Sterically demanding iminopyridine ligands

Article abstract of DOI:10.1002/ejic.200700322

Two sterically demanding iminopyridine ligands, (2,6-diisopropylphenyl)[6- (2,4,6-triisopropylphenyl)pyridin-2-ylmethylene]amine and (2,6- diisopropylphenyl)[6-(2,6-dimethylphenyl)pyridin-2-ylmethylene]amine, were prepared by a two-step process: first, condensation of 6-bromopyridine-2- carbaldehyde with an equimolecular amount of 2,6-diisopropylaniline, and second, Kumada-type coupling of in-situ-formed Grignard compounds of 1-bromo-2,6-dimethylphenyl and l-bromo-2,4,6-triisopropylphenyl. Dichlorido complexes of the ligands were synthesized starting from FeCl₂, [PdCl₂(cod)], [NiCl₂(dme)], and CoCl₂ (cod = 1,5-cyclooctadiene, dme = dimethoxyethane). X-ray crystal structure analyses of a Fe, Pd, and Co complex were determined. Ethylene polymerization/ oligomerization behavior of the dichlorido complexes after activation with methylalumin-oxane or triethylaluminum was studied. Ethylene dimerization selectivity greater than 95 % was observed. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700322

FULL PAPER
DOI: 10.1002/ejic.200700322
Sterically Demanding Iminopyridine Ligands
Torsten Irrgang,[a] Sandra Keller,[a] Heidi Maisel,[a] Winfried Kretschmer,[b] and
Rhett Kempe*[a]
Keywords: Ethylene polymerization / N ligands / Cobalt / Iron / Nickel / Palladium
Two sterically demanding iminopyridine ligands, (2,6-diiso-
propylphenyl)[6-(2,4,6-triisopropylphenyl)pyridin-2-ylmeth-
ylene]amine and (2,6-diisopropylphenyl)[6-(2,6-dimeth-
ylphenyl)pyridin-2-ylmethylene]amine, were prepared by a
two-step process: first, condensation of 6-bromopyridine-2-
carbaldehyde with an equimolecular amount of 2,6-diisopro-
pylaniline, and second, Kumada-type coupling of in-situ-
formed Grignard compounds of 1-bromo-2,6-dimethylphenyl
and 1-bromo-2,4,6-triisopropylphenyl. Dichlorido complexes
of the ligands were synthesized starting from FeCl2,
[PdCl2(cod)], [NiCl2(dme)], and CoCl2 (cod = 1,5-cyclooc-
tadiene, dme = dimethoxyethane). X-ray crystal structure
analyses of a Fe, Pd, and Co complex were determined. Eth-
ylene polymerization/oligomerization behavior of the
dichlorido complexes after activation with methylalumin-
oxane or triethylaluminum was studied. Ethylene dimeriza-
tion selectivity greater than 95% was observed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Olefin polymerization catalyzed by late-transition-metal
complexes is an intensively explored area of research.[1] Out
of the neutral N ligand environments studied so far, diimine
group 10 (Scheme 1, left) and the pyridyl diimine Fe/Co
catalysts (Scheme 1, right) have received most of the atten-
tion.[2] Remarkable changes in the polymerization behavior
especially with regard to the obtained molecular weights of
the polymers have been observed for these complexes if very
bulky 2,6-diisopropylphenyl substituents were introduced.
Scheme 2. Sterically demanding aminopyridinato ligands (left) and
iminopyridines [3a: R1 = CH3, R2 = H; 3b: R1 = R2 = CH-
(CH3)2].
Bianchini and coworkers for instance described phenyl
and naphthalenyl substitution at the pyridine ring by Su-
zuki coupling.[4] It may not be possible to use such a proto-
col to cross couple sterically demanding phenyl derivatives
like 2,6-diisopropylphenyl.[6] We recently described an ef-
ficient protocol to introduce 2,6-diisopropylphenyl substitu-
ents into pyridine rings by Kumada coupling. This chemis-
try was explored to synthesize sterically demanding amino-
pyridines[7] and to use them as aminopyridinato ligands
(Scheme 2, left);[8] a class of ligands we investigated inten-
sively in the past.[9]
Because of the importance of steric protection in late-
metal-catalyzed olefin polymerization chemistry, as well as
other areas of coordination chemistry, we became interested
in the synthesis of bulky iminopyridines such as 3a,b
(Scheme 2, right). We felt that an approach we used to syn-
thesize bulky aminopyridines might by suitable for the syn-
thesis of ligands such as 3a,b as well. The imine function
that is usually sensitive towards the addition of Grignard
reagents might be well enough protected due to the 2,6-
diisopropylphenyl substituent at the imine N atom. Herein
we report the synthesis of sterically demanding iminopyr-
idine ligands, the syntheses and structures of Co, Ni, Fe,
Scheme 1. N-ligand-stabilized late-transition-metal olefin polyme-
rization precatalysts (for instance: M = Ni, Pd; MЈ = Fe, Co; X =
Cl; R = H; R1 = CH3; Ar = 2,6-diisopropylphenyl).
Iminopyridine ligands (Scheme 2, right) might be consid-
ered to be structurally related to both of the above-men-
tioned ligand systems, and consequently, the olefin polyme-
rization behavior of some of these complexes was investi-
gated recently.[3–5] Sterically demanding examples bearing
2,6-diisopropylphenyl substituents in the pyridine ring are
not known to the best of our knowledge.
