Synthesis, structure and catalytic properties of CNN pincer palladium(II) and ruthenium(II) complexes with N-substituted-2-aminomethyl-6-phenylpyridines

Article abstract of DOI:10.1039/c1dt10331h

N-substituted-2-aminomethyl-6-phenylpyridines 2a-c have been easily prepared from commercially available 6-bromo-2-picolinaldehyde in two steps. Reaction of 2a-c with PdCl₂ in toluene in the presence of triethylamine gave the CNN pincer Pd(ii) complexes 3a-c in 18-28% yields. The CNN pincer Ru(ii) complex 5 containing a Ru-NHR functionality could be obtained in a 71% yield by treatment of 2c with a Ru(ii) precursor instead of PdCl ₂. Additionally, the related CNN pincer Ru(ii) complex 7 containing a Ru-NH₂ functionality has been synthesized by the reaction of 2-aminomethyl-6-phenylpyridine with the same Ru(ii) precursor in a 68% yield. All the new compounds were characterized by elemental analysis (MS for ligands), ¹H, ¹³C NMR, ³¹P{¹H} NMR (for Ru complexes) and IR spectra. Molecular structures of Pd complex 3c as well as Ru complexes 5 and 7 have been determined by X-ray single-crystal diffraction. The obtained Pd complexes 3a-c were effective catalysts for the allylation of aldehydes as well as for three-component allylation of aldehydes, arylamines and allyltributyltin and their activity was found to be much higher than a related NCN Pd(ii) pincer in the allylation of aldehyde. On the other hand, the two new CNN pincer Ru(ii) complexes 5 and 7 displayed excellent catalytic activity in the transfer hydrogenation of ketones in refluxing 2-propanol with the latter being much more active. The final TOF values were up to 4510 h^-1 with 0.01 mol% of 5 and 220800 h^-1 with 0.005 mol% of 7, respectively. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10331h

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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
ARTICLES:
Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta
Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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Volume 40 | Number 35 | 21 September 2011 | Pages 8713–9052
Themed issue: Pincers and other hemilabile ligands
ISSN 1477-9226
COVER ARTICLE
Gong, Song et al.
Synthesis, structure and catalytic
properties of CNN pincer Pd(II) and
Ru(II) complexes with N-substituted-2-
aminomethyl-6-phenylpyridines
1477-9226(2011)40:35;1-Q

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PAPER
Synthesis, structure and catalytic properties of CNN pincer palladium(II) and
ruthenium(II) complexes with N-substituted-2-aminomethyl-6-
phenylpyridines†
Tao Wang, Xin-Qi Hao, Xiao-Xue Zhang, Jun-Fang Gong* and Mao-Ping Song*
Received 25th February 2011, Accepted 14th April 2011
DOI: 10.1039/c1dt10331h
N-substituted-2-aminomethyl-6-phenylpyridines 2a–c have been easily prepared from commercially
available 6-bromo-2-picolinaldehyde in two steps. Reaction of 2a–c with PdCl2 in toluene in the
presence of triethylamine gave the CNN pincer Pd(II) complexes 3a–c in 18-28% yields. The CNN
pincer Ru(II) complex 5 containing a Ru-NHR functionality could be obtained in a 71% yield by
treatment of 2c with a Ru(II) precursor instead of PdCl2. Additionally, the related CNN pincer Ru(II)
complex 7 containing a Ru–NH2 functionality has been synthesized by the reaction of
2-aminomethyl-6-phenylpyridine with the same Ru(II) precursor in a 68% yield. All the new compounds
1
were characterized by elemental analysis (MS for ligands), 1H, 13C NMR, 31P{ H} NMR (for Ru
complexes) and IR spectra. Molecular structures of Pd complex 3c as well as Ru complexes 5 and 7
have been determined by X-ray single-crystal diffraction. The obtained Pd complexes 3a–c were
effective catalysts for the allylation of aldehydes as well as for three-component allylation of aldehydes,
arylamines and allyltributyltin and their activity was found to be much higher than a related NCN
Pd(II) pincer in the allylation of aldehyde. On the other hand, the two new CNN pincer Ru(II)
complexes 5 and 7 displayed excellent catalytic activity in the transfer hydrogenation of ketones in
refluxing 2-propanol with the latter being much more active. The final TOF values were up to 4510 h-1
with 0.01 mol% of 5 and 220 800 h-1 with 0.005 mol% of 7, respectively.
modification of the terdentate aryl-based pincer ligands. These
Introduction
factors provide a number of metal-mediated organic transforma-
Transition-metal pincer complexes, in which the terdentate ligands
tions including highly selective asymmetric versions. For example,
coordinate to the metal center in a meridional fashion through
the NCN pincer palladium(II) complexes are found to be effective
catalysts for the stannylation of allyl 2 or propargylic substrates,3
Diels–Alder reaction,4 cross-coupling reactions including Heck,5
Suzuki and Sonogashira5d reactions. The ruthenium(II) complexes
are effective for transfer hydrogenation and hydrogenation of
ketones,6 the iridium(III) complexes for transfer hydrogenation
of ketones7 and the platinum(II) complexes for alkylation of
aldimines8a and aldol reaction of isocyanides and aldehydes.8b On
the other hand, the cyclometalated CNN pincer complexes with a
carbanion from the pendant aryl ring as the C donor atom have
also received much interest in recent years. Particularly, a variety
of complexes including Pt(II),9 Pd(II)10 and Ir(II)11 with 6-phenyl-
2,2¢-bipyridine and its derivatives (Chart 1, structure A) have
been extensively studied due to their intriguing photophysical and
photochemical properties. Baratta and co-workers have reported a
series of achiral and chiral CNN pincer Ru(II) and Os(II) complexes
of 2-primary amine functionalized 6-arylpyridines (structure B),
which are highly active catalysts for the transfer hydrogenation
(TH) of ketones that afford TOF values up to 2.5 ¥ 106 h-1
with very low catalyst loadings (0.005–0.001 mol%) in a short
time.12 The -NH2 group in the CNN ligand backbone is found
one sigma and two dative bonds or three dative bonds to form
two stable 5- or 6-membered metallacycles, have been intensively
investigated and used in organic synthesis, organometallic catal-
ysis, materials science and the related areas.1 Research on these
complexes has been dominated by ECE (the term ECE refers to the
three atoms directly attached to the metal) complexes containing
two identical neutral E, such as P, N, S, or Se donor groups, in
the side arms of the central aryl ring and one carbanion from the
central aryl ring as the central C donor atom. The existence of the
M–C s bond supported by the two neutral E donors makes ECE
complexes exhibit generally high stabilities towards heat, air and
moisture. More importantly, their reactivities, stabilities or other
important properties can be readily tuned via variation of the
metal center nature and ancillary ligands, as well as the structural
Department of Chemistry, Henan Key Laboratory of Chemical Biology
and Organic Chemistry, Zhengzhou University, Zhengzhou, 450052, China.
E-mail: mpsong@zzu.edu.cn, gongjf@zzu.edu.cn
† Electronic supplementary information (ESI) available. CCDC reference
numbers 800253, 804174 and 804175. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt10331h
8964 | Dalton Trans., 2011, 40, 8964–8976
This journal is
The Royal Society of Chemistry 2011
©

Chart 1 Representative examples of CNN pincer metal complexes.
Scheme 1 Synthesis of CNN pincer Pd(II) complexes 3a–c with N-substituted-2-aminomethyl-6-phenylpyridines.
to be crucial in enhancing the activity of the Ru complexes since
replacing the NH2 group by a NMe2 group leads to much lower
activity of the corresponding catalysts. Additionally, the CNN
pincer Pd(II) complexes have been successfully applied to C–C
coupling reactions such as Heck (structure C),13 Suzuki (structure
D)14 and Stille 15 reactions.
