Kinetics and mechanisms of homogeneous catalytic reactions. Part 6. Hydroformylation of 1-hexene by use of Rh(acac)(CO)2/dppe [dppe = 1,2-bis(diphenylphosphino)ethane] as the precatalyst

Article abstract of DOI:10.1016/j.molcata.2007.01.044

A kinetic study of the homogeneous hydroformylation of 1-hexene to the corresponding aldehydes (heptanal and 2-methyl-hexanal) was carried out using a rhodium catalyst formed by addition of 1 equiv. of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh(acac)(CO)₂ under mild reaction conditions (80 °C, 1-7 atm H₂ and 1-7 atm CO) in toluene; in all cases linear to branched ratios were close to 2. The reaction rate is first-order in dissolved hydrogen concentration at pressures below 3 atm, but independent of this parameter at higher pressures. In both regimes (low and high H₂ pressure), the initial rate was first-order with respect to the concentration of Rh and fractional order with respect to 1-hexene concentration. Increasing CO pressure had a positive effect on the rate up to a threshold value above which inhibition of the reaction was observed; the range of positive order on CO concentration is smaller when the total pressure is increased. The kinetic data and related coordination chemistry are consistent with a mechanism involving RhH(CO)(dppe) as the active species initiating the cycle, hydrogenolysis of the acyl intermediate as the rate-determining step of the catalytic cycle at low hydrogen pressure, and migratory insertion of the olefin into the metal-hydride bond as rate limiting at high hydrogen pressure. This catalytic cycle is similar to the one commonly accepted for RhH(CO)(PPh₃)₃ but different from previous proposals for Rh-diphosphine catalysts.

Full text of DOI:10.1016/j.molcata.2007.01.044

Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
Kinetics and mechanisms of homogeneous catalytic reactions
Part 6. Hydroformylation of 1-hexene by use of Rh(acac)(CO) /dppe
2
[dppe = 1,2-bis(diphenylphosphino)ethane] as the precatalyst
a,∗
a
a
Merlin Rosales , Angel Gonz a´ lez , YusbeIi Guerrero ,
a
b
In e´ s Pacheco , Roberto A. S a´ nchez-Delgado
a
La Universidad del Zulia (L.U.Z.), Facultad Experimental de Ciencias, Departamento de Qu ´ı mica,
Laboratorio de Qu ´ı mica Inorg a´ nica, Apdo 526, Maracaibo, Venezuela
b
Chemistry Department, Brooklyn College and The Graduate Center, City University of New York, New York 11210, United States
Received 17 February 2006; received in revised form 29 January 2007; accepted 30 January 2007
Available online 4 February 2007
Abstract
A kinetic study of the homogeneous hydroformylation of 1-hexene to the corresponding aldehydes (heptanal and 2-methyl-hexanal) was carried
out using a rhodium catalyst formed by addition of 1 equiv. of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh(acac)(CO)
2
under mild reaction
◦
conditions (80 C, 1–7 atm H
2
and 1–7 atm CO) in toluene; in all cases linear to branched ratios were close to 2. The reaction rate is first-order in
dissolved hydrogen concentration at pressures below 3 atm, but independent of this parameter at higher pressures. In both regimes (low and high
pressure), the initial rate was first-order with respect to the concentration of Rh and fractional order with respect to 1-hexene concentration.
H
2
Increasing CO pressure had a positive effect on the rate up to a threshold value above which inhibition of the reaction was observed; the range
of positive order on CO concentration is smaller when the total pressure is increased. The kinetic data and related coordination chemistry are
consistent with a mechanism involving RhH(CO)(dppe) as the active species initiating the cycle, hydrogenolysis of the acyl intermediate as the
rate-determining step of the catalytic cycle at low hydrogen pressure, and migratory insertion of the olefin into the metal-hydride bond as rate
limiting at high hydrogen pressure. This catalytic cycle is similar to the one commonly accepted for RhH(CO)(PPh
proposals for Rh-diphosphine catalysts.
3 3
) but different from previous
©
2007 Elsevier B.V. All rights reserved.
Keywords: Hydroformylation; Rhodium complexes; Kinetics; Mechanisms; 1-Hexene
1
. Introduction
polyvinyl chloride (PVC) plasticizers and detergents [1].
