Selective oligomerization of ethylene to linear α-olefins by tetrahedral cobalt(II) complexes with 6-(organyl)-2-(imino)pyridyl ligands: Influence of the heteroatom in the organyl group on the catalytic activity

Article abstract of DOI:10.1021/om030227d

The oligomerization of ethylene to α-olefins has been obtained by MAO activation of tetrahedral Co^II complexes of iminopyridines substituted in the 6-position of the pyridine ring by thiophen-2-yl, furan-2-yl, or phenyl groups. An intriguing heteroatom effect has been noticed on both activity and selectivity, together with a change in the Co^II spin state upon activation of the dichloride precursors with MAO.

Full text of DOI:10.1021/om030227d

Organometallics 2003, 22, 2545-2547
2545
Selective Oligom er iza tion of Eth ylen e to Lin ea r r-Olefin s
by Tetr a h ed r a l Coba lt(II) Com p lexes w ith
6-(Or ga n yl)-2-(im in o)p yr id yl Liga n d s: In flu en ce of th e
Heter oa tom in th e Or ga n yl Gr ou p on th e Ca ta lytic
Activity
Claudio Bianchini,* Giuseppe Mantovani, Andrea Meli, and Fabio Migliacci
Istituto di Chimica dei Composti Organometallici (ICCOM-CNR),
Via J . Nardi 39, 50132 Firenze, Italy
Franco Laschi
Dipartimento di Chimica, Universita` di Siena, Via Aldo Moro, 53100 Siena, Italy
Received March 26, 2003
Summary: The oligomerization of ethylene to R-olefins
has been obtained by MAO activation of tetrahedral CoII
complexes of iminopyridines substituted in the 6-position
of the pyridine ring by thiophen-2-yl, furan-2-yl, or
phenyl groups. An intriguing heteroatom effect has been
noticed on both activity and selectivity, together with a
change in the CoII spin state upon activation of the
dichloride precursors with MAO.
2 and 4 were grown by slow evaporation of the solvent
from CH2Cl2 and CH2Cl2/ethyl acetate solution, respec-
tively. The cobalt atom in each complex is tetrahedrally
coordinated by two nitrogen atoms from the pyridin-
imine ligand and by two chlorine atoms. ORTEP draw-
ings are reported in Figure 1.
The distortion from a regular tetrahedron is remark-
able in the two geometries, as shown by the angle
originated by the chelating dinitrogen ligand N1-Co-
N2, which is much smaller than the regular tetrahedral
angle, while the N2-Co-Cl2 angles are remarkably
large, which reflects the presence of a sterically de-
manding substituent at the 6-position of the pyridine
ring. The Co-Nimine bonds are slightly shorter than the
Co-Npyridine bonds, which is opposite to what is observed
in related 2,6-bis(imino)pyridyl CoII dichlorides.4 The
thiophenyl sulfur atom in 2 does not show any bonding
interaction to cobalt, being actually oriented away from
it.
The use of tetrahedral CoII complexes to catalyze the
insertion polymerization of ethylene is limited to a few
o-bis(imino)benzene precursors reported in a patent and
claimed to produce polyethylene.1,2 The present work
shows, for the first time, that pyridinimine ligands
bearing appropriate substituents in the 6-position of the
pyridine ring can form, in conjunction with CoCl2 and
methylaluminoxane (MAO), very efficient catalysts for
the oligomerization of ethylene to R-olefins. An intrigu-
ing heteroatom effect has been noticed on both oligo-
merization activity and selectivity.
Fu
The new pyridinimines N2Th, N2ThE, and N2 were
The reflectance spectra of all complexes in the visible
and near-IR region, showing three d-d absorption
bands in the spectral regions 5500-6100, 7000-7500,
and 9500-10 100 cm-1, respectively, and three or two
higher intensity bands in the region 14 500-18 000
cm-1, are consistent with a high-spin tetrahedral coor-
dination of the CoII ion.3 The electronic spectra in CH2-
Cl2 solution are fully comparable to the reflectance
spectra, indicating that the primary stereochemistry is
the same in both the solid state and solution. This would
exclude a bonding interaction between cobalt and the
prepared via Stille coupling between 6-bromo-2-((2,6-
diisopropylphenyl)imino)pyridine and thiophen-2-yl stan-
nane, 5-ethylthiophen-2-yl stannane, and furan-2-yl
stannane, respectively (Scheme 1).
Ph
The phenyl-substituted pyridinimine N2 was ob-
tained by a Suzuki reaction between 6-bromo-2-acetyl
ketone and PhB(OH)2, followed by condensation of the
resultant 6-phenyl-2-acetyl ketone with the aniline.
Th
ThE
The CoII complexes CoCl2N2 (1), CoCl2N2
(2),
Fu
Ph
CoCl2N2 (3), and CoCl2N2 (4) were obtained as green
crystals in high yield by reacting 1 equiv of the ligand
with CoCl2‚6H2O in refluxing n-BuOH. All complexes
are high spin in the solid state with µeff at 20 °C ranging
from 4.7 to 4.9 µB, which is typical for a d7 metal ion in
a tetrahedral coordination geometry.3 Single crystals of
1
sulfur atom in 2 even in solution. The H NMR spectra
of 2 and 4 in CD2Cl2, being very similar to each other,
confirm indirectly the nonbonding nature of the sulfur
atom.
As expected, the presence of three unpaired electrons
in each complex makes all compounds EPR silent in the
X-band at room temperature in both the solid state and
solution.5 Surprisingly, however, clean EPR signals
* To whom correspondence should be addressed. E-mail: bianchin@
fi.cnr.it.
(1) Doi, Y.; Fujita, T. J P Patent 10298225 (Mitsui Chemicals Inc.,
J apan), priority date April 25, 1997.
(2) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169.
(3) (a) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; pp
814-835. (b) Morassi, R.; Sacconi, L. J . Chem. Soc. A 1971, 492.
(4) Britovsek, G. J . P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.;
Maddox, P. J .; Mastroianni, S.; McTavish, S. J .; Redshaw, C.; Solan,
G. A.; Stro¨mberg, S.; White, A. J . P.; Williams, D. J . J . Am. Chem.
Soc. 1999, 121, 8728.
(5) Bencini, A.; Gatteschi, D. Transition Met. Chem. 1982, 8, 1.
10.1021/om030227d CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/29/2003

