Synthesis of mixed-valence hexanuclear Mn(II/III) clusters from its Mn(II) precursor: Variations of catecholase-like activity and magnetic coupling

Article abstract of DOI:10.1039/c5dt00709g

One Mn(ii) coordination polymer, [Mn(o-(NO₂)C₆H₄COO)₂(pyz)(H₂O)]_n (1), has been synthesized and oxidized with n-Bu₄NMnO₄ in non-aqueous media to two mixed-valence hexa

Full text of DOI:10.1039/c5dt00709g

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Graphical Abstract
Paramita Kar,Yumi Ida, Takuya Kanetomo, Michael G. B. Drew, Takayuki Ishida
and Ashutosh Ghosh
_
________________________________________________________________________
A pyrazine bridged 1D coordination polymer of Mn(II) has been oxidized by n-Bu NMnO4.
4
to hexanuclear Mn(II/III) complexes. Their catecholase-like activities and magnetic
interactions have been studied.

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Journal Name
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Synthesis of mixed-valence hexanuclear Mn(II/III) clusters from its
Mn(II) precursor: Variations of catecholase-like activity and magnetic
coupling
a
b
b
c
b
Paramita Kar, Yumi Ida, Takuya Kanetomo, Michael G. B. Drew, Takayuki Ishida * and Ashutosh
a*
5
Ghosh
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
One Mn(II) coordination polymer, [Mn(o-(NO )C H COO) (pyz)(H O)] (1) has been synthesized and oxidized by n-Bu NMnO in non-
2
6
4
2
2
n
4
4
III
II
aqueous
media
to
two
mixed-valence
hexanuclear
Mn(II/III)
complexes
[Mn Mn O (pyz)0.61/(MeOH)0.39(o-
2 4 2
III
2
II
1
0
(NO )C H COO) ∙(H O)∙{(CH ) CO} ]∙(CH ) CO (2) and [Mn Mn O (pyz)0.28/(MeCN)3.72(o-(NO )C H COO) ∙(H O)] (3) (where
2
6
4
10
2
3 2
2
3 2
4
2
2
6
4
10
2
pyz = pyrazine). All three complexes were characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction
analyses, and variable-temperature magnetic measurements. The structural analyses reveal that complex 1 is comprised of linear chains
of pyz bridged Mn(II), which are further linked to one another by syn-anti carboxylate bridges, giving rise to a two-dimensional (2D) net.
III
II
Complexes 2 and 3 feature mixed valence [Mn Mn ] units in which each of the six manganese centres reside in an octahedral
2
4
1
5
environment. Apart from the variations in terminal ligands (acetone for 2 and acetonitrile for 3), the complexes are very similar. Using
,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, the catecholase-like activity of the complexes has been studied and it is found that
the mixed valent Mn complexes (2 and 3) are much more active towards aerial oxidation of catechol compared to the Mn(II) complex
3
6
(
1). Variable-temperature (1.8-300 K) magnetic susceptibility measurements showed the presence of antiferromagnetic coupling in all
three complexes. The magnetic data have been fitted with a 2D quadratic model derived by Lines, giving the exchange constant J/k = –
B
2
0
0.0788(5) K for 1. For 2 and 3, antiferromagnetic interactions within the Mn cluster have been fitted with models containing three
6
exchange constants: J /k = –70 K, J /k = –0.5 K, J /k = –2.9 K for 2 and J /k = –60 K, J /k = –0.3 K, J /k = –2.8 K for 3.
A
B
B
B
C
B
A
B
B
B
C
B
II
III
10+
Introduction
and finally [Mn
reductive
[
4
Mn
2
O
2
]
species can be obtained by a
complexes containing the
45
cleavage
of
Among the transition metal ions, manganese-based
coordination compounds have received considerable interest in
the fields of supramolecular chemistry and crystal engineering
not only for their remarkable magnetic properties, but also for
their rich biochemistry and versatile catalytic activities. The
III
IV
12 16+
10
Mn 8Mn O ] core.
4
2
3
3
4
5
0
5
0
The hexanuclear mixed-valence carboxylate based Mn6
clusters possess the following common features: (i) the central
core contains six manganese centres, two Mn(III) and four Mn(II)
1
2
,3
4
II
magnetic properties of such species are fascinating as the number 50 ions; (ii) the metal atoms are located in the vertex of two Mn -
2
III
and symmetry of the magnetic orbitals of the metal ion along
with their possible overlap through the bridging ligand account
for the ferro- or antiferromagnetic nature of the magnetic
Mn tetrahedra sharing their Mn(III)−Mn(III) edge with a μ4
2
oxygen atom in the centre of each tetrahedron and there are four
coordinated solvent molecules to four peripheral Mn(II) centres a
distorted tetrahedral disposition. Recently, our group has replaced
5
coupling. In this regard, the hexanuclear mixed-valence
carboxylate based clusters [Mn O (RCO ) L ] deserve special 55 these solvent molecules by linear linker pyrazine to produce a
6
2
2 10 4
mention for their magnetic properties at very low temperatures
which have given rise to a specific family of single-molecule
magnets (SMMs). Several approaches leading to the isolation of
mixed-valence Mn unit based 3D diamondoid multifunctional
6
(antiferromagnetism, gas adsorption and catechol oxidation)
6
III
II
framework [Mn Mn O (pyz) (C H CH COO) ] . The species
2 4 2 2 6 5 2 10 n
II
III
10+
hexanuclear manganese complexes with the [Mn Mn 2O2]
was obtained by aerial oxidation of its Mn(II) precursor i.e.
4
7
-10
II
II
11
core are known.
