Combined Effects of Backbone and N-Substituents on Structure, Bonding, and Reactivity of Alkylated Iron(II)-NHCs

Article abstract of DOI:10.1021/acs.organomet.8b00466

Iron and N-heterocyclic carbenes (NHCs) have proven to be a successful pair in catalysis, with reactivity and selectivity being highly dependent on the nature of the NHC ligand backbone saturation and N-substituents. Four (NHC)Fe(1,3-dioxan-2-ylethyl)₂ complexes have been isolated and spectroscopically characterized to correlate their reactivity to steric effects of the NHC from both the backbone saturation and N-substituents. Only in the extreme case of SIPr where NHC backbone and N-substituent steric effects are the largest is there a major structural perturbation observed crystallographically. The addition of only two hydrogen atoms is sufficient for a drastic change in product selectivity in the coupling of 1-iodo-3-phenylpropane with (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide due to resulting structural perturbations to the precatalyst. M?ssbauer spectroscopy and magnetic circular dichroism enabled the correlation of covalency and steric bulk in the SIPr case to its poor selectivity in alkyl-alkyl cross-coupling with iron. Density functional theory calculations provided insight into the electronic structure and molecular orbital effects of ligation changes to the iron center. Finally, charge donation analysis and Mayer bond order calculations further confirmed the stronger Fe-ligand bonding in the SIPr complex. Overall, these studies highlight the importance of considering both N-substituent and backbone steric contributions to structure, bonding, and reactivity in iron-NHCs.

Full text of DOI:10.1021/acs.organomet.8b00466

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Combined Effects of Backbone and N‑Substituents on Structure,
Bonding, and Reactivity of Alkylated Iron(II)-NHCs
†
†
Salvador B. Munoz, III, Valerie E. Fleischauer, William W. Brennessel, and Michael L. Neidig*
̃
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
*
S Supporting Information
ABSTRACT: Iron and N-heterocyclic carbenes (NHCs)
have proven to be a successful pair in catalysis, with reactivity
and selectivity being highly dependent on the nature of the
NHC ligand backbone saturation and N-substituents. Four
(
NHC)Fe(1,3-dioxan-2-ylethyl) complexes have been iso-
2
lated and spectroscopically characterized to correlate their
reactivity to steric effects of the NHC from both the backbone
saturation and N-substituents. Only in the extreme case of
SIPr where NHC backbone and N-substituent steric effects
are the largest is there a major structural perturbation
observed crystallographically. The addition of only two
hydrogen atoms is sufficient for a drastic change in product
selectivity in the coupling of 1-iodo-3-phenylpropane with (2-
(
1,3-dioxan-2-yl)ethyl)magnesium bromide due to resulting structural perturbations to the precatalyst. Mossbauer spectroscopy
̈
and magnetic circular dichroism enabled the correlation of covalency and steric bulk in the SIPr case to its poor selectivity in
alkyl−alkyl cross-coupling with iron. Density functional theory calculations provided insight into the electronic structure and
molecular orbital effects of ligation changes to the iron center. Finally, charge donation analysis and Mayer bond order
calculations further confirmed the stronger Fe−ligand bonding in the SIPr complex. Overall, these studies highlight the
importance of considering both N-substituent and backbone steric contributions to structure, bonding, and reactivity in iron-
NHCs.
INTRODUCTION
Scheme 1. Examples of Cross-Coupling Reactions with Iron-
18,27,32
■
NHCs and the Results of IPr vs SIPr on Product Yields
The ability to synthetically manipulate ligand additives for
catalysis has enabled the development of a large selection of
commercially available organometallic precatalysts and ligands
for use in catalytic reactions. N-heterocyclic carbenes (NHCs)
are a particularly popular class of ligand additives and have been
1
,2
used in a vast number of catalytic reactions as organocatalysts,
3
−9
and in tandem with transition metals
and Group 13
1
,2,10
elements.
NHCs are available in the form of stable salts
with a large number of different N-substituents, central ring size,
and even central ring functionalization, making them very easy
2
,11
to access and work with.
Furthermore, “smart NHCs” are
1
3
under development which are redox-switchable, bifunctional, or
explored. Alternatively, Beller and co-workers reported the
first aryl−alkyl cross-coupling reactions having success with
IMes as a ligand additive instead of IPr in the presence of
hemilabile and have even been shown to exhibit backbone
1
2
reactivity. Due to their structural diversity, NHCs have been
utilized in many catalytic transformations including hydro-
silylation, olefin metathesis, C−H activation, and, of particular
1
4
palladium. A simple N-substituent change allowed for the
cross-coupling of two different sets of reagents.
Along with their popularity in precious metal catalysis, NHCs
2
note, cross-coupling reactions.
1
5,16
have been used in a multitude of iron catalyzed reactions.
In
The value of the NHC functionality is seen in some of the
earliest reports of NHC cross-coupling with precious metals.
The first Kumada cross-coupling reaction using NHCs was
developed by Nolan and co-workers and showed a dependence
on the bulkier IPr in catalysis over IMes (Scheme 1, right) for
the successful coupling of aryl iodides with aryl Grignard
reagents using palladium, but no backbone variations were
1
7−20
21
22−30
cross-coupling, aryl−aryl,
aryl−alkenyl, alkyl−aryl,
3
1
32
aryl−alkyl, and alkyl−alkyl transformations have all been
achieved using a variety of NHCs (Scheme 1). In a popular
Received: July 5, 2018
©
XXXX American Chemical Society
A
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Article
example, Nakamura and co-workers reported in 2007 the
Kumada coupling of aryl Grignard reagents with aryl halides
Table 1. Reactivity of NHC Variations in Alkyl−Alkyl Cross-
Coupling with Iron
17,18
using iron, nickel, and cobalt fluorides.
Here, SIPr is the
most effective NHC for catalysis, outperforming IPr with 98%
yield compared to only 25% cross-coupled product (Scheme 1).
In this case, and in several other reactions using NHCs and iron,
27,31
a change in backbone saturation drastically alters selectivity.
To better understand the selectivity of certain NHCs for
different catalytic reactions, previous studies have explored the
reactivity and electronic structure of independently synthesized
iron-NHC compounds. For example, Deng and co-workers have
NHC
A (%)
B (%)
C (%)
IMes·HCl
SIMes·HCl
IPr·HCl
78
77
69
23
15
15
23
56
7
8
8
33−37
investigated aryl and alkyl iron-NHCs,
and Tonzetich and
co-workers have explored the electronic structure and bonding
SIPr·HCl
21
38−40
effects in (NHC)FeX and aryl containing species.
More
n
recently, the first direct observation of an in situ formed reactive
cross-coupled product yield to only 23% is observed. The
change from mesityl to diisopropylphenyl causes a loss of
chelation in one of the alkyl ligands on iron for the isolated
41
iron-NHC complex was reported by the Neidig group. These
studies, however, do not address the combined effects of NHC
backbone structure and N-substitutions on electronic structure
and bonding of reactive iron species in great detail.
