New air-stable uranium(IV) complexes with enhanced volatility

Article abstract of DOI:10.1039/c5nj00647c

Herein we report the synthesis and characterization of new air-stable uranium(iv) complexes based on three different heteroarylalkenolate ligands namely DMOPFB (1) (1-(4,5-dimethyl-oxazol-2-yl)-3,3,4,4,4-pentafluoro-but-1-en-2-ol) with an elongated fluori

Full text of DOI:10.1039/c5nj00647c

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New air-stable uranium(IV) complexes with
enhanced volatility†
Cite this: New J. Chem., 2015,
39, 7571
Jennifer Leduc, Rajitha Ravithas, Lisa Rathgeber and Sanjay Mathur*
Received (in Montpellier, France)
15th March 2015,
Accepted 22nd July 2015
DOI: 10.1039/c5nj00647c
www.rsc.org/njc
Herein we report the synthesis and characterization of new air-stable that suppress intermolecular interaction in the solid-state13,14
uranium(IV) complexes based on three different heteroarylalkenolate and the reduction of the overall molecular weight using a
ligands namely DMOPFB (1) (1-(4,5-dimethyl-oxazol-2-yl)-3,3,4,4,4- smaller tetradentate ligand. The heteroarylalkenolate ligand
pentafluoro-but-1-en-2-ol) with an elongated fluorinated alkyl chain DMOPFB 1 was synthesized using 2,4,5-trimethyloxazole, penta-
compared to DMOTFP (2) (3,3,3-trifluoro-1-(4,5-dimethyloxazol-2-yl) fluoropropionic anhydride and pyridine as base. The reaction
propen-2-ol) and the tetradentate enaminone TFB-en (3) (N,N0-bis- proceeded at room temperature for 12 hours to produce the
(4,4,4-trifluorobut-1-en-3-on)-ethylenediamine). These new com- target ligand that was purified via sublimation (45 1C, 10ꢀ3 mbar)
plexes exhibit sufficiently high volatilities, with respect to previously in 52% yield. It was characterized using 1D and 2D NMR
reported uranium compounds, and are thus promising precursors for spectroscopy. The 1H NMR spectrum of 1 showed signals
chemical vapor deposition (CVD) of uranium oxide materials.
corresponding to the enol and enaminone tautomers: the signals
at 11.55 and 11.48 ppm could be assigned to the –OH and –NH
Investigations on uranium complexes have recently attracted protons, respectively, whereas the singlet at 5.89 ppm could be
substantial attention in fields ranging from subvalent (U(II)) attributed to a vinylic proton indicating the existence of the
compounds, isotope separation to materials science.1–6 Uranium enolic form of 1. Comparison of the sublimation temperatures
complexes with fluoride-,7 borohydride-,8 amide-,9 silazane-,9 with DMOTFP 2 (s.p. 55 1C, 10ꢀ3 mbar)15 showed that 1 was
alkoxide-10,11 and acetylacetonate12 ligands are known to be more volatile (s.p. 45 1C, 10ꢀ3 mbar). Tetradentate enaminone
volatile and have been studied especially for isotope separation ligand 3 was prepared in a straightforward two-step synthesis. In
applications. However, these complexes are mostly not suitable the first step, 1-ethoxy-4,4,4-trifluorobut-1-en-3-one (TFAE) was
for the gas phase synthesis of uranium oxide materials by chemical synthesized according to the procedure of S. Matsuo et al.16,20
vapor deposition (CVD) due to their low vapor pressure,7 uncon- The resulting product was reacted with ethylenediamine17 resulting
trolled thermal decomposition,8 reactivity towards moisture9,10 in TFB-en (N,N0-bis-(4,4,4-trifluorobut-1-en-3-on)-ethylendiamine 3)
and visible and UV light sensitivity11 or unsuitability for safe with an isolated yield of 75%.
long-term storage.7,8 Recently, we have demonstrated the synthesis
The reaction of uranium turnings with 2 equiv. of iodine and
of air-stable, volatile uranium(IV) heteroarylalkenolates as well as 4 equiv. of 1 at 50 1C for 2 days resulted in the formation of
their gas phase conversion to uranium oxide films.5 However, when U(DMOPFB)4 4 as a green solid in 50% yield (Scheme 1A).5,18
applied in a thermal CVD process relatively high precursor tempera- Compound 4 was characterized using 1D and 2D NMR spectro-
tures (B150 1C) were required for the heteroarylalkenolates. In our scopy, elemental analysis and single crystal X-ray diffraction
quest for new uranium complexes with enhanced volatility, we analysis. Compared to the free DMOPFB ligand, the signal of
report here on two alternative synthetic approaches based on the the vinylic proton was strongly shifted downfield to 13.1 ppm,
modification (elongation) of the perfluoroalkyl chain of b-donor and the signals of the methyl groups were shifted upfield to
alkenolates, which are known to enhance the volatility of metal ꢀ2.4 and ꢀ18.0 ppm, respectively, plausibly due to the para-
complexes due to electrostatic repulsions between the CxFy-groups magnetic character of the complex. The strong upfield signal of
ꢀ18.0 ppm was assigned to the methyl group oriented toward
1
the uranium center. Since H NMR data showed only a signal set
for the ligand, it can be deduced that the uranium center maintains
Chair of Materials and Inorganic Chemistry, University of Cologne, Greinstr. 6,
50939 Cologne, Germany. E-mail: sanjay.mathur@uni-koeln.de
the symmetric eight-coordinate environment such as the square
antiprism found in the solid-state structure. In comparison to the
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nj00647c
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Scheme 1 Synthesis of U(DMOPFB)4 4 (A) and U(TFB-en)2 5 (B).
