Multidentate phosphanes as ligands in rhodium catalyzed hydroformylation of 1-hexene

Article abstract of DOI:10.1016/S1381-1169(01)00400-9

Triphenylphosphine derivatives modified with thiomethyl (SCH₃) and methoxy (OCH₃) groups in ortho- and para-position of the phenyl ring(s) were screened in situ Rh₄(CO)₁₂ catalyzed 1-hexene hydroformylation reac

Full text of DOI:10.1016/S1381-1169(01)00400-9

Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
Multidentate phosphanes as ligands in rhodium catalyzed
hydroformylation of 1-hexene
Pekka Suomalainena, Riitta Laitinenb, Sirpa Jääskeläinena, Matti Haukkaa,
Jouni T. Pursiainenb, Tapani A. Pakkanena,∗
Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland
a
b
Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90401 Oulu, Finland
Received 31 May 2001; accepted 14 September 2001
Abstract
Triphenylphosphine derivatives modified with thiomethyl (SCH3) and methoxy (OCH3) groups in ortho- and para-position
of the phenyl ring(s) were screened in situ Rh4(CO)12 catalyzed 1-hexene hydroformylation reaction. The effect of amount
and position of the substituents on the hydroformylation results are discussed in terms of geometric and electronic properties
of the ligands. The ab initio molecular modeling methods were used to calculate the ground state structures of the free ligands
and their higher energy conformers. Cone angle calculations were used to evaluate the steric attributes of the calculated ligand
structures. Additionally, study was made of the reactions between the methoxy substituted phosphanes and Rh2(␮-Cl)2(CO)4.
The crystal structure of trans-Rh(CO)Cl(o-OOP)2 (1) is reported. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: 1-Hexene hydroformylation reaction; Catalytic behavior; Ligand structure
1. Introduction
carbonylation [2], hydroformylation [3], and asym-
metric catalysis [4].
The current paper is a continuation to our ear-
lier studies on ortho-substituted triphenylphosphane
derivatives: (o-thiomethylphenyl)diphenylphosphane,
(o-methoxyphenyl)diphenylphosphane, and (o-N,Nꢀ-
dimethylaminophenyl)diphenyl-phosphine [5,6].
Here, we focus on the corresponding multidentate
triphenylphosphane ligands having methoxy and
thiomethyl groups in ortho- and para-positions of the
phenyl rings.
Through systematic variation in amount, position,
and type of the substituent groups in the ligand, we
hoped to be able to find structure, activity and struc-
ture, regioselectivity trends.
Bi- and multidentate phosphorus ligands with elec-
tronically different substituents, capable of behaving
either as chelating or as open-chain ligands, are well
documented in the context of homogeneous transition
metal catalyzed reactions. It has been proposed that,
during the catalytic cycle, the substrate can replace the
more weakly coordinated donor site, and thus, accel-
erate the catalytic reactions.
Perhaps the most widely studied donor combina-
tions are various P–O ligands, as reviewed extensively
by Bader and Lindner [1]. Mixed P–S ligands are less
studied, although good results have been achieved in
To assist in this, ab initio Hartree–Fock calculations
were performed to obtain the free ligand geometries.
In addition, to incorporate the flexibility and steric
∗ Corresponding author. Tel.: +358-13-2513345;
fax: +358-13-2513344.
E-mail address: tapani.pakkanen@joenssu.fi (T.A. Pakkanen).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00400-9

