  The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into
Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of
the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation
of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an
antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a
nonmagnetic $J=0$ band insulator. First principles calculations reveal a
remarkably flat insertion energy that locally drives the fluorination process
to completion. Band structure calculations support the formation of a band
insulator whose charge gap relies on the strong spin-orbit coupling inherent to
the Ir metal ions of this compound.
