Versatility of 2,6-diacetylpyridine (dap) hydrazones in stabilizing uncommon coordination geometries of Mn(II): Synthesis, spectroscopic, magnetic and structural characterization

Article abstract of DOI:10.1039/b503891j

Five seven- or eight-coordinate manganese complexes of hydrazone ligands have been prepared. Three seven-coordinate neutral Mn^II complexes: [Mn(dapA₂)]_n (1), [Mn(dapB₂)(H ₂O)₂] (2), [Mn(dap

Full text of DOI:10.1039/b503891j

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F U L L P A P E R
Versatility of 2,6-diacetylpyridine (dap) hydrazones in stabilizing
uncommon coordination geometries of Mn(II): synthesis,
spectroscopic, magnetic and structural characterization†
Subhendu Naskar,a Dipankar Mishra,a Shyamal Kumar Chattopadhyay,*a
Montserrat Corbella*b and Alexander J. Blakec
a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah,
711 103, India. E-mail: shch20@hotmail.com; Fax: 91 033 2668 2916
b Department of Inorganic Chemistry, University of Barcelona, Mart´ı i Franque`s 1-11, 08028,
Barcelona, Spain. E-mail: montse.corbella@ub.edu; Fax: (34)-934907725
c School of Chemistry, The University of Nottingham, University Park, Nottingham,
UK NG7 2RD
Received 17th March 2005, Accepted 3rd June 2005
First published as an Advance Article on the web 15th June 2005
Five seven- or eight-coordinate manganese complexes of hydrazone ligands have been prepared. Three
seven-coordinate neutral MnII complexes: [Mn(dapA2)]n (1), [Mn(dapB2)(H2O)2] (2), [Mn(dapS2)(H2O)2] (3) have
been synthesized from the bis-Schiff bases of 2,6-diacetylpyridine: dap(AH)2, dap(BH)2 and dap(SH)2 (AH =
anthraniloyl hydrazide, BH = benzoyl hydrazide, SH = salicyloyl hydrazide), respectively. Two eight-coordinate MnII
complexes: [Mn(dapS)2] (4) and [Mn(dapB)2]·3H2O (5) have been synthesized from the mono-Schiff bases dapBH
and dapSH, respectively. The complexes have been characterized by elemental analyses and by IR, UV-Vis., FAB
mass, EI mass and EPR spectroscopy. The molecular structures of 1, 3·DMF and 4·DMF have been determined by
single-crystal X-ray diffraction. The mono-Schiff bases are monoanionic and the bis-Schiff bases are dianionic. The
octa-coordinated mono-Schiff base complex 4 adopts a dodecahedral geometry, while the hepta-coordinated
bis-Schiff base complex 1 forms a one-dimensional linear polymeric chain. A weak antiferromagnetic exchange
interaction (J = −0.15 cm−1) between the Mn(II) ions in 1 is attributed to weak Mn · · · Mn interaction through the
PhNH2 moiety of the ligand, as indicated by extended-Hu¨ckel molecular orbital calculations. A good simulation of
the EPR spectrum of a frozen solution (DMSO at 4 K) of compound 1 was obtained with g = 2.0, D = 0.1 cm−1, E =
0.01 cm−1. The EPR spectrum of a powdered sample of compound 1 shows a large broadening of the signal, due in
part, to the important zero-field splitting of the hepta-coordinated Mn(II) ion.
X-ray structure of this complex [Mn(dapA2)]n (1) and that
Introduction
of another bis-Schiff base complex [Mn(dapS2)(H2O)2]·DMF
Although interest in the coordination properties of hydrazone
ligands towards transition metal ions and in the biological
(3·DMF) are reported here.
As a diketone, 2,6-diacetylpyridine retains the ability to form
properties of their metal complexes has been growing in
a mono-Schiff base.11 In an attempt to explore this aspect of
recent years,1–6 the magnetic properties7 of polynuclear metal
its chemistry, mono-Schiff bases with salicyloyl hydrazide and
complexes of hydrazones have been only rarely studied.
benzoyl hydrazide were prepared. Here the ligands are typically
2,6-Diacetylpyridine is an excellent precursor for synthesizing
tetradentate and the resultant complexes have a strikingly
hydrazone Schiff bases, its versatile ligating behavior producing
different geometry compared to the bis-Schiff bases, with both
unusual geometries—specifically, high coordination numbers—
ligands forming octa-coordinate complexes with MnII. A single-
in its metal complexes including those of manganese.2,5,8 Gen-
crystal X-ray structure determination on [MnII(dapS)2]·DMF
erally bis-Schiff bases are known for 2,6-diacetylpyridine, and
(4·DMF) shows that MnII has dodecahedral geometry.
