Low- A nd Mid-Valent Uranium Species Supported by Phenyltris(oxazolinyl)borate Ligands

Article abstract of DOI:10.1021/acs.organomet.9b00793

The synthesis and characterization of a family of uranium derivatives featuring the phenyltris(4,4-dimethyl-2-oxazolinyl)borate (To^M) ligand framework was achieved. Uranium(IV) halide compounds, specifically To^MUCl₃(THF) and To^MUI₃, were generated by adding 1 equiv of Tl[To^M] to either UCl₄ or UI₃(THF)₄, respectively. The tetravalent derivatives To^MUCl₂(NPh₂) and To^MUI(a? N-Ad)(THF) were also synthesized, showing that To^M supports uranium amido and imido species. Attempts to make bis(phenyltris(oxazolinyl)borate) compounds were unsuccessful. However, when the smaller hydrotris(pyrazolyl)borate (Tp) ligand was used, a mixed bis(ligand) derivative, TpTo^MUI, was isolated. All of the compounds were characterized by ¹H NMR, infrared, and electronic absorption spectroscopies. Where possible, X-ray crystallography was used to assess structural parameters.

Full text of DOI:10.1021/acs.organomet.9b00793

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Low- and Mid-Valent Uranium Species Supported by
Phenyltris(oxazolinyl)borate Ligands
Caleb J. Tatebe, Ellen M. Matson, Christopher L. Clark, John J. Kiernicki, Phillip E. Fanwick,
Matthias Zeller, and Suzanne C. Bart*
H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a family of
uranium derivatives featuring the phenyltris(4,4-dimethyl-2-
oxazolinyl)borate (ToM) ligand framework was achieved. Uranium-
(IV) halide compounds, specifically ToMUCl3(THF) and ToMUI3,
were generated by adding 1 equiv of Tl[ToM] to either UCl4 or
UI3 (THF)4 , respectively. The tetravalent derivatives
ToMUCl2(NPh2) and ToMUI(N-Ad)(THF) were also synthesized,
showing that ToM supports uranium amido and imido species.
Attempts to make bis(phenyltris(oxazolinyl)borate) compounds were
unsuccessful. However, when the smaller hydrotris(pyrazolyl)borate
(Tp) ligand was used, a mixed bis(ligand) derivative, TpToMUI, was
1
isolated. All of the compounds were characterized by H NMR,
infrared, and electronic absorption spectroscopies. Where possible, X-ray crystallography was used to assess structural
parameters.
Coupled with our previous experiences with hydrotris(3,5-
dimethylpyrazolyl)borate (Tp*)25−40 and hydrotris(3-
phenylpyrazolyl)borate (TpPh)41 ligands on uranium, we
hypothesized that the chemistry of the phenyltris(4,4-
dimethyl-2-oxazolinyl)borate (ToM) ligand framework would
follow suit and may allow access to chiral variants of low-valent
organoactinide complexes. Herein we report our entry into the
synthesis and characterization of uranium(III) and uranium-
(IV) derivatives of the ToM ligand, including their reactivity.
All species were characterized by 1H NMR, infrared, and
electronic absorption spectroscopies. For selected compounds,
structural characterization by X-ray crystallography was
obtained and is also discussed.
INTRODUCTION
■
Scorpionate ligands, first popularized by Trofimenko, have
been used on virtually every metal on the periodic table.1−4
While these ligands are named for their tridentate coordination
mode, scorpionates are considered to be analogous to
cyclopentadienes, as both are monoanionic, six-electron donors
(ionic model) and three-coordinate (one σ bond, two π
bonds). In the years since the report of Trofimenko’s
poly(pyrazolyl)borate ligands,1−3,5 there have been many
variations of this framework developed, including substituted
poly(pyrazolyl)borate,6,7 poly(triazolyl)borate,8−10 poly-
(tetrazolyl)borate,11,12 poly(imazoyl)borate,13 and poly(alkyl
phosphinyl)borate ligands,14,15 to tailor the steric and
electronic properties of the resulting coordination complexes.
