Synthesis, spectral and catalytic activity of some Manganese(II) bis-benzimidazole diamide complexes

Article abstract of DOI:10.1016/j.saa.2011.05.024

Four Mn(II) complexes bound to a neutral bis-benzimidazole diamide ligand N,N′-bis(2-methyl benzimidazolyl 2,2′-oxy-diethanamide) (GBOA) have been synthesized and characterized. Anionic ligand associated with the complexes varies as Cl^- CH₃COO^-, SCN^- and ClO₄^-. X-ray structure of one of the complexes [Mn(GBOA)₂(H₂O)₂]Cl₂·4H ₂O was solved and shows that the Mn(II) ion is hexacoordinate. Two equatorial positions are occupied by benzimidazole imine nitrogen atoms while the other two sites are occupied by amide carbonyl oxygens. The imine nitrogen and carbonyl oxygens are bound to Mn(II) by different arms of the two ligands while axial sites are occupied by two water molecules. Two Cl^- anions are outside the coordination sphere and form an extensive 3D H-bonded network. Axially distorted octahedral geometry is confirmed for all the four complexes by low temperature EPR spectroscopy. Distortion parameter D was found to be similar for [Mn(GBOA)₂(H₂O)₂]Cl ₂·4H₂O and [Mn(GBOA)₂(H ₂O)₂]·(CH₃COO)₂·H ₂O. Cyclic voltammograms have been obtained for all the four complexes and E_1/2 values are dependant on the anionic ligand being in the coordination sphere or outside. [Mn(GBOA)₂(H ₂O)₂]Cl₂·4H₂O and [Mn(GBOA)₂(H₂O)₂]·(CH ₃COO)₂·H₂O carry out the selective oxidation of N-benzyldimethylamine, and 1-methyl-pyrollidine to their respective carbonyl products with catalytic efficiency of 35-50%.

Full text of DOI:10.1016/j.saa.2011.05.024

Spectrochimica Acta Part A 79 (2011) 1634–1641
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral and catalytic activity of some Manganese(II)
bis-benzimidazole diamide complexes
Subash Chandra Mohapatraa, M.S. Hundalb, Pavan Mathura,∗
a Department of Chemistry, University of Delhi, Delhi 110007, India
b Guru Nanak Dev University, Amritsar, India
a r t i c l e i n f o
a b s t r a c t
Four Mn(II) complexes bound to a neutral bis-benzimidazole diamide ligand N,Nꢀ-bis(2-methyl ben-
zimidazolyl 2,2ꢀ-oxy-diethanamide) (GBOA) have been synthesized and characterized. Anionic ligand
associated with the complexes varies as Cl− CH3COO−, SCN− and ClO4−. X-ray structure of one of the
complexes [Mn(GBOA)2(H2O)2]Cl2·4H2O was solved and shows that the Mn(II) ion is hexacoordinate.
Two equatorial positions are occupied by benzimidazole imine nitrogen atoms while the other two
sites are occupied by amide carbonyl oxygens. The imine nitrogen and carbonyl oxygens are bound
to Mn(II) by different arms of the two ligands while axial sites are occupied by two water molecules.
Two Cl− anions are outside the coordination sphere and form an extensive 3D H-bonded network. Axi-
ally distorted octahedral geometry is confirmed for all the four complexes by low temperature EPR
spectroscopy. Distortion parameter D was found to be similar for [Mn(GBOA)2(H2O)2]Cl2·4H2O and
[Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O. Cyclic voltammograms have been obtained for all the four com-
plexes and E1/2 values are dependant on the anionic ligand being in the coordination sphere or outside.
[Mn(GBOA)2(H2O)2]Cl2·4H2O and [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O carry out the selective oxidation
of N-benzyldimethylamine, and 1-methyl-pyrollidine to their respective carbonyl products with catalytic
efficiency of 35–50%.
