Electronic influence of the thienyl sulfur atom on the oligomerization of ethylene by cobalt(II) 6-(thienyl)-2-(imino)pyridine catalysis

Article abstract of DOI:10.1021/om0609665

The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly affects the catalytic activity of the corresponding tetrahedral high-spin dihalide Co^II complexes in the oligomerization of ethylene to α-olefins upon activation with methylaluminoxane (MAO). Complexes with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to 1-butene, while catalysts containing thien-2-yl groups give C₄-C₁₄ a-olefins. In situ EPR experiments showed the occurrence of a spin state changeover with the formation of low-spin Co^II species upon activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems.

Full text of DOI:10.1021/om0609665

726
Organometallics 2007, 26, 726-739
Electronic Influence of the Thienyl Sulfur Atom on the
Oligomerization of Ethylene by Cobalt(II)
6-(Thienyl)-2-(imino)pyridine Catalysis
Claudio Bianchini,*,† Dante Gatteschi,‡ Giuliano Giambastiani,*,† Itzel Guerrero Rios,†
Andrea Ienco,† Franco Laschi,§ Carlo Mealli,† Andrea Meli,† Lorenzo Sorace,‡
Alessandro Toti,† and Francesco Vizza†
Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto
Fiorentino (Firenze), Italy, Dipartimento di Chimica, UniVersita` di Siena, Via Aldo Moro, 53100 Siena,
Italy, and Dipartimento di Chimica and UdR INSTM, UniVersita` di Firenze, Via della Lastruccia 3, 50019
Sesto Fiorentino (Firenze), Italy
ReceiVed October 21, 2006
The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly
affects the catalytic activity of the corresponding tetrahedral high-spin dihalide CoII complexes in the
oligomerization of ethylene to R-olefins upon activation with methylaluminoxane (MAO). Complexes
with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to
1-butene, while catalysts containing thien-2-yl groups give C4-C14 R-olefins. In situ EPR experiments
showed the occurrence of a spin state changeover with the formation of low-spin CoII species upon
activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow
for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems.
polymerization catalysis.4 Understanding the role of the thienyl
group in the oligomerization of ethylene by the present CoII
catalysts would be a more important achievement than address-
ing a chemical curiosity (to date, no other tetrahedral CoII
precursor is known to effectively oligomerize ethylene).5 Indeed,
there are other examples in polymerization catalysis where a
hemilabile ligand, uncoordinated in the precursor, proves to be
of crucial importance for promoting the catalytic reaction as
well as driving the selectivity toward a specific product.6-9
Hessen and co-workers have reported that a mono(cyclopenta-
dienylarene)titanium system can be transformed from a poor
ethylene polymerization catalyst into an efficient and selective
ethylene trimerization catalyst by attachment of a pendant arene
group.7 Theoretical studies have confirmed that the η6 coordina-
tion of the arene to titanium drives out the initially formed alkene
products and stabilizes intermediates for the cyclotrimerization
of ethylene to 1-hexene.8 Even more pertinent to the present
piece of chemistry is a work reported by Huang and co-workers,
who showed that the introduction of a thien-2-yl group on the
cyclopentadienyl ring of a half-sandwich titanium complex
generates an efficient and selective catalyst for ethylene trim-
Introduction
We have recently communicated that tetrahedral dichloride
CoII complexes with (imino)pyridine ligands bearing thiophen-
2-yl or benzo[b]thiophen-2-yl groups in the 6-position of the
pyridine ring (N22Th, N22ThE, and N2
2BT
in Scheme 1) are
effective catalyst precursors for the Schulz-Flory oligomer-
ization of ethylene.1-3 On activation by methylaluminoxane
(MAO), these CoII complexes promote the conversion of
ethylene into short-chain R-olefins with turnover frequencies
(TOFs) as high as 1.4 × 105 mol of C2H4 converted (mol of
Co)-1 h-1 bar-1. Under conditions analogous to those of the
N22Th, N22ThE, and N22BT catalysts, CoII precursors bearing groups
of comparable steric size in the pyridine 6-position, yet with
no pendant thienyl group (furanyl, phenyl, or naphthyl) are 5
times less active and are also selective for ethylene dimerization
(Scheme 1).1-3
Since the sulfur atom in the dichloride CoII precursors does
not show any bonding interaction with the metal center in both
the solid state and solution,1 we postulated that the sulfur atom
might play a relevant role in the catalytic process only after the
tetrahedral precursors are activated by MAO. However, no
evidence whatsoever was obtained for S-coordination, and we
could also not rule out the occurrence of subtle steric effects,
in view of the utmost importance of the latter in insertion
(4) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(c) Rappe´, A. K.; Skiff, W. M.; Casewit, C. S. Chem. ReV. 2000, 100, 1435.
(5) (a) Dawson, D. M.; Walker, D. A.; Thornton-Pett, M.; Bochmann,
M. Dalton Trans. 2000, 459. (b) Wang, M.; Yu, X.; Shi, Z.; Qian, M.; Jin,
K.; Chen, J.; He, R. J. Organomet. Chem. 2002, 645, 127.
(6) (a) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. In Progress in
Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New York, 1999; p 233.
(b) Nandi, M.; Jin, J.; RajanBabu, T. V. J. Am. Chem. Soc. 1999, 121,
9899. (c) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 964.
(7) (a) Hessen, B. J. Mol. Catal. A: Chem. 2004, 213, 129. (b) Deckers,
P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122. (c)
Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int. Ed. 2001,
40, 2516.
* To whom correspondence should be addressed. E-mail:
claudio.bianchini@iccom.cnr.it.
† ICCOM-CNR.
‡ Universita` di Firenze and UdR INSTM-Firenze.
§ Universita` di Siena.
(1) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.; Laschi, F.
Organometallics 2003, 22, 2545.
(2) Bianchini, C.; Giambastiani, G.; Mantovani, G.; Meli, A.; Mimeau,
D. J. Organomet. Chem. 2004, 689, 1356.
(3) Mantovani, G. Doctoral Dissertation, Scuola Normale Superiore di
Pisa, 2002.
(8) (a) Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organometallics
2003, 22, 2564. (b) Tobisch, S.; Ziegler, T. Organometallics 2005, 24, 256.
(9) Huang, J.; Wu, T.; Qian, Y. Chem. Commun. 2003, 2816.
10.1021/om0609665 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/03/2007

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 727
Scheme 1
Scheme 2
Schlenk-type techniques. Anhydrous toluene, THF, and Et2O were
obtained by distillation under nitrogen from sodium-benzophenone
ketyl. Solid MAO for polymerization was prepared by removing
toluene and AlMe3 under vacuum from a commercially available
MAO solution (10 wt % in toluene, Crompton Corp.). The MAO
solution was filtered on a D4 funnel and evaporated to dryness at
50 °C under vacuum. The resulting white residue was heated further
to 50 °C under vacuum overnight. A stock solution of MAO was
prepared by dissolving solid MAO in toluene (100 mg mL-1). The
solution was used within 3 weeks to avoid self-condensation effects
of the MAO. Literature methods were used to synthesize the
erization to 1-hexene.9 Notably, shifting the thienyl sulfur atom
from the 2-position to the 3-position changed the catalyst
selectivity from 1-hexene to polyethylene (PE) (Scheme 2).
A further reason to prove the existence of a “regiochemical
thienyl” effect on the polymerization activity of the present CoII
catalysts is their industrial relevance in ethylene oligomerization
to R-olefins10 as well as in tandem catalysis for the production
of low-density polyethylene.11 Indeed, establishing a clear-cut
relationship between catalytic activity and presence/regiochem-
istry of hemilabile thienyl groups would provide valuable
information for designing improved oligomerization/polymer-
ization catalysts.
In this paper, we report the results of an experimental and
theoretical study of ethylene oligomerization by cobalt(II)
6-(organyl)-2-(imino)pyridine dichlorides/MAO, with particular
regard to the preliminary activation by MAO. Both known and
new complexes have been investigated. Some new complexes
contain (imino)pyridine ligands bearing thiophen-3-yl and
benzo[b]thiophen-3-yl groups, which has allowed us to evaluate
the effect of the position of the sulfur atom in the thienyl group
on both productivity and selectivity. Other new complexes do
not have an organyl group at the pyridine 6-position or have a
group capable of forming a robust bond to the cobalt center.
We are confident to have obtained convincing evidence of the
decisive role exerted by the 6-organyl group and, in particular,
by the position of the sulfur atom in the thienyl ring in steering
the ethylene oligomerization activity of (imino)pyridine CoII
catalysts.
2ThE
tetrahedral cobalt complexes CoCl2N22Th, CoCl2N22BT, CoCl2N2
,
and CoCl2N2 (N2 ) 6-(organyl)-2-(imino)pyridine).1,2 All the
other reagents and solvents were used as purchased from com-
mercial suppliers. Catalytic reactions were performed with a 500
mL stainless steel reactor, constructed at the ICCOM-CNR (Firenze,
Italy), equipped with a magnetic drive stirrer and a Parr 4842
temperature and pressure controller. The reactor was connected to
an ethylene reservoir to maintain a constant pressure throughout
the catalytic runs. Deuterated solvents for NMR measurements were
Ph
org
dried over molecular sieves. H and 13C{1H} NMR spectra were
1
obtained on Bruker ACP 200 (200.13 and 50.32 MHz, respectively)
and Bruker Avance DRX-400 spectrometers (400.13 and 100.62
MHz, respectively). Chemical shifts are reported in ppm (δ) relative
to TMS, referenced to the chemical shifts of residual solvent
resonances (1H and 13C). The multiplicities of the 13C{1H} NMR
spectra were determined with the DEPT 135 technique and quoted
as CH3, CH2, CH, and C for primary, secondary, tertiary, and
quaternary carbon atoms, respectively. X-band EPR spectra were
recorded on a Bruker Elexsys E500 spectrometer equipped with a
4He continuous flow cryostat for operation at variable temperature
(4-300 K). Electronic spectra in both the solid state (powdered
samples; range 250-1800 nm) and CH2Cl2 solution (range 350-
1600 nm) were recorded on a Perkin-Elmer Lamda 9 spectropho-
tometer. Infrared spectra were recorded on a Perkin-Elmer Spectrum
BX FT-IR spectrophotometer using samples mulled in Nujol
between KBr plates. Molar susceptibilities were measured on solid
samples using a Sherwood Scientific MSB AUTO balance.
Elemental analyses were performed using a Carlo Erba Model 1106
elemental analyzer with an accepted tolerance of (0.4 unit on
carbon (C), hydrogen (H), and nitrogen (N). Melting points were
recorded on a Stuart Scientific SMP3 apparatus. GC analyses of
the reaction products were performed on a Shimadzu GC-17 gas
chromatograph equipped with a flame ionization detector and a
Supelco SPB-1 fused silica capillary column (30 m length, 0.25
mm i.d., 0.25 µm film thickness) for the C6-C20 fraction or a
HP-PLOT Al2O3 KCl column (50 m length, 0.53 mm i.d., 15 µm
film thickness) for the C4-C6 fraction. The GC/MS analyses were
Experimental Section
General Considerations. All air- and/or water-sensitive reactions
were performed under nitrogen in flame-dried flasks using standard
(10) Bianchini, C.; Sommazzi, A.; Mantovani, B.; Santi, R.; Masi, F.
(Polimeri Europa) WO 02/34701, 2002.
(11) Bianchini, C.; Frediani, M.; Giambastiani, G.; Kaminsky, W.; Meli,
A.; Passaglia, E. Macromol. Rapid. Commun. 2005, 26, 1218.

