Syntheses and characterizations of iridium complexes containing bidentate phosphine ligands and their catalytic hydrogenation reactions to α,β-unsaturated aldehydes

Article abstract of DOI:10.1016/S1381-1169(01)00294-1

Five iridium complexes containing bidentate phosphine ligands, IrH(CO)(PPh₃)(BPPB) [BPPB = 1,2-bis(diphenylphosphino)benzene] (1), IrH(CO)(PPh₃)(BISBI) [BISBI = 2,2′-bis(diphenylphosphinomethyl)-1,1′-biphenyl] (2), IrH(CO) (PPh₃)(BDNA) [BDNA = 1,8- bis(diphenylphosphinomethyl)naphthalene] (3), IrH(CO)(PPh₃)(BDPX) [BDPX = 1,2-bis(diphenylphosphinomethyl)benzene] (4), and IrHCI(CO)(PCP) [PCP-H = 1,3-bis(diphenylphosphinomethyl)benzene] (5) were synthesized. Their compositions and structures were identified by elemental analysis, FTIR, ³¹P{(¹H}) NMR and ¹H NMR. The molecular structure of IrH(CO)(PPh₃)(BDNA) determined by single crystal X-ray diffraction indicated a trigonal bipyramidal structure with the three phosphorus atoms in the equatorial plane. The crystal belonged to triclinic system, P1 space group, a = 11.47 ?, b = 11,65 ?, c = 19.20 ?, α = 81.95°, β = 75.60°, γ = 70.60°, and Z = 2. The catalytic hydrogenation activities and selectivities of the five complexes as catalysts for citral and cinnamaldehyde were investigated. Complexes 1-4 showed high selectivity for the hydrogenation of C=O group in citral. High selectivity for the hydrogenation of C=O group in cinnamaldehyde catalyzed by complex 4 could be obtained in the presence of excess amount of ligand (BDPX).

Full text of DOI:10.1016/S1381-1169(01)00294-1

Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
Syntheses and characterizations of iridium complexes containing
bidentate phosphine ligands and their catalytic hydrogenation
reactions to ␣,␤-unsaturated aldehydes
Rui-Xiang Lia, Xian-Jun Lia,∗, Ning-Bew Wongb, Kam-Chung Tinb,
Zhong-Yuan Zhouc, Thomas C.W. Makc
Department of Chemistry, Sichuan University, Chengdu, Sichuan, China
a
b
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
c
Department of Chemistry, Chinese University of Hong Kong, Shatin, NT, Hong Kong, China
Received 12 October 2000; received in revised form 18 June 2001; accepted 30 June 2001
Abstract
Five iridium complexes containing bidentate phosphine ligands, IrH(CO)(PPh3)(BPPB) [BPPB = 1,2-bis(diphenyl-
phosphino)benzene] (1), IrH(CO)(PPh3)(BISBI) [BISBI = 2,2ꢀ-bis(diphenylphosphinomethyl)-1,1ꢀ-biphenyl] (2), IrH(CO)
(PPh3)(BDNA) [BDNA = 1,8-bis(diphenylphosphinomethyl)naphthalene] (3), IrH(CO)(PPh3)(BDPX) [BDPX = 1,2-
bis(diphenylphosphinomethyl)benzene] (4), and IrHCl(CO)(PCP) [PCP-H = 1,3-bis(diphenylphosphinomethyl)benzene]
1
(5) were synthesized. Their compositions and structures were identified by elemental analysis, FTIR, 31P{ H} NMR and 1H
NMR. The molecular structure of IrH(CO)(PPh3)(BDNA) determined by single crystal X-ray diffraction indicated a trigonal
bipyramidal structure with the three phosphorus atoms in the equatorial plane. The crystal belonged to triclinic system, P1
space group, a = 11.47 Å, b = 11.65 Å, c = 19.20 Å, α = 81.95◦, β = 75.60◦, γ = 70.60◦, and Z = 2. The catalytic
hydrogenation activities and selectivities of the five complexes as catalysts for citral and cinnamaldehyde were investigated.
