Sequential reduction and alkyl exchange reactions of bis(imino)pyridine dialkyliron(II) with trimethylaluminum

Article abstract of DOI:10.1021/om500167r

The sequence of reactions ensuing when the 2,6-bis(imino)pyridine Fe(II) dialkyl complexes [Fe(CH₂SiMe₃)₂(BIP)] react with trimethylaluminum is described in detail. The first step is the reduction of the dialkyl to the cor

Full text of DOI:10.1021/om500167r

Article
pubs.acs.org/Organometallics
Sequential Reduction and Alkyl Exchange Reactions of
Bis(imino)pyridine Dialkyliron(II) with Trimethylaluminum
́
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M. Angeles Cartes, Antonio Rodrıguez-Delgado, Pilar Palma, Eleuterio Alvarez, and Juan Campora*
́
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Instituto de Investigaciones Quımicas, CSIC-Universidad de Sevilla, c/Americo Vespucio, 49, 41092 Sevilla, Spain
S
* Supporting Information
ABSTRACT: The sequence of reactions ensuing when the
2,6-bis(imino)pyridine Fe(II) dialkyl complexes [Fe-
(CH2SiMe3)2(BIP)] react with trimethylaluminum is de-
scribed in detail. The first step is the reduction of the dialkyl
to the corresponding monoalkyl complex [Fe(CH2SiMe3)-
(BIP)]. In the presence of an excess of trimethylaluminum,
these undergo exchange of the remaining CH2SiMe3 group for
methyl, and ultimately the methyliron fragment is displaced
from the complex, leading to previously known paramagnetic
aluminum compounds. The diethylaluminum complex [AlEt2-
(iPrBIP)] was obtained when triethylaluminum was used
instead of trimethylaluminum.
reaction seems at the first sight unusual, combined structural,
spectroscopic, and theoretical studies have shown that the
aforementioned reduced species [M(R)(BIP)]0/− are properly
described as M(II) complexes containing singly or doubly
reduced BIP radical anion ligands;5b,9,11 therefore, the redox
changes are ligand- rather than metal-centered processes. Since
the electronic states and steric environments of the metal
centers in isoelectronic cationic, neutral, and anionic monoalkyl
species are very similar, it is conceivable that differently charged
species could play some role in the polymerization process.
So far, no stable alkyliron or alkylcobalt complexes have been
isolated from the reactions of iron or cobalt precursors with
organoaluminum reagents. For example, Gambarotta studied
the reaction of the iron complexes [FeCl2(iPrBIP)] and
[Fe(CH2SiMe3)2(iPrBIP)] with trimethyl- or triethylaluminum
(henceforth TMA and TEA, respectively), but this led to the
isolation of [AlMe2(iPrBIP)] and other paramagnetic, NMR-
silent aluminum compounds that are catalytically inactive,
rather than to iron alkyl species.12 Consequently, studies on
activation processes have relied on in situ spectroscopic
studies.13−16 These led to different and sometimes contra-
INTRODUCTION
■
The study of iron and cobalt complexes with 2,6-bis(imino)-
pyridine (BIP) ligands as catalysts for olefin polymerization has
attracted much interest since their discovery in 1998.1 As a
result, the main factors controlling their performance and
selectivity are now reasonably well understood.2 However, the
investigation of the mechanistic aspects of these catalysts has
proved to be very challenging, and the true nature of the active
species involved in these catalytic systems is still being
debated.3
By analogy with other homogeneous olefin polymerization
catalysts, early mechanistic proposals postulated that activation
of [MX2(BIP)] with organoaluminum cocatalysts leads to
coordinatively unsaturated cationic Fe(II) or Co(II) alkyl
species.4 This proposal received a strong experimental support
from Chirik, who synthesized iron5 and cobalt6 complexes of
the type [M(R)(S)(iPrBIP)]+ (S is a solvent molecule or no
ligand; the superscript on the BIP ligand refers to the aryl
substituents on the imine groups; see Scheme 1) and confirmed
that these complexes do in fact behave as single-component
catalysts for ethylene polymerization. However, one of the
difficulties in this classic activation scheme is that, while the
reaction of paramagnetic dihalide complexes [MX2(BIP)] with
organoaluminum compounds is complex and difficult to study,
the better-behaved organolithium or -magnesium reagents
seldom lead to straightforward alkylation.7 Frequently, the
outcome of such reactions is simultaneous alkylation and
dictory results. Thus, using ESI mass spectrometry, Leskela and
̈
co-workers identified the formation of the cationic alkyl species
[FeMe(THF)(iPrBIP)]+ when the corresponding halide pre-
cursor complex was treated with MAO in THF,13 but a
combined Mossbauer and UV−vis analysis of the MAO
activation products in the more usual nonpolar solvents led
Gibson to propose that this process involves oxidation rather
than reduction, producing Fe(III) species.14 This conclusion
has found support in theoretical contributions.17 Using NMR,
reduction, the main products being the unusual neutral or
8−10
anionic monoalkyl species [M(R)(BIP)]0/−
.
