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

Complexes of the Lewis base-free cations ( Me BDI)Mg + and ( t Bu BDI)Mg + with Ph-X ligands (X = F, Cl, Br, I) have been studied ( Me BDI = HC[C(Me)N-DIPP] 2 and t Bu BDI = HC[C( t Bu)N-DIPP] 2 ; DIPP = 2,6-diisopropylphenyl). For the smaller β-diketiminate ligand ( Me BDI) only complexes with PhF could be isolated. Heavier Ph-X ligands could not compete with bonding of Mg to the weakly coordinating anion B(C 6 F 5 ) 4 − . For the cations with the bulkier t Bu BDI ligand, the full series of halobenzene complexes was structurally characterized. Crystal structures show that the Mg X-Ph angle strongly decreases with the size of X: F 139.1°, Cl 101.4°, Br 97.7°, I 95.1°. This trend, which is supported by DFT calculations, can be explained with the σ-hole which increases from F to I. Charge calculation and Atoms-In-Molecules analyses show that Mg F-Ph bonding originates from electrostatic attraction between Mg 2+ and the very polar C δ + -F δ − 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. Complexes of a highly Lewis acidic Mg cation and the full series of Ph-X (X = F, Cl, Br, I) have been structurally characterized. The Mg X-Ph angle decreases with halogen size on account of the growing halogen σ-hole.