Site-selective alkyl dehydrogenation of a coordinated acylphosphine ligand

Article abstract of DOI:10.1021/om300471h

Regio- and stereoselective alkane dehydrogenation is a difficult challenge in organometallic chemistry. Intermolecular reactions of this type typically produce numerous olefin stereo- and regioisomers. Herein, we report our initial investigations into the intramolecular dehydrogenation of a datively bound alkyl ligand, demonstrating the first example of a site-selective dehydrogenation of an unactivated acyclic alkyl group. The alkyl group is located on an acylphosphine ligand that is coordinated to a Cp*IrCl₂ monomer. A mechanistic proposal, guided by the isolation of a dimeric iridium complex and supported by computational results, is also described.

Full text of DOI:10.1021/om300471h

Article
pubs.acs.org/Organometallics
Site-Selective Alkyl Dehydrogenation of a Coordinated
Acylphosphine Ligand
Sean M. Whittemore, Ryan J. Yoder, and James P. Stambuli*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States
S
* Supporting Information
ABSTRACT: Regio- and stereoselective alkane dehydrogen-
ation is a difficult challenge in organometallic chemistry.
Intermolecular reactions of this type typically produce
numerous olefin stereo- and regioisomers. Herein, we report
our initial investigations into the intramolecular dehydrogen-
ation of a datively bound alkyl ligand, demonstrating the first
example of a site-selective dehydrogenation of an unactivated
acyclic alkyl group. The alkyl group is located on an
acylphosphine ligand that is coordinated to a Cp*IrCl2
monomer. A mechanistic proposal, guided by the isolation of a dimeric iridium complex and supported by computational
results, is also described.
intramolecular alkyl dehydrogenations. The use of directing
groups to assist in intramolecular alkane dehydrogenations is
not a new concept.19 Numerous metal complexes have been
shown to activate a C−H bond on a coordinated ligand, and
this C−H activation event typically leads to a stable,
cyclometalated complex. Other C−H activation events that
do not stop at the cyclometalated complex are less prominent.
Results from a number of laboratories that have reported this
chemistry are shown in Chart 1.20−31
The examples in Chart 1 represent a range of complexes that
have undergone alkane or cycloalkane dehydrogenation of a
coordinated ligand. Most of these ligands are strongly basic and
sterically hindered. The steric bulk crowding the metal center
situates the C−H bond closer to the metal so activation of the
inert bond is more facile.32 More importantly, these examples
illustrate that the only reported intramolecular acyclic desaturations
are alkyl groups that do not have any potential selectivity issues, as
any C−H bond will provide the same complex. A single
intermolecular example by Arndtsen and Bergman reported
the selective dehydrogenation of pentane to 1-pentene.33
INTRODUCTION
■
Achieving selectivity in the preparation of organic molecules is
one of the many challenges facing synthetic chemists. To meet
the challenge of developing selective C−H activation reactions,
many groups have prepared exquisite metal complexes wherein
selectivity is controlled via the ancillary ligands on the metal
center.1−8 The genesis of this selectivity can also emanate from
substrate-controlled reactions, which are also mediated by a
metal complex. Much of this selectivity has revolved around
allylic9−11 or aromatic C−H bonds.12,13 Selectivity in reactions
of acyclic alkanes is more challenging to achieve because of the
lack of inherent functionality and the high temperatures
typically required to activate sp3 C−H bonds.14
Another challenge in organic chemistry is that of employing
renewable resource materials in the synthesis of platform
chemicals. Biomass provides excellent sources of carbohydrates,
lignin, and fatty acids; however, most research programs focus
on the conversion of carbohydrates or lignin to value-added
chemicals. Fatty acids are likely avoided because of their lack of
initial functionality and the difficulty to activate C−H bonds in
a selective fashion. We hypothesized that we could develop a
site-selective dehydrogenation of unactivated alkyl groups by
employing modified fatty acids that are able to covalently bind
to the metal center and allow the formation of a single alkene.
Not only would a successful reaction prove that the site-
selective dehydrogenation of an unactivated acyclic alkane is
possible, but it would also promote the use of renewable
resource materials in chemical synthesis.
RESULTS AND DISCUSSION
■
Rather than employ highly basic ligands, the lesser known
acylphosphines were employed as ligands.34 Initially, we
thought to employ less basic ligands so that the coordinated
ligand and olefin would be easier to exchange with an additional
ligand and, hence, allow the possibility for catalyst turnover. We
were intrigued by the limited employment of these types of
ligands in transition metal catalysis, as well as the ability to
directly incorporate fatty acids as starting materials to prepare
the ligand. Ligand preparation was facile, starting from the
Most dehydrogenation studies involving alkanes focus on
intermolecular processes. Despite the development of a
multitude of efficient catalysts,15−18 there are no reports of a
truly site-selective system that does not produce isomeric
products. Rather than focus on the intermolecular reaction, we
explored employing directing groups to attain selectivity in
Received: May 29, 2012
Published: August 15, 2012
© 2012 American Chemical Society
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Chart 1. Previous Examples of Intramolecular Alkane Dehydrogenation Reactions
corresponding saturated carboxylic acids (1−5) (Scheme 1).
