Synthesis and characterisation of an N-heterocyclic carbene with spatially-defined steric impact

Article abstract of DOI:10.1039/c6dt02100j

The synthesis and co-ordination chemistry of a new 'bulky yet flexible' N-heterocyclic carbene ("IPaul") is reported. This carbene has spatially-defined steric impact; steric maps show that two quadrants are very bulky while the other two are quite open.

Full text of DOI:10.1039/c6dt02100j

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Synthesis and characterisation of an N-heterocyclic carbene with
spatially-defined steric impact
Received 00th January 20xx,
Accepted 00th January 20xx
Paul Shaw, Alan R. Kennedy, and David J. Nelson*
The synthesis and co-ordination chemistry of a new ‘bulky yet flexible’ N-heterocyclic carbene (“IPaul”) is reported. This
carbene has spatially-defined steric impact; steric maps show that two quadrants are very bulky while the other two are
quite open. The electronic properties of this carbene are very similar to those of other 1,3-diarylimidazol-2-ylidenes.
Copper, silver, iridium, and nickel complexes of the new ligand have been prepared. In solution, the ligand adopts two
different conformations, while X-ray crystallographic analyses of the transition metal complexes suggest that the syn-
conformer is preferred in the solid state due to intermolecular interactions. The copper(I) chloride complex of this new
ligand has been shown to be highly-active in the hydrosilylation of carbonyl compounds, when compared to the analogous
IPr, IMes, IPr* and IPr*OMe complexes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
N-Heterocyclic carbenes (NHCs) are useful and versatile tools
for a number of applications.1 Their use as ligands for the
stabilisation of reactive catalyst species2 and main group
compounds has drawn considerable attention.3 One of the
reasons for the widespread use of NHCs is that the preparation
of the air stable azolium salt precursors is typically
5
straightforward and scalable,4, without the need for inert
conditions. Free carbenes tend to be at least somewhat air-
and moisture-sensitive, but there exist a variety of methods by
which transition metal complexes can be prepared without the
need to liberate and isolate these reactive species.6
The concept of ‘bulky-yet-flexible’ NHC ligands has
emerged as a rationale for the success of a number of series of
bulky ligands in transition metal catalysis.7-10 Essentially, the
theory is that an ‘ideal’ ligand should be bulky enough to
protect and stabilise coordinatively unsaturated intermediate
species – such as putative ‘NHC-Pd0’ or ‘NHC-Ni0’ complexes
for cross-coupling
– yet flexible enough to allow the
coordination and reaction of substrates at the metal centre
without impediment.9 This theory has been put forward to
explain the successful use in catalysis of various ligands such as
Figure 1. Examples of ‘bulky-yet-flexible NHC ligands for catalysis.
11
12-14
the IBiox series of NHCs,7, IPr* and derivatives,9,
and
IPent8 and its analogues15 Figure 1).§ Straub has shown that
(
ligand in palladium catalysis yet an excellent ligand for the
stabilisation of intermediates in gold catalysis.16, 17 Chaplin has
recently explored the co-ordination chemistry of IBiox ligands
with rhodium and iridium,18-21 and has shown that despite the
fact that it holds some steric bulk near the metal centre, C-H
activation processes do not occur, unlike with the
corresponding ItBu species.22, 23
there are limits to this concept, with IPr** proving to be a poor
WestCHEM Department of Pure and Applied Chemistry, University of Strathclyde,
Thomas Graham Building, 295 Cathedral Street, Glasgow, G1 1XL, UK.
david.nelson@strath.ac.uk.
Electronic Supplementary Information (ESI) available: NMR spectra for all new
compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2016
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DOI: 10.1039/C6DT02100J
Figure 2. The target carbene for this study.
Here we have taken advantage of the flexibility of the
synthetic route to the IPr* family of ligands to prepare a ligand
with spatially-defined steric impact (Figure 2). This structural
motif piqued our curiosity because it would lead to
considerable steric impact within only two of the four
quadrants of the space around the metal centre. This might
lead to interesting co-ordination chemistry or improved
catalysts when coordinated to a metal centre. There is also the
potential for the ligand to exist in two different conformations,
with a considerable barrier to their interconversion. The
systematic name for this ligand is quite cumbersome so the
trivial name “IPaul” has been assigned, in analogy with the
naming conventions of many Buchwald-type ligands
(DavePhos, BrettPhos, JohnPhos, etc.). Here, we describe the
synthesis and co-ordination chemistry of IPaul, and investigate
the catalytic behaviour of its copper(I) chloride complex.
Scheme 1. Synthesis of the new NHC ligand.
Results and Discussion
Synthesis of the imidazolium salt.
Imidazolium salt
1 was prepared by modifying the second
generation synthesis of IPr*·HCl.24 Starting from cheap and
commercially available 2,4-dimethylaniline,25 the condensation
reaction furnished the desired aniline in good yield and could
be scaled up to gram quantities (Scheme 1). The imine
formation proceeded much more quickly than that for IPr*;
the ring-closing step proceeded in 80% yield using standard
conditions to yield the desired imidazolium salt. While the
spectroscopic data for the aniline
2 and diimine 3 are readily
interpreted, the 1H and 13C{1H} NMR spectra of the
imidazolium salt are very complex (see the Supporting
Information). The data suggest that the imidazolium salt
comprises a mixture of two rotamers as depicted in Figure 2
.
Scheme 2. Synthesis of NHC-metal complexes.
