Synthesis, structural features, antibacterial behaviour and theoretical investigation of two new manganese(III) Schiff base complexes

Article abstract of DOI:10.1016/j.poly.2018.05.043

Two discrete manganese(III) coordination compounds, [Mn(L)(OCN)]₂ (1) and [Mn(L′)(H₂O)₂][dnba]·DMF·H₂O (2) bearing in situ generated Schiff base ligands H₂L [H₂L = N,N′-o-phenylenebis(salic

Full text of DOI:10.1016/j.poly.2018.05.043

Accepted Manuscript
Synthesis, structural features, antibacterial behaviour and theoretical investiga-
tion of two new manganese(III) Schiff base complexes
Pranajit Paul, Kanti R.N. Bhowmik, Subhadip Roy, Dibakar Deb, Nandita Das,
Maitri Bhattacharjee, R.N. Dutta Purkayastha, Louse Male, Vickie Mckee,
Raghavaiah Pallepogu, Debasis Maiti, Antonio Bauza, Antonio Fontera,
Alexander M. Kirillov
PII:
S0277-5387(18)30281-X
https://doi.org/10.1016/j.poly.2018.05.043
POLY 13190
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
26 February 2018
18 May 2018
Please cite this article as: P. Paul, K.R.N. Bhowmik, S. Roy, D. Deb, N. Das, M. Bhattacharjee, R.N. Dutta
Purkayastha, L. Male, V. Mckee, R. Pallepogu, D. Maiti, A. Bauza, A. Fontera, A.M. Kirillov, Synthesis, structural
features, antibacterial behaviour and theoretical investigation of two new manganese(III) Schiff base complexes,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.05.043
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Synthesis, structural features, antibacterial behaviour and theoretical
investigation of two new manganese(III) Schiff base complexes
Pranajit Paula, Kanti R. N. Bhowmika, Subhadip Royb, Dibakar Deba, Nandita Dasa, Maitri
Bhattacharjeea, R.N. Dutta Purkayasthaa,*, Louse Malec, Vickie Mckeed, Raghavaiah
Pallepogue, Debasis Maitif, Antonio Bauzag, Antonio Fonterag,*, Alexander M. Kirillovh
aDepartment of Chemistry, Tripura University, Suryamaninagar-799022, Tripura, India
bDepartment of Chemistry, IISER Bhopal, Bhopal Bypass Road, Bhauri, Bhopal-462066, M.
P., India.
cSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
dDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire LE11
3TU, UK
eDepartment of Chemistry, Dr. Harisingh Gour University, Sagar-470003, Madhya Pradesh,
India
fDepartment of Human Physiology, Tripura University, Suryamaninagar-799022, Tripura,
India
gDepartament de Quimica, Universitat de les Illes Balears, Crta. deValldemossa km 7.5,
07122 Palma de Mallorca (Baleares), Spain
hCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisbon, Portugal
* Corresponding authors. Addresses: Department of Chemistry, Tripura University,
Suryamaninagar-799022, Tripura, India. Tel.:+919402137040 (R.N Dutta Purkayastha).
Departament de Quimica, Universitat de les Illes Balears, Crta. deValldemossa km 7.5,
07122 Palma de Mallorca (Baleares), Spain (A. Frontera).
E-mail addresses: rndp09@gmail.com (R.N. Dutta Purkayastha), toni.frontera@uib.es (A.
Frontera).

Abstract
Two discrete manganese(III) coordination compounds, [Mn(L)(OCN)]2 (1) and
[Mn(L′)(H2O)2][dnba]·DMF·H2O (2) bearing in situ generated Schiff base ligands H2L
[H2L=
N,Nʹ-o-phenylenebis(salicylaldimine)]
or
H2L′
[H2L′=
N,N′-
bis(salicylidene)ethylenediamine], were easily prepared by treating Mn(CH3COO)2·4H2O
with salicylaldehyde and o-phenylenediamine (for 1) or ethylenediamine (for 2), and in the
presence of supporting cyanate ligands or 3,5-dinitrobenzoate (dnba) counter anions,
respectively. The obtained products were isolated as air-stable crystalline solids and were
fully characterized by spectroscopic methods, single-crystal X-ray diffraction, topological
analysis, as well as theoretical methods. The X-ray diffraction of the compound 1 reveals that
the structure is dimeric with phenolic oxygen atom acting as bridge between two
symmetrically equivalent Mn(III) atoms. The structure of 2 contains mononuclear cationic
[Mn(L′)(H2O)2] units along with a 3,5-dinitrobenzoate anion and crystallization DMF
molecule. Structural analysis indicates a distorted octahedral geometry for both complexes.
Intermolecular hydrogen bonding and π-π stacking interactions are present in the lattice and
play an important role in cohesion of the structures. Topological analysis of the compound 2
reveals a binodal 3,4-connected underlying 2D net with the 3,4L13 topology. Noncovalent
interactions were investigated in terms of energies and geometries using theoretical
calculations. Fluorescence and antibacterial properties of 1 and 2 were also investigated and
discussed.
Keywords: Manganese; Schiff bases; Crystal structures; Noncovalent interactions; DFT
calculations.

