Electronic effect on rhodium diphosphine catalyzed hydroformylation: The bite angle effect reconsidered

Article abstract of DOI:10.1021/ja981969e

The electronic effect in the rhodium diphosphine catalyzed hydroformylation was investigated. A series of electronically modified thixantphos ligands was synthesized, and their effects on coordination chemistry and catalytic performance were studied. Phosphine basicity was varied by using p-(CH₃)₂N, p-CH₃O, p-H, p-F, p-Cl, or p-CF₃ substituents on the diphenylphosphine moieties. X-ray crystal structure determinations of the complexes (thixantphos)Rh(CO)H(PPh₃) and (p-CH₃O- thixantphos)Rh(CO)H(PPh₃) were obtained. The solutions structures of the (diphosphine)Rh(CO)H(PPh₃) and (diphosphine)Rh(CO)₂H complexes were studied by IR and NMR spectroscopy. IR and ¹H NMR spectroscopy showed that the (diphosphine)Rh(CO)₂H complexes consist of dynamic equilibria of diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium compositions proved to be dependent on phosphine basicity; the ee:ea isomer ratio shifts gradually from almost one for the p-(CH₃)₂N-substituted ligand to more than nine for the p-CF₃-substituted ligand. Assignments of bands to ee and ea isomers and the shifts in wavenumbers in the IR spectra were supported by calculations on (PH₃)₂Rh(CO)₂H, (PH₃)₂Rh(CO)₂D, and (PF₃)₂Rh(CO)₂H complexes using density functional theory. In the hydroformylation of 1-octene and styrene an increase in 1:b ratio and activity was observed with decreasing phosphine basicity. Most remarkably for 1-octene the selectivity for linear aldehyde formation was between 92 and 93% for all ligands. These results indicate that the chelation mode in the (diphosphine)Rh(CO)₂H complexes per se is not the key parameter controlling the regioselectivity. Mechanistic explanations of the effect of the natural bite angle on regioselectivity are reconsidered.

Full text of DOI:10.1021/ja981969e

11616
J. Am. Chem. Soc. 1998, 120, 11616-11626
Electronic Effect on Rhodium Diphosphine Catalyzed
Hydroformylation: The Bite Angle Effect Reconsidered
Lars A. van der Veen,† Maarten D. K. Boele,† Frank R. Bregman,† Paul C. J. Kamer,†
Piet W. N. M. van Leeuwen,*† Kees Goubitz,‡ Jan Fraanje,‡ Henk Schenk,‡ and Carles Bo§
Contribution from the Institute for Molecular Chemistry, UniVersity of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and
Department of Physical and Inorganic Chemistry, UniVersitat RoVira i Virgili,
Plaza Imperial Tarraco 1, 43005 Tarragona, Spain
ReceiVed June 5, 1998
Abstract: The electronic effect in the rhodium diphosphine catalyzed hydroformylation was investigated. A
series of electronically modified thixantphos ligands was synthesized, and their effects on coordination chemistry
and catalytic performance were studied. Phosphine basicity was varied by using p-(CH3)2N, p-CH3O, p-H,
p-F, p-Cl, or p-CF3 substituents on the diphenylphosphine moieties. X-ray crystal structure determinations of
the complexes (thixantphos)Rh(CO)H(PPh3) and (p-CH3O-thixantphos)Rh(CO)H(PPh3) were obtained. The
solutions structures of the (diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)2H complexes were studied
1
by IR and NMR spectroscopy. IR and H NMR spectroscopy showed that the (diphosphine)Rh(CO)2H
complexes consist of dynamic equilibria of diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium
compositions proved to be dependent on phosphine basicity; the ee:ea isomer ratio shifts gradually from almost
one for the p-(CH3)2N-substituted ligand to more than nine for the p-CF3-substituted ligand. Assignments of
bands to ee and ea isomers and the shifts in wavenumbers in the IR spectra were supported by calculations on
(PH3)2Rh(CO)2H, (PH3)2Rh(CO)2D, and (PF3)2Rh(CO)2H complexes using density functional theory. In the
hydroformylation of 1-octene and styrene an increase in l:b ratio and activity was observed with decreasing
phosphine basicity. Most remarkably for 1-octene the selectivity for linear aldehyde formation was between
92 and 93% for all ligands. These results indicate that the chelation mode in the (diphosphine)Rh(CO)2H
complexes per se is not the key parameter controlling the regioselectivity. Mechanistic explanations of the
effect of the natural bite angle on regioselectivity are reconsidered.
Introduction
using rhodium diphosphite catalysts.14 A new highly active and
selective catalyst system based on binuclear rhodium complexes
and tetraphosphine ligands has been developed by Stanley and
co-workers.15 Systematic studies of the correlation between
phosphine structure and catalytic performance are rare, however,
and despite the development of a wide variety of phosphorus
ligands, no detailed understanding concerning how phosphorus
ligands control selectivity has emerged.
The generally accepted dissociative mechanism for the
rhodium-catalyzed hydroformylation as proposed by Wilkinson
is shown in Scheme 1.16 According to this mechanism for the
hydroformylation of 1-alkenes, the selectivity is determined in
the step that converts a five-coordinate L2Rh(CO)H(alkene) into
Rhodium-catalyzed hydroformylation is one of the most
important applications of homogeneous catalysis in industry.1-5
Since the discovery of the rhodium catalysts by Wilkinson,6,7
much effort has been made to enhance the regioselectivity of
the reaction toward the formation of the more desirable linear
aldehyde. High selectivities in the hydroformylation of 1-al-
kenes have been obtained for both diphosphite- and diphosphine-
modified catalysts.8-13 Functionalized alkenes have been hy-
droformylated with high selectivity by Cuny and Buchwald
† Department of Inorganic Chemistry, University of Amsterdam.
‡ Department of Crystallography, University of Amsterdam.
§ Universitat Rovira i Virgili.
(1) Parshall, G. W. Homogeneous Catalysis: the applications and
chemistry of catalysis by soluble transition metal complexes; Wiley: New
York, 1980.
(2) Cornils, B. In New Syntheses with Carbon Monoxide; Falbe, J., Ed.;
Springer-Verlag: Berlin, Heidelberg, 1980; pp 1-225.
(3) Tolman, C. A.; Faller, J. W. In Homogeneous Catalysis with Metal
Phosphine Complexes; Pignolet, L. H., Ed.; Plenum: New York, 1983; pp
81-109.
(4) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpaintner, C. W. J. Mol.
Catal. A: Chem. 1995, 104, 17-85.
(5) Frohning, C. D.; Kohlpaintner, C. W. In Applied Homogeneous
Catalysis with Organometallic Compounds: a comprehensiVe handbook
in two Volumes; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim,
New York, Basel, Cambridge, Tokyo, 1996; Vol. 1, pp 27-104.
(6) Evans, D.; Yagupsky, G.; Wilkinson, G. J. Chem. Soc. A 1968, 2660-
2665.
(7) Brown, C. K.; Wilkinson, G. J. Chem. Soc. A 1970, 2753-2764.
(8) Buisman, G. J. H.; Vos, E. J.; Kamer, P. C. J.; van Leeuwen, P. W.
N. M. Tetrahedron: Asymmetry 1995, 6, 719-738.
(9) van Rooy, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Goubitz,
K.; Fraanje, J.; Veldman, N.; Spek, A. L. Organometallics 1996, 15, 835-
847.
(10) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. Organometallics 1995, 14, 3081-3089.
(11) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. (to Union Carbide Corp.)
U.S. Patent 4.769.498, 1988.
(12) Devon, T. J.; Phillips, G. W.; Puckette, T. A.; Stavinoha, J. L.;
Vanderbilt, J. J. (to Eastman Kodak) U.S. Patent 4,694,109, 1987.
(13) Burke, P. M.; Garner, J. M.; Tam, W.; Kreutzer, K. A.; Teunissen,
A. J. J. M.; Snijder, C. S.; Hansen, C. B. (to DSM/Du Pont de Nemours)
WO 97/33854, 1997.
(14) Cuny, G. D.; Buchwald, S. L. J. Am. Chem. Soc. 1993, 115, 2066-
2068.
(15) Broussard, M. E.; Juma, B.; Train, S. G.; Peng, W.-J.; Laneman, S.
A.; Stanley, G. G. Science 1993, 260, 1784-1788.
(16) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968,
3133-3142.
10.1021/ja981969e CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/03/1998

Rhodium Diphosphine Catalyzed Hydroformylation
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11617
Table 1. Selected Data for Ligands 1-7
Scheme 1
flexibility
c
ligand
R
ân,a deg
range,a deg
øi,b cm-1
σp
1
2
3
4
5
6
7
N(CH3)2
OCH3
CH3
H
F
Cl
109.1
106.9
106.7
106.4
106.6
107.8
109.3
92-124
91-123
91-125
91-127
92-128
91-126
92-128
1.8
3.4
3.5
4.3
5.0
5.6
6.6
-0.83
-0.27
-0.17
0.00
0.06
0.23
CF3
0.54
a The natural bite angle (ân) and the flexibility range were calculated
analogously to the method used by Casey and Whiteker.23 ân is defined
as the preferred chelation angle determined only by ligand backbone
constraints and not by metal valence angles. The flexibility range is
defined as the accessible range of bite angles within less than 3 kcal
mol-1 excess strain energy from the calculated natural bite angle.
b Tolman øi values were abstracted from ref 28. c Hammett σp values
were abstracted from ref 29.
hydroformylation by using a series of p-substituted triphen-
ylphosphines.25 Diphosphines of different basicities were studied
by Unruh and Christenson.26 Both studies showed that less basic
phosphines afford higher reaction rates and higher linear over
branched (l:b) ratios. In a recent study Casey and co-workers
investigated the electronic effect of equatorial and apical
phosphines.27 They concluded that electron-withdrawing sub-
stituents on phosphines in the equatorial position give high l:b
ratios, while electron-withdrawing substituents on phosphines
in the apical position give low l:b ratios. The results of this
study, however, cannot be described in terms of electronic
effects only, since there are major steric differences between
the ee and ea coordinating diphosphines.
either a linear or branched four-coordinate L2Rh(CO)(alkyl).
For the linear rhodium alkyl species this step is irreversible at
moderate temperatures and sufficiently high pressures of CO.
The structure of the alkene complex is therefore thought to play
a crucial role in controlling the regioselectivity.17,18 However,
contrary to the L2Rh(CO)2H precursor complex, the L2Rh-
(CO)H(alkene) intermediate has not been observed directly so
far. Brown and Kent have shown that the PPh3-modified pre-
cursor complex (PPh3)2Rh(CO)2H exists as a mixture of two
rapidly equilibrating trigonal bipyramidal isomers in a diequa-
torial (ee) to equatorial-apical (ea) isomer ratio of 85:15.19,20
To study the exact nature of the electronic effect in the rho-
dium diphosphine catalyzed hydroformylation, we synthesized
a series of thixantphos10 ligands with varying basicity (1-7,
The stereoelectronic properties of phosphorus ligands have
a dramatic influence on the reactivity of hydroformylation cata-
lysts. Tolman introduced the concept of cone angle θ and the
electronic parameter ø as measures of respectively the steric
bulk and the electronic properties of phosphorus ligands.21,22
Casey and co-workers developed the concept of the natural bite
angle as an additional characteristic of diphosphine ligands based
on molecular mechanics calculations.23 The pronounced effect
of the natural bite angle on the selectivity in the hydroformy-
lation was shown by Casey and co-workers as well as by our
group.10,17
While a rational understanding of how ligand structure can
steer the selectivity in the hydroformylation is now unfolding,
only limited attention has been paid to the electronic proper-
ties of phosphine ligands and their influence on catalytic
performance.24 In one of the few detailed studies, Moser and
co-workers investigated the effect of phosphine basicity in the
Table 1) and probed their catalytic performance and coordination
chemistry in rhodium complexes. In this series of ligands, steric
differences are minimal so purely electronic effects can be
investigated. Here we report that the equilibrium between ee
and ea coordination in the (diphosphine)Rh(CO)2H complexes
is dependent on phosphine basicity. Decreasing phosphine
basicity shifts the equilibrium to ee coordination. In the
hydroformylation of 1-octene and styrene decreasing phosphine
basicity gives an increase both in l:b ratio and in activity, while
increasing phosphine basicity gives the opposite effect. Most
remarkably, for 1-octene, selectivity for linear aldehyde remains
virtually the same, 92-93%, due to the enhanced isomerization
activity displayed by the less basic phosphines. These results
show that the electronic properties of the diphosphine ligands
and their coordination mode in the (diphosphine)Rh(CO)2H
complexes do not influence the regioselectivity. Mechanistic
implications and possible explanations for the bite angle effect
are discussed.
(17) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J. A., Jr.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535-5543.
(18) Casey, C. P.; Petrovich, L. M. J. Am. Chem. Soc. 1995, 117, 6007-
6014.
(19) Brown, J. M.; Kent, A. G. J. Chem. Soc., Perkin Trans. 2 1987,
1597-1607.
(24) Oswald, A. A.; Hendriksen, D. E.; Kastrup, R. V.; Mozeleski, E. J.
AdV. Chem. Ser. 1992, 230, 395-418.
(20) Throughout this work, notations ee and ea refer to respectively the
diequatorial and equatorial-apical chelation modes of the diphosphine in
trigonal bipyramidal rhodium complexes.
(25) Moser, W. R.; Papile, C. J.; Brannon, D. A.; Duwell, R. A.;
Weininger, S. J. J. Mol. Catal. 1987, 41, 271-292.
(26) Unruh, J. D.; Christenson, J. R. J. Mol. Catal. 1982, 14, 19-34.
(27) Casey, C. P.; Paulsen, E. L.; Beuttenmueller, E. W.; Proft, B. R.;
Petrovich, L. M.; Matter, B. A.; Powell, D. R. J. Am. Chem. Soc. 1997,
119, 11817-11825.
(21) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.
(22) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2953-2956.
(23) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299-304.

11618 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Van der Veen et al.
Scheme 2a
a (i) n-BuLi/TMEDA/Et2O/25 °C; (ii) ZnCl2/0 °C; (iii) PCl3/-110
°C; (iv) ArMgBr/THF/0 °C.
Results
Synthesis of Thixantphos Ligands Containing Electron-
Donating and Electron-Withdrawing p-Substituents. The
thixantphos ligands were synthesized by reacting the Grignard
reagent of the appropriate p-substituted aryl bromide with 4,5-
bis(dichlorophosphino)-2,7-dimethylphenoxathiin (8) (Scheme
2). Compound 8 was prepared by selective dilithiation of 2,7-
dimethylphenoxathiin followed by reaction with zinc chloride
and reaction of the resulting dizinc compound with an excess
of phosphorus trichloride at low temperature.30 Typical yields
of the ligands varied from 25% for p-CH3O-thixantphos (2) to
54% for p-(CH3)2N-thixantphos (1) based on 8. Table 1 shows
that variation of the substituent has only a minor effect on the
natural bite angle and flexibility range and, therefore, steric
effects can be neglected when evaluating the electronic influence
of these ligands in catalysis.31
Rhodium Complexes. To investigate the influence of
phosphine basicity on chelation behavior, the solution structures
of the (diphosphine)Rh(CO)H(PPh3) complexes and the (diphos-
phine)Rh(H)(CO)2 complexes were studied in great detail for
all ligands. Displacement of PPh3 in (PPh3)3Rh(CO)H by the
diphosphines 1-7 resulted in formation of the (diphosphine)-
Rh(CO)H(PPh3) complexes 9a-15a. Crystals of the (diphos-
phine)Rh(CO)H(PPh3) complexes suitable for X-ray structure
determination were obtained for ligands 2 and 4.
Figure 1. X-ray structure and numbering scheme for (thixantphos)-
Rh(CO)H(PPh3) (12a) (hydrogen atoms have been omitted for clarity)
and optimized geometry of (PH3)3Rh(CO)H (bond lengths are given
in angstroms).
The X-ray crystal structures of (p-CH3O-thixantphos)Rh-
(CO)H(PPh3) (10a) and (Thixantphos)Rh(CO)H(PPh3) (12a) are
very similar so only the latter is shown in Figure 1. Selected
bond lengths and angles of both complexes are given in Table
2. The crystal structures reveal distorted trigonal bipyramidal
complex geometries with all phosphines occupying equatorial
sites. In the structures the Rh atom is located slightly above
the equatorial plane defined by the three phosphorus atoms and
displaced toward the apical carbonyl ligand. The hydride ligand
was located in neither structures, but 1H NMR indicates that it
occupies the apical site trans to the carbonyl ligand (vide infra).
The P-Rh-P bite angles are 111.73(5)° for the thixantphos
ligand and 109.31(7)° for the p-CH3O-thixantphos ligand. The
small difference in observed bite angles between p-CH3O-
thixantphos and the unsubstituted thixantphos confirms our
assumption based on molecular mechanics that p-substitution
on the aryl rings has only minor steric effects. A noticeable
difference between comparable bond lengths in the complexes
is found in the Rh-CO distance. The decreased Rh-CO bond
length for the p-CH3O-thixantphos complex (1.863(8) Å)
compared to the thixantphos complex (1.887(8) Å) is a direct
result of increased back-bonding of the rhodium to the carbonyl
ligand (vide infra).
Table 2. Selected Bond Lengths and Angles for
(Thixantphos)Rh(CO)H(PPh3) (12a) and
(p-CH3O-Thixantphos)Rh(CO)H(PPh3) (10a)
(thixantphos)Rh(CO)H(PPh3) (p-CH3O-thixantphos)Rh(CO)H(PPh3)
(12a)
(10a)
Bond Lengths (Å)
2.327(1) Rh-P(1)
2.318(1) Rh-P(2)
2.295(2) Rh-P(3)
1.887(8) Rh-C(61)O
Rh-P(1)
Rh-P(2)
Rh-P(3)
Rh-C(1)O
2.313(2)
2.328(2)
2.292(2)
1.863(8)
Bond Angles (deg)
P(1)-Rh-P(2) 111.73(5) P(1)-Rh-P(2)
P(1)-Rh-P(3) 121.73(5) P(1)-Rh-P(3)
P(2)-Rh-P(3) 124.53(5) P(2)-Rh-P(3)
109.31(7)
126.65(8)
121.65(7)
94.4(2)
96.9(3)
94.2(3)
P(1)-Rh-C(1)
P(2)-Rh-C(1)
P(3)-Rh-C(1)
94.2(2)
94.9(2)
95.0(2)
P(1)-Rh-C(61)
P(2)-Rh-C(61)
P(3)-Rh-C(61)
Calculated Structure for the (PH3)3Rh(CO)H Complex.
To support the spectroscopic results of the (diphosphine)Rh-
(CO)H(PPh3) complexes (vide infra), calculations were per-
formed on the (PH3)3Rh(CO)H complex using density functional
theory (DFT). The geometry of the (PH3)3Rh(CO)H complex
was optimized under the constraints of the C3V symmetry group
and characterized as a minimum from the vibrational analysis.
The structure is depicted in Figure 1. The calculated Rh-P
and Rh-C bond lengths and the H-Rh-C angle are in good
agreement with the distorted trigonal bipyramidal symmetry
observed in the X-ray structures of complexes 10a and 12a.
Recently Schmid and co-workers also reported on the optimized
(28) Dias, P. B.; de Piedade, M.; Simoes, J. A. M. Coord. Chem. ReV.
1994, 135/136, 737-807.
(29) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(30) Private communication of Dr. H. T. Teunissen and Prof. Dr. F.
Bickelhaupt.
(31) It should be noted that in the MM2 calculations the same
approximate parameters are used for phosphorus and rhodium, despite
electronic changes.

