Synthesis of Pincer Iridium Complexes Bearing a Boron Atom and iPr-Substituted Phosphorus Atoms: Application to Catalytic Transfer Dehydrogenation of Alkanes

Article abstract of DOI:10.1021/acs.organomet.5b00376

An ⁱPr-substituted PBP-pincer ligand was synthesized and introduced to iridium metal to give ⁱPr-(PBP)Ir(H)Cl and ⁱPr-(PBP)Ir(C₂H₄) complexes. Both of these complexes were found to be moderately active for the catalytic transfer dehydrogenation of cyclooctane. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00376

Article
pubs.acs.org/Organometallics
Synthesis of Pincer Iridium Complexes Bearing a Boron Atom and
iPr-Substituted Phosphorus Atoms: Application to Catalytic Transfer
Dehydrogenation of Alkanes
Keita Tanoue† and Makoto Yamashita*,†,‡
†Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, 112-8551
Tokyo, Japan
‡CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
i
ABSTRACT: An Pr-substituted PBP-pincer ligand was synthesized
i
i
and introduced to iridium metal to give Pr-(PBP)Ir(H)Cl and Pr-
(PBP)Ir(C2H4) complexes. Both of these complexes were found to be
moderately active for the catalytic transfer dehydrogenation of
cyclooctane.
Scheme 1. Previously Reported PBP-Pincer Ligands 1a,b
and Their Complexation with Iridium
INTRODUCTION
■
a
Dehydrogenation of alkane is one of the most important
reactions in industrial chemistry, because the resulting alkenes
are also one of the most important resources for commodity
chemicals. For the production of alkene, heterogeneous
catalysts are usually used because of their advantages, such as
easy separation and tolerance to high reaction temperature
(500−900 °C) leading to high catalyst activity. However, the
application of heterogeneous catalysts for dehydrogenation of
alkanes is limited to small alkanes such as ethane,1 propane,2
and ethylbenzene3 because there are selectivity issues for larger
alkanes. Thus, many homogeneous catalysts4 have been
developed to apply to the dehydrogenation of higher alkanes
after the discovery by Crabtree of the Ir complex
(Ph3P)2IrH2(acetone)2, which is capable of stoichiometric
dehydrogenation.5 Among these complexes, benzene-based
PCP-pincer iridium complexes6 are recognized as being the
best catalysts from the viewpoint of catalytic activity.7 The most
recent major advance for this type of catalyst from the
viewpoint of catalytic activity is the development of POCSP-
pincer iridium complexes, which show a TON of 5901 for
transfer dehydrogenation of cyclooctane.8 The replacement of
ligand side arms in PCP-pincer systems has also been reported
in the development of CCC- and PCN-pincer systems for
catalytic dehydrogenation.9 However, the catalytic activity of
these dehydrogenation catalysts is still waiting to be further
improved for future applications in industry.
a
Legend: (a) [Ir(L)Cl]2 (L = 1,5-cycooctadiene for 1a, L = C2H4 for
1b); (b) LiTMP, ethylene (TMP = 2,2,6,6-tetramethylpiperidide).
t
precursors to iridium to form the Bu- or Cy-substituted
(PBP)Ir(H)Cl complexes 2a,b. Further treatment of these
complexes with base in the presence of ethylene afforded the
Ir(I) ethylene complexes 3a,b. Since then, PBP-pincer ligand(s)
have been introduced to Ir, Rh,11 Ru,12 Os,12d Pt,13 Co,14 and
Ni14b,15 to provide a wide variety of PBP-pincer complexes.
Some of the PBP complexes were reported to have catalytic
activity for hydrogenation of aldehyde (Ru),12b hydrosilylation
of alkene (Pt),13a hydrogenation of alkene (Co, Ni),14b and
dehydrogenation of amine−borane (Co).14a However, there
has been no report of the transfer dehydrogenation of alkane by
using PBP-pincer complexes. Since the first report of the PBP-
Ir complexes, we have been making efforts to apply these
complexes toward dehydrogenation of alkanes. However,
complexes 2a,b and 3a,b were not active for catalytic
dehydrogenation. Therefore, we decided to modify the alkyl
On the other hand, we recently developed the hydroborane
precursors 1a,b (Scheme 1) of a related pincer ligand having a
boron atom as one of the coordinating atoms.10 In these
reports, we demonstrated the easy introduction of these
Received: May 3, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00376
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Article
groups on the phosphorus atom to slightly smaller isopropyl
groups. Herein, we report the synthesis of the isopropyl-
substituted PBP-pincer ligand precursor 1c, its complexation
methylene protons between phosphorus and nitrogen atoms
became inequivalent to result in the characteristic AB quartet
pattern. Two phosphorus atoms in 2c resonated as one singlet
signal at δP 73.8 ppm in the 31P NMR spectrum, as expected
from the result of the previously reported 2a,b.10 The boron
nucleus in 2c showed a broad singlet at δB 33.3 ppm, being
i
with iridium to form the Pr-(PBP)Ir(H)Cl complex 2c, its
subsequent derivatization to Ir(I) ethylene complex 3c, and the
application of 3c as a catalyst for dehydrogenation of
cyclooctane.
