Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.06.030

Compound [NbCp′Me₄] (Cp′ = η⁵-C ₅H₄SiMe₃, 1) reacted with several ROH compounds (R = tBu, SiiPr₃, 2,6-Me₂C₆H₃) to give the derivatives [NbCp′Me₃(OR)] (R = tBu 2a, SiiPr ₃ 2b, 2,6-Me₂C₆H₃ 2c). The diaryloxo tantalum compound [TaCp Me₂(OR)₂] (Cp = η⁵-C₅Me₅, R = 2,6-Me₂C ₆H₃ 3) was obtained by reaction of [TaCp Cl ₂Me₂] with 2 equiv of LiOR (R = 2,6-Me₂C ₆H₃). Abstraction of one methyl group from these neutral compounds 1-3 with the Lewis acids E(C₆F₅)₃ (E = B, Al) gave the ionic derivatives [NbCp′Me₂X][MeE(C ₆F₅)₃] (X = Me 4-E. X = OR; R = SiiPr ₃ 5b-E, 2,6-Me₂C₆H₃ 5c-E. E = B, Al) and [TaCp Me(OR)₂][MeE(C₆F₅)₃] (R = 2,6-Me₂C₆H₃ 6-E; E = B, Al). Polymerization of MMA with the aryloxoniobium compound 2c and Al(C₆F₅) ₃ gave syndiotactic PMMA in a low yield, whereas the tetramethylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.

Full text of DOI:10.1016/j.jorganchem.2010.06.030

Journal of Organometallic Chemistry 695 (2010) 2469e2473
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium
and tantalum complexes
,
1
*
*
Javier Sánchez-Nieves , Vanessa Tabernero, Claudimar Camejo, Pascual Royo
Departamento de Química Inorgánica, Universidad de Alcalá, Campus Universitario, E-28871 Alcalá de Henares, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Compound [NbCp0Me4] (Cp0 ¼
h
5-C5H4SiMe3, 1) reacted with several ROH compounds (R ¼ tBu, SiiPr3,
Article history:
2,6-Me2C6H3) to give the derivatives [NbCp0Me3(OR)] (R ¼ tBu 2a, SiiPr3 2b, 2,6-Me2C6H3 2c). The
Received 25 May 2010
Received in revised form
23 June 2010
Accepted 23 June 2010
Available online 13 July 2010
diaryloxo tantalum compound [TaCp*Me2(OR)2] (Cp* ¼
h
5-C5Me5, R ¼ 2,6-Me2C6H3 3) was obtained by
reaction of [TaCp*Cl2Me2] with 2 equiv of LiOR (R ¼ 2,6-Me2C6H3). Abstraction of one methyl group from
these neutral compounds 1e3 with the Lewis acids E(C6F5)3 (E ¼ B, Al) gave the ionic derivatives
[NbCp0Me2X][MeE(C6F5)3] (X ¼ Me 4-E. X ¼ OR; R ¼ SiiPr3 5b-E, 2,6-Me2C6H3 5c-E. E ¼ B, Al) and
[TaCp*Me(OR)2][MeE(C6F5)3] (R ¼ 2,6-Me2C6H3 6-E; E ¼ B, Al). Polymerization of MMA with the arylox-
oniobium compound 2c and Al(C6F5)3 gave syndiotactic PMMA in a low yield, whereas the tetrame-
thylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.
Keywords:
Monocyclopentadienyl
Cationic compounds
Niobium compounds
Alkyl compounds
Ó 2010 Elsevier B.V. All rights reserved.
Methylmethacrylate polymerization
1. Introduction
studied [39e41]. For example, the system [Cp2TaMe3]/2Al(C6F5)3 is
highly active for MMA polymerization, although it is completely
It is well known that early transition metal alkyl complexes
promoted coupling of unsaturated organic substrates, making this
process a relevant strategy for organic synthesis and catalytic
polymerization [1e4]. In particular, niobium and tantalum alkyl
complexes have been applied in CeC coupling reactions via
migratory insertion of coordinated unsaturated molecules into the
metal alkyl bond [5e16]. In addition, alkyne niobium and tantalum
(III) compounds are efficient catalysts for the oligomerization and
polymerization of alkynes [17,18] and the formation of alkylidene
and alkenyl intermediates through this pathway is commonly
accepted [19,20]. However, the number of alkyl mono(cyclo-
pentadienyl) derivatives used for this type of processes is small in
part due to their instability [7,13,21e30].
inactive in other reaction conditions such as catalyst/cocatalyst
ratio or different cocatalysts [40].
