Promotion effect of water on hydroformylation of styrene and its derivatives with presence of amphiphilic zwitterionic phosphines

Article abstract of DOI:10.1016/j.molcata.2015.07.004

Two kinds of novel zwitterionic phosphines of L1 and L2 have been synthesized in which the sulfonate groups (-SO₃^-) are incorporated to render them amphiphilic. Both of them are strong π-acceptor ligands in which the positive-charged imidazoliums with intensive electron-withdrawing effect are vicinal to the coordinating phosphorous atoms. The amino-group of piperidyl is additionally incorporated in L2 to make it behave as a hybrid ligand with potential hemilability. The comparison of the ligands of L1 and L2 in Rh-catalyzed hydroformylation of styrene (and its derivatives) and the effect of water on their behaviors were discussed. Without the involvement of water, the catalytic system of Rh(acac)(CO)₂-L2 exhibited lower activity than that of Rh(acac)(CO)₂-L1 under the same conditions. However, the dramatic promotion effect of water as a suspension was observed especially for Rh(acac)(CO)₂-L2 system with TOF spurred up from 195 to 470 h^-1. The corresponding in situ high pressure FT-IR spectral information showed that the presence of water in Rh(acac)(CO)₂-L2 system greatly facilitated the formation and stability of the active Rh-H species (ν_Rh-H, 2047 cm^-1), which was in charge of the efficient hydroformylation of styrene and its derivatives.

Full text of DOI:10.1016/j.molcata.2015.07.004

Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Promotion effect of water on hydroformylation of styrene and its
derivatives with presence of amphiphilic zwitterionic phosphines
∗
Sheng-Jie Chen, Yong-Qi Li, Peng Wang, Yong Lu, Xiao-Li Zhao, Ye Liu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry & Molecular Engineering, East China Normal University,
Shanghai 200062, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two kinds of novel zwitterionic phosphines of L1 and L2 have been synthesized in which the sulfonate
groups (–SO3 ) are incorporated to render them amphiphilic. Both of them are strong ␲-acceptor ligands
Received 26 March 2015
Received in revised form 30 June 2015
Accepted 2 July 2015
–
in which the positive-charged imidazoliums with intensive electron-withdrawing effect are vicinal to the
coordinating phosphorous atoms. The amino-group of piperidyl is additionally incorporated in L2 to make
it behave as a hybrid ligand with potential hemilability. The comparison of the ligands of L1 and L2 in Rh-
catalyzed hydroformylation of styrene (and its derivatives) and the effect of water on their behaviors were
discussed. Without the involvement of water, the catalytic system of Rh(acac)(CO)2-L2 exhibited lower
activity than that of Rh(acac)(CO)2-L1 under the same conditions. However, the dramatic promotion
effect of water as a suspension was observed especially for Rh(acac)(CO)2-L2 system with TOF spurred
Available online 9 July 2015
Keywords:
Amphiphilic phosphines
Rhodium complexes
Zwitterions
Hydroformylation
In situ high-pressure FT-IR spectra
−1
up from 195 to 470 h . The corresponding in situ high pressure FT-IR spectral information showed that
the presence of water in Rh(acac)(CO)2-L2 system greatly facilitated the formation and stability of the
−1
active Rh–H species (ꢀRh-H, 2047 cm ), which was in charge of the efficient hydroformylation of styrene
and its derivatives.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
Extensive studies show that the activity and selectivity of the
rhodium catalysts for hydroformylation are significantly governed
by the phosphorous-containing ligands such as tertiary phosphines
as the typical ␴-donor ligands or phosphites as the ␲-acceptor lig-
ands. Attractively, the uses of Rh-complexes with the ␲-acceptor
ligands have been reported to be more active and more selective
hydroformylation catalysts [13–19]. Unfortunately, the lability of
Hydroformylation is a reaction route for (functionalized) olefins
toward aldehydes with 100% atom economy and has been devel-
oped into one of the most important homogeneous catalytic
processes [1–3]. In comparison to aliphatic olefins as substrate
of hydroformylation leading to detergent and plasticizers, styrene
as an often applied interesting substrate can be efficiently hydro-
formylated to the branched and linear aldehydes, in which the
regioselectivity to the branched aldehyde is favored due to the for-
phosphites toward hydrolysis (derived from the susceptible P
O
bonds) and a tendency to undergo degradation have limited their
practical uses. Recently, one type of ionic phosphines [20–32], in
which the phosphorous(III) atom is directly vicinal to the posi-
2
mation of a stable benzylic Rh-species induced by the ꢁ -electron
donation from the benzene ring [2b,4]. The linear aldehydes of
styrene (derivatives) can be used as the important intermediated
in organic synthesis for the production of scents and plasticisers
tive charged quaternary ammonium groups through the stable C P
linkages such as the phosphine-functionalized imidazolium-based
ionic liquids reported by our group [23–26] and the imidazolio-
phosphine ligands reported by Chauvin and co-workers [27–32],
can be regarded as the potential alternatives to mimic the behaviors
of ␲-acceptor ligands of phosphites with the apparent advantage
like the robustness against hydrolysis and degradation.
