Supramolecular interactions through lone pair(lp)-π and hydrogen bonding in cobalt(III) and manganese(II) derivatives of N,N′-dimethylvioluric acid: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.ica.2015.06.018

The reactivities of cobalt(II) and manganese(II) salts toward HDMV (N,N′-dimethylvioluric acid monohydrate) were studied. Reaction of cobalt(II) chloride hexahydrate with HDMV in water-methanol medium at pH = 8.0 led to the formation of a cobalt(III) complex [Co^III(DMV)₃]·0.5H₂O (1), whereas the interaction of manganese(II) chloride tetrahydrate with HDMV under similar reaction conditions afforded [Mn^II(H₂O)₆](DMV)₂ (2). Dimethylviolurate anion behaves as a bidentate ligand in 1 and counteranion in 2. Both compounds were characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, cyclic voltammetry and thermogravimetric analysis. Presence of Co(III) in 1 indicates the aerial oxidation of the cobalt(II) precursor. In 1, Co(III) adopts a distorted octahedral [CoN₃O₃] coordination environment through bonding to the three dimethylviolurato anions. In 2, the [Mn(H₂O)₆]²⁺ cations and (DMV)^- anions are assembled by multiple hydrogen bonds into a complex 3D H-bonded network. It was analyzed from the topological viewpoint, disclosing a binodal 4,8-connected underlying net with the flu (fluorite) topology. Electrochemical studies reveal that the compounds undergo a quasi-reversible one electron metal centered redox process. DFT studies were carried out to analyze the supramolecular assemblies in the solid state structures, especially lone pair(lp)-π interactions in 1 and H-bonding network in 2.

Full text of DOI:10.1016/j.ica.2015.06.018

Inorganica Chimica Acta 435 (2015) 178–186
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Supramolecular interactions through lone pair(lp)–
p and hydrogen
bonding in cobalt(III) and manganese(II) derivatives of N,N0-
dimethylvioluric acid: A combined experimental and theoretical study
c
Rupak Banik a,1, Subhadip Roy a,1, Antonio Bauza b, Antonio Frontera b, , Antonio Rodríguez-Diéguez ,
⇑
Juan M. Salas c, Alexander M. Kirillov d, Shubhamoy Chowdhury e, Saroj Kr. Das a, Subrata Das f,
⇑
a Department of Chemistry, National Institute of Technology (NIT) Agartala, 799 046 Tripura, India
b Departament de Quimica, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares), Spain
c Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
d Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
e Department of Chemistry, Tripura University, Suryamaninagar, 799 022 Tripura, India
f Department of Chemistry, National Institute of Technology (NIT) Patna, Ashok Rajpath, Patna 800 005, Bihar, India
a r t i c l e i n f o
a b s t r a c t
The reactivities of cobalt(II) and manganese(II) salts toward HDMV (N,N0-dimethylvioluric acid monohy-
drate) were studied. Reaction of cobalt(II) chloride hexahydrate with HDMV in water–methanol medium
at pH = 8.0 led to the formation of a cobalt(III) complex [CoIII(DMV)3]ꢀ0.5H2O (1), whereas the interaction
of manganese(II) chloride tetrahydrate with HDMV under similar reaction conditions afforded
[MnII(H2O)6](DMV)2 (2). Dimethylviolurate anion behaves as a bidentate ligand in 1 and counteranion
in 2. Both compounds were characterized by single-crystal X-ray diffraction, elemental analysis, IR
spectroscopy, cyclic voltammetry and thermogravimetric analysis. Presence of Co(III) in 1 indicates the
aerial oxidation of the cobalt(II) precursor. In 1, Co(III) adopts a distorted octahedral [CoN3O3] coordina-
tion environment through bonding to the three dimethylviolurato anions. In 2, the [Mn(H2O)6]2+ cations
and (DMV)ꢁ anions are assembled by multiple hydrogen bonds into a complex 3D H-bonded network. It
was analyzed from the topological viewpoint, disclosing a binodal 4,8-connected underlying net with the
flu (fluorite) topology. Electrochemical studies reveal that the compounds undergo a quasi-reversible one
electron metal centered redox process. DFT studies were carried out to analyze the supramolecular
Article history:
Received 30 April 2015
Received in revised form 23 June 2015
Accepted 25 June 2015
Available online 2 July 2015
Keywords:
Cobalt(III)
Manganese(II)
Structure elucidation
Noncovalent interactions
DFT calculations
assemblies in the solid state structures, especially lone pair(lp)–
p interactions in 1 and H-bonding
network in 2.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
formation of phospholipid membranes, ribosomes and micro-
tubules are notable examples of how nature uses self-assembly
which is pivotal for living organisms [5]. The common feature of
all biological self-assembly processes is the ability of taking the
advantage of numerous weak, noncovalent interactions such as
Complex molecular assemblies and supramolecular arrays con-
structed from small and relatively simple building blocks are ubiq-
uitous in the natural world. While the intricacies of nature’s design
are not yet completely understood by us, it is well established that
nature utilizes three distinct processes [1] namely, self-assembly
[2], self-organization [3], and self-replication [4]. Naturally occur-
ring organic building blocks have specific functionalities in config-
urations that allow them to interact in a deliberate manner [5].
