Control of molecular architecture by steric factors: Mononuclear vs polynuclear manganese(iii) compounds with tetradentate N2O 2 donor Schiff bases

Article abstract of DOI:10.1039/c0dt01723j

Three manganese(iii) compounds, [Mn^III(vanoph)(DMF)(H ₂O)]ClO₄ (1), [Mn^III(vanoph)(N ₃)(H₂O)]·2H₂O (2) and [Mn ^III(saloph)(μ_1,3-N₃)]_n (3), where H₂vanoph = N,N′-(1,2-phenylene)-bis(3- methoxysalicylideneimine), H₂saloph = N,N′-(1,2-phenylene)- bis(salicylideneamine) are tetradentate N₂O₂ ligands and DMF = N,N-dimethylformamide, have been prepared and characterised by elemental analysis, IR and UV-Vis spectroscopy and single-crystal X-ray diffraction studies. Compounds 1 and 2 are monomeric but compound 3 consists of a chain system with the repeating unit [Mn^III(saloph)(N₃)] bridged by μ-1,3 azide. Compound 1 crystallises in monoclinic space group P2 ₁/n with cell dimensions of a = 11.1430(2), b = 16.3594(3), c = 15.4001(3) A, β = 108.417(1), Z = 4 whereas compounds 2 and 3 crystallise in orthorhombic space groups Pbca and Pna2₁, respectively, with cell dimensions of a = 16.069(3), b = 15.616(3), c = 18.099(4) A, Z = 8 (for 2) and a = 18.760(9), b = 13.356(5), c = 6.616(3) A, Z = 4 (for 3). In all the compounds, Mn(iii) has a six-coordinated pseudo-octahedral geometry in which O(2), O(3), N(1) and N(2) atoms of the deprotonated di-Schiff base constitute the equatorial plane. In both compounds 1 and 2, water molecules are present in the fifth coordination sites in the apical positions. The sixth coordination sites are occupied by one O atom of a solvent DMF in compound 1 and an N atom of azide in compound 2. The coordinated water initiates hydrogen-bonded networks in both compounds 1 and 2 to form well-isolated supramolecular dimers. At room temperature the χ_MT values for the compounds 1 and 2 remain almost constant until 30 K. Below this temperature, the χ_MT values drastically drop to 0.72 cm ³ mol^-1 K for 1 and 0.52 cm³ mol^-1 K for 2. The best fits were obtained with J = -0.92 cm^-1, D = 2.05 cm^-1, g = 2.0 and R = 8.1 × 10^-4 for 1 and J = -1.16 cm^-1, D = 2.05 cm^-1, g = 2.0 and R = 1.2 × 10 ^-3 for 2. However, in compound 3, two axial positions are occupied by the azide ions. The Mn...Mn repeating distance is 6.616 A along the chain. Magnetic characterisation shows that the μ_1,3-bridging azide ion mainly transmits an antiferromagnetic interaction (J = -6.36 cm ^-1) between Mn(iii) ions. The presence of two methoxy groups increases the steric crowding in the H₂vanoph moiety and thereby inhibits the formation of a polynuclear compound with this ligand.

Full text of DOI:10.1039/c0dt01723j

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PAPER
Control of molecular architecture by steric factors: mononuclear vs
polynuclear manganese(III) compounds with tetradentate N2O2 donor Schiff
bases†
Prasanta Bhowmik,a Hari Pada Nayek,b Montserrat Corbella,*c Nu´ria Aliaga-Alcalde*d and
Shouvik Chattopadhyay*a
Received 8th December 2010, Accepted 9th May 2011
DOI: 10.1039/c0dt01723j
Three manganese(III) compounds, [MnIII(vanoph)(DMF)(H2O)]ClO4 (1), [MnIII(vanoph)(N3)(H2O)]·
2H2O (2) and [MnIII(saloph)(m1,3-N3)]n (3), where H2vanoph = N,N¢-(1,2-phenylene)-bis(3-methoxy-
salicylideneimine), H2saloph = N,N¢-(1,2-phenylene)-bis(salicylideneamine) are tetradentate N2O2
ligands and DMF = N,N-dimethylformamide, have been prepared and characterised by elemental
analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. Compounds 1 and 2
are monomeric but compound 3 consists of a chain system with the repeating unit [MnIII(saloph)(N3)]
bridged by m-1,3 azide. Compound 1 crystallises in monoclinic space group P21/n with cell dimensions
˚
of a = 11.1430(2), b = 16.3594(3), c = 15.4001(3) A, b = 108.417(1), Z = 4 whereas compounds 2 and 3
crystallise in orthorhombic space groups Pbca and Pna21, respectively, with cell dimensions of a =
˚
˚
16.069(3), b = 15.616(3), c = 18.099(4) A, Z = 8 (for 2) and a = 18.760(9), b = 13.356(5), c = 6.616(3) A,
Z = 4 (for 3). In all the compounds, Mn(III) has a six-coordinated pseudo-octahedral geometry in which
O(2), O(3), N(1) and N(2) atoms of the deprotonated di-Schiff base constitute the equatorial plane. In
both compounds 1 and 2, water molecules are present in the fifth coordination sites in the apical
positions. The sixth coordination sites are occupied by one O atom of a solvent DMF in compound 1
and an N atom of azide in compound 2. The coordinated water initiates hydrogen-bonded networks in
both compounds 1 and 2 to form well-isolated supramolecular dimers. At room temperature the cMT
values for the compounds 1 and 2 remain almost constant until 30 K. Below this temperature, the cMT
values drastically drop to 0.72 cm3 mol-1 K for 1 and 0.52 cm3 mol-1 K for 2. The best fits were obtained
with J = -0.92 cm-1, |D| = 2.05 cm-1, g = 2.0 and R = 8.1 ¥ 10-4 for 1 and J = -1.16 cm-1, |D| =
2.05 cm-1, g = 2.0 and R = 1.2 ¥ 10-3 for 2. However, in compound 3, two axial positions are occupied by
˚
the azide ions. The Mn ◊ ◊ ◊ Mn repeating distance is 6.616 A along the chain. Magnetic characterisation
shows that the m1,3-bridging azide ion mainly transmits an antiferromagnetic interaction (J =
-6.36 cm-1) between Mn(III) ions. The presence of two methoxy groups increases the steric crowding in
the H2vanoph moiety and thereby inhibits the formation of a polynuclear compound with this ligand.
