Story of the Pentamethyl Cyclopentadienyl Cation C5Me5

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Story of the Pentamethyl-Cyclopentadienyl
CationC5Me5

• Roger L. DeKock
• Department of Chemistry and Biochemistry
• Calvin College, Grand Rapids, MI
• dekock_at_calvin.edu

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The Stable Pentamethyl-cyclopentadienyl Cation

• Angewandte Chemie Int. Ed. 2002, 41, No. 8,
  1429-1431, by Joseph B. Lambert, Lijun Lin, and
  Vitaly Rassolov.
• Antiaromatic, 2n Aromatic, 2n2

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The Stable Pentamethyl-cyclopentadienyl Cation

• Elusive Carbocation Isolated as a Solid
• Chemical and Engineering News, April 29, 2002.
• Four unusual features
• Stable
• Unequal C-C bond lengths around the ring
• Me-C-C-Me dihedral angle nearly 110
• Singlet state, no unpaired spins

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The Fall-Out

• Corrigendum, Issue No. 10
• Note from the Editors unfortunately the results
  reported in the communication The Stable
  Pentamethylcylopentadienyl Cation by Joseph B.
  Lambert et al. in issue 8/2002 (pp. 1429-1431)
  must be corrected. Guy Bertrand et al. quickly
  discovered that not the pentamethyl-cylopentadieny
  l cation but pentamethyl-cylopentenyl cation was
  prepared ?

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The Fall-Out

• Editorial (Issue No. 13, page 2211)
• And Hückel Was Correct After All?A Withdrawal
  and an Apology by Dr. Peter Gölitz, Editor
• The Stable Pentamethylcyclopentdienyl Cation
  Remains Unknown, by M. Otto, D. Scheschkewitz,
  T. Kato, M. M. Midland, J. B. Lambert, and G.
  Bertrand. Angewandte Chemie Int. Ed. 2002, 41,
  No. 13, 2275-2276.

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The Fall-Out

• Comment on the X-Ray Structure of
  Pentamethylcyclopentadienyl Cation by Thomas
  Müller, Angewandte Chemie Int. Ed. 2002, 41, No.
  13, 2276-2278.
• Statement by Joseph B. Lambert, Angewandte
  Chemie Int. Ed. 2002, 41, No. 13, 2278.

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The Fall-Out

• The crystal structure of pentamethyl-cylopentadi
  enyl cation is that of the pentamethylcyclopentyl
  cation by Jamie N. Jones, Alan H. Cowley, and
  Charles L. B. Macdonald, Chemical Communications,
  2002, 1520-1521.

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Connection to Pedagogy

• Two-week computational chemistry module in
  advanced organic chemistry and in biophysical
  chemistry
• Week 1
• Before lab session students are given the
  original article and the CEN article to read,
  along with some notes
• In lab students complete computational exercises,
  SHMO, Gaussian, Spartan, etc. Start with C5H5,
  later add methyl groups
• Examine nature of important molecular orbitals

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Highest Occupied MO
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Week 1 Laboratory Contd.

• Examine both singlet and triplet electronic
  configurations, especially with respect to
  dihedral angle of interest
• Compare theory and experiment, consider crystal
  packing forces, etc.

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Week 2, Laboratory

• Pointers to subsequent literature
• Complete additional theoretical studies on
  distorted molecule
• Strengths of this approach
• Fit a short segment of computational chemistry
  into a larger course
• Connects to prior study in organic chemistry
• Practice in reading original literature

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Student Learning Outcomes

• Read literature with a critical eye
• Learn about scientific method, how it is self
  correcting
• Use computational chemistry software
• Interpret output from software
• Write a formal report on the results of the two
  week project

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Additional Information www.calvin.edu/academic/che
mistry/

• Teaching materials
• This PowerPoint

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Acknowledgements

• Lee Heeringa
• Calvin College