Multielectron C-O bond activation mediated by a family of reduced uranium complexes

Article abstract of DOI:10.1021/ic500012x

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, ^MesPDI^Me ( ^MesPDI^Me = 2,6-((Mes)N=CMe)₂-C ₅H₃N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp^P (Cp* = 1,2,3,4,5- pentamethylcyclopentadienide, Cp^P = 1-(7,7-dimethylbenzyl) cyclopentadienide), uranium complexes of the type Cp^XUI ₂(^MesPDI^Me) (1-Cp^X; X = * or P), Cp^XUI(^MesPDI^Me) (2-Cp^X), and Cp^XU(^MesPDI^Me)(THF)_n (3-Cp ^X; *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of ^MesPDI^Me reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp^X, which has a doubly reduced ^MesPDI^Me, with furfural results in radical coupling between the substrate and ^MesPDI^Me, leading to C-C bond formation to form Cp^XUI(^MesPDI ^Me-CHOC₄H₃O) (4-Cp^X). Exposure of 3-Cp* and 3-Cp^P, which contain a triply reduced ^MesPDI^Me ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O₂C₂Ph₂H₂)( ^MesPDI^Me) (5-Cp*) and Cp^PU(O ₂C₂Ph₄)(^MesPDI^Me) (5-Cp^P). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1-5 have been characterized by ¹H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [^MesPDI^Me]^- ligand in 1-Cp ^P and 5-Cp^P. Structural parameters of complexes 1-Cp ^P, 2-Cp^X, 4-Cp*, and 5-Cp^X have been elucidated by X-ray crystallography.

Full text of DOI:10.1021/ic500012x

Article
pubs.acs.org/IC
Multielectron C−O Bond Activation Mediated by a Family of Reduced
Uranium Complexes
John J. Kiernicki,† Brian S. Newell,‡ Ellen M. Matson,† Nickolas H. Anderson,† Phillip E. Fanwick,†
Matthew P. Shores,‡ and Suzanne C. Bart*,†
†H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States
‡Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
S
* Supporting Information
ABSTRACT: A family of cyclopentadienyl uranium complexes supported by
the redox-active pyridine(diimine) ligand, MesPDIMe
(
MesPDIMe = 2,6-
((Mes)NCMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthe-
sized. Using either Cp* or CpP (Cp* = 1,2,3,4,5-pentamethylcyclopentadie-
nide, CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of
the type CpXUI2(MesPDIMe) (1-CpX; X = * or P), CpXUI(MesPDIMe) (2-CpX),
and CpXU(MesPDIMe)(THF)n (3-CpX; *, n = 1; P, n = 0) were isolated and
characterized. The series was generated via ligand centered reduction events;
thus the extent of MesPDIMe reduction varies in each case, but the uranium(IV)
oxidation state is maintained. Treating 2-CpX, which has a doubly reduced
MesPDIMe, with furfural results in radical coupling between the substrate and
MesPDIMe, leading to C−C bond formation to form CpXUI(MesPDIMe-
CHOC4H3O) (4-CpX). Exposure of 3-Cp* and 3-CpP, which contain a triply
reduced MesPDIMe ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes
Cp*U(O2C2Ph2H2)(MesPDIMe) (5-Cp*) and CpPU(O2C2Ph4)(MesPDIMe) (5-CpP). The reducing equivalents required for this
coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1−5 have been characterized
by 1H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic)
[
MesPDIMe]− ligand in 1-CpP and 5-CpP. Structural parameters of complexes 1-CpP, 2-CpX, 4-Cp*, and 5-CpX have been
elucidated by X-ray crystallography.
i
methylphenolate), −CH(SiMe3)2, −N(SiMe3)2, PrNC(Me)-
INTRODUCTION
■
NiPr).18 These complexes act as four electron reductants
toward small molecules, such as cyclooctatetraene or
azidoadamantane, forming the corresponding uranium(VI)
products.
Among the first organouranium complexes reported were those
bearing cyclopentadienyl ligands, including Cp3U,1,2 Cp3UCl,1
and Cp4U.3 Since that time, the field of organoactinide
chemistry has been dominated by variations of these frame-
works, with a particular emphasis on the bulky permethylated
derivative, 1,2,3,4,5-pentamethylcyclopentadienide (Cp*).4−12
The bis(Cp*) metallocene framework supports uranium
compounds in varying oxidation states13−17 and provides steric
protection to eliminate unwanted side reactions while focusing
desired reactivity at the metallocene wedge.
By comparison, significantly fewer studies focus on the piano
stool variation for uranium compounds. Using a single Cp ring
provides the desired steric protection at one face of uranium,
while facilitating easy steric and electronic tuning at vacant
coordination sites by introducing non-Cp-based ligands.
Because cyclopentadienyl ligands offer little in the way of
redox flexibility, incorporation of redox-active ligands at these
positions can promote multielectron chemistry in addition to
easier tunability. This was recently demonstrated by Evans with
the synthesis of the family of Cp* uranium dimers that feature a
bridging, two-electron reduced benzene ring, [Cp*(Y)U]2(μ-
η6:η6-C6H6) (Y = −OAr (OAr = 2,6-bis(tert-butyl)-4-
Recently, we adapted this strategy with the report of the
synthesis and characterization of Cp*U(MesPDIMe)(THF) (3-
Cp*), which has a trianionic [MesPDIMe]3− ligand.19 The three
reducing equivalents stored in the pyridine(diimine) ligand,
plus one from uranium, can be used for the four-electron N
N double bond cleavage of azobenzene to produce the
uranium(V) bis(imido) Cp*U(MesPDIMe)(NPh)2, which has a
neutral [MesPDIMe]0. On the basis of the success of 3-Cp* for
bond activation reactions, additional reduced species were
targeted to determine if a family with differing steric and
electronic properties of the Cp ring could be generated. Herein,
we report the sequential reduction of two series of uranium
complexes bearing cyclopentadienyl ancillary ligands and a
redox-active pyridine(diimine). Exposure of these species to
Received: January 7, 2014
Published: March 10, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Article
text; the identical data presented as the thermal dependence of χMT
(cgs units) can be found in Figure S3. Data were corrected for the
magnetization of the sample holder by subtracting the susceptibility of
an empty container and for diamagnetic contributions of the sample by
using Pascal’s constants.28
both aldehydes and ketones results in either ligand or pinacol
radical coupling from ligand-based oxidation events.
EXPERIMENTAL SECTION
■
Synthesis of Cp*UI2(MesPDIMe) (1-Cp*). A 20 mL scintillation
vial was charged with 0.788 g (0.934 mmol) of Cp*UI2(THF)3 and 5
mL of toluene. While stirring, 0.372 g (0.935 mmol) of MesPDIMe was
added, resulting in a gradual color change from blue to brown. After
stirring for 2 h, volatiles were removed in vacuo. The resulting solid
was washed with cold pentane (3 × 10 mL) to afford brown solid
(0.868 g, 0.847 mmol, 91%) assigned as Cp*UI2(MesPDIMe). Elemental
analysis of C37H46N3I2U, calculated: C, 43.37; H, 4.53; N, 4.10. Found:
C, 43.30; H, 4.52; N, 3.98. 1H NMR (C6D6, 25 °C): δ −68.18 (91, 1H,
4-pyr-ArH), −22.61 (16, 12H, o-CH3), −4.03 (3, 4H, m-ArH), −3.98
(3, 6H, CH3), 12.23 (23, 15H, Cp*), 21.50 (14, 2H, 3,5-pyr-ArH),
31.05 (115, 6H, CH3).
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 −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.20 Benzene-d6 was
purchased from Cambridge Isotope Laboratories, dried with molecular
sieves and sodium, and degassed in three freeze−pump−thaw cycles. A
1.0 M solution of sodium triethylborohydride in toluene was
purchased from Sigma-Aldrich and used as received. Benzophenone
was purchased from Sigma-Aldrich and recrystallized from dry diethyl
ether prior to use. Benzaldehyde and furfural were purchased from
Sigma-Aldrich and distilled from magnesium sulfate prior to use.
