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(5)H(6) (HCp), C(5)H(5)‐Si(iPr)(3) (HCp(TIPS))] in diethyl ether smoothly yields heteroleptic [(Et(2)O)Mg(Cp(R))(μ‐Br)](2) (Cp(R)=Cp (1), Cp(TIPS) (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(Cp(R))(2)] (3) and [(Et(2)O)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 [(Et(2)O)(2)Mg(Cp(TIPS))Br] (2’), [(Et(2)O)(n)Mg(Cp(TIPS))(2)] (3’), and [(Et(2)O)(2)MgBr(2)] (4’) coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp)(4)MgBr(2)] (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.