they behave as pentadentate ligands with a N3O2 donor set.2,5,9,10
However, there remains the possibility of generating new molec-
ular architecture of these complexes by suitable substitution
of the phenyl ring of the aroyl hydrazide, in order to provide
Experimental
Physical measurements
additional donor atoms. With this in mind we used the bis-Schiff
base of anthraniloyl hydrazide and salicyloyl hydrazide. In this
paper we demonstrate that in the bis(anthraniloyl hydrazone)
ligand the two –NH2 groups in the ortho positions of the aroyl
rings impart a heptadentate nature to the ligand spanning
three MnII centers resulting in a linear polymeric chain. The
bis(salicyloyl hydrazone) ligand behaves as a pentadentate N3O2
donor spanning only one MnII center. To our knowledge this
is the first report of formation of a linear chain of MnII ions
involving a heptadentate hydrazone ligand. The single-crystal
C, H, N analyses were performed on a Perkin-Elmer 2400
instrument. FTIR spectra (4000–600 cm−1) were recorded as
KBr discs on a JASCO 460 PLUS spectrophotometer. Electronic
spectra were obtained using a JASCO 7850 spectrophotometer.
Variable-temperature (2–300 K) magnetic measurements for
polycrystalline samples of 1 were carried out with a Quantum
Design SQUID MPMS-XL susceptometer, working in the range
2–300 K under a magnetic field of 500 G, at the “Servei
de Magnetoqu´ımica” (Universitat de Barcelona). Pascal’s con-
stants were used to estimate the diamagnetic corrections. The fit
ꢀ
was performed by minimizing the function R = [(vMT)exptl
−
† Electronic supplementary information (ESI) available: Table ST1:
Intermolecular contacts in 3. Fig. SF1: Fourier difference map for 3. Fig.
SF2: View of the hydrogen bonding and other short contacts in 3. Fig.
SF3: MOs for the dimer model. See http://dx.doi.org/10.1039/b503891j
ꢀ
(vMT)calc]2/ (vMT)exptl2. EPR spectra were recorded at X-band
(9.4 G Hz) frequency with a Bruker ESP-300E spectrometer
at the “Servei de Magnetoqu´ımica” (Universitat de Barcelona).
2 4 2 8
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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A program written by Weihe12 was used to simulate the EPR
spectra. The simulation was performed by generating the energy
matrix for each orientation of the molecule relative to the
magnetic field. The resonance condition for each transition was
then found by successive diagonalizations and iterations of the
energy matrix, and the relative intensities were calculated from
the eigenvectors multiplied by the appropriate Boltzmann factor
at 4 K. Summation of all the transitions over the whole space,
where each transition is represented by a differentiated Gaussian
curve, gives the simulated spectrum. The spin-Hamiltonian used
for the simulation include the ZFS parameters D and E. FAB
and EI mass spectra of the samples were recorded on JEOL
JMS600 instrument in m-nitrobenzyl alcohol as matrix.
Diffraction-quality single crystals were obtained by concen-
tration of a DMF solution of the orange material. Found
(calc. for C23H21N7O2Mn): C 57.56 (57.26), H 4.30 (4.36), N
20.22 (20.33)%. Selected IR bands (cm−1): 3376 ms [mNH(NH2)],
1612s [amide I], 1524vs [amide II]. Electronic spectrum in DMF
solution, k/nm (e/dm3 mol−1 cm−1): 425 (13040), 375 (24590).
FAB MS, m/z 483 [Mn(dapA2) + H]+.
[Mn(dapB2)(H2O)2] (2). Yield: 80%. Found (calc. for
C23H23N5O4Mn): C 55.92 (56.56), H 4.62 (4.71), N 14.41
(14.34)%. Selected IR bands (cm−1): 3434 [mOH of H2O], 1625m
[amide I], 1525w [amide II]. Electronic spectrum in DMF
solution, k/nm (e/dm3 mol−1 cm−1): 390 (6244), 335 (13182),
310 (16882). FAB MS, m/z 453 [Mn(dapB2) + H]+.
[Mn(dapS2)(H2O)2] (3)·DMF. Yield: 67%. Found (calc. for
C26H30N6O7Mn): C 52.32 (52.61), H 5.11 (5.06), N 14.20
(14.17)%. Selected IR bands (cm−1): 3400 br [mOH of H2O],
1612vs [amide I], 1500w [amide II]. Electronic spectrum in DMF
solution, k/nm (e/dm3 mol−1 cm−1): 395 (10738), 345 (31021).
FAB MS, m/z 485 [Mn(dapS2) + H]+.
Materials
2,6-Diacetylpyridine was obtained from Aldrich and used
without further purification. Organic solvents were purified
following published procedures.13
Ligand preparations
[Mn(dapS)2] (4)·DMF. Yield: 65%. Found (calc. for
C35H35N7O7Mn): C 58.72 (58.33), H 4.71 (4.86), N 13.42 (13.61).
Selected IR bands (cm−1): 1697s [mCO(ketone)], 1647s [amide I],
1546s [amide II]. Electronic spectrum in DMF solution, k/nm
(e/dm3 mol−1 cm−1): 395 (15345), 348 (44035). FAB MS, m/z
648 [Mn(daps)2 + H]+.
