Hydroxy- and alkoxy-bridged dinuclear uranyl-Schiff base complexes: Hydrolysis, transamination and extraction studies

Article abstract of DOI:10.1039/b800469b

The reaction of uranyl nitrate with 1,3-bis(salicylideneamino)-2-propanol (H₃L1) and 1,3-bis(3,5-di-tert-butylsalicylideneamino)-2-propanol (H₃L2) in the presence of triethylamine (Et₃N) yielded hydroxy- and alkoxy-bridged dinuclear complexes; [(UO₂) ₂(L1)(OH)(MeOH)₂]?(MeOH)₂ (1?(MeOH)₂) and [(UO₂)₂(L2)(OH)(MeOH) ₂]?(MeOH)₂ (2?(MeOH)₂). The crystal structures of 1?(DMF)₂ and 2?(DMF)₂ exhibit an unsymmetrical central U₂O₂ core involving bridging alkoxy- and hydroxy-oxygen atoms. The geometry around the uranium center in 1?(DMF)₂ and 2?(DMF)₂ is that of a distorted pentagonal bipyramid with the solvent molecule occupying the fifth coordination site. The flexible nature of the ligand backbone is more pronounced in 2?(DMF)₂ compared to 1?(DMF)₂, yielding two molecules per unit cell in different conformations. Under similar reaction conditions, using ethylenediamine as a base, the respective Salen-based uranyl compounds, [UO₂(Salen)(MeOH)] (3) and [UO₂(Bu ^t₂-Salen)(MeOH)] (4) are obtained due to transamination of the ligand backbone. Complexes 1?(MeOH)₂ and 2?(MeOH) ₂ when reacted with an excess of ethylenediamine failed to yield the respective Salen-based complexes, 3 and 4, respectively. The new compounds have been characterized using solution (NMR and UV-Vis) and solid-state (IR, X-ray crystallography) techniques. Hydrolysis of 1?(MeOH)₂ and 2?(MeOH)₂ in the pH range 1-14 was studied using UV-Vis spectroscopy and compared with the hydrolysis of 3 and [UO₂(Salophen) (MeOH)] (5). A two-phase extraction study suggests quantitative removal of uranyl ions from the aqueous phase at higher pH conditions. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b800469b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Hydroxy- and alkoxy-bridged dinuclear uranyl–Schiff base complexes:
hydrolysis, transamination and extraction studies†
Mohan S. Bharara, Kathryn Heflin, Stephen Tonks, Kara L. Strawbridge and Anne E. V. Gorden*
Received 10th January 2008, Accepted 31st March 2008
First published as an Advance Article on the web 29th April 2008
DOI: 10.1039/b800469b
The reaction of uranyl nitrate with 1,3-bis(salicylideneamino)-2-propanol (H
,3-bis(3,5-di-tert-butylsalicylideneamino)-2-propanol (H L2) in the presence of triethylamine (Et
yielded hydroxy- and alkoxy-bridged dinuclear complexes; [(UO (L1)(OH)(MeOH)
]·(MeOH)
1·(MeOH) ) and [(UO (L2)(OH)(MeOH) ]·(MeOH) (2·(MeOH) ). The crystal structures of
·(DMF) and 2·(DMF) exhibit an unsymmetrical central U core involving bridging alkoxy- and
hydroxy-oxygen atoms. The geometry around the uranium center in 1·(DMF) and 2·(DMF) is that of
a distorted pentagonal bipyramid with the solvent molecule occupying the fifth coordination site. The
flexible nature of the ligand backbone is more pronounced in 2·(DMF) compared to 1·(DMF)
yielding two molecules per unit cell in different conformations. Under similar reaction conditions, using
3
L1) and
1
3
3
N)
2
)
2
2
2
(
2
2
)
2
2
2
2
1
2
2
2
O
2
2
2
2
2
,
ethylenediamine as a base, the respective Salen-based uranyl compounds, [UO
2
(Salen)(MeOH)] (3) and
t
[
1
UO
·(MeOH)
2
(Bu
2
-Salen)(MeOH)] (4) are obtained due to transamination of the ligand backbone. Complexes
when reacted with an excess of ethylenediamine failed to yield the
2
and 2·(MeOH)
2
respective Salen-based complexes, 3 and 4, respectively. The new compounds have been characterized
using solution (NMR and UV-Vis) and solid-state (IR, X-ray crystallography) techniques. Hydrolysis
of 1·(MeOH)
compared with the hydrolysis of 3 and [UO
suggests quantitative removal of uranyl ions from the aqueous phase at higher pH conditions.
2
and 2·(MeOH)
2
in the pH range 1–14 was studied using UV-Vis spectroscopy and
2
(Salophen)(MeOH)] (5). A two-phase extraction study
Introduction
and transamination by nucleophiles due to the presence of an
11
imine group (C=N). The role of the metal ion as catalyst
in the hydrolysis and transamination of SB ligands, where the
transamination reaction is equilibrium controlled, and the excess
Actinide coordination chemistry has attracted attention due
to the development of nuclear technology for the production
of electricity. The remediation of the resulting nuclear waste
generated, in which uranium is a major contributor to the long-
term radioactivity, is a primary concern. However, the separation
of actinides from lanthanides is difficult due to their similar
oxidation states and ionic radii. Various ligand systems have
been used as extractants, including organic phosphorous oxides,
crown ethers, azacrowns, calixarenes and modified calixarenes,
12,13
of exchanging amine favors the reaction, is well documented.
