Regioselective cyclometallation of N-methyl-N-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)ethanamine with palladium(II) acetate and catalytic reduction of various functional groups

Article abstract of DOI:10.1016/j.ica.2018.09.018

The preparation of novel unsymmetrical, tridentate ligand, N-methyl-N-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl) ethanamine (1), has been described. Ligand 1 reacts with Pd(OAc)₂ to give the corresponding cyclometallated complex, which could not be isolated. The substitution reactions of the in-situ prepared cyclometallated complex with LiCl/LiBr afforded the corresponding palladacycles (3-((methyl(2-(pyridin-2-yl)ethyl)amino)methyl)naphthalen-2-yl)palladium(II) chloride (2) /bromide (3). Both the complexes having one γ′-C–H, N(alkyl) and N(aryl) atom, form square planar complexes with halogens (Cl for 2 and Br for 3) as the fourth ligand. Interestingly, ligand 1 has two ortho-activated (γ-C–H and γ′-C–H) hydrogen atoms of the naphthyl ring, however, only γ′-C–H atom is palladated. The prepared CNN pincer –Pd complex 3 has been investigated for its catalytic property in the reduction of various functional groups under mild reaction conditions. In addition, ligand 2-(2-bromophenyl)-1H-benzo[d]imidazole (4) and its palladium (II) complex bis-[2-(2-bromophenyl)-1H-benzo[d]imidazole)PdCl₂ (5) have also been prepared.

Full text of DOI:10.1016/j.ica.2018.09.018

Accepted Manuscript
Regioselective cyclometallation of N-methyl-N-(naphthalen-2-ylmethyl)-2-
(
pyridin-2-yl)ethanamine with palladium(II) acetate and catalytic reduction of
various functional groups
Puspendra Singh, Gulam Shabbani, Amravati S. Singh, Hari C. Bajaj,
Eingathodi Suresh
PII:
S0020-1693(18)30883-1
https://doi.org/10.1016/j.ica.2018.09.018
ICA 18476
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
4 May 2018
5 September 2018
6 September 2018
Please cite this article as: P. Singh, G. Shabbani, A.S. Singh, H.C. Bajaj, E. Suresh, Regioselective cyclometallation
of N-methyl-N-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)ethanamine with palladium(II) acetate and catalytic
reduction of various functional groups, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.
2
018.09.018
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Regioselective cyclometallation of N-methyl-N-(naphthalen-2-ylmethyl)-2-
pyridin-2-yl)ethanamine with palladium(II) acetate and catalytic reduction of
various functional groups
(
a,b
a
b
,
b
Puspendra Singh* , Gulam Shabbani , Amravati S. Singh Hari C. Bajaj and
c
Eingathodi Suresh
a. Department of Chemistry, Dr. Shakuntala Misra National Rehabilitation University
Lucknow, 226017, Uttar Pradesh India.
b. Inorganic Material and Catalysis Division, Central Salt and Marine Chemical Research
Institute, Bhavnagar 3640002, Gujarat, India.
c. Central Salt and Marine Chemical Research Institute, Bhavnagar 3640002, Gujarat, India.
*
Email: pushpendrasingh0612@gmail.com
Abstract
The preparation of novel unsymmetrical, tridentate ligand, N-methyl-N-(naphthalen-2-
ylmethyl)-2-(pyridin-2-yl) ethanamine (1), has been described. Ligand 1 reacts with Pd(OAc)
2
to give the corresponding cyclometallated complex, which could not be isolated. The substitution
reactions of the in-situ prepared cyclometallated complex with LiCl/LiBr afforded the
corresponding palladacycles (3-((methyl(2-(pyridin-2-yl)ethyl)amino)methyl)naphthalen-2-
yl)palladium(II) chloride (2) / bromide (3). Both the complexes having one ’-C–H, N(alkyl)
and N(aryl) atom, form square planar complexes with halogens (Cl for 2 and Br for 3) as the
fourth ligand. Interestingly, ligand 1 has two ortho-activated (-C–H and ’-C–H) hydrogen
1

