Hydrosilanes as reducing Reagents of copper salts into copper metal particles under remarkably mild conditions

Article abstract of DOI:10.1246/bcsj.81.168

Copper nanoparticles were synthesized using hydrosilanes as a reducing agent. The reaction smoothly occurred under remarkably mild conditions, i.e., room temperature. This reaction depended on the counter anions of the copper salts and the structure of the hydrosilanes, and their role is discussed.

Full text of DOI:10.1246/bcsj.81.168

168 Short Articles
Bull. Chem. Soc. Jpn. Vol. 81, No. 1, 168–170 (2008)
Hydrosilanes as Reducing Reagents
of Copper Salts into Copper Metal
Particles under Remarkably
Mild Conditions
Ã
Kenichi Goto, Hiroshi Watanabe, and Shoji Tamai
Masaki Takahashi, Jun Kamada, Kenji Iwata,
Material Science Laboratory, Mitsui-Chemicals, Inc.,
580-32 Nagaura, Sodegaura 299-0265
Received February 8, 2007; E-mail: Jun.Kamada@mitsui-
chem.co.jp
Fig. 1. X-ray diffraction patterns of products from the
reactions of PhSiH3 with CuI salts; (a) the Cu salt is
Cu(OCOCH3); (b) CuCl; (c) CuBr.
Copper nanoparticles were synthesized using hydro-
silanes as a reducing agent. The reaction smoothly occurred
under remarkably mild conditions, i.e., room temperature.
This reaction depended on the counter anions of the copper
salts and the structure of the hydrosilanes, and their role is
discussed.
Table 1. Reaction of CuI Salts with PhSiH3 in Acetonitrile
Copper salt
(A)
Hydrosilane
(B)
Mol ratio
(B)/(A)
Entry
Producta)
1
2
3
Cu(OCOCH3)
CuCl
CuBr
PhSiH3
PhSiH3
PhSiH3
2
2
2
Cu
Cu, CuCl
CuBr
Organosilanes, especially polysilanes, are known to reduce
noble metal salts and silver salts into the corresponding noble
metal particles due to the high-HOMO-electron of the Si–Si
bonds.1 However, these compounds cannot reduce transition-
metal ions with low standard redox potentials. Hydrosilanes
are known to reduce organic compounds, such as the hydro-
silylation of organic compounds having unsaturated bonds
using a noble metal catalyst,2 but using these compounds to
reduce metal salts is still undeveloped. Anderson has reported
the only example, wherein a copper (Cu) salt is reduced to
metallic Cu using a hydrosilane. However, they obtain only
a trace amount of Cu metal, and the analysis is ambiguous.3
Recently, metal particles, especially nano-sized particles,
have been frequently highlighted for new applications due to
their unique properties. Other metal particles have also been
used in industries, for example, the manufacturing of conduc-
tive pastes for use as materials in conducting lines of a circuit
or for new magnetic materials. Cu nanoparticles are particu-
larly useful because of their catalytic, optical, and conducting
properties and low cost. A number of methods have been de-
veloped for the preparation of Cu nanoparticles, such as ther-
mal decomposition,4 the sonochemical method,5 electrodeposi-
tion,6 chemical reductions,7–10 etc.
In this study, the reactivity of hydrosilanes as reducing
agents for Cu salts was investigated, and the preparation of
Cu nanoparticles was achieved using some hydrosilanes under
quite mild, safe, and facile conditions.
First, the reduction of some CuI salts with phenylsilane
(PhSiH3) were examined. The procedure for this type of reac-
tion involved dispersing Cu salts at 500 mM in acetonitrile in
the presence of the proper mole ratio of PhSiH3 under a nitro-
gen atmosphere at room temperature. When Cu(OCOCH3) or
CuCl were used as the Cu salts, a black precipitate appeared
a) Identified by XRD.
Table 2. Reaction of CuI Acetate with PhSiH3 in Various
Organic Solvents
Copper salts Hydrosilane
(A)
Mol ratio
(B)/(A)
Entry
(B)
Solvent
Producta)
1
4
5
6
7
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
PhSiH3 Acetonitrile
2
2
2
2
2
Cu
Cu
PhSiH3
PhSiH3
PhSiH3
PhSiH3
THF
Hexane
Ethanol
Methanol
Cu
Cu
Cu(OCOCH3),
CuO
a) Identified by XRD.
within 30 min. In that case, the solution after the solvent was
removed by flowing nitrogen, and a residual black powder
was obtained after decantation and drying under vacuum.
