Inorganic Chemistry
Article
(s), 742(s), 752(s), 766(m), 799(w), 821(s), 844(s), 889(w), 900(m),
927(m), 967(w), 958(m, sh), 964(s), 980(w), 992(m, sh), 995(s),
1010(s), 1073(m), 1111(w), 1121(w), 1127(w), 1145(w), 1149(w),
1154(m), 1166(w), 1174(w) cm−1.
uranyl(VI) coordination chemistry and challenging long-held
concepts about the chemistry of {UO2}2+ while increasing the
knowledge of reactivity, bonding, and structure within uranyl
complexes. Exploration of novel uranyl complexes is important
for understanding the seminal role of the {UO2}2+ structural
unit in actinide science that includes current and future nuclear
fuel cycles, environmental remediation, safe storage of nuclear
materials, and the long-term immobilization of nuclear waste.
The electronically tunable nitrogen-donor ligand 2,6-bis(2-
benzimidazolyl)pyridine (BBP), which can be functionalized at
the imidazole positions, has been studied as a sensitizer for
lanthanide luminescence and has also shown potential as a low
valent actinide sequestering agent.24−30 In this instance, the free
base derivative of 2,6-bis(2-benzimidazolyl)pyridine (H2BBP)
has been employed and successively deprotonated with sodium
hydride to produce new mono- (NaHBBP) and disodium
(Na2BBP) substituted salts, and the reactivity of these species
has been explored with uranyl(VI) chloride. This systematic
approach to synthesizing N-donor uranyl complexes has
provided a system with discrete chemical, structural, and
electronic (bonding) properties within a N-donor uranyl
system and a preliminary probe of the uranium 4d edges by
scanning transmission X-ray microscopy is described. The
synthesis and structural characterization of three new {UO2}2+
tridentate N-donor complexes 1−3 is reported herein. In
addition, first-principles DFT-based calculations have been
employed to compare coordination environments and elec-
tronic structures of 1−3 to investigate the origin of the
geometrical differences found in these uranyl complexes.
Synthesis of Water-Free 2,6-Bis(2-benzimidazolyl)pyridine,
[H2BBP·xPy]. In an inert atmosphere box, H2BBP·0.5H2O (0.0950 g,
0.2966 mmol) was dissolved in pyridine (2 mL) and stirred over a
mixture of 4 Å and 13X molecular sieves for 1 h, producing a cloudy
solution. The solution was filtered through Celite supported on glass
wool, and pentane (ca. 10 mL) was added with stirring to precipitate a
white solid. The solid was collected by centrifugation, washed with
pentane (3 × 1 mL), dried under vacuum conditions, and confirmed as
the target H2BBP·0.5Py by NMR (0.101g, 97%). 1H NMR (300 MHz,
25 °C, (CD3)2SO): δH 13.03 (2H, s), 8.61 (1H, d, Py), 8.39 (2H, d),
8.22 (1H, t), 7.85−7.75 (4.5H, brm), 7.45−7.30 (5H, brm) ppm.
Analysis calculated for C21.5N5.5H15.5: C, 73.59; N, 21.95; H, 4.45.
Found: 73.60, 21.41, 4.56.
Synthesis of Monosodium 2,6-Bis(2-benzimidazolyl)-
pyridine, [NaHBBP·xPy]. H2BBP·0.5Py (0.0163 g, 0.0464 mmol)
and NaH (0.0006 g, 0.0250 mmol) were combined and stirred in
pyridine (1 mL) for 60 min, producing a clear solution after initial
bubbling. Pentane (5 mL) was added dropwise to the solution with
stirring, leading to precipitation of a white solid collected by
centrifugation, washed with ether (2 × 1 mL), dried under vacuum
conditions (0.0157 g, 91%), and confirmed as NaHBBP·0.5Py by
NMR. 1H NMR (300 MHz, 25 °C, (CD3)2SO): δH 13.08 (1H, s), 8.61
(1H, d, Py), 8.25 (2H, d), 8.01 (1H, t), 7.82(0.5H, t, Py), 7.62 (4H,
m), 7.42 (1H, t, py), 7.10 (4H, m) ppm. The THF adduct can be
formed by carrying out the reaction in THF to produce NaHBBP·
xTHF.
Synthesis of Disodium 2,6-Bis(2-benzimidazolyl)pyridine,
[Na2BBP·xPy]. H2BBP·0.5Py (0.0084 g, 0.0239 mmol) and NaH
(0.0011 g, 0.0500 mmol) were combined and stirred in pyridine (1
mL) for 60 min, producing a cloudy solution after initial bubbling. The
solution was then centrifuged and the clear supernate collected. To the
supernatant, pentane (5 mL) was added dropwise with stirring that
resulted in the precipitation of a white solid collected by
centrifugation, washed with ether (2 × 1 mL), dried under vacuum
conditions (0.0089 g, 86%), and confirmed as Na2BBP·Py by NMR.
1H NMR (300 MHz, 25 °C, (CD3)2SO): δH 8.57 (2H, d, Py) 8.16
EXPERIMENTAL SECTION
■
Syntheses Caution! 238U is a low specific-activity α-particle emitting
radionuclide, and its use presents hazards to human health. This research
was conducted in a radiological facility with appropriate analyses of these
hazards and implementation of controls for the safe handling and
manipulation of toxic and radioactive materials.
