Acceptorless dehydrogenation of amines to nitriles catalyzed by N-heterocyclic carbene-nitrogen-phosphine chelated bimetallic ruthenium (II) complex

Article abstract of DOI:10.1016/j.jcat.2020.09.005

We have developed a clean, atom-economical and environmentally friendly route for acceptorless dehydrogenation of amines to nitriles by combining a new dual N-heterocyclic carbene-nitrogen-phosphine ligand R(CNP)₂ (R = o-xylyl) with a ruthenium precursor [RuCl₂(η⁶-C₆H₆)]₂. In this system, the electronic and steric factors of amines had a negligible influence on the reaction and a broad range of functional groups were well tolerated. All of the investigated amines could be converted to nitriles in good yield of up to 99% with excellent selectivity. The unprecedented catalytic performance of this system is attributed to the synergistic effect of two ruthenium centers chelated by R(CNP)₂ and a plausible reaction mechanism is proposed according to the active species found via in situ NMR and HRMS.

Full text of DOI:10.1016/j.jcat.2020.09.005

Journal of Catalysis 391 (2020) 378–385
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Acceptorless dehydrogenation of amines to nitriles catalyzed by
N-heterocyclic carbene-nitrogen-phosphine chelated bimetallic
ruthenium (II) complex
⇑
⇑
Xufeng Nie, Yanling Zheng, Li Ji, Haiyan Fu , Hua Chen, Ruixiang Li
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2020
Revised 1 September 2020
Accepted 2 September 2020
Available online 12 September 2020
We have developed a clean, atom-economical and environmentally friendly route for acceptorless dehy-
drogenation of amines to nitriles by combining a new dual N-heterocyclic carbene-nitrogen-phosphine
ligand R(CNP)2 (R = o-xylyl) with a ruthenium precursor [RuCl2(g
6-C6H6)]2. In this system, the electronic
and steric factors of amines had a negligible influence on the reaction and a broad range of functional
groups were well tolerated. All of the investigated amines could be converted to nitriles in good yield
of up to 99% with excellent selectivity. The unprecedented catalytic performance of this system is attrib-
uted to the synergistic effect of two ruthenium centers chelated by R(CNP)2 and a plausible reaction
mechanism is proposed according to the active species found via in situ NMR and HRMS.
Ó 2020 Elsevier Inc. All rights reserved.
Keywords:
Bimetallic Ru(II) complex
Amines
Nitriles
Acceptorless dehydrogenation
Atom economy
1. Introduction
cessful examples have been reported. The original work was
reported by Szymczak’s groups with a ruthenium complex bearing
Nitriles are key motif in the structure of numerous useful com-
pounds [1,2], such as pharmaceuticals, bioactive natural products,
and functional materials. In addition, they also serve as versatile
building blocks in organic synthesis. Nitriles are commonly pre-
pared [3–5] using the Sandmeyer reaction, the Rosenmund-von
Braun reactions, cyanation of aryl or alkyl halides, oxidization of
primary amines, Koble’s nitrile synthesis, and others. However,
the (super)stoichiometric quantities of undesired by-products
which are inevitably produced in these methods may be wasteful,
unsustainable and environmentally deleterious. In past decades,
the Ru-catalyzed acceptorless dehydrogenation of primary amines
aroused great interests among the chemists. In this transformation
the primary amine is directly dehydrogenated twice to the nitrile
with hydrogen gas (a clean fuel) as the sole by-product. Accord-
ingly, it represents the cleanest, atom-economical and environ-
mentally friendly route for nitrile synthesis and meets the
requirement of green and sustainable chemistry.
an amide-derived NNN-type ligand as a catalyst (Fig. 1a) [6a,6b].
This seminal work highlighted thepositive effect of the ligand
on this reaction. Later on, several other type ligands, such as
nitrogen heterocycle carbenes (NHC) [6c], naphthyridinyl
pyrazoles [6d] have been developed for this reaction (Fig. 1b–c).
Nevertheless, these systems still suffer from low reaction
efficiency, limited substrate scope, moderate selectivity and low
TON. To address these problems and meet the the requirement of
the green and sustainable chemistry, we endeavoured to develop
a
more efficient and high selective catalyst system for
acceptorless dehydrogenation of primary amines to nitriles.