Article history:
Received 22 October 2010
Received in revised form 10 May 2011
Accepted 16 May 2011
Keywords:
Benzimidazole diamide
Manganese(II)-Diaqua complex
Hydrogen bonding network
Amine oxidation
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Di-aqua Mn(II) complex bound to a bisbenzimidazolyl diamide
ligand; in this context it may be pertinent to recall the role of
Multidentate ligands have the ability to chelate to metal ions
in a terminating fashion and thus prevent the formation of poly-
meric species. However if some sites are left out during the process
of chelation these could end up forming intermolecular Hydro-
gen bonds and facilitate the formation of extended networks. Such
Hydrogen bonding networks have gained attention in view of their
capability to generate self assembled supramolecular structures
[1–4]. The conventional OH· · ·N and NH· · ·O along with ␲–␲ stack-
ing interactionshave been found to be the most common cementing
force for such supramolecular architectures [5–7]. The present
study reports the synthesis and spectral studies of four new Mn(II)
Complexes. One of the complex [Mn(GBOA)2(H2O)2]Cl2·4H2O has
been structurally characterized, interestingly the anionic ligand Cl−
is found to be outside the coordination sphere, while two water
molecules are coordinated. This leads to extensive intermolecular
H-bonding interactions leading to the formation of helical sheets
and solvent accessible voids. These positively charged sheets are
held by extensive Cl· · ·HO and Cl· · ·HN, H-bonding network in the
chloride ions in photosystem II, where it is proposed that Cl−
ions maintain a proton relay network that allows the transport
and release of protons during the water oxidation reaction [8].
Low temperature EPR study has been used to obtain the axial
distortion parameter D for all the complexes and it is found to
be dependant on whether the anionic ligand is within the coor-
dination sphere or outside. E1/2 values have been obtained from
the respective cyclic voltammograms and they are found to be
related to the D parameter found from EPR work, larger the
D value less anodic the E1/2. [Mn(GBOA)2(H2O)2]Cl2·4H2O and
[Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O are found to carry out the oxi-
dation of amines to carbonyl products in the presence of tert.butyl
hydroperoxide as an external oxidant, and mimics the amine oxi-
dation activity of a Manganese(II) dependant peroxidase sourced
from lignin [9].
2. Experimental
2.1. Materials and physical measurements
∗
Glycine benzimidazole dihydrocloride was prepared by follow-
ing the procedure reported by Cescon and Day [10]. Spectroscopic
Corresponding author. Tel.: +91 1127667725/1381; fax: +91 1127666605.
E-mail address: pavanmat@yahoo.co.in (P. Mathur).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.024

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.

1636
S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
Table 1
Crystal data and structure refinement for [Mn(GBOA)2(H2O)2]Cl2·4H2O complex.
Empirical formula
C40 H52 Cl2 Mn N12 O12
Formula weight
Temperature
Wavelength
1018.78
293(2) K
0.71069 A
˚
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/c
a = 19.992(5) A
b = 10.001(5) A
˛ = 90.000(5)◦
ˇ = 91.343(5)◦
ꢁ = 90.000(5)◦
˚
˚
˚
c = 22.971(5) A
3
˚
Volume
4592(3) A
Z
4
Density (calculated)
Absorption coefficient
F(000)
Theta range for data collection
Index ranges
1.474 Mg/m3
0.479 mm−1
2124
1.02–23.71◦
−22 ≤ h ≤ 22, −11 ≤ k ≤ 11,
−25 ≤ l ≤ 25
Reflections collected
52,992
Fig. 1. TGA spectra of ligand (GBOA), [Mn(GBOA)2(SCN)] (SCN)·4H2O,
[Mn(GBOA)2(ClO4)] (ClO4)·2CH3CN·0.5H2O and [Mn(GBOA)2(H2O)2]·(CH3COO)2·
H2O (inserted figure).
Independent reflections
Completeness to theta = 23.71◦
Absorption correction
Refinement method
6878 [R(int) = 0.0590]
98.7%
None
Full-matrix least-squares
on F2
at 2074 cm−1 confirming the presence of N-bound thiocyanate.
In the complex [Mn(GBOA)2(ClO4)] (ClO4)·2CH3CN, a broad band
is observed at 1135 cm−1. A blow up of this band shows that
the band is split into peaks arising at 1135, 1108 and 1091.
This region covers both ionic/coordinated perchlorate anion [16b].
To further confirm whether both the anions are coordinated in
the thiocyante/perchlorate bound complexes, molar conductance
were obtained in dry DMF. The values found were 71.5 and
108 ꢂ−1 cm2 mole−1 and are in keeping with values for a 1:1 elec-
trolyte [16c].
Thermo grams of parent ligand GBOA and their respective Man-
ganese(II) complexes are shown in Fig. 1. In the case of the free
ligand GBOA, a major mass loss is observed in the temperature
region 260–330 ◦C accounting for nearly 46.5% of the starting
compound, while a mass loss of 4.6% is observed uptill 100 ◦C,
confirming the presence of solvated water molecules.
