Complexes of ortho-nitrophenols with aluminum bromide in nonaqueous solutions

Article abstract of DOI:10.1134/S1070427207060080

The reactions of 2-nitro-, 2-nitro-4,6-dichloro-, 2,4-dinitro-, and 2,4,6-trinitrophenols with aluminum bromide in nitromethane solution were studied by calorimetry and by ¹H and ¹³C NMR.

Full text of DOI:10.1134/S1070427207060080

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 6, pp. 887 890.
Original Russian Text A.V. Golounin, V.A. Sokolenko, O.V. Zakharova, E.A. Shor, M.S. Tovbis, 2007, published in Zhurnal Prikladnoi Khimii, 2007,
Vol. 80, No. 6, pp. 911 914.
Pleiades Publishing, Ltd., 2007.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Complexes of ortho-Nitrophenols with Aluminum Bromide
in Nonaqueous Solutions
A. V. Golounin, V. A. Sokolenko, O. V. Zakharova, E. A. Shor, and M. S. Tovbis
Institute of Chemistry and Chemical Technology, Siberian Division, Russian Academy of Sciences,
Krasnoyarsk, Russia
Received February 2, 2006; in final form, February 2007
Abstract The reactions of 2-nitro-, 2-nitro-4,6-dichloro-, 2,4-dinitro-, and 2,4,6-trinitrophenols with alumi-
num bromide in nitromethane solution were studied by calorimetry and by 1H and 13C NMR.
DOI: 10.1134/S1070427207060080
The reactions of methylated phenols, naphthols,
and polyphenols with strong Lewis acids in non-
aqueous solutions frequently result in tautomeric
transformations [1 4]. Data on the reactions of ni-
trophenols with Lewis acids are searce. For example,
it has been found that the reaction of 4-nitrophenol
with TiCl4 in benzene at room temperature yields
the salt NO2 C6H4 O TiCl3 [5]. In dry methanol,
iron(III) chloride forms a coordination compound with
4-nitrophenol via OH group [6]. Complexes of 2-ni-
trophenol with gallium, aluminum, and boron halides
in organic solutions were studied in [7, 8]. It was
concluded that a coordination bond of metal halides
with the nitro group is formed; however, in [9] this
conclusion was to questioned.
Furthermore, formation of complexes of different
compositions is possible; thus, the pattern becomes
complicated.
To determine the stoichiometry of complexes of
o-nitrophenol I with aluminum bromide, we perform-
ed calorimetric measurements. First, an 0.01 M solu-
tion of o-nitrophenol in bromobenzene was titrated
with an AlBr3 solution at 100 C and then back-titra-
tion was performed. The experimental procedure has
been repeatedly described in the literature [9, 10].
In the dependence of the enthalpy of reaction on
the molar ratio of donor and acceptor mD : mA, there
are two inflections: at the ratios of 1 : 1 and 1 : 2.
The heat effects were evaluated by the procedure de-
scribed in [9], assuming that the enthalpy of reaction
is expressed by the equation
The most complete data on the structure of com-
plexes of nitrophenols with metal halides can be ob-
tained by NMR spectroscopy. Therefore, we syn-
thesized complexes of o-nitrophenols with aluminum
Hc
=
He + 0.5 HAl2Br6
Hsolv ,
1
bromide and recorded H and 13C NMR spectra of
where Hc is the heat of complexation; He, en-
thalpy determined experimentally; HAl2Br6, en-
thalpy of dimer dissociation; and Hsolv, enthalpy
of solvation [4].
these complexes.
It is well known that metal halides are bonded in
nonaqueous solutions with aromatic nitro compounds
via nitro group [10, 11]. In the presence of other donor
groups, such as hydroxy, carbonyl, or amino group,
the coordination site may change. In the case of o-ni-
trophenol, nitro-aci tautomerism I : II is also possible
The first portion of the curve corresponds to Hc =
1
31.5, and the second, to 20.1 kJ mol . The total ther-
1
mal effect is 51.6 1 kJ mol .
To estimate the energy of o-nitrophenol trans-
formation into an aci tautomer, we performed a quan-
tum-chemical calculation. The molecular configura-
tions of I and II were optimized using the restricted
Hartree Fock method. The energies were estimated
by considering correlation effects in the framework of
the Møller Plesset method (MP-2) [13]. In the cal-
H
H
O
N
O
1
O
N
O
6
O
2
O
5
3
4
I
II
887

