Copper(II) methoxide: Direct solventothermal synthesis and X-ray crystal structure

Article abstract of DOI:10.1016/S0277-5387(02)01132-4

Copper methoxide has been prepared under fairly mild solventothermal conditions by direct reaction of copper(II) acetate monohydrate and methanol. The product has been structurally characterized for the first time by single crystal X-ray diffraction and found to possess a novel infinite one-dimensional chain structure in which copper atoms in distorted square planar coordination environments are linked by bridging methoxide ligands. The conversion of copper acetate to copper methoxide is essentially quantitative. The major byproduct of the synthesis is methyl acetate.

Full text of DOI:10.1016/S0277-5387(02)01132-4

Polyhedron 21 (2002) 2017Á2020
/
www.elsevier.com/locate/poly
Copper(II) methoxide: direct solventothermal synthesis
and X-ray crystal structure
Kendric J. Nelson a, Ilia A. Guzei b, Gregory S. Lund a, Robert W. McGaff a,*
a Department of Chemistry, University of WisconsinÁ
b Department of Chemistry, University of WisconsinÁ
Madison, 1101 University Avenue, Madison, WI, 53706 USA
/La Crosse, 1725 State Street, La Crosse, WI 54601, USA
/
Received 15 February 2002; accepted 18 June 2002
Abstract
Copper methoxide has been prepared under fairly mild solventothermal conditions by direct reaction of copper(II) acetate
monohydrate and methanol. The product has been structurally characterized for the first time by single crystal X-ray diffraction and
found to possess a novel infinite one-dimensional chain structure in which copper atoms in distorted square planar coordination
environments are linked by bridging methoxide ligands. The conversion of copper acetate to copper methoxide is essentially
quantitative. The major byproduct of the synthesis is methyl acetate. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Solventothermal synthesis; Crystal structures; Coordination polymers; Copper methoxide; Single crystals
1. Introduction
metallic copper were absent, based upon this unspecified
powder diffraction data. Thus, it was not possible to
compare our results (via a simulated powder pattern) to
any diffraction data in Ref. [2]. Copper(II) methoxide
has also been prepared by adding pyridine to a suspen-
sion of metallic copper in methanol under an oxygen
atmosphere [3]. In addition, copper methoxide has been
identified as an intermediate existing within a zeolite
framework during CuY-catalyzed oxidative carbonyla-
tion of methanol [4].
We now report the direct synthesis of copper(II)
methoxide (1) from copper(II) acetate monohydrate and
methanol under solventothermal conditions. We have
determined the structure of copper(II) methoxide via
single crystal X-ray methods. This is the first definitive
structural characterization of this coordination polymer,
and the synthesis does not require any inert-atmosphere
techniques or drying of solvent until the product
isolation stage. Secondary characterization has been
accomplished by comparison of IR spectra to those
that have been published previously [1,2]. The route by
which copper methoxide forms in the present study has
been established through analysis of the byproducts in
solution after reaction.
The first report describing the synthesis and isolation
in pure form of copper(II) methoxide was made by
Brubaker and Wicholas [1] in 1965. In this report,
copper methoxide was prepared in 89.5% yield by
reaction of lithium methoxide and CuCl2 in methanol
and characterized by elemental analysis and infrared
spectroscopy. A three-dimensional extended structure
for copper(II) methoxide was proposed, based upon
spectral properties and upon its unusual insolubility (for
an alkoxide) in organic solvents and low volatility. In
the same year Costa et al. [2] reported preparation of
copper methoxide via decomposition of CuCH3 in the
presence of air and by reaction of CuBr2 with NaOCH3
in methanol. In this report, copper(II) methoxide was
characterized by elemental analysis and infrared spec-
troscopy. While the authors obtained a powder X-ray
diffraction pattern of products, no unit cell or data was
reported. It was only reported that copper oxide and
* Corresponding author. Tel.: ꢀ
8281
E-mail address: mcgaff.robe@uwlax.edu (R.W. McGaff).
