Multinuclear iron-phenyl species in reactions of simple iron salts with PhMgBr: Identification of Fe4(μ-Ph)6(THF)4 as a key reactive species for cross-coupling catalysis

Article abstract of DOI:10.1039/c8sc02915f

The first direct syntheses, structural characterizations, and reactivity studies of iron-phenyl species formed upon reaction of Fe(acac)₃ and PhMgBr in THF are presented. Reaction of Fe(acac)₃ with 4 equiv. PhMgBr in THF leads to the formation of [FePh₂(μ-Ph)]₂^2- at -80 °C, which can be stabilized through the addition of N-methylpyrrolidone. Alternatively, at -30 °C this reaction leads to the formation of the tetranuclear iron-phenyl cluster, Fe₄(μ-Ph)₆(THF)₄. Further synthetic studies demonstrate that analogous tetranuclear iron clusters can be formed with both 4-F-PhMgBr and p-tolylMgBr, illustrating the generality of this structural motif for reactions of simple ferric salts and aryl Grignard reagents in THF. Additional studies isolate and define key iron species involved in the synthetic pathway leading to the formation of the tetranuclear iron-aryl species. While reaction studies demonstrate that [FePh₂(μ-Ph)]₂^2- is unreactive towards electrophile, Fe₄(μ-Ph)₆(THF)₄ is found to rapidly react with bromocyclohexane to selectively form phenylcyclohexane. Based on this reactivity, a new catalytic reaction protocol has been developed that enables efficient cross-couplings using Fe₄(μ-Ph)₆(THF)₄, circumventing the current need for additives such as TMEDA or supporting ligands to achieve effective cross-coupling of PhMgBr and a secondary alkyl halide.

Full text of DOI:10.1039/c8sc02915f

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Multinuclear Iron-Phenyl Species in Reactions of Simple Iron Salts
µ
with PhMgBr: Identification of Fe4( -Ph)6(THF)4 as a Key Reactive
Species for Cross-Coupling Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Stephanie H. Carpenter, Tessa M. Baker, Salvador B. Muñoz III, William W. Brennessel and Michael
L. Neidig*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first direct syntheses, structural characterizations, and reactivity studies of iron-phenyl species formed upon reaction of
Fe(acac)3 and PhMgBr in THF are presented. Reaction of Fe(acac)3 with 4 equiv PhMgBr in THF leads to the formation of
[FePh2(µ-Ph)]22- at -80 oC which can be stabilized through the addition of N-methylpyrrolidone. Alternatively, at -30 oC this
reaction leads to the formation of the tetranuclear iron-phenyl cluster, Fe4(µ-Ph)6(THF)4. Further synthetic studies
demonstrate that analogous tetranuclear iron clusters can be formed with both 4-F-PhMgBr and p-tolylMgBr, illustrating
the generality of this structural motif for reactions of simple ferric salts and aryl Grignard reagents in THF. Additional studies
isolate and define key iron species involved in the synthetic pathway leading to the formation of the tetranuclear iron-aryl
2-
species. While reaction studies demonstrate that [FePh2(µ-Ph)]2 is unreactive towards electrophile, Fe4(µ-Ph)6(THF)4 is
found to rapidly react with bromocyclohexane to selectively form phenylcyclohexane. Based on this reactivity, a new
catalytic reaction protocol is developed that enables efficient cross-couplings using Fe4(µ-Ph)6(THF)4, circumventing the
current need for additives such as TMEDA or supporting ligands to achieve effective cross-coupling of PhMgBr and a
secondary alkyl halide.
ferric salts and PhMgBr.18-25 Such studies have hypothesized the
formation of mononuclear FeI and Fe0 species in-situ.18-25
Alternatively, extended X-ray absorption fine structure (EXAFS)
Introduction
Iron-catalyzed organic transformations continue to attract
significant interest due to the low cost, improved sustainability
and potential for novel reactivity of iron compared to more
traditional precious metal catalytic systems.1-17 Catalytic
reactions involving simple ferric salts and phenyl nucleophiles
(most extensively phenylmagnesium bromide, PhMgBr) are of
particular interest as this combination has shown to be effective
in catalysis in both iron-catalyzed cross-coupling and iron-
catalyzed C-H functionalization systems.1-17 Unfortunately, the
nature of the in-situ formed and reactive iron species generated
from simple ferric salts and PhMgBr in catalysis remains
ambiguous. Such structural insight is essential for defining the
role of various iron-phenyl species for both productive and off-
cycle reactivity, and for the development of more efficient
catalytic methodologies using these reagents.
