Alkali-metal-ion-directed self-assembly of redox-active manganese(III) supramolecular boxes

Article abstract of DOI:10.1021/ic501842x

The ability to organize functional molecules into higher dimensional arrays with well-defined spatial relationships between the components is one of the major goals in supramolecular chemistry. We report here a new route for the preparation of supramolecu

Full text of DOI:10.1021/ic501842x

Article
pubs.acs.org/IC
Alkali-Metal-Ion-Directed Self-Assembly of Redox-Active
Manganese(III) Supramolecular Boxes
Gustavo Gonzalez-Riopedre,† Manuel R. Bermejo,‡ M. Isabel Fernandez-García,*,†
́
́
Ana M. Gonzalez-Noya,†,‡ Rosa Pedrido,†,‡ M. Jesus Rodríguez-Douton,†,§ and Marcelino Maneiro*,†
́
́
́
†Departamento de Química Inorgan
‡Centro de Investigacion
́
ica, Facultade de Ciencias, Universidade de Santiago de Compostela, 27002 Lugo, Spain
ica e Materiais Moleculares, Universidade de Santiago de Compostela, 15782 Santiago de
́
en Química Biolox
́
Compostela, Spain
S
* Supporting Information
ABSTRACT: The ability to organize functional molecules into
higher dimensional arrays with well-defined spatial relationships
between the components is one of the major goals in
supramolecular chemistry. We report here a new route for
the preparation of supramolecular boxes, incorporating two
types of metal ions: (i) alkali-metal ions, which induce the
supramolecular architecture and essentially play a structural
role in the final compounds; (ii) manganese(III) ions, which
are redox-active systems and give functionality to the new
cages. Our results evidence that the size of the cavity inside the
box can be tuned depending on the alkali metal used, a
characteristic that gives this new family of compounds the
potential to act selectively against different substrates. These
compounds behave as active catalysts for disproportionation of H2O2 or for water photolysis, but they catalyze neither
catecholase reaction nor peroxidase action upon using bulky organic substrates.
Chart 1. Schematic Representation of Our Synthetic
Approach to the Assembly of Manganese(III) Boxes with
Alkali-Metal Ions
INTRODUCTION
■
Metal−organic solids containing cavities or channels with
controlled sizes, shapes, and chemical environments have
become increasingly important because some of these supra-
molecular structures may have functional properties and
specific applications in various fields, including host−guest
chemistry, redox activity, magnetic behavior, photo- and
electrochemical sensing, and catalysis.1−5
a
Metal-directed self-assembly6 constitutes one of the out-
standing strategies to obtaining supramolecular architectures; a
particular advantage of this method is the single-step assembly
process involving simple mixing of metals and ligands rather
than a stepwise approach.7
a
The red dots in the ligand represent the negative charges after
The typical metal-assisted process involves the use of a
transition-metal ion to induce the assembly of the structure, but
the use of alkali-metal cations is relatively rare because it is
more difficult to control the architecture because of the variety
of coordination numbers and weak coordination ability of such
ions in comparison to transition-metal ions (Chart 1).8,9
The electronic component of ligand binding to alkali-metal
ions can generally be considered in terms of simple
electrostatics. In fact, these cations may be characterized as a
sphere of positive charge that generally attracts donors with
little regard for ligand orientation.10
deprotonation of the phenoxo and amide groups. The alkali-metal
cation makes stabilization of this novel supramolecular motif possible.
geometrical shapes, functionality patterns, and distributions of
cohesion energy along the charged objects; i.e., it is “real”
supramolecular chemistry in which the secondary structural
elements support the alignment in a process driven by the
charge−charge coupling.11−13
We have previously studied ionic self-assembly processes to
obtain functional manganese(III)−Schiff base networks using
In contrast to simple salt association and because of the lack
of selectivity and the long-range character of the Coulombic
interactions, ionic self-assembly must also involve distinct
Received: August 2, 2014
Published: February 23, 2015
© 2015 American Chemical Society
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Found: C, 64.0; H, 5.5; N, 9.4. ESI-MS: m/z 301 ({H4L + H}+). IR
(KBr, cm−1): ν(N−H) 3408 (vs), ν(O−H) 3063 (m), ν(amide I, C
O) 1646 (m), ν(amide II, C−N, N−H) 1550 (m), ν(C−O) 1253 (s).
1H NMR (DMSO-d6): δ 2.48 (s, 4H), 6.85 (t, 4H), 7.36 (t, 2H), 7.80
(d, 2H), 8.95 (s, 2H). 13C NMR (DMSO-d6): δ 39.4 (−CH2−), 55.7
(NCH2−), 57.0 (−OCH3), 116.4−134.2 (Car), 161.1 (C−OH),
169.9 (CO).
different counteranions such as perchlorate, nitrate, chloride,
carboxylates, etc.14 The results of these studies show that the
nature of the anion can control the architecture in the final
products from the self-assembly process.
In the work described here, we further explored these simple
and versatile methods in order to establish the first strategy to
connect redox-active manganese(III) complexes through
spherical alkali-metal cations. The self-assembly of manganese
compounds directed by alkali-metal ions could generate novel
supramolecular motifs that improve the important properties of
manganese(III) complexes.
The challenge of controlling the combination of transition-
and alkali-metal ions in an assembled supramolecular structure
was overcome by using the appropriate ligand. For this
purpose, we selected the tetraionic H4L bisamido−bisphenoxy
ligand (Chart 2), which contains six potential donor atoms: two
Synthesis of the Complexes. All of the manganese(III)
complexes were prepared by stirring a methanol (MeOH) solution
(50 mL) of the corresponding ligand and subsequently adding a
MeOH solution of metal hydroxide (30 mL) and a MeOH solution of
Mn(Ac)2·4H2O at room temperature. The initial light color of the
solutions rapidly changed to red-brown. After 3 h of stirring at room
temperature, slow evaporation of the solvent led to the deposition of
brown compounds. The products were collected by filtration, washed
with diethyl ether (2 × 20 mL), and then dried in vacuo.
