Microwave-assisted synthesisof monodisperse nickel nanoparticles using a complex of nickel formate with long-chainamine ligands

Article abstract of DOI:10.1246/bcsj.82.1044

We have successfully prepared face-centered cubic(fcc) Ni nanoparticles by the intramolecular reduction of Ni2+ ion contained inaformate complex having long-chainamine ligands within an extremely short time under microwave irradiation. Formate ion coordinated to Ni²⁺ ion acted as a reducing agent for Ni²⁺ inthis reaction and finally decomposed to hydrogen and carbon dioxide. Ligation of the long-chainamine lowered the reaction temperatures by decreasing the energy barrier required for the reduction of the nickelformate complex. Monodisperse Ni nanoparticles with average sizes of 43, 71, and 106 nm were prepared using the complex of nickelformate with several chain lengths of alkylamines such as oleylamine (=(Z)-9-octadecenylamine), myristylamine (=tetradecylamine), and laurylamine (=dodecylamine), respectively. The relationship between the magnetic properties and the particlesizes of Ni nanoparticles isdiscussed.

Full text of DOI:10.1246/bcsj.82.1044

1044 Bull. Chem. Soc. Jpn. Vol. 82, No. 8, 1044–1051 (2009)
© 2009 The Chemical Society of Japan
Microwave-Assisted Synthesis of Monodisperse Nickel Nanoparticles
Using a Complex of Nickel Formate with Long-Chain Amine Ligands
Tomohisa Yamauchi,1 Yasunori Tsukahara,*1 Tetsuo Sakamoto,2 Takumi Kono,2
Makoto Yasuda,3 Akio Baba,3 and Yuji Wada*4
1Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871
2Nippon Steel Chemical Co., Ltd., 14-1 Sotokanda 4-chome, Chiyoda-ku, Tokyo 101-0021
3Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871
4Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-12 Ookayama, Meguro-ku, Tokyo 152-8552
Received March 19, 2009; E-mail: ytsuka@jrl.eng.osaka-u.ac.jp, yuji-w@apc.titech.ac.jp
We have successfully prepared face-centered cubic (fcc) Ni nanoparticles by the intramolecular reduction of Ni2+
ion contained in a formate complex having long-chain amine ligands within an extremely short time under microwave
irradiation. Formate ion coordinated to Ni2+ ion acted as a reducing agent for Ni2+ in this reaction and finally decomposed
to hydrogen and carbon dioxide. Ligation of the long-chain amine lowered the reaction temperatures by decreasing the
energy barrier required for the reduction of the nickel formate complex. Monodisperse Ni nanoparticles with average sizes
of 43, 71, and 106 nm were prepared using the complex of nickel formate with several chain lengths of alkylamines such
as oleylamine (=(Z)-9-octadecenylamine), myristylamine (=tetradecylamine), and laurylamine (=dodecylamine),
respectively. The relationship between the magnetic properties and the particle sizes of Ni nanoparticles is discussed.
Nanoparticles of ferromagnetic metals such as Fe, Co, and
Ni have been extensively studied for catalysts, permanent
magnets, magnetic fluids, and magnetic recording media.1-9
Their physical and chemical properties depend on their sizes
and shapes.10-12 Among these metals, Ni nanoparticles have
attracted much attention in application to conductive adhesives
and electrodes in multilayer ceramic capacitors,13-15 because of
low cost than silver nanoparticles and high chemical stability
compared to copper nanoparticles. Recently, unagglomerated
pure Ni nanoparticles for technological applications are
required to have fine particle sizes between 50-100 nm with a
narrow size distribution as well as high crystallinity, because
nanoparticles with sizes below 20 nm suffer from low oxidation
resistance and nanoparticles with sizes over 100 nm have a
problem of low dispersibility. Therefore, a number of physical
and chemical techniques have been developed to produce Ni
nanoparticles.
The reactions in gas-solid phases such as chemical vapor
deposition of NiCl2,16 frequently induce agglomeration of the
produced particles, leading to Ni particles having large sizes
between submicro- and micrometer scales. Several solution-
phase reactions such as sonochemical deposition,17 micro-
emulsion synthesis,18 several chemical reduction methods,19-23
polyol methods,24-29 and hydrothermal reactions,30 have been
studied to produce fine particles. In these solution-phase
methods, monodispersed Ni nanoparticles having small sizes
between a few nanometers and 50 nm were obtained by adding
surface modifying agents. Both phase methods have displayed
difficult problems in preparing monodispersed Ni particles with
sizes in the middle range between 50 nm and several hundred
nanometers. A new process is desired for preparation of Ni
nanoparticles having sizes between 50-100 nm.