In recent years, our group has developed N-heterocyclic
carbene-nitrogen-phosphine ligands (CNPP and CNP) [7a,7b],
and the high catalytic performance of their ruthenium complexes
has been proven in the cross-coupling dehydrogenation of primary
alcohols
to
form
cross-esters
[7c],
the
acceptorless
dehydrogenation of alcohols to acids [7d] and the acceptorless
dehydrogenation of diols with amines to N-substituted lactams
[7e]. The excellent performance can be attributed to the
anchoring effect of the NHC moiety and coordination hemilability
of the P and N atoms, as well as the facial coordination mode
adopted in the Ru-CNP complexes. Encouraged by these
achievements, we speculated that such type of ligands would
address the problems associated with the dehydrogenation of
primary amines to nitriles.
However, the strong nucleophilic character of the amines and
the low reactivity of the b-hydride elimination from amino and/
or amido complexes, make amine acceptorless dehydrogenation
to nitrile be a challenging problem [6]. To date, only a few of suc-
⇑
Corresponding authors.
E-mail addresses: scufhy@scu.edu.cn (H. Fu), liruixiang@scu.edu.cn (R. Li).
https://doi.org/10.1016/j.jcat.2020.09.005
0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

X. Nie et al.
Journal of Catalysis 391 (2020) 378–385
Scheme 1. Synthesis of complexes Ru-Ln.
Notably, the characteristic peak of proton in carbene carbon at
9.45 ppm still exists in 1H NMR spectrum, showing that the NHC
did not coordinate with Ru center (SI, Fig. S2). In addition, the
upfield shift of the proton of CH@N in the 1H NMR spectrum
revealed the nitrogen coordination with the ruthenium. Combined
with HRMS results, the structure of ruthenium complex bearing L1
Fig. 1. Ruthenium catalyzed acceptorless dehydrogenation of amines to nitriles.
was attributed as a mononuclear cationic [RuCl(
[PF6][Cl] (Ru-L1) and that bearing L2-L5 were attributed as dica-
tionic diruthenium complexes [(
6-C6H6)ClRu( 2-NPRNP- 2)RuCl
6-C6H6)][PF6]2[Cl]2 (Ru-Ln).
j g
2-NP)( 6-C6H6)]
On the other hand, the bimetallic complexes show generally to
be highly efficient catalysts for a wide range of chemical transfor-
mations, such as olefin hydroformylation [8], the CAH acetoxyla-
tion [9a], dihydroalkoxylation of alkynes [9b], alkyne
polymerization, cyclopropanation reaction [10] and others
[11,12]. By constraining two metal centers in close proximity to
each other, a cooperative action might arise and enhance greatly
the reactivity compared with their anologeous monometallic com-
plexes [13]. However, such a synergistic effect has never been
investigated in amine dehydrogenation reaction. In this regard,
we originally synthesized a series of dual CNP ligands with differ-
ent linkers and envisioned they could bring two ruthenium centers
into relative proximity that may allow for interaction of both
ruthenium atoms with the same amine molecule to achieve the
diruthenium synergistic catalysis for the acceptorless dehydro-
genation of amines. Herein, we report the first successful example
of bimetallic catalyst system for acceptorless dehydrogenation of
primary amines. This bimetallic protocol enabled a variety of ali-
phatic and aromatic primary amines to be converted into corre-
sponding nitriles in excellent yields. In most cases, the nitriles
were obtained in quantitative yields, not only reducing the waste-
ful byproducts but also obviating the need for chromatographic
purification, which comply with the principle of green chemistry.
g
j
j
(g
2.2. Catalytic activity measurement
The catalytic activities of these complexes for the acceptorless
dehydrogenation of primary amines to nitriles were examined by
using 1.0 mol% Ru-L1 and 0.5 mol% Ru-Ln (n = 2–5) (Table 1, entry
1–5), respectively. The reaction proceeded at 100 °C for 24 h, open
to
a N2 atmosphere, with Cs2CO3 as the base, and N,N-
dimethylformamide (DMF) as the solvent. As expected, all the four
bimetallic complexes showed enhanced activities relative to their
analogous monometallic complex (entry 1, L1). In addition, the
yield [15] of the nitrile 1 increased gradually with increasing car-
Table 1
Acceptorless Dehydrogenation of Amines to Nitriles.a
2. Results and discussion
2.1. Catalyst preparation characterization
To assess the relationship between catalyst activity and the
chain length of linker, we designed and synthesized four ligands
R(CNP)2 [R = A(CH2)n (n = 1, 2, 4) (L2-L4) and R = o-xylyl (L5)].
