Isolation, Characterization, and Reactivity of Fe8Me12-: Kochi's S = 1/2 Species in Iron-Catalyzed Cross-Couplings with MeMgBr and Ferric Salts

Article abstract of DOI:10.1021/jacs.6b03760

Iron-catalyzed cross-couplings with simple ferric salts have been known since the 1970s, pioneered by Kochi for cross-coupling using alkylmagnesium nucleophiles including MeMgBr. While Kochi observed the formation of a S = 1/2 iron species in reactions of simple ferric salts with MeMgBr proposed to be an iron(I) species, the identity of this species has remained undefined for nearly 40 years. Herein, we report the isolation and characterization of [MgCl(THF)₅][Fe₈Me₁₂], which combined with EPR and MCD studies is shown to be consistent with Kochi's S = 1/2 species. Reaction studies with β-bromostyrene demonstrate that this species alone displays minimal reactivity but, when combined with additional MeMgBr, leads to rapid and selective formation of cross-coupled product.

Full text of DOI:10.1021/jacs.6b03760

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Communication
8
12-
Isolation, Characterization and Reactivity of FeMe: Kochi's S = 1/2
Species in Iron-Catalyzed Cross-Couplings with MeMgBr and Ferric Salts
Salvador B Munoz, Stephanie L. Daifuku, William W. Brennessel, and Michael L. Neidig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03760 • Publication Date (Web): 26 May 2016
Downloaded from http://pubs.acs.org on June 4, 2016
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Isolation, Characterization and Reactivity of Fe8Me12 : Kochi’s S =
1/2 Species in Iron-Catalyzed Cross-Couplings with MeMgBr and
Ferric Salts
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Salvador B. Muñoz III, Stephanie L. Daifuku, William W. Brennessel and Michael L. Neidig*
Department of Chemistry, University of Rochester, Rochester, NY 14627 USA
Supporting Information Placeholder
remains one of the longest standing mysteries in transition
metal catalyzed cross-coupling.
ABSTRACT: Iron-catalyzed cross-couplings with simple
ferric salts have been known since the 1970s, pioneered by
Kochi for cross-coupling using simple ferric salts and al-
kylmagnesium nucleophiles including MeMgBr. While
Kochi observed the formation of a S = 1/2 iron species in
reactions of simple ferric salts with MeMgBr proposed to
be an iron(I) species, the identity of this species has re-
mained undefined for nearly 40 years. Herein, we report the
isolation and characterization of [MgCl(THF)5][Fe8Me12],
which combined with EPR and MCD studies is shown to
be consistent with Kochi’s S = 1/2 species. Reaction studies
with β-bromostyrene demonstrate that this species alone
displays minimal reactivity but, when combined with addi-
tional MeMgBr, leads to rapid and selective formation of
cross-coupled product.
Towards the goal of identifying the iron species formed
in-situ in this chemistry, Fürstner and co-workers reported
a
homoleptic tetramethyliron(II) ferrate complex
[(Me4Fe)(MeLi)][Li(OEt2)]2 synthesized from reaction of
MeLi with FeCl3 or FeCl2 in Et2O.15-16 This complex exhib-
its a color change from red to yellow as well as reactivity
towards activated electrophiles when dissolved in THF16
and was also found to be active in both stoichiometric and
catalytic ring-opening/cross-coupling reactions with
MeMgBr.17 However, an iron(II) center is not capable of
generating the S = 1/2 EPR signal observed by Kochi ex-
cept in cases where the iron(II) center is part of a mixed
valence multinuclear system. More recent studies from our
group identified the formation of the homoleptic tetralky-
liron(III) ferrate complex [MgCl(THF)5][FeMe4] from the
reaction of FeCl3 with MeMgBr in THF at low temperature
In the 1970s, Kochi demonstrated that simple ferric salts
are effective in the cross-coupling of alkymagnesium nu-
cleophiles and alkenyl halides.1-6 This work has inspired
significant research in the intervening decades on the de-
velopment of iron-catalyed cross-coupling methods,
demonstrating the ability of iron to effectively promote a
variety of cross-couplings including Kumada, Negishi and
Suzuki-Miyaura reactions.7-12 Despite the advances in iron-
based cross-coupling methodologies, mechanistic insight
into these reactions, including the identification of the ac-
tive iron species responsible for catalysis, has largely re-
mained undefined.
