Mn(II) complexes of some acylhydrazones with NNO donor sites: Syntheses, a spectroscopic view on their coordination possibilities and crystal structures

Article abstract of DOI:10.1016/j.poly.2010.09.007

Mn(II) complexes derived from a set of acylhydrazones were synthesised and characterized by elemental analyzes, IR, UV-vis and X-band EPR spectral studies as well as conductivity and magnetic susceptibility measurements. In the reported complexes, the hyd

Full text of DOI:10.1016/j.poly.2010.09.007

Polyhedron 29 (2010) 3318–3323
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Mn(II) complexes of some acylhydrazones with NNO donor sites: Syntheses, a
spectroscopic view on their coordination possibilities and crystal structures
⇑
Neema Ani Mangalam, S.R. Sheeja, M.R. Prathapachandra Kurup
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, Kerala 682 022, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Mn(II) complexes derived from a set of acylhydrazones were synthesised and characterized by elemental
analyzes, IR, UV–vis and X-band EPR spectral studies as well as conductivity and magnetic susceptibility
measurements. In the reported complexes, the hydrazones exist either in the keto or enolate form, as evi-
denced by IR spectral data. Crystal structures of two complexes are well established using single crystal
X-ray diffraction studies. In both of these complexes two equivalent monoanionic ligands are coordinated
in a meridional fashion using cis pyridyl, trans azomethine nitrogen and cis enolate oxygen atoms posi-
tioned very nearly perpendicular to each other. EPR spectra in DMF solutions at 77 K show hyperfine sex-
tets and in some of the complexes the low intensity forbidden lines lying between each of the two
hyperfine lines are also observed.
Received 12 March 2010
Accepted 7 September 2010
Available online 17 September 2010
Keywords:
Hydrazones
Crystal structures
Mn(II) complexes
EPR
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
tion of O2ꢀ radicals catalyzed by superoxide dismutases (SODs) and
water oxidation by photosynthetic enzymes (photosystem II)
The coordination chemistry of transition metals with ligands
from the hydrazine family has been of interest due to different
bonding modes shown by them with both electron rich and elec-
tron poor metals. Acylhydrazones have been extensively investi-
gated by chemists in the synthesis of coordination polymers
owing to their inherent coordination and hydrogen-bonding do-
nor/acceptor functionalities, as well as their biological activities
[1,2]. Recently it was reported that such ligands, like non-symmet-
rical salens, can act as effective catalysts towards alkene epoxida-
tion [3]. In analytical chemistry also, hydrazone ligands find wide
applications as transition metal binders [4]. Studies have shown
that the azomethine N, which has a lone pair of electrons in a
sp2 hybridized orbital, has considerable biological importance.
Acylhydrazone complexes of transition metal ions are known to
provide useful models for elucidation of the mechanism of enzyme
inhibition by hydrazine derivatives and for their pharmacological
applications [5]. Additionally hydrazone complexes have been
the subject of studies over many years for their anti-bacterial
and anti-tumor activities [6,7]. Manganese and its compounds find
very historical importance in medicine and play a significant role
in enzyme activation. It is well known that Mn plays an important
role in many biological redox processes including disproportion-
ation of H2O2 [8] (catalase activity) in microorganisms, decomposi-
[9,10].
This paper is an extension of our earlier studies with transition
metal complexes of acylhydrazones [11,12]. An attractive aspect of
hydrazones is that they are capable of exhibiting keto–enol tautom-
erism and can coordinate in tridentate neutral NNO donor mode
[13], monoanionic NNO donor mode [14], dianionic tridentate
ONO form [15], tetraanionic form [16] and bidentate neutral NO
forms [17] to the metal ions generating mononuclear, dinuclear
or polynuclear species. However, it depends on the reaction condi-
tions, such as metal ion, its concentration, the pH of the medium
and the nature of the hydrazone used. These interesting properties
of the transition metal hydrazone complexes promoted us to syn-
thesize Mn(II) complexes of some acylhydrazones and herein we
report their syntheses, spectral characterization and crystal
structures.
