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Challenging in Many Aspects: A Heavy Analogue of the Cyclopentadienyl Cation

SC-XRD Application Note - APEX2 Software

A heavy analogue App Note, SC-XRD

Although the cyclopentadienyl cation Cp+ is still synthetically out of reach the isoelectronic replacement of most of the carbon atoms by elements from the lithosphere such as silicon and phosphorus, makes the heavier congener accessible. Obviously, the third row elements stabilize the anti-aromatic five-membered ring considerably and phosphorus as an excellent π-donor provides diagonal relationship without mimicking a simple carbon-copy.

Although this anti-aromatic ring is 24 kcal/mol higher in energy than the virtual aromatic anion the silicon and phosphorus atoms offer sufficient stabilization allowing the isolation of the cation. Most surprisingly the counter anion of the five-membered cationic ring system is a simple chloride anion which does not interact with the ring system and therefore does not contribute to the ring’s stability.