Bis(imino)pyridine iron alkyls containing β-hydrogens: Synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation

Article abstract of DOI:10.1021/ja803296f

Bis(imino)pyridine iron alkyl complexes bearing β-hydrogens, ( ^iPrPDI)FeR ((^iPrPDI = 2,6-(2,6-ⁱPr ₂-C₆H₃N=CMe)₂C₅H ₃N; R = Et, ⁿBu, ⁱBu, CH₂ ^cycloC₅H₉; 1-R), were synthesized either by direct alkylation of (^iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, (^iPrPDI)Fe(N ₂)₂ (1-(N₂)₂). In the latter method, the formal oxidative addition reaction produced (^iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d₆ solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of β-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-ⁱBu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial β-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

Full text of DOI:10.1021/ja803296f

Published on Web 08/08/2008
Bis(imino)pyridine Iron Alkyls Containing ꢀ-Hydrogens:
Synthesis, Evaluation of Kinetic Stability, and Decomposition
Pathways Involving Chelate Participation
Ryan J. Trovitch, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca,
New York, 14853
Received May 3, 2008; E-mail: pc92@cornell.edu
Abstract: Bis(imino)pyridine iron alkyl complexes bearing ꢀ-hydrogens, (iPrPDI)FeR ((iPrPDI ) 2,6-(2,6-
iPr2-C6H3NdCMe)2C5H3N; R ) Et, nBu, iBu, CH2cycloC5H9; 1-R), were synthesized either by direct alkylation
of (iPrPDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the
appropriate alkyl bromide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (1-(N2)2). In the latter method,
the formal oxidative addition reaction produced (iPrPDI)FeBr (1-Br), along with the desired iron alkyl, 1-R.
Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements,
structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically
coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based
(one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic
stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the
corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated
with the number of ꢀ-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss
of ethane over the course of three hours, whereas the corresponding 1-iBu compound had a half-life of
over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of
deuterium labeling experiments and support a pathway involving initial ꢀ-hydrogen elimination followed by
cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway.
The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization
reactions is also presented.
Introduction
substituent are selective for R-olefin production with near ideal
Schultz-Flory distributions.1,2,6-8
When activated with methylalumoxane (MAO), aryl-substi-
tuted bis(imino)pyridine iron and cobalt dihalide compounds
exhibit productivities for ethylene polymerization that rival the
most efficient group 4 metallocene catalysts.1-3 Accordingly,
this class of compounds has attracted considerable attention from
both academic and industrial laboratories.4 Systematic evaluation
of aryl substituent effects has established structure-reactivity
relationships that allow tuning of the polymerization activity
by straightforward manipulation of ligand architecture.5 For
example, bis(imino)pyridine iron dihalide precatalysts bearing
two large 2,6-substituents on the aryl ring are known to produce
linear polyethylene whereas those with only a single ortho aryl
Despite the tremendous successes with bis(imino)pyridine iron
catalysts, short catalyst lifetimes and formation of large amounts
of 1-butene during ethylene oligomerization to R-olefins have
been identified as potential obstacles to commercialization.7
Ideally, new catalyst discovery would be guided by well-
understood mechanistic principles - an approach that has proven
invaluable in group 4 metallocene catalyst development and
implementation.9 For example, an understanding of counterion
effects, the nature of the transition structure and the influence
of cyclopentadienyl substituents has resulted in the genesis of
improved catalysts with remarkable selectivities, productivities,
and lifetimes.10
(6) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.;
Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood,
M. R. J. Chem.-Eur. J. 2000, 6, 2221.
(1) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.
1998, 120, 4049.
(7) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones, B. B.; Fish,
B. M.; Schiffhauer, M. F. Organometallics 2008, 27, 1147.
(8) Ionkin, A. S.; Marshall, W. J.; Adelman, D. J.; Fones; B. B.; Fish,
B. M.; Schiffhauer, M. F.; Soper, P. D.; Waterland, R. L.; Spence,
R. E.; Xie, T. J. Polym. Sci., Part A 2008, 46, 585.
(2) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;
McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 849.
(3) Bianchini, C.; Giambastiani, G.; Rios, I. G.; Mantovani, G.; Meli, A.;
Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391.
(4) For early patents see: (a) Bennett, A. M. A. (DuPont) PCT Int. Appl.
WO9827124 A1, 1998; p 68. (b) Brookhart, M. S.; Small, B. L. PCT
Int. Appl. WO 9902472 A1 1999, 54 pp.
(9) (a) Moehring, 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.
(5) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. ReV. 2007, 107, 1745.
(10) Miller, S. A.; Bercaw, J. E. Organometallics 2006, 25, 3576.
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 11631–11640 11631

A R T I C L E S
Trovitch et al.
Figure 1. Neutral bis(imino)pyridine iron alkyls stabilized from ꢀ-hydrogen elimination.
