Effects of Phenyl- and Methyl-Substituents on p-phenylenediamine, an Electrochemical and Spectral Study

Transcription

1 Journal of the Chinese Chemical Society, 2009, 56, Effects of Phenyl- and Methyl-Substituents on p-phenylenediamine, an Electrochemical and Spectral Study Yi-Chun Chung a ( ) and Yuhlong Oliver Su a,b *( ) a Department of Applied Chemistry, ational Chi an University, 1, University Road, Puli, antou Hsien 545, Taiwan, R.O.C. b Department of Chemistry, ational Chung Hsing University, 250, Kuo Kuang Rd, Taichung 402, Taiwan, R.O.C. Two series of substituted p-phenylenediamines have been studied for their electronic effects on redox potential and spectral properties. p-phenylenediamines and,,, -tetramethyl-p-phenylenediamine substituted with different numbers of phenyl groups have been synthesized and their cyclic voltammograms have been obtained. The correlation between the substituent number and the redox potential appears linear. The slope reflects the additive effect of electron-donating methyl and electron-withdrawing phenyl groups. The absorption spectra of the cation radicals indicate that phenyl-substituted ones have broad intervalence-charge transfer bands. The p-phenylenediamines exhibit different properties from triphenylamines in that the oxidized forms are more stable in CH 3 C then those in CH 2 Cl 2. Some of the cation radicals or dications could undergo follow-up chemical reactions and form products that are more easily oxidized. Keywords: Phenylenediamines; Substituent effect; Electrochemistry; Spectroscopy. ITRODUCTIO Triphenylamine (TPA) and carbazole (Cz) possess an electroactive nitrogen atom as the redox center. The oxidized nitrogen-containing compounds have a very strong interaction between the nitrogen atom and the phenyl rings and thus exhibit very diversified electrochemical and spectral properties. 1 In 1952, Brown 2 reported the synthesis and properties of ortho-phenylenediamines. Stamires 3 carried out the electrochemical oxidation of ortho-phenylenediamines. In 1966, Adams 4,5 found that triphenylamine could form tetraphenylbenzidine (TPB) by way of oxidation and dimerization. A series of work on the oxidation of aniline 6 and diphenylamine 7,8 was further pursued later on. Triphenylamines with different substituents have been exhaustively studied for their effect at the para-position of the phenyl rings. It was then found that triphenylamine can undergo carbazole formation when the potential was set at the second oxidation wave. 9 p-phenylenediamine derivatives have been studied for their electrochemical, 10,17-19,23 UV-Vis, 10,17,22,24 photochemical, 16 EPR, 18,19 IR, 23 Resonance Raman 20,24 and electron transfer 21,23 properties. The experiments were carried out in aqueous media or in non-aqueous solutions in the presence of acids. Among them,, -diphenyl-p-phenylenediamine has been most extensively investigated. 10,16,17,20,23 p-phenylenediamine, however, is not stable in its oxidation form and thus has been rarely studied. In this study, we have systematically investigated the electrochemical properties as well as spectra of compounds 1-12 (Fig. 1). Different substituents were screened for stabilization of cation radical and dications. This work is to establish the electrochemical and spectral pattern for p- phenylenediamines in non-aqueous solvents. The redox potential varies with the number of substituent, which includes electron-donating methyl and electron-withdrawing phenyl groups. The stability of cation radicals and dications could be justified by comparing the measurements of the absorbance of the neutral state before and after oxidation. The properties will lay the foundation for polyamine compounds as chromic, voltaic or luminescent materials. * Corresponding author. Tel: ext. 4150; Fax: (Yi-Chun Chung); Tel: ; yosu@ncnu.edu.tw (Yuhlong Oliver Su)

