Manganese- and Lanthanide-Based 1D Chiral Coordination Polymers as an Enantioselective Catalyst for Sulfoxidation

Article abstract of DOI:10.1021/acs.inorgchem.5b02234

The chiral 1D-coordination polymers (CP) {[Ln₂(MnLCl)₂(NO₃)₂(dmf)₆(H₂O)₂]·xH₂O}_n [Ln = Pr (1), Nd (2), Sm (3), and Gd (4)] were synthesized by the reaction of N,N'-bis(4-carboxysalicylidene)cyclohexanediamine (H₄L) with [MnCl₂·4(H₂O)] and [Ln(NO₃)₃·x(H₂O)] in the presence of dmf/pyridine at 90 °C. The polymers consist of manganese-salen-based moieties having carboxylate linkers connected to rare earth atoms in a 1D-chain structure. The polymers are very easily accessible. A one-step synthesis for the ligand and a second step for the preparation of the 1D coordination polymers starting from commercially available material are needed. The solid state structures of 1-4 were established by single-crystal X-ray diffraction. Compounds 1-4 were investigated as heterogeneous catalysts for the sulfoxidation reaction of various alkyl and aryl sulfides. The influence of various solvents and oxidizing agents on the catalytic reaction was examined. It was found that the catalysts were active for more than one reaction cycle without significant loss of activity. For phenylsulfide with 1 mol % of the catalyst 4, a maximum conversion 100% and a chemoselectivity 88% were observed.

Full text of DOI:10.1021/acs.inorgchem.5b02234

Article
pubs.acs.org/IC
Manganese- and Lanthanide-Based 1D Chiral Coordination Polymers
as an Enantioselective Catalyst for Sulfoxidation
Munendra Yadav, Asamanjoy Bhunia, Salil K. Jana, and Peter W. Roesky*
Institut fur Anorganische Chemie, Karlsruher Institut fur Technologie (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The chiral 1D-coordination polymers (CP)
{[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n [Ln = Pr (1),
Nd (2), Sm (3), and Gd (4)] were synthesized by the reaction
of N,N′-bis(4-carboxysalicylidene)cyclohexanediamine (H4L)
with [MnCl2·4(H2O)] and [Ln(NO3)3·x(H2O)] in the
presence of dmf/pyridine at 90 °C. The polymers consist of
manganese-salen-based moieties having carboxylate linkers
connected to rare earth atoms in a 1D-chain structure. The
polymers are very easily accessible. A one-step synthesis for the
ligand and a second step for the preparation of the 1D
coordination polymers starting from commercially available material are needed. The solid state structures of 1−4 were
established by single-crystal X-ray diffraction. Compounds 1−4 were investigated as heterogeneous catalysts for the sulfoxidation
reaction of various alkyl and aryl sulfides. The influence of various solvents and oxidizing agents on the catalytic reaction was
examined. It was found that the catalysts were active for more than one reaction cycle without significant loss of activity. For
phenylsulfide with 1 mol % of the catalyst 4, a maximum conversion 100% and a chemoselectivity 88% were observed.
reactions.2,4,11,24−26,35,38 The stability of the MOF used plays
INTRODUCTION
■
a key role maintaining its structural integrity during catalysis. In
contrast, 1D CPs have been far less studied for catalytic
applications.28,39−41 Their relative ease of formation and the
good accessibility of the catalytic active metal centers make
them promising candidates for heterogeneous catalysis. By
altering the metal within the 1D CPs scaffold, the scope of
catalytic application may easily be varied.
Recently, large research activity has been observed in the area
of metal organic frameworks (MOF’s). MOFs have been widely
used for applications, such as gas adsorption, catalysis, drug
delivery, separation, and imaging.1−21 The structural diversity of
MOFs needed for these applications can be realized by
choosing the desired functionality within the linker or the metal
knots.2,4,8,22−26 Besides the well-structured three-dimensional
MOFs, also one- and two-dimensional coordination polymers
(CP) have been studied intensively.27−29 The structures,
properties, and applications of 1D coordination polymers
have been reviewed by J. J. Vidal recently.28 In contrast to
three-dimensional MOFs, one-dimensional coordination poly-
mers (1D CP) have a much simpler topology. The relative ease
of formation by self-assembly and the simplicity of the 1D CPs
facilitates the incorporation of functional groups at the metal
centers or in the backbone of the organic linkers.28,30 The
variation of the structures of 1D CP ranges from linear chains,
over zig chains and helicates to complicated structures. Their
ease of assembly makes them interesting for material science.
Molecular-based ferromagnets, synthetic metallic conductors,
nonlinear optical materials, and ferroelectrics represent several
applications of low-dimensional coordination polymers.31 1D
CPs were also deposited onto surfaces as single molecule
devices32 and as molecular wires for potential nanoelec-
tronics.29,32
Chiral sulfoxides are valuable subunits for the pharmaceutical
industry37 and biological systems.42’43,44 Thus, considerable
progress has been made developing catalysts for asymmetric
sulfoxidation over the past few decades.45,46 Sulfoxides were
synthesized starting from achiral molecules in the presence of
enantiomeric pure catalyst.37,42,47−51 Mainly homogeneous
catalysts derived from transitional metals, such as titanium,52,53
vanadium,44 chromium,47 manganese,54−57 and iron,47,58
ligated by chiral moieties and a few metal-free systems59 were
used in the past. Exceptional efficiency in the enantioselective
oxidation of sulfides and olefins has been observed by using
these homogeneous systems. They show high activities and
selectivities for most of the organic reactions, but their practical
application remains limited because of problems concerning
catalytic stability, separation, and recyclability.25,37,47,60−63
Therefore, the development of new asymmetric heteroge-
neous catalysts remains a continuing challenge.48,50,61,64 In this
regard, the synthesis of chiral metal organic frameworks
(CMOFs)65,66 and chiral coordination polymers (CCPs)67
The well-structured three-dimensional MOFs have widely
been used in heterogeneous catalysis.2,6,9,11,13,15,33−38 One
advantage of using MOFs for catalysis is the uniform and
homologous pore size, which may allow shape selective
Received: September 29, 2015
© XXXX American Chemical Society
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Table 1. Crystallographic Details of 1−4
1
2
3
4
chemical formula
formula mass
crystal system
a (Å)
C62H84Cl2Pr2Mn2N12O27
1891.99
monoclinic
32.726(7)
10.721(2)
25.497(5)
114.88(3)
8116(3)
150(2)
C62H84Cl2Nd2Mn2N12O27
1898.65
monoclinic
32.763(7)
10.803(2)
25.554(5)
114.82(3)
8210(3)
150(2)
C62H84Cl2Sm2Mn2N12O27
1910.89
monoclinic
32.571(7)
10.734(2)
25.464(5)
114.71(3)
8087(3)
150
C62H84Cl2Gd2Mn2N12O27
1924.67
monoclinic
32.393(7)
10.749(2)
25.390(5)
114.29(3)
8058(3)
150
b (Å)
c (Å)
β (deg)
unit cell vol (Å3)
temp (K)
space group
no. of formula units per unit cell, Z
radiation type
C2
C2
C2
C2
4
4
4
4
Mo Kα
1.629
Mo Kα
Mo Kα
1.882
Mo Kα
2.077
absorption coefficient, (μ/mm−1
no. of reflns measured
no. of independent reflns
Rint
)
1.688
58171
45633
31488
138229
21739
17201
16582
17138
0.0400
0.1013
0.0581
0.0578
final R1 values (I > 2σ(I))
final wR(F2) values (all data)
GOF on F2
0.0755
0.0820
0.0980
0.0642
0.1906
0.1761
0.2409
0.1764
1.255
0.977
1.199
1.048
Flack parameter
0.077(6)
0.186(11)
0.092(15)
0.043(4)
has been an attractive research area.68 One method for the
generation of CMOFs is the use of chiral metalloligands as
linkers between the knots. Among others, enantiomeric pure
salen complexes have been used as metalloligands.25,37,61,69−72
These chiral metalloligands within the CMOFs have been
employed as chiral catalytically active centers to generate
heterogeneous catalysts for various enantioselective reac-
tions.25,35,37,61,69−74 The enantioselective sulfoxidation has
only been studied with some CMOFs.25,34,37,74,75 Mostly,
zinc, copper, or titanium were used as catalytic active centers.
To the best of our knowledge, 1D CCPs have not been
employed as heterogeneous catalysts for sulfoxidation so far.
Previously, we reported a series of 1D achiral coordination
polymers (CPs) based on manganese and lanthanide metal
atoms.40 The polymers consist of manganese-salen-based
metalloligands having carboxylate linkers connected to
lanthanide atoms to form 1D CPs. The CPs were used as
heterogeneous oxidation catalysts for the epoxidation reaction.
Air was used as oxidant. Herein we report now chiral 1D
coordination polymers derived from chiral salen manganese
metalloligands and lanthanide knots. These polymers are very
easily accessible. We present a one-step synthesis for the ligand
starting from commercially available material. Only a second
step is needed to prepare the 1D coordination polymers. These
polymers were used as catalysts for the enantioselective
sulfoxidation reaction of aryl and alkyl sulfides.76
X-ray powder diffraction patterns (XRD) for different samples of
compound 4 were measured on a STOE STADI P diffractometer
(CuKa1 radiation, Germanium monochromator, Debye−Scherrer
geometry) in sealed glass capillaries. The theoretical powder diffraction
pattern was calculated on the basis of the atom coordinates obtained
from single crystal X-ray analysis by using the program package
Mercury by CCDC.
Synthesis of N,N′-Bis(4-Carboxysalicylidene)cyclohexanedi-
amine (H4L). H4L was synthesized by condensation of 4-formyl-3-
hydroxybenzoic acid (99.6 mg 0.6 mmol) and (1R, 2R)-(−)-1,2-
diaminocyclohexane (34.2 mg 0.3 mmol) in 15 mL of ethanol. The
reaction mixture was stirred for 4 h at 80 °C and then cooled to room
temperature. The resulting yellow solid was filtered and washed with
cold ethanol and dried in vacuum. Yield: 75.4 mg (61.3%). EI-MS: m/
z = 410.11. Anal. Calcd for C22H22N2O6: C, 64.38; H, 5.40; N, 6.83.
Found: C, 64.39; H, 5.42; N, 6.18. 1H NMR (300 MHz, DMSO-d6): δ
8.55 (s, 2H, −CHN−), 7.43 (s, 2H aromatic-H), 7.37 (m, 2H,
aromatic-H), 7.31 (m, 2H, aromatic-H), 3.40 (m, 2H, cylohexane),
1.20−1.90 (m, 8H). 13C{1H} NMR (75 MHz, DMSO-d6): δ 167.8,
165.0, 160.3, 136.5, 131.8, 121.5, 119.7, 117.4, 71.8, 32.8, 24.0. IR
(ATR): 2944 (m), 2859 (w), 1689 (m), 1629 (s), 1605 (m), 1561 (s),
1543 (s), 1449 (m), 1384 (m), 1366 (w), 1346 (w), 1286 (s), 1203
(m), 1148 (w), 1133 (w), 1096 (w), 1049 (w), 985 (w), 958 (m), 895
(m), 864 (w), 835 (w), 783 (w), 770 (w), 759 (s), 693 (w), 606 (w),
557 (m), 447 (w) cm−1.
General Procedure for the Synthesis of {[Ln2(MnLCl)2(NO3)2-
(dmf)6(H2O)2]·xH2O}n (1−4). H4L (22.5 mg, 0.055 mmol) and
MnCl2·4(H2O) (10.8 mg, 0.055 mmol) were stirred in dmf (2 mL) for
5 min. Then, Ln(NO3)3·(H2O)m (0.1 mmol) and pyridine (0.15 mL)
were added to the reaction mixture. The resulting black solution was
stirred for 2 h at room temperature and then filtered in a 10 mL glass
vial. To get the single crystals, the reaction solution was heated in an
oven at 90 °C and then was cooled to room temperature. Red block
shaped crystals suitable for single-crystal X-ray diffraction were
collected and washed three times with dmf, followed by diethyl
ether and dried in air for further analysis.
EXPERIMENTAL SECTION76
■
General Considerations. IR spectra were obtained on a Bruker
FTIR Tensor 37 via the attenuated total reflection method (ATR).
Elemental analyses were carried out with an Elementar vario EL or
Vario Micro Cube. NMR spectra were recorded on Bruker Avance II
300 MHz NMR spectrometer. Chemical shifts are referenced to
internal solvent resonances and were reported to tetramethylsilane.
The enantiomeric ratios of sulfoxides were recorded using high
performance liquid chromatography (HPLC) Agilent Technologies
1200 series with a CHIRALPAK-IA column. TGA measurements were
made on a Netzsch STA 429 instrument. Aldehydes, amines, sulfides,
and solvents were purchased from commercial sources and used
without further purification.
{[Pr2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (1). Yield: 20 mg (21% as
single crystalline material). Anal. Calcd for C56H70Cl2Mn2N10Pr2O25
(corresponds to loss of two dmf molecules and x = 1): C, 38.53; H,
4.04; N, 8.02. Found: C, 37.96; H, 4.89; N, 8.52. IR (ATR): 2939 (w),
2861 (w), 1613 (m), 1559 (m), 1515 (m), 1479 (m), 1397 (s), 1338
(s), 1300 (s), 1269 (s), 1198 (m), 1140 (w), 1109 (w), 1022 (w), 977
(m), 895 (w), 862 (w), 819 (w), 777 (s), 736 (w), 637 (w), 606 (s),
520 (w), 493 (w), 467 (w), 441 (w) cm−1.
B
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Scheme 1. Synthesis of 1−4
{[Nd2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (2). Yield: 19 mg (20%
a s s i n g l e c r y s t a l l i n e m a t e r i a l ) . A na l . C a l c d f o r
C59H77Cl2Mn2N11Nd2O26 (corresponds to loss of one dmf molecule
and x = 1): C, 38.82; H, 4.25; N, 8.44. Found: C, 38.12; H, 4.85; N,
8.59. IR (ATR): 2940 (w), 2861 (w), 1647 (m), 1615 (s), 1565 (s),
1521(w), 1476 (w), 1389 (s), 1337 (w), 1299 (m), 1269 (m), 1198
(m), 1139 (w), 1106 (w), 1023 (w), 974 (m), 897 (w), 860 (w), 817
(m), 775 (s), 736 (w), 662 (w), 634 (s), 601 (w), 520 (w), 494 (w),
467 (w), 442 (w), 418 (w) cm−1.
{[Sm2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (3). Yield: 20 mg (21%
based on single crystals). Anal. Calcd for C58H77Cl2Sm2Mn2N11O26
(corresponds to loss of one dmf molecule and x = 1): C, 38.56; H,
4.22; N, 8.38. Found: C, 38.31; H, 4.43; N, 8.91. IR (ATR): 2939 (w),
2861 (w), 1651 (m), 1617 (s), 1564 (s), 1519 (w), 1477 (w), 1391
(s), 1337 (w), 1299 (m), 1269 (m), 1197 (m), 1139 (w), 1106 (w),
1022 (w), 975 (m), 897 (w), 860 (w), 813 (m), 777 (s), 737 (w), 635
(s), 602 (w), 520 (w), 494 (w), 467 (w), 441 (w) cm−1.
97.77 Further non-hydrogen atoms were located from successive
Fourier difference map calculations. The refinements were carried out
using full-matrix least-squares techniques on F2, minimizing the
2
2
function (F0 − Fc)2, where the weight is defined as 4F0 /2(F0 ) and F0
and Fc are the observed and calculated structure factor amplitudes
using the program SHELXL-97.77 The hydrogen atoms were placed in
their calculated positions without refinement. The final values of
refinement parameters are given in Table 1. The locations of the
largest peaks in the final difference Fourier map calculation as well as
the magnitude of the residual electron densities in all case were of no
chemical significance.
RESULTS AND DISCUSSION
■
Synthesis and Structures. The new ligand N,N′-bis(4-
carboxysalicylidene)cyclohexanediamine (H4L) was obtained in
one step from 4-formyl-3-hydroxybenzoic acid and (1R, 2R)-
(−)-1,2-diaminocyclohexane in ethanol. The reaction of H4L
with [MnCl2·4(H2O)] and [Ln(NO3)3·x(H2O)] in the
presence of dmf/pyridine at elevated temperature led to the
formation of chiral 1D-cordination polymers (CPs) {[Ln2-
(MnLCl)2(NO3)2(dmf)6(H2O)2]·H2O}n [Ln = Pr (1), Nd (2),
Sm (3), and Gd (4) in a second step. The resulting products
were obtained as red single crystals. Only single crystals were
used for the subsequent analysis and catalytic reactions. The
solid state structures of 1−4 were established by single-crystal
X-ray diffraction and characterized by standard analytical/
spectroscopic techniques. Compounds 1−4 were insoluble in
common solvents and due to their paramagnetic behaviors no
NMR data could be acquired. The IR spectra for the
carboxylate group show stretching bands at 1613 cm−1 for 1;
1615 cm−1 for 2; 1617 cm−1 for 3 and 1616 cm−1 for 4. The
absence of absorption at 1689 cm−1 also proves the complete
deprotonation of carboxyl groups of the salen ligand.
{[Gd2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n (4). Yield: 18 mg (18%
based on single crystals). Anal. Calcd for C53H63Cl2Gd2Mn2N9O24
(corresponds to loss of three dmf molecules and x = 1): C, 37.33; H,
3.72; N, 7.39. Found: C, 36.75; H, 4.52; N, 7.81. IR (ATR): 2939 (w),
2861 (w), 1647 (m), 1616 (s), 1566 (s), 1521(w), 1477 (w), 1394 (s),
1337 (w), 1300 (m), 1270 (m), 1198 (m), 1139 (w), 1107 (w), 1023
(w), 975(m), 898 (w), 861 (w), 819 (m), 776 (s), 738 (w), 667 (w),
634 (s), 602 (w), 519 (w), 494 (w), 467 (w), 443 (w), 419 (w) cm−1.
General Procedure for the Catalytic Reactions. The catalyst
{[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·H2O}n [Ln = Pr (1), Nd (2),
Sm (3), and Gd (4); x = 1] (1−4) (20 mg) and the affiliated sulfide (1
mmol) were combined in acetonitrile (10 mL) followed by addition of
iodosobenzene (1 mmol). The combined reaction mixture was stirred
for 16 h at room temperature. The reaction progress was monitored by
TLC (eluent = EtOAc/hexane). The catalyst was separated out from
the reaction mixture by ordinary filtration. The catalyst was recycled
and performed in the similar way. The products were analyzed by
NMR and HPLC. The ratio sulfide/sulfoxides/sulfone was determined
1
by integration of the H NMR spectra.
Compounds 1−4 crystallized in the monoclinic chiral space
group C2. All compounds are isostructural (see also Scheme 1
and Table 1). The molecular structure of compound 4 is shown
in Figure 1. The basic building blocks of all structures consist of
X-ray Crystallographic Studies of 1−4. A suitable crystal was
covered in mineral oil (Aldrich) and mounted on a glass fiber. The
crystal was transferred directly to a cold stream of a STOE IPDS II
diffractometer. All structures were solved using the program SHELXS-
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network, the number of solvent molecules could not be
determined without doubt. In the lattice we determined one
water molecule, which corresponds to x = 1 in the sum formula.
Therefore, an uncertainty remains concerning the sum formula
of compounds 1−4. BET measurements show that solvent is in
the void space because the observed N2 uptake curve at 77 K
shows a surface absorption only (Figure S1).
The manganese ions in the salen units [MnLCl(NO3)] are in
the center of a distorted octahedral coordination polyhedron,
which is built by the ONNO atoms of the salen ligand, chloride
and a nitrate ion. The chloride and nitrate occupy the apical
positions. The ONNO atoms of the salen ligand form an
equatorial plane around the Mn(III) ion. 6-fold-coordinated
Mn(III) ions within salen ligands have been reported earlier
{[Ln2(MnLCl)2(NO3)2(dmf)5] (H4L = N,N′-bis(4-carboxy-
salicylidene)ethylenediamine).40 The Mn−salen units [MnLCl-
(NO3)] are similar to each other in the structural parameters
and the environment near to Mn1 and Mn2 ions. Thus, only
the representative structural parameters of complex 4 are
discussed in detail. The Mn1−Cl1 and Mn2−Cl2 bond
distances are similar (Mn1−Cl1 2.600(6) Å and Mn2−Cl2
2.600(4)). The Mn1−Ophenolate bond distances are Mn1−O3
1.880(8) Å and Mn1−O4 1.884(8) Å, which are comparable to
the Mn2−Ophenolate distances Mn2−O9 1.892(8) Å and Mn2−
O10 1.904(8) Å. The Mn1−N1 1.998(8) Å and Mn1−N2
1.997(8) Å bond distances are also close to Mn2−N3 1.997(9)
Figure 1. Solid-state structure of the compound 4. The coordination
arrangement of a gadolinium dimer is shown, and hydrogen atoms are
omitted for clarity.
a {[Ln2(MnLCl)2(NO3)2(dmf)6(H2O)2]·xH2O}n chain. The
asymmetric unit is formed by two Mn-salen units [MnLCl-
(NO3)], two rare earth ions (Ln1 and Ln2), six dmf, and two
water molecules. However, because of the potentially porous
Figure 2. Solid-state structure of compound 4. SBU unit is highlighted (top) and cutout of the polymeric structure (bottom). All hydrogen atoms
and most of the solvent molecules have been omitted for clarity.
D
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Å and Mn2N4 1.980(8) Å. The MnOnitrate bond distances are
Mn1−O16 2.346(10) and Mn2O13 2.36(2). The Mn-salen
unit acts as a linker to form the 1D coordination polymers.
Each carboxylate group of the Mn−salen unit connects two
lanthanide atoms. Thus, the two lanthanide atoms (Ln1 and
Ln2) are coordinated together by four Mn−salen units, each
through four bridging carboxylate groups and six dmf and two
water molecules, as it is expected because of the large size of the
lanthanide atoms. Two of these coordinated dmf molecules and
two water molecules are disordered. The lanthanide atoms Ln1
and Ln2 form the secondary building units (SBUs). Each SBU
is based on a [(Ln)2(μ-O2CR)4(dmf)6(H2O)2] building block.
These SBUs can be considered as a distorted square paddle-
wheel built from two rare earth ions which are bridged by four
carboxylate groups. The basic structural arrangement of
compounds 1−4 is comparable to our previously reported
1D-cordination polymers {[Ln2(MnLCl)2(NO3)2(dmf)5] (H4L
= N,N′-bis(4-carboxysalicylidene)ethylenediamine).40
release of additional three dmf molecules in the temperature
range of 100−295 °C (obsd 11.61%, calcd 11.47%). By
increasing the temperature, the frameworks started to
decompose. For compound 4, a rapid weight loss is found in
the temperature range of 30−70 °C, corresponding to the loss
of one dmf and three water molecules (obsd 5.88%, calcd
6.8%). In the temperature range of 100−136 °C, another dmf is
lost (obsd 3.89%, calcd 3.80%). After that, the four remaining
dmf molecules were released (obsd 16.43%, calcd 15.19%) in
the temperature range of 170−305 °C. Finally, compound 4 is
destroyed by further heating. From the above TGA plot of
compounds 1−4, we concluded that compounds 1−4 released
the solvent molecules in the temperature range of 30−315 °C
and then started to decompose.
Catalytic Studies of Compounds 1−4. Mn(III)-based
salen complexes are known to be highly active catalysts in
epoxidation and sulfoxidation reactions.40,33,37 Previously, we
reported the epoxidation of trans-stilbene with air by using 1D
coordination polymers as catalysts. We observed that Mn-Ln-
based polymers were active for this epoxidation reaction.40
Motivated by these results, we examined the chiral-1D
coordination polymers 1−4 as catalysts for sulfoxidation
reactions. The synthesis of enanatiomerically pure sulfoxides
is challenging as they are useful drugs in some cases.37
Compounds 1−4 were used for the oxidation of aryl and alkyl
sulfides with iodosobenzene, which act as an oxidizing reagent.
We explored the utilization of compounds 1−4 as heteroge-
neous catalysts and the reaction resulted in the formation of
aryl and alkyl sulfoxides as main products and sulfone as side
products.
The thermal stability of compounds 1−4 was proven by
TGA measurements (Figure 3). Although compounds 1−4 are
First, we examined compounds 1−4 for the oxidation of
methyl p-tolylsulfide. Conversions of 71−77% indicate that the
choice of the lanthanide does not have a significant impact on
the sulfoxidation reaction. Therefore, we focused the catalytic
screening test on only one catalyst and used compound 4 for
detailed studies. The reaction parameters were varied to
optimize the conversion and selectivity for the methyl p-
tolylsulfide. The parameters shown in Scheme 2 are the
Figure 3. TGA curve of 1−4 in the temperature range of 20−1000 °C
under N2 atmosphere.
Scheme 2. Sulfoxidation of Aryl and Alkyl Sulfides with
PhIO Catalyzed by Compounds 1−4
isostructural, they show slightly different thermal behavior. The
TGA curve of compound 1 shows a gradual weight-loss step of
3.14% in the temperature range of 30−60 °C, corresponding to
the escape of three water molecules from the frameworks
(calcd. 2.85%) followed by a plateau in the temperature range
of 60−100 °C. A gradual weight loss of 14.65% (calcd 15.45%)
is observed in the temperature range of 100−285 °C, which is
assigned to the loss of four dmf molecules. Another two dmf
molecules are lost rapidly in the temperature range of 290−310
(obsd 8.22%, calcd 7.72%), after that the compound started to
decompose. In compound 2, there is a gradual weight loss of
6.54% between 30 and 70 °C, corresponding to loss of three
water and one dmf molecules (calcd 6.69%). A similar type of
decomposition behavior as observed for compound 1 is seen:
gradual weight loss followed by rapid weight loss due to release
of additional five dmf molecules in the temperature range of
100−315 °C (obsd 18.37%, calcd 19.25%) before the
frameworks starts to collapse. The TGA curve of compound
3 shows a rapid weight loss in the temperature range of 100−
170 °C (obsd 13.16%, calcd 14.30%), corresponding to the loss
of three water and three dmf molecules. Afterward, a gradual
weight loss followed by rapid weight loss is found due to the
optimized conditions. We examined air, tBuOOH, hydrogen
peroxide and iodosobenzene as oxidizing agents (Table S2).
We found that hydrogen peroxide is destroying the polymeric
structure. Compared to the other oxidizing reagents the
conversion was significantly higher by using idosobenzene
(Table S2). By using iodosobenzene as oxidizing agent there is
no difference if the reaction is performed under air or under N2
atmosphere (Table S2, entries 1−6). We thus, conclude that
molecular oxygen from air is not significantly involved in the
reaction. The reaction is almost not running in the absence of 4
(Table 2, entry 14, and Table S1). The catalytic reactions were
also examined in the presence of different solvents, such as
acetone, hexane, dichloromethane and acetonitrile but con-
version was highest when acetonitrile was used as a solvent.
Furthermore, we examined different stoichiometric ratios for
the oxidizing agent. 1eq of idosobenzene was found most useful
for maximum conversion of the substrates. A higher amount of
E
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Inorganic Chemistry
Article
a
(100% conversion; Table 2 entry 5) the yield is significantly
lower. This trend cannot be generalized. The para-brom aryl
ethyl and methyl sulfides (Table 2, entries 7 and 11) showed
almost the same reactivity. When a n-butyl group was
introduced in para position (n-BuPhSEt), the conversion
decreased to 36% (Table 2, entries 12). This is probably due
to the slower mass-diffusion of the bulky substrate in the
potentially porous media. Whereas the yields of the reaction are
very good, the enantioselectvities were found to be quite low.
A comparative experiment was carried out to prove the
heterogeneity of the catalytic reaction and leaching of the
catalyst. By using methyl p-tolylsulfide as substrate, we
performed two catalytic reactions under similar reaction
conditions. For the first reaction, the catalyst was removed
from the reaction mixtures after 4 h, for the second reaction, it
was removed after 8 h. The conversion rates of the 4 and 8 h
reactions were compared with the standard 16 h reaction
(Table 2, entries 4). When the standard reaction was carried
out with reaction time of 4 h, a conversion of 21% with 100%
chemoselectivity was observed. Thereafter, the solid catalyst
was separated out from the reaction and fresh iodosobenzene
was added to the reaction mixture, whereas the other reaction
conditions were maintained as shown in Scheme 2. After an
additional 14 h, the conversion had barely increased (22%
conversion; 100% chemoselectivity). Second, a standard
reaction was carried out for a reaction time of 8 h and a
conversion of 44% with 93% chemoselectivity was obtained.
Then, the solid catalyst was separated out and fresh
idosobenzene was added to the reaction mixture. After an
additional 14 h, the conversion of methyl p-tolylsulfide had
slightly increased (50% conversion; 90% chemoselectivity).
These results undoubtedly proved that the removal of the
catalyst after filtration through a regular filter afforded very little
additional oxidation product. Thus, no active species is leached
out from the reaction system as the conversion rates of the
catalysis after 4, 8, and 16 h under standard conditions were
close to each other. Some blinds tests were performed by
running the catalysis without a catalyst (Table 2, entry 14 and
Table S1), for example, for 4-bromophenyl methyl sulfoxide,
under the standard reaction conditions of Table 2, 4% yield of
sulfoxide was obtained after 16 h without adding a catalyst
(Table 2, entry 14).
The stability of the catalyst by recycling was investigated for
the oxidation of methyl phenylsulfide. The catalyst was recycled
after the first run just by ordinary filtration and reused under
exactly the same reaction conditions. After the first, second, and
third run, the conversion for the methyl phenylsulfide oxidation
was found to be 100%, 98%, and 90%. Thus, the catalyst was
not significantly deactivated under optimized reaction con-
ditions after 3 runs. The reused catalyst was characterized by
PXRD (Figure 4). The experimental powder pattern of
compound 4 shows a good agreement with the simulated
pattern from single crystal, proving the crystalline purity of the
compound. A comparison of the PXRD pattern of compound 4
before and after catalysis shows that the structure remains
unchanged except from some few minor structural rearrange-
ments of the 1D CP, which may be due to the loss of some dmf
and water molecules from its lattice. The reused catalyst was
found to be similar to the dry catalyst except from small
deviations. This data proves the stability of compound 4 during
catalysis.
Table 2. Catalytic Oxidation of Sulfides
a
Reaction conditions: A reaction solution of aryl or alkyl sulfides (1
mmol), PhIO (1 mmol), acetonitrile (10 mL), and catalyst (1 mol %)
was stirred for 16 h. The ee of the resulted sulfoxide was determined
by HPLC with Chiralpak-A column. The conversion and selectivity
*
1
were determined by H NMR. n.d.: sulfoxides not measured for ee.
an oxidizing agent led to sulfone formation. As optimized
conditions we established acetonitrile as solvent, iodosoben-
zene (1 equiv) as an oxidizing agent and 1 mol % of the catalyst
at room temperature for the oxidation of methyl p-tolylsulfide.
All other reactions were performed under these optimized
reaction conditions.
The sulfoxidation of methyl p-tolylsulfide and methyl
phenylsulfide gave for compound 4 a conversion of 76% and
100%, respectively (Table 2, entries 4 and 5), while the 4-
chlorophenyl methyl sulfide and 4-bromophenyl methyl sulfide
led to conversion of 78% and 81%, respectively (Table 2,
entries 6 and 7). When the phenyl ring was functionalized with
the strong electron-withdrawing −NO2 and −CN groups, the
same reaction conditions led to a conversion of 64% and 71%,
respectively (Table 2, entries 8 and 9). Introduction of an ethyl
group in aryl sulfide (PhSEt) instead of a methyl resulted in a
conversion of 79% (Table 2, entry 10). Compared to PhSMe
F
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: roesky@kit.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the DFG-funded transregional
collaborative research center SFB/TRR 88 “3MET”. Ms Sibylle
Schneider is acknowledged for measuring TGA. We would like
to thank Dr. Olaf Fuhr for helpful discussion regarding the X-
ray diffraction solution, and Dr. Andreas Eichhofer for the
̈
PXRD measurement.
Figure 4. Powder X-ray diffractograms of compound 4.
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