R. Chatterjee et al. / Polyhedron 68 (2014) 265–271
267
2CoðNO3Þ2 þ 12Na2WO4 þ C5H5N þ 21HNO3 þ H2O
¼ ½CoðH2OÞ6ꢃ½C5H6NꢃH4½CoW12O40ꢃ ꢀ NO3 ꢀ 3H2O þ 24NaNO3
cobalt complexes [19–21]. Four oxygen atoms lying on the mirror
plane, O12 and those generated by the symmetry operations
(ꢁx + 2, ꢁx + y + 1, ꢁz + 1/3, x, x ꢁ y, ꢁz + 1/3, ꢁx + 2, ꢁy + 1, z), de-
fine the basal plane and the remaining two oxygen atoms, O11 and
the symmetry (ꢁx + 2, ꢁx + y + 1, ꢁz + 1/3) related oxygen atom, ly-
ing on the four-fold axis occupy the axial sites. Selected bond dis-
tances and angles are given in Table 2. The hexa aquo cobalt(II)
units play a decisive role in the 3D supramolecular organization
of the complex. Each hexa aquo cobalt(II) unit acts as a nodal point
of a tetrahedral network (Fig. S1) through strong O–Hꢀ ꢀ ꢀO (Table 3)
hydrogen bonds connecting four adjacent POM units in a tetrahe-
dral geometry. A part of this network propagating along the [001]
direction is shown in Fig. 2. Here centrosymmetrically related co-
balt octahedra and polyoxoanion tetrahedra are connected through
pairs of O12–H12Aꢀ ꢀ ꢀO4 (1ꢁx, 1 ꢁ x + y, 1/3 ꢁ z) hydrogen bonds,
which according to the graph-set notation can be described as an
R66(36) ring (Fig. 3). Intermolecular O11–H11ꢀ ꢀ ꢀO6 (1 ꢁ x, 1 ꢁ y,
z) and O12–H12ꢀ ꢀ ꢀO3 (1ꢁx, 1 ꢁ y, z) hydrogen bonds link the
[Co(H2O)6]2+ cation and the polyoxoanion. Along the [001] direc-
tion, the [Co(H2O)6]2+ units and the POM units are wound in a dou-
2.6. Structural description
6ꢁ
The crystal structure is composed of a Keggin-type [CoW12O40
]
anion, one hexa aqua cobalt [Co(H2O)6]2+ counter cation and three
ꢁ
water molecules of crystallization, along with one NO3 group and
one pyridinium [C5H6N]+ ion. The oxygen atom (O14) of the solvent
water molecules in 1, located near (1, 0, 1/6) was disordered be-
tween two centrosymmetrically related positions. The dodeca-
6ꢁ
tungstocobaltate [CoW12O40
]
consists of four vertex-sharing
trinuclear W3O13 triad units, each of which is made up of three
edge-sharing WO6 octahedra around the CoO4 central tetrahedron
(Fig. 1) [15,16]. According to the different coordination fashions of
the oxygen atoms in the Keggin unit, the O atoms can be classified
into three groups, i.e. unshared or terminal O atoms ‘Ot’, bridging O
atoms (Ob) connecting two W atoms, and O atoms (Oc) of the central
CoO4 unit. The W–Ot bond lengths vary between 1.66(2) and
1.73(2) Å, while the W–Ob and W–Oc bond distances lie in the range
1.88(2)–1.97(2) and 2.13(2)–2.20(2) Å, respectively. In the CoO4
tetrahedron, the metal center (Co1) is seated at the intersection of
three two-fold rotation axes with coordinates (½, 0, 0). The coordi-
nation geometry around the Co atom is distorted tetrahedral, which
is reflected by the central Co–O bond lengths [1.88(2) Å] and the O–
Co–O angles [108.1(14)–111.8(14)°]. From the variation of the W–O
distances and the bond angles around the W atoms, it is evident
that there is considerable distortion in the WO6 octahedra in the
complex. This is consistent with the structural features of polyoxo-