1. Introduction
The coordination chemistry of manganese has been an active area of research for
decades [1]. One of the outstanding traits of manganese concerns a possibility of adopting a
variety of oxidation states that is reflected in its diverse redox functions in several enzymes
[2]. In addition to various in vivo and biomimetic roles, manganese compounds also exhibit
interesting magnetic properties [3], intricate structures [4] and catalytic activity in a number
of organic reactions [5].
Metal complexes of Schiff bases have been recognised as an important class of
compounds with a wide range of applications as catalysts [6], metalloenzyme models [7],
metallomesogens [8], nonlinear optical materials [9], building blocks for supramolecular
structures [10], as well as antimicrobial [11], antiviral [12] or anticancer [13] agents.
Although Schiff base complexes of manganese(III) are well known, they still continue to
generate interest due to their important role in several metalloenzymes [14] and potential
applications in the areas of catalysis [15] and magnetic materials [16].
Thus, to explore manganese(III) Schiff base chemistry and as a continuation of our
interest in this research area, we report herein the synthesis, full characterization,
spectroscopic, fluorescence, structural and topological features, antibacterial properties, as
well as theoretical analysis of two novel manganese(III) complexes bearing in-situ generated
Schiff base ligands, namely [Mn(L)(OCN)]2 (1) and [Mn(L′)(H2O)2][dnba]·DMF·H2O (2),
wherein
H2L=
N,Nʹ-o-phenylenebis(salicylaldimine),
H2L′=
N,N′-
bis(salicylidene)ethylenediamine, dnba= 3,5-dinitrobenzoate, DMF= dimethylformamide.
2. Experimental
2.1. Materials and methods
Chemicals were of reagent grade and used without further purification. Solvents were
purified by distillation prior to use. The FT-IR spectra were recorded as KBr discs using a
Perkin Elmer 100 FT-IR spectrometer. Electronic spectra were recorded on a Perkin-Elmer
model 1800 spectrophotometer. Magnetic susceptibility was measured by Gouy method using
Hg[Co(SCN)4] as standard. Fluorescence spectra were obtained using a Fluorescence
spectrophotometer model Fluorolog-3 Horiba Scientific. Antibacterial activity was evaluated
using a molten agar disc diffusion method [17] and a broth culture method. C, H and N
contents in the obtained compounds were ascertained using a Perkin Elmer CHN analyzer

(2400 series II). Manganese content in the compounds was determined by complexometric
titration with EDTA using Erio-T as an indicator [18].
2.2. Synthesis of complexes
ʹ
2.2.1 Synthesis of [Mn(L)(OCN)]2(1)[H2L= N,N -o-phenylenebis(salicylaldimine)]
A solution containing Mn(CH3COO)2·4H2O (0.2 g, 1.0 mmol) and NaOCN (0.1 g, 2.0
mmol) in 30 cm3 of methanol was prepared (solution A). Another solution was obtained by
refluxing salicylaldehyde (0.1 g, 1.0 mmol) and o-phenylenediamine (0.05 g, 0.5 mmol) in
methanol (30 cm3) for ca. 30 min (solution B). The solutions A and B were then mixed
together and stirred in open air for ca. 2 h, resulting in a formation of a dark brown
precipitate. The solid was collected by filtration and dissolved completely by slow addition of
acetonitrile (25 cm3), then stirred for 2 min and filtered. The filtrate was left undisturbed at
room temperature for slow evaporation in air. The diffraction quality crystals of brown color
were deposited within one week, separated by filtration, washed with methanol to give
compound 1. Yield 74%. Anal. Calc. for C42H28Mn2N6O6: C, 61.32; H, 3.43; N, 10.22; Mn,
13.36. Found: C, 61.21; H, 3.40; N, 10.17; Mn, 13.25%. FTIR (KBr cm-1) 1605sν (C=N),
2175sνas(NCO¯ ), 1376 νs (NCO¯ ), 1313 ν(CO), 753 ν(Mn-O-Mn), 628 δ(NCO), 541 ν(Mn-
N), 386 ν(Mn-O).
2.2.2
Synthesis
of
[Mn(L′)(H2O)2][dnba]·DMF·H2O
(2)
(H2L′=
N,N′-
bis(salicylidene)ethylenediamine, dnba= 3,5-dinitrobenzoate, DMF= dimethylformamide)
Mn(CH3COO)2·4H2O (0.2 g, 1.0 mmol) and 3,5-dinitrobenzoic acid (0.2 g, 1.0 mmol)
were dissolved together in 15 cm3 of aqueous methanol (1:1) to obtain a clear solution
(solution A). Another solution (solution B) was prepared by dissolving together
salicylaldehyde (0.1 g, 1.0 mmol) and ethylenediamine (0.03 g, 0.5 mmol) in 20 cm3 of
methanol, followed by heating the mixture under reflux for ca. 30 min. The solutions A and
B were mixed together and the resulting mixture was stirred in open air for ca. 2 h, resulting
in the precipitation of a brown microcrystalline product. This product was filtered off and
then dissolved in 20 cm3 of DMF-water mixture (1:1). The obtained filtrate was filtered again
and then left undisturbed at room temperature for 4 days. Deep brown crystals suitable for X-
ray diffraction studies were deposited, isolated by filtration, and washed with a small volume
of methanol to give compound 2. Yield 78%. Anal. Calc. for C26H30MnN5O2: Found: C,
47.34; H, 4.56; N, 10.62; Mn, 8.32, calcd: C, 47.35; H, 4.59; N, 10.62; Mn, 8.33. FTIR (KBr

