Kinetics and mechanisms of homogeneous catalytic reactions. Part 16. Regioselective hydrogenation of quinoline catalyzed by dichlorotris(triphenylphosphine)ruthenium(II)

Article abstract of DOI:10.1016/j.mcat.2020.110970

The complex RuCl₂(PPh₃)₃ (1) showed to be an efficient and regioselective precatalyst for the hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) at P = 5.0–6.5 at m and T = 140?167 °C. This reaction showed to be first order on catalyst concentration and fractional order on dissolved hydrogen concentration (1.84). However, different to other Ru, Os, Rh and Ir precatalysts, the reaction showed an inverse fractional order on Q concentration (-0.44). The rate law may be written as: r = {K_oK₁k₂/(K_o + [Q]+ K_oK₁[H₂])}[Ru][H₂]². Coordination chemistry and theoretical DFT studies allowed us to propose a detailed catalytic cycle for this reaction. Under the hydrogenation conditions, the ruthenium precursor 1 is totally transformed to RuCl₂(κN-Q)₂(PPh₃)₂ (3) and subsequently to RuHCl(κN-Q)₂(PPh₃)₂ (4) by heterolytic addition of H₂. Complex 4 reversibly dissociates a Q ligand to generate RuHCl(κN-Q)(PPh₃)₂, which was proposed as the catalytically active species (CAS) entering to the catalytic cycle. The reversible addition of hydrogen yields a species containing a κN-1,2-dihydroquinoline (κN-DHQ) ligand (possibly through the coordination of hydrogen, migration of hydride to the N atom of Q ligand and posterior DHQ reductive elimination); this species suffers a change of coordination mode from κN- to an olefin-like η²-DHQ. Posteriorly, the addition of a second molecule of H₂, considered the rate-determining step, generates a THQ species (probably through migration of hydride to C₃ of DHQ ligand followed by the heterolytic addition of hydrogen). The cycle is completed by the substitution of the THQ ligand by a new Q molecule, therefore regenerating the CAS.

Full text of DOI:10.1016/j.mcat.2020.110970

Molecular Catalysis 490 (2020) 110970
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Molecular Catalysis
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Kinetics and mechanisms of homogeneous catalytic reactions. Part 16.
Regioselective hydrogenation of quinoline catalyzed by dichlorotris
(
triphenylphosphine)ruthenium(II)
a,
a
a
a
b
Merlín Rosales *, Karely Molina , Federico Arrieta , Dángelo Fernández , Pablo J. Baricelli
a
Universidad Del Zulia (L.U.Z.), Facultad Experimental De Ciencias, Departamento De Química, Laboratorios De Química Inorgánica (LQI) and Química Inorgánica
Teórica (LQIT), Venezuela
Universidad De Carabobo, Facultad De Ingeniería, Centro De Investigaciones Químicas. Valencia Venezuela
b
A R T I C L E I N F O
A B S T R A C T
2 3 3
The complex RuCl (PPh ) (1) showed to be an efficient and regioselective precatalyst for the hydrogenation of
Keywords:
Ruthenium
Triphenylphosphine
Quinoline
Hydrogenation
Kinetics
quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) at P = 5.0–6.5 at m and T = 140−167 °C. This reaction
showed to be first order on catalyst concentration and fractional order on dissolved hydrogen concentration
(
1.84). However, different to other Ru, Os, Rh and Ir precatalysts, the reaction showed an inverse fractional
order on Q concentration (-0.44). The rate law may be written as: r = {K /(K + [Q]+ K [H ])}[Ru]
] . Coordination chemistry and theoretical DFT studies allowed us to propose a detailed catalytic cycle for
this reaction. Under the hydrogenation conditions, the ruthenium precursor 1 is totally transformed to RuCl (κN-
Q) (PPh (3) and subsequently to RuHCl(κN-Q) (PPh (4) by heterolytic addition of H . Complex 4 reversibly
dissociates a Q ligand to generate RuHCl(κN-Q)(PPh , which was proposed as the catalytically active species
CAS) entering to the catalytic cycle. The reversible addition of hydrogen yields a species containing a κN-1,2-
o
K
1
k
2
o
o
K
1
2
2
[H
2
2
2
3
)
2
2
3
)
2
2
3 2
)
(
dihydroquinoline (κN-DHQ) ligand (possibly through the coordination of hydrogen, migration of hydride to the
N atom of Q ligand and posterior DHQ reductive elimination); this species suffers a change of coordination mode
2
from κN- to an olefin-like η -DHQ. Posteriorly, the addition of a second molecule of H
2
, considered the rate-
determining step, generates a THQ species (probably through migration of hydride to C
3
of DHQ ligand followed
by the heterolytic addition of hydrogen). The cycle is completed by the substitution of the THQ ligand by a new
Q molecule, therefore regenerating the CAS.
