Hydrogen Bonding Networks and Solid-State Conversions in Benzamidinium Salts

Ten
benzamidinium salts of carboxylic acids, amides, and sulfonamides
have been crystallized from solution. Single-crystal X-ray analyses
revealed various hydrogen bonding motifs which are discussed in terms
of supramolecular synthons and graph sets. Benzamidinium hydrogen
maleate (5a) crystallizes as large needles of up to >3
cm in length. Attempts to influence the crystal habit and size through
a change of solvent and the presence of additives yielded a second
polymorph (5b). The formation of the benzamidinium salts
by mechanochemical reaction was also investigated. Grinding of benzamidine
with nicotinic acid, salicylic acid, p-aminobenzoic
acid, cyanuric acid, pimelic acid, saccharin, and sulfathiazole with
mortar and pestle or using a ball-mill gave compounds identical to
those obtained by crystallization from solution. Time-dependent X-ray
powder patterns of a stoichiometric benzamidine/cyanuric acid mixture
suggested that the mechanochemical salt formation occurred via the
amorphous state. Ball-milling of benzamidine with sulfamerazine generated
amorphous benzamidinium sulfamerazinate that was stable toward crystallization
for at least 2 weeks, when stored at 25% relative humidity.