Catalytic alcoholysis of quinolin-8-yl esters by manganese complexes

Article abstract of DOI:10.1016/j.inoche.2009.04.023

The role of different oxidation states of manganese in manganese(II) acetate promoted methanolysis reaction of p-nitrobenzoic acid quinolin-8-yl ester is presented.

Full text of DOI:10.1016/j.inoche.2009.04.023

Inorganic Chemistry Communications 12 (2009) 569–571
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Catalytic alcoholysis of quinolin-8-yl esters by manganese complexes
*
Dipjyoti Kalita, Rupam Sarma, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781 039 Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The role of different oxidation states of manganese in manganese(II) acetate promoted methanolysis
reaction of p-nitrobenzoic acid quinolin-8-yl ester is presented.
Ó 2009 Elsevier B.V. All rights reserved.
Received 10 January 2009
Accepted 20 April 2009
Available online 3 May 2009
Keywords:
Quinolin-8-yl esters
Alcoholysis
Catalysis
Manganese complexes
Crystal structures
Quinoline derivatives are used as drugs for malaria, arthritis,
and lupus [1]. Control release of such drugs is essential in curing
and there exists a reasonable amount of scope to study and design
probes for monitoring degradation of quinoline derivatives for
such studies [2]. Bio-compatible methods, for such study is of
prime concern [3]. We show here the use of near-IR spectroscopy
as a tool to study the alcoholysis of p-nitrobenzoic acid quinolin-
8-yl ester for its transformation to manganese(III) complex and
also compare such results with other related systems to establish
the catalytic role of manganese.
The reaction between the p-nitrobenzoic acid quinolin-8-yl
ester and methanol or ethanol is catalysed by manganese(II) ace-
tate tetrahydrate under aerobic conditions [4]. In the process the
manganese(II) gets oxidized to manganese(III). The tris-(8-oxyqui-
nolato) manganese(III) is formed during these reactions (Eq. (1)).
quinoline under aerobic condition leads to the formation of tris-
(8-oxyquinolinato)manganese(III) [5]. Manganese(III) complexes
are useful catalysts for hydrolysis reactions of esters [6]. The man-
ganese(II) complexes can be easily oxidized from +2 to +3 oxidation
state in presence of oxygen as oxidizing agent. The complex tris-(8-
oxyquinolinato)manganese(III) has a d4-electronic configuration
and it posses a near-IR absorption at 950 nm due to d–d transition
between dx2
and d2z orbitals arising from Jahn–Teller distortion.
ꢀy2
Similar near IR absorptions are reported in manganese(III) com-
plexes [7]. The formation of tris-(8-oxyquinolinato)manganese(III)
can be monitored by recording the changes in absorbances at
950 nm. It shows that the within 90 min complete formation of
tris-(8-oxyquinolinato)manganese(III) takes place from the reaction
of manganese(II) acetate with 8-hydroxyquinoline in methanol.
Since in the alcoholysis of p-nitrobenzoic acid quinolin-8-yl ester
the complex tris-(8-oxyquinolinato)manganese(III) was formed,
the formation of ester could be monitored by the increase in absor-
bance of this peak. The reactions were monitored at 950 nm and it
was found that complete conversion to tris-(8-oxyquinolinato)man-
ganese(III) from p-nitrobenzoic acid quinolin-8-yl ester on reaction
with manganese(II) acetate (3:1 molar ratio) in methanol took
about 160 min, whereas the similar reaction of benzoic acid quino-
lin-8-yl ester took approximately 1100 min. The change of absorp-
tion at 950 nm in the two cases is illustrated in Fig. 1a and b. The
similar reaction with ethanol was also carried out and observed that
ethanolysis of p-nitrobenzoic acid quinolin-8-yl ester in the pres-
ence manganese(II) acetate (3:1 molar ratio); it took place in
140 min. The alcoholysis in other solvents such as iso-propanol, t-
butanol, were attempted and they were found to be not effective
and in these cases hydrolysis took place rather than alcoholysis
with the traces of moisture present in the system.
R
R
O
N
N
R1OH
Mn
ð1Þ
O
O
+
O
O
N
Mn(II)acetate / O2
N
O
R1O
R1 = CH3, CH3CH2
R = H, NO2
We have also confirmed the formation of tris-(8-oxyquinolato)
manganese(III) by solving its crystal structure. It is reported in liter-
ature that the reaction of manganese(II) acetate with 8-hydroxy-
* Corresponding author.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.04.023

