Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes

Article abstract of DOI:10.1016/j.poly.2016.02.044

A series of new (^tBu2PSCOP^R2)IrHCl iridium complexes with ‘hybrid’ phosphinothious-phosphinite PSCOP ligands ([^tBu2PSCOP^R2?=?1-(SPtBu₂)-3-(OPR₂)-C₆H₄], R?=?tBu, 4a, R?=?Cy, 4b, R?=?iPr, 4c, and R?=?Et, 4d) have been synthesized and characterized. Treatment of complexes 4a–d with sodium tert-butoxide generates the active species for catalytic transfer-dehydrogenation of cyclooctane (COA) or n-octane using tert-butylethylene (TBE) as hydrogen acceptor to form cyclooctene (COE) or octenes, respectively. The catalytic activity of these complexes and the product selectivity in alkane dehydrogenation is greatly influenced by the steric properties of the pincer ligand. In general, the less sterically bulky complex exhibits higher catalytic activity than the more hindered complex. Among the new (PSCOP)Ir-type complexes, the least crowded complex (^tBu2PSCOP^Et2)IrHCl 4d is most active for n-octane/TBE transfer-dehydrogenation. The relatively crowded, less active, complexes (^tBu2PSCOP^tBu2)IrHCl (4a) and (^tBu2PSCOP^Cy2)IrHCl (4b) exhibit high regioselectivity for α-olefin formation at the early stages of the reaction.

Full text of DOI:10.1016/j.poly.2016.02.044

Accepted Manuscript
Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes
Wubing Yao, Xiangqing Jia, Xuebing Leng, Alan S. Goldman, Maurice
Brookhart, Zheng Huang
PII:
S0277-5387(16)00138-8
DOI:
http://dx.doi.org/10.1016/j.poly.2016.02.044
POLY 11860
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
19 January 2016
26 February 2016
Please cite this article as: W. Yao, X. Jia, X. Leng, A.S. Goldman, M. Brookhart, Z. Huang, Catalytic alkane transfer-
dehydrogenation by PSCOP iridium pincer complexes, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.
2
016.02.044
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Catalytic alkane transfer-dehydrogenation by
PSCOP iridium pincer complexes
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a
a
a
b
c,
Wubing Yao , Xiangqing Jia , Xuebing Leng , Alan S. Goldman , Maurice Brookhart *, and
a,
Zheng Huang *
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Science, 345 Lingling Road, Shanghai 200032, China
b
Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854,
United States
c
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North
Carolina 27599-3290, United States
th
On the occasion of his 80 birthday, this article is dedicated to Professor Malcolm Green who
has repeatedly made path breaking and enduring contributions to organometallic and inorganic
chemistry over more than five decades.
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tBu2
R2
ABSTRACT. A series of new ( PSCOP )IrHCl iridium complexes with ‘hybrid’
tBu2
R2
phosphinothious-phosphinite PSCOP ligands ([ PSCOP = 1-(SPtBu )-3-(OPR )-C H ], R =
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tBu, 4a, R = Cy, 4b, R = iPr, 4c, and R = Et, 4d) have been synthesized and characterized.
Treatment of complexes 4a-d with sodium tert-butyloxide generates the active species for
catalytic transfer-dehydrogenation of cyclooctane (COA) or n-octane using tert-butylethylene
(
TBE) as hydrogen acceptor to form cyclooctene (COE) or octenes, respectively. The catalytic
activity of these complexes and the product selectivity in alkane dehydrogenation is greatly
influenced by the steric properties of the pincer ligand. In general, the less sterically bulky
complex exhibits higher catalytic activity than the more hindered complex. Among the new
tBu2
Et2
(
PSCOP)Ir-type complexes, the least crowded complex ( PSCOP )IrHCl 4d is most active for
n-octane/TBE transfer-dehydrogenation. The relatively crowded, less active, complexes
tBu2
tBu2
tBu2
Cy2
(
PSCOP )IrHCl (4a) and ( PSCOP )IrHCl (4b) exhibit high regioselectivity for α-olefin
formation at the early stages of the reaction.
Keywords: alkane · alkene · dehydrogenation · homogeneous catalysis · iridium
1
. Introduction
Alkane dehydrogenation is a reaction with important potential applications. This transformation
converts abundant, inexpensive saturated hydrocarbon feedstocks into olefins that are useful
synthetic intermediates. Industrially, heterogeneous catalytic alkane dehydrogenation is carried
out on enormous scales via hydrocarbon cracking and reforming processes. However, these
processes typically operate at very high temperatures (> 500˚ C), resulting in low energy
[
1]
efficiency and poor product selectivities . In this respect, the development of efficient
homogeneous systems for selective catalytic alkane dehydrogenation under mild conditions is of
great interest.
2

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Chart 1. Structures of (PCP)Ir, (POCOP)Ir, and (PSCOP)Ir Complexes
O
O
S
O
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tBu P
PtBu2
tBu2P
PtBu2
iPr P
PiPr2
2
Ir
Ir
2
Ir
H
H
H
H
Cl
Cl
(tBu2PCPtBu2)IrH2
(tBu2POCOPtBu2)IrHCl
(iPr2PSCOPiPr2)IrHCl
1
2
3
Since the discoveries of homogenous alkane dehydrogenations by Crabtree and Felkin in the
[2-4]
[5-8]
1
980s , numerous soluble molecular dehydrogenation catalysts have been developed
.
Among them, the robust iridium complexes of pincer ligands have proven to be most effective.
Chart 1 shows three examples of pincer iridium systems. The bis(phosphine)-based pincer
(PCP)Ir complex 1 has been extensively investigated by the Kaska, Jensen, and Goldman
[
9-17]
groups , while the bis(phosphinite)-based pincer (POCOP)Ir complex 2 was mainly explored
[
18-21]
by the Brookhart group
. These and other relevant pincer-type iridium catalyst systems can
effect alkane dehydrogenation with or without a hydrogen acceptor, typically at temperatures in
[22-35]
the range of 150-200˚ C
. Recyclable, highly active solid-supported pincer iridium complexes
[
36]
have also been reported for alkane transfer-dehydrogenations . In addition, the pincer iridium
catalysts have been applied to various types of alkane transformations, including alkane
[
37-39]
metathesis via tandem alkane dehydrogenation and olefin metathesis
, conversion of n-
[
40, 41]
alkanes to alkylaromatics via dehydroaromatization
, and regioselective conversion of n-
alkanes to linear alkylsilanes via Ir-catalyzed alkane dehydrogenation and Fe-catalyzed tandem
[42]
olefin isomerization and hydrosilylation .
Although significant progress has been achieved in the field of homogeneous alkane
dehydrogenation in the past two decades, there is room for improvement of known catalytic
systems with respect to catalytic activity and product selectivity in dehydrogenation of linear
[
5]
alkanes . Early studies on pincer-type Ir complexes have shown that the nature of the linkers
[19, 43]
connecting the P atoms and the aryl backbone
can affect the productivity and
regioselectivity in alkane dehydrogenation. In this context, very recently we reported that an
isopropyl-substituted (PSCOP)Ir complex (3) with a hybrid phosphinothious-phosphinite ligand
(see Chart 1), upon activation with NaOtBu, is highly active for transfer-dehydrogenation of
3