Now we have selected the intramolecular reduction of nickel
formate dihydrate to avoid adding other toxic reducing agents
for the synthesis of Ni nanoparticles. Ni particles have been
prepared by thermal decomposition of a neat formate salt at
temperatures above 523 K.31-34 In the intramolecular reduction
method using nickel formate dihydrate, formate ion acted as a
reducing agent for Ni2+ and finally decomposed to hydrogen
and carbon dioxide according to the following reactions.
2HCOOꢀ ! 2CO2 þ H2 þ 2eꢀ
Ni2þ þ 2eꢀ ! Ni0
ð1Þ
This pyrolytic synthesis gave Ni powders with a wide size
distribution in the micrometer range in spite of the molecular
level reduction of Ni2+ ion producing Ni atom in the initial
stage of the reaction. The large particle sizes obtained in this
method could be attributed to aggregation of the particles while
the reaction system was heated at high temperatures. Our
strategies employed for controlling the particle sizes were as
described below: 1) The reaction should be carried out in
reduced concentration of the precursor using a solvent to
decrease the collision frequencies of Ni0 nuclei. 2) The particle
sizes can be controlled by adding surface modifying agents to
prevent excess particle growth. 3) The reaction temperature
should be lowered. Then we have applied the ligation of nickel
formate with alkylamines in order to change the geometric and
electronic structures of the complex.
Reactants are directly heated under microwave irradiation
through the interaction of the oscillating electric and magnetic
Published on the web August 7, 2009; doi:10.1246/bcsj.82.1044

T. Yamauchi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1045
OH2
Ni2+
OH2
R
H
H
O
O
C
C
long-chain amine
393 K
Octanol, MW
Ni2+
O
O
O
O
463 K, 10 min
C
C
O
O
H
H
R
* R = H2NCnHm
[Ni(HCOO)2(H2O)2]
[Ni(HCOO)2(CnHmNH2)2]
Ni nanoparticles
Scheme 1. Synthesis of Ni nanoparticles using nickel formate complex with long-chain amine ligands.
(EYELA MAZALA-Z, Tokyo Rikakikai Co., Ltd.) equipped with
a glass rod having a poly(tetrafluoroethylene) (PTFE) rotor blade at
the end.
fields with substances and are quickly and uniformly heated.
Therefore, nucleus growth should simultaneously and homo-
geneously occur in the entire vessel and particles with a narrow
size distribution can be obtained within a short time.35-37
Recently, preparation of monodispersed Ag and Cu nano-
particles with a narrow size distribution has been demonstrated
by using microwave-assisted alcohol reduction.38-40 Wada et al.
and Tsuji et al. reported that cubic or spherical Ni nanoparticles
with narrow size distribution (7 or 6 nm) were rapidly prepared
by the polyol method under microwave irradiation through
reduction of Ni(OH)2 with ethylene glycol with or without
using Pt catalyst as a nucleation agent, respectively.41,42 We
have selected a microwave-assisted method for the preparation
of Ni nanoparticles in this work.
Combination of the above-described three strategies and the
microwave method has enabled preparation of fcc Ni nano-
particles having the desired particle sizes by the intramolecular
reduction of Ni2+ ion contained in its formate complex with
long-chain amine ligands within a short time at 463 K. The
sizes of the obtained particles were controlled using various
long-chain amine ligands. The coordination environment of
Ni2+ ion should be changed by ligation with the long-chain
amines to nickel formate, enhancing the reactivity of the
complex.
The synthesis of Ni nanoparticles was performed according to
Scheme 1. Nickel formate dihydrate (5 mmol) and oleylamine
(50 mmol) were mixed and then heated at 393 K for 10 min. The
color of the reaction solution changed in the same way as the case
of preparing the complex of [Ni(HCOO)2(C12H25NH2)2] described
above, confirming the complexation of nickel formate with
oleylamine ligands. After cooling to room temperature, 1-octanol
(60 mL) was added to the solution. This solution in a quartz vessel
was heated at a rate of 40 K min¹1 under microwave irradiation and
then left to stand at 463 K for 10 min under nitrogen atmosphere.
