Crystal structure of (Z)-10-(bromoiodomethylene)phenanthren-9(10H)-one: halogen bond and mechanism implication

A novel (Z)-10-(bromoiodomethylene)phenanthren-9(10H)-one crystal structure is reported herein. Its synthetic formation arising from the phenyl shift during the electrophilic iodination of the alkynol of fluorenone provides proof of the early proposed mechanism of the McNelis rearrangement. The implication of the Z-geometry in the mechanism is discussed. Single crystal X-ray diffraction studies on the title compound show the β-vinyl bromine and the β-iodine atoms trans and syn, respectively, to the C = O in the molecule establishing the configuration of the dihaloenone about the alkenyl motif as Z-geometry. The intermolecular interactions primarily involve the iodine and oxygen atoms with a C-I…O = C distance of 3.110 (1) Ả, which describes an I…O halogen bond since the I···O distance is less than the sums of the respective van der Waals radii, 3.50 Å. In this interaction, the iodine acts as a Lewis acid, and the carbonyl oxygen serves as the Lewis base electron donor moiety. Consistent with the strong directional polarization of halogen bonds, the C-I···O angle = 167°. There is an additional intramolecular C-I ∙∙∙O = C distance of 3.147 (1) Å and I∙∙∙I distance of 3.956 (1) Å along the c-axis. These interactions form a one-dimensional, non-planar infinite network along the c-axis.