The uranium-nitrogen bond in U(IV) complexes supported by the hydrotris(3,5-dimethylpyrazolyl)borate ligand

Article abstract of DOI:10.1039/b509229a

Insertion of benzonitrile and acetonitrile into the U-C bond of [U(Tp ^Me2)Cl₂(CH₂SiMe₃)] (Tp^Me2 = HB(3,5-Me₂pz)₃) gives the ketimide complexes [U(Tp ^Me2)Cl₂{NC(R)(CH₂SiMe₃,)}] (R = Ph (1); Me (2)). The identity of complex 1 was ascertained by a single-crystal X-ray diffraction study. In the solid state 1 exhibits octahedral geometry with a short U-N bond length to the ketimide ligand. We also report herein the synthesis and the X-ray crystal structures of the uranium amide complexes [U(Tp^Me2)Cl₂(NR₂)] (R = Et (3); Ph (4)). A detailed comparison of the U-N bond lengths in these compounds with other known U-N (and Th-N) distances in amide and ketimide actinide(IV) complexes is performed, confirming the short character of the U-N bond length in 1. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509229a

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F U L L P A P E R
The uranium–nitrogen bond in U(IV) complexes supported by the
hydrotris(3,5-dimethylpyrazolyl)borate ligand
ˆ
Manuela Silva, Maria Augusta Antunes, Marta Dias, Angela Domingos,
Isabel Cordeiro dos Santos, Joaquim Marc¸alo and Noe´mia Marques*
Departamento de Qu´ımica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953,
Sacave´m, Portugal. E-mail: nmarques@itn.mces.pt
Received 29th June 2005, Accepted 19th August 2005
First published as an Advance Article on the web 1st September 2005
Insertion of benzonitrile and acetonitrile into the U–C bond of [U(TpMe2)Cl2(CH2SiMe3)] (TpMe2 = HB(3,5-Me2pz)3)
gives the ketimide complexes [U(TpMe2)Cl2{NC(R)(CH2SiMe3)}] (R = Ph (1); Me (2)). The identity of complex 1 was
ascertained by a single-crystal X-ray diffraction study. In the solid state 1 exhibits octahedral geometry with a short
U–N bond length to the ketimide ligand. We also report herein the synthesis and the X-ray crystal structures of the
uranium amide complexes [U(TpMe2)Cl2(NR2)] (R = Et (3); Ph (4)). A detailed comparison of the U–N bond lengths
in these compounds with other known U–N (and Th–N) distances in amide and ketimide actinide(IV) complexes is
performed, confirming the short character of the U–N bond length in 1.
Introduction
Results and discussion
During the last years a large amount of organometallic
compounds of uranium and thorium with unprecedented re-
activity patterns has been reported.1 Organouranium com-
plexes with amino, alkyl or hydrogen ligands have proven to
have a remarkable reaction chemistry. Most of this chem-
istry deals with bis(pentamethylcyclopentadienyl) compounds
[An(C5Me5)2R(Cl)] and [An(C5Me5)2R2]2 (An = actinide) or
with compounds with two cyclopentadienyl ligands bridged by
a SiMe2,3 in which the approaching of the two rings leads to
an increase of the coordination space around the metal and
consequently to enhanced reactivity. The pronounced effect that
steric coordination seems to have on the reactivity of the com-
pounds led to the identification of new ancillary ligand systems
capable of stabilizing monomeric uranium and thorium species
while provoking novel reactivity patterns. Our group has a
long standing interest in hydrotris(3,5-dimethylpyrazolyl)borate
uranium and thorium complexes, because of the potential
offered by these ligands for steric and electronic tunability. The
use of TpMe2 (hydrotris(3,5-dimethylpyrazolyl)borate) allowed
the synthesis of the complexes [An(TpMe2)Cl3(THF)],4 that
showed to be good precursors for the preparation of a wide
range of compounds containing An–O, An–N, An–S and
An–C (r and p) bonds.5 The reactivity of the U–C bond
of [U(TpMe2)Cl2{CH(SiMe3)2}] and [U(TpMe2)Cl2(CH2SiMe3)]
towards ketones and aldehydes has been reported.6 Herein
we wish to report that nitriles undergo migratory insertion
into the U–C bond of [U(TpMe2)Cl2(CH2SiMe3)] to yield the
ketimide compounds [U(TpMe2)Cl2{NC(R)(CH2SiMe3)}] (R =
Ph (1), Me (2)). This synthetic route has been used previously
in the preparation of the mono(ketimide) uranium complexes,
[U{(Me3Si)2N}3(NC(Me)(R)] (R = CH3, C3H7)7,8 and recently
in the synthesis of a series of bis(ketimide) thorium and uranium
complexes, [An(C5Me5)2{NC(Ph)(R)}2]9 (R = CH2Ph, Ph). The
structure of 1 has been ascertained by means of a single-
crystal X-ray determination. The short U–N bond distance
Addition of 1 equiv. of benzonitrile or acetonitrile to a toluene
solution of [U(TpMe2)Cl2(CH2SiMe3)] gives the ketimide com-
plexes [U(TpMe2)Cl2{NC(R)(CH2SiMe3)}] (R = Ph (1), Me (2))
(eqn (1))
[U(TpMe2)Cl2(CH2SiMe3)] + RC≡N
→ [U(TpMe2)Cl2{NC(R)(CH2SiMe3)}]: R = Ph (1), Me (2) (1)
Synthetic procedures reported in the literature for the prepa-
ration of actinide ketimide complexes include reductive cleav-
age of benzophenone azine by [U(C5Me5)2Cl2] in the pres-
ence of sodium amalgam10 or metathesis of UI3(DME)2 with
K{NC(But)(C6H2-2,4,6-Me3)} in DME that led to isolation of
[UI{NC(But)(C6H2-2,4,6-Me3)}3(DME)].11 However, migratory
insertion of nitriles into U C,12 U–CH27,8 or An–R8,9 bonds has
=
been more extensively used in the synthesis of actinide ketimide
complexes.
