Syntheses, X-ray structures and properties of a dinuclear cadmium(II)azido and a polymeric cadmium(II)thiocyanato compounds containing bis(tridentate) congregators

Article abstract of DOI:10.1016/j.poly.2007.07.029

One-pot reactions of cadmium(II) perchlorate/nitrate, Schiff bases (pbap/pfap) and pseudohalides (sodium azide/ammonium thiocyanate) in a 2:1:4 molar ratio in MeOH-MeCN solvent mixtures at room temperature result in a dinuclear compound [Cd₂(pbap)(OH₂)₂(N₃)₄] (1) [pbap = N-(1-pyridin-2-ylbenzylidene)-N-[2-(4-{2-[(1-pyridin-2-ylbenzylidene)amino]ethyl}piperazin-1-yl)ethyl]amine] and a polymeric compound [Cd₂(pfap)(μ_1,3-NCS)(μ_1,3-SCN)(NCS)₂]_n (2) [pfap = N-(1-pyridin-2-ylformylidene)-N-[2-(4-{2-[(1-pyridin-2ylformylidene)amino]ethyl}piperazin-1-yl)ethyl]-amine]. X-ray crystal structural analyses reveal a bis(tridentate) congregation behaviour of the hexadentate blocker (pbap/pfap) encapsulating two metal centers. Each cadmium(II) center in 1 and 2 is in a distorted octahedral geometry with CdN₅O and CdN₅S chromophores, respectively. In 1, the dinuclear units participate in intermolecular O-H?N hydrogen bonding between bound water O atoms and terminal azide N atoms, in combination with C-H?π interactions, resulting in a 3D supramolecular network with an intramolecular Cd?Cd distance of 6.473(2) A?. In the crystal lattice, the covalent 1D chain of 2 is further engaged in face-to-face π?π interactions from two terminal pyridine rings, which stabilizes the chain with an intradimer Cd?Cd separation of 6.640(5) A?. Both the complexes display intraligand ¹(π-π^*) fluorescence and intraligand ³(π-π^*) phosphorescence in glassy solutions.

Full text of DOI:10.1016/j.poly.2007.07.029

Polyhedron 26 (2007) 5023–5029
www.elsevier.com/locate/poly
Syntheses, X-ray structures and properties
of a dinuclear cadmium(II)azido and a polymeric
cadmium(II)thiocyanato compounds containing bis(tridentate)
congregators
*
Habibar Chowdhury, Rajarshi Ghosh, Sk. Hafijur Rahaman, Barindra Kumar Ghosh
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India
Received 1 May 2007; accepted 9 July 2007
Available online 5 September 2007
Abstract
One-pot reactions of cadmium(II) perchlorate/nitrate, Schiff bases (pbap/pfap) and pseudohalides (sodium azide/ammonium thiocya-
nate) in a 2:1:4 molar ratio in MeOH–MeCN solvent mixtures at room temperature result in a dinuclear compound [Cd2(pbap)(OH2)2(N3)4]
(1) [pbap = N-(1-pyridin-2-ylbenzylidene)-N-[2-(4-{2-[(1-pyridin-2-ylbenzylidene)amino]ethyl}piperazin-1-yl)ethyl]amine] and a poly-
meric compound [Cd2(pfap)(l1,3-NCS)(l1,3-SCN)(NCS)2]n (2) [pfap = N-(1-pyridin-2-ylformylidene)-N-[2-(4-{2-[(1-pyridin-2ylformylid-
ene)amino]ethyl}piperazin-1-yl)ethyl]-amine]. X-ray crystal structural analyses reveal a bis(tridentate) congregation behaviour of the
hexadentate blocker (pbap/pfap) encapsulating two metal centers. Each cadmium(II) center in 1 and 2 is in a distorted octahedral geometry
with CdN5O and CdN5S chromophores, respectively. In 1, the dinuclear units participate in intermolecular O–Hꢀ ꢀ ꢀN hydrogen bonding
between bound water O atoms and terminal azide N atoms, in combination with C–Hꢀ ꢀ ꢀp interactions, resulting in a 3D supramolecular
˚
network with an intramolecular Cdꢀ ꢀ ꢀCd distance of 6.473(2) A. In the crystal lattice, the covalent 1D chain of 2 is further engaged in
face-to-face pꢀ ꢀ ꢀp interactions from two terminal pyridine rings, which stabilizes the chain with an intradimer Cdꢀ ꢀ ꢀCd separation of
1
3
˚
6.640(5) A. Both the complexes display intraligand (p–p*) fluorescence and intraligand (p–p*) phosphorescence in glassy solutions.