Introduction
2001, respectively. Afterwards, Rosales and coworkers have reported
3
the use of complexes of the type MH(κ −OCOR)(CO)(PPh
3
)
2
(M = Ru
The hydrogenation of quinoline (Q, C
9
H
7
N) can generate 1,2,3,4- or
and Os) [7] as well as Rh and Ir systems containing mono-, bi- and
tridentate phosphine [8] and an allyl functionalized phosphine rhodium
complex [9] as precatalysts for this reaction. Parkin et al. [10] reported
5
,6,7,8-tetrahydroquinoline (C
9
H
11N) [1] as well as the totally hydro-
17N) [2], as depicted in
genated product, decahydroquinoline (C
9
H
Scheme 1. This reaction is of current interest because it is related with
the hydrodenitrogenation (HDN) process [3]. Moreover, 1,2,3,4-tetra-
hydroquinoline, henceforth THQ, is a key structural feature of many
pharmacologically relevant therapeutic agents and biologically active
natural and synthetic products [4]. The catalytic hydrogenation of Q
and benzoquinolines is considered as the most straightforward, pow-
erful, efficient and atom-economic approach to access THQ and related
structures.
4 3 4
that MoH (PMe ) was an active precatalyst for the hydrogenation of Q
and other heteroaromatic nitrogen compounds, which is the only
simple molybdenum complex to perform the catalytic hydrogenation of
Q. In all of these cases, THQ is the only product obtained. However,
2
Chaudret et al. [11] reported that RuH
2
(η -H
2
)
2
(PCy
3
)
2
hydrogenates
the nonheterocyclic ring of Q, although its homogeneity has been put in
doubt [12]; homogeneous hydrogenation of both hetero- and non-het-
eroaromatic rings were achieved by ruthenium dihydride precatalysts
[13]. Most recent studies of this reaction involve the use of an efficient
iridium complex in water without any co-catalyst or additives [14], the
development of non-noble metals (Fe, Co and Mn) precatalysts [15], as
Homogeneous catalysts have been extensively studied for this re-
action by using Ru, Os, Rh and Ir complexes; some of these works were
reviewed by Sánchez-Delgado [5] and Bianchini et al. [6] in 2000 and
⁎
Corresponding author.
E-mail addresses: merlinrosalesaiz@gmail.com, merlin2002@cantv.net (M. Rosales).
https://doi.org/10.1016/j.mcat.2020.110970
Received 17 January 2020; Received in revised form 7 April 2020; Accepted 17 April 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
Scheme 1. Possible products of the catalytic hydrogenation of Q.
well as the use of Ru, Rh and Ir complexes in the asymmetric hydro-
genation of prochiral substituted quinolines [16].
2 3 3
hydrogen at atmospheric pressure to a solution of RuCl (PPh )
(100 mg, 0.25 mmol) in benzene (25 mL) in the presence of triethyla-
mine (0.5 mL, 3.6 mmol) [25].
An aspect of great importance is the study of the kinetics of such
hydrogenation reactions, which have allowed the elucidation of some
characteristics of the mechanisms operating in the reaction in combi-
nation with spectroscopic techniques, isotope labeling, isolation of
stable analogues and theoretical calculations [17]. However, due to its
complexity, only few detailed kinetic studies have been reported in the
literature for this reaction, which involves both C]N as C]C bond
reductions. Kinetic studies of this reaction have been performed both
for Rh [9,18] and Ir [18c] as for Ru [19] and Os catalytic systems [20].
These studies have shown that the reaction is first order on catalyst and
independent on substrate whereas the order on dissolved hydrogen
concentration generally varies from second order for Rh precatalysts to
fractional order (between 1 and 2) for the Ru, Os and Ir systems.
Although the rhodium precatalysts have shown to be more active
than the ruthenium ones for the homogeneous hydrogenation of Q and
Catalytic runs
The catalytic reactions and the kinetic study were carried out as
follow: in a typical experiment, a solution of the precatalyst (complex
1
), the substrate (Q), naphthalene as internal standard and xylene as
solvent (40 ml total volume) were placed in the reactor, and the solu-
tion was carefully deoxygenated. Then, the reactor placed into a pre-
heated oil bath and finally the magnetic stirring was started. When the
reactor reached thermal equilibrium, hydrogen was admitted into the
evacuated system; this point was considered as the zero time of the
reaction. The reaction was thereafter followed by measuring the hy-
drogen pressure drop as a function of time and by gas chromatography.
other
RuCl
3 3
its rhodium analogue, RhCl(PPh )
polynuclear
(PPh (1) [21,22] showed to have a higher catalytic activity than
[22,23]. In order to gain further
heteroaromatic
nitrogen
compounds,
2
3 3
)
Kinetic measurements
insight into the reaction mechanism and explain the high activity of the
Ru precursor as well as other aspects reported in previous studies, in the
present paper we provide detailed kinetic, coordination chemistry and
DFT theoretical studies of the hydrogenation of Q catalyzed by the di-
chloro-ruthenium complex 1.
