Synthesis, characterization and structure assessment of mononuclear and binuclear low-spin manganese(II) complexes derived from oxaloyldihydrazones, 1,10-phenanthroline and 2,2′-bipyridine

Article abstract of DOI:10.1016/j.molstruc.2018.04.043

Low-spin manganese (II) complexes [Mn^II(H₂slox) (bipy)] (1), [Mn^II(H₂slox) (phen)] (2), [Mn^II(H₂slox) (phen)].1.5.H₂O (2a), [Mn^II(H₂nph) (bipy)].H₂O (3) and [Mn^II(H₂nph) (phen)].H₂O (4) were synthesized from oxaloyldihydrazones in methanolic-water medium. The stoichiometry of the complexes was established by analytical, molecular weight and thermoanalytical data. Based on the data obtained from molar conductance, UV–visible, Infrared spectral, magnetic moment and electron paramagnetic resonance spectroscopic studies, the structures of the complexes have been established. From molar conductance it is suggested that all the complexes are non-electrolyte in DMF medium. The complex (2a) is monomeric in nature whereas the complexes (1) to (4) are dimeric based on the molecular weight data. The effective magnetic moment of complexes (1) to (4) show metal-metal interaction while complex (2a) has no metal-metal interaction. The complexes (1)–(4) show two quasi-reversible metal centred electron transfer reaction involving Mn^II/Mn^I/Mn⁰ redox reactions in DMF medium.

Full text of DOI:10.1016/j.molstruc.2018.04.043

Accepted Manuscript
Synthesis, characterization and structure assessment of mononuclear and binuclear
low-spin manganese(II) complexes derived from oxaloyldihydrazones, 1,10-
phenanthroline and 2,2′-bipyridine
Ibanphylla Syiemlieh, Arvind Kumar, Sunshine D. Kurbah, Ram A. Lal
PII:
S0022-2860(18)30480-0
10.1016/j.molstruc.2018.04.043
MOLSTR 25116
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 7 November 2017
Revised Date: 10 April 2018
Accepted Date: 11 April 2018
Please cite this article as: I. Syiemlieh, A. Kumar, S.D. Kurbah, R.A. Lal, Synthesis, characterization and
structure assessment of mononuclear and binuclear low-spin manganese(II) complexes derived from
oxaloyldihydrazones, 1,10-phenanthroline and 2,2′-bipyridine, Journal of Molecular Structure (2018), doi:
10.1016/j.molstruc.2018.04.043.
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ACCEPTED MANUSCRIPT
Synthesis, characterization and structure assessment of mononuclear and binuclear low-
spin manganese(II) complexes derived from oxaloyldihydrazones, 1,10-phenanthroline and
2,2'-bipyridine
Ibanphylla Syiemlieha, Arvind Kumarb, Sunshine D. Kurbaha, and Ram A. Lala*
aCentre for Advanced Studies, Department of Chemistry, North-Eastern Hill University,
Shillong-793022, India.
bDepartment of Chemistry, Faculty of Science and Technology, The University of West-Indies,
St. Augustine, Trinidad and Tobago.
Abstract
Low-spin manganese(II) complexes [MnII(H2slox)(bipy)] (1), [MnII(H2slox)(phen)] (2),
[MnII(H2slox)(phen)].1.5.H2O (2a), [MnII(H2nph)(bipy)].H2O (3) and [MnII(H2nph)(phen)].H2O
(4) were synthesized from oxaloyldihydrazones in methanolic-water medium. The stoichiometry
of the complexes was established by analytical, molecular weight and thermoanalytical data.
Based on the data obtained from molar conductance, UV-visible, Infrared spectral, magnetic
moment and electron paramagnetic resonance spectroscopic studies, the structures of the
complexes have been established. From molar conductance it is suggested that all the complexes
are non-electrolyte in DMF medium. The complex (2a) is monomeric in nature whereas the
complexes (1) to (4) are dimeric based on the molecular weight data. The effective magnetic
moment of complexes (1) to (4) show metal-metal interaction while complex (2a) has no metal-
metal interaction. The complexes (1) – (4) show two quasi-reversible metal centred electron
transfer reaction involving MnII/MnI/Mn0 redox reactions in DMF medium.
Introduction
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Manganese plays an important role in various biological redox processes, viz., in
catalases [1] and in oxygen-evolving photosystem II (PS-II) [2, 3]. As a cofactor of proteins it is
involved in processes including metabolism and wound healing. Manganese is the necessity of
the body for the function of the mitochondria in the form of the antioxidant enzyme manganese
superoxide dismutase (MnSOD). Superoxide and other reactive species, by products of cellular
respiration that can cause oxidative damage when they are not adequately controlled by
antioxidant systems such as MnSOD and its cycloplasmic counterpart CuZnSOD. This redox
imbalance is implicated in a number of diseases including cancer and inflammatory,
cardiovascular and neuro-degenerative diseases [4]. Mn ribonucleotide reductase shows the
conversion of nucleotides to deoxynucleotides [5], and oxalate degradation by oxalate oxidase
[6] and oxalate decarboxylase [7, 8]. Consequently, manganese complexes of polyfunctional
molecules in different oxidation states have attracted attention of inorganic chemists [9-10] in
recent years. Although high-spin manganese(II) and (III) complexes have been extensively
studied, yet low-spin manganese(II) and (III) complexes are very few in number. Manganese(II)
ion has very high-spin pairing energy amongst 3d metal ions, hence ligands with very strong
ligand fields only can induce low-spin character on manganese ion. Thus cyano- [11], phosphine
[12], dithiolate ligand [13] and oxime ligands [14] and some hydrazone ligands [15] have been
reported to give low-spin Mn(II) complexes.