(silver hexafluorophosphate) at 45 °C in CH2Cl2, giving a
complex spectrum containing a small iridium hydride
resonance. The reaction of dicyclohexylphosphine complex 7
at 45 °C with AgPF6 in CH2Cl2 gave a single C−H activation
product in which the cyclohexyl ring on the phosphine was
dehydrogenated. Two-dimensional 1H NMR spectroscopic
experiments did not absolutely verify the position of the
double bond in the ring. To confirm C−H activation occurred
on the cyclohexyl ring and not on the alkyl chain, we prepared a
deuterated acylphosphine-ligated iridium complex of 2 and
submitted this complex to the same reaction conditions. The
Because of the lack of basicity in the phosphine, the electron-
Scheme 1. Preparation of Acylphosphines and
Corresponding Complexes
1
hydride signal was still observed in the H NMR spectrum
(−16.4 ppm, d, J1H‑31P = 32.4 Hz, 1H), confirming that a C−H
bond on the cyclohexyl ring of the phosphine was activated.
This type of C−H activation reaction on the tricyclohex-
ylphosphine ligand has been previously reported (Chart
1).22,25,26,28,31,33 When the acylphosphine complex containing
isopropyl groups (8) was reacted with AgPF6 at 45 °C, complex
rich Cp* (1,2,3,4,5-pentamethylcyclopentadienyl) ligand was
chosen to provide electron density to the complex. Reaction of
the corresponding acylphosphine with [Cp*IrCl2]2 provided
the desired complexes (6−10) in excellent yields.
1
12 was formed in 89% yield, as determined by H NMR
spectroscopy. Submitting the fatty acid acylphosphine complex
The reactivity of these iridium complexes was then
investigated (Scheme 2). Complex 6 reacted with AgPF6
9 to the same reaction conditions similarly gave a single
1
complex (13) in 84% yield. In the H NMR spectra of 12 and
Scheme 2. Reactivity of Ir-Acylphosphine Complexes
13, resonances displaying a single coordinated alkene, as well as
an iridium hydride, were observed. This result differs from the
only other site-selective example by Bergman,33 because we
form an internal olefin, which are less favorable in alkane
dehydrogenation reactions than terminal C−H bonds. Interest-
ingly, we have previously reported that the di-tert-butylacyl-
phosphine complex (10) provided the novel anionic complex
14 in the presence of AgOTf at ambient temperature.35
To confirm the assignment of complex 12, an X-ray quality
crystal was grown by layering a CH2Cl2 solution of the isolated
complex with diethyl ether, followed by cooling to −30 °C. An
ORTEP diagram of the obtained X-ray structure of the
unsaturated ligand complex 12 is shown in Figure 1. The
C19−C20 bond distance was found to be 1.402(9) Å, indicating
a CC double bond. The hydride ligand was not located by
the X-ray analysis; however, a signal in the hydride region of the
1H NMR spectrum displayed a doublet at δ −17.2. This
1
resonance became a singlet upon 31P decoupling of the H
NMR spectrum. An iridium hydride stretch was also observed
in the IR spectrum at 2182 cm−1. Moreover, the sum of the
bond angles around the iridium center (352°) indicated
distorted tetrahedric coordination, as opposed to a trigonal
planar geometry, and thus provided additional evidence for the
presence of an iridium hydride.27 Using density functional
theory (see Supporting Information), computational studies
have shown that the isolated olefin complex 12 is the most
thermodynamically stable of the four possible alkene products,
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During the final stages of the reaction, the resonance at 56.7
ppm disappears, and the product (12) at 72.3 ppm became the
only major signal remaining in the 31P spectrum. Throughout
the course of the reaction, additional resonances from 10 to
−30 ppm were observed due to hydrolysis of the hexafluor-
ophosphate anion. The major hydrolysis product formed in the
crude reaction was PO2F2, which was confirmed by resonances
in the 19F and 31P NMR spectra: −80.6 ppm (d, J19F‑31P= 976
Hz) and −15.9 ppm (t, J19F‑31P = 976 Hz), respectively. The
hydrolysis of the hexafluorophosphate anion was previously
reported to occur in other late transition metal complexes.36,37
The conversion of complex 8 to 12 was also monitored by
1H NMR spectroscopy (Figure 3). These experiments revealed
that an iridium hydride species formed prior to formation of the
olefin product 12. While isolation and full characterization of
this species has remained elusive, we believe that this hydride
may be associated with the unknown signal observed at 56.7
ppm in the 31P NMR spectrum. Currently, we speculate that
the signal at 56.7 ppm corresponds to the iridacycle formed
after the initial C−H activation (19, Scheme 4, vide infra).
Although the isolation of the product that gives rise to the
resonance at 56.7 ppm in the 31P NMR spectrum is still
ongoing, we were able to isolate and crystallize the species
observed at 50.9 ppm in the 31P NMR spectrum during the
reaction between complex 8 and AgPF6 in CH2Cl2 at 45 °C.
This complex was determined to be the unusual dimeric
iridium complex 15, which is bridged by the carbonyl of the
acylphosphine ligand (Ir−OC bond length is 2.148(4) Å).
This structural determination arose from X-ray crystallographic
analysis of a single crystal of the isolated complex (Figure 4).
Interestingly, some batches of this isolated dimeric complex
contained trace amounts of another unknown complex that
resonates at 61.7 ppm in the 31P NMR spectrum. Cooling the
bridged carbonyl dimer 15 to −80 °C did not favor one
resonance over the other (50.9 vs 61.7 ppm), which likely
eliminates the two signals from being conformers.