728 Organometallics, Vol. 26, No. 3, 2007
Bianchini et al.
performed on a Shimadzu QP2010S apparatus equipped with a
column identical with that used for GC analysis.
within 10 min. The mixture was allowed to stand at -78 °C for 1
h and then was slowly warmed to -35 °C and quenched with 30
mL of a saturated NaHCO3 aqueous solution. The aqueous phase
was extracted with 3 × 15 mL of Et2O. The combined organic
layers were dried over Na2SO4. Removal of the solvent under
reduced pressure gave the stannane product as a colorless oil (yield
Synthesis of 1-(6-Bromopyridin-2-yl)ethanone. To a stirred
solution of 2,6-dibromopyridine (7.11 g, 30.0 mmol) in Et2O (130
mL) at -78 °C was added 18.8 mL (30.0 mmol) of a 1.7 M solution
of t-BuLi in pentane within 10 min. After 30 min of stirring at
-78 °C, N,N-dimethylacetamide (3.1 mL, 33.0 mmol) was added
and stirring maintained for 1.5 h. The resulting mixture was warmed
to room temperature and treated with water (30 mL). The formed
layers were separated, and the organic phase was washed with water
(2 × 30 mL). The aqueous layer was extracted with Et2O (3 × 30
mL). The combined organic layers were dried over Na2SO4.
Removal of the solvent under reduced pressure gave a yellow oil
that was dissolved in petroleum ether and cooled to -20 °C. After
6 h, small yellow pale crystals were separated by filtration (yield
72%). Mp: 44 °C. IR (KBr): ν 1695 cm-1 (CdO). 1H NMR
(CDCl3): δ 2.70 (s, 3 H, C(O)CH3), 7.68 (m, 2 H, CH), 7.98 (dd,
J ) 6.5, 2.1 Hz, 1 H, CH). 13C{1H} NMR (CDCl3): δ 26.4 (1 C,
C(O)CH3), 121.1 (1 C, CH), 132.4 (1 C, CH), 139.8 (1 C, CH),
142.0 (1 C, C), 154.9 (1 C, C), 198.5 (1 C, C(O)CH3). Anal. Calcd
for C7H6BrNO (200.03): C, 42.03; H, 3.02; N, 7.00. Found: C,
42.09; H, 2.90; N, 7.02.
1
54%). H NMR (CDCl3): δ 0.43 (s, 9 H, CH3), 7.31-7.40 (m, 2
H, CH), 7.45 (s, 1 H, CH), 7.80 (m, 1 H, CH), 7.98 (m, 1 H, CH).
13C{1H} NMR (CDCl3): δ 5.01 (3 C, CH3), 123.34 (2 C, CH),
124.37 (1 C, CH), 124.58 (1 C, CH), 125.53 (1 C, C), 125.61 (1
C, C), 133.07 (1 C, CH), 137.35 (1 C, C). Anal. Calcd for C11H14-
SSn (296.98): C, 44.48; H, 4.75. Found: C, 44.38; H, 4.70.
Synthesis of [1-(6-thiophen-3-ylpyridin-2-yl)ethylidene](2,6-
diisopropylphenyl)amine (N23Th). A solution of N2Br (0.300 g, 0.84
mmol) and (thiophen-3-yl)trimethylstannane (0.309 g, 1.252 mmol)
in 3 mL of toluene was treated with a solution of Pd(dba)2 (0.014
g, 0.025 mmol) and PPh3 (0.042 g, 0.162 mmol) in toluene (1 mL).
The reaction mixture was refluxed for 20 h, and then cooled to
room temperature and treated with water (10 mL). The formed
layers were separated, and the organic phase was washed with 2 ×
20 mL of water and then dried over Na2SO4. The collected organic
layers were evaporated under reduced pressure, and the crude
product was purified via flash chromatography on neutral alumina
(n-pentane:Et2O ) 97.5:2.5) to afford a pale yellow crystalline
Synthesis of [1-(6-Bromopyridin-2-yl)ethylidene](2,6-diiso-
propylphenyl)amine (N2Br). A solution of 1-(6-bromopyridin-2-
yl)ethanone (2.40 g, 12.0 mmol), 2,6-diisopropylaniline (tech. 90%;
3.54 g, 18.0 mmol), and formic acid (2 drops) in MeOH (20 mL)
was refluxed in an Erlenmeyer flask. After 18 h, the orange solution
was cooled to room temperature and allowed to stand for 24 h.
During this time, yellow crystals precipitated, which were separated
by filtration and washed with MeOH (yield 84%). Mp: 126-128
°C. IR (KBr): ν 1639 cm-1 (CdN). 1H NMR (CDCl3): δ 1.15 (d,
J ) 6.9 Hz, 12 H, CH(CH3)2), 2.20 (s, 3 H, C(N-Ar)CH3), 2.64-
2.77 (m, 2 H, CH(CH3)2), 7.06-7.20 (m, 3 H, CH), 7.59 (dd, J )
8.0, 1.3 Hz, 1 H, CH), 7.65-7.72 (m, 1 H, CH), 8.33 (dd, J ) 7.5,
1.3 Hz, 1 H, CH). 13C{1H} NMR (CDCl3): δ 17.98 (1 C, C(N-
Ar)CH3), 23.55 (2 C, CH(CH3)(CH3)), 23.90 (2 C, CH(CH3)(CH3)),
28.97 (2 C, CH(CH3)(CH3)), 120.73 (1 C, CH), 123.72 (2 C, CH),
124.43 (1 C, CH), 129.89 (1 C, CH), 136.34 (2 C, C), 139.46 (1
C, CH), 141.68 (1 C, CH), 146.80 (1 C, C), 158.09 (1 C, C), 166.62
(1 C, C(N-Ar)CH3). Anal. Calcd for C19H23BrN2 (359.31): C, 63.51;
H, 6.45; N, 7.80. Found: C, 63.48; H, 6.31; N, 7.99.
1
product (yield 63%). IR (KBr): ν 1645 cm-1 (CdN). H NMR
(CDCl3): δ 1.33 (s, 6 H, CH(CH3)(CH3)), 1.35 (s, 6 H CH(CH3)-
(CH3)), 2.46 (s, 3 H, NdCCH3), 2.96 (m, 2 H, CH(CH3)(CH3)),
7.26-7.37 (m, 3 H, CH), 7.58-7.62 (m, 1 H, CH), 7.88-7.96 (m,
2 H, CH), 8.01 (t, J ) 7.6 Hz, 1 H, CH), 8.18 (m, 1 H, CH), 8.43
(d, J ) 7.6 Hz, 1 H, CH). 13C{1H} NMR (CDCl3): δ 17.52 (1 C,
NdCCH3), 23.20 (2 C, CH(CH3)(CH3)), 23.52 (2 C, CH(CH3)-
(CH3)), 28.56 (2 C, CH(CH3)(CH3)), 119.52 (1 C, CH), 121.24 (1
C, CH), 123.29 (1 C, CH), 123.80 (1 C, CH), 123.89 (1 C, CH),
126.57 (1 C, CH), 126.65 (1 C, CH), 136.13 (1 C, CH), 137.48 (1
C, CH), 141.87 (1 C, C), 142.51 (1 C, C), 146.88 (1 C, C), 152.57
(1 C, C), 156.38 (1 C, C), 167.53 (NdCCH3). Anal. Calcd for
C23H26N2S (362.53): C, 76.20; H, 7.23; N, 7.73. Found: C, 76.28;
H, 7.34; N, 7.82.
Synthesis of [1-(6-(Benzo[b]thiophen-3-yl)pyridin-2-yl)eth-
Br
ylidene](2,6-diisopropylphenyl)amine (N23BT). A solution of N2
(0.300 g, 0.84 mmol) and (benzo[b]thiophen-3-yl)trimethylstannane
(0.297 g, 1.00 mmol) in 3 mL of toluene was treated with a solution
of Pd(dba)2 (0.014 g, 0.025 mmol) and PPh3 (0.044 g, 0.167 mmol)
in toluene (1 mL). The reaction mixture was refluxed for 70 h and
then cooled to room temperature and treated with water (10 mL).
The formed layers were separated, and the organic phase was
washed with 2 × 20 mL of water and then dried over Na2SO4. The
collected organic layers were evaporated under reduced pressure,
and the crude product was purified via flash chromatography on
neutral alumina (n-pentane:Et2O ) 95:5) to afford a pale yellow
crystalline product (yield 84%). IR (KBr): ν 1642 cm-1 (CdN).
1H NMR (CDCl3): δ 1.17 (s, 6 H, CH(CH3)(CH3)), 1.20 (s, 6 H
CH(CH3)(CH3)), 2.34 (s, 3 H, NdCCH3), 2.81 (m, 2 H, CH(CH3)-
(CH3)), 7.08-7.23 (m, 3 H, CH), 7.37-7.53 (m, 2 H, CH), 7.80-
7.97 (m, 4 H, CH), 8.37 (dd, J ) 7.59, 1.2 Hz, 1 H, CH), 8.68 (m,
1 H, CH). 13C{1H} NMR (CDCl3): δ 18.22 (1 C, NdCCH3), 23.60
(2 C, CH(CH3)(CH3)), 23.93 (2 C, CH(CH3)(CH3)), 29.00 (2 C,
CH(CH3)(CH3)), 120.20 (1 C, CH), 123.49 (1 C, CH), 123.71 (1
C, CH), 124.07 (2 C, CH), 124.29 (1 C, CH), 125.12 (1 C, CH),
125.35 (2 C, CH), 127.40 (1 C, CH), 136.54 (1 C, C), 137.08 (1
C, C), 137.90 (1 C, CH), 139.70 (1 C, C), 141.74 (2 C, C), 147.16
(1 C, C), 154.26 (1 C, C) 156.80 (1 C, C), 167.93 (1 C, NdCCH3).
Anal. Calcd for C27H28N2S (412.59): C, 78.60; H, 6.84; N, 6.79.
Found: C, 78.49; H, 6.66; N, 6.63.
Synthesis of (Thiophen-3-yl)trimethylstannane. A solution of
3-bromothiophene (0.937 mL, 10.0 mmol) in 35 mL of Et2O was
cooled to -78 °C and treated dropwise with a 1.6 M solution of
n-BuLi in hexanes (12.5 mL, 20.0 mmol). The reaction mixture
was stirred at that temperature for 30 min, and then a solution of
Me3SnCl (2.99 g, 15.0 mmol) in Et2O (15 mL) was added dropwise
within 10 min. The mixture was allowed to stand at -78 °C for 1
h and then was slowly warmed to -35 °C and quenched with 30
mL of a saturated NaHCO3 aqueous solution. The aqueous phase
was extracted with 3 × 25 mL of Et2O. The combined organic
layers were dried over Na2SO4. Removal of the solvent under
reduced pressure gave the stannane product as a crude orange oil.
The product was purified by fractional distillation under reduced
pressure (1.0 × 10-2 mmHg, 50-55 °C) to afford a colorless oil
(yield 50%). 1H NMR (CDCl3): δ 0.32 (s, 9 H, CH3), 7.20 (dd, J
) 4.6, 1.0 Hz, 1 H, CH), 7.39 (dd, J ) 2.5, 1.0 Hz, 1 H, CH), 7.46
(dd, J ) 4.6, 2.5 Hz, 1 H, CH). 13C{1H} NMR (CDCl3): δ 5.05 (3
C, CH3), 125.98 (1 C, CH), 132.05 (1 C, CH), 133.37 (1 C, CH),
139.29 (1 C, C). Anal. Calcd for C7H12SSn (246.92): C, 34.05; H,
4.90. Found: C, 33.96; H, 4.88.
Synthesis of (Benzo[b]thiophen-3-yl)trimethylstannane. A
solution of 3-bromothianaphthene (5.0 mmol) in 20 mL of Et2O
was cooled to -78 °C and treated dropwise with a 1.6 M solution
of n-BuLi in hexanes (6.9 mL, 11.0 mmol). The reaction mixture
was stirred at that temperature for 30 min, and then a solution of
Me3SnCl (2.19 g, 11.0 mmol) in Et2O (15 mL) was added dropwise
Synthesis of 2-(Tri-n-butylstannanyl)pyridine. To a stirred
solution of 2-bromopyridine (3.0 mL, 4.98 g, 31.5 mmol) in 150
mL of Et2O at -78 °C was added 12.6 mL (31.5 mmol) of a 1.6

