Activation of 2,6-bis(imino)pyridine iron(II) chloride complexes by methylaluminoxane: An electrospray ionization tandem mass spectrometry investigation

Article abstract of DOI:10.1021/om050351s

The activation of the [2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl) pyridine]iron(II) chloride complex (1) with methylaluminoxane (MAO) was investigated in tetrahydrofuran (THF) by means of electrospray ionization tandem mass spectrometry (ESI-MS) compl

Full text of DOI:10.1021/om050351s

3664
Organometallics 2005, 24, 3664-3670
Activation of 2,6-Bis(imino)pyridine Iron(II) Chloride
Complexes by Methylaluminoxane: An Electrospray
Ionization Tandem Mass Spectrometry Investigation
Pascal M. Castro,† Petro Lahtinen,† Kirill Axenov,† Jyrki Viidanoja,‡
Tapio Kotiaho,‡ Markku Leskela¨,† and Timo Repo*,†
Laboratory of Inorganic Chemistry and Laboratory of Analytical Chemistry,
Department of Chemistry, A.I. Virtasen Aukio 1, P.O. Box 55, University of Helsinki,
FI-00014 Helsinki, Finland
Received May 4, 2005
The activation of the [2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine]iron(II) chloride
complex (1) with methylaluminoxane (MAO) was investigated in tetrahydrofuran (THF) by
means of electrospray ionization tandem mass spectrometry (ESI-MS) complemented by UV-
visible spectroscopy studies. The four-coordinated cationic methyl iron(II) complex 2 ([LFe-
Me]+, L ) 2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine) was identified in the ESI-
MS experiment. In addition, the cationic monochloride iron(II) complex 3 ([LFe-Cl]+), the
product of R-H transfer from 2 to trimethylaluminum (TMA) [LFe-CH2AlMe2]+ (4), and the
cationic iron hydride complex 5 ([LFe-H]+) were identified. Correlation of the ESI-MS
experiments with UV-visible studies revealed that 2 and 3 are present in the THF solution
as their solvent adducts.
Introduction
locene dichloride by aluminum alkyls present in MAO
and (2) abstraction of the remaining chloride by the
Lewis acidic MAO species. As a result, a monometh-
ylated metallocene cation with a weakly coordinated
counteranion is formed and is accountable for the
catalytic activity in olefin polymerization.3a,5 If excess
amounts of MAO are used, the first step can lead to the
dialkylation product, which undergoes methyl abstrac-
tion by MAO to generate the active species.
With regard to iron complexes, if stable diamagnetic
dialkyl and alkyl halogen complexes are known,6 their
paramagnetic counterparts are prompt to decompose via
reductive elimination,7 which significantly restricts the
possibility to isolate and characterize the catalytically
active components of the polymerization system. Ex-
amples of structurally characterized stable paramag-
netic iron(II) alkyl or halogeno alkyl complexes are
rather scarce and have only been recently reported.8
Therefore, a different pathway was proposed for the
activation of 2,6-bis(imino)pyridine iron(II) chloride
Among the variety of late transition metal complexes
introduced in the field of polymerization catalysis almost
a decade ago,1 2,6-bis(arylimino)pyridine iron(II) chlo-
ride complexes were reported to be efficient catalysts
for the polymerization of ethylene after activation with
methylaluminoxane (MAO).2 By analogy with metal-
locene catalysts,3 it is assumed that the species active
in polymerization is a highly reactive monomethylated
iron(II) cation ([LFe-Me]+) bearing a weakly coordinat-
ing counteranion ([X-MAO]-, X ) Cl, Me);2,4 chain
propagation occurs after olefin coordination to the metal
center and subsequent insertion into the metal-carbon
bond.
As a matter of fact, activation of group 4 metallocene
dichlorides with MAO has been an extensively studied
subject, and the activation process usually proposed
consists of two steps: (1) monoalkylation of the metal-
* To whom correspondence should be addressed. Fax: (+358)-9-
19150198. E-mail: timo.repo@helsinki.fi.
† Laboratory of Inorganic Chemistry.
(5) Kaminsky, W.; Stru¨bel, C. J. Mol. Catal. A: Chem. 1998, 128,
191-200.
‡ Laboratory of Analytical Chemistry.
(1) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428-447. (b) Ittel, S. D.; Johnson, L. K.; Brookhart,
M. Chem. Rev. 2000, 100, 1169-1203. (c) Gibson, V. C.; Spitzmesser,
S. K. Chem. Rev. 2003, 103, 283-315.
(2) (a) 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, 8728-8740. (b) Small, B. L.; Brookhart, M. Macro-
molecules 1999, 32, 2120-2130.