In a recent study, the uranyl ion has been demonstrated to
catalyze the reversible transamination of 2-methylalanine with
1
pyridoxal (Vitamin B
6
) yielding [(UO
2
PmHpyr)
3
(l
3
-O)]Cl·3H
2
O
2
14
(
PmHpyr = pyridoxaminylpiruvate anion). Transamination in
3
uranyl–SB complexes with 8-hydroxy-7-quinolinecarbaldehyde
has also been reported, where the addition of either ethylene-
diamine or diaminobenzene yields symmetrical tetradentate SB
4
,5
6
7,8
hydroxamic acids, and Schiff base ligands. Macrocyclic ligands
and calixarene compounds are either too rigid or too flexible
for complexation and instead result in the formation of uranyl
15
complexes.
In an effort to synthesize stable uranyl–SB complexes which
could be used as model compounds in the investigation of new
means of remediation of uranium from aqueous sources or nuclear
9,10
salts or polymeric arrays with weak interactions. Schiff base
SB) ligands are potential candidates for selective separation
(
as they contain multidentate mixed aza- and oxo-cores and
possess sufficient steric freedom, while lacking the problems
associated with macrocyclic cavity size. One drawback, however,
is that Schiff base compounds are susceptible to hydrolysis
wastes, we here report the synthesis of [(UO
2
)
)
2
(L1)(OH)(MeOH)
(L2)(OH)(MeOH)
2
]·
16
(
(
MeOH)
MeOH)
2
(1·(MeOH)
2
)
and [(UO
2
2
2
]·
2
(2·(MeOH)
2
), which do not undergo facile nucleophilic
addition and/or substitution reactions. Hydrolysis of 1·(MeOH)
2
and 2·(MeOH)
2
in the pH range 1–14 are studied along
Department of Chemistry and Biochemistry, Auburn University, Auburn, AL,
USA 36849. E-mail: gordeae@auburn.edu; Fax: 01-334-844-6959; Tel: 01-
with comparison with alkyl- and aryl-backbone containing
ꢀ
uranyl–SB complexes, [UO
ethylenebis(salicylidenimine)) and [UO
2
(Salen)(MeOH)] (3) (Salen = N,N -
3
34-844-697
†
Electronic supplementary information (ESI) available: Packing diagrams
2
(Salophen)(MeOH)]
ꢀ
of 1·(DMF)
2
and 2·(DMF)
2
, extended tables for bond distances and angles,
(5) (Salophen
=
N,N -Disalicylidene-o-phenylenediamine).
hydrolysis profile of H
3
L2, 4 and 6, UV-Vis profile for extraction at pH 5
Transamination reaction of [UO
2
(Salophen)(MeOH)] (5) and
-Salophen)(MeOH)] (6) is also reported for comparison
with 1·(MeOH) and 2·(MeOH)
using method A. CCDC reference numbers 656873 and 662434. For ESI
and crystallographic data in CIF or other electronic format see DOI:
t
[
UO
2
(Bu
2
1
0.1039/b800469b
2
2
.
2
966 | Dalton Trans., 2008, 2966–2973
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Table 1 Crystal data for 1·(DMF)
2
and 2·(DMF)
2
(refinement method:
Experimental
2
full-matrix least squares on F )
General procedure
1
·(DMF)
2
2·(DMF)
2
UO
2
(NO
3
)
2
·6H
2
O, 1,3-diamino-2-propanol, salicylaldehyde and
Empirical formula
C
23
H
30
N
4
O
10
U
2
87 143 11 23 4
C H N O U
3
,5-di-tert-butyl-2-hydroxybenzaldehyde were purchased from
M
r
998.57
0.71073
Monoclinic
P2 /n
11.797(3)
9.784(2)
24.594(5)
90.00
94.811(4)
90.00
2828.7(1)
4
2663.240
0.71073
Triclinic
¯
P1
16.636(2)
16.641(2)
20.421(3)
104.963(3)
93.128(3)
103.839(3)
5262.2(1)
2
1
13
Acros and used as received. The H and C NMR were recorded
on a Bruker AV 400 spectrometer operated at 400 MHz with d
DMSO as the solvent with tetramethylsilane as the reference.
All melting points were recorded on a Mel-temp II melting
point apparatus, and the values are uncorrected. The IR data
were recorded as KBr pellets on SHIMADZU Inc. IR, Prestige-
k/A˚
6
-
Crystal system
Space group
1
a/A˚
b/A˚
c/A˚
◦
a/
b/
◦
2
4
1 Fourier Transform Infrared Spectrophotometer in the range
00–4000 cm . Electrospray ionization mass spectrometry was
◦
−
1
c /
V/A˚
3
performed on a Micromass QTOF mass spectrometer (Waters
Corp, Milford MA). All UV data was collected using a Cary 50
UV-Vis spectrophotometer with a xenon lamp in the range 200–
Z
−3
D
c
/Mg m
l/mm
F(000)
2.345
11.495
1840
1.682
6.205
2588
−
1
1
200 nm.
