What a difference a 5f element makes: Trivalent and tetravalent uranium halide complexes supported by one and two bis[2-(diisopropylphosphino)-4- methylphenyl]amido (PNP) ligands

Article abstract of DOI:10.1021/ic802061x

The coordination behavior of the bis[2-(diisopropylphosphino)-4- methylphenyl]amido ligand (PNP) toward UI₃(THF)₄ and UCI₄ has been investigated to access new uranium(III) and uranium(IV) halide complexes supported by one

Full text of DOI:10.1021/ic802061x

Inorg. Chem. 2009, 48, 2114-2127
What a Difference a 5f Element Makes: Trivalent and Tetravalent
Uranium Halide Complexes Supported by One and Two
Bis[2-(diisopropylphosphino)-4-methylphenyl]amido (PNP) Ligands
Thibault Cantat, Brian L. Scott, David E. Morris,* and Jaqueline L. Kiplinger*
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Received October 27, 2008
The coordination behavior of the bis[2-(diisopropylphosphino)-4-methylphenyl]amido ligand (PNP) toward UI3(THF)4 and
UCl4 has been investigated to access new uranium(III) and uranium(IV) halide complexes supported by one and two
PNP ligands. The reaction between (PNP)K (6) and 1 equiv of UI3(THF)4 afforded the trivalent halide complex (PNP)UI2(4-
tBu-pyridine)2 (7) in the presence of 4-tert-butylpyridine. The same reaction carried out with UCl4 and no donor ligand
gave [(PNP)UCl3]2 (8), in which the uranium coordination sphere in the (PNP)UCl3 unit is completed by a bridging chloride
ligand. When UCl4 is reacted with 1 equiv (PNP)K (6) in the presence of THF, trimethylphosphine oxide (TMPO), or
triphenylphosphineoxide (TPPO), the tetravalent halide complexes (PNP)UCl3(THF) (9), (PNP)UCl3(TMPO)2 (10), and
(PNP)UCl3(TPPO) (11), respectively, are formed in excellent yields. The bis(PNP) complexes of uranium(III), (PNP)2UI
(12), and uranium(IV), (PNP)2UCl2 (13), were easily isolated from the analogous reactions between 2 equiv of 6 and
UI3(THF)4 or UCl4, respectively. Complexes 12 and 13 represent the first examples of complexes featuring two PNP
ligands coordinated to a single metal center. Complexes 7-13 have been characterized by single-crystal X-ray diffraction
and 1H and 31P NMR spectroscopy. The X-ray structures demonstrate the ability of the PNP ligand to adopt new coordination
modes upon coordination to uranium. The PNP ligand can adopt both pseudo-meridional and pseudo-facial geometries
when it is κ3-(P,N,P) coordinated, depending on the steric demand at the uranium metal center. Additionally, its hemilabile
character was demonstrated with an unusual κ2-(P,N) coordination mode that is maintained in both the solid-state and
in solution. Comparison of the structures of the mono(PNP) and bis(PNP) complexes 7, 9, 11-13 with their respective
C5Me5 analogues 1-4 undoubtedly show that a more sterically congested environment is provided by the PNP ligand.
The electronic influence of replacing the C5Me5 ligands with PNP was investigated using electronic absorption spectroscopy
and electrochemistry. For 12 and 13, a chemically reversible wave corresponding to the UIV/UIII redox transformation
comparable to that for 3 and 4 was observed. However, a ∼350 mV shift of this couple to more negative potentials was
observed on substitution of the bis(C5Me5) by the bis(PNP) framework, therefore pointing to a greater electronic density
at the metal center in the PNP complexes. The UV-visible region of the electronic spectra for the mono(PNP) and
bis(PNP) complexes appear to be dominated by PNP ligand-based transitions that are shifted to higher energy in the
uranium complexes than in the simple ligand anion (6) spectrum, for both the UVI and UIII oxidation states. The near IR
region in complexes 1-4 and 7, 9, 11-13 is dominated by f-f transitions derived from the 5f3 and 5f2 valence electronic
configuration of the metal center. Though complexes of both ligand sets exhibit similar intensities in their f-f bands, a
somewhat larger ligand-field splitting was observed for the PNP system, consistent with its higher electron donating
ability.
Introduction
these elements in a variety of chemical environments. These
can range from inert atmosphere syntheses of well-defined
actinide materials for nuclear fuel applications and materials
science1,2 to addressing purely fundamental questions such
as the involvement of 5f-orbitals in bonding and reactivity
by studying highly reactive actinide molecular complexes
Non-aqueous chemistry of the actinides has proved invalu-
able for gaining insight into the behavior and properties of
*
To whom correspondence should be addressed. E-mail:
kiplinger@lanl.gov.
2114 Inorganic Chemistry, Vol. 48, No. 5, 2009
10.1021/ic802061x CCC: $40.75  2009 American Chemical Society
Published on Web 01/23/2009

TriWalent and TetraWalent Uranium Halide Complexes
Chart 1. Alternatives to Cyclopentadienyl Ligands Employed to Support Non-Aqueous Actinide Chemistry17
in the absence of air and water.3-13 The synthesis of uranium
nitride/carbide materials and coordination complexes featur-
ing tetravalent, pentavalent, and hexavalent uranium-main
group element multiple bonds (i.e., oxo, imido, and phos-
phinidene complexes) represent major achievements in this
area.14,15
The optimal ligand sets for supporting non-aqueous
actinide chemistry must not only coordinate to the metal and
stabilize the complex against disproportionation and dis-
sociation reactions but also support a variety of oxidation
states. Because of the large ionic radii of the actinides, the
ligand framework should also bring enough steric bulk and
electronic saturation so that only a few controlled coordina-
tion sites remain available for chemistry. For these reasons,
the bis(pentamethylcyclopentadienyl) (C5Me5)2 and the tris(cy-
clopentadienyl) (C5H5)3 ligand sets have been enormously
successful as frameworks for organometallic actinide
chemistry.15,16
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In recent years, new heteroatom-based ligand sets spanning
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1) and some have been shown to support reactivity patterns
and structures that are not currently available for the
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all designed using hard donor atoms (oxygen and nitrogen).
Mono-, di- and trianionic backbones with denticity of up to
Inorganic Chemistry, Vol. 48, No. 5, 2009 2115

Cantat et al.
were collected using a Perkin-Elmer Princeton Applied Research
Corporation (PARC) Model 263 potentiostat under computer control
with PARC Model 270 software. All sample solutions were ∼1-2
mM in complex with ∼0.1 M [Bu4N][B(3,5-(CF3)2-C6H3)4] sup-
porting electrolyte in THF solvent. All data were collected with
six have been used to tune the number of remaining reactive
sites on the metal center. Among these, the tris(pyrazolylbo-
rate) ligand (III), which is considered the inorganic comple-
-
ment of C5Me5 , has been utilized the most in supporting
coordination chemistry for both uranium(III) and ura-
nium(IV) with one and two ligands coordinated to the metal
ion. However, a major drawback of the tris(pyrazolylborate)
system is its vulnerability to fragmentation reactions (cleav-
age of the B-N bonds) when coordinated to highly elec-
tropositive metals.18-21 On the whole, none of these ligands
has met with the same success as the cyclopentadienyl-based
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Ligands that combine both hard and soft coordination
environments have proved useful for stabilizing highly
reactive functional groups such as alkylidenes, phosphin-
idenes, and imides on transition metals and lanthanides.23-30
However, soft donor ligands have been largely ignored as
supporting ligands for early actinide chemistry, with the
exception of coordination complexes featuring phosphorus
and chalcogenide ligands prepared to investigate actinide/
lanthanide separation technologies and questions about
covalency in “hard-soft” interactions.31-37 Given that soft
donor ligands provide an excellent opportunity to promote
the low valent chemistry of uranium, we initiated a program
to develop the coordination chemistry of the robust monoan-
ionic bis[2-(diisopropylphosphino)-4-methylphenyl]amido or
PNP ligand toward the uranium halide complexes, UCl4, and
UI3(THF)4. Herein, we report the synthesis and characteriza-
tion of a series of uranium(III) and uranium(IV) halide
complexes supported by one and two PNP ligands and
discuss the structural and electronic consequences of replac-
ing the pentamethylcyclopentadienyl (C5Me5) ligand by the
softer PNP ligand.
Experimental Section
Instrumentation and Sample Protocols. Electronic absorption
spectral data were obtained for toluene solutions of the complexes
over the wavelength range from 280-2000 nm on a Perkin-Elmer
Model Lambda 950 UV-visible-near-infrared spectrophotometer.
Data were collected in 1 mm path length cuvettes loaded in the
Vacuum Atmospheres drybox system described below and run
versus a solvent reference or versus air. In the latter case, the data
were corrected for solvent absorbance post-acquisition. Spectral
resolution was 2 nm. Voltammetric data were obtained in the
Vacuum Atmospheres drybox system described below. All data
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J., Eds.; Springer: The Netherlands, 2006; Vol. 5, pp 2799-2910.