In order to calculate the initial rate of the reaction, the percentage of
conversion of the hydrogenation reactions was restricted to 5–10 %
26]. The hydrogen consumption was converted to molar concentration
by using the ideal gas law and the solution volume; the values of THQ
concentration were calculated by considering the stoichiometry of the
reaction.
[
Experimental
The data for Q hydrogenation were plotted as molar concentration
of THQ versus the time in seconds yielding straight lines, which were
fitted by conventional linear regression programs (r > 0.98). Initial
2
Instruments and materials
o
rates (r ) of the reaction were obtained from the corresponding slopes.
All manipulations were conducted with rigorous exclusion of air
under a N inert atmosphere, using Schlenk techniques, a vacuum line
2
In order to ensure reproducibility of the results, each reaction was re-
peated at least twice. Statistical analyses were carried out using analysis
of variance (ANOVA) to test the difference between means; significant
differences were considered at P-values < 0.05.
and a glove box. All the gases were U.A.P. grade and were purchased
from INVERGAS, Venezuela. Q (Sigma-Aldrich) was purified by stan-
dard method and distilled under reduced pressure. Solvents were dried
and distilled under argon prior to use.
The analysis of the reaction mixtures were performed by gas chro-
matography (GC), employing a 3300 Series VARIAN instrument fitted
with a flame ionization detector (FID) and a 2 m 20 % SP-2100 on a 0.1
In order to study the effect of each component on the reaction rate,
three experiment sets were carried out varying the concentration of
only one of the components within each set (catalyst and substrate
concentrations, as well as hydrogen pressures), while keeping constant
the concentration or pressure of the other components and the tem-
perature (413 K). A fourth set of experiments were performed varying
temperature and keeping constants the reactants and precatalyst con-
centrations. The hydrogen concentrations in the reaction medium were
calculated from solubility data reported in the literature [27]. The
homogeneity of the reaction was corroborated by the mercury test re-
ported by Crabtree and Anton [28].
%
carbowax 100/120 Supelcoport column, using N
2
as carrier gas; the
products were quantified with microcomputer coupled to
a
a
Measurement Computing interphase, equipped with the Cromat 1.2
software.
Complex RuCl
the literature, whereas RuHCl(PPh )
2
(PPh
3
)
3
[24] was prepared as previously described in
was synthetized by bubbling
3 3
2

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
Coordination chemistry related with quinoline hydrogenation
Table 1
Kinetic data for the hydrogenation of Q to THQ catalyzed by RuCl
2
3 3
(PPh ) .
The coordination chemistry experiments were performed by the
(atm) 102 [H
(M)
106
(Ms−1
)
Entry T (K) [Ru]
(mM)
10[Q] (M) pH
2
2
]
r
o
interaction of complex RuCl
2 3 3 3 3
(PPh ) (1) or RuHCl(PPh ) (2) with each
component of the catalytic mixture (i.e. Q and/or hydrogen).
1
2
3
4
5
6
7
8
413
413
413
413
413
413
413
413
413
413
413
413
413
421
427
440
0.78
1.00
1.20
1.41
1.60
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.05
1.05
1.05
1.05
1.05
0.63
0.84
1.27
1.48
1.05
1.05
1.05
1.05
1.05
1.05
1.05
5.9
2.71
2.71
2.71
2.71
2.71
2.71
2.71
2.71
2.71
2.39
2.55
2.85
3.03
2.80
2.88
3.03
1.04 ± 0.01
1.32 ± 0.02
1.54 ± 0.03
1.89 ± 0.01
2.11 ± 0.04
1.67 ± 0.03
1.49 ± 0.03
1.24 ± 0.02
1.17 ± 0.02
0.99 ± 0.04
115 ± 0.09
1.41 ± 0.05
153 ± 0.06
2.29 ± 0.06
2.95 ± 0.05
6.82 ± 0.08
5.9
5.9
5.9
5.9
5.9
5.9
5.9
5.9
5.2
5.5
6.2
6.6
5.9
5.9
5.9
3 3
Interaction of RuHCl(PPh ) with Q
Q (0.5 mL, 4.2 mmol) was added to a benzene (20 mL) solution of
complex 2, prepared in situ from 1 (96 mg, 0.1 mmol) by bubbling hy-
drogen in presence of triethylamine (0.5 mL, 3.6 mmol) at atmospheric
pressure and room temperature. The mixture was vigorously stirred at
room temperature for 30 min and the resulting solution was dried in the
vaccum.
9
10
1
1
1
1
15.
16
1
2
3
4
2 3 3
Reaction of complex RuCl (PPh ) with Q and hydrogen
Complex 1 (96 mg, 0.1 mmol), Q (0.1 mL) and benzene (10 mL)
were placed in a reactor equipped with a glass vessel. The reactor was
manipulated as described in the catalytic runs and finally charged with
hydrogen (6 atm). After 4 h, the solution was immediately transferred
to a Schenk, and evaporated under vacuum to ca. one-third of its initial
volume. A brown precipitate was obtained by addition of n-pentane,
which was washed with pentane and diethyl ether in order to remove
Initial rates (r
o
), xylene solvent.