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P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
requirements of the ligands, conformational analy-
sis and cone angle [7] measurements were carried
out. Furthermore, coordination properties for possi-
bly hemilabile methoxy-substituted phosphanes were
investigated in reaction with Rh2(␮-Cl)2(CO)4.
Extending our previous studies [5,8], we investigated
several multidentate phosphane ligands by molecular
modeling. Geometry optimizations and conforma-
tional analyses were calculated at Hartree–Fock level
using the 3-21G∗ basis set. To classify the ligands by
steric character, cone angle measurements were done
for all conformers.
2. Results and discussion
Table 2 presents the calculated cone angles for en-
ergy minimized structures as well as for higher energy
conformers within 100 kJ/mol of the global minimum.
Cone angles varied widely, from 152 ◦ for p-SP to
262 ◦ for o-SSSP, with steric crowding greatest for
2.1. Ligands
The methoxy and thiomethyl substituted phos-
phanes used in this study are listed in Table 1.
Table 1
Structures and abbreviations for ligands
Abbreviation
Structure (HF/3-21G∗)
Abbreviation
Structure (HF/3-21G∗)
o-SP (o-thiomethylphenyl)-
p-S–o-OOP (p-thiomethylphenyl)-
bis(o-methoxyphenyl)phosphine [19]
diphenylphosphine [18]
o-SSP bis(o-thiomethylphenyl)-
p-O–o-OOP (p-methoxyphenyl)-
phenylphosphine [18]
bis(o-methoxyphenyl)phosphine [19]
o-SSSP tris(o-thiomethylphenyl)-
p-O–o-SSP (p-methoxyphenyl)-
phosphine [18]
bis(o-thiomethylphenyl)phosphine [19]
p-SP (p-thiomethylphenyl)-
o-OOSP (o-thiomethylphenyl)-
diphenylphosphine [18]
bis(o-methoxyphenyl)phosphine [20]
o-OP (o-methoxyphenyl)-
p-S–o-SSP (p-thiomethylphenyl)-
diphenylphosphine [18]
bis(o-thiomethylphenyl)phosphine [19]
o-OOP bis(o-methoxyphenyl)-
o-OOOP tris(o-methoxyphenyl)-
phosphine [18]
phosphine [18]

P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
95
Table 2
energy the phenyl rings begins to rotate, the substituent
groups fold inside the cone, thus, no longer determin-
ing the maximum cone angle. This behavior was sim-
ilar for all the ligands.
Steric requirements of ligands
Ligand
Θ (degree)
Flexibility range
o-SP
o-SSP
o-SSSP
o-OP
o-OOP
o-OOOP
p-SP
p-S–o-SSP
p-O–o-OOP
p-S–o-OOP
p-O–o-SSP
158
223
262
166
183
205
152
196
185
186
198
153–158
166–223
167–262
153–166
165–183
169–205
–
–
–
–
–
2.2. Synthesis and characterization of rhodium
phosphine complexes
Equimolar amounts of Rh2(␮-Cl)2(CO)4 and
o-OOP ligand reacted in methanol solution through
the chloride bridge splitting route to yield a yellow
precipitate, which was characterized crystallographi-
cally as trans-Rh(CO)Cl(o-OOP)2 (1) (Fig. 1).
The analogous trans-Rh(CO)Cl(o-OP)2 structure
was recently reported by us [5]. The IR spectrum of 1
shows a CO stretching vibration at 1968 cm−1, while
the 31P-NMR spectrum gives a doublet at 20.5 ppm
with JRh–P of 137 Hz. The reaction proceeded simi-
larly at lower ligand to rhodium ratio (L/Rh = 1/2),
but resulted in IR absorption at higher wave number,
the ground state “strain-free” conformers. However,
those conformers are unlikely to be present when the
ligand is coordinated to metal, particularly in the case
of bulky ligands. Scheme 1 shows the conformational
variation for the o-SSSP ligand. As moving to higher
Scheme 1. Low lying conformations of o-SSSP ligand.

96
P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
Fig. 1. Ortep drawing of trans-Rh(CO)Cl(o-OOP)2 with thermal ellipsoids at 50% level.
at 1999 cm−1, representative of Rh(CO)Cl(o-OOP) (2)
compound, with the ligand chelating via oxygen and
phosphorus.
Similarly, the reaction between Rh2(␮-Cl)2(CO)4
and o-OOOP (L/Rh = 1/2) resulted in the chelate
structure trans-Rh(CO)Cl(o-OOOP) (3). Both the CO
stretching vibration in IR at 1995 cm−1 and the doublet
in 31P-NMR at 38.2 ppm (165Hz) are indicative of
such species. The elemental analysis of compounds 2
and 3 are consisted with the chelating species as well.
degree of substitution of the ligand; thus, no activ-
ity was observed with the o-SSP and o-SSSP ligands.
Likewise, regioselectivity was affected by the size of
the ligand; the normal to branched ratio (n:i) increased
linearly as a function of the cone angle. However, en-
hanced n:i ratios compared to PPh3 could also be due
to the greater basicity of the ligands [9,10].
In view of the low hydroformylation activity as well
as the high isomerization activity with the o-thioanisyl
ligands, the ligand to rhodium ratio was lowered. At
L/Rh ratio of 2, the conversion levels were roughly
the same with the different catalyst systems, but again
crowding of the ortho-position of the ligand had a neg-
ative effect on the total aldehyde selectivity. Further,
the effect was more pronounced with methoxy deriva-
tives (decrease in conversion from 54 to 37% going
from o-SP to o-SSSP and from 49 to 26% going from
o-OP to o-OOOP) (Fig. 2). Cone angle measurements
of the free ligands showed the thiomethyl substituted
ligands to be sterically more demanding, which would
2.3. Catalysis
Studies of closely related phosphane ligands re-
vealed distinct correlations between the catalytic
behavior and structure of the ligands. Results of the
1-hexene hydroformylation experiments are shown in
Tables 3 and 4.
At L/Rh ratio of 10 (Table 3), the conversion and
the total aldehyde selectivity dropped rapidly with the