Figure 1. ORTEP diagram of olefin complex 12 with atoms drawn at
50% probability. The hydrogen atoms and the counteranion were
omitted for clarity. Selected bond distances and angles: C19−C20
1.402(9) Å, Ir−P = 2.2570(16) Å, Ir−C19 = 2.203(6) Å, Ir−C20
2.184(6) Å, CO = 1.213(8) Å, C18−C19−C20 = 122.0(6)°.
=
=
which could arise from β-hydride elimination of a five- or six-
membered iridalactone. The cis-Δ3,4 alkene product is more
stable than the corresponding trans-alkene by 3.1 kcal/mol,
while also being more stable than the cis- or trans-Δ4,5 alkene by
4.5 to 5.5 kcal/mol.
To better understand the transformation of complex 8 to the
dehydrogenation product 12, closer inspection of the reaction
was undertaken by observing changes in the acylphosphine
resonance by 31P NMR spectroscopy (Figure 2). During the
initial stages of the reaction, two species were formed with 31P
resonances at 50.9 and 61.7 ppm, with the 50.9 ppm resonance
being the dominant species. These two species lessened in
intensity as two new resonances at 56.7 and 72.3 ppm began to
appear. The signal at 72.3 ppm belongs to the monounsatu-
rated olefin complex (12). After nearly two hours, the
resonances at 50.9 and 61.7 ppm were no longer present,
and the major species observed were at 56.7 and 72.3 ppm.
The reactivity of the dimeric complex 15 was examined
further (Scheme 3). One equivalent of the diisopropyl
acylphosphine ligand 3 was added to a solution of dimer 15,
and the mixture was heated to 45 °C. After two hours at this
temperature, a 2.5:1 ratio of the unknown complex (18) (61.7
ppm) and the starting dimer (15) was measured using 31P
NMR spectroscopy. This ratio remained unchanged after 4 h at
45 °C. Interestingly, no olefin product (12) was observed in the
presence of the added ligand 3. The addition of ligand provided
some evidence that the unknown species contains two
Figure 2. 31P NMR spectra of the reaction of diisopropyl acylphosphine complex 8 and AgPF6 in CH2Cl2 at 45 °C.
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1
Figure 3. H NMR spectra of the reaction of diisopropyl acylphosphine complex 8 and AgPF6 in CH2Cl2 at 45 °C.
coordinated acylphosphine ligand. To investigate this, we
prepared the analogous complex bearing a hexyl diisopropyl-
phosphine ligand (i.e., no acyl unit for coordination to Ir).
When the trialkylphosphine-ligated metal complex was
submitted to the reaction conditions, a 1:1 mixture of olefin
complexes was obtained. We surmise that the lack of selectivity
may arise from the inability of the ligand to form a rigid
iridacycle.
Scheme 3. Reactivity of Carbonyl Dimer Complex 15
The proposed reaction mechanism is described in Scheme 4.
The reaction pathways were proposed on the basis of the
observations of isolated complexes and from previously
mentioned computational data regarding the fate of the isolated
dimeric species 15. The addition of two equivalents of AgPF6 to
complex 8 presumably gives the unsaturated cationic iridium
(III) complex 16. Complex 16 reacts with starting material 8 to
form the dimeric complex 17, which can dissociate phosphine
and form the bridged carbonyl dimer 15. We have previously
observed that the abstraction of the first chloride of a similar
acylphosphine ligands in a dimeric form or that 15 may be
cleaved by additional ligand into two equivalents of the
monomeric iridium complex [(Cp*Ir(Cl)(3)] (18, as shown).
Once again, using density functional theory, the formation of
the monomeric complex from the carbonyl dimer was
calculated to be enthalpically favorable. Heating the dimer 15,
without additional ligand in CH2Cl2 at 45 °C for 4 h, provided
a 72% yield of the monounsaturated olefin complex 12.
The exquisite selectivity observed in this reaction is most
likely due to the geometric constraints afforded by the
Scheme 4. Proposed Reaction Mechanism of 8 to 12
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reported literature procedure.34 The X-ray crsystal structure and
elemental analysis for 12 were obtained for the triflate analogue. The
PF6 anion was quantitatively exchanged by stirring 12 in CH2Cl2 with
three equivalents of NaOTf. This was necessary to obtain X-ray quality
crystals. All solvents were dried over CaH2 or activated alumina
columns. All other commercially obtained reagents were used as
received. 1H and 13C NMR spectra were recorded on Bruker
spectrometers and are reported relative to TMS. 31P NMR spectra
were recorded on Bruker spectrometers and are reported relative to
H3PO4. 19F NMR spectra were recorded on Bruker spectrometers and
are reported relative to trifluoromethyl benzene. High-resolution mass
spectra were obtained from the mass spectrometry facility at The Ohio
State University. IR spectra were recorded on a Perkin-Elmer 1600
FT-IR spectrometer.