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[44]
cyclooctane (COA) and n-octane . Herein we report the synthesis and characterization of a
series of (PSCOP)Ir complexes bearing tBu groups on the phosphinothious arm and various alkyl
substituents on the phosphinite arm. The steric demands of the pincer ligands have a large impact
on the catalytic performance of these complexes in transfer-dehydrogenation reactions: the
complexes bearing small phosphino-substituents are more active than those with bulky
substituents for transfer-dehydrogenation of both cyclic and linear alkanes, while the crowded
complexes offer higher regioselectivity for α-olefin formation in dehydrogenation of the linear
alkane, octane.
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2
. Experimental
General Considerations. All manipulations were carried out using standard Schlenk, high
vacuum, and glovebox techniques. THF, hexane, pentane and toluene were distilled from sodium,
then degassed by three freeze-pump-thaw cycles and stored in an argon atmosphere glovebox.
Cyclooctane (COA) (99%), tert-butylethylene (TBE) (98.5%), n-octane (99%) were purchased
from Aldrich and dried over LiAlH overnight, then distilled under vacuum and stored in an
4
argon atmosphere glovebox prior to use. NaH (95%), dicyclohexylchlorophosphine (98+%), and
3-hydroxythiophenol (96%) were purchased from TCI and used as received. Di-tert-
butylchlorophosphine (96%), chlorodiethylphosphine (95%), and chlorodiisopropylphosphine
(
96%) were purchased from Acros and used as received. Triethylamine was dried over LiAlH4
overnight, then distilled under vacuum and stored in an argon atmosphere glovebox prior to use.
All other reagents and solvents mentioned in this text were purchased from commercial sources.
[
45]
tBu4
[9]
tBu4
[19]
[
Ir(COD)Cl]2 , ( PCP)IrH (1) , and ( POCOP)IrHCl (2) were prepared according to
2
previously reported procedures.
NMR spectra were obtained on Aglient 400, Varian 400 MHz, and Varian 300 MHz
1
instruments. H NMR spectra were referenced to residual protio solvent peaks or TMS (0 ppm)
1
3
31
and C NMR spectra were referenced to the solvent resonance. P NMR chemical shifts were
1
referenced to an external H PO standard. H NMR data are recorded as follows: chemical shift
3
4
(
δ, ppm), multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet or unresolved, br =
1
3
broad singlet, coupling constant (s) in Hz, integration). C NMR data are reported in terms of
chemical shift (δ, ppm). Elemental analyses and high resolution mass spectroscopy (HRMS)
were carried out by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS).
4

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GC analysis was acquired on Agilent 7890A gas chromatograph equipped with a flame-
ionization detector. GC-MS analysis was performed on Agilent 7890A gas chromatograph
coupled to an Agilent 5975C inert mass selective detector.
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General procedures for transfer-dehydrogenation of COA with TBE
R
R’
In an argon-filled glovebox, the appropriate complex ( PSCOP )IrHCl (4a-d) (3 µmol) was
dissolved in a solution of COA and TBE (3000 equiv. each relative to iridium, 9.0 mmol).
Sodium tert-butoxide (1.5 equiv. with respect to iridium, 4.5 µmol) was then added to the
solution. The solution was transferred into a 10 mL thick-wall glass tube. The tube was sealed
and heated in a preheated oil-bath at 200 °C. At regular intervals, the tube was cooled to the
room temperature and the sample was analyzed by gas chromatography.
General procedures for transfer-dehydrogenation of n-octane with TBE
R2
R’2
In an argon-filled glovebox, the appropriate complex ( PSCOP )IrHCl (4a-d) (3 µmol) was
dissolved in a solution of n-octane (5728 equiv. with respect to iridium, 17.2 mmol) and TBE
(500 equiv. each relative to iridium, 1.5 mmol). Sodium tert-butoxide (1.5 equiv. with respect to
iridium, 4.5 µmol) was then added to the solution. The solution was transferred into a 10 mL
thick-wall glass tube. The tube was sealed and heated in a preheated oil-bath at 200 °C. At
regular intervals, the tube was cooled to the room temperature and the sample was analyzed by
gas chromatography.
S
OH
tBu P
2
Synthesis of 1-(SPtBu )-3-(OH)-C H . Triethylamine (11.1 mmol, 1.12 g) was added to a
2
6
4
solution of 3-hydroxythiophenol (7.84 mmol, 0.99 g) in 30 mL of THF. The mixture was heated
at 70 °C for 1 h, di-tert-butylchlorophosphine (8.69 mmol, 1.57 g) was then added to the reaction
mixture. The mixture was stirred at room temperature for 1 h. After evaporation of the solvent
under high vacuum, the residue was dissolved in 30 mL of toluene and the extract was cannula
transferred and filtered through a pad of Celite. After evaporation of the solvent under high
vacuum, the flask was heated to 70˚ C for 1 h to remove the residual amounts of di-tert-
butylchlorophosphine. The product was obtained as waxy solid. Yield, 70% (1.48 g, 5.49 mmol).
1
2
2
H NMR (300 MHz, CDCl ) δ 7.62 (s, 1H), 7.41 (d, J = 6.0 Hz, 1H), 7.06 (t, J = 6.0 Hz,
3
HH
HH
5

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1
8
H), 6.93 (d, J = 9.0 Hz, 1H), 1.34 (d, J = 12.0 Hz, 18H). P NMR (121 MHz, CDCl ): δ
HH HH 3
1
3
1
0.7. C{ H} NMR (101MHz, C D ): δ 157.5, 138.9, 129.7, 122.6, 118.3, 113.7, 35.0, 29.6.
6
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+
HRMS (ESI), m/z calculated for C H OPS (M ): 270.1207, found: 270.1220.
1
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S
O
tBu P
PtBu2
2
tBu2
tBu2
Synthesis of ligand PSCOP . A suspension of NaH (11.0 mmol, 0.26 g) in 10 mL of THF
was slowly added via syringe to a solution of the 3-hydroxythiophenol (5.00 mmol, 0.63 g) in 30
mL of THF (caution: hydrogen evolution). The mixture was stirred at room temperature for 1 h,
then di-tert-butylchlorophosphine (10 mmol, 1.80 g) was added via syringe, and the mixture was
stirred at room temperature for an additional 1 h. After evaporation of the solvent under high
vacuum, the residue was dissolved in hexane (50 mL), and the extract was cannula transferred
and filtered through a pad of Celite. After evaporation of the solvent under high vacuum, the
flask was heated to 70˚ C for 1 h to remove the residual amounts of di-tert-butylchlorophosphine.
The crude product was obtained as pale yellow viscous oil and exhibit >95% purity by NMR (see
Supporting Information). The ligand was used without further purification. Yield: 73% (1.51 g,
1
3
.65 mmol). H NMR (400 MHz, C D ): δ 7.87-7.85 (m, 1H), 7.37-7.34 (m, 1H), 7.07-7.03 (m,
6
6
2
2
2
1
H), 6.98 (t, J = 8.0 Hz, 1H), 1.23 (d, J = 12.0 Hz, 18H), 1.10 (d, J = 12.0 Hz, 18H ).
HH
HH
HH
3
1
1
13
1
P{ H} NMR (162 MHz, C D ) δ 154.4, 82.1. C{ H} NMR (101 MHz, C D ) 160.5, 139.5,
6
6
6
6
129.8, 124.3, 121.2, 116.4, 35.8, 35.3, 29.9, 27.6. HRMS (ESI), m/z calculated for C H OP S
2
2
40
2
+
(M ): 414.2275, found: 414.2278.
General procedure for synthesis of ligands PSCOP (R = Cy, iPr, Et)
tBu2
R2
A suspension of NaH (6.00 mmol, 0.14 g) in 10 mL of THF was slowly added via syringe to a
solution of 1-(SPtBu )-3-(OH)-C H (5.0 mmol, 1.35 g) in 30 mL of THF (caution: hydrogen
2
6
4
evolution). The mixture was stirred for 1 h at room temperature, then dialkylchlorophosphine
5.0 mmol) was added via syringe and the mixture was stirred at room temperature for additional
(
1 h. After evaporation of the solvent under high vacuum completely, the residue was dissolved in
pentane (50 mL) and the extract was cannula transferred and filtered through a pad of Celite.
After evaporation of the volatiles, the crude products were obtained as colorless or white oils and
exhibit >95% or ca. 95% purity by NMR (see Supporting Information). The ligands were used
without further purification.
6

