Iron(II) Active Species in Iron-Bisphosphine Catalyzed Kumada and Suzuki-Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides

Article abstract of DOI:10.1021/jacs.5b06648

While previous studies have identified FeMes₂(SciOPP) as the active catalyst species in iron-SciOPP catalyzed Kumada cross-coupling of mesitylmagnesium bromide and primary alkyl halides, the active catalyst species in cross-couplings with phenyl nucleophiles, where low valent iron species might be prevalent due to accessible reductive elimination pathways, remains undefined. In the present study, in situ M?ssbauer and magnetic circular dichroism spectroscopic studies combined with inorganic syntheses and reaction studies are employed to evaluate the in situ formed iron species and identify the active catalytic species in iron-SciOPP catalyzed Suzuki-Miyaura and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl halides. While reductive elimination to form Fe(η⁶-biphenyl)(SciOPP) occurs upon reaction of FeCl₂(SciOPP) with phenyl nucleophiles, this iron(0) species is not found to be kinetically competent for catalysis. Importantly, mono- and bis-phenylated iron(II)-SciOPP species that form prior to reductive elimination are identified, where both species are found to be reactive toward electrophile at catalytically relevant rates. The higher selectivity toward the formation of cross-coupled product observed for the monophenylated species combined with the undertransmetalated nature of the in situ iron species in both Kumada and Suzuki-Miyaura reactions indicates that Fe(Ph)X(SciOPP) (X = Br, Cl) is the predominant reactive species in cross-coupling. Overall, these studies demonstrate that low-valent iron is not required for the generation of highly reactive species for effective aryl-alkyl cross-couplings.

Full text of DOI:10.1021/jacs.5b06648

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Article
Iron(II) Active Species in Iron-Bisphosphine Catalyzed Kumada and Suzuki-
Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides
Stephanie L. Daifuku, Jared L. Kneebone, Benjamin E. R. Snyder, and Michael L. Neidig
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Aug 2015
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Journal of the American Chemical Society
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Iron(II) Active Species in Iron-Bisphosphine Catalyzed Kuma-
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philes and Secondary Alkyl Halides
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Stephanie L. Daifuku, Jared L. Kneebone, Benjamin E. R. Snyder and Michael L. Neidig*
Department of Chemistry, University of Rochester, Rochester, NY 14627 USA
ABSTRACT: While previous studies have identified FeMes (SciOPP) as the active catalyst species in ironꢀSciOPP catalyzed Kuꢀ
2
mada crossꢀcoupling of mesitylmagnesium bromide and primary alkyl halides, the active catalyst species in crossꢀcouplings with
phenyl nucleophiles, where low valent iron species might be prevalent due to accessible reductive elimination pathways, remains
undefined. In the present study, inꢀsitu Mössbauer and magnetic circular dichroism spectroscopic studies combined with inorganic
syntheses and reaction studies are employed to evaluate the inꢀsitu formed iron species and identify the active catalytic species in
ironꢀSciOPP catalyzed SuzukiꢀMiyaura and Kumada crossꢀcouplings of phenyl nucleophiles and secondary alkyl halides. While
6
reductive elimination to form Fe(η ꢀbiphenyl)(SciOPP) occurs upon reaction of FeCl (SciOPP) with phenyl nucleophiles, this
2
iron(0) species is not found to be kinetically competent for catalysis. Importantly, monoꢀ and bisꢀphenylated iron(II)ꢀSciOPP speꢀ
cies that form prior to reductive elimination are identified, where both species are found to be reactive towards electrophile at cataꢀ
lytically relevant rates. The higher selectivity towards the formation of crossꢀcoupled product observed for the monoꢀphenylated
species combined with the undertransmetalated nature of the inꢀsitu iron species in both Kumada and SuzukiꢀMiyaura reactions
indicates that Fe(Ph)X(SciOPP) (X = Br, Cl) is the predominant reactive species in crossꢀcoupling. Overall, these studies demonꢀ
strate that lowꢀvalent iron is not required for the generation of highly reactive species for effective arylꢀalkyl crossꢀcouplings.
37
44
1
. INTRODUCTION
Negishi, and Sonogashiraꢀtype couplings. For ironꢀSciOPP
crossꢀcoupling, it has been proposed that catalysis proceeds
Iron catalyzed CꢀC crossꢀcoupling reactions have attracted
42ꢀ43, 49
through an Fe(II)/Fe(III) radical pathway.
Recent work
significant interest as versatile and cost effective alternatives
to traditional precious metal catalysts, including reactions that
have proven difficult for precious metal catalysts such as
from our group investigating ironꢀSciOPP catalyzed Kumada
crossꢀcoupling of mesitylmagnesium bromide and primary
1
ꢀ6
alkyl halides has identified Fe(Mes) (SciOPP) as the active
crossꢀcoupling of nonꢀactivated alkyl halides. Initially deꢀ
2
50
7ꢀ11
catalytic species. While an analogous mechanism has been
proposed across a range of nucleophiles (e.g. mesityl, phenyl
and alkynyl Grignards, as well as arylꢀborates) with ironꢀ
veloped in the 1970s by Kochi using simple iron salts, mulꢀ
tiple ironꢀbased systems have subsequently been reported
which use reaction additives (e.g. TMEDA, Nꢀheterocyclic
carbenes, NMP), well defined mononuclear iron complexes
42ꢀ44
SciOPP,
the reaction rates, yields, and temperatures vary
significantly between reactions and the possibility that analoꢀ
gous iron(II) active species exist across all of these reactions
has not been evaluated in detail. A fundamental understanding
of the critical differences between these ironꢀbisphosphine
crossꢀcoupling reactions, including iron speciation, sideꢀ
product formation, and the mechanism of catalysis, is critical
to broadening our understanding of the molecular mechanisms
of catalysis in crossꢀcouplings with ironꢀbisphosphines in orꢀ
der to inspire and facilitate the development of improved cataꢀ
lytic protocols.
(
including those ligated by TMEDA and bisphosphine ligands)
or simple iron salts to generate robust catalysts for a wide vaꢀ
riety of reactions including ironꢀcatalyzed Kumada, Negishi
and SuzukiꢀMiyaura crossꢀcoupling.
1
2ꢀ34
Wellꢀdefined ironꢀbisphosphines, as well as the combination
of simple iron salts and bisphosphine ligands, have been
shown to be highly effective crossꢀcoupling systems in the
recent work of Bedford, Chai and Nakamura utilizing dpbz
38ꢀ39
((bisꢀ1,2ꢀ(diphenylphosphino)benzene),
dppe
bearing the dpbz ligand have been shown to be highly efficient
for Negishi coupling and, importantly, recent studies have
isolated a lowꢀspin (S = 1/2) trigonal bipyramidal iron(I) speꢀ
3
5ꢀ37
Xantphos,
40ꢀ41
42ꢀ45
and SciOPP ligands
(Scheme 1A). Preꢀcatalysts
While it has previously been demonstrated in ironꢀSciOPP
catalyzed crossꢀcoupling that mesityl Grignards result in
FeMes (SciOPP) as the active species and similar bisꢀ
2
3
6
phenylated iron(II) species have also been proposed with pheꢀ
nyl nucleophiles, several critical differences exist between
mesitylated and phenylated ironꢀbisphosphines that might lead
to important differences in inꢀsitu iron speciation and, thus, the
underlying mechanisms of catalysis. For example, while sevꢀ
eral isolated bisꢀmesitylated iron(II)ꢀbisphosphine compounds
cies, (dpbz) FeX (X = Cl, Br) (Scheme 1B). This iron(I)
2
species is an effective preꢀcatalyst and, while a specific mechꢀ
anism was not proposed, these results might suggest a potenꢀ
tial Fe(I)/Fe(III) mechanism. Analogous S = 1/2 iron(I) species
have also been identified for this Negishi reaction with dppe.
Nakamura and coꢀworkers have found that the fourꢀcoordinate
40
50ꢀ51
have been reported,
there are notably no reports of the
(
4C) ironꢀbisphosphine complex, FeCl (SciOPP), is an effecꢀ
2
43, 46 42, 47ꢀ48
analogous phenylated iron(II)ꢀbisphosphine compounds,
tive preꢀcatalyst for Kumada,
SuzukiꢀMiyaura,
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Scheme 1. Representative IronꢀBisphosphine Catalyzed Crossꢀ
Coupling Reactions.
species in 2 iron crossꢀcouplings involving phenyl
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nucleophiles.
Recently, our group has established that an experimental
approach combining physical inorganic spectroscopies with
synthetic and reaction studies is a powerful approach for eluꢀ
cidating mechanistic insight in ironꢀbased crossꢀcoupling reacꢀ
50
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tions. Our earlier work demonstrated the utility of Fe
Mössbauer spectroscopy to monitor inꢀsitu formed iron speꢀ
cies at freezeꢀtrapped time points during catalytic and stoichiꢀ
ometric reactions to determine the key components of a cataꢀ
lytic mechanism as well as the kinetics of reaction of specific
iron species with electrophile. Combined with magnetic circuꢀ
lar dichroism (MCD) studies to further define the electronic
structure and the local coordination environment of paramagꢀ
netic iron species, and electron paramagnetic resonance
0
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(EPR), which provides a highly sensitive technique to quantify
the presence of Kramers doublets (S = 1/2, 3/2, etc.) formed
inꢀsitu, this approach is uniquely positioned to provide unꢀ
precedented molecularꢀlevel insight into the nature of ironꢀ
bisphosphine catalyzed crossꢀcoupling with phenyl nucleoꢀ
philes, including the nature of the active iron species and the
potential contributions of disproportionation and reductive
elimination reactions in these systems.