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 729
M solution of n-BuLi in hexanes over 10 min. After 30 min, n-Bu3-
SnCl (9.0 mL, 10.8 g, 31.8 mmol) was added portionwise and
stirring was continued for 2 h. The resulting mixture was warmed
to room temperature and then treated with 60 mL of a saturated
NH4Cl aqueous solution. The organic layer was washed with 2 ×
50 mL of water. The aqueous layer was extracted with 3 × 30 mL
of Et2O, and the combined organic layers were dried over Na2SO4.
Evaporation under reduced pressure of the solvent gave the product
as a pale orange oil (yield 93%). 1H NMR (CDCl3): δ 0.85 (t, J )
7.3 Hz, 9H, CH3), 1.00-1.20 (m, 6H, CH2), 1.26-1.35 (m, 6H,
CH2), 1.42-1.66 (m, 6H, CH2), 7.08 (ddd, J ) 6.4, 4.9, 1.5 Hz,
1H, CH), 7.37 (ddd, J ) 7.5, 1.5, 1.0 Hz, 1H, CH), 7.46 (ddd, J )
7.5, 7.5, 1.9 Hz, 1H, CH), 8.71 (ddd, J ) 2.9, 1.9, 1.0 Hz, 1H,
CH). 13C{1H} NMR (CDCl3): δ 10.41 (3C, Sn-CH2), 14.30 (3C,
CH3), 27.98 (3C, CH2), 29.72 (3C, CH2), 122.60 (1C, CH), 132.97
(1C, CH), 133.86 (1C, CH), 151.13 (1C, CH), 174.70 (1C, C). Anal.
Calcd for C17H31NSn (368.14): C, 55.46; H, 8.49; N, 3.80.
Found: C, 55.36; H, 8.60; N, 3.77.
MeOH (yield 88%). Mp: 186 °C. IR (KBr): ν 1640 cm-1 (CdN).
1H NMR (CDCl3): δ 1.18 (d, J ) 6.8 Hz, 12H, CH(CH3)2), 2.35
(s, 3H, py-C(NR)CH3), 2.81 (sept, J ) 6.8 Hz, 2H, CH(CH3)2),
7.09-7.23 (m, 3H, CH), 7.34 (ddd, J ) 7.5, 4.8, 1.2 Hz, 1H, CH),
7.85 (td, J ) 7.8, 1.8 Hz, 1H, CH), 7.96 (t, J ) 7.9 Hz, 1H, CH),
8.42 (dd, J ) 7.9, 1.0 Hz, 1H, CH), 8.55 (dd, J ) 7.9, 1.1 Hz, 1H,
CH), 8.57 (dt, J ) 8.0, 1.0 Hz, 1H, CH), 8.72 (ddd, J ) 4.9, 0.2,
0.1 Hz, 1H, CH). 13C{1H} NMR (CDCl3): δ 17.93 (1C, C(N-R)-
CH3), 23.60 (2C, CH(CH3)(CH3)), 23.91 (2C, CH(CH3)(CH3)),
28.95 (2C, CH(CH3)(CH3)), 121.77 (1C, CH), 121.87 (1C, CH),
122.61 (1C, CH),123.67 (2C, CH), 124.21 (1C, CH),124.50 (1C,
CH), 136.51 (2C, C), 137.58 (1C, CH), 138.10 (1C, CH), 147.21
(1C, C), 149.86 (1C, C), 155.86 (1C, C), 156.32 (1C, C), 156.71
(1C, C), 167.74 (1C, C(N-R)CH3). Anal. Calcd for C24H27N3
(357.49): C, 80.63; H, 7.61; N, 11.75. Found: C, 80.50; H, 7.56;
N, 11.83.
Synthesis of [1-(6′-Methyl[2,2′]bipyridin-6-yl)ethylidene](2,6-
diisopropylphenyl)amine (N2PyMe). A solution of 1-(6′-methyl-
[2,2′]bipyridin-6-yl)ethanone (0.424 g, 2.00 mmol) and 2,6-
diisopropylaniline (1.68 mL, 8.00 mmol) in 20 mL of MeOH was
refluxed for 24 h. The resulting mixture was concentrated to ca.
Synthesis of 2-Methyl-6-(trimethylstannanyl)pyridine. To a
stirred solution of 2-bromo-6-methylpyridine (1.71 mL, 2.586 g,
15.0 mmol) in 60 mL of THF at -78 °C was added 9.9 mL (15.8
mmol) of a 1.6 M solution of n-BuLi in hexanes over a period of
2 h. After 30 min, Me3SnCl (3.29 g, 16.5 mol) was added
portionwise and stirring was continued for 1 h. The resulting mixture
was warmed to room temperature (ca. 20 min) and then treated
with 60 mL of a saturated NaHCO3 aqueous solution. The organic
layer was washed with 2 × 30 mL of the saturated NaHCO3 solution
and then 2 × 30 mL of water. The aqueous layer was extracted
with 3 × 30 mL of Et2O. The organic layers were combined and
dried over Na2SO4. Removal of the solvent under reduced pressure
gave the product as a pale orange oil (yield 97%). 1H NMR
(CDCl3): δ 0.35 (s, 9H, Sn(CH3)3), 2.57 (s, 3H, Ar-CH3), 6.98
(d, J ) 7.7 Hz, 1H, CH), 7.25 (d, J ) 7.2 Hz, 1H, CH), 7.26-7.39
(m, 1H, CH). 13C{1H} NMR (CDCl3): δ -7.68 (3C, Sn(CH3)3),
25.55 (1C, Ar-CH3), 122.56 (1C, CH), 129.33 (1C, CH), 134.30
(1C, CH), 159.39 (1C, C), 173.16 (1C, C). Anal. Calcd for C9H15-
NSn (255.91): C, 42.24; H, 5.91; N, 5.47. Found: C, 42.13; H,
6.04; N, 5.39.
PyMe
8-10 mL and cooled to -24 °C. After 2 h, yellow crystals of N2
(0.41 g, 1.10 mmol, 55%) were collected by filtration. A second
crop (0.18 g, 0.48 mmol) was obtained from the mother liquor
maintained at -24 °C for 24 h (overall yield: 80%). Mp: 150-
1
152 °C. IR (KBr): ν 1638 cm-1 (CdN). H NMR (CDCl3): δ
1.17 (d, J ) 6.9 Hz, 12H, CH(CH3)2), 2.33 (s, 3H, py-C(N-R)-
CH3), 2.67 (s, 3H, py-CH3), 2.79 (sept, J ) 6.9 Hz, 2H,
CH(CH3)2), 7.04-7.23 (m, 4H, CH), 7.70-7.77 (m, 1H, CH),
7.90-7.98 (m, 1H, CH), 8.35 (ddd, J ) 7.7, 0.9, 0.6 Hz, 1H, CH),
8.39 (dd, J ) 7.8, 1.2 Hz, 1H, CH), 8.57 (dd, J ) 7.8, 1.2 Hz, 1H,
CH). 13C{1H} NMR (CDCl3): δ 17.93 (1C, py-CH3), 23.60 (2C,
CH(CH3)(CH3)), 23.90 (2C, CH(CH3)(CH3)), 25.36 (1C, C(N-R)-
CH3), 28.92 (2C, CH(CH3)(CH3)), 118.71 (1C, CH), 121.64 (1C,
CH), 122.62 (1C, CH), 123.64 (2C, CH), 124.04 (1C, CH),124.16
(1C, CH), 136.52 (2C, C), 137.70 (1C, CH), 137.98 (1C, CH),
147.23 (1C, C), 155.84 (1C, C), 156.10 (1C, C), 156.23 (1C, C),
158.60 (1C, C), 167.86 (1C, C(N-Ar)CH3). Anal. Calcd for
C25H29N3 (371.53): C, 80.82; H, 7.87; N, 11.31. Found: C, 80.65;
H, 7.74; N, 11.21.
General Procedure for the Syntheses of the CoII Catalyst
Precursors. To the blue solution of anhydrous CoCl2 (0.18 mmol)
in THF (3 mL) the appropriate ligand (0.20 mmol) was added in
one portion. The resulting green mixture was stirred at room
temperature for 3 h. During that time a precipitate formed, which
was decanted, transferred onto a sintered-glass frit, washed with
Et2O, and dried under a stream of nitrogen.
Synthesis of 1-(6′-Methyl[2,2′]bipyridin-6-yl)ethanone. A de-
aerated solution of 2-methyl-6-(trimethylstannanyl)pyridine (1.60
g, 6.25 mmol) and 1-(6-bromopyridin-2-yl)ethanone (1.25 g, 6.25
mmol) in 18 mL of toluene was treated with a solution of Pd(dba)2
(0.073 g, 0.13 mmol) and PPh3 (0.236 g, 0.91 mmol) in toluene (2
mL). The reaction mixture was refluxed for 16 h, cooled to room
temperature, and then treated with 50 mL of saturated aqueous
NaHCO3. The organic layer was separated, washed with 2 × 30
mL of a saturated NaHCO3 solution, 2 × 30 mL of water, and 2 ×
50 mL of brine, and dried over Na2SO4. Upon concentration of the
resulting solution to dryness under reduced pressure the product
was obtained in 93% yield. Mp: 85-87 °C. IR (KBr): ν 1693 cm-1
[(1-(6-Bromopyridin-2-yl)ethylidene)(2,6-diisopropylphenyl)-
amine]cobalt(II) Dichloride (CoCl2N2Br). This compound was
obtained as green microcrystals (yield 91%). IR (KBr): ν 1589
cm-1 (CdN). µeff: 4.71 µB (25 °C). Electronic spectra: diffuse
reflectance, 370 sh, 573, 631, 668, 1000, 1398, 1681 nm; CH2Cl2
solution, 362 sh, 571 (ꢀ ) 750), 632 (ꢀ 1030), 676 (ꢀ ) 1680),
1005 (ꢀ ) 130), 1386 nm (ꢀ ) 390). Anal. Calcd for C19H23BrCl2-
CoN2 (489.15) C, 46.65; H, 4.74; N, 5.73. Found: C, 46.50; H,
4.71; N, 5.68.
1
(CdO). H NMR (CDCl3): δ 2.65 (s, 3H, py-CH3), 2.84 (s, 3H,
C(O)CH3), 7.22 (ddd, J ) 7.7, 1.0, 0.6 Hz, 1H, CH), 7.71-7.79
(m, 1H, CH), 7.90-7.98 (m, 1H, CH), 8.05 (dd, J ) 7.8, 1.4 Hz,
1H, CH), 8.33 (ddd, J ) 7.8, 1.0, 0.6 Hz, 1H, CH), 8.66 (dd, J )
7.6, 1.4 Hz, 1H, CH). 13C{1H} NMR (CDCl3): δ 25.29 (1C, CH3),
26.42 (1C, CH3), 118.77 (1C, CH), 121.91 (1C, CH), 124.35 (1C,
CH), 124.99 (1C, CH), 137.79 (1C, CH), 138.72 (1C, CH), 153.57
(1C, C), 156.38 (1C, C), 156.56 (1C, C), 158.72 (1C, C), 201.10
(1C, C(O)CH3). Anal. Calcd for C13H12N2O (212.25): C, 73.57;
H, 5.70; N, 13.20. Found: C, 73.68; H, 5.59; N, 13.31.
[(1-(6-(Thiophen-3-yl)pyridin-2-yl)ethylidene)(2,6-diisopropy-
lphenyl)amine]cobalt(II) Dichloride (CoCl2N23Th). This com-
pound was obtained as green microcrystals (yield 92%). IR (KBr):
ν 1580 cm-1 (CdN). µeff: 4.68 µB (25 °C). Electronic spectra:
diffuse reflectance, 377, 452 sh, 560, 633 sh, 668, 1020, 1334, 1647
nm; CH2Cl2 solution, 373 (ꢀ ) 8000), 562 (ꢀ ) 1400), 671 (ꢀ )
2700), 998 (ꢀ ) 100), 1351 nm (ꢀ ) 380). Anal. Calcd for C23H26-
Cl2CoN2S: C, 56.11; H, 5.32; N, 5.69. Found: C, 56.19; H, 5.29;
N, 5.57.
Synthesis of [1-([2,2′]Bipyridin-6-yl)ethylidene](2,6-diisopro-
Br
pylphenyl)amine (N2Py). A solution of N2 (0.57 g, 1.59 mmol)
and 2-(tri-n-butylstannanyl)pyridine (0.247 g, 1.67 mmol) in 15 mL
of toluene was treated with Pd(PPh3)4 (0.055 g, 0.048 mmol) and
refluxed for 18 h. The reaction mixture was cooled to room
temperature, and the solvent was removed under reduced pressure
to give the product as a yellow solid, which was washed with cold
[(1-(6-(Benzo[b]thiophen-3-yl)pyridin-2-yl)ethylidene)(2,6-di-
isopropylphenyl)amine]cobalt(II) Dichloride (CoCl2N23BT). This

