Synthesis of the cation complex [TaCp*Me3]+ and a comparison of its reactivity with that of [TaCp*Me4] +

Article abstract of DOI:10.1021/om060027c

The new ionic compound [TaCp*Me₃][MeB(C₆F ₅)₃] (1; Cp* = η⁵-C ₅Me₅) was obtained by reaction of [TaCp*Me ₄] with B(C₆P₅)₃. Addition of pyridine to the strongly acidic cation of 1 gave the new pyridine-coordinated cation [TaCp*Me₃(py)]⁺ (2), Hydrolysis of 1 in wet dichloromethane yielded the ionic compound [(TaCp*Me₂) ₂(μ-O)][MeB(C₆F₅)₃]₂ (3), containing a dinuclear dication identified by X-ray diffraction. Similar hydrolysis of cation 1 as well as its pyridine adduct 2 with H ₂O·B(C₆F₅)₃ afforded the neutral oxoborane compound [TaCp*Me₂{O·B(C ₆F₅)₃}] with elimination of B(C ₆F₅)₃ and py·B(C₆F ₅)₃, respectively. Cation 1 inserted 1 equiv of PhC(O)H at room temperature to give the cationic alkoxo complex [TaCp*Me ₂(OCHMePh)]⁺ (4), which reacted further with an additional 1 equiv of PhC(O)H to give a mixture of the rac and meso diastereoisomers of [TaCp*Me(OCHMePh)₂]⁺ (5), whereas only the neutral alkoxo compound [TaCp*Me₃(OCHMePh)] (6) was formed from the reaction of [TaCp*Me₄] with PhC(O)H upon heating. Cation 1 also reacted with 1 and 2 equiv of PhNCO to give [TaCp*Me₂-{OC(Me) NPh}]⁺ (7) and [TaCp*Me{OC(Me)NPh}₂]⁺ (8) by insertion of isocyanate into one or two Ta-C bonds, respectively. However, heating [TaCp*Me₄] with PhNCO gave the dinuclear μ-oxo compound [(TaCp*Me₃)₂(μ-O)] (9). Compound 1 showed rather low activity when tested as an ethylene polymerization catalyst.

Full text of DOI:10.1021/om060027c

Organometallics 2006, 25, 2331-2336
2331
Synthesis of the Cation Complex [TaCp*Me3]+ and a Comparison of
Its Reactivity with That of [TaCp*Me4]†
Javier Sa´nchez-Nieves, Pascual Royo,* and Marta E. G. Mosquera‡
Departamento de Qu´ımica Inorga´nica, UniVersidad de Alcala´, Campus UniVersitario,
E-28871 Alcala´ de Henares, Madrid, Spain
ReceiVed January 10, 2006
The new ionic compound [TaCp*Me3][MeB(C6F5)3] (1; Cp* ) η5-C5Me5) was obtained by reaction
of [TaCp*Me4] with B(C6F5)3. Addition of pyridine to the strongly acidic cation of 1 gave the new
pyridine-coordinated cation [TaCp*Me3(py)]+ (2). Hydrolysis of 1 in wet dichloromethane yielded the
ionic compound [(TaCp*Me2)2(µ-O)][MeB(C6F5)3]2 (3), containing a dinuclear dication identified by X-ray
diffraction. Similar hydrolysis of cation 1 as well as its pyridine adduct 2 with H2O‚B(C6F5)3 afforded
the neutral oxoborane compound [TaCp*Me2{O‚B(C6F5)3}] with elimination of B(C6F5)3 and py‚B(C6F5)3,
respectively. Cation 1 inserted 1 equiv of PhC(O)H at room temperature to give the cationic alkoxo
complex [TaCp*Me2(OCHMePh)]+ (4), which reacted further with an additional 1 equiv of PhC(O)H to
give a mixture of the rac and meso diastereoisomers of [TaCp*Me(OCHMePh)2]+ (5), whereas only the
neutral alkoxo compound [TaCp*Me3(OCHMePh)] (6) was formed from the reaction of [TaCp*Me4]
with PhC(O)H upon heating. Cation 1 also reacted with 1 and 2 equiv of PhNCO to give [TaCp*Me2-
{OC(Me)NPh}]+ (7) and [TaCp*Me{OC(Me)NPh}2]+ (8) by insertion of isocyanate into one or two
Ta-C bonds, respectively. However, heating [TaCp*Me4] with PhNCO gave the dinuclear µ-oxo
compound [(TaCp*Me3)2(µ-O)] (9). Compound 1 showed rather low activity when tested as an ethylene
polymerization catalyst.
described previously16 the versatile products resulting from the
insertion of isocyanides into the tantalum-methyl bonds of the
tantalum derivative [TaCp*Me4],12,14 for which related insertion
reactions of carbon monoxide12 and ketones17 have also been
reported.
The cationic derivatives from these group 4 parent compounds
are not thermally stable,18,19 due to their electron deficiency and
the coordinative unsaturation of their metal centers, which can
be alleviated by coordinating different ligands.20-23 Although
related cationic derivatives of group 5 metals would be expected
to be more stable, these types of species has not been reported
and, in addition, few examples of monocyclopentadienyl
tantalum compounds suitable for ethylene polymerization have
been described,24-27 principally due to the expected lower acidity
of the cationic intermediates compared with that of the corre-
Introduction
The synthesis of stable alkyl cationic complexes of the early
transition metals is an objective of prime importance in
organometallic chemistry, as these compounds are active in
many catalytic processes. The isolation and structural charac-
terization of these types of thermally stable compounds provide
a further insight into the mechanism of the catalytic reactions
and also improves their catalytic efficiency. Monocyclopenta-
dienyl trialkyl group 41-10 and tetraalkyl Nb11 and Ta12-14
compounds are well-known, although the presence of bulky
alkyl groups in the Nb and Ta compounds causes the formation
of alkylidene ligands through R-hydrogen elimination.13,15 We
* To whom correspondence should be addressed. Fax: 00 34 91 885
4683. E-mail: pascual.royo@uah.es.
† This paper is dedicated to Professor V´ıctor Riera on the occasion of
his 70th birthday.