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2116 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
the positive-feedback IR compensation feature of the software/
potentiostat activated to ensure minimal contribution to the volta-
mmetric waves from uncompensated solution resistance (typically
∼1 kΩ under the conditions employed). Solutions were contained
in a PARC Model K0264 microcell consisting of a ∼3 mm diameter
Pt disk working electrode, a Pt wire counter electrode, and a Ag
wire quasi-reference electrode. Scan rates from 20-5000 mV/s were
employed in the cyclic voltammetry scans to assess the chemical
and electrochemical reversibility of the observed redox transforma-
tions. Half-wave potentials were determined from the peak values
in the square-wave voltammograms. Potential calibrations were
performed at the end of each data collection cycle using the
ferrocenium/ferrocene couple as an internal standard. Voltammetric
and electronic absorption data were analyzed using Wavemetrics
IGOR Pro (Version 6.0) software on a Macintosh platform.
General Synthetic Considerations. Unless otherwise noted, all
reactions and manipulations were performed at 20 °C in a
recirculating Vacuum Atmospheres NEXUS Model inert atmo-
sphere (N2) drybox equipped with a 40CFM Dual Purifier NI-Train.
Glassware was dried overnight at 150 °C before use. All NMR
spectra were obtained using a Bruker Avance 300 MHz spectrom-
eter. Chemical shifts for 1H and 13C{1H} NMR spectra were
referenced to solvent impurities and 31P{1H} NMR spectra were
referenced to pure phosphoric acid. Mass spectrometric (MS)
analyses were obtained at the University of California, Berkeley
mass spectrometry facility using either a VG ProSpec (EI) or VG70-
SE (FAB) mass spectrometer. Elemental analyses were performed
at the University of California, Berkeley Microanalytical facility
on a Perkin-Elmer Series II 2400 CHNS Analyzer. X-ray crystal-
lographic data were collected on a Bruker D8 diffractometer, with
an APEX II charge-coupled-device (CCD) detector and a KRYO-
FLEX liquid nitrogen vapor-cooling device. Structural solution and
refinement was achieved using the SHELXL97 program suite (Table
1).38 Details regarding data collection are provided in the CIF files.
Unless otherwise noted, reagents were purchased from com-
mercial suppliers and used without further purification. Celite
(Aldrich), alumina (Brockman I, Aldrich) and 4 Å molecular sieves
(Aldrich) were dried under dynamic vacuum at 250 °C for 48 h
prior to use. Hexane (anhydrous, Aldrich), pentane (anhydrous,
Aldrich), toluene (anhydrous, Aldrich), tetrahydrofuran (anhydrous,
Aldrich), and bis(trimethylsilyl)ether (Aldrich) were dried over KH
for 24 h, passed through a column of activated alumina under
nitrogen, and stored over activated 4 Å molecular sieves prior to
use. Benzene-d6 (anhydrous, Aldrich) and tetrahydrofuran-d8 (Cam-
bridge Isotope Laboratories) were purified by storage over activated
4 Å molecular sieves or sodium metal prior to use. UCl4,39
UI3(THF)4,40 (C5Me5)2UI(THF) (3),41 and (PNP)H (5)42 were
prepared according to literature procedures.
Caution! Depleted uranium (primary isotope 238U) is a weak
R-emitter (4.197 MeV) with a half-life of 4.47 × 109 years;
manipulations and reactions should be carried out in monitored
fume hoods or in an inert atmosphere drybox in a radiation
laboratory equipped with R- and ꢀ-counting equipment.
Synthesis of (PNP)K (6). A 250 mL sidearm flask equipped
with a stir bar was charged with (PNP)H (5) (4.50 g, 10.5 mmol,
1.0 equiv), toluene (25 mL), and hexane (75 mL). To the resulting
solution was added portion-wise potassium bis(trimethylsilyl)amide
(3.15 g, 15.8 mmol, 1.5 equiv) at ambient temperature. The reaction
mixture immediately turned orange and was stirred for 12 h,
yielding a bright yellow suspension. The solid was collected by
filtration through a coarse porosity frit, washed with hexane (25
mL), and thoroughly dried under reduced pressure to afford
analytically pure (PNP)K (6) as a yellow solid (4.52 g, 9.67 mmol,
1
92%). H NMR (THF-d8, 298 K): δ 1.07 (m, 24H, CHMe2), 2.09
3
(s, 6H, Ar-Me), 2.13 (m, 4H, CHMe2), 6.53 (d, JHH ) 8 Hz, 2H,
3
4
Ar-H), 6.76 (s, 2H, Ar-H), 6.86 (dd, JHH ) 8 Hz, JPH ) 5 Hz,
Ar-H). 13C{1H} NMR (THF-d8, 298 K): δ 20.22 (d, JCP ) 11.4
Hz), 21.27 (d, JCP ) 9.1 Hz), 21.55 (s; Ar-CH3), 23.14 (d, JCP
)
11.9 Hz), 117.3 (d, JCP ) 2.8 Hz), 119.5 (s), 122.5 (s), 130.5 (s),
132.8 (s), 161.4 (d, JCP ) 18.7 Hz). 31P{1H} NMR (THF-d8, 298
K): δ -0.31 (s). Anal. Calcd for C26H40KNP2 (mol. wt. 467.65):
C, 66.78; H, 8.62; N, 3.00. Found: C, 66.49; H, 8.73; N, 2.88.
Synthesis of (PNP)UI2(4-tBu-pyridine)2 (7). A 50 mL sidearm
flask equipped with a stir bar was charged with UI3(THF)4 (1.58 g,
1.74 mmol) and THF (10 mL). To the resulting solution was added
a solution of (PNP)K (6) (817 mg, 1.74 mmol, 1.0 equiv) in THF
(10 mL) at ambient temperature. The reaction mixture turned
immediately green, was stirred for 30 min, and filtered through a
Celite-padded coarse frit. The filtrate was collected, an excess of
4-tBu-pyridine (100 mg, 740 mmol) was added, and the resulting
solution was stirred for 10 min. The volatiles were removed under
reduced pressure, and the resulting dark green solid was washed
with 10 mL of cold (-35 °C) pentane to give (PNP)UI2(4-tBu-
pyridine)2 (7) as a dark green solid (2.01 g, 1.69 mmol, 97%). X-ray
quality samples of 7 were obtained by recrystallization from diethyl
ether at -35 °C. 1H NMR (C6D6, 298 K): δ -24.41 (s, 6H), -13.05
(s, 6H), -3.27 (s, 6H), -1.14 (s, 2H), 2.02 (s, 2H), 2.84 (s, 4H),
4.01 (s, 18H), 4.28 (s, 6H), 4.70 (s, 6H), 5.82 (s, 2H), 14.38 (s,
4H), 19.92 (s, 2H), 44.67 (s, 2H). 31P{1H} NMR (C6D6, 298 K):
no signal observed in the range ( 5000 ppm. Anal. Calcd for
C44H66I2N3P2U (mol. wt. 1190.80): C, 44.38; H, 5.59; N, 3.53.
Found: C, 44.79; H, 5.70; N, 3.73.
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Synthesis of [(PNP)UCl3]2 (8). A 50 mL sidearm flask equipped
with a stir bar was charged with UCl4 (812 mg, 2.14 mmol, 1.0
equiv) and THF (10 mL). To the resulting suspension was added
a solution of (PNP)K (6) (1.00 g, 2.14 mmol, 1.0 equiv) in THF
(10 mL) at ambient temperature. The reaction mixture turned
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(36) Roger, M.; Belkhiri, L.; Arliguie, T.; Thuery, P.; Boucekkine, A.;
Ephritikhine, M. Organometallics 2008, 27, 33–42.
(37) Brennan, J. G.; Stults, S. D.; Andersen, R. A.; Zalkin, A. Organo-
metallics 1988, 7, 1329–1334.
(38) (a) SAINT 7.06 Integration Software; Bruker Analytical X-ray Systems:
Madison, WI, 2003. (b) Sheldrick, G. M. SADABS 2.03 Program for
Adsorption Correction; University of Go¨ttingen: Go¨ttingen, Germany,
2001. (c) Sheldrick, G. M. SHELXTL 5.10, Structure Solution and
Refinement Package; University of Go¨ttingen: Go¨ttingen, Germany,
1997.
(39) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organome-
tallics 2002, 21, 5978–5982.
(40) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin,
J. G.; Zwick, B. D. Inorg. Chem. 1994, 2248–2256.
(41) Avens, L. R.; Burns, C. J.; Butcher, R. J.; Clark, D. L.; Gordon, J. C.;
Schake, A. R.; Scott, B. L.; Watkin, J. G.; Zwick, B. D. Organome-
tallics 2000, 19, 451–457.
(42) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23,
326–328.