In the study presented herein, we utilize this physical inorꢀ
ganic approach to elucidate fundamental structureꢀactivity
correlations and mechanistic principles governing ironꢀ
SciOPP catalyzed Kumada and SuzukiꢀMiyaura crossꢀ
couplings of phenyl nucleophiles with secondary alkyl halides.
While lowꢀvalent iron species are formed in this chemistry, it
is found to exhibit minimal reactivity towards electrophile. By
contrast, transiently formed monoꢀ and bisꢀphenylated
iron(II)ꢀSciOPP species are observed inꢀsitu prior to reductive
elimination, where both are reactive towards electrophile at
catalytically relevant reaction rates. The higher selectivity
towards the formation of crossꢀcoupled product observed for
the monoꢀphenylated species combined with the underꢀ
transmetalated nature of the inꢀsitu iron species in both Kumaꢀ
Scheme 2. Proposed Mechanism by Nakamura and coꢀworkers
for FeCl (SciOPP) Catalyzed CrossꢀCoupling of Aryl Nucleoꢀ
2
42ꢀ43
philes and Alkyl Electrophiles.
da
and
SuzukiꢀMiyaura
reactions
indicates
that
Fe(Ph)X(SciOPP) (X = Br, Cl) is the predominant reactive
species in crossꢀcoupling. Thus, these studies demonstrate that
formation of a lowꢀvalent iron species is not required for the
generation of highly reactive ironꢀbisphosphine catalysts for
arylꢀalkyl crossꢀcoupling.
2
. RESULTS AND ANALYSIS
.1 Reactivity of FeCl (SciOPP) with Phenyl Nucleo-
2
2
philes at Room Temperature. Initial studies focused on the
evaluation of the iron species formed upon reaction of
FeCl (SciOPP) (1-Cl ) with phenyl nucleophiles at room temꢀ
suggesting a significant difference in stability and/or reactivity
with phenyl ligation. The lack of stable phenylated iron(II)ꢀ
bisphosphines likely reflects the additional reaction pathways,
including disproportionation and reductive elimination reacꢀ
tions, accessible for iron species derived from less sterically
hindered aryl nucleophiles. While the recent study on crossꢀ
couplings of MesMgBr and primary alkyl halides with
2
2
perature. Reactions of 1-Cl2 with PhMgBr and tBuPhꢀ
borate/MgBr were performed, which represent the relevant
2
nucleophiles for the Kumada and SuzukiꢀMiyaura reactions,
42ꢀ43
respectively.
While crossꢀcouplings using both tBuPhꢀ
borate/MgBr and nBuPhꢀborate/MgBr were reported in the
2
2
literature depending on the secondary alkyl halide (tBuPhꢀ
borate for bromocycloheptane and nBuPhꢀborate for chlorocyꢀ
FeCl (SciOPP) has supported an Fe(II)/Fe(III) redox cycle
2
50
(Scheme 2), the additional reaction pathways available for
42
cloheptane), reaction studies in our group have demonstrated
phenylated iron(II)ꢀbisphosphines would likely yield the forꢀ
mation of additional reduced iron species that are inaccessible
in mesityl chemistry. Therefore, it has remained unclear if a
similar iron(II) active species would be operative. In fact, it
has recently been suggested that lower valent iron species such
as iron(I) might be more relevant candidates for the active
that tBuPhꢀborate/MgBr gives comparable catalytic perforꢀ
2
mance with chlorocycloheptane (vide infra) and is utilized
herein. The addition of 20 equiv tBuPhꢀborate and 6.7 equiv
5
7
57
MgBr to a 3 mM solution of FeCl (SciOPP) ( Feꢀ1-Cl ) in
2
2
2
o
THF (or 1:1 THF:2ꢀMeTHF) at 25 C led to the rapid change
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Journal of the American Chemical Society
An additional iron sideꢀproduct (3) (Figure 1B and Figure
1
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S1, orange component, δ = 0.46 mm/s and ꢁE = 0.65 mm/s)
Q
is also observed by Mössbauer to be present in ~ 5 % yield for
reactions with PhMgBr to form 2. The observed parameters
are similar to those previously reported for the S = 1/2 iron(I)
5
4
complex Fe(dpbz) Cl (δ = 0.43 mm/s and ꢁE = 0.61 mm/s),
2
Q
suggesting 3 might be a S = 1/2 species. This is confirmed by
0 K EPR spectroscopy which indicates the presence of a
1
phosphine hyperfine split, axial S = 1/2 (g ~ 2) iron species
which spin quantitates to ~ 5 % of the iron in solution (Figure
1
B, inset) – consistent with the quantitation by Mössbauer.
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While this species cannot be observed by Mössbauer spectrosꢀ
copy for the corresponding reaction with tBuPhꢀborate/MgBr2,
spin quantitated EPR indicates < 0.5% of 3 is also formed in
this reaction. While a similar S = 1/2 iron species was previꢀ
ously observed using EPR by Bedford and coꢀworkers in reacꢀ
tions of FeBr L (L = SciOPP analog with SiMe groups reꢀ
2
3
4
1
placing the tBu substituents) with 20 equiv PhMgBr at RT,
no spin quantitation was performed, and the results herein
show that this species is formed as only a very minor compoꢀ
nent in solution. While 3 could be consistent with a lowꢀspin
4
1
iron(I) species as proposed by Bedford and coꢀworkers,
a
mixedꢀvalent iron dimer (or other multiꢀnuclear site) is also
possible though the very minor amounts generated in solution
preclude a more detailed characterization.
6
2.2 Evaluation of Reactivity of Fe(η -biphenyl)(SciOPP).
The formation of 2 upon reaction of 1-Cl with phenyl nuꢀ
2
cleophiles represents a lowꢀvalent, inꢀsitu formed species not
considered in previous mechanistic proposals by Nakamura
nor observed as a byꢀproduct in our previous spectroscopic
studies with mesityl nucleophiles. Thus, the potential reactiviꢀ
ty of this species with electrophile and its role in catalysis
were investigated through both catalytic and pseudo single
turnover reaction studies.
Figure 1. Formation of lowꢀvalent iron species upon reaction of
FeCl (SciOPP) (1-Cl ) with phenyl nucleophiles. 80 K Mössbauer
spectra of the inꢀsitu iron species from reaction of Feꢀ1-Cl
2
2
57
2
with (A)
and (B) 2 equiv PhMgBr at
5 C. The minor S = 1/2 component (3) observed upon reaction with
2
2
0 equiv tBuPhꢀborate / 6.7 equiv MgBr
2
o
Catalytic reactions were performed using tBuPhꢀborate/
PhMgBr (~ 5 % of all iron present) is shown in the inset of (B). (C)
6
MgBr
previously reported in the literature (Scheme 1) using both
-Cl and 2 as preꢀcatalysts (Table 1). Using 3 mol % 1-Cl as
2
and chlorocycloheptane following the reaction protocol
Xꢀray crystal structure of Fe(η ꢀbiphenyl)(SciOPP) (2) with thermal
ellipsoids shown at 50 % probability.
1
2
2
o
the preꢀcatalyst in a 4 h reaction at 25 C, an 89% yield (by
GCꢀFID) of the crossꢀcoupled product phenylꢀcycloheptane
was obtained, similar to the 93% yield reported in the literaꢀ
of the solution from pale yellow to orangeꢀred within 1 min
and further evolution to a dark plumꢀpurple colored solution in
~10 min. The Mössbauer spectrum of the inꢀsitu formed iron
species after 1 h of reaction indicates complete conversion to a
42
ture with nBuPhꢀborate/MgBr2. For comparison, an identical
catalytic reaction was performed using 3 mol % 2 as the preꢀ
single iron species with Mössbauer parameters of δ = 0.44
1
catalyst, generated from the reaction of tBuPhꢀborate/MgBr
2
mm/s and ꢁE = 1.75 mm/s (Figure 1A), and inꢀsitu Hꢀ and
Q
3
1
PꢀNMR spectra are consistent with the formation of a diaꢀ
magnetic iron compound with η ꢀaryl ligation (see SI). Analoꢀ
gous Mössbauer studies indicate that the same plumꢀpurple
6
Table 1. SuzukiꢀMiyaura crossꢀcouplings with FeCl (SciOPP)
2
6
(
1-Cl ) and Fe(η ꢀbiphenyl)(SciOPP) (2) preꢀcatalysts.
2
5
7
iron species is formed from the reaction of Feꢀ1-Cl with 2.2
2
equiv of PhMgBr (Figure 1B, δ = 0.44 mm/s and ꢁE = 1.75
Q
o
mm/s) at 25 C for 1 h. A similar result is achieved with 20
equiv of PhMgBr (Figure S1). A single crystal of this iron
species suitable for study by Xꢀray crystallography was isolatꢀ
ed from the reaction of 1-Cl with 2.2 equiv of PhMgBr that
2
permitted the assignment of this iron species. The crystal
6
structure (Figure 1C) reveals the formation of Fe(η ꢀ
biphenyl)(SciOPP) (2), an iron(0) complex with bidentate
coordination by the SciOPP ligand to the iron and axial coorꢀ
6
53
dination provided by an η ꢀbiphenyl ligand. While the disorꢀ
der observed in this structure precludes a more detailed disꢀ
cussion of the structural parameters, the overall connectivity,
geometry and identity of this species are unambiguous.
a
The results with R = nꢀbutyl are from Ref. 42.
