Uranyl Functionalization Mediated by Redox-Active Ligands: Generation of O-C Bonds via Acylation

Article abstract of DOI:10.1021/jacs.8b11302

A series of uranyl compounds with the redox-active iminoquinone ligand have been synthesized, and their electronic structures elucidated using multinuclear NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray crystallography. Characterization and analysis of the iminoquinone (iq⁰) complex, (^dippiq)UO₂(OTf)₂THF (1-iq), the iminosemiquinone (isq^1-) complex, (^dippisq)₂UO₂THF (2-isq), and the amidophenolate (ap^2-) complex, [(^dippap)₂UO₂THF][K(18-crown-6)(THF)₂]₂(3-ap crown) show that reduction events are ligand-based, with the uranium center remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or pivaloyl chloride leads to U-O_uranyl bond scission and reduction of U(VI) to U(IV) concomitant with ligand oxidation along with organic byproducts. ¹⁸O isotopic labeling experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR spectroscopy confirm that the organic byproducts contain oxygen atoms which originate from U-O_uranyl bond activation.

Full text of DOI:10.1021/jacs.8b11302

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Article
Uranyl Functionalization Mediated by Redox-Active
Ligands: Generation of O-C Bonds via Acylation
Ezra J. Coughlin, Yusen Qiao, Ekaterina Lapsheva, Matthias Zeller, Eric J. Schelter, and Suzanne C. Bart
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 Dec 2018
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Uranyl Functionalization Mediated by Redox-
Active Ligands: Generation of O-C Bonds via
Acylation
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†
‡
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‡
Ezra J. Coughlin , Yusen Qiao , Ekaterina Lapsheva , Matthias Zeller , Eric J. Schelter and
Suzanne C. Bart†*
† Department of Chemistry, Purdue University, West Lafayette, Indiana 47901, USA
‡
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of
th
Pennsylvania, 231 South 34 Street, Philadelphia, Pennsylvania 19104, USA.
Abstract: A series of uranyl compounds with the redox-active iminoquinone ligand
have been synthesized and their electronic structure elucidated using multinuclear
NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray
0
crystallography. Characterization and analysis of the iminoquinone (iq ) complex,
(dippiq)UO (OTf) THF (1-iq), the iminosemiquinone (isq ) complex, ( isq) UO THF (2-
1-
dipp
2
2
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-
dipp
isq), and the amidophenolate (ap ) complex, [( ap) UO THF][K(18-crown-6)(THF) ] (3-
2
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2 2
ap crown) show that reduction events are ligand-based, with the uranium center
remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or
pivaloyl chloride leads to U-Ouranyl bond scission and reduction of U(VI) to U(IV)
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concomitant with ligand oxidation along with organic byproducts. O isotopic labeling
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experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR
spectroscopy confirm the organic byproducts contain oxygen atoms which originate
from U-Ouranyl bond activation.
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Introduction
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+
The uranyl ion, [UO ] , is the most recognizable actinide containing species due
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to its prevalence in aqueous environments and in the solid state. Characterized by its
robust trans-oxo ligands and +6 oxidation state, derivatives of this ion form naturally
from interactions of water with uranium ore deposits, as well as anthropogenically from
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depleted uranium targets, nuclear waste streams, and uranium mining. The high
solubility of this ion in water is problematic as its increased mobility in this form causes
environmental contamination. In anaerobic environments, uranyl can be reduced to its
+
5 (unstable) and +4 (stable) forms, greatly reducing its water solubility and facilitating
removal from the environment.5
The persistence of uranyl is attributed to its strong uranium-oxygen bonds,
which are not easily functionalized or cleaved. Taking inspiration from Nature, recent
work in non-aqueous uranyl chemistry has demonstrated that transformation of U-
Ouranyl bonds may be facilitated by reduction events to generate U(IV) ions, in which
pentavalent intermediates are likely involved. One strategy, demonstrated by Duval
and co-workers, involves the photochemical reduction of hexavalent
-
[UO (dppmo) (OPPh )][ OTf] (dppmo = Ph P(O)CH P(O)Ph ) in the presence of BF
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anions, producing the corresponding uranium(IV) difluoride species. Reductive
silylation of the uranyl ion has also been studied7-13 and optimized, while strongly
Lewis acidic titanium14 and borane15 compounds have also proven to be potent
reductants for the generation of uranium (IV) via U-Ouranyl bond activation. P. Arnold
and co-workers have pioneered the use of a specialized dipyrrin ligand and strongly
Lewis acidic atoms to facilitate uranyl functionalization by shifting the U(V) – U(IV)
reduction into an accessible redox range. More recently, work from their lab has also
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demonstrated reduction of the terminal oxos in uranyl using silanes or boranes. In
each case, the corresponding silyl and boryl ethers are released, showing full U-Ouranyl
bond scission.
Our group has reported the utility of redox-active ligands in uranyl
functionalization and U-Ouranyl bond scission as well. Treating Cp*UO2(MesPDI ) (Cp* =
Me
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,2,3,4,5-pentamethylcyclopentadienide; MesPDIMe = 2,6-((Mes)N=CMe) -C H N; Mes =
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,4,6-trimethylphenyl), which contains a singly reduced pyridine(diimine) ligand and a
hexavalent uranyl ion, with stoichiometric equivalents of Me SiI results in stepwise oxo
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Mes
Me 17
silylation to form tetravalent (Me SiO) UI ( PDI ). Additional equivalents of Me SiI
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results in full uranium-oxo bond cleavage and the formation of UI (1,4-dioxane) with
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extrusion of hexamethyldisiloxane. The necessary electron equivalents derive from the
reduced pyridine(diimine) ligand and loss of the Cp* ring in this example.
Much of the work in uranyl functionalization is based on silylation and
borylation of U-Ouranyl bonds, with formation of O-Si and O-B bonds acting as a strong
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thermodynamic driving force. Currently, there are no examples of transformations of
U-Ouranyl bonds where O-C bonds are formed. We hypothesized that U-Ouranyl bonds
could undergo such a conversion with electrophilic carbon-containing substrates when
mediated by reduced redox-active ligands. The iminoquinone family would be effective
for such a task, as this ligand set has mediated multi-electron reactions with low-valent
metals18-23 and would obviate the need for an external reductant by its ability to exist in
a reduced state (Figure 1). The most oxidized form of this ligand, the iminoquinone
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1-
(
iq ), can be reduced to the iminosemiquinone (isq ), which is characterized by a ligand
radical. Further reduction results in the formation of the amidophenolate ligand (ap2-),
which stores two electrons in its * orbitals.24
Figure 1. Redox chemistry demonstrated by the iminoquinone/amidophenolate ligand.
