High-pressure infrared studies of rhodium complexes containing thiolate bridge ligands under hydroformylation conditions

Article abstract of DOI:10.1021/om990003o

In situ high-pressure IR spectroscopy studies of the rhodium catalyst systems [Rh₂{μ-S(CH₂)₃N(Me₂)} ₂(COD)₂]/PR₃ (R = Ph, OPh), [Rh₂{μ-S(CH₂)₂S}(COD)₂]/PPh ₃, [Rh₂{μ-S(CH₂)₄S}-(COD)₂] ₂/PPh₃, [Rh₂{μ-XANTOSS}(COD)₂]₂/PPh₃, and [Rh(acac)(CO)₂]/PR₃ (R = Ph, OPh) revealed the presence of mononuclear rhodium hydride species under hydroformylation conditions (80°C, 5-30 bar). The activities and selectivities, obtained during the hydroformylation of 1-hexene using these systems as catalyst precursors, can be fully accounted for by the mononuclear species observed. Deuterioformylation experiments using dinuclear [Rh₂{μ-S(CH₂)₃N(Me₂)} ₂(COD)₂]/PR₃ systems lent no support to a dinuclear mechanism.

Full text of DOI:10.1021/om990003o

Organometallics 1999, 18, 2107-2115
2107
High -P r essu r e In fr a r ed Stu d ies of Rh od iu m Com p lexes
Con ta in in g Th iola te Br id ge Liga n d s u n d er
Hyd r ofor m yla tion Con d ition s
Montserrat Die´guez, Carmen Claver,* Anna M. Masdeu-Bulto´, and Aurora Ruiz
Departament de Quı´mica Fı´sica i Inorga`nica, Universitat Rovira i Virgili,
Plac¸a Imperial Tarraco 1, 43005 Tarragona, Spain
Piet W. N. M. van Leeuwen* and Gerard C. Schoemaker
Institute for Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Received J anuary 4, 1999
In situ high-pressure IR spectroscopy studies of the rhodium catalyst systems [Rh2{µ-
S(CH2)3N(Me2)}2(COD)2]/PR3 (R ) Ph, OPh), [Rh2{µ-S(CH2)2S}(COD)2]/PPh3, [Rh2{µ-S(CH2)4S}-
(COD)2]2/PPh3, [Rh2{µ-XANTOSS}(COD)2]2/PPh3, and [Rh(acac)(CO)2]/PR3 (R ) Ph, OPh)
revealed the presence of mononuclear rhodium hydride species under hydroformylation
conditions (80 °C, 5-30 bar). The activities and selectivities, obtained during the hydro-
formylation of 1-hexene using these systems as catalyst precursors, can be fully accounted
for by the mononuclear species observed. Deuterioformylation experiments using dinuclear
[Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/PR3 systems lent no support to a dinuclear mechanism.
In tr od u ction
lytic experiments (0.1-1 mM). Furthermore, the gas
volume above the solution is often too small and mass
transport is too slow to allow a fast catalytic reaction
to be monitored for a longer period of time. Infrared
spectroscopy is a better alternative.1a,b,d-f,j,k,3
Rhodium-catalyzed hydroformylation has received a
great deal of attention in recent years both from
academia and industry.1 Despite this fact, there are very
few studies which aim to observe intermediates under
reaction conditions. The two most appropriate tech-
niques, high-pressure NMR and IR spectroscopy, both
have shortcomings and the crucial intermediates may
still escape from direct observation.
Nowadays, high-pressure NMR1i,j,n,o,2 spectroscopy is
used on a routine basis for identifying organometallic
compounds under high pressure. However, in order for
the signal-to-noise ratios to be satisfactory, high con-
centrations (10-100 mM) are required. At these high
concentrations metal/ligand and monomer/dimer equi-
libria deviate considerably from the equilibria in cata-
Two techniques are available: transmission and
reflectance spectroscopy.1b Transmission spectroscopy
gives the highest signal-to-noise ratio and the highest
resolution, but reflectance spectroscopy is a real in situ
technique. One drawback of the transmission cells,
which are usually connected to the autoclave via a pump
and tubing, is that during transport to the cell the
catalyst consumes the gases dissolved and the actual
spectrum is not representative of the contents of the
autoclave. A new transmission cell has been described
that avoids this problem.1k,4 Rhodium carbonyl com-
plexes have strong absorptions for the CO stretching
vibrations. Hence, low concentrations that approach real
catalyst conditions can be used. Mechanistic aspects of
the hydroformylation reaction using high-pressure IR
(HPIR) have been only studied for mononuclear com-
plexes containing triarylphosphines,1b monophosphites,1a,f
and diphosphites.1i,j,k
* To whom correspondence should be addressed. E-mail: C.C.,
claver@quimica.urv.es; P.W.N.M.v.L., pwnm@anorg.chem.uva.nl.
(1) (a) Trzeciak, A. M.; Zio´lkowski, J . J . J . Mol. Catal. 1986, 34,
213. (b) Moser, W. R.; Papile, C. J .; Brannon, D. A.; Duwell, R. A.;
Weininger, S. J . J . Mol. Catal. 1987, 41, 271. (c) Brown, J . M.; Kent,
A. G. J . Chem. Soc., Perkin Trans. 2 1987, 1597. (d) Garland, M.; Bor,
G. Inorg. Chem. 1989, 20, 410. (e) Garland, M.; Pino, P. Inorg. Chem.
1989, 28, 411. (f) J ongsma, T.; Challa, G.; van Leeuwen, P. W. N. M.
J . Organomet. Chem. 1991, 421, 121. (g) Sakai, N.; Mano, S.; Nozaki,
K.; Takaya, H. J . Am. Chem. Soc. 1993, 115, 7033. (h) Buisman, G. J .
H.; Vos, E. J .; Kamer, P. C. J .; van Leeuwen, P. W. N. M. J . Chem.
