Synthesis and reactivity of new niobocene hydride-stibine and hydride-stilbene complexes. X-ray crystal structure of [Nb(η5- C5H4SiMe3)2(H)(trans -η2-C,C-PhCH=CHPh)]

Article abstract of DOI:10.1021/om301171j

The synthesis of a stable hydride triphenylstibine derivative, Nb(η⁵-C₅H₄SiMe₃) ₂(H)(SbPh₃) (1), has been achieved through the formation of the transient coordinatively unsaturated 16-electron species [Nb(η⁵-C₅H₄SiMe₃) ₂(H)] by thermolytic loss of H₂ from Nb(η⁵- C₅H₄SiMe₃)₂(H)₃ (2) followed by the coordination of a triphenylstibine ligand. Low-temperature protonation of 1 with a slight excess of CF₃COOD led to the η²-dihydrogen complex 3 as the monodeuterated H-D isotopomers of [Nb(η⁵-C₅H₄SiMe₃) ₂(η²-HD)(SbPh₃)]⁺CF ₃CO₂^- (3-d₁). When the temperature was increased to room temperature, complex 3 was converted into the transoid dihydride [Nb(η⁵-C₅H₄SiMe₃) ₂(H)₂(SbPh₃)]CF₃CO₂ (4). Surprisingly, when CF₃COOH in deuterated solvents was used in the protonation process, the final η²-dihydrogen product always contained a substantial amount of deuterated isotopomers. Furthermore, a new hydrido olefin niobocene complex, [Nb(η⁵-(C₅H ₄SiMe₃)₂(H)(trans-η²-C,C-PhCH= CHPh)] (5), can be synthesized by elimination of the stibine ligand from 1 followed by the addition of cis or trans stilbene. Complex 5 can also be obtained by reaction of 2 in the presence of cis or trans stilbene by the thermolytic loss of H₂. All of these compounds were characterized by IR and multinuclear NMR spectroscopy, and the molecular structure of 5 was determined by single-crystal X-ray diffraction.

Full text of DOI:10.1021/om301171j

Article
pubs.acs.org/Organometallics
Synthesis and Reactivity of New Niobocene Hydride-Stibine and
Hydride-Stilbene Complexes. X‑ray Crystal Structure of
[Nb(η5‑C5H4SiMe3)2(H)(trans-η2‑C,C-PhCHCHPh)]
Antonio Antinolo,*,† Juan-Carlos Perez-Flores,† Maria Jesus Hervas,† Santiago García-Yuste,*,†
́
́
́
̃
Maria Isabel Lopez-Solera,† Antonio Otero,*,† Ana Rodríguez Fernandez-Pacheco,‡
́
́
and Maria Teresa Tercero-Morales†
†Departamento de Química Inorgan
Castilla-La Mancha, Avenida Camilo Jose
́
ica, Organ
́
ica y Bioquímica, Facultad de Ciencias y Tecnologías Químicas, Universidad de
́
Cela, 10, 13071-Ciudad Real, Spain
‡ETS Ingenieros Industriales, Universidad de Castilla-La Mancha, Avenida Camilo Jose
́
Cela, 3, 13071-Ciudad Real, Spain
S
* Supporting Information
ABSTRACT: The synthesis of a stable hydride triphenylstibine
derivative, Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1), has been achieved
through the formation of the transient coordinatively unsaturated
16-electron species [Nb(η5-C5H4SiMe3)2(H)] by thermolytic loss of
H2 from Nb(η5-C5H4SiMe3)2(H)3 (2) followed by the coordination
of a triphenylstibine ligand. Low-temperature protonation of 1 with
a slight excess of CF3COOD led to the η2-dihydrogen complex 3 as
the monodeuterated H−D isotopomers of [Nb(η5-C5H4SiMe3)2(η2-
HD)(SbPh3)]+CF3CO2 (3-d1). When the temperature was
−
increased to room temperature, complex 3 was converted into the
transoid dihydride [Nb(η5-C5H4SiMe3)2(H)2(SbPh3)]CF3CO2 (4). Surprisingly, when CF3COOH in deuterated solvents was
used in the protonation process, the final η2-dihydrogen product always contained a substantial amount of deuterated
isotopomers. Furthermore, a new hydrido olefin niobocene complex, [Nb(η5-(C5H4SiMe3)2(H)(trans-η2-C,C-PhCHCHPh)]
(5), can be synthesized by elimination of the stibine ligand from 1 followed by the addition of cis or trans stilbene. Complex 5 can
also be obtained by reaction of 2 in the presence of cis or trans stilbene by the thermolytic loss of H2. All of these compounds
were characterized by IR and multinuclear NMR spectroscopy, and the molecular structure of 5 was determined by single-crystal
X-ray diffraction.