The hydroformylation of alkenes is a well-known synthetic
RCH = CH2 + H2/CO
tool for a wide range of organic molecules of high commercial
value, as well as one of the largest scale applications of homo-
geneous catalysis in industry. The major capacity is based on
the low-pressure oxo process (LPO) using phosphine-modified
rhodium carbonyls as catalysts. LPO is used to convert light
olefins (mainly C2 and C3) into C3 and C4 aldehydes by their
reaction with syn-gas (CO + H2) (Eq. (1)); these aldehydes are
mainly converted to Cn>8 alcohols used in the manufacture of
→
R-CH2-CH2-CHO + R-CH(CHO)-CH3,
R = H, CH3
(1)
The most widely used ligand in LPO is triphenylphosphine
and RhH(CO)(PPh3)3 is recognized as one of the most impor-
tant catalysts for the homogeneous hydroformylation of alkenes.
In recent years, new applications for high added-value interme-
diates for fine chemicals and pharmaceuticals have emerged in
literature [2], e.g. the hydroformylation of styrene or its deriva-
tives to obtain aryl propionaldehydes, which on subsequent
oxidation can give aryl propionic acid derivatives (e.g. ibupro-
fen from p-isobutylstyrene). Van Leeuwen and Claver recently
∗
Corresponding author.
E-mail address: merlin2002@cantv.net (M. Rosales).
1
381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.01.044

2
42
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
published a comprehensive review dealing with recent advances
in homogeneous and biphasic hydroformylation of olefins with
rhodium complexes, including kinetic and mechanistic aspects
typical experiment, a solution containing the catalyst, 1-hexene,
n-heptane (as internal standard) and toluene as the solvent (to
30 mL total volume) was placed in the reactor. The solution
was carefully deoxygenated by flushing with argon, stirring was
commenced and the reactor was heated to the desired tempera-
ture. When the reaction temperature was reached, a mixture of
CO and H2 in the required proportion and pressure was intro-
duced into the autoclave; this moment was taken as the zero time
of the reaction. Each run was followed by taking liquid samples
at regular intervals of time, and analyzing them by gas chro-
matography using a 610 Series UNICAM gas chromatograph
fitted with a thermal conductivity detector and a 3 m 10% SE-30
on Supelcoport glass column, with helium as the carrier gas; the
chromatograph was coupled to a UNICAM 4815 data system.
The supply of CO/H2 from a high-pressure reservoir was
maintained at constant pressure and molar ratio throughout the
reaction. Each run was repeated at least twice in order to ensure
reproducibility of the results. All the reactions were carried out
for short periods of time to maintain the conversion in the liquid
phase at no more than ca. 10% in order to perform a kinetic anal-
ysis based on the initial rate method [19]. The data for 1-hexene
hydroformylation were plotted as molar concentration of the
corresponding products vs. time yielding straight lines, which
were fitted by conventional linear regression programs. Initial
reaction rates were obtained from the corresponding slopes. The
hydrogen and carbon monoxide concentrations in the reaction
medium were calculated from solubility data reported in the
literature [20].
[
3].
In spite of the industrial importance of olefin hydroformyla-
tion and the large number of publications in this field published
over several decades, relatively few studies have been devoted
to the kinetics of this reaction, and most of them involve
RhH(CO)(PPh3)3 [4–9]; kinetic and mechanistic studies of
hydroformylation with catalysts containing bulky phosphites
[
10,11], phosphine-phosphite [12] or diphosphites [13], 1,2,5-
triphenyl-1H-phosphole [14] and triphenylarsine [15] as ligands
have also been reported. Of particular relevance to the present
work, hydroformylation by rhodium–diphosphine systems has
been extensively studied over the years and reasonable catalytic
cycles have been proposed [1,3,16]. A recent publication from
the group of Claver on the mechanism of hydroformylation by
rhodium–diphosphine catalysts [17a] and the book on Rh hydro-
formylation by Van Leeuwen and Claver published in 2000 [3]
both point out that the kinetics of hydroformylation with these
systems have not been discussed. Another review by Green et
al. from 2004 [17b] summarizes the accumulated knowledge
about hydroformylation mechanisms by rhodium–diphosphine
catalysts, noting the influence of bite angles on the activity
and selectivity, as well as the agreement of theoretical studies
with previously proposed catalytic cycles; no mention is made
of any previous study on the kinetics of hydroformylation by
Rh–diphosphine systems.
