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

Abstract

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.