One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides |
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| Authors: | Philipp Schüler Dr Helmar Görls Dr Sven Krieck Prof Dr Matthias Westerhausen |
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| Institution: | Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstraße 8, 07743 Jena, Germany |
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| Abstract: | In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes C5H6 (HCp), C5H5-Si(iPr)3 (HCpTIPS)] in diethyl ether smoothly yields heteroleptic (Et2O)Mg(CpR)(μ-Br)]2 (CpR=Cp ( 1 ), CpTIPS ( 2 )). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic Mg(CpR)2] ( 3 ) and (Et2O)MgBr(μ-Br)]2 ( 4 ). Interfering solvation and aggregation as well as ligand redistribution equilibria hamper a quantitative elucidation of thermodynamic data for the Schlenk equilibrium of 2 in toluene. In ethereal solvents, mononuclear species (Et2O)2Mg(CpTIPS)Br] ( 2’ ), (Et2O)nMg(CpTIPS)2] ( 3’ ), and (Et2O)2MgBr2] ( 4’ ) coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of (thp)4MgBr2] ( 5 ). The interpretation of the temperature-dependency of the Schlenk equilibrium constant in diethyl ether gives a reaction enthalpy ΔH and reaction entropy ΔS of −11.5 kJ mol−1 and 60 J mol−1, respectively. |
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| Keywords: | cyclopentadienylmagnesium bromides Grignard reagents magnesocene organomagnesium complexes Schlenk equilibrium |
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