Oligomerisation of ethylene to linear α-olefins by new Cs- and C1-symmetric [2,6-bis(imino)pyridyl]iron and -cobalt dichloride complexes

Article abstract of DOI:10.1002/ejic.200390213

A new family of C_s- and C₁-symmetric 2,6-bis(imino)pyridyl ligands has been synthesised. These ligands, which differ from their known counterparts in the contemporaneous presence of alkyl and aryl substituent at the ketimine nitrogen atoms, form stable, five-coordinate complexes with FeCl₂ and COC12. All ligands and catalysts have been fully characterised both in the solid state and in solution. A single-crystal X-ray analysis of [CoCl₂-{2-[CMe=N-(2,6- dimethylphenyl)]6-(CMe=N-C₆H11)C₅H₃ N}]·H₂O shows the metal centre to adopt a highly distorted square-pyramidal geometry. In combination with methylaluminoxane (MAO), both iron and cobalt compounds act as effective and selective (> 99%) catalysts for the oligomerisation of ethylene to a Schulz-Flory distribution of α-olefins, with the α factor ranging from 0.61 to 0.91. The iron catalysts exhibit TOFs as high as 7.10⁵ mol C₂H₄ (mol of cat × h)^-1 and are much more active than their cobalt analogues. Experiments at different ethylene pressures with the iron catalysts showed that both the propagation rate and the chain transfer rate are first order in ethylene pressure. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200390213

FULL PAPER
Oligomerisation of Ethylene to Linear α-Olefins by new Cs- and C1-Symmetric
[2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
Claudio Bianchini,*[a] Giuseppe Mantovani,[a] Andrea Meli,[a] Fabio Migliacci,[a]
Fabrizio Zanobini,[a] Franco Laschi,[b] and Anna Sommazzi[c]
Keywords: Cobalt / Ethylene / Iron / N ligands / Oligomerisation
A new family of Cs- and C1-symmetric 2,6-bis(imino)pyridyl
ligands has been synthesised. These ligands, which differ
from their known counterparts in the contemporaneous pres-
ence of alkyl and aryl substituent at the ketimine nitrogen
atoms, form stable, five-coordinate complexes with FeCl2
balt compounds act as effective and selective (Ͼ 99%) cata-
lysts for the oligomerisation of ethylene to a Schulz−Flory dis-
tribution of α-olefins, with the α factor ranging from 0.61 to
0.91. The iron catalysts exhibit TOFs as high as 7·105 mol
C2H4 (mol of cat × h)−1 and are much more active than their
and CoCl2. All ligands and catalysts have been fully charac- cobalt analogues. Experiments at different ethylene pres-
terised both in the solid state and in solution. A single-crystal
X-ray analysis of [CoCl2{2-[CMe=N−(2,6-dimethylphenyl)]-
6-(CMe=N−C6H11)C5H3N}]·H2O shows the metal centre to
adopt a highly distorted square-pyramidal geometry. In com-
bination with methylaluminoxane (MAO), both iron and co-
sures with the iron catalysts showed that both the propaga-
tion rate and the chain transfer rate are first order in ethyl-
ene pressure.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2003)
Introduction
metallocenes, constrained-geometry early transition metal
complexes) are multiple, ranging from their ease of prep-
aration and handling to the use of low-cost metals with
negligible environmental impact. Another intriguing feature
of [bis(arylimino)pyridyl]iron and -cobalt precursors is pro-
vided by the facile tuneability of their polymerisation ac-
tivity by simple modification of the ligand architecture. In-
deed, previous studies have shown that the size and regioch-
emistry of the substituents in the iminoaryl groups are of
crucial importance in controlling the polymerisation and
oligomerisation of ethylene, and also the isospecific polym-
erisation of propene.[1Ϫ3] In particular, the presence of sub-
stituents in both ortho positions on each aryl group results
in the formation of high molecular weight PE with molecu-
lar weights depending upon the size of these substituents.[1]
In contrast, the presence of one ortho substituent generates
catalysts that selectively oligomerise ethylene to
ShultzϪFlory distributions of α-olefins.[2] Experimental
and theoretical studies agree in identifying restricted ro-
tation around the two nitrogenϪaryl bonds as the key fac-
tor responsible for disfavouring chain transfer over chain
propagation.[1,3c,4] Only when the aryl groups are very large
(e.g., naphthyl, pyrenyl, 2-benzylphenyl) does restricted ro-
tation apparently operate without the need for two ortho
substituents, PE being formed.[5]
In 1998, Brookhart and Gibson independently discovered
that [2,6-bis(arylimino)pyridyl]iron and -cobalt dihalides,
when activated by MAO, are effective catalysts for the pol-
ymerisation of ethylene to high-density polyethylene (PE)
with productivities as high as those of the most efficient
metallocenes (Scheme 1).[1]
Scheme 1
The advantages of these late transition metal systems
over other types of single-site ZieglerϪNatta catalysts (e.g.,
[a]
Istituto di Chimica dei Composti Organometallici (ICCOM)
Ϫ CNR,
Via J. Nardi 39, 50132 Firenze, Italy
Fax: (internat.) ϩ 39-055/2478366
E-Mail: bianchin@fi.cnr.it
[b]
`
Dipartimento di Chimica, Universita di Siena
In this paper we demonstrate that the presence of two
arylimino groups in the bis(imino)pyridyl backbone is not
a mandatory condition to generate a ligand capable of olig-
omerising ethylene to α-olefins in conjunction with FeCl2/
Via Aldo Moro, 53100 Siena, Italy
Polimeri Europa, Novara Research Centre, Istituto Guido
Donegani,
Via Fauser 4, 28100 Novara, Italy
Fax: (internat.) ϩ 39-0321/447241
[c]
1620  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/03/0408Ϫ1620 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
FULL PAPER
Scheme 2
MAO or CoCl2/MAO. In fact, an aryl group can be re- ultaneous presence of 2,6-disubstituted aryl and chiral alkyl
placed by an alkyl group and the resultant [2-(alkylimino)- groups at the imine nitrogen atoms.
6-(arylimino)pyridyl]FeII or -CoII dichloride complexes will
oligomerise ethylene as efficiently and selectively as the 2,6-
bis(arylimino)pyridyl congeners. It is also shown here that
the size of the alkyl group can control both the catalyst
productivity and the SchulzϪFlory parameters.
Results
Scheme 3
Synthesis and Characterisation of Ligands
All complexes are green, crystalline solids and are fairly
The 2-(alkylimino)-6-(arylimino)pyridyl ligands 3Ϫ6
soluble in polar organic solvents such as THF, dichloro-
have been synthesised via keto-imine intermediates ob-
methane, and 1,2-dichloroethane, in which they behave as
tained by treatment of 2,6-diacetylpyridine with the appro-
non-electrolytes. Their solubilities in toluene and other hy-
priate aniline (0.9 equiv.) in MeOH with formic acid as pro-
drocarbons are very low. All compounds are reasonably
moter (Scheme 2).
thermally stable and air-stable both in the solid state and
An important contribution to the selectivity of this reac-
in solution. For caution’s sake, however, their preparation
tion was made by the low solubility of the monoimine prod-
and manipulation in solution were carried out under dry
ucts, which precipitated upon formation and so suppressed
N2.
further condensation with the aniline. The bis(imino)pyrid-
Magnetic data at room temperature and relevant IR and
ine products were indeed obtained in low amounts (2Ϫ4%),
UV/Vis absorptions for each complex are reported in the
however, and so the crude reaction products were purified
Exp. Sect. The IR spectra of all the compounds each show
by repeated extraction of the (imino)pyridyl ketones with
a red shift of ν(CϭN) by ca. 50Ϫ60 cmϪ1 relative to the
boiling ethanol. Once purified, these products were treated
corresponding free ligand, which reflects the coordination
with the appropriate neat primary amine at 100Ϫ110 °C, in
of the imine nitrogen atoms to the cobalt atom.
the presence neither of solvent nor of acid promoter, to give
All the complexes are high-spin in the solid state, with
the expected 2,6-bis(imine)pyridyl ligands. Under these mild
µeff at room temperature ranging from 4.6 to 4.8 BM, which
conditions, the bis(imino) compounds were obtained selec-
is typical for high-spin cobalt() five-coordinate complexes
tively.
with 2,6-bis(imino)pyridyl ligands.[6] In principle, five-coor-
dinate metal complexes may adopt either square-pyramidal
or trigonal-bipyramidal geometries. In idealised C4v and
Synthesis and Characterisation of the Cobalt(II) Complexes
D3h symmetries, a d7 metal ion may have either a low- or a
The [2,6-bis(imino)pyridyl]CoII complexes illustrated in high-spin configuration, according to whether the energy
Scheme 3 were prepared in acceptable yields by heating of separation between the two highest orbitals in each sym-
an nBuOH solution containing equimolar amounts of an- metry is higher or lower than the spin-pairing energy. A
hydrous CoCl2 and ligand at reflux for ca. 10 min. Crystal- donor atom set consisting of three nitrogen and two chlor-
line compounds were obtained upon cooling to room tem- ine atoms has invariably been found to stabilise the high-
perature. Complexes 7 and 9 exhibit Cs symmetry, while spin configuration of cobalt().[6] As shown by a single-
complexes 8 and 10 are C1-symmetric by virtue of the sim- crystal X-ray analysis on the cyclohexyl/2,6-dimethylphenyl
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631
1621

C. Bianchini et al.
FULL PAPER
derivative 9·H2O (see below), the presence of the rigid chel- atoms vary, these are not sufficient for them to be due to
ating 2,6-bis(imino)pyridyl ligand causes important distor- substantial differences in structure. Such differences are
tions from the idealised geometries. However, these are ap- probably due to the different steric bulks of the substituents
parently not so significant as to favour spin pairing and in the complexes.
give a doublet ground state (S ϭ 1/2).