In the present paper, we describe such a study of the
hydroformylation of 1-hexene using as the precatalyst the
system formed by addition of 1 equiv. of 1,2-bis(diphe-
nylphosphine)ethane (dppe) to Rh(acac)(CO)2 and propose a
mechanism consistent with the kinetic data and with some
related coordination chemistry. This mechanism is discussed in
reference to previous mechanistic proposals for similar cata-
lysts. In the accompanying subsequent paper of this series, a
similar study is reported for the hydroformylation of 1-hexene
2
.3. Study of the interaction of Rh(acac)(CO)2 with each
component of the catalytic mixture
2
.3.1. Interaction of Rh(acac)(CO)2 with dppe
To a solution of Rh(acac)(CO)2 (100 mg, 0.38 mmol) in ben-
zene (10 mL) was added dppe (160 mg, 0.40 mmol) and the
mixture was stirred vigorously under argon at room tempera-
ture producing a yellow solution, which was evaporated under
vacuum to ca. one-third of its initial volume. A yellow precipi-
tate was obtained by addition of n-pentane; it was washed with
n-pentane and diethylether and dried in vacuo.
by cationic complexes [M(COD)(PPh3)2]PF , M = Rh, Ir.
6
2
. Experimental
2
.1. Instruments and materials
2
.3.2. Interaction of Rh(acac)(CO)2 with dppe under
syn-gas
Rh(acac)(CO)2 (103 mg, 0.38 mmol) and dppe (160 mg,
.40 mmol) in toluene (10 mL) were introduced in a
All manipulations were conducted with rigorous exclusion of
air using Schlenck techniques. 1-Hexene was purified by distil-
lation under reduced pressure. Solvents were purified by known
procedures. Rh(acac)(CO)2 was prepared by a published pro-
cedure [18]. NMR and IR spectra were recorded on a Bruker
AM-300 spectrometer and a Shimadzu 8300 FT-IR instrument,
respectively.
0
Fischer–Porter reactor, charged with 5 atm of syn-gas and heated
at 100 C for 2 h; the resulting solution was dried under a stream
of syn-gas.
◦
2.3.3. Interaction of RhH(CO)2(dppe) with PPh3
2
.2. Procedure for kinetic measurements
RhH(CO)2(dppe) was prepared in situ as described before
but 1 equiv. of PPh3 was further added (105 mg, 0.40 mmol). The
resulting solution was dried under a stream of syn-gas. This reac-
tion was also carried out in toluene-d8, the solution transferred
to a NMR tube under a syn-gas atmosphere and immediately
Kinetic experiments were carried out in a high-pressure reac-
tor, supplied by Parr Instruments, which was provided with an
arrangement for sampling of liquid contents, automatic tem-
perature and pressure control and variable stirrer speed. In a
1
31
1
monitored by H and P{ H} NMR.

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
243
Table 1
Table 2
Effect of the total pressure of syn-gas on the hydroformylation of 1-hexene
catalyzed by Rh(acac)(CO)2/dppe
Effect of the dissolved hydrogen concentration on the rate of the hydroformy-
lation of 1-hexene catalyzed by Rh(acac)(CO)2/dppe
5
−1
3
5
−1
p (atm)
ri (×10 M s
)
p (atm)
[H2] (×10 M)
ri (×10 M s
)
1
2
4
6
.4
.8
.1
.9
2.56
5.30
10.20
24.72
1.0
1.4
2.4
3.5
5.0
6.3
2.52
5.30
9.97
15.49
16.81
15.58
15.42
16.00
10.1
13.9
16.6
19.2
21.8
23.9
4
4
5
6
.1
.8
.5
.1
Conditions: [cat] = 1.68 × 10− M, [S] = 0.51 M, 80 C, p(CO)/p(H2) = 1,
3
◦
toluene; ri: initial rate, l/b ratio = 2.0–2.6.
3
. Results
Conditions: [cat] = 1.68 × 10 M, [S] = 0.51 M, [CO] = 8.8 × 10−3
−
3
M
◦
(
1.4 atm), 80 C, toluene; ri: initial rate, l/b ratio = 2.0–2.6.
