Kinetics and mechanisms of homogeneous catalytic reactions. Part 14. Hydroformylation of 1-hexene with formaldehyde catalyzed by a cationic bis(diphosphine)rhodium complex

Article abstract of DOI:10.1016/j.molcata.2016.05.014

Complex [Rh(κ²-P,P-dppe)₂]acac ([1a]acac, dppe: 1,2-bis(diphenylphosphino)ethane) showed to be a highly efficient precatalyst for the hydroformylation of 1-hexene with formaldehyde under relatively mild reaction conditions (353-403 K) in 1,4-dioxane as the solvent; the corresponding C₇ aldehydes were obtained in linear to branched ratios (l/b) close to 2. A kinetic study for this reaction lead to the experimentally determined rate law: r_i = {K₁k₂/(1 + K₁[CH₂O])}[Rh][C₆H₁₂][CH₂O] [K₁ = (1.47 ± 0.04) M^-1 and k₂ = (0.47 ± 0.01) M^-1s^-1 at 373 K). The experimental activation parameters for the overall reaction were also calculated [E_a = (20.1 ± 0.9) kcal mol^-1, ΔH^≠ = (19.5 ± 0.9) kcal mol^-1, ΔS^≠ = (-9.3 ± 0.5) cal K^-1 mol^-1 and ΔG^≠ = (22 ± 2) kcal mol^-1]. The kinetic study together with results on coordination chemistry performed with 1a and with the BF₄ salt of its iridium analogue, [Ir(κ²-P,P-dppe)₂]BF₄ ([1b]BF₄), as well as theoretical DFT-calculations allowed us to propose a mechanism for this reaction. The catalytic cycle involves the reversible oxidative addition of formaldehyde to 1a to yield [Rh(H)(CHO)(κ²-P,P-dppe)₂]⁺, followed by the insertion of the olefin into the Rh-H bond to generate [Rh(alkyl)(CHO)(κ²-P,P-dppe)₂]⁺, which is considered as the rds of the process [Ea₂ = (23.9 ± 0.8) kcalmol^-1]; the cycle is completed by the reductive elimination of the aldehydes to regenerate 1a and restart the cycle.

Full text of DOI:10.1016/j.molcata.2016.05.014

Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
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Kinetics and mechanisms of homogeneous catalytic reactions. Part 14.
Hydroformylation of 1-hexene with formaldehyde catalyzed by a
cationic bis(diphosphine)rhodium complexଝ
Merlín Rosalesa,∗, Homero Péreza, Federico Arrietaa, Rodolfo Izquierdoa,
Cristhina Moratinosa, Pablo J. Baricellib
a Universidad del Zulia (L.U.Z.), Facultad Experimental de Ciencias, Departamento de Química, Laboratorios de Química Inorgánica, de Química Teórica y
Computacional y de Química Inorgánica Teórica, Maracaibo, Venezuela
b Universidad de Carabobo (UC), Facultad de Ingeniería, Centro de Investigaciones Químicas, Valencia, Venezuela
a r t i c l e i n f o
a b s t r a c t
Complex [Rh(␬2-P,P-dppe)2]acac ([1a]acac, dppe: 1,2-bis(diphenylphosphino)ethane) showed to be
a highly efficient precatalyst for the hydroformylation of 1-hexene with formaldehyde under rel-
atively mild reaction conditions (353–403 K) in 1,4-dioxane as the solvent; the corresponding C7
aldehydes were obtained in linear to branched ratios (l/b) close to 2. A kinetic study for this reac-
tion lead to the experimentally determined rate law: ri = {K1k2/(1 + K1[CH2O])}[Rh][C6H12][CH2O]
[K1 = (1.47 0.04) M−1 and k2 = (0.47 0.01) M−1s−1 at 373 K). The experimental activation parameters
Article history:
Received 7 February 2016
Received in revised form 1 May 2016
Accepted 13 May 2016
Available online 14 May 2016
Keywords:
for the overall reaction were also calculated [Ea = (20.1 0.9) kcal mol−1, ꢀH =/ = (19.5 0.9) kcal mol−1
,
Hydroformylation
Formaldehyde
Rhodium
Diphosphine
Kinetics
ꢀS =/ = (−9.3 0.5) cal K−1 mol−1 and ꢀG =/ = (22 2) kcal mol−1]. The kinetic study together with results
on coordination chemistry performed with 1a and with the BF4 salt of its iridium analogue, [Ir(␬2-P,P-
dppe)2]BF4 ([1b]BF4), as well as theoretical DFT-calculations allowed us to propose a mechanism for
this reaction. The catalytic cycle involves the reversible oxidative addition of formaldehyde to 1a to
yield [Rh(H)(CHO)(␬2-P,P-dppe)2]+, followed by the insertion of the olefin into the Rh-H bond to gen-
erate [Rh(alkyl)(CHO)(␬2-P,P-dppe)2]+, which is considered as the rds of the process [Ea2 = (23.9 0.8)
kcalmol−1]; the cycle is completed by the reductive elimination of the aldehydes to regenerate 1a and
restart the cycle.
Mechanisms
1-Hexene
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
triphosphine [10] have been published; they have shown that the
syn-gas hydroformylation is a very complex reaction due to the
The Rh-catalyzed olefin hydroformylation under a CO/H2 mix-
ture (henceforth referred as syn-gas) is a well-known synthetic tool
for a wide range of organic molecules and one of the largest scale
industrial application of homogeneous and aqueous-biphasic catal-
ysis [1]. In recent years, new applications of this reaction to obtain
high value intermediates for fine chemicals and pharmaceuticals
are emerging in the literature [2]. Research on kinetics of this reac-
tion catalyzed by RhH(CO)(PPh3)3 [3], RhH(CO)(AsPh3)3 [4], and
other rhodium systems containing phosphites [5], phospholes [6],
diphosphines [7], diphosphites [8], phosphine-phosphite [9] and
existence of various equilibria, which depend on the concentration
of the reactants (H2, CO and free phosphine ligands).
