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Infrared spectra of some organic charge-transfer complexes
E)pectmchitica Acta. Vol. 298,pp. 835to 838. PergamonPress1973. P&ted in NorthernIreland
Infrared spectra of some organic charge-transfer complexes M. A.
SLIFIUN
Department of Pure and Applied Physics, The University, Salford M6 4WT, U.K. (Received21 Jwne 1972) Abbab-Some solid organic complexes have been prepared and their infrared ape&a determined. The spectra are discussedin terms of charge-transfertheory. INTRODUCTION
there are many hundreds of publications dealing with the visible and U.V. spectra of charge-transfer complexes, there is only a very limited amount of work on their i.r. spectra. Undoubtedly this is due both to the difficulty of obtaining them pure in the solid state and in the case of the weak complexes the very small i.r. changes of the complexes as compared to their components [l-3]. One interesting aspect of the i.r. spectroscopy of quinone charge transfer complexes is that there are three distinct classes of complexes. Those in which the spectrum is that of the ions of the components [a], medium strength quinhydrone complexes characterized by large shifts of C=C bands and C=O bands from the uncomplexed position to about half-way that of the ions [5] and the weak ones already mentioned in which very small shifts of bands occur [l-3]. ‘The purpose of this work is to obtain more i.r. data on charge transfer complexes to see whether similar class%cations exist with other groups of acceptors.
ALTHOUCIH
EXPERIMENTAL
METHOD
Chemicals were obtained from commercial sources and purified by the standard textbook methods. Complexes were prepared by a variety of techniques. These included precipitation from a cooled saturated solution, evaporation to dryness of solutions, the fusion of the two components, the grinding together of the two components in an agate pestle and mortar and the adding of the components in acetone to a pestle and mortar containing KBr and grinding until all the acetone evaporated off. In spite of these varied approaches there were many systems which whilst forming coloured complexes in solution could not be obtained in the solid state. Noteworthy among these were complexes of maleic anhydride. Many of the coloured complexes obtained were contaminated by uncomplexed component. The spectrum of the complex was elucidated by making up solutions of a wide variety of molar ratios usually varying between 4: 1 to 1: 4 to ensure that at least one of the components in the solid would be fully complexed. 1.r. spectra were obtained in KBr discs using a Unicam SPBOOGspectrophotometer. [l] [2] [3] [4] [5]
H. KAINER and W. OTTINO, Chem. Bes-. 88, 1921 (1965). J. W. EASTMAN, G. M. ANDROES and M. CALVIN, J. Chem. Phya. 86, 1197 (1902). M. A. SLIFEIN, Chem. Phya. Lett. 7, 195 (1970). Y. MATSUNAUA, J. Chem. Phya. 41, 1609 (1964). M. A. SLIFKIN and R. H. WALMSLEY, Spects-ochim. Actu 26A, 1237 (1970). 836
836
M. A. SLIFKIN
One of the difficulties in interpreting the spectra is that frequently the absorption spectra of both components in the free state overlap. This was partially overcome by a differential method in which the spectra of two KBr discs in tandem each containing one of the components in the correct weight was compared to a disc of the complex allowing the effect of overlapping of bands to be allowed for and any new bands or the loss of bands to be seen. In spite of all the techniques outlined above the spectra of many of the complexes are difficult to arrive at. The results given are incomplete in many cases, only obvious changes being given. Out of more than 50 systems looked at, useful or meaningful results could only be obtained for the few quoted herein. RESULTS .um DISCOSSION 1. Pyrene complexes
Pyrene is taken as the holotype of the aromatic hydrocarbons as it proved possible to obtain many complexes of this compormd. (a) Benzoquirume . Two different complexes were obtained from this system. The first obtained by evaporation from acetone is orange in colour and shows the small changes already shown for the pyrene tetrachloroquinone complex [3]. The changes are slight red-shifts from 1655 to 1650 cm-1 of the carbonyl band, from 1370 to 1360cm-l for the -C=G stretch band, and from 1085-1070 cm-l for the C-H in plane deformation. The 900 cm-1 band of the C-H out of plane deformation is missing and two new bands are seen at 870 and 855 cm- l. The spectra is interpretable as arising from the formation of a sandwich structure with some oharge transfer leading to the lowering of bond order in the quinone [3]. The second form of complex is obtained by grinding the two compounds and is purple in colour. This shows many of the characteristics of the quinhydrone type complex, the most notable of which is the shift of the