In contrast, few mechanistic details are known for bis(imi-
no)pyridine iron catalyzed polymerizations. The mode of
propagation and the oxidation state of the active species remain
matters of controversy. Initial theoretical studies assumed
formation of cationic monoalkyl iron(II) compounds upon
activation with methylalumoxane (MAO),11,12 which have been
experimentally supported by both NMR spectroscopy13 and ESI
mass spectrometry.14 Alternatively, Mo¨ssbauer spectroscopic
and EPR studies suggested that the iron(II) precatalysts are
oxidized to iron(III) upon treatment with MAO.15 Studies into
the MAO-activation of bis(imino)pyridine iron dihalides by
optical spectroscopy revealed gross spectral changes and a
decrease of the d-d transitions as a function of time, temperature,
and activator concentration and were interpreted as an iron
centered spin transition.16 Both the paramagnetism of the iron
center and well-established redox and chemical participation
of the bis(imino)pyridine ligand complicate characterization of
the active species.17,18
In principle, these ambiguities could be resolved by prepara-
tion of well-defined, single component bis(imino)pyridine iron
catalysts. Bis(imino)pyridine iron alkyl cations are worthy
targets as these compounds may allow study of fundamental
transformations related to chain initiation, growth and termina-
tion. For many years, however, the requisite bis(imino)pyridine
iron dialkyl species remained elusive.19 In 2005, our laboratory
reported that treatment of (iPrPDI)FeCl2 (1-Cl2;iPrPDI ) 2,6-
(2,6-iPr2-C6H3N ) CMe)2C5H3N) with 2 equiv of LiCH2SiMe3
followed by recrystallization from cold pentane furnished the
desired iron dialkyl complex, (iPrPDI)Fe(CH2SiMe3)2 (1-Ns2).20
Shortly thereafter, Gambarrotta and co-workers described a more
detailed investigation into this reaction and provided evidence
for chemical participation of the bis(imino)pyridine chelate.21
Ca´mpora and co-workers later described a more versatile
synthetic method whereby addition of free bis(imino)pyridine
to (pyridine)2Fe(CH2SiMe3)2 furnished the corresponding iron
dialkyls in high yield.22 Our laboratory has since expanded this
procedure to explore the relative stability of (iPrPDI)Fe-
CH2SiMe3 and (iPrPDI)Fe-CH2CMe3 complexes.23
With synthetic routes to 1-Ns2 in hand, the bis(imino)pyridine
iron alkyl cations, [(iPrPDI)Fe-R]+X- (R ) CH2SiMe3, X-
)
BPh4-; CH2SiMe2CH2SiMe3, X- ) MeB(C6F5)3-), were syn-
thesized and shown to be active for ethylene polymerization.24
Although these results demonstrate the catalytic competency
of a formally iron(II) alkyl cation as the propagating species,
they by no means demonstrate that such a compound is formed
from MAO-activation of a bis(imino)pyridine iron dihalide. In
fact, studies by Budzelaar,25 Gambarotta,21,26,27 and Kissin28
with bis(imino)pyridine ferrous dihalide-aluminum alkyl mix-
tures establish that ligand alkylation and transmetalation to
aluminum are likely during polymerization and that certain iron
ethylene polymerization catalysts may be multicentered.
Despite these complexities, preparation of well-defined bi-
s(imino)pyridine iron alkyl compounds may provide insight into
fundamental transformations related to catalytic olefin oligo-
merization and polymerization and allow a deeper understanding
of empirically established structure-reactivity relationships. In
addition, these compounds also allow evaluation of the chemical
and electronic participation of the bis(imino)pyridine chelate
and its role during catalysis. To date, the only bis(imino)pyridine
iron alkyls that have been synthesized are those protected from
ꢀ-hydrogen elimination, thereby limiting the study of this
potentially important chain transfer reaction. Examples of known
iron alkyls with iPrPDI as the bis(imino)pyridine chelate are
presented in Figure 1.
Although these compounds have proven useful to determine
the stability of the iron-carbon bond and as synthons to certain
iron alkyl cations, the lack of ꢀ-hydrogens limits relevance to
the propagating species during olefin polymerization. In addition,
preliminary studies with [(iPrPDI)Fe-CH2SiMe3]+ complexes
indicate slow initiation relative to propagation.24 Seeking to
expand the number of well-defined, single component iron
precatalysts and better mimic the potential propagating species,
we sought to prepare bis(imino)pyridine iron alkyl complexes
(11) Griffiths, E. A. H.; Britovsek, G. J. P.; Gibson, V. C.; Gould, I. R.
Chem. Commun. 1999, 1333.
(12) Khoroshun, D. V.; Musaev, D. G.; Vreven, T.; Morokuma, K.
Organometallics 2001, 20, 2007.
(13) Bryliakov, K. P.; Semikolenova, N. V.; Zudin, V. N.; Zakharov, V. A.;
Talsi, E. P. Catal. Commun. 2004, 5, 45.
(14) Castro, P. M.; Lahtinen, P.; Axenov, K.; Viidanoja, J.; Kotiaho, T.;
Leskelae, M.; Repo, T. Organometallics 2005, 24, 3664.
(15) 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.
´
(22) Ca´mpora, J.; Naz, A. M.; Palma, P.; Alvarez, E.; Reyes, M. L.
Organometallics 2005, 24, 4878.
(16) Tellmann, K. P.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C.
Organometallics 2004, 23, 5503.
(23) Ferna´ndez, I.; Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Organo-
metallics 2008, 27, 109.
(17) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton Trans.
2006, 46, 5442.
(24) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2005, 127, 9660.
(18) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936.
(25) Knijnenburg, Q.; Smits, J. M. M.; Budzelaar, P. H. M. Organometallics
2006, 25, 1036.
(19) Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.; Tellmann, K. P.;
White, A. J. P.; Williams, D. J. Dalton Trans. 2002, 6, 1159.
(20) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406.
(21) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am.
Chem. Soc. 2005, 127, 13019.
(26) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. Orga-
nometallics 2005, 24, 6298.
(27) Scott, J.; Gambarotta, S.; Korobkov, I.; Knignenburg, Q.; de Bruin,
B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204.
(28) Kissin, Y. V.; Qian, C.; Xie, G.; Chen, Y. J. Poly. Sci. A. 2006, 44,
6159.
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Bis(imino)pyridine Iron Alkyls Containing ꢀ-Hydrogens
A R T I C L E S
bearing ꢀ-hydrogens. Here we describe the results of these
efforts and report the synthesis, characterization, and thermal
stability of a family of bis(imino)pyridine iron alkyl complexes.
Studies into the oxidative addition of alkyl bromides establish
ligand rather than iron centered electron loss. The mechanism
of iron alkyl complex decomposition has also been studied and
found to undergo transfer dehydrogenation of the bis(imino)py-
ridine chelate.
troscopic, structural and computational studies have established
this formally iron(0) compound is better described as an
intermediate spin ferrous complex with a bis(imino)pyridine
diradical dianion.30 Thus, oxidative transformations are of
interest, as formal electron loss could be either metal or ligand
based.31
Our studies into the oxidative addition of alkyl halides to
1-(N2)2 began with ethyl bromide. Preparation of a neutral
bis(imino)pyridine iron ethyl complex was of interest given the
utility of this class of compounds in ethylene polymerization.
Treatment of a benzene-d6 solution of 1-(N2)2 with 0.5
equivalents of CH3CH2Br resulted in immediate formation of
two new paramagnetic products. The first was readily identified
as the previously reported bis(imino)pyridine iron bromide,
1-Br,20 while the second compound was assigned as the desired
iron ethyl species, 1-Et (eq 2). The latter compound was
characterized using a combination of 1H NMR spectroscopy as
well as degradation studies. Full details of the spectral assign-
ment and the relative stability of each iron alkyl prepared in
this study are reported in a later section.
