Palladium complexes bearing the dipyridyl ligand: Synthesis, structural studies, and use in the Heck reaction

Article abstract of DOI:10.1007/s11164-015-2256-2

Two new palladium complexes derived from the di(2-pyridinyl)methanone N-(2-pyridinyl)hydrazone (DPMNPH) ligand are reported. The compounds were characterized by elemental analysis, spectroscopic studies, and, for the DPMNPH ligand, single-crystal X-ray di

Full text of DOI:10.1007/s11164-015-2256-2

Res Chem Intermed
DOI 10.1007/s11164-015-2256-2
Palladium complexes bearing the dipyridyl ligand:
synthesis, structural studies, and use in the Heck
reaction
Ismail Warad1 Mohammad Azam2
•
•
Saud I. Al-Resayes2 Mohammad Shahidul Islam2
Sarfaraz Ahmed3 Salim F. Haddad4
•
•
•
Received: 17 March 2015 / Accepted: 29 August 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Two new palladium complexes derived from the di(2-pyr-
idinyl)methanone N-(2-pyridinyl)hydrazone (DPMNPH) ligand are reported. The
compounds were characterized by elemental analysis, spectroscopic studies, and, for
the DPMNPH ligand, single-crystal X-ray diffraction analysis. The DPMNPH
ligand crystallized as orthorhombic with the space group P212121. The H1 atom is
˚
intramolecularly bonded to the pyridinic N4 with N1–H1 = 0.92(3) A, H1ÁÁÁN4 =
˚
˚
1.87(2) A, N1ÁÁÁN4 = 2.615(2) A, and N1–H1ÁÁÁN4 = 137(2)°. Both complexes
were excellent catalysts in the Heck reaction in the presence of base.
Keywords Di-2-pyridyl ketone (DPK) Á Crystal structure of the DPMNPH ligand Á
Palladium complexes Á Heck reaction
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2256-2)
contains supplementary material, which is available to authorized users.
&
Mohammad Azam
azam_res@yahoo.com
1
2
3
4
Department of Chemistry, Science College, AN-Najah National University, P.O. Box 7, Nablus,
State of Palestine
Department of Chemistry, College of Science, King Saud University,
P.O. Box 2455, Riyadh 11451, KSA
Department of Pharmacognosy, College of Pharmacy, King Saud University,
P.O. Box 2457, Riyadh 11451, KSA
Department of Chemistry, Hashemite University, Zarqa 13115, Jordan
123

I. Warad et al.
Introduction
Identification of new metal complexes with interesting properties can be approached
from several perspectives; one is study of the chemistry of a specific ligand in
combination with different metal ions. Di-2-pyridyl ketone (DPK) has attracted
much attention in chemistry since the 1960s when the first work on this ligand was
reported [1, 2]. DPK, a versatile ligand of a variety of transition metal ions, forms
five and six-membered chelate rings in bidentate coordination; it can thus form
mononuclear, binuclear, or polynuclear complexes [3–7]. The most characteristic
feature of DPK is that it can occur as a hydrated gem-diol or as an alcoholated
hemiketal, via metal-promoted hydration and alcoholate reaction, respectively, at
the carbonyl carbon [8]. In recent years, palladium complexes associated with
electron-rich phosphine ligands have attracted much attention because of their
remarkable properties as homogenous catalysts in variety of coupling reactions in
organic synthesis [9, 10]. However, because phosphines have such disadvantages as
toxicity, propensity to oxidize in air, high cost, and problems with synthesis, among
others [11], catalysis under phosphine-free conditions is an extremely important
challenge. In this context, nitrogen-based ligands are advantageous, because they
are usually stable in air, inexpensive, and easier to handle than their phosphine
counterparts. Recently, Schiff bases and their palladium complexes have been
shown to be catalytically active in Mizoroki–Heck and Suzuki–Miyaura cross-
coupling reactions, and could become alternatives to phosphine catalysts [12]. The
palladium-catalyzed Heck reaction is a versatile tool for formation of C–C bonds in
the synthesis of olefins and for preparation of intermediates in the synthesis of
natural products and bioactive compounds [13–15]. Herein, we report two novel
palladium complexes obtained from the di(2-pyridinyl)methanone N-(2-
pyridinyl)hydrazone (DPMNPH) ligand [16], and their use as catalysts in the Heck
reaction. We report, for the first time, catalytic applications of Pd(II) complexes
derived from the DPMNPH ligand; both complexes had good to excellent catalytic
activity in KOH. We also report the crystal structure of the DPMNPH ligand.
Experimental
Materials and methods
All reagents and solvents were of AR-grade quality from E. Merck and used as
received. All manipulations of complex preparation reactions were conducted under
argon by use of Schlenk-line techniques. Elemental analysis was performed with an
Elementar Vario EL analyzer. FTIR (4000–400 cm-1) spectra were acquired as KBr
1
pellets by use of a Perkin–Elmer 621 spectrophotometer. H NMR spectra of the
DPMNPH ligand and its Pd(II) complexes were recorded in d6-DMSO by use of a
Jeol 400 spectrometer. The thermal behavior of the synthesized compounds was
studied by use of an SDTQ-600 (TA Instrument) in a helium atmosphere
(100 ml min-1) at a heating rate of 10 °/min from 0–800 °C. Electronic spectra
123

