Stability of metal-carbon bond versus metal reduction during ethylene polymerization promoted by a vanadium complex: The role of the aluminum cocatalyst

Article abstract of DOI:10.1021/om011040u

The dinuclear and trivalent complex {[(Me₃Si)NCH₂CH₂]₂N(Me₃Si )}₂V₂(μ-Cl)₂ (1) is the precursor to mono- and dinuclear alkyl derivatives that are thermally stable. For example, treatment with MeLi gives a stable methyl derivative, probably isostructural with 1, which upon further treatment with pyridine affords the mononuclear complex {[(Me₃Si)NCH₂CH₂]₂N(Me₃Si )}V(CH₃)(pyridme) (2). However, reaction of 1 with Me₂AlCl, AlMe₃, or PMAO-IP yields the tetrametallic species {[(Me₃Si)NCH₂CH₂]₂N(Me₃Si )}₂V₂(μ-Cl)₂(AlMe₂)₂ (3), where the central core of 1 was preserved except for the vanadium centers, which were reduced to the divalent state. The two Me₂Al residues remained coordinated to the amido ligand. The reduction of vanadium to the divalent state relates to the relatively short life of 1 as an ethylene polymerization catalyst. A similar reaction of 1 with AlCl₃ resulted in disproportionation forming the tetravalent complex {[(Me₃Si)NCH₂CH₂]₂N(Me₃Si )}VCl₂AlCl₃ (4) and the pentanuclear mixed-valent V(II)/V(III) species [(AlCl₂){[(Me₃Si)NCH₂CH₂]₂ N(Me₃Si)}V]₂[(μ-Cl)₆V]·(toluene )₂ (5). The fact that complex 5 contains a divalent vanadium atom stripped of its ligand system indicates that two different reaction mechanisms are operating to reduce the vanadium center and that the differing Lewis acidity of the two aluminum species is the determining factor.

Full text of DOI:10.1021/om011040u

968
Organometallics 2002, 21, 968-976
Sta bility of Meta l-Ca r bon Bon d ver su s Meta l Red u ction
d u r in g Eth ylen e P olym er iza tion P r om oted by a
Va n a d iu m Com p lex: Th e Role of th e Alu m in u m
Coca ta lyst
Khalil Feghali, David J . Harding, Damien Reardon, Sandro Gambarotta,* and
Glenn Yap
Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Qinyan Wang
NOVA Chemicals, 2928 16th Street, N.E. Calgary, Alberta T2E 7K7, Canada
Received December 6, 2001
The dinuclear and trivalent complex {[(Me3Si)NCH2CH2]2N(Me3Si)}2V2(µ-Cl)2 (1) is the
precursor to mono- and dinuclear alkyl derivatives that are thermally stable. For example,
treatment with MeLi gives a stable methyl derivative, probably isostructural with 1, which
upon further treatment with pyridine affords the mononuclear complex {[(Me3Si)NCH2-
CH2]2N(Me3Si)}V(CH3)(pyridine) (2). However, reaction of 1 with Me2AlCl, AlMe3, or PMAO-
IP yields the tetrametallic species {[(Me3Si)NCH2CH2]2N(Me3Si)}2V2(µ-Cl)2(AlMe2)2 (3), where
the central core of 1 was preserved except for the vanadium centers, which were reduced to
the divalent state. The two Me2Al residues remained coordinated to the amido ligand. The
reduction of vanadium to the divalent state relates to the relatively short life of 1 as an
ethylene polymerization catalyst. A similar reaction of 1 with AlCl3 resulted in dispropor-
tionation forming the tetravalent complex {[(Me3Si)NCH2CH2]2N(Me3Si)}VCl2AlCl3 (4) and
the pentanuclear mixed-valent V(II)/V(III) species [(AlCl2){[(Me3Si)NCH2CH2]2N(Me3Si)}V]2-
[(µ-Cl)6V]‚(toluene)2 (5). The fact that complex 5 contains a divalent vanadium atom stripped
of its ligand system indicates that two different reaction mechanisms are operating to reduce
the vanadium center and that the differing Lewis acidity of the two aluminum species is
the determining factor.
In tr od u ction
in the catalyst activation by generating, in addition to
the V-R function, empty coordination sites on vana-
Research into the area of Ziegler-Natta olefin po-
lymerization has traditionally focused on the group 4
metals since these catalysts tend to be both efficient and
selective.1 In addition, most of the industrially success-
ful catalysts are based on Cp and its derivatives, as
these ligands are both robust and highly tunable.
Nevertheless, some nonmetallocene systems have found
industrial applications. Particularly relevant to this
work is the V(acac)3 catalyst used for the commercial
production of ethylene-propylene-diene elastomers
(EPDM).2
(2) See for example: (a) Doi, Y.; Suzuki, S.; Soga, K. Macromolecules
1986, 19, 2896. (b) Ouumi, T.; Soga, K. Makromol. Chem. 1992, 193,
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101, 229. (d) Adisson, E. J . Polym. Sci., Part A: Polym. Chem. 1994,
32, 1033. (e) Davis, S. C.; von Hellens, W.; Zahalka, H. In Polymer
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Miyake, H.; Suuki, S.; Soga, K. Transition Metals and Organometallics
as Catalysts for Olefin Polymerization; Kaminsky, W., Sinn H., Eds.;
Springer-Verlag: Berlin, 1988. (h) Carrick, W. L. J . Am. Chem. Soc.
1958, 80, 6455. (i) Christman, D. L. J . Polym. Sci. 1972, A-1, 471. (j)
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Chem. Soc. 1960, 82, 3883. (l) Lehr, M. H. Macromolecules 1968, 1,
178. (m) Christman, D. L.; Keim, G. I. Macromolecules 1968, 1, 358.
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E. W.; Grant, D.; Horder, J . R.; J enkins, D. K.; Marlow, A. E.; Wallis,
S. R.; Doughty, A. G.; Maradon, J . M.; Skinner, G. A. Eur. Polym. J .
1974, 10, 481. (p) Schuere, S.; Fisher, J .; Kress, J . Organometallics
1995, 14, 2627. (q) Feher, F. J .; Blanski, R. L. J . Am. Chem. Soc. 1992,
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Soc. 1991, 113, 3618. (s) Feher, F. J .; Blanski, R. L. Organometallics
1993, 12, 958. (t) Cucinella, S.; Mazzei A. U.S. Patent 3,711,455, CI.
260-85.3, 1973. (u) Boor, J ., J r.; Youngman, E. A. J . Polym. Sci. A
1966, 4, 1861. (v) Zambelli, A.; Proto, A.; Longo, P. In Ziegler Natta
Catalysis; Fink, G., Mulhaupt, R., Brintzinger, H. H., Eds.; Springer-
Verlag: Berlin, 1995.
A recent study of this system in our group highlighted
a number of problems with the process.3 The investiga-
tion confirmed that the Al cocatalyst plays a pivotal role
(1) See for example: (a) Mohring, P. C.; Coville, N. J . J . Organomet.
Chem. 1994, 479, 1. (b) Erker, G.; Nolte, R.; Tsay, Y. H.; Kruger, C.
Angew. Chem. 1989, 28, 628. (c) Erker, G.; Nolte, R.; Aul, R.; Wilker,
S.; Kruger, C.; Noe, R. J . Am. Chem. Soc. 1991, 113, 7594. (d)
Kaminsky, W.; Kulper, K.; Brintzinger, H. H.; Wild, R. W. P. Angew.
