Methoxy-functionalized N-heterocyclic carbenes

Article abstract of DOI:10.1021/om5001919

The effect of methoxy functionalization of three N-heterocyclic carbene ligands was assessed using a variety of methods. The steric environment of each carbene has been assessed in various coordination environments. The electronic properties, specifically the electron-donating character and π-accepting ability, have been evaluated using nickel and iridium complexes and selenium adducts, respectively. Comparisons with the parent systems have been made with respect to both electronic and steric properties. The carbenes IPr ^OMe, SIPr^OMe, and IPr^OMe have been found to be more electron donating than the parent systems IPr, SIPr, and IPr* and only slightly less π accepting, yet they exhibit similar steric properties.

Full text of DOI:10.1021/om5001919

Article
pubs.acs.org/Organometallics
Methoxy-Functionalized N‑Heterocyclic Carbenes
David J. Nelson, Alba Collado, Simone Manzini, Sebastien Meiries, Alexandra M. Z. Slawin,
David B. Cordes, and Steven P. Nolan*
EaStCHEM School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K.
S
* Supporting Information
ABSTRACT: The effect of methoxy functionalization of three
N-heterocyclic carbene ligands was assessed using a variety of
methods. The steric environment of each carbene has been
assessed in various coordination environments. The electronic
properties, specifically the electron-donating character and π-
accepting ability, have been evaluated using nickel and iridium
complexes and selenium adducts, respectively. Comparisons
with the parent systems have been made with respect to both
electronic and steric properties. The carbenes IPrOMe, SIPrOMe, and IPr*OMe have been found to be more electron donating than
the parent systems IPr, SIPr, and IPr* and only slightly less π accepting, yet they exhibit similar steric properties.
complexes bearing unsymmetrical NHC ligands was interpreted
as evidence that the electronic communication from the N-aryl
substituents to elsewhere on the complex did not occur
exclusively via inductive and/or mesomeric effects over seven
bonds to the metal center. In each case, IMes and SIMes (and
variants thereof with different para substituents) were
considered. More recently, Cavallo and co-workers have
investigated this issue and presented computational evidence
for an interaction between the ipso carbon of the N-aryl
substituent and empty metal d orbitals in NHC-bearing
transition-metal complexes.21
Munz et al. have shown that substitution of the para position
of an NHC N-aryl substituent in chelated bis(NHC)
palladium(II) complexes leads to greater changes in properties
than functionalization at the ortho or meta positions.22 We have
found recently that p-methoxy functionalization of the aryl
rings of the IPr*, IPent, IHept, and INon ligands23,24 (leading
to the new ligands IPentOMe, IHeptOMe, INonOMe, and
IPr*OMe) improved the performance of palladium catalysts in
C−N, C−C, and C−S bond formation reactions.25−28 We
wished to explore whether similar functionalization of IPr and
SIPr might lead to ligands with valuable new properties (Chart
1). Therefore, a number of model systems bearing IPr, SIPr,
IPr*, IPrOMe, SIPrOMe, and IPr*OMe have been studied, to fully
explore the effect of the p-methoxy substituent. In particular,
the effect of this structural modification on the value of a
number of common metrics was explored, to see if these could
be correlated with the improved catalytic activity of metal
complexes bearing these ligands and indeed whether these
metrics could quantify the differences in their properties.
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) are incredibly versatile ligands
in organometallic chemistry1,2 and have been employed with
great success in a variety of applications such as ruthenium-
catalyzed alkene metathesis3,4 and palladium-catalyzed cross
coupling.5 Research within our group is focused on the
synthesis and study of these interesting species, with particular
focus on their application in the field of homogeneous catalysis.
Metrics such as the percent buried volume (%Vbur)6 allow the
steric bulk of NHCs to be quantified, while the Tolman
electronic parameter (TEP)7 allows for the comparison and
quantification of their electronic properties,8 with the aim of
understanding how structure affects the properties of these
ligands and the complexes to which they are coordinated.
However, a detailed and quantitative comparison of these
characteristics is not always straightforward.9,10 Various model
systems have been used to probe and quantify the numerous
facets of the electronic properties of NHCs,8,11−15 while the use
of steric maps, calculated using the SambVca web application,16
has allowed more detailed analysis of how NHCs can influence
the steric environment around metal centers. Such evaluation
and quantification are important, as they allow the appraisal of
properties of new NHCs in the context of the vast body of
literature that exists on the subject.
The variety and flexibility of synthetic routes to (4,5-
dihydro)imidazolium-type NHC precursors has enabled a vast
number of such ligands to be prepared,17 with different
functional groups in various positions. Plenio and co-workers
have shown that modulation of the para substituent of N,N′-
diarylimidazolylidenes can change the electronic character of
the ligands, as determined by measurement of the TEP and by
electrochemical experiments.18 In other studies by the same
group, π-face donation from the N-aryl substituents to the
benzylidene ligand in second-generation ruthenium benzyli-
dene complexes was revealed.19,20 The presence of two distinct
redox processes with different half-wave potentials for
Received: February 22, 2014
Published: April 10, 2014
© 2014 American Chemical Society
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structures of the parent species bearing IPr (1a),34 SIPr (1c),35
and (IPr*) (1e)31 have been reported previously. The new
complexes [AuCl(IPrOMe)] (1b), [AuCl(SIPrOMe)] (1d), and
[AuCl(IPr*OMe)] (1f) were prepared using the recently
disclosed general methodology (Scheme 2).33 Heating the
Chart 1. Some Common NHC ligands and Their Methoxy
Analogues
Scheme 2
NHC precursor salt (IPrOMe·HCl, SIPrOMe·HCl, or IPr*OMe
·
HCl) with [AuCl(DMS)]36 in acetone in the presence of a mild
base (K2CO3) allowed the preparation of the corresponding
analytically pure [AuCl(NHC)] complexes 1b,d,f as white
solids in 72%, 56% and 91% yields, respectively. Complex 1d is
poorly soluble in acetone and dichloromethane, in contrast to
the vast majority of [AuCl(NHC)] complexes, which may be
the cause of the poorer yield of this species. Catalytic
applications of these complexes and their derivatives will be
reported in due course.
Single crystals of the new complexes were grown for X-ray
analysis by slowly diffusing pentane into a dichloromethane (for
1b,d) or benzene (1f) solution (Figure 1). All three present the
expected linear geometry about the gold(I) center.
RESULTS AND DISCUSSION
■
Synthesis of Free Carbenes. The syntheses of IPr, SIPr,
IPr*, and IPr*OMe have been described previously.25,29−31 The
syntheses of the salts IPrOMe·HCl and SIPrOMe·HCl have also
been disclosed;32 the key step in their synthesis is a copper-
catalyzed Ullman coupling to generate the requisite 2,6-
diisopropyl-4-methoxyaniline, which is followed by straightfor-
ward and established steps to yield the carbene salts. The free
carbenes IPrOMe and SIPrOMe were prepared by deprotonation
of the corresponding HBF4 salts (prepared by salt metathesis)
using NaH plus catalytic KOtBu, in an argon-filled glovebox
Nickel carbonyl complexes of the form [Ni(CO)3(NHC)]
have been used to extend the evaluation of the Tolman
electronic parameter,7 traditionally used to compare the
electronic properties of phosphine ligands, to the study of
NHC ligands.8 Infrared spectroscopic analysis of these
complexes indicates the electron density of the metal center
by probing its ability to engage in d to π* back-bonding, which
weakens the carbon−oxygen bond. It is therefore a measure of
a combination of the σ-donating and π-accepting properties of
the NHC. In addition, these complexes provide excellent
examples of tetrahedral NHC-bearing complexes, allowing
further evaluation of the steric behavior of the NHC ligand via
their crystal structures. The syntheses and characterization of
[Ni(CO)3(IPr)] (2a), [Ni(CO)3(SIPr)] (2c),8 [Ni-
(CO)3(IPr*)] (2e),37 and [Ni(CO)3(IPr*OMe)] (2f)25 have
been reported previously. New complexes were prepared via
the addition of [Ni(CO)4] to a THF solution of the free
carbene, yielding the complexes [Ni(CO)3(IPrOMe)] (2b) and
[Ni(CO)3(SIPrOMe)] (2d) in 57% and 93% yields, respectively
(Scheme 3). Crystals suitable for X-ray diffraction analysis were
prepared by slow diffusion of pentane into a saturated solution
of each complex in benzene (Figure 2).
While the TEP has classically been determined via the use of
[Ni(CO)3L] complexes (where L is a phosphine or NHC), the
use of [IrCl(CO)2L] or [RhCl(CO)2L] complexes is more
convenient, is more accessible, and avoids the use of highly
hazardous [Ni(CO)4].11,12,38 Recent work has allowed the
development of useful correlations to translate the stretching
frequencies of the carbonyl ligands in these complexes into
values that can be compared to those determined using the
nickel system.9,10 Here, the use of the iridium system was
explored for two reasons: (i) to provide additional data on the
electronic character of the metal centers in complexes bearing
these new NHCs, via IR spectroscopy, and (ii) to evaluate the
steric bulk of the ligands in a third coordination environment,
namely square planar, which is relevant to palladium(II)
Scheme 1
1
(Scheme 1). These carbenes were characterized by H and
13C{1H} NMR spectroscopy. The key δC(carbene) chemical
shifts for IPrOMe, SIPrOMe, and IPr*OMe were found to be at
221.6 (C6D6), 245.0 (C6D6), and 221.8 (THF-d8) ppm
(compared to 220.6 (C6D6), 244.0 (C6D6), and 220.0 (THF-
d8) ppm for IPr, SIPr, and IPr*).14,25,31 The methoxy
functionality shifts the carbene signals slightly downfield,
suggesting that the electronic effects of methoxy substitution
are not negligible even in the free carbene.
