Synthesis and electronic structure of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes: Evaluation of redox activity in single-component ethylene polymerization catalysts

Article abstract of DOI:10.1021/ja106575b

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Moessbauer spectroscopy, magnelochemistry, and open-shell density functional theory. For the cationic complexes, [(^iPrPDI)Fe-R][BPh₄] (^iPrPDI = 2, 6-(2,6-ⁱPr₂-C₆H₃N=CMe)₂C₅H₃N; R = CH₂SiMe₃, CH₂CMe₃, or CH₃), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S_Fe= 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, (^iPrPDI)Fe(CH₂SiMe₃) or (^iPrPDI)Fe(CH₂CMe₃), is chelatebased, resulting in a bis(imino)pyridine radical anion (S_pDI= V₂) antiferromagnetically coupled to a high spin ferrous ion (S_Fe= 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et₂O)₃][(^iPrPDI)Fe(CH₂CMe₃)N₂], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S=O compound is best described as a low spin ferrous compound (S_Fe= O) with a closed-shell singlet bis(imino)pyridine dianion (Sprji = O), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.

Full text of DOI:10.1021/ja106575b

Published on Web 09/30/2010
Synthesis and Electronic Structure of Cationic, Neutral, and
Anionic Bis(imino)pyridine Iron Alkyl Complexes: Evaluation
of Redox Activity in Single-Component Ethylene
Polymerization Catalysts
Aaron M. Tondreau,† Carsten Milsmann,‡ Andrew D. Patrick,† Helen M. Hoyt,†
Emil Lobkovsky,† Karl Wieghardt,‡ and Paul J. Chirik*,†
Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell UniVersity,
Ithaca, New York 14853, and Max-Planck Institute for Bioinorganic Chemistry,
Stiftstrasse 34-36, D-45470 Mu¨lheim an der Ruhr, Germany
Received August 4, 2010; E-mail: pc92@cornell.edu
Abstract: A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been
prepared, and their electronic and molecular structures have been established by a combination of X-ray
diffraction, Mo¨ssbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the
cationic complexes, [(iPrPDI)Fe-R][BPh4] (iPrPDI ) 2,6-(2,6-iPr2-C6H3NdCMe)2C5H3N; R ) CH2SiMe3,
CH2CMe3, or CH3), which are known single-component ethylene polymerization catalysts, the data establish
high spin ferrous compounds (SFe ) 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron
reduction to the corresponding neutral alkyls, (iPrPDI)Fe(CH2SiMe3) or (iPrPDI)Fe(CH2CMe3), is chelate-
1
based, resulting in a bis(imino)pyridine radical anion (SPDI ) /2) antiferromagnetically coupled to a high
spin ferrous ion (SFe ) 2). The neutral neopentyl derivative was reduced by an additional electron and
furnished the corresponding anion, [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)N2], with concomitant coordination of
dinitrogen. The experimental and computational data establish that this S ) 0 compound is best described
as a low spin ferrous compound (SFe ) 0) with a closed-shell singlet bis(imino)pyridine dianion (SPDI ) 0),
demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine
coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state
change at the iron center from high to low spin. The relevance of the compounds and their electronic
structures to olefin polymerization catalysis is also presented.
Introduction
ortho aryl substituent are selective for R-olefin production with
nearly ideal Schultz-Flory distributions.7-9
Aryl-substituted bis(imino)pyridine iron and cobalt dichloride
complexes, (ArPDI)MCl2 (M ) Fe, Co), when activated with
methylaluminoxane (MAO), exhibit high activities for ethylene
and R-olefin oligomerization and polymerization.1 Since the
initial, independent reports in 1998 by Brookhart2 and Gibson,3,4
many studies have focused on alteration of the modular
bis(imino)pyridine ligand framework5,6 to establish structure-
reactivity relationships. For the iron compounds, precatalysts
bearing two large 2,6-substituents on the aryl ring are known
to produce linear polyethylene, whereas those with only a single
Despite a fairly mature understanding of how catalyst
structure influences the type of oligomer or polymer produced,
the mechanism of olefin polymerization and the nature of the
active species formed upon treatment of (ArPDI)FeCl2 with MAO
remain controversial. Talsi and co-workers have conducted a
series of extensive 1H and 2H NMR spectroscopic investigations
to identify catalytic intermediates following addition of MAO,
AlMe3, AlMe3/B(C6F5)3, AlMe3/[Ph3C][B(C6F5)4], or other
trialkylaluminum compounds to (iPrPDI)FeCl2.10-12 Kinetic
studies have identified that two distinct iron(II) species are
† Cornell University.
‡ Max-Planck Institute for Bioinorganic Chemistry.
(1) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.;
Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391.
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(b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.
1998, 120, 4049.
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C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J.
Chem.sEur. J. 2000, 6, 2221.
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B. M.; Schiffhauer, M. F. Organometallics 2008, 27, 1147.
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9
15046 J. AM. CHEM. SOC. 2010, 132, 15046–15059
10.1021/ja106575b  2010 American Chemical Society

Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
Scheme 1. Synthetic Routes to Bis(imino)pyridine Iron Di- and Monoalkyl Complexes
present during the polymerization that are respectively respon-
sible for forming the low- and high-molecular weight polyethyl-
ene.13-15 EPR and Mo¨ssbauer spectroscopic studies from Gibson
and co-workers postulated formation of an iron(III) species upon
activation with excess MAO,16 and recent DFT studies suggest
that this oxidation state is more active than the Fe(II) alterna-
tive.17 Most recently, Bryliakov, Talsi, and co-workers studied
the interaction of (iPrPDI)FeCl2 with MAO and aluminum
trialkyls by NMR and EPR spectroscopies and identified
[(iPrPDI)Fe(µ-Me)2AlMe2], [(iPrPDI)Fe(µ-iBu) (µ-X)Al(iBu)2] (X
reported methods for their preparation.32 Our laboratory reported
direct alkylation of (iPrPDI)FeCl2 with LiCH2SiMe3 to yield
(iPrPDI)Fe(CH2SiMe3)2, following recrystallization (Scheme 1).33
Shortly thereafter, Gambarotta and co-workers reported that
(13) Barabanov, A. A.; Zakharov, V. A.; Semikolvenova, N. V.; Echevskaja,
L. G.; Matsko, M. A. Macromol. Chem. Phys. 2005, 206, 2292.
(14) Barabanov, A. A.; Bukatov, G. D.; Zakharov, V. A.; Semikolvenova,
N. V.; Mikenas, T. B.; Echevskaja, L. G.; Matsko, M. A. Macromol.
Chem. Phys. 2005, 207, 1368.
(15) Kissin, Y. V.; Qian, C.; Xie, G.; Chen, Y. J. Polym. Sci., Part A:
Polym. Chem. 2006, 44, 6159.
(16) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.; Goodgame,
D. M. L.; McTavish, S. J.; Pankhurst, Q. A. Catal. Commun. 2002, 3,
207.
i
) Bu, Cl), and [(iPrPDI)Fe(µ-Me)2AlMe2][MeMAO] after
activation.18
(17) Cruz, V. L.; Ramos, J.; Martinez-Salazar, J.; Gutierrez-Oliva, S.; Toro-
Labbe, A. Organometallics 2009, 28, 5889.
Single-component olefin polymerization catalysts offer ad-
vantages for mechanistic studies, as characterization of the
resting state, propagating species, and fundamental steps related
to the initiation and termination of chain growth, in principle,
is not complicated by the presence of a large excess of an ill-
defined cocatalyst such as MAO. Such species may prove
particularly insightful for bis(imino)pyridine iron-catalyzed
olefin polymerization, given the ambiguities concerning iron
oxidation state and bis(imino)pyridine participation in the
electronic structure of the molecule and even chemical modi-
fication after treatment with an activator. Formally 14-electron,
cationic metal alkyl species, [LnM-R]+X- (Ln ) supporting
ligand(s); R ) alkyl; X- ) weakly coordinating, low-nucleo-
philicity anion), have proven to be effective single-component
olefin polymerization catalysts in both group 4 transition metal
chemistry19-21 and group 10 metal chemistry.22,23 These
compounds are typically synthesized by treatment of the
corresponding neutral metal dialkyl derivative, [LnMR2], with
Brønsted24,25 or Lewis acids26 or an alkyl abstracting agent such
as [Ph3C][B(C6F5)4].27
(18) Bryliakov, K. P.; Talsi, E. P.; Semikolvenova, N. V.; Zakharov, V. A.
Organometallics 2009, 28, 3225.
(19) (a) Mohring, P. C.; Coville, N. J. Coord. Chem. ReV. 2006, 250, 18.
(b) Coates, G. W. Chem. ReV. 2000, 100, 1223. (c) Resconi, L.;
Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253. (d)
Ewen, J. A. Sci. Am. 1997, 276, 86.
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38, 784. (b) Zhang, W.; Zhang, W. J.; Sun, W. H. Prog. Chem. 2005,
17, 310. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103,
283. (d) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000,
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Commun. 2002, 2316.
Guided by these precedents, the analogous, formally 14-
electron bis(imino)pyridine iron alkyl cations, [(ArPDI)Fe-R]+,
have been long-standing targets for single-component iron
polymerization catalysts.28 Although successful for cobalt,29-31
mono- and dialkyl complexes of bis(imino)pyridine iron re-
mained elusive until 2005, when several groups independently
(31) Tellmann, K. P.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C.
Organometallics 2004, 23, 5503.
(32) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008,
130, 11631.
(33) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406.
9
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A R T I C L E S
Tondreau et al.
