Synthesis and reactivity of new bis(N-heterocyclic carbene) iridium(I) complexes

Article abstract of DOI:10.1021/ic4018773

New complexes of the type trans-[IrCl(η²-COE)(NHC) ₂] (COE = cis-cyclooctene; NHC = N-heterocyclic carbene) have been prepared in one step from the reaction of ca. 4 equiv of NHC or [AgCl(NHC)] with [IrCl(η²-COE)₂

Full text of DOI:10.1021/ic4018773

Article
pubs.acs.org/IC
Synthesis and Reactivity of New Bis(N-heterocyclic carbene)
Iridium(I) Complexes
David J. Nelson, Byron J. Truscott, Alexandra M. Z. Slawin, and Steven P. Nolan*
EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, United Kingdom
S
* Supporting Information
ABSTRACT: New complexes of the type trans-[IrCl(η2-
COE)(NHC)2] (COE = cis-cyclooctene; NHC = N-
heterocyclic carbene) have been prepared in one step from
the reaction of ca. 4 equiv of NHC or [AgCl(NHC)] with
[IrCl(η2-COE)2]2 in benzene. These new complexes have
been characterized by techniques including NMR and IR
spectroscopy, X-ray crystallography, and elemental analysis.
Exposing trans-[IrCl(COE)(IiPrMe)2] to CO yielded trans-
[IrCl(CO)(IiPrMe)2], which is the only bis(NHC) analogue of Vaska’s complex trans-[IrCl(CO)(PPh3)2] known to date. The
synthesis of trans-[Ir(CO)(IiPrMe)2(R)] (R = MeO, PhCC, OSiPh3, O2CPh) complexes has been achieved via deprotonation
reactions involving the new hydroxide species trans-[Ir(OH)(CO)(IiPrMe)2].
position. In addition, the two ligands in COD-bearing
complexes are forced into a cis arrangement. A number of
researchers have explored the use of structural motifs where
COD is not present, such as complexes 1−3 (Chart 1). During
INTRODUCTION
■
The development of new catalytic methods is complemented
by the discovery and exploration of new transition-metal
complexes. The employment of N-heterocyclic carbenes
(NHCs) as ligands represents one of the most important
recent advances in transition-metal chemistry.1 NHCs
encompass a range of structural diversity and present
opportunities for the fine tuning of ligand steric2 and
electronic3 properties using the vast array of synthetic
methodologies that is available.4 Research within our group
has focused on the exploration of the properties of NHCs, the
metals to which they are coordinated, and the application of
such complexes in homogeneous catalysis.
Chart 1. Cyclometalated Iridium(III) Complexes
NHC−iridium(I) complexes of various types have been
explored in the literature for a number of applications. For
example, Kerr and co-workers have applied complexes of the
type [Ir(η4-COD)(NHC)(PR3)][PF6] (COD = 1,5-cyclo-
octadiene; PR3 = PPh3, PMe2Ph, etc.) in H/D exchange5 and
hydrogenation6 reactions, whereas Herrmann has utilized
[Ir(η4-COD)(NHC)2][X] (X = BF4, PF6) complexes for C−
H borylation reactions.7,8 Earlier work from our group has
shown the potential of [Ir(η4-COD)(NHC)(py)][PF6] com-
plexes in hydrogenation,9 transfer hydrogenation, and oxida-
tion.10 Recently, we reported the synthesis and characterization
of [Ir(OH)(η4-COD)(NHC)] complexes, which are highly
versatile reagents for the activation of a variety of bonds.11 In
these species, the hydroxide moiety behaves as an internal base,
capable of deprotonating a wide range of organic substrates,
and this scaffold may have significant future applications in
catalysis.
the synthesis of bis(NHC) complexes from [IrCl(η2-COE)2]2
(COE = cis-cyclooctene), spontaneous insertion of the iridium
center into the C−H bond of ItBu (1,3-bis(tert-butyl)imidazol-
2-ylidene) was observed, generating the iridium(III)−hydride
complex [IrClH(κ2-ItBu)(ItBu)] (1).12 A further period of
stirring in solution resulted in the formation of the
corresponding doubly cyclometalated complex [IrCl(κ2-
ItBu)2] (2), whereas chloride abstraction with AgPF6 led to
the cationic 14-electron complex [IrCl(κ2-ItBu)2][PF6] (3).
Further studies of 1−3 established that the cyclometalation
However, these examples all feature a COD ligand. In some
reactions, this ligand may react with reagents such as hydrogen
to yield cis-cycloctene, which cannot reoccupy two coordination
sites and, therefore, can potentially lead to catalyst decom-
Received: July 20, 2013
Published: October 22, 2013
© 2013 American Chemical Society
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Article
event was reversible and that, under H2, a dihydride complex
stabilized by two agostic interactions could be isolated.13 Under
a D2 atmosphere, the methyl groups of the NHC ligand were
completely deuterated. This reversible reactivity with hydrogen
has led to the use of this family of complexes in low loadings for
H/D exchange14,15 and the liberation of hydrogen from
ammonia−borane.16,17 Aldridge and co-workers have more
recently prepared [IrClH(κ2-IMes)(IMes)] (4; IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)18 and [IrClH(κ2-
IPr)(IPr)] (5; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene)19 from the interaction of IMes and IPr, respectively,
with [IrCl(η2-COE)2]2. However, although the interaction of
bulky alkyl- and aryl-bearing NHCs with [IrCl(η2-COE)2]2 has
been probed, the interactions of smaller alkyl-bearing NHCs
have not been examined. We therefore sought to explore the
synthesis and reactivity of new, COD-free bis(NHC)−iridium-
(I) complexes.