theoretical characterization of η2-dihydrogen complexes,
INTRODUCTION
■
namely, [Nb(η5-C5H4SiMe3)2(HD)(L)]+CF3CO2 (L = phos-
−
Transition metal hydride derivatives are thought to be involved
in a wide variety of organic transformations.1 In a seminal work,
Kubas and co-workers reported2 a new family of hydride
complexes, the dihydrogen derivatives, in which the dihydrogen
molecule is coordinated to a metal center. Currently, the study
of new dihydrogen transition metal complexes [M(η2-H2)] has
experienced an increase in interest, as demonstrated by the
continued appearance of numerous research papers and review
articles regarding topics such as the correlation of structure/
NMR spectroscopic properties,3 aqueous chemistry,4 H2
activation processes involving transition metal complexes,5
and the relevance of these compounds to H2 bioconversion in
hydrogenases and hydrogen storage.6,7
phine, phosphite, isocyanide, or carbonyl).9 All of these
compounds represent good examples of elongated dihydrogen
1
complexes, in agreement with their H NMR-calculated H−H
distances.10
Moreover, after the preparation of the first hydride-olefin
niobocene [Nb(η5-C5H5)2(H)(η2-C2H4)] by Tebbe and
Parshall,11 two alternative processes, namely, the reaction of
metallocene dihalides and trihydrides (M = Nb, Ta) with
various alkylmagnesium halides and olefins, respectively, have
been employed to prepare this class of complex. Bercaw12 and
Green13 and co-workers have investigated in depth the
mechanism for the formation of these compounds. A few
years later, several of us reported an investigation into this kind
of complex that improved our understanding of the mechanistic
and reactivity behavior.14 Surprisingly, in all cases only α-olefins
have been employed, probably because Bercaw and co-
Protonation of metal hydride complexes is generally
recognized as the most common method to prepare [M(η2-
H2)] complexes.6 In this field elegant computational studies
have been reported by Lledos
́
and co-workers,8 and they have
elucidated the ways in which the protonation takes place
between a hydride complex and an acidic species.
One of the longstanding areas of interest within our research
group has concerned the preparation and spectroscopic and
Received: December 4, 2012
Published: January 31, 2013
© 2013 American Chemical Society
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workers12a in 1985 reported that attempts to prepare complexes
of sterically demanding olefins resulted in intractable oils.
The aim of the work reported here was twofold: (i) to
prepare a new hydride-niobocene derivative that incorporates
Sb(C6H5)3 as an ancillary ligand and to study the protonation
process that led to the isolation of a new example of
dihydrogen-niobocene complexes and (ii) to study the
synthesis of new olefin-hydride-niobocene derivatives in
which the olefin has a sterically demanding character.
Nb(η5‐C5H4SiMe3)2(H)(SbPh3)
(1)
+CF3COOH
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Nb(η5‐C5H4SiMe3)(η2‐H2)(SbPh3)]CFCOO
3
(3)
(2)
This process was carried out by addition of a slight excess of
CF3COOH to an acetone-d6 solution of complex 1 at 193 K in
an NMR tube.
The 1H NMR spectra show a broad nonsymmetrical
dihydrogen resonance at 228 K at ca. −4.7 ppm and a broad
line width, which is explained by the rapid dipolar relaxation of
these nuclei. It is well known that in metallocene complexes
with two different substituents in the equatorial plane, namely,
triphenylstibine and dihydrogen ligands, the two hydrogen
atoms might appear to be chemically inequivalent if the
rotation can be frozen.9 Decoalescence was observed on
lowering the temperature, and at 223 K two high-field
resonances were present at 4.7 and 5.4 ppm. On decreasing
the temperature to 193 K, the high-field resonance observed at
−5.4 ppm had a 1:1:1 triplet pattern. On increasing the
temperature to 273 K, the broad signal split into a singlet and a
1:1:1 triplet (see Figure 1, I).
RESULTS AND DISCUSSION
■
Preparation of a Hydride-Stibine Niobocene Com-
plex. The complex Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1) was
readily prepared in high yield (95%) by thermal treatment in
THF of Nb(η5-C5H4SiMe3)2(H)3 (2) in the presence of
triphenylstibine (eq 1). In the preparation of a hydride-stibine
complex, the initial step would involve the formation of the
coordinatively unsaturated 16-electron species [Nb(η5-
C5H4SiMe3)2(H)] by thermolytic loss of H2 followed by
coordination of the triphenylstibine ligand (see eq 1). The
reaction is similar to that previously reported9 for the
preparation of several families of niobocene complexes,
Nb(η5-C5H4SiMe3)2(H)(L) (L = π-acid ligand). Complex 1
was isolated as an air-sensitive, red, oily material of
spectroscopic purity, and it is, as far as we know, the first
example of an organometallic niobocene complex with
triphenylstibine as a ligand.15 In addition, the stability of
complex 1 shows an extraordinary solvent dependence; for
example, at room temperature in acetone the complex evolves
to an intractable mixture of products. This chemical behavior is
in contrast with that found for other analogous hydride
niobocene derivatives [Nb(η5-C5H4SiMe3)2(H)(L)] (L = π-
acid ligand) reported by us.9a
Nb(η5‐C5H4SiMe3)2(H)3 + SbPh3
(2)
−H2/Δ
⎯⎯⎯⎯⎯⎯→⎯ Nb(η5‐C5H4SiMe3)2(H)(SbPh3)
(1)
(1)
Complex 1 was characterized by IR and multinuclear NMR
spectroscopy. A stretching mode, ν(Nb−H), was detected by
infrared spectroscopy for this complex at ca. 1700 cm−1. The
1H NMR spectrum, in C6D6, shows a high-field resonance at ca.
