Isolation of a novel intermediate during unsymmetrical to symmetrical rearrangement of a tetradentate Schiff base ligand in a manganese(III) complex: Catalytic activity of the rearranged product towards alkene epoxidation

Article abstract of DOI:10.1016/j.ica.2010.09.036

An attempt for the synthesis of a Mn^III complex with unsymmetrical tetradentate Schiff base ligand uspen (1:1:1 condensate of salicylaldehyde, 2,4-pentanedione and 1,2-ethanediamine) resulted in the isolation of a novel complex, {[{Mn(salen)}{M

Full text of DOI:10.1016/j.ica.2010.09.036

Inorganica Chimica Acta 365 (2011) 318–324
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Isolation of a novel intermediate during unsymmetrical to symmetrical
rearrangement of a tetradentate Schiff base ligand in a manganese(III) complex:
Catalytic activity of the rearranged product towards alkene epoxidation
a
a
b
a,
⇑
Pampa Mukherjee , Paramita Kar , Sandra Ianelli , Ashutosh Ghosh
a
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
Dipartimento di Chimica Generale e Inorganica, Chimica Analitica, Chimica Fisica, Università di Parma, Viale delle Scienze 17/A, I-43100 Parma, Italy
b
a r t i c l e i n f o
a b s t r a c t
An attempt for the synthesis of a MnIII complex with unsymmetrical tetradentate Schiff base ligand uspen
Article history:
Received 16 July 2010
Received in revised form 20 September
(1:1:1 condensate of salicylaldehyde, 2,4-pentanedione and 1,2-ethanediamine) resulted in the isolation
of a novel complex, {[{Mn(salen)}{Mn(uspen)}(HCOO)]ꢀClO
4 n
}
(1) as an intermediate species that con-
tains both the unsymmetrical and symmetrical tetradentate ligand uspen and salen (2:1 condensate of
salicylaldehyde and 1,2-ethanediamine) respectively. The structure of the complex shows that half of
2
010
Accepted 22 September 2010
Available online 29 September 2010
the unsymmetrical Schiff base, uspen rearranged to its symmetrical analogue, salen. A phenoxo bridged
III
dinuclear Mn complex [Mn(salen)(sal)]
2
ꢀ2H
2
O (2) with only the symmetrical Schiff base was also
Keywords:
obtained. Compound 1 that contains both unsymmetrical and symmetrical Schiff base ligands in one mol-
ecule is unprecedented and provides an insight into the unsymmetrical to symmetrical rearrangement of
tetradentate Schiff base ligand. Complex 2 acts as an efficient catalyst in the alkene ((E)-stilbene, styrene)
Manganese(III)
Schiff-base rearrangement
X-ray structure
Catalytic activity
Epoxidation
epoxidation reaction in presence of two terminal oxidants PhIO and NaOCl in solvents CH
independently and it retains its reactivity with high efficiency for a long time.
3 2 2
CN and CH Cl
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Literature study shows that MnIII complexes containing unsym-
metrical tetradentate Schiff base ligands may undergo acid or base
catalysed hydrolysis resulting rearranged product of corresponding
more stable symmetrical tetradentate Schiff base ligand [31–33].
There are many other factors such as (i) presence of metal ion,
(ii) ligand systems, (iii) solvent, etc. that may also cause the rear-
rangement of the unsymmetrical tetradentate Schiff base ligand
to a symmetric one. Therefore, a first step in order to better under-
stand the factors that may govern the hydrolysis of the unsymmet-
rical ligands and also to establish the reaction pathways, isolation
of an intermediate species that form during hydrolysis of the li-
gand is required. In most of the previous reports, the isolated
products were completely rearranged compounds containing sym-
metrical Schiff base ligands. An intermediate species was isolated
only in one case as cocrystals of both symmetrical and unsymmet-
rical compounds [31]. We are interested in investigating if two
such species can be connected by a bridging ligand to obtain a
molecular species that contains both of them. For this purpose
we have tried with several carboxylate ions keeping in mind their
potentials for bridging the metal ions and fortunately succeeded to
obtain the desired compound using the formate ion that contains
both the unsymmetrical (uspen) and symmetrical tetradentate li-
gands (salen). To the best of our knowledge, this is the first report
of isolation and characterization (structurally) of an intermediate
The coordination chemistry of manganese with a diverse range
of ligands is an active field of research for decades. This is not only
because of the relevance that a number of model manganese com-
plexes have biological significance [1,2], but also due to their fasci-
nating cluster compounds, which exhibit unusual magnetic
properties, and the elegant supramolecular arrays that have been
discovered [3–11]. In addition to the interest from this inorganic
standpoint, there is also considerable interest in the application
of manganese complexes in organic synthesis due to their potent
catalytic properties, particularly in the asymmetric epoxidation
of certain olefins [12–28]. Moreover in nature, the oxygen evolving
complexes of photosystem II are believed to be catalysed by a clus-
ter of manganese ions. Manganese(III)–salen type complexes are
very well studied because they can be synthesized easily and as be-
cause this ligand shows suitable biometric properties that can mi-
mic the structural feature of the active site [29], but the chemistry
III
of Mn complexes with unsymmetrical tetradentate Schiff base li-
gands are relatively unexplored [30].
