Ortho‐Substituent Effects on Halogen Bond Geometry for N‐Haloimide⋯2‐Substituted Pyridine Complexes

Abstract

The nature of (imide)N–X⋯N(pyridine) halogen‐bonded complexes formed by six N‐haloimides and sixteen 2‐substituted pyridines are studied using X‐ray crystallography (68 crystal structures), Density Functional Theory (DFT) (86 complexation energies), and NMR spectroscopy (90 association constants). Strong halogen bond (XB) donors such as N‐iodosuccinimide form only 1:1 haloimide:pyridine crystalline complexes, but even stronger N‐iodosaccharin forms 1:1 haloimide:pyridine and three other distinct complexes. In 1:1 haloimide:pyridine crystalline complexes, the haloimide's N─X bond exhibits an unusual bond bending feature that is larger for stronger N‐haloimides. DFT complexation energies (ΔE_XB) for iodoimide–pyridine complexes range from −44 to −99 kJ mol⁻¹, while for N‐bromoimide–pyridine, they are between −31 and −77 kJ mol⁻¹. The ΔE_XB of I⋯N XBs in 1:1 iodosaccharin:pyridine complexes are the largest of their kind, but they are substantially smaller than those in [bis(saccharinato)iodine(I)]pyridinium salts (−576 kJ mol⁻¹), formed by N‐iodosaccharin and pyridines. The NMR association constants and ΔE_XB energies of 1:1 haloimide:pyridine complexes do not correlate as these complexes in solution are heavily influenced by secondary interactions, which DFT studies do not account for. Association constants follow the σ‐hole strengths of N‐haloimides, which agree with DFT and crystallography data. The haloimide:2‐(N,N‐dimethylamino)pyridine complex undergoes a halogenation reaction resulting in 5‐iodo‐2‐dimethylaminopyridine.