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2 sigma(I)]
R indices (all data)
6878/0/604
1.026
R1 = 0.0370, wR2 = 0.0878
R1 = 0.0681, wR2 = 0.1053
−3
˚
Largest diff. peak and hole
0.327 and −0.281 e A
for Lorentz and polarization effects. No absorption corrections were
applied. The structure was solved by direct methods using SIR-97
[12]. The structure was refined by full-matrix least squares refine-
ment methods based on F2, using SHELX-97 [13]. Hydrogen atoms
were fixed geometrically with their Uiso values 1.2 times their
rider atoms. Hydrogens of the water molecules were located from
difference Fourier synthesis and were not refined. All calculations
were performed using Wingx [14] package. Important crystal and
refinement parameters are given in Table 1.
For the complex [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O a mass
loss of about 5.5% is observed up to a temperature of 180 ◦C. The
corresponding DTA shows two endothermic peaks in this tem-
perature region confirming the loss of coordinated and solvated
water molecules. Further a single mass loss of about 20% with one
endothermic peak is found in the temperature regime 220–330 ◦C.
This indicates the loss of ligand fragment and also the stability of
the Mn(II) complex relative to the free ligand.
For the complexes [Mn(GBOA)2(SCN)] (SCN)·4H2O and
[Mn(GBOA)2(ClO4)] (ClO4)·2CH3CN a loss of 4–5% is observed
respectively, in the temperature range up to 120 ◦C implying loss
of non coordinated water/solvent. While a mass loss of nearly
30% associated with two endothermic peaks, is observed in the
temperature range 220–330 ◦C. This suggests loss of coordinated
anion and ligand.
3. Results and discussion
3.1. Electronic spectroscopy, IR data and thermal studies
The electronic spectra of the ligands have two strong bands in
the UV region 272–279 nm. These bands are assigned to the ␲–␲*
transition characterizing the benzimidazole group. The spectra of
the complexes also show two strong bands in the 273–284 nm cor-
responding to ␲–␲* transition, the bands are slightly blue shifted
with lowered extinction coefficient.
The free ligand GBOA has characteristics IR bands at 1657
amide I(␯ O), 1543 amide II(␯
amide), 1446 (␯
benz-
C
C
N
C
N–C
C
imidazole). The manganese complexes (1–4) have amide carbonyl
coordination rather than amide NH. These bands are in the range
of 1664–1636 cm−1 (amide I) and 1559–1543 cm−1 {amide II} in
the Mn(II) complexes [15,16a]. The IR spectra also reveal charac-
teristic stretching frequencies at 1419, 2074, 1135 cm−1 assigned
for ␯CH3COO, ␯ SCN− and ␯ClO4−, respectively. In the com-
plex [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O, a band is observed at
1419 cm−1 assigned to (␯s(COO)). Such a band has been reported
for ionic acetate [16b]. The (␯a(COO)) is not observed in the
present case, as it merges with a strong broad band at 1544 cm−1
Thus the thermal studies show different mass loss behavior for
the [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O complex compared to the
other two Mn(II) complexes strengthening the proposal of coordi-
nated water in this complex.
3.2. EPR spectra
X-band solid state EPR spectra of the Mn(II) complexes were
recorded at liquid nitrogen temperatures and are shown in Fig. 2.
Spectra are spread over nearly 5000 gauss with several transition
typical for a Mn(II) octahedral complex, undergoing weak tetrago-
nal distortion [17(a,b)]. The main features are a strong transition at
geff = 2 (3300–3400 G) and a relatively weak transition at −geff = 2.8
(2400–2600 G). In the case of Mn(II) complex with ClO4 as co-
due to amide II(␯
amide). It is for this reason that ␯
C
N
a(COO)
band is not being reported for this complex. Further it has
been reported that N-bound thiocyanate shows a band in the
region of 2060–2080 cm−1, while S-bound thiocyanate shows a
band in the region of 2100–2115 cm−1 [16b]. In the present
[Mn(GBOA)2(SCN)] (SCN)·4H2O complex a strong band is observed

S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
1637
SCN-
Cl-
a
2x104
-5
1.0x10
Cl-
1x104
0
0.0
-5
-1.0x10
-1x104
-2x104
-3x104
-5
-2.0x10
-5
-3.0x10
0
2000
4000
[G]
6000
8000
-5
-4.0x10
200
0
-200
-400
Potential (mV)
4.0x103
b
Fig.
