Generation of bis(iminoalkyl)pyridine cobalt(I) cations under metal alkyl free conditions that polymerize ethene

Article abstract of DOI:10.1039/b401122h

Treatment of the bis(iminoethyl)- or bis(iminobenzyl)pyridine cobalt dichlorides 3a,b with "butadiene-magnesium" (a, R = CH₃) or methyllithium (b, R = Ph) yields the corresponding bis(iminoalkyl)pyridine cobalt(i) chloride complexes 7a,b. Chlor

Full text of DOI:10.1039/b401122h

Generation of bis(iminoalkyl)pyridine cobalt( ) cations under metal alkyl
I
free conditions that polymerize ethene†
Winfried Steffen, Tobias Blömker, Nina Kleigrewe, Gerald Kehr, Roland Fröhlich and Gerhard
Erker*
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany.
E-mail: erker@uni-muenster.de; Fax: +49 251 83–36503
Received (in Cambridge, UK) 25th January 2004, Accepted 10th March 2004
First published as an Advance Article on the web 14th April 2004
Treatment of the bis(iminoethyl)- or bis(iminobenzyl)pyridine
cobalt dichlorides 3a,b with Bbutadiene-magnesiumB (a, R =
CH3) or methyllithium (b, R = Ph) yields the corresponding
activities, although they are less active than some of the previously
published systems.
The bis(N-aryliminoethyl)pyridineCoCl2 complex 3a was pre-
pared as previously described by Gibson et al. (Scheme 2)6 The
synthesis of the related substituted bis(N-aryliminobenzyl)pyridine
ligand system was carried out according to a procedure described
by Esteruelas et al.7 Subsequent treatment with CoCl2 in n-butanol
at 80 °C gave 3b (72% isolated).
bis(iminoalkyl)pyridine cobalt( ) chloride complexes 7a,b. Chlo-
I
ride abstraction with Li[B(C6F5)4] in toluene generates the
corresponding cations (9) which are low activity catalysts for the
polymerization of ethene. The pyridine-stabilized [bis(imino-
ethyl)pyridineCo(
)(NC5H5)+] [B(C6F5)42] salt was charac-
I
terized by X-ray diffraction.
The cobalt(II) dichloride complex 3a was reacted with the
“butadiene–magnesium” reagent [(C4H6Mg·2 THF)n].8 In contrast
to similar reactions of chelate bis(imine)NiCl2 complexes,5 the
formation of the corresponding (butadiene)cobalt complex was not
It is commonly assumed that homogeneous Ziegler–Natta catalyst
systems require the presence of a reactive transition metal bonded
s-alkyl ligand at the stage of the active catalyst species. However,
Royo et al. have shown that the doubly silylamido-bridged
cyclopentadienyl zirconium cation complex 1 (Scheme 1), derived
from the corresponding [Zr]–benzyl precursor by treatment with
B(C6F5)3 or with methylalumoxane, actively polymerizes ethene.1
This is remarkable since 1 does not contain a metal s-alkyl ligand.
Subsequently, Chen et al. described a related early metal system (2)
that was obtained from the corresponding [Zr]–CH3 complex and
Al(C6F5)3. He proposed that alkyl transfer from [RAl(C6F5)32] to
the corresponding ethene complex of 2 probably represents the
essential C–C bond forming step.2
observed, but 3a was again cleanly reduced to the cobalt( ) chloride
I
complex 7a.9 Treatment of 3b with methyllithium under conditions
similar to the reactions described by Gal et al. gave 7b (73%).
Treatment of the cobalt( ) chloride complex 7a with lithium
I
tetrakis(pentafluorophenyl)borate in toluene solution (278 °C to
room temperature) led to a clean abstraction of the chloride ligand
with precipitation of LiCl. Subsequent addition of a slight excess of
pyridine gave the corresponding pyridine adduct 8a. Recrystalliza-
tion from bromobenzene furnished single crystals of 8a that were
used for the X-ray crystal structure determination (Fig. 1).9
In the crystal, complex 8a contains well separated metal complex
cations and [B(C6F5)42] anions (and bromobenzene solvent
Similar observations were made in the chemistry of homog-
eneous Ziegler–Natta catalyst systems derived from bis(imino-
ethyl)pyridine complexes of some late metals. Gibson et al. showed
that treatment of the cobalt(II) complex 3a with methyllithium led
to reduction and formation of the corresponding neutral s-methyl
molecules).‡ The cobalt( ) center exhibits a distorted square-planar
I
geometry with a N15–Co bond length of 1.799(3) Å and two longer
adjacent N11–Co (1.946(3) Å) and N22–Co (1.918(3) Å) bonds.
The Co–N1 bond length to the coordinated pyridine ligand amounts
to 1.946(3) Å. The angles around the central cobalt atom were
found at 81.6(1)° (N15–Co–N11), 81.5(1)° (N15–Co–N22),
99.4(1)° (N1–Co–N22), and 97.9(1)° (N1–Co–N11). The plane of
cobalt( ) complex (4a: [Co]–CH3) that subsequently gave the
I
cationic dinitrogen complex (5: [Co]+–N2) or the h -olefin complex
2
(6: [Co]+–CH2 = CH2) when treated with B(C6F5)3 in the presence
2
of N2 or ethene, respectively. Complex 6 (with [CH3B(C6F5)3
anion) slowly polymerized ethene.3 Gal et al. isolated the cobalt(
]
)
I
chloride complex (7a: [Co]–Cl) from the 3a–MeLi reaction mixture
and converted it to a variety of mono-s-alkyl cobalt( ) compounds
I
(4: [Co]–R, R = CH3 (a), R = CH2Ph (b), CH2SiMe3 (c)]. The
systems 4 are catalytically inactive, but polymerize ethene when
treated with excess methylalumoxane.4 In principle, both the Gal4
and the Gibson3 systems contain anions that feature alkyl–[B] or
alkyl–[Al] moieties, so that the mechanism proposed by Chen et
al.2 (see above) could in principle be operative here, although
alternatives were discussed.3,5 We have now prepared closely
related bis(iminoalkyl)pyridine cobalt(
) cation–[B(C6F5)42] sys-
I
tems, which feature no easily transferable simple alkyl groups, and
found that such systems still show some ethene polymerization
Scheme 1
Scheme 2 i) R = CH3, + (butadiene–Mg·2 THF)n, toluene; ii) R = Ph,
MeLi, toluene; iii) R = CH3, Li[B(C6F5)4], toluene, 1 h, then pyridine,
crystals from bromobenzene; iv) R = CH3 or Ph, Li[B(C6F5)4], toluene, rt
1 h, then ethene (2 bar), 220 °C, 1 h.
† Dedicated to Professor Helmut Werner on the occasion of his 70th
birthday.
1188
C h e m . C o m m u n . , 2 0 0 4 , 1 1 8 8 – 1 1 8 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