[KMnL(CH3OH)]2·CH3OH·H2O (1). H4L (0.3 g, 1.00 mmol),
Mn(Ac)2·4H2O (0.25 g, 1.00 mmol), and K(OH) (0.06 g, 1.00
mmol) were reacted. Yield: 0.32 g (71%). Selected data for 1 are as
follows. ESI-MS: m/z 353 ({MnL + 2H}+), 393 ({KMnL + H}+), 782
({[KMnL]2 + H}+). Elem anal. Calcd for C35H38K2Mn2N4O12: C,
46.8; H, 4.1; N, 6.1. found: C, 46.5; H, 4.0; N, 6.1. IR (cm−1): ν(O−
H) 3386 (m), ν(amide I, CO) 1599 (m), ν(amide II, C−N, N−H)
Chart 2. Ligand H4L
1
1530 (m), ν(C−O) 1256 (s). Magnetic moment: μeff = 4.8 μB. H
NMR paramagnetic (DMSO-d6): δ −22.80 (H4), −19.50 (H5). λ (ε)
= 510 nm (700 M−1 cm−1).
[Rb2Mn2L2(CH3OH)(H2O)]·3H2O (2). H4L (0.3 g, 1.00 mmol),
Mn(Ac)2·4H2O (0.25 g, 1.00 mmol), and Rb(OH) (0.10 g, 1.00
mmol) were reacted. Yield: 0.47 g (48%). Selected data for 2are as
follows. ESI-MS: m/z 353 ({MnL + 2H}+), 438 ({RbMnL + H}+),
876 ({[RbMnL]2 + H}+). Elem anal. Calcd for C33H36Mn2N4O13Rb2:
C, 40.5; H, 3.7; N, 5.7. Found: C, 41.1; H, 3.5; N, 5.9. IR (cm−1):
ν(C−O, amide I) 1599, ν(amide II) 1526, ν(C−O) 1261, ν(O−H)
amide nitrogen (Na), two phenoxo (Op), and two amide
oxygen (Oa) atoms. The 2Na2Op core forms an inner cavity
that is able to accommodate one metal ion. The amido donor
groups have the potential to stabilize higher oxidation states of
manganese,15 and, in addition, the configuration of the two
amide oxygen atoms, which point toward the outside of the
inner cavity, facilitates an extension of the dimensionality of the
structure.
1
3421. Magnetic moment: μeff = 4.9 μB. H NMR paramagnetic MnIII
(DMSO-d6): δ −24.2, −16.3. λ (ε) = 512 nm (420 M−1 cm−1).
[CsMnL(H2O)]2 (3). H4L (0.3 g, 1.00 mmol), Mn(Ac)2·4H2O (0.25
g, 1.00 mmol), and Cs(OH) (0.15 g, 1.00 mmol) were reacted. Yield:
0.29 g (68%). Selected data for 3 are as follows. ESI-MS: m/z 353
({MnL + 2H}+), 485 ({CsMnL + H}+), 970 ({[CsMnL]2 + H}+).
Elem anal. Calcd for C32Cs2H28Mn2O10: C, 38.3; H, 2.8 N, 5.6. Found:
C, 38.2; H, 2.6; N, 5.5. IR (cm−1): ν(O−H) 3408 (m), ν(amide I, C
O) 1598 (m), ν(amide II, C−N, N−H) 1531 (m), ν(C−O) 1258 (s).
Magnetic moment: μeff = 4.6 μB. 1H NMR paramagnetic MnIII
(DMSO-d6): δ −22.28 (H4), −19.82 (H5). λ (ε) = 514 nm (380
M−1 cm−1).
Crystallography. Single crystals of the ligand H4L and of the
complexes 1−3 suitable for X-ray diffraction studies were obtained by
slow evaporation of the methanolic solution at room temperature.
Detailed crystal data collection and refinement are summarized in
Tables 1 and 2. Intensity data were collected on a Bruker Smart CCD-
1000 diffractometer employing graphite-monochromated Mo Kα
radiation (λ = 1.54184 Å) for 1 and 3 at 120 K, for 2 at 110 K, and
for H4L at 100 K. The structures were solved by direct methods16 and
finally refined by full-matrix least squares based on F2. An empirical
absorption correction was applied using SADABS.17 All non-hydrogen
atoms were included in the model at geometrically calculated
positions. Drawings of the molecules were generated with the
ORTEP-318 and Mercury 3.1 programs.19
Studies on the Catalase-like Function. A 10 mL flask
containing a solution of the complexes in MeOH (3 mL, 1 mM)
was sealed with a septum and connected to a gas-measuring buret
(precision of 0.1 mL) through a double-ended needle. The solution
was stirred at constant temperature on a water (H2O) bath. Catalysis
was initiated by introducing a hydrogen peroxide (H2O2) solution (1
mL, 2.5 M) using a syringe, and the evolved dioxygen (O2) was
volumetrically measured.
Photolysis Experiments. Irradiation of aqueous solutions of the
complexes was carried out in a colorless two-necked glass flask (1 L)
placed in a methacrylate thermostated H2O bath. A magnetically
stirred solution of the corresponding complex (5 × 10−6 mol) and p-
EXPERIMENTAL METHODS
■
Chemical and Reagents. All of the starting materials (Aldrich)
and solvents (Probus) used for the synthesis were of commercially
available reagent grade and were used without further purification.
Physical Measurements. Elemental analyses were performed on a
Carlo Erba model 1108 CHNS-O elemental analyzer. IR spectra were
recorded as KBr pellets on a Bio-Rad FTS 135 spectrophotometer in
1
the range of 4000−400 cm−1. H NMR spectra were recorded on a
Bruker AC-300 spectrometer using CD3OD (296 K) as the solvent
and SiMe4 as an internal reference. Electrospray ionization mass
spectrometry (ESI-MS) spectra of the compounds were obtained on a
Hewlett-Packard model LC-MSD 1100 instrument (positive-ion
mode, 98:2 CH3OH/HCOOH as the mobile phase, 30−100 V).
