From bent to linear uranium metallocenes: Influence of counterion, solvent, and metal ion oxidation state

Article abstract of DOI:10.1021/om060642g

Reactions of Cp*₂UCl₂ with a slight excess of Me₃SiI or Me₃SiOTf in acetonitrile provide convenient routes to Cp*₂UI₂ (1) or Cp*₂U(OTf) ₂ (2) (Cp* = η-C₅Me₅; OTf = OSO ₂CF₃), which were isolated with excellent yields. Crystals of the bent metallocenes Cp*₂UI₂(NCR) (R = Me (3), ^tBu (4)) have been obtained by addition of excess RCN to a toluene solution of 1. The triflate analogue Cp*₂U(OTf) ₂-(NCMe) (5) contains monodentate OTf ligands. Treatment of Cp*₂UMe₂ with 2 equiv of HNEt₃BPh ₄ in acetonitrile led to the immediate formation of [Cp*₂U(NCMe)₅][BPh₄]₂ (6). The ¹H NMR spectra of 1 and 2 in acetonitrile are similar to that of 6, indicating the presence of the dicationic species [Cp*₂U-(NCMe) ₅][X]₂ [X = I (7), OTf (8)] in this solvent. Compounds 6-8 are the first linear metallocenes of an f-element and exhibit an unprecedented structure with auxiliary donor ligands in the equatorial plane parallel to the cyclopentadienyl rings. When treated with Me₃SiOTf, the uranium(III) species Cp*₂UCl₂-Na(thf)₂ did not afford the trivalent complex Cp*₂U(OTf) (9) but gave crystals of the dimeric uranium-(IV) ate compound [Cp*₂U(OTf) ₃Na(thf)₂]₂·2(CH₂) ₄O (10·2(CH₂)₄O). The first organouranium(III) triflate, 9, was isolated in 90% yield from the reaction of U(OTf)₃ with 2 equiv of KCp* in thf. Dissolution of Cp*₂UI(py) in acetonitrile gave the adduct Cp* ₂UI(NCMe)₂ (11), which, like its cerium analogue, adopts a bent sandwich structure in its crystalline form. The ¹H NMR spectra of 11 and Cp*₂U(OTf)-(NCMe)₂ are similar in benzene and acetonitrile, suggesting that the trivalent linear metallocene [Cp*₂U-(NCMe)₅][X] (X = I or OTf) is not present in pure acetonitrile. These observations are confirmed by the addition of 3 equiv or excess MeCN to a solution of [Cp*₂U(thf)₂] [BPh₄] in thf, which afforded the stable [Cp* ₂U(NCMe)₃][BPh₄] as the sole product.

Full text of DOI:10.1021/om060642g

Organometallics 2006, 25, 5603-5611
5603
From Bent to Linear Uranium Metallocenes: Influence of
Counterion, Solvent, and Metal Ion Oxidation State
Je´roˆme Maynadie´, Jean-Claude Berthet,* Pierre Thue´ry, and Michel Ephritikhine*
SerVice de Chimie Mole´culaire, DSM, DRECAM, CNRS URA 331, Laboratoire Claude Fre´jacques,
CEA/Saclay, 91191 Gif-sur-YVette, France
ReceiVed July 17, 2006
Reactions of Cp*2UCl2 with a slight excess of Me3SiI or Me3SiOTf in acetonitrile provide convenient
routes to Cp*2UI2 (1) or Cp*2U(OTf)2 (2) (Cp* ) η-C5Me5; OTf ) OSO2CF3), which were isolated with
t
excellent yields. Crystals of the bent metallocenes Cp*2UI2(NCR) (R ) Me (3), Bu (4)) have been
obtained by addition of excess RCN to a toluene solution of 1. The triflate analogue Cp*2U(OTf)2-
(NCMe) (5) contains monodentate OTf ligands. Treatment of Cp*2UMe2 with 2 equiv of HNEt3BPh4 in
1
acetonitrile led to the immediate formation of [Cp*2U(NCMe)5][BPh4]2 (6). The H NMR spectra of 1
and 2 in acetonitrile are similar to that of 6, indicating the presence of the dicationic species [Cp*2U-
(NCMe)5][X]2 [X ) I (7), OTf (8)] in this solvent. Compounds 6-8 are the first linear metallocenes of
an f-element and exhibit an unprecedented structure with auxiliary donor ligands in the equatorial plane
parallel to the cyclopentadienyl rings. When treated with Me3SiOTf, the uranium(III) species Cp*2UCl2-
Na(thf)2 did not afford the trivalent complex Cp*2U(OTf) (9) but gave crystals of the dimeric uranium-
(IV) “ate” compound [Cp*2U(OTf)3Na(thf)2]2‚2(CH2)4O (10‚2(CH2)4O). The first organouranium(III)
triflate, 9, was isolated in 90% yield from the reaction of U(OTf)3 with 2 equiv of KCp* in thf. Dissolution
of Cp*2UI(py) in acetonitrile gave the adduct Cp*2UI(NCMe)2 (11), which, like its cerium analogue,
1
adopts a bent sandwich structure in its crystalline form. The H NMR spectra of 11 and Cp*2U(OTf)-
(NCMe)2 are similar in benzene and acetonitrile, suggesting that the trivalent linear metallocene [Cp*2U-
(NCMe)5][X] (X ) I or OTf) is not present in pure acetonitrile. These observations are confirmed by the
addition of 3 equiv or excess MeCN to a solution of [Cp*2U(thf)2][BPh4] in thf, which afforded the
stable [Cp*2U(NCMe)3][BPh4] as the sole product.
to the new starting materials Cp*2UX (X ) I,4 BPh4 ) or Cp*2U-
(C6H6)UCp*2.6 These complexes were found to activate small
unsaturated molecules, revealing unexpected reactions and
new structural features and favoring a renewal of interest for
the chemistry of U3+ reducing agents.7 In contrast, the diffi-
cult or multistep preparations of the uranium(IV) compounds
Cp*2UX2 (X ) I,8 OTf9) certainly prevented their utilization
as starting materials. Here we present new convenient syn-
theses of these complexes, their conversion in acetonitrile
into the dicationic species [Cp*2U(NCMe)5][X]2 and the prep-
aration of [Cp*2U(NCMe)5][BPh4]2. With the aim to extend
the variety of U(III) precursors, we also report the synthesis of
the first organouranium(III) triflate complex Cp*2U(OTf) and
of Cp*2UI(NCMe)2, a rare example of a crystallographically
characterized Lewis base adduct of Cp*2UI. While the neutral
compounds Cp*2UX2(NCMe) (X ) I, OTf) and Cp*2UI-
(NCMe)2 adopt the familiar bent sandwich configuration, the
cationic complexes [Cp*2U(NCMe)5][X]2 (X ) I, OTf, BPh4),
5
Introduction
The nature of the halide and pseudo-halide X ligand in the
starting materials [M]Xn is of crucial importance in inorganic
and organometallic chemistry since it has a strong influence on
the synthesis, solubility, structure, and reactivity of the com-
plexes. The nucleophilic character of the X group has a
significant effect in metathesis reactions, and many efforts have
been devoted to the preparation of useful precursors containing
weakly coordinating anionic ligands and of highly reactive
cationic complexes. In f-element chemistry, such compounds
proved to be of major interest, permitting in particular the
circumvention of the pervasive problem of the formation of “ate”
species related to the retention of X on the metal center.1
The series of bis(pentamethylcyclopentadienyl) complexes,
which are regarded as models in organouranium chemistry,
were most generally synthesized from the dichloride precursor
Cp*2UCl2 and its reduced derivatives [Cp*2UCl]3 and
Cp*2UCl2Na(thf)2.2,3 This situation has recently changed in
uranium(III) chemistry with the discovery of practical routes
(4) Avens, L. R.; Burns, C. J.; Butcher, R. J.; Clark, D. L.; Gordon, J.
