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Reaction of aromatic amines with Cu(ClO4)2 in acetonitrile as a facile route to amine radical cation generation
Chemical Physics Letters 413 (2005) 294–299 www.elsevier.com/locate/cplett
Reaction of aromatic amines with Cu(ClO4)2 in acetonitrile as a facile route to amine radical cation generation q S. Sumalekshmy, K.R. Gopidas
*
Photosciences and Photonics Group, Chemical Sciences Division, Regional Research Laboratory (CSIR), Trivandrum 695 019, India Received 27 May 2005 Available online 19 August 2005
Abstract This Letter describes a very simple and effective method for the generation and study of radical cations of aromatic amines. It is shown that simple mixing of micro molar solutions of aromatic amines with micro molar amounts of Cu(ClO4)2 in acetonitrile solution leads to formation of amine radical cations in good yields. The radical cations thus generated are unequivocally characterized by their absorption and electron spin resonance spectra. It is proposed that the radical cations are formed through the donation of an electron from the amines to Cu2+. 2005 Published by Elsevier B.V.
1. Introduction Radical cations of amines are principal intermediates in a large number of single electron transfer reactions [1–3]. They are formed by the one electron oxidations of amines and are involved in the synthesis of amino acids, alkaloids and several other nitrogen containing compounds of biological and pharmaceutical importance [4,5]. Amine radical cations are also important intermediates in several technological applications such as imaging [6] and photopolymerization [7]. Hence, generation of radical cations and study of their structural and reactivity aspects are extremely important. Several methods are now available for generating radical cations for ESR and other spectroscopic studies in fluid solutions or in solid matrices. These include treatment of the substrates with oxidizing metals, Lewis acids, protic q This is publication number PPD(PRU)-RRLT 190 from the Photosciences and Photonics Group, Regional Research Laboratory (CSIR), Trivandrum. * Corresponding author. Fax: +91 471 2490186. E-mail address: gopidaskr@rediffmail.com (K.R. Gopidas).
0009-2614/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.cplett.2005.06.041
acids, stable radicals and onium salts [8]. Other methods include anodic oxidation [9–11], photoionization [12], c-irradiation [13,14] and inclusion in zeolite cavities [15,16]. In this Letter, we introduce a very simple method for the generation of aromatic amine radical cations in acetonitrile solution. In the recent past, we were involved in studies of intramolecular charge transfer processes in donor– acceptor substituted tetrahydropyrenes [17,18]. During the course of these investigations, we have noted that dimethylamino-substituted tetrahydropyrene derivatives gave intense blue coloration with micro molar amounts of Cu(ClO4)2 in acetonitrile solution. A detailed investigation of this phenomenon has now revealed that the colors are due to the radical cations of the amines and that several aromatic amines are capable of generating their radical cations simply by mixing the amine solutions in acetonitrile with micromolar amounts of Cu(ClO4)2. Results of our investigations are presented here. The amines we employed for this study include N,N-dimethylaniline (DMA), N,N,N 0 N 0 -tetramethyl-p-phenylenediamine (TMPD), tris-(p-bromophenyl)amine (TBPA), N-methylphenothi-
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
H3C
N
H3C
CH3
N
CH3
CH3 N S
H3C DMA
N
CH3
TMPD H3C
Br
N
CH 3
NMPT H3C
N
CH 3
N Br
Br CO2C2H5 TPBA
DMTHP
DMCTHP
Chart 1. Structures of the aromatic amines employed in the study.
azine (NMPT), 2-N,N-dimethylaminotetrahydropyrene (DMTHP) and 2-N,N-dimethylamino-7-carboethoxytetrahydropyrene (DMCTHP) (see Chart 1). 2. Experimental techniques Absorption spectra were recorded on a Shimadzu 2100 UV–Vis–NIR spectrometer. Laser flash photolysis experiments were carried out by employing an Applied Photophysics Model LKS-20 Laser Kinetic Spectrometer using GCR-12 Series Quanta Ray Nd:YAG laser. The ESR experiments were carried out using a Bruker EMX-EPR Spectrometer operating at X band with 100 KHz field modulation. Redox potentials were measured using a BAS CV50 W Cyclic voltammetric analyzer. Amines employed in this study were either purchased from Aldrich or synthesized using reported procedures. These were purified by crystallization or distillation before use. Synthesis of DMTHP and DMCTHP are reported elsewhere [17]. Cu(ClO4)2 and other metal perchlorates were purchased from Aldrich and used as such. Spectroscopic grade acetonitrile was used for all measurements. All experiments were carried out at room temperature (298 K).