(15) de Castro, I.; de la Mata, J.; Go´mez, M.; Go´mez-Sal, P.; Royo, P.;
Selas, J. M. Polyhedron 1992, 11, 1023.
‡ X-ray diffraction studies.
(1) Giannini, U.; Cesca, S. Tetrahedron Lett. 1960, 19.
(2) Cannell, L. G. J. Am. Chem. Soc. 1972, 94, 6867.
(3) Chivers, T.; Ibrahim, E. D. J. Organomet. Chem. 1974, 77, 241.
(4) Green, M. L. H.; Lucas, C. R. J. Organomet. Chem. 1974, 73, 259.
(5) Erker, G.; Berg, K.; Treschanke, L.; Engel, K. Inorg. Chem. 1982,
21, 1277.
(6) Wolczanski, P. T.; Bercaw, J. E. Organometallics 1982, 1, 793.
(7) Mena, M.; Pellinghelli, M. A.; Royo, P.; Serrano, R.; Tiripicchio,
A. J. Chem. Soc., Chem. Commun. 1986, 1118.
(8) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701.
(9) Mena, M.; Royo, P.; Serrano, R.; Pellinghelli, M. A.; Tiripicchio,
A. Organometallics 1989, 8, 476.
(10) Cano, A. M.; Cano, J.; Go´mez-Sal, P.; Manzanero, A.; Royo, P.
Inorg. Chim. Acta 1998, 280, 1.
(16) Galakhov, M. V.; Go´mez, M.; Jime´nez, G.; Pellinghelli, M. A.;
Royo, P.; Tiripicchio, A. Organometallics 1994, 13, 1564.
(17) Fandos, R.; Herna´ndez, C.; Otero, A.; Rodr´ıguez, A.; Ruiz, M. J.;
Terreros, P. Dalton Trans. 2000, 2990.
(18) Gillis, D. J.; Tudoret, M. J.; Baird, M. C. J. Am. Chem. Soc. 1993,
115, 2543.
(19) Wang, Q. Y.; Gillis, D. J.; Quyoum, R.; Jeremic, D.; Tudoret, M.
J.; Baird, M. C. J. Organomet. Chem. 1997, 527, 7.
(20) Amor, J. I.; Cuenca, T.; Galakhov, M.; Royo, P. J. Organomet.
Chem. 1995, 497, 127.
(21) Amor, J. I.; Cuenca, T.; Galakhov, M.; Go´mez-Sal, P.; Manzanero,
A.; Royo, P. J. Organomet. Chem. 1997, 535, 155.
(22) Deckers, P. J. W.; van der Linden, A. J.; Meetsma, A.; Hessen, B.
Eur. J. Inorg. Chem. 2000, 929.
(11) Alcalde, M. I.; de la Mata, J.; Go´mez, M.; Royo, P.; Sa´nchez, F. J.
Organomet. Chem. 1995, 492, 151.
(12) Wood, C. D.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101, 5421.
(13) Messerle, L. W.; Jennische, P.; Schrock, R. R.; Stucky, G. J. Am.
Chem. Soc. 1980, 102, 6744.
(14) Sanner, R. D.; Carter, S. T.; Bruton, W. J. J. Organomet. Chem.
1982, 240, 157.
(23) Deckers, P. J. W.; Hessen, B. Organometallics 2002, 21, 5564.
(24) Mashima, K.; Fujikawa, S.; Nakamura, A. J. Am. Chem. Soc. 1993,
115, 10990.
(25) Mashima, K.; Fujikawa, S.; Tanaka, Y.; Urata, H.; Oshiki, T.;
Tanaka, E.; Nakamura, A. Organometallics 1995, 14, 2633.
(26) Antonelli, D. M.; Leins, A.; Stryker, J. M. Organometallics 1997,
16, 2500.
10.1021/om060027c CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/28/2006

2332 Organometallics, Vol. 25, No. 9, 2006
Sa´nchez-NieVes et al.
a
Scheme 1
Figure 1. ORTEP diagram of the cation of [(TaCp*Me2)2(µ-O)]-
[MeB(C6F5)3]2 (3). Hydrogen atoms have been omitted, and thermal
ellipsoids are shown at the 50% level. Selected bond distances (Å)
and angles (deg): Ta(1)-O ) 1.914(3), Ta(2)-O ) 1.910(3), Ta-
(1)-C(6) ) 2.120(5), Ta(1)-C(7) ) 2.125(5), Ta(2)-C(8) )
2.135(5), Ta(2)-C(9) ) 2.118(5); Ta(1)-O-Ta(2) ) 173.7(2).
Compound 1 was thermally stable under an inert atmosphere
for long periods even in halogenated solvents. We believe that
the remarkable thermal stability of its CH2Cl2 solution is due
to the reduced Lewis acidity of the metal center imposed by
the strongly donating pentamethylcyclopentadienyl ligand and
the inductive effect of the three tantalum-bonded methyl groups,
as was confirmed by the X-ray structure of one of the ionic
complexes obtained in this work (vide infra). Nevertheless,
compound 1 was still a strong Lewis acid that reacted with
pyridine to give the cationic adduct [TaCp*Me3(py)][MeB-
(C6F5)3] (2) (Scheme 1), with no evidence of methyl or C6F5
transfer from the [MeB(C6F5)3]- anion, as usually observed for
compounds in which bonding interactions between such anion
a Legend: (i) B(C6F5)3; (ii) pyridine; (iii) H2O‚B(C6F5)3; (iv) H2O;
(v) C2H4.
sponding group 4 catalysts. However, the related cationic
dialkyltantalocene complexes [TaCp2Me2]+ were easily prepared
28,29
by reaction of the trimethyl derivative with Ph3CBF4
or
oxidation of neutral dialkyltantalocene with AgBF4.30 More
recently it has been reported that activation of [TaCp2Me3] with
B(C6F5)3 or Al(C6F5)3 gave a stable dimethyltantalocene cation,
characterized by X-ray diffraction, which could not coordinate
the [MeM(C6F5)3]- counteranion.31
1
and cation pairs are present. The H and 13C NMR spectra of
compound 2 showed one narrow signal due to the three
equivalent Ta-Me groups and the signals expected for a
symmetric pyridine ligand. This behavior is consistent with a
triangular-bipyramidal geometry, with the pyridine ligand
located in an axial position trans to the Cp* ligand and the three
methyl groups occupying the equatorial coordination sites. The
19F NMR spectrum of 2 showed resonances very similar to those
described for 1 and for the remaining ionic complexes described
in this paper that will not be discussed further.
