Halogen Bonding and CO-Ligand Blue-Shift in Hybrid Organic—Organometallic Cocrystals [CpFe(CO)2X] (C2I4) (X = Cl, Br)

This work is focused on the complex interplay of geometry of I⋯X halogen bonds (HaB) and intermolecular interaction energy in two isomorphic cocrystals [CpFe(CO)2X] (C2I4) (X = Cl (1), Br (2)). Their IR-spectroscopic measurements in solid state and solution demonstrate the blue-shift of CO vibration bands, resulting from I⋯X HaB. The reluctance of their iodide congener [CpFe(CO)2I] to form the expected cocrystal [CpFe(CO)2I] (C2I4) is discussed in terms of different molecular electrostatic potential (MEP) of the surface of iodide ligands, as compared with chloride and bromide, which dictate a different angular geometry of HaB around the metal-I and metal-Br/Cl HaB acceptors. This study also suggests C2I4 as a reliable HaB donor coformer for metal-halide HaB acceptors in the crystal engineering of hybrid metal–organic systems.