Soc., Dalton Trans. 1995, 409. (i) Moasser, B.; Gladfelter, W. L.; Roe,
D. C. Organometallics 1995, 14, 3832. (j) Moasser, B.; Gladfelter, W.
L. Inorg. Chim. Acta 1996, 242, 125. (k) van Rooy, A.; Kamer, P. C. J .;
van Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J .; Velman, N.; Spek,
A. L. Organometallics 1996, 15, 835. (l) Buisman, G. J . H.; van der
Veen, L. A.; Kamer, P. C. J .; van Leeuwen, P. W. N. M. Organometallics
1997, 16, 5681. (m) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya,
H. Organometallics 1997, 16, 1981. (n) Ke´gl, T.; Ko´llar, L.; Radics, L.
Inorg. Chim. Acta 1997, 265, 249. (o) Castellanos-Pa´ez, A.; Castillo´n,
S.; Claver, C.; van Leeuwen, P. W. N. M.; de Lange, W. G. J .
Organometallics 1998, 17, 2543.
Kalck et al. described the advantages of using di-
nuclear bridged rhodium thiolate complexes as catalyst
precursors in the hydroformylation reaction.5 A catalytic
cycle, based on spectroscopic evidence and theoretical
calculations, in which the dinuclear framework is
retained for all steps has been proposed for [Rh2(µ-SR)2-
(3) (a) Rigby, W.; Whyman, R. W.; Wilding, K. J . Phys. E: Sci.
Instrum. 1970, 3, 572. (b) Tinker, K.; Morris, D. E. Rev. Sci. Instrum.
1972, 7, 1024. (c) Walker, W. E.; Cosby, L. A.; Martin, S. T. (Union
Carbide Corp.) U.S. Patent 3 886 364, 1975. (d) Horvath, I. Organo-
metallics 1986, 5, 2333.
(2) (a) Elsevier, C. J . J . Mol. Catal. 1994, 92, 285. (b) Roe, D. C. J .
Magn. Reson. 1985, 63, 388.
(4) Schoemaker, G. C.; van Leeuwen, P. W. N. M. To be submitted
for publication.
10.1021/om990003o CCC: $18.00 © 1999 American Chemical Society
Publication on Web 05/06/1999

2108 Organometallics, Vol. 18, No. 11, 1999
Die´guez et al.
The complexes [Rh2{µ-S(CH2)3N(Me2)}2(COD)2],12 [Rh2{µ-
(CO)2(PR3)2] precursors. Kinetic studies and an inves-
tigation of catalyst crossover reactions provided evidence
that mononuclear species were present in a mixture of
the two complexes [Rh2(µ-SR)2(CO)2(P(OMe)3)2], in which
R ) tBu and Ph.6 More recently, Angelici et al.7
described how rhodium thiolate catalysts were im-
mobilized by tethering them onto silica which remained
active for several cycles. The selectivity and activity
were markedly affected by the phosphine donor. It was
suggested that under the reaction conditions mono-
nuclear thiolate species were formed.
Stanley et al. reported dinuclear species of a different
type, for the complex [Rh2(ndb)2(et,ph-P4)](BF4)2 (nbd
) norbornadiene; et,phP4 ) (Et2PCH2CH2)P(Ph)CH2P-
(Ph)CH2CH2PEt2) with a binucleating tetraphosphine
as a ligand. They proposed a catalytic cycle involving a
dinuclear species based on “in situ” NMR studies and
IR spectroscopic studies.8
S(CH2)2S}(COD)2],9b [Rh2{µ-S(CH2)4S}(COD)2]2, and [Rh2{µ-
9b
XANTOSS}(COD)2]210 were prepared according to established
methods. Oct-1-ene and hex-1-ene were acquired from Aldrich
and were percolated over neutral alumina before use in order
to remove peroxides.
Gas chromatography analyses were performed on a Hewlett-
Packard 5890A gas chromatograph using an Ultra-2 (5%
diphenylsilicone/95% dimethylsilicone) column (25 m length
× 0.2 mm i.d.) to separate the aldehydes.
IR spectra were recorded on a Nicolet 510 FTIR spectro-
photometer with a resolution of 2 cm-1. The GC-MS experi-
ments were recorded on a Hewlett-Packard 5890II gas chro-
matograph equipped with
spectrometer.
a Hewlett-Packard 5971 mass
Sta n d a r d Hyd r ofor m yla tion Exp er im en ts. A solution
of the substrate, the catalyst precursor, and the phosphorus
compound were placed in the evacuated autoclave. The gas
mixture was introduced and the system heated. When thermal
equilibrium was reached, the gas mixture was adjusted to the
desired pressure. After the reaction, the autoclave was cooled
to room temperature and depressurized. Samples were ana-
lyzed by gas chromatography.
Th e “in Situ ” HP IR Exp er im en ts.1k The HPIR spectra
were performed in an SS 316 55 mL autoclave equipped with
ZnS windows (700 cm-1, i.d. 10 mm, optical path length 0.4
mm), a mechanical stirrer, a temperature controller, and a
pressure device. Liquid or dissolved reagents (up to 1 mL) were
In recent years we have used precursor systems with
thiolate or dithiolate bridging ligands to hydroformylate
alkenes.9 We have also published one study on the role
of the dithiolate ligand under hydroformylation condi-
tions.1o In the present work, we report the results of the
study by HPIR spectroscopy of different systems con-
taining a thiolate or dithiolate bridge ligand, [Rh2{µ-
S(CH2)3N(Me2)}2(COD)2]/PR3 (R ) Ph, OPh),9a [Rh2{µ-
S(CH2)2S}(COD)2]/PR3 (R ) Ph, OPh, O-o-tBu),9c [Rh2{µ-
S(CH2)4S}(COD)2]2/PPh3,9c and [Rh2{µ-XANTOSS}-
added from
a separately pressurized reservoir. Rhodium
complexes [Rh2(µ-SR)2(COD)2] (0.04 mmol) and the corre-
sponding phosphorus ligand were dissolved in 15 mL of
2-methyltetrahydrofuran. The autoclave was closed and flushed
several times with the corresponding gas. After the autoclave
was pressurized and the mixture was heated, the autoclave
was placed in the infrared spectrometer and, while the sample
was stirred, the infrared spectra were recorded. Hydroformy-
lation studies were performed using the same equipment, and
the substrate was taken from the reservoir and added to the
reaction mixture by overpressure. Once this addition was
made, the reaction started, as was evidenced by a pressure
drop, and the spectra were recorded. The final solution was
analyzed by GC.
