Synthesis, molecular and crystalline architectures, and properties of novel bis(bidentate) and bis(tridentate) Schiff base bridged dinuclear mercury(II)pseudohalides: Control of coordination numbers by varying denticities

Article abstract of DOI:10.1016/j.poly.2010.02.010

A tetracoordinated [Hg₂(L¹)(SCN)₄] (1) [L¹ = N,N′-(bis-(pyridin-2-yl)benzylidene)-1,2-ethanediamine] and two pentacoordinated [Hg₂(L²)(X)₄] [L² = N-((1-pyridin-2-yl)formylidene)-N′-[2-(4-{2-[((1-pyridin-2-yl)formylidene) amino]ethyl}piperazin-1-yl)ethyl]amine; X =N₃^-, (2) and X = NCS^-, (3)] dinuclear mercury(II) compounds have been prepared and characterized using microanalytical, spectroscopic, thermal and X-ray crystallographic results. Structural analyses reveal bis(bidentate) congregation behaviour of classical tetradentate L¹ and bis(tridentate) character of L² encapsulating two metal centers. Each mercury(II) center in 1 is in a distorted tetrahedral coordination environment with HgN₂S₂ chromophore. Hg(1) is attached with two N atoms of L¹ and two S atoms of two terminal thiocyanates whereas neighbouring Hg(1) is ligated to the other two N donor set of the same Schiff base completing its bis(bidentate) mode and two S atoms of thiocyanates. In 2 and 3 each mercury(II) center adopts a distorted square pyramidal geometry with MN₅ (in 2) and MN₃S₂ (in 3) chromophores and two metal(II) centers are encapsulated by bis(tridentate) congregation behaviour of L² through donation of three N donor set to each metal and the fourth and fifth positions are occupied by two N atoms of terminal azides in 2 and two S atoms of terminal thiocyanates in 3. In solid-state, dinuclear units of 1 pack through cooperative C-H?N and C-H?S hydrogen bonds and C-H?π interactions giving rise to a 3D network. Individual dinuclear units of 2 and 3 self-assemble through cooperative C-H?N hydrogen bondings and C-H?π interactions affording 3D network structure and 2D continuum, respectively. In DMF solutions at room temperature all the complexes display high-energy intraligand ¹(π-π*) fluorescence.

Full text of DOI:10.1016/j.poly.2010.02.010

Polyhedron 29 (2010) 1667–1675
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, molecular and crystalline architectures, and properties
of novel bis(bidentate) and bis(tridentate) Schiff base bridged
dinuclear mercury(II)pseudohalides: Control of coordination
numbers by varying denticities
Soumi Chattopadhyay a, Kishalay Bhar a, Sumitra Das a, Smita Satapathi a, Hoong-Kun Fun b,
Partha Mitra c, Barindra Kumar Ghosh a,
*
a Department of Chemistry, The University of Burdwan, Burdwan 713 104, India
b X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
c Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
tetracoordinated [Hg2(L1)(SCN)4] (1) [L1 = N,N0-(bis-(pyridin-2-yl)benzylidene)-1,2-ethanediamine]
and two pentacoordinated [Hg2(L2)(X)4] [L2 = N-((1-pyridin-2-yl)formylidene)-N0-[2-(4-{2-[((1-pyridin-
2-yl)formylidene) amino]ethyl}piperazin-1-yl)ethyl]amine; X = Nꢀ3 , (2) and X = NCSꢀ, (3)] dinuclear mer-
cury(II) compounds have been prepared and characterized using microanalytical, spectroscopic, thermal
and X-ray crystallographic results. Structural analyses reveal bis(bidentate) congregation behaviour of
classical tetradentate L1 and bis(tridentate) character of L2 encapsulating two metal centers. Each mer-
cury(II) center in 1 is in a distorted tetrahedral coordination environment with HgN2S2 chromophore.