/
1-608-785-8656; fax: ꢀ1-608-785-
/
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 1 3 2 - 4

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K.J. Nelson et al. / Polyhedron 21 (2002) 2017Á2020
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2. Experimental
2.3. Inductively coupled plasma emission spectroscopic
analysis.
Methanol (99.8%, anhydrous), copper(II) acetate
monohydrate (98ꢀ
were obtained from commercial sources and used as
received. UVÁVis spectra were recorded using matched
quartz cells on a Shimadzu UV2101 PC spectrophot-
ometer. IR spectra were recorded on a Mattson Galaxy
5000 FT-IR spectrometer. GC-MS data were obtained
using a Varian Saturn 2100D instrument, and induc-
tively coupled plasma emission data were recorded and
analyzed using a Thermo Jarrell Ash Atom Scan 25
instrument and associated software.
/
%) and nitric acid (trace metal grade)
The synthesis of 1 was carried out using a 24.7 mg
(0.124 mmol) sample of copper(II) acetate monohy-
drate. After reaction, the mother liquor was transferred
to a Schlenk flask that was then subjected to dynamic
vacuum until no solvent was evident. The residue was
dissolved in nitric acid and diluted to 50.00 ml with de-
ionized water. Analysis of this solution against a 10.00
ppm Cu standard indicated a Cu concentration of 0.157
ppm with a standard deviation of 0.005 ppm. This
/
corresponds to 1.24ꢁ
10ꢂ4 mmol Cu in the entire
analyte solution.
/
2.1. Synthesis of copper(II) methoxide
2.4. Crystallography
In a typical experiment, a glass vial was charged with
24.5 mg (0.123 mmol) of copper(II) acetate monohy-
drate and 5.0 ml of methanol. After stirring, the vial was
placed into a Teflon-lined stainless steel autoclave and
heated to 110 8C for three days. Upon opening the
autoclave, dark purple block-shaped crystals of 1 were
observed on the bottom and sides of the vial. No other
solid product was evident, and the supernatant liquid
was colorless. The vial containing the crystals and
mother liquor was capped with a rubber septum and
attached to the Schlenk line via a needle. Under argon,
Crystal evaluation and data collection were per-
formed on a Bruker CCD-1000 diffractometer with
˚
0.71073 A). A purple block-
Mo Ka radiation (lꢃ
/
shaped crystal with approximate dimensions 0.45ꢁ
/
0.14ꢁ0.12 mm, obtained directly from the reaction
/
vial, was mounted under oil on the tip of a glass
capillary. The crystal was mounted in a stream of
nitrogen gas at 173(2) K and centered in the X-ray
beam.
Crystal Data: C2H6CuO2 (formula weightꢃ125.61),
/
˚
tetragonal, space group P42/mbc, aꢃ
/
8.1848(6) A, cꢃ
4, rcalc 2.119 g
5.356 mmꢂ1. Initial
/
the mother liquor was withdrawn and analyzed by UVÁ
/
3
˚
˚
5.8770(4) A, Vꢃ
/
393.71(5) A , Zꢃ
/
ꢃ
/
Vis and GC-MS. These experiments clearly indicated the
presence of methyl acetate in the mother liquor. The
crystals remaining in the vial after removal of the
mother liquor were washed with several 5 ml portions
of methanol under argon. No observable change
occurred during the washing or subsequent evacuation
of the vial.