Sadow and co-workers have explored the utility of a variant
of this class of ligands. Work from that laboratory has
summarized the coordination chemistry of tris(oxazolinyl)-
borate ligands on various elements,16−22 including transi-
tion,17,22 main-group,18 and lanthanide19 metals. These ligands
are readily tuned to suit both electronic and steric demands, as
different oxazoline rings can be easily prepared from
commercially available starting materials.23 Enantiopure
amino acids and/or alcohols allow for the preparation of
chiral variants using inexpensive materials, as these building
blocks are readily available.21,24 Furthermore, the incorpo-
ration of the B−C bonds in the oxazoline-based system is
advantageous over the of B−N bonds typically found in
pyrazole-based scorpionates, as the latter class is prone to
borotropic shifting upon metalation.3
RESULTS AND DISCUSSION
■
Our studies commenced with the use of the ToM ligand, as the
synthesis is straightforward from commercially available
materials.42 As a previous investigation from our laboratory33
has shown difficulty with the separation of trivalent uranium
complexes from lithium salts, we chose to probe the synthesis
of the ToMU derivitives utilizing the thallium salt of the ligand,
Tl[ToM]. This synthetic strategy has also been invoked by
Sadow and co-workers for the coordination of the ToM ligand
to a variety of metal centers.22 Treating a THF solution of
UCl4 with 1 equiv of Tl[ToM] showed the formation of
insoluble thallium chloride after 5 min, turning the solution to
a suspension (Scheme 1, right). After 30 min of stirring and
Received: November 21, 2019
© XXXX American Chemical Society
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Scheme 1. Synthesis of ToMUCl3(THF) and ToMUI3
workup, the resulting green material was analyzed by 1H NMR
spectroscopy (benzene-d6, 23 °C), producing a spectrum that
displayed seven paramagnetically broadened and shifted
resonances ranging from −0.93 to 9.68 ppm, consistent with
metalation of the ligand on U(IV). Two of these resonances
were broad and in the region where free THF would be
observed; thus, the product was assigned as ToMUCl3(THF)
(88% yield). Analysis of this pale green solid by IR
spectroscopy (KBr pellet) revealed an absorption at 1548
cm−1, consistent with the CN stretch of a metal-bound ToM
ligand, as reported by Sadow and co-workers.19
To unambiguously confirm the coordination environment of
the first isolated ToM actinide species, ToMUCl3(THF), X-ray
crystallography was utilized. Pale green crystals were grown
from a concentrated solution of THF and toluene at −35 °C.
Refinement of structural data revealed a seven-coordinate
uranium center with a κ3-ToM ligand, three chlorides, and a
THF ligand in a distorted-monocapped-octahedral geometry
(Figure 1 and Table 1). The U−N distances of
ToMUCl3(THF) range from 2.448(6) to 2.511(5) Å and are
similar to those of other reported U(IV) complexes
(Tp*UCl2[NC(Ph)(CH2SiMe3)], 2.48(2)−2.536(19) Å;43
Tp*UCl2(NEt2), 2.483(10)−2.518(10) Å; TpMesUCl3,
2.43(3)−2.51(3) Å;44 Tp*UCl(OPh)2(THF), 2.475(11)−
2.556(12) Å;45 Tp*UCl3(THF), 2.466(5)−2.497(5) Å).46
The U−OTHF distance of 2.576(7) Å is similar to those of
other U(IV)−THF compounds, including Tp*UCl3(THF)
(2.546(4) Å).46 The U−Cl distances of 2.5915(14),
2.6175(16), and 2.6240(15) Å are consistent with U(IV)−Cl
distances in hydrotris(pyrazolyl)borate uranium trichloride
species with varying pyrazole substituents, such as
Tp*UCl2(NPh2) (2.603(4), 2.608(4) Å),43 Tp*UCl3(THF)
(2.599(2), 2.609(2), 2.613(2) Å),46 and TpUCl3(OP-
(OC2H5)3) (2.610(4), 2.614(3), 2.618(4) Å).47
To assess the generality of this salt metathesis method for
coordination of ToM to uranium(IV), the analogous iodide
species was also synthesized. Addition of 1 equiv of Tl[ToM] to
a solution of UI4(1,4-dioxane)2 in toluene caused a gradual
color change from orange to yellow with concurrent
precipitation of thallium iodide (Scheme 1, left). After workup,
the product was isolated as a yellow solid in moderate yield
(62%), tentatively assigned as ToMUI3.
