Mechanisms of the polyol reduction of copper(ii) salts depending on the anion type and diol chain length

Article abstract of DOI:10.1039/c8dt03034k

The mechanisms of the polyol reduction of copper(ii) compounds were investigated by various systematic experiments. The course of the reduction in ethylene glycol strongly depends on the anion present in solution. Elemental copper can only be obtained in high yields starting from Cu(NO₃)₂, Cu(OAc)₂, or Cu(OH)₂; but not from CuCl₂ or CuSO₄. Intermediate compounds were observed, namely Cu₂O, and the alkoxide compounds Cu(C₂H₄O₂) and Cu₃(OAc)₂(C₂H₄O₂)₂. Cu₃(OAc)₂(C₂H₄O₂)₂ was characterised by single-crystal X-ray diffraction. It is a 2D coordination polymer with bridging bidentate acetate and bidentate deprotonated EG ligands leading to vertex- and edge-linked CuO₅ polyhedra. Alkoxide compounds were also detected for higher glycol ethers. With growing glycol ether chain length the stability of alkoxide intermediates decreases, enhancing the stability range of Cu₂O. Because the majority of copper species is precipitated as copper(ii) alkoxide compounds or Cu₂O before the reduction to elemental copper occurs, only few nuclei are formed in solution leading to microparticles instead of nanoparticles. Much smaller but strongly agglomerated particles were observed under alkaline reaction conditions.

Full text of DOI:10.1039/c8dt03034k

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Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT03034K
Journal Name
Article
Mechanisms of the polyol reduction of copper(II) salts depending
on the anion type and diol chain length
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a,b
Johannes Teichert, Thomas Doert, and Michael Ruck*
DOI: 10.1039/x0xx00000x
The mechanisms of the polyol reduction of copper(II) compounds were investigated by various systematic experiments.
The course of the reduction in ethylene glycol strongly depends on the anion present in solution. Elemental copper can
www.rsc.org/
only be obtained in high yields starting from Cu(NO
compounds were observed, namely Cu O, and the alkoxide compounds Cu(C
Cu (OAc) (C was characterised by single-crystal X-ray diffraction. It is a 2D coordination polymer with bridging
bidentate acetate and bidentate deprotonated EG ligands leading to vertex- and edge-linked CuO polyhedra. Alkoxide
compounds were also detected for higher glycol ethers. With growing glycol ether chain length the stability of alkoxide
intermediates decreases, enhancing the stability range of Cu O. Because the majority of copper species is precipitated as
copper(II) alkoxide compounds or Cu O before the reduction to elemental copper occurs, only few nuclei are formed in
3 2
)
, Cu(OAc)
2
, or Cu(OH)
2
; but not from CuCl
2
4
or CuSO . Intermediate
2
2
H
4 2
O )
3 2
and Cu (OAc) (C
2
H
4
O ) .
2
2
3
2
2
H
4 2 2
O )
5
2
2
solution leading to microparticles instead of nanoparticles. Much smaller but strongly agglomerated particles were
observed under alkaline reaction conditions.
given temperature the redox potential increases with
increasing chain length of the polyol.[ At higher temperature
the chelating properties of polyols have to be considered when
planning a reduction reaction. Coordination compounds with
polyol ligands that might represent intermediate species in the
polyol process have been crystallised for several of the less-
8]
Introduction
Polyalcohols (polyols) can act as mild reducing agents for many
metal cations, yielding metallic micro- or nanoparticles from
easily available metal salts. This so-called polyol process has
been known for over three decades and was successfully
applied to produce a variety of mono- and multimetallic
noble metals. During the reduction of Co(OH)
2 2
or Ni(OH) in EG
a solid phase precipitates before the reduction to the
[1–3]
materials.
In these reactions, the polyol normally acts as
[9]
respective metals occurs. This solid is in equilibrium with
both solvent and reducing agent. Moreover, as a surface-active
substance, it can also attach to the formed particles and affect
the product’s particle size and morphology.[ Polyols such as
ethylene glycol (EG) have good chelating properties for metal
cations, which compensate the lower polarity compared to
water, and therefore the solvation behaviour of metal salts in
water and polyols is quite similar.[4] Applying polyols as high-
boiling reaction media allows synthesis temperatures up to
2+
solvated M ions and is regarded as a metal reservoir
governing the nucleation and growth of the final particles.