730 Organometallics, Vol. 26, No. 3, 2007
Bianchini et al.
Scheme 3
compound was obtained as green microcrystals (yield 87%). IR
(KBr): ν 1590 cm-1 (CdN). µeff: 4.69 µB (25 °C). Electronic
spectra: diffuse reflectance, 381, 452 sh, 559, 623 sh, 667, 1015,
1333, 1642 nm; CH2Cl2 solution, 379 (ꢀ ) 11 000), 568 (ꢀ ) 1400),
665 (ꢀ ) 2600), 1002 (ꢀ ) 110), 1370 nm (ꢀ ) 390). Anal. Calcd
for C27H28Cl2CoN2S: C, 59.78; H, 5.20; N, 5.16. Found: C, 59.60;
H, 5.09; N, 4.99.
[(1-([2,2′]Bipyridin-6-yl)ethylidene)(2,6-diisopropylphenyl)-
amine]cobalt(II) Dichloride (CoCl2N2Py). This compound was
obtained as a pale green powder (yield 88%). IR (KBr): ν 1585
cm-1 (CdN). µeff: 4.75 µB (25 °C). Electronic spectra: diffuse
reflectance, 375 sh, 595, 650, 919, 1220, 1790 nm; CH2Cl2 solution,
380 sh, 590 (ꢀ ) 92), 645 (ꢀ ) 105), 920 (ꢀ ) 20), 1230 nm (ꢀ )
22). Anal. Calcd for C24H27Cl2CoN3 (487.33): C, 59,15; H, 5,58;
N, 8,62. Found: C, 59.03; H, 5.47; N, 8.60.
[(1-(6′-Methyl[2,2′]bipyridin-6-yl)ethylidene)(2,6-diisopropy-
lphenyl)amine]cobalt(II) Dichloride (CoCl2N2PyMe). This com-
pound was obtained as a pale green powder (yield 97%). IR
(KBr): ν 1593 cm-1 (CdN). µeff: 4.80 µB (25 °C). Electronic
spectra: diffuse reflectance, 365 sh, 588, 641, 909, 1219, 1786 nm;
CH2Cl2 solution, 370 sh, 585 (ꢀ ) 90), 641 (ꢀ ) 100), 917 (ꢀ )
15), 1220 nm (ꢀ ) 18). Anal. Calcd for C25H29Cl2CoN3 (501.31):
C, 59.89; H, 5.83; N, 8.38. Found: C, 59.81; H, 5.71; N, 8.40.
General Procedure for Ethylene Oligomerization. A 500 mL
stainless steel reactor was heated to 60 °C under vacuum overnight
and then cooled to room temperature under a nitrogen atmosphere.
A MAO solution, prepared by diluting 1.5 mL of a stock toluene
solution of MAO (2.5 mmol) with toluene (93.5 mL), was
introduced by suction into the reactor, previously evacuated by a
vacuum pump. The system was pressurized with ethylene to 5 bar
and stirred for 5 min. Afterward, the ethylene pressure was released
slowly and 5 mL of a precatalyst solution, prepared by dissolving
the solid precatalyst (12 µmol) in 50 mL of toluene, was syringed
into the reactor. The system was repressurized to 5 bar and stirred
at 1500 rpm. Ethylene was continuously fed to maintain the reactor
pressure at the desired value. The temperature inside the reactor
increased solely from the exothermicity of the reaction and reached
a maximum value within 5-7 min. After 30 min, the reaction was
stopped by cooling the reactor to -20 °C, depressurizing, and
introducing 2 mL of acidic MeOH (5% HCl). Depending on the
oligomerization products, two different procedures were followed.
For reactions yielding mixtures of R-olefins, n-heptane was injected
into the reactor as the GC internal standard and the reactor contents
were stirred for 10 min. The solution was analyzed by GC and
GC-MS. The moles of the C6 and C8 fractions were determined
by GC using calibration curves with standard toluene solutions
containing concentrations of 1-hexene and 1-octene as close as
possible to those in the sample under analysis. Schulz-Flory R
constants were determined by the molar ratio of the couple C8/C6.
The moles of C4 were calculated by the Schulz-Flory R constant
and the moles of C6 by the formula moles of C4 ) (moles of C6)/
R, whereas the moles of the oligomers in the C10-C20 range were
calculated by the Schulz-Flory R constant and the moles of C8
by the formula moles of Ci ) (moles of C8)Ri-8/2. The total moles
of converted ethylene was obtained by the formula moles of C2H4
) ∑(moles of Ci)(i/2) with i ) 4-20; therefore, the TOFs were
obtained. For reactions yielding exclusively butenes, the reactor
contents, after being cooled to -50 °C, were transferred into a flask
immersed in a bath at -50 °C. Oligomers were distilled over a
Vigreux column (70 °C), condensed into a cooling trap (-196 °C)
containing isobutane as the GC internal standard, and analyzed by
GC.
EPR Study of the Activation of the Dichloride Precursors
by MAO. The activation of the dichloride precursors was studied
by X-band EPR spectroscopy, using the following procedures.
(a) In Situ Study. The appropriate CoII complex was suspended
in toluene in a Schlenk tube inside a glovebox (partial dissolution
with formation of a green solution was observed only for
CoCl2N22ThE). An excess of solid MAO (30 equiv) was added, and
a brownish solution was obtained, which was poured into an EPR
tube. This was immediately sealed and frozen by means of a liquid
nitrogen bath. The tube was introduced into the EPR probe, which
was precooled to 5 K. Spectra were acquired in the temperature
range 5-293 K. Thermal cycles from 293 to 5 K were repeated
three times to check the stability of the products.
(b) Operando Study. A toluene suspension of the appropriate
CoII complex in an EPR tube, cooled to 223 K, was saturated with
ethylene. After the addition of solid MAO (30 equiv), the solution
was warmed to 293 K and then the tube was immersed into a liquid
nitrogen bath. The acquisition of the EPR spectra was carried out
as described above.