Herein we report the synthesis and chemical behavior of the
cationic compound [TaCp*Me3]+ obtained by reaction of the
tetramethyl derivative [TaCp*Me4] with B(C6F5)3. We also
report the comparative reactivities of both neutral and cationic
complexes in the insertion reactions of CdO-containing com-
pounds such as benzaldehyde (PhC(O)H) and phenyl isocyanate
(PhNCO) and the structural characterization of the resulting
alkoxo and amidate derivatives. Furthermore, we describe the
catalytic activity observed when the new permethylated cationic
complex was used as an ethylene polymerization catalyst.
The cation of compound 1 was very air sensitive and highly
unstable in the presence of traces of water. Hence, in our attempt
to crystallize compound 1 from CH2Cl2 solutions, yellow
crystals suitable for X-ray diffraction studies of the dinuclear
complex [(TaCp*Me2)2(µ-O)][MeB(C6F5)3]2 (3) were obtained,
as a consequence of hydrolysis of one Ta-Me bond and
formation of an oxo bridge. However, we were unable to isolate
pure complex 3 by hydrolysis reactions of 1 to allow spectro-
scopic characterization. The molecular structure of cation 3
(Figure 1) presents a distorted-tetrahedral environment about
the Ta atom with an almost linear Ta-O-Ta bridge (173.7-
(2)°), indicating the high contribution of the dπ-pπ interaction
to the Ta-O bond. This feature is similar to that found in the
related neutral µ-oxo compound [(TaCp*Me3)2(µ-O)], which
presents an exactly linear Ta-O-Ta bridge (180°).33 Both
Ta-O bond distances (1.910(3) and 1.914(3) Å) are in the
middle of the range of Ta-O bond values of complexes
containing Ta-O-Ta bridges (1.82-2.10 Å) and are very close
to those found in the neutral µ-oxo compound [(TaCp*Me3)2-
(µ-O)] (1.909(7) Å).33 This Ta-O bond in compound 3 is
surprisingly longer than expected, due to the higher electrophi-
licity of the 14-electron Ta atom, with respect to the 16-electron
Ta atom in the neutral complex [(TaCp*Me3)2(µ-O)], assuming
Results and Discussion
Synthesis and Reactivity of Cationic Compounds. Addition
of 1 equiv of the Lewis acid B(C6F5)3 to a toluene solution of
the peralkylated monocyclopentadienyl tantalum compound
[TaCp*Me4] (Cp* ) η5-C5Me5) afforded high yields of an
insoluble brownish oil characterized as the ionic derivative
[TaCp*Me3][MeB(C6F5)3] (1) (Scheme 1). Compound 1 was
soluble in dichloromethane, its 1H and 13C NMR spectra in CD2-
Cl2 showed one resonance for the three equivalent Ta-Me
groups of a pseudo-tetrahedral cation, and one broad signal for
the methyl group of the borate anion was also observed in the
1H NMR spectrum. Furthermore, the resonances in the 19F NMR
spectrum were in agreement with the formation of the [MeB-
(C6F5)3] anion, the difference between the m- and p-C6F5 values
(∆δ ) 2.6) being consistent with the presence of separate
cationic and anionic [MeB(C6F5)3]- moieties.32
(27) Decker, J. M.; Geib, S. J.; Meyer, T. Y. Organometallics 1999, 18,
4417.
(28) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577.
(29) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
(30) Lappert, M. F.; Milne, C. R. C. J. Chem. Soc., Chem. Commun.
1978, 925.
(32) Horton, A. D.; de With, J.; van der Linden, A. J.; van der Weg, H.
Organometallics 1996, 15, 2672.
(31) Feng, S. G.; Roof, G. R.; Chen, E. Y. X. Organometallics 2002,
21, 832.
(33) Jernakoff, P.; de Bellefon, C. D.; Geoffroy, G. L.; Rheingold, A.
L.; Geib, S. J. Organometallics 1987, 6, 1362.

Synthesis and ReactiVity of the [TaCp*Me3]+ Cation
Organometallics, Vol. 25, No. 9, 2006 2333
Scheme 2a
that the bridging oxygen atom is a 3-electron donor for each
metal center. On the other hand, the Ta-Me bond distances
were mainly affected in compound 3 in comparison with
[(TaCp*Me3)2(µ-O)], decreasing from the range 2.223-2.259
Å to the range 2.118-2.135 Å, indicating the higher inductive
donor capacity of the methyl groups. This increased inductive
effect of the methyl groups together with the π-bonding donation
observed for the µ-oxo bridging system compensates the electron
deficiency of the metal center enough to stabilize the dinuclear
dication, keeping the free methylborate anion without any
interionic pairing. Therefore, the free cationic species is stable
and can be isolated not only when three methyl groups are
coordinated to the metal, as in the cation of compound 1, but
also when one methyl group is substituted by the less donating
µ-oxo ligand, leaving only two additional methyl groups.
