Iridium complexes of CCC-pincer N-heterocyclic carbene ligands: Synthesis and catalytic C-H functionalization

Article abstract of DOI:10.1021/om100302g

A series of four meta-phenylene-bridged bis-benzimidazolium chlorides were synthesized as precursors to rigid, monoanionic, CCC-pincer N-heterocyclic carbene ligands. For ligands with mesityl, 3,5-xylyl, or 3,5-di-tert-butylphenyl side groups, reaction with [Ir(1,5-cyclooctadiene)Cl]₂ in acetonitrile with either excess triethylamine or stoichiometric cesium fluoride as base gave neutral, iridium(III) pincer complexes of the formula Ir(CCC)HCl(MeCN), which were purified by chromatography on silica gel. Metalation failed under these conditions for a 2,6-diisopropylphenyl-substituted derivative. In combination with NaO^tBu, these complexes form active catalysts for the acceptorless dehydrogenation of cyclooctane and for arene C-H borylation in neat arene solvent.

Full text of DOI:10.1021/om100302g

Organometallics 2010, 29, 3019–3026 3019
DOI: 10.1021/om100302g
Iridium Complexes of CCC-Pincer N-Heterocyclic Carbene Ligands:
Synthesis and Catalytic C-H Functionalization
Anthony R. Chianese,* Allen Mo, Nicole L. Lampland, Raymond L. Swartz, and
Paul T. Bremer
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346
Received April 14, 2010
A series of four meta-phenylene-bridged bis-benzimidazolium chlorides were synthesized as
precursors to rigid, monoanionic, CCC-pincer N-heterocyclic carbene ligands. For ligands with
mesityl, 3,5-xylyl, or 3,5-di-tert-butylphenyl side groups, reaction with [Ir(1,5-cyclooctadiene)Cl]2 in
acetonitrile with either excess triethylamine or stoichiometric cesium fluoride as base gave neutral,
iridium(III) pincer complexes of the formula Ir(CCC)HCl(MeCN), which were purified by chromato-
graphy on silica gel. Metalation failed under these conditions for a 2,6-diisopropylphenyl-substituted
derivative. In combination with NaOtBu, these complexes form active catalysts for the acceptorless
dehydrogenation of cyclooctane and for arene C-H borylation in neat arene solvent.
Introduction
The selective functionalization of weakly reactive C-H
bonds is a central challenge in organometallic chemistry. The
development of methods for transition-metal-catalyzed C-H
bond functionalization has expanded dramatically in the
past decade, with the most practical and predictable trans-
formations beginning to find routine use in organic synthe-
Figure 1. Known CCC-pincer ligands.
sis.1 The pincer2 ligand motif has proved especially useful for
the stabilization of low-valent metal fragments capable of
C-H activation and subsequent functionalization, espe-
cially in the case of monoanionic PCP-pincer ligands,3
iridium complexes of which have provided exceptionally
active catalysts for the dehydrogenation of alkanes, either
with or without an added hydrogen acceptor.4
Aside from ligands derived from unexpected alkyne chloro-
metalation13 and side-group C-H activation,14 mostreported
CCC-pincer ligands fall into two classes (Figure 1).
In class A,6a,f,h,15 the central aryl fragment is connected to
two imidazole- or benzimidazole-based NHC fragments
through CH2 linkers. The ligand forms two six-membered,
N-Heterocyclic carbenes (NHCs) are now routinely em-
ployed as supporting ligands for catalysis,5 and a diverse
array of pincer ligands bearing the NHC motif have been
synthesized, including those based on neutral CNC,6 CNN,7
PCP,8 SCS,9 and CSC;10 anionic CNC;11 and dianionic
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colgate.edu.
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2010 American Chemical Society
Published on Web 06/16/2010
pubs.acs.org/Organometallics

3020 Organometallics, Vol. 29, No. 13, 2010
Chianese et al.
