Reactivity of (NHC)2FeX2 complexes toward arylborane lewis acids and arylboronates

Article abstract of DOI:10.1021/om401105k

(NHC)₂FeCl₂ complexes undergo methoxide transfer in preference to aryl transfer from [(Aryl)B(OR)₃]^-, while addition of ArylBY₂ (Y = Cl, OR) to (NHC)Fe-methoxide compounds leads only to formation of (

Full text of DOI:10.1021/om401105k

Article
pubs.acs.org/Organometallics
Reactivity of (NHC)2FeX2 Complexes toward Arylborane Lewis Acids
and Arylboronates
Jay J. Dunsford,† Ian A. Cade,† Kathlyn L. Fillman,‡ Michael L. Neidig,‡ and Michael J. Ingleson*,†
†School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: (NHC)2FeCl2 complexes undergo methoxide transfer in
preference to aryl transfer from [(Aryl)B(OR)3]−, while addition of ArylBY2
(Y = Cl, OR) to (NHC)Fe−methoxide compounds leads only to formation of
(NHC)BY2Aryl. The addition of PhBCl2 to (NHC)2FeX2 compounds is
introduced as a method for probing NHC dissociation from iron. Two
expanded ring NHCs also undergo dissociation from iron in the respective
(NHC)2FeCl2 complexes.
Scheme 1. Transmetalation Reactions between Boron
Nucleophiles and Iron Complexes
ell-defined iron(II) complexes have become the subject
Wof growing interest as (pre)catalysts in a number of
important catalytic transformations.1−4 This is particularly true
for carbon−carbon bond formation, where low associated cost
and low toxicity combined with a unique reactivity profile make
iron catalysts an attractive alternative to palladium- and nickel-
based systems.5−7 Important breakthroughs have been recently
reported in this area using both well-defined and in situ
generated iron species primarily bearing phosphine or
monodentate N-heterocyclic carbene (NHC) ligands.8−19
These catalysts have enabled the cross-coupling of alkyl and
aryl halides with a range of organometallic nucleophiles. While
these advances have been significant, challenges in iron-
catalyzed cross-coupling still remain. One in particular is
achieving facile transmetalation, the transfer of the hydrocarbyl
group, from boron-based nucleophiles to iron. This trans-
formation is essential to realize a viable iron-catalyzed Suzuki−
Miyaura reaction. Methodologies that proceed efficiently in
palladium catalysis, specifically aryl boronic acids plus base, do
not effect transmetalation with iron catalysts.20 Instead, anionic
tetraarylborates or aryl boronates activated by coordination of
necessitate investigation with four-coordinate anionic arylbo-
rates and neutral three-coordinate arylboranes, respectively.
Nakamura and co-workers have previously determined that one
successful class of iron catalysts for carbon−carbon cross-
coupling contains an iron center that is coordinatively
unsaturated, is in the +2 oxidation state, and has sufficient
spin density located at the iron center.26 Combining this
precedence with the extensive reports on NHC−Fe complexes
catalyzing C−C cross-coupling,27,28 we have explored the
reactivity of coordinatively unsaturated NHC−FeII complexes
toward boron-based aryl nucleophiles. This study was restricted
to monodentate NHC ligands, as heterocoupling reactions
using (NHC)2FeX2 as catalyst precursors were shown to be
more effectively catalyzed using two monodentate carbenes
than when using a single chelating bidentate biscarbene.17,18
To simplify our transmetalation studies, 2-(p-tolyl)-1,3,2-
dioxaborolane (Tol-Beg; eg = ethylene glycolato,
−OCH2CH2O−) and [Tol-Beg(OMe)]− were utilized as
arylboronic acid surrogates. Arylboronic acids were deliberately
avoided due to their existence in equilibrium with boroxines
and H2O; the latter would lead to rapid hydrolysis of NHC−Fe
bonds. To further facilitate this work, we initially focused on
two known (NHC)2FeCl2 complexes (1, NHC = 5-iPr; 2,
t
organometallic nucleophiles (e.g., BuLi to form [(tBu)ArylB-
(OR)2]−) have proved essential (Scheme 1).21−24 Cocatalysts
such as MgBr2 and Zn(Aryl)2 were also necessary to promote
hydrocarbyl transmetalation from boron to iron even when
using these anionic boron-based nucleophiles.
Because of the considerable potential of an iron-catalyzed
Suzuki−Miyaura reaction that is compatible with conventional
boron-based nucleophiles, we were interested in gaining an
increased understanding of the reactivity of iron compounds
toward common boron transmetalation reagents. In palladium
cross-coupling two competitive transmetalation pathways have
been determined, involving (i) ArylB(OR)2 and a Pd−OR
species and (ii) a Pd−X (X = halide) species and M[ArylB-
(OR)3], with the former being dominant.25 Thus, Fe−X and
Fe−OR species of relevance to C−C bond formation
Received: November 13, 2013
© XXXX American Chemical Society
A
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NHC = IMes; Scheme 2).29,30 The NHCs in 1 and 2 are
attractive, as they have distinct steric properties based on
hexanes. In both cases 1 and 2 were entirely consumed (by 1H
NMR spectroscopy) but the new iron compounds were not
observable in the 1H NMR spectra and frustrated all attempts at
crystallization. Elemental analyses of the products derived from
2 repeatedly gave data that were consistent with NHC: Fe
ratios of <2:1. This suggested NHC dissociation on metathesis,
supported by the observation of free IMes during this reaction
and earlier reactivity studies of 1 and 2 with Na[Tol-
Beg(OMe)]. Related NHC dissociation during metathesis has
previously been observed on combination of (NHC)2FeX2 with
Scheme 2. Previously Reported Iron(II) Complexes of the
General Formula (NHC)2FeCl2 with Calculated %Vbur
NaSMe or MesMgBr.19,32 The 80 K 57Fe Mossbauer spectrum
̈
of the mixture isolated from 2 and 5 equiv of NaOMe
predominantly consisted of three high-spin iron(II) species (all
with isomer shifts >1.0 mm/s; Figure 1A). We attribute these
percent buried volume (%Vbur)31 values calculated from the
reported structures. The ability to vary sterics is important, as it
can have a significant effect on the ability to form the bridging
Fe−(μ-Y)−B species essential for transmetalation.25
Combination of 1 or 2 in THF with Tol-Beg and 5 equiv of
NaOMe added to approximate classic cross-coupling conditions
led in each case to complete consumption of the NHC−Fe
starting material and formation of new iron compounds that
1
were not observable in the H NMR spectra. In the reaction
starting with 2 the only boron-containing species observed in
the 11B NMR spectrum was (IMes)Tol-Beg (the identity of all
boron-containing compounds was confirmed by independent
synthesis) formed by loss of NHC from an iron compound,
which then coordinates to Tol-Beg. However, with 1 both (5-
iPr)Tol-Beg and Na[Tol-Beg(OMe)] were observed in an
approximate 1:1 ratio at short reaction times. On standing this
mixture converted to predominantly (5-iPr)Tol-Beg after 16 h.
