Unusual perchlorate binding in heptacoordinated cadmium(II)thiocyanate containing a pentadentate Schiff base

Article abstract of DOI:10.1016/j.inoche.2005.04.014

A new Schiff base, N,N′-(bis(pyridin-2-yl)benzylidene) diethylenetriamine (bpbd), has been used to synthesise a novel heptacoordinated cadmium(II)thiocyanate complex [Cd(bpbd)(NCS)(ClO₄)] (1). Crystal structure analysis of 1 reveals cadmium(II) centre in a distorted pentagonal bipyramid environment with perchlorate binding, C-H?O hydrogen bonding and C-H?π interaction leading to a 2D sheet. 1 displays intraligand ¹(π-π*) fluorescence and intraligand ³(π- π*) phosphorescence in glassy solutions (MeOH at 77 K).

Full text of DOI:10.1016/j.inoche.2005.04.014

Inorganic Chemistry Communications 8 (2005) 700–703
www.elsevier.com/locate/inoche
Unusual perchlorate binding in heptacoordinated
cadmium(II)thiocyanate containing a pentadentate Schiff base
a
Sk Hafijur Rahaman , Rajarshi Ghosh , Golam Mostafa , Barindra Kumar Ghosh
a
b
a,
*
a
Department of Chemistry, The University of Burdwan, Golapbag, Rajbati, Burdwan 713 104, India
b
Department of Physics, Jadavpur University, Kolkata 700 032, India
Received 9 February 2005; accepted 27 April 2005
Available online 16 June 2005
Abstract
A new Schiff base, N,N0-(bis(pyridin-2-yl)benzylidene)diethylenetriamine (bpbd), has been used to synthesise a novel heptacoor-
dinated cadmium(II)thiocyanate complex [Cd(bpbd)(NCS)(ClO4)] (1). Crystal structure analysis of 1 reveals cadmium(II) centre in a
distorted pentagonal bipyramid environment with perchlorate binding, C–Hꢀ ꢀ ꢀO hydrogen bonding and C–Hꢀ ꢀ ꢀp interaction lead-
1
3
ing to a 2D sheet. 1 displays intraligand (p–p*) fluorescence and intraligand (p–p*) phosphorescence in glassy solutions (MeOH
at 77 K).
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cadmium(II)thiocyanate; Schiff base; X-ray structure; Supramer; Luminescence
The development of coordination polymers [1] with
strong covalent bonds [2] and supramolecular entities
[3] involving weak non-covalent forces [4] is an exciting
area of research that has implications for the rational
design of functional materials [5–7] at molecular level.
Self-assembly [8] of the molecular components is the
most efficient approach towards preparation of varied
frameworks of complex connectivities via malleable
coordination bonds and lateral multiple non-covalent
forces. We are interested [9] in this approach for the con-
struction of the inorganic functional polymers and
supramers through variation of ligand backbone and
metal ion coordination environment. Schiff bases have
recently [9a,10] been focused by the coordination chem-
ists as versatile organic blockers because of their pre-
parative accessibilities, structural varieties and varied
denticities. Pseudohalide like thiocyanate [11] is a puta-
tive ambidentate ligand and cadmium is well-suited
[12–14] to this study as its d10 configuration permits a
wide range of symmetries and coordination numbers.
Report of six-coordinated Cd(II) complex [15] with per-
´
chlorate bindings by Jan Fabry et al. has prompted us to
develop new luminous materials with superstructures via
different non-covalent forces. This paper presents syn-
thesis and X-ray structure of a new seven-coordinated
luminous cadmium(II) complex [Cd(bpbd)(NCS)(ClO4)]
(1) through reaction of Cd(ClO4)2 Æ 6H2O, N,N0-
(bis(pyridin-2-yl)benzylidene)diethylenetriamine (bpbd)
and NH4NCS in 1:1:1 molar ratio in MeOH–MeCN
[16]. The interesting feature of this compound is en-
hanced coordination of the metal centre through per-
chlorate binding via one of its O atoms that ensures
such a conformation to the whole coordination frame
that C–Hꢀ ꢀ ꢀO hydrogen bonding and C–Hꢀ ꢀ ꢀp interac-
tion occur leading to a superstructure.
The air-stable, moisture-insensitive complex is solu-
ble in a range of common organic solvents but is insol-
uble in water. In IR, 1 displays splittings of m3(ClO4)
band at 1144, 1116, 1080 cmꢁ1 and conductance mea-
surement in MeCN solution it shows low KM value
*
Corresponding author. Tel.: +91 342 2558545/6501; fax: +91 342
2530452.
E-mail address: barin_1@yahoo.co.uk (B.K. Ghosh).
1387-7003/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.04.014