atoms of the naphthyl ring, however, only ’-C–H atom is palladated. The prepared CNN pincer
Pd complex 3 has been investigated for its catalytic property in the reduction of various
–
functional groups under mild reaction conditions. In addition, ligand 2-(2-bromophenyl)-1H-
benzo[d]imidazole (4) and its palladium (II) complex bis-[2-(2-bromophenyl)-1H-
benzo[d]imidazole)PdCl (5) have also been prepared.
2
Keywords: Palladacycles; Regioselctive; Hydrogenation; Cyclometallation; Functional Groups
1
. Introduction
Increasing attention has been paid to selective functionalization/metallation of the ortho
1], meta [2] and para [3] C-H bonds of arenes containing multiple C-H bonds with very similar
[
intrinsic reactivity. Selectivity is governed by the presence of specific directing groups and the
modulation of their electronic [4] and steric [5] properties can be achieved by the following
aspects: (a) position of directing group on arenes with respect to desired C-H bond activation, (b)
length of the directing group and (c) nature of the donor atoms (N, P, S, O) in the directing
group. Proper combination of these factors determines the selective functionalization/metallation
through C-H bond activation [6] (Fig. 1). Julia and co-workers have reported that the palladation
of the 2-dimethylaminomethylnaphthalene takes place on 1- and 3-positions in 66:33 ratio,
whereas its 1-isomer undergoes metallation on only 2-position [7]. Recently van Koten and
coworkers have reported the conversion of 1-lithio to 3-lithio-derivative in 2-
dimethylaminomethylnaphthalene [8]. We envisaged that the introduction of one pyridyl group
in the side arm of 2-dimethylaminomethylnaphthalene should lead to regioselective palladation
through C–H activation of naphthyl ring of the pincer ligand due to additional pyridyl N atom
2

interactions and semi-rigidity of the directing group. With these ideas, we have developed a new
ligand, N-methyl-N-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)ethanamine, (1) for regioselective
palladation. Pincer ligand bearing metal complexes have been shown to act as homogeneous
catalysts for reduction of functional groups in several examples involving Ni, Fe [9], Co [10], Ru
[
11], Os [12] and Pd [13]. Herein, we report the synthesis and characterization of a new pincer
ligand 1 and its Pd complexes (3-((methyl(2-(pyridin-2-yl)ethyl)amino)methyl)naphthalen-2-
yl)palladium(II) chloride (2)/bromide(3)). Attempted palladation reaction of 2-(2-bromophenyl)-
1
H-benzo[d]imidazole (4) with Na
2
PdCl
4
in methanol, afforded its Pd(II) complex, bis-[2-(2-
(5). The application of complex 3, as catalyst for
bromophenyl)-1H-benzo[d]imidazole)PdCl
2
the hydrogenation of olefins, alkynes, keto, epoxide and nitro groups to respective products at
milder reaction condition has been investigated.
Fig.1. -position is the preferred position for regioselectivity of the functionalization/metallation
2
. Experimental section
2
.1. Materials and methods.
All manipulations of the complexes were carried out using standard Schlenk techniques
under a nitrogen atmosphere. All solvents were freshly distilled and used. 2-Methylnaphthalene,
lithium chloride, lithium bromide, N-methyl-2-(pyridin-2-yl)ethanamine, and palladiumacetate
(
Sigma Aldrich) were used as received. The starting materials, 2-(bromomethyl)naphthalene [14]
3

and 2-(2-bromophenyl)-1H-benzo[d]imidazole (4) [15], were prepared according to literature
procedures. Microanalysis of the products was carried out on Perkin–Elmer 2400CHNS
analyzer, ElementarVario micro, and Perkin–Elmer Optical Emission Spectrometer Optima 2000
1
13
DV. The H NMR and C NMR spectra were recorded on Bruker 200 MHz and 500 MHz
instruments at ambient temperature and the chemical shifts were measured in ppm relative to
1
13
1
TMS for H and C{ H} NMR. FTIR spectra were recorded on Perkin–Elmer Spectrum GX
spectrophotometer in KBr window. Mass spectra were recorded on a Bruker Micro-TOF QII
quadrupole time-of-flight (Q-TOF) mass spectrometry.
2
2
.2. Synthesis
.2.1. Synthesis of N-methyl-N-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)ethanamine (1).
To a solution of N-methyl-2-(pyridin-2-yl)ethanamine (2.80 ml, 20.0 mmol) and NEt
3
(10
ml) in freshly distilled dry toluene (50 ml), was added drop-wise a solution of 2-
bromomethyl)naphthalene (4.40 g, 20.0 mmol) in the same solvent (50 ml). After 1 h, the
resulting white suspension (HNEt -Br) was filtered and washed with dry toluene (25 mL). The
(
3
filtrate was concentrated by removing the solvent under vacuum. This resulted in a light yellow
semi-solid residue, which was purified by column chromatography to yield 1 as a colourless oil
+
1
(
4.12 g, 74%). ES-MS m/z (relative intensity, nature of peak) 277 (100, [MH] ). H NMR (500
MHz, CDCl , 25 ºC): 2.30 (s, 3H, CH ), 2.86 (t, 2H, CH ), 3.01 (t, 2H, CH ), 3.70 (s, 2H,
CH ), 7.04-7.14 (m, 2H), 7.38-7.45 (m, 3H), 7.50-7.58 (m, 1H), 7.66 (s, 1H), 7.72-7.80 (m 3H),
3
3
2
2
2
1
3
8
1
.46-8.49 (d, 1H). C NMR (500 MHz, CDCl , 25 ºC):36.0, 42.1, 57.3, 62.3, 121.1, 123.2,
3
25.5, 125.9, 127.3, 127.5, 127.6, 127.7, 128.0, 132.8, 133.3, 136.2, 149.2, 160.3.
4

