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The vibrational spectra of the copper(II) formates
Journal of Molecular Structure,
162 (1987)
Elsevier
Amsterdam
Science
Publishers
THE VIBRATIONAL
B.V.,
57-67 -
Printed
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SPECTRA OF THE COPPER(I1) FORMATES
Part IV*. The thermal behaviour of CU(HCOO)~ * 4Hz0 and CU(HCOO)~ * 2HZ0
ANTON
M. HEYNS
Department
of Chemistry,
KLAUS-JURGEN
University of Pretoria, 0002 Pretoria (South Africa)
RANGE
Institute of Inorganic Chemistry, Regensburg (F.R.G.) (Received
University of Regensburg,
Universitiitsstr. 31, D-8400
18 May 1987)
ABSTRACT Several modifications of Cu(HCOO), exist and two, or possibly three, are characterized in the present study by means of Raman measurements. Upon the dehydration of * 4H,O, monoclinic Cu(HCOO), is formed in a topotactical reaction and a Cu( HCOO), Raman study reveals that the environments around the copper and formate ions are very similar in the two compounds. No evidence can be found of the formation of the dihydrate by means of the dehydration of the tetrahydrate, even though DTA/TG measurements show that the dehydration process is complex and occurs in several steps. When monoclinic Cu(HCOO), is further heated it transforms into orthorhombic Cu(HCOO), below 100°C. The dehydration of the dihydrate results in the formation of a modification of Cu(HCOO), which closely resembles the monoclinic modification but behaves differently upon further application of heat. INTRODUCTION
It is well-known that copper(I1) formate tetrahydrate and dihydrate as well as various modifications of the anhydrous salt can be prepared [l-lo]. These various forms have been summarized by Martin and Waterman [ll]. The thermal decomposition of some crystalline modifications of copper(I1) formate has also been reported in the literature [12,13]. The weight loss of copper(I1) tetrahydrate when heated corresponds to four molecules of water and leads to the formation of the blue monoclinic modification of copper(I1) formate [7]. The tetrahydrate and the monoclinic CU(HCOO)~ both have the same space group and the dehydration of the tetrahydrate can therefore be described as a topotactic process [8]. Attempts to recrystallize the dihydrate from the tetrahydrate were unsuccessful [ll] since the crystal structures of the two differ [l, 4, 51. It was once claimed, however, that the *For
Part III see J. Mol. Struct.,
0022-2860/87/$03.50
0 1987
127
(1985)
Elsevier
127. Science
Publishers
B.V.
58
dihydrate could be obtained from the tetrahydrate by heating for one hour at 320 K in air [14]. This behaviour could not be verified in a crystallographic study of the thermal dehydration of the tetrahydrate [7]. The structural relationships between the other various modifications of solid hydrated and anhydrous copper(I1) formate have still not been determined unambiguously. In the present study the effect of heat is reported on the Raman spectra of copper(I1) formate tetrahydrate and dihydrate. The purpose of this work is not only to study the topotactic dehydration of the tetrahydrate or the relationship between the tetrahydrate and the dihydrate, but also to clarify the relationships between various modifications of the anhydrous compound. The corresponding relationships in solution have largely been determined by Martin and Waterman [ll] (Scheme 1) and it will thus be interesting to compare the thermal behaviour of copper(I1) formate in the solid state and in solution. SCHEME 1 The relationships between the various modifications dil. HCOOH r------
+
Cu(HCOO), .4H,O ~==,:.64 BM
Copper carbonate HCooH
.\
+ Blue-green solution containing
of copper(I1) formate [ 111
-H,O ;zs;te)’
Blue Cu(HCOO), P = 1.61 BM
_
Cu(HCOO), .4HCOOH /J = 1.61 BM Heat (100°C)
ICu,(HCOO),l
I
Filtration into ether Cu(HCOO), t 2H,O /J = 1.90 BM
Royal-blue Cu(HCOO), P = 1.90 BM Dehydration
Turquoise Cu(HCOO), /.I = 1.75 BM
EXPERIMENTAL
The high-temperature Raman spectra were recorded on a Z-24 Dilor Raman spectrometer using a high-temperature cell supplied by Dilor Industries, Lille, France. A Coherent Radiation Innova model 90-5 Ar’ laser was used to excite the spectra using both the 488.0 and 514.5 nm lines. Various heating rates were used; in general the results are reported which were obtained with continuous heating at rates of 1°C min-‘. Simultaneous DTA/TG/ DTG measurements were performed using a Rigaku “Thermoflex” apparatus. Isothermal dehydration studies were also undertaken and are compared with the results obtained in the continuous heating studies. Of course, the temperatures at which the hydrated compounds dehydrate are greatly influenced by the heating rates. For example, DTA/TG measurements have shown that at a heating rate of 10°C min-’ CU(HCOO)~ - 4Hz0 dehydrates fully at 95°C; however, isothermal dehydration studies at 50°C have yielded the same results.
