Iridium complexes of bulky CCC-pincer N-heterocyclic carbene ligands: Steric control of coordination number and catalytic alkene isomerization

Article abstract of DOI:10.1021/om300468d

Three new iridium complexes of meta-phenylene-bridged bis-N-heterocyclic carbene CCC-pincer ligands were synthesized and characterized. For a pincer ligand with 2,6-diisopropylphenyl N-substituents, a six-coordinate iridium(III) complex of the formula Ir(CCC)HCl(MeCN) was formed. In contrast, ligands with t-butyl or adamantyl N-substituents gave five-coordinate iridium(III) complexes of the formula Ir(CCC)HCl. These iridium complexes, along with two previously described iridium complexes, were tested for activity in the catalytic transfer-dehydrogenation of n-octane at 150 °C. The new complexes were inactive for this reaction, while two previously reported catalysts were modestly active: a mesityl-substituted derivative gave 12 turnovers, and a 3,5-di-t-butylphenyl-substituted variant gave 10 turnovers. In contrast, these complexes were shown to be highly active catalysts for the isomerization of terminal alkenes, under conditions much milder than those required for transfer-dehydrogenation.

Full text of DOI:10.1021/om300468d

Article
pubs.acs.org/Organometallics
Iridium Complexes of Bulky CCC-Pincer N‑Heterocyclic Carbene
Ligands: Steric Control of Coordination Number and Catalytic Alkene
Isomerization
Anthony R. Chianese,* Sarah E. Shaner, Jennifer A. Tendler, David M. Pudalov, Dimitar Y. Shopov,
Daniel Kim, Scott L. Rogers, and Allen Mo
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
S
* Supporting Information
ABSTRACT: Three new iridium complexes of meta-phenyl-
ene-bridged bis-N-heterocyclic carbene CCC-pincer ligands
were synthesized and characterized. For a pincer ligand with
2,6-diisopropylphenyl N-substituents, a six-coordinate iridium-
(III) complex of the formula Ir(CCC)HCl(MeCN) was
formed. In contrast, ligands with t-butyl or adamantyl N-
substituents gave five-coordinate iridium(III) complexes of the
formula Ir(CCC)HCl. These iridium complexes, along with
two previously described iridium complexes, were tested for
activity in the catalytic transfer-dehydrogenation of n-octane at
150 °C. The new complexes were inactive for this reaction,
while two previously reported catalysts were modestly active: a
mesityl-substituted derivative gave 12 turnovers, and a 3,5-di-t-
butylphenyl-substituted variant gave 10 turnovers. In contrast, these complexes were shown to be highly active catalysts for the
isomerization of terminal alkenes, under conditions much milder than those required for transfer-dehydrogenation.
Despite the success in designing highly active PCP−Ir
catalysts for alkane dehydrogenation, achieving useful selectivity
still remains a challenge. A long-standing goal has been the
synthesis of terminal alkenes by selective dehydrogenation of
linear alkanes. Although catalysts with high initial selectivity for
1-alkenes have been reported, competing isomerization of the
initially produced terminal alkene into a mixture of internal
isomers has been observed in each case.6,8b,c
In this context, we and others have recently begun examining
the iridium chemistry of monoanionic CCC-pincer ligands,
where the phosphorus-donor fragments are replaced with N-
heterocyclic carbenes (NHCs, Figure 1). Iridium complexes of
CCC-pincer ligands are expected to retain the strong binding
and donor power of the PCP ligands, while producing a steric
environment around the metal center that is both distinct from
the PCP ligands and highly modular. Hollis and co-workers
reported the first synthesis of iridium complexes of CCC-pincer
ligands, via transmetalation from zirconium.13 Braunstein and
co-workers reported the direct metalation of the same bis-
imidazolium precursor to iridium: depending on the conditions,
either a hydridoiodide complex or a diiodide complex could be
synthesized in good yield.14 Complexes of this nature were not
found to be active catalysts for alkane dehydrogenation,14
possibly because the limited steric bulk of the ligands allowed
INTRODUCTION
■
The selective activation and functionalization of C−H bonds is
one of the broadest and most important challenges facing
organometallic chemistry.1 One of the simplest C−H bond
transformations is the metal-catalyzed dehydrogenation of a
saturated hydrocarbon or alkyl group to give a new CC
double bond.2 Although this reaction has been known for more
than 30 years,3 only one highly active homogeneous catalyst
motif has been discovered, based on the coordination of a
monoanionic PCP-pincer4 ligand to iridium, as originally
demonstrated by Kaska and Jensen.5 Steric and electronic
modifications of the PCP-pincer ligand, including changing the
phosphorus substituents,6 employing an anthracenyl back-
bone,7 introducing electron-withdrawing or -donating backbone
substituents,8 replacing the CH2 linkers with oxygen atoms,8a,c,9
replacing the aryl backbone with a ferrocenyl or ruthenocenyl
unit,10 or installing electron-withdrawing trifluoromethyl
substituents on phosphorus,11 have resulted in the discovery
of iridium catalysts giving turnover numbers in the thousands
for dehydrogenation involving net hydrogen transfer to a
sacrificial acceptor alkene,6b,8a,9,10 or for acceptorless dehydro-
genation with direct removal of H2.6,8b Notably, PCP−
ruthenium complexes containing trifluoromethyl substituents
on phosphorus were recently reported to be active catalysts for
alkane dehydrogenation, giving turnover numbers approaching
200.12
Received: May 29, 2012
Published: October 23, 2012
© 2012 American Chemical Society
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groups and the imidazole and central aryl rings are linked by
methylene groups.18 A tetrahydride derivative showed catalytic
hydrogen exchange between H2 and deuterated arene solvents,
but no activation of alkanes was observed.