=
Complexes 1–4 showed high selectivity for the hydrogenation of C O group in citral. High selectivity for the hydrogenation
=
of C O group in cinnamaldehyde catalyzed by complex 4 could be obtained in the presence of excess amount of ligand
(BDPX). © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Iridium complex; Bidentate phosphine; Hydrogenation; Citral; Cinnamaldehyde
1. Introduction
plexes [1–7]. Although iridium complexes generally
exhibit lower catalytic hydrogenation activities for
unsaturated compounds than ruthenium and rhodium
complexes, the iridium-phosphine complexes usually
exhibit very good selectivity for the hydrogenations of
␣,␤-unsaturated ketones [8–12] and ␣,␤-unsaturated
aldehydes [13–16] under optimum reaction condi-
tions. However, the studies on syntheses and catalytic
reactions of iridium complexes containing bidentate
phosphines are still very scarce [17–20]. The hydro-
genation of carbonyl group of an organic compound
In recent years, a great number of ruthenium and
rhodium complexes with bidentate phosphines were
synthesized and characterized. These complexes gen-
erally exhibited better activities and selectivities for
hydrogenation and hydroformylation of unsaturated
substrates than the analogous monophosphine com-
∗ Corresponding author. Fax: +86-28-5412006.
E-mail address: scuulixj@mail.sc.cninfo.net (X.-J. Li).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00294-1

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catalyzed by transition metal complex is more difficult
than hydrogenation of an olefinic group; therefore, it is
still a challenging problem to synthesize highly selec-
tive catalysts for the hydrogenation of carbonyl group
in ␣,␤-unsaturated carbonyl compounds. In this paper,
we would like to report a simplified method for syn-
thesizing 1,2-bis(diphenylphosphinomethyl)benzene
(BDPX) [21], a modified method for preparing
IrH(CO)(PPh3)(BISBI) [2,2ꢀ-bis(diphenylphosphino-
methyl)-1,1ꢀ-biphenyl] with high purity and yield [22]
and the syntheses of four new iridium complexes con-
taining biphosphine ligands with different chelating
ring sizes (Fig. 1). The effect of coordination envi-
ronment on hydrogenation activities and selectivities
for the carbonyl group of citral and cinnamaldehyde
were also discussed.
2. Experimental
2.1. Materials
All synthetic reactions were performed with
standard Schlenk technique under nitrogen atmo-
sphere. Solvents were dried over appropriate drying
agents and distilled under nitrogen prior to use.
Reagent-grade PPh3, 1,3-bis(bromomethyl)benzene
and 1,2-bis(bromomethyl)benzene were purchased
Fig. 1. Structures of ligands and complexes.

R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
183
from Aldrich. BPPB [1,2-bis(diphenylphosphino)
benzene] was from Strem, and IrCl3·HCl·xH2O was
from Kunming Nobel Metals, China. Citral (97%) and
cinnamaldehyde (98%) from Riedel-deHaen were dis-
tilled before use. Starting materials IrH(CO)(PPh3)3,
Ir(CO)Cl(PPh3)2 [23], PCP-H [1,3-bis(diphenylphos-
phinomethyl)benzene] [24], BISBI [25] and BDNA
[1,8-bis(diphenylphosphinomethyl)naphthalene] [7]
were prepared according to the reported methods.
at room temperature, and the orange-red solution
was filtered to remove the unreacted lithium. t-Butyl
chloride 2.5 g (27 mmol) in 10 ml THF was slowly
added to the filtrate cooling in an ice-bath, and then
the solution was heated to reflux for 10 min. The so-
lution was cooled to 0 ◦C in an ice-bath again, and
1,2-bis(bromomethyl)benzene 3.6 g (13.6 mmol) in
20 ml THF was added slowly into the cooled solution
over a period of 50 min, while the color of the so-
lution changed from orange-red to pale yellow. The
reaction mixture was heated to reflux for 30 min be-
fore the solvents were evaporated to give a yellow
sticky solid under vacuum. CH2Cl2 (30 ml) and water
(20 ml) were added to dissolve the solid, and then it
was allowed to separate in a separatory funnel. The
organic layer was collected and reduced to ca. 10 ml
under vacuum. Ethanol (30 ml) was added to the
CH2Cl2 solution, and a lot of white needle crystals
were formed immediately. The product was filtered,
washed with ethanol and dried under vacuum to give
BDPX as white needles; mp: 125–126 ◦C.
