The first mixed cyclooctatetraenyl-phospholyl metal complexes. Crystal structure of [U(η-C8H8)(η-C4 Me4P)(BH4)(OC4H8)]

Article abstract of DOI:10.1016/S0022-328X(01)01250-5

Treatment of [U(COT)(BH₄)₂(THF)] (1) or [U(COT)(BH₄)(THF)₂][BPh₄] (2) (COT = η-C₈H₈) with Ktmp (tmp, η-C₄Me₄P) gave [U(COT)(tmp)(BH₄)(THF)] (4),

Full text of DOI:10.1016/S0022-328X(01)01250-5

Journal of Organometallic Chemistry 643–644 (2002) 209–213
www.elsevier.com/locate/jorganchem
The first mixed cyclooctatetraenyl-phospholyl metal complexes.
Crystal structure of [U(h-C8H8)(h-C4Me4P)(BH4)(OC4H8)]
Sophie M. Cendrowski-Guillaume *, Martine Nierlich, Michel Ephritikhine *
Ser6ice de Chimie Mole´culaire, DSM, DRECAM, CNRS URA 331, Bat. 125, CEA Saclay, 91191 Gif-sur-Y6ette, France
Received 29 June 2001; accepted 11 September 2001
Abstract
Treatment of [U(COT)(BH4)2(THF)] (1) or [U(COT)(BH4)(THF)2][BPh4] (2) (COT=h-C8H8) with Ktmp (tmp, h-C4Me4P)
gave [U(COT)(tmp)(BH4)(THF)] (4), the crystal structure of which was determined. Further reaction of Ktmp with 4 afforded the
‘ate’ addition derivative K[U(COT)(tmp)2(BH4)(THF)x]. Complex 4 was transformed into [U(COT)(tmp)(OEt)] (5) in the presence
of NaOEt. The cationic compound [U(COT)(tmp)(HMPA)2][BPh4] (6) (HMPA, hexamethylphosphoramide) was isolated from the
reaction of [U(COT)(HMPA)3][BPh4]2 (3) with Ktmp. The cationic complexes 2 and 3 were recovered upon protonation of 4 and
6 with NEt3HBPh4. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Phospholyl; Cyclooctatetraenyl; Borohydride; Uranium
Comparison of these compounds with their pen-
tamethylcyclopentadienyl analogues showed that the
tmp ligand has a lower electron-donating capacity than
Cp*; in particular, the phospholyl complexes were re-
duced more easily into the corresponding uranium(III)
anions. The tmp ligand was also useful for the synthesis
of polynuclear complexes in which uranium and nickel
atoms are associated via phospholyl bridges in the
m-h1,h5 coordination mode [8].
In this paper, we present a new class of phospholyl
uranium(IV) compounds with [U(COT)(tmp)(BH4)-
(THF)] (COT, h-C8H8; THF, tetrahydrofuran) and its
derivatives. Mixed-ring organoactinide complexes are
rather uncommon [9] and those reported here are the
first ones in which cyclooctatetraenyl and phospholyl
ligands are combined on a single metal center.
1. Introduction
Mathey et al. discovered in the mid 1970s the first
phosphametallocenes, phosphaferrocenes [1] and phos-
phacymantrenes [2] in which a h5-phospholyl ligand
was used as a substitute for the isoelectronic cyclopen-
tadienyl ligand. Since then, a large variety of phospho-
lyl complexes of the main group, d-transition metals
and also lanthanides have been synthesized [3]; these
exhibit unique structural and chemical features, which
result from the aromaticity of the heterocyclic ligand
and the accessibility of the phosphorous lone pair.
In collaboration with the group of F. Mathey, we
have prepared the first phospholyl actinide compounds,
i.e. the bis(tetramethylphospholyl) uranium borohy-
drides [U(tmp)2(BH4)2] and [{U(tmp)2(BH4)}2] (tmp,
2,3,4,5-tetramethylphospholyl) [4]. This family was then
extended to a number of complexes of general formula
[U(tmp)4−nXn] (n=1–3; X=BH4, Cl, alkyl or alkox-
ide) [5–7]; the [U(tmp)3UX] derivatives are unique ex-
amples of tris(phospholyl) metal complexes [7].
