M. Khosravan et al.
Inorganic Chemistry Communications 128 (2021) 108561
Table 1
Three component reaction of dimedone, malononitrile and aromatic aldehydes.
Entry
Ar
Time (min)
Yield (%)
M.p. (◦C)
Found
Reported
1a
1b
1c
1d
1e
C6H5
45
45
50
55
50
97
95
87
88
85
231–233
214–216
216–218
176–178
217–219
231–232 [29]
215–217 [29]
217–219 [31]
175–178 [29]
218–220 [29]
4-Cl-C6H4
3-NO2-C6H4
2,4-(Cl)2-C6H3
4-CH3-C6H4
Table 2
Comparison between previous catalysts used for the synthesis of 2-amino-7,7-
dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile and
[VO2(L)](NHET3) used in this study.
Catalyst
Reaction
Time
Yield
(%)
Reference
conditions
(min)
[VO(L)(phen)]
Triethylbenzylammonium
chloride
EtOH, reflux
20
90
90
[29]
[32]
H2O, 90 ◦
C
240
Fig. 1. Perspective view of [VO2(L)] (NHET3).
geometry of the pentacoordinated complexes, trigonality index (τ5
K3PO4
EtOH, stirred
45
20
20
94
92
92
[31]
[2]
[Cu(L′)(Imi)] (8%mol)a
Tetrabutylammonium
bromide
EtOH, 50 ◦
C
)
EtOH, reflux
[33]
described by Addison et al. is calculated [28]. τ5 = 0 indicates an ideal
square–pyramid while τ5 = 1 indicates an ideal trigonal bipyramid. The
values of τ5 for [VO2(L)] (NHET3) (1) and [VO2(L)] (NHET3) (2) were
calculated to be 0.21 and 0.14, respectively, confirming the distorted
square pyramidal geometry of both molecules in the assyemtric unit. In
the structure of the complex, the coordination sphere of the vanadium
ion are occupied by two oxido groups (in cis-position) and ONO donor
atoms of the dianionic Schiff base ligand. The nitrogen atom of the imino
group and the oxygen atoms of the naphtholate and phenolate groups
are the donor atoms of the deporotonated Schiff base [L2ꢀ ].
[VO2(L)](NHET3)
Acetonitrile,
45
97
This work
50 ◦
C
an initial optimization was conducted on the reaction conditions (tem-
perature, solvent, and catalyst loading). Three different solvents (DI
water, acetonitrile, and ethanol), three different levels of catalyst
loading (5, 10, and 20 mol%), and four levels of temperature (30, 50, 80,
100 ◦C) were used in this optimization. The title V(V) complex acted as a
homogenous catalyst and finally column chromatography was used for
the purification of the product. For a complete list of the results obtained
from this optimization, one can refer to Tables S3–S5. As one can see, the
di-oxido vanadium(V) complex shows an optimum catalytic perfor-
mance (97% yield and the reaction time of 45 min) in acetonitrile at
50 ◦C, using 10 mol% catalyst loading for the synthesis of 2-amino-7,7-
dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carboni-
trile (1a). Using the optimal conditions, various tetrahydro-4H-
chromene derivatives were synthesized [29,31]. Table 1 reports the
obtained reaction time and yield for the synthesis of tetrahydro-4H-
chromenes using the aromatic aldehydes. According to the obtained
results, the nature and position of the substituents on the aromatic ring
have no discernible impact on the success of the reaction. In addition,
the recyclability of the di-oxido vanadium(V) complex was studied by
monitoring the efficiency of the catalyst after three consecutive runs
under the optimal conditions (1a: 97%, 95%, 93%). The efficiency of the
catalyst was reduced by only 4% after three consecutive runs, which
demonstrated the ability of the catalyst for multiple uses.
In the complex structure, the triethylammonium cation is strongly
bonded to the apical oxygen atom via hydrogen bonding. According to
the obtained results (Fig. S2), the FT-IR spectrum of [H2L] ligand
showed a vibrational band at 1630 cmꢀ 1, which can be attributed to the
–
C
N group and approved the formation of a Schiff base ligand. The
–
–
–
tensile vibration of OH and vibration band of C C in the aromatic rings
can be seen at 3447 cmꢀ 1 and 1406–1459 cmꢀ 1, respectively. After
–
–
complexation, the frequency of the mentioned azomethine (C N) band
at 1630 cmꢀ 1 decreased to 1615 cmꢀ 1 [8]. Two absorption bands
observed at 987 cmꢀ 1 and 824 cmꢀ 1 are attributed to the symmetric and
asymmetric stretching vibration bands of cis-VO2, respectively [18]. In
addition, the vibrations appeared at 459 cmꢀ 1, 506 cmꢀ 1 and 2977 cmꢀ 1
– –
–
can be related to the V N, V O, and N H bonds, respectively. Two
absorption bands at 2496 cmꢀ 1 and 2602 cmꢀ 1 were also ascribed to
CH2 and CH3 groups of triethyl amine, respectively [29]. According to
the 1H NMR measurements (Fig. S3), three singlet signals were observed
at 8.3 ppm, 10.8 ppm, and 12.0 ppm, which can be assigned to the
–
–
–
protons of CH N, amine OH, and aldehyde O H, respectively [9]. The
In order to compare the performance of [VO2(L)](NHET3) complex,
the obtained results in this study were compared with those of the
previously catalysts used in the catalytic synthesis of 1a. As can be seen
in Table 2, the yield, reaction time, and temperature of the reaction are
superior or comparable with the other catalysts [2,29,32–33]. The
possible mechanism proposed for the synthesis of tetrahydro-4H-
chromene derivatives catalyzed by the di-oxido-vanadium(V) complex
is shown in Fig. 2.
presence of the azomethine proton confirms the formation of the ligand,
while the protons of the aromatic rings are observed in the range of
7.2–8.1 ppm [2]. In title V(V) complex, the protons of the azomethin
moiety and aromatic rings were spotted at 10.1 and 6.7–9.3 ppm
respectivley. The triplet and quaternary peaks observed at 1.2 ppm and
3.1 ppm are assigned to the CH3 and CH2 groups of triethylamine [30].
In the UV–Vis spectrum of the [H2L] ligand (Fig. S4), three absorption
bands appeared at 320 nm, 450 nm, and 470 nm can be attributed to
* transition in the aromatic rings, * transition in the azome-
thine, and n → * transition in azomethine, respectively. In the case of
π
The antibacterial activity of [H2L] and [VO2(L)](NHET3) complex
against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa,
and Bacillus cereus bacteria were evaluated using inhibition zone and
MIC assays (Fig. 3). According to the obtained results (Table 3), [H2L]
ligand is effective on all of the studied bacteria, except E. coli. However,
the [VO2(L)](NHET3) complex have higher antibacterial activity against
both gram-negative and gram-positive bacteria, especially Staphylo-
→
π
π → π
π
the V(V) complex, the absorption band corresponded to the
π → π*
transition is observed at 345 nm. The absorption peak appeared at 455
nm is related to the charge transfer from the ligand to the metal (LMCT)
[2,6].
The title complex was used in the synthesis process of tetrahydro-4H-
chromene derivatives in order to evaluate its catalytic activity. At first,
coccus aureus. Interestingly, this complex shows
a considerable
2