LC-MS analysis of the compound showed two peaks on the
chromatogram, with the corresponding m/z recorded by mass
spectrometry consistent with two conformers of the desired
product (see the Supporting Information).
These place the ligand in three different co-ordination
environments: linear two-coordinate (Cu, Ag), square planar
four-coordinate (Ir), and half-sandwich (Ni).
Copper-NHC complexes are useful both as catalysts26 and
Synthesis of new metal complexes.
as carbene transfer reagents.27 Copper complex
prepared either by heating
with Cu2O (47% yield)28 or by
4 could be
A series of complexes was prepared using the new NHC salt
1
(Scheme 2). None of these methods require the isolation of
heating
1
with CuCl in the presence of a weak base (87%
the free carbene, and proceed directly from the NHC salt.6
2 | Dalton Trans., 2016, 00, 1-3
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yield);29 NMR analysis yielded complicated 1H and 13C{1H}
spectra due to hindered rotation around the C-N bond slowing
the interchange between conformations. A 2:1 mixture of two
conformations was obtained in CDCl3 solution and many of the
signals for the different rotamers could be distinguished, but
the identity of each could not be assigned. However,
elemental analysis confirmed the purity of the bulk material.
Silver-NHC complexes are primarily used as carbene
transfer reagents, but also have a number of catalytic
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DOI: 10.1039/C6DT02100J
applications.30 Complex
5 was prepared from AgNO3 using the
route recently disclosed by Gimeno,31 yielding a white powder
which was also a mixture of rotamers in solution. The complex
was characterised by NMR spectroscopy elemental analysis.
Nickel half-sandwich complexes are easy to prepare and
can perform catalytic reactions including Suzuki-Miyaura32 and
Figure 3. Molecular structures of complexes
4-7 as determined by X-ray
Buchwald-Hartwig cross-coupling,33 and polymerisation
diffraction. H atoms, minor disorder components, and solvent molecules are
omitted for clarity.
35
reactions.34,
Their synthesis from nickelocene and the
imidazolium salt36 is quite general,37 and tolerates NHC ligands
with various functionality.38 Complex
was prepared by
heating with nickelocene in anhydrous THF overnight, which
6
Table 1. Percent buried volumes for new and known complexes at the
crystallographically-determined M-C distance, with sphere radius 3.5 Å, mesh spacing
0.1 Å and hydrogen atoms excluded.
1
yielded the complex as an analytically-pure pink powder after
work-up. The complex was again obtained as a mixture of
rotamers, this time in a ca. 1:1 ratio in CDCl3 solution.
Complex
IPaul
45.7
43.3
40.0
36.5
IMes
37.9
IPr
48.5
IPr*
52.1
53.5
41.4
37.4
[CuCl(NHC)] (4)
Finally, iridium complex
7 was prepared using Plenio’s
[AgCl(NHC)] (5)
35.1
36.1, 35.4 a,b
32.3, 32.0 b
43.8
methodology,39 with the aim of evaluating the steric impact of
the ligand on a square planar metal centre, and the TEP via the
corresponding dicarbonyl complex (vide infra). In this case, the
reduced symmetry around the metal centre allowed the
mixture of rotamers in CDCl3 solution to be assigned as 2:1 anti
to syn, based on the fact that the latter shows four separate
[NiCl(Cp)(NHC)] (6)
[IrCl(COD)(NHC)] (7)
38.1
35.0, 32.9 b
a Data from two different X-ray crystal structure determinations in the literature.
b Multiple independent molecules in the unit cell.
Buried volumes for these new complexes were compared to
1
13C resonances for COD CH atoms, a more complex H signal
those of known complexes bearing IMes, IPr, and IPr* (Table
).12, 33, 42-50 IPaul has a lower percent buried volume than IPr in
for the CH protons, and two separate resonances for CHPh2.
The anti-isomer has a plane of symmetry perpendicular to the
plane of the imidazolylidene core, and as expected yields only
two 13C resonances for COD CH atoms and one for CHPh2.
Crystal structures for these complexes were obtained to allow
the steric profile of the new carbene ligand to be evaluated in
different metal centre geometries, and compared with
analogues such as IMes, IPr, and IPr*. These structures are
depicted in Figure 3. In all four solid state structures the two
diphenylmethyl groups were found to be on the same side of
the ligand (i.e. syn). Most NMR spectra of the complexes
suggest a mixture of anti and syn species in solution, and in the
iridium case where the two could be identified the anti-isomer
appears to be the major constituent. We postulate that the
syn-conformation is more readily crystallised. NMR analysis of
1
the [CuCl(NHC)] environment but, as expected, IPaul is
significantly bulkier than IMes. For the nickel and iridium
complexes the differences are less pronounced and the steric
bulk decreases in the order IPr* > IPaul > IPr > IMes. This is
likely due to the steric interaction of the NHC ligand with the
rest of the ligand sphere in these complexes. It should be
noted that these data refer only to the syn-isomer, and that
the anti-isomer may have a slightly different steric profile.
The steric map of [CuCl(IPaul)] was compared to those of
[CuCl(IMes)], [CuCl(IPr)] and [CuCl(IPr*)] from the literature
(Figure 4). It can be seen from these steric maps, which
represent a view of the ligand from along the Cl-Cu bond, that
IPr, IMes, and IPr* have the steric impact distributed quite
evenly, while IPaul has two very open quadrants and two very
bulky quadrants. This confirms that the steric profile that was
sought has been achieved.
a single crystal of the copper complex
4 showed an 85:15
mixture of two rotamers, rather than the 2:1 mixture seen
previously or the entirely syn arrangement seen by X-ray
crystallographic analysis, suggesting that rotation around the
C-N bond is feasible on long timescales but can occur in
solution even at room temperature. The fact that all four
complexes show a syn-arrangement of the CHPh2 substituents
suggests that crystal picking is not an issue here.