CO band to 1633 cm-l [5]. Other changes are the loss of the C=C band at 1370 cm-l, the loss of the out of plane G-H band at 900 cm-1 and the C-H in plane deformation at 1085 cm-l and the appearance of new bands at 355, 877 and 1260 cm- 1. This is interpreted as arising from a sandwich type structure but with marked charge transfer between the two components [5]. The appearance of two different complexes from the same compounds suggests the existence of different allotropic forms of the complexes. The loss of the in plane C-H deformation band in the quinhydrone type complex is suggestive of a slightly different structure. (b) Trinitobenzene. This is a yellow complex. The main changes are the redshifting of the following bands. The C-H in plane bend [6] goes from 1080 to 1070 cm-l, the C--C stretch from 1443 to 1437 cm-l and from 1626 to 1620 cm-l and the symmetrical NO, stretch from 1345 to 1340 cm-l. This is interpreted as arising from a small amount of charge transfer from pyrene to trinitrobenzene resulting in a iowering in bond order of the acceptor. (c) Picric acid. This yields a deep red complex. There are a large number of [S] H. F. S~WLL, (1967).
J. A. FANIRAN, E. A. SYMONS and E. BUNCEL, Cm. J. Chem. 45, 117,
Infrared spectra of some organic charge-transfer complexes
837
both blue shifts and red shifts in this complex. The following blue-shifts occur for picric acid. From 790 to 785 cm-l for the C-H rock, from 929 to 921 cm-l for the C-H out of plane deformation, from 1098 to 1086 cm-l for the C-H in plane bend. The following red-shifts are seen. From 1165 to 1174 cm-l, OH deformation, 1350 to 1357 cm-l and from 1618 to 1627 cm-l for the C=C stretch. These latter three bands are ail sensitive to both the substitution position and the electronegativity of the substituent [7]. It might be that these changes also arise from an increase of electronegativity as a result of charge transfer. The following changes in pyrene frequencies are observed. A shift of a G-H rock from 754 to 762 cm-l, and from 846 to 852 cm-l occurs and there is a shift of a C-H bend from 1321 to 1326 cm-l. (d) Tetracyanoethylelze . This complex is deep purple in colour. The main i.r. characteristics are a red shifting of C&N [8] stretch frequencies from 2288 and 2260 to 2240 and 2220 cm-l and loss of the C-H stretch frequency at 3030 cm-l. The pyrene exhibits a red shifting of the C-H rock band from 782 to 777 cm-l. New peaks occur at 717, 855, a particularly strong peak, and at 988 cm-l. 2. Hydroquinone complexe:es (a) Tetracyanoethylene. This gives a pale green complex in the solid which is, however, very dark blue in solution. The only notable features are all changes in the hydroquinone spectrum. The 1525 cm-l C=C stretch moves to 1520 cm-l and the 1480 cm-l C=C stretch goes to 1475, with a removal of the splitting on this peak. A new weak band occurs at 1500 cm-l. As no changes could be detected in the acceptor spectrum it is not possible to come to any valid conclusions about the binding. (b) Trinitrobenzene. A pink coloured complex is formed in the solid. This exhibits a large number of changes in both the donor and acceptor spectra. The hydroquinone shows red shifting of the C--r! stretch from 1480 to 1465 cm-l, the C-O stretch goes from 1265 to 1260 cm-l and from 1250 to 1240 cm-l. The double peak at 830 and 835 cm-l, the out of plane C-H bend goes to 850 and 840 cm-l the latter increasing markedly in intensity. Other changes are the shift of the 812 band to 825 cm-l and the disappearance of the band at 1015 cm-l, the in-plane C-H bend. The following trinitrobenzene bands are affected. The 1080 cm-l C-N stretch splits to give a strong additional band at 1085 cm-l and the 925 cm-l band splits to give an additional band at 930 cm-l. This latter is the C-N bend. These changes are interpretable in terms of a sandwich structure with some charge transfer between the two molecules but much less than occurs with quinones [3]. This might well reflect the poorer acceptor properties of trinitrobenzene. These changes are, however, greater than observed with aromatic hydrocarbons such as pyrene or anthracene [9], much poorer donors than hydroquinones, with trinitrobenzene. (c) Pi&c acid. A yellow complex is obtained in the solid. The main changes are the shifts of the following hydroquinone bands. The 1480 C=C stretch moves to 1465 cm-l, the 1165 C-O stretch goes to 1160 cm-l and the 1250 cm-l C-O stretch somewhat surprisingly goes to 1255 cm- l. The 1225 cm-l C-O stretch band apparently disappears. The hydroquinone band at 1370 cm-r, O-H deformation is not [7] L. J. BELLAMY, The InfraredSpectra of Complex Mokculas, Methuen, London (1958). [8] C. F. LOONEY and J. R. DOWNING,J. Am. Chern. Sot. 80,284O (1958). [9] J. P. LARKINDALEand J. R. DOWNING,Spectrochina. Acta 28A, 485 (1972).