In the view of this result, we tried to hydrolyze compound 1
with the oxo transfer reagent H2O‚B(C6F5)3. Hence, the reaction
of 1 with 0.5 mol of the water adduct in CD2Cl2 was monitored
by 1H NMR spectroscopy; the only reaction products observed
were the neutral oxo-borane derivative [TaCp*Me2{O‚
B(C6F5)3}]34 and the free borane compound B(C6F5)3, together
with the starting compound 1 (Scheme 1). This process is
consistent with the protonolysis of two Ta-Me bonds on the
same metal center to form the undetectable, electron-deficient,
and coordinatively unsaturated cation [TaCp*Me{O‚B(C6F5)3}]+,
which is immediately stabilized by transfer of the methyl group
from the [MeB(C6F5)3]- anion, making evident that the Lewis
acidity of the intermediate (oxoborane)methyltantalum cation
is stronger than that known for the borane B(C6F5)3. This
proposal is supported by the lack of reactivity of [TaCp*Me2-
{O‚B(C6F5)3}] toward the Lewis acid B(C6F5)3. The same
behavior was observed when the pyridine cationic adduct 2 was
treated with the water adduct H2O‚B(C6F5)3, this process also
being consistent with the transfer of the methyl group from the
[MeB(C6F5)3]- anion to the intermediate undetected (oxobo-
rane)methyl cation to give the neutral compound [TaCp*Me2-
{O‚B(C6F5)3}] and the pyridine adduct py‚B(C6F5)3.
Insertion Reactions of Benzaldehyde and Phenyl Isocy-
anate. We have discussed above that the more electrophilic
metal center of cationic compounds is responsible for the
increased inductive effect of the remaining methyl groups, for
which shorter Ta-Me bond distances are observed. It should
be expected that insertion reactions would be favored by a higher
positive charge at the Ta atom that would increase the
electrophilic character of the atom at the coordinated ligand,
which is attacked by the migrating alkyl group. However, it
could be also expected that migration should be hindered for
the more stable metal-alkyl bonds. To find the right answer to
this matter, we decided to develop a comparative study of
insertion reactions of benzaldehyde and phenyl isocyanate into
the Ta-Me bonds of the 12-electron cationic trimethyl com-
pound 1 [TaCp*Me3]+ and the 14-electron neutral tetramethyl
complex [TaCp*Me4].
a Legend: (i) PhC(O)H, 20 °C; (ii) PhC(O)H, 70 °C; (iii) B(C6F5)3.
spectrum showed one resonance at δ 89.2 for the alkoxo carbon
atom. The chirality at the carbon atom of this OCHMePh ligand
is consistent with the presence of two resonances for the
diastereotopic Ta-Me groups in both 1H and 13C NMR spectra.
The same reactivity was observed for compound 4 in the
presence of a second equivalent of PhC(O)H, which after 6 h
at room temperature gave a mixture in an approximately 3:2.5
molar ratio of the rac and meso diastereoisomers of the complex
[TaCp*Me(OCHMePh)2][MeB(C6F5)3] (5) (Scheme 2). This is
as expected for the presence of two stereogenic carbon atoms,
one for each OCHMePh ligand. This mixture could not be
separated, although the small difference in the proportion of
the two components helped us to distinguish some H and 13C
1
NMR resonances for each diastereoisomer. Hence, the Cs
symmetry of each molecule of the meso diastereoisomer (SR/
RS, 5a) gave rise to one doublet for the methyl group (1H NMR)
and only one resonance for the carbon atom (13C NMR) of the
alkoxo group. In contrast, the lack of any symmetry of the rac
diastereoisomer (RR/SS, 5b) was reflected by the presence of
1
two doublets for the methyl group in the H NMR and of two
resonances for the carbon atoms of the OCHMePh groups in
the 13C NMR.
Comparatively much less reactive was the neutral compound
[TaCp*Me4], which reacted with 1 equiv of benzaldehyde upon
heating at 70 °C for 24 h, leading to [TaCp*Me3(OCHMePh)]
(6) (Scheme 2). No further insertion of a second molecule of
PhC(O)H was observed, and heating 6 over 100 °C caused
decomposition. Compound 6 was also a precursor for the
cationic complex 4 upon addition of 1 equiv of the Lewis acid
B(C6F5)3. It seems clear that the higher electron density at the
tantalum atom of [TaCp*Me4] hindered the insertion reaction,
and once the 16-electron alkoxo compound 6 was formed
(considering a 3-electron-donor oxygen atom) the electronic and
steric saturation at the metal center blocked the possible insertion
of PhC(O)H. The 1H and 13C NMR spectra of 6 showed signals
corresponding to the formation of the new alkoxo OCHMePh
ligand, as was the case for 4 and 5 (vide supra).
Similar studies were carried out for insertion reactions of
PhNdCdO. Reaction of the cationic complex 1 with 1 equiv
of phenyl isocyanate at room temperature gave the dimethyl
amidate complex [TaCp*Me2{η2-OC(Me)NPh}][MeB(C6F5)3]
(7) (Scheme 3), as a result of the insertion of the CdO group
into one Ta-Me bond. This reaction was not selective but was
followed by a second insertion into another Ta-Me bond,
affording the monomethyl derivative [TaCp*Me{η2-OC(Me)-
Complex 1 reacted with benzaldehyde (PhC(O)H) to form
the alkoxo compound [TaCp*Me2(OCHMePh)][MeB(C6F5)3]
(4) at room temperature in 1 h (Scheme 2), after a process that
involved coordination of the oxygen atom to the metal center
and further migration of one methyl group to the sp2 C atom.
The main spectroscopic 1H NMR data that confirmed the
formation of 4 were the doublet at δ 1.90 and the quartet at δ
5.86 for the methyl group and the hydrogen atom of the alkoxo
OCHMePh fragment, respectively. Moreover, the 13C NMR
(34) Sa´nchez-Nieves, J.; Frutos, L. M.; Royo, P.; Castan˜o, O.; Herdtweck,
E. Organometallics 2005, 24, 2004.

2334 Organometallics, Vol. 25, No. 9, 2006
Sa´nchez-NieVes et al.