1-(Diphenylphosphino)heptan-1-one (1). Heptanoyl chloride
(160 μL, 1.03 mmol) was added dropwise to a stirring solution of
diphenylphosphine (200 mg, 1.07 mmol) and NEt3 (160 μL, 1.15
mmol) in Et2O. After stirring for 2 h at ambient temperature the
reaction mixture was filtered over a pad of Celite. The filtrate was
1
evaporated to dryness to yield 1 as a yellow oil (300 mg, 97%). H
NMR (C6D6, 400 MHz): δ 7.52 (m, 4H), 7.05 (m, 6H), 2.42 (m, 2H),
1.53 (m, 2H), 1.11 (m, 2H), 1.03 (m, 4H), 0.78 (m, 3H). 13C{1H}
NMR (CD2Cl2, 400 MHz): δ 223.7 (d, J13C‑31P = 40.5 Hz), 135.2 (d,
J13C‑31P = 18.5 Hz), 133.1 (d, J13C‑31P = 7.4 Hz), 129.9, 129.1 (d, J13C‑31P
= 8.1 Hz), 45.8 (d, J13C‑31P = 35.3 Hz), 31.9, 29.1, 24.6 (d, J13C‑31P = 4.1
Hz), 22.8, 14.2. 31P NMR (C6D6, 160 MHz): δ 13.2. IR (cm−1): 1689
(CO). HRMS(ESI): calcd for [M + H]+ 299.1559, found 299.1566.
1-(Dicyclohexylphosphino)heptan-1-one (2). Heptanoyl chlor-
ide (300 μL, 1.93 mmol) was added dropwise to a stirring solution of
dicyclohexylphosphine (384 mg, 1.93 mmol) and NEt3 (300 μL, 2.15
mmol) in Et2O. After stirring for 2 h at ambient temperature the
reaction mixture was filtered over a pad of Celite. The filtrate was
Figure 4. ORTEP diagram of bridged carbonyl dimer 15 with atoms
drawn at 30% probability. The hydrogen atoms and the counteranions
were omitted for clarity. Selected bond distances: Ir1b−Cl1b
=
2.4312(14) Å, Ir1b−Cl2b = 2.4422(14) Å, Ir1b−P1b = 2.3430(14) Å,
Ir2b−Cl1b = 2.4209(14) Å, Ir2b−Cl2b = 2.4015(15) Å, Ir2b−O1b
2.148(4) Å, CO = 1.231(7) Å.
=
iridium complex is slower than the second chloride abstraction,
hence the remaining starting material 8. Dimer 15 then
collapses into the monomeric three-coordinate iridium species
(18), and the reaction pathway for this transformation was
calculated to be exothermic by 48.4 kcal/mol (Supporting
Information). Complex 18 can then form the iridacycle 19 via
C−H activation of the alkyl chain. Hydrolysis of hexafluor-
ophosphate to difluorophosphate, followed by elimination of
difluorophosphoric acid, and regioselective β-hydride elimi-
nation of the alkyl chain would give the olefin complex 12. The
thermodynamics of the reaction mechanism have also been
studied computationally, and details are provided in the
Supporting Information. The hydrolysis of one of the PF6
anions was followed by 31P and 19F NMR spectroscopy, which
displayed the proposed intermediates to this step. The PF6
hydrolysis is not believed to be a thermodynamic driving force
for the overall transformation because the initial hydrolysis step
was calculated to be largely endothermic (Supporting
Information).
1
evaporated to dryness to yield 2 as a yellow oil (553 mg, 93%). H
NMR (CD2Cl2, 400 MHz): δ 2.58 (m, 2H), 1.95 (m, 2H), 1.71 (m,
10H), 1.55 (m, 2H), 1.27 (m, 10H), 1.23 (m, 6H), 0.88 (t, J = 6.8 Hz,
3H). 13C{1H} NMR (CD2Cl2, 400 MHz): δ 227.2 (d, J13C‑31P = 46.0
Hz), 48.2 (d, J13C‑31P = 34.2 Hz), 32.8, 32.7, 31.9, 31.6, 31.5, 30.3, 30.2,
29.2, 27.8, 27.7, 27.6, 27.5, 26.6, 24.4 (d, J13C‑31P = 4.4 Hz), 22.8, 14.1.
31P NMR (CD2Cl2, 160 MHz): δ 30.6. IR (cm−1): 1643 (CO).
HRMS(ESI): calcd for [M + H]+ 311.2498, found 311.2522.
1-(Diisopropylphosphino)heptan-1-one (3). Heptanoyl chlor-
ide (300 μL, 1.93 mmol) was added dropwise to a stirring solution of
diisopropylphosphine (229 mg, 1.93 mmol) and NEt3 (300 μL, 2.15
mmol) in Et2O. After stirring for 2 h at ambient temperature the
reaction mixture was filtered over a pad of Celite. The filtrate was
1
evaporated to dryness to yield 3 as a yellow oil (423 mg, 95%). H
NMR (CD2Cl2, 400 MHz): δ 2.58 (m, 2H), 2.14 (m, 2H), 1.55 (q, J =
7.2 Hz, 2H), 1.27 (m, 6H), 1.12 (m, 12H), 0.87 (t, J = 6.8 Hz, 3H).
13C{1H} NMR (CD2Cl2, 100 MHz): δ 227.1 (d, J13C‑31P = 45.9 Hz),
48.2 (d, J13C‑31P = 34.0 Hz), 31.9, 29.2, 24.3 (d, J13C‑31P = 4.3 Hz), 22.8,
22.7, 20.9, 20.8, 19.9, 19.8, 14.1. 31P NMR (CD2Cl2, 160 MHz): δ
38.5. IR (cm−1): 1685 (CO). HRMS(ESI): calcd for [M + H]+
231.1872, found 231.1888.
