In the Laboratory
A Phthalocyanine Synthesis Group Project for General Chemistry
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Darren K. MacFarland,* Christopher M. Hardin, and Michael J. Lowe Department of Chemistry, Illinois College, Jacksonville, IL 62650; *
[email protected] Phthalocyanines provide a colorful, interesting, and research-relevant subject for an organic experiment in the general chemistry curriculum (1). General chemistry courses often include a token organic chemistry experiment. While the aspirin synthesis (2) fills this niche nicely with a familiar compound, students are sometimes discouraged with the odors (acetyl chloride or anhydride) and the sense that one white powder is being turned into…another white powder. By contrast, phthalocyanines are brightly colored, so a white powder is converted into an intense pigment. As phthalocyanines are aromatic, historically interesting, commercially important (3), and the subject of current research in a wide variety of fields (4), a discussion of the importance of organic chemistry easily follows. We report a simple, reliable synthesis that will fill the need for an organic experiment in the general chemistry curriculum with a compound known to most students—a pigment. Phthalocyanines (Pc’s) are macrocycles, a large class that includes porphyrins (5), crown ethers, and cryptands among many others. Each of these macrocycles has found biomedical applications (6, 7) and is interesting for both size and unusual properties.1 Phthalocyanines are flat and insoluble in most solvents 2 and are used most often commercially as blue or green (when perhalogenated) pigments and dyes (8). Students find the 2-dimensional nature of such a large molecule interesting. In addition, Pc’s pose no health risk (8). Both crown ethers and Pc’s are synthesized using template metals (9). In this scheme, a metal ion coordinates to starting materials and either positions or activates them for cyclization around the metal center (10). The template metal then remains in the “hole” at the center of the product Pc, coordinated to four nitrogen atoms (see general synthesis scheme for phthalocyanines from phthalonitriles, below).
N
N N
CN
M2+
CN
(DMAE) heat
4
N
M2+ N N
N
N
Because the metal ion must fit in the “hole”, the size of the ion is quite important. One that is too large or too small will be less effective or entirely ineffective. Pc’s form a strong bond with metals that are good templates, making their removal difficult.3 Without templates to guide these syntheses,
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yields are either low or zero. Our procedure demonstrates this effect. Phthalocyanines have been synthesized from numerous starting materials, with a variety of solvents and under various conditions (11). Some reports include odd additives such as ammonium molybdate (12). Perhaps this array of synthetic strategies has discouraged the use of phthalocyanines in teaching settings to this point (13). We utilize a simpler, more modern synthesis involving commercially available compounds and solvents. In our experiment (see scheme), phthalonitrile (1,2-dicyanobenzene) is taken into N,N-dimethylaminoethanol (DMAE) and heated to reflux. No reaction occurs because no template is present. When a template metal is added, the reaction proceeds rapidly. We have tested this procedure with a variety of template metals and found a number to be quite reliable (14 ).W Use of solvents other than DMAE is less successful. An alternative synthesis simply heats CuCl2 and phthalonitrile in the absence of solvent. This elicits two responses from students. First, some are intrigued by the idea of a solventless reaction, as their experience consists principally of solution-based reactions. Second, because some of the phthalonitrile sublimes before the reaction is complete, they observe (discover!) sublimation firsthand. We have found this experiment to be suitable for group work, using small groups usually of four students. Within a group, student A demonstrates the need for a template by performing the reaction without a metal salt (the reaction will be unsuccessful), students B and C perform the same reaction as student A but use metal salts as templates (these reactions will be successful), and student D performs the cyclization using the solvent-free procedure. The formulation may be crafted to use metals that are more or less successful, depending on the instructor’s wishes (see the experimental procedure online for more information); larger groups may be accommodated by further diversifying the metal salts tested, or by having different groups within a class test different metal salts.W The group (or class) then compares results to develop an understanding of the need for a template and the desirable size qualities of the template. Little analysis need be done on the products; the success of the reaction can be checked visually, as the products are deep shades of blue and the starting solutions are not highly colored. However, UV–vis spectra may be obtained (in pyridine) if analytical proof of reaction is desired. Discussions of experimental design may be structured around a number of aspects of this reaction. Parameters such as solvent choice, the use of an excess of template (or the use of a template at all), choice of metal salt, and temperature of reaction may be explored. If the group approach described above is utilized, any or all of these variables may be isolated and tested.