838
M. A. SLIEKIN
seen in the complex but it might be masked by the strong picric acid band at 1350 om-l, a C--r! stretch band. The only change observed in the pi&c acid spectrum is the movement of the weak 1322 cm-1 C=C band to 1335 cm-r. A peak is observed in the complex at 750 cm-1 which cannot be assigned to either component. The general features of this complex are similar to that of the previously mentioned trinitrobenzene complex. 3. Tetraphenylenedialninecomplexe:es (a) Benzoquinone. Although this gives a purple complex similar in colour to Wurster’s blue, the tetraphenylenediamine cation, the spectrum is a quinhydrone type with the characteristic shifts of the carbonyl band to 1635 cm-l [3] the disappearance of the C-H out of plane band at 900 cm-l, and C--r! stretch bands at 1592,131O and 1085 cm-l. New bands appear at 670,780,810,1220 and 1260 cm-r. The latter band is perhaps of C-C stretch character due to the donation of an electron into the benzoquinone ring. (b) Tetracyanoethylene. This complex is purple in solution but dries out in the solid state to a blue-green colour. Moistening of the solid complex with water brings back the deep blue colour characteristic of the cation. The noteworthy spectral changes are the shifting of the acceptor C-N band from 2250 to 2205 cm-l and of the 960 band to 950 cm-r. The C&C! stretch band of the donor shifts from 1520 to 1530 cm-r. New bands appear at 1590, 1358 and 770 cm-r. (c) Trinitrobenzene. This is a purple complex but showing only a very slight change. A band at 738, NO, symmetrical deformation of the acceptor, is shifted to 735 cm-l. There is probably a large percentage of uncomplexed material in the samples. From the foregoing certain general patterns can be distinguished. The general classification of quinone complexes as judged by carbonyl and C=C stretch bands shifts into weak, medium and strong complexes [3] holds up. Tetracyanoethylene complexes are characterized by red-shifts of the CGN stretch band from 2250 cm-l. The amount of shift would appear to correlate with the strength of complexing, tetraphenylenediamine being a stronger donor than pyrene. Hydroquinone complexes are as shown characterized by red-shifting of C==C stretch bands and C-O stretch bands. An interesting point about tetraphenylenediamine complexes is that whilst they exist as inner complexes in polar solution [lo], the donor being in its cationic form, in the solids examined they exist as outer complexes. TEH Fo YEN et al. (11) have suggested that in general for pi-pi complexes, the bending vibration of the donor partner shift to higher wavenumbers and those of the acceptor to lower wavenumbers. This would appear to hold true for the complexes examined. The idea of charge transfer is useful in explaining the behaviour of some of the bands of these complexes. Additionally the i.r. spectra in many cases gives evidence of the characteristic sandwich-like structure of these systems. Undoubtedly more data is still required before any firm conclusions can be drawn which in view of the great difficulty experienced in preparing pure solid complexes may well prove elusive. [IO]R. FOSTERand T. J. THOIW~ON, Trans. FaradaySot.68,860 (1962). [l l] T. F. YEN, A. K. T. LEE and J. I. S. TANG, Chem. Commun. 762 (1969).
