In the Laboratory
The Chemistry of Formazan Dyes Synthesis and Characterization of a Stable Verdazyl Radical and a Related Boron-Containing Heterocycle David E. Berry, Robin G. Hicks, and Joe B. Gilroy* Department of Chemistry, University of Victoria, Victoria, British Columbia, V8W 3V6, Canada; *
[email protected] The mandate for those of us responsible for undergraduate laboratory programs is to devise experiments that demonstrate applicable concepts and skills at the same time as inspiring the student with a link to relevant topics in current research. However, much of this research is too time-consuming or too challenging (highly reactive or highly air-sensitive molecules) to be adapted to an undergraduate laboratory setting. This experiment describes the synthesis of a 1,3,5-triarylformazan dye 1, a stable verdazyl radical 2, and a boron–nitrogen heterocycle (boratatetrazine) 3. Each of the compounds is easily prepared under aerobic conditions and are intensely colored (Figure 1), providing an instant appeal in the undergraduate teaching laboratory. Like azo-compounds (1–3), formazans (4) and their metal complexes have been used as dyes (5–8). The chemistry of such dyes has long been of interest in the undergraduate laboratory owing to their connections with everyday life, and efforts towards incorporating dyes into the undergraduate laboratory were made as early as 1925 (9). There have not been many reports of the subsequent chemistry of formazans. However, the blood red compounds have recently been used as ancillary ligands for transition metals (10, 11), and their attempted alkylation led to the discovery of verdazyl radicals (12). In addition they have been widely exploited as redox-based staining agents for cell biology (13). Very few examples of undergraduate experiments incorporating the synthesis and characterization of free radicals have been reported (14). The instability of free radicals associated with their tendency to dimerize, as well as their sensitivity to air, have generally limited their use in the undergraduate laboratory to computational (15–17), electrochemical (16), and spectroscopic (17) studies as well as their generation in situ during halogenation reactions (18, 19) and living radical polymerization chemistry (20, 21). Verdazyl radicals, named for the green color (“ver-”) of early examples, are among the most stable classes of organic radicals (22). Verdazyl 2 can be easily prepared under aerobic conditions and is stable indefinitely in solution and solid state. The “novelty” of isolating a free radical has been received with much enthusiasm by undergraduate students, who
are generally led to believe that radicals are short-lived, highly reactive species. There are few undergraduate laboratory experiments incorporating group 13 elements (23) and heterocycles thereof (24). The reason for their absence in relevant literature relates primarily to the extreme care that must be taken in the preparation and handling of many such compounds. Boratatetrazine 3 can be prepared under aerobic conditions from easy-to-handle starting materials and maintains its purple color indefinitely when exposed to air and moisture in both solution and the solid state, allowing for its study in an undergraduate laboratory. These compounds have been converted to radical anions (25), isolobal to previously discussed verdazyl radicals. The aim of this experiment is to expand upon the conventional bonding theories taught in an undergraduate classroom. This is a rare example where classes of compounds that are generally assumed to be too reactive or too air sensitive are easily isolated and handled in air. The students are asked to consider the UV–vis spectra of these highly colored compounds and to consider the types of electronic transitions that may give rise to their color (26). In addition, both NMR and EPR spectra are interpreted by the students, requiring them to consider the bonding in each compound. The use of crystallography further builds on this, as students are asked to assess the bonding depicted in crystal structures and relate it to the best representation of each molecule. Experiment This experiment is designed to run over two four-hour lab periods. Students can comfortably complete the synthesis and characterization of formazan 1 in one lab period and verdazyl radical 2 and boratatetrazine 3 in the second. Formazan 1, 1,5-diphenyl-3-(4-tolyl)formazan, is prepared by adaptation of the method originally reported by Katritzky (27). The reaction of 1-phenyl-3-(4-tolyl)hydrazone, 4, with phenyldiazonium chloride is performed under biphasic conditions (water/dichloromethane) with a quaternary ammonium salt,
AcO Ph
N
H
N
red
N
Ph
Ph
N 4-tol 1
green
N
N
N
N 4-tol 2
Ph
Ph
N
OAc B
N
purple
N
Ph
N 4-tol 3
Figure 1. The intense colors of formazan-based heterocycles. (Shown in color in the table of contents and in the online PDF.)