During our investigation on NCN, NCN¢, PCP and PCN pincer
complexes with group 10 metals, which contain a central aryl
ring in the pincer ligand backbone,16 we became interested in
the CNN pincer metal complexes where the pendant aryl ring
provides the C donor atom. It is hoped that the CNN complexes
may have different reactivity compared to the NCN analogues
due to the different geometrical disposition of the s-donor carbon
atom. For this purpose, we synthesized the CNN pincer Pd(II)
complexes with N-substituted-2-aminomethyl-6-phenylpyridines
3a–c (Scheme 1) and the related NCN complex 4 (Chart 2).17
The obtained Pd complexes were applied to the allylation of
aldehydes with allyltributyltin and their activities were compared.
Complex 3c was further used in the three-component reaction of
aldehydes, amines and allyltributyltin. In addition, considering the
high activity of Baratta’s CNN pincer Ru(II) complexes in transfer
hydrogenation, the CNN pincer Ru(II) complex 5 with a Ru-
NHR functionality was easily prepared by the reaction of N-(p-
tolyl)-2-aminomethyl-6-phenylpyridine 2c with a Ru(II) precursor
(Scheme 2). For comparison, the Ru complex 7 with a Ru–NH2
functionality which is an analogue of Baratta’s pincer Ru(II)
complex was also obtained (Scheme 3). The catalytic activities
Scheme 2 Synthesis of CNN pincer Ru(II) complex 5 with N-(p-tolyl)-
2-aminomethyl-6-phenylpyridine.
Scheme
3
Synthesis of CNN pincer Ru(II) complex
7 with
2-aminomethyl-6-phenylpyridine.
of the two Ru complexes in the transfer hydrogenation of ketones
were investigated. The results are presented below.
Results and discussion
Synthesis of CNN pincer Pd(II) complexes
The synthesis of the required N-substituted-2-aminomethyl-6-
phenylpyridine ligands 2a–c is easily done starting from com-
mercially available 6-bromo-2-picolinaldehyde in two steps as
shown in Scheme 1. First, the Suzuki coupling of 6-bromo-
2-picolinaldehyde with phenylboronic acid in the presence of
Pd(PPh3)4 in toluene at reflux proceeded smoothly to afford 6-
phenyl-2-picolinaldehyde 1. Then the aldehyde group in com-
pound 1 was converted to imines by reaction with aliphatic or
aromatic amines. Without further purification, the formed imines
were readily reduced to secondary amines by NaBH4 to give the
expected ligands 2a–c in good yields. With the ligands in hand, the
Chart 2 NCN pincer Pd(II) complex 4 with N-isopropyl-1-amino-
methyl-3-(2-pyridyloxyl)benzene.
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Dalton Trans., 2011, 40, 8964–8976 | 8965
©

cyclopalladation reaction was carried out by refluxing the ligand 2
and palladium chloride in the presence of triethylamine in toluene
for 14 h. The three new CNN pincer palladium complexes 3a–
c were successfully isolated as orange solids after purification,
though the yields were not very high (18–28%).‡
phenylpyridine containing a Pd-NH2 functionality.18 The val-
ues of the bond lengths and angles also compare well to
those of the CNN Pd complexes with 6-phenyl-2,2¢-bipyridine10a
(Scheme 1, structure A) and 2-[(2,6-diisopropylphenyl)imino]- 6-
phenylpyridine14 (structure D). Similarly to those reported CNN
All of the pincer Pd complexes are air- and moisture-stable both
in the solid state and in solution. They were fully characterized by
Pd complexes,10a,14,18 the Pd(1)–N(2) bond length (2.241(5) A) in
˚
3c is also significantly longer than that of Pd(1)–N(1) (1.964(4)
1
elemental analysis, H NMR, 13C NMR, and IR spectroscopy.
A), which can be attributed to the higher trans influence of
˚
1
The H and 13C NMR spectra of complexes 3a–c confirm the
the metalated carbanion compared to the chloride ligand. The
angle of N(1)–Pd(1)–Cl(1) (179.0◦) is only slightly less than 180◦,
while the N(1)–Pd(1)–C(1) angles (82.2◦) and N(1)–Pd(1)–N(2)
angles (80.6◦) are small, which shows the geometrical constraints
imposed by the two fused five-membered-ring palladacycles.
occurrence of C-palladation: as compared with the corresponding
free ligands 2a–c, one of the resonances due to the aryl protons
is absent and a new quarternary C-signal appears at around d
153 ppm, which is assigned to the orthometalated carbon atom
and is significantly shifted downfield relative to the parent ligands
(around 130 ppm). Additionally, six carbon signals are observed
for the 6-phenyl group in the complexes due to the formation of the
Pd–C bond, while four carbon signals correspond to the 6-phenyl
group in the ligands. The spectra also indicate the coordination of
the -NHR group and pyridine to the Pd center. For example, the
-CH2 protons appear as a singlet at d 3.99, 4.55 and 4.48 ppm for
ligands 2a–c, respectively. While in the complexes 3a–c, the -CH2
protons appear as two doublet of doublets at d 4.54 and 3.97,
5.34 and 4.38, 5.31 and 4.35 ppm, respectively. In the 13C NMR
spectra of the ligands, the resonances due to the C(2)- and C(6)-
carbon atoms of the pyridine ring appear around d 159 and 157
ppm, respectively. For the pincer Pd complexes, the corresponding
resonances are shifted downfield. The former are observed around
d 162 ppm and the latter around d 161 ppm.
The formation of CNN pincer Pd(II) complexes is unambigu-
ously confirmed by single crystal X-ray diffraction analysis of 3c.
As shown in Fig. 1, the ligand coordinates to the Pd(II) center via
pyridine-N, secondary amino-N and the pendant phenyl-C in a
tridentate manner and the metal center adopts a distorted square-
planar geometry with chloride occupying the fourth coordination
site. All of the bond lengths and angles around the Pd(II) center
in complex 3c are very similar to those found in the related
CNN pincer palladium complex with 2-(2-aminoisopropyl)-6-
Synthesis of CNN pincer Ru(II) complexes
The synthesis of the CNN pincer Ru complex with ligand 2 was
carried out in a way similar to that reported for the CNN Ru(II)
complex with 2-aminomethyl-6-(4-methylphenyl)pyridine by
Baratta.12b Thus, ligand 2c was reacted with [RuCl2(PPh3)(dppb)]
(dppb: 1,4-bis(diphenylphosphino)butane) in 2-propanol at reflux
in the presence of triethylamine for 12 h to afford smoothly the
expected Ru(II) complex 5 containing a Ru-NHR functionality
(Scheme 2). For comparison, the Ru(II) complex 7 containing a
Ru–NH2 functionality was also prepared from the Ru(II) precursor
with 2-aminomethyl-6-phenylpyridine 6, which could be easily
derived from 6-phenyl-2-picolinaldehyde 1 (Scheme 3). The two
new complexes were fully characterized by elemental analysis, 1H
1
1
NMR, 13C NMR, 31P{ H} NMR and IR spectroscopy. The H
NMR spectra of 5 and 7 present two signals for the -CH2N protons,
which is one singlet in the corresponding free ligands. The two
signals include one triplet at d 4.36 and one doublet at d 3.88 ppm
for complex 5 and for 7, they are one doublet of doublets at d 4.11
and one doublet of triplets at d 3.70 ppm. Additionally, in the 13
C
NMR spectra the resonances at d 58.9 and 52.3 ppm, respectively
correspond to the CH2N group for complexes 5 and 7, which
are shifted downfield compared to the parent ligands. The above
results indicate the coordination of the -NHR or -NH2 group to
the Ru(II) center. Also in the 13C NMR spectra of Ru complexes,
the signals for the orthometalated carbon atoms appear at d 175.7
and 181.1 ppm, respectively as a doublet of doublets with 2J(CP) =
16.5, 8.1 and 16.8, 7.9 Hz, which are strongly shifted downfield
1
with respect to the corresponding ligands. Finally, the 31P{ H}
NMR spectrum of 5 exhibits two doublets at d 56.4 and 39.2 ppm
with 2J(PP) = 37.7 Hz and for complex 7, the two doublets appear
at d 56.9 and 42.4 ppm with 2J(PP) = 38.2 Hz.
The solid-state structures of the two Ru complexes 5 and
7 were determined by single crystal X-ray diffraction analysis.