This distinct
reactivity of BIP complexes suggested that activation of iron or
cobalt dihalide precursors by organoaluminum compounds
could also involve redox changes.8,9 Although the intermediacy
of neutral or even anionic intermediates in the polymerization
Received: February 15, 2014
Published: March 17, 2014
© 2014 American Chemical Society
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Scheme 1
Bryliakov and Talsi showed that interaction of iron15 and
cobalt16 [MX2(BIP)] complexes with MAO or aluminum
trialkyls leads to different types of products, depending on the
M/Al ratio and the nature of the organoaluminum compounds.
While MAO gives rise to the cationic bimetallic species [M(μ-
X)(μ-Me)AlMe2(BIP)]+ (where X  Cl, Me), TMA and other
aluminum trialkyls cause the reduction of the complexes,
affording the neutral compounds [M(μ-X)(μ-R)AlR2(BIP)],
which are significantly less stable than the cationic species
generated with MAO and decompose at room temperature to
ultimately yield iron-free aluminum complexes, consistent with
Gambarotta’s results.12,15c Both neutral and cationic bimetallic
species can be seen as adducts of the corresponding complexes
[M(X)(BIP)]+/0 with a trialkylaluminum unit. Considering that
both MAO and aluminum trialkyls are efficient activators, and
the previously mentioned similarity of the metal center in
cationic and neutral monoalkyl complexes, it was suggested that
different cocatalysts could lead to different (cationic or neutral)
active species, at least in the case of iron complexes.5b,15c This
could help explain the strong influence of cocatalysts in the
molecular weight distribution of polyethylenes obtained with
the latter catalysts.
from the reaction of 1a and TMA were initially thwarted by the
facile decomposition of the product during the workup.
However, after a number of attempts, we found that 2a is
obtained in high yield (up to 90%) when stoichiometric
amounts of 1a and TMA are combined in toluene at room
temperature and the resulting mixture is evaporated under
vacuum in order to minimize the exposure of the product to the
aluminum byproducts during the workup. As shown in Figure
1, the NMR spectrum of the crystalline sample of 2a confirms
that this is the same species observed in the NMR experiment.
With the aim of contributing to clarify the role of aluminum
alkyls as reducing agents, we have investigated the interaction
of thermally stable dialkyls [Fe(CH2SiMe3)2(BIP)] with TMA
at room temperature as a well-defined model for the interaction
of iron BIP complexes with aluminum alkyls, a highly relevant
process for olefin polymerization. In this contribution, we show
that these reactions take place through a sequence of
consecutive reduction and alkyl exchange steps involving
neutral iron monoalkyls [Fe(R)(BIP)] (R  CH2SiMe3,
Me), which could be isolated or detected under the appropriate
experimental conditions.
Figure 1. 1H NMR spectrum of 1a (bottom) and of 1a after addition
of 1 equiv of TMA (middle). The spectrum of a pure sample of 2a is
shown at the top. The residual signal of the solvent (C6D6) is marked
with an asterisk.