Analysis of the black powder was carried out using the XRD
technique (Fig. 1). The results, summarized in Table 1, sug-
gest the selectivity is controlled by the counter anions of the
Cu salts. Cu(OCOCH3) was completely reduced to Cu metal
(Entry 1), and CuCl was mainly reduced to Cu metal (Entry
2). Copper salts seem to be easily reduced when Pauling’s
electronegativity of the charged atom of the counter anion of
CuI salt is high. For the reaction of Cu(OCOCH3) and PhSiH3,
the product was analyzed by elemental analysis, and the result
showed that 93% Cu was obtained.
The solvent dependence of the reaction using PhSiH3
and Cu(OCOCH3) is shown in Table 2. This result shows
that Cu(OCOCH3) can be reduced in most solvents, meaning
that the solvent dependence of this reaction is small.
Solid Cu(OCOCH3) was also reduced to Cu (Entries 1 and
Published on the web January 10, 2008; doi:10.1246/bcsj.81.168

Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Ó 2008 The Chemical Society of Japan
169
Table 3. Reaction of CuII Salts with PhSiH3 in Acetonitrile
Copper salt
(A)
Hydrosilane Mol ratio
(B)/(A)
Entry
(B)
Producta)
8
9
10
11
Cu(OCOCH3)2
CuCl2
CuBr2
PhSiH3
PhSiH3
PhSiH3
PhSiH3
4
4
4
4
Cu
CuCl
CuBr
CuSO4
CuSO4
a) Identified by XRD.
Table 4. Reaction of Cu Salts with Hydrosilanes in Aceto-
nitrile
Fig. 2. TEM image and size distribution of Cu nanoparti-
cles at [Cu(OCOCH3)] = 20 mM and [PhSiH3] = 40 mM.
Copper salt
(A)
Hydrosilane
(B)
Mol ratio
(B)/(A)
Entry
Producta)
1
12
13
14
15
16
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
Cu(OCOCH3)
PhSiH3
Ph2SiH2
Ph3SiH
(C2H5)2SiH2
(C2H5)3SiH
2
3
6
3
6
6
Cu
Cu
Cu, Cu2O
Cu, Cu2O
Cu, Cu2O
Cu, Cu2O
Cu(OCOCH3) (CH3O)2MeSiH
2
17
18
19
20
21
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
PhSiH3
Ph2SiH2
Ph3SiH
2
3
6
3
6
6
Cu, CuCl
Cu, CuCl
CuCl
CuCl
CuCl
CuCl
(C2H5)2SiH2
(C2H5)3SiH
(CH3O)2MeSiH
Fig. 3. Electron diffraction patterns of Cu nanoparticles at
[Cu(OCOCH3)] = 20 mM and [PhSiH3] = 40 mM.
8
22
23
24
25
26
Cu(OCOCH3)2
Cu(OCOCH3)2
Cu(OCOCH3)2
Cu(OCOCH3)2
Cu(OCOCH3)2
PhSiH3
Ph2SiH2
Ph3SiH
(C2H5)2SiH2
(C2H5)3SiH
4
6
12
6
12
12
Cu
Cu
Cu (trace)
Cu
N.D.
Cu(OCOCH3)2 (CH3O)2MeSiH
Cu, Cu2O
a) Identified by XRD.
4–6). In the case of methanol, it seems to react with PhSiH3
(Entry 7).
The reduction of CuII salts was attempted using PhSiH3
(Table 3) Though the CuII ion has a very low standard redox
potential, copper(II) acetate was also reduced to Cu metal.
Other CuII salts were reduced to the corresponding CuI salts.
When using CuII salts, a similar selectivity of the salt anion
to the CuI salts was observed.
Fig. 4. Typical TEM images of Cu particles at [Cu-
(OCOCH3)] = 20 mM; (a) [Ph2SiH2] = 60 mM; (b)
[(C2H5)2SiH2] = 60 mM.