S.H. Rahaman et al. / Inorganic Chemistry Communications 8 (2005) 700–703
701
which is consonance with coordination of perchlorate
ion in solid state as well as in solution. The X-ray dif-
fraction study confirms this hypothesis. The solution
of 1 in MeCN is colourless and exhibits absorption at
310 (sh) and 358 (sh) nm. Reflectance spectrum (310
and 359 nm) in nujol and electronic spectrum in MeCN
solution are akin reflecting similar gross structure and
electronic structure in solid state and in solution.
(171.4(2)ꢁ). The equatorial plane consists of N2, N3,
N4, N5, N6 nitrogen atoms of pentadentate schiff base
and the axial position occupied by N1 of thiocyanate
and O1 of perchlorate. Cd–N distances are in the range
˚
(2.353(6)–2.457(5)) A, comparable to the corresponding
values in similar systems [11–14]. The Cd–O1
˚
(2.692(5) A) distance is much larger compared to the
Cd–N distances. Angles subtended by the Schiff base
blocker in the equatorial plane (67.14(17)–72.38(18)ꢁ)
lie very close to the ideal angle (72.00ꢁ).The trans axial
angle N1–Cd–O1 (171.4(2)ꢁ) deviates slightly from the
180.00ꢁ. There are two gauche five-membered rings:
Cd, N5, C17, C16, N6 and Cd, N6, C15, C14, N3 and
two coplanar five-membered rings: Cd, N2, C25, C18,
The structure determination reveals that 1 consists of
2D covalent chain through cooperative weak C–Hꢀ ꢀ ꢀO
hydrogen bonding and C–Hꢀ ꢀ ꢀp interaction. An OR-
TEP diagram with atom numbering scheme of the
mononuclear unit with stoichiometry [Cd(bpbd)(NCS)-
(ClO4)] (1) is shown in Fig. 1, where each cadmium is
uniquely heptacoordinated by a pentadentate Schiff base
(bpbd), a terminal thiocyanate and a coordinating per-
chlorate anion. The coordination polyhedron around
cadmium is best described as distorted pentagonal
bipyramid with CdN6O chromophore. Distortion from
the ideal geometry is due to the asymmetric nature [17]
of the bound Schiff base blocker and the deviations of
the refine angles formed at the metal centre. The coordi-
nation includes the Schiff base ligated by two pyridine
nitrogens (N2, N4), two imine nitrogens (N3, N5), one
NH nitrogen (N6), a terminal thiocyanate nitrogen
(N1) and an oxygen (O1) of the coordinated perchlorate
anion. Five N atoms of bpbd and an N-donation of
ambidentate NCS creates hardness in cadmium(II) cen-
tre to such an extent that it forces the perchlorate anion
to bind the metal via covalent bond within the coordina-
tion sphere. This rare binding mode (Cd–O1) of the
counter anion facilitates a heptacoordination around
cadmium. The degrees of distortion from an ideal pen-
tagonal bipyramidal geometry are reflected in the equa-
torial (67.13(16)–150.84(18)ꢁ) and the axial angles
˚
N5 (rms deviation 0.069 A) and Cd, N3, C7, C6, N4
˚
(rms deviation 0.088 A). The ethylenic parts of the Schiff
base are to some extent puckered. All the bond angles
and bond lengths on the ligand network are as expected
[10]. The thiocyanate is coordinated in a linear fashion
as seen from its N1–C1–S angle (177.4(7)ꢁ). The
˚
N1–C1 length (1.256(12) A) is shorter than C1–S
˚
(1.455(9) A) reflecting N-coordination of the pseudoha-
lide. The closed C–Hꢀ ꢀ ꢀp interaction [4] joins adjacent
two molecules and three C–Hꢀ ꢀ ꢀp interactions (Cg1:
N2–C25–C26–C27–C28–C29; Cg2: N4–C2–C3–C4–
C5–C6; Cg3: N8–C9–C10–C11–C12–C13) construct a
supramoleculer chain (Fig. 2) running along a-axis (D–
Hꢀ ꢀ ꢀp: C9–H9ꢀ ꢀ ꢀCg1(i), C24–H24ꢀ ꢀ ꢀCg3(ii)
,
C27–
H27Pꢀ ꢀ ꢀCg2(ii); Hꢀ ꢀ ꢀp: 3.00, 3.20, 3.23 A; Dꢀ ꢀ ꢀp: 3.730
˚
˚
(6), 3.910 (6), 3.695(6) A; \D–H–p: 136ꢁ, 134ꢁ, 113ꢁ.
The symmetry code: (i) ꢁ1/2 + x, y, 3/2 ꢁ z; (ii)
1/2 + x, y, 3/2 ꢁ z). The chains (Fig. 2) are joined by a
C–Hꢀ ꢀ ꢀO interaction (C12–H12ꢀ ꢀ ꢀO3i) to form a 2D
˚
˚
sheet in ab-plane (Hꢀ ꢀ ꢀA: 2.49 A, Dꢀ ꢀ ꢀA: 3.417(10) A,
\D–H–A: 174ꢁ. The symmetry code: 2ꢁx, 1/2 + y,
Fig. 1. The ORTEP diagram of 1 showing the atom labeling scheme (40% ellipsoids).

702
S.H. Rahaman et al. / Inorganic Chemistry Communications 8 (2005) 700–703
Fig. 2. 2D sheet formed by cooperative C–Hꢀ ꢀ ꢀO hydrogen bonding and C–Hꢀ ꢀ ꢀp interaction in ab-plane.
Acknowledgements
Financial support from DST and CSIR, New Delhi,
India, is gratefully acknowledged.
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,
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´
´
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[16] Synthesis of [Cd(bpbd)(NCS)(ClO4)] (1): An acetonitrile solution
(10 ml) of faint yellow bpbd (0.433 g, 1 mmol) was added
dropwise to a colourless methanolic solution (5 ml) of cad-
mium(II) perchlorate hexahydrate (0.311 g, 1 mmol). To this light
yellow solution, ammonium thiocyanate (0.076 g, 1 mmol) in
methanol (5 ml) was added slowly. The faint yellow solution was
filtered and left for slow evaporation in air. After two days fine
transparent light yellow single crystals of compound 1 separated
out, which were filtered, washed with methanol and dried in vacuo
over CaCl2. The yield was 0.527 g (75%). Found: C, 49.35; H,
collected: 6592, unique reflections: 3603 were recorded using the
x-scan technique. Crystallographic data (excluding structure
factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication nos. CCDC-211534 (1). Copies of this
information can be obtained, free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
1223-336033; E-mail: deposit@ccdc.cam.ac.uk or www:http://
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