10
(COD)2]2/PPh3 (XANTOSS ) 9,9-dimethylxanthene-
4,5-dithiolate) under hydroformylation conditions in
order to obtain information about the species involved
in the catalytic reaction. For comparative purposes we
also studied the system [Rh(acac)(CO)2]/phosphorus
ligands, which is known to give mononuclear active
species [RhH(CO)n(PR3)4-n] (n ) 0-3) under hydro-
formylation conditions.11
Exp er im en ta l Section
Gen er a l Meth od s. All operations were performed under
an atmosphere of nitrogen or argon by using standard Schlenk
techniques. Solvents were dried and distilled before use.
[Rh(acac)(CO)2] was acquired from J ohnson Matthey, and
neutral Al2O3, PPh3, and P(OPh)3 were obtained from Aldrich.
Resu lts a n d Discu ssion
HP IR Stu d y of th e [Rh 2{µ-S(CH2)3N(Me2)}2-
(COD)2]/P P h 3 Syst em u n d er H yd r ofor m yla t ion
Con d ition s. We studied the reactivity of the thiolate
system [Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/PPh3 (1/PPh3)
under hydroformylation conditions. It had previously
been reported that it is an efficient catalyst precursor
for the hydroformylation of several olefins.9a The com-
plex was recovered unchanged after the catalytic reac-
tion.12a We studied the infrared spectra of system 1
under the reaction conditions that have previously been
described as the most suitable for “thiolate” hydro-
formylation catalysts (5-7 bar, 60-80 °C). The in situ
HPIR study involves three steps: (a) the addition of 2.5
bar of CO to 1, (b) the addition of the phosphorus ligand,
and (c) the addition of 2.5 bar of H2. The reactions are
carried out in 2-methyltetrahydrofuran, which due to
(5) (a) Dedieu, A.; Escafre, P.; Frances, J . M.; Kalck, Ph.; Thorez,
A. New J . Chem. 1986, 10, 631. (b) Escaffre, P.; Thorez, A.; Kalck,
Ph.; Besson, B.; Perron, R.; Colleuille, Y. J . Organomet. Chem. 1986,
302, C17. (c) Kalck, Ph. Polyhedron 1988, 7, 2441. (d) Kalck, Ph.; Peres,
Y.; Queau, R.; Molinier, J .; Escaffre, P.; Oliveira, E. L.; Peyrille, B. J .
Organomet. Chem. 1992, 426, C16.
(6) Davis, R.; Epton, J . W.; Southern, T. G. J . Mol. Catal. 1992, 77,
159.
(7) Gao, H.; Angelici, R. J . Organometallics 1998, 17, 3063.
(8) Matthews, R. C.; Howell, D. K.; Peng, W.-J .; Train, S. G.;
Treleaven, W. D.; Stanley, G. G. Angew. Chem., Int. Ed. Engl. 1996,
19, 35.
(9) (a) Polo, A.; Claver, C.; Castillo´n, S.; Ruiz, A.; Bayo´n, J . C.; Real,
J .; Mealli, C.; Masi, D. Organometallics 1992, 11, 3525. (b) Masdeu,
A. M.; Ruiz, A.; Castillo´n, S.; Claver, C.; Hitchcock, P. B.; Chaloner,
P. A.; Bo´, C.; Poblet, J . M.; Sarasa, P. J . Chem. Soc., Dalton Trans.
1993, 2689. (c) Aaliti, A.; Masdeu, A. M.; Ruiz, A.; Claver, C. J .
Organomet. Chem. 1995, 489, 101. (d) Claver, C.; Castillo´n, S.; Ruiz,
N.; Delogu, G.; Fabbri, D.; Gladiali, S. J . Chem. Soc., Chem. Commun.
1993, 1833. (e) Ruiz, N.; Aaliti, A.; Fornie´s-Camer, J .; Ruiz, A.; Claver,
C.; Cardin, C. J .; Fabbri, D.; Gladiali, S. J . Organomet. Chem. 1997,
545, 79. (f) Masdeu, A. M.; Orejo´n, A.; Ruiz, A.; Castillo´n, S.; Claver,
C. J . Mol. Catal. 1994, 94, 149. (g) Masdeu-Bulto´, A. M.; Orejo´n, A.;
Castillo´n, S.; Claver, C. Tetrahedron: Asymmetry 1995, 6, 1885. (h)
Castellanos-Pa´ez, A.; Castillo´n, S.; Claver, C. J . Organomet. Chem.
1997, 539, 1.
(11) Trzeciak, A. M.; Zio´lkowski, J . J .; Aygen, S.; van Eldik, R. J .
Mol. Catal. 1986, 34, 337.
(12) (a) Bayo´n, J . C.; Real, J .; Claver, C.; Polo, A.; Ruiz, A. J . Chem.
Soc., Chem. Commun. 1989, 1056. (b) Ferna´ndez, E.; Ruiz, A.;
Castillo´n, S.; Claver, C.; Piniella, J . F.; Alvarez-Larena, A.; Germain,
G. J . Chem. Soc., Dalton Trans. 1995, 2137.
(10) Die´guez, M. Ph.D. Thesis, Universitat Rovira i Virgili, Tarra-
gona, Spain, 1997.

Rh Complexes Containing Thiolate Bridge Ligands
Organometallics, Vol. 18, No. 11, 1999 2109
Sch em e 1
F igu r e 1. HPIR spectra: (a) reaction of 1 with CO and
PPh3; (b) reaction of 1 with CO, PPh3, and H2 after 2 h; (c)
reaction of 1 with CO, PPh3, and H2 after 18 h; (d) reaction
of 5 with CO, PPh3, and H2.
Ta ble 1. HP IR Resu lts of Ca ta lyst P r ecu r sor s
[Rh 2{µ-S(CH2)3N(Me)2}2(COD)2] (1) a n d
a
[Rh (a ca c)(CO)2] (5) w ith P P h 3
P (bar)
entry no.
1
precursor
CO
2.5
H2
IR (cm-1
)
1
2054 (s),b 1989 (s),b
1977 (s),b 1965 (s)c
2054 (m),b 2040 (w),d
1989 (s),b 1977 (s),b
1965 (s),c 1946 (w)d
2042 (m),d 1992 (s),d
1981 (s),d 1947 (s)d
2
1
5
2.5
5
2.5
5
the low polarity gives higher quality spectra and nar-
rower lines than tetrahydrofuran, the solvent used
previously in the catalytic studies.