Room-temperature magnetic susceptibilities were measured using a
MSB-MKI digital measurement system, calibrated using mercury
tetrakis(isothiocyanato)cobaltate(II) (Hg[Co(NCS)4]) as the suscept-
ibility standard. Variable-temperature magnetic data for 3 were
obtained with a Quantum Design MPMS SQUID susceptometer.
The sample was a 3-mm-diameter pellet molded from a ground
crystalline sample. Magnetic data for 1 and 2 were obtained with a
Quantum Design PPMS susceptometer. Electronic spectra were
recorded on a Cary 230 spectrometer. The conductivities of 10−3
M
solutions in N,N-dimethylformamide (DMF) were measured on a
Crison micro CM 2200 conductivimeter.
Synthesis of the H4L ligand. The ligand H4L was prepared by the
reaction of phenyl salicylate and 1,2-ethylendiamine in a 2:1 molar
ratio at 180−190 °C for 1 h. The product was treated with diethyl
ether to obtain a white powder at almost quantitative yield. Mp: 180
°C. Anal. Calcd for C16H16N2O4 (300.3): C, 64.0; H, 5.4; N, 9.3.
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was placed in a two-necked glass flask: one neck contained the oxygen
electrode and the second a septum. The whole flask arrangement was
immersed in a thermostated bath so that H2O came up to the base of
the necks. Stirring was begun, the complex and quinone were added,
and the septum was fitted to make the system airtight. Four needles
were pushed through the septum, one reaching into the liquid, and a
stream of argon was introduced into the solution until the reading of
the oxygen meter fell to <3%. Once this value was reached, the argon
and purge needles were removed, and the system was left stirring for
10 min to equilibrate. The light was switched on. Only the two needles
connected to the flow cell remained through the septum. Oxygen
readings were recorded as a percentage of dissolved oxygen, where
100% corresponds to a fully saturated aqueous solution at 25 °C (6
cm3 O2 dm−3) and 0% corresponds to no dissolved oxygen.
Reproducible results were obtained with this method provided that
the temperature of the bath remained constant (25.0 0.1 °C) and a
constant stirring rate was maintained.
Peroxidase Probes. Oxidation of 2,2′-azinobis(3-ethylbenzothia-
zoline)-6-sulfonic acid (ABTS) with H2O2 at ca. pH 7 in the presence
of the complexes was tested in the following manner. An aqueous
solution of ABTS (50 μL, 0.009 M, 4.5 × 10−7 mol) and a methanolic
solution of the complex (10 μL, 10−3 M, 10−8 mol) were added to
H2O (3 mL). The intensity of the UV absorption bands of ABTS
started to increase immediately after the addition of an aqueous
solution of H2O2 (50 μL, 10 M, 5 × 10−4 mol). Spectra were recorded
every 30 s, and the data reported in Table 5 correspond to the
maximum observed values.
Oxidation of 3,4-Dimethoxybenzyl Alcohol (Veratryl Alco-
hol, VA). VA (200 mmol, 0.29 mL), previously dissolved in 20 mL of
a dihydrogen phosphate/NaOH buffer at pH 8, was mixed with a
methanolic solution (15 mL) of the catalytic complex (0.5 mmol, ca. 6
mg). The oxidant (600 mmol) was added in three portions at 20 min
intervals. The reaction mixture was stirred for 10 h at 22 °C with an air
flow bubbling through the solution and then filtered through a short
silica gel plug to remove the catalyst and excess oxidant. The reaction
mixture was extracted in CH2Cl2 in the presence of a saturated
aqueous NaCl solution. The organic layer was dried over MgSO4 and
evaporated under reduced pressure. The mixture was then purified by
column chromatography using a 1:1 mixture of ethyl acetate/hexane as
the eluent.
Guaiacol Oxidation Assay. Measurements of the oxidation
activities were performed at 20 °C in H2O. A mixture of the complex
(10−4 M, 29.1 μL, MeOH) and guaiacol (0.1 M, 10 μL, MeOH) in a
total volume of 3.0 mL was used, and then 60 μL of 1 M H2O2 (in
H2O) was added. The initial rate of guaiacol oxidation was determined
from an increase in the absorbance at 470 nm (ε470 = 26600 M−1
cm−1).
Table 1. Crystal Data and Structure Refinement for H4L and
1
H4L
1
empirical formula
fw
C16H16N2O4
300.31
C35H36K2Mn2N4O12
892.76
temperature/K
wavelength/Å
cryst syst
space group
a/Å
100
110
0.71073
triclinic
0.71073
triclinic
P1
̅
P1
̅
6.545(3)
6.964(3)
15.297(8)
94.318(8)
94.205(10)
95.464(16)
689.9(6)
2
9.570(6)
10.427(6)
10.825(6)
113.065(7)
97.273(8)
100.410(8)
954.0 (10)
2
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
D/mg m−3
μ/mm−1
1.446
1.554
0.105
0.947
F(000)
316
458.0
θ range/deg
reflns collected
indep reflns
data/restraints/param
GOF
1.34−25.68
2589
2.10−23.82
5100
2589
2932
2589/0/247
0.884
2932/2/266
0.966
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0708, 0.1799
0.1256, 0.2117
0.0706, 0.1720
0.1307, 0.2036
Table 2. Crystal Data and Structure Refinement for 2 and 3
2
3
empirical formula
fw
C33H3Rb2Mn2N4O13
977.48
C16H14CsMnN2O5
502.14
temperature/K
wavelength/Å
cryst syst
space group
a/Å
120
110
0.71073
monoclinic
P21
0.71073
monoclinic
P21/c
9.6398(15)
17.688(3)
10.5316(17)
90
10.839(2)
15.437(3)
9.3724(19)
90
b/Å
c/Å
α/deg
β/deg
102.232(3)
90
92.052(4)
90
γ/deg
V/Å3
Catecholase Activity. With 3,5-di-tert-butylcatechol (3,5-
DTBCH2) as the substrate, catecholase activity is studied by
monitoring UV−vis spectra because the oxidized product, 3,5-di-tert-
butylquinone (3,5-DTBQ), has a characteristic band at ca. 400 nm. To
check the ability of the complexes to behave as catalysts for catecholase
activity, a 10−5 M solution of the complex was treated with a 100-fold
concentrated solution of 3,5-DTBCH2, and the spectra were recorded
up to 60 min. Experiments were done in both DMF and MeOH. It
may be noted here that a blank experiment without a catalyst does not
show formation of the quinone up to 8 h in DMF and 6 h in MeOH.