ꢀ 2007 Published by Elsevier Ltd.
Keywords: Cadmium(II) azide/thiocyanate; Bis(tridentate) Schiff base; Superstructure; Luminescence
1. Introduction
approach to construct such frameworks. We are interested
[11,12] in this approach in the construction of functional
polymers through variation of ligand backbones and
metal–ion coordination environments. Pseudohalides [13]
have long been known for their versatile coordination
modes. Schiff bases [14] are useful blockers because of their
preparative accessibilities, structural varieties and varied
denticities. The investigation of polynuclear complexes of
cadmium(II) is an important objective because of study
of the electronic and optoelectronic properties [15] of this
4d10 metal ion. This work stems from our interest to pre-
pare different coordination polymers of cadmium(II) in
combination with pseudohalides and Schiff bases. In this
paper we report the synthesis, characterization, structure
The construction [1] of metal–organic coordination
frameworks has been extensively examined for the prepara-
tion of functional materials [2–4] of different shapes and
sizes [5] through control and manipulation of metal–ligand
covalent bonds [6] with malleable coordination spheres and
multiple lateral non-covalent forces like hydrogen bonding
[7], pꢀ ꢀ ꢀp [8] and C–Hꢀ ꢀ ꢀp [9] interactions. Self-assembly
[10] of the building components, such as metal–ion tem-
plates, organic blockers and bridging units, is an effective
*
Corresponding author. Tel.: +91 342 2533913; fax: +91 342 2530452.
E-mail address: barin_1@yahoo.co.uk (B.K. Ghosh).
0277-5387/$ - see front matter ꢀ 2007 Published by Elsevier Ltd.
doi:10.1016/j.poly.2007.07.029

5024
H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
a time-correlated single photon counting (TCSPC) spec-
R
R
N
N
N
N
C
C
trometer 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.
N
N
R = Ph, pbap
R = H, pfap
2.3. Preparation of the Schiff bases
Scheme 1.
2.3.1. N-(1-Pyridin-2-ylbenzylidene)-N-[2-(4-{2-[(1-
pyridin-2-ylbenzylidene)amino]ethyl}piperazin-1-
yl)ethyl]amine (pbap)
and luminescence behaviour of a dinuclear compound of
the type [Cd2(pbap)(OH2)2(N3)4] (1) [pbap = N-(1-pyri-
din-2-ylbenzylidene)-N-[2-(4-{2-[(1-pyridin-2-ylbenzy-
lidene)amino]ethyl}piperazin-1-yl)ethyl]amine (Scheme
1)] and a polymeric compound of the type [Cd2(pfap)-
(l1,3-NCS)(l1,3-SCN)(NCS)2]n (2) [pfap = N-(1-pyridin-2-
ylformy-lidene)-N-[2-(4-{2-[(1-pyridin-2ylformy-lid-
ene)amino]ethyl}piperazin-1-yl)ethyl]amine (Scheme 1)]
in combination with tailored hexadentate N-donor Schiff
bases (pbap/pfap) and azide/thiocyanate.
2-[4-(2-Aminoethyl)piperazin-1-yl]ethylamine (0.172 g,
1 mmol) with 2-benzoylpyridine (0.366 g, 2 mmol) was
refluxed in dehydrated alcohol in a 1:2 molar ratio. After
10 h reflux the reaction solution was evaporated under
reduced pressure to a gummy mass, which was dried and
stored in vacuo over CaCl2 for subsequent use; yield,
1.928 g (70%). IR (KBr disc, cmꢁ1): m(C@N) 1594. UV–Vis
(kmax, nm): 245, 293.
2.3.2. N-(1-Pyridin-2-ylformylidene)-N-[2-(4-{2-[(1-
pyridin-2ylformylidene)amino]ethyl}pipera-zin-1-
yl)ethyl]amine (pfap)
2. Experimental
2.1. Materials
2-[4-(2-Aminoethyl)piperazin-1-yl]ethylamine (0.172 g,
1 mmol) and pyridine-2-carboxaldyde (0.214 g, 2 mmol)
were mixed together and refluxed in anhydrous ethanol
(25 cm3) for 10 h. The yellowish brown solution that resulted
was concentrated on a steam bath and dried in vacuo over
CaCl2 to a yellowish brown semi-solid for subsequent use.
Yield: 1.525 g (75%). IR (KBr disc, cmꢁ1): m(C@N) 1592.
UV–Vis (kmax, nm): 242, 290.
