Self-assembled Transition Metal Coordination Frameworks of ...

Ligands
The ‘H and "C NMR spectra of H;L' in CDCI3 recorded at 400 MHz, along
with their spectral assignments are given in Figs. 2.8 and 2.9. The sharp peaks at 5 =
14.56 and 15.45 ppm are assigned to NH protons as they are singlets and their
considerable decrease in intensity on D20 exchange. The downfield shifts of these
protons are due to possible hydrogen bonding interactions with adjacent nitrogen
atoms. Hydrogen bonding interaction decreases the electron density around the
proton, and thus causes shifting of resonance absorption to a lower field [23]. The
singlet nature of these peaks is attributed to the absence of any nearby protons for
coupling interactions. The '3 C NMR spectrum of H2L' (Fig. 2.9) shows a well distinct
peak at 177.69 ppm assigned to thione carbon, which is absent in the DEPTl35
spectrum (Fig. 2.9). The absence of any peaks at ~4 ppm for —SH proton [21] in 'H
NMR along with these observations suggest the exclusive presence of the thione
tautomeric form of H2L' in solution phase. The absorptions for pyridyl hydrogen
atoms are seen between 7.19 and 8.73 ppm. As the electronegative pyridyl nitrogen
atoms cause a downfield shift for adjacent protons, the peaks at 8.73 (lH, d), 8.62
(1H, d), 8.50 (1H, d) and 8.21 ppm (1H, d) are assigned due to the protons of carbons
near to pyridyl nitrogens. The remaining signals centered at 5 = 7.92(lH, d), 7.84 (1H,
t), 7.75 (3H, m), 7.55 (1H, d), 7.51 (lH, d), 7.38 (1H, t), 7.31 (1H, t), 7.25 (1H, t),
7.20 ppm (2H, d) are assigned as the remaining twelve protons of the four pyridyl
rings. The BC NMR spectrum shows well distinct 21 peaks, of which two are of
double intensity, showing the presence of 23 carbon atoms and l6 of which are
retained in DEPTl35 spectrum as expected.
5l