Lanthanide(lll) and actinide(lll) complexes [M(BH4) 2(THF)5][BPh4] and [M(BH4) 2(18-crown-6)][BPh4] (M = Nd, Ce, U): synthesis, crystal structure, and density functional theory i

Article abstract of DOI:10.1021/ic801685v

Treatment of [M(BH₄)₃(THF)₃] with NEt ₃HBPh₃ in THF afforded the cationic complexes [M(BH ₄)₂(THF)₅][BPh₄] [M = U (1), Nd (2), Ce (3)] which were transforme

Full text of DOI:10.1021/ic801685v

Inorg. Chem. 2009, 48, 221-230
Lanthanide(III) and Actinide(III) Complexes [M(BH ) (THF) ][BPh ] and
4
2
5
4
[
M(BH ) (18-crown-6)][BPh ] (M ) Nd, Ce, U): Synthesis, Crystal
4
2
4
Structure, and Density Functional Theory Investigation of the Covalent
Contribution to Metal-Borohydride Bonding
†
Th e´ r e` se Arliguie, Lotfi Belkhiri, Salah-Eddine Bouaoud, Pierre Thu e´ ry, Claude Villiers,
Abdou Boucekkine,* and Michel Ephritikhine*
‡
‡
†
†
,§
,†
CEA, IRAMIS, SerVice de Chimie Mol e´ culaire, CNRS URA 331, CEA/Saclay, 91191
Gif-sur-YVette, France, Laboratoire de Chimie Mol e´ culaire (LACMOM), D e´ partement de Chimie,
Facult e´ des Sciences, UniVersit e´ Mentouri de Constantine, BP 325, Route de l’A e´ roport Ain El
Bey, 25017 Constantine, Algeria, and UMR CNRS 6226 Sciences Chimiques de Rennes, UniVersite´
de Rennes 1, Campus de Beaulieu, 35042 Rennes, France
Received September 2, 2008
Treatment of [M(BH
U (1), Nd (2), Ce (3)] which were transformed into [M(BH
presence of 18-crown-6; [U(BH (18-thiacrown-6)][BPh ] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-
thiophene. Compounds 1, 3·C S, 4· THF, 5, and 6· THF exhibit a penta- or hexagonal bipyramidal crystal structure
with the two terdentate borohydride ligands in apical positions; the BH groups in the crystals of 7·C S are in
4 3
) (THF)
3
] with NEt
3
HBPh
4
in THF afforded the cationic complexes [M(BH
4 2
) (THF)
5 4
][BPh ] [M )
4 2
)
(18-crown-6)][BPh ] [M ) U (4), Nd (5), Ce (6)] in the
4
)
4 2
4
4 8
H
4
4 8
H
relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms
bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously
+
reported [M(BH
4
)
2
(THF) ] cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural
5
+
data obtained for [M(BH
)
4 2
(18-crown-6)] (M ) U, Ce) by relativistic density functional theory (DFT) calculations
are indicative of a small shortening of the U· · ·B with respect to the Ce· · ·B distance, which is accompanied by
a lengthening of the U-H bonds and an opening of the H -B-H angle (H ) bridging hydrogen atom of the
ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH
M(III) donation is greater for M ) U than for M ) Ce, as well as the overlap population of the M-H bond, thus
showing a better interaction between the uranium 5f orbitals and the H atoms. The more covalent character of the
B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent
the π bonding interactions between U(III) and the three H atoms with significant 6d and 5f orbital contributions.
These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the
f orbitals.
b
b
b
b
3
η -BH
4
4
f
b
b
b
4
Introduction
borohydride bonds in these complexes is a subject of much
debate. These bonds were considered as mostly ionic, in view
of the high coordination numbers, complying with the
principle of maximum occupancy of the coordination sphere
Borohydride complexes of the f-elements are generally
more stable than those of the d transition metals and are
interesting for studying the structural properties and chemical
1
4
and the clear-cut fluxionality of the BH ligands. The
behavior of the BH
4
ligand. The nature of the metal-
similarity of the structural and dynamic properties of the
borohydride complexes of the f-elements and those with
*
To whom correspondence should be addressed. E-mail: abdou.
0
boucekkine@univ-rennes1.fr (A.B.), michel.ephritikhine@cea.fr (M.E.).
central d ions led to the conclusion that f electrons do not
†
CEA/Saclay.
Universit e´ Mentouri de Constantine.
CNRS-Universit e´ de Rennes 1.
‡
4
participate in the formation of the M-BH bonds. Otherwise,
the covalent character of these compounds, suggested by their
§
1
0.1021/ic801685v CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 1, 2009 221
Published on Web 11/19/2008

Arliguie et al.
9
,10
high volatility and solubility in nonpolar solvents, was related
to the covalent nature of the B-H-M three-center two-
electron bridging bond. According to the isolobality concept,
than the corresponding bonds in the lanthanide analogues.
This shortening is explained by a modest enhancement of
covalence in the actinide versus lanthanide-ligand bonding,
a difference which plays an essential role in the selective
complexation of trivalent 5f over 4f ions, finding a particular
application in the reprocessing of spent nuclear fuel. Since
the bonds between the BH ligand and d transition metals
1
2
3
the borohydride ligand with η , η , or η denticity is closely
analogous to the chloride, allyl, or cyclopentadienyl anion,
11
2
respectively.
3
4
The uranium borohydrides U(BH
4 4 4 3 6 6
) , [U(BH ) (C Me )],
4
or f-elements are generally considered to have a significant
degree of covalent character, it was appealing to determine
if, and by what amount, this feature would be more
pronounced in the uranium than in the lanthanide complexes.
We present herein the synthesis and characterization of the
and, more recently, the lanthanide borohydrides [Ln(BH )
4 3
5
(
THF)
organometallic derivatives resulting from reactions of the
BH groups with anionic reagents or proton acidic substrates.
Protonolysis of the M-BH bond with acidic ammonium
salts was devised as a convenient route to cationic complexes,
as shown by the synthesis of [U(BH (THF) ][BPh ] (1)
]. In view of the often remarkable
performances of cationic complexes in catalysis, the lan-
thanide counterparts of 1, [Ln(BH (THF) ][BPh ] (Ln )
Y, Sm, Nd, La), were very recently isolated from the
protonolysis reaction of [Ln(BH (THF) ] and were found
to efficiently activate the ring-opening polymerization of
3
], proved to be valuable precursors to inorganic and
4
4
uranium compound [U(BH
neodymium (2) and cerium (3) counterparts, including the
crystal structure of 3· C S; we also describe the synthesis
4 2
and crystal structures of the crown-ether derivatives [M(BH ) -
4 2 5 4
) (THF) ][BPh ] (1) and its
4
)
2
5
4
6
4 8
H
4 3 3
from [U(BH ) (THF)
(
[
1
18-crown-6)][BPh
U(BH (18-thiacrown-6)][BPh
,4,7,10,13,16-hexathiacyclooctadecane). Finally, we make
4
] [M ) U (4), Nd (5), Ce (6)], and
4
)
2
5
4
4
)
2
4
] (7) (18-thiacrown-6 )
4
)
3
3
use of relativistic density functional theory (DFT) to study
the electronic structure of 4 and 6, to give, for the first time,
a clear insight into the covalent contribution to the metal-
borohydride bond.
7
ε-caprolactone. Similarly, [Nd(BH
synthesized by protonolysis of [Nd(BH
NMe PhH][B(C ] and was found to be an efficient
4 2 5 6 5 4
) (THF) ][B(C F ) ] was
4
)
3 3
(THF) ] with
[
2
6 5 4
F )
8
Experimental Section
precatalyst for isoprene polymerization.