Red atoms denote anionic bonds, while blue atoms engage in dative (neutral) bonding.
Herein, we report a new series of uranyl derivatives containing redox-active
iminoquinone/amidophenolate ligands. Storing electrons in this ligand framework
generates uranyl complexes with unprecedented electronic structures, including an
unusual diradical species. These ligands facilitate multi-electron chemistry at uranyl
centers, allowing the complete functionalization and removal of the robust trans-oxo
ligands through borylation and acylation reactions. Full spectroscopic, structural, and
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magnetic characterization is provided, as well as isotopic labeling experiments to
highlight the transformation of the uranyl moiety.
Experimental Section
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General Considerations. All air- and moisture-sensitive manipulations were performed
using standard Schlenk techniques or in an MBraun inert atmosphere drybox with an
atmosphere of purified nitrogen. The MBraun drybox was equipped with a cold well
used for freezing samples in liquid nitrogen as well as two -35 °C freezers for cooling
samples and crystallizations. Solvents for sensitive manipulations were dried and
deoxygenated using literature procedures with a Seca solvent purification system.25
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Water ( O, 97%) was purchased from Cambridge Isotope Laboratories and used as
received. Benzene-d was purchased from Cambridge Isotope Laboratories, dried with
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molecular sieves and sodium and degassed by three freeze–pump–thaw cycles.
Chloroform-d and pyridine-d were purchased from Cambridge Isotope Laboratories,
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dried with molecular sieves, and degassed by three freeze–pump–thaw cycles. 4,6-di-
tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippiq) , potassium graphite
23
2
6
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(
KC ) , uranyl dichloride ([UO Cl (THF) ] ) , uranyl triflate UO (OTf) (THF) , and
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uranium tetrachloride (UCl ) were prepared according to literature procedures. 18-
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crown-6 (18-C-6) and B-chlorocatecholborane (Cl-BCat) were purchased from Sigma
Aldrich and dried under vacuum on a Schlenk line overnight prior to use. Pivaloyl
chloride (Piv-Cl) was purchased from Sigma Aldrich, degassed by three freeze–pump–
thaw cycles, and dried with molecular sieves prior to use.
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1H NMR spectra were recorded on a Varian Inova 300 or Bruker AV-III-400-HD
spectrometer operating at 299.99 and 400.13 MHz, respectively. All chemical shifts are
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reported relative to the peak for SiMe , using H (residual) chemical shifts of the solvent
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as a secondary standard. C{ H} NMR spectra were recorded on a Bruker AV-III-HD
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spectrometer operating at 100.61 MHz. F NMR spectra were recorded on a Bruker AV-
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III-HD spectrometer operating at 376.50 MHz. B NMR spectra were recorded on a
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Varian Inova 300 spectrometer operating at a frequency of 96.24 MHz. B chemical
shifts are reported relative to the peak for BF •Et O (0.0 ppm). The spectra for all
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paramagnetic molecules were obtained by using an acquisition time of 0.5 s, thus, the
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peak widths reported have an error of ±2 Hz. For all paramagnetic molecules, the H
NMR data are reported with the chemical shift, followed by the peak width at half
height in Hz, the integration value, and, where possible, the peak assignment. All
voltammetric data were obtained under inert atmosphere conditions using external
electrical ports of the MBraun inert drybox. All data were obtained using a Gamry
Instruments Interface 1000 model potentiostat using the Gamry Instruments Laboratory
software. All data were collected on samples in THF with 0.1 M [Bu N][PF ] supporting
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electrolyte concentration, and using an internal resistance compensation of
approximately 2000 ohms. Solutions were analyzed in 10 mL beakers, with a 3 mm
glassy carbon working electrode, a Pt wire counter electrode, and an Ag wire quasi-
reference electrode. Potential corrections were performed at the end of the experiment
+
using the Fc/Fc couple as an internal standard. X-Band EPR spectra were recorded on
a Bruker EMX EPR spectrometer and simulated using the EasySpin toolbox in
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MATLAB. Electronic absorption measurements were recorded at 294 K in THF in
sealed, 1 cm quartz cuvettes with a Cary 6000i UV-vis-NIR spectrophotometer.
Elemental analyses were performed by Midwest Microlab (Indianapolis, IN). Solid state
infrared spectra were recorded using a Thermo Nicolet 6700 spectrophotometer.
Synthesis of (dippiq)UO (OTf) THF (1-iq). A 20-mL scintillation vial was charged with
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UO (OTf) (THF) (0.100 g, 0.176 mmol) and 5 mL diethyl ether. A separate 20-mL
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scintillation vial was charged with dippiq (0.067 g, 0.176 mmol), dissolved in 5 mL
diethyl ether and added dropwise to the stirring UO (OTf) (THF) slurry. A reddish-
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brown solution was observed. After 6 hours, the volatiles were removed in vacuo. The
resulting reddish-purple solid was washed with cold diethyl ether (3 × 10 mL) to
afford a red powder (0.106 g, 0.132 mmol, 75% yield) assigned as 1-iq. X-ray quality
crystals were obtained from a concentrated toluene solution stored at -35 °C. Elemental
analysis of C H NO F S U: Calculated, C, 37.69; H, 4.45; N, 1.37. Found, C, 38.07; H,
3
2
45
10 6 2
1
4
.80; N, 1.29. H NMR (C D , 300MHz, 25 °C):  = 0.64 (s, 9H, C(CH ) ), 0.84 (d, 7, 6H,
6
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3 3
CH(CH ) ), 1.21 (d, 7, 6H, CH(CH ) ), 1.44 (s, 9H, C(CH ) ), 1.93 (br s, 4H, THF), 2.70
3
2
3 2
3 3
(
sept, 7, 2H, CH(CH ) ), 5.26 (br s, 4H, THF), 6.17 (d, 2, 1H, CH), 7.28 (d, 2, 1H, CH), 7.31
3
2
1
(
s, 2H, CH), 7.28 (s, 1H, CH). H NMR (CDCl , 400 MHz, 25 °C):  = 1.03 (d, 6.8, 6H,
3
CH(CH ) ), 1.04 (d, 6.8, 6H, CH(CH ) ), 1.19 (s, 9H, C(CH ) ), 1.72 (s, 9H, C(CH ) ), 2.46
3 2
3 2
3 3
3 3
(
br s, 4H, THF), 2.53 (sept, 6.8, 2H, CH(CH ) ), 5.21 (br s, 4H, THF), 6.21 (s, 1H, CH), 7.45
3
2
1
3
1
(
d, 7.7, 2H, CH), 7.53 (t, 7.7, 2H, CH), 7.28 (s, 1H, CH). C{ H} NMR (CDCl , 100 MHz,
3
2
5 °C):  = 22.66, 24.99, 26.35, 27.89, 29.10, 36.16, 36.50, 116.36, 124.85, 129.64, 138.11,
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43.90, 145.39, 150.40, 160.90, 169.15, 195.99. F NMR (CDCl , 376.5 MHz, 25 °C):  = -
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78.10.