0
0
0
0
.11
.10
.09
.08
−1
1
the free Q and then dried in vacuo. IR (cm ): 1945 (w) [υ(Ru-H)]. H
N.M.R. (CDCl , 298 K): 8.8−7.3 p.p.m. (series of multiplets of Q and
). P{ H} N.M.R. (CDCl , 298 K): 54.4−40.4 (series of small and
3
31
1
PPh
3
3
broad multiplets), 33.6 (s, 2 P) and 30.4 (s, 1 P) p.p.m (see
Supplementary Material SM-I).
0.07
Q
0.06
0.05
0.04
0.03
0.02
0.01
DFT calculations
All the calculations were carried out with the Gaussian-09 (G09)
[
29] computational package by using the functional M06-L [30] of
THQ
ro
Density Functional Theory (DFT) combined with the effective core
potential (ECP) LanL2DZ for Rh and P atoms [31] and the extended
basis set 6−31+G(d,p) for C, H, O, N and Cl ones [32]. Minimum
energy structures (MES) calculations for ruthenium species involved in
the catalytic cycle were performed using simplified PH
3
analogue in
0.00
order to reduce the computational cost. Furthermore, the effect of
solvent (xylene) was included by the subroutine SCRF=(Sol-
vent = xylene) [33]. All the optimized structures were classified ac-
cording to frequency calculations.
0
2000
4000
6000
8000
10000
12000
t (s)
2 3 3
Fig. 1. Profile of the hydrogenation of Q catalyzed by RuCl (PPh ) . Conditions
as entry 16 of Table 1.
Results and discussion
Previously, we reported kinetic and mechanistic studies of this reaction
catalyzed by the cationic ruthenium complex [RuH(CO)
The regioselective hydrogenation of Q to THQ [substrate/catalyst
(
2 3 2 4
NCMe) (PPh ) ]BF [19].
ratio (S/C) = 63–160, 140−167 °C, 5.0–6.5 at m H
performed by using RuCl (PPh (1) as precatalyst in xylene as solvent
Table 1). A material balance by GC of all the liquid compounds in-
2
] can be efficiently
2
3 3
)
(
Kinetic study of Q hydrogenation
dicates that Q and THQ are the only compounds present in the reaction
mixtures (> 97 %). This reaction showed to be genuinely homogeneous
The kinetics of the homogeneous hydrogenation of Q to THQ cat-
alyzed by complex 1 in xylene solution was studied by carrying out runs
at different concentrations of the precatalyst (entries 1–5), substrate
(entries 2 and 6–9) and dissolved hydrogen (entries 2 and 10–13), at
constant temperature (413 K), as may be observed in Table 1.
[
28] and their rate data were in the kinetic regime, implying that mass
transfer effects were negligible. A typical reaction profile is depicted in
Fig. 1, which shows the decrease of the substrate concentration while
increasing the corresponding product concentration as a function of
time. These reactions showed induction periods of 5−30 min, which
were necessary for the formation of catalytically active species (CAS).
Although all the reactions showed a linear behavior until 20–30 %, the
The initial rate of Q hydrogenation showed a kinetic of first order on
the ruthenium concentration (entries 1–5) as confirmed by the plot of
log r
i
vs log [Ru] (Fig. 2), which yielded straight lines with slopes of
with respect to ruthenium
initial rates (r
o
) were measured until 5–10% [26]. Under our reaction
, entry 2 of Table 1), we
o
of 1.32 × 10 M s (40 % conversion in 6 h); Fish et at [21]
0.99. The first order dependence of the r
o
conditions (S/C = 100, 140 °C, 6.0 atm H
2
concentration indicates that catalytically active species is a mono-
nuclear complex. Fish and coworkers [21] have suggested that the
hydrogenation of Q catalyzed by 1 proceeds through the in-situ for-
−
6
-1
found a r
reported a reaction rate of 0.47 % min (∼3.9 × 10
C = 10, 85 °C and 20 atm H , whereas Sánchez-Delgado and González
22] reported a r of 63.3 mmol THQ (mmol Ru h) (∼2.3 × 10 M s
at S/C = 92, 150 °C and 30 atm H
In order to determine the rate expression of this reaction, we per-
formed a kinetic study of the Q hydrogenation catalyzed by 1.
-1
−6
-1
M s ) at S/
2
3 3
mation of RuHCl(PPh ) (2), as has been also proposed for the hydro-
-1
−6
-
[
o
genations of other substrates [34]. However, our coordination chem-
istry experiments seem to indicate that the hydride species 2 was not
formed under the hydrogenation conditions in presence of quinoline
(vide infra).
1
)
2
.