RESULTS AND DISCUSSION
■
Some time ago, we studied the interaction of the iron dialkyl
complex [Fe(CH2SiMe3)2(MesBIP)] (1a) with trimethylalumi-
The essentially quantitative reaction of 1a with TMA
suggested that this reaction could be extended to related
complexes. As mentioned before, Gambarotta studied the
reaction of the bulkier dialkyl complex [Fe-
(CH2SiMe3)2(iPrBIP)] (1b) with a large excess (10 equiv) of
TMA, showing that it leads to the compound [Al-
Me2(iPrBIP)].14 However, we found that if this reaction is
carried out using the procedure devised for the synthesis of 2a,
the previously described mono(trimethylsilyl) derivative [Fe-
(CH2SiMe3)(iPrBIP)] (2b)19 is obtained in good yield.
Since the crystal structure of 2b had not been previously
reported, the identities of both monoalkyl derivatives 2 were
confirmed with X-ray diffraction studies (Figures 2 and 3). The
molecule of 2a has no symmetry elements, but 2b has a
crystallographically imposed symmetry mirror. Apart from that,
num (TMA).18 Using H NMR, we observed that the reaction
1
of 1a with 1 equiv of TMA (i.e., 0.5 Al2Me6) in C6D6 leads to a
single product, 2a, moderately stable in solution at room
temperature. This result was considered relevant because
ethylene was rapidly polymerized when it was bubbled through
the resulting solution. In that report, we suggested that complex
2a could be a bimetallic Fe/Al complex, on the basis of the
1
similarity of some of its main H signals with those of the
species reported by Bryliakov and Talsi.15b However, we later
found closer similarity with the spectra of the monoalkyl
complexes [Fe(CH2SiMe3)(EtBIP)]10a and [Fe(CH2SiMe3)-
(iPrBIP)],19 reported by Chirik, and this led us to conclude
that 2a could be a member of the same family of monoalkyl
compounds, as shown in Scheme 1. Our efforts to isolate 2a
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which explains why the latter become nearly complete in a
short time). A significant decay of the intensity of the 2b signals
was observed at longer reaction times. Increasing the TMA/1b
ratio to 2/1 led to the appearance of a new paramagnetic
species, identified as the monomethyl complex [Fe(Me)-
(iPrBIP)] (3b) by comparison with the spectrum of an
authentic sample of this compound independently prepared
as described in the literature.10a Most likely, this product arises
from the alkyl exchange between 2b and TMA. However,
signals of 2b are almost inappreciable in this experiment, while
significant amounts of 1b (ca. 15%) remained unreacted. This
indicates that the alkyl exchange between 2b and TMA
proceeds more quickly than the initial reduction of 1b to 2b. In
an independent experiment, we reacted a pure sample of 2b
with 1 equiv of TMA in C6D6. The spectrum of the reaction
mixture showed the formation of 3b, confirming that, as
represented in Scheme 1, this complex can be formed from 1b
in a stepwise manner: i.e., via 2b. In contrast, treating the
mesityl derivative 1a with 2 equiv of TMA does not afford the
corresponding methyl derivative 3a. Monitoring this reaction
by NMR showed that the signals of 2a lose intensity and fade
away within minutes. Very likely 3a is initially formed, but it
rapidly decomposes in the presence of free TMA. We
confirmed that also 3b has a similar behavior, as its 1H
spectrum becomes broader and decays in the presence of TMA,
a large excess (Al/Fe = 50) causing its immediate
disappearance. The instability of both monomethyl complexes
in the presence of TMA is consistent with the previously
mentioned results by Gambarotta.12
Figure 2. X-ray crystal structure of compound 2a. Selected bond
lengths (Å) and angles (deg): Fe(1)−N(1), 2.007(5); Fe(1)−C(28),
2.050(5); Fe(1)−N(2), 2.103(5); Fe(1)−N(3), 2.110(6); C(1)−
C(6), 1.440(9); C(6)−N(2), 1.323(8); C(5)−C(8), 1.436(10),
C(8)−N(3), 1.325(7); N(1)−Fe(1)−C(28), 148.5(2); N(2)−
Fe(1)−N(3), 136.68(19).