2
.2.2. Synthesis of (3-((methyl(2-(pyridin-2-yl)ethyl)amino)methyl)naphthalen-2-yl)palladium(II)
chloride(2) / bromide(3)
To a solution of Pd(OCOCH
3
)
2
(0.225 g, 1 mmol) in CH OH (20 ml), was added a
3
solution of 1 (0.280 g, 1.02 mmol) in the same solvent (5 ml) under a nitrogen atmosphere. The
º
yellow mixture turned bright yellow upon slow heating to 80 C. After further stirring for 6 hr at
this temperature, the reaction mixture was allowed to cool to room temperature. The bright
yellow reaction mixture was treated with anhydrous lithium halide (0.5 g; halide = Cl, Br) to
afford bright yellow powdered solids of 2/3. The bright yellow precipitate was filtered off and
washed with 3x20 ml of methanol. The solid was taken up in CHC1 and the solution was
3
allowed to slowly evaporate at room temperature to afford needle shaped yellow crystal of 2/3
respectively.
Data for Compound 2: Yield 0.99 g (96%); yellow, air-stable powder. M.p.: 190 ºC (dec >180
+
+
ºC). ES-MS m/z (relative intensity, nature of peak) 799 (15, [2M-Cl] ; 422 (10, [M-Cl +K] );
+
3
81 (100, [M-Cl] ). Anal. Calc. for C19
H
19ClN
2
Pd: C, 54.69; H, 4.59; N, 6.71. Found: C, 54.18;
, 25 ºC): 2.76 (s, 3H, NCH ), 2.96-3.10 (m, 2H,
-pyridyl), 7.16-7.20 (d,
1
H, 4.54; N, 6.54%. H NMR (500 MHz, CDCl
3
3
CH
2
N), 3.61-3.67 (d, 1H, J 13.2, CH-naphthyl), 3.69-3.83 (m, 2H, CH
2
1
9
H, aryl), 7.33-7.39 (m, 2H, aryl), 7.44 (s, 1H, aryl), 7.62-7.84 (m, 4H, aryl), 8.16 (s, 1H, aryl),
.21-9.23 (d, 1H,aryl).
Data for Compound 3: Yield 0.85 g (90%); yellow, air-stable powder. M.p.: 195 ºC (dec >180
+
+
ºC). ES-MS m/z (relative intensity, nature of peak) 799 (15, [2M-Brl] ; 422 (100, [M-Br +K] );
+
3
81 (20, [M-Br] ). Anal. Calc. for C19
H19BrN
2
Pd: C, 49.43; H, 4.15; N, 6.07. Found: C, 49.50;
H, 4.20; N, 6.30%.
5

2
.2.3. Synthesis of 2-(2-bromophenyl)-1H-benzo[d]imidazole (4).
Ligand 4 was prepared by following the reported procedure with slight modification [15].
To a solution of benzene-1,2-diamine ( 1.08 g, 10 mmol) in methanol (25 ml), was slowly added
-bromobenzaldehyde (1.20 ml, 10 mmol) in the same solvent (10 ml). A yellow solid gradually
2
separated during the addition. After stirring for 5 h, the solid was filtered, washed with methanol
and dried. The resulting light yellow solid was recrystallized from methanol to give yellow
powdered solid of 4. Yield: 2.30 g, 84%. M.p.: 245-247 ºC. Anal. Calc. for C13
H, 3.32; N, 10.26. Found: C, 57.08; H, 3.38; N, 9.99%.
H
9
BrN : C, 57.17;
2
2
.2.4. Reaction of 4 with Na
2
PdCl
4
To a solution of 4 (0.273 g, 1 mmol) in methanol (10 ml), was slowly added Na
2
PdCl
4
(
0.15 g, 0.51 mmol) in the same solvent (10 ml). A bright yellow solid separated immediately,
which was allowed to reflux for 6 h and then allowed to come to room temperature. After
cooling, the solid was filtered, washed with methanol and dried. The resulting bright yellow solid
was recrystallized from dimethylsulfoxide to give yellow needdles shaped crystals of bis-[2-(2-
bromophenyl)-1H-benzo[d]imidazole)PdCl
2
5. Yield: 0.34 g, 94%. M.p.: 280 ºC (dec). Anal.
1
Calc. for C26 Cl
H
18Br
2
2
N
4
Pd: C, 43.16; H, 2.51; N, 7.74. Found: C, 43.50; H, 2.40; N, 7.80%. H
NMR (500 MHz, DMSO-d6, 25 ºC):  13.47 (s, 2H, NH), 8.35 (s, 2H), 7.86-7.90(m, 2H), 7.53-
7
6
1
.56 (m, 2H), 7.39-7.40 (d, 2H), 7.30-7.32 (d, 2H), 7.24-7.27 (m 2H), 7.17-7.18 (d, 2H), 6.94-
13
.97 (m, 2H). C NMR (500 MHz, DMSO-d6, 25 ºC):  112.3, 118.3, 122.8, 123.4, 124.2,
28.2, 130.6, 132.7, 133.1, 133.7, 133.8, 140.1, 150.4.
2
.3. X-ray Crystallographic Study
6

