Tantalum(V)-based metallocene, half-metallocene, and non-metallocene complexes as ethylene-1-octene copolymerization and methyl methacrylate polymerization catalysts

Article abstract of DOI:10.1021/om010702c

The preparation of half-sandwich imido tantalum dichloride directly from the reaction of the neutral constrained-geometry ligand Me₂Si(C₅Me₄H)(^tBuNH) and TaCl₅ in refluxing zone was reported. The dich

Full text of DOI:10.1021/om010702c

832
Organometallics 2002, 21, 832-839
Ta n ta lu m (V)-Ba sed Meta llocen e, Ha lf-Meta llocen e, a n d
Non -Meta llocen e Com p lexes a s Eth ylen e-1-Octen e
Cop olym er iza tion a n d Meth yl Meth a cr yla te
P olym er iza tion Ca ta lysts
Shaoguang Feng and Gordon R. Roof
The Dow Chemical Company, Chemical Sciences, Midland, Michigan 48674
Eugene Y.-X. Chen*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Received August 3, 2001
The half-sandwich imido tantalum dichloride (η5-C5Me4H)Ta(dNtBu)Cl2 is prepared directly
from the reaction of the neutral constrained-geometry ligand Me2Si(C5Me4H)(tBuNH) and
TaCl5 in refluxing toluene. The dichloride is cleanly converted to the corresponding imido
tantalum dimethyl complex (η5-C5Me4H)Ta(dNtBu)Me2 (2). Using the same synthetic
strategy, the neutral bulky chelating diamine ligand ArNH(CH2)3NHAr (Ar ) 2,6-iPr2C6H3)
is reacted with TaCl5 in toluene under mild reflux conditions, affording the six-coordinate
tantalum complex [ArN(CH2)3NHAr]TaCl4, which is conveniently converted to the corre-
sponding non-metallocene tantalum trimethyl complex [ArN(CH2)3NAr]TaMe3 (4) on treat-
ment with an excess of MeMgBr. Alternatively, 4 can be prepared in better than 70% overall
yield based on the diamine ligand from a one-pot synthesis by mixing the neutral diamine
ligand with TaCl5 followed by addition of an excess of MeMgBr. Reaction of Cp2TaMe3 with
1 equiv of B(C6F5)3 in benzene or toluene produces an oily mixture containing both
-
Cp2TaMe2+CH3B(C6F5)3 and Cp2TaMe2+[(C6F5)3B-CH3-B(C6F5)3]- in the solution phase.
Subsequently, the reaction of Cp2TaMe3 with 2 equiv of Al(C6F5)3 cleanly generates the
tantalocene cation-binuclear anion ion pair Cp2TaMe2+[(C6F5)3Al-CH3-Al(C6F5)3]- (5) as
colorless crystals. In bromobenzene-d5, reactions of Cp2TaMe3 with Lewis acids M(C6F5)3
-
(M ) B, Al) in a 1:1 ratio afford the expected cationic species Cp2TaMe2+CH3M(C6F5)3 as
clean products. Reaction of the non-metallocene 4 with M(C6F5)3 is similar to that of Cp2-
TaMe3. When it is activated with 2 equiv of Al(C6F5)3, Cp2TaMe3 is active for syndiospecific
MMA polymerization but inactive for olefin polymerization. In contrast, the non-metallocene
4 has no activity for MMA polymerization but is active for olefin polymerization. Most
interestingly, the half-metallocene 2, when activated with a suitable activator, is very active
(with a catalytic efficiency of 1.2 × 106 g of polymer/((mol of metal) atm h)) for high-
temperature (140 °C) olefin copolymerizations, producing low-density poly(ethylene-co-1-
octene) copolymers with high molecular weight.
In tr od u ction
especially tantalum and niobium complexes, have not
been widely studied and used as effective homogeneous
catalysts for polymerization of vinyl monomers. Never-
theless, noteworthy successes include the discovery by
Mashima, Nakamura, and co-workers2 that the half-
sandwich Ta(III) and Nb(III) diene complexes, i.e.,
isoelectronic structures of the group 4 metallocenes, are
living ethylene polymerization catalysts and are also
active for methyl methacrylate (MMA) polymerization
upon activation with suitable activators. Other types of
group 5 complexes such as half-sandwich imido3 and
amidinate4 compounds, tantalum aminopyridinato com-
The activated forms of the group 4 metallocene and
related complexes are often structurally well-defined,
typically highly active, single-site olefin polymerization
catalysts1 and, therefore, have attracted increasing
attention from both academics and industry. On the
other hand, group 5 metallocene and related complexes,
(1) For recent reviews, see: (a) Erker, G. Acc. Chem. Res. 2001, 34,
309-317. (b) Chum, P. S.; Kruper, W. J .; Guest, M. J . Adv. Mater.
2000, 12, 1759-1767. (c) Gladysz, J . A., Ed. Chem. Rev. 2000, 100,
1167-1682. (d) Marks, T. J ., Stevens, J . C. Top. Catal. 1999, 7, 1-208.
(e) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38, 428-447. (f) J ordan, R. F., Ed. J . Mol. Catal. 1998, 128,
1-337. (g) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98,
2587-2598. (h) Piers, W. E. Chem. Eur. J . 1998, 4, 13-18. (i)
Kaminsky, W.; Arndt, M. Adv. Polym. Sci. 1997, 127, 144-187. (j)
Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255-270. (k)
Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(2) (a) Matsuo, Y.; Mashima, K.; Tani, K. Angew. Chem., Int. Ed.
2001, 40, 960-962. (b) Mashima, K. Macromol. Symp. 2000, 159, 69-
76. (c) Mashima, K.; Fujikawa, S.; Tanaka, Y.; Urata, H.; Oshiki, T.;
Tanaka, E.; Nakamura, A. Organometallics 1995, 14, 2633-2640. (d)
Mashima, K.; Fujikawa, S.; Nakamura, A. J . Am. Chem. Soc. 1993,
115, 10990-10991.
10.1021/om010702c CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/05/2002

Ta(V)-Based Metallocene and Non-Metallocene Complexes
Organometallics, Vol. 21, No. 5, 2002 833
plexes,5 and vanadium diimine/pyridine complexes6
have also been found to be active for ethylene polym-
erization.
alumina and Q-5 catalyst (Englehardt Chemicals Inc.) prior
to use. Deuterated hydrocarbon NMR solvents were dried over
sodium/potassium alloy and distilled and/or filtered prior to
use, whereas CDCl3 and C6D5Br were dried over activated
Davison 4 Å molecular sieves. Methyl methacrylate (MMA)
monomer was degassed and dried over CaH2 overnight and
then freshly vacuum-distilled before use. NMR spectra were
It is argued that the cationic forms of the group 5
catalysts are less electron deficient and thus less active
for olefin polymerizations than the corresponding group
4 catalysts, but they are more tolerant toward polar
functionalities. Therefore, it would be of great interest
to synthesize new group 5 complexes with modified
steric/electronic ligation as well as to investigate their
catalytic activities for olefin polymerization and for
polymerization of polar vinyl monomers. We report here
(a) a convenient approach toward the synthesis of new
half-sandwich imido Ta and bulky chelating diamide Ta
alkyl complexes, (b) methide abstraction reactions7 of
Ta(V)-based trimethyl complexes with the strong or-
1
recorded on either a Varian Inova 300 (FT: 300 MHz, H; 75
MHz, 13C; 282 MHz, 19F) or a Varian Inova 400 spectrometer.
Chemical shifts for 1H and 13C spectra were referenced to
internal solvent resonances and reported relative to tetrameth-
ylsilane. 19F NMR spectra were referenced to external CFCl3.