The complete absence of Beg(OMe) (or its NHC adduct)
and/or [Beg(OMe)2]− precluded aryl transfer to iron in both
reactions. This is consistent with the complete absence of cross-
coupling on addition of cycloheptyl bromide to the mixtures
derived from 1 (or 2), NaOMe, and Tol-Beg. To exclude the
possibility that rapid formation of Fe−OMe species by
metathesis with NaOMe is consuming all Fe−Cl species before
any direct reaction of Fe−Cl with Na[Tol-Beg(OMe)] can
occur, stoichiometric Na[Tol-Beg(OMe)] was added to 1 and
2. In both cases this also resulted in the formation of
(NHC)Tol-Beg as the only new boron-containing species. The
transfer of MeO− to Fe−Cl species therefore occurs in
preference to the transfer of aryl from [Tol-Beg(OMe)]− (eq
1). Selective MeO− transfer is not limited to NHC−Fe
Figure 1. 57Fe Mossbauer spectra at 80 K of (A) the product mixture
from reaction of 2 with 5 equiv of NaOMe and (B) the solid-state
multimeric product formed upon reaction of 5-iPr and 2 equiv of
FeCl2 in THF. For both spectra the raw data (black dots), total fits
(black lines) and individual species fits are given. The best fit to
spectrum (A) contains the individual components δ = 1.28 mm/s,
ΔEQ = 3.06 mm/s (46%, red), δ = 1.17 mm/s, ΔEQ = 2.44 mm/s
(29%, blue), δ = 1.15 mm/s, ΔEQ = 1.93 mm/s (14%, green) and δ =
0.17 mm/s, ΔEQ = 1.30 mm/s (11%, purple). The best fit to spectrum
B contains the individual components δ = 0.88 mm/s, ΔEQ = 1.41
mm/s (77%, red), δ = 0.92 mm/s, ΔEQ = 2.81 mm/s (16%, blue) and
δ = 1.06 mm/s, ΔEQ = 2.20 mm/s (7%, green, attributed to 1 on the
basis of NMR data).
to the partially/fully metathesized monomers/oligomers
(IMes)2FeClx(OMe)2−‑x and {(IMes)FeClx(OMe)2−x}2 (x =
0−2). The reaction of this isolated mixture with PhBCl2 (ca. 2
equiv) was informative, leading to Cl/OMe scrambling between
iron and boron, with IMes(Ph)B(OMe)2 and PhB(OMe)2
observed in the 11B{1H} NMR spectrum (Scheme 3). One
1
new iron compound was visible in the H NMR spectrum,
complexes with addition of stoichiometric Na[Tol-Beg(OMe)]
to dppeFeCl2 (dppe = bis(diphenylphosphino)ethane) result-
ing in complete conversion of Na[Tol-Beg(OMe)] to Tol-Beg
(by 11B NMR spectroscopy) consistent with methoxide transfer
to iron.
Scheme 3. Metathesis of 2 with NaOMe and Subsequent
Reactivity with PhBCl2
With no evidence for transmetalation of aryl to well-defined
Fe−X complexes, the ability to preform Fe−OMe complexes
was investigated for subsequent reaction with Tol-Beg. Both 1
and 2 were reacted with an excess of NaOMe followed by
isolation by filtration to remove unreacted NaOMe (which is
poorly soluble in THF), drying in vacuo, and washing with
B
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identifiable as {(IMes)FeCl2}2 (3).30 Complete conversion of
PhBCl2 to (IMes)PhB(OMe)2 and PhB(OMe)2 confirmed that
significant metathesis of chloride in 2 for methoxide had
originally proceeded.
With partial metathesis confirmed, the transmetalation
propensity of the Fe−OMe-containing complexes isolated
from 2 and 5 equiv of NaOMe was examined by combination
with excess Tol-Beg. No new iron products were visible in the
1H NMR spectra from this reaction, and attempts at isolation
and characterization failed. This reaction did, however, lead to
the rapid formation of new 11B NMR resonances attributable to
(Tol)-Beg(OMe) and (IMes)Tol-Beg. With no products from
aryl transfer observed in the 11B NMR spectra, this precludes
transmetalation for these NHC−iron complexes containing
Fe−OMe moieties. Thus, (NHC)Fe−OMe complexes will
undergo anion exchange with B−Cl but not with B−aryl, while
anionic borates will transfer alkoxide to Fe−Cl species in
preference to an aryl moiety.
Figure 2. ORTEP representation of {(5-iPr)FeCl2}4 (4), with
The dissociation of the NHC ligands continually observed
during metathesis/attempted transmetalation indicated labile
NHC→Fe bonds. NHC dissociation from iron is an
increasingly recognized phenomenon that is particularly
prevalent in sterically crowded Fe−NHC complexes.28,34−37
NHC dissociation significantly complicates identification of the
catalytically active species in NHC−Fe catalyzed cross-
couplings, which are generally run with >1 equiv of NHC
relative to iron. Control of the coordination number at iron is a
crucial parameter, as it can drastically affect reactivity. This is
exemplified by a three-coordinate (NHC)Fe(NHR)2 complex
that does not react with aryl iodides, whereas a related four-
coordinate (NHC)2Fe(NHR)2 complex rapidly reacted.38 With
NHC dissociation from (NHC)2FeX2 observed using two
sterically disparate NHCs (based on %Vbur values of 1 and 2),
we sought a simple chemical method to determine if carbene
dissociation is occurring in NHC−Fe complexes. This is
important to provide insight into what catalytic species are
viable in solution. Furthermore, a simple synthetic probe is
desirable, as it is often challenging to unequivocally identify
NHC dissociation in paramagnetic NHC−Fe complexes by
spectroscopic methods. To probe Fe−NHC bond stability, the
reaction of (NHC)2FeCl2 and PhBCl2 was selected, as it
introduces a useful 11B NMR spectroscopic handle and takes
advantage of previously well-characterized (NHC)xFeCl2
complexes. These include monomeric (NHC)2FeCl2 com-
plexes and the dimer {(IMes)FeCl2}2 (3).30 {(NHC)FeCl2}n
oligomers are relevant, as they are the ultimate byproducts
expected on NHC dissociation from (NHC)2FeCl2.
