Origin of the Bite Angle Effect on Rhodium Diphosphine Catalyzed Hydroformylation

Article abstract of DOI:10.1021/om990734o

The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was investigated in detail. A series of xantphos-type ligands with natural bite angles ranging from 102° to 121° was synthesized, and the effect of the natural bite angle on coordination chemistry and catalytic performance was studied. X-ray crystal structure determinations of the complexes (nixantphos)Rh(CO)H(PPh₃) and (benzoxantphos)Rh(CO)H(PPh₃) were obtained. In contrast to the natural bite angle calculations, approximately the same diphosphine bite angles were observed in both crystal structures. The solution structures of the (diphosphine)Rh(CO)H(PPh₃) and (diphosphine)Rh(CO)₂H complexes were studied by IR and NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh(CO)₂H complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium compositions could not be correlated with the calculated natural bite angles. In the hydroformylation of 1-octene an increase in selectivity for linear aldehyde formation and activity was observed with increasing natural bite angle. For styrene the same trend in selectivity for the linear aldehyde was found. For the first time CO dissociation rates of (diphosphine)Rh(CO)₂H complexes were determined using ¹³CO labeling in rapid-scan high-pressure (HP) IR experiments. The observed CO dissociation rates for three complexes proved to be orders of magnitude higher than the hydroformylation rates and, contrary to the hydroformylation activity, did not reveal a correlation with the natural bite angle. These findings indicate that the bite angle effect on hydroformylation activity is dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)H intermediates with CO and alkene. The bite angle affects the selectivity in the steps of alkene coordination and hydride migration; the structure of the saturated (diphosphine)Rh(CO)₂H complex has only some circumstantial relevance to the selectivity.

Full text of DOI:10.1021/om990734o

872
Organometallics 2000, 19, 872-883
Or igin of th e Bite An gle Effect on Rh od iu m Dip h osp h in e
Ca ta lyzed Hyd r ofor m yla tion
Lars A. van der Veen,† Peter H. Keeven,† Gerard C. Schoemaker,†
J oost N. H. Reek,† Paul C. J . Kamer,† Piet W. N. M. van Leeuwen,*,†
Martin Lutz,‡ and Anthony L. Spek‡
Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, and Bijvoet Center for Biomolecular Research, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
Received September 20, 1999
The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was
investigated in detail. A series of xantphos-type ligands with natural bite angles ranging
from 102° to 121° was synthesized, and the effect of the natural bite angle on coordination
chemistry and catalytic performance was studied. X-ray crystal structure determinations of
the complexes (nixantphos)Rh(CO)H(PPh3) and (benzoxantphos)Rh(CO)H(PPh3) were ob-
tained. In contrast to the natural bite angle calculations, approximately the same diphosphine
bite angles were observed in both crystal structures. The solution structures of the
(diphosphine)Rh(CO)H(PPh3) and (diphosphine)Rh(CO)2H complexes were studied by IR and
NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh(CO)2H
complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial-apical (ea )
isomers. The equilibrium compositions could not be correlated with the calculated natural
bite angles. In the hydroformylation of 1-octene an increase in selectivity for linear aldehyde
formation and activity was observed with increasing natural bite angle. For styrene the
same trend in selectivity for the linear aldehyde was found. For the first time CO dissociation
rates of (diphosphine)Rh(CO)2H complexes were determined using 13CO labeling in rapid-
scan high-pressure (HP) IR experiments. The observed CO dissociation rates for three
complexes proved to be orders of magnitude higher than the hydroformylation rates and,
contrary to the hydroformylation activity, did not reveal a correlation with the natural bite
angle. These findings indicate that the bite angle effect on hydroformylation activity is
dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)H
intermediates with CO and alkene. The bite angle affects the selectivity in the steps of alkene
coordination and hydride migration; the structure of the saturated (diphosphine)Rh(CO)2H
complex has only some circumstantial relevance to the selectivity.
In tr od u ction
modification. High selectivities in the hydroformylation
of terminal alkenes have been reported for both diphos-
phine- and diphosphite-modified catalysts.4-9 Function-
alized alkenes have been hydroformylated with high
selectivity by Cuny and Buchwald using rhodium diphos-
phite systems.10 Stanley has developed a highly active
and selective bimetallic catalyst based on tetradentate
phosphine ligands.11 Novel ligands for the selective
linear hydroformylation of internal alkenes, which is of
Rhodium-catalyzed hydroformylation is one of the
most prominent applications of homogeneous catalysis
in industry.1-3 Since stereoelectronic properties of ligands,
like in most catalytic systems, have a pronounced
influence on the course of the rhodium-catalyzed hy-
droformylation, extensive research has been devoted to
fine-tuning of the catalytic performance by ligand
† Department of Inorganic Chemistry and Homogeneous Catalysis,
University of Amsterdam.
(5) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J . A., J r.; Powell, D. R. J . Am. Chem. Soc. 1992, 114, 5535-
5543.
‡ Department of Crystal & Structural Chemistry, Utrecht Univer-
sity.
(6) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J .; van
Leeuwen, P. W. N. M. Organometallics 1995, 14, 3081-3089.
(7) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. (to Union Carbide)
EP 213,639,1987 [Chem. Abstr. 1987, 107, 7392r].
(8) 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.
(1) Parshall, G. W. Homogeneous Catalysis: The Applications and
Chemistry of Catalysis by Soluble Transition Metal Complexes; Wiley:
New York, 1980.
(2) Tolman, C. A.; Faller, J . W. In Homogeneous Catalysis with Metal
Phosphine Complexes; Pignolet, L. H., Ed.; Plenum: New York, 1983;
pp 81-109.
(3) Frohning, C. D.; Kohlpaintner, C. W. In Applied Homogeneous
Catalysis with Organometallic Compounds: A Comprehensive Hand-
book in Two Volumes; Cornils, B., Herrmann, W. A., Eds.; VCH:
Weinheim, 1996; Vol. 1, pp 27-104.
(4) 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
[Chem. Abstr. 1988, 108, 7890].
(9) Buisman, G. J . H.; van der Veen, L. A.; Klootwijk, A.; de lange,
W. G. J .; Kamer, P. C. J .; van Leeuwen, P. W. N. M.; Vogt, D.
Organometallics 1997, 16, 2929-2939.
(10) Cuny, G. D.; Buchwald, S. L. J . Am. Chem. Soc. 1993, 115,
2066-2068.
(11) Broussard, M. E.; J uma, B.; Train, S. G.; Peng, W.-J .; Laneman,
S. A.; Stanley, G. G. Science 1993, 260, 1784-1788.
10.1021/om990734o CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/04/2000

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 873
Sch em e 1
great interest in both industry and synthetic organic
chemistry, were developed recently.12,13 Systematic
studies of the influence of ligand structure on catalytic
performance in the hydroformylation reaction are rare,
however, and despite the development of a wide variety
of ligands, consistent structure-activity relationships
are still lacking.14
ligands have demonstrated that similar dynamic equi-
libria between ee and ea complex isomers also exist for
diphosphine ligands.14,20 The ee:ea isomer ratio of the
complexes proved to be strongly dependent on phos-
phine basicity; the ee:ea ratio increases with decreasing
phosphine basicity.
The generally accepted mechanism for rhodium-
catalyzed hydroformylation as proposed by Wilkinson
in 1968 is shown in Scheme 1.15 The active catalyst is a
trigonal bipyramidal hydridorhodium complex, which
usually contains two phosphorus ligands. According to
this mechanism, the selectivity is determined in the step
that converts a five-coordinate L2Rh(CO)H(alkene) com-
plex into either a linear or branched L2Rh(CO)(alkyl).
For the linear rhodium alkyl species this step is virtu-
ally irreversible at moderate temperatures and suf-
ficiently high pressures of CO.16,17 The structure of the
alkene complex is therefore thought to play a crucial
role in controlling regioselectivity.5 Contrary to the L2-
Rh(CO)2H complex, the L2Rh(CO)H(alkene) complex has
never been observed directly. Brown and Kent have
shown that the PPh3-modified L2Rh(CO)2H complex
consists of two rapidly equilibrating isomeric structures
in which the phosphine ligands coordinate in a diequa-
torial (ee) and an equatorial-apical (ea ) fashion.18,19
Previous studies on electronically modified diphosphine
ee
ea
As a result of their extensive use in catalysis, the
influence of phosphine structure on catalytic perfor-
mance has been studied in great detail.21 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.22,23
Casey and Whiteker developed the concept of the
natural bite angle as an additional characteristic of
diphosphine ligands based on molecular mechanics
calculations.24 The pronounced influence of the natural
bite angle of diphosphine ligands on activity and selec-
tivity has been shown for several catalytic
reactions.5,6,25-27
How the bite angle affects the activity and selectivity
in the rhodium diphosphine catalyzed hydroformylation
(12) Burke, P. M.; Garner, J . M.; Tam, W.; Kreutzer, K. A.;
Teunissen, A. J . J . M. (to DSM/Du Pont de Nemours) WO 97/33854,
1997 [Chem. Abstr. 1997, 127, 294939r].
(18) Brown, J . M.; Kent, A. G. J . Chem. Soc., Perkin Trans. 2 1987,
1597-1607.
(13) van der Veen, L. A.; Kamer, P. C. J .; van Leeuwen, P. W. N.
M. Angew. Chem., Int. Ed. 1999, 38, 336-338.
(14) 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.
(19) Throughout this work, notations ee and ea refer to respectively
the diequatorial and the equatorial-apical chelation modes of the
diphosphines in trigonal bipyramidal complexes.
(20) van der Veen, L. A.; Boele, M. D. K.; Bregman, F. R.; Kamer,
P. C. J .; van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J .; Schenk,
H.; Bo, C. J . Am. Chem. Soc. 1998, 120, 11616-11626.
(21) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpaintner, C. W. J .
Mol. Catal. A: Chem. 1995, 104, 17-85.
(22) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(23) Tolman, C. A. J . Am. Chem. Soc. 1970, 92, 2953-2956.
(24) Casey, C. P.; Whiteker, G. T. Isr. J . Chem. 1990, 30, 299-304.
(15) Evans, D.; Osborn, J . A.; Wilkinson, G. J . Chem. Soc. (A) 1968,
3133-3142.
(16) Lazzaroni, R.; Uccello-Barretta, G.; Benetti, M. Organometallics
1989, 8, 2323-2327.
(17) Casey, C. P.; Petrovich, L. M. J . Am. Chem. Soc. 1995, 117,
6007-6014.