n
boat-shaped chelate rings, and metal complexes are chiral
with varying barriers to enantiomerization.6f,h Even though class
A has been known since 2001,6a pincer complexes bearing
this structure have only been isolated for palladium6a,f,15a
and the rare-earth metals scandium, lutetium, and samarium.15b
Metalation to palladium has required heating to 125 °C
or above6a,f or the installation of the central M-Caryl bond
via C-Br oxidative addition to palladium(0).6f,15a Under
a wide variety of reaction conditions for metalation to
rhodium and iridium, Braunstein and co-workers observed
only the formation of nonpincer mono- and dinuclear
complexes.15c-e
B ligand precursors (R = Bu, Me, CH2SiMe3)16 and later
described their metalation (R = nBu) to zirconium,17 trans-
metalation from zirconium to rhodium17,18 and iridium,18
and catalytic applications in hydroamination18,19 and hydro-
silylation.20 Braunstein and co-workers have more recently
described the metalation of class B ligand precursors (R =
nBu, Pr) to iridium directly from the imidazolium salt
i
precursors, by refluxing with [Ir(cod)Cl]2,21 NEt3, and KI
in acetonitrile.15c The same group has recently reported
a thorough examination of the metalation of the butyl-
substituted precursor to iridium, including the characteriza-
tion of intermediates and unproductive side reactions, and
has prepared a modified ligand with methyl substituents on
the central aryl group, designed to prevent cyclometalation
at undesired positions on the backbone.22 By employing
stoichiometric Cs2CO3 as base instead of excess NEt3,
octahedral complexes of the formula Ir(CCC)HI(MeCN)
were isolated in good yield. Complexes of this formula are
closely related to five-coordinate Ir(PCP)HCl complexes
that have previously been shown to form highly active alkane
dehydrogenation catalysts upon HCl elimination promoted
by NaOtBu4i-k and might be expected to exhibit similar
reactivity, although Braunstein reported a lack of catalytic
activity for the transfer dehydrogenation of cyclooctane with
tert-butylethylene using this method.22
In class B, no CH2 linker is present, so two five-membered
chelate rings form and a rigid, flat backbone is expected.
Hollis and co-workers initially reported the synthesis of class
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Article
Scheme 1. Synthesis of Benzimidazolium Salts 3a-d21
Organometallics, Vol. 29, No. 13, 2010 3021
Scheme 2. Synthesis of Iridium Complexes 4a, 4c, and 4d
base for the metalation of 3c and 3d, provided that only a
slight excess of base is employed. After heating a mixture of
3d, [Ir(cod)Cl]2, and CsF overnight in acetonitrile at 80 °C,
the hydride complex 4d was the only species observed in
solution and was isolated in 69% yield following chromato-
graphy. Using benzimidazolium salt 3c, complex 4c forms as
part of the mixture and slowly decomposes over the course of
a day. By carefully monitoring the reaction and stopping
after a few hours when 4c comprised about half the ligand-
containing species, complex 4c was isolated in 34% yield
following chromatography. Thus far, we have not been able
to prepare analogous complexes of ligand precursor 3b.
Under the conditions in Scheme 2, using either NEt3 or
CsF as base, we observed consumption of the benzimidazo-
lium precursor and formation of a complex mixture of
products by 1H NMR spectroscopy. Although a small
amount of a product with a hydride signal analogous to that
of 4a, 4c, and 4d was observed, we were not able to isolate this
species.
Solution and Solid-State Structural Characterization of 4a,
4c, and 4d. By 1H and 13C NMR spectroscopy, complexes 4a,
4c, and 4d appear to have averaged Cs symmetry in solution,
as indicated by the chemical equivalence of the two benzi-
midazol-2-ylidene fragments and the nonequivalence of the
ortho- and meta-substituents on the mesityl (4a), 3,5-xylyl
(4c), and 3,5-di-tert-butylphenyl (4d) side groups. Complete
assignment of the 1H NMR spectra was facilitated by phase-
sensitive NOESY, which revealed a correlation between the
iridium-bound hydride and an ortho-methyl group (4a) or an
ortho-hydrogen (4d) on the aromatic N-substituent. Addi-
tionally, NOESY reveals a correlation between the aceto-
nitrile CH3 group and the para-methyl group (4a) or the para-
hydrogen (4c and 4d) on the aromatic N-substituent. Taken
together, these data indicate that the solution geometry is as
Results and Discussion
Ligand Synthesis and Metalation. Benzimidazolium salts
3a-d were synthesized as shown in Scheme 1. Selective
Buchwald-Hartwig24 monoamination of iodobromobenzene
gave the ortho-bromoanilines 1a-d. Palladium-catalyzed cou-
pling with 1,3-diaminobenzene then gave the tetraamines 2a-d,
and cyclization25 in triethylorthoformate with a slight excess of
concentrated hydrochloric acid gave the dicationic ligand pre-
cursors 3a-d.26
Initially, the mesityl-substituted precursor 3a was com-
bined with 0.5 equiv of [Ir(cod)Cl]2 and 3 equiv of triethyl-
amine in acetonitrile, and the mixture was stirred at 80 °C
and monitored by 1H NMR spectroscopy. The formation of
several complexes was observed, including one with a singlet
hydride resonance at -23.16 ppm. After some experimenta-
tion, we found that this hydride-containing complex was the
major product after 24 h if a solution of 30 equiv of
triethylamine was added slowly to a dilute, heated solution
of 3a and [Ir(cod)Cl]2 over the course of an hour. Following
this method, we were able to isolate the hydride complex
after column chromatography on silica gel in 64% yield. By
NMR spectroscopy and X-ray crystallography (vide infra),
the complex was identified as 4a, which contains the fully
metalated pincer ligand, in addition to chloride, hydride, and
acetonitrile ligands (Scheme 2). Complex 4a is stable to air
and moisture, as solutions in untreated CD2Cl2 remain un-
changed for days.
