Synthesis, structural, magnetic, DFT calculations and CShM studies of three new pentanuclear Mn(ii) clusters

Article abstract of DOI:10.1039/c1dt11458a

The ditopic ligand PyPzOAPz (N-[(Z)-amino(pyrazin-2-yl)methylidene]-5- methyl-1-(pyridin-2-yl)-1H-pyrazole-3-carbohydrazonic acid) was synthesized by in situ condensation of methyl imino pyrazine-2-carboxylate with 5-methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide. In this work we have also used two of our earlier ligands PzCAP (5-methyl-N-[(1E)-1-(pyridin-2-yl)ethylidene]- 1H-pyrazole-3-carbohydrazonic acid) (Dalton Trans., 2009, 8215) and PzOAP (N-[(Z)-amino(pyridin-2-yl)methylidene]-5-methyl-1H-pyrazole-3-carbohydrazonic acid) (Dalton Trans., 2007, 1229). These ligands PzCAP, PzOAP and PyPzOAPz were made to react with Mn(ClO₄)₂.6H₂O to produce three pentanuclear Mn(ii) clusters [Mn₅(PzCAP)₆](ClO ₄)₄ (1), [Mn₅(PzOAP)₆](ClO ₄)₄ (2) and [Mn₅(PyPzOAPz)₆] (ClO₄)₄ (3). These complexes have been characterized by X-ray structural analyses and variable temperature magnetic susceptibility measurements. All complexes have a pentanuclear core with trigonal bipyramidal arrangement of Mn(ii) atoms, where, the axial metal centers have a N ₃O₃ chromophore and the equatorial centers have N ₄O₂ with an octahedral arrangement. These Mn ₅(ii) clusters 1, 2 and 3 show the presence of antiferromagnetic coupling within the pentanuclear manganese(ii) core (J = -2.95, -3.19 and -3.00 cm^-1 respectively). Density functional theory calculations and continuous shape measurement (CShM) studies have been performed on these complexes to provide a qualitative theoretical interpretation of the antiferromagnetic behaviour shown by them. The pentanuclear Mn(ii) cluster (1) on reaction with Cu(NO₃)₂.6H₂O in 1:1 mole proportion in CH₃OH:H₂O (60:40) forms a homoleptic [2 × 2] tetranuclear Cu₄(ii) grid [Cu₄(PzCAP) ₄(NO₃)₂](NO₃)₂.8H ₂O (4). The same Cu₄(ii) grid is also obtained from a direct reaction between the ditopic ligand PzCAP with Cu(NO₃) ₂.6H₂O in 1:1 mole proportion. This conversion of a cluster to a grid is a novel observation.

Full text of DOI:10.1039/c1dt11458a

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PAPER
Synthesis, structural, magnetic, DFT calculations and CShM studies of three
new pentanuclear Mn(II) clusters†‡
T. N. Mandal,a S. Roy,a S. Konar,a A. Jana,a K. Das,a S. Ray,a S. Gupta,a R. Saha,b M. S. El Fallah,*c
J. Tercero,d R. J. Butcher,e S. Chatterjee f and S. K. Kar*a
Received 2nd August 2011, Accepted 8th September 2011
DOI: 10.1039/c1dt11458a
The ditopic ligand PyPzOAPz (N-[(Z)-amino(pyrazin-2-yl)methylidene]-5-methyl-1-(pyridin-2-
yl)-1H-pyrazole-3-carbohydrazonic acid) was synthesized by in situ condensation of methyl imino
pyrazine-2-carboxylate with 5-methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide. In this work we
have also used two of our earlier ligands PzCAP (5-methyl-N-[(1E)-1-(pyridin-2-yl)ethylidene]-
1H-pyrazole-3-carbohydrazonic acid) (Dalton Trans., 2009, 8215) and PzOAP
(N-[(Z)-amino(pyridin-2-yl)methylidene]-5-methyl-1H-pyrazole-3-carbohydrazonic acid) (Dalton
Trans., 2007, 1229). These ligands PzCAP, PzOAP and PyPzOAPz were made to react with
Mn(ClO4)2·6H2O to produce three pentanuclear Mn(II) clusters [Mn5(PzCAP)6](ClO4)4 (1),
[Mn5(PzOAP)6](ClO4)4 (2) and [Mn5(PyPzOAPz)6](ClO4)4 (3). These complexes have been characterized
by X-ray structural analyses and variable temperature magnetic susceptibility measurements. All
complexes have a pentanuclear core with trigonal bipyramidal arrangement of Mn(II) atoms, where, the
axial metal centers have a N3O3 chromophore and the equatorial centers have N4O2 with an octahedral
arrangement. These Mn5(II) clusters 1, 2 and 3 show the presence of antiferromagnetic coupling within
the pentanuclear manganese(II) core (J = -2.95, -3.19 and -3.00 cm-1 respectively). Density functional
theory calculations and continuous shape measurement (CShM) studies have been performed on these
complexes to provide a qualitative theoretical interpretation of the antiferromagnetic behaviour shown
by them. The pentanuclear Mn(II) cluster (1) on reaction with Cu(NO3)2·6H2O in 1 : 1 mole
proportion in CH3OH:H2O (60 : 40) forms a homoleptic [2 ¥ 2] tetranuclear Cu4(II) grid
[Cu4(PzCAP)4(NO3)2](NO3)2·8H2O (4). The same Cu4(II) grid is also obtained from a direct reaction
between the ditopic ligand PzCAP with Cu(NO3)2·6H2O in 1 : 1 mole proportion. This conversion of a
cluster to a grid is a novel observation.