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
In the Laboratory W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online. It includes more detailed information on experimental procedures and a list of successfully tested metals.
6.
Notes 1. For example, crown ethers are used as phase-transfer catalysts. In a classic demonstration, KMnO4 dissolves in benzene with the help of 18-crown-6. 2. For cleanup purposes, concentrated sulfuric acid does dissolve residual Pc’s by sulfonating the ring to give a water-soluble species; they are also somewhat soluble in pyridine. 3. To make a metal-free Pc, a metal that activates the phthalonitrile but is not a good fit must be used. To this end, lithium alkoxides are used. Two lithium cations occupy the “hole” of the Pc and are removed by reaction with water to leave H2Pc.
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Literature Cited 1. For a broad review, see Phthalocyanines: Properties and Applications Leznoff, C. C.; Lever, A. B. P., Eds.; VCH: New York, 1989, Vol. I; 1992, Vol. II; 1993, Vol. III; 1996, Vol. IV. 2. See for example, Williamson, K. L.; Little, J. G. Microscale Experiments for General Chemistry; Houghton Mifflin: Boston, 1997; pp 329–336. 3. Copper phthalocyanine was discovered in 1907 by von Braun: von Braun, A.; Tcherniak, J. Chem. Ber. 1907, 40, 2709. Copper phthalocyanine is sold as Pigment Blue 15. 4. For example, phthalocyanine complexes have been used in catalysis: Sorokin, A.; Meunier, B. Eur. J. Chem. 1998, 1269– 1281, and references therein. See also biomedical references below. For a review, see: Hanack, M.; Deger, S.; Lange, A. Coord. Chem. Rev. 1988, 83, 115. 5. For examples of porphyrins in undergraduate laboratories see: Falvo, R. E.; Mink, L. M.; Marsh, D. F. J. Chem. Educ. 1999,
11.
12. 13.
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76, 237; Beeston, R. F.; Stitzel, S. E.; Rhea, M. A. J. Chem. Educ. 1997, 74, 1468; Marsh, D. F.; Mink, L. M. J. Chem. Educ. 1996, 73, 1188. For a recent review containing both cryptands and crown ethers, see: Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. M. Chem. Rev. 1999, 99, 2293–2352. For cryptands only: Volkert, W. A.; Hoffman, T. J. Chem. Rev. 1999, 99, 2269–2292. For a recent review containing porphyrins and phthalocyanines, see: Ali, H.; van Lier, J. E. Chem. Rev. 1999, 99, 2379– 2450. Loebbert, G. In Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I.; Howe-Grant, M., Eds.; Wiley: New York, 1996; Vol. 18, pp 1043–1058. For use of sodium as a crown ether template: Maeda, H.; Furuyoshi, S.; Nakatsuji, Y.; Okahara, M. Bull. Chem. Soc. Jpn. 1983, 56, 212–218. For phthalocyanine templates: Metz, J.; Schneider, O.; Hanack, M. Inorg. Chem. 1984, 23, 1065–1071. A discussion of the template effect can be found in Shriver, D.; Atkins, P. W. H. Inorganic Chemistry, 3rd ed.; Freeman: New York, 1999; pp 222–223. Sheeran, D. J.; Mertes, K. B. J. Am. Chem. Soc. 1990, 112, 1055–1061, and references contained therein. Metz, J.; Schneider, O.; Hanack, M. Inorg. Chem. 1984, 23, 1065–1071. See also Ali, H.; van Lier, J. E. Chem. Rev. 1999, 99, 2379–2450. Ruf, M.; Lawrence, A. M.; Noll, B. C.; Pierpont, C. G. Inorg. Chem. 1998, 37, 1992–1999. The only J. Chem. Educ. references to phthalocyanines are Joesten, M. D.; Higgins, M. M. J. Chem. Educ. 1986, 63, 1097–1098. Moser, F. H.; Thomas, A. L. J. Chem. Educ. 1964, 41, 245–249. This general procedure is derived from work of the Hanack group. For references see: Hanack, M.; Schmid, G.; Sommerauer, M. Angew. Chem., Int. Ed. Engl. 1993, 32, 1422– 1424. Rager, C.; Schmid, G.; Hanack, M. Chem. Eur. J. 1999, 5, 280–288.
JChemEd.chem.wisc.edu • Vol. 77 No. 11 November 2000 • Journal of Chemical Education
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