Crystal size/mm
Reflns collected
Indep. reflns
0.1 × 0.1 × 0.05
0.1 × 0.5 × 0.05
18601
36319
X-Ray crystallography
Crystals of 1·(DMF) and 2·(DMF)
4058
0.0687
0.858
15175
0.1222
0.868
R
int
2
2
2
were obtained in good yield
GOF of F
Final R indices [I > 2r(I)]
R
1
= 0.0272,
R
1
= 0.0451,
by slow evaporation of the precipitates dissolved in DMF at room
temperature. Quality crystals could also be obtained from d
wR
2
= 0.0539
wR
1
2
= 0.901
6
-
R indices (all data)
R
1
= 0.0364,
R
= 0.0686,
DMSO as 1·(DMSO)
2
and 2·(DMSO)
2
. X-Ray diffraction data
wR = 0.0557
wR = 0.967
2
2
◦
−3
were collected at −80 C on a Bruker SMART APEX CCD
X-ray diffractometer unit using Mo-Ka radiation from crystals
mounted in Paratone-N oil on glass fibers. SMART (v 5.624)
was used for preliminary determination of cell constants and data
collection control. Determination of integrated intensities and
global cell refinement were performed with the Bruker SAINT
Software package using a narrow-frame integration algorithm.
Dqmax,min/e A
˚
1.177, −0.691
1.811, −1.632
◦
Table 2 Selected bond distances (A) and angles ( ) for 1·(DMF)
˚
2
U1–O1
U1–O6
U1–O8
U2–O3
U2–O5
U2–O8
U2–O1D1
U1–U2
1.792(4)
2.241(5)
2.327(4)
1.778(4)
2.235(5)
2.343(4)
2.400(5)
3.8693(8)
U1–O2
U1–O7
U1–N1
U2–O4
U2–O7
U2–N2
U1–O1D2
1.783(4)
2.380(5)
2.544(5)
1.794(4)
2.364(4)
2.579(5)
2.461(4)
2
Refinement was performed against F by weighted full-matrix
least square, and empirical absorption corrections (SADABS)
were applied. The program suite SHELXTL (v 5.1) was used for
1
7
1
8
space group determination, structure solution, and refinement.
Hydrogen atoms were placed at calculated positions using suitable
riding models with isotropic displacement parameters derived
from their carrier atoms. The tert-butyl groups on molecule 2b
were disordered over two positions (60 and 40%) and restraints
were applied to make chemically equivalent bonds approximately
equal in the disordered fragments. Crystal data, selected bond
distances and angles, are provided in Tables 1–3.
CCDC reference numbers 656873 and 662434.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b800469b
O1–U1–O2
O2–U1–O8
O7–U1–O8
178.7(2)
91.90(2)
68.84(2)
O1–U1–O6
O6–U1–O8
O3–U2–O4
90.76(2)
154.81(2)
179.5(2)
Furthermore, separate vials containing organic solvent without
2
+
H
3
L2 and 5 mL of aqueous UO
were prepared for blanks.
Method B (sequential extraction): Organic phase containing
0 mL of H L2 (4 mM) and triphenylphosphine oxide (TPPO,
mM) was added to 20 mL of aqueous UO
2
solution at the respective pH
2
6
3
2
+
2
(3 mM) solution
and shaken in a separating funnel and the organic layer was
removed. The aqueous layer was checked for uranyl concentration
by UV-Vis. To the aqueous phase an equivalent amount of fresh
Extraction and hydrolysis studies
Two-phase extraction studies (CHCl
determine the extraction capability for the removal of UO
from aqueous solution. The ligand H L2, quantitatively soluble
in chloroform, was used for extraction studies. Fresh solutions
of UO (NO O were prepared in DI water, and the pH was
·6H
adjusted with HNO
3
/H
2
O) were performed to
organic solvent containing H
added again and the process was repeated until no UO
observed in the UV-Vis spectrum of the aqueous phase. The blank
3
L2 (4 mM) and TPPO (6 mM) was
2
+
2+
2
ion
2
peak was
3
2
+
for this method consists of extraction of 5 mL of UO
with 5 mL of TPPO (6 mM) dissolved in CHCl . The calculations
for the reduction of uranyl ion from the aqueous media were done
2
(3 mM)
2
3
)
2
2
3
3
and KOH (± 0.05). Two different methods
2
+
19
were employed for studying extraction.
Method A (exposure time): 5 mL of aqueous UO
based on the band observed for UO
For hydrolysis studies, stock solutions of 1·(MeOH)
5 (0.1 mM each) were prepared by dissolving the respective
compound in 95 : 5 CHCl –MeOH solution (100 mL each),
whereas a stock solution of 2·(MeOH) (0.1 mM) was prepared
2
at ca. 420 nm.
