Bent thorocene complexes with the cyanide, azide and hydride ligands

Article abstract of DOI:10.1039/c3cc42763c

Reaction of the linear thorocene with NC^-, N₃ ^- and H^- led to the bent derivatives [(Cot) ₂Th(X)]^- (X = CN, N₃) and the bimetallic [{(Cot)₂Th}₂(μ-H)]^-<

Full text of DOI:10.1039/c3cc42763c

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Bent thorocene complexes with the cyanide, azide and
hydride ligands†
Cite this: DOI: 10.1039/c3cc42763c
´
´
Alexandre Herve, Nicolas Garin, Pierre Thuery, Michel Ephritikhine and
Received 15th April 2013,
Accepted 24th May 2013
Jean-Claude Berthet*
DOI: 10.1039/c3cc42763c
www.rsc.org/chemcomm
Reaction of the linear thorocene with NCÀ, N3 and HÀ led to the physico-chemical properties from 1. The greater sensitivity of 2 to
bent derivatives [(Cot)2Th(X)]À (X = CN, N3) and the bimetallic solvolysis and its reputed more ionic Th–Cot bonding even
[{(Cot)2Th}2(l-H)]À, whereas only [(Cot)2U(CN)]À could be formed caused initial doubt about the structural similarity of 1 and 2.
À
from (Cot)2U.
The recent characterisation of the cyanido complex,
[(Cot)2U(CN)][NR4],13 demonstrated for the first time the possi-
bility for (Cot)2U to coordinate a ligand with a change in
geometry from linear to bent. After the cyanide ion, a strongly
coordinating ligand with adapted linear shape, it was of inter-
est to consider other anions and to compare their behaviour
towards uranocene and thorocene.
Here we report the distinct reactivity of (Cot)2U and (Cot)2Th
with a variety of sodium salts Na*X (X = CNÀ, N3À, HÀ; Na* =
Na(18-crown-6)), and the facile formation with thorium of
mono or bimetallic products with unusual open sandwich
structure (Scheme 1).
Apart from the ubiquitous cyclopentadienyl ligands, the Cot
dianion (Cot = Z-C8H8) and its substituted derivatives have been
particularly considered within the f-block since the discovery
of uranocene, (Cot)2U (1).1 Its report, after initial theoretical
prediction,2 gave a formidable impetus to the coordination
chemistry of the f-elements.3 Indeed, in addition to be viewed
as the ferrocene analogue for the large f-elements, it was expected
to open new chemical and theoretical vistas.3 This enthusiasm,
partly related to the ideal overlapping of the C8 ring with the
f-orbitals of these large ions should offer a better understanding
of the metalf–ligand bonding and of the chemical specificities
of the f-elements, and prompted formation of many (Cot)2Mf
(Mf = Th, Pa, Np, Pu, Ce),3–5 and [(Z8-C8HnR8Àn)2Mf]À (Mf = Ln,
U, Th, Pu)1b,6,7 sandwich complexes with unique D8h symmetry.
In contrast to the [(C8H8)2Ln]À anions which behave as ionic
salts of (C8H8)2À and show facile ligand mobility,8 (Cot)2Mf
(Mf = Ce, An) complexes of the Mf4+ cations are much less ionic and
1 is by far the most stable of them.1b,9,10 Its remarkable stability
compared to the Ce4+ and Th4+ analogues was theoretically
explained by the greater covalency due to the larger involvement
of 5f and 6d orbitals in the uranium–ligand bonding.4,10,11
The great stability of 1 was experimentally evidenced by a very
poor reactivity, and its recurrent crystallization in the D8h sym-
metry whatever the coordinating solvent. This inability to coordi-
nate ligand was also explained by the strong U–Cot bonding and
the inaccessibility of the metal centre embedded between the two
Cot ligands. The thought that 1 cannot adopt a bent configu-
ration insidiously widened to the other (Cot)2An species despite
the thorium analogue (Cot)2Th (2)12 was reported to have distinct
Addition of excess NaCN into a 1 : 1 solution of 18-crown-6 and 1
in pyridine gave, after 48 h at 90 1C, a dark-green solution from
which the starting material (Cot)2U crystallized as emerald green
platelets. The 1H NMR spectrum showed however a U–Cot signal at
d À32.4, shifted downfield in comparison with that of 1 (d À37.7),
and attributed to [(Cot)2U(CN)][Na*] (3),13 the formation of which
required harsher conditions than that of [(Cot)2U(CN)][NR4].13
Similar treatment of (Cot)2Th gave a yellow-orange suspension after
1
20 h at room temperature (3 h in the H NMR experiment), from
which [(Cot)2Th(CN)][Na*] (4) could be isolated in pure form after
evaporation of the solvent and extraction in THF. Crystallization in
CEA, IRAMIS, SIS2M, CNRS UMR 3299, CEA/Saclay, 91191 Gif-sur-Yvette, France.