⇑
Corresponding author. Fax: +91 33 2351 9755.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.09.036

P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
319
product that contains both unsymmetrical and symmetrical Schiff
2.2.2. Synthesis of {[{Mn(salen)}{Mn(uspen)}(HCOO)]ClO
[Mn(salen)(sal)] O (2)
ꢀ2H
Mn(ClO O (1.805 g; 5 mmol), dissolved in 10 cm of
ꢀ6H
methanol, was added to a methanolic solution (10 cm ) of the li-
gand (H uspen) (5 mmol) with constant stirring. After ca. 10 min
a methanol solution of formic acid (0.32 g; 5 mmol) was added
to it. Triethylamine (2.10 cm , 15 mmol) in methanol (5 cm ) was
added dropwise to this solution with constant stirring. After ca.
2 h a dark brown microcrystalline compound was precipitated
out. It was filtered. The precipitate was collected and the filtrate
was left to stand overnight in open atmosphere. The precipitate
4 n
} (1) and
base ligands in one molecule and thus provide a clear evidence into
the reaction pathway of unsymmetrical to symmetrical rearrange-
ment of tetradentate Schiff base ligand. We are also able to isolate
2
2
3
4
)
2
2
3
III
and characterize a completely rearranged product Mn –salen.
2
Herein, we report the synthesis, spectral study and structural char-
acterization of these two compounds.
3
3
Among the various metal–salen complexes, the manganese(III)–
salen complexes are the most promising as catalysts in alkene
epoxidation, which is an important reaction in organic synthesis
and as model systems of various metallobiosites, e.g. cytochrome
P-450 [12–28,34]. The interest in manganese complexes as cata-
lysts for alkene epoxidations comes mainly from the relationship
of these catalytic systems to the biologically relevant manganese
porphyrins. In the case of manganese porphyrins as epoxidation
catalysts different oxygen sources have been used, e.g. iodosylben-
zene, sodium hypochlorite, molecular oxygen in the presence of an
electron source, alkyl peroxides and hydroperoxides, N-oxides,
potassium hydrogen persulfate and oxaziridines [35]. In spite of
3
was washed with and redissolved in CH CN. X-ray quality deep-
brown single crystals of compound 1 were obtained by slow evap-
oration of the acetonitrile solution of this compound.
From the filtrate, a deep-red microcrystalline compound ap-
pears after 2 days. The precipitate was filtered and washed with
diethyl ether and redissolved in methanol. Layering of the red solu-
tion with diethyl ether gave well-formed X-ray-quality deep-red
single crystals of 2.
III
huge works, the mechanism of Mn –salen catalyzed epoxidation
Complex 1: (Yield: 0.114 g; 15%) Anal. Calc. for C31
2 4
H29Mn N
of olefins is a topic of great controversy. The general mechanism
of cytochrome P-450 catalyzed oxo transfer may be considered
as the guideline for the mechanistic pathway through which the
O
10Cl (762.91): C, 48.80; H, 3.83; N, 7.34. Found: C, 48.72; H,
3
ꢁ1
ꢁ1
3.87; N, 7.19%. kmax/nm (
(447); IR (cm ):
e
max/dm mol cm ) (methanol), 603
ꢁ1
m
(C@N), 1633,
m
as(C@O), 1591, (C@O), 1439,
m
s
III
ꢁ
ꢁ1
Mn –salen catalyzed epoxidation of olefins may proceed in the
m(ClO4 ), 1089 cm
.
presence of terminal oxidant like PhIO, NaOCl, etc. The stereo-
chemistry of the oxygen transfer from the oxomanganese–salen
complex to the alkene depends on the electronic environment
around the oxomanganese function. Herein, we also describe the
catalytic activity of the dinuclear Mn–salen complex towards
epoxidation of alkenes.
Complex 2: (Yield: 0.28 g; 60%) Anal. Calc. for C46
2 4 10
H42Mn N O
(920.72): C, 60.01; H, 4.60; N, 6.09. Found: C, 60.12; H, 4.57; N,
3
ꢁ1
ꢁ1
6.19%. kmax/nm (
e
max/dm mol cm ) (methanol), 608 (464); IR
ꢁ
1
(cm ):
m
(O–H), 3427, (C@N), 1630.
m
2
.2.3. Alternative method for the synthesis of complex 2
Complex 2 has also been synthesized by reacting of H
2
salen
(1.340 g; 5 mmol) in methanol (10 cm ) with a methanolic solution
3
2
. Experimental
3
(
10 cm ) containing Mn(ClO
4
)
2
ꢀ6H
cylaldehyde (0.525 cm ; 5 mmol) followed by NEt
mmol). (Yield: 1.84 g; 80%).
2
O (1.805 g; 5 mmol) and sali-
3
3
2.1. Starting materials
3
(0.70 cm ;
5
The diamine (1,2-ethanediamine) and triethylamine were dried
over potassium hydroxide and distilled prior to use. Pentane-2,4-
dione was dried over anhydrous sodium sulphate and distilled;
the fraction boiling at 138–139 °C was collected All the other
chemicals were of reagent grade and used without further purifica-
tion. Styrene and (E)-stilbene were purchased from Aldrich and
used in epoxidation experiment without further purification.
2
.2.4. Preparation of iodosylbenzene
This was prepared by hydrolysis of the corresponding diacetate
with aqueous sodium hydroxide as reported in the literature [37].
Freshly prepared PhIO was used in every epoxidation experiment.
2.3. Physical measurements
2
2
.2. Preparations
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin–Elmer 240C elemental analyzer. IR spectra
.2.1. Synthesis of the Schiff-base ligands H
2
salen and H
2
uspen
ꢁ1
in KBr (4500–500 cm ) were recorded using a Perkin–Elmer RXI
FT-IR spectrophotometer. Electronic spectra in methanol (1200–
The symmetrical di-Schiff-base ligand H
reacting 5 mmol methanolic solution (10 cm ) of salicylaldehyde
2
salen was prepared by
3
3
50 nm) were recorded in a Hitachi U-3501spectrophotometer.