2. Results and discussion
Recently, we described the protonolysis reaction of
[U(COT)(BH4)2(THF)] (1) [10] with NEt3HBPh4, which
gave successively the mono- and dication [U(COT)-
(BH4)(THF)2][BPh4] (2) and [U(COT)(HMPA)3]-
[BPh4]2 (3) (HMPA, hexamethylphosphoramide) [11];
these represent the first cationic organometallic borohy-
dride and the first organometallic dication of an
* Corresponding authors. Present address: LCPO, CNRS UMR
5629, ENSCP Bordeaux, 16 Avenue Pey Berland, 33607 Pessac
Cedex, France. Tel.: +33-5-56848488; fax: +33-5-56848487
(S.M.C.-G.); tel.: +33-1-69-086436; fax: +33-1-69-086640 (M.E.).
E-mail
addresses:
guillaume@enscpb.u-bordeaux.fr
(S.M.
Cendrowski-Guillaume), ephri@drecam.cea.fr (M. Ephritikhine).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01250-5

210
S.M. Cendrowski-Guillaume et al. / Journal of Organometallic Chemistry 643–644 (2002) 209–213
as the 3 and 4 positions of the heterocycle likely
resulted from the facile permutation of the THF and
BH4 ligands in the tetrahedral structure of 4 (vide infra)
in relation with the rapid exchange between free and
coordinated THF molecules. This process was slowed
down at low temperature, but the slow-limit spectrum
was not observed. A similar fluxionality was encoun-
tered previously with the bis(phospholyl) compounds
[U(tmp)2(BH4)(THF)] [5]. In the IR spectrum of 4, the
strong singlet at 2417 cm−1 and the broad bands at
2125 and 2074 cm−1 are characteristic of a tridentate
BH4 group [12]. The structure of 4 was confirmed by
X-ray crystallography; a view of the molecule is shown
in Fig. 1 and selected bond distances and angles are
listed in Table 1. The uranium atom is found at the
center of a distorted tetrahedron defined by the O and
B atoms and two rings centroids. The UꢀB distance of
Fig. 1. X-ray crystal structure of 4.
f-element, respectively. Complexes 1–3 are valuable
precursors for the preparation of new derivatives by
reaction with alkali metal salts of anionic ligands.
Treatment of 1 with one equivalent of Ktmp in THF
afforded readily the mixed-ring complex [U(COT)-
(tmp)(BH4)(THF)] (4) according to Eq. (1). After filtra-
tion, the
,
2.67(1) A is intermediate between those of typical tri-
dentate and bidentate borohydride ligands, which are
,
2.55 and 2.80 A, respectively [12]. The BH4 ligand is in
fact not symmetrically oriented, i.e. one of the three
bridging hydrogen atoms is farther from the uranium
,
center (UꢀH(3) 2.772(5) A) than the other two (2.36(5)
,
[U(COT)(BH4)2(THF)]+KtmpT“HF
and 2.47(5) A) and the UꢀBꢀH(4) angle is 167(2)°. A
similar distortion of the tridentate borohydride ligand
was also found in [U(BH4)4] and some of its Lewis base
adducts [13]. The COT ring is planar within 90.027(5)
1
[U(COT)(tmp)(BH4)(THF)]+KBH4
(1)
4
solution was evaporated under vacuum, leaving the
brown powder of [U(COT)(tmp)(BH4)] in 89% yield.
Crystals of 4 were obtained from THF. Complex 4 was
synthesized alternatively in almost quantitative yield by
reaction of 2 with Ktmp (Eq. (2)).
,
A; the UꢀC bond distances are ranging from 2.688(9) to
,
,
2.740(9) A and the average value of 2.72(3) A is at the
upper limit of the range of those found in cyclooctate-
traenyl uranium(IV) compounds, which vary from 2.62
in [U(COT)(SiPr)2]2 [14] and 2.71
A
in
,
,
A
[U(COT)(BH4)(THF)2][BPh4]+KtmpT“HF
[U(COT)(NEt2)(THF)3][BPh4] [15]. The phospholyl lig-
and is pentahapto-bonded to uranium, the tmp ring
2
[U(COT)(tmp)(BH4)(THF)]+KBPh4
(2)
,
being planar within 90.015(5) A with the carbon
4
1
atoms of the methyl groups lying out of this plane by
The H-NMR spectrum of 4 in THF-d8 exhibits, in
addition to the resonances assigned to the BH4 and
COT ligands, two signals of equal intensities which
correspond to the a- and b-methyl groups of the tmp
ligand. The equivalence of the 2 and 5 positions, as well
,
an average distance of 0.2(1) A. The UꢀP bond length
,
is 2.970(8) A and the mean UꢀC(ring) distance is
,
2.93(3) A; these values are similar to those of 2.927(4)
and 2.90(7) A, determined in [U(tmp)3Cl] [7].