Recently, we showed that protonolysis of a TaeMe bond in
complex [TaCp*Me4] with several alcohol and silanol compounds was
a suitable method for the synthesis of new monocyclopentadienyl
complexes [TaCp*Me3(OR)] (R ¼ SiiPr3, 2,6-Me2C6H3, 2,6-iPr2C6H3)
[42]. We also reported their reactions with the Lewis acids E(C6F5)3
(E ¼ B, Al) and studies on polymerization of MMA with the systems
[TaCp*Me3X]/E(C6F5)3, observing that the polymerization was
dependent on the X group (X ¼ Me, OR) and E (E ¼ B, Al). The results
showed that polymerization proceeded only when Al(C6F5)3 was the
cocatalyst, the best X group was the aryloxo ligand with the 2,6-
Me2C6H3 moiety.
The polymerization of functional alkenes such as acrylates,
acrylamides and methacrylates catalyzed by lanthanocenes or
group 4 complexes has attracted great attention due to the high
activity shown by these systems [31e38]. Applications of group 5
alkyl complexes to this type of polymerization have been less
As an extension of these studies, and also analogous with the
non-cyclopentadienyl group 5 complexes [41], and with the aim of
correlating the metal properties to the catalytic behaviour, we have
explored the behaviour of new monocyclopentadienyl alkyl
niobium complexes [NbCp0Me3(OR)] and a new tantalum dialkyl-
diaryloxo compound toward the Lewis acids E(C6F5)3 (E ¼ B, Al),
and also their ability as MMA polymerization catalyst. The unex-
pected results demonstrate that, in spite of their higher Lewis
acidity, the niobium complexes are less active for polymerization
than the corresponding tantalum derivatives.
* Corresponding authors. Tel.: þ34 91 885 4765; fax: þ34 91 885 4683.
E-mail addresses: javier.sancheznieves@uah.es (J. Sánchez-Nieves), pascual.
royo@uah.es (P. Royo).
1
Present address: Centro Investigaciones Biomédicas (Ciber-bbn), Universidad
de Alcalá, Spain.
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.06.030

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2. Results and discussion
2.1. Synthesis of neutral compounds
The synthesis of the permethylated compound [NbCp0Me4] (1)
(Cp0 ¼
h
5-C5H4SiMe3) was achieved by reacting the chloro
compound [NbCp0Cl4] with 4 equiv of MgClMe, following previ-
ously described procedures for the related pentam-
ethylcyclopentadienyl compound [25]. Compound 1 was obtained
in good yield as yellow-brown oil, although it is light sensitive and
also decomposed slowly at ambient temperature, it was however
stable at low temperature for weeks.
Next, we tried the protonolysis reactions of 1 with different
alcohols and silanols ROH (R ¼ tBu, SiiPr3, 2,6-Me2C6H3) to obtain
the corresponding monosubstituted compounds [NbCp0Me3(OR)]
(R ¼ tBu 2a, SiiPr3 2b, 2,6-Me2C6H3 2c). The reaction conditions
were dependent on the alcohol R group. With the most acidic
silanol (iPr3SiOH), the reaction proceeded easily at room temper-
ature, whereas with the bulkier alcohols ROH (R ¼ tBu, 2,6-
Me2C6H3), the substitution was slower and required heating to
complete the process. In all cases, the monosubstituted derivatives
[NbCp0Me3(OR)] (R ¼ tBu 2a, iPr3Si 2b, 2,6-Me2C6H3 2c) were
obtained with moderate yields as brownish oils that were also
moisture and also light sensitive. However, these compounds were
thermally more stable than the starting compound 1.
The NMR behaviour of these compounds varies depending on
the R groups of the OR substituent. At 25 ꢀC, the 1H NMR spectrum
of 2a (R ¼ tBu) and 2b (R ¼ iPr3Si) showed two resonances with
relative intensities of 1:2 for two different MeeNb groups, as
expected for a non-fluxional molecule. However, the 1H NMR
spectrum of complex 2c (R ¼ 2,6-Me2C6H3) showed a broad reso-
nance corresponding to three equivalent methyl groups attached to
niobium, indicating that they had become equivalent in the NMR
time scale by a fluxional process, most likely a Berry pseudorotation
[43e46], more favourable for the derivative with the less donating
siloxo ligand [42,47e49].
