Xiao-Lei Zhao, M. Wang, J. Geng et al.
Polyhedron 202 (2021) 115208
hydrazone hydrogen signal, which can be also observed in com-
pound 1 in DMSO d (see Fig. SI4 in Supporting Information). There
6
seems to be a mixture of two hydrazone isomers for these two
pyrazolone ester based heterocyclic dyes, stemming from two
types of hydrogen bonding [32,33]. Namely, one H-bonding is with
an ester carbonyl side carboxy ester group (E-1 and 5), and the
other is with the ketone bonded to the pyrazole ring (Z-1 and 5)
(Scheme 5). Moreover, the molecular-ion peaks for dyes 1 and 5
appear with 100% abundance in the negative modes in methanol
and acetonitrile (Figs. SI6–7), indicating the presence of proposed
species in the reaction mixture.
The electronic absorption spectra for compounds 1–5 are
recorded in methanol in the range of 200–700 nm. As shown in
Fig. 1, the absorption band at 431 nm in 1 could be recognized as
the
form and the band at 250 nm is related to the n–
heterocyclic moiety. In contrast to 1, the absorption maximum
max) value of 5 exhibits a distinct red shift to 472 nm, which
p–p* transition involving the whole molecule in the hydrazone
p* transition of the
(k
may derive from the embedment of additional phenyl ortho–OH
unit. Furthermore, the kmax values of metal complexes 2–4 are sig-
nificantly shifted to higher wavelengths at 508, 508 and
Fig. 1. Normalized UV–Vis absorption spectra for 1–5 in methanol. Acid/base.
4
93/526 nm, respectively, relative to their metal-free ligand 5
L). This can be derived from two reasons, one is the increased
the p-conjugated system of the whole molecule. Actually, the
deprotonated azo form in dye 5 is comparable to those of com-
plexes 2–3, where both of the kmax values are located at 508 nm,
(
H
2
2
ꢀ
ꢀ
electron density by forming divalent L
or monovalent HL
ligands in the deprotonated azo form, and the other is increased
molecular planarity via shaping a fused five- and six-membered
coordination plane. The presence of shoulder absorption bands in
2
originating from the transformation of hydrazone structure H L
to ligand L . Upon the addition of CF COOH (TFA) to the deproto-
3
2ꢀ
nated azo form solution (Fig. 2b and d), the original peaks centered
at 411 and 445 nm are steadily regained, exhibiting that the hydra-
zone-deprotonated azo tautomeric process is fully reversible. Fur-
thermore, the azo-hydrazone tautomerism can be proved by the
observation of one isosbestic point at 367 nm in dye 1 and
470 nm in dye 5.
II
the case of Co complex 4 is ascribed to the formation of a co-crys-
tal form comprising equimolar amounts of 1:1 and 1:2 mononu-
II
clear Co components. As displayed in Figs. SI13–15, the
crystallographic phase of complexes 2–4 was characterized by
the powder X-ray diffraction patterns (PXRD). As for complex 3,
all the experimental diffraction peaks agree well with the simula-
tive ones. In contrast, some extra peaks can be observed in the
experimental curves of complexes 2 and 4 compared to their sim-
ulative ones, which originate from the influence of quick efflores-
cence of single crystals on their crystalline phase.
Heterocyclic dyes having hydroxyl groups in ortho position may
give rise to two distinct isomers: azo and hydrazone isomers,
which may interconvert under certain experimental conditions.
The investigation of interconversion between azo and hydrazone
isomers is still currently a topic of great interest from structures
to spectroscopic properties by acid-base control. Therefore, acid-
base titration experiments have been carried out for heterocyclic
The above-mentioned azo-hydrazone tautomerism can be fur-
ther verified by the significant changes of hydrazone and aromatic
1
hydrogen signals observed in the H NMR spectra. As shown in
Fig. 3a and b, the N–H (13.64 and 13.80 ppm) and phenyl-H proton
signals (7.35–8.31 ppm) decreased in intensity, and a new set of
peaks at d = 8.23, 7.95, 7.44 and 7.24 ppm emerged with the addi-
tion of 4.6 equiv of Et
hydrazone isomers are partially transformed to corresponding
deprotonated azo forms. When the amount of Et N is increased
3 3 3
N to dye 1 in CD CN-d , indicating that the
3
to 228 equiv (Fig. 3c) the previous N–H proton and phenyl-H pro-
ton signals largely vanish, and the emerging peaks at 8.23, 7.95,
1
7.44 and 7.24 ppm are dominant in the H NMR spectra. The gen-
dyes 1 and 5 in their CH
3
CN solutions with the starting concentra-
eral trend was a upfield shift of the aromatic signals. These shifts
indicate that the hydrazone isomers of dye 1 are partially trans-
formed to corresponding deprotonated azo isomer. This result is
in accordance with the changes of aforementioned UV–Vis spectra.
ꢀ5
tion of 3.9 ꢁ 10 mol/L. As illustrated in Fig. 2a and c, the absorp-
tion peaks centered at 411 and 445 nm, which should be ascribed
to
respectively, gradually disappear and new absorption bands at
05 and 511 nm evolve with the addition of different equivalents
equiv) of Et N, accompanied by a color change of the solution
p–p* transitions of the hydrazone isomers of dyes 1 and 5,
3
Upon the addition of 228 equiv of TFA to the CD CN solution of
5
deprotonated azo isomer, the color of the solution changes back
to light yellow, accompanied by the recovery of two H-bonded
(
3
1
from yellow to light red. The red-shifted absorption spectra in
the alkaline condition for dyes 1 and 5 clearly suggest that the for-
mation of deprotonated azo anionic species, which could enrich
the electron density around the chromophoric azo unit, increases
N–H peaks. The H NMR spectrum of dye 1 (Fig. SI5d) shows the
presence of a large and broad signal at 4.1 ppm, presumably result-
ing from excess TFA [33]. It is concluded that there is a hydrazine-
azo tautomeric equilibrium for dyes 1 and 5 in CH CN and the ratio
3
of the hydrazone form is dominating in acidic and original condi-
tions, while the azo form is overwhelming under the alkaline
environment.
OCH CH
CO C O
H CH
3 2
2
3
O
C
H
N
N
O N
2
N
N
N
N
2.2. Structural descriptions of compounds 2–4
N
O N
2
N
R
O
H
O
ORTEP drawing of complexes 2–4 is shown in Fig. 4, and metal-
R
E-1: R = H
Z-1: R = H
involved bond lengths and angles are given in Table SI1. 2 crystal-
ꢀ
E-5: R = OH
Z-5: R = OH
lizes in the triclinic P 1 space group. As shown in Fig. 4, mononu-
Scheme 5. Two possible types of hydrazone isomers for dyes 1 and 5.
clear complex
2
exhibits
a
five-coordinate pyramidal
4