The air-sensitive compounds 1 and 2 were obtained in good
yields, as dark reddish and dark yellow solids, respectively. Both
compounds were soluble in ethereal and aromatic solvents and
poorly soluble in aliphatic solvents.
The IR spectra of 1 and 2 show the typical m(B–H) stretching
vibration at 2540 cm−1, characteristic of a j3-coordination mode
of the TpMe2 ligand,13 and one strong band at 1605 cm−1 assigned
=
to the m(N C) stretching vibration. This band is located at the
low end of the region where m(NC) is found in the spectra of
lanthanide and actinide ketimide complexes,7,8 and probably
=
reflects a decrease in the C N bond order due to donation
of electron density from the nitrogen lone pair to the uranium
ion. This is in agreement with the slight increase in the C–
N distance observed in the X-ray structure of the compound
(see below).
The 1H NMR spectra of both complexes at room temperature
exhibit four TpMe2 methyl and two H-4 singlets in the ratio 2 :
1, indicating that two pyrazolyl rings are equivalent but the
third is distinct. In addition, the spectra displayed resonances
associated with the ketimide ligand: one single resonance for
the methyl protons and one for the methylenic protons of the
CH2SiMe3 group, and one and three resonances, respectively,
for the methyl and phenyl protons of the inserted nitriles.
Hence, the spectra are consistent with Cs-symmetric structures
˚
to the ketimide ligand (2.04 A) in comparison with other
ketimide uranium complexes,9–11 prompted us to synthesize
and structurally characterize two uranium amide complexes,
[U(TpMe2)Cl2(NR2)] (R = Et (3); Ph (4)), in order to understand
if the short U–N bond found in 1 was a result of steric and/or
electronic effects.
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 , 3 3 5 3 – 3 3 5 8
3 3 5 3
©

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for both compounds, with the ketimide ligand contained in the
mirror plane, in accordance with the solid-state structure of 1
(see below).
The ORTEP diagram of 1 is shown in Fig. 1. A listing of
relevant bond lengths and angles is available in Table 1; selected
crystal and refinement data are given in Table 2. The molecular
structure of 1 reveals a six-coordinate uranium complex having
approximate Cs-symmetry with the metal bound to the TpMe2
ligand in a typical j3-chelation fashion, the two chlorine atoms
and the nitrogen atom of the ketimide ligand, in an octahedral
arrangement. One important structural feature is the U–N(4)
˚
distance of 2.04(2) A that compares with the corresponding
12
˚
distance in [UCp3{NC(Me)CH(PMePh2)}] (2.06(1) A), but is
significantly shorter than those of the structurally characterized
uranium(IV) ketimide complexes [U(C5Me5)2(NCPh2)2],10 (U–N
9
˚
2.179(6) and 2.185(5) A), [U(C5Me5)2{NC(Ph)(CH2Ph)}2] (U–
N 2.184(3) A), [U(I){NC(Bu )(C6H2-2,4,6-Me3)}3(DME)]11 (U–
t
˚
t
˚
N 2.176(12), 2.196(12) and 2.196(13) A) and [U{NC(Bu )(C6H2-
2,4,6-Me3)}3]2(COT)11 (U–N 2.161(3), 2.174(4) and 2.186(3) A).
˚
The marked difference between the U–N distances in bispen-
tamethyl bis(ketimide) complexes and triscyclopentadienyl or
hydrotrispyrazolylborate ketimide complexes raises the question
if it is reliable to compare uranium–nitrogen distances in
systems with different sets of ancillary ligands. As no U–
N bond distances are known for amido complexes based
on the “U(TpMe2)Cl2” core, we decided to synthesize and to
characterize by single-crystal X-ray diffraction analysis two
uranium(IV) amide complexes, [U(TpMe2)Cl2(NEt2)] (3) and
[U(TpMe2)Cl2(NPh2)] (4).
Metathesis of [U(TpMe2)Cl3(THF)] with one equivalent of
either LiN(C2H5)2 or LiN(C6H5)2 proceeds readily and yields
respectively, after simple workup, [U(TpMe2)Cl2(NEt2)] (3) and
[U(TpMe2)Cl2(NPh2)] (4). The compounds are obtained as green
and orange solids, respectively, in modest yield (eqn (2)).
Fig. 1 ORTEP diagram of [U(TpMe2)Cl2{NC(Ph)(CH2SiMe3)}] (1)
using 20% probability ellipsoids.