To study the influence of a second OR ligand in the coordination
sphere of the metal we tried to synthesize compounds of the type
[MCpRMe2(OR)2] (M ¼ Nb, Ta). We choose as R group the phenoxo
2,6-Me2C6H3, which in our previous work seemed to be the best
ligand to stabilize compounds of this type as well as the hypo-
thetical cationic derivatives that will be studied below. Heating
[MCpRMe4] (M ¼ Nb, CpR ¼ Cp0; M ¼ Ta, CpR ¼ Cp*) with 2 equiv of
the corresponding alcohol resulted in the decomposition of the
starting compounds or the respective monoaryloxo derivative
formed in this procedure. Consequently, we tried an alternative
synthetic route consisting of the metathesis reaction of
[MCpRCl2Me2] with LiOAr (Ar ¼ 2,6-Me2C6H3), with this method-
ology only proving successful for the synthesis of the tantalum
compound [TaCp*Me2(OAr)2] (Ar ¼ 2,6-Me2C6H3 3), which was
obtained as a white solid in good yield. The 1H and 13C NMR spectra
of complex 3 showed one singlet for both TaeMe groups and also
one singlet for both methyl groups of Ar moiety. With these data,
two possible dispositions of the Me and OAr ligands about the Ta
atom can be proposed, cis and trans. However, the size of the OAr
ligand could favor a trans disposition (Scheme 1).
Scheme 1. Synthesis of neutral compounds. i) 4ClMgMe, ꢁ78 ꢀC. ii) ROH; r.t., 16 h
(R ¼ tBu, SiiPr3); or 60 ꢀC, 24 h (R ¼ 2,6-Me2C6H3). iii) 2LiOR, ꢁ78 ꢀ
C
(R ¼ 2,6-
Me2C6H3).
corresponding to the free [MeB(C6F5)3]ꢁ anion (Dd(m,p-F) < 3 ppm)
[50]. A similar reaction with Al(C6F5)3 caused 1 to decompose.
In an analogous way to the formation of compound 4, abstrac-
tion of one methyl group of the derivatives 2 with the Lewis acids Al
(C6F5)3 or B(C6F5)3 afforded the related ionic derivatives
[NbCp0Me2(OR)][MeE(C6F5)3] (R ¼ iPr3Si 5b-E, 2,6-Me2C6H3 5c-E.
E ¼ B, Al) (Scheme 2). These ionic compounds were obtained as
dark oils insoluble in toluene but soluble in halogenated solvents.
However, in the particular case of 2a only decomposition was
observed. It has been reported that the tBu group can be activated,
releasing isobutene, although we were not able to detect any
byproduct [51,52]. The 1H and 13C NMR spectra of the cationic
fragments of complexes containing the same R group, 5b-B/5b-Al
and 5c-B/5c-Al showed almost identical 1H and 13C NMR spectra, as
expected for identical cationic moieties [NbCp0Me2(OR)]þ without
an ion pair interaction. The 19F NMR spectrum of 5B confirmed this
behaviour, with Dd(m,p-F) < 3 [53]. All of these ionic complexes 4 and
5 decomposed within several hours in BrC6D5 solutions and much
faster in CD2Cl2, in contrast to the stability observed for related
tantalum derivatives [16,42].
The diaryloxotantalum compound 3 also reacted with 1 equiv of
B(C6F5)3 or Al(C6F5)3 in toluene to give the ionic derivatives
[TaCp*Me(OR)2][MeE(C6F5)3] (R ¼ 2,6-Me2C6H3; E ¼ B, 6-B; E ¼ Al,
6-Al) as brownish oils characterized by 1H, 13C and 19F NMR spec-
troscopy (Scheme 2). Complexes 6-B and 6-Al were insoluble in
2.2. Synthesis of cationic compounds
Mixing [NbCp0Me4] (1) with 1 equiv of B(C6F5)3 in toluene at
ambient temperature formed an oily precipitate, which was
washed with hexane and dissolved in BrC6D5, allowing us to
identify the ionic compound [NbCp0Me3][MeB(C6F5)3] (4). The 1H
NMR spectrum showed a broad singlet at
d 1.05 for the MeB group
and three resonances were observed in the 19F NMR spectrum
Scheme 2. Synthesis of cationic compounds.