(3) For metal complex activation by Lewis acid compounds in
general, see: (a) Chen, E. Y. X.; Marks, T. J. Chem. Rev. 2000, 100,
1391-1434. (b) Coevoet, D.; Cramail, H.; Deffieux, A.; Mladenov, C.;
Pedeutour, J. N.; Peruch, F. Polym. Int. 1999, 48, 257-263.
(4) (a) Griffiths, E. A. H.; Britovsek, G. J. P.; Gibson, V. C.; Gould,
I. R. Chem. Commun. 1999, 1333-1334. (b) Deng, L.; Margl, P.; Ziegler,
T. J. Am. Chem. Soc. 1999, 121, 6479-6487. (c) Khoroshun, D. V.;
Musaev, D. G.; Vreven, T.; Morokuma, K. Organometallics 2001, 20,
2007-2026.
(6) (a) Yamamoto, A.; Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y.;
Misono, A. J. Am. Chem. Soc. 1965, 87, 4652-4653. (b) Yamamoto,
A.; Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y.; Misono, A. J. Am.
Chem. Soc. 1968, 90, 1878-1883. (c) Lau, W.; Huffman, J. C.; Kochi,
J. K. Organometallics 1982, 1, 155-169. (d) Field, L. D.; Messerle, B.
A.; Smernik, R. J.; Hambley, T. W.; Turner, P. Inorg. Chem. 1997, 36,
2884-2892. (e) Bennett, M. A.; Ditzel, E. J.; Hunter, A. D.; Khan, K.;
Kopp, M. R.; Neumann, H.; Robertson, G. B.; Zeh, H. Dalton 2000,
1733-1741.
(7) (a) Ikariya, T.; Yamamoto, A. J. Chem. Soc., Chem. Commun.
1974, 720. (b) Ikariya, T.; Yamamoto, A. J. Organomet. Chem. 1976,
118, 65-78.
(8) (a) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky,
E.; Chirik, P. J. Organometallics 2004, 23, 237-246. (b) Vela, J.;
Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.; Lachicotte,
R. J.; Flaschenriem, C. J.; Holland, P. L. Organometallics 2004, 23,
5226-5239. (c) Hermes, A. R.; Girolami, G. S. Organometallics 1987,
6, 763-768.
10.1021/om050351s CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/18/2005

Activation of Pyridine Iron(II) Chloride Complexes
Organometallics, Vol. 24, No. 15, 2005 3665
complexes: (1) abstraction of one of the chlorides
followed by (2) transmetalation with an alkylaluminum
species, leading to the cationic monomethylated iron
catalyst.9,10 Both monochloride and monoalkyl cationic
species are expected to be present in the solution, and
their respective concentrations depend on the relative
concentration of MAOsan excess of which favors the
formation of the monoalkyl species to the detriment of
the monochloride cation.
the aptitude of electrospray to transfer ionic species
from a sample solution to the gas phase in order to
identify the different products resulting from the activa-
tion of the complex [2,6-bis(1-((2,6-diisopropylphenyl)-
imino)ethyl)pyridine]iron(II) chloride (1) with MAO in
a THF solution by tandem mass spectrometry and to
attest to the existence of the iron methyl cation.
Experimental Section
Regardless of the activation pathway, coordinatively
unsaturated monoalkyl metal cations are likely to be
stabilized by further interactions with other molecules
present in the solution: i.e., solvent or cocatalyst.
Indeed, bimetallic metallocene species of the type
[Cp2M(µ-Me)2AlMe2]+[X-MAO]- (M ) group 4 metal)
have been identified and are believed to be the dormant
state of the catalytically active [Cp2M-Me]+[X-MAO]-
ion pair, the position of the equilibrium between these
two species governing the catalytic activity.3a,11 Cor-
respondingly, in the case of 2,6-bis(imino)pyridine iron-
(II) catalysts, the formation of a heterobinuclear Fe-
Al complex via coordination of trimethylaluminum
(TMA) to the cationic iron(II) center has been evidenced
by paramagnetic 1H NMR in toluene-d8,12 and an
equilibrium between active and inactive species identi-
cal with that described above was confirmed experi-
mentally in the polymerization of ethene.13 The actual
active species, [LFe-Me]+[X-MAO]-, was thus defined
by Britovsek et al. as “a transition state or perhaps a
high energy intermediate.”13 Accordingly, to the best of
our knowledge, the existence of the mononuclear cat-
ionic iron(II) complex [LFe-Me]+[X-MAO]- has never
been shown spectroscopically.14
General Considerations. All the manipulations were done
under an inert argon atmosphere using standard Schlenk
techniques. The hydrocarbon and ethereal solvents were
refluxed over sodium and benzophenone, distilled, and stored
under argon with sodium flakes. All the reagents were
purchased from commercially available sources and used
without further purification. MAO (30 wt % in toluene solu-
tion) was received from Borealis Polymer Oy. The complexes
[2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine]iron-
(II) chloride (1),2 [2,6-bis(1-((2,6-diisopropylphenyl)imino)eth-
yl)pyridine]iron(II) chloride hexafluoroantimonate-acetonitrile
(3SbF6-‚CH3CN),9 and [2,6-bis(1-(isopropylimino)ethyl)pyridi-
ne]iron(II) chloride (6)20 were synthesized according to pub-
lished procedures. 1H NMR spectra were collected at room
temperature in CDCl3 with a Varian 200 NMR spectrometer
at 200 MHz.