The molecules are illustrated in Fig. 2 and Fig. 3. Selected
bond lengths and angles are listed in Table 1. The ruthenium
center in each complex adopts a distorted octahedral environment
with the ligand coordinated to the metal via pyridine-N, amino-
N and the pendant phenyl-C in a tridentate manner and the
diphosphane as well as a chloride ligand occupying the other
three coordination sites. The metal–ligand distances and bond
angles around Ru(II) center for complex 7 with 2-aminomethyl-
6-phenylpyridine are very close to those observed for the
Ru complex with 2-aminomethyl-6-(4-methylphenyl)pyridine.12a
In comparison with 7, complex 5 has an obviously longer
Fig. 1 DIAMOND view of a unit of pincer Pd(II) complex 3c. Hy-
drogen atoms are omitted for clarity. Selected bond lengths (A) and
˚
angles (◦): Pd(1)–N(1) 1.964(4), Pd(1)–C(1) 1.999(5), Pd(1)–N(2) 2.241(5),
Pd(1)–Cl(1) 2.3138(16); N(1)–Pd(1)–C(1) 82.2(2), N(1)–Pd(1)–N(2)
80.61(18), C(1)–Pd(1)–N(2) 162.7(2), N(1)–Pd(1)–Cl(1) 179.00(14),
C(1)–Pd(1)–Cl(1) 97.28(18), N(2)–Pd(1)–Cl(1) 99.84(15).
‡ Note added at proof: It was discovered that much better yields (52, 48 and
50%, respectively, for 3a–c) could be achieved by using 2 equiv. of sodium
bicarbonate instead of triethylamine as the base under similar conditions.
8966 | Dalton Trans., 2011, 40, 8964–8976
This journal is
The Royal Society of Chemistry 2011
©

◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for complexes 5 and
7·CH2Cl2
5
7·CH2Cl2
Ru(1)–N(1)
Ru(1)–C(29)/Ru(1)–C(1)
Ru(1)–N(2)
Ru(1)–Cl(1)
Ru(1)–P(1)/Ru(1)–P(2)
Ru(1)–P(2)/Ru(1)–P(1)
N(1)–Ru(1)–C(29)/N(1)–Ru(1)–C(1)
N(1)–Ru(1)–N(2)
N(2)–Ru(1)–P(1)/N(2)–Ru(1)–P(2)
N(1)–Ru(1)–P(2)/N(1)–Ru(1)–P(1)
C(29)–Ru(1)–P(2)/C(1)–Ru(1)–P(1)
N(2)–Ru(1)–P(2)/N(2)–Ru(1)–P(1)
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–Cl(1)/P(2)–Ru(1)–Cl(1)
C(29)–Ru(1)–N(2)/C(1)–Ru(1)–N(2)
C(29)–Ru(1)–P(1)/C(1)–Ru(1)–P(2)
N(1)–Ru(1)–P(1)/N(1)–Ru(1)–P(2)
C(29)–Ru(1)–Cl(1)/C(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
2.0991(18)
2.041(2)
2.5462(19)
2.4887(6)
2.3095(6)
2.2535(6)
79.46(8)
72.04(7)
105.27(5)
93.24(5)
86.41(6)
104.62(5)
93.98(2)
88.48(2)
149.81(8)
101.79(6)
172.73(5)
92.13(6)
84.31(5)
177.35(2)
75.61(5)
2.065(2)
2.057(3)
2.255(3)
2.5157(8)
2.2938(8)
2.2470(8)
79.76(11)
76.30(11)
99.65(8)
91.82(7)
86.90(8)
102.44(9)
94.41(3)
90.42(3)
154.49(12)
103.27(9)
173.20(7)
90.67(8)
83.42(7)
174.98(3)
78.03(9)
Fig. 2 DIAMOND view of a unit of pincer Ru(II) complex 5 containing
a Ru-NHR functionality. Hydrogen atoms are omitted for clarity.
P(2)–Ru(1)–Cl(1)/P(1)–Ru(1)–Cl(1)
N(2)–Ru(1)–Cl(1)
of various biologically active compounds. Szabo´ and co-workers
have demonstrated that the PCP pincer Pd(II) complexes with
bis(phosphine) and bis(phosphinite) ligands are highly active and
robust catalysts for the allylation of aldehydes and sulfonimines
with allylic stannanes, showing some benefits compared to bisallyl
palladium chemistry.1l,2,19 Subsequent to their findings, several
other groups also used pincer Pd complexes including SeCSe,20
SCS,21 PCP,22 PCS23 and PPP24 pincers as efficient catalysts in
these transformations. In contrast, it was found that NCN Pd
pincers with bis(amine) ligands exhibited low catalytic activity.2,19
Additionally, the asymmetric versions of these reactions have
been recently reported with chiral pincer Pd complexes.25 Based
on the literature results, it seemed to be of interest to evaluate
the effectiveness of the new CNN pincer Pd complexes 3 in the
allylation reactions. To our knowledge, CNN Pd pincers have not
been applied to these reactions so far.
The allylation of benzaldehyde with allyltributyltin was initially
chosen as a model to optimize the reaction conditions. The
reaction was performed in the presence of 5 mol% of the Pd
complex 3c under a N2 atmosphere at 50 ◦C for 24 h. Several
solvents including CHCl3, THF, CH3CN, DMSO and DMF were
tested and DMF was found to be the most appropriate, giving the
alcohol product in a 68.2% yield (Table 2, entries 1–5). Prolonging
the reaction time to 48 h did not give a better result (71.2%, entry
6) and the reaction conducted at room temperature only afforded
a low yield (18.9%, entry 7). In addition, complex 3b showed an
activity comparable to that of 3c, while 3a was less efficient under
the same reaction conditions (entries 8 and 9).
Fig. 3 DIAMOND view of a unit of pincer Ru(II) complex 7·CH2Cl2
containing a Ru–NH2 functionality. Hydrogen atoms and CH2Cl2 are
omitted for clarity.
˚
˚
Ru(1)–N(2) bond length (2.5462(19) A vs. 2.255(3) A) as well
as smaller N(1)–Ru(1)–N(2) (72.04 vs. 76.30◦) and C(29)/C(1)–
Ru(1)–N(2) (149.81 vs. 154.49◦) angles. As in the reported Ru
complex with 2-aminomethyl-6-(4-methylphenyl)pyridine12a and
the Pd complex 3c, the metal-N(2) bond length is significantly
longer than that of metal–N(1) in the two Ru complexes 5
and 7. The small values for N(1)–Ru(1)–C(29)/C(1) and N(1)–
Ru(1)–N(2) angles in these complexes are also typical for pincers
consisting of two five-membered-ring metallacycles and reflect the
relative steric strain of the system.
Reactions of other representative aldehydes with allyltributyltin
were carried out in DMF and typically with a catalyst loading of
5 mol% at 50 ◦C for 24 h to explore the activity scope of the
pincer Pd complexes 3 (Table 3). It was found that allylation of
activated aryl aldehydes bearing an electron-withdrawing group
such as 4-Cl, 4-Br, 4-F, 2-Br, 3-PhO gave higher yields (typically
73.2–85.2%) than that of benzaldehyde under similar reactions
conditions (entries 10–19). Particularly, excellent yields (>90%)
were obtained in the allylation of 4-nitrobenzaldehyde in the
Catalytic studies
Allylation with allyltributyltin catalyzed by CNN and NCN pin-
cer Pd complexes. The palladium-catalyzed allylation of aldehy-
des and imines are also reliable and widely employed methods for
carbon–carbon bond formation, giving the homoallylic alcohols
and amines as important building blocks for the construction
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8964–8976 | 8967
©

Table 2 Optimization of reaction conditions for the allylation of benzaldehyde with allyltributyltin using the Pd–CNN pincer complexes 3 as catalystsa
Entry
Cat.