Scheme 3a
reported for related monocyclopentadienyl group 4 compounds,
taking into account the lower acidity of the 12-electron cationic
tantalum complex [TaCp*Me3]+ in relation to the 10-electron
group 4 metal species [MCpR2]+. The activity of complex 1 is
clearly lower than that reported for other nonmetallocene
tantalum compounds such as [TaCl3(aminopyridinato)2] (1000
g of PE (mmol of Ta)-1 h-1 atm-1; MAO, 60 °C, 1 atm),36
[TaCp*Cl3{MeC(NiPr)2}] (470 g of PE (mmol of Ta)-1 h-1
atm-1; MAO, 60 °C, 8 atm),27 and [TaCp*Cl2(NSitBu3)] (60 g
of PE (mmol of Ta)-1 h-1 atm-1; MAO, 25 °C, 5 atm)26 but
close to that reported for [TaCp*X2(η4-diene)] (X ) Cl, Me;
1-7 g of PE (mmol of Ta)-1 h-1 atm-1; MAO, -60 to -20
°C, 1 atm),24,25 although in this case the polydispersity was ca.
1.0. On the other hand, the alkyl derivatives [TaCp*Me3{MeC-
(NiPr)2}]27 and [TaCp*(CH2Ph)2(NtBu)]37 showed no activity;
meanwhile, the allyl compound [TaCp*(η1-C3H5)(η3-C3H5)-
(NAr)] (Ar ) 2,6-iPr2C6H3) was scarcely active for ethylene
polymerization (38 g of PE (mmol of Ta)-1 h-1 atm-1; [Ph3C]-
[B(C6F5)4]; 25 °C, 1 atm).26
a Legend: (i) PhNCO, 20 °C; (ii) PhNCO, 90 °C; (iii) B(C6F5)3.
Conclusions
NPh}2][MeB(C6F5)3] (8) (Scheme 3), leaving part of the
unreacted starting complex 1. However, the 16-electron com-
pound 8 was isolated as the unique reaction product when 1
was treated with 2 equiv of isocyanate for 3 h in dichlo-
romethane. The most significant NMR feature for compounds
7 and 8 was the resonance in the 13C NMR spectrum at about
δ 185 corresponding to the C-O carbon atom and the high-
field shift (δ 18.6 for 7 and δ 20.2 for 8) of the migrated
Methyl abstraction from [TaCp*Me4] with the strong Lewis
acid B(C6F5)3 gave the thermally stable ionic compound
[TaCp*Me3][MeB(C6F5)3], which remained unchanged in CH2-
Cl2. The 12-electron cation [TaCp*Me3]+ was more reactive
than its 14-electron neutral precursor [TaCp*Me4]. It was shown
to be a strong Lewis acid that coordinated pyridine to give the
14-electron cation [TaCp*Me3(py)]+ and was extremely unstable
in the presence of traces of water to give the dinuclear cation
[(TaCp*Me2)2(µ-O)]2+. However, hydrolysis of [TaCp*Me3]+
and [TaCp*Me3(py)]+ with the oxo transfer reagent H2O‚
B(C6F5)3 afforded the neutral compound [TaCp*Me2{O‚
B(C6F5)3}] by methyl transfer from the borate anion to the
unstable cationic intermediate, which was a stronger Lewis acid.
The tantalum atom of the cationic complex [TaCp*Me3]+ was
highly electrophilic, and insertion at room temperature of 1 and
2 equiv of the CdO-containing compounds PhC(O)H and
PhNCO into one and two Ta-Me bonds was observed, giving
the alkoxo and amidate cations [TaCp*Me2(OCHMePh)]+ and
[TaCp*Me2{η2-OC(Me)NPh}]+ in a first step and the dialkoxo
and diamidate cations [TaCp*Me(OCHMePh)2]+ and [TaCp*Me-
{η2-OC(Me)NPh}2]+ after the second insertion, respectively.
In contrast, only one molecule of PhC(O)H and PhNCO was
inserted into one Ta-Me bond upon heating of the less
electrophilic compound [TaCp*Me4], forming the corresponding
alkoxo derivative [TaCp*Me3(OCHMePh)] and the dinuclear
compound [(TaCp*Me3)2(µ-O)], respectively.
All of the ionic complexes reported in this paper showed δ-
(Fmeta) - δ(Fpara) differences in the 19F NMR spectra lower than
2.8 ppm, demonstrating the presence of separated ion pairs.32
Furthermore, this difference was almost the same for all of the
complexes, whatever the electron density at the metal center,
which changed from 12 electrons in cation 1 to 14 electrons in
2 and 4 (considering the alkoxo group as a 3-electron-donor
ligand) and in 7 and to 16 electrons in 5 and 8. Also, cation 8,
with two bidentate amidate ligands, one methyl group, and one
cyclopentadienyl ring, was coordinatively saturated, preventing
any possible ion-pairing interaction.
1
C-bonded methyl group, which is observed in the H NMR
spectrum as a singlet shifted to low field (δ 2.23 for 7 and δ
1.98 for 8), in comparison with that observed for the starting
complex 1. The acidity of the metal center and the length of
the Ta-O-C-N chain should favor a chelate disposition of
the amidate ligand, as previously observed for early-transition-
metal complexes with this type of ligand.35
Surprisingly, a similar reaction of the tetramethyl complex
[TaCp*Me4] with 1 equiv of PhNCO did not give the expected
insertion product; instead, the dinuclear oxo-bridged complex
[(TaCp*Me3)2(µ-O)] (9) was isolated in moderate yield upon
heating in toluene at 90 °C (Scheme 3). Compound 9 was the
only tantalum-containing product of the reaction, even in the
presence of excess PhNCO. When the experiment was carried
out into a sealed NMR tube and the reaction was monitored by
1H NMR spectroscopy, broadening of signals corresponding to
the phenyl ring was observed and the resonances belonging to
the starting PhNCO disappeared. Unfortunately we could not
identify the organic byproduct. It is important to note that this
process required the addition of an equimolecular amount of
PhNCO to complete the reaction, because in the presence of a
smaller amount of PhNCO the corresponding remnant of starting
[TaCp*Me4] was recovered. Unexpectedly, complex 9 could
not be used as a precursor to isolate the dicationic dinuclear
compound 3 by reaction with 2 equiv of B(C6F5)3, which only
led to an unidentified mixture of reaction products.