41
precatalyst with SIPr compared to IMes. Since backbone
saturation has almost no effect on the reactivity of the mesityl
NHCs, IPr was expected to have similarly poor catalytic
performance to SIPr when used in this alkyl−alkyl cross-
coupling reaction.
However, the use of IPr results in the formation of 69% cross-
coupled product, greatly out-performing SIPr, suggesting that
the loss of reactivity seen with SIPr is more complex than a
simple argument of added N-substituent steric bulk.
Structural Characterization of (NHC)Fe(1,3-dioxan-2-
ylethyl)2 Compounds. The unexpected high selectivity
observed for the IPr/SIPr couple inspired further character-
ization of the precatalysts formed during the reaction protocol to
directly identify any structural changes caused by either
backbone saturation or N-substituent variation. In the alkyl−
alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium
bromide with 1-iodo-3-phenylpropane, (NHC)Fe(1,3-dioxan-
In this study, IMes, SIMes, IPr, and SIPr (Scheme 1, right)
were used for the independent synthesis of iron-alkyl
compounds to create a complete set of variations in both
backbone and N-substituents. The effects of these changes on
reactivity, electronic structure, and bonding are identified using
gas chromatography (GC), X-ray diffraction, 57Fe Mo
ssbauer
̈
spectroscopy, magnetic circular dichroism (MCD), and density
functional theory (DFT) and show that iron-NHC reactivity can
be influenced by steric effects deriving from both N-substituents
and backbone saturation effects.
RESULTS AND DISCUSSION
■
Changes in Catalytic Activity Resulting from NHC
Variations. As described previously, catalytic reactions with
iron have used different NHCs with varying levels of success
depending on N-substituent steric effects or backbone
saturation. Recently, reactivity differences due to changes in
N-substituents were solidified in a study from the Neidig
2-ylethyl) type species were found to form in situ during the
2
precatalyst formation step of the reaction where excess Grignard
reagent is added to iron and NHC·HCl in THF at 50−60 °C.
4
1
IMes
IMes
group. (IMes)Fe(1,3-dioxan-2-ylethyl) (1 ) was identified
The IMes derivative, 1
(Figure 1A), was shown to have two
2
as a reactive iron species with chelating dioxane moieties on the
alkyl ligands in the coupling of (2-(1,3-dioxan-2-yl)ethyl)-
magnesium bromide with 1-iodo-3-phenylpropane. In this
hemilabile alkyl ligands from the Grignard reagent. Therefore,
initial efforts focused on the isolation of SIMes and IPr
derivatives.
41
alkyl−alkyl cross-coupling reaction originally reported by
To form (SIMes)Fe(1,3-dioxan-2-ylethyl) , complex 2SIMes, 4
2
3
2
Car
́
denas, bulkier NHCs like SIPr and even more sterically
equiv of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide was
encumbering bisphosphine ligands resulted in low cross-coupled
added in one portion to 1 equiv each of Fe(OAc) and SIMes·
2
32
product yield (Scheme 1). Therefore, reactivity differences
were yet again solely attributed to the change in NHC N-
substituent. Preliminary results using alternative nucleophiles
were also explored, but poor cross-coupled yields were
observed for all reactions that did not contain a rigid, dioxane-
type chelating moiety and are therefore not discussed in this
NHC focused work.
HCl at room temperature. Following cooling to −80 °C, yellow
crystals suitable for X-ray diffraction were obtained. The
SIMes
structure of 2
(Figure 1B) is described as a distorted 5-
41
coordinate iron complex (τ = 0.43) with two chelating oxygen
5
moieties from the alkyl ligands, a similar geometry to that of
IMes
42
1
(τ = 0.52). Oxygen interactions are at 2.5069(15) and
5
2.4553(14) Å to the iron center, slightly elongated in
IMes
Since initial alkyl−alkyl cross-coupling studies did not include
detailed catalytic data for the use of a series of NHCs with
varying N-substituents and backbone saturation under opti-
mized reaction conditions, the catalytic performances of IPr,
SIPr, IMes, and SIMes were evaluated using a modified
comparison with 1 . Fe−C bonds to the alkyl ligands are
2.106(2) and 2.101(2) Å, and the Fe−SIMes bond is 2.124(2) Å
IMes
in length, consistent with the bischelated compound 1
(Table S1).
The IPr precatalyst was isolated in a similar way to complex
3
2
SIMes
procedure from Car
́
denas and co-workers as reported
2
: 6 equiv of (2-(1,3-dioxan-2-yl)ethyl)magnesium bro-
4
1
previously; Table 1 highlights these results. The use of IMes
as a ligand additive results in the highest amount of cross-
coupled product at 78% yield. Consistent with original reports,
SIMes produces almost identical results as the only perturbation
to ligand structure is in the saturation of the backbone. When an
N-substituent change is made in the case of SIPr, a severe drop in
mide was added to 1 equiv each of Fe(OAc) and IPr·HCl at
2
room temperature. After cooling to −80 °C, yellow crystals
suitable for X-ray diffraction were acquired. Consistent with its
IMes
SIMes IPr
comparable reactivity to 1
and 2
, 3 (Figure 1C) is also
described as a distorted 5-coordinate (τ = 0.49) iron complex
5
with two chelating dioxane alkyl rings with oxygen interactions
B
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Article
Figure 1. X-ray crystal structures of NHC iron-alkyl complexes (A) (IMes)Fe(1,3-dioxan-2-ylethyl) (1IMes), (B) (SIMes)Fe(1,3-dioxan-2-ylethyl)2
4
1
2
SIMes
IPr
41
SIPr
(2
), (C) (IPr)Fe(1,3-dioxan-2-ylethyl) (3 ), and (D) (SIPr)Fe(1,3-dioxan-2-ylethyl)2 (4 ). Hydrogen atoms omitted for clarity except for
2
the NHC backbones and thermal ellipsoids are shown at the 50% probability level. (E) Corresponding steric maps for the iron-alkyl complexes
representing NHC proximity to the iron center. %Vbur was calculated using alkyl ligands as the pseudo xy plane, a radius of 3.5 Å from the iron center,
and included H atoms; axes labels are in Å.
at 2.49068(13) and 2.44736(14) Å. Fe−C bond lengths to the
alkyl groups are at 2.1125(19) and 2.11558(19) Å, and the Fe−
IPr bond length is 2.1571(17) Å (Table S1). The stabilizing
chelation of both alkyl groups through oxygen observed in 1IMes,
Table 2. Quantified Canted Angles of Dioxane Rings
Chelated to Iron in 1−4 Relative to the RMS Plane of the
Dioxane Rings and the Plane Normal to the Fe-C13 Line
SIMes
IPr
complex
dioxane ring O1-O2/C3-C6
dioxane ring O3-O4/C9-C12
2
, and 3 reduces the opportunity for β-H elimination
1IMes
29.2°
39.7°
29.0°
38.6°
28.5°
11.3°
74.1°
n.a.
during catalysis and is consistent with their high selectivity.