2
+
2
solution
2
, 3 and
(
4 mM) was added to the 5 mL of organic phase containing
H
3
L2 (8 mM) in scintillation vials and shaken for 20 s. The
3
absorption spectra of the aqueous phase were determined at 1–8 h.
2
This journal is © The Royal Society of Chemistry 2008
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◦
Table 3 Selected bond distances ( A˚ ) and angles ( ) for 2·(DMF)
[UO
(L2)(OH)(MeOH)
]·(MeOH)
(400 MHz, d
), 3.17 (12H, s, 2DMSO), 4.6, 4.8 and
), 7.4 and 7.6 (4H, d, C ), 9.4 (2H, s,
(400 MHz, d -DMSO): 29, 31, 34 and
(2·(MeOH)
). Yield:
-DMSO, ppm): 1.34
2
2
2
2
2
◦
(
1.0 g, 87%), Mp >200 C, d
H
6
2
a
and 1.79 (18H, s, C(CH
.7 (5H, m, CH –CH–CH
CHN), 12.5 (1H, s, 1H). d
)
3 3
5
2
2
6
H
2
U1–O1
U1–O5
U1–O8
U2–O3
U2–O6
U2–O8
U1–U2
O1–U1–O2
O1–U1–O8
O2–U1–O6
O3–U2–O6
O3–U2–O8
O7–U2–O8
1.762(6)
2.221(6)
2.389(6)
1.766(6)
2.341(6)
2.375(6)
3.9340(7)
179.0(3)
87.8(2)
89.6(3)
90.8(3)
88.9(3)
155.6(2)
U1–O2
U1–O6
U1–N1
U2–O4
U2–O7
U2–N2
1.785(6)
2.383(6)
2.541(7)
1.776(6)
2.254(6)
2.558(7)
C
6
35 (−C(CH )), 46 (CH N), 63 (CHO), 118–158 (phenyl) and 169
3
2
3
−1
−1
(
6
C=N). kmax(DMF)/nm: 270, 350, 400 (e/dm mol cm 2100,
−
1
00 and 230). mmax(KBr)/cm : 2953, 1627, 1531, 1457, 1435,
+
1384, 1214, 1167, 876. m/z (ESI): 1203 (M + 2MeOH) (100%).
O1–U1–O5
O2–U1–O5
O3–U2–O4
O3–U2–O7
O4–U2–O6
88.0(3)
91.1(3)
179.5(3)
89.5(3)
88.7(2)
Transamination reactions
t
Ligands. Ligand (Salophen, Bu
1 mmol) was dissolved in a minimal amount of MeOH–CHCl
50 : 50), and to this, ethylenediamine (1–2 mmol) dissolved in
2
-Salophen, H
3
L1 or H L2)
3
(
(
3
2
b
methanol (2 mL) was added. The resulting solution was stirred
overnight. The solvent was then removed to obtain a precipitate,
U3–O9
1.774(6)
2.227(6)
2.341(6)
1.764(6)
2.352(6)
2.338(6)
3.8470(6)
U3–O10
U3–O14
U3–N3
U4–O12
U4–O15
U4–N4
1.767(6)
2.392(6)
2.549(7)
1.796(6)
2.239(6)
2.561(7)
t
which was washed with cold MeOH to obtain Salen or Bu
2
-Salen.
U3–O13
U3–O16
U4–O11
U4–O14
U4–O16
U3–U4
1
The H NMR indicates formation of the respective compounds
21,22
and agrees with the reported data.
Uranyl-complexes. Compound 5 (1 mmol) was dissolved in a
5
0 : 50 mixture of MeOH–CHCl (10 mL) and to this solution
3
ethylenediamine (1–2 mmol) dissolved in minimum amount of
MeOH was added and stirred overnight. The solvent was then
O9–U3–O10
O9–U3–O14
O10–U3–O16
O13–U3–O14
178.4(3)
88.0(2)
87.1(3)
135.8(2)
O9–U3–O13
O10–U3–O13
O13–U3–O16
O14–U3–O16
92.4(3)
86.8(3)
155.4(2)
68.25(19)
removed to obtain a precipitate and washed with cold CHCl
obtain 3. Compound 6 under similar conditions does not yield
; however, the reaction proceeds to completion when heated to
reflux. The precipitates were removed, washed with cold MeOH
and dried under vacuum. Compound 1·(MeOH) or 2·(MeOH)
1 mmol) was dissolved in 50 : 50 MeOH–CHCl mixture (10 mL)
3
to
4
in 100 mL of CHCl
at pH 1–14 was added to separate vials containing 5 mL of H
3
. An equivalent amount of aqueous solution
L2
(0.1 mM), 3 (0.1 mM)
2
2
3
(
3
(
3 mM), 1·(MeOH)
2
(0.1 mM), 2·(MeOH)
2
and to this ethylenediamine (3–4 mmol) was added, and the
resulting mixture was refluxed for 2–3 h. The solvent was removed
or 5 (0.1 mM) in organic solvent and shaken for 60 s. The solution
was left undisturbed overnight, and the organic layer was isolated
for hydrolysis studies employing UV-Vis. The extent of hydrolysis
at different pH was interpreted relative to the spectra at neutral
pH.
to obtain a precipitate, which was washed with cold MeOH–CHCl
3
1
(50 : 50) and dried under vacuum. H NMR and crystallization
from DMF indicates no transamination observed for 1·(MeOH)
2
or 2·(MeOH)
2
.