Hydrazones are potential multinuclear molecules and are capable of giving monometallic
[16], homo-bi, tri metallic [17], hetero- bi, tri metallic [18] and multimetallic complexes. These
ligands exhibit keto-enol tautomerism in their metal complexes and can exist in staggered, syn-
cis and anti-cis conformation in metal complexes. Because of their multidentate nature, they are
able to provide metal complexes of different stereochemistries depending upon the preferred
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orientation of metal valences in three-dimensional space. Further, the continuous search for new
materials exhibiting higher performance than those used currently are necessity of time in order
to meet the requirements of daily changing technology. In this regard, the design and synthesis of
functional materials with interesting geometric configurations depends upon the choice of
suitable ligands with fixed geometry and flexible coordination capacity. In this connection, it is
imperative to mention that 1,10-phenanthroline ligand has a rigid framework as chelating donor
ligand and exhibits excellent coordination ability with transition metals ions [19]. 2,2'-bipyridine
is also a related ligand although it has flexible framework.
Oxaloyldihydrazones are capable of stabilizing metal ions in higher oxidation states [20,
21] and in combination with 1,10-phenanthroline and 2,2'-bipyridine ligands are capable of
providing more amazing and interesting compounds because of significantly different nature of
N and O donor sites. Thus, 1,10-phenanthroline nitrogen atoms are soft donor atoms and are
capable of stabilizing lower oxidation states of transition metals but the hydrazone ligands
possess relatively hard donor atoms and stabilize transition metal ions in their higher oxidation
states. Hydrazones used in the present study have been derived from condensation of
oxaloyldihydrazine and o-hydroxy aromatic aldehydes. They possess as many as eight oxygen
and nitrogen atoms and offer several alternate modes of bonding.
Literature survey shows that the work on synthesis, characterization and structure of
metal complexes of monohydrazide based ligands and their Schiff bases has been carried out in
some detail [16, 22, 23], the corresponding work on their dihydrazone congeners is quite meager
[17, 18, 24]. Moreover, the work on manganese complexes with dihydrazones in low-spin state is
almost non-existent. Hence, in view of the multidentate nature of oxaloyldihydrazones and their
possibility to offer several alternate modes of bonding in complexes, and almost non-existent
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work on low-spin manganese complexes of dihydrazones, in the present paper are synthesized
and characterized some low-spin manganese(II) complexes from oxaloyldihydrazones viz
disalicyaldehyde oxaloyldihydrazone
(H4slox) and
bis(2-hydroxy-1-naphthaldehyde)
oxaloyldihydrazone (H4nph) (Figure 1). The stoichiometry of the complexes has been established
using elemental analyses molecular weight and thermoanalytical data. Based on the data
obtained from molar conductance, magnetic moment, electronic, infrared spectroscopies and
ESR studies, the structural assessment of the complexes has been carried out. Cyclic
voltammetric studies has been employed to observe the electron transfer reactions in the
complexes.
Experimental Section
All reagents and chemicals were E-Merck or equivalent grade, and all solvents were used
as received.
Physical measurements
Manganese [25] was determined by standard literature procedure. Carbon, hydrogen and
nitrogen analyses were carried out by Perkin-Elmer 2400 CHNS/O Analyser. Molecular weight
of the complexes was determined in spectroscopic grade DMSO solution by the freezing point
depression method. DMSO (dimethyl sulfoxide) was kept over 4 Å molecular sieves prior to use.
The molar conductance was measured on a Direct Reading Conductivitymeter-303 with a dip
type conductivity cell at room temperature at 10-3 M dilution in DMF (N, N'-
dimethylformamide) solution. The loss of weight of the complexes was determined by heating
the compounds at 110 °C, 180 °C and 220 °C for 4 hours in open atmosphere in an electronic
oven. The thermogravimetric analysis (TGA) were carried out on the Perkin-Elmer STA 6000
(Simultaneous Thermal Analyzer) model in a ceramic crucible under dynamic nitrogen
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atmosphere. The DTA (Differential Thermal Analysis) standard used in the experiment is Pt 10
% Rh. Infrared spectra were measured on Perkin-Elmer, model 983 spectrophotometer in the
range 4000 – 400/500 cm-1 with samples investigated as KBr discs. Room temperature magnetic
susceptibility measurements were made on Sherwood Magnetic Susceptibility Balance MSB-
Auto. The susceptibility corrections were done by using Pascals Constant [26]. The electronic
spectra of the complexes at 10-5 M in DMF solution were measured from 200-1100 nm on a
Perkin Elmer Lamda 25 UV/Vis Spectrophotometer at room temperature. The ESR spectra of the
complexes were recorded in powder form as well as in DMSO glass at room temperature and
LNT at X-band frequency using TCNE (Tetracyanoethylene) (g = 2.0027) as an internal marker.
The electron transfer properties of the complexes were studied by cyclic voltammetry using CH
Instrument Electrochemical Analyzer under nitrogen atmosphere. The electrolytic cell comprises
of 3 electrodes, the working electrode was Pt disk while the reference electrode and auxiliary
electrode were Ag/AgCl (3M KCl) separated from the sample solution by a salt bridge. 0.1 M
Tetra-n-butyl ammonium perchlorate (TBAP) was used as the supporting electrolyte.
Molecular weight determination
Molecular weight of the complexes was determined using DMSO (Kf = 4.07) [27] as a
solvent, employing Beckman’s freezing point depression instruments. It consisted of a Beckman
thermometer and a stirrer with a non-conducting handle of wood which was fitted into a tube
through a rubber stopper fixed at its upper end. This tube was supported through a rubber stopper
in a bigger glass tube thoroughly purged with dry dinitrogen and functions as a dry nitrogen
jacket which ensures slow and uniform cooling of the inner tube. The outer tube is immersed in a
1,2-dimethyl-3-nitrobenzene (F.P. 15 °C) freezing bath taken in a glass jar. The jar was covered
with a plastic lid carrying three holes was covered with a jar. The inner tube is inserted through
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one hole, a large metal stirrer and an ordinary thermometer are inserted through second and third
hole. The molecular weight apparatus was enclosed in a dry box that was constantly purged with
dry nitrogen to minimize the error due to highly hygroscopic nature of DMSO. All of the
samples were placed in an entry port and purged overnight with dry nitrogen before bringing
them into the dry box.