3
.1. Catalytic hydroformylation of 1-hexene
The system Rh(acac)(CO)2/dppe is an efficient catalyst pre-
study was carried out within this pressure range. The varia-
◦
tion of the initial hydroformylation rate at 80 C was studied
cursor for the hydroformylation of 1-hexene in toluene solution
under moderate conditions of temperature and pressure (80 C,
for several hydrogen concentrations, while the catalyst, sub-
strate and CO concentrations were kept constant; these results
are collected in Table 2 and in Fig. 2, where a typical satura-
tion curve with respect to [H2] is observed. This corresponds to
◦
1–7 atm of syn-gas) yielding exclusively the corresponding alde-
hydes, heptanal and 2-methyl-hexanal; the linear to branched
ratio (l/b) is practically constant (ca. 2) within the range of reac-
tion conditions used in this work. The initial rate of 1-hexene
hydroformylation was found to be independent of the speed
of stirring in the range 500–800 rpm, indicating that the data
obtained correspond to a kinetic regime and that mass transfer
effects were negligible.
an approximately first-order dependence at hydrogen pressures
2
below 3 atm (log r = −1.5 + 1.2 log[H2], r = 0.98), which tends
i
to zero order at higher H2 pressures (log r = −3.8–0.1 log[H2],
i
2
r = 0.03). In consequence, the rate dependence on the concen-
tration of the other reactants was evaluated at three different
−3
values of [H2]: low (1.4 atm, 6.3 × 10 M), intermediate
−
2
−2
(
3.5 atm, 1.0 × 10 M), and high (6.1 atm, 2.4 × 10 M).
3
.2. Kinetic investigation
At low [H2] (data in Table 3) the rate dependence with
respect to [CO] yielded the “volcano-type” curve shown
in Fig. 3, which indicates that the reaction is inhibited
by CO above a threshold value of about 2.1 atm CO
The kinetics of the hydroformylation of 1-hexene catalyzed
by Rh(acac)(CO)2/dppe in toluene solution were studied by
carrying out runs at different concentrations of catalyst, sub-
−2
(
1.3 × 10 M). At lower CO pressures the dependence
◦
strate, dissolved H2, and dissolved CO at 80 C. First of all,
2
was first-order (log r = −1.8 + 1.1 log[CO], r = 0.99), but it
i
it was observed that the hydroformylation rate varied propor-
tionally to the total syn-gas pressure in the range 1–7 atm, as
shown in Table 1 and Fig. 1 and therefore the rest of the kinetic
becomes of inverse order (−1.6) at higher CO pressures
2
(
log r = −6.9 − 1.6 log[CO], r = 0.98). Under the same con-
i
Fig. 2. Rate dependence on dissolved H2 concentration for the hydro-
Fig. 1. Rate dependence on total syn-gas pressure for the hydroformylation of
formylation of 1-hexene catalyzed by the Rh(acac)(CO) /dppe. Conditions:
2
−
3
−3
−3
◦
1
-hexene catalyzed by Rh(acac)(CO)2/dppe. Conditions: [cat] = 1.68 × 10 M,
[cat] = 1.68 × 10 M, [hex] = 0.51 M, [CO] = 8.8 × 10 M (1.4 atm), 80 C,
◦
[
hex] = 0.51 M, 80 C, toluene.
toluene.

2
44
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
Table 3
Kinetic data for the hydroformylation of 1-hexene catalyzed by Rh(acac)(CO)2/dppe at low p(H2)
3
3
5
−1
)
[
Rh] (×10 M)
[1-Hexene] (M)
p(CO) (atm)
[CO] (×10 M)
ri (×10 M s
1
1
1
1
1
1
1
1
1
0
1
2
3
1
1
1
1
.68
.68
.68
.68
.68
.68
.68
.68
.68
.64
.29
.54
.24
.68
.68
.68
.68
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.13
0.27
0.59
0.67
0.7
1.0
1.4
1.7
2.1
2.8
3.5
4.1
5.5
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
4.4
6.6
8.8
10.9
13.2
17.6
22.0
26.4
35.3
8.8
8.8
8.8
8.8
8.8
8.8
8.8
8.8
2.11
3.87
5.30
6.84
8.28
7.48
6.14
4.11
2.36
1.66
3.59
7.43
10.52
2.24
3.93
6.54
6.85
Conditions: p(H2) = 1.4 atm ([H2] 6.3 × 10− M); T = 80 C; solvent: toluene, ri: initial rate, l/b ratio = 1.9–2.4.