The first one is based on Mn (O CR)
oxidation with oxygen, or MnO ¯. The second involves
60 [Mn (pyz)(C H CH COO) ]. Our group has also reported a
2
2
6
5
2
2
7
a
7b
4
mixed-valence
hexanuclear
complex
II
III
6+
III
II
reduction and subsequent coupling of trinuclear [Mn Mn 2O]
[Mn Mn O (hmt) (OBz) ] (hmt = hexamethylenetetramine
2 4 2 4 10
II
III
10+
8
species resulting in the formation of the [Mn Mn 2O2] core.
and OBz = benzoate) which in acetonitrile medium spontaneously
forms a self-assembled vesicular structure that encapsulated
65 organic dye molecules, and also exhibits substantial catecholase-
4
II
III
10+
The third method involves construction of the [Mn Mn 2O2]
4
III
8+
9
unit by reduction of a species containing the [Mn 4O2] core
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1
2
like activity. These results have inspired us to generate new
when the solution turned to deep brown. After half an hour, it was
Mn6 based materials. To serve this purpose, we use here o- 55 filtered. The resulting clear filtrate gave a brown solid on
nitrobenzoate as carboxylate group. However, o-nitrobenzoate
and pyz based Mn(II) precursors cannot be aerially oxidized to
standing overnight at room temperature. The brown precipitate
was collected by filtration and dissolved in acetone (30 mL) and
brown colored plate-like single-crystals of complex 2 suitable for
X-ray diffraction were obtained by slow evaporation of the
filtrate after several days.
5
mixed-valence Mn clusters unlike their phenylacetate analogues,
6
rather, it requires an oxidizing agent e.g. MnO ¯.
4
6
0
Herein, we report the crystal structures, magnetic properties
and catecholase-like activities of one Mn(II) coordination
polymer, [Mn(o-(NO )C H COO) (pyz)(H O)] (1) and two
Complex 3 was obtained by following a procedure similar to that
of 2, the only difference being that the brown precipitate was re-
dissolved in acetonitrile (30 mL).
2
6
4
2
2
n
1
1
2
2
0
5
0
5
mixed-valence
hexanuclear
Mn(II/III)
complexes
III
II
[
(
[
(
Mn Mn O (pyz)0.61/(MeOH)0.39(o-
2
4
2
Complex 2: Yield: Yield: 0.271 g; 73%. Anal. Calcd. for
NO )C H COO) ∙(H O)∙{(CH ) CO} ]∙(CH ) CO
(2) and
2
III
6
4
II
10
2
3 2
2
3 2
65
C80.98H62.08Mn N11.15O (2259.87): C, 43.04; H, 2.77; N, 6.91
46.17
6
Mn Mn O (pyz)0.28/(MeCN) (o-(NO )C H COO) ∙(H O)]
2
4
2
3.72
2
6
4
10
2
−1
Found: C, 43.11; H, 2.91; N, 6.82. IR (KBr pellet, cm ): 1596
3) (where pyz = pyrazine) which have been synthesized by
ν (COO), 1395 ν (COO).
as
s
oxidation of 1 by n-Bu NMnO in non-aqueous medium i.e.
4
4
methanol and then the oxidized products are recrystallized from
acetone (for 2) and acetonitrile (for 3). In complex 1, linear
Complex 3: Yield: 0.242 g; 68%. Anal. Calcd. for
C78.45H52.23Mn N14.23O42 (2195.84): C, 42.91; H, 2.40; N, 9.08
6
−1
chains of pyz bridged Mn(II) are further linked to one another by 70 Found: C, 42.89; H, 2.47; N, 9.11. IR (KBr pellet, cm ): 1593
syn-anti carboxylate bridges, giving rise to a two-dimensional ν (COO), 1389 ν (COO).
as
s
III
II
(2D) net. Complexes 2 and 3 feature mixed valence [Mn Mn ]
2
4
Alternative Method for the Synthesis of Compounds 2 and 3.
To methanolic solution (20 mL) of Mn(o-
(NO )C H COO) •3H O (2.025 g, 5 mmol) was added pyz (0.400
units. The catalytic activities and the antiferromagnetic exchange
couplings of 2 and 3 are much higher than those of complex 1. It
is notable that although the synthesis and structure of
a
2
6
4
2
2
1
3
[
Mn O (RCOO) ] clusters are quite common, we report here 75 g, 5 mmol) to give a pale yellow solution. This was stirred while
6 2 10
for the first time the generation of mixed-valent Mn clusters by
solid n-Bu NMnO (0.57 g, 1.58 mmol) was added in small
4 4
6
oxidizing the corresponding structurally characterized 1D
portions over approximately half an hour; the solution turned
deep brown. Overnight storage gave a brown precipitate, which
was collected by filtration and dissolved in acetone (30 mL) (for
80 2) and in acetonitrile (30 ml) (for 3). Brown coloured plate-like
single crystals of complexes 2 and 3 suitable for X-ray diffraction
were obtained by slow evaporation of the corresponding filtrates
after several days.
coordination polymer (complex 1) using n-Bu NMnO as oxidant
4
4
and the corresponding changes in the magnetic and catalytic
properties.
3
0
Experimental Section
Mn(o-(NO )C H COO) •H O was synthesized by the procedure
2
6
4
2
2
1
4
described previously. All other chemicals purchased were of
reagent grade and used without further purification.
Physical measurements
8
9
9
5
0
5
Elemental analyses (carbon, hydrogen, and nitrogen) were
performed using a Perkin-Elmer 240C elemental analyzer. IR
spectra in KBr (4500−500 cm ) were recorded using a Perkin-
Synthesis of [Mn(o-(NO )C H COO) (pyz)(H O)] (1).
methanolic (10 mL) solution of pyz (0.160 g, 2 mmol) was added
A
2
6
4
2
2
n
3
5
−1
to a solution of Mn(o-(NO )C H COO) •H O (0.405 g, 1 mmol)
2
6
4
2
2
Elmer RXI FT-IR spectrophotometer. Electronic absorption
in 5 mL of methanol. The resulting mixture was refluxed for
about 2 h, cooled, and filtered. The resulting clear filtrate gave a
pale yellow solid on standing for 1−2 days at room temperature.
spectra (1000−200 nm) were recorded in CH CN with a Hitachi
3
U-3501 spectrophotometer. Magnetic susceptibilities on
polycrystalline samples were measured on a QuantumDesign
SQUID magnetometer (MPMS-7) at applied magnetic fields of
4
0
The yellow solid was re-dissolved in CH OH and filtered. Yellow
3
colored plate-like single-crystals suitable for X-ray diffraction
were obtained by slow evaporation of the mother liquor after
several days.