[a] Lehrstuhl für Anorganische Chemie II, Universität Bayreuth,
95440 Bayreuth, Germany
E-mail: kempe@uni-bayreuth.de
[b] Stratingh Institut for Chemistry, Center for Catalytic Olefin
Polymerization, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
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T. Irrgang, S. Keller, H. Maisel, W. Kretschmer, R. Kempe
FULL PAPER
Scheme 3. Synthesis of 3 [R1 = CH3, R2 = H (2a/3a); R1 = R2 = iPr (2b/3b); cat for Kumada coupling is FeCl2 or MnCl2].
and Pd complexes stabilized by these ligands, and some as-
Single crystals of red paramagnetic complex 4 and yellow
pects of the olefin polymerization/oligomerization behav- crystals of diamagnetic 5[13] were grown from a concen-
iors of a few of these complexes.
trated thf solution at –25 °C. For both complexes, one un-
coordinated thf molecule was found in the crystal lattice,
and for 4, one additional coordinated thf ligand was found.
ORTEP drawings are reported in Figures 1 and 2 for 4 and
5, respectively. Crystallographic details are summarized in
Table 2. In 4, the iron is five coordinate, and the coordina-
tion sphere is best described as distorted trigonal bipyrami-
dal with the equatorial plane being occupied by the imino
N atom and the two chlorine atoms. The axial positions are
coordinated by the pyridine N atom and by the O atom of
the coordinated thf ligand. A Cl–Fe–Cl angle of 131.81(6)°
is observed within the equatorial plane and a “chelating”
angle Npyridine–Fe–Nimino of 77.30(16)°. The Fe–O bond of
the coordinated thf ligand is rather long [2.474(8) Å], which
indicates a very weakly bound thf ligand. The Fe–O bond
lengths of thf-coordinated iron chlorides range from
1.944 Å for [FeCl3(thf)][14] to 2.181 Å for the [Fe2Cl3-
(thf)6]– anion.[15] The coordination of 5 is best described as
Results and Discussion
Syntheses of the Ligands
The sterically demanding iminopyridine ligands 3a,b[10]
were prepared by the two-step process illustrated in
Scheme 3. First, the convenient condensation of 6-bromo-
pyridine-2-carbaldehyde with an equimolecular amount of
2,6-diisopropylaniline leads to 1.[11] The reaction of in-situ-
formed Grignard compounds 2a,b of 1-bromo-2,6-dimeth-
ylphenyl and 1-bromo-2,4,6-triisopropylphenyl with 1 in
the presence of a catalytic amount of FeCl2 leads to imino-
pyridines 3a,b (Scheme 3).[12] We observed similar results by
using MnCl2 as a catalyst. The catalyst was chosen on the
basis of the results that were obtained from screening ex-
periments with various transition-metal halogenides and N
or P ligands. The 2,6-diisopropylphenyl group at the imine
N atom protects the imine well enough if an efficient cata-
lyst is added to accomplish biaryl formation.
square planar with
a Npyridine–Pd–Nimino angle of
80.50(15)° and Cl–Pd–Cl angle of 88.84(5)°.
Compounds 3a,b were obtained as pale yellow crystalline
materials. They differ in the steric demand (R1 and R2) of
the phenyl substituent in the 6-position of the pyridine ring.
Syntheses and Structures of the Complexes
Complexes 4–7 were obtained in high yields in the reac-
tion of 1 equiv. of 3a with anhydrous FeCl2, [PdCl2(cod)],
[NiCl2(dme)], and CoCl2 (cod = 1,5-cyclooctadiene, dme =
dimethoxyethane) in thf at 70 °C for 18 h (Scheme 4).
Figure 1. Molecular structure of 4; selected bond lengths [Å] and
angles [°]: C1–N1 1.271(6), C1–C2 1.486(7), C2–N2 1.357(6), C6–
N2 1.353(6), C6–C7 1.490(7), C15–N1 1.441(6), N1–Fe1 2.140(4),
N2–Fe1 2.201(4), Cl2–Fe1 2.2369(17), Cl3–Fe1 2.2586(17), O2–Fe1
2.473; N1–C1–C2 120.3(5), N2–C2–C1 115.4(5), N2–C6–C7
118.1(4), C1–N1–C15 118.4(4), C1–N1–Fe1 113.9(3), C15–N1–Fe1
126.9(3), C6–N2–C2 117.1(4), C6–N2–Fe1 131.0(3), C2–N2–Fe1
111.3(3), N1–Fe1–N2 77.30(16), N1–Fe1–Cl2 122.05(12), N2–Fe1–
Cl2 105.29(12), N1–Fe1–Cl3 103.30(12), N2–Fe1–Cl3 99.34(12),
Cl2–Fe1–Cl3 131.81(6).
Scheme 4. Reaction of 3a with late-transition-metal halogenides
{[M] = [PdCl2(cod)] (5); [NiCl2(dme)] (6); CoCl2 (7)}.
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Sterically Demanding Iminopyridine Ligands
The coordination of the cobalt centre of 11 is best de-
scribed as slightly distorted tetrahedral. For the three struc-
turally characterized metal complexes we observed three
different coordination environments. The sterically de-
manding N ligands introduced here seem to be quite flexi-
ble with regard to the coordination they adopt with transi-
tion metals, and they do not force metals in energetically
unfavored coordinations because of steric pressure. The
chelating Npyridine–Co–Nimino angle of 11 is 80.24°. The
bulky phenyl substituents in the 6-position of the pyridine
and at the imino N atom are twisted relative to the pyridine
ring with dihedral angles of 99.8° and 97.1°, respectively.
The five-membered chelating C2CoN2 ring is nearly planar
(0.0473 Å mean deviation). Details of the X-ray crystal
structure analysis of 11 are summarized in Table 2. The mo-
lecular structure of 11 is shown in Figure 3.