U(DMOTFP)4 complex 6,5 the chemical shifts were more pro-
nounced. The fluorine signals appeared at ꢀ76.1 (–CF3) and
ꢀ107.3 ppm (–CF2) (Fig. 1).
Time-dependent NMR analysis showed that 4 is slowly
oxidized to the diamagnetic species UO2(DMOPFB)2(H-DMOPFB)
8 in THF-d8 probably due to its reaction with water and oxygen
inadvertently present in the solvent (further information in
the ESI†). Single crystals of U(DMOPFB)4 4 were obtained by
recrystallization from heptane solution.
Compound 4 crystallized in the orthorhombic space group
Pbca, with eight molecules per unit cell, whereas 6 was reported
to crystallize in the monoclinic centrosymmetric space group
C2/c with four molecules per unit cell. The uranium atoms
exhibit a distorted square antiprismatic coordination sphere
(Fig. 2). The two ligands generating one distorted square plane
are aligned trans to each other. The difference in bond lengths
of U–N and U–O is responsible for the distortion of the square
antiprismatic coordination sphere resulting in angles deviating
from ideal 901 by ꢁ151 for N3–N4–O6–O8 and by ꢁ101 for
N1–N2–O2–O4. The mean U–O distances of 4 (ca. 2.25 Å) were
similar to those observed in U(DMOTFP)4 6 (2.24 Å).5
Fig. 2 Molecular structure of U(DMOPFB)4 4. Thermal ellipsoids are
shown at the 50% probability level and hydrogen atoms are omitted
for clarity. Selected bond lengths (Å): U1–O2 2.231(5), U1–O4 2.266(5),
U1–O6 2.236(5), U1–O8 2.253(5), U1–N1 2.586(6), U1–N2 2.619(7),
U1–N3 2.649(6), U1–N4 2.646(6).
representing lower masses probably result from the fragmenta-
tion of the DMOPFB ligand. This proved the stability of 4 in the
gas phase at elevated temperatures.
Thermogravimetric analysis of 4 performed under nitrogen
showed a lower decomposition temperature, when compared
with the DMOTFP complex 6. No mass losses were observed
until the onset of decomposition at 240 1C (Fig. 3). Decomposi-
tion occurs in one step and complete combustion was achieved
at 4290 1C. The experimental weight loss (74%) due to the
formation of UO2 is in good agreement with the theoretical
value (79%).
The higher volatility of 4 was confirmed using electron
impact mass spectrometry (EI-MS). Due to the technical limit
of the mass spectrometer at m/z = 1200, the M+ signal (m/z =
1262) could not be detected. Instead, the radical cation derived
from the loss of one DMOPFB ligand was found to possess the
highest intensity ([U(DMOPFB)3]+; m/z = 1006). Other signals
Complex 5 was synthesized following an analogous pathway
as described for 4 by reacting uranium turnings with 2 equiv. of
Fig. 1 Assignment of 1H (blue), 13C (black) and 19F (green) NMR shifts of
U(DMOPFB)4 4 in THF-d8 exemplarily shown for one of the four DMOPFB
ligands in [ppm].
Fig. 3 Thermogravimetric analysis of U(DMOPFB)4 4 under a nitrogen
atmosphere.
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Letter
and 20 ml THF. The reaction mixture was vigorously stirred for
2 d at 50 1C yielding a brown suspension. The reaction mixture
was cooled to rt and decanted to collect a mixture of green and
black crystals. The mixture was washed with hexane and dried
under reduced pressure. The remaining iodine was removed via
sublimation at 110 1C under reduced pressure and the product
was obtained as a green solid (1.1 g, 50%).
1H-NMR (300 MHz, rt, THF-d8): d [ppm] = 13.1 (s, 4H); ꢀ2.4
(s, 12H); ꢀ18.0 (s, 12H).
Fig. 4 Assignment of 1H (blue), 13C (black) and 19F (green) NMR shifts of
U(TFB-en)2 5 in THF-d8 exemplarily shown for one ligand in [ppm].
19F-NMR (282 MHz, rt, THF-d8): d [ppm] = ꢀ76.1 (s, 12F);
ꢀ107.3 (s, 8F).
13C-NMR (75 MHz, rt, THF-d8): d [ppm] = 176.0, 170.1, 140.2,
139.8, 125.2, 104.5, 68.2, ꢀ0.9, ꢀ21.0.
iodine and 2 equiv. of TFB-en 3 at 50 1C. The reaction was
sluggish and 4 days were needed for full conversion of the
educts into products showing a less pronounced tendency of
TFB-en towards the complexation of uranium. U(TFB-en)2 5 was
isolated after sublimation at 130 1C (10ꢀ3 mbar) in 20% yield.