We recently reported the synthesis of a series of iridium
complexes of CCC-pincer ligands.19 Like the pincer ligand
originally employed by Hollis13 and Braunstein,14 ours featured
a direct linkage of the NHC fragment to the central aryl ring,
giving the pincer ligand a rigid, flat backbone. Instead of
imidazole rings, we have incorporated benzimidazole rings,
where synthesis via successive aryl aminations20 facilitates the
installation of a wide variety of bulky N-substituents. Addi-
tionally, C(4) metalation of the imidazole ring to give abnormal
NHC complexes21 is blocked. Initially, we reported the
syntheses of the complexes shown in Figure 1. One extremely
bulky ligand, with 2,6-diisopropylphenyl N-substituents, failed
to metalate under our original conditions. The iridium
complexes were active catalysts for arene C−H borylation,1f
and two derivatives were moderately active catalysts for alkane
dehydrogenation.
Here, we report the synthesis of new benzimidazolium
precursors to CCC-pincer ligands, with the bulky aliphatic N-
substituents, t-butyl and adamantyl. Metalation to iridium gives
5-coordinate hydridochloride complexes, common in PCP−
iridium chemistry, but in striking contrast to the 6-coordinate
acetonitrile-solvated complexes described in Braunstein’s14 and
our19 prior work. We also find that our previously reported 2,6-
diisopropylphenyl pincer ligand metalates cleanly under more
forcing conditions than previously employed.
We examined the iridium−pincer complexes for catalytic
activity in the transfer-dehydrogenation of n-octane and found
them to be only weakly active. In the course of testing a range
of sacrificial hydrogen acceptors, we found that 1-hexene was
isomerized rapidly to internal isomers. Upon further study, we
demonstrated that these complexes are highly active catalysts
for the isomerization of terminal alkenes to internal isomers,
under significantly milder conditions than those typically
required for alkane dehydrogenation.
Figure 1. Previously reported iridium complexes of CCC-pincer
ligands.
facile bimolecular decomposition of potential catalytic inter-
mediates.6b,15 Braunstein and co-workers have recently
reported the synthesis of imidazolium precursors to CCC-
pincer ligands incorporating bulkier adamantyl side groups;16
metalation gave iridium complexes where one NHC fragment
was bound abnormally.17 This mixed normal/abnormal
iridium−pincer complex was weakly active for alkane transfer-
dehydrogenation, giving up to 3.6 turnovers. Heinekey and co-
workers have described iridium complexes of more flexible
CCC-pincer ligands, where the N-substituents are mesityl
RESULTS AND DISCUSSION
Synthesis of Ligands with Bulky Aliphatic Side
Groups. We previously described a method to synthesize
■
Scheme 1. Synthesis of Bis-benzimidazolium Precursors to CCC-Pincer Ligands
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bis-benzimidazolium precursors to CCC-pincer ligands in three
steps via successive Buchwald−Hartwig aminations on 1,2-
bromoiodobenzene, followed by cyclization with triethyl
orthoformate.19 This method allows the installation of a
range of aromatic N-substituents, including highly bulky mesityl
and 2,6-diisopropylphenyl groups. Because bulky aliphatic
substituents are expected to create a very different steric
environment around the iridium center compared to aromatic
substituents, we sought to synthesize CCC-pincer ligands with
t-butyl and adamantyl side groups.
Scheme 2. Metalation of Aromatic-Substituted CCC-Pincer
Ligands
a
We successfully prepared bis-benzimidazolium chlorides 3-
tBu and 3-Ad in good overall yields after several modifications
of our original19 synthetic procedure (Scheme 1). It was
necessary to start with 1,2-dibromobenzene, as only trace
amounts of 1-Ad were formed in reactions between 1,2-
bromoiodobenzene and 1-adamantylamine under similar
conditions. For the second step, classical ligands for
Buchwald−Hartwig cross-coupling, including DPEPHOS22
and BINAP, were ineffective, but the recently developed
CyPF-tBu ligand23 allowed the synthesis of tetraamines 2-tBu
and 2-Ad in good yields with low catalyst loading. Cyclization
with triethyl orthoformate required heating overnight with an
excess of hydrochloric acid.
a
Previously reported in ref 19.
