Structure and property of unsymmetrical binuclear [(3,5-dimethylpyrazole)2Pd2(μ-3,5-dimethylpyrazolate)2(2,6-dipicolinate)] and mononuclear [Na2(H2O)4Pd(2,6-dipicolinate)2] complexes

Article abstract of DOI:10.1016/j.poly.2006.05.030

This work describes the synthesis and characterization of two Pd(II) complexes [(3,5-Hdmpz)₂Pd₂(μ-3,5-dmpz)₂(2,6-dipic)] (1) and [Na₂(H₂O)₄Pd(2,6-dipic)₂] (2) (3,5-Hdmpz = 3,5-dimethylpyrazole and 2,6-dipic = 2,6-pyridinedicarboxylate). The X-ray molecular structure determination shows that 1 is a binuclear complex bridged by two 3,5-dmpz units and is completely unsymmetric since one palladium atom contains two protonated 3,5-Hdmpz ligands and the other one contains the 2,6-dipic^2- unit that coordinates in a bidentate fashion. The central six-member Pd₂N₄ core shows a boat-like conformation where the palladium atoms are positioned at the vertices of the boat. The molecules are assembled in an extended zigzag one-dimensional network formed through 3,5-Hdmpz-carboxylate (2,6-dipic^2-) hydrogen bonds. The packing of complex 2 is constructed of cationic Na⁺-H₂O and anionic Pd-2,6-dipic^2- units joined together to form an extended two dimensional network. Alternate short and long Na-Na distances are observed in the cationic layer. A detailed quantum-chemical calculation on the Na-chains in 2 has been made. Complex 1 shows a luminescence property and antimicrobial activity against Bacillus subtilis, Escherichia coli and Aspergillus niger where the MIC has been found to be 100 μg/mL for all three species.

Full text of DOI:10.1016/j.poly.2006.05.030

Polyhedron 25 (2006) 3138–3144
www.elsevier.com/locate/poly
Structure and property of unsymmetrical binuclear
[(3,5-dimethylpyrazole)2Pd2(l-3,5-dimethylpyrazolate)2-
(2,6-dipicolinate)] and mononuclear [Na2(H2O)4Pd-
(2,6-dipicolinate)2] complexes
a
b
b
a,
*
Jishnunil Chakraborty , Heike Mayer-Figge , William S. Sheldrick , Pradyot Banerjee
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
Lehrstuhl fu¨r Analytische Chemie, Ruhr-Universita¨t Bochum, Universita¨tsstrasse 150, D-44780 Bochum, Germany
b
Received 12 May 2006; accepted 30 May 2006
Available online 9 June 2006
Abstract
This work describes the synthesis and characterization of two Pd(II) complexes [(3,5-Hdmpz)2Pd2(l-3,5-dmpz)2(2,6-dipic)] (1) and
[Na2(H2O)4Pd(2,6-dipic)2] (2) (3,5-Hdmpz = 3,5-dimethylpyrazole and 2,6-dipic = 2,6-pyridinedicarboxylate). The X-ray molecular
structure determination shows that 1 is a binuclear complex bridged by two 3,5-dmpz units and is completely unsymmetric since one
palladium atom contains two protonated 3,5-Hdmpz ligands and the other one contains the 2,6-dipic2ꢀ unit that coordinates in a biden-
tate fashion. The central six-member Pd2N4 core shows a boat-like conformation where the palladium atoms are positioned at the ver-
tices of the boat. The molecules are assembled in an extended zigzag one-dimensional network formed through 3,5-Hdmpz-carboxylate
(2,6-dipic2ꢀ) hydrogen bonds. The packing of complex 2 is constructed of cationic Na+–H2O and anionic Pd-2,6-dipic2ꢀ units joined
together to form an extended two dimensional network. Alternate short and long Na–Na distances are observed in the cationic layer.