Scheme 2. Cationic Bis(imino)pyridine Iron Alkyl Complexes as Single-Component Ethylene Polymerization Catalysts
chelate alkylation can be competitive with iron alkylation under
certain experimental conditions.34 Ca´mpora and co-workers
described an alternative route to (ArPDI)Fe(CH2SiMe3)2 com-
pounds where the free chelate is added to independently or in
situ prepared (pyridine)2Fe(CH2SiMe3)2 (Scheme 1).35 This
method has proven versatile for the introduction of various
bis(imino)pyridine chelates36 and enantiopure pyridine bis(ox-
azoline) ligands.37 However, simply substituting (pyridine)2-
Fe(CH2SiMe3)2 with the neopentyl analogue resulted in com-
peting alkyl migration and iron alkyl homolysis, suggesting this
route is likely limited to ꢀ-silyl-substituted alkyls.36
Bis(imino)pyridine iron monoalkyl complexes have also been
reported and have been prepared by direct alkylation of the
corresponding monohalide precursor.33,36 In this manner, several
ꢀ-hydrogen-stabilized iron alkyl complexes, such as (iPrPDI)-
FeCH3, (iPrPDI)FeCH2SiMe3, and (iPrPDI)FeCH2CMe3, have
been synthesized.33,36 A route to kinetically unstable ꢀ-hydrogen-
containing neutral iron alkyls such as (iPrPDI)FeCH2CH3 has
also been reported by our laboratory, involving treatment of
the iron bis(dinitrogen) compound, (iPrPDI)Fe(N2)2,38 with the
appropriate alkyl bromide.39 This method unfortunately yields
an equimolar mixture of the desired monoalkyl complex along
with the iron monobromide compound, (iPrPDI)FeBr.
Synthesis of the long-sought-after bis(imino)pyridine iron
alkyl cations has been accomplished by protonation of
(iPrPDI)Fe(CH2SiMe3)2 with [PhNMe2H][BPh4] (Scheme 2).
Crystalline compounds were obtained upon addition of Et2O
and THF, and both [(iPrPDI)Fe(CH2SiMe3)L][BPh4] (L ) Et2O,
THF) have been structurally characterized.40 Addition of the
neutral borane, B(C6F5)3, to (iPrPDI)Fe(CH2SiMe3)2 yielded the
four-coordinate, base-free iron cation, [(iPrPDI)Fe(CH2SiMe2-
CH2SiMe3)][MeB(C6F5)3], arising from alkyl rearrangement
following silicon methide abstraction.40-42 Upon exposure to
ethylene, all three iron compounds serve as single-component
polymerization catalysts and yield linear polyethylene terminated
by olefin end groups arising from ꢀ-hydrogen elimination.
Cationic iron alkyl complexes with bidentate R-diimine43 and
nonconjugated diimine44 supporting ligands have also been
prepared, although ethylene polymerization activity was minimal
in both cases. In the latter example, Bouwkamp’s laboratory
was able to observe, for the first time, reversible ethylene
coordination to an iron alkyl cation. Gambarotta and co-workers
also synthesized and isolated the anionic bis(imino)pyridine iron
methyl complex, [Li(THF)4][(iPrPDI)FeMe], and demonstrated
its activity in ethylene polymerization upon treatment with
excess MAO.45
Why are aryl-substituted bis(imino)pyridines such an effective
ligand class in olefin polymerization46 and other applications47,48
of iron catalysis? The ease of synthesis, air-stability, and ability
to rapidly prepare libraries of ligands are all practical advantages
of bis(imino)pyridines that have likely increased their popularity.
Bis(imino)pyridines are also well-established redox-active ligands
and are known to promote reversible transfer of 1-3 electrons
between the chelate and the transition metal.49-54 This phe-
nomenon gives rise to complexes whose formal oxidation state
assignment can be deceiving but, more importantly, contributes
to the catalytic performance of the complex, as the metal-ligand
combination can smoothly adjust to the electronic requirements
of a particular reaction surface and enable new chemistry.55,56
Key to the continued development of more productive and
selective catalysts is elucidation of the mechanism of the
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P. J. Organometallics 2004, 23, 237.
(44) Volbeda, J.; Meetsma, A.; Bouwkamp, M. W. Organometallics 2009,
28, 209.
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nometallics 2005, 24, 6298.
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- Theochem 2009, 903, 100.
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Chem. Soc. 2006, 128, 9660.
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(b) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J.
J. Am. Chem. Soc. 2006, 128, 13340.
(35) Campora, J.; Naz, A. M.; Palma, P.; Alvarez, E.; Reyes, M. L.
Organometallics 2005, 24, 4878.
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metallics 2008, 27, 109.
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126, 13794.
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Lenthe, J. H. Inorg. Chem. 2001, 40, 4649.
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Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
polymerization reaction and identification of the active species.
For bis(imino)pyridine iron-catalyzed reactions, chelate redox
activity presents an additional challenge for determining the
oxidation state of the metal center and understanding funda-
mental transformations relevant to chain initiation, growth, and
termination. Here we describe a new, general synthetic method
for the preparation of base-free, cationic bis(imino)pyridine iron
alkyl compounds and evaluate the electronic structure of single-
component iron ethylene polymerization catalysts. In addition,
the electronic structure and degree of bis(imino)pyridine chelate
participation is determined for a series of cationic, neutral, and
anionic iron alkyl compounds that differ by three oxidation
states, where redox changes occur at the ligand, not the metal.
or pentane, although isolation as a pure solid was not achieved.40
One-electron oxidation of (iPrPDI)FeCH2SiMe3 with [(η5-
C5H5)2Fe][BPh4] in benzene followed by precipitation by
addition of excess pentane furnished [(iPrPDI)FeCH2SiMe3]-
[BPh4] as a dull gray-red powder in 88% yield (eq 1). This
material served as a precursor to the diethyl ether- and THF-
stabilized cations, [(iPrPDI)Fe(CH2SiMe3)L][BPh4] (L) Et2O,
THF),40 by addition of a slight excess of the appropriate solvent.
The bis(imino)pyridine iron methyl cation, [(iPrPDI)Fe-
(CH3)]+, was also targeted due to its potential relevance to the
active species obtained upon treatment of (iPrPDI)FeCl2 with
MAO or AlMe3 and to explore its role as a single-component
olefin polymerization catalyst. As with the neopentyl and
neosilyl compounds, oxidation of (iPrPDI)FeCH3 with [(η5-
C5H5)2Fe][BPh4] in benzene followed by precipitation with
pentane furnished a dull gray solid identified as [(iPrPDI)Fe-
(CH3)][BPh4] (eq 2). Complete characterization and elucidation
of the electronic structure of these molecules are presented in
a later section of this article.
Results and Discussion
Synthesis of Bis(imino)pyridine Iron Alkyl Cations. Our
laboratory previously communicated the synthesis of cationic
bis(imino)pyridine iron alkyl complexes by protonation or alkyl
abstraction from the corresponding dialkyl precursor.40,44 As
documented previously,33,35,36 only ꢀ-silyl-substituted bis(imi-
no)pyridine iron dialkyl complexes, e.g., (iPrPDI)Fe(CH2SiMe3)2,
are currently synthetically accessible. For example, pyridine
displacement from either (pyridine)2Fe(CH2CMe3)2 or (pyridine)2-
Fe(CH2CMe3)(CH2SiMe3) by addition of free bis(imino)pyridine
ligand resulted in ejection of the neopentyl group and formation
of iron monoalkyl complexes.36 To prepare more polymerization
relevant, pure hydrocarbyl-containing bis(imino)pyridine iron
alkyl cations, new synthetic routes are necessary.
Four-coordinate, neutral bis(imino)pyridine iron alkyls that
are stabilized with respect to ꢀ-hydrogen elimination are
synthetically accessible by straightforward salt metathesis of
the corresponding iron chloride or bromide complex with the
appropriate alkyl lithium reagent.33,36 This route offers an
attractive entry into the desired alkyl cation chemistry via one-
electron oxidation. Treatment of a benzene solution of
(iPrPDI)FeCH2CMe3 with [(η5-C5H5)2Fe][BPh4] resulted in pre-
cipitation of a light red powder, identified as the base-free
bis(imino)pyridine iron neopentyl cation, [(iPrPDI)FeCH2-
CMe3][BPh4] (eq 1). Exposure of this product to either Et2O or
THF furnished the corresponding ligand-stabilized cations,
[(iPrPDI)Fe(CH2CMe3)L][BPh4] (L ) Et2O, THF), as light purple
solids. Generation of [(iPrPDI)FeCH2CMe3][BPh4] in situ in
Synthesis of Bis(imino)pyridine Iron Alkyl Anions. The new
route to various ꢀ-hydrogen-stabilized bis(imino)pyridine iron
alkyl cations prompted exploration of the synthesis of related
anionic alkyl complexes to complete a series of compounds that
vary by three oxidation states that will allow systematic
evaluation of chelate participation. Prior to this study, the only
anionic bis(imino)pyridine iron alkyl complexes known were
[Li(THF)4][(iPrPDI)FeMe], reported by Gambarotta,45 and the
aryl anions, [Li(Et2O)3][(iPrPDI)Fe(C6H4-4-R)N2 ] (R ) H, Me),
synthesized in our laboratory.35 However, the electronic structure
of these compounds and the redox activity of the bis(imino)py-
ridine chelate have not been determined.
Because both neutral and cationic bis(imino)pyridine neosilyl
compounds, [(iPrPDI)FeCH2SiMe3]0/+, have been reported, the
synthesis of the corresponding anion was targeted. Treatment
of a diethyl ether solution of (iPrPDI)Fe(N2)2 with 1 equiv of
LiCH2SiMe3 resulted in a color change from green to purple,
suggesting formation of the desired alkyl anion. However,
attempts to isolate the product in the solid state by either
recrystallization or solvent removal in vacuo resulted in isolation
of the neutral bis(imino)pyridine iron alkyl compound,
(iPrPDI)FeCH2SiMe3.36 Likewise, addition of 2 equiv of
LiCH2SiMe3 to a diethyl ether solution of (iPrPDI)FeBr also
yielded a purple reaction mixture, but again the neutral iron
alkyl, (iPrPDI)FeCH2SiMe3, was isolated. Attempts to intercept
the anion by addition of 12-crown-4 to the purple solutions
(generated by either method) were also unsuccessful.
1
benzene-d6 allowed collection of H NMR data. For many of
the isolated alkyl cations, NMR spectra were obtained in C6D5F,
and the data are reported in the Experimental Section and are
consistent with S ) 2 iron compounds.
The successful synthesis of bis(imino)pyridine iron neopentyl
cations from one-electron oxidation of the neutral alkyl precursor
prompted exploration of the synthesis of the other bis(imino)py-
ridine iron alkyl complexes that were inaccessible from the
previously reported iron dialkyl route. We previously reported
in situ generation of [(iPrPDI)Fe(CH2SiMe3)][BPh4] by treatment
of the corresponding neutral bis(imino)pyridine iron dialkyl with
[PhNMe2H][BPh4] in a non-coordinating solvent such as toluene
(55) For representative examples of this concept in stoichiometric and
catalytic reactions in early transition metal chemistry, see: (a) Zarkesh,
R. A.; Ziller, J. W.; Heyduk, A. F. Angew. Chem., Int. Ed. 2008, 47,
4715. (b) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am.