shown in Scheme 2, with the acetonitrile ligand trans to Caryl
,
and the chloride and hydride ligands mutually trans. In the
1H NMR spectra of 4a and 4d, no resonances are broadened
at room temperature, indicating that potential rotation
about the side-group N-C bonds is slow on the NMR time
scale. By contrast, in the 1H NMR spectrum of 4c, the
resonances for the ortho-hydrogens and meta-methyl groups
of the 3,5-xylyl substituents are slightly broadened, indicat-
ing slow-but-observable rotation about the N-C bond.
Exchange between the two ortho-hydrogens and meta-methyl
groups was confirmed by NOESY/EXSY. The iridium-
bound hydrides appeared at approximately -23.2 ppm, the
carbene carbons appeared between 184.4 and 186.6 ppm,
and the iridium-bound aryl carbons appeared between 143.9
and 145.2 ppm.
Attempts to metalate the ligand precursors 3b-d follow-
ing this protocol were unsuccessful. Using ligand precursors
3c and 3d, the desired hydride complexes were formed as part
of a mixture after a few hours, but continued heating led to
decomposition to several unidentified products. After ex-
perimentation with a variety of bases and iridium sources, we
were pleased to discover that cesium fluoride is an effective
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K. H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575–5580.
(b) Sadighi, J. P.; Harris, M. C.; Buchwald, S. L. Tetrahedron Lett. 1998,
39, 5327–5330.
The structures of 4a, 4c, and 4d were confirmed by X-ray
crystallography. ORTEP diagrams are presented in Figures 2,
3, and 4, respectively. The solution geometries suggested by
NMR spectroscopy are confirmed in the solid state: the
(25) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Org. Lett.
2006, 8, 1831–1834.
(26) See Supporting Information for details of ligand synthesis and
characterization.

3022 Organometallics, Vol. 29, No. 13, 2010
Chianese et al.
Figure 2. ORTEP diagram of 4a, showing 50% probability ellip-
soids. Hydrogen atoms are omitted for clarity.
Figure 4. ORTEP diagram of 4d, showing 50% probability elli-
psoids. Hydrogen atoms are omitted for clarity. Atoms C(17),
C(20), Ir(1), Cl(2), N(3), C(4), and C(5) lie on a crystallographic
mirror plane.
Table 1. Selected Bond Lengths and Angles for 4a, 4c, and 4d
˚
Bond Lengths (A)
4a
4c
4d
Ir(1)—C(20)
Ir(1)—C(6)
Ir(1)—C(29)
Ir(1)—Cl(2)
Ir(1)—N(3)
C(6)—N(7)
C(6)—N(14)
C(29)—N(28)
C(29)—N(21)
1.958(3)
2.008(4)
2.014(4)
2.5036(10)
2.105(3)
1.362(5)
1.388(5)
1.363(5)
1.386(4)
1.971(4)
2.019(4)
2.033(5)
2.5066(10)
2.123(3)
1.351(5)
1.383(5)
1.368(5)
1.377(5)
1.944(5)
2.041(4)
2.5129(13)
2.104(4)
1.350(5)
1.384(4)
Bond Angles (deg)
C(6)—Ir(1)—C(20)
C(29)—Ir(1)—C(20)
C(6)—Ir(1)—N(3)
C(29)—Ir(1)—N(3)
Cl(2)—Ir(1)—C(6)
Cl(2)—Ir(1)—C(20)
Cl(2)—Ir(1)—C(29)
Cl(2)—Ir(1)—N(3)
N(7)—C(6)—N(14)
N(28)—C(29)—N(21)
78.17(16)
78.38(15)
101.79(14)
101.69(13)
93.69(12)
90.55(11)
91.66(10)
89.05(9)
78.37(16)
77.94(16)
99.80(14)
103.56(15)
93.65(11)
98.56(11)
90.77(11)
89.04(10)
105.3(3)
78.33(10)
101.37(10)
Figure 3. ORTEP diagram of 4c, showing 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity. One of two
independent molecules in the asymmetric unit is shown.
93.48(10)
98.47(16)
88.30(13)
105.5(3)
105.1(3)
105.3(3)
acetonitrile ligands are trans to the Caryl donor atom, and the
hydride and chloride ligands are mutually trans. Complexes
4a and 4c have approximate Cs symmetry, while 4d lies on a
crystallographic mirror plane. As would be expected, the
ligand “backbone”, consisting of the central aromatic ring
and the two benzimidazole rings, is nearly flat in each
complex, while the aromatic N-substituents are out-of-plane
to varying extents.