3
Solu tion a : 2.5 ba r of CO. The HPIR spectrum at
80 °C of solution a shows three ν(CO) frequencies at
2072 (m), 2053 (s), and 2004 (s) cm-1 attributed to the
dinuclear rhodium complex [Rh2{µ-S(CH2)3N(Me2)}2-
(CO)4] (2).9a This complex retains the thiolate bridge and
contains two terminal CO ligands coordinated to each
rhodium (Scheme 1). This species was the only one
detected even under 15 bar of CO pressure and after 8
h of reaction.
Solu tion b: 2.5 ba r of CO, Ad d ition of P P h 3.
Subsequently, different excesses of PPh3 (PPh3/Rh ) 2,
5) were added to the solution at 80 °C. The HPIR
spectrum showed ν(CO) frequencies at 2054 (s), 1989
(s), 1977 (s), and 1965 (s) cm-1 (Figure 1a and Table 1,
entry 1). By comparison with data in the literature the
first three signals were attributed to the pentacoordi-
nated compound13 [Rh2{µ-S(CH2)3N(Me2)}2(CO)4(PPh3)2]
(3) (Scheme 1). The signal at 1965 (s) cm-1 was at-
tributed to the trans-dicarbonyl dinuclear mixed com-
plex [Rh2{µ-S(CH2)3N(Me2)}2}2(CO)2(PPh3)2] (4)9a (Scheme
1). It has been established that the decarbonylation of
compound 3, to give species 4, is a reversible process.13
a
Reaction conditions and abbreviations: 2-methyltetrahydro-
furan (15 mL), 0.5 mmol catalyst precursor, s ) strong, m )
b
medium, w ) weak, PPh3/Rh ) 2 and 5, T ) 80 °C. [Rh2{µ-S-
c
(CH2)3N(Me)2}2(CO)4(PPh3)2] (3). trans-[Rh2{µ-S(CH2)3N(Me)2}2-
(CO)2(PPh3)2] (4). d[RhH(CO)2(PPh3)2] (6).
The spectrum was unchanged after 18 h of reaction,
even after the pressure was raised to 10 bar and the
PPh3 was added before the CO gas pressure was
introduced.
Solu tion c: 2.5 ba r of CO, P P h 3, a n d 2.5 ba r of
H2. When the solution was pressurized with 2.5 bar of
H2 at 80 °C (5 bar of CO/H2), new weak absorptions
appeared at 2040 and 1946 cm-1 (Table 1, entry 2;
Figure 1b). The intensities of these bands increased with
time, and after 18 h the signals had become relatively
strong (Figure 1c). Since these new absorptions appear
only under H2 pressure, the most likely new species
contains a hydride ligand.
Th e System [Rh (a ca c)(CO)2]/P P h 3 u n d er Syn ga s
Con d ition s. To identify the new signals at 2040 and
1946 cm-1 in the previous system containing dithiolate
1, we performed the same set of experiments for the
system [Rh(acac)(CO)2]/PPh3 (5/PPh3). This system is
(13) Kalck, Ph.; Poilblanc, R. Inorg. Chem. 1975, 11, 2779.

2110 Organometallics, Vol. 18, No. 11, 1999
Die´guez et al.
Ta ble 2. Resu lts of Hyd r ofor m yla tion of 1-Hexen e w ith Ca ta lyst P r ecu r sor s 1, 10, 11, 17, a n d 5a
entry no.
precursor
t (h)
P (bar)
ligand
P/Rh
conversn (%)b
n (%)c
TOF (h-1
)
1
2
3
4
5
6
7
8
1
1
5
5
1
1
5
5
5
10
5
11
18
5
3
4
0.25
0.25
3
5
5
5
5
5
5
5
5
5
30
30
5
5
5
PPh3
PPh3
PPh3
PPh3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
PPh3
2
5
2
5
2
5
5
2
5
2
2
2
1
1
70
95
90
91
70
26
52
83
83
96
74
96
77
86
77
81
77
82
81
84
85
81
83
73
74
72
77
79
47
47
720
827
47
3
17
0.3
0.5
0.5
6
0.05
19
347
339
1510
8
740
3
9d
10e
11e
12e
13f
14f
PPh3
PPh3
PPh3
PPh3
3
0.75
51
115
a
Reaction conditions: 1-hexene (10 mmol), complex (0.05 mmol), toluene (15 mL), 80 °C, CO/H2 ) 1. Precursors: 1, [Rh2-
{µ-S(CH2)3N(Me)2}2(COD)2]; 5, [Rh(acac)(CO)2]; 10, [Rh2{µ-S(CH2)2S)(COD)2]; 11, [Rh2{µ-S(CH2)4S}(COD)2]2; 18, [Rh2(µ-SANTOSS)(COD)2]2.
b
d
Conversion to aldehyde measured by GC integral ratio without internal standard. c Percentage of linear product. Substrate/Rh ) 1000.
e Conditions: 1-hexene (10 mmol), complex (0.2 mmol). f Conditions: 1-hexene (10 mmol), complex (0.1 mmol), 65 °C. TOF ) mol of aldehyde
[mol of catalyst]-1 h-1
.
known to give mononuclear active species [RhH(CO)n-
the same regioselectivity may indicate that these sys-
tems involve the same active species.
(PR3)4-n] (n ) 0-3) under hydroformylation conditions.11
The argument that the selectivities of two systems
are similar remains circumstantial evidence to prove
that the catalyst is the same. Also, because the selectiv-
ity depends on the concentrations of the rhodium and
free ligand, the latter is influenced by the nonactive
rhodium species involved in metal-ligand equilibria.
This result shows that mononuclear species can be
formed from the dinuclear compound [Rh2{µ-S(CH2)3N-
(Me2)}2(COD)2] in the presence of PPh3 and that they
might participate in catalysis. However, the presence
of mononuclear hydrides is indicated more clearly by
the HPIR study. The lower activity observed for the
complex [Rh2{µ-S(CH2)3N(Me2)}2(COD)2] may be due to
the low concentration of the mononuclear hydrido
species, as detected by HPIR.