Protein folding, nucleic acid assembly and tertiary structure,
hydrogen bonding, charge-charge, donor–acceptor,
p-stacking,
van der Waals, hydrophilic and hydrophobic, etc [5]. It is, therefore,
of fundamental interest to understand and determine the strengths
and directional propensities of such ‘weak interactions’.
Supramolecular chemistry studies the interactions between,
rather than within, molecules – in other words, chemistry that uses
molecules rather than atoms as building blocks [6]. Barbituric acid
and its derivatives are useful building blocks for the construction of
metal supramolecular/coordination frameworks and organo-
inorganic hybrid solids [7]. Violuric acid (H3Vi) and N,N0-
dimethylvioluric acid (Scheme 1) are derivatives of barbituric acid
⇑
Corresponding authors. Tel.: +91 7677417481 (S. Das).
E-mail addresses: toni.frontera@uib.es (A. Frontera), subrataorgchem@gmail.
com (S. Das).
1
R. Banik and S. Roy contributed equally to this work.
http://dx.doi.org/10.1016/j.ica.2015.06.018
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
179
PHB-8 pH meter was used for pH measurements. Magnetic suscep-
tibility measurements were carried out with a Sherwood Scientific
Co., UK magnetic susceptibility balance, and diamagnetic correc-
tions were made using Pascal’s constants. Cyclic voltammetric
(CV) data were acquired on a Bioanalytical Systems Inc. Epsilon
electrochemical workstation (Model: CV-50) on a C3 cell stand at
293 K. Dry and degassed solution of 1 and 2, each of which
contained ꢂ1.0 mM of analyte and 0.1 M tetra-n-butylammonium
perchlorate (TBAP) as supporting electrolyte, were saturated with
nitrogen for 10 min prior to each acquisition. A blanket of nitrogen
gas was maintained throughout the measurements. The measure-
ments were carried out with a three-electrode assembly compris-
ing of a Glassy Carbon (GC) working electrode, a platinum wire
counter electrode and a Ag/AgCl reference electrode. All potentials
reported herein are referenced to Ag/AgCl.
O
OH
N
O
OH
N
H3C
O
N
HN
N
CH3
O
O
N
H
O
a
b
Scheme 1. a = Violuric acid, b = N,N0-dimethylvioluric acid.
containing C@NAOH substituent at the 5-position on the barbitu-
rate ring. This substituent can provide an extra coordination site
compared with barbituric acid itself [8], thus potentially affecting
the type of the resulting structures. In addition to metal binding
properties, the availability of several hydrogen bond donor and
acceptor sites together with the presence of a pyrimidine ring that
favors p-stacking interactions, make these molecules very interest-
2.2. Synthesis of the complexes
ing building blocks in context of supramolecular chemistry.
Furthermore, there is a considerable number and diversity of appli-
cations of violurato derivatives, namely, in analytical chemistry
(determination and identification of metal cations) [9], as potent
bioactive molecules (antibacterial, antifungal, antihypoxic,
antiproliferative and antitumor agents) [10] and redox mediators
(e.g., in enzyme-catalyzed degradation of organic pollutants in
the industrial effluents and bleaching treatment) [11]. Although
various metal complexes of violuric acid [12–18] and N,N0-
dimethylvioluric acid [19–24] have been reported, the investiga-
tion of energetic features of supramolecular assemblies observed
in such structures has not been undertaken in detail. Very recently,
some of our group have reported the structures of N,N0-dimethylvi-
oluric acid monohydrate and two cadmium(II) complexes obtained
from reactions involving HDMV and different N-donor ligands
(benzimidazole and pyridine) [25]. The compounds presented
interesting assemblies in the solid state dominated by hydrogen
2.2.1. [Co(DMV)3]ꢀ0.5H2O (1)
A
methanol solution (15 ml) of N,N0-dimethylvioluric acid
monohydrate (0.203 g, 1.00 mmol) was added dropwise to an
aqueous solution (10 ml) of CoCl2ꢀ6H2O (0.120 g, 0.5 mmol) with
stirring. The pH of the resulting solution was adjusted to 8.0. The
mixture was stirred further for a period of 5 h. The resulting
olive-green solution obtained initially changed to orange-red over
time. The orange-red precipitate was filtered and the filtrate was
allowed to evaporate slowly at room temperature. Diffraction
quality red crystals of 1 were obtained from the filtrate after ca.
