Cyclobutadiene’s Crystal Structure Is … A Square

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.