UV-Visible Spectroscopy. UV-vis spectroscopy mea-
surements were carried out with a HP 8453E spectrophotom-
eter and UV-visible ChemStation software. Samples were
withdrawn from the catalyst solution (630 µmol/L) and trans-
ferred under an argon atmosphere to a gastight rectangular
quartz cuvette (10 mm path length) fitted with a silicon
septum.
ESI-MS Measurements. The electrospray ionization tan-
dem mass spectrometric study was undertaken with an API-
300 triple-quadrupole mass spectrometer from Perkin-Elmer-
SCIEX (Toronto, Ontario, Canada). The ESI-MS spectra were
acquired by infusing the sample solution directly into the Sciex
Turboionspray source at a rate of approximately 30 µL/min.
No heating was applied. Nitrogen was used as the nebulizing
gas to assist the electrospray process and to provide a
relatively inert atmosphere within the ion source. Full MS
spectra were acquired every 2 s. The orifice plate, focusing ring,
and curtain gas flow were maintained at 30 V, 300 V, and
approximately 1.1 L/min, respectively.
The combination of electrospray ionization techniques
and tandem mass spectrometry (ESI-MS) is an intrigu-
ing analytical method for the characterization of organic
and organometallic compounds.15 Due to its relatively
soft ionization mode, this technique has found new
applications in the study of metal complexes,16 metal-
catalyzed reaction mechanisms,17 polymerization cata-
lysts,18 and high-throughput screening of homogeneous
catalysis.19 Accordingly, we thought it pertinent to use
Preliminary experiments with 1/MAO in toluene as the
solvent were not satisfactory, probably due to the nonpolar
nature of toluene, which is not suitable for the electrospray
ionization mode. THF was then chosen as the solvent due to
its higher polarity and because polymerization of acrylates
proceeds in this solvent with MAO-activated 2,6-bis(imino)-
pyridine iron(II) complexes.21 The catalyst solution (63 µmol/
L) was prepared by dissolving the iron complex in dry THF
followed by the addition of 50 molar equiv of the aluminum
activator. The injection system was flushed with dry THF
beforehand, and 50 µL of the catalyst solution was injected
prior to the measurement, to remove traces of water from the
injection line and electrospray source. Caution! The activator-
to-complex ratio was voluntarily limited to 50 in order to avoid
the occlusion of the source and injection system by aluminum
oxide, resulting from the hydrolysis of MAO by inherent traces
of moisture.
(9) Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.; Tellmann,
K. P.; White, A. J. P.; Williams, D. J. J. Dalton 2002, 6, 1159-1171.
(10) Babik, S. T.; Fink, G. J. Mol. Catal. A: Chem. 2002, 188, 245-
253.
(11) (a) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl.
1994, 33, 1634-1637. (b) Tritto, I.; Donetti, R.; Sacchi, M. C.; Locatelli,
P.; Zannoni, G. Macromolecules 1997, 30, 1247-1252. (c) Petros, R.
A.; Norton, J. R. Organometallics 2004, 23, 5105-5107.
(12) (a) Talsi, E. P.; Babushkin, D. E.; Semikolenova, N. V.; Zudin,
V. N.; Panchenko, V. N.; Zakharov, V. A. Macromol. Chem. Phys. 2001,
202, 2046-2051. (b) Bryliakov, K. P.; Semikolenova, N. V.; Zudin, V.
N.; Zakharov, V. A.; Talsi, E. P. Catal. Commun. 2004, 5, 45-48. (c)
Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P.
Organometallics 2004, 23, 5375-5378.
(13) Britovsek, G. J. P.; Cohen, S. A; Gibson, V. C.; van Meurs, M.
J. Am. Chem. Soc. 2004, 126, 10701-10712.
(14) The 1H NMR resonance of methyl groups in positions R to the
paramagnetic iron(II) center turned out to be, in any event, too broad
for the detection of a persistent [LFe-Me]+ species.12a
(15) (a) Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J.