3. Results and discussion Solutions of the amines in acetonitrile were either colorless or pale yellow. Upon addition of micro molar amounts of Cu(ClO4)2, significant color changes occurred and these were studied using absorption spectroscopy. Fig. 1a–f show the changes observed in the absorption spectra of the amines shown in Chart 1 in the presence of increasing amounts of Cu(ClO4)2 in acetonitrile solu-
295
tion. Structures of the aromatic amines employed are shown as inserts in the figures. In Fig. 1a–f, the downward arrows indicate the original absorptions due to the aromatic amines and the upward arrows indicate the new absorptions formed in the presence of micro molar concentrations of Cu(ClO4)2. (We have used Cu(ClO4)2 for these studies, but other Cu2+ salts such as CuCl2 also showed absorption spectral changes as shown in Fig. 1.) In each case addition of Cu(ClO4)2 resulted in the disappearance of the band due to the amine and formation of two new absorption bands (one in the UV region and another in the visible). The absorption bands in the visible region occur at different wavelengths and this has resulted in different colors for the solutions. For example, the DMA/Cu(ClO4)2 solution was yellow, NMPT/ Cu(ClO4)2 solution was pink and the TMPD/Cu(ClO4)2 solution was violet in color. TBPA/Cu(ClO4)2, DMTHP/Cu(ClO4)2 and DMCTHP/Cu(ClO4)2 solutions were blue in color. In the case of NMPT/Cu (ClO4)2 and TPBA/Cu(ClO4)2 systems, the colors persisted for more than a day and for the other systems the colors were stable only for 0.5–2 h. As control experiments, absorption spectra of these amines were also recorded in the presence of perchlorate salts of other divalent ions such as Zn2+, Co2+, Mn2+, Pb2+ and Ni2+. Absorption spectral changes similar to those shown in Fig. 1 were not seen in any of these cases. The new absorption bands formed in the UV and visible regions in Fig. 1a–f can be attributed to the radical cations of the respective aromatic amines formed as a result of an electron transfer reaction between the amine and Cu2+ (vide infra). Absorption spectra of the radical cations of DMA, TMPD, NMPT and TBPA are reported in the literature. DMA radical cation has absorption in the visible with kmax = 470 nm [19a]. In the case of TMPD, the radical cation absorption band is reported to have three peaks at 525, 570 and 630 nm [19b]. The spectrum obtained upon addition of Cu (ClO4)2 to TMPD in acetonitrile shows all these features (Fig. 1b). In the case of NMPT, the radical cation can be generated by photoinduced electron transfer [20] or simply by dissolving NMPT in concentrated sulfuric acid [21–23]. Absorption spectrum of NMPT/conc. H2SO4 system is shown in Fig. 2a. The spectrum is identical to the 520 nm band in Fig. 1c. In the case of TBPA, the radical cation spectrum can be easily obtained by laser flash photolysis of TBPA in the presence of an electron acceptor. In an earlier study [24], we have generated this cation by flash photolysis of azadioxatriangulenium cation/TBPA system in acetonitrile solution and the spectrum obtained is shown in Fig. 2b. The spectrum is nearly identical to the long wavelength band in Fig. 1d. The transient absorption spectrum obtained in the flash photolysis of DMCTHP in the presence of the well-known electron acceptor dicyanobenzene is shown in Fig. 2c. The spectrum is identical to the long
296
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299 0.6
a
H3C
f
0.2
b
CH3
N
f
H3C
CH3
N
Absorbance
a a H3C
CH3
0.1
0.0
0.0 300
400
500
600
700
800
300
0.6
c
CH3 N
f
Absorbance
N
0.3
0.2
400
500
d
600
700
800
Br
f S
0.4
a
N
a Br
0.1
Br
0.2
0.0 0.3
0.0 300
400
500
600
e
H3C
N
CH 3
300 0.3
400
f
500
600
700
800 N(CH3)2
f
Absorbance
f 0.2
a
0.2
CO2C2H5
a 0.1
0.0
0.1
300
400
500
600
700
800
0.0 300
400
500
600
700
800
Wavelength, nm Fig. 1. Absorption spectra of the various aromatic amines in the absence and presence of Cu(ClO4)2. [Amines] were 5–10 lM and [Cu(ClO4)2] were: (a) 0, (b) 2.2, (c) 4.45, (d) 6.65, (e) 8.84, and (f) 11 lM.
wavelength band in Fig. 1f. Thus, we assign the new absorption bands produced upon adding Cu(ClO4)2 to the amine solutions in acetonitrile to the radical cations of the respective amines. In order to further confirm the formation of radical cations, ESR spectra of the solutions were recorded. Fig. 3a shows the ESR spectrum of Cu(ClO4)2 in aceto0.6
a
nitrile solution. Fig. 3b is the ESR spectrum of the pink solution obtained by mixing the Cu(ClO4)2 solution with one equivalent of NMPT. The ESR spectrum of NMPT radical cation is reported to have six peaks with intensities in the ratio 1:4:7:7:4:1 [23] and this is the pattern observed in Fig. 3b. We have also generated NMPT radical cation by dissolving NMPT in concentrated H2SO4 and
b
c 0.0050
0.3
∆OD
∆OD
Absorbance
0.04
0.02
0.0 400
500
Wavelength, nm
600
0.00 600
700
Wavelength, nm
800
0.0025
0.0000 550
600
650
700
750
Wavelength, nm
Fig. 2. (a) Absorption spectrum of NMPT in conc. H2SO4; (b) transient absorption spectrum of azadioxatriangulenium cation (1.7 · 104 M)/TBPA (5 mM) system; and (c) transient absorption spectrum of DMCTHP (2 · 105 M)/1,4-dicyanobenzene (2 · 103 M) system.