−8.08 ppm for the hydride ligand and four multiplets,
corresponding to an ABCD spin system, for the cyclo-
pentadienyl rings, a finding consistent with an asymmetrical
environment for the niobium center (see Experimental
Section).
Figure 1. Variable-temperature 1H NMR spectra of 3 and 3-d1
(acetone-d6) at high field.
We assume that the protonation of 1 gives rise to complex 3
as the major isotopomer together with the monodeuterated
isotopomers 3-d1.
Preparation and Spectroscopic Characterization of a
η2-Dihydrogen-Stibine Niobocene Complex. Low-temper-
ature protonation of neutral hydride complexes to give cationic
dihydrogen species was reported for the first time by Crabtree
and co-workers in 1985.16 Following this method, several
cationic dihydrogen niobocene complexes, [Nb(η5-
C5H4SiMe3)2(η2-H2)(L)]+ (L = phosphine, phosphite, iso-
cyanide), have been prepared by some of us9 by protonation at
low temperature of the corresponding neutral hydride
niobium(III) complexes.
This fact was confirmed by adding CF3COOD to an acetone-
1
d6 solution of 1 at 228 K in an NMR tube. The H NMR
spectrum showed a broad resonance at ca. −4.7 ppm, and on
increasing the temperature to 273 K, a 1:1:1 triplet with a
1
coupling J(H−D) = 20.7 Hz was observed (see Table 1); this
value is consistent with the formation of an H−D bond and
provides compelling evidence for the presence of a dihydrogen
complex in 32−6 (see Figure 1, II). On lowering the
temperature to 193 K, decoalescence of the signal was observed
Complex [Nb(η5-C5H4SiMe3)2(H)(Sb(C6H5)3)] (1) was
easily protonated at low temperature to give the corresponding
dihydrogen-containing complex [Nb(η5-C5H4SiMe3)2(η2-H2)-
(Sb(C6H5)3)]+ (3), as a major isotopomer, according to eq 2.
and the high-field H NMR spectrum displayed two triplets
1
(1:1:1) centered at ca. δ −5.0 ppm, probably due to the
existence of two different rotamers: endo 3a-d1 and exo 3b-d1
1
(Scheme 1). Both isotopomers show a similar JHD coupling of
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Article
rH−H = 1.42 − 0.0167JHD
Table 1
(3)
JHD
rH−H
b
ΔG
In this way, it is possible to calculate a value of rH−H = 1.074
Å for complex 3, and this lies within the range of distances of
other dihydrogen ligands calculated by this method; see Table
1.
After the low-temperature protonation of complex 1 the
temperature was raised to room temperature, at which point
the high-field resonance at −5.0 ppm began to disappear and a
new resonance appeared at −2.22 ppm. This fact can be
explained by the transformation of complex 3 into the transoid
isomer [Nb(η5-C5H4SiMe3)2(H)2(Sb(C6H5)3)]+ (4) through
an irreversible process (see Scheme 2).
a
complex
(Hz)
(Å)
(kcal/mol)
(HD) gas
43.22
30.0
26.4
20.7
18.2
15.0
0.698
0.919
0.979
1.074
1.116
1.169
[Nb(η2-HD)(CNR)]CF3COO9c
8.4−9.1
9d
[{Nb}′(η2-HD)(CO)]BF4
c
[{Nb}(η2-HD)(Sb(Ph)3)]CF3COO
10.8
11.0
11.0
[{Nb}(η2-HD)(PHPh2)]CF3COO9b
[{Nb}](η2-HD)(PMePh2)]
CF3COO9d
a
b
{Nb}′ = Nb(η5-C5H5)2; {Nb} = Nb(η5-C5H4SiMe3)2. Calculated
using rHH = 1.42 − 0.0167JHD. In this article.
c
Complex 4 also gives rise to spectroscopic data that are
consistent with a symmetrical disposition of the niobium
center, with signals for an A2B2 spin system observed in the 1H
NMR spectrum for the cyclopentadienyl ligands.9b
The formation of complex 4 could be considered to be the
result of an oxidative addition of the coordinated dihydrogen
molecule in 3 to give a cisoid dihydride derivative (not
observed) that transforms rapidly into the thermodynamically
more stable transoid isomer.
ca. 20.7 Hz, in agreement with the presence of H−D molecules
coordinated [M-(η2-HD)]9 to the niobium center.
Scheme 1
When the reaction between 1 and CF3COOH in toluene was
performed in a Schlenk tube, initially at low temperature and
then with warming to room temperature in a slow process, the
only isolated product was the transoid dihydride 4.