874 Organometallics, Vol. 19, No. 5, 2000
van der Veen et al.
Sch em e 2a
a
(i) n-BuLi/TMEDA/Et2O/0-25 °C; (ii) PPh2Cl/hexanes/0-
25 °C.
lectivity with increasing natural bite angle. No clear
correlation between the chelation mode and the natural
bite angle was found in the (diphosphine)Rh(CO)2H
complexes. To obtain a better understanding of how the
bite angle affects the hydroformylation activity, we have
developed a method to measure the CO dissociation
rates of (diphosphine)Rh(CO)2H complexes. By using
13CO labeling in rapid-scan high-pressure (HP) IR
experiments we were able to determine the rate con-
stants for three (diphosphine)Rh(CO)2H complexes.
F igu r e 1. The xantphos ligands.
Resu lts
Ta ble 1. Ca lcu la ted Na tu r a l Bite An gle a n d
F lexibility Ra n ge for th e Xa n tp h os Liga n d s
Syn th esis. Ligands 1-9 were all synthesized by
selective dilithiation of the appropriate heterocycle,
followed by reaction with chlorodiphenylphosphine
(Scheme 2). 10,11-Dihydrodibenzo[b,f]oxepine,28 10-phen-
ylphenoxaphosphine,29 10-(tert-butyldimethylsilyl)phe-
noxazine,30 and benzo[k,l]xanthene31 were prepared
according to literature procedures. 10-Isopropylidene-
xanthene (10) was synthesized by a McMurry reaction32
starting from xanthone and acetone. In the synthesis
of phosxantphos (2) phenyllithium was used as meta-
lating agent instead of n-butyllithium to annihilate side
product formation by competing nucleophilic substitu-
tion at the ring phosphorus atom by the lithiating
agent.33 Benzylnixantphos (7) was obtained by depro-
tonation of nixantphos (8) with sodium hydride followed
by reaction with benzyl chloride. Typical yields of the
ligands varied from 62% for phosxantphos (2) to 80%
for homoxantphos (1) based on heterocyclic starting
material. Table 1 shows the effect of the heterocyclic
backbone on the calculated natural bite angle and
flexibility range of the ligands. Comparison of the
carbonyl frequencies of the (diphosphine)Rh(CO)2H
complexes (vide infra) reveals that the electronic effect
of the backbone on the electron density on the rhodium
center is marginal, proving that the ligands are elec-
tronically very similar. Therefore, this series of ligands
is ideally suited for studying only the influence of the
natural bite angle in catalysis.
flexibility
ân,a deg range,a deg
ligand
X
homoxantphos (1)
phosxantphos (2)
sixantphos (3)
thixantphos (4)
xantphos (5)
isopropxantphos (6) CdC(CH3)2
benzylnixantphos (7) NBn
nixantphos (8)
CH2-CH2
PPh
102.0
107.9
108.5
109.6
111.4
113.2
114.1
114.2
92-120
96-127
96-130
96-133
97-133
98-139
99-139
99-141
102-146
Si(CH3)2
S
C(CH3)2
NH
benzoxantphos (9)
fused benzene ring 120.6
a
The natural bite angle (ân) and the flexibility range were
calculated as by Casey and Whiteker.24 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 3 kcal mol-1
excess strain energy from the calculated natural bite angle.
is still not understood in detail.14 To investigate the
exact influence of the natural bite angle of diphosphine
ligands on catalytic performance and coordination chem-
istry in rhodium complexes, we have developed a new
family of diphosphine ligands based on xanthene-type
backbones.6 Variation of the substituent at the 9-posi-
tion of the generic backbone structure enables the
construction of a series of diphosphine ligands having
a wide range of natural bite angles (Figure 1, Table 1).
Moreover, the backbone structure in this type of ligand
ensures that mutual variation in electronic properties
and in steric size within the series of ligands is minimal.
We have previously reported on the first members of
this new family of ligands (sixantphos (3), thixantphos
(4), and xantphos (5)), and we have shown that rela-
tively wide natural bite angles give rise to high selec-
tivities and activities in the rhodium-catalyzed hydro-
formylation.6 On the basis of these results and the work
of Casey and co-workers, we hypothesized that there is
a regular increase in selectivity for linear aldehyde
formation with increasing natural bite angle in the
family of xantphos ligands.5
As was pointed out by a reviewer, it should be noted
that xantphos 1 is not completely isostructural to the
(25) Kranenburg, M.; Kamer, P. C. J .; Vogt, D.; van Leeuwen, P.
W. N. M. J . Chem. Soc., Chem. Commun. 1995, 2177-2178.
(26) Kranenburg, M.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.
Eur. J . Inorg. Chem. 1998, 25-27.
(27) Kamer, P. C. J .; Reek, J . N. H.; van Leeuwen, P. W. N. M.
CHEMTECH 1998, 28, 27-33.
(28) Hess, B. A., J r.; Bailey, A. S.; Bartusek, B.; Boekelheide, V. J .
Am. Chem. Soc. 1969, 91, 1665-1672.
(29) Schlosser, M. In Organometallics in Synthesis: A Manual;
Schlosser, M., Ed.; Wiley: Chichester, 1994; p 115.
(30) Antonia, Y.; Barrera, P.; Contreras, O.; Franco, F.; Galeazzi,
E.; Garcia, J .; Greenhouse, R.; Guzma´n, A.; Velarde, E.; Muchowski,
J . M. J . Org. Chem. 1989, 54, 2159-2165.
(31) Orchin, M. J . Am. Chem. Soc. 1948, 70, 495-497.
(32) McMurry, J . E.; Krepski, L. R. J . Org. Chem. 1976, 41, 3929-
3930.
Here we report the synthesis of new members of the
xantphos series, their coordination chemistry, and their
catalytic performance in the rhodium-catalyzed hydro-
formylation. In this series of ligands there is an almost
regular increase in hydroformylation activity and se-
(33) Levy, J . B.; Walton, R. C.; Olsen, R. E.; Symmes, C. J .
Phosphorus, Sulfur 1996, 109-110, 545-548.

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 875
Ta ble 2. Selected Bon d Len gth s a n d An gles for
(Nixa n tp h os)Rh (CO)H(P P h 3) (18a ) a n d
(Ben zoxa n tp h os)Rh (CO)H(P P h 3) (19a )
(nixantphos)Rh(CO)H(PPh3) (benzoxantphos)Rh(CO)H(PPh3)
(18a )
(19a )
Bond Lengths (Å)
Rh-P(1)
Rh-P(2)
Rh-P(3)
Rh-C(1)
Rh-H
2.3288(7)
Rh-P(1)
Rh-P(2)
Rh-P(3)
Rh-C(1)
Rh-H
2.3616(16)
2.3268(15)
2.2822(15)
1.895(6)
F igu r e 2. Wire models of the minimized structures of
xantphos 1 (left) and xantphos 8 (right).
2.3408(7)
2.2854(8)
1.887(3)
1.65(3)
1.62(6)
Bond Angles (deg)
110.21(3) P(1)-Rh-P(2)
P(1)-Rh-P(2)
P(1)-Rh-P(3)
P(2)-Rh-P(3)
P(1)-Rh-C(1)
P(2)-Rh-C(1)
P(3)-Rh-C(1)
109.16(5)
118.45(5)
129.07(6)
97.9(2)
93.1(2)
97.15(19)
125.12(3)
120.98(3)
93.49(8)
97.88(9)
97.70(9)
P(1)-Rh-P(3)
P(2)-Rh-P(3)
P(1)-Rh-C(1)
P(2)-Rh-C(1)
P(3)-Rh-C(1)
angle calculations indicate, however, that in the rhod-
ium complexes the diphenylphosphine moieties of xant-
phos 1 conform to the same pseudo-Cs symmetry as the
other xantphos ligands.
(Dip h osp h in e)Rh (CO)H(P P h 3) Com p lexes 11a -
19a . To investigate the effect of the natural bite angle
on chelation behavior, the structures of the (diphos-
phine)Rh(CO)H(PPh3) complexes and the (diphosphine)-
RH(CO)2H complexes of all xantphos ligands were
studied in detail.34 Formation of the (diphosphine)Rh-
(CO)H(PPh3) complexes was easily accomplished by
displacement of PPh3 in (PPh3)3Rh(CO)H on the addi-
tion of diphosphines 1-9.56 Crystals of the (diphos-
phine)Rh(CO)H(PPh3) complexes suitable for crystal
structure determinations were obtained for ligands 8
and 9.
The X-ray crystal structures of (nixantphos)Rh(CO)H-
(PPh3) (18a ) and (benzoxantphos)Rh(CO)H(PPh3) (19a )
are shown in Figure 3. Selected bond lengths and angles
of complexes 18a and 19a are given in Table 2. Both
crystal structures reveal distorted trigonal bipyramidal
complex geometries with all the phosphines occupying
equatorial sites. The Rh atom is located slightly above
the equatorial plane defined by the three phosphorus
atoms and displaced toward the apical carbonyl ligand.
The positions of the hydride ligands were determined
from difference Fourier maps. These positions are not
1
accurate, but H NMR spectroscopy also indicates that
the hydrides occupy the apical sites trans to the carbo-
nyl ligands (vide infra).
The observed P-Rh-P bite angles for ligands 8 and
9 in the crystal structures are 110.21(3)° and 109.16-
(5)°, respectively. The observed bite angles are within
the flexibility ranges calculated for the ligands and close
to the bite angle observed in the complex containing
ligand 4 (111.73(5)°).20 The bite angle of approximately
110° found in all three crystal structures is probably
the result of stereoelectronic interactions between the
diphosphine and the large triphenylphosphine ligand.
Furthermore, the crystal structures show several pos-
sibilities for π-π-stacking interactions between the
different phenyl moieties, which could also influence the
final complex geometry.
F igu r e 3. Crystal structures and numbering schemes for
(nixantphos)Rh(CO)H(PPh3) (18a ) (top) and (benzoxant-
phos)Rh(CO)H(PPh3) (19a ) (bottom) (non-hydride hydrogen
atoms have been omitted for clarity). Displacement el-
lipsoids are shown at 50% probability.
other xantphos ligands. With respect to the disposition
of the diphenylphosphine moieties, the conformations
of the xantphos ligands 2-9 can roughly be described
as pseudo-Cs (Figure 2 and see also the crystal struc-
tures in Figure 3). Since the backbone of ligand 1 is
based on a seven-membered ring instead of a six-
membered one, the diphenylphosphine moieties of this
ligand adopt a different conformation that can be called
pseudo-C2 (Figure 2). These structural differences be-
tween xantphos 1 and the other xantphos ligands may
have some effect on the coordination chemistry and
catalytic performance of this ligand. The natural bite
Ligands 1 and 3-9 form similar (diphosphine)Rh-
(CO)H(PPh3) complexes in solution. The presence of the
(34) The complexes with diphosphines 3-5 were already described
by Kranenburg et al.6