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Cantat et al.
immediately red, was stirred for 30 min, and filtered through a
Celite-padded coarse frit. The filtrate was collected, and the volatiles
were removed under reduced pressure to give [(PNP)UCl3]2 (8) as
Ph3P)O), 1160.1 (br s, PNP). Anal. Calcd for C44H55Cl3NOP3U
(mol. wt. 1051.22): C, 50.27; H, 5.27; N, 1.33. Found: C, 49.96;
H, 5.31; N, 1.30.
1
Synthesis of (PNP)2UI (12). A 50 mL sidearm flask equipped
with a stir bar was charged with UI3(THF)4 (930 mg, 1.03 mmol,
1.0 equiv) and THF (10 mL). To the resulting mixture was added
a solution of (PNP)K (6) (963 mg, 2.06 mmol, 2.0 equiv) in THF
(10 mL) at ambient temperature. The reaction mixture turned
immediately green, was stirred for 2 days, and filtered through a
Celite-padded coarse frit. The filtrate was collected, the volatiles
were removed under reduced pressure, and the resulting dark green
solid was washed with 10 mL of cold (-35 °C) hexane to give
(PNP)2UI (13) as a dark green solid (1.22 g, 1.00 mmol, 97%).
X-ray quality samples of 13 were obtained by recrystallization from
a red solid (1.62 g, 1.05 mmol, 98%). H NMR (C6D6, 298 K): δ
-26.40 (s, 6H), -19.99 (s, 6H), -9.66 (s, 2H), -8.53 (s, 6H),
-1.47 (s, 2H), 1.09 (s, 6H), 4.40 (s, 2H), 6.91 (s, 6H), 21.45 (s,
2H), 43.57 (s, 2H). 31P{1H} NMR (C6D6, 298 K): δ 1157.3 (br s).
Anal. Calcd for C52H80Cl6N2P4U2 (mol. wt. 1542.44): C, 40.40; H,
5.22; N, 1.81. Found: C, 40.39; H, 5.62; N, 2.0.
Characterization of (PNP)UCl3(THF) (9). Complex 9 is formed
in equilibrium with 8 (52/48 ratio at 25 °C in THF-d8) upon
dissolution of 8 in THF. Crystals of 9 were obtained by cooling a
concentrated solution of 8 in THF at -35 °C. Upon drying under
reduced pressure, crystals of 9 resort to pure 8. 1H NMR (THF-d8,
298 K): δ -23.62 (s, 6H), -19.20 (s, 6H), -9.35 (s, 2H), -6.77
(s, 6H), -6.16 (s, 2H), -3.21 (s, 2H), 0.16 (s, 6H), 5.54 (s, 6H),
19.03 (s, 2H), 38.04 (s, 2H). 31P{1H} NMR (THF-d8, 298 K): δ
1151.2 (s).
1
hexane at -35 °C. H NMR (THF-d8, 273 K): δ -39.92(s, 1H),
-32.96 (s, 1H), -31.53 (s, 3H), -28.45 (s, 6H), -25.21 (s, 3H),
-22.70 (s, 3H), -21.05 (s, 1H), -16.16 (s, 6H), -13.17 (s, 3H),
-9.27 (s, 1H), -6.34 (s, 3H), -5.94 (s, 3H), -5.66 (s, 3H), -2.30
(s, 3H), -1.73 (s, 6H), 3.04 (s, 3H), 5.63 (s, 1H), 6.29 (s, 1H),
6.47 (s, 1H), 8.11 (s, 6H), 10.13 (s, 3H), 10.88 (s, 1H), 11.11 (s,
1H), 11.34 (s, 3H), 13.73 (s, 1H), 14.07 (s, 1H), 15.67 (s, 1H),
16.85 (s, 1H), 18.88 (s, 1H) 26.85 (s, 1H), 30.38 (s, 3H), 34.14 (s,
1H), 38.75 (s, 1H), 56.41 (s, 1H), 68.12 (s, 1H), 96.80 (s, 1H).
31P{1H} NMR (THF-d8, 298 K): no signal observed in the range
(5000 ppm. Anal. Calcd for C52H80IN2P4U (mol. wt. 1222.03): C,
51.11; H, 6.60; N, 2.29. Found: C, 50.80; H, 6.84; N, 2.15.
Synthesis of (PNP)2UCl2 (13). A 50 mL sidearm flask equipped
with a stir bar was charged with UCl4 (391 mg, 1.03 mmol, 1.0
equiv) and THF (10 mL). To the resulting suspension was added
a solution of (PNP)K (6) (963 mg, 2.06 mmol, 2.0 equiv) in THF
(10 mL) at ambient temperature. The reaction mixture turned
immediately red, was stirred for 2 days, and then concentrated to
10 mL. Pentane (10 mL) was added, and the solution was filtered
through a Celite-padded coarse frit. The filtrate was collected, and
volatiles were removed under reduced pressure to give (PNP)2UCl2
(12) as a red solid (1.19 g, 1.02 mmol, 99%). X-ray quality samples
of 12 were obtained by recrystallization from a mixture of THF
Synthesis of (PNP)UCl3(TMPO)2 (10). A 50 mL sidearm flask
equipped with a stir bar was charged with UCl4 (812 mg, 2.14
mmol, 1.0 equiv) and THF (10 mL). To the resulting suspension
was added a solution of (PNP)K (6) (1.00 g, 2.14 mmol, 1.0 equiv)
in THF (10 mL) at ambient temperature. The reaction mixture
turned immediately red, was stirred for 30 min, and filtered through
a Celite-padded coarse frit. The filtrate was collected, trimeth-
ylphosphine oxide (TMPO, 394 mg, 4.28 mmol, 2.0 equiv) was
added as a solid, and the resulting solution was stirred for 10 min.
The volatiles were removed under reduced pressure to give
(PNP)UCl3(TMPO)2 (10) as a red solid (2.01 g, 2.10 mmol, 98%).
X-ray quality samples of 10 were obtained by recrystallization from
THF and TMS2O at -35 °C. 1H NMR (THF-d8, 323 K): δ -19.75
(s, 6H), -17.29 (s, 6H), -10.63 (s, 2H), -7.15 (s, 2H), -5.04 (s,
6H), -0.14 (s, 6H), 2.94 (s, 2H), 4.86 (s, 6H), 13.62 (s, 18H,
Me3P)O), 17.63 (s, 2H), 34.80 (s, 2H). 31P{1H} NMR (THF-d8,
323 K): δ 80.5 (s, Me3P)O), 1057.9 (br s, PNP). 1H NMR (THF-
d8, 223 K): δ -43.29 (s, 3H), -37.78 (s, 3H), -29.59 (s, 3H),
-25.99 (s, 3H), -24.27 (s, 1H), -19.57 (s, 1H), -17.87 (s, 1H),
-10.97 (s, 1H), -10.25 (s, 3H), -8.01 (s, 1H), -6.41 (s, 1H),
0.15 (s, 3H), 2.29 (s, 9H, Me3P)O), 2.85 (s, 3H), 6.88 (s, 3H),
7.13 (s, 3H), 17.15 (s, 1H), 19.94 (s, 3H), 26.68 (s, 1H), 33.12 (s,
9H, Me3P)O), 34.15 (s, 1H), 57.49 (s, 1H). 31P{1H} NMR (THF-
d8, 223 K): δ 85.3 (s, Me3P)O), 157.4 (s, Me3P)O), 1277.9 (br s,
PNP). Anal. Calcd for C32H58Cl3NO2P4U (mol. wt. 957.09): C,
40.16; H, 6.11; N, 1.46. Found: C, 40.21; H, 5.97; N, 1.31.
Synthesis of (PNP)UCl3(TPPO) (11). A 50 mL sidearm flask
equipped with a stir bar was charged with UCl4 (812 mg, 2.14
mmol, 1.0 equiv) and THF (10 mL). To the resulting suspension
was added a solution of (PNP)K (6) (1.00 g, 2.14 mmol, 1.0 equiv)
in THF (10 mL) at ambient temperature. The reaction mixture
turned immediately red, was stirred for 30 min, and filtered through
a Celite-padded coarse frit. The filtrate was collected, triph-
enylphosphine oxide (TPPO, 595 mg, 2.14 mmol, 1.0 equiv) was
added as a solid, and the resulting solution was stirred for 10 min.
The volatiles were removed under reduced pressure, and the
resulting red solid was washed with 10 mL of pentane to give
(PNP)UCl3(TPPO) (11) as a red solid (2.21 g, 2.10 mmol, 98%).