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Page 4 of 28
ure S2). By contrast, under catalytic reaction conditions a rate
of ~7.5 turnovers/hour is expected for SuzukiꢀMiyuara couꢀ
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42
o
pling at 25 C whereas Kumada crossꢀcouplings proceed on
43
the order of multiple turnovers per minute. Furthermore, GC
analysis indicates that cycloheptene is the predominant prodꢀ
uct formed in this reaction (see SI). From our kinetic analysis,
6
it is clear that Fe(η ꢀbiphenyl)(SciOPP) reacts far too slowly
with electrophile to be the active iron species in catalysis. In
addition, the rate of formation of the iron(0) species from reacꢀ
57
tion of Feꢀ1-Cl with 2 equiv PhMgBr at RT is estimated
2
from freezeꢀtrapped Mössbauer studies to occur at 0.12(2)
0
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ꢀ
1
min (vide infra and SI, Figure S6). Thus, it forms at a rate
much lower than that for turnover in the Kumada crossꢀ
coupling. Lastly, 3 remains unperturbed throughout the reacꢀ
tion with chlorocycloheptane (i.e. it remains ~ 5% of all iron
throughout the 45 min of reaction by Mössbauer and EPR (see
SI)) demonstrating that it is significantly less reactive towards
electrophile than 2 and, hence, not kinetically competent to
serve as the active species in catalysis.
2
.3 Iron Species Formed In-Situ Prior to Reductive Elimi-
nation
.3.1 Bisphenylated Iron(II)-SciOPP Species. While 2 is not
2
kinetically competent for catalysis, its formation from the reꢀ
action of 1-Cl with PhMgBr (or tBuPhꢀborate/MgBr ) sugꢀ
2
2
gests that a bisꢀphenylated iron(II) species is likely formed
prior to reductive elimination. Rapid freezeꢀtrapping of the inꢀ
situ formed iron species from the reaction of a 3 mM solution
57
o
of Feꢀ1-Cl with 2 equiv PhMgBr at 25 C in 1:1 THF:2ꢀ
2
MeTHF (total reaction time ~ 30 s) enabled the evaluation of
the iron species formed prior to reductive elimination. The 80
K Mössbauer spectrum of the resulting frozen solution (Figure
3A) indicates the presence of two dominant iron species that
together comprise ~ 94% of the iron in solution. One major
component (4a) is characterized by Mössbauer parameters of δ
Figure 2. 80 K Mössbauer spectra of the inꢀsitu iron species as a
function of reaction time during the reaction of Fe(η ꢀ
biphenyl)(SciOPP) (2) with chlorocycloheptane. The individual
6
=
0.33 mm/s and ꢁE = 1.50 mm/s (62 %, red component) and
Q
Mossbauer components are identified as 2 (purple), 1-X
2
(beige) and
the second (4b) by δ = 0.32 mm/s and ꢁE = 3.13 mm/s
Q
3 (orange).
and 1-Cl as previously described. With 2 as the preꢀcatalyst,
2
6
3% yield of the crossꢀcoupled product was observed after 4 h
o
of reaction at 25 C, which increased to 83 % yield after a total
reaction time to 6 h. Thus, these results demonstrate that 2 can
serve as an effective preꢀcatalyst for crossꢀcoupling, though an
elongated reaction time is required to achieve high product
yield.
Due to the sluggish results of the catalytic reaction using 2
as preꢀcatalyst, pseudo single turnover studies of the reaction
of 2 with excess electrophile were performed to evaluate its
kinetic competence as a potential active catalyst for crossꢀ
coupling. 2 was formed inꢀsitu through the reaction of 2 equiv
57
of PhMgBr with Feꢀ1-Cl for 1 h at room temperature as
2
confirmed by Mössbauer (Figure 2). After the generation of 2,
excess chlorocycloheptane (20 equiv) was added and the subꢀ
sequent reaction was followed by freezeꢀtrapped Mössbauer
spectroscopy as a function of reaction time (Figure 2). While
consumption of 2 is observed, only ~ 1/3 of the iron has reactꢀ
Figure 3. 80 K Mössbauer spectra of the inꢀsitu generated iron speꢀ
5
7
57
ed and converted to 1-X (X = Cl or Br due to halide exꢀ
cies upon reaction of FeCl
2
(SciOPP) ( Feꢀ1-Cl
2
) with 2 equiv
2
o
change) within 45 min (SI, Table S1). By following the
PhMgBr in 1:1 THF:2ꢀMeTHF at (A) 25 C freezeꢀtrapped after 30 s
of reaction time and (B) 0 C freezeꢀtrapped after 5 min of reaction
o
amount of 2 present in solution as a function of reaction time,
ꢀ
3
ꢀ1
time. The individual Mossbauer components are identified as 4a (red),
an observed rate constant of 9.0(5) x 10 min at room temꢀ
perature can be estimated via a firstꢀorder kinetic fit (SI, Figꢀ
4
b (blue) and 3 (orange).
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Table 2.
bisphoshine complexes.
Mössbauer Parameters of mesitylated iron(II)ꢀ
5
0ꢀ51
1
2
3
4
5
6
7
8
9
δ
ꢀE
Q
Complex
Geometry
sq. planar
Sample
(
mm/s)
(mm/s)
3.58
3.67
4.63
4.53
1.71
1.97
2.12
FeMes
2
(SciOPP)
solid
froz. soln.
solid
0.29
0.28
0.31
0.33
0.39
0.52
0.52
FeMes
FeMes
FeMes
2
2
2
(PEt Ph)
2
2
sq. planar
sq. planar
dist. tetra.
dist. tetra
(dppe)
(depe)
solid
solid
FeMesBr(SciOPP)
solid
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
froz. soln.
amount of 3 initially formed upon reaction with nucleophile (~
6 %) is identical within error to the amount present in solution
upon complete reduction to 2 as previously discussed. A firstꢀ
ꢀ
1
order kinetic fit yields an estimated rate of 0.12(2) min for
o
the formation of 2 at 25 C (SI, Figure S6). An analogous reꢀ
o
sult was obtained for the formation of 2 at 0 C (Figure S5 and
Table S3), though the reaction proceeded much more slowly
(~ 28 % 2 after 2.5 h of reaction).
Identification of the remaining two major iron species (4a
and 4b) which together comprise ~ 94% of all the iron in soluꢀ
tion and are consumed to form 2 was possible using the obꢀ
served Mössbauer parameters combined with MCD spectrosꢀ
copy. Previous Mössbauer studies of mesitylated iron(II)ꢀ
bisphosphine complexes provide insight into the two major
species formed upon reaction of 1-Cl with PhMgBr prior to
2
reduction (Table 2). In our previous work, isolated distortedꢀ
square planar FeMes (SciOPP) and distorted tetrahedral
2
FeMesBr(SciOPP) in frozen solution samples exhibited Mössꢀ
bauer parameters of δ = 0.28 mm/s and ꢁE = 3.67 mm/s and
Q
5
0
δ = 0.52 mm/s and ꢁE = 2.12 mm/s, respectively. Chirik
Q
Figure 4. 80 K Mössbauer spectra of the inꢀsitu iron species as a
o
57
and coꢀworkers have also previously reported that the square
function of reaction time at 25 C from reaction of FeCl
2
(SciOPP)
) with 2 equiv PhMgBr in 1:1 THF:2ꢀMeTHF. The indiꢀ
vidual Mossbauer components are identified as 4a (red), 4b (blue), 3
orange) and 2 (purple).
57
planar complexes Fe(Mes) (PEt Ph) and Fe(Mes) (dppe) are
2 2 2 2
(
Feꢀ1-Cl
2
characterized by Mössbauer parameters of δ = 0.31 mm/s and
ꢁE = 4.63 mm/s and δ = 0.33 mm/s and ꢁE = 4.53 mm/s,
(
Q
Q
51
respectively. Chirik also reported that the bisphosphine comꢀ
plex Fe(Mes) (depe) exhibited a similar isomer shift (δ = 0.39
2
(32 %, blue component). There is a third minor species present
mm/s) but a much lower quadrupole splitting (ꢁEQ = 1.71
mm/s) which was attributed to the distorted tetrahedral geomeꢀ
characterized by δ = 0.46 mm/s and ꢁE = 0.65 mm/s (6 %,
Q
51
orange component). A similar iron distribution can also be
try of this complex. From these parameters, the two major
o
obtained from the analogous reaction performed at 0 C for 5
species freezeꢀtrapped prior to reductive elimination to the
iron(0) product both exhibit isomer shifts in range expected
for bisarylatedꢀiron(II)ꢀbisphosphine complexes. It should be
o
min (Figure 3B). The 5 K Mössbauer spectrum of the 0 C
reaction yields the same iron speciation and distribution (SI,
Figure S3). Addition of free SciOPP ligand had no effect on
the iron species generated inꢀsitu (SI, Figure S4). Spin quantiꢀ
tated EPR spectroscopy indicates that the δ = 0.46 mm/s is the
S = 1/2 species 3 (SI, Figure S16).
o
31
noted that ꢀ30 C P NMR studies indicate that no free Sciꢀ
OPP ligand is present in this mixture (i.e. no phosphine resoꢀ
nances are observed), inconsistent with the formation of
homoleptic ironꢀate complexes.