59
The compounds were prepared according to methods reported in the literature [l, 4, 5, 9, 111. They were analyzed for Cu, H, C and 0 and only those samples that corresponded within 0.2% of the calculated percentages for these elements were used. RESULTS
The thermal behaviour of the copper(I1) formates, as determined in the present study is summarized in Scheme 2. SCHEME
2
The thermal
behaviour
Cu(HCOO),
. 4H,O
of the solid copper(I1)
formates Cu(HCOO),
1
Blue_~~$&Hcoo) ) 9& Green-brown < 150°C
>lOO”C Royal-blue
I
. 2H,O
Cu(HCOO),
<15O”C ____*
Cu + H,
I
product
+ 2C0,
The crystal structure of the copper(I1) formate tetrahydrate was determined by Kiriyama et al. [l] and also by neutron diffraction by Okada et al. [ 21. The crystals belong to the monoclinic space group P~,/u--C:~ with 2 = 2 and, as can be seen in Fig. 1, comprise a layer structure, the copper and formate groups being coplanar in the (001) direction. Each copper ion is surrounded by four oxygen atoms in a plane, the distorted octahedral configuration of the copper being completed by coordination to water groups
d -a Fig. 1. The bonding
arrangement
in Cu(HCOO),
. 4H,O
[ 11.
60
above and below this plane. It was previously shown [ll] that the water molecules are relatively free to diffuse parallel to the layers and a topotactic dehydration can accordingly be explained. However, if the water molecules are removed, then the only way of retaining the (4 + 2) coordination of the copper ions is if the layers move closer together so that the copper ions can be coordinated to oxygen atoms of the formate groups above and below the layers. For this reason, while the a and b axes are virtually identical in the tetrahydrate and blue Cu(HCOO),, the c axis in the latter is approximately half as long as the corresponding one in the tetrahydrate [ll] . The Raman spectra of the tetrahydrate and the blue modification of CU(HCOO)~ have been published [15-171. However, the spectral bands have not been followed when the samples were continuously heated. It has been remarked elsewhere that the Raman spectra of the tetrahydrate and the blue modification of Cu(HCOO), are remarkably similar [15], as can be expected if the topotactic nature of the dehydration process is considered.
I
250 .
I
1 150
I 50
cm-l
Fig. 2. The Raman-active lattice modes of Cu(HCOO), blue Cu(HCOO), (bottom). The shoulder at ca. 138 clearly evident in this figure.
. 4H,O cm-’
(top) and the monoclinic in the bottom part is not
61
The Raman results obtained in the low-frequency range when the tetrahydrate is heated are shown in Fig. 2 and it is evident that a great similarity exists between the low-frequency vibrations of the tetrahydrate and blue CU(HCOO)~. The Raman spectrum of the tetrahydrate agrees with the ones published in the literature [15, 161; however, these spectra have not been reported below 70 cm-’ and the intense mode observed at 41 cm-’ is reported for the first time here. The low-frequency IR modes in CU(HCOO)~ * 4Hz0 have been assigned to vibrations of the layers [18]. On the other hand, the Raman spectrum of the dehydration product obtained at 80°C is virtually identical with that of blue CU(HCOO)~ [15]. Some frequency shifts also . 4Hz0 is dehydrated and they are clearly evident occur when CU(HCOO)~ in Fig. 3 depicting the temperature dependence of the lattice modes of the tetrahydrate. In addition, the intensity of the 76 cm-’ band is also notably higher in the anhydrous compound than in the tetrahydrate. It is not the purpose of the present work to give full and unambiguous assignments of all the vibrational bands; however, it must be noted that a rocking librational mode of the formate groups has been observed in the frequency range of 70-80 cm-’ in both the tetrahydrate [15,16] and in the dihydrate [9] and it is therefore highly likely that the band at 76 cm-’ in blue CU(HCOO)~ represents the rocking librational mode. Isotopic substitution [15] shows that the frequency shift of 1.024 is almost exactly equal to that calculated
Fig. 3. The temperature dependence of the low-frequency modes of Cu(HCOO), . 4H,O (below 5O”C), monoclinic Cu(HCOO), (below 100°C) and orthorhombic Cu(HCOO), (above 100°C). Only the most intense modes of the latter are included in this figure.
