Synthesis, characterization, magnetic and catalytic properties of a ladder-shaped MnII coordination polymer

Article abstract of DOI:10.1002/ejic.201402419

[Mn(LH)(H₂O)]_n {1, where LH^2- is the dianion of N-(4-carboxybenzyl)iminodiacetic acid} has been synthesized and its crystal structure has been determined. The crystal of 1 is built from 1D polymeric ladder-shaped chains that extend to a 3D supramolecular architecture through H-bonds. The compound was characterized with spectroscopic and physicochemical techniques. Variable-temperature magnetic data suggest that there are weak antiferromagnetic interactions. Compound 1 has been evaluated as a heterogeneous oxidation catalyst. It catalyzes alkene epoxidation selectively in relatively high yields.

Full text of DOI:10.1002/ejic.201402419

SHORT COMMUNICATION
DOI:10.1002/ejic.201402419
Synthesis, Characterization, Magnetic and Catalytic
Properties of a Ladder-Shaped MnII Coordination
Polymer
Smaragda Lymperopoulou,[a] Maria Papastergiou,[a]
Maria Louloudi,*[a] Catherine P. Raptopoulou,[b]
Vassilis Psycharis,[b] Constantinos J. Milios,[c] and
John C. Plakatouras*[a]
Keywords: Manganese / Metal–organic frameworks / Ladder polymers / Supramolecular chemistry / Magnetic properties /
Heterogeneous catalysis
[Mn(LH)(H2O)]n {1, where LH2– is the dianion of N-(4-carb-
oxybenzyl)iminodiacetic acid} has been synthesized and its
crystal structure has been determined. The crystal of 1 is built
from 1D polymeric ladder-shaped chains that extend to a 3D
chemical techniques. Variable-temperature magnetic data
suggest that there are weak antiferromagnetic interactions.
Compound 1 has been evaluated as a heterogeneous oxi-
dation catalyst. It catalyzes alkene epoxidation selectively in
supramolecular architecture through H-bonds. The com- relatively high yields.
pound was characterized with spectroscopic and physico-
features of both heterogeneous (e.g. easy catalyst separation
Introduction
and recovery and catalyst recyclability) and homogeneous
(e.g. complex catalytic architecture, asymmetric catalysis)
catalysts. Nevertheless, one of the drawbacks of the use of
MOFs as catalysts is their relatively limited thermal and
chemical stability.[4] Within this context, low hydrolytic sta-
bility results in rapid decomposition of the framework when
the gas or liquid phase contains a small amount of water;
this imposes several limitations on their use in catalytic oxi-
dation reactions, in which water is a major reaction prod-
uct.[5] Successful attempts have been made by utilizing por-
phyrin- or salen-based building blocks for the synthesis of
MOFs that show significant catalytic activity.[6]
For several years, we have directed our efforts towards
the construction of coordination polymers (CPs) by using
ligands with increased flexibility. We have targeted two li-
gand families, open-chain polycarboxylates, such as tri-
carballylic[7] and trans-aconitic acid,[8] and pseudopeptides
of open-chain[9] or aromatic[10] dicarboxylates. Some of the
compounds prepared have very interesting properties, justi-
fying our choice. For example, 3D polymers reversibly bind
to coordinated water and act as heterogeneous catalysts for
the oxidation of catechol.[10] In a different work, a planar
complex can act as a ligand to form mixed-metal CPs;[9b]
this led us to use ditopic asymmetric ligands to pursue such
polymers. Our first choice is the ligand N-(4-carboxybenz-
yl)iminodiacetic acid (LH3), presented in this work. The
iminodiacetate moiety has been extensively studied as a
ligand in coordination chemistry, and it can easily be co-
ordinated in a bis-chelating fashion,[11,12] while the 4-carb-
The hunt for supramolecular coordination compounds,
which started a couple of decades ago, has led to numerous
molecular materials with extraordinary structural charac-
teristics, aesthetically pleasing structures, and interesting
properties.[1] Recently, bioinspired ligands have emerged as
building blocks for constructing nanoporous metal–organic
frameworks (MOFs).[2] Ligands belonging to this class
present increased flexibility, multiple possible coordination
modes, and it is also possible to form H-bonds through
their uncoordinated heteroatoms.