SIMes
2
This series of bisalkyl iron compounds is completed with tShIPer
3
SIPr
IPr
comparison to the structure of the SIPr derivative, complex 4
4
(Figure 1D). This precatalyst was found to have the structure
(
SIPr)Fe(1,3-dioxan-2-ylethyl) with pseudo tetrahedral geom-
2
etry (τ = 0.82) where only one alkyl group is chelated to iron
4
NHC structure can be enough to modify substrate interaction
through oxygen, attributed to the added steric bulk of the
with an iron center. In the case of the second “set” of chelated
IMes
diisopropylphenyl groups as opposed to mesityl in 1
2
and
IPr
dioxane rings, the distortion in 3 has a value of 74.1°, whereas
SIMes
. The Fe−NHC bond length is shortened to 2.0920 (10)
IMes
1
shows an almost identical angle to that of the first chelated
Å, and the single chelated oxygen is at a distance of 2.3130(9) Å
from the iron center. Loss of reactivity was initially assigned to
the steric bulk of the N-substituents, but with the addition of
SIMes
ring. Complex 2
to 11.3°, further confirming that backbone saturation can affect
chelation to iron.
also shows a change in ring tilt from 39.7°
IPr
complex 3 to this series, it is clear that this is not the only
To address the varying degrees of alkyl bischelation in
IMes
SIPr
origin of structural perturbations in alkyl iron complexes. So, in
cross-coupling reactions where NHC backbone variation
correlates to product selectivity such as in the biaryl system
developed by Nakamura, it is important to consider the
electronic and steric effects of backbone variations.
compounds 1
to 4 , the steric components of each iron
compound were quantified as a function of NHC using
43
SambVca 2.0. While the Tolman cone angle is a useful
method for quantifying steric properties in monodentate
phosphine ligands, the percent buried volume (%V ) allows
bur
IPr
Interestingly, although 3 is also a bischelate through the two
for the sterics of the NHC ligand that project toward the metal
5
SIPr
dioxane oxygen atoms, the steric bulk of the diisopropylphenyl
center to be quantified. Complex 4 shows the highest %V
bur
IPr
groups appears to cause one of the dioxane rings to be severely
for the NHC ligand of 44.6%, while 3 has a %Vbur of only
34.1%. When the steric maps are considered (Figure 1E), 4
IMes
SIMes
44
SIPr
canted relative to those of 1
and 2
. When the RMS
(
root-mean-square) plane of the dioxane rings relative to the
clearly shows the distortion of an isopropylphenyl group at
plane normal to the Fe−NHC bond is considered, the tilt of the
dioxane rings in these complexes can be quantified. These results
are shown in Table 2.
approximately 1.4 Å to the iron center which creates the increase
IPr
in %V . 3 lacks this tilted isopropylphenyl ring and shows a
bur
maximum distance of approximately 2.2 Å from the iron center
IMes
SIMes
The first “set” of chelated dioxane rings have similar
orientations relative to the plane described above. In the case
in its steric map. 1
3
and 2
show nearly identical results to
IPr
with respect to maximum distance of the NHC substituents
SIMes
SIPr
of 2
this angle is ∼10° greater than in 1
indicating that the simple addition of two hydrogens to the
and 4 , which both have saturated NHC backbones,
to the iron center and %Vbur.
IMes
IPr
The steric mapping and %Vbur for 4SIPr are consistent with its
loss of alkyl chelation observed crystallographically and
and 3 (Table 2),
C
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Article
subsequent decrease in cross-coupled product selectivity when
compared with 3 . In fact, when the two NHC segments are
variation in side N-substituents on the electronic structure of
these molecules.
IPr
directly superimposed, structural differences can be even more
readily visualized (Figure 2A). Shown in red, the SIPr ligand
In the case of Evans method analysis of the isolated crystalline
SIMes
2
in THF-d , the μ value of 4.9(0.2) μ is consistent with
8
eff
b
IMes
SIPr 41
an S = 2 iron(II) center like in 1
Mo
and 4
.
5 K solid
̈
ssbauer of isolated crystalline material further confirms the
spin state of the iron center with δ = 0.54 mm/s and ΔE = 2.56
mm/s (Table 3, Figure S1). The Evans method of the isolated
Q
Table 3. Summary of 57Fe Mo
for (NHC)Fe(1,3-dioxan-2-ylethyl) Compounds
̈
ssbauer and MCD Parameters
2
Mo
̈
ssbauer
NIR MCD
complex
δ (mm/s)
ΔEQ (mm/s)
LF bands (cm−1)
1IMes
0.57
0.54
0.58
0.44
2.42
2.56
2.36
2.91
5310, 7600
5400, 7630
<5000, 8060
5800, 8480
SIMes
2
IPr
3
SIPr
4
crystalline 3IPr in THF-d results in an artificially low μ value of
8
eff
4
.4(2) μ from the expected 4.9 μ for S = 2 iron(II) due to the
b
b
variable presence of cocrystallized magnesium salts. However, 5
̈
K Mossbauer spectroscopy of isolated crystalline 3 results in a
single quadrupole doublet with parameters of δ = 0.58 mm/s
IPr
Figure 2. Skeletal overlay of (A) IPr (blue) and SIPr (red) and (B)
IMes (yellow) and SIMes (green) ligands extracted from crystallo-
and ΔE = 2.36 mm/s (Table 3, Figure S1), consistent with an S
IMes SIMes IPr
SIPr
Q
graphic structures of 1 , 2
, 3 , and 4 showing distortion in
=
2 iron(II). This is further consistent with the Mossbauer
̈
one of the SIPr side arm groups. NHC backbone hydrogens are
represented translucent spheres of the same color as the NHC.
IMes
SIMes
parameters and high spin iron(II) states for 1 , 2
4
4
, and
SIPr
(Table 3). A slight decrease in isomer shift is observed in
SIPr
which corresponds to the shorter bond lengths and
clearly displays a “bent” side arm isopropyl group that would be
variation in ligand environment observed crystallographically.
IMes
closer to the metal center and potentially block a reactive site on
The 5 K, 7 T NIR MCD spectra of complexes 1
through
complement the electronic structure insights observed in
the Mossbauer data. In the case of the mesityl based NHC
complexes, 1
IMes
SIMes
SIPr
iron. When 1
and 2
are overlaid in the same way, it is
4
clear that the side arm groups are oriented in a much more
̈
SIPr
IMes
planar way than in 4 (Figure 2B) and are very similar to one
shows C-term MCD transitions at 5310 and
−
1
IMes
SIMes
another. These representations show the significance of both
NHC backbone and N-substituent manipulations in the
reactivity, characteristics, and stability of intermediates in iron
catalysis.