[
4–8] and the branched ones can be used for the preparation of
pharmaceuticals and food flavors [9,10]. For example, the oxida-
tion of the branched aldehydes of styrene derivatives provides a
potential method for the synthesis of 2-arylpropionic acids such as
anti-inflammatory ibuprofen [11,12].
Besides of the attraction to the ␲-acceptor phosphines, the
hybrid P,N-ligands containing hard (N-) and soft (P-) donor func-
tions with multiple ligations to the rhodium center also represent a
very promising class of ligands for hydroformylation [33,34]. These
hybrid ligands can coordinate to the metal center in the mode
∗ Corresponding author. Fax: +86 21 62233424.
E-mail address: yliu@chem.ecnu.edu.cn (Y. Liu).
1
2
of ꢂ -P or ꢂ -P,N, and each coordination mode can modulate the
http://dx.doi.org/10.1016/j.molcata.2015.07.004
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
213
Scheme 1. Syntheses of the hydrophilic zwitterionicphosphines of L1 and L2 for Rh(acac)(CO)2 catalyzed hydroformylation of styrene and its derivatives.
nucleophilic character of the metal center to influence the catalytic
performance [35]. In addition, the less strongly bound moiety like
N-hard donor can temporarily associate to the metal center for
protection and then dissociate for the substrate insertion timely
ble not only to water to embody ‘on-water’ effect for Rh-catalyzed
hydroformylation but also to the hydrophobic styrene (derivatives)
for the efficient mass transfer. Herein, a comparison of the ligands
of L1 and L2 in Rh-catalyzed hydroformylation of styrene (and its
derivatives) and the effect of water on their catalytic behaviors
were discussed. Furthermore, the in situ high-pressure FT-IR anal-
ysis was performed to monitor the evolving processes of the active
Rh–H species derived from L1 and L2 involved hydroformylation.
(
hemilability concept) [36–43].
The hydrophilic and amphiphilic ligands are widely used in
aqueous organometallic catalysis (AOC) which has been developed
into a most successful way for product isolation and/or catalyst
recycling [44–49]. For example, the sulfonated phosphine of TPPTS
(
sodium salt of tri sulfonated triphenylphosphine) as one type
2. Experimental
of the most important ligands has been successfully applied as
the water-soluble ligands in the aqueous/organic biphasic hydro-
formylation of propene (Ruhrchemie/Rhone Poulenc process), but
led to an extremely low reaction rate when the hydroformyla-
tion is extended to the highly lipophilic olefins because of their
negligible solubility in water [50–53]. Hence, hydroformylation
of lipophilic olefins is mostly performed in homogeneous organic
phase. However, it has been observed that the involvement of water
as a suspension in the reaction system can substantially accelerate
the reaction of Rh-catalyzed hydroformylation of lipophilic olefins
2.1. Reagents and analysis
The chemical reagents were purchased from Shanghai Aladdin
Chemical Reagent Co. Ltd. and Alfa Aesar China, and used as
received. The IR spectra and in situ IR spectra were recorded on
a Nicolet NEXUS 670 spectrometer. The 1H and P NMR spectra
were recorded on a Bruker Avance 500 spectrometer. The P NMR
spectra were referenced to 85% H PO4 sealed in a capillary tube as
an internal standard. CHN elemental analysis was performed on a
Vario EL III Element Analyzer. Gas chromatography (GC) was per-
formed on a SHIMADZU-2014 equipped with a DM-Wax capillary
column (30 m × 0.25 mm × 0.25 ␮m).
31
31
3
[
[
54–57], which has been denoted as ‘on water’ effect by Sharpless
58]. It has also been found that when the applied complexes or
phosphines are insensitive to moisture, water molecules can act as
the labile ligands to coordinate to the metal center [59–64], as well
as to develop the possible hydrogen bonding network. From these
points of view, the amphiphilic phosphine ligands are believed to be
able to consolidate such ‘on water’ effect in hydroformylation due
to the affinity of the hydrophilic group to water molecules, which
can synergistically make the labile water and the phosphine ligand
to be closely accessed to the Rh-center to facilitate the formation
of Rh–H active species.
2.2. Synthesis of phosphine ligands
ꢀ
2.2.1. 1-(3 -sulfonate group)
propyl-2-diphenylphosphino-3-methyl imidazolium salt (L1)
Under nitrogen atmosphere, the solution of 1-methylimidazole
◦
(2.05 g, 25 mmol) in 50 mL dry THF was cooled down to −78 C,
in which 10.5 mL n-BuLi (2.5 M, in hexane, 26.3 mmol) was added
Highlighted by the moisture-insensitivity of imidazolium-
neighbored phosphines with ␲-acceptor character [23–26], the
hemilability of hybrid P,N-ligands [33–43], and the ‘on-water’ effect
on the hydrophilic phosphine-involved hydroformylation [23,24],
in this paper, two kinds of amphiphilic zwitterionic phosphines
dropwise. After stirring the mixture for 1 h, chlorodiphenylphos-
phine (PPh Cl, 5.8 g, 26.3 mmol) was added dropwise. The resultant
2
mixture was stirred overnight with the reaction temperature
increasing to ambient. After quenching excess n-BuLi with deion-
ized water, the obtained oily mixture was removed of solvent
in vacuo. Then the residue was purified by chromatography on
(
L1 and L2) were synthesized purposely as shown in Scheme 1. The
common features of L1 and L2 are including that (1) they are both
strong ␲-acceptor ligands in which the positive-charged imidazoli-
silica gel (CH COOEt/petroleum ether = 1:3) giving 1-methyl-2-
3
diphenylphosphinoimidazole as a white solid (5.0 g. Yield: 75%).