Hg(1) is attached with two N atoms of L1 and two S atoms of two terminal thiocyanates whereas neigh-
bouring Hg(1) is ligated to the other two N donor set of the same Schiff base completing its bis(bidentate)
mode and two S atoms of thiocyanates. In 2 and 3 each mercury(II) center adopts a distorted square pyra-
midal geometry with MN5 (in 2) and MN3S2 (in 3) chromophores and two metal(II) centers are encapsu-
lated by bis(tridentate) congregation behaviour of L2 through donation of three N donor set to each metal
and the fourth and fifth positions are occupied by two N atoms of terminal azides in 2 and two S atoms of
terminal thiocyanates in 3. In solid-state, dinuclear units of 1 pack through cooperative C–Hꢁ ꢁ ꢁN and
Received 26 November 2009
Accepted 9 February 2010
Available online 13 February 2010
Keywords:
Mercury(II) pseudohalides
Bis(bidentate)/bis(tridentate) Schiff base
Synthesis
Superstructure
Luminescence
C–Hꢁ ꢁ ꢁS hydrogen bonds and C–Hꢁ ꢁ ꢁ
p
interactions giving rise to a 3D network. Individual dinuclear units
interactions
of 2 and 3 self-assemble through cooperative C–Hꢁ ꢁ ꢁN hydrogen bondings and C–Hꢁ ꢁ ꢁ
p
affording 3D network structure and 2D continuum, respectively. In DMF solutions at room temperature
1
*
all the complexes display high-energy intraligand
(
p
–p
) fluorescence.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
the binding of transition and non-transition metal ions by pseud-
ohalides like azide and thiocyanate in combination with neutral N
donor Schiff bases of different denticities. Group 12 metal com-
plexes are of immense importance due to their exciting electronic
and optoelectronic properties [17,31–34]. This work stems from
our interest in the study of coordination behaviour of a classical tet-
radentate [N,N-(bis-(pyridin-2-yl)benzylidene)-1,2-ethanediamine]
(L1) and a hexadentate [N-((1-pyridin-2-yl)formylidene)-N0-[2-(4-
{2-[((1-pyridin-2-yl)formylidene)amino]ethyl}piperazin-1-yl)ethy-
l]amine] (L2) Schiff bases in mercury(II) bound states, that remains
unexplored to date. Mercury(II), because of its 5d10 configuration
may give rise to different coordination numbers and molecular
and crystalline aggregates [35–37]. The purpose of this work is to
afford varied coordination geometries around the metal center by
changing the denticities of different N-donor Schiff bases.
Successfully, we have synthesized and X-ray crystallographically
The coordination behaviour of Schiff bases [1–3] has spawned a
great interest in recent times because of their preparative accessi-
bilities, structural variety, varied denticity and subtle steric and/or
electronic control on their frameworks leading to complexes of dif-
ferent coordination numbers and nuclearities with interesting
molecular and crystalline architectures [4–14] and related proper-
ties [15–18]. Pseudohalides [19–21] are established versatile bridg-
ing unit, which in combination with organic blockers may afford
different monomeric, dimeric, polymeric and suprameric metal–or-
ganic frameworks [22–24]. We are currently investigating [25–30]
* Corresponding author. Tel.: +91 342 2533913; fax: +91 342 2530452.
E-mail addresses: kishalaybhar@yahoo.com (K. Bhar), barin_1@yahoo.co.uk
(B.K. Ghosh).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.02.010

1668
S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
characterized
a
tetracoordinated
dinuclear
compound
used as solvent. Ground state absorption and steady-state fluores-
[Hg2(L1)(SCN)4] (1) and two pentacoordinated dinuclear com-
pounds [Hg2(L2)(N3)4] (2) and [Hg2(L2)(SCN)4] (3). The details of
syntheses, structures, thermal and luminescence behaviours of
these compounds are described here.
cence measurements (in dry DMF) were made with a Jasco model
V-530 UV–Vis spectrophotometer and Hitachi model F-4500 spec-
trofluorimeter, respectively. Time-resolved fluorescence measure-
ments were carried out using a time-correlated single photon
counting (TCSPC) spectrometer Edinburgh Instruments, model
199; a hydrogen filled coaxial flash lamp with a pulse width of
1.2 ns at FWHM and a Philips XP-2020Q photomultiplier tube were
used as the excitation source and the fluorescence detector.
2. Experimental
2.1. Materials and methods
2.3. Preparation of the complexes
High purity pyridine-2-carboxaldehyde (Lancaster, UK), 2-ben-
zoylpyridine (Lancaster, UK), 1,2-ethanediamine (Rankem, India),
2-[4-(2-aminoethyl)-piperazin-1-yl]ethylamine (Aldrich, USA), so-
dium azide (E. Merck, India), ammonium thiocyanate (E. Merck, In-
dia) and mercury(II) acetate (E. Merck, India) were purchased from
respective concerns and used as received. All other chemicals and
solvents were AR grade and used as received. The Schiff bases L1
and L2 were prepared following the reported methods [38,39] de-
scribed elsewhere. The synthetic reactions and work-up were done
in open air.
Complexes 1–3 were prepared using 2:1:4 molar ratios of mer-
cury(II) acetate, Schiff bases (L1 and L2) and pseudohalides (azide
and thiocyanate) in methanolic solutions at room temperature.
2.3.1. [Hg2(L1)(SCN)4] (1)
A methanolic solution (15 cm3) of L1 (0.390 g, 1 mmol) was
added dropwise to a solution of Hg(OAc)2 (0.636 g, 2 mmol) in
the same solvent (10 cm3) followed by NH4NCS (0.304 g, 4 mmol)
in MeOH (35 cm3). The light yellow solution was filtered and the
supernatant liquid was kept in air for slow evaporation. After a
few days single crystals of 1 that separated, were filtered, washed
with toluene and dried in vacuo over silica gel indicator. Yield:
0.77 g (75%). Anal. Calc. for C30H22N8S4Hg2 (1): C, 35.17; H, 2.16;
N, 10.98. Found: C, 35.11; H, 2.14; N, 10.91%. IR (KBr, cmꢀ1):
Caution! Mercury and its compounds are toxic [40]. Azido com-
pounds of metal ions are potentially explosive especially in the
presence of organic ligands. Only a small amount of these materials
should be prepared and handled with care.