Comparison of the crystal structures of a variety of
isomorphous and/or isostructural trivalent lanthanide and
uranium complexes showed that, allowing for the variation
in the ionic radii of the metals, the bonds between the 5f-
element and the soft and/or π-accepting ligands are shorter
All reactions were carried out under argon (<5 ppm oxygen or
water) using standard Schlenk-vessel and vacuum line techniques
or in a glovebox. Solvents were dried by standard methods and
(
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222 Inorganic Chemistry, Vol. 48, No. 1, 2009

Lanthanide(III) and Actinide(III) Complexes
Table 1. Crystal Data and Structure Refinement Details
1
4 8
3·C H S
4·THF
5
6·THF
4 8
7·C H S
chemical formula
M/g mol
crystal system
space group
a/Å
C
44
H
68
B
3
O
5
U
C
48
H
76
B
3
O
5
SCe
C
40
H
60
B
3
O
7
U
C
36
H
52
B
3
O
6
Nd
C
40
H
60
B
3
O
7
Ce
40 60 3 7
C H B S U
-
1
947.44
orthorhombic
Pbca
13.0444(7)
25.1790(8)
27.2213(15)
90
937.70
triclinic
P1j
923.34
triclinic
P1j
757.45
monoclinic
P2 /n
14.2979(5)
14.4497(4)
17.7524(6)
90
93.934(2)
90
3659.0(2)
4
825.43
triclinic
P1j
1035.76
monoclinic
P2 /n
1
1
12.4292(3)
14.9752(3)
15.4239(4)
116.434(2)
101.694(3)
100.072(2)
2399.52(12)
2
1.298
1.035
986
12.7029(11)
13.9070(9)
14.0718(11)
83.944(4)
78.912(3)
67.302(4)
2249.2(3)
2
12.6840(3)
13.9061(4)
14.0616(3)
84.128(2)
78.951(3)
67.239(2)
2243.63(10)
2
1.222
1.056
858
17.9685(12)
12.0218(9)
20.6526(14)
90
93.465(4)
90
4453.1(5)
4
1.545
b/Å
c/Å
R/deg
ꢀ/deg
90
90
γ/deg
V/Å3
Z
8940.7(7)
8
1.408
3.671
3832
3
D
calc/g cm-
1.363
3.650
926
1.375
1.461
1564
-
1
µ(Mo KR)/mm
F(000)
4.001
2076
reflections collected
56365
8412
5552
0.108
478
75230
9058
8644
0.052
523
17318
7858
5985
0.083
505
24898
6924
5519
0.061
416
64198
8432
7616
0.061
505
29698
8408
5064
0.072
460
independent reflections
observed reflections [I > 2σ(I)]
R
int
parameters refined
R1
wR2
S
0.051
0.117
1.019
-0.64
2.32
0.060
0.171
1.116
-1.35
1.93
0.060
0.152
1.010
-1.46
2.01
0.031
0.067
1.014
-0.40
0.52
0.049
0.136
1.065
-0.76
2.47
0.061
0.147
0.997
-0.75
1.11
min/e Å-
max/e Å
3
∆F
∆F
-
3
Table 2. Selected Distances (Å) and Angles (deg) in the [M(BH
4
)
2
(THF)
5
]+ Cations
M(BH (THF)
Nd8
B(C
[
4
)
2
5
][X]
Ce
BPh
M
X
Y7
BPh
Sm7
Sm23
Sm(Cp′)(BH
Nd7
BPh
U
BPh
U5
La24
La(BH ) (THF)
4 4 2
4
BPh
4
)
4 3
4
6
F
5
)
4
4
4
U
2
(C
7
H
7
)(BH
4
)
6
M· · ·B(1)
M· · ·B(2)
2.584(3)
2.569(3)
2.427(21)
2.688(5)
2.728(6)
2.440(18)
2.622(8)
2.623(8)
2.485(8)
2.727(3)
2.740(4)
2.459(19)
2.596(4)
2.641(4)
2.51(3)
2.678(6)
2.704(7)
2.534(14)
2.685(9)
2.697(9)
2.544(15) 2.56(2)
2.72(4)
2.71(4)
2.729
2.729
2.567(7)
177.2
〈
M-O〉
B(1)-M-B(2) 178.41(10) 178.97(16) 176.6(3)
178.56(13) 177.67(13) 176.15(19) 177.6(3)
72(3) 72(2) 72(3) 72(2)
176(1)
72(1)
〈
O-M-O〉 72(1) 72(3) 72(1)
72(1)
distilled immediately before use. IR samples were prepared as Nujol
mulls between KBr round cell windows, and the spectra were
recorded on a Perkin-Elmer FT-IR 1725X spectrometer. The H
NMR spectra were recorded on a Bruker DPX 200 instrument and
referenced internally using the residual protio solvent resonances
relative to tetramethylsilane (δ 0); 11B chemical shifts are relative
NMR (pyridine-d
(s, 8 H, Ph), 7.15 (s, 12 H, Ph), 3.64 and 1.56 (m, 2 × 20 H, THF).
B{ H} NMR (pyridine-d ): δ 232.53 (s, BH ), -5.53 (s, BPh ).
5
): δ 77.4 (br s, w1/2 ) 350 Hz, 8 H, BH
4
), 7.72
1
11
1
5
4
4
IR (Nujol): ν/cm- 2425s (B-H stretching), 2218s and 2162s
1
t
(
B-H
were obtained by crystallization from THF.
Synthesis of [Nd(BH (THF) ][BPh ] (2). Following the same
procedure as for 1, [Nd(BH (THF) ] (296.1 mg, 0.73 mmol)
reacted with [NEt H][BPh ] (337.5 mg, 0.80 mmol) to give a pale
violet powder of [Nd(BH (THF) ][BPh ]. Yield: 333.2 mg (71%).
Anal. Calcd for C32 Nd: C, 60.30; H, 6.96; B, 5.09. Found:
C, 59.95; H, 6.89; B, 4.85. H NMR (THF-d
05 Hz, 8 H, BH ), 7.32 (s, 8 H, Ph), 6.78 (s, 8 H, Ph), 6.64 (s 4
H, Ph). H NMR (pyridine-d ): δ 87.2 (br s, w1/2 ) 350 Hz, 8 H,
BH
), 7.73 (m, 20 H, Ph), 3.66 and 1.61 (m, 2 × 4 H, THF).
B{ H} NMR (pyridine-d ): δ 157.42 (s, BH ), -6.60 (s, BPh ).
IR (Nujol): ν/cm- 2436s (B-H
stretching), 2226s and 2168s
B-H stretching). Pale violet crystals of 2 suitable for X-ray
diffraction were obtained by crystallization from THF.
Synthesis of [Ce(BH (THF) ][BPh ] (3). Following the same
procedure as for 1, [Ce(BH (THF) ] (284.0 mg, 0.71 mmol)
reacted with [NEt H][BPh ] (358.1 mg, 0.85 mmol) to give an off-
white powder of [Ce(BH ) (THF) ][BPh ]. Yield: 372.1 mg (74%).
b
stretching). Brown crystals of 1 suitable for X-ray diffraction
to BF
3
2
·Et O as external reference. The spectra were recorded at
)
4 2
5
4
2
3 °C when not otherwise specified. Elemental analyses were
4
)
3
3
performed by Analytische Laboratorien at Lindlar (Germany). 18-
Crown-6 and 18-thiacrown-6 (Fluka) were dried under vacuum
3
4
)
4 2
2
4
5
b
12
13
before use. [M(BH
4
)
3
(THF)
were synthesized as previously reported.
Synthesis of [U(BH (THF) ][BPh ] (1). A flask was charged
with [U(BH (THF) ] (135.9 mg, 0.27 mmol) and [NEt H][BPh
114.9 mg, 0.27 mmol) and THF (30 mL) was condensed in it.
3
] (M ) Nd, Ce, U ) and
44 3 2
H B O
5
b
[
NEt
3
H][BPh
4
]
1
8
): δ 90.1 (br s, w1/2 )
4
)
2
5
4
3
4
4
)
3
3
3
4
]
1
5
(
4
After stirring for 2 h at 20 °C, the brown solution was filtered and
1
1
1
5
4
4
evaporated to dryness, leaving a brown powder of 1 which was
1
1
t
contaminated with NEt
3
·BH
3
, as shown by the H NMR spectra.