Then the obtained solid (5.0 g, 18.8 mmol) was added to a solu-
tion of 1,3-propane sultone (2.5 g, 20.5 mmol) in acetone and the
ums with intensive electron-withdrawing effect are vicinal to the
–
coordinating phosphorous atoms, (2) the sulfonate groups (–SO3
)
◦
are incorporated to endow them hydrophilicity; and (3) they are
typical zwitterions. The difference of L2 to L1 is that the amino-
group of piperidyl is additionally incorporated to make L2 behave
as a hybrid P,N-ligand with potential hemilability. It was expected
that the amphiphilicity of L1 and L2 could render them accessi-
mixture was stirred at 40 C for 12 h. Since 1,3-propane sultone
is highly toxic, it should be handled carefully in the fume hood
with good ventilation. The precipitated white solids was repeat-
edly washed with acetone and dried under vacuum to give L1
in 75% yield. The sample suitable for X-ray diffraction analysis

2
14
S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
Table 1
was obtained by recrystallization from CH Cl /diethyl ether. FT-
2
2
−1
Crystal data and structure refinement for L1 and L2.
IR (KBr, pellet, cm ): 3453 (m), 3088 (w), 2930 (m), 1631 (w),
1
1
(
480 (w), 1438 (w), 1198 (s), 1019 (s), 744 (m), 525 (w). H NMR
L1·2H2O
L2·2CH2Cl2
+
+
␦, ppm, CDCl ): 7.98 (s, 1H, NCHCHN ), 7.79 (s, 1H, NCHCHN ),
3
Empirical formula
[C19H21N O P S ]·
[C22H32N O P S ]·
2
3
1
1
3
3 1 1
7
.56(m, 6H, para-,meta-H, PPh ), 7.45(m, 4H, ortho-H, PPh ), 4.49
[H O ]
2 4 4
[C H Cl ]
2
2
4
2
3
+
−
(
t, 2H, J = 7 Hz, N CH CH CH SO ), 3.55 (s, 3H, NCH ), 2.63 (t, 2H,
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
408.4
Triclinic
P-1
655.4
Monoclinic
P21/c
2
2
2
−
3
3
3
+
+
−
J = 7 Hz, N CH CH CH SO ), 2.08 (m, 2H, N CH CH CH SO ).
2
2
2
3
2
2
2
3
3
1
P NMR (␦, ppm, CDCl ): −26.4 (s, PPh ). CHN-elemental analy-
3
2
10.8550(6)
13.3972(8)
14.2140
81.894(2)
81.737(2)
87.635(2)
2024.7(2)
4
1.327
0.265
296(2)
0.71073
23398
11.9331(11)
31.431(3)
8.5828(8)
90
101.486(3)
90
3154.6(5)
4
1.380
sis found for L1 (%): C 58.044, H 5.19, N 7.16 (Calcd: C 58.3, H 6.18,
N 7.16).
◦
␣
( )
◦
ꢀ ꢀ ꢀ
.2.2. 1-(2 -piperid-1 -yl-ethyl)-2-diphenylphosphino-3-(3 -
␤ ( )
2
◦
ꢄ ( )
sulfonategroup) propyl imidazolium salt (L2)
3
V (Å )
ꢀ
1
-(2 -(1H-imidazol-1-yl)ethyl) piperidine was prepared accord-
Z
−
3
ing to the published method [25]. Under nitrogen atmosphere,
dcalc (g cm
)
ꢀ
␮ (Mo-K␣) (mm−1)
T (K)
0.525
the solution of 1-(2 -(1H-imidazol-1-yl)ethyl) piperidine(4.5 g,
◦
173(2)
0.71073
35878
5544 (0.1199)
0.1146
0.2779
1368
2
1
5 mmol) in 50 mL dry THF was cooled down to −78 C, in which
␭
(A)
0.5 mL n-BuLi (2.5 M, in hexane, 26.3 mmol) was added drop-
Total reflections
Uniquereflections (Rint
R1 [I > 2ꢅ(I)]
wR2 (all data)
F(000)
wise. After stirring the mixture for 1 h, chlorodiphenylphosphine
PPh Cl, 5.8 g, 26.3 mmol) was added dropwise. The resultant
)
7035 (0.0350)
0.0521
0.1628
848
(
2
mixture was stirred overnight with the reaction temperature
increasing to ambient. After quenching excess n-BuLi with deion-
ized water, the obtained oily mixture was removed of solvent
2
Goodness-of-fit on F
1.079
1.159
in vacuo. Then the residue was purified by chromatography on
ꢀ
silica gel (CH COOEt/petroleum ether = 1:3) giving 1-(2 -piperid-
1
temperature and depressurized carefully. The reaction solution
was analyzed by GC to determine the conversions (n-dodecane as
internal standard) and the selectivities (normalization method).