2 weeks. Yield: 57% based on Co. Elemental Anal. Calc. for
C
18H19CoN9O12.5: C, 34.85; H, 3.09; N, 20.32. Found: C, 34.80; H,
3.07; N, 20.44%. IR (cmꢁ1): 1479
(N@O), 1636 (C@O), 1682
(C@O), 1733 (C@O), 3465 (H2O).
m
m
m
m
m
2.2.2. [Mn(H2O)6](DMV)2 (2)
bonding,
p–p and lp–p interactions. Crystal structures of cobalt
Compound 2 was synthesized in a similar manner as described
for 1. A methanol solution (15 ml) of N,N0-dimethylvioluric acid
monohydrate (0.203 g, 1.00 mmol) was added dropwise to an
aqueous solution (10 ml) of MnCl2ꢀ4H2O (0.125 g, 0.5 mmol) with
stirring. The solution was adjusted to pH = 8.0 and the stirring
was allowed to continue for another 5 h. The resulting pink
solution was filtered to remove any small solid particles and
allowed to evaporate. Pink block crystals of 2 suitable for X-ray
structure determination were collected by filtration after ca.
2.5 weeks. Yield: 44% based on Mn. Elemental Anal. Calc. for
complexes containing violurate as a ligand, [CoII(H2Vi)2(H2O)2]ꢀ
2H2O [26] and [CoIII(H2Vi)3]ꢀ6H2O [27], have been reported previ-
ously. Although a dimethylvioluric acid derivative [CoIII(DMV)3]
is known [28], its crystal structure has not been determined earlier.
Bearing these points in mind and following our interest in the
investigation of supramolecular features in different coordination
compounds derived from HDMV, we report herein the preparation,
crystal structures, thermogravimetric analysis, electrochemical
behavior and DFT studies of two compounds with the formulae
of [CoIII(DMV)3]ꢀ0.5H2O (1) and [MnII(H2O)6](DMV)2 (2).
C
12H24MnN6O14: C, 27.13; H, 4.55; N, 15.82. Found: C, 26.95; H,
4.62; N, 15.44%. IR (cmꢁ1): 633
w(H2O), 778 w(H2O), 1288
(C–O), 1481 (N@O), 1619 (C@O), 1678 (C@O), 3414 (H2O),
3487 (H2O).
q
q
2. Experimental
m
m
m
m
m
m
2.1. General remarks and physical measurements
2.3. X-ray data collection and structure refinement
2.1.1. Materials
All chemicals and solvents were obtained from commercial
sources and used as received. The synthetic reactions and work-
up were done in the open air. HDMV (N,N0-dimethylvioluric acid
monohydrate) was prepared according to a previously described
method [20] by means of acid hydrolysis of 6-amino-1,3-
dimethyl-5-nitroso uracil with HCl, followed by recrystallization
in hot water.
Diffraction data for 1 and 2 were collected at 296(2) K on a
Bruker SMART CCD area-detector diffractometer using graphite
monochromated Mo Ka radiation (k = 0.71073 Å). Intensity data
were reduced using SAINT [29] and absorption correction was per-
formed by multi-scan method implemented in SADABS [30]. The
structures were solved by Charge Flipping using the olex2.solve
[31] structure solution program and refined by Least Squares using
version 2013-2 of SHELXL [32]. All non-hydrogen atoms were refined
anisotropically. Hydrogen atom positions were calculated geomet-
rically and refined using the riding model. There is half a water
molecule present in 1 disordered over two positions with equal
(0.25) site occupancy factors. A summary of the crystallographic
data and structure determination parameters for 1 and 2 is given
in Table 1. Selected bond distances and angles for these com-
pounds are listed in Tables 2 and 3. Hydrogen bonding parameters
2.1.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed on a Perkin Elmer CHN analyzer (2400 series II). Infrared
(IR) spectra were recorded as KBr disks using a Perkin-Elmer
Spectrum 100 FT-IR spectrometer. Thermal behaviors were exam-
ined with a Shimadzu TG 50 thermogravimetric analyzer with a
heating rate of 10 °C minꢁ1 under nitrogen atmosphere. A digital

180
R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
Table 1
Crystallographic data and structure refinement for 1 and 2.