Synthesis of (dippisq) UO THF (2-isq). A 20-mL scintillation vial was charged with
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[UO Cl (THF) ] (0.100 g, 0.103 mmol) and 5 mL THF. A separate 20-mL scintillation
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2 2
vial was charged with dippiq (0.156 g, 0.412 mmol), dissolved in 5 mL THF and added
dropwise to the stirring [UO Cl (THF) ] slurry. After stirring for 15 minutes, KC (0.056
2
2
2 2
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g, 0.412 mmol) was weighed by difference and added over 5 minutes, resulting in a
rapid color change from red to dark green. After 2 hours, the slurry was filtered
through Celite and volatiles were removed in vacuo. The remaining mixture was
recrystallized from n-pentane to afford a green powder (0.152 g, 0.138 mmol, 67% yield)
assigned as 2-isq. Single, X-ray quality crystals were obtained from a concentrated
diethyl ether solution at -35 °C. Elemental analysis of C H N O U: Calculated, C,
56 82
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1.07; H, 7.51; N, 2.54. Found, C, 60.94; H, 7.70; N, 2.22. H NMR (C D , 300MHz, 25 °C):
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
= 0.05 (br s), 1.50 (br s), 2.47 (br s), 3.68 (br s), 3.89 (br s), 6.86 (br s), 7.70 (br s), 16.47 (br
s), 53.64 (br s).
Synthesis of [[(dippap) U][K(THF) ] ] (3-ap) from 2-isq. A 20-mL scintillation vial was
2
2 2 2
charged with 2-isq (0.100 g, 0.091 mmol) and 10 mL THF. While stirring, KC (0.025 g,
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0
.181 mmol) was weighed by difference and added over 5 minutes, resulting in a rapid
color change from dark green to black. After 1 hour, the solution was filtered through
Celite and volatiles were removed in vacuo. The crude mixture was washed with 10 mL
cold n-pentane to afford a tan powder (0.140 g, 0.056 mmol, 62% yield) assigned as 3-ap.
Single, X-ray quality crystals were obtained from a concentrated diethyl ether solution
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at -35 °C. Elemental analysis of C H N O K U : Calculated, C, 57.58; H, 7.25; N, 2.24.
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2
1
Found, C, 57.84; H, 7.34; N, 2.34. H NMR (NC D , 300MHz, 25 °C):  = 0.82 (d, 7, 12H,
5
5
CH(CH ) ), 1.29 (d, 7, 12H, CH(CH ) ), 1.40 (s, 18H, C(CH ) ), 1.83 (s, 18H, C(CH ) ), 4.37
3
2
3 2
3 3
3 3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(sept, 7, 4H, CH(CH ) ), 5.73 (s, 2H, CH), 6.49 (s, 2H, CH), 6.80 (t, 8, 2H, CH), 7.02 (d, 8,
3
2
1
3
1
4
H, CH). C{ H} NMR (NC D , 100 MHz, 25 °C):  = 23.28, 25.48, 26.20, 26.95, 30.80,
5
5
32.39, 34.26, 34.81, 67.50, 107.69, 110.48, 122.19, 122.78, 130.77, 137.50, 148.31, 154.09,
1
56.96.
Synthesis of [(dippap) UO THF][K(18-crown-6)(THF) ] (3-ap crown) from 3-ap. A 20-
2
2
2 2
mL scintillation vial was charged with 3-ap (0.100 g, 0.040 mmol) and 5 mL THF. While
stirring, 18-crown-6 (0.042 g, 0.160 mmol) was weighed by difference and added. After 1
hour, volatiles were removed in vacuo. The crude mixture was washed with 10 mL cold
diethyl ether to afford a white powder (0.121 g, 0.061 mmol, 76% yield) assigned as 3-ap
crown. Single, X-ray quality crystals were obtained from a concentrated THF/toluene
(
1:1) solution stored at -35 °C. Elemental analysis of C H N O K U: Calculated, C,
96 162
2
21
2
5
7.75; H, 8.18; N, 1.40. Found, C, 57.27; H, 8.07; N, 2.14. The EA values for C and N are
off by more than 0.4%, which may be an indication of decomposition as a result of the
1
reactive nature of this complex. H NMR (NC D , 300MHz, 25 °C):  = 1.24 (d, 7, 12H,
5
5
CH(CH ) ), 1.37 (d, 7, 12H, CH(CH ) ), 1.44 (s, 18H, C(CH ) ), 1.55 (s, 18H, C(CH ) ), 3.43
3 2
3 2
3 3
3 3
(
s, 24H, CH ), 4.87 (sept, 7, 4H, CH(CH ) ), 5.90 (d, 2, 2H, CH), 6.56 (d, 3, 2H, CH), 7.06
2
3 2
1
3
1
(
t, 8, 2H, CH), 7.38 (d, 8, 4H, CH). C{ H} NMR (NC D , 100 MHz, 25 °C):  = 23.74,
5
5
2
1
4.67, 25.49, 26.53, 26.90, 30.92, 31.33, 32.63, 33.94, 34.27, 34.96, 67.50, 70.08, 106.69,
08.35, 121.58, 122.64, 130.49, 135.82, 148.59, 153.97, 156.48, 159.31.
