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Structural dependence of absorption spectra of β-diketone chelates—I
Spectrochimica Acta, 1960,
Vol.
16,
pp. 889
to 695.
Pergamon
Press Ltd.
Printed in Northern
Ireland
Structural dependence of absorption spectra of p-diketone chelates-1 I&a-red J. CHARETTE and P. TEYSSIB Departments of Physics and Chemistry, Lovanium University, Leopoldville Belgian Congo (Received5 February 1960) Abstract-The infrared and ultra-violet spectra of series of /?-diketone chelates were recorded and the variations interpreted in terms of type of the bonding involved. The infrared shifts were related to the stability constants of the complexes formed as measured by Bjerrum’s potentiometric method, whereas the ultra-violet spectra seem to depend on the spatial configuration of the orbitals involved. Assignments are proposed for the different absorption bands of the spectra, and resonance effects through the chelate ring are discussed.
Introduction IN THE course of a comparative study of chelates formed by polymeric ligands low molecular weight model compounds, we were interested in determining nature of the complexes formed by polymethacroylacetone (PMA) and its molecular weight analogue; pivaloylacetone (PA), as well as those of corresponding monomer, methacroylacetone (MA).
CH, n
-4
CH,-C--CR,
CH,=C I c=o
LO / CH \
“& c-o
/
I CH, where n = 300 PMA
/ CH \
and the low the
“B c-o I CH,
PA
/
c-o I CH, MA
In order to get a representative variety in the type of bonding, we studied the chelates formed by the above diketones with the following metals: magnesium, manganese, nickel, zinc, copper and uranyl. Both infra-red and ultra-violet spectra showed a definite dependence on the nature of the metallic cation involved in the complex. This paper presents the results of infra-red measurements. 689
J.
CHARETTE and P. TEYSSIB
Experimental * Methacroylacetone and its polymer were prepared from acetone and methyl methacrylate as described earlier [l]. Pivaloylacetone was synthesized following the procedure of ADAMS and HAUSER [2]. The low molecular weight chelates were obtained by partial neutralization of equivalent (l/S) mixtures of metal ion and &ketone. The products were recrystallized in alcohol or alcohol-water mixtures. The polymeric chelates which could not be recrystallized were thoroughly washed with water to eliminate the metallic salts used for preparation. The infra-red spectra were taken between 1800 and 1200 cm-l as Nujol mulls with the Perkin-Elmer 112 G prism-grating spectrometer, giving a wave-number reproducibility of 1 cm- I. In the regions of strong Nujol absorption bands, mulls of hexachlorobutadiene were used. Absorption from atmospheric water vapour was avoided by flushing the apparatus with air dried on activated alumina.
Results and discussion The infra-red spectra are gathered in Figs. l-3. The assignment of the most important absorption bands of similar compounds has already been discussed by many authors [3-71. Although still questioned by several workers, the attribution of the 1600-1560 cm-l band to the C=O vibration and of the 1515 cm-1 band to the C=C vibration seems the most likely one. This point of view is supported by the work of DRYDEN and WINSTON [7], who displaced the 1515 cm-l absorption band by substitution of a bulky gsoup on the C3 carbon of the diketone, and also by the discussion of some Raman spectra given by LECOMTE [3]. These results are still enhanced by the definitely different behaviour of the two bands when changing the nature of the metal. The 1600 cm-l band, attributed to the C=O vibration may undergo three different influences: (a) The three diketones are practically completely in the enol form, which allows a cyclization by hydrogen bonding (see the formulae given above); this is shown by the lack of any significant absorption at 1700 and 1720 cm-l and the presence of a strong band in the 1600 cm-’ region [8]. (b) A