D. Basumatary et al. / Journal of Molecular Structure 1092 (2015) 122–129
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5); 2,20-bipyridine, (bpy, 6); 1,10-phenanthroline, (phen, 7) on
the dihydrazone coordinated metal complex (1). The electron
transfer reactions of the complexes have been studied with the
help of cyclic voltammetry.
Introduction
Studies have shown the ability of hydrazone based complexes
of manganese as potential biomimics for manganese enzymes
[1,3,4]. The present work constitutes the exploration of the coordi-
nation chemistry of manganese with ligand bis(2-hydroxy-1-naph-
thaldehyde)adipoyldihydrazone (npahH4). The ligand is rich and
versatile, but has not yet been fully explored with transition
metals. Our earlier work [2] with this ligand and manganese in
alkaline condition reported Mn(IV) complexes in octahedral
geometry.
Experimental section
All reagents and chemicals were E-Merck or equivalent grade,
and all solvents were used as received.
The ligand derived from condensation of adipoyldihydrazide
with 2-hydroxy-1-naphthaldehyde contains four methylene func-
tions flanked by keto groups in addition to other functional groups
like amide, azomethine and naphthol functions, each in duplicate.
It offers a chemically flexible ligand framework because of free
rotation of the two hydrazone groupings about CAC single bond
and has a potential to offer any set of donor atoms depending upon
the preferred stereochemical disposition of the metal valences and
nature of the bonds formed in the coordination process. It is known
that the metal complex stability increases with the topological
complexity of the ligand recognized in the chelate effect, the
macrocyclic effect, and the cryptate effect. Now, we want to retain
the features of this tetraligand npahH4 and explore the level of sta-
bility when a second monodentate or bidentate ligand coordinates
to the metal center and compare the changes achieved in the
topologically constrained environment of the metal complex.
N-donor ligands being particularly active as functional biomimics
for manganese enzymes [3,4], pyridine and its methyl derivatives
were preferably selected as secondary ligands. Reports show that
there are few known manganese complexes with pyridine bases
and around twenty-five structures of Mn(II) complexes with
1,10-phenanthroline and its derivatives [5] are reported till date.
In this article, we provide examples of these situations.
The divalent high-spin manganese complexes are common and
they do not have strong stereo-chemical preferences and can exist
both in tetrahedral and octahedral coordination environment
depending on the structural demand of the coordinated ligand.
But very few low-spin complexes of divalent manganese are cur-
rently known. Because of its higher spin-pairing energy amongst
bivalent 3d ions, the ligands with very strong ligand fields only
can induce low-spin character on manganese ion. Low-spin
Mn(II) complexes have been known with cyano ligands [6,17],
phosphine ligands [7], oxime ligands [8,19], and dithiochelate
ligands [9]. Recently, a few low-spin manganese(II) complexes
have been reported by various groups [10], and also by us in our
earlier reported work with ligand bis(2-hydroxy-1-naph-
thaldehyde)malonoyldihydrazone [11]. The lower oxidation
state species of manganese(II) are commonly 6-coordinate.
Coordination number four with a distorted square-planar or tetra-
hedral geometry is known for the high-spin Mn(II) [12,13] but
uncommon for low-spin manganese(II). We report herein, a low-
spin Mn(II) complex with square-planar geometry and provide
examples of high-spin Mn(II) complexes with octahedral
geometry.
Physical measurements
Determination of manganese was done following the standard
procedure [14]. C, H and N were determined microanalytically.
The molar conductivity of the complexes at 10ꢁ3 M in DMSO solu-
tion were measured on a Systronics Direct Reading Conductivity
meter-303 with a dip-type conductivity cell at room temperature.
Room temperature magnetic susceptibility measurements were
carried out on a Sherwood Scientific Magnetic Susceptibility
Balance. Experimental magnetic susceptibility values have been
corrected for diamagnetism by the procedures given by Figgis
and Lewis [15]. Infrared (IR) spectra were recorded on a Bomen
DA-8FT-IR spectrophotometer from 450 to 4000 cmꢁ1 in KBr disks.
Electronic spectra of the complexes at 10ꢁ2 M in DMSO solution
were recorded from 200 to 1000 nm in DMSO on a Perkin-Elmer
Lambda 25 UV–Vis spectrophotometer. EPR spectra of powdered
samples as well as in solution were recorded at X-band frequency
on a Varian E-112 E-line century series ESR spectrometer using
TCNQ (g = 2.0027) as an internal field marker. FAB mass spectra
of the complexes were recorded on a JEOLSX102/DA-6000 mass
spectrometer/data systems using Argon/Xenon (6 kV, 10 mA) as
FAB gas. Nitrobenzyl alcohol was used as the matrix. Cyclic voltam-
metric measurements were carried out using CH Instruments
Electrochemical Analyzer under nitrogen atmosphere. The elec-
trolytic cell comprises of 3-electrodes. The working electrode was
a glassy-carbon disk from BAS and the reference electrode was
an aqueous SCE separated from the sample solution by a salt
bridge.
Preparation of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone
Adipoyldihydrazide was prepared by reacting diethyl adipate
(1.00 g) with hydrazine hydrate (0.55 g) in 1:2 molar ratio in
20 mL ethanol under reflux for 30 mins. The product was recrystal-
lized from dilute ethanol.
Bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone (npahH4)
was then prepared by reacting a solution of adipoyldihydrazide
(1.00 g) in 20 mL of ethanol with 2-hydroxy-1-naphthaldehyde
(2.18 g) in 1:2 molar ratio in ethanol over a hot plate at 50 °C with
constant gentle stirring. The yellow precipitate obtained on cooling
the solution was thoroughly washed with ethanol and air dried
(yield of 61.53%; m.p. > 300 °C).
In view of the meagre amount of work on manganese com-
plexes of this dihydrazone, the monometallic complexes of
manganese(II) have been synthesized and characterized. The
composition of isolated metal complexes has been judged mainly
from the elemental analysis, thermoanalytical data and mass spec-
tral data. The structures of Mn(II) complexes have been discussed
in the light of molar conductance, magnetic moment, electronic,
infrared spectroscopic and EPR studies. The EPR spectroscopy and
magnetic susceptibility studies have been used as a probe to study
the molecular distortions caused by pyridine (py, 2) and its deriva-
tives, 2-picoline (2-pic, 3); 3-picoline (3-pic, 4); 4-picoline, (4-pic,
Synthesis of [MnII(npahH2)] (1)
Mn(OAc)2ꢂ4H2O (1.00 g) in methanol (20 mL) was added to a
solution of bis(2-hydroxy-1-naphthaldehyde)adipoyldihydrazone
(1.97 g) in 20 mL methanol accompanied by stirring at 60 °C for
10 mins. The homogenous suspension was stirred vigorously for
another half an hour, a yellow-brownish compound was obtained.
The compound was filtered, washed with hot methanol and air
dried (yield: 76%).