[Mn(dapB)2] (5)·3H2O. Yield: 50%. Found (calc. for
C32H34N6O7Mn): C 56.97 (57.39), H 5.15 (5.08), N 12.72
(12.56)%. Selected IR bands (cm−1): 3388br [mOH of H2O], 1685s
[mCO(ketone)], 1660m [amide I], 1556m [amide II]. Electronic
spectrum in DMF solution, k/nm (e/dm3 mol−1 cm−1): 375
(12365), 340 (15634). FAB MS, m/z 616 [Mn(dapB)2 + H]+.
Aroyl hydrazides were prepared by reaction of hydrazine with
corresponding methyl or ethyl esters using a published method.14
The bis-Schiff bases were prepared according to known
methods.3,15 The mono-Schiff bases were prepared as described
below.
DapSH. 2,6-Diacetylpyridine (0.815 g, 5 mmol) was dis-
solved in excess methanol (150 cm3). Salicyloyl hydrazide
(0.76 g, 5 mmol) dissolved in methanol (75 cm3) was added
dropwise with stirring to the former over a period of 2 h.
After an additional hour of stirring a white solid separated,
which was discarded. The filtrate was allowed to evaporate at
room temperature. The light yellow product was thoroughly
washed with hot methanol (4 × 25 cm3); yield: 0.63 g, 42%.
Found (calc. for C16H15N3O3): C 64.23 (64.65), H 4.96 (5.05), N
14.68 (14.14)%. Selected IR bands (cm−1): 3271w [mNH(amide)],
3200 br [mOH], 1697s [mCO(ketone)], 1647s [mCO(amide I)]. 1H NMR
(DMSO-d6): d 2.52 (s, 3H), 2.72 (s, 3H), 7.02–7.45 (m, 4H), 8.01–
8.39 (m, 3H), 11.52 (s, 1H), 11.84 (s, 1H). FAB MS: m/z 298
[DapSH + H]+.
DapBH. To a methanolic solution (150 cm3) of 2,6-
diacetylpyridine (0.815 g, 5 mmol) a methanolic solution
(75 cm3) of benzoyl hydrazide (0.680 g, 5 mmol) was added
dropwise with stirring over 3 h. After an additional 1 h
of stirring a white solid separated and was filtered off and
discarded. The filtrate was concentrated by evaporation at room
temperature, whereupon a white solid (0.600 g, 40%) appeared
which was purified by column chromatography. The fraction
which was eluted with (MeCN–CHCl3 = 5 : 95) gives the
expected compound DapBH. Found (calc. for C16H15N3O2): C
68.04 (68.33), H 5.15 (5.34), N 15.16 (14.95)%. Selected IR
bands (cm−1): 3188w [mNH(amide)], 1699s [mCO(ketone)], 1670s
[mCO(amide I)], 1533 [dNH + mCN(amide II)]. 1H NMR (CDCl3): d
2.55 (s, 3H), 2.76 (s, 3H), 7.49–7.59 (m, 4H), 7.83–8.03 (m, 3H),
9.15 (s, 1H). EI MS, m/z 281 (DapBH)+.
Crystallography
Single crystals suitable for X-ray diffraction were obtained by
evaporation of solvent from concentrated DMF solution (for 1)
or by slow diffusion of a DMF solution into diethyl ether (for 3
and 4). For 3 and 4 the crystals were obtained as DMF solvates
with formulae 3·DMF and 4·DMF, respectively. Diffraction
data for single crystals of 1, 3·DMF and 4·DMF were collected
using a Bruker SMART CCD area diffractometer with graphite-
monochromated Mo-Ka radiation. The structures were solved
using direct methods and refined by full-matrix least squares on
F2 using SHELXL97.16 The hydrogen atoms were refined using
riding model. Crystallographic data are summarized in Table 1.
CCDC reference numbers 241988 (1), 258624 (3·DMF) and
241987 (4·DMF).
See http://dx.doi.org/10.1039/b503891j for crystallographic
data in CIF or other electronic format.
Results and discussion
Syntheses
The bis-Schiff base ligands dap(BH)2, dap(SH)2 and dap(AH)2
used in this study were prepared according to experimental pro-
cedures developed from earlier methods (Scheme 1).3,15 Owing to
the availability of the two carbonyl groups, 2,6-diacetylpyridine
may leave one carbonyl group free by condensing with only
one hydrazide, resulting in a mono-Schiff base. This can be
achieved by a modification of the method used to prepare the
bis-Schiff base: a dilute methanolic solution of the hydrazide
(1 equivalent) is added dropwise over 2–3 h to a very dilute
solution of 2,6-diacetylpyridine (1 equivalent) with stirring at
room temperature. Prolonged stirring of the solution gives the
mono-Schiff base along with small amounts of the bis-Schiff
base. On concentration of the filtrate the pure mono-Schiff base
is obtained, as confirmed by spectroscopic measurements (IR,
NMR and FAB and EI mass spectra).