conjugation effect arises in the methacroylacetone and its chelates from the usual lowering in frequency is the presence of the 2-isopropenyl group; observed: the centre of the band is at 1584 cm-l for the methacroylacetone while it is at 1605 cm-l for the polymethacroylacetone and at 1607 cm-l for the pivaloylacetone. Shifts occur in the same direction for the corresponding chelates. This effect is greater for the pure diketones (21 cm-l) than for the chelates for which a mean value of 11 cm-l was found, the shift observed decreasing with * With the collaboration of E. MIFUNA. l] P. TEYSSI~ and G. SMETS,Makromol. Chem. 26, 245 (1958). 21 J. T. ADAMS and C. R. HAUSER, J. Am. Chena.Sot. 66, 1220 (1944). .3] J. LECOMTE, Discussions B’aradaySot. 9, 125 (1960). 41 L. J. BELLAMY and L. BEECHER,J. Chem. Sot. 4487 (1954). .51L. J. BELLAMY and R. F. BRANCH, J. Chem. Sot. 4491 (1964). 61 H. F. HOLTZCLAW,JR. and J. P. COLLMAN,J. Am. Chem. Sot. 79, 3318 (1957). .7] R .P. DRYDEN and A. WINSTON, J. Phys. Chem. 62, 635 (1958). 181M .C. DE WILDE-DELVAUX and P. TEYSSII&Spect~ochim. Act% 12, 289 (1958).
690
Structural dependence of absorption spectra of fi-diketone chelates-I
increasing stability of the corresponding chelate. We suggest thst this conjugation effect weakens ss the resonance possibilities increase in the chelate ring. (c) A direct relationship was observed between the extent of the shift of the C=O absorption frequency in the chelate derivatives of a given diketone and the formation constant measured by potentiometric methods [9]. Fig. 4 illustrittes this
Fig. 1. Infra-red spectra of methacroylacetone
complexes.
A similar effect was already considered by BELLAMY [5] and the first author was able to detect such a relation for salicylaldehyde chelates but not for diketone ones, while HOLTZCLAW pointed out a parallelism between frequency and stability of acetylacetone chelates of several metals, with, however, an erratic behaviour for nickel. Each of the three diketones investigated here exhibits the same relationship; the relation holds for all the cations studied excepting manganese for which the frequency shift is a little too high. relationship. *
HOLTZCLAW [6];
l When the C = 0 characteristic absorption band exhibited a more or less pronounced shoulder, the frequency of the centre of the band was retained. [9] M. T. TEYSSI~ and P. TEYSSII~,J. Polymer. Sci. In press.
691
J.
CHARETTEand P. TEYSSI~~
The C=C vibration undergoes a drastic lowering of frequency by replacement of the proton by a metallic cation. In the free diketones, we found this frequency at 1609 cm-i for the methacroylacetone while it appears as a shoulder at 1598 cm-l for polymethacroylacetone and pivaloylacetone. By replacement of the proton by a metallic cation, the frequency shifts to a value located between 1513 and 1523 cm-l, practically irrespective of the nature of both the ligand and the metal.
Fig. 2. Infra-red spectra of polymethacroylacetone complexes.
This important shift due to the presence of a metal may be partly attributed to a mass effect: a very rough calculation was done on a model with three masses corresponding respectively to the 3- and 4-carbon atoms and the OH or OM group: CH=C-OM A fairly reasonable value of 7 x lo5 dyn/cm was taken for the C=C force constant and 5 x lo5 dyn/cm for the C-O force constant. A value of 1570 cm-l was obtained for the C=C vibration frequency of the free diketone (observed 1598) shifting to 1558 cm-l for a chelate with a metal of atomic weight 60 (found 1518), to 1555 cm-1 for a metal of mass 270 and to 1549 cm-l for infinite mass. Hence, the mass effect alone does not account for the whole shift observed. A reasonable 692
Structural dependence of absorption spectra of ,8-diketone chelates-I
--------~
Fig. 3. Infra-red spectra, of pivaloylacetone complexes.
14. 12. 10.
2
Fig. 4. Relationship between the stability of the complexes and the shift of the C=O absorption frequency.
