Kinetics and mechanisms of homogeneous catalytic reactions. Part 7. Hydroformylation of 1-hexene catalyzed by cationic complexes of rhodium and iridium containing PPh3

Article abstract of DOI:10.1016/j.molcata.2007.01.045

Cationic rhodium and iridium complexes of the type [M(COD)(PPh₃)₂]PF₆ (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2-5 bar) and temperatures (60 °C for Rh and 100 °C for Ir) in toluene solution; the linear to branched ratio (l/b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0 and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For both complexes, the reaction proceeds according to the rate law r_i = K₁K₂K₃k₄[M][olef][H₂][CO]/([CO]² + K₁[H₂][CO] + K₁K₂K₃[olef][H₂]). Both complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO)₃(PPh₃)₂]PF₆, M = Rh, 2a; Ir, 2b, and with syn-gas to yield [MH₂(CO)₂(PPh₃)₂]PF₆, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH₂(CO)(PPh₃)₂]PF₆ entering the corresponding catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin promoted by an entering CO molecule is the rate-determining step of the catalytic cycle.

Full text of DOI:10.1016/j.molcata.2007.01.045

Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
Kinetics and mechanisms of homogeneous catalytic reactions
Part 7. Hydroformylation of 1-hexene catalyzed by cationic complexes
of rhodium and iridium containing PPh3
a
a
Merlin Rosalesa,∗, Jesus A. Duran , Angel Gonzalez ,
´
´
´
´
´
a
b
´
Ines Pacheco , Roberto A. Sanchez-Delgado
a La Universidad del Zulia (L.U.Z.), Facultad Experimental de Ciencias, Departamento de Qu´ımica,
Laboratorio de Qu´ımica Inorga´nica, Apdo. 526, Maracaibo, Venezuela
b Chemistry Department, Brooklyn College and The Graduate Center, City University of New York (CUNY), 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
Cationic rhodium and iridium complexes of the type [M(COD)(PPh3)2]PF6 (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation
of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2–5 bar) and temperatures (60 ◦C for Rh and
100 ◦C for Ir) in toluene solution; the linear to branched ratio (l/b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0
and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For
both complexes, the reaction proceeds according to the rate law ri = K1K2K3k4[M][olef][H2][CO]/([CO]2 + K1[H2][CO] + K1K2K3[olef][H2]). Both
complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO)3(PPh3)2]PF6, M = Rh, 2a; Ir, 2b, and with syn-gas to
yield [MH2(CO)2(PPh3)2]PF6, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH2(CO)(PPh3)2]PF6 entering the corresponding
catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin
promoted by an entering CO molecule is the rate-determining step of the catalytic cycle.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hydroformylation; Rhodium and iridium complexes; Kinetics; Mechanisms; 1-Hexene
1. Introduction
mechanistic study of the hydroformylation of 1-hexene using
as precatalyst the system Rh(acac)(CO)2/dppe [15]. Continu-
ing our work on the kinetics and mechanisms of homogeneous
catalytic reactions, we now present a study of the hydroformyla-
tion of 1-hexene catalyzed by the cationic rhodium and iridium
complexes [M(COD)(PPh3)2]PF6. On the basis of the kinetic
evidence and some related preparative chemistry we propose a
catalytic cycle and discuss it in the light of the extensive prior
knowledgeonrhodiumhydroformylationcatalysis[1–15]. Early
reports by Oro et al. [16a] and by Crabtree and Felkin [16b]
described the hydroformylation activity of cationic complexes
of the type [Rh(COD)(PPh3)2]+ first synthesized and used as
hydrogenation catalysts by Schrock and Osborn [16c]. A great
dealofinformationonthechemistryofthesecomplexesandtheir
application to hydrogenation catalysis has accumulated over the
years [17] and much of that knowledge can be extended to hydro-
formylation mechanisms. Reports of catalytic hydroformylation
by iridium complexes are scarce and generally deal with neutral
The hydroformylation of alkenes is one of the most
important applications of transition metal based homogeneous
catalysis [1–3]. The kinetics of olefin hydroformylation are
generally complex and despite the wealth of mechanistic
information available for this reaction catalyzed by rhodium
complexes [1–3], detailed kinetic studies have been per-
formed for relatively few catalysts: RhH(CO)(PPh3)3 [4–9],
rhodium systems with bulky phosphites [10], phosphine-
phosphite and diphosphite [11], and diphosphine [12] systems,
a rhodium precatalyst with 1,2,5-triphenyl-1H-phosphole [13]
and a rhodium-triphenylarsine precatalyst [14]. In the accompa-
nying preceding paper of this series we reported a kinetic and
∗
Corresponding author.