A simple reaction between
7
and carbon monoxide allowed
Scheme ). IR
access to iridium carbonyl complex
8
(
3
spectroscopy of this species allows the degree of d to π*CO
back-bonding to be quantified,45 and therefore provides
valuable information on the overall electron-donating ability of
the NHC.51 Analysis of
8 by IR spectroscopy in DCM solution
Percent buried volume was calculated using the SambVca
tool;40 this quantifies the percentage of a sphere of 3.5 Å in
diameter around the metal centre that the ligand occupies.41
showed signals at 2067 and 1984 cm-1. This corresponds to a
TEP of 2051.6 cm-1,45 compared to 2050.7, 2051.5, and 2052.0
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DOI: 10.1039/C6DT02100J
Si
O
3 mol% [CuCl(NHC)]
5 equiv. HSiEt3
O
toluene, 80 C
Scheme 4. The hydrosilylation test reaction.
Figure 4. Steric maps for [CuCl(IPaul)] (top left), [CuCl(IPr*)] (top right), [CuCl(IMes)]
(bottom left), and [CuCl(IPr)] (bottom right).
Figure 5. Reaction profiles for the hydrosilylation reaction with [CuCl(IPaul)] (red
circles), [CuCl(IPr)] (blue circles), [CuCl(IMes)] (green circles), [CuCl(IPr*)] (green
squares) and [CuCl(IPr*OMe)] (blue squares).
Ph
Ph
Ph
Ph
CO
N
N
N
N
Cl
Ph
Cl
Ph
DCM
rt, 1 h
75%
Ir
Ir
Ph
Ph
IPr*-type complexes performed very poorly, with <10%
conversion after 6 h. IMes and IPr complexes were competent,
with the latter achieving complete conversion in 3 hours. The
new catalyst performed very strongly, achieving complete
conversion within 1 hour. The reaction was repeated with
analysis after 30 minutes, which indicated complete
conversion by GC-FID. This level of performance renders it
comparable in catalytic activity to species such as [CuCl(ICy)]
and [Cu(ICy)2][BF4],57, 58 which bear much less hindered ligands.
The most active complexes can achieve similar results at much
lower temperatures and catalyst loadings (room temperature,
0.25 mol% of an abnormal carbene-copper complex),59 but the
results here demonstrate that IPaul holds promise for
applications in catalysis; further applications of this ligand are
being explored in our laboratory.
O
7
8
O
Scheme 3. Synthesis of iridium carbonyl complex 8.
cm-1 for IMes, IPr, and IPr*, respectively.45, 52, 53 This ligand is
therefore – unsurprisingly – very similar to IPr* in terms of its
electronic character, although the TEP range for imidazol-2-
ylidenes is very narrow unless significantly electron-donating
or electron-withdrawing functional groups are present directly
on the imidazol-2-ylidene core.51 IPaul achieves a very
different steric profile to IPr* – as evidenced in Figure 4 – but
without significant perturbation of the electronic properties.
Catalytic testing in hydrosilylation reactions.
Hydrosilylation achieves the reduction and silyl protection of a
ketone in one step, and provides a benchmark reaction that is
operationally straightforward. Catalytic systems based on
rhodium,54 rhenium,55 nickel,56 and copper,26 can carry out
these reactions. [CuCl(NHC)] species can perform the
hydrosilylation of sterically-hindered ketones, providing an
opportunity to identify whether two beneficial effects can be
combined in [CuCl(IPaul)]: steric bulk will shield and stabilise
the metal centre, but the accessibility of this metal centre is
crucial to allow the reaction of bulky substrates.
Conclusions
A new NHC ligand (“IPaul”) has been prepared, co-ordinated to
four different metal centres, and its steric and electronic
properties have been evaluated. The carbene features
spatially-defined steric impact; two quadrants of the
coordination sphere of the metal are left open and accessible,
while the other two are blocked by diphenylmethyl groups.
The new carbene has very similar electronic properties to IPr*.
The reaction in Scheme 4 was performed; [CuCl(IMes)],
A
benchmark hydrosilylation reaction has shown that
[CuCl(IPr)], and [CuCl(IPr*)] were prepared using literature
methods and tested in the reaction.28,
42
[CuCl(IPaul)] is more active in catalysis than congeners
featuring carbenes that are bulkier or less bulky. Further
applications of this intriguing new ligand and how they can
improve catalytic processes are currently under investigation.