1
The diamagnetic region close to 0 ppm in the H NMR
spectra of solutions of 2a,b and 3b directly generated from 1a,b
and 2b and TMA is complex and consistently shows several
signals that can be reasonably assigned to the AlCH2SiMe3
moiety of silyl-containing aluminum species. For example, the
spectrum of Al(CH2SiMe3)3 consists of two signals at δ 0.0 and
−0.55 ppm.20 The complexity of this region can be explained
by assuming that the hypothetical compound [Al-
(Me)2CH2SiMe3] disproportionates into a mixture of silyl-
containing alkyls. The fact that similar Al-containing products
are observed in the reactions of TMA with compounds 1 and 2
suggests that both processes involve Fe/Al alkyl exchange, but
in the former case the resulting mixed iron alkyls [Fe(Me)-
(CH2SiMe3)(BIP)] undergo Fe−Me homolysis, as depicted in
Scheme 1. This outcome is not surprising, as most complexes
of the type [MR2(BIP)] are unstable and decompose with
either M−C bond homolysis or alkyl migration to the BIP
ligand.19,21,22 The successive formation of complexes 2 and 3
can therefore be seen as a series of alkyl exchange reactions,
probably involving bimetallic intermediates similar to those
observed by Bryliakov and Talsi.15 This raises the question of
why we did not detect such intermediates in our experiments.
These authors showed that 3b reacts with TMA at 253 K,
affording the bimetallic derivative [Fe(μ-Me)2AlMe2(iPrBIP)]
(4b),15c but we did not observe this or any new products when
3b was treated with excess TMA at room temperature. In order
to address this point, we revisited the reaction of 3b with TMA
at low temperature. We confirmed that, as reported by
Bryliakov and Talsi, 3b is cleanly transformed into 4b by
reaction with 10 equiv of TMA at 253 K. However, only partial
conversion was observed with the TMA/3b ratio 5/1, which
indicates that either the formation of 4b is slow or it is a
reversible process requiring excess TMA to shift the
equilibrium. Increasing the temperature should accelerate the
Figure 3. X-ray crystal structure of compound 2b. Selected bond
lengths (Å) and angles (deg): Fe(1)−N(1), 1.992(3); Fe(1)−C(18),
2.025(4); Fe(1)−N(2), 2.190(2); N(2)−C(4), 1.316(3); C(4)−C(1),
1.443(4); N(1)−Fe(1)−C(18), 149.06(14); N(2)−Fe(1)−N(2)′,
142.04(11). Symmetry operation to generate equivalent atoms: x,
−y + 1, z.
the main bond lengths and angles are very similar in both
molecules and are close to those in the related monoalkyl
[Fe(CH2SiMe3)(EtBIP)].19 Thus, they both exhibit lengthened
imino CN and contracted 2,5-pyridyl C−CN bonds with
regard to the free BIP ligand, which can be considered as
diagnostic of monoelectronic reduction to [BIP]•−.5b,11b The
four-coordinated Fe center can be described as approximately
square planar, severely distorted by the departure of the Fe−C
bond from the mean coordination plane. Interestingly, the
varying degree of steric hindrance exerted by the MesBIP,
EtBIP,19 and iPrBIP ligands has little effect on this distortion, as
the angles formed by the central pyridine ring, the iron center,
and the metal-bound carbon atom are almost the same in the
three compounds (ca. 149°).
1
Monitoring the reaction of 1b with TMA in C6D6 by H
NMR improved our understanding of the reaction of the iron
and aluminum alkyls (Scheme 1 and Figure 4). The reaction of
1b with an equivalent amount of TMA proceeds more
sluggishly than in the case of 1a, as the conversion into 2b is
ca. 50% 10 min after mixing the reagents and reaches 60% after
30 min (note that samples in NMR experiments are
significantly more diluted than those in preparative reactions,
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Figure 4. 1H NMR monitoring of the reaction of 1b with 1 and 2 equiv of TMA. From bottom to top: (1) spectrum of 1b with signal assignations;
(2) spectrum of 1b after reaction with 1 equiv of TMA, 30 min after mixing, showing signal assignments for 2b; (3) spectrum of 1b after reaction
with 2 equiv of TMA, 5 min after mixing; (4) spectrum of 3b with signal assigments. Shadowed bars highlight the signals of 1b and 3b in the reaction
mixtures. The residual signal of the solvent (C6D6) is marked with an asterisk.
reaction of 3b and TMA; therefore, a rapid exchange between
iron monoalkyls and bimetallic adducts and a low association
constant (because dissociation is entropy driven) could be the
causes that prevent detection of the latter at room temperature.