J. Sánchez-Nieves et al. / Journal of Organometallic Chemistry 695 (2010) 2469e2473
2471
toluene but soluble in halogenated solvents and thermally stable
4. Experimental section
for long periods in the absence of moisture. The 1H and 13C NMR
spectra of these complexes showed one resonance for the TaeMe
group clearly shifted to higher frequency with respect to those
observed for the corresponding parent compounds 3 and a broad
signal corresponding to the methyl MeeB group was also observed
4.1. General considerations
All manipulations were carried out under an argon atmosphere
and solvents were distilled from appropriate drying agents. NMR
spectra were recorded at 400.13 (1H), 376.70 (19F) and 100.60 (13C)
in the 1H NMR spectra of 6-B at about
d 0.50. When Al(C6F5)3 was
used the resonance for the abstracted methyl group was not
observed in the 1H NMR spectrum. The absence of an ion pair
interaction is consistent with the Dd(m,p-F) ¼ 2.5 found in the 19F
NMR spectra [53].
In all these compounds, the Lewis acid E(C6F5)3 (E ¼ B, Al)
abstracted the Me group, also in the presence of an oxygen atom
bound to the niobium atom, in contrast with the behaviour
observed in oxametallacyclic niobium compounds [54].
MHz on a Bruker AV400. Chemical shifts (d) are given in parts per
million relative to internal TMS (1H and 13C) or external CFCl3 (19F).
Elemental analyses were performed on a PerkineElmer 240C.
Compounds [NbCp0Cl4] [55], [TaCp*Cl2Me2] [56], B(C6F5)3 [57] and
0.5(toluene)$Al(C6F5)3 [39] were prepared by literature methods.
MgClMe (3 M in THF), LiMe (1 M in ether) and ROH were purchased
from Aldrich. The alcohols were degassed and stored under Ar with
molecular sieves (R ¼ tBu, iPr3Si) or sublimed under vacuum
(R ¼ 2,6-Me2C6H3). LiOR (R ¼ 2,6-Me2C6H3) was prepared and used
in situ from reaction of LiBu and ROH in Et2O.
2.3. MMA polymerization
Neutral monoalkoxo compounds [NbCp0Me3X] (X ¼ Me 1,
X ¼ OR; R ¼ tBu 2a, SiiPr3 2b, 2,6-Me2C6H3 2c) were studied as
catalysts for MMA polymerization in the presence of the Lewis
acids E(C6F5)3 (E ¼ B, Al). Only compound 2c showed a certain
degree of activity (T ¼ 40 ꢀC, 16 h, yield 20%) when E ¼ Al while for
E ¼ B no polymerization was observed. The polymer obtained was
high molecular weight PMMA of narrow polydispersity, predomi-
nantly syndiotactic ([rr] 57.9%, [mr] 36.4%, [mm] 5.7%;
Mn ¼ 6.10 ꢂ 105; Mw/Mn ¼ 1.25), as indicated by 1H NMR. The
niobium complexes were clearly less effective catalysts than the
related tantalum compounds [TaCp*Me3X] (X ¼ Me, OR). The
bulkiness of the O(2,6-Me2C6H3) aryloxo ligand of compound 2c
might be the reason for the higher stability of the active species,
favoring the polymerization process [42]. In contrast to the related
tantalum compound [TaCp*Me3X] (X ¼ Me, OR), increasing
temperature did not improve polymerization results with any of
these compounds. Also, neither addition of 2 equiv of Al(C6F5)3 led
to better results.
4.2. [NbCp0Me4] (1)
MgClMe (4 mL, 3 M in THF) was added to [NbCp0Cl4] (1.120 g,
3.00 mmol) in 60 mL of Et2O at ꢁ78 ꢀC. The mixture was then
stirred at room temperature for 3 h. The volatiles were removed
under vacuum and hexane was added (60 mL) to extract the
product. Removal of solvent afforded 1 as a dark oil, which was
stored at ꢁ20 ꢀC (0.70 g, 80%). 1H NMR (C6D6): 0.04 (s, 9H, SiMe3),
1.46 (s, 12H, MeNb), 5.26 (m, 2H, C5H4), 5.68 (m, 2H, C5H4). 13C {1H}
NMR (C6D6): 0.1 (SiMe3), 48.9 (MeNb), 110.6, 120.2 (C5H4), 1226
(Cipso, C5H4). Anal. Calc. for C12H25NbSi (290.32): C, 49.64; H, 8.68.
Found: C, 48.90; H, 8.00.