3
ꢀ
-yl-ethyl)-2-diphenylphosphinoimidazole as a white solid (6.5 g.
Yield 72%). Then the obtained solid was added to a solution of 1,3-
propane sultone (3.2 g, 26.2 mmol) in acetone and the mixture was
◦
stirred at 40 C for 12 h. The precipitated white solids was repeat-
2.5. In situ high-pressure FT-IR spectral analysis
edly washed with acetone and dried under vacuum to give L2 in
7
0% yield. The sample suitable for X-ray diffraction analysis was
All the FT-IR spectra were recorded on a Nicolet NEXUS 670
−
1
obtained by recrystallization from CH Cl /diethyl ether. FT-IR (KBr,
pellet, cm ): 3453 (s), 3088 (m), 2934 (w), 1638 (w), 1569(w), 1487
spectrometer. The spectral resolution was about 4 cm . The mix-
tures including Rh(acac)(CO)2, the ligand L1 or L2 (molar ratio
P/Rh = 6), styrene, water (if required), and syngas were mixed in
the specially designed high-pressure IR cell, in which the cylindrical
CaF2 was used as the sealing sheets. However, in comparison to the
real hydroformylation conditions (as in Table 3), the concentrations
for Rh(acac)(CO)2 and the ligand were much higher for IR spectral
detection and the syngas pressure was lower (2.0 MPa instead of
4.0 MPa) due to the limited pressure endurance capability of the
designed IR cell. The real time monitoring was performed contin-
2
2
−
1
1
(
m), 1438 (m), 1191 (s), 1033(s), 751 (m), 524 (w). H NMR (␦, ppm,
+
+
CDCl ): 8.45 (s, 1H, NCHCHN ), 8.05 (s, 1H, NCHCHN ), 7.52(m,
3
6
H, para-,meta-H, PPh ), 7.35(m, 4H, ortho-H, PPh ), 4.49 (t, 2H,
2
2
3
+
+
−
3
J = 8 Hz, N CH CH -piperidine), 4.46 (br, 2H, N CH CH CH SO ),
2
2
2
2
2
3
3
+
−
2
.60 (t, 2H, J = 7 Hz, N CH CH CH SO ), 2.49 (t, 2H, J = 8 Hz,
2
2
2
3
+
+
N CH CH -piperidine), 2.31 (br, 4H, N CH CH -piperidine), 1.82
2
2
2
2
+
+
−
(
br, 6H, N CH CH -piperidine), 1.66 (m, 2H, N CH CH CH SO ).
2 2 2 2 2
3
3
1
P NMR (␦, ppm, CDCl ): −29.8 (s, PPh ). CHN-elemental analysis
3
2
◦
found for L2 (%): C 59.00, H 6.40, N 8.06(Calcd: C 61.46, H 7.22, N
8
uously while the temperature increased from 25 to 80 C with a
◦
−1
◦
.6).
temperature ramp of 10 C min and then kept at 80 C constantly
for 40 min.
2.3. X-ray crystallography
3. Results and discussion
Intensity data were collected at 296 K for L1 (or 173 K for L2) on
a Bruker SMARTAPEX II diffractometer using graphite monochro-
mated Mo-K␣ radiation (ꢃ = 0.71073 Å). Data reduction included
absorption corrections by the multi-scan method. The structures
were solved by direct methods and refined by full matrix least-
squares using SHELXS-97 (Sheldrick, 1990), with all non-hydrogen
atoms refined anisotropically. Hydrogen atoms were added at their
geometrically ideal positions and refined isotropically. The crystal
data and refinement details are given in Table 1.
3.1. Synthesis and characterization
In our synthesis route, –PPh group is introduced at 2C-position
2
of the substituted-imidazolyl ring through deprotonation by n-BuLi
◦
at −78 C and then 1,3-propane sultone is used as a quaterniz-
ꢀ
ing reagent to introduce propyl-1 -sulfonate group at 3N-position
to obtain L1 or L2, respectively (Scheme 1). L1 and L2 are robust
against hydrolysis and oxidative degradation due to the conjugated
effect of P-atom with the adjacent positive-charged imidazolium
ring. The molecular structures of L1 and L2 determined by the sin-
gle crystals X-ray diffraction are depicted in Fig. 1, each of which is
a typical zwitterionic salt. The environment of the P-atoms is close
to tetrahedral structures and shows no anomalies. The negative
charged sulfonate group projects outward from the imidazolium
ring and the distance of S. . .N [Table 2: L1, S1. . .N2 5.2225(25) Å;
L2, S1. . .N1 5.2186(66) Å] in each example is beyond the typical
range of ionic bonds, indicating the negligible ion-pair electrostatic
force between –SO3– group and the positive-charged imidazolium
unit. However, in L2, the arm of ethylpiperidyl projects toward the
2.4. General procedures for hydroformylation of styrene
In typical experiment, the commercial complex of
a
Rh(acac)(CO)2 (0.01 mmol) and the isolated ligand (0.06 mmol)
were added into styrene (20 mmol) sequentially. The obtained
mixture in a 50 mL sealed Teflon-lined stainless steel autoclave
was purged with syngas (CO/H = 1:1, 1.0 MPa) for two times
2
and then pressured by syngas to 4.0 MPa. The reaction mixture
was stirred vigorously at the appointed temperature for some
time. Upon completion, the autoclave was cooled down to room

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
215
Fig. 1. The single crystal structures of L1 and L2 (the hydrogen atoms are omitted for clarity).