Palladium complexes bearing the dipyridyl ligand: synthesis…
of the complexes were obtained in dichloromethane by use of a Pharmacia LKB-
Biochem UV–visible spectrophotometer at room temperature.
Crystallography data collection and refinement for the DPMNPH ligand
A suitable wedge like section with approximate dimensions 0.45 9 0.38 9 0.05
mm3 was mounted in epoxy resin on a glass fiber, and data were collected at room
temperature by use of an Xcalibur/Oxford Diffractometer with enhanced Mo
˚
radiation, k = 0.71073 A and equipped with and Eos charge-coupled detector [17].
Analytical absorption corrections were applied with minimum and maximum
transmission factors of 0.971 and 0.994, respectively. Cell constants were
determined by use of all observed reflections. Data were reduced by use of Olex2
[18]. The structure of the DPMNPH ligand was solved by use of SHELXTL
software [19], by direct methods, and refined by least squares on F2 to R1 = 0.0391
[I C 2r(I)] with h = 3.12–25.01° and diffraction measurement fraction
hmax = 0.998. Goodness-of-fit on F2 = 1.042. All non-hydrogen atoms were
refined anisotropically, with all hydrogen atoms except H1 constrained and assigned
isotropic thermal values 1.2 times that of the riding atoms. H1, involved in intra
molecular hydrogen bonding, was located and refined fully isotropically. The largest
diffraction peak and hole were 0.115 and -0.172 e A-3, respectively.
˚
Synthesis of di(2-pyridinyl)methanone N-(2-pyridinyl)hydrazine, DPMNPH
An ethanol solution of DPK (0.184 mg, 1 mmol) was added dropwise to an ethanol
solution of 2-hydrazinopyridine (0.109 mg, 1 mmol). The reaction mixture was
heated under reflux for 3 h then left to cool at room temperature. After cooling,
distilled water was added at a volume ratio up to 1:3 (Vorg:Vaq), followed by addition
of few drops of sodium hydroxide solution to neutralize the reaction mixture. The
microcrystalline solid product which separated out was washed with ethanol and
dried under vacuum over fused CaCl2. The isolated solid was dissolved in THF–
methanol (2:2), and the solution was left to evaporate at room temperature. After a
few days, yellow crystals suitable for single crystal X-ray diffraction were obtained.
Yield 88 %, M.P. 107 °C, color, yellow; anal. calc. C, 69.80; H, 4.76; N,
1
25.43 %, found C, 69.56; H, 4.47; N, 25.25 %; H NMR (CDCl3, 400 MHz) d
(ppm) 6.5–9.5 (m, 12H, Py, s), 13.9 (s, 1H, HN); IR (KBr, cm-1) 1640 m(C=N).
Synthesis of Pd(II) complexes 1 and 2
[Pd(PhCN)2Cl2] or [Pd(OAc)2] (1.0 mmol) dissolved in 20 ml dichloromethane was
added gradually to a dichloromethane solution of DPMNPH ligand (1 mmol). The
resulting reaction mixture was stirred for 30 min to give yellow colored solution
which was concentrated to 1 ml followed by addition of 10 ml hexane. The colored
product which precipitated was isolated and recrystallized from dichloromethane–
hexane to give an analytically pure product. Despite our best efforts we could not
obtain a crystal suitable for single-crystal X-ray diffraction.
123