B. Anal. Chem. 1985, 57, 675-679. (b) Fenn, J. B.; Mann, M.; Meng,
C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71. (c)
Vestal, M. L. Chem. Rev. 2001, 101, 361-375.
(16) Lipshutz, B. H.; Stevens, K. L.; James, B.; Pavlovich, J. G. J.
Am. Chem. Soc. 1996, 118, 6796-6797.
(17) (a) Aliprantis, A. O.; Canary, J. W. J. Am. Chem. Soc. 1994,
116, 6985-6986. (b) Sabino, A. A.; Machado, A. H. L.; Correia, C. R.
D.; Eberlin, M. N. Angew. Chem., Int. Ed. 2004, 43, 2514-2518.
(18) Feichtinger, D.; Plattner, D. A.; Chen, P. J. Am. Chem. Soc.
1998, 120, 7125-7126.
Hydrolysis of 1/MAO in THF. A 38 mg portion of [2,6-
bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine]iron(II) chlo-
(19) (a) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832-2847. (b)
Hinderling, C.; Chen, P. Angew. Chem., Int. Ed. Engl. 1999, 38, 2253-
2256.
(20) Castro, P. M.; Lappalainen, K.; Ahlgre´n, M.; Leskela¨, M.; Repo,
T. J. Polym. Sci. Part A: Polym. Chem. 2003, 41, 1380-1389.
(21) Castro, P. M.; Lankinen, M. P.; Uusitalo, A. M.; Leskela¨, M.;
Repo, T. Macromol. Symp. 2004, 213, 199-208.

3666 Organometallics, Vol. 24, No. 15, 2005
Castro et al.
another 1 h of stirring, the organic phase was separated from
the aqueous phase, and the latter was further extracted with
2 × 5 mL of CH2Cl2. The different organic phases were
gathered together and dried over magnesium sulfate, and the
solvent was removed in vacuo to afford nearly 30 mg (99%) of
2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine (yellow
powder). The 1H NMR spectrum matched the values reported
in the literature.2
Results and Discussion
Activation with MAO. A priori, despite the precau-
tions taken during the sample preparation and transfer,
the mass spectrum obtained (Figure 1) reveals that the
species present in the solution seem to be particularly
sensitive to traces of moisture and/or other contami-
nants, as exemplified by the presence of the ligand peak
at m/z 482 (ligand + H+). It is important to notice that,
under the same conditions, 1 alone gave only a small
signal corresponding to the ligand peak and that MAO
alone did not give any significant signal. The origin of
the base peak at m/z 523 and of the second most
prominent signal at m/z 538 will be discussed later in
the paper.
Figure 1. ESI-MS spectrum of 1/MAO in THF.
A peak centered at m/z 552 corresponding to the
molar mass of [2,6-bis(1-((2,6-diisopropylphenyl)imino)-
ethyl)pyridine]Fe+-Me (2) could be reproducibly ob-
served in the ESI-MS spectrum of 1/MAO. The frag-
mentation pattern of this ion was further studied by
collision-induced dissociation (CID) at the collision
energy of 30 eV (Figure 2). The fragment at m/z 537
correlates to the loss of a methyl group, followed by the
loss of one isopropyl group from the ligand (m/z 494).
The nature of the m/z 552 peak was additionally
confirmed by comparing its isotopic fingerprint (Figure
3, left) to a computer-generated theoretical plot (Figure
3, right). Analogously, the m/z 572 ion corresponds
unambiguously to LFe+-Cl (3) (fragments obtained by
CID at m/z 536 (3 - Cl); 521 (-Me); 506 (-Me)).22 The
presence of 3 is explained by the low MAO concentration
used for the ESI-MS experiments.
Figure 2. ESI-MS/MS spectrum of the mass-selected ion
2 from the activation of 1 with MAO in THF. Collision
energy 30 eV.
ride (1; 63 µmol) was dissolved in 100 mL of THF ([Fe] ) 6.3
µmol/L). A 50 molar equiv amount of MAO (30 wt % in toluene,
0.7 mL) was then added to the complex solution. After the
mixture was stirred at room temperature for 1 h, the solvent
was removed in vacuo, the residue was dissolved in 10 mL of
regular CH2Cl2, and 10 mL of 1 M HCl was added. After
MAO is synthesized by controlled hydrolysis of TMA,
which is present to a certain extent as a free or MAO-
bound species in the cocatalyst solution.3a Considering
that both MAO and TMA are efficient alkylating agents,
Figure 3. Isotopic fingerprint of the ion 2 issued from the ESI-MS spectrum of 1/MAO in THF (left) and a computer-
generated theoretical plot corresponding to 2 (right).