Chem., Int. Ed. Engl. 1985, 6, 507. (e) Coates, G. W.; Weymouth, R.
M. J . Am. Chem. Soc. 1991, 113, 6270. (f) Ewen, J . A. J . Am. Chem.
Soc. 1984, 106, 6355. (g) Guerra, G.; Cavallo, L.; Moscardi, G.;
Vacatello, M.; Corradini, P. J . Am. Chem. Soc. 1994, 116, 2988. (h)
J ordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. (i) Brintzinger,
H. H.; Fisher, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143. (j) Bochmann, M. J . Chem. Soc.,
Dalton Trans. 1996, 255.
(3) Ma, Y.; Reardon, D. F.; Gambarotta, S.; Yap, G. P. A. Organo-
metallics 1999, 18, 2773.
10.1021/om011040u CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/05/2002

Stability of Metal-Carbon Bond
Organometallics, Vol. 21, No. 5, 2002 969
dium (otherwise coordinatively saturated) via removal
of one or more of the acac ligands. However, by stripping
the metal of the remaining acac ligands, the cocatalyst
decreases the stability of the V-R bond. Consequently,
the vanadium metal is rapidly reduced to the divalent
state, resulting in complete catalyst deactivation. De-
spite this drawback, vanadium catalysts remain ir-
replaceable in EPDM manufacture. Thus, we were
interested in developing new catalysts that might avoid
the problems encountered with the above system. To
prevent ligand leaching, it was decided to change the
donor set from oxygen to nitrogen atoms. With such a
system one might reasonably expect that the lower
affinity of aluminum for nitrogen would reduce migra-
tion of the ligand from the vanadium center.
A direct consequence of ligand leaching is the reduc-
tion of the metal to V(II), which is a rather common
deactivation pathway for vanadium catalysts. This is
thought to be a result of a variety of events including
â-H elimination of the alkyl polymer growing chain4 and
C-H σ-bond metathesis4 and relates to an intrinsic
instability of the V-C bond once the ligand system has
been abstracted from the vanadium center. Although a
number of V(III) alkyl complexes have been successfully
synthesized,5,6 nevertheless, these complexes generally
employ sterically bulky alkyl ligands or bulky support-
ing ligands. By analogy, we felt that a large chelating
ligand would also be desirable for a vanadium catalyst
since it should extend the catalyst’s life. Moreover, it
was hoped that a polydentate chelating ligand might
also contribute to the aggregation of both vanadium and
aluminum in the same molecular structure, providing
insight into the catalyst/cocatalyst interaction.
Exp er im en ta l Section
All operations were performed under inert atmosphere by
using standard Schlenk type techniques. Polymethylalumox-
ane solution (13.5% Al) in toluene (PMAO-IP, Akzo-Nobel),
AlMe3 (Aldrich), and Me2AlCl (Aldrich) were used as received
without further purification. VCl3(THF)312 and [(Me3Si)NHCH2-
CH2]2N(Me3Si)7 were synthesized according to published pro-
cedures. The corresponding dilithium salt and complex 1 were
prepared by a slight modification of the procedure described
by Cloke.6,8-10 AlCl3 was sublimed under reduced pressure
before use. Infrared spectra were recorded on a Mattson 9000
and Nicolet 750-Magna FTIR instruments from Nujol mulls
prepared in a drybox. Samples for magnetic susceptibility
measurements were weighed inside a drybox equipped with
an analytical balance and sealed into calibrated tubes. Mag-
netic measurements were carried out with a Gouy balance
(J ohnson Matthey) at room temperature. Magnetic moments
were calculated following standard methods,13 and corrections
for underlying diamagnetism were applied to the data.14
Elemental analyses were carried out using a Perkin-Elmer
2400 CHN analyzer. NMR analysis were carried out on Varian
Gemini 200 and Bruker AMX-500 spectrometers using vacuum-
sealed NMR tubes prepared inside a drybox.
P r ep a r a tion of [(Me3Si)NLi2CH2CH2]2N(Me3Si). The
addition of butyllithium (52 mL, 0.13 mol, 2.5 M) to a solution
of [(Me3Si)NHCH2CH2]2N(Me3Si) (21 g, 0.065 mol) in hexane
(150 mL) at -80 °C afforded a white precipitate. The solution
was allowed to warm to room temperature, stirred for 1 h, and
filtered to collect the dilithiated salt (19.3 g, 0.058 mol, 89%).
IR (Nujol mull, KBr, cm-1): ν 1258 (m), 1083 (m), 944 (w),
1
884 (w), 825 (m), 729 (w). H NMR (CDCl3, 200 MHz, 25 °C):
δ 0.15 (s, 9H, TMS), 0.34 (s, 18H, TMS), 2.9 (m, 4H, CHethyl).
P r ep a r a tion of {[(Me3S)NCH2CH2]2N(Me3Si)}2V2(µ-Cl)2
(1). A solution of VCl3(THF)3 (5.8 g, 15.5 mmol) in THF (100
mL) was treated with 1 equiv of [(Me3Si)NLi2CH2CH2]2N(Me3-
Si) (5.2 g, 15.5 mmol) at room temperature. The red-orange
color of the solution immediately darkened upon mixing, and
stirring was continued for 30 min. THF was removed in vacuo,
and the resulting red powder was redissolved in hexane (150
mL). The solution was then filtered to eliminate LiCl and
allowed to stand at 0 °C, for 24 h, upon which dark red crystals
of 1 separated (5.1 g, 6.2 mmol, 80%). IR (Nujol mull, KBr,
cm-1): ν 1377 (m), 1250 (s), 1074 (s), 937 (s), 911 (s), 825 (s).
µeff: 2.93 µB. Anal. Calcd for C26H70N6Si6V2Cl2 (found): C, 38.64
(38.53); 8.73 (8.61); N 10.40 (10.34).
P r ep a r a tion of {[(Me3Si)NCH2CH2]2N(Me3Si)}V(CH3)-
(p yr id in e) (2). A solution of {[(Me3Si)NCH2CH2]2N(Me3-
Si)}2V2(µ-Cl)2 (1.1 g, 1.4 mmol) in ether (100 mL) was treated
with 1 equiv of methyllithium (2 mL, 1.4 M) at room temper-
ature. After mixing, the color of the solution changed to pink-
red and stirring was continued for 30 min. The solvent was
removed in vacuo, the solid residue was redissolved in hexane
(50 mL), and the solution was filtered to eliminate LiCl. A
small portion of the red solid was used for a Toepler pump
degradation experiment (0.5 g of dry solid was dissolved in
toluene (15 mL) and treated with 3 equiv of gaseous HCl;
methane gas was collected with a Topler pump recovering 57
mL at 40 mmHg and 298 K). The addition of dry pyridine (5
mL) turned the color of the solution purple. The mixture was
allowed to stand at -80 °C for 36 h, upon which large dark
In view of these considerations the bulky N,N,N-tris-
(trimethylsilyl)diethylenediamidoamino ligand7 was se-
lected for this study, as it has already been successfully
employed for different purposes, including one spec-
tacular case of dinitrogen cleavage,8 with a range of
metals including V,6 Ti,9 Zr,10 and Nb.11 The resulting
vanadium complexes prepared in the course of this
investigation and their catalytic behavior, including
their interaction with the Al cocatalyst, are now pre-
sented.
