´
ˇ
Z. Neuerova and A. Lycka
Dyes and Pigments 188 (2021) 109149
2.2. NMR spectroscopy
for 5-methyl-2-phenyl-4-(2-phenylhydrazono)-2,4-dihydro-3H-pyrazo-
l-3-one in deuteriochloroform.
The 1H, 13C and 15N NMR spectra were recorded on a Bruker Avance
III HD 400 spectrometer operating at 400.13 MHz for 1H, 100.62 MHz
for 13C and at 40.56 MHz for 15N and using liquid nitrogen-cooled 5-mm
Prodigy cryo probe (Bruker) at 300 K. The samples were dissolved in
deuteriochloroform. The 1H and 13C chemical shifts were referenced to
internal TMS (δ = 0.00). The 15N chemical shifts were referred to the
signal of external nitromethane placed in a co-axial capillary (δ = 0.0).
All 2D experiments (gradient-selected (gs)-COSY, gs-NOESY, gs-HMQC,
gs-HSQC, gs-HSQC-TOCSY, gs-HMBC) and 1D 1H–15N gs-HSQC were
performed using manufacturer’s software (TOPSPIN 3.5) [45–47].
[
ꢀ
)
/
ꢀ ꢀ
) )]
%Hydrazone = 1J 15Nα, 1H exp 1J 15Nα, 1H
*100
(1)
H
The results are shown in Table 2 differing from 63.5% for 1–91.2%
for 5.
Analogously, 15N chemical shifts of Nα and Nβ were used for hydra-
zone form content calculation [13,14,18] using Equation (2)
[ ꢀ
)
ꢀ
)
]/[
ꢀ
)
ꢀ
)
]
%Hydrazone= δ 15Nα/β expꢀ 15Nα/β
H
δ
15Nα/β Aꢀ δ 15Nα/β H *100
(2)
The 15N chemical shifts of Nα and Nβ are again shown in Table 2 to
demonstrate their changes and to calculate hydrazone forms contents.
Weighted average values of hydrazone form content was calculated
using all three results being subsequently used for determination of
equilibrium constants K = [hydrazo form]/[azo form].
3. Results and discussions
Azo-hydrazo tautomerism is a typical feature of azo dyes containing
hydroxy group in an appropriate position(s). In most cases, the influence
of substituents on azo/hydrazo tautomeric equilibria has been studied
for compound being substituted in so called active components (i.e. in
diazonium salts) since a wide variety of substituted anilines is very easily
available.
The calculated (average) hydrazo form contents differ considerable
for compounds 1–5 being strongly substituent dependent. Previously
published influence of substituents in diazonium salts showed that more
electron acceptor types of substituents cause increase of hydrazo forms
[6,32,10,29]. Lin et al. presented [24] an approximate correlation of K
In this paper, we study one- and two-dimensional 15N, 13C and H
1
= [hydrazo form]/[azo form] on
σ
+ Hammett constants. A part of it is
NMR spectra in compounds 1–5 (Scheme 1) prepared by coupling of
benzenediazonium chloride with 4-substituted naphthalen-1-ol. The
substituents were chosen in such a manner so that we could cover non-
substituted naphthalen-1-ol, one strongly electron accepting substituent
(NO2), two halogenes and one very electron donating substituent
(OCH3).
presented in Fig. 1 having positive slope contrary to our data having
negative slope of a curve. The correlation is not too precise, however, the
opposite trends are clearly visible.
Hydrazone form content in compound 1, having nitro group in po-
sition 4 of naphthalene, was ca 64% (Table 2). According to literature
data mentioned above [6,32,10,29], an electron accepting group in
position 4 of diazonium salt should increase hydrazone form content. To
test this expectation also in our series of compounds, we prepared
compound 6:
The one- and two-dimensional 15N, 13C and 1H NMR spectra of
compounds 1–5 were measured and very thoroughly analysed. 2D
gradient-selected (gs)-COSY, gs-NOESY, gs-HMQC, gs-HSQC, gs-HSQC-
TOCSY and gs-HMBC) and 1D 1H–15N gs-HSQC were performed. The
obtained results are collected in Table 1.
Lin et al. [24] performed 1H and 13C NMR study of a set of ten 1-[(E)-
(3- or 4-subst. phenyl)diazenyl]naphtalen-2-ol in several solvents.
Azo/hydrazone forms content estimation is based on changes of 13C
chemical shifts of carbon C(2) = O/C(2)-OH. We cannot use this
approach since 13C chemical shifts of carbon C(1) = O/C(1)-OH in
compounds 1–5 are strongly influenced by substituent chemical shifts
(SCS) effect of substituents R from position 4 (SCS for nitro group is
+6.8 ppm, for bromine ꢀ 0.2 ppm, while SCS for methoxy group is – 7.6
ppm) [48]. After “a correction” of experimental 13C chemical shifts for
C-1, we can obtain the following monotonously increasing values of C
(1) = O/C(1)-OH 165.0, 173.2, 174.2 and 184.6 ppm for compounds 1,
2 and 4, 5. On the other hand, we could use 13C chemical shifts of carbon
C(2′) or C(4′) of phenyl group present in all compounds 1–5, however,
the differences are rather small (Table 1).
The solubility of compound 6 in deuteriochloroform is very low,
however, we succeeded in measuring 1J(15N, 1H) exp and δ(15Nα) by long
term accumulation of NMR spectra.
increased to 82.4 Hz comparedewxpith 63.5 Hz in compound 1 (Table 2)
clearly proving the above-mentioned expectation that hydrazone form
content should increase considerably. The calculated hydrazone form
content is 85.4%. The experimental 15N chemical shift of δ(15Nα) is
ꢀ 154.6 ppm (contrary to ꢀ 108.9 ppm in compound 1), and calculated
The value of 1J(15N, 1H)
coupling constant in compound 6
Much greater changes in 1J(15Nα, 1H) exp coupling constants and 15
N
chemical shifts of Nα and Nb for compounds 1–5 are collected in Table 2.
Hydrazone form content was calculated using equation (1) using 1J
´
(
15Nα, 1H) H = 96.5 Hz as proposed by Bekarek et al. [15,16] measured
Scheme 1. R = NO2 (1), Br (2), Cl (3), H (4), OCH3 (5).
2
Neuerová, Zdeňka