The raw data underpinning this study (NMR spectroscopy
Samples were
withdrawn at time points and the conversion was quantified
by GC-FID analysis (calibrated using authentic samples of
starting material and product; see the Supporting Information
for full details) (Figure 5). The new complex 4 was tested under
data for 1-8, and accurate mass data for 1) is available from
the same conditions, and profiled in the same way. Each
reaction was conducted in at least duplicate.
http://dx.doi.org/10.15129/ac8aeef2-8109-4c20-bb0d-
d8d808e3ceda. Crystallographic data for CCDC 1481901 (
4
),
4 | Dalton Trans., 2016, 00, 1-3
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1481902 ( ), 1481903 (6), and 1481904 (7) can be obtained
ARTICLE
N,N’-bis(2-diphenylmethyl-4,6-dimethylphenyl)-ethaVnieew-1A,r2ti-cle Online
5
DOI: 10.1039/C6DT02100J
2 (5.00 g, 17.4 mmol) was dissolved in
diimine (3). Aniline
free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
dichloromethane (10 mL) in a round bottom flask. Aq. glyoxal
solution (40% w/w) (1.0 mL, 8.72 mmol), formic acid (70 μL,
1.74 mmol) and MgSO4 (2.1 g, 17.4 mmol) were added and the
reaction mixture was stirred at room temperature for 25
hours. The crude mixture was poured onto a sintered frit and
the bright yellow MgSO4 cake was washed with DCM until all
colour had been removed. The solvent was removed in vacuo
and the yellow solid was washed with hot EtOAc (20 mL).
Drying in vacuo afforded a bright yellow powder. Yield: 4.5 g,
7.54 mmol, 87%. 1H NMR (CDCl3, 400 MHz): δH 7.40 (2H, s,
CHN), 7.25-7.13 (12H, m, Ar CH), 6.98-7.03 (8H, m, Ar CH), 6.93
(2H, s, Ar CH), 6.55 (2H, s, Ar CH), 5.43 (2H, s, CHPh2), 2.21 (6H,
s, CH3), 2.00 (6H, s, CH3). 13C{1H} NMR (CDCl3, 101 MHz): δC
164.2 (NCH), 147.4 (Ar C), 143.7 (Ar C), 133.9 (Ar C), 133.6 (Ar
C), 129.9 (Ar CH), 129.7 (Ar CH), 128.4 (Ar CH), 128.1 (Ar CH),
126.3 (Ar CH), 125.2 (Ar C), 51.9 (CHPh2), 21.2 (CH3), 18.4
(CH3). Elemental analysis Calcd (%) for C44H40N2: C, 88.55; H,
6.76; N, 4.69. Found: C, 87.57; H, 6.65; N, 4.52. LRMS (MALDI)
m/z: 597.47.
Experimental
General. NMR spectra were acquired using a Bruker AV3-400,
spectrometer equipped with a cryoprobe. All chemical shifts
1
are reported relative to the residual solvent signal (for H) or
1
relative to the deuterated solvent (for 13C).60 [1H, H] COSY,
[1H, 13C] HSQC, and [1H, 13C] HMBC experiments were used
where necessary to assign signals. GC-MS analyses were
carried out on an Agilent 7890A instrument coupled to an
Agilent 5975C mass spectrometer (chemical ionisation mode,
CH4). GC-FID analyses were carried out on an Agilent 7890A
instrument. Elemental analyses were carried out using a Perkin
Elmer 2400 Series II instrument. X-ray crystallographic analyses
were undertaken with samples mounted in oil at 123(2) K
using an Oxford Diffraction diffractometer equipped with a
CCD detector. All structures were refined against F2 and to
convergence using all unique reflections and the program
Shelxl-97.61
Known complexes [CuCl(IPr)], [CuCl(IMes)], [CuCl(IPr*)],
and [CuCl(IPr*OMe)] were prepared using literature methods.28,
29, 42, 62
1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazolium
chloride (1). Diimine
3
(2.326 g, 3.90 mmol) and
All reagents were obtained commercially and used as
paraformaldehyde (132.4 mg, 4.41 mmol, 1.1 equiv.) were
dissolved in EtOAc (25 mL) in a round bottom flask equipped
with stirrer bar. The reaction mixture was stirred and heated
to 70 °C at which point TMSCl (0.55 mL, 470.8 mg, 4.33 mmol,
1.1 equiv.) was added dropwise. The reaction was heated to
reflux for 2 hours over which time the reaction mixture
changed colour from yellow to light brown. After this time the
reaction mixture was filtered through a sintered frit to collect
an off white solid which had precipitated out of solution. This
solid was washed with Et2O (2 x 20 mL) and dried in vacuo. The
product was obtained as a white powder. Yield: 2.175 g, 3.37
mmol, 86%. The product is obtained as a 2:1 mixture of
rotamers A and B. 1H NMR of rotamer A (CDCl3, 400 MHz): δH
10.96 (1H, s, N(CH)N), 7.30 – 7.16 (12H, m, Ar CH), 7.15 – 7.06
(8H, m, Ar CH), 6.61 (2H, s, Ar CH), 6.52 (2H, d, J = 1.3 Hz,
N(CH)2N), 5.48 (2H, s, CHPh2), 2.26 (6H, s, CH3), 2.12 (6H, s,
CH3). 1H NMR of rotamer B (CDCl3, 400 MHz): δH 11.65 (1H, s,
N(CH)N), 7.30 – 7.16 (12H, m, Ar CH), 7.15 – 7.06 (8H, m, Ar
CH), 6.67 (2H, s, Ar CH), 6.47 (2H, d, J = 1.4 Hz, N(CH)2N), 5.32
(2H, s, CHPh2), 2.26 (6H, s, CH3), 2.16 (6H, s, CH3). 13C{1H} NMR
(CDCl3, 101 MHz): δC 142.6, 142.4, 141.9, 141.7, 141.4, 140.4,
139.9, 135.2, 135.1, 131.1 (Ar CH), 130.8 (Ar CH), 130.5 (Ar
CH), 129.9 (Ar CH), 129.85 (Ar CH), 129.80 (Ar CH), 129.46 (Ar
CH), 129.42 (Ar CH), 128.9 (Ar CH), 128.7 (Ar CH), 127.2 (Ar
CH), 127.1 (Ar CH), 127.00 (Ar CH), 126.97 (Ar CH), 123.9
(N(CH)2N, rot. A), 123.6 (N(CH)2N, rot. B), 51.6 (CHPh2, rot. A),
51.5 (CHPh2, rot. B), 21.6 (CH3), 18.2 (CH3). Elemental analysis
Calcd (%) for C45H41N2Cl: C, 83.76; H, 6.40; N, 4.34. Found: C,
83.23; H, 6.29; N, 0.16. LRMS (LC-MS, MeCN/H2O, ESI+ACPI)
supplied. Anhydrous and oxygen-free solvents (THF, toluene)
were obtained from an Innovative Technologies (Inert)
PureSolv purification system.
