Formation of trivalent U-C, U-N, and U-S bonds and their reactivity toward carbon dioxide and acetone

Article abstract of DOI:10.1021/om200612h

A family of uranium(III) complexes of the form Tp*₂UX (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, X = CCPh (2-CCPh), CCSiMe₃ (3-CCSiMe₃), NHPh (4-NHPh), NHCH₂Ph (5-NHCH₂Ph), SPh (6-SPh)) have been synthesized in quantitative yields by protonation of the benzyl group Tp*2U(CH₂Ph) (1-CH₂Ph) with organic precursors. Full characterization of these complexes, which contain newly formed uranium-carbon, uranium-nitrogen, and uranium-sulfur bonds, was performed using 1H NMR and IR spectroscopies. These data along with the structural parameters of the uranium acetylide (2-CCPh) and uranium thiolate (6-SPh) species are reported. Treating the trivalent uraniumacetylides and amides with 1 atm of carbon dioxide results in its insertion into the uranium-element bond to form the corresponding carboxylate or carbamate moiety in quantitative isolable yields. Reversible insertion of carbon dioxide (1 atm) into the uranium-sulfur bond of Tp*2U(SPh) (6-SPh) was also noted. The new family ofCO₂-inserted uranium(III) derivatives Tp*₂UO₂CX (X = CCPh (7-O ₂CCCPh), CCSiMe₃ (8-O₂CCCSiMe₃), NHPh (9-O₂CNHPh), NHCH₂Ph (10-O₂CNHCH ₂Ph), SPh (11-O₂CSPh)) has been characterized spectroscopically and, in the case of 7-O₂CCCPh, crystallographically. The newly formed U-O bonds were cleaved by addition of trimethylsilyl halides, generating the monohalide derivative. Attempts to insert acetone into the uranium-carbon bonds of 1-CH₂Ph and 2-CCPh or the uranium-nitrogen bond in 4-NHPh resulted exclusively in protonation of the benzyl ligand and isomerization to form a rare enolate complex Tp*2U(OC(CH₃)CH₂) (12-OC(CH₃)CH ₂). This complex has been fully characterized by 1HNMR and infrared spectroscopies as well as X-ray crystallography.

Full text of DOI:10.1021/om200612h

ARTICLE
pubs.acs.org/Organometallics
Formation of Trivalent UÀC, UÀN, and UÀS Bonds and Their
Reactivity toward Carbon Dioxide and Acetone
Ellen M. Matson, Phillip E. Fanwick, and Suzanne C. Bart*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S Supporting Information
b
ABSTRACT:
A family of uranium(III) complexes of the form Tp*2UX (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, X = CCPh (2-CCPh),
CCSiMe3 (3-CCSiMe3), NHPh (4-NHPh), NHCH2Ph (5-NHCH2Ph), SPh (6-SPh)) have been synthesized in quantitative
yields by protonation of the benzyl group Tp*2U(CH2Ph) (1-CH2Ph) with organic precursors. Full characterization of these
complexes, which contain newly formed uraniumÀcarbon, uraniumÀnitrogen, and uraniumÀsulfur bonds, was performed using
1H NMR and IR spectroscopies. These data along with the structural parameters of the uranium acetylide (2-CCPh) and uranium
thiolate (6-SPh) species are reported. Treating the trivalent uranium acetylides and amides with 1 atm of carbon dioxide results in its
insertion into the uraniumÀelement bond to form the corresponding carboxylate or carbamate moiety in quantitative isolable yields.
Reversible insertion of carbon dioxide (1 atm) into the uraniumÀsulfur bond of Tp*2U(SPh) (6-SPh) was also noted. The new
family of CO2-inserted uranium(III) derivatives Tp*2UO2CX (X = CCPh (7-O2CCCPh), CCSiMe3 (8-O2CCCSiMe3), NHPh (9-
O2CNHPh), NHCH2Ph (10-O2CNHCH2Ph), SPh (11-O2CSPh)) has been characterized spectroscopically and, in the case of 7-
O2CCCPh, crystallographically. The newly formed UÀO bonds were cleaved by addition of trimethylsilyl halides, generating the
monohalide derivative. Attempts to insert acetone into the uraniumÀcarbon bonds of 1-CH2Ph and 2-CCPh or the
uraniumÀnitrogen bond in 4-NHPh resulted exclusively in protonation of the benzyl ligand and isomerization to form a rare
enolate complex Tp*2U(OC(CH3)CH2) (12-OC(CH3)CH2). This complex has been fully characterized by 1H NMR and infrared
spectroscopies as well as X-ray crystallography.
’ INTRODUCTION
The small size of carbon dioxide facilitates insertion chemistry,
but examples of insertion using larger carbonyl-containing sub-
strates have also been reported. Multiple ketones and aldehydes
insert into the UÀC bond in ((Me3Si)2N)3UCH3,12 while a
cyclometalated derivative of U(N(SiMe3)2)3, ((Me3Si)2N)2-
U(Me3SiNSi(Me)2CH2),13 readily inserts the carbonyl of
substituted cyclohexanones into the UÀC bond to produce
spirometallacycles. The ansa-thorium metallocene Me2Si(η5-
C5(CH3)4)2Th(CH3)2 dimerizes aldehydes, which is proposed
to proceed through insertion of the carbonyl into the ThÀC
bond.14
Recently, we have reported that the uranium(III) alkyl com-
plex Tp*2U(CH2Ph) (1) undergoes insertion of carbon dioxide
along the uraniumÀcarbon bond, producing the rare uranium-
(III) carboxylate Tp*2U(k2-O2CCH2Ph).15 This insertion is
immediate at ambient or low temperatures with low pressures
of CO2 (e1 atm). Due to the relatively underexplored nature of
Insertion of carbon dioxide and other carbonyl-containing sub-
strates into metalÀelement bonds is a key step in many important
transition-metal-mediated organic transformations,1À3 and acti-
nides undergo similar reactivity. Insertion of carbon dioxide has
been noted for tetravalent uraniumÀcarbon bonds, such as
the uranium dimethyl complex Cp*2UMe2 (Cp* = 1,2,3,4,5-
pentamethylcyclopentadienyl), which forms the corresponding
di(acetate) Cp*2U(k2-O2CMe) upon exposure to low pressures
of CO2.4 Similarly, the uranium bis(acetylide) Cp*2U(CCPh)2
forms Cp*2U(k2-O2CCCPh) by CO2 insertion.5 Amido com-
plexes, such as Cp*2U(NEt2)2,6 [((RArO)3tacn)U(NHMes)]
((RArO)3tacn =1,4,7-tris(3-R-5-tert-butyl-2-hydroxybenzyl)-1,4,
7-triazacyclononane, R = tBu, Ad),7 and other tetrakis(dialkyl-
amido) uranium(IV) derivatives, yield carbamate ligands by inser-
tion of CO2 into the UÀN bond.8À10 Although the insertion
chemistry of uranium(IV) sulfido species has had less attention,
UÀS bonds have been shown to reversibly insert carbon dioxide
and carbon disulfide.11
Received: July 9, 2011
Published: October 17, 2011
r
2011 American Chemical Society
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|

Organometallics
ARTICLE
insertion reactions at uranium(III) centers, we set out to perform
a systematic study of the insertion of carbon dioxide and acetone
along trivalent uraniumÀelement bonds, specifically sp-hybri-
dized UÀC bonds, amido UÀN bonds, and thiolate UÀS bonds.
