Transition Met Chem
Table 2 Selected bond lengths
˚
Co1–O1
2.012(2)
2.115(2)
176.53(8)
93.47(9)
87.17(9)
81.41(9)
94.36(10)
Co1–O6
2.005(2)
2.208(2)
86.07(9)
84.37(9)
84.85(9)
175.66(9)
175.65(9)
Co1–O11
2.187(2)
2.189(2)
95.99(9)
95.83(9)
92.59(9)
94.31(9)
89.95(9)
(A) and angles (ꢁ) for the
complex
Co1–O12
Co1–N3
Co1–N4
O1–Co1–O6
O1–Co1–N3
O6–Co1–O12
O11–Co1–O12
O12–Co1–N3
O1–Co1–O11
O1–Co1–N4
O6–Co1–N3
O11–Co1–N3
O12–Co1–N4
O1–Co1–O12
O6–Co1–O11
O6–Co1–N4
O11–Co1–N4
N3–Co1–N4
section describes the typical synthesis procedure in which
the molar ratio of pyridoxal chloride, sulfanilic acid, and
CoCl2Á6H2O was 1:1:1 and the reaction temperature was
323 K. We have tried other ways to synthesize the title
complex. When the molar ratio of pyridoxal chloride, sul-
fanilic acid, and CoCl2Á6H2O was 2:2:1 or the reaction was
carried out at room temperature, crystals of the same com-
plex could also be obtained. Furthermore, in one experiment,
the cooled solution was kept at room temperature without
filtration. There was some precipitate and crystals were
observed at the bottom of the flask by the next day. The
appearance of the crystals from all these experiments was the
same, and they all gave identical IR spectra.
and sulfonic acid groups and protonation of the pyridine
nitrogen. The complex shows an interesting and rarely
reported configuration in that the Co(II) atom is sand-
wiched at the edge of two Schiff base ligands. As a result,
the imine nitrogen and phenolic oxygen both deviate from
the pyridine ring plane and are inclined to the middle of the
sandwich. A number of crystal structures of Schiff base
complexes derived from pyridoxal with planar [21, 22],
arc-shaped [23–25], and other [15, 26, 27] molecular
configurations have been reported. Most of these com-
plexes are derived from aliphatic amines and few from
aromatic amines. In these structures, the imine nitrogen and
the phenolic oxygen of pyridoxal are normally co-planar
with the pyridine ring plane. Naskar et al. [23] reported an
arc-shaped Cu(II) complex, in which the copper atom was
located at the center of the arc. However, the imine
nitrogen and the phenolic oxygen of pyridoxal moiety were
still co-planar with the pyridine ring plane. In rare cases,
however, these atoms can deviate from the pyridine ring
plane. Back et al. [27] reported three rare earth complexes
that exhibited a cross-like molecular configuration in which
the rare earth atom was located at the center of the cross,
and the imine nitrogen was displaced out of the pyridine
ring plane. However, to the best of our knowledge, a Schiff
base complex derived from pyridoxal with sandwich-like
configuration has not been reported previously. Hence, the
complex represents a new molecular configuration for a
Schiff base complex derived from pyrdoxal.
In the IR spectrum, a broad band centered at 3,396 cm-1
is assigned to the v(O–H) absorptions of water molecules
and the alcohol group of pyridoxal. A peak at 1,614 cm-1
can be ascribed to the v(C=N) absorption of the Schiff
base. The sulfonic acid group gives its characteristic
absorptions at 1,174, 1,122, 1,131, and 1,005 cm-1
.
Absorptions at 748 and 624 cm-1 indicate that the Co(II)
atom is coordinated by the phenolic oxygen and imine
nitrogen. The pyridoxal residue in the complex occurs in
the form of a zwitterion (deprotonation of the phenolic
hydroxyl and sulfonic acid, and protonation of the pyridine
nitrogen), which is manifested in the IR spectrum by the
presence of a v(N–H?) band at 2,849 cm-1 [13, 21], as
well as a blue shift of v(S–O) of the sulfonic acid group
from 1,165 in the free ligand to 1,174 cm-1
.
In the complex, the two Schiff base ligands are almost
parallel to each other, but with opposing orientations
(Fig. 1). The short interactions between rings N1–C1–
C2–C3–C4–C5 (R1), N2–C15–C16–C17–C18–C19 (R2),
C9–C10–C11–C12–C13–C14 (R3), and C23–C24–C25–
C26–C27–C28 (R4), which were calculated using Platon
software [28], are listed in Table 3. The data imply that p–
p stacking interactions play important roles in the forma-
tion of the sandwich-like configuration of the complex
molecule and in the crystal packing, to give a 1D structure.
Furthermore, the alcohol groups of pyridoxal, the water
ligands, the lattice water molecules, and the protonated
pyridine nitrogen form a number of intermolecular hydro-
gen bonds (listed in Table 4). These hydrogen bonds
assemble the 1D structure of the complex into a 3D
structure.
Crystal structure
The results of the structural analysis reveal that the
asymmetric unit of the complex contains one Co(II) atom,
two Schiff base ligands, two water ligands, and three lattice
water molecules. The content and the nature of the water
molecules have been confirmed by thermal stability
investigations (see below). ORTEP drawings of the com-
plex viewed from different directions are shown in Fig. 1.
In the complex, the pyridine ring plane is slightly distorted
from the benzene ring plane. The Co(II) atom is coordi-
nated by two O and two N atoms of two Schiff base ligands
and two O atoms of water, forming a distorted octahedral
coordination environment. The complex molecule is neu-
tral overall, due to deprotonation of the phenolic hydroxyl
123