One‐Step Synthesis and Schlenk‐Type Equilibrium of Cyclopentadienylmagnesium Bromides

Abstract 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( i Pr) 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.