3

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
-
-
-
-
-
-
-
-
5.65
5.70
5.75
5.80
5.85
5.90
5.95
-5.80
log ro = -3.01 + 1.84 log [H2]
log ro = 2.90 + 0.99 log [Ru]
-5.85
-5.90
-5.95
-6.00
6.00
-3.15
-3.10
-3.05
-3.00
-2.95
-2.90
-2.85
-2.80
-2.75
-1.62
-1.60
-1.58
-1.56
-1.54
-1.52
log [Ru]
log [H2]
Fig. 2. Rate dependence of the reaction rate on catalyst concentration for the
hydrogenation of Q catalyzed by RuCl (PPh
Fig. 4. Rate dependence of the reaction rate on dissolved hydrogen con-
centration for the hydrogenation of Q catalyzed by RuCl (PPh
2
3 3
) .
2
3 3
) .
-
-
-
-
-
-
-
-
-
-
-
5.76
5.78
5.80
5.82
5.84
5.86
5.88
5.90
5.92
5.94
[
21].
On the other hand, this reaction yielded kinetics close to second
order on dissolved hydrogen concentration (entries 3 and 6–9). A plot
of log r vs log [H ] (Fig. 4) yielded straight lines with slope 1.84. The
fractional order close to second order on the dissolved hydrogen con-
centration allowed us to infer that the first H addition is reversible (an
log ro = -6.31 - 0.44 log [Q]
i
2
2
equilibrium) and that the second hydrogenation is the determining-rate
step of the process. These results are rather similar to those found for
the reaction catalyzed by a cationic ruthenium complex, namely [RuH
+
(
CO)(NCMe)
2
(PPh
3
)
2
] , reported previously by some of us [19]. Based
on detailed NMR studies and deuteration experiments, Fish et al. [21]
proposed that, for the hydrogenation of Q with 1, the addition of the
first H
1.8 deuterium at C2 and 1.0 deuterium at C3 and C4 when D
instead of H ; 0.5 deuterium was also found at the 2-position of the
2
molecule is reversible because in the THQ product was found
2
was used
2
5.96
-
unreacted Q; our kinetic data are consistent with Fish findings.
1.25 -1.20 -1.15 -1.10 -1.05 -1.00 -0.95 -0.90 -0.85 -0.80
Consequently, the experimental rate law may be written as:
log [Q]
= kcat [Ru]0.99 [Q]
−0.44
[H
]1.84
r
o
2
(1)
(2)
Fig. 3. Rate dependence of the reaction rate on substrate concentration for the
hydrogenation of Q catalyzed by RuCl (PPh
2
3
)
3
.
This equation may be also written as:
a
ro =
[Ru][H2]2
Contrary to the results found for the kinetic studies of hydrogena-
tion of Q with other Ru [19], Os [20], Rh [9,18] and Ir [18c] pre-
catalysts, for which the reaction rate was independent of the substrate
concentration, an inverse dependence of the reaction rate on Q con-
centration (entries 3 and 6–9) was observed for the reaction catalyzed
by 1 (Fig. 3); a negative fractional order (- 0.44) was determined from
b + c [Q] + d[H ]
2
Finally, a fourth set of experiments were performed at different
temperatures (entries 2 and 14–16), from which the activation energy
was determined as 18.9 kcal mol (see Supplementary Material SM-II).
−
1
Coordination chemistry related with Q hydrogenation
o
the slope of the plot log r versus log [Q]. This negative order may be
explained taking into account that there is an equilibrium involving the
dissociation of a Q ligand prior to the rate-determining step of the re-
action. Fish et al. [21] reported that the reduction of Q by 1 was in-
hibited by less sterically hindered heteroaromatic nitrogen compounds,
such as 5,6-benzoquinoline, indole, and 2-methylpyridine, as well as by
THQ product by factors of 0.79, 0.66, 0.85 and 0.34, respectively, at
substrate to precatalyst ratio (S/C) of 10, 85 °C and 20 atm H . In the
2
present work, we found that Q itself also did inhibit its hydrogenation
by a factor of 0.74 (see entries 6 and 8 of Table 1), under other reaction
conditions (S/C = 100, 140 °C and 5 atm H ), a factor similar to that
2
In order to complement the kinetic study of the Q hydrogenation
catalyzed by the Ru precatalyst, experiments on the interaction of the
ruthenium complex 1 with the substrate in presence or absence of hy-
drogen were carried out; the results are summarized in Scheme 2.
The addition of Q to a solution of complex 1 at room temperature in
benzene induced an immediately change of color from purple to reddish
brown. Although this new complex could not be isolated, we anticipate
that this species possibly contains two Q ligands, RuCl
3). Similar complexes with pyridine, bipy, 110-phen [34], imidazole,
methylimidazole [35], pyrazol [36] and 2-(2′-pyridyl)quinoline ligand
37] have been reported. On the other hand, when a solution of 1 was
bubbled by hydrogen at room temperature in the presence of an excess
of NEt (3.6 mmol), a color change from purple to reddish purple was
observed indicating the formation of RuHCl(PPh (2) [25]; the
2 2 3 2
(κN-Q) (PPh )
(
found for 5,6-benzoquinoline [21]. Under these last conditions, we also
found that the addition of small amounts of THQ (0.05 M) decreased
the ro, which may be attributed to the stabilization of an intermediate
containing a coordinated THQ molecule, as also proposed by Fish et al.