(
b
The latter was eliminated after dissolution of the powder in THF
(
20 mL) and evaporation of the solution under vacuum; this
4
)
2
5
4
operation was repeated three times, leading eventually to the
analytically pure product 1. Yield: 237.0 mg (92%). Anal. Calcd
)
4 3
3
3
4
68 3 5
for C44H B O U: C, 55.75; H, 7.18; B, 3.48. Found: C, 55.61; H,
1
4
2
4
4
7
BH
8
.11; B, 3.31. H NMR (THF-d ): δ 9.5 (br s, w1/2 ) 315 Hz, 8 H,
4
1
Anal. Calcd for C H B O Ce: C, 61.80; H, 7.78; B, 4.17. Found:
), 7.68 (s, 8 H, Ph), 6.89 (s, 8 H, Ph), 6.63 (s, 4 H, Ph). H
40 60
3
4
1
C, 61.89; H, 7.85; B, 4.26. H NMR (THF-d
8
): δ 29.3 (br s, w1/2
)
(
12) Roger, M.; Arliguie, T.; Thu e´ ry, P.; Fourmigu e´ , M.; Ephritikhine, M.
Inorg. Chem. 2005, 44, 584.
13) (a) Schlesinger, H. I.; Brown, H. C. J. Am. Chem. Soc. 1953, 75, 219.
b) Baudry, D.; Bulot, E.; Charpin, P.; Ephritikhine, M.; Lance, M.;
Nierlich, M.; Vigner, J. J. Organomet. Chem. 1989, 371, 155.
425 Hz, 8 H, BH
4
), 7.20 (s, 8 H, Ph), 6.79 (s, 8 H, Ph), 6.65 (s, 4
H, Ph). H NMR (pyridine-d ): δ 32.3 (br s, w1/2 ) 365 Hz, 8 H,
BH
), 8.05 (s, 8 H, Ph), 7.25 (s, 12 H, Ph), 3.67 and 1.64 (m, 2 ×
8 H, THF). B{ H} NMR (pyridine-d ): δ 15.34 (s, BH ), -6.49
1
5
(
4
(
1
1
1
5
4
Inorganic Chemistry, Vol. 48, No. 1, 2009 223

Arliguie et al.
1
(
2
s, BPh
170s (B-H
4
). IR (Nujol): ν/cm- 2432s (B-H
t
stretching), 2219s and
S suitable
subsequent Fourier-difference synthesis and refined by full-matrix
2
16
b
stretching). Colorless crystals of 3·C
4
H
8
least-squares on F with SHELXL-97. Absorption effects were
1
7
for X-ray diffraction were obtained from a solution of 3 and 18-
thiacrown-6 in the minimum amount of THF, diluted by tetrahy-
drothiophene (1:7).
corrected empirically with the program DELABS. All non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. The borohydride protons were found on Fourier-difference
maps for all compounds, and the carbon-bound hydrogen atoms
were introduced at calculated positions. All hydrogen atoms were
treated as riding atoms with an isotropic displacement parameter
equal to 1.2 times that of the parent atom. In compounds 4 and 6,
the two THF solvent molecules were given 0.5 occupancy factors
to retain acceptable displacement parameters and/or to account for
their closeness to their image by symmetry, and they were refined
with restraints on bond lengths and displacement parameters (a short
H· · ·H contact involving a THF proton is likely due to the poor
resolution, and resulting imperfect location, of these molecules).
Crystal data and structure refinement parameters are given in
Synthesis of [U(BH
charged with 1 (78.0 mg, 0.082 mmol) and 18-crown-6 (22.0 mg,
.083 mmol) and THF (30 mL) was condensed in it. After stirring
for 2 h at 20 °C, the yellow solution deposited yellow crystals of
. The solvent was evaporated off, and the product was washed
with diethyl ether (10 mL) and dried under vacuum. Yield: 63.5
mg (91%). Anal. Calcd for C36 U: C, 50.79; H, 6.16; B,
4 2 4
) (18-crown-6)][BPh ] (4). A flask was
0
4
52 3 6
H B O
1
3
7
.81. Found: C, 50.53; H, 6.32; B, 3.70. H NMR (pyridine-d
1.1 (br s, w1/2 ) 285 Hz, 8 H, BH ), 8.03 (s, 8 H, Ph), 7.21 (s, 12
5
): δ
4
1
1
1
H, Ph), 4.54 (s, 24 H, 18-crown-6). B{ H} NMR (pyridine-d
5
): δ
). IR (Nujol): ν/cm- 2441s (B-H
stretching), 2206s and 2156s (B-H stretching). Crystals of 4·THF
1
2
75.95 (s, BH
4
), -5.85 (s, BPh
4
t
1
6
b
Table 1. The molecular plots were drawn with SHELXTL.
Computational Details. The calculations were performed using
suitable for X-ray diffraction were obtained by slowly cooling a
hot solution of 1 (7.2 mg, 7.57 µmol) and 18-crown-6 (3.0 mg,
1.3 µmol) in THF (0.4 mL).
Synthesis of [Nd(BH (18-crown-6)][BPh
same procedure as for 4, 2 (85.0 mg, 0.10 mmol) reacted with 18-
crown-6 (26.0 mg, 0.10 mmol) to give a pale blue powder of 5.
1
8
DFT with the ADF2007.01 (Amsterdam Density Functional)
1
8f
1
code. Scalar relativistic effects were introduced within the Zero
19
)
4 2
4
] (5). Following the
Order Regular Approximation (ZORA). The DFT/ZORA method
has been successfully used to investigate both experimental
geometries and electronic structure of heavy f-element compounds
1
9c-e
Yield: 62.2 mg (82%). Anal. Calcd for C36
H
52
B
3
O
6
Nd: C, 57.19;
H, 6.92. Found: C, 57.08; H, 6.93. H NMR (pyridine-d ): δ 125.7
br s, w1/2 ) 450 Hz, 8 H, BH ), 7.82 (s, 8 H, Ph), 7.23 (s, 12 H,
Ph), 0.52 (s, 24 H, 18-crown-6). B{ H} NMR (pyridine-d
with a satisfying accuracy.
Spin-orbit effects were not taken
1
5
into account. Triple-ꢁ Slater-type valence orbitals (STO) augmented
by one set of polarization functions were used for all atoms. The
frozen-core approximation where the core density is obtained from
four-component Dirac-Slater calculations has been applied for all
atoms. The 1s core electrons were frozen for the boron, carbon,
and oxygen atoms during molecular calculations. For heavy
elements, the Ce[4d] and U[5d] valence spaces include the 4f/5s/
5p/5d/6s/6p and 5f/6s/6p/6d/7s/7p shells (11 and 14 valence
(
4
11
1
5
): δ
). IR (Nujol): ν/cm- 2446s (B-H
stretching), 2210s and 2160s (B-H stretching). Crystals of 5
1
2
01.42 (s, BH
4
), -5.67 (s, BPh
4
t
b
suitable for X-ray diffraction were obtained by crystallization from
THF.
Synthesis of [Ce(BH
)
4 2
(18-crown-6)][BPh
same procedure as for 4, [Ce(BH
4
] (6). Following the
] (174.6 mg, 0.22
20a
4 2
)
(THF) ][BPh
4
4
electrons) respectively. The Vosko-Wilk-Nusair functional for
mmol) reacted with 18-crown-6 (98.0 mg, 0.37 mmol) to give an
off-white powder of 6. Yield: 121.5 mg (72%). Anal. Calcd for
the local density approximation (LDA) and the gradient corrections
2
0b-e
for exchange and correlation of Becke and Perdew,
respec-
C
36
H
52
B
3
O
6
Ce: C, 57.40; H, 6.96; B, 4.31. Found: C, 57.21; H,
tively, have been used. Complete ZORA/BP86/TZP geometry
optimization for the ground states of the highest spin-multiplet state
was first carried out and was followed by analytical vibrational
frequencies calculations, to check the nature of the stationary point
(minimum or transition state). Molecular orbital plots and geo-
1
6
8
1
.86; B, 4.07. H NMR (pyridine-d
H, BH ), 7.92 (s, 8 H, Ph), 7.25 (s, 12 H, Ph), 0.65 (s, 24 H,
8-crown-6). IR (Nujol): ν/cm 2445s (B-H
stretching). Crystals of 6·THF suitable for X-ray
diffraction were obtained by crystallization from THF.