Figure 2. Molecular structure of 5; selected bond lengths [Å] and
angles [°]: Pd1–N2 2.012(4), Pd1–N1 2.090(4), Pd1–Cl1 2.273(13),
Pd1–Cl2 2.2746(15), N2–C1 1.272(6), N2–C15 1.464(6), N1–C6
1.347(6), N1–C2 1.355(6), C1–C2 1.451(6), C6–C7 1.489(8); N2–
Pd1–N1 80.50(15), N2–Pd1–Cl1 176.40(13), N1–Pd1–Cl1
99.43(10), N2–Pd1–Cl2 91.01(12), N1–Pd1–Cl2 170.89(11), Cl1–
Pd1–Cl2 88.84(5), C1–N2–C15 117.9(4), C1–N2–Pd1 113.8(3),
C15–N2–Pd1 128.2(3), C6–N1–C2 117.1(4), C6–N1–Pd1 132.8(3),
C2–N1–Pd1 110.0(3), N2–C1–C2 119.1(4), N1–C2–C1 115.3(4),
N1–C6–C7 121.2(4).
Olefin Polymerization/Oligomerization Behavior
Oligomerization results are summarized in Table 1. The
investigated complexes mainly produce oligomeric materi-
als. Nearly no activity is observed for the iron complexes.
The highest activities are observed for the cobalt and nickel
complexes. For both metals, Co and Ni, the changes in
terms of activity and distribution of the oligomers are
rather small. The sterically more demanding ligand shows
essentially the same polymerization behavior, aside from a
little enhanced activity. Noteworthy is that the selectivity
for 6, 7, 10, and 11 towards 1-butene is high. High ethylene
dimerization selectivities were observed for iminopyridine
complexes bearing sterically less-demanding substituents on
the pyridine ring as well.[4] Activation with triethylalumi-
num (TEA), Et3Al, instead of methylaluminoxane (MAO),
leads to reduced activity but suppresses the formation of
polymeric byproducts and increases the dimerization selec-
tivity above 95%. The results obtained with our Co com-
plexes are similar to (or nearly as good as) the results ob-
served by Bianchini and coworkers.[4] They observed for
iminopyridine Co complexes (activated with MAO) carry-
ing a phenyl or naphthyl substituent on the pyridine
ring the formation of oligomerization products and
butenes, especially with conversions of 784 and
630 kgproductmolcat–1h–1 bar–1, respectively. This comparison
indicates that the introduction of steric bulk in the 6-posi-
tion of the pyridine ring does not influence the nature of
the products formed. It was recently demonstrated by these
groups that their tetrahedral high-spin complexes react with
MAO in toluene to give low-spin Co square-planar methyl
complexes.[4e] This species, intercepted by EPR spec-
troscopy, inserts ethylene to form a cobalt–propyl interme-
diate that eliminates propene with the formation of a Co–
H initiator. A similar mechanism is most likely in operation
for our Co (and Ni) complexes; by ethylene insertion into
a Co–H bond, an ethyl complex is formed. If this species
undergoes β-H elimination/transfer we get back to the start-
ing point (Co–H complex and ethylene). If a second inser-
tion of ethylene takes place, a butyl complex is formed and
The reaction of sterically more demanding ligand 3b with
the anhydrous metal salts FeCl2, [PdCl2(cod)],
[NiCl2(dme)], and CoCl2 leads to corresponding complexes
8–11 (Scheme 5). Compounds 8, 10, and 11 are paramag-
netic.
Scheme 5. Synthesis of 8 (M = Fe), 9 (M = Pd), 10 (M = Ni), and
11 (M = Co).
Figure 3. Molecular structure of 11; selected bond lengths [Å] and
angles [°]: Co1–N2 2.052(4), Co1–N1 2.057(4), Co1–Cl2
2.2023(16), Co1–Cl1 2.2007(15), N2–C2 1.346(6), N2–C6 1.353(6),
N1–C1 1.267(6), N1–C22 1.453(6), C1–C2 1.477(7), C6–C7
1.479(7); N2–Co1–N1 80.23(16), N2–Co1–Cl2 111.01(11), N1–
Co1–Cl2 109.89(12), N2–Co1–Cl1 125.20(12), N1–Co1–Cl1
113.17(22), Cl2–Co1–Cl1 112.60(7), C2–N2–C6 118.6(4), C2–N2–
Co1 112.3(3), C6–N2–Co1 128.8(3), C1–N1–C22 120.1(4), C1–N1–
Co1 113.2(3), C22–N1–Co1 126.7(3), N1–C1–C2 118.6(4), N2–C2–
C1 114.7(4), N2–C6–C7 118.6(4).
Eur. J. Inorg. Chem. 2007, 4221–4228
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T. Irrgang, S. Keller, H. Maisel, W. Kretschmer, R. Kempe
FULL PAPER
Table 1. Ethylene polymerization/oligomerization of metal complexes by using different activation protocols.[a] Products were analyzed
by GC with the use of α-olefins as internal standards.
Run
Complex[b]
Activator[c]
Conversion
mPol.
[g]
C-4[d]
[g]
C-6
[g]
C-8
[g]
Higher
[g]
[kgmolcat–1 h–1 bar–1]
1
2
3
4
5
6
7
8
4, Fe
MAO
TEA
MAO
TEA
MAO
TEA
MAO
TEA
MAO
MMAO-3A 136
MAO 156
MMAO-3A 140
MAO
MMAO-3A
MAO
2
0
2
0
358
152
472
172
150
0.05
–
0.06
–
0.06
–
0.06
–
0.06
0.06
0.06
0.06
0.3
0.2
0.5
0.3
–
–
–
–
3.95
1.85
5.00
2.10
1.16
1.10
1.27
1.21
–
–
–
–
–
0.55
0.05
0.80
0.05
0.35
0.15
0.59
0.34
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4, Fe[e]
8, Fe*
8, Fe*[e]
7, Co
7, Co[e]
11, Co*
11, Co*[e]
6, Ni
9
0.23
nd[f]
0.03
nd[f]
–
–
–
0.11
nd[f]
–
10
11
12
13
14
15
16
6, Ni
10, Ni*
10, Ni*
5, Pd
nd[f]
–
12
8
20
5, Pd
9, Pd*
9, Pd*
–
–
–
–
–
–
–
–
–
MMAO-3A 12
–
[a] The general procedure and conditions as described in the Experimental Section were followed by using the catalyst (10 µmol), the
activator (500 equiv.), and toluene (260 mL) at 30 °C, 5 bar ethylene with a 15 min run time. [b] * Indicates the sterically more-demanding
ligand. [c] MMAO-3A: Modified MAO {[Me0.7iBu0.3AlO]n}. [d] Ͼ95% 1-Butene. [e] M/Al, 1:200. [f] Not determined, isoparaffinic solvent
(b.p. 118 °C).