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Synthesis of (dippiq)(dippisq)UO Cl (4-iq/isq). A 20-mL scintillation vial was charged with
2
[UO Cl (THF) ] (0.275 g, 0.283 mmol) and 8 mL THF. A separate 20-mL scintillation
2
2
2 2
vial was charged with dippiq (0.430 g, 1.133 mmol), dissolved in 10 mL THF and added
dropwise to the stirring [UO Cl (THF) ] slurry. After stirring for 15 minutes, KC (0.077
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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3
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8
9
0
1
2
3
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2 2
8
g, 0.567 mmol) was weighed by difference and added over 5 minutes, resulting in a
rapid color change from red to brown. After 2 hours, the slurry was filtered through
Celite and volatiles were removed in vacuo. The brown solid was triturated with diethyl
ether (2 × 10 mL) and n-pentane (2 × 10 mL) then dried in vacuo. The resulting black
powder was washed with cold n-pentane (3 × 10 mL) to afford a greenish brown
powder (0.400 g, 0.376 mmol, 66% yield) assigned as 4-iq/isq. Single, X-ray quality
crystals were obtained from a concentrated diethyl ether solution at -35 °C. Elemental
analysis of C H N O ClU: Calculated, C, 58.66; H, 7.01; N, 2.63. Found, C, 58.91; H,
52 74
2
4
1
7
.13; N, 2.53. H NMR (C D , 300MHz, 25 °C):  = 2.09 (br s), 3.10 (br s), 4.90 (br s).
6
6
Reaction of 2-isq with 4 eq. Cl-BCat. A 20-mL scintillation vial was charged with 2-isq
0.100 g, 0.091 mmol) and 10 mL of THF. The vial was placed in the coldwell and the
(
solution was frozen. Upon thawing, Cl-BCat (0.056 g, 0.363 mmol) was weighed by
difference and added to the stirring solution. After warming to room temperature for 1
hour, volatiles were removed in vacuo. The products of the reaction were identified to
be UCl4dippiq (THF) and BOBCat by H and 11B NMR. The reaction mixture was
1
2
2
2
washed with cold n-pentane (2 × 5 mL) to separate organics from the uranium
product, producing a black powder (0.057 g, 0.063 mmol, 69% yield).
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dipp
Independent synthesis of
iqUCl (THF) (5-iq). A 20-mL scintillation vial was
4
2
charged with UCl (0.100 g, 0.263 mmol) and 10 mL THF. In a separate vial, dippiq (0.100
4
g, 0.263 mmol) was dissolved in 5 mL of THF and added dropwise to the stirring UCl4
solution. The solution immediately changed color from green to black. After 8 hours the
solvent was removed in vacuo and washed with n-pentane (2 × 5 mL) to yield a black
powder (0.184 g, 242 mmol, 92% yield). This sample was submitted for elemental
analysis. Single, X-ray quality crystals were obtained from a concentrated THF solution
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
at -35 °C. Elemental analysis of C H N O Cl U (solvent free): Calculated, C, 41.12; H,
26 37
1
1
4
1
4
1
3
1
1
.91; N, 1.84. Found, C, 40.83; H, 5.02; N, 1.64. H NMR (C D , 300MHz, 25 °C):  = -
6
6
4.89 (4, 1H, CH), -6.49 (3, 9H, C(CH ) ), -5.16 (103, 3H, CH(CH )), -5.07 (35, 4H, THF), -
3 3
3
.79 (106, 3H, CH(CH )),-3.61 (35, 4H, THF), -1.07 (4, 9H, C(CH ) ), -0.54 (4, 1H, CH),
3
3 3
.98 (32, 2H, CH), 9.11 (t, J = 7 Hz, 1H, CH), 10.87 (37, 4H, THF), 11.65 (36, 4H, THF),
2.29 (115, 3H, CH(CH )), 13.99 (118, 3H, CH(CH )), 19.44 (10, 2H, CH).
3
3
Reaction of 2-isq with 4 eq. Piv-Cl. A 20-mL scintillation vial was charged with 2-isq
0.080 g, 0.073 mmol), dissolved in 10 mL benzene and transferred to a 25 mL receiving
(
flask. Pivaloyl chloride (0.035 g, 0.290 mmol) was weighed by difference and added to
the green, stirring solution. The flask was sealed and then heated to 50 °C for 8 hours,
producing a red solution. After cooling to room temperature for 1 hour, volatiles were
1
removed in vacuo. The products of the reaction were identified by H NMR to be
pivaloyl anhydride, and 6-iq. The reaction mixture was washed with cold n-pentane (2
×
5 mL) to separate organics from the uranium product, producing a brown powder
(
0.061 g, 0.053 mmol, 73% yield).
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6
Independent synthesis of dippiq UCl (6-iq). A 20-mL vial was charged with UCl (0.056
2
4
4
g, 0.142 mmol) and 8 mL benzene. In a separate vial, dippiq (0.119 g, 0.312 mmol) was
dissolved in benzene (8 mL) and added dropwise to the stirring UCl slurry. The
4
0
1
2
3
4
5
6
7
8
9
0
1
2
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4
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reddish-brown mixture was then transferred to a receiving flask, sealed, and heated to
5
0 °C for 16 hours. The solvent was removed in vacuo and the resulting brown powder
washed with n-pentane (2 × 10 mL), leaving 6-iq (0.144 g, 0.126 mmol, 89% yield).
Elemental analysis of C H N O Cl U: Calculated, C, 54.83; H, 6.55; N, 2.46. Found, C,
5
2
74
2
2
4
1
5
5.64; H, 6.37; N, 2.59. H NMR (C D , 300MHz, 25 °C):  = -75.30 (19, 2H, CH), -5.66 (t, J
6
6
=
7 Hz, 2H, CH), -2.36 (12, 18H, C(CH ) ), 7.03 (8, 12H, CH(CH ) ), 7.25 (8, 2H, CH), 8.81
3
3
3 2
(
10, 12H, CH(CH ) ), 19.07 (7, 18H, C(CH ) ), 28.96 (27, 4H, CH(CH ) ), 56.68 (d, J = 7 Hz,
3 2
3 3
3 2
4
H, CH).
Synthesis of O-Labeled [UO Cl (THF) ] . This procedure was developed by adapting
1
8
2
2
2 2
literature procedures.27, 30 A 25-mL, borosilicate glass Schlenk flask was charged with
approximately 1 g of UO Cl (H O) (x = 1-3) and purged with argon. Against a flow of
2
2
2
x
18
argon, 1 mL of H O was added via pipette. After 1 minute of purging with argon, the
2
flask was sealed. The yellow solution was gently stirred in front of a UV lamp (365 nm).