5
000 Oe in a temperature range of 1.8–300 K. The magnetic
response was corrected with diamagnetic blank data of the
sample holder obtained separately. The diamagnetic contribution
of the sample itself was estimated from Pascal’s constant.
Complex 1: Yield: 0.361 g; 76%. Anal. Calcd. for
4
5
C H MnN O (485.27): C, 44.55; H, 2.91; N, 11.55 Found: C,
1
8
14
4
9
Crystal data collection and refinement
−1
4
4.31; H, 2.97; N, 11.39. IR (KBr pellet, cm ): 1612, 1531
ν (COO), 1405, 1365 ν (COO).
Suitable single crystals of each complexes were mounted on a
as
s
Bruker SMART diffractometer equipped with
a graphite
III
II
Synthesis
of
[Mn Mn O (pyz)0.61/(MeOH)0.39(o-
2 4 2
1
00 monochromator and 3405, 16903, 16277 independent reflection
data for 1, 2 and 3 were collected with Mo-Kα (λ = 0.71073 Å)
radiation at 293K. The crystals were positioned at 60 mm from
the CCD. 360 frames were measured with a counting time of 10
s. The non-hydrogen atoms were refined with anisotropic thermal
05 parameters. Hydrogen atoms were placed in idealized positions
(
NO )C H COO) ∙(H O)∙{(CH ) CO} ]∙(CH ) CO (2) and
[Mn 2Mn O (pyz)0.28/(MeCN)3.72(o-
4 2
NO )C H COO) ∙(H O)] (3). To a methanolic solution (20
mL) of complex 1 ( 2.43 g, 5 mmol) solid n-Bu NMnO (0.57 g,
2
III
6
4
II
10
2
3 2
2
3 2
5
0
(
2 6 4 10 2
4
4
1
.58 mmol) was added in small portions with constant stirring
1
2
|
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and their displacement parameters were fixed to be 1.2 times
larger than those of the attached non-hydrogen atom. Successful
convergence was indicated by the maximum shift / error of 0.001
oxidation of the complex did not take place in this medium. We
changed solvents from acetonitrile to dichloromethane, acetone
and ethanol but in no case did aereal oxidation occur. By
for the last cycle of the least squares refinement. Absorption 35 comparison of these results with those of Mn(II) benzoate, it is
1
5
5
0
5
corrections were carried out using the SADABS program.
obvious that introduction of the -NO group to the phenyl ring
2
Structures 2 and 3 showed disorder in the solvent molecules
including those bound to the metal. In 3, one of the nitro groups
was disordered over two positions. In such cases, occupation
factors were refined and distance constraints were used where
prevents the aerial oxidation of the Mn(II) complex. Therefore, n-
Bu NMnO , a reagent well known for oxidation of Mn(II) species
4
4
to mixed-valence entity in non-aqueous solvents, was used. To
crystallize the brown colored oxidized amorphous solid obtained
from methanol, it was dissolved in acetonitrile, dichloromethane,
acetone and ethanol and obtained single crystals of complexes 2
and 3 from acetone and acetonitrile, respectively.
40
1
6
1
necessary. All calculations were carried out using SHELXS 97,
1
7
18
19
SHELXL 97,
PLATON 99,
ORTEP-32
and WinGX
2
0
systemVer-1.64. The three structures were refined using
Shelxl97 on F to R1 0.0690, 0.0574 0.0442; wR2 0.1892,
.1282, 0.1049 for 2842, 10549, 12275 data with I>2σ(I). Data
collection and structure refinement parameters and
crystallographic data for the three complexes are given in Table
1
7
2
0
1
1
.
Results and Discussion
2
2
3
0
5
0
Synthesis. The Mn(II) coordination polymer (complex 1) has
been synthesized as a light-yellow crystalline solid by self-
assembly of the primary ligand i.e. the Mn(II) salt of o-
nitrobenzoate together with pyz as the secondary spacer in
methanol solution at ambient temperature in 1:1 molar ratios. For
II
III
10+
the preparation of complexes with the [Mn Mn 2O2] core by
4
aerial oxidation, acetonitrile is the most commonly used
8
a,21
solvent,
Previously, we reported
a
mixed-valence
45
hexanuclear Mn(II/III) cluster which was obtained by aerial
1
1
oxidation of Mn(II) precursor. Following the similar procedure
Scheme 1: Formation of complexes.
here also, we dissolved complex 1 in CH CN, but after one week,
3
a yellow crystalline solid reappeared indicating that aerial
Table 1 Crystal data and structure refinement parameters for complexes 1-3.
1
2
3
Formula
Formula weight
Space group
Crystal system
a/ Å
C H MnN O
C80.98H62.08Mn N11.15O46.17
C78.45H52.23Mn N14.23O42
1
8
14
4
9
6
6
485.27
2259.87
Pbca
2195.84
Pn2 a
P2 /n
1
1
Orthorhombic
8.889(5)
7.407(5)
30.467(5)
90
Orthorhombic
22.618(5)
27.352(5)
30.830(5)
90
Monoclinic
13.408(5)
27.548(5)
24.733(5)
99.406(5)º
9013(4)
b/ Å
c/ Å

/deg
3
2006.0(18)
19073(6)
V/Å
Z
4
8
4
-
3
Calculated density Dcalc/g cm
1.607
1.575
1.618
-
1
Absorption coeff.() mm
F(000)
(MoKα) 0.719
988
(MoKα) 0.873
9169
(MoKα) 0.919
4434
R(int)
0.059
0.067
0.036
θ range (deg)
Total reflections
Unique reflections
Observed data I>2(I)
R1, wR2
1.34 to 25.06
8664
1.32 to 25.34
86170
1.1 to 25.4
62179
3405
16903
16277
2842
10549
12275
0.0690, 0.1892
0.0574, 0.1555
0.0442, 0.1178
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Description of structures
The structure of 1 is shown in Figure 1 together with the atomic
numbering scheme in the metal coordination sphere.
20
(a)
(b)
5
Fig. 1 The coordination environments of the Mn(II) ion in
complex 1 (symmetry codes a = x,1+y,z, b = -1/2+x, y, 1/2-z) with
ellipsoids at 30% probability
The manganese atom occupies
a
distorted octahedral
environment. In the equatorial plane it is bonded to two
1
0
monodentate (o-(NO )C H COO) groups through O(11) and
O(31) at 2.122(4) and 2.171(4) Å and to two pyz ligands through
2
6
4
25
(c)
nitrogen atoms N(51) and N(54) (x,1+y,z) at 2.293(10) and
Fig. 2 Packing in 1, (a) Formation of linear chain by pyrazine
linkers; (b) Formation of linear chain by carboxylate oxygens; (c)
Formation of two-dimensional framework.
2
.325(9) Å respectively. The axial positions are occupied by one
water molecule O(1) at 2.153(5) Å and a bridging oxygen atom
1
5
O(33) (-1/2+x, y, 1/2-z) at 2.152(4) Å from (o-(NO )C H COO)
2
6
4
(
Table 2).
The pyz ligands form a linear chain in the b direction via M-
pyz-M-pyz-M links while the carboxylate oxygen O(33) forms a
Mn-O-Mn-O-Mn link in the a direction via glide plane. The
structure is therefore a two-dimensional polymer.
b
30
Table 2: Bond distances (Å) and angles (º) in the metal
coordination sphere of complex 1:
Atom labels
Mn(1)-O(11)
Mn(1)-O(1)
Distance
2.122(4)
2.153(5)
2.171(4)
Angles
Atom labels
Distance
2.293(10)
2.325(9)
2.152(4)
Angles
Mn(1)-N(51)
a
Mn(1)-N(54)
Mn(1)-O(33)
Atom labels
b
Mn(1)-O(31)
Atom labels
O(11)-Mn(1)-O(33)
b
93.62(16) O(11)-Mn(1)-O(1)
179.0(3) O(11)-Mn(1)-O(31)
85.51(16)
174.27(18)
99.93(15)
91.6(3)
b
O(33) -Mn(1)-O(1)
b
O(33) -Mn(1)-O(31) 80.96(14) O(1)-Mn(1)-O(31)
b
O(11)-Mn(1)-N(51)
O(1)-Mn(1)-N(51)
O(11)-Mn(1)-N(54)
91.6(3)
89.0(3)
93.1(3)
90.8(3)
175.3(2)
O(33) -Mn(1)-N(51)
O(31)-Mn(1)-N(51)
86.7(3)
a
b
a
O(33) -Mn(1)-N(54)
88.7(3)
a
a
O(1)-Mn(1)-N(54)
O(31)-Mn(1)-N(54)
88.7(3)
a
N(51)-Mn(1)-N(54)
Fig. 3 Hydrogen-bonding interactions in compound 1 shown as
35
dotted lines.
a
b
Symmetry code: = x, 1+y, z; = -1/2+x, y, 1/2-z
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The water molecule O(1) forms two donor hydrogen bonds to
also when the dimensions of the structures, which are numbered
equivalently, are compared in Table ST1 (supporting
25 information). Indeed there are very few differences in the
geometry of the Mn (o-(NO )C H COO) moieties. In both
O(13) and O(31) (-1/2+x, y, 1/2-z) with dimensions H...O 1.83,
o
2
.38Å, O-H...O 2.621(7), 2.843(6) Å and O-H...O 153, 115
respectively
6
2
6
4
10
structures, all six metal atoms occupy six-coordinate distorted
octahedral environments. At the centre of Mn6 core of both
complexes there are two Mn(III) atoms, namely Mn(1) and
Mn(2) which form the shortest contacts at 2.810(1) Å in 2 and
5
Both 2 and 3 contain discrete hexanuclear units. The complexes
are completed by four terminal ligands; for 2, these are two
acetones, one water molecule together with one site disordered
between ligand pyz at 61(1)% and acetone at 39(1)% occupancy.
For 3 there are four acetonitriles of which one is disordered with
72(1)% occupancy together with a ligand pyz at 28(1) %
occupancy.
3
3
4
0
5
0
2
.804(1) Å in 3. The remaining four metal atoms in both
structures are Mn(II). In 2, Mn(1) is close to Mn(3) and Mn(6) at
.1878(1) and 3.184(1) Å while Mn(2) is close to Mn(4) and
Mn(5) at 3.180(1), 3.182(1) Å. Mn···Mn distances in 3 are
comparable at 3.198(1), 3.183(1), 3.178(1), 3.190(1)
1
0
3
Å
respectively. Mn(1) and Mn(2) are bridged by O(1) and O(2)
which each bridge two further Mn atoms, Mn(4), Mn(6) in the
case of O(1) and Mn(3), Mn(5) in the case of O(2). In the two
structures, the distances from the bridging oxygen atoms O(1)
and O(2) to Mn(1) and Mn(2) are in the range 1.877(3)-1.899(2)
Å while distances to the other metal atoms are in the range
2
.196(2)-2.242(3) Å.