Herein we present the syntheses of uranium(III) species contain-
ing these moieties, as well as their reactivity toward carbon
dioxide and acetone.
1H NMR (C6D6, 25 °C, ppm): À13.49 (28, 18H, Tp*-CH3), 0.62 (7,
18H, Tp*-CH3), 7.49 (7, 6H, Tp*-CH), 9.95 (17, 2H, phenyl-CH),
13.25, (13, 1H, p-phenyl-CH), 20.64 (21, 2H, phenyl-CH). IR:
2049 cmÀ1 (CtC), 2526, 2554 cmÀ1 (BÀH). Spectroscopic data
matches those previously reported by Takats.19
Tp*2U(CtCSiMe3) (3-CCSiMe3). The remaining blue solid was
washed with cold pentane and dried, leaving a product assigned as
Tp*2U(CtCSiMe3). Yield: 0.101 g (0.107 mmol, 97%). Anal. Calcd for
C35H53N12SiB2U: C, 45.22; H, 5.75; N, 18.01. Found: C, 45.17; H, 5.75;
N, 18.03. 1H NMR (C6D6, 25 °C, ppm): À13.53 (26, 18H, Tp*-CH3),
0.69 (8, 18H, Tp*-CH3), 7.04 (9, 9H, Si-(CH3)3), 7.57 (9, 6H, Tp*-
CH). IR: 1997 cmÀ1 (CtC), 2526, 2555 cmÀ1 (BÀH).
’ EXPERIMENTAL SECTION
General Considerations. All air- and moisture-sensitive manip-
ulations 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 designed
for freezing samples in liquid nitrogen as well as two À35 °C freezers for
cooling samples and crystallizations. Solvents for sensitive manipula-
tions were dried and deoxygenated using literature procedures with a
Seca solvent purification system.16 Benzene-d6 was purchased from
Cambridge Isotope Laboratories, dried with molecular sieves and
sodium, and degassed by three freezeÀpumpÀthaw cycles. Elemental
analyses were performed by Midwest Microlab, LLC, in Indianapolis,
IN. Phenylacetylene, ethynyltrimethylsilane, aniline, benzylamine, and
benzenethiol were purchased from Sigma Aldrich, dried over CaH2, and
vacuum-distilled before use. Acetone was purchased from Sigma Aldrich,
dried over CaCl2, and vacuum-transferred before use. Carbon dioxide
(99.9%, anhydrous) was purchased from Sigma Aldrich and used as
received. Tp*2U(CH2Ph) was prepared according to literature
procedures.15
Tp*2U(NHPh) (4-NHPh). The remaining solid was recrystallized from
a pentane/ether mixture (20/1) at À35 °C, producing a blue-green
crystalline solid assigned as Tp*2U(NHPh). Yield: 0.094 g (0.102 mmol,
98%). Anal. Calcd for C36H50N13B2U: C, 46.77; H, 5.45; N, 19.70.
1
Found: C, 46.51; H, 5.39; N, 19.32. H NMR (C6D6, 25 °C, ppm):
À10.31 (1286, 18H, Tp*-CH3), À2.33 (70, 18H, Tp*-CH3), 0.80 (278,
2H, B-H), 7.20 (55, 6H, Tp*-CH), 14.43 (40, 1H, p-phenyl-CH), 18.84
(85, 2H, phenyl-CH), 34.89 (79, 2H, phenyl-CH), 75.02 (219, 1H,
N-H). IR: 2526, 2551 cmÀ1 (BÀH), 3369 cmÀ1 (NÀH).
Tp*2U(NHCH2Ph) (5-NHCH2Ph). The remaining solid was recrys-
tallized from a pentane/ether mixture (20/1), producing a brown
crystalline solid assigned as Tp*2U(NHCH2Ph). Yield: 0.092 g (0.098
mmol, 94%). Anal. Calcd for C37H50N13O2B2U: C, 47.35; H, 5.58; N,
1
19.40. Found: C, 47.04; H, 5.28; N, 19.46. H NMR (C6D6, 25 °C,
ppm): À7.21 (63, 18H, Tp*-CH3), À3.73 (15, 18H, Tp*-CH3), 4.72
(344, 2H, B-H), 7.64 (20, 6H, Tp*-CH), 12.60 (34, 1H, p-phenyl-CH),
14.48 (31, 2H, phenyl-CH), 28.56 (34, 2H, phenyl-CH), 97.15 (111,
2H, CH2), 103.48 (299, 1H, N-H). IR: 2526, 2551 cmÀ1 (BÀH),
3310 cmÀ1 (NÀH).
1H NMR spectra were recorded on a Varian Inova 300 spectrometer
operating at a frequency of 299.992 MHz. All chemical shifts were
1
Tp*2U(SPh) (6-SPh). The remaining dark blue solid was washed with
pentane and dried, leaving a dark blue powder assigned as Tp*2U(SPh).
Yield: 0.094 g (0.100 mmol, 96%). Anal. Calcd for C36H49N12B2SU: C,
45.92; H, 5.25; N, 17.85. Found: C, 46.07; H, 5.28; N, 17.76. 1H NMR
(C6D6, 25 °C, ppm): À11.55 (23, 18H, Tp*-CH3), À0.53 (6, 18H, Tp*-
CH3), 5.61 (178, 2H, B-H), 7.49 (6, 6H, Tp*-CH), 11.21 (18, 1H,
phenyl-CH), 13.47 (18, 2H, phenyl-CH), 20.89 (30, 2H, phenyl-CH).
IR: 2521, 2548 cmÀ1 BÀH.
reported relative to the peak for SiMe4 using H (residual) chemical
shifts of the solvent as a secondary standard. The spectra for para-
magnetic 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. Solid-state infrared spectra
were recorded using a Perkin-Elmer FT-IR Spectrum RX I spectrometer.
Samples were made by crushing the solids, mixing with dry KBr, and
pressing into a pellet.
Single crystals for X-ray diffraction were coated with poly(isobutylene)
oil in a glovebox and quickly transferred to the goniometer head of a
Rigaku Rapid II image plate diffractometer equipped with a Micro-
Max002+ high-intensity copper X-ray source with confocal optics.