2.2. Physical measurements
m
(C–H) 2965, 2920;
m(N@C@S) 2110, 2096; m(C–S) 760; m(C@N)
1630;
m
(C@C) 1588. KM (DMF, ohmꢀ1 cm2 molꢀ1): 5; UV–Vis (k,
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed on a Perkin–Elmer 2400 CHNS/O elemental analyzer. IR
spectra (KBr discs, 4000–300 cmꢀ1) were recorded using a Per-
kin–Elmer FTIR model RX1 spectrometer. Thermal behaviour was
investigated with a Perkin–Elmer Diamond TG/DT analyzer heated
from 30 to 700 °C under nitrogen. Molar conductances were mea-
sured using a Systronics conductivity meter where the cell con-
stant was calibrated with 0.01(M) KCl solution and dry DMF was
nm): 269.
2.3.2. [Hg2(L2)(N3)4] (2) and [Hg2(L2)(SCN)4] (3)
Synthesis of 2 was carried out by adding slowly a methanolic
solution of (35 cm3) NaN3 (0.260 g, 4 mmol) to a mixture contain-
ing L2 (0.351 g, 1 mmol) and Hg(OAc)2 (0.636 g, 2 mmol) in the
same solvent (25 cm3). The light yellow solution was filtered and
Table 1
Crystallographic data for 1–3.
Crystal parameters
1
2
3
Formula
C30H22N8S4Hg2
1024.48
monoclinic
P21/n
8.8680(19)
11.321(2)
16.570(3)
90
95.383(2)
90
C20H26N18Hg2
919.76
C24H26N10S4Hg2
984.59
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
triclinic
ꢀ
ꢀ
P1
P1
8.1312(2)
8.6701(2)
10.1045(2)
78.7620(10)
81.9090(10)
88.912(2)
691.71(3)
0.71073
2.208
8.7291(2)
8.7673(2)
10.2219(2)
74.2200(10)
79.3340(10)
87.6380(10)
739.77(3)
a
(°)
b (°)
c
(°)
V (Å3)
k (Å)
1656.2(6)
0.71073
2.053
0.71073
2.209
q
(g cmꢀ3
)
Z
2
1
1
Crystal size (mm)
0.12 ꢂ 0.13 ꢂ 0.22
0.40 ꢂ 0.30 ꢂ 0.24
0.40 ꢂ 0.16 ꢂ 0.07
T (K)
296(2)
293(2)
293(2)
l
(mmꢀ1
)
9.545
11.132
10.681
F (0 0 0)
964
432
464
h Ranges (°)
h/k/l
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
2.18–23.31
ꢀ9, 9/ꢀ12, 12/ꢀ18, 18
12 043
2381 (0.079)
2381/0/199
0.975
2.1–30.0
2.1–32.5
ꢀ11, 11/ꢀ12, 12/ꢀ14, 13
ꢀ13, 13/ꢀ13, 13/ꢀ15, 15
12 177
19 744
)
4023 (0.030)
4023/0/181
1.032
5333 (0.027)
5333/0/181
1.088
Final R indices [I > 2
r
(I)]
R = 0.0363, WR = 0.0651
R = 0.0677, WR = 0.0729
0.580 and ꢀ1.104
R = 0.0382, WR = 0.0855
R = 0.0539, WR = 0.0926
1.59 and ꢀ0.45
R = 0.0272, WR = 0.0625
R = 0.0344, WR = 0.0662
2.82 and ꢀ0.93
R indices (all data)
Largest peak and hole (eÅꢀ3
)
h
i
1=2
P
P
P
P
2
2
2
Weighting Scheme:
kFojꢀjFck= jFoj; wR ¼
wðF2o ꢀFc2Þ2= wðFo2
Þ
; Calc:w ¼ 1=½r2ðFo2Þþð0:0304PÞ þ0:0000Pꢃð1Þ; Calc:w ¼ 1=½r2ðF2o Þþð0:0485PÞ þ0:0000Pꢃð2Þ; Calc:w ¼
2
1=½r2ðF2o Þþð0:0343PÞ þ0:6167Pꢃð3Þ whereP ¼ ðFo2 þ2F2c Þ=3.

S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
1669
the supernatant filtrate was kept in open air for slow evaporation.