The solution obtained by adding an excess of PPh3
(P/Rh ) 5) to a solution of [Rh(acac)(CO)2] in 2-meth-
yltetrahydrofuran at 80 °C at 10 bar of syngas was
studied spectroscopically. The HPIR spectrum shows
four absorptions at 2042 (m), 1992 (s), 1981 (s), and 1947
(s) cm-1 (Figure 1d) which are attributed to the com-
pound [RhH(CO)2(PPh3)2] (6). This compound exists as
two trigonal-bipyramidal isomers in equilibrium (6a ,
which contains an equatorial and an apical phosphine
ligand, and 6b, which contains two equatorial phos-
phines) (Table 1, entry 3).1c The bands at 1992 and 1947
cm-1 are attributed to the conformer containing the
hydride trans to PPh3, 6a (Scheme 1), and the absorp-
tions at 2042 (m) and 1981 (s) cm-1 are attributed to
the geometry containing the hydride trans to the CO
ligand, 6b (Scheme 1).1o,14 Thus, we conclude that the
signals at 2040 and 1946 cm-1 in the HPIR of the
system [Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/PPh3 under hy-
droformylation conditions are due to the isomers 6a ,b
of the complex [RhH(CO)2(PPh3)2]. The absorptions
expected at 1992 and 1981 cm-1 for the complex [RhH-
(CO)2(PPh3)2] in the HPIR overlap with the signals at
1989 and 1979 cm-1 of the compound [Rh2{µ-S(CH2)3N-
(Me2)}2(CO)4(PPh3)2] (3).
In summary, the HPIR study in combination with the
results of the hydroformylation experiments using the
systems [Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/PPh3 and [Rh-
(acac)(CO)2]/PPh3 allow us to conclude that mononuclear
hydrido species are responsible for the catalytic activity
in the first of these two systems.
Th e System [Rh 2{µ-S(CH2)3N(Me2)}2(COD)2]/
P (OP h )3 u n d er Syn ga s Con d ition s. In the presence
of phosphites, complex 1 was also shown to lead to active
hydroformylation catalysts.9a Rhodium phosphite com-
plexes are well-known catalysts for the hydroformyla-
tion reaction, and some of them are extremely active.1k
Therefore, the suspicion arises that the activity of the
dimeric rhodium thiolate complexes might be caused by
the formation of traces of mononuclear rhodium hydride
species. Thus, a sequence of reactions similar to the ones
described above was carried out and the IR spectra were
recorded in situ in the high-pressure IR cell. First,
complex 1 was treated with 2.5 bar of CO gas (solution
a, not shown), (b) triphenyl phosphite was added, and
(c) hydrogen was added.
Solu tion b: 2.5 ba r of CO P r essu r e a n d P (OP h )3.
The addition of 2.5 bar of CO to a solution of 2-meth-
yltetrahydrofuran containing [Rh2{µ-S(CH2)3N(Me2)}2-
(COD)2] and P(OPh)3 (P/Rh ) 5) showed a strong band
at 1994 cm-1, which was attributed to the complex
trans-[Rh2{µ-S(CH2)3N(Me2)}2(CO)2(P(OPh)3)2] (7)9a
(Scheme 2; Table 3, entry 4; Figure 2a). We also
Thus, the HPIR experiments show that system 1
under hydroformylation conditions generates dinuclear
thiolate-bridged carbonyl phosphine (3 and 4) and
mononuclear rhodium(I) hydride (6) species.
To determine which of these species are the active
ones in hydroformylation, comparative catalytic activity
studies with precursors 1 and [Rh(acac)(CO)2]/PPh3
were performed under the same conditions. Table 2
shows the results of 1-hexene hydroformylation with the
catalyst precursor [Rh2{µ-S(CH2)3N(Me2)}2(COD)2] (1)
and [Rh(acac)(CO)2] (5) (entries 1-4).
We assume that, the conditions being the same, the
regioselectivities observed are indicative of the nature
of the active species in the catalytic process. Therefore,
the fact that experiments 1-2 and 3-4 (Table 2) provide
(14) van der Veen, L. A.; Boelen, 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.

Rh Complexes Containing Thiolate Bridge Ligands
Organometallics, Vol. 18, No. 11, 1999 2111
Sch em e 2
F igu r e 2. HPIR spectra: (a) reaction of 1 with CO and
P(OPh)3; (b) reaction of 1 with CO, P(OPh)3, and H2; (c)
reaction of 5 with CO, P(OPh)3, and H2.
sure of CO/H2 to 19 bar, these new absorptions increase
immediately. At 50 °C these species appear after 2 h of
reaction, and at 40 °C they appear after 3 h. When the
temperature is kept at 50 °C by applying P/Rh ) 2,
these new species do not appear until after 4 h.
Th e System [Rh (a ca c)(CO)2]/P (OP h )3 u n d er Syn -
ga s Con d ition s. As in the previous PPh3 study, to
compare the system with a conventional rhodium sys-
tem which provides mononuclear species, the spectros-
copy study of [Rh(acac)(CO)2]/P(OPh)3 (P/Rh ) 5) in
methyltetrahydrofuran was carried out at 80 °C under
10 bar of syngas.
The HPIR spectrum showed five signals at 2070,
2053, 2034, 2018, and 1992 cm-1 (Table 3, entry 6,
Figure 2c). The absorptions at 2034 and 1992 cm-1 are
attributed to the trigonal-bipyramidal geometry of the
compound [RhH(CO)2(P)2] (8a ), which has the phospho-
rus ligands in equatorial and axial positions, and the
signals at 2053 and 2018 cm-1 are attributed to the
hydride complex in which the phosphorus ligands are
in equatorial-equatorial positions, 8b1a,11,15 (Scheme 2).
The absorption at 2070 cm-1 corresponds to ν(CO) of
the compound [RhH(CO)(P)3] (9)16 (Scheme 2).
Comparing the frequencies for the catalytic system
[Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/P(OPh)3 (Table 3, entry
5) and the precursor [Rh(acac)(CO)2]/P(OPh)3 (Table
3, entry 6) clearly shows that mononuclear hydrido
pentacoordinate species are present in the solution of
the dinuclear thiolate precursor (2070, 2034, 2018, and
2054 cm-1).
Ta ble 3. HP IR Resu lts of Ca ta lyst P r ecu r sor s
[Rh 2{µ-S(CH2)3N(Me)2}2(COD)2] (1) a n d
a
[Rh (a ca c)(CO)2] (5) w ith P (OP h )3
P (bar)
entry no.