(4) Guram, A. S.; J ordan, R. F. In Comprehensive Organometallic
Chemistry; Abel, E. W., Stone F. G. A., Wilkinson, G., Eds. Pergamon:
New York, 1995, and references therein.
(5) Non-Cp and stable vanadium alkyls are rare compounds: (a)
Seidel, W., Kreiseli, G. Z. Anorg. Allg. Chem., 1977, 435, 146. (b)
Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, G. J . Chem.
Soc., Datlon Trans. 1984, 886. (c) Gambarotta, S.; Mazzanti, M.;
Floriani, C.; Chiesi-Villa, A.; Guastini, G. J . Chem. Soc., Dalton Trans.
1985, 829. (d) Solari, E.; De Angelis, S.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. Inorg. Chem. 1992, 31, 96. (e) Wills, A. R.; Edwards, P. G.
J . Chem. Soc., Dalton Trans. 1989, 1253. (f) Gambarotta, S.; Floriani,
C.; Chiesi-Villa, A.; Guastini, C. J . Chem. Soc., Chem. Commun. 1984,
886. (g) Buijink, J . K.; Meetsma, A.; Teuben, J . H. Organometallics
1993, 12, 2004. (h) Berno, P.; Gambarotta, S.; Richeson, D. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Oxford 1995, and references therein.
(6) Clancey, G. P.; Clark, H. C. S.; Clentsmith, G. K. B.; Hitchcock,
P. B.; Cloke, F. G. N. J . Chem. Soc., Dalton Trans. 1999, 3345.
(7) Gol’din, G. S.; Baturina, L. S.; Gavrilova, T. Zh. Obshch. Khim.
1975, 45, 2189.
purple crystals of 2 separated (0.98 g, 2.1 mmol, 75%). µeff
2.90 µB. The extreme air-sensitivity prevented combustion
analysis determinations.
P r ep a r a t ion of {[(Me3Si)NCH 2CH 2]2N(Me3Si)}2V2(µ-
Cl)2(AlMe2)2 (3). Meth od A. A solution of 1 (0.9 g, 1.14 mmol)
:
(8) Clentsmith, G. K. B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F.
G. N. J . Am. Chem. Soc. 1999, 121, 10444.
(9) (a) Cloke, F. G. N.; Love, J . B.; Hitchcock, P. B. J . Chem. Soc.,
Dalton Trans. 1995, 25. (b) Love J . B.; Clark, H. C. S.; Cloke, F. G. N.;
Green, J . C.; Hitchcock, P. B. J . Am. Chem. Soc. 1999, 121, 6843.
(10) Clark, H. C. S.; Cloke, F. G. N.; Hitchcock, P. B.; Love, J . B.;
Wainright, A. P. J . Organomet. Chem. 1995, 503, 333.
(11) Horton, A. D.; De With, J .; Van de Linden, A. J .; Van de Weg,
H. Organometallics 1996, 15, 2672.
(12) Manzer, L. E. Inorg. Synth. 1982, 21, 138.
(13) Mabbs, M. B.; Machin, D. Magnetism and Transition Metal
Complexes; Chapman and Hall; London, 1973.
(14) Foese, G.; Gorter, C. J .; Smits, L. J . Constantes Selectionne´es
Diamagnetisme, Paramagnetisme, Relaxation Paramagnetique; Mas-
son: Paris, 1957.

970 Organometallics, Vol. 21, No. 5, 2002
Feghali et al.
in hexane (100 mL) was treated with 1 mL of PMAO (13% Al
solution in toluene) at room temperature. Upon addition, a
green precipitate separated from the red solution. It took
roughly 54 equiv of Al (PMAO) to yield a colorless solution.
Crystals of 3 were grown by layering a solution of PMAO in
toluene over a solution of 1 in hexane and subsequent standing
at room temperature for 3 days (0.2 g, 0.22 mmol, 19%). IR
(Nujol, KBr, cm-1): ν 2722 (w), 2701 (w), 1377 (m), 1247 (s),
1197 (s), 1126 (m), 1099 (s), 1054 (s), 904 (s), 836 (s), 765 (s),
rapidly changed the color to dark blue. The solid was then
identified as VCl2(TMEDA)2 (0.24 g, 38%) by comparison of
the analytical and spectroscopic data with those of an analyti-
cally pure sample.
Eth ylen e P olym er iza tion . A bench scale reactor was used
in the polymerization experiments. The reactor uses a pro-
grammable logic control (PLC) system with Wonderware 5.1
software for process control. Ethylene polymerizations were
performed in the 500 mL Autoclave Engineers Zipperclave
reactor equipped with an air-driven stirrer and an automatic
temperature control system. All the chemicals were fed into
the reaction batchwise except for ethylene, which was fed on
demand.
674 (s). µeff: 3.9 µB. Anal. Calcd for C30H82N6Si6V2Al2Cl2
(found): C 39.07(38.88); H 8.96 (8.71); N 9.11 (8.97).
Meth od B. A solution of 1 (1.2 g, 1.6 mmol) in hexane (150
mL) was treated with 2 equiv of a solution of Me3Al in hexane
(3 mL, 2.0 M) and at room temperature. After mixing the color
of the solution changed to dark red and stirring was continued
for 30 min. The mixture was allowed to stand at 4 °C, for 2
days, upon which emerald-green crystals of 3 separated from
a brown solution (0.9 g, 0.99 mmol, 60%).
Conditions for polymerization:
slurry phase
216 mL
catalyst concentration 300 µmol/L
Me2AlCl
reaction temperature 50 °C
reactor pressure 300 psig total
solution phase
216 mL
300 µmol/L
toluene
Meth od C. Same procedure and amount as method A but
using Me2AlCl (yield 21%).
Al/V ) 60 (mol/mol) Al/V ) 60 (mol/mol)
Isola tion of {[(Me3Si)NCH2CH2]2N(Me3Si)}VCl2AlCl3 (4)
a n d [(AlCl2){[(Me3Si)N CH2CH2]2N(Me3Si)}V]2[(µ-Cl)6V]‚
(tolu en e)2 (5). A solution of 1 (1.0 g, 1.23 mmol) in toluene
(150 mL) was treated with 2 equiv of AlCl3 (0.37 g, 2.8 mmol)
at room temperature. After mixing the color of the solution
changed to brown and stirring was continued for 1 h. The
solution was then filtered (to eliminate excess AlCl3), and the
volume was reduced. The mixture was allowed to stand at
room temperature, for 24 h, upon which red crystals of 4
separated (0.2 g, 0.35 mmol, 28%). IR (Nujol, KBr, cm-1): ν
1377 (m), 1255 (s), 1089 (m), 1051 (m), 1001 (m), 912 (s), 891
(s), 840 (s), 759 (m), 721 (w), 669 (w), 634 (w). µeff: 1.83 µB.
Anal. Calcd foc C13H35N3Si3VCl5Al (found): C 27.26 (27.17);
H 6.16 (6.12); N 7.33 (7.28). Further concentration of the
mother liquors afforded another crop of crystalline 4. Among
the red crystals thin green plates were present. X-ray analysis
of the latter confirmed the molecular connectivity of 5.