Cp*U(MesPDIMe)(THF) (3-Cp*),19 KC8,21 Cp*UI2(THF)3,22 potas-
sium 1-(7,7-dimethylbenzyl)cyclopentadienide (KCpP),23
Synthesis of Cp*UI(MesPDIMe) (2-Cp*). A 20 mL scintillation vial
was charged with 1.000 g (0.976 mmol) of Cp*UI2(MesPDIMe) and 10
mL of toluene. While stirring, 0.135 g (0.999 mmol) of KC8 was
added. After 30 min, the dark green solution was filtered over Celite,
and volatiles were removed in vacuo. The resulting solid was washed
with cold pentane (3 × 10 mL) to afford a dark green solid (0.693 g,
0.772 mmol, 79%) assigned as Cp*UI(MesPDIMe). Single, X-ray-quality
crystals were obtained by slow diffusion of hexane into a saturated
toluene solution at −35 °C. Elemental analysis of C37H46N3IU,
calculated: C, 49.50; H, 5.16; N, 4.68. Found: C, 49.46; H, 5.09; N,
UI3(THF)4,24 and MesPDIMe
(
MesPDIMe = 2,6-((Mes)NCMe)2-
C5H3N)25 were prepared according to literature procedures.
1H NMR spectra were recorded on a Varian Inova 300
spectrometer operating at 299.992 MHz. All chemical shifts are
1
reported relative to the peak for SiMe4, using H (residual) chemical
1
4.62. H NMR (C6D6, 25 °C): δ −2.21 (5, 6H, CH3), 0.91 (3, 6H,
shifts of the solvent as a secondary standard. The spectra for
paramagnetic molecules were obtained by using an acquisition time of
0.5 s; thus the peak widths reported have an error of 2 Hz. For
paramagnetic molecules, the 1H NMR data are reported with the
chemical shift, followed by the peak width at half height in hertz, the
integration value, and, where possible, the peak assignment. Elemental
analyses were performed by Midwest Microlab, LLC, Indianapolis,
CH3), 1.83 (d, J = 8, 2H, 3,5-pyr-ArH), 2.00 (3, 6H, CH3), 2.56 (4,
15H, Cp*), 6.16 (5, 2H, ArH), 7.32 (5, 2H, ArH), 7.43 (5, 6H, CH3),
7.96 (t, J = 8, 1H, 4-pyr-ArH).
Alternate Synthesis of Cp*U(MesPDIMe)(THF) (3-Cp*). A 20 mL
scintillation vial was charged with 0.100 g (0.111 mmol) of
Cp*UI(MesPDIMe) and 5 mL of THF. While stirring, 0.016 g (0.118
mmol) of KC8 was added, and the solution color changed from dark
green to brown. After 1 h, the volatiles were removed in vacuo. The
product was extracted with diethyl ether and filtered over Celite,
affording a dark brown solid (0.079 g, 0.094 mmol, 84%) identified as
Indiana (CpP U, CpPUI2(THF)3, 1-CpP, 5-CpP) and Complete
3
Analysis Laboratories, Inc., Parsippany, New Jersey (2-CpP, 3-CpP,
1-Cp*, 2-Cp*, 4-CpP, 4-Cp*, 5-Cp*). Electronic absorption
spectroscopic measurements were recorded at 294 K in sealed 1 cm
quartz cuvettes with a Jasco V-6700 spectrophotometer.
1
Cp*U(MesPDIMe)(THF) by H NMR spectroscopy.
P
Synthesis of Cp3 U. A 20 mL scintillation vial was charged with
Single crystals of 2-Cp* suitable for X-ray diffraction were coated
with poly(isobutylene) oil in a glovebox and quickly transferred to the
goniometer head of a Nonius KappaCCD image plate diffractometer
equipped with a graphite crystal, incident beam monochromator.
Preliminary examination and data collection were performed with Mo
Kα radiation (λ = 0.71073 Å). Single crystals of CpP3U,
CpPUI2(THF)3, 1-CpP, 2-CpP, 4-Cp*, 5-CpP, and 5-Cp* suitable
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 MicroMax002+
high intensity copper X-ray source with confocal optics. Preliminary
examination and data collection were performed with Cu Kα radiation
(λ = 1.54184 Å). Cell constants for data collection were obtained from
least-squares refinement. The space group was identified using the
program XPREP.26 The structures were solved using the structure
solution program PATTY in DIRDIF99.27 Refinement was performed
on a LINUX PC using SHELX-97.26
Magnetic susceptibility (dc) data were collected on compounds 1-
CpP and 5-CpP with a Quantum Design MPMS-XL SQUID
magnetometer in the temperature range 5−300 K at an applied field
of 1000 Oe. Powdered microcrystalline samples were loaded into
gelatin capsules in the glovebox, inserted into a straw, and transported
to the SQUID magnetometer under dinitrogen. The absence of
significant ferromagnetic impurities was confirmed for each sample by
observing a linear relationship between magnetization and applied field
(0.1−0.7 T) at 100 K (Figure S1). Susceptibility data reproducibility
was probed by analyzing several independently synthesized solid
samples: the air-sensitivity of the samples results in slight variation of
the room temperature μeff values, but the qualitative temperature
dependences of the data are reproducible (Figure S2). Data for
representative samples are presented in Figure 6 and discussed in the
0.340 g (0.375 mmol) of UI3(THF)4 and 10 mL of THF. While
stirring, 0.249 g (1.124 mmol) of KCpP was added and stirred for 3 h,
resulting in a green solution. Volatiles were removed in vacuo. The
product was extracted with diethyl ether and filtered through Celite.
Removal of volatiles afforded a green solid (0.265 g, 0.337 mmol, 90%)
P
assigned as Cp3 U. Single, X-ray quality crystals were grown in a
concentrated diethyl ether solution at −35 °C. Elemental analysis of
1
C42H45U, calculated: C, 64.03; H, 5.76. Found: C, 63.60; H, 5.82. H
NMR (C6D6, 25 °C): δ −22.21 (38, 6H, ArH), −13.85 (12, 18H,
CH3), −11.08 (24, 6H, ArH), −2.39 (22, 6H, ArH), 2.70 (21, 3H, p-
phenyl-ArH), 3.76 (34, 6H, ArH).
Synthesis of CpPUI2(THF)3. A 20 mL scintillation vial was charged
with 0.335 g (0.369 mmol) of UI3(THF)4 and 10 mL of THF and
cooled to −35 °C. While stirring, 0.082 g (0.369 mmol) of KCpP was
added and stirred for 3 h, resulting in a blue solution. Volatiles were
removed in vacuo. The product was extracted with diethyl ether and
filtered through Celite. Removal of volatiles afforded a blue solid
(0.265 g, 0.297 mmol, 81%) assigned as CpPUI2(THF)3. Single, X-ray
quality crystals were grown in a concentrated diethyl ether solution at
−35 °C. Elemental analysis of C26H39I2O3U, calculated: C, 35.03; H,
1
4.41. Found: C, 34.83; H, 4.31. H NMR (C6D6, 25 °C): δ −24.06
(20, 2H, ArH), −8.26 (16, 2H, ArH), −5.53 (36, 1H, CpP-p-phenyl-
ArH), −2.36 (548, 6H, CpP−CH3), 0.67 (294, 24H, THF−CH2), 4.79
(45, 2H, ArH), 9.54 (85, 2H, ArH).
Synthesis of CpPUI2(MesPDIMe) (1-CpP). A 20 mL scintillation vial
was charged with 0.469 g (0.517 mmol) of UI3(THF)4, 0.205 g (0.516
mmol) of MesPDIMe, and 15 mL of toluene and stirred for 15 min.
Volatiles were removed, affording a brown solid. The solid was
redissolved in 15 mL of toluene. While stirring, 0.115 g (0.518 mmol)
of KCpP was added and allowed to stir for 30 min. The brown
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Inorganic Chemistry
Article
Scheme 1. Synthesis of MesPDIMe Uranium Derivatives
suspension was filtered over Celite, and volatiles were removed in
vacuo to yield a brown solid (0.553 g, 0.516 mmol, 95%) assigned as
CpPUI2(MesPDIMe). Single, X-ray quality crystals were obtained from a
concentrated toluene/pentane (10:1) solution at −35 °C. Elemental
analysis of C41H46N3I2U, calculated: C, 45.91; H, 4.32; N, 3.92. Found:
C, 45.93; H, 4.42; N, 3.82. 1H NMR (C6D6, 25 °C): δ −47.26 (27, 2H,
ArH), −18.20 (12, 12H, o-CH3), −5.55 (10, 6H, CH3), −3.05 (3, 3H,
CpP−CH3), −2.83 (7, 3H, CpP−CH3), −0.83 (3, 1H, ArH), 11.18 (t, J
= 8, 1H, ArH), 12.91 (t, J = 8, 2H, CpP-ArH), 13.38 (34, 2H, ArH),
17.40 (74, 6H, CH3), 17.95 (11, 2H, ArH), 18.73 (42, 4H, PDI-m-
ArH), 21.81 (d, J = 8, 2H, ArH).