Elemental analyses were performed by Oneida Research
Services, Inc., Whitesboro, NY, and by the CHN Lab at the
University of Michigan, Ann Arbor, MI.
Tris(perfluorophenyl)borane, B(C6F5)3, was obtained as a
solid from Boulder Scientific Inc. and was used without further
purification for preparative reactions or purified by recrystal-
lization from hexane at -35 °C for NMR-scale reactions.
Trimethylaluminum (TMA) in toluene or hexanes, TaCl5
(99.99%), ZnMe2 (1.0 M heptane solution), CpTl, MeMgBr in
diethyl ether, n-BuLi in hexane, 2,6-diisopropylaniline, TME-
DA, and 1,3-dibromopropane were purchased from Aldrich
Chemical Co. and used as received. PMAO-IP was purchased
from Akzo-Nobel. Cp2TaMe3,12 Me2Si(C5Me4H)(tBuNH) (CGC-
gano-Lewis acids B(C6F5)3 and Al(C6F5)3,9 and (c)
8
studies using Ta(V) imido complexes as effective cata-
lysts for olefin copolymerization and using tantalocenes-
(V) for polymerization of MMA.10,11
Exp er im en ta l Section
Gen er a l Rem a r k s. All syntheses and manipulations of air-
sensitive materials were carried out in flamed Schlenk-type
glassware on a dual-manifold Schlenk line or in an argon-filled
glovebox. NMR-scale reactions were conducted in Teflon-valve-
sealed sample J . Young tubes. Solvents were first saturated
with nitrogen and then dried by passage through activated
14
H2),13 and CGC-TiMe2 were prepared according to the
literature procedures, respectively. The bulky chelating di-
amine ligand ArNH(CH2)3NHAr (Ar ) 2,6-iPr2C6H3) was
prepared from LiNHAr and 1,3-dibromopropane according to
a literature procedure.15 [HNMe(C18H37)2]+[(C6F5)3AlNC3H3-
NAl(C6F5)3]- (Al imidazolide) was prepared from 2 equiv of Al-
(C6F5)3 with 1 equiv each of imidazole and dioctadecylmeth-
ylamine in toluene.16 Ph3C+[B(C6F5)4]- was prepared according
to literature procedures.17
(3) (a) 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, 78-87. (b) Witte, P. T.; Meetsma, A.; Hessen, B. Organometallics
1999, 18, 2944-2946. (c) Antonelli, D. M.; Leins, A.; Stryker, J . M.
Organometallics 1997, 16, 2500-2502.
P olym er iza tion P r oced u r es a n d P olym er Ch a r a cter -
iza tion s. Eth ylen e-1-Octen e Cop olym er iza tion . A 2 L
Parr reactor was used in the polymerizations. All feeds were
passed through columns of alumina and Q-5 catalyst prior to
introduction into the reactor. Catalyst and activator solutions
were handled in the glovebox. A 2 L reactor was charged with
about 740 g of mixed alkanes solvent (Isopar E, ExxonMobil
Co.) and 118 g of 1-octene comonomer, and the contents were
stirred. Hydrogen was added as a molecular weight control
agent by differential pressure expansion from a 75 mL addition
tank at 300 psig. The reactor contents were heated to the
polymerization temperature of 140 °C and saturated with
ethylene at 500 psig. Catalysts and activators, as dilute
solutions in toluene, were premixed and transferred to a
catalyst addition tank and injected into the reactor. The
polymerization conditions were maintained for 15 min with
ethylene added on demand. Heat was continuously removed
from the reaction through an internal cooling coil. The result-
ing solution was removed from the reactor, quenched with
isopropyl alcohol, and stabilized by an addition of 10 mL of a
toluene solution containing approximately 67 mg of a hindered
(4) Decker, J . M.; Geib, S. J .; Meyer, T. Y. Organometallics 1999,
18, 4417-4420.
(5) Kakala, K.; Lo¨fgren, B.; Polamo, M.; Leskela¨, M. Macromol.
Rapid Commun. 1997, 18, 635-638.
(6) Reardon, D.; Conan, F.; Gambarottta, S.; Yap, G.; Wang, Q. J .
Am. Chem. Soc. 1999, 121, 9318-9325.
(7) (a) Methide abstraction of Cp2TaMe3 with organo-Lewis acids
was communicated earlier: Chen, E. Y.-X.; Abboud, K. A. Organome-
tallics 2000, 19, 5541-5543. (b) Piers et al. reported that the reaction
of Cp2Ta(dCH2)(CH3) with B(C6F5)3 leads to zwitterionic tantalocene
complexes: Cook, K. S.; Piers, W. E.; Rettig, S. J .; McDonald, R.
Organometallics 2000, 19, 2243-2245. (c) Erker et al. developed a new
cation generating method to obtain the (butadiene)tantalocene cation
by reacting the ion pair Cp2ZrMe+MeB(C6F5)3- with (η5-cyclopentadi-
enyl)(η1-cyclopentadienyl)(η2-butadiene)tantalum: Strauch, H. C.; Erk-
er, G.; Frohlich, R. Organometallics 1998, 17, 5746-5757.
(8) (a) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245-
250. (b) Massey, A. G.; Park, A. J .; Stone, F. G. A. Proc. Chem. Soc.
1963, 212-212.
(9) (a) Lee, C. H.; Lee, S. J .; Park, J . W.; Kim, K. H.; Lee, B. Y.; Oh,
J . S. J . Mol. Catal., A: Chem. 1998, 132, 231-239. (b) Biagini, P.; Lugli,
G.; Abis, L.; Andreussi, P. U.S. Patent 5,602,269, 1997.
(10) For recent references on polymerization of MMA by lantha-
nocenes, see: (a) Yasuda, H. J . Polym. Sci., Part A: Polym. Chem.
2001, 39, 1955-1959. (b) Nodono, M.; Tokimitsu, T.; Tone, S.; Makino,
T.; Yanagase, A. Macromol. Chem. Phys. 2000, 201, 2282-2288. (c)
Qian, C.; Nie, W.; Sun, J . Organometallics 2000, 19, 4134-4140. (d)
Giardello, M. A.; Yamamoto, Y.; Brard, L.; Marks, T. J . J . Am. Chem.
Soc. 1995, 117, 3276-3277. (e) Boffa, L. S.; Novak, B. M. Macromol-
ecules 1994, 27, 6993-6995. (f) Yasuda, H.; Yamamoto, H.; Yamashita,
M.; Yokota, K.; Nakamura, A.; Miyake, S.; Kai, Y.; Kanehisa, N.
Macromolecules 1993, 26, 7134-7143. (g) Yasuda, H.; Yamamoto, H.;
Yokota, K.; Miyake, S.; Nakamura, A. J . Am. Chem. Soc. 1992, 114,
4908-4909.
(11) For recent references on polymerization of MMA by group 4
metallocenes, see: (a) Bolig, A. D.; Chen, E. Y.-X. J . Am. Chem. Soc.
2001, 123, 7943-7944. (b) Frauenrath, H.; Keul, H.; Ho¨cker, H.
Macromolecules 2001, 34, 14-19. (c) Bandermann, F.; Ferenz, M.;
Sustmann, R.; Sicking, W. Macromol. Symp. 2001, 174, 247-253. (d)
Cameron, P. A.; Gibson, V.; Graham, A. J . Macromolecules 2000, 33,
4329-4335. (e) Li, Y.; Ward, D. G.; Reddy, S. S.; Collins, S. Macro-
molecules 1997, 30, 1875-1883. (f) Deng, H.; Shiono, T.; Soga, K.
Macromolecules 1995, 28, 3067-3073. (g) Collins, S.; Ward, S. G. J .