high-dimensional molecular systems with adjustable structures.1,2
Introduction
The design and synthesis of transition-metal coordination poly-
mers bridged by small conjugated ligands (such as cyanide,
azide, oxalate, thiocyanate and nitrite) are currently under intense
investigation in view of their structural diversity and in the context
of molecule-based magnets.3 Recent studies on molecule-based
magnets include bulk materials with spontaneous magnetisation
with high TC, even above room temperature,4 hybrid magnetic
compounds with dual properties exemplified by magneto-chiral
dichroism,5 photomagnetic effects6 and porous magnetic metal–
organic frameworks7 as well as nano-scale magnets such as
single molecule magnets (SMM)8 or single chain magnets (SCM)9
exhibiting slow relaxation of magnetisation. To achieve low-
dimensional magnetic systems such as SMM and SCM, the
choice of appropriate bridging ligands is of ultimate importance
There have been many studies on developing engineered
supramolecular networks in an effective manner and designing
aDepartment of Chemistry, Inorganic Section, Jadavpur University, Kolkata,
700 032, India. E-mail: shouvik.chem@gmail.com
bInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology, En-
gesser str. 15, D-76131, Karlsruhe, Germany
cDepartament de Qu´ımica Inorga`nica, Universitat de Barcelona (UB), Mart´ı
i Franque`s 1-11, 08028, Barcelona, Spain. E-mail: monte.corbella@qi.ub.es
dICREA Junior Researcher & Departament de Qu´ımica Inorga`nica, Univer-
sitat de Barcelona (UB), Mart´ı i Franque`s 1-11, 08028, Barcelona, Spain.
E-mail: nuria.aliaga@icrea.cat
† CCDC reference numbers 801737 (1), 794445 (2) and 794446 (3).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01723j
7916 | Dalton Trans., 2011, 40, 7916–7926
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because they can communicate magnetic couplings between
paramagnetic metal ions and then determine the magnetic strength
and nature. Along these lines, azide is an excellent ligand in
terms of constructing diverse structural topologies from discrete
molecules to three-dimensional networks, and predicting the
magnetic nature depending on the binding mode.10 In general,
the end-on mode gives ferromagnetic interactions and the end-
to-end mode produces antiferromagnetic couplings, with few
exceptions.11 The magnetic interactions in azide-bridged com-
pounds, coupled with a degree of magnetic anisotropy, result in
some basic events involving spin flop,12 spin canting,11a,b and meta-
magnetism.13
Experimental
Materials
All starting materials were commercially available, reagent grade,
and used as purchased without further purification.
CAUTION! Since the azide compounds of metal ions are
potentially explosive, only small amounts of the materials should
be handled with care.
Preparation of the ligands N,N¢-(1,2-phenylene)-bis(3-methoxy-
salicylideneimine) (H2vanoph) and N,N¢-(1,2-phenylene)-
bis(salicylideneimine) (H2saloph)
The individual properties in multidimensional magnetic materi-
als based on azide bridges are often revealed and well characterized
from a magnetostructural point of view.11–14 The Mn(III) ion itself
has a high spin number and Mn(III) Schiff base compounds mainly
within N2O2 basal environments have strong uni-axial magnetic
anisotropy created by the Jahn–Teller effect in an octahedral
ligand field.15 The [MnIII(Schiff base)]+ cations whose apical sites
can be filled by incoming bridges and/or their phenoxides have
been utilized as useful tectons for the construction of coordination
compounds with desirable properties. Polynuclear Mn(III) species
are also found to play important roles in catalytic and biological
fields16,17 especially in the oxygen-evolving compound (OEC) of
photosystem II (PSII). Dinuclear manganese compounds have
been reported for water oxidation to generate dioxygen18 and have
been used to artificially model PSII.19 Studies of spectroscopic
and physicochemical properties along with X-ray structure deter-
mination of model compounds are particularly important to gain
insights into the nature and mode of action of the manganese-
containing prosthetic groups. It has also been known that, upon
visible light irradiation, in the presence of p-quinone, Mn(III)
compounds with tetradentate Schiff base ligands having the salen-
type skeleton generate molecular oxygen20–22 and in the absence of
p-quinone, rearrangement of the coordinated Schiff base ligand
occurs.23
Both the tetradentate Schiff base ligands, H2vanoph and H2saloph,
were synthesized in a similar way by refluxing o-phenylenediamine
(10 mmol, 1.08 g) with 3-methoxysalicyldehyde (10 mmol, 1.52 g)
or salicylaldehyde (10 mmol, 1.05 cm3), respectively, in methanol
solution (20 mL) for ca. 1 h. The ligands were not isolated. The
methanol solutions were used for the syntheses of the compounds.
Synthesis of [MnIII(vanoph)(DMF)(H2O)]ClO4 (1)
To prepare compound 1, a methanol solution (20 mL) of Mn(II)
perchlorate hexahydrate (1 mmol, 0.362 g) was added to a
methanol solution (1 mmol) of H2vanoph. The reaction mixture
was refluxed for 1.5 h. Crystalline compounds started to separate
after cooling the resulting solution. The product was recrystallised
from DMF solution to obtain single crystals suitable for X-ray
diffraction.
Yield: 0.26 g (42%). Anal. Calc. for C25H27ClMnN3O10 (619.89):
C, 48.4; H, 4.4; N, 6.8; Mn, 8.9. Found: C, 48.4; H, 4.4; N, 6.8;
Mn, 8.9%. UV–Vis, lmax/nm (emax/dm3 mol-1 cm-1) (CH3CN), 464
(490), 363 (5468), 314 (5683), 243 (7260).
Synthesis of [MnIII(vanoph)(N3)(H2O)]·2H2O (2)
To prepare compound 2, a methanol solution (20 mL) of Mn(II)
acetate tetrahydrate (1 mmol, 0.245 g) was added to a methanol
solution (1 mmol) of H2vanoph. A solution of NaN3 (1 mmol,
0.065 g) in methanol–water was then added to it and refluxed
for 1.5 h. The resulting solution was then filtered hot. Crystalline
compounds started to separate after cooling the resulting solution
and were collected by filtration after ca. 10 h. Single crystals
suitable for X-ray diffraction were obtained at that time.
Many Mn(III) compounds of the type [MnIII(L)(X)], {where
H2L = H2salen or H2salpn or their derivatives and X = N3}
are well known for many years.24–33 Some of the compounds
are mononuclear25 or dinuclear26–29 and the remainder are one-
dimensional chains.26,30–33 On the other hand, reports on Mn(III)
compounds of the type [MnIII(L)(X)], {where H2L = H2saloph =
N,N¢-(1,2-phenylene)-bis(salicylideneimine) or H2vanoph
=
N,N¢-(1,2-phenylene)-bis(3-methoxysalicylideneimine) and X =
N3} are relatively scanty and to the best of our knowledge
only two such structures ([MnIII(saloph)(N3)(CH3OH)]
and [MnIII(saloph)(N3)(H2O)]) have been reported so far,
both of which contain terminal azides.30,34 No chain
system with the repeating unit [MnIII(saloph)(N3)] or
[MnIII(vanoph)(N3)] bridged by azide are found in the
literature. Although our attempts with H2vanoph affords
mononuclear compounds [MnIII(vanoph)(DMF)(H2O)]ClO4
Yield: 0.27 g (51%). Anal. Calc. for C22H28MnN5O7 (525.4): C,
49.9; H, 5.3; N, 13.2; Mn, 10.4. Found: C, 49.9; H, 5.3; N, 13.2;
Mn, 10.4%. UV–Vis, lmax/nm (emax/dm3 mol-1 cm-1) (CH3CN),
584 (366), 468 (488), 354 (5483), 308 (5562), 244 (7255).