2 494 J. Chin. Chem. Soc., Vol. 56, o. 3, 2009 Chung and Su EXPERIMETAL SECTIO HPLC grade CH 2 Cl 2 and CH 3 C were purchased from Acros and were distilled from CaH 2 under 2 atmosphere for electrochemical use. Tetra-n-butylammonium perchlorate (TBAP) was recrystallized from ethyl acetate twice to form flakes and then stored in a desiccator. Compounds 1, 2, 7, 8 were purchased from Acros, and 4 was purchased from TCI. Synthesis Compounds 3, 10 5, 27 6, 28 9, 10, 13 and were synthesized according to literature methods. These compounds were further identified by MR and MS. -Methyl-,, -triphenyl-p-phenylenediamine (12) 0.13 g (0.3 mmol) of compound 5 was dissolved in 5 ml of DMF in a 50 ml round bottom flask. Iodomethane (0.05 ml, 0.8 mmol) and potassium carbonate (0.11 g) were added. The reaction mixture was heated at reflux temperature under nitrogen atmosphere for 10 hours. After cooling to room temperature, iced saturated salt water was added to form white precipitates. The mixture was then filtered and the filtrate was evaporated under reduced pressure. After silica gel column chromatography by using mixed solvent CH 2 Cl 2 : hexane = 1 : 1, 0.1 g of pale green solid was obtained (yield 74%). EI-MS m/z (rel abundance) Fig. 1. p-phenylenediamine derivatives.

3 Substituent Effects on p-phenylenediamine J. Chin. Chem. Soc., Vol. 56, o. 3, (M +, 100), 336, 167, 351, H MR (300 MHz; CDCl 3 ): 3.3 (s, 3H), (m, 9H), (m, 10H). IR (KBr pellet) 3033 (w), 1590 (m), 1493 (s), 1274 (m), 751 (m), 695 (m), 619 (w), 570 (w), 513 (w) cm -1. Instrumentation Electrochemical and spectral experiments were carried out according to previous reports. 10,26 Cyclic voltammetry was performed with a Bioanalytical System Model CV-27 potentiostat and a BAS X-Y recorder. It was conducted with the use of a three-electrode cell in which a BAS glassy carbon electrode (area = 0.07 cm 2 ) was used as a working electrode. The glassy carbon electrode was polished with 0.05 m alumina on Buehler felt pads and was ultrasonicated for 2 min to remove the alumina residue. A platinum wire was used as the auxiliary electrode. All cell potentials were taken with the use of a homemade Ag/AgCl, KCl (sat d) reference electrode. Ferrocene was used as an external reference for calibration (+0.55 V vs. Ag/AgCl) in CH 2 Cl 2. Spectroelectrochemistry was conducted with an electrolytic cell which was composed of a 1 mm cuvette, a platinum gauze (Aldrich, 100 mesh, 0.9 cm 1.8 cm) thin layer as working electrode, a platinum wire as auxiliary electrode, and a Ag/AgCl reference electrode. Absorption spectra were measured with a HP-8453 UV/vis spectrophotometer. Potential step technique was applied for time-resolved spectra. Electrochemical and spectroelectrochemical characterization of p-phenylenediamine (1) In CH 2 Cl 2, an oxidation wave at E pa = V and a reduction wave at E pc = V were observed in the cyclic scan (Fig. 2A). Continuous cycling renders decreases of the reduction current, probably due to blocking electrode reaction by the adsorption of the products from the degradation of oxidized p-phenylenediamine. In the extension of the oxidation scan, a smaller oxidation wave at E pa = V was observed, and a corresponding small reduction wave at E pc = V appeared. The reduction at E pa = V, however, decreased in magnitude for consecutive scans. In CH 3 C, the first oxidation wave at E pa = V has a corresponding reductive peak at E pc = V. (Fig. 2B). It is interesting to note that there are two small redox couples at E 1/2 = V and 0.16 V, respectively. If the oxidation scan is extended to V, a second redox couple at E 1/2 = V was observed. Another new small redox wave at about E 1/2 = V was also observed. But only one small couple in the vicinity E = 0.0 V remained. It is noteworthy that the oxidized forms are more stable in CH 3 C than in CH 2 Cl 2. The property is different from that of triphenylamine. 1b Malval et al., 22 reported E 1 = 276 mv and E 2 = 825 mv vs. SCE in CH 3 C containing 0.1 M TBAPF 6.Thestability and spectra of 1 +. and 1 2+, however, was not mentioned. To probe the stability of the oxidation products of 1, spectroelectrochemistry was carried out (Fig. 3). The cation radical was stable while the dication lost 20% absorbance after 4 minutes. Electrochemical characterization of -phenyl-p-phenylenediamine (2) In CH 2 Cl 2, the first redox couple appeared reversible at E 1/2 = V. When the potential was swept to V, an oxidation wave at E pa = V was observed (Fig. 4A). RESULTS AD DISCUSSIO Fig. 2. Cyclic voltammograms of 1 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate

4 496 J. Chin. Chem. Soc., Vol. 56, o. 3, 2009 Chung and Su p-phenylenediamine (1) On the reversed scan, however, a huge non-diffusional wave at E pc = V appeared. After scan cycling, a small wave appeared between the two redox couples. The second oxidation apparently produced adsorptive compounds that were more difficult to reduce. In CH 3 C, two clearly reversible couples at E 1/2 = and V, respectively, were observed (Fig. 4B). The oxidation products, conceivably the cation radical and dication, apparently were more stable in CH 3 C than in CH 2 Cl 2. It was reported that compound 2 was oxidized at 345 mv and 512 mv vs. SCE, respectively, in CH 3 C. 22 There is a substantial difference in the second oxidation potential between the two research groups. Electrochemical and spectroelectrochemical characterization of,-diphenyl-p-phenylenediamine (3) In CH 2 Cl 2, two redox couples at and V, respectively, were observed (Fig. 5A). 10 In CH 3 C, the potentials were at and V, respectively. The peak-to-peak separation ( E p )inch 3 C was significantly smaller than that in CH 2 Cl 2, indicating a better reversibility. The absorption spectrum of 3 +. (Fig. 6) in CH 3 C is very similar to that in CH 2 Cl However, the spectrum of 3 2+ in this work is completely different from that in CH 2 Cl The recoveries of 3 +. and 3 2+ in this study are nearly quantitative. Thus, the reported 3 2+ spectrum in CH 2 Cl 2 probably originates from the chemical reaction product of Fig. 3. Time-resolved spectral changes of compound 1 during (A) the first electron oxidation at V (B) the second electron oxidation at V in CH 3 C containing 0.1 M TBAP. Fig. 4. Cyclic voltammograms of 2 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate

5 Substituent Effects on p-phenylenediamine J. Chin. Chem. Soc., Vol. 56, o. 3, Electrochemical characterization of, -diphenyl-p-phenylenediamine (4) In CH 2 Cl 2, two reversible redox couples at and V, respectively, were observed (Fig. 7). In CH 3 C, the redox potentials were at and V, respectively. The redox waves appeared more reversible in CH 3 C ( E p ~70 mv) than in CH 2 Cl 2 ( E p ~0.11 V). Also, the potential separation of the redox couples ( E 1/2 )inch 3 C (0.47 V) is larger than that in CH 2 Cl 2 (0.37 V). Compound 4 was most extensitively studied for its photochemical oxidation in CHCl 3, 16 anodic voltammetry in CHCl 3, 17 resonance Raman scattering in CH 3 C, 20 UV- Vis and IR in CH 2 Cl 2, 23 UV-Vis and Raman spectroscopy in CH 3 C. 24 The investigations were different either in solvents or additives. The results were thus different in stability, redox waves, or characteristic absorption. characteristic for an intervalence charge transfer (IV-CT) band. 25 The recovery was nearly quantitative. These two bands decreases upon further oxidation, and another two new bands at 286 and 556 nm appeared. Electrochemical and spectroelectrochemical characterization of,-dimethyl-p-phenylenediamine (7) Compound 7 exhibited a quasi-reversible redox couple in the first oxidation but the second oxidation was irreversible in CH 2 Cl 2.InCH 3 C, the first redox couple was reversible at E 1/2 = V and the second oxidation underwent a slow reaction generating a small redox couple at,-diphenyl-p-phenylenediamine (3) Electrochemical and spectroelectrochemical characterization of,, -triphenyl-p-phenylenediamine (5) In CH 2 Cl 2, two redox couples at and V, respectively, were observed. In CH 3 C, the redox couple were at and V, respectively (Fig. 8). Again the E p is smaller for the compound in CH 3 C(0.43V)thanin CH 2 Cl 2 (0.51 V). In spectroelectrochemistry using CH 3 C as solvent, the potential was varied by small increments. Two steps of spectral change were observed as show in Fig. 9. At 0.0 V, only the neutral form absorption at 306 nm was observed. During the first oxidation, two bands at 393 and 764 nm gradually increased as the 306 nm band decreased concurrently (Fig. 9). The absorption at 764 nm is broad and is Fig. 5. Cyclic voltammograms of 3 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate Fig. 6. Time-resolved spectral changes of compound 3 during (A) the first electron oxidation at V (B) the second electron oxidation at V in CH 3 C containing 0.1 M TBAP.