3. Cyclic
voltammogram
spectra
of
Mn(GBOA)2(SCN)2
and
[Mn(GBOA)2(H2O)2]·(Cl)2·4H2O in 2:8 (DMSO:AN).
CH3COO -
2.0x103
0.0
3.3. Cyclic voltammetry
The cyclic voltammograms were recorded in 2:8
(DMSO:CH3CN), containing 0.10 M TBAP as supporting elec-
trolyte at a platinum working electrode. Ferrocene was used
as internal standard and potentials are referenced to the Fc+/Fc
couple, shown in Fig. 3. The complexes display a quasi-reversible
redox wave due to Mn(II)/Mn(III) process. The data reveals the
instability of Manganese(II) oxidation state in the complexes, in
the decreasing order ClO4− > SCN− > Cl ∼ CH3COO− [E1/2 vs Ag/Ag+:
0.058, 0.080, 0.095, 0.097 (V)]. The pattern of E1/2 values is related
to the axial distortion parameter D obtained from EPR work. It is
apparent that the E1/2 becomes less anodic as the D value increases.
The nearly similar value of E1/2 for [Mn(GBOA)2(H2O)2]Cl2·4H2O
and [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O further strengthens
similar coordinating ligands.
-2.0x103
-4.0x103
0
2000
4000
[G]
6000
8000
Fig. 2. (a) EPR spectra of [Mn(GBOA)2(H2O)2]·(Cl)2·4H2O in the solid state at
microwave frequency 9.448 GHz and receiver gain 103. (b) EPR spectra of
[Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O in the solid state at microwave frequency
9.448 GHz and receiver gain 103.
anion, a relatively strong geff = 6 (1100 G) is also observed, implying
that this compound undergoes a larger axial distortion in compar-
ison to others.
3.4. Description of crystal structure
An ORTEP plot of the [Mn(GBOA)2(H2O)2](Cl)2·4H2O (1) com-
plex is depicted in Fig. 4. The coordination number around Mn2+
is six. Two trans coordination positions are occupied by two ben-
zimidazole imine nitrogens N1 and N7 belonging to two different
ligands, while the other two trans positions are occupied by amide
oxygens O3 and O6; the remaining two positions are occupied by
two oxygen atoms O1W and O2W belonging to water molecules.
The two Cl− anions are outside the coordination sphere. The
asymmetric unit also contains four lattice water molecules labeled
Zero field splitting parameter D (cm−1) for the manganese
complexes with ꢀ = 0.01 has been obtained using the method of
Dowsing et al. [17(c)] and the values compare well with other
Mn(II) complexes undergoing weak axial distortion [17(d,e)]:
X
D (cm−1
)
−
ClO4
0.18
0.11
0.05
SCN−
Cl−, CH3COO−
The axial distortion for the complexes MnLX2 decreases in the
order ClO4− > SCN− > Cl− ∼ CH3COO−. Thus the two complexes (1
and 2) where the anionic ligand is outside the coordination sphere
and have coordinated water molecules have similar D parameter;
this is also borne out from the similar resonance positions observed
in their EPR spectra (Table 2). However it varies for the complexes
(3 and 4) when the anionic ligand becomes part of the coordination
sphere.
Table 2
Field position found in the EPR spectra of Diaquo-Mn(II) complexes.
Complexes
H1(G) H2(G) H3(G) H4(G) H5(G)
Fig. 4. Showing the ORTEP view of the molecule and the labeling scheme used in
the structure analysis. Only the nitrogen and oxygen atoms have been labeled for
the sake of clarity.
[Mn(GBOA)2(H2O)2]·(Cl)2·4H2O
400
1150
1100
1470
1500
2550
2600
3000
3100
[Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O 400

1638
S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
Table 3
Selected bond lengths and bond angles.