Note, using [UO Cl (THF) ] as the uranium source leads to decomposition of the material, as
2
2
2 2
evidenced by color change from yellow to green and precipitation. After 12 days, the water
was removed from the yellow slurry in vacuo, yielding a yellow powder. From this
1
8
material, anhydrous O-labeled [UO Cl (THF) ] was obtained via a previously
2
2
2 2
2
7
published procedure. IR spectroscopy was employed to confirm the incorporation of
18O into the U-Ouranyl
(Figure S20).
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Results and Discussion
Synthesis and Characterization of Reduction Series. Initially, a neutral dippiq U(VI)
complex was synthesized to provide a baseline for studies of more reduced species and
allow comparison of spectroscopic data. Ligation of uranyl triflate, UO (OTf) (THF) ,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
3
was accomplished by stirring with an equimolar amount of dippiq in diethyl ether (Eq.
1
1
). Upon workup, a red-purple powder was obtained in good yield (76%). The H NMR
spectrum (Figure S1) displayed 9 resonances slightly shifted from those observed for
0
the free ligand, indicative of binding to the diamagnetic, U(VI), f ion. Two additional,
broad resonances, assignable as one bound THF molecule, were also observed at 1.93
and 5.26 ppm.
dipp
dipp
O
tBu
N
O
tBu
OTf
UO OTf2
N
2
(1)
U
O
Et2O
THF
O
tBu
TfO
tBu
dippiq
1
-iq
To unequivocally determine the number of coordinated ligands and examine the
structural parameters of the complex, single crystals of 1-iq, grown from toluene, were
analyzed using X-ray diffraction. Refinement of the data revealed a seven-coordinate
pentagonal bipyramidal uranyl coordination complex with one dippiq ligand, one THF
molecule, and two bound triflate anions in the equatorial plane along with the oxo
ligands in axial positions (Figure 2, left; Table 1). The U-Ouranyl distance of 1.745(5) Å
and Ouranyl-U-Ouranyl angle of 179.1(2)° are as expected based on similar uranyl
compounds. If the reaction is performed with excess dippiq ligand, formation of 1-iq is
31
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still observed, suggesting the triflate anions are not subject to substitution by neutral
dippiq despite the chelation effect of the bidentate ligand.
0
1
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9
0
1
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Molecular structure of 1-iq (left), 2-isq (middle) and 3-ap crown (right) shown
with 30% probability ellipsoids. Hydrogen atoms, co-crystallized solvent molecules,
potassium ions, crown ether molecules and bound triflate ions (except coordinated
oxygen atoms) are omitted for clarity.
Table 1: Selected uranyl bond distances (Å) and angles (°) for 1-iq, 2-isq, 3-ap, 3-ap
crown and 4-iq/isq.
1
-iq
2-isq
3-ap
3-ap crown
1.812(2)
4-iq/isq
1.757(3)
1.758(3)
U-OUranyl
U-OUranyl
1.745(5)
1.745(5)
1.762(4)
1.786(3)
1.824(3)
1.834(3)
1.814(2)
OUranyl-U-
OUranyl
1
79.1(2)
175.42(17)
172.22(12)
174.54(10)
175.09(13)
After successful metalation of the neutral dippiq ligand, the redox-active nature of
the ligand was engaged by stirring two equivalents of dippiq with one equivalent of
UO Cl (THF) followed by slow addition of two equivalents of KC (Scheme 1), causing
2
2
3
8
a color change from red/brown to dark green. After workup, a green powder was
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isolated in moderate yield (67%). The H NMR spectrum (Figure S5) displays broad,
paramagnetically-shifted signals ranging from 0 – 55 ppm. In this case, the
0
paramagnetism can arise either from 1) reduction of the uranium (VI) (f ) center to
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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0
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3
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5
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7
8
9
0
1
uranium (V) (f ) while maintaining a neutral iminoquinone ligand, or 2) from ligand
reduction to generate two radical-containing iminosemiquinone ligands.
dipp
tBu
N
O
2
tBu
dippiq
1
) UO2Cl2THF3
) 2 KC8
2
1) UO2Cl2THF3
-
2 KCl
4 C8
2
) 4 KC8
-
-
2 KCl, 2 C8
+
2
[K (18-crown-6)]
dipp
tBu
dipp
O
U
tBu
K
dipp
O
tBu
O
N
1/2
tBu
O
N
O
2
KC8
2 18-Crown-6
tBu
O
N
N
O
tBu
O
N
N
O
U
O
tBu - 2 C8
U
O
tBu
tBu
THF
O
tBu
dipp
dipp
tBu
THF
tBu
dipp
K
2
2
-isq
Et2O
Et2O
3-ap
3-ap crown
Scheme 1. Synthesis of complexes 2-isq, 3-ap and 3-ap crown.
To further elucidate the electronic structure of 2-isq,17, 32, 33 X-ray crystallography
was employed as structural parameters, including intraligand bond distances, can often
3
4
aid in ligand oxidation state assignment. Crystals of 2-isq grown from diethyl ether at
-
35 °C were analyzed. Refinement of the data revealed a seven coordinate uranyl
species, with two dippiq ligands and a THF molecule coordinated in the equatorial plane
(
Figure 2, middle). As with 1-iq, a pentagonal bipyramidal geometry with the uranyl
oxygen atoms in axial positions was observed. Additionally, the U-Ouranyl bond
distances of 1.762(4) and 1.786(3) Å (Table 1) are within the range of 1.76 – 1.79 Å
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observed for U(VI) uranyl molecules.31, 32, 35 These U-Ouranyl distances suggest the
reduction is ligand-based rather than occurring at the uranium.
To further confirm the presence of ligand radicals, EPR spectroscopy was
0
1
2
3
4
5
6
7
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9
0
1
2
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performed. The EPR spectrum of 2-isq (Figure 3) displays an isotropic signal at g =
1
.993, which is shifted from that of 2.0061 observed for 4,6-di-tert-butyl-2-tert-butyl-
tBu
36
iminosemiquinone (H isq•) . This value agrees well with the g-value previously
observed for hexavalent uranyl species featuring redox-active ligands, such as
Cp*UO2(MesPDI ) (giso
Me
=
1.974), Cp*UO ( Bu- PDI ) (giso =1.936) and
t
Mes
Me
2
tBu
+
(gav =1.997) (tBu-MesPDIMe
[
UO (salophen) (H O)]
=
2,6-((Mes)-N=CMe) -p-
2
2
2
t
t
17
C(CH ) C H N; salophen = N,N’-bis(3- Bu-(5 Bu)-salicylidene)-1,2-phenylenediamine).