NMR (C6D6, 25 °C): δ −63.30 (127, 1H, ArH), −43.63 (33, 1H,
ArH), −23.58 (9, 3H, CH3), −21.80 (12, 3H, CH3), −20.57 (17, 3H,
CH3), −12.67 (61, 1H, ArH), −8.01 (21, 1H, ArH), −7.17 (10, 3H,
CH3), −4.99 (8, 1H, ArH), −3.41 (7, 3H, CH3), −2.66 (7, 3H, CH3),
3.30 (33, 3H, CH3), 5.36 (8, 1H, ArH), 5.82 (t, J = 8, 1H, CpP-p-ArH),
5.93 (t, J = 9, 1H, ArH), 6.26 (t, J = 8, 2H, CpP-m-ArH), 6.76 (25, 1H,
ArH), 6.80 (d, J = 8, 2H, CpP-o-Ar H), 6.88 (25, 1H, ArH), 7.42 (d, J =
8, 1H, ArH), 10.86 (9, 3H, CH3), 13.16 (13, 3H, CH3), 13.51 (8, 1H,
ArH), 15.06 (7, 1H, ArH), 17.89 (12, 3H, CH3), 22.69 (10, 1H, ArH),
42.51 (53, 1H, ArH), 107.80 (40, 1H, ArH).
Synthesis of Cp*UI(Furf)(MesPDIMe) (4-Cp*). A 20 mL
scintillation vial was charged with 0.150 g (0.167 mmol) of
Cp*UI(MesPDIMe) and 5 mL of toluene and stirred. Furfural (0.014
mL, 0.170 mmol) was added via microsyringe, resulting in an
immediate color change from green to brown. Volatiles were then
removed, and the resulting solid was washed with pentane (2 × 10
mL), affording a brown solid (0.120 g, 0.093 mmol, 72%) assigned as
Cp*UI(Furf)(MesPDIMe). Single, X-ray quality crystals were obtained
from a concentrated toluene/pentane (2:1) solution at −35 °C.
Elemental analysis of C42H50N3O2IU, calculated: C, 50.76; H, 5.07; N,
Synthesis of CpPUI(MesPDIMe) (2-CpP). A 20 mL scintillation vial
was charged with 1.072 g (1.000 mmol) of CpPUI2(MesPDIMe) and 15
mL of toluene. While stirring, 1.00 mL (1.000 mmol) of NaHBEt3
solution (1.0 M in toluene) was added, resulting in H2 evolution as
indicated by bubbling. After stirring for 15 min, the dark brown
suspension was filtered over Celite. Volatiles were removed in vacuo to
afford a dark brown solid (0.912 g, 0.964 mmol, 97%) assigned as
CpPUI(MesPDIMe). Single, X-ray quality crystals were obtained from a
concentrated toluene/pentane (1:3) solution at −35 °C. Elemental
analysis of C41H46N3IU, calculated: C, 52.07; H, 4.90; N, 4.44. Found:
C, 51.98; H, 5.07; N, 4.39. 1H NMR (C6D6, 25 °C): δ −65.44 (60, 6H,
CH3), −48.48 (59, 6H, CH3), −16.90 (1, 2H, ArH), −8.73 (12, 2H,
ArH), −8.38 (12, 2H, ArH), −3.70 (5, 6H, CH3), 0.93 (17, 6H, CH3),
8.37 (606, 4H, CpP-Ph-o,p-ArH), 18.10 (20, 1H, ArH), 21.90 (24, 2H,
ArH), 45.50 (31, 1H, ArH), 47.22 (87, 2H, ArH), 70.01 (54, 3H,
CH3), 73.78 (111, 3H, CH3).
1
4.23. Found: C, 50.69; H, 5.13; N, 4.09. H NMR (C6D6, 25 °C): δ
−23.71 (11, 3H, CH3), −21.13 (8, 3H, CH3), −16.87 (9, 3H, CH3),
−7.58 (7, 1H, ArH), −7.24 (7, 1H, ArH), −5.81 (9, 3H, CH3), −4.54
(6, 3H, CH3), −4.32 (7, 3H, CH3), −2.81 (d, J = 8, 1H, ArH), −0.41
(7, 1H, ArH), 1.45 (9, 15H, Cp*), 7.37 (t, J = 8, 1H, ArH), 8.33 (9,
3H, CH3), 8.51 (11, 3H, CH3), 8.78 (t, J = 6, 1H, ArH), 10.29 (d, J =
8, 1H, ArH), 14.34 (8, 1H, ArH), 15.75 (6, 1H, ArH), 24.38 (10, 1H,
ArH), 103.10 (26, 1H, ArH).
Synthesis of CpPU(MesPDIMe) (3-CpP). A 20 mL scintillation vial
was charged with 0.912 g (0.964 mmol) of CpPUI(MesPDIMe) and 15
mL of toluene. While stirring, 0.140 g (1.036 mmol) of KC8 was
added. After stirring for 30 min, the dark brown suspension was
filtered over Celite, and volatiles were removed in vacuo. The resulting
solid was washed with cold pentane (−35 °C) to afford a dark brown
solid (0.568 g, 0.694 mmol, 72%) assigned as CpPU(MesPDIMe).
Elemental analysis of C41H46N3U, calculated: C, 60.14; H, 5.66; N,
Synthesis of CpPU(O2C2Ph4)(MesPDIMe) (5-CpP). A 20 mL
scintillation vial was charged with 0.095 g (0.116 mmol) of
CpPU(MesPDIMe) and 5 mL of toluene. While stirring, 0.043 g
(0.236 mmol) of benzophenone was added. After 15 min, volatiles
were removed in vacuo. The resulting solid was washed with cold
pentane (−35 °C) to afford a dark brown solid (0.099 g, 0.084 mmol,
72%) assigned as CpPU(O2C2Ph4)(MesPDIMe). Single, X-ray quality
crystals were grown by slow diffusion of n-hexane into a concentrated
THF solution at −35 °C. Elemental analysis of C67H66N3O2U,
calculated: C, 68.01; H, 5.62; N, 3.55. Found: C, 67.72; H, 5.64; N,
1
5.13. Found: C, 60.09; H, 5.58; N, 5.08. H NMR (C6D6, 25 °C): δ
−109.93 (164, 1H, ArH), −65.95 (159, 2H, ArH), −42.93 (321, 6H,
CH3), −27.90 (119, 6H, CH3), −19.00 (162, 6H, CH3), −10.20 (22,
2H, ArH), −8.09 (50, 2H, ArH), −1.82 (42, 6H, CH3), 1.12 (71, 2H,
ArH), 17.70 (58, 1H, ArH), 21.53 (94, 2H, ArH), 26.78 (247, 6H,
CH3), 46.73 (189, 2H, ArH), 76.69 (416, 2H, ArH).
1
3.43. H NMR (C6D6, 25 °C): δ −310.96 (76, 1H, ArH), −25.28 (9,
2H, ArH), −7.14 (24, 6H, CH3), −1.51 (24, 6H, CH3), 3.20 (d, J = 8,
2H, o-ArH), 3.66 (12, 6H, CH3), 5.76 (t, J = 8, 2H, m-ArH), 5.91 (t, J
= 6, 1H, p-ArH), 6.68 (646, 8H, O2C2Ph4-ArH), 7.67 (779, 4H,
O2C2Ph4-p-ArH), 8.93 (277, 8H, O2C2Ph4-ArH), 12.04 (7, 2H, ArH),
13.53 (12, 2H, ArH), 15.77 (23, 6H, CH3), 25.51 (12, 2H, ArH),
38.14 (15, 2H, ArH), 177.43 (72, 6H, CH3).