,
Table 1
,
Selected bond distances (A) and bond angles (°) in 4
Bond distances
UꢀC(1)
UꢀC(4)
UꢀC(7)
UꢀC(10)
UꢀP
2.740(9)
2.704(9)
2.726(8)
2.892(8)
2.970(8)
2.36(5)
UꢀC(2)
UꢀC(5)
UꢀC(8)
UꢀC(11)
UꢀO
2.724(9)
2.688(9)
2.730(8)
2.937(8)
2.528(6)
2.772(5)
2.610(8)
UꢀC(3)
UꢀC(6)
UꢀC(9)
UꢀC(12)
UꢀH(1)
UꢀB
2.711(9)
2.732(8)
2.923(9)
2.904(8)
2.47(5)
UꢀH(2)
UꢀH(3)
2.672(10)
UꢀCOT a
2.013(8)
Uꢀtmp a
Bond angles
OꢀUꢀB
BꢀUꢀCOT
82.2(3)
119.4(3)
OꢀUꢀCOT
BꢀUꢀtmp
112.8(3)
96.6(3)
OꢀUꢀtmp
COTꢀUꢀtmp
96.8(3)
135.6(3)
a COT and tmp are the centroids of the COT and tmp rings, respectively.

S.M. Cendrowski-Guillaume et al. / Journal of Organometallic Chemistry 643–644 (2002) 209–213
211
Attempts to prepare new derivatives from 4 have met
a moderate success. Reaction of 4 with Ktmp in THF
did not afford the substitution product [U(COT)(tmp)2]
but gave, as frequently observed in f-element chemistry
[16], the ‘ate’ addition complex K[U(COT)(tmp)2(BH4)-
(THF)x]. This latter was not stable in solution, being
slowly decomposed into unidentified species, and was
NEt3HBPh4 was found to give back the cationic com-
plexes 2 and 3.
Further studies will focus on the reduction of 4 and
6, which would be facilitated by the electron-accepting
properties of the phospholyl ligand. The expected ura-
nium(III) complexes and the corresponding mixed
COT–Cp* derivatives will be compared with their
neodymium counterparts [18] in order to get further
information about the differentiation of analogous
trivalent lanthanide and actinide compounds.
1
characterized only by its H-NMR spectra. The signals
of the phospholyl rings were not visible at 20 °C reveal-
ing, as in the case of 4, the facile intramolecular rear-
rangement of the ligands; however, the slow-limit
spectrum showing eight singlets corresponding to the
inequivalent methyl groups of the tmp ligands was
observed at low temperature. The alkoxide compound
[U(COT)(tmp)(OEt)] (5) was isolated as an orange
powder in 69% yield from the reaction of 4 with NaOEt
in THF. However, similar treatment of 4 with NaStBu
led to the formation of Na[U(COT)(StBu)3] [14] as the
only product identified.
While protonation of borohydride ligands proved to
be an efficient route to cationic compounds [11,17],
reaction of 4 with NEt3HBPh4 did not afford the
cationic phospholyl derivative [U(COT)(tmp)(THF)x]+,
but gave instead the cationic borohydride 2, according
4. Experimental
4.1. General procedures
All reactions were carried out under argon using
standard Schlenk-vessel and vacuum-line techniques or
in a glove box. Solvents were dried and deoxygenated
by standard methods and distilled before use. Deuter-
ated solvents were dried over Na–K alloy. Elemental
analyses were performed by Analytische Laboratorien
at Lindlar (Germany). The 1H-NMR spectra were
recorded on a Bruker DPX-200 instrument and were
referenced internally using the residual protio solvent
resonances relative to Me4Si (l 0). IR spectra were
obtained on a Perkin–Elmer 1725X spectrometer. The
salt NEt3HBPh4 precipitated by mixing NEt3HCl and
NaBPh4 in water; Ktmp [5], [U(COT)(BH4)2(THF)]
[10], [U(COT)(BH4)(THF)2][BPh4] and [U(COT)-
(HMPA)3][BPh4]2 [11] were prepared as described previ-
ously.
to Eq. (3).
THF
[U(COT)(tmp)(BH4)(THF)]+NEt3HBPh4 “
4
[U(COT)(BH4)(THF)2][BPh4]+NEt3+’tmpH’
(3)
2
The cationic mixed cyclooctatetraenyl-phospholyl
compound [U(COT)(tmp)(HMPA)2][BPh4] (6) was,
however, synthesized by treating the dication 3 with
one equivalent of Ktmp in THF, as depicted by Eq. (4).
