The Geometry and Nature of C─I···O─N Interactions in Perfluoroiodobenzene-Pyridine N-oxide Halogen-Bonded Complexes.

The N─Oxide oxygen in the 111 C─I···⁻O─N halogen bond (XB) complexes, formed by five perfluoroiodobenzene XB donors and 32 pyridine N-oxides (PyNO) XB acceptors, exhibits three XB modes: bidentate, tridentate, and monodentate. Their C─I···O XB angles range from 148° to 180°, reflecting the iodine σ-hole's structure-guiding influence. The I···⁻O─N angles range from 87° to 152°. On the contrary, the I···⁻O─N angles have a narrower range from 107° to 125° in stronger monodentate N─I···⁻O─N XBs of N-iodoimides and PyNOs. The C─I···⁻O─N systems exhibit a larger variation in I···⁻O─N angles due to weaker XB donor perfluoroiodoaromatics forming weak I···O XBs, which allows wider access to electron-rich N-O group regions. Density Functional Theory analysis shows that I···O interactions are attractive even when the I···⁻O─N angle is ≈80°. Correlation analysis of structural parameters showed that weak I···O XBs in perfluoroiodobenzene-PyNO complexes affect the C─I bond via n(O)→σ*(C─I) donation less than the N─I bond via n(O)→σ*(N─I) donation in very strong I···O XBs of N-iodoimide-PyNO complexes. This implies that PyNOs' oxygen self-tunes its XB acceptor property, dependent on the XB donor σ-hole strength affecting the bonding denticity, geometry, and interaction energies.