2.2. Analytical methods
1
1
The H and 31P{ H} NMR spectra in CDCl3 were
recorded on a Bruker ARX 300 spectrometer at room
1
temperature, at 300 MHz for H NMR with TMS as
internal standard and at 121.5 MHz for 31P{ H} NMR
1
with 85% H3PO4 as external standard, with downfield
shifts as positive. Elemental analyses were performed
by Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences. FTIR was recorded on a Perkin
Elmer 1600 spectrometer with KBr plates.
2.3. Catalytic hydrogenation
3.2. Complex 1
Appropriate amount of catalyst and substrate were
introduced into a stainless steel autoclave (100 ml)
equipped with a stirrer (Parr 4561 minireactor). The
autoclave was evacuated and flushed consecutively
with high purity hydrogen (99.995%) for five times,
then filled with the hydrogen to the desired pressure.
After the reaction mixture was heated to the desired
temperature, stirring was started (400 rpm) and reac-
tion time was accounted. The reaction was quenched
by immersing the reactor in an ice-bath. The products
were analyzed on a GC (HP 5890 Series II) with FID
and a capillary column (HP-FFAP, 25 m × 0.2 mm ×
0.33 ␮m), and the GC graphs were obtained with an
HP 3396 Integrator. The components were identified
by authentic samples on GC and GC-MS (HP 5890
GC with series mass selective detector).
IrH(CO)(PPh3)3 0.101 g (0.1 mmol) and BPPB
0.046 g (0.1 mmol) were dissolved in 10 ml toluene.
The color of the solution changed eventually to
orange-red during reflux. The 20 ml n-hexane was
added to the reaction solution and caused the forma-
tion of orange-red precipitates after 2 h. The product
was filtered, washed with diethyl ether and dried
under vacuum to give orange-red powder.
3.3. Complexes 2–5
IrH(CO)(PPh3)3 0.080 g (0.08 mmol) and BISBI
0.045 g (0.08 mmol) were dissolved in 10 ml toluene.
The yellow solution was refluxed for 3 h, and then was
concentrated to ca. 2 ml under vacuum. The addition
of 20 ml n-hexane to the solution caused the forma-
tion of yellow precipitates. The product was filtered,
washed with diethyl ether and dried under vacuum
to give yellow powder. The preparation procedures
of complexes 3–5 were the same as complex 2, but
complex 5 was formed by using IrCl(CO)(PPh3)2 as
starting materials.
3. Preparation of ligand and complexes
3.1. 1,2-Bis(diphenylphosphinomethyl)benzene
(BDPX)
Triphenylphosphine 7.9 g (30 mmol) and lithium
0.45 g (65 mmol) in 30 ml THF were stirred for 3 h
The elemental analysis, NMR and FTIR of all com-
1
plexes were listed in Tables 1–3, respectively. 31P{ H}

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R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
Table 1
Elemental analysis data of BDPX and all complexes
Ligand or complex
Calculated
C (%)
Found
C (%)
Yield
H (%)
H (%)
BDPX for C32H28P2
1 for C49H40OP3Ir
2 for C57H48OP3Ir
81.00
63.30
66.20
65.53
63.94
54.21
5.95
4.36
4.68
4.60
4.63
4.00
81.23
63.21
66.16
65.48
63.85
54.71
5.86
4.37
4.69
4.77
4.65
3.95
4.2 (65%)
0.070 (75%)
0.075 (92%)
0.046 (47%)
0.065 (68%)
0.095 (66%)
3 for C55
4 for C51
5 for C33
H46OP3Ir
H44OP3Ir
H28ClOP2Ir
Table 3
FTIR spectra of complexes
NMR of complex 4 showed widened multi-peaks at
20.70 ppm and widened doublet at 0.04 ppm with
indeterminable coupling constants at room temper-
ature. When temperature was decreased to −50 ◦C,
two sets of peaks were observed and the intensity
ratio of the stronger peaks to the weaker peaks was
ca. 2:1. The weaker peaks were a doublet at 0.37 ppm
and a triplet at 19.82 ppm with JP–P= 109.6 Hz. The
stronger peaks were a doublets at −0.52 ppm and a
triplet at 20.86 ppm with JP–P = 115.9 Hz. 1H NMR:
a widened asymmetric multiplet at −11.65 ppm which
could further split into indeterminable multiplets at
−50 ◦C.