1H NMR (C6D6, 298 K): δ -23.72 (s, 6H), -20.42 (s, 6H), -13.04
(s, 2H), -8.17 (s, 2H), -6.59 (s, 6H), -1.57 (s, 6H), 4.79 (s, 2H),
6.58 (s, 6H), 9.47 (s, 3H), 10.07 (s, 6H), 21.30 (s, 2H), 23.82 (s,
6H), 44.89 (s, 2H). 31P{1H} NMR (C6D6, 298 K): δ 127.4 (s,
1
and TMS2O at -35 °C. H NMR (THF-d8, 323 K): δ -35.43 (s,
6H), -21.34 (s, 6H), -19.99 (s, 2H), -7.68 (s, 2H), -4.83 (s,
2H), -2.48 (s, 6H), 4.68 (s, 6H), 5.70 (s, 6H), 6.61 (s, 12H), 7.83
(s, 6H), 9.03 (s, 2H), 9.24 (s, 6H), 10.36 (s, 2H), 11.69 (s, 6H),
13.33 (s, 2H), 14.80 (s, 2H), 17.15 (s, 2H), 41.21 (s, 2H), 49.46 (s,
1
2H). 31P{1H} NMR (THF-d8, 323 K): δ 56.1 (s), 1469.3 (s). H
NMR (THF-d8, 223 K): δ -57.79 (s, 6H), -54.15 (s, 2H), -51.31
(s, 6H), -40.12 (s, 2H), -29.54 (s, 6H), -22.81 (s, 2H), -17.74
(s, 6H), -15.12 (s, 6H), -3.49 (s, 6H), -2.88 (s, 6H), 2.19 (s,
2H), 6.57 (s, 6H), 13.81 (s, 2H), 17.24 (s, 6H), 25.20 (s, 2H), 42.91
(s, 6H), 45.90 (s, 2H), 60.66 (s, 2H), 108.43 (s, 2H), 141.20 (s,
2H). 31P{1H} NMR (THF-d8, 223 K): δ 50.3 (s), 1401.1 (s). Anal.
Calcd for C52H80IN2P4U (mol. wt. 1166.03): C, 53.26; H, 6.92; N,
2.40. Found: C, 53.23; H, 7.04; N, 2.29.
Results and Discussion
Synthesis of Mono(PNP) Uranium(III)/(IV) Complexes.
A major reason for the success of the pentamethylcyclopen-
tadienyl ligand in organometallic uranium chemistry is due
to its ability to stabilize uranium in a wide range (III, IV, V,
and VI) of oxidation states.15 This entire body of chemistry
relies on the low-valent uranium(III) and uranium(IV) halide
complexes as starting materials shown in Chart 2, namely,
the mono(C5Me5) complexes 1-2 and the bis(C5Me5)
complexes 3-4. To develop a new and versatile platform
2118 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
Table 1. Crystallographic Experimental Parameters for 7, 9, 10·2 THF, 12·n-hexane, and 13·THF
7
9
10·2 THF
12·n-hexane
13·THF
formula
a (Å)
b (Å)
C44H66I2N3P2U
14.174(3)
14.912(3)
23.424(5)
90
102.151(2)
90
4839.9(18)
4
C38H64Cl3NO3P2U
11.9601(14)
14.2707(17)
24.803(3)
90
90
90
4233.3(9)
4
C40H74Cl3NO4P4U
15.3705(12)
20.0629(15)
15.7578(12)
90
93.0930(10)
90
4852.3(6)
4
C64H108IN2P4U
19.211(4)
23.405(5)
23.373(6)
90
90
90
12758(5)
8
C56H88Cl2N2OP4U
13.6641(18)
14.1260(18)
17.874(2)
91.607(1)
110.087(1)
113.515(1)
2916.0(7)
2
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
fw
1190.77
P21/n
120(1)
0.71073
1.634
4.726
989.22
P212121
120(1)
0.71073
1.552
4.134
0.0441
0.0978
1101.26
P21/n
120(1)
0.71073
1.507
3.679
1394.33
Pbca
120(1)
0.71073
1.452
3.168
0.0486
0.1070
1238.09
P1
120(1)
0.71073
1.410
3.022
0.0406
0.1090
j
space group
T (K)
λ (Å)
Dcalcd (g cm-3
)
µ (mm-1
)
R1 [I > 2σ(I)]
wR2 (all data)
0.0347
0.0862
0.0312
0.0736
Scheme 1. Synthesis of (PNP)K (6) and the Mono(PNP) Uranium(III) Complex 7
for uranium chemistry based on the PNP ligand framework,
similar mono(PNP) and bis(PNP) uranium(III) and ura-
nium(IV) halide complexes were prepared and characterized.
Entry into the low-valent uranium(III) mono(PNP) system
is illustrated in Scheme 1. Using K[N(SiMe3)2] in THF
solution, the ligand (PNP)H (5) was deprotonated to give
the corresponding potassium compound, (PNP)K (6) in 92%
isolated yield. Subsequent addition of 1 equiv of (PNP)K
(6) to a solution of UI3(THF)4 in THF resulted in a rapid
color change from deep blue to dark green accompanied by
the formation of KI as an off-white precipitate. The 31P{1H}
NMR spectrum of the crude mixture was uninformative as
no signal could be observed in the range (5000 ppm.43
1
Although the H NMR spectrum showed broad resonances
confirming the coordination of the PNP ligand to an
uranium(III) metal center, the structure of the complex could
not be unambiguously determined from these data. Crystal-
line material suitable for X-ray diffraction analysis was
obtained by carrying out the same reaction in the presence
of an excess of 4-tert-butylpyridine from which complex 7
could be isolated as an analytically pure solid in an excellent
97% yield. The 1H NMR spectrum of 7 in benzene-d6 reveals
that the coordination of two 4-tert-butylpyridine ligands is
maintained in solution. The signals for the PNP ligand
protons are all paramagnetically shifted and feature an
integration pattern consistent with overall Cs or C2 symmetry.
Complex 7 represents the first example of a PNP complex
in the actinide series; the molecular structure confirms the
coordination of the PNP ligand to the uranium(III) metal
Chart 2
Inorganic Chemistry, Vol. 48, No. 5, 2009 2119

Cantat et al.
of known uranium(III) amide complexes.49 This is likely a
consequence of the κ3-(P,N,P) coordination mode, which
strengthens the U-P interactions while delocalizing the
nitrogen lone pair over the two phenyl rings leading to a
longer U-Namide bond.50 It is important to also note that
unlike many ligand frameworks,51-59 the PNP ligand avoids
formation of an “ate complex” in 7 as well as in the other
uranium complexes presented herein.
The uranium(IV) mono(PNP) complexes are also easily
accessed. As shown in Scheme 2, addition of 1 equiv of
(PNP)K (6) to UCl4 in THF solution leads to the immediate
formation of complex 8, which was isolated in 98% after
1
removal of the KCl byproduct and drying. The H NMR
Figure 1. Molecular structure of complex 7 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl groups
on the PNP ligand have been omitted for clarity. Selected bond distances
(Å) and angles (deg): U(1)-N(1) 2.417(4), U(1)-N(2) 2.731(5), U(1)-N(3)
2.628(4), U(1)-P(1) 3.029(1), U(1)-P(2) 3.009(1), U(1)-I(1) 3.106(1),
U(1)-I(2) 3.116(1), P(1)-U(1)-N(1) 66.16(11), P(2)-U(1)-N(1) 66.43(11),
I(1)-U(1)-I(2) 175.19(1), N(2)-U(1)-N(3) 68.85(14).
spectrum for 8 also reflects Cs or C2 symmetry in solution
with paramagnetically shifted signals. The 31P{1H} NMR
spectrum consists of a broad singlet at δ 1157.3 ppm
indicating a κ3-(P,N,P) coordinated PNP ligand. Though no
crystals suitable for X-ray diffraction analysis could be
obtained for 8, it is likely that this compound exists as a
dimer, with chlorine bridging atoms, considering the coor-
dinative unsaturation of the (PNP)UCl3 unit.60 In fact,
attempts to crystallize 8 from a THF solution afforded 9,
which was subjected to scrutiny by X-ray diffraction analysis
(Figure 2). The U-P and U-N bond lengths measured in 9
are in agreement with known uranium(IV) phosphine61 and
amide complexes (Table 2).62 The THF ligand in 9 was found
to be labile and drying crystals of 9 under reduced pressure
allowed full recovery of complex 8. Moreover, dissolution
of 8 in THF-d8 yields an equilibrium mixture of the two
complexes 8:9 (48:52 ratio), which allowed for full charac-
center, the coordination sphere of the metal ion being
completed by two iodide and two 4-tert-butylpyridine ligands
(Figure 1).44 The PNP ligand coordinates to the uranium
metal center in a regular κ3-(P,N,P) mode, with the metal
center being roughly located in the plane of the three P, N,
and P donor atoms (δPUNP ) 167.9°). This is in agreement
with known complexes in transition metal and lanthanide
series featuring this PNP ligand.24,30,42,45,46 Because of the
large ionic radius of uranium (UIII 1.18 Å, UIV 1.05 Å),47
the metal center is unable to fit within the P-N-P chelate.
As a consequence, the phosphine donors are pulled back and
the P(1)-U(1)-P(2) angle (131.16(4)°) is far smaller than
the usual 165-170° typically seen for late transition metals.
At 3.02 Å, the average U-P bond distance lies at the low
end of the range for other structurally characterized ura-
nium(III) phosphine complexes,48 whereas the U-Namide
bond distance (2.417(4) Å) lies at the high end of the range
terization of 9 by H and 31P{1H} NMR spectroscopy.