2.4. X-ray crystallographic analyses
After a few days, single crystals of 2 that separated, were processed
as above in 1. Yield: 0.64 g (70%). Complex 3 was prepared using a
similar procedure and reaction stoichiometry as described above
except that NH4SCN (0.304 g, 4 mmol) instead of NaN3 was used
as source of pseudohalide. Yield: 0.72 g (73%). Anal. Calc. for
C20H26N18Hg2 (2): C, 26.12; H, 2.85; N, 27.53. Found: C, 26.19; H,
Single crystals of 1–3 suitable for X-ray analyses were se-
lected from those obtained by slow evaporation of methanolic
solutions of the reaction mixtures at 298 K. Diffraction data were
collected (at 296 K for 1, 293 K for 2 and 3) on a Bruker SMART
APEX-II CCD area-detector diffractometer using graphite mono-
2.81; N, 27.44%. IR (KBr, cmꢀ1):
m
(C–H) 2960, 2917;
m
(N@N@N)
chromated Mo Ka radiation (k = 0.71073 Å). For unit cell deter-
2065, 2053;
m
(C@N) 1637;
m
(C@C) 1592. KM (DMF, ohmꢀ1 cm2
mination, the single crystal was exposed with X-ray for 10 s in
three frames. The detector frames were integrated by use of
the program SAINT and absorption corrections were performed
with SADABS [41]. The structures were solved by direct methods,
using the SHELXS-97 [42] program, and refined by full-matrix
least-squares methods based on F2 using the program SHELXL-97
[42]. All atomic displacement parameters for non-hydrogen
atoms have been refined with anisotropic term. For all struc-
tures, the hydrogen atom positions were calculated and they
were constrained to idealized geometries and treated as riding
where the H atom displacement parameter was calculated from
the equivalent isotropic displacement parameter of the bound
atom. The CIF should read ‘constr’ rather than ‘‘mixed” in the
hydrogen atom treatment flag. All calculations were carried out
using SHELXL-97, SHELXS-97, PLATON [43] and ORTEP-3 [44] programs.
Further details are given in Table 1.
molꢀ1): 5; UV–Vis (k, nm): 266. Anal. Calc. for C24H26N10S4Hg2
(3): C, 29.29; H, 2.66; N, 14.29. Found: C, 29.18; H, 2.61; N,
14.18%. IR (KBr, cmꢀ1):
2095; (C–S) 769;
cm2 molꢀ1): 4; UV–Vis (k, nm): 268.
m
(C–H) 2962, 2917;
m(N@C@S) 2109,
m
m
(C@N) 1635;
m
(C@C) 1592. KM (DMF, ohmꢀ1
3. Results and discussion
3.1. Synthesis and formulation
The tetracoordinated dinuclear compound 1 was formed using
both a 1:1:2 and a 2:1:4 ratios of Hg(OAc)2, L1 and NH4NCS. How-
ever, the latter gives a better yield. In an effort to prepare dicationic
dinuclear species of the composition [Hg(L1)(SCN)]2(ClO4)2 or poly-
cationic polynuclear species of the type [Hg(L1)(SCN)]n(ClO4)n with
use of a 1:1:1:1 molar ratio of Hg(OAc)2, L1, NH4NCS and NaClO4,
produced 1 with lower yield; microanalyses once again showed a
2:1:4 ratio of Hg, L1 and pseudohalide, and in the IR spectrum,
no presence of perchlorate bands was noticed. Changing the tetra-
dentate Schiff base L1 to hexadentate L2 and use of 2:1:4 molar ra-
tios of Hg(OAc)2, L2 and pseudohalides (X = Nꢀ3 , NCSꢀ) afforded 2/3
as is the case in good yield. With a view to getting dicationic dinu-
clear compound of the composition [Hg2(L2)(X)2](ClO4)2 reaction of
Fig. 1. ORTEP diagram of
probability for all the non-hydrogen atoms.
1 with atom numbering scheme and 20% ellipsoid
Fig. 2. A perspective view of 2D sheet structure in 1 formed through cooperative C–Hꢁ ꢁ ꢁN and C–Hꢁ ꢁ ꢁS hydrogen bonds.

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S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
Fig. 3. (a) A view of C–Hꢁ ꢁ ꢁ
p
interactions extended along crystallographic a-axis and (b) 3D network structure formed through cooperative C–Hꢁ ꢁ ꢁN and C–Hꢁ ꢁ ꢁS hydrogen
bonds and C–Hꢁ ꢁ ꢁ
p interactions in 1.
Fig. 4. Molecular structure of 2 (ORTEP, 20% ellipsoids).
a 2:1:2:2 molar ratio of Hg(OAc)2, L2, NaN3/NH4NCS and NaClO4
was performed which once again resulted in 2/3 in albeit lower
yield. All the reactions were reproducible as was evident from
repetitive microanalytical results, solution electrical conductivity
data, thermal results and spectral behaviours. The reproducibility
reflects inherent tendencies towards formation of 1, 2 and 3. The
main synthetic steps are summarized in Eq. (1):
The complexes were characterized using microanalyses (C, H
and N), solution electrical conductivity measurements, spectro-
scopic and thermal investigations. The microanalytical data are in
good conformity with formulations of the compounds. The air-
stable, moisture-insensitive complexes are soluble in a range of
common organic solvents such as methanol, ethanol, dichloro-
methane, acetonitrile, dimethylformamide and dimethylsulphox-
ide, but are insoluble in water. In DMF solutions they behave as
non-electrolytes, as reflected from their low conductivity values
h
ꢀ
ꢁ
i
MeOH
!