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
ESR spectrum of this solution is shown in Fig. 3c. It can be seen that Fig. 3b,c are identical. ESR spectra of DMTHP/Cu(ClO4)2 and DMCTHP/Cu(ClO4)2 systems were also obtained (Fig. 3d,e), but in these cases the resolution was poor due to the instability of the radical cations. Thus, the ESR experiments confirm that the colored species generated in these reactions are indeed the amine radical cations. Fig. 3b,d,e are obtained upon mixing the corresponding amines with one equivalent of Cu2+. These ESR spectra do not show the peaks due to Cu2+, suggesting that formation of the amine radical cation is accompanied by the disappearance of Cu2+. Since the amines are converted to radical cations and Cu2+ is converted to Cu+, it is obvious that an electron transfer (ET) reaction as shown in the following equation has taken place upon mixing the amines with Cu2+: Amine þ Cu2þ ! Amineþ þ Cuþ ð1Þ
Table 1 The oxidation potentials (vs. SCE) of amines and free energy changes (DG) for reaction (1) with the amines Aromatic amine
Eox
DG
DMA NMPT TBPA TMPD DMTHP DMCTHP
0.71 0.86 1.05 0.15 0.59 0.66
0.242 0.092 0.098 0.802 0.362 0.292
ter distance of the donor and acceptor and e is the dielectric constant of the solvent. In polar solvents like acetonitrile, the coulombic term can be neglected. Redox potential of the Cu2+/Cu+ couple is solvent dependent and the value in acetonitrile is 1.194 V vs. NHE (= 0.952 V vs. SCE) [25]. The oxidation potentials of DMA [3], TMPD [26], TBPA [27] and NMPT [22] (vs. SCE) in acetonitrile were known in the literature. The oxidation potential of DMTHP, and DMCTHP (vs. SCE) were determined by cyclic voltammetry. In Table 1, the oxidation potentials of the amines and calculated DG values of the electron transfer reactions are given. An inspection of Table 1 shows that the ET reaction between the amines and Cu2+ is exothermic in all cases except for TBPA, in which case DG value is close to zero. Hence, electron transfer reactions as per Eq. (1)
For the electron transfer reaction in Eq. (1), the free energy change DG can be calculated using the following equation [1–3]: DG ¼ Eox Ered e2 =de
ð2Þ
In Eq. (2), Eox is the oxidation potential of the donor (amines in this case) and Ered is the reduction potential of the acceptor (Cu2+ in this case), d is the center to cen200 150
c
b
a
297
Intensity
100 50 0
200 G
-50 10 G
10 G
-100 -150 -200 2600 2800 3000 3200 3400 3600 3800
G 60
3425
3450
3475
3500
3340 3360 3380 3400 3420 3440
G
G d
e
40
Intensity
20 0 -20 10 G 10 G
-40 -60 3400 3420 3440 3460 3480 3500 3520
3420 3440 3460 3480 3500 3520
G
G
Fig. 3. The ESR spectra obtained at 298 K of: (a) Cu(ClO4)2 (6 mM) in acetonitrile, (b) NMPT and Cu(ClO4)2 (2 mM each) in acetonitrile, (c) NMPT (2 mM) in conc. H2SO4, (d) DMTHP and Cu(ClO4)2 (2 mM each) in acetonitrile, and (e) DMCTHP and Cu(ClO4)2 (2 mM each) in acetonitrile.