subsequently butene can be eliminated. It looks as though ther ethylene insertion may lead to the exclusive formation
β-H elimination/transfer is very fast with regard to chain of the dimerization product. Interestingly, we are able to
growth for our Co complexes and thus we preferentially dimerize selectively with relatively low cocatalyst loadings
obtain butenes. If we activate with TEA, β-H elimination/ and MAO was not needed. Mapolie and coworkers studied
transfer may become enhanced because butene selectivity is the formation of polymeric materials for Pd complexes.[5]
increased. Alternatively, TEA may reduce the Co dichloride Conversions up to 15 kgproduct molcat–1 h–1 bar–1 were ob-
complexes followed by oxidative coupling of two ethylene served, which again is in accordance with the results ob-
molecules including the formation of a cobaltacyclopentene tained for our Pd complexes. Once more, no dramatic influ-
followed by β-H elimination, forming a butenyl hydride that ence is seen due to the introduction of the very bulky sub-
undergoes reductive elimination, and butene is formed. The stituents on the pyridine ring. It is surprising that the elec-
stability of the transient cobaltacyclopropene against fur- tronic situation that the iminopyridines provide dominate
the polymerization chemistry and that the bulkiness of the
ligands on the pyridine ring is of minor relevance despite
the difference in shielding at the metal center. The shielding
is visualized in Figure 4.
Conclusions
Four general conclusions can be drawn from the investi-
gations discussed here. First, Kumada coupling is an ef-
ficient protocol to introduce sterically demanding substitu-
ents in the 6-position of the pyridyl ring of iminopyridines.
Second, the sterically demanding ligands discussed here ac-
complish a variety of coordinations with late transition
metals, such as trigonal bipyramidal, tetrahedral, and
square planar. Third, the cobalt and nickel dichlorido com-
plexes (if activated with MAO or TEA) dimerize ethylene
rather than polymerize it and can show dimerization selec-
tivities greater than 95%. Fourth, the introduction of the
bulky 2,6-dialkylphenyl substituent in the pyridine moiety
does not influence the polymerization behavior of the re-
Figure 4. Visualization (space filling plot with C atoms light grey,
sulting late-transition-metal complexes (activation with
Co atoms dark grey, Cl atoms black) of the steric protection of
MAO) tremendously relative to the significantly less bulky
versions of those ligands.
the 2,4,6-triisopropylphenyl substituent at the pyridine ring (right)
versus shielding provided by a phenyl ring only (left).
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Eur. J. Inorg. Chem. 2007, 4221–4228

Sterically Demanding Iminopyridine Ligands
TEA Ethylene Conversion: The general procedure and conditions
as described above were followed with the use of TEA (M/Al,
1:200) instead of PMAO to charge the reactor. The results are pre-
sented in Table 1.
Experimental Section
Polymerization Experiments
General: Toluene (Aldrich, anhydrous, 99.8%) was passed over col-
umns of Al2O3 (Fluka), BASF R3–11 supported Cu oxygen scaven-
ger and molecular sieves (Aldrich, 4 Å) prior to use. Ethylene
(AGA polymer grade) was passed over BASF R3–11 supported Cu
oxygen scavenger and molecular sieves (Aldrich, 4 Å) prior to use.
TEA (1.6  in toluene, Aldrich), PMAO (4.9 wt.-% in Al, toluene,
Akzo), and MMAO-3A (2.1 wt.-% in Al; isoparaffinic solvent, b.p.
118 °C; Akzo) were used as received.
Syntheses and Structure Analyses
General: All reactions and manipulations with air-sensitive com-
pounds were performed under an atmosphere of dry argon by using
standard Schlenk and glove box techniques. Nonhalogenated sol-
vents were distilled from sodium benzophenone ketyl and haloge-
nated solvents from P2O5. Deuterated solvents were obtained from
Cambridge Isotope Laboratories and were degassed, dried, and dis-
tilled prior to use. All other chemicals were purchased from com-
mercial vendors and used without further purification. NMR spec-
tra were obtained with a Bruker ARX 250 spectrometer. Chemical
shifts are reported in ppm relative to the deuterated solvent. Ele-
mental analyses were carried out with a Vario elementar EL III.
Melting points were measured in sealed capillaries with a Stuart
SMP3 apparatus. Magnetic moments were determined with a mag-
netic susceptibility balance Sherwood Mark 1 MSB at room tem-
perature. X-ray crystal structure analyses were performed by with
a STOE-IPDS II equipped with an Oxford Cryostream low-tem-
perature unit. Structure solution and refinement were accomplished
by using SIR97,[16] SHELXL-97,[17] and WinGX.[18] Crystallo-
graphic details are summarized in Table 2. CCDC-639686 (for 4),
-639684 (for 5), and -639685 (for 11) contain the supplementary
crystallographic data for this publication. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Preparative Procedure: The polymer samples were prepared by dis-
solving the polymer (15 mg) in CD2Cl2 (0.5 mL) at 100 °C for 3 h
before measuring. Gel permeation chromatography (GPC) analysis
was carried out with a Polymer Laboratories Ltd. (PL-GPC210)
chromatograph, equipped with a capillary differential viscometer
(Viscotek), a refractive index (RI) detector, and a two-angle (15
and 90°) light-scattering photometer at 150 °C by using 1,2,4-trich-
lorobenzene as the mobile phase. The samples were prepared by
dissolving the polymer (0.1% weight/volume) in the mobile phase
solvent in an external oven and were run without filtration. The
molecular weight was referenced to polyethylene (Mw
=
50000 gmol–1) and polystyrene (Mw = 100000–500000 gmol–1)
standards. The reported values are the average of at least two inde-
pendent determinations. GC analyses were performed with an HP
6890 instrument equipped with an HP-1 dimethylpolysiloxane col-
umn (19095 Z-123). GC–MS analyses were conducted with an HP
5973 mass-selective detector attached to an HP 6890 GC instru-
ment.
