Synthesis, structure and alkene epoxidation activity of an alternating phenoxido and formato bridged manganese(III)-salen complex

Article abstract of DOI:10.1016/j.ica.2012.10.012

One new complex of Mn(III), {[Mn₂(salen)₂(HCOO)] (ClO₄)(1.5CH₃OH)(0.5H₂O)}_n (1) has been synthesized and characterized by single-crystal X-ray diffraction analyses. It consists of dinuclear

Full text of DOI:10.1016/j.ica.2012.10.012

Inorganica Chimica Acta 395 (2013) 67–71
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Note
Synthesis, structure and alkene epoxidation activity of an alternating phenoxido and
formato bridged manganese(III)–salen complex
⇑
Paramita Kar, Ashutosh Ghosh
Department of Chemistry, University College of Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2012
Received in revised form 8 October 2012
Accepted 10 October 2012
Available online 2 November 2012
One new complex of Mn(III), {[Mn2(salen)2(HCOO)](ClO4)(1.5CH3OH)(0.5H2O)}n (1) has been synthesized
and characterized by single-crystal X-ray diffraction analyses. It consists of dinuclear cationic units
[Mn2(salen)2(HCOO)]+, joined together by weak Mnꢀ ꢀ ꢀO (phenoxido) interactions to form an alternating
phenoxido- and formato-bridged 1D polymer. The complex acts as an efficient catalyst in the alkene ((E)-
stilbene, styrene) epoxidation reaction in presence of the terminal oxidant PhIO in solvents CH3CN and
CH2Cl2 independently and it retains its reactivity with high efficiency beyond three cycles.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Manganese(III)
Salen
Bridging formate
Crystal structure
Alkene epoxidation
1. Introduction
activity towards alkene epoxidation of this type of complexes is
not reported till date.
The coordination chemistry of manganese with a diverse range
of ligands is an active field of research for decades not only due to
their fascinating cluster compounds [1–3], which exhibit unusual
magnetic properties [4–5], and the versatile supramolecular arrays
[6–8] but also for their suitable biomimetic properties that can mi-
mic the structural features of the active sites of several metallopro-
teins [9–12] etc. Binuclear phenoxido-bridged manganese(III)
compounds containing salen-type Schiff bases are of potential
interest due to their easy synthetic methodology and their essen-
tial roles in biological systems [13]. Moreover, topical interest in
this type of compound has increased since Miyasaka et al. reported
a dimeric manganese(III) Schiff base compound, [Mn2(saltmen)2
(ReO4)2][H2saltmen = N,N0-(1,1,2,2-tetramethylethylene)bis(salicy-
lideneimine)] showing single-molecule magnet (SMM) behavior
[14]. Recently our group explored interesting spin-canting behav-
ior exhibited by alternating phenoxido and carboxylato bridged
Mn(III)–salen based compounds [15]. As the manganese coordina-
tion compounds are a potential source of homogeneous catalysts,
we would like to explore the catalytic activity of this type of
compounds.
2. Experimental
2.1. Starting materials
The reagents and solvents used were of commercially available
reagent quality, unless otherwise stated. Styrene and (E)-stilbene
were purchased from Aldrich and used in epoxidation experiment
without further purification. H2salen was prepared by the conden-
sation of salicylaldehyde and 1,2-ethanediamine by reported
method [16].
2.2. Caution!
Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only a small amount of material should be
prepared, and it should be handled with care.
2.3. Synthesis of {[Mn2(salen)2(HCOO)](ClO4)(1.5CH3OH)(0.5H2O)}n
(1)
Herein, we report the synthesis, crystal structure, and alkene
epoxidation property of a new 1D chain {[Mn2(salen)2(HCOO)]
(ClO4)(1.5CH3OH)(0.5H2O)}n where H2salen = N,N0-bis(salicylid-
ene)-1,2-diaminoethane. To the best of our knowledge, catalytic
A 10 mL methanolic solution of H2salen (5 mmol) was added to
a
methanolic solution (10 mL) of Mn(ClO4)2ꢀ6H2O (1.805 g,
5 mmol) with constant stirring. After ca. 15 min HCOOH
(0.096 mL, 2.5 mmol) was added to the mixture. Triethylamine
(0.35 mL, 2.5 mmol) was added drop wise to the solution with con-
stant stirring. The color of the solution turned to dark brown
⇑
Corresponding author. Tel.: +91 9433344484; fax: +91 33 2351 9755.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.10.012

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P. Kar, A. Ghosh / Inorganica Chimica Acta 395 (2013) 67–71
immediately. Slow evaporation of the resulting brown solution
gave dark brown microcrystalline compound for 1. The solid was
filtered and redissolved in CH3OH. Layering of the brown methano-
lic solution with Et2O gave well formed X-ray quality single
crystals.
(ESI-MS positive) spectra were recorded with a Micromass Qtof
YA 263 mass spectrometer.
2.7. Epoxidation of alkenes catalyzed by complex 1
Complex 1: Yield: 1.41 g; 67%. Anal. Calc. for C69H70Cl2Mn
4N8O24 (1685.99): C, 49.15; H, 4.18; N, 6.65. Found: C, 49.10; H,
Under nitrogen atmosphere, alkene (0.300 mmol) and catalyst
(1.00 ꢂ 10ꢁ2 mmol) were treated with 5 cm3 dry acetonitrile or
dichloromethane. Afterwards under nitrogen atmosphere iodosyl-
benzene (0.066 g, 0.300 mmol) was added to the solution. 1H
NMR spectroscopy was used for identification of the product. For
1H NMR experiments epoxides were prepared following the proce-
dure described by Kochi and co-workers [19] when PhIO was used
as oxidant.
4.16; N, 6.60%. IR (KBr pellet, cmꢁ1): 1621
m
(C@N), 1534
mas(C@O),
1441
ms(C@O),
m
(ClO4ꢁ), 1098. Magnetic moment
l
= 4.09 BM.
2.4. Preparation of iodosylbenzene
This was prepared by hydrolysis of the corresponding diacetate
with aqueous sodium hydroxide as reported in the literature [17].
Freshly prepared PhIO was used in every epoxidation experiment.
3. Results and discussion
3.1. IR and UV–Vis spectra of complex 1
2.5. Crystal data collection and refinement
In the IR spectra of complex 1, the moderately strong and sharp
For 1, 16657 data were collected on a Bruker Smart APEX II dif-
fractometer, equipped with graphite monochromated Mo Ka radi-
band at 1620 cmꢁ1 is assigned to the azomethine
m(C@N) group.
The strong bands at 1578 and 1441 cmꢁ1 are likely due to the anti-
symmetric and symmetric stretching modes of carboxylate group,
respectively [20]. The characteristic strong peaks for stretching
vibrations of uncoordinated perchlorate anion have been observed
at 1095 cmꢁ1. A broad band at around 3436 cmꢁ1 is due to the sol-
vent methanol and water molecules. The electronic spectra of the
complex in CH3CN and CH2Cl2 solution and solid state diffuse
reflectance spectra are shown in Fig. 1. A single absorption band
was found at ꢃ405 nm in the visible region for 1 in both CH3CN
and CH2Cl2 solutions and a broad absorption band is observed in
the same wavelength region in solid state, as usually observed in
octahedral Mn(III) complexes [21,22]. But, the nature of the
absorption band in solid is different from that in solution which
indicates that the structural identity is lost on dissolution. This
phenomenon was further supported by electrospray ionization
mass spectrometry.
ation at 150 K, and were corrected for Lorentz-polarization effects.
The structure was solved using direct methods with the SHELXS97
program [18]. The non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms bonded to carbon
were included in geometric positions and given thermal parame-
ters equivalent to 1.2 times those of the atom to which they were
attached. In the structure three methanol and one water solvent
molecules were located and refined with distance constraints and
50% occupancy. The structures were refined on F2 using the
SHELXl97 program [18] to R1 0.0771 and wR2 0.2565 for 6396 inde-
pendent reflections.
