, 4H, C10H6O), 7.40–7.48 (m, 4H,
C6H4, C10H6O), 7.38 (d, J = 8.6 Hz, 2H, C10H6O), 7.25 (t, 2H,
naphthaldehyde. Yield, 95%. MS(ESI+) (MeOH): 451 [M+H+]. IR
(KBr pellets, cmꢂ1):
(sh); ( (C@N) 1630s;
30H30N2O2 C, 79.97; H, 6.71; N, 6.22. Found: C, 80.05; H, 7.18;
m(OAH), 3431m (br);
m(CAH), 3054–2852m
m
m
(CACarom) 1544–1449m. Anal. Calc. for
C
C
10H6O), 6.81 (d, J = 7.3 Hz, 2H, C10H6O), 4.94 (d, J = 9.7 Hz, 4H,
N, 6.25%
CH2). IR (KBr): 3430m, 3067w, 2926w, 2427w, 1638s, 921m cmꢂ1
.
Anal. Calc. for C30H24N4O10U: C, 42.97; H, 2.88; N, 6.68. Found: C,
42.52; H, 2.98; N, 6.97%.
2.4. Synthesis of amine ligands H2L8 and H2L9
2.4.1. a,
a0-Bis(salicylamino)-m-xylene (H2L8)
2.5.2.3. [UO2(H2L3)(NO3)2] (3). Yield, 76%. MS(ESI+) (MeOH): 619
Slow addition of KBH4 (0.44 g, 8.2 mmol) to a stirred solution of
H2L1 (2.13 g, 6.2 mmol) in methanol (20 mL) led to isolation of
crude H2L8 as an oily material which was dissolved in a small
volume of chloroform and the solution shaken with water. The
chloroform phase was separated, dried over anhydrous MgSO4,
then the solvent removed to yield the product as a viscous
yellow–brown oil (1.50 g, 70%). 1H NMR (500 MHz, CDCl3, 25 °C,
TMS): d = 7.32 (t, J = 7.3 Hz, 1H, C6H4), 7.24 (s, 2H, OH), 7.23 (s,
1H, C6H4), 7.22 (d, J = 7.5 Hz, 2H, C6H4O), 7.16 (t, J = 1.6, 1.1 Hz,
2H, C6H4O), 6.97 (d, J = 1.1 Hz 2H, C6H4O), 6.84 (d, J = 0.9 Hz, 2H,
C6H4), 6.77 (t, J = 1.1, 1.0 Hz, 2H, C6H4O), 5.28 (s, 2H, NH), 4.00 (s,
4H, CH2AN), 3.80 (s, 4H, CH2)MS(ESI+) (MeOH): 349 [M+H+]. Anal.
Calc. for C22H24N2O2: C, 75.83; H, 6.94; N, 8.04. Found: C, 75.90; H,
6.98; N, 8.15%.
[UO2(H2L3)
–
H+], 679 [UO2(L3)(NO3)]+ UV–Vis (MeOH): 322
(e = 4000), 376 (e = 5200) nm. IR (KBr): 3433m, 3062w, 2925m,
2856w, 2427w, 1654s, 920s cm–1. Anal. Calc. for C22H26N4O10U: C,
35.49; H, 3.52; N, 7.53. Found: C, 35.05; H, 3.63; N, 7.57%.
2.5.2.4. [UO2(H2L4)(NO3)2] (4). Yield, 90%. MS(ESI+) (MeOH): 719
[UO2(H2L4) – H+], 782 [UO2(L13)(NO3)]+. UV–Vis (MeOH): 338
(e = 15400), 395 (e = 13600), 418 (e = 10600) nm. IR (KBr):
3428m, 3065w, 2926w, 2853w, 2427w, 1640s, 921s cmꢂ1; Anal.
Calc. for C30H30N4O10U: C, 42.66; H, 3.58; N, 6.63. Found: C,
42.67; H, 3.76; N, 6.60%.
