Can X-ray crystal structures obtained from supramolecular species be entirely relied upon when the species being studied is so unstable as to have eluded chemists for decades? A controversial X-ray crystal structure published in Science in July would suggest so, but British and US researchers have carried out calculations on the proposed structure that suggest otherwise.
Yves-Marie Legrand, Arie van der Lee and Mihail Barboiu of the University of Montpellier in France, published work on cyclobutadiene, an intriguingly strained and unstable molecule: the smallest of the conjugated cyclic hydrocarbons. The molecule's strained geometry and electronic structure have meant that its unperturbed crystal structure has remained elusive.
The French team reasoned that trapping a precursor, 4,6-dimethyl-α-pyrone, in the vase-like cavity of a calixarene and then irradiating it with ultraviolet light would generate 4,6-dimethyl-β-lactone, a Dewar intermediate. This, they reasoned, would be stable enough at low temperature in the confined space of the calixarene, and could be pushed further with continued irradiation to 1,3-dimethylcyclobutadiene. The team carried out X-ray diffraction studies of both the intermediate and the final product.
However, according to calculations by three independent teams all is not as it seems with the trapped cyclobutadiene and its CO2 companion. Barboiu and colleagues reported four distinct species trapped in the calixarene cavity - the activated precursor, isomeric Dewar β-lactone, and square and rectangular isomers of 1,3-dimethylcyclobutadiene. However, David Scheschkewitz of Imperial College London, UK, has inspected the CIF (crystallographic information format) file, which represents a refinement of the raw data from the diffractometer. He found that the bond lengths were inconsistent with the published interpretation. Scheschkewitz suggests that a new refinement of the model is needed that uses both enantiomers of the Dewar β-lactone. He has requested the raw data file, the hkl, to allow a new refinement to be carried out independently.
Separate calculations carried out by Imperial College's Henry Rzepa and Igor Alabugin of Florida State University (USA) and their co-workers essentially corroborate the suggestion of Scheschkewitz that the original interpretation is incorrect. Overall, their studies suggest that only the bicyclic β-lactone intermediate in which carbon dioxide remains covalently bound to the cyclobutadiene unit is present in the calixarene, rather than there being two distinct compounds within. Rzepa says that the original research and various chemists' attempts to explain the findings have stirred up "quite a hornet's nest". He explains further: "At issue is whether the two bonds that link the cyclobutadiene and the CO2 moieties are covalent in the normal sense of the word, or whether they are in fact very much weaker, van der Waals contacts, or indeed whether they are essentially non-interacting atoms." If the latter, then Barboiu and colleagues would be vindicated; the two molecules exist as separate species in the calixarene cavity. Further calculations by Rzepa on the spectra of the species thought to be involved, show that an enormous energy shift would be required to selectively stabilize the excited state, with the absorption moving from 230 to 320 nm. This makes the photochemical formation of cyclobutadiene within the calixarene even more implausible.
Barboiu and colleagues have defended their interpretation in a published riposte in Science in mid-November, stating that the guanidinium-sulfonate-calixarene host matrix used in their original study plays an essential role in stabilizing the rectangular-bent cyclobutadiene derivative under confinement and that their interpretation is indeed correct. However, Scheschkewitz is on record as saying that, "It is fully justified to report unsatisfactory X-ray data as long as it is not over-interpreted and not the one and only basis for a totally unexpected claim." Barboiu and colleagues concede that their X-ray data have large error bars: Scheschkewitz points out that this is precisely why claiming the presence of 1,3-dimethylcyclobutadiene as well as two distinct forms of this highly reactive compound in the calixarene is so unreasonable.
Rzepa is convinced that part of the problem that has given rise to this controversy, and allowed it to persist, lies in the way crystallographic data are currently handled. The system is not serving the chemical community well: erroneous interpretations are slipping through the peer-review filter and entering the research literature. "Normally, it is mandatory for authors to supply a CIF file," he explains, "but a CIF file represents only one analysis using a given model, and does not allow alternative models to be evaluated." If the raw hkl data were available then alternative refinement models could be tested. Unfortunately, journals, peer reviewers and crystallographic data centers very rarely request the hkl files and authors almost never submit them.
Rzepa adds that according to his calculations, if the 1,3-dimethyl cyclobutadiene and CO2 were to be formed, they would in fact very quickly recombine, too quickly for their X-ray structure to be determined. Rzepa emphasizes through his paper published in Chemical Communications, that "theory is nowadays sophisticated enough to provide reliable estimates of the lifetimes of two species co-constrained in a cavity, and that this general approach could be used for any systems inside a cavity."
Regardless of the merits or otherwise of the Barboiu work itself, the tale reveals that more data needs to be routinely provided by authors who are proposing structural refinements of molecules. Two decades ago when the first crystal data on cyclobutadiene derivatives were deposited at the Cambridge Crystallographic Data Centre (CCDC), information storage was a problem. Today, computer servers provide scientists with essentially limitless space in which all the available data might be stored, allowing others to assess a given crystal structure interpretation in the raw.
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