Preparation of the complexes
All the complexes were prepared using a general procedure
described below for complex 1. The mono- and bis-Schiff base
complexes were prepared by using metal and ligand ratios of
1 : 2 and 1 : 1, respectively.
[Mn(dapA2)]n (1). Addition of Et3N (0.202 g, 2 mmol) to
a suspension of dap(AH)2 (0.429 g, 1 mmol) in methanol
(30 cm3) produced a clear solution. To this solution a methanolic
(10 cm3) solution of Mn(ClO4)2·6H2O (0.362 g, 1 mmol) was
added and the mixture was refluxed for 2 h. The orange
solid that separated out was filtered off; yield: 0.366 g (76%).
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
2 4 2 9

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Table 1 Details of data collection and structural refinement for 1, 3·DMF and 4·DMF
1
3·DMF
4·DMF
Empirical formula
C23H21N7O2Mn
482.40
150(2)
0.71073
Monoclinic
I2/a
12.663(4)
10.626(3)
16.510(5)
90
C26H30N6O7Mn
593.50
150(2)
0.71073
Triclinic
P-1
7.4635(8)
13.908(2)
15.216(2)
114.421(2)
96.351(2)
102.314(2)
1369.5(3), 2
1.439
C35H35N7O7Mn
720.64
150(2)
0.71073
Monoclinic
P21/c
13.4258(13)
15.749(2)
16.267(2)
90
M
T/K
˚
k/A
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
96.499(5)
90
2207(2), 4
1.452
105.174(2)
90
3319.6(7), 4
1.442
3
˚
U/A , Z
Dc/g cm−3
l/mm−1
0.634
0.500
0.459
Reflections collected:
total, unique
Rint
obs. (I > 2.0r(I))
Final R1, wR2
12157, 9036
0.063
6771
12210, 5886
0.031
4794
30065, 7622
0.037
6379
0.0604, 0.182
0.0391, 0.106
0.0407, 0.115
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1
Mn–O(10)
Mn–N(8)
2.2100(16) Mn–N(1)
2.269(2) Mn–N(16#1)
2.337(2)
2.3611(19)
O(10)–Mn–O(10#2) 86.65(8) O(10)–Mn–N(8)
O(10#2)–Mn–N(8) 154.51(6) N(8)–Mn–N(8#2)
69.86(6)
134.87(9)
67.44(5)
O(10)–Mn–N(1)
136.68(4) N(8)–Mn–N(1)
O(10)–Mn–N(16#1) 82.42(6) O(10#2)–Mn–N(16#1) 97.28(6)
N(8)–Mn–N(16#1)
N(1)–Mn–N(16#1)
89.50(7) N(8#2)–Mn–N(16#1)
90.21(4) N(16#1)–Mn–N(16#3) 179.59(9)
90.65(7)
Symmetry transformations used to generate equivalent atoms: #1 x −
1/2, −y, z; #2 −x + 3/2, y, −z + 2; #3 −x + 2, −y, −z + 2.
Scheme 1
IR Spectra
In all the ligands a broad band around 3300 cm−1 indicates
the presence of an amide proton. Lower values for some of
the ligands may be due to the hydrogen bonding. In 1 and 4
the absence of this band indicate deprotonation of the amide
proton. However, in other complexes (2, 3 and 5), the presence
of a strong broad band (due to water) in this region precludes
any conclusion about the presence or absence of the mNH band.
The mCO(amide I) band of the coordinated ligand is also shifted to
lower wavelength compared to the free ligand due to coordina-
tion (in all the complexes) and deprotonation (in all the com-
plexes except 3, see below). A sharp band around ∼1700 cm−1
indicates the presence of free carbonyl group in the mono-Schiff
base ligands: this band is also shifted to lower energy upon
coordination.
Fig. 1 Ellipsoid plot of the linear chain of 1. Displacement ellipsoids
are drawn at the 50% probability level.
Structures
appreciably from that of an ideal pentagon (72◦). Similarly, the
trans angles are 134.87(9)◦ [N(8)–Mn–N(8#2)] and 154.51(6)◦
[N(8)–Mn–O(10#2)], deviate by about 10◦ from the ideal angle
of 144◦. A strong N(16#1)–H(16B) · · · N(9) hydrogen bond is
an N(16#1)–H(16B) · · · N(9) angle is 128◦ [symmetry code #1:
x − 1/2, −y, z]. Each asymmetric unit is then connected to two
other such units through axial coordination of the NH2 groups,
resulting in a linear chain of [Mn(dapA2)]n.