2.6. X-ray data collection and structure solution
2.4.2.
a,
a0-Bis(2-hydroxy-1-naphthalamino)-m-xylene (H2L9)
X-ray diffraction data for [UO2(H2L1)(NO3)2] (1) was collected
Using a similar procedure to that employed for H2L8 but starting
with H2L2 gave H2L9 as a viscous yellow–brown oil (1.65 g, 85%). 1H
NMR (500 MHz, CDCl3, 25 °C, TMS): d = 7.62 (t, J = 8.3 Hz, 1H, C6H4),
7.54 (s, 2H, OH), 7.53 (s, 1H, C6H4), 7.51 (d, J = 6.5 Hz, 2H, C10H6O),
7.46 (d, J = 7.5 Hz, 2H, C10H6O), 7.32 (t, J = 1.3 Hz 2H, C10H6O), 7.23
(d, J = 0.9 Hz, 2H, C10H6O), 7.20 (t, J = 1.8 Hz, 2H, C10H6O), 7.12 (d,
J = 6.2 Hz, 2H, C10H6O), 5.28 (s, 2H, HNH), 4.72 (s, 4H, CH2AN), 3.9
(s, 4H, CH2). MS(ESI+) (MeOH): 449 [M+H+]. Anal. Calc. for
on a Nonius Kappa CCD with x and u scans at 293(2) K. Data col-
lections were undertaken with COLLECT [38], cell refinement with
Dirax/lsq [39], and data reduction with EvalCCD [40]. Data for
structures [UO2(H2L2)(NO3)2] (2) and [UO2(H2L4)(NO3)2] (4) were
collected at 160(2) and 198(2) K respectively using a Bruker AXS
Kappa APEX II CCD diffractometer with an Oxford Cryosystems
coldhead attached. Data integration and reduction were under-
taken with SAINT and APEX2 [41–43]. Each structure was solved by
direct methods using SHELXS-97 [43] employing graphite-mono-
C27H24N2O2: C, 80.33; H, 6.29; N, 6.25. Found: C, 80.10; H, 6.37;
N, 6.31%.
chromated Mo Ka radiation (0.71073 Å) generated from a sealed
tube. Multi-scan empirical absorption corrections were applied to
all data sets using SADABS [44]. All structures were refined and
extended with SHELXL-97 [45]. In general, ordered non-hydrogen
atoms with occupancies greater than 0.5 were refined anisotropi-
cally. Partial occupancy carbon, nitrogen and oxygen atoms were
refined isotropically. Carbon-bound hydrogen atoms were
included in idealised positions and refined using a riding model.
Oxygen and nitrogen bound hydrogen atoms that were structurally
evident in the difference Fourier map were included and refined
2.5. Synthesis of uranyl(VI) complexes
2.5.1. General procedure for the synthesis of 1–4
Complexes 1–4 of type [UO2(H2L)(NO3)2] (H2L = H2L1–H2L4)
were synthesized by heating the required Schiff base ligand
.
(1 mmol) and UO2(NO3)2 6H2O (1 mmol) at the reflux temperature
for 12 h in methanol. The resulting orange–red precipitates were
filtered, washed with cold methanol, and dried under vacuum.
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4
Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
Table 1
Crystal data and structure refinement details of 1, 2, and 4.