similar to those reported for 2a,b.10 In the H NMR spectrum
1
of 3c, all four protons on the coordinating ethylene were
equivalent to exhibit one singlet at δH 2.69 ppm. The
introduction of a symmetrical ethylene molecule and loss of
hydrogen chloride from 2c led to a C2v-symmetrical pattern in
the 1H spectrum of 3c. Although the 31P NMR chemical shift of
the singlet signal of 3c (δP 79.6 ppm) was similar to that of 2c,
the boron nucleus in 3c was more deshielded, probably due to
the change in the electronic structure on the iridium atom to be
monovalent.
RESULTS AND DISCUSSION
■
Synthesis of the iPr-substituted PBP-pincer ligand precursor 1c
was achieved by a procedure similar to the preparation of 1a,b
(Scheme 2).10 Reaction of o-phenylenediamine (4) with freshly
i
Scheme 2. Synthesis of Pr-Substituted PBP Pincer Ligand
Precursor 1c
i
The obtained Pr-substituted (PBP)Ir complexes 2c and 3c
were also structurally characterized by crystallographic analysis
(Figures 1 and 2, respectively). The selected bond lengths (Å)
i
prepared diisopropylphosphinomethanol, Pr2PCH2OH, from
iPr2PH and paraformaldehyde under neat conditions gave the
phosphine-tethered phenylenediamine derivative 5. Subsequent
reaction of the crude mixture of 5 with BH3·THF induced the
formation of 6, having a benzodiazaborole ring and borane-
protected phosphorus atoms. Deprotection of the phosphine−
borane moiety of 6 with morpholine afforded the hydroborane
precursor 1c.
A simple mixing of the resulting hydroborane precursor 1c
with [Ir(cod)Cl]2 afforded the Pr-substituted (PBP)Ir(H)Cl
complex 2c (Scheme 3). The subsequent treatment of 2c with
i
Figure 1. ORTEP drawing of 2c (50% thermal ellipsoids; hydrogen
atoms except hydride ligand omitted for clarity).
i
Scheme 3. Introduction of Pr-Substituted PBP Pincer
Ligand to Ir Atom
LiTMP (lithium 2,2,6,6-tetramethylpiperidide) under an ethyl-
ene atmosphere gave the (PBP)Ir ethylene complex 3c. Both of
the products were characterized by NMR and IR spectroscopy
and elemental analysis. The hydride complex 2c showed a
characteristic triplet signal at δH −25.34 ppm (2JPH = 13 Hz),
assignable to the hydride ligand in comparison with those of
1
2a,b, in the H NMR spectrum. The chemical shift of the
hydride ligand resonated at lower field in comparison to those
(ca. −40 ppm) of the previously reported (POCOP)Ir(H)Cl
complexes.16 The existence of the hydride ligand was also
confirmed by an IR spectrum, possessing a characteristic
vibration at 2229 cm−1, supported by frequency calculations by
DFT methods (see the Supporting Information). According to
the magnetic inequivalency below and above the PBP-Ir plane,
Figure 2. ORTEP drawing of 3c (50% thermal ellipsoids; hydrogen
atoms, one of the two independent molecules, and cosolvated n-
hexane molecule omitted for clarity).
B
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Article
and angles (deg) of 2c and 3c are summarized in Table 1 with
those of reference compounds 2a,b and 3a,b.10 In the crystal
cyclooctane (Table 2). In all entries, the doubly dehydro-
genated product, cyclooctadiene, was not observed at a reaction
Table 1. Selected Bond Lengths (Å) and Angles (deg) of 2c
and 3c with Those of Reference Complexes 2a,b and 3a,b
Table 2. Catalytic Dehydrogenation of Cyclooctane by using
PBP-pincer Ir Complexes
a
2a
2b
2c
B−Ir
Ir−P
1.971(6)
2.3273(12)
2.3357(12)
2.3963(14)
1.55(6)
1.979(5)
2.3103(13)
2.3109(13)
2.3836(13)
1.51(5)
1.976(6)
2.3087(13)
2.3134(12)
2.3994(13)
1.32(5)
Ir−Cl
Ir−H
B−N
b
entry
PBP-Ir cat.