3
◦
ditions, the rate of hydroformylation shows a first-order
are rather similar to those found at the intermediate hydrogen
concentration.
dependence on rhodium concentration (log r = −1.3 + 1.1 log
i
2
[
Rh], r = 0.99), while it is of fractional order (0.7) with respect
The linear to branched ratio (l/b) of the aldehydes formed
was practically constant (between 2.0 and 2.6) and independent
of the reaction conditions, except for the increment on the syn-
gas and CO pressure where a slightly reduction of l/b ratio was
observed.
to the concentration of the alkene (log r = −4.0 + 0.7 log [1-
i
2
hexene], r = 0.99).
Similarly, the results of the kinetics of the hydroformylation
of 1-hexene catalyzed by Rh(acac)(CO)2/dppe at intermediate
hydrogen pressure (3.5 atm) (Table 4) proved to be approx-
imately first-order with respect to [CO] at low pressures
3
.3. Coordination chemistry related with 1-hexene
2
(
log r = −1.8 + 1.1 log[CO], r = 0.95) while it tends to nega-
i
hydroformylation
tive order at high CO pressures (log r = −6.9 − 2.1 log[CO],
i
2
r = 0.98). The rate is also first-order in rhodium concentra-
In order to complement the kinetic study of 1-hexene hydro-
formylation described above, the interaction of the catalyst
precursor Rh(acac)(CO)2 with each component of the catalytic
mixture (i.e. dppe and syn-gas) was studied independently;
the results are summarized in Scheme 1. The interaction of
Rh(acac)(CO)2 (1) with 1 equiv. dppe in benzene results in the
evolution of CO and the formation of a yellow solution, from
which a yellow solid was isolated whose IR spectrum showed
2
tion (log r = −0.7 + 1.1 log[Rh], r = 0.96) and fractional order
i
in substrate concentration (log r = −3.6 + 0.7 log[1-hexene],
i
2
r = 0.99). At high hydrogen pressures, the results (not shown)
1
31
1
no presence of carbonyl ligands. The H and P{ H} NMR
+
spectra showed the presence of [Rh(dppe)2] (2), formed as the
acac salt, as the only phosphorus-containing species [58.8 ppm
2
(
d, J
= 133 Hz)], together with the starting complex; this
P–Rh
cationic bis(dppe) complex was initially reported by James and
Mahahan in 1979 [21a]. On the other hand, when a solution of
1
reacted with 1 equiv. of dppe under syn-gas (4 atm), a yellow
solution was obtained, from which a brown-yellow solid could
be isolated. IR and NMR spectra indicate the presence of several
species, among which [Rh(CO)(dppe)]2(␮-CO)2 (3) could be
31
1
identified by the signals in P{ H] NMR at 41.5 and 40.3 ppm
ꢀ
ꢀꢀ ꢀꢀꢀ
ꢀ
as part of a AA A A XX spin system [21b]. We believe that
this reaction initially produces RhH(CO)2(dppe) (A) which eas-
ily decomposes to form, among other species, the dimer 3; this
latter compound is analogous to the one claimed by Wilkinson
et al. [22] to be formed as a byproduct in the hydroformylation
of 1-hexene catalyzed by RhH(CO)(PPh3)3 and more recently
Fig. 3. Rate dependence on dissolved CO concentration for the hydro-
formylation of 1-hexene catalyzed by Rh(acac)(CO)2/dppe at low hydrogen
concentration (1.5 atm, 1.0 × 10− M).
2

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
245
Table 4
Kinetic data for the hydroformylation of 1-hexene catalyzed by Rh(acac)(CO)2/dppe at intermediate p(H2)
3
3
5
−1
[
Rh] (×10 M)
[1-Hexene] (M)
p(CO) (atm)
[CO] (×10 M)
ri (×10 M s
)
1
1
1
1
1
1
2
2
3
1
1
1
.68
.68
.68
.68
.68
.68
.07
.66
.27
.68
.68
.68
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.27
0.40
0.59
0.7
2.8
3.5
4.1
4.8
1.4
1.4
1.4
1.4
1.4
1.4
1.4
4.4
17.6
22.0
26.4
30.9
8.8
8.8
8.8
8.8
8.8
3.80
18.00
24.80
18.90
12.40
15.50
20.00
26.40
40.70
10.40
13.00
17.50
8.8
8.8
Conditions: p(H2) = 3.5 atm ([H2] 1.0 × 10− M); T = 80 C; solvent: toluene; ri: initial, l/b ratio = 2.0–2.4.