The presence of three unpaired electrons (S ϭ 3/2) in
An ORTEP drawing of the molecular structure of 9·H2O each complex molecule makes all the CoII compounds X-
is shown in Figure 1. Perusal of the bond angles around the band-EPR-silent at room temperature both in the solid
metal centre shows that the coordination geometry in state and in CH2Cl2 solution. A low-temperature EPR
9·H2O is greatly distorted from any idealised five-coordi- study of the X-band was carried out on derivative 8 in
nate structure, and may be roughly described as a square CH2Cl2 (Scheme 3). At 4 K the spectrum displays a broad
pyramid with one chloride ion (Cl1) occupying the apical and poorly resolved rhombic structure with g1 ϭ 5.06(8) Ͼ
position. The presence of two chemically different imine ni- g2 ϭ 3.03(8) Ͼ g3 ϭ 1.95(8) ϶ gelect [ϽgϾ ϭ 3.35(8); a1
trogen atoms in 9·H2O contributes substantially to the ob- 40(8) G, a2 80(8) G, a3 94(8) G; ϽaϾ 71(8) G], which is
served stereochemical distortions. Noticeable asymmetries typical of an ‘‘S’’ ϭ 1/2 fictitious spin (Figure 2). Indeed,
affect the CoϪN(imine) distances in fact: Co1ϪN1 is un- the spectrum may be interpreted in terms of an ‘‘S’’ ϭ 1/2
˚
usually short [2.185(6) A] and Co1ϪN2 is exceedingly long effective spin Hamiltonian occasioned by large Zero Field
[2.348(6) A]. These differences are most probably due to the Splitting (ZFS) effects.[7Ϫ11] In fact, we observed a revers-
˚
higher nucleophilicity of the alkyl-substituted nitrogen ible temperature dependence of the signal intensity, which
atom N1 relative to the aryl-substituted N2. Indeed, the rules out the occurrence of a spin transition. This behaviour
N1ϪCo1ϪN2 angle of 149.3(2)° allows one to invoke trans is not unusual for S Ͼ 1/2 paramagnetic metal species affec-
influence effects. In contrast, due to the substantial depar- ted by important ZFS effects (S ϭ 3/2) as well as noticeable
tures from idealised square-pyramidal geometry, these ef- spin-orbit coupling in highly distorted coordination geo-
fects cannot account for the CoϪCl distances, which (al- metries.[7,8,11] All gi regions show hyperfine structure appar-
though less pronouncedly) are significantly different from ently due to magnetic interaction with the 59Co nucleus
˚
one another [Co1ϪCl1 2.243(2); Co1ϪCl2 2.269(2) A]. As (S ϭ 7/2). However, the poor resolution and the presence
would be expected,[1dϪ1e][2b][3c] the MϪN(pyridyl) bond in each region of more than eight bands, as expected for
˚
[Co1ϪN3 2.032(6) A] is shorter than both the coupling to cobalt, do not rule out either interactions with
MϪN(imino) bonds, which have retained the imine CϭN other spin-active nuclei in the molecule (e.g., 14N) or the
˚
double bond character [N1ϪC1 1.266(10) A, N2ϪC3 presence of different geometric isomers.
˚
1.276(9) A]. The deviation of the metal from the N3 ligand
˚
plane is 0.06 A. The plane of the 2,6-dimethyl-substituted
aryl ring is oriented essentially orthogonally to the plane of
the backbone [86.5(2)°].
Figure 2. X-band EPR spectrum of cobalt() complexes 8 in
CH2Cl2 at 4 K
1
The H NMR spectrum of complex 7 was acquired in
oxygen-free CD2Cl2 solution at 21 °C on a 500 MHz instru-
ment (Figure 3). All protons resonate at chemical shifts sig-
Figure 1. ORTEP drawing of 9·H2O (hydrogen atoms omitted);
nificantly different from those of the corresponding protons
in the free ligands, which is consistent with the paramag-
˚
selected bond lengths [A] and angles [°]: Co1ϪN1 2.185(6),
Co1ϪN2 2.348(6), Co1ϪN3 2.032(6), Co1Cl1 2.243(2), Co1ϪCl2
2.269(2); N3ϪCo1ϪN1 76.7(2), N3ϪCo1ϪCl1 131.87(18),
1
netic nature of the compound. Analysis of the H NMR
N1ϪCo1ϪCl1
99.91(18),
N3ϪCo1ϪCl2
107.58(18),
resonances suggests that the isotropic shift (i.e., the differ-
ence between the observed chemical shift and the reference
diamagnetic shift) is almost completely attributable to a
Fermi contact contribution.[12] Unambiguous signal assign-
N1ϪCo1ϪCl2 99.80(17), Cl1ϪCo1ϪCl2 120.12(9), N3ϪCo1ϪN2
73.4(2), N1ϪCo1ϪN2 149.3(2), Cl1ϪCo1ϪN1 94.88(15),
Cl2ϪCo1ϪN2 95.72(16)
The reflectance and solution UV/Vis spectra of the [2,6- ment was achieved on this basis (Table 1). Overall, the
bis(imino)pyridyl]CoII complexes are similar to one an- NMR spectrum is similar to that reported by Gibson and
other, indicating that the primary stereochemistry is the co-workers for {2,6-bis[1-(2,6-diisopropylphenylimino)-
same both in the solid state and in solution: intermediate ethyl]pyridine}cobalt dichloride.[1d] As in the spectrum of
between the square pyramid and the trigonal bipyramid.[6b] this more symmetric complex, the 3-, 4-, and 5-hydrogen
Although the spectra show some changes in band shape atoms of the central pyridine ring and the CHMe2 hydrogen
and frequency as the substituents at the imine nitrogen atom exhibit remarkable paramagnetic shifts and line
1622
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
FULL PAPER
broadening as well. The strong coordination of the cyclo-
hexyl-substituted imine arm to the cobalt atom, given in
evidence in the X-ray structure, is confirmed by the large
downfield shift (δ ϭ 143.42 ppm) of the NCH resonance as
well as its broad linewidth. Similarly, the coordination of
both imine nitrogen atoms is demonstrated by the chemical
shifts of the NϭCMe methyl groups. The presence of two
signals for the CHMe protons is consistent with hindered
rotation of the aryl ring about the NϪC axis.[1d]
1
Figure 3. H NMR spectrum of 7 (500.13 MHz, 21 °C, CD2Cl2)
Table 1. 1H NMR assignments for 7 (CD2Cl2, 294 K, 500.13 MHz)
Figure 4. Cyclic voltammograms of the cobalt() and iron() com-
plexes 9 (a) and 13 (b) in CH2Cl2 [6·10Ϫ4 , scan rate 0.2 V·sϪ1
Bu4NPF6 (0.2  as supporting electrolyte)]
;
Nucleus
δ [ppm]
3/5-Py
4-Py
108.18 (s, 1 H); 103.73 (s, 1 H)
25.92 (s, 1 H)
NϭCCH3
m-aryl
p-aryl
Ϫ4.63 (s, 3 H); Ϫ15.41 (s, 3 H)
1.15 (s, 2 H)
Ϫ12.69 (s, 1 H)
CH3 of iPr
CH of iPr
Cy
Ϫ12.14 (br. s, 6 H); Ϫ17.81 (s, 6 H)
Ϫ73.80 (br. s, 2 H)
143.42 (s, 1 H), 17.63 (s, 1 H), 4.1 (br. s,
2 H), 0.25 (s, 1 H), Ϫ3.72 (s, 2 H), Ϫ14.75
(s, 2 H), Ϫ71.05 (s, 2 H)
Scheme 4
A cyclic voltammetry study in CH2Cl2 was performed on
complex 9, containing cyclohexyl and 2,6-dimethylphenyl
groups on the imine nitrogen atoms (Figure 4a). The com-
plex undergoes one irreversible one-electron oxidation at
Ep ϭ ϩ1.18 V and another irreversible one-electron re-
duction at Ep ϭ Ϫ0.99 V even at very high scan rates.
All compounds are only sparingly soluble in aromatic hy-
drocarbons, while dissolving fairly well in THF, MeCN,
CH2Cl2, and 1,2-dichloroethane. In the last of these they
behave as non-electrolytes. The solids are fairly air-stable,
whereas they decompose in solution unless protected by di-
nitrogen or argon.