Results and Discussion
Synthesis of Neutral Bis(imino)pyridine Iron Alkyls Bear-
ing ꢀ-Hydrogens: Oxidative Addition of Alkyl Halides to 1-(N2)2.
The bis(imino)pyridine iron isobutyl complex, (iPrPDI)-
FeCH2CHMe2 (1-iBu), was the initial synthetic target of an iron
alkyl complex bearing ꢀ-hydrogens. Because the synthesis of
ꢀ-H stabilized bis(imino)pyridine iron alkyls (Figure 1) was
accomplished by straightforward salt metathesis of 1-Br with
the appropriate alkyl lithium reagent (e.g., LiCH3, LiCH2SiMe3,
LiCH2CMe3), this route was initially explored. Stirring a diethyl
i
ether solution of 1-Br with one equivalent of BuLi under a
dinitrogen atmosphere for less than 30 min furnished only a
small amount (∼10%) of a paramagnetic compound identified
as 1-iBu (Vide infra). Examination of the diamagnetic region
1
of the H NMR spectrum revealed the major iron product as
the bis(dinitrogen) complex, 1-(N2)2, formed from reduction of
1-Br (eq 1).29 Other minor, unidentified paramagnetic iron
products were also observed using this procedure.
The spectroscopic identification of 1-Et following addition
of ethyl bromide to 1-(N2)2 suggested that alkyl halide
addition was an effective method for the synthesis of
bis(imino)pyridine iron alkyl complexes bearing ꢀ-hydrogens.
Addition of one equivalent of either n-butyl or isobutyl
bromide to a benzene-d6 solution of two equivalents of
1-(N2)2 yielded 1-Br in both cases and the bis(imino)pyridine
iron n-butyl and isobutyl compounds, 1-nBu and 1-iBu,
respectively (eq 3).
Alternative salt metathetical methods using less reducing
Grignard reagents were also explored as potential synthetic
routes to 1-iBu. Addition of iBuMgCl to a diethyl ether solution
of 1-Cl furnished the desired iron monoalkyl in greater yield
and purity than the corresponding lithiation. Only small (∼ 10%)
quantities of 1-(N2)2 were obtained using this procedure.
However, the kinetic instability of the compound (Vide infra)
complicated isolation of 1-iBu on a preparative scale.
Because other bis(imino)pyridine iron alkyls may be shorter
lived than 1-iBu, a more reliable synthetic method that obviated
the complications of work up, titration of Grignard reagents and
possibility for competing reduction reactions was targeted.
Oxidative addition of alkyl halides to 1-(N2)2 was explored as
a possible synthetic route to bis(imino)pyridine iron alkyls
bearing ꢀ-hydrogens. This approach is also of interest because
of the unusual electronic structure of 1-(N2)2. Previous spec-
Vacuum transfer of the volatiles immediately following each
of the aforementioned alkyl bromide addition reactions and
(30) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901.
(31) Bart, S. C.; Lobkovsky, E.; Bill, E.; Chirik, P. J. J. Am. Chem. Soc.
2006, 128, 5302.
(29) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
(32) Lal, D.; Griller, D.; Husband, S.; Ingold, K. V. J. Am. Chem. Soc.
1974, 96, 6355.
9
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A R T I C L E S
Trovitch et al.
1
analysis by H NMR spectroscopy demonstrated formation of
small amounts (25% for n-BuBr, 12% for i-BuBr) of alkane
along with trace amounts of alkene immediately following
mixing of the reagents. Addition of more than 0.5 equivalents
of alkyl bromide to 1-(N2)2 furnished a detectable quantity of
1-Br2 along with other unidentified products. Again, analysis
of the volatile components by 1H NMR spectroscopy following
vacuum transfer established formation of the corresponding
alkane. Because these results suggested the intermediacy of
radicals during the formal oxidative addition event, the addition
of 6-bromo-1-hexene to 1-(N2)2 was studied as the free
5-hexenyl radical is known to cyclize at a rate of approximately
105 sec-1 32
This probe has been used previously by Schwartz
.
Figure 2. Solid state magnetic susceptibility data for 1-Br.
to assay radical chain involvement in the addition of R-X to
[(η5-C5H5)2Zr].33Treatment of a benzene-d6 solution of 1-(N2)2
with 0.5 equivalents of 6-bromo-1-hexene yielded 1-Br along
with the bis(imino)pyridine iron methylcyclopentyl complex,
degradation studies. Because each iron alkyl is paramagnetic,
the 1H NMR spectral assignments are not immediately
obvious. To circumvent this complication, the 1H NMR
spectrum of 1-Br was examined in detail, as this compound
is a product common to all of the oxidative addition reactions.
In addition, 1-Br is indefinitely stable if handled under an
inert atmosphere, and serves as a model of comparison for
the transient iron alkyl complexes. The electronic structure
of 1-Br and isoelectronic 1-R derivatives are also of interest
to determine whether ligand or metal oxidation occurs upon
addition of alkyl halides.
c
1-CH2 Pnt, as the sole iron alkyl product (eq 4). Analysis of
the volatile components immediately following addition yielded
27% organic compounds (86% methylene cyclopentane, 14%
methyl cyclopentane) likely arising from ejection of alkyl
c
radical. It is also possible that 1-CH2 Pnt arises from alkylation
of the metal followed by olefin insertion.
Our laboratory has previously investigated the electronic
structure of the related bis(imino)pyridine iron monochloride,
1-Cl, by Mo¨ssbauer spectroscopy, structural studies, magnetic
measurements, and DFT calculations.30 The experimental and
computational data were consistent with a high spin ferrous
center, antiferromagnetically coupled to a monoreduced bis-
(imino)pyridine anion. Related experiments with 1-Br support
a similar description. For example, variable temperature
SQUID magnetization data establish the S ) 3/2 ground-state
and simple paramagnetic behavior for the compound. A plot
of effective magnetic moment versus temperature (Figure 2)
exhibits µeff values ranging from 3.9 to 4.5 µB (5-300 K)
and is consistent with three unpaired electrons and a modest
orbital contribution arising from the square planar iron center.
Separate runs were conducted on three independently pre-
pared samples to establish reproducibility (see Figure S1,
Supporting Information).