High purity 2-benzoylpyridine (Fluka, Germany), pyri-
dine-2-carboxaldyde (Fluka, Germany), 2-[4-(2-amino-
ethyl)piperazin-1-yl]ethylamine (Aldrich, USA), sodium
azide (Aldrich, USA), ammonium thiocyanate (E. Merck,
India) and cadmium(II) nitrate tetrahydrate (E. Merck,
India) were purchased from the respective concerns and
were used as received. Cadmium(II) perchlorate hexahy-
drate was prepared by treatment of cadmium(II) carbonate
(E. Merck, India) with perchloric acid (E. Merck, India)
followed by slow evaporation on steam-bath, filtration
through a fine glass-frit and preservation in a desiccator
containing concentrated sulfuric acid (E. Merck, India)
for subsequent use. All other chemicals and solvents were
AR grade and were used as received.
2.4. Preparation of the complexes
2.4.1. [Cd2(pbap)(OH2)2(N3)4] (1)
Pbap (0.476 g, 1 mmol) dissolved in methanol (20 cm3)
was added to an acetonitrile solution (20 cm3) of cadmium
perchlorate hexahydrate (0.622 g, 2 mmol) with continuous
stirring. To this, a methanolic solution (20 cm3) of sodium
azide (0.260 g, 4 mmol) was added. The resulting light yellow
solution was filtered and kept for slow evaporation. After 7
days colourless crystals were separated, washed with toluene
and dried in vacuo over silica gel indicator. Yield: 0.823 g
(50%). Anal. Calc. for C32H38N18O2Cd2 (1): C, 41.3; H,
4.0; N, 27.0. Found: C, 41.2; H, 4.1; N, 27.1%. IR (KBr,
cmꢁ1): m(N3) 2070; m(OH) 3300–3500 (broad). UV–Vis
(k, nm) 340. KM (MeOH, X ꢁ1 cm2 molꢁ1): 5.
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were
measured using a Perkin–Elmer 2400 CHNS/O elemental
analyser. IR spectra (KBr discs, 4000–300 cmꢁ1) were
recorded using a Perkin–Elmer FTIR model RX1 spectrom-
eter. Molar conductances were measured using a Systronics
conductivity meter where the cell constant was calibrated
with 0.01 (M) KCl solution and DMF was used as the sol-
vent. Thermal analysis was recorded using a Perkin–Elmer
Diamond TG/DTA system under a flow of nitrogen (30
ml minꢁ1). The sample was heated at a rate of 10 ꢁC minꢁ1
with inert alumina as a reference. Ground state absorption
and steady-state fluorescence measurements were made with
a Jasco model V-530 UV–Vis spectrophotometer and Hit-
achi model F-4010 spectrofluorimeter, respectively. Time-
resolved fluorescence measurements were carried out using
2.4.2. [Cd2(pfap)(l1,3-NCS)(l1,3-SCN)(NCS)2]n (2)
An acetonitrile solution (5 cm3) of pfap (0.175 g, 1 mmol)
was added dropwise to a solution of Cd(NO3)2 Æ 4H2O
(0.308 g, 2 mmol) in methanol (5 cm3). NH4NCS (0.152 g,
4 mmol) in methanol (5 cm3) was added slowly to this solu-
tion. The resultant solution was filtered and kept for slow
evaporation. After 5 days 2 was separated, washed with
methanol and dried in vacuo over CaCl2. Yield: 1.570 g
(60%). Anal. Calc. for C24H26N10S4Cd2 (2): C, 35.7; H, 3.2;

H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
5025
N, 17.4. Found: C, 35.8; H, 3.3; N, 17.3%. IR (KBr, cmꢁ1):
mas(SCN) 2105, 2125. UV–Vis (k, nm) 312. KM (MeOH,
Xꢁ1 cm2 molꢁ1): 5.
The structures were solved by direct methods and the struc-
ture solution and refinement were based on jFj . Of the 11344
2
(1) and 3523 (2) reflections, 4325 and 2381 with I > 2r(I)
were used for the structure solution. All non-hydrogen
atoms were refined with anisotropic displacement parame-
ters whereas hydrogen atoms were placed in calculated posi-
tions when possible and given isotropic U values 1.2 times
that of the atom to which they are bonded. At convergence,
the final residual were R1 = 0.0379 (1) and 0.0476 (2);
wR2 = 0.0794 (1) and 0.1003 (2) with I > 2r(I), goodness-
of-fit = 1.011 (1) and 0.996 (2). The final differences Fourier
map showed the maxiꢁm3um and minimum peak heights ꢁo3f
2.5. Crystallographic data collection and refinement
Single-crystals suitable for X-ray work were grown by
slow evaporation of a methanol–acetonitrile solution of 1
and 2 at 298 K [crystal size: 1, 0.10 · 0.10 · 0.07 mm3; 2,
0.12 · 0.05 · 0.05 mm3]. Data were collected on a Siemens
SMART CCD area diffractometer equipped with a graph-
˚
ite-monochromator Mo Ka radiation (k = 0.71073 A).