Synthesis and Characterisation of the Iron(II) Complexes
Like the cobalt() derivatives, the iron() complexes
Magnetic data at room temperature and relevant IR ab-
could be conveniently prepared by heating of a solution sorptions for the [2,6-bis(imino)pyridyl]FeII compounds are
containing anhydrous FeCl2 and 1 equiv. of the ligand at provided in the Exp. Sect. As in the IR spectra of the CoII
reflux in nBuOH for ca. 10 min (Scheme 4). Crystalline derivatives, ν(CϭN) is generally red-shifted by ca. 50Ϫ60
compounds were obtained upon cooling to room tempera- cmϪ1 relative to the corresponding free ligand, reflecting
ture.
the coordination to the metal centre.
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631
1623

C. Bianchini et al.
FULL PAPER
All the compounds are dark blue and exhibit high-spin bis(imino)pyridyl]FeII complexes and also attributed to
electronic configurations, as shown by their magnetic mo- MLCT.[1d] In general, MLCT bands fall at higher energy
ments µeff, which range from 5.5 to 5.7 BM and are similar than those observed by Gibson, but a reduction in fre-
to those reported for several high-spin iron() five-coordi- quency may occur for highly π-conjugated ligand systems,
nate complexes with quintuplet ground states.[1,6] The quin- which may be the case for C2v-symmetric 2,6-bis(imino)pyr-
tuplet ground state makes the complexes EPR-silent even idyl ligands.[6b]
at 4 K and irrespective of the EPR frequency.
The reflectance spectra of 11Ϫ14 are similar to one an-
Like the [2,6-bis(imino)pyridyl]CoII complexes, the FeII other and fully comparable with the solution spectra, which
derivatives might adopt either square-pyramidal or trig- indicates that the primary stereochemistry of the FeII com-
onal-bipyramidal structures with various degrees of geo- plexes is the same both in the solid state and in solution.
metrical distortion. In either geometry, a d6 metal ion may As an example, the spectra of 12 in the solid state and in
assume a singlet, a triplet or a quintuplet ground state, de- CH2Cl2 solution are given in Figure 5.
pending on the nature of the donor atom set.[14] A donor
atom set comprising three nitrogen and two chlorine atoms
generally stabilises the high-spin quintuplet configuration
for iron(), as is in fact observed for these bis(imino)pyri-
dyl complexes.[6,13]
None of the [2,6-bis(imino)pyridyl]FeII complexes de-
scribed in this work has been studied by X-ray diffraction
methods, but the molecular structures of several FeII deriva-
tives bearing aryl substituents on the imine nitrogen atoms
are available from the literature.[1a,1d,2,3c] In most cases the
geometry at the iron centre can be described as distorted
trigonal-bipyramidal with the pyridyl nitrogen atom and
the two chloride ligands forming the equatorial plane; only
when the aryl rings on the imine nitrogen atoms bear two
bulky substituents Ϫ as in {2,6-[2,6-(iPr)2C6H3NCMe]2-
C5H3N}iron dichloride Ϫ does iron tend to adopt a dis-
torted square-pyramidal coordination geometry.[1c]
The [2,6-bis(imino)pyridyl]FeII complexes studied here
exhibit either C1 or Cs symmetry by virtue of the simul-
taneous presence of substituted aryl groups and either chi-
ral [CH(Me)Ph] or cyclic (C6H11) alkyl groups at the imine
Figure 5. Reflectance spectrum of 12 (a); electronic spectrum of 12
nitrogen atoms. The combined action of the ketimine group
and the two o-alkyl substituents should lock the aryl ring
in an orthogonal conformation with respect to the N3 li-
gand plane both in the solid state and in solution, while the
(b) in CH2Cl2 solution
1
The H NMR spectrum of 11 was acquired in deaerated
alkyl group at the other imine nitrogen atom should rotate CD2Cl2 solutions at 21 °C (Figure 6). All protons resonate
freely about the CϪN axis in solution (vide infra). at chemical shifts significantly different from the corre-
Trigonal-bipyramidal structures for all these iron() com- sponding protons in the free ligands, which is consistent
plexes are supported by the UV/Vis spectra in CH2Cl2 solu- with the paramagnetic nature of the compounds. Band as-
tion. Some detailed studies of the electronic spectra of five- signments are given in Table 2.
coordinate FeII complexes with N3Cl2 donor atom sets were
Despite the presence of the same ligand, the H NMR
1
reported in the late 1960s, mainly by Ciampolini and co- spectrum of the iron() complex 11 is notably different from
workers.[14] On the basis of these studies, a band in the that of the cobalt() analogue 7 (Figure 3), which reflects a
7000Ϫ8400 cmϪ1 region, common to all FeII complexes in- structural divergence in solution. In particular, the reson-
vestigated in this work, can be unambiguously correlated ances of four hydrogen atoms of the cyclohexyl group are
with a spin-allowed transition in trigonal-bipyramidal high- shifted to high field (ca. Ϫ30 and Ϫ37 ppm) and have very
spin FeII complexes such as [Fe(dienMe)Cl2] or [Fe(tren- broad linewidths, which suggests that this cyclohexyl ring is
Me)Cl]Cl [dienMe ϭ bis(2-dimethylaminoethyl)methylam- spatially very close to the paramagnetic iron centre on the
ine; trenMe ϭ tris(2-dimethylaminoethyl)amine].[15] Those NMR timescale. No such linewidth broadening for the
complexes also each showed a band at ca. 4000 cmϪ1, not cyclohexyl resonances was observed for the CoII analogue
detectable for our compounds due to the measuring limit 7. The presence of two relatively narrow signals for the iPr
of our spectrophotometer. Unlike the trenMe and dienMe groups confirms the hindered rotation of the aryl group.
complexes, the spectra of 11Ϫ14 each contain a band at ca.
Unlike the cobalt analogue 9, the iron complex 13 can be
20000 cmϪ1 that may be attributable to a metal-to-ligand reversibly oxidised (E0Ј ϭ ϩ0.43 V) to give the yellow
charge transfer (MLCT). Absorption bands at ca. 14500 iron() derivative [FeCl2{2-[CMeϭN(2,6-dimethylphenyl)]-
cmϪ1 have been reported by Gibson for C2v-symmetric [2,6- 6-(CMeϭNC6H11)C5H3N}]ϩ, which is stable in CH2Cl2
1624
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
FULL PAPER
activities [mol of C2H4 converted (mol of M ϫ h)Ϫ1] were
calculated from the GC readings, by use of extrapolated
values for 1-butene.
Table 3. Ethylene oligomerisation with the iron() and cobalt()
catalysts
Entry[a] Pre-catalyst Pressure [bar] TOF [10Ϫ5 [b]
]
α
β
1
2
3
4
5
6
11
12
13
14
13
13
13
13
7
4.1
4.1
4.1
4.1
4.8
7.6
10.3
4.1
4.1
4.1
4.1
4.1
2.4
5.2
3.8
5.6
4.6
6.0
7.4
3.4
0.74 0.35
0.89 0.12
0.66 0.52
0.91 0.10
0.67 0.49
0.67 0.49
0.68 0.47
0.61 0.63
7
8[c]
9
Ͻ 0.01[d]
0.61
Ͻ 0.01[d]
1.0
Ϫ
Ϫ
10
11
12
8
0.89 0.12
1
Figure 6. H NMR spectrum of 11 (500.13 MHz, 21 °C, CD2Cl2)
9
10
Ϫ
Ϫ
0.91 0.10
[a]
Reaction conditions: complex (12 µmol), toluene (100 mL),
Table 2. 1H NMR assignments for 11 (CD2Cl2, 294 K,
500.13 MHz)
[b]
MAO (250 equiv.), 15 min, 25 °C. TOF: mol of C2H4 converted
(mol of M ϫ h)Ϫ1
.
80 °C. Only butenes.
[c]
[d]
Nucleus
δ [ppm]
The four iron complexes 11Ϫ14 were all active catalysts
for the production of α-olefins with SchulzϪFlory distri-
butions.[17] No appreciable formation either of internal olef-
ins or of odd carbon oligomers was observed (Figure 7).
3/5-Py
4-Py
NϭCCH3
m-aryl
89.11 (s, 1 H); 68.55 (s, 1 H)
35.97 (s, 1 H)
12.79 (s, 3 H); Ϫ33.57 (s, 3 H)
20.69 (s, 2 H)
p-aryl
Ϫ1.70 (s, 1 H)
3.08 (br. s, 6 H); Ϫ5.38 (s, 6 H)
Ϫ14.94 (br. s, 2 H)
Ϫ2.99 (s, 2 H), Ϫ7.85 (s, 2 H), Ϫ29.10 (s, 2 H),
Ϫ37.18 (br. s, 2 H), 1.28 (s, 1 H), Ϫ4.72 (s, 1 H),
Ϫ6.74 (s, 1 H)
CH3 of iPr
CH of iPr
Cy
solution, although no attempt was made to isolate it in the
solid state (Figure 4b). Stable C2-symmetric [2,6-bis(arylim-
ino)pyridyl]FeIII dihalide complexes have been prepared by
Gibson and shown to have the same activity as the FeII
precursors in the oligomerisation of ethylene, as a conse-
quence of instantaneous FeIII reduction to FeII by
MAO.[1d,16] Figure 4b also shows that the FeI derivative, ob-
tained by a partially reversible one-electron reduction of 13
(at E0Ј ϭ Ϫ1.15 V), has a relatively short lifetime (t1/2
ϭ
0.5 s).