2. Experimental
2.1. Materials
2-Benzoylpyridine (Aldrich), di-2-pyridylketone (Aldrich),
quinoline-2-carbaldehyde (Aldrich), benzhydrazide (Aldrich), and
nicotinic hydrazide (Aldrich), manganese(II) acetate tetrahydrate
(E-Merck), manganese(II) chloride tetrahydrate (E-Merck) were
used as supplied and solvents were purified by standard procedure
before use.
⇑
Corresponding author. Tel.: +91 484 2862423; fax: +91 484 2575804.
E-mail address: mrp@cusat.ac.in (M.R.P. Kurup).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.09.007

N.A. Mangalam et al. / Polyhedron 29 (2010) 3318–3323
3319
2.2. Syntheses of hydrazones
except 1 and 2 at room temperature were made using a Vibrating
Sample Magnetometer at the Indian Institute of Technology, Roor-
kee, India and the other two were made using a MSB mk1 magnetic
susceptibility balance from Sherwood Scientific Limited. EPR spec-
tra of complexes in solid state at 298 K and in frozen DMF at 77 K
were recorded on a Varian E-112 spectrometer at X-band, using
TCNE as standard with 100 kHz modulation frequency and
9.1 GHz microwave frequency at SAIF, IIT Bombay, India.
All the hydrazones were synthesised by the procedure reported
by us earlier [11]. The hydrazones under investigation includes
2-benzoylpyridine benzoyl hydrazone (HBPB), di-2-pyridyl ketone
nicotinoyl hydrazone (HDKN), quinoline-2-carbaldehyde benzoyl
hydrazone (HQCB), quinoline-2-carbaldehyde nicotinoyl hydra-
zone (HQCN). The chemical structures and abbreviations for the
ligands are given in Fig. 1.
2.3. Syntheses of complexes
2.5. X-ray crystallography
Small brown needle crystals of 1 (0.20 ꢂ 0.20 ꢂ 0.30 mm3) were
obtained by slow evaporation from its methanol–acetonitrile solu-
tion and the X-ray diffraction experiments were performed on a
Bruker axs kappa apex2 CCD Diffractometer with graphite mono-
2.3.1. Syntheses of [Mn(BPB)2] (1) and [Mn(DKN)2] (2)
To a solution of the appropriate acylhydrazone (1 mmol) in
methanol one drop of triethylamine was added. To this, methanolic
solution of Mn(CH3COO)2ꢁ4H2O (1 mmol) was added and the reac-
tion mixture was refluxed for 4 h. The resulting solution was al-
lowed to stand at room temperature and after slow evaporation,
brown crystalline product was separated, filtered and washed with
ether and dried over P4O10 in vacuo.
chromated Mo K
a radiation (k = 0.71073 Å) at 293 K. The APEX2/
SAINT and SAINT
/XPREP softwares were used for cell refinement and
data reduction, respectively. The structure was solved using SIR92
by direct method and refinement were carried out by the full ma-
trix least squares method on F2 using SHELXL-97. All non-hydrogen
atoms were refined with anisotropic displacement parameters
and the hydrogen atoms were placed at calculated positions and
refined as riding atoms using isotropic displacement parameters.
Intensity data for a brown block shaped crystal of [Mn(DKN)2]