E-mail address: merlin2002@cantv.net (M. Rosales).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.01.045

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
251
monohydride catalysts [1–3]. Whyman applied high-pressure
IR spectroscopy to the study of hydroformylation intermediates
or model complexes using Ir-hydrido-triphenylhosphine com-
plexes [18]. More recently, Godard et al. [19] have reported
NMR studies on the reaction of Ir(CO)(PPh3)2(␩3-C3H5) with
hydrogen and CO, which produces a interesting series of com-
plexes including acyl and alkyl dihydride intermediates that
nicely model the hydroformylation mechanism for an irid-
ium monohydride catalyst. Eisenberg and coworkers studied
Ir–Xantphos complexes that catalyze alkene hydroformylation
and used NMR studies with p-H2 to identify a series of monohy-
dride and trihydride complexes implicated in the catalytic cycle
[20]. Haukka and coworkers reported the use of Ir4(CO)12, IrCl3,
and [IrCl(CO)3]n in the presence of inorganic salts in the hydro-
formylation of 1-hexene [21]. To the best of our knowledge,
there are no prior reports in the open literature of the kinetics of
hydroformylation by cationic dihydrido complexes of rhodium
and iridium.
centration of the corresponding products versus time yielding
straightlines, whichwerefittedbyconventionallinearregression
programs. Initial rates of the reaction were obtained from the
corresponding slopes. The H2 and CO concentrations in toluene
under the reaction conditions were calculated from solubility
data reported in the literature [23,24].
The composition of the reaction mixture was analyzed by
means of a 610 Series UNICAM gas chromatograph fitted with a
thermal conductivity detector and a 3 m 10% SE-30 on Supelco-
portglasscolumnusingheliumascarriergas;thechromatograph
was coupled to a UNICAM 4815 data system.
2.3. Coordination chemistry related to hydroformylation of
1-hexene
Reaction of [M(COD)(PPh3)2]PF6, M = Rh (1a), Ir (1b),
with carbon monoxide: Carbon monoxide was bubbled through
a solution of 1a or 1b (1.0 mmol) in benzene (10 mL) while
the mixture was vigorously stirred under gentle reflux for 3 h.
The solution was evaporated to dryness under a CO atmosphere
to produce [Rh(CO)3(PPh3)2]PF6, (2a)—IR (KBr): 2090 (m),
2025 (vs), 1965 (vs) or [Ir(CO)3(PPh3)2]PF6, (2b)—IR (KBr):
2070 (m), 2001 (vs), 1983 (vs).
2. Experimental
2.1. Instruments and materials
All manipulations were conducted with rigorous exclusion
of air using Schlenck techniques. The substrate (1-hexene)
was purified by distillation under reduced pressure, sol-
vents were purified by standard methods, and complexes
[M(COD)(PPh3)2]PF6 (M = Rh, Ir) were prepared by published
procedures [16c]. NMR and IR spectra were recorded on a
Bruker AM-300 spectrometer and Shimadzu 8300 FT-IR instru-
ment, respectively.