Complex
νCO
νIr-H
1
2
3
4
5
1911.3 (s)
1926.3 (s)
1933.3 (s)
1931.4 (s)
2018.0 (s)
2032.1 (m)
2067.1 (m)
2093.1 (m)
2056.8 (m)
2183.0 (m)
with a rotating-anode generator powered at 50 kV
and 90 mA. Intensity data were collected at 294 K
using graphite-monochromatized Mo K␣ radiation
(λ = 0.7103 Å) by taking 53 oscillation photos in
the range of 0–150◦, ꢀφ = 3◦, and exposing time
was 8 min per frame. Crystal-to-detector distance
was 78.10 mm and background level was −40. The
θ range for data collection was 1.10–22.72◦. A
self-consistent semi-empirical absorption correction
was applied using ABSCOR program. All calculation
were performed with Siemens SHELXTL PLUS (PC
Version) system. Structure refinement was based on
3.4. X-ray crystallographic analysis of
IrH(CO)(PPh3)(BDNA)·1/2CH3COCH3
The yellow prism crystal was grown in acetone.
The crystal (0.20 mm × 0.20 mm × 0.16 mm) cov-
ered with a thin layer of paraffin oil as a precau-
tion against decomposition in air was mounted on
a Rigaku RAXIS IIC imaging-plate diffractometer
Table 2
NMR spectra of complexes
1
Complex
1H NMR
31P{ H} NMR
BDPX
1
2
3.3 (4H, s), 6.9–7.3 (24H, m)
−10.9 (dt), JP1,2-H = 39, JP3–H = 24
−11.5 (dt), JP1,2-H = 24, JP3–H = 15
JPa–Px = 123, JPx–Pb = 120
−11.9 (dt), JP1,2-H = 21, JP3–H = 17
−11.65 (m)
−12.3 (s)
39.3 (d), 10.6 (t), JP–P = 44
14.39 (dd), 13.63 (dd), 9.74 (dd), JPa–Pb = 120,
3
4a
21.5 (t), 13.0 (d), JP–P = 113
19.82 (dd), 0.37 (dt), JP–P = 109.6
20.86 (dd), −0.52 (dt), JP–P = 115.9
25.07 (s)
5
−17.74 (t), JP–H = 13.5
a The spectrum was recorded at −50 ◦C.

R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
185
Table 4
ature, the preparation of BDPX was much easier in
addition to a better yield (62%).
Crystal data and structure refinement for complex 3
Empirical formula
C57H46IrOP3
4.2. Structures of complexes 2–4
Formula weight
Temperature
1032.05
293(2) K
Complexes 1–4 were air sensitive in solution and
in solid state. For complex 1, the intensity ratio of the
doublet at 39.16 ppm to the triplet at 10.56 ppm was
Wavelength
Crystal system
Space group
0.71073 Å
Triclinic
P1
a = 11.468(2) Å, α = 81.95(3)◦
b = 11.647(2) Å, β = 75.60(3)◦
c = 19.197(4) Å, ␥ = 70.60(3)◦
2337.9(8) Å3
Unit cell dimensions
1
about 2:1 in 31P{ H} NMR spectrum. The small cou-
pling constant (JP–P = 44 Hz) indicated that the all
phosphorus were in cis-position to each other. Since
the coupling constant between the phosphines in an
equatorial sites is generally more than 100 Hz and is
much larger than that between the phosphines in an
equatorial and an apical site [26], the small coupling
constant supports one phosphorus atom in an apical
site and two phosphorus atoms in an equatorial site.
BPPB is of a similar natural bite angle with DIPHOS
(1,2-bis(diphenylphosphino)ethane); the small natural
bite angle (85◦) of DIPHOS leads to the structure
where one phosphorus atom occupies an equatorial site
and another phosphorus atom occupies an apical site in
the triangle bipyramidal structure. Similarly, one phos-
phorus atom in BPPB should occupy an apical site and
another phosphorus atom should occupy an equatorial
Volume
Z
Density (calculated)
Absorption coefficient
Transmission factor
F(0 0 0)
Crystal size
θ range for data collection
Index ranges
2
1.466 mg/cm3
2.998 mm−1
0.742–1.000
1036
0.20 mm × 0.20 mm × 0.16 mm
1.10–22.72◦
−12 ≤ h ≤ 11, −12 ≤ k ≤ 0,
−20 ≤ l ≤ 20
Reflections collected
Independent reflections
Maximum and minimum
transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Absolute structure parameter
Extinction coefficient
Largest differential peak
and hole
5695
5695 (Rint = 0.0000)
0.6455 and 0.5854
Full-matrix least-squares on F2
5695/27/1273
1.453
R1 = 0.0568, wR2 = 0.1522
R1 = 0.0572, wR2 = 0.1528
0.019 (9)
1
site in complex 1. H NMR spectrum showed a dou-
blet of triplet with the small P–H coupling constants
(JP1,2-H = 39 Hz, JP3–H = 24 Hz) for the coordi-
nated hydride. Therefore, it was reasonable to deduce
that the hydride was in cis-position to all three phos-
phorus atoms. The proposed molecular structure was
shown in Fig. 1. The small coupling constants and the
splitting pattern in NMR spectra indicated that a fact
fluxional process occurred at room temperature. The
fluxional process was probably a Berry pseudorotation
[26] (Fig. 2).