Synthesis of [MnIII(saloph)(l1,3-N3)]n (3)
To prepare compound 3, a methanol solution (20 mL) of Mn(II)
acetate tetrahydrate (1 mmol, 0.245 g) was added to a hot methanol
solution (1 mmol) of H2saloph. A solution of NaN3 (1 mmol, 0.065
g) in methanol was added to it and refluxed for 1.5 h. The resulting
solution was then filtered hot. X-Ray quality single crystals started
to separate after cooling the resulting solution and were collected
by filtration.
and
[MnIII(vanoph)(N3)(H2O)],
polynuclear
compound
[MnIII(saloph)(m1,3-N3)]n has been furnished using H2saloph.
The structures of the compounds are established by X-ray
crystallographic analysis. In the present paper, we report the
syntheses, spectroscopic characterizations, redox behavior,
magnetic properties and sterically regulated structural variation
in such compounds.
Yield: 0.23 g (56%). Anal. Calc. for C20H14MnN5O2 (411.3): C,
58.4; H, 3.4; N, 17.0; Mn, 13.4; Found: C, 58.4; H, 3.4; N, 17.1;
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Scheme 1
Mn, 13.4%. UV–Vis, lmax/nm (emax/dm3 mol-1 cm-1) (CH3CN),
570 (264), 455 (854), 328 (5225), 297 (5883), 247 (8163).
solved using direct methods (SHELXS97 program).35 The non-
hydrogen atoms were refined with anisotropic thermal parameters.
The hydrogen atoms bonded to carbon and nitrogen were included
in geometric positions and given thermal parameters equivalent
to 1.2 times or 1.5 times (for methyl groups) those of the atom
to which they were attached. The structures of 1, 2 and 3
were refined by full-matrix least-squares refinement against F2,
using SHELXS97 and SHELXL97 software.35 A summary of the
crystallographic data are given in Table 1.
Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was per-
formed using a Perkin–Elmer 240 C elemental analyzer. IR
spectra in KBr (4500–500 cm-1) were recorded using a Perkin–
Elmer RXI FT-IR spectrophotometer. Electronic spectra in
acetonitrile (1200–350 nm) were recorded in a Hitachi U-
3501 spectrophotometer. Magnetic susceptibility measurements
between 2–300 K were carried out in a SQUID magnetometer
Quantum Design Magnetometer, model MPMP at the “Unitat de
Mesures Magne`tiques (Universitat de Barcelona)”. Two different
magnetic fields were used, 300 G (2–30 K) and 3000 G (2–300
K), with superimposable graphs. Pascal’s constants were used to
Results and discussion
Syntheses
The ligands (H2vanoph and H2saloph) were prepared by the 1 : 2
condensation of the o-phenylenediamnine with salicyldehyde and
3-methoxysalicyldehyde, respectively, in methanol following the
literature method.36 Both the ligands have already been used
often to prepare several mono- and polynuclear compounds.30,34,37
In the present work, H2vanoph was made to react with Mn(II)
perchlorate to prepare compound 1 and was recrystallised from
DMF. Compound 2 was synthesised by refluxing the same ligand
with Mn(II) acetate and sodium azide in methanol. The formation
of compounds 1 and 2 is shown in Scheme 1. In both the cases,
mononuclear octahedral compounds containing the tetraden-
tate ligand (vanoph)2- are formed, with one coordinated water
molecule. Compounds with coordinated water molecules are very
important as they open up the possibilities of preparing several
estimate the diamagnetic corrections for the compounds. The fit
ꢀ
was performed by minimising the function R =
[(cMT)exp -
ꢀ
(cMT)calc]2/ [(cMT)exp]2.
X-Ray crystallography
Suitable crystals of compounds 1, 2 and 3 were covered in
mineral oil (Aldrich) and mounted on a glass fiber. The crystals
were transferred directly into the stream of N2 of a Stoe IPDS-
II diffractometer with graphite-monochromated Mo-Ka (l =
◦
˚
0.71073 A) radiation at -123 or -73 C. Subsequent computations
were carried out on an Intel Pentium IV PC. The structures were
7918 | Dalton Trans., 2011, 40, 7916–7926
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Table 1 Crystal data and refinement details of compounds 1, 2 and 3
1
2
3
Formula
Mr
Crystal size/mm
T/K
C25H27ClMnN3O10
619.89
0.3 ¥ 0.2 ¥ 0.2
293
C22H28MnN5O7
525.40
0.14 ¥ 0.15 ¥ 0.17
150
C20H14MnN5O2
411.30
0.2 ¥ 0.2 ¥ 0.3
200
Crystal system
Space group
Monoclinic
P21/n
11.143(2)
16.359(3)
15.400(3)
108.417(10)
4
1.546
0.659
1280
45311
Orthorhombic
Pbca
16.069(3)
15.616(3)
18.099(4)
90.000
Orthorhombic
Pna21
18.760(9)
13.356(5)
6.616(3)
90.000
˚
a/A
˚
b/A
˚
c/A
b/◦
Z
8
4
Dc/g cm -3
1.537
0.636
2176
20572
5474
3296
338
0.100
1.648
0.826
840
7096
2075
3131
254
0.107
m/mm -1
F(000)
Total reflections
Unique reflections
Observed data [I > 2s(I)]
No. of parameters
R(int)
6701
4769
369
0.033
R1, wR2 (all data)
R1, wR2 [I > 2s(I)]
0.0740, 0.1673
0.0509, 0.1478
0.1024, 0.1607
0.0655, 0.1478
0.0801, 0.1954
0.0726, 0.1904
water bridged dimeric compounds. Coordinated water molecules
can also play a significant role in understanding the mechanism of
ligand replacement reactions. However, the sixth coordination site
is satisfied by a solvent DMF in case of compound 1 and a terminal
azide in compound 2. In both the compounds, the hydrogen atoms
of the water molecules are involved in H-bonding.
