On Verdigris, Part III: Crystal Structure, Magnetic and Spectral Properties of Anhydrous Copper(II) Acetate, a Paddle Wheel Chain

Article abstract of DOI:10.1002/zaac.201900125

Pure anhydrous Cu(CH₃COO)₂ was obtained both, by thermal dehydration of Cu(CH₃COO)₂·H₂O and by drying a commercially purchased mixture of Cu(CH₃COO)₂·H₂O and Cu(CH₃COO)₂ in a nitrogen atmosphere using P₂O₅ as drying agent. The crystal structure was solved ab initio from synchrotron X-ray powder diffraction (XRPD) data at 150 °C and from laboratory XRPD data at ambient conditions and found to be isotypic to anhydrous chromium(II), molybdenum(II) and rhodium(II) acetate. Cu(CH₃COO)₂ crystallizes in space group P1 (no. 2) with lattice parameters of a = 5.1486(3) ?, b = 7.5856(6) ?, c = 8.2832(6) ?, α = 77.984(4)°, β = 75.911(8)°, γ = 84.256(6)° at ambient conditions. Cu₂(CH₃COO)₄ paddle wheels with short (2.6 ?) Cu–Cu distances form chains in a direction, which is the main motif in the crystal structure. Due to their identical structural main motif Cu(CH₃COO)₂·H₂O and Cu(CH₃COO)₂ exhibit a similar bluish-green color, almost identical UV/Vis spectra and comparable magnetic properties. The temperature dependent magnetic susceptibility also indicates only weak inter-dimer spin exchange between neighbouring Cu₂(CH₃COO)₄ paddle wheels.

Full text of DOI:10.1002/zaac.201900125

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201900125
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
On Verdigris, Part III: Crystal Structure, Magnetic and Spectral
Properties of Anhydrous Copper(II) Acetate, a Paddle Wheel
Chain
Sebastian Bette,*[a,b] Alice Costes,[b] Reinhard K. Kremer,[a] Gerhard Eggert,[b] Chiu C. Tang,[c] and
Robert E. Dinnebier[a]
Abstract. Pure anhydrous Cu(CH3COO)2 was obtained both, by ther-
mal dehydration of Cu(CH3COO)2·H2O and by drying a commercially
b = 7.5856(6) Å, c = 8.2832(6) Å, α = 77.984(4)°, β = 75.911(8)°, γ =
84.256(6)° at ambient conditions. Cu2(CH3COO)4 paddle wheels with
purchased mixture of Cu(CH3COO)2·H2O and Cu(CH3COO)2 in a ni- short (2.6 Å) Cu–Cu distances form chains in a direction, which is the
trogen atmosphere using P2O5 as drying agent. The crystal structure main motif in the crystal structure. Due to their identical structural
was solved ab initio from synchrotron X-ray powder diffraction main motif Cu(CH3COO)2·H2O and Cu(CH3COO)2 exhibit a similar
(XRPD) data at 150 °C and from laboratory XRPD data at ambient
bluish-green color, almost identical UV/Vis spectra and comparable
conditions and found to be isotypic to anhydrous chromium(II), magnetic properties. The temperature dependent magnetic suscep-
molybdenum(II) and rhodium(II) acetate. Cu(CH3COO)2 crystallizes tibility also indicates only weak inter-dimer spin exchange between
¯
in space group P1 (no. 2) with lattice parameters of a = 5.1486(3) Å, neighbouring Cu2(CH3COO)4 paddle wheels.
rial that was obtained after subsequent purification and
Introduction
recrystallization steps. The action of acetic acid vapors on cop-
per metal and alloys usually leads to a mixture of basic and
neutral verdigris phases[7] that is also denoted as raw verdi-
gris.[8]
In a subsequent step the verdigris was dissolved in strong
acetic acid or ground with strong acetic acid. The resulting
Verdigris is a collective term for green and blue copper
based pigments. From antiquity[1] until the 19th century they
were frequently used for artistic scope[2] as natural green and
blue pigments were rare and due to their anti-oxidant effect[3]
and drying effect[4] on binding media like linseed oil. Some-
times verdigris pigments were also used for medical applica-
tions.[5] Verdigris pigments were often produced by intentional
corrosion of copper metal and alloys by using carboxylic acids
like acetic acid.[1] Numerous recipes for the preparation of ver-
digris pigments have been reported using different starting ma-
terials and variable synthesis conditions, i.e. temperature and
time. A great variety of verdigris pigments differing in color,
resistivity against degradation and chemical composition ex-
ists. Usually verdigris pigments are classified in two catego-
ries: neutral or sometimes distilled or recrystallized verdigris
and basic verdigris. The latter category refers to a group of
copper(II) acetate hydroxide salts, also known as “basic cop-
per(II) acetates“, which are distinguished and denoted accord-
ing to their chemical composition. Such phases were subject
of our previous studies.[6] Neutral verdigris denotes the mate-
neutral
verdigris
was
identified
to
be
mainly
Cu(CH3COO)2·H2O.[9] According to Eastaugh et al.[10] anhy-
drous copper acetate “has probably limited relevance as a pig-
ment, but may be a component of some verdigris-related com-
pounds”, Scott[1] included it as “Compound E“ in his dis-
cussion of the composition and variability of verdigris. Sal-
vadó et al.[11] detected anhydrous copper acetate (in addition
to the monohydrate and to Cu(CH3COO2·3Cu(OH)2·2H2O) in
samples of 15th cent. paintings from Catalonia and Aragon by
Synchrotron Radiation μ-XRD. In their earlier publication, de
la Roja et al.[12] interpret Raman spectra of samples from repli-
cation experiments of historic verdigris recipes (e.g. recipe 106
from the 12th cent. Mappae clavicula: corroding copper for 14
d by vinegar vapor at 40 °C) as indicating the presence of both
the anhydrous and the monohydrate form. In a later study from
the same group which used additionally XRPD the anhydrate
is not mentioned anymore.[13]
* Dr. S. Bette
E-Mail: S.Bette@fkf.mpg.de
The monohydrate crystallizes from saturated aqueous solu-
tion.[14] Under conditions of low activity of free water as in
monolayers on metal surfaces during atmospheric corrosion,
the formation of the anhydrate cannot be ruled out. The
conversion reaction (rate constants, equilibrium) between
anhydrate and monohydrate at temperatures Ͻ 80 °C and vary-
ing relative humidity needs further study. For historic samples,
this would allow to assess if the anhydrate can survive over
centuries at ambient conditions or if the monohydrate dehy-
drates in a dry and hot climate.