Synthesis of Model Complexes. Four classes of model
compounds (1−5) bearing these NHCs (NHC: a, IPr; b,
IPrOMe; c, SIPr; d, SIPrOMe; e, IPr*; f, IPr*OMe) were prepared,
to allow the characterization of their electronic and steric
properties.
Linear Au(I) complexes of the form [AuCl(NHC)] are
excellent models with which to evaluate the steric properties of
new ligands;6 the linear geometry enables NHC ligands to
adopt their preferred conformation without interacting with
other ligands present on the metal center. Notably, these can be
prepared directly from the imidazolium salt, without the need
to employ the free carbene.33 The syntheses and crystal
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Figure 1. X-ray crystal structures of (left) [AuCl(IPrOMe)] (1b), (center) [AuCl(SIPrOMe)] (1d), and (right) [AuCl(IPr*OMe)] (1f). Thermal
ellipsoids are drawn at the 50% probability level, and most H atoms are excluded for clarity.
Scheme 3
Scheme 4
therefore carried out under rigorously air- and moisture-free
conditions using glovebox and Schlenk techniques.
While the TEP quantifies the overall ability of the metal
center to engage in d to π* back-bonding, it does not separate
the contributions from σ and π bonding between the NHC and
the metal center; it requires the assumption that all ligands
being compared are similar in terms of π-accepting ability. The
latter can contribute significantly to the overall bonding
between the NHC and the metal center.9,40 Three methods
have been proposed to assess the contribution of d−π
interactions to the NHC−metal bond. Analysis of δPt (via the
Figure 2. X-ray crystal structures of (top) [Ni(CO)3(IPrOMe)] (2b)
and (bottom) [Ni(CO)3(SIPrOMe)] (2d). Thermal ellipsoids are
drawn at the 50% probability level, and most H atoms are excluded for
clarity.
1
195Pt nuclide) and JPt−C for [PtCl2(DMSO)(NHC)] com-
plexes allows the electron density at the metal and d to π back-
bonding contributions to be investigated separately.13 However,
this requires detection of 195Pt satellites for a signal that is
typically very weak in the 13C{1H} NMR spectrum, thus
necessitating long NMR experiment times and large quantities
of expensive platinum. Bertrand and co-workers recently
proposed the use of phosphinidene adducts [PPh(NHC)],
prepared in two synthetic steps, to probe π back-bonding via
the δP chemical shift on the 31P NMR spectrum.14 Ganter and
co-workers reported the use of selenoureas, prepared in one
synthetic step by deprotonation of the imidazolium salt in the
presence of excess selenium, to probe π back-bonding via the
δSe chemical shift on the 77Se NMR spectrum.15 In both of the
latter examples, adducts might be viewed as existing between
two extremes: one where the NHC−E bond has single-bond
character and one where it has double-bond character. In the
former, there is little π back-bonding, while in the latter this
occurs to a considerable extent (Figure 4). Further studies are
underway within our group to explore this proposed relation-
ship between chemical shift and electronic structure.
complexes,39 also of d8 configuration, that are important
precatalysts and catalytic intermediates. [IrCl(CO)2(IPr)] (4a)
and [IrCl(CO)2(SIPr)] (4c) have been reported previously.11
New [IrCl(COD)(NHC)] complexes were prepared first, by
addition of the carbene to a THF solution of [IrCl(COD)]2.
[IrCl(COD)(IPrOMe)] (3b), [IrCl(COD)(SIPrOMe)] (3d),
[IrCl(COD)(IPr*)] (3e), and [IrCl(COD)(IPr*OMe)] (3f)
were isolated in 63−86% yields as analytically pure yellow
microcrystalline solids (Scheme 4). Single crystals of 3b,d−f
suitable for X-ray diffraction studies were grown (Figure 3).
Exposing these species, in DCM solution, to carbon
monoxide yielded the desired cis-dicarbonyl complexes
4b,4d−f. The cis arrangement of the carbonyl ligands was
confirmed by IR and 13C{1H} NMR spectroscopy. While
complexes of this type are typically air and moisture stable, all
attempts to prepare and purify 4b on the bench unexpectedly
led to decomposition. The synthesis and purification of 4b was
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Figure 3. X-ray crystal structures of (top left) [IrCl(COD)(IPrOMe)] (3b), (top right) [IrCl(COD)(SIPrOMe)] (3d), (bottom left)
[IrCl(COD)(IPr*)] (3e), and (bottom right) [IrCl(COD)(IPr*OMe)] (3f). Thermal ellipsoids are drawn at the 50% probability level, and most
H atoms are excluded for clarity.
evaluated and compared to those of the corresponding parent
carbenes.
Steric Properties. The steric properties were first evaluated
by calculating the percent buried volume (%Vbur)6 for each of
the gold, nickel, and iridium complexes (1−3), using the
SambVca web application (Table 1).16 Crystal structures were
obtained from the literature for [AuCl(IPr)] (1a),34 [AuCl-
Figure 4. Phosphinidene and selenium adducts as probes of π back-
(SIPr)] (1c),35 [AuCl(IPr*)] (1e),31 [Ni(CO)3(IPr)] (2a),
bonding to NHCs.
[Ni(CO)3(SIPr)] (2d),8 [Ni(CO)3(IPr*)] (2e),37 [Ni-
(CO)3(IPr*OMe)] (2f),25 [IrCl(COD)(IPr)] (3a), and [IrCl-
The use of selenium adducts was selected, due to the simpler
synthetic protocol and the stability of the products. A THF
solution of each free carbene was prepared in the glovebox and
added via cannula to a Schlenk flask containing excess selenium
(Scheme 5). The desired adducts were obtained in 66−96%
(COD)(SIPr)] (3c).11 Buried volumes were recalculated for
each of these structures, to ensure accuracy in the comparisons
between ligands.
Comparison of the buried volume for each methoxycarbene
with that of its parent congener typically revealed a very modest
difference in steric impact. For the iridium system, there was
effectively no difference in the steric impact, especially given
that for 3a,c there is a ca. 2% difference in buried volume
between the two independent molecules in each case. For
nickel, methoxylation led to a 2.5% decrease in the bulk of IPr
yet a 1% increase in the bulk of IPr*. In the case of the linear
two-coordinate gold(I) complexes, where the greatest differ-
ences in steric bulk typically are manifested, the biggest
difference was for IPr* to IPr*OMe, where the methoxy
functionalization decreased the buried volume by 3.3%. The use
of solid-state structural data to infer the solution-state
properties is fraught with difficulties, as packing effects can
effect considerable changes in buried volumes. Complexes 2a
and 3a,c contain two independent molecules, between which
buried volumes vary by 0.2−2.1%. If the value of 2% is adopted
as an uncertainty in each buried volume, then only the
differences between 2a/2b and 1e/1f can be considered to be
Scheme 5
yields, as analytically pure white or light tan solids, after work-
up in air using undried/nondegassed solvents. Crystal
structures were obtained for all six adducts, via crystallization
by slow diffusion of pentane into a solution of the selenourea in
chloroform, DCM, or acetone (Figure 5).
Full characterization data for complexes 1−5 can be found in
the Experimental Section. With these data in hand, the steric
and electronic properties of the ligands were systematically
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Figure 5. Crystal structures of selenoureas (top left) [Se(IPr)] (5a), (top middle) [Se(IPrOMe)] (5b), (top right) [Se(SIPr)] (5c), (bottom left)
[Se(SIPrOMe)] (5d), (bottom middle) [Se(IPr*)] (5e), and (bottom right) [Se(IPr*OMe)] (5f). Thermal ellipsoids are drawn at the 50% probability
level, while most H atoms are excluded.
Table 1. Percent Buried Volumes (%Vbur) for Complexes 1−
a
3, Assessed using the SambVca Web Application16
NHC
IPr (a)
[Au] (1)
[Ni] (2)
[Ir] (3)
b
c
d
d
44.5
38.3, 38.1
35.8
33.9, 36.0
33.3
IPrOMe (b)
45.6
e
c
SIPr (c)
47.0
39.0
36.4, 34.5
36.1
SIPrOMe (d)
IPr* (e)
46.5
39.1
f
g
50.4
38.8
37.9
h
IPr*OMe (f)
47.1
39.8
37.9
a
Parameters: 3.5 Å sphere radius, 2.00 Å bond length, 0.10 Å mesh
spacing, H atoms excluded. For multiple independent molecules, %
b
c
Vbur is calculated for each. Structure from ref 34. Structure from ref
8. Structure from ref 11. Structure from ref 35. Structure from ref
31. Structure from ref 37. Structure from ref 25.
d
e
f
g
h
significant, with methoxylation leading to a slight reduction in
buried volume in each case.
The methoxy-NHCs adopt very similar conformations, as
illustrated by overlaying the crystal structures of [AuCl(NHC)]
complexes with the corresponding [AuCl(NHCOMe)] struc-
tures (Figure 6).
Steric maps were prepared to survey the steric environment
around the metal center in more detail. Figure 7 shows those
generated for [AuCl(NHC)] complexes 1a−f, as this environ-
ment should reveal the largest differences in the steric profile of
ligands (steric maps for 2a−f and 3a−f can be found in the
Supporting Information). Again, only very small differences
were obtained, suggesting that the methoxy functionality has
very little influence on the conformation and steric bulk of the
NHC ligand.