Compound
[UO2(H2L1)(NO3)2] (1)
[UO2(H2L2)(NO3)2] (2)
C30H24N4O10
[UO2(H2L4)(NO3)2] (4)
Molecular formula
Mr
C
22H20N4O10
U
U
C30H30N4O10
U
738.45
838.56
844.61
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
monoclinic
C2/c
31.190 (6)
11.082 (2)
17.951 (4)
90
113.32 (3)
90
5698 (2)
1.955
triclinic
P1
ꢀ
ꢀ
9.471 (1)
9.510 (1)
15.251 (2)
72.17 (1)
89.36 (1)
70.70 (1)
1228 (3)
1.997
11.107 (1)
11.424 (3)
13.060 (2)
112.12 (2)
90.058 (10)
101.066 (10)
1501.7 (5)
1.859
a
(°)
b (°)
c
(°)
V (Å3)
Dcalc (g cmꢂ3)
Z
2
8
2
Crystal size (mm)
Crystal colour
Crystal habit
T (K)
0.38 ꢃ 0.28 ꢃ 0.19
orange–red
polyhedron
293(2)
0.14 ꢃ 0.11 ꢃ 0.03
orange–red
polyhedron
160(2)
0.15 ꢃ 0.07 ꢃ 0.04
orange–red
polyhedron
198(2)
k (Mo K
a
)
0.71073
6.673
0.71073
5.765
0.71073
5.47
l
(Mo Ka )
) (mmꢂ1
2hmax (°)
60
54
54
N
67317
31794
50574
Nind (Rmerge
Nobs – (I > 2
)
r
7136 (0.040)
7136
0.021
6059 (0.075)
6059
0.027
0.048
0.79 (ꢂ0.83)
6550 (0.066)
5640
0.032
0.060
1.16 (ꢂ1.08)
(I))
(I))
a
R1 – (I > 2
r
a
wR2 – (all)
0.049
Resid. Extr. (eꢂ Åꢂ3
)
1.32 (ꢂ1.23)
P
P
P
P
||Fo| ꢂ |Fc||/ |Fo| for Fo > 2r(Fo) and wR2 = { [w(F2o ꢂ Fc2)2]/ [w(Fc2)2]}1/2 where w = 1/[
r
2(F2o) + (AP)2 + BP], P = (Fo2 + 2F2c)/3 and A and B are listed in the crystal data
a
R1
=
information supplied.
with bond length and angle restraints. Crystal and structure refine-
ment data for all three structures are summarised in Table 1.
approximately 5% applies to the remaining extractions. The con-
centrations of Eu(III) were measured in both phases radiometrically
using 152Eu (ROTOP Pharmaka) by means of a NaI(TI) scintillation
counter (Cobra II/ Canberra-Packard) [51].
2.7. DFT calculations
Density functional theory (DFT) calculations were carried out to
optimize the structure of [UO2(H2L1)(NO3)2] (1) in methanol, as
well as to compare the stability of an alternative isomeric
arrangement. Geometry optimization and Gibbs energy calcula-
tions were performed at the B3LYP level using CPCM [46] with
UAHF radii [47] using the program package Gaussian 03 [48]. Small
core effective core potentials (ECPs) were used on U, O, C, and N
atoms with the corresponding basis sets [49]. For hydrogen, the
5s functions contracted to 3s were employed [50].
3. Results and discussion
3.1. Ligand and complex synthesis
All Schiff base ligands were obtained as yellow solids by the
usual procedure [52,53] of heating the required diamine and
aldehyde derivatives in a 1: 2 M ratio in methanol or ethanol.
Reduction of H2L1 and H2L2 to give H2L8 and H2L9 was achieved
by treating the respective Schiff bases in methanol with KBH4.
The uranyl(VI) complexes [UO2(H2L1)(NO3)2] (1), [UO2(H2L2)
(NO3)2] (2), [UO2(H2L3)(NO3)2] (3) and [UO2(H2L4)(NO3)2] (4) were
synthesised by heating equimolar amounts of UO2(NO3)2 and the
required ligand in methanol. Microanalytical and ESI-MS data were
consistent with the above formulation of the respective products
incorporating the non-deprotonated forms of H2L1–H2L4. Compar-
ison of the 1H NMR spectra of individual free ligands and their
complexes indicated little shift in the respective CHimine proton
signal in accord with non-coordination of the imine functions in
1–4. A small upfield shift of about 0.45 ppm for the phenolic group
in all the complexes relative to the corresponding free ligands is in
keeping with coordination of the phenolic oxygen with the UO2
centre in each case. Similarly, a strong peak at or near 1645 cmꢂ1
in their infrared spectra (versus ꢀ1632 cmꢂ1 for the free ligands)
[54] is in keeping with a significant interaction of the imine nitro-
gen with the hydrogen of the hydroxyl group; a new (weak) peak
2.8. Solvent extraction experiments
The solvent extraction experiments were performed at 23 1 °C
in microcentrifuge tubes (2 mL) with a phase ratio V(w):V(org) of 1:1
(0.5 mL each). The aqueous phase contained 1 ꢃ 10ꢂ4 M UO2(NO3)2,
and a zwitterionic buffer system (MES/NaOH at pH 5.2 or HEPES/
NaOH at pH 7.2); as a precaution, the pH of the aqueous phase
was monitored before and after each experiment with the aid
