The Effect of Local Crystalline Environment on Hydrogen Atom Behavior in Molecular Complexes of a Proton Sponge

Abstract

Proton behavior within the hydrogen bond (HB) networks of five molecular complexes of the proton sponge DMAN and different organic acids is investigated by single crystal neutron diffraction. The complexes form with either 2:1 (acid:DMAN) or 1:1 stoichiometric ratios and contain common structural motifs. All show proton transfer from an acid to DMAN forming a DMANH + moiety and hydrogen bonded acid dimers; complexes with halobenzoic acids have acid molecules linked by short, strong, charge-assisted HBs, while all complexes contain a short, strong, intramolecular N-H•••N HB in DMANH +. The hydrogen atom behavior within the short, strong HBs, accurately described from the neutron data, is rationalized in terms of weak interactions in the local crystal environment, with the position of the proton within both sets of short, strong HBs affected by a combination of the weak interactions in the vicinity of the HBs. A correlation is also found between the thermal motion of the bound proton in the N-H•••N HB of DMANH + and nearby oxygen atoms when they are sufficiently close to one another. This work shows that all interactions in the local environment combine to determine the behavior of protons within short, strong HBs and that by taking these interactions into account further control over the crystal structure and properties may be achievable.

Engineering Short, Strong, Charge-Assisted Hydrogen Bonds in Benzoic Acid Dimers through Cocrystallization with Proton Sponge

“…There is no evidence for temperature-dependent behavior such as proton migration. The precise behavior of the hydrogen atoms is the subject of related work utilizing neutron diffraction data, reported in an accompanying publication …”

“…The refined hydrogen atom position within the N–H···N hydrogen bond does vary from adopting a more central location to favoring one of the N atoms, but Fourier difference maps show some ambiguity in the location of this atom (see the Supporting Information) and there is no discernible trend with the position of the benzoic acid dimer above this group. Neutron diffraction, where the hydrogen atom positions can be unambiguously identified and refined, is, therefore, necessary to investigate the nature of this weak interaction further and is the subject of a related paper . As a consequence of the positioning of the benzoic acid dimer above the DMANH + molecule, weak C–H···O hydrogen bonds are formed from the four methyl groups of the DMANH + molecules to the oxygen of the dimer (Figure ).…”

“…The ambiguity of hydrogen atom positions derived from X-ray diffraction prevents any conclusions on this to be drawn in this study. This is the subject of a related paper utilizing neutron diffraction for its ability to resolve accurately hydrogen atom positions …”

Tuning charge-assisted and weak hydrogen bonds in molecular complexes of the proton sponge DMAN by acid co-former substitution

“…9,10 Such complexes studied previously include DMAN in combination with benzene-1,2,4,5tetracarboxylic acid, 4,5-di-chlorophthalic acid, 11 maleic acid, 12 benzene-1,2,3-tricarboxylic acid, 13 chloranilic acid, 14 1,2-dichloromaleic acid 15 and a range of halo benzoic acids. 16,17 In these complexes, DMAN typically extracts a proton from the organic acid inducing the formation of O-H⋯O − charge-assisted hydrogen bonds (CAHBs) between acid molecules. 11,15,16,18 The extracted proton is held by the DMAN molecule in an asymmetric [19][20][21][22] intramolecular [N-H⋯N] + hydrogen bond (IHB).…”

“…Finding ways to influence the formation of intermolecular interactions represents progress in materials design allowing structural features to be predictably accessed. Importantly, where structure affects property, the tuning of intermolecular interactions can have implications in host-guest chemistry 31 and accessing proton transfer behaviour; 17,38,39 the latter offers the potential for colour change 40,41 or ferroelectric 42,43 properties in a material.…”