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 731
Scheme 4
Ph
2Th
Reactions of CoCl2N2 and CoCl2N2
with MAO. Proce-
dure A. A 250 mL Schlenk flask equipped with a magnetic stirrer
bar was flamed under vacuum and cooled to room temperature
under an argon atmosphere. A MAO solution, prepared by diluting
1.5 mL of a stock toluene solution of MAO (2.5 mmol, 200 equiv)
with toluene (93.5 mL), was transferred by cannula into the flask.
The solid precatalyst (12 µmol) suspended in 5 mL of toluene was
syringed into the flask. The system was stirred for either 10 or 30
min and then treated with 10 mL of acidic MeOH (5% HCl). The
organic layer was separated from the aqueous phase, dried over
Na2SO4, and evaporated under reduced pressure to give a yellow
pale solid that was authenticated as the intact (imino)pyridine ligand
Ph
Figure 1. Diffuse reflectance spectra of (a) CoCl2N2 and (b)
Py
CoCl2N2
.
used did not influence the coordination geometry around the cobalt
atom as well as the electronic state and the total spin density. The
Cartesian coordinates of the complex cations [Co(CH3)N22Th]+, [Co-
1
by H NMR spectroscopy.
Procedure B (EPR Conditions). A 25 mL Schlenk flask
equipped with a magnetic stirrer bar was flamed under vacuum
and cooled to room temperature under an argon atmosphere. The
solid precatalyst (61 µmol) was suspended in 2 mL of toluene into
the flask. A stock toluene solution of MAO (1.1 mL, 30 equiv,
1.83 mmol) was transferred by cannula into the flask. The system
was stirred for 1 min and then treated with 5 mL of acidic MeOH
(5% HCl). The organic layer was separated from the aqueous phase,
dried over Na2SO4, and evaporated under reduced pressure to give
a yellow pale solid that was authenticated as the intact (imino)-
Py +
(CH3)N2Ph]+, [Co(CH3)N23Th]+, and [Co(CH3)N2
in the Supporting Information.
] are available
Results
Synthesis and Characterization of the (Imino)pyridine
CoII Complexes CoCl2N23Th and CoCl2N23BT. The new (imi-
no)pyridine ligands N23Th and N23BT were isolated as pale yellow
Br
crystals by Stille coupling between N2 and (thiophen-3-yl)-
1
pyridine ligand by H NMR spectroscopy.
trimethylstannane or (benzo[b]thiophen-3-yl)trimethylstannane,
DFT Modeling Study. All of the model structures were
calculated with the unrestricted B3LYP hybrid density functional
method12,13 using the Gaussian 03 program package (Revision
C.02).14 The basis set for the cobalt atom was provided by the
effective core potential of Hay and Wadt15 with the associated
double-ú basis set. For the other atoms, the 6-31G(d,p) basis set
was used. The nature of the optimized point was established by
calculating the vibrational frequencies. In all of the models, the
imino methyl group and the isopropyl phenyl substituents of the
ligands were replaced by hydrogen atoms. A test calculation with
all the atoms for [Co(CH3)N22Th]+ showed that the approximations
respectively. The corresponding CoII complexes CoCl2N23Th and
3BT
Br
Br
CoCl2N2
as well as the N2 derivative CoCl2N2 were
straightforwardly obtained as green crystals in high yield by
reacting 1 equiv of the ligand with anhydrous CoCl2 in THF
(Scheme 3).
All complexes are high spin in the solid state with µeff values
of about 4.7 µB at room temperature, which is typical for a d7
metal ion in a tetrahedral coordination geometry.1,2 Notice that
a distorted-tetrahedral geometry has been previously determined
2ThE
Ph 1
by X-ray methods for both CoCl2N2
and CoCl2N2 .
Consistent with a tetrahedral geometry, the electronic spectra
of the present CoII complexes, both in solution and in the solid
state, showed three d-d absorption bands in the spectral regions
1800-1640, 1430-1330, and 1050-990 nm, respectively, and
three or two higher intensity bands in the region 690-550 nm.1,2
Due to the fast spin-lattice relaxation times of the quartet
(12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(13) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Br
spin state, CoCl2N23Th, CoCl2N23BT, and CoCl2N2 were EPR
silent at room temperature both in the solid state and in CH2-
Cl2 solution.16 Only at very low temperature (10 K) were EPR
signals typical of high-spin tetrahedral CoII species observed
(see the Supporting Information for details). As previously
2ThE
observed for the thien-2-yl regioisomer CoCl2N2
and the
(16) (a) Bencini, A.; Gatteschi, D. In Transition Metal Chemistry; Figgis,
B. N., Melson, G., Eds.; Marcel Dekker: New York, 1982; Vol. 8. (b)
Pilbrow, J. R. Transition Ion Electron Paramagnetic Resonance; Clarendon
Press: Oxford, U.K., 1990.
(15) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.

732 Organometallics, Vol. 26, No. 3, 2007
Table 1. Ethylene Oligomerization with Cobalt(II) 6-(Organyl)-2-(imino)pyridine Dichloride Catalyst Precursorsa
alkene amt (mol %)
productivityb,d
Bianchini et al.
entry
cat. precursor
∆T (°C)
TOFb,c
Rb,e
C4
C6
C8
0.6
C10-14
<0.1
R-olefin selectivityb (%)
2Th
1
2
3
4
5
6
CoCl2N2
CoCl2N2
CoCl2N2
CoCl2N2
12
4
9
20
4
6
665000
113000
407000
927000
115000
183000
3724
633
2279
5191
644
0.08
f
0.10
0.20
f
f
92.0
7.4
90
92
93
91
93
92
3Th
2ThE
90.0
80.0
9.0
16.0
0.9
3.2
0.1
0.8
2BT
3BT
CoCl2N2
Ph
CoCl2N2
1025
a Reaction conditions: catalyst precursor, 1.2 µmol; MAO equiv, 2100; C2H4 pressure, 5 bar; toluene, 100 mL; initial temperature, 23 °C; reaction time,
30 min. b Determined by GC. c In units of mol of C2H4 converted (mol of Co)-1 h-1
.
d In units of g of C2H4 converted (mmol of Co)-1 h-1 bar-1
.
e Schulz-
Flory parameter, R ) rate of propagation (rate of propagation + rate of chain transfer)-1 ) mol of Cn+2 (mol of Cn)-1
butenes produced, with prevalent E configuration for 2-butene.
.
f Not determined. Almost exclusively
described for the related 6-(thienyl)-2-(imino)pyridine complexes
and isolated as paramagnetic pale green crystals with a µeff value
of about 4.8 µB at room temperature. However, unlike the
6-(organyl)-2-(imino)pyridine CoII complexes, the metal center
Py
PyMe
in CoCl2N2 and CoCl2N2
is five-coordinate, which is
consistent with the stronger σ-ligating character of the pyridyl
group vs that of the thienyl or phenyl groups in the other ligands.
Convincing evidence for a five-coordinate complex was pro-
vided by the electronic spectra in both solution and the solid
state, consisting of a pattern of bands with energy and relative
intensity easily distinguishable from those of the tetrahedral CoII
complexes previously described.1,2,19 As an example of the
reliability of electron spectroscopy for distinguishing tetrahedral
and five-coordinate CoII complexes, Figure 1 shows the diffuse
reflectance spectra of CoCl2N2Ph (a) and CoCl2N2Py (b). Notice-
ably, the spectral parameters of the latter complex are strongly
indicative of a square-pyramidal geometry. Indeed, its absorption
and reflectance spectra consist of five well-defined bands at 595,
650, 919, 1220, and 1790 nm and can be correlated with those
of 2,6-bis(imino)pyridine CoII dihalides.17,20
Like related dichloride CoII complexes with 2,6-bis(imino)-
Py
PyMe
pyridine ligands,17 CoCl2N2 and CoCl2N2
did not show
Figure 2. 1H NMR spectra (400.13 MHz, 23 °C, CD2Cl2): (a)
any EPR bands at room temperature, whereas a signal attribut-
able to a broad and poorly resolved rhombic structure was
observed below 10 K in CH2Cl2 frozen solution (g1 ) 5.04(8),
g2 ) 3.00(8), and g3 ) 1.95(8) for CoCl2N2Py; g1 ) 5.03(8), g2
) 3.00(8), and g3 ) 1.94(8) for CoCl2N2PyMe). The temperature
dependence of each spectrum was consistent with the presence
of large ZFS effects and fast spin-lattice relaxation times.16,18
Oligomerization of Ethylene Catalyzed by (Imino)pyridine
CoII Dichloride Complexes Activated by MAO. On activation
CoCl2N22ThE; (b) CoCl2N22ThE after treatment with MAO (6 equiv).
6-phenyl derivative CoCl2N2Ph,1 as well as related dichloride
CoII complexes with 2,6-bis(imino)pyridine ligands,17 the ap-
1
pearance of a signal typical for an S )
/ effective spin
2
Hamiltonian with largely anisotropic g values at low temperature
can be attributed to large zero-field splitting (ZFS) effects of
3
the S ) /2 state.16,18
3BT
In conclusion, the magnetic and spectroscopic characterization
by MAO in toluene, the complexes CoCl2N23Th and CoCl2N2
3Th
3BT
of the thien-3-yl complexes CoCl2N2 and CoCl2N2
and
were active catalysts for the selective dimerization of ethylene
to 1-butene. Since the presence of residual AlMe3 in commercial
toluene solutions of MAO may lead to undesired reaction
paths,21,22 batch oligomerization and model activation reactions
were carried out using solid MAO, obtained by careful removal
Br
of the 6-bromo derivative CoCl2N2 is strongly indicative of
high-spin tetrahedral CoII complexes both in the solid state and
in solution.
The introduction of a pyridyl group in the 6-position of the
pyridine ring of the (pyridin-2-yl)ethylidene-2,6-diisopropyl-
(19) (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.
(20) (a) Sacconi, L.; Morassi, R.; Midollini, S. J. Chem. Soc. A 1968,
1510. (b) Sacconi, L.; Bertini, I.; Morassi, R. Inorg. Chem. 1967, 6, 1548.
(c) Ciampolini, M.; Speroni, G. P. Inorg. Chem. 1966, 5, 45.
(21) (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b)
Busico, V.; Cipullo, R.; Cutillo, F.; Friederichs, N.; Ronca, S.; Wang, B. J.
Am. Chem. Soc. 2003, 125, 12402.
(22) (a) Byun, D.-J.; Kim, S. Y. Macromolecules 2000, 33, 1921. (b)
Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; Maddox, P. J.; van Meurs,
M. Angew. Chem., Int. Ed. 2002, 41, 489. (c) 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.; Stromberg, S.; White, A. J.
P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728. (d) Semikolenova,
N. V.; Zakharov, V. A.; Talsi, E. P.; Babushkin, D. E.; Sobolev, A. P.;
Echevskaya, L. G.; Khysniayarov, M. M. J. Mol. Catal. A: Chem. 2002,
182-183, 283.
Py
benzenamine moiety to give the ligand N2 was achieved by
the procedure illustrated in Scheme 3, while the picolyl-
substituted ligand N2PyMe was obtained by the slightly different
route illustrated in Scheme 4. The corresponding CoII complexes
Py
PyMe
CoCl2N2 and CoCl2N2
were synthesized as previously
(17) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.; Zanobini,
F.; Laschi, F.; Sommazzi, A. Eur. J. Inorg. Chem. 2003, 1620.
(18) (a) Drago, R. S. Physical Methods for Chemists; Saunders College:
New York, 1992. (b) Mabbs, F. E.; Collison, D. Electron Paramagnetic
Resonance of d Transition Metal Complexes; Elsevier: Amsterdam, 1992;
Vol. 16. (c) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance
of Transition Ions; Dover: New York, 1970. (d) Wertz, J. E.; Bolton, J. R.
Electron Spin Resonance: Elementary Theory and Practical Applications;
McGraw-Hill: New York, 1972. (e) Carlin, R. L. Magnetochemistry;
Springer-Verlag: Berlin, 1986.