ellipsoids at 50% probability and hydrogens omitted for clarity.
contrast to the dimeric structures reported for 3 and the DIPP
(2,6-diisopropylphenyl-substituted NHC)30 analogue. This
disparity can be attributed to the decrease in steric demand
associated with 5-iPr relative to IMes and DIPP. Dissolution of
1
bulk 4 produced a H NMR spectrum which consisted of only
three resonances (excluding the 5% intractable impurity of 1),
inconsistent with the solid-state tetrameric structure. The data
are more consistent with a dimer presumably in equilibrium
with the tetramer. The solution magnetic moment (Evans
method)39,40 of 7.7 μB at 294 K for 4 was closely comparable to
the dimer {(DIPP)FeCl2}2 (7.5 μB),30 further supporting a
dimeric solution formulation dominating for 4. The 80 K 57Fe
Mossbauer spectrum of a powder sample exhibits a major
̈
feature with δ = 0.88 mm/s, ΔEQ = 1.41 mm/s (Figure 1B,
77%) and a minor species with δ = 0.92 mm/s, ΔEQ = 2.81
mm/s (16%). These are tentatively attributed to the dimer and
the five-coordinate iron centers of the tetramer, respectively.
The similarity of the coordination environment of terminal Fe
in the tetramer to that of iron in the dimer (both distorted
tetrahedral with 1 × 5-iPr, 1 × Clterminal, and 2 × μ-Cl) may
result in coincidence in the Mossbauer spectrum. This
̈
spectrum is thus consistent with the solution-state observations
of a mixture of complexes in which the dimeric complex is
predominant.
With the NMR spectroscopic handles of both (NHC)2FeCl2
and the oligomeric {(NHC)FeCl2}x forms in hand for both
IMes and 5-iPr, the combination of (NHC)2FeCl2 and PhBCl2
To expand the number of well-characterized oligomeric
species, {(5-iPr)FeCl2}n was first targeted. It is noteworthy that
a stoichiometric combination of 5-iPr and (THF)1.5FeCl2 in
THF led to the formation of both 1 and a new (5-iPr)nFeCl2
1
in C6D6 was investigated (Figure 3). Analysis of the H NMR
spectrum recorded minutes after addition of 1 equiv of PhBCl2
to 1 revealed the disappearance of all signals corresponding to 1
and new signals corresponding to 4. Analysis of the 11B NMR
1
complex in an approximate 2:1 ratio (by H NMR spectros-
11B
copy). The new iron compound could be synthesized as the
spectrum revealed the complete consumption of PhBCl2 (δ
1
major product (ca. 5% of 1 was always present by H NMR
55 ppm) and the presence of a single new resonance at 2.8 ppm
consistent with a four-coordinate environment at boron. The
identity of the new four-coordinate boron species was
confirmed as (5-iPr)PhBCl2 (5) on the basis of a comparison
of the 11B NMR resonances with independently prepared 5.33
No other resonances were observed precluding aryl transfer
which would have formed BCl3 or (5-iPr)BCl3.41
spectroscopy) in 38% isolated yield following combination of 5-
iPr (0.5 equiv) and FeCl2 (1 equiv) in THF.33 The new species
was identified as the desired oligomeric monocarbene iron
species by its solid-state structure (Figure 2), which revealed it
to be {(5-iPr)FeCl2}4 (4). Compound 4 is a linear tetramer
containing two “terminal” distorted-tetrahedral four-coordinate
iron centers and two core five-coordinate iron centers. While
the poor quality of the data prohibits a detailed discussion of
metrics, the tetrameric nature of 4 is unambiguous and is in
The greater steric bulk of IMes vs 5-iPr indicated by the %
bur values in the respective (NHC)2FeCl2 structures suggested
V
that reversible dissociation of IMes from 2 should also be
C
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attempted synthesis of 8 a mixture of 8 and a second
paramagnetic species, presumably a monocarbene Cl-bridged
oligomeric species, {(7-iPr)FeCl2}n, were obtained in an
1
approximate 50:50 ratio (by H NMR spectroscopy). This
suggested that an increase in steric constraint imparted by the
larger seven-membered-ring NHC species leads to an
equilibrium position shifted toward oligomeric monocarbene
iron species. The change in the relative stability of
(NHC)2FeCl2 versus {(NHC)FeCl2}n and free NHC on
increasing sterics has been previously observed for the DIPP
analogue. In this case the considerable steric bulk of DIPP
results in monomeric (DIPP)2FeCl2 being sterically inaccessible
and exclusive formation of the dimer {(DIPP)FeCl2}2.30
Solid-state analysis of 7 and 8 (Figure 5) further established
their formulation. The expected increased steric impact of the
Figure 3. Reaction of (NHC)2FeCl2 with PhBCl2.
occurring. Indeed, combination of 2 and 1 equiv of PhBCl2 in
C6D6 at 20 °C resulted in an outcome analogous to that
observed with 1, generating (IMes)PhBCl2 (6) and the Cl-
bridged dimer 3. Considering the above experiments, it would
appear that mono-NHC dissociation from (NHC)2FeX2
followed by oligomerization of the iron(II) complex occurs
rapidly with NHC ligands of different steric demand. The free
NHC is trapped by the borane Lewis acid, driving the
equilibrium fully toward {(NHC)FeCl2}n. This process
presumably proceeds via an unobserved monomeric (NHC)-
FeCl2 complex, with one such species recently isolated,42 that
rapidly oligomerizes.
In an effort to probe both the generality of NHC dissociation
from iron(II) complexes and the applicability of PhBCl2 as a
chemical probe for this process, we investigated other (NHC)
Fe complexes. We were particularly interested in NHC ligands
with steric demand comparable to that of IMes and 5-iPr, as
this would facilitate formation of (NHC)2FeCl2 complexes
while maximizing the probability of an analogous NHC
dissociation equilibrium. Recently, there have been a number
of reports concerning the synthesis and steric/electronic
properties of expanded-ring NHCs, which are noted to be
stronger σ-donor ligands by comparison to their imidazolium-
based analogues. The greater σ donicity of expanded ring NHC
ligands comes with an increase in their associated steric
parameters (7 > 6 > 5 for a fixed wingtip substituent).43 On the
basis of the respective %Vbur values calculated from the
structures of (NHC)Rh(COD)X (COD = cyclooctadiene, X
= halide, OH),44 the 6-iPr and 7-iPr systems were reasonable
targets (Figure 4), being stronger σ donors than 5-iPr and IMes
but possessing steric bulk intermediate between the two.
