Mg₆F₂(OMe)₁₀(MeOH)₁₄—An Alkoxide Fluoride of an Alkaline Earth Metal

Abstract

Sol-gel synthesis, which was developed for metal oxides, can also be applied to the synthesis of nanoscopic metal fluorides. By using anhydrous hydrogen fluoride (aHF) in a suitable organic solvent, the metal-alkoxide bond undergoes fluorolysis instead of hydrolysis. [1,2] Metal fluorides with remarkably high surface areas (hs-MF x ) are thereby formed. Unlike those obtained by conventional methods, nanoscopic metal fluorides are extremely strong Lewis acids and are of interest for heterogeneous catalysis. Furthermore, the ease of preparation of stable metal fluoride colloidal solutions (sols) opens the way to new applications, such as optical or protective corrosion-resistant coatings. Owing to the highly distorted structures of the nanoscopic phases formed, insight into the mechanism of the process was only available by theoretical calculations and MAS-NMR investigations.[3] We have isolated a monofluorinated intermediate from the aHF-Al-(OiPr) 3 -system, [4] which thus confirmed the theoretical predictions.[3] During our investigations into the synthesis of hsMgF 2[5] from the reaction of Mg(OCH 3 ) 2 with alcoholic aHF solution, it was possible to obtain an intermediate which was identified as a magnesium alkoxide fluoride. Herein we report the selective synthesis of the first alkoxide fluoride of an alkaline earth metal and its double-cubane structure with unusual bridging fluoride ligands.[ À anions by pairs of hydrogen bonds (Figure 1). The cationic complex contains two equivalent trinuclear fragments which are fused together by a dimetalladifluoro four-membered ring (Mg1F) 2 . The MgÀF bonds are 213.8 (1) 3À . [7] The bridging arrangement of the metal atoms in 1 has been found in the structure of the calcium ethoxide [Ca 6 O 2 (OEt) 8 ]·14 EtOH, but in that case, the bridging is with m 4 oxygen atoms instead of fluorine atoms.[8] It is worth noting that both the Mg2 and Mg3 cubane fragments in the hexanuclear magnesium cage of compound 1 stem from the cubane structure of the reactant. Indeed, if no HF is Figure 1. The structure of 1 with a hexanuclear dication and two anionic MeO···HOMe units connected to the cation by a total of four hydrogen bonds. The methyl hydrogen atoms are omitted for clarity.

Synthesis, structure and reactivity of a terminal magnesium fluoride compound, [TpBut,Me]MgF: hydrogen bonding, halogen bonding and C–F bond formation

“…As noted above, there are no similar compounds listed in the CSD, with other magnesium fluoride derivatives exhibiting various types of bridging interactions, which include μ 2 -, 24 – 26 μ 3 -, 27 and μ 4 -modes. 28 , 29 Terminal Mg–F moieties have, nevertheless, been structurally characterized in protein structures. 11a , d , e , h , i…”

“… 27 Correspondingly, magnesium fluoride compounds that feature μ 4 -bridges exhibit even longer Mg–F bonds that range from 2.12 Å to 2.21 Å. 28 , 32 , 33 In addition to the Mg–F bond of [Tp Bu t ,Me ]MgF being shorter than other Mg–F bonds, it is also amongst the shortest Mg–X (X ≠ H) bonds listed in the CSD, as illustrated by the magnesium oxide and alkoxide complexes, [{(THF)[BDI Ar ]Mg} 2 (μ-O)] [1.8080(5) Å], 34 [MesC{(C 4 N)Mes} 2 ]Mg(OBu t )(THF) [1.804(2) Å], 35 and [(ArO)Mg(μ-OAr) 2 ] 2 Mg (Ar = C 6 H 3 Pr i 2 ) [1.785(2) Å and 1.790(2) Å]. 36 …”

Tailor‐Made MgF2‐Based Catalysts by Sol–Gel Synthesis

“…Although the reaction of a stoichiometric amount HF with Mg(OCH 3 ) 2 yields nearly X‐ray‐amorphous material,48 the reaction [Equation (4)] was used for the synthesis of the first single‐crystal structure of an alkoxide fluoride alkaline earth metal compound 49. The partial fluorolysis of magnesium methoxide (reaction mixture with an F/Mg ratio ranging from 0.2 to 0.5) transforms the cubane structure units of crystalline magnesium methoxide into hexanuclear dicubane units containing μ 4 ‐fluorine atoms.…”

Alkali and alkaline-earth metals

“…The hexanuclear, cationic dicubane unit was bridged by unusual m 4 -F atoms and hydrogen-bonded to two methoxide anions, resulting in a triple-ionic structure. 157 Two coordination polymers constructed from magnesium and the tetratopic organic linker 2,5-dihydroxyterephthalic acid have been reported. 158…”