Room-temperature magnetic liquid complexes of 2-hexyldecanoato ligand with Cu(II), Ni(II) and Co(II) ions

Article abstract of DOI:10.14233/ajchem.2014.15714

Three new complexes [Cu₂(L)₄(LH)₂] (1), [Ni₂(L)₄(EtOH)₂(H₂O) ₂].H₂O.CHCl₃ (2) and [Co₂(L) ₄(H₂O)₄].3H₂O (3), were obtained as coloured viscous liquids at room temperature (30 °C) and in good yields from the reaction of sodium 2-hexyldecanoate (NaL) with the corresponding MCl ₂ (M = Cu, Ni or Co). The structural formulae of these complexes were indirectly deduced by a combination of analytical results and additionally for 1, inferred from that of [Cu₂(CH₃(CH₂) ₅COO)₄] (4) obtained as single crystals under similar synthetic conditions. Complexes 1-3 were paramagnetic with weak antiferromagnetic interactions between the two M(II) centres, while 4 has a significantly stronger antiferromagnetic interaction. Complex 3 was also high spin. These complexes were thermally stable: 1 and 2 decomposed at 200 °C, 3 at 150 °C and 4 at 280 °C. However, they were redox inactive in the potential range -1.5 V to + 1.5 V, while only 1 was mesogenic with a hexagonal columnar mesophase (Colh) at -23.2 °C.

Full text of DOI:10.14233/ajchem.2014.15714

Asian Journal of Chemistry; Vol. 26, No. 4 (2014), 987-990
http://dx.doi.org/10.14233/ajchem.2014.15714
Room-Temperature Magnetic Liquid Complexes of
2-Hexyldecanoato Ligand with Cu(II), Ni(II) and Co(II) Ions
*
NORBANI ABDULLAH , YASAMEEN AL-HAKEM, NAZIRAH ABDULLAH, HABIBAH SAMSUDIN and NUR SYAMIMI AHMAD TAJIDI
Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
*Corresponding author: Fax: +60 3 79674193; Tel: +60 3 79674264; E-mail: norbania@um.edu.my
Received: 1 May 2013;
Accepted: 21 June 2013;
Published online: 15 February 2014;
AJC-14684
Three new complexes [Cu2(L)4(LH)2] (1), [Ni2(L)4(EtOH)2(H2O)2].H2O.CHCl3 (2) and [Co2(L)4(H2O)4].3H2O ( 3), were obtained as coloured
viscous liquids at room temperature (30 ºC) and in good yields from the reaction of sodium 2-hexyldecanoate (NaL) with the corresponding
MCl2 (M = Cu, Ni or Co). The structural formulae of these complexes were indirectly deduced by a combination of analytical results and
additionally for 1, inferred from that of [Cu2(CH3(CH2)5COO)4] (4) obtained as single crystals under similar synthetic conditions. Complexes
1-3 were paramagnetic with weak antiferromagnetic interactions between the two M(II) centres, while 4 has a significantly stronger
antiferromagnetic interaction. Complex 3 was also high spin. These complexes were thermally stable: 1 and 2 decomposed at 200 ºC, 3 at
150 ºC and 4 at 280 ºC. However, they were redox inactive in the potential range -1.5 V to + 1.5 V, while only 1 was mesogenic with a
hexagonal columnar mesophase (Colh) at -23.2 ºC.
Keywords: 2-Hexyldecanoate, Cu(II), Ni(II), Co(II), Magnetic, Thermal.
corresponding MCl2. It also reports the single-crystal structure
of [Cu2(CH3(CH2)5COO)4] (4) used to ascertain the structure
of 1. The ligand L was specifically chosen as the branched
long alkyl chains would allow for the formation of low melting
temperature complexes, hence minimising thermal degradation.
The metal(II) ions were chosen as they have unpaired electron(s)
and hence expected to form magnetic complexes to allow for
the use of low energy magnetic field for structural alignment.
Additionally, Cu(II) and Ni(II) centres have readily available
axial positions for the formation of coordination polymers
using ditopic ligands, such as 4,4’-bipyridine and pyrazine
12,13, while Co(II) complexes were potential spin crossover
materials useful as sensors and in information storage14.