693
J. CEARETTEand P. TEYSSI~
explanation could be the enhanced ring vs. the free diketonic ring: \
resonance
possibilities \
‘c-0
-f-f \/‘
\
M
ti
c=o
-C
‘c=o
’ \
in the metal chelate
‘M c-o
/
/ / This resonance could lower the frequency of the C=C vibration in the usual direction and account for the additional shift observed. A set of other characteristic absorption bands received the following assignments: (1) The isopropenyl group of the methacroylacetone is responsible for four sets of characteristic absorption bands. One between 1440 and 1395 cm-r is due The band between 1644 and 1634 cm-l corto the CH, deformation vibration. responds to the C=C stretching. The frequency of these bands seems to be a function of the stability of the chelate ring, the frequency decreasing with the In the 3100 and 900 cm-l regions, absorption bands are increasing stability. found which correspond, respectively, to the CH stretching and the out-of-plane CH vibration. (2) The tertiary butyl group of the pivaloylacetone is responsible for a set of three absorption bands located at about 1376-1411, 1361 and 1285 cm-l. (3) The -CH, vibrations common to the three ligands are found as expected around 1445 and 1375 cm-l. (4) All the products investigated exhibit a medium absorption in the region of 1230 cm-i, where are found usually the C-O absorption bands. This band is located at 1214 cm-i for the methacroylacetone, 1221 cm-l for the pivaloylacetone and 1232 cm-i for the polymethaoroylacetone, while it is found at 1229 f 3 cm-l chelates and for methacroylacetone chelates, 1230 & 4 cm-l for pivaloylacetone 1237 f 10 cm-i for polymethacroylacetone chelates. If the band is to be attributed to the C-O vibration, the absence of important shift between the free ligand and the corresponding chelates could be ascribed to the antagonist character of the two effects involved, namely a mass effect which is due to the metal and lowers the frequency, and a resonance effect which shortens the C-O bond length and increases the frequency. Let us consider as an illustration three chelates of pivaloylacetone. The copper and uranyl chelates have about the same stability: values of 11.06 and 11.11, respectively, were found for the half log of the overall formation constant. As a result the mass effect plays the first role and the frequency shifts from 1234 cm-l for the copper chelates to 1231 for the uranyl. On the other hand, copper and nickel have about the same masses while the nickel chelate is much less stable than the copper one: 7.80 vs. 11.06. As a result, the frequency is lower in the nickel chelate: 1228 cm-l. However, this interpretation is still dubious since X-ray studies of tris-(acetylacetonato)-iron(II1) [lo] and bis-(acetylacetonato)-Gun [ll-131 seems to prove the [IO] [ll] [12] [13]
R. B. ROOF, JR., Acta Cry&. 9,781 (1956). E. A. SHUQAM,Doklady Akad. Nauk, S.S.S.R. 91, 863 (1951). H. KOGAMA, Y. SAITO snd H. KUROYA, J. Inst. Polytech. Osaka City Univ. C 4, 43 (1953). S. SHIBATA and K. SONE, Bull. Chem. Sot. Japan 29, 852 (1956).
694
Structural dependence of absorption spectra of /?-diketone chelates-1
equalization
of the pair of CO distances, \ -c
by an enolate-type \
c=o
’
‘M
\
/
c-o
-c
W
c-o
H \
/
resonance:
M
c=o
/
/ conclusions
A definite relationship is now established between the vibration frequency of the C=O bond and the stability of the complex involved, i.e. the nature of the metal. The shifts observed enhance the possibility of a certain amount of resonance through the metal atom involved in. the ring. However, the equalization of the two CL0 bonds of the ring through this resonance remains questionable; if the few X-ray studies published on the question seem to favour this hypothesis, the shifts observed in the infra-red spectra, although significant, are not important enough to support this point of view. A&nowledgenzenk---One of the writers (Amsterdam) for financial support.
(P. T.)