N1–Mn
2.223(2)
N7–Mn
O1W–Mn
O2W–Mn
O3–Mn
2.223(2)
2.205(2)
2.202(2)
2.183(2)
2.184(2)
177.77(8)
91.18(8)
86.59(7)
85.69(8)
96.53(8)
176.10(7)
90.92(7)
89.10(7)
90.20(8)
87.52(8)
90.70(7)
89.43(7)
93.71(8)
88.67(8)
175.73(8)
O6–Mn
O3–Mn–O6
O3–Mn–O2
O6–Mn–O2
O3–Mn–O1
O6–Mn–O1
O2–Mn–O1
O3–Mn–N7
O6–Mn–N7
O2W–Mn–N7
O1W–Mn–N7
O3–Mn–N1
O6–Mn–N1
O2W–Mn–N1
O1W–Mn–N1
N7–Mn–N1
O3W, O4W, O5W and O6W. The important bond lengths and bond
angles are given in Table 3.
The coordination modes of the ligand are very unusual in the
present complex. The ligand contains four potential coordination
sites, two benzimidazole nitrogens, two amide oxygens. Earlier
structural reports on this class of ligands reveals that this ligand
is capable of forming mononuclear as well as dinuclear com-
plexes with transition metal ions [11,15,16]. The size of the spacer
between two amide groups appears to be the major factor deter-
mining the nature of complexation. If the chain length is long, then
the ligand adopts a helical conformation and it wraps around the
metal ion using all the four coordination sites to form a mononu-
clear complex [16]. If the chain length of spacer is small then the
ligand adopts an extended conformation towards the metal ion and
a dinuclear complex is formed. In both types of complexes all the
four donor sites are used in complexation. It has also been observed
that the two protonated benzimidazole nitrogens do not coordi-
nate with the metal ion since under the reaction conditions these
nitrogen atoms do not get deprotonated.
Fig. 5. Showing ‘S’ shaped conformation of the molecule.
coordination to Mn2+. The remaining benzimidazole nitrogen and
the amide oxygen remain uncoordinated. Probably this is due to
low CFSE in case of Mn2+ and extensive H-bonding observed in the
structure. The two benzimidazole rings of each ligand are in gauche
conformation with respect to each other (the dihedral angles
between them being 59.44(6) and 69.00(7)◦, respectively). Thus the
coordinating rings have rotated inwards to facilitate coordination
to the metal ion. The two coordinating and non-coordinating rings
are parallel and trans to each other thus giving a ‘S’ like conforma-
tion to the molecule shown in Fig. 5, offering two ‘U’ shaped cavities
per molecule.
The present structure analysis reveals a new coordination mode
of the ligand in which the ligand adopts an extended conforma-
tion and surprisingly only half the coordination sites are used for
3.4.1. H-bonding network
The crystal structure of the molecule is very interesting due
to extensive H-bonding interactions involving coordinating and
Fig. 6. Showing one of the helical sheets (a) in the ab plane, (b) in the ac plane (c) in the ac plane as space filling model; the hydrogens, lattice water molecules and chloride
ions have been removed for clarity.

S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
1639
Fig. 7. Showing packing in the ac plane: (a) sheets arranged along c axis and (b) space filling model showing the solvent accessible voids.
lattice water molecules, nitrogens of the benzimidazole moiety,
amide group and chloride ions. Table 3 contains a complete list of
the H-bonding interactions whereas important ones of them have
been shown in the various figures (vide infra). Crystal structure
may be visualized considering the most important intermolecu-
lar H-bonding interactions between coordinating water molecules
O1W and O2W and amide oxygens O5 and O8 respectively (Table 3)
stabilizing the two uncoordinated amide oxygens. These two
interactions form helical sheets of molecules in the ab plane as
shown in Fig. 6. In these sheets the inward turned, coordinating
benzimidazole rings are stabilized due to intermolecular ␲· · ·␲*
interactions between them (centroid to centroid distance 3.791(4)
In this context it may be pertinent to recall the role of chlo-
ride ions in the photosynthetic system, where it is proposed that
Cl− ions maintain a proton relay network that allows the transport
and release of protons during the water oxidation reaction, and
depletion of Cl− ions causes a partial loss of water oxidase activity
[8,19].
3.5. Amine binding and oxidation in the presence of
tritert.butylhydroperoxide
A manganese dependant peroxidase (MnP) isozyme has been
documented to oxidize aromatic amines [9]. Further ammonia and
its analogues have been reported to bind to the Mn site on the
water oxidase cluster [20] and more specifically to the S2 state
of the EPR active Mn-cluster in photosystem-II [21]. Earlier Mn
tetraarylporphyrins have been utilized for the oxidation of primary
aromatic amines to azo compounds using imidazole as co cata-
lyst [21a], Shiffs base formation [21b] and oxidative coupling of
˚
and 3.848(4) A). At the same time the amide nitrogen N4 of the
extended, non-coordinating part of the ligand, is forming H-bonds
with the benzimidazole nitrogen N5 in these helical chains (not
shown). The packing diagram of the molecule shows these sheets
further extended along the c axis creating solvent accessible voids
in them (Fig. 7).