Preliminary examination and data collection were performed with Cu
Kα radiation (λ = 1.541 84 A). Cell constants for data collection were
obtained from least-squares refinement. The space group was identified
using the program XPREP.17 The structures were solved using the
structure solution program PATTY in DIRDIFF99.18 Refinement was
performed on a LINUX PC using SHELX-97.17 The data were collected
at a temperature of 150(1) K.
General Procedure for Preparation of Uranium(III) Com-
plexes by Protonation. A 20 mL scintillation vial was charged with
0.100 g (0.108 mmol) of Tp*2U(CH2Ph) and approximately 3 mL of
tetrahydrofuran, making a dark green solution. After the sample was
cooled to À35 °C, a microsyringe was used to deliver 1 equiv of an
organic substrate with an acidic hydrogen (2, phenylacetylene; 3,
trimethylsilylacetylene; 4, aniline; 5, benzylamine; 6 = benzenethiol)
with stirring. After 5 min, the solvent and toluene (byproduct) were
removed in vacuo, leaving a solid.
General Procedure for Preparation of CO2-Inserted Com-
plexes (7À10). In a glovebox, a THF solution of 0.100 g of the
appropriate uranium(III) precursor was added into a thick-walled glass
vessel and sealed. On the Schlenk line, the sample was frozen with liquid
nitrogen and the head space of the flask evacuated. After the sample was
warmed to room temperature under vacuum, 1 atm of CO2 was added to
the flask with stirring. Reaction progress was monitored by color change
of the sample and uptake of CO2 as measured by the manometer. All
reactions proceeded immediately on the basis of the color change.
However, stirring was continued for an additional 30 min to ensure
conversion. Solvent was then removed in vacuo.
Tp*2U(k2-O2CCCPh) (7-O2CCCPh). A dark green powder assigned as
Tp*2U(k2-O2CCtCPh) was isolated in quantitative yield, with no
further purification necessary (0.104 g, 0.106 mmol, 98%). Anal. Calcd
for C39H49N12O2B2U: C, 47.92; H, 5.05; N, 17.19. Found: C, 48.66; H,
5.38; N, 16.44. 1H NMR (C6D6, 25 °C, ppm): À9.23 (134, 18H, Tp*-
CH3), À0.31 (5, 18H, Tp*-CH3), 2.77 (339, 2H, B-H), 7.39 (9, 6H, Tp*-
CH), 8.15 (t, J = 7.2 Hz, 2H, phenyl-CH), 8.84 (t, J = 7.8 Hz, 1H, p-
phenyl-CH), 11.46 (d, J = 7.2 Hz, 2H, phenyl-CH). IR: 2211 cmÀ1
(CtC); 2522, 2551 cmÀ1 (BÀH).
Tp*2U(k2-O2CCCSiMe3) (8-O2CCCSiMe3). A green powder was re-
covered with no purification necessary and assigned as Tp*2U-
(k2-O2CCtCSiMe3) (0.100 g, 0.103 mmol, 99%). Anal. Calcd for
C36H53N12O2B2SiU: C, 44.41; H, 5.49; N, 17.26. Found: C, 44.38;
Tp*2U(CtCPh) (2-CCPh). The product was washed with pentane to
remove the byproduct toluene, leaving a blue-green crystalline solid
assigned as Tp*2U(CtCPh). Yield: 0.093 g (0.100 mmol, 96%).
1
H, 5.54; N, 17.16. H NMR (C6D6, 25 °C, ppm): À9.14 (132, 18H,
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1
Table 1. H NMR Data (ppm) for 2-CCPh, 3-CCSiMe3, 4-NHPh, 5-NHCH2Ph, and 6-SPh Measured in Benzene-d6 at 25 °C
2-CCPh
3-CCSiMe3
4-NHPh
5-NHCH2Ph
6-SPh
Tp*-CH3
Tp*-CH
B-H
À13.49, 0.62
À13.53, 0.69
À10.31, À2.33
À7.21, À3.73
7.64
À11.55, À1.53
7.49
a
7.57
a
7.20
0.80
14.43
18.84
34.89
75.02
À
7.49
5.61
11.21
13.47
20.89
À
4.72
p-phenyl
m-phenyl
o-phenyl
N-H
9.95
13.25
20.64
À
À
À
À
12.60
14.48
28.56
103.48
À
À
À
À
Si(CH3)3
CH2
a Covered by peak for Tp*-CH.
7.04
À
À
À
À
97.15
Tp*-CH3), À0.34 (13, 18H, Tp*-CH3), 0.63 (304, 2H, BH), 2.34 (10,
9H, Si-(CH3)3), 7.38 (23, 6H, Tp*-CH). IR: 2185 cmÀ1 (CtC); 2524,
2558 cmÀ1 (BÀH).
(7-O2CCCPh, 0.102 mmol; 8-O2CCCSiMe3, 0.103 mmol; 9-
O2CNHPh, 0.103 mmol; 10-O2CNHCH2Ph, 0.101 mmol) and ap-
proximately 5 mL of THF. One equivalent of TMSX (X = Cl, I) was
added to the solution via microsyringe. Within 5 min the solution had
changed to dark blue-purple, indicating the formation of the mono-
halide species (7-O2CCCPh, TMSCl 0.054 g, 0.063 mmol, 62%; TMSI
0.064 g, 0.067 mmol, 66%; 8-O2CCCSiMe3, TMSCl 0.051 g, 0.059
mmol, 57%, TMSI 0.070 g, 0.073 mmol, 71%; 9-O2CNHPh, TMSCl
0.084 g, 0.096 mmol, 93%, TMSI 0.090 g, 0.094 mmol, 91%; 10-
O2CNHCH2Ph, TMSI 0.084 g, 0.084 mmol, 83%, TMSCl 0.084 g,
0.087 mmol, 86%).
Tp*2U(k2-O2CNHPh) (9-O2CNHPh). A green powder assigned as
Tp*2U(k2-O2CNHPh) was isolated in quantitative yield after washing
with pentane (0.104 g, 0.103 mmol, 99%). Anal. Calcd for
C37H50N13O2B2U: C, 45.54; H, 5.20; N, 18.80. Found: C, 45.54; H,
4.90; N, 18.59. 1H NMR (C6D6, 25 °C, ppm): À9.64 (499, 18H, Tp*-
CH3), À0.01 (22, 18H, Tp*-CH3), 3.38 (271 2H, B-H), 7.50 (22, 6H,
Tp*-CH), 9.17 (22, 1H, p-phenyl-CH), 9.87 (23, 2H, phenyl-CH),
16.92 (24, 2H, phenyl-CH), 28.51 (25, 1H, NH). IR: 2522, 2552 cmÀ1
(BÀH), 3279 cmÀ1 (NÀH).