Twenty grams DMSO was taken into the thoroughly cleaned and dried freezing point
tube purged with dry nitrogen. About 0.6 g of the complex was accurately weighed and inserted
into the DMSO solution. The tube was heated to dissolve the compound and then brought to
room temperature. Finally, the upper surface of the liquid in the tube was purged with dry
nitrogen and then adjusted into the outer tube. The freezing point temperatures of the solvent and
the solutions were directly measured with this apparatus through a magnifying glass fitted into
the dry box.
Preparation of disalicyaldehydeoxaloyldihydrazone (H4slox)
The disalicyaldehydeoxaloyldihydrazone (H4slox) was prepared according to the procedure
described in the literature [20a].
Preparation of bis(2-hydroxy-1-napthhaladehyde)oxaloyldihydrazone (H4nph)
Oxaloyldihydrazine was prepared by reacting diethyloxalate (20 mL) and hydrazine
hydrate (13 mL) in 1:2 molar ratios under stirring for 30 minutes. The product thus isolated was
recrystallized from H2O.
Bis(2-hydroxy-1-napthhaladehyde)oxaloyldihydrazone (H4nph) was prepared by
condensing oxaloyldihydrazine (3 g, 25.42 mmol), in hot H2O (100 mL) with 2-hydroxy-1-
naphthaladehyde (8.75 g, 50.84 mmol) in ethanol (20 mL) over hot plate at 70 °C with constant
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stirring for 1 h. The yellow precipitate thus obtained was washed with hot H2O-ethanol and dried
over anhydrous CaCl2.
Preparation of [Mn(H2slox)(bipy)] (1)
The complexes were prepared by two general procedures. Typical procedures are given
below.
A solution of MnSO4.H2O (0.09 g, 0.5 mmol) in 20 mL water was added to a mixed
suspension of H4slox (0.16 g, 0.5 mmol) and 2,2'- bipyridine (0.23 g, 1.5 mmol) in 10 mL MeOH
under stirring to get a homogeneous suspension. The reaction mixture was refluxed for 6 hours, a
light yellow compound was obtained. The compound was filtered, washed with water-methanol,
methanol and then with ether and dried over anhydrous CaCl2. (Yield: 82 %)
The other complexes were also prepared by following the above method in which in
complex (2), instead of 2,2'-bipyridine, 1,10-phenanthroline (0.29 g, 1.5 mmol) was taken. In
complex (3), a solution of MnSO4.H2O (0.09 g, 0.5 mmol) in 20 mL water was added to the
mixed suspension of H4nph (0.21 g, 0.5 mmol) and 2,2'- bipyridine (0.23 g, 1.5 mmol) in 10 mL
MeOH. Similarly, in complex (4), instead of 2,2'- bipyridine, 1,10-phenanthroline was taken.
(Yield: 78 % (2), 70 % (3), 75 % (4))
In the second procedure, the MnSO4.H2O, H4L (H4slox and H4nph) and BB (BB = 2,2'-
bipyridine/1,10-phenanthroline) were taken in the molar ratio of 1:1:1 and isolated as above.
(Yield: 76 % (2a))
Results and discussion
The complexes were prepared by two general methods. In the first method, the
MnSO4.H2O: H4slox/H4nph and 2,2'-bipyridine/1,10-phenanthroline were taken in a 1:1:3 molar
ratio in methanolic-water medium and refluxed for 6 hours. In the second method, the reactants
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were refluxed for 6 hours keeping MnSO4.H2O, H4slox/H4nph and 2,2'-bipyridine/1,10-
phenanthroline at 1:1:1 molar ratio. Both the methods gave complexes having similar
composition and properties except in case of the reaction mixture MnSO4.H2O, H4slox and 1,10-
phenanthroline in which case both the methods gave complexes having different properties from
magnetic and EPR point of view. Thus, the reaction mixtures in 1:1:3 molar ratio gave low-spin
complexes involving metal-metal interaction, that the reaction mixture in 1:1:1 molar ratio gave
complex having normal magnetic moment in case of MnSO4.H2O, H4slox and 1,10-
phenanthroline reaction mixture and in other cases, the same complexes were obtained as in case
of 1:1:3 molar ratio.
The complexes with their colour, decomposition point, elemental analyses, molecular
weight, magnetic moment and molar conductance are given in Table 1 and 2. On the basis of
elemental analyses, molecular weight and thermal analyses, the complexes have been found to
have composition [MnII(H2L)(BB)] (H4L = H4slox, H4nph; BB = 2,2'-bipyridine, 1,10-
phenanthroline). The complexes are either light yellow or yellow in colour. The complexes were
insoluble in common organic solvents but soluble in highly coordinating solvents viz., DMF and
DMSO. In comparison with the elemental analyses (Table 1) for both the found and calculated
percentages for different elements reveal that the compositions of the synthesized complexes
coincide well with the proposed formulae i.e., 1:1:1 molar ratio for Mn:H4L:BB. The
composition of the complexes has also been corroborated from thermoanalytical and molecular
weight data. The complexes (1) and (2) show weight loss neither at 110 °C nor at 180 °C ruling
out the possibility of presence of water or methanol molecules either in the lattice or
coordination sphere. However, the complexes (2a), (3) and (4) show weight loss equal to 1.5, 1
and 1 water molecule, respectively at 110 °C. This reveals that these water molecules are present
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in the lattice structure of complexes. But these complexes show no weight loss at 180 °C ruling
out the presence of water or methanol molecules in their coordination sphere. All complexes
show weight loss corresponding to one 2,2'-bipyridine/1,10-phenanthroline molecule when they
are heated at 220 °C for 4 hours in an electronic oven. This indicated the presence of these
secondary ligands in the first coordination sphere of the complexes.