Chem. Soc. 2008, 130, 2728. (c) Stanciu, C.; Jones, M. E.; Fanwick,
P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400.
(56) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794.
9
J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010 15049

A R T I C L E S
Tondreau et al.
Table 1. Solid State (KBr) Infrared Stretching Frequencies of the
NtN Ligands in Bis(imino)pyridine Iron Alkyl and Aryl Anionsa
Due to the complications associated with isolation of the iron
neosilyl anion, attention was then devoted to the preparation of
the corresponding neopentyl complex. Our laboratory has
reported the synthesis of the neutral variant, (iPrPDI)FeCH2CMe3,
by straightforward salt metathesis of (iPrPDI)FeBr with
LiCH2CMe3.36 Treatment of a diethyl ether solution of
(iPrPDI)Fe(N2)2 with 1 equiv of LiCH2CMe3, followed by
recrystallization at -35 °C, furnished a red-brown solid identi-
fied as [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)N2] (eq 3). This com-
pound was also prepared by addition of 2 equiv of LiCH2CMe3
to a diethyl ether solution of the bis(imino)pyridine iron bromide,
(iPrPDI)FeBr.32 The crown ether derivative, [Li(12-crown-
4)][(iPrPDI)Fe(CH2CMe3)(N2)], was also synthesized by addition
of 12-crown-4 to a diethyl ether solution of the initially
generated anion. The diethyl ether solvate, [Li(Et2O)3][(iPrPDI)-
Fe(CH2CMe3)N2], is an exceedingly reactive compound and
undergoes decomposition in the solid state and in solution. For
subsequent spectroscopic and crystallographic studies (vide
infra), the compound was prepared and used immediately.
compound
ν (NtN) (cm-1
)
[Li(OEt2)3][(iPrPDI)Fe(CH2CMe3)N2]
[Li(Et2O)3][(iPrPDI)Fe(C6H4-4-Me)(N2)]
[Li(12-crown-4)][(iPrPDI)Fe(CH2CMe3)N2]
1948
1948
1996
2046
(
iPrPDI)FeN2
a The neutral, four-coordinate dinitrogen complex, (iPrPDI)FeN2, is
also included for comparison.
observed previously during the preparation and isolation of the
related aryl anions, [Li(Et2O)3][(iPrPDI)Fe(C6H4-4-R)(N2)] (R )
H, Me).35 The infrared stretching frequencies of the NtN bands
for each of these compounds recorded in the solid state (KBr)
are presented in Table 1. The neutral bis(imino)pyridine iron
mono(dinitrogen) compound, (iPrPDI)Fe(N2), is also included
for comparison.38 As expected for the more reduced compounds,
the dinitrogen stretching frequencies of the bis(imino)pyridine
alkyl are significantly lower frequency than those of the neutral
dinitrogen complex, (iPrPDI)Fe(N2). For the same cation, both
the iron tolyl and neopentyl anions contain NtN bands at the
same stretching frequency. As will be presented in a subsequent
section, X-ray diffraction established that interaction of the
[Li(Et2O)3]+ cation with the terminal N2 ligand is the same in
both compounds. The most dramatic shift is observed for the
12-crown-4 compound and is likely a result of a different
interaction of the cation with the anion rather than an indication
of a gross change in electronic structure.
Crystallographic Characterization of Cationic and Anionic
Bis(imino)pyridine Iron Alkyl Compounds. As is now well-
established,49 the metrical parameters of the bis(imino)pyridine
ligand as determined from high-quality X-ray crystal structures
are a reliable indicator of redox activity and hence key in the
assignment of the oxidation states of both the ligand and the
metal center. All metrical parameters for the bis(imino)pyridine
iron compounds relevant to this study are reported in Tables 2
and 3. The solid state structure of the neutral bis(imino)pyridine
iron neopentyl compound, (EtPDI)Fe(CH2CMe3), has been
reported previously.36 Note that the 2,6-aryl substituents on the
bis(imino)pyridine ligand are ethyl groups, not isopropyl as in
the compounds described in this study. However, this substitu-
tion is expected to have little impact on the overall electronic
structure.
Single crystals of [(iPrPDI)Fe(CH2CMe3)][BPh4] were ob-
tained by cooling a concentrated fluorobenzene solution of the
compound to -35 °C. A representation of the molecular
structure is shown in Figure 1, and selected bond distances and
angles are reported in Table 2. As was observed in the solid
state structures of the neutral alkyl, (EtPDI)FeCH2CMe3,36 and
in cationic iron neosilyls, [(iPrPDI)Fe(CH2SiMe3)L][BPh4] (L
) Et2O, THF), and the related complex, [(iPrPDI)Fe(CH2Si-
Me2CH2SiMe3)][MeB(C6F5)3],40 the alkyl group in the cation
is lifted out of the idealized square plane defined by the iron
center and the bis(imino)pyridine chelate. There are no close
contacts between the [BPh4]- anion and the iron alkyl cation.
The bis(imino)pyridine iron methyl cation, [(iPrPDI)Fe(CH3)]-
[BPh4], was also characterized by single-crystal X-ray diffrac-
tion, and a representation of the solid state structure is presented
in Figure 2. As was observed with the neutral derivative,
(iPrPDI)FeCH3, the geometry about iron is planar, with the sum
of the angles around the metal equal to 359.99(27)°. Because
neopentyl and methyl would be expected to have nearly identical
field strengths, the distortion in both the neutral and cationic
As was observed for [Li(THF)4][(iPrPDI)FeMe]45 and [Li-
(Et2O)3][(iPrPDI)Fe(C6H4-4-Me)N2],36 [Li(Et2O)3][(iPrPDI)Fe-
(CH2CMe3)N2] exhibits no observable or assignable H NMR
1
resonances in benzene-d6 at 23 °C. This spectroscopic behavior
contrasts with that of the neutral bis(imino)pyridine iron mono-
and dialkyl complexes, where sharp, paramagnetically shifted
resonances are observed and readily assigned.33,35,32 Solid state
magnetic measurements (magnetic susceptibility balance) on
[Li(solv)n][(iPrPDI)Fe(CH2CMe3)N2] (solv ) Et2O, n ) 3; 12-
crown-4, n ) 1) established diamagnetic molecules. Attempts
were made to measure solution magnetic moments by Evans’s
method; however, the extreme sensitivity of the anions prohib-
ited acquisition of reliable data. Monitoring the solution over
short times in benzene-d6 at 23 °C often revealed formation of
free bis(imino)pyridine ligand as well as the corresponding
neutral iron alkyl compound arising from oxidation of the anion.
We note that these compounds were handled under the most
stringent of air- and moisture-free conditions when the decom-
position and oxidation processes were observed. Accordingly,
the origin of the NMR-silent behavior is not currently understood.
Routine characterization of the bis(imino)pyridine iron neo-
pentyl anions was accomplished by infrared spectroscopy, X-ray
diffraction (solv ) Et2O), Mo¨ssbauer spectroscopy, solid state
magnetic measurements, combustion analysis, and degradation
experiments. The details of these experiments will be presented
in a later section. Addition of excess water furnished free
bis(imino)pyridine ligand, dinitrogen, and neopentane. Integra-
tion of the benzene-d6 NMR spectrum following hydrolysis
established 3 equiv of diethyl ether or 1 equiv of 12-crown-4
(per equivalent of free bis(imino)pyridine) for the different
lithium solvates.
The terminal dinitrogen ligand in [Li(solv)n][(iPrPDI)Fe-
(CH2CMe3)N2] provides the most convenient spectroscopic
handle for routine characterization. Dinitrogen coordination was
9
15050 J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010

Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
Figure 2. Representation of the solid state structure of [(iPrPDI)Fe-
(CH3)][BPh4] at 30% probability ellipsoids. Hydrogens are omitted for
clarity.
Figure 1. Representation of the solid state structure of [(iPrPDI)Fe-
(CH2CMe3)][BPh4] at 30% probability ellipsoids. Hydrogens and isopropyl
aryl substituents are omitted for clarity.
Table 2. Metrical Parameters of Cationic, Neutral, and Anionic
Bis(imino)pyridine Iron Neopentyl Complexes
[(iPrPDI)Fe(CH2CMe3)]-
[BPh4]
(EtPDI)Fe-
(CH2CMe3)c
[(iPrPDI)Fe-
(CH2CMe3)]-
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-C(34)a
2.213(2)
2.110(2)
2.242(2)
2.158(3)
1.986(3)
2.126(3)
1.931(2)
1.832(2)
1.919(2)
1.746(2)
2.079(2)
2.035(2)
2.036(4)
N(1)-C(2)
N(3)-C(8)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(7)-C(8)
N(4)-N(5)
1.284(2)
1.288(2)
1.333(2)
1.336(2)
1.490(3)
1.486(3)
1.314(4)
1.329(4)
1.390(4)
1.366(4)
1.446(5)
1.428(5)
1.361(3)
1.355(3)
1.386(3)
1.386(3)
1.394(3)
1.398(3)
1.138(3)
Figure 3. Representation of the solid state structure of [Li(OEt2)3]-
[(iPrPDI)Fe(CH2CMe3)(N2)] at 30% probability ellipsoids. Hydrogens,
isopropyl aryl substituents, and the disordered diethyl ether ligands are
omitted for clarity.
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-N(3)
N(2)-Fe(1)-N(3)
73.14(6)
142.87(6)
72.83(6)
75.08(10)
136.68(11)
75.19(10)
80.17(8)
154.90(8)
79.85(8)
The overall molecular geometry of [(iPrPDI)Fe(CH2CMe3)-
(N2)]- is best described as pseudo square pyramidal (keeping
in mind the constraints of the chelate which distorts the basal
plane away from an idealized square), with the alkyl ligand in
the apical position and the dinitrogen molecule and the
bis(imino)pyridine chelate defining the basal plane. This ar-
rangement is likely preferred to maximize π-backbonding with
the N2 ligand and is corroborated by the relatively low N2
stretching frequency observed by infrared spectroscopy. It is
also possible that this arrangement is determined by the trans
influence, where the strongest field ligand, the alkyl group, is
apical and avoids the position trans to the N-pyridine, which is
confined by the geometrical constraints of the ligand skeleton.