cm-1) 3108 ν(NH), 1621 ν(C=N), 1635 νas(COO¯ ), 1444 νs(COO¯ ), 1342 ν(CO), 756
ρr(H2O), 466 ν(Mn-N), 384 ν(Mn-O).
2.3. Crystallographic data collection and refinement
The intensity data for complexes 1 and 2 were collected at room temperature using a
Bruker Smart Apex CCD diffractometer with graphite monochromated MoKα radiation
(λ=0.71073Å). For data processing and absorption correction, the packages SAINT [19] and
SADABS [20] were used. The crystal structures were solved by direct methods using
SHELXS [21] and the refinement was carried out by full-matrix least-squares technique using
SHELXL [21]. All non-hydrogen atoms were refined anisotropically. Hydrogen atom
positions were calculated geometrically and refined using the riding model. In 1, the cyanate
group [N(3)--C(21)--O(3) / N(3′)--C(21′)--O(3′)] is disordered over two positions with the
observed occupancy ratio of 0.697(16):0.303(16). Crystallographic data and structure
refinement details for 1 and 2 are summarized in Table 1.
<Table 1>
2.4. Theoretical methods
All calculations were carried out using the TURBOMOLE version 5.9 [22] and the
BP86-D3/def2-TZVP level of theory. To evaluate the interactions in the solid state, we
applied the crystallographic coordinates. This procedure and level of theory were successfully
used to evaluate similar interactions [23]. The interaction energies were computed by
calculating the difference between the energies of isolated monomers and their assembly. The
interaction energies were corrected for the Basis Set Superposition Error (BSSE) using the
counterpoise method [24]. The “atoms-in-molecules” (AIM) [25] analysis was performed at
the BP86-D3/def2-TZVP level of theory. The calculation of AIM properties was done using
the AIMAll program [26].
2.5. Antibacterial activity
Antibacterial activity assay was tested against human pathogenic bacteria including
Klebsiella pneumonia, Shegella Flexneri 16, Shigella dysenteriae 1, Vibrio cholerae non-
0139 (L4), V. cholerae non 0139 (CSK6669), Escherichia coli, Staphylococcus aureus and
Streptococcus pneumonia. The strains were treated with compounds 1, 2 and ligands which
were dissolved in DMSO. In determination of zone of inhibition, 5 mm Watmann filter paper

disc were placed on pre-spread bacteria in nutrient agar plate. After 14 hrs of incubation at 37
oC, the clear zone of inhibition was measured by scale. DMSO was used in the measurement
as negative control. In broth culture, 50 µl of fresh overnight cultured bacterial cells were
inoculated in 3 ml of broth media having different concentration of compounds 1, 2 and
ligands. After 14 hrs of shaking incubation, growth of bacteria was determined by optical
density measurements at 600 nm. Only media with complex or ligands are considered as
negative control (without bacteria).
3. Results and discussion
3.1. Synthesis and spectral characterization
The strategy adopted for synthesis of 1 and 2 was similar. In a typical reaction, the
Schiff
base
ligands
N,N′-o-phenylenebis(salicylaldimine)
(H2L)
and
N,N′-
ethylenebis(saclicylaldimine) (H2L′) were generated in situ by a condensation reaction
between o-phenylenediamine/ethylenediamine and salicylaldehyde in methanol solution,
followed by a reaction with manganese(II) acetate and ancillary ligands, namely KOCN in
case of 1 and 3,5-dinitrobenzoic acid for 2 in methanol solution. The manganese(II) ion
undergoes an aerial oxidation to Mn(III) under the reaction conditions. A metal-assisted
deprotonation of the phenolic moiety occurs during the formation of [Mn(L)(OCN)]2 (1) and
[Mn(L′)(H2O)2][dnba]·DMF·H2O (2). Cyanate ion in 1 and water molecules in 2 present in
the reaction solution get coordinated to the manganese(III) ion to complete a six-coordinate
environment around the metal centres. The newly synthesized complexes were characterized
by elemental analyses, spectral studies and by single crystal X-ray diffraction measurements.
The compounds are air-stable and soluble in organic solvents viz. acetonitrile (1), DMF and
DMSO (1 and 2).
The low molar conductivity (Λm) of 20-30 Ω-1cm2m-1 for compound 1 indicates that
cyanate ion in the complex is coordinated to the metal centre and does not dissociate in the
solution. In case of compound 2, the molar conductivity of 78-81 Ω-1cm2m-1 is consistent
with the values expected for 1:1 electrolyte [27]. FT-IR spectra of compounds 1 and 2 were
analyzed and compared with those of the corresponding free ligands. Both the complexes
show similar patterns, with a strong band observed between 1605-1621 cm-1 due to the imine
ν(C=N) stretching vibration, which is red-shifted compared to the observed absorption for
free ligands. A strong absorption band between 1332-1342 cm-1 due to phenolic C-O
stretching also undergoes a shift to lower frequency with respect to free ligands, indicating a

deprotonation and subsequent coordination of the phenolic oxygen atoms to the metal centre.
Compound 1 also reveals strong bands at ca. 2171 cm-1, 1376 cm-1 and 628 cm-1
corresponding to νas(NCO¯ ), νs(NCO¯ ), and δ(NCO¯ ) modes of coordinated cyanato (NCO¯ )
ligand.
In case of compound 2, additional bands were observed at 1635 cm-1 and 1342 cm-1
assignable to νas(COO¯ ) and νs(COO¯ ) modes of an uncoordinated 3,5-dinitrobenzoate anion.
A band at ca. 3108 cm-1 probably arises from ν(NH) of the DMF moiety as evidenced by X-
ray diffraction study (vide supra). Appearance of a strong band at 755 cm-1 for compound 1 is
assignable to the ‘Mn--O--Mn’ vibration, resulting in a dimeric structure of the complex. The
presence of a broad band in the IR spectrum of 2 at about 3400 cm-1 is associated with
coordinated water molecule. Ligand coordination to the metal centre is further substantiated
by prominent ν(Mn--N) bands between 466-511 cm-1. All other characteristic vibrations due
to the metal bound Schiff base are located in the range of 500-600 cm-1.
3.2. Description of structures
3.2.1. [Mn(L)(NCO)]2 (1)
Brown single crystals of the compound 1, [Mn(L)(NCO)]2, were obtained by slow
evaporation from an acetonitrile solution and used for structural analysis. A schematic view
of the structure is shown in Fig. 1 along with an atom labelling scheme. Compound 1
crystallizes in a monoclinic space group P21/c and contains a binuclear manganese(III) entity,
wherein the two centrosymmetrically related six-coordinate metal centres are interconnected
by two weak phenoxo bridges. In each six-coordinated unit, the tetradentate (N2O2) Schiff
base ligand occupies the equatorial position and the axial coordination takes place through N-
atom of the pseudo-halide cyanate (NCO-) moiety in a bent fashion. The characteristic bond
distances and angles of the dimeric core in 1 are: Mn...Mn* 3.606(1) Å, Mn1–O1 (O1:
intramolecular phenolate oxygen) 1.887(2) Å, Mn1–O1* (O1*: intermolecular phenolate
oxygen) 2.828(2) Å, Mn1–O1*–Mn1* 97.78(7)o and O1–Mn1–O1* 82.22(7)o (*: 1-x,1-y,1-
z). In an equatorial plane (O2N2), donor atoms are almost coplanar; however, the manganese
atom deviates significantly from the equatorial plane by 0.245 Ǻ. The coordination geometry
of each of the Mn(III) ion is distorted octahedral with the elongation (Jahn-Teller distortion)
along the OCN--Mn….O axis [16c,d]. This complex can be classified as a Type-1 out-of-plane
dimer [28] and its bond lengths and angles are comparable to similar derivatives reported in
literature [16d,28,29] (Table S1, Supporting information). Weak intermolecular hydrogen
bonding interaction is present in the crystal lattice of 1 and provides further stabilization of