(0.10 ꢂ 0.22 ꢂ 0.32 mm3) grown from its methanolic solution
was collected on a Bruker P4 X-ray diffractometer using graphite
[Mn(BPB)2] (1): Yield: 85%, Color: brown, km (DMF): 4
X
ꢀ1 cm2
molꢀ1
,
leff (B.M.): 5.48, Elemental Anal. Calc.: C, 69.62; H, 4.30; N,
12.82. Found: C, 69.56; H, 4.22; N, 12.48%.
[Mn(DKN)2] (2): Yield: 75%, Color: brown, km (DMF): 7
X
ꢀ1 cm2
molꢀ1
,
leff (B.M.): 5.73, Elemental Anal. Calc.: C, 61.91; H, 3.67; N,
21.24. Found: C, 61.65; H, 3.61; N, 20.94%.
2.3.2. Syntheses of [Mn(QCB)2] (3) and [Mn(QCN)2] (4)
monochromated Mo Ka radiation (k = 0.71073 Å) at 293 K. The
Complexes 3 and 4 were prepared in a similar manner as com-
plexes 1 and 2 without adding triethylamine. The resulting solu-
tion was allowed to stand at room temperature and after slow
evaporation, the dark blue compounds obtained were washed with
methanol followed by ether and then dried over P4O10 in vacuo.
data were solved using SHELXL-97 [18] by direct method and each
refinement was carried out by full-matrix least-squares on F2 (SHEL-
XL-97) with anisotropic displacement parameters for non-hydrogen
atoms. The Bruker SAINT software was used for data reduction and
Bruker SMART for cell refinement.
Crystal data and refinement details are given in Table 1 and
molecular structure of [Mn(BPB)2] was drawn using the program
DIAMOND version 3.1f [19] with 50% displacement ellipsoids.
[Mn(QCB)2] (3): Yield: 58%, Color: blue, km (DMF): 8
X
ꢀ1 cm2
molꢀ1
,
leff (B.M.): 5.70, Elemental Anal. Calc.: C, 67.66; H, 4.01;
N, 13.92. Found: C, 67.60; H, 4.41; N, 13.78%.
[Mn(QCN)2] (4): Yield: 54%, Color: blue, km (DMF): 10
X
ꢀ1 cm2
molꢀ1
,
leff (B.M.): 5.91, Elemental Anal. Calc.: C, 63.47; H, 3.66;
N, 18.51. Found: C, 63.57; H, 3.26; N, 18.30%.
2.3.3. Syntheses of [Mn(HQCB)Cl2] (5) and [Mn(HQCN)Cl2]ꢁ2H2O (6)
Complexes 5 and 6 were prepared by refluxing equimolar meth-
anolic solutions of appropriate acylhydrazone and MnCl2ꢁ4H2O for
4 h. and then kept at room temperature. The pale yellow com-
pounds of 5 and 6 that separated out were filtered, washed with
ether and dried over P4O10 in vacuo.
[Mn(HQCB)Cl2] (5): Yield: 52%, Color: yellow, km (DMF):
15
X , leff (B.M.): 6.13, Elemental Anal. Calc.: C,
ꢀ1 cm2 molꢀ1
IR (KBr) cm-1: ν(N–H) 2928; ν(C=O)
1689; ν(C=N) 1579.
UV/vis (MeCN) λmax (cm-1): 44840, 37170,
31440.
IR (KBr) cm-1: ν(N–H) 3063; ν(C=O)
1678; ν(C=N) 1571.
UV/vis (MeCN) λmax (cm-1): 42920,
36900, 31050.
50.90; H, 3.27; N, 10.47. Found: C, 50.24; H: 3.41; N: 10.53%.
[Mn(HQCN)Cl2]ꢁ2H2O (6): Yield: 49%, Color: yellow, km (DMF):
20
X , leff (B.M.): 5.67, Elemental Anal. Calc.: C,
ꢀ1 cm2 molꢀ1
43.86; H, 3.68; N, 12.79. Found: C, 44.27; H, 3.59; N, 12.93%.
2.4. Physical measurements
Elemental analyzes of the ligands and the complexes were per-
formed on a Vario EL III CHNS analyzer at SAIF, Kochi, India. The IR
spectra were recorded on a JASCO FT/IR-4100 Fourier Transform
Infrared spectrometer using KBr pellets in the range 400–
4000 cmꢀ1. The electronic spectral analyzes of ligands and their
complexes were carried out in acetonitrile solution at room tem-
perature on a Spectro UV–vis Double Beam UVD-3500 spectrome-
ter in the 200–900 nm range. The molar conductances of the
complexes in DMF (10ꢀ3) M solutions were measured at 298 K
with a Systronic model 303 direct-reading conductivity bridge.