105.6(4)
distorted by the rigid chelate geometry of the CCC-pincer
ligands. Metric parameters are in close agreement with a
related structure recently reported by Braunstein and co-
workers.22 The iridium-Ccarbene distances, ranging from
˚
2.008 to 2.041 A, are well within the range of previously
Selected bond lengths and angles are given in Table 1. All
three complexes possess a pseudo-octahedral geometry,
studied iridium complexes of benzimidazole-derived NHCs.
˚
The iridium-Caryl distances, ranging from 1.944 to 1.971 A,

Article
Organometallics, Vol. 29, No. 13, 2010 3023
Table 2. Comparison of Iridium Precatalysts for the Borylation of
m-Xylene
Table 3. Borylation of a Range of Arenes with Mesityl-Substi-
tuted 4a
precatalyst
additive
conversiona [%]
4a
4a
4a
4c
4d
none
41
57
42b
29
22
4% NaOtBu
4% NaOtBu
4% NaOtBu
4% NaOtBu
a Conversion determined by 1H NMR spectroscopy of the crude
reaction mixture. Conversions are based on boron atom in B2pin2.
b Reaction run with 0.05 M B2pin2.
are among the shorter distances known for iridium bound to
an aryl group: a Cambridge Structural Database27 search
returns 628 structures, with an average Ir-C distance of
˚
2.021 ( 0.043 A (median ( SD). Additionally, 57 crystal
structures have been reported for iridium complexes of
monoanionic P-Caryl-P pincer complexes, using either
methylene-bridged bis(phosphine) or oxygen-bridged bis-
(phosphinite) ligands. For this set, the Ir-Caryl distances
˚
range from 1.986 to 2.124 A, with a median distance of 2.048
˚
( 0.033 A. The uncommonly short Ir-Caryl distances for
CCC-pincer complexes apparently result from the con-
strained nature of the chelate rings.
a Conversion determined by 1H NMR spectroscopy of the crude
reaction mixture. Isolated yield in parentheses. Yields are based on
boron atom in B2pin2. b Approximately 6% yield of the isomeric product
of borylation at the 4-position was observed in the crude 1H NMR
spectrum. c Sodium tert-butoxide was not added.
Catalytic Borylation of Arenes. The iridium-catalyzed bor-
ylation of arene C-H bonds has been developed into one of
the most practically useful methods for C-H functionaliza-
tion.1f Selectivity for more sterically accessible aromatic hy-
drogens is predictable and complementary to classical meth-
ods for the formation of arylboron compounds, which derive
from directing effects in electrophilic aromatic halogenation.
Iridium complexes of mono- and bidentate NHCs have pre-
viously been shown to effect the borylation of arenes with bis-
pinacolatodiboron (B2pin2) and pinacolborane (HBpin), un-
der either thermal28 or microwave29 conditions.
We have found that complexes 4a, 4c, and 4d catalyze the
borylation of m-xylene when a limiting amount of B2pin2 is
employed in neat arene solvent (Table 2). The addition of a
catalytic amount sodium tert-butoxide gave a moderately
higher yield of borylated product, although the origin of this
effect is currently uncertain. Employing more dilute condi-
tions (0.05 M vs 0.25 M) showed no benefit.
NMR even without the addition of NaOtBu. In most cases, we
observed products from borylation at the least sterically hin-
dered arene C-H bond, but approximately 6% yield of the
product of borylation at the more-hindered 4-position, ortho to
one methoxy group, was observed in the borylation of 1,3-
dimethoxybenzene. Initial attempts at borylation using limiting
quantities of arene in solvents such as tetrahydrofuran, dioxane,
and cyclohexane30 were unsuccessful.
Catalytic Dehydrogenation of Cyclooctane. The catalytic
dehydrogenation of alkanes has the potential to enable the
production of value-added alkenes, especially 1-alkenes,
from relatively abundant saturated petroleum feedstocks.
Although a variety of homogeneous transition-metal sys-
tems have been studied,31 the highest catalytic activities have
been achieved with iridium catalysts bound to monoanionic,
As 4a/NaOtBu was the most efficient catalyst system, its
activity for the borylation of a small group of substrates was
examined (Table 3). For m-xylene, 1,3-dimethoxybenzene, and
1,2-dimethoxybenzene, approximately 50% yield of borylated
product was formed, consistent with complete transfer of one
boron atom per molecule of B2pin2, which is likely due to the
sluggish reaction of the intermediate HBpin under our condi-
tions. The electron-deficient substrate 1,3-bis(trifluoromethyl)-
benzene was transformed more efficiently, giving 94% yield by
(30) (a) Ishiyama, T.; Takagi, J.; Hartwig, J. F.; Miyaura, N. Angew.