Synthesis of Schiff bases and uranium complexes
Results and discussion
20
The ligands were synthesized according to the literature methods.
Complexes 1·(MeOH) and 2·(MeOH) were synthesized by
refluxing 50 mL of MeOH–CHCl (50 : 50) solution containing
respective ligands; H L1 (2 mmol, 0.6 g) and H L2 (2 mmol,
N (2 mmol, 0.2 g) and UO (NO O (2 mmol,
·6H
.0 g) for 5–6 h. Uranyl–Salen (3 and 4) and uranyl–Salophen
The ligands, H
3
L1 and H
3
L2 possess five potential donor atoms
2
2
to complex with metal ions. The range of complex structures
resulting from similar ligands reported in the literature varies from
monomeric to polymeric controlled by varying the conditions of
3
3
3
1
1
.0 g), Et
3
2
3
)
2
2
23,24
synthesis.
For instance, H L1 with transition-metal ions such as
3
2+
3+
2+
Cu , Mn and Zn typically yields multinuclear (tri-, tetra- and
(
5 and 6) compounds were synthesized according to the literature
25–28
hexanuclear) alkoxo-bridged clusters.
·(MeOH) and 2·(MeOH)
an equivalent amount of ligand, uranyl nitrate hexahydrate and
The uranyl complexes
21,22
methods.
The resulting precipitates obtained were filtered off,
1
2
2
were synthesized by heating to reflux
washed with 5 mL of cold MeOH–CHCl
under vacuum.
3
(1 : 1) mixture and dried
2+
Et
stoichiometry) under similar reaction conditions only yielded
1·(MeOH) or 2·(MeOH) . Using ethylenediamine as a base
instead of Et N, compounds 3 and 4 were obtained. The catalytic
3
N in MeOH–CHCl
3
(50 : 50). Varying UO
2
: ligand ratio (1–
3
[
(UO
2
)
2
(L1)(OH)(MeOH)
2
]·(MeOH)
2
(1·(MeOH)
2
). Yield:
-DMSO): 2.8 (12H, s,
2
2
◦
(
0.9 g, 98%), Mp >200 C, d
H
(400 MHz, d
6
3
2
DMSO), 4.7, 4.9 and 5.8 (5H, m, CH
2
–CH–CH
2
), 6.7–7.9 (8H,
(400 MHz,
N), 71 (CHO), 115–163 (phenyl), 167
role of uranyl ion in the transamination reaction, followed by
m, 2C
-DMSO): 31 and 36 (CH
and 170 (CHN). kmax(DMF)/nm: 270, 330, 385 (e/dm mol cm
6
H
4
), 9.4 (2H, s, CHN), 11.7 (1H, s, OH). d
C
the formation of 3 or 4 (something similar to the formation
14
d
6
2
of [(UO
in the absence of uranyl ion, ligands H
facile transamination with ethylenediamine to yield Salen and
2
PmHpyr)
3
(l
3
-O)]Cl·3H
2
O) can be ruled out because,
3
−1
−1
3
L1 and H L2 undergo
3
−
1
2
1
2
300, 1000 and 260). mmax(KBr)/cm : 2919, 1629, 1541, 1469,
447, 1383, 1309, 1212, 1151, 1030, 895, 801. m/z (ESI): 980 (M +
MeOH) (10%).
+
t
Bu -Salen, respectively. The diamine exchange is only observed
2
when the exchanging amine is more basic than the amine used
2
968 | Dalton Trans., 2008, 2966–2973
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t
Scheme 1 Scheme depicting the synthesis and transamination associated with ligands and uranyl complexes (R = H (3 and 5) and Bu
solvent, en = ethylenediamine).
2
(4 and 6), sol =
to form the imine compound, in this case ethylenediamine
29
replacing 1,3-diamino-2-propanol. Under similar reaction con-
ditions combination of ethylenediamine with 5 and 6 yields 3
and 4, respectively, however, even with a three-fold excess of
ethylenediamine, 1·(MeOH)
2
and 2·(MeOH)
2
do not undergo
transamination (Scheme 1).
1
In the H NMR, a significant shift in the ppm value of the
imine HC=N bond is observed between free ligand (≈8.5 ppm)
and uranium complexes (≈9.4 ppm) indicating the involvement
of the lone pair on nitrogen with the metal center. A singlet over
1
1 ppm in both 1·(MeOH)
2
and 2·(MeOH)
2
indicates the presence
of a bridging hydroxy group attached to the uranium center. In
the IR spectra of the complexes, a strong peak around 1628 cm
compared to 1640 cm of the free ligand indicates coordinated
imine nitrogen.
