Reduced uranium complexes: Synthetic and DFT study of the role of π ligation in the stabilization of uranium species in a formal low-valent state

Article abstract of DOI:10.1021/ja9002525

Reaction of UCl₄(THF)₄ with 1,3-[2,5-(i-Pr) ₂PhNC(=CH₂)]₂C₆H₄Li ₂ produced a complex formulated as [{1,3-[2,5-(i-Pr) ₂PhNC(=CH₂)]₂C

Full text of DOI:10.1021/ja9002525

Published on Web 07/09/2009
Reduced Uranium Complexes: Synthetic and DFT Study of
the Role of π Ligation in the Stabilization of Uranium Species
in a Formal Low-Valent State
Ilia Korobkov, Serge Gorelsky, and Sandro Gambarotta*
Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Received January 15, 2009; E-mail: Sandro.Gambarotta@uottawa.ca
Abstract: Reaction of UCl4(THF)4 with 1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4Li2 produced a complex formulated
as [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}UCl3][Li(THF)4] (1) that exhibits a nonagostic interaction between one
of the carbon atoms of the central phenyl ring and the U metal center. This interaction leads to significant
weakening of the corresponding C-H bond, thereby facilitating proton removal in consecutive transformations.
Attempts to form trivalent uranium derivatives were carried out by reacting the same ligand dianion with in
situ-prepared “UCl3”. The reaction indeed afforded a trivalent species formulated as {1,3-[2,5-(i-Pr)2-
PhNC(dCH2)]2C6H4}U(µ-Cl)3[Li(THF)2]2 (2). The configuration of the ligand system in this complex is similar to
that in 1, with the same type of arrangement of the central phenyl ring. Further reduction chemistry with a
variety of reagents and conditions was examined. Reaction of 1 with 1 equiv of lithium naphthalenide at 0 °C
did not afford 2 but instead gave a closely related U(III) complex formulated as {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}-
U(THF)(µ-Cl)2[Li(Et2O)2] (3). Both of the trivalent complexes 2 and 3 reacted thermally in boiling THF, undergoing
oxidation of the metal center to afford a new tetravalent compound {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(THF)-
(µ-Cl)2[Li(THF)2] (4) in which the oxidation of the trivalent center occurred at the expense of the central phenyl
ring C-H bond. Reaction of 1 with 3 equiv of lithium naphthalenide at room temperature afforded {{1,3-[2,5-
(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-Cl)(µ-[O(CH2)3CH2])[Li(DME)]}[Li(DME)3] (5). In this species, the tetravalent metal
center forms a six-membered metallacycle ring with a moiety arising from THF ring opening. Reaction in DME
afforded reductive cleavage of the solvent accompanied by reoxidation of U to the tetravalent state. Reduction
of 1 in DME with 2 equiv of potassium naphthalenide at room temperature gave a mixture of two compounds
having very similar structures. The two different species [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}UCl-
(OCH3)][Li(DME)3] (6a) and [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}UCl2][Li(DME)3] (6b) cocrystallized in a ratio
very close to 1:1 within the same unit cell. The methoxide group was generated from cleavage of the DME
solvent. We also attempted the reduction of 1 with a different reducing agent such as NaH in DME. After a slow
reaction, a new species formulated as {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-OCH3)3(µ,η6-Na)[η3-Na(DME)]
(7) was isolated in significant yield. Once again, the crystal structure revealed the presence of several methoxy
groups coordinated to the U center in addition to the metalation of the ligand phenyl ring. To minimize solvent
cleavage, reduction of 1 was also carried out at low temperature (-35 °C) and with a larger amount (4 equiv)
of lithium naphthalenide. After suitable workup, the new species {[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-
[2,5-(i-Pr)2PhNdC(CH3)]2C6H4}][Li(DME)(THF)]}·Et2O (8) was isolated in significant yield. Even in this case,
the uranium atom is surrounded by the expected trianionic, ring-metalated ligand. However, a second ligand
unit surrounds the metal center, being bonded through a part of the π system. Reaction of 1 with excess NaH
in toluene proceeded slowly at room temperature, affording a significant yield of {[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C-
6H3}U{1,3-[2,5-(i-Pr)2PhNdC(CH3)]2C6H4}{Na(DME)2}][Na(DME)3]}·1/2C7H8 (9) after crystallization from DME/
toluene. Similar to 8, the complex still contains one ring-metalated trianionic ligand and one intact ligand that
has regained the H atoms and restored the two imine functions. Although according to their connectivities,
complexes 8 and 9 could be assigned with the formal oxidation states +2 and +1, respectively, density functional
theory calculations clearly indicated that these species contain additional spin density on the ligand system
with the metal center in its more standard trivalent state.
Introduction
There are tangible signs in the literature that outstanding
chemical reactivity, along the line of that discovered for divalent
An extraordinary and unpredictable reactivity is the primary
lanthanides, may also be provided by reduced actinide
complexes.1a,3,4 For example, trivalent uranium complexes are
widely established with a variety of σ- and π-donor ligand
systems,5-7 providing remarkable examples of dinitrogen
activation/reduction5b,e,6 and cleavage,7 C-H bond activation,8
solvent fragmentation,7,8a,9 bonding of small molecules in
characteristic of low-valent complexes of f-block elements.1-4
(1) For reviews of lanthanide reactivity, see: (a) Evans, W. J.; Davis, B. L.
Chem. ReV. 2002, 102, 2119. (b) Edelmann, F. T.; Freckmann,
D. M. M.; Schumann, H. Chem. ReV. 2002, 102, 1851. (c) Bochkarev,
M. N. Chem. ReV. 2002, 102, 2089.
9
10406 J. AM. CHEM. SOC. 2009, 131, 10406–10420
10.1021/ja9002525 CCC: $40.75  2009 American Chemical Society

Reduced Uranium Complexes
A R T I C L E S
unusual bonding modes10 (including a unique case of sp3-C-H
bond coordination8c), and oxidative elimination of H2.4i There-
fore, it is conceivable that an even higher reactivity may be
expected when actinide species are further reduced. The problem
with reduced actinides is that these species embark on a very
substantial electron-transfer interaction with the ligand system
due to an intrinsic instability of the low oxidation state. As an
extreme case, even simple salts such as UI2 and ThI2 have been
regarded as consisting of higher-valent species with f-spin
density transferred into a sort of conduction band.11 Given this
tendency, π-bonded ligands may be particularly effective for
stabilizing reduced species because they provide the possibility
of electron-transfer interactions and consequent delocalization.
This idea is further supported by the observation that the sole
cases of paramagnetic Th(III) derivatives12 have been obtained
exclusively with π-donor ligand systems (Cp and COT) and,
again, are unlikely to contain an authentic low-valent thorium.12c
Furthermore, seminal work by Cummins has shown that it is
possible to prepare inverted sandwich π-arene complexes with
the formal appearance of divalent derivatives.13 Even though
theoretical calculations have clearly indicated that the actual
oxidation state of uranium is in fact substantially higher, the
chemical reactivity remarkably remained that of a genuine two-
electron reductant. Thus, the terms “low-valent synthons” and
“low-valent synthetic equivalents” have been forged for these
reduced species.
The behavior of thorium arenes4a,14 obtained through reduc-
tion of tetravalent compounds is along the same lines. In these
complexes, however, the distortion of the π-bonded aromatic
rings that results from the presence of substantial back-bonding
is very visible and clearly attributes the tetravalent state to the
Th atom. Again, these species act as reactive low-valent
synthons. Simple dissociation of the coordinated arene in its
intact form triggers a variety of processes, including dinitrogen
reduction and cleavage, solvent fragmentation, and deoxy-
genation.4a,15
(2) For recent examples of lanthanide reactivity, see: (a) Evans, W. J.;
Champagne, T. M.; Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc.
2006, 128, 16178. (b) Evans, W. J.; Lee, D. S.; Ziller, J. W.;
Kaltsoyannis, N. J. Am. Chem. Soc. 2006, 128, 14176. (c) Zimmer-
mann, M.; Toernroos, K. W.; Anwander, R. Angew. Chem., Int. Ed.
2007, 46, 3126. (d) Edelmann, A.; Blaurock, S.; Lorenz, V.; Hilfert,
L.; Edelmann, F. T. Angew. Chem., Int. Ed. 2007, 46, 6732. (e) Evans,
W. J.; Champagne, T. M.; Ziller, J. W. Organometallics 2007, 26,
1204. (f) Bowman, L. J.; Izod, K.; Clegg, W.; Harrington, R. W.
Organometallics 2007, 26, 2646. (g) Wiecko, M.; Roesky, P. W.
Organometallics 2007, 26, 4846. (h) Roger, M.; Belkhiri, L.; Arliguie,
T.; Thuery, P.; Boucekkine, A.; Ephritikhine, M. Organometallics
2008, 27, 33. (i) Evans, W. J.; Montalvo, E.; Champagne, T. M.; Ziller,
J. W.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2008,
27, 3582. (j) Meyer, G. Angew. Chem., Int. Ed. 2008, 47, 4962. (k)
Amin, S. B.; Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006. (l)
Evans, W. J.; Montalvo, E.; Champagne, T. M.; Ziller, J. W.;
DiPasquale, A. G.; Rheingold, A. L. J. Am. Chem. Soc. 2008, 130,
16. (m) Evans, W. J.; Schmiege, B. M.; Lorenz, S. E.; Miller, K. A.;
Champagne, T. M.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L.
J. Am. Chem. Soc. 2008, 130, 8555.
Ligand systems that contain a large noncoordinating π system
can also achieve the purpose of stabilizing a reduced species.
For example, the bis(imine)pyridine ligand has shown versatility
16
in the trapping of highly reactive units such as NdI2 and
AlR2.17 This is possible only because this particular ligand
efficiently embarks on metal-to-ligand electron transfer.17 As a
result, the actual oxidation state of the metal in all of these
derivatives is higher. Remarkably, however, the high reactivity
expected for a genuine low-valent metal center is preserved.18-21
This background prompted us to attempt the reduction of
uranium complexes involving a dianionic ligand system based
(8) (a) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Organometallics 2001,
20, 2552. (b) Arliguie, T.; Lescop, C.; Ventelon, L.; Leverd, P. C.;
Thuery, P.; Nierlich, M.; Ephritikhine, M. Organometallics 2001, 20,
3698. (c) Castro-Rodriguez, I.; Nakai, H.; Gantzel, P.; Zakharov, L. N.;
Rheingold, A. L.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 15734. (d)
Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2005,
127, 1338. (e) Evans, W. J.; Miller, K. A.; Ziller, J. W.; DiPasquale,
A. G.; Heroux, K. J.; Rheingold, A. L. Organometallics 2007, 26,
4287. (f) Graves, C. R.; Schelter, E. J.; Cantat, T.; Scott, B. L.;
Kiplinger, J. L. Organometallics 2008, 27, 5371. (g) Lam, O. P.; Feng,
P. L.; Heinemann, F. W.; O’Connor, J. M.; Meyer, K. J. Am. Chem.
Soc. 2008, 130, 2806.