3
3
(
0.525 cm ) and 5 mmol methanolic solution (10 cm ) of 1,2-eth-
1
The H NMR spectra were recorded in CDCl
3
on a Bruker AC300
3
anediamine (0.308 cm ) as reported earlier [36]. For the synthesis
of H
wise to a 25 cm chloroform solution of 1,2-ethanediamine
spectrometer.
3
2
uspen, 2,4-pentanedione (1.10 cm ; 5 mmol) was added drop-
3
3
(
0.31 cm ; 5 mmol). After the completion of addition, the solution
2.4. Epoxidation of alkenes catalyzed by complex 2
was stirred for an additional 3 h and chloroform was evaporated
3
under reduced pressure, yielding a viscous liquid. A 5 cm metha-
Under nitrogen atmosphere, alkene (0.300 mmol) and catalyst
3
ꢁ2
3
nolic solution of salicylaldehyde (1.10 cm ; 5 mmol) was added to
the methanolic solution (5 cm ) of this viscous liquid in 1:1 molar
ratio and refluxed for ca. 2 h. The ligand was isolated and the purity
(1.00 ꢂ 10 mmol) were treated with 5 cm dry acetonitrile or
dichloromethane. Afterwards under nitrogen atmosphere iodosyl-
benzene/NaOCl (0.066 g, 0.300 mmol) was added to the solution.
3
was checked by melting point (130 °C) and NMR spectra as was re-
When NaOCl was used as oxidant the solution was buffered to
1
ported earlier [30].
pH 11 using a NaH
2
PO
4
–NaOH buffer. H NMR spectroscopy was
1
1
2
Ligand (H uspen). H NMR (300 MHz, CDCl
3
): d/ppm 13.07 (br,
used for identification of the product. For H NMR experiments
epoxides were prepared following the procedure described by Ko-
chi and co-workers [38] when PhIO was used as oxidant and adopt-
ing the procedure reported by Jacobsen et al. [12,19,39–42] when
NaOCl was used as oxidant.
1
6
3
3
H, OH(sal)), 8.23 (s, 1H,CH@N), 7.09–7.24 (m, 2H, aromatic), 6.71–
.87 (m, 2H, aromatic), 4.89 (s, 1H, CH), 4.81 (br, 1H, OH(acac)),
.11 (t, J = 5.4 Hz, 2H,CH
2
2
), 3.01 (t, J = 5.8 Hz, 2H, CH ), 2.12 (s,
H, CH (N)), 1.91 (s, 3H, CH
3
3
(O)).

3
20
P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
Table 1
were made using ORTEP-3 [47]. ADDSYM procedure detects this
pseudo centre of symmetry and predicts the structure of 1 to be
C2/c. In this structure element, the environments of two Mn atoms
are different as two different ligands chelates the Mn sites. Since L1
and L2 are almost similar, the search criteria of check cif detect a
pseudo-centre of symmetry. The Mn atoms in the crystal structure
seem to be defining a higher symmetry than the actual one which,
on the other hand, is not supported by the remainder unit cell con-
tents. Refinement in suggested new space group was checked and
the structure solution in the non-centrosymmetric space group Cc
is better than the structure solution in the centrosymmetric space
group C2/c. Significant crystallographic data are summarized in Ta-
ble 1.
Crystal data and structure refinement of complexes 1 and 2.
1
2
Empirical formula
Formula weight
Space group
Crystal system
a (Å)
C
31
H
31Mn
2
N
4
O
10Cl
C
46 2 4 10
H42Mn N O
764.93
Cc
920.72
P2 /c
1
monoclinic
14.915(2)
17.492(3)
13.718(2)
90
115.13(2)
90
3240.2(10)
4
monoclinic
11ꢃ.676(3)
14.149(3)
12.981(3)
90
104.97(2)
90
2071.7(9)
2
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
3
Calculated density (g/cm )
Absorption coefficient (
F(0 0 0)
1.564
1.476
0.676 (Mo K
952
l
) (mmꢁ1) 0.925 (Mo K
1560
a
)
a)
Crystal size
Refinement method
0.20 ꢂ 0.26 ꢂ 0.37
SHELXL-97 on F
1.6, 27.5
0.040
0.05 ꢂ 0.05 ꢂ 0.30
SHELXL-97 on F
1.8–25.5
0.049
3. Results and discussion
2
2
Theta range for data collection (°)
3.1. Synthesis of the complexes
R
int
Number of unique data
Number of data with I > 2
7426
4163
0.0452, 0.1335
3787
3205
0.0497, 0.1116
r(I)
Addition of methanolic solution of Mn(ClO
methanolic solution of H uspen followed by formic acid (in meth-
2
4
)
2
ꢀ6H
2
O to the
1
R , wR
2
anol) and triethylamine in 1:1:1:3 molar ratio, results in the isola-
tion of complex 1 as microcrystalline solid in ca. 2 h. In this
complex, the unsymmetrical Schiff base ligand remains intact
around one Mn centre but is rearranged to the symmetrical one
2.5. Crystal data collection and refinement
(
salen) in the other Mn centre (Scheme 1). Upon keeping the fil-
trate for several days a deep-red microcrystalline solid of complex
appears. In this compound, the Schiff base ligand is only the sym-
The data for complex 1 and 2 were collected at room tempera-
ture on Bruker AMX Smart 1000 equipped with an area detector
diffractometer using graphite monochromated Mo K radiation.