E-mail: jean-claude.berthet@cea.fr
† Electronic supplementary information (ESI) available: Experimental procedure.
CCDC 918802–918805. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc42763c
Scheme 1 Synthesis of the bent actinocene complexes 3–6.
c
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hot THF gave crystals suitable for X-ray diffraction,‡ demonstrating
that 4 has the expected bent sandwich geometry. In pyridine,
1
complex 4 showed a H NMR signal at d +6.60, shifted downfield
relative to the d +6.28 resonance of 2. Its IR spectrum displays a
strong n(CN) stretching frequency at 2108 cmÀ1, a value larger than
2073 cmÀ1 for [(Cot)2U(CN)][NR4],13 but close to the values found
for bridging cyanide.14 The easy synthesis of 4 incited Àus to
compare the behaviour of 1 and 2 toward the anions N3 and
HÀ, which have a suitable size and display weaker coordinating
properties than cyanide. (Cot)2U neither reacted with Na*N3 in THF
or pyridine, nor in the presence of Na*H in THF; only crystals of 1
could be recovered from the green solution. In the latter case, a
broad 1H NMR Cot–U signal at d À35.8, shifted from that of 1, was
observed from the lightly coloured solution. It might be related to
the rapid equilibrium between 1 and its reduced counterpart, since
no U–H signal could be detected between +360 and À60 ppm.
The reaction of 2 with Na*X (X = N3À, HÀ) followed a very
distinct course, affording products that evidence the greater ability
of thorocene to trap anions, in line with the lesser covalent Cot–Th
bond character which favours the mobility of Cot around the metal
centre.9 The azido complex [(Cot)2Th(N3)][Na*] (5) was thus easily
obtained by refluxing 2 and NaN3 in pyridine, in the presence of
1 equiv. of crown ether. After 24 h, the resulting orange suspension
was evaporated to dryness and 5 was obtained pure with a yield of
62% (not optimized). The IR spectrum revealed the presence of
the N3 group with a characteristic intense nasym(N3) band at
2075 cmÀ1, which is in the range of values reported for a series
of uranium azides.15 Slow diffusion of Et2O into a pyridine
solution of 5 gave yellow crystals suitable for X-ray diffraction,
which revealed the monometallic structure of the complex.
Treatment of 2 with 6 equiv. of NaH and 1 equiv. of 18-crown-6
in THF was attempted to prepare either a hydride or the Th(III)
species [(Cot)2Th]À. After 24 h at 20 1C, a yellow-green powder
precipitated from the solution, and was separated from excess NaH
Fig. 1 Views of complexes 4 (left) and 5 (right) with 50% probability displacement
ellipsoids. Hydrogen atoms are omitted. Selected bond lengths (Å) and angles (1):
for 4: Th–C1 2.648(4), C1–N1 1.157(6), Na–N1 2.365(4), ThÁ Á ÁCg 2.09 and 2.10,
CgÁ Á ÁThÁ Á ÁCg 150. For 5: Th–N1 2.518(4), N1–N2 1.179(5), N2–N3 1.159(5),
Na–N3 2.484(4), ThÁ Á ÁCg 2.10, CgÁ Á ÁThÁ Á ÁCg 149, N1–N2–N3 178.3(5).
Views of 4 and 5 are shown in Fig. 1 and the anion
[{(Cot)2Th}2(m-H)]À of 6[Na*(THF)2] is represented in Fig. 2.2
These are unique examples of anionic bent actinocenes, with
[(Cot)2U(CN)][NR4].13 The CNÀ ligand in 4 is best refined with a
Th–C rather than a Th–N linkage, in line with all the uranium
cyanides,1,13,14,15c,19 but in contrast to the only other thorium
cyanide (tBu3C5H2)2Th(OSiMe)3(NC).20 Complex 5 is the first
structurally characterized thorium azide. The structural differences
between the anions [(Cot)2Th(X)]À (CN, N3) and [(Cot)2U(CN)]À
arise from the interaction of the sodium atom, bound to the crown
ether, with the terminal nitrogen atom of the cyanide or the
1,3-coordinated azide. The structure of [{(Cot)2Th}2(m-H)]À resembles
that of [(C5Me5)2ThH2]2,17 and comprises a HÀ anion located
on a two-fold axis of symmetry in 6[Na*(THF)2]; the [Na*(THF)2]
counter-cation, which also admits two-fold symmetry, bridges
two identical bent (Cot)2Th fragments which interlock in an
almost perpendicular position. The Cot ring centroids around
the ThÁ Á ÁTh core are at the apexes of a distorted tetrahedron
and the two planes containing the Th and H1 atoms and the
two ring centroids in each (Cot)2Th fragment intersect with
dihedral angles of 83.71 in 6[Na*(THF)2] and 89.81 in 6[K*].