Reaction of 1b with syn-gas: This reaction was carried out
as described before but using a 1:1 mixture of CO and H2
instead of carbon monoxide, to yield the known compounds
[IrH2(CO)2(PPh3)2]PF6 (3b)—IR (KBr): 2212, 2162, 2088,
2044 cm−1; H RMN (CDCl3, 300 MHz): 7.3–7.6 (m, 30H,
1
1
PPh3), −9.90 ppm (t, JHP = 8.7 Hz, 2H); 31P{ H}RMN (CDCl3,
121 MHz): 4.1 (s, 2PPh3), 143.0 ppm (hept, PF6).
Interaction of 3b with 1-hexene under CO: A mixture of com-
plex 3b (92 mg, 0.1 mmol), 1-hexene (13 ␮L, 0.1 mmol) and
benzene (10 mL) was placed under a CO atmosphere at 100 ◦C
for 1 h. The liquid components were analyzed by GC; the solu-
tion was evaporated to dryness under a CO atmosphere, and the
solid was analyzed by IR spectroscopy.
2.2. Procedure for kinetic measurements
Kinetic experiments were carried out in a high-pressure reac-
tor, supplied by Parr Instruments, which was provided with
arrangements for sampling of liquid contents, automatic tem-
perature and pressure control and variable stirrer speed. In a
typical experiment, a solution containing the catalyst, 1-hexene,
n-heptane (as internal standard) and the solvent (toluene, 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 temper-
ature. When the reaction temperature was reached, a mixture
of CO and H2 in the required ratio was introduced into the
autoclave; this moment was taken as the zero time of the reac-
tion. Each reaction was followed by taking liquid samples at
regular intervals of time, which were analyzed by using gas chro-
matography. Since, the major products formed were isomeric
aldehydes, the CO/H2 mixture was supplied in the appropri-
ate ratio to maintain the same composition of the gases in the
autoclave introduced at the beginning of the reaction.
3. Results and discussion
3.1. Catalytic hydroformylation of 1-hexene
The complexes [M(COD)(PPh3)2]PF6, M = Rh (1a), Ir (1b)
are efficient catalyst precursors for the homogeneous hydro-
formylation of 1-hexene in toluene solution to give exclusively
the corresponding aldehydes (heptanal and 2-methyl-hexanal),
under mild conditions of temperature and pressure. Rhodium
complexes of this type [16a,b] and neutral iridium complexes
[2a,b,18–21] have been previously reported to hydroformylate
alkenes. We now provide a direct comparison of the kinetics of
these cationic systems for both metals under similar reaction
conditions. The rhodium precatalyst was very active at 60 ◦C
and 3 bar of syn-gas, while for Ir the catalysis was carried out at
100 ◦C; the lower hydroformylation activity of iridium as com-
pared to rhodium has been previously documented [2a,b]. The
variation of the linear to branched ratio (l/b) of aldehydes was
small (between 3.0 and 3.7 for Rh and close to 2 for Ir) and did
depend very slightly on the reaction conditions. Although these
Each reaction was repeated at least twice in order to ensure
reproducibility of the results. All the reactions were carried out
at low conversion (close to 10%) in order to perform a kinetic
analysis based on the initial rate method [22]. The data for
the hydroformylation of 1-hexene were plotted as molar con-

252
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
complexes are known to be very active for the hydrogenation
of alkenes [17], no hydrogenation products were detected under
hydroformylation conditions, something that can be accounted
for by the proposed mechanism (vide infra). The initial rates of
hydroformylation of 1-hexene catalyzed by complexes 1a and
1b were practically independent of the speed of stirring, show-
ing that the data obtained correspond to a kinetic regime and
mass transfer effects were negligible.
pressure, but it becomes of inverse order at high CO pressure
(log ri = −6.74 − 1.05 log [CO], r2 = 0.98).
The linear to branched ratio (l/b) of the aldehydes formed
by use of the rhodium precursor varied only slightly (between
3.0 and 3.7), decreasing when the concentrations of precatalyst,
hydrogen and CO dissolved were higher and increasing with the
substrate concentration.