˚
˚
The intensity data were corrected for Lorentz and polariza-
tion effects and an empirical absorption correction was
employed using the SAINT [16] and SADABS [17] programs
for the crystals. Diffraction data for the crystals were mea-
sured using the u- and x-scan techniques. Systematic
absence led to the identification of space groups P2(1)/c
for 1 and C2/c for 2. A summary of the crystallographic data
and structure refinement parameters are given in Table 1.
0.934 and ꢁ0.355 e A for 1, and 1.04 and ꢁ0.42 e A
for 2. All crystallographic calculations were conducted with
SHELXL-97 [18], ORTEP-32 [19] and PLATON [20] programs.
3. Results and discussion
3.1. Synthesis and formulation
The Schiff bases, N-(1-pyridin-2-ylbenzylidene)-N-[2-(4-
{2-[(1-pyridin-2-ylbenzylidene)amino]ethyl}piperazin-1-
yl)ethyl]amine (pbap) or N-(1-pyridin-2-ylformylidene)-
N-[2-(4-{2-[(1-pyridin-2-ylformylidene)amino]ethyl}pipera-
zin-1-yl)ethyl]amine (pfap) were synthesized by refluxing
2-[4-(2-aminoethyl)piperazin-1-yl]ethylamine (1 mol) with
2-benzoylpyridine or pyridine-2-carboxaldehyde (2 mol)
in dehydrated alcohol. The reaction of cadmium(II) chlo-
rate hexahydrate/cadmium(II) nitrate tetrahydrate, pbap/
pfba and sodium azide/ammonium thiocyanate in a
2:1:4 molar ratio in MeOH–MeCN solvent mixture at
room temperature results in a dimeric and a polymeric
compound, as shown in Eqs. (1) and (2):
Table 1
Crystallographic data and structure refinement for [Cd2(pbap)-
(OH2)2(N3)4] (1) and [Cd2(pfap)(l1,3-NCS)(l1,3-SCN)(NCS)2]n (2)
Compound
1
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
C
931.60
200(2)
0.71073
monoclinic
P2(1)/c
14.7967(13)
10.4236(9)
12.4202(10)
100.515(2)ꢁ
1883.5(3)
2
1.643
1.186
936
32H38N18O2Cd2
C24H26N10S4Cd2
807.59
200(2)
0.71073
monoclinic
C2/c
27.549(3)
7.8073(8)
14.6330(15)
106.758(2)
3013.6(5)
4
˚
Space group
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
MeOH-MeCN
2 Cd(ClO4)2.6H2O + pbap + 4 NaN3
298 K
3
˚
V (A )
Z
Dcalc (Mg mꢁ3
)
1.780
1.722
1600
[Cd2(pbap)(OH2)2(N3)4] + 4 NaClO4 + 10 H2O
l (mmꢁ1
F(000)
)
(1)
ð1Þ
Crystal size (mm)
h Ranges (ꢁ)
h/k/l
0.10 · 0.10 · 0.07
2.40 to 28.27
ꢁ19,15/ꢁ11,13/
ꢁ16,16
0.12 · 0.05 · 0.05
2.72 to 26.64
ꢁ35,35/ꢁ10,8/
ꢁ18,18
MeOH-MeCN
2n Cd(NO3)2.4H2O + n pfap + 4n NH4SCN
298 K
Reflections collected
Independent reflections
11344
3523
[Cd2(pfap)(μ1,3-NCS)(μ1,3-SCN)(NCS) ] + 4n NH NO + 8n H
2O
2
n
4
3
4325 (0.0421)
2381 (0.0641)
(2)
(Rint
)
ð2Þ
Completeness to theta
(%)
92.6 (28.27ꢁ)
91.5 (28.27ꢁ)
The air-stable, moisture-insensitive compounds are
powders, soluble in a range of common organic solvents
such as methanol, ethanol, acetonitrile, dimethylformam-
ide and dimethylsulfoxide, but insoluble in water. Microan-
alytical (C, H and N) results and spectroscopic data are
useful to formulate the product. In methanolic solution
they behave as non-electrolytes, as reflected from their
low conductivity values (ꢂ5 ohmꢁ1 cm2 molꢁ1). In the IR
Data/restraints/
parameters
Tmax and Tmin
Goodness-of-fit on F2
Final R indices
[I > 2r(I)]
4325/0/252
2381/0/181
1.000 and 0.794
1.011
1.000 and 0.805
0.996
R1 = 0.0379,
wR2 = 0.0794
R1 = 0.0510,
wR2 = 0.0843
0.934 and ꢁ0.355
R = 0.0476,
wR2 = 0.1003
R = 0.0767,
wR2 = 0.1107
1.04 and ꢁ0.42
R indices (all data)
Largest peak and hole
spectra, 1 shows a strong absorption band at 2070 cmꢁ1
,
ꢁ3
˚
(e A
)
P
P
P
Weighting Scheme: R1 = jFoj ꢁ jFcjj/ jFo, wR2 = [ w(F2o ꢁ Fc2)2/
which is assignable to the stretching vibration of the termi-
nal azide m(N3), and also shows a broad band in the region
3300–3500 cmꢁ1 due to m(O–H), indicating the presence of
P
w(F2o)2]1/2, Calc. w = 1/[r2(Fo2) + (0.0495P)2 + 0.0000P] for 1 and w = 1/
[r2(Fo2) + (0.0341P)2 + 0.0000P] for 2 where P = (Fo2 + 2F2c)/3.