876 Organometallics, Vol. 19, No. 5, 2000
van der Veen et al.
Ta ble 3. Selected IR Da ta for
Ta ble 4. Selected NMR Da ta for
(Dip h osp h in e)Rh (CO)H(P P h 3) Com p lexesa
(Dip h osp h in e)Rh (CO)2H Com p lexesa
complex
ân, deg
ν1, cm-1
ν2, cm-1
1J (Rh,H), 1J (Rh,P), 2J av(P,H), ee:ea
complex ân, deg
Hz
Hz
Hz
ratiob
11a
12a
13a b
14a
15a b
16a
17a
18a
19a
102.0
107.9
108.5
109.6
111.4
113.2
114.1
114.2
120.6
1982 (vs)
1966 (s)
1921 (w)
1925 (m)
1911 (w)
1922 (w)
1910 (m)
1920 (m)
1920 (m)
1920 (m)
1919 (m)
11b
12b
13b
14b
15b
16b
17b
18b
19b
102.0
107.9
108.5
109.6
111.4
113.2
114.1
114.2
120.6
9.5
7.0
7.7
6.6
6.7
6.2
6.1
6.0
7.1
120
127
124
128
126
128
127
128
122
37.7
17.0
22.7
14.7
16.6
13.0
14.5
3:7
7:3
6:4
7:3
7:3
8:2
7:3
8:2
6:4
1997 (vs)
1999 (vs)
1997 (vs)
1990 (vs)
2000 (vs)
1988 (vs)
1989 (vs)
13.0
18.3/24.8c
In KBr, carbonyl region. In benzene.6
a
b
In C6D6 at 298 K. b Calculated from the 2J av(P,H) using a trans
a
c 2
2J (P,H) ) 106 Hz and cis 2J (P,H) e (2 Hz.
J (P,H) ) 21.6 Hz.
av
hydride ligand in the complexes is confirmed by 1H
NMR. The rhodium hydride signals appear as broad
quartets or triplets of doublets at δ ) -9.05 to -9.23
ea isomer ratios of these mixtures were estimated using
the averaged phosphorus-proton (2J (P,H)) coupling
constants.14,35 Although ligand 1 has the narrowest
natural bite angle and the lowest ee:ea isomer ratio,
analysis of the whole series of (diphosphine)Rh(CO)2H
complexes reveals that there is no clear correlation
between the ee:ea isomer ratio and the natural bite
angle. Despite a difference in natural bite angles of more
than 12°, the ee:ea ratios of the complexes containing
ligands 3 (ân ) 108.5°) and 9 (ân ) 120.6°) are ap-
proximately the same (6:4). The low ee:ea isomer ratio
of the complex containing ligand 9 can be the result of
the high rigidity of the ligand backbone induced by the
naphthyl ring. The inequivalence of the two phosphorus
atoms in ligand 9 could also influence the complex
mixture composition. Casey and co-workers have re-
ported that electronically dissymmetric DIPHOS de-
rivatives show an enhanced preference for ea coordi-
nation with the least basic phosphine in the equatorial
position.36 The relatively high ee:ea isomer ratio (7:3)
in the complexes with ligands 2 and 4 can be attributed
to the electron-withdrawing capacities of respectively
the phosphorus moiety (σm(PPh2) ) 0.1137) and the
sulfur moiety (σm(SPh) ) 0.2337) in the ligand back-
bones.20
HP IR Da ta for (Dip h osp h in e)Rh (CO)2H Com -
p lexes 11b-19b. The existence of ee and ea complex
isomers was clearly evidenced by HP IR spectroscopy.
The spectra of the (diphosphine)Rh(CO)2H complexes
all showed four absorption bands in the carbonyl region,
which can be assigned to an ee and an ea isomer (Table
5).20 The relative intensities of the absorption bands are
in good agreement with the ee:ea ratios determined by
NMR spectroscopy. Comparison of the IR frequencies
shows that all bands are within a close range, indicating
that the electronic properties of the ligands are similar.
Despite the fact that the electronic properties of the
ligand backbone have hardly any effect on the IR
frequencies, they seem to have an influence on the ee:
ea isomer ratio, as is also evidenced by the IR spectra.
This implies that the composition of the (diphosphine)-
Rh(CO)2H complex mixture is sensitive to small changes
in ligand structure.
2
(td, J (P,H) ) 12-20 Hz). The rhodium-hydride cou-
pling constants could not be resolved. The low values
of the phosphorus-hydride coupling constants indicate
that the hydrides occupying apical sites that are cis to
three equatorial phosphines in distorted trigonal bipy-
ramidal complexes. The 31P{1H} NMR spectra show that
in all complexes, except in that of ligand 9, the two
phosphine moieties of the diphosphine are equivalent.
In the complex containing ligand 9 the two phosphorus
atoms of the diphosphine are intrinsically inequivalent.
In the 31P{1H} NMR spectrum their signals appear in
a complex ABCX pattern with a diphosphine P-P
coupling constant of 77 Hz. The diphosphine P-PPh3
coupling constants (2J (P,P) ) 118-143 Hz) for all
complexes are in agreement with structures in which
all phosphines occupy equatorial sites.
For complex (2)Rh(CO)H(PPh3) only very broad sig-
nals were observed in the 1H and 31P{1H} NMR spectra,
even at 213 K. This is probably due to the formation of
oligomeric complexes in solution via the displacement
of triphenylphosphine by the phosphine moiety in the
ligand backbone. The presence of a hydride and a
carbonyl ligand in this complex is confirmed by IR
spectroscopy. The IR spectra of complexes 11a -19a in
KBr all have two absorption bands in the carbonyl
region due to combination bands of νRhH and νCO (Table
3).
NMR Da ta for (Dip h osp h in e)Rh (CO)2H Com -
p lexes 11b-19b. The (diphosphine)Rh(CO)2H com-
plexes, the resting state of the catalyst under hydro-
formylation conditions,20 were prepared in situ from
Rh(CO)2(acetylacetonate) (acac) and diphosphine under
an atmosphere of CO/H2 (1:1). For all diphosphine
ligands the formation of the (diphosphine)Rh(CO)2H
complexes was evidenced by 31P{1H} and 1H NMR
spectroscopy.57 The 31P{1H} NMR spectra of the com-
plexes with ligands 1-8 show a characteristic doublet
1
at 19-25 ppm, and in the H NMR spectra the rhodium
hydride signal appears as an apparent triplet of dou-
blets. In the NMR spectra of the complex (9)Rh(CO)2H
a double doublet is observed in the 31P{1H} spectrum
1
and a double double doublet in the H spectrum due to
the inequivalence of the two phosphorus atoms of the
ligand. For all (diphosphine)Rh(CO)2H complexes the
characteristic heteronuclear coupling constants are
depicted in Table 4.
It was shown previously that the (diphosphine)Rh-
(CO)2H complexes consist of mixtures of ee and ea
isomers that are in dynamic equilibrium.18,20 The ee:
Hyd r ofor m yla tion of 1-Octen e. The hydroformy-
lation of 1-octene was carried out at 80 °C and 20 bar
(35) Yagupsky, G.; Wilkinson, G. J . Chem. Soc. (A) 1969, 725-733.
(36) Casey, C. P.; Paulsen, E. L.; Beuttenmueller, E. W.; Proft, B.
R.; Matter, B. A.; Powell, D. R. J . Am. Chem. Soc. 1999, 121, 63-70.
(37) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165-
195.