On the other hand, heterogeneous catalysis was one of
the earliest proposed and demonstrated applications for
crystalline MOFs.[3] Despite the exciting and persuasive
achievements over the years, the area of catalysis based on
MOFs is still in an immature phase, since only a few dozen
reports of chemical catalysis by crystalline, microporous
MOFs have appeared to date.[4] The notion is that MOF-
based catalysts may be able to combine some of the key
[a] Department of Chemistry, University of Ioannina,
451 10 Ioannina, Greece
E-mail: iplakatu@cc.uoi.gr
http://www.chem.uoi.gr
[b] Institute of Advanced Materials, Physicochemical Processes,
Nanotechnology and Microsystems, Department of Materials
Science, N.C.S.R. “Demokritos”,
153 10 Ag. Paraskevi, Attiki, Greece
[c] Department of Chemistry, University of Crete,
Voutes, 710 03 Herakleion, Greece
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402419.
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SHORT COMMUNICATION
oxybenzyl moiety can be utilized for coordination to other with non-coordinating substituents.[13a,16] One of the carb-
metals, since the carboxylate group at the fourth position oxylate groups utilizes the coordinated oxygen atom as a
of the ring does not allow the formation of an extra chelat- single-atom bridge [O(4)] to bond to a symmetry-related
ingring.Themethylenehingeprovidesenoughflexibilityforthe Mn atom by a twofold axis, whilst the other one uses both
system.
oxygen atoms [O(5), O(6)] for coordination bridging two
Several articles have been published to date with the metal ions in a syn/anti fashion to complete the formation
3-[13] or 4-[13c,13e,14] carboxyphenyl analogues of the ligand, of a ladder (Figure 1b). The coordination modes of the
describing mostly CPs. The utilization of LH3 in a reaction iminodiacetate moiety found in the literature are presented
system containing a [Mo3S4]4+ cluster has led to a complex in Scheme 1. Unexpectedly, our coordination motif
that maintains its original core with the bis-deprotonated (Scheme 1e) could not be found in a CSD search.[17]
ligand capping the metal corners.[15]
We have initiated our synthetic studies by using a single
metal in the reaction systems to scan the coordination prop-
erties of LH3, and herein we report a unique MnII CP with
rare properties applicable to catalysis.
Results and Discussion
The ligand N-(4-carboxybenzyl)iminodiacetic acid (LH3)
has been synthesized by a modification of the previously
described procedure[14b] and fully characterized with
potentiometric titration, IR and NMR (1H- and 13C-) spec-
troscopy, and preliminary single-crystal X-ray crystallogra-
phy. Homogeneous samples of 1 can be isolated from hy-
drothermal reactions of LH3 with excess amounts of MnII
salts [i.e. MnCl2·4H2O, MnBr2·4H2O, or Mn(NO3)2·4H2O].
It has a complicated IR spectrum with bands attributable
to both neutral and anionic coordinated carboxylates (1710,
1671, 1594, 1416, 1342 cm–1). The compound is pretty
stable upon heating under N2 flow as indicated by TG/DTA
measurements. It loses the coordinated water molecule be-
tween 249 and 300 °C (found, 6.1; calcd., 5.3%), and the
dehydrated intermediate is stable up to 345 °C, where a
complicated decomposition begins. The final residue is ob-
tained at about 700 °C; weight loss calculations suggest that
this is MnCO3 (calcd. 33.9; found 35.9%).