However, the intermediacy of adducts of type 4 can readily
explain the instability of the methyl complexes in the presence
of TMA (Scheme 2).
Scheme 2
Figure 5. X-ray crystal structure of compound 5. Selected bond
lengths (Å) and angles (deg): Al(1)−N(1), 1.9582(16); Al(1)−N(2),
1.972(2); Al(1)−N(3), 2.1345(18); Al(1)−C(34), 1.972(2); Al(1)−
C(36), 1.984(3); C(6)−N(2), 1.311(2); C(8)−N(3), 1.327(2);
C(1)−C(6), 1.456(3); C(5)−C(8), 1.450(3); N(1)−Al(1)−C(34),
To conclude our study, we briefly investigated the interaction
142.72(10); N(1)−Al(1)−C(36), 103.36(9); N(2)−Al(1)−N(3),
of 1b with 2 equiv of TEA, as a possible route to the
145.36(7); C(34)−Al(1)−C(36), 113.47(11).
corresponding monoethyl derivative [FeEt(iPrBIP)]. This
compound was not obtained; instead, the green, NMR-silent
aluminum complex 5 was isolated in 45% yield (eq 1). The
respectively, in comparison with those in the free iPrBIP ligand.
The main difference between the structures of the dimethyl
complex and 5 is in the geometry of the Al center, which is
trigonal bipyramidal in the former and square pyramidal in the
latter with one of the ethyl groups occupying the apical
position. The facile formation of compound 5 confirms that
displacement of the iron center from BIP complexes by
aluminum alkyls is a quite general process and plays an
important role in the deactivation of iron-based olefin
polymerization catalysts.
crystal structure of 5 is shown in Figure 5. This compound is
analogous to the methyl derivative isolated by Gambarotta from
CONCLUSIONS
■
the reaction with excess of TMA, which is best described as an
Al(III) complex containing a reduced BIP•− radical anion. In
agreement with this proposal, the lengths of the imino CN
bonds and the C−C bonds connecting these to the central Py
ring (C1−C6 and C5−C8) are elongated and contracted,
We have shown that the interaction of thermally stable iron(II)
bis((trimethylsilyl)methyl) complexes 1a,b with TMA involves
three distinct stages. The dialkyl complexes are first reduced to
the mono((trimethylsilyl)methyl) derivatives 2a,b. These are
robust enough and can be isolated in good yields when 1a,b
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crystals were obtained by recrystallization from hexane. Anal. Calcd for
C31H42N3FeSi: C, 68.87; H, 7.83; N, 7.77. Found: C, 68.80; H, 8.17;
N, 7.85. μeff(298 K) = 4.3 μB. UV−vis (Et2O): λmax 360 nm (ε =
4994), λmax 479 nm (ε = 2825), λsh 640 nm (ε = 1285), λsh 687 nm (ε
and TMA are reacted in a 1/1 ratio. Presumably, the initial
reduction of the Fe center is not directly induced by the
organoaluminum compound but is due to the intrinsic
instability of mixed alkyl intermediates arising from
CH2SiMe3/Me exchange, which rapidly decomposes, under-
going Fe−Me bond homolysis. The CH2SiMe3 group that
remains in 2a,b is readily exchanged with another 1 equiv of
TMA, a process that leads to methyl complexes (3). Excess
TMA causes the displacement of iron from the BIP ligand,
affording NMR-silent dialkylaluminum−BIP complexes. The
latter process probably constitutes a main deactivation route for
the iron polymerization catalysts. Complexes 3 react further
with organoaluminum compounds such as TMA or the
products resulting from the alkyl exchange, affording NMR-
silent aluminum species. This prevented their isolation from the
reaction mixtures, although the formation of the more stable
derivative 3b (but not 3a) was observed using NMR
spectroscopy. A similar process takes place when TMA is
replaced with higher aluminum alkyls, and complex 5, the ethyl
analogue of the previously known compound [AlMe2(iPrBIP)],
was isolated from the reaction of 1b with 2 equiv of TEA. The
sequence of reactions of the dialkyls 1 with TMA can be
regarded as a series of alkyl exchanges between the BIP-
containing species and the aluminum alkyls and probably
involve the formation of Fe/Al bimetallic species as
intermediates. As reported previously by Bryliakov, Talsi, et
al., the reduced, electrically neutral Fe/Al bimetallics are
thermally unstable and can only be detected at low temper-
ature. Therefore, the reduced monoalkyl species [Fe(R)(BIP)]
are the most relevant products arising from the interaction
between Fe(II) BIP complexes and aluminum alkyls at the
room temperature and, consequently, they are likely to play
some role in catalyst systems generated from Fe(II) precursors
and TMA or other trialkylaluminum compounds. In an ensuing
paper we will be analyzing the role of these reduced species in
the mechanism of ethylene polymerization catalyzed by iron
complexes.