Similarly, the kinetics of 1-hexene hydroformylation cat-
alyzed by the iridium precursor (Table 2) are near first order
on catalyst concentration (log ri = −1.61 + 0.92 log [Ir],
r2 = 0.97), near first order on substrate concentra-
tion (log ri = −4.09 + 0.93 log [1-hexene], r2 = 0.99), and
fractional order with respect to dissolved CO concen-
tration (log ri = −3.01 + 0.55 log [CO], r2 = 0.98), which
tends to a highly negative order at high CO pressure
(log ri = −10.19–2.23 log [CO], r2 = 0.96). However, in contrast
to the Rh precatalyst, in the case of the iridium complex the
reaction rate varies in accord with a saturation curve with
respect to hydrogen concentration, that is, a first order kinetics
at low hydrogen concentration (log ri = −2.42 + 0.92 log [H2],
r2 = 0.99), which tends to zero order at high H2 concentration
(P > 3 bar) (log ri = −4.06 + 0.16 log [H2], r2 = 0.99).
3.2. Kinetic investigation for the hydroformylation of
1-hexene
The kinetics of the hydroformylation of 1-hexene catalyzed
by the cationic Rh- and Ir-complexes in toluene were studied
by carrying out runs at different catalyst, substrate, dissolved
hydrogen and dissolved carbon monoxide concentrations, and
at constant temperature for each precatalyst. The data for the
hydroformylation reaction catalyzed by the rhodium complex
are collected in Table 1, and the results indicate that:
(1) The initial rate of 1-hexene hydroformylation shows
a direct dependence on the rhodium concentration
(log ri = −1.64 + 0.93 log [Rh], r2 = 0.95).
(2) The initial rate of 1-hexene hydroformylation is first
order with respect to the concentration of substrate
(log ri = −4.19 + 1.02 log [1-hexene], r2 = 0.97).
The linear to branched ratio (l/b) of the aldehydes formed was
practically constant (between 1.6 and 2.0) and independent of
the reaction conditions, except when increasing the syn gas and
CO pressure, when a slight reduction of l/b ratio was observed.
(3) The reaction is of fractional order with respect to dissolved
hydrogen concentration (log ri = −3.04 + 0.65 log [H2],
r2 = 0.96) in the concentration range studied for this
reaction.
3.3. Coordination chemistry related with the catalytic
1-hexene hydroformylation
(4) The dependence of the initial rate with respect to the dis-
solved carbon monoxide concentration was of fractional
order (log ri = −3.46 + 0.47 log [CO], r2 = 0.95) at low CO
In order to gain further understanding of the mechanisms of
1-hexene hydroformylation by the cationic systems, the reaction
Table 1
Kinetic data for the hydroformylation of 1-hexene catalyzed by [Rh(COD)(PPh3)2]PF6
104 [Rh] (M)
10 [olefin] (M)
p(H2) (atm)
103 [H2] (M)
p(CO) (atm)
103 [CO]
105ri (M s−1
)
6.44
8.33
11.00
16.70
20.88
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
8.33
5.08
5.08
5.08
5.08
5.08
2.16
3.18
3.96
5.84
6.57
5.08
5.08
5.08
5.08
5.08
5.08
5.08
5.08
5.08
5.08
5.08
5.08
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
0.7
1.0
2.0
2.7
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
5.7
5.7
5.7
5.7
5.7
5.7
5.7
5.7
5.7
5.7
3.4
4.8
8.0
9.9
5.7
5.7
5.7
5.7
5.7
5.7
5.7
5.7
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
0.3
0.6
1.0
1.1
2.4
2.9
3.4
4.5
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
7.1
1.3
2.8
4.2
6.2
12.9
15.8
18.8
24.7
2.40
3.42
4.62
5.29
8.13
1.44
1.88
2.23
3.51
4.51
2.31
2.63
3.91
4.57
1.44
2.15
2.25
3.16
1.64
1.42
1.35
0.85
Conditions: T = 60 ◦C; solvent: toluene; ri: initial; l/b ratio = 3.0–3.7.