P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
97
Table 3
1-Hexene hydroformylation results (L/Rh = 10)a
Ligand
θ
Conversion
S (hexane)
S (isomers)
S (2-Ep)
S (2-Mh)
S (h)
S (ald)
n/i
–
PPh3
o-SP
o-OP
o-OOP
o-OOOP
o-SSP
o-SSSP
–
86
88
88
86
30
17
0
0
0
0
0
2
3
0
0
80
81
81
86
87
86
0
2
2
1
1
0
0
0
0
8
7
7
5
3
3
0
0
11
10
11
9
8
8
20
19
19
14
10
10
0
1.1
1.2
1.4
1.7
2.7
2.8
–
149
158
166
183
205
223
262
0
0
0
0
0
–
a 15 bar (CO/H2=1); 80 ◦C; 6 h; 1-hexene, 15.5 mmol; toluene, 5 ml; Rh4(CO)12, 1.9×10−2 mmol; L/Rh = 10 S (isomers): total amount
of 2- and 3-hexenes; S (2-Ep): selectivity to 2-ethylpentanal; S (2-Mh): selectivity to 2-methylhexanal; S (h): selectivity to 1-heptanal;
S (ald): total aldehyde selectivity.
suggest the reverse order in selectivities. However, in
our recent studies we found the o-SP ligand to behave
as chelating ligand, even under severe reaction condi-
tions, whereas the analogous methoxy ligand (o-OP)
was observed to bind solely monodentately through
the phosphorus atom. Although, here we were able
to show bidentate coordination mode with methoxy
ligands (o-OOP and o-OOOP), we presume that the
chelate ring is too labile to be present in catalytic en-
vironment. Thus, we conclude that the difference in
selectivity can be explained in terms of the different
binding modes of the methoxy and thiomethyl substi-
tuted phosphanes.
When the substituent group thiomethyl or methoxy
was moved from ortho- to para-position the selectiv-
ity to aldehydes was improved from 37 to 45% go-
ing from o-SSSP to p-S–o-SSP and from 26 to 40%
going from o-OOOP to p-O–o-OOP. Decreased steric
crowding in the vicinity of the phosphorus made the
rhodium center more accessible and active for hydro-
formylation.
However, moving the thiomethyl group from the
o-SP ligand to para position caused the total aldehyde
selectivity to drop dramatically, from 54 to 19%. A
similar observation was made earlier when CF3 mod-
ified phosphanes were used as ligands in 1-hexene
Table 4
1-Hexene hydroformylation results (L/Rh = 2)a
Ligand
Conversion
S (isomers)
S (2-Ep)
S (3-Eh)
S (h)
S (ald)
n/i
–
PPh3
o-OP
o-OOP
o-OOOP
o-SP
o-SSP
o-SSSP
p-SP
p-S–o-SSP
p-S–o-OOP
o-OSSP
p-O–o-OOP
p-O–o-SSP
95
94
94
95
93
96
95
94
91
100
100
94
95
100
56
58
51
66
74
46
54
63
81
55
62
61
60
57
6
6
7
5
2
7
7
5
2
6
5
5
5
6
19
18
21
14
10
23
20
16
7
19
16
17
16
18
19
18
22
15
14
24
19
17
10
19
18
17
19
19
44
42
49
34
26
54
46
37
19
45
38
39
40
43
0.8
0.8
0.8
0.8
1.1
0.8
0.7
0.8
1.1
0.8
0.9
0.8
0.9
0.8
a 20 bar (CO/H2 = 1); 100 ◦C; 4 h; 1-hexene, 15.5 mmol; toluene, 5 ml; Rh4(CO)12, 1.9×10−2 mmol; L/Rh = 2 S (isomers): total amount
of 2- and 3-hexenes; S (2-Ep): selectivity to 2-ethylpentanal; S (3-Mh): selectivity to 3-methylhexanal; S (h): selectivity to 1-heptanal;
S (ald): total aldehyde selectivity.