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 877
Ta ble 5. Selected HP IR Da ta for
(Dip h osp h in e)Rh (CO)2H Com p lexesa
Ta ble 7. Resu lts of th e Hyd r ofor m yla tion of
Styr en e a t 120 °Ca
complex ân, deg ν1, cm-1
ν2, cm-1
ν3, cm-1
ν4, cm-1
ligand ân, deg
l:b ratiob
% linear aldehydeb
tofb,c
11b
12b
13bb
14b
15bb
16b
17b
18b
19b
102.0
107.9
108.5
109.6
111.4
113.2
114.1
114.2
120.6
2040 (m) 1997 (m) 1974 (m) 1951 (s)
1
2
3
4
5
6
7
8
9
102.0 0.68 ( 0.01
107.9 1.13 ( 0.02
108.5 0.99 ( 0.01
109.6 1.22 ( 0.11
111.4 1.45 ( 0.01
113.2 1.45 ( 0.02
114.1 1.78 ( 0.05
114.2 2.04 ( 0.06
120.6 1.78 ( 0.07
40.4 ( 0.6
53.0 ( 0.5
49.8 ( 0.4
54.9 ( 2.3
59.1 ( 0.2
59.1 ( 0.3
64.1 ( 0.6
67.0 ( 0.7
64.1 ( 0.8
5766 ( 696
4960 ( 392
4240 ( 132
2735 ( 203
6384 ( 322
6010 ( 151
7386 ( 701
9148 ( 655
14602 ( 1750
2040 (s)
1999 (m) 1977 (s)
1990 (m) 1966 (m) 1940 (vs)
1999 (m) 1977 (s) 1953 (s)
1991 (s) 1969 (vs) 1941 (vs)
1953 (s)
2040 (s)
2036 (s)
2041 (s)
2038 (s)
2035 (s)
1999 (m) 1976 (s)
1997 (m) 1977 (s)
1996 (m) 1976 (s)
1952 (s)
1951 (s)
1949 (s)
1949 (s)
2036 (m) 1996 (m) 1976 (s)
a
a
In cylohexane at 80 °C and 20 bar CO/H2 (1:1), carbonyl region.
In benzene.6
Conditions: CO/H2 ) 1, P(CO/H2) ) 10 bar, ligand/Rh ) 10,
b
substrate/Rh ) 1746, [Rh] ) 0.50 mM in toluene, number of
experiments ) 3. In none of the experiments was hydrogenation
observed. Linear over branched ratio, percent linear aldehyde,
b
Ta ble 6. Resu lts of th e Hyd r ofor m yla tion of
1-Octen e a t 80 °Ca
and turnover frequency were determined at 20% alkene conver-
sion. c Turnover frequency ) (mol of aldehyde) (mol of Rh)-1 h-1
.
% linear
ligand ân, deg
l:b ratiob
aldehydeb % isomer.b
tofb,c
1
2
3
4
5
6
7
8
9
102.0
8.50 ( 0.16 88.2 ( 0.4 1.4 ( 0.3 36.9 ( 4.8
107.9 14.6 ( 0.9
108.5 34.3 ( 0.6
109.6 56.6 ( 0.2
111.4 52.2 ( 1.4
113.2 49.8 ( 0.3
114.1 50.6 ( 1.3
114.2 69.4 ( 3.2
120.6 50.2 ( 0.4
89.7 ( 0.4 4.2 ( 0.1 74.2 ( 1.6
94.4 ( 0.2 2.9 ( 0.2 76.5 ( 8.8
93.7 ( 0.1 4.7 ( 0.03 94.1 ( 0.4
94.5 ( 0.2 3.7 ( 0.2
94.3 ( 0.1 3.8 ( 0.1
94.3 ( 0.3 3.9 ( 0.3
94.9 ( 0.4 3.7 ( 0.5
187 ( 4
162 ( 7
154 ( 12
160 ( 5
96.5 ( 0.04 1.6 ( 0.02 343 ( 7
a
Conditions: CO/H2 ) 1, P(CO/H2) ) 20 bar, ligand/Rh ) 5,
substrate/Rh ) 637, [Rh] ) 1.00 mM in toluene, number of
experiments ) 3. In none of the experiments was hydrogenation
b
observed. Linear over branched ratio, percent linear aldehyde,
percent isomerization to 2-octene, and turnover frequency were
determined at 20% alkene conversion. c Turnover frequency ) (mol
F igu r e 5. Percentage linear aldehyde (triangles) and tof
(circles) vs the calculated natural bite angle (ân) in the
hydroformylation of styrene (data from Table 7).
of aldehyde) (mol of Rh)-1 h-1
.
(NH2) ) -0.1637) in both backbones.20 The increased
flexibility of ligands 7 and 8 caused by the low barrier
of inversion at nitrogen could also affect the activity.
Hyd r ofor m yla tion of Styr en e. The hydroformyla-
tion of styrene was carried out at 120 °C and 10 bar of
1:1 CO/H2 using a 0.50 mM solution of rhodium diphos-
phine catalyst prepared from Rh(CO)2(acac) and 10
equiv of ligand. For styrene usually a distinct preference
for the formation of the branched aldehyde is observed
due to the stability of the benzylic rhodium species,
induced by the formation of a stable η3 complex.2 The
formation of the linear aldehyde can be enhanced by
using high temperature and low pressure.6,38 The pro-
duction of 1-phenylpropionaldehyde and 2-phenylpro-
pionaldehyde was monitored by gas chromatography.
The results of the experiments with ligands 1-9 are
depicted in Table 7 and Figure 5. Average turnover
frequencies were determined at 20% conversion.
In contrast to our findings in the hydroformylation
of 1-octene, no clear trend in activity was observed in
the hydroformylation of styrene. Increasing the natural
bite angle from 102° to 110° results in a decrease of the
hydroformylation rate, with a minimum activity ob-
served for ligand 4. Upon a further increase of the
natural bite angle to 121°, the rates increase again to
result in a maximum activity observed for ligand 9.
The linear over branched ratio (l:b) and hence the
selectivity for 1-phenylpropionaldehyde do increase with
increasing natural bite angle, resulting in an almost
F igu r e 4. Percentage linear aldehyde (triangles) and tof
(circles) vs the calculated natural bite angle (ân) in the
hydroformylation of 1-octene (data from Table 6).
of 1:1 CO/H2 using a 1.0 mM solution of rhodium
diphosphine catalyst prepared from Rh(CO)2(acac) 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 1-9 are shown in Table 6 and Figure 4.
Averaged turnover frequencies were determined at 20%
conversion.
In the series of xantphos ligands both the selectivity
for linear aldehyde formation and the rate of the
reaction increase with increasing natural bite angle.
Changing the natural bite angle from 102° for ligand 1
to 121° for ligand 9 results in an increase of the
selectivity for linear nonanal from 88% to 97%. More-
over, a rate enhancement of almost an order of magni-
tude is observed. The relatively low activities of ligands
7 and 8 compared to ligand 5 can be explained by the
electron-donating capacity of the nitrogen moiety (σm-
(38) Lazzaroni, R.; Raffaelli, A.; Settambolo, R.; Bertozzi, S.; Vitulli,
G. J . Mol. Catal. 1989, 50, 1-9.

878 Organometallics, Vol. 19, No. 5, 2000
van der Veen et al.
F igu r e 6. Representative difference IR spectrum (a) and kinetic data (b and c) for the 13CO dissociation from (thixantphos)-
Rh(13CO)2H (14a -13CO) in the presence of unlabeled CO at 40 °C.
Sch em e 3
3-fold higher l:b ratio for ligand 8 compared to ligand
1. Apparently, in the hydroformylation of styrene the
selectivity-determining step remains the same, while
the rate is probably influenced by more than one step
in the catalytic cycle going from narrow to wide natural
bite angles.
Kin et ics of 13CO Dissocia t ion fr om (Dip h os-
p h in e)Rh (13CO)2H Com p lexes. The hydroformylation
results of both 1-octene and styrene (vide supra) clearly
show that the natural bite angle has a pronounced
influence on the rate of the reaction. From previous
work and preliminary kinetic experiments it is clear
that the rate-determining step in the hydroformylation
of 1-octene with xantphos-type ligands is in an early
stage in the catalytic cycle.20,39 CO dissociation, alkene
coordination, and/or alkene insertion must be rate-
limiting. So far, no reports have appeared in the
literature in which the separate rate constants for these
reactions are determined. We now have developed an
experimental setup that enables us to obtain the rate
constants for CO dissociation from the (diphosphine)-
Rh(CO)2H complexes. We found that by changing the
common 12CO ligands for isotopically pure 13CO ligands,
the carbonyl absorptions in the IR spectra of the
(diphosphine)Rh(CO)2H complexes shift 30-40 cm-1 to
lower energy. Fortunately, the frequencies shift in such
a way that the absorption bands of the 13CO-labeled
complexes are clearly separated from the ones of the
unlabeled 12CO complexes (Figure 6a). Therefore, it is
possible to observe both complexes individually in IR
experiments.
The rate constants k1 (Scheme 3) for the dissociation
of 13CO from the (diphosphine)Rh(13CO)2H complexes
can be determined by monitoring the exchange of 13CO
for 12CO. This exchange can be accomplished by expos-
ing the 13CO-labeled complexes to a large excess of 12CO,
so any dissociated 13CO will be replaced quantitatively
by 12CO. This will first result in the formation of the
(diphosphine)Rh(13CO)(12CO)H complex40 and eventu-
ally lead to the (diphosphine)Rh(12CO)2H complex. The
large excess of12CO prevents re-formation of(diphosphine)-
Rh(13CO)2H by the k-1 pathway.
(39) Preliminary kinetic experiments with ligand 4 have shown a
first-order dependence of the hydroformylation rate on both the
rhodium and the alkene concentration, an approximately linear inverse
dependence on the CO pressure, and no correlation with the H2
pressure (see Supporting Information).