Tp*2U(k2-O2CNHCH2Ph) (10-O2CNHCH2Ph). A green powder iso-
lated in quantitative yield after washing with pentane (0.105 g, 0.103
mmol, 99%) was assigned as Tp*2U(k2-O2CNHCH2Ph). Anal. Calcd
for C38H52N13O2B2U: C, 46.45; H, 5.33; N, 18.53. Found: C, 46.37; H,
5.22; N, 18.11. 1H NMR (C6D6, 25 °C, ppm): À9.46 (389, 18H, Tp*-
CH3), À0.49 (8, 18H, Tp*-CH3), 2.60 (280, 2H, B-H), 7.14 (9, 6H, Tp*-
CH), 8.41 (t, J = 7.5, 2H, phenyl-CH), 8.97 (t, J = 8.1, phenyl-CH), 13.11
(d, J = 6.9, phenyl-CH), 15.12 (14, 2H, CH2), 26.83 (29, 1H, N-H). IR:
2527, 2550 cmÀ1 (BÀH), 3277 cmÀ1 (NÀH).
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Uranium(III) Organo-
metallic Complexes. To study the formation of uranium(III)
acetylide, amide, and thiolate complexes, a general procedure was
developed for their synthesis. Treating Tp*2U(CH2Ph) (1-
CH2Ph) with an organic reagent containing an acidic hydrogen
(pKa e 25) results in protonation of the benzyl group and
formation of the desired uranium(III) complexes in quantitative
yields (eq 1). In each case, toluene was formed as confirmed by
1H NMR spectroscopy, signifying procession of the reaction.
Tp*2U(k2-O2CSPh) (11-O2CSPh). A J. Young NMR tube was charged
with 0.010 g (0.011 mmol) of Tp*2U(SPh) (6-SPh) and a minimal
amount of benzene-d6, producing a dark blue solution. On the Schlenk
line, the solution was frozen with liquid nitrogen and the head space
evacuated, followed by warming to room temperature. One atmosphere
of CO2 was added to the tube, resulting in an immediate color change
from dark blue to bright green. Due to the reversibility of the reaction,
Tp*2U(k2-O2CSPh) could not be isolated in the solid state. 1H NMR
(C6D6, 25 °C, ppm): À9.51 (42, 18H, Tp*-CH3), À0.22 (6, 18H, Tp*-
CH3), 2.88 (143, 2H, B-H), 7.37 (6, 6H, Tp*-CH), 8.50 (17, 1H, p-
phenyl-CH), 8.94 (18, 2H, phenyl-CH), 13.72 (17, 2H, phenyl-CH).
Tp*2UOC(CH3)CH2 (12-OC(CH3)CH2). A solution of 0.100 g (0.108
mmol) of Tp*2UCH2Ph (1-CH2Ph) was dissolved in approximately
3 mL of THF and cooled to À35 °C. After this time, 1 equiv of acetone
(0.008 mL, 0.108 mmol) was added to the dark green solution via
microsyringe. The solution immediately changed from green to brown.
Volatiles were removed in vacuo, leaving a brown oil. Extraction of the
product with pentane and crystallization at À35 °C over 2 days afforded
a brown solid (0.072 g, 0.081 mmol, 76%). Small, brown, X-ray-quality
crystals were grown from a concentrated solution of pentane and
toluene (8/2 ratio). Anal. Calcd: C, 44.56; H, 5.55; N, 18.89. Found:
C, 44.73; H, 5.49; N, 18.88. 1H NMR (C6D6, 25 °C, ppm): À12.66 (28,
18H, Tp*-CH3), À1.11 (4, 18H, Tp*-CH3), 4.16 (183, 2H, B-H), 7.09
(5, 6H, Tp*-CH), 34.79 (22, 1H, OCÀCH2) 35.64 (11, 1H, OCÀCH2),
36.27 (31, 3H, OCÀCH3). IR: 2528, 2555 cmÀ1 (BÀH).
1
These uranium(III) derivatives show similar trends by H
NMR spectroscopy (Table 1). For all derivatives, there are two
resonances for the sets of methyl peaks that point toward and
away from the uranium center. Those that point toward the
uranium center are shifted drastically, ranging from À14 to À7
ppm. Those directed away from the uranium center are closer to
their expected diamagnetic reference values, located from À4 to
1 ppm. The resonance for the equivalent CÀH peak on the
pyrazole rings appear between 7 and 8 ppm for all compounds.
General Procedure for Reductive Silylation. A 20 mL scin-
tillation vial was charged with 0.100 g of the CO2 -inserted product
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Table 2. Absorption Bands Measured by Infrared Spectros-
copy (cmÀ1) for Complexes 2-CCPh, 3-CCSiMe3, 4-NHPh,
5-NHCH2Ph, and 6-SPh (KBr)
Previous examples of homoleptic uranium(III) amides include
U(N(SiMe3)2)3 reported by Andersen,24 and more recently,
convenient synthetic methodologies for U(N(SiMe2Ph)2)3,25
U(THF)(N[R]Ar)3 (R = tBu, Ar = 3,5-Me2C6H3),26 and
BÀH
CtC
NÀH
[K(THF)2]2[U(NH-2,6-i-Pr2C6H3)5]) THF27 have been de-
3
veloped. Heteroleptic examples of uranium(III) amides are
common,28À30 with examples similar to the system reported
here being Tp*2UNR2 (R = Ph, SiMe3),31 Tp*U(N(SiMe3)2)2,32
and Cp*2UN(SiMe3)2.33
2-CCPh
2526, 2554
2526, 2555
2525, 2554
2526, 2551
2521, 2548
2049
1997
À
À
À
3369
3305
À
3-CCSiMe3
4-NHPh
5-NHCH2Ph
6-SPh
À
The synthesis of a uranium(III) thiolate species was per-
formed by treating 1-CH2Ph with benzenethiol, causing an
immediate color change from green to dark blue. From this
reaction, a dark blue-gray solid assigned as Tp*2U(SPh) (6-SPh)
was isolated in quantitative yield after washing. Characterization
by 1H NMR spectroscopy showed phenyl protons down-
field with chemical shifts of 11.2, 13.5, and 20.9 ppm (Table 1)
along with those previously discussed for the ligand. Trivalent
uranium thiolates are rare; Ephritikhine has isolated the triva-
lent uranium thiolate anions [Cp*2U(SiPr)2]À34 and [Cp*2U-
(dddt)]À (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate),35 as well
as the neutral homoleptic U(SMes*)3 (Mes* = 2,4,6-tri-tert-butyl-
phenyl).36 Unlike U(SMes*)3, which decomposes above À25 °C
in the solid state, 6-SPh is stable at room temperature.
À
A broad singlet is visible for the BÀH moieties at variable
positions, depending on the substitution at the uranium. Infrared
spectroscopy shows two inequivalent absorptions for the BÀH in
the solid state (Table 2); for U(III) acetylide, amide, and thiolate
compounds, one absorption is observed in the range of 2520À
2526 cmÀ1, while the second appears from 2548 to 2555 cmÀ1
.