A detailed quantum-chemical calculation on the Na-chains in 2 has been made. Complex 1 shows a luminescence property and antimi-
crobial activity against Bacillus subtilis, Escherichia coli and Aspergillus niger where the MIC has been found to be 100 lg/mL for all
three species.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Mono- and binuclear palladium(II) complexes; Crystal structures; 1D and 2D hydrogen bonded and coordination network; Computational
study; Luminescence property; Antimicrobial activity
1. Introduction
number of potential properties e.g., catalytic activity
and bioactivity of such compounds have been identified
[7,8]. In this context, the binuclear platinum-group com-
plexes stabilized through a pyrazolate bridge(s) are of
much significance owing to their interesting structural fea-
tures and reactivity toward Lewis acid metal complexes
[9]. The use of palladium pyrazolyl complexes as precur-
sors for the synthesis of new palladium containing rings,
and further use of such rings as molecular subunits of
supramolecular assemblies are well documented in the
recent literature. A recent study reports the use of mono-
and binuclear complexes having terminal 3,5-dmpz
(hydrogen bond acceptor) and 3,5-Hdmpz (hydrogen
Pyrazole-type heterocycles are well known as versatile
ligands that can interact with metal ions in several
fashions: neutral monodentate (pyrazole-N), anionic
monodentate (pyrazolato-N) or exo/endo-bidentate
(pyrazolato-N,N0) [1–5]. Such ligands are extensively used
for designing functional models for bimetallic biosites
whereby the metal atoms are held in close proximity,
allowing a wide range of intermetallic separations [6]. A
*
Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805.
E-mail address: icpb@mahendra.iacs.res.in (P. Banerjee).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.05.030

J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
3139
bond donor) ligands bound to palladium centres [10].
These compounds have been used as starting materials
to achieve organic-organometallic hydrogen bonded net-
works by reaction with carboxylic acids. A study on a
Pd2N4 core incorporating a diazo ligand is also known
[11]. We report herein a new complex [(3,5-Hdmpz)2Pd2-
(l-3,5-dmpz)2(2,6-dipic)] (1) that affords an unsymmetri-
cal 1D chain, and the binuclear units assemble in a zigzag
way propagated through 3,5-Hdmpz-carboxylate (2,6-
dipic2ꢀ) hydrogen bonds. The asymmetric unit of this
compound shows a central Pd2N4 six-member heterocycle
that adopts a distorted boat-like conformation.
We considered it of further interest to use another
substituted pyrazole, 3-methyl-1-phenylpyrazole (3-me-1-
phpz). An earlier report has been made that 3-me-1-phpz
displaces acetonitrile from [PdCl2(CH3CN)2] to form
trans-[Pd(N3)2(3-me-1-phpz)2] via [PdCl2(3-me-1-phpz)2]
[12]. In addition, the unsymmetrical N-substituted pyraz-
oles offer the possibility of formation of cyclometallated
products [13]. With the aim to generate a pyrazole based
product, we carried out the reaction of PdCl2(CH3CN)2,
3-me-1-phpz and NaH2,6-dipic. The compound
[Na2(H2O)4Pd(2,6-dipic)2] (2) is formed without incorpo-
ration of the 3-me-1-phpz ligand. A couple of precedents
of Pd-2,6-dipic2ꢀ complexes are known in the literature
[14,15]. The single crystal X-ray analysis shows that 2 is
stabilized through cationic Na+–H2O and anionic Pd-
2,6-dipic2ꢀ units to form an extended two-dimensional
coordination polymer. A detailed study of the molecular
and electronic structures coupled with experimental
results obtained from IR, UV–Vis, 1H NMR and mass
spectral studies have been made. Data on the lumines-
cence property and antimicrobial activity of 1 are also
presented.
standard at room temperature. The mass spectra were
recorded on a Qtof Micro YA263 mass spectrometer.
Absorption and luminescence spectra were studied on a
Shimadzu UV2100 UV–VIS recording spectrophotometer
and a Perkin–Elmer LS 55 Luminescence spectrometer,
respectively.
2.3. Computational details
A detailed quantum-chemical calculation on the Na-
chain as retrieved from the crystal geometry of 2 was made
at the DFT-level of B3LYP/6-31G+(d,p) [17]. These sets of
calculations have been performed using the GAUSSIAN 03
program suite [18].
