Liquid crystalline salen manganese(III) complexes. Mesomorphic and catalytic behaviour

Article abstract of DOI:10.1039/c0dt01700k

Several salen manganese(III) complexes displaying stable columnar mesophases in a wide range of temperatures have been synthesized. In condensed phases the molecules are assembled into dimers through intermolecular manganese-oxygen interactions and the co

Full text of DOI:10.1039/c0dt01700k

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PAPER
Liquid crystalline salen manganese(III) complexes. Mesomorphic and
catalytic behaviour†
Rube´n Chico,a Cristina Dom´ınguez,a,b Bertrand Donnio,b Silverio Coco*a and Pablo Espinet*a
Received 4th December 2010, Accepted 30th March 2011
DOI: 10.1039/c0dt01700k
Several salen manganese(III) complexes displaying stable columnar mesophases in a wide range of
temperatures have been synthesized. In condensed phases the molecules are assembled into dimers
through intermolecular manganese–oxygen interactions and the columnar structure of the mesophases
consist of the stacking of supramolecular discs formed by the association of two or three dimers,
depending on the number and location of alkoxy chains in the complex. The catalytic activity of the
complexes in solution has been studied, and they behave as efficient homogeneous catalysts in the
epoxidation of styrene with iodosylbenzene as oxidant.
manganese(III) complexes affording columnar mesophases have
Introduction
not yet been reported. We report here the synthesis of hemi-
Metal complexes that give rise to liquid crystal phases (met-
discotic- and polycatenar-like metallomesogens based on salen-
allomesogens) have been systematically investigated in the past
manganese(III) complexes, namely dodecacatenars bearing do-
decades and constitute nowadays a classic field in the area of
materials science.1 Many studies have been carried out show-
decyl or chiral dimethyloctyl chains. In addition, similar salen
manganese complexes with the dialkoxy-o-phenylenediamine lig-
ing that metallomesogens can display physical properties such
and, which would more properly be considered as discotic, have
as thermochromism,2 nonlinear optical behaviour,3 magnetism,4
also been obtained. Both types of complexes display columnar
luminescence,5 or photosensitivity in the mesophase,6 which make
mesophases formed by self-assembling of dimer aggregates, and
them interesting candidates for various applications. Many metals
are efficient homogeneous catalysts in epoxidation of styrene.
and oxidation states of the periodic table have been used to
prepare metallomesogens, but the number of liquid crystals
based on manganese(III) is limited to tris(diketonate) derivatives,7
Results and discussion
tetraphenylporphyrin compounds,8 and a family of calamitic
bidentate Schiff base derivatives [MnCl(L)2] (L = N-alkyl[4-
(4-decyloxybenzoyloxy)]salicylaldiminato) that display smectic
and/or nematic mesophases.9 Tetradentate Schiff bases (salen-
type ligands) have been also used as ligands to produce interesting
liquid-crystalline transition metal complexes,10 and also complexes
that are efficient catalysts for a wide variety of processes,11,12
including epoxidation of alkenes in different conditions.13,14 Enan-
tioselective epoxidation of unfunctionalized olefins using chiral
Mn(III)-salen complexes have been also reported.15
Discotic liquid crystals producing columnar mesophases are
interesting in the design of materials that can display inter-
esting electronic properties, for instance high charge carrier
mobility along the columnar stacks.16 Liquid crystalline salen
Synthesis and characterization
The synthesis of the ligands and their complexes is shown in
Scheme 1. The Schiff bases, H2Ln, were synthesized by condensa-
tion of 3-formyl-4-hydroxyphenyl-3,4,5-tris(dodecyloxy)benzoate
or 3-formyl-4-hydroxyphenyl-3,4,5-tris[(S)-3,7-dimethylocty-
loxy]benzoate with the appropriate diamine. The manganese com-
plexes [MnXLn] (X = Cl, Br) were prepared by refluxing a solution
of the corresponding diimine with manganese(II) acetate in ethanol
under aerobic conditions, as reported for similar complexes.17
They were isolated as brown waxy solids. Enantiomerically pure
complexes were obtained introducing a chiral element either in
the starting diamine ([MnClL3] and [MnClL7]) or in a chain of the
starting acid (Rb in [MnClL5]).
