Homogeneous hydrogenation of imines catalyzed by rhodium and iridium complexes. Kinetics and mechanism of the hydrogenation of N-(β-naphthyl methylene) aniline using [Ir(COD)(PPH3)2]PF6 as catalyst precursor

Article abstract of DOI:10.1016/S1381-1169(01)00197-2

The regioselective hydrogenation of some imines to the corresponding amines is efficiently catalyzed by [Ir(COD)(PPh₃)₂]PF₆ (1) and [Rh(COD)(PPh₃)₂]PF₆ (2) at 60°C and 200 psi of H₂. The kinetics of the hydrogenation of N-(β-naphthyl methylene) aniline (NβNA) with [Ir(COD)(PPh₃)₂]PF₆ in tetrahydrofuran (THF) have been studied. The experimentally determined rate law is r_i = k_cat [Ir][NβNA][H₂] where k_cat = (2.2 ± 0.3) × 10³ M^-2 s^-1 at 323 K and the corresponding activation parameters are: ΔH° = (4.04±0.4)kcal mol^-1; ΔS° = (-32±2) e.u.; ΔG° = (14±2) kcal mol^-1. A catalytic cycle is proposed on the basis of the kinetic experiments and some NMR data.

Full text of DOI:10.1016/S1381-1169(01)00197-2

Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
Homogeneous hydrogenation of imines catalyzed by
rhodium and iridium complexes
Kinetics and mechanism of the hydrogenation of
N-(␤-naphthyl methylene) aniline using
[Ir(COD)(PPh3)2]PF6 as catalyst precursor
V. Herreraa,∗, B. Muñoza, V. Landaetaa, N. Canudasb
Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (IVIC), Caracas 1020-A, Venezuela
a
b
Departamento de Qu´ımica, Universidad Simón Bol´ıvar, Valle de Sartenejas, Estado Miranda, Venezuela
Received 23 January 2001; received in revised form 26 April 2001; accepted 26 April 2001
Abstract
The regioselective hydrogenation of some imines to the corresponding amines is efficiently catalyzed by
[Ir(COD)(PPh3)2]PF6 (1) and [Rh(COD)(PPh3)2]PF6 (2) at 60◦C and 200 psi of H2. The kinetics of the hydrogenation
of N-(␤-naphthyl methylene) aniline (N␤NA) with [Ir(COD)(PPh3)2]PF6 in tetrahydrofuran (THF) have been studied. The
experimentally determined rate law is ri = kcat[Ir][N␤NA][H2] where kcat = (2.2 0.3) × 103 M−2 s−1 at 323 K and the
corresponding activation parameters are: ꢀH◦ = (4.0 0.4) kcal mol−1; ꢀS◦ = (−32 2) e.u.; ꢀG◦ = (14 2) kcal mol−1
.
A catalytic cycle is proposed on the basis of the kinetic experiments and some NMR data. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Hydrogenation of imines; Homogeneous catalysis; Iridium; Rhodium; Kinetics; Mechanism
1. Introduction
action that produces the corresponding amine under
mild conditions is important for organic synthesis and
in the literature we found that drastic conditions are
needed for a complete hydrogenation (40–50◦C and
20–30 atm gas pressure). In homogeneous systems,
only a few catalysts are effective for aldimine and ke-
timine [10,11] reduction but they are mostly, if not all,
also effective for alkene and/or ketone hydrogenation.
Most attention has been focused on iridium
complexes and previous studies from our labora-
tory [12–17] have demonstrated that the complexes
[Ir(COD)(PPh3)2]PF6 (1) and [Rh(COD)(PPh3)2]PF6
(2) are useful catalysts for a variety of hydrogenation
reactions.
The catalytic hydrogenation of imines to amines
has attracted considerable interest in recent years, for
simple substrates and for prochiral ones [1–8]. Ho-
mogeneous hydrogenation of carbon–nitrogen dou-
ble bonds, though more difficult and consequently
less developed than the corresponding reduction of
carbon–carbon and carbon–oxygen double bonds, has
been the focus of much recent attention [9]. This re-
∗ Corresponding author. Tel.: +58-212-5041307;
fax: +58-212-5041775/5041350.
E-mail address: vherrera@quimica.ivic.ve (V. Herrera).
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00197-2