[Mn(dapA2)]n (1). In 1 Mn(II) occupies a distorted pentago-
nal bipyramidal environment (Fig. 1) with the axial bonds much
longer than the equatorial bonds. The equatorial Mn–N and
Mn–O bond distances (Table 2) are similar to those observed for
other complexes having similar geometry.8,9,17,18 The equatorial
plane is somewhat distorted, with Mn, N(8), N(8#2), N(1) along
with N(9) and N(9#2) [symmetry code #2: −x + 3/2, y, −z +
2] constituting a good least squares mean plane, with v2 = 94.3
˚
characterised by an N(9) · · · N(16#1) distance of 2.680(3) A and
˚
0.009(2) A
and the maximum deviation of any atom being
[for N(8#2) and N(8), respectively]. The other two coordinating
[Mn(dapS2)(H2O)2] (3)·DMF. As in 1, the Mn(II) is in a
pentagonal bipyramidal environment (Fig. 2). The basal plane
is occupied by N3O2 donor atoms of the pentadentate bis(aroyl
hydrazone) ligand, while two water molecules occupy the axial
positions. All the metal–ligand bond distances in the basal
˚
atoms O(10) and O(10#2) deviate by 0.262(2) and −0.262(2) A,
respectively, from the above plane.
The bite angles of the chelate rings are within the range of 67–
70◦, whereas the O(10)–Mn–O(10#2) angle [86.65(8)◦] deviates
2 4 3 0
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5

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◦
˚
Table 3 Distances (A) and angles ( ) for 3·DMF
Mn–O(1W)
Mn–O(10)
Mn–N(1)
2.1361(15)
2.2462(15)
2.360(2)
Mn–O(2W)
Mn–O(20)
Mn–N(8)
2.1516(15)
2.2176(17)
2.3174(19)
Mn–N(18)
2.3275(17)
O(1W)–Mn–O(2W)
O(1W)–Mn–O(20)
O(1W)–Mn–N(8)
O(2W)–Mn–O(10)
O(2W)–Mn–N(1)
O(2W)–Mn–N(18)
O(10)–Mn–N(1)
O(10)–Mn–N(18)
O(20)–Mn–N(8)
N(1)–Mn–N(8)
176.87(7)
87.11(6)
87.11(6)
89.88(6)
90.42(6)
88.40(6)
135.95(6)
157.20(6)
156.53(6)
66.43(6)
133.12(7)
O(1W)–Mn–O(10)
O(1W)–Mn–N(1)
O(1W)–Mn–N(18)
O(2W)–Mn–O(20)
O(2W)–Mn–N(8)
O(10)–Mn–O(20)
O(10)–Mn–N(8)
O(20)–Mn–N(1)
O(20)–Mn–N(18)
N(1)–Mn–N(18)
88.74(6)
92.52(6)
91.79(6)
90.02(6)
95.05(6)
87.47(6)
69.68(6)
136.57(6)
69.80(6)
66.80(6)
Fig. 3 Intra and inter-molecular hydrogen bonds in complex 3.
[Mn(dapS)2] (4)·DMF. Unlike complexes 1 and 3 where
a bis(aroyl hydrazone) of 2,6-diacetylpyridine provides five
donor atoms (N3O2) in a basal plane, the ligand used here is
the mono(salicyloylhydrazone) of 2,6-diacetylpyridine, which
can provide four donor atoms (N2O2). The crystal structure
determination of the manganese(II) complex of this ligand shows
Mn(II) in an octa-coordinated environment comprising an N4O4
donor set (Fig. 4). Six of these donor atoms (N4O2) are provided
by the hydrazone part of the Schiff base along with the pyridine
nitrogens. The Mn–O and Mn–N bond distances vary between
N(8)–Mn–N(18)
˚
2.25 and 2.32 A (Table 4), which is similar to the values in
other octa-coordinated Mn complexes.20 The other two bonds
˚
are rather long Mn–O bonds (av. 2.54 A) formed by the acetyl
oxygens. This eight donor atoms form a dodecahedron (Fig. 5)
around the Mn(II) centre. It is interesting to note that whereas
in complex 3 the ligand was found to be deprotonated at the
phenolic oxygens, here the deprotonation has occurred at the
Fig. 2 Ellipsoid plot of 3·DMF, showing the atom numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level.
˚
˚
plane e.g. Mn–Npy (2.360(2) A), Mn–Namide (av. 2.322 A) and
˚
Mn–O (av. 2.232 A) for this complex is larger than those in
complex 1 (Table 3). This is probably due to the fact that,
although in both these complexes the basal plane is occupied
by a doubly deprotonated bis(aroyl hydrazone) ligand, the sites
of deprotonations in these complexes are different. While in
complex 1 the amide protons are deprotonated and the ligand
is coordinated in its tautomeric enolate form, in complex 3 it
is the phenolic protons attached to O(16) and O(26), which
are deprotonated. This is clearly demonstrated by the Fourier
difference map (ESI,† Fig. SF1). The higher C–Namide bond
˚
˚
distance (av. 1.361 A) and the lower C–O (1.245 A) bond
Fig. 4 PLUTON19 diagram of 4·DMF showing the atom numbering
distances in complex 3, compared to the corresponding C–N
scheme.
˚
˚
[1.339(3) A] and C–O [1.270(3) A] bond distances in complex
1 also support this conclusion. The bite angles are in the range
66–70◦, while the trans angles are in the range of 136–157◦. The
pentagonal basal plane is distorted and one can not define a
good least square mean plane containing Mn and the donor
atoms of the basal plane. The best available least squares basal
plane consists of Mn, O(10), O(20), N(18) and N(19) [v2 = 728.5,
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for 4·DMF
Mn–O1
Mn–N3
Mn–N2ꢀ
Mn–O3ꢀ
2.2474(12)
2.3314(13)
2.2747(14)
2.5148(13)
Mn–N2
Mn–O1ꢀ
Mn–N3ꢀ
2.2487(13)
2.2504(12)
2.3073(14)
˚
deviation of Mn = −0.033(1) A], and N(1) deviates from this
O1–Mn–N2
N2–Mn–O1ꢀ
N2–Mn–N2ꢀ
O1–Mn–N3ꢀ
O1ꢀ–Mn–N3ꢀ
O1–Mn–N3
O1ꢀ–Mn–N3
N3ꢀ–Mn–N3
N2–Mn–O3ꢀ
N2ꢀ–Mn–O3ꢀ
N3–Mn–O3ꢀ
70.06(5)
84.33(5)
143.90(5)
85.92(5)
138.68(5)
139.10(5)
82.02(5)
122.91(5)
76.02(4)
135.37(5)
75.75(4)
O1–Mn–O1ꢀ
O1–Mn–N2ꢀ
O1ꢀ–Mn–N2ꢀ
N2–Mn–N3ꢀ
N2ꢀ–Mn–N3ꢀ
N2–Mn–N3
N2ꢀ–Mn–N3
O1–Mn–O3ꢀ
O1ꢀ–Mn–O3ꢀ
N3ꢀ–Mn–O3ꢀ
94.40(5)
87.51(5)
69.13(5)
133.19(5)
69.60(5)
69.05(5)
127.50(5)
94.16(5)
154.39(4)
66.06(5)
˚
plane by −0.103(2) A.
There is strong intra-molecular hydrogen bonding between the
amide hydrogens H(9A) and H(19A) and the phenolate oxygens
O(16) and O(26). In addition, there is strong intermolecular
hydrogen bonding between the hydrogen atoms attached to two
axial water molecules and the phenolate oxygens of the adjacent
complex moieties (ESI,† Table ST1). These intermolecular
hydrogen bonds along with several intermolecular C · · · H,
C · · · C, H · · · H short contacts results in a three-dimensional
supramolecular network (Fig. 3; ESI,†Fig. SF2).
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
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The shape of the vMT vs. T plot is characteristic of a weak
antiferromagnetic coupling, but it could be also due to the
presence of zero-field splitting. In general, for Mn(II) ions,
the zero-field splitting is very small. Nevertheless, in 1, the
coordination polyhedron of each Mn(II) ion is a pentagonal
bipyramid, and the ZFS could be significant. On the other
hand, the magnetic interaction between the Mn(II) ions could
˚
be very small, due to the long Mn · · · Mn distance (6.40 A),
2−
and the type of the bridging ligand (the dapA2 ligand). The
magnetic susceptibility data were analysed in two ways: as
isolated Mn(II) ions with ZFS and as a one-dimensional system.
In the first case, it was not possible to find a good fit of
the experimental magnetic susceptibility data. The magnetic
susceptibility data were analysed using the analytical expression
derived by Fisher21 for an infinite chain of classical spins based
ꢀ
on the Hamiltonian H = −J SiSi+1, for local spin values S =
5/2: vMT = [Ng2b2S(S + 1)(1+U)]/[3kT(1 − U)] where U =
coth[JS(S + 1)/kT] − [kT/(JS(S + 1)]. The best fit corresponds
to J = −0.15 cm−1, g = 2.02 and R = 6.1 × 10−4.
Fig. 5 High-symmetry dodecahedral geometry of complex 4.
˚
amide nitrogen. This is reflected in the longer C–O [av. 1.259 A
for C(1)–O(1) and C(1ꢀ)–O(1ꢀ)] bond length and shorter C–N [av.
ꢀ
ꢀ
The weak magnetic interaction between the Mn(II) centres
could be mediated either by the axially bridging NH2–Ph groups
or through p-stacking interactions between pyridine and phenyl
˚
1.343 A for C(1)–N(1) and C(1 )–N(1 )] bond length in complex
4 compared to those in complex 3 (average of C–O and C–N
˚
bond lengths are 1.245 and 1.361 A, respectively).
˚
rings separated by a centroid–centroid distance of 3.93 A.
Magnetic properties
Extended-Hu¨ckel calculations for 1
The magnetic susceptibility of 1 was measured in the range 2–
300 K (Fig. 6). The vMT value of 4.51 cm3 mol−1 K at room
temperature is in agreement with the expected value for an
isolated Mn(II) ion. On cooling below about 50 K, a more rapid
decrease in the value of vMT could be observed, whereas the vM
vs. T plot increases on cooling, without reaching a maximum in
the temperature range studied.