1 J. Cano, P. Royo, M. Lanfranchi, M. A. Pellinghelli and A. Tiripicchio,
Angew. Chem., 2001, 113, 2563–2565 (Angew. Chem., Int. Ed., 2001,
40, 2495–2497).
2 J. Jin, D. R. Wilson and E. Y.-X. Chen, Chem. Commun., 2002,
708–709.
3 V. C. Gibson, M. J. Humphries, K. P. Tellmann, D. F. Wass, A. J. P.
White and D. J. Williams, Chem. Commun., 2001, 2252–2253.
4 T. M. Kooistra, Q. Knijnenburg, J. M. M. Smits, A. D. Horton, P. H. M.
Budzelaar and A. W. Gal, Angew., Chem., 2001, 113, 4855–4858
(Angew. Chem., Int. Ed., 2001, 40, 4719–4722).
5 J. W. Strauch, G. Erker, G. Kehr and R. Fröhlich, Angew., Chem., 2002,
114, 2662–2664 (Angew. Chem., Int. Ed., 2002, 41, 2543–2546).
6 G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberley, P. J.
Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw, G. A. Solan, S.
Strömberg, A. J. P. White and D. J. Williams, J. Am. Chem. Soc., 1999,
121, 8728–8740.
7 M. A. Esteruelas, A. M. López, L. Méndez, M. Oliván and E. Oñate,
Organometallics, 2003, 22, 395–406 and references cited therein.
8 H. Yasuda, Y. Kajihara, K. Mashima, K. Nagasuna, K. Lee and A.
Nakamura, Organometallics, 1982, 1, 388–396 and references cited
therein.
Fig. 1 Molecular geometry of 8a (the cation is depicted).
9 7a: The reaction of 3a (4.88 g, 7.92 mmol) with 1.94 g (8.75 mmol) of
Bmagnesium-butadieneB in toluene (12 h, 278 °C to room temperature)
gave 0.92 g (56%) of complex 7a after recryst. from toluene at 230 °C.
Anal. calcd. for C33H43N3ClCo (576.1): C 68.80, H 7.52, N 7.29; found:
C 68.50, H 7.53, N 7.19%. Complex 7a was characterized by X-ray
crystal structure analysis (details can be obtained from the authors).7b:
Complex 3b (0.70 g, 0.95 mmol) was treated with methyllithium (1.05
mmol, 0.65 ml of a 1.6 M solution in ether) in toluene (150 ml) for 12
h to yield 0.49 g (73%) of complex 7b after recryst. from toluene at 230
°C. Anal. calcd. for C43H47N3ClCo (700.3): C 73.76, H 6.77, N 6.00;
found: C 73.40, H 6.94, N 5.94%. 1H NMR (d6-benzene, 600 MHz): d
9.53 (t, 1H) and 7.23 (d, 2H, pyr), 7.82 (d, 4H), 7.04 (t, 4H), and 7.32
(t, 2H, o-, m-, p-Ph), 7.46 (t, 2H), 7.25 (d, 4H), 3.66 (sept, 4H), 1.16 (2d,
each 12H, C5H3(CHMe2)2). 13C NMR (d6-benzene, 150 MHz): d 168.1
(C = N), 155.3, 123.8, 117.5 (pyr), 140.0, 123.6, 128.3, 128.0 (ipso-, o-,
m-, p-Ph), 152.0, 140.9, 128.6, 127.1 (ipso-, o-, m-, p-C5H3(CHMe2)2),
29.3 (CHMe2), 24.6 and 23.3 (CH(CH3)2).8a: Complex 7a (100 mg, 396
mmol) was reacted with Li[B(C6F5)4] (308 mg, 450 mmol) in 15 ml of
toluene at room temp. for 1 h. Then dry pyridine (0.03 ml) was added.
Lithium chloride was removed by filtration. Toluene was removed in
vacuo and the deep blue salt 8a dissolved in bromobenzene. Diffusion
of pentane vapor into this solution gave crystals of 8a, which were dried
in vacuo to remove some of the incorporated bromobenzene solvent.
Anal. calcd. for C62H48N4BF20Co (1298.8): C 57.34, H 3.73, N 4.31;
