The distinct affinity of cyclopentadienyl ligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

Article abstract of DOI:10.1039/b419088b

The mono and bis(cyclopentadienyl) compounds [M(C₅H ₄Bu^t)I₂] and [M(C₅H ₄Bu^t)₂I] (M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C₅H₄Bu ^t)₃] and LnI₃ or [UI₃(L) ₄] (L = thf or py) in the molar ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI₃(py)₄] or LnI ₃ (Ln = La, Ce, Nd) with 1 or 2 mol equivalents of LiC ₅H₄Bu^t in thf afforded the [M(C ₅H₄Bu^t)I₂] and [M(C ₅H₄Bu^t)₂I₂]^- compounds, respectively. The X-ray crystal structures of [M(C₅H ₄Bu^t)I₂(py)₃] (M = U, La, Ce, Nd), [{Ce(C₅H₄Bu^t)₂(μ-I)}2] and [M(C₅H₄Bu^t)₂I(py)₂] (M = U, Nd) have been determined; the differences between the average M-C distances in the mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and lanthanide ions while the U-N and U-I bond lengths seem to be smaller than those predicted from a purely ionic bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two series of competing reactions: the ligand exchange reactions between [Ln(C₅H₄Bu ^t)_n′I_3-n′] (Ln = La, Ce, Nd) and [U(C₅H₄Bu^t)_n′I _3-n′] species (1 ≤ n′ + n′ = n ≤ 5), and the addition of n mol equivalents of LiC₅H₄Bu^t (1 ≤ n ≤ 5) to a 1: 1 mixture of LnI₃ and [UI ₃(thf)₄] or [UI₃(py)₄]. The stability of the [M(C₅H₄Bu^t)I₂] species was found to vary in the order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be accounted for by a higher degree of covalency in the U-C₅H₄Bu^t bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419088b

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F U L L P A P E R
The distinct affinity of cyclopentadienyl ligands towards trivalent
uranium over lanthanide ions. Evidence for cooperative ligation and
back-bonding in the actinide complexes
Thouraya Mehdoui, Jean-Claude Berthet,* Pierre Thu e´ ry and Michel Ephritikhine*
Service de Chimie Mol e´ culaire, DSM, DRECAM, CNRS URA 331 Laboratoire Claude
Fr e´ jacques, CEA/Saclay, 91191, Gif-sur-Yvette, France.
E-mail: ephri@drecam.cea.fr, berthet@drecam.cea.fr
Received 20th December 2004, Accepted 22nd February 2005
First published as an Advance Article on the web 4th March 2005
t
t
The mono and bis(cyclopentadienyl) compounds [M(C
formed in thf by comproportionation reactions of [M(C
5
H
4
Bu )I
2
] and [M(C
5
H
4
Bu )
2
I] (M = U, La, Ce, Nd) were
] (L = thf or py) in the molar
t
5
H
4
Bu )
3
] and LnI
] or LnI
] and [M(C
3
or [UI (L)
3
4
ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI
3
(py)
4
3
(Ln = La, Ce, Nd) with 1 or 2 mol
t
t
t
−
equivalents of LiC
5
H
4
Bu in thf afforded the [M(C
5
H
4
Bu )I
2
5
H
4
Bu )
2
I
2
] compounds, respectively. The
t
t
X-ray crystal structures of [M(C
5
H
4
Bu )I
2
(py)
3
] (M = U, La, Ce, Nd), [{Ce(C
5
H
4
Bu )
2
(l-I)}
2
] and
t
[
M(C
5
H
4
Bu )
2
I(py)
2
] (M = U, Nd) have been determined; the differences between the average M–C distances in the
mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and
lanthanide ions while the U–N and U–I bond lengths seem to be smaller than those predicted from a purely ionic
bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two
t
series of competing reactions: the ligand exchange reactions between [Ln(C
5
H
4
Bu )nꢀ
I
3−nꢀ ] (Ln = La, Ce, Nd) and
t
ꢀ
ꢀꢀ
t
[
U(C
5
H
4
Bu )nꢀꢀ
I
3−nꢀꢀ ] species (1 ≤ n + n = n ≤ 5), and the addition of n mol equivalents of LiC
5
H
4
Bu (1 ≤ n ≤ 5) to a
t
1
: 1 mixture of LnI
3
and [UI
3
(thf)
4
] or [UI
3
(py)
4
]. The stability of the [M(C
5
H
4
Bu )I
2
] species was found to vary in the
order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and
tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be
t
accounted for by a higher degree of covalency in the U–C
5
H
4
Bu bond, resulting from the late appearance of
back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.
Introduction
bonding interaction between the uranium atom and the p
accepting ligand. A special attention was then paid to the
The first organometallic compounds of the f-elements,
complexation of trivalent MX species (M = U, La, Ce, Nd; X =
3
1
[
Ln(C
5
H
5
)
3
] (Ln = La, Ce, Pr, Nd, Sm, Gd), were synthe-
I, OSO CF , ClO ) with various neutral polydentate nitrogen
2
3
4
sized by Wilkinson and Birmingham in 1954 and two years
bases, in particular those which were found to be efficient in
5,11–22
after, Reynolds and Wilkinson isolated the first organoactinide
the An(III)/Ln(III) separation.
In this context, we recently
2
complex, [U(C
5
H
5
)
3
Cl]. These authors emphasized that the
showed that in competition reactions of [Ce(C H R) ] and
5
4
3
t
uranium compound, unlike the cyclopentadienides of rare-
earth elements, does not react with ferrous chloride to give
[U(C H R) ] (R = SiMe or Bu ) with a series of azines which are
5
4
3
3
basic units of the polydentate extractant molecules considered
in nuclear industry, the selectivity of the azine in favour of
U(III) is proportional to its p accepting ability, measured by
its reduction potential, and increases with the p donor character
ferrocene, and concluded that the U–C
5
5
H bond has a less ionic
character than the Ln–C bond. Since these beginnings of
5
H
5
organo f-element chemistry, the nature of the bond in actinide
compounds (i.e. ionic vs. covalent, involvement of 6d and 5f
orbitals) has been and is still widely debated in a number of
23
of the metallocene. The electron donating capacity of the X
ligands is effectively beneficial to the selective complexation of
UX over LnX species (Ln = La, Ce, Nd) by increasing the
3–5
experimental and theoretical reports. A major incitement to
these investigations was the discovery of uranocene [U(C
3
3
8
H
8
)
2
]
electron richness and the p back-bonding ability of the actinide
complex. However, these ligands X deserve even more attention
because their own softness or p accepting capacity should also
bring a significant contribution to the relative stability of the
6
by Streitwieser and M u¨ ller-Westerhoff. Uranocene was recog-
nized as the first representative of a new class of metallocene-
like compounds with significant covalent bonding between the
cyclooctatetraene ligand and the uranium atom. The question
of the origin and magnitude of the covalent contribution to
f-element bonding was revived in the search of donor ligands
capable of discriminating between trivalent lanthanide (Ln) and
actinide (An) ions, which represents an important problem for
both its fundamental aspects and its applications, especially
Lewis base adducts of the MX complexes. Since the trivalent
3
metallocenes [M(C H R) ] (M = U, Ce) have been used as
5
4
3
9,10,23
models in the study of An(III)/Ln(III) discrimination,
it
seemed to us interesting to confirm this hypothesis in the case of
X = C H R. Here we present two series of competing reactions:
5
4
t
a) the ligand exchange reactions between [Ln(C H Bu )
5
4
n
ꢀ
I
3−n
ꢀ
]
7
t
ꢀ
ꢀꢀ
in the management of nuclear wastes. The differentiation of
(Ln = La, Ce, Nd) and [U(C H Bu ) ꢀꢀ
n
I
3−nꢀꢀ
] species (1 ≤ n + n =
5
4
t
these ions by selective complexation should originate from the
modest enhancement of covalency in actinide-ligand bonding as
n ≤ 5), and b) the addition of n mol equivalents of LiC H Bu
5
4
(1 ≤ n ≤ 5) to a 1 : 1 mixture of LnI and [UI (L) ] (L = thf or
3 3 4
8
compared to the lanthanides. In a seminal work, Andersen et al.
py), which revealed the presence of cooperativity in the binding
of the cyclopentadienyl groups by U(III), due to late appearance
of back-bonding and leading to a greater stabilization of the bis
and tris(cyclopentadienyl) uranium(III) complexes with respect
to their lanthanide counterparts; we also describe the X-ray
demonstrated by spectroscopic, structural and thermodynamic
9
10
studies that alkyl phosphine, phosphite and isocyanide ligands
have a better affinity for uranium(III) than for cerium(III) in
the trivalent metallocenes [M(C
which should be related to the presence of a stronger p back-
5
H
4
R)
3
] (M = U, Ce), a fact
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2
1 2 6 3

View Online
t
crystal structures of [M(C
5
H
4
Bu )I
2
(py)
3
t
] (M = U, La, Ce, Nd),
I(py)
] (M = U, Nd).
of the alkali metal salt was however possible, as shown by the
t
t
[
{Ce(C
5
H
4
Bu )
2
(l-I)}
2
] and [M(C
5
H
4
Bu )
2
2
isolation of [{Ce(C
5
H
4
Bu )
2
(l-I)}
2
] after extraction in pentane.