Infrared spectra of some organic charge-transfer complexes M. A.
SLIFIUN
Department of Pure and Applied Physics, The University, Salford M6 4WT, U.K. (Received21 Jwne 1972) Abbab-Some solid organic complexes have been prepared and their infrared ape&a determined. The spectra are discussedin terms of charge-transfertheory. INTRODUCTION
there are many hundreds of publications dealing with the visible and U.V. spectra of charge-transfer complexes, there is only a very limited amount of work on their i.r. spectra. Undoubtedly this is due both to the difficulty of obtaining them pure in the solid state and in the case of the weak complexes the very small i.r. changes of the complexes as compared to their components [l-3]. One interesting aspect of the i.r. spectroscopy of quinone charge transfer complexes is that there are three distinct classes of complexes. Those in which the spectrum is that of the ions of the components [a], medium strength quinhydrone complexes characterized by large shifts of C=C bands and C=O bands from the uncomplexed position to about half-way that of the ions [5] and the weak ones already mentioned in which very small shifts of bands occur [l-3]. ‘The purpose of this work is to obtain more i.r. data on charge transfer complexes to see whether similar class%cations exist with other groups of acceptors.
ALTHOUCIH
EXPERIMENTAL
METHOD
Chemicals were obtained from commercial sources and purified by the standard textbook methods. Complexes were prepared by a variety of techniques. These included precipitation from a cooled saturated solution, evaporation to dryness of solutions, the fusion of the two components, the grinding together of the two components in an agate pestle and mortar and the adding of the components in acetone to a pestle and mortar containing KBr and grinding until all the acetone evaporated off. In spite of these varied approaches there were many systems which whilst forming coloured complexes in solution could not be obtained in the solid state. Noteworthy among these were complexes of maleic anhydride. Many of the coloured complexes obtained were contaminated by uncomplexed component. The spectrum of the complex was elucidated by making up solutions of a wide variety of molar ratios usually varying between 4: 1 to 1: 4 to ensure that at least one of the components in the solid would be fully complexed. 1.r. spectra were obtained in KBr discs using a Unicam SPBOOGspectrophotometer. [l] [2] [3] [4] [5]
H. KAINER and W. OTTINO, Chem. Bes-. 88, 1921 (1965). J. W. EASTMAN, G. M. ANDROES and M. CALVIN, J. Chem. Phya. 86, 1197 (1902). M. A. SLIFEIN, Chem. Phya. Lett. 7, 195 (1970). Y. MATSUNAUA, J. Chem. Phya. 41, 1609 (1964). M. A. SLIFKIN and R. H. WALMSLEY, Spects-ochim. Actu 26A, 1237 (1970). 836
836
M. A. SLIFKIN
One of the difficulties in interpreting the spectra is that frequently the absorption spectra of both components in the free state overlap. This was partially overcome by a differential method in which the spectra of two KBr discs in tandem each containing one of the components in the correct weight was compared to a disc of the complex allowing the effect of overlapping of bands to be allowed for and any new bands or the loss of bands to be seen. In spite of all the techniques outlined above the spectra of many of the complexes are difficult to arrive at. The results given are incomplete in many cases, only obvious changes being given. Out of more than 50 systems looked at, useful or meaningful results could only be obtained for the few quoted herein. RESULTS .um DISCOSSION 1. Pyrene complexes
Pyrene is taken as the holotype of the aromatic hydrocarbons as it proved possible to obtain many complexes of this compormd. (a) Benzoquirume . Two different complexes were obtained from this system. The first obtained by evaporation from acetone is orange in colour and shows the small changes already shown for the pyrene tetrachloroquinone complex [3]. The changes are slight red-shifts from 1655 to 1650 cm-1 of the carbonyl band, from 1370 to 1360cm-l for the -C=G stretch band, and from 1085-1070 cm-l for the C-H in plane deformation. The 900 cm-1 band of the C-H out of plane deformation is missing and two new bands are seen at 870 and 855 cm- l. The spectra is interpretable as arising from the formation of a sandwich structure with some oharge transfer leading to the lowering of bond order in the quinone [3]. The second form of complex is obtained by grinding the two compounds and is purple in colour. This shows many of the characteristics of the quinhydrone type complex, the most notable of which is the shift of the CO band to 1633 cm-l [5]. Other changes are the loss of the C=C band at 1370 cm-l, the loss of the