Introduction
One of the challenging topics in the field of inorganic chemistry
is the synthesis of polynuclear metal clusters because of their
aDepartment of Chemistry, University College of Science, University of
useful applications in areas like bioinorganic chemistry, molecular
magnetism and catalysis etc.1 The binding of transition metal
ions to properly designed polytopic ligands has been attracting
continuous scientific interest since the seminal reports of the metal-
mediated self-assembly processes.2–4 They have also emerged as the
focal strategic and logical method for the syntheses of high nucle-
arity transition metal complexes with novel magnetic properties.5,6
It is obvious that the formation of these well-defined architectures
solely depends on the proper balance between coordination sites
of the metal ions and the coordination algorithm of the polytopic
ligands. Hence, with a proper choice of paramagnetic metal ions
interacting with those polytopic ligands, magnetic coupling can be
fine-tuned.
Calcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: skkar_cu@
yahoo.co.in
bDepartment of Physics, Jadavpur University, Kolkata, 700 032, India
cDepartament de Qu´ımica Inorga`nica, and Institut de Nanocie`ncia i Nanotec-
nologia, Universitat de Barcelona, Mart´ı i Franque`s 1-11, 08028, Barcelona,
Spain. E-mail: salah.elfallah@qi.ub.es
dDepartament de Qu´ımica Inorga`nica, and Institut de Qu´ımica Teo`rica i
Computacional, Universitat de Barcelona, Mart´ı i Franque`s 1-11, 08028,
Barcelona, Spain
eDepartment of Chemistry, Howard University, 2400 Sixth Street, N. W.,
Washington, DC, 200 59, U. S. A.
f
Department of Chemistry, Serampore College, Serampore, Hooghly, Pin -
712 201, India
† Electronic supplementary information (ESI) available: EPR spectra of 1
and 2; crystallographic data for 1, 2 and 3. CCDC reference numbers
796250–796252. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt11458a
Manganese is the main choice for assembling high-nuclearity
clusters primarily based on ligands with oxygen donors due to its
‡ Dedicated to the late Dr Golam Mostafa.
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Scheme 1
oxophilicity and the easy availability of at least three oxidation
states, +II (d5), +III (d4), +IV (d3), with the highest number of
unpaired electrons possible in the 3d orbital of the coordinated
metal center. Thus, the studies of manganese clusters have not
only helped us to gain knowledge about the structure and function
of manganese-containing biological systems7 but also provided a
wealth of data for species that are able to function as “single
molecule magnets” (SMMs).8
As a sequel of our long-standing interest in metal clusters
formed by Cu(II) with different polytopic ligands which are
judiciously chosen so as to incorporate various supramolecular
linkers,9 we report here the syntheses of three Mn5(II) clusters
(1, 2 and 3) with the ligands PzCAP, PzOAP and PyPzOAPz
respectively (Scheme 1). There is a striking similarity among
the three structures and also in their magnetic properties. The
2-pyridyl substituent in the 1-position of the pyrazole ring of
PyPzOAPz does not play any role in the self assembly process of
Mn5(II) clusters. All the complexes are structurally characterized
and their magnetic properties are also studied. EPR studies were
done for complexes 1 and 2. Density functional theory (DFT)
calculations and continuous-shape measurement (CShM) studies
have also been performed on all the pentanuclear manganese(II)
complexes to reveal the most plausible mechanism of the anti-
ferromagnetic interaction.
Attempts have been made to prepare tetranuclear Cu(II) [2 ¥
2] grid complexes of the ditopic ligands PzCAP, PzOAP and
PyPzOAPz by the reaction of their Mn5(II) cluster with Cu(II)
nitrate salt solution in 1 : 1 methanol-water solvent using ligand
stripping methodology. In this case we have a new observation. The
Mn5(II) clusters are transformed into the tetranuclear Cu4(II) [2 ¥ 2]
grid structure. We have also synthesized these Cu4(II) [2 ¥ 2] grids in
the usual way by reaction with the ditopic ligands and metal salt
solution in methanol-water medium. Although various types of
self-assembled grid-type complexes reported to date are formed by
reaction of polytopic ligands with metal ions, the transformation
of a pentanuclear Mn5(II) cluster to a tetranuclear Cu4(II) [2 ¥ 2]
square grid is a new observation, and to the best of our knowledge,
such a type of transformation “from cluster to grid” has not been
reported to date.
Experimental
Materials
2-Cyano pyridine, 2-cyanopyrazine and 2-acetyl pyridine were
purchased from Aldrich Chemical Company. Other commercially
available chemicals and solvents were used and purified by
standard procedures.10
414 | Dalton Trans., 2012, 41, 413–423
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Caution! Although we have not encountered any problem, it
should be kept in mind that perchlorate compounds of metal
ions are potentially explosive especially in the presence of organic
ligands. Only a small amount of the material should be prepared
and it should be handled with care.
284, 339 nm. mRT 12.79 B.M. Anal. calc. for C72H72Cl4Mn5N30O22:
C, 40.64; H, 3.39; N, 19.75. Found: C, 40.62; H, 3.37; N, 19.72%;
[Mn5(PzOAP)6](ClO4)4 (2). Into the hot solution of
Mn(ClO4)2·6H2O (0.181 g, 0.5 mmol) in CH3CN/H2O (30 mL)
[1 : 1 v/v], solid PzOAP (0.122 g, 0.5 mmol) was added with
stirring. The resulting suspension was stirred for an additional
1 h at ~60 ◦C until complete dissolution of the ligand occurred. A
pale yellow coloured solution was formed. To it, a dilute solution
of triethyl amine (NEt3/H2O = 1 : 5 v/v) was added to adjust the
pH to ~7 and the resulting solution was further stirred for an
additional half an hour. The deep red solution so obtained was
filtered and left at room temperature. Red crystals suitable for X-
ray diffraction were isolated from the filtrate after standing for
three weeks. Yield: 0.136 g, 45%. IR/cm-1: nNH 3332, 3180; nCO/CN
1645(s), 1544(s); nN-Npz 1050(s); npy 994(s); UV-vis (lmax): 224, 254,
355 nm. mRT 12.64 B.M. Anal. calc. for C66H66Cl4Mn5N36O22: C,
37.15; H, 3.10; N, 23.64. Found: C, 37.18; H, 3.12; N, 23.69%.
Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) of the ligand
and the metal complexes were determined with a Perkin–Elmer
CHN analyzer 2400. Mass spectra were done with a JEOLJMS-
AX 500 mass spectrometer. The electronic spectraof the complexes
were recorded on a Hitachi model U-3501 spectrophotometer.
IR spectra (KBr pellet, 400–4000 cm-1) were recorded on a
Perkin–Elmer model 883 infrared spectrophotometer. Magnetic
susceptibility measurements for the compound were carried out
on polycrystalline samples, at the Servei de Magnetoqu´ımica of
the Universitat de Barcelona, with a Quantum Design SQUID
MPMS-XL susceptometer apparatus working in the range 2–
300 K under a magnetic field of approximately 500 G (2–30 K)
and 1000 G (35–300 K). Diamagnetic corrections were estimated
from Pascal Tables.
[Mn5(PyPzOAPz)6](ClO4)4 (3). PyPzOAPz (0.322 g, 1 mmol)
was added to a hot solution of Mn(ClO4)2·6H2O (0.362 g, 1 mmol)
in H2O-CH3OH (20 mL) [60 : 40 v/v], forming initially a clear light
yellow solution. Addition of a few drops of NEt3 (pH ~ 6) produced
a clear, deep red coloured solution, which was stirred for an hour
and filtered and kept in air. Red-coloured X-ray quality crystals
were collected after five days. Yield 0.479 g, 70%. IR/cm-1: nNH
3435, 3330; nCO/CN 1657(s), 1540(s); nN-Npz 1091(s); npy 1016(s); nClO4
1096. UV-vis (lmax): 224, 254, 355 nm. mRT 12.96 B.M. Anal. calc.
for C90H78Cl4Mn5N48O22: C, 41.53; H, 3.00; N, 25.84. Found: C,
41.50; H, 3.02; N, 25.87%.
Synthesis of the ligands and complexes
5-Methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide was prepared
following our previously reported method.11 The ligands PzOAP
and PzCAP were synthesized as reported previously.9a,b
Synthesis of PyPzOAPz
The methyl ester of iminopyrazine-2-carboxylic acid, was prepared
in situ by reaction of 2-cyanopyrazine (5.25 g, 0.05 mol) with
sodium methoxide solution, produced by dissolving sodium metal
(2.5 g, 0.108 mol) in dry methanol (25 mL). 5-Methyl-1-(2-pyridyl)
pyrazole-3-carbohydrazide (10.85 g, 0.05 mol) in 25–30 mL dry
methanol, was added to the above solution with stirring for 1 h.
The mixture was refluxed for 5 h and cooled to room temperature.
Excess methanol was removed in a rotary evaporator to leave a
yellow coloured oily sticky liquid. It was then mixed with water
(50 mL) and neutralized with AcOH (pH ~ 5) which afforded
a shining yellow powder. The solid was filtered off, washed
thoroughly with water and methanol and dried in vacuo over fused
CaCl2. Yield (8.86 g, 55%). Mp 225 ◦C (decomp.). MS (m/z) 322
(M+, 100%). IR/cm-1: nNH 3350; nCO/CN 1660(s), 1550(s); nN–Npz
1065(s); npy 1010(s). Anal. calc. for C15H14N8O: C, 55.90; H, 4.34;
N, 34.78. Found: C, 55.82; H, 4.30; N, 34.70%.
[Cu4(PyPzOAPz)4](NO3)4 (5). The ligand PyPzOAPz (0.322 g,
1 mmol) was added to a hot solution of Cu(NO3)2·6H2O (0.296
g, 1 mmol) in H2O/CH3OH (30 mL) [60 : 40 v/v]. The resulting
suspension was stirred with heating (~ 60 ◦C) until complete
dissolution of the ligand occurred. The resulting deep green
solution was filtered to remove any undissolved ligand and left at
room temperature. All attempts to prepare X-ray quality crystals
were in vain. Yield 0.371 g, 60%. IR/cm-1: nNH 3365; nCO/CN 1660(s),
1540(s); nN-Npz 1080(s); npy 1030(s). UV-vis (lmax): 670 nm. mRT 3.42
B.M. Anal. calc. for C60H52Cu4N36O16: C, 40.31; H, 2.91; N, 28.22.
Found: C, 40.36; H, 2.95; N, 28.25%.
General method of transformation
Complexes 1 and 3 are transformed into [2 ¥ 2] Cu4(II) grids 4 and 6
by reaction between the respective pentanuclear Mn(II) cluster and
hydrated copper(II) nitrate solution in 1 : 5 molar ratio in 1 : 1 (v/v)
CH3OH/H2O solvent at room temperature. The orange solution
of the manganese cluster gradually converted to an intense green
solution after stirring for 1 h. The solution was filtered and the
filtrate was preserved for crystallization. X-ray quality crystals of
4 were formed after four weeks. (Yield ~ 25%).
Preparation of complexes: For 4: Complex 1 (0.212 g, 0.1 mmol)
and Cu(NO3)2·6H2O (0.148 g, 0.5 mmol). Yield 0.050 g, 25%; For
5: Complex 3 (0.260 g, 0.1 mmol) and Cu(NO3)2·6H2O (0.148 g,
0.5 mmol). Yield 0.045 g, 20%.