additive
tBuOLi
tBuONa
tBuOK
temp (°C)
TON
1.429(7)
1.437(7)
158.12(5)
157.67(17)
105.8(5)
3a
1.428(6)
1.435(6)
157.11(4)
144.03(16)
106.6(4)
3b
1.423(7)
1.435(7)
158.83(5)
138.63(16)
106.7(4)
3c
c
1
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
3c
3c
3c
3c
200
200
200
200
200
200
220
220
220
220
220
220
160
180
200
220
36
c
2
15
P−Ir−P
B−Ir−Cl
N−B−N
c
3
16
4
Me3SiCH2Li
LiTMP
5
4
5
c
6
KHMDS
tBuOLi
tBuONa
tBuOK
33
B−Ir
2.048(3)
2.3100(10)
2.3145(9)
2.205(3)
2.208(3)
2.069(5)
2.069(6)
2.064(6)
2.2856(14)
2.2884(14)
2.2799(15)
2.2807(15)
2.181(5)
2.194(6)
2.178(6)
2.188(5)
1.402(8)
1.439(8)
1.438(7)
1.441(7)
1.429(8)
149.45(5)
148.66(5)
105.1(5)
104.5(5)
c
7
39
c
8
14
Ir−P
2.2808(13)
2.2821(13)
2.181(5)
c
9
14
10
11
12
13
14
15
16
Me3SiCH2Li
LiTMP
8
c
43
c
KHMDS
42
Ir−C
9
10
2.199(5)
c
23
c
32
C−C (C2H4)
B−N
1.393(5)
1.439(4)
1.405(7)
1.431(7)
a
Standard conditions: PBP-Ir cat. (3 μmol), additive (4.5 μmol), coa
b
(10 mmol), tert-butylethylene (10 mmol). The amount of coe was
estimated by gas chromatography. Estimated as an average of two
c
1.443(4)
150.67(3)
103.1(3)
1.434(7)
144.86(5)
104.8(4)
experiments.
P−Ir−P
i
time of 15 h. A combination of Pr-(PBP)Ir(H)Cl complex 2c
t
with 1.5 equiv of BuOLi as a base could catalyze the transfer
N−B−N
dehydrogenation of cyclooctane (coa) at 200 °C in the
presence of tert-butylethylene as a hydrogen acceptor, giving
cyclooctene (coe) with a TON (turnover number) of 36,
estimated by gas chromatography (entry 1). Changing to a
stronger base with a larger alkali-metal cation led to lower
TONs (entries 2 and 3). Using (trimethylsilylmethyl)lithium
and LiTMP afforded even lower TONs (entries 4 and 5).
Potassium hexamethyldisilazide (KHMDS) improves the TON
up to 33 (entry 6). Although the tendency of the TONs with
tert-butoxide base are similar for the reactions at 200 °C, the
TONs were improved by a higher reaction temperature of 220
°C (entries 7−9). Even at 220 °C, (trimethylsilylmethyl)-
lithium gave a poor TON (entry 10). In the case of LiTMP, the
higher temperature showed an improvement of the TON
(entry 11). Using KHMDS afforded the same TON with the
case of LiTMP at 220 °C (entry 12). It should be noted that
the time course of the reaction progress showed that most of
the TON could be achieved around 3 h (Figure 3), indicating
that most of the active catalyst decomposed at 3 h.17
Replacement of the PBP-pincer iridium complex with 3c
enabled catalysis of the reaction even at 160 °C (entry 13). As
the temperature increased, the catalyst system with 3c gave
structure of 2c, the B−Ir bond is similar to those of 2a,b, while
the Ir−P bond in 2c is shorter than that of 2a and similar to
that of 2b, reflecting the steric factors of isopropyl substituents
in 2c. The Ir−Cl bond in 2c is slightly longer than those of
2a,b, probably being related to the B−Ir−Cl bond angles (see
below). Although the B−N bonds and N−B−N angle in the
benzodiazaborole ring and the P−Ir−P angle in 2c are similar
to those of 2a,b, the B−Ir−Cl angle in 2c is smaller than those
of 2a,b. This structural change may come from the change of
t
i
steric factor among Bu, Cy, and Pr groups. In the crystal
structure of 3c, the Ir−B bond length is slightly longer than that
of 3a and similar to that of 3b. In contrast, Ir−P bonds and Ir−
C (ethylene ligand) bonds are shorter than those of 3a and
similar to those of 3b, showing that the ethylene ligand can be
i
closer to the iridium atom due to the less-hindered Cy and Pr
substituents, while the P−Ir−P angle of 3c is between those of
3a and 3b. The CC bond length of the coordinating ethylene
molecule in 3c is similar to those of 3a,b, showing a similar
strength of the back-donation from Ir(I) to the π* orbital of
ethylene. The shape of the benzodiazaborole ring in 3c is
similar to those of 3a,b. Thus, the structural features of 2c and
i
higher TONs (entries 14−16). Thus, the present Pr-(PBP)Ir
complexes 2c and 3c could be applied as catalysts for the
transfer dehydrogenation of cyclooctane for the first time
among PBP complexes. The reason why 2c and 3c are active
for transfer dehydrogenation of alkane, even though 2a,b and
3a,b are not, could be attributed to the sufficient space around
i
3c showed the slightly smaller character of Pr group in
t
comparison to the Bu and Cy groups.