The reaction solution was cooled to room temperature. The
obtained particles were centrifuged and washed in methanol to
remove the residual long-chain amines and dried under vacuum at
334 K for 4 h. Black Ni nanoparticles were obtained (denoted as
A). Temperature profiles of this reaction with time and microwave
(MW) power are shown in Figure S1 (Supporting Information).
Nanoparticles using other long-chain amine ligands such as
myristylamine and laurylamine were obtained under the same
reaction conditions (denoted as B and C, respectively).
Instruments.
Fourier transform infrared spectroscopy was
applied to the nickel complexes in KBr pellets using a Perkin-
Elmer 2000FT-IR spectrometer. UV-vis spectra were obtained
using a V-570 spectrophotometer (JASCO Co.). The sizes and
morphologies of Ni nanoparticles were characterized by a trans-
mission electron microscope (TEM) at 200 kV with a Hitachi H-
800 (Hitachi High-Technologies Co.). The crystal phase of the
powder was analyzed by using a MultiFlex (Rigaku Co.) with a
Cu K¡ radiation source in the range of the 2ª Bragg angles = 20-
90° at 40 kV and 40 mA. The amounts of surface modifying agents
on the surface of Ni nanoparticles were determined by thermo-
gravimetric analysis (TGA) using a Shimadzu TGA-50 analyzer.
Experimental
Materials. Nickel formate dihydrate, 1-octanol, and tetra-
ethylene glycol (=bis[2-(2-hydroxyethoxy)ethyl] ether) were
purchased from Kishida Chemical Co., Ltd. Laurylamine and
myristylamine were purchased from Tokyo Chemical Industry Co.,
Ltd. Oleylamine was purchased from Aldrich Co., Ltd. These
reagents were used as supplied.
Preparation of Ni Complex [Ni(HCOO)2(C12H25NH2)2].
The complex of [Ni(HCOO)2(C12H25NH2)2] was prepared from
nickel formate dihydrate (1 mmol) and laurylamine (10 mmol)
in chloroform (20 mL) under heating at 393 K for 20 min. The
reaction solution changed from a greenish suspending solution to a
deep green homogeneous solution. While cooling to room tem-
perature, the color of the solution changed to turquoise blue.
The reaction solution was left at rest. After several weeks, blue
crystals were collected. Elemental analysis was performed with a
YANAKO CHN coder MT-5 (YANAKO). Analytical data: Calcd
for C26H56N2O4Ni: C, 60.12; H, 10.87; N, 5.39%. Found: C, 59.87;
H, 10.69; N, 5.25%.
¹1
TGA was performed at a heating rate of 5 K min in nitrogen
atmosphere at a flow rate of 100 mL min¹1. Magnetic susceptibility
data were obtained in various applied fields between ¹30 and
30 kOe by using a SQUID susceptometer (MPMS-5S, Quantum
Design Co.).
Density Functional Theory (DFT) Calculation. DFT cal-
culation was performed by using the Spartan’ 06 program
(Wavefunction, Inc. Irvine, CA). Fully optimized geometries were
obtained for the Ni2+ complexes in the ground state at DFT
B3LYP/6-31G** level. Electrostatic potential maps of the Ni2+
complexes in the ground state were obtained in the calculation for
the obtained equilibrium geometry at the DFT B3LYP/6-31G**
level.
Preparation of Ni Nanoparticles. Microwave heating was
carried out by using a multi-mode 2.45 GHz microwave apparatus
at 770 W (® Reactor, Shikoku Instrumentation Co., Ltd.). The
temperatures of the reaction solution were measured with a fiber-
optic thermometer (AMOTH TM-5886, Anritsu Meter Co., Ltd.)
directly inserted into the solution. The agitation control was carried
out at an agitation speed of 200 rpm with an agitator motor
Results
Microwave-Assisted Synthesis of Ni Nanoparticles from
Nickel Formate Complex with Long-Chain Amine Ligands.