3 3
5
2
Spin-orbit coupling of the ligand-based radical with the empty f-orbitals of the uranium
tBu
center is likely the cause of the high-field shift from the g-value observed for H isq•.
Simulation of the spectrum was accomplished with one nitrogen (I = 1) and one
hydrogen atom (I = ½) of the ligand, yielding hyperfine coupling constants of A =
N
1
9.48 and A = 13.97 MHz. These values suggest a stronger interaction with the
H
nitrogen atom than the hydrogen atom of the ligand. This hyperfine pattern agrees with
previously noted results18, 36. Thus, these measurements support the presence of an
iminosemiquinone ligand radical with only a weak interaction with uranium.
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Figure 3. EPR spectrum of 2-isq (n-pentane, 5.8 mM) recorded at 25 °C. giso = 1.993.
Frequency: 9.852 GHz. Power: 10 mW. Modulation: 0.5 G/100 kHz.
Variable-temperature and field-dependent magnetic measurements were
performed to elucidate the uranium and ligand oxidation states in 2-isq (Figure 4). At
room temperature, the μeff value (2.45 μ ) was consistent with that expected for two
B
3
7
isolated, unpaired electrons with g = 2.0 (theoretical μ value = 2.45 μ ). Upon cooling,
eff
B
the μ value slightly increased to 2.52 μ at 4 K. The increase in moment resulted from
eff
B
the ferromagnetic coupling of the two unpaired electrons. The data were fitted by the
3
8
−1
Bleaney−Bowers equation with g = 2.00 and exchange coupling constant J = 0.4 cm
(see the supporting information for details). The magnetization in the field dependent
measurement saturated at 1.94 μ at 2 K, also characteristic of two isolated S = 1/2
B
37
spins. Overall, these magnetic data indicated the presence of one U(VI) cation and two
ligand-centered radicals with negligible ferromagnetic coupling in 2-isq. This value
supports the presence of two non-communicating ligand radicals that were measured
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by EPR and predicted with the metrical parameters deduced by X-ray crystallography
(vide infra). The radicals in 2-isq are relegated to the ligand at all temperatures, with
little interaction with the uranium. This rules out alternate potential electronic
structures for 2-isq (Figure 5). These magnetic data also eliminate formation of
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V
pentavalent (isq )(iq )U O , which would have both uranium based and ligand based
2
0
IV
radicals, as well as (iq ) U O , which would have closed-shell ligands with only
2
2
uranium based electrons unpaired.
Figure 4. Variable temperature molar magnetic data (μ ) for 2-isq and variable field
eff
data collected at 2 K (inset)
Figure 5. Possible electronic structures of 2-isq.
To probe the ability of our system to accommodate additional electrons beyond
monoreduction of both ligands, electrochemical measurements of 2-isq were performed
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in THF (Figure S19). The cyclic voltammetric data for 2-isq showed a quasi-reversible
reduction at -1.58 V, which could be attributed to the reduction of the two ligands from
1-
2-
isq to ap or reduction of uranium from +6 to +5. Encouraged by the electrochemical
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results, 2-isq was reduced chemically by stirring with two equivalents of KC in THF.
8
Following workup, a tan powder that is poorly soluble in hydrocarbon solvents was
1
isolated. H NMR spectroscopic analysis (pyridine-d ) showed a spectrum (Figure S6)
5
with 11 resonances. Of these, nine signals were assigned to the ligand and two signals
as bound THF, none of which were drastically shifted from the diamagnetic region.
These data suggest a closed-shell U(VI), f ion with dippap ligands bound to uranium.
0
Because solution behavior can be complicated by dynamic inner-sphere and
outer-sphere cation exchange, a crown ether was employed to encapsulate the
potassium ion for further studies.18, 39, 40 To achieve this, two equivalents of 18-crown-6
were added to a stirring solution of 3-ap in THF. After workup, a white powder was
1
isolated in good yield (76%). The H NMR spectrum (Figure S8) is quite similar to that
of 3-ap, with minor differences, including an additional resonance at 3.43 ppm
corresponding to the 24 protons associated with 18-crown-6.
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Figure 6. Molecular structure of 3-ap (left) shown with 30% probability ellipsoids. Co-
crystallized solvent molecules and H atoms are omitted for clarity. Schematic
representation of 3-ap (right), with interactions between potassium ions and various
parts of the molecule shown as dashed lines.
Comparison of the structural changes that accompany reduction and
examination of the effect of potassium ion sequestration in 3-ap (Figure 6) and 3-ap
crown (Figure 2, right) were examined using X-ray crystallography. 3-ap crystallizes as
a dimer from diethyl ether or toluene at -35 C with the oxygen atom and aromatic ring
of the dippiq ligand, and uranyl oxygen atoms interacting with the potassium ions. This
dimerization causes out of plane bending of the ligand with the N and O atoms
3
appearing to be sp hybridized to allow for lone pair interaction with the nearby
potassium ions. Additionally, the uranyl bond distances of 1.824(3) and 1.834(3) Å
31
(
Table 1) are quite long, appearing to result from the potassium binding to the Ouranyl.
This cation-capping effect has been thoroughly studied by P. Arnold and coworkers
41
who employed a large, compartmental, N -macrocyclic ligand. In these studies,
8
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reduction of uranyl was facilitated by coordination of a cation, held in place by the
specialized “Pacman” type ligand, to the Ouranyl
.