2-(diphenylmethyl)-4,6-dimethylaniline (2). A round bottom flask
was equipped with stirrer bar and charged with 2,4-dimethyl
aniline (6.6 mL, 52.2 mmol) and benzhydrol (9.61 g, 52.2
mmol). The reaction mixture was heated to 100 oC and stirred,
during which time the benzhydrol liquefied to give
a
homogeneous mixture. ZnCl2 (3.56 g, 26.1 mmol) and conc. aq.
HCl (37% w/w, 4.7 mL, 155.1 mmol) were added and the
o
mixture was heated to 160 C and stirred vigorously. After 0.5
hours the reaction mixture solidified; this solid was dissolved
in DCM (20 mL) and sat. NaHCO3 solution (20 mL) was added
to create a biphasic mixture which was stirred overnight. The
organic phase was extracted and washed with distilled water
(2x 20 mL) then concentrated in vacuo. A white solid was
precipitated from solution by adding EtOH. Yield: 10.56 g, 36.7
mmol, 70%. 1H NMR (CDCl3, 400 MHz): δH 7.41-7.33 (4H, m, Ar
CH), 7.33-7.27 (2H, m, Ar CH), 7.24-7.18 (4H, m, Ar CH), 6.90
(1H, s, Ar CH), 6.44 (1H, s, Ar CH), 5.56 (1H, s, CHPh2), 3.41 (2H,
s, NH2), 2.21 (3H, s, CH3), 2.20 (3H, s, CH3). 13C{1H} NMR (CDCl3,
101 MHz): δC 142.9 (Ar C), 140.0 (Ar C), 129.7 (Ar C), 129.6 (Ar
C), 128.7 (Ar C), 128.6 (Ar C), 128.5 (Ar C), 126.9 (Ar C), 126.7
(Ar C), 122.7 (Ar CH), 52.5 (CHPh2), 20.8 (CH3), 17.8 (CH3).
Elemental analysis Calcd (%) for C21H15N: C, 87.76; H, 7.37; N,
4.87. Found: C, 87.43; H, 7.25; N, 4.98.
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m/z: 609.2, 610.2, 611.2. HRMS (LTQ Orbitrap XL, DCM/MeOH) C45H40N2ClAg: C, 71.86; H, 5.36; N, 3.72. Found:ViCew, A7r2tic.l1e7O;nliHne,
DOI: 10.1039/C6DT02100J
m/z: 609.3274, C45H41N2 requires 609.3264.
5.51; N, 3.53.
(1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazol-2-
(1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazol-2-yl)(η5-
yl)copper (I) chloride (4). NHC.HCl salt 1 (0.91 g, 1.41 mmol), CuCl cyclopentadienyl)nickel(II) chloride (6). In a glovebox, a Schlenk
(0.15 g, 1.55 mmol, 1.1 equiv.) and K2CO3 (0.39 g, 2.82 mmol, 2 flask equipped with a stirrer bar was charged with nickelocene (56.7
equiv.) were dissolved in acetone (5 mL) in a round bottom flask. mg, 0.300 mmol, 1.2 equiv.) and NHC.HCl salt 1 (160.6 mg, 0.249
The reaction mixture was stirred at 60 °C for 2 hours and then the mmol). The flask was removed from the glovebox, connected to the
crude was filtered through a pad of silica. The silica pad was washed Schlenk line, and anhydrous THF (5 mL) was added. The reaction
with DCM and then concentrated in vacuo. Pentane was added to mixture was heated to 70 °C for 16 h. After this time the solvent
precipitate a white solid, which was collected by vacuum filtration was removed and the crude material was dissolved in the minimum
and then washed with pentane and dried in vacuo. Yield: 0.87 g, amount of DCM and passed through a celite/silica column, eluting
1.23 mmol, 87%. The product is a mixture of two rotamers (ca 2:1 the red/pink fraction with DCM. The DCM solution was
1
A:B), which can be distinguished on the 1H NMR spectrum. H NMR concentrated in vacuo and a pink powder was precipitated by
of rotamer A (CDCl3, 400 MHz): δH 7.33 – 6.88 (22H, m, Ar CH), 6.74 addition of pentane. Yield: 98.7 mg, 0.129 mmol, 52%. The product
(2H, d, J = 1.1 Hz, Ar CH), 6.40 (2H, s, N(CH)2N), 5.31 (2H, s, CHPh2), is a mixture of two rotamers, which cannot be distinguished on the
1
2.28 (6H, s CH3), 2.06 (6H, s, CH3). 1H NMR of rotamer B (CDCl3, 400 1H NMR spectrum because they are present in a ca. 1:1 ratio. H
MHz): δH 7.33 – 6.88 (22H, m, Ar CH), 6.64 (2H, s, Ar CH), 6.15 (2H, NMR (CDCl3, 400 MHz): δH 7.49 (4H, app. d, J = 7.6 Hz, Ar CH), 7.41