Figure 5. ORTEP representations of (left) (6-iPr)2FeCl2 (7) and
(right) (7-iPr)2FeCl2 (8) at the 50% ellipsoid probability level.
Hydrogens and metrically similar second molecules of 7 and 8 present
in the respective asymmetric units are omitted for clarity.
expanded ring NHC ligands was confirmed by the %Vbur values
of the (NHC)2FeCl2 complexes, with 5-iPr < 6-iPr < 7-iPr
(Table 1). The CNHC−Fe bond distances also elongate with
increasing heterocycle ring size with 5-iPr < 6-iPr < 7-iPr,
demonstrating the greater steric impact afforded by iPr wingtip
Table 1. Selected Distances (Å) and Angles (deg) for
(NHC)2FeCl2
NHC
a
b
IMes (2)
5-iPr (1)
6-iPr (7)
7-iPr (8)
Fe−C1
Fe−C2
Fe−Cl1
Fe−Cl2
C1−Fe−C2
N1−C1−N2
N3−C2−N4
Cl1−Fe−Cl2
C1−Fe−Cl1
C2−Fe−Cl1
C1−Fe−Cl2
C2−Fe−Cl2
2.140(4)
2.144(3)
2.280(2)
2.300(2)
2.136(2)
2.130(2)
2.304(1)
2.298(1)
102.05
2.174(3)
2.187(3)
2.330(1)
2.310(1)
105.69(10)
117.2(2)
117.3(2)
108.40(3)
100.04(7)
98.59(7)
124.26(7)
121.71(7)
82.0
2.198(5)
2.198(5)
2.328(2)
2.324(1)
110.14(17)
119.7(4)
119.7(4)
105.65(5)
94.49(14)
97.85(7)
126.12(14)
125.35(13)
87.5
126.09(14)
102.6(3)
102.9(3)
107.28(7)
99.74(9)
94.97(10)
112.46(11)
115.77(10)
80.8
104.2(1)
104.0(1)
112.95(2)
104.68(4)
106.85(4)
115.62(4)
114.41(4)
34.5
Figure 4. %Vbur values31 calculated from the structures of (NHC)-
Rh(COD)X (X = halide, OH).
The six- and seven-membered NHC complexes (6-
iPr)2FeCl2 (7) and (7-iPr)2FeCl2 (8) were accessed via the
addition of the appropriate in situ generated free carbene to
0.25 equiv of {(TMEDA)FeCl2}2 in toluene. Compound 7 was
formed as a single compound in solution and displayed a
solution magnetic moment (Evans method) of 5.5 μB,
consistent with a high-spin FeII center. In contrast, in the
c
torsion angle
d
%Vbur
31.5
26.3
29.7
31.5
30.6
26.1
30.7
31.1
a
b
c
From ref 30. From ref 29. Torsion angle defined as the calculated
d
angle between the N−CNHC−N planes of the NHC ancillary ligands.
%Vbur calculated utilizing the SambVca software.31
D
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substituents in the larger ring NHCs due to the widening N−
CNHC−N angle. The increasing steric demand across the iPr
series also manifests itself in a progressively greater distortion
away from an ideal tetrahedral environment at iron toward a
trigonal-pyramidal geometry containing disparate solid-state
chloride environments: pseudoaxial and pseudoequatorial.
While the %Vbur value for the 7-iPr compound 8 is comparable
to that of the IMes derivative 2, the observation of considerably
longer Fe−C bonds for 8 is consistent with the inability to
access this species pure in solution from addition of 1 equiv of
free carbene to 0.25 equiv of {(TMEDA)FeCl2}2.
The long solid-state Fe−C distances in 7 suggested that in
solution this compound is also undergoing reversible NHC
ligand dissociation in an fashion analogous to that for
compounds 1 and 2. Indeed, subjection of complex 7 to
stoichiometric PhBCl2 in C6D6 afforded outcomes identical
with those for the imidazolium-based systems in affording the
Cl-bridged dimer {(6-iPr)FeCl2}2 (9) and the borane adduct
(6-iPr)PhBCl2 (10). Support for the dimeric nature of 9 was
initially forthcoming from NMR studies on an independently
prepared sample, which displayed resonances consistent with
only one 6-iPr environment in solution. Further confirmation
came from X-ray diffraction studies, which revealed a dimeric
structure for 9 (Figure 6) closely comparable to that for 3 and
C6D6 producing (5-iPr)PhBCl2 (along with a small quantity of
unreacted PhBCl2 consistent with starting stoichiometry) and
an insoluble precipitate, most probably {FeCl2}n. It is highly
unlikely that dissociation of the NHC would proceed directly
from the dimeric species {(5-iPr)FeCl2}2, as this would require
the intermediacy of an unprecedented45 three-coordinate
neutral iron(II) center ligated only by chlorides. With no
evidence for solvent complexation (e.g., diamagnetic Fe−η6-
arene complexes)46 NHC dissociation from 4 most probably
involves higher oligomers such as the tetramer {(5-iPr)FeCl2}4.
Close inspection of compound 4 revealed FeII−CNHC bond
lengths (2.094(12) Å for the four-coordinate Fe and 2.114(10)
Å for the five-coordinate iron) that do not indicate any
significant degree of elongation and thus weakening.
Furthermore, while the two 5-iPr ligands coordinated to the
five-coordinate iron centers in 4 are located on the same face,
the yaw angles (the in-plane distortion of the NHC defined as
the difference between the two Fe−C−N angles divided by 2)
are both <1° and the pitch angle (imidazole centroid−C−Fe)
of 171° is actually angling the two 5-iPr ligands toward each
other. When these metrics are combined, they all indicate
minimal steric destabilization of the Fe−C bond in 4, but NHC
dissociation still proceeds. Presumably, oligomerization to form
coordination numbers at iron >4 is key in this case to facilitate
NHC dissociation by avoiding the formation, post NHC
dissociation, of low-coordinate iron complexes possessing little
steric protection. A related oligomerization step prior to NHC
dissociation is precluded in (NHC)2FeCl2 complexes due to
the crowded steric environment around iron imparted by two
NHC ligands. In this case we favor the “steric assisted”
dissociation of the NHC and formation of transient three-
coordinate (NHC)FeX2 species, which then subsequently
oligomerize.
