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Resonance Raman investigation of Co(III) sales nitrophenolate complexes
0564-6539/93 $6.00 + 0.00 @ 1993 Pergamon Press Ltd
SpectmchimiaaActa. Vol. 49A, No. 4, pp. 567-570, 1993 Printed in Great Britain
Resonance Raman investigation of Co(III) salen nitrophenolate complexes LUIZ FERNANDOC. DE OLIVEIRA and PAULO S. SANTOS Instituto de Quimica da Universidade de Sao Paulo, C.P. 20780, CEP 01498, So Paulo, Brazil (Receiued 1 July 1992; in final form 4 August 1992; accepted 6 August 1992) Abstract-The resonance Raman spectra of a series of Co(salen) derivatives containing nitrophenolates in the apical position were investigated. The modes more substantially enhanced are assigned to characteristic modes of the nitrophenolate moiety, i.e. NOr and CN stretching modes, and contrary to what has been suggested in previous work, no enhancement was observed for the mode assigned to Co-O stretching.
INTRODUCTION RESONANCERaman spectroscopy has proved to be the technique of choice in identifying the chromophore moiety in complex molecules. In a typical application of the technique, KANDA et al. [l] investigated the reaction of Co(sal-Hpen) [where sal-Hpen is the dianion of the N-N’-disalicylidene-2-methyl-4-(2-hydroxy-5-methyl-phenyl)butane-1 ,2diamine] with molecular oxygen, a species that gives rise to a strong absorption band at 560 nm, assigned to the adduct Co(sal-pen)Oz. This conclusion was based on the appearance of new Raman bands at 1480, 1260 and 510cm-‘: that at 126Ocm-’ was assigned to the O-O stretching mode and that at 510 cm-’ to the Co-O2 stretching mode. The significant resonance enhancement of those two modes strongly suggests that the Co-O2 chromophore is the one involved in the 560 nm electronic transition. In a subsequent paper [2] the same authors reinterpreted their previous results proposing that in fact the species Co(salpen) is formed instead of the oxygen adduct, and accordingly the 126Ocm-’ Raman band is reassigned to the C-O stretching of the coordinated phenolate, and the 510 cm-’ band to the Co-O stretching mode. The reason for such reinterpretation cannot be easily inferred from the aforementioned papers, and in fact it is well known from the literature that the O-O stretching mode in species of comparable structure gives rise to strong resonance enhancement of a band in the 11CKlcm-’ region [3]. From such observations OKAWA et al. [2] propose that in the series of Co(salen) derivatives (where salen is N,iV’-disalicylideneethylenediamine) containing substituted phenolates in the apical position, the electronic transition in the visible should be assigned to a ligand-to-metal charge transfer transition, i.e. the chromophore is mainly located in the Co(III)-0 bond, where the oxygen atom belongs to the substituted phenolate moiety. Such a conclusion was reached on the basis of the observed linear correlation between the energy of the charge transfer transitions and the pK, values of the substituted phenolates. On the other hand, no resonance Raman evidence supporting such a proposition is presented, since according to the authors [2], such evidence had already been given in their previous investigation of Co(sal-pen). We then decided to investigate the resonance Raman behaviour of two Co(salen) derivatives of the series studied by OKAWA et al. [2], namely the ones with p-nitrophenolate and 2,4dinitrophenolate as apical ligands. In addition the complex with 2,4,6-trinitro-phenolate is now investigated for the first time. From our results it can be inferred that the enhanced modes are located in the nitrophenolate moiety, and contrary to OKAWA et al. ‘s proposition [2], no evidence of a ligand-to-metal charge transfer transition in the visible was obtained. EXPERIMENTAL
reagents and solvents were All the used analytical grade. Co(salen), Co(salen) (p-nitrophenolate) and Co(salen) (2&dinitrophenolate) were prepared according to the WA)Ir:c”
567
568
LUIZFERNANDO C.