The linkers A(CH2)nA and o-xylyl can be readily installed by react-
ing the imidazole with Cl(CH2)nCl or o-(ClCH2)2C6H4, then followed
by an analogous synthetic route to that of the CNP ligand (L1) to
afford the desired ligands L2-L5. All these ligands L1-L5 are fully
characterized with both NMR and mass spectra, and the structures
of L3 and L5 were further identified by single-crystal X-ray diffrac-
tion [SI, Fig. S1].[14]
Entry
Variation of conditions
Conv [%]
Selectivity [%]
Yield(%)b
1a
2c
3c
4c
5c
6d
7
8
9
10
Ru-L1
Ru-L2
Ru-L3
Ru-L4
Ru-L5
Ru-L5
Add 300 eq Hg
Without ligand
Without base
Closed system
46.9
56.1
77.3
87.4
93.0
100
100
0.8
91.4
92.7
92.7
90.6
93.4
97.2
96.4
100
42.7
52.0
71.4
79.2
86.9
97.2
96.4
0.8
100
1.3
27.9
100
27.9
1.3
Then, the corresponding Ru-Ln (n = 1–5) complexes were pre-
pared by reacting the corresponding ligand Ln with [Ru(g
6-C6H6)
(a) Reaction conditions: benzylamine (1.0 mmol), Cs2CO3 (1.0 mmol), Ru-L1 (0.01
mmol), DMF (2 mL), 100 °C for 24 h with N2 balloon. (b) Yields were determined by
GC-MS. (c) Ru-Ln (0.005 mmol). (d) Ru-L5 (0.01 mmol), 24 h.
Cl2]2 in MeOH (Scheme 1). The 31P{1H}NMR resonances of the coor-
dinated phosphines appeared at around 43.5 ppm (SI, Fig. S2).
379

X. Nie et al.
Journal of Catalysis 391 (2020) 378–385
bon chain length of the linker (entry 2–4, L2-L4), and the A(CH2)4A
appeared to be the most favourable to cause the desired bimetallic
cooperative effect. Encouragingly, such a positive effect was fur-
ther enhanced by replacement of the flexible four-carbon linker
with a semirigid linkage o-(CH2)2C6H4 (entry 4 and 5, 79.2% yield
for L4 and 86.9% for L5). Thus, the highest catalyst activity was
obtained with complex Ru-L5. It was found that even if the catalyst
loading was decreased to 0.2 mol%, Ru-L5 still gave a yield of 86.7%
with prolonging reaction time to 72 h, thus affording a high TON of
856. To the best of our knowledge, this is the highest TON among
the reported acceptorless dehydrogenation of amines to nitriles (SI,
Table S1). Using the increased catalyst loading of 1 mol% led to a
very clean and quantitative conversion of amine to nitrile within
24 h.
To further understand the difference between the systems Ru-
L1 and Ru-L5, a series of experiments were conducted with the
same Ru loading to compare the conversion and selectivity of the
reaction by monitoring the reaction profile of the Ru-L1 and Ru-
L5 system (see SI for detail, Pages S15-S17). As can be seen from
Fig. 2, the yield of nitrile obtained with Ru-L5 system at any given
time after 6 h is significantly higher than that obtained with Ru-L1
system. These results further comfirmed that the catalytic capabil-
ity of the active species in the bimetallic system is higher than that
in the monometallic system.
All these results demonstrated that the bimetallic complexes
Ru-L5 is a very efficient and robust catalyst in this reaction. Nota-
bly, the reaction was higher yielding in polar aprotic DMF or
dimethyl acetamide (DMA) than in nonpolar solvents such as
toluene, o-xylene, mestitylene, chlorobenzene, fluobenzene etc
(Fig. 3). This may be due to the enhanced solubility of cationic
Ru-L5 and Cs2CO3 in DMF/DMA (SI, Fig. S4). From environmental
considerations, the usage of DMF or DMA as the reaction solvent
instead of the commonly used toluene would be advantageous,
since toluene presents a number of chronic toxicity hazards.
