N-Iodophthalimide as a halogen bond donor, a comparison with N-iodosuccinimide and N-iodosaccharin†

We investigate here the ability of N-iodophthalimide (NIPht) to act as a halogen bond (HaB) donor, in comparison with well-known HaB donors N-iodosuccinimide (NISucc) and N-iodosaccharin (NISacc). The structure of NIPht itself is reported, together with those of neutral adducts with 4-dimethylaminopyridine (DMAP), 4,4′-bipyridine and 2,2′-bipyridine derivatives. Comparison with analogous adducts involving NISucc and NISacc shows that NIPht behaves essentially like NISucc as a HaB donor, both forming weaker adducts than NISacc with a given Lewis base. A symmetric anionic complex [NPht–I–NPht]⁻ is isolated in the presence of [K(18-crown-6)]⁺. It exhibits N–I distances very close to those observed in the known [NSucc–I–NSucc]⁻ and [NSacc–I–NSacc]⁻ species (2.24–2.26 Å), confirming the 3-center–4-electron (3c–4e) character of the bonding in these species. This similarity confirms the peculiar character of the only other reported salt of [NPht–I–NPht]⁻, namely [Me₄N][NPht–I–NPht], where the longer N–I distances (2.29 Å) are a consequence of a specific solid-state arrangement and C–H⋯O hydrogen bonds.