Magnetic susceptibility measurements for all the complexes
IR (KBr) cm-1: ν(N–H) 3191; ν(C=O) IR (KBr) cm-1: ν(N–H) 3173; ν(C=O)
1655; ν(C=N) 1593. 1656; ν(C=N) 1591.
UV/vis (MeCN) λmax (cm-1): 43790, UV/vis (MeCN) λmax (cm-1): 43920, 41490,
41640, 35860, 32580, 31690, 30430, 35840, 32580, 31750, 30440, 29150.
29130.
Fig. 1. Chemical structures of hydrazone ligands and their abbreviations.

3320
N.A. Mangalam et al. / Polyhedron 29 (2010) 3318–3323
nation mode of the hydrazones, the IR spectra of their complexes
3. Results and discussion
were investigated and were found that ligands have coordinated
either in neutral or monoanionic forms. Molar conductivity data
reveal that the complexes are non-electrolytes, in accordance with
the proposed formulations [20]. Magnetic measurements were re-
corded at room temperature for the complexes and their effective
The synthesised hydrazones and complexes are very stable sol-
ids at room temperature without decomposition for a long time. As
far as solubility is concerned, these hydrazones are fully soluble in
most common organic solvents such as EtOH, MeOH, CHCl3,
CH2Cl2, etc. but the complexes have very low solubility and are
only soluble in CH3CN, DMF and DMSO. The Mn complexes syn-
thesised were characterized by the analytical and spectroscopic
techniques. In order to know further information about the coordi-
magnetic moment (leff) values are given in Section 2.3. The mag-
netic moments of the Mn(II) complexes were observed in the range
5.48–6.13 B.M. [21] which are indicative of a high spin d5 system.
After the syntheses of the complexes, we recrystallised 1 and 2 and
Table 1
Crystal data and structure refinement parameters for complexes 1 and 2.
Parameters
[Mn(BPB)2] (1)
38H28MnN6O2
[Mn(DKN)2] (2)
Empirical formula
Formula weight, M
Temperature, T (K)
Wavelength (Mo K
Crystal system
Space group
Lattice constants
a (Å)
C
C34H24MnN10O2
659.57
150
0.71073
monoclinic
P21
655.60
293(2)
0.71073
triclinic
a
) (Å)
ꢀ
P1
10.4629(3)
12.6639(4)
13.2640(4)
65.9150(10)
84.4190(10)
84.9730(10)
1594.66(8)
2
9.5069(7)
10.1525(7)
16.6511(10)
90.00
100.390(7)
90.00
1580.79(19)
2
1.386
c (Å)
b (Å)
a
(°)
b (°)
(°)
c
Volume, V (Å3)
Z
Calculated density,
Absorption coefficient,
Limiting indices
q
(Mg mꢀ3
l
)
1.365
0.459
(mmꢀ1
)
0.466
ꢀ15 6 h 6 15
ꢀ18 6 k 6 18
ꢀ19 6 l 6 19
44 066 (0.0398)
10 915
ꢀ11 6 h 6 11
ꢀ11 6 k 6 11
ꢀ19 6 l 6 19
11 389 (0.0240)
5359
Reflections collected (Rint
Unique reflections
Refinement method
)
full-matrix least-squares on F2
10915/0/425
full-matrix least-squares on F2
5359/1/425
Data/restraints/parameters
Goodness-of-fit on F2
0.998
1.046
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0479, wR2 = 0.1207
R1 = 0.1092, wR2 = 0.1571
R1 = 0.0438, wR2 = 0.1243
R1 = 0.0514, wR2 = 0.1277
P
P
2
2 1=2
wR2 ¼ ½ wðF2o ꢀ Fc2Þ = wðF2oÞ ꢃ
.
P
P
R1
=
||Fo| ꢀ |Fc||/ |Fo|.
Fig. 2. Molecular structure of [Mn(BPB)2] (1) along with the atom numbering scheme (displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are
shown as small spheres of arbitrary radii).