2.6. Physical measurements
Elemental analyses (C, H and N) were performed using a
Perkin-Elmer 2400 series II CHN analyzer. IR spectra in KBr
(4500–500 cmꢁ1) were recorded using a Perkin-Elmer RXI FT-IR
spectrophotometer. The electronic absorption spectra (1000–
200 nm) of the complexes were recorded in CH3CN with a Hitachi
U-3501 spectrophotometer. The 1H NMR spectra were recorded in
CDCl3 on a Bruker AC300 spectrometer. Room temperature mag-
netic susceptibility was measured with a PAR 155 vibrating sample
magnetometer. The electrospray ionization mass spectrometry
3.2. Description of the structure
The structure contains [Mn2(salen)2(OOCH)]+cation together
with a discrete perchlorate anion and one water and three metha-
nol molecules (all the solvent molecules with 50% occupancy) in
the asymmetric unit as shown in Fig. 2 together with the atomic
numbering scheme in the metal coordination spheres. Selected
Fig. 1. Left: UV–Vis spectra in CH2Cl2 (A) and CH3CN (B). Right: UV–Vis spectra in solid state.

P. Kar, A. Ghosh / Inorganica Chimica Acta 395 (2013) 67–71
69
0
Fig. 3. A view of polymeric structure of complex 1. (symmetry element = ꢁx, ꢁy,
ꢁz + 1).
a slip angle is 20.30°. Similarly the phenyl rings R3 [C(32)–C(33)–
C(34)–C(35)–C(36)–C(37)] and R4 [C(44)-C(45)-C(46)–C(47)–
C(48)–C(49)] are also stabilized by
p–p interactions having a
Fig. 2. The dimeric structure of
1 with ellipsoids at 30% probability. Solvent
centroid–centroid distance of 3.696(5) Å whereas the dihedral
angle between the rings is 6.93° with a slip angle of 21.16°.
A search of the CCDC database shows six compounds similar to
complex 1. Among these, a formato bridged complex in which
unsymmetrical Schiff base, uspen (H2uspen = 2-{[2-(3-Hydroxy-
1-methyl-but-2-enylideneamino)-ethylimino]-methyl}-phenol) and
salen form 1D chain {[{Mn(salen)}{Mn(uspen)}(HCOO)](ClO4)}n
[23]. Rest of the complexes are based on symmetrical tetradentate
Schiff base ligands. Of which, compounds [Mn2(salen)2(CH3-
COO)]ClO4 [24] and [Mn2(7-Me-salen)2(CH3COO)]ClO4 [25] are re-
ported to be acetate-bridged dimers. The association of the
dimeric units via the phenoxido bridge of any of these two com-
plexes was not analyzed, although the deposited CIF associated
with the reference [24] reveals that it is a 1D polymer like the others
[15] with long Mn–O(phenoxido) distances, 2.453 and 2.686 Å,
respectively involving the vacant axial positions of Mn(III). The cor-
responding distances are 2.543(2) and 2.743(3) Å in {[Mn2(salen)2
(C6H5CH@CH–COO)](ClO4)}n [15] and, 2.576(4) and 2.729(4) Å
{[Mn2(salen)2(C6H5CH2COO)](ClO4)}n [15]. Since the Mn–O dis-
tances are within the bonding distance [26], all of these compounds
should be considered as 1D polymer. However, the relatively long
Mnꢀ ꢀ ꢀO distance [3.621(2) Å] {[Mn2(salen)2(C6H5COO)](ClO4)}2
[15] restricts the structure to tetranuclear units. In the present com-
plex, the Mn–O(phenoxido) distances [2.692(2) and 2.945(6) Å] are
rather long. However, even the longer Mnꢀ ꢀ ꢀO distance (2.945(6) Å)
is reported to be within the Mnꢀ ꢀ ꢀO bonding distance [26]. There-
fore the structure is considered as 1D polymer rather than a
tetramer.
molecules have been removed for clarity.
bond lengths and angles are summarized in Supplementary Table
2.