(2H, d), 7.91 (1H, t), 7.78 (1H, t, Py), 7.53 (4H, m), 7.38(2H, t, Py),
6.97 (4H, m). The THF adduct can be formed by carrying out the
reaction in THF to produce Na2BBP·xTHF.
All experiments were performed in a MBraun Labmaster 100 argon
atmosphere glovebox, except for the preparation of BBP·xH2O, which
was performed in a fume hood. All solvents were purchased either
anhydrously from a reputable supplier and/or distilled over an
appropriate desiccant, degassed, stored over a mixture of 4 Å/13X
molecular sieves, and tested with a solution of sodium benzophenone
prior to use. Chemicals were purchased from Aldrich; UO2Cl2(THF)3
was prepared according to literature procedures.4 1H and 13C NMR
were recorded on a Bruker Advance 300 MHz spectrometer and
referenced to residual proton and carbon solvent resonances.
Elemental analyses were performed by the Micro-Mass facility in the
Department of Chemistry at the University of California at Berkeley.
Infrared spectra were recorded with a Mattson Sirius 100
spectrometer.
Synthesis of 1, [UO2(H2BBP)Cl2]. H2BBP·0.5Py (0.0245 g, 0.6980
mmol) and UO2Cl2(THF)3 (0.0350 g, 0.0628 mmol) were stirred in
pyridine (2 mL) for 1 h, producing a transparent yellow solution.
Hexane (10 mL) was added dropwise to the solution with stirring to
precipitate a yellow solid, collected on a medium porosity glass frit,
washed with THF (2 × 1 mL), and dried under vacuum conditions
1
(0.0401 g, 98%). H NMR (300 MHz, 25 °C, (CD3)2SO): δH 13.11
(2H, s), 8.34 (2H, d), 8.18 (1H, t), 7.76 (4H, q), 7.33 (4H, m) ppm.
Analysis calculated for UO2Cl2C19N5H13: C, 34.99; N, 10.73; H, 2.01.
Found; 35.70, 10.65, 1.94. IR spectrum (Nujol): 613(m), 625(m),
642(w), 662(w), 668(m), 675(w), 701(w), 722(s), 743(s), 765(m),
792(w), 802(m), 817(m), 847(m), 871(m), 888(w), 918(s), 930(s),
967(m), 998(s), 1005(s), 1018(w), 1034(m), 1045(w), 1064(m),
1078(w), 1117(m), 1147(m) 1153(m), 1191(w) cm−1. Storage of a
concentrated pyridine solution of 1 at −28 °C for several weeks
yielded yellow block-like crystals of [UO2(BBP)Cl2]·2Py suitable for
X-ray diffraction.
Synthesis of 2,6-Bis(2-benzimidazolyl)pyridine·H2O [H2BBP·
xH2O]. 2,6-Bis(2-benzimidazyl)pyridine was synthesized by modifica-
tion of a previously reported method.31 o-Phenylenediamine (4.33 g,
40 mmol) and 2,6-dicarboxylic acid pyridine (3.35 g, 20 mmol) were
heated in an autoclave at 230 °C for 4 h and then allowed to cool. The
brown solid was extracted into methanol (500 mL) and stirred
overnight, resulting in a brown solution and a white solid. The brown
solution was separated by filtration and the volume reduced under
vacuum conditions to precipitate a cream colored solid of hydrated
2,6-bis(2-benzimidazolyl)pyridine (H2BBP·0.5H2O) subsequently col-
lected by filtration, washed with ether (2 × 10 mL), and air-dried (1.04
Synthesis of 2, [UO2(HBBP)(Py)Cl]·Py. NaHBBP·0.5Py (0.0303
g, 0.0812 mmol) in pyridine (1 mL) was added to a solution of
UO2Cl2(THF)3 (0.0403 g, 0.0723 mmol) in pyridine (1 mL)
dropwise, producing a clear orange solution and was allowed to stir
for 1 h. To the solution, pentane (6 mL) was added dropwise to
precipitate an orange solid, collected by centrifugation, washed with
THF (2 × 2 mL), and dried under vacuum conditions (0.0400 g,
71%). Analysis calculated for UO2ClC24N6H17·Py: C, 45.00; N, 12.66;
1
g, 16%). The water content was determined by NMR. H NMR (300
MHz, 25 °C, (CD3)2SO): δH 12.99 (2H, s), 8.35 (2H, d), 8.18 (1H, t),
7.77 (4H, q), 7.33 (4H, m) and 3.30 (1H, s, H2O) ppm. 13C NMR
(100.6 MHz, 25 °C, (CD3)2SO): δ 151.1, 148.1, 145.2, 141.0, 136.0,
125.1, 123.6, 121.2, 120.1, and 116.8 ppm. IR spectrum (Nujol): 737
1
H, 2.86. Found: 41.98, 11.85, 3.09. H NMR of [UO2(BBP)(Py)Cl]·
2Py (300 MHz, 25 °C, (CD3)2SO): δH 13.11 (1H, s), 8.61 (4H, d),
2507
dx.doi.org/10.1021/ic4026359 | Inorg. Chem. 2014, 53, 2506−2515