Synthesis of [U(BH (18-thiacrown-6)][BPh ] (7). A flask was
5
): δ 54.2 (br s, w1/2 ) 425 Hz,
4
-1
t
stretching), 2210s
and 2162s (B-H
b
1
8f
metries were generated using the ADFview program.
)
4 2
4
Results and Discussion
charged with 1 (63.0 mg, 0.066 mmol) and 18-thiacrown-6 (24.0
mg, 0.066 mmol) in tetrahydrothiophene (30 mL). The reaction
mixture was stirred for 2 h, and the red solution deposited a red
powder of 7 which, after filtration, was dried under vacuum. Yield:
Synthesis of the Complexes. The protonolysis reaction
of a metal-borohydride bond by means of an acidic am-
monium salt represents a convenient access to cationic
complexes which was previously used for the preparation
4
52 3 6
7.7 mg (76%). Anal. Calcd for C36H B S U: C, 45.63; H, 5.53;
S, 20.30. Found: C, 45.87; H, 5.71; S, 19.89. The poor solubility
of 7 in organic solvents prevented the collection of NMR spectra.
(
16) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
17) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
IR (Nujol): ν/cm- 2463s (B-H
1
stretching), 2184s and 2116s
S were picked from the
t
(
(
B-H
b
stretching). Red crystals of 7·C
4
H
8
(18) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41. (b)
Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322. (c) te Velde,
G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84. (d) van Lenthe, E.;
Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996, 105, 6505. (e) te
Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca, G. C.; van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
reaction mixture, before filtration and evaporation to dryness.
Crystallographic Data Collection and Structure Determina-
tion. The data were collected at 100(2) K on a Nonius Kappa-
14
CCD area detector diffractometer using graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). The crystals were introduced
into 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 were processed with
2
2, 931. (f) ADF2007.01, SCM; Theoretical Chemistry, Vrije Uni-
versity: Amsterdam, The Netherlands; http://www.scm.com.
19) (a) Fonseca, G. C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 391. (b) van Lenthe, E.; Ehlers, A.; Baerends, E. J.
J. Chem. Phys. 1999, 110, 8943. (c) Shamov, G. A.; Schreckenbach,
G. J. Phys. Chem. A 2005, 109, 10961. (d) BenYahia, M.; Belkhiri,
L.; Boucekkine, A. J. Mol. Struc. (Theo. Chem.) 2006, 777, 61. (e)
Gaunt, A. J.; Reilly, S. D.; Enriquez, A. E.; Scott, B. J.; Ibers, J. A.;
Sekar, P.; Ingram, K. I. M.; Kaltsoyannis, N.; Neu, M. P. Inorg. Chem.
2008, 47, 29. (f) Ingram, K. I. M.; Tassell, M. J.; Gaunt, A. J.;
Kaltsoyannis, N. Inorg. Chem. 2008, 47, 78524.
(
1
5
HKL2000. The structures were solved by direct methods or by
Patterson map interpretation with SHELXS-97, expanded by
(
(
14) Hooft, R. W. W. COLLECT; Nonius BV: Delft, The Netherlands, 1998.
15) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
224 Inorganic Chemistry, Vol. 48, No. 1, 2009

Lanthanide(III) and Actinide(III) Complexes
23
of [Nd(COT)(THF)
of a cyclooctatetraenyl lanthanide cation, and of [U(COT)-
BH )(THF) ][BPh ] and [U(COT)(L) ][BPh [L ) OPPh
OP(NMe ], which are, respectively, the first cationic
organometallic borohydride and the first organometallic
4
][BPh
4
] (COT ) C
8
H
8
), a first example
of [Sm(BH
(THF) ][La(BH
tively, by partial hydrolysis of [Sm(C
4
)
2
(THF)
5
][Sm(C
5
Me
4
nPr)(BH
2
], which were formed, respec-
4 3 4 2
) ] and [La(BH )
5
24
5
4
4
) (THF)
n
(
4
2
4
3
]
4 2
3
,
5
Me
4
Pr)(BH
4
)
2
)
2 3
(THF)] in toluene and crystallization of [La(BH
from THF.
Treatment of complexes 1-3 with 18-crown-6 in THF led
to the immediate formation of the crown ether derivatives
4 2 4
[M(BH ) (18-crown-6)][BPh ] [M ) U (4), Nd (5), Ce (6)]
which, after evaporation of the solvent and washing with
diethyl ether, were isolated as a yellow (U), pale blue (Nd),
or white (Ce) powder in 91, 82, and 72% yield, respectively.
4 3 3
) (THF) ]
21
dication of an f element. The cationic complexes [M(BH
THF) ][BPh ] [M ) U (1), Nd (2), and Ce (3)] were readily
synthesized by reaction of the neutral precursors
M(BH (THF) ] and [NEt H][BPh ] in THF, according to
)
4 2
(
5
4
[
)
4 3
3
3
4
eq 1. By following a similar procedure, Okuda et al. have
independently isolated 2 and the other derivatives with Ln
7
)
Y, Sm, and La. After elimination of the NEt
3
· BH
3
Crystals of 4·THF, 5, and 6·THF were deposited from THF
+
byproduct by washing with THF or successive evaporations
solutions. The [U(BH
4
)
2
]
cation was previously found
of THF solutions, and drying under vacuum, powders of 1
inserted into the dicyclohexyl-(18-crown-6) ether (dcc) in
the complex [U(BH (dcc)][UCl (BH )] which was obtained
accidentally after partial oxidation of [U (BH (dcc) ] in
dichloromethane.
The IR spectrum of 1, which exhibits the characteristic
(
[
brown), [Nd(BH
4
)
2
(THF)
2
][BPh
4
]
(pale violet), and
)
4 2
5
4
Ce(BH (THF) ][BPh
)
4 2
4
4
] (white) were isolated with yields
3
4
)
9
2
2
5
of 92, 71, and 74%, respectively; these powders were
characterized by their elemental analyses (C, H, B) and their
1
H NMR spectra in THF and pyridine which exhibit, in
4
absorptions of tridentate BH ligands, that is, a strong sharp
-
-1
addition to the resonances of the [BPh
4
] anion, a broad high
band at 2425 cm and two strong bands at 2218 and 2162
-1
field signal attributed to the equivalent BH
4
ligands and the
cm assigned, respectively, to the terminal hydrogen-boron
stretch ν(B-H ) and bridging hydrogen-boron stretch
ν(B-H ), is essentially identical with those of the neody-
peaks corresponding to the free THF molecules which were
displaced from the metal by the solvent. It is likely that the
dissociation of the THF ligands from 2 and 3 under vacuum,
t
b
mium and cerium counterparts 2 and 3; these spectra are
also very similar to those of the 18-crown-6 derivatives 4-6.
Identical borohydride band patterns were observed in the IR
7
which was apparently not observed by Okuda et al., is
related to the formation of zwitterionic complexes with
phenyl groups of BPh
π coordination of tetraphenylborate and related anions to
metal centers is well documented and was observed, for
4
coordinated to the metal atom. Such
4 4
spectra of various pairs of analogous complexes like M(BH )
2
6
27
(M ) Zr, Hf), [M(C
5 5 2 4 2
H ) (BH ) ] (M ) Zr, Hf), or
28
[M(C (BH )] (M ) V, Nb), in line with the very similar
H )
5 5 2
4
example, with the powder of [U(NEt
2 3
)
(η-Ph)
2
BPh
2
] which
3
(THF) ]
geometries and bonding of these compounds; this trend also
indicates that the mass of the metal has insignificant effect
upon the structurally diagnostic vibrations. However, the IR
was isolated after drying the crystals of [U(NEt
2
)
3
2
2
[
4
BPh ] under vacuum.
spectra of [M(C
parison to that of M ) Ti, a marked lowering of ν(B-H
while the vibrations involving the B(H portion of the
5 5 2 4
H ) (BH )] (M ) V, Nb) show, in com-
[
M(BH ) (THF) ] +
4
3
3
b
)
THF
t 2
)
[
NEt H][BPh ]
8 [M(BH ) (THF) ][BPh ] +
4 2 5 4
3
4
complex remain unperturbed. This striking anomaly was
explained by the effects of increasing metal ion distortion
NEt · BH +H
M ) U(1), Nd(2), Ce(3) (1)
Crystals of 1 were obtained by crystallization from THF
while crystals of 3·C S were deposited from a solution
of 3 and 18-thiacrown-6 in a mixture of THF and tetrahy-
drothiophene, in an attempt at the synthesis of [Ce(BH
thiacrown-6)][BPh ] (vide infra). The [U(BH
-
3
3
2
4
of the isolated BH unit and decreasing the ionic character
2
8
of the bonding.