C, H, N analyses for the complexes, yields, and relevant IR data
are given in the supplementary information.† The IR spectra of the
metal complexes display the characteristic n(C N) band from the
imine groups in the range 1603–1629 cm-1 at higher wavenumbers
than for the free ligand, as a consequence of coordination.18 The
UV-Vis spectra show two or three broad bands in the range 244–
480 nm, as reported for related manganese complexes.18,19
aIU CINQUIMA/Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad
de Valladolid, 47071, Valladolid, Castilla y Leo´n, Spain
bInstitut de Physique et Chimie des Mate´riaux de Strasbourg (IPCMS),
UMR 7504 (CNRS-Universite´ de Strasbourg), 23 rue du Loess, BP 43,
F-67034, Strasbourg Cedex 2, France
† Electronic supplementary information (ESI) available: Experimental
details, full characterization data for all new compounds, representative
DSC traces and miscibility tests. See DOI: 10.1039/c0dt01700k
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Table 1 Optical, thermal and thermodynamic data of the salen-Mn
complexes
Compound
Transitiona
Temperatureb (◦C)
DHb (kJ mol-1)
MnClL1
Cr → Cr¢
55
0.6
6.9
Cr¢ → Colh
84d
Colh → I (dec)
Cr→ Colh
180c
71d
MnClL2
MnClL3
MnClL4
3.3
6.2
Colh → I (dec)
Cr → Colh
170c
80d
Colh → I (dec)
Cr → Cr¢
170c
48
3.8
3.1
Cr → Colh
68d
Colh → I (dec)
Cr → Colh
190c
77d
MnBrL4
17.6
Colh → I (dec)
Colh → I (dec)
Cr → Colr
170c
190c
75d
MnClL5
MnClL6
8.3
Colr → I (dec)
Colh → I (dec)
190c
150c
MnClL7
a Cr, crystal; Colh, columnar hexagonal; Colr, columnar rectangular; I,
isotropic liquid. b Data from the first heating scans. c Optical microscopy
data. d The transitions are very broad and data refer to the peak
temperature.
Fig.
1
Polarized optical microscopic texture (¥100) observed for
[MnClL2] at 150 ◦C, upon cooling from the isotropic liquid. The
microphotography was taken in the cooling cycle of a sample that had
been quickly heated to isotropic liquid (175 ◦C) in order to minimize
decomposition.
Scheme 1 Synthesis of manganese(III) complexes [MnXLn]. The chiral
complexes are indicated by (*).
could be observed upon mechanical shear, precluding mesophase
assignment.
All the compounds show wide mesophase temperature ranges
(ca. 100 ◦C). In general the mesophases freeze at low temperatures,
but the exact moment of the fluid-to-solid transition is difficult to
detect, whether optically or thermally (no peak is detected by
DSC either), suggesting that the material is frozen retaining the
order of the mesophase (since this is not a long range order we
will refer to it as a glass liquid crystal state). All the compounds
undergo some decomposition upon◦reaching the clearing point to
the isotropic state (about 170–190 C), possibly due to the high
temperature at which this clearing happens. However, samples in
the glass state free of decomposition can be obtained from the
mesophases avoiding to heat close to the melting point.
Mesomorphic behaviour
All the manganese compounds prepared are thermotropic liquid
crystals and display columnar mesophases, as could have been ex-
pected due to the large number of peripheral divergent chains. The
mesophases were studied by polarized optical microscopy (POM),
differential scanning calorimetry (DSC), and small angle X-ray
scattering (SAXS). The optical, thermal and thermodynamic data
are collected in Table 1.
The optical textures observed with a polarizing microscope for
[MnClLn] (n = 1–5) and for [MnBrL4], upon cooling from the
isotropic liquid, providing a short stay in the isotropic liquid or in
its vicinity to avoid extensive decomposition, are compatible with
hexagonal columnar mesophases and display linear birefringent
defects, large areas of uniform extinction, and fan domains (Fig.
1).20 However, for [MnClLn] (n = 6, 7) only a weak birefringence
It is noticeable that the melting enthalpy of [MnBrL4] is
significantly larger than that of [MnClL4] in spite of the fact that
it is out of the question that they produce the same kind of phase
(see later). This could be due in part to differences in the starting
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Fig. 2 Successive photographs of the evolution of a contact experiment showing the miscibility of the hexagonal columnar mesophase of [MnClL4] and
that of [MnClL3]. (a) Separated mesophases at 125 ◦C on heating; (b) Contact preparation at 125 ◦C on heating; (c) Contact preparation at 130 ◦C on
cooling from the isotropic liquid.
crystalline phase, and also the effect of changing the polarizability
and dipolar moment of the molecule upon changing the halide
ligand.