4.3. [NbCp0Me3(OtBu)] (2a)
Compound
1 (0.200 g, 0.69 mmol) and tBuOH (0.051 g,
0.69 mmol) were stirred in toluene for 16 h at ambient tempera-
ture. Volatiles were then removed under vacuum, hexane was
added (2 ꢂ 20 mL) to extract the product, to give 2a as a brownish
oil (0.131 g, 55%). 1H NMR (C6D6): 0.15 (s, 9H, SiMe3), 1.01 (s, 6H,
MeNb), 1.09 (s, 3H, MeNb), 1.20 (s, 9H, CMe3), 5.55 (m, 2H, C5H4),
5.63 (m, 2H, C5H4). 13C {1H} NMR (C6D6): ꢁ0.2 (SiMe3), 24.6 (CMe3),
42.5 (MeNb), 44.1 (MeNb), 69.8 (CMe3), 113.3 (C5H4), 113.9 (C5H4),
119.3 (Cipso, C5H4). Proper analysis of 2a could not be obtained.
On the other hand, the system [TaCp*Me2(OAr)2]/Al(C6F5)3 was
inactive for MMA polymerization. This result can be interpreted in
terms of the steric hindrance imposed by the presence of two
aryloxo groups that avoid the proximity of cation and anion
necessary for the initiation step, as was also observed for [TaCp*-
Me3(OAr)] (Ar ¼ 2,6-iPr2C6H3) [42].
3. Conclusions
4.4. [NbCp0Me3(OSiiPr3)] (2b)
Protonolysis of a NbeMe bond in complex [NbCp0Me4] has been
proved to be a suitable method for the synthesis of new mono-
cyclopentadienyl complexes of the type [NbCp0Me3(OR)], as
previously shown for analogous tantalum derivatives, whereas the
synthesis of monocyclopentadienyl dialkoxo compounds was only
achieved for the tantalum compound [TaCp*Me2(OR)2] by reaction
of [TaCp*Cl2Me2] with 2 equiv of LiOR (R ¼ 2,6-Me2C6H3). The
electronic and steric characteristics of the ROH compounds have
been shown to exert an important influence on the reaction
conditions.
Following the procedure described above, compound 1 (0.200 g,
0.69 mmol) and iPr3SiOH (0.120 g, 0.69 mmol) afforded 2b as
a brownish oil (0.210 g, 68%). 1H NMR (C6D6): ꢁ0.16 (s, 9H, SiMe3),
1.09 (s, 21H, SiiPr2), 1.19 (s, 6H, MeNb), 1.21 (s, 3H, MeNb), 5.56 (m,
2H, C5H4), 5.63 (m, 2H, C5H4). 13C {1H} NMR (C6D6): ꢁ0.2 (SiMe3),
13.6 (CHMe2),18.4 (CHMe2), 42.4 (MeNb), 44.4 (MeNb),113.3 (C5H4),
113.6 (C5H4), 118.3 (Cipso, C5H4). Anal. Calc. for C20H43NbOSi2
(448.63): C, 53.54; H, 9.66. Found: C, 52.91; H, 9.27.
4.5. [NbCp0Me3(OR)] (R ¼ 2,6-Me2C6H3, 2c)
These new niobium derivatives and also the tantalum diaryloxo
compound wþere precursors of the respective cationic compounds
[NbCp0Me2X] (X ¼ Me, OR) and [TaCp*Me(OR)2]þ after reaction
with the Lewis acids E(C6F5)3 (E ¼ B, Al). The cationic niobium
complexes were clearly less stable than the related tantalum
derivatives, consistent with the lower Lewis acidity of tantalum and
also the higher stability of the TaeC bonds.
In the dry-box an ampoule with a Teflon valve was charged with
toluene (20 mL), compound 1 (0.300 g, 1.03 mmol) and ROH
(0.126 g, 1.03 mmol) and the solution was heated at 60 ꢀC for 18 h.
After this time the solvent was evaporated and the product
extracted into hexane (30 mL). Compound 2c was obtained after
elimination of the solvent as a dark oil and was stored at ꢁ20 ꢀC
(0.36 g, 87%). 1H NMR (C6D6): 0.10 (s, 9H, SiMe3), 1.18 (br s, 9H,
MeNb), 2.22 (s, 6H, 2,6-Me2C6H3), 5.58 (m, 2H, C5H4), 5.66 (m, 2H,
C5H4), 6.78 (m, 1H, p-2,6-Me2C6H3), 6.92 (m, 2H, m-2,6-Me2C6H3).