Systematic reduction experiments of cobalt(II) salts in EG
revealed the importance of the type of anion and the
deprotonation of the polyol for the progress of the reduction
reaction.[
4]
10,11]
Like cobalt and nickel, copper particles can be obtained in
polyols at elevated temperature. Copper and copper oxide
nanomaterials, as cheap alternatives to precious metals, have
found wide attention with respect to their potential
application in catalysis, photovoltaics, and optics in the last
years.[ As copper(I) is fairly stable, its emergence in the
3
20 °C facilitating the formation of crystalline nanoparticles
[
4]
without post-treatment.
The redox potential of a polyol decreases with rising
[
5,6]
temperature.
While noble metal ions are reduced at
12]
comparatively low temperatures, even at room temperature in
course of the reaction is not unexpected: Cu
2
O precipitates,
are heated with
2+ [7]
the case of Pd , the reduction of less noble metals cations
[13]
e.g., as an intermediate when CuO or CuSO
4
2+
2+
2+
such as Co , Ni , or Cu requires higher temperatures. For a
eq. of NaOH[ in EG. Increasing the temperature leads to
14]
3
the formation of the final Cu particles. A solid Cu–EG
compound similar to the Co–EG and Ni–EG complexes was
described in the literature.[ By heating copper acetate in
diethylene glycol (DEG) a Cu–DEG compound was obtained.
However, these compounds were not structurally
a.Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062
Dresden, Germany.
Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187
15]
b.
[16]
Dresden, Germany.
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: PXRD patterns and IR spectra characterised.
of the solid reaction products, DTA-TG measurements and magnetic susceptibility
data of alkoxide compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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The knowledge about copper(II) species and their coordination The reduction of copper(II) precursor salts Viienw ArEtiGcle Ownlianes
DOI: 10.1039/C8DT03034K
modes in polyols is fragmentary. It was shown that the Jahn- monitored at different temperatures. It was observed, that the
2+
3
Teller distorted tris-chelated complex [Cu(EG) ]
is formed in a anion of the copper salt has a major influence on the reduction
3 3 2
solution of Cu(CF SO )
in EG.[ Chloride anions have a higher reaction. The reduction of Cu to Cu does not proceed when
17]
2+
0
coordinating affinity towards copper(II) than triflate anions, starting from copper(II) chloride. Reacting CuCl
2
in EG for 2 h
which is consistent with DFT calculations indicating the at 210 °C yielded no metallic copper but a clear green solution.
formation of the complex [CuCl
2
(EG)
2
]
instead of This can be explained by the coordination affinity of chloride
2+ [18]
2+
[
Cu(EG)
been considered yet.
To establish a general understanding of the coordination and complex has a lower reduction potential than [Cu(EG)
3
] . The coordination ability of other anions has not ligands towards the Cu ion, which leads to the formation of
[18]
2 2
solution species like [CuCl (EG) ]. This comparatively stable
2
+
3
] ,
2+
reduction behaviour of copper(II) in polyols, we carried out a which prevents the reduction of Cu to metallic copper at the
series of systematic experiments. Several copper(II) precursor given temperature. A decreasing reduction potential with
salts were reacted in EG, DEG, triethylene glycol (TrEG), and increasing chloride concentration is known from copper
tetraethylene glycol (TEG) at different temperatures. Solid chloride complexes in aqueous solution.[ Although elemental
reaction intermediates were isolated and structurally copper could not be produced, CuCl was obtained by reacting
21]
characterised.
2
CuCl in DEG at 240 °C and TrEG at 280 °C, respectively.
Powder X-ray diffraction (PXRD) showed no other crystalline
phases. The light green solution turns dark greenish-brown
upon heating, and above 220 °C it abruptly turns into a white
suspension of CuCl.
Copper(II) sulfate can be transformed to elemental copper by
refluxing in EG at 210 °C. Copper was obtained only in low
Results and discussion
Reduction of copper(II) in ethylene glycol
The following reduction potentials, which are only valid for
aqueous solutions, have to be considered for the reduction of
2
yields (max. 40%), and a small amount of Cu O was observed
2
+
0 [19]
Cu to Cu .
Reduction potentials of noble metal cations
as a byproduct even after refluxing for 60 min (Fig. S1).
Conducting the reaction at 240 °C in DEG did not significantly
change the product’s purity or yield.
were found to follow the same order in EG and water.[5,6] It is
reasonable to assume that the order is also the same for the
potentials considered here.