C.; Shake, A. R.; Scott, B. L.; Watkin, J. G.; Zwick, B. D. Organometallics
2000, 19, 451.
* Authors to whom correspondence should be addressed. E-mail:
jean-claude.berthet@cea.fr; michel.ephritikhine@cea.fr.
(5) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W. Organo-
metallics 2002, 21, 1050.
(6) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J. Am.
Chem. Soc. 2004, 126, 14533.
(7) (a) Evans, W. J. J. Organomet. Chem. 2002, 652, 61. (b) Evans, W.
J.; Kozimor, S. A. Coord. Chem. ReV. 2006, 250, 911. (c) Ephritikhine, M.
Dalton Trans. 2006, 2501.
(8) Enriquez, A. E.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2005, 44,
7403.
(1) (a) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. ReV.
1995, 95, 865. (b) Edelmann, F. ComprehensiVe Organometallic Chemistry;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; 1995; Vol. 4, Chapter 2,
p 11. (c) Marks, T. J. ComprehensiVe Organometallic Chemistry; Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds.; 1982; Vol. 3, Chapter 21, p 173.
(2) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks,
T. J. J. Am. Chem. Soc. 1981, 103, 6650.
(3) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S.
H.; Day, V. W. Organometallics 1982, 1, 170.
(9) Berthet, J. C.; Nierlich, M.; Ephritikhine, M. Eur. J. Inorg. Chem.
2002, 850.
10.1021/om060642g CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/10/2006

5604 Organometallics, Vol. 25, No. 23, 2006
Maynadie´ et al.
at δ 18.72 (s, w1/2 20 Hz, 30 H, Cp*), 0.23 (s, w1/2 50 Hz, 45 H,
MeCN).
resulting from total dissociation of the X ligands and satu-
ration of the equatorial girdle, are the first linear metallo-
cenes in the f-element series.10 This finding led us to consider
the possible formation of such complexes in the +3 oxidation
state.
Crystals of Cp*2U(OTf)2(NCMe) (5). An NMR tube was
charged with 2 (17.2 mg, 0.021 mmol) in toluene (0.5 mL), and
MeCN (1.1 µL, 0.021 mmol) was added via a microsyringe. Dark
red crystals of 5 were deposited within a few hours at room
temperature.
Experimental Section
Synthesis of [Cp*2U(NCMe)5][BPh4]2 (6). Method 1. A
flask was charged with Cp*2UMe2 (109.2 mg, 0.20 mmol) and
HNEt3BPh4 (170.9 mg, 0.41 mmol). Acetonitrile (20 mL) was
condensed into it, and an immediate release of gas was observed.
After stirring for 30 min at 20 °C, the solvent was evaporated off,
leaving a brown crystalline powder, which was washed with thf (3
mL) and dried under vacuum. Yield: 220.0 mg (80%). Anal. Calcd
for C78H85B2N5U: C, 69.28; H, 6.34; N, 5.18. Found: C, 69.03;
H, 6.53; N, 5.05.
All reactions were carried out under argon with the rigorous
exclusion of air and water (<5 ppm oxygen or water) using standard
Schlenk-vessel and vacuum-line techniques or in a glove box.
Solvents were dried over KH for pyridine and acetonitrile and over
a mixture of sodium and benzophenone for thf, and distilled imme-
diately before use. IR samples were prepared as Nujol mulls be-
tween KBr round cell windows and the spectra recorded on a
1
Perkin-Elmer FT-IR 1725X spectrometer. The H NMR spectra
were recorded on a Bruker DPX 200 instrument and referenced
internally using the residual protio solvent resonances relative to
tetramethylsilane (δ 0); the spectra were recorded at 23 °C when
not otherwise specified. Elemental analyses were performed by
Analytische Laboratorien at Lindlar (Germany). Me3SiI (Aldrich)
was stored under argon over 3 Å molecular sieves. Me3SiOTf
(Aldrich) was distilled and stored under argon over 3 Å molecular
sieves. KH under mineral oil (Aldrich) was used after washings
with toluene. KCp* was prepared by mixing a slight excess of
C5Me5H with KH in thf. TlBPh4 was precipitated as a white powder
by mixing aqueous solutions of NaBPh4 and TlNO3; the compound
was then washed with hot water followed with diethyl ether and
dried under vacuum for 48 h. Cp*2UCl2,2 Cp*2UCl2Na(thf)2,3
Cp*2UMe2,2 UI3(py)4,11 and U(OTf)312 were synthesized as previ-
ously reported.
Method 2. A flask was charged with Cp*2UI2 (200.0 mg, 0.26
mmol) and TlBPh4 (274.8 mg, 0.52 mmol), and acetonitrile (50
mL) was condensed in. The brown solution was stirred for 2 h at
room temperature, and the yellow precipitate of TlI was filtered
off. The volume of the solution was reduced to 10 mL, and diethyl
ether (50 mL) was slowly condensed into it at -78 °C. The red
crystals that were deposited after 12 h at 20 °C were filtered off
and washed twice with the solution in order to eliminate a fluffy
white contaminant. Yield: 180 mg (51%). Anal. Calcd for
C78H85B2N5U: C, 69.28; H, 6.34; N, 5.18. Found: C, 69.14; H,
1
6.35; N, 5.14. H NMR (acetonitrile-d3): δ 35.3 (s, w1/2 600 Hz,
30 H, Cp*), 7.06-6.95 (m, 40 H, BPh4); (dmf-d7): 36.6 (s, w1/2
385 Hz, 30 H, Cp*), 7.25-6.95 (m, 40 H, BPh4), 2.20 (s, 15 H,
MeCN); (pyridine-d5): δ 35.82 (s, w1/2 33 Hz, 30 H, Cp*), 7.70-
6.32 (m, 40 H, BPh4), -44.50 (s, w1/2 750 Hz, 15 H, MeCN). IR
spectrum: ν(CN) ) 2262 and 2269 cm-1. The dark red solution
of 6 in dmf was evaporated to dryness and the brown residue
dissolved in acetonitrile-d3. The 1H NMR spectrum exhibits signals
at δ 35.0 (s, w1/2 110 Hz, 30 H, Cp*), 7.23-6.90 (m, 5 H + 40 H,
dmf + BPh4), 2.53 (s, w1/2 250 Hz, 30 H, dmf); at -40 °C: δ 43.5
(s, w1/2 260 Hz, 30 H, Cp*), 7.90 (s, w1/2 33 Hz, 5 H, dmf), 7.06-
6.81 (m, 40 H, BPh4), 2.91 (s, 15 H, dmf), 2.76 (s, 15 H, dmf).
Crystals of [Cp*2U(NCMe)5][I]2‚nMeCN (7‚nMeCN) (n ) 1,
2.5) and [Cp*2U(NCMe)5][OTf]2‚MeCN (8‚MeCN). Brown crys-
tals of 7‚MeCN were obtained by slow diffusion of diethyl ether
into an acetonitrile solution of 1. Crystals of 7‚2.5MeCN were
obtained by slow evaporation of an acetonitrile solution of 1. Brown
crystals of 8‚MeCN were deposited from a toluene solution of 2 in
the presence of 15 equiv of MeCN. These crystals were converted
into 1 and 2 upon drying under vacuum.
Synthesis of Cp*2UI2 (1). A flask was charged with Cp*2UCl2
(1061 mg, 1.83 mmol), and acetonitrile (50 mL) was condensed
in. Addition of Me3SiI (782 µL, 5.49 mmol) into the red suspen-
sion led immediately to a dark red solution. After stirring for 1 h
at 20 °C, the solution was filtered and the solvent evaporated
off, affording a brown powder, which was washed with pentane
(10 mL) and dried under vacuum. Yield: 1263 mg (91%). Anal.