4
precursor
CO
2.5
H2
IR (cm-1
)
1
2072 (m),b 2054 (m),b
2004 (m),b 1994 (s)c
2070 (w),e 2054 (m),b,d
2034 (w),d 2018 (m),d
2004 (m),b 1994 (s)c
2070 (m),e 2053 (w),d
2034 (m),d 2018 (m),d
1992 (m)d
5
6
1
5
2.5
5
2.5
5
a
Reaction conditions and abbreviations: 2-methyltetrahydro-
furan (15 mL), 0.5 mmol of catalyst precursor, s ) strong, m )
b
medium, w ) weak, P(OPh)3/Rh ) 2 and 5, T ) 80 °C. [Rh2{µ-
S(CH2)3N(Me)2}2(CO)4] (2). c trans-[Rh2{µ-S(CH2)3N(Me)2}2(CO)2-
d
(P(OPh)3)2] (7). [RhH(CO)2(P(OPh)3)2] (8). e [RhH(CO)(P(OPh)3)3]
(9).
observed weak absorptions that were attributed to the
tetracarbonyl species [Rh2{µ-S(CH2)3N(Me2)}2(CO)4] (2).
Solu tion c: 2.5 ba r of CO, P (OP h )3, a n d 2.5 ba r
of H2. When this solution was pressurized with 2.5 bar
of H2 at 80 °C (5 bar of CO/H2), new signals appeared
at 2070, 2034, and 2018 cm-1 and the intensity of the
band at 2054 cm-1 increased (Table 3, entry 5; Figure
2b). The new absorptions appeared immediately and
increased with time.
The formation of the species leading to these new
absorptions depends on pressure, temperature, and the
ratio of phosphite to rhodium. Thus, if we raise the pres-
(15) (a) Trzeciak, A. M.; Zio´lkowski, J . J . J . Mol. Catal. 1983, 19,
41. (b) Buisman, G. J . H. Ph.D. Thesis, University of Amsterdam, 1995.
(16) Trzeciak, A. M. J . Organomet. Chem. 1990, 390, 105.

2112 Organometallics, Vol. 18, No. 11, 1999
Die´guez et al.
Ta ble 4. HP IR Resu lts of Ca ta lyst P r ecu r sor s
[Rh 2(µ-S(CH2)2S)(COD)2] (10) a n d
The assignment of these frequencies to mononuclear
species is in agreement with the observation that their
concentration increases when the pressure increases but
decreases when the ratio P/Rh or the temperature
decreases.
[Rh 2(µ-S(CH2)4S)(COD)2]2 (11)a
P (bar)
entry no.
7b
precursor
CO
15
H2
IR (cm-1
)
2072 (m),c 2059 (s),c
2018 (s)c
Hydroformylation results confirm the spectroscopic
observations in HPIR, since the regioselectivities (Table
2, entries 5-6 and 7-8) obtained with the systems [Rh2-
{µ-S(CH2)3N(Me2)}2(COD)2]/P(OPh)3 and [Rh(acac)-
(CO)2]/P(OPh)3 are the same. This suggests that the
mononuclear species are the same. Since the HPIR
study shows that the mononuclear species is present
only in a low concentration under hydroformylation con-
ditions, a catalytic experiment with a similarly low
precursor concentration [Rh(acac)(CO)2]/P(OPh)3 was
carried out using a high ratio of substrate to precursor
(entry 9). The considerable activity of this system shows
that the mononuclear species observed in HPIR can fully
account for the activity of the catalyst precursor [Rh2-
{µ-S(CH2)3N(Me2)}2(COD)2].
H P IR St u d y of Din u clea r or Tet r a n u clea r
Br id ged Rh od iu m Dith iola te System s w ith P P h 3
u n d er Hyd r ofor m yla tion Con d ition s. A priori, com-
plexes containing bridging alkanedithiolates could be
more resistant to bridge cleavage than the correspond-
ing monothiolates because of their chelating properties.
Thus, we considered it of interest to study dithiolate-
phosphine systems under hydroformylation conditions.
10
8
9
10
10
15
15
2072 (w),c 2057 (m),c,d
2018 (w),c 1996 (m),d
1982 (m),d 1966 (s)e
2072 (w),c 2057 (m),c,d
2018 (w),c 1996 (m),d
1982 (m),d 1966 (s)e
2073 (w),f 2050 (s),f
2007 (w)f
15
11b
12
11
11
11
2.5
2.5
2.5
2050 (m),h 1988 (m),h
1974 (s),h 1964 (s)g
2050 (m),h 1988 (m),h
1974 (s),h 1964 (s)g
13
2.5
a
Reaction conditions: 2-methyltetrahydrofuran (15 mL), 0.2
mmol catalyst precursor. s ) strong, m ) medium, w ) weak.
b
PPh3/Rh ) 2 and T ) 80 °C. Experiments without PPh3. c [Rh2(µ-
d
S(CH2)2S)(CO)4] (12). [Rh2(µ-S(CH2)2S)(CO)4(P)2] (14). e [Rh2(µ-
g
S(CH2)2S)(CO)2(P)2] (15). f [Rh2(µ-S(CH2)4S)(CO)4] (13). [Rh2(µ-
h
S(CH2)4S)(CO)2(P)2] (16). [Rh2(µ-S(CH2)4S)(CO)4(P)2] (17).
Sch em e 3
The systems studied are known to have different
activities in 1-hexene hydroformylation, and their activ-
ity heavily depends on the ring size of the bridging
dithiolate ligand. Thus, the catalyst precursor [Rh2{µ-
S(CH2)2S}(COD)2]/PPh3 (10) shows significant activity
only at 30 bar and 80 °C and with the ligand-to-metal
ratio P/Rh ) 2, whereas the precursors [Rh2{µ--
S(CH2)4S}(COD)2]2/PPh3 (11) and [Rh2(µ-XANTOSS)-
(COD)2]2/PPh3 (18) are active at 5 bar, 80 °C, and P/Rh
) 2 and at 5 bar, 65 °C, and P/Rh ) 1, respectively.9c,10
H P IR St u d y of [R h 2{µ-S(CH 2)2S}(COD)2]/P P h 3
a n d [Rh 2{µ-S(CH2)4S}(COD)2]2/P P h 3 u n d er Hyd r o-
for m yla tion Con d ition s. To obtain information about
the species present during the hydroformylation reac-
tion, we measured the HPIR spectra of the solutions,
which were prepared as follows: (a) addition of CO, (b)
addition of PPh3, and (c) addition of H2.