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V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
Herein we report a comparison between complexes
(or methanol, or CH2Cl2) (5 ml) were placed into the
Parr reactor. After introduction of the catalysis com-
ponents, the reactor was flushed with H2 to remove
oxygen and then pressurized to the desired pressure at
room temperature, heated to the appropriate temper-
ature, and immediately stirred. After 18 h, the reactor
was cooled to room temperature and slowly depres-
surized. A sample of the solution was withdrawn and
analyzed by GC–MS. The reaction conditions and the
results of these experiments have been collected in
Table 1.
1 and 2 as catalytic precursors in the hydrogenation
of imines and a kinetic and mechanistic study of the
most effective system found.
2. Experimental
2.1. General procedures
All reactions and manipulations were routinely
performed under a nitrogen atmosphere by using
standard Schlenck techniques. Solvents of analyti-
cal grade were distilled from the appropriate drying
agents under N2 immediately prior to use. Aniline
(Aldrich) was purified by reduced pressure distilla-
tion. Hydrogen was purified through two columns
in series containing CuO/Al2O3 and CaSO4, respec-
tively. Tetrahydrofuran (THF) was distilled from
sodium/benzophenone, methanol from CaSO4 and
dichlorometane from P2O5. All of the imines were
synthesized according to the method reported in [18].
All other chemicals were commercial products and
were used as received without further purification.
Literature methods were employed for the synthesis
of [Ir(COD)Cl]2 [19], 1 [20], [Rh(COD)Cl]2 [21], 2
[22], and [Ir(H)2(Me2CO)2(PPh3)2]PF6 [23].
3.2. Kinetic measurements
In a typical experiment, a solution of the catalyst
[Ir(COD)(PPh3)2]PF6 and the substrate in THF was
placed in a glass reactor fitted with a reflux con-
denser kept at −5◦C. The reactor was sealed with
Apiezon wax to a high-vacuum line, and the solution
was carefully degassed by three freeze–pump–thaw
cycles; hydrogen was admitted at this point to the
desired pressure, an electric oven preheated to the
required temperature was placed around the reactor,
and magnetic stirring was immediately turned on. The
reaction was followed by measuring the hydrogen
pressure drop as a function of time. Each run was
repeated at least twice to ensure reproducibility of the
results.
2.2. Physical measurements
The conversion of reactants in the catalytic reac-
tions was generally (although not necessarily) kept be-
low 10% in order to use the initial rates method in our
calculations. The measured ꢀP(H2) values were con-
verted to mmol of amine produced, and the data were
plotted as molar concentration of the product as a func-
tion of time, yielding straight lines. Initial rates were
then obtained from the corresponding slopes. All the
straight lines were fitted by use of conventional linear
regression software to r2 > 0.98. Concentrations of
dissolved hydrogen were calculated using published
solubility data [24].
Infrared spectra were recorded on a Nicolet-FTIR
Magna 590 spectrometer using samples in KBr disk.
Deuterated solvents for NMR measurements were
dried over molecular sieves. 1H, 13C{ H} and 31P{ H}
NMR spectra were obtained on a Bruker AM-300
(300 MHz) spectrometer. 1H NMR shifts are recorded
relative to the residual 1H resonance in the deuterated
solvent. Gas chromatograph (GC)–MS analyses were
performed on a HP-5890 GC equipped with a mass
selective detector HP-5972 and a Carbowax capil-
lary column. Reactions under controlled pressure of
hydrogen were performed with a Parr reactor.
1
1
3.3. NMR experiments
3. Catalytic hydrogenation of imines
A 5 mm NMR tube was charged with a solution of
[Ir(H)2(Me2CO)2(PPh3)2]PF6 (15 mg; 0.0153 mmol)
in acetone-d6 (0.5 ml) under nitrogen. The tube was
cooled at −80◦C, the N-(␤-naphthyl methylene) ani-
line (N␤NA) was added (35 mg; 0.153 mmol) and then
3.1. Parr reactor experiments
In a typical experiment a solution of the catalyst
precursor and a 100-fold excess of the imine in THF