Activation of Pyridine Iron(II) Chloride Complexes
Organometallics, Vol. 24, No. 15, 2005 3667
Figure 4. ESI-MS spectrum of the products of the reaction
of 1 with TMA in THF.
Figure 5. UV-vis spectra of 1 (solid line), 1/MAO (dotted
line), and 1/TMA (dashed line) in THF at room tempera-
ture. [Fe] ) 630 µmol/L; [Al]/[Fe] ) 50.
it is conceivable that 1 will react with TMA to form
neutral monoalkyl chloride and/or dialkyl iron(II)
complexes.7,12a The possibility that 2 and 3 could simply
be the result of in-source CID of these neutral mono-
and dialkylation products was therefore envisaged.
When 50 equiv of TMA was added to a solution of 1 in
THF, a color change was immediately observed (see
below), confirming that a reaction took place. The nature
of the obtained products was subsequently investigated
by ESI-MS (Figure 4). A succession of peaks with
decreasing intensities starting at m/z 541 and separated
by intervals of 73-75 atomic mass units (amu) was
retrieved, which is more likely to be the result of TMA
oligomerization in the presence of traces of moisture,
forming aluminoxane oligomers. Apart from these TMA
oligomers and the ligand signal (m/z 482), none of the
peaks recovered from the ESI-MS of 1/MAO (Figure 1)
were detected, suggesting that no cationic species are
formed by the reaction of 1 with TMA and thus confirm-
ing that 2 and 3 are actual products resulting from the
MAO activation process.
Another control experiment was carried out with the
MAO-activated [2,6-bis(1-(isopropylimino)ethyl)pyridi-
ne]iron(II) chloride complex (6), recently reported to
perform the polymerization of various acrylate mono-
mers.20,21 The sample was prepared in THF and trans-
ferred to the MS in the same way as for 1/MAO.
Similarly, the mass spectrum exhibited a complicated
and variable distribution of products, but a peak cen-
tered at m/z 316 corresponding to the cationic iron(II)
monomethylated complex could be reproducibly ob-
served, as well as peaks at m/z 336 (monochloride
cation), 302, and 287 (analogues of the m/z 538 and 523
peaks obtained with 1, see below). Mass selection of the
m/z 316 ion and its fragmentation by CID gave the loss
of one methyl (m/z 301) followed by the loss of iron (m/z
245).
deep royal blue THF solution turns to light purple after
the addition of MAO, whereas the emerald green color
obtained after the reaction with TMA suggests in this
case the presence of different species. In view of that, a
complementary UV-vis spectroscopy study was under-
taken to confirm the observations made in the ESI-MS
measurements (Figure 5).
In THF, 1 shows a broad absorption band in the
visible region at 715 nm, slightly bathochromic com-
pared to the value reported in CH2Cl2 (695 nm).9 After
the addition of 50 equiv of MAO, the original broad
absorption totally disappears to step aside to a unique
less intense band at 588 nm, indicating that the entire
catalyst precursor 1 was consumed during the reaction.
Conversely, the addition of 50 equiv of TMA did not
witness the disappearance of the 715 nm peak arising
from 1 but resulted in the formation of a new absorption
of similar intensity centered on 550 nm, which is most
plausibly due to a neutral five-coordinated dimethyl
[LFeMe2] or monomethyl [LFeMeCl] complex. Indubi-
tably, different species are formed when 1 is reacted
with TMA as compared to MAO.
Nature of the Activation Products in THF. The
exact nature of the species formed during the activation
process has been discussed by two different groups,12,24
and although experimental results tend to support that
the methyl cation is the actual polymerization cata-
lyst,9,13 direct spectroscopic evidence of its existence has
been lacking. Earlier studies on the structural deter-
mination of the catalytically active species have mainly
1
been carried out by means of paramagnetic H NMR12
and EPR12b,24 spectroscopy, and the latest report states
that the addition of MAO to a 2,6-bis(imino)pyridine
iron(II) chloride complex in toluene forms various cat-
ionic bimetallic species of the type [LFeII(µ-Me)(µ-L*)-
AlMe2]+[Me-MAO]- (L* ) Cl, Me depending on the
amount of MAO), in which iron is exclusively in the +2
oxidation state.12b,c
UV-Visible Spectroscopy. In general, the position
of absorption bands of transition metal complexes in the
visible region is related to a ligand to metal charge
transfer (LMCT).9,23 It is then subject to variations
depending on the electron configuration at the metal
center and also reflects the changes occurring in the
coordination sphere of the metal.23 In the case of 1, the
As a matter of fact, the detection of 2 and 3 by ESI-
MS as the activation products of 1 with a low amount
of MAO (50 equiv) does not give any further indication
concerning the activation pathway for 2,6-bis(imino)-
pyridine iron(II) chloride complexes (see above) but
(22) The m/z value found in the literature for 3SbF6-‚CH3CN
measured by FAB mass spectrometry is 572 amu.9
(23) Coevoet, D.; Cramail, H.; Deffieux, A. Macromol. Chem. Phys.