(3) For a review of actinide reactivity, see: Ephritikhine, M. Chem. ReV.
1997, 97, 2193.
(4) For recent examples of actinide reactivity, see: (a) Korobkov, I.;
Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 2003, 42, 814.
(b) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Chem. Commun. 2005,
4681. (c) Gaunt, A. J.; Scott, B. L.; Neu, M. P. Chem. Commun. 2005,
128, 3215. (d) Roger, M.; Barros, N.; Arliguie, T.; Thuery, P.; Maron,
L.; Ephritikhine, M. J. Am. Chem. Soc. 2006, 128, 8790. (e) Kiplinger,
J. L.; Pool, J. A.; Schelter, E. J.; Thompson, J. D.; Scott, B. L.; Morris,
D. E. Angew. Chem., Int. Ed. 2006, 45, 2036. (f) Stubbert, B. D.;
Marks, T. J. J. Am. Chem. Soc. 2007, 129, 4253. (g) Minasian, S. G.;
Krinsky, J. L.; Williams, V. A.; Arnold, J. J. Am. Chem. Soc. 2008,
130, 10086. (h) Evans, W. J.; Miller, K. A.; Ziller, J. W. Angew. Chem.,
Int. Ed. 2008, 47, 589. (i) Lukens, W. W., Jr.; Beshouri, S. M.; Blosch,
L. L.; Andersen, R. A. J. Am. Chem. Soc. 1996, 118, 901. (j) Karmazin,
L.; Mazzanti, M.; Pe´caut, J. Inorg. Chem. 2003, 42, 5900.
(9) For example, see: (a) Avens, L. R.; Barnhart, D. M.; Burns, C. J.;
McKee, S. D. Inorg. Chem. 1996, 35, 537. (b) Larch, C. P.; Cloke,
F. G.; Hitchcock, P. B. Chem. Commun. 2008, 82.
(10) (a) Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A. L.;
Meyer, K. Science 2004, 305, 1757. (b) Summerscales, O. T.; Cloke,
F. G.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Science 2006, 311,
829.
(11) (a) Clark, R. J.; Corbett, J. D. Inorg. Chem. 1963, 2, 2460. (b) Scaife,
D. E.; Wylie, A. W. J. Chem. Soc. 1964, 5450. (c) Guggenberger,
L. J.; Jacobson, R. A. Inorg. Chem. 1968, 7, 2257. (d) Stowe, K.;
Tratzky, S.; Beck, H. P.; Jungmann, A.; Claessen, R.; Zimmermann,
R.; Meng, G.; Steiner, P.; Huefner, S. J. Alloys Compd. 1997, 246,
101.
(5) For example, see: (a) Korobkov, I.; Gambarotta, S.; Yap, G. P. A.;
Thompson, L.; Hay, P. J. Organometallics 2001, 20, 5440. (b) Cloke,
F. G.; Hitchcock, P. B. J. Am. Chem. Soc. 2002, 124, 9352. (c) Evans,
W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W. Organometallics
2002, 21, 1050. (d) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer,
K. J. Am. Chem. Soc. 2003, 125, 4565. (e) Evans, W. J.; Kozimor,
S. A.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 14264. (f) Nakai,
H.; Hu, X.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. Inorg. Chem.
2004, 43, 855. (g) Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc.
2005, 127, 11242. (h) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.;
Fagin, A. A.; Bochkarev, M. N. Inorg. Chem. 2005, 44, 3993. (i)
Gaunt, A. J.; Scott, B. L.; Neu, M. P. Angew. Chem., Int. Ed. 2006,
45, 1638. (j) Karmazin, L.; Mazzanti, M.; Bezombes, J.-P.; Gateau,
C.; Pe´caut, J. Inorg. Chem. 2004, 43, 5147. (k) Karmazin, L.; Mazzanti,
M.; Pe´caut, J. Chem. Commun. 2002, 654.
(12) (a) Evans, W. J.; Miller, K. A.; Kozimor, S. A.; Ziller, J. W.;
DiPasquale, A. G.; Rheingold, A. L. Organometallics 2007, 26, 3568.
(b) Gilbert, T. M.; Ryan, R. R.; Sattelberger, A. P. Organometallics
1989, 8, 857. (c) Parry, J. S.; Cloke, F. G.; Coles, S. J.; Hursthouse,
M. B. J. Am. Chem. Soc. 1999, 121, 6867.
(13) (a) Diaconescu, P. L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.;
Cummins, C. C. J. Am. Chem. Soc. 2000, 122, 6108. (b) Zi, G.; Jia,
L.; Werkema, E. L.; Walter, M. D.; Gottfriedsen, J. P.; Andersen,
R. A. Organometallics 2005, 24, 4251.
(14) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed.
2003, 42, 4958.
(6) (a) Cloke, F. G.; Green, J. C.; Kaltsoyannis, N. Organometallics 2004,
23, 832. (b) 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.
(15) (a) Arunachalampillai, A.; Crewdson, P.; Korobkov, I.; Gambarotta,
S. Organometallics 2006, 25, 3856. (b) Korobkov, I.; Arunachalampil-
lai, A.; Gambarotta, S. Organometallics 2004, 23, 6248. (c) Korobkov,
I.; Gambarotta, S. Organometallics 2004, 23, 5379.
(7) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed.
2002, 41, 3433.
(16) Sugiyama, H.; Korobkov, I.; Gambarotta, S.; Moeller, A.; Budzelaar,
P. H. M. Inorg. Chem. 2004, 43, 5771.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10407

A R T I C L E S
Korobkov et al.
Preparation of [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}UCl3]-
[Li(THF)4] (1). The dilithium ligand salt {1,3-[2,5-(i-Pr)2PhNC-
(dCH2)]2C6H4}Li2(THF)4 (1.104 g, 1.41 mmol) was solubilized in
THF (5 mL). An emerald-green solution of UCl4(THF)4 (0.945 g,
1.42 mmol) in THF (7 mL) was rapidly added to the dark-orange
reaction mixture. The color of the solution immediately changed
to dark-brown-red, and some insoluble solid appeared. After the
mixture was stirred for an additional 2 h at room temperature, the
insoluble material was separated via centrifugation. The mother
liquor was concentrated to 5 mL, layered with hexane (15 mL),
and allowed to stand for 2 days at room temperature, after which
dark-brown-red crystals of 1 were obtained (1.41 g, 1.26 mmol,
89%). Anal. Calcd (Found) for C50H74N2O4ULiCl3: C, 53.69
(53.37); H, 6.67 (6.52); N 2.50, (2.47). 1H NMR (500 MHz,
benzene-d6, 20 °C) δ 49.72 (1H, ArH), 16.36 (6H, -CH(CH3)2),
15.39 (2H, dCH2), 13.79 (2H, -CH(CH3)2), 10.80 (6H,
-CH(CH3)2), 9.94 (3H, Ar′H), 9.04 (3H, Ar′H), 2.26 (6H,
-CH(CH3)2), 0.51 (16H, (-CH2-)THF), 0.35 (16H,
(-CH2-)THF), -0.83 (2H, ArH), -1.70 (1H, ArH), -3.68 (2H,
-CH(CH3)2), -4.05 (6H, -CH(CH3)2), -14.30 (2H, dCH2).
Preparation of {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}U(µ-Cl)3-
[Li(THF)2]2 (2). A solution of UCl4(THF)4 (0.410 g, 0.61 mmol)
in THF (10 mL) was mixed at room temperature with a THF
solution of potassium naphthalenide prepared from metallic K
(0.024 g, 0.62 mmol) and naphthalene (0.079 g, 0.61 mmol) in
THF (10 mL). The color of the solution instantly changed to dark-
red upon mixing, and a considerable amount of dark-red precipitate
appeared. The reaction mixture was vigorously stirred for 20 min.
A solution of dilithium ligand salt {1,3-[2,5-(i-Pr)2PhNC(d
CH2)]2C6H4}Li2(THF)4 (0.476 g, 0.61 mmol) in THF (10 mL) was
added directly to the reaction mixture with vigorous stirring that
continued for an additional 2 h. After that period, the insoluble
material was separated by centrifugation, and the volume of the
resulting solution was reduced to 5 mL under vacuum. The solution
was layered with 10 mL of hexane and allowed to stand at room
temperature. After 3 days, dark-red crystals of 2 were obtained
(0.593 g, 0.53 mmol, 86%). Anal. Calcd (Found) for
C50H74N2O4ULi2Cl3: C, 53.36 (53.32); H, 6.63 (6.54); N, 2.49
(2.49).
on N donor atoms combined with a π system, in which the π
ligation could be only enforced by steric vicinity. This led to
the selection of the 1,3-bis(methylaryliminatobenzene) ligand
system, 1,3-[2,5-(i-Pr)2PhNdC(CH3)]2C6H4,22 for this particular
study. We expected this ligand to preserve the established
electron-storage capacity of the bis(iminato)pyridine ligand. At
the same time, the replacement of the central pyridine with a
regular aromatic ring was expected to either enforce π ligation
or allow ring metalation via C-H bond activation. In this event,
formation of more robust complexes resilient to further reducing
conditions was anticipated. In any event, trivalent uranium was
chosen as the target oxidation state, since it provides highly
reactive complexes4c,5 and might be used as a starting material
for further reduction.
In this study, we report some unusual observations obtained
during our attempts to prepare low-valent uranium complexes
with the 1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H42- ligand and further
reduce them.
Experimental Section
All of the operations were performed under an inert atmosphere
using standard Schlenk-type techniques or with the use of nitrogen-
filled drybox. Li, NaH, K, naphthalene, and UO3 were purchased
from Aldrich and used as received. UCl4(THF)4 was prepared by
23a
recrystallization of freshly prepared UCl4 from tetrahydrofuran
(THF). Solutions of “UCl3” were prepared according to a literature
procedure23b and used immediately. The bis(imino)benzene ligand
1,3-[2,5-(i-Pr)2PhNdC(CH3)]2C6H4 was prepared according to a
literature procedure22 and then converted to its lithium salt using a
method similar to the published procedure for the bis(imino)pyridine
molecule.16 All of the solvents were dried by passage through
Al2O3-filled columns and degassed prior to use. Elemental analyses
were performed on a PerkinElmer 2400 CHN analyzer. Data for
single-crystal X-ray structure determination were collected with a
Bruker diffractometer equipped with a 1K SMART CCD area
detector. Deuterated solvents were purchased from C/D/N Isotopes
Inc. and dried over freshly activated 4 Å molecular sieves for 7
days before use. NMR spectra were recorded at 20 °C on a Varian
Preparation of {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}U(THF)-
(µ-Cl)2[Li(Et2O)2] (3). A dark-red solution of 1 (0.300 g, 0.27
mmol) in THF (7 mL) was cooled to 0 °C and then treated with a
0 °C solution of lithium naphthalenide in THF (7 mL) prepared
from Li metal (0.002 g, 0.29 mmol) and naphthalene (0.039 g, 0.30
mmol). The reaction mixture was stirred for 8 h at low temperature
and then warmed to ambient temperature. The THF solvent was
removed under vacuum at room temperature and replaced with
diethyl ether (15 mL). Some insoluble material was separated by
centrifugation. The volume of the residual solution was reduced to
7 mL, and the solution was allowed to stand in the freezer at -37
°C for 3 days, after which red crystals of 3 (0.184 g, 0.18 mmol,
67%) were obtained. Anal. Calcd (Found) for C46H70N2O3ULiCl2:
C, 54.44 (54.36); H, 6.95 (6.72); N, 2.76 (2.77).
1
INOVA 500 NMR spectrometer. H and 13C chemical shifts were
referenced to internal solvent resonances and are reported in parts
per million relative to Me4Si.
(17) For example, see: (a) Scott, J.; Gambarotta, S.; Korobkov, I.;
Knijnenburg, Q.; De Bruin, B.; Budzelaar, P. H. M. J. Am. Chem.
Soc. 2005, 127, 17204. (b) Vidyaratne, I.; Scott, J.; Gambarotta, S.;
Duchateau, R. Organometallics 2007, 26, 3201.
(18) Vidyaratne, I.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
Inorg. Chem. 2005, 44, 1187.
(19) For example, see: (a) Archer, A. M.; Bouwkamp, M. W.; Cortez, M. P.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2006, 25, 4269. (b) Bart,
S. C.; Lobkovsky, E.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg. Chem.
2007, 46, 7055. (c) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik,
P. J. J. Am. Chem. Soc. 2007, 129, 7212. (d) Scott, J.; Vidyaratne, I.;
Korobkov, I.; Gambarotta, S.; Budzelaar, P. H. M. Inorg. Chem. 2008,
47, 896. (e) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2008, 130, 11631. (f) Fernandez, I.; Trovitch, R. J.; Lobkovsky,
E.; Chirik, P. J. Organometallics 2008, 27, 109. (g) Bouwkamp, M. W.;
Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 3340.
Preparation of {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(THF)-
(µ-Cl)2[Li(THF)2] (4). Method A. A dark-red solution of 1 (0.350
g, 0.31 mmol) in THF (4 mL) was treated with a THF solution (5
mL) of potassium naphthalenide prepared from K (0.013 g, 0.33
mmol) and naphthalene (0.042 g, 0.33 mmol). When the two
solutions were mixed, no significant color change was observed.
Nevertheless, after the mixture was stirred for 4 h at room
temperature, a small amount of light-colored insoluble material
appeared in the reaction vessel as well as some brightening of the
reaction mixture. The insoluble material was removed by centrifu-
gation, and the mother liquor was concentrated to 5 mL and layered
with hexanes (15 mL). Dark-brown-orange crystals of 4 were
obtained after 3 days (0.178 g, 0.18 mmol, 56%). Anal. Calcd
(Found) for C46H65N2O3ULiCl2: C, 54.71 (54.67); H, 6.49 (6.35);
(20) (a) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252. (b)
Humphries, M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. Organometallics 2005, 24, 2039. (c) Scott, J.;
Gambarotta, S.; Korobkov, I. Can. J. Chem. 2005, 83, 279.
(21) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Budzelaar, P. H. M. Inorg.
Chem. 2007, 46, 7040.
(22) Nueckel, S.; Burger, P. Organometallics 2000, 19, 3305.
(23) (a) Moeller, T. Inorganic Syntheses; Maple Press: York, PA, 1957;
Vol. 5. (b) Moody, D. C.; Odom, J. D. J. Inorg. Nucl. Chem. 1979,
41, 533.
1
N, 2.77 (2.73). H NMR (500 MHz, THF-d8, 20 °C) δ 8.56 (2H,
ArH), 7.87 (1H, ArH), 7.45 (4H, Ar′H), 7.39 (2H, Ar′H), 7.33
9
10408 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
(2H, -CH(CH3)2), 7.16 (1H, dCH2), 7.14 (1H, dCH2), 3.82 (THF,
-CH2-), 3.09 (2H, -CH(CH3)2), 2.18 (1H, dCH2), 2.16 (1H,
dCH2), 1.94 (THF, -CH2-), 1.47 (12H, -CH(CH3)2), 1.44 (12H,
-CH(CH3)2).
1H NMR (500 MHz, THF-d8) δ 8.68 (1H, ArH), 8.21 (2H, ArH),
7.93 (1H, dCH2), 7.84 (1H, dCH2), 7.08 (4H, Ar′H), 6.93 (2H,
Ar′H), 3.38 (DME, -CH2-), 3.22 (DME, -CH3), 2.90 (1H,
dCH2), 2.73 (4H, -CH(CH3)2), 2.34 (1H, dCH2), 2.26 (DME,
-CH2-), 2.12 (DME, -CH3), 1.14 (12H, -CH(CH3)2), 1.13 (12H,
-CH(CH3)2), 3.10 (6H, -OCH3), -4.60 (3H, -OCH3).
Method B. A dark-red solution of 2 (0.350 g., 0.31 mmol) in
THF (15 mL) was placed in a glass high-pressure reactor and heated
to 80 °C in an oil bath. A color change occurred over a period of
6 h. A visible amount of pale-colored insoluble material gradually
appeared. After the reaction mixture was cooled to room temper-
ature, all of the insoluble material was separated by centrifugation.
The residual solution was concentrated to 4 mL and layered with
hexanes (15 mL). After 3 days, dark-brown-orange crystals (0.241
g, 0.24 mmol, 77%) of 4 were collected. The identity of the isolated
Method B. A solution of 6 (0.250 g, 0.22 mmol) in DME (10
mL) was placed over NaH (0.075 g, 3.13 mmol) and stirred for 7
days. The insoluble material was then removed by centrifugation.
The solution was evaporated to ∼4 mL and layered with hexane
(10 mL). After 4 days, the dark-brown-orange crystals of 7 (0.102
g, 0.11 mmol, 49%) that had formed were washed with hexanes
and dried. The identity of this species was confirmed by comparison
1
material was confirmed by both X-ray and H NMR data.
1
of X-ray and H NMR data.
Method C. A dark-red solution of 3 (0.350 g, 0.35 mmol) in
THF (15 mL) was placed in a glass high-pressure reactor and heated
to 80 °C in an oil bath. As for method B, the color of the solution
became visibly lighter after 6 h of stirring. The reactor was cooled
to room temperature, and a very small amount of dark-colored
insoluble material was separated by centrifugation. The residual
solution was concentrated to 4 mL and layered with hexanes (15
mL). Dark-brown-orange crystals of 4 were obtained after 4 days
(0.272 g, 0.27 mmol, 77%). The identity of the complex was
Preparation of {[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-
[2,5-(i-Pr)2PhNdC(CH3)]2C6H4}][Li(DME)(THF)]}·Et2O (8). A
solution of 1 (0.500 g, 0.45 mmol) in DME (5 mL) was cooled to
-37 °C and treated with a cold solution (-37 °C) of lithium
naphthalenide [Li, 0.006 g (0.87 mmol); naphthalene, 0.116 g (0.91
mmol)]. The reaction mixture was stirred at -37 °C for 2 days,
during which the color of the solution gradually changed from dark-
red to dark-brown. Subsequently, DME was removed in vacuo while
the temperature of the reaction mixture was maintained at -3 °C.
The solid residue was resolubilized in cold THF (5 mL, -37 °C).
The insoluble material was eliminated by cold filtration, and the
resulting solution was allowed to stand for 3 days at -37 °C after
it was layered with n-heptane (10 mL). The dark-colored oily
substance that separated out was redissolved in cold diethyl ether
(10 mL) and layered with cold n-heptane (10 mL). After 3 additional
days at -37 °C, dark-brown-red crystals of 8 were obtained (0.283
g, 0.20 mmol, 44%). Anal. Calcd (Found) for C80H113N4O4ULi: C,
66.74 (66.69); H, 7.91 (7.90); N, 3.89 (3.87).
1
confirmed by both X-ray and H NMR data.
Preparation of {{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-Cl)-
(µ-[O(CH2)3CH2])[Li(DME)]}[Li(DME)3] (5). A dark-red solution
of 1 (0.493 g, 0.44 mmol) in THF (4 mL) was treated with a freshly
made THF solution (5 mL) of lithium naphthalenide prepared from
Li (0.009 g, 1.31 mmol) and naphthalene (0.170 g, 1.33 mmol).
The color of the reaction mixture did not change significantly upon
mixing, but some insoluble material appeared after 2 h of stirring
at room temperature. Solvent was removed from the reaction
mixture, and the resulting solid residue was resuspended in
dimethoxyethane (DME) (4 mL). After separation of the insoluble
material by centrifugation, the resulting dark-red DME solution was
layered with n-heptane (15 mL). Dark crystals of 5 were obtained
after 3 days (0.181 g, 0.15 mmol, 34%). Anal. Calcd (Found) for
C54H89N2O9ULi2Cl: C, 54.15 (53.87); H, 7.49 (7.35); N, 2.34 (2.32).
1H NMR (500 MHz, THF-d8, 20 °C) δ 8.30 (1H, ArH), 8.24 (2H,
ArH), 7.13 (4H, Ar′H), 7.03 (2H, Ar′H), 7.19 (1H, dCH2), 6.87
(1H, dCH2), 3.57 (THF, -CH2-), 3.39 (DME, -CH2-), 3.21
(DME, -CH3), 2.77 (4H, -CH(CH3)2), 2.31 (1H, dCH2), 2.17
(2H, cycle, -CH2-), 1.97 (1H, dCH2), 1.73 (THF, -CH2-), 1.15
(24H, -CH(CH3)2), 0.98 (2H, cycle, -CH2-), -4.36 (2H, cycle,
-CH2-), -6.62 (2H, cycle, -CH2-).
Preparation of the Cocrystallite of [{1,3-[2,5-(i-Pr)2-
PhNC(dCH2)]2C6H3}UCl(OCH3)][Li(DME)3] (6a) and [{1,3-
[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}UCl2][Li(DME)3] (6b). A solu-
tion of 1 (0.300 g, 0.27 mmol) in DME (5 mL) was quickly mixed
with a solution of potassium naphthalenide prepared from K (0.021
g, 0.54 mmol) and naphthalene (0.068 g, 0.53 mmol) in DME (5
mL). The color of the reaction mixture did not change significantly,
but some light-colored precipitate appeared after 1 h of stirring at
room temperature. The insoluble material was eliminated by
centrifugation, and the resulting dark-brown-yellow solution was
layered with n-heptane (20 mL). After 3 days, dark-yellow crystals
of the cocrystallite 6a/6b were collected (0.197 g, 0.17 mmol, 63%).
Anal. Calcd (Found) for C50.43H82.29N2O8.43ULiCl1.57: C, 52.56
(52.18); H, 7.20 (7.04); N, 2.43 (2.39).
Preparation of {[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-[2,5-
(i-Pr)2PhNdC(CH3)]2C6H4}{Na(DME)2}][Na(DME)3]} ·1/2C7H8 (9).
A solution of 1 (0.500 g, 0.45 mmol) in toluene (10 mL) was stirred
at room temperature in the presence of NaH (0.104 g, 4.52 mmol).
During the 4 days of stirring at room temperature, the red color of
the mixture gradually darkened. Toluene was removed in vacuo,
and the residual solid was redissolved in DME (10 mL). A small
amount of insoluble material was eliminated via centrifugation. The
resulting solution was layered with hexanes (15 mL). After the
diffusion was completed and no solid appeared in the crystallization
vessel, the solution was cooled to -37 °C and allowed to stand for
24 h at low temperature, affording dark-red crystals of 9 (0.304 g,
0.18 mmol, 40%). Anal. Calcd (Found) for C91.50H139N4O10UNa2:
C, 63.19 (63.03); H, 8.06 (7.92); N, 3.22 (3.19).
Computational Details. All of the density functional theory
(DFT) calculations were performed using the Gaussian 03 package24
with the PBE25 exchange-correlation functional and the SDD and
SDDall26 effective core potential (ECP) basis sets. Calculations with
these two basis sets gave nearly identical results. Tight SCF
convergence criteria were used for all of the calculations. The
converged wave functions were tested to confirm that they
corresponded to the ground-state surface. All of the calculations
for the analysis of the electronic structure, including Mulliken
population analysis27 and the calculation of Mayer 2- and 3-center
Preparation of {1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U-
(µ-OCH3)3(µ,η6-Na)[η3-Na(DME)] (7). Method A. A solution of
1 (0.350 g, 0.31 mmol) in DME (10 mL) was placed over NaH
(0.100 g, 4.17 mmol). The reaction mixture was stirred at room
temperature for 7 days. The insoluble material was separated by
centrifugation. The resulting solution was concentrated to 5 mL
and layered with heptane (15 mL). After 4 days, the dark-orange-
brown crystals of 7 (0.247 g, 0.26 mmol, 84%) that had separated
out were washed with hexane and dried. Anal. Calcd (Found) for
C41H60N2O5UNa2: C, 52.11 (51.93); H, 6.40 (6.32); N, 2.96 (2.89).
(24) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc:
Wallingford, CT, 2004.
(25) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett.
1997, 78, 1396.
(26) Fuentealba, P.; Preuss, H.; Stoll, H.; Szentpaly, L. V. Chem. Phys.
Lett. 1989, 89, 418.
(27) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833.
(28) (a) Mayer, I. Int. J. Quantum Chem. 1986, 29, 73. (b) Sannigrahi,
A. B.; Kar, T. Chem. Phys. Lett. 1990, 173, 569. (c) Gorelsky, S. I.;
Ghosh, S.; Solomon, E. I. J. Am. Chem. Soc. 2006, 128, 278.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10409

A R T I C L E S
Korobkov et al.
bond-order indices,28a,b atomic valences,28a and populations of
fragment orbitals,28c were performed using the AOMix software
package.29
X-ray Crystallography. For all of the compounds, the results
presented are the best of several data collection trials. The crystals
were mounted on thin glass fibers using paraffin oil and cooled to
the data collection temperature. Data were collected on a Bruker-
AXS SMART 1K CCD diffractometer. Data for compounds 1, 2,
5, 7, and 8 were collected with a sequence of 0.3° ω scans at 0,
120, and 240° in ꢀ. To obtain acceptable redundancy data for
compounds 3, 4, 6, and 9, a sequence of 0.3° ω scans at 0, 90,
180, and 270° in ꢀ was used. Initial unit-cell parameters were
determined from 60 data frames collected at the different sections
of the Ewald sphere. Semiempirical absorption corrections based
on equivalent reflections were applied.30 Systematic absences in
the diffraction data set and unit-cell parameters were consistent with
the following space groups: monoclinic P21/m for 1; orthorhombic
Figure 1. Partial thermal ellipsoid diagram of the anion of 1, with thermal
ellipsoids drawn at the 50% probability level. The isopropyl substituents
on the phenyl rings have been omitted for clarity.
j
Pnma for 2; triclinic P1 for 3, 4, 6, and 9; and monoclinic P21/n
for 5, 7, and 8. Solutions in centrosymmetric space groups yielded
chemically reasonable and computationally stable refinement results
for all of the compounds. The structures were solved by direct
methods, completed with difference Fourier synthesis, and refined
with full-matrix least-squares procedures based on F2. The com-
pound molecules were located in special positions (mirror plane)
in the structures of 1 and 2. In all of the other complexes, the
compound molecules were located in general positions. The
structure of complex 8 represents a dimer in which the two
monomeric units are related by inversion center. The carbon atoms
of coordinated THF solvent molecules in 1 and 2, coordinated
diethyl ether solvent molecules in 3, and the cocrystallized toluene
solvent molecule in 9 were refined isotropically because of
significant thermal motion disorder and in order to maintain an
optimal data-to-parameters ratio. In every structure, all of the non-
hydrogen atoms except those mentioned above were refined with
anisotropic displacement coefficients. The structures of all of the
complexes except 7 contain severely disordered solvent molecules,
either in the lattice or coordinated to the alkali cations. This
significant disorder, sometimes coupled with the low quality of the
collected data sets resulting from small crystal sizes, led to the
appearance of several A alerts in the check-CIF files generated by
the International Union of Crystallography online check-CIF routine.
Nevertheless, these disordered solvent molecules are spatially
separated from the uranium-containing moieties and have no effect
on either the refinement stability or the errors in bonds and distances
in key fragments of the structure. All of the hydrogen atoms were
treated as idealized contributions. All of the scattering factors were
contained in several versions of the SHELXTL program library,
with the latest version used being 6.12.31
Figure 2. Partial thermal ellipsoid diagram of 2, with thermal ellipsoids
drawn at the 50% probability level. The isopropyl substituents on the phenyl
rings have been omitted for clarity.
The sixth position is occupied by the C-H moiety of the central
phenyl ring, which is coordinated side-on to the metal center
and reaches a bonding contact with one of the phenyl ring carbon
atoms [U1-C8 ) 2.678(12) Å, U1-H8a ) 3.224(12) Å] in
what geometry-wise might be reminiscent of an agostic interac-
tion. The dianionic character of the ligand system is apparent
from the short C-C bond distances [C1-C2 ) 1.328(16) Å,
C9-C10 ) 1.333(16) Å] formed by the two C atoms attached
to the imino carbons as well as the fairly long C-N distances
[C2-N2 ) 1.442(13) Å, C9-N1 ) 1.411(13) Å]. The central
ring of the ligand is not coplanar with the rest of ligand main
core [C1-C2-C3-C4 ) 41.8(18)°] but is not π-bonded to the
uranium center either. A lithium cation tetrahedrally solvated
by four molecules of THF [Li1-O1 ) 2.00(4) Å, Li1-O2 )
2.01(4) Å, Li1-O3 ) 1.90(4) Å, Li1-O4 ) 1.85(4) Å] and
unconnected with the anionic moiety completes the structure.
{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}U(µ-Cl)3[Li(THF)2]2 (2).
The structure of 2 shows the uranium metal center in a
symmetry-generated distorted octahedral coordination environ-
ment (Figure 2). Similarly to 1, three of the four equatorial
positions are occupied by chlorine atoms [U1-Cl1 ) 2.757(3)
Å, U1-Cl2 ) 2.880(3) Å, U1-Cl3 ) 2.751(3) Å, Cl1-U1-Cl2
) 78.02(9)°, Cl1-U1-Cl3 ) 154.57(10)°]. Two nitrogen atoms
of the bis(imino) ligand reside in axial positions of the distorted
octahedron [U1-N1 ) 2.402(7) Å, N1-U1-N1a ) 118.8(3)°].
The last equatorial position is occupied by one C atom of the
central phenyl ring [U1-C6 ) 2.769(9) Å, U1-H6a ) 2.529(9)
Å, N1-U1-C6 ) 61.95(17)°], with a short contact in the U-C
σ-bonding range. The molecule is bisected by a mirror plane
containing the metal center and the chlorine atoms. The
dianionic nature of the ligand is apparent from the values of
Description of the Crystal Structures
[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}UCl3][Li(THF)4] (1).
The structure of 1 consists of an anionic metalate with an
unconnected, THF-solvated lithium countercation (Figure 1).
The anionic part contains the uranium metal center in a slightly
distorted octahedral environment [Cl1-U1-Cl2 ) 173.3(1)°,
Cl1-U1-Cl3)87.2(2)°,Cl3-U1-Cl2)86.6(2)°,Cl1-U1-N1
) 96.0(4)°, Cl1-U1-N2 ) 92.1(5)°, Cl2-U1-N1 ) 88.8(4)°,
Cl2-U1-N2 ) 88.7(4)°]. Five coordination sites are defined
by three Cl atoms [U1-Cl1 ) 2.622(3) Å, U1-Cl2 ) 2.599(3)
Å, U1-Cl3 ) 2.652(5) Å] and two N atoms of the ligand
[U1-N1 ) 2.363(14) Å, U1-N2 ) 2.276(15) Å] (Figure 1).
(29) (a) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635,
187. (b) Gorelsky, S. I. AOMix, version 6.36; University of Ottawa:
Ottawa, ON, 2008.
(30) Blessing, R. Acta Crystallogr. 1995, A51, 33.
(31) Sheldrick, G. M. SHELXTL Program Library; Bruker AXS: Madison,
WI, 2001.
9
10410 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
Figure 3. Partial thermal ellipsoid diagram of 3, with thermal ellipsoids
drawn at the 50% probability level. The isopropyl substituents on the phenyl
rings have been omitted for clarity.
Figure 4. Partial thermal ellipsoid diagram of 4, with thermal ellipsoids
drawn at the 50% probability level. The isopropyl substituents on the phenyl
rings have been omitted for clarity.
the C-C distances formed by the terminal carbon atoms and
the C-N distances of the former imine functions [C1-C2 )
1.342(13) Å, C2-N1 ) 1.381(11) Å]. As in complex 1, the
interaction of the metal center with the phenyl ring C-H bond
causes a significant deviation of the ligand π system from
planarity [C1-C2-C3-C4 ) 40.3(2)°]. The three chlorine
atoms bridge in pairs two Li cations [Cl1-Li1 ) 2.38(2) Å,
Cl2-Li1 ) 2.39(2) Å, Cl2-Li2 ) 2.41(3) Å, Cl3-Li2 )
2.44(3) Å]. Each Li atom resides in a tetrahedral arrangement,
with two positions occupied by chlorines [Cl1-Li1-Cl2 )
96.2(7)°, Cl2-Li2-Cl3 ) 91.9(11)°] and the other two by
coordinated THF molecules [O1-Li1 ) 1.888(13) Å, O2-Li2
) 1.67(3) Å, O3-Li2 ) 2.06(3) Å, Cl1-Li1-O1 ) 107.4(9)°,
O1-Li1-O1a ) 117.2(11)°, Cl2-Li2-O2 ) 134.1(12)°,
O2-Li2-O3 ) 105.4(19)°]. Two of the THF molecules around
a given Li atom are disordered over two positions with
approximately equal occupancies.
{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}U(THF)(µ-Cl)2[Li(Et2O)2]
(3). Complex 3 consists of a uranium metal center in a distorted
octahedral arrangement very similar to those of 1 and 2 (Figure
3). The coordination of the metal center is defined by two
nitrogens placed in the two axial positions [U1-N1 ) 2.393(10)
Å, U1-N2 ) 2.390(10) Å, N1-U1-N2 ) 123.4(3)°]. Three
of the four equatorial positions are defined by two chlorine atoms
[U1-Cl1 ) 2.759(4) Å, U1-Cl2 ) 2.720(4) Å, Cl1-U1-Cl2
) 80.28(13)°, N1-U1-Cl1 ) 117.0(3)°] and the oxygen atom
of a coordinated THF molecule [U1-O1 ) 2.529(9) Å,
Cl1-U1-O1 ) 78.03(3)°, N1-U1-O1 ) 96.9(4)°]. The same
short contact between the metal center and the C-H bond of
the phenyl ring as observed in 1 and 2 is also present in 3 and
occupies the last equatorial position [U1-C8 ) 2.756(13) Å,
U1-H8a ) 2.456(14) Å, C8-U1-Cl1 ) 158.7(3)°, C8-U1-N1
) 63.6(4)°]. In perfect analogy with the two complexes above,
the dianionic nature of the ligand is confirmed by the values of
the C-C and C-N bond distances [C1-C2 ) 1.335(19) Å,
C9-C10 ) 1.350(18) Å, C2-N1 ) 1.407(17) Å, C9-N2 )
1.373(16) Å]. Also very comparable is the deviation of the
ligand from planarity [C1-C2-C3-C4 ) 36.3(5)°].
{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(THF)(µ-Cl)2[Li(THF)2]
(4). The structure of 4 shows that the ligand is coordinated to
the uranium center in a tridentate fashion and arranged meridi-
onally (Figure 4). Two of the three positions are defined by the
two amido nitrogen donor atoms [U1-N1 ) 2.241(10) Å,
U1-N2 ) 2.273(10) Å, N1-U1-N2 ) 132.4(4)°]. The third
position is occupied by the metalated carbon atom of the central
phenyl ring [U1-C8 ) 2.388(13) Å, N1-U1-C8 ) 66.4(4)°].
The polyanionic character of the ligand is confirmed by the C-C
and C-N bond distances [C1-C2 ) 1.38(2) Å, C9-C10 )
1.34(2) Å, C2-N1 ) 1.41(2) Å, C9-N2 ) 1.38(2) Å]. The
ring metalation is responsible for the restoration of the ligand
planarity [C1-C2-C3-C4 ) 9.0(2)°]. The last equatorial
position is occupied by one of the two chlorine atoms [U1-Cl1
) 2.792(3) Å, Cl1-U1-N1 ) 115.3(3)°, Cl1-U1-C8 )
160.6(3)°]. The second chlorine atom fills the first axial position
of the distorted octahedron [U1-Cl2 ) 2.665(4) Å, Cl1-U1-Cl2
) 80.05(12)°, Cl2-U1-C8 ) 119.3(3)°]. The second axial
position is occupied by the O atom of a coordinated THF solvent
molecule [U1-O1 ) 2.477(9) Å, O1-U1-Cl2 ) 158.5(2)°,
O1-U1-C8 ) 81.9(4)°]. Both chlorine atoms bridge uranium
and lithium, defining two positions of the tetrahedral environ-
ment around the Li cation [Li1-Cl1 ) 2.35(3) Å, Li1-Cl2 )
2.44(3) Å, Cl1-Li1-Cl2 ) 94.3(9)°]. Two THF solvent
molecules complete the coordination geometry of the alkali
cation [Li1-O2 ) 1.84(3) Å, Li1-O3 ) 1.90(3) Å, O2-Li1-O3
) 109.8(13)°, Cl1-Li1-O2 ) 111.4(14)°].
{{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-Cl)(µ-[O(CH2)3CH2])-
[Li(DME)]}[Li(DME)3] (5). Complex 5 is ionic, with a uranium-
containing anionic moiety and a solvated lithium countercation.
The uranium center adopts the usual distorted octahedral
arrangement (Figure 5). Similar to complex 4, the ligand is
tridentate and meridionally arranged, with metalation of the
central aromatic ring. One equatorial coordination site is
occupied by a σ-bonded C atom [U1-C8 ) 2.419(12) Å] from
the central phenyl ring, while the two nitrogen atoms of the
bis(amido) ligand occupy two other equatorial sites [U1-N1
) 2.391(11) Å, U1-N2 ) 2.372(11) Å, N1-U1-N2 )
129.78(7)°]. One of the axial sites is occupied by a chlorine
atom [U1-Cl1 ) 2.846(4) Å, C35-U1-Cl1 ) 153.48(6)°].
The last two coordination sites, one equatorial and one axial,
are occupied by the oxygen [U1-O1 ) 2.244(9) Å, O1-U1-C8
Both chlorine atoms bridge the uranium center to lithium
[Li1-Cl1 ) 2.36(3) Å, Li1-Cl2 ) 2.34(3) Å, Cl1-Li1-Cl2
) 97.2(9)°]. The tetrahedral coordination environment of the
Li cation is completed by the oxygen atoms of two coordinated
molecules of diethyl ether [Li1-O2 ) 1.92(4) Å, Li1-O3 )
1.96(4) Å, O2-Li1-O3 ) 108.7(14)°, Cl1-Li1-O2 )
115.5(17)°].
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10411

A R T I C L E S
Korobkov et al.
95.3(3)°] and by the second chlorine atom in 6b [U1-Cl2 )
2.628(7) Å, Cl2-U1-C8 ) 90.0(3)°].
The overall 6a/6b ratio was determined to be 43:57 on the
basis of the best crystallographic refinement of thermal param-
eters. The cationic fragment consists of a Li atom in an
octahedral coordination arrangement defined by the oxygen
atoms of three DME molecules [Li2-O4 ) 2.085(18) Å,
Li2-O5 ) 2.229(17) Å, Li2-O6 ) 2.114(16) Å, Li2-O7 )
2.210(18) Å, Li2-O8 ) 2.070(17) Å, Li2-O9 ) 2.169(16)
Å].
{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-OCH3)3(µ,η6-Na)-
[η3-Na(DME)] (7). The structure of 7 consists of a symmetry-
generated dimer (Figure 7). Each monomeric unit contains a
uranium metal center in a distorted octahedral environment. The
ligand exhibits the usual tridentate bonding mode by bonding
the metal center through the N atoms and the C atom of the
metalated central phenyl ring [C1-C2-C3-C4 ) 20.1(17)°,
U1-N1 ) 2.440(8) Å, U1-N2 ) 2.418(8) Å, U1-C8 )
2.468(10) Å, N1-U1-N2 ) 130.3(3)°, N1-U1-C8 ) 65.2(3)°].
The trianionic character of the ligand is indicated by the
corresponding C-C and C-N bond distances as well as by
the bonding contact between the uranium metal center and the
carbon atom of the ligand central phenyl ring [C1-C2 )
1.316(13) Å, C9-C10 ) 1.355(15) Å, N1-C2 ) 1.401(12) Å,
C9-N2 ) 1.405(13) Å, U1-C8 ) 2.468(10) Å]. The remaining
equatorial and axial positions of the octahedron are filled by
three CH3O groups [U1-O1 ) 2.178(7) Å, U1-O2 ) 2.296(6)
Å,U1-O3)2.207(6)Å,O1-U1-O2)159.8(3)°,O1-U1-O3
) 89.8(3)°, O3-U1-C8 ) 171.3(3)°].
Each of the three methoxy groups bridges U to one of the
two Na metal centers. The first Na atom is bonded to one
methoxy group [Na1-O1 ) 2.296(9) Å], a portion of the central
aromatic ring of the ligand [Na1-C3 ) 2.844(13) Å, Na1-C7
) 3.060(12) Å, Na1-C8 ) 2.583(11) Å, O1-Na1-C8 )
80.8(3)°], and the two oxygens of one DME molecule [Na1-O4
) 2.259(16) Å, Na1-O5 ) 2.315(16) Å, O4-Na1-O5 )
68.3(6)°, O4-Na1-C8 ) 136.0(6)°]. The second is instead
bonded to the other two methoxy groups [Na2-O2 ) 2.316(7)
Å, Na2-O3 ) 2.328(8) Å, O2-Na2-O3 ) 73.8(2)°]. Ad-
ditional bonding interactions are realized through methoxy group
ofasecondidenticalunit[Na2-O2a)2.402(8)Å,O2-Na2-O2a
) 97.5(2)°] as well as one of the peripheral phenyl rings of the
secondunit[Na2-centroid(1a))2.578(9)Å,O2-Na2-centroid(1a)
) 129.8(3)°], thereby assembling the dinuclear structure.
{[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-[2,5-(i-Pr)2PhNd
C(CH3)]2C6H4}][Li(DME)(THF)]}·Et2O (8). Complex 8 contains
a uranium metal center surrounded by two ligands in an overall
severely distorted heptacoordinate environment (Figure 8). Three
sites are occupied by two nitrogen atoms and one carbon atom
of one ring-metalated ligand system [U1-N1 ) 2.372(4) Å,
U1-N2 ) 2.371(4) Å, U1-C8 ) 2.420(5) Å, N1-U1-N2 )
130.60(14)°, N1-U1-C8 ) 66.32(17)°]. This ligand appears
to be trianionic according to the C-C and C-N bond distances
[U1-C8 ) 2.420(5) Å, C1-C2 ) 1.342(8) Å, C9-C10 )
1.348(7) Å, C2-N1 ) 1.413(6) Å, C9-N2 ) 1.402(6) Å] as
well as the planar configuration [N1-U1-C8-C7 ) 174.0(4)°,
N2-U1-C8-C3 ) 178.6(4)°]. A second ligand is also bonded
to uranium, affording a diene-type η4 interaction with the part
of the extended π system encompassing one imino function and
part of the central aromatic ring [U1-N3 ) 2.260(4) Å,
U1-C40 ) 2.682(5) Å, U1-C41 ) 2.808(5) Å, U1-C43 )
2.678(5) Å, η4-centroid-U1-C8 ) 165.26(17)°, η4-cen-
troid-U1-N1 ) 115.80(16)°]. In fact, the two imino functions
Figure 5. Partial thermal ellipsoid diagram of the anion of 5, with thermal
ellipsoids drawn at the 50% probability level.