1
Analysis of ToMUI3 by H NMR spectroscopy (benzene-d6,
25 °C) revealed five paramagnetically shifted resonances
ranging from −2.41 to 11.66 ppm, with the largest, most
upfield signal (18H) corresponding to the dimethyl moiety.
The remaining resonances of the ToM ligand display a splitting
pattern similar to that of ToMUCl3(THF), with phenyl
resonances located downfield at 8.67, 9.07, and 11.66 ppm
and a resonance integrating to 6H for the −CH2 group of the
oxazoline rings at 3.32 ppm. 11B NMR spectroscopy of ToMUI3
showed a single broad resonance at 1.80 ppm, further
indicative of a single product in solution. IR spectroscopy
revealed a CN stretch at 1554 cm−1, consistent with
coordination of ToM to a metal.19 Unfortunately, suitable
crystals for X-ray diffraction could not be obtained, precluding
characterization of this species in the solid state. However,
combustion analysis confirmed the elemental composition of
ToMUI3.
Figure 1. Molecular structure of ToMUCl3(THF) shown with 30%
probability ellipsoids. Hydrogen atoms and cocrystallized solvent
molecules have been omitted for clarity.
Table 1. Bond Distances of ToM Uranium Derivatives (in Å)
Next, we evaluated the suitability of these new metalated
ToM derivatives as starting materials for further chemistry. A
uranium amide species was first targeted, given the
demonstrated utility of these ligands to support the uranium
centers over years of study of actinide coordination
chemistry.48−50 The most common synthetic route for metal
amides is typically salt metathesis, which was demonstrated
here by treating ToMUCl3(THF) with 1 equiv of KNPh2 in
toluene, affording an orange powder (eq 1). Analysis of this
ToMUCl3(THF)
ToMUI(NAd)(THF)
TpToMUI
U1−Cl1
U1−Cl2
U1−Cl3
U1−N1(1)
U1−N2(1)
U1−N3(1)
U−Nimido
U−I(1)
2.6240(15)
2.6175(16)
2.5915(14)
2.511(5)
2.498(5)
2.448(6)
2.526(15)
2.54(4)
2.645(16)
1.972(14)
3.1312(16)
2.610(7)
2.527(7)
2.607(7)
3.222(6)
2.520(7)
2.557(7)
2.607(8)
U1−N5
U1−N7
U1−N9
B
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powder by 1H NMR spectroscopy (benzene-d6, 23 °C)
showed paramagnetically shifted resonances ranging from
−11 to 27 ppm. Three resonances (6H each) corresponded
to the geminal methyl groups, whereas three resonances (2H)
were assigned to the protons on the 5-position of the oxazoline
rings, suggesting Cs symmetry of the product. ToMUCl2(NPh2)
Following solution characterization, crystals of the red-
orange material suitable for single-crystal X-ray diffraction were
obtained from a concentrated THF solution at −35 °C (Figure
2, left, and Table 1). Refinement of the data revealed a six-
coordinate uranium center with one κ3-ToM ligand, one iodide,
one THF molecule, and an adamantyl imido ligand, ToMUI(
N-Ad)(THF). The U−Noxazolinyl distances (2.526(15)−
2.645(16) Å) are similar to those observed for
ToMUCl3(THF) (Table 1) and Tp*U(Bn)(N-Mes)(THF)
(2.542(6)−2.634(6) Å),32 supporting the uranium(IV)
oxidation state in this species. The U−Nimido bond length of
1.972(14) Å is also similar to previously reported bis(Tp*)
uranium(IV) imido U−Nimido distances (Tp*2U(N-Mes),
1.976(3) Å;29 Tp*2U(N-Bn), 1.972(3) Å).40 Thus, the ToM
ligand framework is able to support metal species with multiply
bonded substituents.