studies have led to the proposed formation of FeII dimers in-situ
in these reactions, whereas mass spectrometry studies have
suggested that iron-phenyl-ate species of even higher
nuclearity can form.26,27 While these studies represent
important contributions towards the identification of some of
the in-situ formed iron-phenyl species, the resulting lack of
consensus on the nature of these species (e.g. oxidation state,
nuclearity) reflects the limited availability of isolated and
structurally defined iron-phenyl species. To date, the only
structurally defined species reported have been a mononuclear
iron(II)-phenyl-ate species formed from the reaction between
simple iron salts and PhLi, as well as a reduced iron(I)-phenyl
species formed using the same nucleophile (Scheme 1).18,24 The
latter was found to be unreactive towards electrophile, likely
representing an unproductive iron-phenyl species for cross-
coupling.24 Notably, no structurally defined iron-phenyl species
Motivated by the critical need to define the iron-phenyl
species involved in catalysis, several recent studies have
employed NMR and electron paramagnetic resonance (EPR)
spectroscopies, as well as density functional theory (DFT)
calculations to investigate iron speciation in reactions of simple
Scheme 1 Structurally defined iron-phenyl species formed using PhLi implicated in
iron-cross coupling reactions
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generated from the reaction of simple ferric salts with PhMgBr below -80 °C under a nitrogen atmosphereVietwoArtpicrleeOvnelinnet
in THF, the reagents and solvents employed in both cross- decomposition. Despite these handliDnOgI: 10li.m10i3t9a/tCio8nSCs,029t1h5eF
coupling and C-H functionalization reactions, have been crystalline material was successfully characterized by SC-XRD
reported. This lack of structural insight greatly contrasts which revealed the identity of the orange crystals as the
analogous reactions with the more sterically encumbered dinuclear iron species, [Mg(acac)(THF)4]2[FePh2(µ-Ph)]2
•
4 THF
mesitylmagnesium bromide (MesMgBr), where both FeMes3 (1a) (Figure 1). This complex exhibits an Fe-Fe distance of
and Fe2Mes4 have been isolated.28
2.5175(9) Å, and has average terminal and bridging Fe-Ph bond
-
In this study, we report the first isolation and lengths of 2.083(4) Å and 2.203(4) Å, respectively. Notably, this
characterization of multinuclear iron-phenyl species formed Fe-Fe distance is significantly smaller than the corresponding
from the reaction of Fe(acac)3 and ArMgBr in THF. Insight into distance in Fe2Mes4 and [Ar*Fe(µ-Ph)]2 (Ar* = C6H3-2,6-(C6H2-
the reaction pathway of formation of both di- and tetranuclear 2,4,6-iPr3)2); Fe-Fe distances of 2.612(1) Å to 2.635(1) Å for
iron-complexes is presented, as well as the generality of the Fe2Mes4 and 2.7207(14) Å for [Ar*Fe(µ-Ph)]2.31-35 Based upon
tetranuclear structure across several aryl Grignard reagents. the crystal structure, both iron sites are formally iron(II), where
Stoichiometric reaction studies are utilized to evaluate the the presence of distorted tetrahedral iron sites suggest the
potential roles of the multinuclear iron-phenyl complexes in presence of high-spin iron(II) centers due to the tetrahedral
catalysis. These combined synthetic and reaction studies have ligand fields. The 57Fe Mössbauer spectrum of crystalline 1a was
led to the identification of a highly reactive iron-phenyl species characterized by a single doublet with an isomer shift (δ) of 0.34
for the selective formation of cross-coupled product, resulting mm/s and a quadrupole splitting (ΔEQ) of 2.28 mm/s. The
in a new ligand- and additive-free reaction protocol for cross- observed quadrupole splitting is consistent with the presence
coupling with Fe(acac)3 and PhMgBr.
of high-spin iron(II) sites and the low isomer shift is in
agreement with previously reported transmetalled iron(II)
complexes corresponding to the highly covalent Fe-C bonds.36,37
Unfortunately, further characterization of pure 1a was not
possible due to the high thermal instability of the complex.
N-methylpyrrolidone (NMP) has been previously employed
as an additive in cross-coupling reactions with simple ferric
salts, and has been demonstrated in the case of MeMgBr to
stabilize [FeMe3]-, favoring its formation over [Fe8Me12]-.38,39
Interestingly, NMP was found to bind to the Mg cation in this
case38 as opposed to binding directly to the iron center as
demonstrated to be possible in earlier studies by Holland and
Results and discussion
Isolation and characterization of [FePh2(µ-Ph)]2 from low
temperature reactions of Fe(acac)3 and PhMgBr
2-
Previous iron-catalyzed cross-coupling studies have indicated
that a minimum of 4 equiv of Grignard reagent is required to
achieve effective catalysis using simple ferric salts and
PhMgBr.29,30 Therefore, synthetic studies focused on the
isolation of iron-phenyl species formed using this ratio of iron
to Grignard reagent in THF. While viable single crystals could not
be isolated directly from reactions at -80 °C, a modified
synthetic procedure involving layering and warming of 4 equiv
of PhMgBr on top of Fe(acac)3 (see experimental section for
further details) produced orange crystalline material suitable
for single crystal X-ray diffraction (SC-XRD). The orange crystals
were found to have extreme air-, moisture-, and temperature-
sensitivities; crystalline material could only be handled at or
co-workers.40 It was hypothesized that the inclusion of NMP as
2-
an additive might enable the stabilization of [FePh2(µ-Ph)]2
.
Therefore, an analogous reaction of Fe(acac)3 with 4 equiv
PhMgBr in THF in the presence of excess NMP (36 equiv) was
performed at -30 °C. This reaction led to the formation of an
analogous dimer [Mg(NMP)6][FePh2(µ-Ph)]2 • 3.5 THF (1b) as
confirmed by SC-XRD (Figure 2). A slight decrease in the Fe-Fe
Figure 1 The crystal structure, selected bond distances and angles, and 80 K 57Fe Mössbauer spectrum of [Mg(acac)(THF)4]2[Fe(Ph)2(µ-Ph)]2 • 4 THF (1a). The hydrogen atoms
are omitted for clarity and the thermal ellipsoids are shown at 50% probability.
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combined with the lack of an EPR signal at 10 K,ViisewcAortnicsleisOtnelinnet
DOI: 10.1039/C8SC02915F
with the assignment of a S = 0 ground state due to
antiferromagnetic coupling of the iron(II) sites in the dimer.