With the aim of clarifying the origin of the magnetic interac-
tion, extended Hu¨ckel calculations with the crystallographic
coordinates for compound 1 were carried out with the CACAO
program.22 Taking in consideration the one-dimensional struc-
ture of this compound, only a dinuclear fragment (two Mn(II)
ions and two ligands) was considered. In each manganese ion
one of the axial ligands (NH2-coordinated) was replaced by a
NH3 group so that the model system used was [MnL(NH3)]2,
where the L ligands “bridge” both Mn(II) ions (Scheme 2).
For each d orbital of the manganese ions, two MOs could
be obtained (symmetric and antisymmetric combination), with
similar contribution of both metal ions. Fig. 7 shows one
MO of each pair, and for clarity, the drawing shows only the
Fig. 6 vMT vs. T plot for compound 1. The solid line is the best fit to
the experimental data.
Scheme 2
Fig. 7 Drawings of the MO fragments around one Mn(II) ion; the axial ligands are omitted for clarity.
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
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fragment around one manganese ion (A complete MO diagram
is deposited in ESI† as Fig. SF3).
them are placed in orbitals with an orientation favourable to the
interaction.
With 10 metal d orbitals, a total of 10 MOs with major
contribution from the metal ion would be expected. However,
we found at least 14 MOs with more than a 10% contribution
from each manganese ion (Fig. 8, Scheme 3), indicative of
some delocalisation of the metal electronic density to the
ligand. There are two pairs of MOs (155/156, 161/162) with
more than 10% of contribution of each Mn(II) ion along with
appreciable concentration of bridging PhNH2 ligands but where
the contributions of the axial NH3 ligands are small. Two other
pairs of MOs, 157/158 and 159/160, correspond to the terminal
Ph–NH2 groups in the studied binuclear system, with 157/158
having appreciable contribution from Mn(II), while 159/160
have only minor contribution from the metal ion. Fig. 8 shows
the energy and composition of the MO pairs.
EPR spectra of 1
EPR spectra recorded on powdered sample show, at room
temperature, a broad signal centred at g = 2.06 (DHpp
=
560 G). On cooling, a considerable broadening of the signal
was observed, reaching a DHpp = 1200 G at 4 K (Fig. 9).
Fig.
9
EPR spectra of the polycrystalline 1 at three different
temperatures.
EPR spectra of one-dimensional systems are difficult to
interpret, even from a qualitative point of view. Several factors
influence the line-width of the EPR spectra,23 e.g. the dipolar
interaction, the exchange interaction and the ZFS. The effect of
the dipolar interaction is the broadening of the main line of the
spectrum. When the exchange interaction is present a narrowing
of the bands may be observed.24 If the dipolar interaction is
more important than the exchange interaction, a broadening of
the spectra is observed. Additional broadening mechanisms are
the hyperfine coupling and single-ion ZFS effects.
For compound 1, several factors favour the broadening of the
EPR band. As the exchange interaction is weak, no narrowing
effect can operate. The presence of hyperfine coupling for the
manganese ions (IMn = 5/2) contributes to a broad spectrum,
whereas the contribution of the dipolar interaction must be
Fig. 8 Energy diagram and composition of the MOs with major
contribution from Mn(II) ions. N–Ph symbolises the N atom of the
ligand coordinated in axial position; N*–Ph symbolises the N atom of
the ligand in a terminal position (not coordinated to the Mn(II) ion in
this study). The percentage values correspond to one Mn(II) ion and one
ligand.
˚
moderate, due to the long Mn · · · Mn distance (6.4 A). On
the other hand, a degree of ZFS could be present due to
the seven-coordination of the Mn(II) ion. A manganese chain
described in the literature, MnMn(EDTA)·9H2O,25 also shows a
seven-coordinated Mn(II) ion, and with quite similar Mn · · · Mn
distances: for this compound the authors found that the most
important effect to the broadening of the bands is the ZFS.25
Therefore, for compound 1, with a small J value and similar
Mn · · · Mn distance, the broadening of the spectra could also be
attributed to the presence of ZFS.
The EPR spectra of a DMSO solution of compound 1
were recorded at different temperatures (Fig. 10). Important
differences, of intensity and position of the bands, are observed
between the spectrum at room temperature and the spectra at
low temperatures. In a general way, we can differentiate three
regions of the spectra: ∼1300, ∼2300 and ∼3300 G. While at
room temperature the most important signal appears at g ∼ 2, at
135 K and lower temperatures, the most important band is found
in the second region of the spectra, ∼2300 G (g ∼ 2.9). Some
differences are also observed between the EPR spectra in the
temperature range of 135 to 4 K. The ratio between the intensity
of the first two peaks, (∼1300 and ∼2300 G) remains constant
in the range 4 to 13 K, but increase at higher temperatures.
