An example of unusual pyridine donor Schiff base uranyl (UO22+) complexes

Article abstract of DOI:10.1039/c7cc02747h

The pentadentate coordination environment of a 2,6-bis[1-[(2-hydroxyphenyl)imino]ethyl] pyridine ligand scaffold was designed to accommodate the larger atomic radius of uranium as the uranyl dioxo cation, while fully occupying its equatorial plane. Here, two new uranyl (UO₂²⁺) complexes utilizing this scaffold have been synthesized from successive condensation reactions and subsequent metal complexation. Surprising Zn fluorescence is also discussed.

Full text of DOI:10.1039/c7cc02747h

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Reactivity differences of Sc
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An Example of Unusual Pyridine Donor Schiff Base Uranyl
Complexes
a
a
a
a*
E. E. Hardy, K. M. Wyss , M. A. Eddy, and A. E. V. Gorden
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The pentadentate coordination environment of a 2,6‐bis[1‐[(2‐
hydroxyphenyl)imino]ethyl] pyridine ligand scaffold was designed
to accommodate the larger atomic radius of uranium as the uranyl
dioxo cation, while fully occupying its equatorial plane. Here, two
2
+
new uranyl (UO
2
) complexes utilizing this scaffold have been
synthesized from successive condensation reactions and
subsequent metal complexation. Surprising Zn fluorescence is also
discussed.
As global energy consumption continues to increase, so do
1
demands on the energy sector. Nuclear power is a major
contributor, and remains an integral asset, to carbon neutral
2
energy production. Even as nuclear power is integrated and
essential to the energy market, there is a threat of
environmental contamination release along with an increasing
3
a,b
problem of nuclear waste storage.
Interest in fundamental
actinide research to better understand the f‐elements has been
on the rise to illuminate their fundamental chemistry and pave
2
Figure 2. Projection of UO 1 is shown without the non‐coordinating
acetonitrile molecule. Hydrogen atoms are shown in white, nitrogen in
blue, oxygen in red, and uranium in green. Inset image is the crystal from
the data collection.
the way for enhanced nuclear fuel remediation techniques.4a‐d
Recently, depleted uranium, a by‐product of enriching
uranium, has been investigated for applications in catalysis to
5
a‐b
complete difficult chemical transformations.
Researchers
have also sought to characterize the degree of actinide
covalency both synthetically and computationally using ligands
6
a‐b
featuring nitrogen donors.
Sessler and co‐workers have
explored changes in ligand aromatic character when bound to
7
an actinide. Examples of uranium coordination have included
pyrrole nitrogen donors in the ligand environment, but only a
8a‐
handful have included the less strongly coordinating pyridine.
c
Figure 1. Synthetic Scheme of UO
2
1 and UO 2
2
Previously, we have described highly conjugated,
tetradentate, mixed nitrogen and oxygen donor systems
a. Department of Chemistry and Biochemistry, Auburn University, 179 Chemistry
Building, Auburn, Alabama 36849, USA. E‐mail: anne.gorden@auburn.edu.
Electronic Supplementary Information (ESI) available. CCDC 1469301, 1497023. For
designed as chemosensors in on‐site detection for uranyl, the
ESI [synthetic details, additional electronic spectroscopy, and crystallographic data] most environmentally available oxidation state of uranium.9
a‐b
see DOI: 10.1039/x0xx00000x
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Other nitrogen donor systems that have been explored in this
effort involve mixed oxygen and nitrogen coordination
environments as well.1
0a‐d
Here, as shown in Figure 1, the 2,6‐
bis [1‐[(2‐hydroxy‐3,5‐ditert‐butylphenyl)imino]ethyl] pyridine
) and 2,6‐bis[1‐[(2‐hydroxynapthyl)imino]ethyl]pyridine
2
ligands and the corresponding UO (VI) complexes (UO , UO 2)
(
1
2
2
1
were designed and synthesized to accommodate the larger
atomic radius of uranium and fully occupy its equatorial plane,
as has been shown previously in alkyl amine Schiff base
1
1
ligands. The ability to change the electronic characteristics of
the ligand environment easily, made these Schiff base ligands
an ideal scaffold for uranyl detection (synthetic details in SI).
The expansion and simultaneous change in π‐overlap of the
ligand in the tetradentate salen‐type Salqu and salphenazine
ligands have been used to differentiate between actinide and
A.
B.
9
transition metals of potential concern. One limitation of the
tetradentate binding geometry is the expansion of the binding
pocket to accommodate the larger atomic radius of uranium
often requires longer detection times because of poor
coordination kinetics. The dioxo cation uranyl has been shown
to prefer a pentagonal bipyramidal binding geometry,
explaining the presence of a coordinating solvent molecule in
numerous reported solid state structures. The observation of
this preference lead to the design of the expanded mixed
oxygen and imine nitrogen coordination environment, coupled
Figure 3a. Projection of UO
in white, nitrogen in blue, oxygen in red, and uranium in green. Inset
image is the crystal from the data collection. Figure 3b. Projections of
2
UO 2, highlighting pi‐pi stacking in the solid‐state.
