LINEAR TRINUCLEAR SCHIFF BASE COMPLEXES
[26]. The degradation reaction starts around 300°C
ing complete degradation of the structure does not oc-
cur. On the other hand, degradation of the aromatic
groups are also included within the temperature range
of NiLDM. Similar results have also been observed
for the trinuclear complexes.
the first product being acetone. Similarly, recent re-
ports, dealing with degradation of mononuclear
Cu(II), Ni(II), Zn(II), Cd(II), Fe(II) Schiff base com-
plexes, have shown the thermal degradation reaction
to start in the range 280–300°C. Another study has
suggested the degradation reaction to proceed via the
neighbouring aliphatic groups to the coordinatively
bonded nitrogen donor atoms [22].
The second thermal degradation observed, in
this study, is the total of the two reactions; degrada-
tion of aliphatic groups of the ONNO type ligand
surrounding the terminal Ni(II) ions and degradation
of the acetate groups, surrounding the central Ni(II)
ion, to acetone and carbondioxide. As specified be-
fore, the mass loss of the NiL trinuclear complexes
are as expected whereas the mass loss of the NiLDM
and NiLOH trinuclear complexes are higher and
lower than expected, respectively.
The benefit of the DSC curves for these com-
plexes, lies in the fact that the bond energies of the
DMF molecules, coordinated to the terminal Ni(II)
ions in the complex, can be approximately calculated.
The DMF molecules abandon the complex structures
between 140–190°C (150–255°C for the NiLOH
trinuclear complexes) without degradation of the
complex. Experiments have shown the DMF mole-
cules to be coordinated to the remnants with re-
crystallization in DMF. The heat absorbed during the
first thermal endothermic reaction was measured and
the coordinative bond energy of the DMF molecules
for each trinuclear complex was calculated according
to the following equation.
The NiLDM complexes contain two methyl
groups attached to the trimethylene bridge. These
are open ended groups which and atoms situated at
open ends of the aliphatic groups show a high de-
gree of vibration whereas methyl groups situated on
aromatic ring structures show limited vibration.
The thermal ellipsoids of open ended groups are al-
ways larger than those on ring structures, a fact fre-
quently observed during X-ray studies. In addition
there is not any interaction between the terminal
Ni(II) ions and the methyl groups. As the vibration
amplitude increases with temperature, the two
methyl groups puts more strain on the six
membered chelate ring of NiLDM compared to
NiLOH. The thermal strain created by the methyl
groups leads to earlier degradation of the aromatic
groups. As a result mass loss due to degradation is
higher in the NiLDM complexes and the reaction
temperature is seen to be spread to a wider range.
On the other hand, trinuclear complexes which in-
clude terminal NiLOH molecules possess an –OH
group attached to the methylene bridge. This –OH
group can interact with the terminal Ni(II) ion.
Many studies in literature, dealing with dinuclear
complexes, show the –OH group to interact with
metal ions as a donor [27]. For this reason, in con-
trast to NiLDM containing complexes, in NiLOH
containing complexes the –OH group makes degra-
dation of the chelate ring difficult and delays the
degradation of the ring. The same situation is ob-
served with the mononuclear complexes.
(coordinative bond energy per DMF molecule)=
(heat measured from DSC curve for first thermal reaction)/2–
–(DHevaporation at boiling point of DMF)/2+(DTCp)/2
where DT is the difference between the final temperature
of the thermal reaction and the boiling point of DMF.
The calculated values have been tabulated in Table 4.
The boiling point of DMF at 1 atm is 149.1°C. If the
heat of evaporation is calculated according to the Trouton
law it is found to be 37.2 kJ mol–1. The heat of absorption
of DMF from DSC studies, after total evaporation at
106°C, was found to be 37.16±2.12 kJ mol–1. The Cp val-
ues were calculated, using the software as
144.27±8.89 J mol–1 between 170–210°Cand used in cal-
culation of the coordinative bond energy of DMF mole-
cules. For example, the final temperature for the first re-
action, from Table 3, for the (NiL)2Ni(AcO)2·DMF2 com-
plex is 193°C and the measured heat of absorption
97.96 kJ mol–1. Taking into consideration the boiling
point of DMF as 149.1°C the coordinative bond energy
per DMF molecule is calculated to be 5.49 kJ mol–1.
(coordinative bond energy per DMF molecule)=
{97960–2[37.16+(193–149.1)×144.27]}/2
The results in Table 4 are reasonable and most of
the coordinative bond energies are seen to be at this
level. The effect of the hydrogen bond between the
terminal DMF molecules and OH group in NiLOH
containing complexes and the two methyl groups in
NiLDM containing complexes can clearly be seen.
The hydrogen bond makes separation of the DMF
molecules from the complex resulting in higher heats
of absorption.
The second thermal degradation reaction tem-
perature range for NiL, NiLDM and NiLOH are
323–410, 337–421 and 275–413°C, respectively. Al-
though the widest range is for NiLOH, a lower degree
of mass loss from the chelate ring is observed, mean-
J. Therm. Anal. Cal., 86, 2006
343
Durmus