62
for this band. The twisting librational modes occur at 159 and ca. 138 cm-l (shoulder) while the mode at 202 cm-’ in CU(HCOO)~ at 80°C could probably be assigned to a wagging librational mode. In accordance with the site group symmetry of C1 of the HCOO- groups, each rotational mode, viz. the wagging, twisting and rocking ones, should split into A,, B,, A,, B, components under C:,, thus explaining the two (Ag/Bg) components observed in the case of some of the lattice modes. Another group-theoretical treatment of CU(HCOO)~ is in terms of chains of [Cu,WC00hl n extending in the a direction with no direct bonding interactions between them. The vibrations so obtained (assuming that the chains have Ci symmetry), show that two rotational modes A, and B, of the chains could be expected in the Raman spectrum, while no translational modes of the chains are Raman-active (the factor group tables in ref. 19 have been used). If it is assumed that the two most intense features in the lowfrequency Raman spectrum of CU(HCOO)~, viz. the ones at ‘76 and 159 cm-’ respectively, represent the A, and B, rotational bands, this approach fails to explain the feature at 202 cm-’ as well as some of the more weakly-defined features. However, if the weak features are ignored, this method can be used as a very rough approximation, showing that the forces between the chains are very weak. On the other hand, in the first approach where the existence of the chains is ignored, the predicted 3A, and 3B, translational modes of the formate groups are also not observed in the Raman spectrum and therefore both models fail to make an accurate prediction of the number of bands in Cu(HCOO),. If the bond distance of the out-of-plane copper-oxygen interactions of 2.65 A (71 is considered in CU(HCOO)~ it is doubtful if real bonds do exist between the copper ions and the oxygen atoms of these formate groups. It is also important to note in this regard that the C-O and C-H modes v2 and vg are identical in the tetrahydrate and the blue Cu(HCOO), [15]. If the water molecules are removed from the tetrahydrate the copper ions must be bonded to formate groups above and below its plane as has been explained before and in this event, the C-O vibrations should indicate this quite clearly. Since v2 (symmetric C-O vibration) is identical in these compounds, it must be concluded that these copper-out-of-plane-oxygen interactions are very weak indeed. Some of the remaining C-O vibrations like, for example, the symmetrical O-C-O bending mode v3 and the asymmetric C-O stretching mode v4 show small frequency shifts when the tetrahydrate is dehydrated [ 151; however, these changes are so insignificant that it can be concluded that the environment of the formate groups is virtually identical in these two compounds. With the very weak forces that are operative between the copper ions and the out-of-plane formate groups, it can be expected that the blue modification of copper(I1) formate will not be stable over a wide temperature range, and this is exactly what has been observed. The dehydration process of the tetrahydrate starts just above room temperature (Fig. 3), and at 100°C the
63
crystals turn dark blue and the Raman spectrum of the product is characterized by the development of a great many lattice modes. It is obvious that a different modification of copper(I1) formate has formed. If the dark blue crystals formed at 100°C when copper(I1) formate tetrahydrate is dehydrated, are indeed identical to the royal-blue Cu(HCOO), reported by Martin and Waterman [ll] , then the transition blue Cu(HCOO), + darkblue CU(HCOO)~ in the solid state is reported here for the first time. The crystal structure of dark-blue Cu(HCOO), has been reported by Barclay and Kennard [ 61 and Burger and Fuess [8]. The crystals belong to the orthorhombic space group Pbca (D:;), 2 = 8 with all the atoms occupying general positions. There are no direct molecules in the crystal which consists of a three-dimensional array of copper ions joined together by the
Fig. 4. Some Cu(HCOO),.
internal
Raman
active
HCOO‘
modes
in
the
dark-blue
orthorhombic
64
formate groups in an anti-syn way. This also explains the existence of crystallographically distinct formate groups linking the copper ions. It could be expected that such a bonding arrangement will give rise to a very complicated Raman spectrum. The Raman spectra obtained of pure samples of dark-blue CU(HCOO)~ are shown in Fig. 4. The symmetrical C-O stretching and the in-plane C-H bending modes vz and vg are indicated in this figure, as well as the asymmetrical C-O stretching mode v4. It is clearly evident that each of these modes is split into two components, representing the distinct formate groups. The C(l)-O(l) = 1.226 A, C(1)-0(3) = 1.261 A and C(l)-H(1) = 1.079 A distances are distinctly different from the C(2)+(2) = 1.265 A, C(2)-0(4) = 1.283 A and C(2)-H(2) = 1.052 A [8] and it is therefore only reasonable to expect these differences in bond lengths to be reflected in the Raman spectra. Furthermore it can be noted that these bands are completely different from the corresponding ones in blue CU(HCOO)~ [15], CU(HCOO)~ * 2Hz0 [9] and CU(HCOO)~ * 4Hz0 [15]. The remaining modes of the formate groups, viz. the out-of-plane C-H bending mode, the symmetrical C-H stretching mode, the symmetrical 00-C-O bending mode were all observed to be split into at least two components.