Synthesis of CpPUI(Furf)(MesPDIMe) (4-CpP). A 20 mL
scintillation vial was charged with 0.096 g (0.102 mmol) of
CpPUI(MesPDIMe) and 5 mL of toluene and stirred. Furfural (0.0085
mL, 0.101 mmol) was added via microsyringe, resulting in an
immediate color change from brown to dark green. Volatiles were then
removed, and the resulting solid was washed with pentane (2 × 10
mL), affording a green solid (0.097 g, 0.093 mmol, 92%) assigned as
CpPUI(Furf)(MesPDIMe). Elemental Analysis of C46H50N3IOU, calcu-
lated: C, 53.86; H, 4.91; N, 4.10. Found: C, 53.98; H, 5.09; N, 4.07. 1H
Synthesis of Cp*U(O2C2Ph2H2)(MesPDIMe) (5-Cp*). A 20 mL
scintillation vial was charged with 0.220 g (0.261 mmol) of
Cp*U(MesPDIMe)(THF) and 5 mL of toluene and frozen. Upon
thawing, 0.053 mL (0.522 mmol) of benzaldehyde was added via
microsyringe. The reaction was stirred for 2 h, and volatiles were
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removed. The crude mixture was recrystallized by slow evaporation of
a concentrated diethyl ether solution to afford a green solid (0.123 g,
0.125 mmol, 48%) assigned as Cp*U(O2C2Ph2H2)(MesPDIMe). Single,
X-ray quality crystals were grown from a concentrated toluene/
pentane solution (4:1) at −35 °C. Elemental analysis of
C51H58N3O2U, calculated: C, 62.31; H, 5.95; N, 4.27. Found: C,
1
62.23; H, 6.10; N, 4.24. H NMR (C6D6, 25 °C): δ −50.93 (15, 2H,
ArH), −49.52 (16, 2H, ArH), −23.81 (24, 2H, ArH), −22.17 (22, 2H,
ArH), −19.85 (d, J = 24, 1H, Pin−CH), −5.91 (10, 1H, ArH), −5.71
(8, 1H, ArH), −5.49 (7, 1H, ArH), −5.26 (d, J = 11, 1H, Pin−CH),
1.54 (6, 1H, ArH), 1.95 (5, 6H, CH3), 2.09 (7, 6H, CH3), 2.14, (7, 6H,
CH3), 3.80 (11, 1H, ArH), 9.67 (32, 1H, ArH), 9.85 (36, 1H, ArH),
12.98 (11, 15H, Cp*), 19.63 (17, 3H, CH3), 22.40 (17, 3H, CH3).
Reactivity of 3-CpP with Benzaldehyde. A 20 mL scintillation
vial was charged with 0.027 g (0.033 mmol) of CpPU(MesPDIMe) and 3
mL of toluene. While stirring, benzaldehyde (6.7 μL, 0.066 mmol) was
added via microsyringe. After 30 min without a noticeable color
change, volatiles were removed in vacuo. Inspection by 1H NMR
spectroscopy revealed decomposition of the starting material without
significant consumption of benzaldehyde.
Figure 1. Molecular structure of Cp*UI(MesPDIMe) (2-Cp*) shown
with 30% probability ellipsoids. Hydrogen atoms as well as one 2,4,6-
trimethylphenyl substituent have been omitted for clarity.
Reactivity of 3-Cp* with Benzophenone. A 20 mL scintillation
vial was charged with 0.035 g (0.042 mmol) of Cp*U(MesPDIMe)-
(THF) and 3 mL of toluene. While stirring, benzophenone (0.016 g,
0.088 mmol) was added, resulting in an immediate color change from
brown to dark red. After 10 min, volatiles were removed. Inspection by
1H NMR spectroscopy revealed significant decompostion of starting
material without consumption of benzophenone.
(1.495(3) Å). On the other hand, the C7−C8 distance of
1.499(12) Å is very close to the free ligand value, indicating no
reduction for this bond, and very little for N3−C8 (1.349(11)
Å) as well (1.349(11) Å). Thus, reduction appears to be
relegated to one part of the ligand in the molecular structure.
This is further evident from inspection of the uranium−
nitrogen bond distances for MesPDIMe. Although not as
thoroughly studied, these U−N distances are a more reliable
parameter for establishing ligand reduction due to the higher
accuracy in these measurements. In 2-Cp*, the long U1−N3
distance of 2.781(8) Å is consistent with a dative interaction, as
would be expected for a neutral [MesPDIMe]0. Upon ligand
reduction, however, the U−N bonds change in character from
dative to monoanionic, which is highlighted by bond
shortening. Thus, the U1−N2 and U1−N1 bond distances of
2.337(8) and 2.156(8) Å, respectively, are on the order of
uranium amides33−35 and support reduction on this side of the
ligand in the solid state. Taken with the intraligand distances,
the metric parameter for 2-Cp* supports a dianionic,
RESULTS AND DISCUSSION
■
Synthesis of Reduced Species. Initial studies focused on
metalation of MesPDIMe for the synthesis of a series of reduced
Cp* uranium derivatives. Treating a stirring blue toluene
22
solution of Cp*UI2(THF)3 with one equivalent of MesPDIMe
resulted in a color change to brown over a 3 h period (Scheme
1, red = anionic bond, blue = dative bond). Following the
1
workup, analysis by H NMR spectroscopy revealed seven
paramagnetically shifted resonances, suggesting metalation and
formation of C2v symmetric Cp*UI2(MesPDIMe) (1-Cp*), with
an intense singlet at 12.23 ppm assigned as the η5-Cp* ligand.
Reduction of 1-Cp* was achieved by treatment with one
equivalent of KC8 (Scheme 1). Following the workup, analysis
[
MesPDIMe]2−. The distances in 2-Cp* are also reminiscent of
those in (MesDABMe)2U(THF) (MesDABMe = [ArNC(Me)-
C(Me)NAr], Ar = 2,4,6-trimethylphenyl (Mes)), which has
been characterized to have dianionic [MesDABMe]2− ligands.36
By analogy to the reduction of 1-Cp*, treating 2-Cp* with
one equivalent of KC8 resulted in ligand reduction to generate
Cp*U(MesPDIMe)(THF) (3-Cp*). Compound 3-Cp* was
previously synthesized by the addition of 2 equiv of KC8 to a
solution of Cp*2UI(THF) and MesPDIMe (1:1). The former
synthetic route to 3-Cp* from 2-Cp* resulted in similar purity
and yields (84%) to the latter but circumvents the loss of a Cp*
ligand. Compound 3-Cp* is established to have a triply reduced
1
of the isolated dark green solid by H NMR spectroscopy
revealed nine paramagnetically broadened resonances spanning
from −2.21 to 7.96 ppm with the largest at 2.56 ppm assigned
as the η5-Cp*. Four resonances integrating to six protons each
corresponding to pairs of methyl groups on the MesPDIMe ligand
signify a Cs symmetric complex in solution, Cp*UI(MesPDIMe)
(2-Cp*).
Structural confirmation and features of 2-Cp* were
elucidated by crystallographic analysis of single, X-ray-quality
crystals obtained from the slow diffusion of hexane into a
saturated toluene solution at −35 °C (Figure 1, Table 1).
Refinement of the data revealed the uranium iodide complex
with both η5-Cp* and MesPDIMe ligands. The U1−I1 and U−Ct
distances (3.1087(8) and 2.386 Å, respectively) are on the
order of those for Cp*2UI2(NCCH3) (3.07(2) and 2.465 Å).29
The uranium center is situated 1.189 Å above the MesPDIMe N−
N−N plane.
Intraligand distances have been established as an important
metric for determining pyridine(diimine) ligand reduction.30−32
A comparison of ligand bond distances in 2-Cp* as compared
to free MesPDIMe confirms localized ligand reduction. The N1−
C2 distance of 1.454(12) Å is severely elongated compared to
that in the free ligand of 1.277(3) Å (Figure S5), whereas the
C2−C3 bond of 1.386(13) Å is contracted versus MesPDIMe
[
MesPDIMe]3− ligand,19 demonstrating that reduction occurs at
MesPDIMe and not the uranium center.
With the Cp* reduction series in hand, synthesis of the same
series using the less sterically demanding 1-(7,7-
dimethylbenzyl)cyclopentadienyl (CpP) ligand was attempted.