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 879
Ta ble 8. Kin etics of 13CO Dissocia tion of
(Dip h osp h in e)Rh (13CO)2H Com p lexesa
It is commonly accepted that CO exchange in (diphos-
phine)Rh(CO)2H complexes proceeds via the dissociative
pathway (Scheme 3).2 This implies that the rate of CO
dissociation should be independent of CO partial pres-
sure and that the reaction follows simple first-order
kinetics when the occurrence of the reverse reaction,
13CO association, is prevented.41 As a result, the overall
rate equation for the reaction simplifies to eq 1 and the
first-order integrated rate law eq 2.
ligand
ân, deg
P(CO) (bar)
r2
k1 (h-1
)
2
2
107.9
25
25
25
25
20
25
30
0.998
0.999
1.000
1.000
0.999
1.000
0.999
177 ( 2
174 ( 1
206 ( 1
200 ( 1
182 ( 1
185 ( 1
181 ( 1
4
109.6
113.2
4b
6
6
6
Reaction conditions: [(diphosphine)Rh(13CO)2H] ) 2.00 mM
in cyclohexane, P(13CO) ) 1 bar, P(H2) ) 4 bar, T ) 40 °C,
diphosphine/Rh ) 5. Values for k1 are least-squares fit of lines
from ln[(diphosphine)Rh(13CO)2H] vs time over the first 25 s,
a
-d[(diphosphine)Rh(13CO)2H]/dt )
k1[(diphosphine)Rh(13CO)2H] (1)
b
calculated using TableCurve 2D.43 [(diphosphine)Rh(13CO)2H]
) 3.00 mM.
ln[(diphosphine)Rh(13CO)2H] )
-k1t + ln[(diphosphine)Rh(13CO)2H]0 (2)
[14a -13CO]) versus time can be obtained (Figure 6c). The
slope of this line is the negative of the first-order rate
constant k1 (eq 2).
To investigate the possible effect of the natural bite
angle on the rate of CO dissociation from the (diphos-
phine)Rh(CO)2H complexes, we have synthesized the
13CO-labeled complexes for ligands 2, 4, and 6. The
(diphosphine)Rh(13CO)2H complexes were prepared in
situ from Rh(CO)2(acac) and diphosphine under an
atmosphere of 13CO/H2 (1:4). Complex formation was
usually complete within 2 h. The exchange of 13CO for
12CO in the (diphosphine)Rh(13CO)2H complexes was
monitored by rapid-scan HP IR spectroscopy at 40 °C.
The 13CO/12CO exchange was initiated by adding a large
excess of 12CO (20-fold or more). The strongest carbonyl
absorptions of the complexes at approximately 1908
cm-1 were taken to calculate the concentrations of the
complexes. These absorption bands, which are assigned
to the symmetric CO vibration of the ea complex
isomers, are the most characteristic and intensive
carbonyl vibrations of the (diphosphine)Rh(13CO)2H
complexes due to the strong vibrational coupling be-
tween the two equatorial 13CO ligands. The difference
between the ee and ea isomers of the (diphosphine)-
Rh(13CO)2H complexes cannot be determined, since the
exchange between ee and ea isomers is several orders
of magnitude faster than the rates of 13CO dissociation.42
Representative kinetic data of the experiments with
ligand 2, 4, and 6 are shown in Figure 6, and the
observed rate constants, k1, are listed in Table 8.
The representative difference IR spectrum displayed
in Figure 6a for one of the experiments with (thixant-
phos)Rh(13CO)2H (14a -13CO) clearly shows that the
13CO-labeled complex is quantitatively converted into
its 12CO analogue. The exponential decay of the inten-
sity of the strongest carbonyl absorption (1908 cm-1) of
complex 14a -13CO versus time is displayed in Figure
6b. The solvent cyclohexane used in the experiment is
responsible for the remaining background absorption of
0.71 au. By converting the background-corrected absor-
bance in the complex concentration the linear plot of
the natural logarithm of the complex concentration (ln-
The decay of the carbonyl resonances of the (diphos-
phine)Rh(13CO)2H complexes in time follows simple
first-order kinetics in all experiments. Experimental
reproducibility is within 3% for duplo experiments, and
plots of the ln[(diphosphine)Rh(13CO)2H)] versus time
are linear for at least 2 half-lives. The r2 coefficients of
the least-squares fits of the lines are more than 0.998.
This clearly confirms that re-formation of the (diphos-
phine)Rh(13CO)2H complex via the k-1 pathway is
suppressed effectively, and all dissociated 13CO is
replaced by 12CO. The experiments with ligand 6 at
different 12CO partial pressure show that the rate of CO
displacement is independent of the CO pressure. Fur-
thermore, the rate is also independent of the (diphos-
phine)Rh(13CO)2H complex concentration, as demon-
strated by the experiments with ligand 4. It can
therefore be concluded that the CO dissociation for these
complexes proceeds by a purely dissociative mechanism
and obeys a first-order rate-law. Employing the de-
scribed experimental setup, the rate constants k1 for CO
dissociation from the (diphosphine)Rh(CO)2H complexes
are obtained for the first time.
Table 8 shows that there is no correlation between
the rate of CO dissociation and the natural bite angle.
The rate constants k1 for ligands 2, 4, and 6 do not differ
significantly and cannot explain the trend in observed
hydroformylation activities. The comparison of the k1
values for ligands 2, 4, and 6 with the turnover
frequencies depicted in Table 4 reveal that the rates of
CO dissociation, measured at 40 °C, are higher than the
hydroformylation rates at 80 °C. Since reaction rates
increase approximately an order of magnitude with a
temperature rise of 20 deg, the CO dissociation rate at
80 °C is about 100 times as fast as the hydroformylation
reaction.
Discu ssion
Within the series of xantphos ligands no clear effect
of the ligand backbone is found on the bite angles
observed in the crystal structures of the (diphosphine)-
Rh(CO)H(PPh3) complexes or on the ee:ea isomer
composition of the (diphosphine)Rh(CO)2H complexes.
These results are in contradiction with studies of Casey
and co-workers in which diphosphines with wider
natural bite angles have an increased preference for ee
chelation.5,14 Evidently, only for flexible diphosphines
(40) The transient (diphosphine)Rh(13CO)(12CO)H complex is not
observed in the IR experiments. The intensities of the carbonyl
absorptions of this complex are considerably lower than those of the
isotopically pure complexes due to the absence of vibrational coupling
between the CO ligands. Furthermore, three complex isomers exist for
this complex, which results in even lower intensities of the six separate
absorption bands.
(41) Cotton, F. A.; Wikinson, G. Advanced Inorganic Chemistry, 5th
ed.; Wiley: New York, 1988.
(42) The energy barrier for the interconversion of the two isomers
20
is very low, probably less than 10 kcal mol-1
.

880 Organometallics, Vol. 19, No. 5, 2000
van der Veen et al.
Sch em e 4
In the present study no influence of the natural bite
angle on the rate of formation of the (diphosphine)Rh-
(CO)H complexes (k1) is found, implying that the activa-
tion energy for the formation of these complexes is not
affected significantly. Therefore, the increase in hydro-
formylation rate with increasing bite angle can originate
(i) from an increase in the concentration of this four-
coordinate complex, i.e., from a decrease of the rate of
CO association (k-1), (ii) from an increase in the rate of
alkene coordination (k2), (iii) from a decrease in the rate
of alkene dissociation (k-2), or (iv) from an increase in
the rate of alkene insertion (k3).
What kind of electronic effect the widening of the bite
angle has on the activation energy for CO and alkene
coordination (k-1 and k2) is unclear, since it depends
on the bonding modes of the ligands.44 Rhodium to CO/
alkene back-donation is promoted by narrow bite angles,
while CO/alkene to rhodium donation is enhanced by
wide bite angles. Sterically, alkene coordination will
experience a much stronger influence of the natural bite
angle of the diphosphine than the association of the
smaller CO ligand. Increasing the bite angle results in
increased steric congestion around the rhodium center
and consequently in more steric hindrance for the ligand
entering the coordination sphere. This could result in a
decrease of k2 and an increase of k-2. On the basis of
the results of the CO dissociation experiments, however,
having either a relatively narrow natural bite angle
(dppe) or a relatively wide one (BISBI) does the natural
bite angle control the chelation mode.5 The rhodium
complexes of the more rigid xantphos-type ligands
having intermediate natural bite angles prove to be
sensitive to small changes in the electronic properties
and the rigidity of the ligand backbone. In these
complexes the chelation mode is only partially imposed
by the natural bite angle. The pronounced effect of
phosphine basicity on chelation behavior has been
demonstrated before.14,20,36
The natural bite angle does have a clear effect on the
selectivity for linear aldehyde formation in the hydro-
formylation of both 1-octene and styrene. This unam-
biguously proves that the ee:ea isomer ratio in the
(diphosphine)Rh(CO)2H catalyst resting states is not a
key parameter controlling regioselectivity. The bite
angle effect on regioselectivity has to operate via
another mechanism. The expansion of the effective
steric bulk of the diphosphine with increasing natural
bite angle is the most likely explanation.5 Increasing
the steric congestion around the rhodium center will
result in more selective formation of the sterically less
hindered linear rhodium alkyl species. This is consistent
with our model suggested previously, in which the four-
coordinate intermediate (diphosphine)Rh(CO)H species
that undergoes the alkene attack plays an important
role in the control of regioselectivity.20 The regioselec-
tivity is finally fixed upon the migration of the hydride
to the coordinated alkene (assuming that this step is
irreversible).
So far it is unclear in which step of the hydroformy-
lation cycle the bite angle affects the activity. The rate
of CO dissociation (k1) or association (k-1), the rate of
alkene coordination (k2) or dissociation (k-2), and the
rate of alkene insertion (k3) could all be influenced by
the bite angle (Scheme 4). Treating the step of CO
dissociation/association as a preequilibrium and em-
ploying the steady-state approximation for the (diphos-
phine)Rh(CO)H(alkene) complex leads to eq 3 as the
general rate expression for the reaction sequence de-
picted in Scheme 4.41 The step of CO dissociation/
association can be treated as a preequilibrium, since
k1[(diphosphine)Rh(CO)2H] is orders of magnitude faster
than the overall hydroformylation rate and the (diphos-
phine)Rh(CO)H complex is not observed under hydro-
formylation conditions or in the CO dissociation studies,
indicating that k-1[CO] is orders of magnitude larger
than k1 and k2. The use of the steady-state approxima-
tion is justified since the (diphosphine)Rh(CO)H(alkene)
complex is also not observed under hydroformylation
conditions.
it is not likely that the rate of alkene dissociation (k-2
)
is affected by the natural bite angle.
The increase of the hydroformylation activity with
increasing natural bite angle may be explained by the
relative stability of the unsaturated four-coordinate
(diphosphine)Rh(CO)H complex.20 In these square-
planar complexes trans coordination of the diphosphines
is energetically more favorable than cis coordination.45
Diphosphines that can accommodate the wide bite
angles required for the formation of the trans square-
planar complexes will therefore lead to more stable
(diphosphine)Rh(CO)H complexes. The concentration of
the four-coordinate complexes is directly related to the
relative stability of these complexes compared to the
five-coordinate (diphosphine)Rh(CO)2H complexes. In
rate equation 3 (Scheme 4) the relative stability of the
four-coordinate complexes is reflected in the equilibrium
constant for CO dissociation K1. Since the natural bite
angle has no regular effect on k1, according to this
explanation, increasing the bite angle will have to result
in a decrease of rate constant k-1, leading to an increase
of K1.
Another explanation for the origin of the bite angle
effect on the activity in hydroformylation is found in the
increase of the rate of alkene insertion (k3). Theoretical
studies on platinum-hydride complexes have shown
that upon alkene insertion the P-Pt-P bite angle
increases.46,47 On the basis of these results it can be
concluded that widening of the natural bite angle
enhances the alkene insertion reaction and, therefore,
can increase the hydroformylation rate. The influence
(43) TableCurve, J andel Scientific.
(44) Li, J .; Schreckenbach, G.; Ziegler, T. Inorg. Chem. 1995, 34,
3245-3252.
(45) Schmid, R.; Herrmann, W. A.; Frenking, G. Organometallics
1997, 16, 701-708.
(46) Thorn, D. L.; Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 2079.
(47) Rocha, W. R.; De Almeida, W. B. Organometallics 1998, 17,
1961-1967.