2 HgðIIÞ salt þ L1=L2 þ 4 Xꢀ
Hg2 L1=L2 ðXÞ4
(ꢄ5
X mas(X) stretches appear as strong
ꢀ1 cm2 molꢀ1) [45]. In IR,
L1 : X ¼ NCSꢀð1Þ
bands at 2110, 2096 cmꢀ1 in 1, at 2065, 2053 cmꢀ1 in 2 and at
2109, 2095 cmꢀ1 in 3. The distinct splittings indicate mutual cis
alignment of the two pseudohalides around each mercury(II)
L2 : X ¼ N3ꢀð2Þ;
X ¼ NCSꢀð3Þ
ð1Þ

S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
1671
Fig. 5. ORTEP representation of 3 with 30% ellipsoid probability.
Fig. 6. Supramolecular 2D layer structure in 2 formed by double C–Hꢁ ꢁ ꢁN hydrogen bondings.
[46]. A band corresponding to the m(C–S) stretching frequency ap-
pears at 760 and 769 cmꢀ1 in 1 and 3, respectively. The bands
belonging to the deformation frequency d(NCS) are found as dou-
blets at 468 and 460 cmꢀ1 in 1 and 463 and 458 cmꢀ1 in 3. The
Schiff bases, L1 and L2, in the metal bound states exhibit
m(C@N)
plus
m
(C@C) stretching vibrations at ꢄ1640 and ꢄ1590 cmꢀ1. Weak
bands in the range 2980–2900 cmꢀ1 are assignable to the aliphatic
C–H stretching frequency. All other characteristic organic ligand
vibrations are seen in the range 1600–600 cmꢀ1. DMF solutions
of the complexes 1–3 are very faint yellow and exhibit absorptions
at 269, 266 and 268 nm, respectively assignable to ligand based n–
p*/p–p* transition. Reflectance spectrum (in nujol) and electronic
spectrum (in DMF) [47] are akin in each complex reflecting similar
gross geometric and electronic structures in solid state and
solution.
3.2. Molecular and crystal structure
Single crystal X-ray structure analyses show that compounds 1–
3 consist of dinuclear (0D) units which are further engaged in
cooperative different kinds of hydrogen bondings (C–Hꢁ ꢁ ꢁN and
C–Hꢁ ꢁ ꢁS in 1 and double C–Hꢁ ꢁ ꢁN in 2 and 3) and C–Hꢁ ꢁ ꢁ
p interac-
Fig. 7. A section of 3D network structure in 2 formed through cooperative C–Hꢁ ꢁ ꢁN
tions among themselves resulting in 3D network structures (in 1
hydrogen bonds and C–Hꢁ ꢁ ꢁp interactions.

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S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
and 2) and 2D continuum (in 3). ORTEP diagrams with atom
numbering schemes and perspective views of the superstructures
in 1–3 are shown in Figs. 1–9. Selected bond distances and bond
angles relevant to the metal coordination spheres and non-cova-
lent bonding parameters are set in Tables 2–4.
geometry is due to asymmetric nature of the Schiff base and
deviations of the refine angles (109.5°). Hg–N(Schiff base) and
Hg–S(thiocyanate) distances lie within a close range of 2.311(7)–
2.440(7) Å and 2.393(3)–2.508(3) Å, respectively. Quasi-linear
thiocyanato groups are ligated to mercury(II) in a bending fashion
with relevant bond angles Hg(1)–S(2)–C(3) 100.3(4)° and
Hg(1)–S(5)–C(6) 96.7(4)°. S(2)–C(3) and S(5)–C(6) bond lengths
[1.661(14) Å and 1.695(13) Å] are larger than C(3)–N(4)
[1.124(13) Å] and C(6)–N(7) [1.115(12) Å], indicating coordination
of S(2) and S(5) to mercury, soft metal center.
3.2.1. [Hg2(L1)(SCN)4](1)
Compound 1 is comprised of neutral [Hg2(L1)(SCN)4] molecules
in which dinucleating tetradentate character of L1 and two pairs of
donor set arrangements bridged through ethylenic arm is evident
in Fig. 1. Each mercury(II) center adopts a distorted tetrahedral
geometry with a HgN2S2 chromophore. Hg(1) is surrounded by
two terminal thiocyanate S atoms [S(2) and S(5)] and two N atoms
[N(8) and N(12)] of the Schiff base (L1), whereas the neighbouring
Hg(1a) is ligated to the other two N donor set [N(8a) and N(12a)] of
the same L1 ligand, completing its bis(bidentate) mode and two
terminal thiocyanate S atoms [S(2a) and S(5a)]. The intramolecular
Hgꢁ ꢁ ꢁHg distance is 6.7923(9) Å. Distortion from ideal tetrahedral
The dinuclear units of 1 self-assemble through cooperative
C–Hꢁ ꢁ ꢁN and C–Hꢁ ꢁ ꢁS hydrogen bonds and C–Hꢁ ꢁ ꢁ
p interactions
(Table 2) affording a 3D supramolecular network. The uncoordi-
nated N atom (N7) and metal bound S atom (S2) of thiocyanate
and one ortho H atom (H13) of aromatic pyridine ring and one
meta H atom (H19) of phenyl ring play a vital role in the formation
of weak C–Hꢁ ꢁ ꢁN and C–Hꢁ ꢁ ꢁS hydrogen bonds leading to a 2D
sheet (Fig. 2) parallel to bc-plane. Edge-to-face C–Hꢁ ꢁ ꢁ
p
interaction
involving one ortho H atom (H22) of phenyl ring and
p-cloud of an
aromatic pyridine ring (Cg2) interconnect molecules of two adja-
cent sheets in an interwoven fashion along crystallographic a-axis
(Fig. 3) to construct 3D network.