V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
143
Table 1
fect of temperature on the conversion to the amine
was investigated. At 100◦C and 200 psi of H2, all of
the imines selected are reduced by Ir complex over
60%. The rhodium system was less efficient but in
some cases it reduces with high conversion rates too.
At 60◦C and the same pressure there is no signifi-
cant difference in amine yields for the reduction of
imines B and D for both catalysts. These imines are
structurally similar, having a minimal steric hindrance
Hydrogenation of imines catalyzed by iridium and rhodium com-
plexes after 18 h at 200 psi H2 initial pressure
Entry
1
Substrate
Catalyst
T (◦C)
Yield (%)
A
1
60
100
60
0
60
0
2
1
100
80
2
B
25
60
100
25
60
100
87
96
93
87
87
90
=
around the C N moiety, making the hydrogenation
reaction easier, like an olefin hydrogenation. While
the reduction in these two cases occurs with mild
conditions, all of the other cases need higher tem-
peratures to obtain good yields of the corresponding
amine. In the particular case of the hydrogenation of
N␤NA (B) catalyzed by 1, we found that at 60◦C
and 200 psi of H2, a conversion of 96% was achieved.
2
3
4
5
6
7
C
D
E
F
1
2
60
100
60
0
95
0
100
70
1
2
60
100
60
90
98
82
89
=
Then, these results would indicate that the rate of C N
moieties reduction decreases with increasing steric
hindrance around them, demonstrating that this effect
100
=
plays an important role in the reduction of C N double
1
2
60
100
60
10
90
2
bonds.
100
88
4.2. Kinetic studies
1
2
60
100
60
5
80
0
The kinetics of the hydrogenation of N␤NA to
the corresponding amine (Eq. (1)) was studied in
100
60
G
1
2
60
100
60
0
100
0
Table 2
Kinetic data for the hydrogenation of N␤NA with 1 as the catalyst
100
60
precursor
104[Ir]
(M)
102[N␤NA] 105[H2]
T
(◦C)
106ri
(M s−1
(M)
(M)
)
4.0
5.0
6.0
7.0
7.7
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.5
2.5
2.5
2.5
2.5
1.5
3.5
4.5
5.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
7.36
7.36
7.36
7.36
7.36
7.36
7.36
7.36
7.36
5.28
6.29
8.17
7.36
7.36
7.36
7.36
323
323
323
323
323
323
323
323
323
323
323
323
309
330
333
341
1.7
2.2
2.5
2.9
3.0
1.0
2.9
3.8
4.2
1.5
2.0
2.5
1.5
2.4
2.5
3.0
0.1
0.3
0.3
0.8
1.0
0.3
0.6
0.4
0.3
0.1
0.2
0.3
0.6
0.3
0.1
0.9
placed into a NMR probe at the same temperature.
The reaction was followed by variable-temperature
1
31P{ H} and 1H NMR spectroscopy over the range of
−80 to +40◦C.
4. Results and discussion
4.1. Autoclave reactions
The imines shown in Scheme 1 are efficiently hy-
drogenated to the corresponding amines catalyzed by
1 and 2 under mild conditions (Table 1). The ef-

144
V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
Scheme 1.
THF by carrying out runs at different catalyst, sub-
strate and hydrogen concentrations and at differ-
ent temperatures. The complete data are listed in
Table 2.
The concentration of complex 1 was varied from
4 × 10−4 to 7.7 × 10−4 M while the amine and dis-
solved hydrogen concentrations (2.5×10−2 and 7.36×
10−5 M, respectively), and the temperature (323 K)
were kept constant. The initial rates of hydrogena-
tion of N␤NA (ri) showed a direct dependence with
(1)