350
560
DE
OLIVEIRA and PAULOS. SANTOS
770
980
1190
?I
1400
WAVELENGTH/ nm
Fig, 1. Reflectance spectra of the complexes [Co(saten)(p-nitrophenolate)] (a), [Co(salen)(2,4dinitrophenolate)] (b) and [Co(salen)(2,4,6-trinitrophenolate)] (c). literature [3,4]; the complex with 2,4,6-trinitrophenolate was prepared by a similar procedure. Chemical analysis for C, H and N confirmed the proposed compositions. Magnetic susceptibility measurements indicate that all the investigated compounds are diamagnetic. The details of the Raman set-up were given in a recent publication [5]. The samples were in the form of homogeneous solid mixture with sodium sulphate (l:lOO), which was employed as an internal standard for measuring relative intensities. Reflectance spectra were obtained with a Guided Wave model 260 spectrophotometer, using MgO as a reference.
RESULTS AND
DISCUSSION
The reflectance spectra of the investigated compounds are shown in Fig. l(a-c). Figure 2(a-d) shows the Raman spectra of Co(salen)(p-nitrophenolate) excited by several laser lines; Fig. 3(a-d) presents the corresponding data for Co(salen(2,4,6-trinitrophenolate). Table 1 lists the Raman band wavenumbers of the investigated complexes together with a tentative assignment. In all cases a rather strong band at ca 410 nm, with shoulders at cu 560 and 800 nm was observed in the reflectance spectra. The comparison with previous data by OKAWA et al. [2] is not straightforward since such authors do not show in their spectra the intense band at cu 410 nm and neither do their spectra in the visible show the other two bands clearly resolved. The strong band at ca 410 nm, present at about the same postion in all the compounds investigated, can be easily assigned to an internal transition of the nitrophenolate moiety, since it agrees well with the one present in the spectrum of the nitrophenolate in aqueous solution. Such assignment is on line with the observed preresonance enhancement observed in the Raman spectra (Figs 2 and 3). In fact, as the Table 1. Raman band wavenumbers (in cm-‘) of the CN and NO2 stretching modes in the [Co(salen)(nitrophenolate)] complexes Compound [Co(salen) (p-nitrophenolate)] [Co(salen) (2,4-dinitrophenolate)] [Co(salen) (2,4,6-trinitrophenolate)]
v(CN)
v (NO,)
859 823 817
1293 1312 1105
cO(III) salen nitrophenolate
d
~
complexes
b
Fig. 2. Raman spectra of [Co(saien)(p-nitrophenolate)] dispersed in Na#Od (a, 647.1 nm; b, 514.5 mn; c, 488.0 am; and d, 457.9 nm). The band marked with a cross is the 980 cm-’ band of SOzj-. Spectral resolution of 7 cm-‘.
WAVENUMBER/cm-1
Fig. 3. Raman spectra of [Co(salen)(2,4,6-trinitrophenolate)] dispersed in Na$O, (a, 514.5 nm; b, 488.0 nm; c, 457.9 nm). The band marked with a cross is the 980 cm-’ band of SO:-. Spectral resolution of 7 cm-‘.
569
570
Lutz FERNANDO C. DE OLWEIRA
and PAIJLOS. SANTOS
exciting line moves towards the blue a considerable enhancement of the bands in the 800-900 cm-’ and 1100-1300 cm-’ regions is observed. These are the regions where the C-N and symmetric NO2 stretching modes are expected to appear, respectively. In addition, it is well known from the literature that these are the modes more conspicuously enhanced in the pre-resonance Raman spectra of nitrophenolates [6,7]. Such modes, as can be seen in Table 1, shift to lower wavenumbers upon coordination, as previously observed for other substituted phenolate complexes [8,9]. It is worth mentioning that in the Raman spectra of the investigated compounds a weak band at ca 525 cm-’ presenting no enhancement is observed and tentatively assigned to the Co-O stretching mode. Summing up, all such facts seem to indicate that the observed pre-resonance enhancement in the investigated Co(salen) derivatives is due to the internal transition of the nitrophenolate moiety at cu 410nm. On the other hand, the absence of a resonance enhancement of the Co-O stretching mode indicates that the previously proposed ligand-to-metal charge transfer transition must be located at much higher energies. With the present results it would be rather speculative to propose a tentative assignment for the weaker bands observed in the optical spectrum of Co(salen)(nitrophenolates), but the absence of any significant enhancement as such transitions are approached precludes their assignments as charge transfer transitions. In this respect it is illustrative to mention the Fe(II1) phenolate complexes, where very intense absorption bands in the visible are present giving rise to substantial resonance enhancement of the Fe-O mode [lo, 111. Acknowledgements-The authors are indebted to Prof. H. E. Toma for the use of the reflectance spectrophotometer and for enlightening discussions. L.F.C.O. is indebted to FAPESP for the grant of a post-doctoral research fellowship. P.S.S. is indebted to FAPESP and CNPq for financial support.