X-Ray diffraction experiments
Temperature-dependent X-ray diffraction experiments were sys-
tematically carried out in order to unambiguously identify the
nature of the mesophase. The X-ray patterns of the MnClL4,
MnBrL4 and MnClL5 complexes, recorded between 50 ◦C and
120 ◦C, exhibited three sharp and intense small angle diffraction
√
√
peaks with a reciprocal d spacing in a 1 : 3 : 4 ratio. In addition,
˚
a strong and diffuse halo at 4.5 A was observed (Fig. 3). These
features are unambiguously assigned to the (10), (11) and (20)
reflections of a columnar phase with a hexagonal lattice and to
the liquid-like order of the molten alkoxy chains, respectively,
in agreement with the POM observations. The other complexes
of the same series ([MnClLn], X = Cl, Ln = L1, L2, L3), except
[MnClL7] displayed the same textures, and similar diffraction
patterns, although only one maximum was observed, suggesting
hexagonal columnar packing. The hexagonal columnar nature of
the mesophases of [MnXLn] (X = Cl, Ln= L1, L2, L3) was further
supported by their miscibility with [MnClL4] (Fig. 2). In contrast,
[MnClL7] was not miscible with [MnClL4]. This result did not
allow us to identify the type of mesophase unambiguously. Finally,
for [MnClL6], X-ray diffraction revealed two sharp small-angle
reflections, indicating the formation of a columnar rectangular
phase. The absence of additional higher order reflections consider-
ably limits absolute mesophase assignment, although, the centred
c2mm planar symmetry will be considered thereafter as the highest
symmetry allowed, with a possible two sets◦of lattice parameters
Fig. 3 XRD pattern of [MnClL5] recorded at 100 ◦C.
interactions (Fig. 4).22,23 Consistent with this, the mass spectrum of
[MnClL1] shows the parent peak (1667.103 [M+]) corresponding to
a dimeric aggregate. Consequently we suggest as a basic structural
model that the dimer is likely preserved in the mesophase, and
thus will constitute the elementary molecular entity, whose mean
˚
thickness is approximately in the range of 10 to 13 A, depending
on the substituents and the diamine bridge (Fig. 5).
˚
(a = 79.0/83.2 and b = 44.9/48.9 A at 150 C), which cannot be
discriminated. Structural data from X-ray diffraction experiments
are collected in Table 2.
Fig. 4 Dimeric structure of salen-Mn complexes formed via mangane-
se–oxygen interactions.
As for other related polycatenar metallomesogens,21 the
supramolecular organisation of the Mn complexes into columnar
mesophases results in the present case from the increasing volume
of the terminal chains not being accommodated by an increasing
tilt of the molecular core (in a layer structure configuration), and
thus limiting long-range lateral molecular ordering (breaking of
the layers). Consequently, molecules will tend to self-associate
into discrete molecular aggregates to form the basic unit of the
supramolecular columnar stack. Furthermore, as far as the present
systems are concerned, the structure of related compounds in
the solid state consists of dimers formed by manganese–oxygen
Provided that this correct periodicity along the column, h, is
known, the effective number of molecules or molecular equivalents
(N) can be determined by the relationship between the columnar
cross-section area (S) and the molecular volume (Vmol), according
to the geometric relationship hS = NVmol (note that for h arbitrarily
chosen to be equal to unity, the number of molecular equivalents
˚
is given per unit length i.e. per A, and is appreciated as a linear
density).24 If the average thickness of the dimeric structure is
considered as being of the same magnitude order of the main
˚
periodicity along the column, arbitrarily considered as 10 A £
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Table 2 Structural data from X-ray diffraction experiments
Compound
Indexation
T/◦C
d
meas/A
hkb
Ic
Parametersd,e
a
a
˚
˚
dcalc/A
MnClL1
100
35.66
4.6
10
VS (sh)
VS (br)
35.66
hch
a = 41.18 A
˚
2
˚
S = 1468 A
3
˚
Vmol = 2828 A
˚
N = 6 for h = 11.56 A
MnClL2
120
35.83
4.7
10
10
VS (sh)
VS (br)
35.83
hch
a = 41.37 A
˚
2
˚
S = 1482 A
3
˚
Vmol = 2917 A
˚
N = 6 for h = 11.81 A
MnClL3
100