2
2 is shown. Hydrogen atoms are shown
9
some examples of
derivatives.
The similarity of observed bond lengths and angles to
a
dipicolinic systems and their
8
a,14a,c
with a pyridine donor to occupy the fifth site. The syntheses of tridentate and tetradentate systems discussed suggest that the
UO Figure 2) and UO
Figure 3a‐b) are described in further preferred 5‐coordinate binding in the equatorial plane is
detail in the supplemental information from a commercially achieved without the necessity of significant ligand pocket
2
1
(
2
2 (
available starting material, 2,6‐pyridinedicarboxylic acid.
expansion or solvent coordination. Thus, the improved
coordination kinetics, within seconds for UO21, can be
‡
2
UO 1 has U1‐N1, U1‐N2, U1‐O1, and U1‐O2 bond lengths of
2
.532(2), 2.508(2), 2.2548(18), and 2.2604(18) respectively. explained by this reduced need for ligand rearrangement. While
These bond lengths are comparable (±0.08 Å) to previously UO
synthesized tetradentate salen‐type uranyl complexes.
2
1
2
incorporated an interstitial acetonitrile solvent molecule,
has π‐π stacking interactions that reduced the void
9
a‐c,12a‐b UO2
The U1‐N3 bond length of 2.623(2) are comparable to other U‐ volume and simultaneously the need for an interstitial solvent
N bonds, and average of 2.59 Å from coordinating pyridine molecule. The parallel offset π‐π stacking interactions (3.76 Å)
moieties in similar ligand environments.1
3a‐b
are highlighted in Figure 3b and are significantly longer than
The bond angles
O1‐U1‐N1, N1‐U1‐N3, N3‐U1‐N2, N2‐U1‐O2, O2‐U1‐O1 of graphene (3.2‐3.5 Å) and most examples of
9.54(7), 62.09(7), 61.81(7), 70.91(7), and 96.61(7) respectively
In the UV‐visible spectra a shift of the maximum of the free
describe a beautiful pentagonal bipyramidal coordination base, 334 nm (εb = 9,070 cm M ), is observed to 356 nm (εb =
π‐π interactions.15
6
‐
1
‐1
‐
1
‐1
geometry around the uranium metal centre. The O3‐U1‐O4 7,005 cm M ) upon the addition of UO
2
as the nitrate salt in
7‐9,12‐13
dichloromethane (Figure 4a). This shift is accompanied by the
angle of 175.83(9) is within previously reported ranges.
‡
growth of a charge transfer band at 419 nm (εb = 3,454 cm‐1
M
‐1
‐
UO
2
2
has U1‐N1, U1‐N2, U1‐O1, and U1‐O2 bond lengths of
1
‐1
)
and a large shoulder centred at 512 nm (εb = 1094 cm M ).
2
.486(3), 2.525(3), 2.264(3), and 2.267(3) respectively. These
These spectral changes are observed upon addition of 36 ppm
of uranyl salt; modifying the ligand in future studies to include
bond lengths are comparable to previously synthesized
9a‐c,12a‐b
tetradentate uranyl chemosensors and to UO
2
1
.
The U1‐
N3 bond length of 2.566(3) is slightly shortened from the t‐butyl further conjugation would allow for a decrease in the limit of
analogue, UO
1, but only slightly shorter than the average U‐N detection. In an effort to increase the extinction coefficient and
2
bond 2.59 Å from other similar coordinating pyridine moieties reduce the limit of detection of the system, the napthyl
in the literature.1
3a‐b
analogue (UO22) was synthesized. Absorbance is observed at
The bond angles O1‐U1‐N1, N1‐U1‐N3, N3‐
‐
1
‐1
‐1
‐1
U1‐N2, N2‐U1‐O2, O2‐U1‐O1 of 69.83(10), 62.68(10), 62.99(9), 220 nm (εb = 14,300 cm
M ), at 322 nm (εb = 4,021 cm M ),
‐
1
‐1
6
9.31(10), and 95.61(10) respectively describe a pentagonal and at 366 nm (εb = 2,710cm M ), in the UV‐visible spectrum
bipyramidal coordination geometry around the uranium centre. of UO
2
2
(figure SI5).
Solution fluorescence spectra were collected from a
solution of mixed with 1 equivalent of metal salt in methanol
Figure 4b). The UO , Cu, Co, Mn, Ni, and Fe metals quenched
The O3‐U1‐O4 angle of 175.96(13) is also within previously
7‐9,12‐13
1
reported ranges.