Semi-empirical absorption corrections were applied using SADABS
43]. The crystal structures of the complexes 1 and 2 were solved
by direct method using SHELXS-97 [44] program and refined by using
SHELXL-97 [44]. The non-hydrogen atoms were refined anisotropi-
cally while the hydrogen atoms, either located from difference
electron density maps or placed geometrically, were refined with
isotropic thermal parameters. Neutral atom scattering factors were
taken from Cromer and Weber [45] and anomalous dispersion ef-
fects were included in Fcalc [46]. The crystallographic drawings
2
a
metrical salen. These observations, thus clearly suggest that the
unsymmetrical Schiff base ligand undergoes rearrangement under
the reaction conditions to yield the final product of symmetrical
salen ligand, 2 and during the course of this rearrangement reac-
tion an interesting intermediate product containing both symmet-
rical and unsymmetrical ligands is separated as complex 1. A CSD
search shows ‘no hit’ for such complexes indicating that the com-
plex is a unique one. Complex 2 has also been synthesized by react-
[
ing methanolic solutions of
salicylaldehyde in 1:1:1 molar ratio.
H
2
salen, Mn(ClO
4
)
2 2
ꢀ6H O and
H
N
N
O
O
O
N
N
O
Mn
O
Mn
O
After 2 hrs
N
N
Mn(ClO ) .6H O
1
4
2
2
After 2 days
HCOOH
OH HO
uspen
H
2
CHO
OHC
N
O
N
N
N
N
O
N
O
O
O
O
Mn(ClO ) .6H O
4
2
2
Mn
Mn
OH HO
salen
salicylaldehyde (sal)
H
2
2
Scheme 1. Formation of the complexes.

P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
321
Table 2
o
Selected bond lengths (Å) and bond angles ( ) in complex 1.
Mn1
Mn2
Mn1–O3
Mn1–O2
Mn1–O1
Mn1–N1
Mn1–O5
Mn1–N2
2.091(10)
1.906(7)
1.876(13)
1.982(9)
2.408(8)
1.990(15)
Mn2–O5
Mn2–O6
Mn2–O4
Mn2–N4
Mn2–N3
1.932(7)
1.866(13)
2.125(10)
1.955(11)
1.960(15)
2.401(8)
0
0
0
Mn2–O2
O3–Mn1–O2
O3–Mn1–O1
O3–Mn1–N2
O3–Mn1–N1
95.5(4)
94.5(5)
92.2(5)
91.6(4)
176.1(4)
94.8(4)
89.4(4)
169.6(5)
81.4(3)
171.7(4)
92.2(5)
88.1(4)
82.7(5)
85.5(4)
91.1(4)
O5–Mn2–O6
O5–Mn2–O4
O5–Mn2–N4
O5–Mn2–N3
O5–Mn2–O2
O6–Mn2–O4
O6–Mn2–N4
O6–Mn2–N3
95.1(4)
95.1(3)
169.6(6)
88.5(4)
81.1(3)
92.7(5)
93.5(6)
174.4(4)
90.8(4)
90.2(4)
91.3(5)
175.1(4)
82.5(6)
93.1(4)
85.5(4)
0
00
O5 –Mn1–O3
O2–Mn1–O1
O2–Mn1–N2
O2–Mn1–N1
0
00
O5 –Mn1–O2
O6–Mn2–O2
O4–Mn2–N4
O4–Mn2–N3
O4–Mn2 –O2
O1–Mn1–N2
O1–Mn1–N1
0
00
O5 –Mn1–O1
N2–Mn1–N1
N4–Mn2–N3
0
0
00
O5 –Mn1–N2
O2 –Mn2–N4
00
O5 –Mn1–N1
O2 –Mn2–N3
0
Symmetry code = 2 ꢁ x, 1 ꢁ y, ꢁz.
ꢁ
Fig. 1. ORTEP-3 view of the asymmetric unit of 1 with ellipsoids at 50% probability.
2
, the moderately strong and sharp band at 1630 cm 1 is assigned
(C@N) group and a broad band centred at
427 cm is due to the (OH) of water molecules.
The electronic spectra of the complexes 1 and 2 were recorded
to the azomethine
3
m
ꢁ1
m
To have an idea regarding the species distribution in the solutions
we recorded the HRMS-ESI after 1 h of mixing the reactants (i.e. be-
fore the precipitation of 1 started; solution 1) and found the pres-
in methanol solution. A single absorption band was found at 603
and 608 nm in the visible region for 1 and 2, respectively, as usu-
ally observed in octahedral Mn(III) complexes [49–51].