Then several experiments were carried out to gain more infor-
mation about our catalyst system (Table 1, entry 7–10). Firstly,
mercury poisoning experimental results revealed the homoge-
neous nature of the catalytic system (entry 7). Subsequently, the
experiments in the absence of either L5 or base resulted in a signif-
icant decrease in yield, demonstrating their vital roles for the reac-
tion (entry 8 and 9). The reaction did not proceed well in a closed
system, and only 1.3% yield of 1 was detected (entry 10). Finally,
Fig. 3. Comparison of different solvents. Reagents and conditions: amine
(1.0 mmol), Cs2CO3 (1.0 mmol), Ru-L5 (0.01 mmol), solvent (2 mL), 100 °C for
24 h with N2 balloon. Yields were determined by GC–MS.
the H2 evolution experiment (SI, Page S12), measured by the drai-
nage method, showed about releasing 1.9 equiv. of H2 during the
reaction, which is fully in line with the stoichiometry of the reac-
tion [6d].
2.3. Substrates scope for acceptorless dehydrogenation reaction
With the optimal conditions in hands (detail see SI, Tables S2–
S9), the scope of amines was investigated. As shown in Table 2, the
dehydrogenation reaction was proven to be rather general regard-
less of the electronic and steric factors of the substrates. A variety
of aryl or alkyl nitriles 1–44 could be generated from their corre-
sponding amines in high yields and a gram scale reaction were also
efficient (1, detail see SI, Page S11). The catalytic system was highly
compatible with various functional groups. For example, all of the
t
substituted benzylamines bearing Me, MeO, EtO, Bu, Ph, NH2, CF3
and halogen were dehydrogenated to their benzonitriles in excel-
lent yields. Notably, in comparison with the previously reported
system, the present protocol showed outstanding reactivity with
an extended substrate scope. For example, low yields of 58% and
22% were reported for m-NH2 and m-Cl substituted benzylamines
[6a], while high yields of 95.3% (14) and 75.4% (16) were
achieved in our system. Furthermore, the bromo-substituted
benzylamines of which the acceptorless dehydrogenation to
nitrile has never been demonstrated in the literature were also
tolerated under the reaction condition and afforded the
corresponding nitriles in the excellent yields of 89.4–93.0% (10,
17, 24). The multisubstituted aryl nitriles 26–29 could be formed
in higher than 96% yields, In particularly, even the sterically
demanding (2,6-dimethylphenyl)methanamine underwent the
reaction very well, affording the product 26 in 98.8% yield.
Furthermore, 1-naphthalenemethanamine and 3,4-(methylene
dioxy)benzyl-amine were almost quantitatively converted into
corresponding nitriles 30 and 31. The catalytic system is also
suitable to the synthesis of heterocyclic nitriles. The primary
amines containing heterocyclic motifs, such as pyridine,
thiophen, furan and indole, could be converted to the
corresponding nitriles 32–35 in good yields. Interestingly, p-
Fig. 2. Rate profiles for the reactions catalyzed with Ru-L1 and Ru-L5 systems.
Reagents and conditions: amine (1.0 mmol), Cs2CO3 (1.0 mmol), Ru-L5 (0.01 mmol)
(or Ru-L1 (0.02 mmol)), solvent (2 mL), 100 °C for 24 h with N2 balloon. Yields were
determined by GC–MS.
380

X. Nie et al.
Journal of Catalysis 391 (2020) 378–385
Table 2
substrates and successfully converted to nitriles (37–43) in 85–
99% yields. Alkyl dinitriles, such as 40 could be obtained from
their corresponding diamines in excellent yield of 95.4%. Again,
the sterically hindered adamantan-1-ylmethanamine could react
smoothly and afforded the product 43 in 85.4% yield. Notably,
indoline, a cyclic secondary amine was converted to indole 44 in
Substrates scope of the reaction.a
96.7% yield, possibly via
tautomerization process.
a dehydrogenation followed by a
As aforementioned, the dehydrogenation of the primary amines
did not work well in a closed system, which might be ascribed to
the generated H2. Whereas the (3-nitrophenyl) methanamine
hydrochloride could react smoothly both under N2 balloon condi-
tion and in a closed system to give the product 46 in 86.8% and
83.7% yields, respectively (Scheme 2, eqs 2 and 3). In sharp contrast
with previously reported Ru/pyrazole [6d], [Ru(C6H6)Cl2]2/HMTA
[6f] and RuCl3ÁnH2O [6g] systems, in which the NO2-substituted
benzylamine was converted into nitrile with the nitro group
being remained (Scheme 2, Eq. 1), in our system the nitro group
was reduced into the ANH2 group by the generated H2,
indicating that Ru-L5 complex also has the potential
hydrogenation ability. Indeed, the hydrogenation reaction of 3-
nitrobenzonitrile with a H2 balloon under the otherwise identical
condition to the dehydrogenation reaction could produce 46,
albeit in relatively low yield (Scheme 2, Eq. 4). We also found
that the 4-vinyl benzylamine was converted to the nitrile with
the vinyl group being reduced to ethyl group. This result further
evidences that the Ru-CNP complexes own dehydrogenation and
hydrogenation properties.