o
The generation of 2 as a function of time at 25 C could be
The observation of nearly identical isomer shifts but quite
distinct quadrupole splittings for 4a and 4b might indicate that
they differ in either their geometries and/or coordination numꢀ
bers. MCD spectroscopy is able to identify and characterize
geometric distortions and differences in coordination number
in more detail than Mössbauer as the iron d orbitals (and,
hence, the LF transitions) are very sensitive to coordination
geometry. The nearꢀinfrared (NIR) MCD spectrum of the iron
followed by freezeꢀtrapped Mössbauer spectroscopy (Figure
4). Over the course of 10 min, 53 % of 2 forms from consumpꢀ
tion of the two major iron species initially present upon
transmetalation (4a and 4b) (Table S2). No additional interꢀ
mediates are observed consistent with direct reduction from
these species to form 2. Importantly, 3 is formed upon initial
reaction with nucleophile and remains constant within error
throughout this reaction by both Mössbauer and EPR (SI, Figꢀ
ure S16), arguing against its intermediacy in the reduction to
form 2. This is also consistent with the observation that the
species formed inꢀsitu from reaction of 1-Cl with 2 equiv of
2
o
PhMgBr, freezeꢀtrapped after 5 min of reaction at 0 C in 1:1
THF:2ꢀMeTHF (Figure 5C) shows the presence of four LF
ꢀ1
transitions at ~ 5600, 6560, 8480 and 13500 cm , consistent
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57
57
Figure 5. Mössbauer and MCD characterization of the inꢀsitu formed iron species from the reaction of FeCl
2
(SciOPP) ( Feꢀ1-Cl
2
) with 2
o
equiv PhMgBr at 0 C. (A, B) The 80 K Mössbauer and (C, D) 5 K, 7T NIR MCD spectra of the inꢀsitu iron species from reaction in (A, C) 1:1
THF:2ꢀMeTHF and (B, D) 1:1 Et O:isopentane. Saturation magnetization data (dots) and best fit (lines) collected at (E) 5260 cm and (F)
2
ꢀ1
ꢀ1
ꢀ1
2
13333 cm in 1:1 THF:2ꢀMeTHF and at (G) 5320 cm in 1:1 Et O:isopentane.
with the presence of at least two different major iron(II) speꢀ
cies in solution (as previously determined by Mössbauer). The
presence of multiple LF transitions at low energy (< 10000
NIR MCD spectrum of this species contains LF transitions at
ꢀ1
~ 5660, 6730 and 8770 cm , consistent with a distorted tetraꢀ
5
hedral iron(II) species where all degeneracy in the T derived
2
ꢀ1
55
cm ) is indicative of the presence of a distorted tetrahedral
excited stated has been removed. Saturation magnetization
5
5ꢀ56
ꢀ1
ꢀ1
highꢀspin iron(II) component.
The band at ~13500 cm is
data collected at 5320 cm are wellꢀfit to a –ZFS S = 2
ꢀ
1
too high in energy to be consistent with a tetrahedral species
and is indicative of a 5ꢀcoordinate distorted squareꢀpyramidal,
groundꢀstate model with parameters (δ = 2.8 ± 0.2 cm and g

ꢀ1
= 9.4 ± 0.2, corresponding to D = ꢀ11 ± 1 cm and E/D =
0.30 ± 0.03) identical within error to those determined for the
low energy component in THF:2ꢀMeTHF (Figure 5G). Comꢀ
bined, the ligandꢀfield energies, ground state parameters and
Mössbauer parameters of this species indicate that it is the
distorted tetrahedral component in the mixture of bisphenylatꢀ
ed iron(II)ꢀSciOPP species formed in THF or THF/2ꢀMeTHF.
Furthermore, the lack of formation of the distorted fiveꢀ
coordinate component in diethyl ether or diethyl
ether:isopentane, indicate that the additional ligand in THF
containing solvents is likely THF.
5
5
highꢀspin iron(II) species. Saturation magnetization data
ꢀ
1
ꢀ1
collected at 5260 cm and 13333 cm are both wellꢀfit to S =
2 ground state models, but with very different ground state
ꢀ
1
parameters (Figures 5E and 5F). The band at 5260 cm is
wellꢀfit to a negative zeroꢀfield split (–ZFS) S = 2 ground state
ꢀ
1
model with δ = 3.0 ± 0.2 cm and g = 9.5 ± 0.2, correspondꢀ

ꢀ
1
ing to D = ꢀ11 ± 1 cm and E/D = 0.31 ± 0.02 whereas the
band at 13333 cm is wellꢀfit to a +ZFS S = 2 ground state
ꢀ
1
ꢀ
1
model with D = 8 ± 1 cm and E/D = 0.22 ± 0.03. The difꢀ
ferent observed groundꢀstates are consistent with each transiꢀ
tion being associated with a distinct highꢀspin iron(II) comꢀ
plex. Combined with the previous analysis of the LF transiꢀ
tion energies, the NIR MCD data indicate that the two major
bisphenylated iron(II) species observed by Mössbauer correꢀ
spond to a distorted tetrahedral iron(II) species and a distorted
square pyramidal iron(II) species. Lastly, the corresponding 5
K, 7 T UVꢀVis MCD spectrum (SI, Figure S15) of the mixture
Thus, these extensive spectroscopic studies indicate that the
two major iron species formed upon reaction of 1-Cl and 2
equiv PhMgBr prior to reductive elimination are
Fe(Ph) (SciOPP) (4a) and Fe(Ph) (THF)(SciOPP) (4b). These
assignments are further supported by the following observaꢀ
tions: (1) both species reduce to form 2 (vide supra), (2) both
species are consumed at the same rate during reduction conꢀ
sistent with a rapid equilibrium due to solvent coordination
and (3) both species are consumed at the same rate upon reacꢀ
tion with electrophile to generate Fe(Ph)X(SciOPP) (5-X)
2
2
2
ꢀ
1
contains charge transfer (CT) bands at ~ 19500 cm and ~
ꢀ
1
24850 cm , which represent new CT transitions due to phenyl
ligation. It should be noted that because the bisphenylatedꢀ
iron(II) species can access two distinct geometries in solution,
multiple CT bands are expected from both complexes, conꢀ
sistent with the observation of multiple, broad CT transitions.
(
vide infra).
2
.3.2 Fe(Ph)(X)(SciOPP). Based upon the proposed mechaꢀ
42ꢀ43 50
nism by Nakamura
and our previous studies, the forꢀ
The observation of a fiveꢀcoordinate iron(II) component
raises the question of the origin of the fifth ligand in this comꢀ
plex. As the reaction occurs in a potentially coordinating solꢀ
vent mixture (THF/2ꢀMeTHF), freezeꢀtrapped Mössbauer
studies of the inꢀsitu formed iron species upon reaction of
mation of a monoꢀphenylated iron(II)ꢀSciOPP species would
also be expected to be accessible. The reaction of
5
7
57
FeBr (SciOPP) ( Feꢀ1-Br ) with 1 equiv PhMgBr in 1:1
2
2
o
THF:2ꢀMeTHF at 0 C for 5 min was performed and found to
generate a major iron species in solution with Mössbauer paꢀ
rameters of δ = 0.50 mm/s and ꢁE = 2.37 mm/s (77 % of all
iron) (Figure 6A, green component), similar to those previousꢀ
ly observed for FeMesBr(SciOPP). The two observed minor
species correspond to 16% of unreacted 1-Br as well as ~ 7%
of 4a. No EPR active iron species were observed from 10 K
EPR studies. A similar major species is also observed upon the
analogous reaction of Feꢀ1-Cl with 1 equiv PhMgBr at 25
5
7
o
Feꢀ1-Cl with 2 equiv PhMgBr at 0 C were performed as a
2
Q
function of solvent. A similar mixture of iron species is
formed in pure THF (Figure S7). By contrast, in diethyl either
or 1:1 diethylether:isopentane (Figure S11 and Figure 5B)
only a single iron species is formed with Mössbauer parameꢀ
2
ters (δ = 0.33 mm/s and ꢁE = 1.36 mm/s) similar to the paꢀ
rameters for 4a observed in THF:2ꢀMeTHF. The 5 K, 7 T
Q
57
2
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45
46
47
48
49
50
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52
53
54
55
56
57
58
59
60
Figure 6. Spectroscopic and structural characterization of Fe(Ph)Br(SciOPP) (5-Br). (A, B) The 80 K Mössbauer and (C, D) 5 K, 7T NIR
o
MCD spectra of the inꢀsitu iron species from reaction of 1-Br
:1 Et
2
with 1 equiv PhMgBr for 3 min at 0 C in (A, C) 1:1 THF:2ꢀMeTHF and (B, D)
O:isopentane. Saturation magnetization data (dots) and best fit (lines) collected at 6100 cm (C, inset) and 6120 cm (D, inset). (E) Xꢀ
ꢀ1
ꢀ1
1
2
ray crystal structure of 5-Br with thermal ellipsoids shown at 50 % probability.
o
o
C for 1 min or 0 C for 3 min (Figure S8). The 5 K, 7 T NIR
at this temperature over the course of 1 h, the 80 K Mössbauer
spectra (SI, Figure S9 and Table S4) indicate the consumption
of 5-X (72% at t = 1 min to 46% at t = 60 min) with the simulꢀ
MCD spectrum of this species is also very similar to that preꢀ
viously reported for FeMesBr(SciOPP), exhibiting two ligꢀ
5
0
ꢀ
1
andꢀfield (LF) transitions at 6530 and 7870 cm (Figure 6C),
consistent with the formation of a distorted tetrahedral S = 2
iron(II) species. The observation of the transitions for the maꢀ
jor species is consistent with its dominant presence in solution
combined with the very large ꢀε value previously observed for
monoꢀarylated, distorted tetrahedral iron(II)ꢀSciOPP species
i.e. FeMesBr(SciOPP) relative to other iron(II)ꢀbisphosphine
species). The saturation magnetization data for this species
collected at 6100 cm is wellꢀfit to a ꢀZFS S = 2 groundꢀstate
model with δ = 1.6 ± 0.2 cm and g = 8.5 ± 0.2, correspondꢀ
ing to D = ꢀ8 ± 1 cm and E/D = 0.27 ± 0.03. Notably, reꢀ
action of Feꢀ1-Br with 1 equiv of PhMgBr in 1:1 diethyl
ether:isopentane yields an analogous major iron species as
confirmed by Mössbauer and NIR MCD (Figures 6B and 6D,
respectively) with Mossbauer parameters of δ = 0.50 mm/s
taneous generation of two new iron species, 2 and 1-X , in
2
33% and 22% yields, respectively, at t = 60 min. The observaꢀ
tion of these iron products (where formation of 2 is consistent
with generation of the bisphenylated iron(II) species and their
rapid reductive elimination to generate the iron(0) product) is
indicative of disproportionation of 5-X. By a secondꢀorder
kinetic fit of the Mössbauer data, an estimated rate of 4.7(6) x
(
50
ꢀ3
ꢀ1
ꢀ1
o
10 mM min for the disproportionation of 5-X at 25 C was
ꢀ
1
o
determined (SI, Figure S10). At 0 C, no disproportionation of
ꢀ
1
this species is observed over the course of 1.5 h (SI, Figure
S11).