Notably, the [AuCl(NHC)] model is favored for the
assessment of the steric impact of the NHC, due to the linear
geometry of the AuCl fragment being in a position to avoid
repulsive interactions with the NHC ligand. With crystal
structure data for selenourea adducts in hand, the buried
volumes of the ligands in these species were assessed using
SambVca (Table 2). A fixed Se−C distance of 1.8 Å was used,
to reflect the shorter Se−C bond in comparison to bonds
between NHCs and transition-metal centers. If these
Figure 6. Overlays of the crystal structures of (i) [AuCl(IPr)] (1a)
and [AuCl(IPrOMe)] (1b), (ii) [AuCl(SIPr)] (1c) and [AuCl-
(SIPrOMe)] (1d), and (iii) [AuCl(IPr*)] (1e) and [AuCl(IPr*OMe)]
(1f).
selenoureas could provide a similar means of assessing steric
impact, the synthesis of gold(I) complexes, which requires
expensive starting materials, might be avoidable. Interestingly,
the buried volumes for each of the NHCs are lower on
selenium compounds 5 than on linear gold(I) species 1,
typically by 3−5%, even with a much shorter distance between
the carbene and the sphere center (1.8 Å vs 2.0 Å). In addition,
the same trend is not evident: when IPr, SIPr, and IPr* are
compared to their methoxy-functionalized analogues, opposite
trends in %Vbur are observed in comparison to the [AuCl-
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decoupling of properties can allow for careful tuning of specific
characteristics.
Electronic Properties. The effect of methoxy substitution
on the electron-donating ability of the carbenes was assessed
using the TEP, which was calculated in two ways. The first
method uses the IR spectroscopic analysis of [Ni-
(CO)3(NHC)] (2), using the A1 vibration as an indication of
how much the carbonyl bond has been weakened by d to π*
back-donation.7,8 The second method relies on the same
principle but utilizes [IrCl(CO)2(NHC)] complexes (4)
11
instead, averaging νCO
.
The TEP can be determined from
the average of νCO by using eq 1.11 Data obtained for 2 and 4
are recorded in Table 3.
TEP = (0.847 × νCO(av)) + 336 cm−1
(1)
Table 3. Tolman Electronic Parameter Values for Methoxy-
Functionalized Carbenes versus the Parent Analogues
a
b
c
NHC
IPr (a)
TEP(Ni) /cm−1
av νCO(Ir) /cm−1
TEP(Ir) /cm−1
d
e
2051.5
2023.9
2050.2
2049.8
2051.0
2050.7
2051.7
2051.4
IPrOMe (b)
2049.9
2023.4
d
e
SIPr (c)
2052.2
2024.9
SIPrOMe (d)
IPr* (e)
2050.3
2024.4
2025.6
2025.3
f
2052.7
g
IPr*OMe (f)
2051.1
a
b
In DCM solution. Average of cis- and trans-CO vibrations, in DCM
solution. Using the linear regression in eq 1, from ref 11. From ref 8.
From ref 11. From ref 37. From ref 25.
c
d
e
f
g
For the iridium complexes 4, the differences in the average
CO stretching frequencies between the methoxy series and the
parent series are very small, typically only 0.3−0.5 cm−1. The
differences between the stretching frequencies of each carbonyl
ligand (cis and trans to the NHC) are approximately the same.
However, the observed differences for the nickel complexes are
larger, at 1.6−1.9 cm−1, suggesting that these two systems are
not completely equivalent for the determination of TEP.
Notably, the former system contains two carbonyl ligands
directly cis and trans to the NHC, while the latter contains three
carbonyl ligands that lie at a ca. 109° angle with respect to the
NHC. The orbitals with which the NHC and carbonyl ligands
interact in each complex are therefore different. In each case the
NHC ligands bearing a p-methoxy substituent were found to
render the metal center more electron-rich, in agreement with
analogous studies of IMes and derivatives.18 It should be noted
that while the TEP is typically quoted to one decimal place,
such accuracy, especially between different spectrometers, is
unlikely to be attained, which is a key weakness of this
approach.
Figure 7. Steric maps of [AuCl(NHC)] complexes 1a−f (NHC =
SIPr, IPr, IPr*, SIPrOMe, IPrOMe, IPr*OMe), generated using the
SambVca software (3.5 Å sphere, 0.10 Å mesh spacing, Bondi radii
scaled by 1.17, H atoms omitted). Areas are shaded from blue to red
according to the steric bulk protruding along the z axis: i.e., from the
NHC ligand toward the gold center.
Table 2. Percent Buried Volumes (%Vbur) for Complexes
a
5a−f, Assessed using the SambVca Web Application
NHC
[Se] (5)
NHC
[Se] (5)
IPr (a)
43.1
44.6
47.8
IPrOMe (b)
SIPrOMe (d)
IPr*OMe (f)
41.0
47.2
48.6
SIPr (c)
IPr* (e)
a
Parameters: 3.5 Å sphere radius, 1.80 Å bond length, 0.10 Å mesh
spacing, H atoms excluded.
(NHC)] system (alternative C−Se bond lengths provide the
same trend; see the Supporting Information). Notably, while
selenium has a smaller covalent radius than gold (1.20 Å versus
1.36 Å),41 it has a larger van der Waals radius (1.90 Å versus
1.66 Å),42 which might explain the decreased %Vbur. Selenium
adducts of this type are therefore not appropriate models to
assess the steric impact of NHCs, and the [AuCl(NHC)]
system remains the preferred option.
It can be seen from this assessment that functionalization of
the para position of the N-aryl substituent can be achieved
without inducing significant differences in the steric properties
of the ligand (typically <2% difference in %Vbur). Such
Bearing in mind the aforementioned limitations of the TEP
regarding σ donation versus π back-bonding, the latter was
assessed separately. By understanding how the π back-bonding
ability of the carbene changes with substitution pattern, the
results from TEP measurements can be placed into context. For
the present work, the single-step procedure utilizing selenium
was most convenient.15 The selenium adducts 5 of all six NHCs
1
were prepared. Characterization by H and 13C{1H} NMR
spectroscopy was carried out in CDCl3, while 77Se{1H} analysis
was performed in acetone-d6 to allow comparison with the
literature values. However, due to poor solubility, 77Se{1H}
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analyses of [Se(IPr*)] (5e) and [Se(IPr*OMe)] (5f) could not
be carried out in acetone-d6; it was not even possible to acquire
publishable H NMR spectra of 5e,f in this solvent. While this
precludes detailed comparisons with literature data recorded in
acetone-d6, it is still possible to compare IPr* and IPr*OMe on
the basis of results obtained in CDCl3. In addition, 5a−d were
analyzed in CDCl3 in order to facilitate interpretation of the
data. The results of the 77Se{1H} NMR spectroscopic analyses
of the selenium adducts can be found in Table 4.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out in an
1
Ar-filled glovebox using dry, degassed solvents unless otherwise stated.
IPr, SIPr,29,30 IPr*OMe·HCl and IPr*OMe
,
IPr*,31 and IPrOMe·HCl
25
and SIPrOMe·HCl32 were prepared according to the reported
procedures. [AuCl(DMS)] was prepared according to the literature
procedures.36 Selenium and [Ni(CO)4] were purchased from Strem
and used as supplied. Caution! [Ni(CO)4] is a highly volatile and toxic
chemical which has to be used with great caution. Prior to use it was cooled
in a freezer (−40 °C) and always used as a cold liquid throughout the
whole experiment. The excess of [Ni(CO)4] as well as all the contaminated
glassware and syringes used in this reaction were quenched thoroughly with
a solution of triphenylphosphine in THF. The vacuum trap was filled with
a solution of triphenylphosphine in toluene. Glovebox manipulations were
carried out using anhydrous/oxygen-free THF and DCM obtained
from an MBraun SPS800 solvent purification system and anhydrous/
oxygen-free pentane obtained by refluxing over P2O5 for several hours
followed by distillation.
Table 4. Measurement of the π-Accepting Properties of
a
NHCs using Selenium Adducts
δSe(parent)/ppm
acetone-d6 CDCl3
δSe(methoxy)/ppm
acetone-d6 CDCl3
NHC
b
IPr
87 (87)
90
190
106
83
177
c
87
185
104
b
SIPr
181 (181)
1H NMR spectra were recorded at 300, 400, or 500 MHz, 13C{1H}
NMR spectra at 75, 101, or 126 MHz, and 77Se{1H} NMR spectra at
76 or 95 MHz using Bruker Avance spectrometers. NMR spectra were
referenced internally to residual solvent resonances (for 1H) and
solvent signals (for 13C{1H})43 or externally to (PhSe)2 (for 77Se-
{1H}). Chemical shifts are reported in ppm. Proton and carbon
chemical shifts are reported relative to SiMe4 (δH, δC 0 ppm), and
selenium chemical shifts relative to (PhSe)2 (δSe 463 ppm). Coupling
constants are reported in Hz. Carbon and selenium resonances were
singlets unless otherwise stated. IR analyses were carried out in
solution cells using a Perkin-Elmer spectrometer, and frequencies are
reported in wavenumbers (cm−1). Elemental analyses were carried out
at the London Metropolitan University and are reported as the average
of two analyses.
X-ray Crystallographic Analyses. Details of the data collection
and refinement can be found in the Supporting Information. X-ray
diffraction data for compounds 1b,d, 2d, and 3b,d were collected by
using a Rigaku MM007 High brilliance RA generator/confocal optics
and Mercury CCD system. Data for compound 2b were collected by
using a Rigaku MM007 High brilliance RA generator/confocal optics
and Saturn CCD system. Data for compounds 1f, 3e,f, and 5c,d,f were
collected by using a Rigaku FR-X Ultrahigh brilliance Microfocus RA
generator/confocal optics and Rigaku XtaLAB P200 system. Data for
compounds 5a,b,e were collected on a Rigaku SCXmini CCD system.