570
D. Kalita et al. / Inorganic Chemistry Communications 12 (2009) 569–571
nese(III) ion, that is responsible for these catalytic reactions. Based
on the above observations the mechanistic path for the catalytic
reactions can be described by Scheme 1. The manganese(III) com-
plexes are generally labile, upon interaction with the ligand they
activate the ester group of the p-nitrobenzoic acid quinolin-8-yl es-
ter for nucleophilic attack by methanol. In the process the manga-
nese(III) oxy-quinolinato complex is formed, and along with this
protons are liberated. The liberated protons further triggers the
catalytic cycle and a catalytic reaction is observed. In the case of
manganese(II) acetate, the manganese(II) gets converted to a man-
ganese(III) by arial oxygen; which in turns further takes part in the
catalytic cycle.
Further proof to the catalytic role of manganese(III) comes from
the catalytic ability of tris-acetylacetonato manganese(III) on the
methanolysis of p-nitrobenzoic acid quinolin-8-yl ester. In this
case we had observed a higher catalytic ability of tris-acetylaceto-
nato manganese(III) over tris-(8-oxyquinolinato)manganese(III).
This ability is attributed to the lability of the two ligands in the
complexes. The acetylacetonato groups of the tris-acetylacetonato
manganese(III) can be replaced by 8-oxyquinolinato group easily.
Thus, the tris-(8-oxyquinolinato)manganese(III) can be easily pre-
pared from a methanolic solution of tris-acetylacetonato manga-
nese(III) as illustrated in Eq. (2):
Fig. 1. The changes in absorbance at 950 nm in the reaction of manganese(II)
acetate with (a) p-nitrobenzoic acid quinolin-8-yl ester; (b) benzoic acid quinolin-
8-yl (1:3 molar ratio) in methanol.
MnðAcAcÞ3 þ HQ M!eOH MnðHQÞ3
ð2Þ
where AcAc = acetylacetonato anion.
In order to understand the reaction further we have checked
the reactivity of benzoic acid naphthalene-1-yl ester and p-nitro-
benzoic acid naphthalene-1-yl ester with methanol in the pres-
ence of manganese(II) acetate. We have tried the catalytic
methanolysis reaction of N-quinolin-8-yl-benzimide and observed
no reactions. However, the reaction of manganese(II) acetate with
8-aminoquinoline and benzoic acid together lead to a trinuclear
complex I (Eq. (3)). The trinuclear complex I (Fig. 3, left) is char-
acterized by solving its crystal structure [8] and similar trinuclear
complexes are well known in literature [9]. The reaction of
methyl cinnamate with ethanol was studied with manganese(II)
acetate catalyst but we have not observed catalytic reaction at
ambient condition. However, in the reaction of cinnamic acid
with manganese(II) acetate we observed the formation of co-ordi-
nation polymer II (Eq. (4)). The polymer comprises of two bridg-
ing carboxylates and two aqua ligands (Fig. 3, right). It has
distorted octahedral geometry around the each manganese center
[8]:
Fig. 2. Plot of
% conversion of p-nitrobenzoic acid quinolin-8-yl ester to 8-
hydroxyquinoline with time: by catalytic amount of (A) tris-(8-oxyquinolina-
to)manganese(III) (5 mol%); (B) tris-acetylacetonato manganese(III) (5 mol%).
Since tris-(8-oxyquinolinato)manganese(III) was easily formed
in these reactions we had independently studied the catalytic reac-
tions of this complex for methanolysis of p-nitrobenzoic acid quin-
olin-8-yl ester. The catalytic methanolysis was monitored by using
GC–MS and the percentage conversion vs time is presented in
Fig. 2. It was found that the reaction is also catalysed by tris-(8-
oxyquinolinato)manganese(III) complex. Thus, it is the manga-
O
O
NO2
+A rial O2
N
MnL2
NO2
N
3+
Mn
O
MeO
-
-
-L
N
O
O
-L
+
O
+H+
3-n
nL
3
+M
Mn
MeOH
O
O
N
3
O2N
O2N
n= 1,2
-
+H +
L
-
-
L= [CH3COCHCOCH3
]
or [O2CCH3]
Scheme 1.