Syntheses of the Ligands and Ligand Precursors
Polymethylaluminoxane (PMAO) Ethylene Conversion: The cata-
lytic ethylene conversion reactions were performed in a stainless
steel 1-L autoclave (Medimex) in the semibatch mode (ethylene was
added by replenishing flow to keep the pressure constant). The re-
actor was temperature, flow, and pressure controlled and equipped
with separated toluene, catalyst, and cocatalyst injection systems
and a sample outlet for continuous reaction monitoring. Up to
15 bar of ethylene pressure multiple injections of the catalyst with
a pneumatically operated catalyst injection system were used. Dur-
ing a polymerization run, the pressure, the ethylene flow, the inner
and the outer reactor temperature, and the stirrer speed were moni-
tored continuously. In a typical semibatch experiment, the auto-
clave was evacuated and heated for 1 h at 125 °C prior to use. The
reactor was then brought to the desired temperature, stirred at
600 rpm and charged with toluene (230 mL), PMAO (2.7 g of a
toluene solution, 4.9 wt.-% Al, 5 mmol), and cyclohexane (10 g) as
an internal standard. After pressurizing with ethylene to reach a
total pressure of 5 bar, the autoclave was equilibrated for 5 min.
Subsequently, the catalyst (10 µmol, M/Al, 1:500) suspended in tol-
uene (30 mL) was injected to start the reaction. During the run the
ethylene pressure was kept constant to within 0.2 bar of the initial
pressure by replenishing flow. After a reaction time of 15 min, a
sample of the reaction mixture was taken to analyze and quantify
the soluble products before the reactor was vented and the residual
aluminum alkyls were destroyed by the addition of ethanol
(100 mL). The polymeric product was collected, stirred for 30 min
in acidified ethanol, and rinsed with ethanol and acetone on a glass
frit. The polymer was initially dried in air and subsequently in
vacuo at 80 °C. The results are presented in Table 1.
1: 6-Bromopyridine-2-carbaldehyde (2.0 g, 10.7 mmol) and 2,6-di-
isopropylaniline (2.03 mL, 1.91 g, 10.7 mmol) were dissolved in
ethanol (30 mL), and the mixture was heated at reflux for 2 h. The
solvent was removed under reduced pressure, and a pale yellow
crystalline material was obtained and used without further purifi-
cation. Yield: 3.59 (97%). M.p. 104 °C. 1H NMR (250.13 MHz,
CDCl3): δ = 8.26–8.25 (d, J = 1.0 Hz, 1 H, CH), 8.23 (s, 1 H,
N=CH), 7.73–7.66 (t, J = 7.8 Hz, 1 H, CH), 7.61–7.57 (dd, J =
7.9, 1.0 Hz, 1 H, CH), 7.24–7.08 (m, 3 H, CH), 3.08–2.92 [sept, J
= 6.9 Hz, 2 H, 2ϫCH(CH3)2], 1.16–1.13 [d, J = 6.9 Hz, 12 H,
2ϫCH(CH3)2] ppm. 13C NMR (62.90 MHz, CDCl3): δ = 161.44
(N=CH), 155.40, 147.85, 141.81, 138.99, 136.99, 129.79, 124.66,
123.03 (Cm, C6H3), 119.86, 27.91 [2ϫCH(CH3)2], 23.37
[2ϫCH(CH3)2] ppm. C18H21BrN2 (345.28): calcd. C 62.61, H 6.13,
N 8.11; found C 62.66, H 6.05, N 8.01.
2a: A solution of 2-bromo-m-xylene (1.39 mL, 1.93 g, 10.4 mmol)
in thf (30 mL) was added to magnesium turnings (0.30 g,
12.5 mmol), and the resulting suspension was stirred and activated
by the addition of 1,2-bromoethane (0.2 mL). An exothermic reac-
tion took place, and an ice bath was used to cool the reaction
mixture when the reaction became too vigorous. The reaction mix-
ture was stirred at r.t. for 2 h and then filtered, and the filtrate was
directly used.
2b: To a suspension of magnesium turnings (0.21 g, 8.69 mmol) in
thf (100 mL) was added by syringe 1-bromo-2,4,6-triisopropylben-
zol (1.82 mL, 7.24 mmol). The resulting suspension was stirred and
activated by the addition of 1,2-bromoethane (0.2 mL). A low exo-
thermic reaction took place. The reaction mixture was stirred at r.t.
for 2 h and then filtered, and the filtrate was directly used.
MMAO-3A Ethylene Conversion: The general procedure and condi-
tions as described above were followed with the use of MMAO-3A
(6.4 g of an isoparaffinic solvent, b.p. 118 °C; 2.1 wt.-% Al, 5 mmol,
M/Al, 1:500) instead of PMAO to charge the reactor. The results
are presented in Table 1.
3a: To a solution of 1 (3.0 g, 8.69 mmol) in dioxane (30 mL) was
added a suspension of FeCl2 (0.11 g, 0.87 mmol) in thf (10 mL).
The Grignard solution 2a was then slowly added to the stirred sus-
Eur. J. Inorg. Chem. 2007, 4221–4228
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4225

T. Irrgang, S. Keller, H. Maisel, W. Kretschmer, R. Kempe
FULL PAPER
Table 2. Details of the X-ray crystal structure analyses of 4, 5, and 11.
4
5
11
Formula
C34H46Cl2FeN2O2
641.48
monoclinic
P21/c
17.386(1)
13.495(1)
14.802(1)
96.69(1)
3449.3(4)
4
C30H38Cl2N2OPd
619.92
monoclinic
P21/c
9.428(1)
13.677(2)
23.309(2)
93.82(1)
2999(1)
4
C45H68Cl2CoN2O3
814.84
hexagonal
P61
13.013(1)
13.013(1)
47.354(4)
M [gmol–1]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
β [°]
V [Å3]
6944.5(1)
6
Z
Crystal size [mm]
ρcalcd. [gcm–3]
Absorption correction
T [K]
0.86ϫ0.10ϫ0.05
1.235
numerical
193(2)
1.92–26.27
45992
6890
3082
0.0695
0.2061
0.33ϫ0.15ϫ0.05
1.373
no
193(2)
1.73–25.64
12594
5400
3630
0.0481
0.0951
0.32ϫ0.12ϫ0.11
1.169
no
193(2)
1.81–21.86
52889
5542
4716
0.0475
0.1183
θ range [°]
Reflections collected
Independent reflections
Observed reflections
R value [IϾ2σ(I)]
wR2 (all data)
pension. The reaction mixture was heated to 70 °C for 48 h. Water
(100 mL) and diethyl ether (80 mL) were added, and the resulting
suspension was transferred to a 500-mL separatory funnel. The
organic phase was collected, and the inorganic phase was washed
twice with diethyl ether and extracted. The combined organic
phases were washed with a saturated NaCl solution and dried with
Na2SO4. The organic phase was concentrated to dryness under vac-
uum, resulting in a brown oily product. The oil was purified by
silica chromatography (pentane) and then crystallized from pentane
at –25 °C to afford a pale yellow crystalline material. Yield: 1.16 g
(39%). M.p. 142.5 °C. 1H NMR (250.13 MHz, CDCl3): δ = 8.36
(s, 1 H, N=CH), 8.29–8.25 (dd, J = 7.9, 1.1 Hz, 1 H, pyridine),
7.95–7.89 (t, J = 7.7 Hz, 1 H, 4pyridine-H), 7.37–7.34 (dd, J = 7.6,
1.1 Hz, 1 H, pyridine), 7.25–7.04 (m, 6 H, 2ϫC6H3), 3.08–2.92
[sept, J = 6.9 Hz, 2 H, 2ϫCH(CH3)2], 2.12 (s, 6 H, 2ϫCH3), 1.20–
1.17 [d, J = 6.9 Hz, 12 H, 2ϫCH(CH3)2] ppm. 13C NMR
(62.90 MHz, CDCl3): δ = 163.29 (N=CH), 159.76 (C), 154.31 (C),
148.35 (C), 139.73 (C), 137.13 (2ϫC), 137.01 (CH), 135.84 (2ϫC),
128.17 (CH), 127.79 (2ϫCH), 126.21 (CH), 124.35 (CH), 122.96
(2ϫCH), 119.08 (CH), 27.90 [2ϫCH(CH3)2], 23.41 [2ϫCH-
(CH3)2], 20.34 (2ϫCH3) ppm. C26H30N2 (370.54): calcd. C 84.28,
H 8.16, N 7.56; found C 83.74, H 8.15, N 7.47.
ppm. 13C NMR (62.90 MHz, CDCl3): δ = 163.61 (N=CH), 160.07
(C), 153.93 (C), 149.05 (C), 148.43 (C), 146.24 (2ϫC), 137.22
(2ϫC), 136.34 (CH), 135.71 (C), 126.60 (CH), 124.35 (CH), 122.99
(2ϫCH), 120.96 (2ϫCH), 118.87 (CH), 34.45 (CH), 30.48 (CH),
27.92 [2ϫ(CH3)2CH-C6H3], 24.26 (CH), 24.09 (CH), 24.00 (CH),
23.49 [2ϫ(CH3)2CH-C6H3] ppm. C33H44N2 (468.72): calcd. C
84.56, H 9.46, N 5.46; found C 84.38, H 9.49, N 6.21.
Syntheses of the Complexes
4: A solution of 3a (0.358 g, 0.97 mmol) and FeCl2 (0.11 g,
0.86 mmol) in thf (30 mL) was heated at 70 °C whilst stirring for
18 h. The solvent was removed under vacuum, and the red residue
was washed with hexane, dissolved in thf, and filtered through
quartz. The filtrate was concentrated, and red crystals were grown
at –25 °C. Yield: 0.434 g (90%). M.p. 240 °C. µeff = 4.44 BM
1
(24.4 °C). H NMR (250.13 MHz, CDCl3): δ = 34.50, 30.98, 8.23,
7.63, 7.14, 5.90, 3.97, 2.89, 2.03, 1.74, 1.23, 1.14, 0.86, 0.05,
–2.57 ppm. C26H30Cl2FeN2 (497.25): calcd. C 62.80, H 6.08, N
5.63; found C 63.26, H 6.05, N 5.24.
5: A stirred solution of 3a (0.229 g, 0.81 mmol) and [PdCl2(cod)]
(0.223 g, 0.79 mmol) in thf (30 mL) was heated at 70 °C for 18 h.
The solvent was removed under vacuum, and the residue was
washed with hexane, dissolved in dichloromethane (50 mL), and
filtered through quartz. The resulting orange solution was concen-
trated, and yellow-orange crystals (needles) were grown at –25 °C.
Yield: 76%. M.p. 255 °C. 1H NMR (250.13 MHz, CD2Cl2): δ =
8.23–8.16 (t, J = 7.5 Hz, 1 H, 4pyridine-H), 8.19 (s, 1 H, N=CH),
7.97–7.94 (dd, J = 7.7, 1.5 Hz, 1 H, pyridine), 7.59–7.55 (dd, J =
8.3, 1.5 Hz, 1 H, pyridine), 7.40–7.09 (m, 6 H, 2ϫC6H3), 3.44–3.31
[sept, J = 6.8 Hz, 2 H, 2ϫCH(CH3)2], 2.30 (s, 6 H, 2ϫCH3), 1.40–
1.37 (d, J = 6.8 Hz, 6 H, 2ϫCH3, isopropyl), 1.19–1.16 (d, J =
6.9 Hz, 6 H, 2ϫCH3, isopropyl) ppm. 13C NMR (62.90 MHz,
3b: To a solution of 1 (2.5 g, 7.24 mmol) in dioxane (30 mL) at
70 °C was added a suspension of FeCl2 (0.092 g, 0.72 mmol) in thf
(30 mL). The Grignard solution 2b was then slowly added to the
stirred suspension. The reaction mixture was stirred at 70 °C for
48 h. Water (100 mL) and hexane (80 mL) were added, and the
resulting suspension was transferred to a 500-mL separatory fun-
nel. The organic phase was collected, and the inorganic phase was
washed twice with hexane and extracted. The combined organic
phases were washed with a saturated NaCl solution and dried with
Na2SO4. The organic phase was concentrated under vacuum, and
at –25 °C pale yellow crystals were obtained. Yield: 2.14 g (86%).
1
M.p. 193.5 °C. H NMR (250.13 MHz, CDCl3): δ = 8.33 (s, 1 H, CD2Cl2): δ = 170.22 (N=CH), 166.21 (C), 154.87 (C), 143.30 (C),
N=CH), 8.29–8.26 (dd, J = 7.8, 0.9 Hz, 1 H, pyridine), 7.92–7.86
(t, J = 7.7 Hz, 1 H, 4pyridine-H), 7.44–7.40 (dd, J = 7.6, 0.9 Hz, 1 H,
140.98 (C), 140.11 (CH), 138.40 (C), 135.93 (CH), 133.78 (CH),
129.55 (CH), 128.76 (CH), 127.59 (CH), 123.49 (CH), 28.97
pyridine), 7.19–7.10 (m, 5 H, C6H2/C6H3), 3.07–2.90 [sept, J = [2ϫCH(CH3)2], 24.27 (2ϫCH3, isopropyl), 22.93 (2ϫCH3, iso-
6.8 Hz, 3 H, 3ϫ(CH3)2CH-C6H2], 2.63–2.46 [sept, J = 6.8 Hz, 2
H, 2ϫ(CH3)2CH-C6H3], 1.28–1.26 [d, J = 7.0 Hz, 12 H, 2ϫ(CH3)2-
CH-C6H3], 1.18–1.12 [q, J = 6.8 Hz, 18 H, 3ϫ(CH3)2CH-C6H2]
propyl), 20.68 [(CH3)2CH-C6H4] ppm. C26H30Cl2N2Pd·0.5CH2Cl2
(590.29): calcd. C 53.92, H 5.29, N 4.75; found C 53.52, H 5.35, N
4.41.
4226
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 4221–4228

Sterically Demanding Iminopyridine Ligands
6: Compound 3a (0.25 g, 0.67 mmol), [NiCl2(dme)] (0.145 g, 17.49, 11.24, 5.51, 3.46, 2.78, 2.53, 2.16, 1.25, 1.17, 1.14, 0.03,
0.66 mmol), and thf (50 mL) were added together in a Schlenk flask
under an atmosphere of nitrogen. The reaction mixture was stirred
at 70 °C for 18 h. The solvent was removed under vacuum, and the
red residue was washed with hexane, dissolved in thf, and filtered
through quartz. The filtrate was concentrated, and orange crystals
were grown at –25 °C. Yield: 0.256 g (59%). µeff = 2.99 BM
(25.9 °C). 1H NMR (250.13 MHz, CDCl3): δ = 32.03, 24.09, 20.65,
10.74, 8.85, 8.34, 8.24, 7.92, 7.14, 5.48, 3.75, 2.99, 2.41, 2.10, 1.85,
1.18, 0.89, 0.06 ppm. C26H30Cl2N2Ni·2C4H8O (644.34): calcd. C
63.38, H 7.20, N 4.35; found C 63.04, H 7.57, N 4.54.
–4.63 ppm. C33H44Cl2N2Ni (598.32): calcd. C 66.24, H 7.41, N
4.68; found C 65.94, H 7.22, N 4.44.
11: Compound 3b (0.283 g, 0.604 mmol) and CoCl2 (0.08 g,
0.604 mmol) were dissolved in thf (40 mL). The resulting dark
green reaction mixture was stirred for 20 h at 60 °C. The solvent
was removed under vacuum. The product was washed with hexane,
dissolved in thf, filtered through quartz, and concentrated. Green
crystals were collected at –25 °C. Yield: 0.21 g (58%). M.p. 272 °C.
µeff = 4.38 BM (26 °C). 1H NMR (250.13 MHz, CDCl3): δ = 28.16,
21.32, 8.29, 8.20, 7.86, 7.12, 7.07, 6.88, 5.05, 3.72, 3.11, 2.93, 2.50,
1.62, 1.39, 1.23, 1.12, 0.82, 0.70, 0.35, 0.04, –1.56, –4.38, –5.26,
–5.43 ppm. C33H44Cl2CoN2 (598.56): calcd. C 66.22, H 7.41, N
4.68; found C 65.89, H 7.62, N 4.51.