Intensity data were collected at low temperature (173 K for 1d, 2d, 3b,
5a−f; 93 K for 1b,f, 2b, 3d−f), using graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å), and ω or both ω and φ steps accumulating
area detector images spanning at least a hemisphere of reciprocal
space. All data were corrected for Lorentz polarization effects. A
multiscan absorption correction was applied by using CrystalClear.44
Structures were solved by Patterson (PATTY)45 or direct methods
(SHELXS97,46 SIR97,47 SIR2004,48 or SIR2011)49 and refined by full-
matrix least squares against F2 (SHELXL-2013).46 Non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined
using a riding model. All calculations were performed using the
CrystalStructure interface.50
IPr*
c
a
77Se{1H} NMR spectroscopy was carried out in acetone-d6 at 300 K.
Values in parentheses from ref 15. Insoluble in acetone-d6.
b
c
It can be seen from these data that IPr*-based ligands are
slightly more π-accepting than IPr-type ligands (ΔδSe = ca. 15
ppm). Notably, the methoxy-substituted carbenes exhibit only a
very slightly lower degree of π-accepting ability in comparison
to the parent ligands (ΔδSe = 2−4 ppm). Further, these results
suggest that differences in TEP between p-methoxy-function-
alized bis(aryl) NHCs and their parent congeners are most
likely a result of changes to the σ-donating ability of the NHC,
rather than including a contribution from π acceptance.
However, a quantitative scale allowing the full deconvolution
of σ and π bonding by experimental means has still not been
developed.
CONCLUSIONS
■
In conclusion, we have synthesized and fully characterized the
new NHC ligands IPrOMe and SIPrOMe. In addition, the related
IPr*OMe ligand was characterized in further detail. At each
point, state-of-the-art techniques have been used to interrogate
the steric and electronic properties of these modified NHCs;
this contribution represents a detailed study of the many
aspects of the character of these NHCs.
The steric impact of the methoxy-NHCs was found to be
almost identical with that of the parent ligands, except for two
examples where the methoxy-NHCs exhibited slightly smaller
buried volumes. Notably, these are derived from solid-state
structures; thus, it is difficult to understand whether this is due
to the effect the methoxy group has on the ligand conformation
or to its effect on the packing in the crystal structure. In
addition, only very small differences were observed in the
electronic properties of these ligands, with <2 cm−1 differences
in TEP. However, in each case, the methoxy-functionalized
analogue was found to have a lower TEP, reflecting its more
electron-donating nature. The use of selenourea adducts to
explore π back-bonding suggest that methoxy functionalization
does not have an effect on this property.
Overall, only very modest differences in properties have been
discovered. The challenge remains to understand how this
structural modification can exert such a considerable effect on
the catalytic activity of palladium complexes.25,28 Work in our
laboratories to further apply these ligands in new complexes in
organometallic chemistry and catalysis is currently underway.
Synthesis of Tetrafluoroborate Salts. The NHC·HCl salt was
dissolved in a 100/1 v/v water/THF mixture and stirred for 15 min.
The almost clear solution was then treated with excess HBF4 (48% w/
w in water) at room temperature, which resulted in the formation of a
yellowish suspension. After the mixture was stirred for a further 20
min, the solid was isolated by filtration. The yellow solid was dissolved
in DCM, dried over anhydrous sodium sulfate, and filtered. The filtrate
was concentrated before the addition of pentane, which caused
precipitation of the pure imidazolium tetrafluoroborate salt; this was
isolated by filtration and dried under vacuum.
IPrOMe·HBF4. IPrOMe·HCl (1.95 g, 4.01 mmol, 1.0 equiv) was treated
with HBF4 solution (1.0 mL, 10.2 mmol, 2.5 equiv) to yield a pale
yellowish powder (2.00 g, 3.73 mmol, 93%). H NMR (300 MHz,
CDCl3): δ 8.72 (1H, t, J = 1.5, N(CH)N), 7.65 (2H, d, J = 1.4,
N(CH)2N), 6.76 (4H, s, ArH), 3.85 (6H, s, OCH3), 2.33 (4H, sept,
1
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3
3JHH = 6.8 Hz, CH(CH3)2), 1.20 (12H, d, JHH = 6.8, CH(CH3)2),
to the low solubility of the complex in acetone and dichloromethane.
1H NMR (500 MHz, CDCl3): δ 6.71 (s, 4H, ArH), 3.99 (s, 4H,
1.13 (12H, d, JHH = 6.8, CH(CH3)2). 13C{1H} NMR (75 MHz,
3
3
CDCl3): δ 162.0 (ArC), 146.7 (ArC), 138.3 (N(CH)N), 126.8
(N(CH)2N), 122.7 (ArC), 110.0 (ArCH), 55.6 (OCH3), 29.3
(CH(CH3)2), 24.3 (CH(CH3)2), 23.8 (CH(CH3)2). Anal. Calcd for
C29H41BF4N2O2: C, 64.93; H, 7.70; N, 5.22; Found: C, 64.66; H, 7.77;
N, 5.14.
N(CH2)2N), 3.85 (s, 6H, OCH3), 3.01 (sept, JH−H = 6.8, 4H,
CH(CH3)2), 1.40 (d, 3JH−H = 6.8, 12H, CH(CH3)2), 1.32 (d, 3JH−H
=
6.9, 12H, CH(CH3)2). 13C{1H}-DEPTQ NMR (126 MHz, CDCl3): δ
197.0 (Au−C), 160.5 (C−OCH3), 148.2 (ArC), 127.3 (ArC), 110.1
(ArCH), 55.4 (OCH3), 53.7 (N(CH2)2N), 29.3 (CH(CH3)2), 25.2
(CH(CH3)2), 24.2 (CH(CH3)2). Anal. Calcd for C29H42AuClN2O2: C,
50.99; H, 6.20; N, 4.10. Found: C, 50.92; H, 6.28; N, 4.16. Crystals
suitable for X-ray diffraction studies were grown by slow diffusion of
pentane into a saturated dichloromethane solution.
SIPrOMe·HBF4. SIPrOMe·HCl (4.00 g, 8.21 mmol, 1.0 equiv) was
treated with HBF4 (1.30 mL, 13.3 mmol, 1.6 equiv) to yield a bright
white powder (4.05 g, 7.52 mmol, 92%). 1H NMR (300 MHz,
CDCl3): δ 7.86 (1H, s, N(CH)N), 6.74 (4H, s, ArH), 4.66 (4H, s,
3
N(CH)2N), 3.84 (6H, s, OCH3), 2.97 (4H, sept, JHH = 6.8 Hz,
[AuCl(IPr*OMe)] (1f). IPr*OMe·HCl (100 mg, 0.102 mmol),
[AuCl(DMS)] (30.0 mg, 0.102 mmol) ,and K2CO3 (14.1 mg, 0.102
mmol). The silica pad was rinsed with dichloromethane (3 × 1.0 mL).
White solid (109 mg, 0.093 mmol, 91%). 1H NMR (400 MHz,
CDCl3): δ 7.23−7.09 (32H, m, ArH), 6.93−6.87 (8H, m, ArH), 6.56
(4H, s, ArH), 5.76 (2H, s, N(CH)2N), 5.27 (4H, s, CHPh2), 3.59 (6H,
s, OCH3). 13C{1H}-DEPTQ NMR (101 MHz, CDCl3): δ 176.0 (Au−
C), 160.2 (C−OCH3), 142.9 (ArC), 142.8 (ArC), 142.2 (ArC), 129.7
(ArCH), 129.4 (ArCH), 129.3 (ArC), 128.7 (ArCH), 128.5 (ArCH),
126.9 (ArCH), 123.4 (N(CH)2N), 115.2 (ArCH), 55.3 (OCH3), 51.5
(CHPh2). Anal. Calcd for C69H56AuClN2O2: C, 70.37; H, 4.79; N,
2.38. Found: C, 70.56; H, 4.76; N, 2.37. Crystals suitable for X-ray
diffraction studies were grown by slow diffusion of pentane into a
saturated benzene solution.
CH(CH3)2), 1.37 (12H, d, J = 6.8, CH(CH3)2), 1.22 (12H, d, J = 6.8,
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 161.7 (ArC), 158.8
(N(CH)N), 148.0 (ArC), 122.1 (ArC), 110.4 (ArCH), 55.6
(N(CH2)2N), 55.0 (OCH3), 29.4 (CH(CH3)2), 25.4 (CH(CH3)2),
23.9 (CH(CH3)2). Anal. Calcd for C29H43BF4N2O2: C, 64.69; H, 8.05;
N, 5.20; Found: C, 64.65; H, 7.95; N, 5.23.
Synthesis of Free Carbenes. Inside the glovebox, the
tetrafluoroborate salt was suspended in THF and NaH was added,
plus a catalytic amount (spatula tip) of KOtBu. The mixture was stirred
overnight at room temperature, before removal of the solid byproducts
by filtration through a pad of Celite on a frit. The mother liquor was
concentrated to a fourth of the starting volume, and pentane was
added. The solution was cooled to −38 °C to promote precipitation of
the product. The solid was then filtered off and washed with pentane.
IPrOMe. IPrOMe·HBF4 (1.00 g, 1.86 mmol) and NaH (50 mg, 2.0
mmol) in THF (250 mL) yielded a white solid (735 mg, 1.64 mmol,
Synthesis of [Ni(CO)3(NHC)] Complexes 2. In the glovebox, a
−40 °C solution of free carbene in THF was treated with an excess of
cold [Ni(CO)4]. After 6 h at room temperature, the reddish solution
was concentrated under vacuum. The resulting reddish crude solid was
dissolved in DCM (3 × 3 mL) and filtered through a pad of Celite and
cotton wool. The combined fractions were then concentrated under
vacuum, affording the desired complex.
