Synthesis and characterization of unique nickel(II) carboxylates and their coordination complexes

Article abstract of DOI:10.14233/ajchem.2018.21057

Direct anodic dissolution of nickel metal and cathodic reduction of carboxylic acids (RCOOH) in acetonitrile has proved to be a simple and efficient one-step route to synthesize unique polymeric nickel(II) carboxylate complexes, {Ni(OOCR)₂}n in H-type glass Pyrex cell. When the oxidation was carried out in the presence of neutral ligands (L) such as 2,2’-bipyridyl or 1,10-phenanthroline, the complexes of type {Ni(OOCR)2.L}n were obtained. Tetrabutylammonium chloride has been used as a supporting electrolyte in order to increase the electrolytic conductivity of the electrochemical system which in turn affects the current efficiency, cell voltage and energy consumption in the electrolytic cell. The complexes have been characterized by vibrational spectra, CHN elemental analysis, solubility and melting points shows a good agreement with the structure. The result also shows that the direct electrochemical synthetic technique has high current efficiency, extra purity and yield.

Full text of DOI:10.14233/ajchem.2018.21057

Asian Journal of Chemistry; Vol. 30, No. 2 (2018), 416-420
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2018.21057
Synthesis and Characterization of Unique Nickel(II) Carboxylates and Their Coordination Complexes
*
SHAVINA and BALJIT SINGH
Department of Chemistry, Punjabi University, Patiala-147 002, India
*Corresponding author: E-mail: baljit_chemz@yahoo.co.in; khanshavina@gmail.com
Received: 22 September 2017;
Accepted: 31 October 2017;
Published online: 31 December 2017;
AJC-18716
Direct anodic dissolution of nickel metal and cathodic reduction of carboxylic acids (RCOOH) in acetonitrile has proved to be a simple
and efficient one-step route to synthesize unique polymeric nickel(II) carboxylate complexes, {Ni(OOCR)2}n in H-type glass Pyrex cell.
When the oxidation was carried out in the presence of neutral ligands (L) such as 2,2’-bipyridyl or 1,10-phenanthroline, the complexes of
type {Ni(OOCR)2.L}n were obtained. Tetrabutylammonium chloride has been used as a supporting electrolyte in order to increase the
electrolytic conductivity of the electrochemical system which in turn affects the current efficiency, cell voltage and energy consumption
in the electrolytic cell. The complexes have been characterized by vibrational spectra, CHN elemental analysis, solubility and melting
points shows a good agreement with the structure. The result also shows that the direct electrochemical synthetic technique has high
current efficiency, extra purity and yield.
Keywords: Synthesis, Sacrificial anode, Electrochemical or Current efficiency, Nickel(II) carboxylate, Coordination complexes.
INTRODUCTION
EXPERIMENTAL
Electrochemical technique in inorganic synthesis is highly
selective in terms of potential, current density, electrode material
and electrolyte composition for controlling the reaction rate.
Electro synthesis is not only the oldest but also the simplest
method for the synthesis of coordination compounds starting
from the zero-valent metal. Electrochemical methods employ
simple machinery and pure starting materials. The fact that
the metal is employed rather than one of its salts avoids the
occurrence of competitive reactions between the anion salts
and the ligand to coordinate to the metal ion. Moreover, this
approach allows the selective transformation of specific groups
in a ligand or in a complex under very mild conditions. In the
last several decades, contributions of electrochemical technique
are of interest not only to inorganic [1-7], organic [8-11], organo-
metallic [12], pharmaceutical [13,14] and neuroscience [15]
but also it became the enabling fabrication route to grow the
multi layered metallic thin films [16], nano-structures [17], nano-
particles [18-20] and a high quality single crystal overlayers
[21]. These new applications make the future of the research
in electrochemical material science apparently quite exciting
aspire. The study of transition metal carboxylates has got prime
interest due to their DNA binding [22,23], catalytic [24] and
anticorrosion properties [25].