Only the aryloxo complex NbCp0Me3(OR) (R ¼ 2,6-Me2C6H3 2c)
polymerized MMA with low yield when activated with Al(C6F5)3,
but not in the presence of B(C6F5)3.

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J. Sánchez-Nieves et al. / Journal of Organometallic Chemistry 695 (2010) 2469e2473
13C {1H} NMR (C6D6): ꢁ0.6 (SiMe3), 18.2 (2,6-Me2C6H3), 46.40
(MeNb), 112.7, 114.8 (C5H4), 118.3 (Cipso, C5H4), 122.4 (2,6-Me2C6H3),
13C {1H} NMR (C6D5Br): ꢁ1.2 (SiMe3), 11.5 (BMe), 17.4 (2,6-
Me2C6H3), 69.2 (MeNb), 119.4 (C5H4), 120.5 (C5H4), 128.5 (2,6-
128.1 (2,6-Me2C6H3), 129.2 (2,6-Me2C6H3), 158.1 (Cipso
,
2,6-
Me2C6H3), 128.7 (2,6-Me2C6H3), 128.8 (2,6-Me2C6H3), 131.5 (Cipso,
Me2C6H3). Anal. Calc. for C19H31NbOSi (396.44): C, 57.56; H, 7.88.
Found: C, 57.02; H, 7.77.
C5H4), 136.4 (C6F5), 137.9 (C6F5), 148.7 (C6F5), 161.6 (2,6-Me2C6H3).
19F NMR (C6D5Br): ꢁ131.2 (o-C6F5), ꢁ163.3 (p-C6F5), ꢁ165.8
(m-C6F5). Anal. Calc. for C37H31BF15NbOSi (908.42): C, 48.92; H,
3.44. Found: C, 47.69; H, 3.03.
4.6. [TaCp*Me2(OR)2] (R ¼ 2,6-Me2C6H3, 3)
The same procedure as above for compound 2c (0.020 g,
0.050 mmol) and Al(C6F5)3 (0.032 g, 0.060 mmol) gave 5c-Al
(0.039 g, 83%). 1H NMR (C6D5Br, 25 ꢀC): ꢁ0.13 (s, 9H, SiMe3), 0.21 (br
s, 3H, MeAl), 1.51 (br s, 6H, MeNb), 1.81 (s, 6H, Me2C6H3), 6.02 (m,
2H, C5H4), 6.11 (m, 2H, C5H4), 6.74 (m, 1H, p-2,6-Me2C6H3), 6.92 (m,
2H, m-2,6-Me2C6H3). 13C {1H} NMR (C6D5Br): ꢁ5.5 (AlMe), ꢁ1.1
(SiMe3), 17.4 (Me2C6H3), 67.7 (MeNb), 118.6 (C5H4), 119.8 (C5H4),
128.2 (2,6-Me2C6H3), 128.7 (2,6-Me2C6H3), 128.9 (2,6-Me2C6H3),
130.7 (Cipso, C5H4), 136.3 (C6F5), 138.1 (C6F5), 148.4 (C6F5), 161.3 (2,6-
Me2C6H3). 19F NMR (C6D5Br): ꢁ120.2 (o-C6F5), ꢁ157.8 (p-C6F5),
ꢁ162.7 (m-C6F5). Anal. Calc. for C37H31AlF15NbOSi (924.59): C,
48.06; H, 3.38. Found: C, 47.59; H, 3.13.
2 equiv of LiOR (1.00 mmol) in Et2O (30 mL) was added to
a solution of [TaCp*Me2Cl2] (0.20 g, 0.48 mmol) in the same solvent
(20 mL) at ꢁ78 ꢀC. The final solution was stirred for 16 h at r.t. when
the solvent was evaporated to ca. half of the volume and hexane
was added (20 mL). The solution was filtered and cooled at ꢁ40 ꢀC,
yielding 3 as a white solid (0.210 g, 75%). 1H NMR (CDCl3): 0.25 (s,
6H, MeTa), 2.09 (s, 12H, Me2C6H3), 2.12 (s, 15H, C5Me5), 6.63 (m, 2H,
p-Me2C6H3), 6.87 (m, 4H, m-Me2C6H3). 13C {1H} NMR (CDCl3): 11.1
(C5Me5), 17.9 (Me2C6H3), 57.1 (MeTa), 119.9 (C5Me5), 121.3
(Me2C6H3), 126.6 (Me2C6H3), 128.5 (Me2C6H3), 158.9 (Cipso, 2,6-
Me2C6H3). Anal. Calc. for C28H39O2Ta (588.56); C, 57.14; H, 6.68.