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S
O
tBu P
PCy2
2
tBu2
Cy2
1
Synthesis of ligand PSCOP . Colorless oil, yield: 74%, >95% purity. H NMR (400 MHz,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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2
2
C D ) δ 7.87 (s, 1H), 7.37 (d, J = 8.0 Hz, 1H), 7.06 (d, J = 4.0 Hz, 1H), 6.97-7.01 (m, 1H),
6
6
HH
HH
2
1
.92-1.95 (m, 2H), 1.75-1.64 (m, 8H), 1.58 (s, 2H), 1.50-1.44 (m, JHH = 12.0 Hz, 2H), 1.24 (d,
2
2
31
1
JHH = 12.0 Hz, 22H), 1.13 (t, J = 8.0 Hz, 4H). P{ H} NMR (162 MHz, C D ) δ 143.0, 80.9.
HH
6
6
1
3
1
C{ H} NMR (101 MHz, C D ) δ 160.4, 139.4, 129.7, 124.3, 121.3, 116.5, 38.5, 34.3, 29.8,
6
6
+
28.4, 27.3, 27.1. HRMS (MALDI/DHB), m/z calculated for C H OP S (M ), 467.2649, found:
2
6
45
2
4
67.2660.
S
O
tBu P
PiPr2
2
tBu2
iPr2
1
Synthesis of ligand
PSCOP . White viscous oil, yield: 65%, >95% purity. H NMR (400
2
2
MHz, CDCl ) δ 7.79 (t, J = 4.0 Hz, 1H), 7.32 (d, J = 4.0 Hz, 1H), 7.01-6.94 (m, 2H), 7.07(d,
3
HH
HH
2
2
2
JHH = 16.0 Hz, 1H), 1.80-1.68 (m, 2H), 1.22 (d, J = 12.0 Hz, 18H), 1.11 (d, J = 8.0 Hz, 3H),
HH
HH
2
2
2
31
1
1
.09 (d, J = 8.0 Hz, 3H), 0.98 (d, J = 8.0 Hz, 3H), 0.94 (d, J = 8.0 Hz, 3H). P{ H}
HH
HH
HH
13
1
NMR (162 MHz, C D ) δ 148.7, 81.7. C{ H} NMR(101MHz, C D ) δ 160.1, 139.4, 129.7,
6
6
6
6
1
24.5 121.3, 116.5, 35.3, 29.9, 28.7, 17.9 35.3, 17.1. HRMS (EI), m/z calculated for C H OP S:
20 36 2
+
386.1962 (M ), found: 386.1960.
S
O
tBu P
PEt2
2
tBu2
Et2
1
Synthesis of ligand PSCOP . Colorless oil, yield: 70%, ca. 95% purity. H NMR (400MHz,
2
2
CDCl ) δ 7.31 (s, 1H), 7.19 (d, J = 8.0Hz, 1H), 7.10-7.14 (m, 1H), 6.86 (d, J = 8.0 Hz, 1H),
3
HH
HH
2
31
1
1.83-1.76 (m, 2H), 1.69-1.61 (m, 2H), 1.29 (d, JHH = 12.0 Hz, 18H), 1.19-1.11 (m, 6H). P{ H}
1
3
1
NMR (162 MHz, CDCl ) δ 140.9, 84.6. C { H} NMR (101 MHz, CDCl ) δ 158.4, 138.9,
3
3
+
1
3
24.7, 121.4, 116.3, 35.4, 29.8, 25.2, 7.9. HRMS (EI), m/z calculated for C H OP S (M ),
18 32 2
58.1649, found: 358.1645.
tBu2
R2
General procedure for synthesis of complexes ( PSCOP )IrHCl 4a-4d
7

1
2
3
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5
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0
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0
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8
9
0
1
2
3
4
5
A rubber septum-capped Schlenk flask was charged with 1 equiv of [Ir(COD)Cl] and 2.0 equiv
2
of the PSCOP ligand. Toluene (10 mL) was added via syringe, and the solution was stirred in an
oil bath at 120 °C for several hours. The volatiles were then removed under high vacuum. The
residue was washed with pentane and dried under high vacuum to yield the pure product.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
S
O
tBu P
PtBu2
2
Ir
H
Cl
tBu2
tBu2
Synthesis of complex ( PSCOP )IrHCl 4a. Following the general procedure, treatment of
tBu2
tBu2
2
42 mg (0.36 mmol) of [Ir(COD)Cl] with 298 mg (0.72 mmol) of PSCOP for 10 h at 120
2
1
°
C in 10 mL toluene afforded 402 mg (0.63 mmol, 87%) of 4a as a dark red solid. H NMR (400
2
2
MHz, C D ): δ 7.13 (t, J = 8.0 Hz, 1H), 6.69 (d, J = 4.0 Hz, 2H), 1.47-1.22 (m, 36H), -
6
6
HH
HH
2
31
1
2
2
41.05 (t, J = 14.0 Hz, 1H). P{ H} NMR (162 MHz, C D ): δ 164.5 (dd, J = 22.7 Hz, J
PH
6
6
PH
PP
2
2
13
1
= 358.0 Hz), 107.4 (dd, JPH = 22.7 Hz, JPP = 335.3 Hz). C { H} NMR (101 MHz, C
D
) δ
6
6
1
67.8, 153.8, 132.9, 125.3, 116.7, 107.9, 43.2, 42.7, 39.7, 39.5, 29.5, 27.7. Elemental analysis,
calcd for C H ClIrOP S (642.23): C, 41.14, H, 6.28; Found: C, 40.83, H, 6.26.
2
2
40
2
S
O
tBu P
PCy2
2
Ir
H
Cl
tBu2
Cy2
Synthesis of Complexes ( PSCOP )IrHCl 4b. Following the general procedure, treatment
tBu2
Cy2
of 188 mg (0.28 mmol) of [Ir(COD)Cl]
2
with 262 mg (0.56 mmol) of
PSCOP for 10 h at
1
1
20 °C in 10 mL toluene afforded 305 mg (0.44 mmol, 79%) of 4b as a dark red solid. H NMR
2
(400 MHz, C
6
D
6
): δ 7.15-7.14 (m, 1H), 6.73-6.71 (m, 2H), 2.64 (t, J = 12.0 Hz, 1H), 2.29-
HH
2
2
2
.20 (m, 1H), 1.81-1.72 (m, 7H), 1.59-1.49 (m, 6H), 1.43 (d, J = 16.0 Hz, 9 H), 1.38 (d, J
HH
HH
2
2
31
1
=
12.0 Hz, 9 H), -39.50 (dd, J = 12.0 Hz, J = 10.0 Hz, 1H). P{ H} NMR (162 MHz,
PH
PH
2
2
2
2
C D ): δ 156.2 (dd, J = 346.7 Hz, J = 14.6 Hz), 110.5 (dd, J = 351.5 Hz, J = 11.3 Hz).
6
6
PP
PH
PP
PH
1
3
1
C{ H} NMR (101 MHz, C
9.6, 32.0, 29.5, 27.7, 27.5, 26.9, 26.6, 26.4, 26.3, 26.0, 23.1, 14.4. Elemental analysis, calcd for
S (694.31): C, 44.98, H, 6.39; Found: C, 45.02, H, 6.46.
6 6
D ) δ 167.1, 153.6, 131.6, 125.2, 116.8, 108.0, 43.5, 39.8 , 39.7,
3
26 2
C H44ClIrOP
8