Ethylene Polymerization. The ionic compound 1 generated
in situ was studied as an ethylene polymerization catalyst. It
exhibited rather poor activity (1.6 g of PE (mmol of Ta)-1 h-1
atm-1 at 50 °C and 5 atm over 1 h), yielding HDPE (Tm ) 135
°C) with a molecular mass of ca. 45 × 103 g mol-1 and a narrow
polydispersity (Mw/Mn ) 2.5). Lower catalytic activity was
observed when lower temperature and ethylene pressure were
used. This behavior is not surprising, in comparison with that
(36) Hakala, K.; Lofgren, B.; Polamo, M.; Leskela, M. Macromol. Rapid
Commun. 1997, 18, 635.
(37) Coles, M. P.; Dalby, C. I.; Gibson, V. C.; Little, I. R.; Marshall, E.
L.; da Costa, M. H. R.; Mastroianni, S. J. Organomet. Chem. 1999, 591,
78.
(35) Braunstein, P.; Nobel, D. Chem. ReV. 1989, 89, 1927.

Synthesis and ReactiVity of the [TaCp*Me3]+ Cation
Organometallics, Vol. 25, No. 9, 2006 2335
Finally, the cation complex [TaCp*Me3]+ was active for
ethylene polymerization, although it showed rather low catalytic
activity.
[TaCp*Me(OCHMePh)2][MeB(C6F5)3] (5). PhC(O)H (0.058
g, 0.54 mmol) was added to a solution of [TaCp*Me3][MeB(C6F5)3]
(1; 0.26 mmol) in CH2Cl2 (5 mL), prepared in situ. After the mixture
was stirred for 6 h, the volatiles were removed under vacuum and
the remaining oil was washed first with toluene and then with
hexane to give 5 (0.26 g, 89%). Data for 5: 1H NMR (CD2Cl2)
0.50 (bs, 6 H, Me-B, 5a and 5b), 1.07 (s, 3 H, Me-Ta, 5a), 1.17
(s, 3 H, Me-Ta, 5b), 1.56 (d, 3 H, 3J(H-H) ) 6.5 Hz, Me-C-O,
Experimental Section
General Considerations. All manipulations were carried out
under an argon atmosphere, and solvents were distilled from
appropriate drying agents. NMR spectra were recorded at 300.13
(1H), 188.31 (19F), and 75.47 (13C) MHz at room temperature on a
Varian Unity 300. The 13C NMR data for the methyl group of the
[MeB(C6F5)3] anion was observed by gHMQC experiments.
Chemical shifts (δ) are given in ppm relative to internal TMS (1H
and 13C) or external CFCl3 (19F). Elemental analyses were performed
on a Perkin-Elmer 240C. The compounds [TaCp*Me4],14 B(C6F5)3,38
and [H2O‚B(C6F5)3]39 were prepared by literature methods. PhC-
(O)H and PhNCO were distilled from CaH2 and P2O5, respectively,
under vacuum and stored under argon.
[TaCp*Me3][MeB(C6F5)3] (1). [TaCp*Me4] (0.100 g, 0.26
mmol) and B(C6F5)3 (0.150 g, 0.29 mmol) were stirred in toluene
(5 mL) for 5 min. The solution was then filtered off, leaving an oil
that was washed with hexane (5 mL), to give 1 as a brownish oil
(0.20 g, 85%). Data for 1: 1H NMR (CD2Cl2) 0.47 (bs, 3 H, Me-
B), 1.36 (s, 9 H, Me-Ta), 2.26 (s, 15 H, C5Me5); 13C NMR
(CD2Cl2) 10.4 (MeB(C6F5)3), 12.2 (C5Me5), 86.8 (Me-Ta), 125.3
(C5Me5), 136.4 (m, C6F5), 137.9 (m, C6F5), 148.7 (m, C6F5); 19F
NMR (CD2Cl2) -130.0 (o-C6F5), -162.1 (p-C6F5), -164.7 (m-
C6F5). Anal. Calcd for C32H27BF15Ta (888.29): C, 43.27; H, 3.06.
Found: C, 43.63; H, 3.14.
[TaCp*Me3(py)][MeB(C6F5)3] (2). Pyridine (0.021 g, 0.26
mmol) was added to a suspension of [TaCp*Me3][MeB(C6F5)3] (1;
0.26 mmol) in toluene (5 mL), prepared in situ. After the mixture
was stirred for 10 min, the solution was filtered off and the
remaining oil was washed with hexane (5 mL) to give 2 (0.21 g,
83%). Data for 2: 1H NMR (CD2Cl2) 0.47 (s, 3 H, Me-B), 0.50
(s, 9 H, Me-Ta), 2.26 (s, 15 H, C5Me5), 7.52 (m, 2 H, m-C5H5N),
7.91 (m, 1 H, p-C5H5N), 8.77 (m, 2 H, o-C5H5N); 13C NMR
(CD2Cl2) 10.7 (MeB(C6F5)3), 11.7 (C5Me5), 76.5 (Me-Ta), 121.5
(C5Me5), 125.8 (p-C5H5N), 135.7 (m, C6F5), 139.0 (m, C6F5), 139.3
(m-C5H5N), 148.2 (m, C6F5), 149.4 (o-C5H5N); 19F NMR (CD2Cl2)
-130.0 (o-C6F5), -162.1 (p-C6F5), -164.7 (m-C6F5). Anal. Calcd
for C37H32BF15NTa (967.39): C, 45.94; H, 3.33; N, 1.45. Found:
C, 45.38; H, 3.10; N, 1.29.
