Exceptionally Short Tetracoordinated Carbon-Halogen Bonds in Hexafluorodihalocubanes

Molecules that contain bonds whose length significantly deviates from the average are of interest in the context of understanding the nature and limits of the chemical bonds. However, it is difficult to disentangle the individual contributions of the multiple factors that give rise to such bond-length deviations as reports on such molecules remain scarce. In the present study, we have succeeded in synthesizing hexafluorodihalocubanes of the type C₈F₆X₂ (2) (X = Cl (2), Br (2), I (2)), which represent a new series of molecules with unusual C(sp³)-halogen bonds. The C(sp³)-halogen bonds of 2, 2, and 2, determined via single-crystal X-ray diffraction analysis, are approximately 0.07-0.09 Å shorter than typical C(sp³)-halogen bonds. In particular, the carbon-iodine bonds of 2 are the shortest C(sp³)-I bonds reported to date. The solution-state structures and electronic states of the C(sp³)-halogen bonds in these hexafluorodihalocubanes were analyzed by X-ray absorption spectroscopy, which revealed detailed information on the length of these C(sp³)-halogen bonds in solution and the solid state as well as on the electron-deficient nature of 2. Detailed theoretical calculations and a comparison with halotrinitromethanes (1), which represent another series of molecules with shortened C(sp³)-halogen bonds, revealed that the factors responsible for the shortening of the C(sp³)-halogen bond vary among the different C(sp³)-halogen bonds, i.e., for C(sp³)-Cl and C(sp³)-Br, the s-character and hyperconjugation effects predominate, whereas for C(sp³)-I, the interatomic Coulombic interaction effect prevails.