RESULTS AND DISCUSSION
■
trans-[IrCl(η2-COE)(NHC)2] Complexes. Complexes bear-
ing five different NHC ligands were prepared: IiPr (6a), IiPrMe
(6b), IsB (6c), ICy (6d), and IDD (6e) (IiPr = 1,3-
bis(isopropyl)imidazol-2-ylidene; IiPrMe = 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene; IsB = 1,3-diisobutylimidazol-2-
ylidene; ICy = 1,3-dicyclohexylimidazol-2-ylidene; IDD = 1,3-
dicyclododecylimidazol-2-ylidene) (Scheme 1). For complexes
Scheme 1. Preparation of Bis(NHC)−Ir Complexes
Figure 1. X-ray crystal structures of 8a−e, showing 50% thermal
ellipsoid probability. H atoms are excluded for clarity. More than one
independent molecule is present in the unit cell for 8a−c; therefore,
only one example is presented here. See the CIF files (Supporting
Information) for all independent molecules.
Table 1. Key Structural Parameters for 8a−e
bearing IiPrMe, ICy, and IDD, the free carbene and [IrCl(η2-
COE)2]2 were stirred together in benzene for 24 h. The free
carbenes IiPr and IsB are more difficult to prepare and purify,
being oils that require distillation before use. Therefore, the
corresponding [AgCl(NHC)] salts were employed as trans-
metalation reagents instead; these were prepared via the
reaction of the corresponding NHC·HCl salt with silver(I)
oxide.20 No cyclometalation occurred during the syntheses of
these complexes, and the corresponding trans-[IrCl(η2-COE)-
(NHC)2] complexes 8a−e were obtained as analytically pure
yellow solids after workup.
CNHC1−Ir−CNHC2
Ir−CNHC1
Ir−CNHC2
a
complex
%Vbur
(deg)
(Å)
(Å)
8a
26.5; 26.1
28.2; 28.3
173
176
172
172
171
171
175
173
176
2.05(2)
2.01(3)
2.02(2)
2.07(2)
2.03(2)
2.05(2)
2.06(2)
1.98(2)
2.04(1)
2.02(2)
2.09(3)
2.01(2)
2.08(2)
2.03(2)
2.04(2)
2.06(2)
2.04(1)
2.04(1)
8b
8c
27.1; 26.8
8d
8e
26.8; 26.8
28.5; 29.4
b
a
1
Full characterization was carried out using 1D and 2D H
Parameters: 3.5 Å sphere radius; 0.1 Å mesh spacing; H atoms
b
and 13C{1H} NMR spectroscopic techniques, X-ray crystallog-
raphy, and elemental analysis. The structures obtained through
X-ray crystallographic analyses were consistent with the results
of the elemental analyses (Figure 1; selected structural data can
be found in Table 1, and experimental data can be found in
Table 2). Although the small size of the crystals led to higher
than desired values of R1 and wR2 and some difficulties in
refinement (see the Experimental Section), sufficient data could
be collected to discern the key structural features of the
excluded. The two NHC ligands adopt considerably different
conformations; see Figure 1.
complexes. As expected, the iridium(I) centers adopt a square-
planar arrangement; the two NHC ligands are trans to each
other, whereas the chloride ligand is located trans to the η2-
bound cis-cyclooctene ligand. The NHC ligands occupy the
same plane, as opposed to 1−3, where the NHC ligands occupy
perpendicular planes.
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a
Table 2. Experimental Data for Single-Crystal X-ray Diffraction Analyses of 8a−e
8a
8b
8c
8d
8e
CCDC no.
formula
948893
948894
948895
948896
948897
C26H46ClIrN4
642.35
C30H54ClIrN4
698.46
C30H54ClIrN4
698.46
C40H68ClIrN4
838.68
C62H110ClIrN4
1139.25
formula wt
color and habit
cryst dimens (mm)
crysta syst
lattice type
a (Å)
yellow prism
yellow prism
yellow needle
yellow prism
yellow prism
0.030 × 0.030 × 0.030 0.120 × 0.120 × 0.030 0.100 × 0.010 × 0.010 0.200 × 0.200 × 0.200 0.100 × 0.030 × 0.030
monoclinic
C-centered
35.964(13)
9.425(3)
monoclinic
primitive
orthorhombic
primitive
orthorhombic
primitive
triclinic
primitive
10.586(2)
14.087(3)
21.972(5)
100.058(7)
101.758(8)
96.621(7)
3119(1)
28.701(5)
27.810(5)
13.601(3)
15.831(2)
23.242(3)
35.736(4)
10.527(2)
26.589(5)
15.428(2)
b (Å)
c (Å)
19.587(8)
α (deg)
β (deg)
119.893(7)
94.994(4)
γ (deg)
V (Å3)
5756(4)
Cc (No. 9)
8
10815(3)
P21/c (No. 14)
12
13149(3)
Pbca (No. 61)
16
4318.3(12)
Pna21 (No. 33)
4
space group
P1 (No. 2)
̅
2
Z
Dcalcd (g cm−3
)
1.482
1.287
1.411
1.290
1.213
1208.00
22.266
11009
613
F000
2592.00
47.638
9018
4272.00
38.089
19206
983
5696.00
41.770
11866
649
1732.00
31.917
25479
443
μ(Mo Kα) (cm−1
)
no. of observns (all rflns)
no. of variables
577
rflns/params
15.63
19.54
18.28
57.51
17.96
0.0616
0.0820
0.1729
1.107
0.000
2.04
R1 (I > 2.00σ(I))
0.0774
0.1043
0.1906
1.088
0.0962
0.1332
0.2364
1.130
0.0862
0.1538
0.2087
1.108
0.0609
0.0789
0.1682
1.110
R1 (all reflections)
wR2 (all reflections)
goodness of fit indicator
max shift/error in final cycle
0.000
0.003
0.001
0.000
max peak in final diff map (e/Å3) 2.07
min peak in final diff map (e/Å3) −1.72
1.72
1.92
2.02
−2.98
−1.99
−3.72
−1.36
a
Further details can be found in the Experimental Section.
