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JCE WebWare: Web-Based Learning Aids
William F. Coleman Wellesley College Wellesley, MA 02481
Visualizing Metal Tris Chelates
Edward W. Fedosky University of Wisconsin–Madison Madison, WI 53715
Visualizations To Examine the Structure and Symmetry of Metal Tris Chelates: Symmetry Operations, Chirality, and Mechanisms (Bailar Twist and Rây-Dutt) That Racemize the ∆ and Λ Isomers
Figure 1. An example of a metal tris chelate: the [tris(phenanthroline) RuII]2+ ion.
by Marion E. Cass, Department of Chemistry, Carleton College, Northfield, MN 55057. Henry S. Rzepa, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom Keywords: Audience: Upper Division Undergraduate. Domain: Inorganic Chemistry. Pedagogy: Internet / Web Based Learning. Topics: Molecular Properties/Structure, Coordination Compounds, Inorganic Chemistry, Bioinorganic Chemistry, Stereochemistry, Enantiomers, Group Theory/Symmetry, Conformational Analysis, Mechanisms of Reactions; Transition Elements Requires Java 1.4 or higher. (Note that the Jmol program supports all major Web browsers. These Web pages have been tested to run on Firefox and Safari.)
Achieving understanding of complicated molecular phenomena from two-dimensional drawings can be a challenge. Three examples we would like to highlight in our animated visualizations, are the C2 and C3 symmetry operations in chiral tris chelates, the assignment of absolute configurations in these molecules and the non-dissociative mechanisms (the Bailar Twist and Rây-Dutt) that interchange Λ and Δ enantiomers. Using three-dimensional representations of the molecules that can be exported from the Cambridge Structural Data Base (1), computational output examining the imaginary frequencies of transition states (2), and the powerful molecular visualization program Jmol (3), we present here a Web site that we have constructed to aid in our teaching of these concepts. In addition, we have included animations that show the parallels between left- and right-handed propellers (4, 5), a discussion of the structural aspects of δ and λ twists in ligand rings (possible in some chelates) (4, 6), and a series of exercises to help students develop their skills in understanding the structural features of these metal tris chelates. Two examples of published research are included to illustrate the stereoselective interactions of synthetically designed and/or naturally occurring chiral metal complexes with relevant biological molecules (7–12). References and simple descriptions of the metal ligand bonding are given for the exercises and examples in linked pages.
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Outline of the Web Pages
A. What Is a Metal Tris Chelate?
B. Symmetry Operations for Metal Tris Chelates
1. With Symmetric Ligands a. C3 Operations b. C2 Operations 2. With Asymmetric Ligands a. C3 Operations 3. Higher Order Symmetries
C. Stereochemical Properties of Metal Tris Chelates
1. Definition of Chirality 2. Δ and Λ Enantiomeric Forms 3. Assigning the Absolute Configuration (Δ and Λ)
a. Method 1 (with your right or left hand)
b. Method 2 (viewing as a screw)
D. Examples of Research Applications
1. Chiral Interactions with DNA 2. Chiral Interactions with Proteins
E. Analogies to Propellers
F. Metal Tris Chelates with Non-Planar Ligands that can have λ or δ Ligand Conformations
1. What is a λ or a δ Ligand Conformation? a. The λ Ligand Conformation b. The δ Ligand Conformation 2. Contrasting the λλλ–Λ and δδδ–Δ Enantiomers of [Co(H2NCH2CH2NH2)3]3+ 3. The Eight Isomeric Forms of [Ni(H2NCH2CH2NH2)3]2+
Journal of Chemical Education • Vol. 85 No. 5 May 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
On the Web
G. Mechanisms that Racemize Λ and Δ Isomers
1. Non-dissociative Mechanisms a. Animation of the Bailar Twist Mechanism
b. Animation of the Rây-Dutt Mechanism
c. Contrasting the Bailar and Rây–Dutt Mechanisms 2. Dissociative Mechanisms H. Exercises 1–8
Literature Cited 1. Allen, F. H. Acta. Cryst. 2002, B58, 380–388. 2. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT, 2004.
3. Jmol: an open-source Java viewer for chemical structures in 3D; http://www.jmol.org/ (accessed Mar 2008). 4. See presentation within the Web pages. Links to other cited Web sites are found within those pages. 5. Coleman, W. F. Optical Isomerism. http://www.wellesley.edu/ Chemistry/Flick/chem341/opticalisomers.html#Scene_1 (accessed Feb 2008). 6. JCE Featured Molecules, August 2006; Copper and Nickel Complex Ions. http://jchemed.chem.wisc.edu/JCEWWW/Features/ MonthlyMolecules/2006/Aug/index.html (accessed Feb 2008). 7. Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999, 99, 2777–2796. 8. Krotz, A. H.; Barton, J. K. Inorg. Chem. 1994, 33, 1940–1947. 9. Kielkopf, C. L.; Erkkila, K. E.; Hudson, B. P.; Barton, J. K.; Rees, D. C. Nat. Struct. Bio. 2000, 7, 117–121. 10. Karpishin, T. B.; Dewey, T. M.; Raymond, K. N. J. Am. Chem. Soc. 1993, 115, 1842–1851. 11. Ecker, D. J.; Loomis, L. D.; Cass, M. E.; Raymond, K. N. J. Am. Chem. Soc. 1988, 110, 2457–2464. 12. Rastetter, W. H.; Erickson, T. J.; Venuti, M. C. J. Org. Chem. 1981, 46, 3579–3590.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/May/abs750.html Full text (HTML and PDF) with links to cited URLs Supplement Find “Visualizations To Examine the Structure and Symmetry of Metal Tris Chelates: Symmetry Operations, Chirality, and Mechanisms (Bailar Twist and Rây-Dutt) That Racemize the ∆ and Λ Isomers” in the JCE Digital Library at http://www. JCE.DivCHED.org/JCEDLib/WebWare/collection/reviewed/ JCE2008p0750WW/index.html
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 5 May 2008 • Journal of Chemical Education
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