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 733
Table 2. 1H NMR Parameters for CoCl2N2
(400.13 MHz,
2ThE
Scheme 5
23 °C, CD2Cl2)
nucleus
3-/5-Py
δ (ppm)a
s, 59.71, 1H; s, 56.16, 1H
s, -10.96, 1H
s, 54.00, 3H
s, -6.15, 1H; s, 1.28, 1H
d, 5.79, J ) 5.0 Hz, 2H
t, -20.02, J ) 5.0 Hz, 1H
s, 2.86, 6H; s, 0.46, 6H
s, 1.59, 2H
4-Py
NdCCH3
3-/4-thiophene
m-Ph
p-Ph
iPr
CH3
CHiPr
information on the initiation step. Moreover, catalytically
relevant species might not be sufficiently long-lived to be
intercepted on the IR and NMR time scale, while tetrahedral
CoII complexes are not particularly amenable to be studied by
NMR spectroscopy due to their paramagnetic nature. Neverthe-
Et
CH2
CH3
q, 1.85, J ) 6.6 Hz, 2H
t, -4.49, J ) 6.6 Hz, 3H
Et
a Key: s, singlet; d, doublet; t, triplet; q, quartet.
Table 3. EPR Parameters Obtained from the Dichloride
Catalyst Precursors with MAO (30 Equiv) in Toluenea,b
1
less, as shown by the H NMR spectra acquired in CD2Cl2
2ThE
solution for CoCl2N2
before (a) and after treatment with
starting complex
gx
gy
gz
ax
ay
az
giso aiso
MAO (6 equiv) (b), some useful information may be obtained
on the MAO-activated product (Figure 2).
CoCl2N2Ph
2.419 2.077 2.001
2.420 2.220 1.990 168
2.195 2.030 2.024 107 166
2.290 2.025 2.017
2.290 2.029 2.015
61
18
36 2.15
38
3Th
CoCl2N2
CoCl2N2
CoCl2N2
CoCl2N2
19 153 2.20 113
2ThE
In the absence of MAO, all protons resonate at chemical shifts
significantly different from those of the corresponding protons
in the free ligand N22ThE, which is consistent with the presence
of three unpaired electrons in the cobalt complex. Unambiguous
signal assignment was achieved on the basis of isotropic shifts
essentially attributable to a Fermi contact contribution (Table
2).23
43 2.07 105
47 2.11
50 2.12
Py
84
81
19
20
50
52
PyMe
a The giso values were obtained from room-temperature spectra. b The gi
and ai (expressed in gauss) values were obtained by simulation of the frozen-
solution spectra. The aiso values were calculated as the average of the three
hyperfine coupling constants.
In line with the coordination of cobalt through the two
nitrogen atoms, the largest paramagnetic shifts were exhibited
by the nuclei in the 3- and 5-positions of the pyridine ring (δ
56.16 and 59.71) and by the protons of the CH3CdN group (δ
54.00). The addition of 6 equiv of MAO in 2 equiv portions
dramatically changed the spectrum. A remarkable line broaden-
ing of the original resonances occurred just after adding the
first 2 equiv of activator. Concomitantly, new, relatively sharp
resonances appeared in the 2-10 ppm region, which were not
present in the 1H NMR spectrum of MAO alone. After the third
portion of MAO was added, no clear signal was observed in
the region downfield to 12 ppm, while new signals appeared at
negative chemical shift values.
Overall, this picture suggests that the addition of MAO does
not convert the parent complex into a diamagnetic species, as
occurs for 2,6-bis(imino)pyridine CoII dihalides.24 Indeed, as
shown also by the following EPR study, as well as in situ
magnetic moment measurements, the activated product main-
tains paramagnetic features, although a decrease in the magnetic
moment may be envisaged in view of the higher concentration
of signals in the region of diamagnetic molecules.
of solvent and volatile impurities from commercial MAO under
reduced pressure.22d
The addition of ethylene to the catalyst/cocatalyst mixture
resulted in a rapid exotherm, indicative of no induction period.
Selected data are reported in Table 1, which also contains
2BT
data obtained with the precursors CoCl2N22Th, CoCl2N2
,
CoCl2N22ThE, and CoCl2N2 under comparable experimental
Ph
conditions.
At 5 bar of C2H4, both the TOF (ca. 1 × 105 mol of C2H4
converted (mol of Co)-1 h-1) and the selectivity shown by
3Th
Ph
3BT
CoCl2N2
and CoCl2N2
were comparable to those of
CoCl2N2 (entries 2, 5, and 6). In contrast, the thien-2-yl
regioisomers CoCl2N22Th, CoCl2N2
and CoCl2N22BT, were
2ThE
5-10 times more active and gave low-molecular-weight R-ole-
fins (C4-C14) with Schulz-Flory distributions and more than
90% selectivity (entries 1, 3, and 4; Table 1). The internal olefins
showed predominantly E conformations (>70%), while no
branched product was observed. Consistent with the use of solid
MAO, neither saturated hydrocarbons nor odd olefins, due to
chain transfer to aluminum, were produced.22 The significant
isomerization to internal alkenes indicates â-H transfer to metal
as an effective chain transfer mechanism.1,2,4 For all catalysts,
the TOF is linear with the C2H4 pressure, which suggests that
the major catalyst resting state is a cobalt alkyl complex.1,2,4
Under the experimental conditions reported in Table 1, no
In Situ EPR Study. The inability of NMR spectroscopy to
provide further information on the MAO-activated species
prompted us to carry out an EPR study. In fact, EPR spectros-
copy is much more amenable than NMR spectroscopy to
intercept and characterize elusive paramagnetic species.
As previously said, tetrahedral (imino)pyridine CoII com-
ethylene oligomerization activity at all was observed using
PyMe
CoCl2N2Br, CoCl2N2Py, or CoCl2N2
as catalyst precursor.
plexes are EPR-silent at room temperature. It was therefore
Activation of the Dichloride Catalyst Precursors by MAO.
Product analysis and studies on the dependence of the activity
and Schulz-Flory R values on ethylene pressure have been
useful tools to elucidate the propagation and chain transfer
mechanisms of ethylene oligomerization by the (imino)pyridine
CoII catalysts previously studied.1,2 It was established that
propagation occurs via a Cossee-Arlman mechanism and chain
transfer proceeds by â-H transfer to metal, although the
concomitant occurrence of â-H transfer to monomer cannot be
ruled out.4c Unfortunately, the large excess of MAO required
to have high oligomerization activity makes in situ spectroscopic
studies, such as NMR and IR, unable to provide detailed
3Th
interesting to find out that CoCl2N2Ph, CoCl2N22ThE, CoCl2N2
,
PyMe
CoCl2N2Py, and CoCl2N2
were transformed into products
already showing EPR signals at room temperature on treatment
with 30 equiv of MAO in toluene. In all cases, the spectra did
not show any hyperfine structure and were unequivocally
(23) (a) Jesson, J. P. NMR of Paramagnetic Molecules: Principles and
Applications; La Mar, G. N., Horrocks, W. D., Holm, R. H., Eds.; Academic
Press: New York, 1973. (b) Holm, R. H.; Phillips, W. D.; Averill, B. A.;
Mayerle, J. J.; Herskovitz, T. J. Am. Chem. Soc. 1974, 96, 2109. (c)
McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 335.
(24) Humphries, M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. Organometallics 2005, 24, 2039.

734 Organometallics, Vol. 26, No. 3, 2007
Bianchini et al.
Chart 1
Scheme 6
1
attributed to low-spin CoII species (S ) /2) with the expected
giso values (Table 3). Identical signals were seen when the EPR
spectra were acquired in the presence of ethylene: i.e., under
truly operando conditions.
The magnetic moments of the species generated by treatment
of the high-spin precursors with MAO were estimated by means
of the Evans method25 using cyclohexane as the inert probe
substance.25e Values of the magnetic moments in the range of
1.8-2.3 µB were calculated, which confirms the quantitative
transformation of the high-spin CoII precursors into low-spin
CoII products.26-28
Before making any comment on and interpretation of the EPR
spectra, it is worthwhile to recall the general mechanism by
which MAO can activate early- and late-metal halide complexes
for alkene insertion. As shown in Scheme 5, a well-established
function of MAO is to replace halide ligands with methyl groups
and ultimately create a ion pair made of a coordinatively
unsaturated complex cation and [MAO]- anions.21
In order to obtain these products, the ligands must be treated
with the appropriate reagents under specific experimental
conditions: AlMe3 in refluxing toluene,31 product A; MeLi in
THF at -50 °C, product B; LiAlH4 in refluxing THF, product
C.32,33
Having established that the activation of the cobalt precursors
with MAO does not affect the structure of the supporting
(imino)pyridine ligand, we need to explain the EPR results. The
simple substitution of the chlorides in the precursors with
stronger methyl ligands by MAO cannot account for the
appearance of an EPR signal at room temperature without a
concomitant change in the coordination geometry from high-
spin tetrahedral to low-spin square planar or five-coordinate.
Five-coordination is much more unlikely than four-coordina-
tion, as the number of available donor atoms in the precursors
and, to a major extent, in the activated species is limited to a
maximum of three or four. Moreover, although not numerous,
some examples of low-spin square-planar CoII complexes with
nitrogen ligands are known,26-28 among which it is worth
highlighting a 1,8-bis(imino)carbazolide complex obtained by
replacing chloride with methyl in a tetrahedral precursor
(Scheme 6).28 Other four-coordinate geometries, such as the
pseudotetrahedral geometry exhibited by the doublet ground
state CoII complex CoI[PhBP3] (PhBP3 ) PhB(CH2PPh2)3-),
can be considered unlikely, as they are strongly related to the
tripodal geometry of the supporting ligand.34
Other relevant roles of MAO, such as reducing agent,
hydrogen abstractor, C-alkylating agent and O-ligand, have been
also reported,21,22,24 yet no evidence of their occurrence in the
present reactions was obtained. In particular, several experiments
have been performed which clearly show that the (imino)-
pyridine ligands are not modified by the activator under catalytic
2Th
Ph
conditions. The complexes CoCl2N2
and CoCl2N2 were
suspended in toluene and then treated with an excess of MAO
(30-200 equiv). After they were stirred for 1-30 min at room
temperature, the resulting reaction mixtures were quenched with
MeOH/HCl. The extracted organic residue was shown by NMR
spectroscopy to be the intact (imino)pyridine ligand. Other
possible modifications of the (imino)pyridine ligands, such as
C-alkylation (A),29 H-abstraction30 (B), and hydride addition
(C), were not observed (Chart 1).
(25) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Evans, D. F.;
Fazakerley, G. V.; Phillips, P. R. F. J. Chem. Soc. A 1971, 1931. (c)
Crawford, H. D.; Swanson, J. J. Chem. Educ. 1971, 48, 382. (d) Grant, D.
H. J. Chem. Educ. 1995, 72, 39. (e) Britovsek, G. J. P.; Gibson, V. C.;
Spitzmesser, S. K.; Tellmann, K. P.; White, A. J. P.; Williams, D. J. Dalton
Trans. 2002, 1159.
In conclusion, the formation of square-planar CoII species
upon activation with MAO of the tetrahedral precursors
3Th
CoCl2N2Ph, CoCl2N22ThE, and CoCl2N2
considered as highly probable.
was immediately
(26) Bodreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley: New York, 1976; pp 211-225.
In an attempt to obtain structural information, a variable-
temperature EPR study was conducted on the MAO-activated
products. Frozen spectra at 5 K are reported in Figure 3 as solid
lines.
(27) (a) Muller, G.; Sales, J.; Torra, I.; Vinaixa, J. J. Organomet. Chem.
1982, 224, 189. (b) Anton, M.; Muller, G.; Sales, J.; Vinaixa, J. J.
Organomet. Chem. 1982, 239, 365. (c) Beattie, J. K.; Hambley, T. W.;
Klepetko, J. A.; Masters, A. F.; Turner, P. Polyhedron 1996, 15, 473. (d)
Jaynes, B. S.; Doerrer, L. H.; Liu, S.; Lippard, S. J. Inorg. Chem. 1995,
34, 5735. (e) Fryzuk, M. D.; Leznoff, D. B.; Thompson, R. C.; Retting, S.
J. J. Am. Chem. Soc., 1998, 120, 10126.
1
The spectra, typical for low-spin CoII species (S ) /2, I )
7/2), show large and resolved g and a anisotropy and are
characterized by a complex structure due to the superposition
of the different components of the anisotropic hyperfine
(28) Gibson, V. C.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J.
Dalton Trans. 2003, 2718.
(29) (a) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White,
A. J. P.; Williams, D. J. Chem. Commun. 1998, 2523. (b) Milione, S.;
Cavallo, G.; Tedesco, C.; Grassi, A. Dalton Trans. 2002, 1839. (c)
Knijnenburg, Q.; Smits, J. M. M.; Budzelaar, P. H. M. Organometallics
2006, 25, 1036.
(30) (a) Khorobkov, I.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H.
M. Organometallics 2002, 21, 3088. (b) Sugiyama, H.; Gambarotta, S.;
Yap, G. P. A.; Wilson, D. R.; Thiele, S. K.-H. Organometallics 2004, 23,
5054. (c) Blackmore, I. J.; Gibson, V. C.; Hitchcock, P. B.; Rees, C. W.;
Williams, D. J.; White, A. J. P. J. Am. Chem. Soc. 2005, 127, 6012. (d)
Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006, 45, 2.
(31) Gibson, C. V.; Redshaw, C.; White, A. J. P.; Williams, D. J. J.
Organomet. Chem. 1998, 550, 453.
(32) Bianchini, C.; Giambastiani, G. Unpublished results.
(33) For MAOs which can contain and/or are able to generate aluminum
hydride species see: (a) Rogers, J. S.; Bazan, G. C. Chem. Commun. 2000,
1209. (b) Hagen, H.; Kretschmer, W. P.; van Buren, F. R.; Hessen, B.; van
Oeffelen, D. A. J. Mol. Catal. A: Chem. 2006, 248, 237.
(34) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters,
J. C. J. Am. Chem. Soc. 2002, 124, 15336.