Am. Chem. Soc. 1992, 114, 5460-5462.
(12) Schrock, R. R.; Sharp, P. R. J . Am. Chem. Soc. 1978, 100, 2389-
2399.
(13) (a) Shapiro, P. J .; Cotter, W. D.; Schaefer, W. P.; Labinger, J .
A.; Bercaw, J . E. J . Am. Chem. Soc. 1994, 116, 4623-4640. (b) Piers,
W. E.; Shapiro, P. J .; Bunnel, E. E.; Bercaw, J . E. Synlett 1990, 2,
74-84.
(14) Stevens, J . C.; Timmers, F. J .; Wilson, D. R.; Schmidt, G. F.;
Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. Eur. Pat. Appl. EP
416 815-A2, 1991.
(15) Scollard, J . D.; McConville, D. H.; Vittal, J . J . Organometallics
1997, 16, 4415-4420.
(16) (a) LaPointe, R. E.; Roof, G. R.; Abboud, K. A.; Klosin, J . J .
Am. Chem. Soc. 2000, 122, 9560-9561. (b) LaPointe, R. E. PCT Int.
Appl. WO 9942467, 1999.
(17) (a) Chien, J . C. W.; Tsai, W.-M.; Rausch, M. D. J . Am. Chem.
Soc. 1991, 113, 8570-8571. (b) Ewen, J . A.; Elder, M. J . Eur. Pat. Appl.
EP 0,426,637, 1991.

834 Organometallics, Vol. 21, No. 5, 2002
Feng et al.
phenol antioxidant (Irganox 1010 from Ciba Geigy Corp.) and
133 mg of a phosphorus stabilizer (Irgafos 168 from Ciba Geigy
Corp.).
Between polymerization runs a wash cycle was conducted
in which 850 g of Isopar-E was added to the reactor and the
reactor was heated to 150 °C. The reactor was then emptied
of the heated solvent immediately before beginning a new
polymerization run.
became clear after being stirred for 5 min at room temperature,
and crystalline solids started to precipitate. The mixture was
stirred at room temperature for 2 h and then filtered. The
crystalline solid collected was washed with hexanes and dried
in vacuo to produce 1.98 g of the product in a pure state. Slow
cooling of the combined filtrates at -35 °C gave an additional
1.20 g of the pure product as a white crystalline solid, after
washing with hexanes and drying in vacuo. The total yield is
3.18 g (95.0%). 1H NMR (C6D6, 23 °C): δ 7.09-6.98 (m, 5H,
C6H5), 2.09 (s, 3H, CH3). 19F NMR (C6D6, 23 °C): δ -123.20
Polymers were recovered by drying for about 20 h in a
vacuum oven set at 140 °C. Density values were derived by
determining the polymer’s mass when in air and the volume
displaced when immersed in methyl ethyl ketone. Micro melt
index values (MMI) were obtained using a Custom Scientific
Instrument Inc. Model CS-127MF-015 apparatus at 190 °C and
calibrated using a series of known I2 (melt index: grams of
PE extruded in 10 min at 190 °C, 2.16 kg) commercial PE
standards. Melting and crystallization temperatures of poly-
mers were measured by differential scanning calorimetry (DSC
2910, TA Instruments, Inc.). Samples were first heated from
room temperature to 180 °C at 100 °C/min. After being held
at this temperature for 4 min, the samples were cooled to -40
°C at 10 °C/min and were then heated to 160 °C at 10 °C/min
after being held at -40 °C for 4 min. High-temperature gel
permeation chromatography (GPC) analyses of polymer samples
were carried out in 1,2,4-trichlorobenzene at 135 °C on a
Waters 150C high-temperature instrument. A polystyrene/
polyethylene universal calibration was carried out using
narrow molecular weight distribution polystyrene standards
from Polymer Laboratories with 2,6-di-tert-butyl-4-methylphe-
nol (BHT) as the flow marker.
Meth yl Meth a cr yla te P olym er iza tion . MMA polymer-
izations were performed in 50 mL Schlenk tubes with a septum
and an external temperature-controlled bath on a Schlenk line
or in a glovebox. In a typical procedure, Cp2TaMe3 (46.7 µmol)
and the activator M(C6F5)3 (M ) B, Al) in a desired ratio were
loaded into the tube in a glovebox and toluene was added (10
mL total volume). The tube was removed from the box and
put on the Schlenk line. MMA (1.00 mL, 0.936 g, 9.35 mmol)
was added through the septum via a gastight syringe after
stirring the catalyst and activator mixture for 10 min. The
polymerization was quenched by adding 2 mL of acidified
methanol after the measured time interval. The polymer
product was precipitated into 50 mL of methanol, filtered,
washed with methanol, and dried in a vacuum oven at 50 °C
overnight to a constant weight.
3
3
(d, J F-F ) 18.7 Hz, 6F, o-F), -150.76 (t, J F-F ) 18.3 Hz, 3F,
p-F), -160.62 (m, 6F, m-F). 1H NMR (C6D5Br, 23 °C): δ 7.37-
7.24 (m, 5H, C6H5), 2.38 (s, 3H, CH3). 19F NMR (C6D5Br,
3
23 °C): δ -120.85 (d, J F-F ) 18.0 Hz, 6F, o-F), -149.58 (t,
3J F-F ) 21.2 Hz, 3F, p-F), -159.87 (m, 6F, m-F).
Syn th esis of (η5-C5Me4H)Ta (dNtBu )Me2 (2). In a glove-
box, Me2Si(C5Me4H)(tBuNH) (CGC-H2, 0.50 g, 2.00 mmol) and
TaCl5 (0.72 g, 2.00 mmol) were mixed in 20 mL of toluene and
the mixture was refluxed for 12 h. The reaction mixture was
filtered, and the solvent of the filtrate was removed under
reduced pressure to give a yellow solid. 1H NMR spectra of
the crude material indicated formation of a mixture of the
products containing (C5Me4H)Ta(dNtBu)Cl2 as a major prod-
uct. Recrystallization of the crude product mixture from
toluene at -40 °C afforded a mixture of yellow and orange
crystals. Yellow crystals were combined with the yellow mother
liquor, and the solvent was removed in vacuo. The residue was
extracted with hexanes (30 mL) and filtered. The solvent of
the yellow filtrate was removed under reduced pressure to
afford 0.40 g of (C5Me4H)Ta(dNtBu)Cl2 (1) as a yellow solid;
1
yield 45.5%. H NMR (C6D6, 23 °C): δ 5.79 (s, 1H, C5Me4H),
t
1.91, 1.80 (s, 12H, C5Me4H), 1.21 (s, 9H, Bu).
In a glovebox, 1 (0.40 g, 0.90 mmol) was dissolved in 20 mL
of diethyl ether and the solution was cooled to -40 °C.
MeMgBr in diethyl ether (0.60 mL, 1.80 mmol) was added via
syringe, and the mixture was stirred for 2 h at room temper-
ature. The solvent was removed, and the residue was extracted
with hexanes (20 mL) and filtered through Celite. The
concentrated solution (10 mL) was left in a freezer at -40 °C
for several days, and no crystals were formed. The cloudy
solution was filtered through Celite, and the solvent of the
filtrate was then removed under reduced pressure and the
residue was dried in vacuo for 2 h to afford 0.33 g of 2 as a
yellow-green oil; yield 91.7%. Anal. Calcd for C15H28NTa: C,
44.67; H, 6.99; N, 3.47. Found: C, 44.28; H, 7.23; N, 3.25.