) 157.86(6)°] and carbon atoms [U1-C35 ) 2.476(13) Å,
C35-U1-O1 ) 78.91(6)°, C35-U1-C8 ) 79.756(6)°],
respectively, of what appears to be the result of THF ring
opening (Figure 5). The ene-bis(amido) ligand adopts a nearly
flat conformation having both amido residues coplanar with the
metalated phenyl ring. The bond distances formed by the ene
carbon atoms [(C1-C2 ) 1.34(2) Å, C9-C10 ) 1.358(18) Å,
C2-N1 ) 1.415(17) Å, C9-N2 ) 1.372(17) Å] confirm the
presence of the C-C double bonds and are in agreement with
those of all the other complexes described above. The oxygen
atom of the cleaved THF molecule and the chlorine atom bridge
uranium to the Li cation [Li1-O1 ) 1.89(3) Å, Li1-Cl1 )
2.30(3) Å]. The remaining two coordination sites of the
coordination tetrahedron of the Li atom are filled by the two
oxygen atoms of a DME molecule [Li1-O2 ) 1.97(3) Å,
Li1-O3 ) 1.94(3) Å]. The unconnected countercation consists
of a second Li cation octahedrally coordinated by six oxygen
atoms from three DME molecules [Li2-O4 ) 2.30(3) Å,
Li2-O5 ) 1.97(4) Å, Li2-O6 ) 2.08(3) Å, Li2-O7 ) 2.12(3)
Å, Li2-O8 ) 2.11(4) Å, Li2-O9 ) 2.18(3) Å].
Cocrystallite of [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}UCl-
(OCH3)][Li(DME)3] (6a) and [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2-
C6H3}UCl2][Li(DME)3] (6b). The structure of the cocrystallite
of 6a and 6b consists of two chemically different but crystal-
lographically very similar compounds cocrystallized within the
same unit cell. The difference between the two species consists
of the replacement of the OCH3 group of 6a with a second Cl
in 6b (Figure 6a,b). In both cases, the structure contains an
anionic fragment with a heptacoordinated (pentagonal bipyra-
midal) U metal center and an unconnected Li countercation
solvated by DME. The coordination sphere of the U atom is
defined by the planar ligand, which occupies three equatorial
coordination sites using two nitrogen atoms and one carbon atom
of the deprotonated phenyl ring [U1-N1 ) 2.395(6) Å, U1-N2
) 2.381(6) Å, U1-C8 ) 2.412(7) Å, N1-U1-N2 ) 132.3(2)°].
The last two equatorial positions are occupied by the two oxygen
atoms of a DME molecule [U1-O2 ) 2.810(6) Å, U1-O3 )
2.844(7) Å, O2-U1-O3 ) 57.5(2)°, O2-U1-N1 ) 84.6(3)°,
O3-U1-C8 ) 150.3(3)°]. A chlorine atom resides in one axial
position of the coordination polyhedron [U1-Cl1 ) 2.668(2)
Å, C8-U1-Cl1 ) 111.5(3)°]. The other axial coordination site
is partly occupied by the oxygen atom of the methoxy group in
the case of 6a [U1-O1 ) 1.976(16) Å, O1-U1-C8 )
9
10412 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
Figure 6. Partial thermal ellipsoid diagrams of the anions in (left) 6a and (right) 6b, with thermal ellipsoids drawn at the 50% probability level.
Figure 9. Partial thermal ellipsoid diagram of 9, with thermal ellipsoids
drawn at the 50% probability level. The isopropyl substituents on the phenyl
rings have been omitted for clarity.
Figure 7. Partial thermal ellipsoid diagram of 7, with thermal ellipsoids
drawn at the 50% probability level. The isopropyl substituents on the phenyl
rings have been omitted for clarity.
107.8(5)°]. One noncoordinated molecule of diethyl ether solvent
per uranium complex completes the structure.
{[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-[2,5-(i-Pr)2PhNd
C(CH3)]2C6H4}{Na(DME)2}][Na(DME)3]} ·1/2C7H8 (9). The struc-
ture of 9 reveals two ligands attached to one U metal center in
an arrangement strikingly similar to that observed for 8 (Figure
9). An analysis of the bond distances around C2 and C9 in the
first ligand system in comparison to the corresponding bond
distances around C36 and C42 in the second ligand system suggests
that the first molecule has retained the trianionic bis(amido)
configuration of the starting material [C2-C1 ) 1.382(16) Å,
C2-N1 ) 1.409(14) Å, C9-C10 ) 1.321(15) Å, C9-N2 )
1.378(14) Å] while the second ligand instead has gained three
hydrogen atoms to become a neutral bis(imine) [C36-C35 )
1.505(14) Å, C36-N3 ) 1.414(12) Å, C42-C43 ) 1.521(16) Å,
Figure 8. Partial thermal ellipsoid diagram of 8, with thermal ellipsoids
C42-N4 ) 1.286(13) Å]. The uranium metal center is sited in a
drawn at the 50% probability level. The isopropyl substituents on the phenyl
distorted trigonal-bipyramidal coordination polyhedron.
rings have been omitted for clarity.
The two axial positions are occupied by two nitrogen atoms of the
first [bis(amido)] ligand system [U1-N1 ) 2.519(8) Å, U1-N2 )
2.359(8) Å, N1-U1-N2 ) 129.5(3)°], while the carbon atom of the
central phenyl ring occupies one equatorial position [U1-C8 )
2.479(11) Å, N1-U1-C8 ) 64.7(3)°]. Two other equatorial positions
are engaged in an interaction with the second ligand molecule. This
molecule is connected to the metal center through the η4-diene type
of π bonding of one of the imino groups [U1-N3 ) 2.325(8) Å,
U1-C36 ) 2.654(10) Å, N3-U1-N1 ) 106.6(3)°] as well as one
aromatic bond of the central phenyl ring [U1-C37 ) 2.846(10) Å,
U1-C38 ) 2.790(10) Å, C38-U1-C8 ) 148.2(3)°]. A sodium
countercation solvated by two molecules of DME [Na1-O1 )
2.597(12) Å, Na1-O2 ) 2.616(12) Å, Na1-O3 ) 2.487(18) Å,
Na1-O4 ) 2.549(14) Å] is also part of the structure, as it bonds in
η4-diene fashion through the π system encompassing one double bond
have been restored and the terminal carbon atom has been
protonated in this second ligand system [C35-C36 ) 1.502(7)
Å, C43-C44 ) 1.508(7) Å, C36-N4 ) 1.292(6) Å, C43-N3
) 1.425(6) Å].
The structure is completed by one Li cation in a square-
pyramidal arrangement coordinated to the second part of the
same extended π system of the second, formally neutral, ligand
unit [Li1-N4 ) 2.120(11) Å, Li1-C38 )2.713(12) Å]. One
disordered molecule of THF and one of DME complete the
coordination geometry of the alkali metal ion [Li1-O2 )
2.083(12) Å, Li1-O3 ) 2.119(12) Å, O2-Li1-N4 ) 109.3(5)°,
O2-Li1-C38 ) 153.4(5)°, O3-Li1-C38 ) 90.7(4)°, Li1-O1
) 2.120(11) Å, O1-Li1-N4 ) 109.3(5)°, O1-Li1-O2 )
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10413

A R T I C L E S
Korobkov et al.
Scheme 1
[C1-C2 ) 1.382(16) Å] and an aromatic bond of the central phenyl
ring [C3-C4 ) 1.396(15) Å, Na1-C1 ) 3.015(15) Å, Na1-C2 )
3.280(15) Å, Na1-C3 ) 3.276(16) Å, Na1-C4 ) 3.157(16) Å] of
the triply deprotonated ligand. One additional Na countercation ensures
the electroneutrality of the structure. This Na atom resides in an
octahedral environment defined by six oxygen atoms of three DME
solvent molecules. [Na2-O5 ) 2.445(19) Å, Na2-O6 ) 2.405(17)
Å, Na2-O7 ) 2.372(13) Å, Na2-O8 ) 2.388(14) Å, Na2-O9 )
2.355(13) Å, Na2-O10 ) 2.389(12) Å]. One toluene solvent molecule
in the lattice with partial occupancy of 50%, completes the structure.
uranium complex [{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H4}UCl3]-
[Li(THF)4] (1) (Scheme 1). The ligand system did not adopt
the anticipated π-bonding mode (Figure 1). The main interaction
between uranium and the aromatic ring appears to be realized
through only the central C atom of the phenyl ring, without
any significant contributions from the corresponding C-H bond.
The presence of this unconventional interaction is fully sup-
ported by the results of the DFT calculations (see below) and
possibly indicates a partial charge transfer from the tetravalent
uranium center to a portion of the aromatic ring. Furthermore,
the interaction of the U metal center with the C atom of the
ring promotes a noticeable weakening of the corresponding
C-H bond (see Electronic Structures, below), which makes the
Results and Discussion
The reaction of the dianion [1,3-[2,5-(i-Pr)2PhNC(dCH2)]2-
C6H4]2- with UCl4(THF)4 afforded the corresponding tetravalent
9
10414 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
Scheme 2
aromatic ring incline toward direct metalation. In spite of the
fact that the geometrical parameters of the U-C-H arrangement
are similar to those observed for the coordination of cyclohexane
to a U(III) metal center,8c it appears that no significant agostic
interaction is present in this case.
give the new tetravalent compound {1,3-[2,5-(i-Pr)2PhNC-
(dCH2)]2C6H3}U(THF)(µ-Cl)2][Li(THF)2] (4), in which the
oxidation of the trivalent center occurred at the expense of
the central phenyl ring C-H bond (Scheme 1). The fate of the
hydrogen atom is unclear, since no hydrogen gas was recovered
from the reaction when the thermolysis was carried out in a
closed vessel connected to a Toepler pump. Also, no significant
distortions in the complex that might indicate the presence of a
hydride could be observed (Figure 4). As a result of the
metalation, the ring becomes almost coplanar with the uranium
center, with the small deviation of the metal from the plane
probably resulting from the steric repulsion between the i-Pr
substituents and the coordinated THF molecule. Despite the line
broadening, the 1H NMR spectrum of complex 4 allowed
recognition of all of the functional groups of the ligand system.
The most striking spectral feature is the significant line
separation for the two nonequivalent dCH2 protons of the two
ene-amino moieties, which are also magnetically nonequivalent.
These resonances are present as a pair of doublets at 7.17 and
7.13 ppm coupled to other doublets at 2.18 and 2.16 ppm,
respectively. The four i-Pr substituents give two distinctive
doublets centered at 1.47 and 1.44 ppm with the correct
integration. These resonances are coupled to two broad mul-
tiplets of the corresponding ipso hydrogens located at 3.09 ppm
and 7.33 ppm, respectively, with proper integration. The large
difference in the chemical shifts is not particularly surprising,
given the very different structural environments experienced by
these hydrogen atoms. In fact, two are oriented against the
coordinated THF molecules while the other two point against a
void space. The resonances of the three aromatic rings are
located in the conventional range between 9.0 and 7.4 ppm.