The pale cream colored crystals of compound 1 crys-
tallize in the monoclinic space group C2/c with Z = 8. The
Figure 1. (a) The asymmetric unit of 1, extended to show the coor-
dination sphere of the metal ion. CH hydrogen atoms have been
crystal structure of 1 reveals 1D, ladder-shaped chains pos- omitted for clarity. (b) The coordination mode of the ligand in 1,
showing the build up of the ladder extending along the b axis. The
sessing a twofold axis of symmetry running along the b axis.
benzoic part of the ligand, the coordinated water molecule, and the
The asymmetric unit comprises one MnII atom, one doubly
CH hydrogen atoms have been omitted for clarity. Ellipsoids are
drawn at the 50% probability level. Symmetry operations to gener-
ate equivalent atoms: #1, x, y – 1, z; #2, –x, y, –z + 1/2; #3, x, y
deprotonated ligand, and one ligated water molecule. The
MnII atom is located at the center of a rather distorted octa-
+ 1, z.
hedron, which consists of one nitrogen atom, belonging to
the ligand, and five oxygen atoms, belonging to three dif-
ferent carboxylate ligands and the coordinated water mole-
The two-legged ladder topology is relatively common in
cule (Figure 1a). The Mn–O bond lengths span the range CP chemistry, and it is mainly formed by nitrogen li-
2.114–2.303 Å, while the Mn–N distance is 2.311 Å; those gands.[18] It is presented for the first time in iminodiacetate
distances are in good agreement with literature data for re- coordination chemistry as a result of the unique coordina-
lated complexes.[11] The ligand utilizes only its iminodiacet- tion of the ligand.
ate part for coordination, leaving the benzoic part available
There is an extended array of H-bonds contributing to
for supramolecular interactions, and it is coordinated in a the stability of the compound and leading to a 3D supra-
different fashion relative to those found up to date in Mn molecular architecture (Figure 2, Table 1). Practically, each
chemistry. It uses the nitrogen atom and two of the carb- ladder is H-bonded to an adjacent one through the neutral
oxylate oxygen atoms to cap a face of the octahedron and benzoic moiety to produce a 2D thick layer; O(2) acts as a
form the fac isomer, as found in iminodiacetate complex- donor to a non-coordinated carboxylate oxygen atom
es,[11c,12] and all N-substituted iminodiacetate complexes [O(3)]. Adjacent layers interact through the H-bonds
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Direct current magnetic susceptibility experiments were
performed on a polycrystalline sample of 1 in the 5–300 K
temperature range under an applied field of 0.1 T, and the
χMT product vs. T plot is shown in Figure 3. The magnetic
unit of the polymer consists of a dimeric [MnII2] unit, in
which the two metal atoms are bridged by two monatomic
Ocarboxylate bridges at about 3.5 Å distance and a bridging
angle of approximately 106°. Furthermore, each dimeric
unit is further bridged by two syn,anti-carboxylates to adja-
cent neighboring dimers at a distance of about 6 Å, thus
forming a “ladder” of repeating [MnII2] units. We managed
to fit the data by adopting two models: (1) by assuming
isolated [MnII2] units, regarding the inter-dimer interaction
negligible, given the large inter-dimeric distance, and (2) by
assuming a dimer-of-dimers [MnII4] magnetic unit, in which
an inter-dimer interaction is operational. Both cases led to
excellent fitting of the data; in addition, in the second case
the inter-dimer exchange was found to be practically zero
(–0.002 cm–1), proving that it can indeed be safely regarded
as negligible. Therefore, we discuss only the former case.
The room-temperature χMT value of 8.72 cm3 mol–1 K is in
very good agreement with the expected value for two non-
Scheme 1. Coordination modes of iminodiacetate ligands found in
Mn chemistry: from ref.[12] (a), ref.[16] (b), ref.[11c] (c), ref.[13a] (d),
and this work (e).
formed by the coordinated water molecule to the anti coor-
dinated carboxylate oxygen atom [O(5)] and to the pre-
viously mentioned O(3).
interacting high-spin MnII ions with
g
=
2.0
(8.75 cm3 mol–1 K). This value remains almost constant un-
til about 55 K, below which it decreases to reach its mini-
mum value of 5.10 cm3 mol–1 K at 5 K, the curvature sug-
gesting the presence of a very weak antiferromagnetic ex-
change interaction within the two metallic centers. Fitting
of the data was performed by using the Hamiltonian [Equa-
tion (1)] and the mathematical formula[19] in Equation (2):
ˆ ˆ
H = –JS1S2
(1)
(2)
β2
χM = (2Ng2 )F(J)
kT
where F(J) = (ex + 5e3x + 14e6x + 30e10x + 55e15x)/(1 + 3ex
+ 5e3x + 7e6x + 9e10x + 11e15x).