Deuterated species 3-d1 was observed as a minor isotopomer
when the low-temperature protonation of 1 was carried out
with CF3COOH in the deuterated solvent CD3COCD3. The
formation of this species could be explained by an H/D isotope
exchange process. Dihydrogen metal complexes have been used
as catalysts for H/D exchange between protic solvents and D2
These rotamers are present due to the fact that the rotation
of the coordinated HD molecule is frozen on the NMR time
scale at this temperature.9 Coalescence is reached at 228 K, and
at this temperature or above, it is not possible to distinguish
between the two isotopomers, because when the temperature is
raised, the rotamers endo H 3a and exo H 3b interconvert by
rotation of the HD molecule, so at 273 K a single resonance
(1:1:1 triplet) is observed at ca. −5.0 ppm.
due to their Bronsted acidity and the rapid and reversible
̈
dissociation of H2.18a In contrast, very few examples of H/D
exchange between η2-H2 complexes and aprotic solvents have
been reported, and a mechanistic interpretation has never been
proposed.18b,c As mentioned above, complex 3 in CD3COCD3
rapidly undergoes an H/D interchange at low temperature. The
isotope exchange reactions are very fast and initially produce
the monodeuterated isotopomer 3-d1, as shown by the
presence of high-field resonances with H−D coupling (see
above). Nevertheless, the formation of other isotopomers must
also be considered, although the bideuterated isotopomer 3-d2
The above phenomenon allowed us to estimate the free
energy of activation of the internal rotation at the coalescence
temperature (ΔG# = 10.8 kcal/(mol K)). Similar values have
been observed previously for the related complexes9 [Nb(η5-
C5H4SiMe3)2(η2-H2)(PMe2Ph)]CF3COO (6) (ΔG# = 11 kcal/
(mol K))9d and [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]CF3COO
(7) (ΔG# = 8.29 kcal/(mol K)).9c As a consequence of this
relatively high barrier, rotation of the HD molecule can be
blocked on the NMR time scale. The value of the 1JHD coupling
constant (20.7 Hz) is in excellent agreement with the presence
2
was not studied by H NMR spectroscopy. The exchange
reaction is a fast process because it was observed even at 193 K.
It seems that different mechanisms can operate in this
process. In the mechanism proposed in Scheme 3, the cationic
η2-H2 complex regenerates the terminal hydride by protonation
of the deuterated acetone solvent (step a), which then
protonates a terminal hydride with deuterium from the terminal
D to give the η2-HD complex and the enol (step b), which
quickly isomerizes to form acetone again (step c). Each step
1
of a stretched dihydrogen ligand (as a comparison, the JHD
value for the HD gas molecule is 43.2 Hz).17 An effective
method to ascertain the approximate distance between the
1
hydrogen atoms of a dihydrogen ligand is through the JHD
value, using eq 3:17
Scheme 2
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Scheme 3
Scheme 4
proposed in Scheme 3 has been observed independently in
previous studies.18,19
Finally, one cannot rule out the formation of the
perdeuterated isotopomer [Nb(η5-C5H4SiMe3)2(η2-D2)-
(SbPh3)]CF3COO (3-d2) under the same reactions conditions
by a second H/D isotope exchange following an analogous
mechanism to that described above.
In an effort to prevent the formation of the monodeuterated
isotopomer derivative, we decided to use CD3CN as solvent in
the protonation reaction at low temperature. Surprisingly, a
rapid proton/deuterium exchange also took place even when
working at a temperature close to the melting point of
acetonitrile.
The H/D exchange in acetone-d6 or acetonitrile-d3 did not
happen with other related niobium complexes,10 and we are
therefore convinced of the crucial role of the stibine ligand in
this process; in this sense some precedent for deuterium
exchange with a phosphine ligand in an iron dihydrogen
complex has been reported.20
Preparation of a Hydride-Olefin-Niobocene Complex,
[Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-PhCHCHPh)] (5).
The complex [Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-PhCH
CHPh)] (5) was readily prepared in high yield (90%) by
thermal treatment in THF of Nb(η5-C5H4SiMe3)2(H)(SbPh3)
(1) in the presence of cis- or trans-stilbene (Scheme 4). In the
preparation of complex 5, the initial step would involve the
formation of the coordinatively unsaturated 16-electron species
[Nb(η5-C5H4SiMe3)2(H)] by thermolytic loss of SbPh3
followed by coordination of the olefin present in solution.
The reaction is similar to that previously described for complex
1 (vide infra). Complex 5 was isolated as an air-sensitive,
yellowish, microcrystalline solid.
and therefore the reaction pathways proposed in Scheme 5
seem reasonable for this process.
It seems that two different pathways can operate in this
process depending on the olefin employed. First, when cis-
stilbene was used, path a is suggested with the formation of the
hydride olefin intermediate I, which quickly evolves with the
formation of an intermediate alkylniobocene species II through
insertion of the CC moiety into the Nb−H bond.
Subsequently, the probable interaction between the phenyl
substituents would generate rotation through the C−C bond to
generate a new, less hindered chain configuration in which the
two bulky groups are as far from each other as possible
(intermediate III). Finally, the favored β-elimination process
would lead to the formation of the thermodynamically more
stable trans-olefin hydride niobocene complex 5. Second, in
path b we propose the typical trapping process of the
unsaturated 16-electron hydride intermediate by a trans-stilbene
compound, with the direct formation of the corresponding
complex 5.
The thermal stability of complex 5 has been demonstrated.
When the temperature was raised to the boiling point of the
solvent, exchange of the hydride ligand with the hydrogen
atoms of the olefin did not take place, in contrast with other
analogous hydride-olefin-niobocene derivatives reported by
some of us.14 Furthermore, a particular inertness toward the
insertion of several unsaturated molecules, namely, CO, CNR,
and CS2, is noteworthy, even under harsh reaction conditions.22
This situation is in contrast to the behavior observed previously
by us for other hydride olefin niobocene complexes.14
The structural characterization of 5 was carried out by
spectroscopic and X-ray diffraction studies.