1
is stable in benzene at room temperature, with H NMR
spectroscopy showing no noticeable decomposition over 1
week. The infrared spectrum of a KBr pellet of this material
showed an absorption at 1545 cm−1, corresponding to ν(C
N).
Structural confirmation of ToMUCl2(NPh2) was performed
through crystallographic analysis of single, X-ray-quality
crystals obtained by slow diffusion of pentane into a
concentrated dichloromethane solution at −35 °C (Figure
S5). While refinement of these data did reveal a uranium ion
with two chloride, a diphenyl amide, and ToM ligand, the
quality of the data was not sufficient to include a meaningful
discussion of bond metrics.
Further reactivity of the uranium(IV) imido species was
probed, given that the uranium−nitrogen multiple bond has
been demonstrated to undergo [2π + 2π] cycloaddition of
carbonyls readily in previously studied bis(Tp*) systems (eq
3). Subjecting a THF solution of ToMUI(N-Ad) to 1 equiv
With successful synthesis of the amide derivative, a
uranium(IV) imido species was also targeted to probe the
stability of these ligands to support multiple-bond chemistry.
Synthesis of such a derivative was achieved by treating a THF
solution of UI3(THF)4 with 1-azidoadamantane (N3Ad)
followed by KC8, yielding a brown solution (eq 2). To this
of p-tolualdehyde caused a color change from red-orange to
light yellow. Following workup and removal of intractable
1
uranium products, the H NMR spectrum of the resulting
residue (benzene-d6) is consistent with the formation of the
organic imine product in good yield.29
Interested in probing the possibility of generating a
bis(scorpionate) species derived of the ToM scaffold, we
sought to generate bis(ToM) uranium complexes. Unfortu-
nately, multiple attempts at synthesis of these species by
adding 2 equiv of Tl[ToM] to UI3(THF)4 or UCl4 or 1 equiv
of Tl[ToM] to ToMUCl3(THF) or ToMUI3 were not
successful.
We hypothesized this could be due to the large cone angle
associated with these tris(oxazolinyl)borate ligands and
undertook an analysis using the Solid-G program51 to
determine the extent of steric crowding of the uranium center.
mixture was added a solution of Tl[ToM] in THF dropwise,
and following workup, a red-orange solid was isolated in high
yield (93%). 1H NMR spectroscopic analysis of this red-orange
solid (benzene-d6, 23 °C) revealed a clean compound with a
complicated spectrum with sharp resonances ranging from −40
to 65 ppm (details in the Experimental Section), as expected
given the complexity of the adamantyl substituent. An
absorption at 1560 cm−1 was noted in the infrared spectrum
of this material (KBr pellet).
Figure 2. Molecular structure of ToMUI(N-Ad)(THF) (left) and TpToMUI (right) shown with 30% probability ellipsoids. Hydrogen atoms and
cocrystallized solvent molecules have been omitted for clarity.
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Solid-G51 calculates the percentage of a metal’s coordination
sphere that is blocked by its ligand(s) on the basis of
parameters obtained from experimental molecular structures,
while also correcting for overlapping of adjacent ligands.
Solid-G analysis of ToMUCl3(THF) shows that the ToM
ligand itself consumes about 45% of the coordination sphere,
whereas each halide consumes about 10% of the coordination
sphere (Table 2). Therefore, two ToM ligands and a halide
stretch for the B−H of Tp (2464 cm−1) and one stretch for
ToM’s CN bond (1562 cm−1).