The dinitrogen ligand is capped by the [Li(Et2O)3]+ cation,
which also likely contributes to the reduced infrared stretching
frequency. A similar overall molecular geometry was observed
in the solid state structure of the previously characterized
bis(imino)pyridine iron tolyl anion, [Li(Et2O)3][(iPrPDI)Fe(C6H4-
4-Me)(N2)].36
N(2)-Fe(1)-N(4)
N(1)-Fe(1)-N(4)
N(4)-Fe(1)-C(34)b
N(2)-Fe(1)-C(34)b
167.76(9)
98.45(9)
87.41(9)
104.82(9)
151.61(8)
142.24(14)
a Fe-C distance of the alkyl ligand. b The numbering scheme is
different for the neutral and cationic alkyls. c Data from ref 36.
neopentyl (including neosilyl and related) complexes must be
steric in origin. As with the base-free neopentyl cation, there
are no close contacts between the [BPh4]- anion and the iron
alkyl cation.
Single crystals of [Li(OEt2)3][(iPrPDI)Fe(CH2CMe3)(N2)] were
obtained from a concentrated diethyl ether solution cooled to
-35 °C, and a representation of the solid state structure is
presented in Figure 3. The crystals were large and fragile and
did not diffract well. Each of the diethyl ether ligands on the
[Li(Et2O)3]+ cation is severely disordered, and they were
successfully modeled. The asymmetric unit also contained half
of either a pentane or a diethyl ether molecule that was
disordered and was removed by the SQUEEZE routine.
The metrical parameters of the bis(imino)pyridine chelate in
cationic, neutral, and anionic iron neopentyl derivatives are
9
J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010 15051

A R T I C L E S
Tondreau et al.
Table 3. Metrical Parameters of Anionic, Neutral, and Cationic
Table 4. Metrical Parameters of Anionic Bis(imino)pyridine Iron
Bis(imino)pyridine Iron Methyl Complexes
Alkyl Complexes and a Neutral (N,N-Dimethylamino)pyridine
Compound for Comparison
[Li(THF)4]-
[(iPrPDI)Fe(CH3)]-
[BPh4]
a
[(iPrPDI)Fe(CH3)]-
(iPrPDI)FeCH3
[(iPrPDI)Fe-
[(iPrPDI)Fe-
(iPrPDI)Fe(DMAP)b
a
(CH2CMe3)N2]- (C6H4-4-Me)N2]-
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-CH3
1.917(6)
1.849(6)
1.915(5)
2.013(9)
1.968(5)
1.893(4)
1.952(5)
2.001(6)
2.216(3)
2.117(3)
2.222(3)
2.006(4)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-C(34)
1.931(2)
1.832(2)
1.919(2)
1.746(2)
2.079(2)
1.927(2)
1.837(2)
1.927(2)
1.752(2)
2.005(2)
1.908(3)
1.821(3)
1.943(3)
1.979(3)
N(1)-C(2)
N(3)-C(8)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(7)-C(8)
1.356(8)
1.377(9)
1.364(10)
1.387(10)
1.413(10)
1.406(10)
1.337(7)
1.332(7)
1.349(7)
1.368(7)
1.432(8)
1.442(8)
1.293(4)
1.286(4)
1.347(4)
1.333(4)
1.478(5)
1.493(5)
N(1)-C(2)
N(3)-C(8)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(7)-C(8)
1.361(3)
1.355(3)
1.386(3)
1.386(3)
1.394(3)
1.398(3)
1.356(3)
1.354(3)
1.382(3)
1.380(3)
1.407(3)
1.402(4)
1.350(5)
1.358(5)
1.390(5)
1.387(5)
1.414(5)
1.406(5)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-N(3)
N(2)-Fe(1)-N(3)
80.6(3)
178.9(4)
80.0(3)
79.1(2)
159.2(2)
80.1(2)
73.29(11)
146.26(11)
72.97(11)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-N(3)
N(2)-Fe(1)-N(3)
80.17(8)
154.90(8)
79.85(8)
79.89(8)
154.89(8)
79.55(8)
81.05(14)
161.38(14)
80.74(14)
a Data from ref 45.
a Data from ref 36. b Data from ref 38. DMAP
dimethylamino)pyridine.
) (N,N-
reported in Table 2 and indicate different ligand rather than
metal oxidation states within the series of compounds. The
cationic compound, [(iPrPDI)Fe(CH2CMe3)][BPh4], is the most
straightforward to interpret, as the Cipso-Cimine and Nimine-Cimine
are relatively unperturbed from the free ligand49 and are
consistent with the neutral form of the bis(imino)pyridine. Thus,
there is no redox activity in this compound, and the chelate is
innocent. For the neutral compound, (EtPDI)Fe(CH2CMe3), the
chelate distances, as reported previously for compounds of this
type,36,49b are consistent with one-electron reduction.
Selected metrical parameters for the iron methyl cation,
[(iPrPDI)Fe(CH3)][BPh4], are reported in Table 3. Also included
in Table 3 are the distances for the neutral compound,
(iPrPDI)FeCH3,33 and Gambarotta’s anionic variant, [Li(THF)4]-
[(iPrPDI)FeCH3].45 As with [(iPrPDI)Fe(CH2CMe3)][BPh4], the
metrical data for the chelate in [(iPrPDI)Fe(CH3)][BPh4] are
consistent with a neutral, redox-innocent bis(imino)pyridine. The
elongated Fe-Nimine bond distances of 2.216(3) and 2.222(3)
Å are indicative of a high spin Fe(II) center.
The bis(imino)pyridine iron neopentyl and methyl anions,
[(iPrPDI)Fe(CH2CMe3)(N2)]- [(iPrPDI)Fe(CH3)]-, exhibit the
largest chelate distortions in the two series and suggest the
greatest degree of ligand reduction. To put these values in
perspective, a second set of comparisons are made in Table 4.
Included in Table 4 are the bond distances for [Li(OEt2)3][(iPrPDI)-
Fe(C6H4-4-Me)(N2)]36 and (iPrPDI)Fe(DMAP). The latter was
included for comparison to an established intermediate spin
ferrous compound with a two-electron-reduced bis(imino)-
pyridine.49b For [Li(OEt2)3][(iPrPDI)Fe(CH2CMe3)(N2)], the
Nimine-Cimine distances of 1.361(3) and 1.355(3) Å are signifi-
cantly elongated, while the Cimine-Cipso lengths of 1.394(3) and
1.398(3) Å are the most contracted of any bis(imino)pyridine
iron compound crystallographically characterized to date. These
values are similar to those reported for both [(iPrPDI)FeMe]-
and [(iPrPDI)Fe(C6H4-4-Me)(N2)]- and suggest either two-
electron or possibly even three-electron57 reduction of the
chelate.
Table 5. Solid State Magnetic Moments of Bis(imino)pyridine Alkyl
Compounds at 23 °C
compound
µeff (µB, 23 °C)
(
(
iPrPDI)Fe(CH2CMe3)
iPrPDI)Fe(CH2SiMe3)
4.0a
3.8a
[(iPrPDI)Fe(CH2CMe3)][BPh4]
[(iPrPDI)Fe(CH2CMe3)(Et2O)][BPh4]
[(iPrPDI)Fe(CH2CMe3)(THF)][BPh4]
[(iPrPDI)FeCH3][BPh4]
4.8b
4.8b
4.9b
5.2b
a Determined by the Evans method in benzene-d6. Data from refs 33
and 36. b Determined by solid state magnetic susceptibility balance.
crown-4)][(iPrPDI)Fe(CH2CMe3)N2], are diamagnetic and will
not be discussed further. A summary of the magnetic moments
determined at 23 °C for representative members of the series is
reported in Table 5.
As reported previously,33 the four-coordinate, neutral bis(imi-
no)pyridine iron alkyls exhibit magnetic moments consistent
3
with three unpaired electrons and S ) /2 ground states. The
cationic alkyl complexes have magnetic moments indicative of
S ) 2 ground states, as described previously.33,40 Variable-
temperature SQUID magnetic data for the base-stabilized
bis(imino)pyridine iron cation, [(iPrPDI)Fe(CH2CMe3)(THF)]-
[BPh4], were also measured and are presented in Figure 4.
Between 40 and 300 K, an essentially temperature-independent
magnetic moment of 4.7 µB was measured, slightly lower than
the expected spin-only value for four unpaired electrons but
nevertheless consistent with an S ) 2 ground state. Modeling
the data established a g value of 1.906 and a D value of 6.6
cm-1
.
Mo¨ssbauer Spectroscopy. The cationic, neutral, and anionic
bis(imino)pyridine iron alkyl complexes were also studied by
zero-field 57Fe Mo¨ssbauer spectroscopy. Representative spectra
for the neopentyl series of compounds are presented in Figure
5, and parameters for all compounds described in this work are
reported in Table 6. Also included in Table 6 are the iron the
iron tolyl and methyl anions, [Li(Et2O)3][(iPrPDI)Fe(C6H4-4-
Me)N2] and [Li(THF)4][(iPrPDI)FeMe], published by our labora-
tory36 and Gambarotta,45 respectively.
Magnetic Measurements. The magnetic ground state of each
class of bis(imino)pyridine iron alkyl compound was also studied
in the solid state by magnetic susceptibility balance, solution
Evans method, and in some cases SQUID magnetometry. The
alkyl anions, [Li(Et2O)3)][(iPrPDI)Fe(CH2CMe3)N2] and [Li(12-
Within the series of bis(imino)pyridine iron neopentyl
compounds, the base-free cation, [(iPrPDI)Fe(CH2CMe3)][BPh4],
exhibits the highest isomer shift, δ ) 0.57 mm s-1, consistent
(57) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew.
Chem., Int. Ed. 2002, 41, 3873.