The powder X-ray diffraction pattern of sample A is shown in

1046 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
Microwave-Assisted Method of Ni Nanoparticles
Figure 1. X-ray diffraction pattern of sample A.
Figure 3. TGA measurement of sample A at a heating rate
¹1
¹1
of 10 K min under nitrogen flow of 100 mL min
.
The selected area electron diffraction (SAED) pattern of
sample A (Figure 2a, inset) was a clear fcc Ni pattern. No
ED pattern for NiO was observed. The average particle sizes
were determined to be 43, 71, and 106 nm for samples A
(oleylamine, C18H37N), B (myristylamine, C14H31N), and C
(laurylamine, C12H27N), respectively, demonstrating that the
particle sizes were controlled by changing the chain lengths of
alkylamines under the same reaction conditions.
In order to determine the contents of the organic components
on the surface of Ni nanoparticles, TGA measurements were
carried out. Figure 3 shows the weight loss curve of sample A.
The contents of the long-chain amines in samples A calculated
on the basis of the weight loss were about 1.5 wt %. The
contents of them in samples B and C were about 1.4 and
1.1 wt %, respectively. These results indicated that the small
amounts of the long-chain amines were contained in the
obtained powders. The amines remaining in the samples should
be strongly adsorbed on the surface of the nanoparticles, since
weakly adsorbed amines on the surface should be easily
removed by the washing process.
For comparing with the above synthesis in the presence of
amines, the nanoparticles were prepared by the reaction of
nickel formate dihydrate (5 mmol) in tetraethylene glycol
(60 mL) under microwave irradiation at 770 W at a heating rate
of 40 K min¹1 without adding amines and left to stand at 503 K
for 10 min under nitrogen atmosphere. This experiment without
addition of amines needed a temperature higher by 40 K due to
the lack of reaction enhancement by adding amines and gave
coarse particles with an average particle size of 250 nm
(denoted as D), as shown in Figure 4a. Therefore, long-chain
amines should act effectively as surface modifying agents to
control the particle growth and to suppress aggregation in the
reaction solution.
Figure 2. TEM images and particle size distributions of
samples A (a), B (b), and C (c). Histograms of the size
distributions of the samples were made by the diameters of
randomly selected 200 particles. Inset of Figure 2a is the
SAED pattern of the whole image of sample A. Debye
rings are assigned to (111) (*), (200) (**), (220) (***), and
(311) (****), respectively.
Figure 1. All the peaks of sample A produced by microwave
irradiation were assigned to the pure phase of fcc Ni, including
no other components such as NiO and Ni3C and showing high
crystallinity. The patterns of samples B and C showed the same
fcc Ni diffraction patterns as well as that of sample A. The
TEM images of samples A, B, and C are shown in Figure 2.
Ni nanoparticles were synthesized in tetraethylene glycol
under the same reaction conditions as sample A using oleyl-
amine (denoted as E). The TEM image of sample E is shown in
Figure 4b. The particles were obtained as an agglomeration of
2-3 particles. Long chain alcohols seem suitable as solvents for
preparing the monodispersed Ni nanoparticles.

T. Yamauchi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1047
Figure 4. TEM images of samples D (a) and E (b).
Figure 6. Temperature dependence of the magnetization
for samples A (43 nm) in a ZFC and a FC process
under various fields. [ (ZFC)- (FC) plots at 100 Oe,
(ZFC)- (FC) plots at 500 Oe, and (ZFC)- (FC) plots
at 1000 Oe].
Figure 5. TEM image and particle size distribution of
sample F. Histogram of the size distribution of sample F
was made by measuring the diameters of randomly
selected 200 particles.
Comparison of Microwave Heating with Conventional
Heating. We examined the differences between the micro-
wave-assisted method and the conventional method in the
synthesis of Ni nanoparticles using oleylamine ligands. In the
case of conventional heating, Ni nanoparticles were prepared in
the same conditions except for the heating rate. The reaction
¹1
solution was heated at a rate of ca. 5 K min in an oil bath,
which was the maximum rate available by heating with an oil
bath. Although Ni nanoparticles were obtained (denoted as
sample F), the resulting solution remained the blue color of the
Ni2+ complex, indicating that the reaction did not proceed to
completion. The yield Ni nanoparticles were 95.6% and 67%
for the microwave-assisted method and the conventional
method, respectively. The TEM image of sample F is shown
in Figure 5. The average particle size was determined to be
about 70 nm (standard deviation · = 32.5 nm). Larger agglom-
erated particles were observed for the particles of sample F
than those obtained under microwave irradiation (43 nm,
· = 9.7 nm). In the microwave-assisted method, the reaction
solution was quickly and uniformly heated under microwave
irradiation. Therefore, nucleus growth should simultaneously
occur in the entire vessel and particles with a narrow size
distribution should be obtained within a short time.41-43
Magnetic Properties. The magnetic properties of the Ni
nanoparticles were carried out using a SQUID susceptometer.