−
1
−
1
30,31
Coordination through the phenolic hydroxy
groups is shown by the shift in the C–O band for uranyl complex
−
1
(
1210 cm ) compared to the band observed for free ligands
Fig. 1 Absorption spectra of ligands (0.25 mM) (H
3
L1 = ᭛; H
3
L2 =
−
1
32
−1
ꢀ) and uranyl complexes (0.1 mM) (1·(DMF) = —; 2·(DMF) = ---) in
(1260 cm ). The strong bands around 900 and 800 cm
2
2
DMF.
represent the asymmetric and symmetric stretch of a linear uranyl
32,33
moiety and are comparable to similar complexes.
The UV-vis spectra for the free ligands and uranyl complexes
in DMF are shown in Fig. 1. The absorption bands for ligands
The DMF adducts of 1 and 2 (Fig. 2 and 3 and Tables 2 and
3) were obtained from supersaturated DMF solutions containing
are observed around 270 and 320 nm (H
L2) arising from p → p* and n → p*, respectively. A red shift
in the uranyl complexes is observed except for the band at 270 nm.
The peak around 330 nm (1·(DMF) ) and 350 nm (2·(DMF)
could be assigned to the LMCT transition of the imine group with
3
L1) and 270 and 330 nm
the uranyl complexes. The crystal structure of 1·(DMF)
2
has been
, two
16
(
H
3
described briefly earlier. In the asymmetric unit of 2·(DMF)
2
molecules with different conformation and noticeable differences
in the interatomic distances are observed (2a (boat) and 2b (chair)).
This might be due to the fluxional behavior of the tert-butyl groups
present on the ligand backbone. The environment around the
uranyl center is similar with equatorial phenolic oxygen, imine
nitrogen, bridging alkoxy and hydroxy group along with the
solvent molecules. The geometry around the uranium center is
2
2
)
15,32
appreciable red shift compared to the n → p* of the free ligands.
Similar bands due to the imine coordination and LMCT (5f ←
2
p oxygen) have been reported for uranyl complexes containing
34
multidentate hydroxy groups (390 and 450 nm).
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2966–2973 | 2969

View Article Online
˚
The bridging alkoxy distances in 1·(DMF)
and 2·(DMF)
accordance with the Ualkoxy distances observed in [UO
2
(av. 2.372(4) A)
˚
2
(av. 2.362(6) (2a) and 2.372 (2b) A) are in
2
(Salophen)]
2
38
39
˚
˚
(
2.387–2.463 A) and [(UO
L = aminoalcoholbis(phenolate)). In contrast, symmetrical
(L1)(l–O CCH –C
2
)(H
2
L)
2
(NO
3
)
2
] (2.360–2.389 A)
(
H
2
distances have been reported for [Cu
2
2
2
6
H –
4
40
˚
CH
2
CO
2
)]·2H
2
O (1.944 A). The bridging hydroxy, U–OOH
˚
(2.375(6)
distances in 1·(DMF)
2
(av. 2.335(4) A) and 2·(DMF)
2
˚
(
2a) and 2.330(6) (2b) A) are unsymmetrical leading to the forma-
tion of an unsymmetrical central U core. The U–OOH distances
2
O
2
on an average are smaller than the U–Oalkoxy distances, indicating
a stronger bond. The bridging hydroxyl distances are comparable
with the corresponding distances observed in the uranyl–oxalate
Fig. 2 Molecular structure of 1·(DMF)
2
(ORTEP with 50% thermal
ellipsoids). Hydrogen atoms are omitted for clarity.
41
complex, [(UO ) (C O ) (OH)Na(H O) ] (av. 2.287 A˚ ); uranyl–
2
2
2
4
2
2
2
pyridine-2,6-dicarboxylato complex, [HNEt
3
]
2
[UO
2
L
2
]·2H
2
O
42
˚
(
2.319–2.357 A); as well as uranyl–inorganic structure such as
43
˚
O (2.303–2.433 A).
[
(UO
2
)
4
O(OH)
6
]·5H
2
˚
(3.869(8) A) and
The interatomic U · · · U distances in 1·(DMF)
2
˚
2
·(DMF)
2
(3.847(6) (2a) and 3.933(7) (2b) A) are too extended
38,44
to be indicative of any metallic interactions.
These, however
fall within the range of distances reported for dinuclear uranyl
˚
compounds (3.779–3.932 A). The unsymmetrical distances in
·(DMF) and 2·(DMF) could be attributed to the flexible nature
of the ligand backbone, which is more pronounced in 2·(DMF)
The distortion observed in the ligand backbone along the plane
defined by the central U core is shown in Fig. 4. The phenyl
1
2
2
2
.
2
O
2
groups are present on the same side of the U
a ‘boat’ conformation.