2 4
Obviously, CuCl and CuSO are not well suited for the
2
+
–
Cu + 2 e
→
→
→
→
Cu
E° = +0.34 V (1)
E° = +0.15 V (2)
E° = +0.52 V (3)
precipitation of metallic copper in boiling EG without further
modifying the process. Syntheses starting from these materials
described in the literature always required the addition of 3–4
eq. of hydroxide.[
2
+
−
+
Cu + e
Cu
Cu
+
−
0
Cu + e
2
Cu O + H O + 2 e
−
2 Cu + 2OH− E° = −0.36 V (4)
0
2
14,18]
[
20]
2+
0
As reported before , the reduction of Cu ions to Cu by
polyols proceeds not in one step according to (1), but in two.
The light blue solution of copper(II) nitrate in EG does not
change up to 160 °C. At 180 °C the formation of nitrous gases
is observed, and after 20 min a brown suspension occurs. The
Initially Cu2 is reduced to Cu (2). Although Cu ions should be
+
+
+
unstable and readily react to Cu0 according to the more
precipitate isolated by centrifugation mainly consists of Cu
2
O
positive standard potential (3), insoluble Cu
precipitates from the reaction solution. Cu
negative reduction potential (4) and gets reduced to Cu at a
higher temperature.
2
O immediately
(Fig. 1). The dispersion is rather stable and difficult to separate
2
O has a more
completely by centrifugation. This agrees with the previously
described colloidal stability of oxide nanoparticles in polyols up
to 20 wt.-%.[ A small reflection in the powder patterns can
0
22]
The reflux temperature which can be reached for a given
reaction mixture strongly depends on the water content.
Polyols are hygroscopic and therefore contain a small amount
of water. In this study the water content was kept low by using
freshly supplied polyols. Water was also introduced by the
hydrated copper salts that were used as starting materials.
When 5 vol.-% of water were added to reaction mixtures of EG
and suitable copper(II) compounds (e.g. acetate and
hydroxide), the boiling point dropped to ca. 160 °C and no
elemental copper was obtained under reflux conditions.
However, microwave-assisted heating of mixtures in a closed
vessel to 210 °C led to the formation of elemental copper.
Hence, a small quantity of water does not change the general
reduction behaviour but limits the boiling temperature. The
water content has to be kept low to enable a complete
reduction to elemental copper under ordinary reflux
conditions.
2 4 2
be assigned to copper(II) oxalate Cu(C O )∙xH O (x = 0–1, PDF
number 21-297). IR spectra confirm the presence of carboxyl
groups (Fig. S2). The formation of the byproduct is enabled by
the oxidation of EG to oxalate under simultaneous reduction of
nitrate to nitrous gases according to (5).
–
+
–
NO
Under reflux conditions at 210 °C a dark red suspension of
pure elemental copper is formed after some minutes. No Cu
3
+ 4 H + 3 e
→
NO + 2 H
2
O
E° = +0.957 V (5)
2
O
was detected by PXRD (Fig. 1). However, the surface of copper
particles is easily oxidised by air if no precautions are taken to
prevent this.[ Such a surface layer of copper oxide is likely to
be present for all the products in this study since the reactions
were carried out under air and without surfactants. The
amorphous surface layer is probably only a few nanometres
thick. Rietveld refinements of some of the finally obtained
copper powders, mixed with a known amount of silicon
standard material, showed no notable amorphous content in
the products.
12]
2
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View Article Online
DOI: 10.1039/C8DT03034K
Fig. 1 PXRD patterns of reaction products of Cu(NO
3
)
2
∙2.5H
2
O in EG at various
Fig. 2 PXRD patterns of reaction products of Cu(OH)
2
in EG at various temperatures.
temperatures.
Copper(II) hydroxide does not dissolve in EG even upon
prolonged heating but transforms into an EG-containing solid.
The PXRD patterns of the products obtained at 120–180 °C
show primarily one intense broad reflection at 2
). A Cu–EG compound with a similar powder pattern was
θ = 10.8° (Fig.
2
obtained from copper acetate in EG and described as a
precursor for the production of nanostructured CuO. However,
this compound was not further characterised.[
15]
The IR spectrum of this precipitate (Fig. 3) shows some
absorption bands similar to those of liquid EG: bands due to
–
1
CH stretching (2950–2850 cm ), C–O stretching (1100–1000
–
1
–1
cm ), C–C stretching and CH
2
deformation (ca. 900 cm ) are
2 4 2 2
Fig. 3 IR spectra of Cu(C H O ) and liquid EG.
clearly visible in the spectrum. The absence of the broad OH
–
1
stretching band at 3600–3000 cm and C–OH deformation
–
1
bands at 1450–1300 cm , on the other hand, indicates fully
deprotonated EG molecules. The CH stretching vibrations are
split as compared to liquid EG most probably because of a
chelating coordination of the EG molecules. In combination
with results from elemental analysis (measured: C 18.1 wt.-%,
H 2.9 wt.-%, calculated: C 19.4 wt.-%, H 3.3 wt.-%) and the Cu
content determined from the CuO residual after heating in air
(
determined: Cu 52.3 wt.-%, calculated: Cu 51.4 wt.-%), we
propose that the solid alkoxide yielded in this study is
ethylene glycolato)-copper(II), Cu(C
).