The metrical parameters determined from high quality
X-ray crystal structures have proven useful for characterizing
the degree of ligand reduction and involvement in the
electronic structure.17,18,30 Because repeated attempts to
obtain X-ray quality crystals of four-coordinate 1-Cl and 1-Br
have been unsuccessful, the solid state structure of
1-Br(THF) was determined. X-ray quality crystals were
obtained from a concentrated pentane solution of the iron
compound in the presence of a small amount of THF. A
representation of the molecular structure is presented in
Figure 3 and selected metrical parameters are reported in
Table 1. The overall molecular geometry is best described
as idealized square pyramidal with the THF ligand occupying
the apical position. The THF ligand and one chelate isopropyl
group are positionally disordered and were successfully
modeled. Because of the disorder, the Fe-O bond distance
should be treated cautiously. The imine CdN bond distances
of 1.299(4) and 1.306(4) Å and the Cimine-Cipso distances of
1.446(5) and 1.433(5) Å are consistent with one electron
chelate reduction.17,18,30 To confirm that coordination of the
THF molecule did not alter the magnetic properties of the
The oxidative addition of secondary and tertiary alkyl
bromides to 1-(N2)2 was also explored. Addition of one
equivalent of 2-bromobutane to a benzene-d6 solution of two
equivalents of 1-(N2)2 furnished 1-Br along with small quantities
(∼5%) of the bis(imino)pyridine iron n-butyl complex, 1-nBu.
Analysis of the volatile products of the reaction mixture by 1H
NMR spectroscopy established formation of cis- (4%) and trans-
2-butene (12%) as the sole alkene products along with butane
(84%). The results implicate fast and irreversible ꢀ-hydrogen
elimination from secondary iron alkyls along with radical
formation (Vide infra). In an analogous experiment, addition of
tert-butyl bromide to a benzene-d6 solution of 1-(N2)2 yielded
1-Br along with 1-iBu. Collection and examination of the
volatiles from this reaction (before significant decomposition
of the iron alkyl, Vide infra) by 1H NMR spectroscopy revealed
that a near equimolar ratio of isobutene and isobutane ac-
companies oxidative addition, consistent with involvement of
tert-butyl radical.
Spectroscopic Characterization of Bis(imino)pyridine Iron
Alkyl Complexes. As will be discussed in the next section,
each of the bis(imino)pyridine iron alkyl complexes bearing
ꢀ-hydrogens is kinetically unstable and must be handled
carefullysand sometimes quicklysin solution under an inert
atmosphere. As a result, characterization of these compounds
1
2
has relied primarily on H and H NMR spectroscopy and
(33) Williams, G. M.; Schwartz, J. J. Am. Chem. Soc. 1982, 104, 1122.
(34) For related ligand-centered oxidation chemistry see: Ketterer, N. A.;
Fan, H. J.; Blackmore, K. J.; Yang, X. F.; Ziller, J. W.; Baik, M. H.;
Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 4364.
(35) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
Soc. 2007, 129, 7212.
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Bis(imino)pyridine Iron Alkyls Containing ꢀ-Hydrogens
A R T I C L E S
Figure 4. 1H NMR spectrum of 1-Br in benzene-d6 at 23 °C.
Figure 3. Solid state structure of 1-Br(THF) at 30% probability ellipsoids.
Hydrogen atoms and disordered positions omitted for clarity.
Table 2. Selected 1H NMR Resonances for 1-Br as a Function of
Added THFa
Table 1. Selected Metrical Parameters for 1-Br(THF)
distance (Å)
angle (deg)
equiv of THF
p-pyr
m-pyr
CH(CH3)2
NdC(CH3)
Fe(1)-Br(1)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-O(1)
N(1)-C(2)
N(3)-C(8)
C(2)-C(3)
C(7)-C(8)
N(2)-C(3)
N(2)-C(7)
2.4165(6)
2.172(3)
2.008(3)
2.150(3)
2.166(5)
1.299(4)
1.306(4)
1.446(5)
1.433(5)
1.361(5)
1.358(4)
N(1)-Fe(1)-N(3)
N(1)-Fe(1)-N(2)
N(2)-Fe(1)-N(3)
N(1)-Fe(1)-O(1)
N(2)-Fe(1)-O(1)
N(3)-Fe(1)-O(1)
N(1)-Fe(1)-Br(1)
N(2)-Fe(1)-Br(1)
N(3)-Fe(1)-Br(1)
146.93(11)
74.59(11)
74.93(11)
96.33(11)
92.64(15)
97.58(16)
103.69(7)
167.89(9)
103.39(8)
0
10
388.94
351.87
69.22
69.27
-108.71
-54.96
-212.74
-223.06
a All values reported in ppm in benzene-d6 at 23 °C.
Because 1-Br(THF), not 1-Br, was crystallographically
characterized, the influence of the added base on the 1H NMR
spectral features of the compound was examined. Addition
of 10 equiv of THF to a 0.035 M benzene-d6 solution of
1-Br resulted in dramatic changes to the isotropic shifts of
certain bis(imino)pyridine protons (Table 2). Particularly
notable is the substantial upfield shifts, by 37 and 10.3 ppm
of the para-pyridine and imine methyl hydrogens, respec-
tively. The isopropyl methine resonance shifts downfield by
over 50 ppm whereas the meta-pyridine hydrogen is largely
unaffected. Removal of the THF resulted in regeneration the
original shifts assigned to four coordinate 1-Br.
The electronic structure of 1-Br, in combination with the
previously reported magnetic and structural data for the
isolable iron alkyls 1-Me20 and 1-Ns,23 provides additional
insight on the oxidative addition of alkyl bromides to 1-(N2)2.