+
+
ence of both Mn(salen) (HRMS (ESI): found m/z M = 299.1924;
+
M
calc = 299.00) and Mn(uspen)
M = 321.1844; Mcalc = 321.00) moieties (Fig. S1a). The intensity of
Mn(uspen) peak was much higher than that of Mn(salen) . In this
spectra, a peak of [{Mn(salen){Mn(uspen)}(HCOO)] (HRMS (ESI):
(HRMS (ESI): found m/z
+
+
+
3.3. Description of the structure of {[{Mn(salen)}{Mn(uspen)}
+
(HCOO)]ꢀClO } (1)
4
n
+
found m/z M = 666.3301; Mcalc = 666.00) (Fig. S1b) was also ob-
served. The HRMS-ESI was again recorded after 4 h of mixing when
the precipitation of 1 was completed (solution 2). In this spectra also
both Mn(salen)+ and Mn(uspen) moieties were present but the
intensity ratio was reversed (Fig. S2a). Moreover, the spectra also
The molecular structure of complex 1 consists of formate
bridged dinuclear units formulated as [{Mn(salen)}{Mn(uspen)}(H-
+
+
ꢁ
COO)] (Fig. 1). The charge is balanced by a non coordinating ClO
4
anion. Selected bond lengths and angles are summarized in Table
III
showed the presence of [Mn(salen)(sal)] (HRMS (ESI): found m/z
2. The asymmetric unit consists of two independent Mn atoms,
+
(
M + Na) = 481.4960; Mcalc = 481.01) fragment (sal = salicylalde-
Mn(1) and Mn(2) both presenting a distorted octahedral environ-
III
hyde) and Mn(acac)
3
(acac = acetylacetonate) (HRMS (ESI): found
ment, but the ligand environment is different around the two Mn
+
+
m/z (M + Na) = 375.2571; (M + Na) calcd = 375.01) (Fig. S2b) indi-
cating that the hydrolysis of the Schiff base took place during the
reaction in agreement with the mechanism proposed by Bermejo
et al. [33]. To have greater yield, we also tried to prepare 1 from a
centres. Mn(1) is coordinated by two phenoxo oxygen atoms [O(1)
and O(2)] and two imine nitrogen atoms [N(1) and N(2)] of depro-
tonated symmetrical tetradentate Schiff base ligand (salen) in the
equatorial site. An oxygen atom O(3) from a formate anion coordi-
nates to one of its axial positions. The same formate ion is coordi-
nated to Mn(2) through the other oxygen atom O(4) to form the
syn-anti carboxylate bridged dimeic entity. On the other hand,
Mn(2) is coordinated by two oxygen atoms O(5) from salicylalde-
hyde moiety and O(6) from acetylacetone moiety of unsymmetri-
1
:1 mixture of H
of 1 and another product which was characterized (elemental anal-
ysis) as [{Mn(salen)} (HCOO)]ClO (Supporting information).
2 2
uspen and H salen but the product was a mixture
2
4
3.2. IR and UV–Vis spectra of the complexes
cal tetradentate Schiff base ligand (H
nitrogen atoms N(4) and N(3) from the ligand (H
equatorial position. The other axial position of Mn(1) and Mn(2)
2
uspen) and two imine
In the IR spectra of complex 1, the attributions of the IR spectra
2
uspen), in the
ꢁ
1
in the 1300–1650 cm region are difficult due to the appearance
of several absorption bands from two different Schiff bases and
the formate ligands. However, by comparing the IR spectra of the
0
0
is weakly bonded to a phenoxo oxygen atom O(5 ) ( = x ꢁ 1/2, -
y + 1/2, z ꢁ 1/2) from the unsymmetrical Schiff base ligand (uspen)
III
00
00
Mn complexes of the similar type of Schiff base ligands but with
and O(1 ) ( = x + 1/2, ꢁy + 1/2, z + 1/2) from the symmetrical Schiff
base ligand (salen) respectively of neighboring dinuclear units to
form a one-dimensional wave-like chain (Fig. 2). The deviations
of the four basal donor atoms from their mean plane are within
±0.002 Å and ±0.018 Å around Mn(1) and Mn(2) respectively. The
Mn(1) and Mn(2) atoms deviate from the respective mean plane
by 0.105(4) and 0.095 (2) Å, respectively towards axial formate.
The axial Mn–O bond distances Mn(1)–O(3) 2.091(10) Å and
other anions (e.g. azide and halides), the moderately strong and
ꢁ
1
sharp band at 1633 cm , is assigned to the azomethine
m(C@N)
ꢁ1
group. The strong band at 1591 cm , is likely due to the antisym-
metric stretching mode of the carboxylate group and the bands at
ꢁ1
s
m (C@O), 1439 cm to the symmetric stretching modes of the car-
boxylates [48]. In this complex, the characteristic strong peaks for
stretching vibrations of uncoordinated perchlorate anion have
ꢁ1
ꢁ1
0
0
been observed at 1089 cm
(m
3
) and 948 cm
(m
4
). For complex
Mn(1)–O(5 ) ( = xꢁ1/2, ꢁy + 1/2, z ꢁ 1/2) 2.408(8) Å around

3
22
P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
Fig. 2. The polymeric structure of 1.
3.4. Description of the structure of [Mn(salen)(sal)]
2
2
ꢀ2H O (2)
The crystal structure of complex 2 is composed of a centrosym-
metric dinuclear unit [Mn(salen)(sal)] as shown in Fig. 3, together
2
with the atomic numbering scheme. Selected bond distances and
III
angles are listed in Table 3. The two Mn atoms Mn(1) and
0
Mn(1 ), have distorted six-coordinate octahedral environment.
The phenol functions of the Schiff-base ligand, O(2) and O(21) have
been deprotonated during the synthesis and occupy the equatorial
plane of the Mn site. The imine nitrogen atoms, N(10) and N(13)
also occupy the equatorial site. The deviations of the four basal do-
nor atoms from their mean plane are within ±0.011 Å. The Mn
atom deviates from the basal plane by 0.189 Å. The out-of-plane
Mn axial positions are occupied by a deprotonated salicylaldehyde
0
molecule and an oxygen atom O(21 ) from the neighboring [Mn(sa-
len)] unit involved in the dinuclear complex. The Mn–Ophe and
Mn–Nimi bond distances (Table 3) are in good agreement with
those observed for similar compounds in the literature [18]. The
axial Mn–O bond distances Mn(1)–O(22) 2.048(2) Å and Mn(1)–
0
O(21 ) 2.643(2) Å are significantly longer than the equatorial bonds
as expected for a Jahn–Teller distortion of Mn ions with a +3 oxida-
III
tion state (Table 3). Within the dimer, the two Mn ions are linked
0
through the phenoxo oxygen atoms O(21) and O(21 ); the angle of
0
III
Mn(1)–O(21)–Mn(1 ) is 108.31(8)° and the two Mn ions are sep-
arated by 3.817(5) Å. The six-member chelate ring around Mn(1)
incorporating the aromatic residue adopts a boat conformation.
