Visualizations to Examine the Structure and Symmetry of Metal Tris Chelates: Symmetry Operations, Chirality, and Mechanisms (Bailar Twist and Rây-Dutt) that Racemize the Δ and Λ Isomers
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
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.
Figure 1. An example of a metal tris chelate: the [tris(phenanthroline)RuII]2+ ion.
Outline of the Web Pages
What Is a Metal Tris Chelate?
Symmetry Operations for Metal Tris Chelates
With Symmetric Ligands
C3 Operations
C2 Operations
With Asymmetric Ligands
C3 Operations
Higher Order Symmetries
Stereochemical Properties of Metal Tris Chelates
Definition of Chirality
Δ and Λ Enantiomeric Forms
Assigning the Absolute Configuration (Δ and Λ)
Method 1 (with your right or left hand)
Method 2 (viewing as a screw)
Examples of Research Applications
Chiral Interactions with DNA
Chiral Interactions with Proteins
Analogies to Propellers
Metal Tris Chelates with Non-Planar Ligands that can have λ or δ Ligand Conformations
What is a λ or a δ Ligand Conformation?
The λ Ligand Conformation
The δ Ligand Conformation
Contrasting the λλλ–Λ and δδδ–Δ Enantiomers of [Co(H2NCH2CH2NH2)3]3+
The Eight Isomeric Forms of [Ni(H2NCH2CH2NH2)3]2+
Mechanisms that Racemize Λ and Δ Isomers
Non-dissociative Mechanisms
Animation of the Bailar Twist Mechanism
Animation of the Rây-Dutt Mechanism
Contrasting the Bailar and Rây–Dutt Mechanisms
Dissociative Mechanisms
Exercises
1–8
Literature Cited
Allen, F. H. Acta. Cryst.2002,B58, 380–388.
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