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 881
of the natural bite angle on the rate of CO insertion has
already been demonstrated both experimentally and
theoretically.48-50
lation cycle may be more important than the absolute
value of the natural bite angle. Since the four-coordinate
(diphosphine)Rh(CO)H complexes have escaped direct
observation so far, we are currently, in collaboration
with Dr. C. Bo from the Universitat Rovira i Virgili in
Tarragona, performing theoretical calculations using
density functional theory to further clarify the effect of
the bite angle of xantphos-type ligands on these com-
plexes.
For styrene a somewhat different trend in the activity
is observed than for 1-octene. This may be explained
by a shift of the rate-limiting step in the ligand series.51
For now it is still unclear what causes the decrease of
the activity with increasing natural bite angle in the
first half of the ligand series. The increase in activity
with increasing natural bite angle in the latter half of
the ligand series can be explained along the same lines
as for 1-octene; that is, the formation of the (diphos-
phine)Rh(CO)H complexes and/or alkene insertion can
be favored by wider natural bite angles.
Exp er im en ta l Section
Com p u ta tion a l Deta ils. The molecular mechanics calcula-
tions were performed using the CAChe WorkSystem version
4.0,52 on a Apple Power MacIntosh 950, equipped with two
CAChe CXP coprocessors. Calculations were carried out
similarly to the method described by Casey and Whiteker,24
using a Rh-P bond length of 2.315 Å. Minimizations were done
using the block-diagonal Newton-Raphson method, allowing
the structures to converge with a termination criterion of a
rms factor of 0.0001 kcal mol-1 Å-1 or less.
Con clu sion
No distinct effect of the natural bite angle on the
coordination mode in the (diphosphine)Rh(CO)H(PPh3)
and (diphosphine)Rh(CO)2H complexes within the series
of xantphos-type ligands was found. Although benzo-
xantphos has a 10° wider calculated natural bite angle
than thixantphos, the observed bite angle in the (ben-
zoxantphos)Rh(CO)H(PPh3) (19a) complex is even smaller
than that of thixantphos in the corresponding complex
(14a ). For the (diphosphine)Rh(CO)2H complexes wider
natural bite angles do not lead to increased ee:ea isomer
ratios. The natural bite angle, however, does affect the
catalytic performance of the xantphos-type ligands. In
the hydroformylation of both 1-octene and styrene the
bite angle correlates well with the selectivity for linear
aldehyde formation. The same trend is also valid for the
activity in 1-octene hydroformylation and to a certain
extent in styrene hydroformylation. The bite angle effect
on selectivity originates from the steps of alkene coor-
dination and hydride migration. The increase in steric
congestion around the rhodium center upon widening
the bite angle leads to more selective formation of the
sterically less demanding linear alkyl rhodium species.
It is still unclear how the bite angle influences the
activity in the catalytic cycle. Both the equilibrium
constant for CO dissociation and the rate of alkene
insertion could be affected. For the first time we have
determined the rate constants for CO dissociation from
(diphosphine)Rh(CO)2H complexes using rapid-scan FT
IR spectroscopy. The kinetic experiments with three
(diphosphine)Rh(CO)2H complexes show that the bite
angle has no regular effect on CO dissociation rates and
that the CO dissociation is approximately 100 times as
fast as the hydroformylation reaction. We suggest that
the bite angle effect on the activity in the hydroformy-
lation with xantphos-type ligands is caused by the
decrease of the CO association rate, resulting in an
increase of the concentration of the intermediate four-
coordinate (diphosphine)Rh(CO)H complexes or by an
increase of the rate of alkene insertion. In this context
it has to be noted that the ability of the ligand to
encompass the whole range of bite angles associated
with the different rhodium species in the hydroformy-
Gen er a l P r oced u r e. All reactions were carried out using
standard Schlenk techniques under an atmosphere of purified
argon. Toluene and TMEDA were distilled from sodium, THF
and diethyl ether from sodium/benzophenone, and hexanes
from sodium/benzophenone/triglym. Methanol, ethanol, and
dichloromethane were distilled from CaH2. Chemicals were
purchased from Acros Chimica and Aldrich Chemical Co.
(PPh3)3Rh(CO)H,53 10,11-dihydrodibenzo[b,f]oxepine,28 10-phen-
ylphenoxaphosphine,29 9-(tert-butyldimethylsilyl)phenoxazine,30
and benzo[k,l]xanthene31,54 were prepared according to litera-
ture 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 spectrometer.
31P and 13C spectra were measured 1H decoupled. TMS was
used as an internal standard for 1H and 13C NMR and H3PO4
for 31P NMR. Mass spectroscopy was measured on a J EOL
J MS-SX/SX102A. Elemental analyses were carried out on an
Elementar Vario EL apparatus. Standard infrared spectra
were recorded on a Nicolet 510 FT IR spectrophotometer, and
rapid-scan (4.5 spectra per second) measurements were per-
formed using a Bio-Rad FTS-60A 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. 13CO (99%) was purchased from
Cambridge Isotope Laboratories. Gas chromatographic analy-
ses were run on an Interscience HR GC Mega 2 apparatus
(split/splitless injector, J &W Scientific, DB1 30m column, film
thickness 3.0 mm, carrier gas 70 kPa He, FID detector)
equipped with a Hewlett-Packard data system (Chrom-Card).
10-Isop r op ylid en exa n th en e (10).32 A 7.0 g sample of
freshly flattened lithium metal (1.0 mol) was added to a
suspension of 44.5 g of titanium chloride (288 mmol) in 300
mL of 1,2-dimethoxyethane. The reaction mixture was heated
at reflux temperature for 1 h. After cooling to room temper-
ature, 8.09 g of xanthone (41.2 mmol) and 3.03 mL of acetone
(41.2 mmol) were added, and the reaction mixture was heated
at reflux temperature for another 22 h. The reaction mixture
was diluted with 250 mL of light petroleum ether, filtered over
Celite, and concentrated to give a brown oil. The product was
(48) Anderson, G. K.; Lumetta, G. J . Organometallics 1985, 4, 1542-
1545.
(49) Dekker, G. P. C. M.; Elsevier, C. J .; Vrieze, K.; van Leeuwen,
P. W. N. M. Organometallics 1992, 11, 1598-1603.
(50) Koga, N.; Morokuma, K. Chem. Rev. 1991, 91, 823-842.
(51) van Rooy, A.; Orij, E. N.; Kamer, P. C. J .; van Leeuwen, P. W.
N. M. Organometallics 1995, 14, 34-43.
(52) CAChe Scientific Inc., 18700 N.W. Walker Road, Building 92-
01, Beaverton, OR 97006.
(53) Ahmad, N.; Levison, J . J .; Robinson, S. D.; Uttley, M. F. Inorg.
Synth. 1974, 15, 59-60.
(54) Rice, J . E.; Cai, Z.-W. Tetrahedron Lett. 1992, 33, 1675-1678.

882 Organometallics, Vol. 19, No. 5, 2000
van der Veen et al.
purified by column chromatography (5% toluene in light
petroleum ether (v:v), Rf ) 0.28). Yield: 5.63 g of a light yellow
oil (61%) that slowly crystallized. Mp: 78-80 °C. 1H NMR
(CDCl3): δ ) 7.37 (dd, 3J (H,H) ) 7.7 Hz, 4J (H,H) ) 1.5 Hz,
2H; CH), 7.26 (m, 4H; CH), 7.18 (dt, 3J (H,H) ) 7.0 Hz, 4J (H,H)
) 2.3 Hz, 2H; CH), 2.18 (s, 6H; CH3). 13C{1H} NMR (CDCl3):
δ ) 154.1 (CO), 130.5, 128.3 (CH), 127.0 (CH), 126.7, 122.5,
122.2 (CH), 116.1 (CH), 23.0 (CH3). IR (KBr, cm-1): 3071 (w),
2939 (w), 2907 (w), 1472 (m), 1448 (s), 1255 (s), 1213 (m), 1197
(m), 755 (s). GC-MS (m/z, rel intensity): 222 (M+, 100), 221
(84), 207 (68), 181 (21), 178 (15), 152 (12), 110 (9.9), 103 (9.7).
Anal. Calcd for C16H14O: C, 86.45; H, 6.35. Found: C, 86.25;
) 37.7 Hz, 1H; RhH). 31P{1H} NMR (C6D6): δ ) 24.7 (d,
1J (Rh,P) ) 120.3 Hz). HP IR (cyclohexane, carbonyl region,
cm-1): 2040 (RhCO), 1997 (RhCO), 1974 (RhCO), 1951 (RhCO).
(P h osxa n tp h os)Rh (CO)H(P P h 3) (12a ). This compound
was prepared similarly to 11a . IR (KBr, carbonyl region, cm-1):
1966 (s, HRhCO), 1925 (m, HRhCO).
(P h osxa n tp h os)Rh (CO)2H (12b). This compound was
prepared similarly to 11b. 1H NMR (C6D6): δ ) 7.75 (bm, 4H;
CH), 7.38 (bm, 4H; CH), 6.78 (bm, 23H; CH), -8.55 (dt,
1J (Rh,H) ) 7.0 Hz, 2J (P,H) ) 17.0 Hz, 1H; RhH). 31P{1H} NMR
(C6D6): δ ) 22.8 (d, 1J (Rh,P) ) 126.5 Hz). HP IR (cyclohexane,
carbonyl region, cm-1): 2040 (RhCO), 1999 (RhCO), 1977
(RhCO), 1953 (RhCO).
(Isop r op xa n t p h os)R h (CO)H (P P h 3) (16a ). This com-
pound was prepared similarly to 11a . 1H NMR (C6D6): δ )
7.83 (quar., 3J (H,H) ) 4.5 Hz, 3J (P,H) ) 4.5 Hz, 4H; CH), 7.61
(m, 6H; CH), 7.50 (bs, 4H; CH), 7.17 (m, 2H; CH), 6.90 (m,
21H; CH), 6.72 (m, 4H; CH), 1.79 (s, 6H; CH3), -9.05 (bm, 1H;
RhH).31P{1H} NMR (C6D6): δ ) 41.8 (td, 1J (Rh,P) ) 166.9 Hz,
2J (P,P) ) 132.4, 1P; PPh3), 25.17 (dd, 1J (Rh,P) ) 148.6 Hz,
2J (P,P) ) 131.9 Hz, 2P). IR (KBr, carbonyl region, cm-1): 1990
(vs, HRhCO), 1920 (m, HRhCO). Anal. Calcd for C59H48O2P3-
Rh: C, 71.95; H, 4.92. Found: C, 71.40; H, 5.13.
H, 6.58. Exact mass (MS): 223.1160 (M + H) (calcd for C16H15
223.1123).
O
Xa n tp h os-Typ e Liga n d s 1-9. In a typical experiment, 6.1
mL of n-butyllithium (2.5 M in hexanes, 15 mmol) was added
dropwise to a stirred solution of 1.0 mL of 10,11-dihydro-
dibenzo[b,f]oxepine28 (6.1 mmol) and 2.3 mL of TMEDA (15
mmol) in 25 mL of diethyl ether at 0 °C. The reaction mixture
was slowly warmed to room temperature and stirred for 16 h.
The orange suspension was cooled to 0 °C, and a solution of
3.0 mL of chlorodiphenylphosphine (17 mmol) in 10 mL of
hexanes was added dropwise. The reaction mixture decol-
orized, and a beige precipitate was formed. After being stirred
for 16 h at room temperature, the reaction mixture was diluted
with 50 mL of THF and hydrolyzed with 25 mL of a 1:1
mixture of brine and dilute hydrochloric acid. The water layer
was removed, and the organic layer was dried over MgSO4.
The solvent was removed in vacuo, and the resulting residue
was washed with 25 mL of hexanes and crystallized from
dichloromethane/EtOH. Yield: 2.75 g of pure white crystals
of homoxantphos (1, 80%). All xantphos-type ligands 1-9 were
fully characterized.55-57 Detailed descriptions of the syntheses
and characterizations are included in the Supporting Informa-
tion.
(Hom oxa n tp h os)Rh (CO)H(P P h 3) (11a ). A solution of
(PPh3)3Rh(CO)H (92 mg, 0.10 mmol) and 1 (62 mg, 0.11 mmol)
in 5 mL of dichloromethane was stirred for 1 h at room
temperature. The solvent was removed in vacuo, and the
resulting yellow solid was washed with methanol.1H NMR
(C6D6): δ ) 7.78 (bs, 10H; CH), 7.51 (bs, 4H; CH), 7.02 (bs,
10H; CH), 6.91 (m, 11H; CH), 6.77 (m, 2H; CH), 6.70 (d,
3J (H,H) ) 6.3 Hz, 2H; H1,8), 6.57 (m, 2H; CH), 2.66 (s, 4H;
CH2), -9.05 (td, 1J (Rh,H) ) 16.5 Hz, 2J (P,H) ) 9.8 Hz, 1H;
RhH).31P{1H} NMR (C6D6): δ ) 44.5 (td, 1J (Rh,P) ) 165.2 Hz,
2J (P,P) ) 117.7 Hz; PPh3), 28.0 (dd, 1J (Rh,P) ) 146.3 Hz,
2J (P,P) ) 117.7 Hz, 2P). IR (KBr, carbonyl region, cm-1): 1982
(vs, HRhCO), 1921 (w, HRhCO).
(Isop r op xa n tp h os)Rh (CO)2H (16b). This compound was
prepared similarly to 11b. 1H NMR (C6D6): δ ) 7.61 (bm, 8H;
CH), 7.16 (dd, 3J (H,H) ) unresolved, 4J (H,H) ) 1.5 Hz, 2H;
3
CH), 6.86 (bm, 12H; CH), 6.68 (t, J (H,H) ) 7.7 Hz, 2H; CH),
6.58 (m, 2H; CH), -8.60 (dt, 1J (Rh,H) ) 6.2 Hz, 2J (P,H) ) 13.0
1
Hz, 1H; RhH). 31P{1H} NMR (C6D6): δ ) 23.5 (d, J (Rh,P) )
128.3 Hz). HP IR (cyclohexane, carbonyl region, cm-1): 2041
(RhCO), 1999 (RhCO), 1976 (RhCO), 1952 (RhCO).
(Isop r op xa n tp h os)Rh (CO)(13CO)H (16b-13CO). 13C{1H}
HP NMR (323 K, 18 bar CO/H2 (1:1), 2 bar 13CO, C6D6): δ )
1
199.8 (bd, J (Rh,C) ) 65.3 Hz; Rh-13CO).
(Ben zyln ixa n tp h os)Rh (CO)H(P P h 3) (17a ). This com-
pound was prepared similarly to 11a . 1H NMR (C6D6): δ )
7.80 (m, 8H; CH), 7.59 (m, 6H; CH), 7.47 (bs, 4H; CH), 6.91
(m, 22H; CH), 6.35 (m, 4H; CH), 6.23 (d, 3J (H,H) ) 7.5 Hz,
2H; CH), 4.46 (s, 2H; CH2), -9.06 (td, 1J (Rh,H) ) 18.6 Hz,
2J (P,H) ) 12.7 Hz, 1H; RhH). 31P{1H} NMR (C6D6): δ ) 42.0
(dt, 1J (Rh,P) ) 167.1 Hz, 2J (P,P) ) 132.7, 1P; PPh3), 25.17
(dd, 1J (Rh,P) ) 148.4 Hz, 2J (P,P) ) 132.6 Hz, 2P). IR (KBr,
carbonyl region, cm-1): 2000 (vs, HRhCO), 1920 (m, HRhCO).
(Ben zyln ixa n tp h os)Rh (CO)2H (17b). This compound was
1
prepared similarly to 11b. H NMR (C6D6): δ ) 7.63 (m, 8H;
CH), 7.00 (m, 12H, CH), 6.27 (m, 6H; CH), 4.46 (s, 2H; CH2),
-8.62 (dt, 1J (Rh,H) ) 6.0 Hz, 2J (P,H) ) 13.0 Hz, 1H; RhH).
1
31P{1H} NMR (C6D6): δ ) 20.2 (d, J (Rh,P) ) 127.6 Hz). HP
IR (cyclohexane, carbonyl region, cm-1): 2040 (RhCO), 1999
(RhCO), 1977 (RhCO), 1953 (RhCO).
(Hom oxa n tp h os)Rh (CO)2H (11b). A solution of Rh(CO)2-
(acac) (2.6 mg, 10 µmol) and 1 (6.2 mg, 11 µmol) in 1.0 mL of
C6D6 was pressurized with 20 bar of CO/H2 (1:1) and stirred
for 2 h at 70 °C. After being cooled to room temperature, the
reaction mixture was depressurized and transferred into a 0.5
cm NMR tube and directly analyzed at atmospheric pressure.
1H NMR (C6D6): δ ) 7.85 (bm, 2H; CH), 7.50 (m, 6H; CH),
7.04 (bs, 4H; CH), 6.90 (bm, 8H; CH), 6.76 (bm, 4H; CH), 6.57
(Nixa n tp h os)Rh (CO)H(P P h 3) (18a ). This compound was
prepared similarly to 11a . Yellow crystals suitable for X-ray
structure determination were obtained from toluene/ethanol.1H
NMR (C6D6): δ ) 7.76 (quar., 3J (H,H) ) 5.9 Hz, 3J (P,H) ) 5.9
Hz, 4H; CH), 7.58 (m, 6H; CH), 7.47 (bs, 4H; CH), 6.89 (m,
21H; CH), 6.54 (t, 3J (H,H) ) 7.8 Hz, 2H; H2,7), 6.38 (m, 2H;
H3,6), 6.08 (d, 3J (H,H) ) 7.5 Hz, 2H; H1,8), 4.52 (s, 1H; NH),
3
(m, 2H; CH), 6.68 (t, J (H,H) ) 7.7 Hz, 2H; CH), 6.58 (m, 2H;
CH), 2.72 (m, 4H; CH2), -8.28 (dt, 1J (Rh,H) ) 9.5 Hz, 2J (P,H)
1
2
-9.17 (td, J (Rh,H) ) 19.8 Hz, J (P,H) ) 12.1 Hz, 1H; RhH).
31P{1H} NMR (C6D6): δ ) 41.1 (bquar., 1J (Rh,P) ) 141 Hz,
1
2
2J (P,P) ) 141, 1P; PPh3), 28.0 (bt, J (Rh,P) ) 137 Hz, J (P,P)
) 137 Hz, 2P). IR (KBr, carbonyl region, cm-1): 1988 (vs,
HRhCO), 1920 (m, HRhCO). Anal. Calcd for C55H43NO2P3Rh:
C, 69.85; H, 4.59; N, 1.48. Found: C, 67.37; H, 4.65; N, 1.46.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion of 18a . 18a
crystallized in the triclinic space group P1h, a ) 13.0018(6) Å,
b ) 14.2404(7) Å, c ) 16.2583(9) Å, R ) 64.041(3)°, â ) 78.834-
(3)°, γ ) 80.269(2)°, V ) 2643.3(2) Å3, and Z ) 1. The data
collection was carried out at 230 K. The structure was solved
by direct methods. The hydrogen atoms were calculated. The
structure was refined to R ) 0.0363 and Rw ) 0.0827, for 8443
observed reflections. Crystal data and collection parameters,
(55) Unfortunately, no satisfying elemental analysis could be ob-
tained for compound 7. However, the molecular formula of 7 is
confirmed by exact mass determination and multinuclear NMR
spectroscopy. 1H, 31P{1H}, and 13C{1H} NMR spectra of 7 are included
in the Supporting Information.
(56) Complexes 11a , 12a , and 16a -19a were made for spectroscopic
purposes only; hence, no yields were determined. No satisfactory
elemental analyses could be obtained for complexes 11a , 12a , 17a , and
18a , since we were unable to strip these complexes from excess
triphenylphosphine and/or solvent, without losing structural integrity.
1H and 31P{1H} NMR spectra of the complexes are included in the
Supporting Information.
(57) Complexes 11b-19b are stable only under an atmosphere of
CO and H2. 1H and 31P{1H} NMR spectra of the complexes 11b, 12b,
and 16b-19b are included in the Supporting Information.