We tried to crystallize the complexes under different experimental conditions using
various solvents, and combination of solvent systems. Both saturated and dilute solutions of the
complexes in various solvent systems such as DMF, DMSO, DMF-H2O, DMSO-CH3CN, DMF-
CH3CN, DMSO-CH2Cl2 and DMF-CH2Cl2 each was kept for 1 and 2 months under observation
at ambient temperature to grow crystals. Further, the solutions were gently evaporated in a hot
air electronic oven at 40 °C, 50 °C and 60 °C to promote crystal growth. Effort was also done to
grow crystals from the reaction mixture by layering a solution of the metal salts with a solution
containing the ligand in methanol solutions. Again, the metal salt solutions mixed with the ligand
solutions in DMSO and DMF were also layered with diethyl ether and resulting solution in a
small beaker was kept in a bigger beaker containing n-hexane. Further, MnSO4.H2O (0.5 mmol)
and a mixture of H4slox (0.5 mmol) and 2,2'-bipyridine (1.5 mmol) were placed in the main arm
of a branched tube (branch tube method). A mixture of CH3CN and DMSO (15 : 85 v/v, 15 mL)
was carefully added to fill the arms, the tube was sealed and reagents containing arms immersed
in an oil bath at 60 °C, while the other arm was kept at ambient temperature. The same procedure
was also applied for crystallizing the complexes (2), (3) and (4) by taking MnSO4.H2O (0.5
mmol) and a mixture of H4slox/H4nph (0.5 mmol) and1,10-phenanthroline/2,2'-bipyridine (1.5
mmol). Similarly, complex (2a) was also tried to crystallized following the same procedure by
taking MnSO4.H2O (0.5 mmol), a mixture of H4slox (0.5 mmol) and1,10-phenanthroline (0.5
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mmol). Unfortunately, in all our efforts, only amorphous compounds precipitated which
prevented analysis of the complexes by X-ray crystallography.
Molar conductance
The molar conductivity (Table 2) of the complexes (1) – (4) falls in the region 0.9-1.7 ꢀ-1
cm2 mol-1 at 10-3 M dilution in DMF. This suggests that all the complexes are non-electrolyte in
this medium.
TGA studies
In order to assess the thermal stabilities of the molecular framework of the complexes,
thermogravimetric analytical study was carried out for all of them. The complexes (1) and (2)
show one kind of thermal behavior while remaining complexes show different kind of thermal
behavior. The TGA graph of complex [MnII(H2slox)(bipy)] (1) and [MnII(H2nph)(phen)].H2O (4)
has been given in (Figure S1 and S2). After losing surface water molecules, complexes (1) and
(2) decompose in two temperatures ranges 314-360; 468-566 and 488-606 °C, respectively. The
weight losses in the temperature range 314-360 °C are 79.26 % and 75.36 %, which corresponds
to loss of one molecule of dihydrazone devoid of four oxygen atoms alongwith one molecule of
2,2'-bipyridine and 1,10-phenanthroline (theo: 81.01 % in complex (1) and 78.71 % in complex
(2)), respectively. The residues obtained at 360 °C are 15.88 % and 20.29 % in complexes (1)
and (2), respectively. The residue in complex (1) corresponds to the formation of MnO2 (theo:
15.71 %) but in case of complex (2), the residue (20.29 %) is higher than the theoretical weight
of MnO2 (theo: 15.56 %) although the IR spectra of the residues in the complexes match with
one another. This suggests the presence of some impurity in the residue, most probably,
carbonaceous impurity, in complex (2). After 360 °C, the residue shows slow weight loss upto
468 and 488 °C, losing weight of about 1.45 % and 1.07 %, respectively, at temperature interval
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of 20 °C. The second moderate weight loss step is initiated at 468 and 488 °C which is
completed at 566 and 607 °C temperatures at which the weight of residue vanishes in both
complexes, respectively.
The complexes (3) and (4) decompose in three major steps as against the complexes (1)
and (2) which decompose in two major steps. They lose about 3.00 % and 3.04 % of weight in
the temperature ranges 80-130 and 80-150 °C, respectively. These weight losses correspond to
loss of one water molecule (theo: 2.76 % in complex (1) and 2.66 % in complex (2)), present
most probably, in the lattice structure. At temperatures 150 °C, the decomposition of complexes
commences, first losing secondary ligands 2,2'-bipyridine and 1,10-phenanthroline molecules,
followed by decomposition of primary dihydrazone ligand. Secondary ligands decompose in two
minor steps in the temperature ranges 130-337 °C and 150-328 °C. The weight losses in these
temperatures ranges are 24.62 % and 16.58 %, respectively. The weight loss of 24.62 % in
complex (3) corresponds to loss of one 2,2'-bipyridine molecule (theo: 23.89 %) but that the
weight loss of 16.58 % in complex (4) is much less than that of one molecule of 1,10-
phenanthroline (theo: 26.63 %). This suggests that the remaining portion of 1,10-phenanthroline
molecule decomposes alongwith the primary dihydrazone ligand in the temperature range 328-
387 °C. This is evident from the fact that the weight loss in the above temperature is 61.11%
which is higher than that of the theoretical weight of dihydrazone ligand (theo: 57.90 %) devoid
of two oxygen atoms by 3.21 %. The weight of the residues obtained at 386 °C is found to be
12.68 % and 19.27 %, respectively. The weight of the residue in the complex (3) suggests the
formation of MnO2 (theo: 13.32 %). However, the weight of the residue in complex (4) is much
higher than that of the theoretical weight of MnO2 (theo: 12.85 %). This might be explained by
the presence of carbonaceous impurities in the residue. The weight of the residue in complex (4)
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corresponds to that of MnO2 at a temperature of 550 °C (12.13 %), but the residues show no
stability at any stage beyond 386 °C until the weight vanishes at 654 °C and 702 °C,
respectively.
The complex (2a) [MnII(H2slox)(phen)].1.5H2O shows more or less similar
decomposition behavior to complexes (3) and (4). The loss of weight observed in the
temperature range 60 - 135 °C is 4.47 % which corresponds to loss of 1.5 water molecules (theo:
4.61 %) present in the lattice. Further, the weight loss observed in the temperature range 298 -
366 °C is (exp: 30.13 %). This weight loss corresponds to loss of one 1,10-phenanthroline
molecule (theo: 30.72 %). The loss of one 1,10-phenanthroline molecule at such a high
temperature reveals its coordination to the metal centre. The coordinated ligand decomposes in
two steps 366- 485 °C and 485- 643 °C. The total weight loss in this temperature range 36.62 %
which is less than the theoretical weight of 49.82 % of H4slox devoid of two oxygen atoms. This
indicates the remaining portion of ligand further decomposes along with generated manganese
oxide until the heating process is stopped at 899 °C. However, even at this temperature, complex
decomposition and weight loss process is not stopped. But the IR spectra of the residue at 899 °C
matches with that of an authentic sample of MnO2 which suggests formation of MnO2 as the end
product. The mass of end product is 9.90 % which is less than the theoretical value of mass of
MnO2 (theo: 14.84 %). This indicates that the newly formed MnO2 vaporizes at higher
temperature.
Molecular weight
The molecular weight of the complexes (Table 2) was determined in DMSO (Kf = 4.07)
[27] as a solvent, employing Beckmans’ freezing point depression instruments. The
experimentally determined value for the complex (2a) is (570 ± 27) which is very near to the
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theoretical molecular weight (559) for monomeric formulation of complex. This suggests that the
complex (2a) is monomeric in nature. This was also corroborated by effective magnetic moment
of the complex which rules out any metal-metal interaction in the structural unit of the complex.
On the other hand, the experimentally determined molecular weights for the complexes
(1) to (4) are (1090±55), (1150±55), (1234±60) and (1370±55), respectively. These molecular
weights are close to the values for dimeric formulation for the complexes. This suggests that the
complexes (1) to (4) are dimeric. This is also corroborated by lower magnetic moment of the
complexes which displays metal-metal interaction in the structural unit of the complexes.
Magnetic moment
The effective magnetic moment values (Table 2) for the complex (2a) is 1.74 B.M at 298
K which is very close to the spin only value for one unpaired electron. This reveals that the
5
complex has low-spin character at ambient temperature and idealized t2 (MnII) configuration
[28]. The µeff value for the complex dismisses the possibility of orbital contribution to the
magnetic moment and any metal-metal interaction in the structural unit of the complex.
In the remaining complexes, the µeff value falls in the region 0.52 – 0.92 B.M. These
values are less than the spin-only magnetic moment (1.73 B.M). This suggests that there is a
considerable magnetic exchange interaction between the metal ions in the structural unit of the
complexes.
The magnetic moment for the complexes (1) to (4) is considerably reduced as compared
to that on the no-interaction basis. Such a decrease in µeff value might occur either due to super
exchange via overlap of the metal orbitals with the orbitals of the bridging oxygen atoms of
phenolate/naphtholate oxygen atoms [29] or due to direct overlap of metal orbitals of one
structural unit with the metal orbitals of other structural unit. An alternative route of metal-metal
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interaction is provided by bridging secondary ligands viz 1,10-phenanthroline or 2,2'-bipyridine
groups. In this case the metal orbitals overlap with the orbitals of 1,10-phenanthroline or 2,2'-
bipyridine and the magnetic interaction is communicated to another metal ion through electron
cloud of pyridyl rings via nitrogen atoms. Such an electron exchange through bridging ligands
has been proposed by McCleverty et al. [30] from their studies on molybdenum complexes
derived from bridging ligands. Hence, in the present case, it is proposed that magnetic interaction
from one manganese ion to second manganese ion in the structural unit of the complexes is
communicated through the secondary bridging ligands 1,10-phenanthroline or 2,2'-bipyridine.
It is imperative to mention that we understand that the Mn(II) and Mn(III) ions form low
spin complexes with strong field ligands like cyanide, phosphine and dithiolate [11-13] but cases
are known where other nitrogen and oxygen donor ligands also form low-spin complexes and
sufficient evidence is available for this contention [14, 15, 31]. Our ligand used in the present
study also bears similarly with reported oxygen and nitrogen donor ligands like oximes giving
low-spin and high-spin complexes. Further, it is our experience also in dihydrazone coordination
chemistry that the formation of low-spin and high-spin complexes depends upon the nature of
metal salts and ligand. Most probably, the use of the manganese(II) sulphate and the ligand in the
present study makes suitable combination to give low-spin complexes under appropriate reaction
conditions.
Electron paramagnetic resonance spectroscopy
The EPR spectra of the complexes (1) and (2a) in powder form at RT and LNT have been
shown in (Figure S3-S6). The EPR parameters for the complexes have been given in Table 3.
The complexes show isotropic spectra in polycrystalline state at RT as well as LNT. The gav
value for all the complexes is ~2.025 at RT while at LNT, it falls in the region 2.020-2.025.
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These values reveal that the complexes have almost similar stereochemistry at RT as well as
LNT in polycrystalline state gav value for complex (1) in polycrystalline state at LNT is 2.020
which is different from that at RT (2.025). This indicates that at LNT, the complex attains more
symmetrical structure than that at RT. This may be due to compression of molecule at LNT. It is
evident from the width of the spectra at RT and LNT, the complex (2a) shows different magnetic
behaviour from the remaining complexes, while the width of the central peak decreases in
complex (2a), in going from RT to LNT, that in the remaining complexes, the width increases.
This fact may be understood in terms of the normal magnetic moment value for complex (2a)
and anomalously low value of magnetic moment for the remaining complexes suggesting metal-
metal interaction in the structural unit of complexes. The complexes show a single signal in the
region ~2.025 G in polycrystalline phase. The appearance of single signal in the region ~2.025 G
reveals that the complexes have a pure doublet ground state at RT as well as low temperature.
The complex (2a) shows an isotropic spectrum with gav value equal to 2.025 G at LNT in
DMSO glass. This value is same as those in the polycrystalline phase at RT and LNT. This
shows that this complex in DMSO glass has the same structure as that in the polycrystalline
phase. The complexes (1), (2) and (4) show weak splitting in DMSO glass at LNT but a fully
resolved spectrum is observed in case of complex (3). Further, in the complexes (3) and (4), the
gav values in DMSO are 2.020 and 2.010 which are different from the values at RT and LNT in
polycrystalline state. This indicates that in these complexes some bonds are broken followed by
formation of new bonds which leads to the formation of more symmetrical structure at LNT
rather than in solid state. The complex (3) shows a fully resolved rhombic spectrum at LNT in
DMSO glass while the complexes (1), (2) and (4) show weakly resolved rhombic spectrum. The
g-values for three components are 2.171, 2.040 and 1.971, respectively, for the complex (3)
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which further split due to hyperfine coupling to give complex spectral features characteristic of
the low-spin state of manganese(II) complexes. On the other hand, only very broad room
temperature spectra are observed over g = 2-5 in the high-spin manganese(II) complexes.
Further, in the distorted metal environment, a very complex spectrum is obtained at low
temperature (77 K) over a very wide range as a result of zero field splitting.
The complexes (1) and (4) show hyperfine splitting at LNT in DMSO glass with
hyperfine coupling constant (55Mn, I = 5/2) falling in the range 85-92 G. The reported values for
hyperfine coupling constant fall in the region 75-100 G, while most of the observed values fall
generally in the region 90-100 G. In our case the experimentally observed AMn value falls in the
region 85-92 G, which lies on the border range of reported values. The border range values of the
experimentally observed hyperfine coupling constant may be result of the moderately strong dπ-
pπ metal-ligand interactions.
UV visible spectra
The electronic spectral bands for ligands and metal complexes have been listed in Table 4
along with their molar extinction coefficients. The electronic spectra of the complexes have been
shown in (Figure S7-S9).
The H4slox ligand shows two bands at 304 and 341 nm which are assigned to π→π* and
n→π* transitions respectively, while the H4nph ligand shows three bands each appearing as a
couplet in the spectrum giving rise to overall six bands. These bands appear at 318, 330; 375,
391; and 433, 486 nm, respectively. The bands at 318 and 330 may be assigned to π→π*
transition, those at 375 and 391 nm to n→π* transition. The bands at 375 and 391 nm are
characteristic of naphthaldimine fraction of ligand as has been observed in many
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monohydrazones [17, 18]. The weak bands at 433 and 486 nm have their origin in some hidden
intra-ligand transitions present on the ligand molecule.
The H4slox bands remain almost unshifted on complexation, while π→π* transition in
H4nph is blue shifted by 4 nm. This shows bonding of dihydrazones to the metal centre. On the
other hand, the n→π* transition in uncoordinated ligand H4nph at 375 and 391 nm merge into
one another giving rise to again blue shift by 2 - 5 nm. The hidden bands at 433 and 486 nm in
(H4nph) are shifted to longer wavelength 2 - 3 nm and 5 - 6 nm, respectively. This shows the
effect of complexation on the ligands. In addition to the ligand bands, the complexes (1), (2), and
show two new bands in the regions 405 - 418 and 473 - 681 nm with molar extinction
coefficients in the region 150-1600 dm3 mol-1 cm-1. Due to high molar extinction coefficients, the
band in the region 405 - 418 nm is assigned to have charge-transfer character, most probably,
from transfer of charge from occupied molecular orbital located on the phenolate ion to
unoccupied molecular orbital located on metal centre. The complexes (3) and (4) show another
new band at ~460 nm which again has very high molar extinction coefficient and hence, it is
attributed to have its origin due to charge-transfer from naphthoxide ion to metal centre.
2
However, this band may also contain contribution due to split components of (2A2g, T1g→2T2g)
transition. Weak band in the region 473 – 613 nm appears in all complexes which is suggested to
be due to another component of split transition (2A2g, 2T1g→2T2g) centred on the Mn(II) ion [32].
IR spectra
The bands for ν(OH+NH); νC=O and νC=N for H4slox and H4nph are observed at 3434,
3278, 3207, 3150; 1669; 1620, 1603 cm-1 and 3622, 3492, 3161; 1682; 1621, 1595 cm-1,
respectively. Both the ligand also show strong bands at 1536; 1276, 1262 cm-1 (H4slox) and 1536
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and 1284 cm-1 (H4nph) which may be attributed to amide II + ν(C-O)(phenolic/naphtholic) and
β(C-O)(phenolic/naphthalolic), respectively.
The complexes (1) to (2a) show strong bands at 3443, 3278, 3207; 3435, 3240, 3208 and
3452, 3280, 3206, 3240; 1669; 1620, 1603; 1618, 1609 and 1619, 1603 cm-1 due to ν(OH+NH),
νC=O and νC=N stretchings [16, 22, 23], respectively. These bands remain either unshifted or
slightly shifted in their position and reveal that the ligand is bonded to the metal centre in keto
form. The band at 1536 cm-1 which is due to the amide II + ν(C-O)(phenolic) band also remains
unchanged in frequency in the complexes which suggests weak bonding between phenolate/
naphtholate oxygen atoms and metal centre [23a]. The β(C-O) band shifts to lower frequency by
~3-4 cm-1 in position in both the complexes (1) to (2a) indicating weaker bonding between
phenolate oxygen atoms and metal centre. Such a bonding pattern of hydrazones with metal ions
has been reported by Manoharan et. al. [33] and established by IR and X-ray crystallography.