Calcd for C20H30I2U: C, 31.51; H, 3.97; I, 33.30. Found: C, 31.24;
1
H, 3.90; I, 33.05. H NMR (benzene-d6): δ 18.1 (s, w1/2 37 Hz);
(thf-d8): δ 18.5 (s, w1/2 35 Hz); (pyridine-d5): δ 18.8 (s, w1/2 33
Hz); (acetonitrile-d3): δ 35.1 (s, w1/2 597 Hz).
Synthesis of Cp*2U(OTf)2 (2). A flask was charged with
Cp*2UCl2 (1270 mg, 2.19 mmol), and acetonitrile (50 mL) was
condensed in. Addition of Me3SiOTf (1190 µL, 6.57 mmol) at 20
°C to the red suspension gave immediately a dark red solution,
which was stirred for 1 h at 20 °C. After filtration, evaporation of
the solvent afforded a red-brown powder, which was washed with
pentane (10 mL) and dried under vacuum. Yield: 1732 mg (98%).
Anal. Calcd for C22H30F6O6S2U: C, 32.75; H, 3.76; F, 14.13.
Found: C, 32.61; H, 3.81; F, 13.97. 1H NMR (toluene-d8): δ 18.38
(s, w1/2 19 Hz); (acetonitrile-d3): δ 35.4 (s, w1/2 715 Hz).
Crystals of Cp*2UI2(NCMe) (3) and Cp*2UI2(NCtBu)‚C7H8
(4‚C7H8). An NMR tube was charged with 1 (15.0 mg, 0.019 mmol)
Synthesis of Cp*2U(OTf) (9). A flask was charged with U(OTf)3
(397.0 mg, 0.579 mmol) and KCp* (222.2 mg, 1.275 mmol), and
thf (20 mL) was condensed in. The color of the solution rapidly
turned dark green, and a white precipitate of KOTf was formed.
The solution was stirred for 90 min at room temperature, and its
volume was reduced to 10 mL. The dark green solution obtained
after addition of pentane (30 mL) was filtered. Evaporation of the
solvent and washing of the residue with pentane (20 mL) afforded
a green powder of 9 after drying under vacuum. Yield: 350 mg
(92%). Anal. Calcd for C21H30F3O3SU: C, 38.39; H, 4.60. Found:
t
in toluene (0.5 mL), and an excess of MeCN or BuCN (ca. 15
equiv) was introduced via a microsyringe. The solution was cooled
at -5 °C, and dark brown crystals were deposited after 15 h.
An NMR tube was charged with 1 (15.0 mg, 0.019 mmol) in
toluene-d8 (0.5 mL), and MeCN (1 µL, 0.019 mmol) was introduced
via a microsyringe. 1H NMR (toluene-d8): δ 18.51 (s, w1/2 10 Hz,
30 H, Cp*), -0.27 (s, w1/2 70 Hz, 3 H, MeCN). After addition of
1
C, 38.13; H, 4.42. H NMR (thf-d8): δ -3.24 (s, w1/2 56 Hz);
(acetonitrile-d3): δ -2.64 (s, w1/2 35 Hz). Evaporation of an
acetonitrile solution of 9 gave a brown powder of Cp*2U(OTf)-
1
(NCMe)2. H NMR (benzene-d6): δ -2.93 (s, w1/2 40 Hz, 30 H,
1
Cp*), -31.6 (s, w1/2 85 Hz, 6 H, MeCN).
15 molar equiv of MeCN, the H NMR spectrum exhibits signals
Crystals of [Cp*2U(OTf)3Na(thf)2]2‚2(CH2)4O (10‚2(CH2)4O).
An NMR tube was charged with Cp*2UCl2 (17 mg, 0.03 mmol)
and 2% Na(Hg) (34 mg, 0.03 mmol of Na), and thf (0.5 mL) was
condensed in. The solution rapidly took the green color character-
istic of the U(III) species Cp*2UCl2Na(thf)2. A large excess of
Me3SiOTf (81.5 µL, 0.45 mmol) was then introduced, and the color
(10) Preliminary communication: Maynadie´, J.; Berthet, J. C.; Thue´ry,
P.; Ephritikhine, M. J. Am. Chem. Soc. 2006, 128, 1082.
(11) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin,
J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248.
(12) Berthet, J. C.; Lance, M.; Nierlich, M.; Ephritikhine, M. Eur. J.
Inorg. Chem. 1999, 2005.

From Bent to Linear Uranium Metallocenes
Organometallics, Vol. 25, No. 23, 2006 5605
of the solution immediately turned red-brown. After 5 min at room
temperature, slow concentration of the solution led to the formation
of red crystals of 10‚2(CH2)4O.
Crystals of Cp*2UI(NCMe)2 (11). Dark brown crystals of 11
were obtained by cooling at 4 °C a toluene solution of Cp*2UI(py)
in the presence of 15 equiv of acetonitrile. Evaporation of an
acetonitrile solution of Cp*2UI(py), under vacuum for 15 h at room
temperature, afforded the stable adduct 11 in quantitative yield.
1H NMR (benzene-d6): δ -1.59 (s, w1/2 43 Hz, 30 H, Cp*), -32.12
(s, w1/2 93 Hz, 6 H, MeCN); (acetonitrile-d3): δ -1.09 (s, w1/2 65
Hz, Cp*).
Behavior of [Cp*2U(thf)2][BPh4] in Acetonitrile. 1H NMR of
[Cp*2U(thf)2][BPh4] (thf-d8, 23 °C): δ 5.59-5.14 (m, 20 H, Ph),
0.10 (s, w1/2 100 Hz, 30 H, Cp*); (acetonitrile-d3): δ 7.17-6.84
(m, 20 H, Ph), 3.76 (s, w1/2 7 Hz, 8 H, thf), 2.02 (s, w1/2 100 Hz,
8 H, thf), -2.59 (s, w1/2 30 Hz, 30 H, Cp*). Evaporation to dryness
of an acetonitrile solution of [Cp*2U(thf)2][BPh4] (15.2 mg, 0.016
mmol) gave a brown powder of [Cp*2U(NCMe)3][BPh4]. 1H NMR
(thf-d8, 23 °C): δ 5.63-5.18 (m, 20 H, Ph), 0.16 (s, w1/2 30 Hz,
30 H, Cp*), the resonances of MeCN are not visible; (thf-d8, -80
°C): δ 4.33, 3.81, 3.19 (s, w1/2 30 Hz, 20 H, Ph), -0.49 (s, w1/2
100 Hz, 30 H, Cp*), -76.7 (s, w1/2 100 Hz, 9 H, MeCN). After
addition of MeCN (2.5 µL, 0.048 mmol), the spectrum at -80 °C
was unchanged, except for the appearance of the signal of free
MeCN at δ 1.82 (w1/2 60 Hz).
Crystallographic Data Collection and Structure Determina-
tion. The data were collected at 100(2) K on a Nonius Kappa-
CCD area detector diffractometer13 with graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). The crystals were introduced
into glass capillaries with a protective “Paratone-N” oil (Hampton
Research) coating. The unit cell parameters were determined from
10 frames and then refined on all data. The data (æ and ω scans
with 2° steps) were processed with HKL2000.14 The structures were
solved by direct methods or by Patterson map interpretation with
SHELXS-97 and subsequent Fourier-difference synthesis and
refined by full-matrix least-squares on F2 with SHELXL-97.15
Absorption effects were corrected empirically with SCALEPACK14
or DELABS.16 All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. In compound 7‚2.5CH3CN, one
acetonitrile molecule is disordered around a binary axis and has
been given an occupancy parameter of 0.5. Restraints on C-F bond
lengths and on displacement parameters were applied for the badly
resolved triflate moiety in compound 8. In compound 10, restraints
on one bond length and some displacement parameters have been
applied for the atoms of one thf molecule. The hydrogen atoms
were introduced at calculated positions and were treated as riding
atoms with an isotropic displacement parameter equal to 1.2 (CH,
CH2) or 1.5 (CH3) times that of the parent atom. Crystal data and
structure refinement details are given in Table 1. The molecular
plots were drawn with SHELXTL.17
Results and Discussion
Synthesis of Cp*2UX2 (X ) I, OTf) and Crystal Structures
of Cp*2UI2(NCR) (R ) Me, tBu) and Cp*2U(OTf)2(NCMe).