1
Spectroscopic data for 2 are as follows. H NMR (C6D6, 23
Glass transition temperatures of polymers were measured
by differential scanning calorimetry (DSC 2920, TA Instru-
ments, Inc.). Samples were first heated from room temperature
to 180 °C. After being held at this temperature for 4 min, the
samples were cooled to -20 °C at 10 °C/min and were then
heated to 160 °C at 10 °C/min after being held at -20 °C for
4 min. Gel permeation chromatography analyses of polymer
samples were carried out at room temperature using THF as
eluent on a Waters 150C instrument and calibrated using
monodispersed PMMA standards at a flow rate of 1.0 mL/min.
Number-average molecular weights and polydispersities of
PMMA were given relative to PMMA standards. 1H and 13C
spectra for the analysis of PMMA microstructures were
recorded in CDCl3 and analyzed according to the literature.18
Mod ified Syn th esis of Tr is(p er flu or op h en yl)a la n e (Al-
(C6F 5)3 a s a 0.5 Tolu en e Ad d u ct). This reagent was prepared
by modification of literature procedures9b (Extra caution should
be exercised when handling this material due to its thermal
and shock sensitivity. Preparation of the unsolvated form of
this reagent should be avoided.). In a glovebox, B(C6F5)3 (3.00
g, 5.86 mmol) was slurried in a solvent mixture (10 mL of
toluene and 30 mL of hexanes) and AlMe3 (2.0 M in hexanes,
2.93 mL, 5.86 mmol) was added via syringe. The suspension
°C): δ 5.22 (s, 1H, C5Me4H), 1.81, 1.75 (s, 12H, C5Me4H), 1.42
t
(s, 9H, Bu), 0.16 (s, 6H, Ta-CH3). 13C NMR (C6D6, 23 °C): δ
119.34, 115.91, 100.51 (C5Me4H), 72.95 (Me3CNd), 42.71 (Ta-
CH3), 33.73 (Me3CNd), 13.12, 10.56 (C5Me4H).
Syn th esis of [Ar N(CH2)3NAr ]Ta Me3 (4). In a glovebox,
the neutral diamine ligand ArNH(CH2)3NHAr (Ar ) 2,6-
iPr2C6H3) (1.55 g, 3.93 mmol) was dissolved in 40 mL of toluene
and cooled to -40 °C. TaCl5 (1.41 g, 3.93 mmol) was added in
small portions, and the mixture was gradually warmed to room
temperature and then heated to 90 °C for 2 h. The orange
suspension was filtered and the solid collected and dried to
produce 1.86 g of yellow solid. The red filtrate was cooled to
-40 °C for 12 h, producing 0.25 g of orange crystals. All peaks
in the 1H NMR spectra of the orange crystals are broad at room
temperature but are sharp and well resolved at 80 °C,
indicating the formation of the six-coordinate tantalum com-
plex [ArN(CH2)3NHAr]TaCl4 (3). 1H NMR (C6D6, 80 °C): δ
7.23-7.02 (m, 6H, Ar), 5.25 (s br, NH), 4.92 (s br, 2H,
-NHCH2CH2), 3.78-3.66 (m, 2H, -NCH2CH2), 3.65-3.59 (m,
4H, -CHMe2), 1.78-1.76 (m, 2H, -NCH2CH2), 1.46 (d, J )
6.6 Hz, 6H, -CHMe2), 1.28 (d, J ) 6.6 Hz, 12H, -CHMe2),
1.08 (d, J ) 6.6 Hz, 6H, -CHMe2).
The yellow solid (1.86 g) initially collected after the filtration
was extracted with 100 mL of toluene and filtered. The filtrate
was cooled to -40 °C for 24 h, affording an additional 0.18 g
(18) Bovey, F. A.; Mirau, P. A. NMR of Polymers; Academic Press:
San Diego, CA, 1996.

Ta(V)-Based Metallocene and Non-Metallocene Complexes
Organometallics, Vol. 21, No. 5, 2002 835
of 3. The combined mother liquors were concentrated and
cooled again to -40 °C, producing a third crop of 3 (0.15 g).
The total yield is 0.58 g (20.2%).
Anal. Calcd for C49H19Al2F30Ta: C, 41.67; H, 1.36. Found: C,
41.99; H, 1.47.
Rea ction of Cp 2Ta Me3 w ith 1 Equ iv of Al(C6F 5)3. In a
small vial, Cp2TaMe3 and Al(C6F5)3‚0.5(toluene) were mixed
in 0.7 mL of bromobenzene-d5 in a 1:1 ratio (0.01 mmol scale),
and the mixture was loaded into a J . Young NMR tube in a
glovebox. The mixture was allowed to react at room temper-
ature for 10 min before the NMR spectra were recorded. A
clear solution was obtained immediately after mixing, and the
NMR spectra indicated the clean formation of the correspond-
ing cationic species Cp2TaMe2+CH3Al(C6F5)3-. 1H NMR (C6D5-
Br, 23 °C): δ 5.79 (s, 10H, C5H5), 0.38 (s, br, 3H, Al-CH3),
0.21 (s, br, 6H, Ta-CH3). 19F NMR (C6D5Br, 23 °C): δ -120.96
(dd, 6F, o-F), -158.02 (t, 3F, p-F), -163.09 (m, 6F, m-F). The
same reaction in benzene-d6 or toluene-d8 produced initially
a yellow suspension which gradually turned to oily precipitates
at the bottom of the NMR tube. NMR spectra of the solution
phase showed a mixture of species, including the unreacted
Cp2TaMe3. The 1H and 19F NMR chemical shifts are very
sensitive to the concentration and temperature, and the
spectra are difficult to reproduce.
Complex 3 (0.188 g, 0.25 mmol) was suspended in 15 mL of
diethyl ether and cooled to -40 °C. MeMgBr (0.425 mL, 3.0
M in Et2O, 1.27 mmol) was added via syringe, and large
precipitates formed during the addition. The mixture was
stirred at room temperature for 45 min after the addition, and
the solvent of the resulting yellow suspension was removed
under reduced pressure to leave a solid residue. The residue
was extracted with hexanes (15 mL) and filtered through
Celite to produce a yellow solution which was filtered again
through Celite. The filtrate was cooled to -40 °C in a freezer
inside the box, and no crystals were obtained. The solvent was
then removed and the residue was dried in vacuo to afford
the crude product as a yellow solid. Further purification was
made by redissolving the solid in hexane, filtering through
Celite, and drying in vacuo to produce the desired product 4
in quantitative yield. Anal. Calcd for C30H49N2Ta: C, 58.24;
H, 7.98; N, 4.53. Found: C, 57.59; H, 7.64; N, 4.23.
Alternatively, 4 can be prepared in better than 70% overall
yield based on the diamine ligand from a one-pot approach by
mixing the neutral diamine ligand with TaCl5 at -30 °C and
stirring at room temperature for another 2 h, followed by an
addition of 5 equiv of MeMgBr.
Rea ction of Cp 2Ta Me3 a n d B(C6F 5)3. In a glovebox, Cp2-
TaMe3 and B(C6F5)3 were mixed in 0.7 mL of benzene-d6 in a
1:1 ratio (0.02 mmol scale), and the solution was loaded into
a J . Young NMR tube. The mixture was allowed to react at
room temperature for 15 min before the NMR spectra were
recorded. The initially formed light yellow suspension turned
gradually to yellow oily precipitates at the bottom of the tube.