Solvent
Temp./◦C
Isolated yield
1
2
3
4
5
6
7
8
9
3c
3c
3c
3c
3c
3c
3c
3b
3a
CHCl3
THF
50
50
50
50
50
50
RT
50
50
41.2
45.6
47.0
52.4
68.2
71.2b
18.9c
70.9
46.6
CH3CN
DMSO
DMF
DMF
DMF
DMF
DMF
a Reaction conditions: benzaldehyde (0.2 mmol), allyltributyltin (0.24 mmol), Cat. (5 mol%), solvent (2 mL), 24 h. b Reaction time was 48 h. c DMF
(5 mL).
presence of 2–5 mol% of CNN Pd complex 3 at room temperature
(entries 1–4). Good to excellent yields could be still obtained with
much reduced loadings of the catalyst (0.02–0.2 mol%) when the
same reaction was performed at 50 ◦C (entries 5–7). In comparison
with 3, the related NCN pincer Pd complex 4 with N-isopropyl-
1-aminomethyl-3-(2-pyridyloxyl)benzene displayed a much lower
catalytic activity. The yields for allylation at room temperature
and 50 ◦C were only 8.3% and 20.9%, respectively, in the presence
of 5 mol% of complex 4 (entries 8 and 9). Reaction of the
heteroaryl aldehyde such as 2-picolinaldehyde with allyltributyltin
catalyzed by complex 3c gave the corresponding homoallylic
alcohol in an excellent yield (90.3%, entry 20), while that of furan-
2-carbaldehyde only afforded a 46.0% yield under the typical
reaction conditions (entry 21). For naphthalene-1-carbaldehyde
and trans-b-phenylacrolein, good yields of the alcohol products
were obtained (68.7 and 61.2%, respectively, entries 22 and 23).
Finally, lower yields (40.3–57.0%) were observed in the case of less
electrophilic aldehydes such as 4-methoxybenzaldehyde (entry 24)
and 4- or 2-methylbenzaldehydes (entries 25 and 26) as well as
an aliphatic aldehyde (entry 27) compared to benzaldehyde and
activated aryl aldehydes.
On the basis of the successful application of the CNN pincer Pd
complexes 3 to the allylation of aldehydes, it was of interest to see
whether they worked well in the related nucleophilic addition of
allyltributyltin to imines. A one-pot, three-component reaction
involving the in situ formation of imines from aldehydes and
amines followed by allylation with allyltributyltin was employed
for ease of handling. Some recent results have indicated that
Brønsted acids26a or Lewis acids such as FeCl3·6H2O26b are very
effective catalysts for the three-component reaction in aqueous
media at room temperature. In contrast, to our knowledge, there
is no report on the use of pincer metal complexes for this
transformation. The catalytic activity of complexes 3 was first
investigated in the reactions of 4-nitrobenzaldehyde, which is an
activated substrate for nucleophilic addition, allyltributyltin and
various amines. Under the typical reaction conditions used in
the allylation of aldehydes (5 mol% of complex 3c in DMF at
50 ◦C for 24 h), 4-methoxyaniline was found to give the expected
homoallylic amine product in a 70.4% yield (Table 4, entry 1). p-
Toluidine gave an excellent yield (entry 2) and a yield of 82.1%
was obtained even when the reaction was conducted at room
temperature (entry 3). However, in the case of arylamines bearing
an electron-withdrawing group such as 4-chloroaniline, as well as
aliphatic amines such as isoproprylamine and cyclohexylamine,
the expected amine products were not produced. For the for-
mer, the homoallylic alcohol product from the allylation of 4-
nitrobenzaldehyde was isolated indicating the slower formation
of the related imine. Subsequently, the catalytic three-component
allylation of various aldehydes, p-toluidine and allyltributyltin was
examined. Aryl aldehydes bearing electron-withdrawing group
such as 4-F, 4-Cl and 4-Br as well as 2-picolinaldehyde gave rise
to the corresponding amine products in satisfactory yields (68.4–
81.3%, entries 4–8) and benzaldehyde as well as the electron-rich
aryl aldehydes such as 4- or 2-methoxybenzaldehyde afforded
lower yields (52.7–48.0%, entries 9–11), which is similar to the
allylation of aldehydes. It seemed that steric hindrance had a
negative effect on the reaction since 2-bromobenzaldehyde gave
a much lower yield compared to 4-bromobenzaldehyde (61.0% vs.
81.3%, entry 12 vs. entry 6). Finally, it was found that a good
yield was obtained in the case of benzenepropanal, which is an
aliphatic aldehyde (68.8%, entry 13). Interestingly, the result was
better than the allylation of benzenepropanal itself (40.3%, Table 3,
entry 27).
Catalytic transfer hydrogenation of ketones catalyzed by Ru–CNN
pincer complexes
The findings that the Baratta’s CNN pincer Ru(II) complexes are
extremely active catalysts for transfer hydrogenation (TH) with
2-propanol as the hydrogen source have induced the synthesis of
the new CNN Ru(II) complexes 5 and 7. Therefore, the catalytic
activity of the two complexes in TH of acetophenone was quickly
investigated and compared under the reaction conditions similar
to those of Baratta. As shown in Table 5, the Ru complex 7
with a Ru–NH2 functionality, which is an analogue of Baratta’s
pincer Ru(II) complex, demonstrated an exceptionally high activity
and its activity was much higher than 5 with a Ru-NHR func-
tionality (entries 1–4). For example, acetophenone was reduced
to 1-phenylethanol in refluxing 2-propanol in a 92% yield with
0.005 mol% of 7 in the presence of 2 mol% of NaOH as the base and
8968 | Dalton Trans., 2011, 40, 8964–8976
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Table 3 The allylation of aldehydes with allyltributyltin using the Pd–CNN pincer complexes 3 and Pd–NCN pincer complex 4 as catalystsa
Entry
Aldehyde
Cat. (mol%)
Temp./◦C
Product
Yield/%b
1
2
3
4
5
6
7
8
9
3a (5)
RT
RT
RT
RT
50
50
50
94.0
99.0
95.3
90.4
99.0c
52.4c
76.0c,d
8.3
3b (5)
3c (5)
3b (2)
3b (0.2)
3b (0.02)
3b (0.02)
4 (5)
RT
50
4 (5)
20.9
10
11
12
3c (2)
3c (2)
3c (5)
RT
50
50
56.0
68.7
76.1
13
14
15
16
3c (2)
3c (5)
3c (2)
3c (5)
RT
RT
50
44.0
59.5
77.0
85.2
50
17
3c (5)
50
73.2
18
3c (5)
50
79.2
19
20
3c (5)
3c (5)
50
50
80.0
90.3
21
22
3c (5)
3c (5)
50
50
46.0
68.7
23
3c (5)
50
61.2
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©

Table 3 (Contd.)
Entry
24
Aldehyde
Cat. (mol%)
Temp./◦C
50
Product
Yield/%b
57.0
3c (5)
25
26
3c (5)
3c (5)
50
50
41.1
40.7
27
3c (5)
50
40.3
a Reaction conditions: aldehyde (0.2 mmol), allyltributyltin (0.24 mmol), DMF (2 mL), 24 h. b Isolated yields. c Reactions were performed with 4-
nitrobenzaldehyde (2 mmol), allyltributyltin (2.4 mmol), DMF (1 mL), 24 h. d Reaction time was 48 h.
a final TOF of 220 800 h-1 was achieved in five minutes. In contrast,
complex 5 only gave a 33.0% yield after 12 h with a final TOF of
Conclusions
550 h-1 under the same conditions. The results provided additional
evidence for the key role of the Ru–NH2 functionality in the
fast transfer hydrogenation. According to Baratta’s investigation,
the effectiveness of the CNN Ru complexes containing an amino
NH2 group could be related to their ability to stabilize the related
intermediates through intramolecular NH ◊ ◊ ◊ O hydrogen bonds.12
Nonetheless, complex 5 was still an active catalyst for TH, with
final TOF values up to 4510 h-1 when both a higher catalyst loading
(0.01–0.05 mol%) and a larger amount of the base (4 mol%)
were used (entries 5–7). In fact, the activity of complex 5 was
much higher than that of the NNC Ru(II) complex bearing a
pyridyl-supported pyrazolyl-N-heterocyclic carbene ligand27a and
the NNN Ru(II) complex bearing a pyridyl-supported pyrazolyl-
amine ligand,27b which gave final TOF values of 245 h-1 and
465 h-1, respectively, in the TH of acetophenone with a catalyst
loading of 0.2 mol% under similar reaction conditions. The
higher activity of complex 5 compared to the above mentioned
NNC27a and NNN27b Ru(II) complexes was also proved by TH
of other ketones. Transfer hydrogenation of several representative
ketones was carried out by using 0.05 mol% of complex 5 as
the catalyst in refluxing 2-propanol or 0.1 mol% of 7 at room
temperature (Table 6). As shown in Table 6, aryl–alkyl, diaryl, a,b-
unsaturated and dialkyl (cyclic and linear) ketones were reduced
to the corresponding alcohols in high yields within 25 min–5 h
with final TOF values up to 4397 h-1 and 1836 h-1, respectively,
for the reactions at 82 ◦C and room temperature. The reduc-
tion of 4-methoxyacetophenone, benzophenone and 2-nonanone
was found to be much slower than other ketones investigated
(entries 1, 4, 7).