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
253
Table 2
Kinetic data for the hydroformylation of 1-hexene catalyzed by [Ir(COD)(PPh3)2]PF6
104 [Ir] (M)
10 [olefin] (M)
p(H2) (atm)
103 [H2] (M)
p(CO) (atm)
102 [CO] (M)
105ri (M s−1
)
6.43
8.36
11.00
13.80
16.60
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
8.36
4.96
4.96
4.96
4.96
4.96
1.30
2.08
2.99
5.79
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
4.96
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.3
1.6
3.0
3.8
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
4.1
5.5
9.8
12.6
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
6.9
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.3
0.6
1.0
1.3
1.6
2.3
2.7
3.0
3.3
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
0.5
1.2
1.6
2.0
2.3
3.1
3.4
3.8
4.2
2.71
3.96
4.65
5.97
6.59
1.32
1.81
2.83
4.99
2.44
3.26
4.15
4.36
1.47
2.47
2.24
2.85
3.32
2.35
2.04
1.79
1.37
Conditions: T = 100 ◦C; solvent: toluene; ri: initial; l/b ratio = 1.6–2.0.
of complexes 1a and 1b with each component of the catalytic
mixture was carried out, with the aim of isolating or detecting
some intermediates of the reaction; the results are summarized
in Fig. 1. When solutions of [M(COD)(PPh3)2]PF6, M = Rh
(1a), Ir (1b), in benzene are treated with carbon monoxide
(1 atm, 60 ◦C), rapid displacement of COD occurs and the tri-
carbonyl species [M(CO)3(PPh3)2]PF6, M = Rh (2a), Ir (2b)
can be isolated from the CO saturated solutions. The rhodium
tricarbonyl complex loses CO only slowly in the solid state
but when dissolved in benzene in the presence of acetonitrile,
CO is rapidly evolved and [Rh(CO)(NCMe)(PPh3)2]PF6 can
be isolated as a crystalline solid. The reaction of complexes 1
with syn-gas in benzene generates [M(H)2(CO)2(PPh3)2]PF6,
M = Rh (3a), Ir (3b). Complexes 3 also can be obtained by
hydrogenation of the corresponding complex 2a or 2b. Com-
plexes 2a, 2b, [Rh(CO)(NCMe)(PPh3)2]PF6 and 3b, have been
isolated and characterized before by James and coworkers
[25] and thus we have identified them by comparison of their
spectroscopic properties with published data. It is important
to note that 2a, 2b and 3b displayed similar catalytic activ-
ities for the hydroformylation of 1-hexene to those of 1a
or 1b, respectively, under the corresponding reaction condi-
tions[(m.s.d. 0.5) × 10−5, P = 0.05]. Finally, theinteractionof
complex 3b with 1 eq. of 1-hexene under a CO atmosphere quan-
titatively yields the corresponding C7 aldehydes (l/b ≈ 2) and
complex 2b.
Fig. 1. Stoichiometric reactions related with the hydroformylation of 1-hexene catalyzed by [M(COD)(PPh3)2]PF6 (M = Rh, Ir).

254
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
Fig. 2. Catalytic cycle for the hydroformylation of 1-hexene catalyzed by [M(COD)(PPh3)2]PF6 (M = Rh, Ir).
3.4. Mechanistic aspects of the 1-hexene hydroformylation
migration and CO coordination as a single step. Insertion of
one of the CO ligands into the metal–alkyl bond of 6 (k5) gen-
erates [M(H)(CO)(acyl)(PPh3)2]PF6 (7). Finally, elimination of
the aldehyde and addition of H2 regenerates the catalytic species
4 and restarts the cycle (k6). At high CO pressure, species 7 can
coordinate CO to generate the peripheral coordinatively satu-
rated [M(H)(CO)2(acyl)(PPh3)2]PF6 (8), through K7. Although
the intimate details of the product elimination step have not
been investigated, it is interesting to speculate that this as might
arise from coordination of a dihydrogen molecule to 7 to form
an 18-electron species [M(H)(H2)(CO)(acyl)(PPh3)2]PF6, fol-
lowed by a rapid elimination of the aldehyde through coupling
of the acyl group with a proton originating from coordinated
H2. Alternatively, reductive elimination of the aldehyde from 7
to yield [M(CO)(solv)(PPh3)2]PF6 followed by rapid oxidative
addition of H could also be envisaged. Any of these sequences of
events would be very fast and therefore do not affect our kinetic
analysis.