Solu t ion a : Ad d it ion of CO. To obtain solutions
similar to the ones used in the catalytic experiments,
15 bar of CO was added to the solution of [Rh2{µ-
S(CH2)2S}(COD)2] and 2.5 bar of CO to the solution of
[Rh2{µ-S(CH2)4S}(COD)2]2 in 2-methyltetrahydrofuran
at 80 °C. The HPIR spectra of the solutions show three
frequencies attributed to the dinuclear tetracarbonyl-
rhodium complexes [Rh2{µ-S(CH2)nS}(CO)4]9c (n ) 2, 12;
n ) 4, 13) (Table 4, entries 7 and 11, respectively).
[Rh2{µ-S(CH2)4S}(COD)2]2. In this case the HPIR shows
a new ν(CO) frequency at 1964 cm-1, which is attributed
to complex 16,9c and three peaks at 2050, 1988, and
1974 cm-1 corresponding to the pentacoordinated com-
plex 17 (Table 4, entry 12; Scheme 3).
Solu tion c: Ad d ition of H2. When the solutions of
the compounds [Rh2{µ-S(CH2)2S}(COD)2] and [Rh2{µ-
S(CH2)4S}(COD)2]2 were pressurized with 15 bar of H2
(total pressure 30 bar of CO/H2) and 2.5 bar of H2 (total
pressure 5 bar of CO/H2), respectively (Table 4, entries
9 and 13), the HPIR spectra showed no change, not even
after 48 h of reaction time (Scheme 3).
For the compound [Rh2{µ-S(CH2)2S}(COD)2]/PPh3
a
Solu tion b: Ad d ition of P P h 3. Subsequently, an
excess of PPh3 (P/Rh ) 2) was added at 80 °C. In the
case of the compound [Rh2{µ-S(CH2)2S}(COD)2] the
HPIR showed new ν(CO) frequencies at 2057, 1996, and
1982 cm-1, which were attributed to the pentacoordi-
nated compound [Rh2{µ-S(CH2)2S}(CO)4}(PPh3)2] (14),
and a strong band at 1966 cm-1 corresponding to the
trans-dicarbonyl dinuclear mixed complex [Rh2{µ-
S(CH2)2S}(CO)2(PPh3)2] (15)9c (Table 4, entry 8; Scheme
3). Analogous species were detected for the compound
similar series of experiments was carried out using a
high ratio of H2 to CO (H2/CO ) 8, total pressure 30
bar). After 2 h, the spectra showed a weak absorption
at 2048 cm-1 that can be attributed to the mononuclear
hydride species 6b.
These HPIR experiments show that under hydro-
formylation conditions only dinuclear species are de-
tected and that, if mononuclear species are present,
their concentration is below the HPIR detection limit.
Thus, while there is not definite proof of the nature of

Rh Complexes Containing Thiolate Bridge Ligands
Organometallics, Vol. 18, No. 11, 1999 2113
Sch em e 4
propensity of these systems to give mononuclear active
species such as the ones found in the HPIR study.
Apparently, the geometry enforced by the XANTOSS
backbone leads to bridged thiolate complexes that are
only moderately stable with respect to mononuclear
hydrido carbonyls.
Deu ter iofor m yla tion Exp er im en ts. In an attempt
to find out the role that dinuclear rhodium thiolate
species play, we conducted a series of experiments using
D2 and mixtures of D2 and H2 in hydroformylation
reactions. Inspection of the mechanisms proposed for
dirhodium thiolate complexes5c and monohydride rhod-
ium species17 reveals that the incorporation of D2/H2
during the hydroformylation reaction may be different
for the two mechanisms. In the generally accepted
mechanism for rhodium hydride catalysts18 the two
hydrogen atoms built into the aldehyde product stem
from two different dihydrogen molecules (Figure 3).
Therefore, in a mixture of H2 and D2 (1/1) a statistical
mixture of un-, mono-, and dideuterated products forms,
which may be disturbed by kinetic isotope effects (an
isotope effect was observed for the bulky rhodium
phosphite catalyst).1f Another provision to be made is
that the addition of hydride to alkene is irreversible,
which is the case if the temperature is sufficiently low.19
Indeed, a reaction carried out using 1-octene (5: 2 PPh3,
H2/D2/CO (1/1/1), at 50 °C, 5 bar total pressure) gave a
statistical mixture of deuterated products.
The mechanism proposed for dinuclear thiolate-
bridged complexes5c is a special case (Figure 4). In this
mechanism, dihydrogen is added to the dimer and both
hydrogens are incorporated in the same product mol-
ecule, provided that no H/D exchange occurs between
the dihydrido complex and free dihydrogen. Thus,
hydroformylation using a mixture of H2 and D2 (1/1)
may lead to products containing either two added
hydrogen atoms or two added deuterium atoms.
The experiments were carried out using the thiolate-
bridged rhodium complex 19a for 3 h at 5 bar of syngas,
50 °C, and a P(OPh)3/Rh ratio of 2. Under these mild
conditions the dimers should be the active species. The
HPIR study indeed showed that the mononuclear spe-
cies only reached the detection limit after 4 h of reaction.
The crude mixture resulting from the hydroformyla-
tion experiments using [Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/
PPh3 (P/Rh ) 2) as the catalyst precursor were analyzed
by GC-MS, either directly, using the aldehydes, or
indirectly, using the alcohols obtained by reduction with
LiAlH4. The results were the same for both methods.
The following experiments were performed: (1) pres-
surization of the system with 5 bar of CO/D2 (1/1) and
subsequent addition of 1-octene and (2) pressurization
of the system with 5 bar of CO/H2/D2 (1/1/1) followed
by addition of 1-octene. The GC-MS trace in experiment
the active species, the hydroformylation results give
some clue about what might be going on. The regiose-
lectivities for the systems [Rh2{µ-S(CH2)2S}(COD)2]/
PPh3 and [Rh(acac)(CO)2]/PPh3 are the same (Table 2,
entries 10 and 11) and are typical of a Rh/PPh3 catalyst.
The activities obtained using precursors 10 and 11 are
extremely low (entries 10-12). The activity of the
dithiolate systems is lower than that of the thiolate
systems [Rh2{µ-S(CH2)3N(Me2)}2(COD)2]/phosphorus
ligand (and precursor 5) and may be attributed to the
high stability toward cleavage of the dithiolate systems,
which gives the active mononuclear hydrido species. In
complexes 10 and 11 the dithiolates act as bridging
ligands, forming five- and six-membered rings with the
metal, which lead to more stable complexes.9b Their
activities amount to only 1% of those of the active
hydride species. Since the detection limit under the
present conditions is 5%, hydride formation cannot be
proven for these systems.