/
L50
LOO
350
300
250
200
150
100
cm-1
Fig. 5. The Raman-active
lattice
modes
in dark-blue
orthorhombic
Cu(HCOO),.
65
The copper ions are coordinated as Jahn-Teller distorted pyramids and it can be added that a sixth oxygen ion is also present if the Cu-0 distance of 2.797 A [8] is considered to be bonding. However, it can be expected that Cu-0 vibrations will be present in the Raman spectrum of this compound, and as can be seen in Fig. 5, several modes are observed in the low-frequency spectrum of this modification of CU(HCOO)~ which can be assigned to Cu-0 modes. In CU(HCOO)~ * 4Hz0 the Cu-0 bands in the Raman spectra were observed in the frequency range 316-414 cm-l, which is virtually identical with the corresponding one in the dark-blue CU(HCOO)~ [ 161. The Cu-0 bands shown in Fig. 5 represent Cu-0 bonds which range from 1.942.41 A [8] compared to the ones in the tetrahydrate from 1.94-2.38 A [l, 21, thereby proving that the Cu-0 modes in these compounds are likely to occur in the same frequency ranges. If the dihydrate CU(HCOO)~ * 2Hz0 is heated, the water molecules are lost at relatively low temperatures (
1357tH)
c
Fig. 6. Raman-active 2H,O.
CH
and
CO
modes
illustrating
the
dehydration
of Cu(HCOO),
.
66
very much simpler spectrum of the reaction product represents quite dramatic evidence of the dehydration process. X-Ray powder diffraction patterns of this anhydrous modification obtained by dehydrating the dihydrate show that it is identical to the monoclinic blue modification of CU(HCOO)~. However, we have failed to obtain the dark-blue orthorhombic modification by the application of further heat, showing that this modification is most probably different from monoclinic Cu(HCOO),. The product obtained by heating gradually turned more green, with brown spots (which could possibly be Cu,O) intermixed with it. Powder diffraction patterns show that this consists of CuzO and an unknown phase. It was reported by Martin and Waterman [ll] that the dehydration of CU(HCOO)~ * 2Hz0 gives rise to a turquoise product; however, the spectroscopic and crystallographic results obtained here show that our product obtained is identical to that of blue Cu(HCOO),, with the exception of a low-frequency mode in the Raman spectra. The powder diffraction pattern obtained from the dehydrated Cu(HCOO), .2Hz0 was also different from the turquoise modification of Martin and Waterman [ll]. However, if this compound were identical to blue Cu(HCOO),, the fact that it is not transformed into the dark-blue modification, could then not be explained. CONCLUSION
The application of heat to CU(HCOO)~ . 4Hz0 and CU(HCOO)~ * 2Hz0 gives rise to the dehydration of these compounds well below 100°C. TG/ DTA measurements show that these dehydration processes proceed in at least three steps in both the tetrahydrate and dihydrate with peaks at 55,63 and 95°C and 68, 76 and 90°C respectively recorded with a heating rate of 10°C min-‘. These temperatures are, however, very much dependent on the heating rate. The Raman measurements have proved that the tetrahydrate decomposes into the monoclinic anhydrous compound which in turn transforms at ca. 100°C into the orthorhombic modification. The dihydrate decomposes to a modification which is closely related to monoclinic CU(HCOO)~. Since the application of further heat does not transform the latter into the orthorhombic modification, it must be concluded that the anhydrous modification of Cu(HCO0) 2 obtained by heating the dihydrate is structurally different from the monoclinic modification. ACKNOWLEDGEMENT
A. M. H. wishes to thank the Alexander financial support to complete this work.
von Humboldt
Foundation
for
REFERENCES 1 R. Kiriyama, H. Ibamoto and K. Matsuo, Acta Crystallogr., 7 (1954) 482. 2 K. Okada, M. I. Kay, D. T. Cromer and I. Almodovar, J. Chem. Phys., 44 (1966)
1648.