The iron alkyls have an identical electronic structure to 1-
Br - overall S ) 3/2 ground states arising from high spin
(SFe ) 2) ferrous centers antiferromagnetically coupled to
bis(imino)pyridine radical anions. Thus, oxidative addition
of alkyl bromides to 1-(N2)2 is formally ligand, rather than
metal, based. The two electron reduced bis(imino)pyridine
chelate in 1-(N2)2 is oxidized by one electron to produce two
new iron products where the iron(II) oxidation state is
preserved (Scheme 1).34 Notably, a spin change from
intermediate to high spin occurs at the iron center upon
chelate oxidation. These findings are as expected from the
principle of microscopic reversibility - the reduction of 1-Br
to yield 1-(N2)2 and related neutral ligand complexes occurs
at the chelate with a spin change at iron(II).30
five-coordinate versus the four-coordinate compound, the
solid state magnetic susceptibility of 1-Br(THF) was deter-
mined. A magnetic moment of 4.5 µB was measured at 23
°C by Gouy balance, in excellent agreement with the SQUID
data at the same temperature for 1-Br. Thus, the metrical
parameters, in combination with the solid state magnetic
susceptibility data, support a monoreduced bis(imino)pyridine
1
chelate (SPDI ) /2) antiferromagnetically coupled to a high
spin iron center (SFe ) 2).30
With a firm description of the electronic structure of 1-Br in
1
hand, the H NMR spectrum of the compound was examined
in more detail. Despite the paramagnetism, the 1H NMR
spectrum is readily assigned and exhibits diagnostic resonances
that can be used to identify this and the related bis(imino)py-
ridine iron alkyl compounds. A representative benzene-d6
solution 1H NMR spectrum, recorded at 23 °C, is presented in
Figure 4. Selected ligand resonances have been tabulated in
Table 2. As expected, the hydrogens in the bis(imino)pyridine
chelate plane exhibit the largest isotropic shifts, while those
orthogonal to the plane of the ligand are closer to their
diamagnetic reference values. The temperature dependence of
the isotropic shifts was examined and found to vary linearly
with the reciprocal of absolute temperature. Representative plots
establishing Curie behavior for selected resonances are presented
in Figure S2 of the Supporting Information. As expected, the
resonances in the plane of the chelatessuch as the imine methyl
and the meta pyridine hydrogenssexhibit the steepest slopes
while those orthogonal to the chelate exhibit a relatively shallow
temperature dependence.
The complete assignment of the 1H NMR spectrum of 1-Br
has proven valuable for the spectroscopic characterization
of the kinetically unstable bis(imino)pyridine iron alkyl
complexes bearing ꢀ-hydrogens. Selected assignments for
each iron alkyl complex prepared in this work along with
the previously reported iron methyl20 and neosilyl23 com-
plexes, are given in Table 3. A representative benzene-d6
9
J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008 11635

A R T I C L E S
Trovitch et al.
Scheme 1
Table 3. Selected 1H NMR Resonances for Bis(imino)pyridine Compounds Reported in This Worka
complex
p-pyr
m-pyr
CH(CH3)2
NdC(CH3)
alkyl
1-Br
388.94
392.79
216.5
369.28
263.35
338.91
350.03
284.47
69.22
94.92
58.9
66.93
63.75
67.37
66.30
64.78
-108.71
-143.50
-72.6
-109.04
-89.75
-120.43
-123.36
-99.21
-212.74
-242.75
-163.8
-206.76
-172.74
-182.34
-185.63
-175.37
N/A
N/A
1-OTf
1-Me20
1-Ns23
1-Et
Not Located
48.80 (SiMe3)
149.69 (CH3)
197.22 (CH), 85.67, 52.53, 46.28, 34.62
200.42 (CH), 73.34 (iPr CH3)
142.49, 60.05, 36.92
1-CH2cPnt
1-iBu
1-nBu
a All values reported in ppm in benzene-d6 at 23 °C.
One of the iron products was identified as the previously
reported bis(imino)pyridine dinitrogen complex, 1-(N2) (or
1-(N2)2, depending on the amount of N2 present).29 The second
iron compound was detected after hydrolysis of the organome-
Figure 5. 1H NMR spectrum of 1-Et (labeled resonances) along with 1-Br
(unlabeled) in benzene-d6 at 23 °C.
1
1H NMR spectrum of 1-Et recorded at 23 °C is presented in
Figure 5. As the spectroscopic properties of each alkyl are
similar, only 1-Et will be discussed in detail. Complete
assignments and other representative spectra are reported in
the Supporting Information. For 1-Et, the imine methyl group
appears the most upfield at -172.74 ppm while the p-pyridine
hydrogen is the most downfield at 263.35 ppm. Other
diagnostic resonances with large isotropic shifts are the
m-pyridine hydrogens located at 63.75 ppm and the isopropyl
methine centered at -89.75 ppm. Unfortunately, the hydro-
gens on the carbon attached directly to the iron have not
tallic products and analysis of the free chelate by H NMR
spectroscopy as well as by D2 addition experiments.35 In addition
to signature resonances for free iPrPDI, multiplets were observed
in the alkene region, indicative of dehydrogenated bis(imino)py-
ridine ligand. These data, in addition to hydrogenation studies,
establish the second iron product as the NMR-silent intramo-
lecular olefin complex, 2. This molecule was previously
observed and characterized following the decomposition of a
bis(imino)pyridine iron diazoalkane complex.35 Thus, the
decomposition of 1-Et can be viewed as a transfer dehydroge-
nation from the ligand to the iron alkyl, liberating ethane and
forming an equimolar mixture of 1-N2 and 2. In the present
case, the liberated ethane was quantified with a Toepler pump
and 92% of the expected gas was collected.
1
been located by H NMR spectroscopy for any iron alkyl
compound prepared in this study.
Kinetic Stability of Neutral Bis(imino)pyridine Iron Alkyls.
With a family of neutral bis(imino)pyridine iron alkyls with
ꢀ-hydrogens in hand, the relative stability of each compound
was assayed in benzene-d6 solution at 23 °C. After 18 h, 1-Et
undergoes exclusive loss of ethane and formation of two new
iron products (eq 5).
The thermal stability and half-lives of the other bis(imino)py-
ridine iron alkyls prepared in this study were also assayed at
23 °C in benzene-d6. The organic products released following
disappearance of the iron alkyl vary and are reported in Scheme
2. In addition, the half-life for each reaction is also reported in
the scheme. In each case, a mixture of iron products, 1-(N2)n
9
11636 J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008

Bis(imino)pyridine Iron Alkyls Containing ꢀ-Hydrogens
A R T I C L E S
than iron catalysts,1,3,38 although in single component iron cases,
ꢀ-hydrogen elimination was indeed dominant.
Scheme 2
A series of deuterium labeling experiments were conducted
to verify the origin of the hydrogen atoms in the alkane products.
Treatment of a benzene solution of 1-(N2)2 with 0.5 equivalents
of CD3CD2Br yielded a mixture of 1*-Et and 1*-Br as judged
by 2H NMR spectroscopy (eq 6). The “*” designates deuterium
incorporation into an isopropyl methyl group. Analysis of the
mixture by 2H NMR spectroscopy within 15 min of alkyl halide
addition revealed peaks centered at 36.4 and 149.7 ppm,
corresponding to the Fe-CD2CD3 and Fe-CD2CD3 deuterons,
respectively. The resonance for the methylene positions was
only observed by 2H NMR spectroscopy; no corresponding peak
has been located in the 1H NMR spectrum of natural abundance
1-Et. Observation of 2H but not 1H NMR peaks is due to better
resolution of deuterium spectra for paramagnetic compounds;
a phenomenon that is a result of the difference in magnetogyric
ratios of the two nuclei.39 Interestingly, the 2H NMR spectrum
of the product mixture also contained four additional peaks
corresponding to deuterium labeling of the isopropyl methyl
groups of both 1-Br and 1-Et.
and 2, was observed and is denoted as “[Fe]” for simplicity.