The details of crystal data and structure refinement for 2, 3 and 5 are given in Table 1.
Single crystals with suitable dimensions were chosen under an optical microscope and mounted
on a glass fiber for data collection. Intensity data for all crystals were collected using MoKα
(
λ=0.71073 Å) radiation on a Bruker SMART APEX diffractometer equipped with CCD area
detector at 293(2)K (for Molecules 2 & 5) and 150(2) K for molecule 3. The data integration
and reduction were processed with SAINT software [16]. An empirical absorption correction
was applied to the collected reflections with SADABS [17]. The structures were solved by direct
methods using SHELXTL and were refined on F2 by the full-matrix least-squares technique
using the program SHELXL-97 [18]. All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were placed geometrically and refined with
relative isotropic displacement parameters. The structures are also available in Cambridge
Crystallographic Database as CCDC Nos. 1518798–1518800 contains the supplementary
crystallographic
free of charge
http://www.ccdc.cam.ac.uk/data_request/cif.
data
for
this
manuscript.
These
data
can
be
obtained
from The
Cambridge Crystallographic Data
Centre via
7

Table 1. Crystallographic Data and Refinement Details for 2, 3, and 5
Compound
empirical formula
formula weight
crystal system
space group
a (Å)
2.CHCl
3
3
5.(CH
Cl
879.82
monoclinic
P2 /c
14.1919(18)
10.1656(12)
13.7541(17)
90.00
3
) SO
2
C
20
H
20Cl
4
N
2
Pd C19
H
19BrN
2
Pd C30
H
30 Br
2
2
N
4
O
2
PdS
2
536.58
triclinic
P-1
461.67
monoclinic
P2 /n
9.2170(11)
12.6644(15)
14.4997(17)
90.00
1
1
9.184(13)
b (Å)
C (Å)
11.053(16)
13.13(2)
67.11(2)
78.53(3)
76.39(2)
1185(3)
2

(deg)
(deg)
(deg)


93.069(2)
90.00
118.642(2)
90.00
3
v (A )
1690.1(3)
4
1741.5(4)
2
z
3
D(calcd) (Mg/m )
T (K)
1.504
293(2)
1.814
1.678
150(2)
293(2)
range of θ (deg)
abs coeff (mm−1)
obsd reflens [I > 2σ]
1.70 to 26.00
1.241
2.14 to 25.00 1.63 to 25.00
3.462
3.147
4584
2963
3057
final R
1
[I > 2σ(I)]
0.0466
0.1225
0.0559
0.1720
2963/0/209
1.051
0.0589
0.1596
3057/0/202
1.075
wR indices [I > 2σ]
2
data/restraints/parameters 4584/0/245
goodness of fit on F
2
1.091
2
.4 Catalytic hydrogenation reaction:
The catalytic hydrogenation reactions were carried out in a 50 mL high-Pressure Parr
reactor (Parr Instrument Company USA). In a typical hydrogenation reaction, a known amount
of substrate, solvent and the synthesized catalyst 3, were charged into the reactor. Typically, 2.1
-2
mmol substrate and 1.08 x10 moles of Pd catalyst were charged in 20 mL methanol and the
reactor was packed. Then the reactor was flushed thrice with nitrogen before pressuring with
hydrogen. The hydrogen gas was introduced into the reactor to the desired pressure after setting
the stirrer at slow stirring and getting exact pressure (20 bar). The reaction was initiated by
8