Dipicolinic acid (2,6‐pyridinedicarboxylic
(
2
acid) is a common coordination agent, and has been applied to
the extraction uranyl from acidic solutions.1
4a‐b
fluorescence exhibited from 365 nm excitation, while Zn, VO,
The observed U‐
N
2 2
py distance in UO 1 and UO 2 is equivalent to, or shorter than,
2
| J. Name., 2012, 00, 1‐3
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which were used in all calculations. The final R
1
was 0.0359
and Th increased fluorescence. Notably, Zn increased
fluorescence by a factor of 5, with a quantum yield of 1.6 %.
(
I>=2u(I)) and wR was 0.0755 (all data). CCDC: 1497023.
2
1
1
1
4000
2000
0000
§
Funding was provided by a grant from the Defence Threat
1
Reduction Agency, Basic Research Award # HDTRA1‐11‐1‐0044 to
Auburn University.
1:UO2 3:1
1
:UO2 3:2
1:UO2 3:3
1
1
1
1
1
1
:UO2 3:4
:UO2 3:5
:UO2 3:6
:UO2 3:7
:UO2 3:8
:UO2 3:9
1
Clean Power Plan, Environmental Protection Agency, Carbon
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8
6
4
2
000
000
000
000
0
2
3
4
A.
B.
290
340
390
440
490
540
L
L:UO2 1:1
L:Zn 1:1
L:Cu 1:1
L:Co 1:1
L:VO 1:1
L:Th 1:1
L:Mn 1:1
L:Ni 1:1
L:Fe 1:1
590
N. H. Anderson, S. O. Odoh, U. J. Williams, A. J. Lewis, G. L.
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5
6
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55; D. P. Halter, F. W. Heinemann, J. Bachmann, K. Meyer,
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3
75
475
575
675
775
Wavelength (nm)
Figure 4a.
dichloromethane solution with an increasing ratios of
UO (NO 6H O, from 3:1 to 3:10 1:UO Figure 4b. Solution
fluorescence of a serial titration of 1 with indicated metal salts, shown
is the 1:1 ration in methanol solution at 365 nm excitation.
UV‐Visible spectra of a serial titration of 1 in
2
3
)
2
2
2
.
9
2 2
In summary, UO 1 and UO 2 complexes with uncommon
pyridine coordination have been synthesized and characterized 10 K. D. Bhatt, B. A. Makwana, D. J. Vyas, D. R. Mishra, V. K. Jain,
in solution and in the solid state. This pentadentate binding
ligand fully occupies the equatorial binding environment,
removing the necessity of a coordinating solvent molecule or
ligand rearrangement allowing to quickly and selectively bind
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in the presence of zinc, inspiring interest in use as a zinc
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‡
UO
2 52 3 4
1 Crystal Data for C40H N O U (M =876.87): orthorhombic,
space group Pccn (no. 56), a = 13.9974(3) Å, b = 23.5111(5) Å, c =
1
4 G. Paolucci, G. Marangoni, G. Bandoli, D. A. Clemente, J.
Chem. Soc., Dalton Trans. 1980, , 1304; J. L. Lapka, A.
3
2
4
3.6244(5) Å, V = 7774.7(3) Å , Z = 8, T = 180(2) K, μ(MoKα) =
.217 mm , Dcalc = 1.498 g/mm , 93148 reflections measured
8
‐
1
3
Paulenova, M. Yu. Alyapyshev, V. A. Babain, R. S. Herbst, J. D.
Law, Radiochim. Acta, 97, 291; J. M. Harrowfield, N. Lugan,
G. H. Shahverdizadeh, A. A. Soudi, P. Thuery, Eur. J. Inorg.
(
3.386 ≤ 2Θ ≤ 61.054), 11882 unique (Rint = 0.0538, Rsigma = 0.0376)
which were used in all calculations. The final R was 0.0305 (I >
σ(I)) and wR was 0.0699 (all data). CCDC: 1469301. UO 2 Crystal
U (M =713.54): monoclinic, space group P2 /c
no. 14), a = 22.8062(10) Å, b = 7.2736(3) Å, c = 14.7805(6) Å, β =
1
2
2
2
Chem. 2006,
5 C. R. Martinez and B. L. Iverson, Chem. Sci., 2012,
6 K. P. Carter, A. M. Young, A. E. Palmer, Chem. Rev. 2014, 114
564; A. T. Aron, K. M. Ramos‐Torres, J. A. Cotruvo, Jr., C. J.
Chang, Acc. Chem. Res. 2015, 48, 2434.
2, 389.
Data for C29
H
21
N
3
O
4
1
1
1
3, 2191.
(
,
3
9
6
7.3343(5)°, V = 2431.78(18) Å , Z = 4, T = 180.45 K, μ(Mo Kα) =
.720 mm , Dcalc = 1.9488 g/mm , 61345 reflections measured
4
‐
1
3
(
3.6 ≤ 2Θ ≤ 63.04), 8098 unique (Rint = 0.0473, Rsigma = 0.0295)
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highlighting the novelty of the work)
New pyridyl Schiff base ligands with pentadentate coordination environments were designed to fully
2+
2
occupy the equatorial plane of uranyl (UO ).