Chem., Int. Ed. 2002, 41, 3056–3058. (b) Ishiyama, T.; Takagi, J.; Yonekawa,
Y.; Hartwig, J. F.; Miyaura, N. Adv. Synth. Catal. 2003, 345, 1103–1106.
(c) Chotana, G. A.; Rak, M. A.; Smith, M. R. J. Am. Chem. Soc. 2005, 127,
10539–10544. (d) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.; Batsanov,
A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int. Ed.
2006, 45, 489–491. (e) Lo, W. F.; Kaiser, H. M.; Spannenberg, A.; Beller, M.;
Tse, M. K. Tetrahedron Lett. 2007, 48, 371–375.
(31) (a) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem.
Soc. 1979, 101, 7738–7740. (b) Baudry, D.; Ephritikhine, M.; Felkin, H.;
Holmes-Smith, R. J. Chem. Soc., Chem. Commun. 1983, 788–789.
(c) Burk, M. J.; Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics
1984, 3, 816–817. (d) Felkin, H.; Fillebeen-Khan, T.; Holmes-Smith, R.;
Yingrui, L. Tetrahedron Lett. 1985, 26, 1999–2000. (e) Burk, M. J.;
Crabtree, R. H. J. Am. Chem. Soc. 1987, 109, 8025–8032. (f) Maguire,
J. A.; Boese, W. T.; Goldman, A. S. J. Am. Chem. Soc. 1989, 111, 7088–
7093. (g) Maguire, J. A.; Goldman, A. S. J. Am. Chem. Soc. 1991, 113,
6706–6708. (h) Maguire, J. A.; Petrillo, A.; Goldman, A. S. J. Am. Chem.
Soc. 1992, 114, 9492–9498. (i) Belli, J.; Jensen, C. M. Organometallics
1996, 15, 1532–1534. (j) Wang, K.; Goldman, M. E.; Emge, T. J.; Goldman,
A. S. J. Organomet. Chem. 1996, 518, 55–68.
(27) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae,
C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. Sect. B 2002,
58, 389–397.
(28) Frey, G. D.; Rentzsch, C. F.; von Preysing, D.; Scherg, T.;
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Chem. 2009, 11, 1610–1617.

3024 Organometallics, Vol. 29, No. 13, 2010
Chianese et al.
Table 4. Transfer Dehydrogenation of Cyclooctanea
from reduced stability of iridium(I) intermediates under the
catalytic conditions when less bulky side groups are employed.
Conclusions
In summary, we have presented the synthesis of a series of
rigid-backbone, CCC-pincer bis(carbene) ligands with aro-
matic side groups of varying steric bulk. Pincer-iridium com-
plexes were formed under mild conditions and were shown to
be active catalysts for the C-H borylation of arenes with
B2pin2, as well as the dehydrogenation of cyclooctane. Further
studies, including catalyst optimization, dehydrogenation of
less “active” alkanes, and attempts to isolate and characterize
catalyst resting states, are in progress.
[Ir]
acceptorb
TON of cyclooctene
4a
4c
4d
4a
4d
NBE
NBE
NBE
TBE
TBE
10c
0
0d
1c
2d
a Reactions performed with 4 μmol of [Ir], 8 μmol of NaOtBu, and
200 μmol of acceptor in 1 mL of cyclooctane. b Abbreviations: NBE =
norbornene; TBE = tert-butylethylene. c Average of four experiments.
d Average of two experiments.
Experimental Section
General Methods. [Ir(cod)Cl]2 was prepared as previously
described.33 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 activated alumina, using an MBraun solvent pur-
ification system. Acetonitrile (AcroSeal Extra Dry) was handled
under argon and used as received. 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 4a, 4c, and 4d, along with
detailed procedures for ligand synthesis and characterization,
are given in the Supporting Information.
Iridium Complex 4a. Benzimidazolium salt 3a (0.791 mmol,
490 mg) and [Ir(cod)Cl]2 (0.395 mmol, 263 mg) were added to a
flame-dried Schlenk flask. Acetonitrile (120 mL) was added, and
the reaction mixture was heated to 75 °C. A solution of
triethylamine (23.7 mmol, 3.3 mL) in acetonitrile (125 mL)
was added dropwise over an hour. The reaction was stirred at
75 °C for 10 h. The solvent was evaporated, and the residue was
purified by flash chromatography, using a gradient of 0 to 30%
ethyl acetate in dichloromethane. Yield: 414 mg, 64%. 1H NMR
Table 5. Acceptorless Dehydrogenation of Cyclooctane
[Ir]
TONa
4a
4c
4d
68
0
26
a Average of 2 runs, 4 mL reaction volume.