[a] Max Planck Institute for Solid State Research
Heisenbergstr. 1
70569 Stuttgart, Germany
[b] State Academy of Art and Design
Am Weißenhof 1
70191 Stuttgart, Germany
[c] High Resolution Powder Diffraction Beamline (I11)
Diamond Light Source Ltd
Harwell Science and Innovation Campus
Didcot, Oxfordshire, OX11 0DE, United Kingdom
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201900125 or from the au-
thor.
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Cu(CH3COO)2·H2O sometimes denoted as 1-0-1 phase the copper metal was brought into direct contact with diluted
according to the x-y-z notation, referring to acetic acid (vinegar) or only with its vapors. According to the
xCu(CH3COO)2·yCu(OH)2·zH2O, is well characterized in medieval texts the intentional corrosion was conducted for sev-
terms of spectroscopic[15] and magnetic properties.[15b,16] Its eral (most likely one to six) months, hence a contact time of
crystal structure also has been determined several times[17] six months was applied for model experiment I and II. As de
leading to identical results with hydrate water terminated la Roja et al.[12] brought the copper metal in contact with vine-
Cu2(CH3COO)4
paddle
wheels
as
main
motif. gar vapors only for 15 days, for model experiment III a contact
Cu(CH3COO)2·H2O also occurs naturally as the mineral ho- time of 14 days was applied. In Figure 1 photographs of the
ganite.[18] The monohydrate can be dehydrated by heat[15c,19] corroded copper plates are presented. While the copper cor-
[16]
storage in vacuo over P2O5
or by refluxing in an acetic rosion by vapors from diluted (5%) acetic acid leads to the
acid – acetic anhydride mixture.[20] As the reported synthesis formation of a cyan-bluish efflorescence phase (Figure 1a and
strategies lead to crystalline powders and in particular in case c), direct contact of the copper with diluted acetic acid does
of the historic routes to multiphase mixtures, anhydrous cop- not cause a significant corrosion of the metal (Figure 1b). After
per(II) acetate has only been poorly characterized so far. Espe- exposure of the copper to acetic acid vapors for six months,
cially its crystal structure is unknown. XRPD patterns[20,21] of the metal plate is covered by a massive layer of the efflores-
Cu(CH3COO)2 can be found in the PDF-database,[22] but these cence phase (Figure 1a), whereas the layer of the corrosion
datasets are marked as questionable. Some studies were pub- phase formed after 14 days, is very thin (Figure 1c).
lished on the IR- and Raman spectra[15c,20,23] but due to the
lack of reliable reference data, the published spectra need to
be verified. Figgis and Martin[16] studied the magnetic proper-
ties of anhydrous copper(II) acetate, which was found to exhi-
bit a similar anomalous paramagnetism as the monohydrate
that points to Cu–Cu δ bonds, but without knowledge of the
crystal structure, a deep interpretation of these data was not
possible. The interpretation of the magnetic properties of
Cu(CH3COO)2·H2O was crucial for the development of mod-
ern theories for antiferromagnetic coupling.[16,24] Therefore,
detailed crystallographic data of the anhydrous phase could be
of fundamental importance. There are divalent chromium,[25]
and rhodium[25b] acetate hydrates which are isostructural to
Cu(CH3COO)2·H2O. There are also reports on crystal struc-
tures of anhydrous, divalent transition metal acetates,
Figure 1. Photographs of copper plates corroded by (a) exposure to
M(CH3COO)2 with M = Cr,[26] Mo,[27] Rh;[28] that could be
acetic acid vapors (ca. 3.45 ppm) at 40 °C for 6 months (model experi-
ment I), (b) by direct contact with 5% acetic acid at 40 °C for six
expected to be structure analogues to Cu(CH3COO)2.
In order to fill the gap of structural knowledge on anhydrous
copper(II) acetate, we synthesized the pure phase by applying
the drying method over P2O5. The crystal structure was solved
from laboratory XRPD data at ambient conditions and con-
firmed by an identical structure that was obtained from an in
situ synchrotron powder diffraction experiment on the thermal
behavior of Cu(CH3COO)2·H2O. The spectral and magnetic
properties of the anhydrous phase were also studied in com-
parison to the monohydrate.
months (model experiment II), and (c) by exposure to acetic acid va-
pors at 40 °C for 14 days (model experiment III).
The efflorescence phase was sampled from different
spots of the metal plates. By XRPD analysis only
Cu(CH3COO)2·H2O was identified as crystalline component of
the corrosion phase (Supporting Information, Figure S1). In
consequence we were not able to reproduce the findings of de
la Roja et al.[12] and San Andrés et al.[13] this might be attrib-
uted to the fact that these authors used vinegar and not diluted
acetic acid, but vinegar, which contains a number of volatile
compounds.[29] Nevertheless the occurrence of anhydrous cop-
per(II) acetate in verdigris pigments was proven by Salvadó et
al.[11] by using synchrotron XRPD. In order to obtain a pure
sample of Cu(CH3COO)2 for the detailed characterization, the
monohydrate was dehydrated.