Scheme 1. Spectroscopic Studies and Kochi’s Mechanistic
Proposal for Iron-Catalyzed Cross-Couplings with
MeMgBr and Simple Ferric Salts
While recent studies have begun to define the iron active
species and mechanisms in iron-bisphosphine catalyzed
cross-couplings,9, 13-14 the nature of the iron active species
and mechanisms of catalysis in cross-couplings with simple
iron salts have remained poorly understood. Early mecha-
nistic studies by Kochi and co-workers identified the for-
mation of a S = 1/2 iron species in reaction of simple ferric
salts and MeMgBr5-6 which led to a proposed reaction
mechanism involving an iron(I) active species and an
FeI/FeIII mechanistic cycle (Scheme 1).6 However, the iden-
tity of this S = 1/2 species has never been determined and
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Figure 1. X-ray crystal structure of [MgCl(THF)5][Fe8Me12] (1) with selected bond lengths and angles. Thermal ellipsoids are
shown at 50% probability. The methyl hydrogens are excluded as their positions could not be unambiguously defined in the differ-
ence Fourier map and the hydrogens on the cation are omitted for clarity. The halide on the Mg cation species is disordered with
both Cl and Br coordination (Cl is the major species).
(Scheme 1).18 Upon warming, this distorted square-planar S
= 3/2 species converts to the S = 1/2 species originally ob-
served by Kochi and co-workers with concomitant for-
mation of ethane, consistent with its intermediacy in the
reduction pathway of FeCl3 to generate the reduced S = 1/2
species.
Fe-Fe distances range from 2.64 to 2.81 Å, which differ
considerably from those in 1, which range from 2.4188(15)
to 2.4514(15) Å. Due to the diamond-shaped faces, the Fe-
Fe distances in 1 across the short face diagonals range from
2.7988(14) to 2.8295(14) Å. Atoms Fe1, Fe2, Fe3, and
Fe4, which sit at the vertices of an approximate tetrahe-
dron, are each ligated facially by three iron atoms and three
methyl groups, with Fe-C bond lengths ranging from
2.135(9) to 2.175(8) Å. Atoms Fe5, Fe6, Fe7, and Fe8,
which also sit at the vertices of a distorted tetrahedron that
interleaves the other, are each ligated facially by three iron
atoms and also have close contact to methyl groups at Fe-C
distances ranging from 2.311(8) to 2.366(9) Å. Unfortu-
nately, the methyl group hydrogen atoms could not be lo-
cated unambiguously in the difference Fourier map and,
hence, they ultimately were not modeled. However, it is
worth noting that models that either minimze H…H close
contacts or that orient the methyl groups to favor agostic
interactions with the more distant of the two iron centers
might be possible (see SI). There are reported structural
precedents in which an asymmetric bridging methyl group
participates in one Fe-C σ bond and one C-H…Fe η2 agos-
tic interaction.24 Unfortunately, other experiments that usu-
ally offer insight into the presence of such agostic interac-
tions, including vibrational analysis, have not been viable
to date due to the highly thermally-sensitive and paramag-
netic nature of this material. Lastly, the electronic structure
of 1 is likely complex due to the direct Fe-Fe bonding and
formally mixed valence nature of the cluster. While calcu-
lations of this complex are beyond traditional DFT meth-
ods, detailed studies of the electronic structure of 1 are an-
ticipated to be of significant interest within the theoretical
community.
Herein we report the isolation and characterization of
[MgCl(THF)5][Fe8Me12] (1) from reaction of FeCl3 with
MeMgBr in THF, identified via EPR and near-infrared
(NIR) magnetic circular dichroism (MCD) as Kochi’s pre-
viously observed S = 1/2 species. This novel iron cluster is
demonstrated to be reactive with electrophile and effective
for the generation of cross-coupled product in the presence
of additional MeMgBr. These results demonstrate the im-
portance of small iron clusters in cross-coupling catalysis,
which represents a new paradigm with regards to the types
of iron species operative in this chemistry.
Our previous studies of the thermal decomposition of
[MgCl(THF)5][FeMe4] indicated that warming towards
room temperature was required in order for reductive elim-
ination of ethane to occur with concomitant formation of
Kochi’s S = 1/2 species. Combined with Kochi’s previous
observation that this S = 1/2 species is ultimately unstable
in solution at room temperature and decays within 15 min,4
we envisioned that initial reaction of FeCl3 with MeMgBr
at low temperature followed by warming to promote the
formation of the S = 1/2 species and then rapid cooling to
disfavor its decomposition would provide a pathway for the
isolation of this species. The reaction of FeCl3 with 5 equiv
o
o
of MeMgBr at - 80 C followed by warming to 0 C for 5
o
min and then immediate cooling to -80 C yielded a yellow
brown solution from which dark brown single crystals of 1
suitable for X-ray diffraction could be obtained upon layer-
ing of the THF solution with pentane at -80 oC.
