Synthesis and characterization of iridium(I) and iridium(III) complexes containing dialkylbiphenylphosphines

Article abstract of DOI:10.1021/om400266d

The synthesis and characterization of iridium complexes containing the PCy₂biPh ligand (PCy₂biPh = 2-(dicyclohexylphosphino) biphenyl), is described. Chloride abstraction from (COD)Ir(PCy ₂biPh)Cl with Na(BAr_F)₄ affords [(COD)Ir(PCy₂biPh)][B(Ar_F)₄] (1, Ar_F = 3,5-bis(trifluoromethyl)phenyl). Heating 1 in benzene results in dehydrogenation of one of the cyclohexyl groups on the phosphine ligand with COD serving as the hydrogen acceptor, leading to the new complex 2, with benzene coordinated in an η⁶ fashion to the metal center. Complex 1 reacts with H₂ to produce the Ir(III) dihydride complex [(PCy ₂biPh)IrH₂][B(Ar_F)₄] (3). Initial studies of transfer dehydrogenation and attempts at benzene hydrogenation using 1 and 3 are described.

Full text of DOI:10.1021/om400266d

Note
pubs.acs.org/Organometallics
Synthesis and Characterization of Iridium(I) and Iridium(III)
Complexes Containing Dialkylbiphenylphosphines
Abby R. O’Connor,*,† Werner Kaminsky,‡ Benny C. Chan,† D. M. Heinekey,‡ and Karen I. Goldberg‡
†Department of Chemistry, The College of New Jersey, 2000 Pennington Road, Ewing, New Jersey 08628-0718, United States
‡Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of iridium complexes
containing the PCy2biPh ligand (PCy2biPh = 2-(dicyclohexylphosphino)-
biphenyl), is described. Chloride abstraction from (COD)Ir(PCy2biPh)Cl
with Na(BArF)4 affords [(COD)Ir(PCy2biPh)][B(ArF)4] (1, ArF = 3,5-
bis(trifluoromethyl)phenyl). Heating 1 in benzene results in dehydrogenation
of one of the cyclohexyl groups on the phosphine ligand with COD serving as
the hydrogen acceptor, leading to the new complex 2, with benzene
coordinated in an η6 fashion to the metal center. Complex 1 reacts with H2 to
produce the Ir(III) dihydride complex [(PCy2biPh)IrH2][B(ArF)4] (3).
Initial studies of transfer dehydrogenation and attempts at benzene
hydrogenation using 1 and 3 are described.
he use of iridium complexes to catalyze hydrogenation
thermally stable; decomposition was observed upon heating at
Tand dehydrogenation reactions is well documented in the 60 °C. To date there have been limited reports of iridium
literature.1 Typically, the iridium catalysts employed contain
complexes containing the PCy2biPh (PCy2biPh = 2-
(dicyclohexylphosphino)biphenyl) ligand.24 In an effort to
phosphine ligands and one or more weakly coordinating ligands
synthesize more robust arene-stabilized complexes for hydro-
genation reactions, we report the synthesis and characterization
of new iridium compounds containing the PCy2biPh ligand.
that can be readily displaced, allowing for substrate binding.
The use of arenes as labile ligands has been investigated for a
variety of transition metals, including Rh,2−5 Ag,6 Ru,7−9 Pd,10
Ni,11 and Ir.12 More recently, Werner and Oro reported an
Ir(III) dihydride complex containing a bulky phosphine ligand
with an alkyl aryl group, which can coordinate to iridium in an
η6 fashion and is less labile than nonchelating arenes.13 This
type of combination ligand is potentially useful for catalytic
hydrogenation.
RESULTS AND DISCUSSION
■
Synthesis of [(COD)Ir(PCy2biPh)][B(ArF)]4 (1). Reaction
of (COD)Ir(PCy2biPh)Cl24 with Na(BArF)4 in CH2Cl2 led to
removal of the chloride ligand and yielded compound 1 as a red
powder (eq 1).