2.4. Synthesis of the complexes
2.4.1. Synthesis of [(3,5-Hdmpz)2Pd2(l-3,5-dmpz)2(2,6-
dipic)] (1)
0.026 g (0.1 mmol) of PdCl2(CH3CN)2 was dissolved in
10 cm3 of methanol. To this deep orange solution was
added
a
10 cm3 methanolic solution of 3,5-Hdmpz
(0.020 g, 0.2 mmol). The resulting mixture was stirred for
10 min at room temperature and filtered off to remove
any suspended material. To the filtrate, 15 cm3 aqueous
solution of NaH2,6-dipic (0.040 g, 0.2 mmol) was added
and the solution was warmed at 50 ꢁC for 30 min. The light
yellow solution was cooled to room temperature. Deep yel-
low crystals suitable for X-ray diffraction were obtained
after 7 days in a 71% yield based on Pd. Anal. Calc. for
1: C, 42.60; H, 4.34; N, 16.57. Found: C, 42.26; H, 4.23;
N, 16.43%. IR spectrum (KBr disk, m/cmꢀ1): 3123 (w),
3090 (w), 1683 (s), 1673 (s), 1621 (s), 1573 (m), 1414 (m),
1375 (s). 1H NMR spectrum (300 MHz, d6-DMSO) d:
2.43 (CH3, s), 1.66 (CH3, s), 6.08 (H, s), 2.43 (CH3, s),
1.86 (CH3, s), 5.93 (H, s); 2.11 (CH3, s), 2.30 (CH3, s),
5.44 (H, s), 12.96 (N–H,s), 2.20 (CH3, s), 2.23 (CH3, s),
5.61 (H, s), 17.21 (N–H, s); 7.90, 7,92 (H, d), 8.21, 8.24,
8.27 (H, t), 7.77, 7.79 (H, d). Mass spectrum (CH3CN/
H2O): 762.28.
2. Experimental
2.1. Materials
Palladium chloride (PdCl2) used in the complex was
obtained
from
Arora-Matthey
Limited
(India).
PdCl2(CH3CN)2 was prepared according to the literature
procedure [16]. 3,5-Dimethylpyrazole was purchased from
Fluka. Pyridine-2,6-dicarboxylic acid and 3-methyl-1-
phenylpyrazole were obtained from Lancaster. Sodium
hydroxide was a Merck product. All solvents were dried
and kept over molecular sieves prior to use. Double dis-
tilled water was used throughout.
2.4.2. Synthesis of [Na2(H2O)4Pd(2,6-dipic)2] (2)
PdCl2(CH3CN)2 (0.052 g, 0.2 mmol) was dissolved in
5 cm3 of DMF. To the deep orange solution was added a
solution of 3-me-1-phpz (0.063 g, 0.4 mmol) in 5 cm3 of
DMF under stirring. After 10 min, an aqueous solution
of NaH2,6-dipic (0.080 g, 0.4 mmol) was added to the mix-
ture and the solution stirred for 1 h. The resulting yellow
solution was filtered off to remove any suspended material.
Yellow crystals suitable for X-ray diffraction were obtained
after 5 days in a 40% yield based on Pd. Anal. Calc. for 2:
C, 30.73; H, 1.10; N, 5.12. Found: C, 30.81; H 1.16; N,
5.14%. IR spectrum (KBr disk, m/cmꢀ1): 3528 (s), 3426
2.2. Physical measurements
Elemental analysis was carried out in a 2400 Series-II
CHN analyzer, Perkin–Elmer, USA. IR spectra were
recorded on a Nicolate Magna IR 750 Series-II IR spectro-
photometer. The 1H NMR spectra were recorded on a
300 MHz Bruker Avance DPX 300 machine, Switzerland
in (CD3)2SO and D2O solvent with TMS as the internal
1
(w), 1683 (s), 1633 (s), 1411 (m), 1376 (s). H NMR spec-
trum (300 MHz, d6-DMSO) d: 7.90, 7.92 (H, d), 8.18,
8.20, 8.23 (H, t), 7.75, 7.72 (H, d). Mass spectrum (H2O):
546.28.