Lastly, it was important to evaluate whether 1b could undergo
additional transformations upon reaction with additional
PhMgBr. 57Fe Mössbauer spectroscopy confirmed that addition
of excess PhMgBr (> 20 equiv) directly to a solution of 1b at -80
°C resulted in no reaction within 5 min (a catalytically relevant
time frame, vide infra). The formation of both 1a and 1b differs
drastically from the formation of Fe2Mes4; Fe2Mes4 is accessible
at RT, illustrating how the sterically bulkyl mesityl ligands
modulate the formation and stability of the resulting iron
dimer.28
Isolation and characterization of tetranuclear iron-aryl species
Due to the thermal instability of 1a, it was critical to
determine the nature of the iron species formed at elevated
temperatures as a more reduced iron-phenyl species might be
obtainable. Reaction of Fe(acac)3 with 4 equiv of PhMgBr in THF
at -30 °C, in the absence of NMP, yielded a brown solution from
which brown crystalline material could be obtained. The overall
connectivity, geometry, and chemical formulation could be
unambiguously assigned by SC-XRD as the tetranuclear iron
cluster Fe4(µ-Ph)6(THF)4 • 2 THF (2a) (Figure 3). This cluster is
more reduced than 1a or 1b, formally containing two iron(I) and
two iron(II) ions. Unfortunately, these crystals diffracted very
weakly and, hence, a more detailed discussion of the structural
parameters of 2a is not possible. It was hypothesized that
changing the aryl group of the Grignard reagent might enable
access to higher quality crystalline material for SC-XRD analysis.
An analogous reaction utilizing p-tolylMgBr yielded high quality
crystals idenitifed by SC-XRD as Fe4(µ-p-tolyl)6(THF)4 • 2 THF •
Figure 2 The crystal structure, selected bond distances and angles, and 80 K 57Fe
Mössbauer spectrum of [Mg(NMP)6][Fe(Ph)2(µ-Ph)]2 • 3.5 THF (1b). The hydrogen
atoms are omitted for clarity and the thermal ellipsoids are shown at 50%
probability.
distance in 1b to 2.4969(8) Å is present compared to 1a, with
average terminal and bridging Fe-Ph bond lengths of 2.075(4) Å
to 2.205(4) Å, respectively. The 57Fe Mössbauer spectrum of 1b
was found to be identical to that of 1a, with parameters of δ =
0.34 mm/s and ΔEQ = 2.28 mm/s. Consistent with previous
studies of [FeMe3]-,39 the change of counterion in the complex
significantly improved the thermal stability of the iron-phenyl
C5H12 (2b) upon crystallization at -80 oC
crystallization at -30 oC yielded the formation of a slightly
perturbed tetranuclear cluster, Fe4(µ-p-tolyl)6(THF)3 • THF (2c)
. Of note, an analogous
dimer at -30 °C, enabling additional characterization of 1b
.
While Evans method NMR coupled with atomic absorption
spectroscopy (AAS; to quantify the amount of 1b dissolved in
solution as the material is too thermally unstable to weigh)
would be an ideal way to calculate the spin state of 1b, the
complex was found to be too unstable to obtain meaningful
NMR data. Instead, magnetic circular dichroism (MCD) was
employed to determine the spin state of 1b. No MCD signal is
observed in both mull and solution samples at 5 K which,
,
which lacks one THF ligand to iron (note that the co-crystallized
THF solvent molecule is > 4.6 Å from the open iron center). Thus
slight structural variation demonstrates the potential lability of
coordinated THF in these clusters in order to generate an open
coordination site for reaction with electrophile (vide infra). The
Fe-Fe bond distances in 2b and 2c range from 2.3751(9) Å to
2.5532(10) Å. The Fe-Fe distances in these clusters are notably
Figure 3 Crystal structures of the tetranuclear iron-aryl clusters, with hydrogen atoms omitted for clarity. Due to weakly diffractly crystals, the structures of Fe4(µ-Ph)6(THF)4
(2a) and Fe4(µ-4-F-Ph)6(THF)4 (2d) are used solely for connectivity and chemical formulation as shown. For structures with p-tolyl groups, Fe4(µ-p-tolyl)6(THF)3 (2b) and Fe4(µ-
p-tolyl)6(THF)4 (2c), adequate intensity data could be obtained and the ellipsoids are shown at 50% probability.
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shorter compared to reported tetranuclear iron species such as C-term MCD spectra in both the near-infraredVieawnAdrticUleVO-nVlinies
[Fe4(CO)12(CCH3)]-, [(h5-C5H4Me)4Fe4(µ3-CH)2(µ3-CNPh)2](PF6)2, regions (see ESI) consistent with the presence of an integer spin
DOI: 10.1039/C8SC02915F
and [Fe4(CO)12(µ3-CO)]2-, where the Fe-Fe bond distances paramagnetic complex. Unfortunately, variable-temperature,
typically range from 2.469 Å to 2.618 Å.41-44 Interestingly, a variable-field (VTVH) MCD studies on the ground state of 2a did
[Fe8Me12]-
cluster has been previously isolated and not enable the assignment of the spin state of this complex,
characterized as a key intermediate in reactions involving likely due to complications from decay to additional
simple ferric salts and MeMgBr.45 The Fe-Fe distances in paramagnetic iron species which complicates both these
[Fe8Me12]- are closer to the Fe-Fe distances in 2b and 2c, ranging measurements and attempted Evans studies.