In addition to these bridging oxygen atoms Mn(1) and Mn(2) are
each bonded to four oxygen atoms from four different o-nitro
benzoates. Of these, two bonds, which are trans to the bridging
oxygen atoms are shorter in the range 1.946(2)-1.978(3) Å and
the other two which are mutually trans are longer in the range
45
2
.204(3)-2.246(3) Å.
The four remaining metal atoms are also bonded to four oxygen
(a)
50 atoms from four o-nitro benzoates, but also to one of the bridging
oxygen atoms, either O(1) or O(2), and a terminal solvent ligand
and these two atoms are mutually trans. Thus in the complexes all
2
0 oxygen atoms of the 10 o-nitro benzoates bridge with at least
two different metal atoms but four namely O(11), O(53), O(61),
O(101) are bonded to three metal atoms. As a consequence the
Mn-O bonds containing these four atoms are among the longest.
5
6
6
5
0
5
The coordination spheres of these four metal atoms are completed
by monodentate solvent ligands. In 2, these are two (CH ) CO
3
2
ligands and one water molecule while the fourth site is disordered
between ligand pyz and methanol. By contrast in 3, there are
three acetonitrile molecules while the fourth site is disordered
between a further acetonitrile and ligand pyz.
In 2, there is an additional (CH ) CO solvent molecule, but 3
3
2
does not contain any unattached solvent. There are additional
hydrogen bonds in 2. In 2 the water molecule O(3) forms strong
donor hydrogen bonds to the free solvent acetone molecule
O(1C) (1-x, -y, -z) and to O(18) from a nitro group with
dimensions 2.727(9), 1.87 Å, 174° and 2.992(7), 2.20 Å, 158°
respectively.
1
5
(b)
Fig. 4 Central [Mn ] cluster in complexes 2 (a) and 3 (b) with
6
ellipsoids at 30% probability. All hydrogen atoms and ortho-nitro
benzoate rings have been omitted for clarity. For disordered
terminal ligands, only the major component is shown. 2 contains
disordered solvent molecules which are not shown.
70
Magnetic properties.
Figure 5 shows the temperature dependence of magnetic
susceptibilities for 1 measured at an applied magnetic field of
2
0
3
-1
5
000 Oe. At 300 K, the χ T value was 4.42 cm K mol , which
m
Apart from these variations in terminal ligands, the complexes are
very similar as is apparent from figures of 2 and 3 (Figure 4), and
II
agrees well with S = 5/2 Mn ion and g = 2. The χ T value
m
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Dalton Transactions
3
-
2+
monotonically decreased on cooling and reached 1.95 cm K mol
orbital of the Mn center and the 2p orbital of the carboxyl
1
at 1.8 K. This behavior indicates that antiferromagnetic 35 oxygen atom would lead to antiferromagnetic interaction through
coupling are dominant. As an empirical approach, the Curie-
Weiss parameters have been determined as shown in Table 3.
the μ1,3-carboxylate bridging ligand and depends on the
coordination mode of the carboxylate ligand. Syn-anti
carboxylato bridges induce smaller J values because of the
expanded metallic core and a mismatch in the orientation of
2
4,25
40
magnetic orbitals
which is in good agreement with the
experimental observations.
Figure 6 shows the temperature dependence of magnetic
susceptibilities for 2 and 3 measured at an applied magnetic field
3
of 5000 Oe. At 300 K, the χ T values were 18.2 and 18.9 cm K
m
-
1
4
5
5
5
0
5
mol for 2 and 3, respectively, which are far below the
3
theoretical spin-only values at high temperature limit (23.5 cm K
-
1
III
II
mol from two S = 2 Mn ions and four S = 5/2 Mn ions with g
2). The χ T values monotonically decreased on cooling and
=
m
3
-1
reached 0.525 and 1.18 cm K mol for 2 and 3, respectively, at
1.8 K. No meaningful dynamic properties were found for 2 or 3
at 2 K. These findings indicate that antiferromagnetic couplings
are dominant. As an empirical approach, the Curie-Weiss
parameters have been determined as shown in Table 3. From the
crystallographic analysis, the interaction can be reasonably
5
Fig. 5. Temperature dependences of the product of the magnetic
susceptibility and temperature (right) and reciprocal magnetic
susceptibility (left) for 1, measured at 5000 Oe. The susceptibility
was reduced to the mononuclear formula basis. Solid line shows
the simulation curve from a 2D model (see the text).
attributed to the intramolecular exchange coupling in the Mn6
1
0
cluster.
Table 3. Curie-Weiss constants of 1, 2 and 3.
Curie-Weiss constants
C/cm K mol
θ/K
1
2
3
3
–1
4.449(2)
–2.50(9)
20.02(3)
–33.4(7)
20.99(5)
–38.0(10)
Temp. range for Curie- 50-300 K
Weiss fitting
50-300 K
50-300 K
3
–1
χT/cm K mol
4.42 (300 K)
18.2 (300 K)
5000
18.9 (300 K)
5000
H (Oe)
5000
G
2.00
1.97
2.00
To evaluate the exchange coupling, we need to identify the
dominant interaction pathways, in the metal organic framework
of complex 1. In the crystal lattice, the Mn(II) ions are doubly
bridged with three atoms of the carboxylate groups, and this
motif is repeated infinitely to give a 1D zigzag chain. The most
important exchange coupling can be assigned to intrachain
Mn∙∙∙Mn interactions with equal spacing. The chains are
connected with pyz, which may afford only auxiliary interactions
because of the long distance (across four atoms). Therefore, the
main exchange channel is assumed to be 1D. From the Fisher
1
2
2
5
0
5
(a)
2
2
model with Ĥ = –2JΣ S ∙S we obtain J/k = –0.143(1) K and g
i
i+1
B
=
2.005(2). Assuming that the exchange coupling occurs in a 2D
2
3
quadratic manner, the Lines equation can be used which gives
J/kB = –0.0788(5) K and g = 2.008(1). We need to consider a
possible zero-field splitting effect, which leads to a decrease of
the  T product at low temperatures and therefore the J values
obtained here may be overestimated
m
60
(b)