Synthesis of CpPUI2(THF)x was targeted first as an entry into
22
the reduction series by analogy to Cp*UI2(THF)3 and was
possible by the addition of a single equivalent of KCpP to a
THF solution of UI3(THF)4. Following the workup, the blue
1
solid showed a paramagnetically shifted H NMR spectrum
with broad resonances, most likely due to the dynamic nature
of the CpP ring in solution. Seven signals spanning from −24.06
to 9.54 ppm were observed, including a resonance at −2.36
ppm for the equivalent methyl protons (6H) and a broad
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Table 1. Bond Distances (Å) for MesPDIMe, 2-Cp*, 1-CpP, 2-CpP, and CpPUI2(THF)3
XN
XO
MesPDIMe
2-Cp*
1-CpP
2-CpP
CpPUI2(THF)3
a
U−X1
U−X2
U−X3
2.156(8)
2.337(8)
2.781(8)
1.454(12)
1.386(13)
1.509(13)
1.353(13)
1.375(13)
1.426(11)
1.379(11)
1.362(11)
1.499(12)
1.349(11)
2.386
2.522(10)
2.368(10)
2.484(9)
2.500(2)
2.234(3)
2.327(3)
1.317(4)
1.441(4)
1.357(5)
1.404(5)
1.354(5)
1.416(5)
1.409(4)
1.427(4)
1.378(5)
1.375(4)
2.492
2.500(12)
2.569(14)
2.490(11)
N1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C7
N2−C3
N2−C7
C7−C8
N3−C8
U−Ct
1.277(3)
1.495(3)
1.386(3)
1.378(3)
1.383(3)
1.388(3)
1.346(2)
1.343(3)
1.494(3)
1.276(3)
1.331(16)
1.480(18)
1.334(16)
1.384(17)
1.387(18)
1.393(16)
1.407(15)
1.385(14)
1.434(16)
1.325(14)
2.500
2.505
U−I1
U1−I2
3.1087(8)
3.1257(9)
3.0318(10)
3.0326(3)
3.1667(16)
3.1562(19)
a
The free ligand, MesPDIMe, is shown for comparison.
Figure 2. Molecular structures of CpPUI2(THF)3 (left), 1-CpP (middle), and 2-CpP (right) shown with 30% probability ellipsoids. Hydrogen atoms
and cocrystallized solvent molecules are omitted for clarity.
singlet (Δ1/2 = 294) at 0.67 ppm assigned as coordinated THF
molecules.
requiring formation of CpP UI(THF)x. Attempts to generate
2
this by adding two equivalents of KCpP to a THF solution of
UI3(THF)4 resulted in an immediate color change from purple
X-ray crystallography was employed to elucidate the
structure and determine the number of coordinated solvent
molecules in CpPUI2(THF)x. Refinement of the data from
single crystals grown from diethyl ether at −35 °C confirmed
the product as CpPUI2(THF)3 (Figure 2, left; Table 1). The
molecular structure shows CpPUI2(THF)3 has a U−Ccentroid
distance of 2.505 Å and U−I distances of 3.1667(16) and
3.1562(19) Å, which are all within range for other uranium(III)
cyclopentadienyl complexes.22,37,38 The phenyl group of the
CpP ring points away from the uranium center, while the two
adjacent methyl groups are angled toward it.
1
to green. While analysis of the isolated product by H NMR
spectroscopy revealed a paramagnetically shifted spectrum with
the six resonances expected for the desired product, X-ray
crystallographic analysis of suitable green crystals grown from
diethyl ether at −35 °C showed the molecular structure to be
CpP U, rather than CpP UI(THF)x (Figure S4). The U−
3
2
Ccentroid distances range from 2.527 to 2.568 Å and are on the
order of those in [{1,3-(Me3Si)C5H3}3U] (U−Ccentroid avg
=
2.54 Å).39 Interestingly, the same H NMR spectrum was
obtained regardless of whether two or three equivalents of
1
Using similar conditions to those established for the
synthesis of 1-Cp*, CpPUI2(THF)3 was treated with one
equivalent of MesPDIMe to generate CpPUI2(MesPDIMe) (1-CpP).
KCpP were used, but CpP U is most efficiently synthesized
3
(90% yield) by the addition of three equivalents of KCpP to
UI3(THF)4. Attempts to use CpP U for the synthesis of 3-CpP
3
1
While analysis by H NMR spectroscopy did show a product
were unsuccessful.
with the expected number of resonances and symmetry for 1-
CpP, inseparable side products were also formed. Attempts to
isolate pure 1-CpP by varying solvents and reaction times were
unsuccessful.
Synthesis of 1-CpP was finally possible by the addition of one
equivalent of MesPDIMe to a stirring toluene solution of
UI3(THF)4, which resulted in an immediate color change
from purple to brown (Scheme 1). Following removal of the
volatiles, the resulting brown solid was redissolved in toluene
and treated with an equivalent of KCpP, causing a color change
With the inability to generate 1-CpP cleanly, direct synthesis
of 3-CpPwas attempted in an analogous manner to 3-Cp*,19
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from dark brown to orange-brown. The desired product, 1-CpP,
was isolated in 95% yield; however, failure to remove volatiles
following the addition of MesPDIMe led to a lower yield and
mixture of products. Analysis of 1-CpP by 1H NMR
spectroscopy showed 13 paramagnetically shifted peaks ranging
from −47.26 to 21.81 ppm. A large resonance at −18.20 ppm
integrating to 12 protons was assigned as o-CH3 mesityl
protons of MesPDIMe, and the presence of five more resonances
for this ligand indicated a symmetric MesPDIMe ligand for 1-CpP.
To confirm the assignment as CpPUI2(MesPDIMe) (1-CpP),
analysis of a single crystal grown from toluene/pentane (10:1)
at −35 °C by X-ray crystallography was performed. Refinement
of the data revealed the structure to be the pyridine(diimine)
uranium bis(iodide) species with the expected η5-CpP ligand
(Figure 2, middle; Table 1). The iodides are oriented trans with
respect to each other, with an angle of 153.9(4)°, and have U−I
distances of 3.1257(9) and 3.0318(10) Å, similar to those
reported for UI3(THF)4 (3.103(2)−3.167(2) Å).40 The
uranium center in 1-CpP sits 0.023 Å above the N−N−N
plane of MesPDIMe.
Figure 3. Effective magnetic moment (versus temperature) for 1-CpP
and 5-CpP. Susceptibility data were collected at a 1000 Oe measuring
field.
Although the intraligand distances are not reliable for ligand
oxidation state assignment, as in the case of 2-Cp*, the U−N
distances provide a reliable metric for gauging ligand reduction.
For MesPDIMe in 1-CpP, there are two long U−Nimine distances
of 2.522(10) (U1−N1) and 2.484(9) Å (U1−N3), which are
assigned as dative bonds and are similar to those reported for
the neutral ligand in Cp*U(MesPDIMe)(NPh)2 of 2.578(5),
2.537(5), and 2.606(5) Å.19 However, a shorter U1−N2
distance of 2.368(10) Å, similar to those of 2.201(3) for the
trianionic chelate in 3-Cp*,19 signifies the ligand has been
reduced by the uranium(III) center upon coordination,
generating [MesPDIMe]1− supported by a uranium(IV) center.
Variable temperature magnetic susceptibility studies were
performed on 1-CpP to further confirm the ligand oxidation
state. The ambient temperature magnetic susceptibility for 1-
CpP (μeff: 3.12 μB) is significantly lower than that calculated
−65.44 to 73.78 ppm. Similar to 2-Cp*, four resonances
integrating to six protons each were observed, indicating a Cs
symmetric molecule in solution and leading to the preliminary
assignment as CpPUI(MesPDIMe) (2-CpP).
X-ray quality crystals of 2-CpP obtained from a concentrated
toluene/pentane (1:3) solution at −35 °C were analyzed.