The iron n-butyl compound, 1-nBu, has a significantly longer
half-life than 1-Et but produces predominantly butane with small
1
amounts of 2-butene. No 1-butene was detected by H NMR
spectroscopy from these experiments.
Bis(imino)pyridine iron alkyls bearing only one ꢀ-hydrogen,
c
1-iBu and 1-CH2 Pnt, were longer lived than either 1-Et or
1-nBu and produced significantly higher fractions of olefins upon
disappearance of the iron alkyl, demonstrating a reduced
propensity for transfer hydrogenation with more sterically
demanding alkyls. The relatively long lifetime of 1-iBu versus
1-nBu and 1-Et, established that stability of the iron alkyls
decreased as the number of ꢀ-hydrogens increase.
The loss of alkane from the compounds presented in Scheme
2 prompted reinvestigation of the thermal stability of 1-Me,
1-Ns, and 1-Np. It is possible that iron-carbon bond homoly-
sis,36 not ꢀ-hydrogen elimination, is responsible for the kinetic
instability of the iron-alkyls prepared in this work. Allowing
benzene-d6 solutions of 1-Me, 1-Ns, or 1-Np to stand at 23 °C
produced no detectable change by 1H NMR spectroscopy.
Because the differences in iron-carbon bond strengths for 1-Et,
1-nBu, 1-iBu, and 1-Np are expected to be similar,37 the relative
stability of the compounds is a result of the presence or absence
of ꢀ-hydrogens. In fact, if iron bond homolysis were indeed
operative, the more hindered 1-Np may be expected to be the
least, not most, kinetically persistent.
2
Monitoring the stability of 1*-Et by H NMR spectroscopy
at 23 °C in benzene-d6 revealed smooth disappearance of the
resonances for the ethyl chain over the course of 3.5 h.
Throughout this experiment, the peaks for the isopropyl methyl
positions of 1*-Et remain, demonstrating deuterium depletion
from the ethyl chain before alkane loss (eq 7).
Deuterium Labeling Studies and Proposed Mechanism for
Alkane Loss. The loss of alkane and formation of 2 from the
bis(imino)pyridine iron alkyl complexes prompted more detailed
studies into the mechanism of thermal decomposition for these
compounds. Understanding these pathways may provide insight
into chain transfer processes related to bis(imino)pyridine iron-
catalyzed olefin oligomerizations and polymerizations. Previous
studies with MAO-activated metal dihalides indicate that chain
transfer by ꢀ-hydrogen elimination is more important for cobalt
2
After 18 h at 23 °C, no H NMR signals assigned to 1*-Et
remained, consistent with the lifetime measured previously for
natural abundance 1-Et determined by 1H NMR spectroscopy.
Because of the complexities associated with isotopic scrambling
(38) 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.; Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem.
Soc. 1999, 121, 8728.
(36) Lau, W.; Huffman, J. C.; Kochi, J. K. Organometallics 1982, 1, 155.
(37) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.;
Lachicotte, R. J.; Flaschenreim, C. J.; Holland, P. L. Organometallics
2004, 23, 5226.
(39) Holm, R. H.; Hawkins, C. J. In NMR of Paramagnetic Molecules:
Principles and Applications; La Mar, G. N., Horrocks, W. D., Holm,
R. H., Eds.; Academic Press: New York, 1973; p 287.
9
J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008 11637

A R T I C L E S
Trovitch et al.
Scheme 3
into the ligand, kinetic isotope effects for the loss of ethane
from isotopologues of 1-Et were not determined.
addition of either CD3CD2Br or CH3CD2Br to 1-(N2)2 is likely
due to rapid cyclometalation and isotopic exchange from the iron
alkyl followed by ligand exchange to form 1*-Br. The ligand
undergoing exchange could either be the iron-bromide or the
bis(imino)pyridine chelate. Although our experiments do not
distinguish these two possibilities (or a combination of the two),
we tentatively favor the bromide exchange process. Recall that
1-(N2)n (n ) 1, 2) is formed from the thermal decomposition of
1-Et and thus ligand exchange can occur throughout the process
of ethylene dissociation and ethane formation.
Evidence for isotopic exchange within the ethyl chain prior to
ethane loss was also obtained from 1H NMR spectroscopy. A 1H
NMR resonance was observed at 149.7 ppm approximately 20 min
following addition of CD3CD2Br to 1-(N2)2. It is likely that
hydrogens are also incorporated into the iron methylene position,
1
however, the inability to observe a signal for this position by H
NMR spectroscopy prohibits obtaining definitive experimental
evidence. These experiments definitively establish “chain
walking”sreversible ꢀ-hydrogen elimination followed by olefin
rotation (or loss) and reinsertionsprior to cyclometalation and
reductive elimination of ethane.
Deuterium labeling experiments were also carried out with 1-nBu
to assay the possibility of alkyl chain walking and confirm
bis(imino)pyridine participation. The deuterium labeled iron dini-
trogen complex, 1*-(N2)2, was prepared by addition of excess D2
gas to 1-(N2)2.29 Addition of 0.5 equivalents of CH3CH2CH2CH2Br
to a benzene solution of 1*-(N2)2 resulted in rapid formation of
1*-Br along with 1*-nBu. Monitoring the disappearance of 1*-
nBu by 2H NMR spectroscopy established deuterium incorporation
into all positions of the butane and 2-butenes (eq 8). Thus, alkyl
isomerization (e.g., chain walking) is operative concomitant with
alkane and alkene loss and chelate cyclometalation.
A benzene solution of 1-(N2)2 was also treated with 0.5
2
equivalents of CH3CD2Br. After 30 min at 23 °C, the H NMR
spectrum of the product mixture established formation of 1*-Br
along with 1*-Et-dn. At this time interval, the alkyl region of the
2H NMR spectrum contained a prominent peak at 38 ppm for the
Fe-CD2CH3 position and a very small peak, barely above the
detection limit of the experiment (e5%), at 145 ppm, suggesting
that only a minimal amount of deuterium was present in the
2
ꢀ-methyl group. Continued monitoring of the compound by H
NMR spectroscopy over the course of 18 h revealed disappearance
of the ethyl complex with continued observation of the peaks for
1*-Br.