Fig. 6. Temperature dependence of the products of the magnetic
susceptibility and temperature for 2 (a) and 3 (b), measured at
3
0
For the three atoms (μ1,3) carboxylate bridges, a syn-anti
bridging configuration is rare by comparison with a syn-syn
configuration. It is expected on the basis of the respective
geometries that an efficient overlap between the 3d magnetic
5
000 Oe. Solid lines show the simulation (see the text).
2
4
6
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Dalton Transactions
Page 8 of 11
To further investigate the magnetic coupling within a Mn6
related [Mn ] systems were reported to show comparable values:
6
cluster, we define a spin exchange Hamiltonian for this system, 40 e.g. J /k = –60 K, J /k = –1.2 K, and J /k = –3.5 K in
A
B
B
B
C
B
7
a
so that the exchange parameters can be evaluated by simulation.
[Mn O (O CPh) (py) (MeCN) ]•2MeCN for example.
6 2 2 10 2 2
The exchange couplings among the Mn ions are classified as
follows:
Even though the structure of the [Mn O (RCOO) ] cluster is
6 2 10
5
quite common, only 18 of the 32 known examples have been
1
1,21,27a-g
magnetically characterized.
All of them show moderate
III
III
(
1) Doubly µ -O bridged Mn ---Mn (the exchange parameter is
4
45
antiferromagnetic exchange couplings. The present study has
shown that the most important feature responsible forthe
magnetic properties is the presence of a strong magnetic coupling
between the central Mn(III) ions, the other exchange pathways
being less relevant. This strong coupling between the central
defined by JA),
III
II
(
2) Doubly µ -O and µ -O bridged Mn ---Mn (by J ),
2 4 B1
III
II
(
3) Singly µ -O bridged Mn ---Mn (by J ), and
4
B2
II
II
1
0
(4) Singly µ -O bridged Mn ---Mn (by J ).
50 Mn(III) ions correlates well with the expected value from
magneto-structural correlations.
2
C
Catecholase-like Activity Study and Kinetics. Catechol oxidase
catalyzes exclusively the oxidation of catechols (i.e., o-diphenols)
to the corresponding quinones, and because of this oxidation
55
process, known as catecholase activity (Scheme 2), catechol
oxidase is able to play a key role for disease resistance in higher
plants.
Fig. 7 Scheme of the magnetic exchange pathways in the [Mn6]
cluster.
Scheme 2. Catecholase activity
There are approximately four different magnetic interactions
III
III
III
II
II
1
5
involved, corresponding to the Mn -Mn , Mn -Mn and Mn -
6
0
Quinones are highly reactive compounds which undergo auto
polymerization to produce melanin, a brown colored pigment,
and this process is most likely responsible for protecting damaged
tissues against pathogens and insects.
II
III
III
Mn pathways. The Mn ---Mn distances are very short, which
may afford the strongest antiferromagnetic exchange coupling in
such Mn6 magnetic clusters. A close inspection at the Mn−O
bond lengths in the oxido bridges shows that the double oxido
bridge (O(1) and O(2)) connecting the two central Mn(III) ions
In most of the catecholase activity studies of model complexes,
3,5-di-tert-butylcatechol (3,5-DTBC) has been chosen as the
substrate. Its low redox potential makes it easy to oxidize, and the
bulky substituents prevent further reactions such as ring-
2
0
65
(
(
=
Mn(1) and Mn(2)) presents much shorter Mn−O bond lengths
Mn(1)−O(1) = 1.878(3) (for 2) and 1.888(2) (for 3) Mn(2)−O(1)
1.900(3) Å) compared with the other Mn−O bond lengths.
2
8
opening. The oxidation product 3,5-di-tert-butylquinone (3,5-
DTBQ) is very stable and exhibits a maximum absorption at 403
nm in pure acetonitrile. The catecholase activity of complexes 1–
To reduce the number of parameters, an approximation of JB1
2
5
= JB2 was introduced, because their characteristics are similar
70
III
II
with respect to the interacting species (Mn and Mn ) and
bridging motifs (Fig. 7). Therefore, the magnetic coupling
3
was studied using 3,5-DTBC as a convenient model substrate,
°
in air saturated acetonitrile solvent at room temperature (25 C).
For this purpose, solutions of the complexes were treated with
parameters are defined as J , J and J , from the following
A
B
C
7
a
Heisenberg spin Hamiltonian. Eq. (1) is utilized for 2 and 3.
1
00 equiv. of 3,5-DTBC and the course of the reaction was
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
75 followed by recording the UV–vis spectra of the mixture at an
interval of 5 min. After addition of substrate 3,5-DTBC to the
solutions of the catalysts 1–3, a gradual increase of the band
corresponding to 3,5-DTBQ was observed at ~400 nm (Fig. 8(a)
and (b) for complexes 1 and 2 respectively and Fig. S1 for
H  2J (S ･ S )  2J (S ･ S  S ･ S

A
Mn1
Mn2
B
Mn1
Mn3
Mn1
Mn4
3
0
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
S
･ S  S ･ S  S ･ S  S ･ S

Mn1
Mn5
Mn1
Mn6
Mn2
Mn3
Mn2
Mn4
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
Mn6
S
･ S  S ･ S )  2J (S ･ S  S ･ S )  (1)
Mn2
Mn5
Mn2
Mn6
C
Mn3
Mn5
Mn4
The parameters were optimized by simulation performed using
80
complex 3).