I or LiI to a thf
As expected, addition of excess NBu
4
t
solution of [M(C
5
H
4
Bu )
2
I] led to the immediate formation
t
−
of the [M(C
5
H
4
Bu )
2
I
2
]
complexes. However, it is important
Results and discussion
to note that these latter complexes were also obtained from
t
Synthesis and characterization of the complexes
the mono(cyclopentadienyl) compound [M(C
5
H
4
Bu )I ] in the
2
presence of excess iodide salt. This equilibrating reaction, which
can be depicted by eqn. (1),
Competition reactions of uranium and lanthanide triiodides
t
with C
5
4
H Bu ligands will give mixtures of mono-, bis and
tris(cyclopentadienyl) complexes and it is essential that these
are unambiguously identified and characterized. The solvated
t
] + I− → [M(C
t
]− + MI
2
[M(C
5
H
4
Bu )I
2
5
H
4
Bu )
2
I
2
3
(1)
←
t
t
t
−
[
M(C
5
H
4
Bu )I
2
] and [M(C
5
H
4
Bu )
2
I] compounds (M = U, La,
likely involves the intermediate [M(C
5
H
4
Bu )I
3
]
which
Ce, Nd) were formed in thf by comproportionation reactions of
would be unstable towards disproportionation. The trans-
t
24
25
t
t
−
[
M(C
La, Nd) and LnI
and 2 : 1, respectively. Each of these ligand exchange
5
H
4
Bu )
3
] (M = U, Ce ) or [Ln(C
5
H
4
Bu )
3
(py)] (Ln =
formation of [M(C
[Yb(C Me )I Li(Et O)
The trivalent lanthanum and neodymium metallocenes
5
H
2
4
Bu )I
3
]
is reminiscent of that of
2
6
29
3
or [UI (thf) in the molar ratio of 1 :
3
4
]
5
5
3
2
] into [Yb(C
5
Me
5
)
2
I
2
Li(Et
2
2
O) ].
2
t
reactions afforded the expected complex as the sole product in
quantitative yield, in contrast to that observed in the reactions
[Ln(C
5
H
4
Bu )
3
] (Ln = La, Nd) were prepared similarly to the
2
5
t
cerium counterpart, by treating LnCl
3
with LiC Bu in thf.
5
H
4
t
of [UI
tris-C
is obviously due to the stabilizing effect of the C
3
(thf)
4
] and [U(C
5
H
5
)
3
(thf)] which gave mono-, bis- and
Synthesis of [M(C
of [UI (thf) ] or LnI
5
H
4
Bu )
3
3
] (M = U, La, Ce, Nd) by reaction
27
t
5
H
5
-substituted uranium(III) complexes. This difference
3
4
and LiC
5
H
4
Bu was hampered by the
t
1
5
H
4
Bu ligand;
tedious elimination of LiI; the H NMR spectra in thf showed
t
t
−
the usefulness of Bu -substituted cyclopentadienyl ligands for
providing enhanced stability and better solubility to lanthanide
the formation of the [M(C
equilibrium with [M(C
The thf adducts [M(C
I(thf) ], as well as [Ln(C
5
t
H
4
Bu )
3
I] species which are in
30
5
H
4
Bu )
H
3
].
2
8
t
t
derivatives is well documented.
5
4
Bu )I
2
(thf)
x
] and [U(C
5
H
4
Bu )
2
-
1
t
t
The
M(C
H
Bu )
NMR spectra of the [M(C
I] species in thf, listed in Table 1, exhibit three
5
H
4
Bu )I
2
]
and
x
5
H
4
Bu )
3
] (Ln = La, Nd) were often ob-
t
[
5
H
4
2
tained as oily materials which are not easy to handle; solid sam-
resonances in relative intensities of 2 : 2 : 9 which are param-
agnetically shifted for M = U, Ce, Nd; the distinct chemical
shifts of these signals will allow the easy recognition of each
compound in a mixture. Only slight modifications were noted
in the NMR spectra of the mono(cyclopentadienyl) uranium
ples of the pyridine adducts were prepared for characterization.
t
The mono(cyclopentadienyl) complexes [M(C
5
H
4
Bu )I
2
(py) ]
3
(M = U, La, Ce, Nd), the bis(cyclopentadienyl) com-
pounds [{Ce(C Bu ) (l-I)} ] and [U(C Bu ) I(py) ], and
the tris(cyclopentadienyl) compounds [Ln(C
t
t
5
H
4
2
2
5
H
4
2
t
2
5
H
4
Bu )
3
(py)] (Ln =
complex when [UI
However, the high field signal of the [U(C
3
(py)
4
] was used in place of [UI
3
(thf)
4
].
La, Nd) were synthesized on a preparative scale and gave satis-
t
5
H
4
Bu )
2
I] species
factory elemental analyses; the mono and bis(cyclopentadienyl)
t
was shifted by −11 ppm and the resonances of pyridine were
complexes and [Nd(C
ray crystallography.
5
H
4
Bu )
2
I(py) ] were characterized by X-
2
broader than those of the free molecule; the same spectrum
t
was obtained after addition of 4 mol equivalents of pyridine to
The mono(C
5
H
4
Bu ) compounds are isomorphous; a view
t
a 2 : 1 mixture of [U(C
5
H
4
Bu )
3
] and [UI
3
(thf)
4
] in thf. These
of the cerium derivative is presented in Fig. 1, and selected
bond lengths and angles are given in Table 2. The mer,trans
pseudo-octahedral structure is very familiar in the series of
features are indicative of coordination of the nitrogen ligand to
the bis(cyclopentadienyl) uranium compound.
t
The mono- and bis(C
5
H
4
Bu ) complexes were also classically
[Ln(cyclopentadienyl)X
ticular by the iodo derivatives [Sm(C
2
L
3
] complexes, being adopted in par-
t
31
synthesized by metathesis reactions of [UI
3
(py)
4
] or LnI
3
(Ln =
5
H
4
R)I
2
(thf)
3
] (R = Bu ,
t
32
33
La, Ce, Nd) and LiC
5
H
4
Bu in thf; here again, the expected
H ) and [Ce(C
5
Me
5
)I
2
(thf) ]. The average Ce–C distance of
3
1
˚
˚
products were formed in quantitative yield. The H NMR
2.80(5) A and the Ce–I distances of 3.1625(5) and 3.2069(5) A
are quite similar to those of 2.80(3), 3.1733(12) and 3.2269(12)
A in the pentamethylcyclopentadienyl compound. The Ce–N
t
spectra of the [M(C
5
H
4
Bu )I ] species obtained by metathesis
2
˚
or comproportionation reactions are quite identical, suggesting
that the released lithium iodide has little influence on the
structure of the complexes in solution. In contrast, the spectra
of the bis(cyclopentadienyl) species obtained from the two
synthetic routes are very different (Table 1), certainly because of
the easy formation of stable anionic or “ate” complexes of the
˚
bond lengths of 2.643(5) and 2.733(5) A corresponding to
the equatorial and axial pyridine ligands can be compared
2
1
˚
˚
(py)] and 2.694(2) A
with those of 2.672(4) A in [CeI
3
(bipy)
2
t
23
in [Ce(C
5
H
4
Bu )
3
(py)]. By comparison with the number of
mono- and bis(cyclopentadienyl) complexes of lanthanides(III),
t
28
28
type [M(C
5
H
4
Bu )
2
(l-I)
2
Li(thf)
n
]. In some cases, elimination
such compounds of uranium(III) are rather rare. The
1
a
Table 1 H NMR spectra of the complexes
t
t
t
−
t
t
−
[
M(C
5
H
4
Bu )I
2
]
[M(C
5
H
4
Bu )
2
I]
[M(C
5
H
4
Bu )
2
I
2
]
[M(C
5
H
4
Bu )
3
]
5 4 3
[M(C H Bu ) I]
Nd
Ce
La
U
−0.25
−0.35
1.50
−3.31
9.64
11.24
−1.76
17.37
18.63
1.20
5.93
6.09
−4.21
−19.0
1.65
−10.00
3.40
15.12
−3.29
11.21
11.72
1.18
5.77
5.93
−7.89
−22.90
−2.49
−4.58
3.60
8.30
1
1
0.11
3.96
17.13
1.90
−1.16
−2.26
b
1
2
4.54
2.57
1.31
3.5
9.32
17.27
1.26
5.90
12.62
1.24
5.69
6
6
.16
.30
6.04
5.73
−0.53
10.78
.02
−0.44
−36.70
1.12
−3.58
−25.03
−2.40
−
1
a
2
◦
t
In [ H
8
]thf at 23 C. The d values for the Bu signals (9H) and for the ring protons (2 × 2H) are given successively. The signals are singlets with
b
◦
half-height widths = 15–170 Hz. Masked by the thf signal; d at 30 C: 1.20 (9H), 3.90 (2H), 17.13 (2H).