Oligomerisation of Ethylene by the Cobalt(II) and Iron(II
)
Dichloride Complexes, Activated by MAO
The [bis(imino)pyridyl]cobalt (7Ϫ10) and -iron (11Ϫ14)
complexes, in combination with the activator MAO, were Figure 7. GC trace relative to Entry 1 in Table 3, showing the α-
olefins distribution from C10 to C26
evaluated as catalysts for the oligomerisation of ethylene
in toluene.
Table 3 reports on the results of reactions carried out un-
The catalytic activity increased with the bulkiness of the
der a variety of experimental conditions. In all cases the alkyl substituent at the imine carbon atom (Entries 1, 2, 4),
addition of ethylene to the catalyst/co-catalyst mixture re- but decreased with the size of the substituents on the aryl
sulted in a rapid exothermic event, indicative of no induc- ring (Entries 1, 3). The naphthyl group containing catalyst
tion period. Initially, all catalysts were tested at 4.1 bar was as active as the C2-symmetric [2,6-bis(arylimino)pyridy-
C2H4 pressure and 25 °C for 15 min, with 250 equiv. of l]iron() precursors described by Brookhart and Gibson.[2]
MAO as both co-catalyst and reactor scavenger. Catalyst Notably, the probability of propagation, indicated by the
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631
1625

C. Bianchini et al.
FULL PAPER
SchulzϪFlory parameter α,[17] increased steadily with the Discussion
overall steric hindrance of the imine substituents.
Experiments with precursor 13 in the C2H4 pressure
The nature and statistical distribution of the α-olefins
range from 4.1 to 10.3 bar showed a linear dependence of produced with the [2-(alkylimino)-6-(arylimino)pyridyl]iron
activity on the pressure, while the α factor did not vary and -cobalt catalysts described in this work are consistent
substantially (Entries 3, 5Ϫ7). No isomeric oligomer with a mechanism involving: (i) metalϪalkyl initiators pro-
formed either by alkene isomerisation or by reincorporation duced by the action of MAO on the dichloride precursors,
of α-olefin products into oligomers later in the reaction was (ii) propagation by migratory insertion of Co(alkyl)(ethy-
observed. The absence of reincorporation mechanisms was lene) (CosseeϪArlman mechanism), and (iii) termination
confirmed by independent experiments in which the olig- by chain transfer (Scheme 5).[2,19]
omerisation of ethylene with 13 was carried out in the pres-
In line with the large majority of catalytic systems for
ence of an excess of 1-undecene. In fact, no odd-numbered SchulzϪFlory oligomerisations of ethylene, the rates of
carbon oligomer was formed in the temperature range from propagation and chain transfer exhibited by the iron cata-
25 to 80 °C.[2a] Similarly, these catalysts did not promote lysts do not differ significantly, the β parameter (β ϭ
head-to-head dimerisation of α-olefins to internal olefins, a rtransfer/rpropagation) varying between 0.10 and 0.52. It has
reaction that [2,6-bis(arylimino)pyridyl]iron complexes are been found that the activity of the iron catalysts shows a
able to catalyse.[18]
linear dependence on the ethylene pressure, while α is inde-
Experiments at relatively high temperature (80 °C) with pendent of the pressure. Accordingly, both chain propa-
precursor 13 showed a slight decrease in productivity and gation and chain transfer are first-order in monomer con-
α factor as well. Both phenomena have precedent for [2,6- centration, as previously observed for [2,6-bis(arylimino)py-
bis(arylimino)pyridyl]iron precursors.[1d] In particular, the ridyl]FeII and -CoII oligomerisation catalysts.[2]
decrease in productivity with temperature has been ascribed
to the combined effects of faster catalyst degradation and fer in ethylene polymerisation by three main mechanisms:
lower ethylene solubility.[1d]
β-hydrogen transfer to the metal atom (A), β-hydrogen
Late transition metal catalysts may undergo chain trans-
The activities of the cobalt catalysts 7Ϫ10 (Entries 9Ϫ12) transfer to the monomer (B), and chain transfer to alu-
were approximately one order of magnitude lower than minium (C) (this generally occurs at low pressure)
those of the analogous iron() derivatives. The sensitivity (Scheme 5).[1d,20] The last mechanism can safely be ruled
to steric effects was even higher for cobalt than for iron, as out in the oligomerisation reactions reported here, as only
shown by the catalyst precursors containing the cyclohexyl α-olefins were produced, with no saturated hydrocarbon. It
substituent (Entries 9, 11), which gave only small amounts is therefore very likely that chain transfer proceeds either
of 1-butene, indicating a prevalence of chain transfer over by path A, involving an associative displacement step fol-
chain propagation.
lowed by β-hydrogen transfer to the metal atom, or by path
Scheme 5. Chain propagation and chain transfer mechanisms proposed for polymerisation of ethylene by late transition metal catalysts
1626 Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
FULL PAPER
B, involving direct β-H transfer to the incoming monomer Conclusion
with no formation of metal hydride. The exclusive pro-
C1- and Cs-symmetric 2-(alkylimino)-6-(arylimino)pyri-
dyl ligands, readily prepared by condensation of chiral and
achiral primary amines with 2-(acetyl)-6-(arylimino)pyrid-
ine intermediates, coordinate to iron() and cobalt() di-
chlorides to give monomeric, paramagnetic complexes. The
cobalt complexes adopt distorted square-pyramidal struc-
tures both in the solid state and in solution, while the coor-
dination geometries of the iron catalysts can be better de-
scribed as trigonal-bipyramidal. All the complexes can be
activated with MAO to give ethylene oligomerisation cata-
lysts with complete selectivity for α-olefins. Unlike [2,6-bi-
s(arylimino)pyridyl]iron precursors, no head-to-head di-
merisation of α-olefins to internal olefins was observed.
Iron-based catalysts can be more than one order of magni-
tude more active than cobalt-based systems. The catalytic
activity increases with the bulkiness of the alkyl substituent
and decreases with the size of the substituent on the aryl
ring. The SchulzϪFlory oligomer distribution factor α is
not sensitive to the metal centre, but it increases with the
size of the alkyl and aryl groups. Both chain propagation
and chain transfer are first order in ethylene concentration,
and chain transfer proceeds exclusively by β-H transfer.
duction of α-olefins with no appreciable formation of in-
ternal olefins might be taken as indirect evidence for the
occurrence of path B, as it does not involve the formation
of the metal hydride necessary for isomerisation. Moreover,
calculations suggest its involvement in ethylene polymeris-
ation by late transition metal systems.[20] We wish to stress,
however, that the chain transfer mechanisms A and B are
kinetically indistinguishable, and so no preference is al-
lowed; nor may one exclude their simultaneous occurrence.
Overall, the performances of these Cs- and C1-symmetric
iron and cobalt catalysts do not diverge substantially from
those of the 2,6-bis(arylimino) complexes studied by Brook-
hart and Gibson, which means that the presence of aryl
groups on both ketimine nitrogen atoms is not a necessary
condition to produce an ethylene oligomerisation catalyst
in conjunction with iron or cobalt dihalides and MAO. Ac-
tually, these catalysts polymerise ethylene only when the
aryl rings are 2,6-substituted by alkyl groups. As a general
trend, α-olefins are produced with one alkyl substituent,
while the presence of two alkyl groups results in the forma-
tion of PE with molecular weight increasing with the size
of these alkyl moieties.[1,2] It has been experimentally and
theoretically demonstrated that the steric interaction be-
tween the ketimine methyl group and the alkyl substituents
increases the barrier to aryl rotation and effectively locks
the rings in an orthogonal conformation with respect to the
N3M plane on the timescale of polymerisation.[1d][3c] The
steric protection provided by the orthogonal aryl rings con-
tributes to retard the chain transfer, which Ϫ irrespective of
the mechanism Ϫ requires access to the metal centre above
or below the N3M plane.[4] Apparently, one 2,6-substituted
aryl moiety is sufficient to slow down the chain-transfer rate
and allow oligomer formation. The high α value (0.91) ob-
tained with the naphthyl-substituted catalysts (Table 3, En-
tries 4 and 12) suggests that polymerisation to high molecu-
lar weight PE might be achieved by further increasing the
size of the CϭNϪalkyl group. Studies in this direction are
currently being carried out in our laboratory.