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P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
Fig. 2. The aldehyde selectivity of triphenylphosphane ligands modified with thiomethyl and methoxy groups.
hydroformylation [6]. It was proposed there, that the
lack of rhodium hydride formation may have affected
the reactivity.
3.2. Metal complexes
3.2.1. Trans-Rh(CO)Cl(o-OOP)2 (1)
Rh2(CO)4Cl2 (100 mg, 0.257 mmol) and o-OOP
(170 mg, 0.53 mmol) were dissolved in methanol in
separate flasks, and the ligand solution was added
drop-wise to the precursor solution. The yellow precip-
itate that formed was filtered, washed with methanol,
and quickly dried under vacuum. Yellow crystals for
X-ray studies were crystallized from a CH2Cl2/hexane
solution. Elemental analysis for 1, C41H38P2O5ClRh:
calcd. % of C, 60.72; H, 4.72%; found % of C,
3. Experimental part
3.1. Computational details
Gaussian’94 [11] and Sybyl [12] programs were
used in modeling. The equilibrium geometries of
the ligands were first determined by optimizing the
structures by ab initio Hartree–Fock method using a
3-21G∗ basis set. Starting from the initial geometries,
conformational analysis was carried out by rotating
the torsional angles of the three phenyl rings. The
unsubstituted phenyl ring(s) was rotated 180 ◦, while
the substituted phenyl ring(s) was rotated 360 ◦ by
increments of 20 ◦, resulting in up to ∼6000 different
conformers. Based on contour maps of the potential
energy surface of the resulting conformers, 20–40
local minima were identified and subsequently opti-
mized by the HF/3-21G∗ method. The energies of the
minima in relation to the ground state are presented
in Scheme 1. For cone angle determinations the metal
(dummy)–phosphorus distance of 2.28 Å and the van
der Waals radii of hydrogen of 1.2 Å were used.
60.11; H, 4.65%. IR (CH2Cl2): υ(CO) = 1968 cm−1
.
31P-NMR: δ = 20.5 ppm, J(Rh–P) = 137 Hz.
3.2.2. Rh(CO)Cl(o-OOP) (2)
Rh2(CO)4Cl2 (25 mg, 0.064 mmol) and o-OOP
(35 mg, 0.109 mmol) elemental analysis for 2,
C21H19PO3ClRh: calcd. % of C, 51.61; H, 3.92%;
found % of C, 51.39; H, 3.99%. IR (CH2Cl2):
υ(CO) = 1999 cm−1
.
3.2.3. Rh(CO)Cl(o-OOOP) (3)
Rh2(CO)4Cl2 (100 mg, 0.257 mmol) and o-OOP
(170 mg, 0.53 mmol) were dissolved in methanol
in separate flasks. Before being added to the pre-
cursor solution, the ligand solution was slightly