2546 Organometallics, Vol. 22, No. 13, 2003
Communications
F igu r e 1. ORTEP drawings for 2 (a) and 4 (b). Selected bond lengths (Å) and angles (deg) for 2 and 4, respectively:
Co-N2 ) 2.074(3), 2.084(4); Co-N1 ) 2.031(2), 2.049(4); Co-Cl1 ) 2.221(1), 2.206(2); Co-Cl2 ) 2.213(1), 2.205(2); N1-
Co-N2 ) 81.53, 80.71; N2-Co-Cl2 ) 127.05, 129.78.
Sch em e 1
2/MAO, giso ) 2.04(5); 3/MAO, giso ) 2.12(8); 4/MAO,
giso ) 2.05(8).
The simple substitution of the chlorides in the precur-
sor with stronger methyl ligands by MAO can hardly
account for the appearance of an EPR signal at room
temperature without a concomitant change in the
coordination geometry from tetrahedral (high spin ir-
respective of the donor atom set) to low-spin square
planar or five-coordinate.5 A square-planar coordination
is more likely in view of the fact that an EPR-active
species was observed also when 4, devoid of any
potential donor atom in the phenyl substituent, was
treated with MAO. Moreover, square-planar CoII com-
plexes with nitrogen ligands such as Co(CN)2(py)2 and
[Co(py)4]Cl(PF6) are low spin (py ) pyridine).3a,6 On the
other hand, it is worth noting that the CoII complex CoI-
[PhBP3] is pseudotetrahedral and exhibits a doublet
ground state (PhBP3 ) PhB(CH2PPh2)3-).7
The thiophenyl-containing CoII complexes 1 and 2
were active catalysts for the production of low-molecu-
lar-weight R-olefins (C4-C12) with Schulz-Flory distri-
F igu r e 2. X-Band EPR spectra of 2 in toluene: (a) in the
presence of 50 equiv of MAO at room temperature under
N2; (b) at 8 K.
appeared on treatment of the dichloride precursors 1-4
with an excess of MAO at room temperature. Figure 2a
shows the X-band EPR spectrum of 2 in toluene at room
temperature in the presence of an excess of MAO.
The spectrum with giso ) 2.00(5) does not show any
hyperfine splitting at room temperature, yet a partially
resolved axial structure was observed at 8 K with g| )
2.33(5) and g ) 1.90(5) ( g ) 2.05(5)) (Figure 2b).
Values of giso for the other systems are as follows:
(6) Beattie, J . K.; Hambley, T. W.; Klepetko, J . A.; Masters, A. F.;
Turner, P. Polyhedron 1996, 15, 473.
(7) J enkins, D. M.; Di Bilio, A. J .; Allen, M. J .; Betley, T. A.; Peters,
J . C. J . Am. Chem. Soc. 2002, 124, 2822.

