Synthesis of low-valent uranium fluorides by C-F bond activation

Article abstract of DOI:10.1039/c5cc05049a

The uranium(iii) alkyl, Tp₂UCH₂Ph (Tp = hydrotris(3,5-dimethylpyrazolyl)borate), activates C-F bonds on a variety of fluorinated substrates. From these reactions two new uranium containing products, Tp₂UF and Tp₂UF₂, were isolated and characterized by ¹H, ¹³C, ¹¹B NMR, infrared and electronic absorption spectroscopies, as well as X-ray crystallography. Formation of the uranium(III) or uranium(iv) product was found to be substrate dependent.

Full text of DOI:10.1039/c5cc05049a

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Synthesis of low-valent uranium fluorides by C–F
bond activation†
Cite this:DOI: 10.1039/c5cc05049a
Christopher L. Clark, Jill J. Lockhart, Phillip E. Fanwick and Suzanne C. Bart*
Received 18th June 2015,
Accepted 27th July 2015
DOI: 10.1039/c5cc05049a
www.rsc.org/chemcomm
The uranium(III) alkyl, Tp*2UCH2Ph (Tp* = hydrotris(3,5-dimethyl- organic products have been confirmed using multinuclear NMR
pyrazolyl)borate), activates C–F bonds on a variety of fluorinated spectroscopy and mass spectrometry.
substrates. From these reactions two new uranium containing
Our studies commenced by treating a THF solution of 1-Bz
products, Tp*2UF and Tp*2UF2, were isolated and characterized by with one equivalent of 2,3,4,5,6-pentafluorotoluene at ambient
1H, 13C, 11B NMR, infrared and electronic absorption spectroscopies, temperature, which resulted in the isolation of a dark brown
as well as X-ray crystallography. Formation of the uranium(III) or solid following work-up (Table 1, entry 1a). The 1H NMR spectrum
uranium(IV) product was found to be substrate dependent.
showed three paramagnetically shifted and broadened resonances
ranging from ꢀ8.54 to 6.91 ppm, indicating equivalent Tp* ligands
C–F bond activation is currently an active area of research due in solution, similar to 1-Bz. Resonances for the endo- and exo-Tp*
to its utility in a variety of synthetic applications.1–4 While this CH3 protons are visible at ꢀ8.54 (18H) and ꢀ2.41 ppm (18H),
process has been accomplished using alkaline earth,5 main respectively. A characteristic singlet appears at 6.91 ppm (6H) for
group6 and transition metals,7 less is known about the ability of the pyrazole CH protons. Given the spectroscopic similarity to the
f-block elements to activate the relatively inert carbon–fluorine Tp*2UX (X = Cl, Br, I) series reported by Takats and co-workers,10,11
bond. As such, the collective knowledge regarding the structural the product was assigned as Tp*2UF (1-F, 94%). Analysis of the
details of complexes containing U–F bonds, especially those in
low oxidation states, is limited. Much of the literature primarily
Table 1 Reactivity of 1-Bz with fluorinated substrates to form 1-F
describes uranium(VI) derivatives due to the utility of UF6 in the
nuclear fuel cycle.8
An early discovery in the field of actinide mediated C–F bond
activation was reported by Andersen and coworkers, who described
the reactivity of the tetravalent uranium alkyl, CpMe3U(CMe3) (CpMe
=
Z5-MeC5H4), with fluorinated substrates. Treating CpMe3U(CMe3)
with two equivalents of hexafluorobenzene in toluene at ambient
temperature produced the uranium(IV) fluoride, CpMe3UF, and the
C–C coupled product, C6F5CMe3.9 Inspired by this significant
result, we sought to determine if similar C–F bond scission would
be possible with the trivalent uranium alkyl, Tp*2UCH2Ph (1-Bz).
Herein, we report the reactivity of 1-Bz with fluorinated substrates,
which results in both C–F bond activation and new low-valent
uranium fluoride complexes. These uranium fluorides have been
characterized using 1H, 13C, and 11B NMR spectroscopies, X-ray
crystallography, and electronic absorption spectroscopy, while the
Entry
1a
Substrate
Organic
Yield (%)
90
1b
1c
94
93
H.C.Brown Laboratory, Department of Chemistry, West Lafayette, IN 47907, USA.