While 1 represents a novel iron-methyl cluster complex
formed upon reaction of FeCl3 with MeMgBr in THF, it
was important to spectroscopically characterize 1 in solu-
tion for comparison to the S = 1/2 species formed in-situ as
previously observed by Kochi.5 The 5 K EPR spectrum of
crystals of 1 in THF (Figure 2, top), prepared from dissolu-
The single crystal X-ray diffraction structure of this high-
ly air and temperature sensitive complex contains a
MgX(THF)5 cation, X = 0.92 Cl and 0.08 Br, and a distort-
-
ed heterocubane Fe8Me12 anion with faces that are dia-
mond-shaped (Figure 1). To the best of our knowledge,
only five other Fe8 cubic structures are reported in the
CSD19-23 and all are near-perfect cubes with square faces
and µ4-bridging sulfido ligands capping the faces. Their
o
tion of crystals of 1 in THF at -80 C to prevent thermal
decomposition, contains a broad S = 1/2 signal, analogous
to that previously observed by Kochi5-6 and confirmed by
our group to form in-situ upon reaction of FeCl3 with 5
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equiv or 20 equiv of MeMgBr (Figure 2, bottom). A similar previously identified S = 1/2 species. The solid state EPR
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broad S = 1/2 EPR signal was also reported by Kochi for
the analogous reaction with Fe(acac)3.5-6 EPR spin quantita-
tion indicates that the S = 1/2 species represents (within
error) effectively all of the iron formed in solution. While
the EPR data is consistent with 1 being the previously uni-
dentified S = 1/2 species, such broad EPR signals alone are
insufficient to definitively prove that these species are iden-
tical. NIR MCD is a much higher resolution probe of geo-
metric and electronic structure and the NIR MCD spectrum
of 1 in THF/2-Me-THF (Figure 3, top), prepared from dis-
and NIR MCD spectra of 1 (see SI) are consistent with the
solution data, displaying only differences in signal broad-
ness (EPR) or individual transition intensities (MCD) in-
dicative of slight differences in the distributions of bond
lengths, angles and/or close contact interactions in solution
versus the solid state. Lastly, while the Mossbauer spec-
trum of 1 could not be obtained due to low crystal yields,
the 80 K Mössbauer spectrum of the in-situ formed S = 1/2
species using Fe(acac)3 is described by a broad doublet
signal (see SI).
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o
solution of crystals of 1 in 1:1 THF:2-MeTHF at -80 C to
Beyond the important demonstration that the in-situ
-
prevent thermal decomposition, is consistent with the anal-
ogous spectrum for the in-situ generated S = 1/2 species
from FeCl3 (Figure 3, middle) or Fe(acac)3 (Figure 3, bot-
tom) in 1:1 THF:2-MeTHF, demonstrating that 1 is Kochi’s
formed S = 1/2 species is Fe8Me12 , the most critical ques-
tion is whether this species can serve as an effective and
reactive species for forming cross-coupled product. Hence,
studies of the reaction of 1 with electrophile were per-
formed in order to evaluate the ability of this cluster to
form cross-coupled product. β-bromostyrene was selected
as the electrophile for this study due to the formation of
non-gaseous cross-coupled product for robust GC analysis
and its ability to serve as an effective electrophile in cross-
couplings with ferric salts (see SI for catalytic data). Prior
to performing the reaction studies, it was first necessary to
evaluate the lifetime of 1 in THF at room temperature.
o
Crystals of 1 previously held at -80 C were added to a
known volume of room temperature THF and, as a function
of time, aliquots were freeze-trapped for EPR analysis. As
shown in Figure 4A, 1 slowly decomposes in room temper-
ature THF with a decay of ~ 5 % per minute over the
course of 5 minutes (Figure 4A inset). Thus, reactions with
β-bromostyrene performed within 2-3 minutes would have
limited decomposition of 1. For electrophile reactions,
crystals of 1 were dissolved in a known volume of RT THF,
an EPR sample was freeze-trapped 40 s after dissolution to
determine the amount of 1 present by spin quantitated EPR,
and then a known amount of β-bromostyrene was added.
After addition of β-bromostyrene, an aliquot of the reaction
was chemically quenched after 40 s for analysis by GC.
