Self-assembled Transition Metal Coordination Frameworks of ...

Zinc(II) & Cadmium(Il) complexes
The IR spectra of Zn(II) and Cd(II) complexes are very much rich with bands
and assignments of these bands are very difficult. However, a comparison of spectra
of all Zn(II) and Cd(II) complexes with their corresponding carbohydrazone and
thiocarbohydrazone ligands also found relevant and resulted some assignments as
given in Table 6.2. All the compounds show the presence of lattice water as evidenced
by the broad bands centered in the region of 3405 to 3493 cm". The v(NH) bands are
seen in the region 3110 to 3327 cm", although the broad stretching bands of water
mask some of them. The shifts in frequencies in these regions may be due to changes
in possible hydrogen bonding interactions. Also none of the spectra exhibit bands
between 2500 and 2700 cm" due to the stretching of SH group [33,34] rules out the
presence of any thiol forms in free or coordinated forms in both Zn(II) and Cd(II)
complexes. Bands ranging from 1600-1400 cm", attributed to v(C=C) and v(C=N)
vibration modes, in the spectra of complexes suffer significant shifts and their mixing
pattems are different from that seen in the spectra of their respective free ligands.
The spectra of 24 and 26 resemble and are different from other spectra of
Zn(II) molecular squares. However the formation of squares cannot be differentiated
with IR or electronic spectral studies alone. The strong v(C=S) bands of free ligands
HZL2 and H2L3 seen at 1225 and 1209 cm" are found to be shifted to 1152 and 1143
cm" in the spectra of 24 and 26 respectively, suggesting the thione sulfur
coordination. Also, strong bands at ~320 cm" in the far IR region of these compounds
are consistent with sulfur coordination like in zinc thiosemicarbazone complexes [4].
The v(N—N) bands seen at ~ll30 cm" for free ligands are shifted to ~l210 cm"
confirming the coordination through azomethine nitrogen. Other uncoordinated N—N
stretching bands are seen at ~lll0 cm". The azomethine coordination is further
supported by bands of v(Zn—N,Zo) at ~4l0 cm" [4]. The v(Zn—Np,) bands expected are
present at ~220 cm" [35,36] in both complexes and changes in the 600-700 cm"
regions compared to their respective ligands may be attributed to the pyridyl or
quinolyl nitrogen coordination to Zn(Il) in 24 and 26.
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