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9
0
1
2
3
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9
0
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8
9
0
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7
8
9
0
1
2
3
4
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
S
O
tBu P
PiPr2
2
Ir
H
Cl
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
tBu2
iPr2
Synthesis of Complexes ( PSCOP )IrHCl 4c. Following the general procedure, treatment
tBu2
iPr2
of 262 mg (0.39 mmol) of [Ir(COD)Cl]
2
and 301 mg (0.78 mmol) of
PSCOP for 10 h at
1
1
20 °C in 10 mL toluene afforded 381 mg (0.62 mmol, 80%) of 4c as a red solid. H NMR (300
MHz, CDCl
3
): δ 7.16-7.14 (m, 1H), 6.73-6.67 (m, 2H), 2.66-2.59 (m, 1H), 2.26-2.18 (m, 1H),
2
2
1
.44 (d, J = 12.0 Hz, 9H), 1.38 (d, J = 12.0 Hz, 9H), 1.25-1.14 (m, 6H), 1.11-1.02 (m, 6H),
HH
HH
2
2
31
1
-39.40 (dd, J = 12.0 Hz, J = 10.0 Hz, 1H). P { H} NMR (162 MHz, C D ): δ 163.0 (dd,
6
6
PH
PH
2
2
2
2
13
1
J = 16.2 Hz, J = 343.4 Hz), 110.7 (dd, J = 11.3 Hz, J = 348.3 Hz). C{ H} NMR
PH
PP
PH
PP
(
101 MHz, C
6
D
6
): δ 166.9, 153.3, 131.1, 125.2, 116.9, 108.0, 43.5, 39.6, 29.8, 29.5, 28.9, 17.5,
S (614.18): C, 39.11, H, 5.91; Found: C,
1
7.3, 16.5. Elemental analysis, calcd for C20
9.15, H, 5.87.
H36ClIrOP
2
3
tBu2
Et2
Synthesis of Complexes ( PSCOP )IrHCl 4d. A Schlenk flask with a Teflon plug was
tBu2
Et2
charged with 235 mg (0.35 mmol) of [Ir(COD)Cl] , 301 mg (0.84 mmol) of PSCOP , and 10
2
mL of toluene. The solution was degassed by three pump-freeze-thaw cycles and then flask was
charged with an atmosphere of H . The flask was sealed and the solution stirred for 4 h at 120
2
°
C. The volatiles were then removed under high vacuum. The residue was dissolved in pentane.
Recrystallization of the crude product afforded 336 mg (0.57 mmol, 82%) of pure product 4d as
1
2
2
dark red solid. H NMR (400 MHz, CDCl ) δ 7.02 (d, J = 8.0 Hz, 1H), 6.76 (t, J = 8.0 Hz,
3
HH
HH
2
1H), 6.50 (d, JHH = 8.0 Hz, 1H), 2.48-2.43 (m, 1H), 2.36-2.28 (m, 1H), 2.27-2.18 (m, 2H), 1.47
2
2
2
(
d, J = 4.0 Hz, 9H), 1.50 (d, J = 4.0 Hz, 9H), 1.26-1.21 (m, 6H), -39.59 (dd, J = 14.0 Hz,
HH HH PH
2
31
1
2
2
J = 10.0 Hz, 1H). P{ H} NMR (162 MHz, C D ) δ 150.7 (d, J = 380.7 Hz), 107.6 (d, J
PP
PH
6
6
PP
13
1
=
380.7 Hz). C{ H} NMR (75 MHz, CDCl ) δ 166.4, 153.4, 131.2, 125.3, 116.9, 108.2, 43.9,
3
3
7.5, 29.4, 25.3, 25.1, 24.3, 24.0. Elemental analysis, calcd for C H ClIrOP S (586.13): C,
18 32 2
36.88, H, 5.50; Found: C, 36.87, H, 5.75.
9

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7
8
9
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1
2
3
4
5
S
O
tBu P
PtBu2
2
Ir
CO
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
tBu2
tBu2
Synthesis of ( PSCOP )Ir(CO) 6. 327 mg (0.51 mmol) of 4a and 54 mg (0.56 mmol) of
NaOtBu were dissolved in nitrogen-free benzene in a rubber septum capped Schlenk flask while
purging the flask with hydrogen. The solution was stirred for 4 h at the room temperature under
an atmosphere of hydrogen. The color of the solution turned to pale orange. Then the solution
was purged with CO. The solution was stirred overnight and the color of the solution turned to
yellow slowly. Then the solution was cannula transferred and filtered through a pad of Celite.
The volatiles were evaporated under high vacuum to give 274 mg (0.43 mmol, 85%) of the
1
2
product 4a as yellow solid. H NMR (400 MHz, CDCl ) δ 7.05 (d, J = 8.0 Hz, 1H), 6.82 (t,
3
HH
2
2
2
2
J = 8.0 Hz, 1H), 6.66 (d, J = 8.0 Hz, 1H), 1.50 (d, J = 16.0 Hz, 18H), 1.38 (d, J = 12.0
HH
HH
HH
HH
3
1
1
2
2
Hz, 18H). P{ H} NMR (162 MHz, CDCl ) δ 188.5 (d, J = 304.6 Hz), 125.8 (d, J = 304.6
3
PP
PP
13
1
Hz). C{ H} NMR (101 MHz, CDCl ) δ 196.5, 169.2, 167.6, 158.6, 127.5, 114.3, 106.6, 41.4,
3
-
1
4
1.2, 29.7, 28.5. IR (pentane, cm ): 1946.4 (ν ). Elemental analysis, calcd for C H Cl IrO P S
CO 23 39 2 2
(633.78): C, 43.59, H, 6.20; Found: C, 43.36; H, 6.24.
3
. Results and discussion
tBu2
R2
tBu2
R2
Synthesis and Characterization of PSCOP ligands and ( PSCOP )IrHCl Complexes.
The synthesis of phosphinothious/phosphinite PSCOP ligands and (PSCOP)IrHCl complexes
(
4a-d) is outlined in Scheme 1. The ligands were conveniently prepared from the readily
available m-mercaptophenol. Deprotonation of m-mercaptophenol with 2.1 equiv of NaH,
tBu2
tBu2
followed by diphosphorylation with di-tert-butylchlorophosphine afforded ligand PSCOP
tBu2
R2
in 73% yield. Ligands PSCOP (R = Cy, iPr, Et) bearing different alkyl substituents on two P
atoms were synthesized in two sequential steps. Using triethylamine as the base,
monophosphorylation of m-mercaptophenol with di-tert-butylchlorophosphine generated the
phosphinothious intermediate, which then underwent the second phosphorylation with the
tBu2
R2
corresponding dialkylchlorophosphine using NaH as the base to form ligands PSCOP in 65-
74% yield.
1
0