3140
J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
Table 1
pound 1 [(3,5-Hdmpz)2Pd2(l-3,5-dmpz)2(2,6-dipic)] has
been obtained as a deep-yellow crystalline solid by addition
of 3,5-Hdmpz and NaH2,6-dipic to PdCl2(CH3CN)2 in
methanol at 50 ꢁC without addition of the hydroxide ion
or other bases. Compound 2 was obtained also as a
well-formed deep-yellow crystalline solid by the addition
of 3-me-1-phpz and an aqueous solution of NaH2,6-dipic
to a DMF solution of PdCl2(CH3CN)2. The formation of
compound 2 instead of a substituted pyrazolate one is the
result of a failed experiment. It clearly points out the rele-
vance of the strength of the bidentate chelate (Hdipicꢀ) in
determining the course of the reaction. In the absence of
3-me-1-phpz, no product is obtained. However, a similar
compound, trans-Na2[Pd(2,6-dipic)2 Æ 2H2O, was obtained
earlier by the addition of K2PdCl4 to a warm aqueous
solution of pyridine-2,6-dicarboxylic acid and maintaining
the pH to 6 with 1 N NaOH solution. The yellow crystal-
Crystal data and details for structure refinement for 1 and 2
Formula
C27H33N9O4Pd2
760.42
C14H6N2Na2O12Pd
546.59
triclinic
Formula weight
Crystal system
Space group
monoclinic
C2/c (no. 15)
29.848(6)
11.649(2)
17.713(4)
90.00
94.18(3)
90.00
6143(2)
ꢀ
P1 ðno: 2Þ
˚
a (A)
6.4521(13)
6.5686(13)
11.661(2)
106.19(3)
93.57(3)
107.21(3)
447.68(19)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
8
Dcalc (g/cm3)
1.645
1.219
3056
293
2.027
1.158
268
294
l (mmꢀ1
F(000)
)
Temperature (K)
˚
k (A) (Mo Ka)
0.71073
2.2–25.0
5511
5403 [0.055]
3429
5403/0/372
0.973
R1 = 0.0602
wR2 = 0.1289
R1 = 0.1108
wR2 = 0.1451
0.71073
3.3–25.0
1631
1541 [0.061]
1522
1541/0/142
1.114
R1 = 0.0437
wR2 = 0.1197
R1 = 0.0442
wR2 = 0.1205
hmin–max (ꢁ)
Reflections collected
1
line product was characterized by elemental analysis, H
Independent reflections [Rint
Observed data [I > 2r(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
]
NMR, IR and UV–Vis spectroscopy [14]. There is no
report of the single crystal X-ray structural study of
this compound. The IR spectrum of complex 1 displays
all the characteristic bands of the coordinated 3,5-Hdmpz
and 2,6-dipic2ꢀ ions. These include a pair of strong
bands at ca. 1683 and 1621 cmꢀ1 corresponding to
R indices (all data)
m
asym(COOꢀ) vibrations of the bonded and free carboxy-
lates, respectively, and another pair for msym(COOꢀ) at
1414 and 1375 cmꢀ1, respectively for the bonded and free
carboxylate groups. The ring-breathing mode for 3,5-
Hdmpz is observed at 1573 cmꢀ1. The relatively low value
of the m(N–H) frequency at 3123 cmꢀ1 indicates that the
N–H ꢁ ꢁ ꢁ O hydrogen bonds observed in the X-ray structure
of the compound must be quite strong. The IR spectrum of
complex 2 is quite simple. The bands at 1683 and
1411 cmꢀ1 correspond to the masym(COOꢀ) and msym(COOꢀ)
vibrations, respectively, of the bonded carboxylates whilst
that for the free carboxylates are observed at 1633 and
1376 cmꢀ1. A strong band at 3528 cmꢀ1 is observed for
the m(O–H) frequency, which reveals the presence of the
solvent water molecules. The ESIMS of compounds 1
and 2 corroborate well with their simulated spectra. The
1H NMR spectrum of 1 at room temperature shows three
sets of signals, one for the bridging 3,5-dmpz (d = 2.43
(CH3, s), 1.66 (CH3, s), 6.08 (H, s), 2.43 (CH3, s), 1.86
(CH3, s) 5.93 (H, s) ppm) and another one for the
terminal 3,5-Hdmpz groups (d = 2.11 (CH3, s), 2.30
(CH3, s), 5.44 (H, s), 12.96 (N–H, s) 2.20 (CH3, s), 2.23
(CH3, s) 5.61 (H, s), 17.21 (N–H, s) ppm). The third set
is for the 2,6-dipic2ꢀ moiety (d = 7.90, 7.92 (H, d), 8.21,
8.24, 8.27 (H, t) and 7.77, 7.79 (H, d) ppm). The 1H
NMR spectrum of 2 at room temperature shows signals
only for the 2,6-dipic2ꢀ unit comparable to the signals
observed in complex 1.
2.5. Crystal structure determination and structural
refinement of complexes 1 and 2
Crystals of dimensions 0.19 · 0.21 · 0.49 mm (1) and
0.19 · 0.26 · 0.55 mm (2) were mounted on a Siemens P4
4-circle-diffractometer which was equipped with graphite
˚
monochromated Mo Ka radiation (k = 0.71073 A). Unit
cell dimensions and intensity data were measured at
293 K for both 1 and 2. All calculations for data reduction,
structure solution and refinement were done by standard
procedures (SHELXS-97) [19], (SHELXL-97) [20]. All non-
hydrogen atoms were refined anisotropically by full-matrix
least-squares techniques on F2. All the hydrogen atoms of 1
and 2 were included at calculated positions and refined iso-
tropically. Information concerning crystallographic data
collection and refinement of the structures are summarized
in Table 1.