Synthesis of the complexes
[Mn5(PzCAP)6](ClO4)4 (1). Into
a
hot solution of
Mn(ClO4)2·6H2O (0.181 g, 0.5 mmol) in CH3OH/H2O (40 mL)
[1 : 1 v/v], solid PzCAP (0.122 g, 0.5 mmol) was added with
stirring. The resulting suspension was stirred until complete
dissolution of the ligand occurred. The resulting reddish yellow
solution was stirred for an additional half an hour and filtered
and kept in air. X-ray quality red crystals were obtained after
three weeks. Yield: 0.121 g, 40%. IR/cm-1: nNH 3345, 3182; nCO/CN
1653(s), 1530(s); nN-Npz 1042(s); npy 1012(s). UV-vis (lmax): 239,
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Table 1 Experimental crystallographic data for 1, 2 and 3
Compound
1
2
3
Empirical formula
Formula weight
T/K
C72 H74 Cl4 Mn5 N30 O23
2144.10
150(2)
C66 H74 Cl4 Mn5 N36 O26
2204.14
150(2)
C90 H84 Cl4 Mn5 N48 O25
2654.77
150(2)
˚
Wavelength/A
0.71073
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions
Triclinic
P1
a = 13.832(6) A
b = 14.491(6) A
c = 25.245(11) A
a = 89.257(6)◦, b = 88.790(6)◦,
g = 61.571(5)◦
4449(3)
Monoclinic
C2/c
Monoclinic
P21/c
¯
˚
˚
˚
˚
˚
˚
a = 34.1672(11) A
a = 15.011(2) A
˚
˚
b = 19.6474(7) A
b = 28.447(4) A
˚
c = 29.3633(9) A
c = 26.921(3) A
a = g = 90◦, b = 111.754(2)◦
a = g = 90◦, b = 107.93(1)◦
3
˚
Volume/A
18307.7(11)
8
1.547
0.875
8760
1.28 to 26.37
-42 £ h £ 42
-24 £ k £ 24
-36 £ k £ 36
1.062
10937(3)
4
1.579
0.752
Z
2
Density (calc)/Mg m-3
Absorption coefficient/mm-1
F(000)
1.587
0.899
2194
0.8 to 30.8
-19 £ h £ 19
-20 £ k £ 20
-35 £ k £ 35
1.100
5365
q Range (◦) for data collection
Index ranges
1.07 to 25.48
-18 £ h £ 18
-34 £ k £ 34
-32 £ k £ 32
1.127
Goodness-of-fit on F2
Completeness to theta = 25.00◦
98.2%
98.2%
99.0%
Independent reflections [Rint
Refinement method
]
27211[0.0259]
Full-matrix least squares on F2
27211/1/1211
50768
18403[0.0799]
Full-matrix least squares on F2
18403/1/1204
171710
20109[0.0810]
Full-matrix least squares on F2
20109/0/1528
71466
Data/restraints/parameters
Reflections collected
Final R indices [I > 2s(I)]
R1 = 0.0587, wR2 = 0.1675
R1 = 0.0689, wR2 = 0.1675
0.795 and -0.957
R1 = 0.0718, wR2 = 0.2153
R1 = 0.1062, wR2 = 0.2153
1.860 and -0.870
R1 = 0.0665, wR2 = 0.1819
R1 = 0.0790, wR2 = 0.1819
0.498 and -0.832
R indices (all data)
Largest difference peak and
-3
˚
hole/eA
Single crystal X-ray crystallography
for structure solution and refinement respectively. For complex
1, 2 and 3 the SQUEEZE program was used to remove the
contributions of disordered solvent water molecules. The full
details of the solvent removed are included in the platon squeeze
details sections of the CIF.
Selected crystal data for 1–3 are given in Table 1 and selected
metrical parameters of these complexes are given in Table 2.
For 1, data collection was made using SMART APEX-II CCD
equipped with graphite monochromated Mo-Ka radiation (l =
˚
0.71073 A) source in j-w scan mode. While, for 2 data collections
Results and discussion
were made using a CCD area detector equipped with graphite
monochromated Mo-Ka radiation (l = 0.71073 A) source in j-w
scan mode. For 3 the data collections were made using a Bruker
˚
Structural descriptions of 1–3
SMART APEX II CCD area detector equipped with graphite
The structures of the pentanuclear cation clusters of 1–3 are
illustrated in Fig. 1(a), 2(a) and 3(a) respectively. The pentanuclear
Mn(II) core is best described as a trigonal bipyramid (pentanuclear
core structures are shown in Fig. 1(b), 2(b) and 3(b) respectively).
In all these complexes (1–3), the equatorial plane is constructed
by Mn1, Mn3 and Mn5 centers and Mn2 and Mn4 centers
occupy the axial positions. In all the complexes the Mn–Mn–
Mn angle of the triangle in the equatorial plane falls in the
range 56–63◦. The unique feature of the structures is that the
three pairs of almost parallelly lying uninegative N3O tetradentate
ligands (PzCAP, PzOAP and PyPzOAPz) are arranged in an
opposed fashion around the pentanuclear, trigonal bipyramidal
manganese(II) core. Each ligand fills five metal coordination sites
providing an alkoxide type bridge between adjacent pairs of
pseudo-octahedral manganese atoms. In all the complexes the
axial metal centers have MnN3O3 and equatorial centers have
MnN4O2 chromophores. The average metal-oxygen distances are
˚
monochromated Mo-Ka radiation (l = 0.71073 A) source in w
scan mode. For 1, cell parameters refinement and data reduction
were carried out using Bruker SMART12 and Bruker SAINT13
respectively. For 2, both cell parameters refinement and data
reduction were carried out using Bruker SAINT software.13 For 3
the structure was solved using the Patterson method by using the
SHELXS-97.14 Subsequent difference Fourier synthesis and least-
square refinement revealed the positions of the remaining non-
hydrogen atoms. Non-hydrogen atoms were refined with indepen-
dent anisotropic displacement parameters. Hydrogen atoms were
placed in idealized positions 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 for the last cycle of the least squares