1998, 199, 1451-1457.
(24) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.;
Goodgame, D. M. L.; McTavish, S. J.; Pankhurst, Q. A. Catal. Comm.
2002, 3, 207-211.

3668 Organometallics, Vol. 24, No. 15, 2005
Castro et al.
Scheme 1. Activation Products of 1 with MAO in THF: 2‚THF and 3‚THF
Scheme 2. r-H Transfer from 2 to TMA Leading to
shows unambiguously that 2 is a relatively stable
species under the current experimental conditions.
Donor molecules such as THF are known to stabilize
cationic metal alkyl25 and hydride25c complexes by
coordination, and in fact, 3 and its cobalt analogue have
been isolated as an acetonitrile and a THF adduct,
respectively, for which single-crystal structure deter-
minations could be undertaken.9
4
Al bimetallic complex.27,28 The peak centered on m/z 608
corresponds to the structure [LFe-CH2AlMe2]+ (4), as
confirmed by CID, giving two prominent fragments at
m/z 593 (loss of CH3) and at m/z 578 (loss of a second
CH3). Seemingly, the R-H transfer from 2 to TMA is a
prominent side reaction under the applied experimental
conditions (Scheme 2). This also indirectly proves that
2 and TMA are in close vicinity, even in THF solution.
The origin of the ions at m/z 538 (5), 523 (base peak
in Figure 1), and 507 is more delicate to establish, as
m/z 538 could arise from two different cations: an iron-
(II) hydride [LFe-H]+ and [(LMe)Al-Me]+ (LMe ) 2-imino-
6-aminopyridine). Since the peak pattern centered on
m/z 538 includes also different species, notably the m/z
541 peak derived from TMA (see above), the isotopic
identification of 5 is rendered awkward. In any event,
the observation of 5 is directly linked to the combined
presence of 1 and MAO in the THF solution, as it does
not appear in the TMA-treated sample (Figure 4).
Selection of the m/z 538 ion in the first quadrupole and
its fragmentation by CID gave signals at m/z 523 and
507 (Figure 6). It is then reasonable to deduce that the
This encouraged us to carry on with complementary
UV-vis measurements to further study the nature of
2 in the present system. In a control experiment,
[2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine]-
iron(II) chloride hexafluoroantimonate-acetonitrile
(3SbF6-‚CH3CN) prepared according to the literature9
was used as a model compound for a four-coordinated
2,6-bis(imino)pyridine iron(II) cation. The UV-vis spec-
tra displayed a broad absorption at 584 nm in THF,
while it is reported to absorb at 540 nm in CH2Cl2.9 It
is reasonable to assume that the red shift is due to
replacement of CH3CN with THF. Furthermore, when
1 was activated with MAO in toluene, the UV-vis
spectrum exhibited a faint broad absorption around 530
nm, in accordance with the value reported in the
literature.9 After the addition of 10 equiv of THF, λmax
was red-shifted to a sharper band of a similar intensity
at 599 nm. This phenomenon cannot be ascribed solely
to the increase of solvent polarity but also to the
different nature of the species observed; if a bimetallic
Fe-Al complex is formed in toluene, a monometallic
solvent adduct is dominating in THF.26 Accordingly,
after activation, 2 and 3 are likely to be present in the
solution as THF adducts, 2‚THF and 3‚THF (Scheme
1), while the application of a declustering potential in
the ion source converts both 2‚THF and 3‚THF to free
2 and 3.18 Undeniably, THF had a significant role in
stabilizing 2 in a monometallic form.
Other Species Detected. Further analysis of the
spectrum in Figure 1 afforded a deeper insight into the
reaction mixture. Apart from the formation of TMA
adducts, another side reaction reported for MAO-
activated metallocene catalysts consists of R-H transfer
from a metallocenium-methyl bond to an Al-Me group,
resulting in the formation of methane and a M-CH2-
(25) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem.
Soc. 1986, 108, 1718-1719. (b) Jordan, R. F.; Bajgur, C. S.; Willett,
R.; Scott, B. J. Am. Chem. Soc. 1986, 108, 7410-7411. (c) Jordan, R.
F.; LaPointe, R. E:; Bradley, P. K.; Baenziger, N. Organometallics 1989,
8, 2892-2903.