Results and Discussion
Model Eperiments on the Corrosion of Copper by Diluted
Acetic Acid
Model experiments on the pigment production by the inten-
tional corrosion of copper metal by diluted acetic acid were
performed based on the experimental procedures of de la Roja
et al.[12] and San Andrés et al.[13] that were inspired by ancient
recipes. The copper was corroded by acetic acid vapors (model
Dehydration of Cu(CH3COO)2·H2O
The dehydration behavior of Cu(CH3COO)2·H2O was
experiments I and III) and by direct contact with diluted (5%) studied by thermal analyses, temperature dependent in situ
acetic acid (model experiment II) in sealed vessels at 40 °C, XRPD analysis, as well as by dehydration in a stationary nitro-
as in the medieval recipes it is not state ambiguously whether gen atmosphere while using P2O5 as a drying agent.
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Thermal Analysis and in situ XRPD Measurements
emerge. The latter phase is a short living intermediate, as at
270 °C there are only Cu and Cu2O related reflections left.
Despite the thermal analysis indicated a three step process for
the decomposition of anhydrous copper(II) acetate, in the
in situ XRPD patterns only one crystalline intermediate,
Cu(CH3COO)1–2yOy, was observed. These differences can be
either attributed to the static atmosphere that can lead to dif-
ferent decomposition kinetics, to the fact that the patterns were
recorded in 10 K intervals and that therefore an additional very
short living intermediate may be skipped or the presence of air
in the capillary.
The thermal analysis was carried out in a dynamic nitrogen
atmosphere; the atmosphere in the capillary used for the tem-
perature dependent XRPD analysis was stationary. Hence both
analysis show comparable temperature dependent effects but
the onset temperature of each effect differs.
During heating in a dynamic nitrogen atmosphere the release
of hydrate water from Cu(CH3COO)2·H2O slowly starts at ca.
100 °C (Figure 2, black and green line). This is associated with
a broad endothermic effect (blue line). The thermal dehy-
dration is completed at ca. 150 °C and the decomposition of
the acetate ions starts at ca. 240 °C and progresses quickly in
three overlapping steps, which are associated with endothermic
effects. At ca. 280 °C the thermal decompositions is com-
pleted. The measured mass loss of the dehydration step of
8.9 wt% is in good accordance with the expected one
(9.0 wt%). After the complete decomposition a total mass loss
of 64.8 wt% was observed. This points to the formation of a
mixture of Cu2O (expected mass loss: 64.2 wt%) and elemen-
tal Cu (expected mass loss 68.2 wt%), which was proven by
in situ XRPD (see below). The thermal decomposition behav-
ior of copper carboxylate compounds under inert atmosphere
is complex.
Figure 3. Temperature dependent in situ XRPD patterns (λ
=
1.5406 Å) of Cu(CH3COO)2·H2O including assignment of the mea-
sured reflections: (a) Cu(CH3COO)2·H2O, (b) Cu(CH3COO)2·H2O +
Cu(CH3COO)2 (c) Cu(CH3COO)2, (d) Cu(CH3COO)yO1–2y + Cu +
Cu2O, (e) Cu + Cu2O.
Dehydration of Cu(CH3COO)2·H2O over P2O5
In order to refine the crystal structure of anhydrous
Cu(CH3COO)2 at ambient conditions, suitable for a detailed
comparison with the crystal structure of the monohydrate and
in order to examine the spectral and magnetic properties of the
anhydrous phase, “anhydrous” Cu(CH3COO)2 was purchased.
The XRPD pattern (Figure 4b), however, reveals the presence
of a significant amount of the monohydrate (Figure 4a and b,
Figure 2. Thermogravimetric (TG, black line), differential thermoana-
lytic (DTA, blue line), and differential thermogravimetric (DTG, green
line) curves of Cu(CH3COO)2·H2O.
During the thermal decomposition of carboxylates, radical black indices) in the solid. Accordingly the purchased mixture
species and carbon monoxide form, that lead to a reduction of of Cu(CH3COO)2 and Cu(CH3COO)2·H2O was successfully
CuO.[19,30]
dried (Figure 4c and d) by applying the method of Figgis and
In the in situ XRPD patterns only Cu(CH3COO)2·H2O re- Martin[16] and the sample could be used to determine the crys-
lated reflections can be observed up to a temperature of 90 °C tal structure of Cu(CH3COO)2 at ambient conditions (see be-
(Figure 3). Subsequently, the slow release of hydrate water oc- low). The refined crystal structure was used to calculate a pow-
curs. In a temperature interval from 100 °C to 130 °C both der pattern of pure Cu(CH3COO)2 at room temperature. The
Cu(CH3COO)2·H2O and Cu(CH3COO)2 related reflections are calculated pattern is in good agreement with reported diffrac-
apparent (Figure 3). During heating the intensity of the tional data of the title compound (Table S1, Supporting Infor-
reflections that are assigned to Cu(CH3COO)2·H2O decreases. mation), that were deposited in the JCPDS database and that
In a temperature interval from 140 °C to 230 °C only were used to identify anhydrous copper(II) acetate in historic
Cu(CH3COO)2 related reflections are present (Figure 3). At verdigris samples.[11] Hence, it can be concluded that these
250 °C the reflections of Cu(CH3COO)2 suddenly disappear datasets are reliable and therefore the presence of this phase in
and Cu, Cu2O, and Cu(CH3COO)1–2yOy related reflections[6] historic verdigris samples, is confirmed.