INTRODUCTION
Most of transition metal(II) carboxylates were reported
to exist as multinuclear complexes due to the multibinding
modes of the ligands1,2. For example, most of the copper(II)
carboxylates were dinuclear with the paddle-wheel structure3-7,
while other metal(II) complexes (Mn, Co, Ni and Zn) were
found to form polymeric structures8-11. Hence, these complexes
are attracting the interests of many researchers in various fields,
such as catalysis, molecular electronics and molecular magne-
tism. The main reason is because they are easily prepared from
relatively low cost, nontoxic and readily accessible starting
materials and readily characterized by conventional analytical
methods. However, these complexes are yet to be commercially
used as functional materials due to physical limitations, such as
insolubility, high-melting and low-decomposition temperatures.
Currently, our research group is focused on designing,
synthesizing and characterizing thermally stable and low melting
temperatures magnetic metal(II) carboxylates to function as
molecular magnetic, photonic and electronic materials and as
precursors for functional coordination polymers.
EXPERIMENTAL
2-Hexyldecanoic acid (HL), heptanoic acid and all other
chemicals were commercially available and used as received.
The elemental analyses were carried out on a Perkin-Elmer
CHNO/S 2400 Series II elemental analyser. The FTIR spectra
were recorded in the 4000–400 cm-1 region for neat samples
sandwiched between two NaCl plates on a Perkin-Elmer Spec-
trum 400 spectrometer. The electronic spectra were recorded
for samples dissolved in CHCl3 on a Shimadzu UV-VIS-NIR
1600 spectrophotometer. The room-temperature magnetic
susceptibility measurements were taken on a Sherwood
automagnetic susceptibility balance by the Gouy method,
This paper reports the synthesis, structural, magnetic,
thermal, optical and redox properties of three complexes of
2-hexyldecanoato ligand (L) with Cu(II) (1), Ni(II) (2) and
Co(II) (3). These complexes were obtained as room-tempe-
rature viscous liquids from the reaction of NaL with the

988 Abdullah et al.
Asian J. Chem.
using Hg[Co(SCN)4] as the calibrant. The molar susceptibility
was corrected for the diamagnetism of the constituent atoms
using Pascal’s constants. The diffraction data for single crystals
was collected at 100(2) K on a Bruker SMARTAPEX II fitted
with MoKα radiation so that λmax was 28.4º. The data set was
corrected for absorption based on multiple scans15 and reduced
using standard methods16. The structure was solved by direct
method with SHELXS-9717 and refined by a full-matrix least-
squares procedure on F2 using SHELXL-97 with anisotropic
displacement parameters for non-hydrogen atoms and a
weighting scheme of the form w = 1/[σ2(Fo2) + (0. 0506P)2 +
0.2108P] where P = (Fo2 + 2Fc2)/3). All hydrogen atoms were
included in the final refinement in their calculated positions.
The thermogravimetric analysis (TGA) was done on a Perkin-
Elmer Pyris Diamond TG/DTA thermal instrument in the
temperature range 50-900 ºC, under N2 at a flow rate of 10
cm3 min-1 and scan rate of 20 ºC min-1. The differential scan-
ning calorimetry (DSC) was done on a Mettler Toledo DSC
822 instrument, with the scans recorded for two consecutive
heating-cooling cycles, from 30-50 ºC and then cooled to
-30 ºC, under N2 at a flow rate of 20 cm3 min-1 and scan rate of
10 ºC min-1. The optical textures were inferred from the
photomicrographs captured on either an Olympus polarizing
or Nikon-H600L eclipse microscopes, both equipped with a
Mettler Toledo FP90 central processor and a Linkam THMS
600 hot stage. The heating and cooling rates were 5 ºC and
2 ºC min-1 respectively and the magnification was 50X. The
cyclic voltammetric scans (CV) were mainly recorded on a
Gamry Instrument Reference 600 potentiostat/galvanostat/
ZRA. A three-electrode cell consisting of glassy carbon as the
working electrode, saturated calomel electrode (SCE) as the
reference electrode and platinum wire as the counter electrode
were used. The supporting electrolyte was tetrabutylammonium
tetrafluoroborate (TBATFB, 0.05 M). The samples (0.005 M)
were dissolved in THF or CHCl3 and the solutions were
bubbled with N2 for ca. 1 min. The potential range was -1.5 V
to +1.5 V and the scan rates were 50-150 mV s-1.