◦
Table 1 Selected bond lengths (A) and angles ( ) for [U(TpMe2)-
˚
Cl2{NC(Ph)(CH2SiMe3)}]
(1),
[U(TpMe2)Cl2(NEt2)]
(3)
and
[U(TpMe2)Cl2(NPh2)] (4)
[U(TpMe2)Cl3] + LiNR2 → [U(TpMe2)Cl2(NR2)]
1
3
4
+ LiCl: R = Et (3); Ph (4)
(2)
U–Cl(1)
U–Cl(2)
U–N(11)
U–N(21)
U–N(31)
U–N(4)
2.600(6)
2.620(7)
2.501(19)
2.48(2)
2.536(19)
2.04(2)
2.629(4)
2.627(4)
2.518(10)
2.487(10)
2.483(10)
2.148(10)
2.942(13)
2.608(4)
2.603(4)
2.474(11)
2.493(12)
2.465(11)
2.225(10)
2.949(13)
Compounds 3 and 4 are soluble in aromatic and ether type
solvents, but poorly soluble in aliphatic hydrocarbons.
The IR spectra of 3 and 4 show the m(B–H) stretching mode at
2540 cm−1 consistent with a j3-coordination mode of the TpMe2
ligand.13
U–C(40)
N(4)–C(4)
1
The room-temperature H NMR spectra of both complexes
1.33(3)
were consistent with a Cs-symmetric coordination sphere, with
the three pyrazolyl rings of the TpMe2 ligand giving rise to two
sets of resonances in the ratio 2 : 1. The amide ligands, NEt2 and
NPh2, give rise to two and three resonances, respectively, with
the expected intensity.
X-Ray quality crystals of 3 and 4 were grown by slow concen-
tration of THF solutions. The ORTEP diagrams of compounds
3 and 4 are shown in Fig. 2 and 3. Both structures consist
of discrete molecular units in which the metal centre is six-
coordinate by the tridentate pyrazolylborate ligand, two chlorine
atoms and the nitrogen atom of the amide ligand. Ignoring the
short U–C(40) contacts in 3 and 4, the coordination geometry
Cl(1)–U–Cl(2)
Cl(1)–U–N(4)
Cl(2)–U–N(4)
N(11)–U–N(21)
N(11)–U–N(31)
N(21)–U–N(31)
U–N(4)–C(4)
U–N(4)–C(40)
U–N(4)–C(42)
U–N(4)–C(46)
C(40)–N(4)–C(42)
C(40)–N(4)–C(46)
99.9(2)
98.2(6)
99.7(6)
75.5(6)
75.0(7)
75.5(7)
174.4(18)
97.08(14)
103.6(3)
103.4(3)
76.3(3)
77.7(4)
73.3(3)
99.37(13)
103.2(3)
105.3(3)
75.0(4)
76.7(4)
74.8(4)
107.1(8)
134.6(9)
105.5(8)
140.2(8)
114.2(11)
118.0(11)
Table 2 Crystallographic data for 1, 3 and 4
1
3
4
Formula
Mr
C26H38BN7Cl2SiU
796.46
Orthorhombic
Pna21
20.948(3)
10.5300(10)
15.0120(10)
C19H32BN7Cl2U
678.26
Monoclinic
C27H32BN7Cl2U
774.34
Triclinic
¯
P1
8.1647(11)
10.2881(11)
18.947(2)
75.667(9)
88.415(11)
78.416(10)
1510.2(3)
2
Crystal system
Space group
P21/n
˚
a/A
10.942(5)
17.310(3)
14.541(5)
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
109.01(3)
3
˚
V/A
3311.4(6)
4
1.598
2603.9(14)
4
1.730
Z
Dc/g cm−3
1.703
5.580
l(Mo-Ka)/mm−1
5.126
6.458
R1
wR2
0.0682
0.09310
0.0624
0.01225
0.0792
0.0855
3 3 5 4
D a l t o n T r a n s . , 2 0 0 5 , 3 3 5 3 – 3 3 5 8

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(163(1)◦) and [U(I){NC(tBu)(C6H2-2,4,6-Me3)}3(DME)]11
(172.8(14), 171.4(17) and 160.4(13)◦).
˚
The U–N(4) distance of 2.148(10) A in 3, is close to
the corresponding distance in [U(OC6H3-2,6-But2)3(NEt2)]
19
20
˚
˚
(2.16 A) and [U{(SiMe2NPh)3-tacn}(NEt2)] (2.146(12) A),
but significantly shorter than those usually found in other
structurally characterized uranium(IV) alkylamido complexes,
21
˚
˚
2.220(5) A in [U{(Me3SiNCH2CH2)3N}(NEt2)], 2.210(4) A
in [U{Me2Si(C5Me4)(tBuN)}(NMe2)2],17 and 2.22(1) A for the
˚
terminal U–N bonds in the dimeric [{U(NEt2)4}2].18
The two ethyl groups of the amide ligand are inequivalent.
The U–N(4)–C(40) is more acute than the U–N(4)–C(42)
angle by 27.5◦. This distortion is a result of a close contact
between the U and the N(4)–C(40) functionality. The U · · · C(40)
˚
distance of 2.942(13) A may be compared with those of
contacts to the methyl groups within the bis(trimethylsylilamide)
ligands in [U(SC6H3-2,6-Me2){N(SiMe3)2}3]22 (3.158 A) and
˚
˚
Fig. 2 ORTEP diagram of [U(TpMe2)Cl2(NEt2)] (3) using 20% proba-
bility ellipsoids.