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interactions between these two types of chains (grey, dashed
bonds). In the anhydrous phase each Cu2(CH3COO)4 paddle
wheel is connected to two neighboring paddle wheels by cor-
ner sharing square-pyramidal coordination spheres, resulting
into infinite [Cu2(CH3COO)4]n chains running in a direction
and exhibiting a staggered arrangement (Figure 6c and d, deep
blue and blue polyhedra).
Figure 5. Main building blocks in the crystal structures of (a)
Cu(CH3COO)2·H2O and (b) Cu(CH3COO)2. The copper–oxygen
bonds of the paddle wheel are presented in orange, the copper–copper
bond as dashed, cyan bond, and the apical copper–oxygen bond as
dashed grey bond.
Figure 4. XRPD patterns (λ = 1.5406 Å) of (a) Cu(CH3COO)2·H2O
including selected reflection indices, (b) purchased “anhydrous”
Cu(CH3COO)2 that contains Cu(CH3COO)2·H2O, as well, (c) anhy-
drous Cu(CH3COO)2 obtained by thermal dehydration of
Cu(CH3COO)2·H2O (Figure 2) and (d) after drying the purchased
“anhydrous” Cu(CH3COO)2 over P2O5 in a nitrogen atmosphere
for 2 months.
Crystal Structure Description
In the crystal structures of Cu(CH3COO)2·H2O and anhy-
drous Cu(CH3COO)2 paddle wheels of Cu2(CH3COO)4 are the
main motif (Figure 5). Each copper ion is coordinated by four
monodentate acetate ions forming a distorted squared planar
coordination sphere (Figure 5, orange bonds). Each acetate ion
bridges two copper ions, which leads to short Cu–Cu distances
of ca. 2.6 Å in both compounds (cyan, dashed bonds). In the
monohydrate a water molecule is present in the apical position
of the paddle wheel (Figure 5a, grey dashed bond) with a
Cu–O distance of 2.1 Å, forming a squared pyramidal coordi-
nation sphere and leading to isolated Cu2(CH3COO)4(H2O)2
complexes. As no hydrate water is present in anhydrous
Cu(CH3COO)2 the acetate related oxygen of a neighboring
Cu2(CH3COO)4 paddle wheel occupies the apical position of
the squared pyramidal coordination sphere with a Cu–O dis-
tance of 2.2 Å (Figure 5b). Both compounds exhibit chain-like
structural motifs (Figure 6). In the monohydrate the hydrate
Figure 6. Packing diagrams of Cu(CH3COO)2·H2O (a, b) and of
Cu(CH3COO)2 (c, d). The squared pyramidal coordination sphere of
copper is indicated by light and deep blue polyhedra.
Comparison of the Crystal Structures of Related Anhydrous
Divalent Transition Metal Acetates
The crystal structure of anhydrous copper(II) acetate was
water molecules mediate interactions between neighboring found to be isotypic to the chromium(II), molybdenum(II), and
paddle-wheels (Figure 6a and b, dashed grey bonds) resulting rhodium(II) analogues (Figure 7). Due to the differences be-
into Cu2(CH3COO)4·2H2O]n paddle wheel chains. There are tween the crystal structures of anhydrous transition metal acet-
¯
two types of chains; one is running in [110] direction (Fig- ates, the usage of the crystal structures of the chromium(II),
ure 6a and b, deep blue polyhedra) and the other in [110] direc- molybdenum(II), and rhodium(II) analogues as a starting
tion (light blue polyhedra). Hydrogen bonds also mediate the model in a Rietveld refinement of the powder pattern of anhy-
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Figure 7. Comparison of the [M2(CH3COO)4]n paddle wheel chains of anhydrous copper(II) (a), chromium(II) (b), molybdenum(II) (c) and
rhodium(II) acetate (d), acetate ions perpendicular to the image plain are displayed as transparent atoms and bonds, selected angles and atom
distances are highlighted (Table 1).
Table 1. Comparison of the crystallographic data and selected bond lengths and angles of anhydrous copper(II) with chromium(II),
molybdenum(II), and rhodium(II) acetate.
Cu(CH3COO)2
Cr(CH3COO)2 [26]
Mo(CH3COO)2 [27b]
Rh(CH3COO)2 [28]
¯
¯
¯
¯
Space group
P1 (no. 2)
P1 (2)
P1 (2)
P1 (2)
a /Å
b /Å
c /Å
α /°
5.1486(3)
7.5856(6)
8.2832(6)
77.98(1)
75.91(1)
84.26(1)
306.48(4)
2.59(1)
5.178(1)*
7.583(2)*
8.688(2)*
77.88(2)*
75.86(2)*
84.23(2)*
310.63(14)
2.29(1)
5.4857(4)*
7.5138(5)*
8.4084(6)*
74.83(1)*
74.01(1)*
84.13(1)*
321.39(4)
2.09(1)
5.2216(5)*
7.5264(6)*
8.3392(7)*
78.10(1)*
75.29(1)*
83.45(1)*
309.51(5)
2.41(1)
β /°
γ /°
V /Å3
d(M–M) /Å
d(M–OЈ) /Å (Figure 6)
ε /° (Figure 6)
η /° (Figure 6)
2.16(8)
163(1)
82(1)
2.37(1)
165(1)
77(1)
2.64(1)
160(1)
70(1)
2.50(2)
162(1)
83(1)
drous copper(II) acetate led to a local minimum. Hence for the
crystal structure solution ab initio methods to perform a global
optimization had to be employed. One of the main differences
is the metal–metal bond that is considerable longer in the cop-
per compound (Table 1). In addition, the distance between the
metal ion and the acetate related oxygen of the neighboring
paddle wheel [d(M–OЈ)] is much shorter in Cu(CH3COO)2
(2.16 Å) than in its chromium (2.37 Å), molybdenum (2.64 Å)
and rhodium (2.50 Å) analogues. In consequence, the interac-
tions between the paddle wheel complexes can be expected to
be more pronounced in anhydrous copper(II) acetate. All
phases exhibit a strong tilting of the M–M bond with respect
to the chain axis, therefore the M2(CH3COO)4 chain links exhi-
bit a dislocation when the chain is oriented parallel to the M–
M bonds (Figure 7). The degree of dislocation is indicated by
the deviation of the MMOЈ angle, where OЈ is the acetate re-
lated oxygen atom of the neighboring paddle wheel (denoted
as “ε” in Table 1 and Figure 7), from 180°. The links of the
paddle wheel chains of both compounds show a similar degree
of dislocation (Table 1).