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 735
Figure 3. Frozen CH2Cl2 solution (5 K) X-band EPR spectra of MAO-activated (a) CoCl2N22ThE, (b) CoCl2N2Ph, (c) CoCl2N23Th, and (d)
Py
CoCl2N2 together with best simulations (dotted lines) obtained with parameters reported in Table 3.
Figure 4. Temperature variation of X-band EPR spectrum of
Figure 5. Temperature variation of X-band EPR spectrum of
CoCl2N23Th/MAO (30 equiv).
CoCl2N2PyMe/MAO (30 equiv).
coupling. Due to the broadness of the hyperfine lines, no
computed spectra of CoCl2N22ThE and CoCl2N23Th. Finally, the
superhyperfine structure was observed, notwithstanding the
simulation of low-spin CoII spectra might be made difficult by
likely delocalization of some unpaired spin density on nitrogen
and/or hydrogen atoms of the (imino)pyridine ligand, as
the fact that the g and a principal directions may be non-
collinear, an effect that has been neglected in the present
indicated by theoretical calculations (vide infra). Each spectrum
approach.18b,36 On the other hand, for all compounds, the
was simulated to get the corresponding anisotropic parameters
experimental giso values were in good agreement with the values
obtained from the average of the three principal gi values (i )
x, y, z) of the frozen-solution spectra, which is an indirect
(Table 3), and the results of the simulations are shown in Figure
3 as dotted lines. To reduce the number of parameters for the
simulation process, no quadrupolar term was introduced in the
confirmation of the overall validity of the simulations.
spin Hamiltonian used for the simulation: H ) âB‚g‚S + S‚
Ph
It is worth noticing that the product generated by CoCl2N2
A‚I.35 In view of the spin Hamiltonian parameters for best
shows a frozen EPR spectrum with hyperfine interactions
simulated curves, it is apparent that CoCl2N22ThE is characterized
2ThE
smaller than those of the products obtained from CoCl2N2
by a nearly axial spectrum, while the spectra of CoCl2N2Ph and
and CoCl2N23Th. The relation between the a values and the
3Th
CoCl2N2 are rhombic.
electronic configuration of CoII metal ions is very subtle, as it
Different factors may account for the observed discrepancies
may depend on as many as 12 parameters.16,36,37 In the absence
of an in-depth study, which is out of the scope of this article,
the only comment we can make is that the energies of the excited
quartet states, which, in a perturbative approach, contribute to
the third order in g values and to the second order in a values,
2ThE
between the experimental and simulated spectra of CoCl2N2
(central part of the spectrum, Figure 3a) and CoCl2N23Th (low-
field part of the spectrum, Figure 3c). An important factor might
be the presence of a second species, likely a residual amount
of the starting complex, whose spectrum superimposes onto the
spectrum of the MAO-activated species. Anomalous line-
broadening effects, connected with low-lying excited states, or
our choice to neglect the quadrupolar term may also have
contributed to the nonperfect agreement of the experimental and
Ph
are substantially different in CoCl2N2 as compared to
2ThE
3Th
CoCl2N2
and CoCl2N2
.
All of the EPR spectra of the MAO-activated complexes
showed identical behavior with temperature. As an example,
(36) Daul, C.; Schlapfer, C. W.; von Zelewsky, A. In Structure and
Bonding; Dunitz, J. D., Ed.; Springer-Verlag: Berlin, 1979; Vol. 36, p 129.
(37) McGarvey, B. R. Can. J. Chem. 1975, 53, 2498.
(35) Jacobsen, C. J. H.; Pedersen, E.; Villadsen, J.; Weihe, H. Inorg.
Chem. 1993, 32, 1216.

736 Organometallics, Vol. 26, No. 3, 2007
Bianchini et al.
Figure 6. Optimized models of (a) [Co(CH3)N22Th]+, (b) [Co(CH3)N23Th]+, (c) [Co(CH3)N2Ph]+, and (d) [Co(CH3)N2Py]+.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the
Four Models
Scheme 8
[Co(CH3)- [Co(CH3)- [Co(CH3)- [Co(CH3)-
2Th
+
3Th
+
Py
+
Ph +
N2
]
N2
]
N2
]
N2 ]
Co-Xa
2.40
2.03
2.00
1.94
2.53
1.99
2.01
1.93
2.36
177
153
37
2.04
1.95
2.04
1.94
2.71
2.01
1.98
1.92
2.33
176
149
22
Co-N(1)
Co-N(2)
Co-C(1)
Co-H
N(1)-Co-C(1)
N(2)-Co-X
Xa-C(2)-C(3)-N(1)
179
159
20
175
160
1.7
a X is the sulfur atom for [Co(CH3)N22Th]+, the agostic carbon atom for
The spectral parameters of the spectrum for this species at
10 K as well as its temperature dependence are in line with
those observed for all the other MAO-activated CoII precursors
(Table 3) and are attributable to a low-spin square-planar CoII-
methyl complex.
and [Co(CH3)N2Ph]+, and the N(3) nitrogen atom for
3Th
+
[Co(CH3)N2
]
[Co(CH3)N2Py]+.
Scheme 7
It is noteworthy that the two MAO-activated complexes that
do not contain a potentially coordinating heteroatom (i.e., [Co-
Ph +
(CH3)N2
]
and [Co(CH3)N23Th]+) have similar gx values
(∼2.42), while the compounds eventually stabilized by a σ-bond
with sulfur or nitrogen (i.e., [Co(CH3)N22Th]+, [Co(CH3)N2Py]+,
and [Co(CH3)N2PyMe]+) exhibit much smaller gx values (2.2-
2.3).
the spectra of CoCl2N23Th taken in the temperature interval from
5 and 230 K are reported in Figure 4.
In conclusion, the EPR study has unambiguously demon-
strated the occurrence of a spin state changeover in the activation
of the catalyst precursors by MAO. The most likely coordination
geometry of the CoII centers in the activated species is square
planar with two nitrogen atoms from the (imino)pyridine ligand,
a carbon atom from a methyl group released by MAO, and a
fourth ligand that might be proVided by the organyl group in
the 6-position of the pyridine ring. Indeed, in the absence of
either ring, as in CoCl2N2Br, no EPR signal appeared on
treatment of the complex in toluene with an excess of MAO in
the temperature range from 293 to 20 K. It is therefore very
likely that the organyl group in the 6-position interacts with
the cobalt center.
Increasing the temperature from 5 to 160 K caused line
broadening as well as a decrease of the spectral intensity;
however, no evidence whatsoever of a spin-state transition to a
high-spin configuration was observed. The major variation was
noticed above 180 K, where the solutions became fluid and the
spectrum lost any hyperfine structure due to a large line
broadening, which suggests the anisotropic hyperfine interac-
tions were not completely mediated.16b
A clear EPR signal was seen already at room temperature by
PyMe
reacting the five-coordinate complex CoCl2N2
in toluene
with MAO. A sequence of variable-temperature EPR spectra
of the system CoCl2N2PyMe/MAO are reported in Figure 5.