refinement. All calculations were carried out using SHELXS-97,14
SHELXL-97,14 PLATON-99,15 ORTEP-3216 and WinGX system
Ver-1.80.05.17 All the structures were solved by conventional direct
methods and refined by full-matrix least square methods using
F2 data. SHELXS-97 and SHELXL-97 programs14 were used
˚
˚
˚
2.186 A for 1, 2.188 A for 2 and 2.184 A for 3. The Mn–
O–Mn bridge angles are between 125–131◦ for all the com-
plexes. The average metal-pyridine-N, metal-azomethine-N and
416 | Dalton Trans., 2012, 41, 413–423
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◦
˚
˚
˚
Table 2 Selected bond distances (A) and angles ( ) data for 1, 2 and 3
in the adjacent equatorial positions are 5.209 A (for 1), 4.760 A
˚
(for 2) and 4.834 A (for 3). The axial Mn ◊ ◊ ◊ Mn separations are
5.002 A (for 1), 5.699 (for 2) and 5.544 A (for 3).
(◦)
˚
Selected Bonds
Value(A)
Selected Angles
˚
˚
1
Mn1–O1
Mn1–O2
Mn2–O2
Mn2–O6
Mn2–O3
Mn3–O3
Mn3–O4
Mn4–O4
Mn4–O1
Mn4–O5
Mn5–O5
Mn5–O6
2.182(3)
2.166(3)
2.166(3)
2.198(3)
2.217(3)
2.189(3)
2.197(3)
2.209(3)
2.199(3)
2.174(3)
2.159(3)
2.181(3)
O2–Mn1–O1
O2–Mn1–N2
O1–Mn1–N2
O2–Mn1–N7
O6–Mn2–O3
O2–Mn2–O6
N14–Mn2–O3
O2–Mn2–N14
O4–Mn3–N17
O3–Mn3–O4
N12–Mn3–O4
O4–Mn3–N11
O1–Mn4–O4
O5–Mn4–N4
O1–Mn4–N4
O5–Mn4–N19
O5–Mn5–O6
O5–Mn5–N27
O6–Mn5–N27
O6–Mn5–N21
98.99(10)
127.66(14)
71.31(12)
70.96(13)
90.86(10)
99.63(11)
74.28(13)
167.66(13)
70.41(13)
98.35(11)
131.13(13)
95.38(13)
93.63(10)
91.92(13)
74.40(12)
164.24(13)
100.54(10)
128.69(14)
71.03(12)
95.70(13)
Magnetic behavior of complexes 1–3
The variable temperature magnetic susceptibility data for com-
plexes 1–3 were recorded between 300–2 K. The plot of cM T vs.
T of Fig. 4 shows typical antiferromagnetic behaviors, decreasing
from 20.304, 19.812 and 20.828 cm3 mol-1 K at 300 K, which are
slightly lower than those expected for five uncoupled S = 5/2 (21.9
with g = 2.0) to 4.306, 4.234 and 4.558 cm3 mol-1 K for 1, 2 and 3
respectively at 2 K. In these three cases, these values correspond to
one uncoupled S = 5/2 (4.38 with g = 2.0) due to the odd number
of spin centers in the complexes.
The magnetization measurements at 2 K up to an external field
of 5 T in form M/Nb vs. H show the saturation of the complexes
1, 2 and 3 tending to 5.09Nb (electrons), 4.91Nb (electrons) and
5.37 Nb (electrons) respectively, corresponding to one uncoupled
S = 5/2 system (inset Fig. 4).
As shown in the crystallographic part, compounds 1, 2 and
3 are made by six ligands bound in parallel pairs in an opposed
arrangement around the Mn5(m-O)6 core with the ligands (PzCAP,
PzOAP and PyPzOAPz respectively) opposed (anti) in each pair
similar to the Mn5 compounds reported by L. K. Thompson et al.18
Thus, one kind of six coupling parameters J must be considered
between two neighboring manganese ions to interpret the mag-
netic properties of 1, 2 and 3 (see scheme interactions in Fig. 5).
We have not considered the slight differences observed between the
angles and bond lengths of two neighboring manganese(II) ions.
On the basis of this assumption, the experimental magnetic data
have been fitted using the isotropic Heisenberg Hamiltonian:
2
N3–Mn1–N1
N3–Mn1–O6
N3–Mn1–O1
O5–Mn2–O1
O5–Mn2–O2
O1–Mn2–O2
O1–Mn2–N5
N9–Mn3–N7
N9–Mn3–O2
N9–Mn3–O3
N15–Mn3–O2
O3–Mn4–O4
O6–Mn4–O3
O6–Mn4–O4
O3–Mn4–N17
N27–Mn5–N21
N21–Mn5–O4
N27–Mn5–O4
N27–Mn5–O5
67.41(17)
127.49(15)
71.98(15)
86.04(13)
92.13(13)
88.12(13)
75.37(15)
67.97(17)
71.92(15)
125.05(15)
131.22(15)
91.12(13)
92.07(13)
87.39(13)
75.97(15)
147.85(18)
71.11(15)
131.94(14)
71.23(15)
Mn1–O1
Mn1–O6
Mn2–O1
Mn2–O2
Mn2–O5
Mn3–O2
Mn3–O3
Mn4–O3
Mn4–O4
Mn4–O6
Mn5–O
2.215(3)
2.201(4)
2.181(4)
2.188(4)
2.165(3)
2.205(4)
2.204(3)
2.153(4)
2.182(3)
2.143(3)
2.210(3)
2.205(3)
Mn5–O5
H = - J (S1S2 + S1S4 + S1S5 + S3S2 + S3S4 + S3S5)
(1)
3
Mn1–O1
Mn1–O5
Mn2–O1
Mn2–O2
Mn2–O6
Mn3–O2
Mn3–O3
Mn4–O3
Mn4–O4
Mn4–O5
Mn5–O4
Mn5–O6
2.182(6)
2.202(6)
2.212(6)
2.142(6)
2.234(6)
2.162(6)
2.157(5)
2.161(6)
2.195(6)
2.184(5)
2.165(6)
2.207(5)
O1–Mn1–O5
O1–Mn1–N1
O5–Mn1–N1
O1–Mn1–N4
O2–Mn2–O1
O2–Mn2–O6
O1–Mn2–O6
O6–Mn2–N6
O3–Mn3–O2
O3–Mn3–N9
N20–Mn3–O3
N12–Mn3–O3
O3–Mn4–O4
O3–Mn4–O5
O5–Mn4–O4
O3–Mn4–N30
N28–Mn5–N25
N28–Mn5–O4
N28–Mn5–O6
N44–Mn5–O6
107.5(2)
139.5(2)
94.0(3)
71.7(3)
87.2(2)
92.7(2)
84.2(2)
87.7(3)
106.0(2)
92.9(3)
72.3(3)
126.1(3)
90.9(2)
91.4(2)
85.9(2)
165.4(3)
68.4(3)
71.9(3)
133.6(3)
72.4(3)
Here, the numbering of the spins follows the numbering of the
Mn(II) atoms in Fig. 5. The best fit parameters obtained value
were J = -2.95 cm-1 and g = 2.02 for 1, J = -3.19 cm-1 and g = 2.00
for 2 and J = -3.00 cm-1 and g = 2.05 for 3 with an error, R = 1.2 ¥
10-5, 3.1 ¥ 10-5 and 1.2 ¥ 10-5 respectively, where R = R[(cmT)exp
-
(cmT)calc]2/R[(cmT)exp]2. The series of calculations were made using
the computer program CLUMAG that uses the irreducible tensor
operator formalism (ITO).19
The low super-exchange parameter values found for these three
complexes can be related to the nature of the bridging among