P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
99
warmed (50 ◦C) to allow total dissolution. The yellow
Table 5
Selected bond lengths and angles for compound 1
precipitate, which formed after a few hours of
mixing, was filtered, washed with methanol, and
quickly dried under vacuum. Elemental analysis
for 3, C22H21PO4ClRh: calcd. % of C, 50.94; H,
4.08%; found % of C, 50.65; H, 4.06%. IR (CH2Cl2):
Atoms
trans-Rh(CO)Cl(o-OOP)2 (1)
Bond distances (Å)
1.808 (2)
2.398 (5)
2.328 (6)
2.326 (6)
1.146 (3)
1.829 (2)
1.825 (2)
1.835 (2)
1.362 (3)
1.428 (3)
1.365 (3)
1.434 (3)
1.829 (2)
1.834 (2)
1.832 (2)
1.371 (3)
1.427 (3)
1.368 (3)
1.428 (3)
Bond angles (degree)
175.17 (2)
91.88 (7)
90.87 (7)
177.17 (8)
88.24 (2)
89.21 (2)
178.7 (2)
118.9 (2)
117.45 (2)
117.91 (2)
117.56 (2)
Rh–C(1)
Rh–Cl(1)
Rh–P(1)
Rh–P(2)
υ(CO) = 1994 cm−1
.
31P-NMR: δ = 38.2 ppm,
J(Rh–P) = 165 Hz.
C(1)–O(1)
P(1)–C(11)
P(1)–C(21)
P(1)–C(31)
C(12)–O(71)
O(71)–C(71)
C(22)–O(72)
O(72)–C(72)
P(2)–C(41)
P(2)–C(51)
P(2)–C(61)
C(52)–O(74)
O(74)–C(74)
C(42)–O(75)
O(75)–C(75)
3.3. X-ray crystallography
X-ray diffraction data were collected with a Non-
ius KappaCCD diffractometer using Mo K␣ radiation
(λ = 0.71073 Å) and ␾-scan data collection mode
with a Collect [13] collection program. Denzo and
Scalepack [14] programs were used for cell refine-
ments and data reduction. Structure was solved by
direct methods using the SIR97 program [15] and
the WinGX [16] graphical user interface. Structure
refinements were carried out with the SHELXL97
[17] program. Hydrogens were constrained to ride
P(2)–Rh–P(1)
C(1)–Rh–P(2)
C(1)–Rh–P(1)
C(1)–Rh–Cl
P(2)–Rh–Cl
P(1)–Rh–Cl
O(1)–C(1)–Rh(1)
C(12)–O(71)–C(71)
C(22)–O(72)–C(72)
C(52)–O(74)–C(74)
C(42)–O(75)–C(75)
on their parent atoms (Carom–H = 0.95 Å, Uiso
=
1.2Ceq, CCH –H = 0.99 Å, Uiso = 1.2Ceq, and
2
CCH –H = 0.98 Å, Uiso = 1.5Ceq). Crystallographic
3
data (excluding structure factors) for the structure
reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC 159195. Copies of
the data can be obtained free of charge on appli-
cation to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: int. code +44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk).
Crystallographic data are summarized in Table 6
and selected bond lengths and angles are presented
in Table 5. The structure of complex 1 is shown
in Fig. 1.
100 ◦C, and after four or 6 h reaction it was cooled and
returned to normal atmospheric pressure. The repro-
ducibility of the system was confirmed by performing
the tests twice.
3.4. Catalysis
A disposable inner Teflon liner was used to avoid
the accumulation of rhodium on the reactor walls.
The purity of the system was also checked with blank
runs before each experiment. The products were ana-
lyzed with a Hewlett-Packard 5890 GC equipped with
a capillary column (HP-1, 1.0 ␮m × 0.32 mm × 60 m)
and a flame-ionization detector. Products were quan-
tified by the internal standard method. In addition,
the aldehydes that formed were identified by GC-MS
analysis.
1-Hexene hydroformylation reactions were con-
ducted in a 100 ml autoclave (Berghof) with 60 ml
Teflon liner. Experiments were carried out in batch
mode with rhodium precursor Rh4(CO)12. The re-
actor was charged under a nitrogen purge with sub-
strate, rhodium catalyst, ligand, and internal standard
(cyclohexane). The autoclave was then sealed and
pressurized using a 1:1 mixture of H2 and CO (MG,
99.997%) to 20 bars. The autoclave was heated to

100
P. Suomalainen et al. / Journal of Molecular Catalysis A: Chemical 179 (2002) 93–100
Table 6
to the greater steric size and to the greater basicity of
the ligands.
Crystallographic data for compound 1
Regioselectivity was also dependent on binding
mode, and thus, n/i ratios were lower with thiomethyl-
than methoxy-modified ligands.
Empirical formula
molecular weight
Crystal size (mm)
Crystalline system
Space group
a (Å)
b (Å)
c (Å)
λ (Å)
C41H38P2O5ClRh
811.06
0.3 × 0.3 × 0.3
Triclinic
P-1
10.1423 (2)
12.8966 (4)
16.1360 (6)
85.196 (10)
79.239 (2)
71.758 (2)
1968.56 (10)
2
1.512
0.763
150 (2)
3.02–27.46
16757
8764
482
0.0320
0.0470
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4. Conclusions
Hydroformylation tests revealed relationships be-
tween the catalytic behavior and structure of the
ligands.
The conversion and the total aldehyde selectivity
decreased as a function of cone angle and degree of
substitution of the ligand. As the substituents were
then moved from ortho- to para-position in the ligand,
so as to decrease the steric hindrance near phospho-
rus and rhodium centers, the selectivity to aldehydes
increased.
However, steric requirements did not explain the
differences in the catalytic behavior of the two donor
types. Higher chemoselectivity of the thiomethyl mod-
ified ligands is suggested to arise from the bidentate
coordination, and thus, from the more accessible metal
center. Although Rh(I) complexes containing methoxy
substituted ligands bearing chelate P–O ring were also
observed, ring formation was not considered to be
strong enough to take place under catalytic environ-
ment.
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Compared with PPh3 the regioselectivity (n/i) in-
creased with all of the studied ligands. This was due