7600 cm (Figure 3(1 ) and Table 3), and 2
at 5400 and
−
1
SIMes
7630 cm (Figure 3(2
energies of these compounds (6455 cm and 6515 cm for 1
), Table 3). The average ligand field
−
1
1
IMes
SIMes
and 2
, respectively) are consistent with their longer Fe−
Another ligand system employed in the alkyl−alkyl cross-
coupling system discussed here is Xantphos (4,5-bis(diphenyl-
phosphino)-9,9-dimethylxanthene). In this case, only 27%
cross-coupled product is observed, similar to the 23% yield
CNHC bond distances observed crystallographically and
SIPr
Mo
̈
ssbauer parameters when compared with 4
(7140
−
1
IMes
SIMes
cm ). The MCD transitions for 1
and 2
both show
negative intensity at similar relative intensities. This indicates
that the change in saturation from double to single bond on the
NHC ring backbone has little effect on the electronic structure
of the iron complex, consistent with a previous study by
Tonzetich and co-workers where backbone manipulations to
3
2
with SIPr, making it clear that ligand bulk plays an important
role in this alkyl−alkyl cross-coupling reaction. Since Xantphos
is utilized in the two other alkyl−alkyl cross-couplings with iron
45
46
reported by Nakamura and Chai, we wanted to confirm that
steric hindrance around an iron center across ligand classes was
cause enough for it to be less productive in the coupling of (2-
38
IMes were explored. Only in the case of 4, where the combined
steric effects of the saturated backbone and isopropylphenyl
groups prevent the chelation of two alkyl oxygens, is there a
significant change in electronic structure and bonding at iron.
(
1,3-dioxan-2-yl)ethyl)magnesium bromide with 1-iodo-3-
47
phenylpropane. When (Xantphos)FeCl2 was treated with
the same %Vbur calculation methods as the NHC-alkyl
complexes reported here, a value of 54.6% was determined
and the steric map indicates two major regions of Xantphos
phenyls within 0.5 Å of the iron center (Figure S4). These ligand
IPr
Complex 3 displays two temperature-dependent, C-term,
−
1
IPr
ligand field transitions at <5000 and 8060 cm (Figure 3(3 ),
Table 3). The SIPr derivative has ligand field transitions at 5800
−
1
SIPr
SIPr
and 8480 cm (Figure 3(4 ), Table 3). In the case of 4 , the
C-term intensities produce pseudo A-terms created by two
slightly overlapping C-terms of opposite intensity which is not
SIPr
distances are even closer than those observed for 4 , and
further demonstrate that ligand systems with sufficient steric
bulk can prevent substrate chelation to iron and decrease
selectivity in this alkyl−alkyl cross-coupling system.
IPr
observed for 3 . The change in iron site geometry from pseudo
IPr
SIPr
5-coordinate, 3 , to pseudo 4-coordinate, 4 , would be
expected to be a sufficient change to cause this shift in ligand
field energy. In fact, the average ligand field energy is
Spectroscopic Characterization of (NHC)Fe(1,3-diox-
an-2-ylethyl) Compounds. With the completion of the
2
SIPr
structural analysis of this series of iron-NHC complexes, Evans
significantly larger for 4 than what can be approximated for
5
7
IPr
−1
method, Fe Mo
̈
ssbauer spectroscopy, and MCD experiments
3
(7140 and 6530 cm , respectively). A larger ligand field
SIPr
were used to investigate the effects of backbone saturation and
energy corresponds with 4
having more covalent ligands.
D
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Article
yl) (3 ), and (SIPr)Fe(1,3-dioxan-2-ylethyl) (4SIPr). All
IPr
2
2
complexes share a similar FMO ordering and composition
around the HOMO/LUMO gap. As a representative example,
IPr
3
has a ground state consisting of one occupied FMO, β180,
with 94% Fe d orbital character (HOMO, d ). The LUMO
xy
shows significant π bonding with the NHC carbene C (β181, dyz,
π carbene). In fact, 26% of the FMO composition is located at
the carbene C. Fe d character is also observed in β187 (LUMO
+
6, 74% d ), β188 (LUMO+7, 67% d 2), β194 (LUMO+13,
xz z
2 2
23% d ), and β199 (LUMO+18, 55% d ). Note that two
yz x −y
FMOs possess dyz character, and in each case also have
significant π interactions with the NHC carbene C.
Mayer bond order and charge donation analyses were carried
out from optimized geometry coordinates to quantify the
bonding interactions across this series of iron compounds. Table
4
highlights the bonding interactions of the metal centers of the
NHC-iron-alkyl complexes with each ligand. The Fe−C bond
orders to the carbons of the alkyl ligands are nearly identical for
the two alkyl ligands in 1
IMes
at 0.691 and 0.692. For each
remaining complex, one of the two Fe−C bonds to the alkyl
ligand is slightly larger than the other. This is most pronounced
SIPr
for 4 where the bond orders are at 0.662 and 0.769, likely due
to the presence of only one metallacycle with a dioxane moiety
when compared with the other three molecules. Consistent with
the comparable Fe−C bonding in these complexes, charge
donation analysis from the alkyl ligands to the Fe-NHC portion
of the molecule is similar for all complexes (Table 4). Fe−O
bond orders however are comparable and very weak for each
SIPr
molecule, including complex 4 . Previously, the monoalky-
lated analog, (IMes)FeBr(THF)(1,3-dioxan-2-ylethyl), was
found to have a Mayer bond order of 0.224 to the oxygen of
41
the hemilabile alkyl ligand, which suggests that the origin of
SIPr
the single chelated alkyl moiety in 4 is due to the total steric
SIPr
effects of the SIPr ligand. 4 continues to stand out when the
Fe−NHC bond is considered as well, with a Mayer bond order
of 0.771 which is nearly 0.1 greater than all other NHCs.
CONCLUSIONS
■
The evaluation of four alkylated iron-NHC compounds has
provided insight on the implications of both backbone and N-
substituent variations of traditional N-heterocyclic carbenes on
Figure 3. 5 K, 7 T NIR MCD spectra of (NHC)Fe(1,3-dioxan-2-
IMes
ylethyl)2 compounds. (1 ) (IMes)Fe(1,3-dioxan-2-ylethyl) ,
2
SIMes
IPr
(2
) (SIMes)Fe(1,3-dioxan-2-ylethyl) , (3 ) (IPr)Fe(1,3-dioxan-
2
SIPr
precatalyst structure. Mossbauer spectroscopy, MCD, and DFT
̈
2
-ylethyl) , and (4 ) (SIPr)Fe(1,3-dioxan-2-ylethyl) . All spectra
2
2
have indicated the effects that are possible because of NHC
structure manipulation on the chelation ability of iron alkyl
complexes. These techniques have shown that changes in NHC
structure do little to affect the electronic structure and bonding
in these types of compounds apart from when the steric effects of
the NHC backbone saturation and N-substituent are combined
were collected on solid state mull samples of crystalline material.
This is observed in the shorter bond lengths from Fe−C and
SIPr
Fe−C , and in the decreased value of Mossbauer parameters
̈
alkyl
compared with the other three complexes studied here.
Electronic Structure and Bonding Analysis. Spin
unrestricted DFT calculations using the B3LYP functional and
def2-TZVP basis set were carried out to further analyze the
SIPr
in the case of 4 . Only in this case is the structure of the
reactive iron complex altered to reduce chelation of the alkyl
substrate and prevent the production of cross-coupled product.