Synthesis of Iridium Complexes. Though NHCs are now
widely employed as supporting ligands for transition-metal
catalysis,24 and numerous methods for the synthesis of metal−
NHC complexes have been developed,25 the metalation of
multidentate NHCs remains challenging, especially when a
chelating ligand contains more than one NHC unit. Often, the
formation of multinuclear complexes competes with chelation;
a primary reason for this difficulty appears to be the kinetic
stability of the M−CNHC bonds under mild conditions, which
prevents the thermodynamic chelate effect from operating,
except at very high temperatures.26 For the CNHC−Caryl−CNHC
pincer ligands discussed here, metalation of the central aryl unit
via C−H activation poses an additional challenge. Direct
metalation of the imidazolium salts, using a weak base in
combination with a low-valent metal precursor, has been
successfully applied for the synthesis of group 9 complexes of a
variety of chelating poly-NHC ligands,26 including CCC-
pincers. This method has proved useful for the metalation of
our ligands as well, although the optimal conditions, especially
the identity and amount of base, are highly dependent on the
steric bulk of the ligand, so that the synthesis of each iridium
complex often requires a moderate amount of experimentation
to successfully complete.
Figure 2. Crystal structure of 4-dipp, showing 50% probability
ellipsoids. Hydrogen atoms, except for the iridium-bound hydride, are
omitted for clarity. Selected metric data (bond lengths in Å and angles
in deg): Ir(1)−C(8), 2.007(3); Ir(1)−C(32), 2.015(3); Ir(1)−C(5),
1.951(3); Ir(1)−Cl(2), 2.4930(9); Ir(1)−H(1), 1.597; C(8)−Ir(1)−
C(5), 78.87(14); C(32)−Ir(1)−C(5), 78.79(14); C(8)−Ir(1)−
C(32), 157.60(13).
We previously reported19 the synthesis of iridium complex 4-
Mes, by treatment of the mesityl-substituted benzimidazolium
salt 3-Mes with [Ir(cod)Cl]2 and an excess of triethylamine in
acetonitrile at 80 °C (Scheme 2). The synthesis of complex 4-
dtbp proceeded optimally using a stoichiometric amount of
cesium fluoride as base. When 3-dipp was treated with excess
triethylamine at 150 °C for 16 h in a sealed vessel, iridium
complex 4-dipp was the major product, isolated following
chromatography in 36% yield. Complex 4-dipp was charac-
terized by NMR, elemental analysis, and X-ray crystallography.
The crystal structure (Figure 2) confirms the geometry shown
in Scheme 2. Analogous to 4-Mes and 4-dtbp, 4-dipp is nearly
octahedral, with an acetonitrile ligand trans to the aryl fragment
of the CCC-pincer ligand.
(Scheme 3). Better yields were obtained when using CsF as
base than with triethylamine. The utility of CsF is worthy of
note: although triethylamine is commonly employed for the
Scheme 3. Metalation of Aliphatic-Substituted CCC-Pincer
Ligands
When benzimidazolium salts 3-tBu and 3-Ad were subjected
to similar reaction conditions, the only products isolated were
the 5-coordinate hydridochloride complexes 4-tBu and 4-Ad
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metalation of chelating NHCs,26 we are not aware of any
examples of the use of CsF for NHC metalation, other than our
previous study.19 For the t-butyl-substituted complex 4-tBu,
metalation in toluene proved to be most effective, though
acetonitrile was a more effective solvent for the synthesis of 4-
Ad. In contrast to 4-dipp and previously described CCC-
pincer−iridium complexes,14,19 4-tBu and 4-Ad lack a solvent-
derived acetonitrile ligand in the coordination sphere, as
verified by NMR spectroscopy and X-ray crystallography.
Both 4-tBu and 4-Ad were structurally characterized using X-
ray crystallography. ORTEP diagrams are shown in Figures 3
Figure 4. Crystal structure of 4-Ad, showing 50% probability
ellipsoids. Hydrogen atoms, except for the iridium-bound hydride,
are omitted for clarity. Selected metric data (bond lengths in Å and
angles in deg): Ir(1)−C(29), 2.029(3); Ir(1)−C(6), 2.024(3); Ir(1)−
C(3), 1.915(3); Ir(1)−Cl(2), 2.4631(7); Ir(1)−H(6), 1.398; C(29)−
Ir(1)−C(3), 80.07(12); C(6)−Ir(1)−C(3), 79.96(12); C(6)−Ir(1)−
C(29), 159.98(11).
4-Ad, which is slightly less than the sum of the van der Waals
radii of 3.1 Å.