0.00500 (17)
0.604 and −0.763 eÅ−3
F2 for all reflections. A final R-factor was 0.0568
(wR = 0.1522). Crystallographic data were summa-
rized in Table 4.
Complex 2 was originally synthesized by Casey
et al. [22]. We had modified the method to give a
high purity and good yield product without recrystal-
4. Results and discussion
1
4.1. Preparation of BDPX
lization. Its H NMR spectrum showed a doublet of
triplet at −11.48 ppm with the small coupling con-
stants (JP1,2-H = 24 Hz and JP3–H = 15 Hz). Al-
though the PPh3 in complex 2 showed a doublet of
The ligand BDPX was very sensitive to air oxidation
in solution, but quite stable in solid form. It was gen-
erally synthesized by reacting ␣,␣ꢀ-dichloro-o-xylene
with NaPPh2 in liquid ammonia [21]. This was a
tedious procedure with unsatisfactory yield (46%).
According to the modified method of reacting triph-
enylphosphine with lithium in THF at room temper-
1
doublets at 9.74 ppm in 31P{ H} NMR, the phospho-
rus atoms of BISBI exhibited two doublets of doublets
at 14.39 and 13.63 ppm. This spectrum was consis-
tent with the ABX splitting pattern. The much larger
P–P coupling constant indicated that all phosphorus

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R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
Fig. 2. Berry peseudorotation of complex 1.
atoms were in equatorial plane If the difference of the
chelating ring was omitted, the steric structure of this
complex was shown in Fig. 1.
3 should be similar to complex 2 (Fig. 1). The pro-
posed structure was also confirmed by the single crys-
tal X-ray diffraction (Fig. 3).
The results of X-ray diffraction of complex 3 was
listed in Tables 4 and 5, and Fig. 3. The structure was
consistent with the structural analysis by NMR. How-
ever, the elemental analysis indicated that an acetone
molecule contained in the crystal cell was removed
under vacuum. The rigid naphthalene ring limited the
1H NMR spectrum of complex 3 showed doublet
of triplets at −11.93 ppm with JP1,2-H = 21 Hz and
JP3–H = 17 Hz, and 31P{ H} NMR spectrum showed
1
a doublet at 12.96 ppm and a triplet at 21.52 ppm with
JP–P = 113 Hz. Thus, the much large coupling con-
stant supported that the steric structure of complex
Fig. 3. An ORTEP drawing of complex 3.

R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
187
Table 5
and 19.82 ppm, and the wide peak at 0.04 ppm was
split into two doublets which were centered at 0.37
and −0.52 ppm. The results strongly indicated that the
complex was composed of two isomers and they could
be converted to each other in solution at room temper-
ature. The intensity ratio of the weaker set of peaks
at 19.82 and 0.37 ppm to the stronger set of peaks
at 20.86 and −0.52 ppm was ca. 1–2. The hydride in
1H NMR spectrum appeared as a widened unsym-
metric multiplet at −11.65 ppm which was further
split into overlapping multiplet as temperature was de-
creased to −50 ◦C. The absence of large H–P coupling
Selected bond lengths (Å) and bond angles (◦)
Bonds
Length (Å) or angle (◦)
Ir–C(55)
Ir–P(1)
Ir–P(2)
Ir–P(3)
C(55)–Ir–P(1)
C(55)–Ir–P(2)
C(55)–Ir–P(3)
P(1)–Ir–P(2)
P(1)–Ir–P(3)
P(2)–Ir–P(3)
C(2)–C(11)–C(10)
C(5)–C(6)–C(7)
1.912 (3)
2.2444 (10)
2.3458 (9)
2.2762 (11)
97.57 (8)
104.95 (9)
93.64 (8)
103.98 (4)
124.69 (3)
124.85 (4)
127.3 (3)
1
constant in H NMR spectrum had indicated that the
109.1 (3)
hydride was in the cis-position to all phosphine lig-
ands. Since the two isomers showed the same split-
ting pattern and the close coupling constant in NMR
spectra, it indicated that the chemical environments of
phosphorus atoms could not be very different. We de-
duced that the difference was from the transformation
of the backbone in bidentate ligand (Fig. 4). The ma-
jor isomer should be B because the stero hindrance
between hydrogen atom and bidentate ligand in struc-
ture B was smaller than in structure A.