Table 2
The selected bond distances and 31P NMR signals for L1 and L2.
Ligand
Bond distance/Å
31P NMR/ppm
P–C
P. . .S
Nimidazolium. . .S
P. . .Npiperidyl
m
L1
L2
1.828(3)
6.2177(14)
5.2225(25)
–
−26.4
1
1
.830(3)
.834(3)
1.823(7)
6.9112(29)
5.2186(66)
3.7562(65)
−29.8
1
1
.830(7)
.831(7)
1
P-atom and the short distance of P1. . .N3 of 3.7562(65) Å indi-
cates the intensive electrostatic interaction between the P-atom
and the N-atom. As claimed in Canac and Chauvin’s work [32], due
to the conjugated effect between the P-atom and the imidazolium,
the P-atom can share the partial positive charge with the adjacent
imidazolium ring to make it behave like a phosphenium. In L2,
such electrostatic interaction between the phosphenium and the
N-atom of piperidyl can account for the cis-positioned P,N-units
and the short distance of them as observed in Fig. 1. Compared to
the tertiary piperidyl group with the naked lone-electron-pair in N-
than PPh3 ( JSe-P = 729 Hz), indicating the significantly stronger ␲-
acceptor ability (i.e. weaker ␴-donor ability) of L1 and L2.
On the other hand, L1 is very soluble in water with the excellent
solubility of 120 g/100 mL determined by the equilibrium method
at 20 C, whereas L2 is nearly insoluble.
◦
3.2. The influence of L1 and L2 on Rh-catalyzed hydroformylation
of styrene
It has been widely known that the electronic and steric effects
of the involved phosphines will dramatically influence the Rh-
complex catalyzed hydroformylation of olefins. In this part, the
–
atom, the lone-electrons and the negative charge in –SO3 group are
greatly delocalized, leading to the negligible electrostatic interac-
tion between the –SO3 group and the phosphenium. Hence, –SO3–
unit in each one is positioned far away from the P-atom [L1, S1. . .P1
–
Se
6
.2177(14) Å; L2, S1. . .P1 6.9112(29) Å].
The P NMR signal of L2 (−29.8 ppm) shifts to the relatively
Ph
P Ph
3
1
H
C
N
N
SO
higher field in comparison to that of L1 (−26.4 ppm), implying the
more enriched electron-density in the P-atom of L2 due to the
electrostatic interaction of the P-atom with the electron-donating
N-atom of piperidyl.
1
JSe-P=781 Hz
23.70
18.88
c
Selenide of L1
Because of the intensive electron-withdrawing effect the
positive-charged imidazoliums on the vicinal P-atoms, L1 and L2
Se
Ph
P
Ph
Ph
are definitely featured with the strong ␲-acceptor ability. The mea-
1
37.81
33.31
JSe-P=729 Hz
surement of magnitude of 1JSe-P in the 77Se isotopomer of the
31
Selenide of PPh3
corresponding phosphine-selenide in P NMR spectra can be used
to evaluate the ␴-donor ability of a phosphine as reported by Tay-
lor et al. [65a]. An increase of 1JSe-P indicates a decrease in the
character of ␴-donor ability but an increase in the character of
b
Se
Ph
P Ph
N
N
N
SO
1
␲
-acceptor ability (acidity) of the phosphine [65]. To estimate the
23.70
JSe-P=781 Hz
Selenide of L2
18.88
␲
-acceptor ability of L1 and L2, the corresponding selenides of L1
a
and L2 were prepared by reacting elemental selenium with L1 in
◦
DMSO-d6 and L2 in CDCl3 at 60 C for 12 h. For comparison, the
50
40
30
20
10
0
-10
-20
-30
-40
-50
Chemical shift (ppm)
selenide of PPh was prepared in parallel. From the magnitudes
3
1
of JSe-P in the selenides of L1, L2, and PPh3 as shown in Fig. 2,
Fig. 2. 31P { H} NMR spectra (162 MHz) of the selenides of L1, L2, and PPh3 (the
1
the value of PPh (729 Hz) is in good agreement with the litera-
3
sample of phosphine was reacted with elemental selenium before test under the
1
1
ture [65e]. ( J
= 781 Hz) and L2 ( J
= 781 Hz) are more acidic
◦ ◦ ◦
conditions of (a)DMSO-d6, 60 C, 12 h; (b) CDCl3, 60 C, 12 h; (c) CDCl3, 60 C, 12 h).