To further clarify the nature of the present system, several
experiments were conducted. Firstly, the competing reactions
[16,17] between PhCH2NH2 and PhCH2ND2, as well as PhCH2NH2
and PhCD2NH2, disclosed a kinetic isotopic effect (KIE) of 1.51
and 1.03, respectively (SI, Scheme S3). This result suggested that
neither the N-H nor the C-H bond cleavage was the rate-
determining step. We suspect that the coordination of Ru center
with H atom (Scheme 5A–B and E–F) was the rate-determining
step because the low electron density of hydrogen atom in CAH
bond of the substrate molecule leads to weak coordination to Ru
center. Especially, the fact that these substrates bearing the
electron-withdrawing groups give the relatively lower yields than
those bearing electron-donating groups probably is a reasonable
evidence.
Then, a Hammett plot, which can give the information about the
reaction sensitivity towards the substituents, was constructed with
benzylamines substituted by Me, MeO, NH2, Cl, Br, F and CF3 in
meta-position and para-position, respectively. In both cases, the
negative slopes of
q
= À0.34 and À0.42 were given in the Hammett
(a) Reaction conditions: amine (1.0 mmol), Cs2CO3 (1eq), Ru-L5 (1 mol %), DMF (2
mL), 100 oC for 24-36 h with N2 balloon. (b) Yields were determined by GC-MS and
isolated yields were reported in parentheses. (c) Closed system under argon
atmosphere. (d) gram scale reaction. amine (10.0 mmol), Cs2CO3 (0.5eq), Ru-L5 (0.5
mol %), DMF (2 mL), 100 oC for 5 days with N2 balloon, (e) 60 oC for 48 h.
xylylenediamine was dehydrogenated into 36 in 87.8% yield with
one CH2NH2 group being transformed into a cyano group and
another one to primary amide group. But the exact reason for
this transformation is not clearly understood at this stage, we
preliminarily suspect that amide was obtained by the interaction
between nitrile formed by dehydrogenation of primary amine
and DMF (SI, Pages S24-S25). The alkyl amines were also viable
Scheme 2. Investigation of dehydrogenation and hydrogenation ability of Ru-L5
system. Reagents and conditions: amine (1.0 mmol), Cs2CO3 (1 eq), Ru-L5 (1 mol %),
DMF (2 mL), 100 °C for 24 h. Yields were determined by GC–MS.
381

X. Nie et al.
Journal of Catalysis 391 (2020) 378–385
plot (SI, Fig. S5). Compared to the
q
value of À1.22 in Bera group
55.6 ppm to complex III, 78.5 ppm to complex IV and 80.5 ppm to
complex V (Scheme 3 and SI, Scheme S4). It is also in good agree-
ment with the fact that the coordination of more chlorides with Ru
center would cause the downfield shift of the coordinated P atom
[18].
system [6d], the more placid negative slope in our system clearly
indicated that our system is less sensitive to the electronic
nature of substituents. This should be the reason why our system
is well suitable for the wide substrate scope.