The ready ring metalation accompanied by the one-electron
oxidation of the metal center under thermal conditions clearly
frustrated the possibility of using π coordination to the phenyl
ring for possible stabilization of the lower oxidation states.
Therefore, further attempts to prepare reduced complexes
focused on the use of 1 as the most convenient starting material.
In carrying out reduction reactions, we compensated for the
consumption of the reductant by the seemingly unavoidable ring
metalation through the use of larger stoichiometric ratios.
Reaction of 1 with 3 equiv of lithium naphthalenide at room
temperature afforded {{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}-
U(µ-Cl)(µ-[O(CH2)3CH2])[Li(DME)]}[Li(DME)3] (5) (Scheme
1). In this species, the tetravalent metal center forms a
six-membered metallacycle ring with a moiety arising from THF
ring opening (Figure 5). Similar ring opening has been observed
previously by Burns as the result of a nonredox insertion reaction
of THF into U-I bonds.9a In the present case, however, the
1
The H NMR spectrum of 1 is characterized by a large
spreading of 16 fairly sharp lines over the range +50 to -14
ppm. Besides the two resonances of the THFs coordinated to
Li, which reside approximately in the expected regions, the other
resonances could not be assigned conclusively. One of the few
clearly recognizable features was the C-H hydrogen of the
central C atom of the central phenyl ring. Because of the close
proximity of paramagnetic U metal center, the signal of this
hydrogen was located at +49.72 ppm and coupled to a broad
carbon resonance at -0.25 ppm. Also clearly recognizable were
the dCH2 hydrogens, which gave two resonances at +13.79
and -14.30 coupled to the same carbon at 134.61 ppm.
The behavior of this ligand system vis-a`-vis the trivalent state
of uranium was examined by reacting the same ligand dianion
with in situ prepared “UCl3” (Scheme 1). The reaction indeed
afforded a trivalent species formulated as {1,3-[2,5-(i-Pr)2-
PhNC(dCH2)]2C6H4}U(µ-Cl)3[Li(THF)2]2 (2) (Figure 2). The
configuration of the ligand system in this complex is similar to
that in 1, with the same type of arrangement of the central phenyl
ring (Figure 2). The slight increase in the bond distances formed
by the uranium metal center as well as the somewhat stronger
ring tilt could be attributed to an increase in the size of the
metal center due to the lower oxidation state. The only
significant difference in the structure of 2 is the presence of
two Li cations connected to the U-ligand moiety through
chlorine bridges, as required by the trivalent state of the metal
center. NMR spectra were uninformative in this case, showing
only very broad resonances. No signals could be observed in
the 13C NMR spectrum.
Further attempts to lower the oxidation state involved reacting
tetravalent 1 with a series of reagents (Scheme 1). The outcomes
depended greatly on both the reducing agent and reaction
conditions. Reaction of 1 with 1 equiv of lithium naphthale-
nide at 0 °C did not afford 2 but instead yielded a closely
related U(III) complex formulated as {1,3-[2,5-(i-Pr)2PhNC-
(dCH2)]2C6H4}U(THF)(µ-Cl)2][Li(Et2O)2] (3) (Figure 3). Com-
plexes 2 and 3 differ only in their extents of retention of LiCl.
The ligand system showed the same arrangement in the two
complexes, forming the same unusual interaction between the
metal center and the central aromatic ring. As in the case of 2,
the NMR spectra were uninformative.
Both of the trivalent complexes 2 and 3 reacted thermally
in boiling THF, affording oxidation of the metal center to
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10415

A R T I C L E S
Korobkov et al.
Figure 10. R-HOMO and R-HOMO-1 orbitals for the optimized structure of complex 1.
ring opening is clearly the result of two-electron reductive
cleavage of the THF O-C bond. Since complex 5 contains
tetravalent uranium, it is tempting to state that a transient
divalent synthon or reduced species was responsible for the
attack. The two ene-amido moieties remained unmodified
during the reduction, while ring metalation occurred instead,
as expected. The 1H NMR spectrum showed all of the aromatic
resonaces located between 9.14 and 7.05 ppm. The four
isopropyl groups displayed only one doublet at 1.15 ppm
coupled to a broad multiplet at 2.77 ppm with the expected
integration. As in compound 4, the protons of the ene-amino
moiety are magnetically nonequivalent, resulting in a large
splitting of one pair of broad signals at 7.19 and 6.87 ppm
coupled to equally broad resonances at 2.31 and 1.97 ppm,
respectively. The metallacycle -CH2- groups appeared as four
distinct broad resonances. Two of these, tentatively assigned
to the U-CH2- and U-O-CH2- groups, experience the most
significant chemical shifts, to -6.62 and -4.36 ppm respec-
tively. The other two -CH2- groups were instead located in
the more normal range at 2.17 and 0.98 ppm.
Overall, the THF ring opening and phenyl ring metalation
that occurred during the formation of 5 required a total of three
electrons, assuming that the aromatic-ring H atom was either
released as hydrogen gas or transferred to some other acceptor.
To better clarify this point, the same reaction was carried out
in a sealed vessel connected to a Toepler pump, but not even a
trace of hydrogen gas was produced during the reaction. In
addition, the fact that the reaction actually consumed 3 equiV
of reductant and both the starting material and final product
contain the uranium metal in the tetravalent state clearly
suggested that a byproduct may have been formed. To remove
the interference from THF cleavage, an identical reduction was
performed in the more robust solvent DME. Reduction of 1 in
DME with 2 equiv of potassium naphthalenide at room
temperature afforded a mixture of two compounds having very
similar structure. The two different species, [{1,3-[2,5-(i-Pr)2-
PhNC(dCH2)]2C6H3}UCl(OCH3)][Li(DME)3] (6a) and [{1,3-
[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}UCl2][Li(DME)3] (6b), coc-
rystallized in a ratio very close to 1:1 in the same unit cell,
providing single crystals of mixed composition (Figure 6a,b).
The salient characteristic of the structure of the two compounds
is again the ring metalation, as observed in the previously
reported compounds. While complex 6a has one chlorine atom
and one methoxy group, complex 6b bears two chlorine atoms.
The methoxy group is likely generated by DME cleavage, and
the uranium is tetravalent in both species. In line with the
previous case, no gaseous hydrogen gas was produced during
the reaction.
Figure 11. Spin density distribution for the optimized structure of complex
1. The isosurface contour value is 0.0008 e Å-3
.
We thus attempted the reduction of 1 with excess of NaH in
DME. After a slow reaction, a new species formulated as {1,3-
[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U(µ-OCH3)3(µ,η6-Na)[η3-
Na(DME)] (7) was isolated in significant yield. Once again,
the crystal structure revealed the presence of several methoxy
groups coordinated to the U center in addition to the metalation
of the ligand phenyl ring (Figure 7). The formation of 7 unfolds
some interesting aspects of the reduction process. The formation
of a methoxy moiety from DME as the result of the attack by
one or two low-valent metal centers was also observed in the
case of 6a and 6b. The presence of three methoxide units in 7
raises two different possibilities. The first consists of the
formation of an “over-reduced” species followed by simulta-
neous or consecutive “three-step, two-electron oxidations”. Such
a large storage of electrons is clearly an improbable event.
Perhaps more realistic is a process involving one-electron
reduction immediately followed by DME attack, which is
repeated three times in what is reminiscent of the beginning of
a catalytic process. 1H NMR data for 7 showed the usual
features. All of the aromatic signals were very well resolved
and located between 8.68 and 6.93 ppm. The ene-amino
protons showed the enhanced magnetic nonequivalency, with
two doublets centered at 7.93 and 7.84 ppm coupled to two
other doublets at 2.90 and 2.34 ppm, respectively. The four
isopropyl groups displayed two doublets centered at 1.11 and
1.09 ppm coupled to the same multiplet at 2.74 ppm. The
positions of the methoxy residue resonances are rather unusual:
two of the three methoxy groups gave a broad resonance at
-3.10 ppm, while the third was observed as a sharper line at
4.60 ppm.
From the seemingly ubiquitous solvent cleavage and ring
metalation, it became apparent that milder reaction conditions
were the only possible way to isolate intermediate reduced
species of such a high reactivity. Thus, reduction of 1 was
carried out at low temperature (-35 °C) with a larger amount
of lithium naphthalenide (4 equiv). After suitable workup, the
new species {[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-[2,5-
(i-Pr)2PhNdC(CH3)]2C6H4}][Li(DME)(THF)]}·Et2O (8) was
Recent findings have shown that NaH may be a convenient
reductant for the preparation of formally low-valent com-
plexes,17b,18,19d,21 including examples of dinitrogen fixation.18,19d,21
9
10416 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
Scheme 3. Simplification of the Structures of Complexes 8 and 9a
a Truncated parts are depicted in red. The fragments shown in black were used in the electronic structure analysis.
Figure 12. R-HOMO and R-HOMO-1 orbitals for the optimized structure of the simplified model of complex 8.
Inspection of the structural parameters clearly indicated that
this second ligand unit has acquired two hydrogen atoms and
that the imine functions have been restored, forming a neutral
bis(imino)benzene unit. The formation of complex 8 by reduc-
tion of tetravalent 1 is the result of a complex series of
transformations. In fact, the presence of one intact neutral ligand
implies that during the reaction, one doubly deprotonated ligand
regained the two hydrogen atoms and then became demetalated
and π-coordinated to a second, “formally reduced” metal center.
While the fate of the demetalated metal center remains unclear,
the origin of the two hydrogen atoms can be tentatively ascribed
to the phenyl rings of two other ligand units that were phenyl-
metalated. Thus, it is possible that ring metalation is not the
result of a direct redox transformation but instead arises from
a C-H σ-bond metathesis process. In turn, this may explain
the absence of hydrogen gas from the reaction mixtures. While
this implies that even more uranium-containing species may be
partner products of this series of transformations, we observe
that from the formal point of view, this reaction can be
interpreted as a two-electron reduction affording a “formally”
U(II) bis(amino)benzene complex. Of course, given the π
coordination of the additional neutral ligand, the oxidation state
is likely to be higher (see below). 13C NMR data did not provide
any useful information because of the very large extent of peak
broadening and overlap. The only recognizable features in the
1H NMR spectrum between +26 and -90 ppm were the methyl
signals of the isopropyl group located in their normal positions
at 1.29 and 1.85 ppm.
Figure 13. Spin density distribution for the simplified model structure of
complex 8. The isosurface contour value is 0.0008 e Å-3
.