D. Kalita et al. / Inorganic Chemistry Communications 12 (2009) 569–571
571
II
I
Fig. 3. The crystal structures of left I (hydrogen atoms in I are omitted for clarity) and right is the crystal structure of polymer II.
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2009.04.023.
NH2
N
Mn(II)acetate
+
[Mn3(AQ)2(BzO)6]
ð3Þ
O
HO
BzOH
AQ
References
I
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350;
OH
R1OH/H2O
O
[Mn(Cin)2(H2O)2]n
ð4Þ
Mn(II)acetate
CinH
II
These results also showed that some of the esters like methyl cinna-
mate or N-quinolin-8-yl-benzimide do not undergo catalytic hydro-
lysis; whereas, cinnamic acid or aminoquinoline which can be
derived from such hydrolysis of such esters lead to stable manga-
nese(II) complexes. On the other hand, in the case of p-nitrobenzoic
acid quinolin-8-yl ester a manganese(III) complex is formed on reac-
tion with manganese(II) acetate, which also can be prepared directly
by reacting manganese(II) acetate with 8-hydroxyquinoline. This
depicts that, the propensity for oxidation of manganese(II) to form
labile manganese(III) chelate complex along with the ease of trigger-
ing of protons from the esters by attack of an alcohol, are the two
important factors that decides the described catalytic process.
(d) F.J. Steemers, W. Verboom, J.W. Hofstraat, F.A.J. Geurts, D.N. Reinhoudt,
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[4] In
a
typical experiment manganese(II) acetate tetrahydrate (0.005 g,
well stirred solution of p-nitrobenzoic acid
0.02 mmol) was added to
a
quinolin-8-yl ester (0.25 g, 1 mmol) in 10 ml methanol, and the reaction
mixture was stirred at room temperature for 12 h. The reaction mixture turned
black. To the reaction mixture 10 ml water was added. The reaction mixture
was further extracted with ethylacetate (30 ml). The ethylacetate extract was
dried over anhydrous sodiumsulphate; the extract was concentrated and
purified by column chromatography using silica gel and using ethylacetate (5%)
in petroleum ether as eluent. On purification methyl-p-nitrobenzoate was
obtained in 98% yield.
[5] A.R. Burns, T.J. Cardwell, R.W. Cattrall, Aust. J. Chem. 24 (1971) 661.
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Acknowledgements
[7] M. Ray, S. Mukherjee, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1990) 3635.
[8] Crystal data for I: C60H46Mn3N4O12, Mr = 1179.83, Triclinic, space group P-1,
The authors thank Department of Science and Technology, New
Delhi, India for financial support. D.J.K. and R.S. thank Council of
Scientific and Industrial Research, New Delhi, India for Research
Fellowships.
a = 10.9221(3) Å,
b = 78.658(2)°,
F(0 0 0) = 605, Abs. Coeff./mmꢀ1 = 0.765, 12132 reflection collected, 4609
unique, indices [I > 2 (I)] = 0.0393, indices (all data) = 0.0512, and
gof = 1.028. For II: C18H18MnO6, Mr = 385.26, Monoclinic, space group, C2/c,
a = 37.038(4) Å, b = 6.4832(6) Å, c = 7.4521(7) Å, = 90.00°, b = 91.787(10)°,
= 90.00°, V = 1788.6(3) Å3, Z = 4, density = 1.431 g/cm3, F(0 0 0) = 796, Abs.
Coeff./mmꢀ1 = 0.768, 6851 reflection collected, 1615 unique,
indices
[I > 2 (I)] = 0.0753, R indices (all data) = 0.0875, and gof = 1.238.
[9] A.M. Baruah, A. Karmakar, J.B. Baruah, Open Inorg. Chem. J. 2 (2008) 62.
b = 11.4069(3) Å,
c = 12.1288(3) Å,
a = 65.8510(10)°,
c
= 78.180(2)°, V = 1338.79(6) Å3, Z = 1, density = 1.463 g/cm3,
R
r
R
a
Appendix A. Supplementary material
c
R
CCDC 677170 and 710241 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
r