The structure dependence of the hydrosilanes in this reac-
tion was examined, and the results are summarized in Table 4.
These results seem to show that the Cu(OCOCH3) is better
reduced when a phenyl group is present in the hydrosilane
(Entries 1, 12, 2, 17, 8, and 22) than when an alkyl group or
alkoxy group is present. However, for the triphenylsilane,
steric hindrance or an electric effect of the phenyl groups
seems to lower the reductivity of the hydrosilane (Entries 13,
18, and 23).
Identification of the products from this reaction from
PhSiH3 was somewhat difficult because of the complexity of
the products; thus, we analyzed the product from triethylsilane.
The reaction solution obtained from Cu(OCOCH3) and tri-
ethylsilane was analyzed by GC-MS. The reaction solution
had the smell of acetic acid. GC-MS peaks assignable to ace-
toxyltriethylsilane, triethylsilanol, and 1,1,1,3,3,3-hexaethyldi-
siloxane were observed. The latter two products are thought to
form from the hydrolysis of acetoxytriethylsilane and the de-
hydrating dimerization of the formed silanol.
Transmission electron micrograph of the resulting precipi-
tate, obtained using 20 mM Cu(OCOCH3) and 40 mM PhSiH3
in ethanol, showed highly dispersed 1–8 nm sized Cu particles
(Fig. 2). The electron diffraction pattern for the resultant nano-
particles, as shown in Fig. 3, showed they are pure metallic
Cu particles. PhSiH3 seems to act as both a reducing agent
and a capping agent. On the other hand, in the case of using
other hydrosilanes, the Cu particles were partially or complete-
ly coherent. Typical TEM images are shown in Fig. 4.
The UV–vis spectrum obtained using 2 mM Cu(OCOCH3)
and 4 mM PhSiH3 in acetonitrile is shown in Fig. 5. The plas-
mon absorption band of Cu nanoparticles appeared at 590 nm,
which is in good agreement with the reported values with
absorption maxima from 570 to 590 nm.10

170 M. Takahashi et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
ratio amount of the hydrosilane was added. The mixture was then
stirred for 30 min.
The sample for the elemental analysis was prepared by decant-
ation and washing of the reacted mixture with THF. The weight of
the product obtained after the reaction using Cu(OCOCH3)
(1.01 g, 8.24 mmol) of and phenylsilane (1.82 g, 16.8 mmol) was
0.54 g. Elemental analysis of this sample showed 89% Cu, 5.7%
silicon and carbon, 5.5% oxygen, with 93% Cu being obtained.
XRD patterns were measured using an X-ray diffractometer
(Rigaku, model Rotaflex, Tokyo) employing Cu Kꢀ radiation.
The XRD patterns were recorded at 2ꢁ from 10ꢀ to 120ꢀ using
a fixed time method with a sampling step of 0.01ꢀ. The identifica-
tion of the products was carried out by comparing the peaks to
authentic samples. The elemental analysis was carried out using
a CHN analyzer (Perkin-Elmer, 2400II, MA) and ICP-AES ana-
lyzer (Seiko Instruments, model VISTA-PRO, Tokyo). The cop-
per particles were observed by TEM using a JEOL model JEM-
2200FS at 200 kV. The samples for the TEM analysis were pre-
pared by diluting a dispersed Cu solution with water, placing a
drop of the solution on a collodion-covered copper grid, and then,
drying it in air at room temperature. UV–vis absorption spectrum
of the reaction solution was measured using a spectrometer
(Shimadzu, model MultiSpec-1500, Kyoto).
Fig. 5. UV–vis spectrum of the reaction solution obtained
at [Cu(OCOCH3)] = 2 mM, [PhSiH3] = 4 mM.
-
H
Ph Si
H
Cu+X-
X
H
Cu+
PhSiH3
H
Ph Si
H
H
Cu+X-
+
+
X
+
HX
2Cu
CuH
X
Ph Si
H
Scheme 1.