Here, it is worth-mentioning that in corresponding cadmium(II)
analogue [38] of 1, each metal center is heptacoordinated through
tetradentate chelation behaviour of the Schiff base and end-to-end
pseudohalide bibridging along with N-coordinated terminal motifs
of thiocyanate. So, interesting change in coordination motifs of the
Schiff base from chelator to bis(bidentate) congregator and of the
pseudohalide from bridging to terminal along with variation of
coordination numbers of central metal ions from 7 to 4 is revealed
through a change from 4d to 5d member of the Group 12 metal
ions. Thus, molecular architectures are completely different which
thereby influences the crystalline architectures [38].
3.2.2. [Hg2(L2)(N3)4] (2) and [Hg2(L2)(SCN)4] (3)
The structures of 2 and 3 consist of neutral [Hg2(L2)(X)4] mole-
cules, in which L2 is placed in a crystallographic inversion center.
Each five coordinated mercury(II) center in 2 and 3 adopts a dis-
torted square pyramidal geometry as exemplified by its tau param-
eter (s = 0.088 in 2 and s = 0.246 in 3) [48] with HgN5 and HgN3S2
chromophores, respectively (Figs. 4 and 5). In the centrosymmetric
dimers, two mercury(II) centers are encapsulated by bis(triden-
tate) congregation behavior of L2 through donation of three N
donor set [N(1), N(2) and N(3)] to each metal; fourth and fifth coor-
dination sites around each metal center are occupied with two N
Fig. 8. Double C–Hꢁ ꢁ ꢁN hydrogen bonded 1D chain in 3 along crystallographic
c-axis.
Fig. 9. A 2D continuum in 3 formed by interwoven C–Hꢁ ꢁ ꢁN hydrogen bonds and C–Hꢁ ꢁ ꢁ
p interactions in ac-plane.

S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
1673
atom [S(1)] in 3. Pentacoordinated mercury(II) centers show con-
siderable deviations from ideal square pyramidal geometry pre-
sumably due to the smaller Schiff base bite angles N(1)–Hg(1)–
N(2) [68.66(16)° in 2 and 68.13(10)° in 3] and N(2)–Hg(1)–N(3)
[73.11(16)° in 2 and 72.83(9)° in 3]. Each mercury(II) center devi-
ates (0.475 Å in 2 and 0.788 Å in 3) from the mean basal plane to-
wards the apical pseudohalide. Among four basal atoms, N(1) and
N(3) deviate 0.377 and 0.320 Å in 2 and 0.293 and 0.276 Å in 3 to-
wards the apical pseudohalide, whereas remaining basal atoms de-
parted [N(2) 0.493 Å and N(4) 0.203 Å in 2 and N(2) 0.402 Å and
S(2) 0.168 Å in 3] opposite to the apical pseudohalide. The quasi-
linear pseudohalides are ligated to mercury in a bending fashion.
N(4)–N(5) and N(7)–N(8) bond lengths [1.190(7) Å and
1.186(8) Å] are larger than that [1.136(7) Å and 1.130(9) Å] in
N(5)–N(6) and N(8)–N(9) confirming coordination of N(4) and
N(7) to the mercury(II) center in 2. S(1)–C(11) [1.650(4) Å] and
S(2)–C(12) [1.668(4) Å] bond lengths are larger than that C(11)–
N(4) [1.150(5) Å] and C(12)–N(5) [1.144(5) Å] indicating coordina-
tion of S(1) and S(2) to soft mercury(II) in 3.
The dinuclear units in 2 pack alongside each other to give a 2D
layer structure (Fig. 6) through double C–Hꢁ ꢁ ꢁN hydrogen bonds
involving non-coordinated N atoms (N6 and N9) of terminal azides,
formylidene (–CH@N–) hydrogen (H6A) and ethylenic hydrogen
(H7B) of L2. These 2D layers are interconnected by C–Hꢁ ꢁ ꢁ
p interac-
tions along a-axis affording a 3D network structure (Fig. 7). One
alkylenic hydrogen (H9A) of bridging ring of L2 and
the pyridine ring (Cg3) play a vital role in the formation of such
interaction. The packing diagram of 3 shows two types
p-cloud of
C–Hꢁ ꢁ ꢁ
p
of C–Hꢁ ꢁ ꢁN hydrogen bonds to form a 1D chain along crystallo-
graphic c-axis (Fig. 8). The adjacent 1D chains connect each other
by edge-to-face C–Hꢁ ꢁ ꢁ
resulting in a 2D continuum in ac plane (Fig. 9). The non-coordi-
nated N atom (N4) of terminal thiocyanate and -cloud of the
p interaction in an interwoven fashion
p
pyridine ring (Cg3) act as potential acceptors whereas formylidene
(–CH@N–) hydrogen (H6A), ethylenic hydrogen (H8A) and one
alkylenic hydrogen (H9A) of bridging ring of L2 are the potential
donors.