298
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
should be spontaneous. Since most aromatic amines have Eox values <1.0 V (vs. SCE), reaction (1) should take place spontaneously with most aromatic amines. Redox potential of the Cu2+/Cu+ couple is solvent dependent. Redox potentials for this couple reported in water (0.09 V vs. SCE) and methanol (0.0 V vs. SCE) [25] are very much lower compared to the value reported in acetonitrile. If we use these values for Eox, then DG values would turn out to be positive for all amines. In agreement with this, we did not observe radical cation formation when the reactions were carried out in methanol. The amines were not soluble in water and hence the experiments could not be done in water. Since the NMPT radical cation was very stable, we have tried to determine the yield of the radical cation by measuring the absorbance due to the radical cation immediately after mixing molar equivalents of NMPT and Cu(ClO4)2 solutions. Using the known extinction coefficient of NMPT+ (9.2 · 103 M1 cm1 at 520 nm) [20], we calculated the concentration of NMPT+ formed. Results showed that nearly 75% of NMPT was converted to NMPT+. In this study, we have reported radical cation formation of tertiary aromatic amines only. We have noted that simple aromatic amines such as aniline, and secondary amines such as N-methylaniline also exhibited color changes under comparable conditions. Details of these studies are not included here. It was very surprising to note that DMA+ prepared by this method is stable for more than 30 min. It is believed that the DMA radical cation is very unstable due to facile dimerization to tetramethylbenzidine (TMB) dication [28]. Since the absorption spectra of DMA radical cation and TMB dication are very similar, it is possible that the observed color is due to TMB dication [28]. In an attempt to determine the amount of TMB dication formed, we have quenched the reaction mixture by adding triethylamine and water and the recovered amine mixture was analyzed by GC–MS. The results show that only about 10% of DMA radical was converted to TMB dication indicating that formation of TMB dication is not a significant reaction pathway under our experimental conditions. It was reported earlier that Fe3+ and to a lesser extent, Cu2+ can catalyze the cyclodimerization of a few aromatic enamines in methanol or methanol–acetonitrile mixtures [29,30]. Although amine radical cations were proposed as probable intermediates in these reactions, reaction with Cu2+ as a general method for the generation and study of amine radical cations has not been documented previously. Amine radical cations, particularly the triarylamine radical cations in which the para positions of the phenyl rings are substituted, are among the most useful oxidants because they are very strong and nearly innocent [31]. These are widely used as catalysts in Diels–Alder and other C–C bond forming reactions [32,33] and are generally prepared
by reaction of the corresponding amines with nitrosonium salts or Ag+ salts in the presence of iodine or SbCl5. In this Letter, we show that these radical cations can be prepared simply by mixing the amines with Cu (ClO4)2 in acetonitrile solution.
4. Conclusions The interactions of a few selected aromatic amines with Cu(ClO4)2 were probed by absorption and ESR spectroscopy. All these amines reacted with Cu2+ ions to give amine radical cations, which are characterized by absorption and ESR spectra. Formation of radical cations was explained by invoking an electron transfer mechanism. This is projected as a general method for the generation and study of aromatic amine radical cations in solution.
Acknowledgements The authors thank the Council of Scientific and Industrial Research (CSIR), Government of India and the Department of Science and Technology (DST), Government of India for financial support. We also thank Dr. T. Ramasami, Director, Central Leather Research Institute, Chennai for allowing access to the ESR spectrometer and Dr. J. Subramanian for helping with the ESR measurements.
References [1] V. Balzani (Ed.), Electron Transfer in Chemistry, vols. 1–5, Wiley–VCH, New York, 2001. [2] M.A. Fox, M. Chanon (Eds.), Photoinduced Electron Transfer, Elsevier, Amsterdam, 1988, Parts A–D. [3] G.J. Kavarnos, N.J. Turro, Chem. Rev. 86 (1986) 401. [4] S. Das, V. Suresh, in: V. Balzani (Ed.), Electron Transfer in Chemistry, vol. 2, Wiley–VCH, New York, 2001, p. 379. [5] C.J. Easton, Chem. Rev. 97 (1997) 53. [6] R.D. Theys, G. Sosnovsky, Chem. Rev. 97 (1997) 83. [7] G.K. Pullen, N.S. Allen, M. Edge, I. Weddel, R. Swart, F. Catalina, S. Navaratnam, Eur. Polym. J. 32 (1996) 943. [8] For a recent review, see: A.G. Davies, J. Chem. Res. (S) (2001) 253. [9] D.A. Fielder, M. Koppenol, A.M. Bond, J. Electrochem. Soc. 142 (1995) 862. [10] F. Gerson, G. Gescheidt, J. Kno¨bel, W.B. Martin, L. Neumann, E. Vogel, J. Am. Chem. Soc. 114 (1992) 7107. [11] G. Gescheidt, R. Herges, H. Neumann, J. Heinze, M. Wollenweber, M. Etzkorn, H. Prinzbach, Angew. Chem., Int. Ed. Engl. 34 (1995) 1016. [12] R.M. Krishna, Z. Chang, H. Choo, K.T. Ranjit, L. Kevan, Phys. Chem. Chem. Phys. 2 (2000) 3335. [13] M.C.R. Symons, Chem. Soc. Rev. 13 (1984) 393. [14] T. Shida, E. Haselbach, T. Bally, Acc. Chem. Res. 17 (1984) 180. [15] H. Garcia, V. Marti, I. Casades, V. Fornes, H.D. Roth, Phys. Chem. Chem. Phys. 3 (2001) 2955.