1
2 6 4
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2

View Online
Fig. 2 Average M–N, M–C and M–I distances as a function of the metal
t
Fig. 1 View of the complex [Ce(C
5
H
4
Bu )I
2
(py)
3
]. The hydrogen atoms
t
ionic radii in the isomorphous compounds [M(C
5
H
4
Bu )I
2
(py)
3
] (M =
are omitted for clarity. Displacement ellipsoids are drawn at the 30%
probability level.
Nd, Ce, La, U) with the regression lines corresponding to the lanthanide
2
complexes (—, r ) and to the lanthanide and uranium complexes ( · · · ,
2
r ).
˚
average U–C and U–I distances of 2.80(5) and 3.17(2) A in
t
[
U(C
5
H
4
Bu )I
2
(py)
)I
3
] are similar to those of 2.78(2) and 3.17(1)
], while the mean U–N distance of
.66(5) A is identical to that measured in [U(C
27
36
37
26
38
˚
A in [U(C Me
5
5
2
(thf)
3
Pr, Ce, U and La, respectively. The shortening of the U–N
t
23
˚
2
5
H
4
Bu )
3
(py)].
distance with respect to Ln–N distances in analogous Ln(III)
Variations in the average M–C, M–I and M–N distances as
and U(III) complexes with aromatic nitrogen bases is now well
3
+
34
5,11–23
˚
a function of the ionic radii r(M ) of the metals in the
documented;
the cations [M(btp)
the largest deviation of 0.1 A was observed in
t
3+
isomorphous complexes [M(C
5
H
4
Bu )I
2
(py)
3
] (M = U, La, Ce,
3
]
(M = U, La, Ce, Nd; btp = 2,6-bis(5,6-
12
Nd) are represented in Fig. 2. While the usual linear relationship
between the Ln–I or Ln–N distances and r(Ln ) is respected,
dialkyl-1,2,4-triazin-3-yl)pyridine). The variations in the M–I
and M–N distances of the mono(cyclopentadienyl) complexes
would indicate that the uranium ion interacts more strongly
than the lanthanide ions with the soft and p accepting iodide
3
+
the U–I and U–N bond lengths seem to be shorter than those
˚
expected from a purely ionic bonding model, by 0.03 A. A similar
trend for the M–I distances was observed in other series of
analogous iodide complexes of lanthanides(III) and uranium(III).
and pyridine ligands. In contrast, the dot corresponding to the
t
average U–C distance in [U(C
5
H
4
Bu )I
2
(py) ] (Fig. 2) is definitely
3
35
In the compounds [MI
2
(OPPh
3
)
4
][I], the mean M–I distances
not significantly displaced from the linear plot of the mean Ln–C
bond lengths versus the ionic radii in the lanthanide analogues.
Thus, the M–C distances do not reveal any covalent contribution
in the uranium–cyclopentadienyl bonding.
˚
are 3.14(5), 3.17(3), 3.151(7) and 3.20(3) A for M = Nd,
Ce, U and La, respectively; these distances in the compounds
˚
[MI
3
(thf)
4
] are 3.12(4), 3.14(4), 3.13(4) and 3.15(4) A for M =
◦
Table 2 Selected bond lengths ( A˚ ) and angles ( )
t
t
t
t
[
Nd(C
5
H
4
Bu )I
2
(py)
3
]
[Ce(C
5
H
4
Bu )I
2
(py)
3
]
[La(C
5
H
4
Bu )I
2
(py)
3
]
5 4 2 3
[U(C H Bu )I (py) ]
M–I(1)
M–I(2)
M–N(1)
M–N(2)
M–N(3)
3.1293(3)
3.1765(3)
2.607(3)
2.707(3)
2.612(3)
2.76(5)
3.1625(5)
3.2069(5)
2.643(5)
2.733(5)
2.643(5)
2.80(5)
3.1830(3)
3.2273(3)
2.671(3)
2.765(3)
2.671(3)
2.82(5)
3.1466(11)
3.1912(11)
2.607(10)
2.724(11)
2.641(11)
2.80(5)
ꢁM–Cꢂ
I(1)–M–I(2)
155.612(8)
152.16(8)
78.19(6)
77.45(6)
76.49(8)
75.81(8)
155.911(16)
152.18(15)
78.25(1)
77.69(11)
76.42(15)
75.91(15)
155.748(9)
151.88(8)
78.24(6)
77.56(6)
76.41(8)
75.67(8)
155.97(3)
N(1)–M–N(3)
I(1)–M–N(2)
I(2)–M–N(2)
N(1)–M–N(2)
N(2)–M–N(3)
152.1(3)
78.7(2)
77.3(2)
76.6(3)
75.7(3)
t
t
t
[{Ce(C
5
H
4
Bu )
2
(l-I)}
2
]
[Nd(C
5
H
4
Bu )
2
I(py)
2
]
5 4 2 2
[U(C H Bu ) I(py) ]
b
Ce(1)–I(1)
Ce(1)–I(2)
Ce(2)–I(3)
Ce(3)–I(3)
3.2014(7)
3.2178(6)
3.2174(7)
3.2149(7)
2.77(3)
ꢁM–Iꢂ
3.159(11)
2.741(9)
2.78(7)
3.174(5)
2.742(16)
2.80(7)
ꢁM–Nꢂ
ꢁM–Cꢂ
ꢁCe–Cꢂ
I(1)–Ce(1)–I(2)
Ce(1)–I(1)–Ce(1 )
85.886(15)
94.43(2)
93.80(2)
86.81(2)
86.89(2)
93.150(16)
ꢁN(1)–M–N(2)ꢂ
ꢁI–M–Nꢂ
162(1)
81.0(8)
162.6(4)
81.4(7)
ꢀ
ꢀ
a
Ce(1)–I(2)–Ce(1 )
ꢀ
ꢀ
I(3)–Ce(2)–I(3 )
I(3)–Ce(3)–I(3 )
Ce(2)–I(3)–Ce(3)
a
b
Symmetry code: ‘ = y, x, −z. Average values for the three independent molecules.
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2
1 2 6 5

View Online
3
+
and Nd . This suggests the presence of a stronger interaction
between the actinide and the iodide and pyridine ligands, as in
the case of the mono(cyclopentadienyl) compounds.
Comparison of the crystal structures of the isomorphous
t
complexes [M(C
5
H
2
4
Bu )I
2
(py)
3
] (M = U, La, Ce, Nd) and
t
[
M(C
5
H
4
Bu )
2
I(py)
] (M = U, Nd) does not allow any assump-
tions on the relative stability of the analogous uranium and
lanthanide compounds. This information will be provided by
t
the competition reactions between UI
ligands in thf solution.
3
and LnI
3
with C
5
4
H Bu
Competition reactions
The objective of this work was to reveal and understand the
possible distinct affinity of cyclopentadienyl ligands towards
trivalent uranium and lanthanide ions. The study consisted of
t
analyzing the distribution of an increasing quantity of C
5
4
H Bu
Fig. 3 View of one of the two independent molecules in the complex
groups onto an equimolar mixture of the 4f and 5f ions. Two
t
[
{Ce(C
5
H
4
Bu )
2
(l-I)}
2
]. Hydrogen atoms are omitted for clarity. Dis-
1
series of reactions were monitored by H NMR spectroscopy: the
placement ellipsoids are drawn at the 20% probability level. Symmetry
t
ligand exchange reactions between [Ln(C
5
H
4
Bu )nꢀ
I
3−nꢀ ] (Ln =
code: ‘ = y, x, −z.
t
ꢀ
ꢀꢀ
La, Ce, Nd) and [U(C
5
H
4
Bu )nꢀꢀ
I
3−nꢀꢀ ] species (1 ≤ n + n = n ≤
t
t
5), and the addition of n mol equivalents of LiC H Bu (1 ≤ n ≤
The crystals of [{Ce(C
5
H
4
Bu )
2
(l-I)}
2
] are composed of
5
4
5
) to a 1 : 1 mixture of LnI
3
and [UI
3
(thf)
4
] or [UI
3
(py) ]. The
4
two independent and very similar iodo-bridged dimers which
both contain a two-fold axis of symmetry, defined by the
I(1) · · · I(2) line in the first one (represented in Fig. 3) and by
the Ce(2) · · · Ce(3) line in the second. This structure, in which
the metal centre is in a pseudo-tetrahedral configuration, is very
difference between the two reactions is the presence of LiI in the
t
latter. The distributions of the C
5
H Bu ligands onto Ln(III) and
4
U(III) and the relative proportions of the complexes are listed in
Table 3.
t
39
Reaction of [Ce(C H Bu )I ], prepared by comproportion-
ation of [Ce(C H Bu ) ] and CeI , with 1 mol equivalent of
common for the bis(cyclopentadienyl) lanthanide chlorides
and bromides and is also adopted by the thulium iodide
5
t
4
2
40
5
4
3
3
t
41
[UI (thf) ] or [UI (py) ] in thf led to the immediate formation
derivative [{Tm(C
5
H
4
SiMe
2
Bu )
2
(l-I)}
2
]. The bridging Ce–I
3
4
3
4
˚
of a mixture of the two mono(cyclopentadienyl) complexes
of cerium and uranium in the molar ratio of 60 : 40. These
bonds, with an average distance of 3.213(7) A, are slightly longer
t
than the terminal ones in [Ce(C
5
H
4
Bu )I
2
(py)
3
].
t
two compounds were obtained in the same proportions from
The neodymiumanduranium complexes [M(C
5
H
4
Bu )
2
I(py) ]
2
t
an equimolar mixture of [U(C
from a 1 : 1 : 1 mixture of CeI
5
H
3
4
Bu )I
2
] and CeI
3
, and also
(
M = Nd, U) are isomorphous and crystallize with three
t
, [UI
3
(py) ] and LiC
4
5
H Bu .