38.09
4.6
VS (sh)
VS (br)
38.09
hch
a = 43.98 A
˚
2
˚
S = 1675 A
3
˚
Vmol = 2917 A
˚
N = 6 for h = 10.45 A
MnClL4
100
30.81
17.85
15.4
4.5
31.02
17.93
15.57
4.6
29.11
16.81
14.47
4.6
41.6
39.5
4.5
10
11
20
VS (sh)
S (sh)
M (sh)
VS (br)
VS (sh)
S (sh)
M (sh)
VS (br)
VS (sh)
M (sh)
W (br)
VS (br)
VS (sh)
VS (br)
30.84
17.81
15.42
hch
31.07
17.94
15.53
hch
29.05
16.77
14.53
hch
41.6
39.5
hch
a = 35.61 A
˚
2
˚
S = 1098 A
3
˚
Vmol = 3520 A
˚
N = 4 for h = 12.81 A
MnBrL4
100
10
11
20
a = 35.88 A
˚
2
˚
S = 1115 A
3
˚
Vmol = 3593 A
˚
N = 4 for h = 12.90 A
MnClL5
100
10
11
20
a = 33.55 A
˚
2
˚
S = 975 A
3
˚
Vmol = 3240 A
˚
˚
N = 4 for h = 13.3 A
MnClL6
150
11/20
20/11
a = 79 A; b = 48.9 A; S = 1933 A
2
˚
˚
2
˚
˚
˚
a = 83.2 A; b = 44.9 A; S = 1867 A
3
˚
Vmol = 2828 A
˚
N = 8 for h = 11.7–12.1 A
MnClL7
100
40.55
4.5
10
VS (sh)
VS (br)
40.55
hch
a = 46.8 A
˚
2
˚
S = 1899 A
3
˚
Vmol = 3104 A
˚
N = 6 for 9.8 A
˚
N = 8 for h = 13.0 A
a dmeas and dcalc stand for measured and calculated periodicity. b Miller indices, hk. c I corresponds to the intensity of the reflections (VS: very strong, M:
medium; br and sh stand for broad and sharp); hch represents the molten chain short-range distance. d Phase parameters: Colh, intercolumnar distance,
√
√
√
√
((a/h)2 +
1
a = 2 ¥ [Rdhk (h2 + k2 + hk)]/Nhk 3; number of reflections, Nhk; columnar cross-section of the hexagonal cell, S = 2 a2 3; Colr, 1/dhk
=
(b/k)2); columnar cross-section in the rectangular cell, S = a ¥ b/2; Vmol, molecular volume, is determined considering a density of 1 according to
MW/(6.022.1023)(10-24), where MW is the molecular weight. The repeating columnar unit h is related to the molecular volume and the cross-section
columnar area by the relationship N ¥ Vmol/S, where N is the number of molecular equivalent per repeat unit (in this case, to maintain the density
along the column constant, N must be an integer or half-integer). e The parameters given for MnClL7 have been calculated on the assumption that their
mesophases were hexagonal columnar (see text).
˚
h £ 13 A, the number of dimers (ND = N/2) constituting the
supramolecular disc of all complexes can be determined. Recall,
nevertheless, that no specific signs have been observed in the XRD
patterns, due to the liquid-like order within the columns, but such
an approximation is helpful in the understanding of the molecular
packing.
as a consequence, for the same columnar thickness, the number
of dimers arranged side-by-side is increased in order to pave the
larger surface area.
Finally, the case of [MnClL6], which is an isomer of [MnClL1]
with the benzoate substituent groups in C4 and C4¢ positions of the
salicylidene ligand, deserves some comments: the bent polycatenar
structure of [MnClL6] differs significantly from that of [MnClL1]
(Fig. 6), which has the benzoate substituents groups arranged
straight along the molecular axis (C3 and C3¢ positions of sali-
cylidene group). As for their homologous vanadyl complexes,21a
they possess similar thermal behaviour, but the symmetry of
the mesophase is changed. The multipolar structure of these
complexes is not so easy to analyze, but it looks reasonable that
the bent structure should be more polar and with stronger core
interactions, which should produce a more efficient molecular
stacking.25 For a corresponding thickness along the column, again
chosen arbitrarily for comparison sake, now four dimers are
required to pave the surface area of the rectangular lattice, which
is also in agreement with the important increase in the columnar
The X-ray data of compounds [MnClLn] (Ln = L1, L2, L3) are
consistent with a disc formed by three pairs in order to fully pave
the hexagonal net, whilst for the bulkier [MnXL4] (X = Br, Cl),
and [MnClL5] the discs are made of two dimers (larger assemblies
seem unfavourable because of the lateral chains) assembled back-
to-back. For the latter, the compounds have eight alkoxy chain
per metal center surrounding the molecular core, and give rise to
a dimer with a discotic shape, however not planar: these dimeric
molecules could easily pack as two dimers in a complementary
arrangement, so as to make a disk (Fig. 5).