References
The most plausible mechanism of the reducing reaction
using PhSiH3 is considered as shown in Scheme 1. Scheme 1
is based on the fact that hyper-coordinated hydrosilanes exhibit
a higher reducing ability than the four-coordinate hydrosilanes
to organic carbonyl or carboxyl compounds.11 In this mech-
anism, the acetate anion of Cu(OCOCH3) coordinates to
the silicon atom of the hydrosilane to form a five-coordinate
hydrosilane, which is expected to have a stronger reducing
ability than the starting the four-coordinate hydrosilane spe-
cies. The five-coordinate hydrosilane reacts with the Cu cation
to form copper hydride (CuH). CuH then reacts with another
Cu(OCOCH3) to form Cu metal and acetic acid. Based on a
strong anion dependence, the mechanism of Scheme 1 seems
to be operative, because the rate determining step of Scheme 1
is considered to be the formation of the five-coordinate silicon
species, in which the anion effect would be quite high. The
phenyl group of the hydrosilane seems to stabilize the five-
coordinate hydrosilane anion by electron delocalization.
In conclusion, some of the Cu salts were reduced to metallic
Cu nanoparticles using hydrosilanes in solvents under very
mild conditions. As a Cu salt, the acetates were the most easily
reduced. PhSiH3 promoted the formation of Cu nanoparticles
with a lower degree of size dispersity.
1
M. Fukushima, Y. Hamada, E. Tabei, M. Aramata, S.
Mori, Y. Yamamoto, Chem. Lett. 1998, 347.
For example: a) I. Ojima, in The Chemistry of Organic
2
Silicon Compounds, Part 2, ed. by S. Patai, Z. Rappoport, John
Wiley & Sons, Inc., Chichester, 1989, Chap. 25, pp. 1479–1526.
b) M. A. Brook, Silicon in Organic, Organometallic, and Polymer
Chemistry, John Wiley & Sons, Inc., New York, 2000, Chap. 7,
pp. 171–188.
3
4
H. H. Anderson, J. Am. Chem. Soc. 1958, 80, 5083.
Y. H. Kim, Y. S. Kang, W. J. Lee, B. G. Jo, J. H. Jeong,
Mol. Cryst. Liq. Cryst. 2006, 445, 231.
R. V. Kumar, Y. Matstai, Y. Diamant, A. Gedanken,
J. Mater. Chem. 2001, 11, 1209.
X. J. Zhou, A. J. Harmer, N. F. Heinig, K. T. Leung,
Langmuir 2004, 20, 5109.
5
6
7
Commun. 2002, 2492.
M. Kogiso, K. Yoshida, K. Yase, T. Shimizu, Chem.
8
Berg, N. Koc, M. Bandyopadhyay, A. Birkner, C. Woll, H. Gies,
a) O. P. Tkachenko, K. V. Klementiev, M. W. E. van den
¨
W. Grunert, J. Phys. Chem. B 2005, 109, 20979. b) Y. Zong,
¨
J. J. Watkins, J. Chem. Mater. 2005, 17, 560.
9
a) A. M. L. Jackelen, M. Jungbauer, G. N. Glavee,
Langmuir 1999, 15, 2322. b) T. Homma, A. Tamaki, H. Nakai,
T. Osaka, J. Electroanal. Chem. 2003, 559, 131.
10 a) A. A. Athawale, P. P. Katre, M. B. Majumdar, J.
Nanosci. Nanotechnol. 2005, 5, 991. b) S.-H. Wu, D.-H. Chen,
J. Colloid Interface Sci. 2004, 273, 165.
11 a) R. J. P. Corriu, J. C. Young, in The Chemistry of Organ-
ic Silicon Compounds, Part 2, ed. by S. Patai, Z. Rappoport, John
Wiley & Sons, Inc., Chichester, 1989, Chap. 20, pp. 1241–1288.
b) M. A. Brook, Silicon in Organic, Organometallic, and Polymer
Chemistry, John Wiley & Sons, Inc., New York, 2000, Chap. 4,
pp. 97–114.
Experimental
All reagents were commercially available and used without
further purification. All of the solvents used in this reaction were
commercially available anhydrous solvents and used without
further purification.
A typical procedure for the preparation of the Cu particles is as
follows. The Cu salt was dispersed in a solvent at 20–500 mM
under a nitrogen atmosphere. To this dispersion, the proper molar