Fig. 10. Thermal behaviours of: (a) compound 1 and (b) compound 3.
Interestingly, in corresponding cadmium(II) analogues [39] of 2
and 3, each metal center is hexacoordinated through bis(triden-
tate) congregation behaviour of the Schiff base along with two ter-
minal azides and one coordinated water molecule in the azido
analogue and two end-to-end bibridging plus one N-coordinated
terminal thiocyanate units in the thiocyanato derivative. Again,
an interesting change in coordination motifs of pseudohalides
and coordination numbers of central metal ions from 6 to 5 is
atoms [N(4) and N(7)] of terminal azides in 2 and two S atoms [S(1)
and S(2)] of terminal thiocyanates in 3. In the X2Hg-L2-HgX2 moi-
eties the intramolecular Hgꢁ ꢁ ꢁHg distances are 6.5880(4) Å in 2 and
6.7234(2) Å in 3. The basal dimeric plane consists of six N atoms of
L2 along with two terminal azide N atoms in 2 and two thiocyanate
S atoms in 3, while the apical position of each mercury(II) center is
occupied by remaining azide N atom [N(7)] in 2 and thiocyanate S
Fig. 11. Emission spectra of: (a) L1 and 1 and (b) L2, 2 and 3 (fluorescence in DMF solutions at 298 K).

1674
S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
Table 2
Table 4
Selected bond distances (Å) and bond angles (°) for 1.
Selected bond distances (Å) and bond angles (°) for 3.
Bond distances
Bond distances
Hg(1)–N(1)
Hg(1)–N(2)
Hg(1)–N(3)
Hg(1)–S(1)
Hg(1)–S(2)
Hg(1)–N(12)
Hg(1)–N(8)
Hg(1)–S(2)
Hg(1)–S(5)
2.311(7)
2.440(7)
2.393(3)
2.508(3)
S(2)–C(3)
S(5)–C(6)
N(4)–C(3)
C(6)–N(7)
1.661(14)
1.695(13)
1.124(13)
1.115(12)
2.554(3)
2.288(3)
2.580(3)
2.5010(10)
2.4943(9)
S(1)–C(11)
N(4)–C(11)
S(2)–C(12)
N(5)–C(12)
1.650(4)
1.150(5)
1.668(4)
1.144(5)
Bond angles
N(12)–Hg(1)–S(2)
N(12)–Hg(1)–N(8)
N(12)–Hg(1)–S(5)
N(8)–Hg(1)–S(5)
N(8)–Hg(1)–S(2)
S(2)–Hg(1)–S(5)
138.1(2)
70.0(2)
95.30(19)
100.96(19)
107.97(19)
125.11(11)
C(6)–S(5)–Hg(1)
C(3)–S(2)–Hg(1)
N(4)–C(3)–S(2)
N(7)–C(6)–S(5)
96.7(4)
Bond angles
100.3(4)
177.3(12)
178.6(12)
S(1)–Hg(1)–S(2)
S(1)–Hg(1)–N(1)
S(1)–Hg(1)–N(2)
S(1)–Hg(1)–N(3)
S(2)–Hg(1)–N(1)
C(11)–S(1)–Hg(1)
N(4)–C(11)–S(1)
112.86(4)
86.60(7)
S(2)–Hg(1)–N(2)
S(2)–Hg(1)–N(3)
N(1)–Hg(1)–N(2)
N(1)–Hg(1)–N(3)
N(2)–Hg(1)–N(3)
C(12)–S(2)–Hg(1)
N(5)–C(12)–S(2)
126.09(8)
99.55(7)
68.13(10)
140.84(9)
72.83(9)
118.51(8)
115.39(7)
100.71(7)
104.95(15)
176.3(4)
100.42(13)
175.3(4)
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
Hydrogen bond distances (Å) and angles (°) for 1
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
C19–H19ꢁ ꢁ ꢁS2a
0.93
0.93
2.86
2.55
3.734(12)
3.260(14)
157
134
C13–H13ꢁ ꢁ ꢁN7b
Hydrogen bond distances (Å) and angles (°) for 3
C6–H6A...N4a
C8–H8A...N4b
0.9300
0.9700
2.4100
2.6200
3.315(5)
3.530(6)
165.00
156.00
C–Hꢁ ꢁ ꢁp Interactions bond donor/acceptor scheme (Å, °) for 1
C22–H22ꢁ ꢁ ꢁCg(2)c
0.93
2.85
3.674(11)
148
C–Hꢁ ꢁ ꢁp Interactions bond donor/acceptor scheme (Å, °) for 2
Symmetry code.
C10–H10A...Cg(3)c
Symmetry code.