Treating 1-CH2Ph with phenylacetylene or ethynyltrimethyl-
silane produces the trivalent uranium acetylides Tp*2U-
(CtCPh) (2-CCPh) and Tp*2U(CtCSiMe3) (3-CCSiMe3).
2-CCPh was previously synthesized by Takats via amide pro-
1
tonation, and characterization of 2-CCPh by H NMR and
infrared spectroscopy (KBr) at 25 °C matches previously re-
ported data (Tables 1 and 2).19 Characterization of 3-CCSiMe3
by 1H NMR spectroscopy (benzene-d6) revealed spectroscopic
features similar to those of 2-CCPh. The resonance for the
trimethylsilyl group appears at 7.04 ppm and is shifted signifi-
cantly, due to proximity to the paramagnetic uranium center.
Infrared absorption bands for the carbonÀcarbon triple bonds in
As demonstrated here, protonation of Tp*2UCH2Ph with
commercially available organic substrates is a convenient entry
into a variety of uranium(III) species with a wide range of steric
and electronic properties. Further, this method is synthetically
easy to perform, yields products in near-quantitative conversion
in extremely short reaction times, and leaves toluene as the only
byproduct, which can be easily removed under vacuum. This
procedure eliminates the need for producing alkali-metal salts of
the organic reagents, which can be difficult for reagents with
functional groups and requires longer reaction times for full
conversion to the organouranium complex.
2-CCPh and 3-CCSiMe3 appear at 2049 and 1997 cmÀ1
,
respectively, indicating that this bond has been slightly reduced
upon coordination to the uranium center as compared to free
phenylacetylene (2110 cmÀ1) and (trimethylsilyl)acetylene
(2034 cmÀ1). To our knowledge, uranium(III) acetylide species
are rare,20 but the infrared spectroscopic data for 2-CCPh
and 3-CCSiMe3 are consistent with those reported for the
CtC bond in similar tetravalent uranium species, including
Cp*2U(CCPh)2 (2056 cmÀ1),21 Cp*2UCCPh(NPh2) (2062cmÀ1),
Cp*2UCC-p-MeC6H4(NPh2) (2059 cmÀ1),22 [N3N]UCCPh
Due to the limited amount of structural data on uranium(III)
acetylide and thiolate compounds, 2-CCPh and 6-SPh were
further characterized by single-crystal X-ray crystallography
(Figure 1 and Table 3). Green block crystals of 2-CCPh suitable
for analysis were grown from a concentrated THF/toluene (2/1)
solution at À35 °C, and analysis confirmed the identity of the
product as Tp*2U(CCPh), which features a seven-coordinate
uranium center with a linear acetylide ligand. Four of the UÀN
distances for Tp* range from 2.544(6) to 2.621(6) Å, while one
UÀN bond in each ligand is significantly longer at 2.656(5) and
2.680(5) Å. This is typically observed for uranium complexes
with the bis(hydrotris(3,5-dimethylpyazolyl)borate) ligand
framework.37,38 The uraniumÀcarbon bond distance is
2.589(9) Å, slightly longer than what has been typically observed
for uranium(IV) acetylides, which range from 2.339 to 2.468 Å,
and is consistent with coordination to a more electron-rich
uranium center. In addition, the CtC triple bond in 2-CCPh
is 1.100(11) Å, shorter than carbonÀcarbon triple bonds ob-
served for uranium(IV) species (1.150À1.289 Å).22,39À42 This
shortening of the carbonÀcarbon triple bond in the crystal
structure of 2-CCPh has also been observed for Cp*2SmCCPh-
(THF).43 Finally, the UÀCÀC angle in Tp*2U(CCPh)
(177.7(8)°) is the closest to linearity reported for all of the
uranium acetylide compounds to date. Compound 2-CCPh is
isostructural with the previously synthesized and crystallogra-
phically characterized samarium compound Tp*2Sm(CCPh),
which features a CtC distance of 1.206(12) Å and an SmÀCÀC
angle of 176.6(8)°.
(2052 cmÀ1),23 and [N3N]UCC-o-Me-C6H4 (2050 cmÀ1
)
t
([N3N] = [(RNCH2CH2)3N)]32À; R = SiMe2 Bu).23
In a similar fashion, bis(Tp*) uranium(III) amide complexes are
synthesized by addition of 1 equiv of aniline or benzylamine to
1-CH2Ph. Washing with pentane affords clean Tp*2U(NHPh)
(4-NHPh) as a blue-green powder, while isolation of pure Tp*2U-
(NHCH2Ph) (5-NHCH2Ph) was accomplished by recrystalliza-
tion, producing a dark brown solid. Characterization by 1H NMR
spectroscopy shows all of the expected resonances for 4-NHPh
(Table 1). In addition to those for the ligand, the para, meta, and
ortho protons for the phenyl rings in both 4-NHPh and 5-
NHCH2Ph show a characteristic pattern, with ortho being the most
downfield. The resonances for the NH functionality are shifted
far downfield to 75.02 and 103.48 ppm, respectively, and are
significantly broadened due to proximity to the metal center.
Although they are close to the uranium center, protons of the CH2
are also visible. These uranium(III) amide complexes are unique,
given the proximity of the NÀH bond to the electron-rich uranium
center. Confirmation of the presence of the NÀH was obtained by
IR spectroscopy for both molecules, which have broad absorp-
tions at 3369 cmÀ1 (4-NHPh) and 3305 cmÀ1 (5-NHCH2Ph),
and are reduced from the values in the free amines (Table 2).
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Figure 1. Molecular structures of 2-CCPh (right) and 6-SPh (left) shown with 30% ellipsoids. Selected hydrogen atoms and cocrystallized solvents are
omitted for clarity.
distance reported for 6-SPh is longer than those reported for
uranium(IV) thiolate complexes with η1 coordination (2.68À
2.75 Å),44À47 as would be expected on the basis of the larger
atomic radius of uranium(III) as compared to uranium(IV).
Carbon Dioxide Insertion Chemistry. The insertion chem-
istry of the trivalent UÀC, UÀN, and UÀS bonds was explored
with carbon dioxide. Exposing 2-CCPh, 3-CCSiMe3, 4-NHPh,
5-NHCH2Ph, and 6-SPh to 1 atm of CO2 produces immediate
color changes to green for all reactions, forming the correspond-
ing uranium(III) carboxylate derivatives Tp*2U(k2-O2CCCPh)
(7-O2CCCPh) and Tp*2U(k2-O2CCCSiMe3) (8-O2CCCSiMe3),
the carbamate derivatives Tp*2U(k2-O2CNHPh) (9-O2CNHPh)
and Tp*2U(k2-O2CNHCH2Ph) (10-O2CNHCH2Ph) (eq 2),
and the carboxylate Tp*2U(k2-O2CSPh) (11-O2CSPh) (eq 3).