5026
H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
Table 2
the coordinated water molecules [21]. Two strong peaks at
2125 and 2105 cmꢁ1, assignable to mas(SCN) stretching
vibrations of the thiocyanate [21], are observed for 2. The
position (>2100 cmꢁ1) and number of signals strongly sug-
gest a mutual cis alignment as well as N- and S-coordina-
tion of the bound thiocyanates. The solutions are
colourless and exhibit absorptions at 340 nm for 1 and
312 nm for 2. Reflectance (in Nujol) and electronic (in
MeOH solution) spectra are akin in each complex, reflect-
ing a similar gross structure and electronic structure in the
solid state and in solution. The transition may correspond
to an intramolecular n–p*/p–p* charge transfer [22].
According to the thermal study (TG-DTA), compound 1
upon heating loses two molecule of water at ꢂ140 ꢁC, sug-
gesting that the water molecules are involved in coordina-
tion and in strong hydrogen bonding interactions.
˚
Selected bond distances (A) and angles (ꢁ) for 1 and 2
Compound 1
Compound 2
˚
Bond distances (A)
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–N(3)
N(4)–N(5)
2.414(3)
2.335(3)
2.459(2)
1.181(4)
1.149(5)
2.335(3)
2.243(3)
2.317(2)
1.181(4)
1.148(4)
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–N(3)
Cd(1)–N(4)
Cd(1)–N(5)
Cd(1)–S(1)
N(2)–C(2)
C(2)–S(2)
2.387(5)
2.250(5)
2.524(4)
2.295(4)
2.395(4)
2.6161(14)
1.121(7)
1.624(5)
N(5)–N(6)
Cd(1)–N(4)
Cd(1)–N(7)
Cd(1)–O(1)
N(7)–N(8)
N(8)–N(9)
Bond angles (ꢁ)
N(1)–Cd(1)–N(2)
N(1)–Cd(1)–N(3)
N(1)–Cd(1)–N(4)
N(1)–Cd(1)–N(7)
N(1)–Cd(1)–O(1)
N(2)–Cd(1)–N(3)
N(2)–Cd(1)–N(4)
N(2)–Cd(1)–N(7)
N(2)–Cd(1)–(O1)
N(3)–Cd(1)–N(7)
N(3)–Cd(1)–N(4)
N(3)–Cd(1)–O(1)
N(4)–Cd(1)–N(7)
N(7)–Cd(1)–O(1)
N(4)–Cd(1)–O(1)
N(8)–N(7)–Cd(1)
N(6)–N(5)–N(4)
N(9)–N(8)–N(7)
68.88(9)
141.95(9)
89.73(10)
95.24(10)
93.14(9)
N(1)–Cd(1)–N(2)
N(1)–Cd(1)–N(3)
N(1)–Cd(1)–N(4)
N(1)–Cd(1)–N(5)
N(1)–Cd(1)–S(1)
N(2)–Cd(1)–N(3)
N(2)–Cd(1)–N(4)
N(2)–Cd(1)–N(5)
N(2)–Cd(1)–S(1)
N(3)–Cd(1)–N(4)
N(3)–Cd(1)–N(5)
N(3)–Cd(1)–S(1)
N(4)–Cd(1)–N(5)
N(4)–Cd(1)–S(1)
N(5)–Cd(1)–S(1)
N(1)*–C(1)–S(1)
N(2)–C(2)–S(2)
177.80(18)
89.13(14)
81.86(15)
87.39(15)
93.77(11)
88.70(16)
97.85(16)
94.57(16)
86.78(12)
73.24(15)
143.66(14)
113.82(10)
70.45(15)
171.78(12)
102.51(10)
178.5(5)
3.2. X-ray crystal structures
93.14(9)
87.95(10)
164.12(10)
96.42(10)
122.47(10)
93.38(9)
86.56(9)
92.14(9)
84.05(10)
175.42(10)
127.5(2)
3.2.1. Crystal structure of [Cd2(pbap)(OH2)2(N3)4] (1)
The dinucleating hexadentate character or alternatively
bis(tridentate) view of the ligand and two pairs of donor
set arrangements through the incorporation of the ring