Compound 2 can also be prepared by slow diffu-
sion of concentrated methanol solutions of NaN3 and
[MnIII(vanoph)(H2O)]ClO4 in the two arms of an H-tube, re-
spectively. On the other hand, even after trying several times to
prepare the azide bridged polynuclear compound with H2vanoph,
by gradually increasing the time of reflux, water coordinated
mononuclear compound is always formed, as confirmed by the
IR and mass spectra and thermo-gravimetric analysis, and not the
desired one. Thus it is evident that the presence of two additional
methoxy groups imposes some extra steric restrictions on the
ligand so that the activation energy cannot be achieved by reflux.
The participation of the methoxy oxygen atoms of H2vanoph in
hydrogen bonding with the H atoms of water may also tilt the
stability in favor of mononuclear compound.
A
one-dimensional compound (3) resulted by refluxing
H2saloph, Mn(II) acetate and sodium azide. In this compound,
Mn(III) is hexa-coordinated by a N2O2 donor Schiff base,
saloph2- and two bridging azides (Scheme 2). However, water-
coordinated mononuclear compound, [MnIII(saloph)(H2O)(N3)]
(similar to compound 2) was prepared by stirring a methanol
solution of H2saloph, Mn(ClO4)2·6H2O and NaN3. The struc-
ture of the compound is reported elsewhere.34 A similar
complex, [MnIII(saloph)(CH3OH)(N3)] was prepared by slow
diffusion of concentrated methanol solutions of NaN3 and
[MnIII(saloph)(H2O)]ClO4 in the two arms of an H-tube,
respectively.30 [MnIII(saloph)(H2O)]ClO4 was prepared by mixing
Mn(III) acetate dihydrate and H2saloph in methanol and anhy-
drous sodium perchlorate in water. After reducing the volume
of the solution by evaporation and then cooling, the resulting
black crystals were collected by suction filtration.38 The same
compound can again be prepared by stirring a methanol or
acetonitrile solution of H2saloph and adding successively a
methanol or acetonitrile solution of Mn(ClO4)2·6H2O and Et3N
(TEA) under stirring.39 Interestingly, compound 3 can also be
synthesised by putting the methanol solution of mononuclear
[MnIII(saloph)(CH3OH)(N3)] or [MnIII(saloph)(H2O)(N3)] under
reflux for several hours. It is to be noted that the related Schiff
bases, viz. 5-OCH3salen or 5-Fsalen, form the azide bridged
polynuclear compounds under cold conditions.30 However, in the
case of H2saloph, the mononuclear structure is favored, which
may be caused by the steric effect of the rigid Schiff base ligand
H2saloph, preventing formation of an infinite chain.30 This steric
factor, in its turn, increases the activation energy for the formation
of the polynuclear compound and in order to overcome the
activation barrier, refluxing condition is essential.
IR and electronic spectra
In the IR spectra of compounds 1, 2 and 3, distinct bands due to the
azomethine (C N) group within 1649–1573 cm-1 are customarily
noticed.40 A broad band around 3400 cm-1 both in compounds 1
and 2 is assigned to the OH stretching vibration of the coordinated
water molecule, which is involved in hydrogen bonding.41 The
sharp, strong, single peak at 1092 cm-1 for compound 1 gives
evidence for the presence of ionic perchlorate.42 For compound
2, the ns(N3) and nas(N3) stretching vibration modes of the azide
group appear at 1320 and 2054 cm-1 respectively, corresponding to
the presence of a terminal azide group43 as confirmed by the X-ray
crystal structure. Compound 3 shows a strong absorption band at
2044 cm-1 and is attributed to the presence of an end-to-end azide
bridge.44
The electronic spectrum of a d4 Mn(III) system under an ideal
5
symmetric octahedral ligand field consists of a 5Tg ← Eg transi-
tion. However, the electronic spectra of most pseudo-octahedral
Mn(III) compounds are not so simple, presumably because both
static and dynamic Jahn–Teller effects perturb the octahedral
symmetry. Electronic spectra of both the Mn(III) compounds in the
present study are similar and consist of three regions of absorption.
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Scheme 2
In the first region, two bands with shoulders have been found
around 610–450 nm. These bands are reasonably assignable to
d–d transition on the basis of their low extinction coefficients.45
The bands around 290–360 nm in the second region may be
attributed to phenolate O(pp)→Mn(dp*) ligand-to-metal charge
transfer by analogy with some similar Mn(III) compounds.46 In
the third region, the remaining bands at ca. 250 nm are most likely
due to p → p* (azomethine) ligand transitions.
O(6)}, one from a DMF molecule and another from a water,
are attached to the metal atom to fill its two axial sites. The
˚
Mn–O(5) and Mn–O(6) distances (2.287(2) and 2.241(2) A) are
much longer than those of the Mn–O(2), Mn–O(3), Mn–N(1) and
˚
Mn–N(2) distances (1.876(2), 1.875(2), 1.983(2) and 1.988(2) A
respectively), which fall within the range observed for structurally
characterized Mn(III) compounds.39a,47 The elongation of axial
˚
bonds (0.412–0.253 A) indicate a clear evidence of Jahn–Teller
distortion, as expected for high-spin Mn(III). The basal bond
angles are all close to 90◦ [O(2)–Mn–O(3), O(2)–Mn–N(1), and
O(3)–Mn–N(2) are 92.27(7), 93.17(8) and 92.25(8)◦ respectively]
except N(1)–Mn(1)–N(2) [82.31(9)◦] which is smaller than
expected.39a The deviation of all the coordinating atoms in the
basal plane O(2), O(3), N(1), N(2) from the mean plane passing
Description of the structures
[MnIII(vanoph)(DMF)(H2O)]ClO4
of compound consists of
(1). The
a discrete mononuclear
structure
1
[MnIII(vanoph)(DMF)(H2O)]+ cation together with an
uncoordinated perchlorate anion. The structural representation
for the cation in 1 is shown in Fig. 1, and important bond lengths
and angles are listed in Table 2. The complex shows a distorted
octahedral geometry involving N2O2 donors as the basal plane
{N(1), N(2), O(2) and O(3)}. Two oxygen atoms {O(5) and
˚
through them is 0.005 A and that of Mn(1) from the same plane
˚
is 0.002 A.