Rhodium Diphosphine Catalyzed Hydroformylation
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11619
Table 3. Selected IR Data for (Diphosphine)Rh(CO)H(PPh3)
Complexes 9a-15aa
ligand
R
ν1, cm-1
ν2, cm-1
9a
10a
11a
12a
13a
14a
15a
N(CH3)2
OCH3
CH3
H
F
Cl
1982 (s)
1989 (s)
1999 (s)
1999 (s)
2001 (s)
2006 (s)
2009 (s)
1909 (m)
1916 (m)
1920 (m)
1922 (w)
1921 (m)
1928 (m)
1931 (m)
CF3
a In KBr, carbonyl region.
structure of the (PH3)3Rh(CO)H complex obtained by DFT
calculations.32 The geometry parameters they obtained at the
LDA level of theory are very similar to the values presented
here.
NMR Data for (Diphosphine)Rh(CO)H(PPh3) Complexes
9a-15a. The presence of hydride ligands in all (diphosphine)-
1
Rh(CO)H(PPh3) complexes is confirmed by H NMR. The
rhodium hydride signals appear as broadened quartets or triplets
of doublets at δ ) -9.09 to -9.60 (td, 2J(P,H) ) 10-21 Hz).
The rhodium-proton coupling constants could not be resolved.
The phosphorus-proton coupling constants are consistent with
hydrides occupying apical positions cis to three equatorial
phosphines in distorted trigonal bipyramidal complex geometries
(vide supra). Evaluation of the 31P{1H} NMR spectra leads to
the conclusion that for all complexes the two phosphine moieties
of the diphosphine in the complex are equivalent. The diphos-
phine P-PPh3 coupling constants (2J(P,P) ) 123-126 Hz) are
also in agreement with structures in which all phosphines occupy
equatorial sites.
IR Data for (Diphosphine)Rh(CO)H(PPh3) Complexes
9a-15a. The IR frequencies of the (diphosphine)Rh(CO)-
H(PPh3) complexes in the carbonyl region display a clear
electronic effect (Table 3). The IR spectra of the complexes in
KBr all have two absorption bands between 1900 and 2100 cm-1
due to combination bands of νRhH and νCO. Both bands show
a regular shift to higher wavenumbers with decreasing phosphine
basicity. The lower electron density on the metal leads to a
decrease in back-bonding from the rhodium to the carbonyl
ligand, and hence, higher CO stretching frequencies are
observed. The experimental results are in good correlation with
the results of the DFT calculations on the (PH3)3Rh(CO)H
complex. The calculated IR spectrum also exhibits two
harmonic frequencies: one at 1911 cm-1 corresponding to the
symmetric combination of the Rh-H and the Rh-CO vibrations
and one at 2007 cm-1 corresponding to the antisymmetric
combination of the same vibrations.
Calculated Structures for (PH3)2Rh(CO)2H and (PF3)2Rh-
(CO)2H Complexes. To model the basicity-dependent features
exhibited by the (diphosphine)Rh(CO)2H complexes (vide infra),
DFT calculations were performed on the (PH3)2Rh(CO)2H and
(PF3)2Rh(CO)2H complexes. Similar calculations of the equi-
librium structures of four- and five-coordinate rhodium hy-
drido carbonyl complexes using PH3 as model ligand have
been reported recently by Schmid and co-workers.32 The ee and
ea isomers of both complexes were fully optimized imposing
Cs symmetry (Figure 2). Bond lengths and angles are listed
in Table 4. A clear dependence on phosphine basicity is
observed for almost all parameters. Upon changing the basic
PH3 ligand for the less basic PF3 ligand, C-O distances de-
crease while Rh-C distances increase as a direct result of the
reduced back-bonding to the carbonyl ligands. A large
Figure 2. Optimized geometries of the (PH3)2Rh(CO)2H and (PF3)2-
Rh(CO)2H complexes together with the relative energy of the ee and
ea isomers. Bond lengths are given in angstroms.
Table 4. Calculated Structural Data for ee and ea Isomers of
Complexes (PH3)2Rh(CO)2H and (PF3)2Rh(CO)2Ha
(PH3)2Rh(CO)2H
ee ea
1.617 1.589
2.343 2.361/2.364 2.244
1.940/1.957 1.939
1.152/1.148 1.153
85.5/179.7 82.4 84.9/177.5
(PF3)2Rh(CO)2H
ee ea
1.607 1.597
2.255/2.266
Rh-H (Å)
Rh-Pe/a (Å)
Rh-Ce/a (Å) 1.906/1.931 1.912
C-Oe/a (Å)
H-Rh-Pe/a
(deg)
1.162/1.159 1.163
84.2
H-Rh-COe/a 82.7/177.4 84.6
82.2/179.5 82.3
112.5 97.6
(deg)
P-Rh-P
110.1
94.8
(deg)
a The subscripts e and a denominate equatorial and apical ligand
positions.
decrease in Rh-P bond lengths (0.1 Å) is observed with
decreasing phosphine basicity, accompanied by a slight increase
in P-Rh-P angle. The increase in P-Rh-P angle is, however,
most likely not an electronic but a steric effect. Unlike the other
parameters the effect of phosphine basicity on the Rh-H bond
length differs for each isomer; for the ee isomer the Rh-H
distance decreases with decreasing phosphine basicity, while
for the ea isomer the opposite occurs. Comparison of the
P-Rh-P angles of the calculated complexes with the flexibility
ranges in Table 1 shows that both ee and ea isomers of the
(diphosphine)Rh(CO)2H complexes could be accessible for the
thixantphos ligands.
HP NMR Data for (Diphosphine)Rh(CO)2H Complexes
9b-15b. For all diphosphine ligands, the formation of the
(diphosphine)Rh(CO)2H complexes was evidenced by the
appearance of a doublet in the 31P NMR spectra and an apparent
1
triplet of doublets for the rhodium hydride in the H NMR
spectra. Table 5 reveals that decreasing phosphine basicity gives
decreasing rhodium-proton (1J(Rh,H)) and phosphorus-proton
(2J(P,H)) coupling constants but increasing rhodium-phospho-
rus (1J(Rh,P)) coupling constants.33
(32) Schmid, R.; Herrmann, W. A.; Frenking, G. Organometallics 1997,
16, 701-708.
(33) 103Rh NMR data will be reported elsewhere.

11620 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Van der Veen et al.
Table 5. Selected NMR Data for (Diphosphine)Rh(CO)2H
Complexes 9b-15ba
2
ligand
R
1J(Rh,H), Hz 1J(Rh,P), Hz Jav(P,H), Hz % eeb
9b N(CH3)2
10b OCH3
11b CH3
8.7
7.5
7.2
6.6
6.3
6.0
4.5
121
124
126
128
130
132
134
27.9
21.6
17.6
14.7
11.0
8.4
44-50
56-62
63-69
69-75
76-83
81-88
89-96
12b
13b
H
F
14b Cl
15b CF3
3.6
2
a In C6D6 at 298 K. b Calculated from the Jav(P,H) using trans
2
2J(P,H) ) 106 Hz and cis J(P,H) e (2 Hz.
The rhodium-proton and phosphorus-proton coupling con-
stants and their dependence on phosphine basicity suggest that
these complexes exist as mixtures of ee and ea isomers that are
in dynamic equilibrium.19 Rapid interconversion of the two
isomers, presumably via pseudo-Berry rotations, interchanges
the phosphorus atoms, and therefore, averaged chemical shifts
and coupling constants are observed. As demonstrated by
Yagupsky and Wilkinson and recently by Casey and co-workers
for related iridium complexes,27,34 the ratio of the two isomers
can be calculated from the averaged observed coupling constant
if all separate values are known. Unfortunately the dynamic
equilibria of the complexes could not be frozen out even at 163
K, so the coupling constants for the individual isomers could
not be determined. Estimations of the complex mixture
compositions at room temperature were achieved using a cis
phosphorus-proton coupling of -2 to 2 Hz, and a trans
phosphorus-proton coupling of 106 Hz.35 From the calculated
complex mixture compositions it can be concluded that the
equilibrium between the ee and ea isomer is greatly influenced
by phosphine basicity. For the strongly electron-donating
N(CH3)2 substituent, the ea isomer is slightly favored, while
for the strongly electron-withdrawing CF3 substituent, the
equilibrium is shifted almost completely to the ee isomer.
The enhanced preference of the basic phosphines for apical
coordination and the preference of the nonbasic phosphines for
equatorial coordination is quantitatively demonstrated by our
DFT calculations on the ee and ea isomers of complexes (PH3)2-
Rh(CO)2H and (PF3)2Rh(CO)2H (Figure 2). The energy dif-
ference between the ee and ea isomer of the complex modified
with PH3 as model ligand for a basic phosphine was calculated
to be 0.1 kcal mol-1 in favor of the ea isomer. This small
energy difference would correspond to an ee:ea equilibrium
Figure 3. HP-IR spectra for (diphosphine)Rh(CO)2H complexes 9b-
15b in 2-Me-THF at 80 °C and 20 bar of CO/H2 (1:1) (carbonyl
region).
equilibrium between ee and ea complex isomers was supplied
by HP-IR spectroscopy. The spectra of the (diphosphine)Rh-
(CO)2H complexes all showed four absorption bands in the
carbonyl region (Figure 3).
composition of approximately 1:1 at room temperature.36
A
slight preference of the ea isomer over the ee isomer for the
PH3 ligand has also been reported by Schmid and co-workers
and by Matsubara and co-workers.32,37 With PF3 as a model
ligand for a nonbasic phosphine the ee isomer was found to be
In an effort to assign the bands to ee and ea isomers the
(thixantphos)Rh(CO)2D complex was measured for comparison.
Upon H/D exchange, only ν1 and ν3 shift to lower wavenumbers
(respectively 18 and 14 cm-1), and therefore, these two bands
are assigned to the carbonyls of the ee complex.38 The two
remaining bands that do not shift, ν2 and ν4, belong consequently
to the ea complex. From the disappearance of a low-frequency
shoulder upon H/D exchange, it can be concluded that one of
the rhodium hydride vibrations is partly hidden under ν4.