3. Results and discussion
3.1. Synthesis and spectroscopic characterization of the
complexes
The complexes reported here are formed by the process
of slow crystallization. Sometimes it takes a long time to
isolate the desired product. Moreover, the reported com-
plexes are sparingly soluble in common organic solvents.
We have tried to crystallize the products from an optimum
dilution so that we could strike a balance between
improved crystal quality and time of crystallization. Com-
3.2. Description of the crystal structures
As can be seen from the asymmetric unit (Fig. 1), 1 is a
binuclear Pd(II) complex that contains two 3,5-Hdmpz,

J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
3141
˚
(PMe2Ph)2] [22] (3.115(1) A) and [{Pd2(CH2C6H4P-
(o-tolyl)2-jC,P)2(l3-3,5-dmpz-N,N0,C4)2Ag(l-ClO4)}2] [4]
˚
(3.2297(7) A). Each palladium atom is in an almost square-
planar coordination environment. The central six-member
[Pd(l-3,5-dmpz)2Pd] core is non-planar with Pd–N3,5-dmpz
˚
distances between 1.981(7) and 2.007(7) A. These fall in the
range found in other Pd and Pt complexes with this type of
ligand [4,22–28]. The N–Pd–N angles are 87.8(3)ꢁ and
86.2(3)ꢁ between the 3,5-dimethylpyrazolate groups. The
Pd2N4 six-member ring reveals a boat-like conformation;
the dihedral angle formed by the best least-squares coordina-
tion planes of the metals being 89.7ꢁ. The metal atoms are
positioned at the vertices of the boat and the angle between
the planes containing the Pd–N–N–Pd fragments is 73.8ꢁ
[29]. The boat folding of the Pd2N4 core has also been
observed earlier for other pyrazolato bridged palladium
complexes [9,30,31]. The mean deviation of the plane con-
structed by the four nitrogen atoms N11, N12, N21 and
Fig. 1. Molecular structure of complex [(3,5-Hdmpz)2Pd2(l-3,5-
dmpz)2(2,6-dipic)] (1). All hydrogen atoms have been omitted for clarity.
˚
N22 is 0.0589 A. The displacement of Pd1 towards N22 is
˚
˚
0.9391 A while that for Pd2 towards N22 is 1.1145 A. All
of the 3,5-Hdmpz, 3,5-dmpz and 2,6-dipic2ꢀ ligands are pla-
nar, within experimental error. Comparable bond lengths
within the 3,5-dimethylpyrazolate rings suggest substantial
delocalization upon deprotonation. The bow angles between
the planes N11–N12–N21–N22 and N12–Pd2–N22,
N11–Pd1–N21 are 40.73ꢁ, 50.12ꢁ, respectively. This differ-
ence reflects the unsymmetrical environment around the
metal centres. The dihedral angles between the monodentate
3,5-Hdmpz ligands N31–N32–C33–C34–C35 and N41–
N42–C43–C44–C45, and bridging 3,5-dmpz ligands
N11–N12–C13–C14–C15 and N21–N22–C23–C24–C25
are 91.1ꢁ and 90.3ꢁ, respectively. The ligand 2,6-dipic2ꢀ coor-
dinates to Pd1 in a bidentate fashion through its pyridine
nitrogen and one carboxylate oxygen atom while the other
carboxylate remains free. To achieve internal charge bal-
ance, the non-coordinated oxygen atom O21 of the free car-
boxylate group must be deprotonated. The complex exhibits
a zigzag chain configuration of the 1D network through
intermolecular 3,5-Hdmpz-carboxylate hydrogen bonds.
This arrangement results from the trans orientation of one
protonated pyrazole and a free carboxylate group of 2,6-
dipic2ꢀ moieties. This network is clearly affected by the inter-
molecular and intramolecular hydrogen bonding occurring
between the atoms N32–H32 ꢁ ꢁ ꢁ O21 and N42–H42 ꢁ ꢁ ꢁ
O22, respectively, which is responsible for the conformation
and stability.