Fig. 3. ORTEP-3 view of the asymmetric unit of 2 with ellipsoids at 50% probability.
3.5. Olefin epoxidation catalyzed by complex 2 in presence of terminal
oxidants PhIO and NaOCl
00
00
Mn(1) and Mn(2)–O(2 ) ( = x + 1/2, ꢁy + 1/2, z + 1/2) 2.401(8) Å
and Mn(2)–O(4) 2.125(10) Å are significantly longer than the equa-
torial bonds as expected for a Jahn–Teller distortion of Mn ions
with a +3 oxidation state. The six-member chelate rings around
Mn(1) and Mn(2) incorporating the aromatic residue is slightly dis-
torted from planarity towards a boat conformation. At Mn(1) and
Mn(2), the five-member chelate rings incorporating the diamine
fragment adopts half-chair and envelope conformation, respec-
tively. The six-membered chelate ring around Mn(1) and Mn(2)
incorporating aromatic fragment adopts boat and screw-boat con-
formation, respectively and the six-membered chelate ring around
Mn(2) incorporating the acetylacetone fragments is essentially pla-
nar with no atoms deviating more than 0.01 Å from their respec-
tive mean planes. The complex remains as a one dimensional
polymeric chain as shown in Fig. 2.
3 2 2
Complex 2 is soluble in both CH CN and CH Cl . Hence, its cat-
alytic activity towards alkene (e.g. (E)-stilbene and styrene) epox-
3 2 2
idation reaction has been investigated in CH CN and CH Cl
solvents at room temperature.
In both of these solvents, complex 2 produces a brown colour
solution after dissolution. The colour is intensified on addition of
the terminal oxidants, PhIO. When substrates (alkenes) are added
the colour starts fading and on stirring the solution for ca. 3 h when
complete consumption of the substrates is assumed, the intensity
of the solution again increases to its original value, indicating com-
pletion of alkene epoxidation and the catalyst regeneration. The
formation of epoxides was confirmed by appearance of the charac-
1
teristic peak in the H NMR spectroscopy of the resultant products
(Figs. S3 and S4 for (E)-stilbene epoxide and styrene epoxide,
respectively).

P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
323
Table 3
Selected bond lengths (Å) and bond angles (°) in complex 2.
Mn(1)–O(2)
Mn(1)–O(21)
Mn(1)–O(22)
Mn(1)–N(10)
Mn(1)–N(13)
1.827(2)
2.047(2)
2.048(2)
2.098(2)
1.961(2)
2.643(2)
0
Mn(1)–O(21 )
O(2)–Mn(1)–O(21)
O(2)–Mn(1)–O(22)
O(2)–Mn(1)–N(10)
O(2)–Mn(1)–N(13)
88.81(9)
95.17(9)
97.72(9)
168.58(9)
91.89(8)
108.62(8)
166.75(9)
94.50(9)
71.69(7)
82.38(9)
94.13(9)
172.95(8)
76.96(10)
96.47(8)
78.84(8)
108.31(8)
0
O(2)–Mn(1)–O(21 )
O(21)–Mn(1)–O(22)
O(21)–Mn(1)–N(10)
O(21)–Mn(1)–N(13)
0
O(21 )–Mn(1)–O(21)
O(22)–Mn(1)–N(10)
O(22)–Mn(1)–N(13)
0
O(21 )–Mn(1)–O(22)
N(10)–Mn(1)–N(13)
0
0
O(21 )–Mn(1)–N(10)
O(21 )–Mn(1)–N(13)
Fig. 4. UV–Vis spectral pattern of complex 2: (a) 2.00 ꢂ 10 M solution of complex
in dry acetonitrile at 25 °C, (b) after 1 h reaction with PhIO (0.300 mmol) (sample
diluted by fivefold) and (c) after 1 day of the reaction (sample diluted fivefold).
ꢁ3
0
Mn(1)–O(21)–Mn(1 )
0
Symmetry code = 2 ꢁ x, 1 ꢁ y, ꢁz.
of the intensity. The species which was obtained after 1 day by
the reaction of 2 with PhIO, was again treated with fresh PhIO
(0.300 mmol) and alkenes. It was found to be as reactive as the ini-
tial compound (i.e. complex 2) and this reactivity was noticed to be
retained beyond four cycles.
Table 4
Yield of epoxides from the reaction of alkenes with PhIO or NaOCl catalysed by
complex 2.