I. Warad et al.
Complex 1: yield 68 %, M.P. 180 °C; molecular formula C16H13N5PdCl, anal.
calc. C 49.0, H 3.14, N 16.78 %; found C 48.68, H 2.99, N 16.52 %; H NMR
1
(d6-DMSO, 400 MHz) d (ppm) 7.2–9.0 ppm (m, 12H, Py), 12 (s, 1H, HN); IR (KBr,
cm-1) 1620 m(C=N).
Complex 2: yield 74 %, M.P. 191 °C; molecular formula C18H16N5O2Pd, anal.
calc. C 49.04, H 3.65, N 15.88 %; found C 48.89, H 3.42, N 15.65 % ;1H NMR
(d6-DMSO, 400 MHz) d (ppm) 7.3–8.9 ppm (m, 12H, Py), 13.9 (s, 1H, HN); IR
(KBr, cm-1) 1630 m(C=N).
General procedure for the Heck reaction
Palladium complex (1 or 2) (1.0 mmol), aryl halide (1.0 mmol), methyl acrylate
(1.2 mol), base (4 mmol), and DMF (10 mL) were placed in a 100-ml Schlenk tube,
and the reaction mixture was subjected to a freeze–thaw cycle before being heated
at 80 °C for 10 h. Completion of the reaction was monitored in situ by use of gas
chromatography after a specific time. After complete conversion, the reaction
mixture was cooled and extracted with ethyl acetate–hexane. The extract was
filtered through a pad of silica gel with copious washing, concentrated, and purified
by flash chromatography on silica gel. Product purity was checked by NMR
spectroscopy.
Results and discussion
Free DPMNPH ligand, obtained by reaction of di-2-pyridyl ketone and 2-hy-
drazinopyridine in 1:1 molar ratio in ethanol, crystallized as chiral orthorhombic
with space group P212121. The ligand acts as a polydentate chelating ligand when
reacted with PdX2 (X = Cl, OAc) in 1:1 molar ratio, and forms mononuclear
complexes (Scheme 1). The complexes were characterized by elemental analysis
and by spectroscopic studies. The pure complexes were also screened for use in the
Heck reaction.
Scheme 1 Schematic representation of synthesis of the Pd(I) complexes
123