9
15052 J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010

Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
Table 6. Zero-Field 57Fe Mo¨ssbauer Parameters Recorded at 80 K
for Cationic, Neutral, and Anionic Bis(imino)pyridine Iron Alkyl and
Dialkyl Compounds
compound
δ (mm s-1
)
∆EQ (mm s-1
)
(
iPrPDI)Fe(CH2CMe3)
(EtPDI)Fe(CH2CMe3)
iPrPDI)Fe(CH2SiMe3)
0.57
0.56
0.54
1.16
1.13
1.55
(
[(iPrPDI)Fe(CH2SiMe3)][BPh4]
0.64
0.88
0.88
0.61
1.35
2.20
2.29
1.37
[(iPrPDI)Fe(CH2SiMe3)(Et2O)][BPh4]
[(iPrPDI)Fe(CH2SiMe3)(THF)][BPh4]
[(iPrPDI)Fe(CH2SiMe2CH2SiMe3][MeB(C6F5)3]
[(iPrPDI)Fe(CH2CMe3)][BPh4]
0.57
0.84
0.84
1.30
2.18
2.18
[(iPrPDI)Fe(CH2CMe3)(Et2O)][BPh4]
[(iPrPDI)Fe(CH2CMe3)(THF)][BPh4]
Figure 4. Temperature-dependent SQUID magnetization data (1 T) for
[(iPrPDI)Fe(CH2CMe3)THF][BPh4] plotted as a function of magnetic moment
(µeff) vs temperature (T). Data are corrected for underlying diamagnetism.
[(iPrPDI)FeCH3][BPh4]
0.53
1.53
[(iPrPDI)Fe(CH2CMe3)N2]-
[(iPrPDI)Fe(CH3)]-
0.06
0.18
0.02
0.86
2.96
0.93
[(iPrPDI)Fe(C6H4-4-Me)N2]-
with a high spin iron(II) center. Coordination of diethyl ether
or THF results in a fairly significant increase in the isomer shift
to 0.84 mm s-1. This effect is a result of the overall increase in
the metal-ligand bond distances as the coordination number
increases from 4 to 5, resulting in an overall weaker ligand field
and hence a decrease in charge density at the iron nucleus.
Comparing the bond distances between the four-coordinate
cation, [(iPrPDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3] versus
the five-coordinate THF derivative, [(iPrPDI)Fe(CH2SiMe3)-
(THF)][BPh4], the Fe(1)-C(34) distances increase from 2.011(6)
to 2.034(3) Å, and the Fe(1)-N(1) and Fe-N(3) bond lengths
increase from 2.217(4) to 2.286(2) Å and from 2.221(4) to
2.328(2) Å, respectively. The Fe(1)-N(2) distance, however,
does not elongate and in fact slightly contracts from 2.121(4)
to 2.089(2) Å. This may be due to the difference in the
coplanarity of the iron and the bis(imino)pyridine between the
two coordination environments. While addition of a coordinating
solvent such as Et2O or THF results in a significant change in
isomer shift, reduction of the base-free neopentyl cation to the
corresponding neutral compound, (iPrPDI)Fe(CH2CMe3), has
almost no effect, as identical isomer shifts were measured. This
observation establishes little change in charge density at the
iron nucleus and the same spectroscopic metal oxidation state
between the two compounds.
ether or THF to [(iPrPDI)Fe(CH2SiMe3)][BPh4] results in a higher
isomer shift of 0.88 mm s-1, while formal one-electron reduction
to the neutral compound, (iPrPDI)Fe(CH2SiMe3), produced only
a small shift to 0.54 mm s-1, suggesting that the redox process
is ligand rather than iron based and the Fe(II) oxidation state is
maintained.
The Mo¨ssbauer isomer shifts of 0.06 and 0.02 mm s-1 for
the bis(imino)pyridine tolyl and neopentyl anions, [Li(Et2O)3]-
[(iPrPDI)Fe(C6H4-4-Me)N2] and [Li(Et2O)3][(iPrPDI)Fe(CH2-
CMe3)N2], are significantly lower than those for the neutral alkyl
complexes and indicate a significant change in charge density
at the iron center. These values are lower than the reported
isomer shift of 0.31 mm s-1 for (iPrPDI)Fe(DMAP), an
intermediate spin (SFe ) 1) iron compound.49b The measured
isomer shifts for the anionic complexes are comparable to the
value of 0.03 mm s-1 reported for (iPrPDI)Fe(CO)2, a highly
covalent molecule with contributions from both low spin Fe(II),
closed-shell [PDI]2-, and low spin Fe(0), d8 canonical forms.49b
Computational Studies. DFT calculations using the B3LYP
functional were also performed to gain additional insight into
the electronic structure of the series of cationic, neutral, and
anionic bis(imino)pyridine iron neopentyl compounds. For
consistency within the computed series, all calculations were
performed using the iPrPDI ligand.
For the base-free cation, [(iPrPDI)Fe(CH2CMe3)]+, a calcula-
tion assuming a simple quintet ground state successfully
reproduced the metrical parameters determined by X-ray dif-
fraction, including the deviation of the neopentyl group from
idealized square planar geometry. The computed structure
The other bis(imino)pyridine iron alkyl cations exhibit similar
isomer shifts, consistent with high spin iron(II) compounds. The
base-free neosilyl cation, [(iPrPDI)Fe(CH2SiMe3)][BPh4], and the
related rearranged alkyl compound, [(iPrPDI)Fe(CH2SiMe2-
CH2SiMe3][MeB(C6F5)3], have isomer shifts of 0.64 and 0.61
mm s-1, respectively. Likewise, for the base-free methyl cation,
[(iPrPDI)FeCH3][BPh4], a value of 0.53 mm s-1 was measured.
As with the analogous neopentyl cations, addition of diethyl
Figure 5. Representative zero-field 57Fe Mo¨ssbauer spectra of anionic (left), neutral (middle), and base-free cationic (right) bis(imino)pyridine iron neopentyl
compounds.
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A R T I C L E S
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Table 7. Computed and Experimental Bond Distances (Å) and
Angles (deg) for Cationic and Neutral Bis(imino)pyridine Iron
Neopentyl Complexes
[(iPrPDI)Fe(CH2CMe3)]+ (EtPDI)Fe(CH2CMe3)
(
iPrPDI)Fe(CH2CMe3)
expt calcd expt calcd
calcd
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-C(34)
2.213(2) 2.302 2.158(3) 2.286
2.210(2) 2.145 1.986(3) 2.033
2.242(2) 2.331 2.126(3) 2.226
2.334
2.040
2.281
2.035(2) 2.034 2.036(4) 2.073
2.070
N(1)-C(2)
N(3)-C(8)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(7)-C(8)
N(4)-N(5)
1.284(2) 1.286 1.314(4) 1.308
1.288(2) 1.285 1.329(4) 1.317
1.333(2) 1.340 1.390(4) 1.370
1.336(2) 1.340 1.366(4) 1.374
1.490(3) 1.497 1.446(5) 1.462
1.486(3) 1.498 1.428(5) 1.452
1.306
1.312
1.377
1.374
1.461
1.455
N(1)-Fe(1)-N(2) 73.14(6) 72.68 75.08(10) 73.28
N(1)-Fe(1)-N(3) 142.87(6) 140.68 136.68(11) 137.79
N(2)-Fe(1)-N(3) 72.83(6) 72.21 75.19(10) 74.38
74.08
140.85
73.17
N(2)-Fe(1)-N(4)
N(1)-Fe(1)-N(4)
N(4)-Fe(1)-C(34)
N(2)-Fe(1)-C(34) 151.61(8) 138.03 142.24(14) 138.88
145.25
lography for (EtPDI)FeCH2CMe3. However, the imine-metal
bond lengths are exceedingly overestimated by up to 0.17 Å.
This is most likely due to the increased steric demand of the
2,6-diisopropyl aryl in the computational model rather than the
ethyl substituents in the experimentally determined X-ray crystal
structure. In fact, a geometry optimization of the 2,6-diethyl
aryl-substituted compounds yielded significantly shorter imine-
metal bonds, while the remaining structural parameters and
electronic structure remained virtually unchanged (Table 7).
Consistent with previous proposals,36,49b the computed electronic
structure corresponds to a broken-symmetry (4,1) solution,
obtained via spontaneous symmetry breaking during the unre-
stricted quartet calculation. The BS(m,n) descriptor refers to a
broken symmetry state with m unpaired spin-up electrons on
fragment 1 and n unpaired spin-down electrons essentially
localized on fragment 2. A qualitative MO diagram and spin
density plot are shown in Figure 7. Antiferromagnetic coupling
between a singly occupied orbital of the high spin iron(II) ion
and the bis(imino)pyridine radical, similar to the previously
computed structure of (iPrPDI)FeCl,49b accounts for the S ) 3/2
ground state. The computed 57Fe Mo¨ssbauer parameters of δ )
0.57 mm s-1 and ∆EQ ) +1.78 mm s-1 are in excellent
agreement with the experimentally determined values (δ ) 0.57
mm s-1; ∆EQ ) |1.16 mm s-1|).
Figure 6. (a) Qualitative molecular orbital diagram for
S ) 2
[(iPrPDI)Fe(CH2CMe3)]+ from a B3LYP DFT calculation. (b) Spin density
plot obtained from a Mulliken population analysis (red, positive spin density;
yellow, negative spin density).
slightly overestimates the metal-ligand bond distances, a typical
occurrence for the B3LYP functional.58 The inclusion of solvent
effects by applying the COSMO solvation model (THF) did
not lead to significant changes of the geometric or electronic
structure. A qualitative molecular orbital (MO) diagram and a
spin density plot derived from these results (no COSMO
solvation) are shown in Figure 6. This solution clearly estab-
lishes a high spin iron(II) (SFe ) 2) configuration with a neutral,
redox-innocent bis(imino)pyridine ligand, consistent with the
metrical parameters of the chelate determined by X-ray dif-
fraction. From this electronic structure description, computed
The final compound in the series, diamagnetic [(iPrPDI)Fe-
(CH2CMe3)N2]-, was also examined computationally. Initial
studies were performed on the anionic component, [(iPrPDI)Fe-
(CH2CMe3)N2]-, without the lithium cation but with application
of a COSMO solvation (THF) model to account for the negative
charge. Two electronic structure descriptions were explored: a
spin-restricted closed-shell structure and an open-shell BS(2,2)
singlet alternative. Analysis of the computed electronic structures
revealed that the closed-shell solution corresponds to a low
spin Fe(II) ion coordinated by a closed-shell dianionic PDI2-
ligand with a doubly filled b2 orbital, while the open-shell
solution corresponds to an intermediate spin Fe(II) ion anti-
ferromagnetically coupled to a triplet PDI2- ligand. The energy
difference between the two solutions was essentially indistin-
guishable, with the open shell being more stable by only 3.5
57Fe Mo¨ssbauer parameters of δ ) 0.58 mm s-1 and ∆EQ
)
-1.81 mm s-1 were obtained that are in excellent agreement
with the experimentally determined values (δ ) 0.57 mm s-1
∆EQ ) |1.30 mm s-1|).