2
2
O core in 2a yielding
Fig. 3 Molecular structure of 2a (top) and 2b (bottom) with 50% thermal
ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity.
best described as pentagonal-bipyramidal with axial O=U=O
moiety. The U=O distances and O=U=O angles in 1·(DMF)
2
◦
◦
˚
˚
(
av. 1.78(4) A; 178 ) and 2·(DMF)
2
(av. 1.77(6) A; 178 (2a) and
Fig. 4 Distortion observed in the backbone of 1·(DMF)
2
, 2a and 2b. The
◦
˚
45
1
.77(6) A); 179 (2b)) are typical of the corresponding distances
tert-butyl groups in 2a and 2b are omitted for clarity.
35
and angles reported for similar uranyl compounds.
The U–Ophen distances are unsymmetrical in both complexes
Such a conformation has been reported for various mononu-
clear uranyl–SB complexes and attributed to the strong uranyl–
ligand interaction, which forces the ligand to conform. How-
˚
with distances in the range 2.223(6)–2.253(6) A, however, these are
34
46
˚
within the range observed in related complexes (2.231–2.296 A).
˚
The average U–N distances in 1·(DMF)
2
(av. 2.561(5) A) are
ever, no correlation could be observed between bond distances,
angles, and conformation of a few selected uranyl-complexes
(Table 4). Such conformation is also reported for various transition
slightly longer than those observed in 2·(DMF)
2
(2.549(7) (2a)
and 2.554(7) (2b) A) and are typical of uranium nitrogen distances
˚
36,37
2+
3+
˚
observed in uranyl–SB complexes (2.54–2.58 A).
metal–SB complexes including Cu , Mn , as well as U(IV)–SB
2
970 | Dalton Trans., 2008, 2966–2973
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Table 4 Comparison of structural parameters of selected dinuclear uranyl–SB complexes. Average distances ( A˚ ) and angle ranges ( ) are reporteda
◦
Conform. (subst.)
U–Ophenyl
U–N
U–Obridge
U · · · U
U
2
O
2
core
Ref
1
2
2
·a(DMF)
b
2
c (H) t
2.237
2.237
2.234
2.222
2.225
2.232
2.225
2.235
2.562
2.548
2.548
2.557
2.528
2.562
2.527
2.571
2.353
2.369
2.351
2.353
2.335
2.358
2.441
2.367
3.869
3.934
3.847
3.879
3.863
3.925
3.876
3.942
68.8–111.9
67.2–112.7
67.9–111.2
68.3–111.0
67.8–112.0
67.8–112.9
69.4–106.3
65.7–112.7
This work
This work
This work
8
8
8
38
44
b (Bu )
t
c (Bu )
[
[
[
[
[
UO
2
2
2
2
2
(HL1)]
(HL2)]
(HL3)]
(L4)]
(HL5)]
2
2
2
b (H)
UO
UO
UO
UO
c (OH)
t
c (Bu )
2
b (H)
c (H)
2
a
c = chair; b = boat; L1/L2/L3 = 3-amino-1,2-propanediol based SB ligands; L4 = Salophen; L5 = SB derived from 4,6-O-ethylidene-b-D-
glucopyranosylamine.
4
0,47–79
complexes.
In 1·(DMF)
2
and 2b, the phenyl groups are
core with a chair
present on the opposite site of the U
2
O
2
conformation. In dinuclear uranyl–SB complexes containing 3-
aminopropane-1,2-diol, a boat confirmation is observed in the
backbone where phenyl groups contain no substituents. With sub-
stituents such as OH and tert-butyl groups, a chair conformation is
observed, which was attributed to the electronic and steric effects.
Scheme 2 Scheme depicting hydrolysis of imine of metal–Schiff base
complexes.
52
This is contrary to the conformations observed in 1·(DMF)
2
and
1
·(MeOH)
complete breakdown of the complex is observed at pH 12–14. In
contrast, for complex 2·(MeOH) formation of the carbinolamine
intermediate is observed at pH 1–3. It is interesting to observe
that at high pH conditions complex 2·(MeOH) appears to remain
intact. This could only be explained due to its extreme solubility
in the organic phase (CHCl ) (Experimental section) compared
, 3 and 5, which required a minimum amount of
2
is indicated by the shift in kmax at pH 1–3; however,
2·(DMF)
2
. One possibility is that the conformation in the ligand
backbone of a SB-complex is governed by crystal packing rather
than steric or electronic effects.
2
2
Hydrolysis and extraction studies
3
The two-phase hydrolysis study of H
3
L2 indicates that this ligand
to 1·(MeOH)
2
hydrolyzes in extreme pH conditions (pH 1, 2 and 13, 14; see ESI†).
MeOH for complete dissolution.