The high degree of flexibility associated with the N
substituents in 8a−e means that they may adopt conformations
far away from the metal center. The tert-butyl groups in 1−3
rotate in very close proximity to the metal center, resulting in
spontaneous C−H insertion processes. This type of behavior is
similar to that observed in the case of the addition of free NHC
ligands to [Ni(CO)4], where IAd (1,3-bis(adamantyl)imidazol-
2-ylidene) and ItBu yielded complexes of the form [Ni-
(CO)2(NHC)] rather than the expected [Ni(CO)3(NHC)]
species, due to the increased steric bulk very close to the metal
center.21 Despite the overall size of IDD, no cyclometalation
occurs, most likely due to the considerable flexibility of this
ligand.22 It can be seen from the X-ray crystal structure that one
ligand in 8e adopts a conformation in which the cycloalkyl rings
are far removed from the metal center, whereas the second
ligand adopts an arrangement where the rings envelop the
metal center. The buried volume percent values (%Vbur)2,23 of
the two IDD ligands are therefore rather different (Table 1);
one has a %Vbur value of 29.4%, the bulkiest example seen here,
whereas the other has a %Vbur value of 28.5%, comparable to
that of the smaller ligands. This fluctuating steric behavior is
consistent with that observed in several organometallic
complexes.22 The ligands of the ICy complex (8d) exhibit
slightly lower values for %Vbur than those in IiPrMe-bearing 8b,
despite the larger N substituent in the former. Similar results
have been reported by us for square-planar [Ir(OH)(COD)-
(NHC)] complexes due to the steric repulsion exerted by
methyl groups on the backbone of the NHC.11,24 The Ir1−C1
bond length in 8d is considerably shorter than the Ir1−C21
distance and shorter than the corresponding bond lengths in
the other complexes. Although no clear explanation for this is
apparent, it should be noted that such differences in bond
lengths were recorded for complexes 2 and 3 and that DFT
calculations suggested that NHC to metal π bonding was the
root cause.12 However, it should be noted that given the R1 and
wR2 values obtained for these crystal structure determinations,
particularly detailed comparisons of bond lengths and buried
volumes are not possible. Instead, the data confirm the
proposed structure and indicate that ligand steric bulk
decreases here in the order IDD > IiPrMe > ICy ≈ IsB ≈ IiPr.
Although %Vbur is a convenient metric for assessing the steric
properties of ligands,2 the construction of steric maps using the
freely available SambVca software23 allows the steric environ-
ment around the metal center to be visualized in three-
dimensional space (Figure 2 shows the maps for the IDD
ligands in 8e). The construction of steric maps for each
complex (see Figures S30−S33 in the Supporting Information
for steric maps for 8a−d) revealed that the NHCs in each
complex have similar steric impacts on the metal center, within
the 3.5 Å sphere considered. Steric bulk is usually focused in
two opposite quadrants, suggesting that, if the cis-cyclooctene
ligand were removed, the metal center should be more
accessible than those of complexes 1−3, due to the
perpendicular arrangement of the latter.
With these new complexes structurally characterized, their
reactivity was probed by exposing them to various reagents and
conditions. The isolation of [IrCl(η2-COE)(NHC)2] com-
plexes was consistent with the results reported by Aldridge et al.
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of complexes 1−3 under the same reaction conditions. These
promising results in catalysis are currently under further
investigation. However, further functionalization of the bis-
(NHC) species was desired, to explore their potential in
organometallic chemistry.
trans-[IrCl(CO)(IiPrMe)2]: A Bis(NHC) Analogue of
Vaska’s Complex. When a benzene-d6 solution of 8b was
exposed to CO at room temperature in an NMR tube fitted
with a J. Young valve, we observed complete dissociation of the
cis-cyclooctene ligand and slight upfield shifting of the NHC
ligand resonances. After a preparative-scale reaction (in pentane
solution) (Scheme 4), full characterization of the resulting pale
Scheme 4. Reaction between 8b and CO
Figure 2. Steric maps of the IDD ligands in 8e, calculated using the
SambVca web application23 (sphere radius 3.5 Å, mesh spacing 0.1 Å,
hydrogen atoms omitted).
during the preparation of [IrClH(κ2-IMes)(IMes)] (4), where
the intermediate [IrCl(η2-COE)(IMes)2] (8f) was isolated and
characterized; this was to date the only complex of this motif
reported in the literature, and its reactivity and potential in
catalysis was not explored. Aldridge et al. advanced 8f to 4 by
heating in THF solution (complete conversion after 24 h at 65
°C in toluene). Although small traces of hydride species were
apparent after prolonged heating of 8a,e (12 h at 90 °C),
further heating (72 h at 90 °C) of 8a led to decomposition,
whereas 8e gave ca. 50% conversion to iridium hydride species
(δH(toluene-d8) −32.3 and −32.4 ppm). The cyclometalation
process is therefore far less facile for 8a−e than for 8f; the
reactivity of 8a−e is therefore likely to be quite different from
that of complexes 1−5.
Unfortunately, further attempts to probe the reactivity of
these species were not particularly successful. Attempts to
remove the chloride, either by substitution using CsOH in
THF11 or by chloride abstraction with NaB(ArF)4 (ArF = 3,5-
bis(trifluoromethyl)phenyl) were not successful and resulted in
recovered starting material or decomposition. Complexes 8a−e
are highly oxygen sensitive, even in the solid state, and rapidly
turn green and then black on exposure to the ambient
atmosphere. When a solution of 8b was exposed to oxygen in
an NMR tube with a J. Young valve, characteristic signals
corresponding to free cis-cyclooctene were apparent, along with
decomposition of the complex. The η2-bound COE ligand
therefore renders 8a−e rather fragile. Exposure to hydrogen
yielded a mixture of iridium hydride species, suggesting
potential future applications of these species in H/D exchange
and hydrogenation reactions. A trial reaction was conducted in
which triphenylsilane successfully underwent H/D exchange
catalyzed by [IrCl(η2-COE)(ICy)2] (8d) in 98% conversion
(Scheme 3). This reactivity is at the very least equivalent to that
yellow solid by NMR spectroscopy, elemental analysis, and IR
spectroscopy confirmed the identity of the species to be trans-
[IrCl(CO)(IiPrMe)2] (9).