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299 [16] J.-V. Folgado, H. Garcia, V. Marti, M. Espla, Tetrahedron 53 (1997) 4947. [17] S. Sumalekshmy, K.R. Gopidas, J. Phys. Chem. B 108 (2004) 3705. [18] S. Sumalekshmy, K.R. Gopidas, New J. Chem. 29 (2005) 325. [19] (a) T. Shida, Electronic Absorption Spectra of Radical Cations, Elsevier, Amsterdam, 1988, p. 209; (b) T. Shida, Electronic Absorption Spectra of Radical Cations, Elsevier, Amsterdam, 1988, p. 221. [20] Y. Moroi, A.M. Braun, M. Gra¨tzel, J. Am. Chem. Soc. 101 (1979) 576. [21] E.R. Biehl, H. Chiou, J. Keepers, S. Kennard, P. Reeves, J. Heterocycl. Chem. 12 (1975) 397. [22] H.J. Shine, D.R. Thompson, C. Veneziani, J. Heterocycl. Chem. 4 (1967) 517. [23] A.R. Katritzky, A.J. Boulton (Eds.), Advances in Heterocyclic Chemistry, vol. 9, Academic Press, New York, 1968, p. 351.
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[24] S. Dileesh, K.R. Gopidas, J. Photochem. Photobiol. A 162 (2004) 115. [25] A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Dekker, New York, 1974, Band 2. [26] C.K. Mann, K.K. Barnes, in: A.J. Bard (Ed.), Electrochemical Reactions in Nonaqueous Systems, Marcel Dekker, New York, 1970. [27] D. Astruc, in: V. Balzani (Ed.), Electron Transfer in Chemistry, vol. 2, Wiley–VCH, New York, 2001, p. 714. [28] J. Wheeler, R.W. Nelson, J. Phys. Chem. 77 (1973) 2490. [29] F.A. Bell, R.A. Crellin, H. Fujii, A. Ledwith, J. Chem. Soc., Chem. Commun. (1969) 251. [30] A. Ledwith, Acc. Chem. Res. 5 (1972) 133. [31] N.G. Connelly, W.E. Geiger, Chem. Rev. 96 (1996) 877. [32] N.L. Bauld, D. Gao, in: V. Balzani (Ed.), Electron Transfer in Chemistry, vol. 2, Wiley–VCH, New York, 2001, p. 133. [33] N.L. Bauld, in: P.S. Mariano (Ed.), Advances in Electron Transfer Chemistry, vol. 2, JAI Press, Greenwich, 1992, p. 1.
Reaction of aromatic amines with Cu(ClO4)2 in acetonitrile as a facile route to amine radical cation generation q S. Sumalekshmy, K.R. Gopidas
*
Photosciences and Photonics Group, Chemical Sciences Division, Regional Research Laboratory (CSIR), Trivandrum 695 019, India Received 27 May 2005 Available online 19 August 2005
Abstract This Letter describes a very simple and effective method for the generation and study of radical cations of aromatic amines. It is shown that simple mixing of micro molar solutions of aromatic amines with micro molar amounts of Cu(ClO4)2 in acetonitrile solution leads to formation of amine radical cations in good yields. The radical cations thus generated are unequivocally characterized by their absorption and electron spin resonance spectra. It is proposed that the radical cations are formed through the donation of an electron from the amines to Cu2+. 2005 Published by Elsevier B.V.
1. Introduction Radical cations of amines are principal intermediates in a large number of single electron transfer reactions [1–3]. They are formed by the one electron oxidations of amines and are involved in the synthesis of amino acids, alkaloids and several other nitrogen containing compounds of biological and pharmaceutical importance [4,5]. Amine radical cations are also important intermediates in several technological applications such as imaging [6] and photopolymerization [7]. Hence, generation of radical cations and study of their structural and reactivity aspects are extremely important. Several methods are now available for generating radical cations for ESR and other spectroscopic studies in fluid solutions or in solid matrices. These include treatment of the substrates with oxidizing metals, Lewis acids, protic q This is publication number PPD(PRU)-RRLT 190 from the Photosciences and Photonics Group, Regional Research Laboratory (CSIR), Trivandrum. * Corresponding author. Fax: +91 471 2490186. E-mail address: gopidaskr@rediffmail.com (K.R. Gopidas).