The hydrolysis profile of compounds 1·(MeOH)
2
, 2·(MeOH)
2
, 3
Similar shifts in the kmax peaks in the hydrolysis profile have been
2
+
ꢀ
and 5 are shown in Fig. 5.
reported for the Ni –2,2 -dipyridylmethylideneaniline complex,
52
In the UV-Vis spectra of 1·(MeOH)
2
, a slight change in the
max(LMCT) is observed for bands at pH 1–3 (330 nm) compared
where metal ion stabilization of SB toward hydrolysis is reported.
k
Protection by the metal ion of the protonation of the imine
nitrogen retards the breakdown of the intermediate leading to
overall inhibition of the hydrolysis of the SB complex. The rate of
hydrolysis of the SB-complex is related to the metal ion electron
to that observed at pH 7.0 (315 nm). A significant change in the
spectra is observed at pH 12 and the characteristic peak around
3
15 nm is absent at pH 13 and 14. In 2·(MeOH)
shift in the kmax is observed at pH 1–3. In compound 3, the
max(340 nm) for pH 10–14 is not observed and a meagre shift
2
, a profound
53
withdrawing abilities from the reaction center (imine bond).
k
It can be postulated that the stronger U–N interaction leading
˚
at pH 1–3 is observed. On the other hand, for 5, a significant
shift in the kmax is observed at pH 1, 2 and 11 and no peak is
to the shorter imine bonds in 1·(DMF)
2
(av. 1.279(8) A) and
˚
˚
2·(DMF)
2
(av. 1.274(1) A) compared to 3 (av. 1.284 A) and
˚
˚
observed at pH 12–14. Compounds, 1·(MeOH)
2
and 2·(MeOH)
2
5 (coordinating solvent = MeOH (1.289 A); DMF (1.289 A)
˚
are stable in a much broader pH range and hydrolyze at extreme
conditions. Kinetics and mechanism studies of the hydrolysis
of imine have been studied extensively with the intermediacy
of carbinolamines [–NH–CH(OH)–] formed by addition of a
water molecule across imine and cleavage of the C–N bond in
and DMSO (1.290 A)) might explain the robust nature of these
complexes toward hydrolysis. Some macrocyclic complexes retain
their imine bonds in water due to the “macrocyclic effect”, but
54,55
still are susceptible to intramolecular transamination reactions.
with 5 and 6, which easily
Comparing 1·(MeOH)
2
and 2·(MeOH)
2
50,51
the N-protonated form.
It is likely that the hydrolysis of
undergo transamination, it can be inferred that the presence of
the bridging alkoxy and hydroxy oxygen in the former compounds
provides additional stability to the complexes.
these complexes would follow the same mechanism (Scheme 2)
with the formation of a carbinolamine intermediate followed by
the formation of [UO
characteristic peak for LMCT is not observed for [UO
acid)](NO , 3 and 5.
) at high pH conditions in 1·(MeOH)
Previously, we have reported that the hydrolyzed product of
2
(salicylic acid)](NO
3
) and free amine. A
The two-phase extraction studies reported here (Fig. 6) are
2
(salicylic
studied at pH 5.0 only since H L2 hydrolyzes in the acidic region,
3
3
2
and it was determined that extractions with SB are more effective
7
at higher pH conditions. Employing ‘method A’, extraction
{
[UO
2
(3-(2-hydroxybenzylideneamino)propane-1,2-diol)]
2
} in the
of uranyl ion from aqueous phase alone with H
3
L2 dissolved
8
presence of Ag(NO
3
) yields [UO
2
(salicylate)](Et
3
NH).
in CHCl indicates 40% reduction of uranyl concentration at
3
The attack of water or hydroxide ion on the protonated SB com-
plex are the rate determining steps in acidic and basic conditions,
respectively. The formation of a carbinolamine intermediate in
pH 5.0. Only the first extraction is effective as UV-Vis indicates
an increase of uranyl ion peak with time. This might be due
53
to the formation of 2·(H
2
O)
2
in the aqueous phase, whose
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Fig. 6 UV-Vis profile for two-phase extraction of uranyl ion from aqueous
phase at pH 5.0 employing a sequential extraction method (blank = ×).
absence of the characteristic peak of uranyl at 420 nm indicates
2
+
complete removal of UO
2
ions from aqueous phase.
Conclusions
Novel uranyl compounds with extended chelation are fully char-
acterized and their reactivity (transamination and hydrolysis) is
reported. The compounds are similar with pentagonal bipyra-
midal geometry around the uranium atom and a central U
core. Under experimental conditions compounds 1·(MeOH) and
·(MeOH) do not undergo nucleophilic addition (hydrolysis)
and substitution reaction (transamination). The hydrolysis of
compared to 3 and 5 indicates the
2
O
2
2
2
2
1
·(MeOH)
2
or 2·(MeOH)
2
robust nature of the backbone of the former in basic conditions.
This could be due to the stronger C=N bonds and an additional
stability provided by the bridging alkoxy and hydroxy groups.
Such compounds could be useful to further explore the uranyl–SB
chemistry toward understanding remediation and speciation, or
for the use to stabilize wastes from nuclear fuel sources stored in
alkaline conditions.
Acknowledgements
This work was supported by the Auburn University, Department
of Chemistry and Biochemistry and through a Ralph E. Power
Junior Faculty Enhancement Award.
Fig. 5 Absorption spectra showing hydrolysis of 1·(MeOH)
and 5 between pH 1–14. The bands indicated by × correspond to spectra
at neutral condition.
2
, 2·(MeOH)
2
,
3
Notes and references
1
2
3
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56
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972 | Dalton Trans., 2008, 2966–2973
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Dalton Trans., 2008, 2966–2973 | 2973