1
88%). H NMR (400 MHz, C6D6): δ 6.91 (4H, s, ArH), 6.67 (2H, s,
3
N(CH)2N), 3.46 (6H, s, OCH3), 2.98 (4H, sept, JHH = 6.9,
CH(CH3)2), 1.31 (12H, d, 3JHH = 6.9, CH(CH3)2), 1.20 (12H, d, 3JHH
= 6.9, CH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 221.6
(NCN), 160.4 (ArC), 147.7 (ArC), 132.4 (ArC), 121.9 (N(CH)2N),
109.1 (ArCH), 54.9 (OCH3), 29.1 (CH(CH3)2), 24.7 (CH(CH3)2),
23.6 (CH(CH3)2).
[Ni(CO)3(IPrOMe)] (2b). IPrOMe (130 mg, 0.27 mmol, 1.0 equiv) in
THF (6 mL) was treated with [Ni(CO)4] (100 μL, 0.77 mmol, 2.9
1
equiv). Light purple solid (149 mg, 0.252 mol, 93%). H NMR (300
SIPrOMe. SIPrOMe·HBF4 (2.00 g, 3.7 mmol) and NaH (100 mg, 4.0
mmol) in THF (500 mL) yielded a white solid (1.1 g, 2.44 mmol, 66%
yield). 1H NMR (400 MHz, C6D6): δ 6.90 (4H, s, ArH), 3.46 (6H, s,
MHz, CDCl3): δ 7.09 (2H, app d, J = 1.7, N(CH)2N), 6.79 (4H, s,
3
ArH), 3.87 (6H, s, OCH3), 2.61 (4H, sept, JHH = 6.9, CH(CH3)2),
3
3
1.26 (12H, d, JHH = 6.9, CH(CH3)2), 1.13 (12H, d, JHH = 6.9,
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 198.0 (CO), 197.4
(Ni−C), 160.5 (ArC), 147.5 (ArC), 131.0 (N(CH)2N), 123.7 (ArC),
109.2 (ArC), 55.5 (OCH3), 28.8 (CH(CH3)2), 25.4 (CH(CH3)2),
22.6 (CH(CH3)2). Anal. Calcd for C32H40N2NiO5: C, 64.99; H, 6.82;
N, 4.74; Found: C, 64.82; H, 6.92; N, 4.81. IR νCO (hexane, cm−1):
2053.7 (vs), 1978.3 (s), 1970.2 (s). IR νCO (CH2Cl2, cm−1): 2049.9
(vs), 1972.9 (s), 1960.9 (s). Crystals suitable for X-ray diffraction
studies were grown by slow diffusion of pentane into a benzene
solution.
3
OCH3) 3.40 (4H, s, N(CH2)N), 3.30 (4H, sept, JHH = 6.8,
3
CH(CH3)2), 1.36 (12H, d, JHH = 6.8, CH(CH3)2), 1.30 (12H, d,
3JHH = 6.8, CH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 245.0
(NCN), 159.9 (ArC), 148.8 (ArC), 132.8 (ArC), 109.5 (ArCH), 54.9
(OCH3), 53.9 (N(CH2)2N), 29.3 (CH(CH3)2), 25.4 (CH(CH3)2),
23.6 (CH(CH3)2).
Synthesis of [AuCl(NHC)] Complexes 1. A vial was charged, in
air, with NHC·HCl, [AuCl(DMS)], and finely ground K2CO3. The
resulting mixture was dissolved in acetone (1.0 mL) and stirred for 3 h
at 60 °C. After this time, the mixture was filtered through silica, which
was rinsed with dichloromethane. The solvent was concentrated, and
pentane (3.0 mL) was added, which brought about precipitation of the
product as a white solid; this was collected by filtration, washed with
further portions of pentane (3 × 1.0 mL), and dried in vacuo.
[AuCl(IPrOMe)] (1b). IPrOMe·HCl (100 mg, 0.206 mmol), [AuCl-
(DMS)] (60.7 mg, 0.206 mmol) and K2CO3 (28.5 mg, 0.206 mmol).
The silica pad was rinsed with dichloromethane (3 × 1.0 mL). White
[Ni(CO)3(SIPrOMe)] (2d). SIPrOMe (130 mg, 0.27 mmol, 1.0 equiv) in
THF (6 mL) was treated with [Ni(CO)4] (100 μL, 0.77 mmol, 2.9
1
equiv). Dark purple solid (92 mg, 0.155 mmol, 57%). H NMR (300
MHz, CDCl3): δ 6.74 (4H, s, ArH), 3.92 (4H, s, N(CH2)N), 3.84
(6H, s, OCH3), 3.10 (4H, sept, 3JHH = 6.9, CH(CH3)2), 1.31 (12H, d,
3JHH = 6.9, CH(CH3)2), 1.27 (12H, d, J = 6.9, CH(CH3)2). 13C{1H}
NMR (126 MHz, CDCl3): δ 223.4 (Ni-C), 197.3 (CO), 159.8 (ArC),
148.5 (ArC), 131.5 (ArC), 109.6 (ArCH), 55.5 (OCH3), 53.4
(N(CH)2N), 28.7 (CH(CH3)2), 26.0 (CH(CH3)2), 23.6 (CH-
(CH3)2). Anal. Calcd for C32H42N2NiO5: C, 64.77; H, 7.13; N, 4.72;
Found: C, 64.84; H, 7.05; N, 4.78. IR νCO (hexane, cm−1): 2054.1
(vs), 1980.5 (s), 1971.6 (s). IR νCO (CH2Cl2, cm−1): 2050.3 (vs),
1973.9 (s), 1962.1 (s). Crystals suitable for X-ray diffraction studies
were grown by slow diffusion of pentane into a benzene solution.
Synthesis of [IrCl(COD)(NHC)] Complexes 3. In the glovebox, a
solution of free carbene in THF was added to [IrCl(COD)]2,
whereupon a pale yellow solution was formed. After it was stirred at
room temperature, the solution gradually became darker. After the
solution was stirred overnight, the solvent was stripped and the residue
was worked up.
1
solid (101 mg, 0.148 mmol, 72%). H NMR (300 MHz, CDCl3): δ
7.11 (2H, s, N(CH)2N), 6.75 (4H, s, ArH), 3.87 (6H, s, OCH3), 2.51
3
3
(4H, sept, JH−H = 6.8, CH(CH3)2), 1.32 (12H, d, JH−H = 6.8,
CH(CH3)2), 1.19 (12H, d, JH−H = 6.9, CH(CH3)2). 13C{1H}-
3
DEPTQ NMR (75 MHz, CDCl3): δ 176.3 (Au−C), 161.0 (C-
OCH3), 147.2 (ArC), 127.2 (ArC), 123.5 (N(CH)2N), 109.7 (ArCH),
55.5 (OCH3), 29.1 (CH(CH3)2), 24.5 (CH(CH3)2), 24.1 (CH-
(CH3)2). Anal. Calcd for C29H40AuClN2O2: C, 51.14; H, 5.92; N, 4.11.
Found: C, 51.17; H, 5.89; N, 4.15. Crystals suitable for X-ray
diffraction studies were grown by slow diffusion of pentane into a
saturated dichloromethane solution.
[AuCl(SIPrOMe)] (1d). SIPrOMe·HCl (100 mg, 0.205 mmol),
[AuCl(DMS)] (60.5 mg, 0.205 mmol), and K2CO3 (28.4 mg, 0.205
mmol). The silica pad was rinsed with dichloromethane (5 × 2.0 mL).
White solid (78 mg, 0.114 mmol, 56%). The low isolated yield is due
[IrCl(COD)(IPrOMe)] (3b). IPrOMe (20.1 mg, 0.045 mmol) in THF (1
mL) was added to [IrCl(COD)]2 (15.0 mg, 0.022 mmol, 0.50 equiv)
and the mixture stirred overnight. The residue was taken up in diethyl
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ether (2 mL) and filtered through a short pad of silica gel, followed by
diethyl ether (3 mL). The combined fractions were evaporated to yield
a sticky yellow oil, which was solidified by the addition of cold (−40
°C) pentane and scratching with a spatula. The pentane was removed
in vacuo, and the resulting yellow powder was carefully washed with
0.3 mL of cold (−40 °C) pentane and dried in vacuo to yield the title
compound (22.2 mg, 0.028 mmol, 63%). 1H NMR (500 MHz, C6D6):
δ 6.91 (4H, s, ArH), 6.62 (2H, s, N(CH)2N), 4.80−4.64 (2H, m,
COD CH), 4.08−3.83 (2H, m, CH(CH3)2), 3.36 (6H, s, OCH3),
3.26−3.14 (2H, m, COD CH), 2.79−2.58 (2H, m, CH(CH3)2), 1.84−
1.74 (4H, m, COD CH2), 1.74−1.64 (6H, m, CH(CH3)2), 1.38−1.32
(4H, m, COD CH2), 1.32−1.12 (6H, m, CH(CH3)2), 1.19−1.07 (6H,
m, CH(CH3)2), 1.06−0.94 (6H, m, CH(CH3)2). 13C{1H} NMR (126
MHz, C6D6): δ 185.0 (Ir−C), 161.1 (ArC), 149.8 (ArC), 147.2 (ArC),
129.9 (ArC), 124.8 (N(CH)2N), 109.5 (ArH), 109.0 (ArH), 83.3
(COD CH), 54.8 (OCH3), 50.9 (COD CH), 34.2 (COD CH2), 29.5
(br, CH(CH3)2), 29.2 (COD CH2), 26.7 (br, CH(CH3)2), 26.6 (br,
CH(CH3)2), 23.7 (br, CH(CH3)2), 22.5 (br, CH(CH3)2). Anal. Calcd
for C37H52N2IrO2: C, 56.65; H, 6.68; N, 3.57. Found: C, 56.80; H,
6.63; N, 3.50. Crystals suitable for X-ray diffraction studies were grown
by slow diffusion of pentane into a diethyl ether solution.