4
independent molecules in the asymmetric unit. One of these
molecules in the neodymium compound is presented in Fig. 4.
The metal is found in a distorted trigonal bipyramidal envi-
ronment, the trigonal equatorial plane being defined by the
I atom and the ring centroids, with the two nitrogen atoms
in apical positions. This structure is also that adopted by
The alkali metal salt appeared to have no effect on the
distribution of the cyclopentadienyl groups; equilibrium (1)
was not observed because of the low concentration of iodide
ions in solution. Similar reactions with the neodymium or
lanthanum in place of the cerium complex also afforded a
mixture of monocyclopentadienyl species, and the molar ratios
t
42
ꢀ
ꢀ
[
Pr(C
5
H
(SiMe
4
Bu )
2
I(thf)
2
]
and the [U(cp )
2
XL
2
] complexes [cp =
t
t
43
44
[Ln(C H Bu )I ]/[U(C H Bu )I ] were then equal to 67/33 and
C
5
H
3
3
)
2
; X = Cl and L = Me
3
SiCN or Me
2
C
6
H
3
NC;
5
4
2
5
4
2
t
45
43/57 for Ln = Nd and La, respectively. A small quantity of the
X = Br and L = Bu NC]. The large standard deviations for the
bis(cyclopentadienyl) uraniumspecies (< 5%) was alsoformedin
average Nd–C and U–C distances, which are respectively equal
t
˚
the reaction of LiC
5
H
4
Bu with LaI
3
and [UI
3
(py) ]. These results
4
to 2.78(7) and 2.80(7) A, prevent any meaningful comparison.
reflect the occurrence of the equilibrium depicted by eqn. (2)
The difference between the average U–N and Nd–N distances,
˚
2
.742(16) and 2.741(9) A, as well as the difference between the
t
t
[
Ln(C
5
H
4
Bu )I
2
] + UI
3
→ LnI
3
+ [U(C
5
H
4
Bu )I
2
]
(2)
˚
average U–I and Nd–I distances, 3.174(5) and 3.159(11) A, are
smaller than the 0.04 A difference between the ionic radii of U
←
3
+
˚
t
and indicate that the stability of the [M(C
5
H
4
Bu )I
2
] species
varies in the order Nd > Ce > U > La. This trend is in accord
with an electrostatic bonding model in which the stability of the
complexes should increase with the effective nuclear charge or
the inverse of the ionic radius of the metal.
The mono(cyclopentadienyl) uranium and cerium complexes
were obtained in equal quantities from a 1 : 1 mixture
t
of [Ce(C
2
5
H
4
Bu )
2
I] and [UI
3
(thf)
4
t
] or [UI
3
(py)
Bu with a 1 : 1 mixture of
] also gave rise to an equal distribution
4
]. Reaction of
mol equivalents of LiC
CeI and [UI (thf)
5
H
4
3
3
4
of the cyclopentadienyl ligands onto Ce(III) and U(III). The
cerium triiodide was here again completely transformed into
t
[
Ce(C
5
H
4
Bu )I
2
], but only 75% of UI was converted into a
3
t
t
−
mixture of [U(C
5
H
4
Bu )I
2
] and [U(C
5
H
4
Bu )
2
2
I ] , in relative
proportions of 50/25. By comparison with the compropor-
t
tionation reaction of [Ce(C
5
H
4
Bu )
2
I] and [UI
3
(thf)
] was trans-
] , following equilibrium (1). These
4
], these
t
percentages show that a part of [U(C
5
H
4
Bu )I
2
t
−
2
formed into [U(C
5
H
4
Bu )
2
I
t
−
results also indicate that formation of [M(C
5
H
4
Bu )
2
I
2
] from
Fig. 4 View of one of the three independent molecules in the
t
t
[M(C H Bu )I ] should be more facile for M = U than for M =
complex [Nd(C
5
H
4
Bu )
2
I(py)
2
]. Hydrogen atoms are omitted for clarity.
5
4
2
Displacement ellipsoids are drawn at the 20% probability level.
Ce. Indeed, stepwise addition of NBu I or LiI to a 1 : 1 mixture
4
1
2 6 6
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2

View Online
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2
1 2 6 7

View Online
t
t
of [Ce(C
5
H
4
Bu )I
2
] and [U(C
5
−
H
4
Bu )I
2
] led to the successive
t
t
−
2
formation of [U(C
5
H
4
Bu )
2
I
2
] and [Ce(C
5
H
4
Bu ) I ] .
2
Most interesting facts emerged from the competition reactions
of Ln(III) and U(III) ions with more than two cyclopentadienyl
t
ligands. Ligand exchange reactions between [U(C
5
H
4
Bu )
3
] and
t
CeI
3
or [Ce(C
5
H
4
Bu )
3
] and [UI
3
(py) ] afforded the mono
4
and bis(cyclopentadienyl) complexes of cerium and uranium
t
t
in inverse molar ratios, i.e. [U(C
5
H
4
Bu )
2
I]/[U(C
5
H
4
Bu )I
2
] =
t
t
t
[
Ce(C
5
H
4
Bu )I
2
]/[Ce(C
5
H
4
Bu )
2
I] = 80/20. Thus, the C
5
H
4
Bu
groups were not equally divided among the metal centres,
0% being found on U(III) and 40% on Ce(III). Replacing
CeI with LaI did not bring significant modifications in the
proportions of the uranium and lanthanide compounds and
6
3
3
t
with the neodymium counterparts, the repartition of C
5
H
4
Bu
Fig. 5 Relative proportions of mono (pale grey), bis (medium grey)
and tris (black) cyclopentadienyl uranium and cerium compounds
ligands was slightly different, 57% for U(III) and 43% for Nd(III),
giving a molar ratio of bis to mono(cyclopentadienyl) complexes
of U(III) equal to 70/30, and inverse for the Nd(III) analogues.
These results can be accounted for by the equilibrium depicted
by eqn. (3)
t
in the ligand exchange reactions between [Ce(C
5
H
4
Bu )nꢀ
I
3−nꢀ ] and
t
ꢀ
ꢀꢀ
[
U(C
5
H
4
Bu )nꢀꢀ
I
3−nꢀꢀ ] species (1 ≤ n + n = n ≤ 5).
t
t
[
Ln(C
5
H
[
4
Bu )
2
I] + [U(C
5
H
4
Bu )I
2
] →
←
t
t
Ln(C
5
H
4
Bu )I
2
] + [U(C
5
H
4
Bu )
2
I]
(3)
which is shifted towards the formation of the more stable
bis(cyclopentadienyl) uranium complex.
t
−
The bis(cyclopentadienyl) uranium complex [U(C
5
H
4
Bu )
2
I
2
]
was the sole uranium species formed upon addition of
t
3
mol equivalents of LiC
CeI and [UI (py) ], while 80% of CeI
Ce(C ] and [Ce(C
tions of 60/20. By comparison with the comproportionation
5
H
4
Bu to a 1 : 1 mixture of
were converted into
in the relative propor-
3
3
4
3
t
t
−
2
[
5
H
4
Bu )I
2
5
H
4
Bu ) I ]
2
t
reaction of [Ce(C
favoured the formation of the [M(C
5
H
4
Bu )
3
] and [UI
3
(py)
4
t
], the presence of LiI
−
5
H
4
Bu )
2
I
2
] species, because
Fig. 6 Relative proportions of mono (pale grey), bis (medium grey)
and tris (black) cyclopentadienyl uranium and cerium compounds in
the reactions of a 1 : 1 mixture of CeI and [UI L ] (L = thf or py) with
t
of equilibrium (1), giving a distinct distribution of the C
5
4
H Bu
ligands: 67% on U(III) and 33% on Ce(III).
3
3
4
t
t
Mixing equimolar amounts of [U(C
5
H
4
Bu )
3
]
and
n mol equivalents of LiC
5
H
4
Bu (1 ≤ n ≤ 5).
t
[
Ce(C
5
H
4
Bu )I ] afforded the bis(cyclopentadienyl) complexes
2
t
−
of cerium and uranium in practically equal quantities (90%),
but the remaining Ce(III) and U(III) species were found to be the
reaction, and of an equimolar mixture of [U(C
5
H
4
Bu )
for n = 4 in the metathesis reaction, show
that the values of K and K are effectively higher than those
of K and K , respectively. However, we have already noted that
2
I
2
] and
t
−
[Ce(C
5
H
4
Bu )
2
2
I ]
mono and tris(cyclopentadienyl) compounds, respectively; thus,
1
2
t
5
3% of the C
5
H
4
Bu groups were coordinated to U(III), and 47%
2
3
t
t
to Ce(III). Addition of 4 mol equivalents of LiC
mixture of CeI and [UI (py) ] gave the two [M(C
complexes (M = U, Ce) in quantitative yield.