within the bridge, is clearly evident and is shown in
Fig. 1. Relevant bond distances and bond angles in 1 are
listed in Table 2. C–Hꢀ ꢀ ꢀp and O–Hꢀ ꢀ ꢀN hydrogen bon-
dings leading to the superstructure formation are shown
in Figs. 2–5. The structure of 1 is a centrosymmetric dimer
linked by the bis(tridentate) congregator. Cd(1) is sur-
rounded by two terminal azide nitrogen atoms [N(4),
N(7)], one coordinated water molecule [O(1)] and three N
atoms [N(1), N(2), N(3)] of the new Schiff base ligand
(pbap), whereas the neighboring Cd(1A) is ligated to the
other three N donor set atoms [N(1A), N(2A), N(3A)] of
the same pbap ligand, completing its bis(tridentate) mode,
two terminal azides [N(4A), N(7A)] and one water [O(1A)]
molecule. The basal dimeric plane consists of a six N-donor
set of the congregator and two terminal azide nitrogen
atoms, while the axial coordination sites of each cadmium
center are occupied by an azide nitrogen [N(4)/N(4A)] and
the coordinated water [O(1)/O(1A)]. The cadmium(II) cen-
ters are six-coordinate (CdN5O) with a substantial depar-
ture from an ideal octahedral geometry [cisoid angles:
178.0(5)
177.7(4)
176.4(5)
68.88(9)–122.47(10)ꢁ;
transoid
angles:
141.95(9)–
175.42(10)ꢁ]. Cd–N(pbap) distances are in the range
˚
2.335(3)–2.459(2) A, and Cd–N(azido) distances are within
˚
the range 2.243(3)–2.335(3) A. It is worth mentioning that
none of the azido anions behave as a bridging ligand; this is
an unusual azido coordination mode [15,16]. The intramo-
˚
lecular Cdꢀ ꢀ ꢀCd distance in 1 is quite large [6.473(2) A].
The azido ligands are quasi-linear with N(4)–N(5)–N(6)
and N(7)–N(8)–N(9) angles of 178.0(5)ꢁ and 177.7(4)ꢁ;
The N4–N5 and N(7)–N(8) bond lengths [1.181(4) A] are
larger than N(5)–N(6) [1.149(5) A] and N(8)–N(9)
[1.148(4) A], indicating coordination of N(4) and N(7) to
˚
˚
˚
the cadmium center.
The C–Hꢀ ꢀ ꢀp interaction joins two adjacent molecules
and constructs a supramolecular 1D chain (Fig. 2) in the
110 direction (D–Hꢀ ꢀ ꢀp: C(3)–H(3A)ꢀ ꢀ ꢀCg(1), D–H:
˚
˚
˚
0.93(3) A, Hꢀ ꢀ ꢀCg: 2.88 A, Xꢀ ꢀ ꢀCg: 3.712(5) A, < X–
Fig. 1. Thermal ellipsoid plot of the dinuclear [Cd2(pbap)(OH2)2(N3)4] (1)
with the atom labelling scheme and 20% probability ellipsoids for all non-
hydrogen atoms.
Fig. 2. C–Hꢀ ꢀ ꢀp interactions in 1 forming a 1D chain in the 110 direction.

H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
5027
D–Hꢀ ꢀ ꢀA: 177(4)ꢁ, symmetry code 2 ꢁ x, y, 2 ꢁ z) form a
2D sheet in the bc plane (Figs. 3 and 4). These hydrogen
bonds and C–Hꢀ ꢀ ꢀp interactions are interwoven to form a
3D network (Fig. 5).