All the carboxylic acids and tetrabutylammonium chloride
were purchased from Merck and Sigma Aldrich respectively
and were used as received without further purification. The
solvent acetonitrile was purchased from Merck and was purified
using the 4A molecular sieves, dried using phosphorous
pentoxide and was further distilled. Nickel electrode (Sigma
Aldrich) of dimensions 0.635 mm diameter, 5 cm length was
used as sacrificial anode and platinum electrode of dimensions
1 cm × 1 cm × 0.05 cm was used as inert cathode. For electrolysis,
H-type pyrex glass cell was used which has two compartments
anodic (larger) and cathodic (smaller) separated by sintered
glass disc of G-3 porosity. Both compartments were sealed
through guard tubes filled with calcium chloride and glass wool
in order to protect the reaction mixture from moisture.
Electrochemical oxidation was conducted by passing the
direct current supplies by electrophoresis power supply of
‘Toshniwal’make indicating 0-300V potential and 0-100 mA
current. The stirring of anodic solution was done using magnetic
stirrer of ‘Perfit’ make. The infrared spectral studies of metal
complexes were realized using KBr pallets on ‘Elmer FTIR
spectrochem’ (RXI) in the range of 4000-400 cm-1. Melting
points of metal complexes were determined using electrical
device with a heating rate of 5 °C per min. Carbon, hydrogen

Vol. 30, No. 2 (2018)
Synthesis and Characterization of Unique Nickel(II) Carboxylates and Their Coordination Complexes 417
and nitrogen (where applicable) analysis were performed using
‘ElementorVario EL’CHNS elemental analyzer. The complexes
were analyzed for their metal content by oxine method [26].
General procedure for synthesis of metal complexes:
Solution of 3 mL of the corresponding carboxylic acid in 250 mL
of acetonitrile solvent containing tetrabutylammonium chloride
as supporting electrolyte was poured into the two compartments
of the H-type pyrex glass cell. The nickel electrode and platinum
electrode were made sacrificial anode and inert cathode by
connecting to the positive and negative terminal of electropho-
resis power supply respectively. The cell was closed from both
openings using guard tubes. The potential across the electrodes
was so adjusted that a current of 20 mA passed through the
solution. The electrolytic cell can be represented as:
these complexes are in good agreement with their formulation
as given in the Table-1. The formulation of these complexes
was based on elemental analysis, melting point, solubility and
FTIR data. The complexes exhibited 1:2 metal-carboxylic
acid stoichiometries in which the carboxylic acid moiety is
coordinated to the metal ion with deprotonation in a bidentate
mode.
The FTIR spectrum of the complexes consists of internal
vibrations due to carboxylate group and methylene group. The
spectrum is integrated by comparing with those of related
complexes available in literature. The IR spectrum of the metal
complexes indicates the absence of broad band in the region
of 3300-3000 cm-1 due to –OH group of carboxylic acid which
shows the deprotonation of carboxylic acid group i.e. metal
ion is coordinated to carboxylate moiety. However, the charac-
teristic vibrational bands appear in the region of 564-440, 942-
660, 1409-1337, 1558-1552 and 2979-2851 cm-1 (Fig. 1).
Ni(+) RCOOH + Bu4NCl + CH3CN Pt(−)
where Ni(+) and Pt(-) are sacrificial nickel anode and inert
platinum cathode respectively.
RCOOH is carboxylic acid used in systems (ethanoic acid,
propionic acid, butyric acid, pentanoic acid, hexanoic acid,
heptanoic acid, octanoic acid, nonanoic acid and decanoic
acid).
90
80
70
9
4
.
60
0
Electrolysis was performed for 8 h with continuous stirring
at constant current of 20 mA. During electrolysis, the solid
compound started accumulating in the anode compartment
which was filtered after the electrolysis was done. The solid
product was purified by washing with hot acetonitrile and dried
using dry diethyl ether.All the efforts were done to protect the
product from air and moisture.