of an InLab423 pH electrode. Constant ionic strength was
maintained in the aqueous phase by the addition of NaNO3
(5 ꢃ 10ꢂ3 M). The CHCl3 phase contained a known concentration
of ligand (1 ꢃ 10ꢂ2 M) and in particular cases also n-octanoic acid
(1 ꢃ 10ꢂ3 M). The extraction experiments involved mechanical
shaking of the two-phase system for 1 h. (T = 23 1 °C) during
which time equilibrium was established (the time to reach equilib-
rium was established by preliminary experiments). The phases
at ꢀ2427 cmꢂ1 is assigned to the
(CH@Nꢁ ꢁ ꢁH) vibration. Further,
m
were then separated, centrifuged and duplicate 100
lL samples
evidence for coordination of the phenolic hydroxyl group is given
removed for analysis. The depletion of the uranyl ion concentration
in the respective aqueous phases was measured using an ICP-MS
(ELAN 9000/Perkin Elmer) spectrometer. Except for low extraction
values which are subject to higher error, an overall error of
by a peak at 1350 cmꢂ1 assigned to
m(CaromAO) for each of 1–4 –
shifted from 1385–1384 cmꢂ1 in the spectra of the corresponding
free ligands. All complexes displayed strong bands at ꢀ920 cmꢂ1
assigned to the mUO2asym stretch [55,56].
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Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
5
Fig. 1. (a) X-ray structure of [UO2(H2L1)(NO3)2] (1) showing the all-oxygen coordination sphere of the U(VI) ion. Thermal ellipsoids are shown at the 30% probability level. (b)
View illustrating the configuration adopted by the bound H2L1 and the bidentate nitrato ligands around the UO2 centre.
2þ
Fig. 2. X-ray structure of (a) [UO2(H2L2)(NO3)2] (2) with thermal ellipsoids shown at the 30% probability level and (b) [UO2(H2L4)(NO3)2] (4) with thermal ellipsoids shown at
the 50% probability level.
Fig. 3. Calculated structures of two uranyl(VI) complexes (M1, M2) of the neutral Schiff base ligand H2L1 optimized for solution in CH3OH at the B3LYP level.
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6
Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
Table 2
All three complexes represent new examples of the rare U(VI)
Selected bond lengths [Å] and angles [°] for 1, 2 and 4.
complex category incorporating bound neutral hydroxyaryl Schiff
base ligands [31]. Selected bond distances and angles are listed
in Table 2.
The crystal packing in all three products is characterised by
extended networks of weak interactions that include hydrogen
[UO2(H2L1)(NO3)2] (1)
U–O(8)
U–O(9)
U–O(2)
1.760(2)
1.763(2)
2.519(2)
2.575(2)
178.09(11)
90.19(9)
90.64(8)
49.25(7)
U–O(1)
U–O(26)
U–O(5)
U–O(6)
O(1)–U–O(26)
O(26)–U–O(8)
O(26)–U–O(9)
O(5)–U–O(6)
2.324(1)
2.311(2)
2.579(3)
2.596(3)
75.49(9)
85.91(10)
95.96(8)
48.02(11)
U–O(3)
bonds,
p–p and CH-p interactions between ligand molecules, the
O(8)–U–O(9)
O(1)–U–O(8)
O(1)–U–O(9)
O(2)–U–O(3)
[UO2(H2L2)(NO3)2] (2)
U–O(7)
U–O(8)
U–O(2)
U–O(3)
O(7)–U–O(8)
O(10)–U–O(7)
O(10)–U–O(8)
O(2)–U–O(3)
uranyl group and nitrate anions. Individual nitrate oxygen atoms
are involved in hydrogen bonds that bridge to adjacent molecules
through hydrogen bond interactions that fall in the ranges 2.44–
2.60 Å (1), 2.35–2.58 Å (2), 2.59–2.60 Å (4) (see also the Supporting
information). Also, in each structure weak intramolecular hydro-
gen bond interactions are present between one of the uranyl oxy-
gens and an adjacent aromatic CAH group; the CAHꢁ ꢁ ꢁO@U (Cꢁ ꢁ ꢁO)
distances are 3.20 Å, 3.30 Å and 3.23 Å in 1, 2 and 4 respectively. In
1.750(3)
1.739(4)
2.504(3)
2.544(3)
178.63(17)
88.69(13)
91.74(13)
50.32(11)
U–O(10)
U–O(9)
U–O(4)
U–O(5)
O(9)–U–O(10)
O(9)–U–O(7)
O(9)–U–O(8)
O(4)–U–O(5)
2.358(3)
2.345(3)
2.519(4)
2.547(4)
71.27(12)
86.77(14)
94.60(15)
50.3(11)
each structure, stacking of the complex molecules occur via
p–p
[3.64 Å(1), 3.76–3.79 Å(2), 3.61–3.79 Å(4)] and CH- [2.61–3.05 Å
p
(1), 2.70–2.92 Å (2), 2.83–2.96 Å (4)] interactions. Further struc-
tural details and packing diagrams for 1, 2 and 4 are presented in
the Supplementary data.