Figure 2. (a) A view of the 2D layer formed in the structure of 1,
parallel to the ab plane; three ladder chains, which interact through
H-bonding interactions (turquoise dotted lines) of the neutral carb-
oxylate group, are shown. (b) A packing diagram of 1 down the b
axis, showing the interactions of two 2D layers through the H-
bonds (green dotted lines) formed by the coordinated water mole-
cule. CH hydrogen atoms have been omitted for clarity.
Table 1. Detailed geometrical characteristics of the H-bonds found
in the crystal structure of 1.
Donor–H···Acceptor[a]
D–H H···A
D···A
D–H···A
O(2)–H(2O)···O(3)#1
0.950 1.698
O(1 W)–H(1WA)···O(5)#2 0.882 1.842
2.644
2.717
2.769
173.6
171.8
167.8
O(1 W)–H(1WB)···O(3)#3 0.766 2.015
Figure 3. Plot of χMT vs. T for complex 1 under an applied dc field
of 1000 G. The solid line represents a fitting of the data in the
temperature range 5–300 K (see text for details).
[a] Symmetry operations to generate equivalent atoms: #1, –x +
1/2, y + 1/2, –z + 1/2; #2, –x, –y + 1, –z + 1; #3, x. –y, z + 1/2.
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Table 2. Alkene epoxidations catalyzed by 1 in the presence of H2O2.[a]
[b]
Substrate[a]
Product
% Conversion
% Selectivity[c]
% Other products[b]
–
TON TOF [h–1]
Cyclooctene
Cyclohexene
1-Methylcyclohexene
Limonene
cis-epoxide
cis-epoxide
cis-epoxide
33.0
330 55
47.0
470 78.3
81.6
816.2 136
38.3
383 64
21.0
210 35
12
120 20
15.5
154.5 25.8
33.0
330 55
26.2
262 43.6
5.0
50 8.3
31.1
100
90.2
99.4
87.7
100
100
100
100
100
100
83.3
ol (3%)
one (1.5%)
ol (0.5%)
1,2 epoxides
(Z-/E-)
trans-epoxide
ol (5.7%)
trans-β-Methylstyrene
Styrene
–
epoxide
–
cis-Stilbene
cis-epoxide
epoxide
–
Cyclopentene
α-Pinene
–
α-epoxide
epoxide
–
Hexene
–
2-Cyclohexen-1-ol
epoxide
one(5.2%)
311 51.8
[a] Conditions: catalyst/H2O2/CH3COONH4/substrate = 1:2000:1000:1000; equivalents of catalyst = 1 μmol of asymmetric unit of 1 in
0.85 mL of CH3COCH3/CH3OH (0.45:0.40). [b] Conversion and yields based on starting substrate and products formed. The mass
balance is 98–100%. Reactions were completed within 6 h. [c] Calculated as [epoxide]/{[epoxide] + [other products]}.
The
best
parameters
obtained
were
J
=
–0.20Ϯ0.002 cm–1 and g = 2.00 (R = 9.5ϫ10–3). Finally,
in order to determine whether effects of ordering occur
within the polymeric species, we performed both FC and
ZFC measurements, which proved identical, thus ruling out
this possibility.
The catalytic activity of 1 toward alkene epoxidation
(Scheme 2) was examined with H2O2 as oxidant. Ammo-
nium acetate was added as an additive,[20a] as in previous
cases;[20b–20e] this is absolutely required in order to produce
an efficient catalytic system. According to the data in
Table 2, compound 1 showed significant catalytic activity
within 6 h. The time-course profile of the epoxidation of
cyclohexene catalyzed by compound 1 is given in Figure 4.