Figure 8. Photographs of powder samples of Cu(CH3COO)2 (a) and
Cu(CH3COO)2·H2O (b), and corresponding UV/Vis reflectance spec-
tra.
visual inspection (Figure 8a and b). Hence, the measured UV/
Vis reflectance spectra (Figure 8c) are almost identical. Both
Spectral Properties
Cu(CH3COO)2·H2O and anhydrous Cu(CH3COO)2 exhibit spectra exhibit two maxima, for the monohydrate at 341 nm
nearly the same color and cannot be easily distinguished by and 490 nm (Figure 8c, blue line) and for anhydrous
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Figure 9. Comparison of the FT-IR spectra of Cu(CH3COO)2·H2O and (b) Cu(CH3COO)2 in the high (a), mid (b) and low (c) wavenumber
region.
Cu(CH3COO)2 at 349 nm and 490 nm (black line). The intense Table 2. Band position, shapes, and assignments[15a,30] in the IR spec-
tra of Cu(CH3COO)2·H2O (1–0–1 phase) and Cu(CH3COO)2 (1–0–0
phase).
maximum at 490 nm that is located at identical position [Fig-
ure 8c, (2)] for both compounds is at similar position as the
reflectance maxima of basic verdigris phases (1–2–0: Band No.
Position/cm–1, shape
Assignment
472 nm,[6a] 2–1–5: 488 nm[6b] and 1–3–2 phase: 517 nm[6b]).
1–0–1 phase
1–0–0 phase
The basic verdigris phases, however, do not show a second
reflectance maximum. Hence this reflectance maximum in the
spectra of Cu(CH3COO)2·H2O and anhydrous Cu(CH3COO)2
must originate in the Cu2(CH3COO)4 paddle wheels, as basic
verdigris phases do not exhibit a comparable structural motif.
A comparison of the FT-IR spectra of Cu(CH3COO)2·H2O
and Cu(CH3COO)2 is shown in Figure 9 and the assignment
of measured bands is presented in Table 2. Overall the IR spec-
tra of the two phases exhibit great similarities, especially in
the mid- and low wavenumber regions that originates in their
identical structural main motifs (Figure 5). The measured spec-
trum of the monohydrate is in complete accordance to pre-
viously published data[15a,30] which were therefore used for
band assignment. In the high wavenumber region (Figure 9a)
there is the most striking difference of the IR spectra: whereas
Cu(CH3COO)2·H2O exhibits three hydrate water related O–H
stretching modes [Figure 9, blue line, (1–3)] there is no band
in the spectrum of the anhydrate at all. In the mid IR-region
there are only small differences (Figure 9b).
In both phases the splitting between the symmetric
and asymmetric acetate related C–O stretching band, (154–
177) cm–1 for Cu(CH3COO)2·H2O and (151–170) cm–1 for
Cu(CH3COO)2, is in the expected range for monodentate coor-
dinating, bridging acetate ions.[31] The slightly smaller splitting
of the bands in the spectrum of the anhydrate reflects week
interaction of the acetate ions with the copper ion in the neigh-
boring element of the [Cu2(CH3COO)4]n paddle wheel chain
(Figure 5b). In the low IR-region (Figure 9c) acetate related
C–C stretching modes (11) and carboxylate related bending
modes (12, 13) are apparent at almost identical positions, only
the position of the C-H bending modes of the methyl group
(9, 10) slightly differs. Another striking difference in the spec-
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
3472, m
3368, m
3270, m
1650, m
1596, s
1441, s
1419, s
1354, s
1051, m
1043, m
946, m
687, s
626, s
≈556, br
≈521, br
≈ 500, br
–
–
–
–
ν(OH) –H2O
δ(HOH) –H2O
νas(CO) –COO–
νs(CO) –COO–
1584, s
1433, sh
1414, s
1351, s
1051, m
1033, sh
946, w
690, s
626, s
–
δ(CH) –CH3
ρ(CH) –CH3
(9)
(10)
(11)
(12)
(13)
(14)
(CC) –CH3–COO
δ(OCO) –COO–
ν, ρ, ω(Cu–O)
–
–
s: strong, m: medium, sh: shoulder, w: weak, br: broad
modes (Table 2). This hump is not present in the spectrum of
the anhydrous phase, therefore it is most likely attributed to
Cu–O(H2O) related bands. As the position of the apical water
molecule is occupied by an acetate related oxygen atom of the
neighboring Cu2(CH3COO)4 paddle wheel (Figure 5b), these
modes are most likely shifted towards the lattice modes in the
spectrum of the anhydrous phase.