(26) Referring to the metallocene literature, it was shown that the
formation of a bimetallic Zr-TMA complex in CH2Cl2 by addition of
TMA to a monometallic zirconocenium species was expressed by a
progressive hypsochromic shift of the MLCT band; see: Coevoet, D.;
Cramail, H.; Deffieux, A. Macromol. Chem. Phys. 1998, 199, 1459-
1464.
Figure 6. ESI-MS/MS spectrum of the mass-selected ion
5 from the activation of 1 with MAO in THF, showing that
the m/z 523 and 507 ions are the products of in-source CID
of 5 (collision energy 30 eV).

Activation of Pyridine Iron(II) Chloride Complexes
Organometallics, Vol. 24, No. 15, 2005 3669
Scheme 3. Proposed Reaction Mechanism of the Ring Opening of THF in 2‚THF and Subsequent â-H
Elimination Leading to the Formation of 5
m/z 523 and 507 ions are formed by in-source CID of
the m/z 538 ion.
â-butyl transfer, at least in the gas phase, leading to
the formation of iron hydride (major product) and iron
butyl cations, respectively.30 In fact, this process is
confirmed by the detection of a small signal at m/z 594
([LFe-butyl]+) present in the peak pattern of the m/z
593 ion (Figure 1).
In these experiments, [LFe-H]+ can be formed via
different pathways from the in situ generated cationic
species. At the latest in the gas phase, the inherent
moisture and protons in the ESI-MS spectrometer are
accountable for the formation of an iron hydride, and
their presence can also explain the intensive ligand-H+
peak in the spectra. As the appearance of [LFe-H]+ is
strictly connected to 1/MAO, other explanations can also
be considered. When THF-d8 was employed as the
solvent, a spectrum similar to Figure 1 was retrieved;
in particular, 2 was observed at m/z 552. The spectrum
also revealed some important differences. In fact, the
m/z 539 ion belonging to the isotopic pattern of 5
centered on m/z 538 witnessed an intensity increase of
about 10% compared to the experiment in regular THF.
Furthermore, signals at m/z 524 and 508 were detected,
matching a 1 amu increase of the 523 and 507 masses.
Accordingly, the origin of the m/z 538 ion can, at least
to a certain extent, be attributed to hydride transfer from
THF, leading to the formation of 5 ([5-D] in THF-d8).
As demonstrated above, the coordination of THF to
the cationic metal center in 2 is dominant. It renders
its R-carbon susceptible to a nucleophilic attack from
the methyl ligand, instigating the formation of a new
cationic 2,6-bis(imino)pyridine iron(II) pentoxide via the
ring opening of the THF cycle (Scheme 3).29 This iron
alkoxide cation is further prone to undergo â-H and/or
On the other hand, ions matching m/z 538 could
originate from aluminum complexes, generated by reac-
tions between TMA and the free ligand. The synthesis
of N,N,N-pyridyliminoamide dimethylaluminum com-
plexes ([(LMe)Al-Me2]) was reported to occur by reacting
TMA with 2,6-bis(imino)pyridine in refluxing toluene.
This stable aluminum complex is prompt to undergo
methyl abstraction in the presence of a strong Lewis
acid, yielding [(LMe)Al-Me]+, a robust catalyst for
ethylene or polar monomer polymerization.31 Bearing
in mind that MAO is able to abstract a methyl group
from methylaluminum species,32 the possibility that the
[(LMe)Al-Me]+ complex could form in THF by reaction
of the free ligand and aluminum species was checked.
1 and 50 equiv of MAO were reacted for 1 h at room
temperature in THF under the same conditions as
during the sample preparation. After hydrolysis of the
solution, only the original 2,6-bis(1-((2,6-diisopropyl-
phenyl)imino)ethyl)pyridine ligand was recovered, with-
out any trace of N,N,N-pyridyliminoamide. Similarly,
a simple ligand-TMA adduct could be formed which,
in the presence of MAO, would generate the [LAl-Me2]+
ion. However, if the appearance of the m/z 538 ion in
(27) (a) Kaminsky, W.; Steiger, R. Polyhedron 1988, 7, 2375-2381.
(b) Kaminsky, W.; Stru¨bel, C. J. Mol. Catal. A: Chem. 1998, 128, 191-
200. (c) Kaminsky, W. J. Polym. Sci., Part A: Polym. Chem. 2004, 42,
3911-3921.
(30) (a) Fiedler, A.; Schro¨der, D.; Scharwz, H.; Tjelta, B. L.; Armen-
trout, P. B. J. Am. Chem. Soc. 1996, 118, 5047-5055. (b) Kirkwood,
D. A.; Stace, A. J. Int. J. Mass Spectrom. Ion Processes 1997, 171, 39-
49.