These voids are filled with four water molecules and two Cl− ions
as shown in Fig. 8 which are involved in extensive H-bonding with
the positively charged sheets and are holding them together, along
the c axis. Fig. 9 illustrates the important H-bonding interactions
among the guests (lattice water and Cl− ions) and cationic layers.
Cl1 anion is accepting H-bonds from coordinating water O2W and
two lattice waters O5W and O6W as well as from two protonated
imidazole nitrogens N6 and N12 whereas Cl2 anion is accepting
four H-bonds from three lattice waters O3W, O5W and O6W and
one imidazole nitrogen N8. At the same time the lattice waters O3W
and O4W are also acting as H-bond donors to the amide nitrogens
N10 and N4 and imidazole nitrogens N11 and N5, respectively.
Both these water molecules are in turn receiving H-bonds from
amide nitrogens N9 and N3, respectively. H-bonds between water
molecules O4W and O6W and O1W and O5W gives rise to two
dimeric water molecules. Some important references were found in
the Cambridge Structure Database search for interactions in Mn(II)
complexes having Cl− ions and water molecules (both free and
coordinating) and some N donor ligand molecules [18(a-d)]. These
showed O(water)· · ·Cl and N· · ·Cl H-bonding interactions lying in
Table 4
◦
˚
Important H-bonding interactions (A, ).
X–H· · ·Y
X· · ·Y
H· · ·Y
∠X–H· · ·Y
O2W–H22W· · ·Cl1
O3W–H32W· · ·Cl2
O5W–H51W· · ·Cl2
O5W–H52W· · ·Cl1
O6W–H61W· · ·Cl2
O6W–H62W· · ·Cl1
O4W–H42W· · ·O6W
N6–H61· · ·Cl1I
3.206(2)
3.139(2)
3.387(3)
3.250(3)
2.945(3)
3.206(3)
2.917(3)
3.164(2)
3.214(4)
2.839(3)
2.760(3)
2.814(3)
2.680(3)
3.050(3)
2.802(3)
3.312(3)
3.245(2)
2.789(3)
3.232(3)
2.825(3)
2.786(3)
3.495(3)
2.21
2.24
2.36
2.31
2.02
2.27
2.01
2.18
2.29
1.93
1.91
1.85
1.74
2.15
1.83
2.43
2.27
1.85
2.82
1.75
1.89
2.98
171
164
163
161
167
156
158
168
151
167
147
175
178
174
156
150
162
166
102
167
175
118
N4–H41· · ·N5I
N2–H21· · ·O6WII
N3–H31· · ·O4WII
O1W–H12W· · ·O5WIII
O2W–H21W· · ·O8IV
N8–H81· · ·Cl2V
N9–H91· · ·O3WV
N10–H101· · ·N11VI
N12–H121· · ·Cl1VII
O1W–H11W· · ·O5VIII
O3W–H31W· · ·N10IX
O3W–H31W· · ·N11IX
O4W–H41W· · ·N5X
O4W–H41W· · ·N4X
˚
the ranges 3.055–3.218 and 3.138–3.298 A, respectively which are
quite comparable to the values found in here (Table 4). They showed
two to four H-bonding interactions with the Cl− ion. In the present
complex however, one of the Cl− ions is involved in five such inter-
actions. Thus this extensive H-bonding stabilizes the anions and
at the same time reduces their nucleophilicity to an extent that a
charge separated complex is formed.
(I) −x + 1, −y, −z + 1 (II) x, +y−1, +z (III) x, −y + 1/2, +z − 1/2. (IV) −x + 2, +y − 1/2,
−z + 1/2. (V) x, −y + 1/2 + 1, +z − 1/2. (VI) −x + 2, −y + 1, −z. (VII) −x + 2, +y + 1/2,
−z + 1/2. (VIII) −x + 1, +y + 1/2, −z + 1/2. (IX) x, −y + 1/2 + 1, +z + 1/2. (X) x, +y + 1, +z.