1
= 1202). H NMR (300 MHz, C6D6, 298 K): δ −187.3 (Δν1/2 = 178
Hz, 6H, MeCN), −50.0 (Δν1/2 = 172 Hz, 12H, o-Memes), −11.3 (Δν1/2
= 42 Hz, 4H, m-CHmes), 14.8 (Δν1/2 = 24 Hz, 6H, p-Memes), 33.7
(Δν1/2 = 214 Hz, 9H, SiMe3), 66.0 (Δν1/2 = 96 Hz, 2H, 3,3′-HPy),
331.8 (Δν1/2 = 234 Hz, 1H, 4-HPy).
[Fe(CH2SiMe3)(iPrBIP)] (2b). To a solution of complex 1b (1.56 g,
2.5 mmol) in 30 mL of toluene, stirred at room temperature, was
added 2.5 mL of a 1 M solution of TMA in the same solvent (2.5 equiv
of “AlMe3”) at room temperature. The mixture changed from purple
to green. After 5 min, the volatiles were removed under vacuum, the
residue was extracted in pentane (3 × 20 mL), and the solution was
filtered through a pad of Celite. The green solution was concentrated
to ca. one-third of its original volume and stored at −20 °C for several
days. The product formed a green microcrystalline material that was
filtered, washed with cold pentane, and dried under vacuum. Yield:
1.15 g, 77%. X-ray-quality crystals were obtained by recrystallization
from toluene/pentane. Anal. Calcd for C37H54N3FeSi·0.5C7H7: C,
72.51; H, 8.71; N, 6.26. Found: C, 72.04; H, 8.17; N, 6.35. μeff(298 K)
= 4.0 μB. IR (Nujol mull): 1581 (νCN). UV−vis (Et2O): λmax 353 nm
(ε = 4919), λmax 465 nm (ε = 2538), λsh 589 nm (ε = 478). 1H NMR
(300 MHz, C6D6, 298 K): δ −203.6 (Δν1/2 = 178 Hz, 6H, MeCN),
−105.6 (Δν1/2 = 421 Hz, 2H, CHMe2), −31.7 (Δν1/2 = 102 Hz, 6H,
CHMeMe), −18.4 (Δν1/2 = 41 Hz, 6H, CHMeMe), −17.4 (Δν1/2
=
28 Hz, 2H, p-CHAr), −10.8 (Δν1/2 = 31 Hz, 4H, m-CHAr), 65.8 (Δν1/2
= 79 Hz, 2H, 3,3′-HPy), 364.2 (Δν1/2 = 211 Hz, 1H, 4-HPy).
[AlEt2(iPrBIP)] (5). A solution of complex 1b (1.60 g, 2.25 mmol) in
toluene (30 mL), stirred at room temperature, was treated with 4.5 mL
of a 1 M solution of TEA in the same solvent (4.5 equiv of “AlEt3”).