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0
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2
3
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6
7
8
9
0
1
2
3
4
5
Iridium complexes 4a-c were obtained in 79-87% yields through metalation of the PSCOP
ligands with [Ir(COD)Cl] in toluene under reflux. Complex 4d was prepared in 82% yield by
2
tBu2
Et2
reaction of [Ir(COD)Cl] with PSCOP in the presence of an atmosphere of H . Complexes
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
4a-c are not very sensitive to air and moisture, thus can be purified by washing with alkane
solvents under air. In contrast, complex 4d is moisture-sensitive, and was purified through
recrystallization in a glovebox.
Scheme 1. Synthesis of PSCOP Ligands and the Corresponding Iridium Complexes 4a-d.
1) 2.1 equiv NaH
THF, RT, 1 h
S
O
HS
OH
2) 2.1 equiv tBu2PCl
tBu P
PtBu2
2
THF, RT, 1 h
tBu2PSCOPtBu2, 73%
1) 1.4 equiv Et N
3
THF, 70 °C, 1 h
2) 1.1 equiv tBu2PCl
THF, RT, 1 h
1) 1.2 equiv NaH
S
O
THF, RT, 1 h
S
OH
tBu
P
PR
2
2
2
) 1 equiv R PCl
THF, RT, 1 h
2
tBu P
2
R = Cy, tBu2PSCOPCy2, 74%
R = iPr, tBu2PSCOPiPr2, 65%
R = Et, tBu2PSCOPEt 70%
2
0.5 equiv [Ir(COD)Cl]2
S
O
S
O
toluene
refluxing, 4-10 h
tBu P
PR2
2
Ir
tBu P
PR2
2
H
Cl
R = tBu, Cy, iPr, Et
R = tBu, 4a, 87%
R = Cy, 4b, 79%
R = iPr, 4c, 80%
R = Et, 4d, 82%
All complexes 4a-d were characterized by NMR spectroscopy and elemental analysis. The
3
1
1
P{ H} NMR spectra of complexes 4a-d exhibits an AB pattern of the two non-equivalent P
1
nuclei. In the H NMR spectra, the characteristic hydridic IrH resonances appear as a triplet in
the range of -35.50 (4b) and -41.05 (4a), similar to that expected for a five-coordinate Ir(III)
hydrido chloride species with the hydride trans to a vacant coordination site. In addition,
complexes 4a, 4b, and 4d were characterized by single-crystal X-ray diffraction (Figure 1). Note
11

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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
that the hydrides in the crystal structures of 4a and 4d could not be located. The selected bond
distances and angles for complexes 4a, 4b, and 4d are listed in Table 1. Complexes 4a and 4b
adopt a square pyramidal geometry with the apical site occupied by the hydride. Notably, the
solid state structure of the least sterically hindered complex 4d adopts a chloride-bridged
dinuclear structure. However, we consider complex 4d as having a mononuclear structure in
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
31
solution because it has only one pair of AB pattern resonance in the P NMR spectrum and one
1
hydride signal at -39.62 ppm in the H NMR spectrum, consistent with a pentacoordinate
[
46]
mononuclear Ir(III) species . Both Ir atoms in the X-ray structure of dimeric 4d appear to have
unoccupied coordination sites where in fact the hydrides are presumably located; these sites are
not positioned where the hydrides could be bridging. In particular the “vacant” sixth coordination
site of Ir2 in 4d is trans to the bridging chloride.
Figure 1. ORTEP plot of complexes 4a, 4b, 4d, and 2 with thermal ellipsoids shown at 50%
probability. The hydrides in 4a and 4d could not be located.
12

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7
8
9
0
1
2
3
4
5
Steric and Electronic Properties of PSCOP Iridium Complexes. The only structural
tBu2
tBu2
tBu2
tBu2
tBu2
tBu2
difference between ( PSCOP )IrHCl (4a), ( POCOP )IrHCl (2), and ( PCP )IrHCl
(1) are the linkers between aryl backbone and the P atoms. Thus, it is of interest to assess the
influence of the linkers on the steric properties of the iridium pincer complexes by comparing the
molecular structures of these isostructural complexes. The single crystal structure of 1 has been
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
[16]
previously reported ; for comparison the structure of complex 2 was determined
crystallographically in this study (Figure 1).
Table 1. Selected Bond Distances (Å) and Angles (deg) for complexes 4a, 4c, 4d, and 2.
4
a
4c
4d
2
Ir1-C17 2.019(3),
Ir1-P1 2.2986(8),
Ir1-P2 2.2980(8),
Ir1-Cl1 2.4149(8),
C18-S1 1.772(7),
C22-O1 1.396(9),
P1-S1 2.123(3),
P2-O1 1.670(7)
Ir1-C1 2.018(4),
Ir1-C1 2.022(7), Ir1-P1 2.2573(19)
Ir1-C10 2.011(5),
Ir1-P1 2.3102(12),
Ir1-P2 2.2563(13),
Ir1-Cl1 2.3970(12),
C2-S1 1.763(5),
C6-O1 1.408(5),
P1-S1 2.1085(18),
P2-O1 1.638(4)
Ir1-P2 2.3224(18), Ir1-Cl1, 2.4066(19),
P2-S1, 2106(3), P1-O1, 1.643(6),
Ir2-C19 2.046(6), Ir2-P3, 2.2588(17),
Ir2-P4, 2.3321(17), Ir2-Cl2 2.4562(17),
Ir2-Cl1 2.5980(19), P4-S2 2.107(2),
P3-O2 1.637(5)
Ir1-P1 2.2971(12),
Ir1-P2 2.2928(12),
Ir1-Cl1 2.4041(12),
C11-O9 1.399(5),
C15-O16 1.394(5),
P1-O9 1.662(3),
P2-O16 1.662(3)
P1-Ir1-P2, 168.32(3), P1-Ir1-P2 169.03(4),
P1-Ir1-P2 169.10(7), P1-Ir1-C1 81.2(2),
P1-Ir1-P2 160.06(4),
P1-Ir1-C10 80.13(14),
P2-Ir1-C10 80.01(14),
C10-Ir1-Cl1
P1-Ir1-C17 84.50(9), P1-Ir1-C1 87.78(13), P2-Ir1-C1 88.2(2), C1-Ir1-Cl1 175.4(2),
P2-Ir1-C17 83.83(9), P2-Ir1-C1 81.40(13)
C2-S1-P2, 100.7(3), C6-O1-P1 115.4(5),
S1-P2-Ir1, 104.45(9), O1-P1-Ir1 104.7(2);
P3-Ir2-P4 162.13(6), P3-Ir2-C19
81.14(18),
C17-Ir1-Cl1
172.97(9),
C1-Ir1-Cl1
169.94(12),
179.11(14),
C17-C18-S1
118.9(4),
C2-S1-P1 99.78(16),
C6-O1-P2 115.0(3)
S1-P1-Ir1 105.08(6),
C11-O9-P1 115.5(3),
C15-O16-P2 115.5(3),
O9-P1-Ir1 104.40(12),
O16-P2-Ir1 104.45(12)
P4-Ir2-C19 85.67(18), C19-Ir1-Cl2
176.52(18),
C17-C22-O1
120.9(6),
O1-P2-Ir1 104.76(13) C24-S2-P4 98.7(2), C20-O2-P3 116.3(4),
S2-P4-Ir2 105.10(9), O2-P3-Ir2 104.5(2),
Ir1-Cl1-Ir2 153.93(10)
S1-P1-Ir1 106.46(7),
O1-P2-Ir1 103.2(3)
As shown in Table 1, complex 4a with the hybrid PSCOP ligand has a P–Ir–P angle of
68.32(3)˚, which is significantly greater than that for the POCOP analogue 2 (160.06(4)˚). The
S-linker in PSCOP with a covalent radius of 1.05 Å is much larger than the O-linker in POCOP
1
1
3