Magnetic Properties
The magnetic properties of copper(II) acetate monohydrate
were analyzed early on by Bleaney and Bowers in terms of
a spin exchange coupled spin S = 1/2 dimer[24a] assuming a
Hamiltonian given by:
tra of the copper acetate phase can be observed in the spectral where J is the intra-dimer spin exchange constant.
region from 600 to 400 cm–1. In the spectrum of the monohy-
They derived an analytical relationship for the temperature
drate there is a broad hump that is caused by several broad, dependence which in turn became well known as the Bleaney–
overlapping bands, which can be assigned to Cu-O related Bowers equation.[32] The temperature dependence of the mag-
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Journal of Inorganic and General Chemistry
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netic susceptibility of a S = 1/2 dimer is characterized by a
very broad hump with a maximum at Tmax which is uniquely
related to the spin exchange J between the S = 1/2 entities by
(kB Tmax) / J ≈ 0.623
and a Curie–Weiss temperature dependence of the suscep-
tibility at temperature kBT ϾϾ J. The massive depression of
the magnetic susceptibility of copper(II) acetate monohydrate
at room temperature as compared to other Cu salts had already
been noted by Lifschitz and Rosenbohm as early as 1915
[Equation (1)].[33]
(1)
may become appropriate, where α = J2/J1 gives the ratio be-
tween intra-dimer exchange coupling, J1, and inter-dimer cou-
pling J2. Here one assumed inter-dimer coupling to one mo-
ment of neighboring dimer only. The case of isolated dimers
is given by J2 = 0.
Quantum Monte Carlo calculations for the magnetic suscep-
tibilities of the system described by the Hamiltonian alternat-
ing chains with discrete ratios 0 Յ α Յ 1 have been reported
which have been approximated by a Padé series allowing to
fit experimental data and to extract general values of the alter-
nation parameter α.[24b,34]
Figure 10. Magnetic susceptibilities measured at 1 Tesla of (a) cop-
per(II) acetate monohydrate and (b) copper(II) acetate corrected for
minute Cu2+ monomer impurities. Concentrations are of the order of
0.2% and add a Curie-tail showing up towards lowest temperatures.
The circles represent the experimental data, the solid red line the fit
with the alternating spin S = 1/2 Heisenberg chain and the blue solid
line the difference between experimental data and the fit multiplied by
a factor of 10.
In order to fit the Padé approximant to experimental data
we have first subtracted a minute contribution from monomers
which typically give rise to a Curie-type increase at low tem-
peratures. The difference was fitted to Equation (1) allowing
the variation of the spin exchange constant J1, the alternation
parameter, α, the g-factor, g, and a temperature independent
contribution, χ0, comprising the diamagnetic contributions
from the electrons in the closed shells and the van Vleck con-
tribution from excitations in the open 3d electron shells [Equa-
tion (2)]:
The experimental data are fitted very well by the magnetic
susceptibility of an alternating chain model. The spin exchange
constants of copper acetate monohydrate and anhydrous cop-
per acetate are identical as may have been expected from the
very similar structural parameters of the paddle wheels in both
compounds. The value of 457 K is also in good agreement
with the value of 446Ϯ21 K reported in literature.[35] We con-
sistently find a finite value for the alternation parameter α,
which appears to indicate some inter-dimer spin exchange. α
is by 50% larger for the anhydrous copper acetate consistent
with the closer approach of the dimer units in the crystal struc-
ture. For anhydrous copper acetate a value of 0.06 indicates an
inter-dimer exchange of ca. 27 K. However, we emphasize that
χ(T) = χspin (J1, α, g, T) + χ0
(2)
The experimental data together with the fits and the differ- the effect of a finite α shows up in the susceptibilities at very
ence between measured and fitted molar susceptibilities are high temperatures only. Other effects which are not considered
displayed in Figure 10, for the χmol·T graphs see also Figure in the fits, as for example a temperature dependence of the
S5 (Supporting Information). The results are summarized in spin exchange or the splitting of the excited spin triplet, lead
Table 3.
to the finite values of α.
Table 3. Results of the fits of the Padé approximant of the magnetic susceptibility of a spin S = 1/2 alternating chain model defined by
Equation (1) to the experimental data of copper acetate monohydrate and copper acetate.
Compound
J1/kB /K
α
g
106 ϫχ0 /cm3·mol–1
Imp. conc. /%
1–0–1 phase
1–0–0 phase
457(1)
457(1)
0.04(2)
0.06(2)
1.941(4)
1.946(3)
–18(1)
–21(1)
0.18
0.22
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Journal of Inorganic and General Chemistry
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Zeitschrift für anorganische und allgemeine Chemie
Consistently, we observe g-factors for both compounds
Experimental Section
which are too small as compared to literature values where
powder-averaged values of ca. 2.16 can be derived from EPR
results.[35] We can rule out significant monomer impurities
which gave rise to minute low-temperature Curie-tails. Their
concentration was of the order of 0.2% or less. Crystalline
impurities in substantial amounts to fudge the sample weight
could also not be detected in the XRD patterns. Non-crystalline
non-magnetic impurities are therefore considered.
Model Experiment: Copper Corrosion by Diluted Acetic Acid: In
order to investigate the possible formation of anhydrous copper(II)
acetate, the medieval pigment production by intentional corrosion of
copper metal by diluted acetic acid (vinegar) was performed in a model
experiment as it already has been done by Roja et al.[12] and San
Andrés et al.[13] For the experiments 120ϫ40ϫ0.5 mm copper
(99.99%) plates were used. Adherent grease was removed in an acet-
one bath. Subsequently the adherent acetone was removed by exposing
the plate to an air stream in the fume hood for 24 h. Afterwards the
plates were placed in ca. 1 L preserving glasses (Supporting Infor-
mation, Figure S2). In total three model experiments were performed:
Conclusions
Experiment I: A 100 mL beaker filled with 50 mL 5% acetic acid (pH
≈ 2.0) was placed in the preservation vessel in a way that there was
no contact between the copper plate and the liquid acid (Figure 11a).
The glass was heated up to 40 °C. After one hour the glass was sealed
and stored for 6 months.