7
2
1
50
40
30
20
10
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm–1)
For the synthesis of coordination compounds of nickel
caboxylates, the ligand (1,10-phenanthroline or 2,2’-bipyridyl)
was added initially to the above substrates before electrolysis.
The cell can be represented as:
Fig. 1. IR spectrum of Ni-heptanoic acid system
It is reported in the literature [3,22,27,28] that the vibra-
tional band due to the free >C=O group of carboxylic moiety
appears in the region of 1725-1700 cm-1 and in metal carbo-
xylate ν(COO–) asymmetric and symmetric stretching vibra-
tions appeared in the region of 1610-1520 and 1460-1300 cm-1.
Also ν(M-O) vibrational bands appeared in the region of 400-
600 cm-1 have been reported [23,29-33] in the literature. The
vibrational bands in the region of 3000-2800 cm-1 have been
reported [3] in literature due to asymmetric and symmetric
vibrations of methylene group.
Ni(+) RCOOH + Bu4NCl + CH3CN + L Pt(−)
RESULTS AND DISCUSSION
All the metal complexes were air stable and possessed
good keeping qualities. All the complexes were insoluble in
commonly used organic solvents like methanol, ethanol,
benzene, dimethyl sulfoxide, dimethyl formamide, carbon
disulphide and pyridine. Melting points of all these products
were determined with the melting point apparatus of 5 °C rate
per min. These products do not melt upto 350 °C but they show
colour change in the range 230-250 °C which shows that they
decompose in this temperature range. The analytical data of
In the present products, the vibrational bands in the region
of 1725-1700 cm-1 are absent which indicates that there is no
free carbonyl group in the carboxylate moiety. This is further
supported by the appearance of ν(Ni-O) broad absorption
TABLE-1
ELEMENTAL ANALYSIS AND OTHER RELATED DATA OF NICKEL(II) CARBOXYLATES
Elemental analysis (%): Calculated (Found)
C
Current efficiency (gram
equivalents per Faraday)
System
m.f.
Ni
H
Ethanoic acid
Propionic acid
Butyric acid
C4H6O4Ni
33.2 (33.1)
28.7 (28.5)
25.2 (25.0)
22.5 (21.9)
20.3 (20.1)
18.5 (18.4)
17.0 (16.8)
15.7 (15.4)
14.6 (14.4)
27.2 (27.0)
35.2 (35.1)
41.2 (41.2)
46.0 (45.8)
49.9 (49.6)
53.0 (52.9)
55.7 (55.5)
57.9 (57.7)
59.9 (59.7)
3.4 (3.2)
4.9 (4.6)
6.0 (5.8)
6.9 (6.8)
7.6 (7.4)
8.2 (8.0)
8.7 (8.5)
9.1 (9.0)
9.5 (9.3)
0.68
0.72
0.61
0.89
0.77
0.71
0.93
0.92
0.72
C6H10O4Ni
C8H14O4Ni
C10H18O4Ni
C12H22O4Ni
C14H26O4Ni
C16H30O4Ni
C18H34O4Ni
C20H38O4Ni
Pentanoic acid
Hexanoic acid
Heptanoic acid
Octanoic acid
Nonanoic acid
Decanoic acid

418 Shavina et al.
Asian J. Chem.
bands in the region of 564-440 cm-1 which confirmed the coor-
dination of the carboxylate moiety through oxygen with metal.