Found: C, 57.10; H, 6.58.
4.10. [TaCp*Me(OR)2][MeE(C6F5)3] (R ¼ 2,6-Me2C6H3;
E ¼ B, 6-B; E ¼ Al, 6-Al)
4.7. [NbCp0Me3][MeB(C6F5)3] (4)
In the dry-box, a vial was charged with 1 (0.020 g, 0.069 mmol)
and B(C6F5)3 (0.038 g, 0.075 mmol) and toluene (2 mL) was then
added. After 20 min the formation of oil was observed. The super-
natant was decanted and the oily product was washed first with
toluene (2 mL) and then with hexane (2 ꢂ 2 mL). Compound 4 was
obtained as dark oil (0.048 g, 87%). 1H NMR (C6D5Br): ꢁ0.09 (s, 9H,
SiMe3),1.09 (br s, 3H, BMe), 1.51 (br s, 9H, MeNb), 5.81 (m, 2H, C5H4),
5.83 (m, 2H, C5H4). 13C {1H} NMR (C6D5Br): ꢁ1.4 (SiMe3), 11.5 (MeB),
77.5 (MeNb), 112.6 (C5H4), 116.2 (C5H4), 132.2 (Cipso, C5H4), 136.4
(C6F5), 137.9 (C6F5), 148.7 (C6F5). 19F NMR (C6D5Br): ꢁ131.6 (o-C6F5),
ꢁ163.4 (p-C6F5), ꢁ165.9 (m-C6F5). Adequate elemental analysis
could not be obtained for this compound.
The same procedure as above for compound 3 (0.022 g,
0.037 mmol) and B(C6F5)3 (0.023 g, 0.045 mmol) afforded 6-B
(0.037 g, 90%). 1H NMR (CD2Cl2, 25 ꢀC): 0.46 (br s, 3H, MeB), 2.07 (s,
3H, Me-Ta), 2.25 (m, 12H, 2,6-Me2C6H3), 2.27 (s, 15H, C5Me5), 7.04
(m, 2H, p-2,6-Me2C6H3), 7.12 (m, 4H, m-2,6-Me2C6H3). 13C {1H} NMR
(CD2Cl2, 25 ꢀC): 10.1 (BMe), 10.4 (C5Me5), 17.4 (Me2C6H3), 52.0
(TaMe), 125.6 (C5Me5), 126.5 (Me2C6H3), 127.7 (Me2C6H3), 129.5
(Me2C6H3), 136.4 (C6F5), 137.9 (C6F5), 148.7 (C6F5), 155.8 (Cipso
,
Me2C6H3). 19F NMR (CD2Cl2): ꢁ130.0 (o-C6F5), ꢁ162.2 (p-C6F5),
ꢁ164.7 (m-C6F5). Anal. Calc. for C46H39BF15O2Ta (1100.54): C, 50.20;
H, 3.57. Found: C, 49.30; H, 3.08.
The same procedure as above for compound
3 (0.020 g,
0.034 mmol) and Al(C6F5)3 (0.021 g, 0.040 mmol) gave 6-Al
(0.033 g, 88%). 1H NMR (CD2Cl2, 25 ꢀC): 0.18 (br s, 3H, MeAl), 2.05 (s,
3H, MeTa), 2.28 (s, 12H, Me2C6H3), 2.30 (s, 15H, C5Me5), 7.06 (m, 2H,
Me2C6H3), 7.16 (m, 4H, Me2C6H3). 13C {1H} NMR (CD2Cl2, 25 ꢀC): 10.7
(C5Me5), 17.5 (Me2C6H3), 52.3 (TaMe), 125.6 (C5Me5), 125.8
(Me2C6H3), 127.7 (Me2C6H3), 129.2 (Me2C6H3), 136.5 (C6F5), 140.5
(C6F5), 149.9 (C6F5), 155.8 (Cipso, Me2C6H3). 19F NMR (CD2Cl2):
ꢁ120.5 (o-C6F5), ꢁ160.0 (p-C6F5), ꢁ165.1 (m-C6F5). Anal. Calc. for
C46H39AlF15O2Ta (1116.71): C, 49.48; H, 3.52. Found: C, 48.76; H,
3.83.