1-(Diisopropylphosphino)hexadecan-1-one (4). Palmitic acid
(260 mg, 1.01 mmol) was refluxed in thionyl chloride (10 mL) for 1 h.
All the thionyl chloride was then removed in vacuo, and the remaining
oil was added dropwise to a stirring solution of diisopropylphosphine
(229 mg, 1.93 mmol) and NEt3 (300 μL, 2.15 mmol) in Et2O. After
stirring for 2 h at ambient temperature the reaction mixture was
filtered over a pad of Celite. The filtrate was evaporated to dryness to
yield 4 as a yellow oil (342 mg, 95%). 1H NMR (CD2Cl2, 400 MHz):
δ 2.58 (m, 2H), 2.14 (m, 2H), 1.56 (m, 2H), 1.26 (m, 26H), 1.15 (m,
12H), 0.88 (m, 3H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 227.0 (d,
J13C‑31P = 46.2 Hz), 48.3 (d, J13C‑31P = 38.0 Hz), 32.5, 30.3, 30.2, 30.1,
30.0, 29.9, 29.9, 29.8, 24.5 (d, J13C‑31P = 4.5 Hz), 23.2, 23.0, 22.9, 21.2,
21.0, 20.1, 20.0, 14.4. 31P NMR (CD2Cl2, 160 MHz): δ 38.9. IR
(cm−1): 1666 (CO). HRMS(ESI): calcd for [M + Na]+ 379.3100,
found 379.3067.
CONCLUSION
■
In summary, we have demonstrated that the mild, site-selective
dehydrogenation of an unactivated alkane is possible with the
use of a directing group and occurs in high yields as a single
diastereomeric olefin complex. An interesting dimeric iridium
complex bridged by a carbonyl group from the acylphosphine
ligand was isolated and shown to be a chemically and kinetically
competent intermediate in this reaction. A reaction mechanism
was proposed using H and 31P NMR spectroscopy in concert
1
with computationally calculated reaction pathways. Efforts to
further elucidate the reaction mechanism, as well as develop a
catalytic variant of this process, are ongoing.
EXPERIMENTAL SECTION
■
General Methods. All manipulations were carried out in an argon
atmosphere using standard Schlenk and drybox techniques. The acyl
phosphine ligands were prepared in ∼95% purity using the previously
Hexyldiisopropylphosphine. A Schlenk flask containing chlor-
odiisopropylphosphine (2.0 g, 13.1 mmol) and Et2O (15 mL) was
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cooled to −78 °C. An ethereal solution of hexylmagnesium bromide
(13.5 mL, 26.3 mmol) cooled to −78 °C was added to the Schlenk
flask dropwise with stirring over 20 min via cannula while maintaining
the temperature. The mixture was then allowed to warm slowly to
ambient temperature over 18 h. Degassed water (∼10 mL) was added
slowly to quench any remaining hexylmagnesium bromide. The
ethereal layer was then transferred via cannula to a Schlenk flask
containing sodium sulfate and equipped with a sintered glass filter and
second Schlenk flask. The sodium sulfate was filtered, and the solids
were washed with additional Et2O. The Et2O was then removed in
vacuo, and the crude product was transferred via cannula to a
distillation setup. The product was distilled under vacuum (45 °C, 0.5
mmHg) to yield the title compound as a colorless oil (1.07 g, 40%).
1H NMR (C6D6, 400 MHz): δ 1.59 (m, 2H), 1.52 (m, 2H), 1.38 (m,
2H), 1.28 (m, 6H), 1.08 (d, J = 7.1 Hz, 3H), 1.05 (d, J = 6.6 Hz, 3H),
1.03 (d, J = 6.6 Hz), 1.01 (d, J = 7.1 Hz), 0.89 (m, 3H). 13C{1H} NMR
(C6D6, 125 MHz): δ 32.0, 31.7 (d, J = 11.6 Hz), 28.7 (d, J13C‑31P = 18.8
Hz), 23.9, 23.8, 23.1, 22.3 (d, J13C‑31P = 19 Hz), 20.5, 20.34, 19.1, 19.0,
14.3. 31P NMR (CD2Cl2, 160 MHz): δ 3.08. HRMS(ESI): calcd for
[M + H]+ 203.1923, found 203.1930.
[Cp*Ir(Ph2P{heptanoyl})Cl2] (6). To a stirring suspension of
[Cp*IrCl2]2 (200 mg, 0.251 mmol) in THF (5 mL) was added 1 (160
mg, 0.536 mmol). The mixture was stirred at ambient temperature for
2 h, and the solvent was then removed under vacuum. The residue was
dissolved in Et2O and cooled to −30 °C to yield 6 as an orange
crystalline solid (315 mg, 90%). 1H NMR (CD2Cl2, 400 MHz): δ 7.60
(t, J = 8.6 Hz, 4H), 7.49 (m, 6H), 2.84 (m, 2H), 1.39 (m, 2H), 1.36 (d,
J1H‑31P = 2.2 Hz, 15H), 1.17 (m, 2H), 1.09 (m, 4H), 0.79 (t, J = 7.2 Hz,
3H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 213.3 (d, J13C‑31P = 14.9
Hz), 135.5 (d, J13C‑31P = 8.9 Hz), 131.5 (d, J13C‑31P = 2.0 Hz), 128.6 (d,
J13C‑31P = 11 Hz), 128.1, (d, J13C‑31P = 49.0 Hz), 93.3 (d, J13C‑31P = 2.0
Hz), 46.6 (d, J13C‑31P = 39.3 Hz), 31.9, 28.9, 23.6 (d, J13C‑31P = 2 Hz),
22.2, 14.1, 8.4. 31P NMR (CD2Cl2, 160 MHz): δ 23.1. IR (cm−1) 1699
(CO). HRMS(ESI): calcd for [M − Cl]+ 661.1965, found 661.1940.
