One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides.

In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes [C H (HCp), C H -Si(iPr) (HCp )] in diethyl ether smoothly yields heteroleptic [(Et O)Mg(Cp )(μ-Br)] (Cp =Cp (1), Cp (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(Cp ) ] (3) and [(Et O)MgBr(μ-Br)] (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 [(Et O) Mg(Cp )Br] (2'), [(Et O) Mg(Cp ) ] (3'), and [(Et O) MgBr ] (4') coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp) MgBr ] (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 and 60 J mol , respectively.