i
The obtained Pr-substituted PBP-pincer iridium complexes
2c and 3c were applied as catalysts for the dehydrogenation of
C
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Scheme 5. Previously Reported Decomposition of PBP
Pincer Ligand through the Loss of Boron Atom13b
t
As noted above, the previously reported Bu- or Cy-
substituted (PBP)Ir complexes 2a,b and 3a,b are not
catalytically active for transfer dehydrogenation, while the
present iPr-(PBP)Ir complexes 2c and 3c are active. One of the
possible reasons for this difference could be attributed to the
difference in the available space around the iridium atom of the
(PBP)Ir fragment in 2a−c and 3a−c. The monovalent (PBP)Ir
fragment was extracted from the crystal structures of 3a−c, and
side views of the three fragments obtained are illustrated in
Figure 4. It is difficult to see the central iridium atom in the tBu-
Figure 3. Time course of the TON for dehydrogenation of
cyclooctane under the condition of entry 12 in Table 2. Conditions:
2c (3 μmol), KHMDS (4.5 μmol), coa (10 mmol), tert-butylethylene
(10 mmol). Each TON was estimated by independent experiments;
the amount of coe was estimated by gas chromatography.
the Ir atom. It is also noteworthy that the present catalytic
system was not active for dehydrogenation of linear n-octane.
A possible mechanism for (PBP)Ir-catalyzed transfer hydro-
genation of cyclooctane is illustrated in Scheme 4, according to
Scheme 4. Possible Catalytic Cycle for Transfer
i
Dehydrogenation of Cyclooctane using Pr-Substituted
(PBP)Ir Complexes 2c and 3c
t
i
Figure 4. Side view of the R-(PBP)Ir fragment (R = Bu, Cy, Pr)
extracted from crystal structures of 3a−c: (blue) iridium; (gray)
carbon; (white) hydrogen.
(PBP)Ir fragment of 3a due to the existence of the tBu groups.
In the case of the Cy-(PBP)Ir fragment in 3b and the iPr-(PBP)
Ir fragment in 3c, the top of the central iridium atom could be
seen; however, the Cy-(PBP)Ir fragment in 3b has additional
remote sterics due to the 3,4,5-positions of the cyclohexyl
group. This structural difference may contribute one of the
steps in the catalytic cycle to realize the catalytic activity of 2c
and 3c in comparison with 2a,b and 3a,b. Even there was no
direct information about the reaction mechanism and rate-
determining step, a further mechanistic study with DFT
calculations and improvement of the ligand structure will be
reported in due course.
the previously studied PCP-pincer Ir systems.7 Treatment of 2c
with base would afford the active species 7 possessing a T shape
and 14 electrons as expected. Simple dissociation of ethylene
ligand from 3c would also give the same active species 7.
Oxidative addition of C−H bond of cycloocatane to 7 would
form the (cyclooctyl)iridium hydride complex 8. The
subsequent β-hydride elimination may lead to the formation
of iridium dihydride 9 with liberation of cyclooctene product.
tert-Butylethylene would insert into the Ir−H bond of 9 to give
the alkyliridium hydride complex 10. The following C−H
bond-forming reductive elimination from 10 would give tert-
butylethane with regeneration of the 14-electron active species
7. Considering our previous report of the loss of a boron atom
from a PBP-Pt complex having the weakly coordinating ligand
NTf2 in the presence of water and ethanol (Scheme 5),13b the
effect of the added base to 2c may be attributed to the
decomposition rate of the catalytically active species.