1754(5)
2
1567.2(5)
4
Z
D/mg m−3
μ/mm−1
1.850
2.128
3.549
3.163
F(000)
980
976
θ range/deg
reflns collected
indep reflns
data/restraints/param
GOF
1.98−27.48
7897
1.88−27.88
28977
7897
3736
7897/6/503
1.095
3736/0/232
1.098
RESULTS AND DISCUSSION
■
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0329, 0.0695
0.0443, 0.075
0.0374, 0.0699
0.0618, 0.0749
The synthesis of the new manganese(III) alkali-metal ion
supramolecular boxes is achieved by the treatment of a
methanolic solution of the bisamido−bisphenoxy ligand H4L
with an alkali-metal hydroxide and a manganese(II) salt, as
detailed in the Experimental Methods section. The alkali-metal
hydroxide plays multiple roles in this new approach. It provides
the basic conditions required to achieve tetradeprotonation of
the ligand and oxidation of manganese(II) to manganese(III) in
the presence of oxygen, with these being the expected roles
benzoquinone (BQ; 2 × 10−4 mol, 21 mg) in deoxygenated and
deionized H2O was irradiated with light (in the 350−2500 nm range)
from a 200 W tungsten lamp for 24 h. The solution was connected to a
UV spectrophotometer through a peristaltic pump and a flow cell.
Quantitative measurements of O2 formed during irradiation were
performed using a dissolved-oxygen probe-type electrode (Crison
Oxi45P). In a typical experiment, deionized and deoxygenated H2O
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already known in classical manganese(III) chemistry. In our
route, the alkali-metal cation also has two additional functions:
it counteracts the negative charge of the [MnLD]− tectons, and
it also has a structural role, both of which are crucial for the
assembly of the final supramolecular boxes (see Chart 1).
Elemental analysis establishes their formulas as [KMnL-
(CH3OH)]2·CH3OH·H2O (1), [Rb2Mn2L2(CH3OH)(H2O)]·
3H2O (2), and [CsMnL(H2O)]2 (3), which are consistent with
the formation of the neutral dimeric species of the formula
[MIMnIIIL(D)]2, where L is the tetradeprotonated ligand, D
represents H2O or MeOH, and M is the alkali-metal ion (K for
1, Rb for 2, and Cs for 3). These formulations are in agreement
with molar conductivity measurements in 10−3 M DMF
solutions, which are at about 20−30 μS cm−1, typical of
nonelectrolyte complexes.20 Furthermore, other spectroscopic
techniques support such formulas and give insight into both the
solid and solution structure of the complexes.
from the H4 protons, while the resonances from δ −15.70 to
−21.50 arise from the H5 protons. Our experience shows that
signals in ortho positions to donor atoms are not observable for
these kinds of complexes by paramagnetic H NMR spectros-
copy.
Crystallographic Studies. The main crystallographic data
for H4L are summarized in Table 1, and bond lengths and
angles are listed in Table S1 in the SI.
The crystal structure of H4L reveals that it exists as discrete
molecules (Figure 1). The C8−N10 and N13−C14 distances
1
All of the complexes share some similar features in the IR
spectra (Figure S7 in the Supporting Information, SI),
exhibiting a strong band between 1605 and 1598 cm−1
characteristic of the ν(amide I, CO) stretching mode,
which is shifted 45−38 cm−1 lower with respect to the free
ligand, and a band 1565−1522 cm−1 of the ν(amide II)
stretching mode, which is shifted 13−8 cm−1 lower with respect
to the free ligand (Figure S3 in the SI). The band attributed to
the ν(C−O) mode is also shifted 7−11 cm−1 to higher
frequencies with respect to the free ligand. These data suggest
coordination of the ligand through the inner phenol oxygen and
the amide nitrogen atoms. Strong bands centered at ca. 3400
cm−1 can be assigned to a combination of the ν(O−H) modes
of coordinated and lattice H2O/MeOH, now present in the
complexes.
ESI-MS spectra registered in MeOH show peaks correspond-
ing to the fragment [MnL]+ for all of the complexes, indicating
coordination of the manganese ion to the deprotonated ligand.
Other signals could be assigned to [MMnL]+ and [M2Mn2L2]+
units, which could be attributed to the presence of dimeric
species (Figure S6 in the SI).
Figure 1. Molecular structure of H4L showing the atomic numbering
scheme. The hydrogen atoms are omitted.
of 1.342(4) and 1.340(4) Å, respectively, are practically
identical and consistent with C−N single bonding. The two
oxygen atoms O1 and O22 are forming phenolic groups, and
the observed O1−C2 and C21−O22 bond distances of
1.360(4) and 1.351(4) Å, respectively, are consistent with the
C−O single bonds.
The external C−O amide bond distances, C14−O15
1.257(2) Å and C8−O9 1.260(3) Å, are smaller than the C−
O phenolic distance and consistent with the C−O double
bonds.25 Intramolecular hydrogen bonds exist between the
hydrogen atoms H1 and H22 of phenolic oxygen atoms O1 and
O22 and the neighboring amidic oxygen atoms O9 and O15,
respectively: [H1···O9 1.57(4) Å, O1−H1···O9 2.502(3) Å;
O1−H1···O9 156(3)°] and [H22···O15 1.67 (4) Å, O22−
H22···O15 2.504(3) Å; O22−H22···O15 152(3)°] (Figure S5
in the SI).