Dialkylbiarylphosphine ligands have been extensively used in
palladium-catalyzed coupling reactions.14 It is proposed that the
Pd catalyst stability is enhanced by metal−arene interactions
with the pendent ring.15 Although these ligands have been
widely utilized in Pd-catalyzed amination chemistry,16 the
application of this class of ligand toward other metal-catalyzed
processes has remained underexplored.17−20
We are interested in exploring whether the arene stabilization
observed in Pd complexes could be utilized at other metal
centers. A few reports have detailed the synthesis of dialkyl
biaryl phosphine Ir complexes and the applications of these
complexes in catalysis. In 2009, Tsuji reported the addition of
aromatic acyl chlorides to terminal alkynes using an iridium
catalyst and a biarylphosphine ligand.21 Dahlenburg described a
neutral Ir(I) COD complex containing a (di-tert-butylphosphi-
no)-2-methylbiphenyl (tBu2PbiPhMe) ligand. Aryl cyclometala-
tion was observed, yielding a four-membered iridacycle species,
consistent with DFT calculations.22 Previously, we reported the
synthesis of Rh(I) and Rh(III) complexes containing
dialkylbiarylphosphines.23 The rhodium complexes were not
X-ray quality red block crystals of 1 were obtained by vapor
diffusion of pentane into a concentrated CH2Cl2 solution and
the molecular structure of 1 was determined by X-ray
diffraction (Figure S1, Supporting Information). The coordi-
nation geometry about iridium is approximately square planar,
with coordination of both olefin bonds in COD, phosphorus,
and an η2-arene moiety. The last group is evidenced by close
contacts of the C1 and C6 aryl carbons with the iridium center
Received: March 30, 2013
Published: July 1, 2013
© 2013 American Chemical Society
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Note
(Ir−C1 = 2.49 Å and Ir−C6 = 2.44 Å). A similar η2 coordination
mode has been reported in related Pd complexes25,26 and was
observed in the analogous complex (COD)Rh(PCy2biPh)+.
Notably the aryl carbon distances to the metal are shorter for Ir
in comparison to Rh (2.53 and 2.60 Å).23 Longer Ir−Colefin
bond distances are observed for the olefin of COD that is trans
to phosphorus (2.24 and 2.23 Å) relative to those trans to the
η2-aryl (2.14 and 2.15 Å) due to the stronger trans influence of
phosphorus.
Two broad singlets (2H each) at 3.88 and 3.36 ppm are
observed for the olefinic hydrogens of the bound COD ligand.
The upfield shift of the doublet (2H) at 6.86 ppm (3JH−H = 5.0
Hz) in the aromatic region suggests arene coordination. The
upfield resonance at 119.7 ppm in the 13C NMR also indicates
that the arene moiety is coordinated η2 to Ir. The 31P NMR
spectrum of complex 1 exhibits a singlet at 33.7 ppm. In
contrast to the Rh analogue, there is no indication of dynamic
behavior of the COD moiety.23
Figure 1. ORTEP drawing of the cationic portion of complex 2. H
atoms have been omitted for clarity.
genation of a cyclohexyl ring in {H2RhPCy3}(closo-
CB11H6Br6),27 and Sabo-Etienne observed dehydrogenation
of the cyclohexyl group of PCy3 in Ru hydride complexes.28
Vrieze observed dehydrogenation of a cyclohexyl ring of PCy3
in an iridium complex.29 In complex 1, the bulky biphenyl
moiety positions a cyclohexyl ring close to the metal, with a
close contact between the iridium center and a C−H bond
(Ir1−H28B = 2.84 Å). This interaction may facilitate the
dehydrogenation reaction.
Thermolysis of Complex 1 in Aromatic Solvents. New
products are observed when complex 1 is heated in benzene or
toluene (eq 2). Complex 1 is sparingly soluble in nonpolar
Reaction of Cationic Complex 1 with H2. Reaction of 1
with H2 in CD2Cl2 at room temperature resulted in a rapid
color change from red to pale yellow. In the 1H NMR
spectrum, the signals for bound COD were no longer observed
and the formation of cyclooctane was seen (1.53 ppm). A
doublet at −14.1 ppm (2JP−H = 23 Hz) was observed, indicative
of a metal hydride complex. Removal of the solvent under
vacuum allowed isolation of complex 3 as an air-stable solid (eq
3).
aromatic solvents at room temperature but is soluble at higher
temperatures. Heating a solution of 1 in C6H6 for 5 h at 100 °C
leads to a color change from deep red to pale yellow. After the
volatiles were removed under vacuum and the residue was
1
redissolved in CD2Cl2, the H NMR spectrum exhibited a
singlet at 6.55 ppm (6H). This signal can be attributed to the
symmetric coordination of benzene to iridium. Two multiplets
were observed at 4.26 (1H) and 4.20 (1H) ppm, which were
assigned to the olefinic hydrogen atoms of the dehydrogenated
cyclohexyl group coordinated to the metal. No hydride signals
were visible, and a new singlet at 56.2 ppm was evident in the
31P NMR spectrum. The NMR data are consistent with
dehydrogenation of a cyclohexyl ring and trapping of an
unsaturated Ir center with benzene. Hydrogen obtained from
the cyclohexyl group was transferred to the COD ligand and
was released as COE. Formation of COE was clearly evident in
1
The dihydride complex 3 was fully characterized. In the H
NMR spectrum, three distinct signals at 6.93 (2H, t, 3JH−H = 6.0
Hz), 6.46 (1H, t, 3JH−H = 6.0 Hz), and 6.37 (2H, d, 3JH−H = 6.1
Hz) ppm, shifted upfield in the aromatic region, were observed,
while the remaining aromatic signals were downfield in the
range of 7.45−7.70 ppm. In the 13C NMR spectrum, three
resonances attributed to a coordinated arene were observed at
103.8 (s), 96.7 (s), and 86.8 (d, JP−C = 8.8 Hz) ppm. These
chemical shifts are consistent with those of other η6-arene
complexes.13 A singlet at 55.1 ppm was observed in the 31P
NMR spectrum.