Therefore, in cross-coupling systems where backbone saturation
affects product selectivity, even the addition of two hydrogen
atoms can be a potential steric effect in addition to any N-
substituent steric effects that might also be present. Extension of
this study to other metal centers and chelating alkyl substituents
will provide further understanding of the powerful effects of
these subtle changes to the metal-NHC complex, enabling
improved catalyst design.
SIMes
IPr
electronic structure and bonding in 2
reported calculations for 1
and 3 . Previously
IMes
SPIr
and 4 were repeated using the
same updated basis set and functional for consistency in the
series and more comparable calculated geometries to crystallo-
graphic data (see the Supporting Information). Molecular
orbitals in the gas phase were also explored, focusing on the
occupied and unoccupied frontier molecular orbitals (FMOs) of
the β orbital manifold to determine major bonding contribu-
tions.
Figure 4 gives the calculated FMO energy diagrams for the β
EXPERIMENTAL SECTION
General Considerations. All general experimental methods and
characterization, including collection and refinement of X-ray data, may
■
IMes
manifolds of (IMes)Fe(1,3-dioxan-2-ylethyl) (1 ), (SIMes)-
2
SIMes
Fe(1,3-dioxan-2-ylethyl) (2
), (IPr)Fe(1,3-dioxan-2-yleth-
2
E
DOI: 10.1021/acs.organomet.8b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 4. Calculated frontier molecular orbital diagrams for NHC-iron-alkyl complexes and representative molecular orbitals from 3IPr
.
electrophile after it was cooled to room temperature. Reactions were
quenched using 0.1 mM HCl in THF and filtered through a Florisil pad.
Cross-coupled product yields were determined using quantitative GC
analysis.
Table 4. Calculated Mayer Bond Order and Charge Donation
Analysis for NHC-Iron-Alkyl Complexes
Mayer bond order
charge donation analysis (α + β)
Preparation of (SIMes)Fe(1,3-dioxan-2-ylethyl) (2SIMes). To a
net donation to Fe−NHC
2
complex Fe−NHC Fe−Calkyl Fe−O
fragment
scintillation vial were added Fe(OAc) (50 mg, 0.29 mmol) and 1,3-
2
1IMes
1.154 e−
dimesitylimidazolinium chloride (98.6 mg, 0.29 mmol) along with a stir
bar. To the solid mass was added (2-(1,3-dioxan-2-yl)ethyl)magnesium
bromide in THF (2.3 mL, 0.5 M, 1.16 mmol) in one portion at room
temperature. The reaction mixture was stirred at room temperature for
0.627
0.682
0.624
0.771
0.691
.692
0.727
.690
0.726
.662
0.769
.662
0.117
0.122
0.109
0.122
0.091
0.133
0.138
0
2SIMes
1.160 e−
0
10 min, followed by cooling to −78 °C. The yellow slurry was then
3IPr
1.223 e−
1.267 e−
warmed on a room temperature stir plate for 5 min while stirring, after
which room temperature pentane (ca. 14 mL) was added to the mixture
and the vial cooled back to −78 °C. The resulting solution was filtered
through Celite (precooled to −78 °C), and the yellow filtrate was
stored at −80 °C for 1 week, rendering yellow crystals. Evans method
0
4SIPr
0
SIMes
8
analysis of crystalline 2
redissolved in THF-d indicates μ
=
eff
be found in the Supporting Information. (IMes)Fe(1,3-dioxan-2-
ylethyl)2 (1 ) and (SIPr)Fe(1,3-dioxan-2-ylethyl) (4 ) were
prepared according to literature procedure.
In Situ Reactivity. Fe(OAc) (0.010 mmol) was combined with
SIMes·HCl or IPr·HCl (0.021 mmol) in THF at 55 °C in 2 mL of THF,
and alkyl Grignard reagent was added quickly (0.122 mmol, 12 equiv to
iron) to form the precatalyst. The reaction was cooled to 23 °C after 20
min, and 1-iodo-3-phenylpropane was added (0.400 mmol). (2-(1,3-
Dioxan-2-yl)ethyl)magnesium bromide in THF (0.500 mmol, 0.5 M)
was added via syringe pump at a rate of 0.16 mL/h for 6.25 h.
Dodecane was added to the reaction to match the molarity of
IMes
SIPr
4.9(0.2) μ . Elemental Anal. Calcd: C, 66.89; H, 8.16; N, 4.73. Found:
b
2
41
C, 65.827; H, 8.187; N, 4.321.
Preparation of (IPr)Fe(1,3-dioxan-2-ylethyl)2 (3 ). To a
IPr
2
scintillation vial were added Fe(OAc) (43.4 mg, 0.25 mmol) and
2
1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (100 mg, 0.25
mmol) along with a stir bar. To the solid mass was added (2-(1,3-
dioxan-2-yl)ethyl)magnesium bromide in THF (3.0 mL, 0.5 M, 1.5
mmol) in one portion at room temperature. The reaction mixture was
stirred at room temperature for 10 min, followed by cooling to −78 °C.
The yellow slurry was then warmed on a room temperature stir plate for
32,41
F
DOI: 10.1021/acs.organomet.8b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
5
min while stirring, after which room temperature pentane (ca. 10 mL)
Electronic Structure Calculations. Spin unrestricted density
was added to the mixture and the vial cooled back to −78 °C. The
resulting solution was filtered through Celite (precooled to −78 °C),
and the yellow filtrate was stored at −80 °C for 1 week, rendering yellow
crystals. Evans method analysis of crystalline 3 redissolved in THF-d
indicates μeff = 4.4 μ due to the presence of excess MgBr (THF) salts
which could not be experimentally separated from the isolated material.
Elemental Anal. Calcd for 4:1, (IPr)Fe(1,3-dioxan-2-ylethyl) :MgBr :
C, 64.99; H, 8.11; N, 3.89. Found: C, 64.966; H, 8.421; N, 3.049.
functional theory (DFT) calculations were performed with the
48
Gaussian 09 package. All geometry optimization calculations were
performed with the B3LYP functional with the def2-TZVP basis set on
IPr
8
49
all atoms in the gas phase. The dispersion correction of Grimme
50
(GD2) was used in geometry optimizations of all complexes. The
geometries of all complexes were fully optimized starting from X-ray
crystal structures. All optimized geometries had frequencies found to be
positive. Energies are given in the Supporting Information and include
zero-point and thermal corrections. Further calculations of molecular
b
n
m
2
2
Preparation of Solid 57Fe Mo
ssbauer Samples. Scintillation
̈
5
1−54
orbitals (MOs) and TD-DFT used the B3LYP functional
with the
vials containing crystalline material were transported quickly into a
glovebox in a liquid nitrogen precooled aluminum pie block. The vials
were then placed in a cold well at −78 °C, and the mother liquor was
decanted and discarded. The residual crystalline material was washed
with cold pentane (−78 °C, 2 mL) and decanted. The washing and
decanting process was repeated five times. A mortar and pestle were
placed on top of an aluminum pie block submerged in liquid nitrogen
for cooling. A liquid nitrogen chilled metal spatula was used to collect
crystalline material from the vial, and the frozen crystalline solids were
chipped off onto the mortar. The crystalline solids were finely ground
49
def2-TZVP basis set on all atoms. The analyses of MO compositions
and Mayer bond orders were performed using the AOMix program.