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
W. Kanda, H. Okawa and S. Kida, J. Chem. Sot. Chem. Commun. 973 (1983). H. Okawa, W. Kanda, S. Kida and K. Nakamoto, Inorg. Chim. Acta 104,77 (1985). K. Badjor, H. Oshio, K. Nakamoto, W. Kanda, H. Okawa and S. Kida, Inorg. C&n. Acra 103,63 (1985). T. G. Appleton, J. C/rem. Educ. 54, 443 (1977). L. F. C. de Oliveira and P. S. Santos, .I. Molec. Struct. 269, 85 (1992). K. Kumar and P. R. Carey, J. Chem. Phys. 63, 3697 (1975). E. D. Schmid, M. Moschallski and W. L. Peticolas, J. Phys. C/rem. 90, 2348 (1986). R. H. Heinstand, II, R. B. Laufer, E. Frikig and L. Que, Jr, 1. Am. Chem. Sot. 104,2789 (1982). J. W. Pyrz, K. D. Karlin, T. N. Sorrel, G. C. Vogel and L. Que, Jr, Inorg. Chem. 23, 4581 (1984). Y. Tominaga, S. Kint and J. R. Scherer, Biochemistry l&4918 (1976). W. J. Pyrz, L. Roe, L. J. Stern and L. Que, Jr, .I. Am. Chem. Sot. 107, 614 (1985).
SpectmchimiaaActa. Vol. 49A, No. 4, pp. 567-570, 1993 Printed in Great Britain
Resonance Raman investigation of Co(III) salen nitrophenolate complexes LUIZ FERNANDOC. DE OLIVEIRA and PAULO S. SANTOS Instituto de Quimica da Universidade de Sao Paulo, C.P. 20780, CEP 01498, So Paulo, Brazil (Receiued 1 July 1992; in final form 4 August 1992; accepted 6 August 1992) Abstract-The resonance Raman spectra of a series of Co(salen) derivatives containing nitrophenolates in the apical position were investigated. The modes more substantially enhanced are assigned to characteristic modes of the nitrophenolate moiety, i.e. NOr and CN stretching modes, and contrary to what has been suggested in previous work, no enhancement was observed for the mode assigned to Co-O stretching.