695
is indebted
to the Van’t
Hoff Foundation
Vol.
16,
pp. 889
to 695.
Pergamon
Press Ltd.
Printed in Northern
Ireland
Structural dependence of absorption spectra of p-diketone chelates-1 I&a-red J. CHARETTE and P. TEYSSIB Departments of Physics and Chemistry, Lovanium University, Leopoldville Belgian Congo (Received5 February 1960) Abstract-The infrared and ultra-violet spectra of series of /?-diketone chelates were recorded and the variations interpreted in terms of type of the bonding involved. The infrared shifts were related to the stability constants of the complexes formed as measured by Bjerrum’s potentiometric method, whereas the ultra-violet spectra seem to depend on the spatial configuration of the orbitals involved. Assignments are proposed for the different absorption bands of the spectra, and resonance effects through the chelate ring are discussed.
Introduction IN THE course of a comparative study of chelates formed by polymeric ligands low molecular weight model compounds, we were interested in determining nature of the complexes formed by polymethacroylacetone (PMA) and its molecular weight analogue; pivaloylacetone (PA), as well as those of corresponding monomer, methacroylacetone (MA).
CH, n
-4
CH,-C--CR,
CH,=C I c=o
LO / CH \
“& c-o
/
I CH, where n = 300 PMA
/ CH \
and the low the
“B c-o I CH,
PA
/
c-o I CH, MA
In order to get a representative variety in the type of bonding, we studied the chelates formed by the above diketones with the following metals: magnesium, manganese, nickel, zinc, copper and uranyl. Both infra-red and ultra-violet spectra showed a definite dependence on the nature of the metallic cation involved in the complex. This paper presents the results of infra-red measurements. 689
J.
CHARETTE and P. TEYSSIB
Experimental * Methacroylacetone and its polymer were prepared from acetone and methyl methacrylate as described earlier [l]. Pivaloylacetone was synthesized following the procedure of ADAMS and HAUSER [2]. The low molecular weight chelates were obtained by partial neutralization of equivalent (l/S) mixtures of metal ion and &ketone. The products were recrystallized in alcohol or alcohol-water mixtures. The polymeric chelates which could not be recrystallized were thoroughly washed with water to eliminate the metallic salts used for preparation. The infra-red spectra were taken between 1800 and 1200 cm-l as Nujol mulls with the Perkin-Elmer 112 G prism-grating spectrometer, giving a wave-number reproducibility of 1 cm- I. In the regions of strong Nujol absorption bands, mulls of hexachlorobutadiene were used. Absorption from atmospheric water vapour was avoided by flushing the apparatus with air dried on activated alumina.
Results and discussion The infra-red spectra are gathered in Figs. l-3. The assignment of the most important absorption bands of similar compounds has already been discussed by many authors [3-71. Although still questioned by several workers, the attribution of the 1600-1560 cm-l band to the C=O vibration and of the 1515 cm-1 band to the C=C vibration seems the most likely one. This point of view is supported by the work of DRYDEN and WINSTON [7], who displaced the 1515 cm-l absorption band by substitution of a bulky gsoup on the C3 carbon of the diketone, and also by the discussion of some Raman spectra given by LECOMTE [3]. These results are still enhanced by the definitely different behaviour of the two bands when changing the nature of the metal. The 1600 cm-l band, attributed to the C=O vibration may undergo three different influences: (a) The three diketones are practically completely in the enol form, which allows a cyclization by hydrogen bonding (see the formulae given above); this is shown by the lack of any significant absorption at 1700 and 1720 cm-l and the presence of a strong band in the 1600 cm-’ region [8]. (b) A conjugation effect arises in the methacroylacetone and its chelates from the usual lowering in frequency is the presence of the 2-isopropenyl group; observed: the centre of the band is at 1584 cm-l for the methacroylacetone while it is at 1605 cm-l for the polymethacroylacetone and at 1607 cm-l for the pivaloylacetone. Shifts occur in the same direction for the corresponding chelates. This effect is greater for the pure diketones (21 cm-l) than for the chelates for which a mean value of 11 cm-l was found, the shift observed decreasing with * With the collaboration of E. MIFUNA. l] P. TEYSSI~ and G. SMETS,Makromol. Chem. 26, 245 (1958). 21 J. T. ADAMS and C. R. HAUSER, J. Am. Chena.Sot. 66, 1220 (1944). .3] J. LECOMTE, Discussions B’aradaySot. 9, 125 (1960). 41 L. J. BELLAMY and L. BEECHER,J. Chem. Sot. 4487 (1954). .51L. J. BELLAMY and R. F. BRANCH, J. Chem. Sot. 4491 (1964). 61 H. F. HOLTZCLAW,JR. and J. P. COLLMAN,J. Am. Chem. Sot. 79, 3318 (1957). .7] R .P. DRYDEN and A. WINSTON, J. Phys. Chem. 62, 635 (1958). 181M .C. DE WILDE-DELVAUX and P. TEYSSII&Spect~ochim. Act% 12, 289 (1958).