Alternatively, 5 can be formed when a THF solution of
Nb(η5-C5H4SiMe3)2(H)3 (2) in the presence of cis- or trans-
stilbene (see Scheme 4) is warmed to 60 °C for 3 h.
This is the first example of a hydride olefin niobocene
complex having been prepared by the reaction of a niobocene
trihydride with a sterically demanding olefin.12−14 Some years
ago some of us reported the synthesis of new hydride olefin
complexes, namely, [Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-
The IR spectrum of 5 shows a weak, broad band at 1730
l
cm−l assigned to ν(Nb−H). The H NMR spectrum in C6D6
shows the hydride resonance at δ −2.67 ppm. The 1H and 13
C
t
ROOC-C(H)C(H)-COOR)] (R = Me, Bu) by reaction of
NMR spectra exhibit eight and 10 resonances for the proton
and carbon atoms, respectively, for the two cyclopentadienyl
ligands, indicating that they are not equivalent. This behavior is
in agreement with the structural disposition (vide infra)
observed for this complex, where the olefinic carbon atoms
are coplanar with the niobium and hydride atoms in such a way
2 with activated alkynes such as ROOC−CC−COOR (R =
t
Me, Bu) in an insertion process of the carbon−carbon triple
bond into the Nb−H bond.21
Only the trans-isomer of complex 5 has been prepared,
regardless of the olefin employed (i.e., cis- or trans-stilbene),
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Scheme 5
a
that their substituents (in a trans-disposition) are situated in a
plane orthogonal to that containing the aforementioned atoms.
This structural disposition leads to the presence of two
resonances for the inequivalent SiMe3 groups. Finally, the
Table 2. Bond Lengths (Å) and Angles (deg) for 5
bond lengths (Å)
bond angles (deg)
Nb(1)−H(1)
1.66(4)
2.084
Ct(1)−Nb(1)−Ct(2)
C(7)−Nb(1)−C(8)
C(8)−C(7)−C(1)
C(8)−C(7)−Nb(1)
C(1)−C(7)−Nb(1)
C(7)−C(8)−C(9)
C(7)−C(8)−Nb(1)
C(9)−C(8)−Nb(1)
135.17
Nb(1)−Ct(1)
Nb(1)−Ct(2)
Nb(1)−C(7)
Nb(1)−C(8)
C(1)−C(7)
C(7)−C(8)
C(8)−C(9)
35.7(1)
1
presence in the H NMR spectra of AB spin systems for the
2.073
124.4(3)
73.69(16)
122.5(2)
122.7(3)
70.6(2)
olefinic protons as two doublets at 3.79 and 3.42 ppm, with a
2.315(3)
2.356(3)
1.481(4)
1.433(4)
1.483(4)
3
coupling constant of JHaHb = 13.73 Hz, must also be
emphasized, as this is in agreement with a trans disposition
(see Experimental Section).
Furthermore, the X-ray crystal structure of 5 was determined.
The molecular structure is shown in Figure 2 along with the
atom-numbering scheme. Selected bond distances and angles
are listed in Table 2.
122.7(2)
a
Ct(1) and Ct(2) are the centroids of C(15)−C(19) and C(23)−
C(27), respectively.
Complex 5 adopts the typical bent-metallocene structure.
The environment around the Nb center shows two η5-
coordinated Cp′ ligands, one η2-coordinated stilbene molecule,
and a hydride ligand. The distances between the metal atom
and the centroids of the Cp′ rings are 2.084 and 2.073 Å, and
the angle Ct(1)−Nb(1)−Ct(2) is 135.37° [Ct(1) and Ct(2)
are the centroids of the two cyclopentadienyl rings]. The Cp′
ligands are nearly staggered, with the two SiMe3 groups
arranged trans to each other. The hydride was located in the
structure in the final ΔF map. The Nb−H distance is 1.66 (4)
Å, and it is comparable to values found in a series of scarce
niobium complexes that contain terminal hydride ligands.21,23
The niobium atom is also bound to the C(7) and C(8) carbon
atoms of the dihapto-coordinated alkene. The olefinic carbon−
carbon bond length [1.433(4) Å] is larger than that of a free
olefin, and this may be indicative of substantial back-donation
from the metal to the olefin, which would introduce some
metallacyclopropane-Nb(V) character into the formally Nb(III)
olefin complex. The chirality found in complex 5 is due to the
configuration of the coordinated C(7) and C(8) atoms, and
both the (S,S) and (R,R) enantiomers are present in the unit
cell. The coordination around the metal atom can be described
as an irregular flattened tetrahedron with the centroids Ct(1),
Ct(2), and M(1) [M(1) is at the midpoint of the olefinic
C(7)C(8) bond] and the hydride at the vertices of the
polyhedron. The Nb(1)M(1)H(1) plane is perpendicular to
the Nb(1)Ct(1)Ct(2) plane (90.42°). The four atoms Nb(1),
C(7), C(8), and H(1) are practically coplanar, and the Nb(1)−
H(1) bond is parallel to the C(7)C(8) bond. In fact, the
angles subtended by the two weighted least-squares lines
Figure 2.
