Conjugated, trans-Spanning Ligands as Models for Multivalent p-Phenyleneethynylenes

A conjugated, pyridine-containing, phenylethynyl ligand that forms complexes with AgI and PdII has been developed. NMR titration studies with PdII reveal a stoichiometric binding of the ligand to the metal atom, while similar studies with AgI reveal a binding that is dynamic on the NMR timescale. Analysis of the NMR spectroscopic data by Job\u27s plot analysis and non-linear curve fitting of a titration curve reveals a 1:1 binding ratio of ligand/silver cation and an association constant of Ka = 53 M “1. X-ray crystal structures of the ligand “metal complexes suggest ample room for the nearly barrierless rotation of the unsubstituted central benzene ring of the para-phenylethynyl chain. Subtle electronic differences in substituted systems provide some evidence of impaired rotation

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]+ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]+ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their 15N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]+ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]+ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations