Synthesis and structural characterization of uranium ansa-metallocene complexes containing organoimido functional groups; Electronic effects of ancillary ligands

Article abstract of DOI:10.1016/S0022-328X(99)00525-2

Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand sets [Me₂Si(η⁵-C₅Me₄) ₂]^2- and [Me₂Si(η⁵-C₅Me₄)(η ⁵-C₅H₄)]^2- have been prepared to examine their utility in generating organoimido complexes of uranium. The chloride complexes [Me₂Si(C₅Me₄)₂]UCl ₂·2LiCl·4(Et₂O) (1) and [Me₂Si(C₅Me₄)(C₅H ₄)]-UCl₂·2LiCl·4(THF) (2) may be generated at room temperature by reaction of UCl₄ with the corresponding bis(cyclopentadienyl) dilithio salt in diethyl ether and tetrahydrofuran, respectively. Complex 1 may be recrystallized from toluene in the presence of TMEDA to yield the complex [Me₂Si(C₅Me₄)₂]U(μ-Cl ₄){Li(TMEDA)}₂ (1·TMEDA). The molecular structure of this complex has been determined by single-crystal X-ray diffraction. Compounds 1 and 2 may be alkylated by Grignard reagents. While the dimethyl complex [Me₂Si(C₅Me₄)₂]UMe₂ (3) may be isolated, the analogous complex and [Me₂Si(C₅Me₄)(C₅H ₄)]UMe₂ appears to be thermally unstable. Benzyl derivatives employing both ligand sets (4, 5) have been prepared. These alkyl complexes are protonated by N,N′-diphenylhydrazine to yield phenylimido-containing products. Surprisingly, while reaction of compounds 3 or 4 with N,N′-diphenylhydrazine yields the expected U(VI) complex [Me₂Si(C₅Me₄)₂]U(NC ₆H₅)₂, reactions employing 5 generate only the U(IV) monoimido dimer [Me₂Si(C₅Me₄)(C₅H ₄)]U(NC₆H₅)]₂ (8). The molecular structure of 8 has been determined; the compound exists as a dimer with asymmetric organoimido bridging ligands. Electrochemical investigations of the chloride compounds 1 and 2 suggest that the ancillary ligands have the capacity to significantly alter the redox activity of the metal center. The use of electron-rich ancillary ligands appears to be important in the isolation of high-valent organouranium complexes.

Full text of DOI:10.1016/S0022-328X(99)00525-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 591 (1999) 14–23
Synthesis and structural characterization of uranium
ansa-metallocene complexes containing organoimido functional
groups; electronic effects of ancillary ligands
R. Chris Schnabel a, Brian L. Scott b, Wayne H. Smith c, Carol J. Burns b,*
a Department of Chemistry, Eckerd College, St. Petersburg, FL 37711, USA
b Chemical Science and Technology Di6ision, MS J514, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
c Nuclear Materials Technology Di6ision, MS E501, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 28 June 1999; accepted 9 September 1999
Abstract
Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand sets [Me2Si(h5-C5Me4)2]2− and [Me2Si(h5-C5Me4)(h5-
C5H4)]2− have been prepared to examine their utility in generating organoimido complexes of uranium. The chloride complexes
[Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]-UCl2·2LiCl·4(THF) (2) may be generated at room temperature
by reaction of UCl4 with the corresponding bis(cyclopentadienyl) dilithio salt in diethyl ether and tetrahydrofuran, respectively.
Complex 1 may be recrystallized from toluene in the presence of TMEDA to yield the complex [Me2Si(C5Me4)2]U(m-
Cl4){Li(TMEDA)}2 (1·TMEDA). The molecular structure of this complex has been determined by single-crystal X-ray diffraction.
Compounds 1 and 2 may be alkylated by Grignard reagents. While the dimethyl complex [Me2Si(C5Me4)2]UMe2 (3) may be
isolated, the analogous complex and [Me2Si(C5Me4)(C5H4)]UMe2 appears to be thermally unstable. Benzyl derivatives employing
both ligand sets (4, 5) have been prepared. These alkyl complexes are protonated by N,N%-diphenylhydrazine to yield phenylimido-
containing products. Surprisingly, while reaction of compounds 3 or 4 with N,N%-diphenylhydrazine yields the expected U(VI)
complex [Me2Si(C5Me4)2]U(NC6H5)2, reactions employing 5 generate only the U(IV) monoimido dimer [Me2Si(C5Me4)(C5H4)]U-
(NC6H5)]2 (8). The molecular structure of 8 has been determined; the compound exists as a dimer with asymmetric organoimido
bridging ligands. Electrochemical investigations of the chloride compounds 1 and 2 suggest that the ancillary ligands have the
capacity to significantly alter the redox activity of the metal center. The use of electron-rich ancillary ligands appears to be
important in the isolation of high-valent organouranium complexes. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Uranium; Organoimido; ansa-Metallocene
have investigated the preparation of oxo and imido
complexes of uranium(IV), (V), and (VI) utilizing the
bis(pentamethylcyclopentadienyl) supporting ligand
framework ((h5-C5Me5)2An, An=Th, U) [3]. Although
the complexes (h5-C5Me5)2U(NR)2 (R=aryl, 1-
adamantyl) are reduced slowly by dihydrogen or silanes
[3,4], neither these species nor the related uranium(IV)
monoimido complexes react with alkenes or alkynes by
cycloaddition, as is commonly observed in the chem-
istry of Group 4 complexes containing organoimido
ligands [5]. Similarly, although CꢀH bond activation of
benzene is commonly observed in reactions of Group 4
imido complexes [6], this reaction has not been ob-
served in thermolyses of uranium imido complexes [7].
1. Introduction
Complexes of the actinides containing terminal
metal–ligand multiple bonds are becoming increasingly
common [1] as researchers seek to compare the reactiv-
ity of these species with those of their early transition
metal cogeners. Although in some cases these func-
tional groups have been implicated as the active sites in
processes catalyzed by organoactinide complexes [2],
isolated uranium or thorium oxo or imido ligands are
not generally found to be as reactive as titanium or
zirconium species toward electrophilic reagents. We
* Corresponding author. Fax: +1-505-667-9905.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00525-2

R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
15
The lack of reactivity could be attributed to a number
of factors, including the greater polarity of actinide-ele-
ment bonds, the potential for enhanced metal–ligand
multiple bonding employing actinide 5f-orbitals, or the
steric congestion accompanying the use of the bulky
bis(pentamethylcyclopentadienyl) ligand set. In an ef-
fort to clarify the steric role of the ancillary ligands, we
have attempted to prepare uranium(VI) organoimido
complexes employing the ‘tied-back’ bis(cyclopentadi-
enyl) ligand sets. The silyl-bridged ligands [Me2Si(h5-
Me4C5)2]2− and [Me2Si(h5-Me4C5)(h5-C5H4)]2− were
chosen to improve the solubility of the resulting com-
plexes. The precursor halide complexes of U(IV) have
been synthesized; these may be alkylated by Grignard
reagents to yield the corresponding dialkyl derivatives
[Me2SiCpCp%]UR2. Reactions of the dialkyl compounds
with N,N%-diphenylhydrazine were examined, with the
previously reported [8]. Metallocene halide complexes
of the f-elements using these ligands are generally pre-
pared by reaction of the metal tetrachloride with the
dilithio salts of the bis(cyclopentadienyl) ligands. We
have examined the comparable reactions of UCl4 with
the lithium salts. We have found that these reactions
provide an effective high yield of the desired U(IV)
halogen complexes, [Me2Si(C5Me4)2]UCl2·2LiCl·4-
(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]UCl2·2LiCl·
4(THF) (2) (Eq. (1)). The complexes 1 and 2 are
isolated as dark-red, air-sensitive solids. As has been
observed for the thorium analog [Me2Si(C5Me4)2]-
ThCl2·2LiCl·2(DME) [8], the reduced steric bulk of the
chelating bis(cyclopentadienyl) ligand set results in the
isolation of lithium chloride adducts from metathesis
reactions. This results in somewhat lower solubility
than observed for the precursor halide complex
(C5Me5)2UCl2 [9]; both are soluble in aromatic hydro-
carbons but insoluble in hexane. The complexes are
static at room temperature in aromatic solvents. The
1H-NMR spectrum of complex 2 consists of two dis-
crete sets of isotropically shifted, narrow resonances for
the (C5Me4), (C5H4), and Me2Si groups, suggesting Cs
symmetry for the molecule comparable to the known
lanthanide complex [Me2Si(C5Me4)(C5H4)]LuCl2·Li-
(Et2O)2 [8b] Complex 1 exhibits C2 symmetry; the reso-
nances are fairly broad, as has been previously ob-
served for (h5-C5Me5)2UCl2.
expectation
that
the
products
would
be
bis(phenylimido)uranium(VI) complexes in all cases, by
analogy with known chemistry of complexes (h5-
C5Me5)2UR2 [3]. It was envisioned that these more
constrained ligand sets would primarily alter the reac-
tivity of the organoimido products by relieving steric
congestion around the metal center. We have deter-
mined instead that the electronic nature of the ligand
set has a more profound impact on the course of
reactions; the more electron-rich permethylated ligand
sets are required to stabilize uranium(VI) products. In
their absence, only uranium(IV) imido complexes are
isolated.
2. Results and discussion
2.1. Synthesis and properties of organouranium halide
complexes
Chelating bis(cyclopentadienyl) metallocene com-
plexes of both actinides and lanthanides have been
(1)
2.2. Molecular structure of
[Me2Si(C5Me4)2]U(v-Cl4){Li(TMEDA)}2 (1·TMEDA)
Recrystallization of complex 1 from toluene contain-
ing tetramethylethylenediamine (TMEDA) generates a
TMEDA adduct that is more suitable for characteriza-
tion by single crystal X-ray diffraction (Fig. 1, Tables 1
and 2). The molecular structure of the complex reveals
a typical bent metallocene complex. The ring centroid–
U–centroid angle (114.1°) is considerably smaller than
Fig. 1. Thermal ellipsoid plot of [Me2Si(C5Me4)2]UCl2·2LiCl-
(TMEDA)2 (1·TMEDA) with atomic numbering scheme.

16
R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
Table 1
tween the metal and the carbons farthest in the ring
from the silyl bridge. This dispersion of U–C bond
distances has been reported in other chelating metal-
locene complexes, and is thought to be due to the strain
implemented by the Me2Si bridge. The uranium is
coordinated to four bridging chloride ligands. Two of
the uranium chloride bond distances are longer than
Crystallographic parameters for the compounds [Me2Si(C5Me4)2]U(m-
Cl4){Li(TMEDA)}2 (1·TMEDA) and {[Me2Si(C5Me4)(C5H4)]U-
(NPh)}2 (8)
(1)·TMEDA
(8)·toluene
Empirical formula
Space group
Temperature (°K)
C32H62Cl4Li2N4SiU
P21/n
173
13.085(2)
22.723(2)
14.111(3)
106.62
4020.3(11)
4
C51H58N2Si2U2
P2/n
178
13.251(1)
10.104(2)
17.251(2)
100.85
2268.4(5)
2
,
the others: U(1)ꢀCl(1)=2.885(3) A, U(1)ꢀCl(2)=
,
,
2.853(3) A; U(1)ꢀCl(3)=2.760(3) A, U(1)ꢀCl(4)=
,
a (A)
,
,
b (A)
2.746(3) A. The longer U–Cl distances correspond to
,
c (A)
those chlorides that are in three-fold bridging positions,
coordinated to both lithium ions. Those chloride lig-
ands that are more closely associated with the uranium
center only bridge to one lithium ion. The U–Cl dis-
tances are longer than typical terminal U–Cl dis-
i (°)
3
,
V (A )
Z
zcalc (Mg m−3
)
,
1.528
1.803
Radiation (u (A))
Mo–Ka
(0.71069)
924.66
Mo–Ka
(0.71069)
1231.23
7.219
,
tances (e.g. U–Cl(avg)=2.696(2) A in (C5Me5)2UCl2-
Fw (g mol−1
)
(N2C3H3)) [11], as might be expected for a bridging
Absorption coefficient
4.359
,
chloride. The average Li–Cl distance is 2.49(2) A and
(mm−1
)
,
the average Li–N distance is 2.17(2) A; these are typi-
Absorption correction
Min./max. trans.
Range (°)
Empirical
0.17/0.59
1.75–22.50
4.78
Empirical
0.31/0.75
2.55–20.00
3.96
cal distances seen for other anionic uranium complexes
such as (C5Me5)2U(NPh)(m-Cl)Li(TMEDA) [3c].
R (%) a
Rw (%) a
9.46
8.37
2.3. Preparation of alkyl deri6ati6es
a R1=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ and wR2=[S[w(Fo2−Fc2)2]/S[w(Fo2)2]]1/2
.
The parameter w=1/[|2(Fo2)+(0.0595*P)2], with P=(Fo2+2Fc2)/3.
The uranium center in complex 1·TMEDA is for-
mally ten coordinate in the solid state, and yet complex
1 efffectively behaves in solution as an eight-coordinate
[Me2Si(C5Me4)2]UCl2 fragment useful as a precursor to
a number of complexes. Alkylation of the halo-precur-
sors 1 and 2 with Grignard reagents in most cases (in
the presence of excess 1,4-dioxane) proceeds smoothly
to give the dialkyl complexes as crystalline solids which
are exceedingly air and moisture sensitive (Eqs. (2) and
(3)).
Table 2
,
Selected bond lengths (A) and angles (°) for [Me2Si(C5Me4)2]U-
(m-Cl4){Li(TMEDA)}2 (1·TMEDA)
Bond lengths
UꢀC(21)
UꢀC(22)
UꢀC(23)
UꢀC(24)
UꢀC(25)
UꢀC(31)
UꢀC(32)
UꢀC(33)
UꢀC(34)
UꢀC(35)
UꢀCl(1)
UꢀCl(2)
UꢀCl(3)
UꢀCl(4)
2.732(10)
2.799(10)
2.899 (10)
2.864 (10)
2.785 (10)
2.764 (9)
2.792(10)
2.845 (11)
2.852(10)
2.781(9)
SiꢀC(1)
SiꢀC(31)
SiꢀC(2)
SiꢀC(21)
Li(1)ꢀN(3)
Li(1)ꢀN(4)
Li(2)ꢀN(1)
Li(2)ꢀN(2)
Li(1)ꢀCl(1)
Li(1)ꢀCl(2)
Li(1)ꢀCl(4)
Li(2)ꢀCl(1)
Li(2)ꢀCl(2)
Li(2)ꢀCl(3)
1.880(11)
1.867(10)
1.868(11)
1.863(10)
2.26 (2)
2.13(2)
2.12(2)
2.17(2)
2.49(2)
2.43(2)
2RMgCl
[Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O)ꢀꢀꢀꢀꢀꢀꢀꢀꢁ
Et O, dioxane
2
1
[Me2Si(C5Me4)2]UR2 (2)
3, R=Me
4, R=CH C H
6 5
2
2.885(3)
2.853(3)
2.760(2)
2.746 (3)
2.50(2)
2.45(2)
2.68 (2)
2.42(2)
2RMgCl
[Me2Si(C5Me4)(C5H4)]UCl2·2LiCl·4(THF)ꢀꢀꢀꢀꢀꢀꢀꢀꢁ
Et O, dioxane
2
2
[Me2Si(C5Me4)(C5H4)]UR2 (3)
5, R=CH C H
6 5
2
Bond angles
CpꢀUꢀCp
Cl(1)ꢀUꢀCl(2)
Cl(1)ꢀUꢀCl(4)
Cl(2)ꢀUꢀCl(4)
114.1
C(1)ꢀSiꢀC(2)
Cl(1)ꢀUꢀCl(3)
Cl(2)ꢀUꢀCl(3)
Cl(3)ꢀUꢀCl(4)
101.2(5)
76.78(8)
75.55(8)
145.98(8)
In contrast to the reported instability of the
analogous thorium dimethyl derivative [8a], the
dimethyl complex 3 is quite stable and can be isolated.
The dimethyl derivative could not be generated from
complex 2, however; alkylation with methyl Grignards
gave rise to insoluble precipitates accompanied by gas
generation. The use of bulkier alkyl groups drastically
improves the isolated yield of the dialkyl products. The
dibenzyl complexes 4 and 5 were obtained in 80–90%
yield as green–black microcrystalline solids. Attempts
to make the alkyl-chloride complexes via conpropor-
tionation of complex 1 with 3 or 4 or complex 2 with 5
did not yield the desired products. The mixed alkyl-
67.24(8)
74.96(8)
76.41(8)
that observed in unconstrained bis(cyclopenta-
dienyl)uranium complexes (133–138°) [10], and is com-
parable to the angle determined for the thorium dialkyl
complex [Me2Si(C5Me4)2]Th(CH2Si(CH3)3)2 (118.4°)
[8a]. The average U–C (Cp) distance is 2.80(1) A,
although there is a wide range of distances (2.73(1)–
,
,
2.90(1) A). The longest U–C distances are those be-

R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
17
chloride complex [Me2Si(C5Me4)2]UCl(CH2C6H5) (7)
sistent with the formulation of
7
as
a
U(VI)
may be obtained in good yield, however, by protonation
of the bis(alkyl) 4 with [HNMe3]Cl (Eq. (4)).
bis(phenylimido) complex. Two resonances are observed
for the inequivalent methyl pairs on the (C5Me4) moei-
ties (l=3.67, 5.67 ppm), and a single resonance is ob-
served for the methyl groups on the dimethylsilyl
backbone (l=1.03 ppm). X-ray data collected for 7
were of poor quality and did not permit adequate refine-
ment of a structural model; full structural details will not
be presented. The atom connectivity was identified, how-
ever, and the identity of the complex as a monomeric
bis(phenylimido)uranium(VI) complex confirmed.
[HNMe ]Cl
3
[Me2Si(C5Me4)2]U(CH2C6H5)2ꢀꢀꢀꢀꢀꢀꢁ
toluene
[Me2Si(C5Me4)2]U(CH2C6H5)Cl
(4)
6
2.4. Synthesis and characterization of organoimido
complexes
Protonation of metallocene dialkyl complexes by sub-
stituted anilines has been employed as a route for the
generation of mono(arylimido) complexes of U(IV) [3c].
In the case of the bis(pentamethylcyclopentadienyl)-
uranium framework, the success in forming mono-
(arylimido) complexes is controlled the steric bulk of the
aryl substituent. The reaction of (C5Me5)2UMe2 with
one equivalent of a bulky aniline in toluene at elevated
temperatures produces the corresponding terminal imido
compound (C5Me5)2U(NAr) (Ar=2,6-di-iso-propyl-
C6H3 or 2,4,6-tri-tert-butyl-C6H2). However, reaction
of (C5Me5)2UMe2 with one equivalent of a less
sterically demanding ligand, such as aniline, yields an
equimolar mixture of (C5Me5)2UMe2 and (C5Me5)2U-
(NHPh)2.
The comparable reaction of the uranium dialkyl com-
plexes incorporating chelating bis(cyclopentadienyl) an-
cillary ligands with anilines was investigated to
determine if the steric bulk of the metal complex had an
effect on the course of the reaction. The reactions of
compounds 3, 4, and 5 with with bulkier anilines such as
2,4,6-tri-tert-butylaniline and 2,6-di-iso-propylaniline
was monitored in situ by 1H-NMR spectrometry. No re-
action was observed with any aniline at room tempera-
ture over the course of 48 h. At elevated temperatures,
decomposition of the bis(alkyl) is observed before proto-
nation of the metal bound alkyl via the aniline occurs.
Total decomposition of 3, 4, and 5 occurs when these
complexes are heated to 80°C in benzene for a period of
12 h.
(5)
Attempts to produce the analogous mixed ring U(VI)
bis(imido) complex by oxidation of 5 yielded an unex-
pected product. The reaction of the bis(benzyl) complex
5 with N,N%-diphenylhydrazine generates the dimeric
uranium(IV) complex {[Me2Si(C5Me4) (C5H4)]U(m-
NPh)]}2 (8) (Eq. (6)). This compound is very similar to
previously reported examples of bridging organoimido
species [12], although there are no examples of this class
with the chelating bis(cyclopentadienyl) ligand set. The
1H-NMR spectrum of compound 8 suggests that the
molecule exists as a symmetric dimer in solution; one set
of phenyl resonances and one set of resonances for the
chelating [Me2Si(C5Me4)(C5H4)] ligand may be iden-
tified. If the reaction is monitored by 1H-NMR, the only
products detected when the bis(benzyl) complex 5 is re-
acted with one equivalent of N,N%-diphenylhydrazine in
C6D6 at room temperature are the dimeric U(IV) com-
plex 8, two equivalents of toluene, and one equivalent of
azobenzene. The generation of both oxidized (azoben-
zene) and reduced (organoimido ligands generated from
hydrazine cleavage) products from hydrazine activation
has been reported previously in the catalytic dispropor-
tionation of N,N%-diphenylhydrazine to azobenzene and
aniline [2b]. The lack of an observable hexavalent
bis(imido) complex suggests either that the metal center
is less susceptible to oxidation in this complex, or that
the hexavalent species reacts much more quickly than
the corresponding complexes 7 or (C5Me5)2U(NPh)2
with reductants in solution (e.g. unreacted N,N%-
diphenylhydrazine).
The lack of success in obtaining U(IV) mono(imido)
complexes through protonation routes prevented their
use as reagents in generating U(VI) bis(imido) com-
pounds by reaction with organoazides [3]. Another route
commonly employed to prepare the uranium(VI) in-
volves reaction of metallocene dialkyl complexes with
N,N%-diphenylhydrazine. In this reaction, two
organoimido ligands are formed at uranium by reductive
cleavage of the hydrazine. The reaction of complexes 3
or 4 with N,N%-diphenylhydrazine proceeds analogously,
yielding the U(VI) bis(phenylimido) complex [Me2Si-
(C5Me4)2]U(NPh)2 (7) as a black microcrystalline solid
1
in near quantitative yield (Eq. (5)). The H-NMR of
complex 7 reveals chemical shifts in a range similar to
that previously observed for (C5Me5)2U(NPh)2 [3a], con-
(6)

18
R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
,
the plane of 0.02 A. The UꢀN bond distances in
2.5. Molecular structure of
{[Me2Si(C5Me4)(C5H4)]U(v-NPh)}2 (8)
,
compound 8 are 2.31(1) and 2.19(1) A; these distances
,
in [(MeC5H4)2U(m-NPh)]2 are 2.315(4) and 2.156(4) A.
The molecular structure of 8 was determined by
single-crystal X-ray diffraction (Fig. 2, Tables 1 and 3).
The complex is composed of dimeric [Me2Si-
(C5Me4)(C5H4)]U(NPh) units with a crystallographic
two-fold axis located in the middle of the U2N2 unit.
This mandates that both of the larger (C5Me4) rings lie
on the same side of the U2N2 plane. The average
UꢀC(C5H4) and UꢀC(C5Me4) distances are both 2.75(1)
The phenyl groups of the phenylimido ligands are
neither parallel nor perpendicular to the U2N2 ring
plane. The phenyl ring is bent significantly toward one
uranium center (UꢀNꢀC(11)=100.3(8)°), resulting in a
,
short contact distance of 2.89(1) A for U…C(11). This
distance is shorter than that found in the complex
,
[(MeC5H4)2U(m-NPh)]2 (3.09(1) A), longer than that
found for the uranium–aryl distance in Cp3U(CH2Ph-
,
,
A, again with some dispersion in the UꢀC bond lengths
p-Me) (2.51(2) A) [13] and comparable to other metal–
,
(range 2.72(1)ꢀ2.80(1) A), and the centroid–uranium–
aryl contact distances found either in benzyl complexes
[14] or in h6-arene complexes [15] of the actinides. As
has been postulated for the complex [(MeC5H4)2U(m-
NPh)]2, the unsymmetrical UꢀNꢀC bond angles and
UꢀN bond lengths in 8 are likely the consequence of
the complex having one resonance form in which the
bridging phenylimido ligands act as h3-azo-benzyl
groups.
centroid angle is 115.5°. The geometry of the central
U2N2 core is similar to that reported for the closely
related complex [(MeC5H4)2U(m-NPh)]2 [12a]. The
phenylimido groups bridge the two uranium centers in
an asymmetric fashion in both complexes. The U2N2
unit is nearly planar, with a maximum deviation from
2.6. Electrochemistry of
[Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1) and
[Me2Si(C5Me4)(C5H4)]UCl2·2LiCl·4(THF) (2)
Although clearly the relatively open coordination
environment promoted by the [Me2Si(C5Me4)(C5H4)]
ligand may play a role in the tendency to form a
tetravalent product (reduction reactions enhanced by
reduced steric constraints), it is also possible that this
ligand alters the relative stability of the +4 and +6
formal oxidation states for the uranium center. This is
supported by the observation that compound 8 is stable
with respect to further oxidation to yield a U(VI)
bis(imido) compound; the compound does not react
with organoazides. It has previously been reported that
introduction of chelating bis(cyclopentadienyl) ligand
sets can have a significant impact on the reactivity of a
transition metal center. In particular, it has been re-
ported that the silyl (Me2Si) linked chelating bis-
(cyclopentadienyl) ligand set can generate more elec-
trophilic metal centers in some complexes, and promote
CꢀH bond activation at metals in other species [16]. In
the present study, however, differences in chemistry are
observed between two uranium complexes which both
contain chelating bis(cyclopentadienyl) ligands. This
suggests that the key electronic difference is not the
introduction of a bridge between the two Cp ligands,
but rather the extent of alkylation of these rings. In
order to examine the effect of the substitution pattern
of the chelating bis(cyclopentadienyl) ligands on the
redox characteristics of the metal center, we have un-
dertaken investigations of the electrochemistry of the
parent complexes 1 and 2. Cyclic voltammetry experi-
ments conducted in THF solution demonstrate a
Fig. 2. Thermal ellipsoid plot of {[Me2Si(C5Me4)(C5H4)]U(m-NPh)}2
(8) with atomic numbering scheme.
Table 3
,
Selected bond lengths (A) and angles (°) for {[Me2Si(C5Me4)(C5H4)]
(m-NPh)}2 (8)
Bond lengths
UꢀC(1)
UꢀC(2)
UꢀC(3)
UꢀC(4)
UꢀC(5)
UꢀC(6)
UꢀC(7)
UꢀC(8)
UꢀC(9)
UꢀC(10)
2.750 (13)
2.718(14)
2.757(15)
2.792(14)
2.750(13)
2.726(13)
2.748(14)
2.796(14)
2.765(14)
2.731(13)
SiꢀC(18)
SiꢀC(17)
NꢀC(11)
UꢀN%
1.855(15)
1.843(13)
1.371(16)
2.189(11)
2.311(10)
UꢀN
Bond angles
NꢀUꢀN(0a)
UꢀNꢀU(0a)
UꢀNꢀC(11)
75.4(4)
104.6(4)
100.3(8)
C(17)ꢀSiꢀC(18)
Cp%ꢀUꢀCp
U(0a)ꢀNꢀC(11)
109.4(7)
115.5
153.2(9)

R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
19
Fig. 3. Cyclic voltammagrams of [Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]UCl2·2LiCl·4(THF) (2) (0.10 M [Bu4N][BPh4]
solution in THF) vs. FeCp2/FeCp2+
.
substantial difference in the redox potentials of com-
pounds. Both compounds display quasi-reversible oxi-
dation waves at scan rates of approximately 0.20 V s−1
(Fig. 3); compound 2 is more difficult to oxidize by
tion of the metallocene dialkyl complexes with substi-
tuted anilines. In some cases the chelating ligand sets
support the formation higher valent organouranium
complexes analogous to species which have been previ-
ously isolated using the more traditional (C5Me5)2U
ligand framework. The electronic characteristics of the
ancillary ligands appear to play a more important role
than has previously been identified, however, in the
stability of these complexes. Paradoxically, the more
electron-rich ancillary ligand sets are required to sup-
port uranium complexes in higher formal oxidation
states. This is demonstrated by the failure to isolate a
U(VI) bis(imido) complex using the less highly alkyl-
ated ligand set and [Me2Si(h5-Me4C5)(h5-C5H4)]2−. It
is still anticipated that within a class of uranium(VI)
compounds, the ‘tied-back’ ligand set will generate a
more reactive complex; with this in mind, we are now
examining the reactivity of [Me2Si(C5Me4)2]U(NC6H5)2
to determine if it will be more reactive than its counter-
part (C5Me5)2U(NC6H5)2.
approximately 0.24 V(1: E1/2= −0.66 V; 2: E1/2
=
−0.42 V versus FeCp2/FeCp+2 added as an internal
standard). The difference in redox potential of the
metal center induced by the different ligands sets of
compounds 1 and 2 is greater than that between
(C5Me5)2Ru (E1/2=0.42 V) and (C5H5)(C5Me5)Ru
(E1/2=0.54) [17].
