R. Bikas et al. / Inorganic Chemistry Communications 62 (2015) 60–63
61
Scheme 1. (a) Hydrazones general formula and carbohydrazone; (b) tautomeric form of the HL.
−
1
i
spectrum of 1 a very strong band appears at 2067 cm which attribut-
Mn1⋯Mn2 distance is 3.370(1) Å. The equatorial positions of the two
independent Mn(II) ions are the same and the only difference in the co-
ordination environment between the two metal centers consists in one
of the axial positions, which is a terminal monodentate azide for Mn1
but a methanol molecule for Mn2. The O–H⋯N hydrogen bonds join
the axial ligands to each other and have considerable effect on distortion
from ideal octahedral arrangement. In this compound, the intramolecu-
¯
3
ed to the azide stretch and coordination of N to the manganese ion [11].
By comparing IR spectrum of complex 1 with that of free ligand (Fig. S1),
it is seen that the C_O band is absent in the complex. This finding sug-
gests enolization of the amide functionality upon coordination to the
metal ions in 1. The absence of ν(C_O) and appearance of a new
band at 1614 cm , which can be assigned to the –C_N–N_C– moiety
12] also confirm this finding. The broad band at 3188 cm in the IR
spectrum of complexes is due to presence of N–H bond and confirms
the presence of amidic hydrogen. Disappearance of the carbonyl
stretching vibration band, ν(C_O), and the presence of N–H band sug-
gest the absence of delocalization in the –N–C_O group in the coordi-
nated ligand (L) [13]. UV–Vis spectrum of HL (Fig. S2) shows three
bands at 214, 296 and 384 nm, based on their extinction coefficients
they can be attributed to π → π (214, 296 nm) and n → π (384 nm)
transitions. The UV–Vis spectrum of compound 1 in methanol (Fig. S2)
shows two broad absorbance bands at 518 and 395 nm due to the
charge transfer (LMCT) transitions [14]. The band at 218 nm and shoul-
der at about 259 nm are intraligand transitions.
The molecular structure of 1 was determined by single crystal X-ray
crystallography (see Table S1). The molecular structure and labeling of
the atoms for compound 1 are displayed in Fig. 1 and selected bond
lengths and angles are given in Table S2. Diffraction studies of 1 reveal
a centrosymmetric structure. The carbohydrazone ligand is coordinated
to the Mn(II) ions as a pentadentate mononegative ligand, (L)¯. The
asymmetric part of the unit cell in 1 contains an enolato bridged dinucle-
ar moiety of Mn(II) which is transformed to tetranuclear complex by an
inversion symmetry operation (i = −x + 1, −y + 1, −z + 1). In com-
pound 1, both Mn(II) cores have distorted octahedral coordination envi-
ronment (Fig. S3) with three nitrogen atoms from azides, oxygen and
two nitrogen atoms provided by the Schiff base ligand. Oxygen atom
of carbohydrazone ligand, O1, bridges adjacent Mn(II) ions. The Mn1–
O1–Mn2 angle is 126.42(5)° and Mn1⋯Mn2 distances through this
line is 3.998(2) Å. Two azide bridging ligands connect Mn1 ion to anoth-
−
1
−
1
i
i
[
lar Mn1⋯Mn1 and Mn2⋯Mn2 distances are 5.256(2) and 5.202(2) Å,
respectively. The Mn–O and Mn–N bond lengths are close to other re-
ported Mn(II) complexes with N O-donor hydrazone based ligands
2
[15]. The C1–N1 and C1–N3 bond lengths are 1.3782(18) and
1.3323(18) Å in 1. Comparison of these bond lengths indicates that the
C–N bond length (in which its hydrogen atom has been eliminated on
complexation) is shorter than C–N bond length in which hydrogen
atom remains on the nitrogen atom. In addition, the length of C1–O1
bond is 1.2742(16) Å in 1 which is considerably longer than the C–O
bond length in free carbohydrazone based ligands (1.217(3) Å [16]).
All of these findings express the enolization through N3–C1–O1. There
are some weak C–H⋯N, C–H⋯O and π-stacking interactions between
neighboring molecules stabilizing the crystal packing of 1 (Fig. S4,
Table S2). Uncoordinated methanol solvent molecules are present in
the crystal packing of 1 which are connected to carbohydrazone ligand
by O–H⋯N and N–H⋯O hydrogen bonds (Fig. 1, Table S3). The red crys-
tals of 1 become powder when they are removed from the mother li-
quor, which is attributed to the loss of the uncoordinated methanol
molecules. Since the finger print IR spectra of the crystals and the corre-
sponding powder are the same, we concluded that the complex is stable
in spite of its change to powder form on drying. All spectroscopic and
magnetic data also confirm the stability of the complex even in the pow-
der form.
‾
⁎
⁎
To examine thermal stabilities of 1, thermogravimetric analysis
(TGA) was made in the static atmosphere of nitrogen. TGA curve of 1
(Fig. S6) shows that the uncoordinated methanol molecules are re-
moved between 40–100 °C [starting mass 13.658 mg, observed
0.752 mg (5.5%), calcd. 0.609 mg (4.45%)]. This curve also indicates
i
er Mn2 ion in end-on fashion. Therefore, the Mn1 ion is connected to
two Mn2 ions via two different bridging groups, EO-azides and enolic
oxygen atoms. The bridging azide groups span the axial positions of
that 1 is stable up to 125 °C, where the coordinated CH
3
OH molecule
is removed [observed 0.613 mg (4.49%), calcd. 0.609 mg (4.45%)]
−
one metal center and the basal position of the other and the Mn
N
2 2
cy-
between 130–175 °C, then at 230 °C the N
3
ligands are removed
clic unit is approximately planar. In this four membered cyclic unit the
[observed 2.399 mg (17.56%), calcd. 2.397 mg, (17.52%)]. Finally