The electronic spectroscopic data recorded are very similar
for all compounds 1−3, indicating that the manganese(III)
complexes are behaving as high-spin octahedral d4 systems,
probably suffering a significant Jahn−Teller distortion, which
affects the spectra and complicates its interpretation. A broad
shoulder at about 509−520 nm (ε = 380−700 M−1 cm−1) is
attributable to a d−d transition, while it is reasonable to assign
the broad band obtained at 480−490 nm (ε = 2600−3300 M−1
cm−1) to the phenolate → manganese(III) charge transfer. The
energy and intensity of the ligand-to-metal charge-transfer and
d−d transitions are in agreement with those reported for
related manganese(III) complexes.21,22
One of the two aromatic rings is arranged almost
perpendicular to the tetradentate cavity O22−N13−N10−O9.
The conformation of the free ligands in the solid state is of
particular interest in relation to that required in a metal
complex. This ligand, with a flexible spacer, produces these
types of open structures, as we have previously found in similar
ligands.26 Clearly, this conformation is not suitable for direct
coordination to a metal ion in the usual arrangement for a
tetradentate ligand. Therefore, significant rearrangement of the
molecule must occur, involving rotation about the C11−C12
bond to give a near-eclipsed conformation, for possible
mononuclear complexation. The amide nitrogen atom is a
strong field ligand favoring N-coordination of the planar
tetradentate ligand, although rare examples of coordination via
the oxygen atom of the deprotonated amide group and
deviation from planarity have been encountered.27
Slow evaporation of the mother liquors allowed us to obtain
brown crystals of 1−3, from which the molecular structure was
determined by X-ray crystallography. The main crystal data and
structure refinement details are collected in Tables 1 and 2. The
crystal structures of the compounds reveal the existence of
dimeric supramolecular boxes. Selected bond lengths and
angles for these structures are collected in Tables 3 and 4.
Different drawings showing the crystal structure of these
supramolecular boxes are shown in Figures 2−4. Figure 2
Paramagnetic 1H NMR studies of the complexes were
registered using DMSO-d6 as the solvent, and the data serve to
substantiate the formation of manganese(III) complexes. The
data interpretation was based on previous findings for
manganese(III) complexes made by Pecoraro and co-workers23
and in our own results.24 The data are collected in the
Experimental Methods section, and Figure S8 in the SI shows
one of these spectra. The spectra contain two upfield proton
resonances, outside the diamagnetic region (δ 0−14), due to
the isotropic shifting of the ligand protons for high-spin
manganese(III) complexes in an octahedral field. The signals
must arise from the H4 and H5 protons of the aromatic
phenoxy rings. The signals between δ −20.73 and −30.52 arise
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a
Table 3. Important Bond Lengths (Å) of Compounds 1−3
1
2A
2B
3
Mn−Op
Mn−Na
1.870(4)
1.878(5)
1.927(6)
1.933(6)
1.874(3)
1.894(3)
1.939(3)
1.960(3)
1.863(3)
1.883(3)
1.925(3)
1.942(3)
1.863(3)
1.889(3)
1.951(3)
1.955(3)
Mn−Oax
Mn···Mn
M···M
Mn−O5 2.230(6)
6.514
Mn2A−O26w 2.175(3)
Mn2B−O27 2.212(3)
Mn−O111 2.164(3)
6.746
6.667
4.731
4.472
4.870
Mn···M
M−O
3.788, 4.177
K−O 2.690−3.200
Mn2A−Rb1A 3.889, Mn2A−Rb1B 4.2735
Rb1A−O 2.873−3.552
Mn2B−Rb1B 3.836, Mn2B−Rb1A 4.344
Rb1B−O 2.864−3.431
4.1417
Cs−O 3.107−3.460
a
Op = Ophenolic; Na = Namidic; Oax = Oaxial; the axial oxygen atom is methanolic for 1 (O5) and 2B (O27); the axial oxygen atom is from H2O for 3
(O111) and 2A (O26w).
a
Table 4. Important Bond Angles (deg) of Compounds 1−3
Compound 1
Compound 2A
Compound 2B
Compound 3
O30−Mn1−O10
O30−Mn1−N39
O10−Mn1−N39
O30−Mn1−N19
O10−Mn1−N19
N39−Mn1−N19
90.5(2)
92.4(2)
171.1(3)
170.8(2)
91.9(2)
84.0(2)
O30−Mn1−O41
O10−Mn1−O41
N39−Mn1−O41
N19−Mn1−O41
O30−Mn1−K2
O10−Mn1−K2
92.7(2)
90.6(2)
97.7(2)
96.1(2)
45.69(15)
47.16(14)
O3−Mn2−O24
O3−Mn2−N12
O24−Mn2−N12
O3−Mn2−N15
O24−Mn2−N15
N12−Mn2−N15
89.67(12)
92.69(13)
172.67(15)
167.00(13)
91.55(13)
84.61(14)
O3−Mn2−O26
O24−Mn2−O26
N12−Mn2−O26
N15−Mn2−O26
O3−Mn2−Rb1
O24−Mn2−Rb1
95.78(12)
89.43(13)
97.23(14)
97.18(13)
46.35(9)
45.42(9)
O3−Mn2−O24
O3−Mn2−N12
O24−Mn2−N12
O3−Mn2−N15
O24−Mn2−N15
N12−Mn2−N15
90.14(12)
92.76(13)
175.20(14)
167.09(14)
92.25(13)
84.09(14)
O3−Mn2−O27
O24−Mn2−O27
N12−Mn2−O27
N15−Mn2−O27
O3−Mn2−Rb1
O24−Mn2−Rb1
94.13(12)
86.90(12)
96.70(14)
98.67(13
46.78(9)
46.41(9)
O431−Mn2−O131
O431−Mn2−N1
O131−Mn2−N1
O431−Mn2−N4
O131−Mn2−N4
N1−Mn2−N4
89.67(12)
166.66(14)
90.90(13)
92.00(13)
165.93(14)
84.30(14)
O431−Mn2−O111
O131−Mn2−O111
N1−Mn2−O111
N4−Mn2−O111
O431−Mn2−Cs1
O131−Mn2−Cs1
98.59(12)
98.59(12)
94.69(13)
100.72(13)
44.70(8)
46.26(8)
a
Op = Ophenolic; Na = Namidic; Oax = Oaxial
.
shows the coordination environment around the manganese
center. In all cases, the geometry around the manganese(III)
ions can be described as distorted square pyramidal. The
coordination sphere around each manganese center comprises
the planar bisamido ligand, which is tightly bound to the metal
ion through the inner N2O2 compartment by the Namido and
O
phenol atoms. A fifth coordination position is completed with a
solvent molecule (MeOH for 1, H2O for 3; in the case of 2, one
manganese of a dimer is bonded to a H2O molecule and the
other manganese to a MeOH molecule). These [MnLD]−
tectons are connected through the alkali-metal ions (M) to give
the neutral dimeric [MMnL(D)]2 boxes.