the structure. It is worth mentioning that both theoretical and experimental attempts have
been directed earlier to correlate magnetic properties of out-of-plane dimeric Mn(III) Schiff
base complexes with structural parameters [30]. It has been shown that structural parameters
such as the Mn-O* distance and Mn-O*-Mn* angle have significant influences on magnetic
coupling (J) with longer Mn-O* distances and lower Mn-O*-Mn* angle (~99˚) facilitating
ferromagnetic interactions [30].
Furthermore, a search in the Cambridge Structural Database (CSD) [31] reveals that
there are 68 structurally characterized manganese complexes derived from the Schiff base
H2L (substitution allowed). Among these, 22 examples feature di-manganese centres. Out of
these, only 2 compounds [32] correspond to phenoxo bridged dimers similar to 1. It indicates
that formation of out-of-plane dimers is not favoured with H2L. As a matter of fact, the
Mn(III) complex with Schiff base H2L and pseudohalide NCS- is monomeric, [Mn(L)(NCS)]
[33].
<Fig. 1>
3.2.2. [Mn(L′)(H2O)2][dnba]·DMF·H2O (2)
The crystal structure analysis reveals that the compound 2 crystallizes in the space
group P21/c and exists as mononuclear cationic species [MnIII(L′)(H2O)2]+ [H2L′=N,N′-
ethylenebis(saclicylaldimine)] together with an uncoordinated 3,5-dinitrobenzoate anion and
a crystallization molecule of DMF and H2O. The structure of the complex (Fig. 2) exhibits a
distorted octahedral geometry around the Mn(III) ion, which is coordinated by the
tetradentate N2O2-donor Schiff base and two aqua ligands. The equatorial plane around the
Mn(III) centre is occupied by two imine nitrogen [N1, N2] atoms and phenolic oxygen [O1,
O2] atoms of Schiff base. The axial positions are taken by the oxygen atoms [O1W, O2W] of
terminal water ligands. The Mn–O1W and Mn–O2W axial bonds of 2.224 (2) Å and 2.260
(2) Å are significantly longer than those of the Mn–O(1), Mn–O(2), Mn–N(1) and Mn–N(2)
distances [1.877(2), 1.864(2), 1.978(3) and 1.971(3) Å, respectively]. These bond distances
are typical for similar complexes [34]. The elongation of the axial bonds is indicative of
Jahn–Teller distortion usually observed for high-spin Mn(III) complexes [34]. The deviation
from the ideal octahedral geometry is also revealed by the values of equatorial and axial bond
angles viz. 82.47(12)° to 93.40(9)°, and O1W–Mn1–O2W of 168.62(10)°, respectively.
<Fig. 2>

One of the aqua ligands is involved in H-bond formation with the DMF molecule
[O1W...O31, 2.676(4) Å], while one of the carboxylate O atoms interacts with the 3,5-
dinitrobenzoate anion [O1W...O22, 2.722 (3) Å]. The other H2O ligand aqua is H-bonded to
symmetry related (1+x,+y,+z) O22 atom of 3,5-dinitrobenzoate [O2W...O3W, 2.668(4) Å] and
the symmetry related (1+x,3/2-y,1/2+z) O3W atom of the water ligand O2W...O22, 2.735(3)
Å] from a neighbouring molecule. The water molecule of crystallization is also engaged in
hydrogen bonding with the other carboxylate O-atom of the 3,5-dinitrobenzoate anion
[O3W...O21, 2.730(4) Å], as well as with a symmetry related (+x,3/2-y,-1/2+z) O1 atom of
DMF from another molecule. The details of the H-bonding distances are summarized in
Table 2. The stability of the resulting H-bonded 2D layers is also reinforced by the π–π
stacking interactions. Similar 2D network has been observed in [MnIII(H2L1)(CH3OH)2]Cl
(H4L1= N,N′-bis(3-hydroxysalicylidene)ethane-1,2-diamine) [34a] where discrete units of
cationic MnIII-cores are self-assembled by chloride anion in the solid state through H-
bonding.
A CSD search shows more than 305 structurally characterized manganese complexes
derived from the Schiff base ligand H2L′ (substitution allowed). Nearly one-third of these
contain mono-manganese centres. Among these, 26 examples represent mononuclear cationic
species with uncoordinated anion similar to compound 2. It has been found that the
dimensionality of the supramolecular array in these compounds is governed by the nature of
–
the anions which induce the self-assembly. Some notable anions include Cl– [34a], PF6 [35a],
–
–
NO3 [35a], ClO4 [35b], [Fe(CN)6]3– [35c], 2,2′-bipyridine-4,4′-dicarboxylate [35d], Keggin-
−
type [SiW12O40]4 [35e], and decavanadate [35f]. It is also important to mention here that the
bulky nature of the 3,5-dinitrobenzoate could prevent the axial coordination to the manganese
centre in comparison with related manganese Schiff base complexes bearing axially bound
benzoate [36a], 3-chlorobenzoate [36b], 4-hydroxybenzoate [36c], or 3-nitrobenzoate [36d].
<Table 2>
3.3. Topological analysis of H-bonded network in compound 2
As mentioned above, the crystal packing of 2 reveals an extensive hydrogen bonding
pattern between the [Mn(L′)(H2O)2]+ cations, 3,5-dinitrobenzoate(1–) anions and water
molecules of crystallization (O3w), thus giving rise to the generation of a 2D H-bonded
network (Fig. 3a). To get further insight into the structure of this network, we carried out its
topological analysis by following the concept of the simplified underlying net [37]. Such a