In summary, transfer of methoxide from anionic arylboro-
nates to iron proceeds in preference to the transmetalation of
aryl groups; the Fe−OMe species generated then do not react
further with ArylB(OR)2 species. These transmetalation studies
were complicated by the surprising tendency of NHC ligands to
dissociate from low-coordinate iron(II) systems. NHC
Figure 6. ORTEP representation of {(6-iPr)FeCl2}2, (9) at the 50%
ellipsoid probability level with hydrogens and a disordered molecule of
C6D6 omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe1−C1 = 2.138(7), Fe−Cl1 = 2.370(2), Fe−Cl1A = 2.414(2), Fe−
Cl2 = 2.253(2); Fe1−Cl1−Fe1A = 90.11(7), Cl1−Fe−Cl1A =
89.89(7).
11B
dissociation can be readily detected using the change in δ
of the boron Lewis acid PhBCl2 on coordination of the NHC.
This phenomenon was found to be general across a range of
NHC−Fe systems regardless of the steric constraint or σ-donor
function imparted by the ancillary NHC ligands employed. This
observation has implications with regard to the application of
(NHC)2FeCl2 systems in catalytic transformations.
{(DIPP)FeCl2}2 containing a Fe2Cl2 core with the 6-iPr ligands
coordinated on opposite faces.30 The most notable differences
between the dimers are the increased Fe−C bond lengths in 9
(2.138(7) Å) in comparison to those in the IMes and DIPP
congeners (2.089(4) and 2.090(2) Å, respectively).30 This
elongation occurs despite the relative %Vbur values, which for
the {(NHC)FeCl2}2 dimers follow the expected order DIPP >
IMes > 6-iPr (34.2%, 32.8%, and 30.7%, respectively).
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out using
standard Schlenk techniques, or in a glovebox, under an atmosphere of
argon. Solvents were distilled from appropriate drying agents (THF
(potassium); hexanes and diethyl ether (NaK)), while toluene was
dried by passage through activated alumina columns and thoroughly
degassed prior to use. All solvents were stored over potassium mirrors,
except for THF, which was stored over activated 3 Å molecular sieves.
d6-Benzene was distilled from potassium, degassed by three
consecutive freeze−pump−thaw cycles, and stored under argon. 5-
iPr,47 IMes,48 6-iPr·HBr,43 7-iPr·HI,43 [Fe(TMEDA)Cl2]2,49
[FeCl2(THF)1.5],50 [Fe(5-iPr)2Cl2] (1),29 [Fe(IMes)2Cl2] (2),3 and
iPr·BCl341 were all prepared according to previously reported literature
procedures. All other compounds were purchased from commercial
sources and used as received, except for sodium methoxide, which was
freshly sublimed prior to use. NMR spectra were recorded on Bruker
AV-400 and Bruker AV-500 spectrometers. Chemical shifts are
In contrast to the (NHC)2FeX2 complexes described above,
the bisphosphine compound (dppe)FeCl2 does not react with
PhBCl2 in C6D6 (by 11B NMR spectroscopy), indicating a more
robust iron−ligand linkage. This is not simply a chelation effect
increasing ligand binding strength, as addition of PhBCl2 to the
previously reported chelating biscarbene complex
32
methylenebis(N-DIPP-imidazole-2-ylidene)FeI2 resulted in a
rapid reaction that led to an intractable precipitate insoluble in
common organic solvents and no observable resonances in the
11B NMR spectra.
Surprisingly, the {(5-iPr)FeCl2}n oligomers are also prone to
NHC dissociation, with addition of 2.2 equiv of PhBCl2 to 4 in
E
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referenced relative to residual protio impurities in the NMR solvent for
1H and 13C{1H}, respectively, while 11B{1H} shifts are referenced
relative to external BF3−etherate. Solution-state magnetic moments
were calculated according to the Evans method39,40 at 298 K on a
Bruker AV-400 spectrometer. Microanalysis was performed by the
London Metropolitan University microanalytical service.
inseparable mixture of 8 and a second compound we assign as its
corresponding Cl-bridged oligomer {(7-iPr)Fe(μ-Cl)Cl}n (ratio 50:50
by 1H NMR). 1H NMR (400 MHz, C6D6, 298 K): for [Fe(7-iPr)2Cl2]
(8): 22.24 (bs), 10.70 (bs), 9.11 (bs), 3.70 ppm (bs); for {Fe(7-
iPr)(μ-Cl)Cl}n, 26.94 (bs), 2.30 (bs), 0.95 (bs), 0.30 ppm (bs).
Crystals of 8 suitable for X-ray diffraction were grown at ambient
temperature from the mixture by layering a C6D6 solution with
hexanes.
{(5-iPr)Fe(μ-Cl)Cl}2 (4). In a glovebox a flame-dried J. Young
ampule was charged with 5-iPr (300 mg, 1.66 mmol) and anhydrous
FeCl2 (420 mg, 3.31 mmol). Outside the glovebox THF (20 mL) was
added to afford a dark yellow homogeneous solution and the reaction
mixture was stirred at 80 °C for 16 h. After this time all volatiles were
removed to afford a dark yellow-brown residue, which was extracted
into warm toluene (3 × 20 mL), furnishing a pale yellow solution. The
solvent was removed to afford 4 as a white free-flowing solid (182 mg,
{(6-iPr)Fe(μ-Cl)Cl}2 (9). A flame-dried Schlenk tube was charged
with 6-iPr·HBr (500 mg, 2.01 mmol) which was dried under vacuum
for ca. 1 h prior to the addition of KHMDS (561 mg, 2.81 mmol) in
the glovebox. Outside the glovebox was added THF (10 mL) and the
suspension was stirred at ambient temperature for 1 h, after which
time the free carbene solution was transferred via oven-dried filter
cannula to a stirred THF solution (10 mL) of FeCl2 (509 mg, 4.02
mmol). Upon addition the solution was seen to immediately turn dark
brown. The reaction mixture was stirred at ambient temperature for 3
h prior to filtration via oven-dried filter cannula and the solvent
removed to afford a brown residue. The residue was washed with
several portions of pentane to afford 9 as a free-flowing off-white solid
1
36%), containing a minor impurity of 1 (ca. 5% by H NMR). Anal.
Calcd for C22H40Cl4Fe2N4: C, 43.03; H, 6.57; N, 9.12. Found: C,
1
42.88; H, 6.61; N, 8.97. H NMR (400 MHz, C6D6, 298 K): 22.01
(12H, bs, CH3), 3.42 (24H, bs, CH(CH3)2), 0.30 ppm (4H, bs,
NCH(CH3)2). μeff (Evans method, d8-THF solution, protio-toluene
capillary, concentration 0.015 g/mL, 298 K): 7.7(1) μB. Crystals
suitable for X-ray diffraction were grown at ambient temperature by
layering a C6D6 solution of 4 with hexanes.