Figure 4. Time dependence of cyclohexene oxidation catalyzed by
1. See Table 2 for further details.
cyclohexene, the detected oxidation products were cis-epox-
ide with 99.4% selectivity and small amounts of 1-methyl-
Scheme 2. Epoxidation of alkenes.
Epoxidation of a wide range of olefins proceeded with 2-cyclohexen-1-ol. The total yield of 1-methyl-cyclohexene
high conversion and selectivity for the epoxide product in oxidation products was 81.6%. The major products de-
most of the cases (Table 2, Figure 5). Oxidation of cyclooc- tected from the oxidation of limonene were two epoxides
tene, catalyzed by 1 provided 100% selectivity for cis-cyclo- (Z- and E-) derived from epoxidation of the 1,2-double
octene epoxide with 33.0% yield corresponding to 330 bond. Additionally, small amounts of products formed by
TONs and 55 TOF (Table 2). Cyclohexene oxidation pro- allylic oxidation of the limonene ring were identified. How-
vided mainly epoxide with 42.4% yield (90.2% selectivity). ever, the epoxidation was clearly the main reaction path re-
In some extent, allylic oxidation also occurred, forming 2- sulting mainly in 1,2-epoxides. The total yield of 1,2-epox-
cyclohexene-1-ol (3.0%) and 2-cyclohexene-1-one (1.5%). ides was 33.6% with 87.7% selectivity; the total yield of
The total yield of cyclohexene oxidation products was oxidation products rose to 38.3%. Styrene and trans-β-
47.0% (Table 2, Figure 5). Methyl-substituted alkenes were methylstyrene oxidation produces only the corresponding
more reactive in epoxidation. Thus, in the case of 1-methyl- epoxides with yields 12.0 and 21.0%, respectively, and
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100% selectivity. cis-Stilbene, cyclopentene, α-pinene, and
Conclusions
hexene-1 showed 100% selectivity for the corresponding ep-
oxides with maximum epoxide yields of 15.5, 33.0, 26.2,
and 5.0%, respectively. In the oxidation of 2-cyclohexen-1-
ol, the major product was the corresponding epoxide with
83.3% selectivity. Moreover, considerable amounts of 2-cy-
clohexen-1-one (5.2%) were also detected. The total yield of
2-cyclohexen-1-ol oxidation products was 31.1% (Table 2,
Figure 5). Comparing our results with previous work,[6] we
can see that though the conversion yields are lower, the
selectivity of 1 is much higher, approaching 100% for most
of the substrates.
A ladder-shaped MnII CP, formed through a unique co-
ordination mode of an iminodiacetate ligand, has been syn-
thesized and characterized. The 1D ladders interact
through H-bonds to form a 3D supramolecular architec-
ture. The pairs of MnII ions, bridged by a single-atom carb-
oxylate oxygen bridge are weakly coupled antiferromag-
netically. The molecular material catalyzes, with high selec-
tivity, the epoxidation of alkenes.
Experimental Section
General Remarks: All purchased chemicals were of reagent grade
and they were used as received without further purification. C, H,
and N analyses were conducted by the University of Ioannina,
Greece, Microanalytical Service. FTIR spectra were recorded in the
range 4000–400 cm–1 with a Perkin–Elmer Spectrum GX spectro-
photometer by using KBr pellets. 1H NMR and 13C NMR spectra
were measured with a Bruker Avance AV-250 NMR spectrometer at
room temperature. All spectra were recorded by using commercially
available [D6]DMSO of 99.6% isotopic purity or better and refer-
enced to a residual solvent. Thermal studies were carried out with
a Shimadzu DTG-60 simultaneous DTA-TG apparatus under N2
flow (50 cm3 min–1) with a heating rate of 5 °Cmin–1. Sample sizes
were about 10 mg. X-ray powder diffraction patterns were recorded
with a Siemens (now Bruker) D8 ADVANCE diffractometer. Vari-
able-temperature, solid-state direct current (dc) magnetic suscep-
tibility data down to 5.0 K were collected with a Quantum Design
MPMS-XL SQUID magnetometer equipped with a 7 T DC mag-
net at the University of Crete. Diamagnetic corrections were ap-
plied to the observed paramagnetic susceptibilities by using Pascal’s
constants. GC analysis was performed with a Shimadzu GC-17A
gas chromatograph coupled with a GC–MS-QP5000 mass spec-
trometer.