increasing the agitation speed to 800 rpm. The consumption of hydrogen gas was monitored
from the pressure drop in the reactor. The reaction was stopped when the gas consumption
ceased. After completion of the reaction, the reactor pressure was released and the liquid sample
was analyzed by gas chromatography (Agilent 7890 B and HP- 5 column), to determine the
identity of products and determining the percentage conversion of substrate by comparing the
1
peak intensities of substrate and products. These products were also confirmed by FT-IR, H
1
3
NMR and C NMR spectroscopy. The obtained results are summarized in the Table-2.
Table-2: Hydrogenation of various reducible functional groups by cat 3.
-
1
Entry
Reactant
Product
Time (min)
Conv. %
Sel.%
TOF,min
1
2
3
.
.
.
3
100
100
64.69
18
8
100
100
100
100
10.78
24.26
4
5
.
.
68
75
100
100
100
100
2
2
.85
.58
a
-2
Reaction conditions: Substrate: 2.1 mmoles; Methanol: 20 mL; Catalyst: 1.082 X 10 mmoles
of Pd in the reaction mixture, with respect to mole of reactant and 20 bar H
Temperature: 27 °C.
2
pressure,
3
. Results and discussion
3
.1. Synthesis
9

Recently we have investigated the metallation reactions of BrC
N(Me)CH CH NMe
the desired lignad 1 was prepared by reacting 2-(bromomethyl)naphthalene, 2-(BrCH
with 1 equiv of N-methyl-2-(pyridin-2-yl)ethanamine, HN(Me)CH CH
6
H
4
-
(
CH
2
2
2
2
)-2 with selenium, tellurium and mercury metal salts [19]. Similarly,
2
)C10
H
7
2
2
(2-pyridyl) in the
presence of a large excess of triethylamine 74% yield. The C,N,N pincer palladium complexes 2
and 3 were synthesized by the reaction of 1 with one equiv of palladium acetate in methanol
followed by treatment of the reaction mixture with anhydrous lithium halide (0.5 g; halide = Cl,
Br). The complexes were isolated as bright yellow powdered solids. Cyclometalation takes place
preferentially at the C(’) position of the naphthyl ring over the C() position that is favoured by
the pyridine-N and semi-regidity of the ligand (vide infra). In addition to this, we also prepared
2
-(2-bromophenyl)-1H-benzo[d]imidazole (4) by the treatment of o-benzene-1,2-diamine with 2-
bromobenzaldehyde in methanol [15]. The attempted palladation reaction of the 4 with Na PdCl
2
4
afforded yellow solid of Pd(II) complex bis-[2-(2-bromophenyl)-1H-benzo[d]imidazole)PdCl
2
(
5) in 94% yield [20] (Scheme 1). Compound 5 shows good solubility in DMSO while it has
very poor solubility in common organic solvents. Compounds 4 and 5 have been characterized
l
13
by H, C NMR spectroscopy and elemental analysis. Molecular structures of 2, 3 and 5 were
further confirmed by single crystal X-ray studies (vide infra).
10

Scheme 1: Synthesis of CNN pincer ligand bearing new palladacycles 2, 3 and bis(2-(2-
bromophenyl)-1H-benzo[d]imidazole)PdCl complex 5.
2
3
.2. Hydrogenation reactions of various functional groups using palladium (II) complex (3) as
catalyst
First, styrene (ST) was selected as a model substrate to study the hydrogenation reaction
of the synthesized catalyst. The hydrogenation of ST was completed (100% conversion) within
just 3 min at room temperature, and 20 bar H pressure gave 100% selectivity to ethylbenzene
2
-
1
with Turnover Frequency (TOF) 64.69 min as the sole product. The same reaction condition
was applied for phenyl acetylene hydrogenation; it also completely converted (100%) in 18 min
with 100% selectivity to ethylbenzene. Furthermore, when styrene epoxide was used as reactant,
-
1
it also 100% converted to 2-phenyl ethyl alcohol with TOF 24.26 min in 8 min at the optimized
condition. Further, nitrobenzene was hydrogenated to aniline. As expected, it took a littile longer
time to complete the reaction asit is well known hydrogenation of nitro group required extreme
reaction conditions. Acetophene was also succefully hydrogenated to 1-phenyl ethanol in 75 min.
Hydrogenation of acetophenone plausibly should yield R and S enantiomer. To prove this, we
11