Fig. 4 Crystal structure of Cu(OH)
b).
2 2 4 2
(a) and a proposed structure model of Cu(C H O )
(
(
2 4 2
H O
Hence, copper hydroxide reacts with EG molecules to form the
alkoxide upon the release of water:
These layers are stacked along the b axis with a distance of 530
pm (Fig. 4a). Corrugated layers of edge-linked CuO octahedra
are also found in copper(II) layered hydroxide salts, e.g.
6
Cu(OH)
2
+ C
2
H
6
O
2
→
Cu(C
2
H
4
O
2
) + 2 H
2
O
(6)
The powder pattern of the produced alkoxide is very similar to
those commonly found for layered hydroxide salts.[ In the
[25]
2 3 2 4 2
Cu (OH) (OAc). Assuming a similar structure for Cu(C H O ),
23]
the d-spacing of the most intense reflection corresponds to an
interlayer spacing of 820 pm, probably due to deprotonated
EG ligands occupying the interlayer space (Fig. 4b). The EG
ligands might show an intralayer–chelating or interlayer–
bridging coordination mode or a mixture of both.
crystal structure of Cu(OH)
2
, strongly Jahn-Teller distorted
CuO
6
octahedra are edge-linked to corrugated layers.[24]
Similar to Cu(C
2
H
4
O
2
), the solid alkoxides Co(C
have been obtained by heating Co(OAc)
in EG, respectively. Both compounds were described
2
H
4
O
2
)[26] and
and
[27]
2
Ni(C H
4
O
2
)
2
Ni(OH)
2
as layered brucite-like structures with turbostratic disorder.
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The interlayer spacing of 830 pm is close to the value for particles are poorly stabilised in EG and tend toVaiegwgAlrotimcleeOrnalitnee
Cu(C
) determined in this study. The reported PXRD rapidly as the reaction time is increased. DOI: 10.1039/C8DT03034K
patterns and IR spectra show some similarities to those of Combining the PXRD and IR results the reaction scheme (7) of
Cu(C ). Cu(OH) with EG can be deducted.
According to DTA-TG measurements in air (Fig. S3), Cu(C Cu(OH) Cu(C Cu
is mainly reduced to Cu O in the first thermal decomposition By adding 2 eq. of NaOH to a suspension of Cu(OH)
step at about 200 °C. The oxidation of Cu
product CuO leads to a mass increase between 220 °C and 400 solution in the form of square-planar [Cu(OH)
2 4 2
H O
2
H
4
O
2
2
2
H
4
O
2
)
2
→
2
H
4
O
2
)
→
2
O
→
Cu
(7)
2
2
in EG, a
O to the final clear, dark blue solution is formed. Cu might exist in this
2+
2
2–
4
]
or
2
–
2–
°C. Cu(C
2
H
4
O
2
) is paramagnetic. The temperature dependence [Cu(C
2
H
4
O
2 2
) ]
complexes. The [Cu(OH)
4
]
species exists in
–
1
[28]
2–
of the inverse susceptibility χ (T) (Fig. S4) can be modelled alkaline aqueous solutions.
[Cu(C
2
H
4
O
2
)
2
]
ions, which
above 100 K by a Curie-Weiss law with the Curie constant Cmol might be more stable in the EG solution because of the chelate
3
−1
=
0.350 cm K mol and the Curie-Weiss temperature Θ = −61 effect, are known from crystals formed in alkaline mixtures of
2+
[29–32]
K, indicating antiferromagnetic coupling. The effective Cu and group 1 or 2 metal ions in EG.
magnetic moment per Cu atom of μeff = 1.67 μ derived from A copper alkoxide intermediate precipitates from the dark blue
mol is in good agreement with the spin-only value of 1.73 solution by heating to 120 °C. IR spectra and TG curves of this
B
C
expected for Cu2 ions in a Jahn–Teller distorted octahedral precipitate are very similar to those of Cu(C
+
H
4
O
2
) obtained
environment. The χ (T) curve shows a steady decrease upon without NaOH, but the PXRD pattern (Fig. 6) is slightly
lowering the temperature to 2 K which might be an indication different. Instead of one dominating reflection at 2 = 10.8°
of magnetic frustration. one pronounced reflection at 10.4° and a less intense one at
When reacting Cu(OH) in EG at 120–180 °C, the main fraction 11.2° are found, corresponding to interlayer distances of 850
2
–
1
θ
2
2+
(
more than 80%) of Cu precipitates as Cu(C
2
H
4
O
2
) and only a pm and 790 pm, respectively. This suggests a structural
minor fraction of Cu species remains dissolved. At 200 °C, variability of Cu(C ), which might arise from different
O are coordination modes of the EG ligands influencing the
broad, indicating small domain sizes. Elemental copper is interlayer spacing. The yield of Cu(C ) (ca. 40%) is much
obtained under reflux conditions at 210 °C. lower than with pure Cu(OH) as the starting material due to a
SEM images (Fig. 5) show drastic changes in the size and higher solubility of copper(II) species in the alkaline solution.
2+
2 4 2
H O
2+
2 2
Cu is reduced to Cu O. The Bragg reflections of Cu
2 4 2
H O
2
2–
morphology of the solids collected from the reaction mixtures. The reduction of [Cu(C
The starting material Cu(OH) consists of prolate nanoparticles. according to the reaction scheme (7) shown above, but at a
The appearance of Cu(C ) changes between 120 °C and significantly lower temperature compared to pure Cu(OH)
00 °C from highly agglomerated almost isotropic According to PXRD results, Cu O begins to form at 160 °C and
nanoparticles (Fig. 5b) to facetted crystals up to 3 µm length quantitatively transforms to Cu at 200 °C. This could be due to
2 4 2 2
H O ) ] to elemental copper proceeds
2
H
2 4
O
2
2
.
2
2
(Fig. 5c).
the solubility of the precursor as well as the increased pH
value.
Fig. 5 SEM pictures of starting material Cu(OH)
Cu O (c), and copper particles (d).
2 2 4 2 2 4 2
(a), Cu(C H O ) (b), Cu(C H O ) and
2
The cubic Cu
2
O particles with edge lengths up to 100 nm are Fig. 6 PXRD patterns of reaction products of Cu(OH)
2
and 2 eq. of NaOH in EG at various
temperatures.
well-stabilised in the reaction medium and rather difficult to
2+
precipitate completely by centrifugation. Due to the low Cu
concentration in solution prior to nucleation, the number of
formed nuclei is limited and therefore copper submicron-sized
particles (Fig. 5d) rather than nanoparticles are formed. These
4
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ARTICLE
Sb(OAc)(C
found in Zn
2
H
4
O
2
).[34] Mono-deprotonated EG molecules were
View Article Online
DOI: 10.1039/C8DT03034K
[35]
2
(OAc)
3
(C
2
H
5
O
2
).
The occurrence of
deprotonated EG is rather unusual, especially regarding the
fact that no base was added to the reaction mixture. Acetic
a a
acid (pK = 4.75) is a much stronger acid than EG (pK =
5.1)[ , and therefore the deprotonation of EG by acetate
19]
1
anions is an unlikely process. However, the stability of the
product compound or the evaporation of acetic acid from the
reaction solution may drive the reaction. The yield of the
precipitated alkoxide is ca. 50%.
DTA-TG measurements of dry Cu
S5) showed that the compound is stable up to 200 °C and
decomposes in an exothermic reaction to Cu O and then to the
3 2 2 4 2 2
(OAc) (C H O ) in air (Fig.
2
3 2 2 4 2 2
final product CuO. Cu (OAc) (C H O ) shows paramagnetic
behaviour. The χ– plot (Fig. S6) can be modelled above 200 K
1
Fig. 7 SEM picture of copper particles obtained from Cu(OH)
at 200 °C.
2
and 2 eq. of NaOH in EG by a Curie-Weiss fit. The obtained Curie constant Cmol = 0.412
3
−1
cm K mol corresponds to an effective magnetic moment μeff
=
1
1.81
.73
B
μ , which is slightly higher than the spin-only value of
expected for Cu ions.
Hydroxide ions are assumed to initiate the reaction by
2+
μ
B
2–
deprotonation of the α-carbon atom of the (C
H
2 4
O
2
)
ligand,
thus leaving two electrons for the reduction of the metal
[18]
cation.
The majority of the finally obtained, severely agglomerated Cu
particles (Fig. 7) have a significantly smaller size compared to
those obtained from pure Cu(OH
)
2
due to a higher
concentration of Cu2+ in the reaction solution prior to
nucleation.