EXPERIMENTAL SECTION
■
General Considerations. All of the preparations and manipu-
lations involving air- and moisture-sensitive compounds were carried
out under an argon atmosphere using standard Schlenk and glovebox
(Miwa MFG, KIYON) techniques. All glassware were dried for 20 min
in the 250 °C oven before use. Ether, CH2Cl2, THF, and n-hexane
were purified by passing through a solvent purification system (Grass
Contour). C6D6 was dried by distillation over sodium benzophenone
followed by vacuum transfer. NMR spectra were recorded on JEOL
1
ECA (500 MHz for H, 160.5 MHz for 11B, 126 MHz for 13C) and
Varian Mercury (400 MHz for 1H) spectrometers. Chemical shifts are
1
reported in δ (ppm) relative to the residual protiated solvent for H,
deuterated solvent for 13C, and external BF3·OEt2 for 11B used as
references. Assignment of all signals in the H and 13C NMR spectra
1
were based on DEPT135, HMQC, and HMBC measurements. High-
resolution mass spectra were obtained by using a Xevo G2-XS QTof
instrument (Waters) with ASAP ionization mode. Elemental analyses
D
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were performed at the A Rabbit Science Co., Ltd. IR spectra were
obtained on IRAffinity-1 (Shimadzu). Melting points (mp) were
determined with a MPA100 OptiMelt instrument (Tokyo Instru-
ments, Inc.) and are uncorrected. X-ray crystallographic analyses were
performed on a VariMax/Saturn CCD diffractometer.
of 2c (170 mg, 68%): 1H NMR (400 MHz, C6D6) δ −24.87 (t, 2JPH
=
13 Hz, 1H, Ir−H), 0.97 (d of vt, 3JPH = 9 Hz, 3JHH = 7 Hz, 6H, CH3),
0.99 (d of vt, 3JPH = 9 Hz, 3JHH = 7 Hz, 6H, CH3), 1.11 (d of vt, 3JPH
9 Hz, 3JHH = 7 Hz, 6H, CH3), 1.38 (d of vt, 3JPH = 9 Hz, 3JHH = 7 Hz,
=
3
2
6H, CH3), 1.93 (qq of vt, JHH = 7 Hz, 7 Hz, JPH = 3 Hz, 2H,
3
2
Synthesis of 6 via the Synthetic Intermediate 5. In a glovebox,
iPr2PH (7.25 g, 61.4 mmol) and paraformaldehyde (2.07 g, 68.9
mmol) were charged in a 50 mL J. Young tube and the resulting
suspension was stirred for 22 h at 60 °C. After the suspension was
cooled to room temperature, CH2Cl2 (10.0 mL) was added to the
mixture. A solution of o-phenylenediamine (4; 3.36 g, 31.1 mmol) in
CH2Cl2 (8.00 mL) was added to the reaction mixture, and the
resulting solution was stirred for 18.5 h at room temperature. The
solvents were evaporated under reduced pressure to give an yellow oil
of 5 (11.0 g, 96%). This material was used for the next step without
further purification.
CH(CH3)2), 2.85 (qq of vt, JHH = 7 Hz, 7 Hz, JPH = 3 Hz, 2H,
CH(CH3)2), 3.25 (d of vt, JHH = 12 Hz, JPH = 2 Hz, 2H, N−CHH-
P), 3.39 (d of vt, JHH = 12 Hz, JPH = 3 Hz, 2H, N−CHH−P), 6.89
(dd, J = 5 Hz, 3 Hz, 2H, aromatic CH), 7.12 (dd, J = 5 Hz, 3 Hz, 2H,
aromatic CH); 31P NMR (202 MHz, C6D6) δ 73.8 (s); 11B{1H} NMR
(160 MHz, C6D6) δ 33.3 (s); 13C{1H} NMR (101 MHz, C6D6) δ 17.3
2
2
2
2
2
(s, CH3), 18.2 (s, CH3), 18.5 (vt, JPC = 2 Hz, CH3), 19.7 (vt, the
coupling constant was too small to be estimated, CH3), 23.8 (vt, 1JPC
=
13 Hz, CH(CH3)2), 25.3 (vt, 1JPC = 13 Hz, CH(CH3)2), 39.3 (vt, 1JPC
= 21 Hz, N-CH2−P), 108.5 (s, ortho to N), 118.4 (s, meta to N), 140.1
3
(4°, vt, JPC = 7 Hz, ipso to N); mp 169.2−172.7 °C dec; IR (KBr)