After 3 h at 20 °C, the solution
4.2. Synthesis of [U(COT)(tmp)(BH4)(THF)] (4)
[U(COT)(HMPA)3[BPh4]2+KtmpT“HF
3
4.2.1. From 1
[U(COT)(tmp)(HMPA)2][BPh4]+KBPh4
(4)
A flask was charged with 1 (466 mg, 1.05 mmol) and
Ktmp (187 mg, 1.05 mmol) and THF (40 ml) was
condensed in. After stirring for 3 h at 20 °C, the dark
brown solution was filtered and the solvent evaporated
off. The brown powder of [U(COT)(tmp)(BH4)] was
obtained after drying under dynamic vacuum overnight
(464 mg, 89%). Anal. Found: C, 38.69; H, 4.84; P, 6.21.
Calc. for C16H24BPU: C, 38.71; H, 4.84; P, 6.25%.
1H-NMR (THF-d8, 20 °C): l 39.7 (4 H, q, J=85 Hz,
BH4), 11.40 (6 H, s, Me), 8.00 (6 H, s, Me), −33.27 (8
H, s, COT). Coalescence of the signals corresponding
to the methyl groups of the tmp ligand occurred at
−45 °C but the slow-limit spectrum was not observed.
IR (Nujol): 2417 (s), 2125, 2074 (br) cm−1. Crystals of
4 suitable for XRD analysis were obtained from a THF
solution.
6
was filtered and evaporated, and the brown powder of
6 was recovered in 81% yield after crystallization from
THF–pentane. Similar to reaction (Eq. (3)), treatment
of 6 with NEt3HBPh4 gave back the dication 3 (65%
yield by NMR) with other unidentified products. Such
formation of cationic complexes by protonation of
phospholyl precursors is, to our knowledge,
unprecedented.
3. Conclusion
The first mixed cyclooctatetraenyl-phospholyl metal
compounds [U(COT)(tmp)(BH4)(THF)] (4) and
[U(COT)(tmp)(HMPA)2][BPh4] (6) were synthesized by
addition of Ktmp to the cationic precursors [U(COT)-
(BH4)(THF)2][BPh4] (2) and [U(COT)(HMPA)3][BPh4]2
(3), respectively. The new reaction of the phospholyl
ligands of 4 and 6 with the proton acidic reagent
4.2.2. From 2
An NMR tube was charged with 2 (5.7 mg, 6.96
mmol) and Ktmp (1.5 mg, 8.42 mml) in THF-d8 (0.3 ml).

212
S.M. Cendrowski-Guillaume et al. / Journal of Organometallic Chemistry 643–644 (2002) 209–213
The spectrum showed the immediate and quantitative
formation of 4.
the brown solution was filtered and evaporated to
dryness. The brown powder of 6 was crystallized from
THF–pentane, filtered off and dried under vacuum
(504 mg, 81%). Anal. Found: C, 54.10; H, 6.49; P, 7.84.
Calc. for C52H76BN6O2P3U: C, 53.89; H, 6.56; P,
8.03%. 1H-NMR (THF-d8, 20 °C): l 6.96 (8 H, br,
o-Ph), 6.63 (8 H, t, J=7 Hz, m-Ph), 6.56 (4 H, t, J=7
Hz, p-Ph), −1.24 (36 H, d, J=9 Hz, HMPA), −5.82
(6 H, s, Me), −8.54 (6 H, d, J=6 Hz, Me), −32.60 (8
H, s, COT).
4.3. Reaction of 4 with Ktmp
An NMR tube was charged with 4 (7.1 mg, 12.4
mmol) and Ktmp (2.2 mg, 12.4 mml) in THF-d8 (0.3 ml).
After a few minutes at 20 °C, the color of the solution
changed from dark brown to dark orange and the
spectrum
showed
the
formation
of
K[U(-
COT)(tmp)2(BH4)(THF)]: l (20 °C) 26.3 (4 H, br, w1/
2=300 Hz, BH4), −33.96 (8 H, s, COT); l (−93 °C)
9.8 (4 H, br, w1/2=500 Hz, BH4), −53.27 (8 H, s,
COT), 76.75, 35.22, −1.79, −4.61, −30.18, −34.38,
−37.08, −38.69 (8×3 H, s, Me). This compound,
which was also obtained by treating 1 or 2 with two
equivalents of Ktmp, was not stable in solution. Encap-
sulating the potassium cation into 18-crown-6 ether did
not improve the stability of the complex.