Structural parameters of TpToMUI were examined through
an X-ray crystallographic analysis on suitable crystals obtained
from slow diffusion of pentane into a concentrated THF
solution at −35 °C (Figure 2, right; Table 1). The molecular
structure obtained from refinement of the data displayed a
seven-coordinate uranium center with an iodide atom, one κ3-
Tp ligand, and one κ3-ToM ligand. For the ToM ligand, U−
Table 2. Solid-G51 Data for ToM Uranium Derivatives
N
oxazolinyl (2.527(7)−2.610(7) Å) bond lengths are in the same
range as for ToMUI(N-Ad)(THF) (Table 1), whereas the
U−Npyrazolyl bond lengths (2.520(7)−2.607(8) Å) of the Tp
ligand are similar to the U−Npyrazolyl bond lengths reported for
TpTp*UBn (Bn = benzyl; 2.556(13)−2.260(11) Å) and
TpCp*U(CH2SiMe3) (Cp* = pentamethylcyclopentadienide;
2.556(13)−2.620(11) Å), which also feature U(III) mixed-
ligand systems.36
ToMUI(=N-Ad)
ToMUCl3(THF)
45.07
(THF)
TpToMUI
43.60
G(ToM), %
G(X), %
45.13
X = Cl: 10.43, 10.38, X = N-Ad: 18.18 X = Tp:
10.68
32.16
X = THF: 12.43
X = I: 10.77
X = THF: 12.53
85.23
X = I: 10.08
A Solid-G analysis of TpToMUI further demonstrates the
steric influence of the ToM ligand (Table 2). In this case, the
ToM ligand still consumes over 40% of the coordination
sphere, but the Tp ligand only uses about 32% in comparison.
Coupled with the ∼10% for the iodide ligand, the total steric
demand for TpToMUI is about 84%, which is in line with the
other ToM derivatives. Thus, it is not sterically feasible to
generate a bis(ToM) uranium species.
In order to assess the electronic structure of these species
and confirm the oxidation states, all molecules were
characterized using electronic absorption spectroscopy. Spectra
were collected for each compound as a THF solution from 300
to 1800 nm at ambient temperature. In the near-infrared
region, the spectra are consistent with those for typical
uranium(IV) compounds,52,53 showing many sharp f−f
transition absorbances that are weak in their molar absorptivity
(∼50 M−1 cm−1) consistent with U(IV) 5f2 complexes (Figure
3, inset).52−54 The visible region shows more intense
absorbances that are very broad, ranging from ∼200 to 800
M−1 cm−1 (Figure 3). ToMUI3 shows an absorption at around
400 nm, which is likely responsible for its yellow appearance.
G(complex), % 86.13
84.80
would take up 100% of the coordination sphere. This is likely
the reason that it has not been possible thus far to coordinate
two ToM ligands to uranium.
Instead, a smaller scorpionate ligand, the parent hydrotris-
(pyrazolyl)borate (Tp) ligand, was chosen to complete the
coordination sphere. Given the smaller steric demands of this
ligand, it should be possible to make a bis(ligand) species with
ToM using this framework. The synthesis of the targeted mixed
ligand species, TpToMUI, was achieved in high yield (86%)
and purity by treating UI3(THF)4 with 1 equiv of KTp (eq 4).
After the mixture was stirred in THF for 30 min, a THF
solution of Tl[ToM] was added dropwise; stirring and workup
yielded a purple solid.
Analysis of a benzene-d6 solution of this purple powder by
1H NMR spectroscopy revealed a paramagnetically shifted
spectrum with eight resonances. A singlet is observed at −9.82
ppm (18H) corresponding to the geminal methyl groups of the
ToM ligand, whereas the methylene protons of the oxazolinyl
backbone are displayed as a singlet (6H) at 3.44 ppm. Three
resonances for the phenyl group on the boron of ToM appear
at 8.97 ppm for the para proton of the phenyl ring (1H triplet),
at 9.37 ppm (2H triplet) for the meta protons, and at 13.01
ppm (2H doublet) for the ortho protons. Three singlets (3H
each) are observed (−3.77, 8.16, 18.31 ppm) and are assigned
as the protons on the pyrazole ring. By 11B NMR spectroscopy
(benzene-d6, 25 °C; Figure S10), two broad resonances are
observed (−14, 40 ppm) that correspond to two different
boron-containing ligands bound to uranium. Analysis of
TpToMUI by IR spectroscopy (KBr pellet) displayed one
Figure 3. Electronic absorption spectra for ToMUI3(red),
ToMUCl3(THF) (green), ToMUCl2NPh2 (orange), ToMUI(N-
Ad)(THF) (purple), and TpToMUI (blue) in the visible region.