1
the H NMR spectrum when the reaction was performed in
benzene-d6 and was also verified by GC-MS. A similar complex
with an η6 -bound toluene was observed when complex 1 was
heated to 100 °C in toluene. Full characterization of benzene
complex 2 and the corresponding toluene complex can be
found in the Supporting Information.
X-ray-quality crystals of 2 were grown at room temperature
from a benzene solution. The molecular structure of 2 was
determined by X-ray crystallography (Figure 1). η2 coordina-
tion of the alkene moiety is identified by close Ir1 to C41 and
C42 contacts (2.14 and 2.15 Å, respectively). The C41−C42
bond length (1.43 Å) is shortened relative to the other C−C
distances in the cyclohexene ring (1.49−1.54 Å). The other
ligands coordinated to the iridium center are the phosphine and
an η6-benzene. The pendent aromatic ring on the phosphine
ligand is uncoordinated. The Ir−Cbenzene bond lengths are in the
range of 2.25−2.32 Å.
The solid-state structure of compound 3 was determined by
X-ray crystallography (Figure 2). X-ray-quality crystals were
grown by vapor diffusion of pentane into a dichloromethane
solution of 3 at −21 °C. Two molecules of 3 were contained
within the unit cell with nearly identical structures. Complex 3
exhibits a three-legged piano-stool geometry, in which the arene
moiety of the phosphine biphenyl group is coordinated in an η6
fashion to Ir. A similar geometry was proposed for the
previously reported cationic Rh(III) dihydride analogue on the
basis of NMR spectroscopy.23 The observed geometry is also
Dehydrogenation of a cyclohexyl moiety has precedent with
PCy3-containing metal complexes. Weller observed dehydro-
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Note
H2 to complex 1 yields the Ir(III) dihydride complex 3 with
hydrogenation of COD. Complexes 1 and 3 catalyze COA
dehydrogenation to COE in the presence of an H2 acceptor. It
is likely that the arene moiety of 3 is strongly bound to the
metal center and is difficult to displace, impeding further
reactivity with H2.
EXPERIMENTAL SECTION
■
General procedures and materials are found in the Supporting
Information. (COD)Ir(PCy2biPh)Cl was previously reported.24 All 1H
NMR spectra were measured in CD2Cl2 at 25 °C (500 MHz).
[(COD)Ir(PCy2biPh)][B(ArF)4] (1). A flask was charged with
(COD)Ir(PCy2biPh)Cl (100 mg, 0.15 mmol) in air. NaB(ArF)4
(140 mg, 0.16 mmol) was added in a glovebox. The solids were
dissolved in CH2Cl2 (10 mL), generating a dark red solution with a
precipitate. The sample was stirred for 1 h and then filtered in air. The
solvent was removed, yielding 1 as a red powder (230 mg, 0.15 mmol,
Figure 2. ORTEP drawing of the cationic portion of complex 3.
Carbon-bound H atoms and solvent are omitted for clarity.
analogous to an Ir(I) η6-coordinated arene complex reported by
Werner.13 The arene ring of complex 3 remains nearly planar.
The Ir−Carene distances range from 2.20 to 2.31 Å, in the typical
range of η6-arene coordination to Ir.13,30 Additionally, the Ir−P
bond length has shortened to 2.24 Å in comparison to the Ir−P
bond length (2.31 Å) of 1, wherein the iridium is formally Ir(I).
The stronger Ir−P bond in 3 highlights the electrophilic nature
of the cationic Ir(III) complex.
Reactivity of Ir(III) Dihydride Complex 3. Thermal
Stability. The thermal stability of the iridium dihydride
complex 3 was evaluated in a variety of solvents. Heating a
sample of 3 at 60 °C in CD2Cl2 resulted in no change. Heating
complex 3 in C2D4Cl2 to at 130 °C resulted in slight
decomposition after 5 days. New aromatic resonances indicate
counteranion decomposition, but no other products were
identified.
