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 III / Ga III ,S i IV /Ge IV /Sn II ;Sn IV and P V /As V /Sb V ) could be envisaged as centers in extended fluoride-bridged structures. The studied systems are all based on the 3dF unit where either Cr III or Mn III coordinates a minimum of one fluorido ligand. The 3dF 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 (3dFns) or lanthanides (3dF4f) were synthesized and characterized structurally and magnetically. A new synthetic route to systems with unsupported, bridging fluorido ligands is established by using kinetically robust Cr III fluorido complexes such as trans-[Cr(py)4F2] + , cis-[Cr(phen)2F2] + , cis-[Cr(bpy)2F2] + and cis-[Cr(phen)2(H2O)(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 (3dFns, 3dF3d and 3dF4f). 3dFns systems: Alkali metal ions largely interact with the robust Cr III –fluoride synthon through second sphere coordination in solution. This interaction may, by suitable choice of a first coordination