N.A. Mangalam et al. / Polyhedron 29 (2010) 3318–3323
3321
obtained suitable single crystals for X-ray diffraction analyzes.
3.1. Crystal structures of [Mn(BPB)2] (1) and [Mn(DKN)2] (2)
From the crystal structures it was found that in both cases the li-
gands HBPB and HDKN coordinate via pyridyl, azomethine nitro-
gens and enolate oxygens.
The manganese atom in [Mn(BPB)2] and [Mn(DKN)2] is coordi-
nated in a distorted octahedral arrangement in which each ligand
binds as a monoanionic N,N,O chelator in a meridional fashion. A
perspective view of 1 along with the atom numbering scheme is
depicted in Fig. 2 and relevant bond lengths and angles of 1 and
2 are listed in Table 2. The coordinated ligands form two five
membered chelate rings, imposing large distortions on the ideally
octahedral coordinate angles. The trans angle N2–Mn–N5 and N3–
Mn1–N8 is much farther from 180°, 164.16(5)° in 1 and 161.95(11ꢁ)
and 2, respectively, compared with similar Cu (178.03°) and Zn
(175.23°) complexes [22], whereas the trans N1–Mn–O1
(142.04(5)°) and N4–Mn–O2 (143.07(5)°) angles in 1 defined by
each meridionally coordinated ligand also deviate markedly from
linearity. The intraligand bite angles are correspondingly more
acute at ꢄ71.49° in 1 and ꢄ71.66° in 2 when compared to other
metal complexes reported with similar ligands. As the coordinate
bonds become longer, the inflexible ligand maintains the same
conformation, leading to the contraction of the coordinate angles.
The crystal structure of HBPB was reported by us earlier [11] and
although the C6–N2 and N2–N3 bond lengths are less affected by
the complexation [ca. 1.287 and 1.369 Å in complex compared
with 1.295 and 1.368 Å in free ligand, respectively], the C13–O1
bond lengthens significantly (by ca. 0.04 Å). Relevant bond length
and bond angles of HBPB [11] are tabulated in Table 2. Overall,
these observations are consistent with the enolate resonance form
of HBPB being dominant in the deprotonated coordinated ligand,
and this is supported by the IR spectral data. In 2 as a consequence
of deprotonation, the C13–O1 and C32–O2 bonds lengthen and the
negative charge of the monoanionic ligand is delocalized over the
DKN moiety. Both the bicyclic chelate rings are close to being
Table 2
Selected bond lengths (Å) and bond angles (°) for HBPB, complexes 1 and 2.
HBPB
[Mn(BPB)2] (1)
[Mn(DKN)2] (2)
Mn1–N2
Mn1–N5
Mn1–O1
Mn1–O2
Mn1–N1
Mn1–N4
N2–N3
N5–N6
N2–C6
N5–C25
C13–O1
N3–C13
–
–