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Organometallics
Article
none. All solvents were deoxygenated prior to use. Deuterated solvents
were dried over 4 Å molecular sieves and degassed prior to use. Nb(η5-
C5H4SiMe3)2(H)3 (2) was prepared as described in the literature.24 cis-
Stilbene, trans-stilbene, and triphenylstibine were used as supplied by
Nb(1)H(1) and C(7)C(8) are 0.56°. The Nb(1)C(7)C(8)
plane bisects the two Cp′ rings [the angles between the
weighted least-squares plane Nb(1)C(7)C(8) and the
C(15)···C(19) Cp′ ring and the C(23)···C(27) Cp′ ring are
23.29° and 23.46°, respectively].
1
Aldrich. H and 13C{1H} NMR spectra were recorded on a Varian
Innova 500 MHz spectrometer at room temperature unless stated
1
otherwise. H and 13C{1H} NMR chemical shifts (δ values) are given
in ppm relative to the solvent signal (1H, 13C) or standard resonances.
IR spectra were recorded on a Perkin-Elmer 883 spectrophotometer as
Nujol/polyethylene mulls. Microanalyses were carried out on a Perkin-
Elmer 2400 microanalyzer.
CONCLUDING REMARKS
■
In this paper we have described the preparation of a stable
hydride stibine niobocene derivative, [Nb(η5-C5H4SiMe3)2(H)-
(Sb(C6H5)3)] (1), by thermolytic loss of H2 from the
trihydride derivative [Nb(η5-C5H4SiMe3)2(H)3] (2) followed
by the coordination of a triphenylstibine ligand. Low-temper-
ature protonation with a slight excess of CF3CO2H leads to the
η2-dihydrogen cationic complex [Nb(η5-C5H4SiMe3)2(η2-H2)-
(Sb(C6H5)3)]+ (3), whose η2-(H2) and monodeuterated η2-
(HD) isotopomers show temperature-dependent NMR spec-
troscopic properties. The free energy of activation for the
dihydrogen internal rotation was estimated to be 10.8 kcal/mol.
Interestingly, it was observed that complex 3 promotes a rapid
H/D interchange. Additionally, a new hydride olefin niobocene
complex, [Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-PhCH
CHPh)] (5), has been prepared, and the molecular structure
was determined by X-ray diffraction. This complex represents a
rare example of this type of niobocene complex bearing a
sterically demanding olefin ligand.
Preparation of Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1). SbPh3 (0.190
g, 0.540 mmol) was added to a tan-colored solution of Nb(η5-
C5H4SiMe3)2(H)3 (2) (0.200 g, 0.540 mmol) in THF (40 mL). The
mixture was stirred at 343 K for 2 h. The resulting red solution was
filtered and evaporated to dryness. Complex 1 was isolated as a red,
oily material after maintaining it under vacuum for a lengthy period of
time (yield: 95%).
1
1: Yield 95%. IR (Nujol/polyethylene, cm−1): 1696 (Nb−H). H
NMR (500 MHz, C6D6): δ (ppm) −8.08 (s, 1 H, Nb−H), 0.19 (s, 18
H, SiMe3), 3.89, 4.18, 4.74, 5.04 (m, 2 H each a complex signal,
C5H4SiMe3,), 7.01−7.71 C6H5. 13C{1H} NMR (125 MHz, C6D6): δ
(ppm) 0.7 (SiMe3), 82.0, 84.6, 87.4, 88.8 (C2−5, exact assignment not
possible, C5H4SiMe3,), 89.2 (C1, C5H4SiMe3), 130.0−140.0 (SbPh3).
Protonation of Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1) (Mixture of
3 and 3-d1 Isotopomers). Either CF3COOH or CF3COOD was
added to an acetone-d6 solution of Nb(η5-C5H4SiMe3)2(H)(SbPh3)
(1) in a 5 mm NMR tube at 183 K, to give either the dihydrogen
complex [Nb(η5-C5H4SiMe3)2(η2-H2)(SbPh3)]CF3CO2 (3) or its
isotopomer-d1 [Nb(η5-C5H4SiMe3)2(η2-HD)(SbPh3)]CF3CO2 (3-d1).
3-d1: 1H NMR (500 MHz, C6D6): δ (ppm) −5.00 (t, 1:1:1, JHD = 20.7
Hz, 1 H, Nb-HD), 0.07 (s, 18 H, SiMe3), 4.38, 5.38, 5.93, 6.27 (m, 2 H
each a complex signal, C5H4SiMe3,), 7.59−7.72 (m, 15 H, SbPh3).
3: 1H NMR (500 MHz, C6D6): δ (ppm) −4.74 (s, br, 2 H, Nb-H2),
0.07 (s, 18 H, SiMe3), 4.38, 5.38, 5.93, 6.27 (m, 8 H, C5H4), 7.59−7.72
(m, 15 H, SbPh3).
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out using standard
Schlenk techniques. Oxygen and water were excluded through the use
of vacuum lines supplied with purified N2. Hexane was distilled from
sodium/potassium alloy. THF was distilled from sodium benzophe-
Table 3. Crystal Data and Structure Refinement for 5
Preparation of [Nb(η5-C5H4SiMe3)2(H)2(SbPh3)]+CF3COO− (4).