;
The neutral bis(imino)pyridine iron neopentyl compound,
(iPrPDI)FeCH2CMe3, was calculated as a spin-unrestricted quartet
3
on the basis of the experimentally determined S ) /2 ground
state. Generally, the optimized geometry is in good agreement
with the structural parameters obtained from X-ray crystal-
(58) Neese, F. J. Biol. Inorg. Chem. 2006, 11, 702.
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A R T I C L E S
3
Figure 7. (a) Qualitative molecular orbital diagram for S ) /2 (iPrPDI)Fe(CH2CMe3) from a B3LYP DFT calculation. S represents the spatial overlap of
the antiferromagnetically coupled magnetic orbitals. (b) Spin density plot obtained from a Mulliken population analysis (red, positive spin density; yellow,
negative spin density).
kcal mol-1. The structural parameters of both solutions are in
reasonable agreement with the experimental values (see Table
8). The most significant differences were found for the
Fe(1)-C(34) bond length, which is overestimated by almost
0.07 Å in the BS(2,2) approach but underestimated by only 0.01
Å in the closed-shell calculation. Similarly, the N(1)-C(2) and
N(3)-C(8) distances are more accurately matched by the closed-
shell singlet.
Table 8. Computed and Experimental Bond Distances (Å) and
Angles (deg) for the Anionic Bis(imino)pyridine Iron Neopentyl
Complex
[(iPrPDI)FeN2(CH2CMe3)]-
calcd calcd
[Li(Et2O)3][(iPrPDI)FeN2(CH2CMe3)]
expt
BS(2,2) RKS
expt
calcd
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-N(4)
Fe(1)-C(34)
1.931(2) 2.023 2.029
1.832(2) 1.875 1.849
1.919(2) 2.015 2.000
1.746(2) 1.823 1.794
2.079(2) 2.147 2.069
1.931(2)
1.832(2)
1.919(2)
1.746(2)
2.079(2)
2.043
1.851
2.033
1.756
2.076
The 57Fe Mo¨ssbauer parameters were calculated for both
electronic structures to further substantiate the ground state of
the molecule. Based on the broken-symmetry solution, values
of δ ) 0.29 mm s-1 and ∆EQ ) +1.63 mm s-1 were computed,
which are in agreement with the data obtained for previously
reported intermediate spin Fe(II) complexes with PDI ligands
(e.g., (iPrPDI)Fe(DMAP)49b but disagree with the experimental
parameters of δ ) 0.06 mm s-1 and ∆EQ ) |0.86 mm s-1|. By
contrast, the closed-shell calculation reproduces the small
quadrupole splitting, with a computed value of ∆EQ ) +1.09
mm s-1, but still overestimates the isomer shift significantly (δ
) 0.25 mm s-1).
N(1)-C(2)
N(3)-C(8)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(7)-C(8)
N(4)-N(5)
1.361(3) 1.371 1.348
1.355(3) 1.372 1.361
1.386(3) 1.377 1.388
1.386(3) 1.377 1.396
1.394(3) 1.414 1.430
1.398(3) 1.415 1.418
1.138(3) 1.117 1.124
1.361(3)
1.355(3)
1.386(3)
1.386(3)
1.394(3)
1.398(3)
1.138(3)
1.353
1.349
1.390
1.387
1.421
1.425
1.137
N(1)-Fe(1)-N(2) 80.17(8) 80.17 79.79
N(1)-Fe(1)-N(3) 154.90(8) 147.73 154.78
N(2)-Fe(1)-N(3) 79.85(8) 80.03 79.93
80.17(8)
154.90(8)
79.85(8)
79.15
154.11
79.44
+
To investigate the influence of the terminal Li(Et2O)3 ion
N(2)-Fe(1)-N(4) 167.76(9) 167.81 163.53
N(1)-Fe(1)-N(4) 98.45(9) 96.69 97.58
N(4)-Fe(1)-C(34) 87.41(9) 88.39 89.68
N(2)-Fe(1)-C(34) 104.82(9) 103.80 106.79
167.76(9)
98.45(9)
87.41(9)
104.82(9)
165.44
98.33
88.79
coordinated to the dinitrogen ligand, the molecule was calculated
as [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)N2] without any truncations.
Surprisingly, all attempts to obtain an open-shell singlet broken-
symmetry solution for this compound failed and converged back
105.77
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A R T I C L E S
Tondreau et al.
to the closed-shell solution. The structural parameters obtained
from the geometry optimization of the full molecule are in
excellent agreement with the experimental values. One notable
exception is the significant overestimation of the iron-imine
distances, which is slightly more pronounced than usually
observed at the B3LYP level of theory. The calculated Mo¨ss-
bauer parameters of δ ) 0.21 mm s-1 and ∆EQ ) +1.12 mm
s-1 match the experimental values more closely than the values
obtained for the truncated molecule (without the Li). However,
the isomer shift is still notably overestimated, which is due to
the elongated iron-imine bonds in the optimized geometry.
Consequently, a spin-restricted closed-shell calculation on the
crystallographic geometry yielded Mo¨ssbauer parameters of δ
) 0.11 mm s-1 and ∆EQ ) -0.96 mm s-1, in good agreement
with experiment.
On the basis of these results, the electronic structure of
[Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)N2] is best described as a
singlet with a low spin Fe(II) ion and a closed-shell iPrPDI2-
dianion. An important feature of this model is the strong covalent
2
interaction of the axial neopentyl substituent with the dz orbital
of the Fe center, which is most likely the reason for the spin
state change from high spin to low spin upon reduction of the
neutral compound to the anion. The corresponding MO diagram
is shown in Figure 8. This description is in agreement with the
structural data, which show very short metal-ligand bonds
expected for low spin Fe(II) complexes and indicate two-electron
ligand reduction.
Electronic Structure Summary. The combined synthetic,
spectroscopic, and computational data obtained in this study
establish a comprehensive view of the electronic structure of
bis(imino)pyridine iron alkyl complexes that vary by three
oxidation states. Importantly, these electronic structures are for
single-component ethylene polymerization catalysts and resolve
some of the controversy surrounding the nature of the active
species upon activation of the dihalide complexes with MAO.
A summary of these findings is presented in Figure 9. The
neutral four-coordinate iron monoalkyls, (iPrPDI-)FeIICH2EMe3
(E ) C, Si), are used as the reference point. These compounds
are best described as high spin ferrous derivatives antiferro-
magnetically coupled to a bis(imino)pyridine radical anion
chelate. This configuration avoids formation of relatively rare
Fe(I) alkyl species.59 One-electron oxidation of the neutral iron
monoalkyl compounds to the corresponding alkyl cations,
[(iPrPDI0)FeII(CH2EMe3)]+ (E ) C, Si), is ligand-based, as both
the experimental and computational data support high spin
ferrous complexes with a redox-innocent neutral bis(imino)py-
ridine chelate. The analogous iron methyl cation, [(iPrPDI0)-
FeII(Me)]+, also has the same electronic structure description,
despite the different geometry at the iron center. Given the
single-component (i.e., no need for an activator) ethylene
polymerization activity previously demonstrated for certain
members of this series,40 redox activity, at least in the initiating
species, is not a prerequisite for polymerization activity. Studies
with different metal-ligand complexes are currently under
investigation to further support this claim.
Figure 8. Qualitative molecular orbital diagram for
S ) 0
[(iPrPDI)Fe(CH2CMe3)N2]- from a B3LYP DFT calculation. The lowest
energy orbital shown highlights the strong covalent interaction of the iron
with the apical carbon donor of the alkyl.
ferrous compound (SFe ) 0) with a closed-shell PDI2- dianion.
The low Mo¨ssbauer isomer shift and the extreme distortions to
the chelate are consistent with this description. Notably, one-
electron reduction of (iPrPDI-)FeII(CH2CMe3) is ligand-based
and is also accompanied by a change in spin state (from high-
to low spin ferrous) at the metal. As previous studies have
shown, reduction of the bis(imino)pyridine ligand changes its
field strength such that neutral and monoanionic forms are
relatively weak field, while two-electron-reduced forms (either
singlet or triplet) are stronger field.49 Additionally, the shift of
the strongly σ-donating neopentyl group to the apical position
in the planar neutral complex to the axial position in the square
pyramidal anion facilitates the change to the low spin config-
uration. For the neopentyl, tolyl, and phenyl derivatives prepared
in our laboratory, the d6 electron configuration of the low spin
ferrous center is well established for π-backbonding and
formation of dinitrogen complexes.
The final member of the series is the monoalkyl anion,
[(iPrPDI2-)FeII(CH2CMe3)(N2)]-. Recall, the neosilyl variant
proved inaccessible under conditions used to prepare the other
examples reported in this work. The experimental and compu-
tational data establish that the electronic structure of
[(iPrPDI2-)FeII(CH2CMe3)(N2)]- is best described as a low spin
(59) Holland, P. L. Acc. Chem. Res. 2008, 41, 905.
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Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
Figure 9. Electronic structure summary of cationic, neutral, and anionic bis(imino)pyridine iron alkyl and neutral iron dialkyl compounds.
spin, d6 compound results and dinitrogen coordination results.
These studies imply that the ferrous oxidation state is maintained
during treatment of (iPrPDI)FeCl2 with MAO and that redox
events are confined to the bis(imino)pyridine chelate.