In conclusion, three CNN pincer Pd(II) complexes with N-
substituted -2-aminomethyl-6-phenylpyridines have been con-
veniently synthesized from commercially available 6-bromo-2-
picolinaldehyde in three steps. The obtained Pd complexes were
effective catalysts for the allylation of aldehydes as well as the three-
component allylationofaldehydes, arylamines andallyltributyltin,
which were catalyzed for the first time by the CNN pincer Pd
complexes. Also, the CNN Pd complexes were found to exhibit
a much higher activity than a related NCN Pd(II) pincer in the
allylation of aldehyde. In addition, two new CNN pincer Ru(II)
complexes containing Ru-NHR or Ru–NH2 functionality have
been easily prepared by the reaction of the related ligands with
the Ru(II) precursor. Both complexes displayed excellent catalytic
activity in the TH of ketones with the latter being much more
active.
Experimental
General
Reactions for the preparation of the pincer Pd and Ru complexes
as well as all the catalytic reactions were carried out under
nitrogen atmosphere. Toluene and triethylamine were distilled
over sodium/benzophenone and CaH2, respectively. Dimethylfor-
mamide was dried over CaCl2 or molecular sieves and distilled
under reduced pressure. [RuCl2(PPh3)3],28 [RuCl2(PPh3)(dppb)],29
phenylboronic acid30 and 6-phenyl-2-picolinaldehyde 131 were
prepared according to the literature methods. All other chemicals
were used as purchased. Melting points were measured on an
8970 | Dalton Trans., 2011, 40, 8964–8976
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Table 4 The synthesis of homoallylic amines by three-component allylation reaction using the Pd–CNN pincer complex 3c as the catalysta
Entry
1
Aldehyde
Amine
Product
Yield/%b
70.4
2
3
91.8
82.1c
4
68.4
5
6
72.1
81.3
7
8
80.9
72.5c
9
52.7
50.2
48.0
10
11
12
13
61.0
68.8
a Reaction Conditions: aldehyde (0.2 mmol), amine (0.2 mmol), allyltributyltin (0.24 mmol), Cat. 3c (5 mol %), DMF (2 mL), 50 ◦C, 24 h. b Isolated yield.
c Reaction was performed at room temperature.
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Table 5 TH of acetophenone catalyzed by Ru–CNN pincer complexes 5 and 7a
Entry
Cat. (mol %)
Base (mol %)
Temp. (◦C)
Time (h)
Yield (%)b
Final TOF (h-1)c
1
2
3
4
5
6
7
7 (0.005)
5 (0.005)
7 (0.1)
NaOH (2)
NaOH (2)
NaOH (2)
NaOH (4)
NaOH (4)
iPrOK (4)
NaOH (4)
82
82
RT
RT
82
82
82
1/12
12
0.5
6
2
3
92.0
33.0
91.2
trace
90.2
80.2
93.8
220800
550
1824
—
4510
2673
3752
5 (0.1)
5 (0.01)
5 (0.01)
5 (0.05)
0.5
a Reaction conditions: acetophenone (2.0 mmol, 0.1 M in 20 mL iPrOH). b Isolated yield. c TOF: turnover frequency (moles of alcohol obtained per mole
of catalyst per hour).
XT4A melting point apparatus and were uncorrected. IR spectra
were collected on a Bruker VECTOR22 spectrophotometer in
7.64 (d, J = 7.8 Hz, 1H, ArH), 7.53 (t, J = 7.0 Hz, 2H, ArH),
7.47 (t, J = 7.2 Hz, 1H, ArH), 7.28 (d, J = 7.6 Hz, 1H, ArH),
7.27–7.22 (m, 2H, ArH), 6.79–6.74 (m, 3H, ArH), 4.97 (br s, 1H,
NH), 4.55 (s, 2H, CH2). 13C NMR (100 MHz, CDCl3): d 158.1,
156.7, 147.9, 139.2, 137.3, 129.2, 128.9, 128.7, 126.9, 119.8, 118.8,
117.4, 113.0, 49.3. IR (KBr, cm-1): u 3409, 3051, 2920, 1598, 1568,
1507, 1448, 1317, 1263, 1155, 984, 746, 693, 509. MS (m/z, ESI+):
261.3 (M+H), 283.2 (M+Na).
1
KBr pellets. 1H, 13C and 31P{ H} NMR spectra were recorded on
a Bruker DPX-400 or Bruker DPX-300 spectrometer in CDCl3
1
with TMS as an internal standard for H, 13C NMR and 85%
1
H3PO4 as the external standard for 31P{ H} NMR. Mass spectra
were performed on the Agilent LC/MSD Trap XCT instrument.
Elemental analyses were measured on a Thermo Flash EA 1112
elemental analyzer.
N-(p-tolyl)-2-aminomethyl-6-phenylpyridine 2c. 224.6 mg,
82% yield, pale orange solid. mp: 72–74 ◦C. 1H NMR (400 MHz,
CDCl3): d 8.02 (dd, J = 1.3, 7.7 Hz, 2H, ArH), 7.67 (t, J = 7.8 Hz,
1H, ArH), 7.58 (d, J = 7.8 Hz, 1H, ArH), 7.49–7.47 (m, 2H, ArH),
7.46–7.41 (m, 1H, ArH), 7.23 (d, J = 7.6 Hz, 1H, ArH), 6.99 (d,
J = 8.1 Hz, 2H, ArH), 6.63 (d, J = 8.3 Hz, 2H, ArH), 4.75 (br
s, 1H, NH), 4.48 (s, 2H, CH2), 2.24 (s, 3H, CH3). 13C NMR (100
MHz, CDCl3): d 158.4, 156.6, 145.6, 139.2, 137.1, 129.6, 128.8,
128.6, 126.8, 126.5, 119.8, 118.6, 113.1, 49.6, 20.3. IR (KBr, cm-1):
u 3368, 3023, 2916, 2855, 1615, 1566, 1522, 1447, 1316, 1263, 990,
803, 762, 697, 512. MS (m/z, ESI+): 275.2 (M+H), 297.1 (M+Na),
313.0 (M+K).
Synthesis of ligands 2
For the preparation of ligand 2a or 2b, 6-phenyl-2-picolinaldehyde
(1 mmol, 183.07 mg) and isopropylamine (1.5 mmol, 88.6 mg) or
aniline (1.2 mmol, 111.7 mg) were stirred at room temperature in
THF (5 mL) for 2 h to form the corresponding imines. For 2c, 6-
phenyl-2-picolinaldehyde (1.0 mmol, 183.07 mg) and p-toluidine
(1.0 mmol, 107.07 mg) were ground under the infrared lamp for
30 min to afford the crude imine as a yellow solid. Then the
formed imines (for 2a and 2b) in THF or the crude imine (for
2c) were reduced to secondary amines by NaBH4 (1.5 mmol,
57.0 mg) in methanol (5 mL, for 2c 15 mL) when the reaction
mixture was gently warmed to 45 ◦C for 6 h. The solvent was
evaporated under reduced pressure, the residue was purified by
preparative TLC on silica gel plates eluting with acetone, CH2Cl2
and CH2Cl2/petroleum ether (2/1) to afford the ligands 2a, 2b
and 2c, respectively.