have carried out a hydrogenation reaction under different conditions and after polarimetric
analysis, we did not find any optical rotation in the obtained product. These result suggests that
catalyst 3 only gave simple hydrogenation. We also observed that our catalyst gave more than
five times better results compared to reported palladium(II)catalysts in literature [21-23]. (Please
see Table S1 Supporting info on page S25) Finally, we have also carried out additional
hydrogenation reaction of acetophenone and nitrobenzene using 5% Pd/C as catalyst, rate and
conversion were similar to that of used in this manuscript.
3
3
.3. Spectroscopic Studies
.3.1 NMR and Mass Spectrometry Studies
l
13
1
Ligand 1, a colourless oil, has been characterized by H, C NMR and ES-MS. Its H
NMR spectrum shows singlets for the benzylic, -NMe protons and two doublet of doublet for the
(
CH
2
)
2
protons. The ES-MS spectrum of 1 shows prominent molecular ion peak (277 MH+). The
H NMR spectrum shows the benzylic proton as double of doublet with a coupling constant of
3.2 Hz and singlet for the –NMe and two set of multiplets for the (CH protons. The ES-MS
1
1
)
2 2
spectrum of these complexes did not show the molecular ion peaks. One intense peak seen at 381
+
for both complexes 2 and 3 was assigned to [M-Cl/Br] . In addition, the other prominent peaks
+
observed for 2 and 3 at 422 were assigned to [M-Cl/Br+K] .
3
.4. Molecular Structures of 2, 3 and 5
Crystal
compounds
data
and
and
structure
are
refinement
in Table
details
1.
for
2,
3,
5
given
ORTEP
views of their molecular structures are depicted in Fig. 2-4. Ligand 1 acted as tridentate ligand,
where nitrogen atoms on the side arms, carbon, and halogen atoms (Cl or Br) were bonded to the
metal centers. For complexes 2 and 3 ligand 1 coordinated to the Pd(II) center via pyridine-N,
12

tertiary amine-N and the pendant naphthyl–C(’) in a tridentate manner in meridional fashion.
They all have distorted-square-planar palladium geometry with halogen atoms (Cl or Br)
occupying the fourth coordination site. The six-membered and five-membered rings in 2 and 3
adopt twist-boat conformations. It should be noted that the Pd−C bond distances for the
tridentate palladium complex 3 (1.970(7) Å) were evidently shorter than those for the tridentate
palladium complex 2 (2.045(5) Å). Notably, the Pd−N bond distances are lengthened {2.131(5)
and 2.208(5) Å} for 2 {2.092(6) and 2.131(7) Å} for 3 relative to the palladium complex 5,
2
.017(5) Å. The Pd−Cl bond distances for 2 and 5 are 2.374(4) and ~2.299(1) Å, respectively.
The bond length, which is trans to the -bonded naphthyl, is significantly greater than that trans
to chlorine. This is probably due to the large trans influence of the -bonded carbon atom.
The crystal packings of 2 and 3 consist of centrosymmetric dimers where the two
palladium atoms are separated by 4.828 and 5.551Å respectively, indicating no bonding
8
8
interaction between the metal ions. In fact, intermolecular d -d short contacts are less common
for mononuclear square-planar palladium-complexes [24], which further prove the lower ability
of Pd∙∙∙Pd interaction in solid state with respect to the corresponding Pt(II) analogues [25].
13

Fig. 2. Molecular structure of 2 showing 50% probability displacement ellipsoids and the atom
numbering scheme. Selected bond distances (Å) and angles (°): Pd1–C18 2.045(5), Pd1–N1
2
8
.208(5), Pd1–N2 2.131(5), Pd1–Cl1 2.374(4), C18–Pd1–N1 172.81(15), C18–Pd1–N2
2.03(19), N1–Pd1–N2 92.80(18), C18–Pd1–Cl1 94.39(15), N2–Pd1–Cl1 172.20(11), N1–Pd1–
Cl1 90.87(14).
14

Fig. 3. Molecular structure of 3 showing 50% probability displacement ellipsoids and the atom
numbering scheme. Selected bond distances (Å) and angles (°): Pd1–C1 1.970(7), Pd1–N1
2
.092(6), Pd1–N2 2.131(7), Pd1–Br1 2.394(1), C1–Pd1–N1 82.0(3), C1–Pd1–N2 173.9(3), N1–
Pd1–N2 92.3(2), C1–Pd1–Br1 93.5(2), N1–Pd1–Br1 173.61(16), N2–Pd1–Br1 92.38(17).
15

Fig. 4. Molecular structure of 5 showing 50% probability displacement ellipsoids and the atom
numbering scheme. Selected bond distances (Å) and angles (°): Pd1–N1 2.017(5), Pd1–Cl1
i
2
.299(1) & 2.300(1), N1–Pd1–N1 179.99(2), N1–Pd1–Cl1 89.94(15) & 90.06(15), Cl1–Pd1–
i
Cl1 180.0.
4
. Conclusions
It is noteworthy that the replacement of one methyl group in 2-
dimethylaminomethylnaphthalene with –CH -CH -(2-pyridine), leads to palladation of only ’-
2
2
C-H atom instead of both -C-H & ’-C-H atoms in the naphthyl ring. Also, the synthesized N-
containing Pd complex 3 worked as an excellent homogeneous catalyst for hydrogenation of
various reducible groups like olefin, acetylene, epoxides, nitro and keto groups at room
16