Ligand exchange reactions between [Ce(C
5
H
4
Bu to a 1 : 1
for n = 2 in the metathesis reaction, a part of [U(C
5
H
4
Bu )I
2
]
t
−
t
−
2
3
3
4
5
H
4
Bu )
2
I
2
]
was transformed into the [U(C
occurrence of equilibrium (1).
5
H
4
Bu ) I ]
2
species, due to
t
5
H
4
Bu )
I] gave the
3
] and
It is remarkable, for n = 4 in the comproportiona-
t
t
t
[
U(C
5
H
4
Bu )
2
I] or [U(C
5
H
4
Bu )
3
] and [Ce(C
5
H
4
Bu )
2
tion reaction, that the two bis(cyclopentadienyl) complexes
t
same equilibrating mixture depicted by eqn. (4)
[M(C
5
H
4
Bu )
2
I] (M = Ce, U), formed in equal quantities, were
t
t
present together with the mono(cyclopentadienyl) cerium and
tris(cyclopentadienyl) uranium compounds, [Ce(C
[
Ce(C
5
H
[
4
Bu )
3
] + [U(C
5
H
4
Bu )
2
I] →
t
5 4
←
t
t
5
H
4
Bu )I
2
]
Ce(C
5
H
4
Bu )
2
I] + [U(C
H
Bu )
3
]
(4)
t
and [U(C
5
H
4
Bu )
3
], which were also found in equal quantities.
t
in which the molar ratio of tris to bis(cyclopentadienyl) uranium
compounds, equal to 70/30, was inverse to that of the cerium
For the uneven values of n (n = 3 and 5), the C
5
H Bu
4
ligands are not equally distributed among U(III) and Ce(III),
exhibiting a better affinity for U(III), as shown by the ratios of
bis to mono(cyclopentadienyl) and tris to bis(cyclopentadienyl)
uranium complexes which are inverse to those of the cerium
analogues [for n = 3 in the metathesis reaction, a part of the
mono(cyclopentadienyl) cerium complex was converted into
the bis(cyclopentadienyl) derivative because of equilibrium (1)].
All these observations clearly demonstrate that the neutral and
anionic bis and tris(cyclopentadienyl) complexes of uranium are
analogues. This ratio was equal to 60/40 when 5 mol equivalents
t
of LiC
UI (py)
The relative proportions of the cyclopentadienyl compounds
5
H
4
Bu were added to a 1 : 1 mixture of CeI
3
and
[
3
4
].
of uranium and cerium obtained either by comproportionation
t
t
reactions of [U(C
5
H
4
Bu )nꢀ
I
3−nꢀ ] and [Ce(C
5
H
3
4
Bu )nꢀꢀ
I
3−nꢀꢀ ] or
metathesis reactions of a 1 : 1 mixture of CeI
and [UI
Bu are represented by the
diagrams shown in Figs. 5 and 6, respectively. In both reactions,
3
(py)
4
]
t
with n mol equivalents of LiC
5
H
4
more stable than their cerium analogues.
t
t
the complexes resulted from the distribution of n C
5
H
4
Bu
While the lower stability of [U(C
5
H
4
Bu )I
2
] with respect to
t
groups (1 ≤ n ≤ 5) among U(III) and Ce(III), the number of
[Ce(C
5
H
4
Bu )I ] can be accounted for by an ionic bonding
2
iodide ligands being equal to 6 − n in the comproportionation
model, stabilization of the bis and tris(cyclopentadienyl) ura-
reactions and to 6 in the metathesis reactions. Increasing the
nium complexes relative to the cerium analogues should reflect
t
t
number of C
5
H
4
Bu ligands led to the stepwise formation of the
the more covalent character of the U–C
5
H
4
Bu bond in these
mono, bis and tris(cyclopentadienyl) complexes, indicating that
for a given metal, the corresponding stability constants in each
compounds. This higher degree of covalency should result
from the late appearance of back-bonding which would emerge
only after the first cyclopentadienyl ligand is bound, giving
the metal centre an improved p donating capacity. The linear
plot of the average M–C distances versus the ionic radii of the
series of reactions follow the order K
1
> K
2
> K . In particular,
3
t
the formation of an equimolar mixture of [U(C
5
H
4
Bu )I
2
]
t
and [Ce(C
5
H
4
Bu )I
2
] for n = 2 in the comproportionation
1
2 6 8
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2

View Online
metals in the isomorphous mono(cyclopentadienyl) complexes
[UI
3
(thf)
4
] or [UI
3
(py)
4
] revealed that the better affinity of the
t
t
[
M(C
5
H
4
Bu )I
2
(py)
3
] (M = U, La, Ce, Nd) is indeed in agreement
C
5
H
4
Bu groups for U(III) over Ln(III) clearly appears only after
with the absence of a covalent contribution in the uranium–
cyclopentadienyl bonding. The presence of back-bonding in the
bis and tris(cyclopentadienyl) uranium compounds, which is less
likely in their lanthanide counterparts, should be related to the
enhancement of electron richness and p donating capacity of the
metal centre. Cooperative ligand binding resulting from the late
emergence of back-bonding has been previously encountered in
the stepwise complexation of aqua metal ions with p acceptor
the first cyclopentadienyl ligand is bound. The stabilization of
the bis and tris(cyclopentadienyl) uranium complexes relative to
the lanthanide analogues is consistent with the late emergence
of back-bonding in these compounds, giving a more covalent
character to the U–C
t
5
H
4
Bu bond. Such cooperativity effects in
ligand binding should play an important role in the differentia-
tion between trivalent lanthanide and actinide ions.
4
6,47
ligands.
For example, the K
1
, K
2
and K values corresponding
3
Experimental
2
+
to subsequent addition of 1,10-phenanthroline to Fe ions
5
5
10 47
All reactions were carried out under argon (< 5 ppm oxygen
or water) using standard Schlenk-vessel and vacuum line
techniques or in a glove box. Solvents were dried by standard
methods and distilled immediately before use; deuterated thf
are 7.3 × 10 , 1.78 × 10 and 1.1 × 10 . In the stepwise
t
coordination of C
5
H
4
Bu ligands to U(III) and Ce(III), the effect
of cooperativity is to increase the stability of the bis and
tris(cyclopentadienyl) actinide compounds with respect to the
lanthanide analogues.
˚
(
Eurisotop) was distilled over Na/K alloy and stored over 3 A
molecular sieves. The lanthanide halides LnX
3
(Ln = La, Ce,
Cooperative ligation in trivalent actinide complexes can play
a major role in An(III)/Ln(III) differentiation. The selective com-
plexation of An(III) over Ln(III) by neutral extractant molecules
could be effective only after the metal has gained a sufficient
electron density by initial coordination of ligands, inducing late
appearance of back-bonding in the actinide compound. The
most efficient extractant molecules should be those displaying
both r donor and pronounced p acceptor properties, as 2,6-
bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridines (btp) which exhibit
remarkable performances in An(III)/Ln(III) separation. That
Nd; X = Cl, I) (Aldrich) were used as received; [UI
3
(thf) ]
24
4
26
t
and [UI
3
(py)
4
]
and the metallocenes [M(C
5
H
4
Bu )
3
] (M = U,
25
1
Ce ) were prepared by published methods. H NMR spectra
were recorded on a Bruker DPX 200 instrument and referenced
internally using the residual protio solvent resonances relative
to tetramethylsilane (d 0). Elemental analyses were performed
by Analytische Laboratorien at Lindlar (Germany).
NMR characterization of the complexes
complexation of UI
to the successive formation of the 1 : 2 and 1 : 3 lanthanide
compounds while the 1 : 3 actinide complex was formed
3
and CeI by these ligands in pyridine led
3
(
a) Comproportionation reactions. In a typical experiment,
t
an NMR tube was charged with [M(C
5
H
4
Bu )
3
] (M = Ln or U)
or [UI
] (L =
]thf (0.4 mL). The tube was immersed in an
ultrasound bath (80 W, 40 kHz) for 10 min, and the spectrum
(
ca. 15 mg) and 0.5 or 2 mol equivalents of LnI
3
3
L
4
12
exclusively, whatever the number of mol equivalents of ligand,
2
thf or py) in [ H
8
would attest the presence of cooperativity in the binding of
btp by uranium(III). On the other hand, the results reported
here indicate that co-ligands and counterions should also have
a significant influence on the relative stability of analogous
Lewis base adducts of trivalent 4f and 5f ions. For example,
t
showed the quantitative formation of the [M(C
M(C
5
H
4
Bu )I ] or
2
t
[
5
H
4
Bu )
2
I] complexes.