out of plane G-H band at 900 cm-1 and the C-H in plane deformation at 1085 cm-l and the appearance of new bands at 355, 877 and 1260 cm- 1. This is interpreted as arising from a sandwich type structure but with marked charge transfer between the two components [5]. The appearance of two different complexes from the same compounds suggests the existence of different allotropic forms of the complexes. The loss of the in plane C-H deformation band in the quinhydrone type complex is suggestive of a slightly different structure. (b) Trinitobenzene. This is a yellow complex. The main changes are the redshifting of the following bands. The C-H in plane bend [6] goes from 1080 to 1070 cm-l, the C--C stretch from 1443 to 1437 cm-l and from 1626 to 1620 cm-l and the symmetrical NO, stretch from 1345 to 1340 cm-l. This is interpreted as arising from a small amount of charge transfer from pyrene to trinitrobenzene resulting in a iowering in bond order of the acceptor. (c) Picric acid. This yields a deep red complex. There are a large number of [S] H. F. S~WLL, (1967).
J. A. FANIRAN, E. A. SYMONS and E. BUNCEL, Cm. J. Chem. 45, 117,
Infrared spectra of some organic charge-transfer complexes
837
both blue shifts and red shifts in this complex. The following blue-shifts occur for picric acid. From 790 to 785 cm-l for the C-H rock, from 929 to 921 cm-l for the C-H out of plane deformation, from 1098 to 1086 cm-l for the C-H in plane bend. The following red-shifts are seen. From 1165 to 1174 cm-l, OH deformation, 1350 to 1357 cm-l and from 1618 to 1627 cm-l for the C=C stretch. These latter three bands are ail sensitive to both the substitution position and the electronegativity of the substituent [7]. It might be that these changes also arise from an increase of electronegativity as a result of charge transfer. The following changes in pyrene frequencies are observed. A shift of a G-H rock from 754 to 762 cm-l, and from 846 to 852 cm-l occurs and there is a shift of a C-H bend from 1321 to 1326 cm-l. (d) Tetracyanoethylelze . This complex is deep purple in colour. The main i.r. characteristics are a red shifting of C&N [8] stretch frequencies from 2288 and 2260 to 2240 and 2220 cm-l and loss of the C-H stretch frequency at 3030 cm-l. The pyrene exhibits a red shifting of the C-H rock band from 782 to 777 cm-l. New peaks occur at 717, 855, a particularly strong peak, and at 988 cm-l. 2. Hydroquinone complexe:es (a) Tetracyanoethylene. This gives a pale green complex in the solid which is, however, very dark blue in solution. The only notable features are all changes in the hydroquinone spectrum. The 1525 cm-l C=C stretch moves to 1520 cm-l and the 1480 cm-l C=C stretch goes to 1475, with a removal of the splitting on this peak. A new weak band occurs at 1500 cm-l. As no changes could be detected in the acceptor spectrum it is not possible to come to any valid conclusions about the binding. (b) Trinitrobenzene. A pink coloured complex is formed in the solid. This exhibits a large number of changes in both the donor and acceptor spectra. The hydroquinone shows red shifting of the C--r! stretch from 1480 to 1465 cm-l, the C-O stretch goes from 1265 to 1260 cm-l and from 1250 to 1240 cm-l. The double peak at 830 and 835 cm-l, the out of plane C-H bend goes to 850 and 840 cm-l the latter increasing markedly in intensity. Other changes are the shift of the 812 band to 825 cm-l and the disappearance of the band at 1015 cm-l, the in-plane C-H bend. The following trinitrobenzene bands are affected. The 1080 cm-l C-N stretch splits to give a strong additional band at 1085 cm-l and the 925 cm-l band splits to give an additional band at 930 cm-l. This latter is the C-N bend. These changes are interpretable in terms of a sandwich structure with some charge transfer between the two molecules but much less than occurs with quinones [3]. This might well reflect the poorer acceptor properties of trinitrobenzene. These changes are, however, greater than observed with aromatic hydrocarbons such as pyrene or anthracene [9], much poorer donors than hydroquinones, with trinitrobenzene. (c) Pi&c acid. A yellow complex is obtained in the solid. The main changes are the shifts of the following hydroquinone bands. The 1480 C=C stretch moves to 1465 cm-l, the 1165 C-O stretch goes to 1160 cm-l and the 1250 cm-l C-O stretch somewhat surprisingly goes to 1255 cm- l. The 1225 cm-l C-O stretch band apparently disappears. The hydroquinone band at 1370 cm-r, O-H deformation is not [7] L. J. BELLAMY, The InfraredSpectra of Complex Mokculas, Methuen, London (1958). [8] C. F. LOONEY and J. R. DOWNING,J. Am. Chern. Sot. 80,284O (1958). [9] J. P. LARKINDALEand J. R. DOWNING,Spectrochina. Acta 28A, 485 (1972).