Another way to carry out this reaction is by the use of formalde-
hyde instead of syn-gas, a particular case of the hydroacylation
reaction [11]. Furthermore, it has several advantages over the
traditional method: (a) it enables the reaction to occur under
atmospheric pressure of an inert gas, (b) it does not require high-
pressure equipments, and (c) it avoids the use of lethal carbon
monoxide. In spite of these advantages, the hydroformylation
of olefins with formaldehyde had been poorly investigated [12].
Indeed, in 1999, Seok et al. [13] reported that RhH(CO)(PPh3)3 was
an active precatalyst for the hydroformylation with formaldehyde
of olefins containing oxygen at the ␤ position to the carbon–carbon
double bond (allyl alcohol, acrolein and methylacrylate); how-
ever, simple olefins, such as propylene, 1-hexene and styrene,
were poorly hydroformylated. On the other hand, some of us
ଝ
In memory of Professor Roberto Sanchez-Delgado.
Corresponding author.
E-mail addresses: merlinrosalesaiz@gmail.com, merlin2002@cantv.net
∗
(M. Rosales).
http://dx.doi.org/10.1016/j.molcata.2016.05.014
1381-1169/© 2016 Elsevier B.V. All rights reserved.

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
123
have reported that the hydroformylation of 1-hexene and other
C6 olefins as well as allyl alcohol using formaldehyde as syn-gas
substitute can be catalyzed by rhodium-phosphine system, being
[Rh(␬2-P,P-dppe)2]acac ([1a]acac; dppe: Ph2P(CH2)2PPh2)2, 1,2-
bis(diphenylphosphino)ethane) the most active precatalyst; this
system can be an efficient catalytic precursor either for both simple
olefins and for those molecules containing oxygen at the ␤ position
to the carbon–carbon double bond [14].
More recently, Morimoto et al. [15] reported a highly linear
regioselective hydroformylation of 1-alkenes using formaldehyde
to give aldehydes by using of rhodium catalysts with two type
of diphosphine: Rh/binap [binap: 2,2ꢀ-bis(diphenylphosphino)-
1,1ꢀ-binaphthyl] for formaldehyde decomposition to give syn-gas
and Rh/xantphos [xantphos: 4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene] for olefin hydroformylation. In addition, Taddei
et al. [16] showed that this reaction may be accelerated under
microwave dielectric heating. Other recent works on the reaction
of olefins, dienes and acetylenes with formaldehyde have been
also reported [17], which lead us to think that formaldehyde is an
excellent surrogate of syn-gas for the hydroformylation of olefins.
Beller et al. [18] have published a comprehensive review dealing
with recent advances on alkene carbonylation reactions performed
without the use of carbon monoxide, including the hydroformyla-
tion of olefins with formaldehyde.
carbowax 100/120 Supelcoport column, using N2 as carrier gas).
Calibration was made with cyclohexane as the internal standard.
Each reaction was repeated at least twice in order to ensure repro-
ducibility of the results.
2.3. Kinetic calculations
All the kinetic runs were carried out at low conversions (in the
range of 5–10%) in order to perform a kinetic analysis based on the
initial rate method [20]. The data of the catalytic reactions were
plotted as total molar concentration of the products (heptanal and
2-methyl-hexanal) versus time yielding straight lines, which were
fitted by conventional linear regression programs (r2 > 0.95); ini-
tial rates of the reaction (ri) were obtained from the corresponding
slopes. The orders of the different components of the reaction (cata-
lyst, substrate and formaldehyde) were determined from the slope
of the plot of the logarithm of ri versus logarithm of the correspond-
ing reagent concentration.
2.4. Coordination chemistry related with the hydroformylation of
1-hexene with formaldehyde
2.4.1. Interaction of [1a]acac with formaldehyde
This reaction was carried by reaction of [1a]acac, which was
formed in situ as reported in a prior work [14a], with a ten-fold
excess of formaldehyde (112 mg, 4 mmol) under reflux for 1 h; the
solution was evaporated until dryness and the solid was washed
Continuing our research program on kinetics and mechanisms
of homogeneous catalytic reactions, in the present paper we report
the data obtained from experimental kinetic and coordination
chemistry studies as well as Density Functional Theory (DFT) calcu-
lations of the hydroformylation of 1-hexene with formaldehyde by
using [1a]acac as precatalyst. To the best of our knowledge, there
are not previous reports in the open literature of the kinetics of the
hydroformylation of olefins by using formaldehyde.
1
with n-pentane. This reaction was also followed by in situ 31P{ H}
NMR using an 1:1 mixture of 1,4-dioxane and THF-d8 as the solvent.
2.4.2. Synthesis of [Ir(ꢁ2-P,P-dppe)2]BF4 ([1b]BF4)
[Ir(␮-Cl)(COE)2]2 [21] (178 mg, 0.2 mmol), dppe (320 mg,
0.8 mmol) and benzene (15 mL) were place under reflux for 2 h.
The resulting solution was evaporated under vacuum, dissolved in
1,4-dioxane (10 mL) and NaBF4 (10 mg, 0.4 mmol) was added; the
solution was heated for 1 h. The solvent was removed until dry-
ness and the product was washed with methanol and n-pentane
and dried in vacuum. Yield: 70%. IR (KBr disk): 1080 cm−1 (vs, ꢂB-
F); 1H NMR (CDCl3, 298 K, 300 MHz,) ␦ = 7.4–7.2 (m, Ph, 40H) and
2. Experimental
2.1. Instruments and materials
All manipulations and reactions were performed under rigorous
exclusion of air using a vacuum line, an argon-filled Schlenk line or
an argon-filled glovebox. 1-Hexene (B.D.H. Laboratory Reagents)
was purified by distillation at reduced pressure and the solvents
were distilled from appropriate drying agents, immediately prior to
use. CDCl3 and THF-d8 (Aldrich) were dried using activated molec-
ular sieves. Formaldehyde was generated in situ in the reaction
medium from paraformaldehyde. Complex [1a]acac was prepared
by published procedures [14a] from Rh(acac)(CO)2 [19]. The IR
spectra of the complexes (in KBr disk) were recorded on a Shimadzu
1
2.1 ppm (br s, CH2-P, 8H); 31P{ H} NMR (CDCl3, 298 K, 121 MHz)
␦ = 48.1 ppm (s).