[Th(NMePh)2{N(SiMe3)2}2]23 (3.073(10) and 3.064(10) A). In-
spection of the angles around the nitrogen atom coordinated
to uranium reveals that the planarity of the substituents is
maintained (sum of angles 359.7◦).
˚
The U–N(4) distance of 2.225(10) A in 4 compares with
24
˚
those of 2.29(1), 2.27(10) and 2.267(6) A in [UCp3(NPh2)],
[U(NPh2)4],18 and [U(C5Me5)2{NH(C6H3-2,6-Me2)2}],15 respec-
tively. The molecular structure of 4 also features one short
contact to the carbon-ipso (C40) of one of the phenyl rings
˚
(U · · · C(40) 2.949 (13) A). This interaction results in a decrease
in the angle U–N(4)–C(40) to 105.5(8)◦ vs. U–N(4)–C(46) of
140.2(8)◦, with the two phenyl groups which are attached to ni-
trogen occupying different positions relative to the “UTpMe2Cl2”
core (Fig. 4).
Fig. 3 ORTEP diagram of [U(TpMe2)Cl2(NPh2)] (4) using 30% proba-
bility ellipsoids.
around the uranium atom may be described as being octahedral.
Comparative bond distances and angles are shown in Table 1.
The metrical parameters for the “UTpMe2Cl2” fragment in
compounds 1, 3 and 4 compare favourably with those observed
for other uranium (IV) complexes supported by one TpMe2
ligand.4,6
˚
As referred above, the U–N(4) distance of 2.04(2) A in 1
is rather short. The bond distance of C(4) to the coordinated
˚
nitrogen N(4) (1.33(3) A) in 1 is slightly longer than the
2
2
14
˚
value accepted for the N(sp )–C(sp ) double bonds (1.28 A),
=
suggesting charge delocalisation from the N C bond to the U–
N bond. These structural features together with the almost linear
U–N(4)–C(4) angle of 174.4◦ may corroborate the assumption
made by other authors that there is significant ligand to metal
p-bonding in the uranium ketimido interaction and, therefore,
that these bonds have a significant uranium–nitrogen multiple-
bond character.9 This assumption was based on the U–N
bond distances to the ketimide ligands [U(C5Me5)2(NCPh2)2],10
Fig. 4 A B–U view of [U(TpMe2)Cl2(NPh2)] (4).
The most striking feature of the structures of the ketimide and
the two amido complexes discussed above is the large differences
of the U–N distances in these compounds.
The difference between the two U–N(4) distances in com-
plexes 3 and 4 may be understood as the result of both steric
and electronic effects: it is recognized that NPh2 is larger than
NEt2, as can be immediately inferred from the dimeric structure
presented by the complex [{U(NEt2)4}2] in contrast with the
monomeric structure of [U(NPh2)4],18 and the charge density
on the N atom in the diethylamide ligand is expected to be
larger than in the diphenylamide ligand due to the charge
delocalisation effect of the phenyl groups.
The shorter U–N(4) distance in the ketimide complex 1
as compared to the corresponding distances in the amido
complexes 3 and 4 may also be the consequence, apart from
steric effects, of the intrinsic nature of the N atom which is
9
˚
(U–N(av.) 2.182(8) A), [U(C5Me5)2{NC(Ph)(CH2Ph)}2] (U–
t
11
˚
N(av.) 2.184(3) A), [U(I){NC(Bu )(C6H2-2,4,6-Me3)3}(DME)]
t
˚
(U–N(av.) 2.189(25) A) and [U{NC(Bu )(C6H2-2,4,6-Me3)3}]2-
(COT)11 (U–N(av.) 2.174(16) A), that are shorter than those in
˚
the amide uranium(IV) complexes [U(C5Me5)2{NH(C6H32,6-
Me2)2}]15 (U–N(av.) 2.267(6) A), [U(C5Me5)(C5Me4CH2NAd)-
˚
{NH(Ad)}]16 (U–N 2.231(6), 2.155(7) A), [U{Me2Si(C5Me4)-
˚
˚
(tBuN)}(NMe2)2]17 (U–N(av.) 2.210(7) A) and [{U(NEt2)4}2]
18
˚
(U–N(av.) 2.22(1) A) and in the large U–N–C angles ([U(C5Me5)2-
(NCPh2)2]10 (173.4(6) and 176.5(5)◦), [U(C5Me5)2{NC-
(CH2Ph)(Ph)}2]9 (162.4(3)◦), [UCp3{NC(Me)CH(PMePh2)}]12
D a l t o n T r a n s . , 2 0 0 5 , 3 3 5 3 – 3 3 5 8
3 3 5 5

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formally sp2, while the N atoms are formally sp3 in the amide
ligands.
compounds were not considered in the original work of the
authors.25 In Table 4 we present the CNS of the ligands not
included in ref. 25.