838
M. A. SLIEKIN
seen in the complex but it might be masked by the strong picric acid band at 1350 om-l, a C--r! stretch band. The only change observed in the pi&c acid spectrum is the movement of the weak 1322 cm-1 C=C band to 1335 cm-r. A peak is observed in the complex at 750 cm-1 which cannot be assigned to either component. The general features of this complex are similar to that of the previously mentioned trinitrobenzene complex. 3. Tetraphenylenedialninecomplexe:es (a) Benzoquinone. Although this gives a purple complex similar in colour to Wurster’s blue, the tetraphenylenediamine cation, the spectrum is a quinhydrone type with the characteristic shifts of the carbonyl band to 1635 cm-l [3] the disappearance of the C-H out of plane band at 900 cm-l, and C--r! stretch bands at 1592,131O and 1085 cm-l. New bands appear at 670,780,810,1220 and 1260 cm-r. The latter band is perhaps of C-C stretch character due to the donation of an electron into the benzoquinone ring. (b) Tetracyanoethylene. This complex is purple in solution but dries out in the solid state to a blue-green colour. Moistening of the solid complex with water brings back the deep blue colour characteristic of the cation. The noteworthy spectral changes are the shifting of the acceptor C-N band from 2250 to 2205 cm-l and of the 960 band to 950 cm-r. The C&C! stretch band of the donor shifts from 1520 to 1530 cm-r. New bands appear at 1590, 1358 and 770 cm-r. (c) Trinitrobenzene. This is a purple complex but showing only a very slight change. A band at 738, NO, symmetrical deformation of the acceptor, is shifted to 735 cm-l. There is probably a large percentage of uncomplexed material in the samples. From the foregoing certain general patterns can be distinguished. The general classification of quinone complexes as judged by carbonyl and C=C stretch bands shifts into weak, medium and strong complexes [3] holds up. Tetracyanoethylene complexes are characterized by red-shifts of the CGN stretch band from 2250 cm-l. The amount of shift would appear to correlate with the strength of complexing, tetraphenylenediamine being a stronger donor than pyrene. Hydroquinone complexes are as shown characterized by red-shifting of C==C stretch bands and C-O stretch bands. An interesting point about tetraphenylenediamine complexes is that whilst they exist as inner complexes in polar solution [lo], the donor being in its cationic form, in the solids examined they exist as outer complexes. TEH Fo YEN et al. (11) have suggested that in general for pi-pi complexes, the bending vibration of the donor partner shift to higher wavenumbers and those of the acceptor to lower wavenumbers. This would appear to hold true for the complexes examined. The idea of charge transfer is useful in explaining the behaviour of some of the bands of these complexes. Additionally the i.r. spectra in many cases gives evidence of the characteristic sandwich-like structure of these systems. Undoubtedly more data is still required before any firm conclusions can be drawn which in view of the great difficulty experienced in preparing pure solid complexes may well prove elusive. [IO]R. FOSTERand T. J. THOIW~ON, Trans. FaradaySot.68,860 (1962). [l l] T. F. YEN, A. K. T. LEE and J. I. S. TANG, Chem. Commun. 762 (1969).