The temperature dependence of the magnetization of sample A
between 10 and 400 K under various fields is shown in
Figure 6. In the case of a zero-field-cooling (ZFC) process, the
magnetization of the Ni nanoparticles under a 1000 Oe field
increased with the rise in temperature. The magnetization
reached the maximum point at 250 K, called the blocking
temperature (TB), and then decreased at temperatures above TB.
The magnetization plots in a field-cooling (FC) process under
Figure 7. Magnetization versus applied fields for sample A
at 300 K.
the same applied field showed different behaviors compared to
those in a ZFC process. At temperatures above TB, the plots in
the FC process overlapped with those in the ZFC process.
When the magnitude of the applied fields for the ZFC-FC
process was varied from 100 to 500 and 1000 Oe, the blocking
temperature shifted down from around 400 to 280 and 250 K,
respectively. Magnetization versus the applied field plot at
300 K for sample A is shown in Figure 7. The average particle
sizes, saturation magnetizations and coercive forces of samples
A, B, and C are listed in Table 1. Sample A showed a
saturation magnetization (·s) 43.7 emu g¹1, Samples B and C
had higher ·s (·s = 48.8 and 50.2 emu g¹1) than those of
sample A, which was understandable taking the relationship of
the average particle sizes of Ni nanoparticles into account.

1048 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
Microwave-Assisted Method of Ni Nanoparticles
Table 1. The Particle Sizes, Coercive Forces Hc, and
Saturation Magnetization ·s of Samples A, B, and C at
300 K
¹1
Particle size
/nm
·s/emu g
Hc/Oe
at 30 kOe
Sample A
Sample B
Sample C
43
71
106
61
55
81
43.7
48.8
50.2
Discussion
The Formation of Nickel Formate Complexes with Long-
Chain Amines. At the beginning of the synthetic process of
Ni nanoparticles having laurylamine as a surface modifying
agent according to Scheme 1, nickel formate dihydrate
(5 mmol) and laurylamine (50 mmol) were heated at 393 K
for 10 min in order to coordinate laurylamine to Ni2+ ion. The
changes of the color of the solution and the UV-vis spectra
of the precursor in solution were coincident with those
observed in the preparation of the isolated complex ([Ni-
(HCOO)2(C12H25NH2)2]), indicating the formation of a com-
mon species. Therefore, the properties of the precursor for the
synthesis of Ni nanoparticles could be examined from those of
the isolated [Ni(HCOO)2(C12H25NH2)2]. We discussed the
relationship between the intramolecular reduction at lower
temperatures and the properties of the precursor with lauryl-
amine ligands.
Figure 8. FT-IR spectra of nickel formate dihydrate (top)
and [Ni(HCOO)2(C12H25NH2)2] (bottom) in KBr pellets.
Determination of the Coordination Environment in
The composition of the isolated complex ([Ni(HCOO)2-
(C12H25NH2)2]) was determined by elemental analysis. How-
ever, the geometric structure of the complex was not
determined by X-ray structure analysis because a single crystal
of the complex could not be obtained. Therefore, we examined
the structure by FT-IR and UV-vis spectra.