Figure 5. Distribution of oxidation products catalyzed by 1 in the
presence of H2O2. Reaction performed in CH3COCH3/CH3OH
(0.45:0.40 mL). See Table 2 for further details.
To confirm the heterogeneous nature of the catalytic re-
action, filtration experiments have been performed; that is,
solid catalyst 1 was removed from the solution by filtration
2 h after initiating the catalytic reaction. The reaction of the
filtrate was then monitored for another 4 h; no measurable
catalytic conversion was detected, indicating that putative
manganese active centers leaching from the solid network
are not responsible for the observed catalytic activity. To
check the recyclability of catalyst 1, after a first use for cy-
clohexene oxidation, it was filtered, washed and dried; sub-
sequently, the recovered solid catalyst was reused for a new
oxidation reaction under the same experimental conditions
noted in Table 2. Compound 1 retained 82.5% of its maxi-
mum reactivity for a second run (388 TONs) and performed
with 50% of its activity in the third run (235 TONs), pro-
viding 1093 TONs in total. The XRD spectrum of the cata-
lyst recovered after the first catalytic run did not show sig-
nificant difference from the corresponding XRD pattern of
unused catalyst 1 (see Figure S7 in the Supporting Infor-
mation), but it appears that the small decomposition that
occurs during reuse and handling affects its activity to some
extent. The present data reveal two main pieces of infor-
mation: (1) Here we provide experimental evidence that the
demonstrated Mn-based CP 1 is a promising oxidation cat-
alyst in the presence of H2O2 for alkene epoxidation. (2)
The observed catalytic activity of 1 can be attributed to its
significant oxidative and hydrolytic stability.
LH3·H2O [N-(4-Carboxybenzyl)iminodiacetic Acid Monohydrate]:
A solution of KOH (4.27 g, 76.1 mmol) in water (20 mL) was added
dropwise to a solution of chloroacetic acid (7.19 g, 76.1 mmol) in
water (20 mL), while keeping temperature below 5 °C with an ice
bath. Separately, a solution of KOH (1.80 g, 32.1 mmol) in water
(10 mL) was added to a solution of 4-aminomethylbenzoic acid
(5.00 g, 33.1 mmol) in water (20 mL) at 60 °C. This was cooled to
room temperature and added slowly to the first solution, keeping
the temperature again below 5 °C. Upon the completion of the ad-
dition, a small amount of white solid formed. The mixture was
heated to 90 °C and stirred for 2 h, while maintaining the pH at
about 10 by adding a concentrated solution of KOH. The resulting
solution was cooled down to room temperature and acidified with
HCl (6 m), until the desired acid precipitated as a white solid (pH
= 3). This precipitate was collected by filtration, washed with water,
recrystallized from water, and dried under vacuum over CaCl2.
Yield 7.17 g, 76%. For LH3·H2O [C12H15NO7 (285.3)]:(calcd. C
50.53, H 5.30, N 4.91; found C 50.37, H 5.91, N 4.76. IR (KBr): ν
˜
= 3454 (s), 3287 (m), 3046 (m), 2852 (m), 2553 (m) ν(O–H), 1733
(s), 1684 (s), 1601 (s) ν(C=O), 1422 (m) δ(CH2), 1402 (s), 1360 (m),
1242 (s) ν(C–O), 1112 (w), 1016 (w), 935 (w), 888 (m), 760 (m), 678
(w), 542 (w), 498 (w) cm–1. 1H NMR (250 MHz, DMSO): δ = 3.40
(s, 4 H), 3.88 (s, 2 H), 7.46 (d, J = 8.25 Hz, 2 H), 7.88 (d, J =
8.00 Hz, 2 H) ppm. 13C NMR (100 MHz, DMSO): δ = 54.5, 57.6,
129.5, 130.1, 130.4, 145.0, 168.1, 173.1 ppm.