Further studies of the crystallographic data have shown that the
coordinated aqua ligand of each mononuclear Mn(III) species is
connected to the free O2O2¢ compartment of a neighbor through
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for compounds 1, 2 and
3
1
2
3
Mn(1)–O(2)
Mn(1)–O(3)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–O(5)
Mn(1)–O(6)
Mn(1)–N(5)*
1.876(2)
1.875(2)
1.983(2)
1.988(2)
—
2.287(2)
2.241(2)
—
1.876(2)
1.879(2)
1.990(3)
1.986(3)
2.268(3)
2.256(3)
—
1.875(3)
1.871(4)
1.993(4)
2.000(4)
2.295(5)
—
—
2.385(5)
—
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
O(5)–Mn(1)–O(6)
O(3)–Mn(1)–O(6)
O(3)–Mn(1)–O(5)
O(2)–Mn(1)–O(6)
O(2)–Mn(1)–O(5)
N(1)–Mn(1)–O(5)
N(2)–Mn(1)–O(5)
O(5)–Mn(1)–N(3)
N(1)–Mn(1)–N(5)*
N(2)–Mn(1)–N(5)*
N(3)–Mn(1)–N(5)*
O(2)–Mn(1)–N(5)*
O(3)–Mn(1)–N(5)*
O(6)–Mn(1)–N(1)
O(6)–Mn(1)–N(2)
N(1)–Mn(1)–O(3)
N(2)–Mn(1)–O(3)
N(3)–Mn(1)–O(3)
N(1)–Mn(1)–O(2)
N(2)–Mn(1)–O(2)
N(3)–Mn(1)–O(2)
O(2)–Mn(1)–O(3)
N(3)–Mn(1)–O(5)
N(1)–Mn(1)–N(5)*
N(2)–Mn(1)–N(5)*
N(3)–Mn(1)–N(5)*
O(3)–Mn(1)–N(5)*
82.31(9)
—
—
82.81(11)
88.17(11)
88.18(11)
—
81.89(16)
86.34(19)
88.80(18)
—
—
—
—
—
—
—
174.65(9)
93.85(8)
91.50(8)
90.24(8)
89.53(9)
88.37(9)
90.74(9)
—
—
—
—
—
—
91.01(10)
—
93.54(10)
86.64(11)
86.80(10)
173.20(11)
—
—
—
—
—
Fig. 1 Perspective view of compound 1 along with the atom numbering
scheme. Perchlorate ion is not shown for clarity.
—
88.36(19)
82.67(18)
170.53(19)
90.62(17)
93.11(19)
—
—
86.30(9)
89.08(9)
174.56(8)
92.25(8)
—
93.17(8)
175.46(8)
—
92.27(7)
—
—
—
—
—
—
—
175.09(11)
92.77(11)
93.83(11)
92.78(11)
175.54(11)
91.11(11)
91.67(10)
173.20(11)
—
174.55(15)
92.67(14)
95.0(2)
92.55(16)
173.08(17)
93.91(18)
92.86(14)
—
88.36(19)
82.67(18)
170.53(19)
93.11(19)
—
—
—
—
Symmetry element: * = x, y, -1 + z.
Fig. 2 Formation of the supramolecular dimer. The methyl groups
attached to O(1) and O(4), the perchlorate anion and the DMF molecules
are not shown for clarity.
◦
˚
Table 3 Hydrogen bond distances (A) and angles ( ) for compound 1
D–H ◊ ◊ ◊ A
D–H
D ◊ ◊ ◊ A
H ◊ ◊ ◊ A
∠D–H ◊ ◊ ◊ A
[MnIII(vanoph)(N3)(H2O)]·2H2O (2). Compound 2 crystallizes
in the orthorhombic space group Pbca. It is a mononuclear MnIII
compoundinwhichMn(1) has a six-coordinate pseudo-octahedral
geometry in which O(2), O(3), N(1) and N(2) atoms of the
deprotonated di-Schiff base {(vanoph)2-} constitute the equatorial
plane. In the apical position the Mn(III) center is coordinated
O(5)–H(5A) ◊ ◊ ◊ O(3)#
O(5)–H(5A) ◊ ◊ ◊ O(4)#
O(5)–H(5B) ◊ ◊ ◊ O(1)#
O(5)–H(5B) ◊ ◊ ◊ O(2)#
0.66(3)
0.66(3)
0.74(5)
0.74(5)
2.46(3)
2.42(3)
2.19(5)
2.46(5)
3.021(3)
3.011(3)
2.897(3)
2.995(3)
145(3)
151(3)
162(4)
131(4)
Symmetry element: # = -x, -y, 2 - z; D, donor; H, hydrogen; A, acceptor.
-
by one N3 ion and one water molecule as terminal ligands,
intermolecular hydrogen bonding interactions creating a well-
isolated supramolecular dimer (Fig. 2). The H atom H(5A),
attached with O(5), forms bifurcated hydrogen bonds with O(3)
and O(4) of the crystallographically related (-x, -y, 2 - z) unit.
The second H atom attached with O(5), viz. H(5B), is also engaged
in bifurcated hydrogen bond formation with the symmetry related
(-x, -y, 2 - z) O(1), O(2) atoms, respectively. The details of H
bonding distances are given in Table 3. Moreover, the stability
of the resulting dimer is reinforced by p–p stacking interactions
established between the aromatic rings of the Schiff base (Fig. 3).48
respectively, finishing its distorted octahedral coordination sphere
(Fig. 4). The bond lengths of Mn(1)–O(2), Mn(1)–O(3), Mn(1)–
N(1) and Mn(1)–N(2) are 1.876(2), 1.879(2), 1.990(3) and 1.986(3)
˚
A, respectively. As expected, the bond distances in the axial
˚
˚
position (Mn(1)–N(3) 2.268(3) A; Mn(1)–O(5) 2.256(3) A) are
significantly longer than those associated with the vanoph2- ligand
due to a Jahn–Teller distortion of the Mn(III) ion. The Mn(1)–N(3)
length is a little longer than that in compound 3 for the azide ion
acts as a terminal ligand in 2 while as a bridging ligand in 3. The
donor atoms, O(2), O(3), N(1), N(2), in the equatorial plane are
approximately coplanar with no atoms deviating from the plane
#
˚
The intermetallic Mn(1) ◊ ◊ ◊ Mn(1 ) distance is 4.969 A.
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◦
˚
Table 4 Hydrogen bond distances (A) and angles ( ) for compound 2
D–H ◊ ◊ ◊ A
D–H
D ◊ ◊ ◊ A
H ◊ ◊ ◊ A
∠D–H ◊ ◊ ◊ A
O(5)–H(5A) ◊ ◊ ◊ O(3)i
O(5)–H(5A) ◊ ◊ ◊ O(4)i
O(5)–H(5B) ◊ ◊ ◊ O(1)i
O(5)–H(5B) ◊ ◊ ◊ O(2)i
O(6)–H(6A) ◊ ◊ ◊ O(7)ii
O(7)–H(7A) ◊ ◊ ◊ O(7)iii
O(6)–H(6)B ◊ ◊ ◊ N(3)
O(7)–H(7B) ◊ ◊ ◊ N(5)
O(6) –H(6B) ◊ ◊ ◊ O(2)
0.83(6)
0.83(6)
0.82(5)
0.82(5)
1.00(8)
0.91(7)
0.89(6)
0.92(4)
0.89(6)
2.881(4)
2.869(4)
2.933(4)
2.899(4)
3.160(8)
3.323(11)
2.932(5)
2.830(8)
3.148(5)
2.32(6)
2.09(6)
2.26(5)
2.17(4)
2.18(8)
2.52(7)
2.13(6)
1.94(5)
2.57(6)
126(5)
157(6)
140(4)
149(4)
168(5)
148(6)
151(5)
161(4)
124(4)
Symmetry elements: i = 1 - x, 1 - y, 1 - z; ii = -1/2 + x, y, 3/2 - z; iii = 1 -
x, 1 - y, 2 - z; D, donor; H, hydrogen; A, acceptor.
is reinforced by p–p stacking interactions established between
the aromatic rings of the Schiff base (Fig. S2, ESI†).48 The
i
˚
intermetallic Mn(1) ◊ ◊ ◊ Mn(1 ) distance is 4.969 A. Details of H
bonding distances are given in Table 4. Finally, a 3-D hydrogen-
bonded structure is formed as shown in Fig. S3 (ESI†).
Fig. 3 p–p Stacking interactions within the supramolecular dimer of
compound 1.
[MnIII(saloph)(l1,3-N3)]n (3). Single-crystal X-ray analysis re-
veals that compound 3 consists of a chain system with the
repeating unit [MnIII(saloph)(N3)] bridged by m-1,3 azide (Fig. 5).
There is only one crystallographically independent Mn(III) atom in
a distorted octahedral geometry, which is coordinated by the N2O2
donor atoms from one (saloph)2- ligand in the equatorial mode and
two N donor atoms from two N3- ions in the axial positions. Each
azide ligand functions as a trans-m1,3 bridge to link monomeric
[MnIII(saloph)]+ units into a one-dimensional zigzag chain along
the crystallographic b axis. In the equatorial plane, the bond
lengths of Mn(1)–O(1), Mn(1)–O(2), Mn(1)–N(1) and Mn(1)–
˚
N(2) are 1.875(3), 1.871(4), 1.993(4) and 2.000(4) A, respectively,
which are close to those in other Mn(III)-salen compounds.11,12 As
expected, the bond lengths in the axial position of Mn(1)–N(3)
˚
and Mn(1)–N(5A) (2.295(5) and 2.385(5) A) are elongated due to
a Jahn–Teller distortion at the high-spin d4 metal center;10–12 the
bond angles of Mn(1)–N(3)–N(4) and Mn(1)–N(5A)–N(4A) are
133.8(4) and 142.5(4)◦, respectively. The Mn(1)–N(3) bond length
is a little shorter than that in compound 2 since the azide ion acts
as a bridging ligand in 3 while as a terminal ligand in 2. Selected
bond lengths and bond angles of compound 3 are given in Table 2.
Deviations of the coordinating atoms in the basal plane, O(1),
Fig. 4 Perspective view of compound 2 showing the atom numbering
of all non-carbon non-hydrogen atoms. Lattice water molecules are not
shown for clarity.
˚
by more than 0.03 A. Selected bond lengths and bond angles of
compound 2 are given in Table 2.
Further studies of the crystallographic data have shown that
each mononuclear Mn(III) species is connected to a neigh-
bor through intermolecular interactions creating well-isolated
supramolecular dimers, similar to compound 1 (Fig. S1, ESI†).
The two hydrogen atoms, H(5A) and H(5B), attached with O(5)
of the coordinated water molecule are engaged in bifurcated
hydrogen bond formation with the symmetry related (1 - x, 1 - y, 1
- z) O(3), O(4) and O(1), O(2) atoms, respectively. H(6B) attached
with O(6) of the lattice water is again engaged in bifurcated
hydrogen bond formation with O(2) of the quadridentate Schiff
base, H2vanoph and N(3) of the coordinated azide ion. H(6A),
attached with O(6), is also involved in H bonding with O(7) of
the lattice water. The free end of the azide ion, N(5) is again
H-bonded with H(7B) of the lattice water whereas H(7A) of
the same water molecule is involved in H bonding with O(7) of
another crystallographically related (1 - x, 1 - y, 2 - z) lattice
water molecule. Moreover, the stability of the resulting dimer
Fig. 5 Single azide bridged polymeric chain structure of the compound 3.
Hydrogen atoms are not shown for clarity; * represents symmetry element
x, y, 1 + z.
7922 | Dalton Trans., 2011, 40, 7916–7926
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O(2), N(1) and N(2), from the least-square mean planes through
in the thermogravimetric analysis of compound 1. There are
two endotherm peaks in the corresponding DSC curve, the first
corresponding to the release of water and the second to the
release of DMF molecule. In the thermogravimetric analysis of
compound 2, on the other hand, a weight loss of 10.3% takes
˚
them are 0.030, 0.030. 0.031 and 0.031 A, respectively, and that of
Mn(III) from the same plane is 0.039 A. As for N3- itself, N(3)–N(4)
˚
and N(4)–N(5A) have almost the same bond lengths (1.186(8) and
◦
˚
1.175(8) A) and the N(3)–N(4)–N(5A) bond angle is 178.9(5) ,
close to 180◦. The manganese–manganese repeating distance is
place in the temperature range 80–110 C, corresponding to the
◦
˚
6.616 A along the chain.
loss of three water molecules (calc. 10.28%). An endothermic peak
is also observed in the corresponding DTA curve as expected.
The dehydrated species does not absorb any water molecules on
exposure to the open atmosphere. In the IR spectra, the broad
band at ca. 3400 cm-1, that was observed for compound 2 due to
an OH stretching vibration, is missing in the dehydrated species,
confirming the elimination of the water molecule on heating.
Cyclic voltammetry
Electrochemical measurements of1, 2 and 3 were performedinace-
tonitrile solution under a dry nitrogen atmosphere in conventional
three-electrode configurations using a Pt disk working electrode, Pt
auxiliary electrode and Ag/AgCl reference electrode, with TBAP
(tetrabutylammonium perchlorate) as supporting electrolyte in
the potential range from -2 to 2 V, and were uncorrected for
junction contribution. The value for the Fc–Fc+ couple under our
conditions is 0.39 V.