1
(49) A Cambridge Structural Database search afforded twelve uranium(III)
amide structures with an average U-N bond length of 2.381 Å (range
0.470 Å). For representative examples of complexes possessing an
U-Namide linkage, see: (a) {U[N(SiMe3)2]4}{K(THF)6}, U-N )
2.431(6), 2.430(6), 2.430(7), 2.439(6) Å:Evans, W. J.; Lee, D. S.;
Rego, D. B.; Perotti, J. M.; Kozimor, S. A.; Moore, E. K.; Ziller,
J. W. J. Am. Chem. Soc. 2004, 126, 14574–14582. (b)
(C5Me5)2U[N(SiMe3)2], U-N ) 2.352(2) Å:Evans, W. J.; Nyce, G. W.;
Forrestal, K. J.; Ziller, J. W. Organometallics 2002, 21, 1050–1055.
(c) [(Me3Si)2N]4U2[µ-N(H)(2,4,6-Me3-C6H2)]2, U-N ) 2.354, 2.324
Å:Stewart, J. L.; Andersen, R. A. New J. Chem. 1995, 19, 587–595.
(50) The U-Namide bond distance of 2.489(9) Å reported for the uranium(III)
bis(arylamide) complex (TpMe2)2U[N(C6H5)2] is long:Antunes, M. A.;
Ferrence, G. M.; Domingos, A.; McDonald, R.; Burns, C. J.; Takats,
J.; Marques, N. Inorg. Chem. 2004, 43, 6640–6643.
(43) The 31P NMR spectra of uranium(III) phosphine complexes are usually
silent because of an extreme broadening of the signals associated with
a large paramagnetic shift. For example, see: Gradoz, P.; Ephritikhine,
M.; Lance, M.; Vigner, D.; Nierlich, M. J. Organomet. Chem. 1994,
481, 69–73.
(44) The coordination chemistry of another type of bis-phosphine-amide
ligand to uranium(IV) and thorium(IV) has been studied in the past;
namely, the bis[2-(diisopropylphosphino)ethyl]- and bis[2-(dieth-
ylphosphino)ethyl]amides. These ligands provided highly fluxional
structures (with κ1-(N), κ2-(P,N) and κ3-(P,N,P) coordination modes)
because of the flexible ethyl fragment in the P-N linkage. see: Coles,
S. J.; Danopoulos, A. A.; Edwards, P. G.; Hursthouse, M. B.; Read,
P. W. J. Chem. Soc., Dalton Trans. 1995, 3401–3408.
(51) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448–
9460.
(52) Blake, P. C.; Hey, E.; Lappert, M. F.; Atwood, J. L.; Zhang, H. M. J.
Organomet. Chem. 1988, 353, 307–314.
(45) Gatard, S.; Celenligil-Cetin, R.; Guo, C. Y.; Foxman, B. M.; Ozerov,
O. V. J. Am. Chem. Soc. 2006, 128, 2808–2809.
(46) Weng, W.; Guo, C. Y.; Celenligil-Cetin, R.; Foxman, B. M.; Ozerov,
O. V. Chem. Commun. 2006, 197–199.
(53) Blake, P. C.; Lappert, M. F.; Atwood, J. L.; Zhang, H. M. J. Chem.
Soc., Chem. Commun. 1988, 1436–1438.
(54) Edelman, M. A.; Lappert, M. F. Inorg. Chim. Acta 1987, 139, 185–
186.
(47) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751–767.
(48) A Cambridge Structural Database search afforded four uranium(III)
phosphine structures with an average U-P bond length of 3.051 Å
(range 0.241 Å). For specific references, see: (a) (C5Me5)2UH(DMPE),
U-P ) 3.211(8), 3.092(8) Å:Duttera, M. R.; Fagan, P. J.; Marks,
T. J.; Day, V. W. J. Am. Chem. Soc. 1982, 104, 865–867. (b) (Me-
C5H4)3U(PMe3), U-P ) 2.972(6) Å:Brennan, J. G.; Zalkin, A. Acta
Crystallogr. 1985, 41C, 1038–1040. (c) (C5H5)3U(Me2P-CH2-CH2-
PMe2)U(C5H5)3, U-P ) 3.020(6), 3.024(6) Å:Zalkin, A.; Brennan,
J. G.; Andersen, R. A. Acta Crystallogr. 1987, 43C, 1706–1708. (d)
(MeC5H4)3U[P(OCH2)3CEt], U-P ) 2.988(6) Å:Brennan, J. G.; Stults,
S. D.; Andersen, R. A.; Zalkin, A. Organometallics 1988, 7, 1329–
1334.
(55) Jantunen, K. C.; Haftbaradaran, F.; Katz, M. J.; Batchelor, R. J.;
Schatte, G.; Leznoff, D. B. Dalton Trans. 2005, 3083–3091.
(56) Korobkov, I.; Gambarotta, S.; Yap, G. P. A.; Thompson, L.; Hay,
P. J. Organometallics 2001, 20, 5440–5445.
(57) Schnabel, R. C.; Scott, B. L.; Smith, W. H.; Burns, C. J. J. Organomet.
Chem. 1999, 591, 14–23.
(58) Secaur, C. A.; Day, V. W.; Ernst, R. D.; Kennelly, W. J.; Marks,
T. J. J. Am. Chem. Soc. 1976, 98, 3713–3715.
(59) Villiers, C.; Thuery, P.; Ephritikhine, M. Chem. Commun. 2007, 2832–
2834.
(60) The absence of any coordinated solvent or the formation of any “ate
complex” was verified by 1H NMR spectroscopy and C,H,N elemental
analyses.
2120 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
Scheme 2. Synthesis of the Mono(PNP) Uranium(IV) Complexes 8-11
Table 2. Selected Metrical Parameters for the (PNP)-Uranium Complexes 7, 9, 10, 12, and 13
complex
U-N (Å)
U-P (Å)
U-X (Å)
torsion angle δ(PUPN)
7 (X ) I)
2.417(4)
2.343(7)
2.411(3)
3.009(1)
2.962(2)
2.989(1)
3.019(1)
3.041(2)
3.116(1)
3.106(1)
2.597(2)
2.726(1)
3.116(1)
2.610(2)
2.687(1)
167.9°
160.5°
155.9°
9 (X ) Cl)
10 (X ) Cl)
12 (X ) I)
13 (X ) Cl)
2.642(2)
2.745(1)
3.109(1)
2.376(5)
2.279(3)
2.439(5)
2.290(3)
3.063(2)
3.076(2)
3.197(2)
3.316(2)
176.2°
78.5°
128.3°
89.3°
2.996(1)
3.004(1)
2.576(1)
2.590(1)
Phosphine oxides proved to be excellent reagents for
cleaving the dimer complex 8 to form well-defined mono(P-
NP) uranium(IV) complexes. Indeed, addition of 2 equiv of
trimethylphosphine oxide (TMPO) yields the mono(PNP)
complex 10, in which the metal ion is stabilized by two
TMPO ligands (Scheme 2). Complex 10 was characterized
using 1H and 31P{1H} NMR spectroscopy, elemental analysis,
and solid-state X-ray crystallography. As seen in Table 2,
the metrical parameters for 10 hardly differ from those of 9.
The molecular structure of complex 10 is presented in Figure
3and reveals that the TMPO ligands are located in different
coordination sites that can be described as pseudoapical
(defined by O(2)) and pseudoaxial (defined by O(1)) posi-
1
tions, with respect to the PNP ligand. The H and 31P{1H}
spectra of 10 in THF-d8 at ambient temperature show
unusually broad resonances, which suggests fluxional be-
havior on the NMR time scale. Variable temperature NMR
studies were performed to determine if this behavior was
due to dissociation of a TMPO ligand or the fast exchange
of the TMPO ligands between the two different coordination
sites. At -50 °C, the 31P{1H} NMR spectrum of 10 shows
three singlets at δ 1277.9, 157.4, and 85.3 ppm. The 1H NMR
spectrum directly derives from that of complex 9 but shows
two additional signals at δ 33.12 and 2.29 ppm for the methyl
groups of the TMPO ligands. In total, these data point to a
rigid structure as shown by X-ray crystallography where the
PNP ligand adopts a κ3-(P,N,P) coordination mode, and the
two TMPO ligands are in different coordination sites. At
high temperature (+50 °C), the resonances for the TMPO
Figure 2. Molecular structure of complex 9 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl groups
have been omitted for clarity. Selected bond distances (Å) and angles (deg):
U(1)-N(1) 2.343(7), U(1)-O(1) 2.485(6), U(1)-P(1) 2.962(2), U(1)-P(2)
3.041(2), U(1)-Cl(1) 2.610(2), U(1)-Cl(2) 2.597(2), U(1)-Cl(3) 2.642(2),
P(1)-U(1)-N(1) 66.99(17), P(2)-U(1)-N(1) 66.33(17), Cl(1)-U(1)-Cl(2)
163.21(7), O(1)-U(1)-Cl(3) 78.22(16).