2
◦
observed by Bianchini et al. [4] by high pressure NMR spec-
troscopy.
Finally, in order to have more evidence for the presence of
RhH(CO)2(dppe) in our reaction medium, we studied the inter-
action of a solution of A with 1 equiv. of PPh3 in toluene-d8
least four complexes: the dimeric complex 3, which is the major
species detected by 31P{ H} NMR and three hydride-containing
1
1
species (detected by H NMR), which were characterized as
RhH(CO) (dppe) (A, triplet of doublets at 8.3 ppm, J = 54 and
2
3
2
11 Hz), RhH(CO)(dppe)(PPh ) (4, broad triplet at 10.1 ppm)
(under a stream of syn-gas), which produced a mixture of at
and [Rh(H)(CO)(dppe)] (␮-dppe) (5, triplet of doublets of dou-
Scheme 1. Stoichiometric reactions related with the hydroformylation of 1-hexene catalyzed by Rh(acac)(CO)2/dppe.

2
46
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
blets at 8.6 ppm, J = 57, 15 and 9 Hz); similar hydride species
containing 1,3-bis(diphenylphospino)propane (dppp) have been
reported by James et al. [23]. The use of Rh-dppe-PPh3 mix-
tures in hydroformylation catalysis has been reported before
isomer excluded for clarity), which in general terms is analogous
to the commonly accepted cycle for the Rh–PPh3 system [3,4]. It
is known that Rh(acac)(CO)2 (1) reacts with syn-gas in presence
ofanexcessofPPh3 togeneratecomplexesRhH(CO)x(PPh3)4−x
(x = 1, 2) [6]. It is known that dppe prefers to bind as a
chelate to one metal center, rather than bridge between two
metals, which is the preferred bonding mode of other diphos-
phines with a longer chain between the phosphorus donor atoms
[16b–d,21,25]. We propose that (1) reacts with 1 equiv. dppe
under syn-gas to form RhH(CO)2(dppe) (A); this species is
unstable in absence of syn-gas to form the carbonyl-bridged
dimer [Rh(CO)(dppe)]2(␮-CO)2 (3). Complex (1) is probably
in equilibrium (K1) with the 16-electron RhH(CO)(dppe) (B),
which initiates the catalytic cycle (see Scheme 2). B reversibly
coordinates the olefin (K2) to produce RhH(CO)(olefin)(dppe)
(C). Then, migratoryinsertionoftheolefinintothemetalhydride
bond takes place (K3) with the concomitant coordination of a CO
molecule to yield Rh(CO)2(alkyl)(dppe) (D); the reversibility
of this reaction has been postulated in other works [8,15]. The
insertion of CO into the Rh-alkyl bond of D, generates the unsat-
urated acyl species Rh(CO)(acyl)(dppe) (E) through K4. Finally,
hydrogenolysis of (E) produces the aldehyde, and regenerates
the catalytically active species (A), which re-starts the cycle.
At high CO pressure, species E can reversibly coordinate CO
[
16]. All these experiments lead to us to propose that under
the hydroformylation conditions, species A is the resting state
of the catalyst; the cationic bis(diphosphine) rhodium complex
(2) was discarded as the active species because of under syn-gas
conditions this species must be transformed in A, as reported by
the groups of Claver and van Leeuwen [24] for a similar rhodium
system with (2S,4S)-bis(diphenylphosphine)pentane (BDPP) as
the chelating phosphine ligand.