888
GOLOUNIN et al.
1H Chemical shifts of nitrophenols and complexes in nitromethane
Nitrophenol
2-Nitrophenol
AlBr3
H-2
H-3
H-4
H-5
H-6
OH
1 : 0
1 : 1
1 : 2
1 : 0
1 : 1
8.12
7.89
7.89
8.14
7.84
7.08
6.92
6.92
7.69
7.75
7.75
7.84
7.81
7.80
7.81
7.80
8.49
7.17
6.92
6.92
10.4
8.37
8.13
10.7
8.54
2-Nitro-4,6-dichlorophenol
2,4-Dinitrophenol
1 : 2
7.84
8.28
1 : 0
1 : 1
9.05
8.79
7.45
7.29
7.24
7.29
7.24
10.7
8.98
8.55 8.62
1 : 2
8.79
8.55 8.62
8.65
2,4,6-Trinitrophenol
Nitrobenzene
1 : 0
1 : 1
1 : 2
1 : 0
1 : 1
9.10
8.91
8.91
7.50
7.48
9.10
8.91
8.91
7.50
7.48
11.0
8.64
8.40
8.11
8.09
7.66
7.72
8.11
8.09
1
culations, we used the 6-31G* basis set. The calcula-
ted Ht was 72.7 kJ mol .
strated for nitrobenzene as an example. The H NMR
spectrum of its complex shows no differences from
that of free nitrobenzene, except a certain broadening
of the signals. This fact suggests that, in coordination
with aluminum bromide, the charge on the nitrogen
atom of the nitro group does not noticeably change
[11].
1
H
H
.
Br Al
O
O_
N+
O
O_
N+
3
.
O AlBr3
O
III
IV
AlBr3
H
O
In the spectrum of a solution of 2-nitrophenol with
AlBr3 the H signals are noticeably shifted upfield
1
O
N+
.
(not downfield, as it is observed for complexes of
methylated phenols and naphthols) relative to the sig-
nals of the initial phenol. Hence, it can be assumed
that the coordination mode of 2-nitrophenol differs
from that of nitrobenzene and phenols. The signals
with chemical shifts of 8.37 and 8.13 ppm (approx-
imately 3 : 1) were assigned to hydroxyl protons of
complex compounds, since they have integral inten-
sities corresponding to one hydrogen atom each.
Accidentally, the chemical shifts of the 1 : 1 and 1 : 2
complexes almost coincide; as a result, all the signals
O
V
To find the structure of the complexes, we
recorded their 1H and 13C NMR spectra. The 1H
chemical shifts of the initial phenols and complexes
are listed in the table. The specific feature of the spec-
tra of free o-nitrophenols is that the signals of hy-
droxylic protons are shifted downfield (10 11 ppm)
due to the presence of intramolecular hydrogen bond
[10], contrary to phenols associated by intermolecular
bonds (4 5 ppm) [2].
1
in the spectrum are broadened. The H signal of 3-H
(8.12 ppm, J3, 4 8.55; J3, 5 1.64 Hz in the initial
phenol) is a doublet at 7.89 ppm; in this case, no
Moler remote coupling is observed. The split triplet
of 5-H gives broadened signal at 7.75 ppm. The hy-
drogen atoms 4-H and 6-H appearing in the form of
a split doublet at 7.17 and triplet at 7.08 in the initial
o-nitrophenol in the complex give an unresolved
multiplet in the range 6.8 7.1 ppm.
Attempts to obtain a complex of 2-nitrophenol with
AlBr3 in inert solvents at room temperature failed,
since the reaction is complicated by the release of
gaseous hydrogen bromide. The most suitable solvent
was nitromethane.
The influence of AlBr3 on the chemical shifts of
nitro compounds bound into in a complex was demon-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 6 2007