[UO2(H2L4)(NO3)2] (4)
U–O(3)
U–O(4)
U–O(5)
U–O(6)
O(3)–U–O(4)
O(2)–U–O(3)
O(2)–U–O(4)
O(5)–U–O(6)
1.765(3)
1.763(3)
2.570(3)
2.546(3)
179.34(13)
93.34(11)
86.73(14)
49.48(9)
U–O(2)
U–O(1)
U–O(8)
U–O(9)
O(1)–U–O(2)
O(1)–U–O(3)
O(1)–U–O(4)
O(8)–U–O(9)
2.335(2)
2.385(3)
2.509(3)
2.534(3)
71.63(9)
92.30(12)
88.34(12)
50.16(11)
3.3. DFT computational study
A DFT investigation of [UO2(H2L1)(NO3)2] (1) in a simulated
methanol environment was performed in order to probe the struc-
ture of the complex in this solvent. Comparative DFT modelling of
2þ
the corresponding hypothetical complex in which the UO2 group
binds to the O2N2-donor set of the neutral ligand (nitrates not
coordinated) was also carried out. The results are shown in
Fig. 3. The optimised structure (M1) based on the crystal structure
of 1 is quite close to that observed in the crystal with, in particular,
the confirmation of L1 essentially maintaining the conformation
observed in the solid state. For M2, the structure incorporating
3.2. X-ray structures
Orange–red crystals of 1, 2 and 4 of type [UO2(H2L)(NO3)2] that
were suitable for X-ray diffraction studies were obtained by slow
diffusion of diethyl ether into a methanol/acetonitrile solution of
the respective complexes. The asymmetric unit of each complex
contains one U(VI) ion, one ligand molecule and two nitrate anions.
The coordination geometries present in all three complexes
(Figs. 1–3) are quite similar, with each complex showing a dis-
torted hexagonal-bipyramidal arrangement about its U(VI) centre,
with an axially oriented O@U@O moiety and six equatorial oxygen
atoms. In all three structures the aryl and naphthyl groups are
present on the same side of the equatorial plane [see, for example
Fig. 1(b)].
UO2 ion bound to all four (O2N2) donor atoms of L1, there is a
2þ
substantial change in the conformation of bound H2L1. In particular,
the backbone 1,3-phenylene ring is rotated significantly further
from the mean O2N2-donor plane of L1 in order to minimise a steric
clash (in the absence of proton loss) [60] that otherwise would
occur between the hydrogen in the 2-position of the ring and the
introduced uranyl(VI) ion if the ligand conformation observed in
M1 was maintained. It is proposed that this steric interference
plays a major role in destabilising structure M2 relative to M1.
The calculations also confirm the presence of strong OHꢁ ꢁ ꢁN
interactions between adjacent sites on each salicylimine moiety
The U@O distances and O@U@O angles in 1 (av. 1.76(2) Å;
178°), 2 (av. 1.75(4) Å; 179°) and 4 (av. 1.76(3) Å; 179°) are typical
of the corresponding distances and angles reported for related
uranyl compounds [26,28,30,57].