Palladium complexes bearing the dipyridyl ligand: synthesis…
The IR spectrum of DPMNPH contained a band at 1640 cm-1, ascribed to the
m(C=N) vibration, which occurred at lower wavenumbers, 1620 and 1630 cm-1, for
complexes 1 and 2, respectively, confirming coordination of the palladium ion with
the iminic nitrogen (Fig. 1). The corresponding vibrations for m(Pd–N) were found
at 582 and 580 cm-1 for complexes 1 and 2, respectively. All other bands were
found at their expected positions [20–22]. The 1H NMR spectra of the free
DPMNPH ligand and its palladium complexes were recorded in d6-DMSO at room
temperature; the spectral data are given in the ‘‘Experimental’’ section. The
1H NMR spectrum of free ligand contains a singlet at 13.9 ppm, ascribed to the –N–H
proton. Pyridine ring proton peaks appeared as multiplets at 6.5–9.0 for the free
ligand (Supplementary Fig. 1). However, there was little shift of the pyridyl ring
protons of the palladium complexes, which appeared at 7.2–9.0 ppm. The N–H
protons of the palladium complexes were shifted upfield and appeared at ca 12 ppm,
indicating the N–H protons remained uncoordinated. An additional resonance signal
ascribed to the acetate proton of complex 2 was observed at 2.85 ppm. The
electronic spectrum of the free ligand, recorded in dichloromethane, contained an
intense band at 290 nm, ascribed to p–p* transitions of the aromatic ring; another
band at 355 nm was ascribed to n–p* transitions of the C=N bond [23, 24]. Both
palladium complexes were diamagnetic with square planar geometry, for which
three spin-allowed d–d transitions are expected, corresponding to transitions from
the three lower-lying d-orbitals [24, 25]. The thermal decomposition behavior of the
complexes was studied by TG/DTA at a heating rate of 10 ° min-1 in a nitrogen
atmosphere over the temperature range 0–800 °C. The TG/DTA profiles of both
complexes revealed two distinct steps of weight loss with similar thermal
decomposition patterns (Supplementary Fig. 2). For complex 2 the first decompo-
sition step occurred at 180 °C and was accompanied by weight loss of 18 %, which
Fig. 1 FTIR spectra; a free ligand, b complex 2, c complex 1
123

I. Warad et al.
corresponds to loss of acetate ion and moisture; DTA revealed an endothermic
process at 160 °C. The second stage of decomposition starts at 181 °C and ends at
545 °C, with one broad DTA peak corresponding to an exothermic process at
331 °C, with 43 % weight loss, ascribed to loss of the pyridyl units. There was no
further weight loss above 545 °C indicating that the final product, Pd–O,
corresponding to a calculated mass of 28 %, has been formed (Scheme 2).
Details of data collection and refinement for the ligand are given in Tables 1, 2, 3
and 4. An Ortep drawing of the asymmetric unit (30 % probability), showing the
naming scheme, is given in Fig. 2. A packing diagram with projection down the
a-axis, and revealing four rather discrete molecular units, C16 H13 N5 per unit cell
without p–p contacts, is shown in Fig. 3. The intra molecular N1–H1ÁÁÁN4 hydrogen
˚
bond of 2.615(2) A is shown in Fig. 4. The plane passing through N1, N2, C1 is
almost coplanar with the pyridinic ring containing N4, with a dihedral angle of
2.7(1)° between them; it is also not far from coplanar 10.8(1)° with the pyridinic
ring containing N3. The dihedral angle N3 pyridine–N4 pyridine is 8.3(1)°. It could
be said that the N1, N2, C1 plane is close to coplanar with the pyridinic rings
containing N3 and N4, which implies the same p system extends over all of the
atoms involved. The plane passing through N1, N2, C1, however, forms a dihedral
angle of 43.7(1)° with the pyridinic ring containing N5, indicating that the p system
of N5 pyridine is independent of that for the rest of the molecule. The dihedral
angles N5 pyridine–N4 pyridine and N5 pyridine–N3 pyridine are 45.9(1)° and
˚
54.1(1)°, respectively. The bond length C1–N2 = 1.305(2) A is mainly a double
˚
˚
bond whereas N1–N2 = 1.346(2) A and N1–C2 = 1.379(3) A are mainly single
bonds. The sum of the bond angles around N1 is 359.8(2)°, further indicating sp2
hybridization and partial double-bond nature for N1–N2 and N1–C2. The H1 atom,
located and refined fully, is intramolecular hydrogen bonded to the pyridinic N4
˚
˚
˚
with N1–H1 = 0.92(3) A, H1ÁÁÁN4 = 1.87(2) A, N1ÁÁÁN4 = 2.615(2) A, and N1–
H1ÁÁÁN4 = 137(2)° (Fig. 4).
Heck reaction catalyzed by palladium complexes 1 and 2
The catalytic activity of newly synthesized palladium complexes 1 was tested for
the Heck coupling reaction of an aryl halide with methyl acrylate. To test the
catalytic activity, aryl halide and methyl acrylate were chosen as model substrates.
Initially, 10–100 mol % Pd(II) complex 1 was used in the presence of KOH
(4.0 eq.) in DMF at 80 °C for 3–10 h (Table 5, entries 1–4). Yields in 10 h
improved substantially, from 32 to 99 %, when catalyst loading was increased from
10 to 100 mol % (Table 5, entries 1–4). It is evident from the results that, taking
Scheme 2 Catalytic Heck reaction of aryl halides with olefins
123