Unfortunately, only small amounts of the complex were
obtained and no further characterization was possible.
P r ep a r a tion of {[(Me3Si)NCH2CH2]2N(Me3Si)}VCl2 (6).
Meth od A. A solution of 1 (1.0 g, 1.2 mmol) in toluene (150
mL) at room temperature was treated with 1 equiv of tri-
phenylphosphium dichloride (0.2 g, 0.6 mmol) per vanadium
dimer. After mixing the color of the solution changed to brown
and stirring was continued for 1 h. The solution was then
filtered and the solvent was reduced to dryness. Freshly
distilled hexane was added (30 mL). The mixture was allowed
to stand at -40 °C, for 24 h, upon which brown crystals of 6
separated (0.2 g, 0.5 mmol, 41%). IR (Nujol, KBr, cm-1): ν 1377
(m), 1255 (s), 1089 (m), 1051 (m), 1001 (m), 912 (s), 891 (s),
840 (s), 759 (m), 721 (w), 669 (w), 634 (w). µeff: 1.83 µB. Anal.
Calcd for C13H35N3Si3VCl2 (found): C 35.52 (35.49); H 8.03
(7.95); N 9.58 (9.53).
Met h od B. A solution of [(Me3Si)NLi2CH2CH2]2N(Me3Si)
(7.1 mmol) in ether (100 mL) was combined via cannula with
a suspension of VCl4(DME) (2 g, 7.1 mmol) in ether (100 mL)
over a period of 30 min. The resulting dark brown solution
was allowed to stir overnight and was then filtered to eliminate
LiCl. The volume of the solution was reduced and allowed to
stand at -20 °C overnight, upon which crystals of 6 separated
(2.1 g, 4.9 mmol, 69%).
Rea ction of Com p lex 1 w ith VCl3(THF )3. A dark red
solution of 1 (0.57 g, 0.7 mmol) in toluene (30 mL) was mixed
with a suspension of VCl3(THF)3 (0.54 g, 1.4 mmol) in the same
solvent (70 mL). The mixture was stirred for 2 h, upon which
the color changed to dark brown. The solvent was evaporated
to dryness and replaced by 50 mL of freshly distilled diethyl
ether. The solution was filtered and allowed to stand at -40
°C for 2 days, affording brown flat crystals of 6 (0.32 g; yield
51%). The ether-insoluble residue was redissolved in toluene
and treated with 0.44 mL of TMEDA (0.34 g; 2.9 mmol), which
140 °C
286 psig total
As summarized above the control temperature was 50 °C for
a slurry polymerization and 140 °C for solution polymerization
experiments. The polymerization time varied from 10 to 30
min. The reaction was terminated by adding 5 mL of methanol
to the reactor, and the polymer was recovered by evaporation
of the toluene in vacuo. The polymerization activities were
calculated on the basis of weight of polymer produced.
Polymer molecular weights and molecular weight distribu-
tions were measured by GPC (Waters 150-C) at 140 °C in 1,2,4-
trichlorobenzene calibrated using polystyrene standards. DSC
was conducted on a DSC 220 C from Seiko Instruments. The
heating rate was 10 °C/min from 0 to 200 °C.
In a typical polymerization experiment, the solvent was
presaturated with ethylene in the reactor at the desired
temperature. The cocatalyst was injected in the reactor fol-
lowed by introduction of the precatalyst. Reproducibility of
catalyst activity and reactor operation was checked by regu-
larly running polymerization experiments using a standard
zirconium catalyst and by repeating the experiments under
identical reaction conditions. Errors and deviation were always
below 5.9%.
X-r a y Cr ysta llogr a p h y. Str u ctu r a l Deter m in a tion of
1, 2, 3, 4, a n d 5. Suitable crystals were selected, mounted on
thin glass fibers using viscous oil, and cooled to the data
collection temperature. Data were collected on a Bruker AX
SMART 1k CCD diffractometer using 0.3° ω-scans at 0°, 90°,
and 180° in φ. Unit-cell parameters were determined from 60
data frames collected at different sections of the Ewald sphere.
Semiempirical absorption corrections based on equivalent
reflections were applied.15
No symmetry higher than triclinic was evident from the
diffraction data of 3 and 5. Solution in the centric option
yielded chemically reasonable and computationally stable
results of refinement. Systematic absences in the diffraction
data and unit-cell parameters were uniquely consistent for the
reported space groups for 1, 2, and 4. The structures were
solved by direct methods, completed with difference Fourier
syntheses, and refined with full-matrix least-squares proce-
dures based on F2. The compound molecules for 3 and 5 are
located on inversion centers. Two toluene solvent molecules
were located cocrystallized in the asymmetric unit of 5. All
non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were treated as ideal-
ized contributions. All scattering factors are contained in the
SHEXTL 5.10 program library (Sheldrick, G. M. Bruker AXS,
(15) Blessing, R. Acta Crystallogr. 1995, A51, 33-38.

Stability of Metal-Carbon Bond
Organometallics, Vol. 21, No. 5, 2002 971
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e An a lysis Resu lts for Com p lexes 1, 2, a n d 3
1
2
3
formula
fw
C
26H70N6Cl2Si6V2
C19H43N4Si3V
462.78
C30H82Al2Cl2N6Si6V2
922.30
808.20
space group
a (Å)
b (Å)
monoclinic, P2(1)/c
10.534(1)
37.931(4)
11.707(1)
monoclinic, P2(1)/n
10.7952(5)
16.6050(8)
14.7725(8)
90
93.228(1)
90
2643.8(2)
4
triclinic, P1h
10.433(2)
11.260(2)
12.639(3)
97.09(2)
105.52(1)
115.10(1)
1246.8(4)
1
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
90
109.701(1)
90
4403.5(8)
4
Z
radiation (KR, Å)
0.71073
223(2)
1.219
7.34
0.71073
203(2)
1.163
5.23
1000
0.71073
203(2)
1.228
6.89
T (K)
Dcalcd (g cm-3
)
µcalcd (cm-1
)
F000
1728
494
2 a
R, Rw
,
GoF
0.0725, 0.1597, 1.167
0.0609, 0.1250, 1.052
0.0639, 0.1962, 1.071
R ) ∑Fo - Fc/ ∑FoRw ) [(∑(Fo - Fc)2/∑wFo2)]1/2
.
a
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e An a lysis
Resu lts for Com p lexes 4 a n d 5
tion geometry is defined by a methyl group [V-C ) 2.118(3)
Å] and the nitrogen atom of one coordinated molecule of
pyridine [V-N ) 2.157(2) Å], while the other three coordina-
tion sites are occupied by the ligand nitrogen atoms. The
equatorial plane is occupied by the two terminal nitrogen
atoms of the amido ligand and by the terminally bonded
methyl group. The pyridine nitrogen atom and the central
ligand nitrogen are placed on the axis [C(19)-V-N(1) )
116.65(6)°, C(19)-V-N(3) ) 113.85(7)°, C(19)-V-N(4) )
92.62(5)°, C(19)-V-N(2) ) 100.94(5)°]. The coordination
geometry around the amido nitrogens is trigonal planar as
usual [C(4)-N(1)-Si(1) ) 112.9(2)°, C(4)-N(1)-V ) 114.2-
(2)°, Si(1)-N(1)-V ) 128.17(13)°, Si(2)-N(2)-V ) 118.73(12)°,
C(10)-N(3)-Si(3) ) 112.6(2)°, C(10)-N(3)-V ) 112.5(2)°],
suggesting a substantial extent of V-N π-donation. In agree-
ment, the V-N distances are comparitively short [V-N(1) )
1.972(2) Å, V-N(3) ) 1.954(2) Å].