1
3
5: H NMR (500 MHz, C6D6) δ 0.99 (d, JHH = 7 Hz, 6H, CH3),
1.01 (d, 3JHH = 7 Hz, 6H, CH3), 1.02 (d, 3JHH = 7 Hz, 6H, CH3), 1.05
(d, 3JHH = 7 Hz, 6H, CH3), 1.66 (dqq, 2JPH = 3 Hz, 3JHH = 7, 7 Hz, 4H,
CH(CH3)2), 3.09 (d, 2JPH = 5 Hz, 4H, N−CH2-P), 3.43 (s, 2H, NH),
6.82 (dd, 3JHH = 6 Hz, 4 Hz, 2H, aromatic CH), 7.03 (dd, 3JHH = 6 Hz,
4 Hz, 2H, aromatic CH); 31P NMR (202 MHz, C6D6) δ 4.4 (s).
A solution of BH3·THF (0.95 mol/L in THF, 97.0 mL, 92.2 mmol)
was added to a solution of 5 (11.0 g, 29.7 mmol) in CH2Cl2 (280 mL)
at 0 °C. The resulting solution was stirred for 16 h at room
temperature. The solvents were evaporated under reduced pressure to
afford a crude product. Recrystallization of the crude product from
THF/Et2O (3/7) at −35 °C gave colorless crystals of 6 (4.99 g, 41%):
2229 cm−1. Anal. Calcd for C20H37BClIrN2P2: C, 39.64; H, 6.15; N,
4.62. Found: C, 39.69; H, 6.02; N, 4.50.
Synthesis of 3c. In a glovebox, a solution of lithium 2,2,6,6-
tetramethylpiperidide (35.8 mg, 0.243 mmol) in benzene (1.20 mL)
was added to a solution of 2c (140 mg, 0.231 mmol) in benzene
(0.900 mL) placed in a 10 mL J. Young tube. After the tube was
brought out from the glovebox, the resulting solution was degassed by
three cycles of freeze−pump−thaw and the tube was back-filled with
ethylene. The reaction mixture was stirred for 18 h at 75 °C, and
volatiles were removed under reduced pressure. The resulting residue
was extracted with hexane, and the suspension was filtered through a
pad of Celite. The filtrate was evaporated under reduced pressure to
3
3
1H NMR (400 MHz, C6D6) δ 0.92 (dd, JPH = 14 Hz, JHH = 7 Hz,
1
give a solid of 3c (107 mg, 0.179 mmol, 78%): H NMR (400 MHz,
C6D6) δ 0.87 (d of vt, 3JPH = 13 Hz, 2JHH = 7 Hz, 12H, CH3), 0.95 (d
2
24H, CH3), 1.09 (d, JPH = 14 Hz, 6H, P−BH3, only observed in
3
of vt, 3JPH = 13 Hz, 2JHH = 7 Hz, 12H, CH3), 2.17 (qq of vt, JHH = 7
1H{11B} spectrum), 1.77 (dqq, 2JPH = 11 Hz, 3JHH = 7 Hz, 3JHH = 7 Hz,
2
3
2
Hz, 7 Hz, JPH = 3 Hz, 4H, CH(CH3)2), 2.69 (t, JPH = 2 Hz, 4H,
4H, CH(CH3)2), 3.84 (d, JPH = 4 Hz, 4H, N−CH2−P), 4.89 (s, 1H,
C2H4), 3.55 (vt, 2JPH = 2 Hz, 4H, N−CH2−P), 7.01 (dd, 3JHH = 5 Hz,
BH, only observed in 1H{11B} spectrum), 7.06−7.08 (m, 4H, aromatic
CH); 13C NMR (400 MHz, C6D6) δ 17.2 (d, 2JPC = 15 Hz, CH3), 21.3
3
3 Hz, 2H, aromatic CH), 7.18 (dd, JHH = 5 Hz, 3 Hz, 2H, aromatic
CH); 31P NMR (202 MHz, C6D6) δ 79.6 (s); 11B{1H} NMR (160
MHz, C6D6) δ 54.6 (s); 13C{1H} NMR (101 MHz, C6D6) δ 18.9 (vt,
1
1
(d, JPC = 30 Hz, CH(CH3)2), 37.6 (d, JPC = 31 Hz, N−CH2−P),
110.1 (s, ortho to N), 120.0 (s, meta to N), 137.2 (s, 4°, ipso to N); 31P
NMR (202 MHz, C6D6) δ 37.1 (broad doublet) probably due to
coupling with 11B (I = 3/2) and 10B (I = 3); 11B{1H} NMR (160
MHz, C6D6) δ − 43.1 (s, BH3), 26.5 (s, BH); mp 130.7−132.0 °C
dec; HRMS (ASAP+) calcd for C20H42B3N2P2 [M] 405.3102, found
405.3113.
