Synthesis, structure and reactivity of a terminal magnesium fluoride compound, [TpBut,Me] MgF: hydrogen bonding, halogen bonding and C-F bond formation

The bulky tris(3-tert-butyl-5-pyrazolyl) hydroborato ligand, [Tp(But,Me)], has been employed to obtain the first structurally characterized example of a molecular magnesium compound that features a terminal fluoride ligand, namely [Tp(But,Me)]MgF, via the reaction of [Tp(But,Me)]MgMe with Me3SnF. The chloride, bromide and iodide complexes, [Tp(But,Me)]MgX (X = Cl, Br, I), can also be obtained by an analogous method using Me3SnX. The molecular structures of the complete series of halide derivatives, [Tp(But,Me)]MgX (X = F, Cl, Br, I) have been determined by X-ray diffraction. In each case, the Mg-X bond lengths are shorter than the sum of the covalent radii, thereby indicating that there is a significant ionic component to the bonding, in agreement with density functional theory calculations. The fluoride ligand of [Tp(But,Me)]MgF undergoes halide exchange with Me3SiX (X = Cl, Br, I) to afford [Tp(But,Me)] MgX and Me3SiF. The other halide derivatives [Tp(But,Me)]MgX undergo similar exchange reactions, but the thermodynamic driving forces are much smaller than those involving fluoride transfer, a manifestation of the often discussed silaphilicity of fluorine. In accord with the highly polarized Mg-F bond, the fluoride ligand of [Tp(But,Me)]MgF is capable of serving as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. [Tp(But,Me)]MgF also reacts with Ph3CCl to afford Ph3CF, thereby demonstrating that [Tp(But,Me)]MgF may be used to form C-F bonds.