[Ni2(L)4(EtOH)2(H2O)2].H2O.CHCl3 (2): The synthetic
method was similar to that of 1, using NaL (6.08 g, 21.9 mmol)
and NiCl2.6H2O (2.59 g, 10.9 mmol). The product was a pale
green viscous liquid and the yield was 3.82 g (62 %). Anal.
calcd. (%) for C70H147O13Cl3Ni2: C, 59.1; H, 10.4 ; found (%):
C, 58.6; H, 10.8. FTIR (cm-1): 3368 (w), 2955 (s), 2855 (s),
1685 (s), 1610 (m), 1587 (s), 1465 (m), 1414 (s), 485 (vs).
[Co2(L)4(H2O)4].3H2O (3): The synthetic method was
similar to that of 1, using NaL (3.01 g, 10.8 mmol) and
CoCl2.6H2O (0.70 g, 5.40 mmol). The product was a dark blue
viscous liquid and the yield was 0.98 g (59 %). Anal. calcd.
(%) for C64H138O15Co2: C, 60.7; H, 11.0; found (%): C, 60.8;
H, 11.1. FTIR (cm-1): 3383 (w), 2925 (s), 2856 (s), 1682 (s),
1607 (m), 1465 (s), 1459 (s), 1410 (s), 446 (vs), 436 (vs).
[Cu2(CH3(CH2)5COO)4] (4): The synthetic method was
similar to that of 1, using CH3(CH2)5COONa (1.64 g, 10.8
mmol) and CuCl2.2H2O (0.92 g, 5.40 mmol). The product was
a greenish-blue powder and the yield was 1.28 g (65 %). The
powder formed greenish-blue crystals on recrystallization from
THF-MeOH (2:1). FTIR (cm-1): 3448 (w), 2927 (s), 2871 (s),
1589 (vs), 1416 (s).
RESULTS AND DISCUSSION
Synthesis and structural elucidation: There were several
methods reported in the literature for the synthesis of metal(II)
carboxylates18-20.All complexes in this paper were obtained in
good yields (> 50 %) from the reaction of sodium alkanoate
(either 2-hexyldecanoate or heptanoate) with the corresponding
metal(II) chloride. The structural formulae for complexes 1-3
(Fig. 1) were deduced from a combination of CHN elemental
analyses, FTIR and UV-VIS spectroscopies and room-tempe-
rature magnetic susceptibility data.Additionally, the structural
formula of 1 was ascertained by comparison with the analytical
data obtained from crystals of 4.
Synthesis
[Cu2(L)4(LH)2] (1): A solution of Na2CO3 (5.42 g, 51.1
mmol) in distilled H2O (150 mL) was added to a solution of
LH (26.22 g, 102.3 mmol) in EtOH (95 %, 100 mL). The
cloudy suspension formed was magnetically stirred and gently
heated on a hot plate until a clear colourless solution formed
(2 h). The solvents were removed on a rotary evaporator to
give NaL as a colourless viscous liquid. The yield was 25.61 g
(90 %). FTIR (cm-1): 3366 (m, br), 2956 (m), 2924 (vs), 2855
(s), 1540 (vs), 1459 (m), 1409 (s).
HO
C
O
O
O
Cu
O
O
O
O
Cu
NaL (12.67 g, 45.5 mmol) was then dissolved in hot
aqueous EtOH (1:1, 200 mL). CuCl2.2H2O (3.88 g, 22.8 mmol)
was gradually added to the solution and the mixture magneti-
cally stirred, heated for 0.5 h and left to cool to room temperature.
The oily dark green product formed was extracted using CHCl3
and the solvent evaporated off on a rotatory evaporator. The
viscous dark green liquid obtained was further dried in an oven
at 60 ºC. The yield was 7.71 g (61 %). Anal. calcd. (%) for
C97H192O12Cu2: C, 69.4; H, 11.4. Found (%): C, 69.5; H, 11.1.
FTIR (cm-1): 3393 (w), 2955 (s), 2855 (s), 1706 (m), 1609 (s),
1580 (m), 1455 (s), 1419 (s), 462 (vs).