from 2.4188(15) to 2.4514(15) Å.45 The bridging Fe-Ph bonds
range from 2.139(3) Å to 2.266(3) Å for 2b and from 2.078(5) Å
Investigation of the iron species involved in the synthetic
pathway for formation of di- and tetranuclear iron-aryl species
to 2.412(5) Å for 2c. Lastly, an analogous tetranuclear iron
Stoichiometric reactions of Fe(acac)3 with varying
complex
fluorophenylmagnesium bromide (4-F-PhMgBr), identified by
SC-XRD as Fe4( -4-F-Ph)6(THF)4 (2d). As with 2a, the isolated
could
also
be
synthesized
using
4-
equivalents of 4-F-PhMgBr enabled the further investigation
into the underlying synthetic pathway leading to formation of
the dinuclear and tetranuclear iron species. When 1 equiv of 4-
F-PhMgBr is slowly added to Fe(acac)3 in THF at -30 °C, a red
color persists. From this reaction, crystalline material of [trans-
µ
crystalline material was weakly diffracting, and hence, the SC-
XRD data was solely used for the unambiguous assignment of
connectivity. Overall, the ability to access analogous
tetranuclear iron-aryl clusters across both electron withdrawing
and electron donating substituents on the aryl ligands
demonstrates the generality of this structural motif in reactions
of simple ferric salts and aryl Grignard reagents.
Complex 2a was the most synthetically robust tetranuclear
complex for larger scale isolation and was utilized for further
characterization and reactivity (vide infra) studies. The 57Fe
Mössbauer spectrum of 2a is a broad quadrupole doublet with
δ = 0.60 mm/s and ΔEQ = 0.84 mm/s (see ESI). Analogous broad
features were also previously observed for [Fe8Me12]-.45 The
tetranuclear complex is not EPR active, but exhibits an intense
Fe(acac)2(THF)2]0.58
•
[trans-Mg(acac)2(THF)2]0.42 (3) was
isolated and characterized by SC-XRD (Figure 4). Similar crystals
were independently isolated from reactions of Fe(acac)3 with 1
equiv of three different aryl Grignard reagents (PhMgBr, 4-F-
PhMgBr, and p-tolylMgBr), consistent with the first equivalent
of ArMgBr reducing the iron(III) starting material to iron(II).
Addition of a second equivalent of aryl Grignard reagent to the
iron solution at -30 °C results in a color change from red to
yellow. Although no usable crystals of the yellow material could
be obtained, one could envision a complex analogous to the
structurally characterized [(di-tBu-acac)Fe(µ-Mes)]2.46 The
yellow iron solution turns orange upon the slow addition of a
Figure 4 Iron species in the synthetic pathway for the formation of tetranuclear iron-aryl species utilizing Fe(acac)3 and 4-F-PhMgBr. Hydrogen atoms omitted for clarity and
thermal ellipsoids are shown at 50 % probability for 1c, 3 and 4.
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third equivalent of aryl Grignard reagent at -30 °C. From this towards electrophile prior to its thermal decomposition in THF.
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solution, orange crystals were obtained at -80 °C suitable for SC- Furthermore, the observation that DNOMI:P10.1s0t3a9b/Cil8izSeCs029t1h5eF
XRD and determined to be [Mg(acac)(THF)4]2[FeBr(4-F-Ph)(µ-4- formation of this unreactive dimer is consistent with previous
F-Ph)]2 • 2.5 THF (4). The Fe-Fe distance was found to be studies by Nakamura and co-workers, where NMP was shown
2.5903(11) Å, which is longer than previously identified to be an unfavorable co-solvent for iron-catalyzed cross-
homoleptic iron(II)-phenyl dimers 1a and 1b. The unique coupling reactions involving simple ferric salts and aryl Grignard
terminal and two bridging Fe-Ph bond lengths, based on the reagents.47 In contrast to the lack of reactivity of 1b, formation
independent dianion without disorder, are 2.072(4), 2.166(4) of cross-coupled product was observed to be generated, albeit
and 2.212(4) Å, respectively. The 80 K 57Fe Mössbauer spectrum in low yield (24%), from the reaction of Fe2Mes4 and
of
4
has parameters of δ = 0.56 mm/s and ΔEQ = 1.81 mm/s (see electrophile.28
In order to evaluate the potential reactivity of the more
ESI). At low reaction temperatures, the addition of a fourth
equiv of 4-F-PhMgBr changes the orange solution to red, reduced tetranuclear iron complex 2a with bromocyclohexane,
resulting in the formation of [Mg(acac)(THF)4]2[Fe(4-F-Ph)2(µ-4- it was again critical to first establish the thermal stability of 2a
F-Ph)]2 • 2 THF (1c). Here, the additional equivalent of Grignard in solution at catalytically relevant temperatures. Fortunately,
reagent is used to provide the last aryl group to each iron 2a was found to be stable at RT for up to 5 min in THF, enabling
center, showing that dimers 1a and 1c can be constructed as stoichiometric reactions to be performed within this time
each equivalent of Grignard reagent is slowly added to the iron frame. Reactions of 2a with 15 equiv bromocyclohexane at RT
solution. The Fe-Fe distance of 2.5847(5) Å in 1c is longer than resulted in the rapid and selective formation of
those in 1a and 1b by ~ 0.08 Å. The average Fe-Ph terminal and phenylcyclohexane (0.95 equiv with respect to 2a within 5 s).
bridging bond lengths in 1c of 2.093(2) and 2.208(2) Å, Thus, 2a is a highly reactive species for the selective formation
respectively, are similar to those in 1a and 1b, although there of cross-coupled product (kobs ~ 12 min-1 for the initial turnover).