To a solution of Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1) (0.300 g, 0.360
mmol) in toluene (30 mL) at 183 K was added a small excess of
CF3COOH (0.33 mL, 0.432 mmol) by syringe. The solution was
allowed to reach room temperature and was stirred for 2 h. A red
precipitate formed. When sedimentation was complete, the solution
was filtered and the residue was washed twice with Et2O (10 mL) and
dried under vacuum. The resulting solution was evaporated to dryness.
Complex 4 was obtained as red solid (yield: 80%).
empirical formula
fw
C30H39NbSi2
548.70
temperature
wavelength
cryst syst
150(2) K
0.71073 Å
monoclinic
P21/c
space group
unit cell dimens
1
4: IR (ν, Nujol/polyethylene): ν (cm−1), 1673 (Nb−H). H NMR
a = 12.6788(9) Å
b = 10.9654(8) Å
c = 19.9135(15) Å
2761.5(3) Å3
4
α = 90°
β = 94.087(4)°
γ = 90°
(500 MHz, C6D6): δ (ppm) −2.22 (s, 2 H, Nb−H), 0.06 (s, 18 H,
SiMe3), 5.23, 6.20 (m, 4 H, each a complex signal, C5H4SiMe3), 7.46,
7.61, 7.75 (SbPh3). 13C{1H} NMR (125 MHz, C6D6): δ (ppm) −0.2
(SiMe3), 96.2, 98.1, 105.3 (C2−5, exact assignment not possible,
volume
Z
2
C5H4SiMe3), 116.1 (q, JCF = 14.24 Hz, CF3COO−), 131.8, 135.9,
density (calcd)
abs coeff
1.320 Mg/m3
0.539 mm−1
1152
1
136.6 (SbPh3), 139.0 (C1, C5H4SiMe3), 158.6 (q, JCF = 32.1 Hz,
CF3COO−). Anal. Calcd for C36H43F3NbO2SbSi2: C, 51.75; H, 5.19.
Found: C, 51.48; H, 5.07.
F(000)
cryst size
0.23 × 0.19 × 0.10 mm3
1.61° to 28.38°
Preparation of [Nb(η5-C5H4SiMe3)2(H)(trans-η2-C,C-PhCH
CHPh)] (5). Method 1. Nb(η5-C5H4SiMe3)2(H)(SbPh3) (1) (0.300
g, 0.360 mmol) was dissolved in THF (40 mL) to form a red solution.
To this solution was added cis- or trans-stilbene, Ph−CHCH−Ph
(0.064 mL, 0.360 mmol). The mixture was stirred at 343 K for 2 h.
The resulting red-brown solution was filtered and evaporated to
dryness. The red-brown, oily residue was extracted with hexane (20
mL). The resulting solution was filtered and evaporated to dryness.
After washing the solid with hexane (10 mL) and recrystallization from
diethyl ether, 5 was isolated as a yellowish microcrystalline solid (yield:
80%).
theta range for data collection
index ranges
−16 ≤ h ≤ 16, −14 ≤ k ≤ 14,
−26 ≤ l ≤ 26
reflns collected
33 336
indep reflns
6872 [R(int) = 0.0803]
99.3%
completeness to theta = 28.38°
abs corr
semiempirical from equivalents
0.9481 and 0.8861
full-matrix least-squares on F2
6872/0/308
max. and min. transmn
refinement method
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole
Method 2. Nb(η5-C5H4SiMe3)2(H)3 (2) (0.253 g, 0.680 mmol) was
dissolved in THF (40 mL) to form a tan solution. To this solution was
added cis- or trans-stilbene, Ph−CHCH−Ph (0.12 mL, 0.680
mmol). The mixture was stirred at 343 K for 2 h. The resulting light
brown solution was filtered and evaporated to dryness. The light
1.090
R1 = 0.0451, wR2 = 0.1079
R1 = 0.0715, wR2 = 0.1276
0.715 and −0.899 e·Å−3
867
dx.doi.org/10.1021/om301171j | Organometallics 2013, 32, 862−868

Organometallics
Article
brown, oily residue was extracted with hexane (20 mL). The resulting
solution was filtered and evaporated to dryness. After washing the solid
with hexane (10 mL) and recrystallization from diethyl ether, 5 was
isolated as a yellowish microcrystalline solid (yield: 85%).
(9) (a) Antinolo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; Fernan
́
dez-
Baeza, J.; García-Yuste, S.; Otero, A. Coord. Chem. Rev. 1999, 193−
195, 43−72. (b) Antinolo, A.; Carrillo-Hermosilla, F.; Fernandez-
̃
̃
Baeza, J.; García-Yuste, S.; Otero, A.; San
́
chez-Prada, J.; Villasenor, E.
̃
5: IR (ν, Nujol/polyethylene): ν (cm−1), 1721 (Nb−H), 1589
Eur. J. Inorg. Chem. 2000, 1437−1443. (c) Antinolo, A.; Carrillo-
Hermosilla, F.; Fajardo, M.; Fernandez-Baeza, J.; García-Yuste, S.;
̃
1
(CC). H NMR (500 MHz, C6D6): δ (ppm) −2.23 (s, 1 H, Nb−
H), 0.15, 0.35 (s, 9 H, SiMe3), 3.42, 3.79 (dd, 2 H, AB system, JHbHa
=
Otero, A.; Camanyes, S.; Maseras, F.; Moreno, M.; Lledos
́
, A.; Lluch, J.