Complexes 4 are closely related, differing only by the
presence or absence of an acetonitrile ligand trans to the aryl
fragment of the pincer ligand. As the electronic impact of
substituting aliphatic N-substituents for aromatic ones on
NHCs is known to be minimal,27 this striking difference is
likely steric in origin. Partial space-filling representations of
complexes 4-Ad and 4-Mes,19 shown in Figure 5, illustrate the
difference in steric accessibility of the site trans to the aryl
fragment. Because the rigid pincer backbone forces the N-
substituent to point directly into the CCC plane, the fourth site
in that plane is much more crowded in complexes with roughly
spherical aliphatic groups than in complexes with flatter
aromatic groups.
Transfer-Dehydrogenation. Although PCP−Ir catalysts
have given high turnover numbers for the transfer-dehydrogen-
ation of linear alkanes, selectivity for terminal alkenes has been
limited by competing catalysis of alkene isomerization.6,8b,c We
examined iridium complexes 4 for potential activity in the
transfer-dehydrogenation of n-octane, in the presence of
norbornene as a hydrogen acceptor. Sodium tert-butoxide,
which promotes dehydrochlorination and reduction from
iridium(III) to iridium(I), was necessary for catalysis. The
results of this study are shown in Table 1.
Figure 3. Crystal structure of 4-tBu, showing 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity. Selected metric
data (bond lengths in Å and angles in deg): Ir(1)−C(13), 2.032(5);
Ir(1)−C(4), 2.026(5); Ir(1)−C(10), 1.926(5); Ir(1)−Cl(2),
2.3961(15); C(13)−Ir(1)−C(10), 79.9(2); C(4)−Ir(1)−C(10),
80.1(2); C(4)−Ir(1)−C(13), 159.9(2).
and 4, respectively. In each complex, the chloride ligand is cis to
the aryl fragment of the pincer ligand. The hydride ligand was
located, and its position was freely refined for 4-Ad, providing
evidence for the square-pyramidal geometry shown in Scheme
1
3. The H NMR chemical shifts (in ppm) for the hydride
ligands are −22.2 for 4-tBu and −22.0 for 4-Ad, which
correspond closely to the values of −23.2 for 4-Mes, −23.2 for
4-dtbp, and −22.7 for 4-dipp. Additionally, the hydride is
unlikely to lie preferentially trans to the aryl fragment, as the
site is sterically crowded in both complexes, and the trans
influence is expected to be greater for the aryl fragment than for
the chloride ligand. Although the evidence is not unambiguous,
we propose that the sites trans to the aryl fragments in 4-tBu
and 4-Ad are vacant. The shortest distances from side-group
hydrogen atoms to iridium are 2.73 Å for 4-tBu and 2.71 Å for
The mesityl-substituted complex 4-Mes and the di-tert-
butylphenyl-substituted complex 4-dtbp showed modest
activity for the transfer-dehydrogenation of n-octane. In the
analogous reaction with cyclooctane and norbornene, 4-Mes
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Figure 5. Crystal structures of 4-Ad (left) and 4-Mes (right), showing the N-substituents in a space-filling representation.
a
Table 1. Transfer-Dehydrogenation of n-Octane at 150 °C
Table 2. Isomerization of 1-Hexene by 4-Mes in n-Octane at
150 °C
a
b
additive
time (min)
total TON
T/C/I
2 mM NaOtBu
2 mM NaOtBu
none
15
60
60
420
730
280
67:29:4
65:26:8
58:38:4
[Ir]
TON
4-tBu
4-Ad
0
0
a
4-Mes
4-dtbp
4-dipp
12
10
0
Reactions performed in duplicate, with 1.0 mM 4-Mes and 1.0 M 1-
b
hexene in 4 mL of n-octane. T/C/I is the ratio of trans-2-hexene to
cis-2-hexene to 3-hexenes.
a
Reactions performed in duplicate, with 200 mM norbornene, 1.0 mM
[Ir], and 10 mM NaOtBu in 4 mL of n-octane. Only internal isomers
internal isomers is much more rapid than the hydrogen-transfer
reactions shown in Table 1. Notably, the conversion of 1-
hexene into a mixture of 2-hexene isomers is much faster than
the following conversion to 3-hexene isomers, as only very
small amounts of the 3-isomers were observed. It is also
noteworthy that the isomerization proceeds with or without
added sodium tert-butoxide, although the base causes a clear
rate enhancement.
Isomerization of 1-Octene under Mild Conditions. The
isomerization of alkenes is catalyzed by a wide variety of
transition-metal catalysts, including Ti,28 Fe,29 Co,30 Ni,30a,31
Ru,32 Rh,29a,30a,33 Pd,29a,30a Ir,34 and Pt.30a Recent noteworthy
examples include Veige’s NCN-pincer−chromium complexes
for selective isomerization of 1-alkenes to 2-alkenes,35 Strukul’s
platinum catalysts for the (E)-selective isomerization of
allylbenzenes,36 and Grotjahn’s bifunctional ruthenium cata-
lysts, capable of moving the alkene double bond selectively over
one position37 or over very many,38 depending on the
conditions. Very recently, Brookhart, Goldman, Krogh-
Jespersen, and co-workers reported a detailed study of the
mechanism of alkene isomerization catalyzed by PCP-pincer
complexes of iridium, which are also highly active catalysts for
alkane dehydrogenation.39 They concluded that their catalysts
facilitate alkene isomerization via π-allyl intermediates.
of octene were observed.
was previously19 shown to give 10 turnovers of cyclooctene,
while 4-dtbp was inactive. All of the newly reported iridium
complexes (4-tBu, 4-Ad, and 4-dipp) show no activity for
transfer-dehydrogenation of n-octane under the conditions
reported. No catalysis was observed on replacing norbornene
with tert-butylethylene, another commonly employed hydrogen
acceptor.