3
5b), 1.67 (d, 3 H, J(H-H) ) 6.5 Hz, Me-C-O, 5b), 1.72 (d, 6
H, 3J(H-H) ) 6.5 Hz, Me-C-O, 5a), 2.02 (s, 15 H, C5Me5, 5a),
2.05 (s, 15 H, C5Me5, 5b), 5.60-5.79 (m, 4 H, H-C-O), 7.15-
7.50 (m, 10 H, C6H5); 13C NMR (CD2Cl2) 10.6 (MeB(C6F5)3), 10.9
(C5Me5), 24.6 (Me-C-O, 5b), 25.0 (Me-C-O, 5b), 25.1 (Me-
C-O, 5a), 43.1 (Me-Ta), 43.7 (Me-Ta), 87.2 (C-O), 87.3 (C-
O), 87.4 (C-O), 124.6, 124.7, 126.0, 129.5, 129.6, 129.7, 129.8,
141.1, 141.2 (C6H5 and C5Me5), 136.8 (m, C6F5), 137.9 (m, C6F5),
148.2 (m, C6F5); 19F NMR (CD2Cl2) -131.3 (o-C6F5), -162.0 (p-
C6F5), -164.7 (m-C6F5). Anal. Calcd for C46H39BF15O2Ta
(1100.54): C, 50.20; H, 3.57. Found: C, 49.95; H, 3.48.
[TaCp*Me3(OCHMePh)] (6). A solution of [TaCp*Me4] (0.300
g, 0.78 mmol) and PhC(O)H (0.120 g, 1.20 mmol) in toluene (5
mL) was heated at 70 °C for 24 h. Afterward, the volatiles were
removed under vacuum and hexane was added (15 mL). The
solution was filtered and the filtrate was concentrated to a volume
of ca. 4 mL and cooled to -40 °C, yielding 6 as yellow crystals
(0.25 g, 65%). Data for 6: 1H NMR (CDCl3) 0.06 (s, 6 H, Me-
Ta), 0.17 (s, 3 H, Me-Ta), 1.55 (d, 3 H, 3J(H-H) ) 6.4 Hz, Me-
3
C-O), 1.86 (s, 15 H, C5Me5), 5.40 (q, 1 H, J(H-H) ) 6.4 Hz,
H-C-O), 7.30 (m, 5 H, C6H5); 13C NMR (CDCl3) 11.0 (C5Me5),
25.4 (Me-C-O), 50.5 (Me-Ta), 50.8 (Me-Ta), 53.2 (Me-Ta),
81.4 (C-O), 115.9 (C5Me5), 125.9, 127.1, 128.1, and 145.3 (C6H5).
Anal. Calcd for C21H33OTa (482.43): C, 52.28; H, 6.89. Found:
C, 52.10; H, 6.70.
[TaCp*Me2{OC(Me)NPh}][MeB(C6F5)3] (7). PhNCO (0.040
mmol) was added to a solution of [TaCp*Me3][MeB(C6F5)3] (1;
0.040 mmol) in CD2Cl2, prepared in situ. The reaction was
monitored by NMR until it was finished after 30 min, showing the
remaining unreacted 1 and the formation of 7 and 8. Data for 7:
1H NMR (CD2Cl2) 0.47 (bs, 3 H, Me-B), 0.95 (s, 6 H, Me-Ta),
2.18 (s, 15 H, C5Me5), 2.23 (s, 3 H, Me-C-O), 7.24-7.60 (C6H5,
underneath with those corresponding with compound 8); 13C NMR
(CD2Cl2) 10.5 (MeB(C6F5)3), 11.0 (C5Me5), 18.6 (Me-C-O), 77.1
(Me-Ta), 124.9, 125.4, 128.5, 130.5 and 139.3 (C6H5 and C5Me5),
135.7 (m, C6F5), 138.9 (m, C6F5), 148.5 (m, C6F5), 187.7 (C-O);
19F NMR (CD2Cl2) -130.5 (o-C6F5), -162.5 (p-C6F5), -165.1 (m-
C6F5).
Reaction of [TaCp*Me3][MeB(C6F5)3] (1) with H2O‚B(C6F5)3.
A Teflon-valved NMR tube was charged with a solution of 1 (0.018
g, 0.020 mmol) and H2O‚B(C6F5)3 (0.005 g, 0.010 mmol) in CD2-
1
Cl2. After 3 h, the H NMR spectrum revealed that the reactants
had been transformed into [TaCp*Me2{O‚B(C6F5)3}]34 and B(C6F5)3,
with minor amounts of other unidentified products.
[TaCp*Me{OC(Me)NPh}2][MeB(C6F5)3] (8). PhNCO (0.056
g, 0.56 mmol) was added to a solution of [TaCp*Me3][MeB(C6F5)3]
(1; 0.26 mmol) in CH2Cl2 (5 mL), prepared in situ. After the mixture
was stirred for 3 h, the volatiles were removed under vacuum and
the remaining oil was washed first with toluene and then with
hexane to give 8 (0.27 g, 89%). Data for 8: 1H NMR (CD2Cl2)
0.47 (bs, 3 H, Me-B), 1.05 (s, 3 H, Me-Ta), 1.98 (s, 6 H, Me-
C-O), 2.27 (s, 15 H, C5Me5), 6.66 (bs, 2 H, C6H5), 7.34 (m, 8 H,
C6H5); 13C NMR (CD2Cl2) 10.5 (MeB(C6F5)3), 11.1 (C5Me5), 20.2
(Me-C-O), 67.3 (Me-Ta), 124.7 (C5Me5), 127.3, 128.3, 130.1,
and 139.9 (C6H5), 135.5, 139.9, and 148.7 (m, C6F5), 184.5 (C-
O); 19F NMR (CD2Cl2) -130.4 (o-C6F5), -162.5 (p-C6F5), -165.1
(m-C6F5). Anal. Calcd for C46H37BF15N2O2Ta (1126.53): C, 49.04;
H, 3.31; N, 2.49. Found: C, 49.35; H, 3.48; N, 2.27.
