Fluoride as Ligand: Chemistry of Some New Terminal and Bridged Systems

Abstract

A comprehensive and unifying theme in this thesis has been the synthesis and characterization of new coordination polymers and metal-organic networks. The common structural organizing element for these systems is the fluoride ion, which, according to the Pearson classification, is a "hard" or "class a" ligand. Accordingly, it is expected that fluoride would be best suited as a bridging ligand in systems with correspondingly "hard" metal ions. The regions of the periodic table of primary relevance would thus be the early transition metals, but also main-group elements from group I and II (as well as Al The studied systems are all based on the 3dÀF unit where either Cr III or Mn III coordinates a minimum of one fluorido ligand. The 3dÀF unit has been studied in proper mononuclear systems but also in polynuclear or polymeric systems connected through unsupported bridging by fluorido ligand(s). This bridging is of both homo-and, for chromium also, heterometallic character. For the latter type of systems, completely novel classes of compounds containing either alkali metals (3dÀFÀns) or lanthanides (3dÀFÀ4f) were synthesized and characterized structurally and magnetically.A + and cis-[Cr(phen) 2 (H 2 O)(F)] 2 + (py = pyridine, phen = 1,10-phenantroline, bpy = 2,2'-bipyridine) as precursors. These metal-containing building blocks (or ligands) can be viewed as synthons for the fluoride-containing part of the final complex. The fluorido ligands in these robust synthons are fixed with respect to configuration, and due to the robustness of the synthons undesirable ligand substitution is avoided. This is particularly important as it prevents or retards metathesis reactions catalyzed by reaction partners, including precipitation reactions involving relatively insoluble simple fluorides. The stereochemical control of the disposition of the fluorido ligands in the starting materials provides the opportunity for controlling the structures of the resulting polynuclear or polymeric products. This fairly rare situation of stereochemical control is contingent on the preferential linear bridging by fluoride, which is a recurrent motif in the systems studied here and in agreement with established chemistry of fluoride as a bridging ligand. The applicability and generality of the method is exemplified by reaction with hard metal ions from different parts of the periodic table. A graphical overview of the investigated systems based on the synthetic route to systems with unsupported, bridging fluorido ligands is given in Scheme 1.The following sections discuss the overall results of the synthetic route to systems with unsupported, bridging fluorido ligands (3dÀFÀns, 3dÀFÀ3d and 3dÀFÀ4f).

Br2F7− and Br3F10−: peculiar anions showing μ2- and μ3-bridging F-atoms

“…In fluorine-containing molecules and molecular ions, the fluorine atom clearly prefers terminal positions, although many coordination compounds incorporate fluorine atoms as bridging ligands. 1 The most common type of F-bridging is the m 2 -connection (either straight mono(m-F), or bent di(m-F)). [2][3][4][5][6][7][8][9][10][11][12][13] The highly strained tri(m-F)-bridges between various elements 1 are scarce with only some examples in the literature.…”

“…1 The most common type of F-bridging is the m 2 -connection (either straight mono(m-F), or bent di(m-F)). [2][3][4][5][6][7][8][9][10][11][12][13] The highly strained tri(m-F)-bridges between various elements 1 are scarce with only some examples in the literature. [2][3][4][5][6][7][8][9][10][11][12][13] Bridges among three atoms (m 3 -F) are even more rare; [14][15][16][17][18][19][20][21] however, m 4 -F 22,23 and even cage-like m 6 -F 22,24,25 coordination types are known.…”

“…However, single crystal X-ray diffraction analyses, vibrational spectroscopy, as well as quantum chemical calculations showed that the Br 3 F 10 À anions are not chain-like but contain m 3 -bridging F atoms. Compounds 1, 2 and 3 were synthesized using stoichiometric amounts of MCl (M = Rb, Cs) and BrF 3 , according to eqn (1), which was previously applied in the synthesis of RbBrF 4 : 38 6MCl + (6n + 2)BrF 3 -6MBr n F (3n+1) + Br 2 + 3Cl 2 (1) RbBr 3 F 10 and CsBr 3 F 10 were obtained as yellowish-colored crystalline solids. Since both compounds are structurally isotypic, we present a detailed structural description only for compound 1 (Rb).…”

“…134.75 (9) 140.27 (6) 130.08(4) F(5)-M(1)-F (1) 175.17 (8) 176.18 (6) 176.0(6) F(7)-M(2)-F (1) 176.04 (8) 176.44 (7) 176.0(6) Tilting angle between F 3 M-m-F-MF 3 CsAu 2 F 7 (which is similar in terms of the anion geometry but is not structurally isotypic) 40 are shown in Table 3. In comparison to the Br 3 F 10 À anions, a shortening of the Br-m-F-Br bridges of circa 0.1 to 0.2 Å is observed which may be due to smaller Coulomb repulsion between the two Br atoms.…”

A tetranuclear fluorido-bridged iron compound: Fluoride abstraction from the tetrafluoridoborate anion

“…The synthesis of transition metal fluoride compounds has received considerable attention in the last decades because of their special structural characteristics and potential application in the field of magnetic materials [1][2][3]. Two main strategies have been employed to synthesize metal fluoride compounds: the first one is based on the addition of fluoride salts like NaF or AsF3 as fluorinating agents; the other one is based on the use of metal tetrafluoridoborate as starting salts, which generate fluoride ions upon decomposition.…”