(b) Metathesis reactions. In a typical experiment, an NMR
tube was charged with LnI or [UI (py) ] (ca. 15 mg) and n mol
the greater stability of [U(C
complexes is obviously related to the p accepting capacity of
5
H
4
R)
3
(L)] vs. [Ce(C
5
H
4
R)
3
(L)]
3
3
4
t
2
equivalents (n = 1–3) of LiC
5
H
4
Bu in [ H ]thf (0.4 mL). After
8
◦
9
10
the Lewis base (L = alkylphosphine, phosphite, isocyanide,
10 min at 20 C, the spectra showed the quantitative formation
23
t
t
−
t
−
azine ) but is also determined by the r donor and p acceptor
properties of the cyclopentadienyl ligands, which are not only
spectators. Finally, it should be noted that the higher degree of
of the [M(C
complexes.
5
H
4
Bu )I
2
], [M(C
5
H
4
Bu )
2
I
2
]
or [M(C
5
H
4
Bu ) I]
3
The colour of the thf solutions of the neutral or anionic
compounds was dark blue, green and brown for the mono,
bis and tris(cyclopentadienyl) uranium complexes, respectively,
yellow, pale yellow and pale blue for the cerium, lanthanum
and neodymium compounds, respectively. The spectra of the
complexes are listed in Table 1.
covalency in the U–C
in these analogous tris(cyclopentadienyl) complexes could have
been perceived from the average U–C distances which are
5
H
4
R relative to the Ce–C
5
4
H R binding
˚
invariably shorter, by ca. 0.02 A, than the Ce–C distances,
˚
while the ionic radius of U(III) is ca. 0.02 A larger than
34
that of Ce(III). If major attention was paid to the difference
between the U–L and Ce–L bond lengths, which is as large
Preparations
˚
as 0.1 A in [M(C
5
H
4
Me)
3
(PMe
3
)] (M = U, Ce), the smallest
t
[
Ln(C
5
H
4
Bu )
3
(py)] (Ln = Nd, La). A flask was charged with
variation in the U–C and Ce–C distances was neglected or
considered as not significant. It is also interesting to note that
in the series of electron rich tris(pentamethylcyclopentadienyl)
t
NdCl
3
(112 mg, 0.45 mmol) and LiC
5
H
4
Bu (172 mg, 1.34 mmol)
and thf (30 mL) was condensed in. After refluxing for 24 h, the
solvent was evaporated off and the green oily residue extracted
in pentane (30 mL). The solution was filtered and its volume
reduced to 10 mL; addition of pyridine (3 mL) induced the
precipitation of the pale green powder of [Nd(C
48
compounds [M(C
5
Me
5
)
3
] (M = U, La, Ce, Pr, Nd, Sm, Gd),
a linear relationship can be observed between the average Ln–
C distances and the ionic radii of the lanthanides, while the
average U–C distance is 0.04 A shorter than the average Ce–C
t
5
H
4
Bu )
3
(py)]
˚
which was filtered off and dried under vacuum (196 mg, 75%)
distance. In contrast, the average U–C and Ce–C distances are
(Found: C, 65.25; H, 7.71; N, 2.50. C32H44NNd requires C, 65.48;
˚
practically identical, 2.78(2) and 2.79(1) A in the mono(C
5
Me
5
)
2
◦
t
H, 7.56; N, 2.39%). d
H
([ H
8
]thf, 23 C) = −9.47 (27H, Bu ), 3.60
27
33
compounds [M(C
5
Me
5
)I
2
(thf)
3
] (M = U, and Ce ). A study of
(
6H, CH), 6.43 (4H, py), 6.98 (1H, py), 14.28 (6H, CH). Similar
the relative stability of analogous pentamethylcyclopentadienyl
lanthanide(III) and uranium(III) complexes in solution is planned
in our laboratory.
t
reaction of LaCl
3
(160 mg, 0.65 mmol) and LiC
5
H
4
Bu (251 mg,
t
1
.96 mmol) afforded a pale yellow powder of [La(C
5
H
4
Bu )
3
(py)]
(264 mg, 70%) (Found: C, 66.28; H, 7.48; N, 2.60. C32
H
44NLa
2
◦
requires C, 66.08; H, 7.63; N, 2.41%). d
H
([ H
8
]thf, 23 C) = 1.18
t
Conclusion
(27H, Bu ), 5.77 (6H, CH), 5.93 (6H, CH), 7.22 (2H, py), 7.62
(
1H, py), 8.52 (2H, py).
t
The ligand exchange reactions between [Ln(C
5
H
4
Bu )nꢀ
I
3−nꢀ
]
t
ꢀ
t
(
Ln = Nd, Ce, La) and [U(C
5
H
4
Bu )nꢀꢀ
I
3−nꢀꢀ ] species (1 ≤ n +
[Nd(C
[Nd(C
0.034 mmol) in [ H
5
H
4
Bu )I
2
(py)
3
]. An NMR tube was charged with
(17.9 mg,
]thf (0.4 mL). The tube was immersed
ꢀꢀ
t
n = n ≤ 5), and the addition reactions of n mol equivalents
5
H
4
Bu )
3
(py)] (10.0 mg, 0.017 mmol) and NdI
3
t
2
of LiC
5
H
4
Bu (1 ≤ n ≤ 5) to a 1 : 1 mixture of LnI
3
and
8
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2
1 2 6 9

View Online
in an ultrasound bath (80 W, 40 kHz) for 1 h. The solvent
was evaporated off and the blue oily product was dissolved in
(30 lL), pentane was carefully layered onto the green solution.
t
Dark green crystals of [U(C
5
H
4
Bu )
2
I(py) ] were obtained after
2
◦
pyridine (0.1 mL). Addition of pentane (0.3 mL) induced the
standing at 6 C. The crystals were filtered off and dried under
t
precipitation of the pale blue powder of [Nd(C
5
H
4
Bu )I
2
(py)
3
]
vacuum (34 mg, 75%) (Found: C, 43.62; H, 4.65. C28
H
36IN U
2
2
◦
which was filtred off and dried under vacuum (23 mg, 90%)
requires C, 43.93; H, 4.74%). d
CH), −0.47 (4H, CH), 1.88 (18H, Bu ), 2.37 (4H, py), 5.58 (4H,
H
([ H
8
]thf, 23 C) = −41.93 (4H,
t
(
Found: C, 37.82; H, 3.88. C24
H
28
I
2
N
3
Nd requires C, 38.10;
2
◦
t
H, 3.73%). d
H
([ H
8
]thf, 23 C) = −0.66 (9H, Bu ), 7.09 (6H,
py), 8.27 (2H, py).
py), 7.52 (3H, py), 8.52 (6H, py), 9.79 (2H, CH), 13.90 (2H,
CH). Crystals were obtained by slow diffusion of pentane into
a pyridine solution at 6 C.
t
Ligand exchange reactions between [Ln(C
5
H
4
Bu )n ꢀ
I
3−n ꢀ ] (Ln =
◦
t
ꢀ
La, Ce, Nd) and [U(C
n = n ≤ 5)
5
H
4
Bu )n ꢀꢀ
I
3−n ꢀꢀ ] species (1 ≤ n +
ꢀ
ꢀ
t
[
Ce(C
5
H
4
Bu )I
2
(py)
3
].
A
flask was charged with CeI
3
t
(
243 mg, 0.46 mmol) and LiC
5
H Bu (59.3 mg, 0.46 mmol) and
4
A typical procedure can be represented by the following experi-
ꢀ
ꢀꢀ
thf (15 mL) was condensed in. The reaction mixture was stirred
ment (Ln = Ce, n = 0 and n = 3). An NMR tube was charged
◦
t
for 4 h at 20 C and the solvent was evaporated off. The yellow
with [U(C
0
5
H
4
Bu )
3
] (13.0 mg, 0.022 mmol) and CeI (11.3 mg,
3
2
oily residue was extracted with toluene (15 mL) and the yellow
solution was filtered. Pyridine (5 mL) was added to the solution
.022 mmol) in [ H
8
]thf (0.4 mL). The tube was immersed in
an ultrasound bath for 10 min, and the spectrum showed the
t
t
which was then evaporated to dryness, leaving a yellow powder of
formation of the [M(C
5
H
4
Bu )I
2
] and [M(C
5
H
4
Bu ) I] species
2
t
[
Ce(C
5
H
4
Bu )I
2
(py)
3
] (128 mg, 35%) (Found: C, 38.06; H, 3.69;
Ce requires C, 38.31; H, 3.75; I, 33.73%). d
(
M = Ce, U). Integration of the signals indicated that 60% of
t
I, 33.88. C24
(
8
H
28
I
2
N
3
H
the C
5
H
4
Bu groups were on U(III) and 40% on Ce(III); 89% of the
2
◦
t
t
[ H
8
]thf, 23 C) = −1.27 (9H, Bu ), 7.12 (6H, py), 7.55 (3H, py),
cyclopentadienyl ligands on U(III) were on [U(C
1
Ce(III) were on [Ce(C
5
H
4
Bu )
2
I] and
t
.23 (6H, py), 14.52 (2H, CH), 22.57 (2H, CH). Yellow crystals
1% on [M(C
5
H
4
Bu )I
2
], 66% of the cyclopentadienyl ligands on
t
t
of the complex were obtained by slow diffusion of pentane into
5
H
4
Bu )I
2
] and 34% on [Ce(C
5
H
4
Bu ) I].
2
◦
the toluene–pyridine solution at 6 C.