exists a slight asymmetry in the core as bridging Fe-Ph bond Prolonged reaction times led to the generation of additional
lengths differ by ~ 0.035 Å. The 80 K 57Fe Mössbauer spectrum cross-coupled product, indicating that the iron products of each
of 1c is characterized by parameters of δ = 0.40 mm/s and ΔEQ cross-coupling are capable of further reaction with electrophile
= 2.21 mm/s (see ESI). Much like 1a, complex 1c exhibits (~ 4 equiv phenylcycohexane after 1 min of reaction (see ESI);
extreme temperature and moisture sensitivity, preventing note that the reaction rate decreases for subsequent
further characterization. Lastly, the reduced tetranuclear iron turnovers). It is noteworthy that 2a can directly react with
species, Fe4(µ-4-F-Ph)6(THF)4 (2d) can be formed from the 4 electrophile to form cross-coupled product, whereas [Fe8Me12]-
equiv reaction simply by warming to -30 °C.
requires the addition of MeMgBr following initial reaction with
electrophile to form product,45 indicating the presence of
different underlying reaction mechanisms for iron-phenyl and
iron-methyl clusters.
Reactivity studies of [FePh2(µ-Ph)]22- and Fe4(µ-Ph)6(THF)4 with
bromocyclohexane
Beyond the unprecendented insight into iron speciation and
structure for reactions of simple iron salts and aryl Grignard
reagents described above, evaluation of the potential reactivity
Cross-coupling catalysis using Fe4(µ-Ph)6(THF)4
Simple ferric salts were previously found by Nakamura and
of these di- and tetranuclear complexes with electrophile is co-workers to perform poorly for catalytic cross-couplings of
critical to understanding the potential roles of isolated species PhMgBr with secondary alkyl halides in the absence of
in cross-coupling catalysis. Due to the extreme thermal TMEDA.47 Interestingly, Bedford and co-workers observed
instability of 1a, only complex 1b could be utilized to evaluate similar reactivity in the presence and absence of TMEDA when
the reactivity of the [FePh2(µ-Ph)]22- dimer (see ESI). Employing MesMgBr was employed, though this system was low yielding
time-resolved, freeze-quenched 57Fe Mössbauer spectroscopy, (~ 35%).28 Hence, we were motivated by the observed reactivity
the decomposition of 57Fe-enriched 1b in solution was of 2a to explore its potential effectiveness for catalytic cross-
monitored at temperatures ranging from -20 °C to 0 °C in order coupling in the absence of TMEDA. The utilization of the same
to define its stability as a function of temperature and time. reaction protocol as described for stoichiometric reactions of 2a
After re-dissolution of 1b in THF at -20 °C, only 70% of 1b was but with the addition of PhMgBr (1:1 with respect to
found to remain in solution after 45 s. As expected, repeating electrophile) resulted in the formation of > 95 % cross-coupled
this experiment at 0 °C shows more rapid decay of the dimer
with only ~ 50% of 1b remaining in solution after 45 s.
Therefore, reaction studies with electrophile were performed at
-20 °C, focusing on a 45 s reaction window in order to minimize
contributions
from
the
decomposition
of
1b.
Bromocyclohexane was selected as an example electrophile for
the reaction studies due to its common use in ferric salt
catalyzed cross-coupling reactions with PhMgBr.47,48 GC-MS
reaction studies showed no consumption of electrophile or
generation of phenylcyclohexane within 45 s of reaction at -20
Scheme 2 Catalytic cross-couplings of PhMgBr and bromocyclohexane using (a)
isolated 2a and (b) in-situ generated 2a.
2-
°C. Thus, the [FePh2(µ-Ph)]2 dimer exhibits no reactivity
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product (Scheme 2a). Because 2a is challenging to synthesize
Journal Name
In this study, the first direct synthesesV,iew sAtrtriculecOtunlrinael
DOI: 10.1039/C8SC02915F
and handle, a modified catalytic method targeting the characterizations, and reactivity studies of iron-phenyl species
o
formation of 2a in-situ at -30 C (a temperature where 2a is formed upon reaction of Fe(acac)3 and PhMgBr in THF have
stable for days) was also evaluated as a potentially more been presented. At -80 °C, this reaction leads to formation of
convenient protocol that utilizes the selective reactivity of 2a [FePh2(µ-Ph)]22-, which was found to be unreactive towards
without the need to isolate it (Scheme 2b). With this method, > electrophile. Alternatively, at -30 °C the formation of a more
95 % of cross-coupled product could be obtained. Interestingly, reduced, tetranuclear iron-phenyl cluster, Fe4(µ-Ph)6(THF)4, is
removal of magnesium salts by filtration at -30 °C following in- observed, where this species is found to rapidily react with
situ formation of 2a was found to be critical to achieve high bromocyclohexane to selectively form cross-coupled product.
yields of product, likely indicating an important role of cations Further synthetic studies demonstrate that analogous
on iron speciation during the initial synthesis of 2a or during tetranuclear iron clusters can be formed with both 4-F-PhMgBr
catalysis. While a proof of concept, this initial evaluation of in- and p-tolylMgBr, illustrating the generality of this structural
situ generated 2a for catalysis will hopefully inspire future motif for reactions of simple ferric salts and aryl Grignard
studies in the area of ligandless cross-coupling catalysis.
reagents in THF. Lastly, Fe4(µ-Ph)6(THF)4 can be utilized for
catalytic cross-coupling of PhMgBr and
While the current study has demonstrated the importance efficient
of dimeric and tetrameric iron-phenyl species in reactions of bromocyclohexane, circumventing the current need for
simple ferric salts and aryl Grignard reagents, additional iron- additives such as TMEDA or supporting ligands to achieve high
phenyl species beyond those isolated herein might also be cross-coupling yields in this reaction.
