S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
1635
grade solvents were used for spectral and electrochemical stud-
ies and the rest were freshly distilled off before use. All other
chemicals were obtained from commercial sources and used as
such. Elemental analysis were obtained from USIC, Delhi Univer-
sity, India. NMR spectra were recorded on a 300 MHz Bruker-spin
instrument at IIT, Delhi. Electronic spectra were obtained on a
Shimadzu UV-VIS-NIR-1601 spectrometer. Infra red spectra were
recorded on Perkin-Elmer FTIR-2000 spectrometer as KBr pellets.
Cyclic voltammetric studies were performed on BAS CV 50W instru-
ment. The cyclic voltammograms were obtained in a mixed solvent
system 2:8 DMSO: Acetonitrile containing 0.1 M TBAP as support-
ing electrolyte. A three electrode configuration composed of a Pt
disk working electrode (3.1 mm2 area), a Pt wire counter electrode,
and a Ag/AgNO3 reference electrode was used for the measure-
ments. All the above measurements were done at the Department
of Chemistry, University of Delhi, Delhi, India. X-band solid state
EPR spectra of the Mn(II) complexes were recorded at liquid nitro-
gen temperatures, at the Department of chemistry, IIT Kanpur,
India. X-ray data for the complex [Mn(GBOA)2(H2O)2](Cl)2·4H2O
was collected at the Department of Chemistry, IIT, Roorke, India.
Mn(II) complexes were subjected to thermo gravimetric analysis
on a Shimadzu-TA-60 WS instrument. Well ground samples were
taken and heated at rate of 10 ◦C/min in inert nitrogen atmosphere
from room temperature to 600 ◦C at the Department of Chemistry,
University of Delhi, Delhi.
UV–vis [ꢀmax (nm) log ε) in MeOH]; 274(3.52), 281(3.50); E1/2 vs
Ag/Ag+: 0.058 (V).
2.2.2.4. [Mn(GBOA)2(SCN)] (SCN)·4H2O (4). A nearly saturated solu-
tion of KSCN in methanol was added dropwise to a methanolic
solution of MnCl2·4H2O (0.48 mmol), until a precipitate started
appearing. This solution was centrifuged and the clear centrifugate
was added to a methanolic solution of the ligand GBOA (0.48 mmol).
Subsequent work up of the reaction was performed as outlined in
Section 2.2.1.
Yield: 45%, m.p. = 222 ◦C, CHN:(cal): C:49.0, H:4.6, N:19.0, S:6.2,
found: C:48.0, H:4.5, N:18.0, S:6.2. Selected IR data (/cm−1): 3394
(due to water), 3295, 1636, 1543, 1447, 2074. UV–vis [ꢀmax (nm)
(log ) in MeOH]; 270(3.54), 275(3.57), 280(4.18), E1/2 vs Ag/Ag+:
0.080 (V).
2.3. Amine binding and oxidation in presence of
tert-butylhydroperoxide
A
typical reaction consisted of the following:
A 25 ml
flask was fitted with reflux condenser. Manganese com-
a
plexes [Mn(GBOA)2(H2O)2]·(Cl)2·4H2O or [Mn(GBOA)2(H2O)2]·
(CH3COO)2·H2O 0.015 mmol and the respective amines (0.3 and
0.15 mmol) in 10 ml CH3CN were stirred at room temperature for
10 min. Tertiary butyl hydroperoxide (TBHP) [0.02 ml, 0.15 mmol]
was then added and reaction mixture stirred for another 15 min.
The temperature of the reaction mixture during this period was
raised to 50–55 ◦C degrees and maintained on a water bath.
Stirring was continued for 2–21/2 h and the reaction was moni-
tored periodically by withdrawing a small volume of the sample
and developing a chromatogram in ethyl acetate:hexane (0.5:10).
Visualization of the product formed during the course of reac-
tion was carried out by spraying the chromatogram with a
solution of 2,4-dinitrophenylhydrazine (DNP). A single spot was
observed at Rf = 0.76–0.79 cm suggesting the formation of the cor-
responding carbonyl compound during the course of the above
oxidation reaction. The reaction product was isolated by using sil-
ica gel column chromatography and respective DNP derivatives
were prepared. Benzaldehyde was obtained upon oxidation of N-
benzyldimethylamine, while 1-methylpyrollidinone formed from
1-methyl-pyrollidine.