4.8. [NbCp0Me2(OSiiPr2)][MeE(C6F5)3] (E ¼ B, 5b-B; Al, 5b-Al)
The same procedure as above from 2b (0.020 g, 0.045 mmol)
and B(C6F5)3 (0.027 g, 0.053 mmol) gave 5b-B (0.035 g, 81%). 1H
NMR (C6D5Br): ꢁ0.04 (s, 9H, SiMe3), 0.87 (s, 21H, SiiPr2), 1.14 (s, 3H,
MeB), 1.42 (s, 6H, MeNb), 6.03 (m, 2H, C5H4), 6.29 (m, 2H, C5H4). 13
C
{1H} NMR (C6D5Br): ꢁ0.9 (SiMe3), 11.0 (BMe), 12.9 (CHMe2), 17.0
(CHMe2), 66.5 (MeNb), 118.1 (C5H4), 120.2 (C5H4), 135.8 (Cipso, C5H4),
136.3 (C6F5), 137.9 (C6F5), 148.5 (C6F5). 19F NMR (C6D5Br): ꢁ131.6 (o-
C6F5), ꢁ163.8 (p-C6F5), ꢁ166.2 (m-C6F5). Adequate elemental anal-
ysis could not be obtained for this compound.
4.11. Polymerization of MMA
The same procedure as above from 2b (0.020 g, 0.045 mmol)
and Al(C6F5)3 (0.031 g, 0.053 mmol) gave 5b-Al (0.034 g, 79%). 1H
NMR (C6D5Br): 0.04 (s, 9H, SiMe3), 0.21 (br s, 3H, MeAl), 0.87 (s, 21H,
[NbCp0Me3(OR)] (R ¼ 2,6-Me2C6H3 2c) (0.08 mmol) and Al
(C6F5)3 (0.12 mmol) were premixed in 4 mL of toluene in a Teflon-
valved ampoule and MMA (1 g, [MMA]:[Ta] ¼ 125:1) was added.
The ampoule was stirred at 40 ꢀC. The polymerization was termi-
nated by adding MeOH/HCl. The isolated polymer was washed first
with MeOH/HCl and then with MeOH/water and dried overnight in
vacuo at 60 ꢀC. A 1H NMR (CDCl3) of the polymer was carried out to
determine its tacticity [58]. Gel permeation chromatography (GPC)
analyses of polymer samples were carried out in THF at 25 ꢀC
(Waters GPCV-2000).
SiiPr2), 1.42 (s, 6H, MeNb), 6.03 (m, 2H, C5H4), 6.29 (m, 2H, C5H4). 13
C
{1H} NMR (C6D5Br): ꢁ5.7 (AlMe), ꢁ1.1 (SiMe3), 13.0 (CHMe2), 17.6
(CHMe2), 66.7 (MeNb), 117.6 (C5H4), 119.8 (C5H4), 118.3 (Cipso, C5H4),
136.3 (C6F5), 137.9 (C6F5), 148.5 (C6F5). 19F NMR (C6D5Br): ꢁ120.6
(o-C6F5), ꢁ158.3 (p-C6F5), ꢁ163.3 (m-C6F5). Adequate elemental
analysis could not be obtained for this compound.
4.9. [NbCp0Me2(OR)][MeE(C6F5)3] (R ¼ 2,6-Me2C6H3; E ¼ B, 5c-B;
Al, 5c-Al)
Acknowledgments
The same procedure as above for compound 2c (0.020 g,
0.050 mmol) and B(C6F5)3 (0.031 g, 0.060 mmol) afforded 5c-B
(0.041 g, 90%). 1H NMR (C6D5Br): ꢁ0.15 (s, 9H, SiMe3), 1.14 (br s, 3H,
BMe), 1.58 (br s, 6H, MeNb), 1.87 (s, 6H, 2,6-Me2C6H3), 6.09 (m, 2H,
C5H4), 6.19 (m, 2H, C5H4), 6.81 (m, 3H, m- and p-2,6-Me2C6H3).
We gratefully acknowledge Ministerio de Educación y Ciencia
(project MAT2007-60997) and DGUI-Comunidad de Madrid (pro-
gramme S-0505/PPQ-0328, COMAL-CM) (Spain) for financial
support. C.C. acknowledges Comunidad de Madrid for a fellowship.

J. Sánchez-Nieves et al. / Journal of Organometallic Chemistry 695 (2010) 2469e2473
2473
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Supplementary material associated with this article can be found
in the online version, at doi:10.1016/j.jorganchem.2010.06.030.
[28] M.V. Galakhov, M. Gómez, P. Gómez-Sal, P. Velasco, Organometallics 24
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