accessible in such reactions. For example, previous EPR studies
by Bedford indicated the in-situ formation of a S = 1/2 species
in reactions of FeCl3 and p-tolylMgBr at -30 oC though,
unfortunately, this species was never spin quantified.21,28 We
have observed the formation of the same S = 1/2 species in
Experimental
General Considerations
All reagents were purchased from commercial sources. All air-
and moisture-sensitive manipulations were carried out in an
MBraun inert-atmosphere (N2) glovebox equipped with a direct
liquid nitrogen feed through inlet line. All anhydrous solvents
were freshly dried using activated alumina/4 Å molecular sieves
and stored under an inert atmosphere. Gas chromatography
mass spectrometry was performed using a Shimadzu GCMS QP
2010. Atomic absorption spectroscopy (AAS) analysis was
performed using a Shimadzu AAS 7000. Details on low
temperature crystal manipulations, sample preparations for
spectroscopy and MCD and EPR spectroscopy are given in the
ESI.
reaction of FeCl2 and 4 equiv PhMgBr at -30 °C, though spin
quantitated EPR indicated that it is a very minor species in
solution (< 5% of all iron) (see ESI).21,28 Additionally, Bedford and
co-workers also suggested that a monomeric iron(II) species,
[Fe(p-tolyl)3]- can also form based on 1H NMR studies of
reactions of FeCl2 with p-tolylMgBr,28 where the resonances
assigned to this mononuclear species are significantly downfield
shifted compared to those observed for 2b (see ESI). Again,
however, the amount of the mononuclear species in solution
was not quantified and it remains unclear whether it is formed
significantly in solution compared to other iron-p-tolyl species.
Future studies should continue to define the diverse iron-aryl
57Fe Mössbauer Spectroscopy
species accessible in such reactions as
a function of
Solution samples for 57Fe Mössbauer spectroscopy were
prepared from 57Fe(acac)3 to enable data collection from dilute,
freeze-trapped solution samples; solids samples were made
from non-enriched Fe(acac)3. All samples were prepared in an
inert atmosphere glovebox equipped with a liquid nitrogen fill
port to enable sample freezing to 77 K within the glovebox. Each
concentration, solvent, aryl nucleophile (i.e. ArMgBr, ArLi,
Ar2Zn, etc.), temperature, and reaction time.
Lastly, it is interesting to consider the origin of the reactivity
differences observed for [FePh2(µ-Ph)]22- and Fe4(µ-Ph)6(THF)4.
Since previous studies have proposed an Fe(I) active species for
cross-coupling with PhMgBr and simple ferric salts (such as
[PhFeI(acac)(THF)]-),18-25 the presence of two formally iron(I)
sites in the mixed valent tetrameric iron complex 2a might
suggest iron reduced below iron(II) is important for reactivity in
the isolated multimetallic complexes. Specifically, the THF
ligation differences between complexes 2b and 2c demonstrate
the ability of the tetrameric complexes to lose a THF ligand to
generate an open coordination position for reaction with
electrophile. This ability to readily form an open coordination
site might be equally significant in facilitating reactivity,
whereas the dimeric complexes would require a more
significant geometric distortion in order to react with
electrophile.
sample was loaded into
a Delrin Mössbauer cup for
measurements and loaded under liquid nitrogen. Low
temperature 57Fe Mössbauer measurements were performed
using a See Co. MS4 Mössbauer spectrometer integrated with a
Janis SVT-400T He/N2 cryostat for measurements at 5 K, 80 K,
and 150 K. Isomer shifts were determined relative to an
a-Fe at
298 K. All Mössbauer spectra were fit using the program WMoss
(SeeCo). Errors of the analyses are d ± 0.02 mm/s and DEQ ± 3%.
Magnetic Circular Dichroism Spectroscopy
All samples were prepared in an inert atmosphere glovebox
equipped with a liquid nitrogen filling port to enable sample
freezing to 77 K. Low temperature near-infrared (NIR) MCD
experiments were conducted using
a
JASCO J-730
spectropolarimeter and a shielded S-20 photomultiplier tube.
Conclusions
6 | J. Name., 2012, 00, 1-3
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Chemical Science
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Journal Name
ARTICLE
Both instruments have a modified sample compartment, which order to remove an Mg salts. The solution was stoVrieewdAarttic-le3O0n°liCne.
DOI: 10.1039/C8SC02915F
incorporates focusing optics and an Oxford Instruments Crystalline material of 2a formed over the course of several
SM4000-7T superconducting magnetic/cryostat. This set-up weeks.
allows for measurements from 1.6 K to 290 K, with magnetic
fields up to 7 T. A calibrated Cernox sensor directly inserted in
the copper sample holder is used to measure the temperature
at the sample to 0.001 K. All MCD spectra were baseline-
corrected against zero-field scans.
Preparation of Fe4(µ-p-tolyl)6(THF)4
Solid Fe(acac)3 (75 mg, 0.21 mmol) was dissolved in THF (10 mL).
The solution was then cooled to -30 C, where p-tolylMgBr in
THF (1.0 M, 849 µL, 0.849 mmol) was added dropwise at 0.33
mmol/min. The brown solution was allowed to react at -30
for 5 minutes at 620 rpm. The brown solution was then filtered
2 THF C5H12 (2b)
× ×
°
°
C
Electron Paramagnetic Resonance Spectroscopy
A cold spatula was used to transfer material to a vial containing through cold Celite. Cold pentane (3 mL) was then layered on
a known amount of THF at -80 C. A cold pipette was then used top of the THF solution, and was then stored at -80 C until
°
°
to transfer the redissolved crystalline material to a precooled crystalline material was observed.