2.2. Synthesis
2.2.1. Preparation of ligand: N,Nꢀ-bis(2-methyl
benzimidazolyl-2yl)-2,2ꢀ oxy diethanamide (GBOA)
The bis-benzimidazole diamide ligand N,Nꢀ-bis(2-methyl
benzimidazolyl-2yl)-2,2ꢀ-oxydiethan amide (GBOA) was prepared
as described before [11].
H
N
H
N
O
O
O
N
NH
HN
N
Structural formula of GBOA
A blank experiment was conducted under identical conditions
as reported above in the absence of the Mn(II) complexes and it was
found that only 5–7% of the amine was converted to the respective
carbonyl product. This therefore confirms the role of the Mn(II)
complexes in the oxidation reaction.
2.2.2. Preparation of Mn(II) complexes
A methanolic solution of MnX2 (0.48 mmol) {X = Cl−, CH3COO−,
ClO4−} was added to the ligand GBOA (0.48 mmol) dissolved in
methanol. The solution was stirred for 1 h. The reaction mixture was
than concentrated to half of its volume on a water bath. Dropwise
addition of a cooled solution of diethyl ether, to the reaction mix-
ture resulted in a white product. This was centrifuged and washed
with small amount of acetonitrile and dried over P2O5.
Identification of the carbonyl compounds formed was confirmed
from 1H NMR of their respective DNP derivatives:
2.3.1. Benzaldehyde-2-4-DNP-derivative
1H NMR (CDCl3): ı (ppm) 11.4(s,1H), 9.1(s,1H), 8.41–8.37(d,1H),
8.16–8.11(dd,2H), 8.00–7.97(d,2H), 7.50–7.48(t,3H).
2.2.2.1. [Mn(GBOA)2(H2O)2].(Cl)2·4H2O(1). Yield:
45%,
m.p. =
190 ◦C; CHN:(cal): C:47.1, H:5.1, N:16.5, found: C:47.8, H:4.6,
N:16.1; selected IR data (/cm−1), 3370 (due to water), 3298, 3100,
1644, 1557, 1449. UV–vis [ꢀmax (nm) (log ε) in MeOH]: 244(4.08),
274(4.12), 282(3.96); E1/2 vs Ag/Ag+: 0.098 V.
2.3.2. 1-Methyl pyrrolidinone-2-4-DNP-derivative
1H NMR (CDCl3):
ı
(ppm) 11.06(s,1H), 9.1(s,1H),
8.32–8.30(d,1H), 8.00–7.96(d,1H), 2.20(t,2H), 2.1(s,3H), 1.26(t,2H),
0.89(q, 2H).
2.2.2.2. [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O(2). Yield: 50%, m.p. =
230 ◦C; CHN:(cal): C:52.2, H:5.14, N:16.6, found: C:51.5, H:5.0,
N:15.7. Selected IR data (/cm−1): 3373 (due to water), 3198, 1664,
1544, 1439, 1419. UV–vis [ꢀmax (nm) (log ε) in MeOH]; 253(3.96),
275(4.02), 281(3.99); E1/2 vs Ag/Ag+: 0.097 V.
2.4. X-ray crystallography
Crystals of the complex [Mn(GBOA)2(H2O)2]Cl2·4H2O (1) were
grown by dissolving the complex synthesized earlier, in methanol
and allowing slow diffusion of diethyl ether to this solution in a
sealed tube. X-ray data for the complex (1) was collected at the
Department of Chemistry, IIT, Roorke, India. The data was collected
on a Nonius CCD using Mo K␣ (ꢀ = 0.71069). The data was corrected
2.2.2.3. [Mn(GBOA)2(ClO4)] (ClO4)·2CH3CN(3). Yield: 55%, m.p. =
215 ◦C; CHN:(cal): C:47.1, H:4.1, N:17.5, found: C:48.0, H:5.1,
N:16.8. Selected IR data (/cm−1): 3280, 1639, 1559, 1450, 1135.