Saturday, June 8, 2013

Pyramid Power!

Atop the energy barrier between hardcore synthetic jocks and dyed-in-the-wool physical chemists lives a dedicated group: the molecular architects. A craving for perfect Platonic shapes, combined with the need to push the limits of bond and orbital theory, drive them towards projects like bullvalenes, cubane, or buckyballs. Despite the obvious challenges - bullvalenes shift and change, cubanes feel some strain - the structures' aesthetic appeal keeps us coming back for more.

Well, another class of shapely compounds seems poised to fall. Enter Pyramidanes*:

Source: JACS | Lee, Sekiguchi
Reported a few days back in JACS by a collaboration between Japanese, French, and Russian scientists, the  near-tetrahedral compounds are within shouting distance of an all-carbon version, never synthesized by human hands. I gotta say, they look pretty awesome. Just staring at it brings up so many questions: what's the hybridization of that top atom? How much strain energy? Could you make some schnazzy ligands from it? How labile is that Sn, anyway? Does this thing blow apart in TBAF?

The crew provides an in-depth discussion of frontier MO theory**, bond orders (fractional all around,
from 0.4-0.7), and NMR properties, concluding that there's a decent structural contribution from the ionic resonance form (see below).

The authors even suggest that the "cyclobutadiene" moiety - the pyramid "base" - can be stuck onto other metals; stable pyramidanes might work as strained-ring storage capsules.

Now I'm excited for the all-carbon version, which (theory predicts) has a lone pair at the apex...crazy!

*For the absurdly technical or IUPAC-minded, the authors offer alternative names such as tetracyclo-[] pentane, or (my favorite) []fenestrane

**N.B. I'm no p-chemist, so someone else can go through the nitty-gritty there...

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