PCP-pincer bis(phosphine)4a-h,l,m,o or bis(phosphinite)4i-k,n
ligands. A useful test substrate is cyclooctane, whose positive
standard enthalpy of dehydrogenation is unusually low, at
24 kcal mol .
-1 32 We have examined the potential activity of
3
4a, 4c, and 4d for the dehydrogenation of cyclooctane, both
with and without an added hydrogen acceptor. Results for
transfer dehydrogenation are shown in Table 4, and results
for acceptorless dehydrogenation are shown in Table 5.
For the transfer dehydrogenation of cyclooctane using
norbornene (NBE) as hydrogen acceptor, we found that 4a
was a weakly effective precatalyst when initiated with NaOt-
Bu, giving 10 turnovers in a 24 h period. Complexes 4c and 4d
gave no cyclooctene under these conditions. The common
hydrogen acceptor tert-butylethylene (TBE), which has gi-
ven turnover numbers in the thousands for several Ir-PCP
systems,4j,m,n was completely ineffective under our condi-
tions, which is consistent with Braunstein’s recent report.22
For acceptorless dehydrogenation of cyclooctane under
reflux conditions (bp = 151 °C), where H2 was continually
removed from the system by passing argon over the reflux
condenser, we observed encouraging reactivity for com-
plexes 4a and 4d, activated in situ by the addition of catalytic
NaOtBu (Table 5). For 4a, 68 turnovers were observed, while
26 turnovers were observed for 4d. For comparison, Ir-PCP
systems have given up to 190 turnovers for this transforma-
tion4c,e and up to 3050 turnovers for the acceptorless dehydro-
genation of cyclodecane (bp = 201 °C).4c,e,l,o Complex 4c was
completely inactive under these conditions, which may result
(CD2Cl2): δ 8.20 (d, 2H, 3JHH = 8.1 Hz); 7.71 (d, 2H, 3JHH
=
7.9 Hz); 7.47 (t, 2H, 3JHH = 8.1 Hz); 7.35 (t, 1H, 3JHH = 7.9 Hz);
7.31 (t, 2H, 3JHH = 8.1 Hz); 7.08 (s, 2H); 7.04 (s, 2H); 6.98 (d,
2H, 3JHH = 8.1 Hz); 2.38 (s, 6H); 2.25 (s, 6H); 1.99 (s, 6H); 1.47
(s, 3H); -23.16 (s, 1H). 13C NMR (CD2Cl2): δ 186.6 (Ir-
C
carbene), 147.0, 145.2 (Ir-Caryl), 139.0, 138.8, 137.6, 136.0,
133.6, 132.9, 129.4, 128.8, 123.7, 123.0, 121.5, 116.0, 111.4,
110.8, 108.3, 21.3, 18.4, 18.1, 2.9. Anal. Calcd for C40H37ClIrN5:
C, 58.92; H, 4.57; N, 8.59. Found: C, 58.68; H, 4.31; N, 8.35.
Iridium Complex 4c. Benzimidazolium salt 3c (0.169 mmol,
100 mg), [Ir(cod)Cl]2 (0.085 mmol, 57 mg), and CsF (0.371
mmol, 56 mg) were added to a flame-dried Schlenk flask.
Acetonitrile (100 mL) was added, and the reaction mixture
was stirred at 80 °C for four hours. The solvent was evaporated,
and the residue was purified by flash chromatography, using a
gradient of 0 to 30% ethyl acetate in 99:1 dichloromethane/
acetonitrile. Yield: 45 mg, 34%. 1H NMR (CD2Cl2): δ 8.20 (d,
2H, 3JHH = 8.1 Hz); 7.99 (br s, 2H); 7.73 (d, 2H, 3JHH = 7.9 Hz);
7.48 (m, 2H); 7.13-7.37 (m, 5H); 7.28 (br s, 2H); 7.16 (s, 2H);
2.50 (s, 6H); 2.45 (s, 6H); 1.49 (s, 3H); -23.26 (s, 1H). 13C NMR:
δ 184.4 (Ir-Ccarbene), 146.5, 143.9 (Ir-Caryl), 139.2, 138.4, 137.6,
136.7, 132.3, 130.1, 128.3, 124.7, 123.5, 122.5, 121.0, 116.6, 111.2,
110.8, 108.1, 21.1, 21.0, 2.3. Anal. Calcd for C38H33ClIrN5: C,
57.97; H, 4.22; N, 8.89. Found: C, 57.84; H, 4.46; N, 8.68.
(32) Afeefy, H. Y.; Liebman, J. F.; Stein, S. E. Neutral Thermoche-
mical Data. In NIST Chemistry WebBook, NIST Standard Reference
Database Number 69; Linstrom, P. J.; Mallard, W. G., Eds.; National
Institute of Standards and Technology: Gaithersburg, MD, http://webbook.
nist.gov (retrieved April 9, 2010).
