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Potassium and Well-Defined Neutral and Cationic Calcium Fluoroalkoxide Complexes: Structural Features and Reactivity

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posted on 27.10.2014, 00:00 authored by Sorin-Claudiu Roşca, Thierry Roisnel, Vincent Dorcet, Jean-François Carpentier, Yann Sarazin
The fluorinated aminoether alcohols (1-aza-12-crown-4)­CH2C­(CF3)2OH ({RO1F}­H), (MeOCH2CH2)2NCH2C­(CF3)2OH ({RO2F}­H), and (MeOCH2CH2)­(Me)­NCH2C­(CF3)2OH ({RO3F}­H) have been synthesized and used to prepare the heteroleptic calcium amido complexes {ROxF}­Ca­(N­(SiMe2R)2) (47; x = 2, 3, R = H, Me). The ability to form stable complexes varies with the chelating and electron-donating ability of the aminoether alkoxide ligand, as exemplified by our failure to isolate cleanly the elusive {RO1F}­Ca­(N­(SiMe2R)2). X-ray diffraction studies show that, in the solid state, intramolecular Ca···F interactions help reach coordinative saturation in the dimeric [{RO2F}­Ca­(N­(SiMe3)2)]2 ([4]2), [{RO2F}­Ca­(N­(SiMe2H)2)]2 ([5]2), [{RO3F}­Ca­(N­(SiMe3)2)]2 ([6]2), and [{RO3F}­Ca­(N­(SiMe2H)2)]2 ([7]2), which crystallized free of solvent coligands. Similar stabilizing K···F patterns were found in the polymetallic potassium fluoroalkoxides [{RO1F}­K]2 ([1]2), [{RO2F}­K]4 ([2]4), and [{RO3F}­K]4 ([3]4); [2]4 and [3]4 form heterocubanes in the solid state. Examination of the XRD data for [1]2[7]2 shows that metal···F interactions can be favored over binding of Oether atoms for calcium and potassium. Pulse-gradient spin–echo NMR spectroscopy shows that the complexes remain aggregated in aromatic solvents. The solvent-free salts [{ROxF}­Ca+]·[H2N­{B­(C6F5)3}2] (x = 1 (8), 2 (9), 3 (10)) are obtained by treating 47 with [H­(OEt2)2+]·[H2N­{B­(C6F5)3}2] or by reacting Ca­(N­(SiMe3)2)2 with [{ROxF}­HH+]·[H2N­{B­(C6F5)3}2]; the solid-state structures of [8·H2O]2 and [9·H2O]2 again showed the presence of Ca···F contacts. Complexes 57 are promising catalysts for the regiospecific anti-Markovnikov hydrophosphination of styrene with diphenylphosphine, affording TOF values as high as 52 molsubst molCa–1 h–1 with up to 400 equiv of substrates within 1–2 h at 60 °C.