IR analysis of this novel complex revealed the stretching
frequency of the carbonyl ligand (νCO(CH2Cl2) 1917 cm−1;
νCO(n-hexane) 1929 cm−1, at room temperature). This complex
is the only bis(NHC) structural analogue of Vaska’s complex25
trans-[IrCl(CO)(PPh3)2] (10; νCO(CHCl3) = 1967 cm−1)
reported to date. Liu and co-workers have reported [IrCl-
(CO)(NHC)(PR3)] (PR3 = PPh3, PCy3; NHC = 1,3-
dibenzylimidazol-2-ylidene, 1,3-dibenzyl-4,5-dihydroimidazol-
2-ylidene, 1,3-diethyl-4,5-dihydroimidazol-2-ylidene, 1,3-bis-
(methoxyethylene)-4,5-dihydroimidazol-2-ylidene, 1,3-bis(o-
methoxybenzyl)-4,5-dihydroimidazol-2-ylidene) complexes.26,27
IR measurements showed these mixed-ligand species to be
approximately as electron rich as the corresponding bis-
(phosphine) species (Figure 3). 10 is a key model complex
in organometallic chemistry, which has been used to study
many fundamental processes.25 Notably, the iridium center is
more electron rich in 9 than in 10, as determined from the
lower νCO of the latter, due to back-bonding from the iridium d
orbitals into the σCO* antibonding orbital. The value obtained
is similar to that measured for [IrCl(CO)(PCy3)2] (11;
νCO(CH2Cl2) 1922 cm−1).28 Therefore, although 9 is
structurally related to Vaska’s complex, its reactivity is likely
to be quite different, due to the considerable steric differences
between IiPrMe and PPh3, as well as the measured difference in
the electronic nature of the iridium center.
Complex 9 was then exposed to some model reagents to
understand its reactivity. Reaction with ca. 1 atm of H2 at room
temperature, 40 °C, or −78 °C did not yield any iridium
hydride species despite the presence of a sharp (ϕ1/2 < 1 Hz at
room temperature) 1H resonance on the NMR spectrum
corresponding to hydrogen (Scheme 5). This suggests that the
equilibrium constant for oxidative addition of hydrogen (Keq) is
very low indeed. In the case of 10, Keq is ca. 105.25 Vaska’s
complex is also known to react reversibly with O2 to yield the
Scheme 3. H/D Exchange on Silanes Catalyzed by 8d
1
dioxygen adduct. When 9 was exposed to oxygen, H NMR
signals corresponding to a new species appeared upfield of the
signals for 9, albeit very slowly (ca. 7−10 days under 1 atm of
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Figure 3. Comparison of IR spectroscopic data for a range of [IrCl(CO)(L)2] complexes (L = PR3, NHC); the medium used is indicated in
brackets.26−28
amides, alkoxides, and alkynes.11 We therefore sought to
Scheme 5. Reactivity of 9 and 10 toward H2 and O2
prepare a bis(NHC)−iridium(I) complex to evaluate the
potential of this framework in bond activation reactions. The
reaction between 8b and CsOH often resulted in decom-
position with the formation of free cis-cyclooctene. Hoping that
replacement of the labile alkene with a carbonyl ligand might
concentrate reactivity at the chloride, we exposed 9 to CsOH in
THF in a manner analogous to that used previously.11 In
addition, the carbonyl ligand also provides a convenient
spectroscopic handle. Gratifyingly, the desired hydroxide 13
could be obtained (Scheme 6a). A bond activation study was
Scheme 6. Preparation of Ir Hydroxide 13 from 9 and
Reactivity of 13 toward Organic Substrates
O2 in CD2Cl2). This new complex may be the dioxygen
complex [IrCl(CO)(O2)(IiPrMe)2] (12); however, although key
1H NMR spectroscopic features could be identified, decom-
position to a black precipitate on a similar time scale precluded
isolation and further characterization. These data are sufficient
to show that 9 is very much less reactive than Vaska’s complex
10.
This sluggish reactivity of 9 can be rationalized by comparing
the structure with that of Vaska’s complex 10 and PCy3-bearing
analogue 11. NHC ligands are known to be capable of
accepting π-electron density29 and are therefore perhaps too
soft to stabilize the iridium(III) products. This is despite the
higher electron density at the metal center of 9, as evidenced by
the much lower value of νCO. However, Vaska and Chen
reported that [IrCl(CO)(P(o-MeC6H4)3)2] was almost com-
pletely unreactive with O2, despite having electronic properties
similar to those of 10, suggesting that steric properties play a
key role in these types of complexes.30 11 is also known to be
poorly reactive with O2, despite the increased electron richness
of the iridium center in this species.30 The steric bulk of 9−11
then undertaken with some representative substrates. Although
deprotonation reactions with [Ir(OH)(COD)(NHC)] were
found to be clean and rapid (e.g., Scheme 6b),11 often
occurring within mixing time, the reactions with 13 were far
more sluggish. Reactions were typically conducted neat, over ca.
24 h at room temperature.
was assessed using buried volumes (%Vbur) calculated with
SambVca, using a DFT-derived structure of 9 (M06L/SDD,
using Gaussian;31 see the Supporting Information for details)
and published structures of 10 and 11.32,33 The buried volumes
were found to be 27.3%, 28.6%, and 29.0%, respectively
(average of the two ligands in each complex), pointing to only
marginal differences in steric impact. However, these measure-
ments are based on solid state (or in the case of DFT
calculations, gas phase) data and do not necessarily reflect
solution state behavior.