[Mn(LH)(H2O)]n (1): MnCl2·4H2O (120 mg, 0.6 mmol) and
LH3·H2O (115 mg, 0.4 mmol) were dissolved in H2O (8 mL), and
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the solution was placed in a 15 mL screw-capped (polypropylene)
glass vial. This was heated at 100 °C for 48 h and then cooled
slowly at room temperature. The pale cream colored crystals
formed were filtered, washed with water, ethanol, and diethyl ether,
and dried in air. Some of them were suitable for single-crystal X-ray
data collection. Yield 83 mg, 61%. For 1 [MnC12H13NO7 (338.18)]:
calcd. C 42.62, H 3.87, N 4.14; found C 42.37, H 3.99, N 4.11. IR
Program: Thales. Investing in knowledge society through the Euro-
pean Social Fund).
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(KBr): ν = 3296 (m), 3194 (m) ν(O–H), 2932 (w) ν(C–H), 1708 (s),
˜
1670 (m) ν(C=O), 1594 νas(CO2), 1444 νs(CO2), 1418 (m) δ(CH2),
1342 (m), 1276 (m), 1252 (m), ν(C–O), 1186 (w), 1114 (w), 1094
(w), 980 (w), 940 (w), 868 (w), 792 (m), 746 (m), 702 (w), 642 (w),
564 (w) cm–1.
Catalytic Reactions: Typical conditions employed in catalytic reac-
tions were: 1 equiv. of catalyst, 2000 equiv. of H2O2 30% (w/w),
and 1000 equiv. of substrate and additive. The alkene (1 mmol),
acetophenone or bromobenzene (internal standard, 1 mmol), cata-
lyst (1 μmol of asymmetric unit if 1), and additive (typically 1 mmol
of CH3COONH4) in an acetone/MeOH (450 μL/400 μL) solvent
mixture were cooled to 0 °C. H2O2 (2 mmol) was added by a digi-
tally controlled syringe pump (type SP101IZ WPI) over 1 h whilst
stirring. 10 min later, the test tube was removed from the ice bath
and warmed to room temperature (26Ϯ1 °C). The progress of the
reaction was monitored by GC–MS, by removing small samples
of the reaction mixture. GC analysis of the solution provided the
substrate conversion and product yield relative to the internal stan-
dard integration. To establish the identity of the epoxide product
unequivocally, the retention time and spectroscopic data were com-
pared to those of an authentic sample. Blank experiments showed
that without Mn catalyst or CH3COONH4, epoxidation reactions
do not take place.
Single-Crystal X-ray Structure Analyses: Details of crystal data,
data collection, and refinement are given in Table S1. Diffraction
measurements were made with a Rigaku R-AXIS SPIDER Image
Plate diffractometer by using graphite-monochromated Cu-Kα radi-
ation (λ = 1.54178 Å). Data collection (ω-scans) and processing
(cell refinement, data reduction, and empirical absorption correc-
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The structure was solved by direct methods by using SHELXS-97
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1: 2θmax = 130°; reflections collected/unique/used, 13722/2090 [Rint
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/
(Δρ)min = 0.517/–0.634 eÅ–3; R1/wR2 (for all data), 0.0523/0.1128.
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cle): Spectra and figures related to the characterization of ligand
LH3 (IR, NMR, molecular structure) and complex
[Mn(LH)(H2O)] (IR, TG, DTA, and XRD), as well as and crystal-
lographic tables.
Acknowledgments
This research has been co-financed by the European Union (Euro-
pean Social Fund - ESF) and Greek national funds through the
National Strategic Reference Framework (NSRF) (Operational
Program “Education and Lifelong Learning” – Research Funding
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Received: May 12, 2014
Published Online: July 15, 2014
Eur. J. Inorg. Chem. 2014, 3638–3644
3644
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim