2,6-diiminopyridine iron(II) dialkyl complexes. Interaction with aluminum alkyls and ethylene polymerization catalysis

Article abstract of DOI:10.1021/om050645d

Bis(pyridine)iron dialkyl complexes FeR₂Py₂ (R = CH₂Ph, CH₂CMe₂Ph, and CH₂SiMe ₃) react with 2,6-diiminopyridine ligands, affording the corresponding organoiron complexes. The complex Fe(CH₂SiMe ₃)2-(C₅H₃N-2,6-(C(=N-mesityl)Me)₂, 4a, becomes an active ethylene polymerization catalyst upon treatment with Al or Zn alkyls. A catalytically active 1:1 Fe/Al adduct has been detected in solution by ¹H and UV-vis spectroscopies.

Full text of DOI:10.1021/om050645d

4878
Organometallics 2005, 24, 4878-4881
2,6-Diiminopyridine Iron(II) Dialkyl Complexes.
Interaction with Aluminum Alkyls and Ethylene
Polymerization Catalysis
Juan Ca´mpora,* A. Marcos Naz, Pilar Palma, and Eleuterio AÄ lvarez
Instituto de Investigaciones Quı´micas, CSIC-Universidad de Sevilla, c/ Ame´rico Vespucio,
49, 41092, Sevilla, Spain
Manuel L. Reyes
Centro de Tecnologı´a Repsol-YPF, Carretera de Extremadura NV, Km 18, 28931,
Mo´stoles, Madrid, Spain
Received July 28, 2005
Summary: Bis(pyridine)iron dialkyl complexes FeR2Py2
(R ) CH2Ph, CH2CMe2Ph, and CH2SiMe3) react with
2,6-diiminopyridine ligands, affording the corresponding
organoiron complexes. The complex Fe(CH2SiMe3)2-
(C5H3N-2,6-(C(dN-mesityl)Me)2, 4a, becomes an active
ethylene polymerization catalyst upon treatment with Al
or Zn alkyls. A catalytically active 1:1 Fe/Al adduct has
been detected in solution by 1H and UV-vis spec-
troscopies.
investigations, we decided to extend this methodology
to iron. Thus, the reaction of FeCl2Py4 with alkylmag-
nesium reagents Mg(R)Cl (R ) CH2SiMe3, CH2Ph, or
CH2CMe2Ph) in Et2O gives rise to deep purple solutions
that contain the desired dialkyliron complexes (eq 1).
A standard workup, followed by crystallization from
hexane, allows the isolation of crystals of benzyl (1) and
neophyl (2) complexes of composition FeR2Py2. The
extreme solubility of the trimethylsilyl derivative 3
prevented its isolation in pure form, but the analysis of
the solution contents by 1H NMR showed paramagneti-
cally shifted resonances corresponding to the SiMe3 (ca.
11.2 ppm) and pyridine ligands (35.5 and 17.4 ppm, for
H3 and H4, respectively). The NMR spectra of 1 and 2
display signals corresponding to the pyridine H3 and
H4 signals at roughly the same positions as those found
in the spectrum of 3, and other resonances that can be
assigned to the benzyl or neophyl groups.6
Iron bis(imino)pyridine complexes are extremely ac-
tive catalysts for ethylene polymerization.1 The active
species involved in the catalysis are Fe(II) alkyl com-
plexes that are generated in situ from the corresponding
dihalide complexes and a suitable cocatalyst such as
MAO or aluminum trialkyls.2 However, the isolation of
iron alkyl complexes containing bis(imino)pyridine
ligands that could serve as a model of the catalytic
species has proven elusive until recently.3,4 Herein we
describe the synthesis of three Fe(II) (trimethylsilyl)-
methyl complexes containing 2,6-diiminopyridine ligands
by means of ligand exchange reactions. During the
course of this work, Chirik described the preparation
of (diiminopyridine)iron alkyls, by the direct alkylation
of the precursor chloro complexes.4
We have shown that alkyl complexes of Ni and Pd
stabilized by labile pyridine ligands undergo facile
ligand exchange reactions with R-diimine and other
nitrogen ligands, and therefore they are valuable pre-
cursors for the systematic preparation of otherwise
difficult to synthesize complexes that are relevant in
polymerization chemistry.5 As a continuation for these
(1) (a) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem.
Soc. 1998, 120, 4049. (b) Britovsek, G. J. P. Gibson, V. C.; Kimberly,
B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.;
Williams, D. J. Chem. Commun. 1998, 849. (c) Gibson, V. C.; Spitzmess-
er, S. K. Chem. Rev. 2003, 103, 283. (d) Britovsek, G. J. P.; Bruce, M.;
Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.;
McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stro¨mberg, S.; White, A.
J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728.
(2) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1999, 121,
6479.
(3) Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.; Tellmann,
K. P.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans.
2002, 1159.
(4) (a) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2005, 127, 9660. (b) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E.
J.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. Chem. Commun. 2005,
3406.
The identity of compounds 1-3 has been confirmed
by the crystal structure of the neophyl derivative (Figure
1). This compound exhibits a distorted tetrahedral
coordination, similar to that found in Fe(Mes)2(Py)27 and
(5) Ca´mpora, J.; Conejo, M. M.; Mereiter, K.; Palma, P.; Perez, C.;
Reyes, M. L.; Ruiz, C. J. Organomet. Chem. 2003, 683, 220.
(6) 1H NMR data (C6D6, 25 °C): 1, δ -80.28 (116 Hz, 2H, p-CHar),
-58.03 (456 Hz, 4H, m-CHar), 34.04 (132 Hz, 4H, H3-py), 11.02 (2H,
H4-py); 2, δ 5.42 (48 Hz, m-CHar), 11.77 (330 Hz, 2H, p-CHar), 12.92
(117 Hz, 2H, H4-py), 25.23 (462 Hz, 12H, CMe2), 35.27 (174 Hz, 4H,
H3-py), 119.99 (1350 Hz, 4H, o-CHar).
(7) Klose, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re,
C. J. Am. Chem. Soc. 1994, 116, 9123.
10.1021/om050645d CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/02/2005

Communications
Organometallics, Vol. 24, No. 21, 2005 4879
Table 1. Ethylene Polymerization Dataa
catalyst (µmol)
activator (equiv)
P (bar)
yield (g)
activityb
Mn
PDI
Me/1000C
9
vinyl/1000C
2.1
iPr/1000C
FeCl2‚a (5)
4a (50)c
4a (4)
MMAO (2000)
B(C6F5)3 (0.90)
MMAO (1000)
TiBA (100)
TMA(100)
TMA (50)
2
3
5
5
5
5
5
5
5.34
0.85
3.68
5.33
4.65
4.73
0.35
1.14
1600
19
1840
90300
4730
3130
9770
14700
6250
12900
2.2
2.4
2.2
25d
10
5.5
9.7
3.0
5.6
550
800
700
710
55
17
1.8
1.0
1.0
0.9
0.4
1.3
2.5
4
4a (4)
4.8
4.0
2.7
4.6
1.5
4a (4)
0
4a (4)
0
4a (4)
ZnMe2 (100)
AlMe(OAr)2 (50)
0
4a (4)
170
0
a Experimental conditions: solvent, toluene, 50 mL, 30 °C, 20 min. b kg PE/mol Fe‚h‚bar. c Temperature: -80 °C to RT. d Bimodal
distribution: Mp ) 1200, 20400.
0.05 Å shorter in 4a on average), but more appreciable
for the Fe-N(pyridine) bond (2.0056(18) Å in 4a;
2.110(6) and 2.103(6) Å in the FeCl2 and FeBr2 deriva-
tives, respectively).
Complexes 4a-c give rise to typical paramagnetic 1H
spectra that can be assigned on the basis of the signal
intensities and by comparison with those of the analo-
gous halide derivatives. Together with the ligand reso-
nances, they display a single characteristic signal of
relative intensity 18 H, corresponding to the two SiMe3
groups. The chemical shifts of the NMR signals of 4a
show the expected linear 1/T dependency, but below 258
K, those corresponding to the SiMe3 and the mesityl
o-methyl groups split in two, indicating that a dynamic
process, responsible for their averaging at room tem-
perature, becomes slow at that temperature. This
process probably corresponds to the swinging oscillation
of the alkyl ligands between the axial and equatorial
positions. The coalescence temperatures for the SiMe3
(270 K) and o-Me signals (250 K) allow an estimation
of ca. 10 kcal/mol for the exchange energy barrier.10
The availability of compounds 4a-c led us to inves-
tigate the capability of iron alkyls to catalyze the
polymerization of ethylene (Table 1). As expected, these
high-spin compounds fail to react with ethylene.9a,11 To
convert compound 4a into a catalytically active species,
it was reacted with B(C6F5)3 at room temperature under
2 bar of ethylene, but no polymer was formed under
these conditions, despite the relatively large load of 4a
(50 µmol) that was used. A marginal polymerization
activity was observed when 4a and B(C6F5)3 were mixed
at -80 °C and then allowed to warm to room temper-
ature. The use of specially purified ethylene (stored over
a solution of triisobutylaluminum (TiBA) for 15 days)
did not change these results. However, high catalytic
activity levels, comparable to those obtained with the
conventional FeCl2 catalyst precursor and MMAO, were
recorded in the presence of various aluminum alkyls.
Under similar experimental conditions, 4a becomes
catalytically active in the presence of trimethylalumi-
num (TMA) or triisobutylaluminum (TiBA) at relatively
low Fe:Al ratios. ZnMe2, or even the aluminum alkyl-
aryloxide MeAl(OAr)2 (Ar dOC6H2But3-2,4,6), consid-
ered a good water scavenger but inert as a cocatalyst,12
also activates the iron complex, albeit not as efficiently
as the aluminum alkyls. Not surprisingly, the activity
of 4a in the presence of MMAO is very high, comparable
Figure 1. ORTEP view of compound 2. Selected bond
lengths (Å) and angles (deg): Fe(1)-C(1), 2.0905(18);
Fe(1)-C(11), 2.0895(19); Fe(1)-N(1), 2.1569(16); Fe(1)-
N(2): 2.1414(16); N(1)-Fe-N(2), 91.70(6); C(1)-Fe(1)-
C(11), 131.31(8).
in the related amide compound Fe(N(SiMe3)2)2(Py)2.8 As
observed in the latter compound, the angle N-Fe-N
formed by the pyridine nitrogen atoms and the metal
center is rather acute, approaching 90°. The Fe-C bond
lengths (ca. 2.09 Å) are similar to those found in other
tetracoordinated Fe(II) alkyls.9
Compounds 1-3 react rapidly with 2,6-diiminopyri-
dine ligands in hexane at low temperature, but only the
bis(trimethylsilyl)methyl derivatives 4a-c have been
isolated, as analytically pure dark violet solids (eq 2).
They display µeff values of 5.1 µB in the solid state,
consistent with a high-spin electronic configuration. The
crystal structure of compound 4a is shown in Figure 1.
The iron atom is in a five-coordinated environment, with
an approximately square-pyramidal geometry, as ob-
served in the recently reported Fe(II) alkyl complexes
with the bulky N,N′-bis(2,6-diisopropylphenyl)diimi-
nopyridine ligand.4b Despite the fact that the two alkyl
groups occupy axial and equatorial positions, the Fe-C
distances are virtually identical (ca. 2.077(2) and
2.075(2) Å, respectively). Somewhat surprisingly, the
Fe-N bonds are shorter in 4a than in the electron-
poorer FeX2 complexes of the same tridentate ligand.1d
The difference is small for the Fe-N(imine) bonds (ca.
(10) At 400 MHz. The chemical shifts of the signals at the coales-
cence temperature were corrected for their 1/T dependence by extrapo-
lation of the values recorded at lower temperatures.
(11) Harvey, J. N.; Poli, R.; Smith, K. M. Coord. Chem. Rev. 2003,
238, 347.
(12) Stapleton, R. A.; Galan, B. R.; Collins, S.; Simons, R. S.;
Garrison, J. C.; Youngs, W. J. J. Am. Chem. Soc. 2003, 125, 9246.
(8) Panda, A.; Stender, M.; Olmstead, M. M.; Klavins, P.; Power, P.
P. Polyhedron 2003, 22, 69.
(9) (a) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky,
E.; Chirik, P. J. Organometallics 2003, 23, 237. (b) Hermes, A. R.;
Girolami, G. S. Organometallics 1987, 6, 763.