INTRODUCTION RESONANCERaman spectroscopy has proved to be the technique of choice in identifying the chromophore moiety in complex molecules. In a typical application of the technique, KANDA et al. [l] investigated the reaction of Co(sal-Hpen) [where sal-Hpen is the dianion of the N-N’-disalicylidene-2-methyl-4-(2-hydroxy-5-methyl-phenyl)butane-1 ,2diamine] with molecular oxygen, a species that gives rise to a strong absorption band at 560 nm, assigned to the adduct Co(sal-pen)Oz. This conclusion was based on the appearance of new Raman bands at 1480, 1260 and 510cm-‘: that at 126Ocm-’ was assigned to the O-O stretching mode and that at 510 cm-’ to the Co-O2 stretching mode. The significant resonance enhancement of those two modes strongly suggests that the Co-O2 chromophore is the one involved in the 560 nm electronic transition. In a subsequent paper [2] the same authors reinterpreted their previous results proposing that in fact the species Co(salpen) is formed instead of the oxygen adduct, and accordingly the 126Ocm-’ Raman band is reassigned to the C-O stretching of the coordinated phenolate, and the 510 cm-’ band to the Co-O stretching mode. The reason for such reinterpretation cannot be easily inferred from the aforementioned papers, and in fact it is well known from the literature that the O-O stretching mode in species of comparable structure gives rise to strong resonance enhancement of a band in the 11CKlcm-’ region [3]. From such observations OKAWA et al. [2] propose that in the series of Co(salen) derivatives (where salen is N,iV’-disalicylideneethylenediamine) containing substituted phenolates in the apical position, the electronic transition in the visible should be assigned to a ligand-to-metal charge transfer transition, i.e. the chromophore is mainly located in the Co(III)-0 bond, where the oxygen atom belongs to the substituted phenolate moiety. Such a conclusion was reached on the basis of the observed linear correlation between the energy of the charge transfer transitions and the pK, values of the substituted phenolates. On the other hand, no resonance Raman evidence supporting such a proposition is presented, since according to the authors [2], such evidence had already been given in their previous investigation of Co(sal-pen). We then decided to investigate the resonance Raman behaviour of two Co(salen) derivatives of the series studied by OKAWA et al. [2], namely the ones with p-nitrophenolate and 2,4dinitrophenolate as apical ligands. In addition the complex with 2,4,6-trinitro-phenolate is now investigated for the first time. From our results it can be inferred that the enhanced modes are located in the nitrophenolate moiety, and contrary to OKAWA et al. ‘s proposition [2], no evidence of a ligand-to-metal charge transfer transition in the visible was obtained. EXPERIMENTAL
reagents and solvents were All the used analytical grade. Co(salen), Co(salen) (p-nitrophenolate) and Co(salen) (2&dinitrophenolate) were prepared according to the WA)Ir:c”
567
568
LUIZFERNANDO C.
350
560
DE
OLIVEIRA and PAULOS. SANTOS
770
980
1190
?I
1400
WAVELENGTH/ nm
Fig, 1. Reflectance spectra of the complexes [Co(saten)(p-nitrophenolate)] (a), [Co(salen)(2,4dinitrophenolate)] (b) and [Co(salen)(2,4,6-trinitrophenolate)] (c). literature [3,4]; the complex with 2,4,6-trinitrophenolate was prepared by a similar procedure. Chemical analysis for C, H and N confirmed the proposed compositions. Magnetic susceptibility measurements indicate that all the investigated compounds are diamagnetic. The details of the Raman set-up were given in a recent publication [5]. The samples were in the form of homogeneous solid mixture with sodium sulphate (l:lOO), which was employed as an internal standard for measuring relative intensities. Reflectance spectra were obtained with a Guided Wave model 260 spectrophotometer, using MgO as a reference.
RESULTS AND
DISCUSSION
The reflectance spectra of the investigated compounds are shown in Fig. l(a-c). Figure 2(a-d) shows the Raman spectra of Co(salen)(p-nitrophenolate) excited by several laser lines; Fig. 3(a-d) presents the corresponding data for Co(salen(2,4,6-trinitrophenolate). Table 1 lists the Raman band wavenumbers of the investigated complexes together with a tentative assignment. In all cases a rather strong band at ca 410 nm, with shoulders at cu 560 and 800 nm was observed in the reflectance spectra. The comparison with previous data by OKAWA et al. [2] is not straightforward since such authors do not show in their spectra the intense band at cu 410 nm and neither do their spectra in the visible show the other two bands clearly resolved. The strong band at ca 410 nm, present at about the same postion in all the compounds investigated, can be easily assigned to an internal transition of the nitrophenolate moiety, since it agrees well with the one present in the spectrum of the nitrophenolate in aqueous solution. Such assignment is on line with the observed preresonance enhancement observed in the Raman spectra (Figs 2 and 3). In fact, as the Table 1. Raman band wavenumbers (in cm-‘) of the CN and NO2 stretching modes in the [Co(salen)(nitrophenolate)] complexes Compound [Co(salen) (p-nitrophenolate)] [Co(salen) (2,4-dinitrophenolate)] [Co(salen) (2,4,6-trinitrophenolate)]
v(CN)
v (NO,)
859 823 817
1293 1312 1105
cO(III) salen nitrophenolate
d
~
complexes
b
Fig. 2. Raman spectra of [Co(saien)(p-nitrophenolate)] dispersed in Na#Od (a, 647.1 nm; b, 514.5 mn; c, 488.0 am; and d, 457.9 nm). The band marked with a cross is the 980 cm-’ band of SOzj-. Spectral resolution of 7 cm-‘.