V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
145
Fig. 1. Rate dependence of N-(␤-naphthyl methylene) aniline hy-
drogenation with respect to catalyst concentration.
Fig. 3. Rate dependence of N-(␤-naphthyl methylene) aniline hy-
drogenation with respect to hydrogen concentration.
respect to the catalyst concentration. The plot of log ri
versus log[Ir] (Fig. 1) yields a straight line of slope
0.99, which confirms the first-order dependence.
The effect of N␤NA concentration on the reac-
tion rate was studied varying from 1.5 × 10−2 to
5.5 × 10−2 M, keeping the iridium and dissolved hy-
drogen concentration and temperature constant. Fig. 2
shows a first-order dependence of the hydrogenation
rate with respect to substrate concentration (the slope
value of the plot log ri versus log[N␤NA] = 1.07).
The variation of the initial rates with hydrogen concen-
tration indicates a first-order dependence at low pres-
sures. This first-order dependence was confirmed by
the slope value of plot log ri versus log[H2] (slope =
1.13) (Fig. 3). Consequently, the experimental rate law
can be written as
ri = kcat[Ir][H2][N␤NA]
(2)
Eq. (2) allowed us to calculate an experimental
value for the catalytic rate constant at 50◦C [kcat
(2.2 0.3) × 103 M−2 s−1].
=
The effect of the temperature on the rate constant
was studied in the range 309–341 K for concentrations
of N␤NA at 2.5 × 10−2 M, catalyst at 5.0 × 10−4 M,
and dissolved hydrogen at 7.36 × 10−5 M. Within the
range of conditions used, the variation of the solubil-
ity of hydrogen with temperature is negligible. A plot
of ln kcat versus 1/T (Fig. 4) allowed us to evaluate the
activation energy Ea, the frequency factor A, the ex-
trapolated value of the rate constant at 298 K, and the
values of enthalpy, entropy and free energy of activa-
tion (calculated from the equations ꢀH◦ = Ea − RT,
ꢀS◦ = R ln(hA/e2kBT), and ꢀG◦ = ꢀH◦ − T ꢀS◦);
these values are listed in Table 3.
Fig. 2. Rate dependence of N-(␤-naphthyl methylene) aniline hy-
drogenation with respect to substrate concentration.

146
V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
Fig. 5. Effect of solvent on the N-(␤-naphthyl methylene) aniline
hydrogenation rate.
Fig. 4. Effect of temperature on the N-(␤-naphthyl methylene)
aniline hydrogenation rate.
4.3. Coordination chemistry related to the
hydrogenation of NβNA
In order to complement the kinetic study of the hy-
drogenation of N␤NA, we have tried to establish some
of the active species implicated in the catalysis. It is
well known that complex 1 reacts with hydrogen in
coordinating solvents to give a dihydridobis(solvento)
complex [Ir(H)2(S)2(PPh3)2]PF6 (S = solvent) (3)
[23] (Eq. (3)).
The effect of the solvent on the kinetic reactions
was investigated using 1,2-dichloroethane, THF,
dichloromethane and acetone. As we can observe in
Fig. 5, the catalytic rates decrease with the coordinat-
ing capacity of the solvent: the poorly coordinating
1,2-dichloroethane results in the higher hydrogena-
tion rate, followed by THF and dichloromethane, and
the stronger coordinating acetone inhibits the catal-
ysis. We presume that this effect would indicate the
presence of the coordinatively unsaturated species
within the kinetically important catalytic cycle, and
these species are probably stabilized through coor-
dination of solvent molecules in a competition of
these with the substrate for the vacant coordination
sites.
H2
[Ir(COD)(PPH3)2]PF6So→lvent[Ir(H)2(S)2(PPh3)2]PF6
1
3
(3)
Complex 3 reacts with N␤NA at −80◦C to yield
an equilibrium mixture of 3 plus two new complexes,
which are presumably the mono- and di-substitution
products [Ir(H)2(S)(N␤NA)(PPh3)2]PF6 (4) and
[Ir(H)2(N␤NA)2(PPh3)2]PF6 (5).
The high-field 1H NMR spectra for 5 shows a
triplet at −16.9 ppm (acetone-d6, 300 MHz), while
Table 3
the corresponding 31P{ H} spectra consists of a sin-
1
Activation parameters for the hydrogenation of N␤NA with 1 as
the catalyst precursor
glet at 18.5 ppm. These features indicate the presence
of two equivalent hydrides coupled with two equiv-
alent phosphines, corresponding to a stereochemistry
involving mutually trans-phosphines, and mutually
cis-pairs of hydride and imine ligands. The high-field
1H NMR spectra for 4 shows two triplets of dou-
blets at −16.2 and −16.5 ppm (2J(H–H) = 3 Hz,
Ea (kcal mol−1
A (M−2 s−1
)
4.6
(3.0
1178.45
4.0
−33
14
0.4
)
1.0) × 106
kcat (25◦C) (M−2 s−1
)
ꢀH◦ (kcal mol−1
ꢀS◦ (e.u.)
ꢀG◦ (kcal mol−1
)
0.4
2
2
)