1640
S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
Fig. 8. Showing packing diagram in the ac plane: (a) the voids occupied by lattice water molecules (red) and Cl− ions (green) and (b) the same as space filled models. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
amines to imines in the presence of oxidant ButOOH [21c]. In view
of these reports and since the basicity of tertiary amines is close to
Table 5
Product yields and turn over number.
that of ammonia, we were prompted to study the reaction of ter-
tiary amines with the present Mn(II) complex in the presence of
an external oxidant i.e. tri-tert.butyl hydroperoxide as explained in
the experimental section. It has been established earlier that, in the
absence of protic solvent the hydroperoxides cleave by homolysis
reaction [16a,22,23]. It is assumed that in the present case, simi-
lar reaction sequences are operative, wherein, the Manganese(II)
catalyst promotes the homolysis of tri tert-butyl hydroxide, lead-
ing to the generation of tert butyl and hydroxyl radical. The tert
butyl radical abstracts hydrogen atom from the benzylic (in the
case of aromatic amines) or the aliphatic (in the case of pyrrolidone)
methylene group, forming R3COH and allowing the simultaneous
attack of the OH• on the electron deficient carbon atom resulting
Compound
Product
Complex (1)
Complex (2)
Yield (%)
Turn
Yield (%)
Turn
over
over
N-benzyldimethyl
amine
Pyrollidine
Benzaldehyde
Pyrollidinone
33.3
49.4
8a
30
45
7
24
22
aThe initial amount of N-benzyl dimethyl amine was 0.1505 mmol.
in the oxygenation of the substrate. This is depicted in the reaction
Scheme 1. The percentage yield of the carbonyl products and the
turnover of the Manganese(II) catalyst are given in Table 5.
Scheme 1.

S.C. Mohapatra et al. / Spectrochimica Acta Part A 79 (2011) 1634–1641
1641
Acknowledgment
We gratefully acknowledge financial support from the Defence
Research Development Organization, Directorate of Extramural
Research and Intellectual Property Rights, India, and to Prof. U.P.
Singh, Department of Chemistry, IIT Roorkee, Roorkee, India for
collecting the X-ray diffraction data.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.05.024.
References
Fig. 9. Showing the H-bonding interactions around two anions Cl(1) and Cl(2) and
the water molecules.
[1] J.M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995.
[2] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375–1406.
[3] H.J. Choi, T.S. Lee, M.P. Suh, Angew. Chem. Int. Ed. 38 (1999) 1405–1408.
[4] C.S. Liu, X.S. Shi, J.R. Li, J.J. Wang, X.H. Bu, Cryst. Growth Des. 6 (2006) 656–663.
[5] J.M. Lehn, Angew. Chem. Int. Ed. Engl. 29 (1990) 1304–1319.
[6] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629–1658.
[7] S. Changyan, Z. Xiangjun, J. Linpei, J. Mol. Struct. 646 (2003) 201–210.
[8] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104
(2004) 3981–4026.
[9] H. Wariishi, K. Valli, M.H. Gold, J. Biol. Chem. 267 (1992) 23688–23695.
[10] L.A. Cescon, A.R. Day, J. Org. Chem. 27 (1962) 581–586.
[11] M. Gupta, S.K. Das, P. Mathur, A.W. Cordes, Inorg. Chim. Acta 353 (2003)
197–205.
[12] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi,
A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 32 (1999) 115–119.
[13] G.M. Sheldrick, Programs for Crystal Structure Analysis (Release 97-2) Institüt
für Anorganische Chemie der Universität, Tammanstrasse 4, D-3400 Gottingen,
Germany, 1998.
[14] L.J. Farrugia, J. Appl. Cryst. 32 (1999) 837–838.
[15] S.C. Mohapatra, S. Tehlan, M.S. Hundal, P. Mathur, Inorg. Chim. Acta 361 (2008)
1897–1907.
4. Conclusion
Four new Manganese(II) complexes with a bis-benzimidazolyl
diamide ligand have been synthesized and one of them
[Mn(GBOA)2(H2O)2]Cl2·4H2O has been structurally characterized.
In this complex the bis benzimidazolyl diamide ligand provides
only two of the four potential binding sites to the Mn(II) ion
leading to a stoichiometry of 1:2 for Mn:Ligand, respectively.
The uncoordinated arms of the two ligands participate in inter-
molecular H-bonding leading to the formation of U-type cavities.