0009-2614/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.cplett.2005.06.041
acids, stable radicals and onium salts [8]. Other methods include anodic oxidation [9–11], photoionization [12], c-irradiation [13,14] and inclusion in zeolite cavities [15,16]. In this Letter, we introduce a very simple method for the generation of aromatic amine radical cations in acetonitrile solution. In the recent past, we were involved in studies of intramolecular charge transfer processes in donor– acceptor substituted tetrahydropyrenes [17,18]. During the course of these investigations, we have noted that dimethylamino-substituted tetrahydropyrene derivatives gave intense blue coloration with micro molar amounts of Cu(ClO4)2 in acetonitrile solution. A detailed investigation of this phenomenon has now revealed that the colors are due to the radical cations of the amines and that several aromatic amines are capable of generating their radical cations simply by mixing the amine solutions in acetonitrile with micromolar amounts of Cu(ClO4)2. Results of our investigations are presented here. The amines we employed for this study include N,N-dimethylaniline (DMA), N,N,N 0 N 0 -tetramethyl-p-phenylenediamine (TMPD), tris-(p-bromophenyl)amine (TBPA), N-methylphenothi-
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
H3C
N
H3C
CH3
N
CH3
CH3 N S
H3C DMA
N
CH3
TMPD H3C
Br
N
CH 3
NMPT H3C
N
CH 3
N Br
Br CO2C2H5 TPBA
DMTHP
DMCTHP
Chart 1. Structures of the aromatic amines employed in the study.
azine (NMPT), 2-N,N-dimethylaminotetrahydropyrene (DMTHP) and 2-N,N-dimethylamino-7-carboethoxytetrahydropyrene (DMCTHP) (see Chart 1). 2. Experimental techniques Absorption spectra were recorded on a Shimadzu 2100 UV–Vis–NIR spectrometer. Laser flash photolysis experiments were carried out by employing an Applied Photophysics Model LKS-20 Laser Kinetic Spectrometer using GCR-12 Series Quanta Ray Nd:YAG laser. The ESR experiments were carried out using a Bruker EMX-EPR Spectrometer operating at X band with 100 KHz field modulation. Redox potentials were measured using a BAS CV50 W Cyclic voltammetric analyzer. Amines employed in this study were either purchased from Aldrich or synthesized using reported procedures. These were purified by crystallization or distillation before use. Synthesis of DMTHP and DMCTHP are reported elsewhere [17]. Cu(ClO4)2 and other metal perchlorates were purchased from Aldrich and used as such. Spectroscopic grade acetonitrile was used for all measurements. All experiments were carried out at room temperature (298 K).
3. Results and discussion Solutions of the amines in acetonitrile were either colorless or pale yellow. Upon addition of micro molar amounts of Cu(ClO4)2, significant color changes occurred and these were studied using absorption spectroscopy. Fig. 1a–f show the changes observed in the absorption spectra of the amines shown in Chart 1 in the presence of increasing amounts of Cu(ClO4)2 in acetonitrile solu-
295
tion. Structures of the aromatic amines employed are shown as inserts in the figures. In Fig. 1a–f, the downward arrows indicate the original absorptions due to the aromatic amines and the upward arrows indicate the new absorptions formed in the presence of micro molar concentrations of Cu(ClO4)2. (We have used Cu(ClO4)2 for these studies, but other Cu2+ salts such as CuCl2 also showed absorption spectral changes as shown in Fig. 1.) In each case addition of Cu(ClO4)2 resulted in the disappearance of the band due to the amine and formation of two new absorption bands (one in the UV region and another in the visible). The absorption bands in the visible region occur at different wavelengths and this has resulted in different colors for the solutions. For example, the DMA/Cu(ClO4)2 solution was yellow, NMPT/ Cu(ClO4)2 solution was pink and the TMPD/Cu(ClO4)2 solution was violet in color. TBPA/Cu(ClO4)2, DMTHP/Cu(ClO4)2 and DMCTHP/Cu(ClO4)2 solutions were blue in color. In the case of NMPT/Cu (ClO4)2 and TPBA/Cu(ClO4)2 systems, the colors persisted for more than a day and for the other systems the colors were stable only for 0.5–2 h. As control experiments, absorption spectra of these amines were also recorded in the presence of perchlorate salts of other divalent ions such as Zn2+, Co2+, Mn2+, Pb2+ and Ni2+. Absorption spectral changes similar to those shown in Fig. 1 were not seen in any of these cases. The new absorption bands formed in the UV and visible regions in Fig. 1a–f can be attributed to the radical cations of the respective aromatic amines formed as a result of an electron transfer reaction between the amine and Cu2+ (vide infra). Absorption spectra of the radical cations of DMA, TMPD, NMPT and TBPA are reported in the literature. DMA radical cation has absorption in the visible with kmax = 470 nm [19a]. In the case of TMPD, the radical cation absorption band is reported to have three peaks at 525, 570 and 630 nm [19b]. The spectrum obtained upon addition of Cu (ClO4)2 to TMPD in acetonitrile shows all these features (Fig. 1b). In the case of NMPT, the radical cation can be generated by photoinduced electron transfer [20] or simply by dissolving NMPT in concentrated sulfuric acid [21–23]. Absorption spectrum of NMPT/conc. H2SO4 system is shown in Fig. 2a. The spectrum is identical to the 520 nm band in Fig. 1c. In the case of TBPA, the radical cation spectrum can be easily obtained by laser flash photolysis of TBPA in the presence of an electron acceptor. In an earlier study [24], we have generated this cation by flash photolysis of azadioxatriangulenium cation/TBPA system in acetonitrile solution and the spectrum obtained is shown in Fig. 2b. The spectrum is nearly identical to the long wavelength band in Fig. 1d. The transient absorption spectrum obtained in the flash photolysis of DMCTHP in the presence of the well-known electron acceptor dicyanobenzene is shown in Fig. 2c. The spectrum is identical to the long
296
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299 0.6
a
H3C
f
0.2
b
CH3
N
f
H3C
CH3
N
Absorbance
a a H3C
CH3
0.1
0.0
0.0 300
400
500
600
700
800
300
0.6
c
CH3 N
f
Absorbance
N
0.3
0.2
400
500
d
600
700
800
Br
f S
0.4
a
N
a Br
0.1
Br
0.2
0.0 0.3
0.0 300
400
500
600
e
H3C
N
CH 3
300 0.3
400
f
500
600
700
800 N(CH3)2
f
Absorbance
f 0.2
a
0.2
CO2C2H5
a 0.1
0.0
0.1
300
400
500
600
700
800
0.0 300
400
500
600
700
800
Wavelength, nm Fig. 1. Absorption spectra of the various aromatic amines in the absence and presence of Cu(ClO4)2. [Amines] were 5–10 lM and [Cu(ClO4)2] were: (a) 0, (b) 2.2, (c) 4.45, (d) 6.65, (e) 8.84, and (f) 11 lM.