General procedure for the synthesis of CNN pincer palladium
complexes
To a stirred solution of 2a–c (0.20 mmol) and triethylamine
(93 mL, 0.66 mmol) in toluene (15 mL) was added PdCl2 (43 mg,
0.24 mmol) under N2 atmosphere, and the reaction mixture was
refluxed for 14 h. After cooling, filtration, and evaporation, the
residue was purified by preparative TLC on silica gel plates eluting
with CH2Cl2/acetone (20/1-10/1) to afford the corresponding
CNN pincer Pd complexes 3a–c.
N-isopropyl-2-aminomethyl-6-phenylpyridine 2a. 176.3 mg,
78% yield, orange oil. 1H NMR (400 MHz, CDCl3): d 8.02–7.99
(m, 2H, ArH), 7.70 (t, J = 7.8 Hz, 1H, ArH), 7.59 (d, J = 7.8
Hz, 1H, ArH), 7.49–7.45 (m, 2H, ArH), 7.43–7.39 (m, 1H, ArH),
7.24 (t, J = 7.6 Hz, 1H, ArH), 3.99 (s, 2H, CH2), 2.97–2.91 (m,
1H, CH(CH3)2), 2.89 (br s, 1H, NH), 1.17 (d, J = 6.2 Hz, 6H,
CH(CH3)2). 13C NMR (100 MHz, CDCl3): d 159.0, 156.7, 139.3,
137.1, 128.8, 128.6, 126.9, 120.7, 118.7, 52.5, 48.4, 22.7. IR (KBr,
cm-1): u 3424, 2963, 1574, 1447, 1173, 1075, 760, 695. MS (m/z,
ESI+): 227.1 (M+H).
[2-(isopropylamino)methyl]-6-phenylpyridin-1-ylpalladium(II)-
chloride (3a). 20.5 mg, 28% yield, orange solid. mp: 273–274 ◦C.
1H NMR (400 MHz, CDCl3): d 7.54–7.52 (m, 1H, ArH), 7.26 (t,
J = 7.9 Hz, 1H, ArH), 6.93–6.90 (m, 2H, ArH), 6.88–6.85 (m,
2H, ArH), 6.52 (d, J = 7.8 Hz, 1H, ArH), 5.40 (br s, 1H, NH),
4.54 (dd, J = 7.5, 17.2 Hz, 1H, CH2), 3.97 (dd, J = 7.1, 17.2 Hz,
1H, CH2), 3.55–3.51 (m, 1H, CH(CH3)2), 1.45 (d, J = 6.6 Hz, 3H,
CH3CHCH3), 1.31 (d, J = 6.6 Hz, 3H, CH3CHCH3). 13C NMR
(100 MHz, CDCl3): d 162.1, 161.3, 153.4, 147.4, 137.3, 134.7,
N-phenyl-2-aminomethyl-6-phenylpyridine 2b. 187.2 mg, 72%
yield, pale orange solid. mp: 44–46 ◦C. 1H NMR (400 MHz,
CDCl3): d 8.08–8.06 (m, 2H, ArH), 7.72 (t, J = 7.7 Hz, 1H, ArH),
8972 | Dalton Trans., 2011, 40, 8964–8976
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Table 6 Catalytic TH of ketones (i) with 5 at 82 ◦C and (ii) with 7 at room temperaturea
Entry
1
Ketone
Alcohol
Time (h)
Yield (%)b
Final TOF (h-1)c
7/3
(2.5)
92.3
(90.6)
791
(362)
2
3
5/12
(0.5)
91.6
(91.8)
4397
(1836)
0.5
(2/3)
91.3
(90.5)
3652
(1358)
4
5
6
7
3
(3)
88.6
(87.3)
591
(291)
1
91.4d
1828
0.5
(1)
86.4
(90.6)
3456
(906)
3
(5)
87.6
(81.2)
584
(162)
a Reaction conditions: ketone (2.0 mmol, 0.1 M in 20 mL iPrOH). (i) ketone/cat./NaOH = 2000/1/80, 82 ◦C, 5 (0.05 mol%); ii) ketone/cat./NaOH =
1000/1/20, room temperature, 7 (0.1 mol%). Results from the room temperature reduction of the ketones are given in parentheses. b Isolated yield. c TOF:
turnover frequency (moles of alcohol obtained per mole of catalyst per hour). d GC-MS yield of the corresponding alcohol.
129.0, 123.7, 122.7, 117.1, 115.6, 53.0, 51.3, 20.9. IR (KBr, cm-1):
u 3429, 3171, 3049, 2966, 1602, 1573, 1480, 1429, 1386, 1171, 1138,
938, 758, 728. Anal. calcd for C15H17ClN2Pd: C 49.07, H 4.67, N
7.63. Found: C 48.75, H 4.73, N 7.49%. MS (m/z, ESI+): 331.6
(M–Cl).
1H, CH2). 13C NMR (100 MHz, CDCl3): d 162.0, 161.0, 152.8,
147.5, 147.3, 137.8, 135.2, 129.2, 129.1, 124.3, 124.0, 123.0, 120.2,
117.1, 116.0, 60.2. IR (KBr, cm-1): u 3441, 3197, 3049, 1601, 1573,
1488, 1428, 1209, 1169, 757, 692. Anal. calcd for C18H15ClN2Pd:
C 53.89, H 3.77, N 6.98. Found: C 53.32, H 3.87, N 7.37%. MS
(m/z, ESI+): 364.6 (M–Cl).
[2-(phenylamino)methyl]-6-phenylpyridin-1-ylpalladium(II)chlo-
ride (3b). 14.4 mg, 18% yield, orange solid. mp: 224–227 ◦C. 1H
NMR (400 MHz, CDCl3): d 7.50 (d, J = 7.2 Hz, 1H, ArH), 7.34–
7.28 (m, 3H, ArH), 7.22 (d, J = 7.8 Hz, 2H, ArH), 7.08 (t, J = 7.3
Hz, 1H, ArH), 7.00 (d, J = 8.0 Hz, 1H, ArH), 6.94 (d, J = 7.4 Hz,
1H, ArH), 6.86–6.84 (m, 2H, ArH), 6.55 (d, J = 7.8 Hz, 1H, ArH),
5.34 (dd, J = 7.8, 17.7 Hz, 1H, CH2), 4.38 (dd, J = 4.0, 17.7 Hz,
[2-(p-tolylamino)methyl]-6-phenylpyridin-1-ylpalladium(II)chlo-
ride (3c). 20.7 mg, 25% yield, orange solid. mp: 248–251 ◦C. 1H
NMR (400 MHz, CDCl3): d 7.49 (d, J = 7.2 Hz, 1H, ArH), 7.32
(t, J = 7.9 Hz, 1H, ArH), 7.22 (br s, 1H, NH), 7.14–7.08 (m, 4H,
ArH), 6.99–6.93 (m, 2H, ArH), 6.85–6.81 (m, 2H, ArH), 6.53 (d,
J = 7.8 Hz, 1H, ArH), 5.31 (dd, J = 8.1, 17.5 Hz, 1H, CH2), 4.35
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Dalton Trans., 2011, 40, 8964–8976 | 8973
©

(dd, J = 4.3, 17.6 Hz, 1H, CH2), 2.29 (s, 3H, CH3). 13C NMR
(100 MHz, CDCl3): d 162.0, 160.9, 153.0, 147.5, 144.8, 137.8,
135.2, 133.8, 129.7, 129.2, 124.0, 123.0, 120.2, 117.1, 115.9, 60.6,
20.8. IR (KBr, cm-1): u 3432, 3185, 3136, 3049, 2917, 1603, 1574,
1512, 1480, 1429, 1406, 1209, 1169, 816, 759, 729. Anal. calcd for
C19H17ClN2Pd: C 54.96, H 4.13, N 6.75. Found: C 54.89, H 4.12,
N 6.53%. MS (m/z, ESI+): 379.4 (M–Cl).
