Magnesium-halobenzene bonding: mapping the halogen sigma-hole with a Lewis-acidic complex

Complexes of the Lewis base-free cations ((BDI)-B-Me)Mg+ and ((BDI)-B-tBu)Mg+ with Ph-X ligands (X = F, Cl, Br, I) have been studied ((BDI)-B-Me = HC[C(Me)N-DIPP](2) and (BDI)-B-tBu = HC[C(tBu)N-DIPP](2); DIPP = 2,6-diisopropylphenyl). For the smaller beta-diketiminate ligand ((BDI)-B-Me) only complexes with PhF could be isolated. Heavier Ph-X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C6F5)(4)(-). For the cations with the bulkier (BDI)-B-tBu ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the MgMIDLINE HORIZONTAL ELLIPSISX-Ph angle strongly decreases with the size of X: F 139.1 degrees, Cl 101.4 degrees, Br 97.7 degrees, I 95.1 degrees. This trend, which is supported by DFT calculations, can be explained with the sigma-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that MgMIDLINE HORIZONTAL ELLIPSISF-Ph bonding originates from electrostatic attraction between Mg2+ and the very polar C delta+-F delta- bond. For the heavier halobenzenes, polarization of the halogen atom becomes increasingly important (Cl < Br < I). Complexation with Mg leads in all cases to significant Ph-X bond activation and elongation. This unusual coordination of halogenated species to early main group metals is therefore relevant to C-X bond breaking.

Automated summary

Complexes of Lewis base-free cations with Ph-X ligands were studied, showing that smaller ligands only form complexes with PhF while heavier ligands cannot compete with weakly coordinating anions, leading to Ph-X bond activation and elongation. The coordination of halogenated species to early main group metals is relevant to C-X bond breaking, and the Mg-X-Ph angle decreases with the size of X due to increasing sigma-hole from F to I.