O
O
O
C
OH
(a)

Vol. 26, No. 4 (2014)
Magnetic Liquid Complexes of 2-Hexyldecanoato Ligand with Cu(II), Ni(II) and Co(II) Ions 989
O
OH2
OH2
M
O
O
O
O
M
X
O
O
X
O
(b)
(a)
Fig. 1. Proposed structural formulas: (a) 1 ([Cu2(L)4(LH)2], (b) 2 (M =
Ni(II), X = EtOH) and 3 (M = Co(II), X = H2O. The solvent
molecules are not shown
For 1, the chemical formula deduced from the elemental
analytical data is [Cu(L)2(LH)]. Its FTIR spectrum shows peaks
(in cm-1) for -CH2 at 2955 (νasym) and 2855 (νsym), -COO at
1609 (νasym) and 1455 (νsym) and for intramolecularly H-bonded
-COOH at 3393 and 1706. Accordingly, the ∆COO value (∆COO
= νas,COO - νs,COO) is 154 cm-1, suggesting a bridging carboxylate
ligand1,2 when compared to that of NaL (∆COO = 131 cm-1). Its
UV-visible spectrum shows a broad d-d band at 667 nm (εmax
= 436 M-1 cm-1), a weak shoulder at 860 nm (ε = 131 M-1 cm-1)
and another shoulder at 375 nm (ε = 144 M-1 cm-1). The result
suggest a square pyramidal binuclear Cu(II) complex21. The
electronic transitions, based on the C4v point group at each
Cu(II) centre, are assigned to 2B2 → 2B1, 2A1 → 2B1 and 2E →
2B1, respectively19. The value of its effective magnetic moment
(µeff) was 2.6 BM. It was calculated using the equation: µeff =
(b)
Fig. 2. (a) Crystal structure and (b) packing diagram of
[Cu2(CH3(CH2)5COO)4] (4)
TABLE-1
CRYSTAL DATA AND STRUCTURE
REFINEMENT DETAILS FOR COMPLEX 4
2.83[T(χMcorr - Nα)]1/2, where χM is the corrected molar
corr
Formula
Formula weight
Crystal system
Space group
C14H26O4Cu
321.89
Triclinic
P-1
100(2)
magnetic susceptibility, T is the absolute temperature (298 K)
and Nα is the temperature-independent paramagnetism of
Cu(II) (120 × 10-6 cm3 mol-1 per dinuclear complex). The
experimental value is slightly lower than the expected spin-
only value for two unpaired electrons (2.83 BM) from a
dinuclear Cu(II) complex (d9), suggesting a weak antiferro-
magnetic interaction between the two Cu(II) centres. From
the above analytical results, it is proposed that 1 may adopt
the paddle-wheel structure similar to other Cu(II) carboxy-
lates18,19, but with a 2-hexyldecanoic acid molecule at each
axial position (Fig. 1a).
T (K)
a (Å)
b (Å)
c (Å)
α (º)
β (º)
5.1486(3)
8.3829(5)
18.6623(11)
81.5410(10)
83.5910(10)
76.1890(10)
2
γ (º)
Z
V (Å3)
771.21(8)
F(000)
342
In order to ascertain the proposed structural formula of 1,
a reaction was carried out between CH3(CH2)5COONa and
CuCl2·2H2O under similar conditions. Bluish-green crystals
of [Cu2(CH3(CH2)5COO)4] (4) deposited from a solution of
the powder obtained in THF-MeOH (2:1). Its structure and
packing diagram are shown in Fig. 2 and the crystal data and
structure refinement details are shown in Table-1.
The X-ray data for complex 4 clearly shows the expected
paddle-wheel structure. Its FTIR (∆COO = 173 cm-1) and UV-
visible (λmax = 671 nm, εmax = 375 M-1 cm-1) spectral results are
similar to those of 1. Hence, we are quite confident that both
complexes have similar structure. The significantly lower µeff
value for 4 (1.6 BM) compared to 1 (2.6 BM) may arise from
the coordination of 2-hexyldecanoic acid molecules at the two
axial positions in the latter complex, leading to a more planar
geometry at both Cu(II) centres.
1.11 - 28.37
1.140
R1 = 0.0231, wR2 = 0.0726
R1 = 0.0274, wR2 = 0.0836
θ range (º)
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
For complex 2, the structural formula proposed from the
combined analytical data is [Ni2(L)4(EtOH)2(H2O)2].H2O.CHCl3
(Fig. 1b). Its FTIR spectrum shows the expected peaks (in cm-1)
for -OH at 3368, -CH2 at 2955 (νasym) and 2855 (νsym), -COO
at 1610 (νasym), 1587 (νasym), 1465 (νsym) and 1414 (νsym).Accor-
dingly, the ∆COO values are 122 and 196 cm-1, suggesting
bidentate chelating and monodentate bridging carboxylate,
respectively. Its UV-visible spectrum shows a weak d-d band
at 681 nm (εmax = 7.3 M-1 cm-1), suggesting a highly symmetrical
octahedral geometry at Ni(II) centres 22. The µeff value of 4.6
BM at 298 K is slightly lower than the expected spin-only

990 Abdullah et al.
Asian J. Chem.