The thorough chemical-physical study of the catalyst pre-
cursors described in this work suggests that cobalt() pre-
fers to be square-pyramidal, while iron() prefers to be trig-
onal-bipyramidal. This behaviour of cobalt() and iron()
metal ions is amply recognised in the literature.[6a,21] With
no intention of minimising the importance of electronic ef-
fects related to the different d-electron count, it is possible
that these structural differences (with implications on the
metal spin state),[6a] if maintained during catalysis, may
contribute to rendering the cobalt catalysts less active than
the iron ones. Square-pyramidal coordination, for example,
generally produces less steric congestion at the metal centre
than trigonal-bipyramidal coordination and requires less
energy to attain the octahedral geometry that seems to fea-
ture in relevant transition states in ethylene polymerisation
catalysed by five-coordinate [2,6-bis(arylimino)pyridyl]FeII
and -CoII complexes.[4]
Experimental Section
General Procedures: All air- and water-sensitive reactions were per-
formed in flame-dried flasks under dinitrogen. Solid compounds
were collected on sintered glass frits and washed with appropriate
solvents before being dried in a stream of dinitrogen. Anhydrous
THF and Et2O were obtained by distillation from sodium/benzo-
phenone ketyl, while CH2Cl2 and MeOH were distilled from CaH2
and Mg, respectively. All the other reagents and solvents were used
as purchased from commercial suppliers. Analytical TLC was per-
formed on pre-coated silica gel 60 F254 and/or neutral Al2O3 N/
UV254 plates and developed in an appropriate solvent system. The
products were viewed with the aid of UV light (254 nm), I2 vapour,
2.5% (NH4)6Mo7O24 ϩ 1% (NH4)4Ce(SO4)4 in 10% H2SO4 and
warming. Deuterated solvents for NMR measurements were dried
with molecular sieves. 1H and 13C{1H} NMR spectra were ob-
tained with a Bruker ACP 200 instrument working at 200.13 and
50.32 MHz, respectively. All chemical shifts are reported in ppm
(δ) relative to tetramethylsilane, referenced to the chemical shifts
of residual solvent resonances (1H and 13C). The multiplicity of the
13C{1H} NMR spectra were determined by the DEPT 135 tech-
nique and are quoted as: CH3, CH2, CH, and C for primary, sec-
ondary, tertiary, and quaternary carbon atoms, respectively. Two-
dimensional 1H and 13C NMR spectra were recorded at 21 °C with
a Bruker Avance DRX 500 spectrometer operating at 500.132 and
125.77 MHz, respectively, and equipped with a variable-tempera-
ture control unit accurate to Ϯ0.1 °C. This instrument was also
employed to study the paramagnetic metal complexes. Chemical
shifts are relative to 0.03% (v/v) tetramethylsilane. The assignments
of the signals was achieved with the aid of 1D spectra, 2D 1H
1
DQF-COSY, 1H NOESY, and proton detected H,13C correlations
(HMQC and HMBC) using non-spinning samples. 2D NMR spec-
tra were recorded with pulse sequences suitable for phase-sensitive
1
representations by TPPI. H DQF-COSY experiments[22] were re-
corded with 1024 increments of size 2 K (with 16 scans each) cover-
1627
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

C. Bianchini et al.
FULL PAPER
ing the full range in both dimensions and with a relaxation delay in refluxing ethanol and the resulting mixture was filtered while
1
of 1.5 s. H NOESY measurements[23] were recorded with 1024 in-
crements of size 2 K (with 16 scans each) covering the full range in
both dimensions and with mixing times of 800 ms and a relaxation
still hot. The solvent was removed from the filtrate under reduced
pressure to give pure 1 as pale yellow crystals in 67% yield (2.17 g,
6.73 mmol). M.p. 182Ϫ184 °C. IR: ν(CϭN) 1648 cmϪ1, ν(CϭO) 1698
delay of 1.5 s. H,13C HMQC correlations[24] were recorded by use
cmϪ1. H NMR (200.13 MHz, CDCl3): δ ϭ 1.16 [d, J ϭ 6.9 Hz, 6
1
1
of the standard sequence with no decoupling during acquisition,
1024 increments of size 2 K (with 16 scans each) were collected,
covering the full range in both dimensions with a relaxation delay
H, CH(CH3)(CH3)], 1.17 [d, J ϭ 6.9 Hz, 6 H, CH(CH3)(CH3)],
2.28 [s,
3 H, C(NAr)CH3], 2.74 [sept, J ϭ 6.9 Hz, 2 H,
CH(CH3)(CH3)], 2.81 [s, 3 H, C(O)CH3], 7.08Ϫ7.22 (m, 3 H, CH
of 1 s. 1H,13C HMBC correlations[25] were recorded by the standard Ar), 7.96 (t, J ϭ 7.8 Hz, 1 H, CH Ar), 8.16 (dd, J ϭ 7.7, 1.2 Hz,
sequence with no decoupling during acquisition and a low-pass J-
filter to suppress one-bond correlations, 1024 increments of size
2 K (with 32 scans each) were collected, covering the full range in
1 H, CH Ar), 8.58 (dd, J ϭ 7.9, 1.2 Hz, 1 H, CH Ar) ppm. 13C{1H}
NMR (50.32 MHz, CDCl3): δ ϭ 17.7 [1 C, C(NAr)CH3], 23.5 [2 C,
CH(CH3)(CH3)], 23.9 [2 C, CH(CH3)(CH3)], 26.4 [1 C, C(O)CH3],
both dimensions, with a relaxation delay of 0.8 s. EPR spectra were 29.0 [2 C, CH(CH3)(CH3)], 123.2 (1 C, CH Ar), 123.7 (2 C, CH
recorded for all the compounds at the X-band frequency
(9.23 GHz) with a Varian ESR9 spectrometer equipped with a con-
tinuous-flow 4He cryostat to work at 4 K. Elemental analyses were
performed with a Carlo Erba Model 1106 elemental analyzer. UV/
Vis spectra were recorded with a PerkinϪElmer Lamda 9 spectro-
photometer. Infrared spectra were recorded with a PerkinϪElmer
1600 Series FT-IR spectrophotometer with samples mulled in Nu-
jol between KBr plates. GC analyses were performed with a Shim-
adzu GC-14 A gas chromatograph equipped with a flame ionis-
ation detector and a 30-m (0.25 mm i.d., 0.25 µm film thickness)
SPB-1 Supelco fused silica capillary column. GC/MS analyses were
performed with a Shimadzu QP 5000 apparatus equipped with a
column identical to that used for GC analysis. Molar susceptibility
analyses were performed with a Sherwood Scientific MSB AUTO
balance. Materials and apparatus for electrochemistry have been
described elsewhere.[26] Potential values are referred to the Satu-
rated Calomel Electrode (SCE). Under the adopted experimental
conditions the one-electron oxidation of ferrocene occurs at
E0Ј ϭ ϩ0.39 V.
Ar), 124.4 (1 C, CH Ar), 125.2 (1 C, CH Ar), 136.4 (2 C, C Ar),
138.4 (1 C, CH Ar), 142.9 (1 C, C Ar), 153.1 (1 C, CH Ar), 156.2
(1 C, C Ar), 167.8 [1 C, C(NAr)CH3], 200.8 [1 C, C(O)CH3] ppm.
C21H26N2O (322.45): calcd. C 78.22, H 8.13, N 8.69; found C
78.39, H 8.17, N 8.84.
N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-
diisopropylaniline (3): A mixture of 1 (0.170 g, 0.53 mmol) and
cyclohexylamine (0.910 mL, 0.789 g, 7.95 mmol) was heated at 100
°C without stirring for 20 h. Elimination of excess cyclohexylamine
by distillation under reduced pressure gave an oily material that
was dissolved in 3Ϫ4 mL of MeOH and cooled to 0 °C. After 6 h,
yellow crystals of the desired product were isolated by filtration
(0.153 g, 0.38 mmol, yield 72%). M.p. 186Ϫ188 °C. IR: ν(CϭN) 1636
1
cmϪ1. H NMR: (200.13 MHz, CD2Cl2): δ ϭ 1.16 [d, J ϭ 6.9 Hz,
12 H, CH(CH3)2], 1.25Ϫ1.95 (m, 10 H, CH2), 2.26 (s, 3 H, CH3),
2.45 (s, 3 H, CH3), 2.77 [sept, 2 H, J ϭ 6.9 Hz, CH(CH3)2],
3.55Ϫ3.66 (m, 1 H, CH), 7.05Ϫ7.25 (m, 3 H), 7.81 (t, J ϭ 7.8 Hz,
1 H, CH Ar), 8.21 (dd, J ϭ 7.8, 1.1 Hz, 1 H, CH Ar), 8.36 (dd,
J ϭ 7.8, 1.1 Hz, 2 H, CH Ar) ppm. 13C{1H} NMR (50.32 MHz,
1-{6-[(2,6-Dimethylphenyl)ethanimidoyl]-2-pyridinyl}-1-ethanone (1): CDCl3): δ ϭ 14.1 [1 C, C(N-alkyl)CH3], 17.8 [1 C, C(N-Ar)CH3],
Formic acid (50 µL) was added by syringe at 0 °C to a stirred
solution of 2,6-diacetylpyridine (0.490 g, 3.0 mmol) and 2,6-di-
methylaniline (0.33 mL, 0.327 g, 2.7 mmol) in MeOH (7 mL). Stir-
23.6 [2 C, CH(CH3)(CH3)], 23.9 [2 C, CH(CH3)(CH3)], 25.48 (2 C,
CH2), 25.49 (1 C, CH2); 28.50 [2 C, CH(CH3)(CH3)], 34.1 (2 C,
CH2); 61.0 (1 C, CH), 121.9 (1 C, CH Ar), 122.7 (1 C, CH Ar),
ring was stopped, and the resulting solution was maintained at 0 123.7 (2 C, CH Ar), 124.1 (1 C, CH Ar), 136.5 (2 C, C Ar), 147.2
°C for 24 h. The desired product precipitated as large, yellow crys-
tals, which were separated by filtration (0.447 g, 1.680 mmol). A
(1 C, CH Ar), 155.4 (1 C, CH Ar), 157.8 (1 C, C Ar), 164.4 [1 C,
C(N-alkyl)CH3], 167.8 [1 C, C(N-Ar)CH3] ppm. C27H37N3
second crop (0.148 g, 0.556 mmol) was obtained from the mother (403.61): calcd. C 80.34, H 9.24, N 10.41; found C 80.54, H 9.37,
liquor after cooling to 0 °C for 48 h. Overall yield 75%. M.p.