The structure of 3-ap crown shows that sequestering the potassium ions with
crown ethers permits the amidophenolate ligands to return to their planar geometry,
isostructural to 2-isq. Interestingly, despite the absence of the interaction of potassium
atoms with the uranyl moiety in 3-ap crown, the U-Ouranyl bond distances remain long
1
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(1.814(2) and 1.812(2) Å), suggesting ligand reduction plays a role in the elongation of
the U-Ouranyl bond. This implies that cation coordination is not the only contributing
factor in the U-Ouranyl bond elongation observed in 3-ap. U-Ouranyl bond lengthening has
been previously observed by Hayton and co-workers when they described tris- and
t
tetrakis-ketimide
complexes
[Li(THF)(TMEDA)][UO (N=C BuPh) ]
and
2
3
t
42
[
Li(THF)(Et O)] [UO (N=C BuPh) ]. These ketimide uranyl molecules display U-O
2
2
2
4
uranyl
bond lengths of 1.830(5) and 1.804(5) Å (tris-ketimide) and 1.838(4) Å (tetrakis-
ketimide). The authors attribute this elongation to the presence of four strong donors in
the equatorial plane, which act to weaken the U-Ouranyl bonds via electrostatic repulsion
and -donation to the uranium center. Further evidence of this elongation can be found
43
44
in the imido family of uranyl analogues. Studies of uranium tris- and tetrakis(imidos)
featuring strong -donating imido groups in the equatorial plane – have shown that
–
imido bonds lengthen and weaken with each additional -donor. The dippap ligand has
2-
2
4
been studied as a -donating ligand for high valent metals previously. As its high-
lying  orbitals have the appropriate symmetry to donate electron density to the
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uranium VI center, this is likely a factor in U-Ouranyl bond elongation and activation in 3-
ap crown.
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Figure 7. Bond distance comparison (Å) for 1-iq, 2-isq, and 3-ap crown. Carbon atoms
shown in black, nitrogen atoms shown in blue, oxygen atoms shown in red, and
uranium atom shown in magenta.
The bond metrics vary based on the ligand oxidation state, and can therefore
provide insight into the electronic structure of 1-iq, 2-isq, and 3-ap crown (Figure 7).
The neutral dippiq ligand of 1-iq displays relatively long uranium-oxygen (2.432(4) Å)
and uranium-nitrogen (2.630(4) Å) bonds that are indicative of a datively bound ligand.
Furthermore, the short C-O (1.237(7) Å) and C-N (1.287(8) Å) bonds suggest the ligand
retains double bond character. In contrast, the iminosemiquinone ligands of 2-isq
feature shortened U-O bonds (2.295(3) and 2.362(3) Å) and U-N bonds (2.563(4) and
2
.536(4)) accompanied by longer C-O (1.313(5) and 1.296(6) Å) and C-N bonds (1.339(5)
and 1.337(6) Å), indicating reduction of the ligand.
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The bond distances of 3-ap crown are not as simple to analyze as those of the
other compounds of the series. While the expected shortening trend of the U-E (E = O,
N) bond is observed for one “E” atom of each ligand, the other “E” atom bond distance
is nearly identical to that of 2-isq. This could be attributed to steric hindrance with the
1
1
1
1
1
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t
bulky dipp or Bu groups that prevent the U-E bonds from contracting towards the
metal center upon reduction. In this case, a stronger indication of reduction can be
found in the intraligand bond distances (Figure 7). The C-O and C-N bonds in each
ligand lengthen, signifying the conversion from a double bond to a single bond.
Corroborating evidence can also be found in the C-C bond distances of the rings in the
ligands. As the ligand is reduced, the distances throughout the ring become more
similar, suggesting aromaticity is present.
As the bond distances in molecules 1-iq, 2-isq and 3-ap crown change with
ligand oxidation state, the bond angles also indicate ligand reduction. In particular, this
effect is noticeable in the N -U-O angles of the iminoquinone ligand. As the ligand
iq
iq
becomes more reduced, the shortening of the U-O and U-N bonds causes the N -U-
iq
iq
iq
O bite angle of the ligand to increase from 61.83° (1-iq) to 64.44° (2-isq) and finally to
iq
6
5.26° (3-ap crown).
Another useful technique for examining the ligand and metal oxidation states in
uranium/uranyl systems is electronic absorption spectroscopy. For the current study,
this technique is particularly advantageous as 1-iq, 2-isq, and 3-ap all exhibit distinctly
different colors (Figure S16 and S17). Spectra were recorded in THF at 25 °C from 375 –
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-1
-1
1
800 nm. The spectrum of 1-iq displays an intense absorption at 385 nm (41,711 M cm )
-1
-1
with a weaker absorption at 511 nm (4,075 M cm ), giving rise to the observed red-
-1
-1
purple color. 2-isq shows an intense absorption at 310 nm (26,600 M cm ) and a much
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-1
-1
less intense, broad absorption at 747 nm (2,130 M cm ), responsible for the observed
-1
-1
green color. Finally, 3-ap shows only one absorption at 313 nm (15,313 M cm ) and
does not absorb significantly in the visible region, consistent with its pale color.
Interpretation of these data suggests that the major, color-producing peaks of the
spectra can be attributed to the ligand oxidation state, as similar colors have been
20
18
observed for complexes of early transition metals , lanthanides , and low valent
uranium.21
dipp
dipp
Cl
tBu
O
U
O
U
O
tBu
2
N
tBu
N
O
O
N
1 KC8
Cl
Cl
THF
THF
(2)
THF
+
O
-
1 KCl, C8
tBu
tBu
tBu
O
dipp
dippiq
4-iq/isq
Synthesis and Characterization of a Mixed Ligand Species. The extent of radical
delocalization in the uranyl iminoquinone complexes was tested by synthesizing a
mixed iminoquinone/iminosemiquinone ligand species. We hypothesized that rather
than full delocalization of the radical across both ligands, the radical would remain
localized on one of the iminosemiquinone ligands and not the other. Preparation of
(dippiq)(dippisq)UO Cl (4-iq/isq) was achieved by stirring two equivalents of the
2
iminoquinone ligand with one equivalent of uranyl chloride in the presence of one
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equivalent of potassium graphite (Eq. 2). Upon workup, a greenish-brown powder was
isolated in moderate yield (66%).
1
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Figure 8. Molecular structure of 4-iq/isq shown with 30% probability ellipsoids. Co-
crystallized solvent molecules and H atoms are omitted for clarity.
X-ray crystallographic analysis of green-brown crystals grown from diethyl ether show a
pentagonal bipyramidal uranium center similar to that of 2-isq but with a chloride atom instead
of a THF molecule (Figure 8, Table 1). Compound 4-iq/isq shows asymmetric ligands in the
solid state, with each ligand’s oxidation state undoubtedly assignable based on bond distances
(Figure 9). The U-O distances of 2.493(3) and 2.312(3) Å and U-N distances of 2.670(3) and
2
.498(3) Å highlight the asymmetric nature of the two ligands, with the shorter contact belonging
to the isq ligand and the longer to the iq ligand. Further evidence can be found in the C-O and C-
N distances, as well as the intraligand C-C distances. For the iminoquinone ligand, C-O and C-N
bonds of 1.230(5) and 1.290(5) Å are indicative of double bond character, as observed for 1-iq.