The halide or pseudo-halide silane Me3SiX molecules are useful
substituting reagents in organic, inorganic, and organometallic
chemistry.18 Their efficiency in the conversion of low- and high-
valent actinide compounds was also demonstrated.19,20 In par-
ticular, we reported that the transformation [U]-Cl f [U]-I
with Me3SiI was rapid and quantitative in acetonitrile, which
(13) Kappa-CCD Software; Nonius BV: Delft, 1998.
(14) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(15) Sheldrick, G. M. SHELXS97 and SHELXL97; University of Go¨t-
tingen, 1997.
(16) Spek, A. L. PLATON; University of Utrecht, 2000.
(17) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker AXS Inc.:
Madison, WI, 1999.

5606 Organometallics, Vol. 25, No. 23, 2006
Maynadie´ et al.
Figure 1. 1H NMR spectra of Cp*2UI2 in pyridine, thf, and acetonitrile.
proved to be more suitable than thf or other ethereal or aromatic
solvents in these metathesis reactions.20 Treatment of Cp*2UCl2
with a slight excess of Me3SiX (X ) I, OTf) in acetonitrile at
20 °C led to an immediate change of color from red to dark red
and the formation of the corresponding Cp*2UX2 compounds
(eq 1). After stirring for 1 h to ensure complete reaction, gram
quantities of pure Cp*2UI2 (1) or Cp*2U(OTf)2 (2) were obtained
in almost quantitative yields. Complex 1 was previously
characterized by NMR spectroscopy in the oxidative reaction
of Cp*2UCl with I2 or alkyl iodides or the metathesis exchange
between Cp*2UCl2 and BI3.21 Recently, the benzonitrile adduct
Cp*2UI2(NCPh) was isolated in 36% yield from the reaction of
UI4(NCPh)4 and Cp*MgCl(thf) in toluene.8 We reported that 2
can be obtained by protonolysis of Cp*2UX2 (X ) Me, NMe2)
with HpyOTf in thf.9 The syntheses presented here are more
practical and straightforward routes to 1 and 2, which are
isolated with better yields.
Complexes 1 and 2 were characterized by their elemental
1
analyses and H NMR spectra, which indicate the absence of
(18) (a) Leigh, G. J.; Sanders, J. R.; Hitchcock, P. B.; Fernandes, J. S.;
Togrou, M. Inorg. Chim. Acta 2002, 330, 197. (b) Kirss, R. U. Inorg. Chem.
1992, 31, 3451. (c) Kamigaito, M.; Sawamoto, M.; Higashimura, T. J.
Polym. Sci. A, Polym. Chem. 1991, 29, 1909. (d) Feringa, B. L. Synth.
Commun. 1985, 15, 87. (e) Renard, P. Y.; Vayron, P.; Mioskowski, C. Org.
Lett. 2003, 5, 1661. (f) Kamigaito, M.; Sawamoto, M.; Higashimura, T.
Macromol. Chem. Phys. 1993, 194, 727.
(19) (a) Berthet, J. C.; Nierlich, M.; Ephritikhine, M. Chem. Commun.
2004, 870. (b) Rabinovich, D.; Bott, S. G.; Nielsen, J. B.; Abney, K. D.
Inorg. Chim. Acta 1998, 274, 232. (c) Rabinovich, D.; Chamberlin, R. M.;
Scott, B. L.; Nielsen, J. B.; Abney, K. D. Inorg. Chim. Acta 1997, 36, 4216.
(d) Lukens, W. W.; Beshouri, S. M.; Blosch, L. L.; Stuart, A. L.; Andersen,
R. A. Organometallics 1999, 18, 1235. (e) Roussel, P.; Alcock, N. W.;
Boaretto, R.; Kingsley, A. J.; Munslow, I. J.; Sanders, C. J.; Scott, P. Inorg.
Chem. 1999, 38, 3651. (f) Van der Heijden, H.; Schavieren, C. J.; Orpen,
A. G. Organometallics 1989, 8, 255.
free or coordinated acetonitrile molecules. They are both quite
soluble in benzene, thf, and pyridine, and the NMR spectra
exhibit a single paramagnetic signal corresponding to the Cp*
ligands around δ 18. These values are characteristic of neutral
Cp*2UX2 complexes with a bent sandwich geometry for which
the Cp* resonance in these solvents is visible in the δ 5-18
region.2 Most strikingly, the spectra of 1 and 2 in acetonitrile
show a significant shift of the Cp* signal at δ 35.1 and 35.4,
respectively (Figure 1). In contrast, the chemical shifts of the
Cp* signal of Cp*2UCl2 are very close in benzene and
acetonitrile, with δ values of 13.6 and 12.7, respectively. These
differences in the NMR spectra strongly suggest that 1 and 2
were converted into new compounds in acetonitrile.
(20) Berthet, J. C.; Thue´ry, P. Ephritikhine, M. Inorg. Chem. 2005, 44,
1142.
(21) Finke, R. G.; Hirose, Y.; Gaughan, G. J. Chem. Soc., Chem.
Commun. 1981, 232.
The NMR spectrum of an equimolar mixture of 1 and MeCN
in toluene indicated that the nitrile molecule is coordinated to

From Bent to Linear Uranium Metallocenes
Organometallics, Vol. 25, No. 23, 2006 5607
neutral ligand L is located between the two chloride groups.
Compounds 3 and 4 possess a plane of symmetry containing
the uranium center and the atoms N(1), I(1), and I(2). The U-C
bond lengths in 3-5 are unexceptional, with average values of
2.75(2), 2.75(1), and 2.72(3) Å, respectively. The U-(ring
centroid) distances and the (ring centroid)-U-(ring centroid)
angles [2.465 Å and 135.8° in 3, 2.474 Å and 139.2° in 4, and
2.435 Å and 135.2° in 5] are close to those measured in
Cp*2UI2(NCPh) [2.45(2) and 2.48(2) Å, 136.7(5)°] or Cp*2U-
(OTf)2(H2O) [2.439 and 2.469 Å, 134.7°]. The U-N distances
of 2.515(14), 2.541(9), and 2.523(3) Å in 3-5 can be compared
with those found in Cp*2UI2(NCPh) [2.53(1) Å],8 UI4(NCPh)4
[2.56(1) Å], or [UI2(NCMe)7][UI6] [2.53(1) Å average value].20
In both 3 and 4, the two U-I bond lengths are quite iden-
tical, with mean values of 3.07(2) and 3.072(1) Å, respectively,
which is in contrast to what is observed in Cp*2UI2(NCPh),
where the two U-I bond lengths are quite distinct [2.942(3)
and 3.092(2) Å].8 These U-I bond lengths are in the range of
distances reported for uranium(IV) iodide compounds, in
particular [UI2(NCMe)7][UI6] [3.001(2) Å] and UI4(py)3 [3.00-
(4) Å average value].20 The U-O(OTf) distances of 2.375(2)
and 2.371(2) Å in 5 are similar to those found in the other bis-
(cyclopentadienyl) uranium(IV) compounds with monodentate
triflate ligands, Cp*2U(OTf)2(H2O) [2.36(1) and 2.40(1) Å],22
Cp*2U(OTf)(η2(N,N′)-MeNNdCPh2) [2.395(14) Å],25 and Cp2U-
(OTf)2(py)2 [2.395(4) and 2.385(4) Å].9
Figure 2. View of Cp*2UI2(NCMe) (3). The hydrogen atoms have
been omitted for clarity. Displacement ellipsoids are drawn at the
20% probability level.
The nature of the complexes present in acetonitrile solutions
of 1 and 2 was intriguing. In view of the high polarity of
acetonitrile and the high lability of the iodide and triflate
counterions, the most plausible hypothesis was the formation
of cationic species.
Syntheses and Crystal Structures of the Linear Uranium-
(IV) Metallocenes [Cp*2U(NCMe)5][X]2 (X ) I, OTf, BPh4).
Cationic complexes proved to be valuable precursors in
inorganic and organometallic chemistry and active species in
catalysis due to their strong Lewis acidic character. Protonolytic
cleavage reactions of [An]-X bonds (An ) Th, U; X ) H,26
alkyl,27 BH4,28 NR229) with HNR3BPh4 represent convenient
routes to such actinide compounds. This method was particularly
efficient for the synthesis of monocationic complexes but much
less for the preparation of polycationic derivatives.29 In the
bisCp* series, the monocationic metallocenes [Cp*2ThMe(thf)2]-
[BPh4]27 and [Cp*2UX(thf)][BPh4] (X ) NMe2, NEt2)30 were
easily obtained from Cp*2ThMe2 and Cp*2UX2, whereas
formation of the dication [Cp*2U(L)n][BPh4]2 by subsequent
protonolysis of the U-C or U-N bond could not be achieved
in thf.29 However, this reaction was possible only by using
MeCN as the solvent. Treatment of Cp*2UMe2 with 2 equiv of
HNEt3BPh4 in acetonitrile led to the straightforward formation
Figure 3. View of Cp*2U(OTf)2(NCMe) (5). The hydrogen atoms
have been omitted for clarity. Displacement ellipsoids are drawn
at the 30% probability level.
the metal center, its broad signal being visible at δ -0.27. After
addition of 15 molar equiv of MeCN, this signal became less
broad and was shifted to the diamagnetic region at δ 0.23,
reflecting the rapid exchange between free and coordinated
molecules. Whatever the quantity of MeCN, the Cp* ligands
give rise to a single signal, which is little displaced from δ 18,
showing that in these conditions the geometry of the Cp*2UI2
fragment is not greatly modified upon coordination of the
nitrile ligand. Dark brown crystals of Cp*2UI2(NCMe) (3) and
Cp*2UI2(NCtBu)‚C7H8 (4‚C7H8) suitable for X-ray diffraction
analysis were obtained by addition of an excess of the
corresponding nitrile into solutions of 1 in toluene. A solution
of an equimolar mixture of 2 and MeCN in toluene rapidly
deposited dark red crystals of Cp*2U(OTf)2(NCMe) (5). The
lability of the nitrile ligand in compounds 3, 4, and 5 was
evidenced by its ready dissociation upon drying the crystals
under vacuum, giving back 1 and 2.
The crystal structures of 3 and 5 are shown in Figures 2 and
3, respectively; the structure of 4 is very similar to that of 3.
Selected bond distances and angles are listed in Table 2. The
complexes adopt the familiar bent sandwich configuration with
an asymmetric arrangement of the RCN and I or OTf ligands
in the equatorial plane. This geometry is similar to that found
in Cp*2UI2(NCPh)8 or Cp*2U(OTf)2(H2O)22 but differs from
that of Cp*2UCl2L (L ) pyrazole,23 HNPPh324), where the
(22) Berthet, J. C.; Lance, M.; Nierlich, M.; Ephritikhine, M. Chem.
Commun. 1998, 1373.
(23) Eigenbrot, C. W.; Raymond, K. N. Inorg. Chem. 1982, 21, 2653.
(24) Cramer, R. E.; Roth, S.; Gilje, J. W. Organometallics 1989, 8, 2327.
(25) Kiplinger, J. L.; John, K. D.; Morris, D. E.; Scott, B. L.; Burns, C.
J. Organometallics 2002, 21, 4306.
(26) Berthet, J. C.; Le Mare´chal, J. F.; Lance, M.; Nierlich, M.; Vigner,
J.; Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1992, 1573.
(27) (a) Lin, Z.; Le Mare´chal, J. F.; Sabat, M.; Marks, T. J. J. Am. Chem.
Soc. 1987, 109, 4127. See also: (b) Yang, X.; Stern, C.; Marks, T. J.
Organometallics 1991, 10, 840. (c) Yang, X.; King, W. A.; Sabat, M.;
Marks, T. J. Organometallics 1993, 12, 4254. (d) Lia, J.; Yang, X.; Stern,
C.; Marks, T. J. Organometallics 1994, 13, 3755.
(28) Cendrowski-Guillaume, S. M.; Lance, M.; Nierlich, M.; Ephritikhine,
M. Organometallics 2000, 19, 3257.
(29) Berthet, J. C.; Ephritikhine, M. Coord. Chem. ReV. 1998, 178-
179, 83.
(30) Berthet, J. C.; Boisson, C.; Lance, M.; Vigner, J.; Nierlich, M.;
Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1995, 3027.

5608 Organometallics, Vol. 25, No. 23, 2006
Maynadie´ et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in the Bent Trivalent and Tetravalent Metallocenes 3-5 and 11
Cp*2UI2(NCMe)
Cp*2UI2(NCtBu)‚C7H8
(4‚C7H8)
Cp*2U(OTf)2(NCMe)
Cp*2UI(NCMe)2
(3)
(5)
(11)
U-C
2.75(2)
2.465
2.75(1)
2.474
U-C
2.72(3); 2.72(2)
2.435
2.437
U-C
2.79(1)
2.514
U-Cp*
U-Cp*1
U-Cp*
U-Cp*2
U-I(1)
3.0536(13)
3.0920(12)
3.07(2)
2.515(14)
1.18(3)
135.8
152.7(3)
72.5(3)
3.0707(8)
3.0732(8)
3.072(1)
2.541(9)
1.134(14)
139.2
U-O(1)
2.375(2)
2.371(2)
2.373(2)
2.523(3)
1.142(5)
135.2
146.58(9)
70.77(9)
75.82(9)
169.0(3)
U-I
3.2076(7)
U-I(2)
U-O(4)
U-I
U-O
U-N(1)
U-N(1)
U-N(1)
2.595(6)
1.119(7)
134.9
74.10(12)
148.2(2)
N(1)-C(1)
Cp*-U-Cp*a
N(1)-U-I(1)
N(1)-U-I(2)
I(1)-U-I(2)
U-N(1)-C(1)
N(1)-C(3)
Cp*1-U-Cp*2
N(1)-U-O(1)
N(1)-U-O(4)
O(1)-U-O(4)
U-N(1)-C(3)
N(1)-C(1)
Cp*-U-Cp*
N(1)-U-I
N(1)-U-N(1′)
71.7(2)
155.0(2)
83.29(2)
171.1(9)
80.13(3)
170.2(14)
U-N(1)-C(1)
156.5(5)
a Cp* is the centroid of the cyclopentadienyl ring. Symmetry code: ′ ) -x, y, 1.5 - z.