Com p lex 3. The complex is a tetrametallic cluster with two
vanadium and two aluminum atoms arranged in two identical
subunits each containing one vanadium and one aluminum
(Figure 3). The vanadium and aluminum atoms within each
subunit are linked together by the silylamino(disilylamido)
ligand, while the link between the two subunits is realized by
two bridging chlorine atoms [V(1)-Cl(1) ) 2.510(2) Å, V(1)-
Cl ) 2.4815(17) Å, V(1)-Cl(1)-V(2) ) 99.36(6)°, Cl-V(1)-Cl(1)
) 80.64(6)°]. Each vanadium atom displays a slightly distorted
square pyramidal coordination geometry [Cl-V(1)-N(1) )
98.89(12)°, Cl-V(1)-N(3) ) 167.9(2)°, Cl-V(1)-Cl(1) ) 80.64-
(6)°, Cl(1)-V(1)-N(3) ) 95.33(16)°]. Two bridging chlorine
atoms and two amido nitrogen atoms define the basal plane,
while the amino nitrogen atom from the ligand is located on
the apical position [N(2)-V(1)-Cl ) 110.50(14)°, N(2)-V(1)-
N(3) ) 81.6(2)°, N(2)-V(1)-N(1) ) 81.1(2)°]. Surprisingly, all
three V-N bond lengths are quite similar [V(1)-N(1) ) 2.217-
(5) Å, V(1)-N(2) ) 2.224(7) Å, V(1)-N(3) ) 2.233(5) Å] despite
the different nature of the three nitrogen atoms within each
ligand. Two additional dimethylaluminum units are each
bound to one silylamino(disilylamido)V moiety via the bridging
amido nitrogen atoms [Al-N(1) ) 1.958(5) Å, Al-N(3) ) 1.954-
(6) Å]. The tetrahedral coordination geometry of each alumi-
num center is thus defined by two terminally bonded methyl
groups and two amido nitrogen atoms of the tridentate ligand
[N(1)-Al-N(3) ) 97.3(2)°, N(1)-Al-C(14) ) 112.4(3)°, N(1)-
Al-C(15) ) 111.3(4)°].
4
5
formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C
13H35AlCl5N3Si3V
C40H86Al2Cl10N6Si6V3
1380.97
triclinic, P1h
10.476(5)
11.647(7)
15.177(6)
69.75(3)
87.53(5)
73.54(5)
1663(1)
1
572.88
monoclinic, P2(1)/c
9.373(5)
20.215(9)
14.794(7)
90
97.950(5)
90
2776(2)
4
radiation (KR, Å) 0.71073
0.71073
203(2)
1.379
9.82
717
T (K)
203(2)
1.371
10.06
1188
Dcalcd (g cm-3
)
µcalcd (cm-1
)
F000
R, Rw2,a GoF
0.0509, 0.1094, 1.065 0.0638, 0.1049, 1.055
R ) ∑Fo - Fc/ ∑FoRw ) [(∑(Fo - Fc)2/ ∑wFo2)]1/2
.
a
Madison, WI, 1997). Crystallographic data and relevant bond
distances and angles are given in Tables 1-4.
Com p lex 1. The crystal structure of 1 (Figure 1) shows a
symmetry-generated bimetallic complex with two distorted
trigonal bipyramidal vanadium(III) centers bridged by two
chlorides [V(1)-Cl(1)-V(2) ) 97.36(6)°, V(1)-Cl(2)-V(2) )
97.44(6)°, Cl(2)-V(1)-Cl(1) ) 82.03(6)°, Cl(2)-V(2)-Cl(1) )
82.11(6)°]. The equatorial plane of the trigonal bipyramidal
vanadium is defined by the ligand’s two terminal nitrogens
and a bridging chlorine atom [N(1)-V(1)-N(3) ) 117.91(26)°,
N(1)-V(1)-N(2) ) 84.69(22)°, N(1)-V(1)-Cl(1) ) 119.98(19)°,
N(1)-V(1)-Cl(2) ) 100.98(17)°, N(4)-V(2)-N(6) ) 118.03-
(24)°, N(4)-V(2)-N(5) ) 84.76(21)°, N(4)-V(2)-Cl(1) ) 100.72-
(17)°, N(4)-V(2)-Cl(2) ) 121.96(18)°]. The central ligand
nitrogen atom and the second bridging chlorine atom [N(2)-
V(1)-Cl(2) ) 174.21(16)°, N(5)-V(2)-Cl(1) ) 173.79(15)°]
occupy the axial positions. The V-Cl distances are effectively
equivalent and fall into the expected range for bridging
chlorides [V(1)-Cl(1) ) 2.373(2) Å, V(1)-Cl(2) ) 2.459(2) Å,
V(2)-Cl(1) ) 2.462(2) Å, V(2)-Cl(2) ) 2.373(2) Å]. The V-N
bond lengths are as expected for the silylamido donors [V(1)-
N(1) ) 1.906(5) Å, V(1)-N(3) ) 1.899(5) Å, V(2)-N(4) ) 1.905-
(5) Å, V(1)-N(6) ) 1.902(5) Å], while the longer V-N bond
length formed by the central nitrogen is indicative of a
coordinatively bound nitrogen [V(1)-N(2) ) 2.179(5) Å, V(2)-
N(5) ) 2.193(5) Å]. All other bond distances and angles are as
expected.
Com p lex 4. The complex is a dinuclear vanadium/alumi-
num complex with the two metal centers linked by a single
bridging chlorine atom [V-Cl(1)-Al ) 118.0(6)°] (Figure 4).
The coordination geometry around the vanadium atom is
slightly distorted trigonal bipyramidal. The equatorial plane
is comprised of two terminal nitrogens from the silylamino-
(disilylamido) ligand and one terminal chlorine atom [N(1)-
Com p lex 2. The complex is monomeric and features a
trigonal bipyramidal vanadium atom (Figure 2). The coordina-

972 Organometallics, Vol. 21, No. 5, 2002
Ta ble 3. Selected Bon d Dista n ces a n d An gles for Com p lexes 1, 2, a n d 3
Feghali et al.
