Cocrystal or Salt: Solid State-Controlled Iodine Shift in Crystalline Halogen-Bonded Systems

The distinction between cocrystals
and salts is usually investigated
in hydrogen-bonded systems as A–H···B ⇆
[A]−···[H–B]+,
where the position of the hydrogen atom actually defines the ionicity
of the complex. The same distinction, but in halogen-bonded systems,
is addressed here, in complexes formed out of N-iodoimide derivatives
as halogen bond donors, and pyridines as halogen-bond acceptors, anticipating
that the position of the iodine atom in these A–I···B
⇆ [A]−···[I–B]+ systems will also define their degree of ionicity. We show
that the crystalline halogen-bonded complexes of N-iodosuccinimide
(NIS) with pyridine, 4-methylpyridine, and 4-dimethylaminopyridine
can be described as “close-to-neutral” cocrystals while
the crystalline halogen-bonded complex of N-iodosaccharin (NISac)
with 4-dimethylaminopyridine adopts a “close-to-ionic”
structure. Theoretical calculations were performed (i) in gas phase
on isolated NIS···Py-NMe2 and NISac···Py-NMe2 complexes, and (ii) on the periodic crystal phases, and combined
with the topological analysis of the electron density distribution
ρ­(r). We demonstrate unambiguously that the crystal
environment actually plays a crucial role in the stabilization of
the “close-to-ionic” structure of the NISac···Py-NMe2 complex. An external homogeneous electric field ε applied
to this complex (all atoms frozen at the crystalline geometry, except
iodine) in either gas phase (ε = 3.7 GV m–1) or periodic pseudo-isolated configuration (ε = 2.8 GV m–1) is able to shift the iodine atom at the crystal
geometry, miming the polarization effect induced by the local crystal
electric field. The strong influence of the crystalline environment
on the iodine position is demonstrated by using plane wave DFT periodic
calculations on optimized NIS·Py-NMe2 and NISac·Py-NMe2 crystal structures, as well as by applying this plane wave
basis set formalism to a hypothetical solid where the halogen-bonded
complexes are pushed apart from each other within a periodic system.