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 737
Chart 2
Scheme 9
DFT Modeling Study of the MAO-Activated Species. In
an attempt to elucidate the structure of the low-spin CoII species,
we built up four models of the low-spin complex cations [Co-
(CH3)N22Th]+, [Co(CH3)N2Ph]+, [Co(CH3)N23Th]+, and [Co-
Py +
(CH3)N2
]
generated by the reaction of MAO with the
corresponding dichloride precursors. The optimized structures
are reported in Figure 6, while selected bond distances and
angles are given in Table 4. Models for high-spin species were
not considered, in light of the experimental EPR and NMR
(Evans method) evidence for the exclusive formation of low-
spin species.
Discussion
The presence of an aromatic group bearing a weakly
coordinating donor atom (e.g., a thienyl sulfur such as in
In all optimized structures, the cobalt atom turned out to be
at the center of a distorted square plane with three coordination
positions occupied by the N2C donor atom set. Neither the
geometry nor the stereochemistry of this portion of the complex
cations changes in the three models. In contrast, the four models
differ in the atom occupying the fourth position of the square
plane. This position is occupied by the thien-2-yl sulfur atom
CoCl2N2 and CoCl2N22BT) in the 6-position of the pyridine
2Th
ring of the supporting ligand and the low-spin d7 configuration
of the MAO-activated cobalt center seem to be key factors for
promoting the oligomerization activity of the present (imino)-
pyridine dichloride CoII precursors.6 The importance of ligand
hemilability in assisting insertion polymerization by cobalt
catalysis has been previously demonstrated by Brookhart.40 This
author showed that the substitution of a tertiary phosphine/
phosphite with a hemilabile thioether ligand increases remark-
ably the ability of cyclopentadienyl CoIII complexes to catalyze
the polymerization of ethylene to high-density polyethylene
(HDPE) (Scheme 8). It was suggested that the sulfur atom
lowers the barrier to ethylene insertion through the stabilization
of the electron-deficient transition state for migratory insertion.
No useful information was found in the relevant literature,
which might account for the role exerted by the low-spin d7
electronic configuration observed in the square-planar CoII
propagating alkyls. Nor we have a reliable explanation for it.
On the other hand, neither the tetrahedral high-spin d6 FeII
in [Co(CH3)N2 ]
2Th + or by a nitrogen atom in [Co(CH3)N2Py]+,
whereas an agostic C-H group would complete the square-
Ph +
planar coordination in [Co(CH3)N2
]
and [Co(CH3)N23Th]+.
The Co-Hagostic distance is quite the same in the latter
complexes, whereas, due to the different internal angles of the
thien-3-yl and phenyl rings, the Co-Cagostic distance in [Co-
Ph +
(CH3)N2
]
is almost 0.2 Å longer than in [Co(CH3)N23Th]+.
2Th +
The complex [Co(CH3)N2
]
is appreciably more stable
(around 6 kcal/mol) than the regioisomer [Co(CH3)N23Th]+. The
interaction between the metal and the atom in the fourth position
of the square plane was estimated by allowing the organyl
groups in the pyridine 6-position to rotate about the C(2)-C(3)
bond. For [Co(CH3)N22Th]+ and [Co(CH3)N2Py]+ the maximum
energy was found for the organyl group perpendicular to the
(imino)pyridine plane and the energy gap between this con-
former and the optimized structure was around 15 and 38 kcal/
mol, respectively. The same conformers of [Co(CH3)N2Ph]+ and
[Co(CH3)N23Th]+ were destabilized with respect to the optimized
geometry by 6 and 9 kcal/mol, respectively. Notably, the energy
barriers to the rotation of the thien-3-yl and phenyl rings around
the C(2)-C(3) bond are the same in the free ligands (5.5 kcal/
mol), which confirms that it is the interaction with the metal
that makes the complexes different from each other.
Ph
2Th
complexes FeCl2N2 and FeCl2N2
nor the square-planar
diamagnetic d8 NiII complexes NiCl2N2Ph and NiCl2N22Th (Chart
2) have been found to catalyze the oligomerization/polymeri-
zation of ethylene on treatment with MAO under the experi-
mental conditions of Table 1.3
Incorporation of these data with the experimental and
theoretical pieces of evidence suggests that the precursors
CoCl2N22Th, CoCl2N22BT, CoCl2N2Py, and CoCl2N2
are
PyMe
transformed by MAO in toluene into square-planar CoII
complexes where a coordination position is occupied by sulfur
or nitrogen (Scheme 9).
It is worth mentioning that ortho metalation, the potential
fate of any C-H-metal agostic interaction, is a feasible reaction
opportunity for the ligand N2Ph, provided there is a suitable metal
fragment (Scheme 7).38
Once the square-planar methyl complex cations are formed,
they will have to bind a cis ethylene molecule to undergo methyl
migratory insertion and produce R-olefins by successive ethylene
trapping/migratory insertion cycles, followed eventually by â-H
For all complexes, the calculated total spin density is mainly
transfer termination. The lack of any activity of CoCl2N2Py and
located on cobalt (the value of the spin density for the cobalt
PyMe
CoCl2N2
may be therefore ascribed to the strong bonding
interaction between cobalt and the nitrogen atom of the
Ph +
atom is between 1.17 for [Co(CH3)N2
] and 1.23 for [Co-
(CH3)N2Py]+; see the Supporting Information), while the methyl
bonded to the metal has a negative spin density of about 0.2.
(39) Bianchini, C.; Giambastiani, G.; Guerrero Rios, I.; Mantovani, G.;
Meli, A.; Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391.
(40) Daugulis, O.; Brookhart, M.; White, P. S. Organometallics 2003,
22, 4699.
(38) Bianchini, C.; Lenoble, G.; Oberhauser, W.; Parisel, S.; Zanobini,
F. Eur. J. Inorg. Chem. 2005, 4794

738 Organometallics, Vol. 26, No. 3, 2007
Bianchini et al.
Scheme 10
Scheme 11
peripheral pyridine ring, which does not allow for monomer
activation.
Apparently, the presence of a thien-2-yl sulfur in the
propagating alkyl species is not an obstacle to monomer
coordination, as both CoCl2N22Th and CoCl2N22BT are very active
for ethylene oligomerization, yielding C4-C14 R-olefins. In a
sense, the role of the sulfur atom in the thien-2-yl CoII precursors
may be similar to that proposed for the sulfur in the thioether
CpCoIII complex shown in Scheme 8. Just the production of
C4-C14 R-olefins with a Schulz-Flory distribution indicates
that the thien-2-yl sulfur competes with ethylene for coordination
to cobalt, retarding chain transfer vs chain propagation. After
only six to seven consecutive insertions does the chain transfer
rate definitely prevail over the propagation rate, likely due to
steric reasons. The square-planar structure of the propagating
alkyl, its character of catalyst resting state, and the occurrence
of olefin isomerization are strongly reminiscent of the catalytic
mechanism of diamagnetic R-diimine NiII and PdII complexes
in which the termination proceeds via associative displacement,
irrespective of the chain transfer mechanism (â-H transfer to
metal or monomer) (Scheme 10).4c,41 In this picture, the role of
the thien-2-yl sulfur atom would be that of disfavoring ethylene
coordination, destabilizing the transition state to R-olefin
elimination.
Conclusions
The position of the sulfur atom in the thienyl group of
6-(thienyl)-2-(imino)pyridine ligands has been found to control
both the catalytic activity and selectivity of the corresponding
tetrahedral dihalide CoII complexes in the oligomerization of
ethylene to R-olefins upon activation with MAO. Complexes
with the sulfur atom in the 3-position of the thienyl ring catalyze
the selective conversion of ethylene to 1-butene, while catalysts
containing thien-2-yl groups give C4-C14 R-olefins. Given the
comparable sizes of the thiophen-3-yl and benzo[b]thiophen-
3-yl groups as compared to thiophen-2-yl and benzo[b]thiophen-
2-yl groups, steric factors cannot account for the remarkably
different selectivity of the corresponding catalysts.
In situ EPR experiments have shown that all of the high-
spin CoII precursors undergo a spin state changeover with
formation of low-spin CoII propagating alkyls of the formula
[CoN2(alkyl)]+. DFT calculations on the latter compounds are
consistent with the coordination of the sulfur atom only for the
thien-2-yl derivatives. The fastening/unfastening of the sulfur
atom in the thien-2-yl complexes is suggested, on one hand, to
stabilize the electron-deficient transition state for migratory
insertion and, on the other hand, to retard chain transfer by
destabilizing the transition state to R-olefin elimination. The
results of the DFT study of the [CoN2(alkyl)]+ complexes
bearing thien-3-yl and phenyl substituents are much less
conclusive, as the stabilizing energy of possible C-H agostic
species is too low (6-9 kcal/mol) to draw out a reliable
mechanistic rationale for the observed selectivity. On the other
hand, the presence of an aromatic group in the 6-position of
the central pyridine ring is mandatory for ethylene oligomer-
The energy barriers to rotation of the phenyl and thien-3-yl
rings calculated for the square-planar methyl complexes [Co-
(CH3)N2Ph]+ and [Co(CH3)N23Th]+, although in line with agostic
interactions, are too low (6 and 9 kcal mol-1, respectively) to
be taken as unambiguous proof for propagating alkyls with the
structure shown in Scheme 11. Indeed, under real catalytic
conditions, ethylene, toluene, and even [MAO]- might compete
with the agostic interaction.
Ph
3Th
It is a fact, however, that both CoCl2N2 and CoCl2N2
are as active as the thien-2-yl substituted catalysts but are
selective for 1-butene production, which is consistent with a
much faster â-H transfer from the butenyl group as compared
to its migratory insertion into the monomer. Since the termina-
tion occurs by associative displacement of ethylene (the chain
transfer rate is first order in ethylene concentration),1,2 it is very
likely that the phenyl and thien-3-yl groups in the 6-position
are sterically and electronically appropriate to stabilize the
transition state to R-olefin elimination just after the second
monomer insertion.
(41) (a) Killian, C. M.; Johnson, L. K.; Brookhart, M. J. Am. Chem.
Soc. 1995, 117, 6414. (b) Killian, C. M.; Johnson, L. K.; Brookhart, M.
Organometallics 1997, 16, 2005. (c) Svejda, S. A.; Brookhart, M.
Organometallics 1999, 18, 65.

Cobalt(II) 6-(Thienyl)-2-(imino)pyridine Catalysis
Organometallics, Vol. 26, No. 3, 2007 739
ization, as (imino)pyridine ligands with no substituent in this
position are catalytically inactive. Likewise, a strong donor atom
in the 6-position, as in the 2-pyridyl substituted ligands, inhibits
the catalytic activity.
support of Ente Cassa Risparmio di Firenze and of Minis-
tero dell’Istruzione, dell’Universita` e della Ricerca (PRIN
2005).
Supporting Information Available: Figures giving powder
2Th
EPR spectra of CoCl2N2Br, CoCl2N23Th, and CoCl2N2 at 10 K
Acknowledgment. Thanks are due to the European Com-
munity (STREP Project No. 516972-NANOHYBRID; Network
of Excellence IDECAT) and the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (FIRB Project No. RBNE03R78E-
NANOPACK) for financial support. The computations were
carried out at the CINECA of Bologna under the agreement
CINECA-CNR. L.S. and D.G. acknowledge the financial
and tables giving Cartesian coordinates and R spin densities of the
model cations [Co(CH3)N22Th]+, [Co(CH3)N23Th]+, [Co(CH3)N2Py]+,
and [Co(CH3)N2Ph]+. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM0609665