2
6
the MAGPACK software.
superimposed in Figure 6, and the experimental values were well
reproduced. The best fit parameters were: J /k = –70 K, J /k
0.5 K, J /k = –2.9 K, and g = 1.97 for 2 and J /k = –60 K, J /k
C B A B B B
0.3 K, J /k = –2.8 K, and g = 2.00 for 3. Possible errors in the
C B
calculation of J parameters are estimated to be below 5%. The 85 slowly converts to the quinone as observed in Figure 8. A
The resultant curves are
This observation can be rationalised as follows. The oxidation of
3,5-DTBC to 3,5-DTBQ catalyzed by the complex, proceeds via
the formation of a catalyst-substrate (CS) adduct whose λmax is
close to that of the complex itself. After a few minutes, the adduct
–
B =
A
B
B
3
5
–
=
2
9
present parameters are reasonable for such Mn clusters, as highly
control experiment has been carried out using Mn(o-
6
(
NO )C H COO) ·H O instead of the present complexes under
2
6
4
2
2
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Dalton Transactions
analogous conditions to study the possible catalytic activity of a
simple Mn(II) salt in the oxidation of 3,5-DTBC to 3,5-DTBQ.
Within 1 h of reaction no appreciable amounts of 3,5-DTBQ were 35 such as the Michaelis−Menten constant (KM = 1.7(3)×10 for all
therefore applied and linearized by means of a Lineweaver−Burk
plot (double reciprocal) to calculate various kinetic parameters
−4
observed using UV−vis spectroscopy (supporting information,
Fig. S2).
three complexes) and maximum initial rate (Vmax = 5.20
−4
−1
-5
-1
5
(3)×10
M min for complex 1, 7.2(3) ×10 M.min for
-
5
complex 2 and 6.9(3)×10 for complex 3). The turnover number
of the complex (k = 177 h for complex 1, 432 h for complex
−1
−1
cat
−1
40
2 and 426 h for complex 3 per metal centre) is calculated by
dividing the Vmax value by the concentration of the complex.
(a)
(a)
(b)
1
0
Fig. 8 Increase of absorption spectra after addition of 100 equiv
−4
of 3,5-DTBC to a solution containing complex 1 (1.00×10 M)
−5
(
Fig. 8(a)) and complex 2 (0.166×10 M) (Fig. 8(b)) in
acetonitrile. The spectra were recorded after every 5 min up to 1 h
in CH CN.
3
1
2
2
5
0
5
Kinetic Study. The kinetic study of the oxidation of 3,5-DTBC
to 3,5-DTBQ by complexes 1, 2 and 3 were carried out by
monitoring the growth of the absorbance at ~400 nm by the initial
rates method. To determine the dependence of the rates on the
substrate concentration and various kinetic parameters, solutions
of complexes
1 and 2 were prepared with increasing
concentrations of 3,5-DTBC (from 10 to 100 equiv) under
−4
45
(b)
aerobic conditions at complex concentrations of 1.00×10 and
−6
1
.66×10 M respectively. Complex 3 exhibits almost similar
Fig. 9 Plot of initial rates vs substrate concentration for the
oxidation reaction catalyzed by complex 1 (Fig. 9(a)) and
complex 2 (Fig. 9(b)). Inset shows the Lineweaver−Burk plot.
behavior to that of complex 2. A first-order dependence was
observed at low concentrations of the substrate, whereas
saturation kinetics was found at higher concentrations of the
substrate, as shown in Figure 9 for complexes 1 and 2 (supporting
information, Fig. S3 for complex 3).
The oxidation process of 3,5-DTBC to 3,5-DTBQ involves two
electrons. A literature survey reveals that higher valent metal
centers (e.g., Cu , Mn and Mn )
50
II
III
IV 30,31
are usually involved in
This dependence on the substrate concentration indicates that
catalyst-substrate binding is the initial step in the catalytic
mechanism. A treatment on the basis of the Michaelis−Menten
approach, originally developed for enzyme kinetics, was
catechol oxidation, while reports on catecholase activities of
3
2
3
0
manganese(II) complexes are relatively rare. The manganese
centers are in +2 and +3 oxidation states in complexes 2 and 3.
Therefore, it is reasonable to consider that in the catalytic cycle
55
8
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Dalton Transactions
Page 10 of 11
performed by complexes 2 and 3 , Mn(III) undergoes reduction to
Mn(II) with concomitant oxidation of 3,5-DTBC to 3,5-DTBQ in
the presence of molecular oxygen. Turnover number of the
†Electronic supplementary information (ESI) available: The electronic
supplementary information file contains Figure S1-S2. Table ST1. CCDC
1
049356-1049358 for 1-3. For ESI and crystallographic data in CIF or
−1
−1
55 other electronic format see DOI: 10.1039/
complex 2 and 3 (432 h and 426 h per metal centre) is higher
3
3,34
5
than those of Krebs et al. and Rajak et al.
and comparable to
3
5
29
those reported by Vittal et al. and Das et al. On the other hand,
catechol oxidation catalyzed by Mn(II) complexes has been
proposed to occur through the formation of Mn(III) and Mn(IV)
species . The turnover number of complex 1 (177 h per metal
centre) is comparatively lower than that reported by Das et al.
1
(a) E. Cremades, J. Cano, E. Ruiz, G. Rajaraman, C. J. Milios
and E. K. Brechin, Inorg. Chem., 2009, 48, 8012–8019; (b) N.
Lima, A. Caneschi, D. Gatteschi, M. Kritikos and L. G. Westin,
60 Inorg. Chem., 2006, 45, 2391–2393; (c) A. J. Tasiopoulos, W.
Wernsdorfer, K. A. Abboud and G. Christou, Angew. Chem., Int.
Ed., 2004, 43, 6338–6342; (d) C. C. Stoumpos, R. Inglis, O.
Roubeau, H. Sartzi, A. A. Kitos, C. J. Milios, G. Aromi, A. J.