wavelength band in Fig. 1f. Thus, we assign the new absorption bands produced upon adding Cu(ClO4)2 to the amine solutions in acetonitrile to the radical cations of the respective amines. In order to further confirm the formation of radical cations, ESR spectra of the solutions were recorded. Fig. 3a shows the ESR spectrum of Cu(ClO4)2 in aceto0.6
a
nitrile solution. Fig. 3b is the ESR spectrum of the pink solution obtained by mixing the Cu(ClO4)2 solution with one equivalent of NMPT. The ESR spectrum of NMPT radical cation is reported to have six peaks with intensities in the ratio 1:4:7:7:4:1 [23] and this is the pattern observed in Fig. 3b. We have also generated NMPT radical cation by dissolving NMPT in concentrated H2SO4 and
b
c 0.0050
0.3
∆OD
∆OD
Absorbance
0.04
0.02
0.0 400
500
Wavelength, nm
600
0.00 600
700
Wavelength, nm
800
0.0025
0.0000 550
600
650
700
750
Wavelength, nm
Fig. 2. (a) Absorption spectrum of NMPT in conc. H2SO4; (b) transient absorption spectrum of azadioxatriangulenium cation (1.7 · 104 M)/TBPA (5 mM) system; and (c) transient absorption spectrum of DMCTHP (2 · 105 M)/1,4-dicyanobenzene (2 · 103 M) system.
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
ESR spectrum of this solution is shown in Fig. 3c. It can be seen that Fig. 3b,c are identical. ESR spectra of DMTHP/Cu(ClO4)2 and DMCTHP/Cu(ClO4)2 systems were also obtained (Fig. 3d,e), but in these cases the resolution was poor due to the instability of the radical cations. Thus, the ESR experiments confirm that the colored species generated in these reactions are indeed the amine radical cations. Fig. 3b,d,e are obtained upon mixing the corresponding amines with one equivalent of Cu2+. These ESR spectra do not show the peaks due to Cu2+, suggesting that formation of the amine radical cation is accompanied by the disappearance of Cu2+. Since the amines are converted to radical cations and Cu2+ is converted to Cu+, it is obvious that an electron transfer (ET) reaction as shown in the following equation has taken place upon mixing the amines with Cu2+: Amine þ Cu2þ ! Amineþ þ Cuþ ð1Þ
Table 1 The oxidation potentials (vs. SCE) of amines and free energy changes (DG) for reaction (1) with the amines Aromatic amine
Eox
DG
DMA NMPT TBPA TMPD DMTHP DMCTHP
0.71 0.86 1.05 0.15 0.59 0.66
0.242 0.092 0.098 0.802 0.362 0.292
ter distance of the donor and acceptor and e is the dielectric constant of the solvent. In polar solvents like acetonitrile, the coulombic term can be neglected. Redox potential of the Cu2+/Cu+ couple is solvent dependent and the value in acetonitrile is 1.194 V vs. NHE (= 0.952 V vs. SCE) [25]. The oxidation potentials of DMA [3], TMPD [26], TBPA [27] and NMPT [22] (vs. SCE) in acetonitrile were known in the literature. The oxidation potential of DMTHP, and DMCTHP (vs. SCE) were determined by cyclic voltammetry. In Table 1, the oxidation potentials of the amines and calculated DG values of the electron transfer reactions are given. An inspection of Table 1 shows that the ET reaction between the amines and Cu2+ is exothermic in all cases except for TBPA, in which case DG value is close to zero. Hence, electron transfer reactions as per Eq. (1)
For the electron transfer reaction in Eq. (1), the free energy change DG can be calculated using the following equation [1–3]: DG ¼ Eox Ered e2 =de
ð2Þ
In Eq. (2), Eox is the oxidation potential of the donor (amines in this case) and Ered is the reduction potential of the acceptor (Cu2+ in this case), d is the center to cen200 150
c
b
a
297
Intensity
100 50 0
200 G
-50 10 G
10 G
-100 -150 -200 2600 2800 3000 3200 3400 3600 3800
G 60
3425
3450
3475
3500
3340 3360 3380 3400 3420 3440
G
G d
e
40
Intensity
20 0 -20 10 G 10 G
-40 -60 3400 3420 3440 3460 3480 3500 3520
3420 3440 3460 3480 3500 3520
G
G
Fig. 3. The ESR spectra obtained at 298 K of: (a) Cu(ClO4)2 (6 mM) in acetonitrile, (b) NMPT and Cu(ClO4)2 (2 mM each) in acetonitrile, (c) NMPT (2 mM) in conc. H2SO4, (d) DMTHP and Cu(ClO4)2 (2 mM each) in acetonitrile, and (e) DMCTHP and Cu(ClO4)2 (2 mM each) in acetonitrile.
