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

The bulky tris(3--butyl-5-pyrazolyl)hydroborato ligand, [Tp], has been employed to obtain the first structurally characterized example of a molecular magnesium compound that features a terminal fluoride ligand, namely [Tp]MgF, the reaction of [Tp]MgMe with MeSnF. The chloride, bromide and iodide complexes, [Tp]MgX (X = Cl, Br, I), can also be obtained by an analogous method using MeSnX. The molecular structures of the complete series of halide derivatives, [Tp]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]MgF undergoes halide exchange with MeSiX (X = Cl, Br, I) to afford [Tp]MgX and MeSiF. The other halide derivatives [Tp]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]MgF is capable of serving as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and CFI. [Tp]MgF also reacts with PhCCl to afford PhCF, thereby demonstrating that [Tp]MgF may be used to form C-F bonds.