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Journal of Chemical Education • Vol. 86 No. 1 January 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
tetrabutylammonium bromide, acting as a phase-transfer catalyst (Scheme I). The phenyldiazonium chloride salt is prepared in situ by treatment of aniline with sodium nitrite under acidic conditions and then added at low temperature to the above mixture. A fairly simple workup of adding water and passing the organic layer through a short alumina column is followed by evaporation to dryness to yield the formazan. Verdazyl radical 2, 1,5-diphenyl-3-(4-tolyl)verdazyl, is prepared according to previously reported methods (28). Formazan 1 is reacted with formaldehyde in a basic dimethylformamide (DMF) medium in air to afford the corresponding verdazyl radical via oxidation of a leuco-verdazyl intermediate 5 (Scheme II). The green coloration of the reaction mixture appears within about 30 minutes and the reaction should be complete in less than 1.5 hours. The solid product can be recovered by ether extraction followed by evaporation of the solvent. During workup, the first separation of the organic and aqueous layers can be difficult to distinguish because of the intense color. Boratatetrazine 3, 1,1-diacetato-2,6-diphenyl-4-(4-tolyl) boratatetrazine, is prepared by adaptation of previously reported methods (25). The reaction of boron triacetate (prepared in situ) with 1 yields the corresponding boratatetrazine (Scheme III). The solid product can be obtained by ether extraction, but this time the product is deep purple. However, the purple color does not become apparent until water is added at the completion of the reflux. The intensity of the color makes the organic/aqueous boundary difficult to see until the level in the separatory funnel drops to the thinner part of the glassware’s neck. Crystallographic data for 1 and 3 are available from the Cambridge Crystallographic Data Centre (29; Refcode: PEVWIX for 1 and PEVWET for 3) or as supporting information for ref 25.
Discussion In a class of approximately 50 third-year undergraduate students, 10 students performed this experiment in each of two years. These students had completed the pre-requisite introductory inorganic chemistry course as well as a rigorous course in practical spectroscopy. This experiment is best suited for fourth-year undergraduate students majoring in chemistry.
Ph
H
MeOH
N
H
4-tol
4-tol 4
Cl
Ph
H
N
N
Ph
N
N
N
Na2CO3 Bu4NBr
Ph
CH2Cl2 / H2O
CH2Cl2 / H2O
N 4-tol 1
Scheme I. Synthesis of formazan 1.
Ph
H
N N
Hazards Please see the appropriate MSDS before working with any of the chemicals listed below. Also, please note the cautionary comments on the preparation of boron triacetate (30, 31). Glacial acetic acid, acetic anhydride, hydrochloric acid, and sodium hydroxide are all corrosive as vapors and liquids, to varying degrees. Aniline, dichloromethane, and aqueous formaldehyde are harmful or toxic by inhalation, contact with skin, or ingestion and are possible carcinogens. Boric acid may impair fertility and ingestion may cause gastric disturbances and electrolytic imbalance. Diethyl ether is extremely flammable and may form explosive peroxides. It is harmful if swallowed and vapors may cause drowsiness. Dimethylformamide is harmful by inhalation and in contact with skin. It is irritating to the eyes. It may cause harm to the unborn child and is a possible carcinogen. Anhydrous magnesium sulfate and sodium carbonate monohydrate are irritants. Methanol is highly flammable and toxic by inhalation, by contact with skin, and if swallowed. Phenylhydrazine is toxic by inhalation, by contact with skin, and if swallowed and is a suspected carcinogen. Sodium nitrite is toxic if swallowed. In contact with combustible material, it may cause a fire. Tetrabutylammonium bromide is irritating to eyes, respiratory system, and skin and can be absorbed through the skin in harmful amounts. 4-Tolualdehyde is harmful if swallowed and is an irritant to eyes and skin.
O
PhNHNH2
H
N
N
Ph
CH2O NaOH
Ph
N
Ph
N
N
N
NH
DMF
4-tol
4-tol
1
5 O2 Ph
N
N
N
N
Ph
4-tol 2 Scheme II. Synthesis of verdazyl 2.