Table 3. Structural Parameters (Distances in Å and Angles in
deg) for 2-CCPh and 6-SPh
2-CCPh
6-SPh
U1ÀN11
U1ÀN21
U1ÀN31
U1ÀN61
U1ÀN71
U1ÀN81
U1ÀC41
C41ÀC42
U1ÀS40
2.656(5)
2.558(6)
2.621(6)
2.680(5)
2.544(6)
2.601(6)
2.589(9)
1.100(11)
À
2.670(5)
2.566(5)
2.496(4)
2.674(5)
2.571(5)
2.586(5)
À
À
2.857(15)
U1ÀC41ÀC42
U1ÀS40ÀC41
177.7(8)
À
À
113.8(2)
The identity of 6-SPh was confirmed by X-ray diffraction, with
suitable crystals grown from a concentrated THF/toluene (3/1)
solution. Analysis showed the expected seven-coordinate
uranium(III) η1-thiolate species Tp*2USPh (Figure 1, Table 3).
The UÀN distances for the hydro(trispyrazolyl)borate ligands
are consistent with those previously observed for 2-CCPh and
other bis(ligand) derivatives.19,37,38 The UÀS distance in 6-SPh of
2.857(15) Å is similar to the UÀS distances in the uranium(III)
anions [Cp*2U(SiPr)2]À (2.78(1) Å)34 and [Cp*2U(dddt)]À
(2.773(7) Å).35 In comparison, U(SMes*)3 has three thiolate
ligands in an η3 bonding mode, which gives rise to a shorter
average UÀS distance of 2.720(5) Å and UÀSÀC angle of
81.6°.36 The same angle for the η1-thiolate ligand in 6-SPh is
113.8(2)°; thus, the differences in the structural parameters can
be attributed to the different hapticities of the thiolate ligands.
The UÀSÀC angles for 6-SPh are more appropriately com-
pared to those of tetravalent η1 derivatives, such as U(SPh)4-
(pyridine)3.44 This molecule features a sterically crowded seven-
coordinate uranium center that does not have open sites for
η3 coordination of the thiolate group. On average, the UÀS
Analysis of the products from the reactions with carbon
dioxide by 1H NMR spectroscopy (benzene-d6, Table 4)
showed similar features for the entire family of compounds. The
increase in the distance between the paramagnetic uranium(III)
center and the seventh ligand that results from insertion of
carbon dioxide into the uraniumÀelement bond causes a shifting
of the resonances for the carboxylate substituent toward their
diamagnetic reference values. Resonances for the methyl groups
of the hydrotris(pyrazolyl)borate ligands appear between À9
and À10 ppm and between 0 and À0.5 ppm, while those
assignable to the pyrazole CÀH are found in the range of 7À8
ppm. A broad singlet for the B-H is present between 2 and 4 ppm.
For those species with phenyl groups, these resonances appear
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Table 4. 1H NMR Data for CO2-Inserted Compounds 7-O2CCCPh, 8-O2CCCSiMe3, 9-O2CNHPh, 10-O2CNHCH2Ph, and 11-
O2CSPh Measured in Benzene-d6 at 25 °C
7-O2CCCPh
8-O2CCCSiMe3
9-O2CNHPh
10-O2CNHCH2Ph
11-O2CSPh
Tp*-CH3
Tp*-CH
B-H
À9.23, À0.31
À9.14, À0.34
À9.64, À0.01
7.50
À9.46, À0.49
7.14
À9.51, À0.22
7.39
2.75
8.15
8.84
11.46
À
7.38
a
7.37
2.88
8.50
8.94
13.72
À
3.38
2.60
p-phenyl
m-phenyl
o-phenyl
N-H
À
9.17
8.41
À
9.87
8.97
À
16.92
28.51
À
13.11
26.83
À
À
Si(CH3)3
À
À
2.34
À
À
À
CH2
À
15.12
a Obscured by the Si(CH3)3 peak.
Table 5. Absorption Bands (cmÀ1) As Measured by Infrared
Spectroscopy for Complexes 7-O2CCCPh, 8-O2CCCSiMe3,
9-O2CNHPh, and 10-O2CNHCH2Ph (KBr)
Table 6. Structural Parameters (Distances in Å and Angles in
deg) for 7-O2CCCPh
U1ÀN11
U1ÀN21
U1ÀN31
U1ÀN61
U1ÀN71
U1ÀN81
2.656(10)
2.569(10)
2.629(11)
2.568(10)
2.646(11)
2.639(10)
U1ÀO41
U1ÀO42
O41ÀC40
O42ÀC40
C42ÀC43
2.539(9)
2.512(8)
1.248(16)
1.292(16)
1.197(19)
complex
B-H
CtC
N-H
7-O2CCCPh
2522, 2551
2524, 2558
2522, 2552
2527, 2550
2211
2184
À
À
À
3279
3277
8-O2CCCSiMe3
9-O2CNHPh
10-O2CNHCH2Ph
À
O42ÀU1ÀO41
O41ÀC41ÀO42
161.2(10)
121.8(11)
C41ÀC42ÀC43
178.0(15)
respectively (Table 5). This trend has been observed by Evans,
who has recently reported Cp*2U(k2-O2CCCPh)2, the product
from insertion of CO2 into both UÀC bonds in Cp*2U(CCPh)2
at 80 psi.5 Insertion of carbon dioxide changes the infrared
absorption band for the CtC from 2056 cmÀ1 to 2214 cmÀ1
,
similar to the shifts observed for 7-O2CCCPh and 8-O2CC-
CSiMe3. This shifting in the IR absorption band for the carbonÀ
carbon triple bond in 7-O2CCCPh is complicated by its con-
jugation with the π system of both the CO2 fragment and the
phenyl ring, as displayed by the small dihedral angle (15.32 (
3.07°) between these two planes. The insertion of CO2 into
the uraniumÀcarbon bonds of 2-CCPh and 3-CCSiMe3 is
reminiscent of that observed by our group for the formation of
Tp*2U(k2-O2CCH2Ph)15 and by Marks for Cp*2UMe2 to form
the uranium(IV) diacetate Cp*2U(O2CCH3)2.4 In both cases, CO2
insertion proceeds readily and quantitatively at low pressures, even
at less than 1 atm. The crystal structure of Cp*2U(O2CCH3)2
was not obtained. However, it was proposed that the acetate
ligand adopts either a bidentate or bridging coordination mode.
The solid-state structure of 7-O2CCCPh was explored by
X-ray diffraction of suitable crystals grown from a concentrated
THF solution with a trace of toluene at À35 °C (Figure 2 and
Table 6). The UÀN distances are all as expected and are
consistent with those observed for the previously synthesized
bis(hydrotris(pyrazolyl)borate) uranium(III) carboxylate com-
plex Tp*2U(k2-O2CCH2Ph).15 The UÀO distances of 2.512(8)
and 2.539(9) Å in 7-O2CCCPh are equivalent within error, as are
the CÀO distances of 1.248(16) and 1.292(16) Å. The
O41ÀC41ÀO42 bond angle is 121.8(11)°, supporting sp2
hybridization of the carboxylate carbon. Comparison of the
structural data of 7-O2CCCPh with those of the previously
characterized Tp*2U(k2-O2CCH2Ph) supports the assignment
of a uranium(III) center with a monoanionic carboxylate ligand.