Inset: electronic absorption data in the near-infrared and infrared
regions. Solvent overtones are present between 1700 and 1800 nm.
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Orange ToMUCl2NPh2 shows very weak absorbances around
450 nm as expected, whereas the absorbance for ToMUI(N-
Ad)(THF) is shifted more toward ∼500 nm, which is
consistent with its red-orange appearance. In the case of
TpToMUI, the uranium is trivalent rather than tetravalent, with
an f3 configuration; thus, the spectrum is strikingly different. In
this case, there are broad absorptions throughout the visible
region, including an intense one at ∼650 nm which is likely
responsible for the intense blue color. In the near-infrared
region, TpToMUI has characteristic absorptions for U(III) in
which the absorbances are clearly higher in molar absorptivity
in comparison to those for the U(IV) derivatives.
Laboratories, Inc., Parsippany, NJ, or Midwest Microlab, Indianapolis,
IN. Solid-state infrared spectra were recorded using a Thermo Nicolet
iS5 spectrophotometer; samples were made by crushing the solids,
mixing with dry KBr, and pressing into a pellet. Electronic absorption
spectroscopic measurements were recorded at 294 K in sealed 1 cm
quartz cuvettes with a Cary 6000i UV−vis−NIR spectrophotometer.
Single-crystal data for ToMUCl3(THF), ToMUI(N-Ad)(THF),
and TpToMUI were collected using a Rigaku R-axis curved image
plate diffractometer as described in the Supporting Information. Full
details, in CIF format, have been deposited with the Cambridge
Crystallographic Data Centre (CCDC) with file numbers 1949524−
1949526.
Synthesis of ToMUCl3(THF). A 20 mL scintillation vial was
charged with UCl4 (0.500 g, 1.316 mmol) and approximately 7 mL of
THF. A second vial was charged with Tl[ToM] (0.772 g, 1.316 mmol)
and approximately 3 mL of THF. The Tl[ToM] solution was added
dropwise to the stirred UCl4 solution at room temperature. TlCl was
produced within 5 min, and the reaction mixture was stirred for an
additional 30 min. The resulting suspension was filtered, and the
volatiles were removed in vacuo. The green solid was washed with
pentane and dried under vacuum (0.768 g, 0.961 mmol, 73% yield).
X-ray-quality crystals were grown from a concentrated solution of
THF and toluene at −35 °C. Anal. Calcd for C25H37N3O4BUCl3: C,
37.59; H, 4.67; N, 5.26. Found: C, 38.36; H, 5.05; N, 5.46. 1H NMR
(C6D6, 25 °C): δ −0.93 (5, 18H, (CH3)2), 1.38 (48, 8H, THF), 3.31
(4, 6H, CH2), 3.45 (53, 8H, THF), 7.96 (8, 1H, Ph-CH), 8.24 (t, J =
9, 2H, Ph-CH), 9.68 (d, J = 9, 2H, Ph-CH). IR: 1548 cm−1 (CN).