ꢀ
1
57
2
2.3.3 Iron Species Formed In-Situ Utilizing Borate Nucleo-
phile. While the previous evaluation of the phenylated
iron(II)ꢀSciOPP species formed inꢀsitu prior to reductive elimꢀ
ination focused on reactions with PhMgBr (relevant to Kumaꢀ
da crossꢀcoupling), investigation of the species formed upon
and ꢁE = 2.32 mm/s (77 % of all iron) and LF bands at ~
640 and 8260 cm . The small observed differences in these
Q
ꢀ
1
6
reaction with tBuPhꢀborate/MgBr prior to reduction is also
2
parameters are consistent with minor geometric distortions as
a function of solvent. The saturation magnetization data for
important in order to evaluate if similar species are formed
upon transmetalation with nucleophiles relevant to Suzukiꢀ
ꢀ
1
this species collected at 6120 cm is wellꢀfit to a negative ꢀ
Miyaura crossꢀcoupling. The 80 K Mössbauer spectrum of the
ꢀ1
57
ZFS S = 2 groundꢀstate model D = ꢀ7 ± 1 cm and E/D =
freezeꢀtrapped reaction of Feꢀ1-Cl with 20 equiv tBuPhꢀ
2
o
0
.28 ± 0.03, within error of the ground state parameters in
borate / 6.7 equiv MgBr at 25 C for 6 min is shown in Figure
2
THF:2ꢀMeTHF. Based on the Mössbauer and MCD data, the
major iron species formed can be assigned as
Fe(Ph)Br(SciOPP) (5-Br). This assignment is confirmed by
Xꢀray crystallography of single crystals obtained from the
7A. This spectrum contains contributions from four iron speꢀ
cies with identical Mössbauer parameters to species already
investigated: 5-X (62 %, green component), 4a (12 %, red
component), 4b (12 %, blue component) and 1-X (9%, beige
2
o
reaction of 1-FeCl with 1 equiv PhMgBr in diethyl ether. The
component). The analogous reaction at 0 C proceeds much
2
Xꢀray crystal structure (Figure 5E) reveals the formation of the
distorted tetrahedral complex Fe(Ph)Br(SciOPP) (5-Br). The
isolation of the brominated species demonstrates that halide
exchange is facile, a result which is consistent with our previꢀ
ous characterization of monoꢀmesitylated iron(II)ꢀSciOPP
more slowly and after 2 h of reaction 48 % of 5-X is present
by Mössbauer as well as 32 % and 20 % of 4a and 4b, respecꢀ
tively (Figure 7B). Lastly, while not observable by Mössbauer,
3 is detectable by EPR at both reaction temperatures and quanꢀ
titates to < 0.5 % of all iron in solution ꢀ consistent with the
amount of this component observed in solution upon reduction
to form 2 (vide supra). Importantly, these studies demonstrate
that (a) the same phenylated ironꢀSciOPP species are formed
prior to reductive elimination with phenyl nucleophiles for
50
species.
Upon identification of monoꢀphenylated iron(II)ꢀSciOPP,
the stability of the complex was studied as a function of time
o
at 25 C to evaluate its potential disproportionation. By generꢀ
both Kumada and SuzukiꢀMiyaura reactions and
(b)
ating 5-X inꢀsitu (X = Br or Cl due to halide exchange, vide
5
7
transmetalation with tBuPhꢀborate/MgBr occurs much more
supra) by addition of 1 equiv of PhMgBr to Feꢀ1-Cl in 1:1
2
2
o
THF:2ꢀMeTHF at 25 C and monitoring its change with time
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0
Figure 7. 80 K Mössbauer spectra of the inꢀsitu formed iron species
57
57
upon reaction of FeCl
2
(SciOPP) ( Feꢀ1-Cl
freezeꢀtrapped after reaction at (A) 25 C for
2
) with 20 equiv tBuPhꢀ
o
borate / 6.7 equiv MgBr
2
o
5
min and (B) 0 C for 2 h. The individual Mossbauer components are
2
identified as 4a (red), 4b (blue), 5-X (green) and 1-X (beige).
Figure 8. 80 K Mössbauer spectra of the iron species in solution
slowly than the corresponding transmetalation with PhMgBr
as a significant amount of undertransmetalated iron is present
during catalysis upon freezeꢀtrapping during (A) the Kumada reaction
at t = 5 min and (B) the SuzukiꢀMiyaura reaction at t = 45 min of
reaction time. The individual Mossbauer components are identified as
o
even after 6 min of reaction at 25 C with excess tBuPhꢀ
borate/MgBr2.
2
2 (purple), 5-X (green) and 1-X species (beige/orange).
2
.4 Iron Species Present In-Situ During Catalysis
at 45 min into the 4 h reaction, the 80 K Mössbauer spectrum
indicates the presence of 6 % 2 (purple component), 30 % 5-X
The previously described spectroscopic and synthetic studꢀ
ies have defined the iron species formed inꢀsitu upon reaction
(green component) and 60 % of 1-Cl as well as ~ 5 % of a
2
of 1-Cl with PhMgBr and tBuPhꢀborate/MgBr (Table 3).
second 1-X species (X = Br, Cl due to halide exchange) (Figꢀ
2
2
2
Evaluation of the iron species present during catalysis is imꢀ
portant in order to ascertain if any additional iron species may
form and to determine the extent of transmetalation present
during pseudo steadyꢀstate turnover. The 80 K Mössbauer
spectrum of the Kumada catalytic reaction freezeꢀtrapped at 5
min into the 30 min reaction is shown in Figure 8A. Only iron
species corresponding to 2 (22 % of iron, purple component)
ure 8B). 10 K EPR spectroscopy indicates that ~ 0.5 % of 3 is
also present in the SuzukiꢀMiyaura reaction. Thus, for both
the Kumada and SuzukiꢀMiyaura reactions it is clear that durꢀ
ing catalysis iron is highly undertransmetalated, consistent
with the slow rate of nucleophile addition (e.g. Grignard via
syringe pump or the slowly transmetalating borate nucleoꢀ
phile) used in the reported reaction protocols.
and 1-X (X = Br or Cl due to halide exchange, δ = 0.81 mm/s
2
and ꢁE = 2.60 mm/s (21 %), δ = 0.88 mm/s and ꢁE = 3.00
Q
Q
2
.5 Evaluation of the Reactivity of Phenylated-Iron(II)-
mm/s (58 %), brown and orange components, respectively) are
present and no phenylated iron species are observable. 10 K
EPR spectroscopy does indicate the presence of ~ 5 % of 3
which is not observable by Mossbauer within the noise of the
data. For the SuzukiꢀMiyaura catalytic reaction freezeꢀtrapped
SciOPP Species with Electrophile
The previous reaction studies of 2 with electrophile indicatꢀ
ed that this species is not kinetically competent to be the active
catalyst species. In addition, these studies also demonstrated
that 3 is even less reactive towards electrophile at RT. Thereꢀ
fore, evaluation of the potential reactivity of the phenylated
iron(II)ꢀSciOPP species towards electrophile was performed in
order to evaluate their catalytic relevance. While the low staꢀ
bility of these species precluded direct reaction studies with
electrophile using isolated solids of the phenylated species, the
previous freezeꢀtrapped Mössbauer studies demonstrated that
Table 3. Mössbauer Parameters of inꢀsitu formed iron species
with phenyl nucleophiles in 1:1 THF:2ꢀMeTHF.
Complex
Spin State
δ (mm/s)
Q
ꢀE (mm/s)
a
FeX
2
(SciOPP) (1-X
6
2
)
S = 2
S = 0
0.94
0.44
2.80
1.75
o
o
Fe(η ꢀbiphenyl)(SciOPP) (2)
S = 1/2 species (3)
these species could be formed inꢀsitu at both 25 C and 0 C
S = 1/2
S = 2
0.46
0.33
0.32
0.65
1.50
3.13
from reactions of 1-Cl with 1 or 2 equiv PhMgBr, with stabilꢀ
2
o
Fe(Ph)
Fe(Ph)
2
2
(SciOPP) (4a)
ities ranging from > 30 s and > 1 min at 25 C for the bisꢀ and
monoꢀphenylated species, respectively (vide supra) to > 10
(THF)(SciOPP) (4b)
S = 2
o
57
min for both species at 0 C. Following the reaction of Feꢀ1-
a
Fe(Ph)X(SciOPP) (5-X)
S = 2
0.51
2.35
o
Cl with 1 equiv PhMgBr at 25 C for 1 min to generate 5-X,
2
2
0 equiv of bromocycloheptane was added and the subsequent
a
X = Cl or Br due to halide exchange
reaction was quenched after reaction for 5, 30 and 90 s
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Journal of the American Chemical Society
Table 4. GC Analysis of the products formed upon reaction of inꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
situ generaeted monoꢀ and bisꢀphenylated iron(II)ꢀSciOPP with
o
bromocycloheptane at 25 C.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Yields are with respect to iron. A minor amount of biphenyl is also observed (~ 3 %
and 10% for the 1 equiv and 2 equiv PhMgBr reactions, respectively)
Figure 9. 80 K Mössbauer spectra of the iron species in solution (A)
5
7
57
2 2
following reaction of FeCl (SciOPP) ( Feꢀ1-Cl ) with 1 equiv
o
PhMgBr at 25 C in 1:1 THF:2ꢀMeTHF for 1 min and (B) after subꢀ
sequent addition of 20 equiv bromocycloheptane and reaction for 15
s. The individual Mossbauer components are identified as 4a (red), 4b
(
Table 4). GC analysis indicated that the reaction was comꢀ
plete within 5 s, generating crossꢀcoupled product phenylcyꢀ
cloheptane (84 % with respect to iron) as well as some cycloꢀ
heptene (Table 4), consistent with the reported catalytic Kuꢀ
mada reaction with this substrate (84ꢀ92 % phenylcyclohepꢀ
(
2
blue), 5-X (green) and 1-X (beige).