chelating angle of P(1)–Ir–P(2) to 103.98◦, which
was much smaller than 120◦. As a result, 124.69◦ of
P(1)–Ir–P(3) and 124.85◦ of P(2)–Ir–P(3) were larger
than 120◦. The 97.58◦of C(55)–Ir–P(1), 104.95◦
of C(55)–Ir–P(2) and 104.95◦ of C(55)–Ir–P(3) in-
dicated that phosphine ligands were close to the
hydride in the trans-position of carbonyl in order to
reduce the tension caused by the interaction between
the carbonyl group and the phosphine ligands. There-
fore, Ir did not located in the plane formed by P(1),
P(2) and P(3). Because of the distorted tension force,
the bond angle of C(2)–C(11)–C(10) (127.3◦) was
larger than 120◦, and that of C(5)–C(6)–C(7) (109.1◦)
was smaller than 120◦.
4.3. Structure of complex 5
An equimolar mixture of IrCl(CO)(PPh3)2 and
ligand (PCP-H) was refluxed in toluene to produce
the yellow complex IrHCl(CO)(PCP). The singlet of
1
31P{ H} NMR spectrum at 25.07 ppm showed that the
The structural property of the complex 4 was dif-
two phosphorus atoms of PCP were equilvalent and
occupied the two apical positions of the octahedron.
1
ferent from the previous complexes. 31P{ H} NMR
1
showed two wide peaks at about 0.04 and 20.70 ppm
at room temperature. When the temperature was de-
creased to −50 ◦C, the two wide peaks were sharp-
ened and split, the broad peak at 20.70 ppm was trans-
formed into two triplets which were centered at 20.86
The triplet of a hydride at −17.74 ppm in H NMR
spectrum confirmed that the orthometallation reaction
had happened and the hydride was in cis-position to
two phosphorus atom. The orthometallation occurred
by the oxidation addition of C–H bond in the phenyl
Fig. 4. Structural transformation of complex 4.

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R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
ring of the ligand to iridium during the reaction. It
has been extensively observed that the bisphosphine
ligands 1,3-(R2PCH2)2C6H4, where R = But , Ph, Cy,
Me and Et, could undergo cyclometallation reactions
with Ni, Pd, Pt, Rh, Ru and Ir to form complexes of
tridentate [(2,6-(R2PCH2)2C6H3)−] [24,27–29]. The
NMR spectra of the complex was consistent with an
octahedron structure. Since H− and C− in benzene
ring have a stronger trans-influence than Cl− and
CO, structure of complex 5 was proposed as shown
in Fig. 1.
oxidation state and the back-donating electron from
∗
iridium(III) to ␲ orbital of carbonyl was less. The
high wavenumber (2183 cm−1) of Ir–H bond also re-
vealed that the bonding between iridium and hydride
was stronger in Ir(III) than Ir(I) complex.
4.5. Hydrogenation of citral and cinnamaldehyde
by iridium complexes
The hydrogenation routes and results of citral and
cinnamaldehyde were shown in Table 6 and Fig. 5.
When two triphenylphosphines in IrH(CO)(PPh3)3
were replaced with a bidentate phosphine, all com-
plexes exhibited higher catalytic activities than the
analogous complex with triphenylphosphine. Among
these complexes, the complex 4 exhibited the highest
catalytic activity and selectivity for the hydrogena-
tion of carbonyl group in citral. The selectivities of
complexes 2–4 to form allylic alcohol were over
92%. However, all complexes showed unsatisfactory
selectivity for the hydrogenation of cinnamaldehyde
to form cinnamyl alcohol. According to the results
in Table 6, complex 4 showed the highest activities
for the hydrogenations of citral and cinnamaldehyde.