3. Conclusions
The chelating bis(cyclopentadienyl) ligand sets
[Me2Si(h5-Me4C5)2]2− and [Me2Si(h5-Me4C5)(h5-C5-
H4)]2− have been introduced to complexes of uranium-
(IV) in hopes of providing more ‘open’ and more
reactive coordination environments to promote the re-
activity of organoimido functional groups bound to the
metal center. While structural evidence supports the
contention that these ligands generate less steric bulk at
the uranium center, their use is not conducive to the
isolation of certain classes of compounds, such as
U(IV) monoimido compounds produced by protona-
4. Experimental
All operations were performed using standard
Schlenk techniques under UHP-grade argon, or under

20
R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
helium in a Vacuum Atmospheres HE-553-2 drybox
with a MO-40-2 Dri-Train. Tetrahydrofuran (THF),
diethyl ether (Et2O), hexane, TMEDA, and toluene
were dried and distilled under nitrogen from sodium
benzophenone. Benzene-d6, toluene-d6, and THF-d8
were degassed and passed down a short column (5–6
cm) of activated alumina prior to use. Nuclear mag-
netic resonance (NMR) spectra were recorded on an
Bruker 300 MHz spectrometer. Chemical shifts were
referenced to protio solvent impurities (l 7.15, C6D6;
1.73, THF-d8; 2.09, toluene-d8), and are reported in
ppm downfield of Me4Si. Infrared spectra were ob-
tained in Nujol mulls between KBr plates on a Bio-Rad
Digilab FTS 40 spectrophotometer. Elemental analyses
were performed on a Perkin–Elmer 2400 CHN ana-
lyzer utilizing sealed aluminum capsules for delivery.
UCl4 [18], [Me2Si(C5Me4)2]Li2(THF)4 [8d] and
[Me2Si(C5Me4)(C5H4)]Li2(THF)4 [8b] were made fol-
lowing published procedures. Grignard reagents and
N,N%-diphenylhydrazine were obtained from Aldrich
and used as received.
(cm−1): 1300 m, 1259 m, 1200 w, 1095 w, 1042, w, 890
w, 813 m, 757 m, 721 m. Anal. Calc. for complex
(1·TMEDA) C32H62SiUCl4Li2N4: C, 41.57; H, 6.76; N,
6.06. Found: C, 41.40; H, 6.75; N, 6.03.
4.3. [Me2Si(C5Me4)(C5H4)]UCl2·2LiCl·4(THF) (2)
In the drybox, a 250 ml Erlenmeyer flask equipped
with a magnetic stir bar was charged with 2.72 g (7.16
mmol) of UCl4 and 3.81
g
(7.16 mmol) of
[Me2Si(C5Me4)(C5H4)]Li2(THF)4. The solids were dis-
solved in 120 ml of THF and the reaction mixture was
allowed to stir at r.t. for 12 h. The mixture was filtered
through Celite; removal of the solvent under reduced
pressure yielded 5.98 g (90%) of a dark red micro-crys-
1
talline solid. H-NMR (300 MHz, tol-d8): l −60.6 (s,
1H, (C5H4)), −52.0 (s, 1H, (C5H4)), −41.2 (s, 3H,
(C5Me4)), −29.4 (s, 3H, (C5Me4)), −16.5 (s, 3H,
Me2Si), −14.9 (s, 3H, Me2Si), −11.48 (s, 16 H, THF),
−5.99 (s, 16H, THF), 63.7 (s, 3H, (C5Me4)), 65.8 (s,
3H, (C5Me4)), 90.7 (s, 1H, (C5H4)), 96.2 (s, 1H, (C5H4)).
IR (cm−1): 1299 m, 1269 m, 1110 w, 1075 w, 1052, w,
990 w, 813 m, 727 m, 701 m. Anal. Calc. for
C32H54SiUCl4Li2O4: C, 41.57; H, 5.89. Found: C, 41.40;
H, 5.88.
4.1. Electrochemistry
Cyclic voltammetry experiments were carried out in
an inert-atmosphere drybox with a PAR/EG&G model
270 electrochemical analysis system. All measurements
were conducted in THF solution utilizing 0.10 M
[Bu4N][BPh4] (Ph=C6H5) as the supporting electrolyte.
The working electrode was a gold disk, and the counter
electrode was a platinum wire. A silver wire, separated
from the bulk solution by a fine-porosity fritted glass
disk, was used as a pseudo-reference electrode. Solution
resistance was compensated by electronic positive feed-
back. Potentials are referenced to the ferrocene/ferroce-
nium couple used as an internal standard. The
measured half-wave potential (E1/2) for the ferrocene/
ferrocenium internal standard is +0.705 V versus the
4.4. [Me2Si(C5Me4)2]U(CH3)2 (3)
In a drybox, a 250 ml Erlenmeyer flask was charged
with 1.09 g (1.10 mmol) of 1, 120 ml Et2O, and 1 ml
dioxane. At room temperature 0.85 ml (2.3 equivalents,
2.53 mmol) of ClMgCH3 (3M, in THF) was added
dropwise, resulting in the formation of a orange solu-
tion. Stirring was continued for 2 h, during which time
a white suspension formed. The solution was filtered
through Celite and the volatiles were removed under
reduced pressure to yield a bright orange powder. Re-
crystallization at −35°C in pentane afforded 0.425 g
pseudo-reference electrode at a scan rate of 0.20 V s−1
.
1
(68%) of orange needles. H-NMR (300 MHz, C6H6): l
−49.7 (s, 6H, UMe2), −7.18 (s, 12H, (C5Me4)),
−4.37 (s, 6H, Me2Si), 11.0 (s, 12H, (C5Me4)). IR
(cm−1): 1312 w, 1250 w, 1022 w, 935 w, 877 w, 833 m,
764 m, 673 m. Anal. Calc. for C22H36SiU: C, 46.63; H,
6.40. Found: C, 46.30; H, 6.56.
4.2. [Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1)
In the drybox, a 250 ml Erlenmeyer flask equipped
with a magnetic stir bar was charged with 2.05 g (5.32
mmol) of UCl4 and 3.19
g
(5.32 mmol) of
[Me2Si(C5Me4)2]Li2(THF)4. The solids were dissolved in
120 ml of Et2O and the reaction mixture was allowed to
stir at room temperature (r.t.) for 48 h. The mixture
was filtered through Celite; removal of the solvent
under reduced pressure yielded a dark red–brown solid.
