The progression of strong and weak hydrogen bonds in a series of ethylenediammonium dithiocyanate derivatives--a new bonding protocol for macromolecules?

The experimental charge densities for a series of sym-N-methyl-substituted ethylenediammonium dithiocyanate salts have been investigated based on low-temperature and high-resolution X-ray diffraction data. This series of organic dications provides both strong and weak hydrogen bonding networks that vary depending on the N-H : SCN(-) (donor/acceptor) ratios. The number of N-HN hydrogen bonds connected to each cation increases (linear to bifurcated) as the number of N-H donor groups increases. The bifurcated thiocyanate anions also form a less energetic N-HS hydrogen bond. The presence of more than one hydrogen bond acceptor on each thiocyanate anion results in a competition between the sulfur and nitrogen atoms in forming both strong and weak hydrogen bonds. The formation of a significant number of weak hydrogen bonds is shown to play a crucial role in stabilizing these organic ionic crystals. The progression of these organic dications (smaller to larger N-H : SCN(-) ratios) results in the weaker hydrogen bonds playing a smaller role in stabilizing the crystalline structures. In addition, the electron density along the saddle point has been shown to vary significantly from weak hydrogen bonds to van der Waals interactions. This has led to a better understanding of the progression of hydrogen bonding in the crystalline states of sym-N-methyl substituted ethylenediammonium dithiocyanate salts and provides insight into the relationship between strong and weak hydrogen bonds in organic ionic crystals.