Fine-tuning halogen bonding properties of diiodine through halogen–halogen charge transfer – extended [Ru(2,2′-bipyridine)(CO)2X2]·I2 systems (X = Cl, Br, I)

Abstract

The current paper introduces the use of carbonyl containing ruthenium complexes, [Ru(bpy)(CO)₂X₂] (X = Cl, Br, I), as halogen bond acceptors for a I₂ halogen bond donor. In all structures, the metal coordinated halogenido ligand acts as the actual halogen bond acceptor. Diiodine, I₂, molecules are connected to the metal complexes through both ends of the molecule forming bridges between the complexes. Due to the charge transfer from Ru–X to I₂, formation of the first Ru–X⋯I₂ contact tends to generate a negative charge on I₂ and redistribute the electron density anisotropically. If the initial Ru–X⋯I_A–I_B interaction causes a notable change in the electron density of I₂, the increased negative charge accumulates on the second iodine, I_B. The increased negative charge on I_B reduces its ability to act as a halogen bond donor. With the [Ru(bpy)(CO)₂Cl₂] complex, the electron density of the I₂ molecule remains isotropic and it acts as a symmetrical halogen bond donor for two metal complexes. With [Ru(bpy)(CO)₂Br₂] and [Ru(bpy)(CO)₂I₂], the Ru–X⋯I_A–I_B⋯X–Ru bridges are unsymmetrical with a stronger and shorter Ru–X⋯I_A contact and a weaker and longer Ru–X⋯I_B contact. In these cases, the negative charge is accumulated on the more weakly bonded I_B atoms. QTAIM calculations were used to analyze the strength of the interactions and charge distribution in the metal complex and I₂ molecule in detail. In accordance with the experimental data, the QTAIM analyses show that the charge difference between the two ends of the I₂ molecule is increased in the order Ru–Cl⋯I₂ < Ru–Br⋯I₂ < Ru–I⋯I₂.