neighbouring Mn(II) ions, mainly the angle values which are
varying between 125.3◦–128.5◦ in 1, 128.7◦–131.2◦ in 2 and
125.8◦–131.7◦ in 3, and the Mn ◊ ◊ ◊ Mn bond lengths which are
˚
in the range 3.825–4.054 A (see Table 3). The J values observed in
the three compounds have the same sign and magnitude of those
obtained for the Mn5 complexes reported by L. K. Thompson
et al.18
˚
metal-pyrazole-N distances are 2.303, 2.195 and 2.207 A respec-
tively for complex 1, while, for 2, the same are 2.458, 2.147
and 2.223 A respectively. The average metal-pyrazine-N, metal-
EPR spectra of 1 and 2
˚
azomethine-N and metal-pyrazole-N distances are 2.380, 2.148
EPR spectra of complexes 1 and 2 have been included as Electronic
Supplementary Information†.
˚
and 2.275 A respectively for 3. The average Mn ◊ ◊ ◊ Mn distance
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Fig. 1 (a) Structural representation and atom numbering scheme of the cationic complex 1. (H-atoms were omitted for clarity). (b) Structural
representation and atom numbering scheme of the penta-nuclear core in 1.
Fig. 2 (a) Structural representation and atom numbering scheme of the cationic complex 2 (H-atoms were omitted for clarity). (b) Structural
representation and atom numbering scheme of the penta-nuclear core in 2.
CShM and DFT calculations (B3LYP functional) studies of 1–3
We have also carried out continuous shape measurements for
the five manganese atoms and the six oxygen atoms of the alkoxo
bridge of the compounds 1, 2 and 3. Low measures of trigonal
bipyramid have been obtained (BPT-5) for the Mn atoms and
a close octahedron (OC-6) is measured in the case of the oxygen
atoms (see Scheme 2 and Fig. 7). Focusing on these results one can
conclude the following: i) the manganese atoms located in the axial
positions of the BPT-5 have an octahedral geometry while those
located in the equatorial positions have a trigonal prism geometry.
ii) The six magnetic interactions are always among axial and
equatorial positions of the manganese BPT-5. iii) The polyhedral
BPT-5 formed by the manganese atoms and OC-6 formed by the
alkoxo oxygen atoms are quite similar to each other. These results
suggest that all the magnetic interactions between the manganese
ions should be similar in the three compounds (1, 2 and 3); there
In order to examine the above results and understand the
mechanism of magnetic coupling, firstly, we have studied the
geometry of the manganese atoms of the three pentanuclear
systems using the CShM studies proposed by Avnir, Alvarez and
co-workers.20 According to this theory, if a structure Q is fully
coincident in shape with reference polyhedron P, then SQ(P) = 0
[The SQ(P) expression is given below in the computational details
section]; deviation of the value from 0 measures the departure from
the ideal shape of the P polyhedron. To compare the measures
obtained for OC-6 (octahedron) and for the distortion to TBPR-6
(trigonal prism), we have made a shape map (Fig. 6). The plot also
represents the minimal distortion interconversion path between
the two polyhedra reference considered.
418 | Dalton Trans., 2012, 41, 413–423
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Fig. 3 (a) Structural representation and atom numbering scheme of the cationic complex 3 (H-atoms were omitted for clarity). (b) Structural
representation and atom numbering scheme of the penta-nuclear core in 3.
Fig. 4 Plots of the cM T ns. T and M/Nb ns. H at T = 2 K (inset) for
1, 2 and 3. Solid lines correspond to the best fit of cM T ns. T and the
simulation M/Nb ns. H at T = 2 K with g = 2 respectively (see text). The
experimental magnetization curves of the complexes 1, 2 and 3 were fitted
Fig. 5 Scheme interactions in complexes 1, 2 and 3.
to the Brillouin equation giving the following values T = 2.00, S = 2.55 and
g = 2.00 for 1, T = 2.02, S = 2.51 and g = 1.98 for 2 and T = 2.07, S = 2.56
and g = 2.09 for 3.
These values agree with the experimental ones being close to
-3 cm-1 and can be correlated to the average Mn ◊ ◊ ◊ Mn distances
is a certain likeness between the coupling constants in the three
studied complexes.
˚
˚
˚
(3.913 A for 1, 3.961 A for 2 and 3.939 A for 3) indicating that the
antiferromagnetic interaction in a certain way decreases with the
Mn ◊ ◊ ◊ Mn separation.
To complete the study, DFT calculations have been performed
using, initially, in order to reduce the computational cost, the
atomic coordinates of two dinuclear models Mn1–2 and Mn2–3
which are two 1 structure fragments (Fig. 8). The distances
Mn–Mn and angles Mn–O–Mn in the two models are slightly
different (Table 3). The calculated J values (details are described
in the computational methodology section) are found to be -3.1
and -3.2 for Mn1–2 and Mn2–3 models, respectively. Such as we
predicted, the obtained values are very close: similar local and
molecular symmetry and quasi-spherical spin density (Fig. 9) that
correspond to MnII distribution (one electron for each d orbital).