To be able to make a more thorough comparison of the U–N
(and eventually Th–N) bond lengths in compounds containing
ketimide and amide ligands, we need to take into account the
different steric and/or electronic requirements of these ligands
and also of the ancillary ligands. One way to sort out the steric
effects that may contribute to the metal–ligand distances in a
complex is to use a model proposed some years ago by Marc¸alo
and Pires de Matos, based on a new definition of coordination
number.25 In that work, the authors defined a steric coordination
number (CNS) based on the size of a ligand (as measured by the
solid angle comprising the van der Waals’ spheres of the atoms
of the ligand) and used it to obtain the CNS of a compound,
as the sum of the CNS of all the ligands. From the analysis
of a large number of structurally characterized lanthanide and
actinide compounds, Marc¸alo and Pires de Matos derived a set
of ligand effective radii r(N), which reflect the electronic and/or
electrostatic contribution of the ligand to the metal–ligand bond
length. These ligand effective radii together with the CNS of a
compound and Shannon’s ionic radii r(An4+)26 can be used in the
comparison of the metal–ligand distances in compounds with
different sets of ligands.
Several aspects can be highlighted from an inspection of the
ligand effective radii (r(N)) presented in Table 3. The dipheny-
˚
lamido and arylamido ligands (average r(N) = 1.28(3) A) form,
as expected, longer bonds with the An metal centres as compared
Table 4 Steric coordination numbers (CNS) of new ligandsa
Ligand
CNS
(SiMe2NPh)3-tacn
(Me2ButSiNCH2CH2)3N
Me2Si(C5Me4)(ButN)
C5Me4CH2NAd
NHPh
NH(C6H4-4-Cl)
NH(C6H3-2,6-Me2)
NMePh
NHAd
NMe2
NC(Ph)(CH2SiMe3)
NC(But)(C6H2-2,4,6-Me3)
NC(Ph)(CH2Ph)
NCPh2
5.97
6.89
4.57
4.17
1.45
1.45
1.72
1.63
1.59
1.46
1.52
1.56
1.37
1.44
1.47
In Table 3 we show the results of an analysis, based on the
model of Marc¸alo and Pires de Matos,25 of a significant number
of U(IV) and Th(IV) complexes containing amide and ketimide
ligands. It should be noted that the large majority of these
NC(Me)(CHPMePh2)
a Not considered in original work; ref. 25
Table 3 Steric analysis of U–N and Th–N bond lengths in U(IV) and Th(IV) amide and ketimide complexes
r(An4+)/A
r(N)/A
Ref.
˚
˚
˚
Complex
d(An–N)/A
CNS
[UTpMe2Cl2(NPh2)]
[U(NPh2)4]
[UCp3(NPh2)]
[U(C5Me5)2(NHPh)2]
2.225(10)
2.27(10)
2.29(1)
7.34
7.16
7.91
7.88
8.42
7.43
7.60
7.31
8.92
8.12
7.78
7.93
7.60
7.22
7.64
8.90
8.56
7.49
8.25
8.64
9.29
8.25
8.74
8.40
7.07
7.46
8.12
8.12
7.79
7.86
7.79
7.86
7.59
0.964
0.954
0.994
0.993
1.021
0.969
1.023
0.962
1.048
1.005
1.032
1.040
1.023
0.958
0.980
1.047
1.029
0.972
1.012
1.033
1.067
1.012
1.038
1.020
0.950
0.970
1.005
1.005
0.988
0.992
1.033
1.036
0.977
1.26
1.32
1.30
1.26
1.25
1.27
1.28
1.27
1.24
1.23
1.30
1.28
1.29
1.20
1.17
1.12
1.19
1.24
1.14
1.15
1.13
1.16
1.14
1.20
1.09
1.22
1.17
1.22
1.20
1.19
1.22
1.23
1.08
This work
a
b
c
d
e
f
2.250(4)
2.267(6)
2.237(3)
2.302(10)
2.237(9)
2.285(12)
2.235(12)
2.335(25)
2.32(2)
2.314(20)
2.154(10)
2.146(12)
2.162(5)
2.220(5)
2.210(7)
2.155(7)
2.18(3)
2.202(20)
2.175(15)
2.18(2)
2.22(1)
2.04(2)
2.189(25)
2.174(16)
2.225(5)
2.184(3)
2.182(8)
2.256(8)
2.262(8)
2.06(1)
[U(C5Me5)2{NH(C6H3-2,6-Me2)}]2
[U(C5Me5)2Cl{NH(C6H4-4-Cl)}]
[Th{N(SiMe3)2}2(NMePh)2] (Th–NMePh)]
[UH{N(SiMe3)2}3]
g
h
i
[U(OC6H3-2,6-But2){N(SiMe3)2}3]
[UCl2(DME){N(SiMe3)2}2]
[Th(COT){N(SiMe3)2}2]
j
k
f
[Th(BH4){N(SiMe3)2}3]
[Th{N(SiMe3)2}2(NMePh)2] (Th–N(SiMe3)2)
[UTpMe2Cl2(NEt2)]
This work
l
[U{(SiMe2NPh)3-tacn}(NEt2)]
[U(OC6H3-2,6-But2)3(NEt2)]
[U{(Me2ButSiNCH2CH2)3N}(NEt2)]
[U{Me2Si(C5Me4)(ButN)}(NMe2)2]
[U(C5Me5)(C5Me4CH2NAd)(NHAd)]
[U(NEt2)3(THF)3][BPh4]
m
n
o
p
q
q
r
[U(NEt2)2(py)5][BPh4]2
[U(C5Me5)(NEt2)2(THF)2][BPh4]
[U(COT)(NEt2)(THF)3][BPh4]
[U(C5Me5)2(NMe2)(CNBut)2][BPh4]
[UTpMe2Cl2{NC(Ph)(CH2SiMe3)}]
[UI{NC(But)(C6H2-2,4,6-Me3)}3(DME)]
[U{NC(But)(C6H2-2,4,6-Me3)}3]2(COT)
[U(COT){NC(But)(C6H2-2,4,6-Me3)}3][Na(OEt2)]
[U(C5Me5)2{NC(Ph)(CH2Ph)}2]
[U(C5Me5)2(NCPh2)2]
s
t
This work
u
u
u
v
w
v
[Th(C5Me5)2{NC(Ph)(CH2Ph)}2]
v
[Th(C5Me5)2(NCPh2)2]
x
[UCp3{NC(Me)(CHPMePh2)}]
a Ref. 18. b Ref. 24. c R. C. Schnabel, D. S. Arney and C. J. Burns, manuscript in preparation, cited in D. S. J. Arney and C. J. Burns, J. Am Chem.