Anal. Calcd for C29H38Cl2IrOP: C, 49.99; H, 5.50. Found: C, 49.69; H,
5.29.
washed with cold pentane to yield 9 as an orange powder (284 mg,
99%). H NMR (CD2Cl2, 400 MHz): δ 3.01 (m, 2H), 2.95 (m, 2H),
1
1.57 (m, 2H), 1.48 (d, J1H‑31P = 1.6 Hz, 15H), 1.47 (m, 6H), 1.36 (d, J
= 7.2 Hz, 3H), 1.33 (d, J = Hz, 3 H), 1.26 (m, 24H), 0.87 (t, J = 6.8
Hz, 3H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 216.3 (d, J13C‑31P
=
12.7 Hz), 91.9 (d, J13C‑31P = 2.4 Hz), 44.5 (d, J13C‑31P = 31.6 Hz), 32.0,
29.6, 29.5, 29.5, 29.4, 29.3, 29.0, 26.1 (d, J13C‑31P = 24 Hz), 26.0, 22.6.
22.4, 21.3, 21.2, 19.7, 13.8, 8.0. 31P NMR (CD2Cl2, 160 MHz): δ 24.6.
IR (cm−1): 1670 (CO). HRMS(ESI): calcd for [M − 2Cl + H]+
685.4085, found 685.4046. Anal. Calcd for C32H60Cl2IrOP: C, 50.91;
H, 8.01. Found: C, 51.02; H, 7.86.
[Cp*Ir(iPr2P{hexyl})Cl2]. To a stirring suspension of [Cp*IrCl2]2
(200 mg, 0.25 mmol) in THF (5 mL) was added hexyldiisopropyl-
phosphine (110 mg, 0.54 mmol). The mixture was stirred at ambient
temperature for 2 h, and the solvent was then removed under vacuum.
The residue was washed with cold pentane to yield the title compound
1
as an orange powder (260 mg, 86%). H NMR (C6D6, 400 MHz): δ
2.47 (m, 2H), 2.40 (m, 2H), 1.65 (m, 2H), 1.35 (d, J1H‑31P = 1.6 Hz,
15H), 1.27 (m, 6H), 1.20 (d, J = 7.1 Hz, 3H), 1.17 (d, J = 7.3 Hz, 3H),
1.15 (d, J = 7.4 Hz, 3H), 1.11 (d, J = 7.2 Hz, 3H), 0.87 (t, J = 6.9 Hz,
3H). 13C{1H} NMR (C6D6, 100 MHz): δ 92.2 (d, J13C‑31P = 2.4 Hz),
32.0 (d, J13C‑31P = 12.5 Hz), 25.9 (d, J13C‑31P = 35.5 Hz), 25.3, 25.2,
23.1, 20.7 (d, J13C‑31P = 37.4 Hz), 19.5, 19.4, 19.2, 14.3, 9.4. 31P NMR
(CD2Cl2, 160 MHz): δ 3.08. HRMS(ESI): calcd for [M − 2Cl + H]+
531.2727, found 531.2725. Anal. Calcd for C22H42Cl2IrP: C, 43.99; H,
7.05. Found: C, 44.00; H, 6.95.
[Cp*Ir(iPr2P{(Z)-1-(diisopropylphosphino)hept-3-en-1-one})-
H]PF6 (12). Silver hexafluorophosphate (42 mg, 0.17 mmol) and 8
(50 mg, 0.78 mmol) were combined in CH2Cl2 (1 mL) and stirred at
40 °C for 4 h. The mixture was then cooled to ambient temperature
and filtered through a pad of Celite. The resulting solution was
concentrated to ∼0.5 mL, and pentane was added to precipitate a
white powder, which was collected by vacuum filtration and dried in
vacuo to yield 12 (36 mg, 66%). 1H NMR (CDCl3, 400 MHz): δ 3.42
(t, J = 8.4 Hz, 1H, CHCH), 3.12 (m, 1H, CHCH), 2.85 (dd, J =
19.6 Hz, J = 8.8 Hz, 1H, αCH2), 2.46 (q, J = 6.8 Hz, 2H, iPrCH), 2.05
(s, 15H), 2.05 (m, 1H, αCH2), 1.80 (m, 1H, δCH2), 1.48 (m, 2H),
1.23 (m, 6H), 1.00 (m, 3H), 0.99 (m, 3H), 0.90 (m, 3H), 0.58 (m,
3H), −17.21, (d, J1H‑31P = 32 Hz, 1H). 13C{1H} NMR (CDCl3, 100
MHz): δ 225.7 (d, J13C‑31P = 16.8 Hz), 101.2, 58.5 (CHCH), 42.5
(CHCH), 40.4 (d, J13C‑31P = 48.3 Hz, αCH2), 36.5, 24.7, 23.4 (d,
J13C‑31P = 21.2 Hz), 22.6 (d, J13C‑31P = 35.7 Hz), 18.7, 18.1, 16.6, 16.5,
13.8, 9.9. 31P NMR (CDCl3, 160 MHz): δ 72.3 (d, J31P‑1H = 2.9 Hz),
143.8 (sept, J31P‑19F = 713 Hz). 19F NMR (CDCl3, 376 MHz): δ −73.3
(d, J19F‑31P = 711 Hz). IR (cm−1): 2182 (Ir−H), 1700 (CO).