two 3,5-dmpz and a 2,6-dipic unit. Pd2 contains two Hdmpz
units as coordinating ligands whereas Pd1 contains only one
2,6-pyridinedicarboxylate unit. The two 3,5-dmpz units
bridge the metal centres in a bidentate fashion. Table 2 lists
selected bond distances and angles. Despite the presence of
bridging 3,5-dmpz groups between the metal atoms, the
˚
intermetallic separation (3.397 A) indicates no metal–metal
bond. The intermetallic distance is comparable to [{Pd2(l-
˚
3,5-dmpz)(3,5-dmpz)(3,5-Hdmpz)}2] [9] (3.4020 A) and [Pd
3
˚
(l-3,5-dmPz)(g -C3H4)]2 [21] (3.343 A) but is longer than
the corresponding distances in [Pd2Cl2(l-3,5-dmpz)2
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) in complex 1 with esds in
parentheses
˚
Bond lengths (A)
Pd1–O61
Pd1–N1
2.018(6)
2.059(7)
2.007(7)
1.996(7)
2.002(6)
1.981(7)
2.029(7)
Pd2–N41
N11–N12
N21–N22
N31–N32
N41–N42
N32–H32
N42–H42
2.053(6)
1.368(10)
1.375(10)
1.364(10)
1.360(10)
0.8604
Pd1–N11
Pd1–N21
Pd2–N12
Pd2–N22
Pd2–N31
0.8597
Bond angles (ꢁ)
O61–Pd1–N1
O61–Pd1–N11
O61–Pd1–N21
N1–Pd1–N11
N1–Pd1–N21
N11–Pd1–N21
N12–Pd2–N22
N12–Pd2–N31
N12–Pd2–N41
N22–Pd2–N31
N22–Pd2–N41
N31–Pd2–N41
81.1(2)
91.1(3)
177.1(2)
169.1(3)
99.5(3)
87.8(3)
86.2(3)
91.0(3)
170.5(3)
176.9(3)
91.2(3)
91.7(3)
Pd1–O61–C60
Pd1–N1–C2
Pd1–N1–C6
111.9(6)
132.2(6)
107.7(5)
123.0(5)
127.4(6)
116.6(5)
134.4(6)
120.6(5)
131.5(5)
120.4(5)
127.7(6)
Pd1–N11–N12
Pd1–N11–C15
Pd2–N12–N11
Pd2–N12–C13
Pd1–N21–N22
Pd1–N21–C25
Pd2–N22–N21
Pd2–N22–C23
The X-ray structure and numbering scheme for complex
2 are shown in Fig. 2. Selected bond lengths and angles are
listed in Table 3. The crystal is built up by centrosymmetric
+
2ꢀ
Pd-2; 6-ðdipicÞ2 anions and Na cations hexacoordinated
by four water molecules and two carboxylate oxygens
belonging to the dipic2ꢀ moiety. The 2,6-dipicolinate ion
chelates in a bidentate fashion to the palladium atom and
the resulting coordination around the palladium is square
planar with Pd–N1 and Pd1–O1 bond distances of 2.028
˚
Hydrogen bonds (A, ꢁ)
D–H–A
D–A
H–A
1.8800
2.0200
\D–H–A
173.00
138.00
˚
N32–H32–O21
N42–H42–O22
2.740(10)
2.720(10)
and 2.004 A, respectively. The arrangement of the oxygen
atoms around the Na+ ion is approximately octahedral

3142
J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
Fig. 2. Molecular structure of complex [Na2(H2O)4Pd(2,6-dipic)2] (2). All
hydrogen atoms have been omitted for clarity.
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) in complex 2 with esds in
parenthesesa
˚
Bond lengths (A)
Pd1–O61
Pd1–N1
Na1–OW1
Na1–OW2
2.004(3)
2.028(4)
2.478(5)
2.411(4)
Na1–O21
2.384(5)
2.410(4)
2.541(5)
2.509(5)
Na1–O2_a
Na1–OW1_b
Na1–OW2_d
Bond angles (ꢁ)
O61–Pd1–N1
81.43(16)
180.00
106.28(17)
77.99(16)
169.71(17)
171.5(2)
84.01(15)
170.04(18)
82.11(16)
89.85(16)
95.99(15)
O61_c–Pd1–N1
N1–Pd1–N1_c
98.57(16)
180.00
O61–Pd1–O61_c
OW1–Na1–OW2
OW1–Na1–O2_a
OW1–Na1–OW2_d
O2_a–Na1–OW2
OW2–Na1–OW2_d
OW1_b–Na1–O21
OW1_b–Na1–O2_a
OW1_b–Na1–OW2_d
Na1–OW2–Na1_d
a
Fig. 3. The perspective view of the packing of [Na2(H2O)4Pd(2,6-dipic)2]
(2) comprising of cationic Na+–H2O and anionic Pd-2,6-dipic2ꢀ units.