The redox properties of the Mn(III) compounds exhibit grossly
similar features consisting of a reversible or quasi-reversible
MnIII/MnII reduction and a quasi-reversible MnIII/MnIV oxi-
dation. The criteria of reversibility were checked by observing
constancy of peak–peak separation (DEp = Epa - Epc) and the ratio
of peak heights (ipa/ipc ~ 1) with variation of scan rates.49 The
single-electron nature of the voltammograms has been confirmed
by the comparison of current heights for the compounds and that
of a simple [Fe(bipy)3]2+ compound under identical conditions.46
It has been observed that no well defined oxidative or reductive
responses could be observed on running further in the positive
or negative potential. The E1/2 and DEp values for these redox
couples are given in Table 5. The results of cyclic voltammetry
also closely resemble that of other similar reported compounds,
which serve as further evidence for similar structural and electronic
properties.39a,46 All the redox signals remain virtually invariant
under different scan rates (0.01–1.0 V s-1) in the temperature range
300–280 K. Solvent dependent shift and change in electrochemical
reversibility of redox couples are not significant.
Magnetic properties
Magnetic susceptibility measurements of compounds 1 and 2 were
recorded from room temperature to 2 K using 0.02 T (from
2 to 30 K) and 0.3 T (from 2 to 300 K). Both compounds
show a very similar cMT vs. T plot (Fig. 6 and Fig. S4, ESI†
respectively); at room temperature the cMT value for the two
compounds is ~6.2 cm3 mol-1 K and it remains almost constant
until 30 K. Below this temperature the cMT values drastically
drops to 0.72 cm3 mol-1 K for 1 and 0.52 cm3 mol-1 K for 2. Indeed,
this is a common factor in mononuclear complexes and in the
present work (1 and 2) mostly due to a significant axial anisotropy
(D) and relevant intermolecular interactions. In general, Mn(III)
ions exhibit elongated Jahn–Teller distortions, displaying relatively
high zero-field splitting values. On the other hand, compounds 1
and 2 show a number of intermolecular interactions among the
mononuclear entities. These connections are disposed in a way that
Thermogravimetric analysis
The heating of perchlorate salts are potentially explosive. Al-
though noproblemwas encountered duringthe thermogravimetric
experiment, only small portions of the samples should be heated
carefully. The thermal behaviour of the compounds was studied
in a dynamic nitrogen atmosphere (20 mL min-1) at a heating
◦
rate of 10 C min-1 using thermogravimetric (TG) technique. A
weight loss of 3% at 105 ◦C and another weight loss of 11.3% in
the temperature range 130–140 ◦C, corresponds to a loss of water
(calc. 2.9%) and a DMF molecule (calc. 11.29%), respectively,
Fig. 6 cMT vs. T plot for compound 1 between 2.0 and 300.0 K. The
experimental data is shown as circles and the red line corresponds to the
theoretical values.
Table 5 Electrochemical data for the oxidation and reduction of the manganese(III) centre of the compounds 1, 2 and 3
MnIII/MnII MnIII/MnIV
pa/V pa/V
Compound
E
Epc/V
E
1/2/V
DEp/mV
E
Epc/V
E
1/2/V
DEp/mV
1
2
3
-1.08
-1.65
-1.64
-1.02
-1.54
-1.52
-1.05
-1.60
-1.58
60
110
120
1.23
1.26
1.54
1.11
1.15
1.42
1.17
1.21
1.48
120
110
120
E1/2 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials; DEp = (Epa - Epc).
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supramolecular dimers are formed affecting the overall magnetic
response. As a matter of fact, attempts to fit the experimental data
using the equation for a S = 2 system (and taking into account D)
were unsuccessful. However, when the magnetic data was analyzed
considering a dinuclear system that contained two mononuclear
complexes related to each other through a magnetic coupling
constant J (using the Hamiltonian H = -JS1·S2 and including
the D parameter of each ion) suitable results were obtained. The
above data were analyzed using the Magpackfit50 program and
the best fit were obtained with J = -0.92 cm-1, |D| = 2.05 cm-1,
g = 2.0 and R = 8.1 ¥ 10-4 for 1 and J = -1.16 cm-1, |D| =
2.05 cm-1, g = 2.0 and R = 1.2 ¥ 10-3 for 2. D values are in
the range observed in other mononuclear Mn(III) compounds,
reported by Bocˇa,51 and as expected, the J values are small but
not negligible, in agreement of the hydrogen bond interactions
between the mononuclear entities.
The magnetic response of compound 3 was recorded from room
temperature to 2 K using 0.03 T (from 2 to 30 K) and 0.3 T (from 2
to 300 K) fields. The resulting cMT vs. T plot is depicted in Fig. 7.
At 300 K, a cMT value of 2.62 cm3 mol-1 K was found. This is
lower than the expected for a well-isolated high-spin MnIII system
(3.0 cm3 mol-1 K), pointing the presence of an antiferromagnetic
coupling for the system. The cMT value also decreases when the
temperature drops, corroborating the antiferromagnetic nature of
the interactions among the MnIII ions within the chain. A plot of
Fig. 7 cMT vs. T and cM vs. T (inset) plots for compound 3 between 2.0
and 300.0 K. The experimental data is shown as circles and the red line
corresponds to the theoretical values.
Manganese one-dimensional systems, with single m1,3-azido
bridge, are scarce; all the species reported so far in the literature
are summarized in Table 6. In these systems, the Jahn–Teller
axis of the Mn(III) ions is located in the azido bridge direction,
2
2
2
with one electron in the dz orbital; the dx
orbital remains
-y
empty pointing to the capping ligand. For the Mn(II) systems,
the extra electron is located in this orbital, but its contribution
to the magnetic exchange can be neglected due to the orientation
in the system and therefore comparison between both types of
systems is appropriate.
Escuer et al.58 analysed the effects of the structural parameters
for Mn(II) end-to-end azido bridge systems, with respect to their
magnetic interactions, concluding that the exchange couplings
are always antiferromagnetic, being the result of two types of
c
M vs. T is shown in Fig. 7 (inset). The maximum observed for the
molar susceptibility data (cM) at approximately 26 K also proves
the antiferromagnetic behaviour already mentioned. To provide
an accurate assessment on the strength of the interaction, the cMT
data (and also cM data, values in parenthesis) were fitted following
ꢀ
the law of a chain for classical spins S = 2, with H = -J ij Si·Sj.52
As a result, the best fit parameters were J = -6.45 cm-1, g = 1.99
and R = 3.6 ¥ 10-5 (J = -6.46 cm-1, g = 1.99 and R = 4.8 ¥ 10-5). The
Mn(III) ions in 3 shows a similar environment as that in compounds
1 and 2 and consequently a similar ZFS value of the ions should
be expected. However, the antiferromagnetic coupling between the
ions here is quite strong and therefore the effect of this parameter
could be negligible.