HRMS(ESI): calcd for [M − 2Cl + H]+ 557.2520, found 557.2527.
Anal. Calcd for C24H41F3IrO4PS: C, 40.84; H, 5.85. Found: C, 40.65;
H, 5.65.
[Cp*Ir(Cy2P{heptanoyl})Cl2] (7). To a stirring suspension of
[Cp*IrCl2]2 (100 mg, 0.125 mmol) in THF (5 mL) was added 2 (82
mg, 0.264 mmol). The mixture was stirred at ambient temperature for
2 h, and the solvent was then removed under vacuum. The residue was
washed with cold pentane to yield 7 as an orange powder (147 mg,
1
83%). H NMR (CD2Cl2, 400 MHz): δ 2.90 (m, 2H), 2.64 (m, 4H),
1.82 (m, 6H), 1.72 (m, 2H), 1.65 (m, 2H), 1.53 (m, 2H), 1.47 (d,
J1H‑31P = 1.6 Hz, 15H), 1.35 (m, 14H), 0.88 (m, 3H). 13C{1H} NMR
(CD2Cl2, 100 MHz): δ 216.9 (d, J13C‑31P = 12.8 Hz), 92.2 (d, J13C‑31P
=
2.1 Hz), 44.7 (d, J13C‑31P = 31.4 Hz), 37.8, 37.6, 32.0, 31.9, 29.9, 29.1,
28.5, 28.4, 28.3, 28.2, 26.7, 22.8 (d, J13C‑31P = 5.5 Hz), 14.2, 8.5. 31P
NMR (CD2Cl2, 160 MHz): δ 18.3. IR (cm−1): 1670 (CO).
HRMS(ESI): calcd for [M − 2Cl + H]+ 639.3303, found 639.3301.
Anal. Calcd for C29H50Cl2IrOP: C, 49.14; H, 7.11. Found: C, 49.27; H,
6.97.
[Cp*Ir(iPr2P{(Z)-1-(diisopropylphosphino)hexadec-3-en-1-
one})H]PF6 (13). Silver hexafluorophosphate (75 mg, 0.29 mmol) and
9 (75 mg, 0.099 mmol) were combined in CH2Cl2 (1 mL) and stirred
at 40 °C for 8 h. The mixture was then cooled to ambient temperature
and filtered through a pad of Celite. The resulting solution was
concentrated to ∼0.5 mL, and pentane was added to precipitate a
white powder, which was collected by vacuum filtration and dried in
vacuo to yield 13 (50 mg, 61%). 1H NMR (CDCl3, 400 MHz): δ 3.41
(t, J = 8.4 Hz, 1H, CHCH), 3.10 (m, 1H, CHCH), 2.85 (dd, J =
19.4 Hz, J = 8.8 Hz, 1H, αCH2), 2.46 (q, J = 6.8 Hz, 2H, iPrCH), 2.06
(s, 15H), 2.06 (m, 1H, αCH2), 1.79 (m, 1H, δCH2), 1.43 (m, 2H),
1.25 (m, 24H, CH2 + CH3), 1.00 (dd, J1H‑31P = 17.2 Hz, J = 6.8 Hz,
3H), 0.88 (m, 3H), 0.90 (m, 3H), 0.58 (dd, J1H‑31P = 18.4 Hz, J = 6.4
Hz), 3H), −17.2 (d, J1H‑31P = 32 Hz). 13C{1H} NMR (CDCl3, 100
MHz): δ 225.6 (d, J13C‑31P = 17.2 Hz), 101.2, 58.6 (CHCH), 42.4
(CHCH), 40.4 (d, J13C‑31P = 47.6 Hz, αCH2), 34.5, 32.0, 31.4, 29.7,
29.6, 29.6, 29.5, 29.4, 29.3, 23.4 (d, J13C‑31P = 21.3 Hz), 22.8, 22.5 (d,
J13C‑31P = 35.4 Hz), 18.7, 18.2, 16.6, 16.5, 14.2, 9.9. 31P NMR (CD2Cl2,
160 MHz): δ 72.4 (d, J31P‑1H = 5.3 Hz), −143.8 (sept, J31P‑19F = 712
Hz). 19F NMR (CDCl3, 376 MHz): δ −73.3 (d, J19F‑31P = 712 Hz). IR
(cm−1): 2188 (Ir−H), 1700 (CO). HRMS(ESI): calcd for [M −
PF6]+ 683.3929, found 683.3924.