N 10.39.
1
117Ϫ118 °C. IR: ν(CϭN) 1645 cmϪ1, ν(CϭO) 1698 cmϪ1. H NMR
2,6-Diisopropyl-N-[(E)-1-(6-{[(1R)-1-phenylethyl]ethanimidoyl}-2-
pyridinyl)ethylidene]aniline (4): A mixture of 1 (0.170 g, 0.53 mmol)
and (R)-(ϩ)-(1-phenylethyl)amine (0.473 mL, 0.450 g, 3.71 mmol)
was heated at 100 °C without stirring for 18 h. Elimination of the
excess of amine by distillation under reduced pressure gave an oily
material that was dissolved in 3Ϫ4 mL of MeOH and cooled to 0
°C. After 2 d, yellow microcrystals of the product were isolated by
filtration (0.153 g, 0.36 mmol, yield 68%). M.p. 91Ϫ93 °C. IR: ν(Cϭ
(200.13 MHz, CD2Cl2): δ ϭ 2.02 (s, 6 H, CH3), 2.22 [s, 3 H,
C(NAr)CH3], 2.76 [s, 3 H, C(O)CH3], 6.89Ϫ6.97 (m, 1 H, CH Ar),
7.05Ϫ7.10 (m, 2H CH Ar), 7.96 (m, 1H CH Ar), 8.10 (dd, J ϭ 7.7,
1.2 Hz, 1 H, CH Ar), 8.56 (dd, J ϭ 7.7, 1.2 Hz, 2 H, CH Ar) ppm.
13C{1H} NMR (50.32 MHz, CDCl3): δ ϭ 17.0 [1 C, C(NAr)CH3],
18.6 (2 C, CH3), 26.4 [1 C, C(O)CH3], 123.3 (1 C, CH Ar), 123.9
(1 C, CH Ar), 125.2 (1 C, CH Ar), 126.0 (2 C, C Ar), 128.6 (2 C,
CH Ar), 138.0 (1 C, CH Ar), 149.2 (1 C, C Ar), 153.1 (1 C, CH
Ar), 156.2 (1 C, C Ar), 167.3 [1 C, C(NAr)CH3], 200.7 [1 C,
C(O)CH3] ppm. C17H18N2O (266.34): calcd. C 76.67, H 6.81, N
10.53; found C 76.45, H 6.54, N 10.41.
1633, 1644 cmϪ1 1H NMR (200.13 MHz, CDCl3): δ ϭ
.
N)
1.14Ϫ1.19 [m, 12 H, CH(CH3)2], 1.58 (d, J ϭ 6.6 Hz, 3 H,
CHCH3), 2.26 (s, 3 H, CH3), 2.50 (s, 3 H, CH3), 2.73 (m, 2 H,
CH), 4.97 (q, J ϭ 6.6 Hz, 1 H, CHCH3), 7.07Ϫ7.77 (m, 8 H, CH
1-{6-[(2,6-Diisopropylphenyl)ethanimidoyl]-2-pyridinyl}-1-ethanone Ar), 7.85 (m, 1 H, CH Ar), 8.36Ϫ8.39 (m, 2 H, CH Ar) ppm.
(2): Formic acid (150 µL) was added by syringe at room tempera- 14.4 [1 C, C(N-
13C{1H} NMR (50.32 MHz, CDCl3):
ture to stirred solution of 2,6-diacetylpyridine (1.630 g, alkyl)CH3], 17.9 [1 C, C(N-Ar)CH3], 23.6 [2 C, CH(CH3)(CH3)],
δ
ϭ
a
10.0 mmol) and 2,6-diisopropylaniline (1.70 mL, 1.596 g,
9.0 mmol) in MeOH (15 mL). After 12 h, a yellow solid had
formed, and this was filtered off and washed with cold (0 °C)
MeOH. The precipitate was characterised as a mixture of the ex-
pected product (97%) and the corresponding 2,6-bis(imino)pyridyl
compound (3%) (1H NMR integration). The solid was suspended
23.9 [2 C, CH(CH3)(CH3)], 25.5 (1 C, CH3), 29.0 [2 C,
CH(CH3)(CH3)], 61.0 (1 C, CH), 122.2 (1 C, CH Ar), 122.9 (1 C,
CH Ar), 123.7 (2 C, CH Ar), 124.2 (1 C, CH Ar), 127.4 (1 C, CH
Ar), 127.4 (2 C, CH Ar), 129.1 (2 C, CH Ar), 136.5 (2 C, C Ar),
137.4 (2 C, CH Ar), 146.7 (1 C, CH Ar), 147.3 (1 C, CH Ar), 155.4
(1 C, CH Ar), 157.5 (1 C, C Ar), 165.4 [1 C, C(N-alkyl)CH3], 167.8
1628
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
[1 C, C(N-Ar)CH3] ppm. C29H35N3 (425.62): calcd. C 81.83, H [N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-
FULL PAPER
8.29, N 9.87; found C 82.04, H 8.47, N 9.85.
dimethylaniline]cobalt Dichloride (9): This complex was obtained as
green crystals. Yield 84%. IR: ν(CϭN) 1590 cmϪ1. µeff ϭ 4.85 BM
(25 °C). Diffuse reflectance spectra (40000Ϫ5550 cmϪ1): 25000 sh,
16950, 15400, 10500, 8150, 5700. Solution spectra (CH2Cl2,
28500Ϫ6250 cmϪ1): 25000 sh, 17000 (ε 460), 15450 (ε 740), 11000
(ε 42), 7800 (ε 90). C23H29Cl2CoN3 (477.34): calcd. C 57.87, H
6.12, N 8.80; found C 57.61, H 6.25, N 8.61.
N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-di-
methylaniline (5): A mixture of 2 (0.131 g, 0.49 mmol) and cyclo-
hexylamine (0.485 g, 4.9 mmol) was heated at 100 °C without stir-
ring for 24 h. Elimination of the excess of cyclohexylamine by dis-
tillation under reduced pressure gave an oily material that was dis-
solved in 2 mL of MeOH and cooled to Ϫ24 °C. After 3 d, small
yellow needles of the product were isolated by filtration (0.13 g,
0.38 mmol, yield 78%). M.p. 98Ϫ100 °C. IR: ν(CϭN) 1634 cmϪ1. 1H
NMR (200.13 MHz, CD2Cl2): δ ϭ 1.25Ϫ1.95 (m, 10 H, CH2), 2.01
(s, 6 H, CH3), 2.20 (s, 3 H, CH3), 2.42 (s, 3 H, CH3), 3.55Ϫ3.66
(m, 1 H, CH), 6.88Ϫ7.08 (m, 3 H, CH Ar), 7.81 (m, 1 H, CH Ar),
8.18 (dd, J ϭ 7.7, 1.1 Hz, 1 H, CH Ar), 8.34 (dd, J ϭ 7.7, 1.1 Hz,
1 H, CH Ar) ppm. 13C{1H} NMR (50.32 MHz, CDCl3): δ ϭ 14.1
[1 C, C(N-alkyl)CH3], 17.8 [1 C, C(N-Ar)CH3], 18.6 (2 C, CH3),
25.5 (2 C, CH2), 26.5 (1 C, CH2); 34.2 (2 C, CH2); 61.0 (1 C, CH),
121.9 (1 C, CH Ar), 122.8 (1 C, CH Ar), 123.6 (1 C, CH Ar), 126.1
(2 C, C Ar), 128.5 (2 C, CH Ar), 137.3 (1 C, CH Ar), 149.5 (1 C,
C Ar), 155.3 (1 C, CH Ar), 157.8 (1 C, C Ar), 164.4 [1 C, C(N-
alkyl)CH3], 168.1 [1 C, C(N-Ar)CH3] ppm. C23H29N3 (347.50):
calcd. C 79.50, H 8.41, N 12.09; found C 79.78, H 8.65, N 12.15.
[2,6-Diisopropyl-N-{(E)-1-[6-{[(1S)-1-(2-naphthyl)ethyl]-
ethanimidoyl}-2-pyridinyl]ethylidene}aniline]cobalt Dichloride (10):
This complex was obtained as green crystals. Yield 88%. IR: ν(Cϭ
1592 cmϪ1. µeff ϭ 4.87 BM (25 °C). Diffuse reflectance spectra
N)
(40000Ϫ5550 cmϪ1): 25100 sh, 17400 sh, 15600, 10300, 8000, 5650.
Solution spectra (CH2Cl2, 28500Ϫ6250 cmϪ1): 25000 sh, 17300 sh,
14900 (ε 700), 10350 (ε 40), 7800 (ε 90). C33H37Cl2CoN3 (477.34):
calcd. C 65.46, H 6.16, N 6.90; found C 65.58, H 6.03, N 6.93.
[N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-
diisopropylaniline]iron Dichloride (11): This complex was obtained
as dark blue microcrystals. Yield 81%. IR: ν(CϭN) 1589, 1580 cmϪ1
.