(33) Lin, Y.; Nomiya, K.; Finke, R. G. Inorg. Chem. 1993, 32, 6040–
6045.

Article
Organometallics, Vol. 29, No. 13, 2010 3025
Table 6. Crystal Data and Structure Refinement for 4a, 4c, and 4d
4a
4c
4d
color, shape
empirical formula
fw
yellow, rod
C42H40N5Cl5Ir
984.30
yellow, block
C38H33N5Cl1Ir
786.38
yellow, block
C51H58N5Cl3Ir
1039.63
radiation
temperature (K)
cryst syst
Mo KR, 0.71073
110
monoclinic
P21/c (No. 14)
Cu KR, 1.5418
110
triclinic
Mo KR, 0.71073
110
orthorhombic
Pnma (No. 62)
space group
unit cell dimens
P1 (No. 2)
˚
a (A)
12.8180(3)
7.93105(16)
39.8998(11)
90.0
94.406(2)
90.0
4044.23(17)
4
1.616
14.1102(4)
14.3525(5)
21.3146(7)
77.637(3)
78.668(2)
76.019(3)
4043.6(2)
4
1.292
7.269
0.16 Â 0.08 Â 0.04
4.08 < θ < 67.31
72 312, 14336
0.0379
18.6843(2)
26.8186(3)
9.11513(12)
90.0
90.0
90.0
4567.46(17)
4
1.512
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
Dcalc (g cm-3
)
μ(Mo KR) (mm-1
cryst size (mm)
)
3.663
3.139
0.14 Â 0.11 Â 0.02
3.34 < θ < 29.23
32 928, 9671
0.0434
0.19 Â 0.13 Â 0.13
3.73 < θ < 29.44
26 510, 5735
0.0299
θ range for data coll. (deg)
total, unique no. of refl.
Rint
no. of params, restraints
R, wR2 for all data
R, wR2 for I > 2σ
GOF
478, 0
957, 276
311, 48
0.0625, 0.0642
0.0378, 0.0616
1.033
0.0386, 0.0798
0.0273, 0.0776
0.9488
0.0435, 0.0878
0.0328, 0.0855
0.9618
-3
˚
resid density (e A
)
-4.56, 2.51
-0.58, 1.15
-1.85, 3.66
Iridium Complex 4d. Benzimidazolium salt 3d (0.217 mmol,
167 mg), [Ir(cod)Cl]2 (0.109 mmol, 73 mg), and CsF (0.456
mmol, 69 mg) were added to a flame-dried Schlenk flask.
Acetonitrile (100 mL) was added, and the reaction mixture
was stirred at 80 °C for 24 h. The solvent was evaporated, and
the residue was purified by flash chromatography, using a
gradient of 0 to 70% ethyl acetate in 98:2 dichloromethane/
acetonitrile. Yield: 142 mg, 69%. 1H NMR (CD2Cl2): δ 8.20 (d,
2H, 3JHH = 8.1 Hz); 8.04 (t, 2H, 4JHH = 1.6 Hz); 7.74 (d, 2H,
were initially refined using distance and angle restraints and were
fixed in place for the final refinement cycles. Disordered fragments
were modeled using a refined group occupancy parameter; their
relative geometries were restrained using the SAME command,
and relative displacement parameters were restrained using the
SIMU command. Details of data collection and refinement are
given in Table 6.
X-ray Structure Determination of 4a. Diffraction-quality
crystals were obtained by slow evaporation of a solution in
dichloromethane and hexanes. Two molecules of dichloro-
methane were present in the asymmetric unit. The iridium-
bound hydride was not located in the difference map.
3JHH = 8.0 Hz); 7.58 (t, 2H, 4JHH = 1.6 Hz); 7.48 (t, 2H, 3JHH
=
8.1 Hz); 7.45 (t, 2H, 4JHH = 1.6 Hz); 7.36 (t, 1H, 3JHH = 8.0 Hz);
7.32 (t, 2H, 3JHH = 8.1 Hz); 7.19 (d, 2H, 3JHH = 8.1 Hz); 1.44 (s,
18H); 1.40 (s, 18H); 1.28 (s, 3H); -23.23 (s, 1H). 13C NMR: δ
185.1 (Ir-Ccarbene), 153.1, 151.8, 146.9, 144.4 (Ir-Caryl), 137.8,
137.8, 132.6, 126.0, 123.8, 122.9, 122.8, 121.2, 121.2, 116.1,
111.5, 111.1, 108.4, 35.5, 35.3, 31.9, 31.7, 2.9. Anal. Calcd for
C50H57ClIrN5: C, 62.84; H, 6.01; N, 7.33. Found: C, 62.10; H,
5.86; N, 6.79. Though a satisfactory analysis was not obtained
X-ray Structure Determination of 4c. Diffraction-quality crys-
tals were obtained by slow evaporation of a solution in dichloro-
methane and hexanes. Two independent molecules of 4c were
present in the asymmetric unit. The molecule shown in Figure 3
shows no disorder, while both the 3,5-xylyl groups of the second
molecule were disordered over two positions. Highly disordered
solvent was also present; correction for this residual density was
performed using the option SQUEEZE in the program package
PLATON.37 A total of 196 electrons per unit cell were removed,
after multiple attempts, 4d appeared to be pure by 1H and 13
C
NMR spectroscopy (see images in the Supporting Information).