43
It is noteworthy that the time dependent product formation
tene, 9ꢀ14 % cycloheptene). The analogous experiment at 0
o
in reactions using 1 equiv PhMgBr and chlorocycloheptane
(see SI) indicate that cycloheptene is formed at early time
points and does not further increase with reaction time, in conꢀ
trast to the continued increase in the amount of phenylcycloꢀ
heptane generated with time. Furthermore, the freezeꢀtrapped
Mössbauer studies of the inꢀsitu iron speciation (see SI) indiꢀ
cates that 4a and 4b are consumed rapidly (consistent with its
higher reactivity, vide supra), where its consumption correꢀ
lates with the time period of cycloheptene product. Furtherꢀ
more, the total amount of cycloheptene produced in the 1
equiv PhMgBr reaction correlates within error to the amount
of bisꢀphenylated iron present prior to electrophile addition.
Overall, these observation suggest that the cycloheptene
formed in these reactions likely derives predominately from
reactions with bisꢀphenylated iron(II)ꢀSciOPP.
C with bromocycloheptane was found to form a similar prodꢀ
uct distribution after reaction for 5 min (see SI). Reactions
o
o
with chlorocycloheptane at both 25 C and 0 C also yielded
phenylcycloheptane and slightly less cycloheptene, though
extended reaction times were required (see SI). Freezeꢀtrapped
solution Mössbauer spectroscopy for the reaction with 1 equiv
o
PhMgBr and 20 equiv bromocycloheptane at 25 C demonꢀ
strates the generation of 1-X as the iron product formed upon
2
reaction of 5-X with electrophile (Figure 9). The same iron
transformation is also observed over an elongated reaction
o
time with 20 equiv chlorocycloheptane at 0 C (SI, Figure
S12).
By contrast, the inꢀsitu generation of bisꢀphenylated
iron(II)ꢀSciOPP (4a and 4b) from reaction of 1-Cl with 2
2
o
equiv of PhMgBr at 25 C for 30 s followed by addition of 20
Lastly biphenyl is also observed as a product in our GC
studies. While contributions from reductive elimination are
possible, the large amounts of biphenyl observed in reactions
where incomplete conversion had occurred (SI, Table S9) led
to the hypothesis that the chemical quenching of unreacted
phenylated iron(II) species might also lead to biphenyl generaꢀ
equiv bromocycloheptane was found to be less selective toꢀ
wards crossꢀcoupled product, yielding an ~ 1:1 mixture of
crossꢀcoupled product to cycloheptene (Table 4). The total
product yield indicates ~ 2 turnovers per iron. A similar result
o
was obtained for the analogous reaction at 0 C following a
reaction time of 5 min (see SI). As was observed for reactions
with 1 equiv PhMgBr, reactions with chlorocycloheptane at
o
tion. By preꢀforming 4a and 4b at 0 C as previously described
o
o
and directly quenching with either dilute HCl in THF or aqueꢀ
both 25 C and 0 C also yielded phenylcycloheptane and
slightly less cycloheptene (see SI). By following the reaction
ous NaHSO (i.e. no electrophile was added), a significant
4
o
amount of biphenyl (~ 50 % with respect to iron) was obꢀ
served to form from the bisꢀphenylated iron(II) species. Since
no reduction to 2 had occurred prior to quenching (~ 94 % of
iron was iron(II), vide supra), this result demonstrates that the
quenching process itself can lead to biphenyl formation from
the decomposition of phenylated iron(II)ꢀbisphosphines, conꢀ
tributing to the biphenyl observed in incomplete reactions.
Such artificial contributions to biphenyl generation independꢀ
ent of the presence of reduced iron species in solution are imꢀ
portant to be aware of since biphenyl counting methods to
estimate the average oxidation state of iron in solution have
by freezeꢀtrapped Mossbauer at 0 C upon reaction with 2
equiv chlorocycloheptane (in order to reduce the rate of reacꢀ
tion to allow for resolution of the iron transformations), both
bisꢀphenylated iron(II)ꢀSciOPP species are observed to be
consumed at the same approximate rate (i.e. their ratio remains
constant) to generate 5-X (X = Cl or Br due to halide exꢀ
change) (SI, Figure S13). At long reaction times, further reacꢀ
tion to form 1-X is also observed, consistent with the further
2
reaction of the 5-X product species with electrophile. Reacꢀ
tions with chlorocycloheptane are consistent with the rate of
reactivity of the bisꢀphenylated species being higher than that
for the monoꢀphenylated species.
36, 57
been previously employed in the literature.
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3
. DISCUSSION
dominately cycloheptene during reaction with electrophile
and, hence, represents an off cycle species in catalysis. Furꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Obtaining insight into the inꢀsitu formed iron species and
thermore, the observed rate of formation of 2 from reaction of
mechanisms of catalysis in ironꢀSciOPP catayzed crossꢀ
coupling reactions with phenyl nucleophiles represents a sigꢀ
nificant characterization challenge as potential phenylated
iron(II)ꢀSciOPP species may be shortꢀlived and undergo reꢀ
ductive elimination and disproportionation reactions that can
contribute to inꢀsitu speciation. Furthermore, the crossꢀ
coupling reactions with phenyl nucleophiles such as PhMgBr
can be very rapid with multiple turnovers per minute obꢀ
served. Despite these substantial challenges, herein we have
demonstrated that our experimental methodology combining
freezeꢀtrapped Mössbauer, MCD and EPR investigations with
synthetic and reaction studies can enable the elucidation of the
inꢀsitu formed iron species, including the identification of the
catalytically active iron species, with phenyl nucleophiles. The
major iron species identified to form inꢀsitu and their observed
transformations are summarized in Scheme 3.
o
ꢀ1
1-Cl with PhMgBr at 25 C (k = 0.12(2) min ) is slow relaꢀ
2
obs
tive to the average turnover time in the Kumada crossꢀ
coupling reaction, inconsistent with it serving an active role in
the catalytic cycle. 3, which forms immediately upon
transmetalation and is not an intermediate in the reduction
pathway to form 2, exhibits no observable reactivity towards
electrophile at catalytically relevant reaction times, consistent
with it also being an off cycle species. While the very low
yields of 3 preclude a more detailed characterization of this
species, it may represent an iron(I)ꢀSciOPP complexes as sugꢀ
gested by the observed phosphorus hyperfine in EPR and its
similar Mössbauer parameters to previously characterized,
43
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
54
isolable iron(I)ꢀdpbz complexes.
Phenylated iron(II)ꢀSciOPP species formed inꢀsitu upon reꢀ
action of 1-Cl and phenyl nucleophiles (both Kumada and
2
SuzukiꢀMiyaura) could be identified using a combination of
freezeꢀtrapped Mössbauer and MCD spectroscopies for reacꢀ
Lowꢀvalent iron species are readily accessible upon reaction
of FeCl (SciOPP) (1-Cl ) with phenyl nucleophiles (PhMgBr
or tBuPhꢀborate/MgBr ) at 25 C ꢀ the temperature employed
in catalysis. Fe(η ꢀbiphenyl)(SciOPP) (2) is found to be the
dominant lowꢀvalent iron species formed with ironꢀSciOPP,
consistent with reductive elimination of a bisꢀphenylated
iron(II)ꢀSciOPP species. Formation of this iron(0) species is
in stark contrast to ironꢀdpbz and ironꢀdppe chemistry where S
= 1/2 iron(I) is the dominant lowꢀvalent iron species observed
2
2
o
o
o
tions at both 25 C and 0 C. These species include the distortꢀ
ed tetrahedral S = 2 species Fe(Ph)X(SciOPP) (5-X) (X = Br
or Cl due to halide exchange) as well as the bisphenylated S =
2 species Fe(Ph) (SciOPP) (4a) and Fe(Ph) (THF)(SciOPP)
2
6
2
2
(4b). While Fe(Ph)Br(SciOPP) could be further characterized
by Xꢀray crystallography, the reduced stability of the bisꢀ
phenylated species (even at low temperature) precluded their
structural characterization. However, the combination of deꢀ
tailed spectroscopic studies, solvent effects on inꢀsitu iron
speciation and reaction studies confirm their assignments.
Specifically, the bisꢀphenylated iron(II)ꢀSciOPP species 4a
and 4b are observed to directly reduce to form 2 and both speꢀ
cies are consumed at the same rate to form 5-X upon reactions
36, 40
to form upon reductive elimination with aryl nuclophile.