Refinement of the data confirmed the pyridine(diimine)
uranium iodide complex with an η5-CpP ligand (U1−Ct =
2.492 Å; Figure 2, right; Table 1). The U1−I1 bond distance of
3.032(3) Å is within error of other U(IV) complexes with
iodide ligands.24,29 The uranium center sits 0.815 Å above the
plane of MesPDIMe. For the pyridine(diimine) chelate, ligand
reduction is evident from examination of the intraligand bond
distances in 2-CpP. These data highlight a notable difference
between the two C−Nimine distances, where the N1−C2
distance of 1.317(4) Å is significantly shorter than for N3−C8
of 1.375(4) Å. Thus, while the former maintains double bond
character, the latter has undergone reduction. This is confirmed
by the adjacent carbon−carbon bond distances, where C2−C3
(1.441(4) Å) is similar to free MesPDIMe, and C7−C8 (1.378(5)
Å) is contracted due to reduction. Alternating short (C3−C4
and C5−C6) and long (C4−C5 and C6−C7) distances in the
pyridine ring signify a loss of aromaticity which can accompany
a doubly reduced closed shell ligand.48 Therefore, 2-CpP
displays a greater extent of ligand reduction as compared to
with the Lande
́
formula for either bona fide U(III) (4I9/2, 3.62
μB) or U(IV) ions (3H4, 3.58 μB), respectively; however, it is in
the range observed for complexes in either formal oxidation
state. Compound 1-CpP shows qualitatively similar magnetic
behavior (Figure 3), with gradual and monotonic decreases in
μeff as the temperature decreases, with a slightly steeper
downturn below 20 K. At 5 K, the effective magnetic moment
for 1-CpP is 1.25 μB. The low temperature moment is
significantly smaller than that observed in true U(III)
complexes (2.5−3 μB at 2 K)41−44 and larger than found in
unambiguously U(IV) complexes (0.4−0.8 μB at 2 K) with
orbital singlet ground states.45,46 In the scenario where the
excited spins of a U(IV) ion couple antiferromagnetically with a
ligand-centered radical anion, we might expect the low-
temperature effective moment to be somewhat smaller than
the 1.73 μB value calculated for an S = 1/2, g = 2 spin center.
Thus, the data are consistent with a formal U(IV), 5f2 site
weakly interacting with a ligand radical.
1-CpP, supporting a ligand based reduction event and
2−
formation of dianionic [MesPDIMe
]
upon exposure to
NaHBEt3. Lending further support to this formulation are the
U−N bond distances in 2-CpP, where there are two short U−N
bonds (U1−N2 = 2.234(3); U1−N3 = 2.327(3) Å) consistent
with uranium-amide linkages formed from ligand reduction and
one longer U1−N1 bond (2.500(2) Å) for a dative interaction
similar to 2-Cp*.
Following successful metalation of MesPDIMe, the reduction
chemistry of 1-CpP was probed. While multiple reagents were
effective for reduction, the addition of a single equivalent of
sodium triethylborohydride to a stirring toluene solution of 1-
CpP gave the cleanest product in highest yields.47 Evolution of
H2 as indicated by effervescence and a darkening of the solution
was noted over the course of the reaction. Following the work
Further reduction of 2-CpP was attempted to investigate the
generation of an analogous compound to 3-Cp*. The addition
of an equivalent of KC8 to a solution of 2-CpP resulted in the
isolation of a dark brown powder. Analysis by 1H NMR
spectroscopy again showed 14 broad, paramagnetically shifted
peaks ranging from −109.93 to 76.69 ppm for the Cs symmetric
1
up, analysis by H NMR spectroscopy showed a new product
1
with 14 paramagnetically shifted resonances ranging from
compound. The H NMR data unambiguously establishes the
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presence of both the CpP and MesPDIMe ligands, leading to the
assignment as CpPU(MesPDIMe) (3-CpP). The absence of
additional resonances indicates that no THF is coordinated to
the uranium center. While repeated attempts at X-ray
crystallographic characterization were unsuccessful, similar
spectroscopic features to 3-Cp* support formulation of this
species as containing a [MesPDIMe]3− as well.
= tolyl; R = H, R′ = furanyl). Unfortunately, single, X-ray-
quality crystals of 4-CpP have remained elusive.
Analogous reactivity occurred with the pentamethylcyclo-
pentadienyl complex, 2-Cp*. Treating a green solution of 2-
Cp* with one equivalent of furfural resulted in an immediate
color change to brown (eq 1). Inspection with infrared
spectroscopy revealed the disappearance of the carbonyl
stretch, and 1H NMR spectroscopy again revealed a completely
asymmetric paramagnetic spectrum ranging from −23 to 103
ppm with a large resonance at 1.45 ppm assigned as the Cp*
protons. Eight inequivalent methyl protons from the MesPDIMe
and remaining 11 single-proton resonances led to the product
assignment as Cp*UI(Furf)(MesPDIMe) (4-Cp*).
The uranium derivatives of the CpP ligand reported here are
the first actinide complexes to utilize this framework, as this
ligand has previously been used for transition metals49−52 and
lanthanides.53 For all of the CpPU complexes, the standard η5-
coordination mode is the only one observed, in contrast to the
titanium(IV) compound, [CpPTi(CH3)2][B(C6F5)4], which
shows η6-coordination of the phenyl ring to the titanium
center as well.54 The family of reduced uranium complexes of
both Cp* and CpP reported here also highlights the ability of
MesPDIMe to exist in multiple oxidation states, serving to
maintain the valency at the uranium center. Additionally,
although there are clearly steric and electronic differences of the
two cyclopentadienyl ligands, it appears as though there is little
influence of the cyclopentadienyl ligand on the ability to isolate
analogous species in both series.
Single, X-ray quality crystals of 4-Cp* obtained from a
toluene/pentane solution (2:1, −35 °C) were analyzed to
determine the nature of the reductive coupling (Figure 4).
Reactivity with Carbonylated Substrates. The isolation
of the family of reduced uranium complexes provides an
opportunity to determine if the electrons stored in the ligand
can be used to promote bond formation or activation reactions.
Additionally, the influence of the steric and electronic variability
of the cyclopentadienyl ligand on further reactivity can be
evaluated. For these studies, C−O bond activation of ketones
and aldehydes was explored.
Exposure of 2-CpP to an equivalent of furfural resulted in an
immediate color change from brown to green. Infrared
spectroscopy showed no absorption assignable to a carbonyl,
1
and analysis by H NMR spectroscopy revealed a complicated
(28 resonances) paramagnetically shifted spectrum with peaks
ranging from −63 to 107 ppm. Resonances integrating to three
(× 10), two (× 2), and single (× 16) protons were identified,
indicating an asymmetric molecule in solution, save the CpP-
phenyl protons. These data are consistent with reduction of the
carbon−oxygen double bond; however, formation of a stable
charge separated ketyl radical product was ruled out as Cs
symmetry in solution would be expected for such a species.
The more likely scenario is carbonyl reduction, where the
reducing equivalent is derived from [MesPDIMe]2−, followed by
radical coupling of the newly formed ketyl with the remaining
Figure 4. Molecular structure of 4-Cp* shown with 30% probability
ellipsoids. Selected hydrogen atoms, aryl groups, and solvent
molecules have been omitted for clarity.
Refinement of the data revealed a uranium iodide supported by
an alkoxy-amide MesPDIMe ligand and capped with an η5-Cp*
(U1−Ct = 2.510 Å) in a pseudo-octahedral geometry. The
molecular structure confirms the carbon−carbon coupling
between the furfural ketyl-carbon and the MesPDIMe imine-
carbon (C8−C10 = 1.575(7) Å) and shows the resultant
alkoxide ligand is trans to the iodide. For the MesPDIMe ligand,
two nitrogens are coordinated to uranium in a dative fashion
(U1−N1 = 2.769(4); U1−N2 = 2.534(2) Å), and the other
nitrogen has a short U−N bond (U1−N3 = 2.293(4) Å)
consistent with an amide linkage. The imine opposite of the
coupling shows no reduction (N1−C2 = 1.283(8) Å), and the
pyridine ring has rearomatized. The uranium−oxygen distance
of 2.137(4) Å is as expected for a uranium(IV) alkoxide
interaction, while the carbon−oxygen distance of the substrate
shows the expected elongation (O1−C10 = 1.403(6) Å) for the
reduction to a single bond. The furan ring does not interact
with the uranium center. The bond distances account for 4-
Cp* to be in the uranium(IV) oxidation state (as in the starting
material, 2-Cp*), indicating all the redox chemistry has been
achieved by use of ligand electrons. Further, weakly intense,
sharp transitions (ε ∼ 100 M−1 cm−1) detected in the near-
infrared region for both 4-CpP and 4-Cp* suggest both metal
centers reside in the tetravalent oxidation state (Figure S21).