Within the asymmetric unit both metal atoms can be consid-
ered as five-coordinate with square pyramidal (4 + 1) environ-
ments taking into account only the five strong bonds. However,
both have weak bonds in the other axial position which make
the effective coordination number six with tetragonal geometry
and allow the formation of 1D chain. The two metal atoms Mn(1)
and Mn(2) are coordinated by the four donor atoms of the depro-
tonated tetradentate Schiff base ligand (salen) in the equatorial
sites. The bond lengths are as expected with Mn–O and Mn–N in
the ranges 1.852(6)–2.692(6) Å and 1.951(7)–1.978(7) Å respec-
tively. The two oxygen atoms O(1) and O(3) of the bridging formate
coordinate to the axial positions of Mn(1) and Mn(2) at distances of
2.142(6) and 2.114(6) Å respectively to form a syn-anti formate
bridge between the two Mn ions. These axial bonds are signifi-
cantly longer than the equatorial bonds as expected for a Jahn-Tell-
er distortion of Mn ions in the +3 oxidation state.
The r.m.s. deviations of the four basal donor atoms from their
mean plane are 0.036 and 0.033 Å around Mn(1) and Mn(2) respec-
tively. The Mn(1) and Mn(2) atoms deviate from the respective
mean plane by 0.161(1) and 0.213(1) Å, respectively in the direc-
tion of the bridging axial formate oxygen atom. The equatorial
planes formed by four basal donor atoms around Mn(1) and
Mn(2) intersect at an angle of 51.90(3)°.
In addition to these five bonds, the metal atoms show weak
interactions in the other axial position to phenoxido oxygen atoms
of salen from the neighboring dinuclear units with Mn(1)–O(30)
2.692(2) Å (ꢁx, ꢁy, ꢁz + 1) and Mn(2)–O(31) 2.945(6) Å
(ꢁx, ꢁy + 1, ꢁz + 2) thus forming centrosymmetric bridging
arrangements as shown in Fig. 3. These two alternating bridging
systems i.e. phenoxido- and syn-anti formate constitute the 1D
chain of the compound.
The crystal packing of the complex is further stabilized by offset
or slipped stacking p–p interactions observed between the phenyl
rings of phenoxido bridged dimeric units as shown in Fig. 4. The for-
mate bridged dimeric units are stacked with each other by phenyl
rings [ring R1 = C(12)–C(13)–C(14)–C(15)–C(16)–C(17) with ring
R2 = C(24)–C(25)–C(26)–C(27)–C(28)–C(29)] having centroid–
3.3. Olefin epoxidation catalyzed by complex 1 in presence of terminal
oxidant PhIO
Complex 1 is soluble in both CH3CN and CH2Cl2. Hence, its cat-
alytic activity towards alkene (e.g. (E)-stilbene and styrene) epox-
idation reaction has been investigated in CH3CN and CH2Cl2
solvents at room temperature. In both of these solvents, complex
1 produces a brown color solution after dissolution. The color is
intensified on addition of the terminal oxidant, PhIO. When sub-
strates (alkenes) are added, the color starts fading and on stirring
the solution for ca. 2 h when complete consumption of the sub-
strates is assumed, the intensity of the solution again increases
to its original value, indicating completion of alkene epoxidation
and the catalyst regeneration. The formation of epoxides was con-
firmed by the appearance of the characteristic peak in the 1H NMR
centroid distance of 3.725(6) Å. The
p–p interactions are quite
strong because the dihedral angle between the rings is 13.62° with

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P. Kar, A. Ghosh / Inorganica Chimica Acta 395 (2013) 67–71
Fig. 4. Supramolecular p–p stacking interaction between the phenyl rings of complex 1.
[30]. Previously our group reported the catalytic epoxidation of
Mn(III)–salen based compound with comparable activity [23].
To examine the nature of the catalytic process, 1.00 ꢂ 10ꢁ3
M
acetonitrile (5 cm3) solution of complex 1 was treated with
0.300 mmol PhIO. The results for are shown in Fig. 5. It shows spec-
tra before addition of PhIO (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 1 produced spec-
trum (c) which is identical to its original one (spectrum (a),
Fig. 5) except for the lowering of the intensity. The species which
was obtained after 1 day by the reaction of 1 with PhIO, was again
treated with fresh PhIO (0.300 mmol) and alkenes. It was found to
be as reactive as the initial compound (i.e. complex 1) and this
reactivity was noticed to be retained beyond three cycles.
To have an idea regarding the species distribution in the solu-
tions we recorded the ESI spectra both in CH3CN and CH2Cl2 and
found the presence of base peak of Mn(salen)+ (ESI+): found m/z
M+ = 320.92; Mcalc = 321.04) (Figs. S4a and S5a). In this spectra,
peak of [{Mn(salen)}2(ClO4)]+ ((ESI+): found m/z M+ = 740.77;
Mcalc = 741.03) (Figs. S4a and S5a) was also observed. These two
peaks were obtained in both of the solvents. Whereas in CH3CN
solution a peak of solvent coordinated species [Mn(salen)(CH3-
CN)2]+ (ESI+): found m/z M+ = 403.06; Mcalc = 403.10) (Fig. S5b)
was obtained, but such solvent coordinated species was absent in
CH2Cl2. Again [{Mn(salen)}2(HCOO)]+ (Fig. S4b) ((ESI+): found m/z
M+ = 686.90; Mcalc = 687.08) (Fig. S1b) species is found in CH2Cl2
solution, but not observed in CH3CN.
Fig. 5. UV–Vis spectral pattern of complex 1: (a) 1.00 ꢂ 10ꢁ3 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).
spectroscopy of the resultant products (Supplementary Figs. S2
and S3).
Supplementary Table 3 shows the maximum isolated yield (%)
for (E)-stilbene and styrene epoxidation. On comparing the yield
of oxidation it may be stated that solvent plays a crucial role in
the alkene epoxidation process – the isolated yields of (E)-stilbene
(86% in CH3CN and 72% in CH2Cl2) and styrene epoxide(43% in CH3-
CN and 37% in CH2Cl2) (Supplementary Table 3) indicate that CH3-
CN is a better solvent than CH2Cl2 for epoxidation of both the
substrates.
In recent years, a few similar studies on epoxidation activity of
olefins catalysed by Mn(III)–salen complexes were undertaken by
several groups. For example, Kochi and co-workers have shown
that various types of olefins, including substituted styrenes, stilb-
enes, and cyclic and acyclic alkenes, are epoxidized in 50–75%
yields [19]. Chatterjee and Mitra reported epoxidation of cyclohex-
ene, cyclooctene and 1-hexene with relatively lower yield (2–12%)
[27]. Yang and Nocera reported epoxidation of 1,2-dihydronaptha-
lene with several terminal oxidants with comparable efficiency
[28]. Das and co-workers reported epoxidation of (E)-stilbene
and styrene with high yields of corresponding epoxides [11]. Freire
and co-workers reported very stable and reusable Mn(III)–salen
based catalyst of styrene with high catalytic efficiency [29]. Murray
and co-workers reported electrocatalytic epoxidation with dioxy-
gen as terminal oxidant with 48% yield of corresponding olefin
4. Conclusions
This report presents the synthesis of one new salen based Mn(III)
complex that forms one-dimensional chain by alternating phenoxi-
do and formato bridge. In the complex, the dinuclear cationic units
[Mn2(salen)2(HCOO)]+ are joined together by weak Mnꢀ ꢀ ꢀO (phe-
noxido) interactions and
p–p supramolecular interactions. The
complex is found to be a good catalyst for alkene epoxidation.
Acknowledgments
We thank Council of Scientific and Industrial Research (CSIR),
India for awarding a senior research fellowship [Sanction No. 09/
028(0733)/2008-EMR-I] to P. Kar.
Appendix A. Supplementary material
CCDC No. 880391 contains the supplementary crystallographic
data for compound 1. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via

P. Kar, A. Ghosh / Inorganica Chimica Acta 395 (2013) 67–71
71
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