None of our CCC−Ir complexes show activity for transfer-
dehydrogenation that is comparable to previously studied
PCP−Ir complexes, which give TONs in the thousands under
ideal conditions.6b,8a,9,10 However, the impact of steric bulk is
noteworthy, especially as a seemingly subtle change from
mesityl substituents (4-Mes) to 2,6-diisopropylphenyl sub-
stituents (4-dipp) eradicates any observable dehydrogenation
of n-octane.
Because both of the commonly employed hydrogen
acceptors, norbornene and tert-butylethylene, are significantly
bulkier than n-octane, we considered the possibility that the
steric bulk of the acceptor was inhibiting turnover. To examine
this possibility, we attempted the transfer-dehydrogenation of
n-octane using 1-hexene as hydrogen acceptor. Goldman and
co-workers found that PCP−Ir complexes gave up to 111
turnovers of octene isomers for this substrate pair, along with
some isomerization of the 1-hexene acceptor and the initial
product, 1-octene.6b When using 1-hexene as the potential
acceptor with complex 4-Mes, we observed no transfer-
dehydrogenation. Instead, 1-hexene was rapidly isomerized to
internal isomers (Table 2). The conversion of 1-hexene to
Transfer-dehydrogenation of alkanes has only been shown to
occur at appreciable rates at temperatures of 150 °C or higher.
In contrast, our CCC-pincer iridium complexes efficiently
catalyze the isomerization of 1-octene into internal isomers at
much lower temperatures. Although samples of precatalysts 4
purified by chromatography alone appeared to be pure by
NMR spectroscopy, they were found to be much more active
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for alkene isomerization without added base than samples of
the same precatalysts that were purified further by recrystalliza-
tion. This indicates that a small quantity of an unidentified
impurity, present after chromatographic purification, is a highly
active isomerization catalyst. For all of the experiments
described in Table 3, as well as Tables 1 and 2 above, the
iridium precatalysts were recrystallized twice following
chromatographic purification.
a
Table 3. Isomerization of 1-Octene in Toluene at 100 °C
b
Figure 6. Isomerization of 1-octene catalyzed by 4-Mes at 100 °C. The
reaction was performed with 2.5 mM 4-Mes, 25 mM NaOtBu, and
1.25 M 1-octene in 2.0 mL of toluene. Curves are plotted only as a
guide to the eye.
[Ir]
additive
25 mM NaOtBu
TON (T/C/I)
4-tBu
4-tBu
4-Ad
487 (47:15:38)
15 (n.d.)
none
25 mM NaOtBu
none
488 (46:13:41)
16 (n.d.)
4-Ad
4-Mes
4-Mes
25 mM NaOtBu
484 (75:22:3)
24 (n.d.)
none
a
Reactions performed with 2.5 mM [Ir] and 1.25 M 1-octene in 2.0
b
mL of toluene. T/C/I is the ratio of trans-2-octene to cis-2-octene to
the remaining internal isomers (3-octenes and 4-octenes). Not
determined in experiments with no NaOtBu additive.
In the absence of added base, all three iridium complexes
screened were minimally active for the isomerization of 1-
octene at 100 °C. Because the activity decreases on repeated
recrystallization, it is possible that even the activity observed
after two recrystallizations is due to an unidentified residual
impurity. The addition of 10 equiv of sodium tert-butoxide
relative to the precatalyst resulted in substantial activity in all
cases, which was not diminished on repeated recrystallization of
the iridium precatalysts. In 24 h, the level of 1-octene was
reduced to 2−3% of the reaction mixture. Precatalyst 4-Mes
primarily gave a mixture of cis- and trans-2-octene, while a
substantial fraction of 3- and 4-octene isomers was produced by
4-tBu and 4-Ad. On the basis of available thermodynamic
data,40 we have estimated that a fully equilibrated mixture of
linear octene isomers at 100 °C would contain 1% 1-octene,
with a T/C/I ratio of 36:9:54, implying that reactions catalyzed
by 4-tBu and 4-Ad have nearly reached equilibrium after 24 h.