To verify the empirical peak assignments, the theoretical IR
spectra of the (PH3)2Rh(CO)2H and -D and (PF3)2Rh(CO)2H
complexes were calculated by DFT methods. Recently several
reports have been published concerning the calculation of
favored by 2.4 kcal mol-1
. This corresponds to an ee:ea
equilibrium ratio of approximately 50:1 at room temperature.
The calculated shift of the equilibrium upon changing the basic
PH3 ligand for the less basic PF3 ligand is consistent with the
observed shift in isomer composition with decreasing phosphine
basicity in the series of substituted thixantphos ligands.
HP-IR Data for (Diphosphine)Rh(CO)2H Complexes 9b-
15b. Unambiguous evidence for the existence of a dynamic
(34) Yagupsky, G.; Wilkinson, G. J. Chem. Soc. A 1969, 725-733.
(35) A trans phosphorus-proton coupling of 106 Hz was observed in
1H NMR spectrum of the ea isomer of (DPEphos)Rh(CO)2H at 173 K
(unpublished results).
(36) By the calculation of the complex mixture composition entropy terms
are neglected.
(38) H/D exchange will only effect the ee isomer because in this isomer
there is a trans relationship between the hydride and a carbonyl ligand which
results in coupling of the vibrations. The disappearance of resonance
interaction upon H/D exchange will lead to frequency shifts of the carbonyl
bands of the ee isomer.
(37) Matsubara, T.; Koga, N.; Ding, Y.; Musaev, D. G.; Morokuma, K.
Organometallics 1997, 16, 1065-1078.

Rhodium Diphosphine Catalyzed Hydroformylation
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11621
Table 6. Calculated Rh-H, Rh-D, and C-O Stretching Harmonic Frequencies, IR Intensities, and Assignments for the ee and ea Isomers
of Complexes (PH3)2Rh(CO)2D, (PH3)2Rh(CO)2H, and (PF3)2Rh(CO)2Ha
(PH3)2Rh(CO)2D
(PH3)2Rh(CO)2H
intensity,
km/mol
(PF3)2Rh(CO)2H
frequency,
cm-1
intensity,
km/mol
frequency,b
cm-1
frequency,
intensity,
km/mol
assignment
assignment
cm-1
ee isomer
2006
1963
1414
ea isomer
638
924
14
(C-O)e+(C-O)a
(C-O)e-(C-O)a
Rh-D
2041 (2087)
1983 (2043)
1938 (1961)
532
705
371
(Rh-H)-(C-O)e-(C-O)a
(Rh-H)+(C-O)e+(C-O)a
(Rh-H)-(C-O)e+(C-O)a
2099
2036
2004
423
555
279
2069 (2049)
1985 (2061)
1948 (2028)
138
478
1224
Rh-H
(C-O)+(C-O)
(C-O)-(C-O)
2062
2042
2010
52
374
1035
1985
1948
1470
477
1223
61
(C-O)+(C-O)
(C-O)-(C-O)
Rh-D
a Assignments based on the normal modes analysis, the subscripts e and a denominate equatorial and apical ligand positions, and the signs
indicate equal (+) or different (-) phase between the vibration of each bond. b The values in parentheses were reported by Schmid and co-
workers.32
Table 7. Results of the Hydroformylation of 1-Octene at 80 °Ca
harmonic frequencies of carbonyl ligands in metal-carbonyl
ligand
R
σp
l:b ratiob % selectb % isomerb tofb,c
complexes and we have used the same methodology as described
in some of them.39-41 The calculated harmonic vibrational
frequencies, intensities, and the corresponding assignments based
on the normal modes analysis of the carbonyl vibrations are
depicted in Table 6. Comparison of the frequencies of (PH3)2-
Rh(CO)2H and -D shows that H/D exchange only effects the
carbonyl frequencies of the ee isomer of the complex. The
calculated shift to lower frequency of the vibrations of the ee
isomer is in perfect agreement with the observed spectra and
confirms the empirical peak assignment to ee and ea isomers.
The fit of the absolute values of the calculated frequencies of
(PH3)2Rh(CO)2H with the measured values of the series
(thixantphos)Rh(CO)2H complexes (Figure 3) is striking. Only
the calculated value for ν3 (1983 cm-1) is out of the range of
ν3 values obtained experimentally (1960-1982 cm-1).
The effect of decreasing phosphine basicity on the theoretical
IR spectra is an overall increase in frequency and a decrease in
intensity of the vibrations. The only vibration that does not
experience a positive shift of approximately 60 cm-1 upon
exchanging PH3 for PF3, is the Rh-H frequency of the ea
isomer. Instead the decreased Rh-H bond length (vide supra)
results in a shift of 7 cm-1 to lower frequency. The low
calculated intensities for the Rh-H vibrations of the ea isomers
explain the absence of the corresponding Rh-H bands in the
spectra of the different (thixantphos)Rh(CO)2H complexes
(Figure 3). The calculated frequencies and intensities of the ee
isomers are in good agreement with the assignment of low-
frequency shoulders of ν4 to Rh-H signals in the observed
spectra (Figure 3).
1
2
3
4
5
6
7
N(CH3)2 -0.83
44.6
36.9
44.4
50.0
51.5
67.5
86.5
93.1
92.1
93.2
93.2
92.5
91.7
92.1
4.8
5.3
4.7
4.9
5.7
6.9
6.8
28
45
78
110
75
66
OCH3
CH3
H
-0.27
-0.17
0.00
0.06
0.23
F
Cl
CF3
0.54
158
a Conditions: CO/H2 ) 1, P(CO/H2) ) 20 bar, ligand/Rh ) 5,
substrate/Rh ) 637, [Rh] ) 1.00 mM, number of experiments ) 3. In
none of the experiments was hydrogenation observed. b Linear over
branched ratio, percent selectivity for linear aldehyde, percent isomer-
ization to 2-octene, and turnover frequency were determined at 20%
alkene conversion. c Turnover frequency ) (mol of aldehyde) (mol of
Rh)-1 h-1
.
respective (diphosphine)Rh(CO)2H complexes are identical. The
ee:ea equilibria are not influenced, and no other carbonyl
complexes are observed. The observation that the (diphosphine)-
Rh(CO)2H complexes are the only rhodium carbonyl species
present during hydroformylation implies that the rate-determin-
ing step is located early in the catalytic cycle. Either CO
dissociation or, more likely, alkene coordination will be rate
determining (Scheme 1).26 Evidence for rate-limiting alkene
coordination can be found in increasing reaction rates with
decreasing CO pressure, increasing alkene concentration, and
decreasing phosphine basicity (vide infra).
Hydroformylation of 1-Octene. Hydroformylation of
1-octene was carried out at 80 °C and 20 bar of 1:1 CO/H2
using a 1.0 mM solution of rhodium diphosphine catalyst pre-
pared from Rh(CO)2(dipivaloylmethanoate) (dpm) and 5 equiv
of ligand. The production of octene isomers, nonanal, and
2-methyloctanal was monitored by gas chromatography. The
results of the experiments with ligands 1-7 are shown in Table
7. Turnover frequencies were determined at 20% conversion.
Table 7 shows that, with the exception of ligands 5 and 6,
the rate of the reaction increases with decreasing phosphine
basicity. An explanation for the deviant behavior of 5 and 6
can be incomplete catalyst formation or deactivation of the
catalyst.
The shift in isomer composition that was found in the NMR
spectra is confirmed and clearly visualized when comparing the
IR spectra of the series of complexes (Figure 3). Going from
electron-donating substituents to electron-withdrawing substit-
uents, the intensities of the absorption bands of the ea isomer
are diminished in favor of those of the ee isomer. Another trend
observed in the series is a regular shift to higher wavenumbers
of all four absorption bands with decreasing phosphine basicity.
These shifts are comparable to the ones found for the (diphos-
phine)Rh(CO)H(PPh3) complexes.
The inverse dependency of the reaction rate on the phosphine
basicity is often found in the rhodium-catalyzed hydroformy-
lation.25,26 Decreasing phosphine basicity facilitates CO dis-
sociation from the (diphosphine)Rh(CO)2H complex and en-
hances alkene coordination to form the (diphosphine)Rh(CO)-
H(alkene) complex, and therefore, the reaction rate increases.
Weakening of the rhodium-carbonyl bond in the rhodium
carbonyl complexes by electron-withdrawing substituents on the
The IR spectra of the rhodium species present during actual
hydroformylation experiments with ligands 2, 4, and 7 and the
(39) Ehlers, A. W.; Frenking, G.; Baerends, E. J. Organometallics 1997,
16, 4896-4902.
(40) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg.
Chem. 1997, 36, 5031-5036.
(41) Barckholtz, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1998, 120,
1926-1927.

11622 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Van der Veen et al.
Table 8. Results of the Hydroformylation of Styrene at 120 °Ca
Scheme 3
ligand
R
σp
l:b ratiob
% selectb
tofb,c
1
2
3
4
5
6
7
N(CH3)2
OCH3
CH3
H
F
Cl
-0.83
-0.27
-0.17
0.00
0.06
0.23
0.75
0.85
1.1
1.3
1.3
1.4
1.8
43
46
53
56
56
59
64
1100
1800
2800
3100
2900
3200
3400
CF3
0.54
a Conditions: CO/H2 ) 1, P(CO/H2) ) 10 bar, ligand/Rh ) 10,
substrate/Rh ) 1746, [Rh] ) 0.50 mM, number of experiments ) 3.