Orbitals from the Gaussian calculations were plotted with the
55,56
57
ChemCraft program.
SambVca. 2.0 Calculations. All values were computed using
SambVca. 2.0 - A Web Application for the Buried Volume and Steric
44
43,58
Maps Calculation. The percent buried volume (%Vbur
)
of the first
coordination sphere around iron (3.5 Å radius) occupied by the NHC
ligands was computed for the B3LYP/def2-TZVP optimized structures
of 1−4. Bondi radii and sphere radius were set to standard values of 1.17
and 3.5 Å, respectively. H atoms were included in the steric calculations
due to the unique combination effect of backbone saturation and side
with a cold pestle. A Delrin Mossbauer sample cup was placed on the
̈
cold aluminum pie block surface, and a cold spatula was used to transfer
the frozen mull material into the cup. The cup was then sealed with an
inner cup lined with silicon grease and stored in liquid nitrogen.
44
arm N-substituent groups on the structure. Steric maps indicate 0.00
Å on their axes as the sphere radius of 3.5 Å from the metal center.
Positive values (red) represent areas of high steric hindrance (into the
sphere), and negative values (blue) represent distance away from the
sphere radius.
57
57
Fe Mo
ssbauer Spectroscopy. Solid samples for Fe Mossbauer
̈ ̈
spectroscopy were made with isolated complexes based on natural
abundance of Fe or from Fe-enriched samples of isolated complexes.
57
57
5
7
Frozen solution samples for Fe Mo
̈
ssbauer spectroscopy were
prepared from 57Fe(OAc) or isolated Fe-enriched complexes. All
57
2
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
samples were prepared in an inert atmosphere glovebox with a liquid
nitrogen fill port to freeze each sample at 77 K within the glovebox. Each
sample was loaded into a Delrin Mo
■
*
S
̈
ssbauer sample cup for
measurements and loaded under liquid nitrogen. Low temperature
ACS Publications website at DOI: 10.1021/acs.organo-
57
Fe Mo
Mossbauer spectrometer integrated with a Janis SVT-400T He/N2
cryostat for measurements at 5 K. Isomer shifts were determined
relative to α-Fe at 298 K. All Mossbauer spectra were fit using the
program WMoss (SEE Co).
̈
ssbauer measurements were performed using a SEE Co. MS4
met.8b00466.
̈
57
Supplementary data including Fe Mo
̈
ssbauer and NMR
spectra, details of DFT calculations, and X-ray crystal
structure data (PDF)
Cartesian coordinates (XYZ)
̈
Preparation of Solid MCD Samples. Scintillation vials containing
crystalline material were transported quickly into a glovebox in a liquid
nitrogen precooled aluminum pie block. The vials were then placed in a
cold well at −78 °C, and the mother liquor was decanted and discarded.
The residual crystalline material was washed with cold pentane (−78
Accession Codes
CCDC 1855763 and 1855764 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
°C) and decanted. The washing and decanting process was repeated
five times. A mortar and pestle were placed on top of an aluminum pie
block submerged in liquid nitrogen for cooling. A liquid nitrogen chilled
metal spatula was used to collect crystalline material from the vial, and
the frozen crystalline solids were chipped off onto the mortar. A few
drops of cold isopentane (−80 °C) were added to the mortar. The
mortar was lifted from the aluminum pie block long enough to allow the
mixture to thaw, and the solids were promptly ground with the cold
pestle and refrozen. The process was repeated until a fine mull was
obtained. A quartz disc was placed on the cold aluminum pie block
surface, and a cold spatula was used to transfer the frozen mull material
onto the disc. The quartz disc was lifted from the aluminum pie block
long enough to allow for the material to thaw and spread thinly over the
disc. A second precooled quartz disc was quickly set on top of the first
disc and the material refrozen. All components of the MCD cell were
assembled while submerged in liquid nitrogen.
AUTHOR INFORMATION
Corresponding Author
E-mail: neidig@chem.rochester.edu.
■
*
ORCID
William W. Brennessel: 0000-0001-5461-1825
Michael L. Neidig: 0000-0002-2300-3867
Author Contributions
̃
S. B. Munoz III and V. E. Fleischauer contributed equally to this
work.
†
Magnetic Circular Dichroism (MCD) Spectroscopy. NIR MCD
experiments were conducted using a Jasco J-730 spectropolarimeter
and a liquid nitrogen cooled In-Sb detector. The spectral range
accessible was 2000−600 nm. The instrument utilizes a modified
sample compartment incorporating focusing optics and an Oxford
Instruments SM4000-7T superconducting magnet/cryostat allowing
measurements from 1.6 to 290 K with magnetic fields up to 7 T. A
calibrated Cernox sensor inserted directly in the copper sample holder
was used to measure the temperature of the sample ±0.001 K. All MCD
spectra were baseline corrected using zero-field scans.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a grant from the National Institutes
of Health (R01GM111480 to M.L.N.), by a Ruth L. Kirchstein
National Research Service award (F32GM120823 to S.B.M.),
and by the NSF MRI program for acquisition of an X-ray
diffractometer (CHE-1725028).
G
DOI: 10.1021/acs.organomet.8b00466
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Reagent via Fe-Catalyzed C-O Bond Activation. J. Am. Chem. Soc. 2009,
REFERENCES
■
1
(
31, 14656−14657.
(
1) Marion, N.; Díez-Gonzal
Carbenes as Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2988−
000.
2) Herrmann, W. A. N-Heterocyclic Carbenes: A New Concept in
Organometallic Catalysis. Angew. Chem., Int. Ed. 2002, 41, 1290−1309.
3) Arevalo, R.; Espinal-Viguri, M.; Huertos, M. A.; Perez, J.; Riera, L.
́
ez, S.; Nolan, S. P. N-Heterocyclic
22) Agata, R.; Iwamoto, T.; Nakagawa, N.; Isozaki, K.; Hatakeyama,
T.; Takaya, H.; Nakamura, M. Iron Fluoride/N-Heterocyclic Carbene
Catalyzed Cross Coupling between Deactivated Aryl Chlorides and
Alkyl Grignard Reagents with or without β-Hydrogens. Synthesis 2015,
3
(
4
(
7, 1733−1740.
(
23) Agrawal, T.; Cook, S. P. Iron-Catalyzed Cross-Coupling
Dearomatization of transition metal-coordinated N-heterocyclic
ligands and related chemistry. Adv. Organomet. Chem. 2016, 65, 47−
Reactions of Alkyl Grignards with Aryl Sulfamates and Tosylates.
Org. Lett. 2013, 15, 96−99.
1
(
14.
4) Díez-Gonzal
Carbenes in Late Transition Metal Catalysis. Chem. Rev. 2009, 109,
612−3676.