The structure of complex 2 (Figure 3) was found to consist
of two different coordination environments for the manganese
ion of the dimer because different solvent molecules (H2O or
MeOH) are bonded to the manganese ion.
Figure 2. Coordination environment around the manganese metal
center in 1, showing the square-pyramidal geometry for this ion.
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crystallize together. The supramolecular structures induced by
potassium(I), rubidium(I) and cesium(I) arise from multiple
M−O−Mn bridges. For instance, in the case of 3 (see Figure
4), the cesium cation is surrounded by six oxygen atoms and
occupies the space between two [MnLD]− tectons. This
cesium(+) ion is coordinated by two phenoxo oxygen atoms
(which are also bonded to a manganese ion), a phenoxo oxygen
atom and a methanolic oxygen atom, both of which are bound
to the second manganese ion and two amido oxygen atoms of
the neighboring units. All of the Cs−O distances are in the
range 3.107−3.460 Å. The relatively broad ranges of the Cs−O
distances (and, in general, MI−O distances) are associated with
increased structural flexibility and an increased capacity for
close contacts between the alkali-metal ion and aromatic
rings.31 The Mn−Cs distance of 4.1417 Å is similar to those
previously reported in the literature.32 Intermolecular amido
interactions connect the supramolecular boxes through cesium
ions to form a 3D aggregate.
The solved crystallographic structures for 1 and 2 again show
coordination of the manganese(III) ion to the inner N2O2
compartment of the tetraanionic L4 ligand. The supra-
−
Figure 3. Molecular structure for the supramolecular box 2.
molecular boxes are induced by the potassium (in 1) or
rubidium (in 2) cation, which is able to establish a net of M−
O−Mn bridges. The configuration of these boxes is depicted in
Figure 5.
The size of the cavity inside the box is slightly different
depending on the alkali-metal ion and increases from potassium
to cesium as the size of the ion increases (see Figure 5). Thus,
the distances between the manganese ions range from 6.51 Å
(in 1) to 6.75 Å (in 3). The alkali metal−alkali metal distances
also increase from 4.47 Å (in 1) to 4.87 Å (in 3).
Magnetic Studies. The values for the magnetic moments
at room temperature for 1 and 2 are very close to the spin-only
value of 4.9 μB, as expected for a high-spin magnetically diluted
d4 manganese(III) ion, and also in agreement with the large
distances (6.5−6.7 Å) between the manganese ions shown by
the molecular structures described above. The magnetic
moment at room temperature for 3 is 4.6 μB, slightly lower
than the expected value for an isolated high-spin manganese-
(III) ion.
Magnetic studies at variable temperature have been done for
1−3 (between 300 and 2 K) in a 0.8 T applied magnetic field.
Antiferromagnetic behavior has been found for the three
complexes.
Figure 4. Stick diagram of the supramolecular box 3 (cesium cation in
green, manganese ion in orange, oxygen in red, nitrogen in blue, and
carbon in gray).
The cocrystallization phenomenon is rather abundant in
manganese coordination chemistry,28−30 and it points out the
ability of these complexes to evolve into different structures in
solution, driven by slight variations in the media. Cocrystalliza-
tion usually arises when the initial and final complexes
The magnetic behavior for 3 is shown in Figure 6 as the
thermal variation of the χMT versus T plot. χM is the molar
magnetic susceptibility per Mn2 unit corrected for the
Figure 5. Supramolecular box core for 1 (left), 2 (center), and 3 (right) showing the intermetallic distances between the manganese and alkali-metal
ions (values in angstroms) depicted in the middle of the dashed line connecting the pair of ions (Op = phenolic oxygen atom; Om = methanolic
oxygen atom; Ow = water oxygen atom).
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The best fit to the experimental data (Figure 7) was obtained
with the latter model with a formula that includes some
Figure 6. Temperature dependence of χMT on T for 3. The black line
corresponds to observed data and the red line to the best data fit, with
χM being the molar magnetic susceptibility.
Figure 7. Temperature dependence of χMT on T for 1. The blue
circles correspond to observed data and the red line to the best data fit,
with χM being the molar magnetic susceptibility.
diamagnetism of the ligands.33 It decreases smoothly from 5.22
cm3 mol−1 K at 300 K to 4.37 cm3 mol−1 K at 100 K. Upon
further lowering of the temperature, χMT decreases to 0.23 cm3
mol−1 K at 2 K. This behavior suggests the operation of
antiferromagnetic interactions between manganese(III) ions.
According to the X-ray structure of 3, a simple system of two
exchange-coupled manganese(III) (S = 2) cations was
considered (H = −2JSMn1SMn2) to fit these experimental data
as the first step.34 Attempts to fit the data with this simple
model yielded only average results with discrepancies at lower
temperatures. The introduction of a zero-field-splitting term D,
common for both metal sites (D1 = D2 = D), brought about
significant improvements to the fit. The estimation of the
coupling constant was performed using the Hamiltonian H =
impurity (f) arising from manganese(III) monomeric species. A
satisfactory fit for 1 was obtained with J = −7.0 cm−1, g = 1.95,
D = −8 cm−1 (assumed to be the same for dimers and
monomers), and f = 0.24. The best fit of the experimental data
for 2 is achieved with the following parameters: J = −1.10 cm−1,
g = 1.95, D = −8 cm−1 for dimers and D = −16 cm−1 for
monomers, and f = 0.30. Figures S13 and S14 in the SI collect
the temperature dependence of the magnetic data for 2. The
discrepancy between the magnetic susceptibilities for these
complexes would arise from the presence of different amounts
of mononuclear paramagnetic manganese(III), and J values for
1 and 2 should be taken with caution.