The carbon atoms of the C8 rings in 4–6 are planar with a
rms deviation of 0.050 Å at most. The CgÁ Á ÁThÁ Á ÁCg angles
(Cg = ring centroid) significantly deviate from linearity, with values
of 1501, 1491 and 148–1491 in 4, 5 and 6, respectively, and can
be compared with the angle found in [(Cot)2U(CN)]À (1531).13
1
by extraction in THF. The H NMR spectrum in THF showed the
Cot and hydride resonances at d 6.30 and d 13.4, respectively, in a
32 : 1 ratio, suggesting the formation of the unexpected bimetallic
hydride [{(Cot)2Th}2(m-H)]À, which was confirmed by X-ray crystallo-
graphy. Compound [{(Cot)2Th}2(m-H)] [Na*(THF)] (6[Na*(THF)]) was
isolated pure in good yield (85%). Its strong reactivity was
evidenced by the immediate release of H2 in the presence of
HNEt3BPh4 in THF, to give back 2 whereas it proved to be unstable
in pyridine, splitting into 2 and an unidentified (Cot)2Th derivative
1
in the 1 : 1 ratio. The H NMR spectrum in pyridine revealed the
presence of one residual THF molecule in the analytical sample. By
analogy with the (RCp)3U series,16 attempts to get the monometallic
hydride [(Cot)2Th(H)][Na*] by addition of excess NaH and 18-crown-6
in refluxing THF were unsuccessful. The Th–H resonance of the
anion 6 is shifted upfield relative to the signal at d 19.2 of the dimer
[(C5Me5)2ThH2]217 and is within the range of d 12.81–15.4 found for
a series of (RnC5H5Àn)3ThH complexes.18 Interestingly, in some
attempts at the formation of [(Cot)2ThIII][K*] by reduction of 2 with
a mixture of KC8 and 18-crown-6 in THF or toluene, reproducible
crystallisation of the hydride [{(Cot)2Th}2(m-H)][K*] (6[K*]) was
observed. In the absence of THF on K*+, the K atom interacts with
CQC bonds in the two Cot ligands (shortest KÁÁÁC distances
3.068(4) and 3.119(4) Å) (see ESI†).2
Fig. 2 View of the anion of [{(Cot)2Th}2(m-H)][Na*(THF)2], 6[Na*(THF)2], with
30% probability displacement ellipsoids. Hydrogen atoms are omitted, except
for H1. Symmetry code: i = 1 À x, Ày, z. Selected bond lengths (Å) and angles (1):
Th–H1 2.323(7), ThÁ Á ÁCg 2.09 and 2.10, CgÁ Á ÁThÁ Á ÁCg 148.
c
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a = 15.2646(3), c = 22.1864(8) Å, V = 5169.6(3) Å3, Z = 4. Refinement of
287 parameters on 7863 independent reflections out of 123 984 measured
reflections (Rint = 0.037) led to R1 = 0.031, wR2 = 0.061, Drmin = À1.62,
Drmax = 0.50 e Å–3, Flack parameter 0.491(7). Crystal data for 6[K*]:
C44H57KO6Th2, M = 1185.08, monoclinic, space group C2/c, a =
37.0796(18), b = 9.8234(3), c = 25.6897(13) Å, b = 118.992(2)1, V =
8184.8(6) Å3, Z = 8. Refinement of 478 parameters on 12 469 independent
reflections out of 148 463 measured reflections (Rint = 0.072) led to
These angles are quite similar denoting a weak steric pressure
in the hydride 6.
The mean Th–C(Cot) distances in the two 11-coordinate com-
plexes 4 and 5 are identical and equal to 2.78(2) Å, a value larger
than 2.701(4) Å in the linear 10-coordinate thorocene.12 This
difference reflects the distinct charges and coordination numbers
of the complexes. These distances are also similar to those reported
in some mono-Cot compounds,21 such as [(Cot)ThCl2(THF)2]
(2.72(2) Å)21a or [(Cot)Th(N{SiMe3}2)2] (2.75(2) Å),21b and can be
compared with the average U–C(Cot) distance in [(Cot)2U(CN)]À
(2.73(2) Å), the latter smaller distance being due to the ionic
radius of U4+ being smaller than that of Th4+ by ca. 0.05 Å.22 The
Th–C(Cot) distances in 6 are quite similar to those in 4 and 5,
averaging 2.77(3) Å in 6[Na*(THF)2] and 2.79(3) Å in 6[K*]. All the
ThÁ Á ÁCg distances are in the narrow range of 2.09–2.12 Å.