Se-P
Se-P

2
16
S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
Table 3
3
.3. Detection and characterization of active Rh–species by in situ
The Rh-catalyzed homogeneous hydroformylation of styrene with the different lig-
high-pressure IR spectroscopy
ands free of solvent.a
−1
Entry
Ligand
H2O
Conv. (%)b
Sel. (B/L,%)b,c
TOF (h )d
In situ IR spectroscopy is one of the important techniques for
the research of hydroformylation of olefins because of the valuable
information on the active Rh–H species, which is formed under the
actual reaction conditions and in charge of the on-site transforma-
tion of olefins to aldehydes [66–73]. The dramatic promoting effect
of water evoked us to understand the influence of water on the
formation, longevity, derivation of the active Rh–H species or the
other rhodium complex species in L1 or L2 involved hydroformy-
lation of styrene. The continuously recorded high pressure FT-IR
spectra over time were presented in Fig. 3.
1
2
3
4
L1
L2
L1
L2
–
–
1 mL
1 mL
84
39
92
94
100 (81/19)
100 (91/9)
100 (73/27)
100 (79/21)
420
195
460
470
a
S/C = 2000, Rh(acac)(CO)2 0.01 mmol, ligand 0.06 mmol, styrene 20.0 mmol,
◦
CO/H2 (1:1) 4.0 MPa, temperature 80 C, time 4 h.
b
Determined by GC.
c
B, the branched aldehyde (2-phenylpropanal); L, the linear aldehyde (3-
phenylpropanal).
d
−l −l
h )].
TOF, turnover frequency to the aldehydes [mol of aldehydes (mol of Rh)
−
1
As can be seen in Fig. 3A, the peaks at 2081 and 2010 cm
attributed to CO vibrations in Rh(acac)(CO)2 gradually decayed
hydroformylation of styrene was investigated as a model reaction
to compare the behaviors of L1 and L2 as the auxiliary ligands
◦
as the temperature increased from 25 to 80 C. The broad
−1
and intensive absorptions at ca. 1988 cm
(br) were ascribed
(
Table 3).
Firstly, the catalyst precursor of Rh(acac)(CO)2 and the ligand
L1 or L2) were mixed at ambient temperature in a 50 mL sealed
to the CO vibrations in the Rh-complex intermediates of
Rh(acac)(CO)(L1) [74] and Rh(CO)x(L1)5 after the rapid com-
−x
(
plexation of Rh(acac)(CO)2 with L1 in the syngas atmosphere.
Teflon-lined stainless steel autoclave which was then pressured by
syngas (CO/H = 1:1) to 4.0 MPa. At the optimal reaction conditions
◦
While the temperature kept at 80 C for 1 min, an absorption peak
−1
2
at 2060 cm
2
gradually grew along with a very weak peak at
◦
(
Rh concentration 0.05 mol%, P/Rh molar ratio = 6, 80 C, and 4 h),
the relatively low TOFs were observed over L1 and L2 (Entries 1 and
). However, when 1 mL of water was added additionally, the cor-
responding TOFs were increased dramatically. Especially for L2, the
TOF boosted from 195 up to 470 h , which was 2.4-fold increase
in comparison to the water-free system (Entry 2 vs 4). Compara-
tively, the activity promotion by water over L1 was not as effective
as that over L2 (Entry 1 vs 3). It was also found that with the
presence of water in the reaction system, although the aqueous-
−1
045 cm , the latter which appeared momently and then rapidly
decayed to void in the afterward monitoring was identified as the
catalytic active Rh-H (rhodium hydride) species according to the
previously reported work [65–73]. When water was introduced
2
−1
in this system (Fig. 3B), besides of the broad absorption at ca.
