Synthesis and Characterization of Novel Fluorinated Poly(oxomolybdates)

Two novel poly(oxofluoromolybdate) clusters, [Mo6O18F6]6- and [Mo7O22F3]5-, have emerged from systematic field
studies on the hydrothermal fluorination of poly(oxometalates). They are accessible via the hydrothermal treatment
of Mo(VI)-based precursors in the presence of MF additives (M = Li, Na, Cs, NMe4). The new fluorinated polyanions
are stabilized by specific alkali-cation combinations, and their packing motifs can be tuned through the careful
choice of cations and synthetic parameters. The [Mo6O18F6]6- anion can be described as a cored and fluorinated
form of the Anderson−Evans cluster type. It is stabilized by the interaction of two alkali cations with the fluorinated
faces of the ring-shaped anion. The partial replacement of these capping alkali cations by the bulky, organic NMe4+
cation leads to the formation of [Mo7O22F3]5--based compounds. Thus, the extent of fluorination can be controlled
through the polarizability of the cationic environment:  in the [Mo7O22F3]5- anion, half of the fluoride atoms of the
[Mo6O18F6]6- anion are replaced by a capping MoO4 tetrahedron, thereby rendering this anion a fluorinated, lacunar
derivative of the α-[Mo8O26]4- octamolybdate anion. All compounds have been structurally characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. The templating role of the alkali cations is discussed
and explained with the help of electrostatic calculations.