The solid was washed with hexane, filtered, and dried
under vacuum. Yield 3.92 g (70%). 1H-NMR (300
MHz, C6D6): l −27.3 (bs, w1/2=2400 Hz, 12H,
(C5Me4)), −18.8 (bs, w1/2=570 Hz, 6H, Me2Si), 1.04
(bt, J=7.6 Hz, 24H, Et2O), 3.14, (bq, J=7.6 Hz, 16H,
Et2O), 52.3 (bs, w1/2=2400 Hz, 12H, (C5Me4)). IR
4.5. [Me2Si(C5Me4)2]U(CH2C6H5)2 (4)
In a drybox, a 250 ml Erlenmeyer flask was charged
with 2.02 g (2.04 mmol) of 1, 120 ml toluene, and 1 ml
dioxane. At r.t., 4.6 ml (2.3 equivalents, 4.69 mmol) of
ClMgCH2C6H5 (1 M, in Et2O) was added dropwise,
resulting in the formation of a dark green solution.
Stirring was continued for 2 h, during which time a
white suspension formed. The solution was filtered
through Celite and the volatiles were removed under

R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
21
reduced pressure to yield a dark black–green powder.
4.8. [Me2Si(C5Me4)2]U(NC6H5)2 (7)
The solid was washed with a minimum amount of
hexane, filtered, and dried in vacuo. Yield: 1.22 g
(84%). H-NMR (300 MHz, C6D6): l −10.33 (d, J=
6.3 Hz, 4H, ortho), −8.00 (s, 6H, Me2Si), −2.40 (s,
12H, (C5Me4)), 1.74 (t, J=7.6 Hz, 2H, para), 7.38 (t,
J=6.3 Hz, 4H, meta), 15.8 (s, 12H, (C5Me4)), 22.63
(bs, 4H, CH2C6H5). IR (cm−1): 1576 w, 1320 w, 1250
m, 1109 m, 1077 m, 874 m, 831 m, 803 m, 769 m, 665
w. Anal. Calc. for C34H44SiU C, 56.81; H, 6.17. Found:
C, 56.83; H, 6.38.
In a drybox, a scintillation vial was charged with
0.502 g (0.886 mmol) of 3 and 0.163 g (0.886 mmol) of
C6H5NHNHC6H5; the solids were dissolved in 12 ml of
toluene. The reaction was allowed to stir at r.t. for 2
days during which time the color changed from deep
black–green to black. The solution was filtered through
Celite and the volatiles removed under reduced pres-
sure. The resultant black oil was triturated several times
with hexane to give a black powder. Yield 0.588 g
(92%). 1H-NMR (300 MHz, C6D6): l 1.03 (s, 6H,
Me2Si), 1.77 (t, J=7.3 Hz, 2H, para), 3.60 (d, J=8.3
Hz, 4H, ortho), 3.67 (s, 12H, (C5Me4)), 4.67 (s, 12H,
(C5Me4)), 8.90 (t, J=7.3 Hz, 4H, meta). IR (cm−1):
1916 w, 1583 s, 1251 s, 1170 m, 1080 w, 1022 w, 987 w,
876 m, 836 m, 756 s, 696 m. Anal. Calc. for
C32H40SiUN2: C, 53.47; H, 5.61; N, 3.90. Found: C,
53.26; H, 5.72; N, 3.62.
1
4.6. [Me2Si(C5Me4)(C5H4)]U(CH2C6H5)2 (5)
In a drybox, a 250 ml Erlenmeyer flask was charged
with 2.02 g (2.18 mmol) of 2, 120 ml toluene, and 1 ml
dioxane. At r.t., 5.0 ml (2.3 equivalents, 5.03 mmol) of
ClMgCH2C6H5 (1 M, in Et2O) was added dropwise
resulting in the formation of a dark green solution.
Stirring was continued for 2 h, during which time a
white suspension formed. The solution was filtered
through Celite and the volatiles were removed under
reduced pressure to yield a dark black–green powder.
The solid was washed with a minimum volumn of cold
hexane, filtered, and dried in vacuo. Yield: 1.22 g
(83%). 1H-NMR (300 MHz, C6D6): l −21.6 (d, J=7.5
Hz, 4H, ortho), −19.4 (s, 2H, (C5H4)), −4.66 (s, 6H,
Me2Si), −3.33 (t, J=7.6 Hz, 2H, para), −0.63 (s, 6H,
(C5Me4)), 2.32 (t, J=7.6 Hz, 4H, meta), 4.84 (s, 6H,
(C5Me4)), 9.94 (s, 2H, (C5H4)), 29.5, 24.7 (s, 4H,
CH2C6H5). IR (cm−1): 1676 w, 1333 w, 1252 m, 1006
m, 1037 m, 974 w, 854 m, 813 m, 709 m, 625 m. Anal.
Calc. for C34H36SiU: C, 57.46; H, 5.11. Found: C,
57.40; H, 5.15.
4.9. [Me2Si(C5Me4)(C5H4)]U(NC6H5)]2 (8)
In a drybox, a scintillation vial was charged with
0.502 g (0.706 mmol) of 5 and 0.130 (0.706 mmol) of
C6H5NHNHC6H5; the solids were dissolved in 12 ml of
toluene. The reaction was allowed to stir at r.t. for 2
days during which time the color changed from deep
black–green to black. The solution was filtered through
Celite and the volume of the filtrate was reduced under
reduced pressure to 2 ml. Cooling the solution to
−34°C for 2 days afforded 0.521 g (65%) of black
1
crystals. H-NMR (300 MHz, C6D6): l −57.8 (s, 2H,
(C5H4)), −22.6 (s, 6H, (C5Me4)), −12.4 (t, J=7.5 Hz,
para), −9.92 (bs, 2H, ortho), 2.13 (s, 6H, Me2Si), 7.02
(t, J=7.5 Hz, 2H, meta), 13.08 (s, 6H, (C5Me4)), 29.81
(s, 2H, (C5H4)). IR (cm−1): 1923 w, 1251 m, 1175 w,
1109 m, 1076 m, 1043 m, 1021 m, 875 w, 831 m, 806 w,
775 m, 690 w. Anal. Calc. for C22H27SiNU: C, 46.23;
H, 4.76; N, 2.45. Found: C, 46.29, H, 4.56, N, 2.24.