690
Structural dependence of absorption spectra of fi-diketone chelates-I
increasing stability of the corresponding chelate. We suggest thst this conjugation effect weakens ss the resonance possibilities increase in the chelate ring. (c) A direct relationship was observed between the extent of the shift of the C=O absorption frequency in the chelate derivatives of a given diketone and the formation constant measured by potentiometric methods [9]. Fig. 4 illustrittes this
Fig. 1. Infra-red spectra of methacroylacetone
complexes.
A similar effect was already considered by BELLAMY [5] and the first author was able to detect such a relation for salicylaldehyde chelates but not for diketone ones, while HOLTZCLAW pointed out a parallelism between frequency and stability of acetylacetone chelates of several metals, with, however, an erratic behaviour for nickel. Each of the three diketones investigated here exhibits the same relationship; the relation holds for all the cations studied excepting manganese for which the frequency shift is a little too high. relationship. *
HOLTZCLAW [6];
l When the C = 0 characteristic absorption band exhibited a more or less pronounced shoulder, the frequency of the centre of the band was retained. [9] M. T. TEYSSI~ and P. TEYSSII~,J. Polymer. Sci. In press.
691
J.
CHARETTEand P. TEYSSI~~
The C=C vibration undergoes a drastic lowering of frequency by replacement of the proton by a metallic cation. In the free diketones, we found this frequency at 1609 cm-i for the methacroylacetone while it appears as a shoulder at 1598 cm-l for polymethacroylacetone and pivaloylacetone. By replacement of the proton by a metallic cation, the frequency shifts to a value located between 1513 and 1523 cm-l, practically irrespective of the nature of both the ligand and the metal.
Fig. 2. Infra-red spectra of polymethacroylacetone complexes.
This important shift due to the presence of a metal may be partly attributed to a mass effect: a very rough calculation was done on a model with three masses corresponding respectively to the 3- and 4-carbon atoms and the OH or OM group: CH=C-OM A fairly reasonable value of 7 x lo5 dyn/cm was taken for the C=C force constant and 5 x lo5 dyn/cm for the C-O force constant. A value of 1570 cm-l was obtained for the C=C vibration frequency of the free diketone (observed 1598) shifting to 1558 cm-l for a chelate with a metal of atomic weight 60 (found 1518), to 1555 cm-1 for a metal of mass 270 and to 1549 cm-l for infinite mass. Hence, the mass effect alone does not account for the whole shift observed. A reasonable 692
Structural dependence of absorption spectra of ,8-diketone chelates-I
--------~
Fig. 3. Infra-red spectra, of pivaloylacetone complexes.
14. 12. 10.
2
Fig. 4. Relationship between the stability of the complexes and the shift of the C=O absorption frequency.