1
(374 mg, 60%). H NMR (400 MHz, C6D6, 298 K): 32.45 (bs), 4.15
(bs), 2.12 (bs), 0.13 ppm (bs). Crystals of 9 suitable for X-ray
diffraction were grown at ambient temperature by layering a C6D6
solution with pentane. Attempts to obtain analytically pure bulk
material were repeatedly unsuccessful, with microanalytical data on
material pure by NMR spectroscopy being repeatedly low on C, H,
and N, suggesting FeCl2 impurities.
(5-iPr)PhBCl2 (5).51 In a glovebox a J. Young NMR tube was
loaded with 5-iPr (15 mg, 0.083 mmol) and C6D6 (0.8 mL). Outside
the glovebox PhBCl2 (10.8 μL, 0.083 mmol) was added under an
argon atmosphere and the reaction mixture agitated for 2 min at
1
ambient temperature to cleanly afford 5. H NMR (400 MHz, C6D6,
298 K): 8.16 (2H, d, 3JHH = 7.5 Hz, o-CH), 7.33 (2H, t, 3JHH = 7.5 Hz,
m-CH), 7.19 (1H, t, 3JHH = 7.8 Hz, p-CH), 5.85 (2H, sept, 3JHH = 5.8
Hz, CH(CH3)2), 1.50 (6H, s, CH3), 0.96 ppm (12H, d, 3JHH = 6.8 Hz,
CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6, 298 K): 133.4, 128.9,
127.2, 126.6, 50.5, 21.2, 10.6 ppm. 11B{1H} NMR (128.4 MHz, C6D6,
298 K): 2.7 ppm.
(6-iPr)PhBCl2 (10). A flame-dried Schlenk tube was charged with 6-
iPr·HBr (75 mg, 0.30 mmol) which was dried under vacuum for ca. 1 h
prior to the addition of KHMDS (84 mg, 0.42 mmol) in the glovebox.
Outside the glovebox was added THF (5 mL) and the suspension was
stirred at ambient temperature for 1 h, after which time the free
carbene solution was transferred via oven-dried filter cannula to a
separate flame-dried Schlenk tube and all volatiles were removed. The
resulting white solid was taken up in hexanes (5 mL) and PhBCl2 (55
μL, 0.30 mmol) added, whereupon an immediate pale yellow
suspension was observed and stirring was continued at ambient
temperature for 1 h. The yellow suspension was isolated by filtration
and dried to afford 10 as a pale yellow free-flowing solid (64 mg, 65%).
1H NMR (400 MHz, C6D6, 298 K): 7.96 (2H, bs, o-CH), 7.141 (2H,
bs, m-CH), 7.31 (1H, bs, p-CH), 5.40 (2H, m, CH(CH3)2), 2.29 (4H,
t, 3JHH = 5.2 Hz, NCH2), 1.12 (2H, m, NCH2CH2), 0.75 ppm (12H, d,
3JHH = 5.7 Hz, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 298 K;
N−CNHC−N not observed): 131.8, 127.7, 126.1, 53.9, 39.3, 21.0, 20.1
ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 3.0 ppm.
(IMes)PhBCl2 (6). In a glovebox a J. Young NMR tube was loaded
with IMes (25 mg, 0.082 mmol) and C6D6 (0.8 mL). Outside the
glovebox PhBCl2 (10.7 μL, 0.082 mmol) was added under an argon
atmosphere and the reaction mixture agitated for 2 min at ambient
temperature. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 2.3 ppm.
(6-iPr)2FeCl2 (7). A flame-dried Schlenk tube was charged with 6-
iPr·HBr (400 mg, 1.61 mmol) which was dried under vacuum for ca. 1
h prior to the addition of KHMDS (449 mg, 2.25 mmol) in a
glovebox. Outside the glovebox was added THF (10 mL) and the
resulting suspension was stirred at ambient temperature for 1 h, after
which time the in situ generated free carbene solution was transferred
via oven-dried filter cannula to a stirred THF solution (10 mL) of
{Fe(TMEDA)Cl2}2 (194 mg, 0.40 mmol). Upon addition the pale
yellow {Fe(TMEDA)Cl2}2 solution immediately darkened and the
reaction mixture was stirred at ambient temperature for 1 h. After this
time removal of all volatiles afforded a dark yellow residue, which was
subsequently washed with hexanes (3 × 20 mL) to afford 7 as a free-
flowing white solid (192 mg, 52%). Anal. Calcd for C20H40Cl2FeN4: C,
51.85; H, 8.70; N, 12.09 Found: C, 51.62; H, 8.81; N, 11.93. 1H NMR
(400 MHz, C6D6, 298 K): 26.56 (8H, bs, NCH2), 14.57 (4H, bs), 9.94
(24H, bs, NCH(CH3)2), 9.22 ppm (4H, bs). μeff (Evans method, d8-
THF solution, protio-toluene capillary, concentration 0.014 g/mL, 298
K): 5.5(6) μB. Crystals suitable for X-ray diffraction were grown at
ambient temperature by layering a C6D6 solution of 7 with hexanes.
(7-iPr)2FeCl2 (8) + {(7-iPr)Fe(μ-Cl)Cl}n. A flame-dried Schlenk
tube was charged with 7-iPr·HI (500 mg, 1.61 mmol) which was dried
under vacuum for ca. 1 h prior to the addition of KHMDS (449 mg,
2.25 mmol) in a glovebox. Outside the glovebox was added THF (10
mL) and the resulting suspension was stirred at ambient temperature
for 1 h, after which time the in situ generated free carbene solution was
transferred via oven-dried filter cannula to a stirred THF solution (10
mL) of {Fe(TMEDA)Cl2}2 (194 mg, 0.40 mmol). Upon addition the
pale yellow {Fe(TMEDA)Cl2}2 solution immediately darkened and
the reaction mixture was stirred at ambient temperature for 1 h. After
this time removal of all volatiles afforded a dark yellow residue, which
was washed with diethyl ether (3 × 20 mL) to afford 141 mg of an off-
white free-flowing solid that after 1H NMR analysis was found to be an
(7-iPr)PhBCl2 (11). A flame-dried Schlenk tube was charged with 7-
iPr·HI (100 mg, 0.32 mmol) which was dried under vacuum for ca. 1 h
prior to the addition of KHMDS (90 mg, 0.45 mmol) in the glovebox.