Figure 2. Molecular structure of 7-O2CCCPh shown with 30% ellip-
soids. Selected hydrogen atoms and solvent are omitted for clarity.
downfield of the benzene reference peak and show distinct splitting;
the ortho CH group is shifted the most downfield, with the meta and
para CH groups located within approximately 0.5 ppm of each
other. Analysis by infrared spectroscopy reveals that all carboxylate
compounds still show two inequivalent B-H’s in the same ranges as
those for the starting materials, and no absorptions for carbonyls.
Treating 2-CCPh and 3-CCSiMe3 with 1 atm of carbon dioxide
produces the forest green complexes Tp*2U(k2-O2CCCPh) (7-
O2CCCPh) and Tp*2U(k2-O2CCCSiMe3) (8-O2CCCSiMe3)
in quantitative yields. Most significantly, the infrared spectra have
sharp absorption bands at 2211 and 2184 cmÀ1 assignable to the
carbonÀcarbon triple bonds in 7-O2CCCPh and 8-O2CCCSiMe3,
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Figure 3. Stacked 1H NMR spectra of 6-SPh (top) and 11-O2CSPh (bottom) in benzene-d6 at 25 °C.
CO2 insertion into the UÀN bond proceeds in a fashion similar to
that for homoleptic uranium(IV) amides.10,49
Likewise, the insertion chemistry of carbon dioxide into the
uranium(III)Àsulfur bond was probed. Upon addition of 1 atm
of CO2 to a solution of 6-SPh in THF, the dark blue solution
turns deep green (eq 3). Stirring followed by removal of the
volatiles leaves a dark blue residue. Analysis by 1H NMR spectros-
copy shows that only 6-SPh remains, indicating that insertion
of carbon dioxide into the uranium(III)Àsulfur bond is rever-
sible. Conducting the reaction of 6-SPh in a J. Young NMR tube
allowed for the CO2-inserted product Tp*2U(k2-O2CSPh) (11-
O2CSPh) to be observed spectroscopically (Figure 3), showing a
1H NMR spectrum similar to those of the other carboxylate
products. The green solution characteristic of the carboxylate
derivative persists under 1 atm of CO2. The para, meta, and ortho
phenyl peaks of the sulfide ligand are shifted to 8.50, 8.94, and
13.72 ppm, due to the increased distance from the paramagnetic
uranium center. Analogous reactivity has been observed for the
tetravalent Cp3USiPr.11,50 Insertion of carbon dioxide into the
UÀS bonds of Cp*2U(StBu)2 is irreversible, allowing isolation of
Cp*2U(k2-O2CSC(CH3)3)2 as a microcrystalline solid.11
The insertion chemistry observed with the family of uranium-
(III) complexes presented is in sharp contrast to previous studies
with uranium(III). Addition of 1 atm of carbon dioxide to the
sterically hindered uranium(III) complex [((AdArO)3tacn)U] pro-
duces a charge-separated uranium(IV) species, [((AdArO)3tacn)-
U(CO2)], where a radical is located on the carbon dioxide ligand.51
Reducing the size of the ortho alkyl substituent to tert-butyl does
not create an isolable charge-separated species; instead, two
uranium centers each reduce carbon dioxide by one electron,
creating the μ-oxo dimer [((tBuArO)3tacn)U]2(μ-O). Thus, the
trivalent derivatives presented here are unique in that the trivalent
The similarity in metrical parameters between the two carbox-
ylate compounds indicates that substitution at the carboxylate
carbon has little influence on the overall structure at uranium.
Further evidence for the monoanionic character of the carbox-
ylate ligand is obtained by comparison to uranium(VI) carbox-
ylate species, which has similar bond distances and angles.48
Although studied for uranium(IV) amide complexes,6,7 exam-
ples of the insertion of carbon dioxide into U(III)Àamide bonds
is sparse. Stirring either 4-NHPh or 5-NHCH2Ph in the
presence of small amounts of CO2 causes an instant conversion
to the respective carbamate products Tp*2U(k2-O2CNHPh)
(9-O2CNHPh) and Tp*2U(k2-O2CNHCH2Ph) (10-O2CNH-
CH2Ph) (eq 2). Confirmation by infrared spectroscopy shows
shifts in the NÀH peaks to 3279 cmÀ1 for 9-O2CNHPh and
3277 cmÀ1 for 10-O2CNHCH2Ph (from 3369 cmÀ1 for 4-NHPh
and 3305 cmÀ1 for 5-NHCH2Ph) along with an increase in
intensity upon insertion of CO2. The similarity of the spectro-
scopic features of these compounds to those of the acetylide
derivativessupports a trivalent uranium centerwitha monoanionic
carbamate ligand. Formation of the uranium(III) carbamate by
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uranium center is maintained throughout the reaction and is not
oxidized by the CO2.
As previously demonstrated,15 carboxylate ligands are effec-
tively released from trivalent uranium centers using trimethylsilyl
halides. Addition of trimethylsilyl iodide and trimethylsilyl
chloride to 9-O2CNHPh and 10-O2CNHCH2Ph effectively
cleaves the uraniumÀoxygen bond, generating the uranium
monoiodide Tp*2UI52 Tp*2UCl,19 respectively. The same reac-
tivity was observed with 7-O2CCCPh and 8-O2CCCSiMe3;
however, the reaction products were impure.
Reactivity with Acetone. To determine if insertion of a more
sterically demanding carbonyl substrate is possible, the reactivity
of the uranium(III) derivatives with acetone was explored.
Acetone is an ideal substrate because it has been previously
demonstrated to perform insertion with uranium(IV) organo-
metallics.53,54 Treating Tp*2UCH2Ph, Tp*2UCtCPh, and
Tp*2UNHPh with 1 equiv of dry acetone caused an immediate
color change to brown (eq 4). Interestingly, analysis by 1H NMR
spectroscopy shows the same product regardless of the starting
material. The typical pattern observed for the phenyl groups that
would be expected if insertion happened was noticeably absent.
Further, the presence of toluene, phenylacetylene, and aniline was
observed, indicating protonation had occurred. Addition of acet-
one-d6 to the same compounds produces deuterated organics, as
evidenced by 2H NMR spectroscopy.
Figure 4. Molecular structures of 12-OC(CH3)CH2 shown with 30%
ellipsoids. Selected hydrogen atoms are omitted for clarity.