Compound 5 was characterized using 1H, 13C as well as 19F NMR
spectroscopy (Fig. 4) and mass spectrometric analysis.
CHNS (found/calculated) [%]: C (34.09/34.24); N (4.21/4.44);
H (2.25/2.24).
EI-MS (20 eV, 115 1C): m/z = 1005 [M]+ (100%), 257 [M-(CF2CF3)]+
(8%), 138 [C6H7NO]+ (20%).
Bis[g2-N,O-N,N0-bis-(4,4,4-trifluorobut-1-en-3-on)
ethylenediamine] uranium(IV) (5)
The proton and carbon signals of the ethylene bridge were
1
strongly upfield shifted (ꢀ38.2 ppm, H and ꢀ43.4 ppm, 13C).
Oxide-coated uranium turnings (0.62 g, 2.61 mmol) were stirred
for 5 min in 5 ml of concentrated nitric acid and rinsed with
deionized water and acetone to remove the native oxide. A 100 ml
Schlenk flask was charged with freshly obtained oxide-free
uranium metal turnings, iodine (1.33 g, 5.22 mmol), N,N0-bis-
(4,4,4-trifluorobut-1-en-3-on)-ethylendiamine (TFB-en) 3 (1.59 g,
5.22 mmol) and 25 ml THF. The reaction mixture was vigorously
stirred for 4 d at 50 1C yielding a brown suspension. The solvent
was directly removed under reduced pressure. The remaining
iodine was removed via sublimation at 110 1C under reduced
pressure. The crude product was purified via sublimation at
130 1C (10ꢀ3 mbar) and the product was obtained as brownish
crystals (0.42 g, 20%).
The vinylic protons displayed a downfield shift to 15.2 ppm and
14.4 ppm. As observed for 4, EI-MS showed that 5 also exhibited
enhanced volatility compared to our previously published
uranium(IV) complexes.5 The M+ signal (m/z = 842) with the
highest intensity was detected at 110 1C. Elemental analysis as
well as EI-MS data showed that the compound still contained
slight impurities of iodine, which was removed by additional
washing with heptane and crystallization of 5 from toluene at
ꢀ30 1C. However, the resulting green/brownish solid was found
by 1H NMR analysis to be a mixture of 5 and 3. Further attempts
to purify the compound like solvent extraction or recrystalliza-
tion were unsuccessful and only resulted in decomposition of 5.
Similar to the derivative 4, 5 decomposed in undried THF-d8 to give
TFB-en 3 and a diamagnetic, bright yellow compound from NMR
signals, which could be best assigned to UO2(TFB-en)(THF) 9.19
In summary, we could show that the length of the fluorinated
alkyl chain in the oxazole-ligand and the decrease in the molecular
weight of the complexes by using a tetradentate enaminone
ligand represent viable approaches for increasing the volatility
of uranium(IV) compounds. Both complexes are stable as solids
in air; however only 4 may prove as the useful precursor for CVD
processes since 5 could not be completely purified due to its
instability. Further efforts are currently underway.
1H-NMR (300 MHz, rt, THF-d8): d [ppm] = 15.2 (d, 4H); 14.4
(d, 4H); ꢀ38.2 (s, 8H).
19F-NMR (282 MHz, rt, THF-d8): d [ppm] = ꢀ67.7 (s, 12F).
13C-NMR (75 MHz, rt, THF-d8): d [ppm] = 200.8, 160.1, 128.2,
81.6, ꢀ43.4.
CHNS (found/calculated) [%]: C (27.51/28.52); N (6.36/6.65);
H (2.32/1.91).
EI-MS (70 eV, 110 1C): m/z = 842 [M]+ (100%), 152
[C5H5F3NO]+ (62%), 82 [C4H5NO]+ (25%).
Acknowledgements
We thank the FCI (‘‘Fonds der chemischen Industrie’’) and
the University of Cologne for providing financial assistance.
Prof. Dr. Gerd Meyer (Iowa State University, Ames) and Prof.
Dr. William J. Evans (University of California, Irvine) are gratefully
acknowledged for fruitful discussions.
Experimental
Tetrakis[g2-N,O-1-(4,5-dimethyl-oxazol-2-yl)-3,3,4,4,4-pentafluoro-
but-1-en-2-olato] uranium(IV) (4)
Oxide-coated uranium turnings (0.40 g, 1.70 mmol) were stirred
for 5 min in 5 ml of concentrated nitric acid and rinsed with
deionized water and acetone to remove the native oxide. A 100 ml
Schlenk flask was charged with freshly obtained oxide-free uranium
metal turnings, iodine (0.90 g, 3.50 mmol), 1-(4,5-dimethyl-oxazol-2-
yl)-3,3,4,4,4-pentafluorobut-1-en-2-ol (DMOPFB) 1 (1.80 g, 7.00 mmol)
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7574 | New J. Chem., 2015, 39, 7571--7574
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