NMR spectra of the solution phase indicated formation of a
1
Spectroscopic data for 4 are as follows. H NMR (C6D6, 23
°C): δ 7.18-7.11 (m, 6H, Ar), 3.61-3.50 (m, 8H, -CHMe2,
-NCH2CH2), 2.15 (m, 2H, -NCH2CH2), 1.37 (d, J ) 6.9 Hz,
12H, -CHMe2), 1.25 (d, J ) 6.9 Hz, 12 H, -CHMe2), 1.02 (s,
-
mixture of two products, Cp2TaMe2+CH3B(C6F5)3 and Cp2-
1
9H, Ta-CH3). H NMR (C6D6, 80 °C): δ 3.65 (t, 4H, -NCH2-
TaMe2+[(C6F5)3B-CH3-B(C6F5)3]-, in a ca. 1:2 ratio, along
with unreacted Cp2TaMe3. However, this ratio became ca. 2:1
after this phase stood in the NMR tube at room temperature
CH2), 3.57 (sept, 4H, -CHMe2).1H NMR (C6D5Br, 23 °C): δ
7.34 (m, 6H, Ar), 3.89 (t, J ) 5.7 Hz, 4H, -NCH2CH2), 3.70
(sept, J ) 6.9 Hz, 4H, -CHMe2), 2.66 (m, 2H, -NCH2CH2),
1.53 (d, J ) 6.6 Hz, 12H, -CHMe2), 1.45 (d, J ) 6.6 Hz, 12H,
-CHMe2), 1.04 (s, 9H, Ta-CH3). 13C NMR (C6D6, 23 °C): δ
144.87, 127.40, 126.82, 124.56 (Ar), 65.55 (-NCH2CH2), 55.94
(Ta-CH3), 28.34, 27.02, 25.99, 24.72 (-CHMe2, -CHMe2,
-NCH2CH2).
1
for 1 h. H NMR (C6D6, 23 °C): δ 5.05 (s, br, 10H, C5H5), 1.33
(s, br, 3H, B-CH3-B), 0.95 (s, br, 3H, B-CH3), -0.40 (br, Ta-
3
CH3). 19F NMR (C6D6, 23 °C): δ -131.99 (d, J F-F ) 21.2 Hz,
3
12F, o-F, binuclear anion), -132.30 (d, J F-F ) 21.2 Hz, 6F,
o-F, mononuclear anion), -163.98 (t, 3J F-F ) 21.4 Hz, 6F, p-F,
3
binuclear anion), -164.05 (t, J F-F ) 21.4 Hz, 3F, p-F,
Rea ction of Cp 2Ta Me3 w ith 2 Equ iv of Al(C6F 5)3 a n d
Isola tion of Cp 2Ta Me2+[(C6F 5)3Al-CH3-Al(C6F 5)3]- (5).
NMR-scale reactions were carried out in J . Young NMR tubes,
the samples being loaded into the NMR tubes in a glovebox
after mixing Cp2TaMe3 (0.02 mmol) and Al(C6F5)3‚0.5(toluene)
(11.4 mg, 0.04 mmol) in 0.7 mL of C6D6. The mixture was
allowed to react at room temperature for 10 min before the
NMR spectra were recorded. The solution became cloudy
immediately, and colorless crystals started to precipitate onto
the wall of NMR tubes. NMR spectra of the solution phase
indicated formation of the clean product 5. 1H NMR (C6D6, 23
mononuclear anion), -166.57 (t, 12F, m-F, binuclear anion),
-166.64 (t, 6F, p-F, mononuclear anion). The 1:2 Cp2TaMe3/
B(C6F5)3 reaction produced a yellow oil containing the same
mixture of species in the solution phase but with substantially
sharper peaks for the Cp and Ta-CH3 resonances. A substan-
tial amount of the unreacted B(C6F5)3 was also present in the
solution.
The reactions were repeated in bromobenzene-d5 (0.01 mmol
scale) to afford clear solutions. NMR spectra of the 1:1 ratio
-
.
1H
reaction indicated formation of Cp2TaMe2+CH3B(C6F5)3
NMR (C6D5Br, 23 °C): δ 5.78 (s, 10H, C5H5), 1.29 (s, br, 3H,
B-CH3), 0.21 (s, 6H, Ta-CH3). 19F NMR (C6D5Br, 23 °C): δ
-131.95 (d, 3J F-F ) 21.2 Hz, 6F, o-F), -163.50 (t, 3J F-F ) 21.0,
3F, p-F), -166.12 (m, 6F, m-F). NMR spectra of the 1:2
reaction showed formation of the same product, along with
unreacted B(C6F5)3 and a small amount of MeB(C6F5)2.
Rea ction of Cp 2Ta Me3 a n d P h 3C+[B(C6F 5)4]-. In a small
vial, Cp2TaMe3 and Ph3C+[B(C6F5)4]- were mixed in 0.7 mL
of bromobenzene-d5 in a 1:1 ratio (0.01 mmol scale) and the
mixture was loaded into a J . Young NMR tube in a glovebox.
The mixture was allowed to react at room temperature for 10
min before the NMR spectra were recorded. A clear yellow
solution was obtained immediately after the mixing. NMR
spectra indicated the clean formation of the corresponding
cationic species Cp2TaMe2+[B(C6F5)4]- and 1 equiv of Ph3CCH3.
1H NMR (C6D5Br, 23 °C): δ 5.78 (s, 10H, C5H5), 2.24 (s, 3H,
Ph3CCH3), 0.21 (s, 6H, Ta-CH3). 19F NMR (C6D5Br, 23 °C): δ
°C): δ 4.96 (s, 10H, C5H5), 1.77 (s, br, Al-CH3-Al), -0.40 (s,
3
6H, Ta-CH3). 19F NMR (C6D6, 23 °C): δ -122.66 (d, J F-F
)
18.3 Hz, 12F, o-F), -154.58 (s, br, 6F, p-F), -162.35 (m, 12F,
m-F). The crystals collected were redissolved in bromobenzene-
d5, and the NMR spectra indicated a species identical with
that in the solution (vide infra).
This reaction was repeated in bromobenzene-d5 (0.01 mmol
scale) to afford a clear solution. NMR spectra indicated
1
formation of the clean product 5. H NMR (C6D5Br, 23 °C): δ
5.78 (s, 10H, C5H5), 2.21 (s, br, 3H, Al-CH3-Al), 0.23 (s, 6H,
3
Ta-CH3). 19F NMR (C6D5Br, 23 °C): δ -122.83 (d, J F-F
)
18.3 Hz, 12F, o-F), -154.39 (s, br, 6F, p-F), -162.19 (t,
3J F-F ) 18.1 Hz, 12F, m-F). 13C NMR (C6D5Br, 23 °C): δ 112.10
1
1
(C5H5, J CH ) 175.5 Hz), 57.05 (Ta-CH3, J CH ) 125.3 Hz).
The preparative isolation of the complex was carried out in
the glovebox by mixing Cp2TaMe3 (0.018 g, 0.05 mmol) and
Al(C6F5)3‚0.5(toluene) (0.057 g, 0.1 mmol) in 10 mL of toluene
and stirring at room temperature for 5 min. The solvent was
removed under reduced pressure, and the residue was washed
with 2 mL of hexane and dried in vacuo, affording 0.067 g of
the clean product as a white crystalline solid. Yield: 94.9%.
3
-131.62 (s, br, 6F, o-F), -161.40 (t, J F-F ) 21.0, 3F, p-F),
-165.35 (s, br, 6F, m-F). The 1:2 reaction produced the same
product, even after heating at 100 °C.