s, N(CH)2N), 5.47 (2H, s, CHPh2), 2.28 (6H, s CH3), 2.06 (6H, s, CH3). (4H, app. d, J = 7.6 Hz, Ar CH), 7.34 (4H, app. t, J = 7.8 Hz, Ar CH),
13C{1H} NMR (CDCl3, 101 MHz): δC 143.1, 142.5, 140.5, 139.9, 135.3, 7.30 – 7.22 (6H, m, Ar CH), 7.22 – 7.13 (8H, m, Ar CH), 7.12 – 7.04
134.9, 130.5, 129.8, 129.7, 129.6, 129.5, 128.7, 128.6, 128.5, (10H, m, Ar CH), 6.97 – 6.88 (4H, m, Ar CH), 6.87 – 6.78 (8H, m, Ar
126.81, 122.7 (N(CH)2N), 122.5 (N(CH)2N), 51.7 (CHPh2), 51.4 CH), 6.60 (s, 2H, CHPh2), 6.41 (s, 2H, CHPh2), 5.71 (s, 2H, N(CH)2N),
(CHPh2), 21.6 (CH3), 18.1 (CH3). Elemental analysis Calcd (%) for 5.65 (s, 2H, N(CH)2N), 4.74 (s, 5H, Cp CH), 4.67 (s, 5H, Cp CH), 2.37
C45H40N2ClCu: C, 76.36; H, 5.70; N, 3.96. Found: C, 75.78; H, (s, 6H, CH3), 2.35 (s, 6H, CH3), 1.96 (s, 6H, CH3), 1.75 (s, 6H, CH3).
5.66; N, 3.84.
13C{1H} NMR (CDCl3, 101 MHz): δH 167.6 (NCN), 164.6 (NCN), 144.6
(Ar C), 143.9 (Ar C), 143.8 (Ar C), 143.3 (Ar C), 143.0 (Ar C), 142.0 (Ar
(1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazol-2-yl)silver C), 138.9 (Ar C), 137.0 (Ar C), 136.8 (Ar C), 136.7 (Ar C), 135.8 (Ar C),
(I) chloride (5). NHC.HCl salt 1 (190 mg, 0.294 mmol) and AgNO3 (50 130.7 (Ar CH), 130.3 (Ar CH), 129.9 (Ar CH), 129.8 (Ar CH), 129.6 (Ar
mg, 0.294 mmol) were dissolved in DCM (10 ml) and stirred for 5 CH), 129.3 (Ar CH), 128.41 (Ar CH), 128.37 (Ar CH), 128.3 (Ar CH),
minutes before addition of K2CO3 (690.0 mg, 4.99 mmol, 17 equiv.). 128.2 (Ar CH), 128.15 (Ar CH), 128.09 (Ar CH), 126.5 (Ar CH), 126.4
The reaction was stirred at room temperature for 2 hours and then (Ar CH), 126.2 (Ar CH), 126.1 (Ar CH), 124.9 (N(CH)2N), 124.5
filtered over celite and concentrated in vacuo. A white powder was (N(CH)2N), 92.8 (Cp CH), 92.3 (Cp CH), 51.5 (CHPh2), 50.9 (CHPh2),
obtained after trituration with Et2O. Yield: 170.0 mg, 0.220 mmol, 21.7 (CH3), 19.2 (CH3), 18.3 (CH3). Elemental analysis Calcd (%)
75%. The product is a mixture of two rotamers (ca. 1.3:1), which for C50H45N2ClNi: C, 78.19; H, 5.91; N, 3.65. Found: C, 77.98; H,
1
can be distinguished on the 1H NMR spectrum. H NMR of rotamer 6.27; N, 3.47.
A (CDCl3, 400 MHz): δH 7.33 – 7.11 (12H, m, Ar CH), 7.07 – 6.86
(10H, m, Ar CH), 6.65 (2H, d, J = 1.2 Hz, Ar CH), 6.33 (2H, d, J = (1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazol-2-
2.0 Hz, N(CH)2N), 5.38 (2H, s, CHPh2), 2.29 (6H, s, CH3), 2.00 yl)(η2,η2-1,5-cyclooctadienyl)iridium(I) chloride (7). A vial was
(6H, s, CH3). 1H NMR of rotamer B (CDCl3, 400 MHz): δH 7.33 – charged with NHC.HCl salt 1 (137.7 mg, 0.21 mmol), [IrCl(cod)]2
7.11 (12H, m, Ar CH), 7.07 – 6.86 (10H, m, Ar CH), 6.73 (2H, d, J (68.5 mg, 0.10 mmol, 0.48 equiv.) and K2CO3 (264.6 mg, 1.93 mmol,
= 1.2 Hz, Ar CH), 6.52 (2H, d, J = 2.0 Hz, N(CH)2N), 5.23 (2H, s, 9 equiv.). The mixture was dissolved in acetone (6 mL) and stirred
CHPh2), 2.29 (6H, s, CH3), 2.02 (6H, s, CH3). 13C{1H} NMR (CDCl3, for 24 h at 60 °C. The solvents were removed and the residue was
101 MHz): δC 183.9 (NCN, rot. B), 183.8 (NCN, rot. A), 143.2 (Ar taken up in the minimum amount of DCM. This solution was passed
C), 143.0 (Ar C), 142.3 (Ar C), 142.2 (Ar C), 140.6 (Ar C), 140.5 through a pad of silica, eluting with dichloromethane until the
(Ar C), 140.0 (Ar C), 139.9 (Ar C), 135.4 (Ar C), 135.3 (Ar C), yellow fraction was removed. The solution was concentrated in
135.1 (Ar C), 135.0 (Ar C), 130.5 (Ar CH), 129.7 (Ar CH), 129.6 vacuo. The addition of pentane yielded a yellow solid, which was
(Ar CH), 129.5 (Ar CH), 128.91 (Ar CH), 128.87 (Ar CH), 128.6 washed with pentane (2 x 3 mL) and dried under high vacuum.