0.93
2.97
3.658(4)
129
a
x, ꢀ1 + y, z.
b
3/2ꢀx, 1/2 + y, 3/2 ꢀ z.
c
a
1 ꢀ x, ꢀy, 2 ꢀ z.
x, y, ꢀ1 + z.
b
2 ꢀ x, ꢀy, 1 ꢀ z.
c
1 + x, y, z.
Table 3
Selected bond distances (Å) and bond angles (°) for 2.
Table 5
Bond distances
Photophysical data.
Hg(1)–N(1)
Hg(1)–N(2)
Hg(1)–N(3)
Hg(1)–N(4)
Hg(1)–N(7)
2.540(5)
2.274(4)
2.586(5)
2.158(5)
2.270(6)
N(4)–N(5)
1.190(7)
1.136(7)
1.186(8)
1.130(9)
N(5)–N(6)
N(7)–N(8)
N(8)–N(9)
Sample
Absorption (k, nm)
Emission (k, nm)
Life times (ns)
2.52
L1
1
272
269
271
266
268
378
373
387
372
375
L2
2
Bond angles
2.56
2.54
N(1)–Hg(1)–N(2)
N(1)–Hg(1)–N(3)
N(1)–Hg(1)–N(4)
N(1)–Hg(1)–N(7)
N(2)–Hg(1)–N(3)
N(5)–N(4)–Hg(1)
N(8)–N(7)–Hg(1)
68.66(16)
139.96(15)
98.5(2)
N(2)–Hg(1)–N(4)
N(2)–Hg(1)–N(7)
N(3)–Hg(1)–N(4)
N(3)–Hg(1)–N(7)
N(4)–Hg(1)–N(7)
N(6)–N(5)–N(4)
N(9)–N(8)–N(7)
134.7(2)
112.1(2)
117.1(2)
91.6(2)
111.6(3)
175.1(6)
176.2(7)
3
91.9(2)
73.11(16)
120.3(4)
117.0(4)
decomposition behaviours of 2 and 3 are similar, and both are sta-
ble upto 200 °C. TG curve (Fig. 10b) shows release of four pseud-
ohalides and one L2 molecule at 200–455 °C with exothermic
effects at 140 °C, 210 °C and 446 °C. In summary, the thermogravi-
metric analyses reveal that 2 and 3 have greater thermal stability
than 1.
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
Hydrogen bond distances (Å) and angles (°) for 2
C6–H6A...N6a
C7–H7B...N9b
0.93
0.97
2.41
2.57
3.336(8)
3.423(11)
171
146
C–Hꢁ ꢁ ꢁp Interactions bond donor/acceptor scheme (Å, °) for 2
3.4. Luminescence properties
C9–H9A...Cg(3)c
Symmetry code.
0.93
2.82
3.5257(67)
130
In DMF solutions at room temperature the photoluminescence
spectra of free ligands (L1 and L2) and corresponding mercury(II)
pseudohalide complexes (1–3) are depicted in Fig. 11 and spectro-
scopic data are listed in Table 5. Upon photoexcitation at the cor-
responding absorption bands in DMF solutions free ligands
exhibit broad fluorescent emission centered on 378 nm (for L1)
and 387 nm (for L2); whereas, the corresponding mercury(II)pseu-
dohalide complexes 1–3 show albeit blue-shifted but more intense
photoluminescence with the main emissions, respectively at 373,
372 and 375 nm. The lifetimes are in the range 2.52–2.56 ns. The
a
x, y, ꢀ1 + z.
b
x, ꢀ1 + y, z.
c
1 + x, y, z.
observed through such change in 4d–5d member of group 12.
Consequently, molecular and crystalline architectures are com-
pletely different.
appearance of luminescence [49,50] in the complexes may be
3.3. Thermogravimetric analysis
1
attributed to the intraligand
(p–p*) transition and the blue shift
as well as greater intensity may presumably be due to the increase
in conformational rigidity of the ligands upon coordination.
To examine thermal stabilities of the compounds 1–3, thermo-
gravimetric (TG) and differential thermal analyses (DTA) were
made between 30 and 700 °C in the static atmosphere of N2. Com-
pound 1 is stable upto 128 °C at which temperature decomposition
commences. The TG curve of compound 1 indicates the release of
one thiocyanate anion (weight loss: observation 6.71% and calc.
5.68%) at 128–136 °C in the first step and remaining three thiocy-
anate anions and L1 molecule at 200–440 °C in the second step
with one exothermic effect at 220 °C (Fig. 10a). The thermal
4. Conclusion
Three new photoluminous coordination molecules have been
synthesized and X-ray crystallographically characterized where
classical tetra- and hexa-dentate Schiff bases respectively behave
as bis(bidentate) and bis(tridentate) congregators bridging two

S. Chattopadhyay et al. / Polyhedron 29 (2010) 1667–1675
1675
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Financial support from the DST, CSIR and UGC, New Delhi, India
is gratefully acknowledged. SC is grateful to the UGC and KB, SD
and SS, to the CSIR, New Delhi, India for fellowships. The authors
also acknowledge the use of DST-funded National Single Crystal
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