[TaCp*Me2(OCHMePh)][MeB(C6F5)3] (4). PhC(O)H (0.028
g, 0.26 mmol) was added to a solution of [TaCp*Me3][MeB(C6F5)3]
(1; 0.26 mmol) in CH2Cl2 (5 mL), prepared in situ. After the mixture
was stirred for 1 h, the volatiles were removed under vacuum and
the remaining oil was washed first with toluene (5 mL) and then
with hexane (5 mL) to give 4 (0.23 g, 87%). Data for 4: 1H NMR
(CD2Cl2) 0.47 (bs, 3 H, Me-B), 0.93 (s, 3 H, Me-Ta), 1.02 (s, 3
H, Me-Ta), 1.90 (d, 3 H, 3J(H-H) ) 6.5 Hz, Me-C-O), 2.09 (s,
3
15 H, C5Me5), 5.87 (q, 1 H, J(H-H) ) 6.5 Hz, H-C-O), 7.38
(m, 2 H, m-C6H5), 7.46 (m, 3 H, p-C6H5 and o-C6H5); 13C NMR
(CD2Cl2) 10.5 (MeB(C6F5)3), 11.2 (C5Me5), 24.7 (Me-C-O), 67.1
(Me-Ta), 67.2 (Me-Ta), 89.2 (C-O), 125.3, 126.1, 129.8, 130.3
(C6H5 and C5Me5), 135.7 (m, C6F5), 139.0 (m, C6F5), 140.5 (o-
C6H5), 148.2 (m, C6F5); 19F NMR (CD2Cl2) -131.3 (o-C6F5),
-162.0 (p-C6F5), -164.7 (m-C6F5). Anal. Calcd for C39H33BF15-
OTa (994.41): C, 47.10; H, 3.34. Found: C, 47.20; H, 3.48.
[(TaCp*Me3)2(µ-O)] (9). A solution of [TaCp*Me4] (0.300 g,
0.78 mmol) and PhNCO (0.095 g, 0.78 mmol) in toluene (5 mL)
was heated at 90 °C for 24 h. Afterward, the volatiles were removed
under vacuum and hexane was added (10 mL). The solution was
filtered off, leaving a pale orange solid that was identified as 933
(0.24 g, 80%). Data for 9: 1H NMR (CDCl3) 0.36 (s, 3 H, Me-
(38) Lancaster, S. In www.syntheticpages.org 2003, 215.
(39) Doerrer, L. H.; Green, M. L. H. Dalton Trans. 1999, 4325.

2336 Organometallics, Vol. 25, No. 9, 2006
Sa´nchez-NieVes et al.
Ta), 0.39 (s, 6 H, Me-Ta), 2.07 (s, 15 H, C5Me5). Anal. Calcd for
C26H48OTa2 (482.43): C, 42.28; H, 6.55. Found: C, 42.10; H, 6.70.
Polymerization of C2H4 with [TaCp*Me3][MeB(C6F5)3] (1).
A sealed glass ampule containing a solid mixture of [TaCp*Me4]
(0.030 g, 0.08 mmol) and B(C6F5)3 (0.042 g, 0.08 mmol) was
introduced into a Bu¨chi reactor, and 60 mL of toluene containing
10 × 0.08 mmol of AliBu3 was added to the reactor. The system
was purged with argon for 30 min and then ethylene for 10 min at
the required temperature and pressure, and then the ampule was
broken and the suspension stirred for 60 min. Afterward the
polymerization was terminated by closing the ethylene supply and
venting the autoclave to the atmosphere. The isolated polymer was
washed first with MeOH/HCl (dilute) and then with MeOH/water
and dried overnight in vacuo at 80 °C. Melting and crystallization
temperatures of polymers were measured by differential scanning
calorimetry (DSC, Perkin-Elmer DSC6). High-temperature gel
permeation chromatography (GPC) analyses of polymer samples
were carried out in 1,2,4-trichlorobenzene at 135 °C (Waters GPCV-
2000).
2, Fcalcd ) 1.779 g cm-3, F(000) ) 1792, Mo KR radiation (λ )
0.710 73 Å), µ ) 3.306 mm-1. Data collection was performed at
200(2) K on a Nonius KappaCCD single-crystal diffractometer. The
crystal structure was solved by direct methods and refined using
full-matrix least squares on F2. All non-hydrogen atoms were
anisotropically refined, except for a disordered benzene molecule.
Hydrogen atoms were geometrically placed and left riding on their
parent atoms. The final cycle of full-matrix least-squares refinement
based on 15 731 reflections and 922 parameters converged to final
values of R1(F2 > 2σ(F2)) ) 0.0404, wR2(F2 > 2σ(F2)) ) 0.0673,
R1(F2) ) 0.0968, and wR2(F2) ) 0.0782. Final difference Fourier
maps showed no peaks higher than 1.135 or deeper than -0.820 e
Å-3
.
Acknowledgment. We gratefully acknowledge the Minis-
terio de Educacio´n y Ciencia (Project No. MAT2004-02614)
and DGUI-Comunidad de Madrid (Programme S-0505/PPQ-
0328, COMAL-CM) (Spain) for financial support.
X-ray Structure Determination of 3‚C6H6. Yellow crystals
suitable for X-ray diffraction studies were obtained by cooling a
dichloromethane/C6H6 solution of 1 at 0 °C. Crystal data for 3‚
C6H6: C68H54B2F30OTa2, Mr ) 1840.63, triclinic, space group P1h,
a ) 13.5223(12) Å, b ) 15.006(3) Å, c ) 17.416(4) Å, R ) 81.144-
(18)°, â ) 88.494(9)°, γ ) 79.818(15)°, V ) 3436.8(11) Å3, Z )
Supporting Information Available: Crystallographic data for
3 as a CIF file. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM060027C