2.4.3. Reaction of [1b]BF4 with formaldehyde: synthesis of cis-
and trans-[Ir(H)2(ꢁ2-P,P-dppe)2]BF4
[1b]BF4 (195 mg, 0.2 mmol) and a ten-fold excess of formalde-
hyde (56 mg, 2 mmol) were dissolved in 1,4-dioxane as the solvent
(10 mL) and the reaction mixture was place under reflux for 2 h.
The solution was evaporated under vacuum to about 1/4 of its initial
volume and the product was precipitated by addition of n-pentane,
filtered and dried in vacuum. Yield: 80%. IR (KBr disk): 2090 and
2080 cm−1 (m, ꢂIr-H), 1080 cm−1 (vs, ꢂB-F); 1H NMR (CDCl3, 298 K,
300 MHz,): ␦ = 8.5–6.9 (m, Ph, 40H), 2.3 (m, CH2-P, 8H), −10.5
(quint, 2J(H-P) = 15 Hz, Ir-H), −11.8 ppm (dm, 2J(H-P) = 112 Hz, Ir-
1
8300 FT-IR instrument. 1H and 31P{ H} NMR spectra were recorded
on a Bruker AM-300 spectrometer; chemical shift are expressed in
p.p.m. upfield from Me4Si and H3PO4, respectively.
2.2. General procedure for kinetic runs
1
H); 31P{ H} NMR (CDCl3, 298 K, 121 MHz): ␦ = 38.9 (s), 32.9 (t,
The reactions were carried out in a Parr Instrument high-
pressure reactor, which was provided with an arrangement for
sampling of liquid contents in order to take reaction mixture
aliquots at regular intervals of time, besides of automatic temper-
ature, pressure and variable stirrer speed controls. In a typical run,
a solution of the catalyst, 1-hexene, formaldehyde, cycloheptane
as internal standard (1.0 mL, 8.3 mmol) and 1,4-dioxane (total vol-
ume 30 mL) was placed in the reactor. The solution was carefully
deoxygenated with argon and charged with this gas at atmospheric
pressure, and the reactor was heated to the desired temperature;
the reaction was monitored by using GC (3300 Series VARIAN with
a flame ionization detector fitted to a 2 m 20% SP-2100 on a 0.1%
2J(P-P) = 7 Hz) and 21.1 ppm (t, 2J(P-P) = 7 Hz); elemental analysis
calcd (%) for IrC52H50BF4P4: C 57.94, H 4.68; found: C 58.12, H 4.59.
2.5. Computational details
All the calculations were carried out with the Gaussian-09
(G09) [22] computational package by using the hybrid functional
M06L [23] of Density Functional Theory (DFT) combined with the
effective core potential (ECP) LanL2DZ for Rh and P [24] and the
extended basis set 6–31 + G(d,p) for C, H and O were used [25].
Minimum energy structures (MES) and transition states (TS) cal-
culations for all rhodium species involved in the catalytic cycle

124
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
Table 1
Table 3
Effect of the solvent on the hydroformylation of 1-hexene with formaldehyde cat-
Kinetic data for the hydroformylation of 1-hexene with formaldehyde catalyzed by
alyzed by [Rh(␬2-P,P-dppe)2]acac ([1a]BF4).
[Rh(␬2-P,P-dppe)2]acac ([1a]BF4) at 373 K.
Entry
Solvent
TON[b]
l/b
Entry
103 [Rh]/M
[C6H12]/M
[CH2O]/M
104 ri/Ms−1[a]
1
2
3
4
5
1,4-dioxane
THF
Diglyme
Acetonitrile
2-methoxyethanol
201
143
89
9
9
7
5
1
1
1.7 0.2
1.8 0.2
2.1 0.3
1.5 0.1
1.8 0.2
1
2
3
4
5
6
7
8
1.03
1.68
2.59
3.36
4.13
1.68
1.68
1.68
1.68
1.68
1.68
1.68
1.68
1.68
1.68
1.68
0.50
0.50
0.50
0.50
0.50
0.24
0.37
0.64
0.77
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.22
0.33
0.44
0.67
0.78
1.00
1.25
1.12 0.06
1.77 0.09
2.56 0.10
3.33 0.09
4.02 0.40
0.87 0.11
1.37 0.10
2.36 0.08
3.02 0.15
0.99 0.04
1.24 0.15
1.68 0.07
2.00 0.09
2.07 0.12
2.08 0.06
2.15 0.14
4
Conditions: [Rh] = 3.3 mM, [1-hexene] = 1.0 M, [CH2O] = 2.5 M, total volume = 15 mL,
T = 403 K; TON: mol products mol cat−1 in 4 h; l/b: linear to branched ratio.
9
10
11
12
13
14
15
16
Table 2
Effect of the presence of H2 and CO on the rate of 1-hexene hydroformylation with
formaldehyde catalyzed by [Rh(␬2-P,P-dppe)2]acac ([1a]BF4).
Entry
Conditions
105 ri/Ms−1
9.90 0.40
5.18 0.11
6.65 0.18
1.28 0.09
10.90 0.11
1
2
3
4
5
Formaldehyde
3 atm 1:1H2:CO
Formaldehyde/3 atm H2
Formaldehyde/3 atm CO
Formaldehyde/3 atm 1:1H2:CO
Conditions: solvent: 1,4-dioxane; total volume = 30 mL.
other hand, other series of reactions were performed in presence
of syn-gas, hydrogen or carbon monoxide (Table 2). The hydro-
formylation of 1-hexene with formaldehyde catalyzed by [1a]acac
is faster (entry 1) than that carried out under a low pressure of syn-
gas without formaldehyde (3 atm, entry 2); the presence of 3 atm
of either molecular hydrogen (entry 3) or carbon monoxide (entry
4) in the hydroformylation with formaldehyde retards the reaction,
whereas in the presence of syn-gas (3 atm, entry 5) the reaction is
slightly faster than that without syn-gas. These results are in accord
with those reported by Seok et al. [13] for the hydroformylation
with formaldehyde of propylene, 1-hexene and styrene catalyzed
by RhH(CO)(PPh3)3, and they are consistent with a mechanism that
does not involved decomposition of formaldehyde into H2 and CO.