Rea ction of [Ar N(CH2)3NAr ]Ta Me3 (4) a n d B(C6F 5)3. In
a small vial, [ArN(CH2)3NAr]TaMe3 and B(C6F5)3 were mixed

836 Organometallics, Vol. 21, No. 5, 2002
Feng et al.
in 0.7 mL of bromobenzene-d5 in a 1:1 ratio (0.01 mmol scale)
and the mixture was loaded into a J . Young NMR tube in a
glovebox. The mixture was allowed to react at room temper-
ature for 10 min before the NMR spectra were recorded. A
clear pale yellow solution was obtained immediately after the
mixing. NMR spectra indicated the clean formation of the
corresponding cationic species [ArN(CH2)3NAr]TaMe2+CH3-
Sch em e 1
-
1
B(C6F5)3 (6). H NMR (C6D5Br, 23 °C): δ 7.49-7.32 (m, 6H,
Ar), 4.24 (t, J ) 6.0 Hz, 4H, -NCH2CH2), 3.09 (sept, J ) 6.9
Hz, 4H, -CHMe2), 2.53 (m, 4H, -NCH2CH2), 1.39 (d, J ) 6.6
Hz, 12H, -CHMe2), 1.35 (d, J ) 6.6 Hz, 12H, -CHMe2), 1.31
(s, br, 3H, -BCH3), 1.12 (s, 6H, Ta-CH3). 19F NMR (C6D5Br,
3
23 °C): δ -131.95 (d, J F-F ) 18.5 Hz, 6F, o-F), -164.10 (t,
3J F-F ) 20.9 Hz, 3F, p-F), -166.62 (m, 6F, m-F).
The reaction of [ArN(CH2)3NAr]TaMe3 and B(C6F5)3 in a 1:2
ratio produced the same product, along with unreacted B(C6F5)3
and a small amount of MeB(C6F5)2.
Rea ction of [Ar N(CH2)3NAr ]Ta Me3 (4) a n d Al(C6F 5)3.
In a small vial, [ArN(CH2)3NAr]TaMe3 and Al(C6F5)3‚0.5-
(toluene) were mixed in 0.7 mL of bromobenzene-d5 in a 1:1
ratio (0.01 mmol scale) and the mixture was loaded into a J .
Young NMR tube in a glovebox. The mixture was allowed to
react at room temperature for 10 min before the NMR spectra
were recorded. A clear pale yellow solution was obtained
Sch em e 2
1
immediately after the mixing. H NMR spectra indicated the
clean formation of the cation [ArN(CH2)3NAr]TaMe2+, but 19F
NMR spectra showed the presence of both the mononuclear
-
and binuclear anions CH3Al(C6F5)3 (7; ∼80%) and [(C6F5)3-
Al-CH3-Al(C6F5)3]- (8; ∼20%). 1H NMR (C6D5Br, 23 °C): δ
7.38-7.24 (m, 6H, Ar), 4.31 (t, J ) 6.0 Hz, 4H, -NCH2CH2),
2.93 (sept, J ) 6.9 Hz, 4H, -CHMe2), 2.49 (m, 4H, -NCH2CH2),
1.36 (d, J ) 6.6 Hz, 12H, -CHMe2), 1.33 (d, J ) 6.6 Hz, 12H,
-CHMe2), 1.15 (s, 6H, Ta-CH3), 0.50 (s, br, 3H, -AlCH3). 19
F
3
NMR (C6D5Br, 23 °C) for 7: δ -121.30 (d, J F-F ) 18.3 Hz,
6F, o-F), -158.00 (s, br, 3F, p-F), -163.25 (m, 6F, m-F). 19F
3
NMR (C6D5Br, 23 °C) for 8: δ -121.63 (d, J F-F ) 18.3 Hz,
3
6F, o-F), -157.10 (t, J F-F ) 19.5 Hz, 3F, p-F), -163.25 (m,
6F, m-F).
The same reaction, but with 2 equiv of Al(C6F5)3, generated
a clear pale yellow solution, and NMR spectra indicated the
clean formation of the corresponding cation paired with a
binuclear anion, [ArN(CH2)3NAr]TaMe2+[(C6F5)3Al-CH3-Al-
(C6F5)3]- (8). The 1H NMR spectrum is identical with that
obtained from the 1:1 reaction, except that the broad Al-CH3-
Al signal is now shifted to 0.19 ppm.
1
peaks in the H NMR spectrum of 3 are very broad and
unassignable at room temperature but are well-resolved
at high temperatures. Thus, the H NMR spectrum of
1
3 at 80 °C shows two doublets for the inequivalent
isopropyl methyl groups on the phenyl ring at the
covalent bond (Ta-N) site but only one doublet for
equivalent isopropyl methyl groups at the dative bond
(TarNH) site, presumably due to dissociation and
Resu lts a n d Discu ssion
Syn th esis of (η5-C5Me4H)Ta (dNtBu )Me2 (2). In a
one-step fashion, reaction of the neutral constrained-
geometry ligand Me2Si(C5Me4H)(tBuNH) with TaCl5 in
refluxing toluene conveniently affords the half-sandwich
imido tantalum dichloride 1 in better than 45% yield
(Scheme 1).19 Details about the reaction mechanism are
currently unknown. Nevertheless, the dichloride 1 can
be cleanly converted to the corresponding dimethyl
complex 2 using 2 equiv of MeMgBr. Spectroscopic
features of 1 and 2 are similar to those structurally
related imido tantalum complexes reported in the
literature.3c,20
(19) When carried out in the presence of 2 equiv of Et3N, the same
reaction produced an ammonium salt of a chloride-bridged dimeric
tantalum anion in low yield, the structure of which was confirmed by
X-ray diffraction analysis but was only partially refined with the
following selected bonding distances (Å) for the structure:
Syn th esis of [Ar N(CH2)3NAr ]Ta Me3 (Ar ) 2,6-
iP r 2C6H3) (4). To avoid the potential reduction of Ta-
(V) species to lower valent species when organometallic
reagents such as organolithium reagents are used for
the synthesis, the neutral bulky chelating diamine
ligand ArNH(CH2)3NHAr (Ar ) 2,6-iPr2C6H3)15 was
used to directly react with TaCl5 in toluene under mild
reflux conditions (Scheme 2). The six-coordinate, toluene-
soluble complex 3 was isolated as orange crystals. All
(20) (a) Royo, P.; Sa´nchez-Nieves, J . J . Organomet. Chem. 2000, 597,
61-68. (b) Gibson, V. C.; Poole, A. D.; Siemeling, U.; Williams, D. N.;
Clegg, W.; Hockless, D. C. R. J . Organomet. Chem. 1993, 462, C12-
C14.

Ta(V)-Based Metallocene and Non-Metallocene Complexes
Organometallics, Vol. 21, No. 5, 2002 837
Sch em e 3
recombination of the TarNH bond at this temperature
resulting in racemization at the nitrogen center. In an
attempt to increase the yield of 3 by refluxing the
neutral ligand and TaCl5 in toluene in the presence of
2 equiv of triethylamine, only the unreacted ligand was
recovered.
The six-coordinate complex 3 can be quantitatively
converted to the corresponding Ta trimethyl species 4
by reacting 3 with an excess of MeMgBr. Alternatively,
4 can be prepared in better than 70% overall yield from
a one-pot approach by mixing the neutral diamine
ligand with TaCl5 at -30 °C and stirring at room
temperature for another 2 h, followed by addition of an
excess of MeMgBr.