4.8. Reaction of 6 with NEt3HBPh4
An NMR tube was charged with 6 (7.1 mg, 6.1 mmol)
and NEt3HBPh4 (2.6 mg, 6.1 mmol) in THF-d8 (0.3 ml).
After 1.5 h at 20 °C, an orange precipitate was ob-
tained and the spectrum of the brown solution showed
the formation of unidentified products. The precipitate
1
was identified as 3 by its H-NMR spectrum in pyri-
4.4. Synthesis of [U(COT)(tmp)(OEt)] (5)
dine-d5 (65% yield).
A flask was charged with 4 (215.8 mg, 0.38 mmol)
and NaOEt (26.5 mg, 0.39 mmol) and THF (40 ml) was
condensed in. The mixture was stirred for 3 h at 20 °C;
the dark orange solution was filtered and the solvent
evaporated off. The residue was extracted with toluene
(20 ml) and the red–orange powder of 5 was obtained
after evaporation and drying under vacuum (141 mg,
69%). Anal. Found: C, 41.22; H, 4.97; P, 5.93. Calc. for
C18H25OPU: C, 41.06; H, 4.75; P, 5.89%. 1H-NMR
(THF-d8, 20 °C): l 131.5 (2 H, s, CH2), 45.88 (3 H, s,
Me), 14.98 (6 H, s, Me), −12.44 (6 H, s, Me), −33.14
(8 H, s, COT).
4.9. X-ray crystal structure of 4
Selected single crystals were introduced into thin-
walled Lindemann glass tubes in the glove box. Data
were recorded on a Nonius Kappa-CCD area-detector
diffractometer using graphite-monochromatized Mo–
Ka radiation. The crystal-to-detector distance was set to
28 mm and the unit cell was determined from all the
reflections measured on ten frames with b-rotation
steps of 2°. A 180° b-range was scanned during data
recording (90 frames, b-rotation=2°, exposure time=
30 s by frame). The data were corrected for Lorentz
polarization and absorption [19] effects and processed
with the HKL package [20]. The structure was solved by
the heavy atom method with SHELXS-86 [21] and refined
on F2 with SHELXTL [22] with anisotropic thermal
parameters for all non-H atoms. H atoms of the BH4
ligand were found in the last Fourier difference map
and refined with constrained BꢀH and HꢀH distances
and isotropic displacement parameter equal to 1.2 times
that of the B atom. The other H atoms were introduced
at calculated positions as riding atoms with an isotropic
displacement parameter equal to 1.2 (CH, CH2) times
that of the parent atom.
4.5. Reaction of 4 with NaStBu
An NMR tube was charged with 4 (4.4 mg, 7.8 mmol)
and NaStBu (10.8 mg, 9.7 mml) in THF-d8 (0.3 ml).
After 30 min at 20 °C, the spectrum of the orange
solution showed the formation of Na[U(COT)(StBu)3]
[14] (ca. 25% yield) with other unidentified products.
4.6. Reaction of 4 with NEt3HBPh4
An NMR tube was charged with 4 (6.1 mg, 10.7
mmol) and NEt3HBPh4 (4.5 mg, 10.7 mml) in THF-d8
(0.3 ml). After 1 h at 20 °C, the spectrum of the orange
solution showed the formation of 2 (65%) along with 4
(23%) and 1 (12%).
Crystal data for 4: formula C20H32BOPU, crystal
dimensions 0.20×0.20×0.20 mm, monoclinic, space
group P21/n, a=8.668(2), b=20.501(4), c=11.955(2)
3
,
,
A, i=109.27(3)°, V=2005.4(7) A , Z=4, zcalc.
=
4.7. Synthesis of [U(COT)(tmp)(HMPA)2][BPh4] (6)
1.882 g cm−3, 6.2B2qB41.6°, v(Mo–Ka)=8.178
cm−1, F(000)=1088, T=123 K. Reflections (19 225)
collected, 2045 unique, 1584 with I\2|(I), R=0.028,
A flask was charged with 3 (811.7 mg, 0.53 mmol)
and Ktmp (93.0 mg, 0.52 mmol) and THF (40 ml) was
condensed in. The mixture was stirred for 3 h at 20 °C;
wR=0.058. Difference Fourier analysis showed no
peaks beyond 0.571 or −0.955 e A
−3
,
.

S.M. Cendrowski-Guillaume et al. / Journal of Organometallic Chemistry 643–644 (2002) 209–213
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Director, CCDC, 12 Union Road, Cambridge CB2
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