Anal. Calcd for C37H54N2IrO2: C, 74.04; H, 5.49; N, 2.24. Found: C,
74.23; H, 5.68; N, 2.37. Crystals suitable for X-ray diffraction studies
were grown by slow diffusion of pentane into a benzene solution.
[IrCl(COD)(IPr*OMe)] (3f). IPr*OMe (102.5 mg, 0.108 mmol, 2.1
equiv) and [IrCl(COD)]2 (35.0 mg, 0.052 mmol) in THF (3 mL),
stirred overnight. The residue was washed with diethyl ether (3 × 1
mL) and pentane (3 × 1 mL) and dried in vacuo to yield a yellow
powder (105.8 mg, 0.083 mmol, 79%). 1H NMR (500 MHz, CDCl3):
δ 7.55−6.48 (44H, m, ArH), 6.44 (2H, s, CHPh2), 5.54 (2H, s,
CHPh2), 4.80 (2H, s, N(CH)2N), 4.70−4.63 (2H, m, COD CH), 3.56
(6H, s, OCH3), 3.29−3.22 (2H, m, COD CH), 2.11−1.99 (2H, m,
COD CH2), 1.72−1.61 (2H, m, COD CH2), 1.60−1.50 (2H, m, COD
CH2), 1.33−1.18 (2H, m, COD CH2). 13C{1H} NMR (126 MHz,
CDCl3): 179.7 (Ir−C), 158.8 (ArC), 145.3 (ArC), 144.4 (ArC), 144.1
(ArC), 143.7 (ArC), 143.5 (ArC), 142.5 (ArC), 131.9 (ArC), 130.9
(ArCH), 130.2 (ArCH), 129.6 (ArCH), 129.3 (ArCH), 128.3
(ArCH), 128.2 (ArCH), 128.1 (ArCH), 127.9 (ArCH), 126.7
(ArCH), 126.6 (ArCH), 126.2 (ArCH), 126.1 (ArCH), 123.7
(N(CH)2N), 115.4 (ArCH), 114.6 (ArCH), 83.7 (COD CH), 55.2
(OCH3), 53.1 (COD CH), 51.7 (CHPh2), 51.3 (CHPh2), 33.5 (COD
CH2), 28.9, (COD CH2). Anal. Calcd for C77H68N2IrClO2: C, 72.19;
H, 5.35; N, 2.19. Found: C, 71.77; H, 5.20; N, 2.43. Crystals suitable
for X-ray diffraction studies were grown by slow diffusion of pentane
into a benzene solution.
[IrCl(COD)(SIPrOMe)] (3d). SIPrOMe (20.4 mg, 0.0453 mmol) in
THF (1 mL) was added to [IrCl(COD)]2 (15.0 mg, 0.0223 mmol,
0.50 equiv) and the mixture stirred overnight. The residue was taken
up in diethyl ether (2 mL) and filtered through a short pad of silica gel,
followed by the addition of diethyl ether (3 mL). The combined
fractions were evaporated in vacuo to yield a sticky yellow oil, which
was solidified by the addition of cold (−40 °C) pentane and scratching
with a spatula. The pentane was removed in vacuo, and the resulting
yellow powder was carefully washed with 0.3 mL of cold (−40 °C)
pentane and dried in vacuo to yield the title compound (23.7 mg,
Synthesis of [IrCl(CO)2(NHC)] Complexes 4. The corresponding
[IrCl(COD)(NHC)] complex (3) was dissolved in DCM and exposed
to carbon monoxide (4b in a Schlenk flask, avoiding air and moisture;
4d−f in vials). Over this time, the reaction turned from dark golden
yellow to a pale straw yellow. The solvent was then removed in vacuo,
and the residue was washed with pentane and dried in vacuo.
[IrCl(CO)2(IPrOMe)] (4b). 3b (30.2 mg, 0.0385 mmol) was dissolved
in DCM (2.5 mL) in a flask fitted with a J. Young tap. The flask was
removed from the glovebox, the solution was frozen, and the
headspace was evacuated. The flask was then filled with carbon
monoxide and stirred at room temperature for 6 h. The flask was
placed under static vacuum and reintroduced into the glovebox. The
residue was washed with cold (−40 °C) pentane (3 × 0.3 mL) and
dried in vacuo to yield a pale yellow powder (19.1 mg, 0.026 mmol,
0.030 mmol, 66%). 1H NMR (500 MHz, C6D6): δ 6.91 (2H, d, 3JHH
=
3
2.8, ArH), 6.89 (2H, d, JHH = 2.8, ArH), 4.75−4.64 (2H, m, COD
2
CH), 4.28 (2H, sept, JHH = 6.8 Hz, CH(CH3)2), 3.74−3.68 (2H, m,
N(CH2)2N), 3.51−3.46 (2H, m, N(CH2)2N), 3.38 (6H, s, OCH3),
3.29−3.11 (4H, m, COD CH and CH(CH3)2), 1.78 (6H, d, 2JHH = 6.8
Hz, CH(CH3)2), 1.74−1.62 (4H, m, COD CH2), 1.36 (6H, d, 2JHH
=
6.8 Hz, CH(CH3)2), 1.32−1.24 (4H, m, COD CH2), 1.22 (6H, d, 2JHH
2
1
= 6.8 Hz, CH(CH3)2), 1.10 (6H, d, JHH = 6.8 Hz, CH(CH3)2).
68%). H NMR (300 MHz, C6D6): δ 6.86 (4H, s, ArH), 6.62 (2H, s,
13C{1H} NMR (126 MHz, C6D6): δ 211.2 (Ir−C), 160.5 (ArC), 151.2
(ArC), 147.9 (ArC), 130.4 (ArC), 110.0 (ArH), 109.4 (ArH), 84.0
(COD CH), 54.8 (OCH3), 54.4 (N(CH2)2N), 50.9 (COD CH), 29.5
(CH(CH3)2), 29.4 (CH(CH3)2), 28.9 (COD CH2), 27.1 (CH-
(CH3)2), 26.9 (CH(CH3)2), 24.6 (CH(CH3)2), 23.2 (CH(CH3)2).
Anal. Calcd for C37H54N2IrO2: C, 56.50; H, 6.92; N, 3.56. Found: C,
56.63; H, 6.83; N, 3.53. Crystals suitable for X-ray diffraction studies
were grown by slow evaporation of a diethyl ether solution.
2
N(CH)2N), 3.30 (6H, s, OCH3), 3.13 (4H, sept, JHH = 6.7,
CH(CH3)2), 1.51 (12H, d, 3JHH = 6.7, CH(CH3)2), 1.05 (12H, d, 3JHH
= 6.7, CH(CH3)2). 13C{1H} NMR (75 MHz, C6D6): δ 180.8 (CO),
180.4 (Ir−C), 169.9 (CO), 161.6 (ArC), 147.9 (ArC), 128.0 (ArC),
125.1 (N(CH)2N), 109.8 (ArCH), 54.8 (OCH3), 29.5 (CH(CH3)2),
26.4 (CH(CH3)2), 22.8 (CH(CH3)2). [1H, 13C] HMBC was used to
locate the signal at 128.0 ppm and to assign the Ccarbene signal. Anal.
Calcd for C31H40N2ClIrO4: C, 50.84; H, 5.51; N, 3.83. Found: C,
50.96; H, 5.33; N, 3.98. IR νCO (CH2Cl2, cm−1): 2066.3 (vs), 1980.4
(vs).
[IrCl(COD)(IPr*)] (3e). IPr* (224.7 mg, 0.246 mmol, 2.2 equiv) and
[IrCl(COD)]2 (75.7 mg, 0.113 mmol) in THF (3.5 mL), stirred for 36
h (time not optimized). The residue was dissolved in ethyl acetate (5
mL) and filtered through a pad of silica, which was washed with ethyl
acetate (10 mL). The solvent was removed in vacuo, and the resulting
residue was dissolved in benzene and filtered using a syringe filter. The
benzene was removed in vacuo, and the resulting yellow powder was
washed with pentane and dried under high vacuum to yield the title
complex (243.0 mg, 0.195 mmol, 86%). 1H NMR (500 MHz, CDCl3):
δ 7.66−7.54 (4H, m, ArH), 7.39−7.19 (16H, m, ArH), 7.19−7.00
(12H, m, ArH), 6.95 (2H, s, ArH), 6.90 (2H, s, ArH), 6.85−6.78 (4H,
m, ArH), 6.76−6.69 (4H, m, ArH), 6.50 (2H, s, CHPh2), 5.70 (2H, s,
CHPh2), 4.97 (2H, s, N(CH)2N), 4.82−4.76 (2H, m, COD CH),
3.37−3.29 (2H, m, COD CH), 2.29 (6H, s, OCH3), 2.20−2.09 (2H,
m, COD CH2), 1.79−1.68 (2H, m, COD CH2), 1.68−1.59 (2H, m,
COD CH2), 1.34−1.24 (2H, m, COD CH2). 13C{1H} NMR (126
MHz, CDCl3): δ 178.8 (Ir−C), 144.6 (ArC), 144.4 (ArC), 143.9
(ArC), 143.7 (ArC), 143.2 (ArC), 140.7 (ArC), 138.1 (ArC), 136.4
(ArC), 131.0 (ArCH), 130.9 (ArCH), 130.2 (ArCH), 129.7 (ArCH),
129.7 (ArCH), 129.3 (ArCH), 128.3 (ArCH), 128.1 (ArCH), 127.8
(ArCH), 126.5 (ArCH), 126.0 (ArCH), 126.0 (ArCH), 123.7
(N(CH)2N), 83.7 (COD CH), 53.2 (COD CH), 51.4 (CHPh2),
51.0 (CHPh2), 33.4 (COD CH2), 28.9 (COD CH2), 21.9 (ArCH3).