Isopropyl methyl groups on the phenyl ring in 4
appear as two doublets, attributable to the restricted
rotations about the N-Cipso bond (assuming the two
nitrogen centers are sp2 hybridized). However, three
tantalum methyl groups have the same chemical shift,
in sharp contrast to Cp2TaMe3, in which two lateral
methyl groups are separated from the central methyl
group by +0.04 ppm (downfield shift) in benzene-d6 and
+0.18 ppm in bromobenzene-d5. The equivalency of the
three Ta methyl groups in 4 is not disrupted down to
-80 °C, presumably due to fast interconversion on the
NMR time scale between two prototype structures
(trigonal bipyramid and square pyramid) for a five-
coordinate complex.
of B(C6F5)3 was used in the reaction with Cp2TaMe3 in
an attempt to isolate the tantalocene cation-dinuclear
anion pair and characterize it in the pure state. How-
ever, the reaction still produced an oily product mixture
containing a significant amount of the unreacted
B(C6F5)3. Next, the use of the sterically more accessible,
24
stronger organo-Lewis acid Al(C6F5)3 was tried, but
it profoundly changed the reaction outcome as well as
the stability of the resulting Me-bridged binuclear anion.
Thus, mixing Cp2TaMe3 with 2 equiv of Al(C6F5)3 in
benzene or toluene results in immediate precipitation
of a clean product as colorless crystals in quantitative
yield. Spectroscopic and analytical data as well as X-ray
crystal diffraction studies confirm the formation of the
desired ion pair Cp2TaMe2+[(C6F5)3Al-CH3-Al(C6F5)3]-
(5)7a (Scheme 3).
Meth id e Abstr a ction of Cp 2Ta Me3 a n d 4 w ith th e
Str on g Or ga n o-Lew is Acid s M(C6F 5)3 (M ) B, Al)
a n d P h 3C+[B(C6F 5)4]-. The product of the reactions
of Cp2TaMe3 with Lewis acids M(C6F5)3 is very sensitive
to the reaction medium. In toluene or benzene, unlike
the reaction of Cp′2ZrMe2 with B(C6F5)3, which cleanly
generates the corresponding isolable and X-ray crystal-
lographically characterizable cationic complexes Cp′2-
The 1:1 reactions of Cp2TaMe3 with Lewis acids
M(C6F5)3 (M ) B, Al) in polar NMR solvents such as
bromobenzene-d5 afford clear solutions and produce the
-
expected cationic species Cp2TaMe2+CH3M(C6F5)3 as
clean products. The reaction with 2 equiv of the borane
produces the same product along with the unreacted
borane, whereas the reaction with 2 equiv of the alane
cleanly generates the tantalocene cation-binuclear
anion pair (5). The reaction of Cp2TaMe3 with Ph3C+-
[B(C6F5)4]- produces the corresponding cationic species
Cp2TaMe2+[B(C6F5)4]-. Attempts to abstract the second
Ta-CH3 group for generating possibly a tantalum
dication with 2 equiv of Ph3C+[B(C6F5)4]- afforded the
same product as in the 1:1 ratio reaction, even with
heating at 100 °C.
Reaction of the nonmetallocene Ta complex 4 with
strong Lewis acids M(C6F5)3 (M ) B, Al) in toluene-d8
results in oily precipitates, and the NMR spectra of the
solution phase showed formation of a mixture of several
cationic species. However, the reactions in bromoben-
zene-d5 are clean and afford clear solutions. Thus,
reaction of 4 with B(C6F5)3 in C6D5Br cleanly generates
the corresponding cationic species 6 (Scheme 4). In
comparison with the neutral species 4, the -CHMe2
resonance in 6 is substantially high-field shifted by 0.61
ppm, while the resonances for -NCH2CH2 and Ta-CH3
are downfield shifted by 0.35 and 0.08 ppm, respectively,
suggesting the cationic nature around the metal center.
The observed large high-field shift in chemical shifts for
the meta fluorines and especially for the para fluorines,
ZrMe+MeB(C6F5)3 (Cp′ ) Cp and substituted deriva-
-
tives),21 the reaction of Cp2TaMe3 with 1 equiv of
B(C6F5)3 produces an oily product mixture containing
both Cp2TaMe2+CH3B(C6F5)3- and Cp2TaMe2+[(C6F5)3B-
CH3-B(C6F5)3]- in the solution phase, along with un-
reacted Cp2TaMe3.
An interesting aspect of this reaction is that, due to
the weakly Lewis acidic Cp2TaMe2+ countercation, the
-
enhanced nucleophilicity of the free anion CH3B(C6F5)3
,
once generated, attacks B(C6F5)3 to form the Me-bridged
binuclear anion [(C6F5)3B-CH3-B(C6F5)3]-.22 This chem-
istry is different from what has been demonstrated for
the group 4 metallocenes, where the weakly coordinat-
ing feature of the anion promotes the reactivity of the
countercation to form the Me-bridged dinuclear cations
[Cp′2ZrMe-CH3-MeZrCp′2]+.23 Subsequently, 2 equiv
(21) (a) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015-10031. (b) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem.
Soc. 1991, 113, 3623-3625.
(22) The preparation of trityl or ammonium salts of dinuclear anions
having a general formula of [(C6F5)3M-LG-M(C6F5)3]-, where the
linking group LG ) cyanide and M ) B22a and LG ) cyanide, azide,
dicyanamide, imidazolide and
M )
B, Al,22b has been recently
reported: (a) Lancaster, S. J .; Walker, D. A.; Thornton-Pett, M.;
Bochmann, M. Chem. Commun. 1999, 1533-1534. (b) See ref 16.
(23) (a) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J . J . Am. Chem.
Soc. 1996, 118, 12451-12452. (b) Beck, S.; Prosenc, M.-H.; Brintzinger,
H.-H.; Goretzki, R.; Herfert, N.; Fink, G. J . Mol. Catal. 1996, 111, 67-
79. (c) Bochmann, M.; Lancaster, S. L. Angew. Chem., Int. Ed. Engl.
1994, 33, 1634-1637. (d) Bochmann, M.; Lancaster, S. J . J . Organomet.
Chem. 1992, 434, C1-C5.
(24) Chen, E. Y.-X.; Kruper, W. J .; Roof, G.; Wilson, D. R. J . Am.
Chem. Soc. 2001, 123, 745-746.

838 Organometallics, Vol. 21, No. 5, 2002
Feng et al.
F igu r e 1. 1H NMR of 4 (bottom) and the reaction product of 4 and Al(C6F5)3‚toluene (top) bromobenzene-d5 at 23 °C. See
Experimental for peak assignments.
Sch em e 4
Sch em e 5
using the three distinctive classes of Ta(V) complexes
activated with various activators are summarized in
Table 1. It can be seen from the table that Cp2TaMe3
has no olefin polymerization activity (but is active for
MMA polymerization with suitable activators; vide
infra) even with an excess of Lewis acid activators (runs
2 and 3). The non-metallocene Ta complex 4 is active
but exhibits low polymerization activity toward copo-
lymerization of ethylene and 1-octene upon activation
with [HNMe(C18H37)2]+[(C6F5)3AlNC3H3NAl(C6F5)3]- or
PMAO-IP (runs 4 and 5). The most active catalyst of
this series is the half-sandwich imido Ta dimethyl
complex 2. Thus, upon activation with [HNMe(C18H37)2]+-
[(C6F5)3AlNC3H3N(Al(C6F5)3]-, 50 g of copolymer was
produced within 15 min using 5.0 µmol of 2, giving an
activity of 1.2 × 106 g of polymer/((mol of Ta) atm h)
(run 6). This copolymer has a low density of 0.898, a
high molecular weight of 127 000, and a molecular
weight distribution of 2.29, indicative of large octene
incorporation in the copolymer. Complex 2 is still very
active at a polymerization temperature of 160 °C, giving
an activity of 5.4 × 105 g of polymer/((mol of Ta) atm h)
(run 7). Other activators such as PMAO-IP give lower
polymerization activity (run 8).
as well as the small ∆δ (δm - δp) value of 2.52 ppm in
19F NMR chemical shifts of the anion, indicate the
solvent-separated, nonassociated free anion MeB(C6-
- 25
F5)3
.