The resulting organoiridium(I) complexes 14−17 were fully
1
characterized using H and 13C{1H} NMR spectroscopy and
elemental analysis. Although the new hydroxide species 13 is
less reactive than the corresponding [Ir(OH)(COD)(NHC)]
species, the isolation of the former complex suggests that the
synthesis of a range of iridium(I) hydroxides should be possible
from the corresponding chloride species. Complex 13 therefore
allows access to a new range of organoiridium(I) species,
bearing two NHC ligands.
Applying [Ir(OH)(CO)(IiPrMe)2] in the Synthesis of New
Organoiridium(I) Complexes. We have previously shown
that iridium(I) hydroxide complexes are powerful intermediates
for the preparation of a range of organoiridium(I) complexes,
bearing various substituents including siloxides, aryl groups,
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10H, COE), 1.66 (d, 3JHH = 6.9, 12H, CH(CH3)2), 1.51 (d, 3JHH = 7.2,
12H, CH(CH3)2). 13C{1H} NMR (CD2Cl2, δ): 182.0 (Ir−C), 123.1
(N(CCH3)2N), 52.1 (CH(CH3)2), 35.1 (COE), 32.2 (COE), 30.5
(COE), 26.7 (COE), 22.2 (CH(CH3)2), 21.5(CH(CH3)2), 10.2
(N(CCH3)2N). Anal. Calcd for C30H54N4ClIr: C, 51.59; H, 7.79; N,
8.02. Found: C, 51.61; H, 7.64; N, 7.97. Crystals suitable for X-ray
diffraction studies were grown by chilling a pentane solution to −40
°C.
CONCLUSIONS
■
A series of complexes has been prepared and characterized,
showing how the interaction between small NHC ligands and
[IrCl(η2-COE)2]2 reproducibly yields trans-[IrCl(η2-COE)-
(NHC)2] complexes. Unlike the previously reported complexes
1−5, where larger ligands such as IMes, ItBu, and IPr are
employed, cyclometalation is far from facile.
[IrCl(η2-COE)(IsB)2] (8c). Benzene (5 mL) was added to solid
[IrCl(η2-COE)2]2 (100.0 mg, 0.111 mmol) and [AgCl(IsB)] (154.7
mg, 0.478 mmol, 4.3 equiv) and stirred for 48 h at room temperature,
with protection from light. Upon completion, the solution was filtered
through Celite, and the solvent was removed in vacuo to yield an
orange-yellow solid. This solid was extracted into pentane (3 × 5 mL),
and the solution was filtered through Celite before being concentrated
in vacuo. The residue was carefully washed with cold (−40 °C)
pentane (2 × 1 mL) and dried in vacuo. Yield: 58.4 mg (0.084 mmol,
The trans-[IrCl(η2-COE)(NHC)2] complexes thus formed
were somewhat resistant to further functionalization, which is
proposed to be due to facile dissociation of the η2-COE ligand.
Exposure of trans-[IrCl(η2-COE)(IiPrMe)2] to CO yielded
trans-[IrCl(CO)(IiPrMe)2], which is the only bis(NHC)
analogue of Vaska’s complex reported to date. Although the
reactivity of this complex was very limited, attributed to the soft
NHC ligands being less able than phosphanes to stabilize
iridium(III) products, it was robust enough to be straightfor-
wardly converted to the corresponding hydroxide complex.
This hydroxide species allowed access to a number of
bis(NHC) organoiridium(I) complexes via established depro-
tonation routes, suggesting that it may be a useful reagent for
the preparation of a range of new Ir(I) complexes.
1
38%) of a bright yellow powder. H NMR (CD2Cl2, δ): 6.80 (s, 4H,
i
N(CH)2N), 5.02−4.85 (m, 4H, NCH2 Pr), 4.01−3.86 (m, 4H,
i
3
NCH2 Pr), 2.40 (sept, JHH = 6.8, 4H, CH(CH3)2), 2.15−2.03 (m,
2H, COE), 1.81−1.70 (m, 2H, COE), 1.46−1.17 (m, 6H, COE), 0.99
3
(dd, JHH = 6.8, 0.8, 24H, CH(CH3)2), 1.15−0.56 (m, 4H, COE).
13C{1H} NMR (CD2Cl2, δ): 184.6 (Ir−C), 119.0 (N(CH)2N),
58.3(CH2CH(CH3)2), 35.7 (COE), 30.8 (CH2CH(CH3)2), 29.5
(COE), 27.0 (COE), 20.8 (CH2CH(CH3)2), 20.7(CH2CH(CH3)2).
Crystals suitable for X-ray diffraction studies were grown by chilling a
pentane solution to −40 °C.
Further investigations into the reactivity and applications of
these novel complexes are currently underway in our
laboratories.
[IrCl(η2-COE)(ICy)2] (8d). Benzene (2 mL) was added to solid
[IrCl(η2-COE)2]2 (50.1 mg, 0.056 mmol) and ICy (52.7 mg, 0.277
mmol, 4.05 equiv) and stirred for 24 h at room temperature. The
solution was filtered through Celite, the solvent was removed in vacuo,
and the residue was washed carefully with cold (−40 °C) pentane (3 ×
1 mL) and dried in vacuo to give a yellow solid. Yield: 69.3 mg (0.086
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out in an
Ar-filled glovebox using dry, degassed solvents unless otherwise stated.