In fact, if the reaction occur by the conventional syn-gas hydro-
formylation mechanism, the reaction rate should increase or keep
constant when hydrogen is added to the reaction medium due to
the first order found on hydrogen concentration at low H2 pressure
and zero order at pressure higher than 3.5 atm for the 1-hexene
hydroformylation with syn-gas catalyzed by theRh/dppe system
[7a].
Conditions: [Rh] = 1.68 mM, [1-hexene] = 0.50 M, [CH2O] = 0.22 M; solvent: 1,4-
dioxane; total volume = 30 mL, T = 373 K.
were performed using simplified dpe diphosphine analogue (dpe:
1,2-bis(diphosphino)ethane, H2P(CH2)2PH2) and ethylene as olefin
model in order to reduce the computational cost. Furthermore,
the effect of solvent 1,4-dioxane is included by the subroutine
SCRF = (Solvent = 1,4-Dioxane) [26]. All the optimized structures
were classified in accord to frequency calculations. These fre-
quencies permit us to distinguish between the MES (with all the
vibrational frequencies positives) associated to the intermediate of
reaction and the first order saddle points (with only one imaginary
frequency) associated to the TS. All of the TS were obtained with the
synchronous transit-guide quasi-Newton (STQN) methods, using
the keyword QST2 and/or QST3 implemented in G09. Moreover; the
intrinsic reaction coordinates (IRC) method; which examines the
reaction paths going down from the TS on the potential energy sur-
face (PES); was carefully analyzed to confirm the right connectivity
between all TS and their associated reaction immediate (MES).
Although the reaction mixtures were homogeneous solutions
with no evidence of metallic components, the homogeneity of
the reaction was further established by the method of addition of
mercury, which is known to deactivate metallic catalysts but not
homogeneous systems [27]; no effect of Hg on the catalysis was
observed. This fact, combined with the excellent reproducibility of
the kinetic measurements, is strongly indicative that the hydro-
formylation of 1-hexene with formaldehyde catalyzed by [1a]acac
was truly homogeneous and if any metallic rhodium was pro-
duced during the reaction, it was not responsible for the catalysis.
Finally, the initial rates (ri) of the 1-hexene hydroformylation with
formaldehyde were found to be independent of stirring speed in
the range of 600–800 rpm, therefore it was chosen a stirring speed
of 700 rpm.
3. Results and discussion
The hydroformylation reaction of 1-hexene with formalde-
hyde to generate heptanal and 2-methyl-hexanal was efficiently
catalyzed by using [Rh(␬2-P,P-dppe)2]acac ([1a]acac) as the precat-
alyst (Eq. (1)). In Table 1, we collect the experimental values about
the effect of several solvents on the activities and regioselectivi-
ties, i.e. the linear to branched ratio (l/b), for this reaction at 403 K.
The turnover numbers (TON) were higher in ether solvents (1,4-
dioxane > THF > diglyme) than in more polar ones (acetonitrile and
2-methoxyethanol), in which 1-hexene was poorly hydroformy-
lated; the regioselectivities (l/b ratio) slightly varies from 1.5 and
2.1; in non-polar solvents (toluene or xylene), [1a]acac was catalyt-
ically inactive.
3.2. Kinetic investigation
(1)
Although at 403 K and high reactant concentrations [1a]acac
showed a high catalytic activity in 1,4-dioxane, the reaction can be
performed under lower temperature and reactant concentrations
(entry 1, Table 2) which allowed us to measure the initial reac-
tion rate, ri [20]. A material balance of all the products indicated
that 1-hexene and the aldehydes are the main compounds present
in the reaction mixtures (>95%), observing only small amounts of
the isomerization products (<5%), under these conditions. On the
The kinetics of the hydroformylation of 1-hexene with
formaldehyde catalyzed by [1a]acac at 373 K was performed by
monitoring the reaction by GC; the data are listed in Table 3. We had
carried out three series of experiments at different catalyst (entries
1–5), substrate (entries 2 and 6–9) and formaldehyde concentra-
tions (entries 2 and 10–16).
The reaction was found to be first-order with respect to catalyst
and substrate concentrations (0.92 and 1.05, respectively) and it
varies in accord to a saturation curve with respect to formaldehyde

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
125
80 ◦C. It is probably that cationic complex 1a reversibly reacts
with formaldehyde to produce [RhH(CHO)(␬2-P,P-dppe)2]+ (2a),
which in absence of the olefin returns to the original complex; this
behaviour could explain the fractional order found for the reaction
rate on formaldehyde concentration. Other possibility is that 2a
decomposes forming H2 and CO regenerating 1a. However, the cat-
alytic experiments in presence of 3 atm of H2 and theoretical DFT
studies of decomposition of coordinated formaldehyde in the dpe
analogue of intermediate 2a (vide infra) indicate that aldehydes are
generated by the hydroacylation mechanism rather than through
the conventional hydroformylation one, this is decomposition of
formaldehyde to form H2 and CO and subsequent hydroformylation
as it was reported by Morimoto et al. [15].