Communications
Organometallics, Vol. 22, No. 13, 2003 2547
Ta ble 1. Eth ylen e Oligom er iza tion w ith th e CoII Ca ta lystsa
overall
TOFb
TOFc
(×10-5
C8,C6
sel
R-olefins
(%)
precatalyst/
amt (µmol)
pressure
(psi)
)
entry
(×10-5
)
Rd
âd
1
2
3
4
5
6
7
8
1/0.8
1/12
2/0.8
2/12
3/0.8
4/0.8
1/0.8
1/0.8
2/12
2/12
60
60
60
60
60
4.62
1.20
5.10
1.33
1.10e
1.12e
8.70
15.2
2.70
0.52
0.42
0.20
0.48
0.24
0.03
<0.01
0.80
1.37
0.44
0.10
0.06
0.11
0.09
0.13
-
15.6
8.1
10.1
6.7
90
66
93
67
f
60
-
f
120
200
60
0.06
0.06
0.11
0.16
15.6
15.6
8.1
94
95
71
61
9g
10h
60
5.2
a
b
Reaction conditions: toluene 100 mL, MAO 200 equiv, 30 min, 25 °C. Mol of C2H4 converted (mol of Co)-1 h-1 determined by GC at
appropriate temperatures. c Mol of C2H4 converted (mol of Co)-1 h-1 into C6 and C8 olefins. Schulz-Flory parameters: R ) mol of Cn+2
d
(mol of Cn)-1; â ) (1 - R)/R. e Almost exclusively butenes produced. f Not determined. 5 min. 180 min.
g
h
butions.4,8 Selected data are reported in Table 1. In all
cases, the addition of ethylene to the catalyst/cocatalyst
mixture resulted in a rapid exotherm, indicative of no
induction period. Experiments with precursor 1 in the
C2H4 pressure range from 60 to 200 psi (entries 1, 7,
and 8) showed a linear dependence of activity with the
pressure, while the R factor was independent of the
pressure, indicating that the propagation and chain-
transfer rates are first order in ethylene.4,8 The TOF
observed at 200 psi (1.52 × 106 mol of C2H4 converted
(mol of Co)-1 h-1) is remarkable, at least 2 orders of
magnitude higher than those reported for 2,6-bis(imino)-
pyridyl CoII catalysts under comparable conditions.4,9
The furanyl- and phenyl-substituted precursors 3 and
4 were much less active than either thiophenyl-
substituted catalyst and produced essentially butenes
(entries 5 and 6). In the reactions catalyzed by 1 or 2,
the selectivity in R-olefins decreased with time (entries
4, 9, and 10) due to isomerization to internal olefins
(predominantly with E configuration), while the R factor
increased due to the reincorporation of R-olefin products
into oligomers made later in the reaction.8 The occur-
rence of both olefin isomerization and reincorporation
was confirmed by independent experiments in which the
oligomerization of ethylene with 1 was carried out in
the presence of an excess of 1-undecene. Odd-carbon
oligomers were formed together with various undecenes,
in fact. No saturated hydrocarbon was produced, which
indicates the absence of chain transfer to aluminum.4
Given the comparable size of the furanyl and phenyl
groups as compared to thiophenyl, steric factors cannot
account for the remarkably different activity and selec-
tivity of 1 and 2 vs 3 and 4. It is therefore likely that
the sulfur atom plays an active role during the oligo-
merization of ethylene. In view of the EPR study
discussed above, the oligomerization process may in-
volve square-planar CoII initiators. Interestingly, theo-
retical calculations suggest the occurrence of a spin state
changeover in the propagation and chain transfer of
ethylene polymerization by 2,6-bis(imino)pyridyl FeII
catalysis.10 It is also worth mentioning that square-
planar CoI complexes, formed by reduction of 2,6-bis-
(imino)pyridyl CoII precursors with MAO, can initiate
the polymerization of ethylene, yet it is not clear
whether the active catalysts are CoI or CoIII species.11,12
Ack n ow led gm en t. This work was supported by the
Polimeri Europa and the European Commission (Con-
tract HPRN-CT-2000-00010).
Su p p or tin g In for m a tion Ava ila ble: Text giving the
synthesis and characterization of the pyridinimine ligands
N2Th, N2ThE, and N2 and of the CoII complexes 1-4, text giving
Fu
details of the oligomerization procedure, and text and tables
giving crystallographic data for compounds 2 and 4. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM030227D
(10) Khoroshun, D. V.; Musaev, D. G.; Vreven, T.; Morokuma, K.
Organometallics 2001, 20, 2007.
(11) Gibson, V. C.; Humphries, M. J .; Tellmann, K. P.; Wass, D. F.;
White, A. J . P.; Williams, D. J . Chem. Commun. 2001, 2252.
(12) Kooistra, T. M.; Knijnenburg, Q.; Smits, J . M. M.; Horton, A.
D.; Budzelaar, P. H. M.; Gal, A. Angew. Chem., Int. Ed. 2001, 40, 4719.
(8) Small, B. L.; Brookhart, M. J . Am. Chem. Soc. 1998, 120, 7143.
(9) Bennett, A. M. A. (DuPont) WO 98/27124.