The weak band in the region 557 - 576 cm-1 is assigned to ν(M-O)(phenolate) [33, 34].
The complexes (3) and (4) show similar spectral features (Table 5) to that of
uncoordinated ligand suggesting coordination of dihydrazone to the metal centre in keto form
[18]. The ν(OH + NH) bands in the region 3034 - 3466 cm-1 in both the complexes (3) and (4),
remain unshifted or slightly shifted in their position. But we have refrained from drawing any
conclusion regarding bonding of -OH and -NH groups to the metal centre. However, the strong
νC=O band at 1682 cm-1 in free ligand splits into two bands in both the complexes (3) and (4).
One band is slightly shifted by 1 (complex 3) and 3 cm-1 (complex 4) and appears at 1683, 1662
and 1685, 1663 cm-1, respectively, while the other band shifts to lower frequency by 21 (complex
3) and 22 cm-1 (complex 4) and appears at 1662 and 1663 cm-1, respectively. However, on an
average, still they fall (~1674 cm-1) in the region in which uncoordinated νC=O groups have
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been reported to absorb. Hence, it is suggested that νC=O groups remain uncoordinated in the
complexes. Sufficient evidence is available for this contention in literature on hydrazone
complexes established by IR and X-ray crystal structure [17f]. The strong couplet at 1621 and
1594 cm-1 (νC=N) shifts to higher frequency and appears at 1622 cm-1 in the complexes (3) – (4).
The shift of νC=N band to higher position by 1 cm-1 shows that there is a flow of naphthly ring
electron density to metal centre through azomethine nitrogen atoms which is consistent with the
coordination of azomethine nitrogen atom to manganese metal centre [17]. Both the complexes
show slightly shifted or unshifted bands at 1536 and 1284 cm-1 due to amide II + ν(C-
O)(naphtholic) and β(C-O) vibrations when compared to the free ligand spectra. This is due to
weak coordination of naphtholate oxygen atoms to the metal centre and non-coordination of
>C=O group.
The complexes (1) and (3) show new medium intensity bands at 731, 654 cm-1 and 744,
656 cm-1, respectively. These bands are attributed to out-of-plane motion of the hydrogen atoms
as expected for two identical groups of four hydrogens each [35] and in plane ring deformation
mode of 2,2'-bipyridine [36]. The complexes (2), (2a) and (4) show a medium to strong intensity
bands at 828, 828 and 820 cm-1. This band may be assigned to the hydrogen atoms on the central
ring. Further, these complexes show strong band at 756, 755 and 744 cm-1 which are attributed to
the out-of-plane motion of the hydrogen atoms on the heterocyclic rings of 1,10-phenanthroline.
Cyclic voltammetry
The cyclic voltammograms for the complexes (1) to (3) have been shown in (Figure S10-
S12). The cyclic voltammetric data for the complexes have been presented in Table 4.
All of the complexes display three reductive waves in the forward scan and three
oxidative waves in the anodic scan except the complex (1) which shows four reductive waves in
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the forward scan. Further, the complex (2) also shows only one oxidative wave at +0.65 V. All
complexes invariably show two redox couples in the region (E1/2 = +0.33 to +0.39 V; ꢁE (130-
250 mV) and (E1/2 = -0.81 to -1.00 V; ꢁE (140-350 mV), respectively. A comparison of the
cyclic voltammograms of the complexes with those of the free dihydrazones reveals that there is
no redox wave in the region +0.33 to +0.39 V in the free dihydrazone. Hence, first redox couple
in the region +0.33 to +0.39 V may be attributed to arise due to metal centred redox reaction
MnII/MnI. The ꢁE for this redox couple falls in the region 130-250 mV which reveals its quasi-
reversible nature. When the position of the second redox couple in the region -0.81 to -1.00 V
(E1/2) in the complexes is compared with the cyclic voltammograms of the free dihydrazones, it
is found that it is very near to the reductive waves at -1.32 V and -0.98 V in H4slox and H4nph,
respectively. Hence, it is suggested that the second redox couple has some contribution from
both metal centred electron transfer reactions as well as ligand centred electron transfer reaction.
This metal centred electron transfer reaction may be attributed to the electrode reaction MnI/Mn0.
Apart from these metal centred electron transfer reactions, the complexes show two irreversible
reductive waves in the region -0.14 to -0.34 V and -1.54 to -1.64 and an oxidative wave in the
region +0.72 to +1.57 V, respectively. In the free dihydrazone H4nph, there is a reductive wave
at -0.34 V but there is no wave in this region in free ligand H4slox. Hence, the reductive wave in
the region -0.14 to -0.34 V in the complexes is attributed to arise due to some ligand centred
electron transfer reaction. As there is no reductive wave in this region in the free ligand H4slox, it
seems that ligand gets excited on complexation to give electron transfer reaction. Similarly, the
reductive wave in the region -0.81 to -1.00 V also arises due to activation of ligand on
complexation as there is a reductive wave at this position in free dihydrazones. It is imperative to
mention that these reductive waves do not have their counterparts in the anodic scan either in the
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free ligand or in the complexes. This is consistent with the reductive product undergoing
chemical reactions over the longer time scales. This suggests that the species corresponding to
the wave is unstable in DMF and does not survive long in DMF solution either in free ligand or
in metal complexes. Hence, the reductive wave in the region -0.81 to -1.00 V corresponds to
generation of the ligand based phenoxyl radical [37-39]. Hence, this irreversible wave is also
considered to arise from ligand centred electron transfer reaction. It is worth noting that with the
highly negatively charged dihydrazone ligands bonded to the metal centre, it is expected to help
make the reduction at the metal centre unfavorable, leading to quite negative Epc values [40].