Scheme 4. Mayer Bond Orders (in Red) and MPA-Derived Atomic
Charges (au, in Blue) for the π-Bonded Bis(imino)benzene
Fragment in 8
Even though complex 8 gives no evidence of having been
directly involved in solvent cleavage, another reduction was
carried out at ambient temperature but in noncoordinating
solvent. Reaction of 1 with excess NaH in toluene proceeded
slowly at room temperature and afforded a significant yield of
{[{1,3-[2,5-(i-Pr)2PhNC(dCH2)]2C6H3}U{1,3-[2,5-(i-Pr)2PhNd
isolated in significant yield (Scheme 2). Even in this case, the
uranium atom is surrounded by the much-expected trianionic,
ring-metalated ligand (Figure 8). However, a second ligand unit
is bonded to the metal center through a part of the π system.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10417

A R T I C L E S
Korobkov et al.
Figure 14. R-HOMO and R-HOMO-1 orbitals for the optimized structure of the simplified model of complex 9.
Electronic Structures. The X-ray structures of complexes
1-3 revealed the presence of a close contact between the U
metal center and the C-H bond of the central phenyl ring
of the ligand. At first glance, the relative positions of these
three atoms may suggest the presence of a three-center, two-
electron agostic U-H-C interaction. However, DFT calcula-
tions performed using the crystallographic atomic coordinates
revealed a rather different scenario. A full-molecule geometry
optimization was performed prior to the electronic structure
evaluation and provided a satisfactory match with the X-ray
structural data for all of the bonds and angles (see the
Supporting Information).
The calculations established that the three-center bond-order
index for the U-C-H interaction in complex 1 is equal to 0,
which rules out the presence of agostic interactions. Furthermore,
the calculations predicted only a minor elongation of the C-H
bond from 1.094 to 1.103 Å, which is smaller than the typical
+0.07 Å C-H bond expansion normally expected for a
metal-H-C agostic interaction.32 Accordingly, the U-H bond
order is only 0.07. Conversely, the U-C interaction was
calculated to be much stronger, displaying a Mayer bond order
of 0.23. Such an interaction may be described in terms of weak
π bonding and an electron density transfer of 0.06 from the
portion of the π orbital located over C8 to the U(IV) atom. The
spin density on the U atom was calculated as 2.22, which reveals
the presence of two unpaired 5f electrons (Figures 10 and 11).
The uranium valence index of 6.9 supports this assumption,
indicating the existence of two free valences in addition to five
conventional σ bonds.
DFT calculations were also performed on both 8 and 9 to
evaluate the most realistic oxidation state of the metal center.
From the formal point of view, the two compounds contain
the U metal center in the unrealistically low U(II) and U(I)
oxidation states, respectively. Both 8 and 9 possess two ligand
systems. In each compound, the two ligands bonded to the
metal have conceptually different electronic structures. One
possesses three negatively charged donor atoms coordinated
to the U metal center through three σ bonds. The other
displays structural parameters typical of the neutral molecule
and is connected to the U metal center through a series of π
interactions. In complex 8, the same π-coordinated ligand
system also hosts an alkali cation (Scheme 3). This alkali
cation is retained through several π interactions and is
situated on the remote part of the ligand system, having no
significant interactions with U metal center. Therefore, in
Figure 15. Spin density distribution for the simplified model structure of
complex 9. The isosurface contour value is 0.0008 e Å-3
.
Scheme 5. Mayer Bond Orders (in Red) and MPA-Derived Atomic
Charges (au, in Blue) of the π-Bonded Bis(imino)benzene
Fragment in 9
C(CH3)]2C6H4}{Na(DME)2}][Na(DME)3]} · 1/2C7H8 (9) after
crystallization from DME. Similar to 8, the complex still
contains one ring-metalated trianionic ligand and one intact
ligand with the H atoms regained and the two imine functions
restored (Figure 9). The metric parameters of the anionic
moiety in 9 compare well with those of 8. The main
difference consists of the presence of two solvated Na atoms
instead of one Li cation. This assigns a formal monovalent
state to uranium. Such a low oxidation state is of course
unrealistic for uranium, which most likely embarks in a
substantial back-bonding interaction with the π system of
the neutral ligand. In any event, this complex appears to be
a species reduced to the largest extent observed to date. The
NMR data for this compound were completely uninformative
because of substantial line broadening and signal overlap.
Although 8 and 9 are formally highly reduced species, the
reactivity of the metal center seems to have been substantially
quenched by the acquisition of the second ligand system.
These species no longer reacted with ethers and upon
exposure to a wide range of mild oxidating agents at high
temperature gave only intractable materials.
(32) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908.
9
10418 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

Reduced Uranium Complexes
A R T I C L E S
Figure 16. Highest-occupied fragment orbitals (HOFOs) of the dianionic ligand that interact with the U(IV) ion in complex 8.
order to optimize calculation time for compounds 8 and 9,
the calculations were performed on truncated models obtained
by replacing the distant half of the bis(imino)benzene
molecules by hydrogen atoms. Such a simplification is
justified in light of the remote position of this moiety,
resulting in total absence of any interaction with U metal
center. The geometry of the ligands in both cases prevents
extensive conjugation as well as delocalization of the
electrons through π-system interactions. It should be also
taken into consideration that employing this simplification
led to the removal of the alkali cation associated with the
truncated part of the molecule in the case of compound 8.
Such cation removal causes the appearance of one additional
negative charge on the model structure, for which the
presence of the positively charged metal center compensates
in the full structure.
Another model simplification involved replacing three out
of the four i-Pr groups in complex 8 with H atoms. The last
i-Pr group was not simplified because of its close proximity
to the U metal center and possible influence in stabilization
of the oxidation state through agostic interactions. All of the
i-Pr substituents in the model of 9 were preserved for similar
reasons. All of the alkali cations and their coordinated solvent
molecules were removed from both structures. All of the other
heavy-atom coordinates were imported directly from the
X-ray data and used without further geometry optimization.
All of the C-H bond distances were set to a length of
1.08 Å.
Calculations were performed for different multiplicities
corresponding to hypothetical U(II) f4 and U(IV) f2 configu-
rations for complex 8 and U(I) f5 and U(III) f3 configurations
for complex 9. For complex 8, the possibilities of singlet (S
) 0), triplet (S ) 1), and quintet (S ) 2) states were
evaluated. The results clearly indicated that the triplet state
is the ground state for 8, in further agreement with the strong
paramagnetism already suggested by the NMR data. For
complex 9, three possibilities were analyzed, namely, the
doublet (S ) 1/2), quartet (S ) 3/2), and sextet (S ) 5/2) ground
states. The calculations indicated that the doublet ground state
has the lowest total electronic energy, with ∆Etot ) 152.4
and 23.8 kcal/mol required to reach the quartet and sextet
states, respectively. The net electron spin densities on the
uranium metal center were determined to be 2.24 for complex
8 and 1.27 for complex 9. Mulliken population analysis
(MPA) for compound 8 revealed two unpaired electrons with
nearly pure 5f character (91 and 94% for the R-spin HOMO
and HOMO-1, respectively; Figure 12), accounting for the
spin density distribution shown in Figure 13. In summary,
the most realistic oxidation state of the uranium center in
this species can be assigned as +4, with the ligand molecule
playing the role of storage of electron density. The electrons
hosted in the π-bonded ligand fragment are used for diene-
type bonding with the U metal center (Scheme 4).
Similar to complex 8, MPA of 9 indicated that the complex
has three electrons in three nearly degenerate U 5f nonbonding
orbitals (89% U for the R-HOMO, 92% U for the R-HOMO-1,
and 100% U for the ꢁ-HOMO; Figure 14), resulting in the spin
density distribution shown in Figure 15. In this case, the most
realistic oxidation state of the metal center could be regarded
as +3, with the ligand hosting two “additional” electrons in
the delocalized π system as presented in Scheme 5.
Calculations on the π-bonded moiety holding an overall
charge of -2 indicated that for complex 8 as well as complex
9, two additional electrons are hosted mainly in the three highest-
occupied fragment orbitals (HOFOs) of the π-coordinated ligand
(Figure 16).
Overall, the donor-acceptor interactions between the uranium
ion and the dianionic π-bonded moiety in complex 8 are limited
to charge transfer to the metal from the HOFOs in Figure 16.
This charge transfer is evident from the changes in the orbital
occupancies upon complex formation:28c R-HOFO, 40.4%;
ꢁ-HOFO, 27.2%; R-HOFO-1, 13.3%; ꢁ-HOFO-1, 11.3%;
R-HOFO-2, 13.1%; ꢁ-HOFO-2, 11.4%. As can be seen from
the changes in occupancies, the HOFOs of the ligand play the
most important role in charge transfer to the U(IV) ion. Since
the nature of these orbitals in complexes 8 and 9 is nearly
identical, the same orbitals contribute to covalent bonding
between the metal and the ligand in complex 9: R-HOFO,
43.0%; ꢁ-HOFO, 26.6%; R-HOFO-1, 14.1%; ꢁ-HOFO- 1,
11.4%; R-HOFO-2, 12.1%; ꢁ-HOFO-2, 10.2%. These large
changes in the donor-orbital occupancies highlight the impor-
tance of metal-ligand covalent interactions in the stabilization
of 8 and 9.
The most significant observed difference between 8 and 9
involves the interactions of the metal center with the aliphatic
carbons of the isopropyl groups. In the case of 8, such
interactions are completely negligible. However, in the case of
9, several of the C-H bonds of the isopropyl groups are oriented
to interact with the U atom, with one ipso C-H interaction being
noticeably stronger. The significance of these interactions is
confirmed by the Mayer bond orders for the corresponding U-C
and U-H contacts, for which the largest values (0.35 and 0.21,
respectively) were found for U1-C32 and U1-H32a. Although
they contribute to the stabilization of the complex, these
interactions do not cause significant changes in the C-H bond
lengths.
Conclusions
Use of the bis(imino)benzene ligand made possible the isolation
of several new U complexes. Reduction reactions mainly led to
different solvent fragmentation products. In every case, the solvent
fragmentation suggested transient formation of low-valent or
reduced species. In contrast, the ligand system appears to be quite
resilient, as its only involvement in the reactivity of the metal center
is the direct metalation of the aromatic ring. Decreasing the
temperature of the reduction or using noncoordinating solvents
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10419

A R T I C L E S
Korobkov et al.
Acknowledgment. This work was supported by the Natural
Science and Engineering Research Council (NSERC) of Canada.
allowed the isolation of two complexes in which the U metal center
coordinates to two anionic radical ligands. In these species, the
metal center deceivingly appeared to be unusually low-valent.
However, DFT calculations unequivocally confirmed that the actual
oxidation states of the U metal centers in these two complexes
could be assigned as U(IV) and U(III), respectively, and that the
U centers were coupled to dianionic forms of the ligand, with
electron storage performed by the π-coordinated systems.
Supporting Information Available: Complete ref 24, crystal-
lographic data (CIF), and tables of computational data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA9002525
9
10420 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009