We cannot rule out the possibilty of inorganic impurities,
though this seems unlikely given the chromatographic purifica-
tion procedure employed.
3
˚
from a total potential solvent-accessible void of 964.2 A . The
iridium-bound hydride was not located in the difference map.
X-ray Structure Determination of 4d. Diffraction-quality
crystals were obtained by slow evaporation of a solution in
dichloromethane and hexanes. Complex 4d and one molecule of
dichloromethane lie on a crystallographic mirror plane. One
tert-butyl group in the asymmetric unit was disordered over two
positions; only the major component is shown in Figure 4. The
iridium-bound hydride was not located in the difference map.
Arene C-H Borylation Experiments (Tables 2,3). In an argon-
filled glovebox, a vial was charged with an iridium complex
(5 μmol), sodium tert-butoxide (0.96 mg, 10 μmol), and bis-
(pinacolato)diboron (64 mg, 0.25 mmol). The appropriate arene
(1 mL) was added, the vial was capped, and the mixture was
stirred at 100 °C for 24 h. A small aliquot was removed for
analysis by 1H NMR. For experiments where the product
was isolated, the reaction mixture was transferred directly to a
silica gel column, and products were purified by gradient flash
chromatography, generally using solutions of ethyl acetate in
X-ray Crystallography, General Methods. Structure determi-
nations were performed on an Oxford Diffraction Gemini-R
diffractometer, using Mo KR 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.34 Direct methods structure
solution was accomplished using SIR92,35 and full-matrix
least-squares refinement was carried out using CRYSTALS.36
All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in calculated positions. Hydrogen positions
(34) Xcalibur CCD system and CrysAlisPro software system, version
1.171.32; Oxford Diffraction Ltd., 2007.
(35) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435–436.
(36) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(37) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.

3026 Organometallics, Vol. 29, No. 13, 2010
Chianese et al.
hexanes. For the volatile product of borylation of 1,3-bis-
(trifluoromethyl)benzene, a solution of pentane and ether was
employed as eluent. All products were pure by H NMR and
8 μmol), and 4.0 mL of cyclooctane. A condenser was attached,
and the solution was vigorously refluxed under argon in an oil
bath heated to 175 °C, while the atmosphere above the con-
denser was purged with a steady stream of argon. After 20 h, the
mixture was cooled, and a 100 μL aliquot was transferred to
700 μL of a standard solution of 1,3,5-trimethoxybenzene in
CDCl3. Cyclooctene was quantified by integrating the 1H NMR
signals against the standard.
1
were identified by comparison with previously reported data.38
Cyclooctane Transfer-Dehydrogenation Experiments. In an
argon-filled glovebox, a 4 mL medium-pressure screw-cap tube
was charged with a stir-bar, an iridium complex (4 μmol),
sodium tert-butoxide (0.77 mg, 8 μmol), 1.0 mL of cyclooctane,
and 0.200 mmol of either norbornene or tert-butylethylene. The
tube was capped, removed from the glovebox, and heated to
150 °C in an oil bath for 24 h. The mixture was cooled to room
temperature, and a 25 μL aliquot was transferred to 975 μL of
a standard solution of 1,3,5-trimethoxybenzene in CDCl3.
Cyclooctene and unconsumed hydrogen acceptor were quantified
by integrating the 1H NMR signals against the standard.
Acknowledgment. We are grateful to the ACS Petro-
leum Research Fund (46754-GB3) and the National Science
Foundation Major Research Instrumentation program
(CHE-0819686) for generous support of this work.
Supporting Information Available: Details of the synthesis
and characterization of new compounds, including images of 1H
and 13C NMR spectra. CIF files giving X-ray diffraction data,
atomic coordinates, thermal parameters, and complete bond
distances and angles. This material is available free of charge via
the Internet at http://pubs/acs/org.
Acceptorless Cyclooctane Dehydrogenation Experiments. An
oven-dried 25 mL round-bottom flask was charged with a stir-
bar, an iridium complex (4 μmol), sodium tert-butoxide (0.77 mg,
(38) Tse, M. K.; Cho, J.-Y.; Smith, M. R. Org. Lett. 2001, 3, 2831–
2833.