[Ni(HCOO)2(C12H25NH2)2] by UV-Vis Spectra:
The
coordination environments of Ni2+ complexes can be deter-
mined by comparing the three absorption bands of the first
spin-allowed, the second spin-allowed, and the spin-forbidden
3
3
d-d transitions (3A2 ¼ 3T2, A2 ¼ 3T1, and A2 ¼ 1E) with
those of Ni2+ complexes having known structures. Figure 9
shows UV-vis spectra of nickel formate dihydrate in water and
[Ni(HCOO)2(C12H25NH2)2] in dichloromethane. The spectrum
of nickel formate dihydrate closely resembled that of NiSO4¢
6H2O, showing that both complexes of nickel formate
dihydrate and NiSO4¢6H2O have the same coordination
environment of six oxygen atoms (denoted as O6 type). In
contrast, the spectrum of [Ni(HCOO)2(C12H25NH2)2] was
different from both spectra of nickel formate dihydrate and
NiSO4¢6H2O. The first spin-allowed d-d transition band
(3A2 ¼ 3T2) of [Ni(HCOO)2(C12H25NH2)2] shifted to high
Determination of Ligands Contained in [Ni(HCOO)2-
(C12H25NH2)2] by FT-IR Spectra:
Figure 8 shows the
differences in the FT-IR spectra between nickel formate
dihydrate itself and [Ni(HCOO)2(C12H25NH2)2]. The spectrum
¹1
of nickel formate dihydrate showed a peak at 1570 cm
attributed to the C=O asymmetric stretching vibration of
formate ion, and a broad peak at 3100-3400 cm¹1 attributed to
the O-H stretching vibration of crystal water. In the case of
[Ni(HCOO)2(C12H25NH2)2], the peak of the O-H stretching
vibration of crystal water disappeared. The sharp peaks at
¹1
¹1
2950-2850 and at 3325 and 3283 cm were assigned to the
energy by ca. 664 cm compared to that of nickel formate
stretching vibration of the aliphatic C-H group and the N-H
asymmetric/symmetric stretching vibration of the laurylamine
ligands, respectively. A sharp peak at 1620 cm was assigned
dihydrate (see Supporting Information Table S2). Meanwhile,
the spectrum of [Ni(HCOO)2(C12H25NH2)2] closely resembled
that of [Ni(acac)2(tmen)],44-46 having the coordination environ-
ment of two nitrogen and four oxygen atoms (denoted as N2O4
type), (acac = acetylacetonato, tmen = N,N,N¤,N¤-tetramethyl-
ethylenediamine). Therefore, the similar spectra indicated that
the complex of [Ni(HCOO)2(C12H25NH2)2] had an N2O4
coordination environment. The results of elemental analysis,
FT-IR, and UV-vis spectra made it clear that the isolated
complex ([Ni(HCOO)2(C12H25NH2)2]) was a six-coordinated
complex composed of two bidentate formate ions and two
monodentate laurylamines.
¹1
to the N-H bending vibration of the laurylamine ligands. It
indicated that laurylamine ligands were coordinated to the Ni2+
ion. In addition, the shift of the C=O asymmetric stretching
¹1
vibration to high wavenumber by ca. 15 cm was observed
for [Ni(HCOO)2(C12H25NH2)2]. The result of the FT-IR
spectra confirmed that the isolated complex ([Ni(HCOO)2-
(C12H25NH2)2]) contained the ligands of formate ion and
laurylamine. Furthermore, by considering both results of
elemental analysis and FT-IR spectra, the structural formula
of the isolated complex was determined to be [Ni(HCOO)2-
(C12H25NH2)2].
Long-chain amine providing a pair of electrons from the
nitrogen at the end of an alkyl chain could be strongly

T. Yamauchi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1049
19.5 wt %). The second weight loss (48.5 wt %) observed at
520-553 K was attributed to the decomposition of formate ion
(the theoretical value, 48.7 wt %). The residual weight content
(31.4 wt %) corresponded to the theoretical Ni content
(31.8 wt %) in nickel formate dihydrate.
Nickel powder was formed according to the following.32-34
NiðHCOOÞ 2H O ! NiðHCOOÞ þ 2H O
ð2Þ
ð3Þ
2ꢁ
2
2
2
NiðHCOOÞ2 ! Ni0 þ 2CO2 " þH2 "
In the case of [Ni(HCOO)2(C12H25NH2)2], the weight
(87.3 wt %) continuously decreased up to 530 K. The residual
weight (12.7 wt %) corresponded approximately to the theoret-
ical Ni content (11.6 wt %). The decomposition temperature, at
which the organic molecules in [Ni(HCOO)2(C12H25NH2)2]
were removed and only Ni0 remained, was lower by 23 K than
that of nickel formate dihydrate. These results indicated that
long-chain amine ligands coordinated to Ni2+ ion enhanced the
decomposition of formate ion and decreased the energy
required for the intramolecular reduction of nickel formate
complexes.