E-mail: sbart@purdue.edu
† Electronic supplementary information (ESI) available: Experimental proce-
dures, multinuclear NMR data, X-ray crystallographic data. CCDC 1406897 and
1406898. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5cc05049a
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Scheme 1 Synthesis of 1-F2 from 1-Bz and pentafluorobenzene. 1-F is
produced via two competing pathways. Tetrafluorobenzyne is trapped by
anthracene to form 1,2,3,4-tetrafluorotriptycene.
Fig. 1 Molecular structures of 1-F (left) and 1-F2 (right), with ellipsoids displayed
at 30% probability level. Selected hydrogen atoms and co-crystallized solvent
molecules have been omitted for clarity. Select bond distances for 1-F in Å:
U1–F1 = 2.156(3), U1–N11 = 2.552(3), U1–N21 = 2.640(4), U1–N31 = 2.644(3),
U1–N61 = 2.542(4), U1–N71 = 2.739(3), U1–N81 = 2.604(4). Select bond
distances for 1-F2 in Å: U1–F1 = 2.090(6), U1–F2 = 2.086(6), U1–N11 =
in the formation of 1-F and the C–C coupled product, 1,2,3,4,5-
pentafluoro-6-benzyl-benzene, in high yields (94%) (Table 1, entry 1b).
Similar reactivity was observed for 1-Bz with perfluorotoluene, which
once again produced 1-F (89%) and the product from C–C coupling,
1,2,4,5-tetrafluoro-6-benzyl-3-(trifluoromethyl)benzene, in 93% yield
2.596(10), U1–N21
= 2.678(9), U1–N31 = 2.545(9), U1–N61 = 2.677(9),
U1–N71 = 2.582(9), U1–N81 = 2.615(10). F–U–F angle for 1-F2: 89.7(3)1.
organics by 1H and 19F NMR spectroscopy showed resonances (Table 1, entry 1c). Interestingly, no reaction was observed for
consistent with formation of 2,3,5,6-tetrafluoro-4-benzyl-toluene, a,a,a-trifluorotoluene or perfluorohexane, even after heating to
which was isolated in high yield (90%) (Table 1, entry 1a). This 55 1C; only degradation of 1-Bz was noted.
product is consistent with C–F bond activation at the para position
Additional studies were carried out using non-perfluorinated
of the substrate, followed by C–C bond formation.
aromatic substrates. Treating a dark green solution of 1-Bz with
To determine if 1-F exists as a monomer or dimer in the solid one equivalent of pentafluorobenzene produced an immediate
state, analysis by X-ray crystallography was employed. Dark green color change to dark purple, indicative of 1-F formation
blocks of 1-F grown by cooling a concentrated THF/pentane/ (Scheme 1). However, continued stirring caused additional
benzene (10/1/0.5) solution to ꢀ35 1C were analyzed. Refinement colour changes to dark blue, followed by translucent light
of the data revealed a seven coordinate uranium monomer with brown, which persisted upon standing. Removal of the volatiles
1
a monocapped octahedral geometry (Fig. 1, left). The U–N bond in vacuo yielded a tan solid. The H NMR spectrum of the new
lengths, ranging from 2.542(4) to 2.739(3) Å, are in agreement with uranium species had three paramagnetically shifted resonances
typical U–N bond lengths seen for bis-Tp* supported uranium(III) ranging from ꢀ7.61 to 19.31 ppm, indicative of C2v symmetry.
species.12–14 As expected, the terminal U–F distance of 2.156(3) Å is Resonances for the endo- and exo-Tp* CH3 protons appeared at
shorter than the analogous U–X distances reported for Tp*2UCl ꢀ7.61 (18H) and 19.31 ppm (18H), respectively. The pyrazole CH
(2.698 Å) and Tp*2UI (3.220 Å). To the best of our knowledge, resonance was visible at 6.87 ppm (6H), which led to the hypothe-
this complex represents the first crystallographically characterized sis that two C–F bond activation events occurred to form Tp*2UF2
1
terminal uranium(III) fluoride. The molecular structure of 1-F is (1-F2, 77%). The H and 19F NMR spectra of the isolated organic
similar to that reported by Takats for Tp*2SmF, which has a Sm–F product showed formation of the C–C coupled product, 1,2,4,5-
distance of 2.090 Å.15 The difference in M–F bond length is on the tetrafluoro-3-benzyl-benzene, isolated in low yield (27%).
order of the difference between the ionic radii of the two metals
An X-ray crystallographic analysis of 1-F2 was performed on
(Sm(III) = 0.96 Å, U(III) = 1.03 Å). Monomeric 1-F is reminiscent of light brown crystals grown by slow diffusion of a hexane/pentane
the symmetric U(III) dimer, [Cp00 UF]2 (Cp00 = 1,3-(Me3Si)2Z5-C5H3), (2 : 1) solution into a concentrated 1-F2/THF solution at ꢀ35 1C.