3.2.2. Crystal structure of [Cd2(pfap)(l1,3-NCS)(l1,3
-
SCN)(NCS)2]n (2)
A single crystal X-ray diffraction study of 2 shows that
the crystal lattice consists of a [Cd2(pfap)(l1,3-NCS)(l1,3
-
SCN)(NCS)2]n unit (Fig. 6). The layer structure for 2,
through pꢀ ꢀ ꢀp interactions, is displayed in Fig. 7. Selected
bond distances and bond angles relevant to the metal ion
coordination sphere are given in Table 2. The polynuclear
unit of 2 contains two symmetry related cadmium(II) cen-
ters [Cd(1), Cd(1*)] connected by a Schiff base acting as a
bis(tridentate) ligand, and two end-to-end (l1,3ꢁ) bi-
bridged thiocyanate/isothiocyanate intermediaries and
one N-coordinated isothiocyanate are attached to each of
the two cadmium(II) centers completing the hexacoordina-
tion of the metal ions with a distorted octahedral CdN5S
coordination environment, which propagates forming the
1D polymeric chain. The N-coordination of the terminal
NCSꢁ ligands indicates that the three N-atoms of the Schiff
base (pfba) and the N-donation of the l1,3ꢁ bridged NCSꢁ
ligand create a hardness in cadmium(II) center that
enforces the ambidentate ligand to be bound in such a fash-
ion. Cd(1) is attached by three nitrogen atoms, viz. the
piperazine nitrogen [N(3)], the imine nitrogen [N(4)] and
the pyridine nitrogen [N(5)], of the Schiff base and two
end-to-end (l1,3ꢁ) bi-bridged nitrogen [N(1)] and sulfur
[S(1)] atoms and one terminal nitrogen [N(2)] atom of the
thiocyanate ligands, whereas the neighbouring symmetry
related Cd(1*) is ligated to the other three N donor set of
the same Schiff base, completing its bis(tridentate)mode,
and the counter ends [S(1*) and N(1*)] of two end-to-end
(l1,3ꢁ) bi-bridged thiocyanate ligands and the one terminal
nitrogen [N(2*)] of the pseudohalide. Each metal center
contains one eight-membered bridging loop and two five-
membered chelate loops. The eight-membered Cd–
(SCN)2–Cd bridging loops adopt a chair confirmation.
The equatorial plane of Cd(1) consists of three nitrogen
atoms [N(3), N(4), N(5)] of the bis(tridentate) Schiff base
and one sulfur atom [S(1)] of a bridging thiocyanate,
whereas the axial positions are occupied by a bridging
nitrogen atom [N(1)] and a terminal nitrogen atom [N(2)]
of the thiocyanate ligands. The maximum deviation of
Fig. 3. O–Hꢀ ꢀ ꢀN hydrogen bonding in 1 forming a sheet in the bc plane.
Fig. 4. Sheet formation in 1 in the bc plane through hydrogen bonding
(omitting the ligand frame for clarity).
˚
the Cd(1) center from the mean plane is 0.1713 A. The
degrees of distortion from an ideal octahedral geometry
are reflected in the cisoid [70.45(15)–113.82(10)ꢁ] and tran-
soid [143.66(14)–177.80(18)ꢁ] angles. The Cd–N [2.250(5)–
Fig. 5. O–Hꢀ ꢀ ꢀN hydrogen-bonded sheet interwoven by C–Hꢀ ꢀ ꢀp inter-
actions in 1 forming a 3D network.
˚
2.524(4) A] bond lengths are shorter than the bridging
˚
Cd–S [2.6161(14) A] distance, as expected, and are consis-
Hꢀ ꢀ ꢀCg: 147ꢁ, symmetry code: 1 ꢁ x, y, 2 ꢁ z). Two types
of O–Hꢀ ꢀ ꢀN hydrogen bonds (O(1)–H(1)ꢀ ꢀ ꢀN(4):
tent [15] with the corresponding values of the cadmium(II)
thiocyanato bridging system. The Cd(1)–N(3) [piperazine]
˚
˚
˚
Hꢀ ꢀ ꢀA:2.02(4) A, Dꢀ ꢀ ꢀA: 2.767(4) A, angle D–Hꢀ ꢀ ꢀA:
177(3)ꢁ, symmetry code x, 1/2 ꢁ y, ꢁ1/2 + z and O(1)–
distance [2.524(4) A] is longer than the Cd(1)–N(5) [pyri-
˚
dine] distance [2.395(4) A] and Cd(1)–N(4) (imine) bond
˚
˚
˚
H(2)ꢀ ꢀ ꢀN(7): Hꢀ ꢀ ꢀA: 0.72(4) A, Dꢀ ꢀ ꢀA: 2.840(4) A, angle
length [2.295(4) A]. The intradimer Cdꢀ ꢀ ꢀCd separation

5028
H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
Fig. 6. Thermal ellipsoid plot of the polynuclear [Cd2(pfap)(l1,3-NCS)(l1,3-SCN)(NCS)2]n (2) with the atom labelling scheme and 30% probability
ellipsoids for all non-hydrogen atoms.
and N(2) from the terminal isothiocyanate are in the merid-
ional position of the distorted octahedron. The thiocya-
nates are coordinated in a linear fashion as seen from the
N(1*)–C(1)–S(1) and N(2)–C(2)–S(2) angles [178.5(5) and
˚
176.4(5)ꢁ]. The N(2)–C(2) length [1.121(7) A] is shorter
˚
than C(2)–S(2) [1.624(5) A], reflecting N-coordination of
the terminal thiocyanate.