Inorganic Chemistry, Vol. 48, No. 5, 2009 2121

Cantat et al.
able to coordinate to the metal center of the (PNP)UCl3
fragment (see 10), this indicates that a more sterically
congested environment is provided by the PNP ligand.
It is interesting to note that the syntheses of the
mono(C5Me5) complexes 1 and 2 were originally developed
to access sterically and electronically unsaturated uranium
centers with enhanced reactivities compared to the
bis(C5Me5) systems. However, this strategy did not meet with
the expected success as the electronically unsaturated metal
center coupled with the presence of extra available coordina-
tion sites results in unwanted reactions with solvent mol-
ecules.63 In fact, most of the mono(C5Me5) complexes
reported to date were obtained from reactions of bis(C5Me5)
precursors. From this perspective, the PNP ligand set shows
great promise for expanding the chemistry of uranium
supported by a single anionic ligand.
Figure 3. Molecular structure of complex 10 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl groups
on the PNP ligand have been omitted for clarity. Selected bond distances
(Å) and angles (deg): U(1)-N(1) 2.411(2), U(1)-O(1) 2.341(2), U(1)-O(2)
2.343(2), U(1)-P(1) 2.989(1), U(1)-P(2) 3.116(1), U(1)-Cl(1) 2.687(1),
U(1)-Cl(2) 2.745(1), U(1)-Cl(3) 2.726(1), P(1)-U(1)-N(1) 64.78(7),
P(2)-U(1)-N(1) 62.61(7), Cl(1)-U(1)-Cl(2) 140.54(3), Cl(1)-U(1)-Cl(3)
97.59(3), Cl(2)-U(1)-Cl(3) 98.47(3), O(1)-U(1)-O(2) 78.22(16).
Synthesis of Bis(PNP) Uranium(III)/(IV) Complexes.
Considering the extensive body of work developed with the
(C5Me5)2U framework using (C5Me5)2UI(THF) (3) and
(C5Me5)2UCl2 (4), it was of interest to access the isostructural
complexes with the PNP ligand. Despite the steric hindrance
of the PNP ligand, reaction of 2 equiv of (PNP)K (6) with
UI3(THF)4 in THF solution for 2 days at room temperature
gave the bis(PNP) uranium iodide complex 12, which was
isolated in 97% isolated yield after workup (Scheme 3). This
complex represents the first example of the coordination of
ligands collapse into a singlet at δ 80.5 ppm, whereas the
signal for the two coordinated phosphines are found at δ
1057.9 ppm. The 1H NMR spectrum shows a unique
resonance δ 13.62 ppm for the 18 protons of the TMPO
ligands. No free TMPO is observed in either the 31P{1H} or
1H spectra ruling out any dissociation processes.
The addition of excess triphenylphosphine oxide (TPPO)
to a THF solution of 8 similarly yields the monomeric
uranium(IV) complex 11 (in 98% isolated yield), in which
only one phosphine oxide is coordinated to the metal center.
No exchange with free TPPO is observed by NMR.
Complexes 9 and 11 are the mono(PNP) analogues of the
mono(C5Me5) complexes 2a and 2b, respectively. However,
two ligands (THF or TPPO) are required to stabilize the
(C5Me5)UCl3 moiety in 2a and 2b, but only one can bind to
the metal center in 9 and 11. Since two neutral donors are
1
two PNP ligands on the same metal center. The H NMR
spectrum of 12 reveals a dynamic process at room temper-
ature; however, at 0 °C it shows the coordination of two
PNP ligands with an overall C1 symmetry, which is lower
than observed for complexes 7-11. Given that even at -50
°C the κ3-(P,N,P)-coordinated PNP ligand presents a local
Cs or C2 symmetry (see 10), we propose that the reduction
(61) A Cambridge Structural Database search afforded five uranium(IV)
phosphine structures with an average U-P bond length of 3.063 Å
(range 0.205 Å). For specific references, see: (a) U(OC6H5)4(DMPE)2,
U-P ) 3.102(6), 3.116(6), 3.105(6), 3.097(6) Å:Edwards, P. G.;
Andersen, R. A.; Zalkin, A. J. Am. Chem. Soc. 1981, 103, 7792–
7794. (b) U(CH3)(CH2C6H5)3(DMPE), U-P ) 3.020(5), 3.010(2)
Å:Edwards, P. G.; Andersen, R. A.; Zalkin, A. Organometallics 1984,
3, 293–298. (c) U(BH3Me)4(DMPE), U-P ) 3.029(2), 3.017(2)
Å:Brennan, J.; Shinomoto, R.; Zalkin, A.; Edelstein, N. Inorg. Chem.
1984, 23, 4143–4146. (d) U{κ2-(P,P)-[P(CH2CH2PMe2)2]}4, U-P )
3.109(2), 2.988(2), 2.976(2), 2.980(2) Å:Edwards, P. G.; Parry, J. S.;
Read, P. W. Organometallics 1995, 14, 3649–3658. (e) UCl2{κ2-(P,N)-
[N(CH2CH2PEt2)2]}2, U-P ) 3.103(1), 3.155(1), 3.115(1), 3.181(1),
3.085(1), 3.097(1) Å:Coles, S. J.; Danopoulos, A. A.; Edwards, P. G.;
Hursthouse, M. B.; Read, P. W. J. Chem. Soc., Dalton Trans. 1995,
3401–3408.
Scheme 3. Synthesis of Bis(PNP) Uranium(III) Complex 12 and
Uranium(IV) Complex 13
(62) A Cambridge Structural Database search afforded 46 uranium(IV)
amide structures with an average U-N bond length of 2.245 Å (range
0.351 Å). For representative examples of complexes containing a
U-Namide linkage, see: (a) Cp3U[N(C6H5)2], U-N ) 2.29(1) Å:Cramer,
R. E.; Engelhardt, U.; Higa, K. T.; Gilje, J. W. Organometallics 1987,
6, 41–45. (b) (C5Me5)2U[NH(2,6-Me2-C6H3)]2, U-N ) 2.267(6)
Å:Straub, T.; Frank, W.; Reiss, G. J.; Eisen, M. S. J. Chem. Soc.,
Dalton
Trans.
1996,
2541–2546.
(c)
[Me2Si(h5-
C5Me4)(tBuN)]U(NMe2)2, U-N ) 2.278(4), 2.207(4), 2.212(4) Å:S-
tubbert, B. D.; Stern, C. L.; Marks, T. J. Organometallics 2003, 22,
4836–4838.
(63) Larch, C. P.; Cloke, F. G. N.; Hitchcock, P. B. Chem. Commun. 2008,
82–84.
2122 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
Figure 5. Molecular structure of complex 13 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl groups
have been omitted for clarity. Selected bond distances (Å) and angles (deg):
U(1)-N(1) 2.290(3), U(1)-N(2) 2.279(3), U(1)-P(1) 2.996(1), U(1)-P(2)
3.004(1), U(1)-Cl(1) 2.590(1), U(1)-Cl(2) 2.576(1), P(1)-U(1)-N(1)
64.20(9),P(2)-U(1)-N(2)64.58(9),Cl(1)-U(1)-Cl(2)101.37(4),Cl(1)-U(1)-
P(1) 78.89(3), Cl(2)-U(1)-P(2) 81.04(3).
Figure 4. Molecular structure of complex 12 with thermal ellipsoids
projected at the 50% probability level. Hydrogen atoms and methyl groups
have been omitted for clarity. Selected bond distances (Å) and angles (deg):
U(1)-N(1) 2.439(5), U(1)-N(2) 2.376(5), U(1)-P(1) 3.076(2), U(1)-P(2)
3.063(2), U(1)-P(3) 3.316(2), U(1)-P(4) 3.197(2), U(1)-I(1) 3.109(1),
P(1)-U(1)-N(1) 65.22(14), P(2)-U(1)-N(1) 63.43(14), P(3)-U(1)-N(2)
60.29(13), P(4)-U(1)-N(2) 61.86(13), I(1)-U(1)-P(1) 87.40(4), I(1)-U(1)-
P(4) 76.87(4).
yields a bis(PNP) complex (13) in excellent yield after
workup. The new uranium(IV) dichloride complex was
in symmetry observed for 12 is due to the decoordination of
at least one arm of the phosphine donor in solution.