1
2
3
V(1)-N(1) ) 1.903(6) Å
V(1)-N(2) ) 2.179(5) Å
V(1)-N(3) ) 1.902(5) Å
V(2)-N(4) ) 1.906(5) Å
V(2)-N(5) ) 2.188(5) Å
V(2)-N(6) ) 1.909(5) Å
V(1)-Cl(1) ) 2.371(2) Å
V(1)-Cl(2) ) 2.4592(19) Å
V(2)-Cl(1) ) 2.459(2) Å
V(2)-Cl(2) ) 2.373(2) Å
V-C(19) ) 2.118(3) Å
V-N(1) ) 1.972(2) Å
V-N(2) ) 2.264(2) Å
V-N(3) ) 1.954(2) Å
V-N(4) ) 2.157(2) Å
V(1)-Cl(1) ) 2.510(2) Å
V(1)-Cl ) 2.4815(17) Å
V(1)-N(1) ) 2.217(5) Å
V(1)-N(2) ) 2.224(7) Å
V(1)-N(3) ) 2.233(5) Å
Al-N(1) ) 1.958(5) Å
Al-N(3) ) 1.954(6) Å
Al-C(14) ) 1.970(10) Å
Al-C(15) ) 1.991(9) Å
N(1)-V(1)-N(3) ) 117.91(26)°
N(1)-V(1)-N(2) ) 84.69(22)°
N(1)-V(1)-Cl(1) ) 119.98(19)°
N(1)-V(1)-Cl(2) ) 100.98(17)°
N(4)-V(2)-N(6) ) 118.03(24)°,
N(4)-V(2)-N(5) ) 84.76(21)°
N(4)-V(2)-Cl(1) ) 100.72(17)°
N(4)-V(2)-Cl(2) ) 121.96(18)°
V(1)-Cl(1)-V(2) ) 97.36(6)°
V(1)-Cl(2)-V(2) ) 97.44(6)°
Cl(2)-V(1)-Cl(1) ) 82.03(6)°
Cl(2)-V(2)-Cl(1) ) 82.11(6)°
C(19)-V-N(1) ) 116.65(6)°
C(19)-V-N(3) ) 113.85(7)°
C(19)-V-N(4) ) 92.62(5)°
C(19)-V-N(2) ) 100.94(5)°
C(4)-N(1)-Si(1) ) 112.9(2)°
C(4)-N(1)-V ) 114.2(2)°
Si(1)-N(1)-V ) 128.17(13)°
Si(2)-N(2)-V ) 118.73(12)°
C(10)-N(3)-Si(3) ) 112.6(2)°
C(10)-N(3)-V ) 112.5(2)°
N(2)-V(1)-Cl ) 110.50(14)°
N(2)-V(1)-N(3) ) 81.6(2)°
N(2)-V(1)-N(1) ) 81.1(2)°
Cl(1)-V(1)-N(1) ) 167.9(2)°
Cl(1)-V(1)-N(3) ) 95.33(16)°
N(1)-V(1)-Cl ) 98.89(12)°
Cl-V(1)-N(3) ) 167.9(2)°
Cl-V(1)-Cl(1) ) 80.64(6)°
V(1)-Cl(1)-V(2) ) 99.36(6)°
N(1)-Al-N(3) ) 97.3(2)°
N(1)-Al-C(14) ) 112.4(3)°
N(1)-Al-C(15) ) 111.3(4)°
Ta ble 4. Selected Bon d Dista n ces a n d An gles for
Com p lexes 4 a n d 5
silylamine nitrogen atom occupy the axial positions [N(2)-V-
Cl(1) ) 170.63(8)°]. The nearly regular tetrahedral environ-
ment around Al is defined by three terminal and one bridging
chlorine atom [Cl(1)-Al-Cl(3) ) 107.13(7)°, Cl(1)-Al-Cl(4)
) 103.46(7)°, Cl(1)-Al-Cl(5) ) 106.32(8)°]. The V-N distances
[V-N(1) ) 1.815(3) Å, V-N(3) ) 1.832(3) Å] are normal and
compare well with those of complex 1. The vanadium sily-
lamine distance is instead substantially longer [V-N(2) )
2.294(3) Å]. The V-Cl distances [V-Cl(1) ) 2.4649(14) Å,
V-Cl(2) ) 2.2441(13) Å] differ slightly from each other, with
4
5
V-N(1) ) 1.815(3) Å
V-N(2) ) 2.294(3) Å
V-N(3) ) 1.832(3) Å
V-Cl(1) ) 2.4649(14) Å
V-Cl(2) ) 2.2441(13) Å
Al-Cl(3) ) 2.120(2) Å
Al-Cl(4) ) 2.1070 Å
Al-Cl(5) ) 2.108(2) Å
V(1)-Cl(1) ) 2.412(2) Å
V(1)-Cl(2) ) 2.389(2) Å
V(1)-Cl(3) ) 2.389(2) Å
V(2)-Cl(1) ) 2.453(2) Å
V(2)-Cl(2) ) 2.456(2) Å
V(2)-Cl(3) ) 2.460(2) Å
V(1)-N(1) ) 2.150(5) Å
V(1)-N(2) ) 2.158(5) Å
V(1)-N(3) ) 2.216(5) Å
Al-N(1) ) 1.911(6) Å
Al-N(3) ) 1.914(5) Å
N(1)-V-N(3) ) 124.86(14)° Cl(1)-V(1)-N(1) )95.37(15)°
N(1)-V-N(2) ) 79.80(12)° Cl(1)-V(1)-N(2) ) 178.53(15)°
N(2)-V-Cl(2) ) 117.60(11)° Cl(1)-V(1)-N(3) ) 97.83(15)°
N(2)-V-Cl(1) ) 95.71(10)°
V-Cl(1)-Al ) 118.0(6)°
Cl(1)-V(1)-Cl(3) ) 86.03(8)°
Cl(1)-V(2)-Cl(2) ) 82.74(7)°
Cl(1)-Al-Cl(3) ) 107.13(7)° Cl(1)-V(2)-Cl(3) ) 83.62(8)°
Cl(1)-Al-Cl(4) ) 103.46(7)° Cl(1)-V(2)-Cl(1)#1 ) 180.00(1)°
Cl(1)-Al-Cl(5) ) 106.32(8)° Cl(4)-Al-N(1) ) 113.12(19)°
Cl(4)-Al-N(3) ) 113.57(18)°
Cl(4)-Al-Cl(5) ) 195.97(11)°
V-N(3) ) 124.86(14)°, N(1)-V-N(2) ) 79.80(12)°, N(2)-V-
Cl(2) ) 117.60(11)°, N(2)-V-Cl(1) ) 95.71(10)°]. The chlorine
atom that bridges vanadium to aluminum, and the central
F igu r e 2. ORTEP plot of 2. Thermal ellipsoids are drawn
at 30% probability level.
F igu r e 1. ORTEP plot of 1. Thermal ellipsoids are drawn
at 30% probability level. Carbon atoms of the trimethylsilyl
groups have been omitted for clarity.
F igu r e 3. ORTEP plot of 3. Thermal ellipsoids are drawn
at 30% probability level. Carbon atoms of the trimethylsilyl
groups have been omitted for clarity.

Stability of Metal-Carbon Bond
Organometallics, Vol. 21, No. 5, 2002 973
geometry [Cl(4)-Al-N(1) ) 113.12(19)°, Cl(4)-Al-N(3) )
113.57(18)°, Cl(4)-Al-Cl(5) ) 195.97(11)°].
Resu lts a n d Discu ssion
The preparation of [{V(Me3Si)N{CH2CH2N(SiMe3)}2}2-
(µ-Cl)2] (1) as a red-violet crystalline material was
carried out according to the procedure recently described
by Cloke6,8 via room-temperature reaction of Li2[(Me3-
Si)N{CH2CH2N(SiMe3)}2] with VCl3(THF)3 in THF. The
structure unequivocally confirmed the dinuclear nature
of 1 with the two trigonal bipyramidal vanadium centers
linked by two bridging chlorine atoms (Figure 1).