Palladium complexes bearing the dipyridyl ligand: synthesis…
Table 1 Crystal data and structure refinement for di(2-pyridinyl)methanone N-(2-pyridinyl)hydrazone
Identification code
Empirical formula
Formula weight
Temperature
exp_622
C16 H13 N5
275.31
293(2) K
˚
0.71073 A
Wavelength
Crystal system
Space group
Orthorhombic
P212121
˚
a = 7.9169(6) A
Unit cell dimensions
a = 90°
b = 90°
c = 90°
˚
b = 10.6433(13) A
˚
c = 16.5348(14) A
3
˚
Volume
1393.3(2) A
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.313 mg/m3
0.083 mm-1
576
0.4528 9 0.3770 9 0.0518 mm3
Crystal size
Theta range for data collection
Index ranges
3.12–25.01°
-6 B h B 9, -7 B k B 12, -19 B l B 12
3615
Reflections collected
Independent reflections
Completeness to theta = 25.01°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/variables
Goodness-of-fit on F2
Final R indices (I [ 2sigma(I))
R indices (all data)
Absolute structure variable
Largest diff. peak and hole
2270 [R(int) = 0.0178]
99.8 %
Analytical
0.994 and 0.971
Full-matrix least-squares on F2
2270/0/194
1.042
R1 = 0.0391, wR2 = 0.0813
R1 = 0.0519, wR2 = 0.0885
-1(3)
-3
˚
0.115 and -0.172 e A
˚
Table 2 Bond lengths (A) and
N(4)–C(8)
1.337(3)
1.348(2)
1.305(2)
1.346(2)
1.332(3)
1.379(3)
1.394(3)
1.338(3)
1.339(3)
angles (°) for di(2-
pyridinyl)methanone N-(2-
pyridinyl)hydrazone
N(4)–C(7)
N(2)–C(1)
N(2)–N(1)
C(2)–N(3)
C(2)–N(1)
C(2)–C(3)
N(5)–C(12)
N(5)–C(13)
Symmetry transformations were
used to generate equivalent
atoms
123

I. Warad et al.
Table 3 Torsion angles (°) for
di(2-pyridinyl)methanone N-(2-
pyridinyl)hydrazone
N(1)-C(2)-N(3)-C(6)
C(3)-C(2)-N(3)-C(6)
C(8)-N(4)-C(7)-C(11)
C(8)-N(4)-C(7)-C(1)
N(3)-C(2)-C(3)-C(4)
C(11)-C(7)-C(1)-N(2)
N(4)-C(7)-C(1)-C(12)
177.3(2)
-1.9(3)
0.2(3)
178.3(2)
1.1(3)
Symmetry transformations were
used to generate equivalent
atoms
176.52(19)
177.52(17)
˚
Table 4 Hydrogen bonds for di(2-pyridinyl)methanone N-(2-pyridinyl)hydrazone (A and °)
D–HÁÁÁA
N(1)–H(1)ÁÁÁN(4)
d(D–H)
0.92(3)
d(HÁÁÁA)
1.87(2)
d(DÁÁÁA)
2.615(2)
\(DHA)
137(2)
Symmetry transformations were used to generate equivalent atoms
Fig. 2 Ortep drawing of free DPMNPH ligand
into consideration the cost of the catalyst, 50 mol % catalyst is the optimum
amount for the 10-h reaction (Table 5, entries 3). To discover the optimum
conditions, reactions were performed in different solvents and with several bases.
With 50 mol % complex 1 at 80 °C and 10-h reaction (Table 5, entries 5–15) the
best result was obtained by use of 4 eq. KOH in DMF (Table 5, entry 3). No product
was obtained by use of ACN or toluene (Table 5, entries 13, 14).
To investigate substrate scope, the Heck coupling reaction was performed with
bromobenzene and chlorobenzene with methyl acrylate, using the same set of
123