The Th–Ccyanide distance of 2.648(4) Å is close to 2.626(4) Å in
[(Cot)2U(CN)]À, whereas the Th–Nazide bond length of 2.518(4) Å
can be compared with U–Nazide distances which span the range
2.219(6)–2.564(12) Å15b,c and to the Th–NC bond length of
2.454(4) Å in (tBu3C5H2)2Th(OSiMe)3(NC).20
R1 = 0.036, wR2 = 0.053, Drmin = À1.41, Drmax = 0.93 e ÅÀ3
.
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in 6[K*]) can be compared to values reported in the literature, which
vary within a large range depending on whether the hydride is
terminal or bridging [1.99(5)–2.6(1) Å]. The present values are
however close to those in (C5Me5)3ThH (2.33(13) Å)18b and in
[(C5Me5)2ThH2]2 [2.03(1) (terminal); 2.29(3) (bridging) Å].17
In summary, comparison of the reactivity of the actinocenes
(Cot)2An towards the addition of a variety of anions evidences
distinct chemical behaviour between Th and U. Both 1 and 2 trap
the cyanide ion, but only thorocene reacts with the weaker ligands
´
´
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N3 and HÀ. The monometallic complexes [(Cot)2Th(X)]À are
À
´
13 J. C. Berthet, P. Thuery and M. Ephritikhine, Organometallics, 2008,
27, 1664.
obtained with CNÀ and N3À, while an unexpected bimetallic
complex is formed with HÀ. All these species show a unique
bent thorocene fragment. These results clearly dismiss the
long-held belief that actinocenes are poorly reactive species
and are devoid of any coordinating ability, and they demon-
strate that at least one attainable coordination site remains on
the actinide centre. That thorocene is much more reactive than
uranocene is also clearly evidenced here despite the lower Lewis
basicity of the Th4+ ion.23 This may originate from the reputedly
less covalent Cot–Th bonding which favours the mobility of
Cot2À on the metal centre and makes easier the bending of the
(Cot)2Th fragment upon interaction of a ligand.
´
´
14 J. Maynadie, J. C. Berthet, P. Thuery and M. Ephritikhine, Organo-
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This work was financially supported by CEA, CNRS and the
´
RBPCH program of the Direction de l’Energie Nucleaire (CEA).
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R. A. Andersen, Organometallics, 2005, 24, 4251; (b) J. Maynadie,
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26, 5485; (c) J. C. Berthet, P. Thuery and M. Ephritikhine, Chem.
‡ Crystallographic data. Crystal data for 4: C29H40NNaO6Th, M = 753.65,
Commun., 2007, 604 and references therein.
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13.4858(4) Å, V = 2927.20(16) Å3, Z = 4. Refinement of 344 parameters on 21 (a) C. Le Vanda, J. P. Solar and A. Streitwieser, J. Am. Chem. Soc., 1980,
8618 independent reflections out of 68 974 measured reflections (Rint
=
,
102, 2128; (b) T. M. Gilbert, R. R. Ryan and A. P. Sattelberger, Organo-
0.022) led to R1 = 0.027, wR2 = 0.064, Drmin = À1.07, Drmax = 0.95 e ÅÀ3
metallics, 1988, 7, 2514; (c) T. R. Boussie, R. M. Moore, A. Streitwieser,
´
Flack parameter 0.487(7). Crystal data for 5: C28H40N3NaO6Th, M =
769.66, monoclinic, space group P21/c, a = 12.7844(4), b = 19.3693(6),
c = 13.0254(4) Å, b = 115.214(2)1, V = 2918.11(16) Å3, Z = 4. Refinement
of 352 parameters on 8909 independent reflections out of 123 806
measured reflections (Rint = 0.034) led to R1 = 0.035, wR2 = 0.091,
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Drmin = À1.43, Drmax = 2.70 e ÅÀ3. Crystal data for 6[Na*(THF)2]: 22 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.
%
C52H73NaO8Th2,
M
=
1313.17, tetragonal, space group P421c, 23 D. D. Schnaars and R. E. Wilson, Inorg. Chem., 2012, 51, 9481.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.