(Ar CH), 128.5 (Ar CH), 126.92 (Ar CH), 126.88 (Ar CH), 126.8 Yield: 155.6 mg, 0.16 mmol, 78%. The product is a mixture of two
(Ar CH), 123.1 (N(CH)2N), 123.04 (N(CH)2N), 122.98 (N(CH)2N), rotamers in a 2:1 ratio (A:B), for which some signals can be
122.9 (N(CH)2N), 51.6 (CHPh2, rot. A), 51.3 (CHPh2, rot. B), 21.6 distinguished; rotamer A is the anti-isomer, while B is the syn-
(CH3), 18.3 (CH3, rot. A), 18.0 (CH3, rot. B). Imidazol-2-ylidene isomer, on the basis of the number of COD CH and CHPh2 signals
C2 carbons were identified via [1H, 13C] HMBC correlation (3JCH
)
observed in the NMR spectra for each. 1H NMR (CDCl3, 400 MHz):
to the C4/C5 protons. Elemental analysis Calcd (%) for δH 7.52 – 6.53 (m, Ar CH plus CHPh2 for rot. A) 6.21 (2H, s,
CHPh2, rot. B), 5.73 (1H, s, N(CH)2N, rot. A), 5.58 (1H, s,
6 | Dalton Trans., 2016, 00, 1-3
This journal is © The Royal Society of Chemistry 2016
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N(CH)2N, rot. A), 5.56 (2H, s, N(CH)2N, rot. B), 4.79 – 4.66 (2H, rot A.), 51.2 (CHPh2, rot. B), 21.7 (CH3), 19.0 (CH3, rot. A), 18.7
View Article Online
-1
DOI: 10.1039/C6DT02100J
m, COD CH, rot. B), 4.59 – 4.45 (2H, m, COD CH, rot. A), 3.35 – (CH3, rot. B). IR (DCM solution): 2067, 1984 cm .
3.22 (2H, m, COD CH, rot. B), 3.17 – 2.97 (2H, m, COD CH, rot.
A), 2.30 (6H, s, CH3), 2.27 (3H, s, CH3), 2.20 (3H, s, CH3), 1.98 Procedure for the hydrosilylation reaction. A vial equipped with a
(3H, s, CH3), 1.91 (3H, s, CH3), 1.94 – 1.11 (8H, m, COD CH2). stirrer bar and a septum-fitted cap was charged with
13C{1H} NMR (CDCl3, 101 MHz): δH 182.5 (NCN, rot. B), 177.8 benzophenone (0.18 g, 1 mmol), KOtBu (0.02 g, 0.2 mmol, 20
(NCN, rot. A), 144.6 (Ar C), 144.1 (Ar C), 143.8 (Ar C), 143.7 (Ar mol%), and [CuCl(NHC)] (0.03 mmol, 3 mol%). The vial was
C), 143.3 (Ar C), 143.0 (Ar C), 142.7 (Ar C), 141.1 (Ar C), 138.7 evacuated and refilled with nitrogen. Dry toluene (2 mL) was
(Ar C), 138.5 (Ar C), 138.3 (Ar C), 136.9 (Ar C), 136.2 (Ar C), added via syringe and the mixture was stirred at 80 °C for 10
135.9 (Ar C), 135.4 (Ar C), 130.74, 130.66, 130.6, 130.1, 129.81, minutes. Triethylsilane (0.79 mL, 5 mmol, 5 equiv.) was added
129.75, 129.6, 129.5, 129.34, 129.28, 128.5, 128.2, 128.0, via syringe and the reaction was monitored by withdrawing
127.9, 127.8, 126.5, 126.4, 126.1, 126.0, 125.9, 124.5 aliquots at time points. Conversion was assessed by GC-FID
(N(CH)2N, rot. A), 123.53 (N(CH)2N, rot. B), 123.48 (N(CH)2N, analysis; the GC-FID was calibrated for starting material and
rot. A), 84.0 (COD CH, rot. A), 83.7 (COD CH, rot. B), 82.8 (COD product (see the Supporting Information).
CH, rot. A), 54.4 (COD CH rot. A), 51.6 (COD CH rot. A), 51.4
(CHPh2, rot. B), 51.2 (COD CH, rot. B), 50.6 (CHPh2, rot. A), 50.5
Acknowledgements
(CHPh2 rot. A), 34.6 (COD CH2), 33.7 (COD CH2), 32.7 (COD
CH2), 30.2 (COD CH2), 29.0 (COD CH2), 27.5 (COD CH2), 21.6
(CH3), 20.5 (CH3), 18.6 (CH3), 18.5 (CH3). Elemental analysis
Calcd (%) for C51H52N2IrCl: C, 67.39; H, 5.55; N, 2.97. Found: C,
67.69; H, 5.61; N, 2.83.