cmꢂ3, F(000)ꢃ
/
252, m (Mo Ka)ꢃ
/
cell constants were obtained from three series of v scans
at different starting angles. Each series consisted of 50
frames collected at intervals of 0.3 in a 158 range about
v with an exposure time of 20 s per frame. The final cell
constants were calculated from a set of 1084 strong
reflections. Data were collected by use of the hemisphere
data collection routine. A total of 3205 data were
harvested by collecting three sets of 30-s frames with
0.38 scans in v. Data were corrected for Lorentz and
polarization effects. An absorption correction was
performed and was based on fitting a function to the
empirical transmission surface as sampled by multiple
equivalent measurements [5]. Systematic absences in the
diffraction data were consistent with the space groups
P42bc and P42/mbc. The E-statistics strongly suggested
the centrosymmetric space group P42/mbc that yielded
chemically reasonable and computationally stable re-
finement results [6]. A solution by direct methods
provided most of the non-hydrogen atoms from the E-
map. The remaining non-hydrogen atoms were located
in an alternating series of least-squares and difference
Fourier maps. Non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included in the struc-
ture factor calculation at idealized positions and were
allowed to ride on the neighboring atoms with relative
isotropic displacement coefficients. A total of 3205
2.2. Spectroscopic characterization of copper(II)
methoxide
A sample of the washed crystalline material described
above was ground under methanol to produce a fine
slurry. Several drops of the slurry were placed on a
sodium chloride plate in an argon-filled chamber that
was subsequently evacuated. When the methanol had
evaporated from the slurry, the chamber was filled with
argon and brought to the IR spectrometer, the sample
cavity of which had been purged with nitrogen gas. The
salt plate was mounted in the spectrometer and a
spectrum matching that of copper(II) methoxide [1,2]
was recorded. The sample cavity was then opened to the
air and spectra were recorded at approximately five-
minute intervals, during which time the bands ascribed
to copper methoxide were replaced by a broad OÁ
band typical of a hydrogen-bonded OÁH stretch and
other unidentified signals.
/
H
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K.J. Nelson et al. / Polyhedron 21 (2002) 2017Á
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reflections were collected, 224 of which were indepen-
dent [Rint 0.0300] in the range 3.528BU B26.388. The
data were 100% complete over this range. The final least
squares refinement on F2 yielded R1ꢃ
0.0204 (wR2ꢃ
0.0713) for 179 reflections having I ꢀ2s(I), where
R1ꢃaD/a(Fo),
ꢃ
/
/
/
/
/
/
2
2
2 2 1/2
wR2ꢃ
/
S[w(Fo ꢂ
/
Fc )]/a[(wFo ) ]
;
/
Dꢃ Fc)j. The Cu atom occupies three crystal-
/
j(Foꢂ
/
lographic twofold axes while the carbon and oxygen
atoms reside on a crystallographic mirror plane.
3. Results and discussion
Fig. 1. A section of the infinite one-dimensional chain in 1, with
ellipsoids drawn at the 50% probability level.
We have found that treatment of copper(II) acetate
monohydrate with methanol for three days at 110 8C
produces exceptionally well-formed deep purple block-
shaped crystals of copper(II) methoxide with no other
solid products. These crystals are indefinitely stable
under the mother liquor or pure methanol at room
temperature. They also appear to be stable for at least
several hours under argon or vacuum. In the air in
absence of methanol, the crystals decompose rapidly,
undergoing an irreversible color change to light blue.
Interestingly, if the amount of copper acetate reacting in
5.0 ml of methanol is raised above 25 mg, the yield of
purple crystals is drastically decreased and a large
amount of uncharacterized brown/black solid forms.
Further, no reaction is observed if the same starting
material is left to stand in methanol under argon for 11
days, indicating that solventothermal conditions are
necessary for the facile synthesis of 1 by this reaction.
3.1. Description of the crystal structure of copper(II)
methoxide
Fig. 2. A view down the crystallographic c-axis in the lattice of 1.
After preliminary identification of the identity of the
purple crystalline product as copper(II) methoxide,
confirmation was achieved via single crystal X-ray
methods. In the structure of 1, infinite one-dimensional
chains of copper atoms with bridging methoxide ligands
run parallel to the crystallographic c-axis in the tetra-
gonal space group P42/mbc. The copper atoms in 1 are
each coordinated by four oxygen atoms in a distorted
square planar fashion. The methyl groups on either side
of the one-dimensional chain tip in alternating direc-
tions away from the mean copperÁ
/
oxygen plane in a
‘zigÁzag’ arrangement, and the copper atoms are ar-
/
Fig. 3. A view down the crystallographic a-axis in the lattice of 1.
ranged in a linear fashion with respect to one another. A
segment of this coordination polymer is shown in Fig. 1.