Outside the glovebox was added THF (5 mL) and the suspension was
stirred at ambient temperature for 1 h, after which time the free
carbene solution was transferred via oven-dried filter cannula to a
separate flame-dried Schlenk tube and all volatiles were removed in
vacuo. The white solid residue was taken up in hexanes (5 mL) and
PhBCl2 (34 μL, 0.26 mmol) added, whereupon an immediate pale
yellow suspension was observed and stirring was continued at ambient
temperature for 1 h. The yellow suspension was isolated by filtration
and dried to afford 11 as a pale yellow free-flowing solid (84 mg, 77%).
3
1H NMR (400 MHz, C6D6, 298 K): 8.01 (2H, d, JHH = 7.5 Hz, o-
3
CH), 7.33 (2H, t, JHH = 7.5 Hz, m-CH), 7.18 (1H, m, p-CH), 5.19
3
3
(2H, sept, JHH = 6.4 Hz, CH(CH3)2), 2.65 (4H, t, JHH = 5.3 Hz,
3
NCH2), 1.18 (4H, m, NCH2CH2), 0.75 ppm (12H, d, JHH = 6.7 Hz,
CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 298 K; N−CNHC−N
not observed): 132.7, 127.9, 126.6, 54.3, 44.4, 22.9, 20.7 ppm. 11B{1H}
NMR (128.4 MHz, C6D6, 298 K): 3.5 ppm.
2-(p-tolyl)-1,3,2-dioxaborolane (p-TolBeg). This compound
was prepared according to a slightly modified literature procedure.52
Under ambient conditions with no additional precautions taken to
exclude air and moisture, p-tolylphenylboronic acid (600 mg, 4.41
mmol) was suspended in reagent grade dichloromethane (15 mL)
prior to the addition of MgSO4 (531 mg, 4.41 mmol) and ethylene
F
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Organometallics
Article
glycol (271 μL, 4.85 mmol). The reaction mixture was stirred at
ambient temperature for 1 h prior to filtration and the removal of all
volatiles to afford 2-(p-tolyl)-1,3,2-dioxaborolane as a white micro-
crystalline solid of sufficient purity to be used without further
purification (642 mg, 90%). 1H NMR (400 MHz, CDCl3, 298 K): 7.73
(2H, d, 3JHH = 7.8 Hz, ArH), 7.22 (2H, d, 3JHH = 7.8 Hz), 4.38 (4H, s,
OCH2), 2.39 ppm (3H, s, p-CH3). 13C{1H} NMR (100 MHz, CDCl3,
298 K): 141.7, 134.8, 128.7, 65.9, 21.7 ppm. 11B{1H} NMR (128.4
MHz, C6D6, 298 K): 31.5 ppm.
(11) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K.; Nakamura, M. J.
Am. Chem. Soc. 2009, 131, 11949.
(12) Hatakeyama, T.; Nakamura, M. J. Am. Chem. Soc. 2007, 129,
9844.
(13) Guisan-Ceinos, M.; Tato, F.; Bunuel, E.; Calle, P.; Cardenas, D.
J. Chem. Sci. 2013, 4, 1098.
(14) Cahiez, G.; Gager, O.; Buendia, J.; Patinote, C. Chem. Eur. J.
2012, 18, 5860.
(15) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.;
Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104.
(16) Mo, Z.; Zhang, Q.; Deng, L. Organometallics 2012, 31, 6518.
(17) Gao, H.; Yan, C.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.;
Zhang, Y. Organometallics 2010, 29, 4189.
(18) Meyer, S.; Orben, C. M.; Demeshko, S.; Dechert, S.; Meyer, F.
Organometallics 2011, 30, 6692.
(19) Xiang, L.; Xiao, J.; Deng, L. Organometallics 2011, 30, 2018.
(20) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2012, 111,
1417.
(5-iPr)-p-TolBeg. In a glovebox a J. Young NMR tube was loaded
with iPr (11.2 mg, 0.062 mmol), 2-(p-tolyl)-1,3,2-dioxaborolane (p-
TolBeg) (10 mg, 0.062 mmol), and C6D6 (0.8 mL). The colorless,
homogeneous solution was agitated at ambient temperature for ca. 5
1
min to cleanly afford the title compound 5-iPr-p-TolBeg. H NMR
(400 MHz, C6D6, 298 K): 7.81 (2H, bs, ArH), 7.26 (2H, d, 3JHH = 6.8
Hz, ArH), 6.35 (2H, bs, CH(CH3)2), 2.29 (4H, bs, eg), 1.56 (6H, s,
3
CH3), 1.30 (3H, bs, CH3-p-Tol), 1.10 ppm (12H, d, JHH = 6.8 Hz,
CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6, 298 K): 128.2, 127.9,
124.4, 65.8, 49.3, 21.6, 21.6, 10.1 ppm. 11B{1H} NMR (128.4 MHz,
C6D6, 298 K): 6.4 ppm.
(21) Bedford, R. B.; Hall, M. A.; Hodges, G. R.; Huwe, M.;
Wilkinson, M. C. Chem. Commun. 2009, 6430.
(IMes)-p-TolBeg. In a glovebox a J. Young NMR tube was loaded
with IMes (25 mg, 0.082 mmol), 2-(p-tolyl)-1,3,2-dioxaborolane (13.3
mg, 0.082 mmol), and C6D6 (0.8 mL). 11B{1H} NMR (128.4 MHz,
C6D6, 298 K): 5.4 ppm.
(22) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.; Fujiwara, Y.; Seike,
H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc.
2010, 132, 10674.
(23) Hatakeyama, T.; Hashimoto, T.; Kathriarachchi, K. K. A. D. S.;
Zenymo, T.; Seike, H.; Nakamura, M. Angew. Chem., Int. Ed. 2012, 51,
8834.
ASSOCIATED CONTENT
* Supporting Information
■
(24) Hashimoto, T.; Hatakeyama, T.; Nakamura, M. J. Org. Chem.
2012, 77, 1168.
S
Text, figures, a table, and CIF files giving full experimental and
crystallographic details and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(25) For the mechanism of Pd-catalyzed Suzuki−Miyaura cross-
coupling see: (a) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011,
133, 2116. (b) Lennox, A. J. J.; Lloyd-Jones, G. C. Chem. Soc. Rev.
2014, 43, 412.
(26) Noda, D.; Sunada, Y.; Hatakeyama, T.; Nakamura, M.;
Nagashima, H. J. Am. Chem. Soc. 2009, 131, 6079.
(27) (a) See references in: Bezier, D.; Sortais, J.-B.; Darcel, C. Adv.
