Novel Fluorine-Bridged Polyfluorinated Iodine Structures. Presence of Fluorine as the Central Atom in a Five-Center Six-Electron Bond

Article abstract of DOI:10.1021/ja00233a022

The reaction of tris(dialkylamino)sulfonium perfluoro-2-methyl-2-pentyl carbanion with perfluoro iodides (R_fI) leads to novel structures of the form f-I-F-I-R_f>^-.The crystal structure for R_fI = pentafluorophenyl iodide reveals discrete ion pairs.The crystal structure for R_fI = 1,4-diiodooctafluorobutane shows an extended polymeric structure with (CF2)4 groups bridged by ^- units.The I-F bond distances are 0.3 - 0.4 Angstroem longer than normal fluorine-iodine distances but are > 1.0 Angstroem shorter than the sum of the van der Waals radii.The R_f-I-F bond angles are quasi-linear wheras the I-F-I bond angles deviate from linearity by 10 degree - 20 degree.High-level ab initio calculations of c-I-F>^- and c-I-CF3>^- are presented.The free ions are calculated to have linear structures at I and at the central fluorine.The R_f-I bond distances are calculated to be slightly longer than those in a free R_f-I compound, and the I-F_c bond distances are significantly elongated as compared to free I-F.The vibrational analysis shows a very low bending frequency of 28 cm^-1 at the central fluorine in ^-.Analysis of the wave functions shows that the bonding in these structures is best described by a five-center, six-electron (5c 6e) hypervalent ? bond and not as an ion-dipole complex.The 5c 6e bond is stabilized by more electronegative elements at the central and terminal positions and less electronegative elements at the other two positions.The negative charge is found at the central and terminal positions.The alkyl fluorides stabilize the excess negative charge at the terminal position by negative anionic hyperconjugation.