Effect of Ligation of the Long-Chain Amines on the
Figure 9. UV-vis spectra of nickel formate dihydrate (i),
nickel sulfate hexahydrate (ii), [Ni(acac)2(tmen)] (iii), and
[Ni(HCOO)2(C12H25NH2)2] (iv) in water (top) and di-
chloromethane (bottom), respectively (acac = acetylace-
tonato, tmen = N,N,N¤,N¤-tetramethylethylenediamine).
Decomposition of the Nickel Formate Complex:
The
formate ion is ordinarily stabilized by the counter cation (Ni2+
ion). Therefore, diminishment of the binding power between
Ni2+ and formate ions is required for decomposition at lower
temperatures. The lengthening of the bond distance of Ni-
O(formate ligand) in the Ni2+ complex is needed for the
reaction at lower temperatures because the bond energy of
Ni-O(formate ligand) is decreased by increase of the bond
distance.
Baba and Yasuda et al. have reported that bromide
coordination to tin enolate increases the nucleophilicity and
decreased Lewis acidity of the metal center promoting the
reaction of enolate ligand with organic halide.47,48 The
coordination of halide ligand as an electron donor to the tin
enolate complex increased the electron density the metal center
and the bond distance of Sn-O(enolate ligand). Their obser-
vation led to an idea that the coordination of the electron-
donating ligands to nickel formate complex increased the
electron density of Ni2+ ion and the bond distance of Ni-
O(formate ligand). Ivaniková et al. have succeeded in prepa-
ration of nickel formate complexes containing N-donor base
ligands and reported that the coordination of imidazole ligands
increased the distance of Ni-O(formate ligand).49 Now, the
effect of long-chain amine ligands as N-donor base ligands on
the nickel formate complexes can be understood as increasing
the distance of Ni-O(formate ligand) and lowering the
decomposition temperature of the formate complex. We have
calculated the bond distance of Ni-O(formate ligand) of the
nickel formate complexes by DFT calculation.
Figure 10. TGA measurement of nickel formate dihydrate
(a) and [Ni(HCOO)2(C12H25NH2)2] (b) at a heating rate of
¹1
¹1
10 K min under a nitrogen flow of 100 mL min
.
coordinated with Ni2+ ion to form the octahedral Ni2+
complex. Furthermore, the amine could convert very insoluble
nickel formate dihydrate into a soluble complex. As a result,
the unagglomerated particles were obtained by preparing in
homogeneous solution.
Low-Temperature Thermal Decomposition of Formate
Ion in the Ni2+ Complex with Long-Chain Amine Ligands.
Experimental Evidence for Low-Temperature Thermal
Decomposition: Figure 10 shows the weight loss curves of
nickel formate dihydrate and [Ni(HCOO)2(C12H25NH2)2] under
nitrogen atmosphere in TGA measurements. The thermal
decomposition of nickel formate dihydrate consisted of two
stages. The first weight loss (20.2 wt %) observed at 410-480 K
could be attributed to dehydration (the theoretical value,
DFT Studies of the Structures of the Nickel Complexes.
The DFT calculations were carried out to estimate the charge
densities and the bond distances of Ni-L(ligand) in both
[Ni(HCOO)2(C12H25NH2)2] and nickel formate dihydrate. The
three-dimensional electrostatic potential views and the struc-
tures of both complexes obtained by the DFT calculations at the
B3LYP/6-31-G** level are depicted in Figure 11. The bond
distances obtained from the DFT calculations are listed in
Table 2. As shown in Figure 11, there was a large difference in

1050 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
Microwave-Assisted Method of Ni Nanoparticles
thank Dr. T. Sakata and Prof. H. Mori, Research Center for
Ultra-High Voltage Electron Microscopy, Osaka University and
Ms M. Takahashi at Wavefunction, Inc. Irvine, CA for
supporting DFT calculation.
Supporting Information
Time profiles of temperature and MW power in the synthesis of
sample A and a table of the absorption energies of Ni complexes.
This material is available free of charge on the web at http://
www.csj.jp/journals/bcsj/.
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