Journal of the Chinese Chemical Society, 2008, 55, 13-15
13
Communication
A Novel Inorganic Clathrate Generated through Arene-Arene Interactions
Shih-Hsuan Maoa (
Fen-Ling Liaob (
a
), Shih-An Liua (
) and Kom-Bei Shiua* (
),
)
Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan, R.O.C.
b
Instrument Center, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C.
The direct mixing of CoCl2, sodium 2,6-pyridinedicarboxylate (Na2(2,6-pda)), KSCN, and 1,2-bis(4pyridyl)ethane (bpye) in water at the ambient temperature afforded purple crystals readily. The crystal
structure, as determined by X-ray diffraction crystallography, consists of discrete inorganic [Co2(h3-2,6pda) 2(m-SCN) 2(H 2O) 2] and organic bpye molecules. Adjacent dicobalt molecules are connected via
intermolecular hydrogen bonds, to form one-dimensional hydrophobic channels in the solid state, which
serve as a host framework to incarcerate bpye guests inside. Every organic molecule attaches to four adjacent inorganic species via arene-arene interactions. The facile approach in assembling the dicobalt and
bpye molecules together into an inorganic clathrate is demonstrated.
Key words: Clathrate; Cobalt; Pyridinedicarboxylate; 1,2-Bis(4-pyridyl)ethane; Structure.
The noncovalent synthesis of clathrates in the field of
supramolecular chemistry has been continuing to capture
scientists’ imagination since the first such compound reported in the beginning of the nineteenth century, due to the
increasing industrial applications, such as recognition,
transport, and catalysis.1 However, most clathrates, repoted
in the literature, are organic, containing host lattices of organic molecules and one or more solvent molecules as
guests. These clathrates are often discovered purely by
chance, via recrystallization of an organic compound from
suitable solvent systems. Our interest in molecular materials2,3 led us to explore systematically the facile synthesis of
inorganic clathrates from water. Herein, we wish to demonstrate an easy, reproducible, route with an inorganic clathrate example. In this typical example, a host lattice, composed of discrete inorganic molecules, contains one-dimensional hydrophobic channels, incorporating organic
guest molecules inside (Fig. 1). The inorganic molecules
interconnect each other with directional, stronger, hydrogen bonds, whereas the organic molecules attaches to adjacent inorganic species via arene-arene p-p stacking interactions (Fig. 2). The inorganic clathrate structure is hence
reminiscent of those found for the commonly observed
organic clathrates.4a
Typically, a suspension of CoCl2·6H 2O (0.0493 g,
0.199 mmol), 1,2-bis(4-pyridyl)ethane (bpye) (0.0185 g,
0.101 mmol), and H 2O (8 mL) was first added with an
aqueous solution of KSCN (0.0098 g, 0.0998 mmol) in 1
mL of H 2O, and then added with an aqueous solution of
2,6-pyridinedicarboxylic acid (H 2pda) (0.0175 g, 0.104
mmol) and NaOH (0.080 g, 2.0 mmol) in 1 mL of H2O, to
result in a purple, transparent solution. Copious purple
single-crystals of the dicobalt(II) complex [Co 2(h 3-2,6pda) 2(m-SCN) 2(H 2O) 2]·bpye (1) then formed after two
weeks. Yield 0.44 g (56%).5
The x-ray analysis 6 of a single crystal reveals that
there is a crystallographic center of symmetry imposed on 1
and the asymmetric unit contains a half dicobalt and a half
bpye molecules. The respective ORTEP drawings are
Fig. 1. Perspective packing of 1 along c axis to reveal
that the dicobalt units interconnect each other
via hydrogen bonds (as dashed lines) to form
one-dimensional channels with bpye inside.
14
J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
shown in Figs. 3 and 4, respectively. A tridentate 2,6-pda2ligand and one water molecule bind with each cobalt atom.
Two thiocyanate anions also bridge two cobalt atoms with
both cobalt centers separated by 5.590(1) Å. The coordination geometry about the Co(II) ion can hence be described
as a distorted octahedron (Fig. 3). Although the single crystals form from water, the crystals do not contain any free
water molecules in the lattice. Since the two halves of bpye
are symmetrically related, the two phenyl rings are parallel
to each other (Fig. 4). All atom-atom bond distances of
(a)
(b)
Fig. 2. Views of the p-stacked sheets for 1 as (a) balland-stick and (b) space-filling representations.
(50% probability thermal ellipsoids. Atoms for
the dicobalt unit are shown as empty spheres
and those for bpye are shown as gray spheres.
Mao et al.
bpye in 1 are similar to those found for the free bpye in its
crystal structure, 7 indicating the weak arene-arene p-p
stacking interactions between dicobalt and bpye molecules; the mean interplanar separations between the bpye
(i.e., the pyridyl plane of C10, C11, C12, C13, C14, and
N3) and one closest plane of the host molecule (C1, C2, C3,
C4, C5, and N1), adopting a symmetry code of (3/2-x,
3/2-y, 1-z), is 3.47 Å, shown as the black arrows in Fig. 2a.
The respective planes form an angle of 3.89° and the center-to-center distance is 4.41 Å. (Although the bpye plane
is also close to another symmetry-related host plane, adopting a symmetry code of (1-x, 1-y, 1-z), at a distance of 3.52
Å, shown as the white arrows, the respective center-to-center distance is 6.92 Å). Two strong intermolecular hydrogen bonds are located between adjacent dicobalt units with
d(O2---H5B) = 1.997 Å and d(O4---H5A) = 1.780 Å, both
H atoms adopting a symmetry code of (3/2-x, 1/2+y, 3/2-z)
Fig. 3. Molecular structure of the dicobalt molecule of
1. Bond distances (Å): Co1-N1, 2.037(3);
Co1-N2, 2.002(3); Co1-O1, 2.159(2); Co1-O3,
2.110(2); Co1-O5, 2.126(4); Co1-S1, 2.670(4);
S1-C8, 1.636(4); C8-N2, 1.154(5). Bond angles
(°): N1-Co-N2, 171.55(10); S1A-Co1-O5,
176.33(5); O1-Co1-O3, 151.85(7); N1-Co1O1, 74.97(7); N1-Co1-O3, 76.88(8).
Fig. 4. Molecular structure of the bpye molecule of 1.
Bond distances (Å): C9-C9A, 1.477(5); C9C(10), 1.505(6); C(10)-C(11), 1.390(4); C(11)C(12), 1.437(6); C(12)-N(3), 1.323(5), N(3)C(13), 1.300(5), C(13)-C(14), 1.355(6); C(14)C(10), 1.378(10). Bond angle (°): C9A-C9C10, 112.1(3).
Inorganic Clathrate
(Figs. 1 and 2). However, the closest distance between water hydrogen atom, H5B (with a symmetry code: 3/2-x,
3/2-y, 1-z), and the bpye nitrogen atom, N3, is at 4.054 Å,
too long to be considered with significant hydrogen bonding interaction, supporting the assigned clathrate structure
for 1.4b
J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
4.
ACKNOWLEDGEMENT
Financial supports for this work via a research grant
for gifted high-school students from the Ministry of Education of the Republic of China, administered by the Science
Education Center, National Taiwan Normal University, and
via another research grant from the National Science Council of the Republic of China are gratefully acknowledged.
5.
Received May 30, 2007.
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(a) For the well known gas hydrates, many gases can form
crystalline hydrates with water at low temperatures and high
pressures. The water molecules form polyhedral cages held
together by hydrogen bonding. A variety of structures were
solved to reveal that the cages can contain Cl2, ClO2, SO2,
CO2, N2O, CHCl3, and saturated and unsaturated hydrocarbons. Among these gas hydrates, gas (methane) hydrates,
formed either at 0 °C and 26 atm or at 10 °C and 76 atm, have
recently attracted lots of attention for the potentially useful
resources to provide water and fuel. (b) Compounds with hydrogen bonding, such as aldehydes, ketones, and nitriles,
cannot form gas hydrates. They have to be discarded as guest
components as they disturb the lattice structure and the cohesion of the water host molecules.1a
IR (KBr, cm-1): 3320 (m, n OH), 3079 (m, n OH), 2104 (s, n CN),
1651 (s), 1626 (s), 1589 (s), 1556 (s), 1495 (m), 1435 (m),
1398 (s), 1372 (s), 1286 (m), 1195 (m), 1156 (w), 1080 (m),
1033 (w), 991 (w), 916 (m), 838 (m), 774 (m), 735 (s), 690
(m), 672 (m), 653 (w), 585 (w), 540 (m), 518 (m), 443 (w),
422 (w).
Crystal data of 1: M f = 392.25, monoclinic, space group
C2/c, a = 19.6352(16) Å, b = 10.3417(8) Å, c = 16.9082(14)
Å, b = 115.317(2)°, V = 3103.6(4) Å3, Z = 8, Dcalcd = 1.679 g
cm -3 , m = 1.271 mm -1 , F(000) = 1592, q range = 2.28 –
28.33°, 217 variables refined with 2293 reflections to final R
indices (I > 2s(I)) of R1 = 0.0356 and wR2 = 0.0853, and GOF
= 0.947. The crystal graphic data have been deposited with
the Cambridge Crystallographic Data Centre, CCDC No.
648901.
Ide, S.; Karacan, N.; Tufan, Y. Acta Cryst. 1995, C51, 2304.