[
3
3 3
)
4

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
2 3 3
Scheme 2. Coordination chemistry study related with hydrogenation of Q catalyzed by RuCl (PPh ) .
2 3 3
Scheme 3. Proposed general mechanism of hydrogenation of Q catalyzed by RuCl (PPh ) .
addition of Q caused the rapid decomposition of complex 2. Our results
indicate that, under the reaction conditions, triphenylphosphine dis-
placement by Q yielding a dichlorobis(quinoline) complex 3 occur more
rapidly than the reaction with hydrogen to yield complex 2.
54.4 and 40.2 ppm plus two singlets in 2:1 ratio at 33.6 and 30.4 ppm;
this last signal is attributed to triphenylphosphine oxide. We think that
complex 4 is unstable in solution when it is not expose to a high
pressure of molecular hydrogen generating other species by some or-
ganometallic reaction: dissociation of one of the Q ligands, migration of
the hydride ligand to one of the Q ligands, some reaction of reductive
elimination or by the formation of binuclear ruthenium species con-
taining hydride or chloride bridged ligands as propose by other authors
[25,38].
All of these results are consistent with the formation of labile species
involving rapid equilibria, which is usually expected from a highly
active catalytic system. On the basis of these experimental results, we
have suggested that 1 reacts with Q to yield initially 3, which suffers a
Finally, the reaction of 1 with Q at 140 °C and 6 atm of H
a brown complex showing a weak band at 1945 cm
2
generated
in the IR spec-
trum, which is characteristic of the υRu-H; this signal may be assigned
to the complex RuHCl(κN-Q) (PPh (4) by comparison with the
analogue bipyridine complex, RuHCl(κN-bipy)(PPh (υRu-H at
in Nujol mull) [25]. However, in the H NMR spectra only
were mainly observed the aromatic protons of Q and PPh ligands; no
hydride signal was observed at high field. On the other hand, in the
−
1
2
3 2
)
3 2
)
−1
1
1
930 cm
3
31
P
1
{
H} NMR were observed at least four broad multiplet signals between
5

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
3 2 3 2
Scheme 4. Possible intermediates for the reaction of RuHCl(κN-Q)(PPh ) (A) with hydrogen to yield a DHQ intermediate RuHCl(κN-DHQ)(PPh ) (B).
heterolytic addition of hydrogen at high pressure of hydrogen to gen-
erate the monohydride species 4 plus quinolinium chloride. Although it
has been reported that dihydride species may be active species for ru-
thenium catalyzed hydrogenation reactions [13,39], it is unlikely that
such species are involved in the mechanism of hydrogenation of Q
3 2
complex RuHCl(κN-THQ)(PPh ) (C); this step is considered to be the
rate-determining step of the mechanism. Finally, the catalytic cycle is
completed with the reversible substitution of a THQ ligand by a new Q
molecule regenerating the catalytically active species (A) and restarting
the catalytic cycle.
catalyzed by RuCl
hydride species hydrogenate Q to tetrahydroquinolines (1,2,3,4- and
,6,7,8-) and decahydroquinoline [13], whereas 1,2,3,4-tetra-
hydroderivative was obtained as the only product in our catalytic re-
actions.
2
(PPh
3
)
3
(1) because it has been found that such di-
As the addition of the second dihydrogen molecule is the rate-de-
termining step, a rate law can be derived as represented by Eq. 3:
5
ro = k3 [B][H2]
(3)
o 1
Considering the equilibria showed in K and K , we have:
[
4][H2]2
Q]
The mechanism of Q hydrogenation
ro = KoK1k2
[
(4)
On the basis of the experimental results presented above, we pro-
pose that the mechanism of hydrogenation is constituted by the set of
reactions showed in Scheme 3. The bis(quinoline)ruthenium complex 4
o
Assuming that the mass balance for the metal is [M] = [4] + [A]
+
[B] + [C], the concentration of 4 may be expressed as:
[
Ru]o [H2]
(
possibly the resting state of the catalytic cycle) reversibly dissociates a
Q ligand generating RuHCl(κN-Q)(PPh (A), considered by us as the
CAS of the reaction, which is rapidly and reversibly hydrogenated to
form a dihidroquinoline (DHQ) species, RuHCl(κN-DHQ)(PPh (B).
Reaction of B with a second molecule of dihydrogen produces the THQ
[
4] =
Ko [THQ]
K3[Q]
3
)
2
Ko + [Q] + KoK1 [H2] +
2
(5)
3
)
2
Substituting [4] from Eq. (5) in Eq. (4) allow us to rewrite the rate
expression as:
6

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
parameter of these species are included in Supplementary Material SM-
III.