V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
147
Table 4
The subsequent reaction of 4 to release the prod-
uct amine and produce the unsaturated species
[Ir(S)2(PPh3)2]PF6 (6) is the rate determining step as
shown in Eq. (5).
NMR dataa (δ, ppm) for compounds 3–5 in acetone-d6
1H
31P{ H}
1
Complex
3
4
−27.3 (t)b
31.3 (s)
−16.2 (td)b,c
18.4 (s)
[Ir(H)2(S)(N␤NA)(PPh3)2]PF6
−16.5 (td)
4
5
−16.9 (t)b
18.5 (s)
k2
→[Ir(S)2(PPh3)2]PF6 + amine
(5)
6
a At −80◦C.
b J(H–P) = 15.8 Hz.
c 2J(H–H) = 3 Hz.
The catalytic cycle is completed with the oxidative
addition of hydrogen to regenerate 3 (Eq. (6)).
1
and J(H–P) = 15.75 Hz) and the 31P{ H} spectrum
consists of a singlet at 18.4 ppm, indicating that the
two equivalent phosphines remain in trans-disposition
and two different cis-hydrides are trans to an imine
K4
[Ir(S)2(PPh3)2]PF6 + ꢀ[Ir(H)2(S)2(PPh3)2]PF6 (6)
6
3
A rate law corresponding to this mechanism can
be derived, taking into account that the rate of amine
production is
and solvent ligands. H and 31P{ H} NMR data are
contained in Table 4. It is interesting to note that all
complexes prefer to adopt the more sterically crowded
stereochemistry containing one or two cis-imines
rather than an all trans-disposition of the six ligands,
most probably as a result of the large trans effect
typical of hydride ligands.
1
1
d[amine]
r =
= k2[4]
(7)
dt
Considering the equilibrium showed in Eqs. (4) and
(6), and the mass balance for iridium, and substituting
[4] in Eq. (7), the rate expression becomes
The results are summarized in Scheme 2.
k2K1K4[Ir]0[H2][N␤NA]
r =
(8)
4.4. Mechanism of NβNA hydrogenation
1 + K4[H2] + K1K4[H2][N␤NA]
Eq. (8) can be inverted and reorganized as
On the basis of the results presented above, we
propose that under the conditions of the catalytic re-
action, complex 1 reacts with hydrogen and solvent
to give 3, which is proposed as the initial species
entering the catalytic cycle. Then, 3 coordinates one
imine molecule to form 4 through the equilibrium K1
shown in Eq. (4).
[Ir]0[N␤NA]
1
1+K1[N␤NA]
=
+
(9)
r
k2K1K4[H2]
k2K1
A plot of the reciprocal of the rate of N␤NA hy-
drogenation versus the reciprocal of hydrogen concen-
tration yields a straight line from which the value of
k2K1K4 (2.8 × 103 M−2 s−1) can be obtained. If the
term (K4[H2]+K1K4[H2][N␤NA]) ꢀ 1 (Eq. (8)), the
rate expression can be approximated to
[Ir(H)2(S)2(PPh3)2]PF6 + N␤NA
3
K1
ꢀ[Ir(H)2(S)(N␤NA)(PPh3)2]PF6
(4)
r = k2K1K4[Ir]0[H2][N␤NA]
(10)
4
Scheme 2.

