Halogen Bond-Driven Ligand Displacement: Co-Crystal Lattice Versus Coordination Bonds

Coordination bonds are generally stronger than halogen bonds; however, the Jahn–Teller effect in d⁹ Cu(II) and the trans influence of the oxo-ligand in vanadyl (V═O) acetylacetonates can weaken N→Cu/V bonds, bringing them closer to the upper range of halogen bond strength. The study investigates the interactions between transition metal acetylacetonate complexes, M(acac)₂(L) (M─Cu(II), V(IV) = O; L = amine ligands), and halogen bond (XB)-donor co-formers, particularly 1,4-diiodotetrafluorobenzene (1,4-DITFB). The co-crystallization experiments reveal an unusual ligand displacement phenomenon wherein the expected M(acac)₂(L)·1,4-DITFB complexes fail to form, instead yielding separate M(acac)₂·1,4-DITFB and L·1,4-DITFB co-crystals. Computational studies reveal that while XB interactions alone may be insufficient to disrupt the M─N coordination bond, they can induce ligand displacement when amplified by the lattice stabilization of the resulting halogen-bonded co-crystals, particularly in Jahn–Teller distorted d⁹ Cu(II) and trans-influenced V(IV) = O complexes interacting with halogen bond donors.