Synthesis and Characterization of Novel Fluorinated Poly(oxomolybdates)

Abstract

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 alpha-[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.

Hydrothermal Synthesis of Molybdenum Oxide Based Materials: Strategy and Structural Chemistry

“…From the theoretical point of view, we have investigated the detailed templating effect of the alkali cations on the polyoxomolybdate framework in terms of electrostatic calculations 89. 90 For preparative purposes, high‐throughput hydrothermal experiments provide an efficient pre‐screening of a given alkali halide/Mo‐precursor system for new structural motifs 82…”

“…This led to the formation of the new (M/M′) 6 Mo 6 O 18 F 6 ⋅ n H 2 O (M=Na, Li; M′=K–Cs, NH 4 ) hexafluorohexamolybdates, which are formed within rather narrow synthetic parameter windows, for example, carefully adjusted MF/M′F‐ratios at a reaction temperature of 220 °C. All hexafluorohexamolybdates have the [Mo 6 O 18 F 6 ] 6− anion with idealised D 3 d symmetry in common (Figures 8 and 10 below) 89…”

“…Figure 8), the former sustains the packing motif of the ring moieties. These different roles of the cations are furthermore clearly reflected in their large electrostatic potential differences 89…”

A binuclear molybdenum oxyfluoride: μ-oxido-bis[(2,2′-bipyridyl)fluoridodioxidomolybdenum(VI)]

“…For oxidofluoridomolybdates and -vanadates, see: Adil et al (2010); Burkholder & Zubieta (2004); Jones et al (2010); Michailovski et al (2006Michailovski et al ( , 2009 Table 1 Hydrogen-bond geometry (Å , ). (5) 3.199 (6) 120 (4) Symmetry codes: (i) x À 1; y; z; (ii) Àx; Ày þ 1; Àz þ 1; (iii) Àx; y þ 1 2 ; Àz þ 1 2 ; (iv) Àx þ 1; y þ 1 2 ; Àz þ 1 2 ; (v) Àx þ 1; Ày þ 1; Àz.…”

“…One area of oxide chemistry that has witnessed considerable activity is that of zeolitic materials, compositions forming open-framework structures consisting of metal oxide components and organic moieties acting as charge compensating cations, structure-directing agents or ligands. While the majority of these materials are simple oxides or oxyanion based, the introduction of fluoride to substitute for some oxo-groups provides a novel class of oxyfluorometalates (Adil et al, 2010;Jones, et al, 2010;Michailovski, et al, 2006 and2009;Burkholder and Zubieta, 2004).…”