Experimental Section
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard vacuum line, Schlenk,
and cannula techniques or in an MBraun inert atmosphere drybox
containing an atmosphere of purified nitrogen. Solvents for air- and
moisture-sensitive manipulations were initially dried and deoxy-
genated using literature procedures.60 Hydrogen and deuterium gas
were passed through a column containing manganese oxide
supported on vermiculite and 4 Å molecular sieves before admission
to the high-vacuum line. Benzene-d6 and toluene-d8 were purchased
from Cambridge Isotope Laboratories and dried over 4 Å molecular
sieves or titanocene, respectively. (iPrPDI)Fe(CH2SiMe3),36 (iPrPDI)-
Fe(CH2CMe3),36 (iPrPDI)FeCH3,33 [(iPrPDI)Fe(CH2SiMe3)(Et2O)]-
[BPh4],40 [(iPrPDI)Fe(CH2SiMe3)(THF)][BPh4],40 [(iPrPDI)Fe(CH2-
SiMe2CH2SiMe3)][MeB(C6F5)3],40 and [Li(Et2O)3][(iPrPDI)Fe(C6H4-
4-R)(N2)]35 were prepared according to literature procedures.
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.76, 399.78, and
500.62 MHz, respectively. All 1H NMR chemical shifts are reported
relative to SiMe4, using 1H (residual) chemical shifts of the solvent
as a secondary standard. Peak width at half-height is given for
paramagnetically broadened resonances. Elemental analyses were
performed at Robertson Microlit Laboratories, Inc., Madison, NJ.
Solution magnetic moments were determined by the method61
of Evans using a ferrocene standard and are the average value of
at least two independent measurements. Solid state magnetic
susceptibility measurements were performed with a Johnson Mat-
they magnetic susceptibility balance (MSB) that was calibrated with
HgCo(SCN)4 or using SQUID magnetometry. SQUID magnetiza-
tion data of crystalline powdered samples were recorded with a
SQUID magnetometer (Quantum Design) at 10 kOe between 5 and
300 K for all samples. Values of the magnetic susceptibility were
corrected for the underlying diamagnetic increment by using
tabulated Pascal constants and the effect of the blank sample holders
(gelatin capsule/straw). Samples used for magnetization measure-
ment were recrystallized multiple times and checked for chemical
composition by 1H NMR spectroscopy. The program julX, written
by E. Bill, was used for (elements of) the simulation and analysis
of magnetic susceptibility data.62
Figure 10. Proposed electronic structures for intermediates detected from
treatment of (iPrPDI)FeCl2 with MAO and AlMe3 (see ref 18).
The elucidation of the electronic structure of the cationic,
neutral, and anionic bis(imino)pyridine iron monoalkyl com-
plexes allows evaluation of the iron species proposed to form
upon treatment of (iPrPDI)FeCl2 with MAO or AlMe3 (Figure
10).18 Recently, Bryliakov, Talsi, and co-workers reported
observation of (iPrPDI)Fe(µ-X)(µ-CH3)Al(CH3)2 (X ) Cl, CH3)
and [(iPrPDI)Fe(µ-X)(µ-CH3)Al(CH3)2]+ by 1H NMR and EPR
spectroscopies.18 The identity of X was determined by the
relative ratio of MAO or AlMe3 to the iron dihalide. For both
the neutral and cationic complexes, these species were only
observed in situ and were not isolated; hence, no crystal-
lographic, magnetic, or Mo¨ssbauer data are available. However,
on the basis of analogy to compounds prepared and studied in
this work, electronic structures for both classes of compounds
are proposed. The neutral compound, (iPrPDI-)FeII(µ-X)(µ-
CH3)Al(CH3)2, is best described as a ferrous compound with a
bis(imino)pyridine radical anion. The cationic complex, [(iPrPDI0)-
FeII(µ-X)(µ-CH3)Al(CH3)2]+, is likely a high spin ferrous
compound (SFe ) 2) with a redox-innocent, neutral bis(imi-
no)pyridine chelate. Notably, our data suggest that treatment
of (iPrPDI)FeCl2 with MAO or trialkylaluminums does not result
in redox change at the iron centersall oxidation/reduction events
occur at the bis(imino)pyridine ligand.
Concluding Remarks
Bis(imino)pyridine iron alkyl complexes have been long-
sought-after targets due to the likely intermediacy of such
species in olefin polymerization catalysis. Here we establish that
bis(imino)pyridine iron monoalkyl complexes having three
formal oxidation states (cation, neutral, and anion) all contain
ferrous centers, and the redox chemistry is confined to the
chelate. For neopentyl and neosilyl derivatives, the neutral and
monoanionic forms of the bis(imino)pyridine are sufficiently
weak field that high spin ferrous compounds result. In the case
of the anion, the field strength increases sufficiently that a low
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop, and
then quickly transferred to the goniometer head of a Bruker X8
APEX2 diffractometer equipped with a molybdenum X-ray tube
(λ ) 0.71073 Å). Preliminary data revealed the crystal system. A
(60) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(61) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(62) http://ewww.mpi-muelheim.mpg.de/bac/logins/bill/julX_en.php.
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A R T I C L E S
Tondreau et al.
hemisphere routine was used for data collection and determination
of lattice constants. The space group was identified and the data
were processed using the Bruker SAINT+ program and corrected
for absorption using SADABS. The structures were solved using
direct methods (SHELXS), completed by subsequent Fourier
synthesis, and refined by full-matrix least-squares procedures.
Quantum-Chemical Calculations. All DFT calculations were
performed with the ORCA program package.63 The geometry
optimizations of the complexes and single-point calculations on
the optimized geometries were carried out at the B3LYP level64-66
of DFT. This hybrid functional often gives better results for
transition metal compounds than pure gradient-corrected functionals,
especially with regard to metal-ligand covalency.67 The all-electron
Gaussian basis sets were those developed by the Ahlrichs group.68,69
Triple-ꢁ-quality basis sets TZVP with one set of polarization
functions on the metal and on the atoms directly coordinated to
the metal center were used.69 For the carbon and hydrogen atoms,
slightly smaller polarized split-valence SV(P) basis sets were used
that were of double-ꢁ quality in the valence region and contained
a polarizing set of d-functions on the non-hydrogen atoms.68
Auxiliary basis sets used to expand the electron density in the
resolution-of-the-identity (RI) approach were chosen70-72 to match
the orbital basis.
The SCF calculations were tightly converged (1 × 10-8 Eh in
energy, 1 × 10-7 Eh in the density change, and 1 × 10-7 in
maximum element of the DIIS error vector). The geometry
optimizations for all complexes were carried out in redundant
internal coordinates without imposing symmetry constraints. In all
cases, the geometries were considered converged after the energy
change was less than 5 × 10-6 Eh, the gradient norm and maximum
gradient element were smaller than 1 × 10-4 and 3 × 10-4 Eh
bohr-1, respectively, and the root-mean-square and maximum
displacements of all atoms were smaller than 2 × 10-3 and 4 ×
10-3 bohr, respectively.
CP(PPP) basis set78 for iron. The Mo¨ssbauer isomer shifts were
calculated from the computed electron densities at the iron centers
as previously described.79
Preparation of [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)N2]. A 50 mL
round-bottom flask was charged with 0.150 g (0.25 mmol) of
(iPrPDI)Fe(N2)2 and approximately 20 mL of diethyl ether. The
contents of the flask were cooled to -35 °C. A scintillation vial
was charged with 0.020 g (0.25 mmol) of neopentyl lithium and
approximately 10 mL of diethyl ether, and the resulting solution
was cooled to -35 °C. The flask containing the iron compound
was stirred, and the neopentyl lithium solution was added dropwise
over the course of 5 min. The reaction was warmed to room
temperature and stirred. After 0.5 h, the reaction mixture was filtered
through Celite. The filtrate was collected, and the volatiles were
removed. The resulting residue was dissolved in a minimal amount
of diethyl ether, and the resulting solution was placed in a
scintillation vial and cooled overnight at -35 °C for recrystalliza-
tion. The resulting solid was collected on a glass frit and yielded
0.078 g (34%) of [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)(N2)]. Analysis
for C50H84N5FeO3Li, calcd: C, 69.34; H, 9.78; N, 8.09. Found: C,
69.73; H, 8.92; N, 8.84. Magnetic susceptibility (benzene-d6, 23
°C): µeff ) 0 µB. IR (KBr): ν(N2) ) 1948 cm-1
.
Alternative Preparation of [Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)-
(N2)]. This molecule was prepared using a procedure identical to
that described above, with the exception that 0.100 g (0.14 mmol)
of (iPrPDI)FeBr and 0.022 g (0.28 mmol) of neopentyl lithium
were used as the reagents. Recrystallization from diethyl ether at
-35 °C furnished 0.052 g (41%) of [Li(Et2O)3][(iPrPDI)Fe-
(CH2CMe3)(N2)].
Preparation of [Li(12-Crown-4)][(iPrPDI)Fe(CH2CMe3)(N2)].
A scintillation vial was charged with 0.05 g (0.06 mmol) of
[Li(Et2O)3][(iPrPDI)Fe(CH2SiMe3)(N2)] and approximately 5 mL of
diethyl ether. A solution containing 0.02 g (0.12 mmol) of 12-
crown-4 in diethyl ether was added to the stirring solution of iron
compound. The volume of the solution was reduced to ap-
proximately 5 mL, and the vial was placed in a -35 °C freezer.
The solvent was decanted, and the solid was dried under reduced
pressure to yield 0.030 g (64%) of a dark red-brown solid, identified
as [Li(12-Crown-4)][(iPrPDI)Fe(CH2CMe3)(N2)]. Analysis for
C46H70N5FeO4Li, calcd: C, 67.39; H, 8.61; N, 8.54. Found: C, 67.06;
H, 8.42; N, 8.34. Magnetic susceptibility: µeff ) 0 µB. IR (KBr):
Throughout this paper we describe our computational results by
using the broken-symmetry (BS) approach described by Ginsberg73
and Noodleman.74 Because several BS solutions to the spin-
unrestricted Kohn-Sham equations may be obtained, the general
notation BS(m,n)75 has been adopted, where m (n) denotes the
number of spin-up (spin-down) electrons at the two interacting
fragments. Canonical and corresponding76 orbitals as well as spin
density plots were generated with the program Molekel.77 Non-
relativistic single-point calculations on the optimized geometries
were carried out to predict Mo¨ssbauer spectral parameters (isomer
shifts and quadrupole splittings). These calculations employed the
ν(N2) ) 1996 cm-1
.