tungstates exhibiting Keggin structures [16–18]. Another part of
compound 1 is the presence of a discrete cobalt complex, where
the metal atom is surrounded by six molecules of water forming
the cation. The Co (Co2) atom is in a nearly regular octahedral coor-
dination geometry, with Co–O distances between 1.99(4) and
2.07(4) Å, and angles ranging from 87.4(12)° to 92.6(12)°,which
are comparable with the reported values for related hexa aqua
ble helical way (Fig. 4a), generating
a channel along the
crystallogꢁraphic c-axis (Fig. 4b) which encapsulates a unique 1D
H2O–NO3 double helical assembly (Fig. 4c). Adjacent [Co(H2-
2ꢁ
O)6]2+–H4[CoW12O40
]
double helices are fused with each other
and a 3D supramolecular chiral tetrahedral network is formed. It
should be pointed out that due to the relatively larger size of the
POM units than the hexa aquo cobalt(II) units, even after fusion of
successive helices there remains an inter-groove vacant space
which accomodates pyridinium ions. The pyridinium ion acts as a
suitable H-donor for the Keggin terminal O acceptors, simulta-
neously donating to four O atoms of the same Keggin POM unit that
vertically land on the centre of the cap of the Keggin unit (Fig. 5).
The pyridinium ion also establishes a bifurcated CHꢀ ꢀ ꢀO bond with
two other Keggin units on the other sides, leading to 2D layers
(Fig. 6) and interact with [Co(H2O)6]2+ units in a plane with the Keg-
gin units lying above and below this plane (Fig. S2). Double helical
compounds generally consist of the same coordination, supramo-
lecular or covalent chains [22], and here we observed the unique
double helix constructed through supramolecular assemblies
(Fig. S3). It is noted that the achiral ligand and metal ion may not
produce a helical structure that is exclusively right-handed or
left-handed, which always results in a racemic mixture of right-
and left-handed helical products. In the present structure, the
hexa-aquo cobalt cation is responsible for induction of chirality in
the assembly, resulting a racemic mixture. The coordinated water
molecules to Co in the [Co(H2O)6]2+ ion act as hydrogen bond do-
nors for the Keggin POM units. In the octahedral coordination
geometry of Co, two adjacent water molecules, one attached axially
Table 2
Selected bond lengths (Å) and angles (°) with esd’s in parentheses for 1.
W(1)–O(10)
W(1)–O(2)
W(1)–O(4)
W(1)–O(7)#1
W(1)–O(8)
W(1)–O(1)
W(2)–O(3)
W(2)–O(8)#2
W(2)–O(4)
W(2)–O(5)#3
W(2)–O(6)
O(1)#1–Co(1)–O(1)#2
O(1)–Co(1)–O(1)#1
1.66(2)
1.92(2)
W(2)–O(1)
W(3)–O(9)
W(3)–O(7)
W(3)–O(5)
W(3)–O(6)
W(3)–O(2)
W(3)–O(1)
Co(1)–O(1)
Co(2)–O(11)
Co(2)–O(12)
2.13(2)
1.68(2)
1.89(2)
1.90(2)
1.91(2)
1.97(2)
2.17(2)
1.88(2)
1.99(4)
2.07(4)
1.92(3)
1.95(2)
1.95(2)
2.20(2)
1.73(2)
1.88(2)
1.91(2)
1.92(2)
1.95(3)
108.1(14)
111.8(14)
O(11)#4–Co(2)–O(12)#4
O(11)–Co(2)–O(12)#4
87.4(12)
92.6(12)
Symmetry codes: #1 x ꢁ y, ꢁy, ꢁz #2 ꢁx + 1, ꢁy, z #3 ꢁx + y + 1, y, ꢁz #4 ꢁx + 2,
ꢁx + y + 1, ꢁz + 1/3.
Fig. 1. ORTEP view (30% ellipsoidal probability) of the title compound 1.