Tasiopoulos, V. Nastopoulos, E. K. Brechin and S. P. Perlepes,
Inorg. Chem., 2010, 49, 4388–4390; (e) C. C. Stoumpos, R.
Inglis, G. Karotsis, L. F. Jones, A. Collins, S. Parsons, C. J.
Milios, G. S. Papaefstathiou and E. K. Brechin, Cryst. Growth
Des., 2009, 9, 24–27; (f) S. G. Baca, I. L. Malaestean, T. D.
Keene, H. Adams, M. D. Ward, J. Hauser, A. Neels and S.
Decurtins, Inorg. Chem., 2006, 47, 11108–11119; (g) P. Kar, R.
Biswas, M. G. B. Drew, Y. Ida, T. Ishida and A. Ghosh,
Dalton Trans., 2011, 40, 3295-3304; (h) S. Naiya, S. Biswas, M.
G. B. Drew, C. J. Gómez-García and A. Ghosh, Inorg. Chem.,
3
2b
−1
3
1a
1
0
3
3
and comparable to those reported by Krebs et al.
Conclusions
II
A
2D
coordination
polymer
of
Mn ,
Mn(o-
6
7
7
5
0
5
(
NO )C H COO) ∙3H O has been prepared by reacting pyz with
2
6
4
2
2
1
2
2
3
3
5
0
5
0
5
Mn(o-(NO )C H COO) ∙3H O in 1:1 molar ratio. The compound
2 6 4 2 2
did not undergo aerial oxidation in acetonitrile solution unlike its
benzoate or phenyl acetate analogues; instead it needed oxidizing
agent, n-Bu NMnO , to produce hexanuclear Mn(II/III)
4
4
III
II
4
10+
complexes based on [Mn Mn O ]
units. The product was
2
2
crystallized from two solvents (acetonitrile and acetone) and
structural analysis of both showed that only one of the four
III
II
10+
solvent molecules of [Mn Mn O ]
units was partially
2
4
2
2
012, 51, 5332–5341; (i) P. Kar, P. M. Guha, M. G. B. Drew, T.
replaced by the pyrazine molecule. The results are in sharp
contrast to that of the phenyl acetate analogue where all four
solvent molecules have been replace by pyrazine to form a
diamondoid metal-organic framework. Variable-temperature (1.8-
Ishida and A. Ghosh, Eur. J. Inorg. Chem., 2011, 2075–2085; (j)
P. Kar, R. Biswas, Y. Ida, T. Ishida, and A. Ghosh, Cryst. Growth
Des., 2011, 11, 5305–5315.
2
(a) K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and
3
00 K) magnetic susceptibility measurements showed the
S. Iwata, Science., 2004, 303, 1831−1838; (b) A. W. Rutherford
and A. Boussac, Science, 2004, 303, 1782−1784; (c) S.
Mukhopadhyay, S. K. Mandal, S. Bhaduri and W. H. Armstrong,
Chem. Rev., 2004, 104, 3981−4026; (d) F. Rappaport and B. A.
Diner, Coord. Chem. Rev., 2008, 252, 259−272; (e) M. Carboni
and J.-M. Latour, Coord. Chem. Rev., 2011, 255, 186−202.
presence of antiferromagnetic coupling through the syn-anti
carboxylate bridge in compound 1. The modelling of the
magnetic results of 2 and 3 revealed that the magnetic coupling
8
0
III
between the central Mn ions is strong and other exchange
pathways are less relevant, a result that is usual for such
complexes. All three complexes showed catecholase-like activity
and the Michaelis–Menten approach to enzyme catalysis has been
applied to rationalize the kinetic parameters. Considerably higher
turnover numbers of 2 and 3 compared to 1 clearly indicate that
85
3 (a) D. Lieb, A. Zahl, T. E. Shubina and I. Ivanovic-
Burmazovic, J. Am. Chem. Soc., 2010, 132, 7282−7284; (b) I.
Kani, C. Darak, O. ꢀahin and O. Bꢁyꢁkgꢁngꢂr, Polyhedron,
III
the presence of higher valent manganese (Mn ) enhances their
catecholase-like activity.
2
008, 27, 1238−1247; (c) G. Maayan and G. Christou, Inorg.
Chem., 2011, 50, 7015−7021; (d) P. Mukherjee, P. Kar, S. Ianelli
and A. Ghosh, Inorg. Chim. Acta, 2011, 365, 318−324.
90
Acknowledgements
4
(a) D. Shen, C. Miao, S. Wang, C. Xia and W. Sun, Eur. J.
4
0
We thank DST-FIST, India funded Single Crystal Diffractometer
Facility at the Department of Chemistry, University of Calcutta,
Kolkata, India. P.K. is thankful to DST-PURSE, India for
funding.
Inorg. Chem., 2014, 5777–5782; (b) S. Kal, L. Ayensu-Mensah
and P. H. Dinolfo, Inorg. Chim. Acta, 2014, 423, 201–206; (c) T.
Katsuki, Coord. Chem. Rev., 1995, 140,189-214.
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5 (a) P. Mahata, D. Sarma and S. Natarajan, J. Chem. Sci., 2010,
Notes and references
1
22, 19–35; (b) X.-Y. Wang, H.-Y. Wei, Z.-M. Wang, Z.-D.
a
4
5
Department of Chemistry, University College of Science, University of
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Calcutta, 92, A.P.C. Road, Kolkata-700 009, India; E-mail:
ghosh_59@yahoo.com
b
Department of Engineering Science, The University of Electro-
1
Communications, Chofu, Tokyo 182-8585, Japan
c
5
0
School of Chemistry, The University of Reading, P.O BOX 224,
Whiteknights, Reading RG6 6AD, U.K.
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DOI: 10.1039/C5DT00709G
Page 11 of 11
Dalton Transactions
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0
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5
0
5
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