Empirical formula
C18H19CoN9O12.5 (1)
C12H24MnN6O14 (2)
Formula weight
T (K)
Crystal system
Space group
a (Å)
620.35
296(2)
531.31
296(2)
triclinic
monoclinic
P21/n
14.4634(7)
11.0688(5)
15.4830(7)
90
98.671(2)
90
2450.4(2)
4
ꢀ
P1
7.3029(5)
8.2993(6)
9.9486(7)
93.777(4)
103.920(4)
112.198(4)
533.58(7)
1
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g/cm3)
1.682
0.785
1.653
0.702
l
(mmꢁ1
)
F(000)
1268.0
275.0
Crystal size (mm)
Radiation
0.27 ꢃ 0.23 ꢃ 0.22
0.22 ꢃ 0.20 ꢃ 0.19
Mo K
a
(k = 0.71073)
Mo Ka (k = 0.71073)
2H
range for data collection (°)
4.542–51.994
5.384–59.924
Index ranges
ꢁ17 6 h 6 17, ꢁ13 6 k 6 13, ꢁ19 6 l 6 19
ꢁ10 6 h 6 10, ꢁ11 6 k 6 11, ꢁ13 6 l 6 13
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
39520
10739
4813 [Rint = 0.0437, Rsigma = 0.0262]
4813/0/375
1.028
3049 [Rint = 0.0287, Rsigma = 0.0260]
3049/9/177
1.042
Final R indexes [I P 2
r
(I)]
R1 = 0.0370, wR2 = 0.1024
R1 = 0.0469, wR2 = 0.1123
0.65 and ꢁ0.39
R1 = 0.0403, wR2 = 0.1152
R1 = 0.0484, wR2 = 0.1219
0.81 and ꢁ0.27
Final R indexes [all data]
Largest difference in peak and hole (e Åꢁ3
)
Table 3
of compound 2 are presented in Table 4. The molecular graphics
Selected bond lengths (Å) and angles (°) for 2.
were prepared using MERCURY program [33].
Bond lengths (Å)
Mn1–O1W
Mn1–O2W
Mn1–O3W
O10–C13
2.2197(15)
2.1886(12)
2.1221(12)
1.2210(19)
1.2175(18)
1.2636(19)
Mn1–O1W1
Mn1–O2W1
Mn1–O3W1
O13–C12
2.2197(15)
2.1886(12)
2.1221(12)
1.2269(18)
1.3401(19)
2.4. Theoretical methods
All calculations were carried out using the TURBOMOLE version
5.9 [34] using the BP86-D3/def2-TZVP level of theory. For the cal-
culations we have used the BP86 functional with the latest avail-
able correction for dispersion (D3) [35]. To evaluate the
interactions in the solid state, we have used the crystallographic
coordinates. This procedure and level of theory have been success-
fully used to evaluate similar interactions [36]. The theoretical
study is devoted to the analysis of the supramolecular assemblies
observed in the solid state of compounds 1 and 2, giving the possi-
bility to evaluate the different contributions to molecular recogni-
tion and to assign discrete energy values to them. The aim of the
manuscript is not to find the minimum energy structure for a given
complex, instead is to evaluate the binding energy of a given inter-
action as it is in the crystal structure. The interaction energies were
computed by calculating the difference between the energies of
isolated monomers and their assembly. The interaction energies
O12–C11
O11–N10
N10–C10
O11–N10–C10
119.38(13)
180.0
N11–C11–C10
116.24(12)
90.73(5)
90.73(5)
125.82(14)
112.67(13)
89.39(7)
O1W1–Mn1–O1W
O2W1–Mn1–O1W
O2W–Mn1–O1W1
O2W–Mn1–O2W1
O3W–Mn1–O1W
O10–C13–N12
O2W1–Mn1–O1W1
O2W–Mn1–O1W
N10–C10–C13
89.27(5)
89.27(5)
180.00(3)
89.39(7)
119.57(13)
90.61(7)
90.11(5)
89.89(5)
89.89(5)
120.00(14)
N10–C10–C11
O3W1–Mn1–O1W1
O3W–Mn1–O1W1
N12–C13–C10
90.61(7)
O3W1–Mn1–O1W
O3W1–Mn1–O2W
O3W–Mn1–O2W
O3W1–Mn1–O2W1
O12–C11–N11
115.27(13)
120.18(13)
121.47(14)
90.11(5)
O13–C12–N12
O13–C12–N11
O3W–Mn1–O2W1
O3W–Mn1–O3W1
180.0
1
1ꢁx, 2ꢁy, ꢁz.
Table 4
Hydrogen bonds for 2.
Table 2
D
H
A
d(D-H)/Å
d(H-A)/Å
d(D-A)/Å
D-H-A/°
Selected bond lengths (Å) and angles (°) for 1.
O1W
O1W
O2W
O2W
O3W
O3W
H1WA
H1WB
H2WA
H2WB
H3WA
H3WB
O131
O132
O103
N104
O113
O115
0.838(9)
0.839(9)
0.849(10)
0.842(9)
0.847(10)
0.843(9)
2.042(11)
1.978(13)
2.095(13)
2.107(12)
1.989(18)
1.823(10)
2.8662(18)
2.7881(18)
2.8546(17)
2.8713(18)
2.7519(18)
2.6629(18)
168(3)
162(2)
149(2)
151(2)
149(2)
174(3)
Bond lengths (Å)
Co1–O10
Co1–O30
Co1–N20
1.9582(16)
1.9626(15)
1.890(2)
Co1–O20
Co1–N10
Co1–N30
1.9380(17)
1.892(2)
1.8828(19)
Bond angles (°)
O10–Co1–O30
O20–Co1–O30
N10–Co1–O20
N20–Co1–O10
N20–Co1–O30
N30–Co1–O10
N30–Co1–O30
N30–Co1–N20
C23–O20–Co1
O11–N10–Co1
C20–N20–Co1
96.16(7)
91.20(7)
O20–Co1–O10
N10–Co1–O10
N10–Co1–O30
N20–Co1–O20
N20–Co1–N10
N30–Co1–O20
N30–Co1–N10
C13–O10–Co1
C33–O30–Co1
C10–N10–Co1
O31–N30–Co1
89.77(7)
84.09(8)
87.66(8)
84.69(9)
96.82(10)
93.82(8)
92.34(8)
109.64(16)
109.19(13)
112.96(17)
123.39(15)
1
2
3
4
5
1 ꢁ x, 2 ꢁ y, 1 ꢁ z.
ꢁ1 + x, +y, ꢁ1 + z.
1 ꢁ x, 2 ꢁ y, ꢁz.
1 ꢁ x, 1 ꢁ y, ꢁz.
+x, 1 + y, +z.
173.59(8)
87.48(8)
174.51(8)
176.39(8)
84.21(7)
92.41(8)
were corrected for the Basis Set Superposition Error (BSSE) using
the counterpoise method [37]. The molecular electrostatic poten-
tial (MEP) surfaces have been computed at the B3LYP/6-31 + G⁄
level of theory by means of the Spartan software [38].
109.50(16)
124.43(17)
112.31(17)

R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
181
3. Results and discussion
3.1. Synthesis and characterization
The literature reported synthesis of [CoIII(DMV)3] was carried
out by reacting Na3[Co(NO2)6] with 1,3-dimethylbarbituric acid
at 50 °C for 24 h [28]. However, in the present study a different
synthetic route was adopted. Reaction of HDMV with CoCl2ꢀ6H2O
in water–methanol medium and controlled pH resulted in the
formation of the cobalt(III) complex [CoIII(DMV)3]ꢀ0.5H2O (1), also
showing the aerial oxidation of precursor cobalt(II) ions.
However, such oxidation (CoII to CoIII) is not unprecedented and
was previously observed with violurate as a ligand [9,27]. On the
other hand, the reactivity of MnCl2ꢀ4H2O with HDMV in similar
conditions was also explored, leading to the isolation of
[MnII(H2O)6](DMV)2 (2).
The IR spectra of 1 and 2 (Figs. S1 and S2 in the Supporting
Information) show an expected set of bands [19,25]. For 1, a broad
band at ca. 3465 cmꢁ1 is attributable to O–H stretching of water of
crystallization. Three bands at ca. 1733, 1682 and 1636 cmꢁ1 may
Fig. 1. Mercury perspective of the [Co(DMV)3] unit in 1. Water molecule and
hydrogen atoms are omitted for clarity; 50% probability ellipsoids are shown.
be assigned to coupled
band appears at 1479 cmꢁ1. In the IR spectrum of 2, two vibrations
with maxima at 3487 and 3414 cmꢁ1 are due to
(H2O) bands of
m(C@O) vibrations, whereas the m(N@O)
m
water ligands. In addition, two weak bands at ca. 778 and
generating a trimeric unit. Further H-bonding interactions,
633 cmꢁ1 may be assigned to the Mn-OH2 rocking and wagging
involving also the O1W water ligands, lead to the generation
of a three-dimensional supramolecular network (Fig. 3A), which
is held together by several hydrogen bonds in the 2.663–2.924 Å
range.
mode, respectively. For dimethylviolurate moieties, m(C@O) bands
due to two carbonyl groups appear at about 1619 and
1678 cmꢁ1. The bands at 1481 and 1288 cmꢁ1 correspond to the
m
(N@O) and
m
(C–O) vibrations, being in agreement with the
To get further insight into an intricate 3D H-bonded network in
2, we have carried out its topological analysis by applying the con-
cept of the simplified underlying net [43,44]. Such a net was gen-
erated by contracting [Mn(H2O)6]2+ cations and (DMV)ꢁ anions to
the respective centroids maintaining their connectivity via conven-
tional hydrogen bonds [DꢁHꢀ ꢀ ꢀA, wherein the Hꢀ ꢀ ꢀA < 2.50 Å,
Dꢀ ꢀ ꢀA < 3.50 Å, and \(DꢁHꢀ ꢀ ꢀA)>120°; D and A stand for donor
and acceptor atoms] [43]. The obtained underling framework
(Fig. 3B) can be topologically classified as a binodal 4,8-connected
3D net with the flu (fluorite) topology. It is defined by the point
symbol of (412.612.84)(46)2, wherein the (412.612.84) and (46)
notations correspond to the 8-connected [Mn(H2O)6]2+ and
presence of anionic non-coordinated dimethylviolurate.
The cobalt(III) complex 1 is diamagnetic. The room temperature
(300 K) magnetic moment of the solid 2 is 5.90 B.M. This value is in
agreement with the magnetically dilute high spin d5 Mn(II)
complex [39].
3.2. Description of the crystal structures of [Co(DMV)3]ꢀ0.5H2O (1) and
[Mn(H2O)6](DMV)2 (2)
The structure of 1 consists of [Co(DMV)3] units and half water
4-connected (DMV)ꢁ nodes, respectively.
A few manganese
molecule.
A perspective view of the mononuclear fragment
together with the atom numbering scheme is shown in Fig. 1. In
this complex, cobalt(III) atom is six-coordinated by three
dimethylviolurato anions acting as bidentate ligands leading to a
[CoN3O3] chromophore in a fac disposition. This coordination mode
has been previously observed in other metal complexes containing
dimethylviolurato anions as ligands [22,40]. The coordination of
each bidentate ligand forms a five-membered chelate ring, exhibit-
ing very significant differences between the Co–N (ꢂ1.88 Å) and
Co–O (ꢂ1.95 Å) distances. The coordination environment of the
cobalt(III) can be considered as a slightly distorted octahedral.
Half crystallization water molecule is disordered in two positions
and involved in hydrogen bond with the oxygen atom O13 from
the anionic ligand (2.932 Å).
coordination compounds with the present type of topology have
been reported [45].
3.3. Electrochemistry
The redox properties of compounds 1 and 2 have been exam-
ined in acetonitrile and DMF solution, respectively, at GC electrode
under a N2 atmosphere; electrochemical data are summarized in
Table 5. The cyclic voltammograms (CVs) of 1 and 2 (Fig. 4) show
a redox couple at ꢁ0.059 and 0.532 V (versus Ag/AgCl), respec-
tively. The peak current ratio, ipa/ipc, is 0.60 and 1.20 for 1 and 2,
respectively. The peaks potential differences (DE) of 1 and 2 are
The molecular structure of 2 is shown in Fig. 2. Its asymmetric
unit consists of half hexaaquamanganese(II) cation and one
dimethylviolurato anion (DMV). The cations are located in special
positions at the inversion center. The Mn1 center is six-coordi-
nated in a slightly distorted octahedral fashion with water mole-
cules filling the coordination spheres. The lengths of Mn–O bonds
lay between 2.1221(12) and 2.2197(15) Å, agreeing with typical
values for these type of compounds [41,42]. In the crystal struc-
ture, dimethylviolurato anions are positioned forming strong
H-bond interactions (2.752 and 2.855 Å) between the oxygen
atoms (O11 and O10) with the O3W and O2W coordination water
molecules pertaining to the hexaaquamanganese(II) cation, thus
253 and 103 mV, respectively. Since HDMV is electrochemically
inert in the potential range of interest, these electrochemical pro-
cesses can be assigned as metal centered. Comparison of the
voltammetric peak currents with those of the ferrocene–ferroce-
nium couple under the same experimental conditions establishes
that the redox responses in both compounds involve one electron
transfer process. In general, the peak current, ip, increases with
the square root of scan rate (m
1/2) but does not follow in propor-
tionality. Thus, the metal centered redox processes for compounds
1 and 2 are quasi-reversible in nature [46]. The electrochemical
response for compound 1 and 2 can be ascribed to the CoIII/CoII
[47] and MnII/MnIII [48] couple, respectively.