Copper(II) acetate completely dissolves in EG at ca. 80 °C
yielding a bright turquoise solution. Further heating to 120 °C
leads to the precipitation of a dark green crystalline compound
(
Fig. 8). The IR spectrum of this compound (Fig. 9) shows CH
–
1
–
stretching (2950–2850 cm ), C–O stretching (1100–1000 cm
1
–1
)
2
, C–C stretching and CH deformation (ca. 900 cm ), but no
OH stretching band, indicating fully deprotonated EG groups.
Bands in the range 1600–1300 cm–1 are due to COO stretching
and CH
The structure determination of the obtained product by single-
crystal X-ray diffraction revealed the coordination polymer single-crystal structure data.
Cu (OAc) (C . It crystallises in the triclinic space group
and shows a 2D network structure generated by bridging
3
deformation because of the presence of acetate ions.
Fig. 8 PXRD patterns of reaction products of Cu(OAc) O in EG obtained at various
2 4 2 2
temperatures. The reference pattern for Cu (OAc) (C H O ) was simulated from the
2
∙H
2
3
2
3
�
1
2
2 4 2 2
H O )
P
bidentate acetate and deprotonated EG ligands (Fig. 10). The
three crystallographically independent Cu atoms have
approximately square-pyramidal coordination environments of
O atoms. Four O atoms are located in distances of 192–200 pm
forming a distorted square-planar array.
The apical O atoms are more remote with distances between
24.6 pm and 251.4 pm. Valence-bond sums[ of 2.11 (Cu1)
33]
2
and 2.04 (Cu2, Cu3) unambiguously indicate copper(II). The
CuO pyramids are linked via their edges and vertices forming
bands along [100] that consist of alternating Cu and Cu
5
6
O
6
4 4
O
rings. The longest Cu–O distance of 251.4 pm (at Cu2) links the
chains to a strongly corrugated layer parallel (001). The layers
are stacked along the c axis and not linked via hydrogen bonds.
The mixed coordination of metal cations to solely double
deprotonated EG and acetate ligands, which occurs in
3 2 2 4 2 2
Cu (OAc) (C H O ) , was also reported for the compound
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ARTICLE
Journal Name
2 2
Fig. 9 IR spectra of reaction products from Cu(OAc) ∙H O in EG obtained at different
reaction (Fig. S7). Na
has to be removed from the solid product by multiple washing
cycles.
2
SO
4
is only sparingly soluble in EG and
View Article Online
DOI: 10.1039/C8DT03034K
temperatures.
Addition of 2 eq. of NaOAc to solutions of CuCl
Cu(NO produces bright turquoise solutions which show the
same reaction behaviour as those of pure Cu(OAc)
Remarkably, CuCl can be reduced to elemental copper at 210
C (Fig. S7). Because of its chelating ability, acetate seems to
2 4
, CuSO , and
3 2
)
2
.
2
°
2+
be a stronger ligand for Cu than chloride.
The addition of 2 eq. of NaCl prevents the reduction of
Cu(NO
3 2 4
) or CuSO to elemental copper. Clear solutions
without any solid products were obtained after 30 min at 210
2
+
°
C. Chloride acts as a Lewis base and directly binds to Cu ,
probably forming the solution species [CuCl (EG) ], which leads
to the same reaction behaviour as for CuCl
Based on these results, we propose a sequence of decreasing
2
2
2
.
2+
affinity towards Cu ions in EG solutions: hydroxide > acetate
chloride > nitrate/sulfate.
>
Reduction of copper(II) in higher glycol ethers
Besides EG, the reduction of copper(II) hydroxide and acetate
was also monitored in the longer-chain diols DEG, TrEG, and
TEG. The resulting solid phases starting from copper hydroxide
and acetate are summarised in Tables 1 and 2, respectively.
3 2 2 4 2 2
Fig. 10 Crystal structure of Cu (OAc) (C H O )
2
and coordination environment of the On reacting Cu(OH) with DEG, an alkoxide compound is
three independent Cu atoms. The longest Cu–O distances are represented by dashed obtained between 120 °C and 200 °C. PXRD patterns (Fig. 11)
lines. Hydrogen atoms are omitted for clarity. Ellipsoids represent a probability density
of 90%.
show an intense reflection at 2θ = 11.0° and weaker reflections
at 16.2° (d = 550 pm) and 32.7° (d = 270 pm). IR spectra
indicate the absence of acetate in this compound as they only
show bands associated with the DEG molecules.