6 498 J. Chin. Chem. Soc., Vol. 56, o. 3, 2009 Chung and Su about +0.6 V. The neutral form has an absorption peak at 258 nm while the cation radical has three sharp peaks at 323, 517 and 554 nm, respectively (Fig. 11). Electrochemical and spectroelectrochemical characterization of,,, -tetramethyl-p-phenylenediamine (8) In CH 2 Cl 2, the first redox couple at E 1/2 = V was observed. But the second oxidation at E pa = V was a sharp peak, indicating oxidation product deposition onto the electrode surface. In the reverse scan, a reduction sharp peak corresponding to desorption at V was observed (Fig. 12). In CH 3 C, two well-defined redox couples were observed at E 1/2 = V and V, respectively. However, a very small reduction wave at about V was observed after the second oxidation wave. Oxidation in CH 3 C containing 0.1 M TBAPF 6, however, exhibited two redox couples at E 1/2 = mv and mv, respectively. 22 The E 1/2 (598 mv) is close to 0.59 V in the presence of TBAP as the electrolyte. Oxidation at E app = V generates a spectrum of 8 +. with sharp peaks at 327, 566 and 614 nm, which were also observed in CHCl 3 ; 17 the dication 8 2+ has a relatively small absorbance at about 255 and 305 nm (Fig. 13).,, -triphenyl-p-phenylenediamine (5) Fig. 7. Cyclic voltammograms of 4 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate Fig. 8. Cyclic voltammograms of 5 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate Fig. 9. Time-resolved spectral changes of compound 5 during (A) the first electron oxidation at V (B) the second electron oxidation at V in CH 3 C containing 0.1 M TBAP.

7 Substituent Effects on p-phenylenediamine J. Chin. Chem. Soc., Vol. 56, o. 3, The CV and spectra results of compounds 6, 9, 10, 11 and 12 are provided as supporting materials. Analysis of the redox potential Table 1 lists the redox potentials of the compounds 1-6 in CH 3 C and CH 2 Cl 2 (parentheses). The difference in potential between the second and first redox reactions reflects the degree of stabilization by the solvent for each oxidation state and the degree of IV-CT. The effect of phenyl group substitution is substantial in the oxidation potential (Fig. 14A). Each phenyl group substitution causes the first oxidation potential to increase by about 73 mv. The second oxidation, corresponding to dication formation, is increased by 47 mv per substituent, a little smaller than the first one. Table 2 lists the redox potentials of p-phenylenediamines substituted with methyl and phenyl groups in CH 3 C and CH 2 Cl 2. The redox potentials were plotted as a function of number of phenyl groups (Fig. 14B). Both first and second oxidation potentials exhibited good linearity. The first exhibited a larger slope (111 mv) than the second (77 mv), indicating a higher sensitivity for cation radical formation. Compounds 10 and 11 are isomers. For compound 11, two nitrogen atoms are equivalent because their substituents are the same. For compound 10, however, one is substituted with two methyl groups while another has two phenyl groups. The first oxidation for compound 10 probably lies on the methylated nitrogen and the potential is more cathodic than for compound 11 by about 30 mv (Table 2). Analysis of absorption spectra Table 3 lists the peak wavelength of the absorption bands of phenyl substituted p-phenylenediamines in this,-dimethyl-p-phenylenediamine (7) Fig. 10. Cyclic voltammograms of 7 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate Fig. 11. Time-resolved spectral changes of compound 7 during (A) the first electron oxidation at V (B) the second electron oxidation at V in CH 3 C containing 0.1 M TBAP.