net (Fig. 3b) was constructed by eliminating the terminal DMF moieties and contracting
cations, anions, and solvent H2O molecules to respective centroids, maintaining their
connectivity via strong hydrogen bonds [D−H···A, wherein the H···A <2.50 Å, D···A <3.50 Å,
and ∠(D−H···A) >120°; D and A stand for donor and acceptor atoms] [37a]. Topological
analysis of this net reveals a binodal 3,4-connected underlying layer with the 3,4L13
topology. It is defined by the point symbol of (4.62)2(42.62.82) wherein the (4.62) and
(42.62.82) indices correspond to the 3-connected H2O and 3,5-dinitrobenzoate(1–) nodes
(topologically equivalent) and the 4-connected [Mn(L′)(H2O)2]+ nodes, respectively.
<Fig. 3>
3.4. Theoretical studies
The theoretical study is devoted to the analysis of the interesting noncovalent
interactions observed in the solid state of the synthesized compounds 1 and 2. As mentioned
above, in the dimeric unit of 1, the two centrosymmetrically related Mn(III) centres are held
together by two phenoxo bridges. We were interested to compute the interaction energy of
this dimer (dimerization energy). Hence, in compound 1, we focused our attention on the
interactions that lead to an infinite 1D ladder; these interactions are relevant to explain the
crystal packing (Fig. 4A). Such an assembly is formed by the successive stacking of self-
assembled Mn(III) dimers. The molecular electrostatic potential surface (MEP) for compound
1 has been computed and the most positive part of the surface is located close to the Mn(III)
centre at the opposite side of the pseudohalide ligand. In addition, the negative potential at
the coordinated phenolate O atom explains the formation of the self-assembled dimer. We
have also evaluated the dimerization energy, which is large and negative (∆E1 = –36.9
kcal/mol), confirming the importance of the self-complementary Mn···O interactions.
Furthermore, we also evaluated energetically the π-stacking complex (Fig. 4C) that is
responsible for the formation of the infinite ladder. In addition to the π-stacking interaction,
two unconventional C–H/π interactions are also established in which the π-system of the
pseudohalide is involved. Electrostatically, this interaction is favoured since the potential
value at the aromatic H-atom is positive (36 kcal/mol) due to an enhanced acidity of this
hydrogen atom as a consequence of the coordination of the ligand to the Mn(III) centre. In
addition, the electrostatic potential at the central C atom of the pseudohalide ligand is
negative (–25 kcal/mol, Fig. 4B). The interaction energy of this complex (∆E2 = –30.4

kcal/mol) combines π–π and two C–H/π interactions (Fig. 4D). In an effort to evaluate the
contribution of each interaction, we have computed an additional theoretical model where the
pseudohalide ligands have been oriented oppositely (Fig. 4E). As a result, the interaction
energy is reduced to ∆E3 = –13.8 kcal/mol and corresponds to the π–π interaction. This
interaction is larger than expected for this type of noncovalent bonding due to the presence of
an extended π-system (two aromatic rings) and also owing to a large dipole–dipole term due
to the metal coordination that increases the magnitude of the dipole and the antiparallel
stacking mode. The contribution of each C–H/π interaction can be estimated as ½×(∆E2 –
∆E3 ) = 8.3 kcal/mol.
<Fig. 4>
In compound 2, we have studied the formation of infinite 1D H-bonded motifs in the
solid state, which is governed by π–π stacking interactions and a hydrogen bonding network
with participation of the uncoordinated water molecule (Fig. 5A). In particular, these water
molecules interconnect the mononuclear Mn(III) units into 2D H-bonded layers (Fig. 3A).
The dinitrobenzoate counter anion and DMF interact with the aromatic ligands by means of a
combination of H-bonding and π–stacking interactions forming an interesting assembly. We
have evaluated energetically both stacking interactions (Fig. 5, right). That is, using the
Mn(III) complex interacting with the counter anion and the DMF molecules at one side of the
aromatic ligand (Fig. 5B), we have computed the binding energies for the formation of the π–
complexes at the opposite side, which are shown in Figs. 5C and 5D (3,5-dinitrobenzoate and
DMF, respectively). The interaction with 3,5-dinitrobenzoate is very strong (∆E4 = –40.8
kcal/mol) due to the contribution of strong electrostatic effects between the anion and the
cationic Mn(III) complex and also to the formation of a strong H-bonding interaction
between the coordinated water molecule (enhanced acidic proton) and the anionic
carboxylate group. We have also used a theoretical model (Fig. 5E) where this coordinated
water molecule has been eliminated. If the same complex with 3,5-dinitrobenzoate is
computed, the interaction is reduced (∆E’4 = –26.5 kcal/mol) and corresponds to the stacking
interaction, which can be also viewed as an anion–π interaction due to the anionic nature of
–
the π-system. The difference ∆E4 – ∆E’4 = –14.3 kcal/mol corresponds to the strong RCO2
···H2O–Mn(III) H-bonding interaction. The π-interaction and H-bond with DMF present
smaller interaction energy (Fig. 5D, ∆E5 = –18.6 kcal/mol) due to the neutral nature of DMF.
In case of using the theoretical model where the coordinated water molecule is eliminated,

the interaction energy is reduced to ∆E’5 = –9.6 kcal/mol that corresponds to the π–π stacking
interaction between DMF and the complex, and the difference (∆E5 – ∆E’5 = –9.0 kcal/mol)
is the contribution of the H-bond.
<Fig. 5>
In compound 2, we have also studied the existence of self-complementary antiparallel
NO···NO interactions (blue dashed lines in Fig. 6A). These interactions in conjunction with
bifurcated H-bonds are responsible for the formation of tetrameric units in the solid state. The
importance of similar NO3···NO3 interactions between coordinated nitrato ligands has been
highlighted in the solid state structure of hetero bimetallic Cu(II)–uranyl complexes with
Schiff-base ligands [38] and nitrato co-ligands and more recently between nitrato co-ligands
of N-benzimidazolyl-pyrimidine–Cu(II) complexes [39]. We have evaluated the interaction
energy (∆E5 = −12.5 kcal/mol) of this tetramer as a dimer; only the antiparallel NO···NO and
bifurcated H-bonds were evaluated (red and blue dashed lines). In an effort to evaluate the
contribution of the antiparallel NO···NO interaction, we have computed a theoretical model
(Fig. 6B) where only the 3,5-dinitrobenzoate moieties were considered. As a result, the
interaction energy is reduced to ∆E6 = −4.7 kcal/mol that is a rough estimate of the
antiparallel NO···NO interaction.
Finally, we have used Bader's theory of “atoms-in-molecules”, which provides an
unambiguous definition of chemical bonding, to characterize the non covalent NO···NO
interactions described above. In Fig. 6C we show the AIM analysis of the dimer and the
NO···NO interaction is characterized by the presence of one bond critical point that connects
the symmetrically related oxygen atoms of the nitro substituents, thus confirming the
existence of the interaction. The value of the Laplacian of the charge density computed at the
bond critical point is positive, being common in closed-shell interactions.
<Fig. 6>
3.5. Electronic spectra
Electronic spectra (10-4 M) of the H2L and H2L′ ligands were measured in methanol
solution. Ligand H2L shows absorptions at ca. 316-328 nm (Ԑm= 13920-15230 mol-1cm-1L-1)
and a medium intensity band at ca. 290 nm (Ԑm= 8560 mol-1cm-1L-1), whereas H2L′ shows an

absorption band at ca. 331nm (Ԑm= 4940 mol-1cm-1L-1). The observed absorption for H2L′ is
assignable to intra-ligand charge transfer (ILCT) transition. However lower intensity
absorption for H2L′ arises due to intra-ligand π-π* transition.
The UV visible spectra (10-4 M) for the complexes 1 and 2 were recorded in
acetonitrile and dimethylformamide solutions, respectively (Fig. S1, Supporting information).
Absorptions for compound 1 were observed in the range of 317-330 nm (Ԑm= 1087 mol-1cm-
1L-1). For compound 2, an absorption band was observed at ca. 405 nm (Ԑm= 3540 mol-1cm-
1L-1). The observed transition for the complexes can be assigned to phenolate (pπ)→Mn(III)
(dπ) charge transfer by analogy with similar Mn(III) complexes [15c].
3.6. Room temperature magnetic study
Complexes 1 and 2 show room temperature magnetic moments of 4.84 BM (per Mn
centre) and 4.76 BM, respectively, which are close to the spin only value for high spin d4
manganese(III) ion. Although a detailed investigation of magnetic properties of 1 and 2
including variable temperature magnetic measurements is out of the scope of the present
investigation, compound 1 may show interesting magnetic features. Previous reports indicate
that MnIII Schiff base out-of-plane dimers are promising candidates for single-molecule
magnets as the phenoxide pathway can mediate ferromagnetic exchange couplings, leading to
a spin ground state of ST = 4 [40].
3.7. Fluorescence studies
Photoluminescence behaviour of the ligands (H2L and H2L′) and compounds (1 and 2)
was investigated in DMSO solution. Steady state fluorescence studies were employed as
independent evidence of complexation between the ligands and metal ions. The ligand H2L
displays prominent emission at 459 nm on excitation at 328 nm, while ligand H2L′ exhibits
emission at 434 nm on excitation at 330 nm. It is important to mention here that compound 2
is nonfluorescent, while compound 1 shows a broad emission at 421 nm on excitation at 330
nm (Fig. S2, Supporting information). The observed emissions for the ligands are assignable
to π(L)→π*(L) transitions and possess an intraligand charge transfer (ILCT) character. The
broad nature of emission band of 1 indicates a charge transfer nature of the transition.
The emission for 1 is blue-shifted in comparison to free ligand and assignable to intra-
ligand π-π* transition [41]. The fluorescence quantum yield of compound 1 is found to be
higher than that of the ligand H2L (Table S2, Supporting information). Appreciable difference

in position of emission maximum and higher fluorescence quantum yield in 1 support a
complexation process. The quenching of fluorescence of ligand by transition metal ion during
the complexation is a common phenomenon, which is explained by processes such as
magnetic perturbation, redox-activity andelectronic energy transfer [42a,b]. The
nonfluorescent nature of compound 2 might be due to any of the processes mentioned above.
3.8. Antibacterial activity
The obtained antibacterial activity data are shown in Table 3 in terms of zone of
inhibition and minimum inhibitory concentration (MIC). The activity of free ligands and
Mn(III) complexes was screened against eight human pathogenic bacteria including
Klebsiella pneumonia, Shegella Flexneri 16, Shigelladysentriae 1, Vibrio cholerae non. 0139,
Escherichia coli, Staphylococcus aureus and Streptrococcus pneumonia using disk diffusion
method and broth culture method. The results of antibacterial screening indicate that H2L and
H2L′ are not active. However, complex 1 exhibits an appreciable antibacterial activity. The
inhibition zone data show that 1 is active against Shigella dysentriae 1, Vibrio cholera non.
0139, Escherichia coli, and Staphylococcus aureus; the MIC values are 400µg/ml in all
cases. Compound 2 is less active showing MIC values of 500 µg/ml for Vibrio cholera non.
0139, Escherichia coli, and Streptrococcus pneumonia; for other human pathogenic strains
investigated herein, the inhibition of bacterial growth is not very significant.
<Table 3>
4. Conclusions
In the present work, we have synthesized two new manganese(III) complexes (1 and
2) that bear two different N2O2-Schiff base ligands generated in situ. The obtained
compounds have been fully characterized, by spectroscopic methods, single-crystal X-ray
diffraction, topological analysis and DFT calculations. Two centrosymmetrically related
manganese(III) centres are held together by the phenoxy bridges resulting in a dimeric
structure in the complex 1, while the compound 2 represents a mononuclear cationic
complex. The solid state structures of both products show participation of the Schiff base
ligand and anionic co-ligands in concurrent hydrogen bonding, π-hole and π-stacking
interactions that control crystal packing patterns. The difference in Schiff base ligands and

co-ligands employed during the synthesis as well as diverse noncovalent interactions present
in the compounds have led to the structural diversity between 1 and 2.
In compound 1, the ancillary Mn(III)...O and π-stacking interactions have been
energetically studied and confirmed their importance in the solid state. In compound 2, the
nitro substituents of the 3,5-dinitrobenzoate are involved in antiparallel NO...NO interaction
that has been confirmed by AIM analysis of critical points and bond paths. This experimental
investigation also supports the importance of π-stacking and NO...NO interactions in solid
state chemistry. In addition, the computational study has highlighted the impact of these
unconventional interactions on the final structure. Such analyses of supramolecular
arrangements in manganese Schiff base complexes are significantly less explored in literature
[43]. The computation of the energetic features of the different noncovalent interactions is
important to gain knowledge about the processes that govern supramolecular chemistry and
crystal packing.
Besides, the results of antibacterial screening indicate that the compound 1 shows an
appreciable antibacterial activity against different types of pathogenic bacteria. Further
research aiming at the synthesis of related types of manganese(III) compounds and search for
their functional applications (e.g., magnetic behavior) will be pursued.
Acknowledgements
Authors R.N.D.P, K.R.N.B, P.P, D.D, N.D and M.B are thankful to the authority of
Tripura University, Suryamaninagar for providing infrastructure facility. S.R acknowledges
SERB, Government of India (File number PDF/2017/001188) for National Post-doctoral
fellowship. A.B and A.F thank DGICYT of Spain (project CONSOLIDER INGENIO
CSD2010-00065, FEDER funds) for funding. We also thank the CTI (UIB) for free
allocation of computer time. A.M.K. acknowledges the Foundation for Science and
Technology
(FCT)
and
Portugal
2020
(LISBOA-01-0145-FEDER-029697,
UID/QUI/00100/2013). We also thank the reviewers for their helpful comments at the
revision stage.
Appendix A. Supplementary material
CCDC 857299 and 938779 contain the supplementary crystallographic data for 1 and
2,
respectively.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.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.
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Table 1. Crystallographic data and structure refinement for 1 and 2.
Empirical formula
Formula weight
C42H28Mn2N6O6 (1)
822.58
C26H30MnN5O12 (2)
659.49
Temperature/K
293(2)
298(2)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
a/Å
b/Å
c/Å
α/°
10.0161(5)
14.4552(7)
13.1972(6)
90.00
7.2860(10)
29.489(4)
13.6274(18)
90.00
β/°
106.7480(10)
90.00
1829.70(15)
2
1.493
0.749
90.039(2)
90.00
2928.0(7)
4
1.496
0.523
γ/°
Volume/Å3
Z
ρcalcg/cm3
µ/mm-1
F(000)
840.0
1368.0
Crystal size/mm3
Radiation
0.20 × 0.20 × 0.10
MoKα (λ = 0.71073)
6.44 to 50.06
0.42 × 0.30 × 0.08
MoKα (λ = 0.71073)
2.76 to 52.08
2θ range for data collection/°
-11 ≤ h ≤ 11, -17 ≤ k ≤ 17,
-15 ≤ l ≤ 15
-8 ≤ h ≤ 8, -36 ≤ k ≤ 36,
-16 ≤ l ≤ 16
Index ranges
Reflections collected
21539
30135
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
3217 [Rint = 0.0571]
3217/60/281
1.015
5735 [Rint = 0.0616]
5735/9/417
1.108
Final R indexes [I≥ 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
R1 = 0.0384, wR2 = 0.0815
R1 = 0.0615, wR2 = 0.0916
0.26/-0.28
R1 = 0.0631, wR2 = 0.1375
R1 = 0.0864, wR2 = 0.1476
0.47/-0.28
Table 2. Hydrogen bonding parameters in compound 2.
D
O1W
O1W
H
H1WA
H1WB
A
O22
O31
d(D-H)/Å
0.811(18)
0.808(18)
d(H-A)/Å
1.914(18)
1.868(18)
d(D-A)/Å D-H-A/°
2.722(3)
2.676(4)
173(4)
179(5)
O2W
O2W
H2WA
H2WB
O221 0.819(18)
O3W2 0.827(18)
1.92(2)
1.87(2)
2.735(3)
2.668(4)
170(4)
161(4)
O3W
O3W
H3WA
H3WB
O13
O21
0.816(19)
0.828(19)
2.14(2)
1.90(2)
2.941(4)
2.730(4)
170(6)
176(6)
11+x,+y,+z; 21+x,3/2-y,1/2+z; 3+x,3/2-y,-1/2+z

Table 3. Antibacterial activity of compounds 1 and 2.
(a) Effect of complex 1
Bacterial strain
Concentration of complex 1 (µg/disc or µg/ml)
MIC
(µg/ml)
Klebsiella pneumonia
400
Zone of Inhibition (mm) 10.3 0.57 8.6 0.89
O.D. at 600 nm No growth
Zone of Inhibition (mm) 8.6 1.15
O.D. at 600 nm No growth
Zone of Inhibition (mm) 9.0 1.0
O.D. at 600 nm No growth
Vibrio cholerae non-0139 Zone of Inhibition (mm) 9.3 1.52
100
10
0
6.2 0.44
0.66 0.21 1.13 0.17 1.09 0.2
400
-
Shegella Flexneri 16
Shigella dysenteriae 1
8
1.6
0.46 0.3
8.2 0.44
6.75 0.95
1.08 0.13 1.11 0.24
6.0 0.0
0.54 0.07 1.08 0.13 0.74 0.33
9.0 2.44 6.75 0.95
0.49 0.04 0.93 0.43 1.0 0.39
7.5 0.7 6.5 0.7
0.35 0.11 0.92 0.14 1.18 0.36
7.6 1.3 6.0 0.0
0.25 0.17 0.65 0.12 0.97 0.11 0.97 0.1
-
(L4)
O.D. at 600 nm
Zone of Inhibition (mm) 8.0 1.52
O.D. at 600 nm No growth
Zone of Inhibition (mm) 8.3 2.3
No growth
400
-
V. cholerae non 0139
(CSK6669)
Escherichia coli
O.D. at 600 nm
400
400
400
Staphylococcus aureus
Zone of Inhibition (mm)
O.D. at 600 nm
Zone of Inhibition (mm) 10.0
No growth
0.41 0.31 0.94 0.09 1.1 0.15
8.5 0.7 6.6 1.15
0.61 0.39 1.05 0.18 1.13 0.15
Streptococcus
pneumoniae
0
O.D. at 600 nm No growth
(b) Effect of complex 2
Bacterial strain
Concentration of complex 2 (µg/disc or µg/ml)
MIC
(µg/ml)
Klebsiella pneumonia
400
100
10
0
Zone of Inhibition (mm) 10.0 0.0
8.25 0.5
6.25 0.5
O.D. at 600 nm
Zone of Inhibition (mm) 10.3 3.21
O.D. at 600 nm 0.43 0.08 0.49 0.11 0.85 0.32 0.75 0.27
Zone of Inhibition (mm) 9.6 0.57 8.6 0.54 6.0 0.0
O.D. at 600 nm No growth
Zone of Inhibition (mm) 11.0 0.0
0.24 0.09 0.48 0.16 0.78 0.18 0.74 0.23
-
-
Shegella Flexneri 16
Shigella dysenteriae 1
9
1.4 7.0 1.15
0.62 0.21 0.79 0.19 0.88 0.31
10.5 0.58 6.75 0.95
-
Vibrio cholerae non-
0139 (L4)
O.D. at 600 nm
No growth
0.53 0.15 0.78 0.17 0.78 0.28
500
-
V. cholerae non 0139
(CSK6669)
Zone of Inhibition (mm)
O.D. at 600 nm
8.0 0.0
0.14 0.18 0.77 0.2
Zone of Inhibition (mm) 9.0 1.4 7.5 1.3
6.0 0.0
0.96 0.17 0.98 0.15
6.3 0.57
Escherichia coli
O.D. at 600 nm
Zone of Inhibition (mm)
O.D. at 600 nm
No growth
0.65 0.19 0.83 0.22 0.96 0.27
500
-
Staphylococcus aureus
No growth
0.67 0.18 0.88 0.09 0.79 0.15
Streptococcus
pneumoniae
Zone of Inhibition (mm) 10.0
O.D. at 600 nm 0.25 0.23 0.61 0.12 1.11 0.19 1.07 0.11
0
9.0 1.0
6.0 0.0
500

Fig.1. Crystal structure of [Mn(L)(NCO)]2 (1). Colour codes: Mn (violet balls), C (grey), O
(red), N (blue), H (pale blue). Only the major component of the disordered cyanate ligand
is shown.

Fig. 2. Crystal structure of [Mn(L′)(H2O)2][dnba]·DMF·H2O (2). Colour codes: Mn (violet
balls), C (grey), O (red), N (blue), H (pale blue). H bonds are shown as thin dotted lines.

Fig. 3. Structural fragments of 2: (a) 2D H-bonded network and (b) its topological
representation showing a binodal 3,4-connected underlying net with the 3,4L13 topology
(views along the b axis). Further details: (a) H atoms (apart from those participating in H-
bonds) and DMF moieties were omitted for clarity, colour codes: Mn (cyan balls), C (grey),
O (red), N (blue), H (dark gray); (b) centroids of 4-connected [Mn(L′)(H2O)2]+ cations (cyan
balls), centroids of 3-connected 3,5-dinitrobenzoate(1–) anions (grey), centroids of 3-
coonected H2O molecules of crystallization (red).

Fig. 4. (A) Structural fragment of compound 1. (B) MEP surface of complex 1. (C–D)
Theoretical models used to evaluate the noncovalent interactions.
Fig. 5. (A) Structural fragment of compound 2. (B–E) Theoretical models used to evaluate
the noncovalent interactions.

Fig. 6. (A) Structural fragment of compound 2. (B) Theoretical models used to evaluate the
NO···NO interaction. (C) AIM analysis of compound 2. Bond and ring critical points are
represented by red and yellow spheres, respectively. The bond paths connecting bond critical
points are also represented.

Synopsis
Manganese(III)
Schiff
base
compounds,
[Mn(L)(OCN)]2
(1),
[Mn(L′)(H2O)2][dnba]·DMF·H2O (2) [H2L= N,Nʹ-o-phenylenebis(salicylaldimine)] or H2L′
[H2L′= N,N′-bis(salicylidene)ethylenediamine], were prepared and characterized by
spectroscopic and X-ray diffraction analysis. Theoretical study analyses noncovalent
interactions in the complexes. Topological analysis of 2 reveals existence of binodal 3,4-
connected 2D net with the 3,4L13 topology. Fluorescence and antibacterial properties of the
compounds were also explored.
Graphical Abstract