Due to the impossibility of isolate or detect any hydrido-
formyl rhodium species by interaction of 1a with formaldehyde,
we proceed to prepare the more robust bis(dppe)iridium complex,
[Ir(␬2-P,P-dppe)2]+ (1b), by reaction of [Ir(␮-Cl)(COE)2]2 with four
equivalents of dppe in toluene under reflux, generating the corre-
spondent chloride salt, [1b]Cl; chloride counterion may be replaced
by BF4 one by addition of sodium tetrafluoroborate in 1,4-dioxane
solvent. The reaction of [1b]BF4 with a ten-fold excess of formalde-
hyde produce a pale yellow solid. The 1H NMR of this solid showed,
besides the signal of phenyl and methylene groups, two high
field signals in ratio 1:4: a quintet at −10.5 ppm (2J(HP) = 15 Hz)
and a doublet of multiplets (dm) at −11.8 ppm (2J(HPcis) ≈ 15 Hz,
2J(HPtrans) = 114 Hz); the Ph:CH2:hydrides ratio was 20:4:1. On the
other hand, the 31P{ H} NMR spectrum shows a singlet at 38.9 ppm
1
and two triplets at 32.9 and 21.1 ppm, 2J(PP) ≈ 7 Hz, also in ratio 1:4.
These data may be interpreted through the formation of two iso-
mers of a dihydride species [Ir(H)2(␬2-P,P-dppe)2]BF4, one in which
the two hydride ligands are in mutually cis positions (major species)
and other in which they are in mutually trans (minor species).
The chloride salt of cis-[Ir(H)2(␬2-P,P-dppe)2]+ [28] and its ana-
logue of 1,2-bis(di-isopropylphosphino)ethane (dippe) [29] have
been previously reported, and their spectra agree well with those
of cis-[1b]BF4.
Scheme 1. Coordination chemistry related with the hydroformylation of 1-hexene
with formaldehyde catalyzed by [M(␬2-P,P-dppe)2]+ (M = Rh, Ir).
concentration; at low formaldehyde concentrations (<0.78 M), the
reaction presented a fractional order dependence (0.63) whereas
at higher concentrations, the reaction rate was independent (zero
order) of this parameter (Supplementary Material SM-I). The first
order found with respect to 1-hexene concentration does indicate
that the substrate is probably involved in the rate-determining step
(rds) of the reaction, whereas the fractional order on formaldehyde
concentration implies that this component is involved in a previous
equilibrium to the rds.
Complexes [Ir(H)2(␬2-P,P-dppe)2]BF4 (cis- and trans-) could be
originated by the initial oxidative addition of formaldehyde on
[1b]BF4 to yield the hydrido-formyl species cis-[Ir(H)(HCO)(␬2-
P,P-dppe)2]BF4 (cis-2b), which decarbonylates to produce the
corresponding dihydride complexes plus carbon monoxide or by
hydrogen transfer from 1,4-dioxane solvent [30]; the same reaction
was carried out in absence of formaldehyde but the corresponding
dihydrido species were not obtained, which discard the second pos-
sibility. Examples of hydrido-formyl species of iridium generated
by oxidative addition of formaldehyde have been published in the
literature [31]. The formation of trans-[Ir(H)2(␬2-P,P-dppe)2]BF4
is an additional proof that the hydride ligands proceeds from
formaldehyde decomposition [32]. Some trans-dihydride iridium
complexes have been reported in the literature including some
complexes formed by cis-trans isomerization [32,33].
Finally, the interaction of either complex [1a]acac or [1b]BF4
with formaldehyde and 1-hexene yields the corresponding C7 alde-
hydes (l/b ≈ 2); the only metal species observed at the end of the
reaction were the corresponding bis(dppe) complexes. Complex
[1b]BF4 showed be less active for the hydroformylation of 1-hexene
with formaldehyde (TON = 46 and l/b = 2.1 in 8 h) than [1a]acac
(TON = 201 and l/b = 1.8 in 4 h) at 130 ◦C (conditions as in Table 1); in
the reaction of hydroformylation with formaldehyde catalyzed by
the iridium precatalyst [1b]BF4 was observed significant amounts
of the hydrogenation product (n-hexane) in accord with the coor-
dination chemistry of [1b]BF4 with formaldehyde, which generates
cis- and trans- isomers of a dihydride complex.
In view of the above kinetic observations, the catalytic hydro-
formylation of 1-hexene with formaldehyde by using [1a]acac
proceeds according to the rate law:
a
ri =
Rh C H
] [
CH O
12] [
(2)
[
]
6
2
b + c CH O
[
]
2
3.3. Coordination chemistry related with the hydroformylation
with formaldehyde
Cationic complex 1a can be formed easily as its acetylaceto-
nate (acac) salt by the addition of 2 equivalents of dppe to the
complex Rh(acac)(CO)2 in 1,4-dioxane solution under reflux [14a].
In order to gain some understanding on the mechanism of the
reaction of 1-hexene with formaldehyde catalyzed by [1a]acac,
the reaction of this complex and the analogue of iridium, [Ir(␬2-
P,P-dppe)2]BF4 ([1b]BF4), with each component of the catalytic
mixture (i.e. formaldehyde, 1-hexene and 1,4-dioxane) was car-
ried out with the aim of isolating or detecting some intermediates;
the results are showed in Scheme 1. The interaction of [1a]acac
with an excess of formaldehyde ([CH2O]/[Rh] = 10) in 1,4-dioxane
under reflux produces a color change from yellow to orange,
1
from which a yellow solid was isolated; the 1H and 31P{ H}
NMR spectra indicate the presence of only the starting complex.
The same reaction was performed on a NMR tube by using 1,4-
1
dioxane/THF-d8, and monitored by 31P{ H}; it was impossible to
detect any other species different to [1a]acac by heating until

126
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
3.4. DFT-calculations on the thermodynamic parameters of the
elementary steps of the mechanism of hydroformylation of
1-hexene with formaldehyde
In order to obtain detailed insight into the mechanisms of
the hydroformylation of 1-hexene with formaldehyde catalyzed
by [1a]acac, various pathways were calculated via DFT using
the simplified diphosphine model 1,2-bis(diphosphino)ethane,
H2P(CH2)2PH2 (dpe), in which the phenyl groups of the dppe
ligands were replaced by hydrogen atoms; we also used ethy-
lene as a model for the substrate. In a previous work, some
of us reported theoretical DFT-calculations for the [Rh(␬2-P,P-
dpe)2]+ (A), performed with B3LYP hybrid method of density
functional theory (DFT) and the 6–31 + G(d, p) basis sets [14b].
