Magnesiacycloalkanes with Different Ring Sizes

Di-Grignard compounds are prepared via the direct synthesis of magnesium with Br2C­{Si­(CH3)3}2, α,ω-dihalides of XCH2YCH2X (X: Cl, Br; Y: Si­(CH3)2, (CH2)n with n = 4, 5, 6), and with ClC­(CH3)2(CH2)n(CH3)2CCl (n = 3, 4) in tetrahydrofuran (THF) or diethyl ether. After removal of the sparingly soluble magnesium halides after addition of 1,4-dioxane (dx) the corresponding magnesium alkanediides remain in solution. Only the geminal di-Grignard complex (BrMg)2(μ-C­{Si­(CH3)3}2) forms the corresponding dioxane adduct [{(dx)2MgBr}2(μ-C­{Si­(CH3)3}2)] (1). The magnesium alkanediides can be recrystallized from THF and characterized with X-ray crystallography and NMR spectroscopy. Preferably dinuclear rings of the types [(thf)2Mg­(μ-CH2-Y-CH2)}2] (Y: Si­(CH3)2 (2), (CH2)6 (5)) and [({thf}2Mg­{μ-C­(CH3)2­(CH2)4C­(CH3)2})2] (7) crystallize. From a 1 M solution the strand-like coordination polymer [({thf}2Mg­{μ-(CH2)6})∞] (3) precipitates. Only [(thf)2Mg­{C­(CH3)2­(CH2)3(CH3)2C}] (6) crystallizes as a mononuclear compound. The NMR spectra of 2, 3, 5, and 6 show only one set of resonances for the alkanediyl moieties in [D8]­THF solutions. The NMR spectra of 7 showed also at 50 °C two signal sets with the intensity ratios depending on the measurement temperature. Complex 7 shows a temperature-dependent monomer–dimer equilibrium in THF solution.