8 500 J. Chin. Chem. Soc., Vol. 56, o. 3, 2009 Chung and Su Table 1. Oxidation potentials of phenyl p-phenylenediamine derivatives in CH 3 CandinCH 2 Cl 2 (parentheses) containing 0.1 M TBAP 2nd 1st E 1/ (0.60) 0.33 (A) (A) (0.93a) 0.41 (0.49) 0.47 (0.44) (1.07) 0.48 (0.56) 0.46 (0.51) (0.99) 0.48 (0.62) 0.47 (0.37) (1.09) 0.58 (0.58) 0.43 (0.51) (1.10) 0.61 (0.65) 0.45 (0.45) A: not available a: estimated value study. The absorption exhibited a bathochromic shift as the number of the phenyl group increases for both p-phenylenediamines and,,, -tetramethyl-p-phenylenediamines series in the neutral state. As opposed to the simple pattern for spectral analysis of neutral compounds, cation radicals and dications exhibit distinctly different spectra for these with or without phenyl substituents. Compound 1 has these absorption peaks at 200, 251 and 320 nm, respectively. Its cation radical (1 +. ), however, exhibits a peak at 319 nm, and a very broad band at 465 nm with a shoulder extending to 720 nm. Compound 5 has an absorption peak at 308 nm in its neutral states. Its cation radical follows the same pattern as other phenyl p-phenylenediamines. amely, 5 +. exhibits a UV peak at 393 nm and a very broad band at 764 nm. During the second oxidation, the cation radical peak at 393 and 764 nm of 5 +. decreases while there are two peaks at 286 and 556 nm. Compound 6, on the other hand, has only one UV absorption peak at 308 nm. Also, its cation radical exhibits a peak at 404 nm, and a very broad band (FWHM 300 nm) at max = 826 nm. This pattern is observed for all other phenyl-containing phenylenediamine derivatives. For compound 8, these peaks at 202, 265 and 330 nm are present in,,, -tetramethyl-p-phenylenediamine (8) Fig. 12. Cyclic voltammograms of 8 in (A) CH 2 Cl 2 and (B) CH 3 C containing 0.1 M TBAP. Scan rate Fig. 13. Time-resolved spectral changes of compound 8 during (A) the first electron oxidation at V (B) the second electron oxidation at V in CH 3 C containing 0.1 M TBAP.

9 Substituent Effects on p-phenylenediamine J. Chin. Chem. Soc., Vol. 56, o. 3, Table 2. Oxidation potentials of methyl and phenyl p-phenylenediamine derivatives in CH 3 CandinCH 2 Cl 2 (parentheses) containing 0.1 M TBAP 2nd 1st E 1/ (A) (0.37) 0.57 (A) (A) (0.20) 0.59 (A) (1.02) 0.30 (0.42) 0.53 (0.60) (1.05) 0.41 (0.49) 0.50 (0.56) (1.02) 0.44 (0.48) 0.50 (0.54) (1.05) 0.51 (0.55) 0.47 (0.50) (1.10) 0.61 (0.65) 0.45 (0.45) A: not available its neutral state. However, two distinctive new peaks in the visible region grow upon one electron oxidation. All the absorption peaks are relatively sharper than those of phenyl-substituted p-phenylenediamines. COCLUSIOS Two series of substituted p-phenylenediamine have been systematically investigated for their electrochemical and spectral properties. The first series in Table 1 exhibits +73 and +47 mv shifts in first and second redox potential, respectively, for each phenyl substitution. The second se- Fig. 14. Plots of oxidation potentials of p-phenylenediamine derivatives as a function of phenyl substituents (A) compound 1-6 (B) compounds 8-12,and6 in CH 3 C.

10 502 J. Chin. Chem. Soc., Vol. 56, o. 3, 2009 Chung and Su Table 3. Absorption peak wavelength of p-phenylenediamine derivatives in CH 2 Cl 2 Compound UV/Vis Compound UV/Vis H 2 H nm H nm H H nm nm H nm 271 nm 3 9 H H 303 nm 295 nm 4 10 H 306 nm 304 nm nm 308 nm 6 12 ries in Table 2 has larger shifts, 111 and 77 mv, respectively. It is ascribed to the additive electron-donating effect Table 4. Absorption peak wavelength (nm) of p-phenylenediamine derivatives in CH 3 C Compound eutral Cation radical Dication 1 200,251, , , ,291,493, , , , , , ,517, , ,566, , , , , , ,380, , ,555 from the methyl substituents. But the substituent effect is weaker for the second oxidation. The intervalence-charge transfer band has a bathochromic shift as the number of the phenyl group increases. Solvent is important for the stabilization of the cation radical and dication, and the oxidized forms are more stable in CH 3 C than in CH 2 Cl 2. ACKOWLEDGMETS This work was supported by the ational Science Council of the Republic of China. Received December 31, REFERECES 1. (a) Schäfer, H. J. In Organic Electrochemistry,4 th ed.; Lund,