Although all three catalysts gave nearly complete con-
sumption of 1-octene after 24 h at 100 °C, the substantial
production of 3- and 4-octene isomers by 4-tBu and 4-Ad
implies that they are more active for alkene isomerization than
4-Mes. To probe this distinction, the isomerization of 1-octene
was monitored over time. In a reaction employing 0.2% 4-Mes
at 100 °C (Figure 6), the consumption of 1-octene required
several hours. During the first 4 h of the reaction, the ratio of
trans-2-octene to cis-2-octene remained nearly constant at about
4:1, while the buildup of 3- and 4-octene isomers was minimal
even after 24 h. In a reaction employing 0.2% 4-Ad at 60 °C
(Figure 7), 1-octene was fully converted to a mixture of cis- and
trans-2-octene in approximately 30 min, which further evolved
to give an increasing fraction of 3- and 4-octene isomers over
the 24 h course of the experiment. These experiments
demonstrate that 4-Ad is a substantially more active precatalyst
for 1-octene isomerization than 4-Mes.
Figure 7. Isomerization of 1-octene catalyzed by 4-Ad at 60 °C. The
reaction was performed with 2.5 mM 4-Ad, 25 mM NaOtBu, and 1.25
M 1-octene in 2.0 mL of toluene. Curves are plotted only as a guide to
the eye.
SUMMARY
■
Three new CCC-pincer iridium complexes were synthesized
and characterized. These complexes, as well as two previously
described complexes, were tested as precatalysts for the
transfer-dehydrogenation of n-octane. Although substantial
activity for transfer-dehydrogenation was not observed, iridium
complexes of the formula (CCC)IrHCl(MeCN) or (CCC)-
IrHCl appear to be generally active catalysts for the
isomerization of terminal alkenes to internal isomers with
activation by sodium tert-butoxide. As these catalysts operate
efficiently at 100 °C, while temperatures of 150 °C or higher
have uniformly been required to achieve high activity in alkane
dehydrogenation, it is unlikely that iridium complexes of this
nature will facilitate the selective dehydrogenation of linear
alkanes to produce linear α-olefins. Detailed studies on the
catalysis of alkene isomerization by these complexes are in
progress, focusing on substrate scope, kinetic selectivity, and
mechanism.
EXPERIMENTAL SECTION
■
General Methods. [Ir(cod)Cl]2 was prepared as previously
described.41 All other materials were commercially available and
were used as received, unless otherwise noted. Solvents were generally
purified by sparging with argon and passing through columns of
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activated alumina, using an MBraun Solvent Purification System. Flash
chromatography using solvent gradients was performed using a
Combiflash RF system. NMR spectra were recorded at room
temperature on a Bruker spectrometer operating at 400 MHz (1H
NMR) and 100 MHz (13C NMR) and referenced to the residual
solvent resonance (δ in parts per million, and J in Hz). Elemental
analyses were performed by Robertson Microlit, Madison, NJ. Detailed
NMR assignments for complexes 4-tBu, 4-Ad, and 4-dipp, along with
procedures for ligand synthesis and characterization, are given in the
Supporting Information.
Hz), 0.97 (d, 6H, 3JHH = 6.9 Hz), 0.88 (d, 7H, 3JHH = 6.9 Hz), −22.76
(s, 1H). 13C NMR (CD2Cl2): δ 186.9, 149.3, 148.4, 146.8, 146.3,
137.9, 133.1, 132.7, 130.2, 124.2, 124.0, 123.7, 122.7, 121.5, 111.7,
111.3, 108.5, 28.57, 28.59, 25.6, 24.7, 24.0, 23.7, 2.48. The signal for
the NCCH3 carbon, expected at ∼116 ppm by analogy to 4-Mes
and 4-dtbp, was too weak to identify unambiguously. Anal. Calcd. for
C46H49ClIrN5: C, 61.42; H, 5.49; N, 7.79. Found: C, 61.22; H, 5.30;
N, 7.61.
Transfer-Dehydrogenation of n-Octane. In an argon-filled
glovebox, a 15 mL, medium-pressure screw-cap tube was charged with
a stir bar, an iridium complex (4.0 μmol), sodium tert-butoxide (3.8
mg, 40 μmol), norbornene (75 mg, 0.80 mmol), and 4.0 mL of n-
octane. The tube was capped, removed from the glovebox, and heated
to 150 °C in an oil bath for 20 h. The mixture was cooled to room
temperature, and a 100 μL aliquot was transferred to 700 μL of a
standard solution of 1,3,5-trimethoxybenzene in CDCl3. Only internal
isomers of octene were observed. The total quantity of octene was
Iridium Complex 4-tBu. Benzimidazolium salt 3-tBu (0.646
mmol, 320 mg), [Ir(cod)Cl]2 (0.323 mmol, 215 mg), CsF (1.94
mmol, 294 mg), and toluene (80 mL) were added to an oven-dried,
medium-walled pressure vessel in the glovebox. The mixture was
heated at 120 °C for 19 h, and then allowed to cool to room
temperature. The solvent was evaporated, and the residue was purified
by chromatography on activated alumina, first washing with dichloro-
methane, then eluting the yellow product with 30% ethyl acetate in
dichloromethane. After removing the solvent under vacuum, the
complex was analytically pure. Yield: 178 mg, 42%. Recrystallization
for catalytic trials was performed at room temperature in an argon-
filled glovebox by layering a solution in dichloromethane with pentane.