−1
1
980 cm
ascribed to Rh(acac)(CO)(L1) and Rh(CO)x(L1)5−x, the
−1
CO vibration peaks at 2081 and 2010 cm for Rh(acac)(CO)2 were
completely unobservable even at the initial temperature of 25 C,
◦
indicating that with the involvement of water the complexation of
soluble L1 could make the pre-catalyst of Rh(acac)(CO) completely
2
Rh(acac)(CO) with the water-soluble L1 to afford Rh(acac)(CO)(L1)
2
dissolve in the aqueous phase at the initial stage after mixing
Rh(acac)(CO)2 and L1 at the molar ratio of 1/6, upon completion
of the reaction, all the resultant Rh-complexes were transferred
into the organic phase with the indication of the non-detectable
Rh-concentration (detected by the ICP analysis) in the left aqueous
phase. This phenomenon indicated that the hydroformylation of
and [Rh(CO)x(L1)5−x] instantaneously happened as they are mixed
at ambient temperature. Subsequently, the absorption of the active
−1
Rh–H species at 2045 cm
8
appeared faintly but persistently at
◦
0 C, implying the improved longevity of the formed active Rh–H
species. The in situ FT-IR spectral information over Rh(acac)(CO)2-
L1 system indicated that without the presence of water the active
Rh–H species could be formed but suffered from the very poor sta-
bility (longevity), and that the involvement of water then facilitated
the formation and stability of such Rh–H species, which was reason-
ably correlated to the improved catalytic activity with the presence
styrene over Rh(acac)(CO) -L1 also truly happened in the organic
2
phase like in the case of Rh(acac)(CO) -L2 (which was insoluble in
2
water). The formed product of aldehydes with high polarity could
extract the Rh-complexes into the organic phased completely to
facilitate the rapid conversion of the lipophilic styrene to the alde-
hydes. These results suggested that the promoting effect of water
on the activities of L1 and L2 not be correlated to their aqueous
solubility, since the catalytic hydroformylation truly happened in
organic phase. The change in the coordination mode of the involved
phosphine fragments in L1 and L2 to the metal center with or with-
out the presence of water molecules was the essential factor for
their various catalytic performance. As for L2 in comparison to
L1, the more bulky steric hindrance around the phosphine frag-
ment made it too crowded and encapsulated (as shown in Fig. 1)
to be accessed to Rh-center, leading to the sluggish reaction rate
for styrene transformation (Table 3, Entry 2 vs 1). However, with
the presence of water, besides of water’s ligand effect on Rh-center,
the hydrophilic affinity of sulfonate and/or peperidyl groups with
water molecules made the hydrophobic phosphine fragments fully
exposed with better accessibility to Rh-center, resulting in the
dramatically improved activities both over L1 and L2. And the addi-
tional hemilability of L2 as a P,N-hybrid ligand was also potentially
attributed to its better catalytic performance than that over L1
−1
−1
of water (Table 3, Entry 3 vs 1: TOF of 460 h vs 420 h ).
In contrast, as for Rh(acac)(CO) -L2 system as shown in Fig. 3C,
2
without the involvement of water only happened the complexa-
−1
tion of Rh(acac)(CO)2 (ꢀCO, 2081 and 2010 cm ) with L2 to afford
−1
Rh(acac)(CO)(L2) because of the sharp absorption at 1982 cm
.
And the continuous derivation to the Rh–H species was barely
observed during the overall monitoring process. When water
was introduced, not only the complexation of Rh(acac)(CO)2 with
L2 was dramatically expedited to afford many complex inter-
mediates including Rh(acac)(CO)(L2) and Rh(CO)x(L2)5−x(ꢀCO, br
−1
1
2
8
990–1960 cm ), but also the derived active Rh–H species (ꢀRh-H,
−1
047 cm ) was clearly observed after monitoring the reaction at
0 C for 18 min. Afterwards, the active Rh–H species grew to reach
the maximum absorption intensity after holding 40 min at 80 C,
along with the slight redshift from 2047 to 2043 cm
◦
◦
−1
(Fig. 3D)
probably due to the exchange of the different ligands (L2, CO,
and H O). The obtained in situ FT-IR spectral information over
2
Rh(acac)(CO) -L2 system indicated that without the presence of
2
water the activation of Rh(acac)(CO) -L2 in syngas for the forma-
2
(Table 3, Entry 4 vs 3).
tion of the active Rh–H species was more difficult in comparison
Unexceptionally, in all case, the kinetic favored branched prod-
to that over Rh(acac)(CO) -L1 under the same reaction condition,
2
uct of 2-phenylpropanal is predominantly formed due to the
which was correlated to the lower hydroformylation reaction rate
over L2 than that over L1 (Table 3, Entries 4 vs 3). However,
the involvement of water definitely gave rise to the activation
2
formation of a stable benzylic Rh-species induced by the ꢁ -
electron donation from the benzene ring [2b,4,8].

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
217
A: Rh(acac)(CO) -L1-no water
2
B: Rh(acac)(CO) - L1 - with water
2
2045
061
2
2081
2045
2061
2100
2050
2000
1950
1900
1850
2
100
2050
2000
1950
1900
1850
-1
Wavenumber (cm )
-1
Wavenumber(cm )
C: Rh(acac)(CO) - L2 - no water
D: Rh(acac)(CO) - L2- with H O
2
2
2
2061
2047~2043
2061
1982
2081
2
081
2010
2000
2
010
2000
2100
2050
1950
1900
1850
2100
2050
1950
1900
1850
-1
-1
Wavenumber (cm )
Wavenumber (cm )
◦
Fig. 3. In situ high-pressure FT-IR spectra continuously recorded over time as the temperature increasing from 25 to 80 C [upon mixing Rh(acac)(CO)2, L1 (or L2), H2O (if
required), and styrene in 2.0 MPa syngas (CO/H2 = 1)].
Rh H bond (ꢀRh-H 2047 cm 1) [74]. However, when styrene was
−
of Rh(acac)(CO) -L2 for the formation of the active Rh–H species
2
with the better stability, which corresponded to the highest TOF
used as the substrate, the intensive absorption of C C bond at ca.