–
–
–
–
2.190(2)
2.194(2)
2.120(1)
2.096(1)
2.287(2)
2.284(2)
1.369(2)
1.370(2)
1.287(2)
1.292(2)
1.267(2)
1.332(2)
1.334(2)
125.97(2)
164.16(5)
71.75(5)
97.76(6)
71.75(5)
71.14(5)
71.33(5)
101.40(5)
142.04(5)
143.07(5)
94.19(5)
103.69(6)
120.01(1)
114.57(2)
125.26(2)
Mn1–N3
Mn1–N8
Mn1–O1
Mn1–O2
Mn1–N1
Mn1–N6
N3–N4
N8–N9
N3–C6
N8–C23
C12–O1
N4–C12
2.204(3)
2.195(3)
2.133(3)
2.125(3)
2.313(3)
2.268(3)
1.375(4)
1.373(4)
1.280(5)
1.295(5)
1.261(4)
1.343(5)
1.326(5)
126.80(3)
161.95(1)
72.41(1)
97.38(1)
72.15(1)
71.23(1)
70.85(1)
102.48(1)
143.59(1)
142.15(1)
96.48(1)
97.08(1)
120.20(3)
115.10(3)
125.50(3)
1.3682(2)
–
1.2955(2)
–
1.2213(1)
1.3610(2)
–
124.67(1)
–
–
–
–
–
–
–
N6–C32
N9–C29
O1–C13–N3
N2–Mn1–N5
N2–Mn1–O1
N1–Mn1–N4
N5–Mn1–O2
N1–Mn1–N2
N4–Mn1–N5
O1–Mn1–O2
O1–Mn1–N1
O2–Mn1–N4
N2–Mn1–N4
N5–Mn1–N1
C6–N2–N3
N2–C6–C5
N2–C6–C7
O1–C12–N4
N3–Mn1–N8
N3–Mn1–O1
N1–Mn1–N6
N8–Mn1–O2
N1–Mn1–N3
N6–Mn1–N8
O1–Mn1–O2
O1–Mn1–N1
O2–Mn1–N6
N3–Mn1–N6
N8–Mn1–N1
C6–N3–N4
N3–C6–C5
N3–C6–C7
–
–
–
–
117.38(1)
127.69(1)
114.35(1)
Table 3
H-bonding,
p–p and C–Hꢁ ꢁ ꢁp interaction parameters in [Mn(BPB)2] and [Mn(DKN)2].
[Mn(BPB)2]
H-bonding interactions
D–Hꢁ ꢁ ꢁA (Å)
D–H
0.93
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA (°)
C8–H(8)ꢁ ꢁ ꢁN(3)a
2.57
3.332
140
Equivalent position code
a = 1 ꢀ x, 1 ꢀ y, 2 ꢀ z
p–p interactions
Cg(I)ꢁ ꢁ ꢁCg(J)
Cg–Cg (Å)
3.731
a
0.03
(°)
b (°)
18.63
Cg(6)ꢁ ꢁ ꢁCg(6)b
Equivalent position code
b = 2 ꢀ x, ꢀy, 2 ꢀ z
Cg(6) = N(4), C(20), C(21), C(22), C(23), C(24)
C–Hꢁ ꢁ ꢁ
p interaction
X–Hꢁ ꢁ ꢁCg(J)
Hꢁ ꢁ ꢁCg (Å)
X–Hꢁ ꢁ ꢁCg (°)
Xꢁ ꢁ ꢁCg (Å)
C(9)–H(9)ꢁ ꢁ ꢁCg(1)c
Equivalent position code
c = 1 ꢀ x, 1 ꢀ y, 2 ꢀ z
2.74
139
3.492
Cg(1) = Mn (1), O(1), C(13), N(3), N(2)
[Mn(DKN)2]
H-bonding interactions
D–Hꢁ ꢁ ꢁA (Å)
D–H
0.93
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA (°)
C16–H(16)ꢁ ꢁ ꢁN(4)d
Equivalent position code
d = ꢀx, ꢀ1/2 + y, ꢀz
2.60
3.523
176
p–p interactions
Cg(I)ꢁ ꢁ ꢁCg(J)
Cg–Cg (Å)
3.751
a
(°)
b (°)
13.44
Cg(1)ꢁ ꢁ ꢁCg(6)e
17.49
Equivalent position code
e = 1 ꢀ x, ꢀ1/2 + y, ꢀz
Cg(1) = N(4), C(20), C(21), C(22), C(23), C(24); Cg(6) = Mn(1), O(1), C(12), N(4), N(3)
C–Hꢁ ꢁ ꢁ
p interaction
X–Hꢁ ꢁ ꢁCg(J)
Hꢁ ꢁ ꢁCg (Å)
X–Hꢁ ꢁ ꢁCg (°)
Xꢁ ꢁ ꢁCg (Å)
C(28)–H(28)ꢁ ꢁ ꢁCg(2)f
Equivalent position code
f = 1 ꢀ x, ½ + y, 1 ꢀ z
2.65
146
3.462
Cg(2) = Mn(1), O(2), C(29), N(9), N(8)
D, donor; A, acceptor; Cg, centroid; a, dihedral angles between planes I and J; b, angle Cg(1)–Cg(J) vector and normal to plane I.