7: 101.4 mg, 68% yield, orange solid. mp: 280–282 ◦C. 1H NMR
(400 MHz, CDCl3): d 8.12 (t, J = 8.2 Hz, 2H, ArH), 7.85–7.82
(m, 3H, ArH), 7.46–7.23 (m, 12H, ArH), 7.14 (t, J = 7.7 Hz, 1H,
ArH), 7.08 (d, J = 8.0 Hz, 1H, ArH), 6.95–6.93 (m, 2H, ArH),
6.82 (t, J = 7.2 Hz, 1H, ArH), 6.62–6.58 (m, 3H, ArH), 5.96 (t, J =
8.1 Hz, 2H, ArH), 5.29 (s, 2H, CH2Cl2), 4.11 (dd, J = 3.8, 15.1 Hz,
1H, CH2N), 3.70 (dt, J = 3.7, 15.5 Hz, 1H, CH2N), 3.52 (br s, 1H,
NH2), 3.08–2.98 (m, 2H, CH2), 2.37–2.32 (m, 1H, CH2), 2.17–2.10
(m, 1H, CH2), 1.96–1.70 (m, 4H, NH2 and CH2), 1.10–1.08 (m, 1H,
CH2). 13C NMR (100 MHz, CDCl3): d 181.1 (dd, J(C,P) = 16.8,
7.9 Hz, CRu), 163.5 (s, NCC), 155.7 (s, NCCH2), 149.5–116.0 (m,
ArC), 53.5 (s, CH2Cl2), 52.3 (s, CH2N), 32.9 (d, J(C,P) = 24.9 Hz,
CH2P), 30.1 (d, J(C,P) = 31.6 Hz, CH2P), 26.4 (s, CH2), 21.5 (s,
Synthesis of ligand 6
6-phenyl-2-picolinaldehyde (0.5 mmol, 91.54 mg) and hydroxy-
lamine hydrochloride (1 mmol, 69.0 mg) were stirred at room
temperature in methanol (20 mL) for 4 h. The solvent was
evaporated under reduced pressure, the residue was purified by
preparative TLC on silica gel plates eluting with CH2Cl2 to afford
6-phenyl-2-picolinaldoxime. To a stirred solution of the oxime
(0.5 mmol, 99.0 mg) in THF (20 mL) was added LiAlH4 (1.5 mmol,
57.0 mg) under N2 atmosphere, and the reaction mixture was
refluxed for 10 h. After cooling, filtration, and evaporation, the
residue was purified by preparative TLC on silica gel plates eluting
with CH3OH to afford 2-aminomethyl-6-phenylpyridine 6, which
is a known compound.
1
CH2). 31P{ H} NMR (162 MHz, CDCl3): d 56.9 (d, J(P,P) = 38.2
Hz), 42.4 (d, J (P,P) = 38.2 Hz). IR (KBr, cm-1): u 3434, 3298,
3245, 3048, 2917, 2845, 1599, 1573, 1476, 1432, 1400, 1384, 1091,
1001, 813, 737, 697, 512. Anal. calcd for C40H39ClN2P2Ru·CH2Cl2
(831.15): C 59.25, H 4.97, N 3.37. Found: C 59.25, H 5.15, N
3.23%.
Allylation of aldehydes by CNN pincer Pd complexes 3 and 4
A Schlenk flask was charged with aldehyde (0.2 mmol), pincer Pd
complex (0.01 mmol) and DMF (2 mL) under N2 atmosphere.
Allyltributyltin (74.7 mL, 0.24 mmol) was then added via syringe.
The resulting mixture was stirred at room temperature or 50 ◦C for
24 h. After cooling (for reactions at 50 ◦C), the reaction mixture
was evaporated and the residue was purified by preparative TLC
on silica gel plates to give the known homoallyl alcohol, which
6-phenyl-2-picolinaldoxime. 94.0 mg, 95% yield, white solid.
mp: 142–144 ◦C. 1H NMR (400 MHz, CDCl3): d 8.41 (br s, 1H,
OH), 8.37 (s, 1H, CH ), 8.00 (d, J = 7.3 Hz, 2H, ArH), 7.76–7.68
(m, 3H, ArH), 7.49–7.42 (m, 3H, ArH). 13C NMR (100 MHz,
CDCl3): d 157.5, 151.6, 151.4, 138.8, 137.2, 129.2, 128.8, 127.0,
120.9, 119.1. IR (KBr, cm-1): u 3427, 3179, 3085, 2999, 2876, 2773,
1568, 1494, 1447, 1389, 1328, 1290, 1257, 1163, 1077, 993, 821, 765,
700, 654, 622. MS (m/z, ESI+): 199.3 (M+H), 221.3 (M+Na).
1
were identified by H NMR or/and 13C NMR spectra and their
analytical data are given in the ESI.†
Synthesis of homoallylic amines catalyzed by CNN pincer Pd
complex 3c
General procedure for the synthesis of CNN pincer Ru complexes 5
and 7
Allyltributyltin (74.7 mL, 0.24 mmol) was added to a mixture of
pincer complex 3c (4.2 mg, 0.01 mmol), aldehyde (0.2 mmol),
primary amine (0.2 mmol) and DMF (2 mL) under N2 atmosphere.
After it was stirred at 50 ◦C for 24 h, the reaction mixture was
cooled and the solvent was removed under reduced pressure. The
residue was purified by TLC on silica gel plates to afford the
pure product, which was identified by 1H NMR or/and 13C NMR
spectra. The analytical data of the known products are given in
the ESI.†
To a stirred solution of 2c or 6 (0.24 mmol) and triethy-
lamine (279 mL, 1.98 mmol) in 2-propanol (20 mL) was added
[RuCl2(PPh3)(dppb)] (169.2 mg, 0.2 mmol) under N2 atmosphere,
and the reaction mixture was refluxed for 12 h. After cooling and
filtration the yellow precipitate was purified by silica gel column
chromatography eluting with CH2Cl2/acetone 10/1 to afford the
corresponding CNN pincer Ru complexes 5 or 7.
5: 118.5 mg, 71% yield, orange solid. mp: 266–268 ◦C. 1H NMR
(400 MHz, CDCl3): d 8.13 (br s, 2H, ArH), 7.53 (t, J = 7.3 Hz,
1H, ArH), 7.45–7.18 (m, 10H, ArH), 7.13–7.06 (m, 3H, ArH),
6.92–6.83 (m, 4H, ArH), 6.71–6.66 (m, 4H, ArH), 6.61 (d, J = 7.7
Hz, 3H, ArH), 6.32 (d, J = 8.1 Hz, 2H, ArH), 6.08 (br s, 2H, ArH),
4.36 (t, J = 12.9 Hz, 1H, CH2N), 3.88 (d, J = 13.6 Hz, 1H, CH2N),
3.00–2.90 (m, 1H, CH2), 2.80 (t, J = 13.4 Hz, 1H, CH2), 2.27 (s, 3H,
CH3), 2.13–2.03 (m, 2H, CH2), 1.90–1.62 (m, 3H, CH2), 1.14–1.10
(m, 1H, CH2). 13C NMR (100 MHz, CDCl3): d 175.7 (dd, J(C,P) =
16.5, 8.1 Hz, CRu), 163.9 (s, NCC), 154.0 (s, NCCH2), 148.8–116.4
(m, ArC), 58.9 (s, CH2N), 33.3 (d, J(C,P) = 25.0 Hz, CH2P), 30.4
(d, J(C,P) = 30.6 Hz, CH2P), 26.3 (s, CH2), 22.5 (s, CH2), 20.8
4-Methyl-N-[1-(4-nitrophenyl)but-3-enyl]aniline. 1H
NMR
(400 MHz, CDCl3): d 8.17 (d, J = 8.8 Hz, 2H), 7.55 (d, J = 8.7
Hz, 2H), 6.90 (d, J = 8.3 Hz, 2H), 6.40 (d, J = 8.2 Hz, 2H),
5.74–5.65 (m, 1H), 5.21–5.16 (m, 2H), 4.43 (dd, J = 5.4, 7.9 Hz,
1H), 2.67–2.61 (m, 1H), 2.55–2.50 (m, 1H), 2.19 (s, 3H). 13C
NMR (100 MHz, CDCl3): d 151.0, 147.1, 143.5, 133.3, 129.7,
127.3, 123.9, 123.6, 119.3, 114.1, 57.6, 42.5, 20.3.