5) Farmer, J. L.; Pomepeo, M.; Organ, M. G. Pd-N-Heterocyclic
(24) Bica, K.; Gaertner, P. An Iron-Containing Ionic Liquid as
́
ez, S.; Marion, N.; Nolan, S. P. N-Heterocyclic
Recyclable Catalyst for Aryl Grignard Cross-Coupling of Alkyl Halides.
Org. Lett. 2006, 8, 733−735.
3
(
(25) Deng, H.-N.; Xing, Y.-L.; Xia, C.-L.; Sun, H.-M.; Shen, Q.; Zhang,
Y. Ionic iron(III) complexes of bis(phenol)-functionalized imidazolium
cations: synthesis, structures and catalysis for aryl Grignard cross-
coupling of alkyl halides. Dalton Trans. 2012, 41, 11597−11607.
Carbene Complexes in Cross-Couping Applications. In Ligand Design
in Metal Chemistry: Reactivity and Catalysis, 1st ed.; Stradiotto, M.,
Lundgren, R. J., Eds.; John Wiley & Sons, Ltd.: Toronto, Ontario
Canada, 2016; pp 134−175.
6) Fortman, G. C.; Nolan, S. P. N-Heterocyclic carbene (NHC)
ligands and palladium in homogeneous cross-coupling catalysis: a
(26) Gao, H.-H.; Yan, C.-H.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.;
Zhang, Y. Synthesis of Anionic Iron(II) Complex Bearing an N-
Heterocyclic Carbene Ligand and Its Catalysis for Aryl Grignard Cross-
Coupling of Alkyl Halides. Organometallics 2010, 29, 4189−4192.
(27) Ghorai, S. K.; Jin, M.; Hatakeyama, T.; Nakamura, M. Cross-
Coupling of Non-activated Chloroalkanes with Aryl Grignard Reagents
in the Presence of Iron/N-Heterocyclic Carbene Catalysts. Org. Lett.
2012, 14, 1066−1069.
(28) Perry, M. C.; Gillett, A. N.; Law, T. C. An unprecedented iron-
catalyzed cross-coupling of primary and secondary alkyl Grignard
reagents with non-activated aryl chlorides. Tetrahedron Lett. 2012, 53,
4436−4439.
(
perfect union. Chem. Soc. Rev. 2011, 40, 5151−5169.
7) Gafurov, Z. N.; Kantyukov, A. O.; Kagilev, A. A.; Balabayev, A. A.;
(
Sinyashin, O. G.; Yakhvarov, D. G. Nickel and palladium N-heterocyclic
carbene complexes. Synthesis and application in cross-coupling
reactions. Russ. Chem. Bull. 2017, 66 (9), 1529−1535.
(
8) Marion, N.; Nolan, S. P. N-Heterocyclic carbenes in gold catalysis.
Chem. Soc. Rev. 2008, 37, 1776−1782.
9) Sipos, G.; Dorta, R. Iridium complexes with monodentate N-
heterocyclic carbene ligands. Coord. Chem. Rev. 2017, In press.
DOI: 10.1016/j.ccr.2017.10.019.
10) Huynh, H. V. Electronic Properties of N-Heterocyclic Carbenes
and Their Experimental Determination. Chem. Rev. 2018, Article
ASAP. DOI: 10.1021/acs.chemrev.8b00067.
̈
11) Furstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. Effective
(
(29) Xia, C.-L.; Xie, C.-F.; Wu, Y.-F.; Sun, H.-M.; Shen, Q.; Zhang, Y.
Efficient cross-coupling of aryl Grignard reagents with alkyl halides by
recyclable ionic iron(III) complexes bearing a bis(phenol)-function-
alized benzimidazolium cation. Org. Biomol. Chem. 2013, 11, 8135−
(
(
8
(
144.
30) Silberstein, A. L.; Ramgren, S. D.; Garg, N. K. Iron-Catalyzed
Alkylations of Aryl Sulfamates and Carbamates. Org. Lett. 2012, 14,
796−3799.
31) Agata, R.; Iwamoto, T.; Nakagawa, N.; Isozaki, K.; Hatakeyama,
Modulation of the Donor Properties of N-Heterocyclic Carbene
Ligands by “Through-Space” Communication within a Planar Chiral
Scaffold. J. Am. Chem. Soc. 2007, 129, 12676−12677.
3
(
(
12) Peris, E. Smart N-Heterocyclic Carbene Ligands in Catalysis.
Chem. Rev. 2017, Article ASAP. DOI: 10.1021/acs.chemrev.6b00695.
13) Huang, J.; Nolan, S. P. Efficient Cross-Coupling of Aryl
Chlorides with Aryl Grignard Reagents (Kumada Reaction) Mediated
by a Palladium/Imidazolium Chloride System. J. Am. Chem. Soc. 1999,
T.; Takaya, H.; Nakamura, M. Iron Fluoride/N-Heterocyclic Carbene
Catalyzed Cross Coupling between Deactivated Aryl Chlorides and
Alkyl Grignard Reagents with or without β-Hydrogens. Synthesis 2015,
(
4
(
7, 1733−1740.
1
(
21, 9889−9890.
32) Guisan-Ceinos, M.; Tato, F.; Bunuel, E.; Calle, P.; Cardenas, D. J.
́
̃
́
14) Frisch, A. C.; Rataboul, F.; Zapf, A.; Beller, M. First Kumada
Fe-catalysed Kumada-type alkyl−alkyl cross-coupling. Evidence for the
intermediacy of Fe(I) complexes. Chem. Sci. 2013, 4, 1098.
reaction of alkyl chlorides using N-heterocyclic carbene/palladium
catalyst systems. J. Organomet. Chem. 2003, 687, 403−409.
(33) Liu, Y.; Wang, L.; Deng, L. Three-Coordinate Iron(II) Dialkenyl
(
15) Bezier, D.; Sortais, J.-B.; Darcel, C. N-Heterocyclic Carbene
́
Compound with NHC Ligation: Synthesis, Structure, and Reactivity.
Ligands and Iron: An Effective Association for Catalysis. Adv. Synth.
Organometallics 2015, 34, 4401−4407.
Catal. 2013, 355, 19−33.
16) Ingleson, M. J.; Layfield, R. A. N-Heterocyclic carbene chemistry
of iron: fundamentals and applications. Chem. Commun. 2012, 48,
579−3589.
17) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K.; Nakamura, M.
Highly Selective Biaryl Cross-Coupling Reactions between Aryl
Halides and Aryl Grignard Reagents: A New Catalyst Combination
of N-Heterocyclic Carbenes and Iron, Cobalt, and Nickel Fluorides. J.
Am. Chem. Soc. 2009, 131, 11949−11963.
18) Hatakeyama, T.; Nakamura, M. Iron-Catalyzed Selective Biaryl
Coupling: Remarkable Suppression of Homocoupling by the Fluoride
Anion. J. Am. Chem. Soc. 2007, 129, 9844−9845.