67 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
K. Osaki, Y. Nakai and T. Watanabe, J. Phys. Sot. Jpn., 18 (1963) 919. M. Bukowska-Strzyzewska, Acta Crystallogr., 19 (1965) 357. M. I. Kay, I. Almodovar and S. F. Kaplan, Acta Crystallogr., Sect. B, 24 (1968) 1312. G. A. Barclay and C. H. L. Kennard, J. Chem. Sot., (1961) 3289. J. R. Giinter, J. Solid State Chem., 35 (1980) 43. N. Burger and H. Fuess, Solid State Commun., 34 (1980) 699. A. M. Heyns, J. Mol. Struct., 127 (1985) 9. A.M. Heyns, J. Mol. Struct., 127 (1985) 127. R. L. Martin and H. Waterman, J. Chem. Sot., (1958) 1359. A. K. Galway, D. M. Jamieson and M. E. Brown, J. Phys. Chem., 78 (1974) 2664. P. Baraldi, Spectrochim. Acta, Part A, 35 (1979) 1003. Y. Masudu, Thermochim. Acta, 39 (1980) 325. A. M. Heyns, J. Mol. Struct., 18 (1973) 471. J. Berger, J. Phys. C, 8 (1975) 2903. R. P. Canterford and F. Ninio, J. Phys. C, 8 (1975) 385. J. Hiraishi, Bull. Chem. Sot. Jpn., 45 (1972) 128. D. M. Adams and D. C. Newton, Tables for Factor Group Analysis, Beckman-RIIC Ltd., Croydon, England, 1970.
162 (1987)
Elsevier
Amsterdam
Science
Publishers
THE VIBRATIONAL
B.V.,
57-67 -
Printed
in The Netherlands
SPECTRA OF THE COPPER(I1) FORMATES
Part IV*. The thermal behaviour of CU(HCOO)~ * 4Hz0 and CU(HCOO)~ * 2HZ0
ANTON
M. HEYNS
Department
of Chemistry,
KLAUS-JURGEN
University of Pretoria, 0002 Pretoria (South Africa)
RANGE
Institute of Inorganic Chemistry, Regensburg (F.R.G.) (Received
University of Regensburg,
Universitiitsstr. 31, D-8400
18 May 1987)
ABSTRACT Several modifications of Cu(HCOO), exist and two, or possibly three, are characterized in the present study by means of Raman measurements. Upon the dehydration of * 4H,O, monoclinic Cu(HCOO), is formed in a topotactical reaction and a Cu( HCOO), Raman study reveals that the environments around the copper and formate ions are very similar in the two compounds. No evidence can be found of the formation of the dihydrate by means of the dehydration of the tetrahydrate, even though DTA/TG measurements show that the dehydration process is complex and occurs in several steps. When monoclinic Cu(HCOO), is further heated it transforms into orthorhombic Cu(HCOO), below 100°C. The dehydration of the dihydrate results in the formation of a modification of Cu(HCOO), which closely resembles the monoclinic modification but behaves differently upon further application of heat. INTRODUCTION
It is well-known that copper(I1) formate tetrahydrate and dihydrate as well as various modifications of the anhydrous salt can be prepared [l-lo]. These various forms have been summarized by Martin and Waterman [ll]. The thermal decomposition of some crystalline modifications of copper(I1) formate has also been reported in the literature [12,13]. The weight loss of copper(I1) tetrahydrate when heated corresponds to four molecules of water and leads to the formation of the blue monoclinic modification of copper(I1) formate [7]. The tetrahydrate and the monoclinic CU(HCOO)~ both have the same space group and the dehydration of the tetrahydrate can therefore be described as a topotactic process [8]. Attempts to recrystallize the dihydrate from the tetrahydrate were unsuccessful [ll] since the crystal structures of the two differ [l, 4, 51. It was once claimed, however, that the *For
Part III see J. Mol. Struct.,
0022-2860/87/$03.50
0 1987
127
(1985)
Elsevier
127. Science
Publishers
B.V.
58
dihydrate could be obtained from the tetrahydrate by heating for one hour at 320 K in air [14]. This behaviour could not be verified in a crystallographic study of the thermal dehydration of the tetrahydrate [7]. The structural relationships between the other various modifications of solid hydrated and anhydrous copper(I1) formate have still not been determined unambiguously. In the present study the effect of heat is reported on the Raman spectra of copper(I1) formate tetrahydrate and dihydrate. The purpose of this work is not only to study the topotactic dehydration of the tetrahydrate or the relationship between the tetrahydrate and the dihydrate, but also to clarify the relationships between various modifications of the anhydrous compound. The corresponding relationships in solution have largely been determined by Martin and Waterman [ll] (Scheme 1) and it will thus be interesting to compare the thermal behaviour of copper(I1) formate in the solid state and in solution. SCHEME 1 The relationships between the various modifications dil. HCOOH r------
+
Cu(HCOO), .4H,O ~==,:.64 BM
Copper carbonate HCooH
.\
+ Blue-green solution containing
of copper(I1) formate [ 111
-H,O ;zs;te)’
Blue Cu(HCOO), P = 1.61 BM
_
Cu(HCOO), .4HCOOH /J = 1.61 BM Heat (100°C)
ICu,(HCOO),l
I
Filtration into ether Cu(HCOO), t 2H,O /J = 1.90 BM
Royal-blue Cu(HCOO), P = 1.90 BM Dehydration
Turquoise Cu(HCOO), /.I = 1.75 BM
EXPERIMENTAL
The high-temperature Raman spectra were recorded on a Z-24 Dilor Raman spectrometer using a high-temperature cell supplied by Dilor Industries, Lille, France. A Coherent Radiation Innova model 90-5 Ar’ laser was used to excite the spectra using both the 488.0 and 514.5 nm lines. Various heating rates were used; in general the results are reported which were obtained with continuous heating at rates of 1°C min-‘. Simultaneous DTA/TG/ DTG measurements were performed using a Rigaku “Thermoflex” apparatus. Isothermal dehydration studies were also undertaken and are compared with the results obtained in the continuous heating studies. Of course, the temperatures at which the hydrated compounds dehydrate are greatly influenced by the heating rates. For example, DTA/TG measurements have shown that at a heating rate of 10°C min-’ CU(HCOO)~ - 4Hz0 dehydrates fully at 95°C; however, isothermal dehydration studies at 50°C have yielded the same results.