The mixture changed from purple to green. After it was stirred for 5
min, the solution was taken to dryness, and the residue was extracted
in pentane. The solution was filtered through a pad of Celite,
concentrated to ca. one-third of the original volume, and stored at −20
°C. The product formed crystals that were appropriate for X-ray
diffraction. Yield: 570 mg, 45%. Anal. Calcd for C37H53N3Al: C, 78.40;
H, 9.48; N, 7.41. Found: C, 78.41; H, 9.92; N, 7.22. IR (Nujol mull):
1581 (νCN).
EXPERIMENTAL SECTION
■
NMR Monitoring of the Reactions of Iron Alkyl Complexes
with TMA. General Procedure. Samples of the iron complexes were
weighed in the glovebox, dissolved in the required amount of C6D6,
and placed in NMR tubes capped with septa. A 0.1 M stock solution of
TMA (containing 1 equiv of “AlMe3”/mL) was prepared in the same
All manipulations were carried out under an oxygen-free argon
atmosphere using conventional Schlenk techniques or in a nitrogen
-filled glovebox. Solvents were rigorously dried and degassed before
using. TMA and TEA were purchased from Aldrich and used as
received. Compounds 1a,b were prepared according to literature
procedures.18,19 Microanalyses were performed by the Microanalytical
1
solvent, and a sample was placed in a septum-capped vial. The H
NMR spectra of the pure samples were recorded. The required
amount of the TMA solution was taken with a gastight syringe from its
vial and injected into the tube of the corresponding complex. The
NMR tube was sealed with a small amount of silicone grease and
Parafilm and gently shaken before placing into the NMR probe.
Monitoring the Reaction of Complexes 1−3 with TMA at Room
Temperature. Samples containing 10 μmol of the complexes in C6D6
were prepared as described and treated with 100 or 200 μL of the
TMA solution (Al/Fe = 1 or 2, respectively). For the reaction of 3b
with 50 equiv of TMA, a 1 N solution of this reagent was used.
Monitoring the Reaction of 3b with TMA at Low Temperature.
Two samples containing 4.5 mg of 3b (8 μmol) in toluene-d8 were
prepared in the glovebox and placed in septum-capped NMR tubes. A
0.2 N solution of TMA in the same solvent was similarly prepared.
The 3b samples were cooled to 193 K (−80 °C) in an acetone−dry ice
bath, and the prescribed amount of the TMA solution (200 or 400 μL,
Al/Fe = 5 or 10, respectively) was added. The septa were sealed with
silicone grease and Parafilm, and the tubes were transferred to the
precooled NMR probe.
́
Service of the Instituto de Investigaciones Quımicas (Sevilla, Spain).
Infrared and UV−vis spectra were recorded respectively on Bruker
Vector 22 and Perkin-Elmer Lambda 12 and Lambda 750
spectrophotometers and NMR spectra on Bruker DRX 300, 400,
and 500 MHz spectrometers. The resonances of the solvent were used
as the internal standard, but chemical shifts were reported with respect
to TMS. Magnetic susceptibilities were measured at 298 K using a
Sherwood Model MSB-Auto magnetic balance and were corrected for
the diamagnetic contributions estimated from Pascal’s constants.23
[Fe(CH2SiMe3)(MesBIP)] (2a). To a solution of complex 1a (627
mg, 1 mmol) in toluene (30 mL), stirred at room temperature, was
added dropwise 1.1 mL of a 1 M solution of TMA in toluene (1 mmol
of “AlMe3”/mL, 1.1 equiv). During the addition, the mixture changed
from purple to green. The stirring was continued for 5 min, and then
the solution was evaporated to dryness. The residue was extracted in
pentane (2 × 30 mL), and the solution was filtered through a pad of
Celite. The resulting green solution was concentrated to one-third of
the original volume (approximately 20 mL) and stored at −20 °C. The
product was isolated as a green solid by filtration, washed with cold
pentane, and dried under vacuum. Yield: 500 mg, 90%. X-ray-quality
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Organometallics
Article
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17206.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, a table, and CIF files giving experimental details for the X-
ray crystal structure analyses and crystallographic data for 2a,b
and 5. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Government of Spain (project
number CTQ2012-30962), the Junta de Andalucıa (project
number FQM5074), and the European Union (EU) (FEDER
funds). M.A.C. thanks the CSIC for a predoctoral research
fellowship (I3P program).
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