While activated by 60 equiv of Me2AlCl, Me3Al, or
PMAO-IP, complex 1 displayed a substantial activity
as an ethylene polymerization catalyst. The activity
obtained with PMAO-IP activation (237 g of PE/mmol/
h) was 20 times higher than in the case of Me3Al, while
the highest activity was obtained with Me2AlCl. The
polymers obtained from the different activators and
under comparable reaction conditions were very similar.
The catalytic activity recorded for polymerization ex-
periments carried out with Me2AlCl at 50 °C and under
300 psig of ethylene was 660 g of PE/mmol/h. The
polyethylene showed catalyst unimodality with average
molecular weight of 721 000 and polydispersity of 2.3
(see Supporting Information Figure S1). The melting
point of the polymer (133.95 °C) indicated that the
polyethylene was essentially linear. Not surprising for
a vanadium catalyst, the activity decreased at higher
temperature (237 g of PE/mmol/h at 140 °C, 286 psig).
Also the polymer displayed lower average molecular
weight (135 700) but similar polydispersity (2.7) and the
presence of a significant fraction of higher molecular
weight polymer (see Supporting Information Figure S2),
indicating that catalyst bimodality takes place at higher
temperatures.
In common with other vanadium catalysts, 1 is short-
lived at either 35 or 140 °C, typically not remaining
active for more than 20-30 min, the longest catalyst
life having been observed at 50 °C. Such deactivation
is usually attributed to reduction of the vanadium center
to the inactive divalent state. The reduction is thought
to involve loss of the polymeric alkyl chain and is
believed to be due to an intrinsic instability of the
vanadium-carbon bond. However, we have reported
that amide ligands may stabilize tetravalent vanadium
alkyls,16 while, by using the silylamino(disilylamido)
ligand, Cloke has isolated a mononuclear trimethyl-
silylmethyl derivative and successfully hydrogenolyzed
it to produce the corresponding dinuclear and trivalent
vanadium hydride.8 In all these reactions there was no
indication of reduction of the vanadium center, suggest-
ing that amide ligands indeed provide sufficient stabi-
lization to the V-C function.
F igu r e 4. ORTEP plot of 4. Thermal ellipsoids are drawn
at 30% probability level.
F igu r e 5. ORTEP plot of 5. Thermal ellipsoids are drawn
at 30% probability level. Carbon atoms of the trimethylsilyl
groups have been omitted for clarity.
the bridging chlorine forming a longer distance. A similar trend
is observed for the Al-Cl distances with terminal chlorine
atoms having slightly shorter Al-Cl distances [Al-Cl(3) )
2.120(2) Å, Al-Cl(4) ) 2.1070 Å, Al-Cl(5) ) 2.108(2) Å] than
the bridging Al-Cl distance [Cl(1)-Al ) 2.1971(18) Å].
Com p lex 5. The structure is a symmetry-generated pen-
tametallic cluster composed by three vanadium atoms and two
aluminum residues located at the periphery of the molecule
(Figure 5). The three hexacoordinated vanadium atoms are
arranged to form a linear trimetallic unit. The central vana-
dium, located on the inversion center, is surrounded by six
chlorides [V(2)-Cl(1) ) 2.453(2) Å, V(2)-Cl(2) ) 2.456(2) Å,
V(2)-Cl(3) ) 2.460(2) Å], which fill the six coordination sites
and provide the metal with a rather regular antiprismatic
geometry [Cl(1)-V(2)-Cl(2) ) 82.74(7)°, Cl(1)-V(2)-Cl(3) )
83.62(8)°, Cl(1)-V(2)-Cl(1)#1 ) 180.00(1)°]. The other two
vanadium atoms are located at either side of the central
vanadium and share three chlorine atoms each [V(1)-Cl(2) )
2.389(2) Å, V(1)-Cl(3) ) 2.389(2) Å] in a slightly distorted
octahedral coordination geometry [Cl(1)-V(1)-N(1) )95.37-
(15)°, Cl(1)-V(1)-N(2) ) 178.53(15)°, Cl(1)-V(1)-N(3) )
97.83(15)°, Cl(1)-V(1)-Cl(3) ) 86.03(8)°]. The equatorial plane
is defined by two of the three bridging chlorine atoms and the
two terminal amido nitrogen atoms [V(1)-N(1) ) 2.150(5) Å,
V(1)-N(3) ) 2.216(5) Å]. The third chlorine atom [V(1)-Cl(1)
) 2.412(2) Å] and the central nitrogen atom [V(1)-N(2) )
2.158(5) Å] of the ligand occupy the two axial positions. The
fact that one chlorine atom is in the axial versus the equatorial
position with respect to the peripheral vanadium atom deter-
mines the antiprismatic distortion of the octahedral geometry
of the central vanadium atom. Two AlCl2 units are located at
the exterior of the trivanadium unit each connected to one of
the two external vanadium atoms. Each aluminum atom is
bonded to the two amido ligand nitrogen atom bonds [Al-N(1)
) 1.911(6) Å, Al-N(3) ) 1.914(5) Å], while two terminally
bonded chlorine atoms complete the distorted tetrahedral
Nevertheless, the rapid catalyst failure and the fact
that the alkyl derivatives of this system reported before
are particularly bulky prompted us to further investi-
gate the ability of this versatile ligand system to support
more standard vanadium-alkyls. Complex 1 undergoes
a rapid reaction with MeLi in hexane resulting in the
formation of a grayish precipitate (LiCl) and yielding a
(16) Desmangles, N.; Gambarotta, S.; Bensimon, C.; Davis, S.;
Zahalka, H. J . Organomet. Chem. 1997, 562, 53.

974 Organometallics, Vol. 21, No. 5, 2002
Feghali et al.
Sch em e 1
complex that was tentatively assigned as a methyl-
bridged dimer [{V(Me3Si)N{CH2CH2N(SiMe3)}2}2(µ-
Me)2] probably of very similar structure to 1. Unfortu-
nately, despite repeated attempts we were unable to
obtain crystals suitable for an X-ray diffraction study.
However, given the release of nearly a stoichiometric
amount of methane gas during degradation experiments
carried out with gaseous HCl in a closed vessel con-
nected to a Toepler pump, a structure with methyl
bridges seems the most likely. Furthermore, treatment
of the reaction solution with AlCl3 re-formed the starting
material 1 in good yield (Scheme 1), and simple addition
of pyridine gave red crystals of [(Me3Si)N{CH2CH2N-
(SiMe3)}2V(Me)(Py)] (2) in good yield. The X-ray crystal
structure revealed that 2 is mononuclear and that it has
a terminally bonded Me group (Figure 2). Complex 2
and its precursor enlarge the meager list of examples
of stable V(III)-alkyl complexes.5,8 Both are thermally
robust since no decomposition was observed in boiling
toluene, and their unusual stability is likely to be
ascribed to the particular nature of the amide ligand.
The versatility of the silylamino silyldiamide ligand
to stabilize the vanadium carbon bond reiterates ques-
tions about the factors determining the catalyst failure.