Reaction of 4 with 2 equiv of B(C6F5)3 does not
effect the abstraction of the second tantalum methyl
group, forming the same product as the 1:1 reaction
except for the presence of the unreacted borane.
The products of the reaction between 4 and Al(C6F5)3
in C6D5Br depend on the ratio of the two reactants. In
a 1:1 ratio, the reaction at room temperature produces
Ta cations with a mixture of the mononuclear anion
CH3Al(C6F5)3- (7, ∼80%) and the binuclear anion [(C6F5)3-
Al-CH3-Al(C6F5)3]- (8, ∼20%). Due to the nonassocia-
tive nature between the cation and anion, the cation
1
portion is spectroscopically identical in H NMR for both
species, whereas in 19F NMR two different anions can
be seen. When a second equivalent of Al(C6F5)3 is added,
8 is formed quantitatively by NMR (Scheme 5). The
spectroscopic features of the tantalum cation are similar
to those observed from the reaction with the borane, and
the sharp differences between the neutral 4 and the
cation in 7 or 8 are illustrated in Figure 1.
P olym er iza tion Stu d ies. Results from high-tem-
perature ethylene-1-octene copolymerization studies
Half-sandwich Ta(III) and Nb(III) diene complexes
are active for controlled MMA polymerization when
activated with suitable activators,2 whereas the non-
metallocene Ta(V) complex 4 exhibits no polymerization
activity upon activation with various activators. The
(25) (a) Klosin, J .; Roof, G.; Chen, E. Y.-X.; Abboud, K. A. Organo-
metallics 2000, 19, 4684-4686. (b) Horton, A. D.; de With, J .; van der
Linden, A. J .; van de Weg, H. Organometallics 1996, 15, 2672-2674.

Ta(V)-Based Metallocene and Non-Metallocene Complexes
Organometallics, Vol. 21, No. 5, 2002 839
Ta ble 1. High -Tem p er a tu r e Eth ylen e a n d 1-Octen e Cop olym er iza tion Resu lts a n d P olym er P r op er ties
Ca ta lyzed by Ta (V) Com p lexes w ith Va r iou s Activa tor sa
efficiency
MMI
amt of catalyst/ exotherm yield
(g of polymer/
density (micro melt
b
entry
catalyst
activator
activator (µmol)
(°C)
(g) ((mol of metal) atm h)) (g/mL)
index)
Mw
Mw/Mn
1c
2
3
4
5
CGC-TiMe2 B(C6F5)3
Cp2TaMe3 B(C6F5)3
Cp2TaMe3 Al(C6F5)3
2.0/2.0
1.0/4.0
1.0/4.0
2.0/2.0
20/10 000
5.0/5.0
8.1
0.5
0.4
1.3
1.1
1.4
1.3
1.2
138.0
trace
trace
3.0
8 288 000
0
0.892
0.900
3.7
77 500 1.94
0
4
4
2
2
2
Al imidazolided
180 000
11 000
1 206 000
538 000
89 000
0.2
PMAO-IP
1.9
6
Al imidazolide
Al imidazolide
PMAO-IP
50.2
22.4
14.8
0.898
0.900
0.917
0.9
1.8
0.2
127 000 2.29
125 000 2.72
7e
8f
5.0/5.0
20/10000
a
b
Conditions: 140 °C, 118 g of octene, 5 mmol of H2, 500 psig of ethylene, 740 g of isopar-E, 15 min. GPC relative to polystyrene
standards. c A control run. [HNMe(C18H37)2]+[(C6F5)3AlNC3H3N(Al(C6F5)3]-. e Carried out at 160 °C. f Carried out at 130 °C.
d
Ta ble 2. MMA P olym er iza tion Resu ltsa
I*c (%)
Tg (°C)
[rr]d (%)
[mr] (%)
[mm] (%)
b
entry
complex
Tp (h)
yield (%)
Mn
Mw/ Mn
1
2
Cp2TaMe3/B(C6F5)3
Cp2TaMe3/Al(C6F5)3
12
12
0
0
3
4
Cp2TaMe3/2Al(C6F5)3
(i) 2Al(C6F5)3
10
10
100
100
158 600
347 300
5.42
3.22
12.6
5.7
120.5
72.9
75.9
25.8
23.4
1.3
0.7
(ii) Cp2TaMe3
5
Cp2TaMe3/2B(C6F5)3
12
0
a
Polymerization conditions: 46.7 µmol of initiator (I); mole ratio MMA/I ) 200 with target Mn ) 20 000; 10 mL of toluene; 25 °C.
Catalyst and activator are mixed in situ the the ratio indicated in the complex column. For run 4, MMA monomer was first mixed with
Al(C6F5)3 followed by addition of Cp2TaMe3. GPC relative to polystyrene standards. c Initiator efficiency: I* ) Mn(calcd)/Mn(exptl).
b
Determined by 1H NMR spectroscopy.
d
metallocene complex Cp2TaMe3 is also inactive for MMA
polymerization in toluene when activated with 1 equiv
of B(C6F5)3 or Al(C6F5)3. However, active polymerization
systems are generated if 2 equiv of Al(C6F5)3 was used
for the activation, regardless of addition sequence. Thus,
the activated catalyst polymerization by first mixing
Cp2TaMe3 with 2 equiv of Al(C6F5)3 before adding MMA
produced PMMA in 100% yield at a MMA/initiator ratio
of 200 for 10 h (Table 2, run 3). A similar activity is
observed via activated monomer polymerization by first
mixing MMA with 2 equiv of Al(C6F5)3 before adding
Cp2TaMe3 (run 4). Interestingly, activation with 2 equiv
of B(C6F5)3 did not generate an active MMA polymeri-
zation system (run 5). These findings seem to be
consistent with the observations that the reactions of
Cp2TaMe3 with 1 equiv of Al(C6F5)3 or with 1 or 2 equiv
of B(C6F5)3 in toluene produce a mixture of species,
while the reaction with 2 equiv of Al(C6F5)3 cleanly
generates Cp2TaMe2+[(C6F5)3Al-CH3-Al(C6F5)3]- (vide
supra).
In summary, neutral routes by direct reaction of the
neutral ligands with TaCl5 have been developed for the
synthesis of half-metallocene and non-metallocene tan-
talum complexes. Activation of tantalum trimethyl
complexes with 1 equiv of strong Lewis acids M(C6F5)3
affords the corresponding cationic species paired with
-
anions MeM(C6F5)3 in polar solvents; however, the
reaction in benzene or toluene produces a mixture of
products. The reaction of Cp2TaMe3 with 2 equiv of Al-
(C6F5)3 cleanly generates a cation paired with a bi-
nuclear anion, Cp2TaMe2+[(C6F5)3Al-CH3-Al(C6F5)3]-,
which is active for MMA polymerization. Among the
three classes of complexes included in this study, the
half-sandwich imido complex was established as an
efficient olefin copolymerization catalyst. These results
extend the olefin copolymerization catalyst library for
producing low-density elastomers by including group 5
half-metallocene imido complexes.
Number-average molecular weights of the PMMA
produced from the tantalocene system are significantly
higher than the calculated values, and molecular weight
distributions are also broad (PDI from 3.22 to 5.42).
Initiator efficiencies are low (from 5.6% to 12.6%), and
the polymers are syndiotactic, with [rr] triads ranging
from 73 to 76%.
Ack n ow led gm en t. We thank Drs. J . Klosin, W. J .
Kruper, and P. N. Nickias of The Dow Chemical Co. for
helpful discussions and Dr. Khalil A. Abboud of the
University of Florida for X-ray diffraction analysis.
OM010702C