Synthesis of ToMUI3. A 20 mL scintillation vial was charged with
UI4(dioxane)2 (0.100 g, 0.108 mmol) and 7 mL of toluene. A second
vial was charged with Tl[ToM] (0.064 g, 0.108 mmol) in 3 mL of
toluene. The Tl[ToM] solution was added dropwise to the stirred UI4
solution at ambient temperature. TlI was observed immediately and
after stirring for 30 min. The solution was filtered over Celite and
washed with toluene to separate TlI. Solvents were removed under
reduced pressure, leaving the product, ToMUI3, as a yellow powder
(0.068 g, 0.068 mmol, 62%). Anal. Calcd for C25H37BI3N3O4U (THF
adduct): C, 27.98; H, 3.48; N, 3.92. Found: C, 28.46; H, 3.05; N,
4.12. 1H NMR (benzene-d6, 25 °C): δ −2.41 (3, 18H, (CH3)2), 3.32
(2, 6H, CH2), 8.67 (3, 1H, Ph-CH), 9.07 (2, 2H, Ph-CH), 11.66 (4,
2H, Ph-CH). No THF resonances are visible, likely due to the
fluxionality of THF association/dissociation. 11B NMR (CDCl3, 25
°C): δ 1.80. IR: 1554 cm−1 (CN).
CONCLUSIONS
■
In summary, we have demonstrated that phenyltris(oxazolinyl)
borate ligands are suitable for supporting uranium centers in
both the +3 and +4 oxidation states. Using common uranium
starting materials, metalation was achieved with the thallium
derivative of the ToM ligand. Further reactivity was studied to
assess the similarity of these ligands to traditional pyrazole-
based scorpionate ligands. Both uranium(IV) amide and imido
derivatives showed that these ToM ligands are sterically
protective and robust to stay metalated during salt metathesis.
This study has shown, however, that the large cone angle
associated with the ToM ligand does not allow synthesis of the
bis(ligand) derivative on a large metal such as uranium, in
comparison to the bis(tris(pyrazolyl)borate) ligand architec-
tures that are popular in the literature. Instead, the smaller
unsubstituted hydrotris(pyrazolyl)borate, Tp, is required to
generate a mixed bis(ligand) species. The brief steric analysis
carried out here also demonstrates that this is a useful
predictive tool for designing new ligand architectures. Future
studies will build on this work with the synthesis and
exploration of chiral variants of the tris(oxazolinyl)borate
ligands.
EXPERIMENTAL SECTION
■
Synthesis of ToMUCl2(NPh2). A 20 mL scintillation vial was
charged with ToMUCl3(THF) (0.125 g, 0.156 mmol) in 4 mL of
toluene. To this light green solution was added KNPh2 (0.032 g,
0.156 mmol), and the resulting suspension changed from yellow to
orange. After the mixture was stirred for 2 h, KCl was removed by
filtration and the orange solution was then concentrated under
reduced pressure. Crystals were obtained by slow diffusion of pentane
into a concentrated dichloromethane solution at −35 °C (Figure S5)
but were not of sufficient quality to obtain a publishable data set.
Anal. Calcd for C33H39BCl2N4O3U: C, 46.12; H, 4.57; N, 6.52.
Found: C, 45.97; H, 4.63; N, 6.38. 1H NMR (C6D6, 25 °C): δ −10.52
(3, 6H, ToM-(2 × CH3)), −3.96 (d, 2H, ToM-CH2), −3.57 (d, 2H,
ToM-CH2), −0.17 (3, 6H, ToM-(2 × CH3)), 5.53 (t, 2H, ortho-CH),
5.80 (t, 1H, para-CH), 7.87 (9, 10H, N-Ph), 18.24 (3, 2H, ToM-
CH2), 26.91 (5, 6H, ToM-(2 × CH3)). IR (KBr): νC=N 1545 cm−1.