In conclusion, the [M(COD)(PPh3)2]PF6-catalyzed olefin
hydroformylationseemstooperatethroughamechanisminvolv-
ing dihydrido intermediates, clearly related to well-understood
hydrogenation mechanisms for these complexes [17]. The fact
that no hydrogenation products were observed indicates that CO
insertion into the metal–alkyl bond (k4) proceeds much faster
than reductive elimination of the corresponding alkanes from 5,
in line with other mechanisms for the Rh-catalyzed hydroformy-
lation of alkenes [1,2].
On the basis of the experimental findings presented above
and the extensive previous knowledge on hydroformylation
mechanisms [1–15], the catalytic cycles depicted in Fig. 2
are proposed for the hydroformylation of 1-hexene catalyzed
by complexes [M(COD)(PPh3)2]PF6, M = Rh (1a), Ir (1b).
Although similar in the main features, the two systems display
notable differences in the values of the constants of the equi-
libria involved (vide infra). Complexes 1 react with syn-gas to
yield [M(CO)3(PPh3)2]PF6 (2) with concomitant hydrogena-
tion and/or hydroformylation of the COD ligand; species 2
are in equilibrium with [M(H)2(CO)2(PPh3)2]PF6 (3) through
oxidative addition of dihydrogen accompanied by dissociation
of one CO ligand (K1). Complexes 3 can reversibly dissoci-
ate a carbonyl ligand (K2) to form the unsaturated reactive
intermediates [M(H)2(CO)(PPh3)2]PF6 (4), which are believed
to be the active species initiating the catalytic cycle. Com-
plexes 4 can coordinate the olefin through the equilibrium K3,
to produce [M(H)2(CO)(olefin)(PPh3)2]PF6 (5). The hydride
ligand of 5 is then thought to migrate to the olefin (k4)
with simultaneous coordination of a CO molecule, to yield
[M(H)(CO)2(alkyl)(PPh3)2]PF6 (6);thisissuggested astherate-
determining step of the catalytic cycle. With the data available
it is impossible to ascertain whether this is a single concerted
elementary step or a rapid succession of two independent events,
but for our treatment of the kinetic data we assume the hydride

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 270 (2007) 250–256
255
In the cycle described in Fig. 2, if the CO-promoted migration
of the hydride to the olefin is assumed to be the rds, a rate-law can
be derived by applying the equilibrium approximation. Taking
into account that the overall hydroformylation rate is given by
the rate of this step, the rate equation may be expressed as:
controlled by concentrations of gaseous reactants that influence
the reaction rates. On the basis of these results, a catalytic cycle
is proposed for this reaction, in which [M(H)2(CO)(PPh3)2]PF6
is considered the active species, and the migration of the hydride
ligand to the olefin is the rate determining step.
ri = k4[5][CO]
(1)
Acknowledgements
Considering the equilibria K1, K2 and K3, [3] = K1[2][H2]/
[CO], [4] = K1K2[2][H2]/[CO]2, and [5] = K1K2K3[2][olef]
[H2]/[CO]2, we have:
Financial support from FONACIT for Project CONIPET 97-
3777, Programa CYTED Project V.9, and Consejo de Desarrollo
´
´
Cientıfico y Humanıstico (CONDES) of the Universidad del
K1K2K3k4[2][olef][H2]
Zulia (L.U.Z.) for the acquisition of a gas chromatograph are
ri =
(2)
[CO]
´
gratefully acknowledged. We thank Dr. Ysaıas Alvarado for his
valuable help to record the IR spectra.
Assuming that 6 and 7 are rapidly transformed to 4, the mass
balance for the metal is [M]o = [2] + [3] + [5] and therefore, the
concentration of 2 may be expressed as:
References
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[2] =
(3)
[CO]2 + K1[H2][CO] + K1K2K3[olef][H2]
Substituting [2] in Eq. (2) allows us to rewrite the rate expres-
sion as:
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K1K2K3k4
[CO]2K1[H2][CO] + K1K2K3[olef][H2]
× [M]o[olef][H2][CO]
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(5)
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