693
J. CEARETTEand P. TEYSSI~
explanation could be the enhanced ring vs. the free diketonic ring: \
resonance
possibilities \
‘c-0
-f-f \/‘
\
M
ti
c=o
-C
‘c=o
’ \
in the metal chelate
‘M c-o
/
/ / This resonance could lower the frequency of the C=C vibration in the usual direction and account for the additional shift observed. A set of other characteristic absorption bands received the following assignments: (1) The isopropenyl group of the methacroylacetone is responsible for four sets of characteristic absorption bands. One between 1440 and 1395 cm-r is due The band between 1644 and 1634 cm-l corto the CH, deformation vibration. responds to the C=C stretching. The frequency of these bands seems to be a function of the stability of the chelate ring, the frequency decreasing with the In the 3100 and 900 cm-l regions, absorption bands are increasing stability. found which correspond, respectively, to the CH stretching and the out-of-plane CH vibration. (2) The tertiary butyl group of the pivaloylacetone is responsible for a set of three absorption bands located at about 1376-1411, 1361 and 1285 cm-l. (3) The -CH, vibrations common to the three ligands are found as expected around 1445 and 1375 cm-l. (4) All the products investigated exhibit a medium absorption in the region of 1230 cm-i, where are found usually the C-O absorption bands. This band is located at 1214 cm-i for the methacroylacetone, 1221 cm-l for the pivaloylacetone and 1232 cm-i for the polymethaoroylacetone, while it is found at 1229 f 3 cm-l chelates and for methacroylacetone chelates, 1230 & 4 cm-l for pivaloylacetone 1237 f 10 cm-i for polymethacroylacetone chelates. If the band is to be attributed to the C-O vibration, the absence of important shift between the free ligand and the corresponding chelates could be ascribed to the antagonist character of the two effects involved, namely a mass effect which is due to the metal and lowers the frequency, and a resonance effect which shortens the C-O bond length and increases the frequency. Let us consider as an illustration three chelates of pivaloylacetone. The copper and uranyl chelates have about the same stability: values of 11.06 and 11.11, respectively, were found for the half log of the overall formation constant. As a result the mass effect plays the first role and the frequency shifts from 1234 cm-l for the copper chelates to 1231 for the uranyl. On the other hand, copper and nickel have about the same masses while the nickel chelate is much less stable than the copper one: 7.80 vs. 11.06. As a result, the frequency is lower in the nickel chelate: 1228 cm-l. However, this interpretation is still dubious since X-ray studies of tris-(acetylacetonato)-iron(II1) [lo] and bis-(acetylacetonato)-Gun [ll-131 seems to prove the [IO] [ll] [12] [13]
R. B. ROOF, JR., Acta Cry&. 9,781 (1956). E. A. SHUQAM,Doklady Akad. Nauk, S.S.S.R. 91, 863 (1951). H. KOGAMA, Y. SAITO snd H. KUROYA, J. Inst. Polytech. Osaka City Univ. C 4, 43 (1953). S. SHIBATA and K. SONE, Bull. Chem. Sot. Japan 29, 852 (1956).
694
Structural dependence of absorption spectra of /?-diketone chelates-1
equalization
of the pair of CO distances, \ -c
by an enolate-type \
c=o
’
‘M
\
/
c-o
-c
W
c-o
H \
/
resonance:
M
c=o
/
/ conclusions
A definite relationship is now established between the vibration frequency of the C=O bond and the stability of the complex involved, i.e. the nature of the metal. The shifts observed enhance the possibility of a certain amount of resonance through the metal atom involved in. the ring. However, the equalization of the two CL0 bonds of the ring through this resonance remains questionable; if the few X-ray studies published on the question seem to favour this hypothesis, the shifts observed in the infra-red spectra, although significant, are not important enough to support this point of view. A&nowledgenzenk---One of the writers (Amsterdam) for financial support.
(P. T.)
695
is indebted
to the Van’t
Hoff Foundation