2
reported by Andersen, which contains two bridging fluorine atoms.16 Refinement of the data confirmed the assignment of 1-F2 (Fig. 1,
However, the terminal U–F bond of 1-F is shorter (0.175 Å) than the right). As in the case of 1-F, the U–N bond lengths were typical
shortest of the four bridging U–F bonds (2.331(3) Å) reported for for Tp*2U complexes, ranging from 2.545(9) to 2.678(9) Å. 1-F2
[Cp00 UF]2. The monomeric nature of 1-F versus the dimeric [Cp00 UF]2 represents the only example of a crystallographically characterized
2
2
is likely due to the greater steric demand of the Tp* ancillary ligand bis-Tp* supported uranium complex with two additional ligands
over the Cp-based system.17
completing the coordination sphere. The two fluoride ligands
In order to understand the generality of the C–F bond activation prove to be small enough to allow for symmetric Tp* ligands in
and C–C coupling reactions, the reactivity of 1-Bz with additional the solution phase, while the larger chlorides of Tp*2UCl2 impart
1
fluorinated substrates was tested. Treating 1-Bz with one equivalent asymmetry in the solution H NMR spectrum.18 The U–F bond
of hexafluorobenzene under the same reaction conditions resulted lengths of 2.090(6) and 2.086(6) Å are shorter than those of 1-F,
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Thus, benzyne dissociation occurs readily, as seen by the paucity
of benzyne coordinated uranium complexes.30,31 Concomitant
formation of 1,2,4,5-tetrafluoro-3-benzyl-benzene and the substi-
tuted benzyne supported the hypothesis that two reaction pathways
were occurring for the initial C–F activation. Interestingly, 19F NMR
spectroscopy showed unreacted substrate (460%) remained in the
NMR tube experiment despite complete conversion to 1-F2. Using
a sub-stoichiometric amount (0.25 equiv.) of pentafluorobenzene
again resulted in full conversion to 1-F2, albeit at a slower rate. Thus,
multiple C–F activation events occur at each substrate molecule.
Monitoring the reaction between 1-Bz and pentafluorobenzene
1
by H, 11B, and 19F NMR spectroscopy showed complete conver-
sion of 1-Bz to a mixture of 1-F and an intermediate species (1-F*)
within fifteen minutes. The in situ 1H NMR spectrum of 1-F*
shows 7 broad resonances (ꢀ32.13 to 12.58 ppm), as well as a new
signal at 4.83 ppm in the 11B NMR spectrum. Two broad
resonances in the 19F spectrum at ꢀ141.64 ppm and ꢀ236.63,
and a triplet at ꢀ143.81 ppm support a rotating perfluorophenyl
group in 1-F*. Based on this characterization, we hypothesize 1-F*
is Tp*2U(C6F5). This is consistent with the observed extrusion of
Fig. 2 Electronic absorption spectra of 1-F (purple) and 1-F2 (tan)
recorded in THF at 23 1C. Solvent overtones between 1670–1750 nm have
been removed.
reflecting the difference in the ionic radii of the U(III) and U(IV) toluene during the reaction, and b-fluoride elimination from 1-F*
centers (B0.1 Å).19 The U–F bond lengths in 1-F2 are shorter than to give both 1-F and 3,4,5,6-tetrafluorobenzyne (vide supra). When
those in Kiplinger’s uranium(IV) difluoride Cp*2UF2(NC5H5) the sample had turned dark blue, multinuclear NMR experiments
(U–F = 2.146 Å),20 likely due to the steric demand of the pyridine showed 1-F*, 1-F, and 1-F2 were present in solution.