298
S. Sumalekshmy, K.R. Gopidas / Chemical Physics Letters 413 (2005) 294–299
should be spontaneous. Since most aromatic amines have Eox values <1.0 V (vs. SCE), reaction (1) should take place spontaneously with most aromatic amines. Redox potential of the Cu2+/Cu+ couple is solvent dependent. Redox potentials for this couple reported in water (0.09 V vs. SCE) and methanol (0.0 V vs. SCE) [25] are very much lower compared to the value reported in acetonitrile. If we use these values for Eox, then DG values would turn out to be positive for all amines. In agreement with this, we did not observe radical cation formation when the reactions were carried out in methanol. The amines were not soluble in water and hence the experiments could not be done in water. Since the NMPT radical cation was very stable, we have tried to determine the yield of the radical cation by measuring the absorbance due to the radical cation immediately after mixing molar equivalents of NMPT and Cu(ClO4)2 solutions. Using the known extinction coefficient of NMPT+ (9.2 · 103 M1 cm1 at 520 nm) [20], we calculated the concentration of NMPT+ formed. Results showed that nearly 75% of NMPT was converted to NMPT+. In this study, we have reported radical cation formation of tertiary aromatic amines only. We have noted that simple aromatic amines such as aniline, and secondary amines such as N-methylaniline also exhibited color changes under comparable conditions. Details of these studies are not included here. It was very surprising to note that DMA+ prepared by this method is stable for more than 30 min. It is believed that the DMA radical cation is very unstable due to facile dimerization to tetramethylbenzidine (TMB) dication [28]. Since the absorption spectra of DMA radical cation and TMB dication are very similar, it is possible that the observed color is due to TMB dication [28]. In an attempt to determine the amount of TMB dication formed, we have quenched the reaction mixture by adding triethylamine and water and the recovered amine mixture was analyzed by GC–MS. The results show that only about 10% of DMA radical was converted to TMB dication indicating that formation of TMB dication is not a significant reaction pathway under our experimental conditions. It was reported earlier that Fe3+ and to a lesser extent, Cu2+ can catalyze the cyclodimerization of a few aromatic enamines in methanol or methanol–acetonitrile mixtures [29,30]. Although amine radical cations were proposed as probable intermediates in these reactions, reaction with Cu2+ as a general method for the generation and study of amine radical cations has not been documented previously. Amine radical cations, particularly the triarylamine radical cations in which the para positions of the phenyl rings are substituted, are among the most useful oxidants because they are very strong and nearly innocent [31]. These are widely used as catalysts in Diels–Alder and other C–C bond forming reactions [32,33] and are generally prepared
by reaction of the corresponding amines with nitrosonium salts or Ag+ salts in the presence of iodine or SbCl5. In this Letter, we show that these radical cations can be prepared simply by mixing the amines with Cu (ClO4)2 in acetonitrile solution.
4. Conclusions The interactions of a few selected aromatic amines with Cu(ClO4)2 were probed by absorption and ESR spectroscopy. All these amines reacted with Cu2+ ions to give amine radical cations, which are characterized by absorption and ESR spectra. Formation of radical cations was explained by invoking an electron transfer mechanism. This is projected as a general method for the generation and study of aromatic amine radical cations in solution.
Acknowledgements The authors thank the Council of Scientific and Industrial Research (CSIR), Government of India and the Department of Science and Technology (DST), Government of India for financial support. We also thank Dr. T. Ramasami, Director, Central Leather Research Institute, Chennai for allowing access to the ESR spectrometer and Dr. J. Subramanian for helping with the ESR measurements.
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