11 Substituent Effects on p-phenylenediamine J. Chin. Chem. Soc., Vol. 56, o. 3, H.; Hammerich, O. Eds.; Marcel Dekker: ew York, 2001; Chapter 22. (b) Adams, R.. Electrochemistry at Solid Electrodes; Marcel Dekker: ew York, Brown, H. C.; elson, K. L. J. Am. Chem. Soc. 1953, 75, Stamires, D..; Turkevich, J. J. Am. Chem. Soc. 1963, 85, Adams, R..; elson, R. F. J. Am. Chem. Soc. 1966, 88, elson, R. F.; Marcoux, L. S.; Adams, R.. J. Phys. Chem. 1967, 71, Bacon, J. R.; Adams,. J. Am. Chem. Soc. 1968, 90, Leedy,D.W.;Adams,R.. J. Am. Chem. Soc. 1970, 92, Hand, R. L.; elson, R. F. J. Am. Chem. Soc. 1973, 96, Reynold, R.; Line, L. L.; elson, R. F. J. Am. Chem. Soc. 1974, 96, Chiu,K.Y.;Su,T.X.;Li,J.H.;Lin,T.H.;Liou,G.S.; Cheng, S. H. J. Electroanal. Chem. 2005, 575, Cheng,S.H.;Hsiao,S.H.;Su,T.H.;Liou,G.S. Macromolecules 2005, 38, Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, Ciminale, F.; Ciardo, A.; Francioso, S.; acci, A. J. Org. Chem. 1999, 64, Weller, H.; Grellmann, K. H. J. Am. Chem. Soc. 1983, 105, Kung, A. C.; McIlroy, S. P.; Falvey, D. E. J. Org. Chem. 2005, 70, Fitzgerald, E. A.; Wuelfing, P.; Richtol, H. H. J. Phys. Chem. 1971, 75, Cauquis, G.; Serve, D. Anal. Chem. 1972, 44, Male, R.; Allendoerfer, R. D. J. Phys. Chem. 1988, 92, Petr,A.;Dunsch,L. J. Phys. Chem. 1996, 100, Boyer, M. I.; Quillard, S.; Cochet, M.; Louarn, G.; Lefrant, S. Electrochimca Acta 1999, 44, Lu, J.-M.; Wen, X.-L.; Wu, L.-M.; Liu, Y.-C.; Liu, Z.-L. J. Phys. Chem. A 2001, 105, Malval, J. P.; Morand, J. P.; Lapouyade, R.; Rettig, W.; Jonusauskas, G.; Oberlé, J.; Trieflinger, C.; Daub, J. Photochem. Photobiol. Sci. 2004, 3, ishiumi, T.; omura, Y.; Chimoto, Y.; Higuchi, M.; Yamamoto, K. J. Phys. Chem. B 2004, 108, Santana, H.; Quillard, S.; Fayad, E.; Louarn, G. Synth. Met. 2006, 156, Yeh, S. J.; Tsai, C. Y.; Huang, C.-Y.; Liou, G.-S.; Cheng, S.-H. Electrochem. Commun. 2003, 5, Chiu, K. Y.; Su, T. X.; Huang, C. W.; Liou, G. S.; Cheng, S. H. J. Electroanal. Chem. 2005, 578, Selby, T. D.; Stickley, K. R.; Blackstock, S. C. Org. Lett. 2000, 2, Yano, M.; Ishida, Y.; Aoyama, K.; Tatsumi, M.; Sato, K.; Shiomi,D.;Ichimura,A.;Takui,T. Syn. Met. 2003, 137, Weller, H.; Grellmann, K. H. J. Am. Chem. Soc. 1983, 105, 6268.