182
R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
Fig. 2. Mercury perspective of 2; 50% probability ellipsoids are shown.
3.4. Thermal study
To estimate the stability of 1 and 2, thermogravimetric analyses
(TGA) (Figs. S3 and S4 in the Supporting Information) were carried
out between 40 and 800 °C in a static atmosphere of nitrogen. The
TGA study of 1 reveals that solvent water molecule is lost below
ꢂ112 °C. After that, gradual decomposition of the desolvated sam-
ple occurs in the 165–660 °C temperature range, corresponding to
the pyrolysis of dimethylviolurate molecule. The remaining residue
most likely corresponds to the formation of Co3O4 (obsd: 13.1%;
calcd: 12.9%).
Compound 2 shows a weight loss of 20.1% in the 95–155 °C
interval, corresponding to the release of six coordinated water
molecules (calcd: 20.3%). An anhydrous sample then decomposes
in the 275–700 °C range, probably leading to the formation of
MnO2 (obsd: 16.1%; calcd: 16.4%).
3.5. Computational study
The theoretical study was focused to analyze the interesting
noncovalent interactions observed in the solid state architectures
of both compounds, with a special interest to the lone pair(lp)–
p
interactions in compound 1 and hydrogen bonding network in
compound 2. Compound 1 exhibits an intricate combination of
very short lp–p contacts in the solid state that are responsible of
the solid state architecture and the very compact crystal packing.
A representative dimer is represented in Fig. 5A, where the
dimethylviolurate ligand acts as lp donor and acceptor simultane-
ously. The interaction energy computed for this dimer is very large
and negative (
short lp– distances, which is likely due to the electronic nature
of the six membered ring of the ligand ( -acidity). As a matter of
D
E1 = ꢁ30.2 kcal/mol), being in agreement with the
p
p
fact, we have represented in Fig. 5B the molecular electrostatic
potential surface computed for a theoretical model of compound
1 where two dimethylviolurate ligands have been simplified.
That is, the two bidentate ligands have been replaced by two 2-
(hydroxyimino)acetaldehyde molecules that are also used as a
O,N-bidentate ligands. This simplification not only saves computa-
tional time, it also allows a better visualization of the positive
potential isosurface over the violurate ring plane in the MEP plot.
Table 5
Cyclic voltammetric data for compound 1 and 2.
Parameter
Epa(ipa
)
Epc(ipc
)
D
E
ipa/ipc
E
½
Fig. 3. Structural fragments of 2. (A) Three-dimensional H-bonded network and (B)
its topological representation showing an underlying binodal 4,8-connected net
with the flu (fluorite) topology; (A, B) view along the b axis; (B) color codes:
centroids of 8-connected [Mn(H2O)6]2+ (magenta) and 4-connected (DMV)ꢁ (gray)
nodes. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Compound 1
Compound 2
0.068(9.46)
0.583(18.57)
ꢁ0.185(15.68)
253
103
0.60
1.20
0.059
0.532
0.480(15.54)
Epa = anodic peak potential, V; Epc = cathodic peak potential, V; ipa = anodic peak
current, A; ipc = cathodic peak current, A; E = 0.5(Epc + Epa) V;
E = Epa ꢁ Epc, mV.
l
l
D
½

R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
183
observed between two dimethylviolurate ligands. In particular,
we have used two theoretical models (see Fig. 6) based on the
dimer shown in Fig. 5B and using the crystallographic coordinates.
In one model (Fig. 6A), we evaluate the double lp–p interaction and
an additional contribution from a C–Hꢀ ꢀ ꢀO interaction involving a
hydrogen atom of the methyl group and one oxygen atom (see
red dashed line, Fig. 6A). As a result, the binding energy computed
for this model shows that the interaction is very favorable
(
D
E2 = ꢁ22.7 kcal/mol) confirming the strong ability of this ligand
to participate in lp– interactions due to coordination to the
p
Co(III). In the second theoretical model (Fig. 6B), we have substi-
tuted one dimethylviourate ligand by violurate in the Co(III) com-
plex with the purpose to eliminate this hydrogen bonding
contribution. As a result, the interaction energy is slightly reduced
to
D
E3 = ꢁ21.7 kcal/mol, indicating that the contribution of this
hydrogen bond is very small, which agrees with the long distance
(2.82 Å) and small C–Hꢀ ꢀ ꢀO angle (97.2°).
The formation of strong hydrogen bonds by coordinated water
molecules is a topic of recent interest [49]. In the theoretical study
of compound 2 we have focused our attention to evaluate the
hydrogen bonding network that is observed in the crystal packing.
This hydrogen bonding network generates interesting supramolec-
ular assemblies in the solid state where the equatorial water mole-
cules of the [Mn(H2O)6]2+ moiety act as strong hydrogen bonding
donors and the dimethylviourate molecules as hydrogen bonding
acceptors. Two different binding modes are observed in the forma-
tion of the pentameric assembly where one central [Mn(H2O)6]2+
moiety interacts with four coplanar dimethylviolurate rings (see
Fig. 7A). Starting from two different trimeric units (formed by
one [Mn(H2O)6]2+ and two dimethylviolurates), we have analyzed
the energetic features of both binding modes. We have started
from these trimers because they are neutral and the strong and
non-directional electrostatic effects are minimized. In both
binding modes three H-bonds are formed, see green and blue
dashed lines in Fig. 7. In the first one, both atoms of the N–O bond
participate in the formation of two hydrogen bonds and the other
one is formed with one C@O group of the ring (blue lines). The
interaction energy that corresponds to these three H-bonds is
Fig. 4. Cyclic voltammogram of compound 1 in acetonitrile (red) and compound 2
in DMF (blue) at a scan rate of 100 mV sꢁ1. The concentrations of 1 and 2 were
0.893 ꢃ 10ꢁ3 and 1.011 ꢃ 10ꢁ3 M, respectively. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
If the calculation of the MEP is performed using the whole com-
plex, the electrostatic potential over the ring does not change, thus
validating the model. A small dark blue region of positive electro-
static potential over center of the ring (MEP value 20 kcal/mol) can
be observed. In addition, there is a large bluish surface over the
ring where the potential ranges from 14 to 18 kcal/mol. This anal-
ysis is useful to understand the compact crystal packing provoked
by the network of lp–p interactions between the three dimethylvi-
olurate ligands of the Co(III) complex, since the ligands have
several lone pair donor groups and concurrently, they have a
strong ability to p-interact with electron rich entities.
We have used simplified theoretical models to evaluate the lp–
p
interactions in compound 1 and to estimate the binding strength
of the characteristic double intermolecular lp–p interaction
Fig. 5. Dimer of compound 1 (A) and MEP surfaces of a model of 1 (B). Distances in Å (left). Hydrogen atoms have been omitted for the sake of clarity.

184
R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
Fig. 6. Theoretical models used to evaluate the double lp–
p
interaction in compound 1. Distances in Å.
½ ꢃ
D
E4 = ꢁ20.5 kcal/mol. In the second binding mode (green
addition to these two binding modes that are responsible for the
formation of the supramolecular assembly in the solid state of 2,
the apical water molecules also participate in H-bonding interac-
tions as connectors of supramolecular assemblies (see Fig. 7B).
We have also evaluated the interaction energy of this complex that
lines), one C@O group forms a bifurcated hydrogen bond and the
exocyclic N–O group also establishes an N–Oꢀ ꢀ ꢀH bond. This bind-
ing mode is considerably more favorable ½ ꢃ
D
E5 = ꢁ33.5 kcal/mol
than the other one, in agreement with the shorter distances. In
Fig. 7. Partial view of the crystal packing of compound 2 with indication of the different noncovalent interactions. The equations used to evaluate the energetic contributions
are also shown. Distances in Å.

R. Banik et al. / Inorganica Chimica Acta 435 (2015) 178–186
185
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Acknowledgements
Financial assistance from Department of Biotechnology,
Government of India [vide sanction No BCIL/NER-BPMC/2012.1549]
is gratefully acknowledged. One of the authors, R.B. is grateful to
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