1
determined by integrating the H NMR signals against the standard.
Isomerization of 1-Hexene. In an argon-filled glovebox, a 15 mL
medium-pressure screw-cap tube was charged with a stir bar, iridium
complex 4-Mes (3.3 mg, 4.0 μmol), sodium tert-butoxide (0.77 mg, 8
μmol), 1-hexene (500 μL, 4.0 mmol), and 3.5 mL of n-octane. The
tube was capped, removed from the glovebox, and heated to 150 °C in
an oil bath for either 15 or 60 min. The mixture was cooled to room
temperature, and a 100 μL aliquot was transferred to 700 μL of a
standard solution of 1,3,5-trimethoxybenzene in CDCl3. The
conversion of 1-hexene to internal isomers was determined by
integrating the 1H NMR signals against the standard. Separately, a 100
μL aliquot of the reaction mixture was added to 2 mL of toluene,
filtered, and analyzed by GC-MS to determine the ratios of internal
hexene isomers formed.
1H NMR (CD2Cl2): δ 8.16 (d, 2H, 3JHH = 7.8 Hz), 7.90 (d, 2H, 3JHH
8.0 Hz), 7.50 (d, 2H, 3JHH = 7.9 Hz), 7.48 (t, 2H, 3JHH = 7.5 Hz), 7.43
=
3
3
(t, 2H, JHH = 7.6 Hz), 7.26 (t, 1H, JHH = 7.9 Hz), 2.12 (s, 18H),
−22.16 (s, 1H). 13C NMR (CD2Cl2): δ 194.9, 145.1, 135.1, 134.3,
123.5, 122.3, 121.4, 115.0, 111.4, 108.3, 60.3, 30.2. The Ir−Caryl
carbon, expected at ∼124 ppm by analogy to compound 4-Ad, was not
observed and is likely isochronous with the H−Caryl signal at 123.5
ppm. Anal. Calcd. for C28H30ClIrN4: C, 51.72; H, 4.65; N, 8.62.
Found: C, 51.49; H, 4.79; N, 8.50.
Iridium Complex 4-Ad. Benzimidazolium salt 3-Ad (0.614 mmol,
400 mg), [Ir(cod)Cl]2 (0.307 mmol, 205 mg), CsF (1.843 mmol, 280
mg), and acetonitrile (90 mL) were added to an oven-dried, medium-
walled pressure vessel in the glovebox. The flask was brought outside
of the glovebox, and the reaction mixture was stirred at 110 °C for 22
h behind a blast shield. After cooling to room temperature, the solvent
was evaporated. The residue was purified by flash chromatography
using a gradient of 0−25% ethyl acetate in dichloromethane. Yield:
292 mg, 59%. Recrystallization was performed at room temperature in
an argon-filled glovebox by layering a solution in dichloromethane
with pentane. Repeated attempts at analysis gave a low value for
carbon; this appears to be due to retention of approximately 10% by
mass of CH2Cl2 even upon prolonged storage under vacuum, which is
observed by H NMR. H NMR (CD2Cl2): δ 8.11 (d, 2H, JHH = 8.0
Hz), 7.97 (d, 2H, 3JHH = 8.1 Hz), 7.46 (d, 2H, 3JHH = 8.0 Hz), 7.41 (t,
2H, 3JHH = 7.7 Hz), 7.36 (t, 2H, 3JHH = 7.7 Hz), 7.21 (t, 1H, 3JHH = 7.9
Hz), 2.97 (d, 6H, 2JHH = 11.5 Hz), 2.72 (d, 6H, 2JHH = 11.5 Hz), 2.34
(s, 12H), 1.96 (d, 6H, 2JHH = 12.3 Hz), 1.88 (d, 6H, 2JHH = 12.3 Hz),
−22.01 (s, 1H,). 13C NMR (CD2Cl2): δ 194.7, 145.2, 134.8, 134.3,
124.9, 123.3, 122.0, 121.3, 115.4, 111.5, 108.3, 61.6, 42.5, 36.6, 30.7.
Anal. Calcd. for C40H42ClIrN4: C, 59.57; H, 5.25; N, 6.95. Found: C,
58.69; H, 5.28; N, 6.62.
Isomerization of 1-Octene. In an argon-filled glovebox, a vial was
charged with a stir bar, an iridium complex (5.0 μmol), 1-octene (392
μL, 2.5 mmol), 1.6 mL toluene, and optionally sodium tert-butoxide
(4.8 mg, 50 μmol). The vial was capped and heated in the glovebox
while stirring. At the time points indicated, aliquots were removed for
analysis. A 50 μL aliquot was transferred to 700 μL of a standard
solution of 1,3,5-trimethoxybenzene in CDCl3. The conversion of 1-
1
octene to internal isomers was determined by H NMR integration
against the standard. Separately, a 50 μL aliquot was added to 2 mL of
toluene, filtered, and analyzed by GC-MS to determine the ratios of
internal octene isomers formed.