Fig. 2 is a view down the crystallographic c-axis
showing the orientation of the chains with respect to
one another. Fig. 3 presents a view looking down the
crystallographic a axis, illustrating the orientation of the
methyl groups in 1 with respect to the linear arrange-
ment of copper atoms. Selected bond distances and
angles are reported in Table 1.
The bridging of copper atoms by methoxide ligands is
a common structural motif, but the infinite chain
structure determined for 1 is novel. The structure of 1
can be thought of as the most basic of all methoxide-
bridged Cu coordination compound structures. A search
of the Cambridge Structural Database [7] for com-
pounds containing methoxide ligands bridging copper

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K.J. Nelson et al. / Polyhedron 21 (2002) 2017Á2020
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Table 1
˚
Selected bond lengths (A) and bond angles (8) for 1
Bond lengths
Cu(1)Á
Bond angles
O(1)ÁCu(1A)Á
O(1)ÁCu(1A)Á
/
O(1)
1.915(1)
O(1)Á
/
C(1)
1.417(3)
Scheme 1.
/
/
O(1A)
O(1D)
100.5(1)
79.80(9)
C(1)Á
/
O(1)Á
/
Cu(1)
124.22(8)
reported synthetic conditions lead to products that are
suitable for complete structural characterization by X-
ray methods.
/
/
The observed stability of 1 under methanol, even in
the presence of a significant amount of water (present
because of the use of hydrated copper acetate as starting
material) is somewhat surprising and worthy of note.
The observation that a sample of 1 identified in the
nitrogen-purged sample cavity of an IR spectrometer
decomposes rapidly upon the introduction of air is,
however, consistent with the known air sensitivity of
copper methoxide in the absence of solvent.
atoms, limited to structures with final R values of 5% or
lower and no disorder yielded 26 hits. Among these
˚
structures, the average CuÁO distance was 1.93(2) A
/
˚
and the average CuÁ
/
Cu separation 3.0(1) A. Both the
CuÁO bond distance and CuÁ
/
/
Cu separation in 1,
˚
1.915(1) and 2.9385(2) A, respectively, compare well
with these parameters.
3.2. Discussion of the synthetic reaction and products
The only solid product of the synthesis is 1, as
confirmed by IR spectroscopy. Analysis of the mother
liquor by inductively coupled plasma emission spectro-
scopy showed only a minute trace of copper, corre-
sponding to about 0.1% of the copper in the starting
material. This indicates that the conversion of copper(II)
acetate to copper(II) methoxide is essentially quantita-
tive, as there are no reasonable volatile copper-contain-
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 178855. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax:
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.a-
ꢀ44-1223-336033; e-mail: depos-
/
ing products. A UVÁVis spectrum of the mother liquor
/
was transparent in the visible region, showing only an
absorbance attributable to methyl acetate, the presence
of which was confirmed by GC-MS. We believe that the
methyl acetate forms via a classical condensation
reaction whereby methanol and acetic acid react with
elimination of water and concomitant formation of the
ester. This observation strongly suggests that acetate
ions, present in solution due to the use of copper acetate
as the copper source, are responsible for deprotonation
of methanol. It is not clear what role, if any, copper
atoms may play in this process. Nonetheless, the overall
pathway by which 1 and methyl acetate form can be
summarized as shown in Scheme 1.
c.uk).
Acknowledgements
Acknowledgement is made to the donors of The
Petroleum Research Fund, administered by the ACS,
for support of this research. The authors thank Dr. Paul
J. Taylor and Dr. Paul D. Miller of UW-La Crosse for
assistance with ICP analysis and GC-MS, respectively.
This work has confirmed initial suppositions of a
methoxide-bridged polymeric structure for copper(II)
methoxide [1], well supported from the beginning by
observed physical properties. What is not clear is
whether the initial suggestion of a three-dimensional
coordination polymer (as opposed to the crystallogra-
phically-determined one-dimensional structure) was in
fact in error, or rather that the present conditions result
in the formation of a different phase of copper
methoxide than those that had been prepared and
characterized previously. This question will be difficult
to answer conclusively, as none of the previously
References
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