H
4 8
The THF ligands of 1 were not displaced with the hexathia
macrocycle 18-thiacrown-6 in THF but, in tetrahydrothiophene,
4
)
2
(18-
1 was readily transformed into [U(BH
6)][BPh ] (7) which crystallized as the red solvate 7·C
Complex 7 is, after the iodide compounds [MI (9-thiacrown-
2
3)(MeCN) ] (M ) U, La), a novel example of a crown
4
)
2
(18-thiacrown-
+
4 8
H S.
4
4
)
2
(THF)
5
]
4
cation was previously encountered in the crystals of the
inverse cycloheptatrienyl sandwich complex [U(BH
THF) ][(BH U(µ-C )U(BH ] which were obtained
fortuitously during purification of K[(BH U(µ-C
]. In addition to the aforementioned crystals of
3
1
0e
4
)
2
(
5
)
4 3
H
7 7
4
)
3
thioether complex of an f element. Attempts at the synthesis
of the lanthanide analogues of 7 were unsuccessful; in
contrast to 1, complexes 2 and 3 are poorly soluble in
tetrahydrothiophene, and their THF ligands could not be
4
)
3
7 7
H )
6
4 3
U(BH )
7
[Ln(BH
[Nd(BH
[Ln(BH
4
)
2
(THF)
5 4
][BPh ] (Ln ) Y, Sm, Nd), and
8
4
)
2
(THF)
5
][B(C
6
F
5
)
4
], the lanthanide cations
+
4
)
2
(THF)
5
] (Ln ) Sm, La) were found in the crystals
(23) Bonnet, F.; Visseaux, M.; Hafid, A.; Baudry-Barbier, D.; Kubicki,
M. M.; Vigier, E. Inorg. Chem. Commun. 2007, 10, 690.
24) Bel’skii, V. K.; Sobolev, A. N.; Bulychev, B. M.; Alikhanova, T. K.;
(
(
20) (a) Vosko, S. D.; Wilk, L.; Nusair, M. Can. J. Chem. 1990, 58, 1200.
b) Becke, A. D. J. Chem. Phys. 1986, 84, 4524. (c) Becke, A. D.
Phys. ReV. A 1988, 38, 3098. (d) Perdew, J. P. Phys. ReV. B 1986, 33,
Kurbonbekov, A.; Mirsaidov, U. Koord. Khim. 1990, 16, 1693.
(25) Dejean, A.; Charpin, P.; Folcher, G.; Rigny, P.; Navaza, A.; Tsoucaris,
G. Polyhedron 1987, 6, 189.
(26) Marks, T. J.; Kennelly, W. J.; Kolb, J. R.; Shimp, L. Inorg. Chem.
1972, 11, 2540.
(27) Johnson, P. L.; Cohen, S. A.; Marks, T. J.; Williams, J. M. J. Am.
Chem. Soc. 1978, 100, 2709.
(28) Marks, T. J.; Kennelly, W. J. J. Am. Chem. Soc. 1975, 97, 1439.
(
8
822. (e) Perdew, J. P. Phys. ReV. B 1986, 34, 7406.
(21) Cendrowski-Guillaume, S. M.; Lance, M.; Nierlich, M.; Ephritikhine,
M. Organometallics 2000, 19, 3257.
(
22) Berthet, J. C.; Boisson, C.; Lance, M.; Vigner, J.; Nierlich, M.;
Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1995, 3019.
Inorganic Chemistry, Vol. 48, No. 1, 2009 225

Arliguie et al.
4 2 5 4
Figure 1. View of the cation of [U(BH ) (THF) ][BPh ] (1). The hydrogen
atoms have been omitted, except those of the borohydride ligands. The
displacement ellipsoids are drawn at the 30% probability level.
4 2 4
Figure 2. View of the cation of [U(BH ) (18-crown-6)][BPh ] (4). The
hydrogen atoms have been omitted, except those of the borohydride ligands.
The displacement ellipsoids are drawn at the 30% probability level.
exchanged with sulfur-containing molecules. This difference
could reflect the softer (less hard) character of the 5f versus
the phosphorus complexes [M(C
5
H
4
Me)
3
(L)] [M ) Ce or
and in the tris(btp)
(M ) La, Ce, Sm, or U; btp ) 2,6-
4
f trivalent ion, leading to a better affinity of soft ligands
for the actinide. It is during these attempts that crystals of
·C S were obtained.
Crystal Structures. The [M(BH
10a
U; L ) PMe
compounds [M(btp)
dialkyl-1,2,4-triazin-3-yl)pyridine), and ∆ ) 0.2 Å in the
9k
3
or P(OCH
2 3
) CEt]
]I
3 3
3
H
4 8
9d
]+ cations of
4 2
) (THF)
5
5 5 2
terpyridine compounds [M(C Me ) (terpy)]I (M ) Ce, U).
complexes 1 and 3 adopt a pentagonal bipyramidal confi-
guration with the oxygen atoms of the THF ligands in the
equatorial plane and the borohydride ligands in apical
positions. This structure is also that adopted by a series of
These greatest deviations were explained by the softer
character and better π-accepting ability of the phosphorus-
and nitrogen-containing ligands. However, examining the
bond length variations in the present series appears incon-
clusive when the (3σ confidence interval is taken into
+
29
[
LnX
of the uranium complex 1 is shown in Figure 1; selected
bond lengths and angles in the cations of 1 and 3· C
are listed in Table 2, together with those of the previously
2 5
(THF) ] cations (X ) Cl or I). A view of the cation
account. Okuda et al. already noted that the Sm · · ·B distances
4 8
H S
7
in [Sm(BH
the cation of [Sm(BH
difference is even larger between the Nd · · · B distances of
4 2
) (THF)
5
][BPh
4
] differ from those measured in
n
23
4 2 5 5 4 4 3
) (THF) ][Sm(C Me Pr)(BH ) ]; the
+
7
7,23
7,8
reported [M(BH
4 2
) (THF)
5
] cations (M ) Y, Sm,
La, U ) for comparison. The M-O distances are unex-
ceptional, and the short M · · ·B distances indicate a tridentate
Nd,
2
4
5
7
8
[
Nd(BH
also be seen that the difference of -0.03 Å between the
Nd· · · B and Ce · · · B distances of [Ln(BH (THF) ][BPh
Ln ) Nd, Ce) is not of the expected sign. These discre-
4
)
2
(THF)
5
][X] with X ) BPh
4 6 5 4
or B(C F ) . It can
4
ligation mode of the BH ligands, in keeping with the
positions found for the hydrogen atoms.
4
)
2
5
4
]
(
Variations in the strength of M-X bonds in analogous
uranium(III) and lanthanide(III) complexes (X ) C, N, P, I,
S), showing the more covalent character of the U-X bond,
have previously been detected through the deviations ∆
corresponding to the differences [〈U-X〉 - 〈Ln-X〉] and
pancies can be explained by the compounds being not
isostructural, the counteranions being not identical, and the
crystal data having been collected under different experi-
mental conditions.