The incorporation of deuterium into the isopropyl methyl groups
of both the iron alkyl and the iron bromide compounds clearly
establishes chemical participation of the bis(imino)pyridine chelate.
Additional experiments were carried out to determine the origin
of the deuterium in 1*-Br. Because it is known that the isopropyl
methyl groups of 1-(N2)2 are rapidly deuterated upon exposure to
four atm of D2 gas, an analogous experiment was conducted with
isolated 1-Br. No deuterium incorporation was observed over the
course of 24 h at 23 °C, demonstrating that isopropyl methyl group
cyclometalation is not operative in this compound.
One possibility for the formation of 1*-Br involves ligand
exchange from 1*-R. Because 1*-Br was observed immediately
following addition of bromoethane to 1-(N2)2, the product iron alkyl
must undergo rapid ꢀ-hydrogen (deuterium) elimination and
cyclometalation to form 1*-(N2), which then would participate in
ligand exchange. Crossover experiments were conducted to evaluate
this possibility (Scheme 3). The imine methyl groups of the
bis(imino)pyridine ligand were deuterium labeled to track the
chelate compounds by 1H and 2H NMR spectroscopy. The
compounds with deuterium labeled imine methyl groups are
designated by “**”. Thus, 1**-Br was prepared in a straightforward
manner from NaBEt3H reduction of 1**-Br2.20
The oxidative addition of CD3OTf to 1-(N2)2 was also studied
as a control experiment to verify that the ꢀ-carbon position was
the source of the deuterium during competing cyclometalation.
Treatment of 1-(N2)2 with 0.5 equivalents of CD3OTf cleanly
furnished the deuterated isotopologue of the previously reported
iron methyl complex, 1-CD3,20 along with the bis(imino)pyridine
iron triflate, 1-OTf. The latter complex was independently
prepared by addition of Me3SiOTf to 1-(N2)2 and has been fully
characterized and has diagnostic 1H NMR shifts, similar to 1-Br
2
(Table 3). Importantly, analysis of the product mixture by H
NMR spectroscopy provided no evidence for deuterium incor-
poration into the isopropyl methyl groups of either 1-CD3 or
1-OTf, confirming the ꢀ-position as the source of isotopic
exchange.
In addition to deuterium labeling experiments, olefins were
added to 1-(N2)2 to further assay cyclometalation and the transfer
hydrogenation step. Addition of one equivalent of 1-hexene to
a benzene-d6 solution of 1-(N2)2 at 23 °C resulted in clean and
quantitative formation of 2 and hexane over the course of five
days (eq 9). The iron-olefin compound, 2, and ethane were also
Preparation of a benzene solution containing an equimolar
mixture of 1**-Br and 1-(N2)2 at 23 °C resulted in immediate
formation of 1**-(N2)2 as judged by 2H NMR spectroscopy
(Scheme 3). Performing a similar experiment with 1**-Br and
1-Np produced no evidence for formation of 1**-Np, suggesting
that ligand exchange does not occur between iron halide and
persistent iron alkyl complexes. Thus, observation of 1*-Br from
9
11638 J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008

Bis(imino)pyridine Iron Alkyls Containing ꢀ-Hydrogens
A R T I C L E S
Scheme 4
Scheme 5
obtained following treatment of 1-(N2)2 with excess ethylene.
However, an unidentified red precipitate was also formed.
Despite this complication, these experiments clearly establish
the role of the chelate isopropyl methyl groups in transfer
hydrogenation of the iron alkyl.
olefin for iron alkyl exchange studies were also conducted.
Addition of one equivalent of isobutene to a benzene-d6 solution
c
of 1-CH2 Pnt resulted in a near equimolar mixture of 1-iBu
c
and 1-CH2 Pnt along with the corresponding free olefins
(Scheme 4). Similar exchange reactions were observed in related
bis(imino)pyridine cobalt complexes where treatment of
n
n
(PDI)CoR (R ) Pr, Bu) with ethylene yielded the corre-
sponding cobalt ethyl and free olefins.40
Based on all of the experimental data, a mechanism for alkane
loss from bis(imino)pyridine iron alkyl complexes is presented
in Scheme 5, using 1-Et as a representative example. ꢀ-hydrogen
elimination from the iron alkyl forms the bis(imino)pyridine
iron olefin hydride compound. Olefin dissociation and reinsertion
(or olefin rotation) is fast relative to subsequent steps, as isotopic
exchange between the ꢀ-CH3 and R-CH2 positions is faster than
liberation of alkane. If ethylene loss occurs, a putative bis(imi-
no)pyridine iron hydride is formed, which is known to undergo
rapid loss of H2 to form either the iron dihydrogen or N2
complex, depending on the relative concentration of the respec-
tive gases.29 Formal oxidative addition of an isopropyl methyl
C-H bond affords the bis(imino)pyridine cyclometalated hy-
dride. A similar species has been implicated in H/D exchange
To further substantiate the claim of facile ꢀ-hydrogen
elimination and olefin dissociation prior to alkane formation,
9
J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008 11639

A R T I C L E S
Trovitch et al.
compounds, stabilized by a redox active bis(imino)pyridine
chelate, promotes C-H bond oxidative addition leading to
cyclometalation and irreversible formation of alkane. These
studies demonstrate the viability of this processes as a chain
termination pathway in bis(imino)pyridine iron catalyzed olefin
polymerization reactions. Experiments designed to evaluate this
possibility are currently under investigation.
Experimental Section41
Characterization of (iPrPDI)Fe(CH2CH3) (1-Et). Using a
calibrated gas bulb, 0.021 mmol of either bromoethane or bromo-
ethane-d5 was transferred to a J. Young Tube containing 0.025 mg
(0.042 mmol) of 1-(N2)2 and approximately 0.7 mL of benzene-d6.