4880 Organometallics, Vol. 24, No. 21, 2005
Communications
Figure 3. Central section of the 1H NMR spectrum of 4a
and 4a + 0.9 equiv of AlMe3, displaying the signals
corresponding to the mesityl, trimethylsilyl, and Al-Me
groups. Signals in the diamagnetic region have been
omitted for clarity.
Figure 2. ORTEP view of compound 4a. Selected bond
distances (Å) and angles (deg): Fe(1)-C(28), 2.075(2);
Fe(1)-C(32), 2.077(2); Fe(1)-N(1), 2.044(19); Fe(1)-N(3),
2.2284(19); Fe(1)-N(2), 2.0056(18); C(28)-Fe(1)-C(32),
117.23(9); C32-Fe(1)-N(2), 106.11(8); C(28)-Fe(1)-N(2),
136.66(8); N(1)-Fe(1)-N(3), 144.11(7).
pendant AlMe2 unit connected to the Fe moiety by
bridging Me groups in Bryliakov’s system. Measurement
of the magnetic moment of this species (Evans’ method)
afforded values close to 5.0 µB in the full temperature
range studied by NMR, indicating that the oxidation
state of Fe has not changed. These results give support
to the occurrence of 4a‚TMA as a high-spin, neutral
Fe(II) complex. To examine the species formed in
conditions more akin to the real catalysis setup, the 1H
NMR spectra of mixtures of 4a with increasing amounts
of TMA (up to 10 equiv) were obtained. These display
new signals, indicating the formation of one or more
different species, and are significantly broader. Al-
though these spectra are difficult to interpret, the UV-
vis spectra of the 4a/TMA mixtures show no important
variations for different Fe:Al ratios, suggesting that the
new species formed are structurally similar, probably
arising from alkyl exchange processes.
to that of the analogous chloro complex. The polymers
produced with MMAO and TiBA display isopropyl
branches, indicating that Fe-Al chain transfer occurs
in the process, as is normally observed with iron
catalysts.1d
The relatively low number of vinyl terminal groups
(systematically less than 1 per molecule) indicates that
chain transfer to the cocatalyst competes with â-H
elimination as the chain termination processes.
These results prompted us to investigate the interac-
tion of 4a with trimethyl aluminum, as a functional
model of the catalytic reaction. It was noticed that,
under the highly diluted conditions used in catalysis (4
µmol Fe in 50 mL of toluene), a minimal Fe:Al ratio of
1:25 is required to generate an active system, even using
purified ethylene. However, in more concentrated solu-
tions (ca. 0.02 M), 4a reacts rapidly with 1 equiv of TMA,
with a color change from deep violet to greenish-yellow.
If ethylene is bubbled through these solutions, precipi-
The fact that 4a develops very low activity levels on
treatment with B(C6F5)3 (and this only when low tem-
perature is used) is somewhat surprising in view of
the recent results of Chirik, who showed that the
latter Lewis acid reacts with an Fe(II) dialkyl related
to 4a containing the very bulky N,N′-bis(2,6-diiso-
propylphenyl)-2,6-diiminopyridine ligand, to afford a
stable cationic complex that acts as a single-compo-
nent polymerization catalyst.4a A possible explanation
for our observation is that the mesityl-based pyri-
dinediimine ligand does not provide enough steric
stabilization to such cationic species under the actual
polymerization conditions. In any case, the formation
of the Fe(II)R2-AlR3 adduct is very important under the
usual catalysis conditions and contributes to the stabi-
lization of the catalytic species in its resting state. It is
conceivable that these adducts could dissociate into
1
tation of PE takes place. The H NMR spectrum of a
1:0.9 4a/TMA mixture in C6D6 shows the formation of
a major product (Figure 2) displaying one single signal
for the four ortho-methyl groups of the aryl substituents,
which indicates an effective C2v symmetry. No evidence
for fluxional processes was found when the spectra were
recorded in the temperature range 300-210 K (toluene-
d8). This feature makes it difficult to propose a specific
binding mode for TMA, since alkyl-bridged structures
such as those proposed by Bryliakov13 for the species
formed upon reaction of FeCl2(diiminopyridine) com-
plexes with AlMe3 would decrease the molecular sym-
metry. Even so, the 4a/TMA spectrum bears some
resemblance to the latter. For example, the signal at
ca. 36 ppm is almost identical to that assigned to the
-
Fe(II)R+AlR4 ion pairs, with the anion playing a
stabilizing role that can be considered analogous to that
of phosphines (or other secondary ligands) in nickel-
catalyzed olefin oligomerization or polymerization reac-
tions.14
(13) Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Talsi,
E. P. Organometallics 2004, 23, 5375.

Communications
Organometallics, Vol. 24, No. 21, 2005 4881
In summary, we have developed a new method for the
preparation of Fe(II) alkyl complexes with nitrogen
ligands and showed that these compounds can be
employed in the investigation of the mechanism of the
Fe-catalyzed ethylene polymerization reactions. Our
results suggest that aluminum cocatalysts may have an
important role in the stabilization of the catalyst resting
state and in the modulation of its activity. We are
currently extending our methodology to the synthesis
of other types of Fe(II) alkyls that could favor the
isolation and characterization of catalytically active Fe-
Al complexes.
Acknowledgment. Financial support from the DGI
(Project PPQ2003-00975) and Junta de Andaluc´ıa is
gratefully acknowledged. A.M.N. is grateful for a re-
search fellowship from the Consejo Superior de Inves-
tigaciones Cient´ıficas (I3P program). We thank Prof. E.
Carmona for helpful discussions.
Supporting Information Available: Text giving experi-
mental procedures and characterization data for all new
complexes and tables of crystallographic X-ray data for 2 and
4a, and NMR and UV-vis spectra of 4a + TMA. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) (a) Pietsch, J.; Braunstein, P.; Chauvin, Y. New J. Chem. 1998,
22, 467. (b) Heinicke, J.; He, M. Z.; Dal, A.; Klein, H. F.; Hetche, O.;
Keim, W.; Florke, U.; Haupt, H. J. Eur. J. Inorg. Chem. 2000, 431. (c)
Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs,
R. H.; Bansleben, D. A. Science 2000, 287, 460. (d) Jenkins, J. C.;
Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827.
OM050645D