Olefins
E)-stilbene
Terminal oxidant
PhIO
Solvent
Isolated yield of epoxides (%)
(
CH
3
CH
2
CH
3
CH
2
CN
Cl
CN
Cl
85
70
60
54
2
4
. Conclusion
NaOCl
2
Existence of MnIII complexes with both unsymmetrical (uspen)
Styrene
PhIO
CH
CH
CH
CH
3
2
3
2
CN
Cl
CN
Cl
40
35
34
30
2
and symmetrical (salen) tetradentate ligands were proved earlier
by isolating them in the form of cocrystals [31]. In the present
study, we have shown that these two different species can be
joined together through a formate bridge to yield an unprece-
NaOCl
2
III
dented one-dimensional chain of alternating Mn –salen and
III
Mn –uspen. It is to be noted that probability of linking two differ-
Table 4 shows the maximum isolated yield (%) for (E)-stilbene
and styrene epoxidation, respectively, using PhIO and NaOCl as ter-
minal oxidants in both CH Cl and CH CN. On comparing the yield
2 2 3
of oxidation it may be stated that (i) PhIO is more efficient as ter-
minal oxidant than NaOCl towards epoxidation of both (E)-stilbene
and styrene resulting higher epoxide yield; (ii) solvent plays a cru-
cial role in the alkene epoxidation process – the isolated yields of
ent species by formate is twice as much as that of any one of the
III
similar species considering the equal concentration of Mn –salen
III
and Mn –uspen. Therefore, formation of compound 1 is not sur-
prising. The completely rearranged product, which is a phenoxo
III
bridged dinuclear Mn complex (2) acts as an efficient catalyst
in the epoxidation of alkenes, e.g. (E)-stilbene, styrene, etc and it
retains its reactivity with high efficiency for a long time.
(
E)-stilbene and styrene epoxide (Table 4) in presence of both ter-
minal oxidants PhIO and NaOCl indicate that CH CN is a better sol-
vent than CH Cl for epoxidation of both the substrates. The slight
improvement in epoxidation yield of alkenes with sodium hypo-
chlorite as the oxygen source on changing the solvent from CH Cl
to CH CN is most probably related to higher solubility of the cata-
lysts in CH CN. When PhIO is used as oxygen source in addition to
3
2
2
Acknowledgments
2
2
We thank CSIR, India for awarding Junior Research Fellowship
Sanction no. 09/028 (0663)/2006-EMR-I] and [Sanction no. 09/
28(0733)/2008-EMR-I] to P.M. and P.K. respectively. We are
thankful to Dr. D. Das and Ms A. Banerjee for their suggestions dur-
ing catalytic study. We also thank Dr. G. Mustafa, Jadavpur Univer-
sity for his help in solving crystal structure.
3
[
0
3
the solubility effect, the minimization of the formation of benzal-
dehyde and other chlorinated products in styrene epoxidation in
CH
3
CN are also to be taken in account to explain the better effi-
CN as solvent for the epoxidation of styrene [52–54].
ciency of CH
3
To examine the nature of the complex 2 in the catalytic process,
we studied the UV–Vis spectral changes for complex 2 in the pres-
Appendix A. Supplementary material
ꢁ
3
3
ence of PhIO (1.00 ꢂ 10 mmol complex in CH
2 2
Cl (5 cm ) was
treated with 0.300 mmol PhIO). The result for the complex has
been shown in Fig. 4. It shows spectra before addition of PhIO
CCDC 742805 and 742806 contain the supplementary crystallo-
graphic data for complexes 1 and 2. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.09.036.
(
a), after 1 h of reaction (b) and after 1 day of reaction (c). PhIO
was completely reacted after 1 day in a solution of the complex.
After 1 day, complex 2 produced spectrum (c) which is identical
to its original one (spectrum (a), Fig. 4) except for the lowering

3
24
P. Mukherjee et al. / Inorganica Chimica Acta 365 (2011) 318–324
[
[
[
[
[
29] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104
2004) 3981.
30] N. Matsumoto, Z.J. Zhong, H. Okawa, S. Kida, Inorg. Chim. Acta 160 (1989) 153.
and references therein.
31] A.G. Deibe, M.R. Bermejo, A. Sousa, C.A. McAuliffe, P.M. Peter, T. Ndifonb, R.G.
Pritchard, J. Chem. Soc., Dalton Trans. (1993) 1605.
32] A.G. Deibe, A. Sousa, M.R. Bermejo, P.P. Mac Rory, C.A. McAuliffe, R.G. Pritchard,
M. Helliwell, J. Chem. Soc., Chem. Commun. 1 (1991) 728.
References
(
[
1] V.L. Pecoraro (Ed.), Manganese Redox Enzymes, VCH Publishers, New York,
992.
1
[
[
2] G. Christou, Acc. Chem. Res. 22 (1989) 328.
3] R. Sessoli, H.-L. Tsai, A.R. Schake, S. Wang, J.B. Vincent, K. Folting, D. Gatteschi,
G. Christou, D.N. Hendrickson, J. Am. Chem. Soc. 115 (1993) 1804.
4] R. Sessoli, D. Gatteschi, A. Caneschi, M.A. Novak, Nature 365 (1993) 141.
5] J.R. Friedman, M.P. Sarachik, J. Tejada, R. Ziolo, Phys. Rev. Lett. 76 (1996) 3830.
6] W. Wernsdorfer, R. Sessoli, Science 284 (1999) 133.
[
[
[
[
33] M.R. Bermejo, A.G. Deibe, M. Rey, J. Sanmartin, A. Sousa, N. Aurangzeb, C.E.
Hulme, C.A. McAuliffe, R.G. Pritchard, M. Watkinson, M. Helliwell, J. Chem.
Soc., Dalton Trans. (1994) 1265.
7] C. Boskovic, E.K. Brechin, W.E. Streib, K. Folting, J.C. Bollinger, D.N.
Hendrickson, G. Christou, J. Am. Chem. Soc. 124 (2002) 3725.
[
34] T. Chattopadhyay, S. Islam, M. Nethaji, A. Majee, D. Das, J. Mol. Catal. A: Chem.
267 (2007) 255. and references therein.
[
[
8] E.K. Brechin, M. Soler, G. Christou, J. Davidson, D.N. Hendrickson, S. Parsons, W.