59
The compounds were prepared according to methods reported in the literature [l, 4, 5, 9, 111. They were analyzed for Cu, H, C and 0 and only those samples that corresponded within 0.2% of the calculated percentages for these elements were used. RESULTS
The thermal behaviour of the copper(I1) formates, as determined in the present study is summarized in Scheme 2. SCHEME
2
The thermal
behaviour
Cu(HCOO),
. 4H,O
of the solid copper(I1)
formates Cu(HCOO),
1
Blue_~~$&Hcoo) ) 9& Green-brown < 150°C
>lOO”C Royal-blue
I
. 2H,O
Cu(HCOO),
<15O”C ____*
Cu + H,
I
product
+ 2C0,
The crystal structure of the copper(I1) formate tetrahydrate was determined by Kiriyama et al. [l] and also by neutron diffraction by Okada et al. [ 21. The crystals belong to the monoclinic space group P~,/u--C:~ with 2 = 2 and, as can be seen in Fig. 1, comprise a layer structure, the copper and formate groups being coplanar in the (001) direction. Each copper ion is surrounded by four oxygen atoms in a plane, the distorted octahedral configuration of the copper being completed by coordination to water groups
d -a Fig. 1. The bonding
arrangement
in Cu(HCOO),
. 4H,O
[ 11.
60
above and below this plane. It was previously shown [ll] that the water molecules are relatively free to diffuse parallel to the layers and a topotactic dehydration can accordingly be explained. However, if the water molecules are removed, then the only way of retaining the (4 + 2) coordination of the copper ions is if the layers move closer together so that the copper ions can be coordinated to oxygen atoms of the formate groups above and below the layers. For this reason, while the a and b axes are virtually identical in the tetrahydrate and blue Cu(HCOO),, the c axis in the latter is approximately half as long as the corresponding one in the tetrahydrate [ll] . The Raman spectra of the tetrahydrate and the blue modification of CU(HCOO)~ have been published [15-171. However, the spectral bands have not been followed when the samples were continuously heated. It has been remarked elsewhere that the Raman spectra of the tetrahydrate and the blue modification of Cu(HCOO), are remarkably similar [15], as can be expected if the topotactic nature of the dehydration process is considered.
I
250 .
I
1 150
I 50
cm-l
Fig. 2. The Raman-active lattice modes of Cu(HCOO), blue Cu(HCOO), (bottom). The shoulder at ca. 138 clearly evident in this figure.
. 4H,O cm-’
(top) and the monoclinic in the bottom part is not
61
The Raman results obtained in the low-frequency range when the tetrahydrate is heated are shown in Fig. 2 and it is evident that a great similarity exists between the low-frequency vibrations of the tetrahydrate and blue CU(HCOO)~. The Raman spectrum of the tetrahydrate agrees with the ones published in the literature [15, 161; however, these spectra have not been reported below 70 cm-’ and the intense mode observed at 41 cm-’ is reported for the first time here. The low-frequency IR modes in CU(HCOO)~ * 4Hz0 have been assigned to vibrations of the layers [18]. On the other hand, the Raman spectrum of the dehydration product obtained at 80°C is virtually identical with that of blue CU(HCOO)~ [15]. Some frequency shifts also . 4Hz0 is dehydrated and they are clearly evident occur when CU(HCOO)~ in Fig. 3 depicting the temperature dependence of the lattice modes of the tetrahydrate. In addition, the intensity of the 76 cm-’ band is also notably higher in the anhydrous compound than in the tetrahydrate. It is not the purpose of the present work to give full and unambiguous assignments of all the vibrational bands; however, it must be noted that a rocking librational mode of the formate groups has been observed in the frequency range of 70-80 cm-’ in both the tetrahydrate [15,16] and in the dihydrate [9] and it is therefore highly likely that the band at 76 cm-’ in blue CU(HCOO)~ represents the rocking librational mode. Isotopic substitution [15] shows that the frequency shift of 1.024 is almost exactly equal to that calculated
Fig. 3. The temperature dependence of the low-frequency modes of Cu(HCOO), . 4H,O (below 5O”C), monoclinic Cu(HCOO), (below 100°C) and orthorhombic Cu(HCOO), (above 100°C). Only the most intense modes of the latter are included in this figure.