2
contributions: s (through the dz orbitals) and p (through the
dxz and dyz orbitals). In general, the exchange coupling constant
depends of three structural parameters: the Mn–N distance (d),
the Mn–N1–N2 angle (a) and the Mn–N1–N3–Mn torsion angle
(t). In this way, systems with short Mn–N distances, a angles close
ꢀ
Table 6 Selected structural and magnetic parameters for reported 1D manganese compounds with a single m1,3-azido bridge (H = -J ij Si·Sj); a =
Mn–N–N angle; t = Mn–N1–N3-Mn torsion angle
a/◦
t/◦
J/cm-1
Ref
˚
d(Mn–N)/A
Mn(III)
3
[Mn(phox)2(N3)]n
[Mn(Etphox)2(N3)]n
[Mn(5-Fsalen)(N3)]n
[Mn(5-MeOsalen)(N3)]n
[Mn(salen)(N3)]n
[Mn(5-Brsalen)(N3)]n
[Mn(L)2(N3)]n
2.34 (av.)
2.35
2.30
2.31
2.31
2.31
2.20
2.26
2.34
138.3 (av.)
144.2
120.5
116.6
118.4
116.5
119.3
134.1
127.6
131.3
172.6 (av.)
180.0
103.7
110.6
104.7
109.8
106.5
156.0
126.8
63.1
-6.5 (av.)
-6.8
This work
53
53
30
30
31
26
54
a
b
-2.7
-8.8
-6.0
-5.2
-6.5
c
-11.7
-6.2
[Mn(salpn)(N3)]
[Mn(acac)2(N3)]n
32–33
55
2.24
-10.6
Mn(II)
[Mn(L)2(N3)ClO4]n
[Mn(L)(N3)PF6]n
d
2.18
2.29
142.8
122.0
95
143.3
-6.6
-4.8
56
57
e
a Hpox = 2-(4,5-dihydrooxazol-2-yl)phenol. b HEtphox = 2-(4-ethyl-4,5-dihydrooxazol-2-yl)phenol. c HL = bidentate Schiff base obtained from the
condensation of salicyladehyde with 4-methoxy aniline. d L = bidentate Schiff base obtained from the condensation of pyridine-2-carbaldehyde with
4-methoxyl aniline. e L = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo{12.3.1}octadeca-1(18),2,12,14,16-pentaene.
7924 | Dalton Trans., 2011, 40, 7916–7926
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to 110◦ and t torsion angles similar to 0◦ (or 180◦) should show
the most antiferromagnetic interactions.
The sign of the exchange coupling agrees with previous work
and as expected, every compound reported in Table 6, including
K. Koikawa, N. Achiwa and H. Okawa, J. Am. Chem. Soc., 1992, 114,
6974; (c) B. Q. Ma, S. Gao, G. Su and G. X. Xu, Angew. Chem., Int.
Ed., 2001, 40, 434; (d) J. Larionova, J. Sanchiz, S. Gohlen, L. Ouahab
and O. Kahn, Chem. Commun., 1998, 953; (e) M. Yuan, F. Zhao, W.
Zhang, F. Pan, Z. M. Wang and S. Gao, Chem.–Eur. J., 2007, 13, 2937.
4 (a) H.-Z. Kou, S. Gao, J. Zhang, G.-H. Wen, G. Su, R. K. Zheng and
X. X. Zhang, J. Am. Chem. Soc., 2001, 123, 11809; (b) M. Ohba, N.
Usuki, N. Fukita and H. Okawa, Angew. Chem., Int. Ed., 1999, 38,
1795.
5 (a) H. Imai, K. Inoue, K. Kikuchi, Y. Yoshida, M. Ito, T. Sunahara
and S. Onaka, Angew. Chem., Int. Ed., 2004, 43, 5618; (b) K. Inoue, H.
Imai, P. S. Ghalsasi, K. Kikuchi, M. Ohba, H. Okawa and J. V. Yakhmi,
Angew. Chem., Int. Ed., 2001, 40, 4242.
6 (a) O. Sato, T. Iyoda, A. Fujishima and K. Hashimoto, Science, 1996,
272, 704; (b) T. Hozumi, K. Hashimoto and S. Ohkoshi, J. Am. Chem.
Soc., 2005, 127, 3864; (c) S. Ohkoshi, S. Ikeda, T. Hozumi, T. Kashiwagi
and K. Hashimoto, J. Am. Chem. Soc., 2006, 128, 5320; (d) J. M.
Herrera, V. Marvaud, M. Verdaguer, J. Marrot, M. Kalisz and C.
Mathoni’ere, Angew. Chem., Int. Ed., 2004, 43, 5468.
compound 3, show antiferromagnetic behaviour, with J values
ꢀ
in the range -2.7 to -11.7 cm-1(H = -J ij Si·Sj). However,
comparing the three factors mentioned before, it is not possible
to find direct magneto-structural correlations among them; it
appears that the three parameters are relevant but none of them
leads the strength of the exchange. From all of the above, one may
conclude that compound 3 shows a long Mn–N distance, and one
the greatest a values found for this type of Mn(III) systems, but
the Mn–N1–N2–N3–Mn core is almost planar (t ~ 180◦), and as a
result exhibits a moderate antiferromagnetic behaviour.
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Conclusion
Although the room-temperature preparation of m-1,3 azide
bridged Mn(III) complexes with salen or salpn type Schiff bases is
common in the literature, introduction of a phenylene diamine
moiety in the ligand prevented the formation of polynuclear
compounds as a result of molecular rigidity, and mononuclear
compounds resulted. In the present study, we have demonstrated
for the first time that polynuclear compounds can also be
prepared, using the ‘so-called’ rigid ligands, simply by increasing
the temperature of the system which helps to overcome the
activation energy. Thus the work opens up a new vista in
preparing polynuclear complexes which was hitherto believed to
be impossible to prepare. However, we have also showed that if
sterically demanding substituents are adjacent to the phenolic
oxygen of the N2O2 donor Schiff bases, the transition state becomes
practically inaccessible and refluxing is not sufficient to overcome
the kinetic barrier of polymerisation. On the other hand, magnetic
studies do illustrate the relevance of the supramolecular features
for compounds 1 and 2, where their magnetic susceptibility data
are better described in terms of dinuclear species. Complex 3 shows
an antiferromagnetic behavior typical of a 1D system where the
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Acknowledgements
This work was supported by CSIR, India (Fellowship for Pras-
anta Bhowmik, Sanction No. 09/096(0607)/2010-EMR-I, dated
27.1.10) and University Grants Commission, New Delhi (Minor
Research Project sanctioned to S. Chattopadhyay, F. No. 37-
577/2009 (SR) dated 12.01.2010). The work was also supported
by the Ministerio de Educacio´n y Ciencia (CTQ2009-07264/BQU
and CTQ2009-06959/BQU), the Comissio´ Interdepartamental de
Recerca i Innovacio´ Tecnolo`gica de la Generalitat de Catalunya
(CIRIT) (2009SGR1454) and ICREA (Institucio´ Catalana de
Recerca i Estudis Avanc¸ats).
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