The cavities host Cl− anion and water molecules as guests. Two
Cl anions remain uncoordinated to the Mn(II) ion but remain
in the lattice. They participate in a uncommon Cl· · ·HN and
Cl· · ·OH H-bonding mode resulting in a extended H-bonding net-
work. Axial distortion parameter D has been obtained from EPR
work for all the complexes and it is found to be dependant on
whether the anionic ligand is in the coordination sphere or out-
side. Axial distortion for the Mn(II) complexes decreases in the
order ClO4− > SCN− > Cl− ∼ CH3COO−. Axial distortion D is smaller
for complexes having two axial water molecules in the coordination
sphere while the value is larger when the aquo ligand is replaced
by an anionic ligand The complexes display a quasi-reversible
redox wave due to Mn(II)/Mn(III) process. The data reveals the
instability of Manganese(II) oxidation state in the complexes, in
the decreasing order ClO4− > SCN− > Cl ∼ CH3COO−. E1/2 values are
found to be related to the D parameter found from EPR work, larger
the D value less anodic the E1/2. [Mn(GBOA)2(H2O)2]Cl2·4H2O
and [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O carry out the selective
oxidation of N-benzyldimethylamine, and 1-methyl-pyrollidine
to their respective carbonyl products with catalytic efficiency of
35–50%.
[16] (a) S. Tehlan, M.S. Hundal, P. Mathur, Inorg. Chem. 43 (2004) 6589–6595;
(b) K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
compounds, 5th edition, Wiley, New York, 1997;
(c) W.J. Geary, Coord. Chem. Rev. (1971) 122–781.
[17] (a) M. Singla, P. Mathur, Spectrochim. Acta Part A 74 (2009) 536–543;
(b) S.J. Brudenell, L. Spiccia, A.M. Bond, G.D. Fallon, D.C.R. Hockless, G. Lazarev,
P.J. Mahon, E.R.T. Tiekink, Inorg. Chem. (2000) 39881–39892;
(c) R.D. Dowsing, J.F. Gibson, M. Goodgame, P.J. Hayward, J. Chem. Soc. (A)
(1969) 187–193;
(d) P. Mathur, M. Crowder, G.C. Dismukes, J. Am. Chem. Soc. 109 (1987)
5227–5233;
(e) C. Hureau, L. Sabater, E.A. Mallart, M. Nierlich, M.F. Charlot, F. Gonnet, E.
Riviere, G. Blond, Chem. Eur. J. 10 (2004) 1998–2010.
[18] (a) T.O. Vardosanidze, V.I. Sokol, A.N. Sobolev, A.E. Shvelashvili, M.A. Porai-
Koshits, Russ. J. Inorg. Chem. 30 (1985) 1745–1751;
(b) S. Wang, T.D. Westmoreland, Inorg. Chem. 48 (2009) 719–727;
(c) P. Bonhote, M. Ferigo, H. Stoeckli-Evans, W. Marty, Acta Cryst. C 49 (1993)
2102–2107;
(d) B.B. Levaton, M.M. Olmstead, Acta Cryst. E 66 (2010) m1226–m1227.
[19] K. Olesen, L.-E. Andreasson, Biochemistry 42 (2003) 2025–2035.
[20] P.O. Sandusky, C.F. Yocum, Biochim. Biophys. Acta 766 (1984) 603–611.
[21] (a) F.B. Warren, C. de Paula Julio, W.B. Gary, J. Am. Chem. Soc. 108 (1986)
4018–4022;
(b) A. Rezaeifard, M. Jafarpour, S. Rayati, R. Shariati, Dyes Pigments 80 (2009)
80–85;
The oxidation of amines by the complex along with the role of Cl
anion in the formation of a extended H-bonding network is reminis-
cent of roles in a Mn(II) dependant peroxidase from lignin and the
Manganese containing Oxygen evolving complex of Photosystem-
II.
(c) S. Tollari, A. Fumagalli, F. Porta, Inorg. Chim. Acta 247 (1996) 71–74;
(d) Q. Lan Yuan, X. Tai Zhou, H. Bing Ji, Catal. Commun. 12 (2010) 202–206.
[22] D. Chin, G.N. Mar La, A.L. Balch, J. Am. Chem. Soc. 120 (1980) 5945–5947.
[23] R.D. Arasasingham, A.L. Balch, L. Latos-Grazynski, J. Am. Chem. Soc. 109 (1987)
5846.