Synthesis of ToMUI(N-Ad)(THF). A 20 mL scintillation vial
was charged with UI3(THF)4 (0.100 g, 0.110 mmol) and 2 mL of
THF. A second vial was charged with 1-azidoadamantane (0.020 g,
0.110 mmol) and 2 mL of THF, and this colorless solution was added
to the first vial, giving a purple solution. While this mixture was
stirred, KC8 (0.015 g, 0.110 mmol) was slowly added to the purple
solution; an additional 15 min of stirring yielded a brown solution. At
this juncture, a solution of Tl[ToM] (0.065 g, 0.110 mmol) in 3 mL of
THF was added dropwise to the brown solution. After the mixture
was stirred for 2 h, KI, TlI, and graphite were filtered out and volatiles
were removed in vacuo. The resulting red-orange solid was washed
with pentane (2 × 5 mL) and dried under vacuum (0.098 g, 0.102
mmol, 93% yield). Single X-ray-quality crystals were obtained from a
General Considerations. All air- and moisture-sensitive manip-
ulations were performed using standard Schlenk techniques or using
an MBraun inert-atmosphere drybox with an atmosphere of purified
nitrogen. The MBraun drybox was equipped with a cold well designed
for freezing samples in liquid nitrogen, as well as two −35 °C freezers
for cooling samples and crystallizing compounds. Diethyl ether,
pentane, tetrahydrofuran, and toluene for sensitive manipulations
were dried and deoxygenated using literature procedures with a Seca
solvent purification system.55 Benzene-d6 and chloroform-d were
purchased from Cambridge Isotope Laboratories, dried with
molecular sieves (C6D6, CDCl3) and sodium (C6D6), and degassed
by three freeze−pump−thaw cycles. 1-Azidoadamantane was
purchased from Sigma-Aldrich and used as received. Potassium
graphite,56 Tl[ToM],22 KTp,57 UCl4,58 UI3(THF)4,59 and UI4(1,4-
dioxane)259 were prepared using previously reported methods. KNPh2
was prepared by treating diphenylamine with benzylpotassium.
1H NMR spectra were recorded on a Varian Inova 300
spectrometer operating at 299.992 MHz. 11B NMR spectra were
recorded on a Varian Inova 300 spectrometer operating at a frequency
of 96.24 MHz. All chemical shifts are reported relative to the peak for
SiMe4, using 1H residual chemical shifts of the solvent as a secondary
standard. 11B chemical shifts are reported relative to the peak for BF3·
Et2O. The spectra for paramagnetic molecules were obtained using an
acquisition time of 0.5 s; thus, the peak widths reported have an error
of 2 Hz. For paramagnetic molecules, the 1H NMR data are
reported with the chemical shift, followed by the peak width at half-
height in hertz, the integration value, and, where possible, the peak
assignment. Elemental analyses were performed by Complete Analysis
E
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concentrated solution of THF at −35 °C. 1H NMR (C6D6, 25 °C): δ
−40.07 (8, 6H, ToM -(2 × CH3)), −25.21 (d, 2H, ToM-(2 × H)),
−24.03 (d, 2H, ToM-(2 x H)), −9.5 (6H, ToM-(2 × CH3)), 7.5 (t,
1H, para-CH), 7.62 (t, 2H meta-CH), 7.80 (d, 2H ortho-CH), 20.79
(d, 3H, Ad-CH), 26.32 (d, 3H, Ad-CH), 28.48 (13, 3H, Ad-CH),
56.3 (9, 6H, Ad-CH), 59.24 (5, 2H, ToM-(2 × H), 65.41 (9, 6H,
ToM-(2 × CH3)). 11B NMR (C6D6, 25 °C): δ 26.07. IR (KBr): νC=N
1560 cm−1.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00793.
Additional NMR and IR data, connectivity image of
ToMUCl2(NPh2), and tabulated X-ray parameters
(PDF)
Accession Codes
CCDC 1949524−1949526 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for S.C.B.: sbart@purdue.edu.
ORCID
Caleb J. Tatebe: 0000-0002-7817-388X
Ellen M. Matson: 0000-0003-3753-8288
Matthias Zeller: 0000-0002-3305-852X
Suzanne C. Bart: 0000-0002-8918-9051
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(NSF) (CHE-1665170, S.C.B.). This material is also based
upon work supported by the NSF through the Major Research
Instrumentation Program under Grant CHE 1625543. The
authors also acknowledge Derek Jorgensen for his contribu-
tions to THE preparation of LiToM.
F
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