A hexanuclear manganese(ii) complex: Synthesis, characterization and catalytic activity toward organic sulfide oxidation

Article abstract of DOI:10.1039/c4nj01159g

Herein we report on the synthesis and characterization of a hexanuclear [Mn^II₆] wheel-like assembly with naphthalene-1,8-dicarboxylate and 1,10-phenanthroline as ligands. Overall antiferromagnetic behavior of the magnetic properties is observed. The complex acts as a catalyst toward organic sulfide oxidation in the presence of H₂O₂ with good selectivity (98-100%). This journal is

Full text of DOI:10.1039/c4nj01159g

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A hexanuclear manganese(II) complex: synthesis,
characterization and catalytic activity toward
organic sulfide oxidation†
Cite this: New J. Chem., 2014,
38, 5069
Mohammad Mahdi Najafpour,*ab Mojtaba Amini,*c Małgorzata Hołynska,*d
´
Maryam Zaree and Emad Aminia
Received (in Porto Alegre, Brazil)
10th July 2014,
Herein we report on the synthesis and characterization of a hexanuclear [MnII6] wheel-like assembly
with naphthalene-1,8-dicarboxylate and 1,10-phenanthroline as ligands. Overall antiferromagnetic
behavior of the magnetic properties is observed. The complex acts as a catalyst toward organic sulfide
oxidation in the presence of H2O2 with good selectivity (98–100%).
Accepted 15th August 2014
DOI: 10.1039/c4nj01159g
www.rsc.org/njc
nature. In recent years, some transition-metal catalysts such as
vanadium,6 rhenium,7 iron,8 manganese9 and titanium10 have
Introduction
Oxidation reactions to produce useful chemicals are among been widely used to catalyze the selective oxidation of sulfides
important reactions on both small and industrial scales.1 The to sulfoxides in the presence of hydrogen peroxide.
application of transition metal complexes as catalysts in organic
Manganese compounds are not only low cost and environ-
synthesis is an exciting field of research, and they are extensively mentally friendly, but also used by Nature in a variety of metallo-
employed in a wide range of areas of preparative organic chemistry.2 enzymes especially for oxidation reactions11 Thus, manganese
Among those reactions, the selective oxidation of sulfides is complexes with different ligands were used as catalysts to promote
an important transformation in organic synthesis, leading to oxidation reactions.12
sulfoxides, which play the role of essential intermediates.3
In this paper we present a new hexanuclear Mn(II) complex
Sulfur-containing compounds play an important role in the (1) active as a catalyst towards oxidation reactions, in particular
pharmaceutical industry and particularly as antibacterial of sulfides.
agents with sulfur at an elevated oxidation state.4 To obtain
many of these agents, oxidation of a sulfide is often required.
Among different sulfide oxidation reactions, the oxidation of a
sulfide to a sulfoxide is especially important. The oxidation
Experimental
Synthesis
reaction performances have been tested with a very wide variety
of reagents and catalysts.5 Hydrogen peroxide is a commonly
used oxidant due to its low-cost and environmentally friendly
A mixture of MnCl2ꢀ4H2O (395.82 mg, 2 mmol), naphthalene-
1,8-dicarboxylic anhydride (396.36 mg, 2 mmol), NaOH (80.4 mg,
2 mmol) and phenanthroline (396.48 mg, 2 mmol) in acetonitrile–
water (30 mL, v/v, 1 : 2) was sealed in a 100 mL stainless steel reactor
with a Teflon liner and heated at 393 K for 24 h. Then, the reactor
was cooled slowly to room temperature and the mixture was
filtered, yielding yellow block-shaped single crystals of 1 suitable
for X-ray analysis after one week (yield 24%).
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan, 45137-66731, Iran. E-mail: mmnajafpour@iasbs.ac.ir;
Tel: +98 241 415 3201
b Center of Climate Change and Global Warming, Institute for Advanced Studies in
Basic Sciences (IASBS), Zanjan, 45137-66731, Iran
c Department of Chemistry, Faculty of Science, University of Maragheh, Golshahr,
P.O. Box 55181-83111731, Maragheh, Iran. E-mail: mamini@maragheh.ac.ir;
Structure 1
Tel: +98 421 2278900
d
X-ray diffraction experiment. A single crystal of 1 in the form
of a yellow polyhedron was mounted on a Stoe IPDS2 diffract-
ometer equipped with a MoKa radiation source and an image
plate detector (see Table 1 for basic crystallographic data).
Details of structure refinement. The crystal structure of 1
was solved by direct methods using SHELXS97 software and
refined in SHELXL97.13
¨
Fachbereich Chemie, Wissenschaftliches Zentrum fur Materialwissenschaften,
¨
Philipps-Universitat Marburg, Hans-Meerwein-Strasse, D-35043 Marburg,
Germany. E-mail: holynska@staff.uni.marburg.de; Tel: +49 6421282 25615
e Department of Basic Sciences, Golpayegan University of Technology,
P.O. Box 8771765651, Golpayegan, Iran
† Electronic supplementary information (ESI) available. CCDC 1004129. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4nj01159g
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Table 1 Selected X-ray data for 1
Other measurements
MIR spectra of the samples prepared in the form of KBr pellets
were recorded on a Genesis FT-Spectrometer (ATI Mattson) in
the range between 400 and 4000 cmꢁ1. Measurements of
magnetic properties were carried out using a Quantum Design
SQUID magnetometer for the sample of 1 (6.83 mg) in scotch
tape holder within a temperature range of 2–300 K by applying
external fields of 500 and 1000 Gs. Diamagnetic corrections
were introduced using Pascal constants.14
1
Formula
Mn6C144H108N12O36ꢀ18.12(H2O)
Formula weight
Temperature [K]
l [Å]
Crystal system
Space group
a [Å]
3220.25
100(2)
0.71073
Trigonal
R3
19.708 (3)
30.193 (4)
10 156 (3)
3, 1.580
%
c [Å]
V [Å3]
Z, rcalc [g cmꢁ3
]
m [mmꢁ1
F(000)
]
0.65
4971
General procedure for sulfide oxidation
To a solution of sulfide (0.2 mmol, see Table 2), chlorobenzene
(0.2 mmol) as an internal standard and 1 (0.005 mmol) in a 1 : 1
mixture of CH3OH/CH2Cl2 (1 mL) was added 0.2 mmol H2O2 as
an oxidant. For the analysis of products, after stirring at room
temperature for 2 h, the solution was subjected to ether
extraction (3 ꢂ 10 mL), and the extract was also concentrated
down to 1.0 mL by distillation in a rotary evaporator at room
temperature. Then, a sample (2 mL) was taken from the solution
and analyzed by gas chromatography (GC). The retention times
of the peaks were compared with those of commercial standards,
and chlorobenzene was used as an internal standard for the GC
yield calculation.
Crystal size [mm]
y range [1]
Reflns: total/unique
R(int)
0.19 ꢂ 0.19 ꢂ 0.18
1.37–24.99
62 327/3977
0.075
Numerical
0.985, 0.990
3977/0/335
1.00
0.029
0.041
0.35, ꢁ0.30
Abs. corr.
Min., max. transmission factors
Data/restraints/params
GOF on F2
R1 [I 4 2s(I)]
wR2 (all data)
Max., min. Drelect [e Åꢁ3
]
Disordered water molecules of solvation were localized.
O2W/O22W are cooperatively disordered with the refined occupancy
factors of 0.72(1)/0.28(1), respectively. The occupancies of O3W/
O33W/O23W/O4W were refined and subsequently fixed at
the refined values of 0.38(1), 0.19(1), 0.17(1) and 0.28(1),
Results and discussion
respectively. O11W lies in a special position on a three-fold 1 was synthesized by a simple method, and by the reaction of
symmetry axis with symmetry-induced disorder of H atoms, MnCl2ꢀ4H2O (2 mmol), naphthalene-1,8-dicarboxylic anhydride
taken into account by setting the corresponding occupancies. (2 mmol), NaOH (2 mmol) and phenanthroline (2 mmol) in
All H atoms, excluding those bonded to most of the disordered acetonitrile–water and in a steel reactor with a Teflon liner at
water molecules, were found on difference Fourier maps. In the 393 K for 24 h. 1 is a hexanuclear [MnII6] wheel-like assembly of
case of water H atoms a DFIX restraint was used to set the O–H B22.8 Å outer diameter with naphthalene-1,8-dicarboxylate
bond length at 0.82 Å. Subsequently the H atom parameters and 1,10-phenanthroline acting as bridging and terminal
were constrained with Ueq = 1.5/1.2Ueq (parent atom) for water/ ligands, respectively (Fig. 1). Chen reported on an analogous
aromatic C-bonded H atoms, respectively. On the final difference tetranuclear assembly with 2,20-bipyridyl acting as a terminal
Fourier map the highest rest peak of 0.35 e Åꢁ3 is located at ligand and naphthalene-1,8-dicarboxylate forming carboxylate
0.04 Å from Mn1.
bridges between the Mn2+ ions.15a In the reported literature six
a
Table 2 Oxidation of sulfides catalyzed by 1/H2O2
Entry
Catalyst (amount of catalyst)
Substrate
Conversionb (%) (TON)c
Selectivityd (%)
1
2
3
4
5
6
7
8
—
Ph–S–CH3
Ph–S–CH3
Ph–S–CH3
Ph–S–CH3
12
33
45
51
63 (25)
65 (26)
61 (24)
63 (25)
54 (22)
49 (20)
43 (17)
39 (16)
87
93
89
93
99
99
99
98
100
100
100
100
Mn(OAc)2ꢀ4H2O (0.005 mmol)
Mn(OAc)2ꢀ4H2O (0.03 mmol)
Mn(OAc)2ꢀ4H2O (0.03 mmol + 0.03 mmol bpy)
1 (0.005 mmol)
Ph–S–CH3
1 (0.005 mmol)
1 (0.005 mmol)
1 (0.005 mmol)
1 (0.005 mmol)
1 (0.005 mmol)
1 (0.005 mmol)
1 (0.005 mmol)
Ph–S–C2H5
Ph–S–CH2Ph
PhCH2–S–CH2Ph
Ph–S–Ph
C2H5–S–C2H5
C4H9–S–C4H9
C8H17–S–C8H17
9
10
11
12
a
The molar ratios for 1 : substrate : oxidant are 1 : 40 : 40. The reactions were performed in a (1 : 1) mixture of CH2Cl2/CH3OH (1 mL) under air at
b
c
room temperature. The GC yields (%) are measured relative to the starting sulfide. TON = (mmol of sulfoxide + mmol of sulfone)/mmol of the
catalyst. Selectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone)) ꢂ 100.
d
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Scheme 1 Coordination mode of the naphthalene-1,8-dicarboxylate
ligand in 1.
structures of Mn complexes with the naphthalene-1,8-dicarboxylate
ligand are known.15 Complexes with this ligand have also been
reported for other transition metals: Cd, Cu, Zn and Ag.16
The complex molecule lies in a special position on a three-
fold inverse axis, so that only one Mn2+ ion, two water ligands, one
terminal 1,10-phenanthroline ligand and one bridging naphthalene-
1,8-dicarboxylate ligand are symmetry-independent (Fig. 1a).
A coordination sphere of each Mn2+ ion includes two phenan-
throline N atoms, two carboxylate O atoms from two different
naphthalene-1,8-dicarboxylate ligands, and two water ligands.
The corresponding geometric parameters are listed in Table S1
(ESI†). The analysis of the bond lengths, including BVS calcula-
tions (Table S2, ESI†) and charge balance considerations con-
firms the Mn(II) oxidation state. The coordination mode of the
bridging ligand is illustrated in Scheme 1. The Mn-coordinated/
non-coordinated carboxylate groups are twisted with respect to
the plane of the naphthyl ring by 48.2(2)/49.0(2)1, respectively.
It is interesting to note that only one carboxylate group is
involved in Mn–O coordination bonds (syn–anti arrangement),
whereas the second carboxylate group participates solely in the
formation of intramolecular O–Hꢀ ꢀ ꢀO hydrogen bonds (Table S3,
ESI†). In these hydrogen bonds the O12–water ligand acts as a
donor to the neighbouring carboxylate group and as an acceptor
to the neighbouring water ligand. The O22–water ligand is an
O–Hꢀ ꢀ ꢀO hydrogen bond donor to the neighbouring carboxylate
and water ligand. Thus two stacked puckered S99(24) graph-set
motifs17 are formed (Fig. 1b). On the other hand, in all the
reported transition metal complexes with the naphthalene-1,8-
dicarboxylate ligand15,16 both carboxylate groups participate in
the formation of coordination bonds.
Within the molecular wheel structure the shortest Mnꢀ ꢀ ꢀMn
distance is of 5.267(1) Å. The neighbouring 1,10-phenanthroline
and naphthalene-1,8-dicarboxylate rings are involved in a weak
stacking interaction with a centroidꢀ ꢀ ꢀcentroid distance of
B3.7 Å.
The complex molecules in 1 form overlapped columns
extending along [001] (Fig. 1c). Voids between the columns
are occupied by disordered water molecules. Inside each molecular
wheel heavily disordered water molecules form ‘‘droplets’’ of B3.7 Å
diameter interacting with the hydrophilic interior through O–HꢀꢀꢀO
hydrogen bonds.
Fig. 1 (a) Structure of compound 1 with the atom labeling scheme and
bonds to Mn2+ ions highlighted in black. The non-labeled part is generated
by the symmetry operation x, ꢁy, z. Thermal ellipsoids are plotted at 30%
probability level. C-bonded H atoms are omitted, C atoms are shown as
sticks for clarity. (b) Hydrogen-bonded core with two S99(24) graph-set
motifs, one of which is denoted in black. Hydrogen bonds are shown as
dashed lines. (c) Overlapped columns of complex molecules extending
along [001]. H atoms are omitted, C atoms are shown as sticks.
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entries 10–12) sulfides underwent clean and selective oxidation
to the corresponding sulfoxides under air, with impressive
selectivities (98–100%). In a few cases, minor amounts of
byproducts resulting from overoxidation to sulfone were found
(entries 1–5). It was observed that aromatic sulfides undergo
oxidation reactions more easily than aliphatic substrates. More-
over, in the case of dialkyl sulfides (Table 2, entries 10–12), no
overoxidation to sulfone was observed. As shown in Table 2, the
catalytic oxidation of sulfides showed moderate efficiency in
yield but excellent in selectivity. The highest and the lowest
conversions were obtained for ethylphenylsulfide (65%) and
dioctyl sulfide (39%), respectively (Table 2, entries 5 and 12).
After sulfide-oxidation reaction, FTIR spectra (Fig. S1, ESI†)
shows that all organic functions are still present but XRD
patterns (Fig. S2, ESI†) cannot give a definitive clue that the
Fig. 2 wT vs. T dependence at 1000 Gs for 1.
The IR spectrum of 1 indicates peaks at 3400–3500 cmꢁ1 structure is maintained. The results show that although 1
(OH groups), 3100–2900 cmꢁ1 (aromatic C–H stretching vibra- indicates good activity toward sulfide oxidation, Mn(OAc)2ꢀ
tions), 1600–1550 cmꢁ1 (n(CQN), n(CQO) and n(CQC) stretches), 4H2O or Mn(OAc)2ꢀ4H2O/bpy show very similar activity for
1470–1020 cmꢁ1 (n(C–C) + n(C–N) vibrations), and 810–710 cmꢁ1 sulfide oxidation. We cannot relate the activity of 1 to leaking
(aromatic C–H deformation vibrations) (Fig. S1, ESI†).
of a few Mn(II) ions to solution, because even equal mol of Mn(II)
Magnetic properties of 1 were studied at a broad tempera- shows higher activity for 1 in the organic sulfide-oxidation
ture range at the fields of 500 and 1000 Gs, respectively. Fig. 2 reaction (entries 3 and 5, 0.005 mmol from 1 have similar mol
shows the temperature dependence of the wT product at 1000 Gs. Mn(II) ions in 0.03 mmol Mn(OAc)2ꢀ4H2O).
The wT values are essentially not dependent on the field value.
Fu’s group reported that hexadentate 8-quinolinolato
A dominating antiferromagnetic behavior is confirmed by the wT manganese(III) complexes in the presence of NH4OAc, HOAc and
value at 1000 Gs at B295 K of 20.78 cm3 molꢁ1 K, which is lower aqueous solution of hydrogen peroxide were found to efficiently
than theoretically expected for the presence of six non-interacting catalyze the mild oxidation of thioanisole in environmentally
Mn2+ ions (26.25 cm3 molꢁ1 K, S = 5/2). These values are constant benign acetone–water media. The achieved conversion was from
until B50 K and then rapidly decrease to B7 cm3 molꢁ1 K at 2 K. 21 to 93% and the corresponding selectivities were 34–86%.20
The Mn2+ ions in 1 interact through carboxylate bridges, displaying Bagherzade’s group showed that efficient catalytic systems for the
syn–anti coordination modes, which can lead to only weak oxidation of sulfides with urea hydrogen peroxide (UHP) with
magnetic interactions.18 Additional magnetic exchange pathways short reaction times (5 min) and under mild conditions are
may involve hydrogen bonds. Westerhausen et al. also investigated manganese(III) oxazoline complexes. The conversion was from 65
a wheel-like [MnII ] complex with the bis(8-amidoquinoline) ligand to 90% and the selectivities were 45–85%.21 The group also
6
showing antiferromagnetic coupling of the Mn2+ ions, consistent investigated a polymeric manganese(II) complex with iminodi-
with results of DFT calculations.19 On the other hand, an analogous acetate. This complex exhibited an excellent catalytic activity and
[FeIII6] complex was reported to display close-lying ferromagnetically selectivity for oxidation of various sulfides to the corresponding
and antiferromagnetically coupled states.
sulfoxides using UHP as an oxidant under air at room tempera-
Initially, oxidation processes of methylphenylsulfide (MPS) ture.22 Najafpour et al. reported that a di-manganese complex
were examined without a catalyst and with Mn(OAc)2ꢀ4H2O or 1 with ‘‘back-to-back’’ 1,4-bis(2,20:6,200-terpyridin-40-yl) benzene
to study the effects of different catalysts on the oxidation of ligand catalyzes oxidation of various sulfides to the corres-
MPS. As reported in Table 2, the maximum activity has been ponding sulfoxides with conversion 30–72% and selectivity of
observed with 1 and oxidation of MPS without a catalyst gives very 100% in the presence of H2O2.23 The group also reported on sulfide
low conversion. Further screening with 0.03 mmol of Mn(OAc)2ꢀ to sulfoxide oxidation by mononuclear manganese(II) complexes
4H2O was also carried out, but none of them provided the with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz). The achieved conversion
product in a yield as high as that obtained in the presence of 1. and selectivities were 26–57% and 94–100%, respectively.
The compound 1 was found to be a catalyst for oxidation of
For 1 the obtained conversion and selectivity were 33–51% and
various sulfides. Without the catalyst, low sulfide oxidation was 96–100%. A conversion of 39–63%, and selectivity of 98–100%
observed (Table 2, entry 1). However, Mn(II) acetate shows places 1 among most selective catalysts toward sulfide oxidation.
catalytic activity for sulfide oxidation (Table 2, entries 2 and 3).
bpy improves conversion and selectivity of Mn(II) acetate
(Table 2, entry 4). A series of various types of structurally diverse
sulfides were subjected to the oxidation reaction using 1
Conclusions
complex as a catalyst and H2O2 as an oxidant. Arylalkyl A new hexanuclear [MnII6] wheel-like assembly with naphthalene-
(Table 2, entries 5 and 6), arylbenzyl (Table 2, entry 7), dibenzyl 1,8-dicarboxylate and 1,10-phenanthroline has been synthesized
(Table 2, entry 8), diaryl (Table 2, entry 9) and dialkyl (Table 2, and characterized. The solid state studies further evidenced that
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coordination sphere of each Mn2+ ion includes two phenanthroline
N atoms, two carboxylate O atoms from two different naphthalene-
1,8-dicarboxylate ligands and two water ligands. We demonstrated
the effectiveness of this complex as a catalyst for the oxidation of
sulfides to the corresponding sulfoxides. Conversion of 39–63%,
and selectivity of 98–100% is observed for the compound as a
catalyst. Although we found that Mn(OAc)2ꢀ4H2O shows activity
toward organic sulfide oxidation but both conversion and selectivity
are lower than for 1 under these conditions. An easy preparation
method, mild reaction conditions, high yields of the products,
short reaction times and high selectivities make this catalytic
system very useful for oxidation reactions.
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Acknowledgements
MMN and EA are grateful to the Institute for Advanced Studies
in Basic Sciences and the National Elite Foundation for financial
support. MH gratefully acknowledges the help of Prof. Dr S. Dehnen
(generous support and helpful discussions) and C. Pietzonka (SQUID
measurements).
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