As suggested by the physical data, the X-ray diffraction studies
in each case confirm that the uranyl ion is bound to the phenolic
OH groups of the respective neutral ligands (H2L1, H2L2 and H2L4)
and two bidentate nitrate ions. The UAOphenol bond lengths appear
unremarkable for each of 1 (2.311 and 2.324 Å), 2 (2.345 and
2.358 Å) and 4 (2.333 and 2.341 Å), being slightly longer than the
distances (2.202–2.298 Å) observed in related uranyl(VI) salicy-
lideneimine complexes in which the coordinated phenolic group
is deprotonated [26,28,30,58]. The UAOnitrate bonds are longer at
2.519–2.596 Å (1), 2.504–2.547 Å (2), 2.509–2.570 Å (4). The imine
2þ
nitrogens do not bind to the UO2 cation, but in each case are
associated with an intramolecular hydrogen bond (OAHꢁ ꢁ ꢁN@C)
with the nearest phenolic hydroxy function [59]; the bond dis-
tances are (1.98 and 1.94 Å), (1.81 and 1.85 Å) and (1.88 and
1.99 Å) for 1, 2, and 4 respectively [shown as dotted lines in
Figs. 1–3]. Large thermal displacements of the C atoms in the
central cyclohexyl ring in 4 were found and the ring was hence
modelled as positionally disordered over two sites. However, for
the sake of clarity, only one position is shown in Fig. 2(b).
1
Fig. 4. UO2 extraction by H2L –H2L9 at pH 5.2 and 6.8 as well as in the presence of
2þ
n-octanoic acid at pH 5.2(⁄); [U(VI)] = 1 ꢃ 10ꢂ4 M, [NaNO3] = 5 ꢃ 10ꢂ3 M;
[L] = 1 ꢃ 10ꢂ2 M, ⁄[n-octanoic acid] = 1 ꢃ 10ꢂ3 M in CHCl3; t = 60 min, T = 23 1 °C.
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Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
7
Fig. 5. The pH dependence of UO2 extraction with the ligands H2L1 and H2L8. The downturn in extraction at pH values above ꢀ7 is almost certainly reflects the occurrence
2þ
of metal hydrolysis in the aqueous phase.
Table 3
influenced by addition of the above acid. Under comparable condi-
tions Eu(III) extraction with all ligands was negligible (61%).
Clearly the extraction efficiency of these ligands strongly favours
Comparison of selected bond lengths [Å] for 1 obtained from the DFT study with those
obtained from the X-ray diffraction determination.
M1 (calcd.)
1.776, 1.782
2.611, 2.615
2.613, 2.615
2.322, 2.324
1 (X-ray)
U(VI) over Eu(III) both in the absence and in the presence of
n-octanoic acid.
U–Oaxial
U–Onitrate
1.760(2), 1.763(2)
2.519(2), 2.579(3)
2.579(3), 2.596(3)
2.324(1), 2.311(2)
The influence of the pH of the aqueous phase on extraction has
been investigated. A significant increase of extraction with rising
pH was found especially for the amine-containing ligands H2L8
and H2L9. For the latter ligands, this observation is in accord with
both the greater flexibility of their backbone structures and the
higher basicity of these amine-containing derivatives. Further-
more, the higher extraction efficiency of H2L9 relative to H2L8 is
in accord with the former’s higher lipophilicity.
U–Ophenolic
of metal-bound H2L1. Comparison of the calculated coordination
bond lengths for [UO2(H2L1)(NO3)2] (M1) in methanol obtained
by DFT are in good agreement with the corresponding X-ray data
obtained for [UO2(H2L1)(NO3)2] (1) (Table 3).
The pH dependence of UO2 extraction by the Schiff base H2L1
2þ
and the structure-related secondary amine ligand H2L8 is illus-
trated in Fig. 5. In accord with the above, for a given pH the extrac-
tion efficiency of the latter ligand is substantially higher. Related
pH dependence studies for H2L3–H2L4 also indicated that the
highest extraction is achieved at pH ꢀ7; the respective values (at
pH 6.8) are shown in Fig. 4.
4. Solvent extraction studies
The extraction of U(VI) and Eu(III) using H2L1-–H2L9 involved
parallel sets of experiments in both the absence and presence of
n-octanoic acid; the results for UO2 are summarised in Fig. 4.