While a S = 1/2 iron species (3) is also observed with ironꢀ
SciOPP and phenyl nucleophiles, it represents a very minor
species in solution (~ 5 % with PhMgBr and < 0.5 % with
tBuPhꢀborate/MgBr ), demonstrating the critical importance of
spin quantitation when using EPR due to the high sensitivity
of this method to low concentrations of EPR active species
2
o
with electrophile at 0 C, consistent with a rapid equilibrium
between the solvent coordinated and uncoordinated species.
(including S = 1/2 iron species). The increased amount of this
species with PhMgBr (which results in more rapid transmetaꢀ
lation than tBuPhꢀborate/MgBr ) and its immediate formation
Pseudo single turnover studies of the reaction of 5-X and
o
4a/4b with electrophile at 25 C indicate that both monoꢀ and
2
upon transmetalation suggest that it may represent an initial
kinetic product in transmetalation reactions. It has been previꢀ
ously suggested that lowꢀvalent iron species are likely candiꢀ
dates for the active catalytic species in ironꢀbisphosphine
crossꢀcoupling reactions with phenyl nucleophiles. However,
reaction studies with electrophile clearly demonstrate that 2
reacts far too slowly to be catalytically relevant, forms preꢀ
bisꢀphenylated iron(II)ꢀSciOPP species are exceptionally reacꢀ
tive towards electrophile to generate crossꢀcoupled product
o
(complete reaction in < 5 s at 25 C with bromocycloheptane
ꢀ
1
corresponding to an observed rate of > 12 min ). However,
reactions using 1 equiv PhMgBr (to form 5-X) yield predomiꢀ
nately crossꢀcoupled product whereas reactions with 2 equiv
5
2
o
PhMgBr at 25 C (to form 4a and 4b) are less selective and
2 2
Scheme 3. The major iron species formed inꢀsitu and their reaction pathways for the FeCl (SciOPP) (1-Cl ) catalyzed Kumada and Suzukiꢀ
o
Miyaura crossꢀcouplings of phenyl nucleophiles and secondary alkyl halides. Estimated rates are given for the transformations at 25 C where
determined in the present study.
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Journal of the American Chemical Society
to track biphenyl formation was found to be inherently unreliꢀ
yield an ~ 1:1 ratio of crossꢀcoupled product and cyclohepꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tene. Thus, while both 5-X and 4a/4b react with electrophile at
catalytically relevant rates suggesting both could contribute to
catalysis, the higher crossꢀcoupled product selectivities obꢀ
served with 5-X indicate that this is likely the dominant active
species in both the Kumada and SuzukiꢀMiyaura crossꢀ
coupling reactions. This is consistent with the observation of
highly undertransmetalated ironꢀSciOPP species during pseuꢀ
do steadyꢀstate catalysis. Furthermore, the slow addition of
PhMgBr for Kumada crossꢀcoupling (~ 1 equiv every 5 s)
combined with the relatively slow rate of transmetalation from
the borate nucleophile in the SuzukiꢀMiyaura reaction would
serve to maximize the formation of 5-X while minimizing the
formation of the less selective bisꢀphenylated species in order
to yield highly selective crossꢀcouplings. The observation of
able as such quenchings of phenylatedꢀiron(II)ꢀSciOPP species
were found to yield biphenyl due to quenching. While biaryl
quantitation methods have been used to estimate iron oxidaꢀ
tion states present in solution during reactions of iron species
36, 57
with aryl nucleophiles,
such contributions to biaryl forꢀ
mation from chemical quenching methods could lead to overꢀ
estimation of the amount of reduced iron present in solution.
Due to this challenge, direct spectroscopic characterization of
the actual iron species present in solution represents a direct
and more reliable method for the evaluation of the inꢀsitu
formed species in crossꢀcoupling.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4. CONCLUSION
In the present study, a combination of spectroscopic methꢀ
ods (Mössbauer, MCD, EPR) and synthetic and reaction studꢀ
ies have enabled elucidation of the inꢀsitu formed iron species,
identification of the active iron species and insight into the
mechanism of catalysis in ironꢀcatalyzed Kumada and Suzukiꢀ
Miyaura crossꢀcouplings of phenyl nucleophiles and secondꢀ
ary alkyl halides. While low valent iron species are observed
to form in this chemistry, these species do not react with elecꢀ
trophile at catalytically relevant rates and predominately yield
cycloheptene. Monoꢀ and bisꢀphenylated iron(II)ꢀSciOPP speꢀ
cies are identified that form prior to reductive elimination and
exhibit reactivity with electrophile at catalytically relevant
rates. The higher selectivity towards the formation of crossꢀ
coupled product observed for the monoꢀphenylated species
combined with the undertransmetalated nature of the inꢀsitu
iron species in both Kumada and SuzukiꢀMiyaura reactions
indicates that Fe(Ph)X(SciOPP) (X = Br, Cl) is the predomiꢀ
nant reactive species in crossꢀcoupling. Overall, these studies
demonstrate that lowꢀvalent iron is not required for the generaꢀ
tion of highly reactive species for effective arylꢀalkyl crossꢀ
couplings. The further application of this research methodoloꢀ
gy to additional ironꢀbased crossꢀcouplings systems should
evaluate the broader relevance of iron(II) active species in
these reactions.
5
-X by Mössbauer during the steadyꢀstate SuzukiꢀMiyaura
reaction is consistent with the slower rate of reaction of 5-X
with the chlorocycloheptane employed in the SuzukiꢀMiyaura
reaction compared to reactions with bromocycloheptane. Furꢀ
thermore, these results represent an interesting contrast to the
Kumada crossꢀcoupling of MesMgBr and primary alkyl halꢀ
ides with 1-Cl , where slow Grignard addition was found to be
2
ꢀ
important for minimizing inꢀsitu formation of FeMes (which
3
is reactive but not highly selective in catalysis) through Sciꢀ
50
OPP ligand dissociation in the presence of excess MesMgBr.
Lastly, the observation that reaction of 5-X with electrophile
results in the formation of 1-X and crossꢀcoupled product is
2
consistent with previous mechanistic proposals involving an
Fe(II)/Fe(III) redox cycle in ironꢀSciOPP catalyzed crossꢀ
39, 43, 50
coupling with aryl nucleophiles.
While previous studies demonstrated that FeMes (SciOPP)
2
was the active catalytic species in Kumada crossꢀcouplings of
50
MesMgBr with primary alkyl halides, the bulky mesityl nuꢀ
cleophile and the stability of the inꢀsitu formed
FeMesBr(SciOPP) and FeMes (SciOPP) species have led to
2
suggestions that less bulky aryl nucleophiles might more likely
result in lowꢀvalent iron(I) or iron(0) iron active species in
5
2
catalysis. Consistent with this hypothesis, iron(I)ꢀdpbz comꢀ
plexes including the arylated species (dpbz) Fe(4ꢀtolyl) have
2
5
. EXPERIMENTAL
been isolated which can serve as effective preꢀcatalysts for
Negishi crossꢀcoupling with (4ꢀtolyl) Zn nucleophile. Howevꢀ
5.1 General Considerations. All reagents were purchased from
2
er, the observation of highly reactive phenylated iron(II)ꢀ
SciOPP species in the present study that react with electroꢀ
phile to form crossꢀcoupled products at rates exceeding that
for reductive elimination to form 2 clearly demonstrates that
lowꢀvalent iron is not required to generate highly reactive aryꢀ
latedꢀironꢀbisphosphine species for effective crossꢀcoupling.
While it is possible that ironꢀdpbz crossꢀcoupling does involve
iron(I) active species due to the prevalence of this system for
reduction to iron(I) compared to reduction to iron(0) with the
more sterically bulky SciOPP ligand, it is clear that iron(II)ꢀ
bisphosphines complexes can serve as active species for crossꢀ
couplings with phenyl nucleophiles. Therefore, iron(II) comꢀ
plexes as well as lowꢀvalent iron species must also be considꢀ
ered as potential active species in mechanistic studies of ironꢀ
based crossꢀcoupling reactions.
commercial sources. All air and moisture sensitive manipulations
were carried out in an MBraun inertꢀatmosphere (N ) dry box
2
equipped with a direct liquid nitrogen inlet line. All anhydrous solꢀ
vents were further dried using activated alumina/4Å molecular sieves
31
and stored under inertꢀatmosphere over molecular sieves. P NMR
resonances are referenced to the external standard phosphorus signal
57
57
of 85 % H
3 4 2 2
PO . FeCl (SciOPP) and FeBr (SciOPP) were prepared
57 57
following literature methods from FeCl
2
ꢀ 1.5THF and FeBr
FeCl2•1.5THF and FeBr were synthesized following
58
2
, reꢀ
42 57
57
spectively.
2
57
literature procedures using Fe metal (95% enriched) purchased
from Isoflex. Nꢀbutylphenyl pinacolborate (Phꢀborate) was syntheꢀ
4
2
sized according to literature methods.
6
5
.2 Synthesis of Fe(η -biphenyl)(SciOPP) (2). A 0.978 mmol
(
1.00 g) sample of FeCl (SciOPP) was added to approximately 25 mL
2
of a 6:1 (v:v) solution of THF and 1,4ꢀdioxane. The resulting pale
yellow solution was chilled to ꢀ30 °C on a cold plate prior to the slow
addition of 2.2 equiv of phenylmagnesium bromide (2.15 mL of 1.0
M THF solution) with stirring. As the addition progressed, the reacꢀ
tion mixture turned orange, with precipitation of magnesium salts.