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
[47]
with a covalent radius of 0.66 Å . The longer S–C (1.772(7) Å) and S–P (2.123(3) Å) bonds in
4
a compared to O–C (1.39 Å) and O–P (1.66) Å) bonds in 2 (Table 1), combined with the larger
size of the S-linker versus O, results in the phosphine substituents being “pushed forward” in 4a.
Consequently, the gap between the trans-di-tert-butyl phosphino groups of 4a is smaller than
that of 2. The P–Ir–P angle in complex 4a (168.32(3)˚) is even larger than that in the PCP
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
[16]
complex 1 (164.27(4)˚). The data indicate that for those iridium complexes containing the
same phosphino-substituents, the degree of crowding follows the trend, (PSCOP)Ir > (PCP)Ir >
(POCOP)Ir.
NaOtBu
S
O
CO
S
O
(1)
tBu P
PtBu2
tBu P
PtBu2
2
Ir
toluene
2
Ir
H
Cl
CO
4a
6, 88%
One possible method for evaluation of the electronic property of the pincer Ir complexes is the
νCO stretching frequencies of the corresponding carbonyl complexes. Complex 6,
tBu2
tBu2
(
PSCOP )Ir(CO), was obtained in 88% yield by reaction of 4a with NaOtBu in the presence
-
1
of CO (eq 1). The ν stretching frequency of complexes 6 in a pentane solution (1954 cm ) is
CO
tBu2
tBu2
-1 [18]
slightly blue-shifted from that of ( POCOP )Ir(CO) (1949 cm ), suggesting that the
PSCOP)Ir fragment is slightly more electron-deficient than the (POCOP)Ir fragment. However,
it has been shown that electrostatic factors also, independently, affect the ν stretching
(
CO
[14]
frequency which is therefore not a definitive measure of electron density at the Ir center .
Catalytic transfer-dehydrogenation of COA using complexes 4a-d. The PSCOP iridium
complexes were first tested as precatalysts for transfer-dehydrogenation of COA with tert-
butylethylene (TBE). COA-to-TBE hydrogen transfer is considered a “benchmark” reaction, and
is thermodynamically favorable by ~6 kcal/mol. With the PCP iridium complex 1 as the catalyst,
TONs up to 1000 can be achieved at 200° C, but inhibition of catalysis by TBE requires
[
9]
portionwise addition of the hydrogen acceptor to the reaction mixture . The POCOP iridium
system is less subject to inhibition by TBE; with a COA:TBE:catalyst ratio of 3300:3300:1, the
[19]
best POCOP catalyst gives 2200 TOs after 40 h at 200 °C . The iPr-substituted (PSCOP)Ir
1
4

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
complex (3) appears to be more active than complexes 1 and 2 for COA/TBE transfer-
dehydrogenation; TONs up to 5900 have been obtained after 15 h at 200 °C with a
[
44]
COA:TBE:catalyst ratio of 6000:6000:1 . However, to assess the effect of the linkers on the
catalytic activity, it is more useful to compare complex 4a to 1 and 2 because these all contain
tBu substituents on the P atoms.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
Table 2. TONs for the Transfer-Dehydrogenation of COA/TBE Catalyzed by Complexes 4a-d in
a
the Presence of NaOtBu.
1.3 mM [Ir]
.0 mM NaOtBu
2
+
+
2
00° C
COA
3.9 M
TBE
3.9 M
COE
TBA
time
4a
4b
56
4c
4d
1
0 min
0 min
69
607
679
694
707
732
472
503
505
509
519
3
100
101
102
106
109
139
164
164
1
h
4 h
8
h
a
Average of three runs, based on conversion of TBE determined by GC.
Upon activation with NaOtBu, all PSCOP iridium complexes 4a-d are active for COA/TBE
transfer-dehydrogenation, but their catalytic activities vary substantially. A system containing
1
.3 mM iridium precatalyst, 2.0 mM NaOtBu, 3.9 M COA, and 3.9 M TBE (3000 equiv relative
to Ir), was heated at 200 °C under argon in a sealed vessel. The results are summarized in Table
tBu2
tBu2
2
. The most sterically hindered complex, ( PSCOP )IrHCl (4a), bearing tBu groups on both
P atoms exhibits low activity. After 8 h, catalysis by 4a converted only 106 equiv of TBE to
TBA. This result, together with the data obtained in COA/TBE transfer-dehydrogenation
[
9, 19, 44]
reactions using 1 and 2 as the catalyst precursors (vide supra)
, indicate that the sterically
more congested PSCOP complex is much less efficient than the tBu-substituted POCOP and
PCP analogues.
1
5

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
Reducing the size of the phosphino-substituents has an advantageous effect on the catalytic
tBu2
Cy2
activity. Complex ( PSCOP )IrHCl 4b with cyclohexyl substituents on one P atom shows
very slightly enhanced activity relative to 4a, giving 164 TOs after 8 h. Complex
tBu2
iPr2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
(
PSCOP )IrHCl 4c provides a more active catalyst than 4a and 4b; dehydrogenation with 4c
gave 732 TOs after 8 h (entry 3). However, the catalytic activity does not increase by using the
tBu2
Et2
least hindered analogue 4d, ( PSCOP )IrHCl. The run with 4d gave 519 TOs after 8 h. The
tBu2
Et2
solid-state structure of 4d reveals that the iridium species with the PSCOP ligand tends to
form a dinuclear cluster (vide supra). Thus we attribute the reduced catalytic activity of complex
4d relative to 4c to facile catalytic deactivation via formation of clusters. Notably a similar
decomposition pathway has been reported for a sterically undemanding complex
tBuMe
tBuMe
[15]
(
PCP
)IrH4 . It should also be noted that more than 95% of overall TONs occur within
the first 4 hours for catalysis with all the PSCOP complexes investigated. To summarize the
results in this section, the catalytic activity of PSCOP complexes in COA/TBE transfer-
iPr2
iPr2
tBu2
iPr2
dehydrogenation follows the trend: ( PSCOP )IrHCl (3) > ( PSCOP )IrHCl (4c) >
tBu2
Et2
tBu2
Cy2
tBu2
tBu2
(
PSCOP )IrHCl (4d) > ( PSCOP )IrHCl (4b) > ( PSCOP )IrHCl (4a).
Catalytic transfer-dehydrogenation of n-octane using complexes 4a-d. Next, the PSCOP
iridium catalysts were tested for transfer-dehydrogenation of linear alkane. A system containing
the catalyst precursor (1.0 mM), NaOtBu (1.5 mM), TBE (0.5 M, 500 equiv relative to Ir) in n-
octane was heated at 200° C under argon. The results, as summarized in Table 3, are compared to
tBu2
tBu2
tBu2
tBu2
those using the PCP complex, ( PCP )IrH
(1), POCOP complex, ( POCOP )IrHCl (2),
2
iPr2
iPr2
and the iPr-substituted PSCOP complex ( PSCOP )IrHCl (3). Consistent with the catalytic
activity observed in COA/TBE transfer-dehydrogenation reactions, the more sterically
tBu2
tBu2
tBu2
Cy2
demanding complexes ( PSCOP )IrHCl (4a) and ( PSCOP )IrHCl (4b) are less effective
tBu2
iPr2
tBu2
Et2
than ( PSCOP )IrHCl (4c) and ( PSCOP )IrHCl (4d) for dehydrogenation of n-octane.
Runs with 4a and 4b gave only 32 and 40 TOs after 60 min at 200° C, respectively (entries 1 and
2). A comparison between 4a and the closely related complexes 1 (60 min, 135 TOs) and 2 (60
min, 143 TOs), also bearing tBu substituents, reveals that the (PSCOP)Ir complex is much less
[
44]
efficient than the POCOP and PCP systems (entries 5 and 6) . The reaction with
tBu2
iPr2
(
PSCOP )IrHCl (4c) gave 277 TOs after 60 min (entry 3). To our satisfaction, the least
tBu2
Et2
crowded complex ( PSCOP )IrHCl 4d is markedly active, giving an initial turnover frequency
-
1
of 21 min with a total TO number of 475 after 60 min (entry 4). As a comparison, the activity of
1
6