Contrary to the structures of the [M(BH
4
)
2
(THF)
5
]+
3
+
3+
3+
[
r(U ) - r(Ln )], r(M ) being the ionic radius of the
30
+
cations, those of the [M(BH
4 2
) (18-crown-6)] cations (M )
metal. These deviations are generally small and hardly
U, Nd, Ce), and especially the uranium and cerium deriva-
tives 4· THF and 6· THF, which are isomorphous, would
permit a rigorous comparison of their geometrical parameters.
A view of the cation of 4 is shown in Figure 2 and selected
bond lengths and angles in complexes 4-6 are listed in Table
3. The metal center is in a slightly distorted hexagonal
bipyramidal environment, with the six oxygen atoms of the
significant (0.02-0.05 Å), but they are as high as 0.1 Å in
(
29) (a) Willey, G. R.; Woodman, T. J.; Carpenter, D. J.; Errington, W.
J. Chem. Soc., Dalton Trans. 1997, 2677. (b) Willey, G. R.; Aris,
D. R.; Errington, W. Inorg. Chim. Acta 2001, 318, 97. (c) Evans, W. J.;
Shreeve, J. L.; Ziller, J. W.; Doedens, R. J. Inorg. Chem. 1995, 34,
576. (d) Willey, G. R.; Meehan, P. R.; Woodman, T. J.; Drew, M. G. B.
Polyhedron 1997, 16, 623. (e) Deacon, G. B.; Feng, T.; Junk, P. C.;
Skelton, B. W.; Sobolev, A. N.; White, A. H. Aust. J. Chem. 1998,
crown ether in the equatorial plane and the BH
4
groups at
5
1, 75. (f) Roesky, P. W. Organometallics 2002, 21, 4756. (g) Anfang,
+
S.; Karl, M.; Faza, N.; Massa, W.; Magull, J.; Dehnicke, K. Z. Anorg.
Allg. Chem. 1997, 623, 1425. (h) Huebner, L.; Kornienko, A.; Emge,
T. J.; Brennan, J. G. Inorg. Chem. 2004, 43, 5659. (i) Xie, Z.; Chiu,
K. Y.; Wu, B.; Mak, T. C. W. Inorg. Chem. 1996, 35, 5957. (j) Izod,
K.; Liddle, S. T.; Clegg, W. Inorg. Chem. 2004, 43, 214.
2
the apexes. Such [LnX (18-crown-6)] cations are, to the
best of our knowledge, limited to X ) NO
crown-6)] [Ln(NO ) ] (Ln ) Nd,
3 3 6
3
in [Ln(NO
3
)
2
(18-
Gd ); the chloride
analogues have an additional ligand in their coordination
3
1a
31b
(
(
30) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
31) (a) B u¨ nzli, J. C. G.; Klein, B.; Wessner, D.; Schenk, K. J.; Chapuis,
G.; Bombieri, G.; De Paoli, G. Inorg. Chim. Acta 1981, 54, L43. (b)
Nicolo, F.; B u¨ nzli, J. C. G.; Chapuis, G. Acta Crystallogr. Sect. C
3
1c
sphere, like [LnCl
of the cations of 4-6 are quite similar to those of the neutral
complexes [M(BH (18-crown-6)] (M ) Sr, Ba) in which
the crown features crystallographically imposed D3d sym-
2 2
(18-crown-6)(H O)]Cl. The structures
4 2
)
1
988, 44, 1733. (c) Rogers, R. D.; Rollins, A. N.; Etzenhouser, R. D.;
Voss, E. J.; Bauer, C. B. Inorg. Chem. 1993, 32, 3451.
226 Inorganic Chemistry, Vol. 48, No. 1, 2009

Lanthanide(III) and Actinide(III) Complexes
Table 3. Selected Distances (Å) and Angles (deg) in the
+
[
M(BH
4
)
2
(18-crown-6)] Cations
[
4
M(BH )
2
(18-crown-6)][BPh
4
]
M
Nd
Ce
U
M· · ·B(1)
M· · ·B(2)
2.652(3)
2.608(3)
2.640(17)
2.43(4)
0.9(1)
2.684(7)
2.692(7)
2.643(14)
2.46(5)
1.15(8)
1.09(5)
175.5(2)
60.2(4)
43(3)
2.650(12)
2.698(12)
2.653(9)
2.57(4)
1.10(5)
1.19(2)
175.7(4)
60.2(6)
41.5(9)
111(3)
〈
〈
〈
〈
M-O〉
M-H
b
〉
B-H
B-H
b
〉
t
〉
1.0(1)
B(1)-M-B(2)
174.27(10)
60.1(4)
36(4)
105(7)
113(5)
92(7)
〈
〈
〈
〈
〈
O-M-O〉
H
H
H
b
b
b
-M-H 〉
b
-B-H
-B-H
b
〉
104(9)
114(3)
88(2)
t
〉
107(2)
83(2)
M-H
b
-B〉
4 2 4
Figure 3. View of the cation of [U(BH ) (18-thiacrown-6)][BPh ] (7). The
hydrogen atoms have been omitted, except those of the borohydride ligands.
The displacement ellipsoids are drawn at the 30% probability level.
3
2
metry. The average M-O distances in 4 and 6 are 0.109(7)
Å larger than those in 1 and 3, respectively, reflecting the
larger steric hindrance in the former compounds, while the
M· · ·B distances are quite identical, showing that the greater
number of equatorial donor atoms has no effect on the apical
positions. In this case also, the values of the estimated
standard deviations do not permit to draw any conclusion
about the possible small variations of the M-O and M· · · B
bond lengths; the differences of +0.01 Å between the U-O
and the Ce-O distances and of -0.02 Å between the U · · ·B
and the Ce · · ·B distances are less than 3 times the standard
deviation of the individual distances.
Table 4. Selected Distances (Å) and Angles (deg) in the
+
[
4
U(BH )
2
(18-thiacrown-6)] Cation
U· · ·B(1)
U· · ·B(2)
U-S(1)
U-S(2)
U-S(3)
U-S(4)
U-S(5)
U-S(6)
2.616(14)
2.605(13)
3.084(3)
3.059(3)
3.064(3)
3.050(3)
3.013(3)
3.101(3)
B(1)-U-B(2)
S(1)-U-S(2)
S(2)-U-S(3)
S(3)-U-S(4)
S(4)-U-S(5)
S(5)-U-S(6)
S(1)-U-S(6)
99.1(5)
65.11(7)
66.00(7)
63.05(7)
68.75(7)
64.69(8)
66.56(8)
Deviation of the geometrical parameters of the coordinated
-
The relationship of the M · · ·B distance to metal ion size
for a number of borohydride complexes was already con-
sidered to discuss what factors govern these distances as the
metal ion and tetrahydroborate ligation geometry are
changed. For each series of compounds with bidentate and
tridentate BH ligands, the overall correlation between M · · ·B
distances and ionic radii was found amazingly linear,
considering the large range of metals with distinct oxidation
states and coordination numbers, the variety of supporting
ligands, and the different methods, temperatures, and phases
for which crystal structures were determined. It was therefore
pointed out that for most complexes, the variations in M · · ·B
distances correspond to those expected from the ionic
bonding model, and no conclusion about covalent bonding
between the metal atom and the borohydride group could
be drawn solely from structural considerations based on the
M· · ·B distance. However, disparities were noted within the
bidentate structures, with M · · ·B distances varying more than
might be expected for comparable ionic radii. In particular,
4 4
BH ligand from the tetrahedral symmetry of the free BH
anion might give information on the degree of covalency of
the metal-ligand bonding. Unfortunately, the position of the
hydrogen atoms in the presence of heavy atoms cannot be
measured with a good accuracy by X-ray diffraction, and
any variation should be regarded with caution and criticism.
33
4
Considering again the pair of [M(C
(M ) Ti, Nb),
5
H
5
)
2
(BH
it was noted that while the Nb · · ·B
distance is smaller than the Ti · · ·B distance, the Nb-H bond
length is larger than the Ti-H bond length, and the
-B-H angles are larger in the niobium compound. In
4
)] complexes
3
4,35
b
b
H
b
b
the present case, the nature of the metal-borohydride bonding
in complexes 4 and 6, being beyond the reach of our
experimental approach, was analyzed by relativistic DFT
(vide infra).