Soc., 1995, 117, 9448. d Ref. 15. e R. G. Peters, B. L. Scott and C. J. Burns, Acta Crystallogr., Sect. C, 1999, 55, 1482. f Ref. 23. g R. A. Andersen,
A. Zalkin and D. H. Templeton, Inorg. Chem., 1981, 20, 622. h J. M. Berg, D. L. Clark, J. C. Huffman, D. E. Morris, A. P. Sattelberger, W. E. Streib,
W. G. Van Der Sluys and J. G. Watkin, J. Am. Chem. Soc., 1992, 114, 10811. i L. G. McCullough, H. W. Turner, R. A. Andersen, A. Zalkin and
D. H. Templeton, Inorg. Chem. 1981, 20, 2869. j T. M. Gilbert, R. R. Ryan and A. P. Sattelberger, Organometallics, 1988, 7, 2514. k H. W. Turner,
R. A. Andersen, A. Zalkin and D. H. Templeton, Inorg. Chem., 1979, 18, 1221. l Ref. 20. m Ref. 19. n Ref. 21. o Ref. 17. p Ref. 16. q J. C. Berthet,
C. Boisson, M. Lance, J. Vigner, M. Nierlich and M. Ephritikhine, J. Chem. Soc., Dalton Trans., 1995, 3019. r J. C. Berthet, C. Boisson, M. Lance,
J. Vigner, M. Nierlich and M. Ephritikhine, J. Chem. Soc., Dalton Trans. 1995, 3027. s C. Boisson, J. C. Berthet, M. Ephritikhine, M. Lance
and M. Nierlich, J. Organomet. Chem., 1996, 522, 249. t C. Boisson, J. C. Berthet, M. Lance, M. Nierlich and M. Ephritikhine, J. Organomet. Chem.,
1997, 548, 9. u Ref. 11. v Ref. 9. w Ref. 10. x Ref. 12.
3 3 5 6
D a l t o n T r a n s . , 2 0 0 5 , 3 3 5 3 – 3 3 5 8

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with the dialkylamido and alkylamido ligands (average r(N) =
1950; dH (300 MHz, C6D6, Me4Si) 55.8 (2H, CH2SiMe3), 42.5
(1H, H-4), 35.8 (3H, Me-pz), 34.8 (2H, doublet, H-o), 18.7 (3H,
Me-pz), 13.7 (2H, triplet, H-m), 12.7 (1H, triplet, H-p), 11.2 (9H,
CH2SiMe3), −5.8 (6H, Me-pz), −10.7 (2H, H-4), −24.5 (6H,
Me-pz). Dark orange crystals of 1 suitable for X-ray diffraction
analysis were grown by slow diffusion of hexane into a toluene
solution.
˚
1.17(4) A), and the disilylamido ligand forms also longer bonds
˚
(average r(N) = 1.27(3) A), probably reflecting a lower basicity
as compared to the dialkylamides (this difference was already
apparent in the early work of Marc¸alo and Pires de Matos,25
which included also lanthanide compounds). According to
this analysis, compounds 3 and 4 seem to have unexceptional
U–N bond lengths to the diphenylamido and diethylamido
ligands. Surprisingly, the ketimide ligands, with the exceptions
of compound 1 and of [UCp3{NC(Me)(CHPMePh2)}],12 seem
to form bonds that are slightly longer (excluding the indicated
compounds) than the ones formed by dialkylamido and alky-
[U(TpMe2)Cl2{NC(Me)(CH2SiMe3)}] 2. The compound was
synthesized as described for 1 by using 135 mg (0.19 mmol)
of [U(TpMe2)Cl2(CH2SiMe3)] and 9 lL (0.19 mmol) of ace-
tonitrile. Compound 2 (121 mg, 85%) was obtained as a
dark yellow microcrystalline solid (Found: C, 35.2; H, 5.2;
N, 12.8%. UCl2BSiC21H36N7 requires C, 34.3; H, 5.0; N,
˚
lamido ligands, as an average r(N) = 1.21(2) A is obtained for
the ketimide compounds and, as indicated above, an average
13.3%); mmax(film)/cm−1 2540 (B–H), 1605 (C N), 255 (U–Cl);
=
˚
r(N) = 1.17(4) A is obtained for the dialkylamide and alkylamide
kmax(toluene)/nm: 570, 600, 650 (sh), 660 (sh), 680, 840, 935,
995 (sh), 1060 (sh), 1110, 1145 (sh), 1210; dH (300 MHz, C6D6,
Me4Si) 55.1 (2H, CH2SiMe3), 47.5 (3H, CH3CN), 43.7 (1H, H-
4), 40.5 (3H, Me-pz), 18.3 (3H, Me-pz), 11.2 (9H, CH2SiMe3),
−6.7 (6H, Me-pz), −11.2 (2H, H-4), −24.6 (6H, Me-pz).
complexes.