2JPC = 2 Hz, CH3), 19.0 (s, 27.0, CH3) (vt, JPC = 3 Hz, CH(CH3)2),
3
37.8 (s, C2H4), 42.2 (vt, 1JPC = 21 Hz, N−CH2−P), 108.2 (s, ortho to
3
N), 117.8 (s, meta to N), 140.9 (vt, JPC = 8 Hz, 4°, ipso to N); mp
109.9−126.7 °C dec; IR (KBr) 1287 cm−1. Anal. Calcd for
C22H40BIrN2P2: C, 44.22; H, 6.75; N, 4.69. Found: C, 44.26; H,
6.61; N, 4.61.
Synthesis of 1c. A suspension of 6 (752 mg, 1.85 mmol) and
morpholine (2.50 mL, 28.7 mmol) was stirred for 6 h at 130 °C. The
reaction mixture was evaporated under reduced pressure, and the
residue was filtered through a pad of silica gel with Et2O/hexane (1/1)
eluent. The filtrate was evaporated under reduced pressure to give a
crude product. Recrystallization of the crude product from hexane at
General Procedure for Catalytic Dehydrogenation of Cyclo-
octane by using Complex 2c with Base. In a glovebox, a 10 mL J.
t
Young tube was charged with base (ca. 4.5 μmol, 0.4 mg for BuOLi,
0.4 mg for tBuONa, 0.5 mg for tBuOK, 0.7 mg for LiTMP, 0.4 mg for
Me3SiCH2Li, 0.9 mg for KHMDS) and cyclooctane (1.202 mL, 9
mmol). In a small vial, 2c (18.2 mg, 30.0 μmol) was dissolved in tert-
butylethylene (11.65 mL, 90.0 mmol). An aliquot (1.165 mL,
containing 3.00 μmol of 2c and 9.00 mmol of tert-butylethylene) of
the resulting solution was placed in the J. Young tube, and then the
tube was brought out from the glovebox. The reaction mixture was
stirred for 15 h at 200 or 220 °C with an aluminum-block heating
stirrer. After the mixture was cooled to room temperature, dodecane
(227.1 μL, 1.000 mmol) as an internal standard for GC was added to
the reaction mixture. The resulting solution was analyzed by GC to
estimate the yield on the basis of the independently calculated GC
factor by using authentic samples.
1
−35.0 °C gave colorless crystals of 1c (447 mg, 64%): H{11B} NMR
(500 MHz, C6D6) δ 0.99 (d, 3JHH = 7 Hz, 6H, CH3), 1.00 (d, 3JHH = 7
3
Hz, 6H, CH3), 1.01 (d, JHH = 7 Hz, 12H, probably two signals were
2
3
overlapped, CH3), 1.59 (dqq, JPH = 2 Hz, JHH = 7, 7 Hz, 4H,
2
CH(CH3)2), 3.83 (d, JPH = 3 Hz, 4H, N−CH2−P), 5.11 (br s, 1H,
B−H), 7.16 (dd, 3JHH = 6 Hz, 3 Hz, 2H, aromatic CH), 7.40 (dd, 3JHH
= 6 Hz, 3 Hz, 2H, aromatic CH); 13C NMR (400 MHz, C6D6) δ 19.4
(d, 2JPC = 12 Hz, CH3), 19.8 (d, 2JPC = 14 Hz, CH3), 23.3 (d, 1JPC = 16
Hz, CH(CH3)2), 40.5 (d, 1JPC = 19 Hz, N−CH2−P), 110.5 (d, 4JPC = 6
Hz, ortho to N), 119.4 (s, meta to N), 138.4 (d, 3JPC < 1 Hz, 4°, ipso to
N); 31P NMR (202 MHz, C6D6) δ −3.8 (s); 11B{1H} NMR (160
MHz, C6D6) δ 26.5 (s); mp 45.8−47.2 °C dec. Anal. Calcd for
C20H37BN2P2: C, 63.50; H, 9.86; N, 7.41. Found: C, 63.65; H, 9.77; N,
7.42.