(28) Additionally, interactions between Cp2ZrMe+ and TMA were
observed in the gas phase by Fourier transform ion cyclotron resonance
mass spectroscopy; see: Siedle, A. R.; Newmark, R. A.; Schroepfer, J.
N.; Lyon, P. A. Organometallics 1991, 10, 400-404.
(31) (a) It was, however, demonstrated that this aluminum complex
is not the species active in the 1/MAO-catalyzed ethylene polymeriza-
tion,2a as it produces low molar mass polyethylene with low catalytic
activity; see: Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.;
White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 2523-2524. (b)
Baugh, L. S.; Sissano, J. A. J. Polym. Sci., Part A: Polym. Chem. 2002,
40, 1633-1651.
(29) According to our unpublished results, MAO-activated 2,6-bis-
(imino)pyridine iron(II) chloride complexes are able to provide, al-
though to a minor extent, the ring-opening polymerization of THF in
the presence of acrylate monomers, which further supports the
feasibility of the reaction discussed here.
(32) Kim, J. S.; Wojcinski, L. M., II; Liu, S.; Sworen, J. C.; Sen, A.
J. Am. Chem. Soc. 2000, 122, 5668-5669.

3670 Organometallics, Vol. 24, No. 15, 2005
Castro et al.
Figure 1 is connected to the formation of an aluminum
complex, the CID spectrum from m/z 538 should exhibit
a signal of reasonable intensity at m/z 508 (ligand +
Al). Whereas such a signal is indeed observed in Figure
6, it is of minor intensity compared to the ion at m/z
507, indicating that formation of an aluminum complex
in the gas phase is not totally excluded but is of lesser
importance.
an iron hydride species [LFe-H]+ (5) formed, at least
partly, via hydride transfer from THF to 2. On the basis
of these facts, it can assumed that 2‚THF is a major
activation product of 1 by MAO in THF: in this
particular case of relative MAO concentration, polar
donor solvent, and gas-phase mass spectrometry. It is
interesting to notice that the presence of 5 in the ESI-
MS spectrum gives also a direct proof of the existence
of such a species, which is considered to be a central
reaction intermediate in the catalytic cycle of olefin
polymerization, and has so far only been deduced from
the analysis of polymer end groups2 or from the addition
of H2 to a propylene polymerization reaction yielding
to lowered molar mass and increased activity.10 Despite
the fact that 2 and 3 can be described as solvent-
stabilized ion pairs rather than bimetallic TMA adducts
as established by UV-vis, the presence of the bimetallic
species [LFe-CH2-AlMe2]+ (4) is an indirect proof that
the iron methyl cation and aluminum alkyls can come
into close vicinity in THF.
Conclusion
Electrospray ionization tandem mass spectrometry
represents a powerful and straightforward tool for the
analysis of complicated mixtures of reaction products,
in particular when traditional separation and analytical
methods are not applicable because of the instability of
the products. In the present study, the use of ESI-MS
facilitated a direct insight into the various products
formed by the reaction of a 2,6-bis(arylimino)pyridine
iron(II) chloride complex with MAO in THF. Several
conclusions can be drawn from these experiments. First
of all, the cationic iron methyl complex [LFe-Me]+ (2),
claimed to be the actual catalytically active species in
the polymerization of olefins,4 was identified. To the best
of our knowledge, the detection of 2 by ESI-MS is the
first direct spectroscopic evidence of the existence of
such a cationic 2,6-bis(imino)pyridine iron(II) methyl
complex after MAO activation.33 It was also confirmed
that, at low MAO/Fe ratios, the activation reaction of 1
by MAO is not complete, as the cationic monochloride
complex [LFe-Cl]+ (3) was detected.
Acknowledgment. This work has been financed by
the University of Helsinki and the Academy of Finland,
projects 0204408 and 77317 (Centre of Excellence, Bio-
and Nanopolymer Research Group).
OM050351S
(33) The ability of 2‚THF to promote olefin polymerization is not
addressed here. In any event, isolated cationic methyl zirconocenes
stabilized by coordination with THF were found to catalyze the
polymerization of olefins.25 Similarly, diethyl ether adducts of cationic
nickel and palladium R-diimine methyl complexes are active catalysts
for olefin polymerization.1b In addition, olefin polymerization with
cationic metallocenes in the presence of coordinating ligands such as
PPh3 has been reported; see: Beck, S.; Prosenc, M. H.; Brintzinger,
H. H. J. Mol. Catal. A: Chem. 1998, 128, 41-52.
The signals at m/z 538 and 523 are the most promi-
nent peaks in the ESI-MS spectrum and appear only
with 1/MAO. The signal at m/z 538 was attributed to