148
V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
Scheme 3.
Which is identical to the experimental rate
sites trans to the hydride ligands to form the mono-
law kcat
=
k2K1K4 and the value of k2K1K4
and di-substitution products 4 and 5 (Scheme 2).
In the literature has been proposed that imines can
coordinate in two different modes to the metal cen-
ter [1]: by the nitrogen atom (␩1-N) or by the dou-
(2.8 × 103 M−2 s−1) is in agreement with the experi-
mental value obtained (2.2 × 103 M−2 s−1).
Also, Eq. (8) can be reorganized again as
2
2
=
ble bond C N (␩ -N, C), where the ␩ -bonding mode
is more likely to occur in the hydrogenation reac-
tion (Scheme 4). With the NMR data obtained we
have not been able to distinguish how the imine is
coordinated to the iridium center, but probably the
[Ir]0[H2]
1 + K4[H2]
[H2]
=
+
(11)
r
k2K1K4[N␤NA]
k2
A plot of the reciprocal of the rate of N␤NA hydro-
genation versus the reciprocal of N␤NA concentration
yields a straight line in agreement with the kinetic re-
sults. From these plots the values of k2 (0.052 s−1), K1
(22.14 M−1) and K4 (2.4×103 M−1) can be obtained.
Although, the details of each step involved in
the mechanism have not been elucidated, a general
schematic cycle for the [Ir(COD)(PPh3)2]PF6-cataly-
zed hydrogenation of N␤NA is proposed (Scheme 3)
in accordance with the experimentally determined
rate law.
We believe that once complex 1 generates the
species 3, the incoming N␤NA would occupy the
Scheme 4.

V. Herrera et al. / Journal of Molecular Catalysis A: Chemical 174 (2001) 141–149
149
mono-coordinated species 4 corresponds to an equi-
librium mixture of the two forms; where the ␩1-N
mode is out of the cycle and the ␩2-bonding mode
is the olefin-like intermediate that undergoes selective
for his useful comments and suggestions, and Mr.
A. Fuentes for his assistance in the experimental
work.
=
hydrogenation of the C N bond (Scheme 3).
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4735.
2
=
the ␩ -(C N)-imine coordination in a bis-imine com-
plex would not be possible for steric reasons.
Then, the complex 4 would evolve by successive
transfer the hydrides to the imine. Considering that
Ir-hydrides generally have a very hydridic character,
we presume that the first transfer will go to the car-
bon atom to yield the iridium-alkylaminium interme-
diate. Reductive elimination of the amine generates
the unsaturated species [Ir(S)2(PPh3)2]PF6, which re-
acts with hydrogen to produce 3 and restart the cycle.
5. Conclusions
We have studied the hydrogenation of imines
using rhodium and iridium complexes as catalyst
precursors, finding that the iridium complex is
the better catalyst precursor. The kinetic study of
the hydrogenation of N␤NA with 1 in THF was
done and we have established the experimental rate
law as ri = kcat[Ir][H2][N␤NA], where kcat
=
(2.2 0.3) × 103 M−2 s−1. Besides, we have sug-
gested a mechanistic scheme for this reaction, which
is consistent with the kinetic and spectral data. The
initial species entering the catalytic cycle is 3 and we
proposed a rate determining step corresponding to the
first hydride transfer.
Further work using similar iridium and rhodium
complexes for kinetic and mechanistic studies for the
same reaction are in progress, and some extensions of
this work with prochiral imines are programmed to be
done.
[22] R. Schrock, J.A. Osborn, J. Am. Chem. Soc. 93 (1971)
2397.
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Parnell, J.M. Quirk, G.E. Morris, J. Am. Chem. Soc. 104
(1982) 6994.
Acknowledgements
[24] IUPAC, Solubility DATA Series: Hydrogen and Deuterium,
Vol. 5/6, p. 219
The authors are grateful to CONICIT for financial
support (S1-95000693), to Dr. R. Sánchez-Delgado