To clarify the bimetallic synergistic effect in the reaction pro-
cess, in-situ NMR spectroscopic studies were performed to monitor
the structure transformation of complexes in Ru-L1 and Ru-L5 sys-
tem. For Ru-L1 system, upon addition of 20 equiv of Cs2CO3, the
disappearance of the carbene CAH resonance (9.45 ppm) in 1H
NMR (SI, Fig. S6, a) and the signal at 43.5 ppm (II) in 31P NMR
(Fig. 4a) indicated that the NHC in L1 coordinated to Ru with the
assistance of base. At the same time, three new species in 31P
NMR were present in downfield at 55.6 ppm (III), 78.5 ppm and
80.5 ppm, respectively (Fig. 4a and Scheme 3). When the reaction
time was extended, the peak at 55.6 ppm gradually diminishes
and two species at 78.5 ppm and 80.5 ppm become more promi-
nent. In addition, 1H NMR monitoring of the solution showed the
signal of free benzene which dissociated from the coordinated
For Ru-L5 (Fig. 4b), in the early stage, each of the two halves of
L5 coordinated independently with ruthenium in a similar manner
to that of L1. But in the late stage, these analogous species
(Scheme 4, VI, VII and VIII) with Ru-L1 gradually disappeared, sev-
eral new species (Fig. 4b) appeared in the upfield region in 31P
NMR. Especially, a singlet at 11 ppm emerged as the major species,
which evidences a symmetric structure. In addition, the high-
resolution mass spectrometry (HRMS) analysis of the solution
(Scheme
z = 1177.0705. So we tentatively assign this as a bimetallic Ru spe-
cies, formulated as Ru2(L5)Cl2( -Cl) (IX) or Ru2(L5)( -Cl)3 (X)
4 and SI, Scheme S5) showed a peak at m/
l
l
(Scheme 4). The linkage of ligand L5 forces the two Ru center closer
to each other and forms a symmetric chloro-bridged bimetallic
complex. This structure should be relevant with its higher catalytic
activity than Ru-L1 system. Notably, only a low nitrile yield of 30%
was detected at the beginning 12 h and the major metal species
g
6-C6H6 with ruthenium (Fig. 4c). Furthermore, the addition of LiCl
caused the increase of peak intensity at 80.5 ppm (Fig. 4d), which
evidences two species at 78.5 ppm and 80.5 ppm build up an equi-
librium in solution. Hence, it is reasonable to attribute the peak at
[(g j j g
6-C6H6)RuCl( 2-CNP-(o-xylyl)- 2-CNP)RuCl( 6-C6H6)](PF6)2
(VI) was detected in situ NMR during this time. The result indi-
cated that complex VI was less active for this reaction. However,
after the catalytic system was pretreated without benzylic amine
under the standard condition for 12 h, the complex VI was gradu-
ally converted into species IX or X (Fig. 4, b and Scheme 4). Subse-
quently, benzylic amine was added to this system and reacted for
12 h. In this case the dehydrogenation proceeded about 1.7 times
faster than nonpretreated system and the yieldof nitrile increased
up to 50%. These results indicated the IX or X might be the more
reactive species.
The Ru-H species are believed to be the key intermediate for the
dehydrogenation of benzylic amine [6,19]. After the benzylic
amine (10 equiv) was added, the in-situ 1H NMR studies on the
Ru-L1 system (Fig. 5a) showed that two doublets at À10.34 and
À10.25 ppm with JHP = 118.9 Hz and 113.4 Hz, respectively. The
large coupling constant suggested that the hydride and phospho-
rus atom were mutually located in the trans position. However,
the peaks of Ru-H appeared at d À10.19 ppm (JHP = 25.4 Hz) and
d À12.64 ppm (JHP = 15.0 Hz) for Ru-L5 system. The small coupling
constants indicated that the hydride located in the cis position of
the phosphorus atom (Fig. 5, b). Combining with HRMS data, we
assumed that Ru-L5 finally form a chloro-bridged bimetallic spe-
cies, in which the hydride was located in the cis-position of phos-
phorus atom in the catalytic cycling.
Fig. 4. NMR spectra of Ru-L1 and Ru-L5 system.
On the basis of above results and literature reports, a plausible
bimetallic mechanism for the dehydrogenation of benzylic amine
Scheme 3. The possible active species in Ru-L1 system.
Scheme 4. The possible bimetallic species in Ru-L5 system.
382

X. Nie et al.
Journal of Catalysis 391 (2020) 378–385
ized. These novel ligands enable us to develop a highly efficient
bimetallic catalyst system (Ru-L5) for the acceptorless dehydro-
genation of primary amines to nitriles. It exhibits enhanced cat-
alytic reactivity compared with its analogous monometallic
counterpart. The present system displays much broader substrate
scope and functional group compatibility than other known accep-
torless amine dehydrogenation systems. The excellent catalytic
performance can be attributed to a bimetallic synergistic effect,
in which one Ru center combines with nitrogen atom of the amine,
and the another abstracts the CAH proton of the benzylic amine via
a six-membered ring transition state. This is more thermodynam-
ically favourable than the four-membered ring involved in the
monometallic Ru-L1 system.
Fig. 5. 1H NMR evidence for the formation of Ru-H species in Ru-L1 and Ru-L5
system.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 21572137 and 21871187) and the Opening Project of Key
Laboratory of Green Chemistry of Sichuan Institutes of Higher Edu-
cation (LZJ1901) and the Key Program of Sichuan Science and Tech-
nology Project (2019YFG0146) for their financial support.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2020.09.005.
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