Apart from these waves, all complexes invariably show a new oxidative wave in the region
+0.72 to +1.57 V, which, most probably, arises due to oxidation of >C=N group [41]. The
difference between reductive and oxidative waves falls in the region 140-350 mV which is much
more than one electron uncomplicated electron transfer reaction. This reveals that the metal
centred electron transfer reaction is either quasi-reversible or irreversible. The large peak
separation originates, most probably, from a slow heterogeneous one electron exchange rather
than from intervening homogeneous reaction [42]. The metal centred electron transfer reaction
may be written as
+e
-e
+e
-e
[H2LMnI(BB)]-.nH2O
[H2LMnII(BB)].nH2O
[H2LMn0(BB)]2-.nH2O
On the other hand, the complex (2) shows only one redox couple (Epc = +0.13 V, Epa =
+0.65 V, E1/2 = +0.39 V, ꢁE = 520 mV). This may be assigned to metal centred electron transfer
reactions. The irreversible reductive waves at -0.38 and -1.15 V are attributed to ligand centred
electron transfer reaction. The species produced corresponding to these reduction reactions are
unstable and do not survive long in DMF solution and return to original species. The metal
centred electron transfer reaction may be written as
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+e
-e
[MnII(H2slox)(phen)]
[MnI(H2slox)(phen)]-
On the basis of various physico–chemical and spectroscopic studies the complexes have,
tentatively, been proposed to have the following structures (Figure 2 and 3).
Conclusion
In the present paper, we have synthesized some low-spin manganese(II) complexes from
oxaloyldihydrazones derived from condensation of oxaloyldihydrazine with salicyaldehyde and
2-hydroxy-1-naphthaldehyde. The complexes have been characterized by various physico-
chemical techniques and their structures have been discussed in the light of spectroscopic
techniques. In all complexes, the principal dihydrazone ligand is present in keto-form. The
complex (2) is monomeric while the remaining complexes are dimeric. In complex (2), the
principal dihydrazone ligand is present in anti-cis configuration and secondary ligand 1,10-
phenanthroline as a chelating bidentate ligand. In the remaining complexes the principal
dihydrazone ligands are present in syn-cis configuration. The dihydrazone ligands and the
secondary bidentate ligands are coordinated to the metal centre as a bridging bis(monobasic
bidentate) and bidentate ligands, respectively. In all complexes, the manganese(II) centre is
present in distorted octahedral geometry. The complex (2) has normal magnetic moment
corresponding to low-spin Mn(II) complexes while remaining complexes have anomalously low
value of magnetic moment exhibiting metal-metal interaction in their structural unit. All
complexes show two metal centred electron transfer reactions, while one electrode reaction is
purely metal centred, the second electrode reaction has contribution from ligand centred electron
transfer reaction along with metal centred electron transfer reaction.
Acknowledgements
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Authors are thankful to the Head, SAIF, IIT, Bombay for recording EPR Spectra.
Authors would also like to thank UGC, New Delhi for the award of NFHE Fellowship.
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Table 1. Complex, colour, decomposition point and elemental analysis for manganese(II) complexes
derived from oxaloyldihydrazones
Sl.
Complex and colour
M.P/ Elemental Analysis: Found (calcd.) %
No.
D.P
(°C)
Mn
C
H
N
1
2
[MnII(H2slox)(bipy)]
Light yellow
˃
300
10.58
57.95
3.78
15.29
(10.26) (58.33) (3,76) (15.70)
[MnII(H2slox)(phen)]
Yellow
>250
10.19
(9.82)
8.97
60.49
(60.11) (3.60) (15.02)
56.99 3.96 14.75
(57.34) (3.95) (14.33)
3.58
14.87
2a
[MnII(H2slox)(phen)].1.5
H2O
˃
280
(9.37)
Light yellow
3
4
[MnII(H2nph)(bipy)].H2O
Yellow
˃
˃
300
300
8.05
(8.40)
8.45
62.81
(62.49) (4.01) (12.86)
64.18 3.85 12.04
(63.81) (3.87) (12.40)
4.03
12.44
[MnII(H2nph)(phen)].H2O
Yellow
(8.11)
a, synthesized from reaction of MnSO4.H2O, H4slox and 1,10-phenanthroline in molar ratio 1:1:1.
Table 2: Molecular weight, magnetic moment and molar conductance for manganese(II) complexes
derived from oxaloyldihydrazones
Sl.
Complex
Molecular Magnetic
Molar
No.
weight
(Exp.)
Moment Conductance
µeff(BM) ꢀ-1cm2mol-1
{Calcd.}
(1090±55)
{535}
(1150±55)
{559}
1
2
[MnII(H2slox)(bipy)]
[MnII(H2slox)(phen)]
0.92
0.78
1.74
0.52
0.81
0.9
1.2
1.3
1.7
1.5
2a [MnII(H2slox)(phen)].1.5H2O (570±27)
{586}
3
[MnII(H2nph)(bipy)].H2O
(1234±60)
{653}
(1370±55)
{667}
4
[MnII(H2nph)(phen)].H2O
Table 3: EPR for manganese(II) complexes derived from oxaloyldihydrazones
Sl.
No.
1
Compounds
Phase Temperature
gav
Amn Width
(G)
(∆H)
[MnII(H2slox)(bipy)]
Solid
DMSO
Solid
RT
LNT
RT
LNT
RT
2.025
2.020
-
-
-
-
100
110
-
2.025 88
-
2
[MnII(H2slox)(phen)]
2.025
2.025
-
-
-
-
105
120
-
LNT
RT
DMSO
LNT
RT
2.021 85
-
90
2a [MnII(H2slox)(phen)].1.5H2O Solid
2.025
-