Basing on the previous studies of the shape measures and
the DFT calculations, we have been able to reproduce the
DFT calculation on the complete structures with six identical J
interactions (scheme interaction in Fig. 5). The values obtained in
this case were -4.1, -2.8 and -3.7 cm-1 for 1, 2 and 3 respectively.
On the other hand, it is extremely difficult to relate the magnetic
interaction and shape measurements of these systems although
we see for 2, which is antiferromagnetically the least one, two
octahedral Mn distortions differents to those of compound 1 and
3 (see Fig. 6). These differences are not significant if we consider
the isotropic Mn(II) interaction. Compounds 1 and 3 have lower
shape measurements in the octahedral polyhedron formed by the
oxygen bridge atoms. In contrast these compounds have higher
shape measurements in the trigonal bipyramid polyhedron formed
by the five manganese atoms (Fig. 7).
Continuous shape measurement (CShM) details
These measurements provide a way of quantitatively evaluating
the loss of shape or symmetry, to estimate the degree of distortion
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Table 3 Selected geometrical parameters, continuous shape measurements and exchange coupling constants of 1, 2 and 3
a(Mn–O–Mn)/◦
d(Mn ◊ ◊ ◊ Mn)/A
CshMjb Mni, Mnj
J
B3LYP/cm-1 (full structure)jc
J
Exp../cm-1
j
a
d
˚
Complex
Mni–j
1
Mn1–2
Mn1–4
Mn1–5
Mn3–2
Mn3–4
Mn3–5
Mn1–2
Mn1–4
Mn1–5
Mn3–2
Mn3–4
Mn3–5
Mn1–2
Mn3–2
Mn3–4
Mn1–4
Mn3–5
Mn1–5
128.4
128.0
125.6
125.3
127.6
128.5
129.6
128.7
131.2
129.0
130.6
129.7
130.2
127.2
127.7
131.6
131.7
125.8
3.943
3.953
3.851
3.843
3.952
3.935
3.974
3.918
3.964
3.940
3.995
3.970
3.976
3.895
3.876
4.006
4.054
3.825
2.1, 4.8
2.1, 4.9
2.1, 4.7
1.9, 4.8
1.9, 4.9
1.9, 4.7
3.4, 4.2
3.4, 4.1
3.4, 4.5
2.9, 4.2
2.9, 4.1
2.9, 4.5
2.6, 5.9
2.6, 5.1
2.6, 6.7
2.8, 5.9
2.8, 5.1
2.8, 6.7
-4.1(1)
-2.8(2)
-3.7(1)
-3.0
-3.2
-3.0
2
3
a Mnij, i and j indicate the manganese number considered in the exchange coupling (see Fig. 9). b The CShM measurements correspond to the lowest
values obtained for Mn(II) octahedral and trigonal prism coordination for Mni and Mnj respectively. c The number in parentheses indicates the standard
deviation of the least-squares fitting (see text). d JExp is determined by the fit of the experimental cTM data by using the Hamiltonian (see text).
Fig. 7 Representation of S(OC-6) for oxygen bridge atoms and S(BPT-5)
for manganese system atoms ns JB3LYP for 1, 2 and 3 indicated by hexagons,
triangles and squares symbols respectively. A filled and open symbol
Fig. 6 The minimal distortion path between the octahedron (OC-6) and
the trigonal prism (TPR-6) is represented by the continuous line. The
hexagons, the triangles and the squares symbols indicate the position
(shape measures) of the manganese polyhedral coordination for 1, 2 and
3 complexes, respectively.
represents octahedral and trigonal bipyramidal geometry, respectively.
N
⎡
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎤
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎦
ꢁ
ꢁ
2
2
qi − pi
∑
i=1
(2)
S (P) = min
.100
Q
N
ꢁ
ꢁ
qi −q0
∑
i=1
of a particular coordination polyhedron from some chosen ideal
polyhedron. The calculation of continuous shape measurements
of the coordination sphere of a given atom (polyhedron Q), relative
ꢀꢀꢁ
to an ideal polyhedron, P, (Scheme 3) requires the knowledge of
In eqn (2),
is the position vector of the geometric center of Q.
q0
ꢀꢀꢁ
the N vectors
describing the 3N Cartesian coordinates qi, as
The CShM values must lie within 0 and 100. If a structure has the
desired shape, SQ(P) = 0, the shape measurement values increase
as it departs from P-polyhedron reaching a maximum value (not
necessarily 100). Stereochemical studies of polyhedral structures
having between four and ten vertices on the basis of CShM have
been published by Alvarez et al.20
qi
ꢀꢁ
well as the corresponding vectors containing the coordinates
pi
of the ideal polyhedron. The ideal shape is then rotated, translated,
and scaled in such a way so as to minimize the distance function
in the equation 7, which then gives the shape measurement of the
investigated structure, Q, relative to the ideal shape P, SQ(P).
420 | Dalton Trans., 2012, 41, 413–423
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Scheme 2 Structural representation for 1, 2 and 3 showing the trigonal
bipyramid (BPT-5) or octahedron (OC-6) polyhedron formed for man-
ganese atoms and the alkoxo oxygen atoms bridged ligands respectively.
In parenthesis the local geometry of manganese atoms, trigonal prism
(TPR-6) or octahedron (OC-6) is indicated.
Fig. 9 Spin density distribution for Mn dinuclear model corresponding
to the S = 0 ground state single determinant B3LYP solution. Positive and
negative values are represented as white and dark surfaces, respectively.
lations for those pentanuclear systems, we have employed a spin
Hamiltonian (eqn (1)) with six equal J values (scheme interaction
in Fig. 5).
For the evaluation of the n different coupling constants Jij
present in a polynuclear complex, we need to carry out calculations
for at least n + 1 different spin configurations.23 Thus, solving the
system of n equations obtained from the energy differences we
can obtain the n coupling constants. In the case that more than n
spin distribution was calculated, a fitting procedure to obtain the
coupling constants must be used and the standard deviation of
the least square fitting is presented in parenthesis (Table 3). In the
specific case of the studied Mn5 complex, {Mn1, Mn2, Mn3 Mn4,
Mn5}, three spin configurations have been employed to obtain the
six indicated J values: {+, +, + +, +}, {-, +, + +, +}, {+, +, - +,
+}, where the negative sign corresponds to the axial manganese of
the trigonal bipyramid polyhedron formed by the five manganese
ions.