Catalytic Studies. The ability to vary the size of the cavities
in these new redox-active materials may provide access to
tunable frameworks that can be matched with the desired
application. We believe that the approach reported herein could
open new perspectives for the design of selective catalysts in
different redox processes.
These supramolecular boxes incorporate the redox-active
manganese(III) ion, and they could be used as catalysts for
different processes. For example, catalase-like activity could be
one of the most interesting applications. Manganese catalases
are able to disproportionate H2O2 into H2O and O2. These
enzymes protect organisms against oxidative stress by removing
appreciable levels of H2O2, which is a byproduct of
respiration.38
Excessive concentrations of reactive oxygen species can have
serious adverse effects on living systems, including peroxidation
of membrane lipids, hydroxylation of nucleic acid bases, and
oxidation of sulfhydryl groups and other sensitive moieties in
proteins.39 If uncontrolled, mutations and cellular death
result.40,41 Owing to their potencial application as therapeutic
agents against oxidative stress, various biomimetic mononuclear
−JSMn1SMn2 + DMn[S2
+ S2Mn,z ], which includes the axial
Mn,z1
1
single-ion local anisotropy of the manganese(III) ions,
according to the solved crystal structure.35 This model
reproduced the experimental curve very well (Figure 6), except
for small disagreements at low temperature, which may be due
to the simultaneous operation of intermolecular interactions
between adjacent dinuclear molecules through weak contacts.
These intermolecular interactions are expected to be weaker
than J because of the significantly greater distances involved.36
In fact, because the introduction of an intermolecular
interaction according to the mean-field Hamiltonian J =
−2zJ⟨Sz⟩Sz affords a marginal improvement to the fit of 3,
only zero-field splittings were considered in order to avoid
overparametrization.
The best fit of the experimental susceptibility data with the
above Hamiltonian led to the following parameters: J = −4.6
cm−1, D = 1.22, the mean Lande
́
factor g = 1.95, and an
agreement factor R (=∑[(χT)obs − (χT)cal]2/∑(χobsT)2) of 1.3
× 10−4. Compound 3 is characterized by large Mn···Mn
distances of about 6.75 Å. The small absolute value of J agrees
with overlap through the 2p oxygen orbitals. The J value,
showing small antiferromagnetic behavior, and the D value, are
similar to those observed in other manganese(III) dimers.37
Attempts to fit the magnetic data for 1 and 2 with the
previous model using the simple system of two exchange-
coupled manganese(III) (H = −2JSMn1SMn2) or the Hamil-
manganese complexes have been selected for pharmaceutical
•−
uses as O2
scavengers,42 and a number of dinuclear
manganese-based complexes has been investigated as low-
molecular-weight catalytic scavengers of H2O2.43
We evaluated the disproportionation of H2O2 catalyzed by
1−3 by volumetric determination of the evolved O2. The good
solubility of these supramolecular boxes in different solvents
facilitates these experiments in biological media. The
tonian with the axial single-ion local anisotropy of the
2
manganese(III) ions (H = −JSMn1SMn2 + DMn(SMn,z
+
1
SMn,z 2) were unsuccessful to simulate the experimental results.
1
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experimental setup was evaluated by the measurements for the
catalytic decomposition of H2O2 (2.5 × 10−3 mol) with MnO2:
ca. 30 mL of O2 evolution was measured by a buret, which
agrees with the calculated value based on the assumption that
catalytic decomposition of H2O2 to H2O and O2 takes place.
Table 5 collects the results of the catalase activities. All three
complexes showed significant rates of catalase activity, with
Table 5. Catalytic Activity for 1−3 in the Catalase-like
Function, Peroxidase Action, and Capacity of Oxidation of
VA
catalase activity (in
MeOH)
Figure 8. Evolution of the UV spectra during the water photolysis
experiments for 1, showing a decrease of the 246 nm band
(disappearance of BQ) and an increase of the 290 nm band
(formation of H2BQ).
peroxidase
activity
oxidation
a
complex
23 °C
667
36 °C
of VA
b
1
turnovers
1
2
6
3
5
3
667
100
1
2
% activity
turnovers
% activity
turnovers
% activity
100
162
25
2
1
1
0.5
0.3
0.5
nob
nob
nob
2
3
368 22
55
384
58
7
8
4
cation ABTS•+,45 which presents characteristic absorption
bands at 415, 650, 735, and 815 nm (Figure S17 in the SI).
The extent of the reaction can be measured quantitatively at λ =
650 nm because ε = 12000 M−1 cm−1 has been determined.
The oxidation potential of ABTS to provide ABTS•+ is
invariable over a wide range of pH values. In the absence of
the complex, a solution of ABTS and H2O2 is stable for several
hours without showing any formation of ABTS•+. Complexes
1−3 do not show significant peroxidase-like activity because the
UV absorbances corresponding to the ABTS•+ radical cation
are negligible in the assays with these complexes.
Oxidation of guaiacol (2-methoxyphenol) by manganese
peroxidases in the presence of H2O2 produces tetraguaiacol.
This process can be monitored by absorbance at 470 nm.46
Complexes 1−3 show a significant rate of guaiacol oxidation
because their UV−vis spectra is hardly altered during the
guaiacol oxidation assay. In contrast, monomeric manganese
complexes show relevant peroxidase activity. Figures S18 and
S19 in the Supporting Information compare the results of this
oxidation assay with 1−3 and with a monomeric [MnL(H2O)-
(OAc)](H2O) complex (where L = N,N′-bis(3-OMe-salicyli-
dene)-1,2-diphenylenediamine), previously reported by us.14
An increase in the tetraguaicol absorbance at 470 nm is clearly
observed by catalysis with this monomeric system but not with
complexes 1−3.
110
17
b
a
nob = not observable. Turnover numbers after 60 min, except for
the catalase activity of 1, which catalyzes the decomposition of 100%
H2O2 at 36 °C in 20 min in the cited experimental conditions.