The absorption bands in the region of 1558-1552 and 1409-
1337 cm-1 may be assigned due to asymmetric and symmetric
ν(COO–) stretching vibrations respectively. The difference
between ν(COO–) asymmetric and symmetric is a parameter
that identifies the type of bonding between the metal and
carboxylate moiety. It is reported in the literature [22,34] that
the value of ∆ν above 250 cm-1 signify the monodentate coordi-
nation, a value in the range of 120-250 cm-1 signify the bridging
mode while a difference below 120 cm-1 correspond to a chelate
nature. In the present compounds the values of ∆ν for the pro-
ducts lie in the range of 149-168 cm-1. The value of ∆ν suggests
bridging bidentate mode due to the incarceration of interaction
with both oxygen atoms of the carboxylate group. The
vibrational frequencies observed in the region of 2851-2880
and 2921-2979 cm-1 are assigned to symmetric and asymmetric
stretching vibrations of the –CH2 group respectively. The bands
observed in 942-660 cm-1 are due to the bending vibrations of
the –CH group. The fascinating characteristic of the bands
appearing in the region of 564-440 cm-1 is that the bands are
comparatively broad which indicates the bridging structure
and polymeric nature of these compounds. Thus the broad
bands, high melting points and insolubility in various organic
solvents confirm that these compounds are unique polymeric
nickel(II) carboxylates.
The reaction scheme for the formation of nickel(II)
carboxylates is given as:
At cathode
2RCOOH + 2e– → 2RCOO– + H2
At anode
2RCOO– + Ni(+) → Ni(OOCR)2 + 2e–
Coordination complexes of nickel(II) carboxylates: In
order to prepare coordination complexes of nickel carboxy-
lates, the synthesized parent nickel(II) carboxylates have been
refluxed with the ligands 1,10-phenanthroline or 2,2’-bipyridyl
in the solvents like methanol, ethanol, acetonitrile and benzene
for 48 h. However, the analytical and IR data showed that the
ligand moiety could not be attached to the parent nickel(II)
carboxylates. It may be due to the reason that the metal atom
in these nickel(II) carboxylates have already achieved its
favourable coordination number through bridging, therefore
further expansion of coordination sphere due to addition of
ligand could not be achieved. However, the ligand molecule
may be added to these compounds before these form carboxy-
late bridging and get polymerized. The coordination compounds
have been prepared electrochemically by adding 1 g of ligand
(1,10-phenanthroline or 2,2’-bipyridyl) to the above substrates
and then electrolyzing the reaction mixture for 8 h.After elec-
trolysis, the solid compound in the anodic compartment have
been filtered, washed with hot acetonitrile and dried with dry
diethyl ether. The elemental analysis data for nickel (oxine
method), carbon, hydrogen and nitrogen for these adducts have
been given in Table-2. The data shows that the complexes exhibit
1:2:1 stoichiometry corresponding to metal, carboxylic acid
and ligand moiety respectively and conforms to the general
formula Ni(OOCR)2·L.
The probable structure of the unique polymeric nickel(II)
carboxylates is shown below:
R
C
O
O
Ni
IR spectra of these metal complexes shows absorption
bands in the region of 598-468, 1411-1338, 1494-1442, 1579-
1560 and 1615-1597 cm-1. There is no absorption peak corres-
ponding to free carbonyl group in the region of 1725-1700
cm-1 and the hydroxyl group in the region of 3300-3000 cm-1
of carboxylic acid moiety which indicates the deprotonation.
Ni
O
O
C
R
TABLE-2
ELEMENTAL ANALYSIS AND OTHER RELATED DATA OF COORDINATION COMPLEXES OF NICKEL(II) CARBOXYLATES
Current efficiency
(gram equivalents
per Faraday)
Elemental analysis (%): Calculated (Found)
System
m.f.
