Crystallography squared organically

Cyclobutadiene, the smallest cyclic hydrocarbon having alternating double bonds has finally succumbed to X-ray crystallography at least in terms of the determination of an immobilized derivative of the compound.

Ever since chemists began to figure out chemical bonding, molecular geometry has been high on the agenda, after all it is shape that helps underpin how and with what any given molecule will react. From Kekule's tail-biting snake that is the benzene ring to the carbon football that fell to Earth, buckminsterfullerene (featured in the SpectroscopyNOW IR channel this week). Of particular interest are those crowded or strained geometries in organic molecules in which bonds between carbon atoms are bent tighter than they would like to be and locked into rings, triangles and squares.

Among the most intriguing is perhaps cyclobutadiene (CBD), at its simplest, nothing more than four carbon atoms at the corners of a squat rectangle, with a couple of double bonds and attendant hydrogen atoms filling the valencies. But, the apparent simplicity of C₄H₄, the smallest annulene, belies its true complexity. CBD is an extremely unstable hydrocarbon with a lifetime of the order or mere seconds when left alone.

But, why so unstable? Hückel's rules of aromatic stability would suggest that, like benzene , the alternating double bonds around the system would make it more stable than if those double bonds were not present. But, of course, CBD is no square, its rectangular structure verified by infrared spectroscopy confirms that it fails Hückel's rule because the ring has just four pi-electrons, and four is not twice an odd number (six pi-electrons in benzene, double three). Indeed, CBD is the archetypal Hückel anti-aromatic molecule, where electronic destabilization is associated with networks of p electrons delocalized over an even number of alternating cyclic double bonds.

Chemists, of course, are never satisfied to leave even an unstable molecule uninvestigated and have synthesised organocmetallic cyclobutadiene derivatives that gain stability by virtue of an electron pair from the metal atom. Others have chilled precursors to close to absolute zero and then carefully triggered reactions to nudge them towards the elusive rectangle. This work has led to new insights into CBD's character, but one aspect that has remained elusive is its crystal structure.

Now, researchers in France have immobilized a precursor, 4,6-dimethyl-alpha-pyrone, in a supramolecular guanidinium-sulfonate-calixarene (G₄C) crystalline network that confines its guest through a combination of CH-pi and hydrogen-bond interactions. With a little photochemical tweaking using ultraviolet irradiation they can transform the trapped guest into a 4,6-dimethyl-beta-lactone Dewar intermediate that is relatively stable at 175 Kelvin. Additional irradiation nudges the reaction to completion allowing them to obtain an X-ray structure of the 1,3-dimethylcyclobutadiene product, the nearest neighbour to a bare-naked CBD molecule.

"Our data support experimental observation of square-planar (Me₂CBD^S) and rectangular-bent (Me₂CBD^R) geometries in the host matrix," team leader Mihail Barboui of the Adaptive Supramolecular Nanosystems Group, at the Institut Européen des Membranes, Ecole Nationale Supérieure de Chimie de Montpellier-Université Montpellier II-UMR-CNRS explains.. "The hydrogen-bonded, dissociated carbon dioxide co-product interacts more strongly with Me₂CBD^S than with Me₂CBD^R.

"There is tremendous geometric strain associated with squeezing olefinic carbons down from their traditional 120° bonding angle to the 90° motif dictated by a closed square ring," the researchers add, which makes this molecule all the more fascinating.

"The structures reported in this paper: the beta-Dewar intermediate, square-CO₂ stabilized CBD and rectangular CBD have not been crystallized previously and only theoretical calculations provided structural parameters for the geometrical features of these very unstable molecules," Barboiu told SpectroscopyNOW. He points out that until now only spectroscopic data (IR, NMR, and Raman) had supported the mechanism of formation and structure elucidation.

"The reported structural data are in agreement with some previous theoretical calculations giving the first experimental details on the 3D structure of these molecules," Barboiu adds. "The automerization reaction of CBD, previously predicted by modelling is experimentally supported by the crystal structure of determined here. These structures might serve for more complex calculations for determining their energies in confined conditions."