Fig. 8 Dinuclear structural representation model (Mn1–2) with exchange
coupling constant, J, in 1. There are six similar interactions in the
compound.
All calculations were performed appealing to the density
functional theory approach, using the hybrid functional B3LYP24
and the triple-z all electron basis set was used for all the atoms.25
The guess function was generated using Jaguar 7.0 code.26 Total
energy calculations were performed using Gaussian 03 code.27 The
use of the non-projected energy of the broken symmetry solution
as the energy of the low spin state within the DFT framework
provides good results because it avoids the cancellation of the
non-dynamic correlation effects as stated in previous works.28
Scheme 3 Coordination sphere of a given atom (polyhedron Q), relative
to an ideal polyhedron P.
DFT computational Details
Kinetic studies
The use of density functional theory calculations provides a good
estimation of the exchange coupling constants taking into account
the tiny involved energy differences.21,22
The cluster to grid transformation process is substantiated by time
dependent UV-vis spectroscopic studies of reactions between the
10-6 M solutions of 1–3 and 10-6 M of hydrated Cu(II) nitrate
salt solution. The conversion kinetics are monitored by following
the reactant (Mn5(II) cluster) LMCT band and the product
(Cu4(II) [2 ¥ 2] grid) d-d band using absorption spectroscopy. The
compounds (1–3) show absorption maxima at 335, 347 and 350
nm respectively. On addition of Cu(NO3)2·6H2O solution (1 ¥
10-6 M) to the solution (1 ¥ 10-6 M complex solution of 1), there
is a gradual change in spectral pattern (Fig. 10 for 1 and Fig.
Initially we have calculated the coupling constant, J, of two
dinuclear models of complex 1. The exchange coupling constant
are calculated employing a phenomenological Heisenberg Hamil-
tonian H = - J Si Sj (i and j are the different paramagnetic centers)
to describe the interactions between the two MnII present in the
dinuclear models.
Considering the continuous shape measurements, the experi-
mental geometrical parameters and the dinuclear models calcu-
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11 for 2, other figures of the kinetic studies are in the ESI†). The
maximum peak at 347 nm gradually decreases in intensity with the
appearance of an isosbestic point at 388 nm (at room temperature
for 4 h). The absorbance of the broad d-d band of the copper(II)
complex gradually increases and reaches a maxima at 795 nm.
The occurrence of an isosbestic point strongly suggests that the
original reactant is being replaced by one product or if by more
than one product that these are always in a strictly constant ratio.
The absence of appreciable amounts of reaction intermediates
can thus be implied.29 This fact is substantiated by the isolation
and X-ray structure determination of the reaction product which
is a Cu4(II) grid. In the case of complex 3, the reaction goes to
completion within a short time (~10 min). Because of this, only
a few spectral curves are obtained during the kinetic study of
complex 3.
Fig. 11 Absorbance spectral change for the LMCT band of the complex
2 (1 ¥ 10-6 M) after addition of Cu(NO3)2·6H2O (1 ¥ 10-6 M) spectrum
recorded at 10 min intervals and d-d band of the Cu(II) complex (inset).
Curves 1 → 8 correspond to a decrease in absorbance of the LMCT band
of 2 and 8 → 1 correspond to an increase in absorbance of the d-d band
of the Cu complex.
conversion of Mn5(II) clusters to Cu4(II) [2 ¥ 2] grids follows the
Irving-Williams series.
Acknowledgements
Financial support [Sanc. No. SR/S1/IC-27/2008, dated
13.03.2009] from Department of Science & Technology (DST),
New Delhi, India, is gratefully acknowledged. T. N. M. thanks
University Grants Commission, New Delhi, India, for a fel-
lowship. The Spanish (CTQ2009-07264/BQU and CTQ2008–
06670-C02–01) and Catalan (2009 SGR-1454 and 2009SGR-
1459) financial support are acknowledged. J. Tercero is grateful
to the Centre de Computacio´ de Catalunya (CESCA) with a grant
provided by Fundacio´ Catalana per a la Recerca (FCR) and the
Universitat de Barcelona. X-ray Crystallography was performed
at the DST funded National Single Crystal Diffractometer facility
at the Department of Chemistry, University of Calcutta.
Fig. 10 Absorbance spectral change for LMCT band of the complex
1 (1 ¥ 10-6 M) after addition of Cu(NO3)2·6H2O (1 ¥ 10-6 M) spectrum
recorded at 10 min intervals and d-d band of Cu(II) complex (inset). Curves
1 → 8 correspond to decrease in absorbance of LMCT band of 1 and 8 →
1 correspond to increase in absorbance of d-d band of 4.
Conclusion
The ligands PzCAP, PzOAP and PyPzOAPz formed pentanuclear
Mn5(II) clusters with (BPT-5) Mn(II) core. Density functional
theory calculations based on the single crystal X-ray structures
for the Mn(II) complexes 1, 2 and 3 have been used to model
the exchange processes leading to the experimentally determined
values which reveal a typical antiferromagnetic interaction in
them. The CShM studies have been performed to qualitatively
evaluate the loss of shape or symmetry, so as to estimate the degree
of distortion in the Mn5(II) coordination polyhedron for an ideal
polyhedron. Low measures of the trigonal bipyramid have been
obtained (BPT-5) for the Mn-atoms and a close octahedron (OC-
6) measures in the case of oxygen atoms. The measurement infers
that the Mn-atom located in the axial position of the BPT-5 has
an octahedral geometry, while, those located in the equatorial
positions have a trigonal prism geometry. Overall, a reasonable
theoretical interpretation of the magnetic behavoiur shown by the
complexes has been provided from the results of the studies. The
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