Uranyl stabilized Schiff base complex

Article abstract of DOI:10.1039/b712322a

Uranyl Schiff base complex [(UO₂)₂(Salpro)(OH) (Solvent)₂] (1) in the presence of excess of ethylenediamine (EDA) does not undergo nucleophilic addition (hydrolysis) and substitution (transamination) reactions due to an extended chelation [2N, 3O + OH] by the flexible backbone. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712322a

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Uranyl stabilized Schiff base complex{
Mohan S. Bharara, Stephen A. Tonks and Anne E. V. Gorden*
Received (in Austin, TX, USA) 10th August 2007, Accepted 9th September 2007
First published as an Advance Article on the web 14th September 2007
DOI: 10.1039/b712322a
Uranyl Schiff base complex [(UO
2
)
2
(Salpro)(OH)(Solvent)
2
] (1)
of the uranyl ion in the transamination reaction, followed by the
12
in the presence of excess of ethylenediamine (EDA) does not
undergo nucleophilic addition (hydrolysis) and substitution
formation of 2?(MeOH) (similar to the formation of 3), can be
2
+
ruled out because in the absence of UO
facile hydrolysis and transamination with EDA to yield Salen
N,N-ethylenebis(salicylidenimine)) (Scheme 1). The reaction of
?(MeOH) with an excess of EDA (3 equivalents) under similar
2
3
SalproH undergoes
(transamination) reactions due to an extended chelation [2N,
(
3O + OH] by the flexible backbone.
1
2
Schiff base (SB) compounds are versatile ligands for the
complexation of various metal ions and important intermediates
reaction conditions (heating at reflux temperature in polar solvent)
failed to yield 2?(MeOH). This is probably due to blockade by the
uranium atom of the protonation of the imine nitrogen atom,
inhibiting the hydrolysis of the imine bond that leads to
1–3
in many enzymatic reactions. Transamination involving the SB
is a key step of biological reactions involving, for example,
4 5,6
pyridoxal phosphate, or semicarbazide-sensitive amine oxidase.
19
transamination. Preliminary studies with uranyl-SB complexes
In a transamination reaction, the stronger base replaces the weaker
containing [2N, 2O] coordination sites, including the uranyl-
base; in this case, the imine group (CLN) of the SB is susceptible to
hydrolysis by nucleophiles. Metal ion catalysis in transamination
Salophen complex (Salophen = N,N-disalicylidene-o-phenylene-
20
diamine), suggest facile transamination under mild conditions.
7
8,9
reactions of SB compounds is well documented. With transition
The crystals of 1?(S) were obtained from either DMSO or
2
2
+
2+
metal ions including Cu , Ni , Zn and Fe , the transamina-
2+
2+
DMF and pyridine (in case of 2?(S)) from supersaturated solutions
tion reaction is equilibrium controlled, and the excess of
containing precipitates.§ The structures of 1?(DMF) and 2?(Py)
2
7,10
exchanging amine favors the reaction. Presumably, the multiple
charge of the SB is more effective than the lone charge of the
proton at stabilizing the carbanion formed upon heterolytic
along with selected bond distances and angles are shown in Fig. 1
and 2, respectively. The geometry around the uranium atoms in
1?(DMF)2 and 2?(Py) could be best described as pentagonal
1
1
cleavage. In a recent study, the uranyl ion has been demonstrated
to catalyze the reversible transamination of 2-methylalanine with
bipyrimidal with axial OLULO moieties. In 1?(DMF) , the
2
coordination around the metal center is completed by the imine
N, aryl O, the bridging oxo-group, a bridging hydroxyl group, and
a solvent molecule. The phenyl moieties are distorted and present
pyridoxal (Vitamin B
6 2 3 3 2
), yielding [(UO PmHpyr) (m -O)]Cl?3H O
12
(3) (PmHpyr = pyridoxaminylpiruvate anion). Transamination
in uranyl-SB complexes with 8-hydroxy-7-quinolinecarboxalde-
hyde is also reported, where the addition of EDA or diamino-
on the opposite sides of the plane defined by the U
plane. Similar distortion has been observed in the dinuclear
UO (ASB)] complexes (ASB = 3-amino, 1,2-propanediol based
asymmetric SB ligands).
The ULO distances (avg 1.786(4) s) and OLULO angles (avg
79(2)u) are typical of the corresponding distances and angles
2 2 2
(m -O)(m -OH)
13
benzene yields symmetrical tetradentate Schiff base complexes.
[
2
2
14
In an effort to synthesize stable uranyl-SB complexes to be used
as model compounds in the investigation of new means of
remediation of heavy metals from aqueous sources or nuclear
wastes, we here report a dinuclear hydroxyl-bridged uranyl-SB
complex, which does not undergo nucleophilic addition or
substitution reactions. Recently, we have reported unsymmetrical
dinuclear uranyl-SB complexes, found to be resistant to simple
1
21
reported for the uranyl compounds in the literature. The U–N
distances (2.544(5) and 2.579(5) s) are unsymmetrical, and in the
2
2
range usually observed in uranyl-SB complexes (2.54–2.58 s).
The average CLN distance in 1?(S) (1.279(8) s) is shorter than the
2
14
nucleophilic addition and/or substitution reactions.
The reaction of SalproH (1,3-bis(salicylideneamino)-2-propa-
nol) and uranyl nitrate in the presence of an equivalent amount
of triethylamine (TEA) as base in MeOH–CHCl yielded
?(MeOH) .{ Similar reactions of SalproH with metal ions such
3
3
1
2
3
2+
3+
2+
as Cu , Mn or Zn typically yield multinuclear (tri-, tetra- and
15–18
hexanuclear) alkoxo-bridged clusters.
conditions using EDA as the base instead of TEA, the compound
Under the same reaction
[
UO (Salen)MeOH] (2?(MeOH)) was obtained.{ The catalytic role
2
Auburn University, Auburn, AL, USA. 36849.
E-mail: gordeae@auburn.edu; Fax: 01-334-844-6959;
Tel: 01-334-844-6973
Scheme 1 Formation and reactivity of 1 (Ar = phenyl group, solvent
{
Electronic supplementary information (ESI) available: Experimental
procedure, NMR and mass spectra, structure of SalproH , crystal data,
selected bond distances and angles. See DOI: 10.1039/b712322a
attached to the uranyl group is not shown). Reaction steps: a: MeOH,
3
2
2
+
mild conditions; b: UO
and triethylamine; c: reflux with excess EDA.
4
006 | Chem. Commun., 2007, 4006–4008
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few discrepancies in the bond distances between 2?(Py) and
2
2?(H O/EtOH/DMSO) and similar uranyl-SB (with Py as
coordinating solvent) have been observed. For instance, the
average U–O distance in 2?(Py) (2.236(3) s) is smaller than those
reported for 2?(H
2
O/EtOH/DMSO) (avg 2.281, 2.267 and 2.248 s,
respectively). Similarly, the U–Npy distance in 2?(Py) (2.617(6) s)
is shorter than those observed in [UO (tert-butylSalen)(Py)]
2.640 s) and [UO (Salpn)(Py)] (2.632 s) (Salpn
28
2
(
=
2
28
N,N-propylenebis(salicylidenimine). The OLULO angle and
U–N distances are, however, within the range observed in these
28
cases.
In conclusion, we have synthesized a novel dinuclear uranyl-SB
complex (1?(S) ), which does not undergo nucleophilic addition
2
and substitution reactions, contrary to SB complexes with
transition metal ions. The extended chelation due to the flexible
backbone as well as the bridging hydroxyl group provides stability
to the overall complex. Such compound(s) could be useful to
further explore the uranium chemistry from remediation as well as
speciation points of view. This may also be of interest in stabilizing
wastes from nuclear fuel sources in alkaline solutions. We are
continuing with the investigation of similar reactivity with different
uranyl-SB complexes with variable or extended backbones.