The acetate–alkoxide coordination polymer is not stable at
reaction temperatures of 160 °C and higher. Products isolated
after 30 min at 180 °C and 200 °C show PXRD patterns (Fig. 8)
and IR spectra (Fig. 9) characteristic for the already described
No OH stretching band can be observed, although in some
–
1
samples there is a broad signal around 3000 cm arising from
adsorbed water or DEG molecules. The formula deducted for
2 4 2
compound Cu(C H O ). The solid alkoxide is formed in high
4 8 3
this alkoxide compound is Cu(C H O ) which is in agreement
2+
yields and only a small amount of Cu remains in solution.
Elemental copper is obtained under reflux conditions at 210
with results from elemental analysis (measured: C 28.7 wt.-%,
H 4.8 wt.-%, calculated: C 28.7 wt.-%, H 4.8 wt.-%) and the Cu
content calculated from the CuO residual after heating in air
°
C. Cu
small stability range in this particular reaction system, which is
accompanied by the high stability of Cu(C ).
2
O could not be detected after 30 min because of its
(determined: 36.8 wt.-%, calculated: 37.9 wt.-%). The
2 4 2
H O
compound described here had already been synthesised and
two structural models, containing either mono-deprotonated
or a mixture of double-deprotonated and neutral DEG
molecules, had been proposed.[ However, no or only very
few OH groups might be present in the structure according to
By combining the PXRD and IR results, reaction scheme (8) can
be deducted for the reduction of copper acetate in EG.
Cu(OAc) ∙H
2 2
O
→
→
Cu
Cu(C
3
(OAc)
2
(C
2
H
4
O
Cu
2
)
2
16]
2
H
4
O
2
)
→
(8)
2
our results. The starting compound Cu(OH) can still be found
Coordination affinity of anions to copper(II) in ethylene glycol
in the solid products obtained with TrEG and TEG up to 140 °C
and 160 °C, respectively. Similar to EG and DEG, an alkoxide
compound is formed with TrEG. The PXRD pattern (Fig. 11)
Experiments including more than one anion in the solution
showed some trends from which a general sequence of affinity
2+
towards Cu could be deducted. Addition of NaOH to mixtures
of CuCl , CuSO , Cu(NO , or Cu(OAc) in EG first yields a light
shows intense broad reflections at 2
3.7° (d = 645 pm).
θ = 10.8° (d = 820 pm) and
2
4
3
)
2
2
1
blue precipitate (2 eq. of NaOH), which subsequently dissolves
resulting in a dark blue solution (4 eq. of NaOH). The reaction
In contrast to the previously discussed polyols TEG does not
2+
form an alkoxide compound with Cu . Only Cu(OH)
2
, Cu
2
O and
behaviour of these solutions is very similar to that of Cu(OH)
2
copper can be detected in the obtained solid by PXRD. IR
spectra show weak CH and C–O stretching bands, possibly due
to TEG molecules adsorbed to the particle surface.
2–
2–
4 2 4 2 2
or [Cu(OH) ] /[Cu(C H O ) ] . With 4 eq. of NaOH, the
complete reduction of the starting material to elemental
copper can be achieved at 180 °C. Seemingly, anions other
than hydroxide do not affect the reduction. However, with
2 4
sulfate anions Na SO was formed as byproduct during the
6
| J. Name., 2012, 00, 1-3
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ARTICLE
Table 1 Stability ranges (in °C) of obtained solid compounds starting from Cu(OH)
2
after
2 2
Table 2 Stability ranges (in °C) of obtained solid compounds starting from Cu(OAc) ∙H O
View Article Online
a reaction time of 30 min.
after a reaction time of 30 min.
DOI: 10.1039/C8DT03034K
Acetate–
Polyol
Polyol
EG
Cu(OH)
2
Alkoxide
120–200
120–200
120–180
Cu
2
O
Cu
Alkoxide
Cu
2
O
Cu
alkoxide
200
≥ 210
≥ 220
≥ 220
≥ 200
EG
120–140
120
160–200
140–200
≥ 210
≥ 220
≥ 240
≥ 240
DEG
TrEG
TEG
180–200
180–200
140–220
DEG
TrEG
TEG
200–220
140–240
120–240
120
120–160
Conclusions
Systematic experiments were carried out to study the anion-
dependent course of the reduction of copper(II) salts in
polyols. Elemental copper can only be obtained in high yields
starting from Cu(NO
CuCl or CuSO . With EG, the intermediate compounds
Cu (OAc) (C , Cu(C ), and Cu O were observed
3 2 2 2
) , Cu(OAc) , or Cu(OH) ; but not with
2
4
3
2
2
H
4
O
2
)
2
H
2 4
O
2
2
during the reaction. Experiments with higher glycol ethers
show that increasing the polyol chain length decreases the
stability of alkoxides und therefore enhances the stability
range of Cu
phase-pure Cu
range. The formation of Cu
2
O. In the case of TEG, no alkoxide compound but
O was detected in a very wide temperature
O or alkoxide compounds reduces
2
2
2+
the Cu concentration in the reaction solution and therefore
leads to a small number of nuclei resulting in micro- rather
than nano-sized copper particles. Under alkaline reaction
conditions, e.g. by addition of NaOH, the lowest reduction
temperature and the smallest particle sizes are achieved due
2
Fig. 11 PXRD patterns of alkoxides synthesised from the reaction of Cu(OH) with EG,
DEG, and TrEG.