We thank the University of Strathclyde for funding access to
analytical facilities. High resolution mass spectra were
collected by the EPSRC UK National Mass Spectrometry Facility
(NMSF) at Swansea University. DJN thanks the University of
Strathclyde for a Chancellor’s Fellowship. We are grateful to
Mr Craig Irving, Dr John Parkinson, Ms Patricia Keating, Mr
Alexander Clunie and Mr Gavin Bain for their assistance with
various technical facilities. We thank Professor Jonathan Percy
for allowing us access to his laboratory equipment, Dr Mark
Spicer for supplying us with IMes.HCl, and Professor Rein Ulijn
for providing us with AgNO3. We are grateful to Dr Adrián
Gómez Suárez (Westfälische Wilhelms-Universität Münster)
for helpful discussions.
(1,3-bis(2-diphenylmethyl-4,6-dimethylphenyl)-imidazol-2-
yl)dicarbonyliridium(I) chloride (8). Complex
7 (40.7 mg, 0.0431
mmol) was dissolved in DCM (2 mL) and CO was bubbled
through the solution for 5 minutes. The solution turned from
dark yellow to pale yellow. The flask was closed with a septum,
and a balloon of CO was attached. The solution was stirred for
1 h, during which time the product precipitated from solution.
The solvent was then removed and the light yellow solid was
washed with pentane (2 x 2 mL) and dried under high vacuum.
Yield: 29.0 mg, 0.0325 mmol, 75%. The product is a mixture of
two rotamers in a 1.2:1 ratio in C6D6 and a 1.5:1 ratio in CDCl3.
Satisfactory 13C{1H} NMR spectra could not be obtained in
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
C6D6, CDCl3, THF-d8, or MeCN-d3 due to poor solubility, but matter under discussion, limited experimental and spectral data,
and crystallographic data.
§ Abbreviations for NHC ligands referred to in this manuscript
some key signals could be identified. 1H NMR for rotamer A
(C6D6, 400 MHz): δH 7.72 (4H, d, J = 7.2 Hz, Ar CH), 7.24 – 6.93
(16H, m, Ar CH), 6.92 (2H, d, J = 1.3 Hz, Ar CH), 6.72 (2H, s, Ar
CH), 6.68 (2H, s, CHPh2), 5.47 (2H, s, N(CH)2N), 1.98 (6H, s,
CH3), 1.85 (6H, s, CH3). 1H NMR for rotamer B (C6D6, 400 MHz):
δH 7.66 (4H, d, J = 7.5 Hz, Ar CH), 7.24 – 6.93 (16H, m, Ar CH),
6.85 (2H, s, Ar CH), 6.81 (2H, s, Ar CH), 6.65 (2H, s, CHPh2), 5.43
(2H, s, N(CH)2N), 2.13 (6H, s, CH3), 1.89 (6H, s, CH3). 1H NMR
for rotamer A (CDCl3, 400 MHz): δH 7.36 – 7.02 (20H, m, Ar
CH), 6.98 – 6.90 (4H, m, Ar CH), 6.55 (2H, d, J = 1.2 Hz, Ar CH),
6.22 (2H, s, CHPh2), 5.72 (2H, s, N(CH)2N), 2.29 (6H, s, CH3),
2.02 (6H, s, CH3). 1H NMR for rotamer B (CDCl3, 400 MHz): δH
7.36 – 7.02 (20H, m, Ar CH), 6.89 – 6.82 (4H, m, Ar CH), 6.67
(2H, s, Ar CH), 6.24 (2H, s, CHPh2), 5.87 (2H, s, N(CH)2N), 2.28
(6H, s, CH3), 2.12 (6H, s, CH3). Partial 13C{1H} NMR (CDCl3, 101
MHz): δC 144.1, 143.4, 143.2, 143.0, 141.7, 139.5, 139.4, 136.0,
135.7, 135.0, 134.9, 130.5, 130.3, 130.2, 129.8, 129.7, 129.6,
129.4, 128.5, 128.3, 128.2, 128.1, 126.51, 126.46, 126.4,
123.85 (N(CH)2N, rot. A), 123.77 (N(CH)2N, rot. B), 51.4 (CHPh2,
are as follows: IPr*
methylphenyl)imidazol-2-ylidene; IPent
pentylphenyl)imidazol-2-ylidene;
ItBu
ditertbutylimidazol-2-ylidene;
tertbutylphenyl)methyl-4-methylphenyl)imidazol-2-ylidene;
IPaul 1,3-bis(2-diphenylmethyl-4,6-dimethyl-
phenyl)imidazol-2-ylidene; ICy = 1,3-dicyclohexylimidazol-2-
ylidene.
=
1,3-bis(2,6-diphenylmethyl-4-
1,3-bis(2,6-diiso
1,3-
1,3-bis(2,6-di(p-
=
-
=
IPr**
=
=
1.
2.
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62.
O. Santoro, F. Lazreg, Y. Minenkov, L. Cavallo and C. S. J.
Cazin, Dalton Trans., 2015, 44, 18138-18144.
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DOI: 10.1039/C6DT02100J
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DOI: 10.1039/C6DT02100J
Synthesis and characterisation of an N-heterocyclic carbene with spatially-defined steric impact
Paul Shaw, Alan R. Kennedy, and David J. Nelson*
Table of Contents Entry
The synthesis and co-ordination chemistry of a new N-heterocyclic carbene (“IPaul”) with spatially-
defined steric impact is reported.