In crystal lattice the 1D covalent chain of 2 forms a layer
structure with a face-to-face pꢀ ꢀ ꢀp interaction from two ter-
(a)
minal pyridine rings [Ring(4)-ring(4)]: Cg(4)–Cg(4)
˚
sep-
˚
aration: 5.1732(3) A, vertical displacement of Cg: 4.873 A;
(a)1/2 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z; Cg(4) = N(5)–C(8)–C(9)–
C(10)–C(11)–C(12)] (Fig. 7).
3.3. Luminescence spectra
The fluorescence behaviour of 1 and 2 has been exam-
ined. The fluorescent emission was measured in MeOH
solution at 298 K. Upon photoexcitation at 340 and
312 nm an intense fluorescent band was observed with a
maximum at 465 and 410 nm and a lifetime of 2.63 and
2.58 ns for 1 and 2, respectively. The higher lifetime may
be due to the presence of stronger weak interactions, which
are mainly responsible [23,24] for fluorescence. In glassy
solutions (77 K) a red shift is observable at 535 and
Fig. 7. Layer structure in 2 through pꢀ ꢀ ꢀp interactions.
˚
[6.640(5) A], through the bis(tridentate) bridge of the Schiff
3
510 nm for 1 and 2, which is presumably due to (p–p*)
˚
base is larger than the Cdꢀ ꢀ ꢀCd separation [5.849(5) A]
phosphorescence.
through the bi-bridged thiocyanates. The angle between
the terminal and bridging thiocyanates [S(1)–Cd(1)–N(2)]
is 86.78(12)ꢁ, reflecting the cis alignment of the N- and S-
coordination of the bound thiocyanates. N(1) and S(1)
from two bridging isothiocyanate and thiocyanate ligands,
4. Conclusion
In conclusion, X-ray crystallographic characterization
of new dimeric cadmium(II) azido and polymeric cad-

H. Chowdhury et al. / Polyhedron 26 (2007) 5023–5029
5029
(b) G.R. DesirajuT. Steiner, The Weak Hydrogen Bond in
Structural Chemistry and Biology, Oxford University Press,
Oxford, 1999;
mium(II) thiocyanato compounds is reported herein. To
the best of our knowledge, this the first report of a Schiff
base containing polymeric cadmium(II) pseudohalide com-
pound, where the versatility of the tailored hexadentate
blocker is realized through its bis(tridentate) congregation
behaviour encapsulating two metal ions instead of the clas-
sical hexacoordination binding a single metal ion. Again,
intermolecular cooperative [25] forces are harnessed to
engineer novel crystalline architectures. Both the com-
pounds are examples of good luminous materials.
(c) M. Meot-Ner, Chem. Rev. 105 (2005) 213.
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Acknowledgements
(b) S.H. Rahaman, R. Ghosh, G. Mostafa, B.K. Ghosh, Inorg.
Chem. Commun. 8 (2005) 700;
(c) S.H. Rahaman, R. Ghosh, G. Mostafa, B.K. Ghosh, Inorg. Chem.
Commun. 8 (2005) 1137.
We are thankful to the Department of Science and Tech-
nology (DST) and the Council of Scientific and Industrial
Research (CSIR), New Delhi, India for financial support.
The authors thank Prof. M. J. Zaworotko, The University
of South Florida, USA for the X-ray study.
[12] (a) D. Bose, S.H. Rahaman, G. Mostafa, R.D.B. Walsh, M.J.
Zaworotko, B.K. Ghosh, Polyhedron 23 (2004) 545;
(b) S.H. Rahaman, D. Bose, H. Chowdhury, G. Mostafa, H.-K. Fun,
B.K. Ghosh, Polyhedron 24 (2005) 1837.
5. Supplementary material
[13] (a) A.M. Golub, H. Kohler, V.V. Skopenko (Eds.), Chemistry of
Pseudohalides, Elsevier, Amsterdam, 1986;
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references cited therein;
CCDC 205911 and 205912 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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