These salts were removed by filtering the reaction mixture through a
pad of Celite and, as the filtrate warmed to room temperature, it took
on a deep, plumꢀpurple color. The filtrate was concentrated to nearꢀ
dryness under vacuum, and the resulting residue, which is exceedingꢀ
ly soluble in every common hydrocarbon solvent, was reꢀdissolved in
Lastly, the identification of highly reactive iron(II) species
such as 5-Br in Kumada and SuzukiꢀMiyaura crossꢀcoupling
of phenyl nucleophiles with secondary alkyl halides was posꢀ
sible due to the combination of freezeꢀtrapped spectroscopic
studies sensitive to iron speciation and reaction/GC studies
derived from reactions of electrophile with known iron distriꢀ
butions present inꢀsitu. By contrast, chemical quenching alone
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5
mL of a ~1:1 v:v solution of hexane and pentane solution and reꢀ
band EPR spectra were recorded on a Bruker EMXplus spectrometer
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
filtered through celite. The filtrate was concentrated to dryness under
equipped with a 4119HS cavity and an Oxford ESRꢀ900 helium flow
cryostat. The instrumental parameters employed for all samples were
as follows: 1 mW power; time constant 41 ms; modulation amplitude
8 G; 9.38 GHz (5K spectra)/9.83 GHz (298 K spectra); modulation
frequency 100 kHz. Spin integration was performed on samples exꢀ
hibiting S = 1/2 EPR spectra and were spin integrated using a 3 mM
1
vacuum to yield a purple solid. H NMR (500 MHz, THFꢀd
8
: δ 7.40 –
7
.30 (m, 14H), 7.09 (br, 2H), 6.88 – 6.76 (m, 5H), 5.66 (s, 1H), 4.48
31
(
s, 4H), 1.21 (s, 72H). P NMR (202 MHz, THFꢀd
0 K Mössbauer: (powder) δ = 0.44 mm/s, ꢀE = 1.81 mm/s; (1:1
THF:2ꢀMeTHF) δ = 0.44 mm/s, ꢀE = 1.75 mm/s. Calcd for
Fe with half an equivalent of 1,4ꢀdioxane: 79.42 C, 8.94 H.
8
): δ 88.98 (s).
8
Q
Q
CuSO
tation parameters were used for both the iron and standard samples.
.7 Reactions of FeCl (SciOPP) and 1 or 2 equiv of PhMgBr
with Electrophile for In-situ Spectroscopic Studies. As an example
4
standard under nonꢀsaturating conditions. Identical instrumenꢀ
74 98 2
C H P
Found: 79.36 C, 9.03 H. Dissolution of the isolated powder in 5 mL
of a ~1:1 v:v solution of hexane and benzene yielded, after 3 days at ꢀ
5
2
3
0 °C, a moderately airꢀsensitive purple, crystals suitable for Xꢀray
57
of the general procedure employed, the reaction of FeCl
2
(SciOPP)
diffraction.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
with 1 equiv of PhMgBr and 20 equiv of bromocycloheptane is deꢀ
5
.3 Synthesis of Fe(Ph)Br(SciOPP) (5-Br). A 20 mL scintillation
5
7
scribed. To a 1:1 THF:2ꢀMeTHF solution of FeCl
2
(SciOPP) was
2
vial was charged with FeCl (SciOPP) (49 mg, 0.048 mmol) and 7 mL
added dropwise 1.0 equiv PhMgBr at RT to generate 3 mM
diethyl ether. The resulting colorless solution was chilled with stirꢀ
5
7
o
FePhCl(SciOPP) in solution. The solution was stirred for 30 s and
ring to ꢀ30 C and 1.0 equiv of phenylmagnesium bromide (96 μL of a
then a Mössbauer sample was prepared and frozen in liquid nitrogen
0
.5 M solution in diethyl ether) was added dropwise over 30 sec. The
o
within an anaerobic glove box (samples prepared at 0 C were stirred
resulting orange solution was allowed to stir for an additional 5 min
o
for 5 min prior to electrophile addition). To the remaining solution
was added 20 equiv of bromocycloheptane and a sample was immediꢀ
ately frozen in liquid nitrogen to freeze trap the reaction at the desired
time point (15 s).
3
0 sec, and with the temperature maintained at ꢀ30 C the solvent
volume was reduced in vacuo to ~ 1 mL. At this time 1 mL more of
o
chilled diethyl ether (ꢀ30 C) was added to the mixture and the susꢀ
o
pension filtered through a pad of celite (preꢀchilled to ꢀ30 C). The
5
.8 Reaction of FeCl
2
(SciOPP) and 1 equiv of PhMgBr with
(SciOPP) (0.012 mmol)
concentrated brownꢀorange filtrate was allowed to slowly evaporate at
o
Bromocycloheptane. To a solution of FeCl
2
ꢀ
30 C resulting in the formation of highly temperature and air sensiꢀ
in 1:1 THF:2ꢀMeTHF (3.16 mL) was added dodecane (240 ꢂL, 0.1 M
in THF) and 1 equiv of PhMgBr (120 ꢂL, 0.1 M in THF) and stirred
for 30 s at RT. Then bromocylcoheptane (480 ꢂL, 0.5 M in THF) was
added quickly at room temperature. At 5, 30 and 90 s time points, an
tive pale yellow needles suitable for Xꢀray diffraction analysis within
2
2
0
Q
4 hrs. 80 K Mössbauer (1:1 THF:2ꢀMeTHF): δ = 0.50 mm/s, ꢀE =
ꢀ1
.37 mm/s. VTVHꢀMCD: S = 2, D = ꢀ8 ± 1 cm , E/D = 0.27 ±
ꢀ1
.03. NIR MCD: dꢀd bands at 6530 and 7870 cm .
5.4 Mössbauer Spectroscopy. Solution samples for Fe Mössꢀ
ꢀ5
57
aliquot of the reaction mixture was quenched with 10 M HCl in THF
57
4
(or with aqueous 1 M NaHSO ; no difference in product yields was
bauer spectroscopy were prepared from 3 mM FeCl
2
(SciOPP) or
5
7
observed between quenching solvents) and diluted with THF and
filtered through a pad of Florisil (<200 mesh, Sigma Aldrich). Product
yields and recovery of bromocycloheptane were determined by quanꢀ
titative GC analysis using dodecane as an internal standard.
FeBr
2
(SciOPP) in 1:1 (v:v) THF:2ꢀMeTHF to enable the simultaneꢀ
ous preparation of Mössbauer and MCD samples or in only THF for
catalytic reaction measurements. All samples were prepared in an
inert atmosphere glove box equipped with a liquid nitrogen fill port to
enable sample freezing to 77 K within the glove box. Each sample
was loaded into a Delrin Mössbauer sample cup for measurements
ASSOCIATED CONTENT
57
and loaded under liquid nitrogen. Low temperature Fe Mössbauer
measurements were performed using a See Co. MS4 Mössbauer specꢀ
Supplementary figures and data including Mössbauer, MCD,
EPR, NMR and GC data. This material is available free of charge
via the Internet at http://pubs.acs.org.
2
trometer integrated with a Janis SVTꢀ400T He/N cryostat for measꢀ
urements at 80 K with a 0.07 T applied magnetic field. Isomer shifts
were determined relative to αꢀFe at 298 K. All Mössbauer spectra
were fit using the program WMoss (SeeCo). Errors of the fit analyses
AUTHOR INFORMATION
were the following: δ ± 0.02 mm/s and ꢁE
component fits the quantitation errors were ± 3% (e.g. 70 ± 3%).
.5 Magnetic Circular Dichroism Spectroscopy. All samples for
Q
± 3%. For multiꢀ
Corresponding Author
neidig@chem.rochester.edu.
5
MCD spectroscopy were prepared in an inert atmosphere glove box
equipped with a liquid nitrogen fill port to enable sample freezing to
77 K within the glove box. MCD samples were prepared in 1:1 (v:v)
THF:2ꢀMeTHF (to form low temperature optical glasses) in copper
cells fitted with quartz disks and a 2 mm gasket. Low temperature
MCD experiments were conducted using two Jasco spectropolarimeꢀ
ters. Both instruments utilize a modified sample compartment incorꢀ
porating focusing optics and an Oxford Instruments SM4000ꢀ7T suꢀ
perconducting magnet/cryostat. This setꢀup permits 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. UVꢀvisible MCD
spectra were collected using a Jasco Jꢀ715 spectropolarimeter and a
shielded Sꢀ20 photomultiplier tube. Nearꢀinfrared (NIR) data were
collected with a Jasco Jꢀ730 spectropolarimeter and a liquid nitrogen
cooled InSb detector. All MCD spectra were baselineꢀcorrected
against zeroꢀfield scans. VTVHꢀMCD spectra were analyzed using
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by a grant from the National Institutes
of Health (R01GM111480 to M.L.N.). The authors thank Dr.
William W. Brennessel for assistance in the collection and analyꢀ
sis of Xꢀray crystallographic data.
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9
0
1
2
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9
0
9
7x129mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 21 of 28
Journal of the American Chemical Society
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8x134mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 28
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1
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0
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0
1
02x130mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 23 of 28
Journal of the American Chemical Society
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8
6x113mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 28
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1
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0
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0
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0
1
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9
0
6
4x49mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 25 of 28
Journal of the American Chemical Society
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02x83mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 26 of 28
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0
9
6x76mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 27 of 28
Journal of the American Chemical Society
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8x76mm (300 x 300 DPI)
ACS Paragon Plus Environment

Iron(II)-SciOPP Active Species
Freeze-Trapped Mössbauer Spectroscopy
Journal of the American Chemi Pc a gl Se o2 c8 i eo t fy 28
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+
ꢀꢀ
Br
t < 5 s
at RT
Fe(Ph)X(SciOPP)
(X = Cl
AC
Br)S Paragon Plus Environment
or
+
ꢀꢀ
FeX (SciOPP)
2