Synth. Catal. 2013, 355, 19. (b) For an example after this review see:
Wu, J.; Dai, W.; Farnaby, J. H.; Hazari, N.; Le Roy, J. J.; Mereacre, V.;
Murugesu, M.; Powell, A. K.; Takase, M. K. Dalton Trans. 2013, 42,
7404.
(28) Ingleson, M. J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579.
(29) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479.
(30) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. Organometallics
2012, 31, 3264.
(31) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.;
Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759.
(32) Zlatogorsky, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.; Ingleson,
M. J. Organometallics 2011, 30, 4974.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.J.I.: Michael.ingleson@manchester.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Royal Society (M.J.I. for the
award of a University Research Fellowship), the ERC (J.J.D.),
and the EPSRC (I.A.C., grant number EPJ000973/1) for
financial support. M.L.N. acknowledges the University of
Rochester for financial support.
REFERENCES
(33) See the Supporting Information.
(34) Layfield, R. A.; McDouall, J. J. W.; Scheer, M.; Schwarzmaier,
C.; Tuna, F. Chem. Commun. 2011, 47, 10623.
(35) Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2011, 50, 268.
■
(1) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
3317.
(2) Plietker, B. Iron Catalysis in Organic Chemistry; Wiley-VCH:
Weinheim, Germany, 2008.
(36) Day, B. M.; Pugh, T.; Hendriks, D.; Guerra, C. F.; Evans, D. J.;
Bickelhaupt, F. M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135, 13338.
(37) Musgrave, R. A.; Tubervill, R. S. P.; Irwin, M.; Goichoechea, J.
M. Dalton Trans. 2013, DOI: 10.1039/C3DT52638K.
(38) Wang, X.; Mo, Z.; Xiao, J.; Deng, L. Inorg. Chem. 2013, 52, 59.
(39) Evans, D. F. J. Chem. Soc. 1959, 2003.
(40) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(41) Muthaiah, S.; Do, D. C. H.; Ganguly, R.; Vidovic, D.
Organometallics 2013, 32, 6718.
(42) Pranckevicius, C.; Stephan, D. W. Organometallics 2013, 32,
2693.
(43) For recent papers on the steric/electronic properties of
expanded ring NHCs see: (a) Dunsford, J. J.; Cavell, K. J. Dalton
Trans. 2011, 40, 9131. (b) Dunsford, J. J.; Cavell, K. J.; Kariuki, B. M.
Organometallics 2012, 31, 4118. (c) Dunsford, J. J.; Tromp, D. S.;
Cavell, K. J.; Elsevier, C. J.; Kariuki, B. M. Dalton Trans. 2013, 42,
7318.
(3) Hashimoto, T.; Urban, S.; Hoshino, R.; Ohki, Y.; Tatsumi, K.;
Glorius, F. Organometallics 2012, 31, 4474.
(4) Li, H.; Castro, L. C. M.; Zheng, J.; Roisnel, T.; Dorcet, V.; Sortais,
J.-B.; Darcel, C. Angew. Chem., Int. Ed. 2013, 52, 8045.
(5) Sherry, B. D.; Fuerstner, A. Acc. Chem. Res. 2008, 41, 1500.
(6) Czaplik, W. M.; Mayer, M.; Cvengros, J.; von Wangelin, A. J.
ChemSusChem 2009, 2, 396.
(7) Nakamura, E.; Yoshikai, N. J. Org. Chem. 2010, 75, 6061.
(8) Adams, C. J.; Bedford, R. B.; Carter, E.; Gower, N. J.; Haddow,
M. F.; Harvey, J. N.; Huwe, M.; Angeles-Carter, M.; Mansell, S. M.;
Mendoza, C.; Murphy, D. M.; Neeve, E. C.; Nunn, J. J. Am. Chem. Soc.
2012, 134, 10333.
(9) Bedford, R. B.; Carter, E.; Cosgwell, P. M.; Harvey, J. N.; Gower,
N. J.; Haddow, M. F.; Harvey, J. N.; Murphy, D. M.; Neeve, E. C.;
Nunn, J. Angew. Chem., Int. Ed. 2013, 52, 1285.
(10) Kawamura, S.; Ishizuka, K.; Takaya, H.; Nakamura, M. Chem.
Commun. 2010, 46, 6054.
G
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Article
(44) For the structures of (NHC)Rh(COD)X see: (a) IMes: Evans,
P. A.; Baum, E. W.; Fazal, A. N.; Pink, M. Chem. Commun. 2005, 63.
(b) 5-iPr: Truscott, B. J.; Slawin, A. M.; Nolan, S. P. Dalton Trans.
2013, 42, 270. (c) 6-iPr: Mayr, M.; Wurst, K.; Ongania, K.-H.;
Buchmeiser, M. R. Chem. Eur. J. 2004, 10, 1256. (d) 7-iPr: Iglesias,
M.; Beetstra, D. J.; Stasch, A.; Horton, P. N.; Hursthouse, M. B.; Coles,
S. J.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2007, 26,
4800. The %Vbur value for 7-iPr is estimated from the 7-cyclohexyl
structure, which sets an approximate upper limit for the 7-iPr value.
(45) No neutral three-coordinate iron(II) compounds ligated only by
chlorides are reported in the Cambridge Structural Database (Oct
2013).
(46) (NHC)Fe(η6-arene) complexes have been recently reported
and are robust in a range of oxidation states: (a) Zlatogorsky, S.;
Ingleson, M. J. Dalton Trans. 2012, 41, 2685. (b) Blom, B.; Tan, G.;
Enthaler, S.; Inoue, S.; Epping, J. D.; Driess, M. J. Am. Chem. Soc.
2013, 135, 18108.
(47) Kuhn, N.; Kratz, T. Synthesis 1993, 561.
(48) Bantreil, X.; Nolan, S. P. Nat. Protoc. 2011, 6, 69.
(49) Davies, S. C.; Hughes, D. L.; Leigh, G. J.; Sanders, J.; de Souza,
J. S. J. Chem. Soc., Dalton Trans. 1997, 1981.
(50) Winter, G.; Thompson, D. W.; Loehe, J. R. Inorg. Synth. 1973,
99.
(51) This compound has been previously reported as an
intermediate, but no physical data were reported; see: Wang, Y.;
Robinson, G. H. Inorg. Chem. 2011, 50, 12326−12337.
(52) Iwai, Y.; Gligorich, K. M.; Sigman, M. S. Angew. Chem., Int. Ed.
2008, 47, 3219.
H
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