We proposed that species 3 suffers heterolytic addition of dihy-
2 3 2
drogen to yield quinolinium chloride plus RuHCl(κN-Q) (PH ) (4),
considered as the resting state of the catalytic hydrogenation. The Q
located trans to the hydride ligand (dRu-N = 2.39 Å) is more weakly
coordinated to the ruthenium center than Q in cis position (dRu-
N = 2.12 Å), therefore we propose that 3 dissociates the trans Q ligand
3 2
to generate RuHCl(κN-Q)(PH ) (A), which is considered by us as the
corresponding and true CAS of the Q hydrogenation process catalyzed
by complex 1.
Once A is formed in the reaction medium by dissociation of a Q
ligand of species 4, which is thermodynamically favorable, the first
hydrogenation should occur either by coordination of hydrogen in the
vacant coordination site and subsequent hydride transfer to the Q li-
gand or by the direct hydride transfer to Q followed by addition of the
first hydrogen molecule. In both cases, the migration of the hydride
ligand to the Q ligand may occur toward either its nitrogen or C(2) atom
generating a β-amino species or an amide one, respectively (Scheme 4).
2
Although Wieglie et al. [40] reported the importance of a κ N,C(2) co-
ordination mode of the Q ligand in the hydrogenation of this substrate,
such species could not be optimized by DFT calculations.
The thermodynamic parameters (see Scheme IV) indicate that the
1
initial coordination of hydrogen molecule to yield the I intermediate is
Scheme 5. Possible products of the migration of the hydride ligand to the DHQ
ligand of RuHCl(κN-DHQ)(PPh ) (B).
3 2
less disfavored with respect to the direct hydride migration both to the
N atom of Q ligand, yielding a species containing a κ N,C(2)-2-mono-
hydroquinolinyl ligand [dRu-N 2.17 Å, dRu-C = 2.08 Å, dN-
2
KoK1k2
[Ru][H2]2
2
ro =
Ko [THQ]
K3[Q]
Ko + [Q] + KoK1 [H2] +
2
(6)
C
2
= 1.44 Å)] (I
agostic interaction between Ru and the methylene group (I
coordinated the H molecule, the migration of the hydride pre-
ferentially occurs toward the N atom of Q yielding a dihydride species
), which is thermodynamically favored with respect to the migration
to the C(2) to generate the dihydride intermediate (I ’); the formation of
1
’), or to the C
2
one, yielding an amide species with an
This rate expression is valid for all the concentration range of Q and
THQ. However, under the kinetic reaction conditions (conversions <
1
’’). Once
2
1
0 %), the THQ concentration is very small (concentration of [3] is
small), therefore the fourth term of the denominator in Eq. (6) may be
neglected, and rewritten as:
(I
2
2
2
KoK1k2
the 18 electron κ N,C(2)-monohydroquinolinyl complex is perhaps the
ro =
[Ru][H2]2
motif force driving the reaction. These results are in concordance with
Ko + [Q] + KoK1[H2]
(7)
theoretical DFT calculations reported by Fogg et al. [41] for the hy-
This theoretical rate law is in good accord with the experimental
i
drogenation of ethylene catalyzed by RuHCl(PH
3
)(P Pr
3
)
2
but contrary
0 1 2 0 0 1
finding (Eq. 2) with a = K K k , b = K , c = 1 and d = K K . The
to those found by Rosales et al. [42] for the hydrogenation of Q cata-
lyzed by [RuH(CO)(NCMe) (PPh ]BF , who reported that the migra-
tion of the hydride occurs toward the C(2) atom of Q. Reductive elim-
ination of DHQ generates the intermediate RuHCl(κN-DHQ)(PH (B).
Since DHQ is a weakly coordinating ligand, we suggest that species
reaction order on dissolved hydrogen concentration close to 2 may be
explained considering that K have a small value at low H pressure.
2
3
)
2
4
1
2
3 2
)
Theoretical DFT calculations related with Q hydrogenation
2
B is in equilibrium with an olefin-like isomer, RuHCl(η C
3 4
,C -DHQ)
In order to obtain detailed insight into the mechanisms of the hy-
drogenation of Q catalyzed by 1, various pathways were calculated via
DFT by using the simplified phosphine (PH ) model, in which the
3
phenyl groups of the triphenylphosphine ligands were replaced by hy-
drogen atoms.
(
PH (B´), similar to those reported by Gladysz et al. [43] for rhenium
3 2
)
complexes; this change is exothermic and thermodynamically favored.