µeff ϭ 5.72 BM (25 °C). Diffuse reflectance spectra (40000Ϫ5550
cmϪ1): 18200 sh, 7400. Solution spectra (CH2Cl2, 28500Ϫ6250
cmϪ1): 20230 (ε 240), 7170 (ε 6). C27H37Cl2FeN3 (530.36): calcd.
C 61.15, H 7.03, N 7.92; found C 60.97, H 6.91, N 7.88.
2,6-Diisopropyl-N-{(E)-1-[6-{[(1S)-1-(2-naphthyl)ethyl]-
ethanimidoyl}-2-pyridinyl]ethylidene}aniline (6): A mixture of 1
(0.170 g, 0.53 mmol) and (S)-(Ϫ)-(1-naphthalenylethyl)amine
(0.33 mL, 0.350 g, 2.04 mmol) was heated at 100 °C without stir-
ring for 17 h. The resulting mixture was diluted with 2Ϫ3 mL of
MeOH and cooled to 0 °C. After 5 h, pale yellow microcrystals of
the product were isolated by filtration (0.118 g, 0.36 mmol, yield
[2,6-Diisopropyl-N-{(E)-1-[6-{[(1R)-1-phenylethyl]ethanimidoyl}-2-
pyridinyl]ethylidene}aniline]iron Dichloride (12): This complex was
obtained as a blue powder. Yield 87%. IR: ν(CϭN) 1584 cmϪ1. µeff ϭ
5.56 BM (25 °C). Diffuse reflectance spectra (40000Ϫ5550 cmϪ1):
18000 sh, 14600, 8100. Solution spectra (CH2Cl2, 28500Ϫ6250
cmϪ1): 20300 (ε 250), 7700 (ε 8). C29H35Cl2FeN3 (552.37): calcd.
C 63.06, H 6.39, N 7.61; found C 62.68, H 6.28, N 7.59.
69%). M.p. 66Ϫ69 °C. IR: ν(CϭN) 1643 cmϪ1
.
1H NMR
(200.13 MHz, CD2Cl2): δ ϭ 1.10Ϫ1.17 [m, 12 H, CH(CH3)2], 1.72
(d, J ϭ 6.5 Hz, 3 H, CHCH3), 2.26 (s, 3 H, CH3), 2.50 (s, 3 H,
CH3), 2.73 [m, 2 H, CH(CH3)2], 5.67 (q, J ϭ 6.5 Hz, 1 H, CHCH3)
7.03Ϫ7.20 (m, 3 H, CH Ar), 7.45Ϫ7.77 (m, 7 H, CH Ar), 7.88 (m,
1 H, CH Ar), 8.32Ϫ8.43 (m, 2 H, CH Ar) ppm. C33H37N3 (475.68):
calcd. C 83.32, H 7.84, N 8.83; found C 83.64, H 7.82, N 8.77.
[N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-
dimethylaniline]iron Dichloride (13): This complex was obtained as
dark blue crystals. Yield 81%. IR: ν(CϭN) 1589 cmϪ1. µeff ϭ 5.68
BM (25 °C). Diffuse reflectance spectra (40000Ϫ5550 cmϪ1): 18800
sh, 8330 cmϪ1. Solution spectra (CH2Cl2, 28500Ϫ6250 cmϪ1):
19050 (ε 230), 8350 (ε 12). C23H29Cl2FeN3 (474.26): calcd. C 58.25,
H 6.16, N 8.86; found C 58.13, H 6.29, N 8.68.
Synthesis of Metal Complexes: All iron() and cobalt() complexes
with the 2,6-bis(imino)pyridyl ligands were prepared by the follow-
ing procedure. The ligand (1.0 equiv.) was added under nitrogen to
a solution of the metal halide (1.2 equiv.) in nBuOH and the re-
sulting mixture was heated at reflux for 10 min. After the mixture
had cooled to room temperature, a crystalline solid was obtained
within 30Ϫ60 min. This was filtered through a sintered-glass frit,
washed with nBuOH and petroleum ether and then dried in a
stream of nitrogen.
[2,6-Diisopropyl-N-{(E)-1-[6-{[(1S)-1-(2-naphthyl)ethyl]-
ethanimidoyl}-2-pyridinyl]ethylidene}aniline]iron Dichloride (14):
This complex was obtained as blue microcrystals. Yield 79%. IR:
ν(CϭN) 1590 cmϪ1. µeff ϭ 5.77 BM (25 °C). Diffuse reflectance spec-
tra (40000Ϫ5550 cmϪ1): 18300 sh, 8000. Solution spectra (CH2Cl2,
28500Ϫ6250 cmϪ1): 20000 (ε 240), 7900 (ε 8). C33H37Cl2FeN3
(602.43): calcd. C 65.79, H 6.19, N 6.98; found C 65.90, H 6.10,
N 6.84.
[N-{(E)-1-[6-(Cyclohexylethanimidoyl)-2-pyridinyl]ethylidene}-2,6-
diisopropylaniline]cobalt Dichloride (7): This complex was obtained
as green microcrystals. Yield 70%. IR: ν(CϭN) 1590, 1587 cmϪ1
.
Oligomerisation Reactions: A 0.5-L stainless steel reactor was
heated at 100 °C under vacuum for 3 h, solid precatalyst (12 µmol)
was introduced into the autoclave at room temperature, and the
system was evacuated. A solution of oxygen-free toluene (100 mL)
and MAO (sol. 10% w/w, 1.6 mL) was then introduced into the
reactor and the mixture was stirred at room temperature for
10 min. The system was then heated or thermostatted to the desired
temperature, pressurised with ethylene, and stirred at 1500 rpm.
The reaction was terminated by cooling to 5 °C and slow depress-
urisation, followed by addition of MeOH (2 mL) and n-heptane
(internal standard, 200 µL).
µeff ϭ 4.82 BM (25 °C). Diffuse reflectance spectra (40000Ϫ5550
cmϪ1): 25000 sh, 17000 sh, 15600, 10500, 8200, 5700. Solution
spectra (CH2Cl2, 28500Ϫ6250 cmϪ1): 25000 sh, 17000 sh, 14950 (ε
830), 11100 (ε 67), 7800 (ε 100). C27H37Cl2CoN3 (533.45): calcd.
C 60.79, H 6.99, N 7.88; found C 60.55, H 7.01, N 8.72.
[2,6-Diisopropyl-N-{(E)-1-[6-{[(1R)-1-phenylethyl]ethan-
imidoyl}-2-pyridinyl]ethylidene}aniline]cobalt Dichloride (8): This
complex was obtained as green crystals. Yield 85%. IR: ν(CϭN) 1590
cmϪ1. µeff ϭ 4.95 BM (25 °C). Diffuse reflectance spectra
(40000Ϫ5550 cmϪ1): 25000 sh, 17200 sh, 15500, 10500, 8300, 5600.
Solution spectra (CH2Cl2, 28500Ϫ6250 cmϪ1): 25000 sh, 17100 sh,
X-ray Structure Determination of 9·H2O: Single crystals of 9·H2O
14800 (ε 710), 10500 (ε 40), 8000 (ε 75). C29H35Cl2CoN3 (555.46): were grown by slow diffusion of n-hexane into a CH2Cl2 solution of
calcd. C 62.71, H 6.35, N 7.57; found C 62.80, H 6.29, N 7.60.
9. A summary of crystal and intensity data is presented in Table 4.
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631
1629

C. Bianchini et al.
FULL PAPER
Experimental data were recorded at 20 °C with an EnrafϪNonius
CAD4. A set of 25 carefully centred reflections in the range 8° Յ
θ Յ 10.25° was used for determining the lattice constants. As a
general procedure, the intensity of three standard reflections were
measured periodically every 200 reflections for orientation and
intensity control. This procedure did not reveal any appreciable
decay in intensities. The data were corrected for Lorentz and polar-
isation effects. Atomic scattering factors were those tabulated by
Cromer and Waber,[27] with anomalous dispersion corrections
taken from ref.[28] An empirical absorption correction was applied
by use of the program XABS with correction factors in the range
1.3518Ϫ0.394. The computational work was carried out by inten-
sive use of the program WINGX. Final atomic coordinates of all
atoms and structure factors are available on request from the au-
thors and are provided. A dark green parallelepiped crystal with
dimensions 0.375 ϫ 0.300 ϫ 0.225 mm was used for the data collec-
tion. The structure was solved by direct methods by use of the
SIR97 program.[29] Refinement was performed by full-matrix, least-
squares calculations, initially with isotropic thermal parameters
and then with anisotropic thermal parameters for all the atoms
other than the oxygen atoms. In the final stage of the refinement,
two atoms were detected and were assigned as oxygen of solvent
molecules and successfully refined. The phenyl ring was treated as
a rigid body with D6h symmetry, and the hydrogen atoms were al-
lowed to ride on the attached carbon atoms. Hydrogen atoms were
introduced at calculated positions. CCDC-195274 contains the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.)
ϩ 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
[1]
[1a] B. L. Small, M. Brookhart, A. M. A. Bennett, J. Am. Chem.
Soc. 1998, 120, 4049Ϫ4050. [1b] G. J. P. Britovsek, V. C. Gibson,
B. S. Kimberley, P. J. Maddox, S. J. McTavish, G. A. Solan, A.
J. P. White, D. J. Williams, Chem. Commun. 1998, 849Ϫ850. [1c]
A. M. A. Bennett (DuPont), WO 98/27124. Chem. Abstr. 1998,
[1d]
129, 122973x.