The incorporation of the very bulky tert-butyl group at the C5-
and C5¢-positions of the salicylidene ligand [MnClL7] produces
a substantial increase in the hexagonal lattice parameter (a) and
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Table 3 Catalytic results in the epoxidation of styrene catalyzed by chiral
catalysts, in acetonitrile and using iodosylbenzene as oxidant
Catalyst (3.4-mol%)
(hexane solution)
Entry
Yield (%)a
ee (%)a
Blank
—
0
—
2
2
1
2
3
[MnClL3]
[MnClL5]
[MnClL7]
68
78
89
11
a Determined by GC on a LIPODEX-E chiral column. ee values are given
for the major enantiomer (Rt = 11.4 min).
Catalytic behaviour
Salen-manganese(III) complexes are efficient catalysts in the oxi-
dation of olefins to epoxides.26 The catalytic activity of [MnClL2]
was studied in the epoxidation of styrene with iodosylbenzene in
acetonitrile (Scheme 2), under liquid–liquid biphasic conditions
using a solution of the catalyst in hexane (which is immiscible with
acetonitrile). The epoxidation of styrene proceeded in moderate
yield (60% GC yield after 24 h) with a catalyst loading of 3.4 mol%.
A blank test showed that no reaction occurs without catalyst under
the same conditions.
Fig. 5 Schematic molecular model for the mesophase structure of Mn
complexes.
The potential of the complexes for enantioselective epoxidations
of alkenes under the same conditions, using chiral catalysts,
was tested on [MnClL3], [MnClL5] and [MnClL7]. The results
are shown in Table 3. The three catalysts produced good yields
but, unfortunately, no enantioselectivity was found for [MnClL3]
or [MnClL5]. Even for [MnClL7], which bears bulky tert-butyl
substituents at C5- and C5¢-positions of the salicylidene lig-
and (as usually required to induce enantioselectivity),27–29 only
poor enantiomeric excess was observed (Table 3, entry 3).
Similar yields and low enantioselectivities have been reported
for catalytic epoxidations of styrene with related polymeric
chiral Mn(III) complexes,30 and for biphasic epoxidation of
1,2-dihydronaphthalene with iodosylbenzene in the presence of
pyridine N-oxide and chiral(salen)manganese complexes bearing
six perfluoroalkyl substituents.31
The induction of enantioselectivity in the epoxidation of
unfunctionalized olefins highly depends on the proximity of
the stereocenters to the active catalytic centre, thus one could
expect higher enantioselectivity with [MnClL7] (as observed) and
[MnClL3] (which is not observed). This difference in favour of
the former might be associated to the presence in L7 of a bulky
substituent, which was meant for this purpose as said before.
In summary, we have prepared metallomesogens based on salen-
Mn(III) complexes, which display columnar mesophases in a wide
range of temperatures, varying in the nature of the chains (dodecyl
and dimethyloctyl), the nature of the diamine (ethenediamine,
cyclohexene diamine, and dialkoxybenzene amine) and shape
Fig. 6 Structural differences between [MnClL6] and [MnClL1] complexes.
2
1
˚
cross-section area (ca. 25%), from 1468 A for [MnClL ] to ca. 1900
2
6
˚
A for [MnClL ]. At this stage, and due to the limited information
gained from X-ray diffraction, the exact details of the intimate
supramolecular ordering within the columns cannot be proposed
further.
Finally, insertion of chiral chains (MnClL4 versus MnClL5) or
chiral diamine (MnClL2 versus MnClL3) in the various molecular
structures has no or little effect on the thermal behaviour, and no
sign of chiral phases was detected.
Scheme 2 Epoxidation of styrene with iodosylbenzene in acetonitrile, under liquid–liquid biphasic conditions, using a solution of the Mn catalyst in
hexane.