A view of the [U(BH
+
4
)
2
(18-thiacrown-6)] cation of 7 is
presented in Figure 3, and selected bond lengths and angles
are listed in Table 4. The average U · · ·B distance of 2.611(6)
Å of the tridentate borohydride ligands, which are in relative
cis positions, is about 0.07 Å smaller than that of the trans
3
4
the Nb· · ·B distance in [Nb(C
5 5 2 4
H ) (BH )] is 0.11 Å shorter
than the Ti · · ·B distance in the isostructural titanium coun-
4
diaxial BH groups in the bipyramidal complexes 1 and 4.
3
5
3+
terpart, while the ionic radius of Nb is 0.05 Å larger
3
+
30
(36) (a) Hartman, J. R.; Wolf, R. E.; Foxman, B. M.; Cooper, S. R. J. Am.
Chem. Soc. 1983, 105, 131. (b) Wolf, R. E.; Hartman, J. R.; Storey,
J. M. E.; Foxman, B. M.; Cooper, S. R. J. Am. Chem. Soc. 1987, 109,
than that of Ti . This discrepancy was tentatively ex-
plained by the more covalent character of the Nb-BH bond,
in line with the greater volatility of the niobium complex
and the unusual features in its IR spectrum (vide supra).
4
4
328.
(
(
(
37) Junk, P. C.; Atwood, J. L. Supramol. Chem. 1994, 3, 241.
38) Fyles, T. M.; Gandour, R. D. J. Inclusion Phenom. 1992, 12, 313.
39) (a) Hartman, J. R.; Cooper, S. R. J. Am. Chem. Soc. 1986, 108, 1202.
(b) Helm, M. L.; Helton, G. P.; VanDerveer, D. G.; Grant, G. J. Inorg.
Chem. 2005, 44, 5696. (c) Shaw, J. L.; Wolowska, J.; Collison, D.;
Howard, J. A. K.; McInnes, E. J. L.; McMaster, J.; Blake, A. J.;
Wilson, C.; Schr o¨ der, M. J. Am. Chem. Soc. 2006, 128, 13827.
(
32) Bremer, M.; N o¨ th, H.; Thomann, M.; Schmidt, M. Chem. Ber. 1995,
28, 455.
1
(
(
33) Edelstein, N. Inorg. Chem. 1981, 20, 297.
34) Kirilova, N. I.; Gusev, A. I.; Struchkov, Y. T. J. Struct. Chem. 1974,
1
5, 622.
(40) Willey, G. R.; Lakin, M. T.; Alcock, N. W. J. Chem. Soc., Dalton
Trans. 1992, 1339.
(41) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(
35) Melmed, K. M.; Coucouvanis, D.; Lippard, S. J. Inorg. Chem. 1973,
2, 232.
1
Inorganic Chemistry, Vol. 48, No. 1, 2009 227

Arliguie et al.
Table 5. Mean Calculated and Experimental (in Square Brackets) Bond Lengths (Å) and Angles (deg) in the [M(BH
4
)
2
(18-crown-6)]+ Cations
M, spin state
M-O
M-B
M-H
b
B-H
t
B-H
b
M-B-H
b
H
b
-B-H -B-H
t
H
b
b
Ce, doublet
2.727
2.631
2.430
1.200
1.244
67
113
105
[
2.643(14)]
2.734
2.653(9)]
[2.688(6)]
2.583
[2.67(3)]
[2.46(5)]
2.484
[2.57(4)]
[1.09(5)]
1.201
[1.19(2)]
[1.15(8)]
1.248
[1.10(5)]
[66(2)]
71
[73(2)]
[114(3)]
111
[107(2)]
[104(9)]
109
[111(3)]
U, quartet
[
The U-S bond lengths vary from 3.013(3) to 3.101(3) Å
the four other sulfur atoms. This is at variance with the
central position occupied by the metal in a series of
and average 3.06(3) Å, a value which is identical to that of
1
0e
3
.05(3) Å measured in [UI
3
(9-thiacrown-3)(MeCN)
2
]. The
octahedral complexes of general formula [M(18-thiacrown-
n+ 39
1
8-thiacrown-6 ligand adopts a conformation which can be
3
6)] . More similitude is found in the complex [BiCl (18-
characterized by the torsion angle sequence for the six groups
thiacrown-6)], in which the metal ion is located between the
thiacrown and the three chlorine atoms, and is nearly
coplanar with three sulfur atoms.
+
+
+
of S-C, C-C, and C-S bonds, which is (a g a, ac g
+
-
-
-
-
-
-
+
-
-
40
g , ac g a, g g g , a g ac , g g a) where a is anti
(150 to 180°), g gauche (30 to 90° or -30 to -90°) and
(
Molecular Geometry Optimizations. The molecular
+
ac anticlinal (90 to 150° or -90 to -150°). The S-C-C-S
angles are all close to the ideal gauche value (in the range
structures of the [M(BH
4
)
2
(18-crown-6)] cations (M ) Ce
and U) have been fully optimized at the ZORA/BP86/TZP
level (see Computational Details). The highest spin multi-
plicity has been considered for the U(III) complex bearing
5
1.3-66.0°), whereas the C-S-C-C angles span a wide
range, from gauche to anti, with intermediate anticlinal,
values. In contrast with crown ethers, thiacrowns show a
marked tendency to adopt conformations giving the maxi-
mum number of gauche arrangements around C-S bonds
3
the f configuration, that is, the quartet state. The spin state
1
of the Ce(III) species is obviously a doublet (f configuration
of the metal). Such high spin states are well described by
the single determinant configuration Kohn and Sham
(
as a result of the larger length of C-S versus C-O bonds)
1
8
and this often results in exodentate positioning of the sulfur
approach.
atoms; conversely, anti arrangements around C-C bonds are
The most relevant calculated bond distances and angles
are reported in Table 5 together with experimental data, for
comparison; the optimized molecular geometries are dis-
played in Figure 4. These calculations give U · · · B distances
smaller than Ce · · · B, and U-O bond lengths larger than
3
6
favored in thioethers. However, this rule suffers some
3
6,37
exceptions in free 18-thiacrown-6,
with, for example,
+
-
+
+
-
+
+
the sequence (g a g , ac g ac , ac a g )
forms known, and uranium complexation appears to result
2
for one of the
36
in a reversing of these trends in 7 [for comparison, the torsion
Ce-O. When the BH
only two deviations in bond lengths are larger than 0.09 Å,
that is, 0.11 Å for the B-H distance in the cerium complex
and 0.15 Å for the B-H distance in the uranium counterpart;
the deviations in bond angles vary from 0.6 to 2.0°, except
that concerning the H -B-H angle in the U(III) complex,
which amounts to 4.3°. Although the U · · · B distance is
smaller than the Ce · · · B distance, the U-H bond is longer
4
coordination geometry is considered,
+
angles sequence for 18-crown-6 in complexes 4-6 is (a g
-
a, a g a)
3
, which corresponds to the very common D3d
t
3
8
geometry. The conformation adopted in complex 7 results
in the molecule having roughly a saddle shape, with the
diametrically opposite atoms S3 and S6 bound to uranium
in trans positions [S3-U-S6 174.52(8)°]. The metal atom
is thus held on this S3-S6 axis above the basket formed by
b
b
t
b
4 2
Figure 4. DFT geometries, calculated and experimental (in brackets) bond distances (Å) and angles (deg) of the [M(BH )
(18-crown-6)]+ cations (M ) Ce,
U). The crown-ethers have been omitted for clarity.
228 Inorganic Chemistry, Vol. 48, No. 1, 2009

Lanthanide(III) and Actinide(III) Complexes
f3 states of these atoms in the complexes, implying that no
metal-to-ligand back-donation exists at this level. As ex-
q
pected, the Mulliken metallic net charges M for both the
cerium(III) and uranium(III) borohydride complexes are
much smaller than their formal +3 oxidation state. These
electronic features reflect a significant ligand-to-metal dona-
tion. As given by NBO, this trend is more pronounced for
the uranium(III) complex whose metallic net charge, +2.01,
is appreciably smaller than that of the cerium(III) analogue,
Figure 5. Modifications in the BH
4
coordination geometry when passing
from the cerium (black) to the uranium (red) complexes, from DFT analysis.
+
2.42. It must be pointed out that the charges of the oxygen
than the Ce-H
with the opening of the U-B-H
Ce-B-H angle (70.7 versus 66.9°), and the larger
-B-H and smaller H -B-H angles in the uranium
compound. These variations in the geometrical parameters
are represented in Figure 5, and they will be discussed further
in the light of the electronic structure analysis.
b
bond (2.484 versus 2.430 Å), in relation
atoms of the 18-crown-6 ligand are the same in the two
complexes, with a value of -0.62 (NBO). However, the
BH
atoms is greater in the uranium compound, as shown by the
smaller NPA global negative charge of a BH moiety, that
is, -0.65 versus -0.88 in the Ce(III) analogue. The
difference between the total charges of the metals is likely
due to these distinct BH
to the same conclusions. Furthermore, the sum of the three
MPA overlap populations of the M-H bonds is significantly
greater in the uranium(III) complex, 0.107 versus 0.073 for
Ce(III), and presents a good correlation with the opening of
b
with respect to the
b
-
4
f U(III) donation through the bridging H
b
hydrogen
H
b
b
b
t
4
Electronic Structure Calculations. The results of Mul-
liken Population Analysis (MPA) and Natural Bond Orbital
-
4
f M(III) donations. MPA leads
(
NBO) analysis are given in Table 6. Although MPA can
41
b
provide correct global trends, NBO is much more reliable.
MPA spin-unrestricted analysis provides metallic spin densi-
ties F and atomic net charges; the value of F is the
M
M
the H
b b
-B-H angle, thus showing a better directional
difference between the total R and ꢀ electronic populations
of the metal. Table 6 also contains the MPA R+ꢀ spin-
unrestricted overlap populations relative to the metal-ligand
bonding.
interaction between uranium 5f orbitals and the bridging
hydrogen atoms. These results highlight the significant
Ce(III)/U(III) differentiation by the borohydride ligand and
sustain the hypothesis of stronger covalent interactions
between this ligand and the uranium(III) center.
The MPA spin density borne by cerium and uranium, 1.03
1
and 3.11 respectively, correspond to the doublet f and quartet
Figure 6. Spin-unrestricted frontier MO diagrams of the Ce(III) and U(III) complexes in their doublet and quartet states, respectively.
Inorganic Chemistry, Vol. 48, No. 1, 2009 229

Arliguie et al.
Table 6. Mulliken and NBO (NPA) Analysis
metal
ligand
spin dens.
net charge
atom-atom overlap population
]- M-B
M-(H
NPA
[BH
structure
F
M
Mq
Bx
H
t/b
[BH
4
b
)
3
Mq
Bx
4
]-
H
t/b
1
Ce(III) (4f ) 1.03
+1.99 +0.77 -0.29/-0.32 -0.52
+0.73 +0.87 -0.25/-0.28 +0.25
-0.061
-0.116
+0.073
+0.107
+2.42 -0.52 -0.88
+2.01 -0.46 -0.65
+0.08/-0.13
+0.08/-0.09
3
U(III) (5f )
3.11
-
Molecular Orbital (MO) Analysis. The stronger interac-
5 5 3
[M(C Me )(SBT) ] (M ) La, Ce, Nd, U; SBT ) 2-mer-
9n
tion between the borohydride ligands and the uranium(III)
capto benzothiazolate). Furthermore, the splitting of the
5f block highlights the major participation of these orbitals
in the metal-ligand bonding. This synergetic more covalent
interaction is obviously unique to the U(III) system, leading
to the Ce(III)/U(III) differentiation. It is also noteworthy that
the MO diagrams indicate no direct M-B bonding and
confirm the fact that this interaction remains mainly elec-
trostatic as indicated by the NBO boron atomic net charges,
-0.52 and -0.46 in the cerium and uranium complexes,
respectively (Table 6). Moreover MPA indicates an anti-
bonding interaction between the metal and boron atoms, the
M-B overlap population being more negative for the
uranium than for the cerium compound.
3
ion through bridging hydrogen atoms according to an η
coordination mode could be exalted by the ability of valence
5
f orbitals of uranium(III) to participate in covalent bonding,
and this raises the question of the role of 5f orbitals in these
B-H -M three-center two-electron bridging bonds and their
b
influence on the cerium(III)/uranium(III) differentiation. To
address these issues, we have performed a molecular orbital
analysis with a particular emphasis on these B-H -M
b
interactions.
Comparative frontier spin-unrestricted MO diagrams of
the Ce(III) complex and U(III) counterpart in their doublet
and quartet states, respectively, are displayed in Figure 6.
The percentages %(d/f/M/BH
orbital contributions to the MOs, as well as the global
contribution of the metal center and of the two BH ligands.
4
) represent the d and f metal
Conclusion
4
The protonolysis reaction of a M-BH bond by means of
4
the acidic ammonium salt NEt HBPh proved to be an
4
The diagrams show that there are two important sets of
frontier MOs for the Ce(III) and U(III) systems. The highest
occupied molecular orbitals, i.e., singly occupied molecular
orbital (SOMO, MO # 67) for the former and SOMO,
SOMO-1 and SOMO-2 (MOs # 69, 68, and 67) for the latter,
are almost 4f or 5f orbitals. As expected from the Mulliken
analysis, there is no evidence for metal-to-ligand back-
donation effects. The MOs immediately below the SOMOs
appear to be rather different in the two compounds; the
ligand-to-metal donation is present in the uranium(III)
complex and dominates with the MOs # 59, 58, and 56 below
SOMO-2 as shown by the percentage composition %(d/f/
3
efficient route to the cationic complexes [M(BH
[BPh ] from the neutral precursors [M(BH (THF)
U, Nd, Ce), further reaction with 18-crown-6 giving the
adducts [M(BH (18-crown-6)][BPh ]. The DFT analysis of
these compounds showed that the M-BH interaction has a
4
)
2
(THF)
5
]
4
)
4 3
3
] (M )
)
4 2
4
4
more covalent character for M ) U than for M ) Ce. This
Ce(III)/U(III) differentiation revealed that the increase of the
covalent contribution to the metal-borohydride bonding is
accompanied by variations in the coordination geometry of
the BH
4
ligand, that is, the lengthening of the M-H
b
bonds,
the opening of the H
b
-B-H angle, in addition to the
b
U/BH
4
). Indeed, these MOs represent the π bonding interac-
shortening of the M · · · B distance. These structural differ-
ences, which are, however, too small to be within reach of
the X-ray diffraction experiments, are related to the partici-
tions between the central metal and the three bridging
hydrogen atoms with significant 6d and 5f uranium(III)
orbital contributions. On the contrary, these MOs in the
cerium(III) complex exhibit a much less metallic weight with
practically no participation of the 4f orbitals. For example,
pation of the uranium 5f orbitals to the B-H -M three-
b
center two-electron bond, which is much greater than that
of the cerium 4f orbitals.
the (d/f/M/BH
4
) percentages are (0/1.1/1.1/36.7) and (0/6.7/
6
.7/68.5) for the R-MO # 59 in the cerium and uranium
Supporting Information Available: Tables of crystal data,
atomic positions and displacement parameters, anisotropic displace-
ment parameters, and bond lengths and bond angles in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
compounds, respectively. That an increase in f orbital
participation is responsible for the enhancement of covalency
in U-X bonds compared to Ln-X bonds was also recently
demonstrated by DFT analysis of the complexes
1
9e,f
[
M{N(EPR)
2 3
} ] (M ) La, U; E ) O, S, Se, Te)
and
IC801685V
230 Inorganic Chemistry, Vol. 48, No. 1, 2009