X-ray Crystallography, General Methods. Structure determi-
nations were performed on an Oxford Diffraction Gemini-R
diffractometer, using Mo Kα radiation. Single crystals were mounted
on Hampton Research Cryoloops using Paratone-N oil. Unit cell
determination, data collection and reduction, and analytical absorption
correction were performed using the CrysAlisPro software package.42
Direct methods structure solution was accomplished using SIR92,43
and full-matrix least-squares refinement was carried out using
CRYSTALS.44 All non-hydrogen atoms were refined anisotropically.
Unless otherwise noted, hydrogen atoms were placed in calculated
positions, and their positions were initially refined using distance and
angle restraints. All hydrogen positions were fixed in place for the final
refinement cycles.
X-ray Structure Determination of 4-tBu. X-ray quality crystals
of 4-tBu were grown by slow evaporation of a solution in CH2Cl2 and
hexanes. One molecule of dichloromethane was present in the
asymmetric unit. The iridium-bound chloride was disordered over two
positions. The site occupancy was freely refined, and no geometric
restraints were employed. The occupancy of the major component was
0.782; the structural data listed in Figure 3 refer to this component.
The iridium-bound hydride, also expected to be disordered over two
positions, was not located in the difference map.
1
1
3
Iridium Complex 4-dipp. Benzimidazolium salt 3-dipp19 (0.568
mmol, 400 mg), [Ir(cod)Cl]2 (0.284 mmol, 191 mg), acetonitrile (92
mL), and triethylamine (17.0 mmol, 2.38 mL) were added to an oven-
dried medium-walled pressure vessel in an argon-filled glovebox. The
flask was brought outside of the glovebox, and the reaction mixture
was stirred at 150 °C for 16 h behind a blast shield. After cooling to
room temperature, the solvent was evaporated, and the residue was
purified by flash chromatography, using a gradient of 0−35% ethyl
acetate in dichloromethane. After removing the solvent under vacuum,
the complex was analytically pure. Yield: 186 mg, 36%. Recrystalliza-
tion for catalytic trials was performed at room temperature in an
argon-filled glovebox by layering a solution in dichloromethane with
1
3
X-ray Structure Determination of 4-Ad. X-ray quality crystals of
4-Ad grew directly from a slowly cooled crude reaction mixture. The
iridium-bound hydride was located in the difference map, and its
position was refined freely before being fixed in place for the final
refinement cycles.
pentane. H NMR (CD2Cl2): δ 8.21 (d, 2H, JHH = 8.0 Hz), 7.74 (d,
2H, 3JHH = 8.0 Hz), 7.47 (m, 4H), 7.40 (dd, 2H, 3JHH = 7.9 Hz, 4JHH
1.5 Hz), 7.36 (t, 1H, 3JHH = 8.0 Hz), 7.31 (m, 4H), 6.96 (d, 2H, 3JHH
=
=
3
3
8.0 Hz), 3.18 (sept, 2H, JHH = 6.9 Hz), 2.44 (sept, 2H, JHH = 6.9
Hz), 1.34 (s, 3H), 1.27 (d, 6H, 3JHH = 6.9 Hz), 1.07 (d, 6H, 3JHH = 6.9
7365
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Organometallics
Article
X-ray Structure Determination of 4-dipp. X-ray quality crystals
of 4-dipp were grown by slow evaporation of a solution in CH2Cl2 and
hexanes. A molecule of dichloromethane was present in the
asymmetric unit. The iridium-bound hydride was located in the
difference map, and its position was refined freely before being fixed in
place for the final refinement cycles.
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF file giving crystallographic data for complexes 4-tBu, 4-Ad,
and 4-dipp; detailed NMR assignments for complexes 4-tBu, 4-
Ad, and 4-dipp; and complete experimental procedures for
ligand synthesis and characterization. This material is available
free of charge via the Internet at http://pubs.acs.org.
(16) Zuo, W. W.; Braunstein, P. Organometallics 2010, 29, 5535−
5543.
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643.
AUTHOR INFORMATION
Corresponding Author
*E-mail: achianese@colgate.edu..
Notes
■
(18) Schultz, K. M.; Goldberg, K. I.; Gusev, D. G.; Heinekey, D. M.
Organometallics 2011, 30, 1429−1437.
(19) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer,
P. T. Organometallics 2010, 29, 3019−3026.
The authors declare no competing financial interest.
(20) Rivas, F. M.; Riaz, U.; Giessert, A.; Smulik, J. A.; Diver, S. T.
Org. Lett. 2001, 3, 2673−2676.
ACKNOWLEDGMENTS
We thank the National Science Foundation (Grant Nos. CHE-
1057792 and CHE-0819686) for financial support.
■
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