General Procedure for Catalytic Dehydrogenation of Cyclo-
octane by using Complex 3c. In a glovebox, a 10 mL J. Young tube
was charged with cyclooctane (1.202 mL, 9 mmol). In a small vial, 3c
(10.8 mg, 18.1 μmol) was dissolved in tert-butylethylene (6.99 mL,
54.0 mmol). An aliquot (1.165 mL, containing 3.02 μmol of 3c and
9.00 mmol of tert-butylethylene) of the resulting solution was placed in
the J. Young tube, and then the tube was brought out from the
glovebox. The reaction mixture was stirred for 15 h at the indicated
temperature with an aluminum-block heating stirrer. After the mixture
was cooled to room temperature, dodecane (227.1 μL, 1.000 mmol) as
an internal standard for GC was added to the reaction mixture. The
resulting solution was analyzed by GC to estimate the yield on the
Synthesis of 2c. In a glovebox, a solution of 1c (51.4 mg, 0.136
mmol) in THF (2 mL) was added to [Ir(cod)Cl]2 (155 mg, 0.409
mmol) placed in a 4 mL vial, and the resulting solution was stirred for
20 min at room temperature to afford an orange suspension. Volatiles
were removed from the suspension, and the resulting residue was
dissolved in a minimum amount of THF. The THF extract was
filtered, and the remaining residue was rinsed with hexane (1 mL).
The resulting filtrate was evaporated to afford the crude product.
Recrystallization from THF/hexane at −35.0 °C gave colorless crystals
E
DOI: 10.1021/acs.organomet.5b00376
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
basis of the independently calculated GC factor by using authentic
samples.
Japan. We thank Prof. Hiyama, T., Chuo University, for
providing the X-ray diffractometer.
Analysis of the Decomposed Product from Active Catalyst.
In a glovebox, a 10 mL J. Young tube was charged with 2c (30.6 mg,
50.5 μmol) and KHMDS (10.5 mg, 52.6 μmol). In the J. Young tube
were placed cyclooctane (1.349 mL, 10.1 mmol) and tert-butylethylene
(1.308 mL, 10.1 mmol), and then the tube was brought out from the
glovebox. The reaction mixture was stirred for 6 h at 220 °C with an
aluminum-block heating stirrer. After the mixture was cooled to room
temperature, all of the volatiles were removed under reduced pressure.
After the J. Young tube was brought back to the glovebox, the resulting
residue was triturated with hexane. The hexane-soluble content of the
resulting suspension was collected by filtration, and the solvent was
evaporated from the filtrate to give a crude product. The crude
product was dissolved in C6D6 to obtain NMR spectra. The 1H NMR
spectrum of the crude product (Figure S14 in the Supporting
Information) showed several signals around 6 ppm, indicating the
existence of π-allylic type protons.
Details of the X-ray Crystallographic Study. Details of the
crystal data and a summary of the intensity data collection parameters
for 1c, 2c, 3c, and 6 are given in Table S1 in the Supporting
Information. A suitable crystal was mounted with mineral oil on the
glass fiber and transferred to the goniometer of a Rigaku VariMax
Saturn CCD diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71070 Å). In the following procedure for analysis,
Yadokari-XG 2009 was used as a graphical interface.18 The structure
was solved by direct methods (SIR-2014)19 and refined by full-matrix
least-squares techniques against F2 (SHELXL-2014).20 The intensities
were corrected for Lorentz and polarization effects. The non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were placed using
AFIX instructions except for the hydrogen atom of the hydride ligand
in 2c and the BH3 moiety in 6, which were found in differential
Fourier maps as residual peaks and were refined isotropically.
Details for Computation. All calculations were performed with
the Gaussian 09 (rev. D.01) software package.21 The molecular
geometries of 2c and 3c were both optimized without constraints at
the B3LYP level with LanL2DZ/6-31G(d) basis sets. Figure S13 in the
Supporting Information shows the characteristic Ir−H vibration in 2c
and C2H4 vibration in 3c. The optimized geometries are available in
the Supporting Information in XYZ format.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
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NMR spectra of new compounds and crystal structures
of synthetic intermediates (PDF)
Crystallographic data for 1c, 2c, 3c, and 6 (CIF)
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Cartesian coordinates of the calculated structure (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.Y.: makoto@oec.chem.chuo-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
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Commun. 2014, 50, 15718−15721.
ACKNOWLEDGMENTS
■
(16) Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
̈
This work was supported by Grants-in-Aid for Scientific
Research on Innovative Areas “Stimuli-Responsible Chemical
Species for the Creation of Functional Molecules” (No.
24109012) and CREST area “Establishment of Molecular
Technology towards the Creation of New Functions” (No.
14529307) from the JST. The computations were performed at
the Research Center for Computational Science, Okazaki,
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essentially the same TONs.
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