OW1–Na1–O21
OW1–Na1–OW1_b
OW2–Na1–O21
OW1_b–Na1–OW2
O2_a–Na1–O21
OW2_d–Na1–O21
O2_a–Na1–OW2_d
Na1–OW1–Na1_b
99.90(18)
89.99(16)
85.61(16)
90.51(16)
101.00(16)
80.63(17)
91.80(15)
90.01(17)
2,6-dipic2ꢀ layer links two adjacent parallel Na-layers
through the oxygen atoms (O2 and O21) to form the two
dimensional network. No short contacts between Pd(II)
˚
atoms exist, the minimum Pd–Pd distance being 6.569 A.
A detailed analysis of the crystal geometries reveals an
interesting phenomenon in compound 2. The 1D Na-chain
in the crystal shows a significant distortion with short and
long alternating Na–Na distances [Na–Na (type
Symmetry transformations used to generate equivalent atoms: a = ꢀx,
1 ꢀ y, 1 ꢀ z; b = ꢀx, 2 ꢀ y, 2 ꢀ z; c = 1 ꢀ x, 1 ꢀ y, 1 ꢀ z; d = 1 ꢀ x,
2 ꢀ y, 2 ꢀ z.
˚
˚
1) = 3.550 A; Na–Na (type 2) = 3.655 A]. Examples of
Na–H2O chains are known in the literature whereby alter-
nating Na–Na distances in a particular system do not show
any difference. The Na–Na distance in polyhedral
as shown in Fig. 2. The cation coordination can be ratio-
nalized in terms of bond valence [32,33], which assumes
that the total charge of the cations has to be saturated by
the summation of the separated bond valence si of each
coordinated atom or ion. The quantity s can be calculated
by the expression s = exp[(r0 ꢀ r)/B], where r is the actual
Na–O distance, and r0, B are parameters empirically deter-
[Na3(O)4(H2O)10]3+ is 3.477(8) A [35] and that in the cat-
˚
ionic Na–H2O-malonate chain is 3.786 A [36]. A marginal
˚
˚
difference in Na–Na distances (3.566(3) and 3.534(4) A)
has been reported in [(L3VO2Na)2(H2O)7]ꢂ (L3 = a schiff
base ligand) [37]. Our observation is very similar to that
known in the famous case of Peierls-distortion in trans-
polyacetylene that leads formation of a gap near the
Fermi-energy and leads to insulating behavior of polyacet-
ylene [38,39]; for trans-polyacetylene, C@C (type
1) = 1.355 A; C–C (type 2) = 1.54 A. Thus, our system
with a 1D Na-chain also exhibits a Peierls-distortion. To
quantify the hypothesis, we performed a detailed quan-
tum-chemical calculation on the Na-chain as retrieved
from the crystal geometry at the DFT-level of B3LYP/6-
31G+(d,p). The calculations are performed on a finite sized
mined (r0 = 1.803 and B = 0.37) [34]. In the present struc-
P
ture, the value of 1.04 for
si obtained for the sodium
cation is in good agreement with the expected charge value
of 1.00. The packing arrangement of 2 consists of cationic
Na+-water and anionic Pd-2,6-dipic2ꢀ units to form a two-
dimensional coordination-polymeric network as shown in
Fig. 3. In the cationic part, solvent water molecules OW1
and OW2 link the Na atoms to form a –Na1–H2O–Na–
zigzag chain where there are two types of Na–Na distances,
˚
˚
˚
3.656 and 3.550 A. The chains line up in the ab-plane and
are parallel to the Pd-2,6-dipic2ꢀ sheet. Finally, every Pd-

J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
3143
Fig. 4. Highest molecular orbital plot for the Na-chain: clearly seen electron localization over alternate Na–Na distances leading to alternate short and
long distances supporting the Peierls-distortion in 2.
(10 atoms) Na-atom chain. The Na atom, having one
unpaired electron in the 3s1 orbital, shares a similarity to
the polyacetylene chain which has one unpaired electron
in the pp orbital. The highest occupied molecular orbital
(HOMO) indeed shows a localization of electrons on each
alternating short bond and thereby supports the existence
of alternating short and long Na–Na bonds with a Na–
Na–Na angle of 127.11ꢁ (Fig. 4). It is true that the Na-
chains do not have conventional bonding in terms of
organic molecules. The Na–Na distance in the BCC Na-
solution of compound 2 is very weakly emissive with a
UF value of 6.0 · 10ꢀ3 (measured against p-cresol in
cyclohexane).