Palladium complexes bearing the dipyridyl ligand: synthesis…
Fig. 3 Structure of the free
ligand, packed along the a-axis
Fig. 4 Structure of the free
ligand accompanied by
intramolecular N1–H1ÁÁÁN4
hydrogen bonding
conditions (Table 5, entries 16, 17) and the results showed that bromobenzene
resulted in excellent yields (93 %), in contrast with chlorobenzene (4 %).
Unfortunately, the catalysts are oxygen-sensitive and must be kept under an inert
atmosphere to avoid deactivation and to maintain high activity. In addition, no
conversion was observed if non-degassed DMF was used as solvent (Table 5, entry
18). Therefore, DMF was dried and deoxygenated before use, and the reactions were
performed under argon. A control experiment revealed that coupling reactions did
not occur in the absence of base (Table 5, entry 19). When palladium complex 2
was used for Heck coupling of aryl halides with methyl acrylate the trends were
almost the same as for catalyst 1 (Table 5, entries 20–23). These results are similar
to those reported elsewhere [26, 27].
123

I. Warad et al.
Table 5 The Heck coupling reaction of aryl halides with methyl acrylate
No. PhX Complex no. mol % Base Solvent (dry) Yield after 3 h Yield after 10 h
1
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhBr
PhCl
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
10
20
50
100
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
10
20
50
50
KOH
KOH
KOH
KOH
KOH
KOH
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
25
36
52
55
14
21
21
24
7
32
43
97
99
41
68
71
65
12
19
26
53
–
2
3
4
5a
6b
7
8
Na2CO3 DMF
9
TEA
DMF
DMF
DMF
DMAc
ACN
10
11
12
13
14
15
16
17
NaOAc
K3PO4
KOH
KOH
KOH
KOH
KOH
KOH
KOH
–
15
18
22
–
Toluene
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
–
–
31
34
0
59
93
4
18c PhI
19d PhI
0
0
0
0
20
21
22
23
PhI
PhI
PhI
PhI
KOH
KOH
KOH
K2CO3
15
26
55
10
29
63
92
35
Reaction conditions: Pd(II) 10–50 mol %, aryl halide 1 mmol, methyl acrylate 1.2 mmol, base 4 mmol,
in 10 mL DMF at 80 °C for 10 h, isolated yields are based on aryl halide, TON = 1000, all reactions
were monitored by GC
a
Base KOH 1.5 eq. was used
b
Base KOH 3.0 eq. was used
c
Non-degassed solvent was used
d
No-base was added
Conclusion
In this work we prepared and characterized two new palladium complexes with
DPMNPH ligand. The structure of the ligand was determined by single crystal
X-ray diffraction analysis, and both the complexes were studied by elemental
analysis and by spectroscopic studies. Both the new palladium complexes were
tested for catalytic activity, and were found to be highly efficient homogeneous
catalysts of the Heck reaction.
123

Palladium complexes bearing the dipyridyl ligand: synthesis…
Supplementary materials
CCDC 1035466 contains the supplementary crystallographic data for the ligand
DPMNPH. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments The authors would like to extend their sincere appreciation to the Dean of Scientific
Research at King Saud University for funding this work through research group number RG -1435-091.
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