Attempted Preparation of [Li(Et2O)3][(iPrPDI)Fe(CH2SiMe3)-
(N2)]. Attempts to synthesize this compound were carried out using
the methods described above to successfully prepare
[Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)(N2)]. In a typical experiment,
0.300 g (0.50 mmol) of (iPrPDI)Fe(N2)2 and 0.047 g (0.53 mmol)
of LiCH2SiMe3 were used. Recrystallization from diethyl ether at
-35 °C furnished 0.24 g (51%) of red crystals, identified as
(iPrPDI)Fe(CH2SiMe3), containing about 5% (as judged by Mo¨ss-
bauer spectroscopy) of the desired [Li(Et2O)3][(iPrPDI)Fe-
(63) Neese, F. Orcasan ab initio, DFT and Semiempirical Electronic
Structure Package, Version 2.7, Revision 0; Institut fu¨r Physikalische
und Theoretische Chemie, Universita¨t Bonn: Bonn, Germany, August
2009.
(64) Becke, A. D. J. Chem. Phys. 1986, 84, 4524.
(65) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(66) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(67) Neese, F. Solomon, E. I. In Magnetism: From Molecules to Materials;
Miller, J. S., Drillon, M., Eds.; Wiley: New York, 2002; Vol. 4, p
345.
(CH2SiMe3)(N2)]. IR (KBr): ν(N2) ) 1938 cm-1
.
Preparation of [(iPrPDI)Fe(CH2CMe3)][BPh4]. A 20 mL scin-
tillation vial was charged with 0.100 g (0.164 mmol) of
(iPrPDI)Fe(CH2CMe3), 0.083 g (0.164 mmol) of [Cp2Fe][BPh4], and
a stir bar. Approximately 7 mL of benzene was added to the mixture
of solids with stirring. The stirring rate was increased as the reaction
mixture thickened and a precipitate formed. After 5 min, an equal
volume of pentane was added, and the stirring was continued for
another 10 min. The solid was collected on a glass frit and washed
four times with ∼20 mL of pentane. The solid was dried under
vacuum and yielded 0.143 g (93%) of a dull gray-red powder,
identified as [(iPrPDI)Fe(CH2CMe3)][BPh4]. Analysis for
C62H74N3FeB, calcd: C, 80.25; H, 8.04; N, 4.53. Found: C, 80.41;
H, 7.84; N, 4.21. Magnetic susceptibility (MSB, 23 °C): µeff ) 4.8
(68) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(69) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(70) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem.
Acc. 1997, 97, 119.
¨
(71) Eichkorn, K.; Treutler, O.; Ohm, H.; Ha¨ser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 240, 283.
¨
(72) Eichkorn, K.; Treutler, O.; Ohm, H.; Ha¨ser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 242, 652.
(73) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111.
(74) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J. M. Coord.
Chem. ReV. 1995, 144, 199.
(75) Kirchner, B.; Wennmohs, F.; Ye, S.; Neese, F. Curr. Opin. Chem.
Biol. 2007, 11, 134.
(76) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
(77) Molekel, Advanced Interactive 3D-Graphics for Molecular Sciences;
available under http://www.cscs.ch/molkel/.
(78) Neese, F. Inorg. Chim. Acta 2002, 337, 181.
(79) Sinnecker, S.; Slep, L. D.; Bill, E.; Neese, F. Inorg. Chem. 2005, 44,
2245.
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15058 J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010

Redox Activity in Ethylene Polymerization Catalysts
A R T I C L E S
µB. 1H NMR (benzene-d6): δ -124.31 (569 Hz, 2H), -43.32 (470
Hz, 12H), -17.35 (258 Hz, 12H,), -0.70 (316 Hz, 4H), 4.61 (64
Hz, 4H), 12.33 (101 Hz, 8H), 23.25 (64 Hz, 8H), 101.95 (1042
mmol) of [(iPrPDI)Fe(CH2SiMe3)][BPh4]. The addition of ether
resulted in an immediate color change of the solid to blue and
yielded 0.050 g (98%) of a light blue powder, identified as
[(iPrPDI)Fe(CH2SiMe3)(OEt2)][BPh4] as reported previously.40
Preparation of [(iPrPDI)FeCH3][BPh4]. This molecule was
prepared in a manner similar to that used for [(iPrPDI)Fe-
(CH2CMe3)][BPh4], but with 0.112 g (0.202 mmol) of (iPrPDI)-
FeCH3 and 0.100 g (0.200 mmol) of [Cp2Fe][BPh4]. The light red
solid was collected on a glass frit and washed four times with ∼20
mL of pentane. The solid was dried under vacuum and yielded
1
Hz, 6H). H NMR (fluorobenzene-d5): δ -132.53 (588 Hz, 2H),
-44.57 (237 Hz, 12H), -17.84 (116 Hz, 12H,), -0.01 (115 Hz,
4H), 5.89 (158 Hz, 4H), 10.60 (158 Hz, 8H), 28.98 (98 Hz, 8H),
73.41 (1138 Hz, 2H), 106.58 (528 Hz, 6H), 133.40 (611 Hz, 1H).
Preparation of [(iPrPDI)Fe(CH2CMe3)(THF)][BPh4]. A 20 mL
scintillation vial was charged with 0.050 g (0.054 mmol) of
[(iPrPDI)Fe(CH2CMe3)][BPh4]. A minimum amount of THF was
added to dissolve the solid, forming a blue solution. The blue
solution was layered with approximately 1.5 mL of pentane, and
the vial was placed in a -35 °C freezer. Over the course of 8 h, a
blue solid formed. The mother liquor was decanted, and the solid
was dried at room temperature under vacuum, yielding 0.045 g
(84%) of a light blue solid, identified as [(iPrPDI)Fe(CH2-
CMe3)(THF)][BPh4]. Analysis for C66H82N3OFeB, calcd: C, 79.27;
H, 8.26; N, 4.20. Found: C, 78.78; H, 7.96; N, 3.94. Magnetic
susceptibility (MSB, 23 °C): µeff ) 4.9 µB.
0.153
g (89%) of a dull gray-red powder, identified as
[(iPrPDI)FeCH3][BPh4]. Single crystals suitable for X-ray diffraction
were grown by dissolving 0.020 g (0.036 mmol) of (iPrPDI)FeCH3
in 1 mL of C6H5F and adding 0.016 g (0.032 mmol) of
[Cp2Fe][BPh4]. After being stirred for 1 min with a glass pipet, the
resulting solution was filtered through a glass frit into an NMR
tube and layered with n-hexane. The layers were allowed to diffuse
together over the course of 36 h at room temperature, yielding
translucent blocks with a reddish-brown hue. Analysis for
C58H66N3FeB, calcd: C, 79.90; H, 7.63; N, 4.82. Found: C, 79.62;
H, 7.86; N, 4.57. Magnetic susceptibility (MSB, 23 °C): µeff ) 5.2
Preparation of [(iPrPDI)Fe(CH2CMe3)(OEt2)][BPh4]. This
compound was prepared in a manner similar to that used for
[(iPrPDI)Fe(CH2CMe3)(THF)][BPh4] by dissolving 0.050 g (0.054
mmol) of [(iPrPDI)Fe(CH2CMe3)][BPh4] in approximately 5 mL of
diethyl ether. Recrystallization from diethyl ether at -35 °C
furnished 0.045 g (84%) of a light blue solid, identified as
[(iPrPDI)Fe(CH2CMe3)(OEt2)][BPh4]. Analysis for C66H84N3OFeB,
calcd: C, 79.11; H, 8.45; N, 4.19. Found: C, 78.78; H, 7.96; N,
3.94. Magnetic susceptibility (MSB, 23 °C): µeff ) 4.8 µB.
Alternative Preparation of [(iPrPDI)Fe(CH2SiMe3)][BPh4].
This molecule was prepared in a manner similar to that used for
[(iPrPDI)Fe(CH2CMe3)][BPh4] with 0.150 g (0.240 mmol) of
(iPrPDI)Fe(CH2SiMe3) and 0.121 g (0.240 mmol) of [Cp2Fe][BPh4]
and yielded 0.199 g (88%) of a dull gray-red powder, identified as
[(iPrPDI)Fe(CH2SiMe3)][BPh4] as reported previously.40
1
µB. H NMR (benzene-d6): δ -115.45 (1715 Hz), -49.31 (1665
Hz), -17.53 (216 Hz), -8.15 (380 Hz), 1.95 (102 Hz), 8.40 (114
Hz), 9.31 (455 Hz), 20.40 (102 Hz), 81.49 (1850 Hz), 131.31 (1053
Hz). 1H NMR (fluorobenzene-d5): δ -117.93 (985 Hz, 2H), -46.66
(873 Hz, 12H), -18.36 (454 Hz, 12H), -0.71 (284 Hz, 4H), 3.73
(186 Hz, 4H), 8.73 (198 Hz, 8H), 12.21 (221 Hz, 8H), 68.61 (730
Hz, 2H), 185.38 (855 Hz, 6H), 128.03 (513 Hz, 1H).
Acknowledgment. We thank the U.S. National Science Foun-
dation and Deutsche Forschungsgemeinschaft for a Cooperative
Activities in Chemistry between U.S. and German Investigators
grant. We also thank Dr. Marco Bouwkamp and Chantal Stieber
for sample preparation and for preliminary Mo¨ssbauer studies.
Alternative Preparation of [(iPrPDI)Fe(CH2SiMe3)(THF)]-
[BPh4]. This molecule was prepared in a manner similar to that
used for [(iPrPDI)Fe(CH2CMe3)(THF)][BPh4] with 0.050 g (0.240
mmol) of [(iPrPDI)Fe(CH2SiMe3)][BPh4] and yielded 0.049 g (96%)
of a light blue powder, identified as [(iPrPDI)Fe(CH2SiMe3)-
(THF)][BPh4] as reported previously.40
Supporting Information Available: Crystallographic details
for [(iPrPDI)Fe(CH2CMe3)][BPh4], [(iPrPDI)FeMe][BPh4], and
[Li(Et2O)3][(iPrPDI)Fe(CH2CMe3)(N2)] in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Alternative Preparation of [(iPrPDI)Fe(CH2SiMe3)(OEt2)]-
[BPh4]. This molecule was prepared in a manner similar to that
used for [(iPrPDI)Fe(CH2CMe3)(OEt2)][BPh4] with 0.050 g (0.240
JA106575B
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