Ni
C
H
N
Ethanoic acid + 2,2’-bipyridyl
Propionic acid +2,2’-bipyridyl
Butyric acid + 2,2’-bipyridyl
C14H14N2O4Ni
C16H18N2O4Ni
C18H22N2O4Ni
C20H26N2O4Ni
C22H30N2O4Ni
C24H34N2O4Ni
C26H38N2O4Ni
C28H42N2O4Ni
C30H46N2O4Ni
C16H14N2O4Ni
C18H18N2O4Ni
C20H22N2O4Ni
C22H26N2O4Ni
C24H30N2O4Ni
C26H34N2O4Ni
C28H38N2O4Ni
C30H42N2O4Ni
C32H46N2O4Ni
17.6 (17.4)
16.2 (16.0)
15.1 (15.0)
14.1 (14.0)
13.2 (13.0)
12.4 (12.1)
11.7 (11.6)
11.1 (11.0)
10.5 (10.3)
16.4 (16.2)
15.2 (15.0)
14.2 (14.1)
13.3 (13.2)
12.5 (12.3)
11.8 (11.6)
11.1 (10.8)
10.6 (10.4)
10.1 (9.8)
50.5 (50.3)
53.2 (53.0)
55.5 (55.3)
57.6 (57.4)
59.3 (59.1)
60.9 (60.8)
62.3 (62.1)
63.5 (63.2)
64.6 (64.4)
53.8 (53.6)
56.1 (55.9)
58.1 (57.7)
59.9 (59.5)
61.4 (60.8)
62.8 (62.6)
64.0 (63.8)
65.1 (64.8)
66.1 (66.0)
4.2 (4.0)
4.9 (4.8)
5.7 (5.6)
6.2 (6.0)
6.7 (6.6)
7.2 (7.0)
7.6 (7.5)
7.9 (7.8)
8.3 (8.1)
3.9 (3.6)
4.7 (4.6)
5.3 (5.1)
5.9 (5.7)
6.4 (6.3)
6.8 (6.6)
7.2 (7.1)
7.6 (7.4)
7.9 (7.7)
8.4 (8.2)
7.8 (7.7)
7.2 (7.1)
6.7 (6.6)
6.2 (6.0)
5.9 (5.7)
5.6 (5.4)
5.3 (5.1)
5.0 (4.9)
7.8 (7.6)
7.3 (7.2)
6.8 (6.6)
6.4 (6.2)
5.9 (5.7)
5.6 (5.4)
5.3 (5.1)
5.1 (5.0)
4.8 (4.6)
0.72
0.93
0.60
0.89
0.61
0.64
0.82
0.98
0.71
0.78
0.44
0.81
0.72
0.64
0.62
0.74
0.90
0.83
Pentanoic acid + 2,2’-bipyridyl
Hexanoic acid + 2,2’-bipyridyl
Heptanoic acid + 2,2’-bipyridyl
Octanoic acid + 2,2’-bipyridyl
Nonanoic acid + 2,2’-bipyridyl
Decanoic acid + 2,2’-bipyridyl
Ethanoic acid + 1,10-phenanthroline
Propionic acid + 1,10-phenanthroline
Butyric acid + 1,10-phenanthroline
Pentanoic acid + 1,10-phenanthroline
Hexanoic acid + 1,10-phenanthroline
Heptanoic acid + 1,10-phenanthroline
Octanoic acid + 1,10-phenanthroline
Nonanoic acid + 1,10-phenanthroline
Decanoic acid + 1,10-phenanthroline

Vol. 30, No. 2 (2018)
Synthesis and Characterization of Unique Nickel(II) Carboxylates and Their Coordination Complexes 419
The absorption bands appeared in the region of 598-468 cm-1
may be assigned due to ν(Ni-O) stretching vibrations. How-
ever, the absorption bands appeared in the region of 1411-
1338 and 1579-1560 cm-1 may be assigned due to ν(COO–)
symmetric and asymmetric stretching vibrations. All these
absorption bands showed a blue shift of 10-20 cm-1 than the
parent carboxylate complexes because of the complex formation
with ligand molecule. In addition to these vibrational bands,
the complexes showed the absorption bands in the region of
1494-1442 and 1615-1597 cm-1 due to the ν(C=C) and ν(C=N)
stretching vibrations [33,35-37] which were absent in the
parent nickel carboxylate and hence confirms that the ligand
molecule has been attached to these nickel carboxylate complex.