Table 7. Structural Parameters (Distances in Å and Angles in
deg) for 12-OC(CH3)CH2
U1ÀN111
U1ÀN121
U1ÀN131
U1ÀN161
U1ÀN171
2.687(9)
2.677(9)
2.548(9)
2.685(9)
2.551(9)
U1ÀN181
2.679(9)
2.198(9)
1.388(19)
1.46(2)
U1ÀO140
O140ÀC141
C141ÀC142
C141ÀC143
1.32(2)
U1ÀO140ÀC141
O140ÀC141ÀC142
161.2(10)
119.0(19)
O140ÀC141ÀC143
C141ÀC142ÀC143
118.5(17)
122(2)
1
Analysis of 12-OC(CH3)CH2 by H NMR spectroscopy in
benzene-d6 shows a spectrum typical for uranium(III) derivatives
of this ligand set. Two resonances for the methyl peaks on the
ligands pointing toward and away from the uranium center are
observed at À12.66 and À1.11 ppm, respectively. As expected,
the CH on the pyrazole ring appears at 7.09 ppm, while that for
the BH appears as a broad singlet at 4.16 ppm. The resonances
for the enolate ligand are upfield of the benzene solvent peak,
with the methyl group visible at 36.27 ppm and the methylene
protons at 34.79 and 35.64 ppm.
The formation of the enolate product is significant, given that
metal complexes of this type have been invoked as intermediates
in metal-mediated aldol condensations. Upon exposure of 12-
OC(CH3)CH2 to additional amounts of acetone, no further
progression to the aldolate occurred. This is in contrast to the
case of the related uranium(III) dichloride Tp*UCl2, which readily
forms the aldolate.15,54 Insertion of acetone into the UÀC bond
of Tp*U(Cl2)CH2SiMe353 has also been observed, forming the
aldolate derivative with excess acetone. In contrast to both Tp*
uranium examples, addition of acetone to the unsubstituted
hydrotris(pyrazolyl)borate uranium(IV) dimethyl compound
Tp2UMe2 results in insertion of acetone into both methyl groups
to form the uranium bis(tert-butoxide) Tp2U(OtBu)2.54 In
comparison, Tp*2UCH2Ph is much more sterically hindered
than Tp*UCl2, Tp2UMe2, and Tp*U(Cl2)CH2SiMe3, indicating
X-ray-quality crystals were obtained from cooling a solution of
pentane and toluene (4/1) to À35 °C. Refinement confirmed
that insertion had not occurred as anticipated, but rather a new
uranium(III) alkoxide species, Tp*2UOC(CH3)CH2 (12-OC-
(CH3)CH2), resulted (Figure 4). The unit cell contains two
independent molecules, labeled with U1 and U2 centers (only
the molecule containing U1 is presented). The UÀN distances
for the Tp* ligand for both molecules in the unit cell are within
the range of those expected for others with this bis(ligand)
framework.37,38 The U1ÀO140 bond is 2.198(9) Å, similar to
those observed for other uranium(III) alkoxides (Table 7).29
Analysis of the O140ÀC141 bond distance of 1.388(19) Å indicates
that the carbonyl group has been reduced to a single bond.
Furthermore, the distance between C141 and the two adjacent
carbons are significantly different, with distances of 1.46(2) and
1.32(2) Å. The shortened CÀC bond distances establish that the
original methyl group has been deprotonated to form a methy-
lene and is confirmed by the refinement of two hydrogen atoms.
Additionally, the central carbon of the alkoxide group forms a
planar conformation between the oxygen and two carbons,
showing sp2-hybridized character has been maintained as a result
of the newly formed carbonÀcarbon double bond.
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Scheme 1
Scheme 2
that acetone insertion may not be possible due to its hindered
approach to the uranium center.
’ CONCLUSIONS
The studies presented herein demonstrate the synthesis of
uranium(III) acetylide, amide, and thiolate species via the
protonation of a uranium(III) alkyl complex. These complexes
readily insert carbon dioxide into their uraniumÀelement bonds
to produce the corresponding carboxylate derivatives. Interest-
ingly, these species maintain their trivalent oxidation state, in
contrast to previously reported work.51,57,58 When the more
sterically demanding acetone was utilized, protonation of the
uraniumÀelement bond followed by rearrangement resulted in
the formation of a new uranium enolate compound. In the
presence of excess acetone, no formation of the aldolate product
was observed, which is in sharp contrast to previous reports for
f-block elements,55,56 where aldolate products are commonly
observed.
The formation of aldolate ligands at f-block centers has been
studied, with differing mechanisms being proposed for the lanthanides
and actinides (Scheme 1). For the former, Teuben has proposed
that no carbonyl insertion occurs for Cp*2Ln(CH(SiMe3)2)
(Ln = La, Ce).55 Instead, protonation of the lanthanide alkyl
followed by rearrangement forms an inisolable enolate inter-
mediate, which then reacts by CÀC coupling in an aldol fashion
to generate the product. For the actinides, Marks also invokes
protonation of Cp*2Th(Cl)Ru(Cp)(CO)2 to generate CpRu-
(CO)2H and a new thorium alkyl, but this undergoes insertion of
acetone into the ThÀC bond to produce the aldolate product.56
Thus, insertion is not observed for the lanthanides but is favored
for the actinides. Although the oxidation states of these species
differ by one electron, these examples both use the bis(Cp*) ligand
framework, indicating that their steric environments are similar.
Similar to these pathways, 12-OC(CH3)CH2 has resulted
from the initial protonation of the alkyl, acetylide, or amide ligand
by the acidic hydrogen on the acetone (Scheme 1), in accordance
with both mechanisms discussed above (Scheme 2). Unlike the
mechanism proposed by Marks,56 once the new alkyl is formed,
no further insertion of acetone takes place, presumably due to steric
crowding from the bulky bis(hydrotris(3,5-dimethylyrazolyl)-
borate) ligand framework. Instead, like the lanthanide chemistry,
rearrangement to the enolate occurs; however, no further CÀC
coupling is noted, as Tp*2UO(CH2)CH3 is the sole product.
Thus, the protonation product is presumably the kinetically
preferred one, while the enolate is the thermodynamically most
stable. This is most likely because formation of the strong
uraniumÀoxygen bond acts as a significant driving force. When
the uranium thiolate 6-SPh was stirred with 1 equiv of acetone,
no reaction was observed by 1H NMR spectroscopy. It is possible
that a reversible process is occurring for the thiolate complex in
analogy to the chemistry of carbon dioxide; however, there is no
spectroscopic evidence to support this.
’ ASSOCIATED CONTENT
Supporting Information. Figures giving 1H NMR spec-
S
b
tra, CIF files giving crystallographic data for the crystal structures
determined in this paper, and text giving experimental proce-
dures for X-ray crystallography. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sbart@purdue.edu.
’ ACKNOWLEDGMENT
We wish to acknowledge Purdue University for funding.
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