7: In a closed Schlenk flask, a stirred solution of 3a (0.362 g,
0.98 mmol) and CoCl2 (0.12 g, 0.92 mmol) in thf (30 mL) was
heated at 70 °C for 18 h. The solvent was removed under vacuum,
and the red residue was washed with hexane, dissolved in thf, and
filtered through quartz. The filtrate was concentrated, and green
needles were grown at –25 °C. Yield: 0.43 g (77%). M.p. 300 °C.
µeff = 4.17 BM (25.9 °C). 1H NMR (250.13 MHz, CDCl3): δ =
34.70, 32.45, 19.13, 7.16, 5.21, 3.75, 3.34, 2.33, 2.15, 2.09, 1.80,
1.43, 1.22, 1.17, 1.14, 1.10, 0.85, 0.82, 0.04, –2.40, –3.37,
–8.80 ppm. C26H30Cl2CoN2·C4H8O (572.47): calcd. C 62.94, H
6.69, N 4.89; found C 63.23, H 6.22, N 5.18.
Acknowledgments
Financial support from the Fonds der Chemischen Industrie is
gratefully acknowledged.
[1] S. Mecking, Angew. Chem. 2001, 113, 550–557; Angew. Chem.
Int. Ed. 2001, 40, 534–540.
8: In
a dry box, 3b (0.243 g, 0.52 mmol), FeCl2 (0.066 g,
[2] a) G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem.
1999, 111, 448–468; Angew. Chem. Int. Ed. 1999, 38, 428–447;
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Chem. 2003, 1620–1631.
0.52 mmol), and thf (50 mL) were combined in a Schlenk flask un-
der an atmosphere of nitrogen. The resulting solution was stirred
at 60 °C for 48 h. The solvent was removed, and the precipitate was
washed with hexane, dissolved in thf, and filtered through quartz.
The filtrate was concentrated to dryness under vacuum, resulting
in a dark red solid product. Yield: 0.11 g (32%). µeff = 6.16 BM
(25.1 °C). 1H NMR (250.13 MHz, CDCl3): δ = 39.31, 8.23, 7.86,
7.09, 2.92, 2.50, 1.56, 1.13, 0.05 ppm. C33H44Cl2FeN2·C4H8O
(667.57): calcd. C 66.57, H 7.85, N 4.20; found C 66.38, H 7.55, N
3.92.
[3] a) D. M. Haddleton, C. B. Jasieczek, M. J. Hannon, A. J.
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9: A stirred solution of 3b (0.252 g, 0.538 mmol) and [PdCl2(cod)]
(0.138 g, 0.483 mmol) in thf (50 mL) was heated at 60 °C for 72 h.
The solvent was removed under vacuum, and the residue was
washed with hexane, dissolved in toluene (50 mL), and filtered
through quartz. The solvent was then removed, and the yellow
product was dried under vacuum. Yield: 0.18 g (47%). M.p. 161 °C.
1H NMR (250.13 MHz, CD2Cl2): δ = 8.18 (s, 1 H, N=CH), 8.17–
8.11 (t, J = 8.0 Hz, 1 H, 4pyridine-H), 7.92–7.89 (d, J = 7.5 Hz, 1 H,
pyridine), 7.64–7.61 (d, J = 7.9 Hz, 1 H, pyridine), 7.38–7.32 (t, J
= 7.2 Hz, 1 H, p-CH-C6H3), 7.22–7.18 (d, J = 8.0 Hz, 2 H, 2ϫm-
CH-C6H3), 7.06 (s, 2 H, C6H2), 3.40–3.28 [sept, J = 6.8 Hz, 2 H,
2ϫCH(CH3)2], 3.02–2.88 [sept, J = 7.0 Hz, 1 H, p-CH(CH3)2],
2.62–2.50 [sept, J = 6.8 Hz, 2 H, 2ϫCH(CH3)2], 1.59–1.56 (d, J =
6.8 Hz, 6 H, 2ϫCH3), 1.37–1.36 (d, J = 6.8 Hz, 6 H, 2ϫCH3),
1.29–1.26 (d, J = 6.9 Hz, 6 H, 2ϫCH3), 1.18–1.15 (d, J = 6.9 Hz,
6 H, 2ϫCH3), 1.10–1.08 (d, J = 6.8 Hz, 6 H, 2ϫCH3) ppm. 13C
NMR (62.90 MHz, CD2Cl2): δ = 170.22 (N=CH), 166.71, 154.88,
151.11, 146.18, 143.42, 141.13, 139.20, 134.39, 134.16, 128.90,
127.70, 123.70, 121.24, 34.74, 31.72, 29.07, 25.18, 24.53, 24.18,
24.11, 23.18 ppm. C33H44Cl2N2Pd·C4H8O (718.15): calcd. C 61.88,
H 7.30, N 3.90; found C 61.40, H 7.33, N 3.61.
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10: Compound 3b (0.24 g, 0.512 mmol), [NiCl2(dme)] (0.113 g,
0.512 mmol), and thf (50 mL) were added together in a Schlenk
flask under an atmosphere of nitrogen. The reaction mixture was
stirred at 70 °C for 18 h. The solvent was removed, and the beige
precipitate was washed with hexane, dissolved in thf, and filtered
through quartz. After concentration, beige crystals were grown at
–25 °C and dried under vacuum. Yield: 0.044 g (15%). µeff
=
2.57 BM (26 °C). 1H NMR (250.13 MHz, CDCl3): δ = 39.53, 24.43,
Eur. J. Inorg. Chem. 2007, 4221–4228
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4227

T. Irrgang, S. Keller, H. Maisel, W. Kretschmer, R. Kempe
FULL PAPER
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Received: April 12, 2007
Published Online: July 18, 2007
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