Anhydrous copper(II) acetate and copper(II) acetate mono-
hydrate are components of historic verdigris pigments, in par-
ticular of neutral or crystallized verdigris. The formation of
this phase by intentional corrosion of copper metal by using
vapors of diluted acetic acid could not be proven. A pure sam-
ple of the title compound was obtained by thermal dehydration
of Cu(CH3COO)2·H2O and drying of a Cu(CH3COO)2·
H2O–Cu(CH3COO)2 mixture in nitrogen atmosphere using
P2O5 as drying agent, samples of pure anhydrous
Cu(CH3COO)2 were obtained. The crystal structure was solved
ab initio using in situ synchrotron XRPD data at 150 °C and
laboratory XRPD data at ambient conditions. Both
Cu(CH3COO)2·H2O
and
Cu(CH3COO)2
contain
Cu2(CH3COO)4 paddle wheels with short Cu–Cu distances of
2.6 Å as main motif. Hence, both compounds show a similar
bluish-green color and almost identical UV/Vis reflectance
spectra. Due to the δ bonding anhydrous Cu(CH3COO)2, like
the monohydrate, exhibits an antiferromagnetic dimer behav-
ior. In Cu(CH3COO)2·H2O the coordination sphere of copper
is filled by hydrate water. By removal of the water molecules,
the void in the coordination sphere is filled by the acetate re-
lated oxygen of the neighboring Cu2(CH3COO)4 paddle wheel,
leading to the formation of one dimensional [Cu2(CH3COO)4]n
paddle wheel chains. Despite the chain-like motif the IR spec-
trum of anhydrous copper(II) acetate exhibits vast similarities
with the spectrum of the monohydrate, in particular in the re-
gion of CO related modes. Due to the interaction of neigh-
boring Cu2(CH3COO)4 paddle wheels the splitting of the
asymmetric and symmetric CO stretching modes is smaller in
the spectrum of the anhydrate than in the spectrum of the
monohydrate but only to a small extent, which shows that
these modes are governed by the paddle wheel itself. Enforced
inter-dimer interactions in copper acetate as compared to the
monohydrate are also found by the magnetic susceptibility data
which appear to indicate increased inter-dimer spin exchange.
With the complete characterization of anhydrous copper(II)
acetate, a reliable database for the substance identification by
XRPD and IR spectroscopy is available now. Reference data
for the powder pattern of the title compound that have been
already published and that were used to identify anhydrous
copper(II) acetate in historic verdigris samples, can be evalu-
ated as reliable and therefore the presence of this phase in
historic verdigris samples, is confirmed.
Figure 11. Set up of (a) model experiment I and III and of (b) model
experiment II.
Experiment II: The preservation glass was filled with 700 mL 5% ace-
tic acid (pH ≈ 2.0) in order to provide direct contact between the cop-
per plate and the liquid acid (Figure 11b). The glass was heated up to
40 °C. After one hour the glass was sealed and stored for 6 months.
Experiment III: In this experiment the same set up as in experiment I
was used. The glass was stored at 40 °C for 14 d.
After the storage the glasses were opened and the metal coupons dried
for 24 h in the air-stream of the fume hood. Afterwards the corrosion
phases were sampled from different positions of the plates and investi-
gated by XRPD.
Synthesis of Cu(CH3COO)2: Cu(CH3COO)2 was obtained by heating
Cu(CH3COO)2·H2O (Merck, p.A.) in a borosilicate capillary using a
hot air blower (Oxford Cryosystem) up to 150 °C. A sample of
Cu(CH3COO)2 was also purchased from Alfa Aesar (p.A.). As initial
investigation by XRPD revealed the purchased sample to be a mixture
of Cu(CH3COO)2 and Cu(CH3COO)2·H2O, the powder was stored in
a desiccator over P2O5 (ABCR chemicals, p.A.) in a glove box in a
nitrogen atmosphere at room temperature for 2 months. Afterwards the
solid was found to be completely dehydrated.
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Journal of Inorganic and General Chemistry
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Phase Characterization: Thermal analysis was carried out using a
fined in z-matrix notation and translated and rotated freely through the
TG/DTA 22 of Seiko instruments (reference substance: Al2O3, unit cell. A merging radius of 0.7 Å[42] was applied to account for
open platinum crucible, nitrogen flow 50 mL·min–1, heating rate
atoms sitting on symmetry centers. After few hours the positions of
2 K·min–1). The magnetic measurement of a powder sample enclosed all atoms were found. The procedure was carried out several times
in gel capsules was carried out in the temperature range from 1.8 to and in result the simulated annealing process always yielded identical
300 K in a DC mode using a SQUID magnetometer (MPMS XL,
Quantum Design) at 0.1, 1.0, 3.0, 5.0 and 7.0 T. Infrared spectra of
Cu(CH3COO)2 and of Cu(CH3COO)2·H2O were recorded on in attenu-
structural models that were independent from the starting parameters,
i.e. the number of atoms that were initially put into the unit cell. For
the final Rietveld refinement,[43] all profile and lattice parameters were
ated total reflection (ATR) geometry with a Perkin–Elmer Spectrum released iteratively and positions of the copper ions were subjected to
Two equipped with a diamond crystal. The background spectrum was
measured separately and subtracted.