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
iPr2
iPr2
4
d is slightly lower than the highly active precatalyst ( PSCOP )IrHCl (3) (entry 7, 60 min,
5
00 TOs), but much higher than PSCOP complexes 4a-c. For n-octane/TBE transfer-
dehydrogenation reactions, the catalytic activity of the new PSCOP complexes is correlated to
iPr2
iPr2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
the steric properties of the pincer ligands and follows the trend: ( PSCOP )IrHCl (3) >
tBu2
Et2
tBu2
iPr2
tBu2
Cy2
(
(
PSCOP )IrHCl (4d) > ( PSCOP )IrHCl (4c) > ( PSCOP )IrHCl (4b) >
tBu2
tBu2
PSCOP )IrHCl (4a).
a
Table 3. Transfer-dehydrogenation of n-octane/TBE Catalyzed by complexes 4a-d at 200 °C.
1.0 mM [Ir]
1.5 mM NaOtBu
n-octane + TBE
octenes
+
TBA
200 °C
0
.5 M
a
b
Entry
1
Cat.
Time
min)
TONs
1-oct
%
trans-2
cis-2
(
[mM]
5
[mM]
1
[mM]
4a
4b
4c
4d
1
5
1
6
83
55
40
34
71
60
40
38
32
24
10
8
0
0
4
6
0
11
6
5
30
25
10
11
10
14
6
5
0
32
2
3
4
7
5
1
1
10
10
6
3
1
3
0
35
14
15
21
22
22
22
19
20
18
12
35
39
37
27
5
9
6
60
5
40
12
23
34
79
98
39
78
149
116
20
32
48
64
9
8
65
13
19
42
50
23
42
71
43
10
16
23
27
4
1
0
89
30
213
277
106
212
448
475
69
6
5
1
3
0
18
9
0
0
4
60
5
3
c,d
5
51
42
31
20
26
19
7
10
92
3
6
5
1
0
0
120
135
19
d
6
2
0
26
5
12
31
47
5
30
83
6
14
21
6
0
143
7
5
1
7

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
d
7
3
5
1
114
192
495
500
38
37
13
10
33
19
3
44
20
37
65
50
0
77
30
137
110
6
0
2
a
b
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
Average of three runs, based on conversion of TBE determined by GC. The fraction of 1-
c
d
octene relative to the total octenes. No NaOtBu was added. Data obtained from Ref [12].
The selective formation of terminal olefin via alkane dehydrogenation is a goal of great
importance. Transfer-dehydrogenation of n-octane using bulky complexes 4a and 4b gave high
regioselectivity for 1-octene formation at the early stages of the reactions. For example, after 5
min at 200° C, the terminal olefin (5 mM) obtained in the run with 4a constitutes 83% of total
octenes (entry 1). The combined 1-octene (11 mM) and 2-octenes (20 mM) observed in the
reaction with 4a after 60 min comprises >95% of total octenes, indicating catalytic
isomerization of 2-octenes to 3- or 4-octenes is much slower than isomerization of 1-octene to 2-
octenes. The more active and less hindered PSCOP precatalysts 4c and 4d afforded low fractions
of 1-octene (entries 3 and 4). For instance, the reactions with 4d gave 18% 1-octene (19 mM)
after 106 turnovers in 5 min, and 3% of 1-octene (12 mM) after 475 turnovers in 60 min (entry
4
). A substantial amount of 3- and 4-octenes was formed in this case, implying very rapid olefin
isomerization concomitant with transfer-dehydrogenation.
4. Conclusions
Using readily available m-mercaptophenol as the building block, we have prepared a series of
phosphinothious-phosphinite (PSCOP)Ir pincer complexes with varying steric properties. The S-
for-O substitution of the pincer ligand leads to a sterically more hindered, and electronically
more deficient iridium center in comparison with the parent (POCOP)Ir system. We also
demonstrated that replacement of tBu groups with sterically less congested units has a substantial
favorable impact on catalytic transfer-dehydrogenation activity.
ASSOCIATED CONTENT
Author information
Corresponding Author
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*
E-mail: mbrookhart@unc.edu; huangzh@sioc.ac.cn
Notes
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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6
The authors declare no competing financial interest.
Acknowledgements
We gratefully acknowledge the financial support from the National Basic Research Program of
China (2015CB856600), the National Natural Science Foundation of China (21432011,
21422209), and the National Science Foundation as part of the Center for Enabling New
Technologies through Catalysis (CENTC), Phase II Renewal, CHE-1205189.
Appendix A. Supplementary data
CCDC 1444862-1444865 contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
REFERENCES
1] K. Weissermel, H.-J. Arpel, Industrial Organic Chemistry; Wiley-VCH, Weinheim,
Germany, 2003; pp 59–89.
2] M.J. Burk, R.H. Crabtree, J. Am. Chem. Soc. 109 (1987) 8025.
[3] M.J. Burk, R.H. Crabtree, C.P. Parnell, R.J. Uriarte, Organometallics 3 (1984) 816.
4] H. Felkin, T. Fillebeen-khan, R. Holmes-Smith, L. Yingrui, Tetrahedron Lett. 26, (1985)
1999.
[
[
[
[
5] J. Choi, A.H.R. MacArthur, M. Brookhart, A.S. Goldman, Chem. Rev. 111 (2011) 1761.
6] Y. Zhang, W. Yao, H. Fang, A. Hu, Z. Huang, Sci. Bull. 60. (2015) 1316.
[
[7] M. Findlater, J. Choi, A.S. Goldman, M. Brookhart, In Alkane C-H Activation by Single-
Site Metal Catalysis, ed. P.J. Perez, Springer, Netherlands, 38, (2012) 113.
́
[8] J. Choi, A.S. Goldman, in Iridium Catalysis, ed. P.G. Andersson, Springer,
Berlin/Heidelberg, 2011, 139-167.
[9] M. Gupta, C. Hagen, R.J. Flesher, W.C. Kaska, C.M. Jensen, Chem. Commun. (1996)
2083;
[
10] M. Gupta, W.C. Kaska, C.M. Jensen, Chem. Commun. (1997) 461.
11] W. Xu, G.P. Rosini, K. Krogh-Jespersen, A.S. Goldman, M. Gupta, C.M. Jensen, W.C.
[
Kaska, Chem. Commun. (1997) 2273.
[12] D.W. Lee, W.C. Kaska, C.M. Jensen, Organometallics 17 (1998) 1.
1
9