The molar ratio of bis to mono(cyclopentadienyl) complexes of
U(III) was thus equal to 80/20, and was inverse to that of the
t
[
La(C
5
H
4
Bu )I
3
2
(py)
3
]. An NMR tube was charged with
(31 mg,
]thf (0.4 mL). The tube was immersed in an
t
cerium analogues.
The distributions of the C
[
0
La(C
5
H
4
Bu )
(py)] (17.0 mg, 0.029 mmol) and LaI
.058 mmol) in [ H
3
t
2
5
H
4
Bu ligands onto Ln(III) and
8
U(III) and the relative proportions of the complexes are listed in
Table 3 and represented in Fig. 5.
ultrasound bath for 1 h. The solvent was evaporated off and the
yellow oily product was dissolved in pyridine (0.1 mL). Addition
of pentane (0.3 mL) induced the precipitation of the pale yellow
t
Reactions of a 1 : 1 mixture of LnI
3
and [UI
3
L
4
] (L = thf or py)
powder of [La(C
5
H
4
Bu )I
2
(py) ] which was filtred off and dried
3
t
with n mol equivalents of LiC
5
H
4
Bu (1 ≤ n ≤ 5)
under vacuum (39 mg, 88%) (Found: C, 38.19; H, 3.50; I, 34.05.
2
C
2
(
24
H
28
I
2
N
3
La requires C, 38.37; H, 3.76; I, 33.79%). d
H
([ H ]thf,
8
In a typical experiment, an NMR tube was charged with CeI
6.7 mg, 0.013 mmol), [UI (py) ] (12.0 mg, 0.013 mmol) and
Bu (4.9 mg, 0.039 mmol) in [ H
tube was immersed in an ultrasound bath for 10 min, and
3
◦
t
3 C) = 1.17 (9H, Bu ), 6.18 (2H, CH), 6.23 (2H, CH), 7.23
(
3
4
6H, py), 7.62 (3H, py), 8.64 (6H, py). Crystals were obtained
t
2
LiC
5
H
4
8
]thf (0.4 mL). The
◦
by slow diffusion of pentane into a pyridine solution at 6 C.
t
t
the spectrum showed the formation of the [Ce(C
5
H
4
Bu )I ] and
2
[
U(C
U(C
.027 mmol) in [ H
5
H
4
Bu )I
3
2
(py)
3
]. An NMR tube was charged with
(py) ] (25 mg,
]thf (0.4 mL). The tube was immersed in the
t
−
t
[
M(C
5
H
4
Bu )
2
I
2
] species (M = Ce, U). Integration of the signals
[
0
5
H
4
Bu )
] (8.0 mg, 0.013 mmol) and [UI
3
4
t
2
indicated that 67% of the C
5
H
4
Bu groups were on U(III) and 33%
8
on Ce(III); 60% of the cyclopentadienyl ligands on Ce(III) were
ultrasound bath for 15 min. After addition of pyridine (30 lL),
pentane was carefully layered onto the dark blue solution.
t
t
−
on [Ce(C
ratio of bis to mono(cyclopentadienyl) complexes of Ce(III) was
thus equal to 20/60, and 20% of Ce(III) was in the form of CeI
5
H
4
Bu )I
2
] and 40% on [Ce(C
5
H
4
Bu )
2
I
2
] . The molar
t
Dark blue crystals of [U(C
5
H
4
Bu )I
2
(py) ] were obtained after
3
◦
3
.
standing at 6 C for 4 d. The crystals were filtered off and
dried under vacuum (26 mg, 78%) (Found: C, 33.39; H, 3.36.
U requires C, 33.88; H, 3.29%). d
8.21 (2H, CH), −0.86 (9H, Bu ), 0.98 (2H, CH), 7.16 (6H, py),
t
The distributions of the C
5
4
H Bu ligands onto Ln(III) and
2
◦
U(III) and the relative proportions of the complexes are listed in
Table 3 and represented in Fig. 6.
C
24
H
28
I
2
N
3
H
([ H
8
]thf, 23 C) =
t
−
8
.0 (6H, py), 8.74 (3H, py).
Crystallography
t
[
{Ce(C
5
H
4
Bu )
2
(l-I)}
2
]. A flask was charged with CeI
3
t
(
100 mg, 0.19 mmol) and LiC
5
H
4
Bu (49.1 mg, 0.38 mmol) and
The data were collected at 100(2) K on a Nonius Kappa-CCD
area detector diffractometer using graphite-monochromated
49
thf (15 mL) was condensed in. The reaction mixture was stirred
◦
˚
for 12 h at 20 C and the yellow solution was evaporated to
Mo-Ka radiation (k = 0.71073 A). The crystals were introduced
dryness, leaving a pink residue. The product was extracted with
pentane (20 mL) and after filtration, the solvent was evaporated
in glass capillaries with a protecting “Paratone-N” oil (Hampton
Research) coating. The unit cell parameters were determined
from ten frames, then refined on all data. The data (u-scans or
combination of u- and x-scans) were processed with DENZO-
t
off giving a pink powder of [{Ce(C
5
H
4
Bu )
2
(l-I)}
2
] (24.5 mg,
2
4
3
5%) (Found: C, 42.32; H, 5.17; I, 24.72. C18
H
26ICe requires C,
2
◦
t
50
2.44; H, 5.14; I, 24.91%). d
.93 (4H, CH), 17.13 (4H, CH). Pink platelets of the complex
were obtained by crystallization from pentane at 6 C.
H
([ H
8
]thf, 30 C) = 1.21 (18H, Bu ),
SMN. The structures were solved by direct methods with
SHELXS-97 and subsequent Fourier-difference synthesis and
refined by full-matrix least-squares on F with SHELXL-97.
◦
2
51
Absorption effects were corrected empirically with the program
t
[
Nd(C
5
H
4
Bu )
2
I(py)
2
]. An NMR tube was charged with NdI
3
52
DELABS from PLATON. In all compounds, all non-hydrogen
atoms were refined with anisotropic displacement parameters.
The hydrogen atoms were introduced at calculated positions
and were treated as riding atoms with a displacement parameter
t
(
10 mg, 0.019 mmol) and LiC
pyridine (0.4 mL). The tube was immersed in an ultrasound bath
for 15 min. Pentane was carefully layered on the blue solution
and blue crystals of [Nd(C
5
4
H Bu (4.9 mg, 0.038 mmol) in
t
5
H
4
Bu )
2
I(py) ] were obtained after
2
◦
equal to 1.2 (CH) or 1.5 (CH
atom. The structure of [{Ce(C
corresponding to a racemic twin with a Flack parameter value of
3
) times that of the parent
standing at 6 C.
t
5
H
4
Bu )
2
(l-I)}
2
] was refined as
t
[
U(C
U(C
.02 mmol) in [ H
5
H
4
4
Bu )
2
I(py)
2
]. An NMR tube was charged with
(py) ] (20.0 mg,
]thf (0.4 mL). The tube was immersed in
t
53
t
[
0
5
H
Bu )
3
] (25.7 mg, 0.04 mmol) and [UI
3
4
0.49(4). In the case of [U(C
5
H
4
Bu )
2
I(py) ], the very low crystal
2
2
8
quality and the large size of the asymmetric unit comprising
three independent molecules permitted only a rough model to
an ultrasound bath for 15 min. After addition of pyridine
1
2 7 0
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2

View Online
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2
1 2 7 1

View Online
be determined; in spite of the unsatisfactory refinement which
required the use of restraints on displacement parameters for
the more badly behaving atoms and the high residual electron
density peaks located near the heavy atoms (U and I), the con-
nectivity can be considered as determined unambiguously, which
is confirmed by the structure of the isomorphous neodymium
complex, for which far better results were obtained. Crystal
data and structure refinement details are given in Table 4. The
molecular plots were drawn with SHELXTL.
CCDC reference numbers 258864–258870.
See http://www.rsc.org/suppdata/dt/b4/b419088b/ for cry-
stallographic data in .cif or other electronic format.
21 C. Rivi e` re, M. Nierlich, M. Ephritikhine and C. Madic, Inorg. Chem.,
2001, 40, 4428.
2
2
2
2 J. C. Berthet, M. Nierlich, Y. Miquel, C. Madic and M. Ephritikhine,
Dalton Trans., 2005, 369.
3 T. Mehdoui, J. C. Berthet, P. Thu e´ ry and M. Ephritikhine, Dalton
Trans., 2004, 579.
4 S. D. Stults, R. A. Andersen and A. Zalkin, Organometallics, 1990,
9, 115.
25 S. D. Stults, R. A. Andersen and A. Zalkin, Organometallics, 1990,
9, 1623.
54
2
6 L. R. Avens, S. G. Bott, D. L. Clark, A. P. Sattelberger, J. G. Watkin
and B. D. Zwick, Inorg. Chem., 1994, 33, 2248.
2
7 L. R. Avens, C. J. Burns, R. J. Butcher, D. L. Clark, J. C. Gordon, A. R.
Schake, B. L. Scott, J. G. Watkin and B. D. Zwick, Organometallics,
2
000, 19, 451.
2
8 (a) F. T. Edelmann, in Comprehensive Organometallic Chemistry, eds.