This premise is supported by the solid-state molecular
structure of complex 12, which is given in Figure 4. Although
the two PNP ligands coordinate to the uranium(III) metal
center in a κ3-(P,N,P) fashion, they present very different
features. The first PNP ligand (identified by P(1), N(1) and
P(2)) is identical to the PNP ligand in the mono(PNP)
complex 7 and features the same U-P and U-N bond
distances. Moreover, as observed in complexes 7-11, the
uranium center lies in the (P(1), N(1), P(2)) plane with δPUNP
) 171.9° and d(U-PNP) ) 0.20 Å. In contrast, the second
PNP ligand (identified by P(3), N(2) and P(4)) is highly
twisted with the metal being out of the (P,N,P) plane of the
ligand, with δPUNP ) 128.3° and d(U-PNP) ) 2.00 Å. This
situation is not observed in complexes 7-11 nor in the
known transition metal and lanthanide PNP complexes and
demonstrates the ability of the PNP ligand to adopt multiple
coordination geometries.64 Interestingly, this geometry alters
the U-P and U-N bond distances. The U-N(2) bond length
of 2.376(5) Å is considerably shorter than that for U-N(1)
(2.439(5) Å). In addition, U-P(3) ) 3.316(2) Å and U-P(4)
) 3.197(2) Å are about 6% longer than the U-phosphine
bond distances observed for U-P(1) (3.076(2) Å) and
U-P(2) (3.063(2) Å). Clearly, these data reflect increased
steric hindrance around the metal center, which weakens the
U-phosphine interactions (where the steric crowding is
the greatest) and reinforces the U-amide bond for one of
the PNP ligands. This is in agreement with the absence of
any coordinated solvent in the coordination sphere of the
metal.
1
characterized by a combination of H and 31P{1H} NMR
spectroscopy and X-ray diffraction. The solid state structure
of 13 is shown in Figure 5 and reveals a new κ2-(P,N)
coordination motif of PNP binding to the uranium(IV) center.
The U-N and U-P bond distances (U(1)-N(1) 2.290(3),
U(1)-N(2) 2.279(3), U(1)-P(1) 2.996(1), U(1)-P(2)
3.004(1)) are in agreement with those found in the UIV
complex 9 (Table 2). To the best of our knowledge, κ2-(P,N)
coordination of a PNP ligand has not been observed to date
for any transition metal or lanthanide PNP complex. Contrary
to previous assumptions, this κ2-(P,N) coordination mode
demonstrates the ability of the PNP ligand to act as a
hemilabile ligand and further confirms our proposed analysis
of the solution structure of complex 12; that being, decoor-
dination of a PNP phosphine arm.
To determine whether this coordination mode is main-
1
tained in solution, variable temperature H and 31P{1H}
experiments were performed on a THF-d8 solution of
complex 13. At -50 °C, the 31P{1H} spectrum consists of
two singlets at δ 1401.1 and 50.3 ppm revealing the presence
of uncoordinated phosphine donors.65 The integration pattern
for the 1H signals is consistent with a complex of overall Cs
or C2 symmetry, pointing to a structure equivalent to that
observed in the solid state (Figure 5) with two κ2-(P,N)-
coordinated PNP ligands. At higher temperature (+50 °C),
the same structure could be derived from the 1H and 31P{1H}
spectra.
Structural Evaluation of the Mono and Bis(PNP)
Uranium Complexes. Comparison of the X-ray structures
of the mono and bis(PNP) complexes (7, 9, 10, 12, and 13)
reveals that the U-phosphine bond distances are not notice-
ably influenced by the oxidation state of the metal center or
the number of coordinated PNP units. The same finding
As depicted in Scheme 3, the reaction between UCl4 and
2 equiv of (PNP)K (6) at room temperature for 2 days also
(64) A similar situation (δPUNP ) 139.8°) was observed for the thalium
complex (PNP)Tl, see: DeMott, J. C.; Basuli, F.; Kilgore, U. J.;
Foxman, B. M.; Huffman, J. C.; Ozerov, O. V.; Mindiola, D. J. Inorg.
Chem. 2007, 46, 6271–6276.
(65) The large difference in chemical shifts between these two signals
precluded the determination of their relative integration by 31P{1H}
NMR.
Inorganic Chemistry, Vol. 48, No. 5, 2009 2123

Cantat et al.
Scheme 4. Geometric Differences between the C5Me5 and PNP Ligands upon Coordination to Uranium
prevails for the U-Namide bond lengths, which are more
sensitive to the total coordination number of the complex as
seen for the mono(PNP) uranium (IV) complexes 9 (U-N(1)
) 2.343(7) Å) and 10 (U-N(1) ) 2.411(3) Å), which have
coordination numbers of 7 and 8, respectively. Clearly, the
oxidation state of the metal center cannot be deduced from
these metric parameters.
density at the metal center, the magnitude of the crystal-
field induced splitting of the 5f electronic manifold derived
from the 5f2 (U4+) and 5f3 (U3+) valence electronic configu-
rations, and the extent of covalency in the metal-ligand
Further, the electronic influence of replacing the C5Me5
ligands in 1-4 with PNP to yield 7, 9, 10, 12, and 13 cannot
be simply tracked by comparing the U-Cl or U-I bond
distances. Therefore, the structural data do not allow a
detailed comparison between the C5Me5 and PNP ligand or
their impact on the electronic structure of the corresponding
uranium complexes, which were investigated using electronic
absorption spectroscopy and electrochemistry (vide infra).
The κ3-(P,N,P) coordinated PNP ligand was found to be
more bulky than the η5-C5Me5. Scheme 4 shows that the
C5Me5 and PNP coordination motifs do present interesting
geometric discrepancies that should lead to different chemical
behaviors. The C5Me5 ligand exhibits a single η5-coordination
mode upon coordination to actinides, and the related
bis(C5Me5) complexes generally adopt a bent metallocene
structure, with the exception of a few recent examples of
uranium linear metallocene complexes.66,67 In contrast, the
present work demonstrates that the PNP framework is more
versatile and able to adopt two different coordination modes:
κ2-(P,N) and κ3-(P,N,P) with pseudo-meridional or pseudo-
facial geometries.
Electrochemistry and Electronic Spectroscopy. To as-
sess the electronic structure in this new class of low- and
midvalent uranium complexes, we have obtained voltam-
metric data for complexes 3, 12 and 13, and UV-visible-
near-infrared electronic absorption spectral data for com-
plexes 3, 7, 10-13, and compared these data to previously
published results for analogous C5Me5 complexes.68 Previous
work, principally on the bis(C5Me5) systems, has demon-
strated that metal-centered redox potentials, electronic transi-
tion energies, and electronic transition intensities all provide
insight into the ligand-derived variations in the electron
Figure 6. Cyclic voltammograms for (PNP)2UI (12) and (PNP)2UCl2 (13)
in ∼0.1 M [Bu4N][B(1,3-(CF3)2-C6H3)4]/THF solution at a Pt work-
ing electrode. The vertical arrows indicate the rest potentials of each
solution.
Table 3. Comparison of UIV/UIII Redox Potentials for Homologous
PNP vs C5Me5 Complexesa
E1/2 [UIV/UIII] |∆E1/2(1)|b
|∆E1/2(2)|c
(mV)
complex
(V)
(mV)
(PNP)2UCl2 (13)
-2.19
340
(C5Me5)2UCl2 (4)d
(PNP)2UI (12)
-1.85
-1.47
-1.12
720 (PNP),730 (C5Me5)
350
(C5Me5)2UI(THF) (3)
a Potentials vs [(C5H5)2Fe]+/0 in ∼0.1 M [Bu4N][B(C6F5)4]/THF. b ∆E1/2(1)
) E1/2(PNP complex) - E1/2(C5Me5 complex). c ∆E1/2(2) ) E1/2(UIV complex)
- E1/2(UIII complex). d Data from ref 68.
2124 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
Figure 7. UV-visible-near IR electronic absorption spectral data for toluene solutions of (C5Me5)2UI(THF) (3), (PNP)K (6), (PNP)UI2(4-tBu-py)2 (7), and
(PNP)2UI (12). The left ordinate scale pertains to the UV-visible region (blue, purple, and black spectra) and the right ordinate scale to the near-IR region
(red and green spectra).
bonding. By comparison of data for structural homologues
in which the C5Me5 framework is replaced by a PNP
framework, an evaluation of changes in electronic structural
properties derived from the framework change should be
possible, and correlations in molecular/electronic structure
should enable greater insight and predictive capability
regarding trends in complex stability and reactivity patterns.
In general, we find that the electrochemical and spectroscopic
behavior for analogous C5Me5 and PNP complexes of UIII
and UIV is quite similar, albeit with some important perturba-
tions described in detail below.
