Halogen bonding between metal-bound I3− and unbound I2: the trapped I2⋯I3− intermediate in the controlled assembly of copper(i)-based polyiodides

Abstract

Crystallization of [CuI(CNXyl)₃] (1) with I₂ (exhibiting strong halogen bond donor properties), at different molar ratios between the reactants, resulted in a series of (XylNC)Cu^I crystal polyiodides formed along with gradual accumulation of iodine, namely [Cu(I₃)(CNXyl)₃] (two crystalline polymorphs 2^I and 2^II), [Cu(I₃)(CNXyl)₃]·½I₂ (2·½I₂) and [Cu(CNXyl)₃](I₅) (3); all these compounds were studied by X-ray diffractometry. Molecular electrostatic potential (MEP) surface plots were also calculated using density functional theory (DFT) for isolated molecules of 2 and I₂, showing electrophilic and nucleophilic sites. Halogen bonding in 2·½I₂ was additionally elucidated for both crystal and cluster models, including combined quantum theory of atoms-in-molecules (QTAIM) and one-electron potential (OEP) projections. For model clusters, DFT energetic analysis, quantum theory of atoms-in-molecules, combined with the noncovalent interaction index plot (QTAIM/NCIplot), natural bond orbital (NBO) donor–acceptor charge transfer analysis, and Wiberg bond index (WBI) analysis were used. In the structure of 2·½I₂, the presence of an I₂⋯I₃⁻ halogen bonded linkage gives a key toward the understanding of the precise mechanism for the generation of I₅⁻ (and then I₈²⁻) ligands from I₂ and metal-coordinated I₃⁻.