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
[
13] F. Liu, E.B. Pak, B. Singh, C.M. Jensen, A.S. Goldman, J. Am. Chem. Soc. 121 (1999)
4
1
086.
[14] K. Zhu, P.D. Achord, X. Zhang, K. Krogh-Jespersen, A.S. Goldman, J. Am. Chem. Soc.
26 (2004) 13044.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
[15] S. Kundu, Y. Choliy, G. Zhuo, R. Ahuja, T.J. Emge, R. Warmuth, M. Brookhart, K.
Krogh-Jespersen, A.S. Goldman, Organometallics 28 (2009) 5432.
[16] B. Punji, T.J. Emge, A.S. Goldman, Organometallics 29 (2010) 2702.
[17] A. Kumar, T. Zhou, T.J. Emge, O. Mironov, R.J. Saxton, K. Krogh-Jespersen, A.S.
Goldman, J. Am. Chem. Soc. 135 (2015) 9894.
[
18] I. Göttker-Schnetmann, P.S. White, M. Brookhart, Organometallics 23 (2004) 1766.
19] I. Göttker-Schnetmann, P.S. White, M. Brookhart, J. Am. Chem. Soc. 126 (2004) 1804.
[
[20] I. Göttker-Schnetmann, M. Brookhart, J. Am. Chem. Soc. 126 (2004) 9330.
21] D. Morales-Morales, D. R. Redón, R. C. Yung, C.M. Jensen, C. M. Inorg. Chim. Acta 357
2004) 2953.
22] M.W. Haenel, S. Oevers, K. Angermund, W.C. Kaska, H.-J. Fan, M. B. Hall, Angew.
Chem. Int. Ed. 40 (2001) 3596.
23] S.A. Kuklin, A.M. Sheloumov, F.M. Dolgushin, M.G. Ezernitskaya, A.S. Peregudov, P.V.
Petrovskii, A.A. Koridze, Organometallics 25 (2006) 5466.
24] A.R. Chianese, A. Mo, N.L. Lampland, R.L. Swartz, P.T. Bremer, Organometallics 29
2010) 3019.
[
(
[
[
[
(
[
[
[
25] J.J. Adams, N. Arulsamy, D.M. Roddick, Organometallics 31 (2012) 1439.
26] W. Zuo, P, Braunstein, Organometallics 31 (2012) 2606.
27] Y. Shi, T. Suguri, C. Dohi, H. Yamada, S. Kojima, Y. Yamamoto, Chem. Eur. J. 19
(
2013) 10672.
28] D. Bézier, M. Brookhart, ACS Catal. 4 (2014) 3411.
[29] A.V. Polukeev, R. Gritcenko, K.J. Jonasson, O.F. Wendt, Polyhedron 84 (2014) 63.
30] D.F. Brayton, P.R. Beaumont, E.Y. Fukushima, H.T. Sartain, D. Morales-Morales, C.M.
Jensen, Organometallics 33 (2014) 5198.
[
[
[
31] X. Jia, L. Zhang, C. Qin, X. Leng, Z. Huang, Chem. Commun. 50 (2014) 11056.
32] X. Jia, Z. Huang, Sci. China Chem. 58 (2015) 1340.
[
[33] K. Tanoue, M. Yamashita, Organometallics 34 (2015) 4011.
34] B.C. Gruver, J.J. Adams, S.J. Warner, N. Arulsamy, D.M. Roddick, Organometallics 30
2011) 5133.
35] Y. Zhang, H. Fang, W. Yao, X. Leng, Z. Huang, Organometallics 35 (2016) 181.
[36] Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B.
[
(
[
C. Adv. Synth. Catal. 2009, 351, 188.
[37] A.S. Goldman, A.H. Roy, Z. Huang, R. Ahuja, W. Schinski, M. Brookhart, Science 312
2006) 257.
(
20

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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8
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2
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0
1
2
3
4
5
[
38] Z. Huang, E. Rolfe, E.C. Carson, M. Brookhart, A.S. Goldman, S.H. El-Khalafy, A.H.R.
MacArthur, Adv. Synth. Catal. 352 (2010) 125.
[39] M.C. Haibach, S. Kundu, M. Brookhart, A.S. Goldman, Acc. Chem. Res. 45 (2012) 947.
[40] R. Ahuja, B. Punji, M. Findlater, C. Supplee, W. Schinski, M. Brookhart, A.S. Goldman,
Nat. Chem. 2011, 3, 16.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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4
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4
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4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
[
[
[
41] S. Kundu, T.W. Lyons, M. Brookhart, ACS Catalysis 3 (2013) 1768.
42] Jia, X.; Huang, Z. Nat. Chem. 8, (2016) 157.
43] S. Biswas, Z. Huang, Y. Choliy, D.Y. Wang, M. Brookhart, K. Krogh-Jespersen, A.S.
Goldman, J. Am. Chem. Soc. 134 (2012) 13276.
[44] W. Yao, Y. Zhang, X. Jia, Z. Huang, Angew. Chem. Int. Ed. 53 (2014) 1390.
[45] J.L. Herde, J.C. Lambert, C.V. Senoff, M.A. Cushing, Inorg. Synth. 15 (1974) 18.
[46] An alternate explanation is that there is a rapid equilibrium between monomer and dimer
on an NMR timescale; however, an average hydride shift at -39.6 ppm seems inconsistent with
such an explanation. The complexes all reveal a single far-upfield chemical shift that is highly
characteristic of the hydride ligand of a coordinatively unsaturated, five-coordinate, Ir(III)
complex, viz. 4a, δ -41.05; 4b, δ -39.50; 4c, δ -40.32, and 4d, δ -39.62. Crystallography reveals
that 4a and 4b are monomeric in the solid state; 4c should be similar to 4c in terms of steric
effects (with two cyclohexyl groups vs. two iPr groups on the phosphinite group), therefore it is
presumably monomeric in the solid too. Thus the solution-phase chemical shift of 4d, although it
is dimeric in the crystalline form, strongly indicates that it too is monomeric in solution.
Likewise, the J coupling constants of 4d, are close to those of monomeric 4a, 4b and 4c.
2
HP
[47] B. Cordero, V. Gomez, A.E. Platero-Prats, M. Reves, J. Echeverria, E. Cremades, F.
Barragan, S. Alvarez, Dalton Trans. (2008) 2832.
21

tBu2
R2
New ( PSCOP )IrHCl iridium complexes ligated by hybrid phosphinothious-phosphinite PSCOP
ligands have been synthesized and characterized. The steric properties of the pincer ligands prove to
have a marked impact on catalytic activities of these complexes in transfer-dehydrogenation of cyclic
and linear alkanes.

tBu2
R2
New ( PSCOP )IrHCl iridium complexes ligated by hybrid phosphinothious-phosphinite PSCOP
ligands have been synthesized and characterized. The steric properties of the pincer ligands prove to
have a marked impact on catalytic activities of these complexes in transfer-dehydrogenation of cyclic
and linear alkanes.