62
for this band. The twisting librational modes occur at 159 and ca. 138 cm-l (shoulder) while the mode at 202 cm-’ in CU(HCOO)~ at 80°C could probably be assigned to a wagging librational mode. In accordance with the site group symmetry of C1 of the HCOO- groups, each rotational mode, viz. the wagging, twisting and rocking ones, should split into A,, B,, A,, B, components under C:,, thus explaining the two (Ag/Bg) components observed in the case of some of the lattice modes. Another group-theoretical treatment of CU(HCOO)~ is in terms of chains of [Cu,WC00hl n extending in the a direction with no direct bonding interactions between them. The vibrations so obtained (assuming that the chains have Ci symmetry), show that two rotational modes A, and B, of the chains could be expected in the Raman spectrum, while no translational modes of the chains are Raman-active (the factor group tables in ref. 19 have been used). If it is assumed that the two most intense features in the lowfrequency Raman spectrum of CU(HCOO)~, viz. the ones at ‘76 and 159 cm-’ respectively, represent the A, and B, rotational bands, this approach fails to explain the feature at 202 cm-’ as well as some of the more weakly-defined features. However, if the weak features are ignored, this method can be used as a very rough approximation, showing that the forces between the chains are very weak. On the other hand, in the first approach where the existence of the chains is ignored, the predicted 3A, and 3B, translational modes of the formate groups are also not observed in the Raman spectrum and therefore both models fail to make an accurate prediction of the number of bands in Cu(HCOO),. If the bond distance of the out-of-plane copper-oxygen interactions of 2.65 A (71 is considered in CU(HCOO)~ it is doubtful if real bonds do exist between the copper ions and the oxygen atoms of these formate groups. It is also important to note in this regard that the C-O and C-H modes v2 and vg are identical in the tetrahydrate and the blue Cu(HCOO), [15]. If the water molecules are removed from the tetrahydrate the copper ions must be bonded to formate groups above and below its plane as has been explained before and in this event, the C-O vibrations should indicate this quite clearly. Since v2 (symmetric C-O vibration) is identical in these compounds, it must be concluded that these copper-out-of-plane-oxygen interactions are very weak indeed. Some of the remaining C-O vibrations like, for example, the symmetrical O-C-O bending mode v3 and the asymmetric C-O stretching mode v4 show small frequency shifts when the tetrahydrate is dehydrated [ 151; however, these changes are so insignificant that it can be concluded that the environment of the formate groups is virtually identical in these two compounds. With the very weak forces that are operative between the copper ions and the out-of-plane formate groups, it can be expected that the blue modification of copper(I1) formate will not be stable over a wide temperature range, and this is exactly what has been observed. The dehydration process of the tetrahydrate starts just above room temperature (Fig. 3), and at 100°C the
63
crystals turn dark blue and the Raman spectrum of the product is characterized by the development of a great many lattice modes. It is obvious that a different modification of copper(I1) formate has formed. If the dark blue crystals formed at 100°C when copper(I1) formate tetrahydrate is dehydrated, are indeed identical to the royal-blue Cu(HCOO), reported by Martin and Waterman [ll] , then the transition blue Cu(HCOO), + darkblue CU(HCOO)~ in the solid state is reported here for the first time. The crystal structure of dark-blue Cu(HCOO), has been reported by Barclay and Kennard [ 61 and Burger and Fuess [8]. The crystals belong to the orthorhombic space group Pbca (D:;), 2 = 8 with all the atoms occupying general positions. There are no direct molecules in the crystal which consists of a three-dimensional array of copper ions joined together by the
Fig. 4. Some Cu(HCOO),.