100% H2O2 decomposition achieved in 20 min at 36 °C upon
using compound 1 with a peroxide/catalyst ratio of 667.
Complex 1 appears to be stable over 1200 catalytic turnovers.
The time course of oxygen production is similar for each of the
complexes. The reaction proceeds quickly during the first 5−10
min, and over time, the total amount of oxygen evolved
approaches an asymptotic limit.
Complexes 1−3 also show catalytic activity in H2O oxidation
and subsequent O2 production under light irradiation and in
the presence of a hydrogen acceptor (BQ). The molecule of
H2O is a small substrate that is able to enter into the cavity of
the supramolecular box. Light absorption at 425 nm in an
aqueous solution causes a π* ← n transition, and this is
responsible for hydrogen abstraction from H2O by optically
excited BQ (BQ*).44 The course of the water photolysis
experiments was followed in two ways: quantitative oxygen
evolution and variation of the electronic spectrum of quinone
during photolysis.24 The concentration of O2 in the solutions
during the experiments increased from about 3% dissolved
oxygen, to 13.1% for photolysis catalyzed by complex 1, to 10%
for photolysis catalyzed by complex 2, and to 10.2% for
photolysis catalyzed by complex 3. The reduction of BQ to
hydroquinone (H2BQ) can be easily followed by UV
spectroscopy (Figure 8). BQ in H2O has a major absorption
at 246 nm (ε = 2.2 × 104 M−1 cm−1), which decreased during
the experiments, while a characteristic H2BQ peak at 290 nm
developed. The time courses of the absorbances at 246 and 290
nm are collected in Figures S15 and S16 in the SI.
The peroxidase activity using the bulky ABTS substrate of
the same monomeric [MnL(H2O)(OAc)](H2O) complex and
employing an analogous experimental procedure gives a
conversion rate of about 33%, 30-fold higher than that for
the activity measured for 1−3. Nevertheless, the catalase
activity reported for this monomeric manganese complex is
lower than that for 1−3. A comparison of the catalytic behavior
of compounds 1−3 and monomeric complexes reinforces the
selectivity of the catalytic processes driven by the supra-
molecular boxes.
Additional studies highlighted how the supramolecular box
structure of these compounds increases the selectivity of the
catalytic processes. Thus, we demonstrated the lack of activity
of these complexes in peroxidase action upon using bulky
organic substrates such as ABTS [the diammonium salt of 2,2′-
azinobis(3-ethylbenzothiazoline-6-sulfonic acid)], guaiacol, or
the lignin analogue VA.
VA is a commonly used lignin model.47 Oxidative
degradation of VA by manganese peroxidases yields veratralde-
hyde, which absorbs strongly at 310 nm (ε = 9300 M−1
cm−1).48 This reaction may then be followed by UV
spectroscopy through variation of the absorbance at this
wavelength. We have also purified the resulting mixture by
column chromatography using a procedure already reported by
us.49 The main product was characterized by NMR spectros-
copy and identified as VA [1H NMR (300 MHz): δ 7.02 (s,
ABTS is colorless, and it reacts readily with H2O2 in the
presence of a peroxidase catalyst to yield a stable green radical
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1H), 6.79 (d, 1H), 6.53 (d, 1H), 4.61 (s, 2H), 3.83 (s, 6H)].
Veratraldehyde was not detected by UV or NMR spectroscopy.
Complexes 1−3 do not show catecholase activity using 3,5-
DTBCH2 as a different bulky substrate. The catecholase activity
is the catalytic process of oxidation of o-diphenolcatechol to
3,5-DTBQ coupled with 2e−/2H+ reduction of O2 to H2O.50
Regarding manganese compounds, some mononuclear, dinu-
clear, and oligonuclear systems are known to act as catalysts for
catecholase activity.51 In order to contrast this activity, we have
also carried out the catecholase experiments using a di-μ-
oxomanganese dimer, [Mn(3,5-Brsalpn)O]2, previously re-
ported by us.52 UV−vis spectra of complexes 1−3 and
[Mn(3,5-Brsalpn)O]2 in the presence of 3,5-DTBCH2 are
shown in Figure S20 in the SI. A band with a maximum in the
region 380−390 nm, corresponding to the formation of 3,5-
DTBQ, is only generated using [Mn(3,5-Brsalpn)O]2 as the
catalyst. The plot of variation of the absorbance at 380 nm
versus time for 1−3 in the presence of 3,5-DTBCH2 (Figure
S21 in the SI) clearly shows the lack of catecholase activity for
these complexes.
The square-pyramidal geometry of the manganese ion in
complexes 1−3 is optimal for catalytic activity because the
potential catalyst has a vacancy in the coordination sphere
where the substrate molecule may be located. Our findings
show that these compounds behave as active catalysts for water
photolysis and for disproportionation of H2O2, but they do not
catalyze peroxidase action upon using bulky organic substrates
such as ABTS, guaiacol, or VA. Catecholase activity is not
found for these supramolecular boxes. A detailed kinetic study
of this catalytic process merits further investigation in order to
propose the reaction mechanism for these catalase mimics and
also to determine whether the fact that the bulky substrates are
unable to enter into the cavity of the supramolecular box is the
key factor in their catalytic performance.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: misabel.fernandez.garcia@usc.es.
*E-mail: marcelino.maneiro@usc.es.
Present Address
§M.J.R.-D.: Dipartamento di Farmacia, Universita
Pisa, 56126 Pisa, Italy.
́
degli Studi di
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Spanish Ministerio de Ciencia e
Innovacion
́
(Grant CTQ2010-19191) and Xunta de Galicia
(Grant 10PXIB262132PR) is acknowledged.
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■
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CONCLUSION
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format, selected bond
(13) Lorena-Cortez, M.; Pallarola, D.; Ceolín, M.; Azzaroni, O.;
Battaglini, F. Chem. Commun. 2012, 48, 10868−10870.
1
distances and angles for H4L, H/13C NMR spectra for H4L,
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