Facile triphenylborane-based syntheses of the sterically crowded tris(pentamethylcyclopentadienyl) complexes (C5Me5) 3UMe and (C5Me5)3UCl

Article abstract of DOI:10.1021/om050230s

An atom- and time-efficient synthetic route to sterically crowded organoactinide (C₅Me₅)₃-UX complexes involving the in situ formation of borate salts is reported. Addition of BPh₃ to (C₅Me₅)₂UMeCl followed by KC ₅Me₅ provides a much improved synthesis of (C ₅Me₅)₃UCl that presumably proceeds through a [(C₅Me₅)₂UCl] [MeBPh₃] intermediate borate salt. Attempts to make (C₅Me₅)₃UMe, the first tris(pentamethylcyclopentadienyl) metal alkyl complex, by reaction of BPh₃ with (C₅Me₅)₂UMe₂ followed by KC₅Me₅ were also successful, and the product was characterized by X-ray crystallography. In this reaction system, the formation of the borate intermediate, [(C₅Me₅) ₂UMe][MeBPh₃], was confirmed by variable-temperature NMR spectroscopy and by X-ray crystallography of the THF adduct.

Full text of DOI:10.1021/om050230s

Organometallics 2005, 24, 3407-3412
3407
Facile Triphenylborane-Based Syntheses of the
Sterically Crowded Tris(pentamethylcyclopentadienyl)
Complexes (C5Me5)3UMe and (C5Me5)3UCl
William J. Evans,* Stosh A. Kozimor, and Joseph W. Ziller
Department of Chemistry, University of California, Irvine, California 92697-2025
Received March 24, 2005
An atom- and time-efficient synthetic route to sterically crowded organoactinide (C5Me5)3-
UX complexes involving the in situ formation of borate salts is reported. Addition of BPh3
to (C5Me5)2UMeCl followed by KC5Me5 provides a much improved synthesis of (C5Me5)3UCl
that presumably proceeds through a [(C5Me5)2UCl][MeBPh3] intermediate borate salt.
Attempts to make (C5Me5)3UMe, the first tris(pentamethylcyclopentadienyl) metal alkyl
complex, by reaction of BPh3 with (C5Me5)2UMe2 followed by KC5Me5 were also successful,
and the product was characterized by X-ray crystallography. In this reaction system, the
formation of the borate intermediate, [(C5Me5)2UMe][MeBPh3], was confirmed by variable-
temperature NMR spectroscopy and by X-ray crystallography of the THF adduct.
Introduction
the unconventional bond distances.1 This includes an
η1-C5Me5 alkyl-like reactivity, (C5Me5)--based reduction
called sterically induced reduction (SIR),12 and unex-
pectedly facile (C5Me5)- displacement.13 In contrast, the
(C5Me5)- ligands in complexes displaying “normal” M-C
bond distances are generally inert ancillary ligands.
Devising synthetic routes to these highly reactive,
sterically disfavored, (C5Me5)3M and (C5Me5)3MX com-
plexes is challenging since less crowded alternative
products prefer to form if any pathway is available.
Currently the highly reactive (C5Me5)3M species are
prepared in reactions that produce stable byproducts
and leave the three (C5Me5)- ligands as well as the
trivalent metal with no alternative but to form the
sterically crowded complexes.9,14-16 One of the most
general syntheses involves addition of KC5Me5 to a
(BPh4)- salt, which eliminates KBPh4 and adds a
(C5Me5)- ligand, eq 1.
Recent synthetic advances in f element organometallic
chemistry have shown that an entire family of tris-
(pentamethylcyclopentadienyl) metal complexes can be
isolated in which all of the metal ligand bonds are longer
than normal.1 This was unexpected since metal-ligand
distances in f element complexes are generally quite
regular from one complex to another.2-6 Prior to the
isolation of (C5Me5)3Sm,7 it also seemed sterically
impossible to put three (C5Me5)- groups around one
metal center. The subsequent isolation of (C5Me5)3UCl8
demonstrated that, in addition to three (C5Me5)- groups,
a metal could also accommodate a fourth ligand. In each
of the subsequently isolated (C5Me5)3MX (X ) H,9 F,8
Cl8) and (C5Me5)3ML (L ) CO,10 N211) complexes, the
fourth ligand is either a single atom or a small,
cylindrical molecule that could fit into the space along
the C3 axis perpendicular to the plane defined by the
metal and the three ring centroids, Figure 1.
These long bond organometallic complexes are highly
reactive since unusual (C5Me5)- reactivity accompanies
* To whom correspondence should be addressed. E-mail: wevans@
uci.edu.
(1) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 2119, and
references therein.
The [(C5Me5)2M][(µ-Ph)2BPh2] starting materials for
these syntheses contain loosely ligated (BPh4)- anions
that stabilize the cationic metallocenes, but interact only
through long M-C(arene) distances.15,16
We report here that this borate route for (C5Me5)3M
synthesis can also be applied to the synthesis of
(2) Raymond, K. N.; Eigenbrot, C. W., Jr. Acc. Chem. Res. 1980, 13,
276.
(3) Marks, T. J.; Ernst, R. D. Comprehensive Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Perga-
mon: Oxford, 1982; Chapter 21.
(4) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283.
(5) Evans, W. J.; Foster, S. E. J. Organomet. Chem. 1992, 433, 79.
(6) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem.
Rev. 2002, 102, 1851.
(7) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc.
1991, 113, 7423.
(12) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc.
1998, 120, 9273.
(8) Evans, W. J.; Nyce, G. W.; Johnston, M. A.; Ziller, J. W. J. Am.
Chem. Soc. 2000, 122, 12019.
(13) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J.
Am. Chem. Soc. 2004, 126, 14533.
(9) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Organometallics 2001,
20, 5489.
(14) Evans, W. J.; Forrestal, K. J.; Leman, J. T.; Ziller, J. W.
Organometallics 1996, 15, 527.
(10) Evans, W. J.; Kozimor, S. A.; Nyce, G. W.; Ziller, J. W. J. Am.
Chem. Soc. 2003, 125, 13831.
(15) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 6745.
(11) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. J. Am. Chem. Soc.
2003, 125, 14264.
(16) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W.
Organometallics 2002, 21, 1050.
10.1021/om050230s CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/03/2005

3408 Organometallics, Vol. 24, No. 14, 2005
Evans et al.
Figure 1. Crystallographically characterized (C5Me5)3MX and (C5Me5)3UL complexes.
(C5Me5)2UMeCl.21 After 5 min, a solution of BPh3 (35 mg, 0.144
mmol) in benzene (5 mL) was added to the reaction vessel.
After the mixture was stirred for 3 h, it was transferred to a
silylated flask that had been charged with KC5Me5 (36 mg,
0.207 mmol). After the mixture was stirred for an additional
20 h, a white precipitate was removed by centrifugation. The
solvent was removed by rotary evaporation, and the previously
characterized (C5Me5)3UCl8 was isolated as a red powder (96
(C5Me5)3UX (X ) Cl, Me) complexes. In this case, the
borate salts are generated in situ by addition of BPh3
to the appropriate conventional bis(pentamethylcyclo-
pentadienyl) uranium(IV) alkyl complexes. The basis for
this approach has actually been in the literature since
Fischer and co-workers analyzed the addition of BPh3
to (C5H5)3UMe in 1988.17 We describe below the first
example of this type of reaction applied to the synthesis
of long bond organometallic species. The results dem-
onstrate that (MeBPh3)- salts, formed in situ, react
analogously to the (BPh4)- salts as precursors to steri-
cally crowded complexes.
This approach was initially tested as a route to the
previously characterized (C5Me5)3UCl8 and was subse-
quently used to make the first (C5Me5)3UX complex with
a polyatomic X, namely, (C5Me5)3UMe. Since alkyl
abstraction from metallocenes by Lewis acids to form
borate and aluminate salts is extensively studied in
polymerization chemistry,18,19 many variations of the
approach demonstrated here can be envisaged based on
the cationic complexes available in the literature.
1
mg, 97%) and identified by H NMR spectroscopy.
[(C5Me5)2UMe][MeBPh3], 2. A red solution of (C5Me5)2-
UMe2 (202.4 mg, 0.376 mmol) in toluene (7 mL) was added to
BPh3 (91 mg, 0.376 mmol) in toluene (7 mL). After the mixture
was stirred for 12 h, the mixture was centrifuged and [(C5-
Me5)2UMe][MeBPh4] (291 mg, 99%) was isolated as a red solid
1
upon removal of the solvent by rotary evaporation. H NMR
(C6D6, 298 K): 7.6 (m, br BPh3), 7.2 (m, br ∆ν1/2 ) 90 Hz, BPh3),
5.2 (s, 30H, ∆ν1/2 ) 90 Hz, C5Me5), -133.2 (s, 6H, ∆ν1/2 ) 190
Hz, Me) ppm. 11B NMR (C6D6): δ 67.3 ppm. IR: 2961s, 2907s,
2860s, 2725w, 1594s, 1567w, 1494m, 1432s, 1378m, 1355w,
1320s, 1282s, 1239s, 1185m, 1158w, 1096m, 1069m, 1027s,
999m, 884s, 803m, 776m, 745s, 699s, 645s cm-1. Anal. Calcd
for C40H54BU: C, 61.57; H, 6.54; B, 1.38; U, 30.51. Found: C,
61.38; H, 6.48; B, 1.30; U 30.75. X-ray quality crystals of the
THF adduct of 2, [(C5Me5)2UMe(THF)][MeBPh3], 2‚THF, formed
from a saturated solution of 2 in THF at -35 °C in a nitrogen-
filled glovebox.
Experimental Section
General Experimental Procedures. The synthesis and
manipulations of these extremely air- and moisture-sensitive
compounds were conducted with rigorous exclusion of air and
water by Schlenk, vacuum line, and glovebox techniques.
Unless otherwise specified, the compounds were handled under
argon with rigorous exclusion of coordinating solvents. Glass-
ware was treated with Siliclad (Gelest) to avoid formation of
oxide decomposition products. Toluene, benzene, and THF
were saturated with Ar and passed through a GlassContour
column.20 Benzene-d6 and toluene-d8 (Cambridge Isotope
Laboratories) were distilled over NaK alloy and benzophenone
and were degassed by three freeze-pump-thaw cycles. BPh3
(Aldrich) was sublimed (65 °C at 6 × 10-6 Torr) before use.
(C5Me5)2UCl2,21 (C5Me5)2UMe2,21 and C5Me5K13 were prepared
as previously described. NMR experiments were conducted
with Bruker 400 or 500 MHz spectrometers, and 11B NMR data
were referenced to an external standard of BF3‚OEt2. Elec-
tronic absorption measurements were made in benzene and
conducted using a Perkin-Elmer Lambda 900 UV/vis/NIR
spectrophotometer in Teflon sealable 1 cm quartz cells. IR
samples were analyzed as thin films from benzene using an
ASI ReactIR1000.22 Elemental analyses were provided by
Analytische Laboratorien, Lindlar, Germany.
(C5Me5)3UMe, 3, from (C5Me5)2UMe2. Following the pro-
cedure for 1, BPh3 (142 mg, 0.587 mmol) in benzene (7 mL),
(C5Me5)2UMe2 (322 mg, 0.598 mmol) in benzene (7 mL), and
KC5Me5 (125 mg, 0.718 mmol) were combined to produce
(C5Me5)3UMe, which was isolated as a red powder (261 mg,
66%). Hexagonal crystals of 3 suitable for crystallographic
analysis were grown from benzene solutions at room temper-
ature in an NMR tube by slow evaporation. The synthesis can
also be carried out in toluene. 1H NMR (C6D6): δ 9.2 (s, 45H,
C5Me5, ∆ν1/2 ) 10 Hz); -204 (s, 3H, CH3, ∆ν1/2 ) 70 Hz) ppm.
13C NMR (C6D6): δ -30.5 (s, C5Me5), 260.4 (s, C5Me5) ppm,
assignments confirmed by HMQC. IR (thin film): 2964s, 2910s,
2856s, 2725vw, 1436m, 1378m, 1262s, 1069s, 1019s, 949w,
864w, 799s, 698w, 671m cm-1. Anal. Calcd for C31H48U: C,
56.52; H, 7.29. Found: C, 57.65; H, 6.88.
(C5Me5)3UMe, 3, from 2. A solution of [(C5Me5)2UMe]-
[MeBPh3] (298 mg, 0.382 mmol) in benzene (10 mL) was added
to silylated flask containing KC5Me5 (86 mg, 0.494 mmol). The
mixture darkened in color as it stirred for 12 h. A white solid
was separated from the red solution by centrifugation. Upon
removal of solvent by rotary evaporation, as described above,
(C5Me5)3UMe, 3, was isolated as a red powder (154 mg, 61%).
(C5Me5)3UCl, 1. A solution of (C5Me5)2UMe2 (39 mg, 0.072
mmol) in benzene (5 mL) was added to a stirred solution of
(C5Me5)2UCl2 (42 mg, 0.072) in benzene (5 mL) to generate
X-ray Data Collection, Structure Solution, and Re-
finement [(C5Me5)2UMe(THF)][MeBPh3], 2‚THF. A red
crystal of approximate dimensions 0.08 × 0.10 × 0.12 mm was
mounted on a glass fiber and transferred to a Bruker CCD
platform diffractometer. The SMART23 program package was
used to determine the unit-cell parameters and for data
collection (25 s/frame scan time for a sphere of diffraction data).
The raw frame data were processed using SAINT24 and
(17) Aslan, H.; Fo¨rster, J.; Yu¨nlu¨, K.; Fischer, R. D. J. Organomet.
Chem. 1988, 355, 79.
(18) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev.
2000, 100, 1253.
(19) Chen, E. Y.; Marks, T. J. Chem. Rev. 2000, 100, 1391.
(20) THF and diethyl ether were dried over activated alumina and
sieves. Toluene and hexanes were dried over Q-5 and molecular sieves.
For more information on the drying system see www.glasscontour.com.
(21) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.;
Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650.
(22) Evans, W. J.; Johnston, M. A.; Ziller, J. W. Inorg. Chem. 2000,
39, 3421.
(23) SMART Software Users Guide, Version 5.1; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1999.
(24) SAINT Software Users Guide, Version 6.0; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1999.

Facile Triphenylborane-Based Syntheses
Organometallics, Vol. 24, No. 14, 2005 3409
Table 1. Crystal Data and Structure Refinement
for [(C5Me5)2UMe(THF)][MeBPh3], 2‚THF, and
(C5Me5)3UMe, 3
Scheme 1
2‚THF
3
empirical formula
C
44H59BOU
C31H48
658.72
P63/m
10.0075(12)
10.0075(12)
15.452(4)
90
90
120
1340.2(4)
2
0.71073
1.632
6.072
1.263
0.0294
0.0719
U
fw
852.75
P1h
space group
a (Å)
b (Å)
c (Å)
9.299(2)
13.881(4)
16.137(4)
69.988(5)
83.060(5)
74.169(5)
1882.0(8)
2
mixing (C5Me5)2UMe2 and its precursor (C5Me5)2UCl2
with a slight modification of the previously described
procedure.21
R (deg)
â (deg)
γ (deg)
volume (Å3)
Z
Addition of 1 equiv of BPh3 to a silylated vessel
charged with (C5Me5)2UMeCl generated a solution that
1
had a H NMR spectrum similar to the starting mate-
λ (Å)
0.71073
1.505
4.344
1.045
0.0657
0.1834
densitycalc (Mg/m3)
rial, but contained broadened resonances. After 3 h,
addition of KC5Me5 resulted in precipitation of a white
solid, presumably KMeBPh4, which was removed by
centrifugation from the red solution. (C5Me5)3UCl,8 1,
was isolated from the solution as a red powder in >90%
yield, Scheme 2.
abs coeff (mm-1
)
goodness-of-fit on F2
Ra [I>2σ(I)]: R1
Rb (all data): wR2
SADABS25 to yield the reflection data file. Subsequent calcula-
tions were carried out using the SHELXTL26 program. There
were no systematic absences nor any diffraction symmetry
other than the Friedel condition. The centrosymmetric triclinic
space group P1h was assigned and later determined to be
correct.
Scheme 2
The structure was solved by direct methods and refined on
F2 by full-matrix least-squares techniques. The analytical
scattering factors27 for neutral atoms were used throughout
the analysis. Hydrogen atoms were included using a riding
model. The uranium atom was refined anisotropically. All
remaining atoms were included using isotropic thermal pa-
rameters. At convergence, wR2 ) 0.1834 and GOF ) 1.045
for 194 variables refined against 6363 data (0.85 Å). As a
comparison for refinement on F, R1 ) 0.0657 for those 4980
data with I > 2.0σ(I), Table 1.
(C5Me5)3UMe, 3. A red crystal of approximate dimensions
0.07 × 0.13 × 0.14 mm was handled as described above for
2‚THF. The systematic absences were consistent with the
hexagonal space group P63/m, which was later determined to
be correct. The structure was solved by direct methods and
refined on F2 by full-matrix least-squares techniques. The
molecule was located on a site of 6h symmetry. Carbon atom
C(7) was disordered and was included with a site-occupancy
factor of 1/6. At convergence, wR2 ) 0.0719 and GOF ) 1.263
for 57 variables refined against 1156 data. As a comparison
for refinement on F, R1 ) 0.0294 for those 971 data with I >
2.0σ(I), Table 1.
Synthesisof[(C5Me5)2UMe][MeBPh4]and(C5Me5)3-
UMe. The synthesis of (C5Me5)3UMe, via the method
of Scheme 2, was subsequently attempted to determine
if it would provide the first (C5Me5)3MX or (C5Me5)3ML
complex in which the fourth ligand is more than a single
Results
Synthesis of (C5Me5)3UCl. The most convenient
previously reported synthesis of (C5Me5)3UCl proceeds
from (C5Me5)2UMe2 in four steps in 62% overall yield
via the sequence shown in Scheme 1.8 Although each
step in Scheme 1 proceeds in high yield, the preparation
typically required 7 or 8 days.
To shorten this procedure, an attempt was made to
synthesize (C5Me5)3UCl in analogy with the KC5Me5/
[(C5Me5)2U][(µ-Ph)2BPh2] synthesis of (C5Me5)3U in eq
1 by reacting KC5Me5 with [(C5Me5)2UCl][MeBPh3],
generated in situ. The precursor to the necessary borate
salt, (C5Me5)2UMeCl,21 was conveniently prepared by
(25) Sheldrick, G. M. SADABS, Version 2.05; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 2001.
(26) Sheldrick, G. M. SHELXTL Version 6.12; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 2001.
(27) International Tables for X-Ray Crystallography, Vol. C; Kluwer
Academic Publishers: Dordrecht, 1992.
Figure 2. Thermal ellipsoid plot of [(C5Me5)2UMe(THF)]-
[MeBPh3], 2‚THF, drawn at the 50% probability level.

3410 Organometallics, Vol. 24, No. 14, 2005
Evans et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for [(C5Me5)2UMe(THF)][MeBPh3], 2‚THF, and
(C5Me5)3UMe, 3
2‚THF
3
U(1)-O(1)
2.419(8)
2.739(11)
2.733(11)
2.717(12)
2.670(12)
2.703(11)
2.428
U(1)-C(21)
2.393(12)
2.751(12)
2.733(11)
2.704(11)
2.687(12)
2.702(12)
2.435
U(1)-C(7)
U(1)-C(1)
U(1)-C(2)
U(1)-C(3)
2.66(2)
U(1)-C(1)
U(1)-C(11)
2.904(6)
2.832(4)
2.802(4)
U(1)-C(2)
U(1)-C(12)
U(1)-C(3)
U(1)-C(13)
U(1)-C(4)
U(1)-C(14)
U(1)-C(5)
U(1)-C(15)
U(1)-Cnt(1)
Cnt(1)-U(1)-O(1)
C(21)-U(1)-Cnt(1)
O(1)-U(1)-C(21)
U(1)-Cnt(2)
Cnt(2)-U(1)-O(1)
C(21)-U(1)-Cnt(2)
Cnt-U(1)-Cnt
U(1)-Cnt
2.418
106.5
106.6
100.0
94.9(3)
98.8
140.0
C(7)-U(1)-Cnt
Cnt-U(1)-Cnt
90
120
atom (X ) H,9 Cl,8 and F8) or a cylindrical diatomic (L
to the analogous 2.72(2)-2.739(6) Å distances in (C5Me5)2-
) CO8 or N2 ). In this case, the intermediate borate salt
UCl2 and (C5Me5)2UMe2.32 The 2.393(12) Å U-C(21)
8
31
was isolated and fully characterized.
distance in 2‚THF is similar to other tetravalent
U-C(alkyl) distances, which are typically 2.4 Å in
length.33 The 2.419(8) Å U-O(THF) distance is also
comparable to other eight-coordinate U(IV)-O(THF)
distances, i.e., 2.449(9), 2.444(6), and 2.449(8) Å in
[(Me3Si)3C5H2]UCl2(THF)(µ-Cl)2Li(THF)2,41 (C9H7)UCl3-
(THF)2,42 and (C5MeH4)UCl3(THF)2,43 respectively. This
length is approximately 0.1 Å shorter than analogous
distances in U(III) complexes, e.g., 2.511(8), 2.55(2),
and 2.55(1) Å in [(C5Me5)2U(THF)2](BPh4),28 (C5Me5)-
U(NMe2)3(THF),44 and (C5H5)3U(THF),45 respectively.
The (MeBPh3)- anion is structurally similar to that in
the previously characterized pyridinium triphenyl-
methylborate.46
Addition of BPh3 to (C5Me5)2UMe2 to make the
putative borate [(C5Me5)2UMe][MeBPh3] in situ caused
substantial broadening of 1H NMR resonances without
a significant change in chemical shift, as observed in
the (C5Me5)2UMeCl reaction in Scheme 2. The room-
temperature 11B NMR spectrum of this mixture con-
tained a single broad resonance at 67 ppm consistent
with a BPh3 standard. Variable-temperature NMR
experiments in toluene-d8 suggested that a reversible
equilibrium exists between (C5Me5)2UMe2 and BPh3 and
the methyl abstraction product, [(C5Me5)2UMe][MeBPh3],
eq 2. At 268 K the dominant peak in the 11B NMR
Addition of KC5Me5 instead of THF to the in situ
generated [(C5Me5)2UMe][MeBPh3] salt at room tem-
perature in benzene precipitated white solids and left
a red solution from which (C5Me5)3UMe, 3, was isolated
in 66% yield, Scheme 3. The identity of 3 as the first
(C5Me4R)3UMe complex was established by X-ray crys-
tallography, Figure 3 and Table 1.
[(C Me ) UMe][MeBPh ]
(C5Me5)2UMe2 + BPh3 h
5
5 2
4
2
(2)
spectrum is still that of the free BPh3. However, a
resonance at -46 ppm that is similar to the -43 ppm
shift of [(C5Me5)2U][(µ-Ph)2BPh2] is also observed. In the
1H NMR spectrum at this temperature, the most intense
resonances are those of (C5Me5)2UMe2 and BPh3, but a
small peak at 14.1 ppm is also observed that is attribut-
able to the (C5Me5)- ligands of [(C5Me5)2UMe][MeBPh3].
As the sample is cooled, the resonances of (C5Me5)2UMe2
The 1H NMR spectrum of (C5Me5)3UMe has a (C5Me5)-
resonance at 9.2 ppm which displays 13C-1H satellites
(31) Spirlet, M. R.; Revizant, J.; Apostolidis, C. Acta Crystallogr. C
1992, 48, 2135.
(32) 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.
and BPh3 in the H and 11B NMR spectra decrease in
1
intensity and the resonances assigned to [(C5Me5)2UMe]-
[MeBPh3] increase in intensity until at 238 K only the
resonances associated with [(C5Me5)2UMe][MeBPh3] are
observed.
Removal of solvent from this equilibrium mixture and
crystallization from THF gave crystals of [(C5Me5)2UMe-
(THF)][MeBPh3], 2‚THF, suitable for X-ray diffraction,
Figure 2 and Table 1. The trapping of “[(C5Me5)2UMe]-
[MeBPh4]” by THF is similar to the addition of THF to
[(C5Me5)2U][(µ-Ph)2BPh2],16 which generates the sol-
vated complex [(C5Me5)2U(THF)2](BPh4).28 Mono- and
di-solvated thorium analogues of 2 are also known:
[(C5Me5)2Th(Me)(THF)]{tBuCH2CH[B(C6F5)2]2H}29 and
[(C5Me5)2Th(Me)(THF)2][B(C6F5)4].30
(33) (C5H5)3U(C4H9),34 2.426(23) Å; (C5H5)3U(η1-2-methylallyl),35
2.48(3) Å; (C5H5)3U(p-MeC6H4CH2), 2.541(15) Å;35 (PhCH2)3UMe-
(DMPE),36 2.41(1) Å; Li(tBu2CHO)4UMe,37 2.465(7) Å; [C5H3(SiMe3)2]2-
UMe2,38 2.42(2) Å; [(3,5-C6H3Me2)(tBu)N]3UMe,39 2.446(7) Å; (C5Me5)U-
(CH2C6H5)3,40 2.477(18) to 2.53(2) Å; (C5Me5)2UMe2 2.424(7) Å.32
(34) Perego, G.; Cesari, M.; Farina, F.; Lugli, G. Acta Crystallogr.
B 1976, 32, 3034.
(35) Halstead, G. W.; Baker, E. C.; Raymond, K. N. J. Am. Chem.
Soc. 1975, 97, 3049.
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1984, 3, 293.
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L.; Andersen, R. A. Organometallics 1999, 18, 1235.
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Organometallics 2001, 20, 4993.
(40) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J.
Organometallics 2002, 21, 5978.
Complex 2‚THF displays conventional bond distances
for an eight-coordinate U(IV) metallocene, Table 2. The
2.71(3) Å U-C(C5Me5) average bond distance is similar
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Salazar, K. V. J. Organomet. Chem. 1983, 254, 305.
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Soc. 1987, 109, 4127.

Facile Triphenylborane-Based Syntheses
Organometallics, Vol. 24, No. 14, 2005 3411
Scheme 3
similar to the second class of organouranium complexes
identified by Morris and co-workers.55
The solid state structure of 3 is shown in Figure 3.
The complex crystallizes in the same P63/m space group,
as do all the other related complexes, (C5Me5)3U,56
(C5Me5)3MX (M ) U; X ) Cl,8 F;8 M ) Th, Z ) H9), and
(C5Me5)3UL (L ) CO,10 N211), Table 1. Complex 3 has
similar unit cell constants to these other sterically
crowded complexes despite the presence of the larger
methyl ligand (van der Waals radii of F, Cl, and Me are
1.35, 1.8, and 2.0 Å, respectively57). (C5Me5)3UMe is also
like these complexes in that its bond distances and
angles are unusual, Table 2. The (C5Me5 ring centroid)-
U-(C5Me5 ring centroid) angles in 3 are rigorously 120°,
as in (C5Me5)3U, despite the presence of the methyl
group and in contrast to the 140° (C5Me5 ring centroid)-
U-(C5Me5 ring centroid) angle in 2‚THF. The (C5Me5
ring centroid)-U-C(7) angles in 3 are rigorously 90°,
as are the (C5Me5 ring centroid)-M-fourth ligand
angles in the (C5Me5)3MX and (C5Me5)3UL complexes.
The 2.802(4)-2.904(6) Å U-C(C5Me5) distances and
2.569 Å U-centroid length are longer than those in
conventional uranium cyclopentadienyl complexes. For
example, the U(IV)-C(C5Me4H) distances in (C5Me4H)3-
UCl58 range from 2.658(11) to 2.911(10) Å and the
U-centroid is 2.52 Å. Analogous distances in [C5H3-
(SiMe3)2]3UCl are 2.718(8)-2.81(1) and 2.49 Å, respec-
tively.59
Despite the high quality of the crystallographic data
on 3, the refinement yielded an elongated anisotropic
thermal ellipsoid for the uranium atom along the
U-C(7) axis. In light of this, it is difficult to compare
the 2.66(2) Å U-C(7) distance with other tetravalent
U-C(alkyl) distances, which typically are around 2.4
Å.33 Long uranium to fourth ligand distances have been
observed with all the other (C5Me5)3UX and (C5Me5)3-
UL complexes characterized to date, but elongated
ellipsoids were identified only in complexes containing
anionic ligands. Refinements in lower space groups were
examined as well as models in which the uranium was
disordered on either side of the trigonal plane. These
did not give any additional definitive information on this
issue.
Figure 3. Thermal ellipsoid plot of (C5Me5)3UMe, 3, drawn
at the 50% probability level. The disordered portion of the
C7 methyl carbon atom has been omitted.
(J ) 125 Hz) despite the paramagnetism of the complex.
The spectrum also contains a resonance at -204 ppm
consistent with a U(IV) methyl complex. For example,
the U(IV) methyl complexes (C5H5)3UMe,47 (C5Me5)-
21
{O[SiMe2(tBuN)]2}UMe,48 and (C5Me5)2UMe2 have
resonances at δ -202, -146, and -124 ppm, respec-
tively.
The near-IR spectrum of 3 in the 16,700 to 6300 cm-1
region, Figure 4, contains absorptions consistent with
a 5f2 uranium complex.49-55 The intensities of these
absorptions (ꢀ ≈ 30 to 130 M-1 cm-1) are greater than
those observed for (C5Me5)2UMe2, (C5Me5)2UCl2, and
(C5Me5)2UMeCl (ꢀ ≈ 10 to 80 M-1 cm-1), but are less
than those observed in pentamethylcyclopentadienyl
uranium complexes containing imido and ketimido
ligands (ꢀ ≈ 40 to 400 M-1 cm-1). The absorption
intensities displayed by 3 are most similar to metal-
locenes that contain two hydrazonato ligands (ꢀ ≈ 30
to 120 M-1 cm-1). This makes (C5Me5)3UMe most
Discussion
The in situ borate route to (C5Me5)3UCl, 1, Scheme
2, is a much improved route to this compound compared
to the previously reported synthesis, Scheme 1. Both
routes start with (C5Me5)2UMe2, but Scheme 1 avoids
the reduction, protonolysis, and PhCl re-oxidation steps.
The Scheme 2 synthesis occurs without change in metal
oxidation state and with intermediates that do not need
to be isolated for successful synthesis.
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Actinide Elements, 2nd ed.; Chapman and Hall: New York, 1986; Vols.
1 and 2, and references therein.
The value of this in situ borate approach for preparing
(C5Me5)3UX compounds was further demonstrated by
making (C5Me5)3UMe, 3. The synthesis of this com-
(50) Zanella P.; Rossetto, G.; De Paoli, G. Inorg. Chim. Acta 1980,
44, L155.
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K.; Fischer, R. D. Organometallics 1983, 2, 344.
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sity Press: Ithaca, NY, 1960.
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Chem. 2001, 40, 6725.
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Organometallics 1994, 13, 2895.
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Chem. 1998, 551, 261.
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3412 Organometallics, Vol. 24, No. 14, 2005
Evans et al.
Figure 4. UV-vis-NIR electronic absorption spectrum of a 3.15 mM solution of (C5Me5)3UMe, 3, in benzene. Left: The
entire spectrum. Right: The near-infrared region.
pound was challenging since even (C5Me5)3UBr8 has yet
to be crystallographically confirmed due to its instability
and Me has a larger van der Waals radius (2.0 Å) than
Br (1.85 Å).57 Hence, it was uncertain if 3 would form
for steric reasons. In addition, all previous (C5Me5)3MX
and (C5Me5)3ML complexes had X and L ligands that
could remove electron density from the metal center.
The electronic difficulties in making (C5Me4R)3MX
complexes with electron-donating X ligands such as H
and Me have previously been discussed and can also be
substantial.58,60,61 The successful isolation of 3 showed
that neither steric nor electronic constraints prevent its
formation.
demonstrates that the steric limits of tris(pentameth-
ylcyclopentadienyl) complexes are still expanding and
that (C5Me5)3MX and (C5Me5)3ML complexes can be
obtained with X or L ligands that are electron donating
and are more than monatomic ions or cylindrical di-
atomics. These syntheses also demonstrate that in
addition to the (BPh4)- displacements done in the past,
displacement by (C5Me5)- of (BPh3Me)- borates gener-
ated in situ can be accomplished. The breadth of this
approach to uranium alkyls remains to be determined,
but it is likely that it could apply to complexes with
conventional bond distances as well as these long bond
organometallics.
Conclusion
Acknowledgment. For support of this research, we
thank the National Science Foundation.
The facile syntheses of 1 and 3 described here provide
the necessary synthetic methodology to readily prepare
sufficient quantities of (C5Me5)3UCl and (C5Me5)3UMe
for reactivity studies. The isolation of (C5Me5)3UMe
Supporting Information Available: 1H NMR spectrum
of 3. X-ray diffraction data, atomic coordinates, thermal
parameters, and complete bond distances and angles; listing
of observed and calculated structure factor amplitudes for
compounds 2‚THF and 3 (PDF and CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(60) Gradoz, P.; Boisson, C.; Baudry, D.; Lance, M.; Nierlich, M.;
Vigner, J.; Ephritikhine, M. J. Chem. Soc., Chem. Commun. 1992,
1720.
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M.; Vigner, J. J. Organomet. Chem. 1994, 466, 107.
OM050230S