4-Methyl-N-[1-(4-fluorophenyl)but-3-enyl]aniline. 1H NMR
(400 MHz, CDCl3): d 7.35–7.31 (m, 2H), 6.99 (t, J = 8.6 Hz, 2H),
6.91 (d, J = 8.1 Hz, 2H), 6.48 (d, J = 7.8 Hz, 2H), 5.74–5.67 (m,
1H), 5.17–5.11 (m, 2H), 4.33 (t, J = 6.7 Hz, 1H), 2.62–2.55 (m,
2H), 2.20 (s, 3H).
1
(s, CH3). 31P{ H} NMR (121 MHz, CDCl3): d 56.4 (d, J(P,P) =
37.7 Hz), 39.2 (d, J(P,P) = 37.7 Hz). IR (KBr, cm-1): u 3441, 3180,
3049, 2911, 1575, 1513, 1478, 1433, 1234, 1177, 1092, 998, 811, 746,
696, 511. Anal. calcd for C47H45ClN2P2Ru·0.25CH2Cl2 (857.58): C
66.18, H 5.35, N 3.27. Found: C 66.48, H 5.60, N 3.12%.
4-Methyl-N-[1-(4-bromophenyl)but-3-enyl]aniline. 1H NMR
(400 MHz, CDCl3): d 7.41 (d, J = 8.4 Hz, 2H), 7.23 (d,
8974 | Dalton Trans., 2011, 40, 8964–8976
This journal is
The Royal Society of Chemistry 2011
©

Table 7 Crystal data and structure refinement parameters for Pd complex 3c as well as Ru complexes 5 and 7·CH2Cl2
3c
5
7·CH2Cl2
Empirical formula
Mr
C19H17ClN2Pd
415.20
C47H45ClN2P2Ru
836.31
C41H41Cl3N2P2Ru
831.12
T/K
291(2)
0.71073
293(2)
0.71073
292(2)
0.7107
Triclinic
˚
l/A
Crystal system
cryst size/mm
Tetragonal
0.20 ¥ 0.20 ¥ 0.18
14.309(2)
14.309(2)
16.961(3)
90
Monoclinic
0.20 ¥ 0.20 ¥ 0.20
10.4744(2)
13.7238(3)
29.0038(7)
90
106.250(2)
90
4002.69(15)
4
P21/c
1.388
0.574
2.93–26.37
1728
0.20 ¥ 0.18 ¥ 0.16
9.1716(3)
11.2294(6)
19.1796(9)
104.722(4)
90.241(4)
95.615(4)
1900.46(15)
2
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
90
90
3
˚
V/A
3472.8(10)
8
P4(3)2(1)2
1.588
1.222
1.86–27.52
1664
13831
3977
0.0524
Z
¯
Space group
P1
Dc/g cm-3
1.452
m/mm-1
0.740
3.07–26.37
852
15657
7742
q range/◦
F(000)
no. of data collected
no. of unique data
R(int)
18008
8161
0.0264
0.0277
Final R indices (I > 2s(I))
R1 = 0.0482
wR2 = 0.1038
R1 = 0.0584
wR2 = 0.1095
0.328 and -0.569
R1 = 0.0341
wR2 = 0.0698
R1 = 0.0491
wR2 = 0.0753
0.425 and -0.300
R1 = 0.0404
wR2 = 0.0908
R1 = 0.0536
wR2 = 0.0976
0.667 and -0.634
R indices (all data)
-3
˚
Largest diff peak and hole/e A
J = 8.3 Hz, 2H), 6.89 (d, J = 8.1 Hz, 2H), 6.40 (d, J = 8.2 Hz,
2H), 5.76–5.65 (m, 1H), 5.18–5.12 (m, 2H), 4.28 (dd, J = 5.4, 7.7
Hz, 1H), 2.60–2.54 (m, 1H), 2.50–2.42 (m, 1H), 2.18 (s, 3 H).
13C NMR (100 MHz, CDCl3): d 144.2, 142.4, 134.1, 131.6, 129.6,
128.2, 127.3, 120.6, 118.7, 114.0, 57.2, 42.9, 20.4.
to afford the alcohol product. The room temperature reduction of
the ketones with Ru complex 7 (0.002 mmol) were carried out in
a similar way. The purified products were identified by 1H NMR
spectra and their analytical data are given in the ESI.†
4-Methyl-N-[1-(2-bromophenyl)but-3-enyl]aniline. 1H NMR
(400 MHz, CDCl3): 7.54 (d, J = 7.9 Hz, 1H), 7.47 (d, J = 7.5
Hz, 1H), 7.25–7.0 (m, 1H), 7.10–7.06 (m, 1H), 6.89 (d, J = 8.2 Hz,
2H), 6.39 (d, J = 8.0 Hz, 2H), 5.81–5.74 (m, 1H), 5.21–5.14 (m,
2H), 4.76 (dd, J = 4.5, 8.2 Hz, 1H), 2.74–2.68 (m, 1H), 2.45–2.37
(m, 1H), 2.17 (s, 3H). 13C NMR (100 MHz, CDCl3): d 143.7, 141.4,
134.2, 133.0, 129.6, 128.6, 127.9, 127.8, 126.9, 123.0, 118.6, 114.0,
56.6, 40.7, 20.4.
Crystal structure determination and data collection
Crystals of 3c, 5 and 7 were obtained by recrystallization from
CH2Cl2/n-hexane at ambient temperature. The data of 3c were
collected with Rigaku-IV imaging plate area detector and those
of 5 and 7 were collected on a Oxford diffraction Gemini E
diffractometer with graphite-monochromated Mo-Ka radiation
at ambient temperature. The diffraction data were corrected for
Lorentz and polarization factors. The structures were solved by
direct methods and expanded using Fourier techniques and refined
by full-matrix least-squares methods. The non-hydrogen atoms
were refined anisotropically, and the hydrogen atoms were included
but not refined. Their raw data were corrected and the structures
were solved using the SHELXS-97 program.32 Crystal data and
structure refinement parameters for Pd complex 3c as well as
Ru complexes 5 and 7 are summarized in Table 7. The dimeric
structures of the three complexes formed by hydrogen bonds are
given in ESI (see Fig. S1-3†). CCDC numbers 800253, 804175 and
804174 contain the crystallographic data for complexes 3c, 5 and
7, respectively.†
4-Methyl-N-[1-(2-methoxy)but-3-enyl]aniline. 1H
NMR
(400 MHz, CDCl3): d 7.32 (dd, J = 1.0, 7.3 Hz, 1H), 7.25–7.19 (m,
1H), 6.90 (d, J = 8.2 Hz, 4H), 6.45 (d, J = 8.3 Hz, 2H), 5.82–5.75
(m, 1H), 5.19–5.10 (m, 2H), 4.78 (dd, J = 5.1, 7.7 Hz, 1H), 3.90
(s, 3H), 2.71–2.65 (m, 1H), 2.52–2.47 (m, 1H), 2.19 (s, 3H). 13C
NMR (100 MHz, CDCl3): d 156.7, 144.8, 135.4, 130.8, 129.5,
127.8, 127.2, 126.5, 120.7, 117.6, 113.7, 110.4, 55.3, 51.9, 40.5,
20.4.
Catalytic transfer hydrogenation of ketones by CNN pincer Ru
complexes 5 and 7
Under nitrogen atmosphere, the ketone (2.0 mmol) was dissolved
in 2-propanol (18 mL), and the solution was stirred at 82 ◦C for 5
min. The Ru complex 5 (1 mL, 0.001 M in iPrOH) and 0.8 mL of
0.1 M NaOH solution in 2-propanol were then added to initiate
the reduction of the ketone. The reaction was monitored by GC
analysis. After the reaction was complete, the reaction mixture was
evaporated and the residue was purified by TLC on silica gel plates
Acknowledgements
The authors are grateful to the National Natural Science Founda-
tion of China (no. 20872133 and 21072177) for financial support
of this work.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8964–8976 | 8975
©

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8976 | Dalton Trans., 2011, 40, 8964–8976
This journal is
The Royal Society of Chemistry 2011
©