Cyclic and square-wave voltammetric data were obtained
for bis(PNP) complexes of uranium(III) (12) and uranium(IV)
(13) in ∼0.1 M [Bu4N][B(1,3-(CF3)2-C6H3)4]/THF solution
at a Pt working electrode. (All potentials are referenced to
the [(C5H5)2Fe]+/0 couple.) Typical cyclic voltammograms
are illustrated in Figure 6, and the potential data determined
from the peaks in the square-wave voltammograms (not
shown) are summarized and compared to similar data for
the C5Me5 systems in Table 3. An attempt was also made to
collect voltammetric data for mono(PNP) complexes 7 and
11, but both complexes degraded completely as evidenced
by loss of solution color within a few minutes in the solvent/
supporting electrolyte solution. The defining characteristic
in the voltammetry of complexes 12 and 13 is a chemically
reversible wave attributed in both cases to the UIV/UIII redox
transformation. For both systems there is also evidence for
a chemical process coupled to this metal-based redox
transformation as evidenced by the new cathodic peak in
the voltammograms of 12 at ∼-1.9 V and the new anodic
peak in the voltammograms of 13 at ∼-1.6 V. This extra
feature is only observed in conjunction with the metal-based
couple as illustrated in the green traces in Figure 6, and the
current magnitude of the extra feature is diminished at faster
scan rates as indicated in the red traces; both observations
are consistent with the assignment of this extra feature to a
coupled chemical process. Given the hemilability of the PNP
ligand noted above, it is plausible that this new product
formed in minor amounts in conjunction with the redox
transformation is a complex of different PNP denticity that
has slightly different redox energetics. As gleaned from the
1H and 31P NMR studies on complexes 12 and 13, it is indeed
likely that upon reduction of 13 the coordination mode of
one PNP ligand changes from κ2-(P,N) to κ3-(P,N,P) to
stabilize the larger uranium(III) center (a decrease of the
denticity of one PNP ligand is conversely expected upon
oxidation of 12). Interestingly, this coordination of a soft
donor (P) leads to an ∼150-300 mV shift of the UIV/UIII
couple to more positive potentials, therefore stabilizing the
uranium(III) oxidation state. The only other voltammetric
features (not illustrated in the data in Figure 6) found in these
studies are two irreversible oxidation waves in the data for
13 at ∼-0.1 and 0.2 V. These are ascribed to oxidations of
the coordinated chloride ions. The electrochemical behavior
for the bis(PNP) complexes 12 and 13 is analogous to that
for the bis(C5Me5) congeners 3 and 4 in that the principal
redox process is a reversible UIV/UIII couple (Table 3), and
no other metal-based redox transformation (e.g., UV/UIV) are
observed.
Variations in these metal-based redox potentials for the
same UIV/UIII oxidation state transformation primarily reflect
the differing degrees of electron density at the metal center
Inorganic Chemistry, Vol. 48, No. 5, 2009 2125

Cantat et al.
Figure 8. UV-visible-near IR electronic absorption spectral data for toluene solutions of (C5Me5)2UCl2 (4), (PNP)UCl3(TMPO)2 (10), (PNP)UCl3(TPPO)
(11), (PNP)2UCl2 (13), and (C5Me5)U(CH2C6H5)3 (14). The left ordinate scale pertains to the UV-visible region (blue, purple, and black spectra) and the
right ordinate scale to the near-IR region (red and green spectra). Data for 14 are reproduced from ref 68.
as a result of changes in the electron-donating (withdrawing)
ability of the ligand(s) and the number of such ligands. The
more substantial energetic effect for both bis(PNP) and
bis(C5Me5) systems results from the replacement of a single
iodide ligand by two chloride ligands. In both systems, this
structural change shifts the UIV/UIII couple by ∼700 mV
(∆E1/2(2), Table 3) to more negative potentials as a result of
stabilization of the higher oxidation state (or, conversely,
destabilization of the lower oxidation state).
The influence of the bis(PNP) framework relative to the
bis(C5Me5) framework can be seen in the ∆E1/2(1) values in
Table 3. The energetic consequence of this structural change
for both the uranium(III) iodide and the uranium(IV)
dichloride is a ∼350 mV shift of the UIV/UIII couple to more
negative potentials on substitution of bis(C5Me5) by bis(P-
NP). This leads to the somewhat counterintuitive conclusion
that the PNP ligand induces greater electron density at the
metal center than does the C5Me5 ligand. The overall
comparability in the magnitude of these ∆E1/2(1,2) values
points to the importance of electrostatic effects in determining
the redox energetics in these systems dominated by σ-donor
and/or dative metal-ligand interactions.
The electronic absorption spectra in the UV-visible region
(E > ∼15,000 cm-1) for the PNP complexes of uranium(III)
(Figure 7) and uranium(IV) (Figure 8), as well as that of the
simple ligand salt, (PNP)K (6) (Figure 7, bottom panel), are
all quite similar and appear to be dominated by PNP ligand-
based transitions that are shifted to higher energy in the
uranium complexes than in the simple ligand anion spectrum.
The assignment of these broad, high intensity transitions in
the uranium complexes to PNP ligand-based processes is
supported by the similarity in the data for the free ligand
anion, and by comparison to the UV-visible data for most
of the C5Me5 congeners for which the UV-visible transition
intensities are diminished 5-10 times relative to that in the
PNP systems suggesting very different origins. The notable
exception to this intensity difference is found for
(C5Me5)U(CH2C6H5)3 (14), for which the intensities in the
UV-visible bands are comparable to those in this spectral
region for the PNP complexes. In contrast, the electronic
2126 Inorganic Chemistry, Vol. 48, No. 5, 2009

TriWalent and TetraWalent Uranium Halide Complexes
spectral data in the near IR (NIR) spectral region for the
PNP systems show a good qualitative correlation with the
data for their C5Me5 congeners. This region is dominated
by f-f transitions derived from the 5f3 (U3+) and 5f2 (U4+)
valence electronic configurations. The density of ligand-field
states from which these NIR transitions derive is very large
for both valence electronic configurations,69 and specific
assignments are not possible. There are, however, some
noteworthy trends in both energies and intensities across the
comparative series illustrated in Figures 7 and 8.
The energies and relative intensities of the f-f transitions
for the two uranium(III) PNP complexes (Figure 7, com-
plexes 7 and 12) are quite similar. The NIR spectrum for
the (C5Me5)2UIII congener (3) is similar to that of the PNP
analogues, although there does appear to be a compression
of the spectral bands in the region from ∼10,000-5,000
cm-1 into a narrower energy window for 3 suggesting that
the ligand-field splitting is somewhat less in the C5Me5
complex than in the PNP complexes (as would be expected
based on the electrochemical result demonstrating that PNP
donates greater electron density than C5Me5). The intensities
in the f-f bands for all uranium(III) complexes are also
comparable, and do not depend to a large degree on the
structural framework (PNP vs C5Me5). Similar energetic and
intensity arguments can be made regarding the comparisons
in the NIR region for the uranium(IV) systems (Figure 8).
In particular, although there are clearly differences in the
splitting of the f-f bands for the different ligand sets, there
are no wholesale variations in the intensities for the PNP
versus C5Me5 systems.
(PNP)UCl3(THF) (9), (PNP)UCl3(TMPO)2 (10), and
(PNP)UCl3(TPPO) (11). For the first time, bis(PNP) com-
plexes have been accessed giving (PNP)2UI (12) and
(PNP)2UCl2 (13). Interestingly, these complexes reveal that
new coordination modes are available for the PNP ligand,
with a denticity of two (κ2-(P,N)) or three (κ3-(P,N,P)) and
two different geometries (pseudomeridional vs pseudofacial)
for the κ3-(P,N,P) coordination mode.
The influence of replacing the C5Me5 ligands in 1-4 with
PNP (7, 9, 11-13) on the geometry and electronic structure
of the complexes was investigated using X-ray analysis,
electronic absorption spectroscopy, and electrochemistry. The
PNP ligand was shown to provide a more sterically congested
environment in complexes 7-13. Both the bis(C5Me5) and
bis(PNP) systems present a chemically reversible wave
corresponding to the UIV/UIII redox transformation (for 3, 4,
12-13). However, a ∼350 mV shift of this couple to more
negative potentials was observed on substitution of the
bis(C5Me5) by the bis(PNP) framework, therefore pointing
to greater electronic density at the metal center in the PNP
complexes. The UV-visible regions of the electronic spectra
for the mono(PNP) and bis(PNP) complexes are dominated
by PNP ligand-based transitions. The near IR region in
complexes 1-4 and 7, 9, 11-13 is dominated by f-f
transitions, and although both ligand sets exhibit similar
intensities, a somewhat greater ligand-field splitting was
noticed for the PNP system, consistent with its higher
electron donating ability. Studies aimed at exploiting the
novel electronic and geometric features provided by these
PNP uranium halide systems are currently underway to
support novel structures and promote new reactivity patterns
for the actinide series.
Conclusions
The first actinide complexes of the bis[2-(diisopropylphos-
phino)-4-methylphenyl]amido ligand (PNP) have been pre-
pared and have demonstrated new coordination modes for
this ligand set. The PNP scaffold is able to stabilize both
uranium(IV) and uranium(III) halide complexes, namely
(PNP)UI2(4-tert-butylpyridine)2 (7), [(PNP)UCl3]2 (8),
Acknowledgment. For financial support of this work, we
acknowledge LANL (Director’s PD Fellowship to T.C.), and
the Division of Chemical Sciences, Office of Basic Energy
Sciences, Heavy Element Chemistry program. We are also
thankful to Prof. Oleg V. Ozerov (Brandeis University) for
providing the initial samples of (PNP)H (5) used in this work.
This work was carried out under the auspices of the National
Nuclear Security Administration of the U.S. Department of
Energy at Los Alamos National Laboratory under Contract
DE-AC5206NA25396.
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Supporting Information Available: Crystallographic data in
CIF file format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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