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(linear versus bent). These modifications hardly modify the overall
mesomorphic behaviour of the samples. The columnar structures
of the mesophases have been determined by X-ray diffraction
experiments. The columnar structure consists of the staking of
discs formed by either four or six molecules of the complexes,
associated into dimers through intermolecular manganese–oxygen
interactions, two or three (respectively) of these dimers in each disc.
Their catalytic activity has been studied and they behave as effi-
cient catalysts in the epoxidation of styrene with iodosylbenzene
as oxidant.
General procedure for the epoxidation of styrene with
iodosylbenzene as the oxidant
To a mixture thermostated at 25 ◦C, formed by the catalyst
(0.036 mmol) dissolved in hexane (3 mL) and acetonitrile (5 mL),
were added successively under nitrogen styrene (1 mmol) and
iodosylbenzene (2 mmol). After stirring for 24 h, the hexane was
evaporated under vacuum and the solid catalyst was removed by
filtration. The resulting acetonitrile solution was analyzed by GC.
Acknowledgements
Experimental section
This work was sponsored by the D.G.I. (Project CTQ2008-
03954/BQU and INTECAT Consolider Ingenio-2010 (CSD2006-
0003) and the Junta de Castilla y Leo´n (Projects VA248A11-2
and GR169). R. C. and C. D. thank the Ministerio de Ciencia e
Innovacio´n for a studentship.
Experimental conditions for the analytical, spectroscopic and
diffraction studies were as reported elsewhere.32 Mass spectra
were recorded on an MicroTOF, Bruker Daltonics. Analyti-
cal gas chromatography was performed on a Hewlett Packard
5890 Series II machine, equipped with a LIPODEX-E (25 m
¥ 0.25 mm ¥ 0.40 mm) capillary chiral column. The reten-
tion times were: for styrene: t = 2.2 min; for styrene epox-
ides: t1 = 9.7 min, t2 = 11.4 min. GC: isotherm at 90 ◦C.
Literature methods were used to prepare iodosylbenzene,33
tert-butyl-2,5-dihydroxybenzaldehyde,34 1,2-bis(dodecyloxy)-4,5-
diaminobenzene,35 and (S)-3,7-dimethyloctylbromide.36
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All salen-type ligands were synthesized similarly to that
described for 4-hydroxy-3-[({2-[((E)-{2-hydroxy-5-[(3,4,5-trido-
decyloxybenzoyl) oxy] phenyl}methylidene) amino] ethyl} imino) -
methyl]phenyl 3,4,5-tris(dodecyloxy)benzoate (H2L1),37 starting
from 3-formyl-4-hydroxyphenyl-3,4,5-tris(dodecyloxy)benzoate,
4-formyl-5-hydroxyphenyl-3,4,5-tris(dodecyloxy)benzoate or 3-
formyl - 5 - tert-butyl-4-hydroxyphenyl-3, 4, 5-tris[dodecyloxy]-
benzoate and the corresponding diamine. A typical procedure is
as follows:
´
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To a solution of 3-formyl-4-hydroxyphenyl-3,4,5-tris(dode-
cyloxy)benzoate (1.28 g, 1.61 mmol) in 100 mL of toluene was
added the corresponding diamine (0.80 mmol) and 5 drops of
glacial acetic acid. The mixture was refluxed for 3 h and water
formed by the reaction was removed azeotropically using a
Dean–Stark apparatus. Then the mixture was cooled to room
temperature and the solvent was removed at reduced pressure.
The crude product was purified by recrystallization from absolute
ethanol and it was isolated as a yellow solid.
Preparation of salen-Mn(III) complexes (MnXLn)
All the compounds were synthesized using a reported method.38
Salen ligand (0.186 mmol) and manganese(II) acetate tetrahydrate
(0.069 g, 0.28 mmol) were added to 20 mL of ethanol, and the
mixture was refluxed for 3 h in air. Upon cooling, 3 mL of an
aqueous saturated sodium chloride solution was added, and the
mixture was extracted with dichloromethane (2 ¥ 15 mL). The
combined organic portions were then washed with 15 mL of
water and dried over MgSO4. The solvent was removed by rotary
evaporation to yield the brown product that was recrystallized in
dichloromethane–ethanol.
´
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5982 | Dalton Trans., 2011, 40, 5977–5983
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