Rh Diphosphine Catalyzed Hydroformylation
Organometallics, Vol. 19, No. 5, 2000 883
atomic coordinates, bond lengths, bond angles, and thermal
parameters are included in the Supporting Information.
(Nixa n tp h os)Rh (CO)2H (18b). This compound was pre-
pared similarly to 11b. 1H NMR (C6D6): δ ) 7.61 (bs, 4H; CH),
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(acac) (5 or 10 µmol) in 8.5 mL of toluene. The autoclave
was pressurized to the appropriate pressure (6 or 16 bar),
heated to the 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 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.
HP F T IR Exp er im en ts. In a typical experiment the HP
IR autoclave was filled with 2-5 equiv of ligand, 4 mg of Rh-
(CO)2(acac), and 15 mL of cyclohexane. The autoclave was
purged three times with 15 bar CO/H2 (1:1), pressurized to
approximately 18 bar, and heated to 80 °C. Catalyst formation
was monitored in time by FT-IR and was usually completed
within 1 h.
Ra p id -Sca n HP F T IR Exp er im en ts. In a typical experi-
ment the HP IR autoclave was filled with 75 µmol of ligand,
30 or 45 µmol of Rh(CO)2(acac), and 15 mL of cyclohexane.
The autoclave was purged four times with 15 bar of H2,
pressurized to 3 bar with 13CO, and consequently to 10 bar
with H2, and heated at 40 °C for approximately 2 h. Next a
gas reservoir containing 50 bar of unlabeled CO is opened,
which resulted in a total pressure in the autoclave of 30 bar.
At the same time the rapid-scan FT IR experiment (53 scans
min-1 for a total acquisition time of 2.5 min) is started.
3
7.52 (bm, 2H; CH), 6.89 (bm, 14H; CH), 6.51 (t, J (H,H) ) 7.5
3
Hz, 2H; H2,7), 6.26 (m, 2H; H3,6), 6.12 (dd, J (H,H) ) 7.8 Hz,
4J (H,H) ) 1.2 Hz, 2H; CH), 4.62 (bs, 1H; NH), -8.69 (dt,
1J (Rh,H) ) 6.1 Hz, 2J (P,H) ) 14.5 Hz, 1H; RhH). 31P{1H} NMR
(C6D6): δ ) 19.8 (d, 1J (Rh,P) ) 127.2 Hz). HP IR (cyclohexane,
carbonyl region, cm-1): 2038 (RhCO), 1997 (RhCO), 1977
(RhCO), 1951 (RhCO).
(Ben zoxa n tp h os)Rh (CO)H(P P h 3) (19a ). This compound
was prepared similarly to 11a . Red crystals suitable for X-ray
structure determination were obtained from toluene/ethanol.
3
1H NMR (C6D6): δ ) 7.75 (t, J (H,H) ) 9.4 Hz, 1H; CH), 7.65
(t, 3J (H,H) ) 8.7 Hz, 1H; CH), 7.52 (m, 6H; CH), 7.25 (dd,
2J (P,H) ) 11.7 Hz, 3J (H,H) ) 7.6 Hz, 1H; PCCH), 6.91 (m, 19H;
3
1
CH), 6.70 (t, J (H,H) ) 7.5 Hz, 1H; CH), -9.23 (td, J (Rh,H)
) 18.9 Hz, 2J (P,H) ) 12.3 Hz, 1H; RhH). 31P{1H} NMR
(C6D6): δ ) 42.1 (dt, 1J (Rh,P) ) 170.0 Hz, 2J (P,P) ) 142.9,
1P; PPh3), 21.8 (AB-system: δA ) 21.8 (t, 1J (Rh,P) ) 147.5
Hz, 2J (P,P) ) 147.5 Hz), δB ) 19.2 (t, 1J (Rh,P) ) 142.9 Hz,
2J (P,P) ) 142.9 Hz), J AB ) 77.3 Hz, 2P). IR (KBr, carbonyl
region, cm-1): 1989 (vs, HRhCO), 1919 (m, HRhCO). Anal.
Calcd for C59H44O2P3Rh: C, 72.25; H, 4.52. Found: C, 72.17;
H, 4.71.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion of 19a . 19a
crystallized in the monoclinic space group P21/c, a ) 25.3330-
(6) Å, b ) 18.7936(5) Å, c ) 11.7296(2) Å, R ) 90°, â ) 92.6944-
(14)°, γ ) 90°, V ) 5578.3(2) Å3, and Z ) 4. The data collection
was carried out at 150 K. The structure was solved by direct
methods. The hydrogen atoms were calculated. The structure
was refined to R ) 0.0590 and Rw ) 0.1237, for 6496 observed
reflections. Crystal data and collection parameters, atomic
coordinates, bond lengths, bond angles, and thermal param-
eters are included in the Supporting Information.
Ackn owledgm en t. Financial support from the Tech-
nology Foundation (STW) of the Netherlands Organiza-
tion for Scientific Research (NWO) is gratefully acknowl-
edged, and we thank J . van Slageren for his assistance
with the rapid-scan FT IR experiments.
(Ben zoxa n tp h os)Rh (CO)2H (19b). This compound was
1
prepared similarly to 11b. H NMR (C6D6): δ ) 7.58 (quar.,
3
3J (H,H) ) 10.1 Hz, J (P,H) ) 10.1 Hz, 4H; CH), 7.40 (m, 6H;
CH), 7.24 (m, 4H; CH), 7.00 (m, 11H; CH), 6.70 (m, 3H; CH),
Su p p or tin g In for m a tion Ava ila ble: Detailed description
of the syntheses and characterizations of the xantphos-type
ligands 1, 2, 6-9, 1H, 31P{1H}, and 13C{1H} NMR spectra of 7,
1H and 31P{1H} NMR spectra of complexes 11a , 12a , 17a , 18a
and complexes 11b, 12b, 16b-19b, preliminary kinetic results
of the hydroformylation of 1-octene with ligand 4, and tables
of crystal data and collection parameters, atomic coordinates,
bond lengths, bond angles, thermal parameters, and H atom
coordinates for 18a and 19a . This material is available free of
charge via the Internet at http://pubs.acs.org.
1
2
2
-8.74 (ddd, J (Rh,H) ) 7.1 Hz, J (P,H) ) 24.8 Hz, J (P,H) )
18.3 Hz, 1H; RhH). 31P{1H} NMR (C6D6): δ ) 16.4 (dd,
1J (Rh,P) ) 122.2 Hz, 2J (P,P) ) 20.8, 1P), 13.1 (dd, 1J (Rh,P) )
122.2 Hz, 2J (P,P) ) 21.0 Hz, 1P). HP IR (cyclohexane, carbonyl
region, cm-1): 2036 (RhCO), 1996 (RhCO), 1977 (RhCO), 1967
(RhCO).
Hyd r ofor m yla tion Exp er im en ts. Hydroformylation reac-
tions were carried out in an autoclave, equipped with a 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
OM990734O