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 [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.