1
Scheme 1. Syntheses of the Linear Metallocenes
The H NMR spectrum of 6 in acetonitrile shows a peak at
δ 35.3 corresponding to the Cp* ligands; the MeCN resonance
is observed at its diamagnetic position due to the rapid exchange
between free and coordinated nitrile molecules. While 6 is
insoluble in thf, it dissolves readily in pyridine, and the presence
of five coordinating acetonitrile molecules was confirmed. The
MeCN ligands are not displaced by pyridine, and two NMR
signals of relative intensities 30:15 are visible at δ 35.82 and
-44.50, respectively. In contrast, a singlet corresponding to free
MeCN was observed when 6 was dissolved in dmf, with the
signal of the Cp* ligands at δ 36.6. Evaporation of a dmf
solution of 6 afforded a brown powder of [Cp*2U(dmf)5][BPh4]2,
which was characterized by its 1H NMR spectrum in acetonitrile;
broad resonances corresponding to the dmf molecules at their
diamagnetic position indicate that these molecules have been
almost totally displaced by the solvent.
[Cp*2U(NCMe)5][X]2 (X ) BPh4, I, OTf)
The observation that the chemical shifts of the Cp* signals
of 1 and 2 in MeCN (δ 35.1 and 35.4) are very close to those
of 6 in acetonitrile, dmf, and pyridine (δ 35.3-36.6) strongly
suggests that the iodide and triflate complexes were converted
into the dicationic compounds [Cp*2U(NCMe)5][X]2 (X ) I
(7), OTf (8)) in acetonitrile. The easy formation of these
complexes highlights the major influence of both the solvents
and the counterions. Though acetonitrile is known to dissociate
the U-I bond of U(IV) and U(III) complexes,20,33 thus favoring
the formation of polycationic species, the weaker lability of
Cl- versus I- and OTf- explains why the dication [Cp*2U-
(NCMe)5]2+ could not be obtained from Cp*2UCl2. However,
in contrast to 6, which is stable toward desolvation, complexes
1 and 2 were recovered after evaporation of their acetonitrile
solutions, due to elimination of the MeCN ligands and back-
coordination of the iodide and triflate ions.
The determination of the structure of the dication [Cp*2U-
(NCMe)5]2+ was exciting since a linear geometry of the Cp*2U
fragment, which would permit it to accommodate a large num-
ber of auxiliary ligands, could be anticipated, and this type of
linear metallocene was unknown. Dark red crystals of 6 and
brown crystals of [Cp*2U(NCMe)5][I]2‚MeCN (7‚MeCN) were
obtained by slow diffusion of diethyl ether into an acetoni-
trile solution of 6 and 1, respectively, and brown crystals of
7‚2.5MeCN were deposited upon concentration of an acetonitrile
solution of 1. Brown crystals of [Cp*2U(NCMe)5][OTf]2‚MeCN
(8‚MeCN) were readily formed upon addition of 15 equiv of
MeCN into a toluene solution of 2; formation of 8 was thus
more facile than that of 7, which crystallized only from pure
of a compound analyzed as [Cp*2U(NCMe)5][BPh4]2 (6) with
concomitant evolution of CH4 (Scheme 1). After evaporation
of the solvent, washing with thf, and drying under vacuum, the
analytically pure brown powder of 6 was isolated in 80%
yield. The visible release of methane clearly indicates that the
synthesis of 6 proceeds by protonolysis of the U-Me bonds of
Cp*2UMe2 before the latter was transformed into the bisketimine
derivative Cp*2U(NdCMe2)2 by insertion of the nitrile molecule
into the U-Me bonds.31 The synthesis and the reactivity of the
bisketimine compound will be reported elsewhere. Complex 6
was alternatively synthesized from 1 by reaction with 2 equiv
of TlBPh4 in acetonitrile; the precipitate of TlI was filtered off
and the crystals of 6 that slowly deposited upon addition of
diethyl ether were, after usual workup, recovered with a lower
yield (51%).
The infrared spectrum of 6 exhibits two bands at 2262 and
2269 cm-1, corresponding to the vibrational frequencies of
bound nitriles; the increase of the nitrile CN stretching frequen-
cies with respect to free MeCN (2254 cm-1) reflects the σ-donor
character of the acetonitrile ligand.32
(32) Beeckman, W.; Goffart, J.; Rebizant, J.; Spirlet, M. R. J. Organomet.
Chem. 1986, 307, 23
(33) Enriquez, A. E.; Matonic, J. H.; Scott, B. L.; Neu, M. P. Chem.
Commun. 2003, 892.
(31) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.;
Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L.
Organometallics 2004, 23, 4682.

From Bent to Linear Uranium Metallocenes
Organometallics, Vol. 25, No. 23, 2006 5609
In addition to representing a synthetic challenge, changing the
geometry of a metallocene complex from bent to linear or vice
versa is of major significance for the control of distinct
physicochemical properties and/or reactivity and for the evalu-
ation of the relative importance of covalent, electrostatic, and
steric interactions in the metal-cyclopentadienyl bonding.36
Bent metallocenes of the main group and d transition metals
were forced to be linear by increasing the steric bulk of the
t
ring substituents. Thus, the plumbocene (C5Me4SiMe2 Bu)2Pb
or the titanocenes (C5Me4R)2Ti (R ) SiMe3, SiMe2But) exhibit
parallel cyclopentadienyl rings, and the latter are reluctant to
form bent derivatives of the type (C5H5)2TiL2 (L ) PF3, PMe3,
CO).37,38 The full filling of the equatorial girdle of the Cp*2U
fragment with donor molecules represents a new strategy for
making bent metallocenes linear, which is exclusive to f-element
sandwich compounds.
Behavior of the Uranium(III) Metallocenes Cp*2UX (X
) I, OTf, BPh4) in Acetonitrile. From the above results, it
was of interest to determine if the linear trivalent metallocenes
[Cp*2U(NCMe)5]X could be obtained by dissolving the ura-
nium(III) compounds Cp*2UX (X ) I, OTf, BPh4) in acetoni-
trile.
The syntheses of Cp*2UI(L) (L ) thf,4 py39), [Cp*2U(thf)2]-
[BPh4],40 and [Cp*2U][BPh4]5 were previously reported. At-
tempts to synthesize Cp*2UOTf (9) by treating Cp*2UI(py) or
Cp*2UCl2Na(thf)2 with Me3SiOTf were unsuccessful. In one
of these reactions of uranium(III) chloride, red crystals were
obtained and found by X-ray diffraction analysis to represent a
unique example of an “ate” complex of the type Cp*2UX3Na.
The crystals of [Cp*2U(OTf)3Na(thf)2]2‚2(CH2)4O (10‚2(CH2)4O)
are composed of the tetranuclear Na2U2 complex and the organic
polymer resulting from thf ring opening reaction. The cen-
trosymmetric structure of the dimeric complex is shown in
Figure 5, and selected bond distances and angles are listed in
Table 4. The two Cp*2U(OTf)3 fragments, which feature a
typical bent metallocene framework, are linked to two Na(thf)2
moieties through the three equatorial triflate ligands. The two
lateral OTf groups are bidentate, whereas the central one is
tridentate in bridging position between the two sodium atoms,
giving the first example of a µ3-coordination mode of a triflate
ligand with an f-element. The U and Na atoms and SO2 groups
in µ2 position are almost coplanar, with an rms deviation of
0.306 Å, and the O(4) and S(2) atoms of the tridentate triflate
ligand are at 0.893(5) and 1.399(3) Å from this mean plane.
The U-O distances of the bridging triflates, which range from
2.381(5) to 2.408(6) Å [mean value 2.39(1) Å], are only slightly
larger than those of the monodentate triflate in 5, in agree-
ment with the presence of σ U-O and dative Na-O bonds.
The Na-O(OTf) and Na-O(thf) distances are unexceptional.
Figure 4. View of the cation [Cp*2U(NCMe)5]2+ in 6. The
hydrogen atoms have been omitted for clarity. Displacement
ellipsoids are drawn at the 30% probability level.
acetonitrile solutions. Crystals of 6 were found to be stable in
air, an unusual feature for U(IV) complexes, which can be
explained by the steric saturation of the coordination sphere
that protects the metal center against oxidation and hydrolysis.
