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G. Grivani et al. / Polyhedron 68 (2014) 144–150
oxidants H2O2, tert-butylhydroperoxide (TBHP) and KHSO5, H2O2
was the best oxidant in this reaction (Table 3). The salt effect
was also studied for the oxidative bromination reaction of 2-nitro-
phenol. The catalyst was employed using H2O2 as an oxidant and
MBr (M = Na, K, NH4) as a bromine source (Table 3). Among the
three salts, KBr was found to be the most efficient bromine source
and gives a monoselective product. Sodium bromide and ammo-
nium bromide showed less efficiency and they did not produce
regioselective products. In the presence of homogeneous and en-
zyme catalysts, the liberation of Br2 from KBr by H2O2 is well
known [38,56,57]. Thus it was expected that the same situation
may also prevail in the present case and could be well explained
by the proposed mechanism where the generation of Br2 is much
easier in this case. In the absence of catalyst, the reaction mixture
did not produced any brominated product, demonstrating the cat-
alytic role of the oxidovanadium(IV) complex of 1. The effect of the
oxidant and salt amount is given in Fig. 4 and 5, respectively and 2
and 4 mmol were chosen as the optimal amounts, respectively. All
of the above experiments were investigated at room temperature.
The influence of temperature was also studied. The reaction was
monitored at a temperature 80 °C, but there was no appreciable ef-
fect on the conversion and selectivity of 2-nitrophenol, similar to
Ref. [56]. With an increase in temperature the conversion and
selectivity of 2-nitrophenol was almost the same. Then the oxida-
tive brominations of some organic phenyl substituted compounds
were studied (Table 4). A comparison of the TOFs (mol of substrate/
mol of catalyst  time (h)) of recent vanadyl complexes [57–60] or
other metal mediated catalysts [57,61] with the TOF of the titled
vanadyl complex (1) shows the high activity of 1 in the oxidative
bromination reaction. As seen, almost all of the given organic com-
pounds in Table 4 are brominated effectively and selectively to give
only the mono brominated compounds. 2-Nitrophenol, 2-chloro-
phenol, 2-chloro benzaldehyde, salicylaldehyde, 2-hydroxy-3-
methoxy benzaldehyde and 2-hydroxy acetophenone were
selectively and efficiently converted into their monobromo prod-
ucts in the para-position (with respect to the –OH group), whereas
phenol and benzaldehyde were converted to their monobromo
products in the para- and meta-positions.
Fig. 4. The effect of the oxidant amount in the oxidative bromination of 2-
nitrophenol by 1 in the presence of H2SO4 as an acid source and KBr as a bromide
source. Reaction conditions at room temperature: substrate (1 mmol), catalyst
(0.0073 mmol), H2O2 (1 mmol, 2 mmol, 3 mmol), H2SO4 (2 mmol), KBr (4 mmol),
solvent (4 mml).
Fig. 5. The effect of the salt amount in the oxidative bromination of 2-nitrophenol
by 1 in the presence of H2SO4 as an acid source and H2O2 as an oxidant. Reaction
conditions at room temperature: Substrate (1 mmol), catalyst (0.0073 mmol), H2O2
(2 mmol), H2SO4 (2 mmol), KBr (2 mmol, 3 mmol, 4 mmol), solvent (4 mml).
3.4. Thermal study
Thermal gravimetric analysis (TGA) was carried out to examine
the thermal stability of 1 and the nature of the produced vanadium
oxide. The TG profile of 1 in air with a heating rate of 10 °C minÀ1 is
shown in Fig. 6. The absence of any weight loss up to 100 °C indi-
cates that there are no water molecules in the crystalline solid. The
TGA curve of 1 shows two stages of thermal decomposition. The
first stage occurs within the temperature range 220–470 °C, with
a weight loss of 18.67% due to the loss of one NCH2CH2Br unit from
the Schiff base ligand (calcd. 17.83). The second stage, in the
good reactivity of the metal center for H2O2, with the formation of
the peroxovanadium(IV) complex, and the interaction with hypo-
acid HOBr or BrÀ3 , the lipophilicity of the solvent should be im-
proved [38]. Three inorganic acids were used in the oxidative
bromination of 2-nitrophenol and the results showed that a high
yield was obtained in the case of H2SO4 (Table 3). For the homoge-
neous vanadium bromoperoxidase mimics, appreciable catalytic
activity is invariably observed in strong acid [56]. Among the
Table 4
Oxidative bromination of the organic substrates by catalytic amounts of 1 under optimal conditionsa at room temperature.
Substrate
Time (min)
Product
Selectivity (%)
% Yields
TOFb
2-Nitrophenol
2-Chlorophenol
Phenol
20
80
80
4-Bromo-2-nitrophenol
4-Bromo-2-chlorophenol
4-Bromophenol,
100
100
76
100
89.5
56.5
411.5
102.9
102.9
2-Bromophenol
24
Salicylaldehyde
Benzaldehyde
130
140
5-Bromo-2-hydroxy benzaldehyde
4-Bromo benzaldehyde
100
69
100
100
63.6
58.8
2-Bromo benzaldehyde
31
2-Chloro benzaldehyde
2-Hydroxy-3-methoxy-benzaldehyde
2-Hydroxy acetophenone
120
10
20
4-Bromo-2-chloro benzaldehyde
4-Bromo-2-hydroxy-3-methoxy benzaldehyde
4-Bromo-2-hydroxy acetophenone
100
100
100
43.5
100
100
68.5
826.4
411.5
a
Reaction conditions: substrate (1 mmol), catalyst (0.0073 mmol), oxidant (H2O2: 2 mmol), H2SO4 (2 mmol), KBr (4 mmol), solvent (1:3 ratio of H2O/MeOH, 4 mml).
TOF = mol of substrate/mol of catalyst  time (h).
b