G. Wilkinson, F. G. A Stone and E. W. Abel, Pergamon, Oxford,
1995, vol. 4, p. 11; (b) H. Schumann, J. A. Meese-Marktscheffel and
L. Esser, Chem. Rev., 1995, 95, 865.
References
1
G. Wilkinson and J. M. Birmingham, J. Am. Chem. Soc., 1954, 76,
210.
6
2
3
L. T. Reynolds and G. Wilkinson, J. Inorg. Nucl. Chem., 1956, 2, 246.
29 (a) P. L. Watson, J. Chem. Soc., Chem. Commun., 1980, 652; (b) P. L.
Watson, J. F. Whitney and R. L. Harlow, Inorg. Chem., 1981, 20,
3271.
30 (a) J. F. Le Mar e´ chal, C. Villiers, P. Charpin, M. Nierlich, M. Lance,
J. Vigner and M. Ephritikhine, J. Organomet. Chem., 1989, 379, 259;
(b) J. C. Berthet, C. Villiers, J. F. Le Mar e´ chal, B. Delavaux-Nicot,
M. Lance, M. Nierlich, J. Vigner and M. Ephritikhine, J. Organomet.
Chem., 1992, 440, 53.
31 V. K. Bel’skii, Y. K. Gun’ko, E. B. Lobkovskii, B. M. Bulychev and
G. L. Soloveichik, Metalloorg. Khim., 1991, 4, 420.
32 D. Stellfeldt, G. Meyer and G. B. Deacon, Z. Anorg. Allg. Chem.,
1999, 625, 1252.
33 P. N. Hazin, J. C. Huffman and J. W. Bruno, Organometallics, 1987,
6, 23.
(a) C. J. Burns and B. E. Bursten, Comments Inorg. Chem., 1989, 9,
6
1; (b) B. E. Bursten, L. F. Rhodes and R. J. Strittmatter, J. Am.
Chem. Soc., 1989, 111, 2756; (c) B. E. Bursten and R. J. Strittmatter,
Angew. Chem., Int. Ed. Engl., 1991, 30, 1069.
N. Kaltsoyannis, Chem. Soc. Rev., 2003, 32, 9.
4
5
(a) M. Mazzanti, R. L. Wietzke, J. Pecaut, J. M. Latour, P. Maldivi
and M. Remy, Inorg. Chem., 2002, 41, 2389; (b) D. Guillaumont,
J. Phys. Chem. A, 2004, 108, 6893.
6
7
(a) A. Streitwieser and U. M u¨ ller-Westerhoff, J. Am. Chem. Soc.,
1
968, 90, 7364; (b) D. Seyferth, Organometallics, 2004, 23, 3562.
(a) Actinides and Fission Products Partitioning and Transmutation.
Status and Assessment Report. NEA/OECD Report, NEA/OECD,
Paris, 1999; (b) Actinides and Fission Products Partitioning and
Transmutation, Proceedings of the Fifth International Information
Exchange Meeting, Mol, Belgium, 25–27 Nov 1998, NEA/OECD
Report, Paris, 1999.
34 R. D. Shannon, Acta Crystallogr., Sect A, 1976, 32, 751.
35 J. C. Berthet, M. Nierlich and M. Ephritikhine, Polyhedron, 2003,
22, 3475.
8
9
(a) K. L. Nash, Solvent Extr. Ion Exch., 1993, 11, 729; (b) K. L.
Nash, in Handbook on the Physics and Chemistry of Rare earths. Lan-
thanides/Actinides: Chemistry, ed. K. A. Gschneidner, Jr., L. Eyring,
G. R. Choppin and G. H. Lander, Elsevier Science, Amsterdam, 1994,
vol. 18, p. 197.
J. G. Brennan, S. D. Stults, R. A. Andersen and A. Zalkin,
Organometallics, 1988, 7, 1329.
36 K. Izod, S. T. Liddle and W. Clegg, Inorg. Chem., 2004, 43, 214.
37 C. Rivi e` re, J. C. Berthet, M. Nierlich and M. Ephritikhine, unpub-
lished results.
38 A. A. Trifonov, P. Van de Weghe, J. Collin, A. Domingos and I.
Santos, J. Organomet. Chem., 1997, 527, 225.
39 (a) H. Lueken, J. Schmitz, W. Lamberts, P. Hannibal and K.
Handrick, Inorg. Chim. Acta, 1989, 156, 119; (b) T. Aknoukh, J.
M u¨ ller, K. Qiao, X. F. Li and R. D. Fischer, J. Organomet. Chem.,
1991, 408, 47.
1
0 M. del Mar Conejo, J. S. Parry, E. Carmona, M. Schultz, J. G.
Brennann, S. M. Beshouri, R. A. Andersen, R. D. Rogers, S. Coles
and M. Hursthouse, Chem. Eur. J., 1999, 5, 3000.
1 J. C. Berthet, C. Rivi e` re, Y. Miquel, M. Nierlich, C. Madic and M.
Ephritikhine, Eur. J. Inorg. Chem., 2002, 1439.
40 (a) W. Lamberts, H. Lueken and U. Elsenhans, Inorg. Chim. Acta,
1986, 121, 81; (b) H. Lueken, W. Lamberts and P. Hannibal, Inorg.
Chim. Acta, 1987, 132, 111.
1
1
2 (a) J. C. Berthet, Y. Miquel, P. B. Iveson, M. Nierlich, P. Thu e´ ry, C.
Madic and M. Ephritikhine, J. Chem. Soc., Dalton Trans., 2002, 3265;
41 P. B. Hitchcock, M. F. Lappert and S. Tian, Organometallics, 2000,
19, 3420.
(
b) P. B. Iveson, C. Rivi e` re, D. Guillaneux, M. Nierlich, P. Thu e´ ry,
42 Q. Shen, M. Qi, J. Guan and Y. Lin, J. Organomet. Chem., 1991, 406,
M. Ephritikhine and C. Madic, Chem. Commun., 2001, 1512.
3 M. G. B. Drew, M. J. Hudson, P. B. Iveson, M. L. Russell, J. O.
Liljenzin, M. Sk a˚ lberg, L. Spjuth and C. Madic, J. Chem. Soc., Dalton
Trans., 1998, 2973.
353.
1
1
1
43 A. Zalkin and S. M. Beshouri, Acta Crystallogr., Sect. C, 1989, 45,
1219.
44 A. Zalkin and S. M. Beshouri, Acta Crystallogr., Sect. C, 1989, 45,
4 M. G. B. Drew, P. B. Iveson, M. J. Hudson, J. O. Liljenzin, L. Spjuth,
1080.
P. Y. Cordier, A˚ . Enarsson, C. Hill and C. Madic, J. Chem. Soc.,
45 S. M. Beshouri and A. Zalkin, Acta Crystallogr., Sect. C, 1989, 45,
1221.
Dalton Trans., 2000, 821.
5 I. Hagstr o¨ m, L. Spjuth, A˚ . Enarsson, J. O. Liljenzin, M. Sk a˚ lberg,
46 P. Franck, P. Ghosh, K. O. Hodgson and H. Taube, Inorg. Chem.,
M. J. Hudson, P. B. Iveson, C. Madic, P. Y. Cordier, C. Hill and N.
Fran c¸ ois, Ion Exch. Solvent Extr., 1999, 17, 221.
6 P. Y. Cordier, C. Hill, P. Baron, C. Madic, M. J. Hudson and J. O.
Liljenzin, J. Alloys Compd., 1998, 271, 738.
2002, 41, 3269.
47 H. Irving and D. H. Mellor, J. Chem. Soc., 1962, 5222.
48 W. J. Evans, Chem. Rev., 2002, 102, 2119.
49 Kappa-CCD Software, Nonius BV, Delft, The Netherlands,
1998.
1
1
1
1
7 G. Y. S. Chan, M. G. B. Drew, M. J. Hudson, N. S. Isaacs and P.
Byers, Polyhedron, 1996, 15, 3385.
50 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276,
8 R. Wietzke, M. Mazzanti, J. M. Latour and J. P e´ caut, Inorg. Chem.,
307.
1
999, 38, 3581.
51 G. M. Sheldrick, SHELXS-97 and SHELXL-97, University of
G o¨ ttingen, Germany, 1997.
52 A. L. Spek, PLATON, University of Utrecht, The Netherlands, 2000.
53 H. D. Flack, Acta Crystallogr., Sect A, 1983, 39, 876.
54 G. M. Sheldrick, SHELXTL, Version 5.1, Bruker AXS Inc., Madi-
son, WI, USA, 1999.
9 (a) Z. Kolarik, U. M u¨ llich and F. Gassner, Ion Exch. Solvent Extr.,
1
999, 17, 23; (b) Z. Kolarik, U. M u¨ llich and F. Gassner, Ion Exch.
Solvent Extr., 1999, 17, 1155.
2
0 M. J. Hudson, M. G. B. Drew, M. S. StJ. Foreman, C. Hill, N. Huet,
C. Madic and T. G. A. Youngs, Dalton Trans., 2003, 1675.
1
2 7 2
D a l t o n T r a n s . , 2 0 0 5 , 1 2 6 3 – 1 2 7 2