The EPR spectrum in solution remains unchanged after a week,
indicating the stability of the complex in DMSO.
Scheme 3
In the actual one-dimensional compound, this terminal
group is coordinated to another Mn(II) ion. Then, in the real
system these MO pairs must also participate in the Mn · · · Mn
interaction through the axial ligand. Substantial difference in the
Mn–Naxial overlap population was observed between the NH3
group (0.053) and the NH2–Ph ligand (0.099), in spite of the
same Mn–N distance. Greater values of overlap population with
the ligands in the equatorial plane were observed (0.2–0.33) in
agreement with the smaller Mn–L distances.
Using these results, we can attribute the variation of the vMT
vs. T as being due to a weak Mn · · · Mn interaction, through
the ligand. However, of the five Mn(II) electrons, only three of
Taking into consideration the weak Mn · · · Mn antiferro-
magnetic coupling, the spectra in solution could be due to an
isolated Mn(II) ion in a pentagonal-bipyramidal geometry. For
D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
2 4 3 3

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ZFS parameters are also comparable (D ∼ 0.11–0.13 cm−1
and E/D ∼ 0.03), while for the one-dimensional system
[Mn(bpy)(H2O)(TDB)]n,27 the X-band EPR spectrum show the
six-line manganese hyperfine pattern centered at g = 2.
Conclusions
Mn(II) is normally found in an octahedral coordination environ-
ment, and structurally characterized hepta- and especially octa-
coordinated Mn(II) complexes are relatively uncommon. Most
of the structurally characterized octa-coordinated Mn(II) com-
plexes involve macrocyclic ligands and there is no report of any
such complex using aroyl hydrazones. Furthermore, the mono-
hydrazones of 2,6-diacetylpyridine comprise a rarely studied
system, and there is only one report11 of an iron(III) complex
of mono (theonyl hydrazone) of 2,6-diacetylpyridine, though
the compound was not structurally characterized. Thus ours is
the first report of a structurally characterized Mn(II) complex of
mono-Schiff base of 2,6-diacetylpyridine, and moreover it is an
octa-coordinated species.
A survey of the structurally characterized metal complexes of
bis-Schiff bases 2,6-diacetylpyridine reveals that in most of the
cases discrete seven-coordinated complexes are obtained. Two
binuclear Ni(II) complexes9 and a binuclear Fe(III) complex11
represent the only known examples of binuclear complexes
using such ligands, but magnetic studies were reported only
for the Ni(II) complexes. In fact the magnetochemistry of aroyl
hydrazone complexes is not very well studied, and thus the
effect of various factors on the magnetic properties of such
metal complexes is yet to be fully understood. Again our
paper for the first time reports that, using a bis-Schiff base
of aroyl hydrazone as the only ligand, with suitable ortho
substituents in the phenyl ring of the aroyl hydrazide moiety,
it is possible to molecular engineer a one-dimensional chain of
Mn(II) compounds, in which Mn(II) ions are coupled by weak
antiferromagnetic interactions. In fact, we have demonstrated
here that by changing the ortho substituents in the phenyl
ring of the aroyl hydrazide moiety, e.g., replacing –NH2 by –
OH, one may control the supramolecular architecture, from a
one-dimensional covalently linked chain to a three-dimensional
hydrogen bonded network.
Fig. 10 EPR spectra of a DMSO solution of compound 1 at different
temperatures.
a Mn(II) ion, with a very small ZFS (D ∼ 0) the five tran-
sitions |5/2,−5/2ꢁ → |5/2,−3/2ꢁ; |5/2,−3/2ꢁ → |5/2,−1/2ꢁ;
|5/2,−1/2ꢁ → |5/2,+1/2ꢁ; |5/2,+1/2ꢁ → |5/2,+3/2ꢁ and
|5/2,+3/2ꢁ → |5/2,+5/2ꢁ occur in the same region, and only
a broad band could be observed at g ∼ 2. For small D values
(D ꢂ gbH) the position of the five transitions differs and these
transitions in x, y and z directions could also appear at different
positions.
A quite good simulation12 of the EPR spectrum at 4 K has
been obtained, assuming an isolated spin state S = 5/2, with
isotropic g = 2.0, D = 0.1 cm−1 and E = 0.01 cm−1, with a
bandwidth of 300 G (Fig. 11). The broadness of the bands and
absence of hyperfine splitting could be due to dipole–dipole
interaction and ZFS, similar to that in the solid state.
Acknowledgements
This work has been supported by grants from AICTE and
UGC, Government of India, New Delhi, and from Ministerio de
Educacion y Cultura (BQ2003-00538) (Spain). S. N. acknowl-
edges UGC, Government of India, and D. M. acknowledges
CSIR, Government of India, for financial support. We also
acknowledge the use of IR and Uv-Vis spectrophotometers of
our department purchased from a DST (India)-FIST grant.
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D a l t o n T r a n s . , 2 0 0 5 , 2 4 2 8 – 2 4 3 5
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