Although the steric coordination number model is approxi-
mate due to a number of limitations,25 particularly the average
nature of the CNS of the ligands (which can be affected
for instance by the presence of agostic interactions like in
compounds 3 and 4), we think that it is reliable enough to show
that we are in the presence of significant differences among the
several structurally characterized An(IV) complexes containing
ketimide ligands, that may be not steric in origin.
[U(TpMe2)Cl2(NEt2)] 3. A solution of LiNEt2 (23 mg,
0.29 mmol) in toluene was slowly added to a suspension of
[U(TpMe2)Cl3(THF)] (206 mg, 0.29 mmol) in the same solvent.
The mixture was stirred for 3 h during which time the light-green
suspension became bright green. The solution was centrifuged
and the toluene solution was concentrated under reduced
pressure (5 ml). Addition of n-hexane results in deposition of
3 (81 mg, 42%) as a green powder (Found: C, 34.6; H, 4.8;
N, 14.5%. UCl2BC19H32N7 requires C, 33.6; H, 4.7; N, 14.5%);
mmax(film)/cm−1: 2540 (B–H), 255 (U–Cl); kmax(toluene)/nm: 610,
647, 664, 730, 780, 935, 993, 1070, 1080 (sh), 1120, 1150 (sh),
1275; dH (300 MHz, C6D6, Me4Si): 118.52 (4H, CH2(NEt2)),
74.66 (3H, Me-pz), 47.43 (1H, H-4), 38.98 (6H, CH3(NEt2)),
13.40 (3H, Me-pz), −11.18 (6H, Me-pz), −12.38 (2H, H-4),
−26.44 (6H, Me-pz). Green needles of 3 suitable for X-ray-
diffraction analysis were grown by slow concentration of a THF
solution.
Conclusions
Nitriles insert into the U–C bond of [U(TpMe2)Cl2(CH2SiMe3)]
to give the ketimide complexes [U(TpMe2)Cl2{NC(R)-
(CH2SiMe3)}]. The molecular structure of the compound
with R = Ph revealed a short U–N bond length to the ketimide
ligand that was compared to the U–N bond distances in
amide complexes based on the same ligand arrangement, also
prepared and structurally characterized in this work. A detailed
analysis of the U–N bond lengths in these compounds and of
other known U–N (and Th–N) distances in amide and ketimide
actinide(IV) complexes confirmed the short character of the
U–N bond length in [U(TpMe2)Cl2{NC(Ph)(CH2SiMe3)}] and
showed that the U–N bond distances in some of the U(IV) and
Th(IV) complexes containing ketimide ligands described in the
literature are unexpectedly long.
[U(TpMe2)Cl2(NPh2)] 4. Compound 4 was synthesized by
slow addition of a solution of LiNPh2 (61 mg, 0.35 mmol) in
THF to a solution of [U(TpMe2)Cl3(THF)] (250 mg, 0.35 mmol)
in the same solvent. The green solution turned immediately to
brown–red. After stirring for 2 h the solvent was removed under
vacuum and the resulting solid extracted in toluene to separate
the LiCl. Removal of the solvent gave an orange solid that was
further washed with hexane and vacuum dried to yield 4 (162 mg,
60%) (Found: C, 42.3; H, 4.3; N, 12.1%. UCl2BC27H32N7 requires
C, 41.9; H, 4.1; N, 12.7%); mmax(film)/cm−1: 2540 (B–H), 265 (U–
Cl); kmax(toluene)/nm: 623, 648, 664, 671, 683, 728, 774, 932,
946, 989, 1020, 1060, 1098, 1136, 1164; dH (300 MHz, C6D6,
Me4Si): 43 (3H, Me-pz), 33 (1H, H-4), 21 (4H, H-o), 12.4 (4H,
H-m), 9.6 (3H, Me-pz), 7 (2H, H-p), −6.2 (6H, Me-pz), −10
(2H, H-4), −14.4 (6H, Me-pz). Golden crystals of 4 were grown
by slow concentration of a THF solution.
Experimental
General considerations
All preparations and subsequent manipulations were carried
out using standard Schlenk line and dry-box techniques in an at-
mosphere of dinitrogen. Benzonitrile, acetonitrile, THF, toluene
and n-hexane were dried by standard methods and degassed
prior to use. LiNPh2 and LiNEt2 were synthesized by addition of
n-BuLi to solutions of the amines in n-hexane, at 0 ◦C. Benzene-
d6 was dried over Na and distilled. [U(TpMe2)Cl3(THF)]4,
and [U(TpMe2)Cl2(CH2SiMe3)]6 were prepared as previously
reported. 1H NMR spectra were recorded on a Varian
INOVA-300 spectrometer at 300 MHz. Spectra were referenced
internally using the residual proton resonances relative to
tetramethylsilane (benzene-d6, 7.15 ppm). Carbon, hydrogen
and nitrogen analyses were performed in-house using a EA110
CE Instruments automatic analyser.