2
Fig. 1 Molecular structure of 1?(DMF) (ORTEP drawing with 50%
thermal ellipsoids). The selected bond distances (s) and angles (u) are
U1–O1, 1.792(4); U1–O2, 1.783(4); U1–O6, 2.241(5); U1–O7, 2.380(5);
U1–O8, 2.327(4); U1–O1D2, 2.461(5); U1–N1, 2.544(5); U1–U2, 3.869(8);
O1–U1–O2, 178.7(2).
Notes and references
2
9
{
1
SalproH
?(MeOH)
hexa-hydrated uranyl nitrate and triethylamine in MeOH–CHCl
Compound 2?(MeOH) was obtained by addition of equivalent amounts of
SalproH , ethylenediamine and uranyl nitrate in MeOH–CHCl (50 : 50)
mixture. Salen could be obtained by the addition of equivalent amounts of
SalproH and EDA in MeOH (see ESI{). Orange crystals of 1?(DMF) and
?(Py) suitable for X-ray analysis were obtained by slow evaporation of
DMF, DMSO and Py solutions containing precipitates.
3
was synthesized according to the reported method. Complex
was synthesized by refluxing equivalent amounts of SalproH
(50 : 50).
2
3
,
3
3
3
3
2
2
§
C
Crystal data for 1?(DMF)
23 30
H
2
:
[(UO
2
)
2
(Salpro)(OH)(DMF)
, M = 998.57, monoclinic, space group P2 /n, a =
2
],
N
4
O
10
U
2
1
1
1.797(3), b = 9.784(2), c = 24.594(5) s, b = 94.811(4)u, V =
3
2
1
2
1
0
828.7(1) s , T = 193(2) K, Z = 4, l = 0.71073 nm, m = 11.495 mm
8 601 reflections measured, 4058 unique, Rint = 0.0687, R1 [I . 2s(I)] =
.0272, wR2 = 0.0539, maximum/minimum residual electron density: 1.177
,
Fig. 2 Molecular structure of 2?Py (ORTEP drawing with 50% thermal
ellipsoids). The selected bond distances (s) and angles (u) are U1–O1,
2
3
1.783(4); U1–O3, 2.242(4); U1–O4, 2.230(3); U1–N1, 2.544(5); U1–N3,
and 20.691 (e s ); CCDC 656873. Crystal data for 2?(Py):
[(UO )(Salen)(Py)], C21 U, M = 615.42, orthorhombic, space
group Pca2 , a =18.427(2), b = 9.0712(1), c = 11.8143(1) s, V =
2.617(6); O1–U1–O2, 177.3(2).
2
12 3 4
H N O
1
3
21
1
974.8(4) s , T = 193(2) K, Z = 4, l = 0.71073 nm, m = 8.252 mm , Flack
parameter = 0.001(10), 10 662 reflections measured, 2838 unique, Rint
0.0569, R1 [I . 2s(I)] = 0.0254, wR2 = 0.0574, maximum/minimum
corresponding distances observed in [UO (Salophen)(S)] (S =
2
=
22,23
MeOH (1.30 s), DMF (1.289 s), and DMSO (1.290 s)).
The
23
residual electron density: 1.146 and 20.605 (e s ); CCDC 656874. For
crystallographic data in CIF or other electronic format, see DOI: 10.1039/
b712322a
presence of a stronger CLN bond, which is most probably due to
the flexible backbone resulting in better chelation around the metal
ion, might explain its inhibition toward hydrolysis.
The bridging oxo distances (avg U–O7 = 2.372(4) s) are
in accordance with the U–Ooxo distances observed in
1
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26
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2
7
[(UO ) O(OH) ]?5H O (2.303–2.433 s).
2
4
6
2
2872.
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2
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008 | Chem. Commun., 2007, 4006–4008
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