temperature with 100 % conversion and excellent turnover frequency. These results suggest that
the synthesized complex is versatile to hydrogenation of various groups efficiently.
Acknowledgment
PS is grateful to the Science and Engineering Research Board (SERB), New Delhi, India
for Fast Track Young Scientist Fellowship (No. SB/FT/CS-036/2012). AS is thankful to UGC
New Delhi for JRF fellowship. PS is heartily thankful to Prof. Harkesh B. Singh IIT Bombay for
valuable suggestion to improve the quality of article.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/...............
References:
[
1]
a) X. Chen, K.M. Engle, D.-H. Wang, J.-Q. Yu, Angew. Chem. Int. Ed. 48 (2009) 5094–
115; b) O. Daugulis, H.-Q. Do, D. Shabashov, Acc. Chem. Res. 42 (2009) 1074–1086;
5
c) D. A. Colby, R.G. Bergman, J.A. Ellman, Chem. Rev. 110 (2010) 624–655; d) T.W.
Lyons, M.S. Sanford, Chem. Rev. 110 (2010) 1147–1169; e) S.H. Cho, J.Y. Kim, J.
Kwak, S. Chang, Chem. Soc. Rev. 40 (2011) 5068–5083; f) C.S. Yeung, V.M. Dong,
Chem. Rev. 111 (2011) 1215–1292; g) P.B. Arockiam, C. Bruneau, P.H. Dixneuf, Chem.
Rev. 112 (2012) 5879–5918; h) K.M. Engle, T.-S. Mei, M. Wasa, J.-Q. Yu, Acc. Chem.
Res. 45 (2012) 788–802; i) C. Wang, Y. Huang, Synlett. 24 (2013) 145–149; j) S. De
Sarkar, W. Liu, S.I. Kozhushkov, L. Ackermann, Adv. Synth. Catal. 356 (2014) 1461–
17

1
479; k) F. Zhang, D.R. Spring, Chem. Soc. Rev. 43 (2014) 6906–6919; l) M. Zhang, Y.
Zhang, X. Jie, H. Zhao, G. Li, W. Su, Org. Chem. Front. 1 (2014) 843–895; m) H. Sun,
Y. Huang, Synlett 26 (2015) 2751–2762; n) J.J. Topczewski, M.S. Sanford, Chem. Sci. 6
(
2015) 70–76.
[
2]
D. Leow, G. Li, T.-S. Mei, J.-Q. Yu, Nature 486 (2012) 518–522; b) S. Lee, H. Lee, K.L.
Tan, J. Am. Chem. Soc. 135 (2013) 18778–18781; c) L. Wan, N. Dastbaravardeh, G. Li,
J.-Q. Yu, J. Am. Chem. Soc. 135 (2013) 18056–18059; d) M. Bera, A. Modak, T. Patra,
A. Maji, D. Maiti, Org. Lett. 16 (2014) 5760–5763; e) R.-Y. Tang, G. Li, J.-Q. Yu,
Nature 507 (2014) 215–220; f) G. Yang, P. Lindovska, D. Zhu, J. Kim, P. Wang, R.-Y.
Tang, M. Movassaghi, J.-Q. Yu, J. Am. Chem. Soc. 136 (2014) 10807–10813; g) M.
Bera, A. Maji, S. K. Sahoo, D. Maiti, Angew. Chem. Int. Ed. 54 (2015) 8515–8519; h) L.
Chu, M. Shang, K. Tanaka, Q. Chen, N. Pissarnitski, E. Streckfuss, J.- Q. Yu, ACS Cent.
Sci. 1 (2015) 394–399; i) Y. Deng, J.-Q. Yu, Angew. Chem. Int. Ed. 54 (2015) 888–891;
j) S. Li, H. Ji, L. Cai, G. Li, Chem. Sci. 6 (2015) 5595–5600.
[
[
[
3]
4]
5]
a) S. Bag, T. Patra, A. Modak, A. Deb, S. Maity, U. Dutta, A. Dey, R. Kancherla, A.
Maji, A. Hazra, M. Bera, D. Maiti, J. Am. Chem. Soc. 137 (2015) 11888–11891; b) T.
Patra, S. Bag, R. Kancherla, A. Mondal, A. Dey, S. Pimparkar, S. Agasti, A. Modak, and
D. Maiti, Angew. Chem. Int. Ed. 55 (2016) 7751–7755.
a) C.-L. Ciana, R. J. Phipps, J. R. Brandt, F.-M. Meyer, M. J. Gaunt, Angew. Chem. Int.
Ed. 50 (2011) 458–462; b) T. Imahori, T. Tokuda, T. Taguchi, H. Takahata, Org. Lett. 14
(
2012) 1172–1175; c) W. Liu, L. Ackermann, Org. Lett. 15 (2013) 3484–3486; d) Z. Wu,
F. Luo, S. Chen, Z. Li, H. Xiang, X. Zhou, Chem. Commun. 49 (2013) 7653–7655.
S. Oi, S.-i. Watanabe, S. Fukita, Y. Inoue, Tetrahedron Lett. 44 (2003) 8665–8668.
18