[IrCl(CO)2(SIPrOMe)] (4d). 3d (26.4 mg, 0.034 mmol) in DCM (1.5
mL) was sparged with CO for ca. 5 min. The reaction mixture was
stirred overnight under a balloon of CO. The residue was washed with
pentane (3 × 1 mL) and dried in vacuo to yield a pale yellow powder
1
(17.8 mg, 0.0247 mmol, 72%). H NMR (300 MHz, CDCl3): δ 6.76
(4H, s, ArH), 4.03 (4H, s, N(CH2)2N), 3.86 (6H, s, OCH3), 3.34 (2H,
3
3
sept, JHH = 6.8, CH(CH3)2), 1.44 (6H, d, JHH = 6.8, CH(CH3)2),
1.27 (6H, d, JHH = 6.8, CH(CH3)2). 13C{1H} NMR (75 MHz,
3
CDCl3): δ 205.4 (Ir−C), 180.4 (CO), 168.8 (CO), 160.1 (ArC),
148.7 (ArC), 128.1 (ArC), 109.8 (ArCH), 55.3 (OCH3), 54.6
(N(CH2)2N), 29.2 (CH(CH3)2), 26.9 (CH(CH3)2), 23.6 (CH-
(CH3)2). Anal. Calcd for C31H40N2ClIrO4: C, 50.70; H, 5.77; N,
3.81. Found: C, 50.62; H, 5.86; N, 3.74. IR νCO (CH2Cl2, cm−1):
2067.2 (vs), 1981.6 (vs).
[IrCl(CO)2(IPr*)] (4e). 3e (75.1 mg, 0.060 mmol) in DCM (3 mL)
was sparged with CO for ca. 10 min. The residue was washed with
pentane (3 × 1 mL) and dried under high vacuum to yield a pale
yellow solid (65.8 mg, 0.055 mmol, 92%). 1H NMR (500 MHz,
CDCl3): δ 7.38−7.21 (20H, m, ArH), 7.14−7.05 (12H, m, ArH), 6.80
(4H, s, ArH), 6.77−6.73 (8H, m, ArH), 5.90 (4H, s, CHPh2), 5.02
(2H, s, N(CH)2N), 2.23 (6H, s, CH3). 13C{1H} NMR (126 MHz,
2056
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Organometallics
Article
CDCl3): δ 179.8 (CO), 176.0 (C−Ir), 168.0 (CO), 144.3 (ArC),
143.3 (ArC), 141.4 (ArC), 139.1 (ArC), 134.6 (ArC), 130.6 (ArCH),
130.5 (ArCH), 129.3 (ArCH), 128.4 (ArCH), 128.2 (ArCH), 126.6
(ArCH), 126.5 (ArCH), 123.5 (N(CH)2N), 51.7 (CHPh2), 22.0
(CH3). IR νCO (CH2Cl2, cm−1): 2067.6 (vs), 1983.6 (vs). Anal. Calcd
for C71H56ClN2O2Ir: C, 71.25; H, 4.72; N, 2.34. Found: C, 71.37; H,
4.88; N, 2.41.
13C{1H} NMR (126 MHz, CDCl3): δ 184.2 (CSe), 147.3 (ArC),
135.4 (ArC), 129.5 (ArCH), 124.6 (ArCH), 51.5 (N(CH2)2N), 29.3
(CH(CH3)2), 24.9 (CH(CH3)), 24.5 (CH(CH3)2). 77Se{1H} NMR
(95 MHz, CDCl3): δ 189.5. 1H NMR (400 MHz, acetone-d6): δ 7.43−
7.36 (2H, m, ArH), 7.31−7.26 (4H, m, ArH), 4.16 (4H, s,
3
N(CH2)2N), 3.21 (4H, sept, JHH = 6.9, CH(CH3)2), 1.38 (12H, d,
3
3JHH = 6.9, CH(CH3)2), 1.33 (12H, d, JHH = 6.9, CH(CH3)2).
[IrCl(CO)2(IPr*OMe)] (4f). 3f (38.3 mg, 0.030 mmol) in DCM (2
mL) was sparged with CO for ca. 10 min. The residue was washed
with pentane (3 × 0.5 mL) and dried in vacuo to yield a pale yellow
77Se{1H} NMR (75 MHz, acetone-d6): δ 181.1. Anal. Calcd for
C27H38N2Se: C, 69.06; H, 8.16; N, 5.97. Found: C, 68.94; H, 8.16; N,
5.98. Crystals suitable for X-ray diffraction analysis were grown by slow
diffusion of pentane into an acetone solution. Data are consistent with
that reported by Ganter and co-workers.15
1
solid (33.7 mg, 0.027 mmol, 92%). H NMR (500 MHz, CDCl3): δ
7.37−7.17 (20H, m, ArH), 7.15−7.06 (12H, m, ArH), 6.78 (8H, app
d, J = 7.0, ArH), 6.50 (4H, s, ArH), 5.90 (4H, s, CHPh2), 4.95 (2H, s,
N(CH)2N), 3.57 (6H, s, OCH3). 13C{1H} NMR (126 MHz, CDCl3):
179.8 (CO), 176.4 (Ir−C), 168.1 (CO), 159.3 (ArC), 144.0 (ArC),
143.4 (ArC), 143.1 (ArCH), 130.6 (ArCH), 129.2 (ArCH), 128.4
(ArCH), 128.3 (ArCH), 126.7 (ArCH), 126.5 (ArCH), 123.5
(N(CH)2N), 115.2 (ArCH), 55.1 (OCH3), 51.9 (CHPh2). IR νCO
(CH2Cl2, cm−1): 2067.4 (vs), 1983.2 (vs). Anal. Calcd for
C71H56N2IrClO4: C, 69.39; H, 4.59; N, 2.28. Found: C, 69.24; H,
4.47; N, 2.15.
Synthesis of Selenoureas 5. In the glovebox, a solution of free
carbene in THF was prepared. Outside the glovebox, under argon, this
THF solution was transferred onto excess selenium, and the mixture
was stirred overnight at room temperature. After this time, the THF
was removed, the residue was dissolved in DCM, and this solution was
passed through a pad of Celite. The pad was washed with further
DCM. The solvent was removed and the residue was washed with
pentane or hexane (2 × 1 mL) and dried in vacuo.
[Se(SIPrOMe)] (5d). SIPrOMe (85.2 mg, 0.189 mmol) was in THF (15
mL) added to selenium (70.2 mg, 0.889 mmol, 4.7 equiv). Off-white
1
solid (78.5 mg, 0.148 mmol, 78%). H NMR (500 MHz, CDCl3): δ
6.76 (4H, s, ArH), 3.98 (4H, s, N(CH2)2N), 3.84 (6H, s, OCH3), 3.01
3
3
(4H, sept, JHH = 6.9, CH(CH3)2), 1.38 (12H, d, JHH = 6.9,
CH(CH3)2), 1.31 (12H, d, JHH = 6.9, CH(CH3)2). 13C{1H} NMR
3
(126 MHz, CDCl3): δ 185.0 (CSe), 159.9 (ArC), 148.6 (ArC), 128.6
(ArC), 109.9 (ArH), 55.2 (OCH3), 51.5 (N(CH2)2N), 29.4
1
(CH(CH3)2), 24.8 (CH(CH3)2), 24.4 (CH(CH3)2). H NMR (500
MHz, acetone-d6): δ 6.78 (4H, s, ArH), 4.06 (4H, s, N(CH2)2N), 3.83
(6H, s, OCH3), 3.12 (4H, sept, 3JHH = 6.9, CH(CH3)2), 1.34 (12H, d,
3
3JHH = 6.9, CH(CH3)2), 1.28 (12H, d, JHH = 6.9, CH(CH3)2).
77Se{1H} NMR (95 MHz, acetone-d6): δ 176.8. Anal. Calcd for
C29H42N2O2Se: C, 65.77; H, 7.99; N, 5.29. Found: C, 65.57; H, 8.11;
N, 5.21. Crystals suitable for X-ray diffraction analysis were grown by
slow diffusion of pentane into an acetone solution.
[Se(IPr*)] (5e). IPr* (200.1 mg, 0.219 mmol) was in THF (15 mL)
added to selenium (52.8 mg, 0.669 mmol, 3.1 equiv). Beige solid
(168.7 mg, 0.170 mmol, 78%). 1H NMR (500 MHz, CDCl3): δ 7.42−
7.37 (8H, m, ArH), 7.25−7.20 (8H, m, ArH), 7.20−7.14 (4H, m,
ArH), 7.13−7.07 (12H, m, ArH), 6.86 (4H, s, ArH), 6.85−6.80 (8H,
m, ArH), 5.43 (4H, s, CHPh2), 5.37 (2H, s, N(CH)2N), 2.22 (6H, s,
CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 160.9 (CSe), 143.5
(ArC), 142.9 (ArC), 141.6 (ArC), 139.6 (ArC), 133.9 (ArCH), 130.4
(ArCH), 130.2 (ArCH), 129.5 (ArCH), 128.3 (ArCH), 128.2
(ArCH), 126.6 (ArCH), 126.4 (ArCH), 121.0 (N(CH)2N), 51.8
(CHPh2), 22.0 (ArCH3). 77Se{1H} NMR (95 MHz, CDCl3): δ 105.8.
Anal. Calcd for C69Hx56N2Se: C, 83.53; H, 5.69; N, 2.82. Found: C,
83.38; H, 5.75; N, 2.91. Crystals suitable for X-ray diffraction analysis
were grown by slow diffusion of pentane into an acetone solution.