NMR spectra were recorded on Bruker 300, 400, or 500 MHz (1H
observe frequency) NMR spectrometers and referenced to residual
solvent resonances.34 All chemical shifts are given in ppm, and
coupling constants are given in Hz. [IrCl(η2-COE)2]2 was prepared
according to the literature method35 and stored at −40 °C in the
glovebox. [AgCl(NHC)] complexes were prepared according to the
literature method.20 IR spectroscopy was carried out using a Perkin-
Elmer Paragon series 1000 FTIR spectrometer.
1
mmol, 77%). H NMR (CD2Cl2, δ): 6.75 (s, 4H, N(CH)2N), 5.66
(app t, 3JHH = 11.7, 4H, NCH(CH3)2), 2.39−0.59 (m, 62H). 13C{1H}
NMR (CD2Cl2, δ): 182.7 (Ir−C), 116.1 (N(CH)2N), 59.0 (CH-
(CH3)2), 36.0, 35.0, 34.2, 32.2, 30.9, 27.0, 26.7, 26.1. Anal. Calcd for
C38H70N4IrCl: C, 56.87; H, 7.79; N, 6.98. Found: C, 56.74; H, 7.87;
N, 6.89. Crystals suitable for X-ray diffraction studies were grown by
chilling a pentane solution to −40 °C.
[IrCl(η2-COE)(IDD)2] (8e). Benzene (3 mL) was added to solid
[IrCl(η2-COE)2]2 (99.5 mg, 0.111 mmol) and IDD (191.8 mg, 0.479
mmol, 4.31 equiv) and stirred for 5 days at room temperature. The
solution was filtered through Celite, the solvent was removed in vacuo,
and the residue was washed carefully with cold (−40 °C) pentane (3 ×
1 mL) and dried in vacuo to give a yellow solid. Yield: 171.5 mg (0.150
[IrCl(η2-COE)(IiPr)2] (8a). Benzene (5 mL) was added to solid
[IrCl(η2-COE)2]2 (99.3 mg, 0.111 mmol) and [AgCl(IiPr)] (141.9
mg, 0.480 mmol, 4.3 equiv) and stirred for 36 h at room temperature.
Upon completion, the solution was filtered through Celite, and the
solvent was removed in vacuo. The solid was washed carefully with
cold (−40 °C) pentane (3 × 1 mL) and dried in vacuo. Yield: 104.5
mg (0.163 mmol, 74%) of a bright yellow powder. 1H NMR (CD2Cl2,
1
mmol, 68%). H NMR (CD2Cl2, δ): 6.77 (s, 4H, N(CH)2N), 5.91−
δ): 6.86 (s, 4H, N(CH)2N), 6.06 (sept, 3JHH = 6.8, 4H, NCH(CH3)2),
2.17−2.08 (m, 2H, COE), 1.98−1.89 (m, 2H, COE), 1.50 (d, JHH
5.79 (m, 4H, NCHR2), 2.24−1.18 (m, 98H), 1.16−1.10 (m, 2H),
0.85−0.71 (m, 2 H). 13C{1H} NMR (CD2Cl2, δ): 181.9 (Ir−C), 116.3
(N(CH)2N), 57.5 (NCH), 34.9, 30.5, 30.3, 29.9, 26.7, 25.6, 25.4, 23.2,
23.0, 22.9, 22.8. Anal. Calcd for C62H110N4IrCl: C, 65.37; H, 9.73; N,
4.92. Found: C, 65.38; H, 9.61; N, 5.06. Crystals suitable for X-ray
diffraction studies were grown by chilling a pentane solution to −40
°C.
3
=
6.8, 12H, NCH(CH3)2), 1.47−1.43 (m, 2H, COE), 1.37−1.22 (m, 4H,
COE), 1.18−1.07 (m, 2H, COE), 0.93−0.77 (m, 2H, COE). 13C{1H}
NMR (CD2Cl2, δ): 182.4 (Ir−C), 115.4 (N(CH)2N), 51.4 (CH-
(CH3)2), 35.9 (COE), 32.3 (COE), 31.0 (COE), 27.0 (COE), 24.3
(CH(CH3)2), 23.0 (CH(CH3)2). Anal. Calcd for C26H46N4ClIr: C,
48.62; H, 7.22; N, 8.72. Found: C, 48.65; H, 6.87; N, 8.75. Crystals
suitable for X-ray diffraction studies were grown by chilling a pentane
solution to −40 °C.
[IrCl(CO)(IiPrMe)2] (9). 8b (248.7 mg, 0.356 mmol) was dissolved in
pentane (50 mL) in a flask fitted with a J. Young tap. The flask was
closed, removed from the glovebox, and attached to the Schlenk line.
The solution was frozen, the headspace was removed in vacuo, and the
solution was thawed under carbon monoxide. This process was
repeated an additional two times. The flask was closed and removed
from the Schlenk line, and the solution was stirred at room
temperature. Within a few hours, the bright yellow solution became
very pale and precipitated a pale yellow solid. After 3 days, the solvent
volume was carefully reduced to ca. 10 mL in vacuo, and the flask was
returned to the glovebox and chilled to −40 °C. The pentane was
carefully decanted, and the solid was washed with cold (−40 °C)
pentane (2 × 1 mL) and dried in vacuo to yield a pale yellow solid.
Yield: 165.8 mg (0.269 mmol, 76%). 1H NMR (C6D6, δ): 6.68 (app br
[IrCl(η2-COE)(IiPrMe)2] (8b). Benzene (2 mL) was added to solid
[IrCl(η2-COE)2]2 (68.7 mg, 0.077 mmol) and IiPrMe (58.1 mg, 0.322
mmol, 4.19 equiv) and stirred for 20 h at room temperature. The
solvent was removed in vacuo, and the resulting solid was carefully
washed with cold (−40 °C) pentane (3 × 1 mL) and dried in vacuo.