(in liquid nitrogen) 4 mm OD suprasil quartz EPR tube from
Wilmad Labglass. The solution in the EPR tube was immediately
Preparation of Fe4(µ-p-tolyl)6(THF)3 THF (2c)
×
Solid Fe(acac)3 (76 mg, 0.22 mmol) was dissolved in THF (2 mL).
The solution was then cooled to -30 C, where p-
tolylmagnesium bromide (p-tolylMgBr) in THF (1.0 M, 861 µL,
0.861 mmol) was added dropwise at 0.33 mmol/min. The
solution was allowed to react at -30 °C for 5 minutes at 620 rpm.
The brown solution was then filtered through cold Celite. Cold
pentane (2 mL) was then layered on top of the THF solution, and
the solution was stored at -30 °C until crystalline material was
frozen in liquid nitrogen. The remaining solution not used for
the EPR sample was saved for AAS, so that spin integration of
any EPR signal could be completed. All X-band EPR spectra were
collected on a Bruker EMXplus spectrometer containing a
4119HS cavity and an Oxford ESR-900 helium flow cryostat. All
EPR spectra were collected at 10 K, 9.38 GHz.
°
Preparation of [Mg(acac)(THF)4]2[FePh2(µ-Ph)]2 4 THF (1a)
×
Solid Fe(acac)3 (70.5 mg, 0.2 mmol) was added to a frozen observed.
solution of THF (2 mL) and PhMgBr in THF (1.0 M, 800 µL, 0.8
Preparation of Fe4(µ-4-F-Ph)6(THF)4 (2d)
mmol) in a 20 mL scintillation vial. The frozen solution
containing the solid Fe(acac)3 was allowed to stir at -30 C for 5
minutes before quickly transferring the vial to -80 C. At -80 C,
the solution was allowed to sit for 30 min at prior to filtering the
solution through cold Celite. Following the filtration, cold
Solid Fe(acac)3 (70 mg, 0.2 mmol) was dissolved in THF (2 mL).
The solution was then cooled to 0 °C, where 4-F-PhMgBr in THF
°
°
°
(1.0 M, 794 µL, 0.794 mmol) was added dropwise at 0.33
mmol/min. The brown solution was then filtered through cold
Celite. Cold toluene (1 mL) was then layered on top of the THF
solution, and the sample was stored at -30
material formed.
toluene (1 mL) was added to the solution at -80
solution was allowed to sit at -80 C for an additional 15 minutes
before filtering through cold Celite. The vial was then sealed
with Apiezon N-grease and stored in a -80 C freezer. Red
°C. The red
°
C until crystalline
°
°
Preparation
Mg(acac)2(THF)2]0.42 (3)
Solid Fe(acac)3 (66 mg, 0.19 mmol) was dissolved in THF (5 mL).
The solution was then cooled to -30 C, where 4-F-PhMgBr in
THF (1.0 M, 186 µL, 0.186 mmol) was added dropwise at 0.33
mmol/min. The orange solution was allowed to stir at -30 C for
of
[trans-Fe(acac)2(THF)2]0.58
×
[trans-
crystals of 1a formed within a few days. Note that 1a is
extremely air and temperature sensitive, precluding further
analyses. Analogous complications exist for the other di- and
tetrameric iron species whose synthesis are described below.
°
°
Preparation of [Mg(NMP)6][FePh2(µ-Ph)]2 3.5 THF (1b)
×
5 minutes at 620 rpm prior to filtering through cold Celite. Cold
pentane (10 mL) was then layered on top of the solution, and
Solid Fe(acac)3 (100 mg, 0.28 mmol) was dissolved in a solution
of THF (10 mL) and 1-methyl-2-pyrrolidone (NMP) (0.98 mL,
the sample was stored at -80
formed.
°C until crystalline material was
10.1 mmol). The solution was then cooled to 0
PhMgBr in THF (1.0 M, 1.128 mL, 1.128 mmol) was added at
0.33 mmol/min. The orange solution was allowed to stir at 0
°C, where
°
C
Preparation of [Mg(acac)(THF)4]2[FeBr(4-F-Ph)(µ-4-F-Ph)]2
THF (4)
×
2.5
for 5 minutes at 780 rpm. The orange solution was then filtered
through cold Celite and cold pentane (3 mL) was layered on top
Solid Fe(acac)3 (63 mg, 0.18 mmol) was dissolved in THF (5 mL).
The solution was then cooled to -30 C, where 4-F-PhMgBr in
of the THF solution and stored at -30
observed the next day.
°C. Red crystals of 1b were
°
THF (1.0 M, 535 µL, 0.535 mmol) was added dropwise at 0.33
mmol/min. The dark orange solution was allowed to stir at -30
Preparation of Fe4(µ-Ph)6(THF)4
Solid Fe(acac)3 (100 mg, 0.28 mmol) was dissolved in THF (2
mL). The solution was then cooled to -30 C, where PhMgBr in
THF (1.0 M, 1.133 mL, 1.133 mmol) was added dropwise at 0.33
mmol/min. The brown solution was allowed to react at -30
for 5 minutes at 620 rpm prior to filtering through cold Celite.
The brown solution was allowed to sit at -30 C an additional 20
×
2 THF (2a)
°
C for 5 minutes at 620 rpm. The dark orange was then filtered
through cold Celite. Cold pentane (2 mL) was then layered on
°
top of the solution, and the sample was stored at -80
crystalline material was formed.