The lipophilicities of the Schiff base ligands in the above series
are similar as shown by measuring their distribution between
water and n-octanol. In general, the extraction efficiency of the
Schiff bases H2L1-–H2L5 for U(VI) is quite limited [at pH 5.2 only
1–9% of the U(VI) in the aqueous phase was extracted] but this
was somewhat increased with rising pH (at pH 6.8 the extraction
lies between 9 and 29%).
2+
The increased extraction on rising the pH of the aqueous phase
is in accord with the deprotonation of the phenolic OH functions of
the ligands occurring as the pH is increased, ultimately resulting in
the formation of neutral uranyl complexes; however, it is noted
that a ‘‘slope analysis’’ [51] involving a log D versus pH plot, where
2þ
D is the distribution ratio for UO2 between the aqueous and
Of all the Schiff bases studied (that is, H2L1–H2L7) only H2L6
(95%) and the analogous 1,2-cyclohexane linked ligand H2L7
(43%) exhibited substantial extraction efficiencies at pH 5.2. This
significantly enhanced extraction by H2L6 and H2L7 undoubtedly
reflects that, relative to the other Schiff base ligands in the series,
both these ligands are capable of yielding stable neutral chelate
ring patterns with N2O2-donor coordination on 1:1 complexation
with UO22þ. This behaviour contrasts with the preferred O2-donor
coordination of H2L1–H2L4 (along with the binding of two nitrate
anions).
organic phase at equilibrium, did not correspond to the expected
release of two protons – in keeping with the presence of competing
equilibria rather than simple 1:1 complex behaviour (with loss of
two protons) occurring in the organic phase.
5. Conclusion
Unusual uranyl complexes of neutral phenolic Schiff base ligands
have been synthesised and characterised by a range of physical
methods that includes X-ray crystallography. These complexes do
not use their imine functions for binding to the UO22þ, an outcome
attributed to steric hindrance from the proton in the 2-position of
the phenylene ring in the backbone bridging unit. The extraction
properties of the ligands H2L1–H2L5 towards U(VI) are quite similar.
However, the distribution ratios of UO22þ with these 1,3-phenylene
bridged derivatives are significantly lower than in case of the related
1,2-bridged compounds H2L6 and H2L7 which almost certainly use
their O2N2 donor sets for binding the uranyl ion (coupled with con-
comitant loss of both their phenol protons). The preference of the
1,3-linked bis(2-hydroxyaryl)imine species H2L1–H2L4 to act as
As expected, the related secondary amine ligands H2L8 (65%
extraction) and H2L9 (92% extraction) give rise to higher extraction
efficiencies relative to the Schiff bases. This observation is in accord
with both the consequence of the greater flexibility of their back-
bone structures and the higher basicities of these amine derivatives.
Furthermore, the higher extraction efficiency of H2L9 relative to
H2L8 is in accord with the former’s enhanced lipophilicity.
Addition of ten equivalents (relative to the metal ion concentra-
tion) of n-octanoic acid to the organic phase resulted in the extrac-
tion of UO2 by H2L1–H2L4 being markedly enhanced at pH 5.2
2þ
2þ
(from a few percent to >75% extraction) reflecting a remarkably
large synergistic effect in each case; no extraction (61%) was
observed with the acid alone under comparable conditions. Only
the extraction by H2L5 (possessing different relative positions of
its two imine function in comparison to the other ligands) is not
neutral ligands towards UO2 in such complexes in conjunction
with nitrato ligands provides a rationale for the strong synergistic
effect observed in the presence of n-octanoic acid (at pH 5.2). That
is, the hydrophobic carboxylate anion likely replaces the nitrato
ligands in the coordination sphere leading to a higher complex
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8
Harold B. Tanh Jeazet et al. / Polyhedron xxx (2015) xxx–xxx
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references therein).
lipophilicity and, consequently, improved extraction efficiency.
Finally, all ligands studied favour the extraction of U(VI) over Eu(III)
– an outcome that points the way towards their possible use in rare
earth/uranium separation processes.
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The authors thank the German Federal Ministry of Education
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Research Council (ERC) (SynPhosproject number 307616) for
financial support. J. Mizera gratefully acknowledges the European
Social Fund for a Ph.D. grant.
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