[Se(IPr*OMe)] (5f). IPr*OMe (165.1 mg, 0.175 mmol) was in THF
(10 mL) added to selenium (56.5 mg, 0.716 mmol, 4.1 equiv). Beige
[Se(IPr)] (5a). IPr (102.4 mg, 0.264 mmol) was in THF (10 mL)
added to selenium (59.5 mg, 0.754 mmol, 2.9 equiv). White solid
(118.2 mg, 0.253 mmol, 96%). H NMR (500 MHz, CDCl3): δ 7.48
1
3
3
(2H, t, JHH = 7.9, ArH), 7.31 (4H, d, JHH = 7.9, ArH), 7.01 (2H, s,
3
N(CH)2N), 2.69 (4H, sept, JHH = 6.9, CH(CH3)2), 1.34 (12H, d,
3JHH = 6.9, CH(CH3)2), 1.20 (12H, d, JHH = 6.9, CH(CH3)2).
3
13C{1H} NMR (126 MHz, CDCl3): δ 162.2 (CSe), 146.2 (ArCH),
134.5 (ArC), 130.3 (ArC), 124.4 (N(CH)2N), 121.2 (ArCH), 29.1
(CH(CH3)2), 24.5 (CH(CH3)2), 23.5 (CH(CH3)2). 77Se{1H} NMR
1
(95 MHz, CDCl3): δ 90.4. H NMR (400 MHz, acetone-d6): δ 7.47
(2H, s, N(CH)2N), 7.47 (2H, t, 3JHH = 7.9, ArH), 7.33 (4H, d, 3JHH
=
7.9, ArH), 2.75 (4H, sept, 3JHH = 6.9, CH(CH3)2), 1.30 (12H, d, 3JHH
= 6.9, CH(CH3)2), 1.19 (12H, d, JHH = 6.9, CH(CH3)2). 77Se{1H}
3
NMR (76 MHz, acetone-d6): δ 86.7. Anal. Calcd for C27H36N2Se: C,
69.36; H, 7.76; N, 5.99. Found: C, 69.17; H, 7.88; N, 5.93. Crystals
suitable for X-ray diffraction analysis were grown by slow diffusion of
pentane into an acetone solution. Data are consistent with those
reported by Ganter and co-workers.15
1
solid (155.4 mg, 0.152 mmol, 87%). H NMR (500 MHz, CDCl3): δ
7.46−7.39 (8H, m, ArH), 7.30−7.09 (34H, m, ArH), 6.92−6.85 (8H,
m, ArH), 6.61 (4H, s, ArH), 5.46 (4H, s, CHPh2), 5.36 (2H, s,
N(CH)2N), 3.60 (6H, s, OCH3). 13C{1H} NMR (126 MHz, CDCl3):
δ 161.7 (CSe), 159.8 (ArC), 143.5 (ArC), 143.3 (ArC), 142.6 (ArC),
130.1 (ArCH), 129.4 (ArCH), 128.4 (ArCH), 128.2 (ArCH), 126.7
(ArCH), 126.5 (ArCH), 121.1 (N(CH)2N), 115.1 (ArCH), 55.2
(OCH3), 52.0 (CHPh2). 77Se{1H} NMR (95 MHz, CDCl3): δ 103.9.
Anal. Calcd for C69H56N2O2Se: C, 80.92; H, 5.51; N, 2.74. Found: C,
80.68; H, 5.40; N, 2.61. Crystals suitable for X-ray diffraction analysis
were grown by slow diffusion of pentane into an acetone solution.
[Se(IPrOMe)] (5b). IPrOMe (100.2 mg, 0.223 mmol) was in THF (15
mL) added to selenium (64.2 mg, 0.813 mmol, 3.6 equiv). Salmon-
1
colored solid (104.2 mg, 0.197 mmol, 89%). H NMR (500 MHz,
CDCl3): δ 6.97 (2H, s, N(CH)2N), 6.80 (4H, s, ArH), 3.86 (6H, s,
OCH3), 2.65 (4H, sept, 3JHH = 6.9, CH(CH3)2), 1.33 (12H, d, 3JHH
=
3
6.9, CH(CH3)2), 1.19 (12H, d, JHH = 6.9, CH(CH3)2). 13C{1H}
NMR (126 MHz, CDCl3): δ 163.0 (CSe), 160.59 (ArC), 147.6 (ArC),
127.7 (ArC), 121.4 (N(CH)2N), 109.7 (ArH), 55.3 (OCH3), 29.3
1
(CH(CH3)2), 24.4 (CH(CH3)2), 23.4 (CH(CH3)2). H NMR (500
MHz, acetone-d6): δ 7.39 (2H, s, N(CH)2N), 6.84 (4H, s, ArH), 3.87
(6H, s, OCH3), 2.70 (4H, sept, 3JHH = 6.9, CH(CH3)2), 1.29 (12H, d,
ASSOCIATED CONTENT
3
3JHH = 6.9, CH(CH3)2), 1.18 (12H, d, JHH = 6.9, CH(CH3)2).
■
77Se{1H} NMR (95 MHz, acetone-d6): δ 83.4. Anal. Calcd for
C29H40N2O2Se: C, 66.02; H, 7.64; N, 5.31. Found: C, 65.89; H, 7.75;
N, 5.21. Crystals suitable for X-ray diffraction analysis were grown by
slow diffusion of pentane into an acetone solution.
S
* Supporting Information
Figures, tables, and CIF files giving NMR spectra for new
complexes, crystallographic data for 1b,d,f, 2b,d, 3b,d−f, and
5a−f, steric maps for 1, 2, and 4, and buried volumes for 5. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data for CCDC 987701−987715
(1b,d,f, 2b,d, 3b,d−f, 5a−f) can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[Se(SIPr)] (5c). SIPr (109.8 mg, 0.281 mmol) was in THF (10 mL)
added to selenium (74.1 mg, 0.938 mmol, 3.3 equiv). White solid
1
(86.7 mg, 0.185 mmol, 66%). H NMR (500 MHz, CDCl3): δ 7.43
3
3
(2H, t, JHH = 7.7, ArH), 7.27 (4H, d, JHH = 7.7, ArH), 4.05 (4H, s,
3
N(CH2)2N), 3.08 (4H, sept, JHH = 6.9, CH(CH3)2), 1.40 (12H, d,
3JHH = 6.9, CH(CH3)2), 1.34 (12H, d, JHH = 6.9, CH(CH3)2).
3
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Article
(21) Credendino, R.; Falivene, L.; Cavallo, L. J. Am. Chem. Soc. 2012,
134, 8127.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for S.P.N.: snolan@st-andrews.ac.uk.
(22) Munz, D.; Allolio, C.; Doring, K.; Poethig, A.; Doert, T.; Lang,
̈
H.; Straßner, T. Inorg. Chim. Acta 2012, 392, 204.
(23) Organ, M. G.; Çalimsiz, S.; Sayah, M.; Hoi, K. H.; Lough, A. J.
Notes
Angew. Chem., Int. Ed. 2009, 48, 2383.
The authors declare no competing financial interest.
(24) Meiries, S.; Le Duc, G.; Chartoire, A.; Collado, A.; Speck, K.;
Athukorala Arachige, K. S.; Slawin, A. M. Z.; Nolan, S. P. Chem. Eur. J.
2013, 19, 17358.
(25) Meiries, S.; Speck, K.; Cordes, D. B.; Slawin, A. M. Z.; Nolan, S.
P. Organometallics 2012, 32, 330.
(26) Bastug, G.; Nolan, S. P. Organometallics 2014, 33, 1253.
(27) Bastug, G.; Nolan, S. P. J. Org. Chem. 2013, 78, 9303.
(28) Le Duc, G.; Meiries, S.; Nolan, S. P. Organometallics 2013, 32,
7547.
(29) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55,
14523.
(30) Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121, 9889.
́ ́ ́
(31) Gomez-Suarez, A.; Ramon, R. S.; Songis, O.; Slawin, A. M. Z.;
Cazin, C. S. J.; Nolan, S. P. Organometallics 2011, 30, 5463.
(32) Meiries, S.; Nolan, S. P. Synlett 2014, 25, 393.
(33) Collado, A.; Gomez-Suarez, A.; Martin, A. R.; Slawin, A. M. Z.;
Nolan, S. P. Chem. Commun. 2013, 49, 5541.
ACKNOWLEDGMENTS
■
We thank the ERC (Advanced Investigator Award “FUNCAT”
to S.P.N.) and the EPSRC for funding. S.P.N. is a Royal Society
Wolfson Merit Award Holder. Umicore are thanked for gifts of
auric acid and [IrCl(COD)]2.
ABBREVIATIONS
■
COD, 1,5-cyclooctadiene; DMS, dimethyl sulfide; IHept, 1,3-
bis(2,6-diisoheptylphenyl)imidazol-2-ylidene; IHeptOMe, 1,3-
bis(2,6-diisoheptyl-4-methoxyphenyl)imidazol-2-ylidene; IMes,
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; INon, 1,3-
bis(2,6-diisononylphenyl)imidazol-2-ylidene; INonOMe, 1,3-bis-
(2,6-diisononyl-4-methoxyphenyl)imidazol-2-ylidene; IPent,
1,3-bis(2,6-diisopentylphenyl)imidazol-2-ylidene; IPentOMe
,
1,3-bis(2,6-diisopentyl-4-methoxyphenyl)imidazol-2-ylidene;
(34) Fructos, M. R.; Belderrain, T. R.; de Frem
́
ont, P.; Scott, N. M.;
IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPrOMe
,
́
Nolan, S. P.; Dıaz-Requejo, M. M.; Per
́
ez, P. J. Angew. Chem., Int. Ed.
2005, 44, 5284.
1,3-bis(2,6-diisopropyl-4-methoxyphenyl)imidazol-2-ylidene;
́
(35) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P.
Organometallics 2005, 24, 2411.
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