1
100%). H NMR: δ 7.61 (t, JH−H = 6.9 Hz, 1H), 7.51 (m), 6.88 (m,
1H), 6.84 (d, 3JH−H = 7.1 Hz, 2H), 3.88 (br s, 2H, COD HC), 3.36
(br s, 2H, COD HC), 2.34 (br q, JH−H = 9.7 Hz), 2.02 (m), 1.87
(m), 1.75 (m), 1.58 (m), 1.33 (m), 1.18 (m). Anal. Calcd for
C64H55BF24IrP: C, 50.74; H, 3.66. Found: C, 50.04; H, 3.50.
Thermolysis of 1 in Benzene. In air, a J. Young style NMR tube
was charged with 6 mg of 1, and then benzene (420 mg) was added by
vacuum transfer. The dark red suspension was degassed and heated
(100 °C) overnight, affording a pale yellow solution. The solvent was
removed to yield a pale yellow powder. COE was detected by NMR
and GC-MS. Characteristic 1H NMR signals for 2: δ 8.46 (ddd, J = 18
Hz, J = 6 Hz, J = 3 Hz, 1H), 7.48−7.15 (Ar of biphenyl, 6H), 6.53 (s,
bound C6H6), 5.61 (COE), 4.25 (m, 1H, olefinic), 4.21 (m, 1H,
olefinic), 2.67 (m), 1.45 (COE), 2.13−0.64 (m, Cy).
[H2Ir(PCy2biPh)][B(ArF)4] (3). (COD)Ir(PCy2biPh)Cl (100 mg,
0.15 mmol) was added to a dry flask in air. Na(BArF)4 (150 mg, 0.17
mmol) was added in the drybox. The solids were dissolved in dry
CH2Cl2 (6.0 mL), yielding a red solution with a precipitate. H2 was
then bubbled through the sample for 15 min, giving a near-colorless
solution. The solvent was removed to yield a pale yellow powder,
which was dissolved in a minimal amount of CH2Cl2 and filtered in air
to give a pale yellow powder (210 mg, 0.15 mmol, 100%). 1H NMR: δ
Transfer Dehydrogenation. Complexes 1 and 3 were
screened for transfer dehydrogenation of cyclooctane with
TBE as a hydrogen acceptor (eq 4). At 155 °C, 13 turnovers
were achieved after 5 days with 0.2 mol % of complex 3 as a
catalyst. With complex 1, 22 turnovers were observed after 5
days at 155 °C.
3
7.70 (overlapping with ArF Ho, 1H), 7.64 (br t, JH−H = 7.4 Hz, 1H),
7.59 (overlapping with ArF Hp 1H), 7.47 (t, 3JH−H = 8.1 Hz, 1H), 6.93
(t, 3JH−H = 6.0 Hz, 2H, H2), 6.46 (t, 3JH−H = 6.0 Hz, 1H, H1), 6.37 (t,
3JH−H = 6.1 Hz, 2H, H3), 2.11 (m, 2H), 1.85 (br m, 4H), 1.76 (br m,
2H, Cy), 1.68 (br m, 2H), 1.48 (br m, 2H), 1.37−1.14 (m, 8H), 0.99
2
(m, 2H), −14.1 (d, 2H, JP−H = 23 Hz, H4). Anal. Calcd for
C56H45F24BPIr: C, 47.77; H, 3.22. Found: C, 47.71; H, 3.09.
Arene Hydrogenation. Heating a CD2Cl2 solution of 3 (7.4
mol % loading) in the presence of C6H6 resulted in only 5%
conversion to cyclohexane after 3 days at 60 °C. After 3 days
the sample darkened substantially to an orange-brown from
near colorless but no precipitate was observed. The major
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, tables, figures, and CIF files giving experimental details,
X-ray crystallographic data for complexes 1−3, and details of
crystallographic data collection. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
species observed by H NMR was the dihydride complex 3.
Heating a neat solution of anisole at 125 °C with 1.7 or 0.8 mol
% loading of 3 under 90 psi of H2 resulted in no change in the
1H NMR spectrum. Complex 3 is still present in solution after
AUTHOR INFORMATION
■
1
2.5 days at 125 °C, as identified by H NMR spectroscopy.
Corresponding Author
*E-mail for A.R.O.: oconnora@tcnj.edu.
Notes
Decomposition of complex 3 was observed after 1 day upon
raising the temperature to 145 °C. Arene coordination prevents
the generation of open coordination sites at the metal center.
The synthesis and characterization of Ir(I) and Ir(III)
complexes containing the ligand PCy2biPh have been
described. The structure of the cationic Ir(I) complex 1
includes an η2 interaction of a pendent phenyl ring.
Dehydrogenation of one of the cyclohexyl rings is observed
upon heating 1 in benzene to yield the new Ir(I) complex 2,
which exhibits a three-legged piano-stool geometry. Addition of
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF as part of the Center for
Enabling New Technologies through Catalysis (CENTC)
(CHE-0650456 and CHE-1205189). XRD data for 2 were
collected with support from NSF-MRI #0922931.
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