When the pressure is released, the dinuclear com-
pounds [Rh2(µ-thiolate)2(CO)4(PPh3)2] are re-formed in
solution.5c,9c,12a
HP IR St u d y of t h e [R h 2(µ-XANTOSS)(COD)2]2/
P P h 3 Ca ta lytic System u n d er Hyd r ofor m yla tion
Con d ition s. This system is very active for the hydro-
formylation of 1-hexene under very mild conditions (5
bar of CO/H2 and 65 °C).
The addition of 2.5 bar of CO to a solution of [Rh2(µ-
XANTOSS)(COD)2]2 (18) (Scheme 4) gives an HPIR
spectra which shows six signals assigned to the com-
pound [Rh2(µ-XANTOSS)(CO)4]2 (19).10
The subsequent addition of PPh3 (P/Rh ) 1) to the
solution at 65 °C showed three new peaks at 1992 (s),
1982 (s), and 1970 (m) cm-1, which are attributed to
the complex [Rh2(µ-XANTOSS)(CO)2(PPh3)2]2 (20).
When the solution was pressurized with 2.5 bar of
H2 at 65 °C (total pressure 5 bar of CO/H2), the HPIR
spectrum immediately showed new, strong signals at
2040 and 1947 cm-1 characteristic of the mononuclear
species [RhH(CO)2(PPh3)2]. The regioselectivity obtained
in the corresponding hydroformylation experiment is the
same as that for [Rh(acac)(CO)2] (Table 2, entries 13
and 14). This is in agreement with the presence of the
mononuclear pentacoordinate rhodium species. The
catalyst precursor [Rh2(µ-XANTOSS)(COD)2]2/PPh3 is
more active than the precursor systems [Rh2{µ-S-
(CH2)2S}(COD)2]/PPh3 and [Rh2{µ-S(CH2)4S}(COD)2]2/
PPh3 (Table 2, entries 10, 12, and 13). This reflects the
(17) (a) O’Connor, C.; Yagupsky, G.; Evans, D.; Wilkinson, G. J .
Chem. Soc. D 1968, 420. (b) Evans, D.; Yaguspsky, G.; Wilkinson, G.
J . Chem. Soc. A 1968, 2660. (c) Evans, D.; Osborn, J . A.; Wilkinson,
G. J . Chem. Soc. A 1968, 3133. (d) Brown, C. K.; Wilkinson, G. J . Chem.
Soc. A 1970, 2753.
(18) van Leeuwen, P. W. N. M.; van Koten, G. In Catalysis: An
Integrated Approach to Homogeneous, Heterogeneous and Industrial
Catalysis, 2nd ed.; van Santen, R. A., Moulijn, J . A., van Leeuwen, P.
W. N. M., Eds.; Elsevier: Amsterdam, 1995; Chapter 6.
(19) (a) Uccello-Baretta, G.; Lazzaroni, R.; Settambolo, R.; Salvadori,
P. J . Organomet. Chem. 1991, 417, 111. (b) Casey, C. P.; Petrovich, L.
M. J . Am. Chem. Soc. 1995, 117, 6007. (c) Horiuchi, T. E.; Shirakawa,
E. K.; Nozaki, K. H.; Takaya, H. Organometallics 1997, 16, 2981.

2114 Organometallics, Vol. 18, No. 11, 1999
Die´guez et al.
F igu r e 3. Presently accepted mechanism of hydroformylation catalyzed by [RhH(CO)2(PPh3)2].
F igu r e 4. Catalytic cycle of hydroformylation proposed for dinuclear thiolate complexes [Rh2(µ-SR)2(CO)2L2].
1 shows only one peak M, for the dideuterated com-
pound, and no signals were observed at M - 1 or M +
1, which indicates that no isomerization has taken place.
For experiment 2, we obtained a statistical distribution
of approximately 1/2/1 for M - 1, M, and M + 1,
corresponding to the peaks of alcohol, monodeuterio
alcohol, and dideuterio alcohol. This result is in ac-
cordance with the presence of a mononuclear rhodium

Rh Complexes Containing Thiolate Bridge Ligands
Organometallics, Vol. 18, No. 11, 1999 2115
hydride catalyst. If the reaction mechanism involved
dinuclear species,5c only dideuterated alcohols or dihy-
drogenated alcohols would be expected. In a separate
experiment we tried to observe the putative dihydrido-
dirhodium intermediate. The dinuclear [Rh2(µ-SC3H7)2-
(CO)4], which is similar to complex 2, was studied using
HP NMR spectroscopy under pressures as high as 50
PPh3 and P(OPh)3 are added to the thiolate compounds
1 under hydroformylation conditions, mononuclear spe-
cies are formed, which are responsible for the hydro-
formylation activity.
HPIR spectroscopic studies of the more stable dithio-
late compounds indicate that their behavior under
hydroformylation conditions depends on their stability
toward syngas. Thus, complexes 10 and 11 are more
resistant to the dithiolate cleavage than is compound
18. In all cases the activity and selectivity observed
correlates with the concentration of monomeric rhodium
hydride species. For 10 and 11 no hydride was observed
under the conditions applied, but the activitiy of these
systems was correspondingly low.
1
bar of H2, but no change was detected in the H or the
31P spectra in a temperature range of 25-80 °C.20
In summary, the deuterioformylation experiments
indicate that mononuclear species formed under hydro-
formylation conditions are most probably responsible for
the activity of the catalyst, thus supporting the outcome
of the HPIR spectroscopy and hydroformylation experi-
ments.
Ack n ow led gm en t. We thank the Ministerio de
Educacio´n y Ciencia and the Generalitat de Catalunya
for financial support (Grant No. QFN-95-4725-C03-2;
CICYT-CIRIT). We thank Oscar Pamies and Wim G. J .
de Lange for their valuable assistance.
Con clu sion s
HPIR spectroscopic studies and hydroformylation
experiments revealed that, when phosphorus ligands
(20) Die´guez, M.; Mhammedi, S.; Bayo´n J . C.; van Leeuwen, P. W.
N. M. Unpublished results.
OM990003O