Hydride transfer toward the C(3) or C(4) of DHQ ligand generate the
3 3
1,2,3- or 1,2,4-trihydroquinolinyl intermediates (I or I ’, respectively),
which are shown in Scheme 5. The calculated thermodynamic para-
meters do not allow us to discern between both possibilities. However,
In the first instance, we calculated and optimized the structures of
the initial species RuCl (PH ) , RuHCl(PH ) and RuCl (κN-Q) (PH ) ,
2 3 3 3 3 2 2 3 2
3
the formation of the intermediate containing a η -allyl ligand, involving
simplified analogues of the catalyst precursor 1, of the proposed CAS
for the olefin hydrogenation 2 (also proposed by Fish et al. [21] as the
CAS for the Q hydrogenation catalyzed by 1) and 3, a specie im-
mediately generated by reaction of 1 with Q, respectively; structural
( )
the C(4), C 4′ and C(5) atoms of the 1,2,3-trihydroquinolinyl fragment,
generated by the migration of hydride to C(3) of DHQ ligand is ther-
modynamically lesser disfavored than the formation of the intermediate
Table 2
Thermodynamic parameters (kcal mol−1) of the most feasible elementary steps of the cycle for the RuCl
(PPh ) -catalyzed hydrogenation of Q to THQ.
2 3 3
Elementary step of the catalytic cycle
Reaction
ΔE
ΔH
ΔG
Formation of the CAS, RuHCl(κN-Q)(PPh
3
)
2
(A)
4 ↔ A + Q
13.2
14.0
−3.2
7.0
−0.9
13.2
−15.6
3.1
−8.2
−9.4
Coordination of the H
2
molecule to A
A + H
2
↔ I
1
−4.4
−0.5
13.8
−16.1
2.7
−5.1
−0.5
13.8
−16.1
2.7
Migration of hydride to N atom of Q ligand of intermediate I
Reductive elimination of DHQ: formation of RuHCl(κN-DHQ)(PPh
Rearrangement of the DHQ ligand of B from κN- to η -
Migration of hydride to C(3) atom of the DHQ ligand of B
1
I
I
1
↔ I
↔ B
2
3
)
2
(B)
2
2
B ↔ B’
B’ ↔ I
+ H
C + Q ↔ A + THQ
2
Addition of the second H
2
molecule: formation of RuHCl(κN-THQ)(PPh
3
)
2
(C)
I
2
2
↔ C
−18.0
−6.8
−18.8
−6.8
Substitution of THQ ligand of C by a new Q molecule to regenerate the CAS
7

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
2 3 3
Scheme 6. Detailed catalytic cycle for the hydrogenation of Q catalyzed by RuCl (PPh ) .
2 3 3
Fig. 5. Optimized geometries for the most feasible intermediates for the hydrogenation of Q catalyzed by RuCl (PPh ) .
8

M. Rosales, et al.
Molecular Catalysis 490 (2020) 110970
I
2
’, formed by migration of the hydride toward C(4) atom of DHQ; this
Declaration of Competing Interest
last intermediate also present an agostic interaction between the me-
thylene group in C(4) and the ruthenium atom.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper
The addition of a second molecule of H
2
to generate RuHCl(κN-
THQ)(PH (C) is exothermic and exergonic; this THQ species was
3 2
)
1
detected by H NMR in a previous study [21]. Finally, the substitution
of THQ ligand by a new Q molecule regenerates the CAS and restarts
the catalytic cycle; this reaction is also exothermic and exergonic.
Table 2 shows the thermodynamic parameters of the more feasible
elementary steps of the catalytic cycle of the hydrogenation of Q cat-
alyzed by complex 1. The structural parameters of optimized geometry
for all of the possible intermediates involved in the catalytic cycle are
included in Supplementary Material SM-IV.
CRediT authorship contribution statement
Merlín Rosales: Conceptualization, Investigation, Methodology,
Writing - review & editing. Karely Molina: Investigation, Methodology.
Federico Arrieta: Methodology, Investigation. Dángelo Fernández:
Investigation. Pablo J. Baricelli: Conceptualization, Investigation,
Writing - review & editing.
Now we may propose a detailed schematic catalytic cycle for the
hydrogenation of Q by complex 4 (Scheme 6), in accordance with our
experimental and theoretical DFT calculations. Species C may be de-
hydrogenated to produce the imine intermediate D (ΔE = 13.4,
ΔH = 14.2 and ΔG =4.2 kcal/mol) or ortho-metalated to yield E (ΔE =
ΔH = ΔG =16.3 kcal/mol). Species D and E (Supplement Material SM-
V) were postulated by Fish et al. [21] in concordance with their deu-
teration experiments, although ours theoretical DFT calculations in-
dicated that E is a dihydrogen species instead of a dihydride one. When
an excess of THQ is added to the reaction medium or produced by high
conversion of the Q substrate, species C, D and E should accumulate in
the reaction medium which reduce substantially the reaction rate as
observed by Fish et al. [21] and by us in the present work.
Acknowledgements
This work was supported by the Consejo de Desarrollo Científico y
Humanístico, Universidad del Zulia (CONDES-L.U.Z.).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.110970.
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