Using the spin-quantitated EPR of 1 prior to electrophile
addition, it was determined that these reactions
Figure 2. 5 K EPR Spectra of 1 (top) and Kochi’s in-situ
formed S = 1/2 species (bottom).
Figure 3. 5 K, 7 T Near-Infrared MCD Spectra of 1 (top) and
Kochi’s in-situ Formed S = 1/2 Iron Species from Reaction of
20 equiv MeMgBr with FeCl3 (middle) or Fe(acac)3 (bottom).
All spectra were collected in 1:1 THF:2-MeTHF.
Figure 4. EPR studies of (A) the thermal stability in THF at
RT and (B) the changes in the amount of 1 in solution in reac-
tion studies with electrophile. The inset in (A) shows a linear
fit of the decay of 1 in RT THF over the course of 5 min.
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iron species. Future studies to further elucidate the un-
Scheme 2. Reactions of 1 with β-bromostyrene
1
2
3
4
derlying mechanism on the reactions of 1 with electro-
phile will further develop our understating of iron-
catalyzed cross-coupling reactions using ferric salts.
ASSOCIATED CONTENT
5
6
7
8
9
Supporting Information. Experimental methods and sup-
plementary data including EPR, MCD, Mössbauer and X-
ray crystallographic data. The Supporting Information is
available free of charge on the ACS Publications website.
involved ~ 15 mM of 1 and ~ 0.5 equiv of β-bromostyrene
for each run. While cross-coupled product does form in
this reaction, only ~ 5% of β-methylstyrene (with respect to
β-bromostyrene) was found to form after 40 s of reaction
(Scheme 2). No significant increase in product was ob-
served at extended reaction time (~ 6 % after 120 s) and no
additional side products were observed.
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AUTHOR INFORMATION
Corresponding Author
neidig@chem.rochester.edu
Notes
Despite the minimal formation of cross-coupled prod-
uct following 40 s of direct reaction of 1 with β-
bromostyrene, EPR analysis of the reaction freeze-
trapped 40 s after electrophile addition indicated signifi-
cant consumption of 1 upon addition of electrophile
(Figure 4B) though no new EPR active species are ob-
served to be generated. Interestingly, in Kochi’s original
studies it was suggested that reaction of the iron active
species with electrophile might first form an initial in-
termediate species which then required reaction with
additional MeMgBr to form cross-coupled product.6 To
test this hypothesis for 1, an experiment was also per-
formed where 40 s after the addition of β-bromostyrene
additional MeMgBr was added (1.1 equiv with respect
to β-bromostyrene) and the reaction quenched following
an additional 50 s of reaction time. With the addition of
MeMgBr, essentially complete conversion of the elec-
trophile to selectively form cross-coupled product was
observed (99% yield with respect to electrophile)
(Scheme 2). Furthermore, EPR of the reaction solution
freeze-trapped 50 s after MeMgBr addition demonstrates
the near-quantitative re-formation of 1 in-situ (see SI).
Overall, these results are consistent with 1 being an ef-
fective reactive species for the formation of cross-
coupled product in the presence of additional MeMgBr.
While challenging, future studies will be directed to-
wards the identification of the initial intermediate spe-
cies formed upon reaction of 1 with electrophile in order
to further probe the underlying mechanism of this reac-
tion.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was support by a grant from the National Insi-
tutes of Health (R01GM111480 to M.L.N.) and by an Al-
fred P. Sloan Fellowship to M.L.N.
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In summary, the S = 1/2 species formed in-situ in reac-
tions of simple ferric salts and MeMgBr originally ob-
served by EPR in the 1970s by Kochi has been isolated
and identified as [MgCl(THF)5][Fe8Me12]. While the
direct reaction of this species with electrophile generates
minimal cross-coupled product, reaction with electro-
phile followed by addition of MeMgBr leads to rapid
reaction to selectively form cross-coupled product. Im-
portantly, the identification of an iron cluster as a reac-
tive species in iron-catalyzed cross-coupling represents a
new paradigm in catalyst structure for such reactions,
contrasting previous proposals centered on mononuclear
22. Goh, C.; Segal, B. M.; Huang, J.; Long, J. R.; Holm, R. H., J. Am.
Chem. Soc. 1996, 118, 11844-11853.
23. Marsh, R. E.; Clemente, D. A., Inorg. Chim. Acta 2007, 360,
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24. Dawkins, G. M.; Green, M.; Orpen, A. G.; Stone, F. G. A., J.
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Journal of the American ChemiPcaagl eSo1c2ieotfy12
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