3.4. Antimicrobial activity
The antimicrobial activity of compound 1 was deter-
mined against Bacillus subtilis, Escherichia coli and Asper-
gillus niger in a medium of nutrient and Czapek-dox
broth, respectively for bacteria and mold. 0.1 mL of cell
suspension (5.0 · 105 cfu/mL) from 18 h cell culture of bac-
teria grown on nutrient agar at 37 ꢁC was added per 5 ml of
the medium. For A. niger, spore suspension was added
instead of cell suspension. The compound was dissolved
in a mixture of 1:1 DMF and absolute ethanol. The tubes
were incubated at 37 ꢁC for 24 h for the growth of the bac-
teria and at 30 ꢁC for 48 h for mold. The lowest concentra-
tion of the compound that resulted in complete inhibition
of the visible growth after incubation was recorded as the
minimum inhibitory concentration (MIC). The MIC of
the compound against B. subtilis, E. coli and A. niger has
been found to be 100 lg/mL.
˚
crystal is 3.72 A. Of course, Na-crystal is bound through
bonding interactions albeit weak in the solid state. One
cannot argue that Na-atoms are packed with just hard-
sphere packing with no Na–Na interactions. Anyway, in
our system the Na–Na bonds are smaller than that in
Na-crystal, rendering the possibility of weak interactions
between the Na-atoms in the 1D-chains. Otherwise, one
could see such localization on each atom and not on the
bonds.
3.3. Absorption spectra and luminescence properties
Luminescent Pd(II) complexes are exceedingly rare if
compared to the analogous Pt(II) luminophores [40]. With
few notable exceptions [41,42], most systems emit only at
low temperature and with low efficiency [43–46]. It has
been also recognized that when incorporated in polynu-
clear complexes, Pd(II) centers may lead to partial quench-
ing of luminescence. Nevertheless despite these unfavorable
premises, several photoactive palladium(II) complexes
recently have been the subject of accurate photophysical
studies. The absorption spectrum of complex 1 in DMF
solution is dominated by an intense band with the maxima
at 267 nm in the ultraviolet region (e = 1.56 ·
104 Mꢀ1 cmꢀ1) which is assigned to a spin-allowed p–p*
transition. At longer wavelength in the range 320–
400 nm, a shoulder is obtained which may be attributed
to a metal-to-ligand charge transfer (MLCT) transition.
Complex 2 also exhibits a strong band centered at
4. Conclusion
In conclusion, we have here shown two palladium deriv-
atives, [(3,5-Hdmpz)2Pd2(l-3,5-dmpz)2(2,6-dipic)] and
[Na2(H2O)4Pd(2,6-dipic)2] which form hydrogen bonded
1D and 2D coordination networks, respectively. Com-
pound 1 has been obtained as a homobimetallic complex
bridged by two 3,5-dmpz units without addition of any
external base. The central Pd2N4 core adopts a boat-like
conformation. Compound 2 on the other hand, forms a
2D coordination polymeric network constructed of cationic
Na+–water and anionic Pd-2,6-dipic2ꢀ units. Quantum-
chemical calculation shows that alternate short and long
Na–Na distances observed are due to Peierls-distortion in
the Na-chains.
The binuclear compound 1 has been investigated as an
inhibitory agent against microbial organisms and shows
luminescence property with a UF value of 0.04.
274 nm with
a
molar absorption coefficient of
7.19 · 103 Mꢀ1 cmꢀ1. In addition, the lower energy band
in the range 320–450 nm is assigned to the spin allowed
MLCT transition. In the fluorescence emission spectrum
of a solution of 1 in DMF (5.0 · 10ꢀ6 M) at room temper-
ature, on excitation at 270 nm, the emission peaks are
observed at 410 and 430 nm in the visible region and these
are independent of the concentration of the solution. A
value for quantum efficiency UF = 0.04 has been calculated
using p-cresol in cyclohexane as a standard. The aqueous
Acknowledgements
We thank Prof. A.K. Guha, Department of Biological
Chemistry, IACS, Kolkata for his advice in the study of
antimicrobial activity. We thank Mr. A. Datta of JNCASR,
Bangalore for discussions on theoretical modelling.

3144
J. Chakraborty et al. / Polyhedron 25 (2006) 3138–3144
B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, GAUSS-
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