Crystals of 6 and solvates of 7 and 8 are composed of discrete
[Cp*2U(NCMe)5]2+ cations and BPh4-, I-, or OTf- anions in
the ratio 1:2 and free solvent molecules. The crystal structure
of the cation of 6, which is quite identical to those in the other
compounds, is shown in Figure 4, while selected bond dis-
tances and angles are listed in Table 3. The planar Cp* rings
are almost parallel, with a dihedral angle varying from 0.5(2)°
in 7‚2.5MeCN to 2.8(2)° in 7‚MeCN, and are equidistant from
and parallel to the plane defined by the uranium and five
nitrogen atoms (rms deviation 0.003-0.042 Å). The Cp* ligands
are eclipsed, in a staggered conformation with respect to the
pentagon of nitrogen atoms, thus minimizing the intramolecular
steric interactions. The displacements of the methyl substituents
away from the metal and out of the Cp* ring plane seem to be
slightly more important in the linear than in the bent metal-
locenes, with average values of 0.225(18) and 0.18(8) Å,
respectively. However, the maximum displacement of 0.244 Å
is much less than the value of 0.42 Å, which was considered as
the upper limit for cyclopentadienyl complexes with conven-
tional M-C(C5R5) bond distances and angles and no unusual
ring reactivity.34 The complexes adopt a pentagonal bipyramidal
configuration, with the Cp* ligands, considered as monodentate,
in apical positions and the five acetonitrile molecules in the
equatorial plane; this geometry is reminiscent of that of the
uranyl(VI) complexes [UO2(L)5]2+ 35
. The mean U-C distance
in 6-8 is 0.05-0.1 Å larger than in the bent metallocenes 3-5,
as expected from the increase of the coordination number by
two units, whereas the average lengthening of the U-N distance
is only 0.02 Å, likely reflecting the higher Lewis acidic character
of the metal center in the dicationic complex.
Eventually, the triflate derivative 9 was prepared in a manner
similar to that of Cp*2UI(thf), by treating U(OTf)3 with 2 equiv
of KCp* in thf (eq 2), and was isolated after usual workup as
a green powder in 90% yield. The absence of coordinating thf
Complexes 6-8 are the first linear metallocenes of an
f-element. Their structure is novel since they are unique
examples of linear metallocenes containing auxiliary ligands.
(36) (a) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin,
G.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.;
Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525. (b) Dinnebier, R. E.;
Van Smaalen, S.; Olbich, F.; Carlson, S. Inorg. Chem. 2005, 44, 964.
(37) Constantine, S. P.; Hitchcock, P. B.; Lawless, G. A. Organometallics
1996, 15, 3905.
(38) (a) Hora´cˇek, M.; Kupfer, V.; Thewalt, U.; Sˇtepnicˇka, P.; Pola´sˇek,
M.; Mach, K. Organometallics 1999, 18, 3572. (b) Hitchcock, P. B.; Kerton,
F. M.; Lawless, G. A. J. Am. Chem. Soc. 1998, 120, 10264.
(39) Mehdoui, T.; Berthet, J. C.; Thue´ry, P.; Salmon, L.; Ephritikhine,
M. Chem. Eur. J. 2005, 11, 6994.
(34) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Inorg. Chem. 2005, 44,
7960.
(35) (a) Harrowfield, J. M.; Kepert, D. L.; Patrick J. M.; White A. H. J.
Chem. Soc., Dalton Trans. 1983, 393. (b) Harrowfield, J. M.; Skelton, B.
W.; White, A. H. C. R. Chim. 2005, 8, 169. (c) Clark, D. L.; Conradson, S.
D.; Donohos, R. J.; Keogh, D. W.; Morris, D. E.; Palmer, P. D.; Rogers,
R. D.; Tait, C. D. Inorg. Chem. 1999, 38, 1456. (d) Mikhailov, Y. N.;
Kanishecheva, A. S.; Zemskova, L. A.; Mistryukov, V. E.; Kuznetsov, N.
T.; Solutsev, K. A. Zh. Neorg. Khim. 1982, 27, 2343. (e) Rogers, R. D.;
Kurihara, L. K.; Benning, M. M. J. Inclusion Phenom. Macrocycl. Chem.
1987, 5, 645.
(40) Boisson, C.; Berthet, J. C.; Ephritikhine, M.; Lance, M.; Nierlich,
M. J. Organomet. Chem. 1997, 533, 7.

5610 Organometallics, Vol. 25, No. 23, 2006
Table 3. Selected Bond Lengths (Å) and Angles (deg) in the Linear Metallocenes 6-8
Maynadie´ et al.
[Cp*2U(NCMe)5][BPh4]2
[Cp*2U(NCMe)5][I]2‚
MeCN (7‚MeCN)
[Cp*2U(NCMe)5][I]2‚
2.5MeCN (7‚2.5MeCN)
[Cp*2U(NCMe)5][OTf]2‚
MeCN (8‚MeCN)
(6)
U-C
2.80(1)
2.540
2.528
2.521(4)
2.576(3)
2.529(3)
2.570(3)
2.537(3)
2.55(2)
2.81(2)
2.549
2.531
2.81(1)
2.539
2.541
2.547(4)
2.548(4)
2.535(4)
2.551(4)
2.556(4)
2.547(7)
1.14(1)
2.809(8)
2.541
2.532
2.558(10)
2.563(8)
2.539(7)
U-Cp*1a
U-Cp*2
U-N(1)
2.545(3)
2.577(3)
2.549(2)
2.518(2)
2.562(3)
2.55(2)
1.141(3)
178.0
72.23(8)
73.81(8)
74.22(8)
69.21(8)
70.54(8)
U-N(2)
U-N(3)
U-N(4)
U-N(5)
U-N
2.55(1)
1.12(1)
179.5
72.04(16)
71.9(2)
72.2(3)b
N-C
1.142(7)
178.6
Cp*1-U-Cp*2
N(1)-U-N(2)
N(2)-U-N(3)
N(3)-U-N(4)
N(4)-U-N(5)
N(5)-U-N(1)
179.9
71.21(11)
73.10(11)
72.16(11)
70.48(11)
73.07(12)
72.49(12)
71.47(12)
72.51(12)
71.09(12)
72.53(11)
b
a Cp* is the centroid of the cyclopentadienyl ring. N(3)-U-N(3′); symmetry code: ′ ) x, 0.5 - y, z.
Figure 6. View of Cp*2UI(NCMe)2 (11). The hydrogen atoms have
been omitted for clarity. Displacement ellipsoids are drawn at the
30% probability level. Symmetry code: ′ ) -x, y, 1.5 - z.
Figure 5. View of [Cp*2U(OTf)3Na(thf)2]2 (10). The hydrogen
atoms have been omitted for clarity. Displacement ellipsoids are
drawn at the 20% probability level. Symmetry code: ′ ) -x, 1 -
y, -z.
is, to the best of our knowledge, the first organouranium(III)
triflate to have been isolated.