This result was perhaps related to its ligand structure.
The selectivity for the hydrogenation of cin-
namaldehyde to form cinnamyl alcohol using these
complexes as catalysts were unsatisfactory with the
formation of saturated aldehyde (phenylpropanal).
When cinnamyl alcohol was used as substrate under
the similar reaction conditions, the saturated aldehyde,
4.4. IR spectra of complexes
Compared with the analogous iridium complex
with triphenylphosphine, the IR absorption peak of
the carbonyl group in complex 1 shifted to the lower
wavenumber 1911 cm−1 and the carbonyl absorp-
tion of complexes 2–4 shifted to higher wavenumber
1926, 1933 and 1931 cm−1, respectively. The data
suggested that more d electron of iridium in com-
∗
plex 1 was back-donated to ␲ orbital of CO than in
IrH(CO)(PPh3)3. The result was consistent with the
proposed structure because a strong electron-donating
ligand coordinated to the trans-position of CO and
led to a more back-donating electron transforming
∗
to ␲ orbital ∗of CO. In contrast, the back-donating
electron to ␲ orbital of CO from iridium in com-
plexes 2–4 were less than in IrH(CO)(PPh3)3. The
high wavenumber (2018 cm−1) of carbonyl group
in complex 5 revealed that iridium was in higher
Table 6
Catalytic hydrogenation of ␣,␤-unsaturated aldehydesa
Complexes
Citralb
Cinnamaldehydec
Conversion (%)
Selectivity (%)c
Conversion (%)
Selectivity (%)d
IrH(CO)(PPh3)3
3.1
7.7
11.6
19.3
58.8
13.9
70.5
61.2
92.2
95.8
96.4
42.5
11.4
27.3
44.6
20.6
58.1
52.0
35.0
19.3
13.1
77.4
9.0
IrH(CO)(PPh3)(BPPB)
IrH(CO)(PPh3)(BISBI)
IrH(CO)(PPh3)(BDNA)
IrH(CO)(PPh3)(BDPX)
IrHCl(CO)(PCP)
1.3
a Reaction conditions: H2 pressure 50 kg/cm2, catalyst concentration 1.0 × 10−3 M, citral or cinnamaldehyde 2.0 ml, toluene 8.0 ml,
temperature 100 ◦C.
b Reaction time: 5 h.
c Reaction time: 3 h.
d Allylic alcohol.

R.-X. Li et al. / Journal of Molecular Catalysis A: Chemical 178 (2002) 181–190
189
Fig. 5. Hydrogenation routes of citral and cinnamaldehyde.
Table 7
Hydrogenation of cinnamaldehyde in the presence of IrH(CO)(PPh3)(BDPX) and BDPX liganda
Ligand/complex
(molar ratio)
Conversion (%)
Distribution of products (%)
Phenylpropanal
Phenylpropanol
Cinnamyl alcohol
0b
1
2
58.1
70.9
20.9
24.4
72.6
67.0
6.1
18.3
19.8
7.5
9.0
13.2
86.4
92.2
5
3.3
4.5
a Reaction conditions: reaction time 5 h, others were same as in Table 3.
b Reaction time: 3 h.
which could be formed by the isomerization of the
double bond in cinnamyl alcohol, was not detected in
the reaction products. This result indicated that the
hydrogenation of cinnamaldehyde to form phenyl-
mechanisms in which an active species with two phos-
=
phorus atoms complex was responsible for C C bond
reduction, whereas carbonyl group reduction was re-
lated to a species with three coordinated phosphorus
atoms [30,31] because the high ratio of BDPX to Ir
was favorable for the formation of the species with
three coordinated phosphorus atoms.
=
propanal occurred directly on the C C double bond
=
rather than on the C O bond to form cinnamyl al-
cohol followed by its isomerization to the saturated
aldehyde as reported by Chin et al. [16].
When the complex 4 was used as catalyst in the
presence of larger excess BDPX (Ir/BDPX = 5:1 in
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a much higher selectivity for cinnamyl alcohol was
obtained. These results were in agreement with the
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