value for four unpaired electrons (4.89 BM) from a dinuclear
Ni(II) octahedral complex (d8). This indicates a weak antiferro-
magnetic interaction between the two metal centres. The
analytical results seem to suggest that 2-hexyldecanoato ligand
exerted a weak ligand field effect on the d orbitals of Ni(II) in
this complex.
As similarly argued for 1 and 2, the proposed structural
formula for 3 is [Co2(L)4(H2O)4]·3H2O. The ∆COO values of
147 and 217 cm-1 from its FTIR spectrum suggest bidentate
chelating and monodentate bridging carboxylate, respectively.
The broad d-d band at 617 nm (εmax = 215 M-1cm-1) from its
UV-visible spectrum suggest that Co(II) has similar geometry
as 2 (Fig. 1b). The µeff value of 6.46 BM at 298 K is slightly
lower than the expected spin value for six unpaired electrons
(6.93 BM) for a dinuclear high-spin Co(II) octahedral complex
(d7). Accordingly, there is also a weak antiferromagnetic
interaction between the two Co(II) centers22.
solvents (THF and CHCl3) with different concentrations, under
different experimental conditions and using two different
instruments. Surprisingly in all cases, neither cathodic nor
anodic peaks were detected. It is infered from these results
that the structures of these complexes were stable in these
solvents and that the 2-hexyldecanoato ligand was an effective
‘insulating shield’, preventing the central metal(II) ions from
being electrochemically reduced or oxidized.
Conclusion
Complexes formed from 2-hexyldecanoato ligand with
Cu(II) (1), Ni(II) (2) and Co(II) (3) ions existed as coloured
viscous liquids at room temperature (30 ºC). These complexes
were dinuclear, paramagnetic with weak antiferromagnetic
interactions between the two metal(II) centres and thermally
and electrochemically stable. Complex 3 was high spin, while
1 was mesogenic exhibiting a hexagonal columnar mesophase
(Colh) at -23.2 ºC.
Thermal properties: The TGA trace for 1 shows that its
decomposition temperature was 200 ºC, with a total weight
loss of 89.7 % (calculated, 89.8 %). The excellent agreement
between the experimental and calculated values further supports
the proposed formula for the complex. In comparison, 4 was
found to decompose at a higher temperature of 280 ºC, which
was similar to that of [Cu2(C6H5COO)4(H2O)2] (Tdec = 280 ºC)
reported by Siqueira et al.23. The lower thermal stability of 1
is likely due to the condensation of H2O from the axially coordi-
nated 2-hexyldecanoic acid molecules. As expected from a
viscous liquid sample, its DSC trace does not show any peaks
in the temperature range -30 ºC to 50 ºC. However, it is noted
that a needle-like growth developed in the liquid after six
months at room temperature. This may be due to crystallization
from the isotropic liquid. Viewed under a polarising optical
microscope (POM), the photomicrographs of the needles and
the Colh mesophase of the viscous liquid at -23.2 ºC are shown
in Fig. 3.
ACKNOWLEDGEMENTS
This work was funded by the Fundamental Research Grant
Scheme from Malaysia Ministry of Higher Education (FP008-
2011A) and the Universiti Malaya High Impact Research Grant
(UM.C/625/1/HIR/MOHE/5).
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at -23.2 ºC
The complex 2 was found to decompose at about the same
temperature as 1 (200 ºC), with a total weight loss of 88.2 %
(calculated: 89.5 %). It is noted that its decomposition tempe-
rature is lower than that reported for [Ni(C6H5COO)2] (Tdec
=
230 ºC)23. In contrast, 3 decomposed at a lower temperature
(150 ºC) with a total weight loss of 92.3 % (calculated: 92.4
%). The DSC traces for both complexes do not show any peaks,
while the POM did not detect any mesophases on cooling from
room temperature to -30 ºC. These results further support the
proposed isotropic structures as shown in Fig. 1b.
Redox properties: The cyclic voltammetric studies (CV)
for complexes 1-4 were performed repeatedly in two different