internal
Raman
active
HCOO‘
modes
in
the
dark-blue
orthorhombic
64
formate groups in an anti-syn way. This also explains the existence of crystallographically distinct formate groups linking the copper ions. It could be expected that such a bonding arrangement will give rise to a very complicated Raman spectrum. The Raman spectra obtained of pure samples of dark-blue CU(HCOO)~ are shown in Fig. 4. The symmetrical C-O stretching and the in-plane C-H bending modes vz and vg are indicated in this figure, as well as the asymmetrical C-O stretching mode v4. It is clearly evident that each of these modes is split into two components, representing the distinct formate groups. The C(l)-O(l) = 1.226 A, C(1)-0(3) = 1.261 A and C(l)-H(1) = 1.079 A distances are distinctly different from the C(2)+(2) = 1.265 A, C(2)-0(4) = 1.283 A and C(2)-H(2) = 1.052 A [8] and it is therefore only reasonable to expect these differences in bond lengths to be reflected in the Raman spectra. Furthermore it can be noted that these bands are completely different from the corresponding ones in blue CU(HCOO)~ [15], CU(HCOO)~ * 2Hz0 [9] and CU(HCOO)~ * 4Hz0 [15]. The remaining modes of the formate groups, viz. the out-of-plane C-H bending mode, the symmetrical C-H stretching mode, the symmetrical 00-C-O bending mode were all observed to be split into at least two components.
/
L50
LOO
350
300
250
200
150
100
cm-1
Fig. 5. The Raman-active
lattice
modes
in dark-blue
orthorhombic
Cu(HCOO),.
65
The copper ions are coordinated as Jahn-Teller distorted pyramids and it can be added that a sixth oxygen ion is also present if the Cu-0 distance of 2.797 A [8] is considered to be bonding. However, it can be expected that Cu-0 vibrations will be present in the Raman spectrum of this compound, and as can be seen in Fig. 5, several modes are observed in the low-frequency spectrum of this modification of CU(HCOO)~ which can be assigned to Cu-0 modes. In CU(HCOO)~ * 4Hz0 the Cu-0 bands in the Raman spectra were observed in the frequency range 316-414 cm-l, which is virtually identical with the corresponding one in the dark-blue CU(HCOO)~ [ 161. The Cu-0 bands shown in Fig. 5 represent Cu-0 bonds which range from 1.942.41 A [8] compared to the ones in the tetrahydrate from 1.94-2.38 A [l, 21, thereby proving that the Cu-0 modes in these compounds are likely to occur in the same frequency ranges. If the dihydrate CU(HCOO)~ * 2Hz0 is heated, the water molecules are lost at relatively low temperatures (
1357tH)
c
Fig. 6. Raman-active 2H,O.
CH
and
CO
modes
illustrating
the
dehydration
of Cu(HCOO),
.
66
very much simpler spectrum of the reaction product represents quite dramatic evidence of the dehydration process. X-Ray powder diffraction patterns of this anhydrous modification obtained by dehydrating the dihydrate show that it is identical to the monoclinic blue modification of CU(HCOO)~. However, we have failed to obtain the dark-blue orthorhombic modification by the application of further heat, showing that this modification is most probably different from monoclinic Cu(HCOO),. The product obtained by heating gradually turned more green, with brown spots (which could possibly be Cu,O) intermixed with it. Powder diffraction patterns show that this consists of CuzO and an unknown phase. It was reported by Martin and Waterman [ll] that the dehydration of CU(HCOO)~ * 2Hz0 gives rise to a turquoise product; however, the spectroscopic and crystallographic results obtained here show that our product obtained is identical to that of blue Cu(HCOO),, with the exception of a low-frequency mode in the Raman spectra. The powder diffraction pattern obtained from the dehydrated Cu(HCOO), .2Hz0 was also different from the turquoise modification of Martin and Waterman [ll]. However, if this compound were identical to blue Cu(HCOO),, the fact that it is not transformed into the dark-blue modification, could then not be explained. CONCLUSION
The application of heat to CU(HCOO)~ . 4Hz0 and CU(HCOO)~ * 2Hz0 gives rise to the dehydration of these compounds well below 100°C. TG/ DTA measurements show that these dehydration processes proceed in at least three steps in both the tetrahydrate and dihydrate with peaks at 55,63 and 95°C and 68, 76 and 90°C respectively recorded with a heating rate of 10°C min-‘. These temperatures are, however, very much dependent on the heating rate. The Raman measurements have proved that the tetrahydrate decomposes into the monoclinic anhydrous compound which in turn transforms at ca. 100°C into the orthorhombic modification. The dihydrate decomposes to a modification which is closely related to monoclinic CU(HCOO)~. Since the application of further heat does not transform the latter into the orthorhombic modification, it must be concluded that the anhydrous modification of Cu(HCO0) 2 obtained by heating the dihydrate is structurally different from the monoclinic modification. ACKNOWLEDGEMENT
A. M. H. wishes to thank the Alexander financial support to complete this work.
von Humboldt
Foundation
for
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