4
. Discussion
Despite the fact that Rh-catalyzed hydroformylation is prac-
ticed successfully in industry and has been the object of intense
studies for several decades, information on the kinetics of this
reaction is still relatively scarce and often contradictory, mainly
becausethesereactionsareextremelysensitivetotheexperimen-
tal conditions employed. Most of the kinetic studies available
have dealt with the Rh–PPh3 system, for which the reaction has
been generally found to be first-order with respect to olefin and
rhodium concentration and zero order on hydrogen pressure,
implying that the slow step is either the coordination of the sub-
stratetoRhorthemigratoryinsertionofthealkeneintotheRh–H
bond. However, theoxidative additionof hydrogenhas been con-
sidered as the rate-determining step in other reports employing
the same catalytic system under different reaction conditions
on the basis of the acceleration of the hydroformylation rate
observed upon increasing the hydrogen pressure [3–9]. Never-
theless this is considered by some authors [3] to be most likely
an artifact due to the presence of inactive dimeric species which
react with hydrogen to regenerate monomeric rhodium hydrides,
thereby increasing the effective concentration of active rhodium
in solution. It is claimed that under “standard” catalytic condi-
to generate Rh(CO)2(acyl)(dppe) (F) through K , and a further
5
CO ligand through K , to form RhH(CO)3(acyl)(dppe*) (G), by
6
dissociation of one of the P-atoms of dppe.
Our results indicate that, depending on the dissolved hydro-
gen concentration, different rate-determining steps may be
operating in this cycle. The first-order kinetics observed at
low hydrogen pressure is consistent with the hydrogenolysis
of the acyl intermediate E being the rds, whereas the zero order
dependence on [H2] at higher pressures points to a rds in the
early stages of the cycle, most likely the migratory insertion
of the olefin into the Rh H bond. Hydroformylation is clearly
first-order in rhodium concentration, irrespectively of the gas
pressures employed. Claver et al. [17] reported a similar obser-
vation for the hydroformylation of styrene catalyzed by the
rhodium/BDPP system and interpreted it in terms of dinuclear
species not being involved in the rate-limiting step, which would
result in fractional order with respect to metal concentration.
Although we favor a similar explanation in our case, the impli-
cation of dinuclear species in the catalytic process cannot be
definitively ruled out with the available data. On the other hand,
thefractionalkineticsobservedasafunctionofsubstrateconcen-
tration may be the result of operating in the intermediate regime
of a saturation kinetics, similarly to what was found by Cavalieri
d ˇı Oro et al. for the hydroformylation of propylene catalyzed by
RhH(CO)(PPh3)3 [5]. Saturation kinetics in olefin hydroformy-
lation catalyzed by this precursor have also been reported by
Kiss et al. for ethene [6], and by Chaudhari et al. for 1-hexene
[7], for 1-decene [8], and for 1-dodecene. Chaudhari et al. also
have reported a zero order with respect to olefin for styrene
hydroformylation catalyzed by the same precursor [9]. Finally,
the inhibition of the rate of hydroformylation of 1-hexene at high
CO pressures may be explained by the reactions occurring in the
catalytic cycle of Scheme 2 (k−1, k and k ) leading to inactive
◦
tions (i.e. “industrial operating conditions” 70–120 C, 5–25 atm
CO, 5–25 atm H2, ca. 1 mM Rh, 0.1–2 M alkene) the reaction
is first-order in rhodium and alkene concentration, zero order in
olefin and negative order in CO or phosphine concentrations [3].
Pioneering work by Cavalieri d’Oro [5] on the hydroformylation
of propene reported a rate law:
r = k[alkene]0.6[PPh3]−0.7[CO]−0.1[Rh] [H2]0
1
A detailed study has also been carried out with catalysts con-
taining bulky phosphite ligands [10,11], which are characterized
by the fact that only one phosphorus ligand is coordinated to
rhodium; in this case, the rate is independent of the alkene con-
centration, first-order in hydrogen and rhodium concentrations
and inverse first-order in CO pressure; the rate determining step
is thought to be the hydrogenolysis of the acyl intermediate [11].
For bulky diphosphites the reaction exhibited a first-order depen-
dence on alkene concentration, zero order in hydrogen pressure
and negative order in CO pressure, consistent with an rds early
in the catalytic cycle [13].
Our results can be accommodated by the mechanism depicted
in Scheme 2 for the formation of the linear aldehyde (branched
5
6

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
247
Scheme 2. Catalytic cycle for the Rh(acac)(CO)2/dppe-catalyzed 1-hexene hydroformylation.
coordinatively saturated polycarbonyl species, similarly to what
has been reported for hydroformylation with RhH(CO)(PPh3)3
viz. HRh(CO)2(PPh3)2. In contrast, our data indicate that if only
one equivalent of diphosphine is used, HRh(CO)2(P–P) is pref-
erentially formed and in consequence, the mechanism is closely
related to that of Rh–PPh3. This illustrates well how sensitive the
kinetics and mechanisms of hydroformylation are to seemingly
small variations of the reaction conditions.