found: C 56.75, H 3.77, N 3.74. X-Ray crystal structure analysis: Crystal
data for [C38H48CoN4]+[B(C6F5)4]2·C6H5Br, M = 1455.79, mono-
clinic, space group P21/c (No. 14), a = 20.027(1), b = 12.241(1), c =
25.895(1) Å, b = 93.75(1)°, V = 6334.6(7) Å3, Dc = 1.526 g cm23, m
= 10.07 cm21, Z = 4, l = 0.71073 Å, T = 198 K, 20215 reflections
the single pyridine ligand is rotated by 64.6° from the average
coordination plane of the Co( ) center. The 2,6-diisopropylphenyl
I
substituents at the imino-nitrogen atoms of the chelate ligand are
both rotated almost perpendicular to the central ligand plane.
Complex 7a (50 mg) was treated with a ca. 5 fold excess of
Li[B(C6F5)4] at ambient temperature in toluene to generate 9. The
mixture was then cooled to 220 °C and purged with ethene (2 bar).
After 1 h the mixture was quenched with methanol. Workup gave
170 mg of linear polyethylene (mp 128 °C), which corresponds to
a catalyst activity of ca. 2 g PE mmol[Co]21 bar (ethene)21. The
analogous polymerization reaction was carried out with the 7b–
Li[B(C6F5)4] system (50 mg/300 mg employed) to yield 350 mg of
polyethylene (a ≈ 5 g PE mmol [Co]21 bar (ethene)21).
The activation of these catalysts was carried out in the absence of
any initial transferable alkyl group. We have neither employed an
alkyl-boron or -aluminium activator (nor their corresponding Lewis
acid components) nor did the transition metal component contain an
abstractable alkyl group as in the previous cases.1–5,10 Never-
theless, the 7–Li[B(C6F5)4] systems were active in ethene polymer-
ization, albeit with rather low catalyst activities. We must,
therefore, conclude that there seem to be additional mechanistic
pathways of transition metal catalyzed olefin polymerization that
follow other than the established pathways of concurrent C–C and
M–C bond formation. Whether metallacyclopentane formation is
involved,3,11 or pathways utilizing more than one metal center or
other mechanistic alternatives may be favoured is an open question
at this time.
collected (±h, ±k, ±l), [(sinq)/l] = 0.59 Å21, 11147 independent (Rint
=
0.063) and 6954 observed reflections [I 4 2s(I)], 866 refined
parameters, R = 0.056, wR2 = 0.114. Spectroscopic characterisation of
9 was attempted, but has remained inconclusive so far due to solubility
problems.
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowl-
edged.
10 B. Temme, G. Erker, J. Karl, H. Luftmann, R. Fröhlich and S. Kotila,
Angew. Chem., 1995, 107, 1867–1869 (Angew. Chem., Int. Ed., 1995,
34, 1755–1757). Reviews: G. Erker, Acc. Chem. Res., 2001, 34,
309–317; G. Erker, Chem. Commun., 2003, 1469–1476 (feature
article).
Notes and references
‡ CCDC 230096. See http://www.rsc.org/suppdata/cc/b4/b401122h/ for
crystallographic data in .cif or other electronic format.
11 U. Dorf, K. Engel and G. Erker, Angew. Chem., 1982, 94, 916–919
(Angew. Chem., Int. Ed., 1982, 21, 914–915).
C h e m . C o m m u n . , 2 0 0 4 , 1 1 8 8 – 1 1 8 9
1189