3322
N.A. Mangalam et al. / Polyhedron 29 (2010) 3318–3323
perpendicular to each other with a dihedral angle of 86.24° and
86.13° in 1 and 2, respectively. The M–N(pyridine), M–N(imine) and
M–O bond lengths observed here are within the range reported
for other similar complexes of divalent metal ions [23,24].
No classical hydrogen bond was found in the crystal structure of
1. The crystal packing is determined by intermolecular hydrogen
tion of the p-orbital on the double bond with the d-orbital on the
metal ion with the reduction of the force constant [27]. This sup-
ports the participation of imine group of these ligands in binding
to the metal ion. The appearance of a new band due to the new
m
(C@N) formed as a result of enolization of the ligands at
ꢄ1585 cmꢀ1 may be taken as additional evidence for the participa-
tion of the imine nitrogen in coordination [28]. The low energy
pyridine ring out-of-plane vibrations observed in the spectra of li-
gands at ꢄ650 cmꢀ1 are shifted to higher frequencies in the case of
complexes, which are good indication of the coordination of the
heterocyclic nitrogen atom. The ligand coordination is substanti-
ated by two bands appearing at ꢄ487, 436 cmꢀ1 for the complexes;
bonds,
p
–
p
interactions and C–Hꢁ ꢁ ꢁ
p interactions (Table 3). In
the crystal structure of 2, the asymmetric units are linked by
intermolecular interactions (Table 3) and gives stability to the
complex.
3.2. Infrared spectra
these are mainly attributed to m(M–N) and m(M–O), respectively.
The far IR spectra of the complexes 5 and 6 display peaks at 303
and 321 cmꢀ1 indicating the terminal coordination of chlorine
atom [29]. From the above observations it was found that the
hydrazones under study could coordinate both in keto and in eno-
late form.
The significant IR frequencies of the ligands are shown in Fig. 1.
The main stretching frequencies of the IR spectra of the complexes
and their tentative assignments are tabulated (see Supplementary
material). Comparing the IR spectra of the ligands with that of the
complexes, we observed that m(NH) and m(C@O) stretching vibra-
tions of the ligands disappeared in the IR spectra of their com-
plexes except in 5 and 6, which showed that the frame H–N–
C@O has transformed to N@C–O–H form coordinating to metal
(Mn) in enolate form [25]. Additionally a new C–O absorption band
appeared in the range 1348–1368 cmꢀ1 in all the complexes except
3.3. Electronic and EPR spectra
The absorption bands of the ligands are shown in Fig. 1 and that
of complexes are tabulated (see Supplementary material). The
ground state of high-spin octahedral Mn(II) complex is A1g and
6
5 and 6. Both of the m(NH) and m(C@O) bands were retained in the
as there are no excited terms of sextet spin multiplicity, d–d tran-
sitions are doubly forbidden. However, some forbidden transitions
occur and consequently, these transitions have an extremely low
molar extinction coefficient value. The electronic spectra of the
manganese(II) complexes exhibit four weak intensity absorption
bands in the ranges 17 280–18 950; 23 923–28 380; 28 450–
metal complexes 5 and 6 but were shifted towards lower frequen-
cies, indicating that the hydrazone ligand has not undergone eno-
lization but has coordinated to the metal center in keto form [26].
The m(C@N) stretching bands are strongly affected by chelation and
shifted to lower wavenumbers which is attributed to the conjuga-
28 980; 31 055–31 600 cmꢀ1, which may be assigned to the transi-
4
tions: T1g (4G) 6A1g
,
4A1g (4G), 4Eg 6A1g
,
4Eg (4D) 6A1g
,
4T1g
(4P) 6A1g, respectively. For octahedral Mn(II) complexes, elec-
tronic spectra normally show two bands ca. 18 000 and
4
4
20 000 cmꢀ1, which are assigned to T1g (G) 6A1g and T2g
(G) 6A1g, respectively [30]. But the high intense charge transition
tailing into the visible region, obscure the very weak d–d absorp-
tions of the Mn(II) complexes.
The electron spin properties of manganese have long been of
interest as a spectroscopic probe of manganese centers in manga-
nese proteins. The Mn(II) oxidation state possesses Kramers’
ground-state doublets and exhibits characteristic spin transitions
in the normal mode X-band regime.The spin Hamiltonian,
ꢀ
ꢁ
^
SðS þ 1Þ
H ¼ gbBS þ IAS þ D S2z ꢀ
þ EðS2x ꢀ S2yÞ
3
where B is the magnetic field vector, g is the spectroscopic splitting
factor, b is the Bohr magneton, D is the axial zero field splitting
parameter, E is rhombic zero field splitting parameter and S is the
electron spin vector. The first two terms represent the electronic
Zeeman and the electron nuclear hyperfine interactions, respec-
tively, whereas the last two terms define the zero-field splitting
interaction with D and E gauging the axial and the rhombic parts.
Mn2+ (d5) being an odd electron system, the zero-field splitting
produces three doubly degenerate spin states Ms = 5/2, 3/2, 1/2
(Kramers’ degeneracy). Each of these is split into two singlets by
the applied field, producing six levels. As a result of this splitting,
five transitions (ꢀ5/2 ? ꢀ3/2, ꢀ3/2 ? ꢀ1/2, ꢀ1/2 ? 1/2, 1/2 ? 3/
2, 3/2 ? 5/2) are expected. But in practice usually only one signal
is observed since they are of equal energy. The spectrum will fur-
ther split by the nuclear hyperfine interaction with the Mn nucleus
(I = 5/2) which would give rise to thirty peaks in the spectrum.
EPR spectra of [Mn(DKN)2] and [Mn(HQCN)Cl2]ꢁ2H2O in poly-
crystalline state at 298 K gave a broad signal with g = 2.083 and
Fig. 3. Simulated and experimental best fits of EPR spectra of [Mn(DKN)2] (above)
and [Mn(HQCB)Cl2] (below) in frozen DMF at 77 K.

N.A. Mangalam et al. / Polyhedron 29 (2010) 3318–3323
3323
2.053, respectively, without any resolved hyperfine splitting. The
Appendix A. Supplementary data
broadness of the spectra may be due to immobilization of Mn(II)
ion in the complex or may be due to dipolar interactions and en-
hanced spin lattice relaxation. However, when recorded in DMF
at 77 K, both spectra exhibit sixfold well-resolved hyperfine split-
ting pattern with hyperfine coupling constants 9.3 and 8.97 mT,
respectively, arising due to the hyperfine interaction between the
unpaired electron with the 55Mn nucleus (I = 5/2) [31]. The frozen
solution spectrum of 2 and 5 in DMF at 77 K was simulated using
EasySpin [32]. The observed g values are very close to the free elec-
tron spin value of 2.0023 which is consistent with the typical
Mn(II) and also suggestive of the absence of spin orbit coupling
CCDC 715900 and 767043 contain the supplementary crystallo-
graphic data for compounds [Mn(BPB)2] and [Mn(DKN)2]. These
data can be obtained free of charge via http://www.ccdc.cam.a-
c.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.poly.2010.09.007.
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Acknowledgments
M.R.P. Kurup is thankful to KSCSTE, Thiruvananthapuram, India
for financial assistance and N.A. Mangalam thanks the Cochin Uni-
versity of Science and Technology for the award of Senior Research
Fellowship. S.R.S. thanks CSIR, New Delhi for the award of Senior
Research Fellowship. The authors are thankful to the SAIF, Cochin
University of Science and Technology, Kochi, Kerala, India for ele-
mental and IR analyses. We are thankful to IIT Bombay, India for
EPR spectra. We also thank Prof. M.V. Rajasekharan, School of
Chemistry, University of Hyderabad and Dr. Babu Varghese, SAIF,
IIT Madras for providing single crystal XRD data.