With copper acetate as precursor material (Table 2), only with
EG and DEG alkoxides are formed. Similar to the reaction
sequence in EG, an acetate–alkoxide compound is formed
when copper acetate is heated in DEG to 120 °C. Both DEG and
carboxylate vibration bands are visible in the IR spectrum of
the precipitate. The powder diffractogram shows one intense
2+
to a higher Cu concentration prior to nucleation.
This study shows, that the anion-dependent reduction
behaviour of copper in polyols is similar to the less noble
metals cobalt and nickel, although the occurrence of the stable
oxidation state +I is a peculiarity of the copper system. Less
noble metals should be treated under alkaline conditions to
and sharp reflection at 2
desert-rose shaped particles of Cu(C
S11). Cu(C ) transforms to Cu O and finally to copper. No
θ
= 9.4° (d = 940 pm). At 140 °C
ensure
a complete reduction at the lowest possible
4 8 3
H O
) are obtained (Fig.
temperature.
4
8
H O
3
2
alkoxide compounds could be detected for TrEG and TEG.
However, the reduction to Cu+ starts at a much lower
temperature and the stability range of Cu O is significantly
2
Experimental
enhanced. There is a small amount of a byproduct at 180 °C for
TrEG and at 160 °C for TEG (Fig. S8). IR spectra of the samples
show intense carbonyl bands at 1650 cm–1 (Fig. S14).
Elemental copper is produced at higher temperatures when
compared to copper acetate as the starting material. This is
Chemicals
Freshly supplied polyols were used as received: ethylene glycol
(
Fluka, 99.5%), diethylene glycol (Aldrich, 99%), triethylene
glycol (Alfa Aesar, 99%), and tetraethylene glycol (Acros,
9.5%). Hydrated copper(II) salts were used for all syntheses:
9
due to the stronger alkalinity of Cu(OH)
deprotonation of EG molecules.
2
which leads to
CuCl
Cu(NO
Cu(OH)
2
∙2H
2
O
(Aldrich, 99%), CuSO (Aldrich, 99%),
4
∙5H
2
O
)
3 2
∙2.5H O (ABCR, 99.5%), Cu(OAc) ∙H O (Aldrich, 99%).
was prepared by precipitation of an aqueous solution
∙5H O with 2 eq. of NaOH. The light blue powder was
2
2 2
2
of CuSO
4
2
filtrated, washed several times with water and ethanol, and
dried under vacuum at ambient temperature overnight. For
some experiments, solutions of NaOH (Grüssing, 99%),
2
NaOAc∙3H O (Grüssing, 99.5%), or NaCl (VWR Prolabo, 99.9%)
in EG with a concentration of 1 mol/l were used.
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ARTICLE
Synthesis
Journal Name
Acknowledgements
View Article Online
DOI: 10.1039/C8DT03034K
For each experiment, the respective copper(II) compound (0.5
mmol) was mixed with 5 ml of the polyol in a flask equipped
with a magnetic stirring bar and a reflux condenser. In some
experiments a specified amount of NaOH, NaOAc∙3H O, or
2
NaCl solution was added, maintaining the total volume of 5 ml.
Heating was performed with a CEM Discover-SP microwave
We thank Philipp Lange for elemental analysis, Dr. Ilka Kuhnert
for DTA-TG measurements, Dr. Martin Kaiser for assistance
with the single-crystal measurement, and Dr. Jens Hunger for
measuring magnetic data and helpful discussions.
device under open vessel conditions. The reaction Notes and references
temperature was controlled with the in-built infrared sensor.
The mixture was heated to the desired temperature in 3 min,
annealed for 30 min, and rapidly cooled down to 60 °C with
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Dalton Transactions
Page 10 of 10
View Article Online
DOI: 10.1039/C8DT03034K
Table of contents
Systematic experiments were carried out to identify the main factors influencing the polyol reduction
of copper(II) compounds to elemental copper.