Cyclobutadiene’s Crystal Structure Is … A Square | July 19, 2010 Issue - Vol. 88 Issue 29 | Chemical & Engineering News
Volume 88 Issue 29 | p. 33 | Concentrates
Issue Date: July 19, 2010

Cyclobutadiene’s Crystal Structure Is … A Square

French scientists finally succeed in getting chemistry’s smallest conjugated ring to hold still for a picture
Department: Science & Technology
Keywords: cyclobutadiene, crystal structure
A representation of the elusive crystal structure of a cyclobutadiene.
Credit: Science
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A representation of the elusive crystal structure of a cyclobutadiene.
Credit: Science

Cyclobutadiene may seem like kindergarten chemistry, but for crystallographers the molecule has been a highly sophisticated headache. For more than 40 years, the smallest cyclic hydrocarbon with conjugated double bonds has defied crystal structure analysis. The main problem has been that the molecule’s awkward 90° bonds are too unstable for the molecule to crystallize. The first step toward this milestone was achieved in 1991 when researchers figured out how to confine cyclobutadiene in a carcerand organic cage molecule—but the molecule still refused to crystallize. Researchers led by Mihail Barboiu at the European Institute of Membranes, in Montpellier, France, have finally succeeded. The researchers trapped 4,6-dimethyl-α-pyrone in a guanidinium-sulfonate-calixarene network, then zapped the complex repeatedly with ultraviolet light to convert the precursor pyrone into 1,3-dimethylcyclobutadiene. They were able to crystallize the dimethyl analog in the presence of CO2 and subsequently solve its crystal structure (Science 2010, 329, 299). The structure is, not surprisingly, square planar when stabilized by a CO2 molecule. But there are some slightly stretched, slightly bent rectangular states in the lattice. The French team’s structural analysis is a fundamental fait accompli for an ever-so-finicky molecule.

 
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