Now, we corroborate this structure by DFT employing the M06-
L functional/LANL2DZ/6–31 + +G(d,p) basis set. The theoretically
calculated structure satisfactorily reproduces that determined by
X-ray diffraction for [Rh(dppe)2]+ [34] and for its Cy2P(CH2)2PCy2
analogue (Cy = cyclohexyl) [35], although some distance and angle
values were sub- or overestimated. This agreement between the
calculated structural parameters and the experimental values
shows that the method used in this work can provide reliable
geometries for the species involved along the entire catalytic cycle.
Here, we report DFT-calculations of thermodynamic properties,
such as energy (ꢀE◦), enthalpy (ꢀH◦) and free energy (ꢀG◦) for
the possible elementary steps of the catalytic cycle at 373 K and
1 atm, in order to simulate the experimental conditions. The struc-
tures of intermediates are shown in Fig. 1 and the results of the
thermodynamic parameter for the elementary step of the cycle are
shown in Scheme 2.
As the order of the reaction rate on formaldehyde concentration
was fractional, we thought that the reversible oxidative addition of
formaldehyde to the bis(dpe)rhodium complex (A) is the first step
of the mechanism. This reaction may form two octahedral isomers,
one containing the hydride and the formyl ligands coordinate in
mutually cis positions (B in Fig. 1) and other in which these two
ligands are in trans positions (D in Fig. 1). The state functions of the
A + CH2O ꢀ B step (formation of cis isomer) indicate that the pro-
cess is exothermic and favored thermodynamically, whereas the
A + CH2O ꢀ D step (formation of trans isomer) is endothermic and
disfavored thermodynamically; therefore we thought that in the
catalytic reaction only the cis complex is formed.
On the other hand, as the reaction is first order on olefin, we
believe that the reaction of cis-[Rh(H)(COH)(␬2-P,P-dpe)2]+ (B)
with ethylene should be considered as the possible rds of the pro-
cess; this reaction may occur by direct insertion of the olefin in the
Rh-H bond (B + C2H4 → C) to produce the octahedral ethyl-formyl
intermediate cis-[Rh(C2H5)(COH)(␬2-P,P-dpe)2]+ (C in Fig. 1) or
through the coordination of the olefin by dissociation of one of the
arm of a diphosphine ligand (B + C2H4 → E), and posterior migra-
tion of the hydride to the olefin (E → C). We consider that reaction
of B with ethylene occurs through the direct insertion of ethylene
in the Rh-H bond, which is exothermic and thermodynamically
favored (ꢀG◦ = −6.2 kcal mol−1), whereas the process B + C2H4 → E
is endothermic (ꢀH◦ = 5.3 kcal mol−1) and thermodynamically dis-
favored (ꢀG◦ = 17.7 kcal mol−1). The insertion of ethylene in the
Rh-formyl bond of the intermediate B (do not shown in Scheme 1)
Fig. 1. Structures of the intermediates involve in the catalytic cycle of hydroformy-
lation of ethylene with formaldehyde catalyzed by [Rh(␬2-P,P-dpe)2]+.
ethylene, propanal and C) are included in Supplementary Material
SM-II.
3.5. The mechanism of the hydroformylation of 1-hexene with
formaldehyde
was also thermodynamically disfavored (ꢀE◦ = −6.6 kcal mol−1
,
On the basis of the experimental findings (kinetics and coor-
dination chemistry) and DFT-calculations, we propose that the
mechanism of the hydroformylation of 1-hexene with formalde-
hyde catalyzed by [1a]acac involves the sequence of reactions
showed in Scheme 3 (branched isomer excluded for clarity).
The first step consists in the reversible oxidative addition of
formaldehyde to the cation 1a to yield the corresponding species
cis-[Rh(H)(CHO)(␬2-dppe)2]+ (cis-2a), followed by the insertion of
the olefin into the Rh-H bond to generate [Rh(C6H13)(CHO)(␬2-
ꢀH◦ = −7.4 kcal mol−1 and ꢀG◦ = 8.8 kcal mol−1).
Finally, the reductive elimination of the formyl and ethyl groups
of intermediate C to yield propanal and regenerate the active
species (A) is exothermic (ꢃH = −3.2 kcal/mol) and thermodynam-
ically favored (ꢃG = −17.9 kcal/mol).
The structural parameters of optimized geometry for all of the
intermediates involved in the catalytic cycle (A, formaldehyde, B,

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
127
Taking into account the K1 equilibrium, the 2a concentration
may be established as [2a] = K1[1a][CH2O], and therefore:
ri = K1k2[1a][C6H12][CH2O]
(4)
Assuming that species 3a is rapidly transformed into 1a, the
mass balance for the rhodium is [Rh]0 = [1a] + [2a], and therefore
the concentration of 1a may be expressed as:
Rh
[
]
o
1a =
]
(5)
[
1 + K1 CH O
[
]
2
Substituting [1a] in eq. (4) allows us to rewrite the rate expres-
sion as:
K1k2
1 + K1 CH O
ri =
Rh C H
] [
CH O
]
2
(6)
[
12] [
6
[
]
2
This theoretical rate law is in good accord with the experimental
finding, Eq. (2), with a = K1k2, b = 1 and c = K1[CH2O], and explains
why the dependence of the reaction rate with respect to formalde-
hyde concentration tends to change from fractional to zero order
on going from low to high concentration of CH2O.
Eq. (6) can be inverted and reorganized as:
Rh C H
] [ ]
12
1
k2
1
Scheme 2. Thermodynamic parameters of the elementary steps of the catalytic
cycle for the ethylene hydroformylation with formaldehyde catalyzed by [Rh(␬2-
P,P-dpe)2]+ (values in brackets were calculated at MP2 level).
[
6
=
+
(7)
ri
K1k2[CH2O]
A plot of [Rh]0[C6H12]/ri versus the reciprocal of formaldehyde
concentration yields a straight line (Supplementary Material SM-
III), in agreement with the kinetic results, from which the values
of K1 and k2 [(1.47 0.04) M−1 and (0.47 0.01) M−1 s−1, respec-
tively] can be obtained at 373 K.