19) Chua, Y.-Y.; Duong, H. A. Iron(II) triflate/N-heterocyclic
carbene-catalysed cross-coupling of arylmagnesiums with aryl chlorides
and tosylates. Chem. Commun. 2016, 52, 1466−1469.
20) Teng, Q.; Wu, W.; Duong, H. A.; Huynh, H. V. Ring-expanded
(34) Wang, X.; Zhang, J.; Wang, L.; Deng, L. High-Spin Iron(II)
(
Alkynyl Complexes with N-Heterocyclic Carbene Ligation: Synthesis,
Characterization, and Reactivity Study. Organometallics 2015, 34,
3
(
2
(
775−2782.
35) Xiang, L.; Xiao, J.; Deng, L. Synthesis, Structure, and Reactivity of
Organo-Iron(II) Complexes with N-Heterocyclic Carbene Ligation.
Organometallics 2011, 30, 2018−2025.
(
36) Zhang, Q.; Xiang, L.; Deng, L. Dinuclear Iron−Imido Complexes
with N-Heterocyclic Carbene Ligation: Synthesis, Structure, and Redox
(
Reactivity. Organometallics 2012, 31, 4537−4543.
(37) Liu, Y.; Luo, L.; Xiao, J.; Wang, L.; Song, Y.; Qu, J.; Luo, Y.; Deng,
L. Four-Coordinate Iron(II) Diaryl Compounds with Monodentate N-
Heterocyclic Carbene Ligation: Synthesis, Characterization, and Their
Tetrahedral-Square Planar Isomerization in Solution. Inorg. Chem.
2015, 54, 4752−4760.
(
(
N-heterocyclic carbenes as ligands in iron-catalysed cross-coupling
reactions of arylmagnesium reagents and aryl chlorides. Chem.
Commun. 2018, 54, 6044−6047.
(38) Fillman, K. L.; Przyojski, J. A.; Al-Afyouni, M. H.; Tonzetich, Z.
J.; Neidig, M. L. A combined magnetic circular dichroism and density
functional theory approach for the elucidation of electronic structure
and bonding in three- and four-coordinate iron(ii)-N-heterocyclic
carbene complexes. Chem. Sci. 2015, 6, 1178−1188.
(
21) Li, B.-J.; Xu, L.; Wu, Z.-H.; Guan, B.-T.; Sun, C.-L.; Wang, B.-Q.;
Shi, Z.-J. Cross-Coupling of Alkenyl/Aryl Carboxylates with Grignard
H
DOI: 10.1021/acs.organomet.8b00466
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
39) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. J. Complexes of
(55) Gorelsky, S. I. AO Mix: Program for Molecular Orbital Analysis,
version 6.8.5; 2014.
Iron(II) and Iron(III) Containing Aryl-Substituted N-Heterocyclic
Carbene Ligands. Organometallics 2012, 31, 3264−3271.
(56) Gorelsky, S. I.; Lever, A. B. P. Electronic structure and spectra of
ruthenium diimine complexes by density functional theory and INDO/
S. Comparison of the two methods. J. Organomet. Chem. 2001, 635,
187−196.
(
40) Przyojski, J. A.; Veggeberg, K. P.; Arman, H. D.; Tonzetich, Z. J.
Mechanistic Studies of Catalytic Carbon-Carbon Cross-Coupling by
Well-Defined Iron NHC Complexes. ACS Catal. 2015, 5, 5938−5946.
(
41) Fleischauer, V. E.; Mun
̃
oz, S. B., III; Neate, P. G. N.; Brennessel,
(57) Chemcraft: Graphical software for visualization of quantum
chemistry computations. https://www.chemcraftprog.com.
W. W.; Neidig, M. L. NHC and nucleophile chelation effects on reactive
(58) Poater, A. R. F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan, S. P.;
iron(ii) species in alkyl-alkyl cross-coupling. Chem. Sci. 2018, 9, 1878−
Cavallo, L.; Ragone, F. Thermodynamics of N-heterocyclic carbene
dimerization: The balance of sterics and electronics. Organometallics
1
(
891.
42) Yang, L.; Powell, D. R.; Houser, R. P. Structural Variation in
2008, 27, 2679−2681.
Copper(I) Complexes with Pyridylmethylamide ligands: Structural
Analysis with a New Four-coordinate Geometry Index, τ . Dalton.
4
Trans. 2007, 9, 955−964.
(
43) Poater, A. C. B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.;
Cavallo, L.; Cosenza, B. SambVca: A Web Application for the
Calculation of the Buried Volume of N-Heterocyclic Carbene Ligands.
Eur. J. Inorg. Chem. 2009, 2009, 1759−1766.
(
44) Poater, A.; Ragone, F.; Mariz, R.; Dorta, R.; Cavallo, L.
Comparing the Enantioselective Power of Steric and Electrostatic
Effects in Transition-Metal-Catalyzed Asymmetric Synthesis. Chem. -
Eur. J. 2010, 16, 14348−14353.
(
45) Hatakeyama, T.; Hashimoto, T.; Kathriarachchi, K. K.; Zenmyo,
T.; Seike, H.; Nakamura, M. Iron-catalyzed alkyl-alkyl Suzuki-Miyaura
coupling. Angew. Chem., Int. Ed. 2012, 51, 8834−7.
(
46) Dongol, K. G.; Koh, H.; Sau, M.; Chai, C. L. L. Iron-Catalysed
sp3−sp3 Cross-Coupling Reactions of Unactivated Alkyl Halides with
Alkyl Grignard Reagents. Adv. Synth. Catal. 2007, 349, 1015−1018.
(
47) Kneebone, J. L.; Fleischauer, V. E.; Daifuku, S. L.; Shaps, A. A.;
Bailey, J. M.; Iannuzzi, T. E.; Neidig, M. L. Electronic Structure and
Bonding in Iron(II) and Iron(I) Complexes Bearing Bisphosphine
Ligands of Relevance to Iron-Catalyzed C−C Cross-Coupling. Inorg.
Chem. 2016, 55, 272−282.
(
48) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G.
A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.;
Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang,
W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark,
M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith,
T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.;
Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene,
M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma,
K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Rev. B.01;
Gaussian, Inc.: Wallingford, CT, 2016.
(
49) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence,
triple zeta valence and quadruple zeta valence quality for H to Rn:
Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7,
3
(
297−305.
50) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping
function in dispersion corrected density functional theory. J. Comput.
Chem. 2011, 32, 1456−1465.
(
51) Becke, A. D. Density-functional thermochemistry. III. The role of
exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(
52) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti
correlation-energy formula into a functional of the electron density.
Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.
(
53) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent
electron liquid correlation energies for local spin density calculations: a
critical analysis. Can. J. Phys. 1980, 58, 1200.
(
54) Stephens, P. J.; Devlin, F. J.; Chabalowski, M. J.; Frisch, M. J. Ab
Initio Calculation of Vibrational Absorption and Circular Dichroism
Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98,
1
1623−11627.
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DOI: 10.1021/acs.organomet.8b00466
Organometallics XXXX, XXX, XXX−XXX