OAc
AcO Ph
N
H
N
N N
Ph
Ph B(OAc)3 AcOH / Ac2O
N
B
N
N
Ph
N
4-tol
4-tol
1
3
Scheme III. Synthesis of boratatetrazine 3.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 1 January 2009 • Journal of Chemical Education
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In the Laboratory
In most cases, students successfully obtained all three compounds, with their purity depending heavily on the experience of the student. A summary of student results is available in the online material. During the first term this experiment was run, samples of formazan 1 were commonly found to contain significant quantities of unreacted 4-tolualdehyde. As a result of the contamination, samples of verdazyl 2 and boratatetrazine 3 were not clean, and in some cases the products were not isolated. To overcome these problems, we decided to supply the students with hydrazone 4 in the second term the experiment ran. This allowed the students more time to complete the experiment and significantly improved their results. The following summary of results is based on the second term in which this experiment ran, as the first term was not representative of the experiment as it currently stands. The yields obtained for each compound in this experiment were variable. The formazan was isolated in 22–49% yield, while the verdazyl radical and the boratatetrazine were isolated in 22–70 and 15–40% yield, respectively. The low yields of the latter examples relate to inefficient extraction of the compounds, while the low yield of the formazan relates to decomposition of the diazonium salt and inefficient purification (column chromatography). The NMR spectra collected for 1 and 3 were often complicated by the presence of solvent signals. However, in most cases, students were able to observe equivalency of the N-substituents. This equivalency prompts them to consider the bonding within
3460
3480
3500
3520
3540
3560
Magnetic Field / G Figure 2. The EPR spectrum of 2 in degassed CH2Cl2 (g = 2.0040).
F / (103 L molź1 cmź1)
30 25
compound
20
1 15
2
10
3
Ph
Ph
N
N 4-tol
to understand the EPR spectrum. The SOMO—containing two nodal planes—is an antibonding orbital spanning the four nitrogen atoms; that is, the unpaired electron is coupled to all four nitrogen atoms. The UV–vis spectra of compounds 1, 2, and 3 (Figure 3) are relatively insensitive to impurity as a result of the large extinction coefficients associated with each electronic transition. In formazan 1 and boratatetrazine 3 the low-energy transitions (1, λmax = 491 nm, 3, λmax = 558 nm) result from intramolecular charge transfer on the basis of the large extinction coefficients. Most students explain the origin of the electronic transitions well, but fail to explain the red shifting observed in 3 (fixed conformation and anionic character). The low-energy absorption in 2 (λmax = 721 nm) is the result of electronic transitions involving the SOMO. To study the crystal structures of 1 and 3, we require our students to use the free program Mercury (34) to view the 3D representation and to report relevant bond lengths and angles. Typically, students comment on the bond lengths of the different N–N and C–N bonds and the asymmetry of the N–H....N. In fact, the differences in many of the bond lengths are not as great as one might expect—signalling the effect of delocalizing the π electrons around the conjugated system. In general, most students enjoy this portion of the experiment, quickly making the connection between solution and solid-state properties. As is often the case with crystal structures, students are excited by the chance to visualize the molecule and to measure its bond lengths and angles directly, quickly realizing that the bonding is highly delocalized. In reality, the structures show some bond alternation within their frameworks. However, when compared to isolated single and double bonds it becomes obvious that these compounds are best represented as delocalized. In addition to the bonding arguments, compound 3 crystallizes with two molecules in the repeat unit, further introducing students to fundamentals of crystallography in the undergraduate laboratory (35). Conclusion
5 0 300
400
500
600
Wavelength / nm Figure 3. UV–vis spectra of 1, 2, and 3.