WAVENUMBER/cm-1
Fig. 3. Raman spectra of [Co(salen)(2,4,6-trinitrophenolate)] dispersed in Na$O, (a, 514.5 nm; b, 488.0 nm; c, 457.9 nm). The band marked with a cross is the 980 cm-’ band of SO:-. Spectral resolution of 7 cm-‘.
569
570
Lutz FERNANDO C. DE OLWEIRA
and PAIJLOS. SANTOS
exciting line moves towards the blue a considerable enhancement of the bands in the 800-900 cm-’ and 1100-1300 cm-’ regions is observed. These are the regions where the C-N and symmetric NO2 stretching modes are expected to appear, respectively. In addition, it is well known from the literature that these are the modes more conspicuously enhanced in the pre-resonance Raman spectra of nitrophenolates [6,7]. Such modes, as can be seen in Table 1, shift to lower wavenumbers upon coordination, as previously observed for other substituted phenolate complexes [8,9]. It is worth mentioning that in the Raman spectra of the investigated compounds a weak band at ca 525 cm-’ presenting no enhancement is observed and tentatively assigned to the Co-O stretching mode. Summing up, all such facts seem to indicate that the observed pre-resonance enhancement in the investigated Co(salen) derivatives is due to the internal transition of the nitrophenolate moiety at cu 410nm. On the other hand, the absence of a resonance enhancement of the Co-O stretching mode indicates that the previously proposed ligand-to-metal charge transfer transition must be located at much higher energies. With the present results it would be rather speculative to propose a tentative assignment for the weaker bands observed in the optical spectrum of Co(salen)(nitrophenolates), but the absence of any significant enhancement as such transitions are approached precludes their assignments as charge transfer transitions. In this respect it is illustrative to mention the Fe(II1) phenolate complexes, where very intense absorption bands in the visible are present giving rise to substantial resonance enhancement of the Fe-O mode [lo, 111. Acknowledgements-The authors are indebted to Prof. H. E. Toma for the use of the reflectance spectrophotometer and for enlightening discussions. L.F.C.O. is indebted to FAPESP for the grant of a post-doctoral research fellowship. P.S.S. is indebted to FAPESP and CNPq for financial support.
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll]
W. Kanda, H. Okawa and S. Kida, J. Chem. Sot. Chem. Commun. 973 (1983). H. Okawa, W. Kanda, S. Kida and K. Nakamoto, Inorg. Chim. Acta 104,77 (1985). K. Badjor, H. Oshio, K. Nakamoto, W. Kanda, H. Okawa and S. Kida, Inorg. C&n. Acra 103,63 (1985). T. G. Appleton, J. C/rem. Educ. 54, 443 (1977). L. F. C. de Oliveira and P. S. Santos, .I. Molec. Struct. 269, 85 (1992). K. Kumar and P. R. Carey, J. Chem. Phys. 63, 3697 (1975). E. D. Schmid, M. Moschallski and W. L. Peticolas, J. Phys. C/rem. 90, 2348 (1986). R. H. Heinstand, II, R. B. Laufer, E. Frikig and L. Que, Jr, 1. Am. Chem. Sot. 104,2789 (1982). J. W. Pyrz, K. D. Karlin, T. N. Sorrel, G. C. Vogel and L. Que, Jr, Inorg. Chem. 23, 4581 (1984). Y. Tominaga, S. Kint and J. R. Scherer, Biochemistry l&4918 (1976). W. J. Pyrz, L. Roe, L. J. Stern and L. Que, Jr, .I. Am. Chem. Sot. 107, 614 (1985).