At high formaldehyde concentrations, K1[CH2O] » 1 in Eq. (6),
and therefore the rate expression can be approximated to:
P
+
P
Rh
P
K1
O
R
P
r = k2 RH C H
]0 [
(8)
[
]
12
6
i
CH2O
1a
which explains why the order with respect to formaldehyde
concentration is zero under these conditions. At low formalde-
hyde concentrations, K1[CH2O] ≈ 0.32-1.14, which explains the
fractional order on formaldehyde concentration under these con-
ditions.
fast
CH2O
O
O
As may be observed, although the kinetics and mechanisms
of the hydroformylation reactions with syn-gas catalyzed by
phosphine-modified rhodium catalysts are very complex due to
the existence of various equilibria (which depend on the H2, CO
and free phosphine concentrations), both the kinetics as the mech-
anism of the alternative reaction with formaldehyde are simpler
compared with those carried out under syn-gas conditions.
H
H
C
C
H
P
+
+
R
P
Rh
P
P
Rh
P
P
P
P
k2
3a
2a
3.6. Effect of temperature on the reaction rate
R
In order to calculate the activation parameters, the effect of
the temperature (353–393 K) on the rate constant was studied for
the hydroformylation of 1-hexene with formaldehyde (Table 4,
entries 1–5), keeping the catalyst (1.68 mM), 1-hexene (0.50 M)
and formaldehyde (0.56 M) concentrations constant; the plot of
ln kcat versus 1/T(K) for this reaction yielded a straight line
(Supplementary Material SM-IV), from which the corresponding
activation energy (Ea = 20.1 0.9 kcal mol−1) and frequency factor
were evaluated; the values of enthalpy, entropy and free energy of
Scheme 3. Proposed catalytic cycle for the hydroformylation of 1-hexene with
formaldehyde catalyzed by [Rh(␬2-P,P-dppe)2]acac ([1a]BF4).
dppe)2]+ (3a), which is considered as the rds of the catalytic cycle;
the cycle is completed by the reductive elimination of the corre-
sponding aldehyde, regenerating 1a and restarting the cycle.
According to this mechanism, the rate law for the hydroformy-
lation of 1-hexene with formaldehyde catalyzed by [1a]acac can be
derived by applying the Equilibrium Approximation. As the inser-
tion of the olefin into the rhodium-hydride bond of 2a is the rds of
the cycle, a rate law can be derived as follows:
activation were also determinated [ꢀH =/ = (19.5 0.9) kcal mol−1
,
ꢀS =/ = (−9.3 0.5) cal K−1 mol−1 and ꢀG =/ = (22 2) kcal mol−1].
The negative activation entropy obtained for this reaction supports
an ordered transition state leading to catalytic hydroformylation.
Other set of experiments were also carried out at two different
temperatures, 383 (Table 4, entries 4 and 6–9) and 393 K (Table 4,
entries 5 and 10–13), and at several formaldehyde concentra-
tions, keeping the catalyst and 1-hexene concentrations constants
ri = k2[2a][C6H12
]
(3)

128
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
Table 4
Effect of the temperature on the hydroformylation of 1-hexene with formaldehyde
catalyzed by [Rh(␬2-P,P-dppe)2]acac ([1a]BF4).
Entry
T/K
[CH2O]/M)
104 ri/Ms−1
1
2
3
4
5
6
7
8
373
353
363
383
393
383
383
383
383
393
393
393
393
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.56
0.22
0.33
0.44
0.67
1.12 0.06
1.77 0.09
2.56 0.10
3.33 0.09
4.02 0.40
0.87 0.11
1.37 0.10
2.36 0.08
3.02 0.15
0.99 0.04
1.24 0.15
1.68 0.07
2.00 0.09
9
10
11
12
13
Conditions: [Rh] = 1.68 mM, [C6H12] = 0.50 M; solvent: 1,4-dioxane, total vol-
ume = 30 mL.
(1.68 mM and 0.5 M, respectively) in order to determine the val-
ues of K1 and k2 at these temperatures. Making use of Eq. (6),
the values of K1 and k2 were determined; a plot of ln k2 versus
1/T(K) allowed us to calculate the activation energy of the rds,
Ea2 = (23.9 0.8) kcal mol−1, and the other activation parameters
of this elementary step (Supplementary Material SM-V).
If the limit case of the expression rate is considered
at low formaldehyde concentrations (first order kinetics,
ri = K1k2[Rh]o[C6H12][CH2O]), it is possible to extract inter-
esting conclusions. Considering the rate constant at low [CH2O] in
the logarithm form (ln kcat = ln K1 + ln k2), making use of the direct
and inverse constants (k1 and k-1), and substituting each one of
the terms of the k’s by the corresponding Arrhenius expression,
may be established:
Eacat = Ea1 − Ea−1 + Ea2
(9)
As the activation energies of the overall reaction and the rds
were 20.1 (Eacat) and 23.9 kcal mol−1 (Ea2), respectively, it may be
supposed that the activation energy for the forward reaction of the
first elementary step (Ea1) is smaller than the one for the reverse
reaction (Ea-1) in 3.8 kcal mol−1
.
3.7. DFT-calculations of the potential energy surfaces (PES) for
the intermediates of the mechanism
As
the
reactions
A + CH2O ꢀ B,
B + C2H4 → C
and
C → A + propanal are the thermodynamically favored steps of
the catalytic cycle, we proceeded to explore the potential energy
surfaces (PES) that connects to this step sequence of the catalytic
process; the transition states (TS) are shown in Fig. 2.
Fig. 2. Structures of the TS involved in the catalytic cycle of hydroformylation with
formaldehyde catalyzed by [Rh(␬2-P,P-dpe)2]+.