[
5–9].
It is important to compare our catalytic cycle with earlier
mechanistic proposals by workers from Celanese on closely
related Rh–diphosphine systems [16c,d]. These workers found
that the [diphosphine]:[Rh] ratio had a strong influence on the
activity and selectivity of the reaction. The most selective sys-
tems for the production of linear aldehydes contained 2 or more
equivalents of diphosphine and mechanistic studies were con-
centrated on those systems. The crucial species under such
conditions are of the type HRh(CO)(P–P)(P–P*), where P–P
represents a bidentate and P–P* a monodentate diphosphine,
and the catalytic cycle derived from such species is substan-
tially different from the one commonly accepted for PPh3,
where the key species contains only two phosphorus donors,
4.1. Rate expressions for the hydroformylation of 1-hexene
catalyzed by Rh(acac)(CO) /dppe
2
According to the cycle described in Scheme 2, the rate law
for the hydroformylation of 1-hexene with Rh(acac)(CO) /dppe,
2
under conditions for which the hydrogenolysis of the acyl inter-
mediate E is the rds, can be derived by applying the Equilibrium
Approximation. Taking into account that the overall hydro-
formylation rate is given by the individual rate of this step, the

2
48
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 241–249
rate equation may be expressed as
RhH(CO)2(dppe) is the resting state and RhH(CO)(dppe) is con-
sidered the active species. This mechanism is similar to the one
generally accepted for Rh-PPh3 catalysts but different from the
one proposed for the Rh system containing 2 equiv. of dppe.
The hydrogenolysis of the acyl intermediate or the migration of
the olefin into the metal hydride bond are the rate determining
steps at low and high hydrogen pressures, respectively. Under
high CO pressures ([CO]/[H2] > 2), the stable catalytically
r = k7[E][H2]
(2)
Considering the equilibria involved between K1 and K6
and the mass balance for rhodium as [Rh]o = [A] + [C] + [D] +
[F] + [G] (coordinatively saturated species which can be in sig-
nificant concentrations under the catalytic reaction conditions),
the rate expression becomes:
K1K2K3K4K7[Rh] [S][H2][CO]
o
r =
(3)
2
3
[
CO] + (K1K2 + K1K2K3[CO][S] + K1K2K3K4K [S][CO] + K1K2K3K4K K [S][CO] )
5
5
6
where S is the substrate (1-hexene). This rate law is consistent
with our proposed mechanism for the hydroformylation of 1-
hexene under low hydrogen pressures, indicating a first-order
dependence on [H2] and [Rh], first-order in CO at low pres-
sure and negative order at high pressures and fractional order in
olefin concentration; this rate law is also rather similar to that
found by Chaudhari et al. for the hydrogenation of styrene cat-
alyzed by RhH(CO)(PPh3)3 [9]. It is impossible with our data
to further discuss the details of the hydrogenolysis step, which
may be a concerted reaction or a two stage process involving
the oxidative addition of hydrogen (rds) followed by reductive
elimination of the product aldehyde. On the other hand, if the
CO-promoted migration of hydride to the olefin (k3) is consid-
ered as the (irreversible) rate-determining step, then the rate law
becomes:
inactive polycarbonyl species Rh(acyl)(CO)2(dppe), and possi-
bly Rh(acyl)(CO)3(dppe*) are present in appreciable concen-
trations outside the productive cycle, which explains the high
negative order of the reaction rate on [CO] under these condi-
tions.
Acknowledgements
Financial support from FONACIT for Project CONIPET
97-3777, CYTED for Project V.9, and Consejo de Desarrollo
Cient ´ı fico y Human ´ı stico (CONDES) of the Universidad del
Zulia (L.U.Z.) for Project 0369-04 and for the acquisition of a
gas chromatograph are gratefully acknowledged. We thank Dr.
Y. Alvarado for his valuable help in recording the i.r. spectra.
r = k3[C][CO]
(5)
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