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

Abstract

International audienceThe 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 ?(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 syste

NX⋯Y halogen bonds. Comparison with NH⋯Y H-bonds and CX⋯Y halogen bonds

“…The XB angles in the final column of Table 7 are all close to linearity, both experimental and computed. The results of Makhotkina et al 48 offer additional support for our calculated finding that I-saccharin forms a stronger I-bond with pyridine than does I-succinimide. The results of Makhotkina et al 48 offer additional support for our calculated finding that I-saccharin forms a stronger I-bond with pyridine than does I-succinimide.…”

“…Puttreddy et al 50 reported the solid state geometrical parameters for complexes of I-succinimide and I-saccharin with pyridine-N-oxide, and Makhotkina et al 48 reported analogous quantities for pyridine. Puttreddy et al 50 reported the solid state geometrical parameters for complexes of I-succinimide and I-saccharin with pyridine-N-oxide, and Makhotkina et al 48 reported analogous quantities for pyridine.…”

“…[38][39][40] Most of this data is structural in nature and derives from crystal studies, as recently summarized by Troff et al 41 The N-X XB bond in halosuccinimides [42][43][44] shows up as a short intermolecular contact. 47 Halosaccharins have also been observed to engage in XBs, 48 including with water and pyridine as halogen acceptor. 47 Halosaccharins have also been observed to engage in XBs, 48 including with water and pyridine as halogen acceptor.…”

“…[41][42][43][44][45][46][47][48][49][50] As described below, a wide range of electron donor molecules is considered, including both lone pair and p-donors, and molecules of varying donor ability. We take as a starting point systems where there is available a significant amount of experimental data to serve as a check on the validity of the calculations.…”

Modeling halogen bonding with planewave density functional theory: Accuracy and challenges

“…Beyond isolated dimers, XB‐assembled crystalline systems have received strong interest in the applications of crystal engineering, photoluminescence, and porous materials. Theoretical investigations using planewave DFT periodic calculations on halogen‐bonded crystals have been reported recently . It is therefore pertinent to study a few reported periodic systems to test the transferability of our dimeric benchmarking results.…”