1400–1500 cm badly overlapped the Rh–D absorption. Hence 1-
−
1
−1
of 470 h
for hydroformylation of styrene in water suspension
(Table 3, Entry 4).
octene was used in place of styrene while D O was introduced
2
In order to further verify the adsorption band at ca. 2047 cm−1
into Rh(acac)(CO) -L2 system to monitor the formation of Rh–D
2
as observed in Fig. 3D was truly attributed to formation of Rh–H
species, D O was used instead of H O in the evolving processes
species. As shown in Fig. 4A, while the absorption of Rh–H species at
−
1
2
2
2047 cm was observed (due to the presence of H in syngas), the
2
−
1
of Rh(acac)(CO) -L2 system in 2.0 MPa syngas (H /CO). Since the
band at 1445 cm appeared concurrently. The latter was very con-
2
2
vibration frequency is inversely proportional to the square root
of the quality, the vibration frequency of Rh D bond is theo-
retically expected to redshift to ca.1448 when D replaces H of
sistent to the theoretical vibration frequency of Rh D bond (ꢀRh-D
1448 cm ). In contrast, under the same conditions, the introduc-
−
1
tion of H O into the same Rh(acac)(CO) -L2 system only led to the
2
2

2
18
S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
2
047
(
Rh-H)
2083
2013
1
460
1
445
1
434
1982
(Rh-D)
2100
2050
2000
1950
1900
1850
1500
1450
1400
1350
1300
-1
Wavenumber (cm )
2
047
(
Rh-H)
2012
1458
1434
1982
2
100
2050
2000
1950
1900
1850
1500
1450
1400
1350
1300
-1
Wavenumber (cm )
◦
Fig. 4. In situ high-pressure FT-IR spectra continuously recorded over time as the temperature increasing from 25 to 80 C [upon mixing Rh(acac)(CO)2, L2, D2O (or H2O),
and 1-octene in 2.0 MPa syngas (CO/H2 = 1)].
−
1
absorption of Rh–H species at 2047 cm without the appearance
The substrates with the electron-donating groups such as −Me
or −OMe in para-position of the phenyl ring could be more
rapidly converted to the aldehydes than the one with electron-
withdrawing groups (Entries 4 and 5 vs 2 and 3), due to the more
favored coordination of the electron-richer C C bond to the Rh–H
species for the substrate activation. When ortho-methyl styrene
was used as the substrate, in comparison to para-methyl styrene
−1
of the peak at 1445 cm (Fig. 4B). The simultaneous observations
of the peaks of 2047 cm (ꢀRh-H) and 1445 cm (ꢀRh-D) with the
involvement of D O confirmed that the observed adsorption at
047 cm was indeed attributed to the formation of Rh–H species
and that water truly participated its formation as the efficient addi-
tive.
−1
−1
2
−
1
2
−
1
(
Entry 4, TOF = 470 h ), the reaction rate was decreased greatly
−
1
3
.4. Generality of Rh(acac)(CO) -L2 as the pre-catalyst for
(Entry 6, TOF = 132 h ) due to the bulky steric hindrance from
ortho-methyl to inhibit the accessibility of C C bond to the Rh–H
species. In all the cases, the selectivity to the branched aldehyde
was not varied obviously because of the formation of the kinetic
stable benzylic Rh-species for the preference to the branched alde-
hyde. The catalytic system of Rh(acac)(CO)2-L2 was also applicable
to the hydroformylation of aliphatic olefin of 1-octene with TOF of
2
hydroformylation of styrene derivatives
The generality of Rh(acac)(CO) -L2 as the pre-catalyst for the
2
hydroformylation of styrene derivatives was examined in Table 4.
The electronic properties and the steric effect of the substituents
showed obvious influences on the reaction rates of hydroformyla-
tion.
−
1
420 h and preference to linear product (Entry 7).

S.-J. Chen et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 212–220
219
Table 4
Generality of Rh(acac)(CO)2-L2 as the pre-catalyst for hydroformylation of olefins.
a
−1
Entry
1
Substrate
Conv.(%)b
Sel. to ald. (%)b
B/Lc
TOF (h )d
Styrene
94
84
90
94
98
33
92
100
100
100
100
100
100
91
79/21
85/15
83/17
81/19
82/18
86/14
30/70
470
336
450
470
490
132
420
e
2
Para-fluoro styrene
Para-chloro styrene
Para-methyl styrene
Para-methoxystyrene
Ortho-methyl styrene
1-Octene
3
4
5
e
6
7
a
◦
Rh(acac)(CO)2 0.01 mmol, L2 0.06 mmol, substrate 20.0 mmol (S/C = 2000), H2O 1 mL, CO/H2 (1:1) 4.0 MPa, 80 C, time 4 h.
Determined by GC.
b
c
The ratio of branched aldehyde to linear one.
TOF, turnover frequency to the aldehydes [mol of aldehydes (mol of Rh)
Time 5 h.
d
e
−1 −1
.
]
4
. Conclusions
The zwitterionic phosphines of L1 and L2 with strong ␲-
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(
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[
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acceptor ability were synthesized, in which the sulfonate groups
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–
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(
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(
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2
[
[
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−
1
ence of water can make the TOF increased from 195 to 470 h
.
It was found that the presence of water definitely gave rise to the
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2
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Appendix A. Supplementary data
CCDC-1024206 (L1) and CCDC-1024207 (L2) contain the sup-
plementary crystallographic data in this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data request/cif. Supplementary
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