[
[
[
6]
7]
8]
J. Ye, M. Lautens, Nature Chem. 7 (2015) 863–870.
M. Julia, M. Duteil, J. Y. Lallemand, J. Organomet. Chem. 102 (1975) 239–243.
J.T.B.H. Jastrzebski, A.M. Arink, H. Kleijn, T.W. Braam, M. Lutz, A.L. Spek, G. van
Koten, J. Am. Chem. Soc. 135 (2013) 13371–13378.
[
[
[
9]
S. Chakraborty, P. Bhattacharya, H. Dai, H. Guan, Acc. Chem. Res. 48 (2015) 1995–
2
003.
10] K. Junge, B. Wendt, A. Cingolani, A. Spannenberg, Z. Wei, H. Jiao, M. Beller, Chem.
Eur. J. 24 (2018) 1046–1052.
11] a) W. Baratta, G. Chelucci, S. Magnolia, K. Siega, P. Rigo, Chem. Eur. J. 15 (2009)
7
26–732; b) S. Facchetti, V. Jurcik, S. Baldino, S. Giboulot, H.G. Nedden, A. Zanotti-
Gerosa, A. Blackaby, R. Bryan, A. Boogaard, D.B. McLaren,
E. Moya, S. Reynolds, K.S. Sandham, P. Martinuzzi, W. Baratta, Organometallics 35
2016) 277–287.
12] W. Baratta, M. Ballico, G. Chelucci, K. Siega, P. Rigo, Angew. Chem. 120 (2008) 4434
4437.
(
[
[
–
13] a) S.A. Burgess, A. Kassie, S.A. Baranowski, K.J. Fritzsching, K. Schmidt-Rohr, C.M.
Brown, C.R. Wade, J. Am. Chem. Soc. 138 (2016) 1780–1783; b) B. Ding, Z. Zhang, Y.
Liu, M. Sugiya, T. Imamoto, W. Zhang, Org. Lett. 15 (2013) 3690–3693.
[
[
[
14] B.S. Furniss, A.J. Hannaford, P.W.G. Smith, A.R. Tatchell, Vogel’s Text Book of
Practical Organic Chemistry, 5th ed.; Addison Wesley: Reading, MA, 1989; p 1003.
15] B. Das, B.S. Kanth, K.R. Reddy, A.S. Kumar, J. Heterocyclic Chem., 45 (2008) 1499–
1
502.
16] G.M. Sheldrick, SAINT 5.1 ed., Siemens Industrial Automation Inc., Madison, WI, 1995.
19

[
[
[
17] SADABS, Empirical Absorption Correction Program, University of Göttingen,
Göttingen, Germany, 1997.
18] a) G. M. Sheldrick, Acta Crystallogr. A64 (2008) 112–122.; b) G. M. Sheldrick, Acta
Crystallogr. C71 (2015) 3–8.
19] P. Singh, A.K. Gupta, S. Sharma, H.B. Singh, R.J. Butcher, Inorg. Chim. Acta, 483
(
2018) 218-228.
[
[
[
[
[
[
20] K.R. Reddy, G.G. Krishna, Tetrahedron Lett. 46 (2005) 661–663.
21] S.O. Ojwach, A.O. Ogweno, M.P. Akerman, Catal. Sci. Technol., 6 (2016) 5069–5078.
22] T.A. Tshabalala, S.O. Ojwach, Inorg. Chim. Acta 483 (2018) 148–155.
23] W.R Cullen, N.F. Han, Appl. Organomet. Chem. I (1987) 1–6.
24] G. Aullon, S. Alvarez, Chem. Eur. J. 3 (1997) 655–664.
25] V.H. Houlding, V.M. Miskowski, Coord. Chem. Rev. 111 (1991) 145–152.
20

Highlights:

Preparation of novel unsymmetrical, tridentate ligand, N-methyl-N-(naphthalen-2-
ylmethyl)-2-(pyridin-2-yl) ethanamine (1) is described.


Regioselective C–H activation of the naphthyl ring in presence of ortho directing group.
Finally we also explored the application of synthesized palladium complex from ligand 1,
as excellent catalyst for the hydrogenation of various functional groups.

In addition we also prepared the 2-(2-bromophenyl)-1H-benzo[d]imidazole and its
palladium (II) complex bis-[2-(2-bromophenyl)-1H-benzo[d]imidazole)PdCl .
2
Graphics:
21