Table 4. Selected Bond Lengths (Å) and Angles (deg) in
[Cp*2U(OTf)3Na(thf)2]2‚2(CH2)4O (10‚2(CH2)4O)
U-C
2.732(6); 2.75(3)
2.453
2.468
U-Cp*1a
U-Cp*2
U-O(1)
2.401(6)
2.381(5)
2.408(6)
2.39(1)
2.453(7)
2.325(7)
2.416(6)
2.431(7)
2.41(5)
75.06(18)
75.72(19)
150.75(18)
131.2
U-O(4)
U-O(7)
Evaporation of an acetonitrile solution of Cp*2UI(py) gave
a brown powder of Cp*2UI(NCMe)2 (11), and dark brown
crystals of 11 were obtained by cooling a toluene solution of
Cp*2UI(py) in the presence of 15 equiv of MeCN. In contrast
to the U(IV) complexes 3-5, but as observed with the cerium
counterpart Cp*2CeI(NCMe)2, which was obtained by crystal-
lization of the “ate” complex Cp*2CeI2K(thf)2 in acetonitrile,42
the nitrile ligands of 11 are not removed by evacuation. A view
of 11, which is a rare example of a crystallographically
characterized Lewis base adduct of Cp*2UI,39 is shown in Figure
6; the complex is isostructural to Cp*2CeI(NCMe)2 and will
not be discussed further. It is only noteworthy that the U-C,
U-I, and U-N distances (Table 2) seem to be slightly smaller
than the corresponding distances in the cerium counterpart, by
0.01-0.03 Å, while the ionic radius of U3+ is 0.01 Å larger
U-O
Na-O(2)
Na-O(5)
Na-O(6′)
Na-O(8′)
Na-O(OTf)
O(1)-U-O(4)
O(4)-U-O(7)
O(1)-U-O(7)
Cp*1-U-Cp*2
a Cp* is the centroid of the cyclopentadienyl ring. Symmetry code: ′ )
-x, 1 - y, -z.
in the product, confirmed by the elemental analysis and the 1H
NMR spectrum, suggests that 9 is likely oligomeric in the solid
state with bridging OTf ligands, as observed, for example, in
the polymeric compound [U(OTf)3(MeCN)3]n.41 Compound 9
than that of Ce3+ 43
; this trend has been observed in a variety of
(41) Natrajan, L.; Mazzanti, M.; Bezombes, J.-P.; Pe´caut, J. Inorg. Chem.
2005, 44, 6115.
(42) Hazin, P. N.; Lakshminarayan, C.; Brinen, L. S.; Knee, J. L.; Bruno,
J. W.; Streib, W. E.; Folting, K. Inorg. Chem. 1988, 27, 1393.

From Bent to Linear Uranium Metallocenes
Organometallics, Vol. 25, No. 23, 2006 5611
3+ ions are larger than that of U4+ 43
. It is likely that the number
analogous uranium(III) and lanthanide(III) compounds and is
explained by the more covalent character of the uranium-ligand
bond.44
M
of f-electrons in the valence shells and the availability of empty
f-orbitals in the metal-ligand interactions have a major influence
on the bent or linear structure of the metallocenes. Recently,
relativistic DFT calculations revealed that the 5f orbitals give
an important contribution to the metal-cyclopentadienyl bond-
ing in the hypothetical [Cp2U]2+ complex, predicting that the
more stable configuration of this dication is linear with D5h
symmetry.45
The 1H NMR spectrum of 11 in benzene exhibits two signals
at δ -1.59 and -32.12 attributed to the Cp* and MeCN ligands,
respectively; the Cp* resonance is slightly shifted at δ -1.09
1
in acetonitrile. In the H NMR spectra of Cp*2CeI in thf and
acetonitrile, the signals corresponding to the Cp* ligands are
visible at δ 5.3539 and 4.95,42 respectively. These observations,
in contrast to those made with Cp*2UI2, suggest that the
geometry of the Cp*2M fragment (M ) U, Ce) in the trivalent
metallocenes is not modified by changing the solvent and that
the cationic complex [Cp*2M(NCMe)5]I is not formed. The
same conclusion can be drawn from the NMR spectra of 9 in
thf and MeCN, with the Cp* signals at δ -3.24 and -2.64,
respectively. Evaporation of an acetonitrile solution of 9 gave
a brown powder of the triflate analogue of 11, Cp*2U(OTf)-
(MeCN)2, which is also stable toward desolvation and exhibits
in benzene two 1H NMR resonances in the ratio 30:6 at δ -2.93
(Cp*) and -31.6 (MeCN). A greater shift of the Cp* resonance
was observed in the case of [Cp*2U(thf)2][BPh4] in thf or
acetonitrile, with δ values of 0.10 and -2.59 respectively.
Unfortunately, no crystal suitable for X-ray diffraction analysis
could be obtained from an acetonitrile solution of [Cp*2U(thf)2]-
[BPh4]. However, evaporation to dryness of this solution
afforded a brown powder of a product analyzed as [Cp*2U-
(NCMe)3][BPh4] since the 1H NMR spectrum in thf at -80 °C
shows two broad signals at δ -0.49 and -76.7, in relative
intensities of 30:9, corresponding to the Cp* and MeCN ligands;
these resonances were not affected upon addition of acetonitrile
(3 equiv), which gave rise to the signal of the free ligand in the
diamagnetic region. Although the structure of the cation [Cp*2U-
(NCMe)3]+ is not determined unambiguously, these results
clearly indicate that the linear uranium(III) metallocene [Cp*2U-
(NCMe)5][BPh4] was not formed.
Conclusion
The easy syntheses of Cp*2UI2 and Cp*2U(OTf)2 by treatment
of Cp*2UCl2 with Me3SiX (X ) I, OTf) in acetonitrile provide
new entries into organouranium chemistry. The combined use
of weakly coordinating anions (I- and OTf-) and of acetonitrile
as the polar solvent led to the discovery of the first linear
metallocenes of the f-elements, [Cp*2U(NCMe)5][X]2 (X ) I,
OTf), which exhibit a novel type of structure, with the presence
of five additional ligands in the equatorial girdle of the linear
Cp*2U fragment. The use of acetonitrile as solvent was also
beneficial for the double protonolysis reaction of Cp*2UMe2,
affording the dicationic species [Cp*2U(NCMe)5][BPh4]2. In
contrast, the MeCN adducts of Cp*2UX (X ) I, OTf, BPh4), as
well as that of Cp*2CeI, retain a bent configuration, suggesting
that the stability of linear metallocenes is likely related to the
electronic configuration of the f-element and the role of the
f-orbitals. Further work devoted to the enlargement of this new
class of linear metallocenes, by changing the nature of either
the auxiliary ligands, or the cyclopentadienyl groups, or the
metal ion and/or its oxidation state, together with theoretical
studies, will be presented in forthcoming papers.
Acknowledgment. We thank the Nuclear Fission Safety
Program of the European Union for support under the EURO-
PART (FI6W-CT-2003-508854) contract and the postdoctoral
situation of J.M. We also thank the Direction de l’Energie
Nucle´aire of the Commissariat a` l’Energie Atomique for its
financial support.
The formation of the linear uranium(IV) metallocenes 6-8,
which contain five MeCN ligands in the equatorial girdle, could
be related, in addition to the role of the counterion and the
solvent, to the large size of the actinide ion and the small steric
hindrance of the acetonitrile molecule. These sole steric effects
are, however, not determinant, as shown by the results obtained
with the trivalent compounds Cp*2MX (M ) U and X ) I,
OTf, BPh4; M ) Ce and X ) I), where the ionic radii of the
Supporting Information Available: Tables of crystal data,
atomic positions, and displacement parameters, anisotropic dis-
placement parameters, and bond lengths and bond angles in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
(43) Shannon, R. D. Acta Crystallogr. Sect. A 1976, 32, 751.
(44) (a) Berthet, J. C.; Nierlich, M.; Ephritikhine, M. Polyhedron 2003,
22, 3475. (b) Berthet, J. C.; Nierlich, M.; Miquel, Y.; Madic, C.;
Ephritikhine, M. Dalton Trans. 2005, 369. (c) Mehdoui, T.; Berthet, J. C.;
Thue´ry, P.; Ephritikhine, M. Dalton Trans. 2005, 1263.
OM060642G
(45) Belkhiri, L.; Bencharif, M.; Boucekkine, A. J. Soc. Alger. Chim.
2005, 15, 25.