1
Yield: 61.0 mg (0.087 mmol, 57%) of a bright yellow powder. H
NMR (CD2Cl2, δ): 6.60 (sept, JHH = 7.0, 4H, NCH(CH3)), 2.17 (s,
3
12H, N(C(CH3))2N), 2.15−2.07 (m, 2H, COE), 1.96 (app d, J = 9.0,
3
2H, COE), 1.64−1.25 (m, 6H, COE), 1.59 (d, JHH = 7.0, 12H,
CH(CH3)2), 1.45 (d, 3JHH = 7.0, 12H, CH(CH3)2), 1.22−1.08 (m, 2H,
COE), 1.02−0.84 (m, 2H, COE). 1H NMR (C6D6, δ): 6.96 (sept, 3JHH
= 7.0, 4H, NCH(CH3)), 2.49 (app d, 2H, J = 12.9, COE), 2.38 (app d,
J = 10.0, 2H, COE), 1.81 (s, 12H, N(C(CH3))2N), 1.77−1.06 (m,
3
s, 4H, CH(CH)3), 1.77 (s, 12H, N(CMe)2N), 1.59 (d, JHH = 7.2,
3
12H, CH(CH3)2), 1.47 (d, JHH = 7.2, 12H, CH(CH3)2). 13C{1H}
12679
dx.doi.org/10.1021/ic4018773 | Inorg. Chem. 2013, 52, 12674−12681

Inorganic Chemistry
Article
NMR (C6D6, δ): 179.0, 173.5, 123.9 (N(C(CH3))2N), 53.5
(NCH(CH3)2), 21.9 (NCH(CH3)2), 21.8 (NCH(CH3)2), 10.1
(N(C(CH3))2N). IR (νCO, CH2Cl2): 1916.6 cm−1. IR (νCO, hexane):
1929.5 cm−1. Anal. Calcd for C23H40N4ClIrO: C, 44.83; H, 6.54; N,
9.09. Found: C, 44.71; H, 6.70; N, 9.16.
1927.1 cm−1. Anal. Calcd for C31H45N4IrO: C, 54.60; H, 6.65; N, 8.22.
Found: C, 52.02; H, 5.83; N, 8.90.
[Ir(CO)(IiPrMe)2(OSiPh3)] (17). 13 (20 mg, 0.034 mmol) and
triphenylsilane (8.8 mg, 0.034 mmol) were stirred in toluene (0.5 mL)
at room temperature in the glovebox for 16 h. The reaction mixture
was concentrated in vacuo to a white solid, which was washed with
cold (−40 °C) pentane (3 × 1 mL) and dried in vacuo. Yield: 19.3 mg
(0.023 mmol, 66%). 1H NMR (C6D6, δ): 7.9 (d, 3JHH = 7.8, 2H, ArH),
[IrCl(CO)(O2)(IiPrMe)2] (12). 9 (6.3 mg, 0.010 mmol) was dissolved
in CD2Cl2 (0.6 mL) in an NMR tube fitted with a J. Young tap. The
flask was closed, removed from the glovebox, and attached to the
Schlenk line. The solution was frozen, the headspace was removed in
vacuo, and the solution was thawed. Oxygen was then introduced into
the NMR tube, and the tube was repeatedly inverted using an NMR
tube spinner. The sample was analyzed periodically by 1H NMR
spectroscopy. Key signals for this species are recorded below. 1H NMR
(CD2Cl2, δ): 5.69 (app br s, 4H, CH(CH)3), 2.25 (s, 12H,
7.73−7.63 (m, 6H, ArH), 7.30 (t, 2JHH = 7.4, 1H, ArH), 7.08 (t, 2JHH
=
3
7.4, 1H, ArH), 6.83 (sept, JHH = 7.0, 4H, CH(CH3)2), 1.78 (s, 12H,
N(C(CH3))2N), 1.45 (d, 2JHH = 7.0, 12H, CH(CH3)2), 1.21 (d, 2JHH
=
7.1, 12H, CH(CH3)2).13C{1H} NMR (C6D6, δ): 185.9, 181.0, 143.8
(ArC), 135.9 (ArCH), 127.1 (ArCH), 123.7 (N(C(CH3))2N), 53.5
(CH(CH3)2), 22.3 (CH(CH3)2), 21.5 (CH(CH3)2, 10.3 (N(C-
(CH3))2N). IR (νCO, hexane): 1918.0 cm−1. Anal. Calcd for
C41H55N4IrO2Si: C, 57.51; H, 6.47; N, 6.54. Found: C, 57.39; H,
6.35; N, 6.34.
3
3
N(CMe)2N), 1.60 (d, JHH = 7.2, 12H, CH(CH3)2), 1.55 (d, JHH
6.8, 12H, CH(CH3)2).
=
[Ir(CO)(IiPrMe)2(OH)] (13). 9 (93 mg, 0.15 mmol) was dissolved in
THF (4 mL), and the solution was stirred over CsOH (45 mg, 0.30
mmol) at room temperature in the glovebox for 24 h. The reaction
mixture was filtered through Celite, the solvents were removed, and
the product was washed with cold (−40 °C) pentane (2 × 1 mL) and
dried in vacuo to give a pale yellow solid. Yield: 78 mg (0.13 mmol,
X-ray Crystal Structure Determinations. Table 2 gives details of
the data collection and refinement. Full details can be found in the
Supporting Information, which includes CIF and INS data. Data were
collected at −180 1 °C using a Rigaku MM007 high-brilliance RA
generator with a Saturn 70 CCD area detector using ω scans, using
Mo Kα radiation (λ = 0.71075 Å). Intensities were corrected for
Lorentz−polarization and for absorption. For 8a, the structure was
solved by direct methods and expanded using Fourier techniques. For
8b−e, the structure was solved by heavy-atom Patterson methods and
expanded using Fourier techniques. In all cases, the non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined
using the riding model.
