Self-assembled Transition Metal Coordination Frameworks of ...

Chapter 2
10.92 ppm, attributed to hydrogen bonded NH protons, and a peak at 3.45 ppm,
assigned to —SH proton are indicative of the presence of both thione and thiol
tautomeric forms in the solution phase. The presence of more than thirtyfive peaks in
the '3 C NMR spectrum and the disappearance of more than thirteen peaks in the
DEPTl35 spectrum (Fig. 2.13) also support the presence of both tautomeric fonns.
The possibility of syn and anti forms of thiol tautomer is also clear from the number
of peaks. However, the intensities of various peaks confirm that the percentage of
thione form is more compared to thiol forms in CDCI3 solution. The ‘H NMR
spectrum of its oxygen analogue HZLS (Fig. 2.14) is also rich in peaks, but with the
signs of much less amounts of enol tautomer compared to the tautomeric forms in
HZLZ solution. Here the downfield singlets at 5= 14.27, 12.66, 12.25, 9.83 and 8.02
ppm are found to loss its intensity considerably on D20 exchange showing the
exchangeability of corresponding protons. The last one may be attributed due to —OH
proton of the enol form and 12.25, 9.83 ppm peaks are assigned to hydrogen bonded
NI-I protons of the ketone form, while the peak at 14.27 ppm is assigned to the same of
the enol tautomer. The signal at 12.66 ppm is of insignificant intensity may be
indicating both syn and anti confirmation of the enol form. The signals of the pyridine
and phenyl hydrogens are seen in between 6.93 and 8.70 ppm. The '3 C NMR
spectrum (Fig. 2.15), however requires a highly concentrated solution to show all
signals of both tautomers, not showed much resolved signals which might be due to
the low concentration of the enol tautomer in solution. The peak at 155.42 ppm is
assigned to carbonyl group, and is absent in the DEPTl35 spectrum. The DEPTl35
spectrum retains more than 18 signals, with a new peak at 129.48 ppm, suggests the
presence of the enol tautomer, and makes assignments difficult with the unequal
intensities.
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