Pure Appl. Chem., 2008, Vol. 80, No. 3, pp. 547-553
doi:10.1351/pac200880030547
Ultralong C-C bonds in hexaphenylethane derivatives*
Takanori Suzuki1, Takashi Takeda1, Hidetoshi Kawai1,2, and Kenshu Fujiwara1
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan; 2PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama 332-0012, Japan
Abstract:
The longer C-C bond than the standard (1.54 Å) is so weakened that it is cleaved easily, as found in the parent hexaphenylethane (HPE). However, the compounds with an ultralong C-C bond (1.75 Å) can be isolated as stable solids when the bond-dissociated species does not undergo any reactions other than bond reformation. This is the central point in designing the highly strained HPEs, which were obtained by two-electron reduction of the corresponding dications. Steric repulsion of "front strain" is the major factor to expand the central C-C bond of HPEs. During the detailed examination of the ultralong C-C bond, the authors discovered the intriguing phenomenon of "expandability": the C-C bond length can be altered over a wide range by applying only a small amount of energy (1 kcal mol-1) supplied by crystal packing force. This observation indicates that the much longer C-C bond than the shortest nonbonded contact (1.80 Å) will be realized under the rational molecular design concept.
Keywords: covalent bonds; hexaphenylethanes; long bonds; redox system; strained molecules; X-ray structures.
*Paper based on a presentation at the 12th International Symposium on Novel Aromatic Compounds (ISNA-12), 22-27 July 2007, Awaji Island, Japan. Other presentations are published in this issue, pp. 411-667.