COMPLEXES OF ortho-NITROPHENOLS WITH ALUMINUM BROMIDE
889
On heating, hydrogen bromide is released from
the solution as a result of formation of 2-NO2 C6H4
O AlnBr3n 1, and the intensities of the signals at 8.37
and 8.13 ppm decrease. All other signals remain un-
changed. This fact indicates that the chemical shifts of
the complexes and aluminum salts are close. A similar
fact has been noted previously [14]. After recording
the spectrum, hydrogen bromide was passed with cool-
ing through the resulting solution until its saturation.
As a result, the intensities of signals of hydroxyl
protons increased to the initial value. The absence of
A similar pattern is observed in the reaction of
2,4,6-trinitrophenol with AlBr3. It is the common
opinion that the yellow color of 2,4,6-trinitrophenol
solution is caused by appearance of an aci structure
1
(type II) [16]. However, both H and 13C NMR sig-
nals correspond to picric acid in the phenolic form.
Upon addition of AlBr3, the color of the solution
becomes deeper and the signal of aromatic protons is
insignificantly shifted upfield. In parallel, two signals
appear at 8.65 and 8.40 ppm, whose intensity corre-
sponds to one proton.
1
any other changes in the H NMR spectrum after
Thus, the NMR spectra show that, irrespective of
the structure of o-nitrophenols, two complexes in
which the aromaticity of the benzene ring is preserved
are formed with AlBr3.
saturation of the solution with HBr suggests that
the complex compounds are protonated neither via
the ring nor via the functional groups.
To elucidate whether the complexes obtained are
actually complexes of aci form II, we recorded
the 13C NMR spectrum of a solution of 2-nitrophenol
with AlBr3. We did not find a signal at 170 190 ppm
(coordinated carbonyl group [15]), which suggests
the absence of a system of double bonds >C=O and
>C=N . This fact is in good agreement with calcula-
tions and calorimetric measurements.
The whole set of the results obtained is well
accounted for by formation of chelate complexes.
When considering a model of chelate complex 1 : 1
V (complex 1 : 2 has apparently the same structure
excluding the fact that aluminum bromide is dimer),
it is seen that AlBr3 should be situated between OH
and NO2 groups. This arrangement of AlBr3 will re-
sult in rupture of the intramolecular hydrogen bond.
Due to the formation of a chelate, the fractional pos-
itive charge on the oxygen atom of the hydroxy group
will be lower than that in the oxonium complex;
therefore, the shift of hydroxyl proton in the NMR
spectra will be intermediate between the shift in
free phenols and oxonium complexes, which is actu-
ally observed. The upfield shift of aromatic protons in
complexes is apparently caused by the anisotropic ef-
fect of aluminum atom in the resulting six-membered
ring.
The effect of substituents on the complexation of
o-nitrophenols was determined in the reactions of
2-nitro-4,6-dichloro-, 2,4-dinitro-, and 2,4,6-trinitro-
phenols with AlBr3 in nitromethane.
In the spectrum of a solution of 2-nitro-4,6-dichlo-
rophenol with AlBr3, the signal of 3-H protons is
shifted upfield from 8.14 to 7.84 ppm, the signal of
5-H proton is split into two (7.81 and 7.80 ppm) but
remains virtually at the same place. In addition, in-
stead of the low-field (10.7 ppm) signal of the hy-
droxyl proton, two signals whose intensity corre-
sponds to one proton appear in higher field at 8.54
and 8.28 ppm (ratio approximately 3 : 1).
The signal of 3-H in the spectrum of a solution of
2,4-dinitrophenol with AlBr3 is shifted upfield from
9.05 to 8.79 ppm. The doublet of 6-H (7.45 ppm) is
shifted to 7.25 ppm. The doublet of 5-H (8.49 ppm)
transforms into a complex set of signals in the range
8.55 8.62 ppm, suggesting the existence of several
complexes in the solution. Two signals of hydroxyl
protons appear in the spectrum: at 8.86 ppm and in
the range of the 5-H signal at 8.65 ppm. In the 13C
NMR spectrum of the 2,4-dinitrophenol complex,
there are signals at 162.8 (C1), 131.0 (C2), 134.3 (C3),
137.0 (C4), 125.3 (C5) and 125.3 ppm (C6). As in
the preceding case, the signals in the range 170
190 ppm corresponding to the coordinated C=O group
[15] were not found.
The model suggested is additionally confirmed by
the 17O NMR spectra obtained previously for the com-
plex 2-NO2 C6H4 OH BCl3 [8]. In the spectrum of
the complex, two signals appear at 560 and 406 ppm,
instead of one signal of two oxygen atoms of nitro
group (569 ppm). The signal of oxygen atom of
the hydroxy group is shifted after coordination from
94 to 144 ppm, i.e., one oxygen atom of nitro group
and oxygen atom of hydroxy group are involved in
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890
GOLOUNIN et al.
the coordination. This fact can be explained by as-
suming that a chelate complex of o-nitrophenol with
BCl3 is formed.
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1
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1
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