free unconstrained refinement. A small amount of residual
Cu(CH3COO)2·H2O (3.4 wt%) was detected, as well and included into
the final Rietveld refinement as a second phase. The bond lengths and
angles of the rigid bodies were refined, restraining them to reasonable
Temperature Dependent in situ X-ray Powder Diffraction Experi-
ments (XRPD): Powder diffraction experiments were performed with values and the hydrogen positions were omitted due to the limits of the
a D8-Advance diffractometer (Bruker, Cu-Kα1 radiation from primary powder diffraction method. In order to derive structural information at
Ge(111)-Johannson-type monochromator, Lynx Eye position sensitive room temperature the Cu(CH3COO)2 sample was used that was ob-
detector (Bruker)) in Debye–Scherrer set-up using a water cooled fur-
tained by drying over P2O5 over 2 months. The cell parameters were
nace (mri capillary heater, 25–1000 °C) for heating the capillary. The determined by a Pawley fit[37] and the crystal structure determined at
sample was sealed in a 0.7 mm diameter quartz glass capillary (Hilgen-
berg), which was spun during the measurement. The patterns were
150 °C was used as a starting model for a Rietveld refinement.[43] For
constraining the atomic positions of the acetate related atoms, rigid
collected with a scan range of (5.0 to 50.0) °2θ, employing a step size bodies defined in z-matrix notation were introduced. The bond lengths
of 0.005 and a total scan time of 4 h. The XRPD patterns were re-
corded in a temperature interval from 30 to 290 °C in 10 K steps.
and angles of the acetate ions were refined restraining them to reason-
able values. In an alternative approach the crystal structures of the
Between the steps a heating rate of 0.5 K·s–1 was applied. In order related transition metal acetates were used a starting models for a Riet-
to ensure thermal equilibration a delay time of 15 min before each veld refinement,[43] which led to considerably worse agreement factors.
measurement was used. For crystal structure determination a powder The final agreement factors are listed in Table S2 (Supporting Infor-
pattern was recorded at the High Resolution Powder Diffraction Beam-
line (I11) at the Diamond Light Source (UK) with a wavelengths of
mation), the atomic coordinates and selected bond lengths are given
in Tables S3 and S4 (Supporting Information), a figure with atom lab-
0.8260(1) Å (E ≈ 15.01 keV) in transmission geometry using multi- els is shown in Figure S1, the fits of the whole powder patterns are
analysing crystals (MAC) detectors.[36] For the measurement, the sam-
ple was sealed in a 0.5 mm glass capillary (Hilgenberg, Glass No.
0014) that was spun during the measurement. The capillary was heated
with an Oxford Cryosystem hot air blower to 150 °C. Calibration of
the X-ray beam was carried out using a Si640c NIST standard, which
was measured in a 0.5 mm borosilicate capillary, as well. A Pawley
refinement[37] returned the exact wavelengths and the 2θ zero-offset.
For determination of the instrumental resolution function a diffraction
pattern of the NIST line profile standard Si640D was recorded using
the same experimental conditions. For refining the crystal structure at
room temperature a laboratory powder diffractometer in Debye–
Scherrer set-up [Stadi P-Diffractometer (Stoe), Cu-Kα1 radiation from
primary Ge(111)-Johann-type monochromator, Mythen 1 K detector
(Dectris)]. The sample was sealed in 0.5 mm diameter borosilicate
glass capillary (Hilgenberg glass No. 0140) in a nitrogen atmosphere
in a glove box. The capillary was spun during the measurement. The
XRPD pattern was collected using the same device applying a scan
range from (5.0 to 110)° 2 θ and a total scan time of 20 h.
shown in Figures S3 and S4 (Supporting Information).
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-1859444 and CCDC-1859445. (Fax: +44-1223-336-
033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk)
Supporting Information (see footnote on the first page of this article):
The supporting information contain additional graphs and figures on
the measured XRPD patterns of the corrosion phases obtained by the
model experiments and the experimental setup, a comparison of the
calculated and the reported XRPD pattern of Cu(CH3COO)2, detailed
crystallographic information, the graphical results of the final
Rietveld refinements and χmol vs. T plots of Cu(CH3COO)2 and
Cu(CH3COO)2·H2O.
Crystal Structure Solution: The program TOPAS 6.0[38] was used to
determine and refine the crystal structure of Cu(CH3COO)2 at 150 °C
and ambient conditions. Indexing of the phase was carried out by an
iterative use of singular value decomposition (LSI)[39] leading to a
primitive triclinic unit cell with lattice parameters given in Table S2
Acknowledgments
M.Sc. Julia Kröger and M.Sc. Alexander M. Pütz from the Max Planck
Institute for Solid State Research are acknowledged for measuring the
UV/Vis and the IR spectrum, Eva Brücher from the Max Planck Insti-
tute for Solid State Research for performing the SQUID measurements.
Funding by DFG for the project “In search of structure“ (grant EG
137/9–1) is gratefully acknowledged. We also thank Diamond Light
Source for access to beamline I11 that contributed to the results pre-
sented here.
¯
(Supporting Information) and P1(2) was determined as most probable
space group. The overall cell metric was found to be similar to anhy-
drous M(CH3COO)2 with M = Cr,[26] Mo,[27b] Rh[28] (Table 1). The
peak profile and the precise lattice parameters were determined by
LeBail[40] fits applying user defined convolutions for the description
of the instrumental resolution function (see above). The background
was modeled by employing Chebychev polynomials of 6th order. The
crystal structure of Cu(CH3COO)2 was solved by applying the global
optimization method of simulated annealing (SA) in real space as it is
implemented in TOPAS.[41] Rigid bodies for the acetate ions were de-
Keywords: Copper acetate; Structure elucidation; Carboxyl-
ate ligands; X-ray diffraction; Magnetic properties
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Journal of Inorganic and General Chemistry
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Zeitschrift für anorganische und allgemeine Chemie
[23] a) I. M. Bell, R. J. H. Clark, P. J. Gibbs, Spectrochim. Acta Part
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Received: June 14, 2019
Published Online: ᭿
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
S. Bette,* A. Costes, R. K. Kremer, G. Eggert, C. C. Tang,
R. E. Dinnebier ............................................................... 1–11
On Verdigris, Part III: Crystal Structure, Magnetic and Spectral
Properties of Anhydrous Copper(II) Acetate, a Paddle Wheel
Chain
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