ligand. The distances in 1-F2 are on the order of those in other
The reaction of 1-Bz with 2,3,5,6-tetrafluorotoluene proceeds
neutral uranium(IV) complexes ([Cp002UF2]2,16 Cp*2U(O-2,6-iPr2- through the analogous blue phase, and monitoring the reaction
C6H3)(F),21 Cp3UF,22 Cp*3UF,23) which range from 2.073–2.429 Å. by 19F NMR spectroscopy shows the presence of an intermediate
Further characterization of the unique tri- and tetravalent analogous to 1-F* with two broad resonances at ꢀ126.97 and
uranium fluorides was accomplished using electronic absorption ꢀ241.82 ppm (Fig. 3). These data support the formation of an
spectroscopy (Fig. 2). Data were collected in THF at ambient intermediate with a rotating tetrafluorotoluene ligand Tp*2U(C6F4-
temperature in the range of 300–2100 nm. For 1-F, low- to mid- p-Me), and by analogy, the proposed identity of 1-F*. Furthermore,
intensity, color-producing d–f transitions can be seen in the reaction of 1-Bz with perfluorodecalin or perfluorocyclohexene,
visible and near IR regions of the spectrum, as is typical for which have no protons, proceeds directly to 1-F2 with no visible or
uranium(III) species.24,25 In contrast, the visible region of the spectroscopic evidence to support formation of a 1-F*-analogue.
spectrum for 1-F2, which features a uranium(IV) center, shows Instead, 1-Bz reacts quickly with these cyclic substrates, cleanly
much less intense bands, consistent with the light brown color. generating 1-F2 in good yields (68, 74%, respectively) in one hour.
The near-infrared regions of electronic absorption spectra show In these cases, extrusion of bibenzyl was noted. Interestingly, full
characteristic features for uranium(III) and uranium(IV) ions, conversion to 1-F2 was successful with sub-stoichiometric quan-
confirming the valency. In the case of 1-F, this part of the tities of these substrates, once again supporting that multiple C–F
spectrum shows f–f transitions with molar absorptivities up to activation events occur per substrate molecule.
100 Mꢀ1 cmꢀ1 in the range of 1100–1400 nm and 1900–2100 nm,
Under these conditions, 1-Bz readily activates both sp2 and sp3
which is commonly observed for other uranium(III) ions.26,27 The hybridized C–F bonds, with the exception of those in the benzylic
corresponding region for 1-F2 shows much weaker (o20 cmꢀ1 Mꢀ1
)
position of a,a,a-trifluorotoluene, or with linear fluorinated alkyls,
Laporte-forbidden f–f transitions, as is typically observed for such as perfluorohexane and 1-fluoropentane. This is notable
uranium(IV) species with an f2 electronic configuration.24,25,28
given that sp2 hybridized C–F bonds have higher bond strengths
The reaction to generate 1-F2 from pentafluorobenzene and than their sp3 hybridized counterparts. For instance, complex 1-Bz
1-Bz was repeated in a sealed NMR tube (THF-d8) to confirm reacts rapidly with hexafluorobenzene (C–F bond strength =
formation of volatile organic products. Analysis by 1H NMR 154 kcal molꢀ1), but not at all with a,a,a-trifluorotoluene or
spectroscopy showed the presence of toluene, indicating protona- perfluorohexane (C–F bond strengths o125 kcal molꢀ1).32,33 An
tion of the U–C bond in 1-Bz with concurrent formation of 1-F2. increase in the degree of fluorination in aromatic substrates
During the reaction, the deprotonated pentafluorobenzene under- facilitates the C–F bond activation by 1-Bz, despite the fact that
goes b-fluoride elimination to generate the benzyne intermediate, this results in increased C–F bond strengths. This observation
3,4,5,6-tetrafluorobenzyne,29 which can be trapped by repeating is likely due to the decrease in reduction potential of the substrate
the reaction in the presence of an excess of anthracene to success- that accompanies the increase in electron-withdrawing fluorine
fully form 1,2,3,4-tetrafluorotriptycene. This was confirmed by 1H substituents.34,35 Similar reactivity was noted by Andersen for the
and 19F NMR spectroscopy and mass spectrometry (Scheme 1). lanthanide complex Cp*2Yb, which activates the C–F bonds of
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plexes were generated, 1-F and 1-F2, and their formation was
found to be substrate dependant. Isolation and characterization
of 1-F is significant, as this is the first example of a terminal
trivalent uranium fluoride.
This material is based upon work supported by the National
Science Foundation under Grant No. 1149875 (CAREER Award
to SCB). We also acknowledge Dr Ellen Matson for performing
preliminary experiments.
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