X-Ray crystallographic analysis
Crystals were mounted in thin-walled glass capillaries in a
nitrogen-filled glove-box. Data were collected at r.t. on an Enraf-
Nonius CAD4-diffractometer with graphite-monochromatized
˚
Mo-Ka radiation (k = 0.71069 A) in the x–2h scan mode. Data
Synthetic procedures
were corrected27 for Lorentz and polarization effects, and for
absorption by empirical corrections based on W scans.
[U(TpMe2)Cl2{NC(Ph)(CH2SiMe3)}] 1. Benzonitrile (21 lL,
0.20 mmol) was added to a solution of [U(TpMe2)Cl2(CH2SiMe3)]
(141 mg, 0.20 mmol) in toluene. The solution was stirred
for 18 h. The solvent was removed under vacuum to yield 1
(137 mg, 86%) as a dark reddish solid (Found: C, 38.7; H,
5.0; N 12.2%. UCl2BSiC26H38N7 requires C, 39.2; H, 4.8; N,
The structure of 1 was solved using Patterson methods and
successive difference Fourier techniques and refined by full-
matrix least squares refinements on F2 using SHELX-97.28 The
structures of 3 and 4 were solved by direct methods using
SIR9729 and refined by full-matrix least-squares refinements on
F2 using SHELXL-9728 and the winGX software package.30 The
contributions of the hydrogen atoms were included in calculated
positions. The drawings were made with ORTEP-3.31
12.3%); mmax(film)/cm−1 2540 (B–H), 1605 (C N), 260 (U–Cl);
=
kmax(toluene)/nm: 540, 605, 640 (sh), 670 (sh), 680, 840, 880, 940,
1000 (sh), 1040 (sh), 1070 (sh), 1095, 1110, 1150 (sh), 1320, 1580,
D a l t o n T r a n s . , 2 0 0 5 , 3 3 5 3 – 3 3 5 8
3 3 5 7

View Article Online
A summary of the crystallographic data is given in Table 2.
CCDC reference numbers 276941 (1), 276942 (3) and 276943
(4).
See http://dx.doi.org/10.1039/b509229a for crystallographic
data in CIF or other electronic format.
10 J. L. Kiplinger, D. E. Morris, B. L. Scott and C. J. Burns,
Organometallics, 2002, 13, 3073.
11 P. L. Diaconescu and C. C. Cummins, J. Am. Chem. Soc., 2002, 124,
7660.
12 R. E. Cramer, K. Panchanatheswaran and J. W. Gilje, J. Am. Chem.
Soc., 1984, 106, 1853.
13 (a) M. Akita, K. Ohta, Y. Takahashi, S. Hikichi and Y. Moro-oka,
Organometallics, 1997, 16, 4121; (b) A. Carvalho, A. Domingos, P.
Gaspar, N. Marques, A. Pires de Matos and I. Santos, Polyhedron,
1992, 12, 1481; (c) Y. Sun, J. Takats, T. Eberspacher and V. Day,
Inorg. Chim. Acta, 1995, 229, 315.
Acknowledgements
Financial support from Fundac¸a˜o para a Cieˆncia e a Tecnolo-
gia (POCTI/QUI/36015/2000, POCTI/QUI/46179/2002) is
gratefully acknowledged. M. A. A. thanks FCT for a PhD grant
(SFRH/BD/4827/2001).
14 J. March, Advanced Organic Chemistry, McGraw-Hill, New York,
1977, vol. 2, p. 24.
15 T. Straub, W. Frank, G. J. Reiss and M. S. Eisen, J. Chem. Soc.,
Dalton Trans., 1996, 2541.
16 R. G. Peters, B. P. Warner, B. L. Scott and C. J. Burns,
Organometallics, 1999, 18, 2587.
17 B. D. Stubbert, C. L. Stern and T. J. Marks, Organometallics, 2003,
22, 4836.
18 J. G. Reynolds, A. Zalkin, D. H. Templeton, N. Edelstein and L. K.
Templeton, Inorg. Chem., 1976, 15, 2498.
19 P. B. Hitchcock, M. F. Lappert, A. Singh, R. G. Taylor and D. Brown,
J. Chem. Soc., Chem. Commun., 1983, 561.
20 M. A. Antunes, M. Dias, B. Monteiro, A. Domingos, I. C. Santos and
N. Marques, presented in part at the 36th International Conference
on Coordination Chemistry, Merida, Mexico, July, 2004.
21 P. Roussel, N. W. Alcock, R. Boaretto, A. J. Kingsley, I. J. Munslow,
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23 D. M. Barnhart, D. L. Clark, S. K. Grumbine and J. G. Watkin,
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24 R. E. Cramer, U. Engelhardt, K. T. Higa and J. W. Gilje,
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3 3 5 8
D a l t o n T r a n s . , 2 0 0 5 , 3 3 5 3 – 3 3 5 8