°C until
°
C
Preparation of [Mg(acac)(THF)4]2[Fe(4-F-Ph)2(µ-4-F-Ph)]2
THF (1c)
×
2
°
min. The solution was filtered through cold Celite once more in
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Page 8 of 10
ARTICLE
Journal Name
Solid Fe(acac)3 (75 mg, 0.20 mmol) was dissolved in THF (5 mL). amount of THF at -30
The solution was then cooled to -30
°
C using a cold spatulVaie.wOArnticcleeOntlhinee
DOI: 10.1039/C8SC02915F
°
C, where 4- crystallined was completely dissolved, the solution was quickly
fluorophenylmagnesium bromide (4-F-PhMgBr) in THF (1.0 M, transferred to a vial containing THF, bromocyclohexane (13
850 µL, 0.85 mmol) was added dropwise at 0.33 mmol/min. The equiv wrt 2a), PhMgBr (1.0 M in THF,13 equiv wrt 2a), and a stir
dark red solution was allowed to stir at -30 °C for 5 minutes at bar at RT. The crystalline material was allowed to react with
780 rpm. The dark red solution was then filtered through cold electrophile and excess nucleophile, and aliquots were taken
Celite. Cold toluene (3 mL) was then layered on top of the THF over the course of 20 minutes. The aliquots were quenching in
solution, and the solution was stored at -80
material was formed.
°
C until crystalline MeOH (50 µL). A known amount of dodecane was then added
to the quenched samples, and the samples were diluted to 1
mM prior to filtering though silica. AAS was used to determine
the concentration of 2a in solution.
Thermal stability of 1b and 2a in solution
Crystalline material was collected as described in the ESI.
Crystalline material was transferred to a vial containing a known
Catalytic reaction protocol targeting 2a formation in-situ
amount of THF at -80
spatula. Once the crystalline material was completely cooled to -30
°
C for 1b and at -30
°
C for 2a using a cold Fe(acac)3 (4 mg, 0.011 mmol) was dissolved in 2 mL THF and
C. PhMgBr in THF (1.0 M, 45 µL, 4 equiv) was
°
redissolved, the solution was transferred to a vial containing a added dropwise at 0.33 mmol/min. The resulting brown
known amount of THF and stir bar at a warmer temperature solution was allowed to mix for 5 min at 620 rpm prior to
using a cold pipette. Aliquots of the decaying solution were filtering through cold Celite. The solution was allowed to sit at -
taken at various points over 20 minutes. Aliquots were taken 30 °C for an additional 20 min, and filtered through cold Celite
using a cold pipette, and were transferred into a Delrin once more in order to remove any Mg salts. The solution was
Mössbauer sample cup. Samples were immediately frozen in immediately transferred to a vial at RT containing THF,
liquid nitrogen. This process was repeated for various bromocyclohexane, PhMgBr, and a stir bar. Aliquots were taken
temperatures, including, -20
°C and 0 °C. AAS was used to at various time points over a course of 20 min, and quenched
determined to the concentration of the respective complexes in with MeOH (50 µL). A known amount of dodecane was then
solution.
added to the quenched samples, and the samples were diluted
to 1 mM prior to filtering through silica.
Reaction of 1b with bromocyclohexane
Dark red blocks of 1b were collected as described in the ESI.
Crystalline material was transferred to a vial containing a known
Conflicts of interest
There are no conflicts to declare.
amount of THF at -80
°C using a cold spatula. Once the
crystalline material was completely redissolved, the solution
was transferred to a vial containing THF, bromocyclohexane,
PhMgBr, and stir bar at a warmer temperature using a cold
pipette. The crystalline material was allowed to react, and
aliquots were taken at various points over 20 minutes. The
aliquots were quenched in 1:1 (v:v) THF:MeOH solution. A
known amount of dodecane was then added to the quenched
samples, and the samples were diluted to 1 mM prior to filtering
through silica. AAS was used to determine the concentration of
1b in solution.
Acknowledgements
This work was supported by a grant from the National Institutes
of Health (R01GM111480 to M.L.N.), funding from the National
Science Foundation (CHE-1454370 to M.L.N. for initial dinuclear
iron-phenyl synthetic work), an NSF Graduate Research
Fellowship (T.M.B.) and a Ruth L. Kirchstein National Research
Service award (F32GM120823 to S.B.M. III). The NSF is gratefully
acknowledged for support for the acquisition of an X-ray
diffractometer (CHE-1725028).
Reaction of 2a with bromocyclohexane
Crystalline material was collected as described in the ESI.
Crystalline material was transferred to a vial containing a known
Notes and references
amount of THF at -30
°C using a cold spatula. Once the
crystalline material was completely dissolved, the solution was
transferred to a vial containing THF, bromocyclohexane (15
equiv wrt 2a), and a stir bar at a warmer temperature using a
cold pipette. The crystalline material was allowed to react, and
aliquots were taken at various points over 20 minutes. The
aliquots were quenched in MeOH (50 µL). A known amount of
dodecane was then added to the quenched samples, and the
samples were diluted to 1 mM prior to filtering through silica.
AAS was used to determine the concentration of 2a in solution.
1
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DOI: 10.1039/C8SC02915F
The first direct syntheses, structural characterizations, and reactivity studies of multinuclear iron-phenyl
species formed upon reaction of Fe(acac)3 and PhMgBr in THF are described, where Fe4(µ-Ph)6(THF)4 is
identified as a reactive species for rapid and selective formation of cross-coupled product.