[
MesPDIMe]1−, to form CpPUI(Furf)(MesPDIMe) (4-CpP) (eq 1).
Radical coupling with reduced carbonyl moieties has been
observed for trivalent Cp2*U(2,2′-bpy) (2,2′-bpy =2,2′-
bipyridine), which contains a monoanionic bipyridine ligand.55
Upon the addition of carbonylated substrates, the carbonyls are
reduced by one electron and couple with the bipyridine ligand
to generate a family of compounds, Cp2*U[(NC5H4)-
NC4H3CH-CRR′O] (R = R′ = Ph; R = R′ = Me; R = H, R′
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Complex 4-CpP displays an intense transition (ε = 1011 M−1
cm−1, λmax = 600 nm) assigned as the color producing band.
Isolation of the analogous 4-Cp* and 4-CpP complexes
supports that there is little variation in reactivity between the
Cp* and CpP systems in this case, despite the differences in
sterics and electronics. To further probe the role of Cp
substituent, the reactivity of the compounds with the triply
reduced MesPDIMe ligands was explored with similar substrates.
Treating 3-CpP with one equivalent of benzophenone
bond. On the basis of the reaction stoichiometry and
spectroscopic data, 5-CpP is assigned as the uranium(IV)
pinacolate complex, CpPU(O2C2Ph4)(MesPDIMe).
To elucidate structural parameters of 5-CpP, single, X-ray-
quality crystals obtained by slow diffusion of n-hexane into a
concentrated THF solution at −35 °C were analyzed.
Refinement of the data confirmed the assignment of 5-CpP,
which is coordinated by pyridine(diimine) and an η5-CpP ligand
(U1−Ct = 2.575 Å; Figure 5, left; Table 2). The U−O
Table 2. Structural Parameters for 4-Cp*, 5-CpP, and 5-Cp*
4-Cp*
5-CpP
5-Cp*
U−N1
U−N2
U−N3
2.769(4)
2.532(4)
2.293(4)
1.283(8)
1.493(8)
1.389(9)
1.372(10)
1.377(9)
1.400(8)
1.345(7)
1.340(7)
1.509(8)
1.492(7)
2.510
2.679(4)
2.434(4)
2.607(4)
1.305(6)
1.440(7)
1.384(7)
1.385(8)
1.370(8)
1.388(7)
1.383(7)
1.386(7)
1.435(7)
1.321(6)
2.575
2.527(3)
2.523(4)
2.568(4)
1.315(6)
1.441(6)
1.473(6)
1.397(7)
1.419(7)
1.479(7)
1.343(6)
1.344(6)
1.437(6)
1.311(6)
2.592
N1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C7
N2−C3
N2−C7
C7−C8
N3−C8
U−Ct
maintained the dark brown solution color (eq 2). Analysis by
1H NMR spectroscopy showed unreacted starting material as
well as the formation of a new product, 5-CpP, in a 1:1 ratio. To
reach full conversion, two equivalents of benzophenone were
added to a stirring solution of 3-CpP, which successfully
generated 5-CpP as the exclusive product. The 1H NMR
spectrum revealed 17 paramagnetically shifted resonances
ranging from −310.96 to 177.43 ppm. Deuterium labeling
experiments using benzophenone-d10 facilitated the assignment
of three broad resonances (6.68, 7.67, and 8.93 ppm)
corresponding to the protons derived from benzophenone.
Analysis of a KBr pellet of 5-CpP by infrared spectroscopy did
not show a carbonyl stretch, supporting reduction of the CO
U−O1
U−O2
U−I1
2.137(4)
2.118(3)
2.155(3)
2.151(3)
2.133(3)
3.0860(4)
C27−C34
1.647(6)
1.554(6)
distances of 2.118(3) and 2.155(3) Å are similar to those found
in the uranium(IV) bis(pinacolate), U(O2C2Ph4)2(THF)2,
reported by Ephritikhine and co-workers.56 Due to the extreme
steric hindrance in the molecule, the uranium center is pulled
out of the plane of the pyridine(diimine) ligand by 1.099 Å.
Examination of the intraligand bond distances indicates that
MesPDIMe is reduced by a single electron in 5-CpP. In
Figure 5. Molecular structure of 5-CpP (left) and 5-Cp* (right) shown with 30% probability ellipsoids. Hydrogen atoms, selected 2,4,6-
trimethylphenyl groups, and cocrystallized solvent molecules have been omitted for clarity.
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comparison to the free ligand, both C−Nimine distances are
elongated, while the adjacent C−C distances are contracted,
supporting ligand reduction. Furthermore, the Npyr−C
distances of 1.383(7) and 1.386(7) Å are elongated as
compared to the free ligand but are on the order of those in
1-CpP. This reduction pattern is further evident in the
respective U1−N1 and U1−N3 distances of 2.607(4) and
2.679(4) Å for MesPDIMe in 5-CpP, which are on the order of
dative uranium−nitrogen bonds, while the U1−N2 distance
(2.434(4) Å) is shorter, supporting electron occupation of the
ligand similar to that in 1-CpP for the assignment as
monoanionic [MesPDIMe]1−.
brown to green (eq 3). 1H NMR spectroscopy gave a spectrum
of a single, paramagnetically shifted product, 5-Cp*, with a
singlet at 12.98 ppm assigned as the equivalent Cp* protons.
Analysis of the crude material by IR spectroscopy confirmed
the absence of an absorption for a CO double bond (Figure
S20).
To confirm the identity of 5-Cp*, single, X-ray-quality
crystals obtained from a concentrated toluene/pentane (4:1)
solution cooled to −35 °C were analyzed. Refinement of the
data revealed an η5-Cp* ligand (U1−Ct = 2.592 Å) bound to a
uranium pinacolate supported by a MesPDIMe ligand (Figure 5,
right; Table 2). The formerly aldehydic protons on the
pinacolate ligand were also refined. The molecular structure
shows the pinacolate ligand adopts a meso conformation with
U−O distances (2.151(3) and 2.133(3) Å) similar to those in
5-CpP and U(O2C2Ph4)2(THF)2, supporting the hypothesis
that steric bulk precludes formation of the benzophenone
derivative. Similar to the case of 5-CpP, the uranium is situated
1.032 Å above the plane of the MesPDIMe ligand.
Formulation of [MesPDIMe]1− in 5-CpP is further confirmed
by variable temperature magnetic susceptibility studies (Figure
3). The ambient temperature magnetic susceptibility of 5-CpP
is 3.27 μB, which is slightly larger than for 1-CpP (μeff: 3.12 μB)
but in the range expected for uranium(III) or (IV) compounds.
As in the case of 1-CpP, there is a gradual and monotonic
decrease in μeff for 5-CpP as the temperature decreases, with a
final effective magnetic moment of 1.44 μB at 5 K. While the
low temperature moment is intermediate of U(III) and U(IV)
complexes with orbital singlet ground states,45,46 based on the
similarity to 1-CpP, the low temperature magnetic data are
consistent with a formal U(IV), 5f2 site weakly interacting with
a ligand radical.
Examination of the metrical parameters for MesPDIMe in 5-
Cp* indicates a potentially differing electronic structure than
for 5-CpP. The N1−C2 and N3−C8 distances of 1.315(6) and
1.311(6) Å are similar to free MesPDIMe, supporting their double
bond character, while the adjacent C−C distances of 1.441(6)
(C2−C3) and 1.437(6) Å (C7−C8) show only a slight
deviation from the free MesPDIMe, indicating little, if any,
reduction. However, ligand reduction is most clear in the
pyridine ring, where the C3−C4 (1.473(6) Å) and C6−C7
(1.479(6) Å) distances have been significantly elongated to the
degree of C−C single bonds, while C4−C5 (1.397(7)) and
C5−C6 (1.419(7)) possess multiple bond character. These
parameters are in contrast to those noted for 5-CpP, where the
bond distances within the pyridine ring are similar to an
aromatic system. A consequence of the unusual electronic
structure of MesPDIMe observed for 5-Cp* is that the uranium−
nitrogen distances show three dative linkages spanning
2.523(4)−2.568(4) Å. Thus, the metrical parameters suggest
that a charge separated radical anion resonance form of the
The presence of this ligand radical in 5-CpP supports that the
two electrons to reduce both equivalents of OCPh2 to
[OCPh2]• were derived from trianionic [MesPDIMe]3− in 3-
CpP. Thus, one reducing equivalent remains and is localized in
MesPDIMe. Generating radical character in both benzophenone
ligands results in radical coupling to form the pinacolate
product, 5-CpP, which has been oxidized to a radical
1−
[
MesPDIMe
]
ligand and uranium(IV) center. Thus, the
tetravalent uranium center is maintained throughout the
reaction, with redox events promoted by the ligand.