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, D. J.
Williams, J. Am. Chem. Soc. 1999, 121, 8728Ϫ8740. [1e] G. J. P.
Britovsek, V. C. Gibson, B. S. Kimberley, S. Mastroianni, C.
Redshaw, G. A. Solan, A. J. P. White, D. J. Williams, J. Chem.
Soc., Dalton Trans. 2001, 1639Ϫ1644.
[2] [2a]
B. L. Small, M. Brookhart, J. Am. Chem. Soc. 1998, 120,
7143Ϫ7144. [2b] G. J. P. Britovsek, S. Mastroianni, G. A. Solan,
S. P. D. Baugh, C. Redshaw, V. C. Gibson, A. J. P. White, D. J.
Williams, M. R. Elsegood, Chem. Eur. J. 2000, 6, 2221Ϫ2231.
[3] [3a]
B. L. Small, M. Brookhart, Polym. Prepr. (Am. Chem. Soc.
[3b]
Div. Polym. Chem.) 1998, 39, 213.
M. Brookhart, B. L.
Small (UNC), World Patent Application 98/30612, 1998.
[3c]
Chem. Abstr. 1998, 129, 149375r.
B. L. Small, M. Brook-
hart, Macromolecules 1999, 32, 2120Ϫ2130.
[4] [4a]
P. Margl, L. Deng, T. Ziegler, Organometallics 1999, 18,
[4b]
5701Ϫ5708.
L. Deng, P. Margl, T. Ziegler, J. Am. Chem.
[4c]
Soc. 1999, 121, 6479Ϫ6487.
D. V. Khoroshun, D. G. Mu-
saev, T. Vreven, K. Morokuma, Organometallics 2001, 20,
2007Ϫ2026.
[5]
[6]
A. S. Abu-Surrah, K. Lappalainen, U. Piironen, P. Lehmus, T.
M. Repo, J. Leskelä, J. Organomet. Chem. 2002, 648, 55Ϫ61.
[6a] R. Morassi, I. Bertini, L. Sacconi, Coord. Chem. Rev. 1973,
11, 343Ϫ402. [6b] L. Sacconi, R. Morassi, S. Midollini, J. Chem.
Soc. A 1968, 1510Ϫ1515. [6c] L. Sacconi, J. Chem. Soc. A 1970,
[6d]
248Ϫ256.
492Ϫ499.
R. Morassi, L. Sacconi, J. Chem. Soc. A 1971,
[7] [7a]
R. S. Drago, Physical Methods for Chemists, Saunders Col-
lege Publishing, New York, 1992. [7b] F. E. Mabbs, D. Collison,
‘‘Electron Paramagnetic Resonance of d Transition Metal
Compounds’’, in: Studies in Inorganic Chemistry, vol. 16, Elsev-
ier Publ., New York, 1992.
Table 4. Selected crystallographic data for 9·H2O
[8]
A. Bencini, D. Gatteschi, ‘‘ESR spectra of Metal Complexes
of the First Transitions Series in Low-Symmetry Environ-
ments’’, in: Transition Metal Chemistry, vol. 8, Marcel Dekker
Inc., New York, 1982.
Empirical formula
Formula mass
T [K]
C23H31Cl2CoN3O
495.36
293(2)
orthorhombic
Pcab (No. 61)
8.5557(10)
22.0942(17)
27.468(8)
5192(2)
Crystal system
Space group
[9] [9a]
A. Abragam, B. Bleaney, Electron Paramagnetic Resonance
[9b]
˚
of Transition Ions, Dover Publ. Inc., New York, 1970.
J.
a [A]
˚
E. Wertz, J. R. Bolton, ‘‘Electron Spin Resonance: Elementary
Theory and Practical Applications’’, series in: Advanced Chem-
istry, McGraw-Hill Book Company, New York, 1972.
J. R. Pilbrow, Transition Ion Electron Paramagnetic Resonance,
Claredon Press, Oxford, 1990.
b [A]
˚
c [A]
3
˚
V [A ]
[10]
[11]
Z
8
ρcalcd. [g·cmϪ3
]
1.262
0.884
0.38 ϫ 0.30 ϫ 0.22
0.8258/0.7327
2.66Ϫ24.97
4546/0/265
4546
Absorption coefficient [mmϪ1
Crystal size [mm]
Transmission, min./max.
2θ range [°]
Data/restraints/parameters
Reflections collected
Independent reflections
R1[a]; wR2
]
R. L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1986.
[12] [12a]
J. P. Jesson, NMR of Paramagnetic Molecules: Principles
and Applications (Eds.: G. N. La Mar, W. D. Horrocks, R. H.
[12b]
Holm), Academic Press, New York, 1973.
R. H. Holm, W.
D. Phillips, B. A. Averill, J. J. Mayerle, T. Herskovitz, J. Am.
Chem. Soc. 1974, 96, 2109Ϫ2117. [12c] H. M. McConnell, Proc.
Natl. Acad. Sci. U. S. A. 1972, 69, 335Ϫ341.
4546
7.75; 23.09
[13]
L. Sacconi, Pure Appl. Chem. 1968, 17, 95Ϫ127.
[14] [14a]
M. Ciampolini, N. Nardi, Inorg. Chem. 1966, 5,
[14b]
[a]
1150Ϫ1154.
1966, 5, 45Ϫ49.
M. Ciampolini, G. P. Speroni, Inorg. Chem.
I Ͼ 2σ(I).
[15]
[16]
M. Ciampolini, Struct. Bonding 1969, 6, 52Ϫ93.
T. M. Kooistra, Q. Knijnenburg, J. M. M. Smits, A. D. Horton,
P. H. M. Budzelaar, A. W. Gal, Angew. Chem. 2001, 113,
4855Ϫ4858; Angew. Chem. Int. Ed. 2001, 40, 4719Ϫ4722.
Acknowledgments
Polimeri Europa and the European Commission (contract no.
HPRN-CT-2000-00010, COST Action D17 Working Group 0003/
2000) are thanked for financial support. We are grateful to Mr.
Dante Masi (ICCOM-CNR) for technical assistance in X-ray data
collection and to Prof. Piero Zanello (University of Siena) for CV
measurements.
[17] [17a]
[17b]
[17c]
P. J. Flory, J. Am. Chem. Soc. 1940, 62, 1561Ϫ1565.
G. V. Schulz, Z. Phys. Chem. Abt. B 1935, 30, 379Ϫ385.
G. V. Schulz, Z. Phys. Chem. Abt. B 1939, 43, 25Ϫ31.
[17d]
G.
´
´
Henrici-Olive, S. Olive, Adv. Polym. Sci. 1974, 15, 1Ϫ19.
[18]
B. L. Small, A. J. Marcucci, Organometallics 2001, 20,
5738Ϫ5744.
1630
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631

Cs- and C1-Symmetric [2,6-Bis(imino)pyridyl]iron and -cobalt Dichloride Complexes
FULL PAPER
[19]
S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000,
100, 1169Ϫ1204.
L. Deng, T. K. Woo, L. Cavallo, P. M. Margl, T. Ziegler, J.
Am. Chem. Soc. 1997, 119, 6177Ϫ6186.
2093Ϫ2094. [25b] M. F. Summers, L. G. Marzilli, A. Bax, J. Am.
Chem. Soc. 1986, 108, 4285Ϫ4294.
F. Fabrizi de Biani, F. Laschi, P. Zanello, G. Ferguson, J. Trot-
ter, G. M. O’Riordan, T. R. Spalding, J. Chem. Soc., Dalton
Trans. 2001, 1520Ϫ1523.
D. T. Cromer, J. T. Waber, Acta Crystallogr. 1965, 18, 104Ϫ109.
International Tables of Crystallography, vol. IV, Kynoch Press,
Birmingham, UK, 1974.
A. Altomare, M. C. Burla, M. Camalli, G. Cascarano, C. Gia-
covazzo, A. Guagliardi, G. Polidori, R. Spagna, J. Appl. Crys-
tallogr. 1999, 32, 115Ϫ119.
[20]
[21]
[26]
A. F. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann,
Advanced Inorganic Chemistry, 6th ed., Wiley-Interscience,
New York, 1999, pp. 780Ϫ786 and 814Ϫ822.
[27]
[28]
[22]
A. Derome, M. Williamson, J. Magn. Res. 1990, 88, 177Ϫ186.
[23] [23a]
[29]
V. Sklener, H. Miyashiro, G. Zon, H. T. Miles, A. Bax,
[23b]
FEBS Lett. 1986, 208, 94Ϫ98.
J. Jeener, B. H. Meier, P.
Bachmann, R. R. Ernst, J. Chem. Phys. 1979, 71, 4546Ϫ4553.
A. Bax, R. H. Griffey, B. L. Hawkins, J. Magn. Reson. 1983,
55, 301Ϫ315.
[24]
Received November 25, 2002
[I02624]
[25] [25a]
A. Bax, M. F. Summers, J. Am. Chem. Soc. 1986, 108,
Eur. J. Inorg. Chem. 2003, 1620Ϫ1631
1631