1
84%). H NMR (C6D6, δ): 6.83 (app br s, 4H, CH(CH3)2), 1.79 (s,
12H, N(C(CH3))2N), 1.53 (d, JHH = 6.7, 24H, CH(CH)3).13C{1H}
2
NMR (C6D6, δ): 182.1, 179.3, 123.5 (N(C(CH3))2N), 53.3
(CH(CH3)2), 21.9 (CH(CH3)2), 10.2 (N(C(CH3)2N). IR (νCO
,
hexane): 1905.0 cm−1. Anal. Calcd for C23H41N4IrO2: C, 46.21; H,
6.91; N, 9.37. Found: C, 46.38; H, 6.87; N, 9.32.
[Ir(CO)(IiPrMe)2(O2CPh)] (14). 13 (19.8 mg, 0.032 mmol) and
PhCO2H (4.3 mg, 0.035 mmol, 1.1 equiv) were dissolved in toluene
(0.5 mL), and the solution was stirred at room temperature in the
glovebox for 3 days. The solvent was removed, and the product was
washed with cold (−40 °C) pentane (2 × 0.5 mL) and dried in vacuo.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF and INS files giving general
experimental information and NMR and IR spectra, steric
maps, and crystallographic data for 8a−e. This material is
available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data for CCDC-948893 (8a), CCDC-948894
(8b), CCDC-948895 (8c), CCDC-948896 (8d), CCDC-
948897 (8e) can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
1
Yield: 20.3 mg (0.029 mmol, 90%). H NMR (C6D6, δ): 8.52 (br s,
3
1H, ArH), 8.36 (d, JHH = 6.2, 2H, ArH), 7.15−7.00 (m, 2H, ArH),
6.77 (app br s, 4H, CH(CH3)2), 1.70 (s, 12H, N(C(CH3))2N), 1.62
2
2
(d, JHH = 6.9, 12H, CH(CH3)2), 1.54 (d, JHH = 6.9, 12H,
CH(CH3)2).13C{1H} NMR (C6D6, δ): 178.9, 176.9, 169.9 (PhCO2Ir),
138.7 (Ar C), 130.2 (Ar CH), 129.7 (Ar CH), 127.6 (Ar CH) 123.8
(N(C(CH3))2N), 53.6 (CH(CH3)2), 22.1 (CH(CH3)2), 10.1 (N(C-
(CH3)2N). IR (νCO, hexane): 1931.4 cm−1. Anal. Calcd for
C30H45N4IrO3: C, 51.33; H, 6.46; N, 7.98. Found: C, 51.46; H,
6.34; N, 7.91.
[Ir(CO)(IiPrMe)2(OMe)] (15). 13 (20 mg, 0.034 mmol) was stirred
in methanol (0.5 mL) at room temperature in the glovebox for 24 h.
The reaction mixture was concentrated in vacuo and azeotroped with
pentane until an orange solid resulted. The solid was washed with cold
(−40 °C) pentane (3 × 1 mL) and dried in vacuo. Yield: 20.0 mg
(0.033 mmol, 96%). 1H NMR (C6D6, δ): 6.75 (br s, 4H, CH(CH3)2),
4.01 (s, 6H, OMe), 1.80 (s, 12H, N(C(CH3))2N), 1.67−1.44 (m,
24H, CH(CH3)2). 13C{1H} NMR (C6D6, δ): 182.0, 178.3, 123.4
(N(C(CH3))2N), 61.6 (OMe), 53.4 (CH(CH3)2), 21.9 (CH(CH3)2),
10.2 (N(C(CH3))2N). IR (νCO, hexane): 1906.0 cm−1. Anal. Calcd for
C24H43N4IrO2: C, 47.11; H, 7.08; N, 9.16. Found: C, 46.98; H, 6.90;
N, 9.08.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.P.N.: snolan@st-andrews.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC, the ERC (Advanced Investigator Award
“FUNCAT” to S.P.N.), and Sasol (stipend to B.J.T.) for
financial support. S.P.N. is a Royal Society Wolfson Merit
Award holder. We thank Dr. Tomas Lebl and Mrs. Melanja
Smith for assistance with NMR spectroscopy and Dr. Alba
[Ir(CO)(IiPrMe)2(CCPh)] (16). 13 (20 mg, 0.034 mmol) was stirred
in phenylacetylene (0.5 mL) at room temperature in the glovebox for
24 h. The reaction mixture was concentrated to an orange oil in vacuo.
Cold (−40 °C) pentane (1 mL) was added, and an orange precipitate
formed. The liquid was decanted, and the resultant solid was washed
with cold (−40 °C) pentane (3 × 1 mL) and dried in vacuo. Yield:
Collado, Scott Patrick, Dr. Anthony Chartoire, and Adrian
Gomez-Suarez for helpful discussions. This work has made use
́
́
́
of the resources provided by the EaStCHEM Research
Computing Facility.
1
10.3 mg (0.015 mmol, 45%). H NMR (C6D6, δ): 7.42−7.36 (m,
2H,ArH), 6.97 (t, 3JHH = 7.7, 2H, ArH), 6.82 (tt, 3JHH = 7.4, 1.28, 1H,
ArH), 6.69 (br s, 4H, CH(CH3)2), 1.79 (s, 12H, N(C(CH3))2N),
1.66−1.58 (m, 12H, CH(CH3)2), 1.54−1.44 (m, 12H, CH-
(CH3)2).13C{1H} NMR (C6D6, δ): 187.9, 176.5, 131.4 (o-ArCH),
130.26 (Ir−CC−Ph), 130.1 (ArC), 128.0 (m-ArCH), 124.1 (p-ArCH),
123.7 (N(C(CH3))2N), 111.3 (Ir−CC−Ph), 53.5 (CH(CH3)2), 21.8
(CHCH3), 21.6 (CHCH3), 10.2 (N(C(CH3))2N). IR (νCO, hexane):
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