Radical coupling mediated by electron-rich uranium species
is well established, especially for substrates such as CO,57 NO,58
and diphenylacetylene.13 For carbonyl substrates, Ephritikhine
reports early examples with the formation of UCl2(O2C2Ph4)
and U(O2C2Ph4)2(THF)2 by treating UCl4 with varying
amounts of Na/Hg in the presence of benzophenone.56
Meyer demonstrates that trivalent [((AdArO)3tacn)U] can
reduce benzophenone to form the transient uranium(IV) charge
separated benzophenone radical species, [((AdArO)3tacn)-
UIV(OC·Ph2)].59 This species is unstable and can undergo
radical coupling either with a second equivalent through the
para-phenyl carbon of activated benzophenone to form dimeric
[((AdArO)3tacn)UIV(OCPhPh−CPh2O)UIV(AdArO)3tacn)] or
with an H· source to form [((AdArO)3tacn)UIV(OC·Ph2)].
Benzaldehyde coupling by Cp3U(THF) has also been reported,
which produced tetravalent Cp3UOC(Ph)H-PhCHOCp3U.60
Interestingly, 3-CpP was found to decompose when exposed to
benzaldehyde, indicating that clean C−O bond activation by 3-
CpP cannot be extended to aldehydes.
Reactivity of 3-Cp* with benzophenone was tested under the
same conditions as for 3-CpP. The addition of two equivalents
of benzophenone resulted in immediate decomposition of 3-
Cp* without the formation of tractable uranium products. This
decomposition is most likely due to the increased steric bulk of
the Cp* ligand as compared to CpP, which prevents substrate
access and formation of the pinacolate derivative. Thus,
compound 3-Cp* was treated with two equivalents of
benzaldehyde, which caused an immediate color change from
[
MesPDIMe]− ligand is the dominant form in 5-Cp* (Figure 4,
right; Table 2). As in the case of 5-CpP, the trianionic MesPDIMe
ligand in 3-Cp* is the source of the reducing equivalents for
benzaldehyde activation to form 5-Cp*, which maintains a
uranium(IV) center.
Examination of the electronic absorption spectra of 1-Cp*,
1-CpP, 5-CpP, and 5-Cp* further highlights electronic structure
differences. Complexes 1-Cp*, 1-CpP, and 5-CpP are all dark
brown and display similar, ill-defined, relatively strong
transitions (ε = 200−888 M−1 cm−1) throughout the near-
infrared region (Figure 6). These transitions are thought to
result from intramolecular electron transfer (IET) between the
[
MesPDIMe]− radical and the uranium(IV) center. Bands of
similar energy and intensity in the near-infrared region have
been observed in Me8Fc-PTM, which is an octamethylferrocene
linked through a conjugated vinylene bridge to a perchloro-
triphenylmethyl (PTM) radical. In this case, bands assigned to
the IET result from an electron transfer from the Me8Fc
(donor) to the PTM (acceptor). These bands in the near-
infrared region are absent for the valence tautomeric species,
Me8Fc+−PTM−.61 Instead, this zwitterionic form displays a
diagnostic absorbance at 530 nm (ε ∼ 11 000 M−1 cm−1)
assigned as the PTM anion charge transfer.62 The intense near-
infrared bands observed for 1-Cp*, 1-CpP, and 5-CpP are in
sharp contrast to 5-Cp*, which displays a typical U(IV)
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resonance form that is confirmed by the electronic absorption
spectrum.
CONCLUSIONS
■
In conclusion, a series of reduced pyridine(diimine) uranium-
(IV) species with either the Cp* or CpP framework have been
generated from uranium(III) starting materials. The entries in
the series differ in the extent of pyridine(diimine) ligand
reduction, which was made possible by treatment with
potassium graphite. While analogous members of each series
appear to be quite similar in their electronics and metrical
3−
parameters, the reactivity of the trianionic [MesPDIMe
]
compounds with carbonylated substrates differs. With the
sterically smaller CpP derivative, a new pinacolate compound is
formed by benzophenone coupling, whereas reactivity with
aldehydes results in decomposition. For the larger Cp*
derivative, benzophenone is too bulky to react, but analogous
coupling chemistry is possible with benzaldehyde. In the latter
case, an unusual electronic structure for [MesPDIMe]− is noted
and formulated based on the metrical parameters.
The findings presented here highlight the ability of the
pyridine(diimine) ligand to exist in multiple oxidation states,
which in turn maintains the +4 valency of the uranium center.
The trianionic ligands serve as electron sources to reduce C−O
bonds and are oxidized over the course of the reactions. The
redox-active MesPDIMe ligand facilitates multielectron chemistry
at uranium, while investigating the effect of steric variation of
the cyclopentadienyl ligand in the coordination sphere.
Additionally, changing the electron donation ability of the
cyclopentadienyl substituent dictates the electronic structure of
the pyridine(diimine) ligand in the products. Ongoing studies
are currently focused on the reactivity of this family of reduced
uranium(IV) compounds for small molecule activation.
Figure 6. Electronic absorption spectra of complexes 1-Cp*, 1-CpP, 5-
CpP, and 5-Cp* recorded in THF at 25 °C. Solvent overtones
between 1700 and 1800 nm have been removed for clarity.
absorbance spectrum with sharp, weakly intense (ε = 20−50
M−1 cm−1) transitions in the near-infrared region. The lack of
the IET band supports that 5-Cp* can be thought of as a
zwitterionic complex. In addition to the differences in the near-
IR region, bright green 5-Cp* also contains two dominant
features in the visible region assigned to the charge transfer of
the pyridine anion (ε = 36 430 M−1 cm−1, 431 nm) and to the
color producing band (5560 M−1 cm−1, 602 nm), the latter of
which is analogous to that observed for green 4-CpP.
The variation of the sterics and electronics of the
cyclopentadienyl ligands is highlighted in the reactivity of 3-
CpP and 3-Cp*, as these rings ultimately determine substrate
preference and electronic structures of the final products, 5-
Cp* and 5-CpP. In the case of the CpP ligand, ketones can be
reduced and coupled since the −Me2Ph substituent can rotate
away from the uranium center, which serves to relieve the steric
pressure. For the larger Cp*, only activation of aldehydes is
possible, since substrates are small enough to approach the
uranium center. The dichotomy in the electronic structures of
5-CpP and 5-Cp* can be correlated to the electron donating
ability of the cyclopentadienyl ring (Scheme 2). In the case of
CpP, the pinacolate product features an anionic U−Npyr bond,
forcing a [MesPDIMe]−1 radical. This occurs because the electron
donating effect of the methyl groups is mediated by the
withdrawing effect of the adjacent phenyl group on the Cp
ligand. In contrast, the five alkyl substituents of Cp* create a
highly electron donating ligand, such that the anionic charge of
the U−Npyr bond is pushed onto the pyridine(diimine) ligand,
converting it to a dative bond. This generates the zwitterionic
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra and crystallographic details. This material is
available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
*E-mail: sbart@purdue.edu.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors acknowledge partial support from the Donors of
the American Chemical Society Petroleum Research Fund, as
■
Scheme 2. Influence of Cp Ligand on Pinacolate Product Electronic Structure
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Inorganic Chemistry
Article
(29) Maynadie, J.; Berthet, J.-C.; Thuery, P.; Ephritikhine, M.
Organometallics 2006, 25, 5603.
(30) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton
well as from the Division of Chemical Sciences, Geosciences,
and Biosciences, Office of Basic Energy Sciences of the U.S.
Department of Energy through Grant DE-AC02-12ER16328
(SCB). The National Science Foundation (NSF-CHE-
1058889, grant to MPS) is also acknowledged. SCB is a
Cottrell Scholar funded by the Research Corporation.
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3741
dx.doi.org/10.1021/ic500012x | Inorg. Chem. 2014, 53, 3730−3741