The peaks in the region of 598-468 cm-1 are comparatively
broad which indicates that these compounds may be polymeric
in nature (Fig. 2).
ved have been determined by electrolyzing the above reaction
mixture for exactly 2 h at constant current of 20 mA. After 2 h
of electrolysis, the solution in the anodic compartment have
been filtered and distilled in film evaporator (Buchi) till 10 mL
solution is left. The solution was transferred to the beaker and
dried to dryness. In dry mass, the nickel content was deter-
mined volumetrically by oxine method. The current efficiencies
of all these systems have been determined and listed in Tables
1 and 2.
The current efficiencies of all these systems have been
found to be very high except propionic acid + 1,10-phenan-
throline system. Low value of current efficiency for this system
is due to the corrosion inhibiting behaviour of this system to
nickel anode. In order to elucidate the phenomenon of
corrosion inhibition, the current efficiency of this system has
been determined at different intervals of time and the data is
enlisted in Table-3.
Melting points of all these compounds have been deter-
mined which were found to be higher than 350 °C. These com-
pounds are insoluble in various organic solvents like methanol,
ethanol, benzene, pyridine, dimethyl sulfoxide, dimethyl form-
amide and carbon disulphide. High melting point, insoluble
nature and broad IR peaks confirm the polymeric nature of
these compounds.
TABLE-3
CURRENT EFFICIENCIES OF PROPIONIC
ACID + 1,10-PHENANTHROLINE SYSTEM AT
DIFFERENT INTERVALS OF TIME
System
Time (h)
Current efficiency
1.0
1.5
2.0
0.75
0.60
0.44
The reaction scheme for the formation of adducts of
nickel(II) carboxylate is given below:
At cathode
Propionic acid +
1,10-phenanthroline
2RCOOH + 2e– → 2RCOO– + H2
The content in Table-3 shows that the current efficiency
is high in the initial stage and then decreases with the passage
of time. The decrease in current efficiency with passage of
time indicates that the product of this system forms a protective
layer on nickel anode which inhibits the further dissolution of
metal and hence shows the corrosion inhibition phenomenon.
At anode
2RCOO– + Ni(+) + L → Ni(OOCR)2·L + 2e–
Electrochemical efficiencies or current efficiencies: The
electrochemical efficiency (gram equivalents of metal dissolved
per Faraday of electricity passed) is the ratio of experimental
amount of metal dissolved to the theoretical amount of nickel
dissolved by passing the current of 20 mA for 2 h. Theoretical
amount of nickel dissolved have been determined by Faraday’s
First law of electrolysis. Experimental amount of nickel dissol-
Conclusion
The preparative electrochemistry is an effective, single
step, potential controlled, cost efficient, sustainable and selec-
tive method for the preparation of nickel complexes. The electro-
104.4
100
3834.103
500.103
3972.102
2169.102
1942.101
1905.99
193,98
2001 798
90
468.97
1874.97
2346.95
1074.94
799.89
80
70
60
50
40
30
20
10
608.87
563.81
999.86
2572.86
2732.78
838.82
976.81
1056.80
1024.76
1201.66
1186.61
1174.62
1671.63
1220.60
665.57
3070.51
3032.52
3106.51
1493.54
7747.532.52
1253.51
1269.45
3051.53
1112.47
926.42
909.38
3329.42
1351.34
1316.26
2857.22
3431.22
1443.21
2956.16
2928.13
1394.11
0
1564.6
-5.0
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450
Wavelength (cm–1)
Fig. 2. IR spectrum of Ni-heptanoic acid + 2,2’-bipyridyl system

420 Shavina et al.
Asian J. Chem.
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or reductants so this method is highly eco-friendly for the
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ACKNOWLEDGEMENTS
One of the authors (Shavina) is highly thankful to University
Grants Commission, New Delhi for the financial assistance
under Maulana Azad National Fellowship Program.
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