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each compound in solution. In some (futile) cases, students attempt to collect NMR data for the verdazyl radical 2, leading to a discussion of EPR spectroscopy and paramagnetism. Interpretation of EPR spectra serves as a challenging exercise for undergraduate students (32, 33). The EPR spectrum of 2 (Figure 2) consists of a nine-line pattern due to four nearlyequivalent nitrogen atoms with nuclear spin of 1 (2nI + 1, n = 4, I = 1). Although verdazyl radicals are commonly drawn with the unpaired electron localized on a single atom, students must consider the singly occupied molecular orbital (SOMO),
700
800
This experiment is an effective demonstration of how ideas and concepts taught in the classroom need not always apply in a laboratory setting. The compounds produced are easily isolated and handled under aerobic conditions, and the experi-
Journal of Chemical Education • Vol. 86 No. 1 January 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
ment incorporates a wide range of spectroscopic techniques, with particular emphasis placed on the effects of bonding on spectroscopic properties. The isolation of a free radical has been met with much enthusiasm, and students generally respond well to the introduction of new concepts and techniques relating to the unpaired electron. Literature Cited 1. Alfare, K.; Lee, D.; Scharrer, E.; Van Arman, S. A. Chem. Educator 2004, 9, 89–90. 2. Gung, B. W.; Taylor, R. T. J. Chem. Educ. 2004, 81, 1630– 1632. 3. Mosher, M. W.; Ansell, J. M. J. Chem. Educ. 1975, 52, 195– 196. 4. Nineham, A. W. Chem. Rev. 1955, 55, 355–483. 5. Ishiyama, M.; Miyazono, Y.; Shiga, M.; Sasamoto, K.; Ohkura, Y.; Ueno, K. Anal. Sci. 1996, 12, 515–519. 6. Galil, F. M. A.; Khalifa, F. A.; Abdin, T. S. Dyes and Pigments 1990, 12, 49–56. 7. Tescan, H.; Ozkan, N. O. Dyes and Pigments 2003, 56, 159– 166. 8. Gok, Y. Dyes and Pigments 1989, 11, 101–107. 9. Williams, K. R. J. Chem. Educ. 1999, 76, 154–155. 10. Gilroy, J. B.; Otieno, P. O.; Ferguson, M. J.; McDonald, R.; Hicks, R. G. Inorg. Chem. 2008, 47, 1279–1286. 11. Gilroy, J. B.; Patrick, B. O.; McDonald, R.; Hicks, R. G. Inorg. Chem. 2008, 47, 1287–1294. 12. Kuhn, R.; Trischmann, H. Monatsh. Chem. 1964, 95, 457–479. 13. Seidler, E. Prog. Histochem. Cytochem. 1991, 24, 1–86. 14. Morey, J. J. Chem. Educ. 1988, 65, 627–628. 15. Haddy, A. J. Chem. Educ. 2001, 78, 1206–1208. 16. Heffner, J. E.; Raber, J. C.; Moe, O. A.; Wigal, C. T. J. Chem. Educ. 1998, 75, 365–367. 17. Beck, R.; Nibler, J. W. J. Chem. Educ. 1989, 66, 263–266. 18. Breton, G. W. Chem. Educator 2005, 10, 298–299.
19. Gilow, H. M. J. Chem. Educ. 1991, 68, A122–A124. 20. Beers, K. L.; Woodworth, B.; Matyjaszewski, K. J. Chem. Educ. 2001, 78, 544–547. 21. Mazza, R. J. J. Chem. Educ. 1975, 52, 476–478. 22. Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321–1338. 23. Thurston, J. T. J. Chem. Educ. 1929, 6, 550–552. 24. Payne, D. A., Jr.; Eads, E. A. J. Chem. Educ. 1964, 41, 334–336. 25. Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrick, B. O.; Hicks, R. G. Chem. Commun. 2007, 126–128. 26. Orna, M. V. J. Chem. Educ. 1978, 55, 478–484. 27. Katritzky, A. R.; Belyakov, S. A.; Cheng, D.; Durst, H. D. Synthesis 1995, 577–581. 28. Gilroy, J. B.; McKinnon, S. D. J.; Koivisto, B. D.; Hicks, R. G. Org. Lett. 2007, 9, 4837–4840. 29. Cambridge Crystallographic Data Centre. http://www.ccdc.cam. ac.uk/ (accessed Oct 2008). 30. Lerner, L. M. Chem. Eng. News 1973, 51 (34), 42. 31. Ritscher, J. S.; Kowalski, D. C.; McKeon, J. E. J. Chem. Educ. 1974, 51, 688. 32. Bunce, N. J. J. Chem. Educ. 1987, 64, 907–914. 33. McMillan, J. A. J. Chem. Educ. 1961, 38, 438–440. 34. Mercury Home Page. http://www.ccdc.cam.ac.uk/free_services/ mercury/ (accessed Oct 2008). 35. Hoggard, P. E. J. Chem. Educ. 2002, 79, 420–421.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Jan/abs76.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Instructions for the students
Notes for the instructor
Summary of student results
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