One possibility, although unlikely, is that the ligand
might be leached out of the metal center by the
aluminum cocatalyst with a consequent decrease of the
stability of the V-C bond. To rule out this possibility,
we studied the interaction of 1 with the aluminum
cocatalyst in the hope of trapping some vanadium-
aluminum species. The reaction of 1 with either PMAO-
IP, AlMe3, or Me2AlCl in hexane initially gave a red
solution, which after a few days afforded emerald-green
crystals of the tetrametallic ({[(Me3Si)(AlMe2)NCH2-
CH2]2N(SiMe3)}2V2(µ-Cl)2) (3) (Scheme 2). As expected,
the best yield was obtained with Me3Al. The X-ray
crystal structure clearly showed that the ligand remains
triply bound to the vanadium center. The main frame
of the complex is virtually the same as in 1, with the
two ({[(Me3Si)NCH2CH2]2N(SiMe3)}V) moieties held
together by two bridging chlorines. However, there are
two major differences. First, two Me2Al residues are now
coordinated each to one ligand’s amide nitrogen atom.
Second, the vanadium center has been reduced to the
inactive divalent state, thus accounting for the catalyst
failure. The magnetic moment of 3.9 µB/vanadium is as
expected for the high-spin d3 electronic configuration of
a divalent vanadium center.
The isolation and characterization of 3 indicates that
no ligand leaching occurs in the present catalytic system
but rather aggregation with the cocatalyst. Yet, the
vanadium center was reduced to the inactive divalent
state. Thus, questions arise about how the reduction
occurred given that simple chorine replacement by Me
groups afforded thermally stable trivalent methyl de-
rivatives.
In Ziegler-Natta catalysis, PMAO or AlR3 in general
plays a dual role as both an alkylating agent and a
Lewis acid. To separately analyze these two different
aspects of the aluminum derivatives, we reacted 1 with
AlCl3, a pure Lewis acid, with no possibility of function-
ing as a reducing agent. The reaction was carried out
in toluene (Scheme 2) and afforded a brown solution,
from which a mixture of two different compounds was
obtained. X-ray diffraction of the crystals of the major
component revealed the structure to be the hetero-
bimetallic and tetravalent [(Me3Si)N{CH2CH2N-
(SiMe3)}2V(Cl)(µ-Cl)AlCl3] (4), which is basically the
result of the coordination of AlCl3 to the tetravalent
[(Me3Si)N{CH2CH2N(SiMe3)}2VCl2] via sharing of one
of the two vanadium chlorine atoms (Figure 4). The
tetravalent oxidation state of vanadium was also con-
firmed by the measurement of the magnetic moment,
which is consistent with a d1 electronic configuration
(1.83 µB).
The second component of the reaction mixture is
particularly intriguing. The complex is a mixed-valence
V(II)/V(III) hetero-bimetallic pentamer [{[(SiMe3)(AlCl2)-
NCH2CH2]2(Me3Si)NV}(µ-Cl)3V(µ-Cl)3[(SiMe3)(AlCl2)-
NCH2CH2]2(Me3Si)NV}]‚(toluene)2 (5), in which there

Stability of Metal-Carbon Bond
Organometallics, Vol. 21, No. 5, 2002 975
Sch em e 2
are two vanadium atoms formally in the +3 oxidation
state and one in the +2 (Figure 5). Unfortunately, while
complex 4 was obtained in analytically pure form,
samples of 5 were invariably contaminated by variable
amounts of 4, thus preventing a satisfactory analytical
characterization.
and a poorly soluble residue, which was redissolved in
boiling THF and precipitated upon treatment with
TMEDA and identified as VCl2(TMEDA)2.17 Finally,
complex 6, which was conveniently obtained via oxida-
tion of 1 with Ph3PCl2, reacted with AlCl3 to afford 4.
Thus, the isolation of both tetra- and divalent vana-
dium species in the reaction of 1 with AlCl3 clearly
indicates that a disproprotionation is at the basis of the
reduction of the vanadium center. However, the fact
that a vanadium atom in 3 has retained its ligand
system and yet has been reduced strongly suggests that
no ligand leaching occurred during the reaction of 1 with
AlMe3, as it can be expected given its lower Lewis
acidity. Thus, the only possibility to explain the reduc-
tion of the metal center in the case of 3 is to assume
that the addition intermediate (Me2Al){(Me3Si)N[CH2-
CH2N(SiMe3)]2}VClMe, which is also likely to be the
catalytically active species, has an intrinsic instability
of the V-C bond (Scheme 4). Accordingly, a substantial
amount of methane and a small amount of ethane was
identified in the GC of the reaction mother liquor.
The simultaneous formation of both 4 and 5 clearly
indicates that AlCl3 triggers a disproportionation reac-
tion through which V(II) and V(IV) species are formed.
The fact that the central vanadium atom in complex 5
has been stripped of the ligand system clearly indicates
that ligand leaching still occurred despite the ligand
tridenticity and the higher affinity of vanadium for
nitrogen donor atoms compared to aluminum. However,
the fact that two AlCl2 residues are connected to the
periphery of the trimetallic cluster in a fashion that is
identical to the coordination of the AlMe2 residues in 3
indicates that (1) the ligand leaching is only partial and
probably governed by an equilibrium; (2) there is a
common trend in the behavior of 1 with both Me3Al and
AlCl3.
Complex 5 may be regarded as resulting from the
formal aggregation of 1 with AlCl3 and VCl2 residues
obviously generated as a partner of the oxidation to 4.
Thus, it is tempting to rationalize the formation of the
mixture of 4 and 5 in terms of initial partial reaction of
1 with AlCl3 to afford VCl3 and [(Me3Si)N{CH2CH2-
N(SiMe3)}2Al(Cl)] (Scheme 3). The association of VCl3
with 1 triggers a disproprotionation to the tetravalent
[(Me3Si)N{CH2 CH2N(SiMe3)}2VCl2 (6) and “VCl2”. This
residue may react with both unreacted 1 and AlCl3 to
form 5. In agreement with this proposal, reaction of 1
with anhydrous VCl3 in toluene indeed afforded a
mixture of 6 (isolated from crystallization from hexane)
Con clu sion s
This work has allowed the isolation of a few novel
vanadium aluminum clusters supported by the trim-
ethylsylilamino(trimethylsilyldiamido) ligand. While the
ligand system is capable of aggregating vanadium and
aluminum residues, the complete abstraction of ligand
from vanadium, responsible for triggering dispropor-
tionation toward di- and tetravalent vanadium, is
(17) Edema, J . J . H.; Stauthamer, W.; Gambarotta, S.; van Bolhuis,
F.; Smeets, W. W. J .; Spek, A. Inorg. Chem. 1990, 29, 1302.

976 Organometallics, Vol. 21, No. 5, 2002
Feghali et al.
Sch em e 3
Sch em e 4
determined by the Lewis acidity of the aluminum
cocatalyst, i.e., the chlorine content. Despite the ability
of this ligand system to stabilize both mono- and
dinuclear trivalent vanadium alkyl derivatives and even
in the event where ligand leaching did not occur, the
vanadium atom can still be reduced. This is likely due
to an intrinsic instability of the V-C bond of an alkyl/
halide intermediate. Thus, the design of long-lasting
vanadium catalysts remains determined by the under-
standing of the factors affecting the stability of the V-C
bond.
Ack n ow led gm en t. This work was supported by the
National Science and Engineering Research Council of
Canada (NSERC) and by NOVA Chemicals (Calgary,
Canada). We are also indebted to Prof. S. Collins
(Waterloo) for making available his HT-GPC facility.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
tables for all the compounds reported in this work, additional
figures, and figure for the differential molecular weight
distribution. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM011040U