Wernsdorfer, Polyhedron 22 (2003) 1771.
9] M. Soler, W. Wernsdorfer, K. Folting, M. Pink, G. Christou, J. Am. Chem. Soc. 126
[
[
[
[
[
35] K.A. Joergensen, Chem. Rev. 89 (1989) 431.
36] P. Mukherjee, C. Biswas, M.G.B. Drew, A. Ghosh, Polyhedron 26 (2007) 3121.
37] H. Saltzman, J.G. Sharefkin, Org. Synth. Coll. V 5 (1973) 658.
38] K. Srinivasan, P. Michaud, J.K. Kochi, J. Am. Chem. Soc. 108 (1986) 2309.
39] H. Fu, G.C. Look, W. Zhang, E.N. Jacobsen, C.-H. Wong, J. Org. Chem. 56 (1991)
(
2004) 2156.
[
[
[
10] L.F. Jones, E.K. Brechin, D. Collison, A. Harrison, S.J. Teat, W. Wernsdorfer,
Chem. Commun. (2002) 2974.
11] A.J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K.A. Abboud, G. Christou, Angew.
Chem., Int. Ed. 43 (2004) 2117.
6497.
[
[
40] N.H. Lee, E.N. Jacobsen, Tetrahedron Lett. 32 (1991) 6533.
41] N.S. Finney, P.J. Pospisil, S. Chang, M. Palucki, R.G. Konsler, K.B. Hansen, E.N.
Jacobsen, Angew. Chem., Int. Ed. Engl. 36 (1997) 1720.
12] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, J. Am. Chem. Soc. 112 (1990)
2
801.
[
42] M. Palucki, N.S. Finney, P.J. Pospisil, M.L. Guler, T. Ishida, E.N. Jacobsen, J. Am.
Chem. Soc. 120 (1998) 948.
[
[
[
[
[
[
13] A.R. Oki, D.J. Hodgsen, Inorg. Chim. Acta 170 (1990) 65.
14] C. Bowers, P.K. Dutta, J. Catal. 122 (1990) 271.
15] W. Zhang, E.N. Jacobsen, J. Org. Chem. 56 (1991) 2296.
16] Q.-H. Xia, H.-Q. Ge, C.-P. Ye, Z.-M. Liu, K.-X. Su, Chem. Rev. 105 (2005) 1603.
17] R. Irie, Y. Ito, T. Katsuki, Synlett (1991) 265.
[43] Bruker, SADABS, Bruker Axs Inc., Madison, Wisconsin, USA, 2001.
[44] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[45] D.T. Cromer, J.T. Weber, International Tables for X-ray Crystallography, vol. IV
(
Table 2.2A), The Kynoch Press, Birmingham, UK, 1994. p. 2.
18] R. Irie, K. Noda, Y. Ito, N. Matsumoto, T. Katsuki, Tetrahedron: Asymmetry 2
[
[
46] J.A. Ibers, W.C. Hamilton, Acta Crystallogr., Sect. 17 (1964) 781.
47] L.J. Farrugia, ORTEP-3 for WINDOWS, University of Glasgow, Scotland, UK,
(
1991) 481.
[
[
[
19] E.N. Jacobsen, W. Zhang, M.L. Guler, J. Am. Chem. Soc. 113 (1991) 6703.
20] N. Hosoya, R. Irie, T. Katsuki, Synlett (1993) 261.
21] X.-H. Lu, Q.-H. Xia, H.-J. Zhana, H.-X. Yuan, C.-P. Ye, K.-X. Su, G. Xu, J. Mol. Catal.
A: Chem. 250 (2006) 62. and references therein.
1
999.
48] P. Mukherjee, M.G.B. Drew, C.J. Gomez-García, A. Ghosh, Inorg. Chem. 48
2009) 4817.
[
[
[
(
49] L. Lecren, W. Wernsdorfer, Y.-G. Li, A. Vindigni, H. Miyasaka, R. Clérac, J. Am.
Chem. Soc. 129 (2007) 5045.
50] R. Karmakar, C.R. Choudhury, G. Bravic, J.-P. Sutter, S. Mitra, Polyhedron 23
[
[
[
22] H. Sasaki, R. Irie, T. Katsuki, Synlett (1993) 300.
23] N. Hosoya, A. Hatayama, K. Yanai, H. Fuji, R. Irie, T. Katsuki, Synlett (1993) 641.
24] J.F. Larrow, E.N. Jacobsen, Y. Gao, Y. Hong, X. Nie, C.M. Zepp, J. Org. Chem. 59
(
2004) 949.
(
1994) 1939.
[
[
[
51] Z. Lu, M. Yuan, F. Pan, S. Gao, D. Zhang, D. Zhu, Inorg. Chem. 45 (2006) 3538.
52] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85.
53] W. Adam, K.J. Roschmann, C.R. Saha-Möller, D. Seebach, J. Am. Chem. Soc. 124
[
[
[
[
25] T. Hamada, R. Irie, T. Katsuki, Synlett (1994) 479.
26] T. Hamada, T. Fukuda, H. Imanishi, T. Katsuki, Tetrahedron 52 (1996) 515.
27] T. Linker, Angew. Chem., Int. Ed. Engl. 36 (1997) 2060.
(
2002) 5068.
28] R. Irie, T. Hashihayata, T. Katsuki, M. Akita, Y. Moro-oka, Chem. Lett. (1998)
[
54] J.P. Collman, L. Zeng, J.I. Brauman, Inorg. Chem. 43 (2004) 2672.
1
041.