4.7. [Me2Si(C5Me4)2]U(CH2C6H5)Cl (6)
In a drybox, a scintillation vial was charged with
0.502 g (0.698 mmol) of 4 and 0.067 g (0. 698 mmol) of
[HNMe3]Cl; the solids were dissolved in 12 ml of
toluene. The reaction was allowed to stir at r.t. for 12 h.
The solution was filtered through Celite and the
volatiles removed under reduced pressure. The resultant
green powder was recrystallized from toluene to give 6
5. Structural determinations
5.1. [Me2Si(C5Me4)2]U(v-Cl4){Li(TMEDA)}2
(1·TMEDA)
1
as dark green needles. Yield 0.412 g (89%). H-NMR
Complex 1 was dissolved in a minimum amount of
toluene, excess TMEDA was added, and the solution
was placed in a −40°C refrigerator. After several days,
large red plates of complex 1·TMEDA were obtained.
A dark red rectangular plate was attached to a thin
glass fiber using silicone grease. The crystal was them
immediately placed under a liquid N2 stream on a
Siemens P4/PC diffractometer. Data were collected
with graphite monochromatized Mo–Ka radiation (u=
(300 MHz, C6D6): l −35.2 (d, J=7.6 Hz, 2H, ortho),
−13.8 (bs, CH2C6H5), −12.2 (s, 3H, SiMe26 ), −10.3
(t, J=7.6 Hz, 1H, para), −9.02 (s, 3H, SiMe2), −3.10
(t, J=7.6 Hz, 2H, meta), 1.78 (s, 6H, (C5Me4)), 7.74 (s,
6H, (C5Me4)), 11.6 (s, 6H, (C5Me4)), 14.8 (s, 6H,
(C5Me4)). IR (cm−1): 1506 w, 1344 w, 1266 m, 1001 m,
977 m, 884 m, 832 m, 799 m, 769 m, 644 w. Anal. Calc.
for C27H37SiUCl: C, 48.90; H, 5.62. Found: C, 50.01;
H, 5.66.
,
0.71069 A). The lattice parameters were optimized from

22
R.C. Schnabel et al. / Journal of Organometallic Chemistry 591 (1999) 14–23
a least-squares calculation on 32 carefully centered
reflections of high Bragg angles. Three check reflections
monitored every 97 reflections showed no systematic
variation of intensities. Lattice determination and data
collection were carried out using XSCANS Version 2.10b
software. All data reduction, including Lorentz and
polarization corrections and structure solution and
graphics were performed using SHELXTL PC Version
4.2/360 software. The structure refinement was per-
formed using SHELX 93 software [19]. Absorption cor-
rections were performed using the laminar option in the
XEMP facility of SHELXTL PC. Data collection parame-
ters are given in Table 1.
5.4. Structure solution and refinement
The structure was solved in space group P2/n using
Patterson techniques, which revealed the uranium and
silicon atom positions. The carbon and nitrogen atoms
appeared in subsequent Fourier synthesis. All hydrogen
atoms were refined using the riding model in the HFIX
facility in SHELXL 93. Hydrogen atoms had their
isotropic temperature factors fixed at 1.2 (phenyl) or 1.5
(methyl) times the equivalent isotropic U of the carbon
atom they were bonded to. One toluene molecule per
dimer was found in the lattice and refined with
isotropic temperature factors and no hydrogen atoms.
The final refinement included anisotropic thermal
parameters on all non-hydrogen atoms (except the
toluene) and converged to R1=0.0396 and wR2=
0.0837.
5.2. Structure solution and refinement
The space group P21/n was uniquely determined by
the systematic absences. Patterson techniques were used
to locate the uranium and chlorine atom positions. The
remaining atoms appeared in subsequent Fourier syn-
thesis. All hydrogen atoms were fixed in positions
corresponding to a CꢀH distance of 0.96 using the
HFIX facility in SHELXL 93. The final refinement in-
cluded anisotropic thermal parameters on all non-hy-
drogen atoms. Hydrogen atoms had their isotropic
temperature factors fixed at 1.2 (ethyl) or 1.5 (methyl)
times the equivalent isotropic U of the carbon atom
to which they were bonded. The final refinement in-
cluded anisotropic thermal parameters on all non-hy-
drogen atoms and converged to R1=0.0478 and
wR2=0.0946.
6. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 127518 and 127519 for
1·TMEDA and 8, respectively. Copies of this informa-
tion may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ [fax:
+44-1223-336-033 or e-mail deposit@ccdc.cam.ac.uk
or http://www.ccdc.cam.ac.uk].
Acknowledgements
5.3. {[Me2Si(C5Me4)(C5H4)]U(NC6H5)}2 (8)
This work was performed under the auspices of the
Division of Chemical Services, Office of Energy Re-
search, the US Department of Energy. We wish to
thank Dr Robert K. Rosen for a generous gift of
C5Me4H.
A black, elongated hexagonal plate was attached to a
thin glass fiber using silicone grease. The crystal, which
was mounted from a crystallizing dish bathed in argon,
was then immediately placed under a liquid N2 stream
on a Siemens P4/PC diffractometer. Data were col-
lected with graphite monochromatized Mo–Ka
References
,
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showed no systematic variation of intensities. Lattice
determination and data collection were carried out
using XSCANS Version 2.10b software. All data reduc-
tion, including Lorentz and polarization corrections
and structure solution and graphics were performed
using SHELXTL PC Version 4.2/360 software. The
structure refinement was performed using SHELX 93
software [19]. Absorption corrections were performed
using the laminar option in the XEMP facility of
SHELXTL PC. Data collection parameters are given in
Table 1.
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