13.73 Hz, C5H6−CHCH−C5H6), 3.69, 3.71, 3.86, 4.05, 4.38, 4.63,
4.75, 5.03 (m, 1 H, each a complex signal, C5H4SiMe3), 6.87−7.48 (m,
10 H, C5H6). 13C{1H} NMR (125 MHz, C6D6): δ (ppm) 0.4 (SiMe3),
34.3, 34.6 (CC−H), 90.8, 97.1, 97.2, 98.7, 99.8, 101.0, 102.7, 102.8,
102.9, 106 (C5H4SiMe3), 122.4, 122.8, 126.9, 128.0, 152.6, 153.6
(C6H5). Anal. Calcd for C30H39NbSi2: C, 65.67; H, 7.16. Found: C,
65.50; H, 7.11.
M. J. Am. Chem. Soc. 1997, 119, 6107−6114. (d) Jalon
́
, F.; Otero, A.;
Manzano, B.; Villasenor, E.; Chaudret, B. J. Am. Chem. Soc. 1995, 117,
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3795.
X-ray Structure Determination for 5. Data were collected on a
Bruker X8 APEX II CCD-based diffractometer, equipped with a
graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å).
The crystal data and data collection, structural solution, and
refinement parameters are summarized in Table 3. Data were
integrated using SAINT,25 and an absorption correction was
performed with the program SADABS.26 The structure was solved
by direct methods using SHELXTL27 and refined by full-matrix least-
squares methods based on F2. All non-hydrogen atoms were refined
with anisotropic thermal parameters. Hydrogen atoms were placed
using a “riding model” and are included in the refinement at calculated
positions.
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C.; Bercaw, J. E. Organometallics 2003, 22, 188−194.
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Organometallics 1994, 13, 2761−2766.
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Chem. Soc. 1996, 118, 5396−5407. (b) Luther, T. A.; Heinekey, D. M.
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ASSOCIATED CONTENT
■
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
(18) (a) Albeniz, A. C.; Heinekey, D. M.; Crabtree, R. H. Inorg.
Chem. 1991, 30, 3632−3635. (b) Mediati, M.; Tachibana, G. N.;
Jensen, M. C. Inorg. Chem. 1992, 31, 1827−1832. (c) Bautista, M. T.;
Earl, K. A.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc. 1988, 110,
4056−4057.
AUTHOR INFORMATION
■
Corresponding Author
(19) Bianchini, C.; Linn, K.; Masi, D.; Peruzzini, M.; Polo, A.; Vacca,
A.; Zanobini, F. Inorg. Chem. 1993, 32, 2366−2376.
*E-mail: Antonio.Antinolo@uclm.es; Antonio.Otero@uclm.es.
Web: http://www.uclm.es/.
(20) Gusev, D. G.; Hubener, R.; Burger, P.; Orama, O.; Berke, H. J.
̈
Am. Chem. Soc. 1997, 119, 3716−3731.
Notes
(21) Antinolo, A.; Carrillo-Hermosilla, F.; Fajardo, M.; García-Yuste,
̃
The authors declare no competing financial interest.
S.; Lanfranchi, M.; Otero, A.; Pellinghelli, M. A.; Prashar, S.;
Villasenor, E. Organometallics 1996, 5, 5507−5513.
̃
ACKNOWLEDGMENTS
(22) Antinolo, A.; del Hierro, I.; Fajardo, M.; García-Yuste, S.; Otero,
̃
■
A.; Blacque, O.; Kubicki, M. M.; Amaudrut, J. Organometallics 1996,
The authors gratefully acknowledge financial support from the
15, 1966−1971.
́
Ministerio de Economia y Competividad (MINECO), Spain
(23) (a) Alonso-Moreno, C.; A. Antinolo, A.; García-Martínez, J. C.;
̃
(Grant Nos. Consolider-Ingenio 2010 ORFEO CSD 2007-
00006, CTQ2009-09214, CTQ2011-22578/BQU), and the
European Union (COST CM0802). We would also like to
thank the Consejo Superior de Investigaciones Cientificas
(CSIC) of Spain for the award of a license to use the
Cambridge Crystallographic Data Base (CSD).
Garcia-Yuste, S.; Lopez-Solera, I.; Otero, A.; Perez-Flores, J. C.; Lopez-
Solera, I.; Tercero-Morales, M. T. Eur. J. Inorg. Chem. 2012, 1139−
1144. (b) Antinolo, A.; Garcia-Yuste, S.; Otero, A.; Perez-Flores, J. C.;
̃
́
Lopez-Solera, I.; Rodriguez, A. M. J. Organomet. Chem. 2007, 692,
3328−3339. (c) Wilson, R. D.; Koetzle, T. F.; Hart, D. W.; Kvick, A.;
Tipton, D. L.; Bau, R. J. Am. Chem. Soc. 1977, 99, 1775−1781.
(d) Guggenberger, L. J.; Tebbe, F. N. J. Am. Chem. Soc. 1971, 93,
5924−5925. (e) Guggenberger, L. J. Inorg. Chem. 1973, 12, 294−301.
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dx.doi.org/10.1021/om301171j | Organometallics 2013, 32, 862−868