However, the C-O and C-N bonds of the other ligand are significantly longer at 1.297(5) and
1.341(5) Å, agreeing well with the ligands of 2-isq. Additional evidence can be found in the C-C
bonds of each ring, with the iminoquinone ligand featuring two short bonds and three long ones,
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whereas the iminosemiquinone ligand has bonds more similar in length. Both of these trends
match well with the ligands of compounds 1-iq and 2-isq, respectively.
0
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9
0
Figure 9. Bond distance comparison (Å) for 4-iq/isq. Carbon atoms shown in black,
nitrogen atoms shown in blue, oxygen atoms shown in red, chloride atoms shown in
green and uranium atom shown in magenta.
The presence of a ligand radical in 4-iq/isq was confirmed using EPR spectroscopy
(Figure S18). In a toluene solution, an isotropic signal is observed at g = 1.989. This is
comparable to the g-value of 2-isq (1.993). Variable temperature SQUID magnetometry provides
additional evidence of the ligand radical (Figure S25), which establishes a room temperature μeff
value of 1.86 μ . The UV-Vis/NIR spectrum of 4-iq/isq displays absorbance in multiple regions
B
(Figure S16 and S17), which explains the observed brown color. Intense absorbances are noted at
-1
-1
-1
-1
-1
-1
307 nm (15,188 M cm ) and 388 nm (5759 M cm ), while signals at 528 nm (259 M cm )
-1
-1
and 800 nm (839 M cm ) are broad and less intense. The number of absorbance maxima is
likely the product of the ligands existing in two oxidation states, absorbing at different
wavelengths.
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Uranyl Functionalization
Reactivity of 2-isq with B-Chlorocatecholborane. Boron is well-known for its Lewis
acidic properties, making it an excellent candidate to examine reactivity with electron-
rich 2-isq. B-chlorocatecholborane (Cl-BCat) features a B-Cl bond that can be
heterolytically cleaved by the chlorophilic uranium. Treating 2-isq with four
equivalents of Cl-BCat causes the solution to change color from green to brown
1
1
1
1
1
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1
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1
immediately. Resonances in the H NMR spectrum of the reaction mixture (Figure S11)
range from -20 to +20 ppm, suggesting that the uranyl was reduced to paramagnetic
2
U(IV), f , a transformation that likely involves uranyl U-O
bond activation (Eq. 3).
uranyl
1
1
Further inspection of the data, coupled with B NMR data (Figure S12), revealed that
dicatecholatobisborylether (BOBCat ) was produced during the reaction. In sealed-tube
2
NMR experiments in benzene-d and THF-d , free ligand is also observed, but does not
6
8
integrate to one full equivalent. Based on the stoichiometry of Cl-BCat and the number
1
and integration values of resonances in the H NMR spectrum, the resulting uranium
product was hypothesized to be UCl4(dippiq ) (n = 1, 2).
0
n
dipp
tBu
O
O
O
4
Cl
B
dipp
Cl
U
tBu
O
N
Cl
THF
THF
Cl
N
O
tBu
N
O
U
(3)
tBu
THF
THF
Cl
tBu
O
tBu
dipp
O
O
O
O
2
-isq
B
O
B
5-iq
2
Identification of the U(IV) product from borylation of 2-isq was achieved by an
independent synthesis. Treating UCl with an equivalent of dippiq in THF produced a
4
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2
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5
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
dark green solution (Eq. 4), and H NMR data confirmed the product was the same as
that observed in the uranyl borylation reaction (Figure S14). The material from the
independent synthesis was crystallized from THF and subsequent X-ray analysis
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
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9
0
revealed the identity to be UCl4dippiq(THF) (5-iq) (Figure S23). The ligands are
2
confirmed to be in the iminoquinone form with C-O and C-N distances of 1.234(3) and
1
.298(3) Å, signifying double bonds and matching the distances observed in 1-iq and 4-
iq/isq. Further evidence can be found in the U-O and U-N distances of 2.4878(16) and
2
.8283(18) Å, which are longer than the contacts reported for iminosemiquinone-
tBu
containing U(IV) complexes ( isq) UI (THF) (U-O = 2.160(10) and 2.164(10), U-N =
2
2
2
1
2
.514(12) and 2.567(14) Å). These longer contacts highlight the dative nature of the
ligand binding in 5-iq.
dipp
+
dipp
Cl
Cl
Cl
U
tBu
N
O
Cl
Cl
6 Hours
THF
Cl
THF
THF
Cl
tBu
N
O
U
(4)
tBu
Cl
tBu
dippiq
5-iq
To corroborate that the organic products observed were the result of U-Ouranyl
bond cleavage and that the extracted oxygen atoms had originated from the U-Ouranyl
,
isotopic labeling studies were performed. 18O-labeled uranyl dichloride
1
8
([U O Cl (THF) ] ) was synthesized through photolysis of uranyl chloride hydrate in
2
2
2 2
18O-labeled water.31
18O incorporation was confirmed by IR spectroscopy, with the
expected OUO asymmetric stretching frequency reducing from 942.4 cm-1
1
6
-1
18
(
[U O Cl (THF) ] ) to 892.3 cm ([U O Cl (THF) ] ) (Figure S20). Synthesis of
2
2
2 2
2
2
2 2
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(dippisq) U O THF (2-isq ) was accomplished in an analogous manner to 2-isq, using
18
18
2
2
1
8
[U O Cl (THF) ] as the uranyl source. Successful incorporation was confirmed by IR
2
2
2 2
spectroscopy (Figure S21).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
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5
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5
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0
1
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0
1
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8
9
0
Figure 10. Electron ionization mass spectra (EI-MS) of BOBCat from the reaction mixture
2
1
8
of ClBCat and 2-isq (top) and 2-isq (bottom).
18
Treating 2-isq with Cl-BCat under the same conditions as previously described
for the unlabeled isotopomer yielded analogous results as identified by H and 11B
1
18
NMR spectroscopies. Analysis of the organic bisborylether, B OBCat , by EI-MS gave
2
•
+
an increase in the M peak appearing at m/z = 256, corresponding to the incorporation
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