Upon sitting at room temperature for 20 min, the reaction mixture
was found to contain 1-Br and 1-Et (or 1-Et-d5) by 1H NMR
spectroscopy. 1H NMR (benzene-d6, 293 K): δ ) 263.35 (221 Hz,
1H, p-pyr), 149.69 (555 Hz, 3H, CH2CH3), 63.75 (103 Hz, 2H,
m-pyr), -4.50 (100 Hz, 4H, m-aryl), -15.14 (64 Hz, 12H,
CH(CH3)2), -27.88 (123 Hz, 12H, CH(CH3)2), -89.75 (308 Hz,
4H, CH(CH3)2), -172.74 (214 Hz, 6H, C(CH3)), Fe-CH2 and
p-aryl resonance not located. Degradation of 1-Et was monitored
over the course of 24 h at ambient temperature. Transfer of the
volatiles and analysis by 1H NMR spectroscopy confirmed a 15%
yield of solely ethane (based on bromoethane) as judged by
integration against 1 µL of cyclohexane. A second degradation
experiment yielded 92% of the ethane expected by Toepler pump
analysis.
Figure 6. Kinetic stability of bis(imino)pyridine versus ꢀ-diketiminate
ferrous alkyl complexes.
in the preparation of 1*-(N2)2 from D2 gas.29 Insertion of
ethylene into the iron hydride bond yields the cyclometalated
ethyl compound, which can undergo reversible or productive
ꢀ-hydrogen elimination to furnish ethane and 2.
It is interesting to compare the relative stability of the
bis(imino)pyridine iron alkyl compounds studied in this work
to the three coordinate ꢀ-diketiminate (BDI) iron alkyls reported
by Holland and co-workers (Figure 6).37 Because the 1-R series
of compounds are best described as containing high spin ferrous
ions antiferromagnetically coupled to bis(imino)pyridine chelate
radical anions, both classes of molecules contain a high spin
(SFe ) 2) iron coordinated to a monoanionic ligand. Furthermore,
both ligand environments possess orthogonal 2,6-diisopropyl
aryl substituents and to a first approximation (ignoring differ-
ences in chelate bite angles), have similar steric environments.
In a comprehensive study of iron(II) alkyl stability, Holland
and co-workers prepared a broad spectrum of (BDI)Fe-R
compounds bearing ꢀ-hydrogens, including secondary and
tertiary iron alkyls. Isomerization of the iron alkyl and olefin
for alkyl exchange reactions were observed, demonstrating that
reversible ꢀ-hydrogen elimination is operative. Irreversible
formation or decomposition of these compounds by alkane loss
was not reported. Why then are the bis(imino)pyridine iron
alkyls not kinetically persistent while the ꢀ-diketiminate com-
pounds are?
The difference in reactivity may be traced to the competing
cyclometalation in the bis(imino)pyridine compounds. The
instability of the putative product of ꢀ-hydrogen elimination,
1-H, to H2 loss and formation of formally low-valent [(iPrP-
DI)Fe] compounds that undergo C-H activation provide a
pathway to irreversible alkane formation and hence decomposi-
tion of the iron alkyl. No analogous compound or pathway has
been reported in the ꢀ-diketiminate chemistry and hence
kinetically persistent iron alkyls result. It should be noted that
in the bis(imino)pyridine examples, chain walking via ꢀ-hy-
drogen elimination, olefin dissociation (or rotation) followed
by reinsertion is operative followed by a slower cyclometalation-
insertion sequence.
Characterization of (iPrPDI)Fe(CH2CHMe2) (1-iBu). In a
manner similar to bromoethane, gas bulb addition of 0.017 mmol
of 1-bromo-2-methylpropane to 0.020 g of 1-(N2)2 yielded a mixture
of 1-Br and 1-iBu in benzene-d6, as judged by 1H NMR
spectroscopy. This complex was additionally observed by 1H NMR
spectroscopy, along with 1-Br, from the addition of 0.002 g (2
µL, 0.017 mmol) of 2-bromo-2-methylpropane to 0.020 g (0.034
mmol) of 1-(N2)2 in benzene-d6. A third method of preparation was
achieved through the addition of 0.008 g (32 µL of a 2.0 M solution
in ether, 0.065 mmol) of isobutylmagnesium chloride to a J. Young
1
tube containing 0.025 g (0.044 mmol) of 1-Cl in benzene-d6. H
NMR (benzene-d6, 293 K): δ ) 350.03 (583 Hz, 1H, p-pyr), 200.42
i
(7533 Hz, 1H, FeCH2CH(CH3)2), 73.34 (301 Hz, 6H, Pr CH3),
66.30 (85 Hz, 2H, m-pyr), -9.19 (42 Hz, 4H, m-aryl), -13.38 (38
Hz, 2H, p-aryl), -19.74 (54 Hz, 12H, CH(CH3)2), -31.10 (118
Hz, 12H, CH(CH3)2), -123.36 (336 Hz, 4H, CH(CH3)2), -185.63
1
(209 Hz, 6H, C(CH3)), two peaks not located. As judged by H
NMR spectroscopy, a 55% yield of the organic degradation products
(52% 2-methylpropene, 48% 2-methylpropane) was achieved upon
vacuum transfer of the volatiles after 1 week and integration against
1 µL of cyclohexane.
Acknowledgment. We thank the David and Lucile Packard
Foundation and the donors of the Petroleum Research Fund
administered by the American Chemical Society for financial
support and Dr. Suzanne Bart and Professor Karsten Meyer
(Erlangen) for obtaining the SQUID data for 1-Br.
Supporting Information Available: Complete experimental
procedures, selected NMR spectroscopic data for bis(imino)py-
ridine iron alkyls, magnetic data and crystallographic data for
1-Br(THF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Concluding Remarks. Neutral bis(imino)pyridine iron alkyl
complexes bearing ꢀ-hydrogens have been synthesized either
by salt metathesis reactions or by oxidative addition of an
appropriate alkyl bromide to 1-(N2)2. In the latter reactions,
studies into the electronic structures of the 1-R and 1-Br
products establish that formal electron loss occurs from the
bis(imino)pyridine ligand, not the metal. Evaluation of the
kinetic stability of the iron alkyl species revealed decomposition
by transfer hydrogenation of an isopropyl methyl substituent
on the chelate aryl rings. The ability to access reduced iron
JA803296F
(40) Gibson, V. C.; Tellmann, K. P.; Humphries, M. J.; Wass, D. F. Chem.
Commun. 2002, 2316.
(41) Representative procedures for one specific class of compounds are
reported. Full details on all of the remaining compounds, including
general considerations, are described in the Supporting Information.
9
11640 J. AM. CHEM. SOC. VOL. 130, NO. 35, 2008