In none of the experiments was hydrogenation observed. b Linear over
branched ratio, percent selectivity for linear aldehyde, and turnover
frequency were determined at 20% alkene conversion. c Turnover
frequency ) (mol of aldehyde) (mol of Rh)-1 h-1
.
ligand is evidenced by an increase in carbonyl stretching
frequencies (vide supra).
selectivity for linear aldehyde. Although this may seem
paradoxical at first, this is not in disagreement with the results
obtained in the hydroformylation of 1-octene. Of course styrene
behaves rather different in the hydroformylation compared to
1-octene due to the formation of a stable η3 complex. The
reactions were not performed under identical conditions, but
the results obtained with both substrates can still be ex-
plained similarly. As found for 1-octene the increase in l:b ratio
with decreasing phosphine basicity is probably caused by
enhanced â-hydrogen elimination. However, for styrene, this
does not result in isomerization since â-hydrogen elimination
will always result in reformation of styrene. The only effect
of enhanced â-hydrogen elimination is an increase in l:b ratio
and selectivity for linear aldehyde, as was already explained
by Lazaroni and co-workers for the deuterioformylation of
styrene.44,45
Surprisingly, no electronic effect of the ligands on the
selectivity for linear aldehyde is observed.42 The selectivities
for linear aldehyde are all between 92 and 93%. The l:b ratio
and the isomerization to 2-octene, however, both increase with
decreasing phosphine basicity. The increase in l:b ratio with
decreasing phosphine basicity can be attributed completely to
an increased tendency of the branched alkyl rhodium species
to form 2-octene instead of branched aldehyde (Scheme 3). The
increasing electrophilicity of the rhodium center leads to a higher
reactivity of the rhodium alkyl species toward CO dissociation
and â-hydrogen elimination.26 The total amount of all other
products (branched aldehyde and 2-octene) is 7-8% for all
ligands.
The constant selectivity for linear aldehyde in the hydro-
formylation of 1-octene implies that for the basic ligands the
l:b ratio reflects the regioselectivity of the formation of the
rhodium alkyl species. For the less basic ligands the increase
in l:b ratio results from the different behavior of branched and
linear rhodium alkyls toward â-hydrogen elimination, as was
already reported by Lazaroni and co-workers for the deuterio-
formylation of 1-hexene.43 The linear alkyl is mainly converted
to linear aldehyde, while the branched alkyl partially generates
2-octene. Since 2-octene is far less reactive in the hydroformy-
lation, its formation is essentially irreversible when 1-octene is
still present. We conclude that for our catalytic system the ratio
of linear to branched rhodium alkyl species formed is determined
not by phosphine basicity but by steric constraints only. Since
the natural bite angles for all ligands are virtually the same,
this steric parameter seems to explain best the observed constant
selectivity for linear aldehyde in this series of ligands.
Hydroformylation of Styrene. The hydroformylation of
styrene was carried out at 120 °C and 10 bar of CO/H2 (1:1)
using a 0.50 mM solution of rhodium diphosphine catalyst
prepared from Rh(CO)2(dpm) and 10 equiv of ligand. Styrene
is a substrate having a distinct preference for the formation of
the branched aldehyde due to the stability of the benzylic
rhodium species, induced by the formation of a stable η3
complex.3 The formation of the linear aldehyde can be enhanced
by using high temperature and low pressure.10,44 The production
of 1-phenylpropionaldehyde and 2-phenylpropionaldehyde was
monitored by gas chromatography. The results of the experi-
ments with the ligands 1-7 are shown in Table 8. Turnover
frequencies were determined at 20% conversion.
Discussion
Our results are in contradiction with the earlier studies of
Moser and co-workers and Unruh and Christenson.25,26 This can
be explained by the fact that in their ligand systems a strict
separation of steric and electronic effects may not have been
obtained. In the series of substituted thixantphos ligands steric
differences are minimal and only electronic effects are displayed.
In the present study we find that there is no influence of
phosphine basicity on linear aldehyde selectivity.
Casey and co-workers correlated the regioselectivity of the
hydroformylation both with the natural bite angle and the
chelation mode of diphosphines in the five-coordinate hydride
complexes.17,18 No plausible explanation, however, has yet been
found for the observed trends.27 From our results it can be
concluded that the coordination mode in the five-coordinated
hydrido complexes by itself is not crucial in the determination
of the regioselectivity of the reaction. Regioselectivity cannot
be correlated with the (predominant) chelation mode of the
ligands 1-7 in the (diphosphine)Rh(CO)2H complexes, since
the ratio of ee and ea isomers varies from almost 1 for ligand
1 to more than 9 for ligand 7, while the selectivity for linear
aldehyde remains the same for all ligands. Instead, the natural
bite angle (obtained from MM2) seems to display a higher
correlation with the regioselectivity! Naturally, the observed
structures of the (diphosphine)Rh(CO)2H complexes need not
be identical to the structures of the (diphosphine)Rh(CO)H-
(alkene) complexes, which escape direct observation.
In the hydroformylation of styrene a regular increase in rate
with decreasing phosphine basicity is observed. The increase
in activity is accompanied with an increase in l:b ratio and
(43) Lazzaroni, R.; Uccello-Barretta, G.; Benetti, M. Organometallics
1989, 8, 2323-2327.
(44) Lazzaroni, R.; Raffaelli, A.; Settambolo, R.; Bertozzi, S.; Vitulli,
G. J. Mol. Catal. 1989, 50, 1-9.
(42) Selectivity for linear aldehyde is the percentage of 1-nonanal of all
products including 2-octene.
(45) Lazzaroni, R.; Settambolo, R.; Raffaelli, A.; Pucci, S.; Vitulli, G.
J. Organomet. Chem. 1988, 339, 357-365.

Rhodium Diphosphine Catalyzed Hydroformylation
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11623
Scheme 4
that formation of linear C is favored over that of branched C.
Modeling studies are currently in progress to substantiate this
further.
The existence of a transient four-coordinate L2Rh(CO)H
species in the hydroformylation was already postulated by
Wilkinson and co-workers.7,16,46 Yoshida and co-workers have
shown that coordinatively unsaturated trans L2Rh(CO)H com-
plexes can be stabilized and isolated by the use of sterically
bulky phosphine ligands (L ) PCy3 or P-i-Pr3).47 The X-ray
crystal structure of (PCy3)2Rh(CO)H was later reported by
Freeman and Young.48 The crystal structure has a square-planar
complex geometry with the two phosphines in a trans relation
and a P-Rh-P angle of 164.1°. Morokuma and co-workers
have calculated the potential and free energy profile of the full
cycle of ethylene hydroformylation catalyzed by the (PH3)2Rh-
(CO)H complex using ab initio molecular orbital theory.37,49
The optimized structure of the intermediate (PH3)Rh(CO)2H
complex was found to be square planar, and the structures of
the transition states between the (PH3)Rh(CO)2H and (PH3)-
Rh(CO)2H(C2H4) complex were found to be square pyramidal.
Herrmann and co-workers have recently calculated the equi-
librium structures of both the square-planar four-coordinate
(PH3)Rh(CO)2H and (PH3)2Rh(CO)H complex using DFT
theory.32 The ideal geometry of the (PH3)2Rh(CO)H complex
was found to have the phosphine groups positioned in a trans
relationship and bent toward the hydride.
Evidence for the assumption that even a thixantphos ligand
with calculated flexibility range for rhodium of 92-124° can
accommodate a trans square-planar geometry can be found in
the crystal structure of the (p-(CH3)2N-thixantphos)Ni(CN)2
complex.50,51 In this distorted square-planar d8 complex
p-(CH3)2N-thixantphos spans two trans positions with a P-Ni-P
angle of 151.5°. In the proposed transition states B (Scheme
4), the thixantphos or BISBI ligands also span trans positions
with P-Rh-P angles around 150-160° and the phosphorus
atoms are bent toward the hydride. This complex geometry
induced by the ligand backbone and the bite angle controls the
orientation of the substituents on the phosphorus atoms, which
on their turn determine the relative stabilities of the transition
states B and/or of the isomers C.
The assumption that four-coordinate (diphosphine)Rh(CO)H
complexes play a key role in the hydroformylation not only
explains the effect of the bite angle on the regioselectivity but
also explains the effect on the activity of the reaction. The
natural bite angle influences the relative stabilities of the
transient four-coordinate complexes and thus the activity of these
intermediates. A diphosphine with a natural bite angle close
to 90° cannot form a trans square-planar complex but will give
an energetically less favorable cis complex.32 A diphosphine
that can accommodate bite angles up to 160°, such as BISBI,17
can lead to the formation of a trans square-planar complex of
lower relative energy (two monodentate phosphines will of
course give the most stable trans complex due to the lack of
bite angle constraints). A higher stability of the transient four-
coordinate (diphosphine)Rh(CO)H intermediate can result in a
The fact that in our system the ee:ea isomer ratio does not
influence regioselectivity suggests that CO dissociation from
both isomeric (diphosphine)Rh(CO)2H complexes gives the
same reactive four-coordinate (diphosphine)Rh(CO)H interme-
diate (A) in the hydroformylation cycle (Scheme 4). In this
context it is important to note that the isomerization reaction
of ee and ea isomers is very fast. We were unable to freeze
the fluxional behavior in the NMR spectrum even at 163 K,
which indicates that the energy barrier for the interconversion
of the two isomers is very low, probably less than 10 kcal mol-1
.
Following CO dissociation from the (diphosphine)Rh(CO)2H
complexes, the regioselectivity in the hydroformylation will be
determined by two consecutive steps of alkene coordination and
hydride migration. Here we suggest that the regioselectivity is
controlled by the alkene attack on the four-coordinate intermedi-
ate A via a square-pyramidal transition state B (Scheme 4).
Increasing the bite angle of the diphosphine in intermediate A
results in increased embracing of the rhodium center by the
ligand and consequently in more steric hindrance for the alkene
entering the coordination sphere. The hindrance is different
for the pro-linear and pro-branched transition states B. The
preference for the formation of pro-linear over pro-branched B
is not only determined by the magnitude of the bite angle in
intermediate A. Although two PPh3 ligands will induce a larger
P-Rh-P angle in intermediate A, selectivities for linear
aldehyde are much higher for diphosphines such as thixantphos
and BISBI, that will probably give bite angles of only (150°
(vide infra). Inspection of the molecular models shows that
the backbone of the diphosphine plays a key role in the
determination of the relative stabilities of linear and branched
isomers of complex C. The backbone constrains the orientation
of the phenyl substituents of the diphosphines in such a way
(46) Yagupsky, G.; Brown, C. K.; Wilkinson, G. J. Chem. Soc. A 1970,
1392-1401.
(47) Yoshida, T.; Okano, T.; Ueda, Y.; Otsuka, S. J. Am. Chem. Soc.
1981, 103, 3411-3422.
(48) Freeman, M. A.; Young, D. A. Inorg. Chem. 1986, 25, 1556-1560.
(49) Koga, N.; Jin, S. Q.; Morokuma, K. J. Am. Chem. Soc. 1988, 110,
3417-3425.
(50) Goertz, W. Ph.D. Thesis, Rheinisch-Westfa¨lischen Technischen
Hochschule Aachen, 1998.
(51) Goertz, W.; Keim, W.; Vogt, D.; Englert, U.; Boele, M. D. K.; van
der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem.
Soc., Dalton Trans. 1998, 2981-2988.

11624 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Van der Veen et al.
sistently in energy calculations as well as in the geometry optimiza-
tions.58 Geometries were fully optimized under the constrains of the
Cs or C3V symmetry group. Second derivatives were evaluated
numerically with a two point formula.
b. MM2 Calculations. The molecular mechanics calculations were
performed using the CAChe WorkSystem version 3.9,59 on an Apple
Power Macintosh 950, equipped with two CAChe CXP coprocessors.
Calculations were carried out similarly to the method described by
Casey and Whiteker,23 using a Rh-P bond length of 2.315 Å.
Minimization’s were done via the block-diagonal Newton-Raphson
method, allowing the structures to converge fully with a termination
criterion of a rms factor of 0.0001 kcal mol-1Å-1 or less.
higher concentration of the intermediate and/or in a lower
activation energy for its formation. If the decrease in activation
energy for CO dissociation is accompanied by only a small
increase in activation energy for alkene coordination to the four-
coordinate complex, then both the higher reactant concentration
and the lower activation energy can explain the observed
increase in hydroformylation activity with increasing natural
bite angle.10,17
Conclusion
A pronounced effect of phosphine basicity on the chelation
mode of the ligands in the (diphosphine)Rh(CO)2H complexes
was observed. The ee:ea isomer ratio, observed in the IR and
1H NMR spectra, showed a regular increase with decreasing
phosphine basicity. The spectroscopic results were supported
by theoretical calculations using DFT. The electronic ligand
effect on catalysis is reflected in both hydroformylation rate
and rate of â-hydrogen elimination. Most remarkably, in the
hydroformylation of 1-octene, the overall selectivity for linear
aldehyde is not influenced by phosphine basicity. Apparently,
the chelation mode in the (diphosphine)Rh(CO)2H complexes
per se does not determine the regiochemistry in the hydro-
formylation reaction and the selectivity for linear aldehyde
formation correlates better to the calculated natural bite angle.
Up to now the role of the four-coordinate L2Rh(CO)H
complexes in the hydroformylation has been underestimated.
In finding explanations for ligand effects on catalytic perfor-
mance, attention is focused mainly on the step that converts
the more stable five-coordinate L2Rh(CO)H(alkene) complexes
into four-coordinate L2Rh(CO)(alkyl) complexes. However, as
stated by Casey and co-workers, “The regioselectiVity of
hydroformylation is goVerned by a complex web of electronic
and steric effects that haVe so far defied unraVeling.” 27 Our
results clearly indicate that an explanation for the effect of the
bite angle on the regioselectivity and activity in the rhodium
diphosphine catalyzed hydroformylation is not found in the
structure of the five-coordinate (diphosphine)Rh(CO)2H com-
plexes. The role of the four-coordinate (diphosphine)Rh(CO)H
complexes in the reaction can lead to an alternative plausible
explanation of the bite angle effect.
General Procedure. All reactions were carried out using standard
Schlenk techniques under an atmosphere of purified argon or nitrogen.
Toluene and TMEDA were distilled from sodium, THF and diethyl
ether from sodium/benzophenone, and hexanes from sodium/benzophe-
none/triglym. Methanol, ethanol, 1-propanol, and dichloromethane
were distilled from CaH2. Chemicals were purchased from Acros
Chimica and Aldrich Chemical Co. (PPh3)3Rh(CO)H,60 Rh(CO)2-
(dpm),61 and 2,7-dimethylphenoxathiin62 were prepared according to
literature procedures. Silica gel 60 (230-400 mesh) purchased from
Merck was used for column chromatography. Melting points were
determined on a Gallenkamp MFB-595 melting point apparatus in open
capillaries and are uncorrected. NMR spectra were obtained on a
Bruker AMX 300 or a DRX 300 spectrometer. 31P and 13C spectra
were measured H decoupled. TMS was used as a standard for H
and 13C NMR and H3PO4 for 31P NMR. Mass spectroscopy was
measured on a JEOL JMS-SX/SX102A. Elemental analyses were
carried out on an Elementar Vario EL apparatus. Infrared spectra were
recorded on a Nicolet 510 FT-IR spectrophotometer. HP-IR spectra
were measured using a 20-mL homemade stainless steel autoclave
equipped with mechanical stirring and ZnS windows. Hydroformylation
reactions were carried out in a 200-mL homemade stainless steel
autoclave. Syn gas (CO/H2, 1:1, 99.9%) and CO (99.9%) were
purchased from Air Liquide. D2 was purchased from Hoekloos. Gas
chromatographic analyses were run on an Interscience HR GC Mega
2 apparatus (split/splitless injector, J&W Scientific, DB1 30-m column,
film thickness 3.0 mm, carrier gas 70 kPa He, FID detector) equipped
with a Hewlett-Packard Data system (Chrom-Card).
4,5-Bis(dichlorophosphino)-2,7-dimethylphenoxathiin (8).30 At 0
°C, 55.0 mL of n-butyllithium (2.5 M in hexanes, 137 mmol) was added
dropwise to a stirred mixture of 12.5 g of 2,7-dimethylphenoxathiin
(54.8 mmol) and 21.0 mL of TMEDA (137 mmol) in 250 mL of diethyl
ether. The reaction mixture was slowly warmed to room temperature
and stirred for 16 h. Then the dark yellow solution was cooled to 0
°C, and 74.0 mL of zinc chloride (1.85 M in diethyl ether, 137 mmol)
was added dropwise. The reaction mixture decolorized, and a white
precipitate was formed. After being stirred for 2 h at room temperature,
the reaction mixture was cooled to -196 °C using liquid nitrogen. Then
275 mL of phosphorus chloride (3.2 mol) was distilled onto the frozen
reaction mixture. The reaction mixture was warmed to -110 °C, and
mechanical stirring was started as soon as possible. The temperature
was allowed to rise slowly, and at -50 °C, the colorless reaction mixture
became yellow. At room temperature, the reaction mixture was
evaporated in vacuo and the resulting residue was extracted with diethyl
ether. The solvent was removed in vacuo, and the obtained product
was crystallized from hexanes. Yield: 14.3 g of light yellow crystals
(61%). Mp: 124-126 °C. 1H NMR (CDCl3): δ ) 7.60 (bs, 2H, H1,8),
7.08 (bs, 2H, H3,6), 2.36 (s, 6H, CH3). 31P{1H} NMR (CDCl3): δ )
155.4. 13C{1H} NMR (C6D6): δ ) 149.7 (t, J(P,C) ) 28.8 Hz, OC),
1
1
Experimental Section
Computational Details. a. DFT Calculations. All DFT calcula-
tions were performed with the Amsterdam Density Functional (ADF)
program.52-54 The valence molecular orbitals were expanded in a large
basis set of Slater’s type orbitals (STOs), whereas the frozen core
approximation was used to treat the core electrons. The frozen core
orbitals have been taken from atomic calculations in a very large basis
set, and quasirelativistic corrections using the Pauli formalism were
applied to obtain the frozen core shells. For rhodium, the atomic
orbitals up to 3d shell were kept frozen and a triple-ú basis set with
one p polarization function was used. For phosphorus, carbon, oxygen,
and fluor, the triple-ú basis set with one d polarization function was
selected, keeping the 2p for phosphorus and the 1s for C, O, and F
frozen. The basis set used for hydrogen was also a triple-ú with one
p function for polarization. For the energy calculations the local
exchange-correlation potential (LDA) parametrized by Vosko, Wilk,
and Nusair55 was used, in combination with the gradient-corrected
(GGA) Becke’s56 functional for exchange and Perdew’s57 functional
for correlation (BP86). The GGA approach was applied self-con-
(56) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(57) Perdew, J. P. Phys. ReV. B 1986, 33, 8822-8824.
(58) Fan, L.; Versluis, L.; Ziegler, T.; Baerends, E. J.; Ravenek, W. Int.
J. Quantum Chem., Quantum Chem. Symp. 1988, S22, 173.
(59) CAChe Scientific Inc., 18700 N.W. Walker Road, Building 92-01,
Beaverton, OR 97006.
(52) Guerra, C. F.; Visser, O.; Snijders, J. G.; te Velde, G.; Baerends,
E. J. In Methods and Techniques in Computational Chemistry, METECC-
95; Clementi, E., Corongiu, G., Eds.; Cagliari: Stef, 1995; p 307.
Baerends, E. J.; Ros, P. Chem. Phys. 1973, 2, 41.
(54) te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.
(55) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(60) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg.
Synth. 1974, 15, 59-60.
(61) Coolen, H. K. A. C.; van Leeuwen, P. W. N. M.; Nolte, R. J. M. J.
Org. Chem. 1996, 61, 4739-4747.
(62) Suter, C. M.; McKenzie, J. P.; Maxwell, C. E. J. Am. Chem. Soc.
1936, 58, 717-720.

Rhodium Diphosphine Catalyzed Hydroformylation
J. Am. Chem. Soc., Vol. 120, No. 45, 1998 11625
135.43, 130.9, 129.0, 128.6 (t, J(P,C) ) 64.9 Hz, C4,5), 119.5. IR (KBr,
cm-1): 2923 (w), 2416 (w), 1436 (m), 1428 (m), 1141 (s), 1104 (s),
987 (s).
) 21.7 (d, 1J(Rh,P) ) 124 Hz). HP-IR (2-MeTHF, cm-1): 2034
(RhCO), 1990 (RhCO), 1966 (RhCO), 1942 (RhCO), 1595 (CO-dpm).
(p-CH3-Thixantphos)Rh(CO)H(PPh3) (11a). This compound was
prepared similarly to 9a. Orange crystals were obtained from toluene/
ethanol. 1H NMR (C6D6): δ ) 7.73 (m, 10H, PPh3), 7.46 (bm, 4H,
PPh3), 7.21 (s, 2H, H1,8), 7.08 (m, 1H, PPh3), 6.98 (m, 8H, PCCH),
6.81 (d, 3J(H,H) ) 8.2 Hz, 4H, CH3CCH), 6.75 (d, 3J(H,H) ) 7.9 Hz,
4H, CH3CCH), 6.69 (s, 2H, H3,6), 2.12 (s, 12H, CH3 tolyl), 2.04 (s,
12H, CH3 tolyl), 1.78 (s, 6H, CH3), -9.23 (dt, 2J(P,H) ) 20 Hz, 2J(P,H)
) 12 Hz, RhH). 31P{1H} NMR (C6D6): δ ) 43.6 (dt, 1J(Rh,P) ) 167
Hz, 2J(P,P) ) 123 Hz; PPh3), 26.8 (dd, 1J(Rh,P) ) 148 Hz, 2J(P,P) )
124 Hz). IR (KBr, cm-1): 1999 (s, HRhCO), 1920 (m, HRhCO).
(p-CH3-Thixantphos)Rh(CO)2H (11b). This compound was pre-
pared similarly to 9b. 1H HP-NMR (C6D6): δ ) 7.51 (bs, 8H, PCCH),
Thixantphos Ligands 1-7. In a typical experiment, a solution of
3.52 g of 4-bromo-N,N-dimethylaniline (17.6 mmol) in 20 mL of THF
was added dropwise to a stirred mixture of 0.64 g of magnesium
turnings (26.4 mmol), activated with 0.05 mL of 1,2-dibromoethane
(0.6 mmol), in 5 mL of THF at room temperature. The reaction mixture
was stirred for three h. The reaction mixture was filtered and added
to a stirred solution of 1.50 g of 8 (3.52 mmol) in 15 mL of THF at 0
°C. The reaction mixture was slowly warmed to room temperature
and stirred for another 3 h. The beige reaction mixture was hydrolyzed
with 5 mL of water, and the solvent was removed in vacuo. The
resulting brown residue was dissolved in CH2Cl2 and washed with dilute
HCl. The organic layer was separated, and the aqueous layer was
extracted CH2Cl2. The combined organic layers were dried with
MgSO4, and the solvent removed in vacuo. The resulting white powder
was washed with hexanes and recrystallized from toluene. Yield: 1.43
g of pure p-(CH3)2N-thixantphos (1) as a white microcrystalline powder
(53%). All thixantphos ligands 1-7 were fully characterized. Detailed
descriptions of the syntheses and characterizations are included in the
Supporting Information.
3
6.68 (d, J(H,H) ) 8.1 Hz, CH3CCH), 6.56 (bs, 2H, H1,8), 6.42 (d,
2J(P,H) ) 5.1 Hz, H3,6), 1.93 (s, 12H, CH3 tolyl), 1.60 (s, 6H, CH3),
1
2
-8.52 (dt, J(Rh,H) ) 7.2 Hz, J(P,H) ) 17.6 Hz, 1H, RhH). 31P-
{1H} HP-NMR (C6D6): δ ) 22.7 (d, J(Rh,P) ) 126 Hz). HP-IR
1
(2-MeTHF, cm-1): 2035 (RhCO), 1992 (RhCO), 1969 (RhCO), 1943
(RhCO), 1595 (CO-dpm).
(Thixantphos)Rh(CO)H(PPh3) (12a). This compound was pre-
pared similarly to 9a and was already described by Kranenburg and
co-workers.10 Orange yellow crystals suitable for X-ray structure
determination were obtained from toluene/ethanol.
(p-(CH3)2N-Thixantphos)Rh(CO)H(PPh3) (9a). A solution of
(PPh3)3Rh(CO)H (92 mg, 0.10 mmol) and 1 (85 mg, 0.11 mmol) in 3
mL of toluene was stirred for 0.5 h at 70 °C. The solvent was removed
X-ray Crystal Structure Determination of 12a. 12a crystallized
in the triclinic space group P1, a ) 10.9776(6) Å, b ) 11.0845(9)
Å, c ) 11.730(3) Å, R ) 113.77(1)°, â ) 94.81(1)°, γ ) 108.435(5)°,
V ) 1201.3(4) Å3, and Z ) 1. The data collection was carried out
at room temperature. The structure was solved by direct methods.
The hydrogen atoms were calculated. The structure was refined to
R ) 0.030 and Rw ) 0.038, for 4931 observed reflections. Crystal
data and collection parameters, atomic coordinates, bond lengths,
bond angles, and thermal parameters are included in the Supporting
Information.
1
in vacuo ,and the resulting yellow solid was washed with ethanol. H
NMR (C6D6): δ ) 7.77 (m, 10H, PPh3), 7.50 (m, 4H, PPh3), 7.12 (m,
1H, PPh3), 6.99 (m, 10H, PCCH, H1,8), 6.81 (bs, 2H, H3,6), 6.29 (d,
3J(H,H) ) 8.6 Hz, 4H, (CH3)2NCCH), 6.24 (d, 3J(H,H) ) 8.7 Hz, 4H,
(CH3)2NCCH), 2.51 (s, 12H, (CH3)2N), 2.43 (s, 12H, (CH3)2N), 1.78
(s, 6H, CH3), -9.09 (dt, 2J(P,H) ) 21 Hz, 2J(P,H) ) 11 Hz, 1H, RhH).
1
2
31P{1H} NMR (C6D6): δ ) 43.4 (dt, J(Rh,P) ) 167 Hz, J(P,P) )
1
2
125 Hz; PPh3), 24.7 (dd, J(Rh,P) ) 147 Hz, J(P,P) ) 125 Hz). IR
(KBr, cm-1): 1982 (s, HRhCO), 1909 (m, HRhCO).
(p-(CH3)2N-Thixantphos)Rh(CO)2H (9b). A solution of Rh(CO)2-
(acac) (13 mg, 50 µmol) and 1 (39 mg, 50 µmol) in 1.0 mL of C6D6
was pressurized with 20 bar of CO/H2 (1:1) and left overnight at 80
°C. 1H HP-NMR (C6D6): δ ) 7.61 (m, 8H, PCCH), 6.75 (s, 2H, H1,8),
(Thixantphos)Rh(CO)2H (12b). This compound was prepared
similarly to 9b and was already described by Kranenburg and
co-workers.10
(Thixantphos)Rh(CO)2D (12-Db). This compound was prepared
in situ from 12b and D2. HP-IR (2-MeTHF, cm-1): 2019 (RhCO),
1994 (RhCO), 1958 (RhCO), 1947 (RhCO), 1587 (CO-dpm).
(p-F-Thixantphos)Rh(CO)H(PPh3) (13a). This compound was
prepared similarly to 9a. 1H NMR (C6D6): δ ) 7.54 (m, 10H, PPh3),
7.28 (m, 4H, PPh3), 7.09 (m, 1H, PPh3), 6.96 (m, 8H, PCCH), 6.82
(bs, 2H, H1,8), 6.61 (m, 8H, FCCH), 6.40 (bs, 2H, H3,6), -9.49 (dt,
2J(P,H) ) 18 Hz, 2J(P,H) ) 11 Hz, 1H, RhH). 31P{1H} NMR (C6D6):
2
3
6.63 (d, J(P,H) ) 6.0 Hz, H3,6), 6.22 (d, J(H,H) ) 8.1 Hz, (CH3)2-
NCCH), 2.39 (s, 24H, (CH3)2N), 1.65 (s, 3H, CH3), -8.25 (dt, 1J(Rh,H)
2
) 8.7 Hz, J(P,H) ) 27.9 Hz, 1H, RhH). 31P{1H} HP-NMR (C6D6):
1
δ ) 19.8 (d, J(Rh,P) ) 121 Hz). HP-IR (2-MeTHF, cm-1): 2027
(RhCO), 1983 (RhCO), 1960 (RhCO), 1935 (RhCO), 1596 (CO-dpm).
(p-CH3O-Thixantphos)Rh(CO)H(PPh3) (10a). This compound
was prepared similarly to 9a. Orange yellow crystals suitable for X-ray
structure determination were obtained from toluene/ethanol. 1H NMR
(C6D6): δ ) 6.68 (m, 10H, PPh3), 7.41 (m, 4H, PPh3), 7.04 (m, 1H,
1
2
δ ) 42.7 (dt, J(Rh,P) ) 167 Hz, J(P,P) ) 124 Hz, PPh3), 26.5 (dd,
1J(Rh,P) ) 149 Hz, J(P,P) ) 124 Hz). IR (KBr, cm-1): 2002 (s,
2
4
PPh3), 6.93 (m, 8H, PCCH), 6.80 (d, J(H,H) ) 1.6 Hz, 2H, H1,8),
HRhCO), 1921 (m, HRhCO). Anal. Calcd for C57H42F4O2P3RhS: C,
64.41; H, 3.99; S, 3.02. Found: C, 64.88; H, 4.28; S, 2.68.
(p-F-Thixantphos)Rh(CO)2H (13b). This compound was prepared
similarly to 9b. 1H HP-NMR (C6D6): δ ) 7.19 (bs, 8H, PCCH), 6.70-
6.36 (ar), 1.58 (s, 6H, CH3), -8.97 (dt, 1J(Rh,H) ) 6.3 Hz, 2J(P,H) )
11.0 Hz, 1H, RhH). 31P{1H} HP-NMR (C6D6): δ ) 23.3 (d, 1J(Rh,P)
) 130 Hz). HP-IR (2-MeTHF, cm-1): 2041 (RhCO), 1997 (RhCO),
1975 (RhCO), 1950 (RhCO), 1588 (CO-dpm).
3
6.61 (bs, 2H, H3,6), 6.51 (d, J(H,H) ) 8.6 Hz, 4H, CH3OCCH), 6.47
3
(d, J(H,H) ) 8.7 Hz, 4H, CH3OCCH), 3.32 (s, 6H, CH3O), 3.25 (s,
6H, CH3O), 1.77 (s, 6H, CH3), -9.33 (dt, 2J(P,H) ) 20 Hz, 2J(P,H) )
1
11 Hz, 1H, RhH). 31P{1H} NMR (C6D6): δ ) 43.4 (dt, J(Rh,P) )
163 Hz, 2J(P,P) ) 124 Hz; PPh3), 25.7 (dd, 1J(Rh,P) ) 148 Hz, 2J(P,P)
) 124 Hz). IR (KBr, cm-1): 1989 (s, HRhCO), 1916 (m, HRhCO).
Anal. Calcd for C61H54O6P3RhS: C, 65.95; H, 4.90; S, 2.89. Found:
C, 66.14; H, 5.03; S, 3.23.
(p-Cl-Thixantphos)Rh(CO)H(PPh3) (14a). This compound was
3
prepared similarly to 9a. 1H NMR (C6D6): δ ) 7.47 (dd, J(H,H) )
X-ray Crystal Structure Determination of 10a. 10a crystallized
in the triclinic space group P1, a ) 10.7820(7) Å, b ) 11.707(3)
Å, c ) 23.322(2) Å, R ) 86.737(9)°, â ) 78.718(6)°, γ ) 68.24(1)°,
V ) 2682.1(9) Å3, and Z ) 2. The data collection was carried out
at room temperature. The structure was solved by direct methods.
The hydrogen atoms were calculated. The structure was refined to R
) 0.067 and Rw ) 0.085, for 9235 observed reflections. Crystal
data and collection parameters, atomic coordinates, bond lengths,
bond angles, and thermal parameters are included in the Supporting
Information.
3
11 Hz, J(H,H) ) 8.3 Hz, 6H, PPh3), 7.36 (m, 4H, PPh3), 7.17 (bs,
4H, PPh3), 7.02 (m, 1H, PPh3), 6.87 (m, 16H), 6.74 (d, 4J(H,H) ) 1.2
Hz, 2H, H1,8), 6.36 (bs, 2H, H3,6), 1.69 (s, 6H, CH3), -9.60 (q, 2J(P,H)
) 14 Hz, 1H, RhH). 31P{1H} NMR (C6D6): δ ) 42.8 (dt, 1J(Rh,P) )
165 Hz, 2J(P,P) ) 123 Hz; PPh3), 26.6 (dd, 1J(Rh,P) ) 149 Hz, 2J(P,P)
) 123 Hz). IR (KBr, cm-1): 2006 (s, HRhCO), 1928 (m, HRhCO).
Anal. Calcd for C57H42Cl4O2P3RhS: C, 60.66; H, 3.75; S, 2.84.
Found: C, 61.14; H, 4.13; S, 2.78.
(p-Cl-Thixantphos)Rh(CO)2H (14b). This compound was prepared
(p-CH3O-Thixantphos)Rh(CO)2H (10b). This compound was
prepared similarly to 9b. 1H HP-NMR (C6D6): δ ) 7.46 (bs, 8H,
PCCH), 6.73 (s, 2H, H1,8), 6.43 (bs, 8H, CH3OCCH), 6.30 (bs, 2H,
similarly to 9b. 1H HP-NMR (C6D6): δ ) 7.25-6.95 (ar), 6.83-6.68
1
2
(ar), 1.59 (s, 6H, CH3), -9.04 (dt, J(Rh,H) ) 6.0 Hz, J(P,H) ) 8.4
Hz, 1H, RhH). 31P{1H} HP-NMR (C6D6): δ ) 23.7 (d, J(Rh,P) )
1
1
H3,6), 3.20 (s, 12H, CH3O), 1.64 (s, 6H, CH3), -8.25 (dt, J(Rh,H) )
132 Hz). HP-IR (2-MeTHF, cm-1): 2042 (RhCO), 1999 (RhCO), 1977
(RhCO), 1952 (RhCO), 1574 (CO-dpm).
2
7.5 Hz, J(P,H) ) 21.6 Hz, 1H, RhH). 31P{1H} HP-NMR (C6D6): δ

11626 J. Am. Chem. Soc., Vol. 120, No. 45, 1998
Van der Veen et al.
(p-CF3-Thixantphos)Rh(CO)H(PPh3) (15a). This compound was
prepared similarly to 9a. Orange crystals were obtained from toluene/
ethanol. 1H NMR (C6D6): δ ) 7.43 (m, 10H, PPh3), 7.19 (m, 4H,
PPh3), 7.02 (t, J ) 7.2 Hz, 8H, PCCH), 6.94 (m, 1H, PPh3), 6.85 (m,
HP-FT-IR Experiments. In a typical experiment the HP-IR
autoclave was filled with 2-5 equiv of ligand, 5 mg of Rh(CO)2(dpm),
and 15 mL of 2-MeTHF. The autoclave was purged three times with
15 bar of CO/H2 (1:1), pressurized to approximately 18 bar, and heated
to 80 °C. Catalyst formation was followed in time by FT-IR and was
usually complete within 1 h.63
HP-NMR Experiments. In a typical experiment 0.4 mL of a
solution of 50 µmol of Rh(CO)2(acac) and 50 µmol of ligand in 1.0
mL of C6D6 was transferred into a 0.5-cm sapphire NMR tube. The
tube was pressurized with 20 bar of CO/H2 (1:1) and left overnight at
80 °C. For VT experiments half the amounts of rhodium and ligand
were used in 0.5 mL of THF-d8.
4
8H, CF3CCH), 6.75 (d, J(H,H) ) 1.6 Hz, 2H, H1,8), 6.22 (bs, 2H,
2
H3,6), 1.66 (s, 6H, CH3), -9.43 (q, J(P,H) ) 14 Hz, 1H, RhH). 31P-
1
2
{1H} NMR (C6D6): δ ) 41.8 (dt, J(Rh,P) ) 164 Hz, J(P,P) ) 122
Hz, PPh3), 27.8 (dd, 1J(Rh,P) ) 149 Hz, 2J(P,P) ) 122 Hz). IR (KBr,
cm-1): 2009 (s, HRhCO), 1931 (m, HRhCO). Anal. Calcd for
C61H42F12O2P3RhS: C, 58.02; H, 3.35; S, 2.54. Found: C, 57.92; H,
3.47; S, 2.78.
(p-CF3-Thixantphos)Rh(CO)2H (15b). This compound was pre-
pared similarly to 9b. 1H HP-NMR (C6D6): δ ) 7.23 (bs, 8H, PCCH),
3
2
Acknowledgment. Financial support from SON/STW is
gratefully acknowledged, and we thank Dr. H. T. Teunissen
and Prof. Dr. F. Bickelhaupt for providing the experimental
procedure for the synthesis of compound 8. DFT calculations
were carried out on workstations purchased from funds provided
by the Spanish Direccio´n General de Investigacio´n Cient´ıfica
y Te´cnica (DGICYT) under Project PB95-0639-C02-02 and
by the CIRIT of the Generalitat de Catalunya under Project
SGR97-17.
7.00 (d, J(H,H) ) 8.1 Hz, 8H, CF3CCH), 6.85 (d, J(P,H) ) 6.0 Hz,
2H, H3,6), 6.71 (s, 2H, H1,8), 1.59 (s, 6H, CH3), -9.07 (dt, 1J(Rh,H) )
4.5 Hz, 2J(P,H) ) 3.6 Hz, 1H, RhH). 31P{1H} HP-NMR (C6D6): δ )
24.7 (d, 1J(Rh,P) ) 134 Hz). HP-IR (2-MeTHF, cm-1): 2046 (RhCO),
2004 (RhCO), 1982 (RhCO), 1957 (RhCO), 1607 (CO-dpm).
Hydroformylation Experiments. Hydroformylation reactions were
carried out in an autoclave, equipped with glass inner beaker, a substrate
inlet vessel, a liquid sampling valve, and a magnetic stirring rod. The
temperature was controlled by an electronic heating mantle. In a typical
experiment, the desired amount of ligand was placed in the autoclave
and the system was evacuated and heated to 50 °C. After 0.5 h, the
autoclave was filled with CO/H2 (1:1) and a solution of Rh(CO)2(dpm)
(5 or 10 µmol) in 8.5 mL of toluene. The autoclave was pressurized
to the appropriate pressure (6 or 16 bar), heated to reaction temperature,
and stirred for 0.5 h (120 °C) or 1.5 h (80 °C) to form the active catalyst.
Then 1.0 mL of substrate (filtered over neutral activated alumina to
remove peroxide impurities) and 0.5 mL of the internal standard
n-decane were placed in the substrate vessel, purged three times with
10 bar CO/H2 (1:1), and pressed into the autoclave with 10 or 20 bar
CO/H2 (1:1). At the 20% conversion level the reaction was stopped
by adding 0.25 mL of tri-n-butyl phosphite and cooling on ice. Samples
of the reaction mixture were analyzed by temperature-controlled gas
chromatography.
Supporting Information Available: Detailed description
of the syntheses and characterizations of thixantphos ligands
1-7, HP-IR spectrum of 12-Db (carbonyl region), and tables
of crystal data and collection parameters, atomic coordinates,
bond lengths, bond angles, thermal parameters, and H-atom
coordinates for 10a and 12a (39 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
JA981969E
(63) Castellanos-Pa´ez, A.; Castillo´n, S.; Claver, C.; van Leeuwen, P. W.
N. M.; de Lange, W. G. J. Organometallics 1998, 17, 2543-2552.