[Cp*Ir(iPr2P{heptanoyl})Cl2] (8). To a stirring suspension of
[Cp*IrCl2]2 (825 mg, 1.04 mmol) in THF (15 mL) was added 3 (500
mg, 2.17 mmol). The mixture was stirred at ambient temperature for 2
h, and the solvent was then removed under vacuum. The residue was
washed with cold pentane to yield 8 as an orange powder (1.29 g,
1
i
97%). H NMR (C6D6, 400 MHz): δ 3.14 (m, 2H, PrH), 3.05 (m,
2H, αCH2), 1.67 (quint, J = 7.2 Hz, 2H), 1.56 (d, J = 7.2 Hz, 3H), 1.52
(d, J = 7.2 Hz, 3H), 1.27 (m, 4H), 1.23 (d, J1H‑31P = 1.6 Hz, 15H), 1.19
(d, J = 7.2 Hz, 3H), 1.16 (d, J = 7.2 Hz, 3H), 0.87 (t, J = 6.8 Hz, 3H).
13C{1H} NMR (CD2Cl2, 100 MHz): δ 216.8 (d, J13C‑31P = 12.7 Hz)
92.3 (d, J13C‑31P = 2 Hz), 44.9 (d, J13C‑31P = 13.2 Hz, αCH2), 32.0, 29.0,
26.5 (d, J13C‑31P = 24 Hz), 22.8, 21.7, 21.6, 20.0, 14.1, 8.4. 31P NMR
(CD2Cl2, 160 MHz): δ 25.8. IR (cm−1): 1670 (CO). HRMS(ESI):
calcd for [M − 2Cl + H]+ 559.2676, found 559.2658. Anal. Calcd for
C23H42Cl2IrOP: C, 43.94; H, 6.73. Found: C, 44.01; H, 6.70.
[Cp*Ir(iPr2P{palmitoyl})Cl2] (9). To a stirring suspension of
[Cp*IrCl2]2 (150 mg, 0.188 mmol) in THF (5 mL) was added 4 (140
mg, 0.393 mmol). The mixture was stirred at ambient temperature for
2 h, and the solvent was then removed under vacuum. The residue was
6129
dx.doi.org/10.1021/om300471h | Organometallics 2012, 31, 6124−6130

Organometallics
Article
[(Cp*IrCl)2(iPr2P{heptanoyl)](PF6)2 (15). Silver hexafluorophos-
phate (85 mg, 0.336 mmol) and 8 (100 mg, 0.155 mmol) were
combined in CH2Cl2 (1 mL) and stirred at ambient temperature for
10 min. The mixture was then filtered through a pad of Celite. The
resulting solution was concentrated to ∼0.5 mL, layered with Et2O,
and cooled to −30 °C to yield 15 as an orange crystalline solid (83 mg,
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85%). 1H NMR (CD2Cl2, 500 MHz): δ 3.11 (td, J = 7.0 Hz, J1H‑31P
=
3.5 Hz, 2H, αCH2), 2.94 (m, 2H, iPrH), 1.82 (m, 2H), 1.62 (s, 15H),
1.60 (d, J1H‑31P = 2 Hz, 15H), 1.40 (m, 18H, CH2 + CH3), 0.93 (m,
3H). 13C{1H} NMR (CD2Cl2, 125 MHz): δ 236.1 (d, J13C‑31P = 7.8
Hz), 97.3, 90.5, 50.6 (d, J13C‑31P = 12.8 Hz, αCH2), 31.5, 30.2 (d,
J13C‑31P = 20 Hz), 28.2, 24.8, 22.4, 19.5, 18.5, 13.7, 9.5, 9.0. 31P NMR
(CD2Cl2, 202 MHz): δ 50.9, 143.8 (sept, J31P‑19F = 710 Hz). 19F NMR
(CD2Cl2, 376 MHz): δ −73.1 (d, J19F‑31P = 710 Hz). IR (cm−1): 1611
(CO).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and structural characterization,
1
computational results, and H and 31P NMR spectroscopic
studies. This material is available free of charge via the Internet
at http://pubs.acs.org.
(29) Johnson, J. A.; Sames, D. J. Am. Chem. Soc. 2000, 122, 6321.
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R.; Kociok-Kohn, G.; Moxham, G. L.; Weller, A. S.; Wondimagegn, T.;
Vadivelu, P. Chem.Eur. J. 2008, 14, 1004.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: stambuli@chemistry.ohio-state.edu.
(32) da Piedade, M. E. M.; Simoes, J. A. M. J. Organomet. Chem.
1996, 518, 167.
(33) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
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Organomet. Chem. 2003, 667, 112.
Notes
The authors declare no competing financial interest.
(35) Whittemore, S. M.; Gallucci, J.; Stambuli, J. P. Organometallics
2011, 30, 5273.
(36) Fernandez-Galan, R.; Manzano, B. R.; Otero, A.; Lanfranchi, M.;
Pellinghelli, M. A. Inorg. Chem. 1994, 33, 2309.
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219.
ACKNOWLEDGMENTS
■
J.P.S. and S.M.W. gratefully acknowledge the Air Force Office
of Scientific Research (FA9550-10-1-0532) and The Ohio State
University for funding this research. The authors thank Prof.
Christopher Hadad (The Ohio State University) for directing
the computational studies performed. The authors thank Judith
Gallucci for the X-ray crystallographic results.
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