The first step of the proposed mechanism, the oxidative addition
of CH2O to the active species (A), should overcome an activa-
tion energy of 8.4 kcal mol−1 to produce intermediate B; this step
occurs through the three-centre transition state (TS1), in which one
of the C H bonds of formaldehyde is lengthened (1.442 Å) when
compared with the corresponding distances in free formaldehyde
(1.100 Å) whereas the Rh H (1.693 Å) and Rh C bonds (2.142 Å)
are being formed.
Complex B reacts with ethylene (direct insertion) through the
four-centered transition state (TS2), which requires overcome an
activation energy of 40.0 kcal mol−1 (Ea2) to form the ethyl-formyl
intermediate C; in this TS, the Rh-H and C C bond distances are
lengthened (2.078 and 1.445 Å) when compared with the corre-
rds of the overall process, as we had proposed on the basis of our
experimental results.
Finally, the reductive elimination of propanal from C to regen-
erate A occurs through the TS3, which requires 23.4 kcal mol−1. The
structural parameters of optimized geometry for the TS involved in
the catalytic cycle are included in Supplementary Material SM-VI.
A diagram of energy of the hydroformylation process is repre-
sented in Scheme 4; the activation energies estimated through this
methodology, bothfor theoverallreactionas for the rds (Eacat = 31.5
and Ea2 = 40.0 kcal mol−1, respectively) are higher than the values
experimentally determined (Eacat = 20.1 and Ea2 = 23.9 kcal mol−1
,
sponding ones in B (1.596 Å) and ethylene (1.331 Å); the C
(1.445 Å) and C Rh bonds (2.594 Å) are being formed. This step
has the highest activation energy, which implicates that it is the
H
respectively) probably due to the absence of phenyl groups in the
diphosphine ligands; however, qualitatively the calculated energy
profile is in accord with our experimental results.

M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
129
Ea = 36.5 Kcal/mol
CO
33.1
38.3
8.4
E
12.3
F + C2H4
A + CH2O
8.7
B
G + CO
17.8
A + C3H6O
C
H
Scheme 4. Calculated energy profile of the catalytic cycle for the hydroformylation
of ethylene with formaldehyde catalyzed by [Rh(␬2-P,P-dpe)2]+.
RC
CH3CH2C(O)H
Scheme 6. Calculated energy profile for the ethylene hydroformylation with
formaldehyde catalyzed by [Rh(␬2-P,P-dpe)2]+ through of syn-gas mechanisms.
P
Rh
P
P
C(O)CH2CH3
H
+
P
+
P
P
Rh
P
P
structural parameters of optimized geometry for intermediates and
transition states involved in this catalytic cycle are listed in Supple-
mentary Material SM-VII.
The results indicate that the calculated barrier for the
hydroformylation of ethylene with the syn-gas originated from
formaldehyde coordinated in the intermediate B (Scheme 6) is
higher (36.5 kcal mol−1) than those for the hydroacylation mech-
anism (31.5 kcal mol−1, see Scheme 4).
Experimental results reported in the present work on the hydro-
formylation of 1-hexene with formaldehyde in the presence of
3 atm of hydrogen as well as the results of a prior compara-
tive study of the 1-hexene hydroformylation either under syn-gas
conditions or with formaldehyde catalyzed by Rh/diphos sys-
tems, diphos = Ph2P(CH2)nPPh2)2 (n = 2, 3 and 4), corroborates that
hydroformylation of 1-hexene with formaldehyde does proceed via
hydroacylation mechanism [14b].
H
A
CH2O
CO
P
CH2CH3
H
P
Rh
P
+
P
P
Rh
P
C(O)H
H
+
P
P
G
P
Rh
P
B
H
H
+
P
P
CH2=CH2
CO
F
4. Conclusions
Scheme 5. Catalytic cycle for the ethylene hydroformylation by decomposition of
formaldehyde catalyzed by [Rh(␬2-P,P-dpe)2]+.
The results showed that complex [1a]acac is an efficient cata-
lyst for the 1-hexene hydroformylation with formaldehyde under
mild reaction conditions. Experimental results and theoretical
DFT-calculations allowed us to propose a catalytic cycle via hydroa-
cylation mechanism, in which the insertion of the olefin into the
Rh-H bond of species [Rh(H)(CHO)(␬2-dppe)2]acac (generated by
oxidative addition of CH2O to [1a]acac) is considered as the rds.
Finally, the kinetics and mechanism of the reaction with formalde-
hyde catalyzed by [1a]acac is simpler than that carried out under
syn-gas conditions using rhodium catalysts containing phospho-
rous ligands. To the best of our knowledge, the present work is
the first complete experimental and theoretical study on kinet-
ics and mechanism of the hydroformylation of an olefin by using
formaldehyde as syn-gas substitute.
Our DFT-calculations differ from those reported by Meng et al.
[36] for the hydroformylation of propylene with formaldehyde cat-
alyzed by a Rh(PH3)2 complex, in which the oxidative addition of
formaldehyde was the rds of the mechanism for the formation of
linear aldehyde and the reductive elimination was the rds for the
formation of isoaldehyde. In our case, for the reaction of ethylene
with formaldehyde catalyzed by [Rh(dppe)2]+, the insertion of the
olefin into the Rh-H bond is the rds of the mechanism.
Finally, in order to discard if the reaction proceeds by the clas-
sic mechanism through to the decomposition of formaldehyde in
carbon monoxide and molecular hydrogen and subsequent hydro-
formylation, we performed DFT-calculations of the elementary
steps of this mechanism (Scheme 5): decomposition of hydrido-
formyl species B to generate a dihydrido rhodium complex F plus
carbon monoxide, ethylene insertion in one of the Rh-H bond of
F to yield hydride-ethyl complex G, insertion of CO to produce a
hydrido-acyl intermediate (H) and, finally, the reductive elimina-
tion of the aldehyde for regenerate A and restart the cycle. The
Acknowledgements
Financial support from FONACIT, CYTED and Consejo de Desar-
rollo Científico y Humanístico (CONDES) of the Universidad del
Zulia (L.U.Z.) are gratefully acknowledged.

130
M. Rosales et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 122–130
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the online version, at http://dx.doi.org/10.1016/j.molcata.2016.05.
014.
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